**Kinetic Study of the Partial Hydrogenation of 1-Heptyne over Ni and Pd Supported on Alumina**

M. Juliana Maccarrone, Gerardo C. Torres, Cecilia Lederhos, Carolina Betti, Juan M. Badano, Mónica Quiroga and Juan Yori

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/48699

### **1. Introduction**

158 Hydrogenation

Philadelphia, pp 543-549.

Catal. A: Gen. 289: 256-266.

[56] Purcell KF, Kotz JC (1977) Inorganic Chemistry, Holt-Saunders International Editions:

[57] Papp A, Molnár A, Mastalir A (2005) Catalytic investigation of Pd particles supported on MCM-41 for the selective hydrogenations of terminal and internal alkynes. Appl.

> Selective hydrogenation reactions are industrially used for the partial hydrogenation of unsaturated organic compounds in order to form more stable products or intermediate materials for different processes. The production of final organic products of high added value or intermediate compounds for the synthesis of fine chemicals is both of industrial and academic importance [1]. Alkenes are much appreciated products used in the food industry (flavours), the pharmaceutical industry (sedatives, anesthetises, vitamins) and in the perfumes industry (fragrances). They are also used for the production of biologically active compounds [2], resins, polymers and lubricants, etc.

> The partial hydrogenation of acetylenic compounds using homogeneous or heterogeneous metallic catalysts provides a very viable and economically feasible way for the obtaining of these olefinic compounds. Selective catalysts and optimum operational conditions are necessary in order to avoid the complete hydrogenation of the unsaturated bond. Certain transition metals anchored on different solids have demonstrated to be very active and selective catalysts for this type of reaction. They also have the advantage that they can be operated under milder reaction conditions. It is well documented that palladium is a highly active catalyst for hydrogenation [3]. In this sense, Lindlar catalyst (Pd/CaCO3, 5 wt % of Pd modified with Pb(OAc)2) has been used since 1952 as an excellent commercial catalyst for this type of reactions [4]. The argued reasons for the differences in reactivity of Pd indicate that when the metal is electron deficient it becomes less active because alkynes are more weakly adsorbed [5].

> During decades a lot of research has been carried out modifying this type of catalysts in order to increase the activity and selectivity: different supports as alumina, coal, silica [5-7] have been tried while modified palladium [8], or nanoparticles of Pd have also been

© 2012 Maccarrone et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Maccarrone et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

investigated [9-11]. In a recent work of our group very high activities and selectivities of W monometallic and W-Pd bimetallic catalysts were found during the partial hydrogenation of a long chain terminal alkyne [12]. In this contribution it was reported that important variations in the activity and selectivity were produced when the employed pretreatment conditions were changed. The published reports do not give a clear explanation of the effect of each metal.

Kinetic Study of the Partial Hydrogenation of 1-Heptyne over Ni and Pd Supported on Alumina 161

instrument with a H2/Ar (5% v/v) flow during 10 min. The spectra were obtained using pass energy of 30 eV and a Mg anode operated at 200 W. During the tests the pressure was less

X-Ray Diffraction (XRD) measurements of the samples were obtained using a Shimadzu XD-D1 instrument with CuKα radiation (λ = 1.5405A) in the 15 < 2θ < 85° at a scan speed of 1° min−1. Samples were powdered and reduced under a hydrogen flow, then they were immediately put into the chamber of the equipment and the data acquisition was started.

The tests were performed in an Ohkura 2002 S apparatus equipped with a thermal conductivity detector. A cold water trap was placed before the thermal detector to condense water. Before the TPR tests the samples were dried *in situ* at 673 K for 30 min in an oxygen flow (AGA purity 99.99%). After that the samples were cooled down up to 298 K in an argon flow. Then the temperature was increased up to 1223 K at 10 K min-1 in a H2/Ar (5% v/v) gas

Hydrogen chemisorption was performed by means of the dynamic pulse method using a Micromeritics Auto Chem II apparatus equipped with a thermal conductivity detector. 0.2 g of the samples were reduced 1 h *in situ* at the above mentioned temperatures using a H2/Ar (5% v/v) flow. The samples were degassed *in situ* for 2 h under an argon flow (AGA, 99.99%) at a temperature equal to that of the corresponding reduction step and then cooled down to room temperature. In the case of the palladium catalysts they were cooled down to 373 K to make the formation of palladium hydride negligible [16]. After that the chemisorption of

The partial hydrogenation of 1-heptyne was carried out in a stainless steel stirred batch reactor equipped with a magnetically coupled stirrer with two blades in counter-rotation that was operated at 800 rpm. The inner wall of the reactor was completely coated with PTFE lining in order to prevent the catalytic action of steel reported by other authors [17]. The tests were performed using the following conditions: partial pressure of H2 equal to 1.4, 1.9, 2.4 bar; reaction temperature: 293, 303, 323 K; initial 1-heptyne concentration of 0.1019, 0.1528, 0.2038 mol L-1; particle size: fractions of 60 or 120 mesh and 1500 μm. 1-heptyne (Fluka, Cat. No. 51950, >98%), the reactant, was dissolved in toluene (Merck, Cat. No. TX0735-44, >99%). 75 mL of the stock solution and catalyst samples of 0.3 g of Ni/Al2O3 and 0.03 g of Pd/Al2O3 previously reduced in hydrogen for 1 h at 673 and 573K, respectively,

than 2 10-12 MPa.

flow.

*2.2.3. X-Ray Diffraction (XRD)* 

*2.2.5. Hydrogen chemisorption* 

**2.3. Catalytic tests** 

were used in the different catalytic tests.

*2.2.4. Temperature Programmed Reduction (TPR)* 

hydrogen was performed until total saturation of the samples.

Providing an economic, active and selective metallic catalyst for the production of alkenes via selective hydrogenation is a very important challenge. Besides the bibliography on the kinetics of the reaction of heavy alkynes is scarce [13-15]. In this sense, the kinetic study of the liquid phase hydrogenation reaction of 1-heptyne using monometallic nickel and palladium catalysts anchored on -alumina made in the current work had three principal objectives: i) to gain knowledge about the reaction system, ii) to obtain kinetic expressions which enable the design of partial hydrogenation reactors, and iii) to give an explanation of the catalytic effect of different metals. The kinetic modelling was used in this work to shed light on these issues.

### **2. Experimental**

### **2.1. Catalyst preparation**

The catalysts were prepared by the incipient wetness technique using Ketjen CK300 γ-Al2O3 as support (cylinders of 1.5 mm, calcined at 823K during 4 h, 180 m2 g-1 BET surface area). To study the influence of metals, acidic solutions with pH=1 were prepared using NiCl2 (Merk Cat. No. 806722), and Pd(NO3)2 (Fluka, Cat. No. 76070) to prepare the monometallic catalysts. The concentration of the solutions was adjusted in order to obtain metal loading of 4 and 0.4 wt % of Ni and Pd, respectively. The impregnated solids were dried during 24 h at 373 K and then they were calcined in an air flow at 823 K during 3 h. Prior to reaction, the catalysts were reduced in hydrogen flow (105 mL min-1) during 1 h at 673 K for Ni/Al2O3 and at 573 K for Pd/Al2O3.

### **2.2. Characterization**

### *2.2.1. Chemical analysis*

The metal loadings of the catalysts were determined by digesting the samples and then analyzing the liquors by inductively coupled plasma analysis (ICP) in an OPTIMA 2100 DV Perkin Elmer equipment.

### *2.2.2. X-Ray Photoelectron Spectroscopy (XPS)*

XPS determinations were made in Multitech UniSpecs XR-50 equipment with a dual Mg/Al X-ray source. A SPECS PHOIBOS 150 hemisphere analyzer was used in the Fixed Analyzer Transmission mode (FAT). The samples were treated *ex situ* with a hydrogen flow during 1 h at the corresponding reduction temperature and *in situ* in the load camera of the instrument with a H2/Ar (5% v/v) flow during 10 min. The spectra were obtained using pass energy of 30 eV and a Mg anode operated at 200 W. During the tests the pressure was less than 2 10-12 MPa.

### *2.2.3. X-Ray Diffraction (XRD)*

160 Hydrogenation

of each metal.

light on these issues.

**2. Experimental** 

**2.1. Catalyst preparation** 

and at 573 K for Pd/Al2O3.

**2.2. Characterization** 

*2.2.1. Chemical analysis* 

Perkin Elmer equipment.

*2.2.2. X-Ray Photoelectron Spectroscopy (XPS)* 

investigated [9-11]. In a recent work of our group very high activities and selectivities of W monometallic and W-Pd bimetallic catalysts were found during the partial hydrogenation of a long chain terminal alkyne [12]. In this contribution it was reported that important variations in the activity and selectivity were produced when the employed pretreatment conditions were changed. The published reports do not give a clear explanation of the effect

Providing an economic, active and selective metallic catalyst for the production of alkenes via selective hydrogenation is a very important challenge. Besides the bibliography on the kinetics of the reaction of heavy alkynes is scarce [13-15]. In this sense, the kinetic study of the liquid phase hydrogenation reaction of 1-heptyne using monometallic nickel and palladium catalysts anchored on -alumina made in the current work had three principal objectives: i) to gain knowledge about the reaction system, ii) to obtain kinetic expressions which enable the design of partial hydrogenation reactors, and iii) to give an explanation of the catalytic effect of different metals. The kinetic modelling was used in this work to shed

The catalysts were prepared by the incipient wetness technique using Ketjen CK300 γ-Al2O3 as support (cylinders of 1.5 mm, calcined at 823K during 4 h, 180 m2 g-1 BET surface area). To study the influence of metals, acidic solutions with pH=1 were prepared using NiCl2 (Merk Cat. No. 806722), and Pd(NO3)2 (Fluka, Cat. No. 76070) to prepare the monometallic catalysts. The concentration of the solutions was adjusted in order to obtain metal loading of 4 and 0.4 wt % of Ni and Pd, respectively. The impregnated solids were dried during 24 h at 373 K and then they were calcined in an air flow at 823 K during 3 h. Prior to reaction, the catalysts were reduced in hydrogen flow (105 mL min-1) during 1 h at 673 K for Ni/Al2O3

The metal loadings of the catalysts were determined by digesting the samples and then analyzing the liquors by inductively coupled plasma analysis (ICP) in an OPTIMA 2100 DV

XPS determinations were made in Multitech UniSpecs XR-50 equipment with a dual Mg/Al X-ray source. A SPECS PHOIBOS 150 hemisphere analyzer was used in the Fixed Analyzer Transmission mode (FAT). The samples were treated *ex situ* with a hydrogen flow during 1 h at the corresponding reduction temperature and *in situ* in the load camera of the X-Ray Diffraction (XRD) measurements of the samples were obtained using a Shimadzu XD-D1 instrument with CuKα radiation (λ = 1.5405A) in the 15 < 2θ < 85° at a scan speed of 1° min−1. Samples were powdered and reduced under a hydrogen flow, then they were immediately put into the chamber of the equipment and the data acquisition was started.

### *2.2.4. Temperature Programmed Reduction (TPR)*

The tests were performed in an Ohkura 2002 S apparatus equipped with a thermal conductivity detector. A cold water trap was placed before the thermal detector to condense water. Before the TPR tests the samples were dried *in situ* at 673 K for 30 min in an oxygen flow (AGA purity 99.99%). After that the samples were cooled down up to 298 K in an argon flow. Then the temperature was increased up to 1223 K at 10 K min-1 in a H2/Ar (5% v/v) gas flow.

### *2.2.5. Hydrogen chemisorption*

Hydrogen chemisorption was performed by means of the dynamic pulse method using a Micromeritics Auto Chem II apparatus equipped with a thermal conductivity detector. 0.2 g of the samples were reduced 1 h *in situ* at the above mentioned temperatures using a H2/Ar (5% v/v) flow. The samples were degassed *in situ* for 2 h under an argon flow (AGA, 99.99%) at a temperature equal to that of the corresponding reduction step and then cooled down to room temperature. In the case of the palladium catalysts they were cooled down to 373 K to make the formation of palladium hydride negligible [16]. After that the chemisorption of hydrogen was performed until total saturation of the samples.

### **2.3. Catalytic tests**

The partial hydrogenation of 1-heptyne was carried out in a stainless steel stirred batch reactor equipped with a magnetically coupled stirrer with two blades in counter-rotation that was operated at 800 rpm. The inner wall of the reactor was completely coated with PTFE lining in order to prevent the catalytic action of steel reported by other authors [17]. The tests were performed using the following conditions: partial pressure of H2 equal to 1.4, 1.9, 2.4 bar; reaction temperature: 293, 303, 323 K; initial 1-heptyne concentration of 0.1019, 0.1528, 0.2038 mol L-1; particle size: fractions of 60 or 120 mesh and 1500 μm. 1-heptyne (Fluka, Cat. No. 51950, >98%), the reactant, was dissolved in toluene (Merck, Cat. No. TX0735-44, >99%). 75 mL of the stock solution and catalyst samples of 0.3 g of Ni/Al2O3 and 0.03 g of Pd/Al2O3 previously reduced in hydrogen for 1 h at 673 and 573K, respectively, were used in the different catalytic tests.

The evolutions of reactant and products concentrations with reaction time were followed by Gas Chromatography using a flame ionization detector (FID) and a capillary column 30 m J&W INNOWax 19091N-213.

Kinetic Study of the Partial Hydrogenation of 1-Heptyne over Ni and Pd Supported on Alumina 163

peaks, with maximums at 855.4 eV and 862.6 eV. The former can be associated to Ni2+ species [19-22]; the other peak corresponds to the shake up of Ni(II) [25,28,29]. According to the literature, the maximum BE of the Pd 3d5/2 peak of the monometallic palladium catalyst

The XRD difractograms of the catalysts only present the -alumina peaks at 2θ = 37.7, 46.0 and 67.0º [24-26]. For this reason the difractograms are not shown. Because of the low amount of Pd or Ni on the Pd/Al2O3 or Ni/Al2O3 catalysts, the crystalline phases of palladium or nickel were undetectable. Several authors stated that high charges of nickel (>>15 wt %) are necessary to observe the peaks at 2θ = 43.3°; 63.0°; 75.5° and 79.5° of NiO

Figure 3 shows the TPR traces of the studied catalysts. The TPR trace of the Pd monometallic catalyst had a main hydrogen consumption peak at 287 K, which can be attributed to the reduction of PdO species and to the formation of palladium hydrides [29]. This catalyst also has an inverted reduction peak at 339 K that could be related to the decomposition of the β-PdH phase [20,21,24]. As these species interact weakly with the support the palladium hydrides are completely eliminated during reduction. Figure 3 also shows the profile of reduction of Ni/Al2O3 catalyst, the principal reduction peak begins at 700 K, finishes over 900 K and presents a maximum at 802 K which corresponds to the reduction of NiO (Ni2+ species) interacting with the alumina [30]. The second peak of minor intensity is observed at higher temperatures (>1000 K), and corresponds to the reduction of nickel aluminates [31- 32]. At the reduction temperature used during the preparation of the catalysts, the obtained TPR spectra suggest the presence of species Ni2+ and Pd0 on Ni/Al2O3 and Pd/Al2O3 catalysts,

400 600 800 1000 1200

**Temperature (K)**

It must be noted that the characterization techniques suggest that, after the pretreatment conditions employed during the preparation of the catalysts, only one type of active site is

**Pd/Al2 O3**

**Ni/Al2 O3**

respectively. These results are in total agreement with the XPS results.

**H2 uptake (a.u)**

**Figure 3.** TPR traces of the Ni/Al2O3 and Pd/Al2O3 catalysts.

present on each catalytic system.

at 334.9 eV, with its 3d3/2 doublet peak at 340.1 eV, corresponds to Pd0 [18-19,22-23].

[27,28].

### **3. Results and discussion**

### **3.1. Catalysts characterization**

The metal loadings of the catalysts determined by ICP were 3.5 and 0.35 wt % of Ni and Pd, respectively.

For the monometallic Ni/Al2O3 catalyst no hydrogen consumption was detected during the chemisorption analysis, in total accordance with previously reported results [18]. For the Pd/Al2O3 catalyst a chemisorption value of 18 μmol H2 gcat-1 was measured.

Figures 1 and 2 show the XPS spectra of Ni 2p3/2 and Pd 3d for Ni/Al2O3 and Pd/Al2O3 catalysts, respectively. It can be seen that the monometallic nickel catalyst presents two

**Figure 1.** XPS Ni 2p3/2 spectra of Ni/Al2O3.

**Figure 2.** XPS Pd 3d spectra of Pd/Al2O3

peaks, with maximums at 855.4 eV and 862.6 eV. The former can be associated to Ni2+ species [19-22]; the other peak corresponds to the shake up of Ni(II) [25,28,29]. According to the literature, the maximum BE of the Pd 3d5/2 peak of the monometallic palladium catalyst at 334.9 eV, with its 3d3/2 doublet peak at 340.1 eV, corresponds to Pd0 [18-19,22-23].

162 Hydrogenation

respectively.

J&W INNOWax 19091N-213.

**3. Results and discussion** 

**3.1. Catalysts characterization** 

The evolutions of reactant and products concentrations with reaction time were followed by Gas Chromatography using a flame ionization detector (FID) and a capillary column 30 m

The metal loadings of the catalysts determined by ICP were 3.5 and 0.35 wt % of Ni and Pd,

For the monometallic Ni/Al2O3 catalyst no hydrogen consumption was detected during the chemisorption analysis, in total accordance with previously reported results [18]. For the

Figures 1 and 2 show the XPS spectra of Ni 2p3/2 and Pd 3d for Ni/Al2O3 and Pd/Al2O3 catalysts, respectively. It can be seen that the monometallic nickel catalyst presents two

Ni 2p3/2

satellite peak

845 850 855 860 865 870

**Binding Energy (eV)**

325 330 335 340 345

**Binding Energy (eV)**

Pd/Al2O3 catalyst a chemisorption value of 18 μmol H2 gcat-1 was measured.

**Counts/s**

**Counts/s**

**Figure 1.** XPS Ni 2p3/2 spectra of Ni/Al2O3.

**Figure 2.** XPS Pd 3d spectra of Pd/Al2O3

The XRD difractograms of the catalysts only present the -alumina peaks at 2θ = 37.7, 46.0 and 67.0º [24-26]. For this reason the difractograms are not shown. Because of the low amount of Pd or Ni on the Pd/Al2O3 or Ni/Al2O3 catalysts, the crystalline phases of palladium or nickel were undetectable. Several authors stated that high charges of nickel (>>15 wt %) are necessary to observe the peaks at 2θ = 43.3°; 63.0°; 75.5° and 79.5° of NiO [27,28].

Figure 3 shows the TPR traces of the studied catalysts. The TPR trace of the Pd monometallic catalyst had a main hydrogen consumption peak at 287 K, which can be attributed to the reduction of PdO species and to the formation of palladium hydrides [29]. This catalyst also has an inverted reduction peak at 339 K that could be related to the decomposition of the β-PdH phase [20,21,24]. As these species interact weakly with the support the palladium hydrides are completely eliminated during reduction. Figure 3 also shows the profile of reduction of Ni/Al2O3 catalyst, the principal reduction peak begins at 700 K, finishes over 900 K and presents a maximum at 802 K which corresponds to the reduction of NiO (Ni2+ species) interacting with the alumina [30]. The second peak of minor intensity is observed at higher temperatures (>1000 K), and corresponds to the reduction of nickel aluminates [31- 32]. At the reduction temperature used during the preparation of the catalysts, the obtained TPR spectra suggest the presence of species Ni2+ and Pd0 on Ni/Al2O3 and Pd/Al2O3 catalysts, respectively. These results are in total agreement with the XPS results.

**Figure 3.** TPR traces of the Ni/Al2O3 and Pd/Al2O3 catalysts.

It must be noted that the characterization techniques suggest that, after the pretreatment conditions employed during the preparation of the catalysts, only one type of active site is present on each catalytic system.

### **3.2. Catalytic results**

### *3.2.1. Partial hydrogenation of 1-heptyne*

Before considering kinetic expressions or comparing catalyst performances it is necessary to check whether the selected reaction system proceeds in kinetic regime. The possibility of external and internal diffusional limitations during the catalytic tests was thus experimentally assessed.

Kinetic Study of the Partial Hydrogenation of 1-Heptyne over Ni and Pd Supported on Alumina 165

0 20 40 60 80 100 120

**Time (min)**

**Figure 5.** CB vs. reaction time. PH2 =1.4 bar, C0A= 0.1528 mol L-1, T= 323 K, 800 rpm, Wcat= 0.3 gNi/Al2O3 or

A series-parallel reaction network was proposed for partial hydrogenation of 1-heptyne [14], as indicated in Figure 6.a. This is composed of three hydrogenation reactions that can be a priori considered reversible. The equilibrium constant for each of the previous reactions were calculated using Joback's group contribution method [34]. The values at 323 K were calculated as K1=3.35 1021, K2=1.87 1035 and K3=5.58 1013. These values indicate that the individual reactions in Figure 6.a can be considered as irreversible. The experimentally

**Figure 6.** Reaction scheme for 1-heptyne (a) reversible hydrogenation and (b) irreversible hydrogenation.

(b)

(a)

**k1**

**k3**

1-heptyne 1-heptene

1-heptyne 1-heptene n-heptane

**1 2**

**3**

H2C CH3 H3C CH3

H2C CH3

H3C CH3

n-heptane

Experimentally, it is observed that while 1-heptyne is present in the reaction medium, 1 heptene concentration always increases, showing higher concentration than n-heptane.

obtained values of total conversion of 1-heptyne confirmed this prediction.

CH3

CH3

HC

**Ni/Al2 O3**

**Pd/Al2 O3**

0,00 0,02 0,04 0,06 0,08 0,10 0,12

**1-Heptene concentration ( mol.L-1 )**

0.03 g Pd/Al2O3, S/Ni = 64 and S/Pd = 29385.

**4. Kinetic modeling** 

**4.1. Reaction network** 

HC

### *3.2.1.1. Experimental verification of the absence of external and internal mass transfer limitations*

In order to eliminate external diffusional limitations, experiences were carried out using different stirring speeds in the range of 180-1400 rpm. It was found that at stirring rates higher than 500 rpm, 1-heptyne conversion values remained constant, indicating that external gasliquid limitations were absent. A stirring rate of 800 rpm was therefore chosen for all the kinetic tests. On the other hand and in order to ensure that the catalytic results were not influenced by external and intraparticle mass transfer limitations, the catalyst pellets were milled to samples of different particle size: a fraction bigger than 100 mesh (<150 μm), a fraction of 60-100 mesh (250-150 μm) and pellets of 1500 μm (not milled). The obtained values of 1-heptyne conversion were the same for the two milled fractions indicating the absence of internal diffusional limitations. Then particles with sizes smaller than 250 μm were used in all tests.

#### *3.2.1.2. Catalytic activity results*

The catalytic activity results for the partial hydrogenation of 1-heptyne are shown in Figures 4 and 5, where it is represented the variation of 1-heptyne (CA) and 1-heptene (CB) concentration as a function of the reaction time for the Ni/Al2O3 and Pd/Al2O3 catalysts.

It can be clearly seen that Pd is more active than Ni, even when using one tenth of the catalyst mass of the Ni catalyst. Reasons for the differences in reactivity can be found in the literature. Most authors report that when the metal is more electron deficient it becomes less active because alkynes are more weakly adsorbed [5,33]. Nothing however is commented about the role of hydrogen. The published reports do not give a clear explanation of the effect of each metal and for this reason kinetic modelling was used in this work to shed light on these issues.

**Figure 4.** CA vs. reaction time. PH2 =1.4 bar, C0A= 0.1528 mol L-1, T= 323 K, 800 rpm, Wcat= 0.3 gNi/Al2O3 or 0.03 g Pd/Al2O3, S/Ni = 64 and S/Pd = 29385.

**Figure 5.** CB vs. reaction time. PH2 =1.4 bar, C0A= 0.1528 mol L-1, T= 323 K, 800 rpm, Wcat= 0.3 gNi/Al2O3 or 0.03 g Pd/Al2O3, S/Ni = 64 and S/Pd = 29385.

### **4. Kinetic modeling**

164 Hydrogenation

**3.2. Catalytic results** 

experimentally assessed.

*3.2.1.2. Catalytic activity results* 

*3.2.1. Partial hydrogenation of 1-heptyne* 

Before considering kinetic expressions or comparing catalyst performances it is necessary to check whether the selected reaction system proceeds in kinetic regime. The possibility of external and internal diffusional limitations during the catalytic tests was thus

*3.2.1.1. Experimental verification of the absence of external and internal mass transfer limitations* 

limitations. Then particles with sizes smaller than 250 μm were used in all tests.

In order to eliminate external diffusional limitations, experiences were carried out using different stirring speeds in the range of 180-1400 rpm. It was found that at stirring rates higher than 500 rpm, 1-heptyne conversion values remained constant, indicating that external gasliquid limitations were absent. A stirring rate of 800 rpm was therefore chosen for all the kinetic tests. On the other hand and in order to ensure that the catalytic results were not influenced by external and intraparticle mass transfer limitations, the catalyst pellets were milled to samples of different particle size: a fraction bigger than 100 mesh (<150 μm), a fraction of 60-100 mesh (250-150 μm) and pellets of 1500 μm (not milled). The obtained values of 1-heptyne conversion were the same for the two milled fractions indicating the absence of internal diffusional

The catalytic activity results for the partial hydrogenation of 1-heptyne are shown in Figures 4 and 5, where it is represented the variation of 1-heptyne (CA) and 1-heptene (CB) concentration as a function of the reaction time for the Ni/Al2O3 and Pd/Al2O3 catalysts.

It can be clearly seen that Pd is more active than Ni, even when using one tenth of the catalyst mass of the Ni catalyst. Reasons for the differences in reactivity can be found in the literature. Most authors report that when the metal is more electron deficient it becomes less active because alkynes are more weakly adsorbed [5,33]. Nothing however is commented about the role of hydrogen. The published reports do not give a clear explanation of the effect of each metal and for this reason kinetic modelling was used in this work to shed light on these issues.

0 20 40 60 80 100 120 0,00

**Time (min)**

**Ni/Al2 O3**

**Pd/Al2 O3**

**Figure 4.** CA vs. reaction time. PH2 =1.4 bar, C0A= 0.1528 mol L-1, T= 323 K, 800 rpm, Wcat= 0.3 gNi/Al2O3 or

0,02 0,04 0,06 0,08 0,10 0,12 0,14 0,16

1**-Heptyne concentration ( mol.L-1 )**

0.03 g Pd/Al2O3, S/Ni = 64 and S/Pd = 29385.

#### **4.1. Reaction network**

A series-parallel reaction network was proposed for partial hydrogenation of 1-heptyne [14], as indicated in Figure 6.a. This is composed of three hydrogenation reactions that can be a priori considered reversible. The equilibrium constant for each of the previous reactions were calculated using Joback's group contribution method [34]. The values at 323 K were calculated as K1=3.35 1021, K2=1.87 1035 and K3=5.58 1013. These values indicate that the individual reactions in Figure 6.a can be considered as irreversible. The experimentally obtained values of total conversion of 1-heptyne confirmed this prediction.

**Figure 6.** Reaction scheme for 1-heptyne (a) reversible hydrogenation and (b) irreversible hydrogenation.

Experimentally, it is observed that while 1-heptyne is present in the reaction medium, 1 heptene concentration always increases, showing higher concentration than n-heptane. After all 1-heptyne was consumed, 1-heptene concentration begins to decrease very slowly and n-heptane concentration equally increases. These profiles are consistent with two reaction schemes: i) two parallel irreversible reactions (steps 1 and 3), and ii) series-parallel irreversible reactions, with a k1/k2 value higher than 100. The latter considerations, allows us to disregard step 2. Therefore a simplified network of parallel reactions for 1-heptyne hydrogenation can be assumed in Figure 6.b.

Kinetic Study of the Partial Hydrogenation of 1-Heptyne over Ni and Pd Supported on Alumina 167

*P .C <sup>r</sup> P K .C K .C* 2 2 <sup>13</sup> *\* P K .C H H*

*P k .C* <sup>14</sup> *A S*

2

2 2H

<sup>C</sup> A S AS K

2 2

AS H S AH S S K

<sup>1</sup> C C BS B S

<sup>1</sup> C C CS C S

dC dt r r A 13 */* (15)

dC dt r B 1 */* (16)

H S H S K

2 \* H S

C C*.* (8)

C C*.* (9)

*.* (11)

B BS

C CS

K C *.* (14)

K C *.* (13)

H S C

A S

2 2 <sup>15</sup> <sup>1</sup> *\* \* P K .K .C .K HH A*

16 1 *\* P k .K .K .C .C AH H S*

17 3 *\* \* P k .K .K .K .C .C A HHS*

2 2

2

2 2 22 2

2

AS H S C C

\* AH S S

C C *.*

del Hypothesis of the model Simplified Rate Parameters

13 15 1

13 15 1

H2 dissociative adsorption. H2 non dissociative adsorption.

C C*.* (1) <sup>S</sup>

C C*.* (2) AS <sup>A</sup>

2 2 AH S S

C C *.*

AS HS C C

AS 2HS BS 2S K <sup>1</sup> (3) AS H S BS S K 2 1 (10)

AH S 2 HS CS 2S K 2 3 (5) AH S H S CS S K 2 2 <sup>3</sup> (12)

*.* (4) <sup>2</sup>

*.* (6) B S

*.* (7) C S

reaction scheme of Figure 6.b, for 1-heptyne (A), 1-heptene (B) and n-heptane (C):

**Table 2.** Elementary steps with H2 dissociative or non-dissociative adsorption reaction mechanism.

The following mass balances for components in the liquid phase were considered for the

*P .C <sup>r</sup>*

<sup>13</sup> 1

<sup>14</sup>

*A BB CC*

 16 1 2

 17 3 2

*P P .C K .C K .C*

*P .C <sup>r</sup> P P .C K .C K .C*

*A A BB CC*

*A A BB CC*

Mo-

Controlling step: adsorption

dissociative adsorption of H2 [43]. Competitive adsorption of 1-heptyne and H2. The active sites are not completely

Controlling step: surface chemical reaction. Non dissociative adsorption of H2 [43]. Competitive adsorption of 1-heptyne and H2. The active sites are not completely

**Table 1.** LHHW kinetic models.

<sup>C</sup> H 2S 2 HS K

<sup>C</sup> A S AS K

AS 2HS AH S 2S K

<sup>1</sup> C C BS B S

<sup>1</sup> C C CS C S

**4.3. Mass balances** 

2 H 2

2

AS <sup>A</sup> A S

2

2 2

B S B BS

> C S C CS

K C

K C

H S

2 HS

of 1-Heptyne. Non

covered.

covered.

V

VI

### **4.2. Langmuir-Hinshelwood-Hougen-Watson (LHHW) models**

Models of heterogeneous reactions were outlined using the Langmuir-Hinshelwood-Hougen-Watson formalism (LHHW models). Taking into account the previously presented characterization results of Ni/Al2O3 and Pd/Al2O3, in all the models only one type of active sites was considered to be present. Six different models with their respective basic hypotheses are presented in Table 1. The elementary steps with H2 dissociative or nondissociative adsorption reaction mechanism are presented in Table 2.



**Table 1.** LHHW kinetic models.

Mo-

I

II

III

IV

covered.

hydrogenation can be assumed in Figure 6.b.

Controlling step: adsorption of H2. Dissociative adsorption of H2 [42]. Competitive adsorption of 1-heptyne and H2. Total coverage of active sites.

Controlling step: adsorption of 1-Heptyne. Dissociative adsorption of H2 [42]. Competitive adsorption of 1 heptyne and H2. Total coverage of active sites.

Controlling step: surface chemical reaction.

Dissociative adsorption of H2 [42]. Competitive adsorption of 1-heptyne and H2. Total coverage of active sites.

Controlling step: adsorption of H2. Non dissociative adsorption of H2 [43]. Competitive adsorption of 1 heptyne and H2. The active sites are not completely

**4.2. Langmuir-Hinshelwood-Hougen-Watson (LHHW) models** 

dissociative adsorption reaction mechanism are presented in Table 2.

After all 1-heptyne was consumed, 1-heptene concentration begins to decrease very slowly and n-heptane concentration equally increases. These profiles are consistent with two reaction schemes: i) two parallel irreversible reactions (steps 1 and 3), and ii) series-parallel irreversible reactions, with a k1/k2 value higher than 100. The latter considerations, allows us to disregard step 2. Therefore a simplified network of parallel reactions for 1-heptyne

Models of heterogeneous reactions were outlined using the Langmuir-Hinshelwood-Hougen-Watson formalism (LHHW models). Taking into account the previously presented characterization results of Ni/Al2O3 and Pd/Al2O3, in all the models only one type of active sites was considered to be present. Six different models with their respective basic hypotheses are presented in Table 1. The elementary steps with H2 dissociative or non-

del Hypothesis of the model Simplified Rate Parameters

 3

 6 4 5 1 *A B C*

 10 1 3 7 45 1

 11 3 3 7 45 1

 <sup>12</sup> 1 *<sup>A</sup> A BB CC*

*<sup>P</sup> <sup>r</sup> K .C K .C K .C* 2 2 <sup>12</sup> *' P k .C .C HHS*

*P .C P .C P .C*

*P .C <sup>r</sup> P .C P .C P .C*

*P .C <sup>r</sup>*

*A ABC*

*A ABC*

*ABC* 1 2

2

*<sup>P</sup> <sup>r</sup> C P .C P .C* <sup>1</sup> *<sup>B</sup>*

*P .C <sup>r</sup> P .C P .C* 2 2

*A <sup>K</sup> <sup>P</sup> <sup>K</sup>* <sup>2</sup>

3 2 *HHS A k .C .C*

*<sup>K</sup> <sup>P</sup> K .C*

*<sup>K</sup> <sup>P</sup>*

*<sup>K</sup>*

*P*

4

5

6

7

10

*P*

1

*k .K .C <sup>P</sup> K .C*

3 11

*k .K .K.C <sup>P</sup> K .C*

*P*

2 2

*B H H*

*C H H*

. . *A S H H*

*k C*

*K C*

*K .C* ..

2 2

2 2

2 2

2 2

2 2

*A S H H*

*A S H H*

1 *HH A H H K.K .C .K*

*K .C*

3

2 2

3

*C A*

*<sup>K</sup> <sup>P</sup> <sup>K</sup>*

2


**Table 2.** Elementary steps with H2 dissociative or non-dissociative adsorption reaction mechanism.

#### **4.3. Mass balances**

The following mass balances for components in the liquid phase were considered for the reaction scheme of Figure 6.b, for 1-heptyne (A), 1-heptene (B) and n-heptane (C):

$$\text{dC}\_{\text{A}} / \text{dt} = \ -\mathbf{r}\_{\text{1}} - \mathbf{r}\_{\text{3}} \tag{15}$$

$$\text{dC}\_{\text{B}} / \text{dt} = \text{r}\_{\text{l}} \tag{16}$$

$$\text{dC}\_{\text{C}} / \text{dt} = \text{r}\_{\text{\text{\textdegree}}} \tag{17}$$

Kinetic Study of the Partial Hydrogenation of 1-Heptyne over Ni and Pd Supported on Alumina 169

*EA*

*R.T k A.e* 

Several tests were performed at varying hydrogen partial pressures (1.4, 1.9 and 2.4 bar) keeping all other variables constant (C0A= 0.1528 mol L-1, T= 303 K, 800 rpm, Wcat= 0.3 g Ni/Al2O3 or 0.03 Pd/Al2O3). The partial pressure of hydrogen was calculated as the difference between the total pressure and the partial pressure of the solvent, since the partial pressures of 1-heptyne and the products was negligible. The pressure of the solvent at 303 K

Experimental values of conversion of 1-heptyne as a function of time are plotted in Figures 7.a and 7.b. Both Figures show that as PH2 rises, 1-heptyne total conversion increases for Ni/Al2O3 catalyst, but decrease for Pd/Al2O3. The reaction order with respect to hydrogen (β)

> *AA H* <sup>2</sup> *ln(r ) ln k. C .ln(P )*

The initial reaction rates of 1-heptyne were calculated by polynomial differentiation of the traces of Figures 7.a and 7.b and extrapolation to zero reaction time. From the plot of ln(r0A) vs. ln(PH2), shown in Figures 8.a and 8.b, reaction order in hydrogen of 1.3 and -2.6 were calculated for Ni/Al2O3 and for Pd/Al2O3, respectively. The results indicate that: a) for

(23)

0 0

(21)

(22)

3. The amplitude of the confidence interval must be lower than the value of the estimated

The final model is selected from the set of models complying the above 1 to 3 conditions, as the model with the lowest SCD, the summation of squares of the deviations. Another condition is that the standard deviation is smaller than the value of the parameter. If differences are not big, then the model selection criterion (MSC) should be used.

Preliminary tests were performed in order to check the influence of the different variables on the reaction rate. These results will be used later in the model selection stage. The variables screened were the partial pressure of hydrogen, the initial concentration of 1 heptyne and the reaction temperature. In order to analyze the influence of each variable a pseudo homogeneous reaction model was proposed in which the reaction rate was assumed

to follow a potential law. The initial reaction rate should thus be written as:

An Arrhenius dependent was supposed for the specific constant of reaction:

0 0 *A AH* <sup>2</sup> *r k.(C ) .(P )* 

Appropriate models should have a MSC value greater than 4.

**4.6. Kinetic models for the reaction** 

*4.6.1. Influence of the partial pressure of hydrogen* 

was determined using Antoine's equation (0.048 bar).

was calculated from the linearized form of equation (21).

parameter.

initial conditions were: t = 0 min, C0A = 0.1528 mol L-1, C0B = C0C = 0 mol L-1.

#### **4.4. Numerical resolution and statistics**

The system of differential equations (15)-(17) was solved numerically using the Runge-Kutta-Merson algorithm. The model parameter estimation was performed by a non-linear regression, using a Levenberg-Marquardt algorithm which minimized the objective function:

$$\text{SCD} = \sum\_{j}^{n} \left( \mathbf{C}\_{i,j} - \mathbf{C}\_{i,j}^{\text{CALC}} \right)^{2} \tag{18}$$

where *Ci,j* and *CALC Ci,j* are the experimental and the predicted concentration values, respectively, *"i"* is the chemical compound and *"j"* is the reaction time.

The model adequacy and the discrimination between models were determined using the model selection criterion (MSC), according to the following equation:

$$\text{MSC} = \ln \left( \frac{\sum\_{j}^{n} \left( \mathbf{C}\_{i,j} - \overline{\mathbf{C}}\_{i} \right)^{2}}{\sum\_{j}^{n} \left( \mathbf{C}\_{i,j} - \mathbf{C}\_{i,j}^{\text{CALC}} \right)^{2}} \right) - \left( \frac{\mathbf{2}p}{n} \right) \tag{19}$$

where*Ci* is the average relative concentration; p is the amount of parameters fitted and n is the number of experimental data. In order to compare different models, the selected one is that leading to the highest MSC value.

The Standard Deviation (S) was calculated with the following equation:

$$S = \sqrt{s^2} = \sqrt{\frac{\sum\_{j}^{n} \left(\mathbf{C}\_{i,j} - \mathbf{C}\_{i,j}^{\text{CALC}}\right)^2}{n - p}} \tag{20}$$

#### **4.5. Model discrimination**

The first main requisite for appropriateness of a model should be that of physical significance. A priori this means that the model parameters adopt feasible real values. A second requisite is that of adequate statistical confidence, i.e. the parameters should lie in one as small as possible confidence interval.

The practical criteria for the selection of the kinetic models were:


3. The amplitude of the confidence interval must be lower than the value of the estimated parameter.

The final model is selected from the set of models complying the above 1 to 3 conditions, as the model with the lowest SCD, the summation of squares of the deviations. Another condition is that the standard deviation is smaller than the value of the parameter. If differences are not big, then the model selection criterion (MSC) should be used. Appropriate models should have a MSC value greater than 4.

#### **4.6. Kinetic models for the reaction**

168 Hydrogenation

function:

dC dt r C 3 */* (17)

The system of differential equations (15)-(17) was solved numerically using the Runge-Kutta-Merson algorithm. The model parameter estimation was performed by a non-linear regression, using a Levenberg-Marquardt algorithm which minimized the objective

> *<sup>n</sup>* <sup>2</sup> *CALC i,j i,j <sup>j</sup>*

where *Ci,j* and *CALC Ci,j* are the experimental and the predicted concentration values,

The model adequacy and the discrimination between models were determined using the

*C C <sup>p</sup> MSC ln*

*<sup>i</sup> i,j <sup>j</sup> <sup>n</sup> CALC i,j i,j <sup>j</sup>*

*n*

2

*<sup>n</sup> C C*

<sup>2</sup>

*n p*

*<sup>n</sup> CALC i,j i,j <sup>j</sup> C C*

The first main requisite for appropriateness of a model should be that of physical significance. A priori this means that the model parameters adopt feasible real values. A second requisite is that of adequate statistical confidence, i.e. the parameters should lie in

1. The estimated values of the parameters must be positive and different from zero. 2. The upper and lower extremes of the confidence interval (95%) must be positive.

2

2

(20)

(19)

where*Ci* is the average relative concentration; p is the amount of parameters fitted and n is the number of experimental data. In order to compare different models, the selected one is

*SCD C C* (18)

initial conditions were: t = 0 min, C0A = 0.1528 mol L-1, C0B = C0C = 0 mol L-1.

respectively, *"i"* is the chemical compound and *"j"* is the reaction time.

model selection criterion (MSC), according to the following equation:

The Standard Deviation (S) was calculated with the following equation:

*S s*

The practical criteria for the selection of the kinetic models were:

2

**4.4. Numerical resolution and statistics** 

that leading to the highest MSC value.

**4.5. Model discrimination** 

one as small as possible confidence interval.

Preliminary tests were performed in order to check the influence of the different variables on the reaction rate. These results will be used later in the model selection stage. The variables screened were the partial pressure of hydrogen, the initial concentration of 1 heptyne and the reaction temperature. In order to analyze the influence of each variable a pseudo homogeneous reaction model was proposed in which the reaction rate was assumed to follow a potential law. The initial reaction rate should thus be written as:

$$r\_A^0 = k.(\mathbb{C}\_A^0)^\alpha.(\mathbb{P}\_{H2})^\beta \tag{21}$$

An Arrhenius dependent was supposed for the specific constant of reaction:

$$k = A.e^{\frac{-E\_A}{R.T}}\tag{22}$$

#### *4.6.1. Influence of the partial pressure of hydrogen*

Several tests were performed at varying hydrogen partial pressures (1.4, 1.9 and 2.4 bar) keeping all other variables constant (C0A= 0.1528 mol L-1, T= 303 K, 800 rpm, Wcat= 0.3 g Ni/Al2O3 or 0.03 Pd/Al2O3). The partial pressure of hydrogen was calculated as the difference between the total pressure and the partial pressure of the solvent, since the partial pressures of 1-heptyne and the products was negligible. The pressure of the solvent at 303 K was determined using Antoine's equation (0.048 bar).

Experimental values of conversion of 1-heptyne as a function of time are plotted in Figures 7.a and 7.b. Both Figures show that as PH2 rises, 1-heptyne total conversion increases for Ni/Al2O3 catalyst, but decrease for Pd/Al2O3. The reaction order with respect to hydrogen (β) was calculated from the linearized form of equation (21).

$$\ln(r\_A^0) = \ln\left[k.\left(\mathbf{C}\_A^0\right)^{\alpha}\right] + \beta.\ln(\mathbf{P}\_{H2})\tag{23}$$

The initial reaction rates of 1-heptyne were calculated by polynomial differentiation of the traces of Figures 7.a and 7.b and extrapolation to zero reaction time. From the plot of ln(r0A) vs. ln(PH2), shown in Figures 8.a and 8.b, reaction order in hydrogen of 1.3 and -2.6 were calculated for Ni/Al2O3 and for Pd/Al2O3, respectively. The results indicate that: a) for

Kinetic Study of the Partial Hydrogenation of 1-Heptyne over Ni and Pd Supported on Alumina 171

0,0

0,2

0,4

**1- Heptyne Conversion**

0,6

0,8

1,0

**Figure 9.** Effect of C0A on the catalytic activity for (a) Ni/Al2O3 and (b) Pd/Al2O3. PH2=1.4 bar, T=303 K, 800 rpm, Wcat= 0.3 gNi/Al2O3 or 0.03 g Pd/Al2O3, S/Ni = 42, 64 and 85, S/Pd =19596, 29335 and 39192.

(a) **Time ( min )** (b)

**Figure 10.** Initial reaction rate of 1-heptyne concentration for (a) Ni/Al2O3 and (b) Pd/Al2O3. PH2=1.4 bar, T=303 K, 800 rpm, Wcat= 0.3 gNi/Al2O3 or 0.03 g Pd/Al2O3, S/Ni = 42, 64 and 85, S/Pd =19596, 29335 and 39192.

The value of the reaction order in 1-heptyne (α) can be calculated along the lines described

 0 0 *AH A* <sup>2</sup> *ln(r ) ln k. P .ln(C )* 

The graph of ln(r0A) vs. ln(C0A), Figures 10.a and 10.b, yields value of order of reaction of 1 heptyne equal to -0.22 and -1.5 for Ni/Al2O3 and Pd/Al2O3, respectively. The results indicate that an increase in the initial concentration of 1-heptyne is detrimental to the reaction rates.

Catalytic tests were performed varying the reaction temperature: 293, 303 and 323 K, and keeping the rest of the variables constant (PH2=1.4 bar, C0A = 0.1528 mol L-1, 800 rpm, Wcat =


**ln (ro**

**A)**

(24)

0 25 50 75 100 125 150 175 200 225


**ln (Co A)**

 **C0**

 **C0**

 **C0**

**1- Heptyne = 0.1019 mol.L-1**

**1- Heptyne = 0.1528 mol.L-1**

**1- Heptyne = 0.2038 mol.L-1**

in the previous section:


**ln (ro**

**A)**

0,0

0,2

0,4

0,6

**1-Heptyne Conversion** 

0,8

1,0

*4.6.3. Influence of the reaction temperature* 

0 25 50 75 100 125 150 175 200 225

Co

Co

Co

1-Heptyne= 0,2M

1-Heptyne= 0,15M

1-Heptyne= 0,1M

**Time (Min)**


(a) (b)

**ln (Co A)**

0.3 g Ni/Al2O3 or 0.03 Pd/Al2O3).

**Figure 7.** Effect of PH2 on the catalytic activity for (a) Ni/Al2O3 and (b) Pd/Al2O3. C0A= 0.1528 mol L-1, T=303 K, 800 rpm, Wcat= 0.3 gNi/Al2O3 or 0.03 g Pd/Al2O3, S/Ni = 64 and S/Pd =29385.

Ni/Al2O3 catalyst, high hydrogen partial pressures are beneficial for the reaction kinetics, probably both adsorption and surface reaction elementary steps could be enhanced; and b) an increase in the partial pressure of hydrogen negatively affects the reaction rate for Pd/Al2O3.

**Figure 8.** Initial reaction rate as a function of hydrogen pressure for (a) Ni/Al2O3 and (b) Pd/Al2O3. C0A=0.1528 mol L-1, T=303 K, 800 rpm, Wcat= 0.3 gNi/Al2O3 or 0.03 g Pd/Al2O3 , S/Ni = 64 and S/Pd =29385.

#### *4.6.2. Influence of the initial concentration of 1-heptyne*

Catalytic tests were performed varying the initial concentration of 1-heptyne: 0.1019, 0.1528 and 0.2038 mol L-1, keeping all the rest of the variables constant (PH2=1.4 bar, T= 303 K, 800 rpm, Wcat = 0.3 g Ni/Al2O3 or 0.03 Pd/Al2O3).

The obtained values of conversion of 1-heptyne as a function of time are plotted in Figures 9.a and 9.b. It can be seen that for both catalysts the catalytic activity is decreased when the initial concentration of 1-heptyne is increased.

**Figure 9.** Effect of C0A on the catalytic activity for (a) Ni/Al2O3 and (b) Pd/Al2O3. PH2=1.4 bar, T=303 K, 800 rpm, Wcat= 0.3 gNi/Al2O3 or 0.03 g Pd/Al2O3, S/Ni = 42, 64 and 85, S/Pd =19596, 29335 and 39192.

**Figure 10.** Initial reaction rate of 1-heptyne concentration for (a) Ni/Al2O3 and (b) Pd/Al2O3. PH2=1.4 bar, T=303 K, 800 rpm, Wcat= 0.3 gNi/Al2O3 or 0.03 g Pd/Al2O3, S/Ni = 42, 64 and 85, S/Pd =19596, 29335 and 39192.

The value of the reaction order in 1-heptyne (α) can be calculated along the lines described in the previous section:

$$\ln(r\_A^0) = \ln\left[k.\left(P\_{H.2}\right)^{\beta}\right] + \alpha.\ln(\mathbb{C}\_A^0) \tag{24}$$

The graph of ln(r0A) vs. ln(C0A), Figures 10.a and 10.b, yields value of order of reaction of 1 heptyne equal to -0.22 and -1.5 for Ni/Al2O3 and Pd/Al2O3, respectively. The results indicate that an increase in the initial concentration of 1-heptyne is detrimental to the reaction rates.

#### *4.6.3. Influence of the reaction temperature*

170 Hydrogenation

Pd/Al2O3.

**ln (ro**

**A**)


0,0

0,2

0,4

0,6

**1-Heptyne Conversion** 

0,8

1,0

**Figure 7.** Effect of PH2 on the catalytic activity for (a) Ni/Al2O3 and (b) Pd/Al2O3. C0A= 0.1528 mol L-1,

**Figure 8.** Initial reaction rate as a function of hydrogen pressure for (a) Ni/Al2O3 and (b) Pd/Al2O3. C0A=0.1528 mol L-1, T=303 K, 800 rpm, Wcat= 0.3 gNi/Al2O3 or 0.03 g Pd/Al2O3 , S/Ni = 64 and S/Pd =29385.

Catalytic tests were performed varying the initial concentration of 1-heptyne: 0.1019, 0.1528 and 0.2038 mol L-1, keeping all the rest of the variables constant (PH2=1.4 bar, T= 303 K, 800

The obtained values of conversion of 1-heptyne as a function of time are plotted in Figures 9.a and 9.b. It can be seen that for both catalysts the catalytic activity is decreased when the

*4.6.2. Influence of the initial concentration of 1-heptyne* 

(a) (b)

0,3 0,4 0,5 0,6 0,7 0,8 0,9

**ln (PH2** )

rpm, Wcat = 0.3 g Ni/Al2O3 or 0.03 Pd/Al2O3).

initial concentration of 1-heptyne is increased.

Ni/Al2O3 catalyst, high hydrogen partial pressures are beneficial for the reaction kinetics, probably both adsorption and surface reaction elementary steps could be enhanced; and b) an increase in the partial pressure of hydrogen negatively affects the reaction rate for

0,0

3,5 4,0 4,5 5,0 5,5 6,0

l**n (ro**

**A)**

0,2

0,4

**1- Heptyne Conversion**

0,6

0,8

1,0

0 25 50 75 100 125 150 175 200 225

**Time ( min )**

0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 3,0

**ln (PH2 )**  **PH2 = 1.4 bar PH2 = 1.9 bar PH2 = 2.4 bar**

T=303 K, 800 rpm, Wcat= 0.3 gNi/Al2O3 or 0.03 g Pd/Al2O3, S/Ni = 64 and S/Pd =29385.

 P= 1,4 bar P= 1,9 bar P= 2,4 bar

0 25 50 75 100 125 150 175 200 225

(a) (b)

**Time (Min)**

Catalytic tests were performed varying the reaction temperature: 293, 303 and 323 K, and keeping the rest of the variables constant (PH2=1.4 bar, C0A = 0.1528 mol L-1, 800 rpm, Wcat = 0.3 g Ni/Al2O3 or 0.03 Pd/Al2O3).

The experimental values of conversion of 1-heptyne as a function of time at different temperature values are plotted in Figures 11.a and 11.b. As expected the activity of the catalyst is increased while the reaction temperature is raised up.

**Figure 11.** Effect of T on the catalytic activity for (a) Ni/Al2O3 and (b) Pd/Al2O3. PH2=1.4 bar, 800 rpm, Wcat= 0.3 gNi/Al2O3 or 0.03 g Pd/Al2O3, S/Ni = 64 and S/Pd = 29385.

When equation (21) is linearized a value of "apparent" activation energy (EA) can be got, as indicated in Eq. (25):

$$\ln \text{(r}\_A^0\text{)} = \ln \left[ A. \left( P\_{H.2} \right)^{\beta}. \left( \text{C}\_A^0 \right)^{\alpha} \right] - \frac{E\_A}{R.T} \tag{25}$$

Kinetic Study of the Partial Hydrogenation of 1-Heptyne over Ni and Pd Supported on Alumina 173

meter

P1 P2

P3

P2=0 P1

P3

KA KB KC P12

KA KB= 0 KC P12

KA KB= 0 KC= 0 P12

KA= 0 KB= 0 KC= 0 P120

Parameter sign

> (+) (+) (+)

> (+)

(+)

(+) (-) (+) (+)

(-)

(-) (+)

(-)

(+)

Discrimination Viabi-

IC 0, CL VE IC 0, CL VE

IC 0, CL VE

IC > 0, CL < VE

IC > 0, CL < VE

IC 0, CL VE IC 0, CL VE IC 0, CL VE IC 0, CL VE

IC 0, CL VE

IC 0, CL VE IC 0, CL VE

IC < 0, CL > VE

IC < 0, CL > VE

A=0.1528 mol.L-1, S/Ni = 64.

lity

Not viable

Viable

Not viable

Not viable

Not viable

Viable

Models II, III, V and VI of Table 1 were discarded because they could not explain the negative and positive orders in 1-heptyne and hydrogen obtained experimentally. The parameters estimated for the models I and IV are indicated in Table 3. A statistical analysis was performed to discriminate between the different models, by means of the selection criteria described in Section 4.5. The results of this analysis are detailed in Table 3. It can be concluded that the best fit is achieved with model I- B. In this model the value of P2 is equal to zero, indicating that n-heptane is not adsorbed. The model IV-D also shows a good fit of the experimental data, but a pseudo homogeneous kinetic expression is obtained with

2.1.10-3 3.7

2.10-3 4.5

2.47.10-4 5.17

2.47.10-4 5.23

2.47.10-4 5.21

*4.6.4. Model discrimination for Ni/Al2O3 catalyst* 

different reaction orders than those previously calculated.

P1 = 4.4823237168.14985673 P2 = 3.44577501 335.92334499 P3 = 0.00012112225870.00003376836

P2 = 0 P1 = 5.931084340.97784253 P3 = 0.0001137204720.000026730661

KA= 8.34221367355.9610964 KB= -1097.261877313.33134 KC= 2384.3931612837.26474 P12= 0.006544811170.15591798

KA= -6.364432523.54535551 KB= 0 KC= -17.123787350.0933591 P12= 7.82119826.10-51.570645.10-3

KA= -5.148583640.29479977 KB= 0 KC= 0 P12= 0.0006130579243.87.10-5

KA= 0 KB= 0 KC= 0 P120 A pseudohomogeneous model is obtained.

Reaction conditions: PH2=1.4 bar, T=323 K, wcat=0.3 g, stirring rate=800 rpm, C0

**Table 3.** Estimated parameters and model discrimination for Ni/Al2O3.

(\*) 95% confidence interval. IC: confidence interval; CL: confidence level; VE: estimated parameter value

Figure 13 contains experimental values of the concentration of 1-heptyne, 1-heptene and nheptane along with theoretical values (solid line) estimated with model I-B, as a function of

tion Estimated parameter (\*) SCD MSC Para-

Model

I

IV

Op-

A

B

A

B

C

D

The initial reaction rates of 1-heptyne were calculated as in the previous sections. The value of the apparent activation energy were obtained from the plots presented in Figure 12 of ln(r0A) as a function of 1/T. The calculated values were 24 and 18 KJ mol-1 for Ni/Al2O3 and Pd/Al2O3, respectively. These values have not a real physical meaning and are only apparent.

**Figure 12.** Temperature dependence of the reaction rate for (a) Ni/Al2O3 and (b) Pd/Al2O3. PH2=1.4 bar, T=303 K, 800 rpm, Wcat= 0.3 gNi/Al2O3 or 0.03 g Pd/Al2O3, S/Ni = 64 and S/Pd = 29385.

#### *4.6.4. Model discrimination for Ni/Al2O3 catalyst*

172 Hydrogenation

The experimental values of conversion of 1-heptyne as a function of time at different temperature values are plotted in Figures 11.a and 11.b. As expected the activity of the

**Figure 11.** Effect of T on the catalytic activity for (a) Ni/Al2O3 and (b) Pd/Al2O3. PH2=1.4 bar, 800 rpm,

**Time ( min )** (a) (b)

When equation (21) is linearized a value of "apparent" activation energy (EA) can be got, as

The initial reaction rates of 1-heptyne were calculated as in the previous sections. The value of the apparent activation energy were obtained from the plots presented in Figure 12 of ln(r0A) as a function of 1/T. The calculated values were 24 and 18 KJ mol-1 for Ni/Al2O3 and Pd/Al2O3, respectively. These values have not a real physical meaning and are only

**Figure 12.** Temperature dependence of the reaction rate for (a) Ni/Al2O3 and (b) Pd/Al2O3. PH2=1.4 bar,


**ln (ro**

**A)**

T=303 K, 800 rpm, Wcat= 0.3 gNi/Al2O3 or 0.03 g Pd/Al2O3, S/Ni = 64 and S/Pd = 29385.

3,05 3,10 3,15 3,20 3,25 3,30 3,35 3,40 3,45

**1/T (K) X 10-3**

(a) (b)

*A*

(25)

0 20 40 60 80 100 120

3,05 3,10 3,15 3,20 3,25 3,30 3,35 3,40 3,45

**1/T (K) X 10-3**

 **T = 293 K T = 303 K T = 323 K**

0,0

0,2

0,4

**1- Heptyne Conversion**

0,6

0,8

1,0

 0 0 2

*A HA <sup>E</sup> ln(r ) ln A. P .(C ) R.T* 

catalyst is increased while the reaction temperature is raised up.

 T= 323K T= 303K T= 293K

Wcat= 0.3 gNi/Al2O3 or 0.03 g Pd/Al2O3, S/Ni = 64 and S/Pd = 29385.

0 25 50 75 100 125 150 175 200

**Time (Min)**

indicated in Eq. (25):

0,0

0,2

0,4

0,6

**1-Heptyne Conversion** 

0,8

1,0

apparent.

**ln (ro**

**A)**



Reaction conditions: PH2=1.4 bar, T=323 K, wcat=0.3 g, stirring rate=800 rpm, C0A=0.1528 mol.L-1, S/Ni = 64. (\*) 95% confidence interval. IC: confidence interval; CL: confidence level; VE: estimated parameter value

**Table 3.** Estimated parameters and model discrimination for Ni/Al2O3.

Figure 13 contains experimental values of the concentration of 1-heptyne, 1-heptene and nheptane along with theoretical values (solid line) estimated with model I-B, as a function of time. A good fit between the two sets of values can be seen. The same regression with model I-B was done with experimental data obtained at other reaction temperatures in the 293-323 K range. In all cases and as a consequence of the fit, parameters different from zero were obtained for a confidence interval of 95% and with values of the MSC parameter greater than 4.0. The thermodynamic consistency of the P1 and P3 parameters was graphically evaluated by plotting lnP1 and lnP3 as a function of 1/T. In both cases a straight line was obtained (Figure 14) indicating that the constants have an Arrhenius dependence on temperature.

Kinetic Study of the Partial Hydrogenation of 1-Heptyne over Ni and Pd Supported on Alumina 175

2 2

<sup>1</sup> 17 91 KJ mol *HH . B A* (28)

<sup>1</sup> *E .H H A* <sup>2</sup> 2 58 KJ mol (29)

(26)

(27)

*B B B A*

*H A* 2 *HH S H H S*

*C .k .C C .A .C E .H*

In equations (26) and (27) the enthalpies of adsorption of 1-heptyne and 1-heptene were expressed in absolute values. The obtained values from the slopes of the lines in Figure 14

Considering that *HB* ≈ 0, in accord with the experimental results, a value of the enthalpy of adsorption for 1-heptyne can be obtained from equation (28): *HA* -17.91 KJ mol-1.

1. Model I-B that supposes dissociative adsorption of hydrogen as the rate-controlling step of reaction and a single type of active sites with total coverage, is the one that best fits

2. The model does not allow to directly obtain the enthalpies of adsorption of 1-heptyne

3. From equation (28) it can be inferred that the enthalpy of adsorption of 1-heptyne is greater than that of 1-heptene, in agreement with the information available in the

4. This model also supposes that the alkane is not adsorbed. If we additionally suppose that the enthalpy of adsorption of 1-heptene is negligible, in accordance with experimental results, a value of the enthalpy of adsorption of 1-heptyne and activation

Then, it can be concluded that the dissociative adsorption of H2 is the rate limiting step for the Ni/Al2O3 catalyst, and that the active sites are preferentially occupied by 1-heptyne.

Models I, II, IV and V of Table 1 were discarded because they could not explain the negative orders in 1-heptyne and hydrogen obtained experimentally. The parameters estimated for the models III and VI are indicated in Table 4. A statistical analysis was performed to discriminate between the different models, by means of the selection criteria described in Section 4.5. The results of this analysis are detailed in Table 4; these results indicate that model III-C gives the best fit of the experimental data. In this model the value of the

Introducing this value in equation (29) a value of EH2 of 22.2 KJ mol-1 can be estimated.

experimental data with statistical and thermodynamic consistency.

literature on the partial hydrogenation of alkynes [34].

*4.6.5. Model discrimination for the Pd/Al2O3 catalyst* 

and 1-heptene and the activation energy for the adsorption of hydrogen.

energy for the H2 adsorption can be obtained from eqs. (28) and (29).

*K A H H <sup>P</sup> .Exp K A R.T* 

22 2 2 2

*<sup>P</sup> .Exp K A R.T*

1

3 2 2

From the results it could be concluded that:

were:

*A A*

*A A*

**Figure 13.** Fit of the experimental data of Ci as a function of time with model I-B for Ni/Al2O3 catalyst. PH2=1.4 bar, C0A=0.1528 mol L-1, 323 K, Wcat=0.3 g, S/Ni = 64.

**Figure 14.** Temperature dependence of parameters P1 and P3 for Ni/Al2O3 catalyst

The slopes of the straight lines correspond to the values of the enthalpies of adsorption of 1 heptyne and 1-heptene and the value of the energy of activation for the dissociative adsorption of hydrogen on the active sites. Considering the definition of P1 and P3, the following equations can be obtained:

Kinetic Study of the Partial Hydrogenation of 1-Heptyne over Ni and Pd Supported on Alumina 175

$$P\_1 = \frac{K\_B}{K\_A} = \left(\frac{A\_B}{A\_A}\right).Exp\left[\frac{|\Delta H\_B| - |\Delta H\_A|}{R.T}\right] \tag{26}$$

$$P\_3 = \frac{\mathbb{C}\_{H2} \cdot k\_{H2} \cdot \mathbb{C}\_S^2}{K\_A^2} = \left(\frac{\mathbb{C}\_{H2} \cdot A\_{H2} \cdot \mathbb{C}\_S^2}{A\_A^2}\right) \cdot \exp\left[\frac{-\left(E\_{H2} + 2\left\lfloor \Delta H\_A \right\rfloor\right)}{R.T}\right] \tag{27}$$

In equations (26) and (27) the enthalpies of adsorption of 1-heptyne and 1-heptene were expressed in absolute values. The obtained values from the slopes of the lines in Figure 14 were:

$$\left|\Delta H\_B\right| - \left|\Delta H\_A\right| = -17.91 \text{ KJ mol}^{-1} \tag{28}$$

$$\left|E\_{H2} + \text{2.}\right| \Delta H\_A \Big| = 58 \text{ KJ mol}^{-1} \tag{29}$$

Considering that *HB* ≈ 0, in accord with the experimental results, a value of the enthalpy of adsorption for 1-heptyne can be obtained from equation (28): *HA* -17.91 KJ mol-1. Introducing this value in equation (29) a value of EH2 of 22.2 KJ mol-1 can be estimated.

From the results it could be concluded that:

174 Hydrogenation

temperature.

time. A good fit between the two sets of values can be seen. The same regression with model I-B was done with experimental data obtained at other reaction temperatures in the 293-323 K range. In all cases and as a consequence of the fit, parameters different from zero were obtained for a confidence interval of 95% and with values of the MSC parameter greater than 4.0. The thermodynamic consistency of the P1 and P3 parameters was graphically evaluated by plotting lnP1 and lnP3 as a function of 1/T. In both cases a straight line was obtained (Figure 14) indicating that the constants have an Arrhenius dependence on

0 20 40 60 80 100

**Figure 13.** Fit of the experimental data of Ci as a function of time with model I-B for Ni/Al2O3 catalyst.

**Time (Min)**

3,0 3,1 3,2 3,3 3,4 3,5 3,6 3,7 3,8

 **)**

**1/T\*1000 ( K-1**

The slopes of the straight lines correspond to the values of the enthalpies of adsorption of 1 heptyne and 1-heptene and the value of the energy of activation for the dissociative adsorption of hydrogen on the active sites. Considering the definition of P1 and P3, the

 Ln (P1 ) Ln (P3 )

 % (1-heptyne) % (1-heptene) % (n-heptane)

0,00 0,02 0,04 0,06 0,08 0,10 0,12 0,14 0,16

> -12 -10 -8 -6 -4 -2 0 2

following equations can be obtained:

**Figure 14.** Temperature dependence of parameters P1 and P3 for Ni/Al2O3 catalyst

**Concentration (mol.L-1)**

PH2=1.4 bar, C0A=0.1528 mol L-1, 323 K, Wcat=0.3 g, S/Ni = 64.

**Ln (P)**


Then, it can be concluded that the dissociative adsorption of H2 is the rate limiting step for the Ni/Al2O3 catalyst, and that the active sites are preferentially occupied by 1-heptyne.

#### *4.6.5. Model discrimination for the Pd/Al2O3 catalyst*

Models I, II, IV and V of Table 1 were discarded because they could not explain the negative orders in 1-heptyne and hydrogen obtained experimentally. The parameters estimated for the models III and VI are indicated in Table 4. A statistical analysis was performed to discriminate between the different models, by means of the selection criteria described in Section 4.5. The results of this analysis are detailed in Table 4; these results indicate that model III-C gives the best fit of the experimental data. In this model the value of the

parameters P4 and P5 are equal to zero. Therefore the only species being adsorbed on the Pd/Al2O3 catalyst are 1-heptyne and hydrogen.

Kinetic Study of the Partial Hydrogenation of 1-Heptyne over Ni and Pd Supported on Alumina 177

 **= 1-Heptyne = 1-Heptene = n-Heptane**

 Ln (P7 ) Ln (P10) Ln (P11

)

The heterogeneous model VI-E also produces a good fit of the experimental data but its parameters KB, KC, P13 and P15 are equal to zero, thus transforming into a pseudo homogeneous reaction rate expression in which the reaction orders in 1-heptyne and hydrogen are positive. This is not in agreement with the observed results, so this model was

Figure 15 contains values of concentration of 1-heptyne, 1-heptene and n-heptane as a function of time, experimentally (symbols) or theoretically obtained (solid line) with the III-

0 20 40 60 80 100 120

**Time (min)**

3,0 3,1 3,2 3,3 3,4 3,5

The same fit with model III-C was repeated with experimental data at other reaction temperatures in the 293-323 K range. In all cases and as a consequence of the fit, parameters

**Figure 16.** Temperature dependence of parameters P7, P10 and P11 for Pd/Al2O3 catalyst

**1/T.1000 ( K-1 )**

**Figure 15.** Fit of the experimental data of Ci as a function of time with model III-C for Pd/Al2O3 catalyst.

0,00


0,04

0,08

 **Concentration ( mol.L-1 )**

PH2=1.4 bar, T=323 K, 800 rpm, Wcat= 0.03 g, S/Pd = 29385.

**Ln ( P )**

0,12

0,16

0,20

discarded.

C model. There is an excellent fit.


Reaction conditions: PH2=1.4 bar, T=323 K, wcat=0.03 g, stirring rate=800 rpm, C0A=0.1528 mol.L-1, S/Pd = 29385. (\*) 95% confidence interval. IC: confidence interval; CL: confidence level; VE: estimated parameter value.

**Table 4.** Estimated model parameters and model discrimination for Pd/Al2O3

The heterogeneous model VI-E also produces a good fit of the experimental data but its parameters KB, KC, P13 and P15 are equal to zero, thus transforming into a pseudo homogeneous reaction rate expression in which the reaction orders in 1-heptyne and hydrogen are positive. This is not in agreement with the observed results, so this model was discarded.

176 Hydrogenation

Model

III

VI

Op-

A

B

C

A

B

C

D

E

Pd/Al2O3 catalyst are 1-heptyne and hydrogen.

P4 = -53.08660553399.8626255 P5 = 221.3489717134.846189 P7 = 28.7488806370.8944874 P10 = 4.68720094147.04798306 P11 = 1.9215935860.33982722

P4 = 0 P5 = 73.3086809919.4831331 P7 = 23.0235065284.1617705 P10 = 2.7560322879.34470502 P11 = 1.1297818532.52404635

P4 = 0 P5 = 0 P7 = 0.5542285000.330992359 P10 = 0.03687780350.0032648682 P11 = 0.01509404730.0014453604

KB = -16.3714023661.1021023 KC = 48.41328462509.3986254 P13 = 6.35365882135.97712018 P15 = 7.14818320324.0314528 P16 = 2.1326332447.39641486 P17 = 0.87414365419.422821246

KB = 0 KC = 53.73942846350.0751416 P13 = 3.53653444247.34456656 P15 = 19.82314331876.7295967 P16 = 1.7112967050.9810815 P17 = 0.70203972620.910298274

KB = 0 KC = 0 P13 = 2.9394173918.79975681 P15 = 1.8678261310.08702077 P16 = 0.5413420905.18514542 P17 = 0.2216424482.121511182

KB = 0 KC = 0 P13 = 0 P15 = 0.6104457950.825496865 P16 = 0.03571509180.0052744265 P17 = 0.01464128330.0023224598

KB = 0 KC = 0 P13 = 0 P15 = 0 P16 = 0.03205488390.0012299287 P17 = 0.01309650760.0007651406

tion Estimated parameter (\*) SCD MSC Para-

parameters P4 and P5 are equal to zero. Therefore the only species being adsorbed on the

2.2.10-4 5.206

2.2.10-4 5.266

2.2.10-4 5.145

2.2.10-4 5.206

2.2.10-4 5.26

2.2.10-4 5.327 P15

2.4.10-4 5.303 P16

Reaction conditions: PH2=1.4 bar, T=323 K, wcat=0.03 g, stirring rate=800 rpm, C0A=0.1528 mol.L-1, S/Pd = 29385. (\*) 95% confidence interval. IC: confidence interval; CL: confidence level; VE: estimated parameter value.

**Table 4.** Estimated model parameters and model discrimination for Pd/Al2O3

9.1.10-5 6.23 P7

meter

P4 P5 P7 P10 P11

P5 P7 P10 P11

P10 P11

KB KC P13 P15 P16 P17

KC P13 P15 P16 P17

P13 P15 P16 P17

P16 P17

P17

Parameter sign

> (-) (+) (+) (+) (+)

> (+) (+) (+) (+)

> (+) (+) (+)

> (-) (+) (+) (+) (+) (+)

> (+) (+) (+) (+) (+)

> (+) (+) (+) (+)

> (+) (+) (+)

> (+) (+)

Discrimination Viabi-

IC 0, CL VE IC 0, CL VE IC 0, CL VE IC 0, CL VE IC 0, CL VE

IC 0, CL VE IC 0, CL VE IC 0, CL VE IC 0, CL VE

IC 0, CL VE IC 0, CL VE IC 0, CL VE

IC 0, CL VE IC 0, CL VE IC 0, CL VE IC 0, CL VE IC 0, CL VE IC 0, CL VE

IC 0, CL VE IC 0, CL VE IC 0, CL VE IC 0, CL VE IC 0, CL VE

IC 0, CL VE IC 0, CL VE IC 0, CL VE IC 0, CL VE

IC 0, CL VE IC 0, CL VE IC 0, CL VE

IC 0, CL VE IC 0, CL VE lity

Not viable

Not viable

Viable

Not viable

Not viable

Not viable

Not viable

Viable

Figure 15 contains values of concentration of 1-heptyne, 1-heptene and n-heptane as a function of time, experimentally (symbols) or theoretically obtained (solid line) with the III-C model. There is an excellent fit.

**Figure 15.** Fit of the experimental data of Ci as a function of time with model III-C for Pd/Al2O3 catalyst. PH2=1.4 bar, T=323 K, 800 rpm, Wcat= 0.03 g, S/Pd = 29385.

**Figure 16.** Temperature dependence of parameters P7, P10 and P11 for Pd/Al2O3 catalyst

The same fit with model III-C was repeated with experimental data at other reaction temperatures in the 293-323 K range. In all cases and as a consequence of the fit, parameters were obtained with values different from zero in a confidence interval of 95% and with values of the MSC parameter greater than 4.0. The thermodynamic consistence of parameters P7, P10 and P11 was evaluated by plotting lnP7, lnP10 and lnP11 as a function of 1/T. In all cases straight lines were obtained confirming the hypotheses of Arrhenius dependence with respect to temperature (Figure 16).

The slopes of the straight lines obtained correspond to the enthalpies of adsorption of 1 heptyne and H2 and the energies of activation of the surface reactions of hydrogenation. Taking into account the definition of the parameters P7, P10 and P11 (Table 1) the following equations can be obtained:

$$\mathbf{P}\_{7} = \frac{(\mathbf{1} + \mathbf{K}.\mathbf{K}\_{\mathrm{H}2}.\mathbf{C}\_{\mathrm{H}2}).\mathbf{K}\_{\mathrm{A}}}{\sqrt{\mathbf{K}\_{\mathrm{H}2}.\mathbf{C}\_{\mathrm{H}2}}} \tag{30}$$

Kinetic Study of the Partial Hydrogenation of 1-Heptyne over Ni and Pd Supported on Alumina 179

<sup>1</sup> E H 15 84 KJ mol <sup>3</sup> *.* (37)

The enthalpy of H2 adsorption on Pd has been reported in Ref. [35] (-75.31 KJ mol-1), then from equation (34): HA 19.64 KJ mol-1. Replacing eq. (34) into eq. (35): E1 18.58 KJ

5. Model III-C that poses the surface chemical reaction as the limiting step is the one that best fits the experimental data. The model also poses the dissociative adsorption of H2 and the competition with 1-heptyne for the active sites. The model presents statistical

6. The model enables obtaining directly the values of the enthalpies of adsorption of 1 heptyne and the activation energy for the hydrogenation of 1-heptyne to 1-heptene (E1). Neither the calculation of the activation energy for the hydrogenation of 1-heptyne to n-

7. The model indicates that 1-heptyne and H2 are the only species adsorbed on the active sites. The enthalpy of adsorption of 1-heptyne over Pd (-19.64 KJ mol-1) is not much different from the value reported by Semagina et al [36] for the hydrogenation of 1-

8. Equation (34) shows that the enthalpy of adsorption of hydrogen is higher than that of 1-heptyne over the Pd/Al2O3 catalyst. This suggests that there are not thermodynamic limitations for the adsorption of H2. This was confirmed by the tests of hydrogen chemisorptions as Pd is able to chemisorb an important amount of H2 at room temperature, suggesting that there is not a kinetic impediment as that observed for Ni. Consequently, the dissociative adsorption of hydrogen is fast and then the controlling

9. The value obtained for the activation energy of the hydrogenation reaction of 1-heptyne to 1-heptene (E1) turned out to be quite low (18.58 KJ mol-1). This coincides with the fact

10. The kinetic modelling of the reactions over the Ni and Pd catalysts gives an explanation of the different reactivity of the catalysts. Pd is more active than Ni for partial hydrogenation reactions because there is a kinetic limitation for the adsorption of hydrogen on Ni. Hydrogen is more strongly chemisorbed on Pd, so there is a great coverage of the surface by H2, therefore making the surface reaction step as the rate-

Pd/Al2O3 was more active and selective than Ni/Al2O3 for the partial hydrogenation of 1-

In order to analyze the influence of the different variables (hydrogen partial pressure, initial concentration of 1-heptyne and reaction temperature) on the reaction rate a pseudo

mol-1. From equations (34) and (36):

From the results it can be concluded that:

and thermodynamic consistency.

hexyne over Pd nanoparticles.

step is the surface chemical reaction.

that the reaction can proceed at low temperatures.

heptane (E3).

controlling one.

**5. Conclusions** 

heptyne to 1-heptene.

Supposing that H2 H2 1 KK C *. .* (this assumption is made for the purpose to estimate some values of the involved constants).

The previous equation is reduced to:

$$\mathbf{P}\_{7} = \frac{\mathbf{K}\_{\text{A}}}{\sqrt{\mathbf{K}\_{\text{H2}} \mathbf{C}\_{\text{H2}}}} = \frac{\mathbf{A}\_{\text{A}}}{\sqrt{\mathbf{A}\_{\text{H2}} \mathbf{C}\_{\text{H2}}}} \text{.} \exp\left[\frac{-\left(\frac{1}{2} \left| \Delta \mathbf{H}\_{\text{H2}} \right| - \left| \Delta \mathbf{H}\_{\text{A}} \right|\right)}{\mathbf{R} \cdot \mathbf{T}}\right] \tag{31}$$

$$\mathbf{P}\_{10} = \frac{\mathbf{k}\_{1}, \mathbf{K}\_{\text{A}}, \mathbf{C}\_{\text{S}}^{3}}{\sqrt{\mathbf{K}\_{\text{H}2}, \mathbf{C}\_{\text{H}2}}} = \frac{\mathbf{A}\_{1}, \mathbf{A}\_{\text{A}}, \mathbf{C}\_{\text{S}}^{3}}{\sqrt{\mathbf{A}\_{\text{H}2}, \mathbf{C}\_{\text{H}2}}}.\text{Exp}\left[\frac{\cdot\left(\mathbf{E}\_{1} - \left|\Delta \mathbf{H}\_{\text{A}}\right| + \frac{1}{2}.\left|\Delta \mathbf{H}\_{\text{H}2}\right|\right)}{\mathbf{R}\_{\text{T}}}\right] \tag{32}$$

$$\mathbf{P}\_{\rm{11}} = \frac{\mathbf{k}\_3 \mathbf{K}\_{\rm{A}} \mathbf{K}\_{\rm{C}} \mathbf{C}\_{\rm{S}}^3}{\sqrt{\mathbf{K}\_{\rm{H2}} \mathbf{C}\_{\rm{H2}}}} = \frac{\mathbf{A}\_3 \mathbf{A}\_{\rm{A}} \mathbf{A}\_{\rm{A}} \mathbf{C}\_{\rm{S}}^3}{\sqrt{\mathbf{A}\_{\rm{H2}} \mathbf{C}\_{\rm{H2}}}} \times \exp\left[\frac{\cdot \left(\mathbf{E}\_3 - \left|\Delta \mathbf{H}\_{\rm{A}}\right| + \Delta \mathbf{H} + \frac{1}{2} \cdot \left|\Delta \mathbf{H}\_{\rm{H2}}\right|\right)}{\mathbf{R} \cdot \mathbf{T}}\right] \tag{33}$$

In equations (31), (32) and (33) the enthalpies of adsorption of 1-heptyne and H2 are expressed in absolute values. The values of the respective slopes were calculated as follows:

$$\frac{\left|\Delta\text{H}\_{\text{H}2}\right|}{2} - \left|\Delta\text{H}\_{\text{A}}\right| = 18.02 \text{ KJ mol}^{-1} \tag{34}$$

$$\left| \text{E}\_1 + \frac{\left| \Delta \text{H}\_{\text{H}2} \right|}{2} - \left| \Delta \text{H}\_{\text{A}} \right| = 36.6 \quad \text{KJ} \text{ mol}^{-1} \tag{35}$$

$$\left| \mathbf{E}\_3 + \boldsymbol{\Delta H} \mathbf{I} + \frac{\left| \boldsymbol{\Delta H}\_{\rm H2} \right|}{2} - \left| \boldsymbol{\Delta H}\_{\rm A} \right| = 2.18 \quad \mathbf{KJ} \text{ mol}^{-1} \tag{36}$$

The enthalpy of H2 adsorption on Pd has been reported in Ref. [35] (-75.31 KJ mol-1), then from equation (34): HA 19.64 KJ mol-1. Replacing eq. (34) into eq. (35): E1 18.58 KJ mol-1. From equations (34) and (36):

$$\text{E}\_3 + \Delta \text{H} = -15.84 \text{ KJ} \cdot \text{mol}^{-1} \tag{37}$$

From the results it can be concluded that:

178 Hydrogenation

with respect to temperature (Figure 16).

equations can be obtained:

values of the involved constants).

The previous equation is reduced to:

7

10

11

were obtained with values different from zero in a confidence interval of 95% and with values of the MSC parameter greater than 4.0. The thermodynamic consistence of parameters P7, P10 and P11 was evaluated by plotting lnP7, lnP10 and lnP11 as a function of 1/T. In all cases straight lines were obtained confirming the hypotheses of Arrhenius dependence

The slopes of the straight lines obtained correspond to the enthalpies of adsorption of 1 heptyne and H2 and the energies of activation of the surface reactions of hydrogenation. Taking into account the definition of the parameters P7, P10 and P11 (Table 1) the following

> 1 KK C K <sup>P</sup> K C *( . . ). .*

Supposing that H2 H2 1 KK C *. .* (this assumption is made for the purpose to estimate some

<sup>1</sup> H H K A <sup>2</sup> <sup>P</sup> Exp KC AC R.T

<sup>1</sup> -E H H kK C AA C <sup>2</sup> <sup>P</sup> Exp KC AC R.T *. .. .. . . .* 

<sup>1</sup> -E H H H k K KC A A AC <sup>2</sup> <sup>P</sup> Exp KC AC R.T *. . .. . .. . . .* 

> 1 A H

3 A H

In equations (31), (32) and (33) the enthalpies of adsorption of 1-heptyne and H2 are expressed in absolute values. The values of the respective slopes were calculated as follows:

> H2 1 A

H2 1

H2 1

3 3 1 A H2

3 3 3 A H2

<sup>H</sup> H 18 02 KJ mol <sup>2</sup> *.* (34)

<sup>E</sup> H 36 6 KJ mol <sup>2</sup> *.* (35)

E H H 2 18 KJ mol <sup>2</sup> *.* (36)

7

A A

1AS 1AS

3A S 3A S

H2 H2 H2 H2

H2 H2 H2 H2

H2 H2 H2 H2

*. . .*

H2 H2 A

(30)

H2 A

(31)

(32)

(33)

*.*

H2 H2


### **5. Conclusions**

Pd/Al2O3 was more active and selective than Ni/Al2O3 for the partial hydrogenation of 1 heptyne to 1-heptene.

In order to analyze the influence of the different variables (hydrogen partial pressure, initial concentration of 1-heptyne and reaction temperature) on the reaction rate a pseudo homogeneous model was proposed based on power law kinetics. Reaction orders for hydrogen and 1-heptyne were obtained as well as the apparent activation energy. For the Ni/Al2O3 catalyst, reaction orders of 1.3 in hydrogen and -0.22 in 1-heptyne, and apparent activation energy of 24 KJ mol-1 were obtained. For the Pd/Al2O3 catalyst, reaction orders of - 2.6 and -1.5 in hydrogen and 1-heptyne, respectively, and apparent activation energy of 18 KJ mol-1 were obtained.

Kinetic Study of the Partial Hydrogenation of 1-Heptyne over Ni and Pd Supported on Alumina 181

Juan M. Badano and Mónica Quiroga\*

*University of Litoral (UNL), Santa Fe, Argentina* 

and CONICET of Argentina are greatly acknowledged.

AA: preexponential factor of the heat of adsorption of 1-heptyne. AB: preexponential factor of the heat of adsorption of 1-heptene. AH2: preexponential factor of the heat of adsorption of hydrogen. A1: preexponential factor of the specific reaction rate constant, k1. A3: preexponential factor of the specific reaction rate constant, k3.

CH2: concentration of hydrogen in the liquid phase (mol L-1).

EH2: activation energy for the hydrogen adsorption (KJ mol-1).

HH2: absolute value of the enthalpy of hydrogen adsorption (KJ mol-1). HA: absolute value of the enthalpy of adsorption of 1-heptyne (KJ mol-1). HB: absolute value of the enthalpy of adsorption of 1-heptene (KJ mol-1).

: order of reaction with respect to 1-heptyne. : order of reaction with respect to hydrogen.

CA0: initial concentration of 1-heptyne (mol L-1). CA: concentration of 1-heptyne (mol L-1). CB: concentration of 1-heptene (mol L-1). CC: concentration of n-heptane (mol L-1).

A: Arrhenius preexponential factor.

CS: concentration of free sites.

H: reaction heat (KJ mol-1)

Corresponding Author

 \*

k: specific reaction rate constant.

EA: apparent activation energy (KJ mol-1).

*Santa Fe, Argentina* 

*Santa Fe, Argentina* 

*Santa Fe, Argentina* 

**Acknowledgement** 

Juan Yori

**Notation** 

*Institute of Catalysis and Petrochemistry Research, INCAPE (CONICET- UNL),* 

*Institute of Catalysis and Petrochemistry Research, INCAPE (CONICET- UNL),* 

*Departament of Chemical Engineering Reactions, Faculty of Chemical Engineering,* 

*Inorganic Chemistry, Departament of Chemistry, Faculty of Chemical Engineering National* 

The experimental assistance of C. A. Mázzaro and the financial support provided by UNL

E1: activation energy of the reaction of hydrogenation of 1-heptyne to 1-heptene (KJ mol-1). E3: activation energy of the reaction of hydrogenation of 1-heptyne to n-heptane (KJ mol-1).

In order to elucidate the role of Ni and Pd on the reaction rate, the kinetic data were fitted with six different heterogeneous LHHW models. The results obtained indicate that for the Ni/Al2O3 catalyst the controlling step is the dissociative adsorption of hydrogen over the metal active sites and the reaction rate can be expressed by:

$$r = \frac{P\_3}{\left[\text{C}\_A + P\_1 \text{C}\_B\right]^2}$$

If it is assumed that the adsorption enthalpy of 1-heptene can be considered negligible, a value of -17.91 KJ mol-1 is obtained for the adsorption enthalpy of 1-heptyne. In the same way, the value of activation energy for the hydrogen adsorption is 22.2 KJ mol-1.

For the Pd/Al2O3 catalyst the controlling steps are the surface hydrogenation reactions (1-heptyne to 1-heptene and 1-heptyne to n-heptane). The corresponding reaction rates are:

$$r\_1 = \frac{P\_{10} \cdot C\_A}{\left[1 + P\_7 \cdot C\_A\right]^3} \qquad\qquad\qquad\qquad r\_3 = \frac{P\_{11} \cdot C\_A}{\left[1 + P\_7 \cdot C\_A\right]^3}$$

Using a value for the adsorption enthalpy of hydrogen on Pd reported in literature, a value of -19.64 KJ mol-1 is obtained for the adsorption enthalpy of 1-heptyne (HA) over Pd. Besides, the value obtained for the activation energy for the hydrogenation reaction of 1 heptyne to 1-heptene (E1) was 18.58 KJ mol-1. This coincides with the fact that the reaction can proceed at low temperatures.

The different activity levels of the Pd/Al2O3 and Ni/Al2O3 catalysts are due to a kinetic limitation for the adsorption of hydrogen on Ni. In the case of Pd this limitation does not exist.

### **Author details**

M. Juliana Maccarrone, Cecilia Lederhos and Carolina Betti *Institute of Catalysis and Petrochemistry Research, INCAPE (CONICET- UNL), Santa Fe, Argentina* 

Gerardo C. Torres *Departament of Chemical Engineering Reactions, Faculty of Chemical Engineerin, Santa Fe, Argentina* 

Juan M. Badano and Mónica Quiroga\* *Institute of Catalysis and Petrochemistry Research, INCAPE (CONICET- UNL), Santa Fe, Argentina Inorganic Chemistry, Departament of Chemistry, Faculty of Chemical Engineering National University of Litoral (UNL), Santa Fe, Argentina* 

Juan Yori *Institute of Catalysis and Petrochemistry Research, INCAPE (CONICET- UNL), Santa Fe, Argentina Departament of Chemical Engineering Reactions, Faculty of Chemical Engineering, Santa Fe, Argentina* 

### **Acknowledgement**

The experimental assistance of C. A. Mázzaro and the financial support provided by UNL and CONICET of Argentina are greatly acknowledged.

### **Notation**

180 Hydrogenation

are:

exist.

**Author details** 

*Santa Fe, Argentina*  Gerardo C. Torres

*Santa Fe, Argentina* 

KJ mol-1 were obtained.

homogeneous model was proposed based on power law kinetics. Reaction orders for hydrogen and 1-heptyne were obtained as well as the apparent activation energy. For the Ni/Al2O3 catalyst, reaction orders of 1.3 in hydrogen and -0.22 in 1-heptyne, and apparent activation energy of 24 KJ mol-1 were obtained. For the Pd/Al2O3 catalyst, reaction orders of - 2.6 and -1.5 in hydrogen and 1-heptyne, respectively, and apparent activation energy of 18

In order to elucidate the role of Ni and Pd on the reaction rate, the kinetic data were fitted with six different heterogeneous LHHW models. The results obtained indicate that for the Ni/Al2O3 catalyst the controlling step is the dissociative adsorption of hydrogen over the

> 3

If it is assumed that the adsorption enthalpy of 1-heptene can be considered negligible, a value of -17.91 KJ mol-1 is obtained for the adsorption enthalpy of 1-heptyne. In the same

For the Pd/Al2O3 catalyst the controlling steps are the surface hydrogenation reactions (1-heptyne to 1-heptene and 1-heptyne to n-heptane). The corresponding reaction rates

Using a value for the adsorption enthalpy of hydrogen on Pd reported in literature, a value of -19.64 KJ mol-1 is obtained for the adsorption enthalpy of 1-heptyne (HA) over Pd. Besides, the value obtained for the activation energy for the hydrogenation reaction of 1 heptyne to 1-heptene (E1) was 18.58 KJ mol-1. This coincides with the fact that the reaction

The different activity levels of the Pd/Al2O3 and Ni/Al2O3 catalysts are due to a kinetic limitation for the adsorption of hydrogen on Ni. In the case of Pd this limitation does not

*C P .C*

*r*

way, the value of activation energy for the hydrogen adsorption is 22.2 KJ mol-1.

 10 1 3 <sup>7</sup> 1

M. Juliana Maccarrone, Cecilia Lederhos and Carolina Betti

*Institute of Catalysis and Petrochemistry Research, INCAPE (CONICET- UNL),* 

*Departament of Chemical Engineering Reactions, Faculty of Chemical Engineerin,* 

*P .C*

*P .C*

*r*

can proceed at low temperatures.

*A A* *A B* 1 *P*

2

*r*

 11 3 3 <sup>7</sup> 1

*P .C*

*P .C*

*A A*

metal active sites and the reaction rate can be expressed by:

: order of reaction with respect to 1-heptyne.

: order of reaction with respect to hydrogen.

A: Arrhenius preexponential factor.

AA: preexponential factor of the heat of adsorption of 1-heptyne.

AB: preexponential factor of the heat of adsorption of 1-heptene.

AH2: preexponential factor of the heat of adsorption of hydrogen.

A1: preexponential factor of the specific reaction rate constant, k1.

A3: preexponential factor of the specific reaction rate constant, k3.

CA0: initial concentration of 1-heptyne (mol L-1).

CA: concentration of 1-heptyne (mol L-1).

CB: concentration of 1-heptene (mol L-1).

CC: concentration of n-heptane (mol L-1).

CH2: concentration of hydrogen in the liquid phase (mol L-1).

CS: concentration of free sites.

EA: apparent activation energy (KJ mol-1).

EH2: activation energy for the hydrogen adsorption (KJ mol-1).

E1: activation energy of the reaction of hydrogenation of 1-heptyne to 1-heptene (KJ mol-1).

E3: activation energy of the reaction of hydrogenation of 1-heptyne to n-heptane (KJ mol-1). H: reaction heat (KJ mol-1)

HH2: absolute value of the enthalpy of hydrogen adsorption (KJ mol-1).

HA: absolute value of the enthalpy of adsorption of 1-heptyne (KJ mol-1).

HB: absolute value of the enthalpy of adsorption of 1-heptene (KJ mol-1).

k: specific reaction rate constant.

 \* Corresponding Author

K: equilibrium constant (equation (6)).

KA: adsorption constant for 1-heptyne.

KB: adsorption constant for 1-heptene

KC: adsorption constant for n-heptane

KH2: adsorption constant for hydrogen

kA: direct rate specific constant for 1-heptyne adsorption.

kH2: direct rate specific constant for H2 dissociative adsorption.

k' H2: direct rate specific constant for H2 non disociative adsorption.

k1: direct rate specific constant for 1-heptyne to 1-heptene surface hydrogenation reaction.

Kinetic Study of the Partial Hydrogenation of 1-Heptyne over Ni and Pd Supported on Alumina 183

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k3: direct rate specific constant for 1-heptyne to n-heptane surface hydrogenation reaction. PH2: partial pressure of hydrogen (bar).

R: ideal gas universal constant (0.082 L.atm.mol-1.K-1).

r1: rate of step 1 (mol L-1.h-1).

r3: rate of step 3 (mol L-1.h-1).

r0A: 1-heptyne initial hydrogenation rate (mol L-1.h-1).

T: absolute temperature (K)

t: time.

### **6. References**


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182 Hydrogenation

k'

t: time.

**6. References** 

K: equilibrium constant (equation (6)). KA: adsorption constant for 1-heptyne. KB: adsorption constant for 1-heptene KC: adsorption constant for n-heptane KH2: adsorption constant for hydrogen

PH2: partial pressure of hydrogen (bar).

r1: rate of step 1 (mol L-1.h-1). r3: rate of step 3 (mol L-1.h-1).

T: absolute temperature (K)

Synth 46: 89–92.

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kA: direct rate specific constant for 1-heptyne adsorption. kH2: direct rate specific constant for H2 dissociative adsorption.

R: ideal gas universal constant (0.082 L.atm.mol-1.K-1).

r0A: 1-heptyne initial hydrogenation rate (mol L-1.h-1).

H2: direct rate specific constant for H2 non disociative adsorption.

k1: direct rate specific constant for 1-heptyne to 1-heptene surface hydrogenation reaction. k3: direct rate specific constant for 1-heptyne to n-heptane surface hydrogenation reaction.

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**Section 2** 

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**Chemistry and Renewable Energy** 


**Hydrogenation Reactions in Environmental Chemistry and Renewable Energy** 

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**Chapter 8** 

© 2012 Sotelo-Boyás et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 Sotelo-Boyás et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons

**Hydroconversion of Triglycerides** 

Due to the depletion of crude reserves and the increasing demand for clean hydrocarbon fuels the production of renewable materials-based fuels has emerged to solve at least partially this problem in the past decade and it is expected to continue [1,2]. Green fuels can be classified as naphtha, jet fuel, and diesel. In the case of green diesel, its increasing

The common ways to produce diesel-type fuel from biomass are a) by transesterification of triglycerides to obtain biodiesel, and b) by hydroprocessing to synthesize green diesel. Biodiesel is a mixture of fatty acid methyl esters (FAME) while green diesel is a mixture of hydrocarbons, mainly heptadecane and octadecane. Both fuels can be used as additives to petro-diesel. FAME can be used to enhance the lubricity of petro-diesel while green diesel can boost the cetane number. Biodiesel can be obtained with an alkaline liquid catalyst at 60 °C and atmospheric pressure, while green diesel requires a bifunctional solid catalyst (acid/metal), temperatures around 300 °C and ca. 5 MPa of hydrogen in a continuous flow

The technology for producing green fuels from triglycerides has been used in petroleum refineries for about 60 years, for instance the same catalyst, reactor type and separation equipment used in the hydrotreating of vacuum gas oil can effectively be used for hydrotreatment of fats and vegetable oils. Thus, during the last decade it has been an increasing interest in research and development for their optimal production, trying to find the best catalyst and the most favorable operating conditions. Many countries and major petroleum companies are nowadays considering the use of vegetable oils and fats as raw materials for production of hydrogenated green fuels. The main drawbacks seem to be the availability of raw material and the large consumption of hydrogen during the process,

**into Green Liquid Fuels** 

and Felipe de Jesús Hernández-Loyo

http://dx.doi.org/10.5772/48710

**1. Introduction** 

process.

Rogelio Sotelo-Boyás, Fernando Trejo-Zárraga

Additional information is available at the end of the chapter

demand could reach 900 million tons by 2020 [3].

### **Chapter 8**
