**4. Results and discussion**

### **4.1. Catalyst characterization**

SBA-15 and PW-NH2-SBA-15 catalysts showed a typical IV adsorption isotherm with H1 hysteresis loop as defined by IUPAC. It was observed that the specific surface area and total pore volume of SBA-15 decreased with the immobilization of PW on SBA-15 (Table 1).


(a)HPA load determined by ICP analysis

(b) BET

(c) (p/p°) = 0.98

**Table 1.** Physicochemical characterization of SBA-15 and PW-NH2-SBA-15

X-ray diffraction patterns of the SBA-15 and PW-NH2-SBA-15 catalysts are shown in Figure 1. All three reflections are still detectable after PW immobilization, suggesting that hexagonal pore structure of the support is retained.

Valorization of Waste Cooking Oil into Biodiesel over Heteropolyacids Immobilized on Mesoporous Silica… http://dx.doi.org/10.5772/59584 291

Surface

X-ray diffraction patterns of the SBA-15 and PW-NH2-SBA-15 catalysts are shown in Figure 1. All three reflections are still detectable after PW immobilization, suggesting that hexagonal pore

VT c

(cm3 /g)

areab (m2 /g)

Figure 1. X-ray diffractograms of SBA-15 (A) and PW-NH2-SBA-15 (B). **Figure 1.** X-ray diffractograms of SBA-15 (A) and PW-NH2-SBA-15 (B).

PW load<sup>a</sup>

(wt%)

(a)HPA load determined by ICP analysis

structure of the support is retained.

(b) *BET* 

(c) (p/pº) = 0.98

SBA-15 - 1050 1.38

PW-NH2-SBA-15 5.6 735 0.62

Transmission electron microscopy (TEM) analyses were performed on a Hitachi S-2400

The catalytic experiments were carried out in a stirred batch reactor at 60°C. In a typical experiment, the reactor was loaded with 30 mL of methanol and 0.2 g of catalyst. Reactions

Stability tests of the catalyst were carried out by running four consecutive experiments, using the same reaction conditions. Between the catalytic experiments, the catalyst was separated from the reaction mixture by filtration, washed with acetone and methanol and dried at 70° C

In order to study the reusability, the PW-NH2-SBA-15 catalyst was filtered from the reaction mixture. After this operation, it was soaked in hexane overnight and it was dried overnight. Undecano was used as the internal standard. Samples were taken periodically and analyzed by GC, using a Hewlett Packard instrument equipped with a 30 m x 0.25 mm HP-5 column.

SBA-15 and PW-NH2-SBA-15 catalysts showed a typical IV adsorption isotherm with H1 hysteresis loop as defined by IUPAC. It was observed that the specific surface area and total pore volume of SBA-15 decreased with the immobilization of PW on SBA-15 (Table 1).

SBA-15 - 1050 1.38 PW-NH2-SBA-15 5.6 735 0.62

X-ray diffraction patterns of the SBA-15 and PW-NH2-SBA-15 catalysts are shown in Figure 1. All three reflections are still detectable after PW immobilization, suggesting that hexagonal

**Surface areab (m2 /g)**

**VTc (cm3/g)**

**PW loada (wt%)**

**Table 1.** Physicochemical characterization of SBA-15 and PW-NH2-SBA-15

scanning electron microscope, at a current voltage of 25 kV.

were started by adding 2.5 ml waste cooking oil (WCO).

**3.3. Catalytic experiments**

290 Biofuels - Status and Perspective

**4. Results and discussion**

**4.1. Catalyst characterization**

(a)HPA load determined by ICP analysis

pore structure of the support is retained.

(b) BET

(c) (p/p°) = 0.98

overnight.

Catalyst acidity was measured by means of potentiometric titration [53].

Figure 2 shows the FT-IR spectra of SBA-15 and PW-NH2-SBA-15 samples. For SBA-15 bands were observed at 1084, 812 and 476 cm-1, which are attributed to asymmetric stretching, symmetric stretching and bending modes of Si–O–Si, respectively. For PW-NH2-SBA-15 catalyst the characteristic bands of SBA-15 are present along with the bands at 1083, 982, 890 and 808 cm-1, which are the fingerprint of Keggin structure of Figure 2 shows the FT-IR spectra of SBA-15 and PW-NH2-SBA-15 samples. For SBA-15 bands were observed at 1084, 812 and 476 cm-1, which are attributed to asymmetric stretching, symmetric stretching and bending modes of Si–O–Si, respectively. For PW-NH2-SBA-15 catalyst the characteristic bands of SBA-15 are present along with the bands at 1083, 982, 890 and 808 cm-1, which are the fingerprint of Keggin structure of HPW. However, some bands typical of the Keggin-type HPA structures are overlapped or partially overlapped with the bands of the SBA-15 matrix framework, probably due to the low loading of PW. Similar results were also observed by Liu et al. [52].

HPW. However, some bands typical of the Keggin-type HPA structures are overlapped

Figure 2. FT-IR spectra of catalysts: (A) SBA-15 and (B) PW-NH2-SBA-15. **Figure 2.** FT-IR spectra of catalysts: (A) SBA-15 and (B) PW-NH2-SBA-15.

remains after immobilization of PW on SBA-15 (Fig. 3).

TEM image of PW-NH2-SBA-15 catalyst was carried out. The morphology of the support remains after immobilization of PW on SBA-15 (Fig. 3).

TEM image of PW-NH2-SBA-15 catalyst was carried out. The morphology of the support

Figure 3. Transmission electron microscopy (TEM) image of PW-NH2-SBA-15.

Table 2 shows the initial electrode potential (Ei) of the materials. The Ei indicates the

maximum acid strength of the surface sites [53]. It can be observed that Ei increased

**Figure 3.** Transmission electron microscopy (TEM) image of PW-NH2-SBA-15.

Table 2 shows the initial electrode potential (Ei) of the materials. The Ei indicates the maximum acid strength of the surface sites [53]. It can be observed that Ei increased with the amount of PW immobilized in SBA-15, which can be due to the increase in the amount of protons with PW loading of the SBA-15 support.


**Table 2.** Initial electrode potential (mV) of materials

### **4.2. Catalytic experiments**

The biodiesel production from WCO was carried out over PW-NH2-SBA-15 catalyst at 60°C.

Different catalytic experiments were carried out at different stirring speeds to study the influence of external resistances to mass transfer. It was observed that experiments carried out with 700 rpm have got a good mix of the compounds and eliminate possible mass transfer problems.

### **4.3. Effect of the nature of alcohol**

The alcohols most frequently used in biodiesel production are methanol and ethanol. Due to the low cost of methanol, this alcohol is the first choice for the esterification/transesterification reactions of WCO. However, for biodiesel production to be more environment friendly, ethanol is the ideal candidate for the synthesis of a fully biogenerated biodiesel, since ethanol is derived from agricultural products (renewable sources) [3].

Figure 4 shows the effect of the nature of alcohol (methanol, ethanol and 1-propanol) on the fatty acid ester concentrations obtained over PW-NH2-SBA-15 catalyst, at 60°C. It was observed that the ester concentrations obtained with 1-propanol led to lower conversion, when com‐ pared with methanol and ethanol. Similar results were also observed by Sreeprasanth et al. [54], on the transesterification of rubber seed with ethanol over Fe-Zn-1 catalyst. This behavior can be explained not due to the reaction rate with ethanol is slower than with methanol, as ethyl nucleophile is less reactive than methyl nucleophile. be explained not due to the reaction rate with ethanol is slower than with methanol, as ethyl nucleophile is less reactive

Figure 4. Biodiesel production from WCO with methanol over PW-NH2-SBA-15 catalyst. Effect of alcohol nature. FAME concentration (mol.dm-3) *versus* time (h): ( ) Methanol; ( ) Ethanol; ( ) 1-Propanol. **Figure 4.** Biodiesel production from WCO with methanol over PW-NH2-SBA-15 catalyst. Effect of alcohol nature. FAME concentration (mol.dm-3) *versus* time (h): (○) Methanol; (▲) Ethanol; (□) 1-Propanol.

In order to study the influence of the initial free fatty acid amount in the WCO, three different amounts of fatty acid (as a

#### **4.4. Effect of the initial amount of free fatty acid 4.4. Effect of the initial amount of free fatty acid**

0 0.05 0.1 0.15 0.2

CFAME

than methyl nucleophile.

**Figure 3.** Transmission electron microscopy (TEM) image of PW-NH2-SBA-15.

PW loading of the SBA-15 support.

292 Biofuels - Status and Perspective

**Table 2.** Initial electrode potential (mV) of materials

**4.2. Catalytic experiments**

problems.

Table 2 shows the initial electrode potential (Ei) of the materials. The Ei indicates the maximum acid strength of the surface sites [53]. It can be observed that Ei increased with the amount of PW immobilized in SBA-15, which can be due to the increase in the amount of protons with

**Sample Ei**

SBA-15 +110 PW-NH2-SBA-15 +408

The biodiesel production from WCO was carried out over PW-NH2-SBA-15 catalyst at 60°C. Different catalytic experiments were carried out at different stirring speeds to study the influence of external resistances to mass transfer. It was observed that experiments carried out with 700 rpm have got a good mix of the compounds and eliminate possible mass transfer

 **(mV)**

model) was added to the reaction mixture. The catalytic experiments were carried out at different amounts of initial palmitic acid in the WCO, over PW-NH2-SBA-15 catalyst, while the initial concentration of WCO (0.088mol.dm-3) and the catalysts loading (m=0.2 g) were kept constant. Figure 5 shows the influence of the initial amount of FFA in WCO on the biodiesel production. It was observed that the initial reaction rate increases with the amount of FFA. When the initial amount of FFA increases, a slight increase on the conversion was observed. Similar results were also observed by Marchetti and Errazu [16]. Therefore, this effect could also be seen on the total FAME production since the final amount of biofuel will be produced from the triglycerides as well as from the fatty acids present in the reaction mixture. 0.25 0.3 0.35 (mol.dm-3)In order to study the influence of the initial free fatty acid amount in the WCO, three different amounts of fatty acid (as a model) was added to the reaction mixture. The catalytic experiments were carried out at different amounts of initial palmitic acid in the WCO, over PW-NH2- SBA-15 catalyst, while the initial concentration of WCO (0.088mol.dm-3) and the catalysts loading (m=0.2 g) were kept constant. Figure 5 shows the influence of the initial amount of FFA in WCO on the biodiesel production. It was observed that the initial reaction rate increases with the amount of FFA. When the initial amount of FFA increases, a slight increase on the conversion was observed. Similar results were also observed by Marchetti and Errazu [16]. Therefore, this effect could also be seen on the total FAME production since the final amount

0 20 40 60 80 100

Time (h)

than methyl nucleophile.

0

0.05

0.1

0.15

Cesters (mol.dm-3)

0.2

0.25

of biofuel will be produced from the triglycerides as well as from the fatty acids present in the reaction mixture. the biodiesel production. It was observed that the initial reaction rate increases with the amount of FFA. When the initial amount of FFA increases, a slight increase on the conversion was observed. Similar results were also observed by Marchetti and Errazu [16]. Therefore, this effect could also be seen on the total FAME production since the final amount

0 20 40 60 80 100 120

Time (h)

(mol.dm-3) *versus* time (h): ( ) Methanol; ( ) Ethanol; ( ) 1-Propanol.

**4.4. Effect of the initial amount of free fatty acid**

be explained not due to the reaction rate with ethanol is slower than with methanol, as ethyl nucleophile is less reactive

Figure 4. Biodiesel production from WCO with methanol over PW-NH2-SBA-15 catalyst. Effect of alcohol nature. FAME concentration

model) was added to the reaction mixture. The catalytic experiments were carried out at different amounts of initial palmitic acid in the WCO, over PW-NH2-SBA-15 catalyst, while the initial concentration of WCO (0.088mol.dm-3) and the catalysts loading (m=0.2 g) were kept constant. Figure 5 shows the influence of the initial amount of FFA in WCO on

of biofuel will be produced from the triglycerides as well as from the fatty acids present in the reaction mixture.

**Figure 5.** Biodiesel production from WCO with methanol over PW-NH2-SBA-15 catalyst. Effect of the amount of pal‐ mitic acid. FAME concentration (mol.dm-3) *versus* time (h): (○) 0%; (□) 4%; (▲) 12 % ; (×) 27 %.

#### **4.5. Catalyst stability** Figure 5. Biodiesel production from WCO with methanol over PW-NH2-SBA-15 catalyst. Effect of the amount of palmitic acid. FAME

In order to study the catalytic stability of the PW-NH2-SBA-15, different batch runs with the same catalyst sample and under the same conditions were carried out. Figure 6 shows the catalytic activity of PW-NH2-SBA-15 at different batch runs. It was observed that the catalytic activity decreases only about 10%, after the fourth use. **4.5. Catalyst stability** In order to study the catalytic stability of the PW-NH2-SBA-15, different batch runs with the same catalyst sample and under the same conditions were carried out. Figure 6 shows the catalytic activity of PW-NH2-SBA-15 at different batch

runs. It was observed that the catalytic activity decreases only about 10%, after the fourth use.

concentration (mol.dm-3) *versus* time (h): ( ) 0%; ( ) 4%; ( ) 12 % ; (×) 27 %.

Figure 6. Catalytic stability of PW-NH2-SBA-15 catalyst in biodiesel production with methanol. The initial activities are taken as the maximum observed reaction rate, which was calculated from the maximum slope of the methyl ester kinetic curve. **Figure 6.** Catalytic stability of PW-NH2-SBA-15 catalyst in biodiesel production with methanol. The initial activities are taken as the maximum observed reaction rate, which was calculated from the maximum slope of the methyl ester ki‐ netic curve.

A simple kinetic model can be established based on the following assumptions:

4. Triglycerides are consumed according to the consecutive reaction network:

The reaction rate of these three pseudo elementary reaction are expressed as:

2. First order kinetics with respect to the reactants is assumed. The forward and reverse reactions follow second order

where T represents triglycerides, D represents diglycerides, M represents monoglycerides, A represents alcohol, G

3. Due to the excess of methanol used, the reverse reaction could be minimized and it was not considered in the

**4.6. Kinetics modeling**

overall kinetics;

Bez Broja 1*<sup>k</sup> TA DE*

Bez Broja 2*<sup>k</sup> DA ME*

Bez Broja 3 *<sup>k</sup> MA GE*

1 1T A r =k C .C (1)

2 2D A r =k C .C (2)

reaction rate.

1. Isothermal and isobaric reaction conditions;

represents glycerol and E represents esters of fatty acids;

### **4.6. Kinetics modeling**

of biofuel will be produced from the triglycerides as well as from the fatty acids present in the

0 20 40 60 80 100

Time (h)

Figure 5. Biodiesel production from WCO with methanol over PW-NH2-SBA-15 catalyst. Effect of the amount of palmitic acid. FAME

In order to study the catalytic stability of the PW-NH2-SBA-15, different batch runs with the same catalyst sample and under the same conditions were carried out. Figure 6 shows the catalytic activity of PW-NH2-SBA-15 at different batch

Figure 6. Catalytic stability of PW-NH2-SBA-15 catalyst in biodiesel production with methanol. The initial activities are taken as the

2. First order kinetics with respect to the reactants is assumed. The forward and reverse reactions follow second order

where T represents triglycerides, D represents diglycerides, M represents monoglycerides, A represents alcohol, G

3. Due to the excess of methanol used, the reverse reaction could be minimized and it was not considered in the

maximum observed reaction rate, which was calculated from the maximum slope of the methyl ester kinetic curve.

A simple kinetic model can be established based on the following assumptions:

1st use 2nd use 3rd use 4th use

**Figure 6.** Catalytic stability of PW-NH2-SBA-15 catalyst in biodiesel production with methanol. The initial activities are taken as the maximum observed reaction rate, which was calculated from the maximum slope of the methyl ester ki‐

4. Triglycerides are consumed according to the consecutive reaction network:

The reaction rate of these three pseudo elementary reaction are expressed as:

**Figure 5.** Biodiesel production from WCO with methanol over PW-NH2-SBA-15 catalyst. Effect of the amount of pal‐

In order to study the catalytic stability of the PW-NH2-SBA-15, different batch runs with the same catalyst sample and under the same conditions were carried out. Figure 6 shows the catalytic activity of PW-NH2-SBA-15 at different batch runs. It was observed that the catalytic

runs. It was observed that the catalytic activity decreases only about 10%, after the fourth use.

concentration (mol.dm-3) *versus* time (h): ( ) 0%; ( ) 4%; ( ) 12 % ; (×) 27 %.

mitic acid. FAME concentration (mol.dm-3) *versus* time (h): (○) 0%; (□) 4%; (▲) 12 % ; (×) 27 %.

0 20 40 60 80 100 120

Time (h)

(mol.dm-3) *versus* time (h): ( ) Methanol; ( ) Ethanol; ( ) 1-Propanol.

**4.4. Effect of the initial amount of free fatty acid**

than methyl nucleophile.

0

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

activity decreases only about 10%, after the fourth use.

**4.5. Catalyst stability**

**4.6. Kinetics modeling**

0

2

Catalytic activity x 104

(mol/h.gcat)

4

6

8

overall kinetics;

Bez Broja 1*<sup>k</sup> TA DE*

Bez Broja 2*<sup>k</sup> DA ME*

Bez Broja 3 *<sup>k</sup> MA GE*

1 1T A r =k C .C (1)

2 2D A r =k C .C (2)

reaction rate.

1. Isothermal and isobaric reaction conditions;

represents glycerol and E represents esters of fatty acids;

CFAME

(mol.dm-3)

0.05

0.1

0.15

Cesters (mol.dm-3)

0.2

0.25

be explained not due to the reaction rate with ethanol is slower than with methanol, as ethyl nucleophile is less reactive

Figure 4. Biodiesel production from WCO with methanol over PW-NH2-SBA-15 catalyst. Effect of alcohol nature. FAME concentration

In order to study the influence of the initial free fatty acid amount in the WCO, three different amounts of fatty acid (as a model) was added to the reaction mixture. The catalytic experiments were carried out at different amounts of initial palmitic acid in the WCO, over PW-NH2-SBA-15 catalyst, while the initial concentration of WCO (0.088mol.dm-3) and the catalysts loading (m=0.2 g) were kept constant. Figure 5 shows the influence of the initial amount of FFA in WCO on the biodiesel production. It was observed that the initial reaction rate increases with the amount of FFA. When the initial amount of FFA increases, a slight increase on the conversion was observed. Similar results were also observed by Marchetti and Errazu [16]. Therefore, this effect could also be seen on the total FAME production since the final amount of biofuel will be produced from the triglycerides as well as from the fatty acids present in the reaction mixture.

reaction mixture.

294 Biofuels - Status and Perspective

**4.5. Catalyst stability**

netic curve.

A simple kinetic model can be established based on the following assumptions:


$$\begin{aligned} T + A &\xrightarrow{k\_1} D + E \\\\ D + A &\xrightarrow{k\_2} M + E \\\\ M + A &\xrightarrow{k\_3} G + E \end{aligned}$$

where T represents triglycerides, D represents diglycerides, M represents monoglycerides, A represents alcohol, G represents glycerol and E represents esters of fatty acids;

The reaction rate of these three pseudo elementary reaction are expressed as:

$$\mathbf{r\_1=k\_1C\_T.C\_A} \tag{1}$$

$$\mathbf{k\_2 = k\_2 C\_D, C\_A} \tag{2}$$

$$\mathbf{r}\_3 = \mathbf{k}\_3 \mathbf{C}\_{\mathbf{M}} \mathbf{C}\_{\mathbf{A}} \tag{3}$$

For batch reactor the mole balance equations may be written as

$$\frac{\mathbf{d}\mathbf{C}\_{\rm T}}{\mathbf{d}\mathbf{t}} = -\frac{\mathbf{W}}{\mathbf{V}} \begin{pmatrix} \mathbf{r}\_{\rm i} \end{pmatrix} \tag{4}$$

$$\frac{\mathbf{dC}\_{\Delta}}{\mathbf{d}t} = -\frac{\mathbf{W}}{\mathbf{V}} (\mathbf{r}\_1 + \mathbf{r}\_2 + \mathbf{r}\_3) \tag{5}$$

$$\frac{\mathbf{dC\_D}}{\mathbf{dt}} = \frac{\mathbf{W}}{\mathbf{V}} \cdot \left(\mathbf{r\_i} - \mathbf{r\_2}\right) \tag{6}$$

$$\frac{\mathbf{dC\_M}}{\mathbf{dt}} = \frac{\mathbf{W}}{\mathbf{V}} \cdot \left(\mathbf{r\_2} - \mathbf{r\_3}\right) \tag{7}$$

$$\frac{\text{dC}\_{\text{G}}}{\text{dt}} = \frac{\text{W}}{\text{V}} \left(\text{r}\_{\text{3}}\right) \tag{8}$$

$$\frac{\mathbf{dC\_E}}{\mathbf{dt}} = \frac{\mathbf{W}}{\mathbf{V}} \left(\mathbf{r\_i} + \mathbf{r\_2} + \mathbf{r\_3}\right) \tag{9}$$

The differential equations system was integrated using the Euler Method. The optimization was carried out by the *SOLVER* routine in a *Microsoft Excel* spreadsheet.

Figure 7 shows the concentration of fatty acid methyl ester (FAME) *versus* time (h) on the transesterification of WCO with methanol. The solid line represents the model fitted to the data points. It was observed that the kinetic model fits experimental concentration data quite well. The model parameters, k1, k2 and k3, have got the value of 0.00979, 0.01348 and 0.01956 dm6 .mol-1.h-1.gcat-1, respectively. It was observed that k1<k2<k3, which can be explained due to the molecular size of monoglycerides, diglycerides and triglycerides and due to the textural characteristics of PW-NH2-SBA-15. The size of monoglycerides is smaller than that of digly‐ cerides and triglycerides. Consequently, it is expected that, near the active sites of catalyst, the amount of monoglycerides is higher than the amount of diglycerides and triglycerides. As the reaction rate is dependent on reactant concentration, a high concentration of monoglycerides leads to high reaction rates.

**Figure 7.** Concentration of FAME (mol.dm-3) versus time (h).

### **5. Conclusions**

Biodiesel production from WCO with methanol was carried out over tungstophosphoric acid immobilized on SBA-15 by grafting technique, at 60°C. After PW immobilization, the mor‐ phology of the support remained.

In order to optimize the conditions, the influence of various reaction parameters, the nature of alcohol and the amount of initial free fatty acid on the transesterification of WCO in the presence of PW-NH2-SBA-15 catalyst were carried out. The catalytic stability of the material was also studied.

The esterification/transesterification of WCO with propanol and ethanol led to lower concen‐ tration of fatty acid esters than with methanol.

When different amounts of free fatty acids (palmitic acid) were added to the WCO, a slight increase on the concentration of FAME was observed.

In order to study the catalytic stability of PW-NH2-SBA-15 catalyst, four consecutive batch runs were carried out. It was observed that the catalytic activity of PW-NH2-SBA-15 tends to stabilize.
