**Hydrotreating Catalytic Processes for Oxygen Removal in the Upgrading of Bio-Oils and Bio-Chemicals**

Iñaki Gandarias and Pedro Luis Arias

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/52581

#### **1. Introduction**

In a future sustainable scenario a progressive transition by the chemical and energy indus‐ tries towards renewable feedstock will become compulsory. Energy demand is expected to grow by more than 50% by 2035 [1], with most of this increase in demand emerging from developing nations. Clearly, increasing demand from finite petroleum resources cannot be a satisfactory policy for the long term. The transition to a more renewable production system is now underway; however, this transition needs more research and investment in new tech‐ nologies to be feasible.

Biomass appears as the only renewable source for liquid fuels and most commodity chemi‐ cals [2]. This is the reason why, in the near future, bio-refineries in which biomass is catalyti‐ cally converted to pharmaceuticals, agricultural chemicals, plastics and transportation fuels will take the place of petrochemical plants [3]. Indeed, biomass represents 77.4% of global renewable energy supply [4]. Current technologies to produce liquid fuels from biomass are typically multistep and energy-intensive processes, including the production of ethanol by fermentation of biomass derived glucose [5],bio-oils by fast pyrolysis or high pressure lique‐ faction of biomass [6,7], polyols and alkanes from hydrogenolysis of biomass derived sorbi‐ tol [8],and biodiesel from vegetable oils [9].Biomass can also be gasified to produce CO and H2(synthesis gas), which can be further processed to produce methanol or liquid alkanes through Fischer–Tropsch synthesis [10].

The so-called "First Generation" biofuels, such as sugarcane ethanol in Brazil, corn ethanol in US, oilseed rape biodiesel in Germany, and palm oil biodiesel in Malaysia,already present mature commercial markets and well developed technologies. Nonetheless, there is a world‐ wide increasing awareness against the use of edible oils and seeds to generate transporta‐ tion fuels, and critical voices have aroused questioning the actual sustainability of these

"First Generation" biofuels. In fact, nowadays 95 % of biodiesel is made from edible oil [9]. This means that possible food resources are being used as automotive fuels when some part of the World's population is suffering from hunger. Therefore, large-scale production of bio‐ diesel from edible oils may bring about a global imbalance in the food supply market. An‐ other significant concern of using "First Generation" technologies is the deforestation and the destruction of ecosystems. Indeed, the expansion of oil-crop plantations for biofuel pro‐ duction on a large scale has caused deforestation in countries such as Malaysia, Indonesia and Brazil because more and more forest has been cleared for plantation purposes. In addi‐ tion to this, in developing countries energy crops are powerful competitors for scarce water resources [11].

residence time 0.5-5s). In order to obtain that fast heating rates, it is essential to use reactors that provide high external heat transfer (such as fluidized bed reactors) and to guarantee an efficient heat transfer through the biomass particle, using biomass particle size of less than 5 mm [7]. Fast pyrolysis produce 60-75 wt% of liquid bio-oil, 15-25 wt% of solid char, and 10-20 wt% of non condensable gases, depending on the feedstock. In slow pyrolysis biomass is heated to around 500ºC at much lower heating rates than those used in fast pyrolysis. The vapor residence times are much longer; they vary from 5 min to 30 min. As a consequence of the lower heating rate and of the longer vapor residence time, lower yields to pyrolysis oils and higher yields to char and gas products are obtained (Figure 2). As a result of all this, for

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Bio-oils are dark-red brown color liquids. They are also known as pyrolysis oils, bio-crude oil, wood oil or liquid wood. Bio-oils usually have higher density, viscosity and oxygen con‐ tent compared to fuel-oil. While the sulfur and nitrogen content is usually smaller (Table 1). The high oxygen content of bio-oils generates some negative characteristics like low heating value (HV), immiscibility with conventional fuels and high viscosity. A serious problem of bio-oils is their instability during storage, as their viscosity, HV and density are affected. This is because some of the organic compounds present in bio-oils are highly reactive. For instance, ketones, aldehydes and organic acids react to form ethers, acetals and hemiacetals respectively [15]. Therefore, bio-oils need to be upgraded to reduce their oxygen content in order to increase their stability, to be miscible with conventional oil, and to increase their H/C ratio. This upgrading can be carried out through three different routes: (i) catalytic hy‐ drotreating, usually known as hydrodeoxygenation (HDO), which consists mainly on decar‐ boxylation, hydrocracking, hydrogenolysis and hydrogenation reactions, (ii) zeolite upgrading or (iii) through esterification reactions. Zeolite upgrading is carried out without external hydrogen sources, and therefore the resulting oil has lower HV and H/C than con‐ ventional fuels. Esterification can significantly increase the chemical and physical properties

bio-oil production from biomass, fast pyrolysis processes are preferred.

**Figure 2.** Product spectrum from pyrolysis. Data from [14].

Being the non-edible portion of the plant and the most abundant source of biomass, ligno‐ cellulosic biomass materials are attracting growing attention as sustainable and renewable energy sources. The so-called "Second Generation" technologies for the production of fuels and chemicals can use a wide range of lignocellulosic biomass residues such as agricultural, industrial, and forest wastes, and also energy crops (willow, switchgrass) that do not com‐ pete with food crops for available land. The average composition of lignocellulosic material is as follows: 50% cellulose, 25% hemicellulose, and 20% lignin [12]. Cellulose is a linear pol‐ ysaccharide with β-1,4 linkages of D-glucopyranose monomers (Figure 1). Hemicellulose is a more complex polymer containing five different sugar monomers: five carbon sugars (xy‐ lose and arabinose) and six carbon sugars (galactose, glucose, and mannose). Lignin is a highly branched aromatic polymer, that consists of an irregular array of variously bonded "hydroxy-" and "methoxy-" substitutedphenylpropane units. Lignin is mainly found in woody biomass. Lignocellulosic materials can be converted into liquid fuels by three pri‐ mary routes, including (i) syngas production by gasification, (ii) bio-oil production by pyrol‐ ysis or liquefaction, and (iii) acid hydrolysis reactions [13].

**Figure 1.** Chemical structure of cellulose.

In the pyrolysis process, biomass feedstock is heated in the absence of oxygen, forming a gaseous product, which after cooling condenses. Depending on the operating conditions that are used, pyrolysis processes are known as slow or fast pyrolysis. Fast pyrolysis proc‐ esses are characterized by high rates of particle heating (heating rate > 1000ºC/min) to tem‐ peratures around 500ºC, and rapid cooling of the produced vapors to condense them (vapor residence time 0.5-5s). In order to obtain that fast heating rates, it is essential to use reactors that provide high external heat transfer (such as fluidized bed reactors) and to guarantee an efficient heat transfer through the biomass particle, using biomass particle size of less than 5 mm [7]. Fast pyrolysis produce 60-75 wt% of liquid bio-oil, 15-25 wt% of solid char, and 10-20 wt% of non condensable gases, depending on the feedstock. In slow pyrolysis biomass is heated to around 500ºC at much lower heating rates than those used in fast pyrolysis. The vapor residence times are much longer; they vary from 5 min to 30 min. As a consequence of the lower heating rate and of the longer vapor residence time, lower yields to pyrolysis oils and higher yields to char and gas products are obtained (Figure 2). As a result of all this, for bio-oil production from biomass, fast pyrolysis processes are preferred.

**Figure 2.** Product spectrum from pyrolysis. Data from [14].

Bio-oils are dark-red brown color liquids. They are also known as pyrolysis oils, bio-crude oil, wood oil or liquid wood. Bio-oils usually have higher density, viscosity and oxygen con‐ tent compared to fuel-oil. While the sulfur and nitrogen content is usually smaller (Table 1). The high oxygen content of bio-oils generates some negative characteristics like low heating value (HV), immiscibility with conventional fuels and high viscosity. A serious problem of bio-oils is their instability during storage, as their viscosity, HV and density are affected. This is because some of the organic compounds present in bio-oils are highly reactive. For instance, ketones, aldehydes and organic acids react to form ethers, acetals and hemiacetals respectively [15]. Therefore, bio-oils need to be upgraded to reduce their oxygen content in order to increase their stability, to be miscible with conventional oil, and to increase their H/C ratio. This upgrading can be carried out through three different routes: (i) catalytic hy‐ drotreating, usually known as hydrodeoxygenation (HDO), which consists mainly on decar‐ boxylation, hydrocracking, hydrogenolysis and hydrogenation reactions, (ii) zeolite upgrading or (iii) through esterification reactions. Zeolite upgrading is carried out without external hydrogen sources, and therefore the resulting oil has lower HV and H/C than con‐ ventional fuels. Esterification can significantly increase the chemical and physical properties of bio-oil, however it requires using high amounts of alcohols, which are highly demanded. Catalytic hydrotreating appears to have the greatest potential to obtain high grade oils which are compatible with the already available infrastructure for fossil fuels.

derived oils differs from processing petroleum because of the importance of deoxygenation as compared to nitrogen or sulfur removal. Bio-oil hydrodeoxygenation (HDO) process im‐ plies complex reaction networks that includes cracking, decarbonylation, decarboxylation, hydrocracking, hydrogenolysis, hydrogenation and polymerization. The upgrading process should yield a product with lower amount of water and oxygen, decreased acidity and vis‐ cosity, and higher HV. The complexity of the reactions and the high variety of oxygenated compounds make the evaluation of bio-oil upgrading difficult and has brought the use of model compounds such as phenol, guaicol, 2-ethylphenol, methyl heptanoate or benzofuran to test different catalysts and to understand the main characteristics of the HDO process. El‐ liot [17] has reported the HDO reactivity of different organic compounds that are typically present in bio-oils (see Figure 3). Olefins, aldehydes and ketones can easily be reduced by H2 at temperatures as low as 150–200 °C. Alcohols react at 250–300 °C by hydrogenation and thermal dehydration to form olefins. Carboxylic and phenolic ethers react at around 300 °C. Regarding the operating pressures, due to the low solubility of hydrogen in organic and aqueous solutions, high pressures are required to guarantee high availability of hydrogen in

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the vicinity of the catalyst (80-300 bar of H2 pressure) [15].

**Figure 3.** Reactivity scale of organic components under HDO conditions. Adapted from [17].

HDO is a process closely related to hydrodesulphurization (HDS), which is highly devel‐ oped in the oil-refinery industry. In both processes, hydrogen is used to remove the heteroa‐

**2.1. Catalysts and reaction mechanisms**


**Table 1.** Typical Properties of Wood Pyrolysis Bio-Oil, and Heavy Fuel Oil [13].

Not only fuels, but also commodity chemicals are nowadays derived from petroleum-based resources. Commodity chemicals are involved in the production of a wide variety of prod‐ ucts and thus are an essential and integral part of the modern societies. Hence, in the search for a sustainable scenario, it is crucial to also look towards alternative biorenewable sources for these chemicals. In the case of platform chemicals coming from biomass, such as glucose, levulinic acid, 5-(hydroxyl-methyl furfural), sorbitol, or glycerol,they usually have higher O/C ratio than most commodity chemicals. Therefore, the conversion of these platform chemicals into value-added chemicals usually requires O removal reactions.

This book chapter summarizes the main aspects involved in the catalytic hydrotreating processes for the oxygen removal from bio-oils and from biomass based platform chemicals.

#### **2. Hydrotreating catalytic processes in bio-oil upgrading**

As it has been stated in the introduction, a general characteristic of bio-oils coming from the pyrolysis of biomass is their high oxygen content (35-40 wt%). More than 300 compounds have been identified in bio-oil, most of them containing oxygen atoms. The exact composi‐ tion of the bio-oil depends on the type of biomass fed. These compounds can be classified in five broad categories: (i) hydroxyaldehydes, (ii) hydroxyketones, (iii) sugars and dehydrosu‐ gars, (iv) carboxylic acids, and (v) phenolic compounds [16]. Hydroprocessing of biomassderived oils differs from processing petroleum because of the importance of deoxygenation as compared to nitrogen or sulfur removal. Bio-oil hydrodeoxygenation (HDO) process im‐ plies complex reaction networks that includes cracking, decarbonylation, decarboxylation, hydrocracking, hydrogenolysis, hydrogenation and polymerization. The upgrading process should yield a product with lower amount of water and oxygen, decreased acidity and vis‐ cosity, and higher HV. The complexity of the reactions and the high variety of oxygenated compounds make the evaluation of bio-oil upgrading difficult and has brought the use of model compounds such as phenol, guaicol, 2-ethylphenol, methyl heptanoate or benzofuran to test different catalysts and to understand the main characteristics of the HDO process. El‐ liot [17] has reported the HDO reactivity of different organic compounds that are typically present in bio-oils (see Figure 3). Olefins, aldehydes and ketones can easily be reduced by H2 at temperatures as low as 150–200 °C. Alcohols react at 250–300 °C by hydrogenation and thermal dehydration to form olefins. Carboxylic and phenolic ethers react at around 300 °C. Regarding the operating pressures, due to the low solubility of hydrogen in organic and aqueous solutions, high pressures are required to guarantee high availability of hydrogen in the vicinity of the catalyst (80-300 bar of H2 pressure) [15].

**Figure 3.** Reactivity scale of organic components under HDO conditions. Adapted from [17].

#### **2.1. Catalysts and reaction mechanisms**

HDO is a process closely related to hydrodesulphurization (HDS), which is highly devel‐ oped in the oil-refinery industry. In both processes, hydrogen is used to remove the heteroa‐ tom in the form of H2O and H2S respectively. This is the reason why several works on bio-oil HDO use catalytic systems already used in HDS processes, such as Co-Mo or Ni-Mo based catalysts. These catalysts are active in their sulphide form, so they need to be pretreated with H2S before operation to obtain Co-MoS2 or Ni-MoS2 active sites. Romero et al. [18] us‐ ing Co-MoS2 type catalysts for the HDO of 2-ethylphenol at 340ºC and 7 MPa of hydrogen pressure proposed the reaction mechanism described in Figure 4. It is suggested that the oxygen from the molecule adsorbs on a vacancy of a MoS2 matrix. At the same time, the H2 from the feed dissociatively adsorbs on the catalyst surface forming S-H species. The addi‐ tion of a proton to the adsorbed oxygenated molecule leads to an adsorbed carbocation. This intermediate can directly undergo a C–O bond cleavage and the aromatic ring is regenerat‐ ed leading to ethylbenzene. The vacancy is afterwards recovered by elimination of water.

inhibitory effect of H2S, leading to a decrease in phenol conversion and not preventing cata‐ lyst deactivation. This was ascribed to the competitive adsorption between phenol and H2S [21]. Moreover, the formation of sulfur-containing compounds such as dimethyl sulfide, di‐ heptyl sulfide, hexanethiol and heptanethiol was observed in the HDO of aliphatic oxygen‐ ates over Co-MoS2 catalysts, even in the absence of sulfiding agents [22]. Therefore, the use of MoS2 type catalysts in bio-oil HDO seems challenging, becouse sulfur free bio-oil can be contaminated by sulfur, and because wood-based bio-oils contain high amounts of phenolic

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Another alternative is the use of bi-functional catalysts formed by the combination of transi‐ tion metals and oxophilic metals, such as MoO3, Cr2O3,WO3 or ZrO2. In this case, the oxo‐ philic metal acts as a Lewis acid site. The oxygen ion pair of the target molecule is attracted by the unsaturated oxophilic metal. The second step of the mechanism is hydrogen dona‐ tion. In this case, the hydrogen molecule is dissociatively adsorbed and activated on the transition metal. Finally, the activated hydrogen is transferred to the adsorbed molecule.

Regarding the support, γ-Al2O3 is the most commonly used one. Nonetheless, it has to be taken into account the structural changes that γ-Al2O3 might suffer under the typical oper‐ ating conditions in HDO. In contact with hot water (T > 350ºC), γ-Al2O3 is converted into a hydrated boehmite (AlOOH) phase with a significant decrease in the acidity and sur‐ face area [23]. Moreover, the relatively high surface acidity of Al2O3 is thought to pro‐ mote the formation of coke precursors. In fact, coke formation is one the main factors affecting the stability of the catalyst. Therefore, the use of less acidic or neutral support like active carbon or SiO2 is an interesting alternative [24]. For instance, Echeandia et al. [25] using Ni-WO3 on active carbon for the HDO of 1 wt% phenol in n-octane at 150-300ºC and 15 bar observed lower coke formation on the surface of the active carbon with re‐ spect to alumina support. Based on product analysis, they also concluded that HDO of phenol occurs via two separate pathways: one leading to aromatics through a direct hy‐ drogenolysis route, and the other one to cyclohexane, through a hydrogenation-hydroge‐ nolysis route (see Figure 5). In terms of obtaining a final product with high octane number and reducing the consumption of hydrogen, direct hydrogenolysis reaction is preferred. Nonetheless, aromatics are harmful to human health and its content in transportation fuels is limited by legislation. Therefore, it is important to understand which sites are responsi‐ ble of each route, in order to obtain an upgraded product with the desired aromatic con‐ tent. CeO2 and ZrO2 supports have also shown to give good results in the HDO of different molecules. ZrO2-supported noble metal catalysts (Rh, Pd and Pt) [26] were com‐ pared with the conventional sulfided CoMo/Al2O3 catalyst in the HDO of Guaiacol in the presence of H2 at 300 °C. Sulfided CoMo/Al2O3 deactivated due to carbon deposition, and the products were contaminated with sulfur, however, neither problem was observed with the ZrO2-supported noble metal catalysts. As a conclusion, a good support for HDO should provide high affinity for the oxygen-containing molecule while presenting moder‐

compounds that would compete with H2S for the active sites of the catalyst.

ate acidity in order to minimize the formation of coke deposits.

**Figure 4.** Proposed mechanism of HDO of 2-ethylphenol over a schematic Co-MoS2 catalyst Adapted from [18].

The problem of using MoS2 type catalysts for HDO of bio-oils is that during prolonged oper‐ ation sulfur stripping and oxidation of the surface of the catalyst occurs, causing deactiva‐ tion of the catalyst. The reason is that as compared to conventional oil, the sulfur content of bio-oil is very low (less than 0.1 wt % [19]). One alternative to avoid this problem is the cofeeding of H2S to the system, in order to regenerate the sulfide sites. For instance, in the HDO of alyphatic esters over a CoMoS2/Al2O3 and NiMoS2/Al2O3 catalysts a promoting ef‐ fect was observed in the activity of the catalyst when co-feeding H2S, however this co-feed‐ ing did not prevent from catalyst deactivation. This promoting effect was related to the increase in Brönsted acidity in the presence of H2S [20]. Nonetheless, the use of H2S has also some drawbacks. In the HDO of phenol over a Ni-MoS2-Al2O3 catalyst, it was observed an inhibitory effect of H2S, leading to a decrease in phenol conversion and not preventing cata‐ lyst deactivation. This was ascribed to the competitive adsorption between phenol and H2S [21]. Moreover, the formation of sulfur-containing compounds such as dimethyl sulfide, di‐ heptyl sulfide, hexanethiol and heptanethiol was observed in the HDO of aliphatic oxygen‐ ates over Co-MoS2 catalysts, even in the absence of sulfiding agents [22]. Therefore, the use of MoS2 type catalysts in bio-oil HDO seems challenging, becouse sulfur free bio-oil can be contaminated by sulfur, and because wood-based bio-oils contain high amounts of phenolic compounds that would compete with H2S for the active sites of the catalyst.

Another alternative is the use of bi-functional catalysts formed by the combination of transi‐ tion metals and oxophilic metals, such as MoO3, Cr2O3,WO3 or ZrO2. In this case, the oxo‐ philic metal acts as a Lewis acid site. The oxygen ion pair of the target molecule is attracted by the unsaturated oxophilic metal. The second step of the mechanism is hydrogen dona‐ tion. In this case, the hydrogen molecule is dissociatively adsorbed and activated on the transition metal. Finally, the activated hydrogen is transferred to the adsorbed molecule.

Regarding the support, γ-Al2O3 is the most commonly used one. Nonetheless, it has to be taken into account the structural changes that γ-Al2O3 might suffer under the typical oper‐ ating conditions in HDO. In contact with hot water (T > 350ºC), γ-Al2O3 is converted into a hydrated boehmite (AlOOH) phase with a significant decrease in the acidity and sur‐ face area [23]. Moreover, the relatively high surface acidity of Al2O3 is thought to pro‐ mote the formation of coke precursors. In fact, coke formation is one the main factors affecting the stability of the catalyst. Therefore, the use of less acidic or neutral support like active carbon or SiO2 is an interesting alternative [24]. For instance, Echeandia et al. [25] using Ni-WO3 on active carbon for the HDO of 1 wt% phenol in n-octane at 150-300ºC and 15 bar observed lower coke formation on the surface of the active carbon with re‐ spect to alumina support. Based on product analysis, they also concluded that HDO of phenol occurs via two separate pathways: one leading to aromatics through a direct hy‐ drogenolysis route, and the other one to cyclohexane, through a hydrogenation-hydroge‐ nolysis route (see Figure 5). In terms of obtaining a final product with high octane number and reducing the consumption of hydrogen, direct hydrogenolysis reaction is preferred. Nonetheless, aromatics are harmful to human health and its content in transportation fuels is limited by legislation. Therefore, it is important to understand which sites are responsi‐ ble of each route, in order to obtain an upgraded product with the desired aromatic con‐ tent. CeO2 and ZrO2 supports have also shown to give good results in the HDO of different molecules. ZrO2-supported noble metal catalysts (Rh, Pd and Pt) [26] were com‐ pared with the conventional sulfided CoMo/Al2O3 catalyst in the HDO of Guaiacol in the presence of H2 at 300 °C. Sulfided CoMo/Al2O3 deactivated due to carbon deposition, and the products were contaminated with sulfur, however, neither problem was observed with the ZrO2-supported noble metal catalysts. As a conclusion, a good support for HDO should provide high affinity for the oxygen-containing molecule while presenting moder‐ ate acidity in order to minimize the formation of coke deposits.

tive and unstable compounds are transformed into more stable ones at low temperature (270ºC, 136 atm H2) and without a catalyst. In the second step, a deeper HDO is carried out at higher temperatures (400ºC, 136 atm H2) and using hydrotreating catalysts. The two-step hydrotreatment allows 13% reduction in hydrogen consumption for equivalent oil yield. Nonetheless, the reported octane number of the upgraded bio-oil, 72, is still lower than that

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Environmental aspects should also be taken into account. Aromatic compounds have on one hand high octane number; however, they are also harmful to health. Indeed, environmental standards for aromatics in transportation fuels are becoming more restrictive. Thus, it seems challenging to achieve an agreement between obtaining oils with high octane number while

**3. Hydrogenolysis reactions in the valorization of platform chemicals**

Biomass components have a great potential as building block intermediates. Indeed, sugars, vegetable oils and terpenes can be employed for synthesizing products with a high added value, such as chemicals and fine chemicals. There are hundreds of different processes to ob‐ tain chemicals from biomass origin building blocks. This chapter deals with those processes involving hydrotreating for the removal of oxygen. In the first part of this section, some ex‐ amples of significant hydrogenolysis reactions in the valorization of platform chemicals will be given, while the last part will be focused on one of the most studied hydrogenolysis proc‐

As it has been previously stated, platform chemicals coming from biomass usually contain higher O/C ratio than most commodity chemicals; thus main valorization processes require the removal of oxygen. One widely used process to remove oxygen is hydrogenolysis. Hy‐ drogenolysis is a type of reduction that involves chemical bond dissociation in an organic substrate and simultaneous addition of hydrogen to the resulting molecular fragments [33]. Therefore, reaction for oxygen removal involves the cleavage of the C-O bond and the addi‐ tion of hydrogen (oxygen is removed in the form of H2O). This is a significant aspect, be‐ cause, in those processes where the starting and target molecule have the same number of carbons it is important to use catalytic systems that present high activity in C-O bond hydro‐

Two types of sugars are present in biomass: hexoses (six-carbon sugars), of which glucose is the most common one, and pentoses (five-carbon sugars), of which xylose is the most com‐ mon one. Glucose and xylose can be easily hydrogenated to yield sorbitol [29] and xylitol [30] respectively. These two molecules can undergo C-C and C-O hydrogenolysis in the presence of hydrogenation catalysts, leading mainly to a mixture of ethyleneglycol, glycerol, and 1,2-propanediol. Other products such as butanediols, lactic acid, methanol, ethanol, and propanol can also be formed (Figure 6). Ni is known to show high hydrogenolysis activity

of gasoline [17].

fulfilling aromatic content policies.

cesses; the conversion of glycerol into propanediols (PDO).

genolysis while low activity in C-C bond hydrogenolysis.

**3.1. Hydrogenolysis of sugars**

**Figure 5.** Scheme of phenol HDO. Adapted from [25].

#### **2.2. Upgrading of real bio-oils**

An important aspect in the HDO of bio-oils is the required degree of deoxygenation. It is assumed that the upgraded oil should contain less than 5 wt% oxygen so that the viscosi‐ ty is decreased to that required for fuel applications [17]. However, during the hydrotreat‐ ing, not only the oxygen is removed in the form of water, but also the saturation of double bounds occurs. This saturation has two significant negative effects. The first one is relat‐ ed to the quality of the upgraded oil, because the saturation of the aromatic components has a highly detrimental effect in the octane number. For instance, the octane number of toluene (119) decreases to 73 when the aromatic ring is hydrogenated [10]. The second negative effect is related to the consumption of hydrogen. According to Venderbosh et al. [27] in order to achieve 50% of deoxygenation 16 g H2/Kg of bio-oil is required, which is close to the expected stoichiometry value. Nonetheless, if the aim is to obtain the total re‐ moval of oxygen, the H2 consumption increases to 50 g H2/Kg of bio-oil; which means that the H2 consumption is 56% higher than the stoichiometry value. Some other studies sug‐ gest even higher H2 consumption requirements, 62 g H2/Kg of bio-oil [28]. This deviation of the H2 consumption from the stoichiometry value is explained on the basis of the differ‐ ent reactivity of the oxygenated compounds present in the bio-oil. High reactive com‐ pounds, such as ketones, are easily converted with low hydrogen consumption. However, more complex molecules, such as phenols, might suffer the hydrogenation/saturation of the molecule and therefore the hydrogen consumption exceeds the stoichiometric predic‐ tion at the high degree of deoxygenation.

In order to obtain high degrees of HDO but minimizing the hydrogenation of aromatics in bio-oil, two step hydrogenating processes have been developed. In the first stage, high reac‐ tive and unstable compounds are transformed into more stable ones at low temperature (270ºC, 136 atm H2) and without a catalyst. In the second step, a deeper HDO is carried out at higher temperatures (400ºC, 136 atm H2) and using hydrotreating catalysts. The two-step hydrotreatment allows 13% reduction in hydrogen consumption for equivalent oil yield. Nonetheless, the reported octane number of the upgraded bio-oil, 72, is still lower than that of gasoline [17].

Environmental aspects should also be taken into account. Aromatic compounds have on one hand high octane number; however, they are also harmful to health. Indeed, environmental standards for aromatics in transportation fuels are becoming more restrictive. Thus, it seems challenging to achieve an agreement between obtaining oils with high octane number while fulfilling aromatic content policies.

### **3. Hydrogenolysis reactions in the valorization of platform chemicals**

Biomass components have a great potential as building block intermediates. Indeed, sugars, vegetable oils and terpenes can be employed for synthesizing products with a high added value, such as chemicals and fine chemicals. There are hundreds of different processes to ob‐ tain chemicals from biomass origin building blocks. This chapter deals with those processes involving hydrotreating for the removal of oxygen. In the first part of this section, some ex‐ amples of significant hydrogenolysis reactions in the valorization of platform chemicals will be given, while the last part will be focused on one of the most studied hydrogenolysis proc‐ cesses; the conversion of glycerol into propanediols (PDO).

As it has been previously stated, platform chemicals coming from biomass usually contain higher O/C ratio than most commodity chemicals; thus main valorization processes require the removal of oxygen. One widely used process to remove oxygen is hydrogenolysis. Hy‐ drogenolysis is a type of reduction that involves chemical bond dissociation in an organic substrate and simultaneous addition of hydrogen to the resulting molecular fragments [33]. Therefore, reaction for oxygen removal involves the cleavage of the C-O bond and the addi‐ tion of hydrogen (oxygen is removed in the form of H2O). This is a significant aspect, be‐ cause, in those processes where the starting and target molecule have the same number of carbons it is important to use catalytic systems that present high activity in C-O bond hydro‐ genolysis while low activity in C-C bond hydrogenolysis.

#### **3.1. Hydrogenolysis of sugars**

Two types of sugars are present in biomass: hexoses (six-carbon sugars), of which glucose is the most common one, and pentoses (five-carbon sugars), of which xylose is the most com‐ mon one. Glucose and xylose can be easily hydrogenated to yield sorbitol [29] and xylitol [30] respectively. These two molecules can undergo C-C and C-O hydrogenolysis in the presence of hydrogenation catalysts, leading mainly to a mixture of ethyleneglycol, glycerol, and 1,2-propanediol. Other products such as butanediols, lactic acid, methanol, ethanol, and propanol can also be formed (Figure 6). Ni is known to show high hydrogenolysis activity towards C-C and C-O bond hydrogenolysis, this is the reason why, the use of Ni on differ‐ ent acid supports seems an interesting alternative for this process. For instance, Ni support‐ ed on NaY zeolite gave 68% sorbitol conversion with 75% combined selectivity to 1,2-PDO and glycerol at 220ºC and 60 bar H2 pressure after 6 h [8]. The addition of Pt to the catalyst did not influence its activity and selectivity significantly. However, in the case of 20 wt% Ni/Al2O3 prepared by coprecipitation, the addition of 0.5 wt% of Ce significantly increased sorbitol conversion (from 41% to 91%) and the stability of the catalyst [31]. It seems that the addition of Ce considerably reduces Ni leaching, and hence improves the stability of the cat‐ alyst. Other catalytic systems have also been reported besides the Ni acid-support ones. For instance, Ru supported on carbon nanofiber and graphite felt composite catalysts gave 68% sorbitol conversion and 79% propylene glycol selectivity at 220ºC and 8.0 MPa hydrogen pressure [32].

**Figure 7.** Reaction scheme for the conversion of sugars into 2,5-dimethylfuran. Adapted from [36]

Biodiesel is currently obtained from the transesterification reaction of vegetable oils. A pos‐ sible drawback of this technology is that large investment is required to build up new bio‐ diesel plants. An interesting alternative is to directly feed the vegetable oil into the hydrotreating unit of a petroleum refinery, for instance, vegetable oil can be co-fed with heavy vacuum oil HVO. Under typical hydrotreating conditions (300-450ºC, 50 bar H2 pres‐ sure, sulfidedNiMo/Al2O3 catalyst), vegetable oils are transformed into alkanes through three different pathways: decarboxylation, decarbonylation and HDO. The straight chain al‐ kanes can undergo isomerization and cracking to produce lighter and isomerized alkanes (Figure 8) [37]. It was reported that mixing the sunflower oil with HVO does not decrease the rate of desulfurization. Moreover, the rate of vegetable oil hydrotreating is faster that the rate of HVO desulfurization. For industrial application, corrosion problems should be taken into account and the formation of waxes should be minimized, as they can plug the reactor.

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Fatty alcohols can be obtained by catalytic hydrogenolysis of fatty acid methyl esters. Smallchain fatty alcohols are used in cosmetics and food and as industrial solvents or plasticizers, while the large-chain fatty alcohols are important as biofuels and as nonionic surfactants or

**3.3. Hydrotreating of vegetable oils and hydrogenolysis of fatty acids**

**Figure 8.** Reaction pathway for conversion of tri-glycerides to alkanes [37].

**Figure 6.** Reaction products of catalytic hydrogenolysis of sorbitol over supported Ni catalyst in the aqueous phase. Adapted from [31].

#### **3.2. Hydrogenolysis of 5-Hydroxymethyl-Furfural (HMF)**

5-Hydroxymethyl-furfural (HMF) can be obtained in a biphasic reactor from the acid-cata‐ lyzed dehydration of hexoses[33]. HMF by itself cannot be used as motor fuel due to its high boiling point (283ºC). However, it can be transformed to 2,5-dimethylfuran (DMF) through a two consecutive hydrogenolysis reactions (see Figure 7). DMF not only decreases the boiling point to a value suitable for liquid fuels, but also attains the lowest water solubility and the highest octane number (RON) of the mono-oxygenated C6 compounds, while preserving a high energy density 30 kJ cm-3, which is 40% higher that the energy density of bio-ethanol and comparable to the one of gasoline (35 KJ cm-3) [34]. Roman-Leshkov et al. [34] used CuRu/C catalysts (prepared by incipient wetness impregnation) in a flow reactor using 5 wt % HMF in a 1-butanol solution at 220 ºC and 6.8 bar H2 pressure. Yields to DMF of 71% were measured. An important aspect in their process is that the catalyst should be chloride-resist‐ ant, because, NaCl was used in the dehydration step of hexoses to HMF to increase their sol‐ ubility in water. Very recently, Luijkx et al. [35] reported the production of 2,5-DMF by the hydrogenolysis of 5-HMF over a Pd/C catalyst in 1-propanol. Due to simultaneous alcoholy‐ sis, significant amount of ethers products were formed.

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**Figure 7.** Reaction scheme for the conversion of sugars into 2,5-dimethylfuran. Adapted from [36]

#### **3.3. Hydrotreating of vegetable oils and hydrogenolysis of fatty acids**

Biodiesel is currently obtained from the transesterification reaction of vegetable oils. A pos‐ sible drawback of this technology is that large investment is required to build up new bio‐ diesel plants. An interesting alternative is to directly feed the vegetable oil into the hydrotreating unit of a petroleum refinery, for instance, vegetable oil can be co-fed with heavy vacuum oil HVO. Under typical hydrotreating conditions (300-450ºC, 50 bar H2 pres‐ sure, sulfidedNiMo/Al2O3 catalyst), vegetable oils are transformed into alkanes through three different pathways: decarboxylation, decarbonylation and HDO. The straight chain al‐ kanes can undergo isomerization and cracking to produce lighter and isomerized alkanes (Figure 8) [37]. It was reported that mixing the sunflower oil with HVO does not decrease the rate of desulfurization. Moreover, the rate of vegetable oil hydrotreating is faster that the rate of HVO desulfurization. For industrial application, corrosion problems should be taken into account and the formation of waxes should be minimized, as they can plug the reactor.

**Figure 8.** Reaction pathway for conversion of tri-glycerides to alkanes [37].

Fatty alcohols can be obtained by catalytic hydrogenolysis of fatty acid methyl esters. Smallchain fatty alcohols are used in cosmetics and food and as industrial solvents or plasticizers, while the large-chain fatty alcohols are important as biofuels and as nonionic surfactants or emulsifiers. Fatty alcohols are produced by hydrogenolysis, in the presence of Cu based het‐ erogeneous hydrogenation catalysts, operating under H2 pressures between 20 and 30 bar and temperatures in the range of 97-197ºC [38]. High hydrogen pressures are required to in‐ crease the solubility of hydrogen in the reaction mixture, in order to boost the availability of H2 at the catalyst surface and to reduce mass transport limitations [39].The stoichiometry of the reaction is presented below:

*3.4.1. Reaction mechanisms*

**ii.** dehydration– hydrogenation,

**iii.** direct glycerol hydrogenolysis.

**i.** Glyceraldehyde route

tivity to 1,2-PDO [45].

Glycerol hydrogenolysis to PDOs consists of hydrogen addition and removal of one oxygen atom in the form of H2O. In order to design efficient catalysts, it is fundamental to under‐ stand the mechanism of this reaction. Three main reaction mechanisms have been proposed in the literature, depending on whether the reaction runs on acid or basic catalytic sites and

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One of the first studies related to glycerol hydrogenolysis was developed by Montassier et al. [40] in the late 1980s. They suggested that over Ru/C catalyst glycerol is first dehydrogen‐ ated to glyceraldehyde on the metal sites. Next, a dehydroxylation reaction takes place by a nucleophilic reaction of glyceraldehyde with water or with adsorbed -OH species. Finally, hydrogenation of the intermediate yields 1,2-PDO (Figure 9). The main controversial point of this mechanism is the initial dehydrogenation step, which is thermodynamically unfa‐ vored due to the high hydrogen pressures used [41]. Therefore, in order to shift the equili‐ brium, glyceraldehyde dehydration should be faster than glycerol dehydrogenation. Otherwise glyceraldehyde would be hydrogenated back to glycerol on the metal sites. Sev‐ eral authors observed that the addition of a base notably increased glycerol conversion, and this was related to the fact that bases enhance glyceraldehyde dehydration [42-44]. It is inter‐ esting to point out that when glycerol hydrogenolysis is carried out under alkaline condi‐

Apart from 1,2-PDO, other products stemming from C-C bond cleavage were also report‐ ed when glycerol hydrogenolysis is conducted under alkaline conditions; mainly, ethyl‐ ene glycol (EG), methanol and methane. It is suggested that glyceraldehyde can either undergo dehydration or retro-aldolization reactions.The so formed intermediates are hy‐ drogenated in the last step to yield the products of C-C bond cleavage. Because both the glyceraldehyde dehydration and glyceraldehyde retro-aldol reaction are catalyzed by OH-

the addition of a base increases the glycerol reaction rate but does not improve the selec‐

,

**i.** dehydrogenation – dehydration – hydrogenation (glyceraldehyde route),

with or without the formation of intermediate compounds:

Below, the main features of each mechanism will be discussed

tions, marginal 1,3-PDO selectivities are measured.

**Figure 9.** PDO formation from glycerol under alkaline conditions.

R-COOCH3+ 2H2→R-CH2OH + CH3OH

#### **3.4. Hydrogenolysis of glycerol**

In the last years, much attention has been devoted to the valorization of glycerol. Glycerol is obtained as byproduct in the transesterification reaction of fatty acids to produce biodiesel. With the significant increase of worldwide biodiesel production, there is also an important in‐ crease in glycerol availability. Due to the increments in biodiesel manufacture, important amounts of glycerol have been placed in the market, and glycerol has become a waste difficult to handle. The volumes of glycerol remaining unsold in recent years are a clear example of wasted energy and material resources. This is the reason why intense research activity has started worldwide in order to find an exit to the big amounts of glycerol produced. Glycerol price has experimented constant reduction during the last years. Low glycerol prices allow new interesting applications like the production of high added value chemicals. Effective val‐ orization of glycerol will enable to make more cost effective biodiesel production and to re‐ place fossil fuels as the raw material for the production of commodity chemicals.

Among the different possible transformations of glycerol, the hydrogenolysis to propane‐ diols (PDO) presents special interest due to the big number of applications of both 1,2 and 1,3-propanediol (PDO). 1,3-PDO has traditionally been considered a specialty chemical; it has been used in the synthesis of polymers and other organic chemicals, but its market has been quite small. However, over the past years this situation has changed significantly. 1,3- PDO is a starting material in the production of polyesters. It is used together with tereph‐ thalic acid to produce polytrimethylene terephthalate (PTT), which is in turn used for the manufacture of fibers and resins. This polymer is currently manufactured by Shell Chemical (Corterra polymers) and DuPont (Sorona 3GT).1,2-PDO is a major commodity chemical tra‐ ditionally derived from propylene oxide, and hence also based on fossil feedstock. It is a widely used commodity chemical that plays a significant role in the manufacture of a broad array of industrial and consumer products, including unsaturated polyester resins, plasticiz‐ ers and thermoset plastics, antifreeze products, heat-transfer and coolant fluids, aircraft and runway deicing products, solvents, hydraulic fluids, liquid detergents, paints, lubricants, cosmetics and other personal care products. Today, the industry estimates a global demand for 1,2- PDO between 2.6 and 3.5 billion lb/yr [48]. One of the future main markets for 1,2- PDO shall be the substitution of ethylene glycol (EG) in cooling water systems to prevent freezing, as ethylene glycol is harmful to health.

#### *3.4.1. Reaction mechanisms*

Glycerol hydrogenolysis to PDOs consists of hydrogen addition and removal of one oxygen atom in the form of H2O. In order to design efficient catalysts, it is fundamental to under‐ stand the mechanism of this reaction. Three main reaction mechanisms have been proposed in the literature, depending on whether the reaction runs on acid or basic catalytic sites and with or without the formation of intermediate compounds:


Below, the main features of each mechanism will be discussed

**i.** Glyceraldehyde route

One of the first studies related to glycerol hydrogenolysis was developed by Montassier et al. [40] in the late 1980s. They suggested that over Ru/C catalyst glycerol is first dehydrogen‐ ated to glyceraldehyde on the metal sites. Next, a dehydroxylation reaction takes place by a nucleophilic reaction of glyceraldehyde with water or with adsorbed -OH species. Finally, hydrogenation of the intermediate yields 1,2-PDO (Figure 9). The main controversial point of this mechanism is the initial dehydrogenation step, which is thermodynamically unfa‐ vored due to the high hydrogen pressures used [41]. Therefore, in order to shift the equili‐ brium, glyceraldehyde dehydration should be faster than glycerol dehydrogenation. Otherwise glyceraldehyde would be hydrogenated back to glycerol on the metal sites. Sev‐ eral authors observed that the addition of a base notably increased glycerol conversion, and this was related to the fact that bases enhance glyceraldehyde dehydration [42-44]. It is inter‐ esting to point out that when glycerol hydrogenolysis is carried out under alkaline condi‐ tions, marginal 1,3-PDO selectivities are measured.

Apart from 1,2-PDO, other products stemming from C-C bond cleavage were also report‐ ed when glycerol hydrogenolysis is conducted under alkaline conditions; mainly, ethyl‐ ene glycol (EG), methanol and methane. It is suggested that glyceraldehyde can either undergo dehydration or retro-aldolization reactions.The so formed intermediates are hy‐ drogenated in the last step to yield the products of C-C bond cleavage. Because both the glyceraldehyde dehydration and glyceraldehyde retro-aldol reaction are catalyzed by OH- , the addition of a base increases the glycerol reaction rate but does not improve the selec‐ tivity to 1,2-PDO [45].

**Figure 9.** PDO formation from glycerol under alkaline conditions.

#### **ii.** Dehydration-hydrogenation route

Dasari et al. [46] observed the formation of acetol (hydroxyacetone) together with 1,2-PDO using copper-chromite catalyst at 473 K and 15 bar hydrogen pressure. Moreover, glycerol hydrogenolysis to 1,2-PDO occurred even in the absence of water. Since the copper-chromite catalyst was reduced in a stream of hydrogen prior to the reaction, no surface hydroxyl spe‐ cies were present to take part in the reaction. Therefore, the mechanism suggested by Mon‐ tassier et al. (Figure 9) was not able to explain these results. Dasari et al. proposed a new mechanism in which glycerol is first dehydrated to acetol, which is further hydrogenated to 1,2-PDO (Figure 10). Based on their findings, a two step process was developed [47]. In the first step, acetol is generated from glycerol dehydration by a reactive distillation process, op‐ erating at 513 K, slight vacuum and using copper-chromite catalyst. The acetol obtained is then hydrogenated at 15 bar H2 pressure using the same catalyst. The process was patented in the USA in 2005 [48].

propoxide; it is suggested that the formation of 2,3-dihydroxypropoxide is preferred as it re‐ quires a smaller adsorption cross-section than 1,3-dihydroxyisopropoxide [52]. Next, the hydride attack to the 2-position of 2,3-dihydroxypropoxide gives 1,3-PDO, while the hy‐ dride attack to the 3-position of 2,3-dihydroxyisopropoxide yields 1,2-PDO. The higher se‐ lectivity to 1,3-PDO obtained (1,3-PDO/1,2-PDO ratio = 2.7) is explained on the basis of the higher stability of the six membered-ring transition state that leads to the formation of 1,3- PDO as compared to the stability of the seven membered-ring transition state that leads to

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**Figure 11.** Model structures of the transition states of the hydride attack to the adsorbed substrate in the glycerol

A different direct glycerol hydrogenolysis mechanism was established by Chia et al. [53] try‐ ing to explain the hydrogenolysis of different polyols and cyclic ethers over a Rh-ReOx/C catalyst. They concluded from DFT calculations that the -OH groups on Re associated with Rh are acidic. The acidic nature of ReOx was also reported before [54]. Such acidic Re sites can donate a proton to the reactant molecule and form carbenium ion transition states. In the case of glycerol hydrogenolysis, the first step involves the formation of a carbocation by pro‐ tonation-dehydration reaction. This carbocation is stabilized by the formation of a more sta‐ ble oxocarbenium ion intermediate resulting from the hydride transfer from the primary - CH2OH group. Final hydride transfer step leads to 1,2-PDO or 1,3-PDO [53]. The authors also reported that the secondary carbocation is more stable than the primary carbocation. Nevertheless, higher selectivity to 1,2-PDO was obtained (1,3-PDO/1,2-PDO ratio = 0.65).

**Figure 12.** Reaction mechanism for direct glycerol dehydrogenation. Adapted from[55].

the formation of 1,2-PDO (Figure 11).

hydrogenolysis [52].

#### **Figure 10.** PDO formation via the dehydration-hydrogenation route.

According to Schlaf, acid-catalyzed hydrogenolytic cleavage of -OH group occurs through an initial protonation of the hydroxyl group that leads to the formation of a carbocation and water [49]. Thermodynamically, the formation of a secondary carbocation is more favored than the formation of a primary carbocation. Therefore, operating under acid conditions should bring about higher selectivity to 1,3-PDO. The fact that product distribution is usual‐ ly shifted towards 1,2-PDO seems to be a complex function of operating conditions, catalyst and starting materials. Ethylene glycol, ethanol, methanol and methane are usually reported as degradation products. Ethylene glycol and methanol are formed from the C-C bond cleavage reaction of glycerol, while ethanol stems from the further hydrogenolysis of ethyl‐ ene glycol.

#### **iii.** Direct glycerol hydrogenolysis

A direct glycerol hydrogenolysis mechanism was recently proposed by Yoshinao et al. [50]. The experiments were carried out using Rh-ReOx/SiO2 and Ir-ReOx/SiO2 catalysts at 393 K and 80 bar H2 pressure. The low reaction temperature implies that the dehydration-hydroge‐ nation route was not further possible, due to the endothermic character of glycerol dehydra‐ tion and the required activation energy, and suggests the energetically more favored direct hydrogenolysis reaction [51]. They suggested a direct hydride proton mechanism. The se‐ lected catalysts are able to activate hydrogen easily and to form hydride species. It is pro‐ posed that glycerol is adsorbed on the surface of ReOx clusters to form alkoxide species. Glycerol can form two adsorbed alkoxides: 2,3-dihydroxypropoxide and 1,3-dihydroxyiso‐ propoxide; it is suggested that the formation of 2,3-dihydroxypropoxide is preferred as it re‐ quires a smaller adsorption cross-section than 1,3-dihydroxyisopropoxide [52]. Next, the hydride attack to the 2-position of 2,3-dihydroxypropoxide gives 1,3-PDO, while the hy‐ dride attack to the 3-position of 2,3-dihydroxyisopropoxide yields 1,2-PDO. The higher se‐ lectivity to 1,3-PDO obtained (1,3-PDO/1,2-PDO ratio = 2.7) is explained on the basis of the higher stability of the six membered-ring transition state that leads to the formation of 1,3- PDO as compared to the stability of the seven membered-ring transition state that leads to the formation of 1,2-PDO (Figure 11).

**Figure 11.** Model structures of the transition states of the hydride attack to the adsorbed substrate in the glycerol hydrogenolysis [52].

A different direct glycerol hydrogenolysis mechanism was established by Chia et al. [53] try‐ ing to explain the hydrogenolysis of different polyols and cyclic ethers over a Rh-ReOx/C catalyst. They concluded from DFT calculations that the -OH groups on Re associated with Rh are acidic. The acidic nature of ReOx was also reported before [54]. Such acidic Re sites can donate a proton to the reactant molecule and form carbenium ion transition states. In the case of glycerol hydrogenolysis, the first step involves the formation of a carbocation by pro‐ tonation-dehydration reaction. This carbocation is stabilized by the formation of a more sta‐ ble oxocarbenium ion intermediate resulting from the hydride transfer from the primary - CH2OH group. Final hydride transfer step leads to 1,2-PDO or 1,3-PDO [53]. The authors also reported that the secondary carbocation is more stable than the primary carbocation. Nevertheless, higher selectivity to 1,2-PDO was obtained (1,3-PDO/1,2-PDO ratio = 0.65).

**Figure 12.** Reaction mechanism for direct glycerol dehydrogenation. Adapted from[55].

#### *3.4.2. Catalytic systems*

#### **i.** Noble metals

Hydrogenolysis reactions involve the addition of hydrogen to an organic molecule. There‐ fore, hydrogenolysis catalysts must be able to activate hydrogen molecules. Noble metals are known to be active for the dissociation of hydrogen molecules and are widely used in hydrogenation reactions. The first studies on glycerol hydrogenolysis were carried out using Ru based catalysts [56]. Feng et al. [57] studied the effect of different supports (TiO2, SiO2, NaY, γ-Al2O3) on Ru based catalysts. The TiO2 supported catalyst exhibited the highest ac‐ tivity giving a glycerol conversion of 90.1%; however, it also favored the production of eth‐ ylene glycol over 1,2-PDO. In contrast, Ru/SiO2 showed the lowest activity, but resulted in much higher selectivity to 1,2-PDO. They also performed blank reactions with the supports, achieving no significant conversions; which indicated that the supports cannot catalyze the reaction independently. Ru particle size was affected by the type of support, and a correla‐ tion was established between the size of the Ru particle and the activity of the catalyst, being higher with decreasing Ru particle size.

PDO, is the use of bifunctional noble metal-acid catalysts. Different Bronsted acids like sulfonated zirconia, zeolites, homogeneous H2SO4 and Amberlyst 15 were tested together with Ru/C [60,61]. Acid-type cation-exchange resin Amberlyst 15 was the most effective cocatalyst. Nevertheless, a weak point in the system of Ru/C with Amberlyst 15 is that the re‐ action temperature is limited to 393 K. At higher temperatures sulfur compounds such as SO2 and H2S, which are formed by the thermal decomposition of the sulphonic groups of the resins, poison the catalyst. Using Amberlyst 70 the reacting temperature can be increased to

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**mgcat/gglyc Time**

Ru/TiO2, 5wt% 50 180 20 96 12 90.1 1,2-PDO (21), EG (41) [57]

Cu/Al2O3, 60wt% 1 120 -200 30 - 0.066 h−1 a 100 1,2-PDO (96.9), acetol (1.4) [64] Cu/SiO2, 30wt% 90 180 80 62.5 12 32.7 1,2.PDO (98), EG (1) [65]

Pd0.04Cu0.4Mg5.6−Al2(OH) 20 180 75 166 10 77 1,2-PDO (98), EG (1.6) [66]

**Table 2.** Selected examples of hydrogenolysis of aqueous glycerol over heterogeneous catalysts. PDO: Propanediol,

The use of more stable inorganic salts can avoid the temperature problems related to ionexchange resins. Balaraju et al. [67] used the combination of Ru/C catalyst with different in‐ organic salts such as niobia, zirconia-supported 12-tungstophosphoric acid or acid caesium 12-tungstophosphate in glycerol hydrogenolysis at 453 K. The best results were achieved with those co-catalysts presenting a high number of medium strength acid sites. Particular‐ ly, with niobia as co-catalyst 62.8% glycerol conversion and 66.5% 1,2-PDO selectivity were reported. Another option is the use of a noble metal on acid supports. Vasiliadou et al. [68] investigated glycerol hydrogenolysis on Ru-based (γ-Al2O3, SiO2, ZrO2) catalysts at 513 K and 80 bar. The nature of the oxidic support was found to influence the ability of the catalyst to both activate the glycerol substrate and selectively convert it to propanediol. The charac‐ terization of the catalytic materials revealed a correlation between catalytic activity for the

**(h)**

40 200 1 233 5 40 1,2-PDO (71), lacticacid (19),

80 120 20 112.5 10 79.3 1,2-PDO (75), 1-PO(8), 2-PO

80 180 20 12.2 10 48.8 1,2-PDO (70), 1,3-PDO (1.3),

30 180 75 166 20 91 1,2-PDO (96), EG (3) [44]

80 120 20 37.5 36 81.0 1,2-PDO (4.2), 1,3-PDO

**Conv. (%)**

**(%)**

EG (9)

(2), EG (7)

1-PO (7.1), EG (8.3)

(46.3), 1-PO (41.2)

**Product Selectivity**

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343

[42]

[63]

[62]

[72]

453 K before observing thermal decomposition [62].

**Temp. (ºC)**

**Glyc. Conc. (wt.%)**

**(bar)**

**Catalyst H2**

Pt/C, 3wt% + CaO 0.8 M

Ru/C, 5wt% + Amberlyst 15

Ru/C, 5wt% + Amberlyst 70

Cu0.4/Mg5.6Al2O9 + NaOH

Ir–ReO*x*/SiO2, 4wt% (Re/Ir = 1)

WHSV (weight hour space velocity)

PO: Propanol, EG: Ethylene Glycol.

a

Apart from Ru, other noble metals have also been studied. For instance, Furikado et al. [58] compared the activity of various supported noble-metal catalysts (Rh, Ru, Pt and Pd over C, SiO2 and Al2O3). Among all the catalysts, the best results in terms of 1,2-PDO selectivity were achieved with Rh/SiO<sup>2</sup> at low reacting temperature and low glycerol conversions (7.2). Nevertheless, the selectivities to 1,2-PDO obtained were rather low, due to the over-hydro‐ genolysis of 1,2- and 1,3-PDO to 1 and 2-PO.

The use of noble metal-base bifunctional catalytic systems has also been reported. As it was previously described in the glyceraldehyde based mechanism, the dehydration of glycerol to glyceraldehyde, and further dehydration of glyceraldehyde to pyruvaldehyde are both thought to be catalyzed by adsorbed hydroxyls.The effect of different base additives on the performance of Ru/TiO2was reported [45]. The addition of Li or Na hydroxides dramatically increased the glycerol hydrogenolysis activity of Ru/TiO2and the selectivity to 1,2-PDO. The highest conversion of glycerol (89.6%) and the highest selectivity to 1,2-PDO (86.8%) were observed with LiOH. The selectivity to 1,2-PDO was similar with all the bases added, which showed that the selectivity to 1,2-PDO is independent of base concentration within a certain range. However, the selectivity to ethylene glycol decreased no matter which base was add‐ ed. Almost no reaction was observed in the absence of Ru/TiO2, indicating that the presence of metal is required in order to take place glycerol hydrogenolysis. The lower selectivity to ethylene glycol with increasing base addition to the reacting solution was explained by the fact that ethylene glycol presented higher affinity to adsorb in the surface of the catalyst and to suffer the attack of hydroxyl groups, whose concentration was higher at elevated pH val‐ ues [59].

Noble metal-acid catalytic systems have also been used. According to the mechanism in Fig‐ ure 10, glycerol is firstly dehydrated to acetol, which is then hydrogenated to 1,2-PDO. The first dehydration step is supposed to be catalyzed by acid sites while the second one by met‐ al sites. Therefore, one interesting option to increase the selectivity to target product, 1,2PDO, is the use of bifunctional noble metal-acid catalysts. Different Bronsted acids like sulfonated zirconia, zeolites, homogeneous H2SO4 and Amberlyst 15 were tested together with Ru/C [60,61]. Acid-type cation-exchange resin Amberlyst 15 was the most effective cocatalyst. Nevertheless, a weak point in the system of Ru/C with Amberlyst 15 is that the re‐ action temperature is limited to 393 K. At higher temperatures sulfur compounds such as SO2 and H2S, which are formed by the thermal decomposition of the sulphonic groups of the resins, poison the catalyst. Using Amberlyst 70 the reacting temperature can be increased to 453 K before observing thermal decomposition [62].


a WHSV (weight hour space velocity)

**Table 2.** Selected examples of hydrogenolysis of aqueous glycerol over heterogeneous catalysts. PDO: Propanediol, PO: Propanol, EG: Ethylene Glycol.

The use of more stable inorganic salts can avoid the temperature problems related to ionexchange resins. Balaraju et al. [67] used the combination of Ru/C catalyst with different in‐ organic salts such as niobia, zirconia-supported 12-tungstophosphoric acid or acid caesium 12-tungstophosphate in glycerol hydrogenolysis at 453 K. The best results were achieved with those co-catalysts presenting a high number of medium strength acid sites. Particular‐ ly, with niobia as co-catalyst 62.8% glycerol conversion and 66.5% 1,2-PDO selectivity were reported. Another option is the use of a noble metal on acid supports. Vasiliadou et al. [68] investigated glycerol hydrogenolysis on Ru-based (γ-Al2O3, SiO2, ZrO2) catalysts at 513 K and 80 bar. The nature of the oxidic support was found to influence the ability of the catalyst to both activate the glycerol substrate and selectively convert it to propanediol. The charac‐ terization of the catalytic materials revealed a correlation between catalytic activity for the hydrogenolysis reaction and total acidity, as the yield to hydrogenolysis products increased with the concentration of the acid sites. However, increased acidity was also responsible for the promotion of the excessive hydrogenolysis of the desired 1,2-propanediol to propanols.

Rh/SiO2. In a more recent work, an Ir–ReO*x*/SiO2 (Re/Ir = 1) catalyst prepared by a similar method to that for Rh–ReO*x*/SiO2, catalyzed the hydrogenolysis of glycerol to 1,3-PDO in a more effectively way (1,3-PDO/1,2-PDO ratio = 11) [72]. Based on characterization results, the authors suggested that oxidized low-valence Re clusters are attached to the Ir or Rh met‐ al particles. Glycerol is adsorbed on the surface of MO*x* species (M = Mo, Re and W) at the OH group to form alkoxide. Hydrogen is activated on the noble-metal (Rh or Ir) surface. The alkoxide located on the interface between MO*x* and the noble-metal surface is attacked by the activated hydrogen species, and the C–O bond neighboring to the C–O–M group is dis‐ sociated. The hydrolysis of the resulting alkoxide releases the product (see Figure 11). One of the weak point of these catalytic systems is that they are also active in the further hydro‐

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In summary, Cu based catalysts are active and selective for the production of 1,2-PDO from glycerol. However, if the aim is to produce the more valuable 1,3-PDO, different approaches are required. The used of noble metals combined with low-valence metal oxide seems to be a promising alternative. Nonetheless, there is still room for improvement; both in catalyst design and in process engineering, as PDOs further hydrogenolysis significantly affect the

In the previous sections the significance that hydrogenolysis reactions have and will have in the future bio-refineries has been highlighted. In fact, they will be essential in fuel and chemical manufacturing. Hydrogenolysis involves chemical bond dissociation in an organic substrate and simultaneous addition of hydrogen. Therefore, hydrogen is required as reac‐ tant in all hydrogenolysis reactions. This is the reason why, most of the literature works re‐ ferred to hydrogenolysis report experiments conducted under molecular hydrogen (H2) atmosphere. Nevertheless, the use of molecular hydrogen has some important drawbacks:

**i.** Liquid phase processes are preferred to gas phase processes as they are more ener‐

safety measures, as hydrogen is easily ignited and shows high diffusivity.

**ii.** Most of the nowadays available hydrogen gas is produced from fossil fuels by ener‐

**iii.** The low density and high diffusivity of hydrogen make problematic and expensive

gy efficient. However, H2 presents really low solubility on aqueous or organic solu‐ tions. As a consequence, when operating in liquid phase it is necessary to operate at elevated hydrogen pressures to obtain significant hydrogen concentrations near the catalysts. This, on one hand, notably increases the cost of design and building of the future plants, and on the other hand, increases the operating cost related to

gy intensive processes. Therefore, if sustainability is the goal it is a contradiction that the main reactant in most of the biorefinery processes is based on fossil resources.

its transportation and storage. This problem is more relevant for small size biomass

**4. Main alternatives to the use of molecular hydrogen**

genolysis of both 1,2 and 1,3-PDO to 1-PO.

final yields to target products.

conversion facilities.

#### **ii.** Cu based catalysts

Cu has been extensively investigated in the glycerol hydrogenolysis reactions. Although its hydrogenation activity is generally lower than that of noble metals, its much lower price and its ability to catalyze C-O bond but not C-C bond hydrogenolysis make Cu catalysts at‐ tractive for this process. There are some works in the literature that report the use of other transition metals like Ni or Co, however, Cu based catalysts are predominant. Vapor phase glycerol dehydration reaction was studied by Sato et al. [69] over different copper catalysts at 513 K and atmospheric N2 pressure. They observed that basic MgO, CeO2, and ZnO sup‐ ports showed low acetol selectivity, while acidic supports, such as Al2O3, ZrO2, Fe2O3, and SiO2, effectively promoted acetol formation. The best results were obtained with Cu/Al2O3 catalyst. Increments in copper content lead to increments in acetol selectivity. Moreover, the activity of the Al2O3 support alone was rather low, which indicates that copper metal sites play a significant role in glycerol dehydration. Continuing with vapor phase processes, Akiyama et al. [64,70] studied glycerol hydrogenolysis in a fixed-bed down-flow glass reac‐ tor at temperatures between 340 and 473 K, atmospheric hydrogen pressure, and using Cu/Al2O3 catalysts. In the two step reaction they observed that glycerol dehydration to ace‐ tol was favored at relatively high temperatures. However, acetol hydrogenation to 1,2-PDO was favored at lower temperatures, because it is an exothermic reaction and the dehydro‐ genation of 1,2-PDO occurs preferentially at high temperatures. Based on these findings, they developed a reactor with gradient temperatures, at the top of the reactor glycerol dehy‐ dration reaction occurred at 453 K while at the bottom of the reactor acetol was hydrogenat‐ ed to 1,2-PDO at 418 K. Really high 1,2-PDO yields (94.9%) were reported.

Some of the best results in terms of glycerol conversion and 1,2-PDO selectivity were recent‐ ly reported using Cu on base supports. For instance, Yuan et al. [44] developed a Cu based solid catalyst (Cu0.4/Mg5.6Al2O8.6)via thermal decomposition of the as-synthesized Cu0.4Mg5.6Al2(OH)16CO3 layered double hydroxides. This bifunctional highly dispersed Cusolid base catalyst was effective for hydrogenolysis of aqueous glycerol. The measured con‐ version of glycerol reached 80.0% with a 98.2% selectivity of 1,2-propanediol at 180 °C, 30 bar H2 and 20 h. The addition of Pd to the same catalytic system notably increased the activi‐ ty of the catalyst [71]. It was suggested that the hydrogen spill over from Pd to Cu favored glycerol hydrogenolysis to 1,2-PDO.

#### **iii.** Metal oxide modified-noble metal

As stated above, the use of acid or base as a co-catalyst gives 1,2-PDO as a main product. To obtain more valuable 1,3-PDO, the most effective approach has shown to be the use of noble metal (Ir, Rh or Pt) combined with oxophilic metals. Shinmi et al. [52] modified Rh/SiO2 cat‐ alyst with Re, W and Mo. Re addition showed the largest enhancing effect on catalytic activ‐ ity and also increased the selectivity to 1,3-PDO. The Rh–ReOx/SiO2 (Re/Rh = 0.5) exhibited 22 times higher glycerol conversion (79%) and 37 times higher 1,3-PD yield (11%) than Rh/SiO2. In a more recent work, an Ir–ReO*x*/SiO2 (Re/Ir = 1) catalyst prepared by a similar method to that for Rh–ReO*x*/SiO2, catalyzed the hydrogenolysis of glycerol to 1,3-PDO in a more effectively way (1,3-PDO/1,2-PDO ratio = 11) [72]. Based on characterization results, the authors suggested that oxidized low-valence Re clusters are attached to the Ir or Rh met‐ al particles. Glycerol is adsorbed on the surface of MO*x* species (M = Mo, Re and W) at the OH group to form alkoxide. Hydrogen is activated on the noble-metal (Rh or Ir) surface. The alkoxide located on the interface between MO*x* and the noble-metal surface is attacked by the activated hydrogen species, and the C–O bond neighboring to the C–O–M group is dis‐ sociated. The hydrolysis of the resulting alkoxide releases the product (see Figure 11). One of the weak point of these catalytic systems is that they are also active in the further hydro‐ genolysis of both 1,2 and 1,3-PDO to 1-PO.

In summary, Cu based catalysts are active and selective for the production of 1,2-PDO from glycerol. However, if the aim is to produce the more valuable 1,3-PDO, different approaches are required. The used of noble metals combined with low-valence metal oxide seems to be a promising alternative. Nonetheless, there is still room for improvement; both in catalyst design and in process engineering, as PDOs further hydrogenolysis significantly affect the final yields to target products.

#### **4. Main alternatives to the use of molecular hydrogen**

In the previous sections the significance that hydrogenolysis reactions have and will have in the future bio-refineries has been highlighted. In fact, they will be essential in fuel and chemical manufacturing. Hydrogenolysis involves chemical bond dissociation in an organic substrate and simultaneous addition of hydrogen. Therefore, hydrogen is required as reac‐ tant in all hydrogenolysis reactions. This is the reason why, most of the literature works re‐ ferred to hydrogenolysis report experiments conducted under molecular hydrogen (H2) atmosphere. Nevertheless, the use of molecular hydrogen has some important drawbacks:


Hydrogen from non fossil origin will surely be a reality in the oncoming years, as reforming processes from various renewable compounds (like biomethane, glycerol or ethanol) and water splitting processes using solar light are being intensively developed. Nonetheless, the problems of transportation, storage and low solubility in liquid solutions will remain. One interesting option that could solve the problems associated to the use of molecular hydrogen is to directly generate the required hydrogen in the active sites of the catalyst.

**4.2. Hydrogenolysis with in-situ generation of hydrogen**

One interesting option to in situ generate the required hydrogen for hydrogenolysis reac‐ tions is through aqueous phase reforming (APR). APR is a quite well known process in which a polyol is converted to hydrogen and CO2 in the presence of water. The hydrogen generated can be further used in the hydrogenolysis reaction. The specific case for combined glycerol APR and hydrogenolysis to 1,2-PDO is shown in Figure 14. If the process is perfect‐ ly balanced, glycerol is fully converted into 1,2-PDO, being CO2 and H2O the only byprod‐ uct. Tailored metal-acid bifunctional catalysts or combination of catalysts are required to obtain high yields to 1,2-PDO. Indeed, there must be a proper balance between the C-C bond cleavage reactions that lead to the production of hydrogen, and the C-O bond cleavage reactions that lead to the formation of PDOs [76]. While Pt is known to be active in C-C bond cleavage, its combination with other metals active in C-O bond hydrogenolysis, like Ni, Sn or Ru, over acidic supports appears as promising formulations to obtain high yields to 1,2-PDO [77]. However, glycerol APR itself runs at elevated pressure and therefore the advantage over conventional hydrogenolysis at high hydrogen pressure is marginal with re‐

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The same benefits that have been previously addressed for the use of *in situ* generated hy‐ drogen in glycerol hydrogenolysis can be applied to the conversion of other higher polyols, like sorbitol or xylitol. However, the considerable research effort that has been directed to the conversion of glycerol yet has not been paid to other biomass based polyols. Therefore, the amount of works related to high polyol hydrogenolysis with *in situ* generation of the re‐ quired hydrogen is quite scarce. As a consequence of this, it is a really interesting and open

*4.2.1. Aqueous Phase Reforming (APR)*

gard to equipment and safety costs.

**Figure 14.** Combined glycerol APR and hydrogenolysis to 1,2-PDO.

research field.

#### **4.1. Bio-oil upgrading using hydrogen donating solvents**

One interesting approach to reduce the consumption of molecular hydrogen during the HDO of bio-oils is to use hydrogen donating solvents. For instance, Elliott has reported that when the bio-oil upgrading is carried out in the presence of a hydrogen donor solvent (tetra‐ lin, 1-1 ratio with bio-oil feedstock) the oxygen removal increases from 70 to 85% and less deactivation of the catalyst was observed. Some of the components already present in the bio-oil, such as alcohols or acids, may also provide hydrogen for the deoxygenation reac‐ tions [10]. Traditional catalysts active in hydrogen transfer reactions, such as Pd, Ni or Cu should be used in this process [73].

Another attractive option is to use hydrogen donating solvents during the hydrotreating of biomass. The idea is to obtain a bio-oil with a lower oxygen content, and therefore, easier to upgrade. This concept has been mainly applied in the pyrolysis of lignin. If a hydrogen do‐ nor molecule is added during the pyrolysis, both depolymerization and hydrogenation oc‐ cur simultaneously. Remarkable results have been obtained using hydrogen-donating solvents, such as tetralin or 9,10-dihydroanthracene [74]. However, a major drawback is the need for large quantities of these solvents. At this point, formic acid appears to be a promis‐ ing donor molecule, as it can be obtained together with levulinic acid from the hydrolysis of biomass. On heating, formic acid decomposes completely into CO2 and two active hydrogen atoms, which are efficient scavengers of any radical species formed in the lignin. By succes‐ sive homolytic cleavage of the covalent linkages of the lignin, including aromatic rings, most of the oxygen is removed as water and hydrocarbons are formed (Figure 13). When pyroly‐ sis is carried out with formic acid, lignin can be converted into hydrogen-rich, oxygen de‐ pleted products with no added catalyst [75].

**Figure 13.** Schematic picture of the products formed upon the pyrolysis of lignin in thepresence of formic acid [75].

#### **4.2. Hydrogenolysis with in-situ generation of hydrogen**

#### *4.2.1. Aqueous Phase Reforming (APR)*

One interesting option to in situ generate the required hydrogen for hydrogenolysis reac‐ tions is through aqueous phase reforming (APR). APR is a quite well known process in which a polyol is converted to hydrogen and CO2 in the presence of water. The hydrogen generated can be further used in the hydrogenolysis reaction. The specific case for combined glycerol APR and hydrogenolysis to 1,2-PDO is shown in Figure 14. If the process is perfect‐ ly balanced, glycerol is fully converted into 1,2-PDO, being CO2 and H2O the only byprod‐ uct. Tailored metal-acid bifunctional catalysts or combination of catalysts are required to obtain high yields to 1,2-PDO. Indeed, there must be a proper balance between the C-C bond cleavage reactions that lead to the production of hydrogen, and the C-O bond cleavage reactions that lead to the formation of PDOs [76]. While Pt is known to be active in C-C bond cleavage, its combination with other metals active in C-O bond hydrogenolysis, like Ni, Sn or Ru, over acidic supports appears as promising formulations to obtain high yields to 1,2-PDO [77]. However, glycerol APR itself runs at elevated pressure and therefore the advantage over conventional hydrogenolysis at high hydrogen pressure is marginal with re‐ gard to equipment and safety costs.

**Figure 14.** Combined glycerol APR and hydrogenolysis to 1,2-PDO.

The same benefits that have been previously addressed for the use of *in situ* generated hy‐ drogen in glycerol hydrogenolysis can be applied to the conversion of other higher polyols, like sorbitol or xylitol. However, the considerable research effort that has been directed to the conversion of glycerol yet has not been paid to other biomass based polyols. Therefore, the amount of works related to high polyol hydrogenolysis with *in situ* generation of the re‐ quired hydrogen is quite scarce. As a consequence of this, it is a really interesting and open research field.

Huber et al. [78] studied the production of renewable alkanes (C1-C6) from the aqueous phase reforming of sorbitol using a Pt/SiO2-Al2O3catalyst.They suggested a multistep bifunc‐ tional reaction pathway. The first step involves the formation of CO2 and H2 on the Pt sites, and the dehydration of sorbitol on the acid sites of the silica-alumina support. These initial steps are followed by hydrogenation of the dehydrated reaction intermediates on the metal catalyst (Scheme 9). 64 % alkane selectivity at 92% sorbitol conversion were recorded at 498 K and 39.6 bar. When hydrogen was co-fed, alkane selectivity significantly increased up to 91%. Glucose showed to be less active than sorbitol over a Pt/Al2O3 catalyst at 538 K and 52.4 bar of N2pressure, achieving moderate alkane selectivities (49.5%) [79]. Therefore, it seems that initial hydrogenation of glucose to sorbitol and subsequent aqueous phase reforming of the sugar is more effective than direct aqueous phase reforming of glucose.

Ni-Cu alloy notably reduced formation of products <C3. This was related to the fact that C-C bond cleavage reactions are ensemble size sensitive and that the formation of a Cu-Ni alloy causes a decrease in the Ni ensemble size. Therefore, the presence of both metals is required for obtaining high 1,2-PDO yields: Ni to provide high hydrogenolysis activity and Cu to shift the selectivity towards C-O bond cleavage. It was also observed that above a certain metal content, further increments led to a decrease in glycerol conversion. This was correlat‐ ed to the total acidity of the catalyst that also decreased with increasing metal content. A di‐

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**Figure 15.** Proposed mechanism for glycerol hydrogenolysis by CTH using formic acid as hydrogen donor molecule [84].

Bio-oils coming from the pyrolysis of biomass feedstocks and biomass based platform chem‐ icals present a common limiting feature: their high oxygen content. This oxygen can be re‐ moved by catalytic hydrotreating in the form of H2O. Intensive research is required in this field in order to develop catalytic systems active and stable under the hard operating condi‐ tions used: high temperatures and pressures, and high concentrations of sub-critical water. The required bifunctional catalysts must have Brönsted acidity to catalyze dehydration reac‐ tions or/and Lewis acid sites to attract the oxygen ion pair of the target molecule; but also metal sites that show the ability to activate hydrogen molecules. In this sense, the combina‐ tion of oxophilic metals (Re, Mo or W) with Ni or noble metals has shown to be a promising approach. In the case of bio-oil upgrading, the developed catalysts should promote hydro‐ deoxygenation reactions against hydrogenation reactions that lead to higher hydrogen con‐ sumption and reduction in the octane number of the oil. In order to avoid coke formation under the hard operating conditions used, neutral supports appear as an interesting option. In the case of catalysts for platform chemical valorization, C-C bond cleavage reactions should be avoided. Therefore, for some applications, like glycerol hydrogenolysis to 1,2- PDO, Cu based catalysts have to be considered due to the high selectivity of Cu for C-O

Hydrogenolysis processes for oxygen removal require the use of large amounts of hydro‐ gen, which is commonly supply by operating under high molecular hydrogen pressures. Nonetheless, this might be a problem because nowadays, most technologies to obtain hydro‐ gen are energy intensive and non-renewable. An interesting alternative might be to in-situ

rect glycerol hydrogenolysis mechanism was also proposed (Figure 15).

**5. Conclusions**

bond cleavage reactions.

#### *4.2.2. Catalytic Transfer Hydrogenation*

Catalytic transfer hydrogenation (CTH) is a process in which hydrogen is transferred from a hydrogen donor molecule to an acceptor [80]. CTH reactions can be of industrial importance as the renewable production, transportation and storage of hydrogen donors can be cheaper than those for molecular hydrogen. For CTH, it has been reported that adjacent sites may be necessary for donor and acceptor molecules [73]. Therefore, the first criterion to be fulfilled by the selected hydrogen donor molecules is to be soluble in the compound to be hydro‐ treated. Moreover, in order to improve the yield of desired products, reactions other than dehydrogenation of the donor should be minimized under the operating conditions. The best hydrogen donors for heterogeneous CTH include simple molecules like cyclohexene, hydrazine, formic acid and formates [81]. Alcohols like 2-propanol (2-PO) or methanol can also be used as hydrogen donors; primary alcohols are generally less active than the corre‐ sponding secondary alcohols due to the smaller electron-releasing inductive effect of one al‐ kyl group as against two [82]. The most active catalysts for heterogeneous transfer reduction are based on palladium metal. Other noble metals such as Pt and Rh are also widely utiliz‐ ed. Sometimes, other transition metals such as Ni and Cu have also been reported but for operation at higher temperature [73].

In this area, the most studied process has been the conversion of glycerol into 1,2-PDO. Mu‐ solino et al. [83] studied glycerol hydrogenolysis by transfer hydrogenation under 5 bar in‐ ert atmosphere, using ethanol and 2-PO as solvents and hydrogen donor molecules over 10PdFe2O3 catalyst at 453 K. They observed that complete glycerol conversion and high se‐ lectivities to 1,2-PDO could be obtained when the hydrogen came from the dehydrogenation of the solvent. Formic acid has also been used as a hydrogen donor molecule in the glycerol hydrogenolysis process using Ni-Cu/Al2O3 catalysts [84]. Under the operating conditions used, formic acid was readily converted into CO2 and H2, therefore, a semi-continuous setup was used to continuously pump formic acid to the glycerol water solution, in order to ensure a constant supply of hydrogen at an appropriate rate [85]. For a constant metal con‐ tent of 35 wt-% (Ni+Cu), increasing Ni proportion caused an increase in glycerol conversion but also an increase in C-C bond cleavage reactions. Cu is known to be active in the C-O bond cleavage but not in the C-C bond cleavage. The presence of Cu and the creation of a Ni-Cu alloy notably reduced formation of products <C3. This was related to the fact that C-C bond cleavage reactions are ensemble size sensitive and that the formation of a Cu-Ni alloy causes a decrease in the Ni ensemble size. Therefore, the presence of both metals is required for obtaining high 1,2-PDO yields: Ni to provide high hydrogenolysis activity and Cu to shift the selectivity towards C-O bond cleavage. It was also observed that above a certain metal content, further increments led to a decrease in glycerol conversion. This was correlat‐ ed to the total acidity of the catalyst that also decreased with increasing metal content. A di‐ rect glycerol hydrogenolysis mechanism was also proposed (Figure 15).

**Figure 15.** Proposed mechanism for glycerol hydrogenolysis by CTH using formic acid as hydrogen donor molecule [84].

#### **5. Conclusions**

Bio-oils coming from the pyrolysis of biomass feedstocks and biomass based platform chem‐ icals present a common limiting feature: their high oxygen content. This oxygen can be re‐ moved by catalytic hydrotreating in the form of H2O. Intensive research is required in this field in order to develop catalytic systems active and stable under the hard operating condi‐ tions used: high temperatures and pressures, and high concentrations of sub-critical water. The required bifunctional catalysts must have Brönsted acidity to catalyze dehydration reac‐ tions or/and Lewis acid sites to attract the oxygen ion pair of the target molecule; but also metal sites that show the ability to activate hydrogen molecules. In this sense, the combina‐ tion of oxophilic metals (Re, Mo or W) with Ni or noble metals has shown to be a promising approach. In the case of bio-oil upgrading, the developed catalysts should promote hydro‐ deoxygenation reactions against hydrogenation reactions that lead to higher hydrogen con‐ sumption and reduction in the octane number of the oil. In order to avoid coke formation under the hard operating conditions used, neutral supports appear as an interesting option. In the case of catalysts for platform chemical valorization, C-C bond cleavage reactions should be avoided. Therefore, for some applications, like glycerol hydrogenolysis to 1,2- PDO, Cu based catalysts have to be considered due to the high selectivity of Cu for C-O bond cleavage reactions.

Hydrogenolysis processes for oxygen removal require the use of large amounts of hydro‐ gen, which is commonly supply by operating under high molecular hydrogen pressures. Nonetheless, this might be a problem because nowadays, most technologies to obtain hydro‐ gen are energy intensive and non-renewable. An interesting alternative might be to in-situ generate the required hydrogen. Among all the alternatives, the use of hydrogen donor mol‐ ecules that can be obtained from biomass in a renewable way, such as formic acid, appears as a promising approach.

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[20] Şenol Oİ, Viljava T-, Krause AOI. Effect of sulphiding agents on the hydrodeoxyge‐ nation of aliphatic esters on sulphided catalysts. Applied Catalysis A: General 2007;

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#### **Author details**

Iñaki Gandarias\* and Pedro Luis Arias

\*Address all correspondence to: inaki\_gandarias@ehu.es

Department of Chemical and Environmental Engineering, University of the Basque Country (UPV/EHU) Alameda Urquijo s/n, Bilbao, Spain

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**Chapter 11**

**Synthesis of Biomass-Derived Gasoline**

Armando T. Quitain, Shunsaku Katoh and

Additional information is available at the end of the chapter

Motonobu Goto

**1. Introduction**

oxygenate.

http://dx.doi.org/10.5772/52539

**Fuel Oxygenates by Microwave Irradiation**

Recent concerns about climate change and problems associated with the use of fossil-de‐ rived fuels and nuclear energy have inspired researchers to seriously explore environmen‐ tally benign and economically viable renewable energy and fuels. As potential solution to reduce fossil-derived carbon dioxide (CO2) emissions from gasoline-run automobiles, addi‐ tion of biomass-derived oxygenates was proposed. Bioethanol has been considered, howev‐ er, ether oxygenates such as ethyl *tert*-butyl ether (hereby referred to as ETBE), has gained popularity over ethanol (EtOH) due to its superior properties which blend well with gaso‐ line [1]. ETBE also outranks MTBE as an octane enhancer due to its low blending Reid vapor pressure. Moreover, ETBE is a better option because it is derived from EtOH which can be obtained from biomass. ETBE is produced from the reaction of isobutene (IB) and EtOH, however, the current supply of IB, which is mostly derived from non-renewable crude oil, may not be sufficient to cope up with the expected high demand in the future. For this rea‐ son, alternative routes for its synthesis are also currently being explored. *tert*-Butyl alcohol (TBA), which can also be derived from biomass can be employed instead of IB [2]. Research for the development of efficient and energy-saving methods for the production of these gas‐ oline oxygenates had gained significant momentum over the past few years. The application of microwave technology was proposed for the synthesis of the above mentioned gasoline

Microwave technology relies on the use of electromagnetic waves to generate heat by the os‐ cillation of molecules upon microwave absorption. Unlike the conventional heating, the heat is generated within the material, thus rapid heating occurs. Other than the advantages of rapid heating, microwave effects on reaction likely occur, thus obtaining dramatic increase

> © 2013 Quitain et al.; licensee InTech. This is an open access article 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.

© 2013 Quitain 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.

