Hydrolysis

Lignocellulosic materials are complex mixtures of natural polymers – cellulose (35–50%), hemicelluloses (25–30%), and lignin (15–30%) – tightly bonded by physical and chemical interactions (Rubin, 2008). Since cellulose is the major component of lignocellulosic materials, efficient processes for hydrolysis or depolymerization of cellulose seem to be interesting entry points for the production of biofuels and biochemicals (Fig. 6.1).

However, its chemical transformation presents a serious challenge due to the problems associated with its mechanisms for structural protection, which make this polymer recalcitrant towards efficient chemical and biological transformations. One of the most important bottlenecks of commercializing lignocellulosic bioethanol is the discovery of a cost-effective hydrolysis of cellulose (Banerjee et al., 2010; Kumar et al., 2009). The β-glycosidic linkages of the sugar molecules contained in cellulose or lignocellulose are strongly protected by the tight packing of cellulose chains in microfibers, making hydrolysis challenging. Accordingly, the hydrolysis of cellulose requires harsh conditions such as the use of dilute strong acids at high temperatures. The hydrolysis rate of cellulose and cellobiose shows a clear dependence on the pK_a of the acid used in the reaction (Morales-Delarosa et al., 2012; Vanoye et al., 2009). For this reason, the hydrolysis of lignocellulosic materials requires the use of strong acids as catalysts (pK_a < 0). Typically the catalysts are based on sulfuric, hydrochloric and p-toluenesulfonic acids (Murzin and Simakova, 2011; Rinaldi and Schüth, 2009a). The use of homogeneous catalysts has some problems of separation, reuse, and neutralization.

The application of solid acid catalysts to biomass transformation into transportation biofuels and chemicals has been receiving much attention (Rinaldi and Schüth, 2009a). The ease of catalyst separation after the reaction, which enables their reuse, is an advantage of heterogeneous catalysis for biorefineries. Furthermore, solid acid catalysts are typically less aggressive to the industrial plants than liquid mineral acids. However, the hydrolysis of solid biomass with heterogeneous catalysts must occur in two steps. Firstly, a partial hydrolysis of biomass proceeds, involving Brønsted acidic species either released by the solid material or formed in the reaction medium (Rinaldi and Schüth, 2009b). Subsequently, the reactions involving the solid catalyst take place when the oligomers are small enough to access the pore system. Thus, the porosity of the solid catalyst can play a fundamental role in the catalytic activity and selectivity, because it determines to what extent the initial hydrolysis needs to proceed in homogeneous phase before starting the heterogeneously catalysed reactions. There is also the possibility that the reaction starts on the external active sites of the catalysts. However, due to the fact that a dispersion system with two solid phases exists, such contributions are probably small. Transformation of lignocellulose into biofuels and chemicals involves water either as a reagent, a product, or even as a solvent. Only a number of materials are suitable for these applications (in terms of acidity, stability, and insolubility). Some catalysts that fulfill these characteristics are functionalized activated carbons, functionalized resins, functionalized silica, zeolites, some solid heteropoly compounds, niobic acid, MoO₃-ZrO₂, zirconium tungstates, zirconium phosphates, lanthanum phosphates, niobium phosphates, and some other materials (Okuhara, 2002).

Another alternative is the use of enzymes (Murzin and Simakova, 2011), through which the hydrolysis is produced with high selectivity. But the use of enzymes has the drawbacks of high cost and low activity. Enzyme catalysis demands the use of highly specific cellulases and is actually a heterogeneous process that depends on such physicochemical properties as crystallinity, the degree of polymerization, surface conditions, and the presence of lignin and hemicellulose, if cellulose is not used as the raw material. All this, as was mentioned above, results in a low reaction rate.

A breakthrough in lignocellulose chemistry was the finding that alkylmethylimidazolium ionic liquids (IL) can dissolve cellulose or even wood (Pinkert et al., 2009; Zhu et al., 2006; Sun et al., 2011; Olivier-Bourbigou et al., 2010). The dissolution process disrupts the fibers of cellulose, leaving the hydroxyl groups and β-glycosidic bonds accessible for the hydrolysis. It appears that the ‘physical’ barrier can be overcome through the formation of a cellulose solution that facilitates the acid-catalyzed hydrolysis under mild reaction conditions and a lower catalyst loading. Chemical deconstruction of cellulose to produce glucose in ionic liquids has resulted in only moderate yields (Rinaldi et al., 2008, 2010; Li et al., 2008), which contrasts with the high yields obtained from cellulose in concentrated acids and other cellulose solvents (Kumar et al., 2009). This low yield is due to the small amount of water added in the reaction in the presence of IL; a large amount of water leads to the precipitation of unreacted cellulose. However, a large excess of water is essential to reach higher sugar yields dissolved in ionic liquids because it favors the hydrolysis of cellulose and retards the dehydration of glucose. This effect is described by the ‘Le Chatelier’s Principle’. By adding water successively during the reaction, this issue was elegantly overcome (Binder and Raines, 2010; Morales-Delarosa et al., 2012), producing a very high yield of sugars. The recovery and the reuse of ionic liquids – in a continuous process – is a big challenge to industrialization of new technologies. Given the high costs of ionic liquids, efficient recycling is mandatory for the development of commercially viable processes.

Hydrogenolysis

Because the rates of hydrolysis of cellulose and degradation of glucose are close (Vanoye et al., 2009; Ruppert et al., 2012), one method to reduce the contribution from degradation could be combining hydrolysis with other processes, e.g., hydrogenation, leading to the formation of sugar alcohols, which are more stable than sugars. This concept was studied some time ago with the hydrolytic hydrogenation of cellulose, hemicellulose, and wood with Ru, Pd, and Pt catalysts in the presence of phosphoric and sulfuric acids (Murzin and Simakova, 2011), but recently there has been a renaissance of reactions that use only water, without the use of diluted acid solutions (Fukuoka and Dhepe, 2006; Yan et al., 2006; Luo et al., 2007; Ji et al., 2008; Jollet et al., 2009; Zheng et al., 2010; Zhu et al., 2010; Ruppert et al., 2012). In these studies the acidic function of the solution was substituted with the acid function of the carrier that accelerates hydrolysis and is associated with the metallic function that produces hydrogenation. In addition to cellulose, hydrolytic hydrogenation can take place for the mixtures of cellulose and hemicellulose (Käldström et al., 2011). Sorbitol is obtained and, correspondingly, the product of xylose hydrogenation, namely, xylitol.

Transesterification

Lipids can be transformed in biodiesel by transesterification. Biodiesel, a clean renewable fuel, has recently been considered as the best candidate for a diesel fuel substitute because it can be used in any compression ignition engine without the need for modification. Chemically, biodiesel is a mixture of methyl esters with long-chain fatty acids and is typically made from nontoxic, biological resources such as vegetable oils, animal fats, or even used cooking oil. The plant oils usually contain free fatty acids, phospholipids, sterols, water, odorants and other impurities. They have high viscosities and boiling points so the oil cannot be used as fuel directly because it produces serious problems in the diesel engine (especially in direct injection engine). The most common way to produce biodiesel is through catalyzed transesterification (Fig. 6.4).

Transesterification or alcoholysis is the displacement of alcohol from an ester by another in a process similar to hydrolysis, except that alcohol is used instead of water. Methanol is mainly used in commercial biodiesel production, although some longer alcohols have also been applied, such as ethanol, butanol, propanol, and amyl alcohol. Methanol is used most often because of its low cost and the fact that it is readily available. This process has been widely used to reduce the high viscosity of triglycerides (Fig. 6.5). Transesterification is one of the reversible reactions and proceeds essentially by mixing the reactants. However, the presence of a catalyst (a strong acid or base) accelerates the conversion.

Catalysts used for the transesterification of triglycerides are classified as alkali, acid, or enzyme, among which alkali catalysts such as sodium hydroxide, sodium methoxide, potassium hydroxide, and potassium methoxide are more effective. Alkali-catalyzed transesterification occurs at a reaction temperature above alcohol boiling point (70°C) at atmospheric pressure, and an alcohol and triglyceride ratio 6:1 to drive the equilibrium to maximum ester yield (Lestari et al., 2009a; Leung et al., 2010). Chemical transesterification using an alkaline catalysis process gives high conversion levels of triglycerides to their corresponding methyl esters in short reaction times. However, the reaction has several drawbacks: it is energy intensive, recovery of glycerol is difficult, the alkaline catalyst has to be removed from the product, and alkaline waste water requires treatment. Additionally, the quality of the feedstock must be very high, the oil has to be free fatty acid (FFA) and the alcohol anhydrous. Because of the presence of these impurities, the yield to esters is reduced, leading to partial saponification, producing soap that makes the separation and purification steps more difficult.

Acid catalysts require higher temperature and pressure at a comparable amount of catalyst compared to alkaline processes, higher alcohol/oil ratio, and yields slower reaction rate. But, these catalysts may be applied for feedstock with high FFA content and are less sensitive to water content in alcohol.

Utilization of a lower-grade feedstock such as grease, beef tallow, and used frying oils from the food industry may reduce production costs, but on the other hand this requires processing of feeds with a high content of FFAs. In this case, a homogeneous acid catalyst is usually applied at the pre-treatment stage of lower grade feedstocks to reduce the FFA content. Therefore, an integrated process comprising acid-catalyzed pre-esterification of FFAs followed by base-catalyzed transesterification of triglyceride is the preferred way of dealing with lower-quality feedstocks.

Enzymatic catalysts like lipases are able to effectively catalyze the transesterification of triglycerides in either aqueous or non-aqueous systems, which can overcome the problems mentioned above. In particular, glycerol can be easily removed without any complex process, and also the free fatty acids contained in waste oils and fats can be completely converted to alkyl esters. On the other hand, in general the production cost of a lipase catalyst is significantly greater than that of an alkaline one.

An efficient heterogeneous catalyst would provide economic benefits as, unlike homogeneous catalysts, it does not require a separation procedure. In most cases, catalysts can be recycled and re-used for long periods, providing an opportunity for a large-scale continuous process. Many heterogeneous catalysts that are based on both acid and alkaline solid catalysts have recently been proposed in the literature (Meher et al., 2006; Lopez Granados et al., 2007, 2009a, 2009b, 2010, 2011; Martin Alonso et al., 2007, 2009a, 2009b, 2010b; Kotwal et al., 2009; Lestari et al., 2009a; Leung et al., 2010; Fu et al., 2011; Toda et al., 2005; Marchetti et al., 2007). Activity comparison with a homogeneous catalyst showed that the homogeneous catalyst gave higher ester yield, using the same reaction conditions, compared to the heterogeneous catalysts. This effect is due to a double effect on the mass transfer, the presence of alkali helps the mixture of oil and alcohol (Lestari et al., 2009a), and the presence of a solid introduces a third phase transfer limitations. Basic catalysts include Na/NaOH/γ-Al₂O₃, KOH/γ-Al₂O₃, hydrotalcites, CaO, Li/CaO, and basic resins, while acid solids employed include C-SO₃H, WO₃/ZrO₂, acid resins, sulfonic functionalized silica, and heterogenized acidic polymers. The main part of these heterogeneous catalysts suffers leaching and deactivation during its use in reaction (Alba-Rubio et al., 2010; Lopez Granados et al., 2009b; Martin Alonso et al., 2007, 2009b); only some examples seem to be reasonably stable in reaction: poly(styrenesulfonic) acid (Lopez Granados et al., 2011), sulfonic functionalized carbon (Toda et al., 2005), and basic resins (Marchetti et al., 2007).

Hydrogenation

Hydrotreating of triglycerides and fatty acids to hydrocarbon middle distillates are suitable as alternative diesel fuels (Fig. 6.6). Hydroprocessing has advantages in terms of the flexibility of feedstock, because the process can utilize low-quality feeds such as grease, used frying oil, animal fats, or tall oils from the Kraft pulping industry. The separation stage of the byproducts is not as complex as in the transesterification reaction. The main product, which consists mostly of paraffinic hydrocarbons, is superior for application in combustion engines (Table 6.1).

Hydrotreating is usually used in a petroleum refinery to remove sulfur, nitrogen, and metals from petroleum-derived feedstocks. Conventional hydrotreating catalysts containing sulfided mixed oxides such as NiMo, NiW, and CoMo, can be used in the process. Application of this process for upgrading vegetable oils involves three main pathways: hydrogenation of double bonds in the alkyl chain of fatty acids; decarboxylation and decarbonylation; and hydrodeoxygenation (or dehydration/hydrogenation) to produce alkanes (Lestari et al., 2009a). The main products are n-aliphatic hydrocarbons of the corresponding fatty acids, and propane from the triglyceride molecules. The oxygen content in the triglyceride is released either as carbon monoxide or carbon dioxide, together with the formation of water.

Coprocessing with the existing refinery reactor was initially preferred in the hydroprocessing of vegetable oil. However, further studies have indicated some problems due to the presence of trace metal contaminants in the vegetable oil, such as phosphorous, sodium, potassium, and calcium. Furthermore, the presence of water and carbon oxide affected the catalyst lifetime and caused problems in the separation of carbon oxides from the recycle gas. Considering all these potential issues, the refinery seems to favor the construction of a dedicated or stand-alone unit that is optimized for vegetable oil processing because of the unique nature of the feedstock (Lestari et al., 2009a).

Despite the hydrotreating of pure vegetable oil in the conventional hydrotreating catalyst, NiMo/Al₂O₃, presulfided with H₂S/H₂, hydrotreating of vegetable oil and hydrodesulfurization probably occurred on different catalytic sites, and as the feed vegetable oil did not contain sulfur, it was pointed out that part of the sulfur could leach out from the catalyst and there would be a need to add sulfur to the feed (Lestari et al., 2009a).

Deoxygenation

An interesting option is to convert bioderived feedstocks by deoxygenation (Mäki-Arvela et al., 2006, 2011; Snåre et al., 2006; Lestari et al., 2009a). The advantage of this technology is that no hydrogen is required in the process compared to the hydrotreating process, thus eliminating the additional cost of hydrogen. This reaction is catalyzed by noble metals supported on carbon. The reaction occurs at temperatures around 300°C, and a low partial pressure of hydrogen was beneficial in increasing the turnover frequency (TOF) and final conversion, compared to pure hydrogen or an inert reaction atmosphere (Kubičková et al., 2005). The presence of hydrogen also diminished the consecutive aromatization, which is undesirable because aromatics are unsuitable as diesel constituents and they accelerate catalyst deactivation. It should be noted that catalytic deoxygenation gives a higher selectivity than hydrotreating. The results of metal catalyst testing on similar supports by normalizing the results with metal content revealed the following order: Pd > Pt > Ni > Rh > Ir > Ru > Os (Snåre et al., 2006). Some authors propose that activated carbons possess catalytic activity for the hydrothermal decarboxylation of fatty acids (Fu et al., 2011).

A plausible reaction path for the production of linear hydrocarbons from fatty acids via direct deoxygenation combines liquid phase reactions and gas-phase reactions (Fig. 6.7). Fatty acids of vegetable oils can be directly decarboxylated or decarbonylated. Direct decarboxylation removes the carboxyl group by releasing carbon dioxide and paraffinic hydrocarbon, while direct decarbonylation produces an olefinic hydrocarbon via removal of the carboxyl group by forming carbon monoxide and water, as illustrated by the first two reactions in Fig. 6.7, respectively. Additionally, fatty acids can be deoxygenated by adding hydrogen; in this case the production of linear hydrocarbon can occur via direct hydrogenation or indirect decarboxylation. The gas-phase reactions are those involved in the production of CO, CO₂, hydrogen, and water during decarboxylation/decarbonylation. Deoxygenation of unsaturated renewables, such as oleic acid, methyl oleate, and linoleic acids also leads to saturated diesel-fuel-range hydrocarbons. The reaction occurs initially via hydrogenation of double bonds and subsequent deoxygenation of the corresponding saturated feed (Lestari et al., 2009a). The reaction rates of different reactants were independent of the carbon chain length of its fatty acids (Simakova et al., 2009).

Comparison of the deoxygenation activity was performed in batch, semibatch and tubular reactors. The lowest productivity was found in tubular reactors attributed to mass transfer limitations (Snåre et al., 2008). Furthermore, a better performance was observed in a semibatch reactor compared to a batch reactor due to the use of a flowing purging gas in the former reactor, thus flushing the gaseous products formed, CO and CO₂, away from the reactor (Simakova et al., 2009).

Catalysts can suffer deactivation during the deoxygenation of fatty acids and their derivatives due to both coking and poisoning by CO and CO₂. Furthermore, it was observed that the use of lower-boiling-point solvents, such as decane and mesitylene, slightly enhanced the catalyst stability (Lestari et al., 2009a). The stable performance of the continuous deoxygenation of neat stearic acid over a mesoporous Pd supported on Sibunit (mesoporous carbon) was recently demonstrated (Lestari et al., 2009b).

Furans platform

The conversion of cellulose into furans, 5-hydroxymethylfurfural (HMF) and 2-furaldehyde (furfural) and its derivatives, a versatile feedstock not only for the production of polyesters and other plastics, but also diesel-like fuels and pharmaceuticals (Fig. 6.8) is a very interesting process. Polysaccharides can be hydrolyzed to their constituent monomers, which can subsequently be dehydrated by protonic acid as well as by Lewis acid catalysts (Zhao et al., 2007; West et al., 2008; Tong et al., 2010; Lam et al., 2011) to furan compounds with a carbonyl group such as HMF, 5-methylfurfural, or furfural. Furthermore, furans can be produced from both cellulose and hemicellulose fractions of biomass; thus, furan platforms utilize a large fraction of the available lignocellulosic feedstock.

Furfural and HMF can be produced by dehydration with good selectivity (e.g., 90%) from xylose and fructose, respectively, in biphasic reactors (Chheda et al., 2007b), whereas yields are lower for glucose (42% at low concentrations of 3 wt%) (Martin Alonso et al., 2010a). The addition of aprotic solvents, such as dimethyl sulfoxide (DMSO) or N,N-dimethylacetamide, improves the selectivity to HMF from fructose, with final yields of over 90% (Binder and Raines, 2009; Martin Alonso et al., 2010a). Reducing the water concentration is critical to the selective preparation of HMF, because it is readily hydrated in water to form levulinic acid and formic acid.

To minimize the incidence of side reactions, such as condensation, furfural compounds can be extracted from the aqueous layer using organic solvents (Roman-Leshkov et al., 2006; Chheda et al., 2007a; West et al., 2008), and by addition of salts to the aqueous phase (Roman-Leshkov et al., 2007), which decreases the solubility of organic species in water. The use of DMSO and an extracting solvent increases HMF selectivity to 55% when glucose is used as the feed, compared to 11% in water (Martin Alonso et al., 2010a). A potential drawback of this approach is the use of solvents; the presence of solvents requires a downstream separation step, which increases the total cost of the process; this point is very important when high boiling point solvents are employed, like DMSO. The energy requirements of downstream purification can be reduced by using other solvents like 2-butanol and methyl isobutyl ketone (MIBK) (Martin Alonso et al., 2010a). The HMF yield can be increased by the combination of metal chlorides and strong acids (HCl), especially in the transformation of glucose, cellulose, and lignocellulosic materials. The best results have been obtained using CrCl₂ as catalyst (Zhao et al., 2007; Tong et al., 2010; Lima et al., 2011). An interesting option is the combination of the dissolution capability of lignocellulosic materials by ionic liquids and a biphasic reactor (Lima et al., 2011; Wang et al., 2011).

HMF can be upgraded to liquid fuels employing different methods (Fig. 6.9) (Kazi et al., 2011; Jin and Enomoto, 2011). HMF can be transformed by hydrogenolysis to 2,5-dimethyl furan (DMF) with copper-based catalysts (Cu-Ru/C or CuCrO₄) (Roman-Leshkov et al., 2007). DMF is not soluble in water and can be used as blender in transportation fuels. A second alternative is the formation of hydrocarbons, as it has been shown that various carbohydrate-derived carbonyl compounds such as furfural, HMF, dihydroxyacetone, acetone, and tetrahydrofurfural can be condensed in aqueous and organic solvents to form larger molecules (C₇–C₁₅) that can subsequently be converted into components of diesel fuel (Barrett et al., 2006; Chheda and Dumesic, 2007). To form larger hydrocarbons, HMF and other furfural products can be upgraded by aldol condensation with ketones, such as acetone, over basic catalysts (NaOH, MgO/ZrO₂), or acid catalysts (West et al., 2008; Sádaba et al., 2011a, 2011b; Murzin and Simakova, 2011). Single condensation of HMF and acetone produces a C9 intermediate, which can react with a second molecule of HMF to produce a C15 intermediate. Aldol condensation can be coupled with hydrogenation steps using a bifunctional catalyst like Pd/MgO–ZrO₂, leading to high yields of condensation products (Chheda and Dumesic, 2007; Chheda et al., 2007a; Geboers et al., 2011; Martin Alonso et al., 2010a). By selective hydrogenation, HMF and furfural can be converted to 5-hydroxymethyltetrahydrofurfural (HMTHDA) and tetrahydrofurfural (THF2A) that, after self-condensation and hydrogenation/dehydration steps, produce C12 or C10 alkanes, respectively.

Levulinic acid platform

Levulinic acid is produced during the hydrolysis of sugars to furans, as a decomposition of unstable HMF (Fig. 6.8). High yield to levulinic acid can be obtained by adjusting the reaction conditions using acid minerals as catalyst (Bozell et al., 2000; Martin Alonso et al., 2010a). Levulinic acid can be upgraded into different liquid fuels (Fig. 6.10).

Levulinic acid (LA) can be converted to methyltetrahydrofuran (MTHF) by hydrogenation using Ru(acac)₃ and NH₄PF₆, or Pd-Re/C in liquid phase or nickel-promoted copper/silica in gas phase (Bozell et al., 2000; Mehdi et al., 2008; Upare et al., 2011). MTHF can be blended up to 70% with gasoline without modification of current internal combustion engines. The lower heating value of MTHF compared with gasoline is compensated by its higher specific gravity, which results in similar mileage to that achieved with gasoline. Direct conversion of levulinic acid to MTHF is possible; however, improved yields can be achieved through indirect routes, which proceed through the production of γ-valerolactone as an intermediate (see next section).

Another processing option for LA is the production of methyl and ethyl esters that can be blended with diesel fuel. This esterification can be carried out at room temperature during LA storage in the presence of methanol or ethanol. Various acid catalysts have been studied to increase yields and reaction rates (Timokhin et al., 1999).

γ-Valerolactone platform

Derivatives of γ-valerolactone (GVL) (Lange et al., 2010) are an alternative class of promising biofuels that can be produced from cellulose (Fig. 6.11). γ-Valerolactone can be used in a number of applications, ranging from direct use as a fuel additive or solvent to diverse upgrading strategies for the production of fuels and chemicals. There are limitations to its direct application as a transportation fuel in the present infrastructure, such as low energy density, blending limits, and high solubility in water.

Because GVL is derived from levulinic acid in aqueous media, the direct use of GVL as a fuel necessitates purification of the GVL, by separation/purification steps and distillation/extraction methods that remove water, but increase the overall cost of the process. Another alternative is to directly process the aqueous solutions of GVL to produce hydrophobic liquid alkanes with the appropriate molecular weight to be used as liquid fuels (Fig. 6.12).

GVL can be upgraded by liquid fuels by two strategies: (a) ring opening and hydrogenation and (b) ring opening and decarboxylation.

GVL can be converted by ring opening to pentenoic acids, and this mixture of pentenoic acids can subsequently be hydrogenated to produce pentanoic acid. These two reactions can be performed using Pd/Nb₂O₅ or Pt/HZSM-5 (Serrano-Ruiz et al., 2010; Lange et al., 2010). Pentanoic acid can be upgraded to 5-nonanone by ketonization over CeZrO_x (Martin Alonso et al., 2010a) or to alkyl esters by esterification with alcohols with acid catalyst (Lange et al., 2010). This 5-nonanone can be hydrogenated/dehydrated to nonane over Pt/Nb₂O₅ (West et al., 2008). Another alternative is to hydrogenate the ketones over Ru/C at 423 K and 50 bar to produce alcohols which can subsequently be dehydrated over an acid catalyst, such as Amberlyst 70 (423 K), to produce nonene, which can be coupled by acid catalyzed oligomerization (Martin Alonso et al., 2010c). In this case, smaller ketones would also be converted to alkenes that would undergo oligomerization along with nonene to produce C6–C27 alkenes that can be hydrogenated over Pt/Nb₂O₅ to liquid alkanes to be used as a jet fuel or diesel blenders. The molecular weight range for the final alkanes can be modified by varying reaction conditions of temperature, pressure, or WHSV (Martin Alonso et al., 2010c).

Alternatively, GVL can be upgraded by ring opening and decarboxylation to produce an equimolar mixture of butenes and CO₂ (Bond et al., 2010). Both reactions take place over a solid acid catalyst, HZSM-5 or SiO₂/Al₂O₃, with good butene yields. These butene monomers are coupled by oligomerization over an acid catalyst (Amberlyst 70 or ZSM-5) to form C8 + alkenes that can be used as jet fuel upon hydrogenation. This process requires separated reactors because the butene oligomerization is favored at elevated pressures, while the GVL decarboxylation is favored by low pressures.

Deoxygenation by cracking

Deoxygenation cracking rejects oxygen as CO₂, as summarized in the conceptual overall reaction:

${\text{C}}_1{\text{H}}_{1.33}{\text{O}}_{0.43}+0.26{\text{O}}_2\to 0.65{\text{CH}}_{1.2}+0.34{\text{CO}}_2+0.27{\text{H}}_2\text{O}$

The deoxygenation upgrading can operate on the liquid or vapors within or close coupled to the pyrolysis process, or they can be decoupled to upgrade either the liquids or re-vaporized liquids. Deoxygenation cracking does not require co-feeding of hydrogen and can therefore be operated at atmospheric pressure. The bio-oil is converted into at least three phases in the process: oil, aqueous, and gas. Typically, reaction temperatures in the range from 300 to 600°C are used for the process. An increased temperature resulted in a decrease in the oil yield and an increase in the gas yield. This is due to an increased rate of cracking reactions at higher temperatures, resulting in the production of smaller volatile compounds. However, in order to decrease the oxygen content to a significant degree, the high temperatures were required. In conclusion, it is crucial to control the degree of cracking. A certain amount of cracking is needed to remove oxygen, but if the rate of cracking becomes too high, at increased temperatures, degradation of the bio-oil to light gases and carbon will occur instead.

The catalysts employed are based on zeolites and mesoporous materials: ZSM-5, H-Y, MCM-41, SBA-15, FCC catalysts, etc. (Czernik and Bridgwater, 2004; Zhang et al., 2007; Mortensen et al., 2011; Bridgwater, 2012). A report on hydrocarbon processing for the future of FCC and hydroprocessing in modern refineries states that ‘Biomass-derived oils are generally best upgraded by HZSM-5 or ZSM-5, as these zeolitic catalysts promote high yields of liquid products and propylene. Unfortunately, these feeds tend to coke easily, and high TANs (Total Acid Number) and undesirable by-products such as water and CO₂ are additional challenges’ (Bridgwater, 2012).

Hydrodeoxygenation

Hydrodeoxygenation (HDO) rejects oxygen as water by catalytic reaction with hydrogen. The process can be depicted by the following conceptual reaction:

${\text{C}}_1{\text{H}}_{1.33}{\text{O}}_{0.43}+0.77{\text{H}}_2\to {\text{CH}}_2+0.43{\text{H}}_2\text{O}$

where ‘CH₂’ represents an unspecified hydrocarbon product.

HDO is closely related to the hydrodesulfurization (HDS) process from the refinery industry, used in the elimination of sulfur from organic compounds (Mortensen et al., 2011). Both HDO and HDS use hydrogen for the exclusion of the heteroatom, forming respectively H₂O and H₂S. Water is formed in the conceptual reaction, so (at least) two liquid phases will be observed as product: one organic and one aqueous. The appearance of two organic phases has also been reported, which is due to the production of organic compounds with densities less than water. In this case a light oil phase will separate on top of the water and a heavy one below. The formation of two organic phases is usually observed in instances with high degrees of deoxygenation, which will result in a high degree of fractionation in the feed.

Regarding operating conditions, a high pressure is generally used, which has been reported in the range from 75 to 300 bar in the literature (Czernik and Bridgwater, 2004; Zhang et al., 2007; Mortensen et al., 2011; Bridgwater, 2012). The high pressure has been described as ensuring a higher solubility of hydrogen in the oil and thereby a higher availability of hydrogen in the vicinity of the catalyst. This increases the reaction rate and further decreases coking in the reactor. High degrees of deoxygenation are favored by high residence times.

The catalysts originally tested were based on sulfided CoMo or NiMo supported on alumina or aluminosilicate and the process conditions are similar to those used in the desulfurization of petroleum fractions (Mortensen et al., 2011). However a number of fundamental problems arose, including that the catalyst supports of typically alumina or aluminosilicates were found to be unstable in the high water content environment of bio-oil and the sulfur was stripped from the catalysts requiring constant re-sulfurization (Bridgwater, 2012). More recently, attention has turned to precious metal catalysts on less susceptible supports, and considerable academic and industrial research has been initiated in the last few years, for instance: Pd on C (Wildschut et al., 2009; Zhao et al., 2009; Crossley et al., 2010), Ru on C (Zhao et al., 2009), and Pt (Fisk et al., 2009).

Oxidation

The oxidation of glycerol is conducted mainly using supported noble metal nanoparticles such as Pd, Pt, and Au as catalysts (Zhou et al., 2008). Noble metal catalysts prepared using sol–gel immobilization techniques performed better than catalysts prepared by impregnation or incipient wetness methods (Porta and Prati, 2004). Several supports have been employed: different carbons (i.e., carbon black, activated carbon, and graphite) and oxides (TiO₂, MgO, CeO₂, and Al₂O₃), all of which were active for the heterogeneously catalyzed liquid-phase oxidation of glycerol under atmospheric pressure conditions. However, for the same reaction conditions and using comparable metal particle size, the carbon supported catalysts showed high activity for the liquid phase oxidation of glycerol. The pH of the solution directs the selectivity to the different products, when a basic reaction solution is used, the oxidation of the primary alcohol function is promoted, whereas acidic conditions promoted the oxidation of the secondary alcohol function (Smits et al., 1987; Mallat and Baiker, 1995; Abad et al., 2006).

Hydrogenolysis

Hydrogenolysis is a catalytic chemical reaction that breaks a chemical bond in an organic molecule with the simultaneous addition of a hydrogen atom to the resulting molecular fragments. Through the selective hydrogenolysis of glycerol in the presence of metallic catalysts and hydrogen, 1,2-propanediol (1,2-PD), 1,3-propanediol (1,3-PD), or ethylene glycol (EG) could be obtained (Ruppert et al., 2012).

First studies of the glycerol hydrogenolysis use Raney metals (Ni, Rh, Ru, Ir, Cu) as catalysts. Mainly methane was produced except for Cu catalyst, where 1,2-PD was the main reaction product (Zhou et al., 2008). This phenomenon was attributed to the low hydrogenolytic activity toward C–C bonds of copper, but high activity for C–O bond hydrogenation and dehydrogenation. However, the reaction conditions employed for glycerol hydrogenolysis using Cu Raney catalysts are very hard (P > 10 MPa). This drawback was avoided using supported metal catalysts (Miyazawa et al., 2007). Several combination support metals have been studied, Cu, Ru, Pd, and Rh catalysts supported on ZnO, C, and alumina, for instance (Chaminand et al., 2004; Dasari et al., 2005; Vila et al., 2012), reaching a 100% selectivity to 1,2-PD during the hydrogenolysis of glycerol in water using CuO/ZnO catalysts (Chaminand et al., 2004). The hydrogenolytic activity toward C–C bonds of different metals can be modulated by the addition of different compounds, and interesting results have been obtained by sulfur-modified Ru catalysts (Casale and Gomez, 1994; Lahr and Shanks, 2005), Ni/Re catalysts (Werpy et al., 2003), Ni/Ce catalysts (Jiménez-Morales et al., 2012) or a combination of cobalt, copper, manganese, molybdenum, and an inorganic polyacid (Schuster and Eggersdorfer, 1996). The addition of acid to metal catalysts enhances the glycerol conversion (Zhou et al., 2008). During the hydrogenolysis reaction, the acid dehydrates the glycerol to 1-hydroxyacetone, and then the metallic function hydrogenates more easily to 1-hydroxyacetone (Miyazawa et al., 2006). An excellent combination is Ru/C with an acidic resin, that yields better results than other metals (Rh/C, Pd/C and Pt/C) and acids (zeolites, sulfated zirconia, H₂WO₄, and liquid H₂SO₄) (Kusunoki et al., 2005; Miyazawa et al., 2006, 2007).

Despite much research effort, the potential importance of the glycerol hydrogenolysis reaction is limited to the laboratory scale as the common drawbacks of high temperature and pressure, dilute solutions and the low selectivity toward propylene glycol still require further investigation.

Dehydration

Acid dehydration of glycerol to acrolein could offer an alternative for the currently commercial catalytic petrochemical process based on propylene (Fig. 6.14).

This reaction was studied using reactive distillation in the presence of organic or mineral acids (Waldmann and Petrū, 1950), and in gas phase over solid acids (Schwenk et al., 1933; Haas et al., 1994). However, this process is energy intensive because it is necessary to evaporate glycerol and water present in crude glycerol. An alternative is the use of liquid phase dehydration. Sub- (HCW) and supercritical water (SCW) reactions can be used for the production of acrolein from glycerol (Ramayya et al., 1987; Bühler et al., 2002; Ott et al., 2006). The yield to acrolein in SCW is very low, but can be increased by the presence of acids (Watanabe et al., 2007). The rate constant of acrolein decomposition was always higher than that of acrolein formation in the absence of an acid catalyst, but the rate constant of acrolein formation could overcome that of acrolein decomposition by the addition of an acid in supercritical condition. The use of SCW is only promising if acid is added, but this produces tremendous corrosion problems, because SCW itself induces corrosion, and the presence of an acid compound intensifies the corrosive effect. Several authors confirmed that the formation of acrolein from glycerol was controlled by ionic species (such as proton) and can be increased by the presence of an acid and HCW conditions (Bühler et al., 2002; Watanabe et al., 2007). A catalytic alternative is the use of zinc sulfate, which is an effective catalyst for the acrolein synthesis from glycerol in HCW (573–663 K, 25–34 MPa, 10–60 s), achieving an acrolein selectivity of 75% at 50% of glycerol conversion (Ott et al., 2006). These results indicate that an improvement in the technology is necessary to find any practical application in the formation of acrolein from glycerol.

Carboxylation

Glycerol carbonate is a new and interesting material in the chemical industry. Inexpensive glycerol carbonate could serve as a source of new polymeric materials for the production of polycarbonates and polyurethanes. Several procedures of glycerol carbonate have been described in the literature, but the most interesting is the direct reaction with carbon dioxide (Vieville et al., 1998; Aresta et al., 2006; Zhou et al., 2008). First studies focused on the use of basic zeolites and resin catalysts (Vieville et al., 1998). Under supercritical CO₂ and in the presence of ethylene carbonate, the glycerol carbonate can be formed by direct reaction with carbon dioxide. Another alternative is the use of metal alcoxides, especially Sn catalysts. The carbonate was formed with an appreciable rate until a 1.14:1 molar ratio of carbonate to catalyst was reached.

Methanol synthesis

Commercially, methanol is produced from natural gas or coal via syngas, mainly containing CO and H₂ along with a small amount of CO₂, which has been developed over the last century (Klier, 1982; Chinchen et al., 1988; Lange, 2001). Methanol synthesis catalyst can be produced from a mixture of copper, zinc and aluminum nitrates by coprecipitation with sodium carbonate followed by filtration, washing, drying, and calcination of the purified precipitate (Klier, 1982; Chinchen et al., 1988). Ternary Cu–Zn–Al oxide catalyst produces methanol at 5.0–10.0 MPa and 473–523 K. However, the ternary catalyst that was active for CO-rich feedstock was not so active for the CO₂-rich sources (syngas from biomass). In fact, a small quantity of CO₂ (3–5%) is present in the conventional synthesis of methanol (Chinchen et al., 1988; Liu et al., 2003). For this reason, only modifications of traditional Cu/ZnO catalysts have been proposed to improve the catalytic activity. Cu/ZnO-based catalysts have been modified with Al₂O₃, Ga₂O₃, ZrO₂, and Cr₂O₃ (Saito and Murata, 2004; Yang et al., 2006); they showed a better stability and a slight increase in methanol productivity. But the most interesting results have been obtained with multicomponent catalysts (Cu/ZnO/ZrO₂/Al₂O₃ and Cu/ZnO/ZrO₂/Al₂O₃/Ga₂O₃). Multicomponent catalysts were more active than the ternary or the binary catalyst. In addition, the multicomponent catalysts were found to be highly active even after the treatment of the catalysts in flowing H₂ at 723 K, indicating that their thermal stabilities were extremely high (Yang et al., 2006). Methanol synthesis from biomass syngas is a technology ready to be implanted at industrial scale (Phillips et al., 2011b).

Fischer–Tropsch (FT) synthesis

In 1922, Hans Fischer and Franz Tropsch proposed the synthol process, which gave, under high pressure (> 100 bar), a mixture of aliphatic oxygenated compounds via reaction of carbon monoxide with hydrogen over alkalized iron chips at 673 K. This product was transformed after heating under pressure into ‘Synthine’, a mixture of hydrocarbons (Khodakov et al., 2007). Since its discovery, the significance of the process has been amply demonstrated by the enormous amount of research and development effort achieved concomitantly to the oil price increase, that is, more than 4,000 papers on FT were published between 2000 and 2011, and a great number of patents dealing with FT synthesis can be found in the literature, and have been thoroughly reviewed (Iglesia et al., 1993; Dry, 2002a, 2002b; Khodakov et al., 2007; de Smit and Weckhuysen, 2008; Abelló and Montané, 2011; Klerk, 2011; De la Peña O’Shea et al., 2003, 2005; González Carballo et al., 2011; Ojeda et al., 2004; Pérez-Alonso et al., 2007).

All group VIII metals have noticeable activity in the hydrogenation of carbon monoxide to hydrocarbons. But only ruthenium, iron, cobalt, and nickel have catalytic characteristics which allow them to be considered for commercial production. Nickel catalysts under practical conditions produce too much methane. Ruthenium is too expensive; moreover, its worldwide reserves are insufficient for large-scale industry. Thus, cobalt and iron are the metals which were proposed by Fischer and Tropsch as the first catalysts for syngas conversion. Both cobalt and iron catalysts have been used in the industry for hydrocarbon synthesis. A brief comparison of cobalt and iron catalysts is given in Table 6.2.

Iron-based catalysts are especially suited for the production of liquid hydrocarbon products from syngas derived from sources, such as coal (CTL) and biomass (BTL), which typically have too low a H₂ to CO ratio to stoichiometrically produce longer chain hydrocarbon products. Because they have unique water–gas shift (WGS) capabilities, they catalyze the reaction between carbon monoxide and water to form hydrogen and carbon dioxide. Iron-based FTS catalyst precursors consist of nanometer-sized Fe₂O₃ crystallites to which often promoters are added to improve the catalyst performance. A typical catalyst contains promoters like copper to enhance catalyst reducibility, potassium to improve CO dissociation, along with some silica or zinc oxide to improve the amount of iron atoms interacting with the gas phase (i.e., catalyst dispersion). The catalyst is treated in H₂, CO, or syngas to convert it to its active form. During FTS, a complex mixture of iron phases is formed. The nature of active sites in iron-based catalysts is still a subject of debate. Several forms of iron oxides and iron carbides (Fe_xC_y) may co-exist during the FT reaction: α-Fe, γ-Fe, Fe₃O₄, coexisting with ε-Fe₂C, ε′-Fe_2.2C, Fe₅C₂ (Hägg carbide), Fe₇C₃, and θ-Fe₃C (Pérez-Alonso et al., 2007; de Smit and Weckhuysen, 2008; González-Carballo and Fierro, 2010; Abelló and Montané, 2011).

The BTL process is one of the most important and sustainable paths to produce liquid fuels, such as gasoline and diesel, and chemicals from renewable resources. However, a still unresolved major problem of FTS is to achieve good selectivity control toward certain products of interest. Such control depends mainly on the nature of active ingredients in the catalyst, the presence of promoters, and the choice of an adequate support. Accordingly, tailoring the catalyst to achieve specific product distributions is a major objective for current investigations.

Biomass syngas feeds exhibit low conversion efficiency owing to their H₂-deficient or CO₂-rich nature. Many studies on the carbon monoxide hydrogenation with regard to hydrocarbon production have been performed by using iron-based catalysts independent of the H₂/CO ratio in the syngas feed. The flexibility of the WGS reaction in such cases makes it possible to achieve the production of hydrocarbons by H₂/CO ratios < 2 in the syngas feed.

Catalytic cracking

The cracking or hydrocracking of lignin can be performed on the same catalysts used at oil refineries. The catalysts used in hydrocracking are predominantly bifunctional, combining a support active in cracking with a (noble) metal for the hydrogenation reaction. The hydrogenation catalyst is typically composed of noble metal, cobalt, tungsten, or nickel, and the cracking component typically consists of zeolites or amorphous silica-alumina of various compositions. Lignin can also be treated with hydrocracking catalysts, which leads to cleavage of the β-O-4 bond and relatively unstable carbon–carbon bonds. The resulting low molecular weight aromatic compounds are then susceptible to further conversion to valuable products (Thring and Breau, 1996). The acidic function of the catalysts is proportioned by zeolites or silica alumina. H-ZSM-5 and H-mordonite produced more aromatic than aliphatic hydrocarbons from fast pyrolysis bio-oil, whereas H-Y, silicalite, and silica-alumina produced more aliphatic than aromatic hydrocarbons.

Hydrolysis

Hydrolysis of lignin is catalyzed by bases. This process has been studied in detail, because delignification is one of the main processes in the manufacture of cellulose and paper. The reaction was favored by strong bases, and combinations of bases gave either positive synergistic effects, such as with NaOH and Ca(OH)₂, or negative synergistic effects, such as with LiOH or CsOH with Ca(OH)₂ (Miller et al., 1999; Zakzeski et al., 2010). The obtained products consist in a mixture of products derived from phenol (Miller et al., 1999; Nenkova et al., 2008; Zakzeski et al., 2010). Several solvents have been used for the hydrolysis of lignin, mainly polar solvents under standard conditions (Nenkova et al., 2008) or supercritical conditions (Miller et al., 1999; Wahyudiono et al., 2007, 2009). Among them, some attention has been focused on supercritical water, which is able to hydrolyze lignin in the absence of catalysts (Wahyudiono et al., 2007, 2009). However, the high temperature necessary to obtain supercritical water (374°C) can yield coke formation (Zakzeski et al., 2010).

Reduction

For lignin reductions, typical reactions involve the removal of the extensive functionality of the lignin subunits to form simpler monomeric compounds such as phenols, benzene, toluene, or xylene. These simple aromatic compounds can then be hydrogenated to alkanes or used as platform chemicals for use in the synthesis of fine chemicals using technology already developed in the petroleum industry.

The first catalysts studied in the hydrogenation of lignin are based on metal-supported Raney Ni, Pd/C, Rh/C, Rh/Al₂O₃, Ru/C, Ru/Al₂O₃. Employing these catalysts, a significant amount of the original lignin was converted into the monomeric products 4-propylguaiacol and dihydroconiferyl alcohol under mild conditions (3.4 MPa, 468 K) (Pepper and Hibbert, 1948; Pepper and Lee, 1969; Pepper and Fleming, 1978; Pepper and Supathna, 1978). However, the majority of studies have been focused on the use of catalysts based on industrial hydrotreatment catalysts (cobalt- and nickel-promoted molybdenum catalysts) and their modification; sulfided Co-Mo catalyst provided the best results (Zakzeski et al., 2010). This phenomenon was explained by the systematic study of C–O bond hydrogenolysis of diphenyl ether of a series of sulfided M-Mo/Al₂O₃ catalysts (M = Cr, Fe, Co, Ni, Ru, Rh, Pd, Re, Ir, or Pt, at 623 K, 13.8 MPa H2), where Co-Mo, Rh-Mo, and Ru-Mo catalysts showed the highest hydrogenolysis activity in this order.

Some disadvantages that are associated with conventional hydrodeoxygenation catalysts are possible contamination of products by incorporation of sulfur, rapid deactivation by coke formation, and potential poisoning by water. These issues arise especially with biomass feedstocks and thus have prompted efforts to explore alternative hydrogenation catalysts (Zhao et al., 2009). Ni-W/SiO₂-Al₂O₃, Ni-Cu/ZrO₂, Ni-Cu/CeO₂, transition metal carbides, and noble metals on carbon have been proposed as alternative catalysts. The supported platinum group catalysts are known to be more active than the sulfided Mo-based ones, and can therefore be used at lower temperatures, and non-alumina supports such as carbon avoid water instability associated with Al₂O₃ (Elliott and Hart, 2008).

Oxidation

For lignin oxidation, lignin is converted to more complicated platform chemicals with extensive functionality or converted directly to target fine chemicals. Oxidative catalysts have played an important role in the pulp and paper industry as a means to remove lignin and other compounds from wood pulps in order to increase the quality of the final paper product.

Some studies have focused on the use of heterogeneous catalysts, using different catalysts and oxidants. Some systems are based on the use of titanium oxide as photocatalyst, or its modification with Pt or Fe in the lignin oxidation at room temperature (Portjanskaja and Preis, 2007; Ma et al., 2008; Portjanskaja et al., 2009). Other authors propose the use of copper-based catalysts operating at higher temperature (373 K); these catalysts have only Cu, Cu-Ni, Cu-Mn, Cu-Fe, etc. (Bhargava et al., 2007; Zhang et al., 2009). Some authors have studied the use of alternative oxidants like hydrogen peroxide. In this case, they propose the use of hydrogen peroxide with methylrhenium trioxide catalysts immobilized on poly(4-vinyl pyridine) or polystyrene (Crestini et al., 2006; Herrmann et al., 2000).

Several studies have focused on the use of homogeneous catalysts; these catalysts are based in coordination compounds of transition metals. The oxidation of lignin by homogeneous catalysts represents one of the most promising approaches toward the production of fine chemicals from lignin and lignin pulp streams. Several homogeneous catalysts that are capable of performing selective oxidation of lignin have been reported in the literature. Homogeneous catalysts offer several advantageous properties that make them particularly suitable for lignin oxidation, especially the ability to use a wide range of ligands, the electronic and steric properties which drastically influence the activity, stability, and solubility of the catalyst. It thus becomes possible to tune the reactivity and selectivity of the homogeneous catalyst to the oxidation of specific lignin linkages or functionalities with appropriate choice of ligands.

Generally, the homogeneous catalysts used for lignin oxidation can be subdivided into several categories depending on the ligand set employed: metalloporphyrins, Schiff-base catalysts, polyoxometalates (POM), simple metal salts, and other kind of catalysts. A significant disadvantage of using the porphyrin complexes is the susceptibility to degradation in the presence of excess oxidant, particularly H₂O₂, or through the formation of catalytically inactive μ-oxo species (Zakzeski et al., 2010). In contrast, the Schiff-base catalysts have several advantages over the metalloporphyrin complexes discussed above in that they are often cheaper, easier to synthesize, and relatively stable (Zakzeski et al., 2010). Similarly to the case of metalloporphyrins, the original objective for the design of POMs focused on the ability to selectively degrade lignin rather than cellulose and other materials in the paper industry. That is, active catalysts rapidly oxidized lignin to carbon dioxide and water with minimal degradation of the polysaccharides, leaving a lignin-free white pulp suitable for paper production. For this reason, a modification of POM composition is necessary for the selective oxidation of Kraft pulps to chemicals (Voitl and von Rohr, 2008). In general, the order of activity for the homogeneous catalysts was Cu^2 + > Fe^2 + > Mn^2 + > Ce^2 + > Bi^2 + > Co^2 + > Zn^2 + > Mg^2 + Ni^2 + (Bhargava et al., 2007) when the same category of catalysts was studied.

Trends in biomass depolymerization

New, flexible biomass pre-treatment processes should be developed and tailored to suit improved biomass feedstocks in order to obtain fully functional fractions (e.g., lignin and carbohydrates from lignocellulose). Current research on lignocellulose breakdown must be accelerated, by improving existing technologies to develop efficient and cost-effective processes.

For gasification and pyrolysis processes, research should focus on scaling up and integrating them into existing production units, together with end-product quality improvement (e.g., syngas purification for catalytic conversion and pyrolysis oil upgrading and fractionation).

Trends in product upgrading

Biomass products contain a large number of oxygenated functional groups. The number of these groups must be reduced by different reactions. There is therefore a need to perform research in order to develop novel catalysts that are able to perform these reductions of oxygenated group reactions, in contrast to the oxidation reactions typical of the conventional chemical industry. Processing these highly functionalized, biomass-derived molecules leads to selectivity issues (with selective hydrogenation being needed to modify some groups but not others). The catalytic processes need to remove this functionality selectively, often to generate ‘bi-functional’ molecules that can be used as building blocks for bulk chemicals and bulk bio-based polymers. At this point the development of bi- or even multi-functional catalysts is a critical issue.

Further research needs to be focused on the development of catalysts that are capable of selectively transforming biomass-derived monomers (sugars, fatty acids, etc.) to platform molecules, or catalyzing the reaction from these intermediates to final products. At the same time, the work should also focus on the possible modification of catalyst surface properties to improve their functionality in the presence of water. R&D is needed to understand catalyst functionality and to revise the structure/morphology/function approach to catalyst development using new tools (such as process spectroscopy, etc.). The high reactivity of biomass-derived molecules leads to catalyst coking issues (rapid deactivation of catalysts by accumulation of carbonaceous compounds on their surface). Solutions to these issues can be found via the development of more robust catalytic formulations and also in process design, by designing new reactors and introducing small doses of oxygen in the reaction medium, optionally with the addition of an oxygen-splitting function on the catalyst surface to facilitate the process.

Trends in syngas transformations

Syngas made by biomass gasification can be used to produce hydrogen (via the catalytic water–gas shift reaction to H₂ and CO₂), biofuels (e.g., synthetic diesel by Fischer–Tropsch synthesis), or chemicals (mainly short-chain alcohols). The main problems arise from contamination of the syngas by impurities that ‘poison’ or inactivate the catalyst. Future catalysts for syngas conversion have to be developed which have a greater resistance to poisoning, allowing syngas purification costs to be reduced.