Phenol, water, acetic acid, acetaldehyde, hydroxyacetone, D-glucose and 2-hydroxymethylfuran were mixed together and used as a model bio-oil to react with 1-octene/1-butanol at 120 °C for 3 h over each of the five catalysts: Cs25/K10, A15, A36, DX2 and SSA. Table 3 shows the 1-octene, 1-butanol and phenol conversions as well as the 1-octene isomerization and O-alkylation selectivities of these reactions [22]. 1-Octene conver­sions differed significantly over these catalysts and followed the order: SSA («60%) > DX2 (40-50%) > A15 (27%) > A36 (14%) > Cs25/K10 (10%). Similar differences occurred for both phenol conversion and 1-oc — tene isomerization. The phenol conversion was higher with SSA (64.1%) verses DX2 (37.3%), A15 (27.6%), A36 (6.1%) and Cs25/K10 (1.2%). The 1-octene isomerization activities of these five catalysts are 87.9% (SSA), 55.5% (DX2), 54.1% (A15), 13.5% (A36) and 1.9% (Cs25/K10). These follow the same order and show the higher activity of the SSA catalyst. Higher phenol conversion was accompanied by higher 1-octene isomeri­zation activity and higher 1-octene conversions. SSA is the most active catalyst. This is because it is a stronger acid than the three resin sulfonic acids. Compared with the resin sulfonic acids (P-C6H4-SO3H), where the S atom has 3 O atoms attached, the S atom in SSA (SiO2-OSO3H) has 4 O atoms attached. This causes the weaker basicity of — O-SO3- verses that of Ph-SO3-. Thus, SSA is the strongest acid. The stronger the acid, the more 1-octene protonation is favored. Hence, more octyl cations are generated. With the increase in octyl cation concentration, both phenol alkylation
(phenolic oxygen attack on the carbocation) and 1-octene isomerization reaction (loss of proton from the carbocation) would speed up accompa­nied with faster consumption of 1-octene. This is consistent with higher phenol conversion and both 1-octene isomerization activity and conver­sion to other products with SSA.

aMaterial ratio: 1-octene: 1-butanol: phenol: water: acetic acid: acetaldehyde: hydroxyacetone: D-glucose: 2-hydroxymethylfuran (g) = 1.35: 0.15: 0.94: 0.15: 0.15: 0.12: 0.12: 0.15: 0.15, catalyst: 0.15g; b1-Octene conversion = 100% x (1- GC area% of unreacted octenes versus the sum of the GC area% of alkylated phenols, octanols, dioctyl ethers, oligomers and octyl acetates); cPhenol conversions = 100% x GC area% of phenol alkylates versus the sum of the GC area% of unreacted phenol and phenol alkylates; dPercent of 1-octene isomerization = 100% x (1- GC area% of 1-octene versus the sum of the GC area% of 1-octene and 1-octene isomers); ‘1-Butanol conversion = 100% x (1- GC area% of unreacted 1-butanol versus the sum of the GC area% of butyl acetate, dibutyl ether, 1,1-dibutoxyethane and butyl levulinate); fO-Alkylates selectivity = 100% x GC area% of O-alkylates versus the sum of the GC area% of all phenol alkylates.

Stronger acids also promote both esterification and acetal formation rates. This can be observed from the higher 1-butanol conversion (97.4%) with SSA catalyst. Except for the modest 1-butanol conversion (68%) formed over Cs2.5/K10, 1-butanol conversions with the three resin sulfonic acids catalysts all exceeded 90%. Carboxylic acid esterifications and alde­hyde/ketone acetal formation with 1-butanol are reversible or equilibrium reactions. The desirable forward reaction products like esters and acetals were produced accompanied by formation of water. That water and the original water present in bio-oil would inhibit the forward reactions, limit­ing further formation of more esters and acetals. Water removal by acid catalyzed hydration of 1-octene helped to shift these equilibria forming esters and acetals toward completion.

Phenol alkylates (C — and O-) are desired because of their high octane number and high heating values [15]. The O-alkylated products are espe­cially desirable because the acidic phenolic hydroxyl group is converted to an ether lowering product acidity and decreasing hydrophilicity. Moreover,

O-alkylated phenol ethers are readily combusted. Except for Cs25/K10, all the catalysts gave high O-alkylation selectivity (>60%). Compared with the three resin sulfonic acids, SSA gives more C-alkylates because that stronger acid promotes conversion of O-alkylates into the thermodynamic phenol C-alkylates by enhancing O-alkylate protonation.

SSA exhibited a higher water-tolerance than other catalysts based on the model systems shown in Table 3. Desulfonation of the three resin sulfonic acids catalyst occurred progressively at 120 °C over time, leading to partial deactivation of these catalysts. Cs25/K10 lost almost all catalytic activity.

One of the most widely used chemical absorption techniques for carbon capture and storage/sequestration (CCS) is CO2 adsorption by ceramic materials. Once CO2 has been captured-fixed, it can be converted into value-added products such as precursors in chemical transformation reac­tions. CO2 is extensively used for enhanced oil recovery, as a monomer feedstock for urea and polymer synthesis, in the food and beverage indus­try as a propellant, and in production of chemicals. Therefore, the capture — fixation of CO2 would make a system suitable for accomplishing chemical transformation of CO2. The utilization of carbon dioxide is also very at­tractive because it is environmentally benign [105-115]. CO2 conversion to fuel and value-added products is an ideal route for CO2 utilization due to the simultaneous disposal of CO2 and the benefit that many products can be used as alternate transportation fuels [116]. CO2 chemical transfor­mation methods include (i) reverse water-gas shift, (ii) hydrogenation to hydrocarbons, alcohols, dimethyl ether and formic acid, (iii) reaction with hydrocarbons to syngas, (iv) photo — and electrochemical/catalytic conver­sion, and (v) thermo-chemical conversion [100-122].

CO2 can be catalyzed to valuable organic or inorganic compounds, where some basic catalytic materials (containing alkaline or alkaline-earth elements) are used. The activation of CO2 by alkali metals has received considerable attention in various surface science studies, which have demonstrated the formation of intermediate CO2, dissociation of CO2 and formation of oxalate and carbonate alkali compounds [118-121]. Carbon dioxide has been identified as one such potential vector molecule (through reduction to syngas, methanol, methane, formic acid, formaldehyde, di — methylether (DME) and short-chain olefins) [117-118, 120-122]. CO2 is a kinetically and thermodynamically stable molecule, so CO2 conversion reactions are endothermic and need efficient catalysts to obtain high yield. CO2 conversion to carbon monoxide (CO) looks like the simplest route for CO2 reduction [121]. CO is a feedstock or intermediate product for the production of methanol and hydrocarbon fuels via Fischer-Tropsch synthesis of CH4/CO2 reforming to form syngas (CO/H2) [122]. CO2 re­forming with CH4 is an example of CO2 being used as a soft oxidant, where the dioxide is dissociated into CO and surface oxygen, and oxygen abstracts hydrogen from methane to form water via the water-gas shift reaction (WGS) (Eqs. 11 and 12) [100-103, 121]. The catalytic chemis­try of the reverse water-gas shift reaction and the following transforma­tion to methanol/DME (or hydrocarbons via Fischer-Tropsch synthesis), and the subsequent production of gasoline (methanol-to-gasoline or diesel via hydrocracking of the alkanes produced in the Fisher-Tropsch process) are well established [102, 117-122]. On the other hand, methanol can be produced directly from carbon dioxide sources by catalytic hydrogena­tion and photo-assisted electrochemical reduction. A wide variety of CO2 photo-reduction methods have been performed to oxygenate products, in­cluding formic acid (HCOOH) and formaldehyde (HCHO). HCOOH and

HCHO are the simplest oxygenates produced from the reduction of CO2 with H2O (or proton solvents) [121]. Furthermore, CO2 can be utilized as a monomeric building block to synthesize various value-added oxygen-rich compounds and polymers under mild conditions. As an example, chemical conversion of CO2 through C-N bond formation can produce value-added chemicals such as oxazolidinones, quinazolines, carbamates, isocyanates and polyurethanes [105]. These commodity chemicals have been synthe­sized from green methods and have important applications in the pharma­ceutical and plastic industries. The chemisorption of CO2 based on C-N bond formation could be one of the most efficient strategies, utilizing liq­uid absorbents such as conventional aqueous amine solutions, chilled am­monia, amino-functionalized ionic liquids, and solid absorbents including amino-functionalized silica, carbon, polymers and resins. The processes by which chemicals for CO2 capture are manufactured should also be con­sidered in terms of their energy requirements, efficiencies, waste products, and CO2 emissions [105, 123]. In that sense, dimethyl carbonate (DMC) is a promising target molecule derived from CO2 catalyzed by inorganic dehydrating agents such as molecular sieves [107]. Dimethyl carbonate has received much attention as a safe, non-corrosive, and environmentally friendly building block for the production of polycarbonates and other chemicals, an additive to fuel oil owing to its high octane number and an electrolyte in lithium batteries due to its high dielectric constant. It can be synthesized through a two-step transesterification process utilizing CO2 as raw material [105, 107].

As a complementary technology to carbon sequestration and storage (CSS), the chemical recycling of carbon dioxide to fuels is an interesting opportunity. Chemical compounds such as alkane products (CnH(2n+2)) are un-branched hydrocarbons suitable for diesel fuel and jet fuel [121]. In this regard, biofuels or biodiesel, catalyzed using ceramic materials, can provide a significant contribution in energy independence and mitigation of climate change [109-127]. Today the main renewable biofuels are bio­ethanol and biodiesel. Biodiesel is a liquid fuel consisting of mono alkyl esters (methyl or ethyl) of long-chain fatty acids derived from vegetable oils, animal fats or micro and macro algal oils [127]. Biodiesel is a sustain­able, renewable, non-toxic, biodegradable diesel fuel substitute that can be employed in current diesel engines without major modification, offering an interesting alternative to petroleum-based diesel [106, 111-115, 124­128]. Besides this, it is free from sulfur and aromatic components, making it cleaner burning than petroleum diesel. Biodiesel has a high flash point, better viscosity and caloric power similar to fossil fuels. It can be mixed with petroleum fossil fuel at any weight ratio or percentage, and it can be used without blending with fossil fuel (B100) as a successful fuel [127, 128]. It has similar properties (physical and chemical) to petroleum die­sel fuel. Recently, transesterification (also called alcoholysis) has been re­ported as the most common way to produce biodiesel with lipid feedstock (such as vegetable oil or algal oil) and alcohol (usually methanol or etha­nol), in presence of an acid or base catalyst. Transesterification is the best method for producing higher-quality biodiesel and glycerol [108, 110-115, 124-132]. The reaction is facilitated with a suitable catalyst [129-131]. The catalyst presence is necessary to increase both, the reaction rate and the transesterification reaction conversion yield. The catalysts are clas­sified as homogeneous or heterogeneous. Homogeneous catalysts act in the same liquid phase as the reaction mixture. Conversely, if the catalyst remains in a different phase, the process is called heterogeneous catalytic transesterification [113, 127-131]. Heterogeneous catalysts are mostly ap­plied in transesterification reaction due to many advantages such as easy catalyst separation and reusability, improved selectivity, fewer process stages, no water formation or saponification reaction, including in green technology, and cost effectiveness [127, 132]. The heterogeneous catalysts increase the mass transfer rate during the transesterification reaction [127, 131]. Various ceramic materials have been investigated for the production of biodiesel [106, 109-115, 124-179]. Some ofthese solid catalysts include alkali and alkaline-earth metal carbonates and oxides such as magnesium oxide (MgO), calcium oxide (CaO), barium oxide (BaO), strontium ox­ide (SrO) [124-131, 133-143]; lithium base ceramics (Li4SiO4 and Li2SiO3 [144-146]); sodium silicate (Na2SiO3 [147]); transition metal oxides and derivatives (titanium oxide, zinc oxide, mixed oxides catalysts [148-149]); ion exchange resin type acid heterogeneous catalysts [150]; MCM-metal impregnated materials [114]; layered double hydroxides (hydrotalcite-like hydroxides) [151-154]; hydrocalumite-like compounds [110,155]; sup­ported bases [156-163]; and zeolites [164-165].

FIGURE 6: Schematic representation of the membrane reactor concept using a CO2 permselective ceramic membrane: (a) CO2 dry methane reforming and (b) water-gas shift reaction with hydrogen purification wherein CO2 capture promotes the separation process.

Among the alkaline earth metal oxides, CaO is a promising basic het­erogeneous catalyst for synthesizing biodiesel at mild temperatures (be­low the boiling point of methanol, MeOH) and at atmospheric pressure due to its plentiful availability and low cost, but it is rapidly hydrated and carbonated upon contact with room temperature air. CaO is the most widely used catalyst for transesterification and produces a high yield of 98% of fatty acid methyl esters (FAME) during the first cycle of reaction [130]. Granados et al. [142] used activated CaO as a solid base catalyst in the transesterification of sunflower oil to investigate the role of water and carbon dioxide on the deterioration of the catalytic performance upon contact with air for different periods. The study showed that CaO was rapidly hydrated and carbonated in air. Consequently, the reusability of the catalyst for subsequent steps is a big question mark. Di Serio et al. [170] reported a 92% biodiesel yield with MgO catalyst, using 12:1 methanol to oil molar ratio with 5.0wt% of the catalyst at methanol supercritical con­dition for 1 h. Wen et al. [171] carried out transesterification from waste cooking oil with methanol at 170 °C for 6 h with 10wt% of MgO/TiO2 and 50:1 M ratio of MeOH and oil. Guo et al. [172] studied the methyl ester yield produced via transesterification of soybean oil using sodium silicate as a catalyst. Sodium silicate was an effective catalyst for the microwave — irradiated production of biodiesel and hydrothermal production of hydro­gen from by-product glycerol combined with Ni catalyst. The optimum reaction conditions obtained were 7.5:1 M ratio of alcohol/oil, 3wt% cata­lyst amount, 1 h reaction time and 60 °C reaction temperature. The FAME yield was ~100%. On the other hand, microwave-assisted transesterifica­tion of vegetable oil with sodium silicate is an effective and economical method for the rapid production of biodiesel. The reused catalyst after transesterification process for four cycles was recovered. Overall, sodium silicate was fully used in biodiesel production and glycerol gasification, and this co-production process provided a novel green method for bio­diesel production and glycerol utilization [172].

Several techniques have been investigated for the transesterification reaction using heterogeneous catalysts for biodiesel production, as fol­lows: transesterification via radio frequency microwaves, alcohol reflex temperature, alcohol supercritical temperature and ultrasonication [127, 173-177]. Recently, the use of ultrasonic irradiation has gained interest in biodiesel production [173-177]. Ultrasonic energy can emulsify the reactants to reduce the catalyst requirement, methanol-oil ratio, reaction time and reaction temperature and also provides the mechanical energy for mixing and the required activation energy for initiating the transesterifica­tion reaction [173-176]. The ultrasound phenomenon has its own physical and chemical effects on the liquid-liquid heterogeneous reaction system through cavitation bubbles, according to the following principles [175]:

(1) the chemical effect, in which radicals such as H+ and OH — are produced during a transient implosive collapse of bubbles (in a liquid irradiated with ultrasound), which accelerates chemical reaction in the bulk medium; and

(2) the physical effect of emulsification, in which the microturbulence generated due to radial motion of bubbles leads to intimate mixing (ho­mogenizing the mixture) of the immiscible reactants. Accordingly, the in­terfacial region between the oil and alcohol increases sharply, resulting in faster reaction kinetics and higher conversion of oil and biodiesel yield [127]. In 2000, the ultrasonication reactor was first introduced by Hiel — scher Ultrasonic GmbH for biodiesel production. Nishimura et al. [175] studied the transesterification of vegetable oil using low-frequency ultra­sound (28-40kHz). An excellent yield (~98%) was obtained at a 28 kHz ultrasound while a significant reduction of reaction time was obtained by using 40 kHz ultrasound. Salamatinia et al. [176] used ultrasonic assisted transesterification to improve the reaction rate. In this study, they used SrO and BaO as heterogeneous catalysts in the production of biodiesel from palm oil. The results showed that the basic properties of the catalyst were the main cause for their high activity. The low-frequency ultrasonic assisted transesterification process had no significant mechanical effects on SrO, but BaO catalyst study confirmed that the ultrasound treatment significantly improved the process by reducing the reaction time to less than 50 min at a catalyst loading of 2.8wt% to achieve biodiesel yield higher than 95%. Another study of alkali earth metals was carried out by Mootabadi et al. [177]. They reported the effect of ultrasonic waves at 20 kHz and 200W on the regenerated catalyst and compared mechanical stir­ring and ultrasonic irradiation. They investigated the optimum conditions, using palm oil for biodiesel production with catalysts such as CaO, SrO and BaO. They concluded that catalyst leaching was the main cause for the catalyst inactivity in the case of the re-used catalyst. BaO catalyst was found to be stable during the leaching. At the optimized condition, 95.2% yield was achieved with 60 min of reaction time for both BaO and SrO catalysts. For CaO catalyst, 77.3% yield was achieved with the same con­ditions. The use of ultrasound showed great enhancement of the reaction parameters in terms of the obtained yield and reaction time. The obtained yields were 30 to 40% higher in comparison to the corresponding results obtained using a conventional stirring reactor system without ultrasonica — tion. Deng et al. [178] prepared nano-sized mixed Mg/Al oxides. Due to their strong basicity, the nanoparticles were further used as catalyst for biodiesel production from jatropha oil. Experiments were conducted with the solid basic catalyst in an ultrasonic transesterification reaction. Un­der the optimum conditions, biodiesel yield was 95.2%. After removing the glycerol on the catalyst surface, the nano-sized mixed Mg/Al oxides were reused eight times. The authors concluded that calcination of hydro — talcite nanocatalyst under ultrasonic radiation is an effective method for the production of biodiesel from jatropha oil. The activity of base solid catalysts is associated to their basic strength, such that the most basic cata­lyst showed the highest conversion. In another work, Deng et al. [179] reported optimum conditions for biodiesel production in the presence of base solid catalysts. They studied BaO and Ca-Mg-Al hydrotalcite (the most effective). The 95% biodiesel yield from jatropha oils and Ca-Mg — Al hydrotalcite was established with 30 min of reaction time. Ca-Mg-Al hydrotalcite could be reused twelve times after washing of the adsorbed glycerol from the catalyst surface with ethanol. Other types of heteroge­neous catalysts under ultrasonic irradiation were used for transesterifica­tion by Georgogianni et al. [114]. They studied a wide range of catalysts including Mg-MCM-41, Mg-Al hydrotalcite and K+-impregnated zirco­nium oxide. They mixed frying oils, methanol and the catalyst in a batch reactor with mechanical stirring for 24 h and with ultrasonication for 5 h. The results suggested that the basic strength was the cause of the good activity of the catalysts. Mg-Al hydrotalcite achieved the highest reaction conversion of 87% at a reaction temperature of 60 °C. Overall, ultrasonic irradiation significantly enhanced the reaction rate, causing a reduction in reaction time, and the biodiesel yield increased [114]. Consequently, a better understanding of the use of ultrasonic sound waves to accelerate the transesterification process could lead to substantial future improvement of both batch and continuous production systems, to obtain a more sustain­able biodiesel production process [127].

The search for sustainable energy resources is one of this century’s great challenges. Biofuels (fuels produced from biomass) have emerged as one of the most promising renewable energy sources, offering the world a so­lution to its fossil-fuel addiction. They are sustainable, biodegradable, and contain fewer environmental contaminants than fossil fuels.

One of the biggest difficulties with biofuel production, however, is grabbing carbon for fuel while also removing oxygen from biomass. Un­like fossil fuel sources, biomass is rich in oxygen, which makes its trans­formation into fuel challenging. Catalysis offers a cheap, efficient, and sustainable way to remove oxygen from biomass, increasing its potential usefulness to the world’s energy needs.

Heterogeneous catalysis has a long history of facilitating energy-effi­cient selective molecular transformations. It already contributes to most chemical manufacturing processes and to many industrial products. Ca­talysis also plays a central role in overcoming the barriers to economical and sustainable biofuel production.

Technical advances in catalyst design and processes are even more es­sential for second-generation biofuels produced from non-food feedstocks. As much oxygen as possible must be removed to gain the maximum ener­gy density. In addition, the costs of oxygen removal need to be minimized.

Research that identifies inexpensive and efficient catalysts is crucial to the world’s energy needs. Additionally, we need catalysts and catalytic processes that minimize hydrogen consumption, increase overall process activity, and gain high fuel yields. The research gathered in this compen­dium contributes to this vital field of investigation.

Juan Carlos Serrano-Ruiz, PhD

Concerns over the economics of proven fossil fuel reserves, in concert with government and public acceptance of the anthropogenic origin of ris­ing CO2 emissions and associated climate change from such combustible carbon, are driving academic and commercial research into new sustain­able routes to fuel and chemicals. The quest for such sustainable resources to meet the demands of a rapidly rising global population represents one of this century’s grand challenges. In Chapter 1, Lee discusses catalytic solutions to the clean synthesis of biodiesel, the most readily implemented and low cost, alternative source of transportation fuels, and oxygenated organic molecules for the manufacture of fine and speciality chemicals to meet future societal demands.

Chapter 2, by Bezergianni, argues that catalytic hydrotreatment of liq­uid biomass is a technology with the potential to overcome the limitations of biomass fuel productions. The author points to a wide range of new alternative fuels that are being developed using this technology, arguing that they are more useful than those developed using older methods, and points to catalytic hydrotreatment as the future of biofuels.

Chapter 3, by Murzin and Holmbom, describes some of the contem­porary methods for the chemical analysis of biomass-derived chemicals. All available methods could not have been treated in this review, therefore the focus was mainly on chromatographic methods. A more comprehen­sive overview of analytical methods was published several years ago by one of the authors [31,33]. In the current work, detailed procedures were discussed for only a few cases as the emphasis was laid more on general approaches.

Concerns about diminishing fossil fuel reserves along with global warming effects caused by increasing levels of CO2 in the atmosphere are driving society toward the search for new renewable sources of energy that can substitute for coal, natural gas and petroleum in the current energy system. Lignocellulosic biomass is abundant, and it has the potential to significantly displace petroleum in the production of fuels for the trans­portation sector. Ethanol, the main biomass-derived fuel used today, has benefited from production by a well-established technology and by partial compatibility with the current transportation infrastructure, leading to the domination of the world biofuel market. However, ethanol suffers from important limitations as a fuel (e. g., low energy density, high solubility in water) than can be overcome by designing strategies to convert non-edible lignocellulosic biomass into liquid hydrocarbon fuels (LHF) chemically similar to those currently used in internal combustion engines. Chapter 4, by Serrano-Ruiz and Dumesic, describes the main routes available to carry out such deep chemical transformation (e. g., gasification, pyroly­sis, and aqueous-phase catalytic processing), with particular emphasis on those pathways involving aqueous-phase catalytic reactions. These latter catalytic routes achieve the required transformations in biomass-derived molecules with controlled chemistry and high yields, but require pretreat — ment/hydrolysis steps to overcome the recalcitrance of lignocellulose. To be economically viable, these aqueous-phase routes should be carried out with a small number of reactors and with minimum utilization of external fossil fuel-based hydrogen sources, as illustrated in the examples present­ed here.

Catalytic refining of bio-oil by reacting with olefin/alcohol over solid acids can convert bio-oil to oxygen-containing fuels. In Chapter 5, Zhang and colleagues studied the reactivities of groups of compounds typically present in bio-oil with 1-octene (or 1-butanol) at 120 °C/3 h over Dowex50WX2, Amberlyst15, Amberlyst36, silica sulfuric acid (SSA) and Cs25H05PW12O40 supported on K10 clay (Cs25/K10, 30 wt. %). These compounds include phe­nol, water, acetic acid, acetaldehyde, hydroxyacetone, D-glucose and 2-hy — droxymethylfuran. Mechanisms for the overall conversions were proposed. Other olefins (1,7-octadiene, cyclohexene, and 2,4,4- trimethylpentene) and alcohols (iso-butanol) with different activities were also investigated. All the olefins and alcohols used were effective but produced varying product selectivities. A complex model bio-oil, synthesized by mixing all the above­stated model compounds, was refined under similar conditions to test the catalyst’s activity. SSA shows the highest hydrothermal stability. Cs25/K10 lost most of its activity. A global reaction pathway is outlined. Simultane­ous and competing esterification, etherfication, acetal formation, hydration, isomerization and other equilibria were involved. Synergistic interactions among reactants and products were determined. Acid-catalyzed olefin hy­dration removed water and drove the esterification and acetal formation equilibria toward ester and acetal products.

A newly designed downdraft wood stove achieved low-emission heat­ing by integrating an alumina-supported mixed metal oxide catalyst in the combustion chamber operated under high temperature conditions. In the first step in Chapter 6, by Bindig and colleagues, a catalyst screening has been carried out with a lab-scale plug flow reactor in order to iden­tify the potentially active mixed metal oxide catalysts. Mixed metal ox­ide catalysts have been the center of attention because of their expected high temperature stability and activity. The catalyst has been synthesized through two novel routes, and it has been integrated into a downdraft wood stove. The alumina-supported mixed metal oxide catalyst reduced the volatile hydrocarbons, carbon monoxide and carbonaceous aerosols by more than 60%.

As part of a programme aimed at exploiting lignin as a chemical feed­stock for less oxygenated fine chemicals, several catalytic C-C bond form­ing reactions utilising guaiacol imidazole sulfonate are demonstrated in Chapter 7, by Leckie and colleagues. These include the cross-coupling of a Grignard, a non-toxic cyanide source, a benzoxazole, and nitromethane. A modified Meyers reaction is used to accomplish a second constructive deoxygenation on a benzoxazole functionalised anisole.

5-Halomethylfurfurals can be considered as platform chemicals of high reactivity making them useful for the preparation of a variety of im­portant compounds. In Chapter 8, by Gao and colleagues, a one-pot route for the conversion of carbohydrates into 5-chloromethylfurfural (CMF) in a simple and efficient (HCl-H3PO4/CHCl3) biphasic system has been in­vestigated. Monosaccharides such as D-fructose, D-glucose and sorbose, disaccharides such as sucrose and cellobiose and polysaccharides such as cellulose were successfully converted into CMF in satisfactory yields under mild conditions. Our data shows that when using D-fructose the optimum yield of CMF was about 47%. This understanding allowed us to extent our work to biomaterials, such as wood powder and wood pulps with yields of CMF obtained being comparable to those seen with some of the enumerated mono and disaccharides. Overall, the proposed (HCl — H3PO4/CHCl3) optimized biphasic system provides a simple, mild, and cost-effective means to prepare CMF from renewable resources.

Chapter 9, by de Canck and colleagues, describes how a Periodic Mes- oporous Organosilica (PMO) functionalized with sulfonic acid groups has been successfully synthesized via a sequence of post-synthetic modifica­tion steps of a trans-ethenylene bridged PMO material. The double bond is functionalized via a bromination and subsequent substitution obtaining a thiol functionality. This is followed by an oxidation towards a sulfonic acid group. After full characterization, the solid acid catalyst is used in the acetylation of glycerol. The catalytic reactivity and reusability of the sulfonic acid modified PMO material is investigated. The catalyst showed a catalytic activity and kinetics that are comparable with the commercially available resin, Amberlyst-15, and furthermore the catalyst can be recy­cled for several subsequent catalytic runs and retains its catalytic activity.

Chapter 10, by Ramirez-Morenoe et al., offers an alternative method to reduce CO2 emissions through the use of alkaline and/or alkaline-earth oxide ceramics. These are able to selectively trap CO2 under different con­ditions and suggests the feasibility of these kinds of solid for being used with different capture technologies and processes, such as: pressure swing adsorption (PSA), vacuum swing adsorption (VSP), temperature swing adsorption (TSA) and water gas shift reaction (WGSR). Therefore, the fundamental study regarding this matter can help to elucidate the whole phenomena in order to enhance the sorbents’ properties.

Chromatographic and spectroscopic methods are widely used today for analytical purposes. Chromatographic techniques are applied not only for off-line analysis, but also for the on-line determination of minute amounts, as well as large-scale preparative separations. In fact, not only monomers, but also polymers and oligomers can be separated by chro­matography, although in the former case it is essentially a group separa­tion. There are several forms of chromatography using different mobile and stationary phases, with the two main forms of instrumental chroma­tography being liquid (LC) and gas chromatography (GC). According to IUPAC definition, chromatography is a physical method of separation in which the components to be separated are distributed between two phases, one of which is stationary, while the other (mobile) moves in a definite direction.

The selected monoliths, primarily composed of aluminum oxide (Al2O3) were coated with mixed metal oxide as an active catalytic phase and later inserted into the walls of the stove in the lower combustion chamber. These Al2O3 foams (porosity of10 ppi) consist of 92% a-Al2O3 along with the trace phases of mullite and cordierite. The results revealed that the catalyst was found to be quite active in terms of oxidation of harmful pollutants e. g. CO and VOC. In addition, two different synthesis routes for mixed metal oxides on the alumina foam were discussed. It was found that, the “Technique 1” proved to be promising as the catalyst showed higher emission reductions, as compared to the one synthesized through the “Technique 2”. Perhaps, it can be attributed to the comparatively high temperature handling of the pre­cursor, thus allowing a more mature crystallization of the active phase struc­ture. Furthermore, the aging experiments were performed with three differ­ent wall catalysts, each consisting of mixed metal oxides but synthesized via different methods. It is quite obvious that each of the three catalysts showed a “thermal activation effect” during the long-term/aging experiments, but this assumption cannot be yet supported due to the lack of catalyst charac­terization, which is planned to be carried out as soon as possible.

Several types of reactions take place during catalytic hydrotreatment of liquid biomass, based on the type of biomass processed, operating condi­tions and catalyst employed. The types of reactions that liquid biomass undergoes during catalytic hydroprocessing include: a) cracking, b) satu­ration, c) heteroatom removal and d) isomerization, which are described in more detail in the following section.

4.4.1 BIOMASS TO LIQUIDS (BTL)

Biomass to liquids can be described as a renewable version of fossil fuel — based technologies like coal to liquids (CTL) and gas to liquids (GTL), involving the integration of two different processes: biomass gasification to syngas (H2/CO) and F-T synthesis. Even though both technologies are well known and relatively mature, integration remains a challenge in BTL, because the utilization of lignocellulosic biomass as a feedstock (in substi­tution for classical carbon sources such as coal and natural gas) introduces new difficulties in the overall process. Biomass gasification is achieved by treatment at high temperatures (e. g., 1100-1500 K) under a well-controlled oxidizing atmosphere (e. g., air, steam, oxygen). Control over the composi­tion of the outlet gaseous stream is difficult and depends on a variety of factors including the oxidizing agent, biomass particle size and gasifier design. [52] In this respect, research indicates that utilization of pure oxy­gen atmosphere, small particle sizes (lower than 1 mm diameter), and a combination of high temperatures, high pressures and low residence times favors the production of syngas versus producer gas (a mixture of CO, H2, CO2, CH4, and N2 used for heat and electricity production). [53-55]

The direct integration of biomass gasification and F-T synthesis re­quires an intermediate gas-cleaning system, because the gaseous stream delivered from the gasifier typically contains a number of contaminants that need to be removed before the F-T unit, which is highly sensitive to impurities. Thus, tars (condensable high molecular weight hydrocarbons produced by incomplete biomass gasification), volatile species such as NH3, HCl, and sulfur compounds (produced by gasification of lignocellu — lose impurity components), fine particles, and ashes typically accompany CO and H2 in the outlet gaseous stream. The high number of contami­nants, along with the strict cleaning standards imposed by the F-T unit, [56] require the use of multiple steps and advanced technologies [52] that contribute significantly to the complexity and cost of the BTL plant. Ad­ditionally, because biomass contains higher amounts of oxygen compared to coal, the syngas delivered from lignocellulosic sources is typically en­riched in CO (H2/CO % 0.5), and F-T synthesis requires syngas with a H2/ CO ratio closer to 2.57, [58]. By providing sufficient water co-feeding, theH2/CO ratio can be adjusted by means of an intermediate water gas — shift (WGS, CO + H2O / CO2 + H2) reactor situated between the gasifier and the F-T unit.

The F-T reactor, the last unit of the BTL plant, achieves conversion of syngas to a distribution of alkanes over Co-, Fe-, or Ru-based catalysts in a well-developed industrial process. [58] However, the hydrocarbons produced by the direct route range from Cj to C50, and neither gasoline nor diesel fuels can be produced selectively without generating a large amount of undesired products. Indirect approaches involve initial produc­tion of heavy hydrocarbons (waxes), followed by controlled cracking of the heavy compounds to diesel and gasoline components to overcome this limitation. [59]

The cost of producing the biofuel, negatively affected by the complex­ity of the process, is the main factor limiting the commercialization of BTL technologies. Application of economies — of-scale allows for improvements in the economics of the process at the expense of having large centralized facilities that, as indicated in a previous section, lead to higher costs for transporting the low energy density biomass. BTL profit margins can be in­creased by co-producing, along with liquid hydrocarbon fuels, higher-value chemicals such as methanol [60] and hydrogen [6j,62] from lignocellulose — derived syngas. Another positive aspect of BTL is its versatility. Thus, since any source of lignocellulose can be potentially gasified, BTL technologies are not constrained to a particular biomass feedstock or fraction.

A volume of 25 mL H2SO4 (2.5 mol L-1) was added to 0.50 g of EP — (CH2)3-SH and stirred at room temperature. After 1 h, the solids were fil­tered and washed thoroughly with water and acetone. Finally, the material was dried at 90 °C for 16 h under vacuum (~0.1 Pa) and referred to as EP-(CH2)3-SO3H.

9.3.2 DETERMINATION OF THE AMOUNT OF REACHABLE THIOLS

A titration is performed as described in the literature [44]. A solution of silver nitrate with a known concentration is added to 0.050 g of material

and left to stir until the equilibrium is reached. The excess of silver is ti­trated with potassium thiocyanate and FeNH4(SO4)212H2O in 0.3 mol L-1 HNO3 as indicator.

Vegetable oils are the main feedstock for the production of first genera­tion biofuels, which can offer several CO2 benefits and limit the con­sumption of fossil fuels. Raw vegetable oils consist of fatty acid triglyc­erides, the consistency of which depends on their origin (i. e. plant type) as shown in Table 2. Their production, however, is competing for the cultivated areas that were originally dedicated for the production of food and feed crops. As a result the production and utilization of vegetable oils for biofuels production has instigated the “food vs. fuel” debate. For this reason traditional energy crops (soy, cotton, etc) with low oil yield per hectare are being substituted by new energy crops (eg. jatropha, palm, castor etc).

Catalytic hydrotreatment was explored for conversion of vegetable oils in the early 90’s. The investigation of the hydrogenolysis of various vegetable oils such as maracuja, buritimtucha, and babassu oils over a Ni-Mo/y-Al2O3 catalyst as well as the effect of temperature and pressure on its effectiveness was firstly investigated [16][17]. The reaction prod­ucts included a gas product rich in the excess hydrogen, carbon monoxide, carbon dioxide and light hydrocarbons as well as a liquid organic product of paraffinic nature. In more detail these studies showed the conversion of triglycerides into carboxyl oxides and then to high qualityhydrocar — bons via decarboxylation and decarbonylation reactions. Rapeseed oil hy­droprocessing was also studied in lab-scale reactor for temperatures 310° and 360°C and hydrogen pressures of 7 and 15 MPa using three different Ni-Mo/alumina catalysts [18]. These products contained mostly n-hep- tadecane and n-octadecane accompanied by low concentrations of other n-alkanes and i-alkanes [19].

In order to prove the feasibility of this upgrading process, more clearly outline the complicated reaction mechanism and probe the causes for cata­lyst deactivation, additional model reactions were investigated. Figure 1 shows the phenol conversions of phenol/1-octene reactions at 120 °C for 3 h over all five catalysts both with and without water present. All the catalysts exhibit high activities in neat phenol/1-octene reactions based on the high phenol conversions (>80%). Water significantly lowered the phenol conversions of phenol/1-octene reactions over DX2 (42.1%), A36 (38.3%), A15 (15.8%) and Cs25/K10 (19.9%). However, a good phenol conversion (74.5%) was still obtained over SSA. This further illustrated the high activity of SSA under hydrothermal conditions.

Phenolic compounds are present in bio-oils, primarily derived from lignin species. These acidic phenolic fractions are prone to oligomeriza­tion reactions with other bio-oil components [25]. Friedel-Crafts-type al­kylations of phenol with 1-octene over solid catalysts leads to a mixture of O — and C-alkylated phenols (Table 4). Isomeric octyl phenyl ethers (O-al — kylates) and octyl phenols (ortho/para-C-alkylates) were formed (Scheme 1), indicating that the 2-octyl carbocation undergoes 1, 2-hydride shifts to generate the 3- and 4-octyl cations in competition with O — and C-alkyla — tion. O-Alkylation is faster than C-alkylation but O-alkylation is revers­ible and the initially generated O-alkylated products can be increasingly converted to C-alkylated (thermodynamic) products as a function of reac­tion conditions. All catalyst used gave high phenol conversions (Figure 1).

Water is the most abundant compound in raw bio-oil. It is difficult to remove due to its miscibility with hydrophilic thermolysis products pres­ent from cellulose and hemicellulose [5,6]. Phenol conversions from the reactions of 1-octene with water/phenol over solid acid catalysts were summarized in Figure 1. The lower phenol consumption with all catalysts when water was present is most likely due to water solvation of the sul­fonic acid sites which lowers the Bronsted acidity or to mass transport effects due to phase separation. The sulfonic acid resins showed higher

phenol conversions than Cs25/K10. This could be due to swelling of resins. This swelling allows a distribution of all reactants to access a larger frac­tion of the internal acid sites of this macroreticular resin. However, par­tial Amberlyst15 decomposition occurred. Product distributions of these reactions are shown in Table 5. Obviously, competition between water and phenol for 1-octene occurred because 1-octanol and its isomers were formed by water uptake. Intermolecular reaction of these octanols further formed ethers (Scheme 2). Meanwhile, the significant increase in the con­centration of octanols with increasing water concentration affirmed the water consumption by olefin hydration [18]. Olefin acid-catalyzed hydra­tion removes water. This is the key reason for the success of this upgrading process.

Carboxylic acids, like acetic and propanoic acids, make bio-oil cor­rosive, especially at an elevated temperature [7]. 1-Octene was reacted with phenol/water/acetic acid solutions and their phenol conversions and octyl acetates yields were shown in Table 6. Along with the 1-octene hy­dration and phenol alkylation, simultaneous conversion of acetic acid to octyl acetates occurred by addition across 1-octene (Scheme 3), generat­ing three groups of improved fuel components in one operation without water generation. Trace amount of phenyl acetate were formed. The phe­nyl acetate yield decreased with increasing temperatures from 60 to 100 °C [17]. Both SSA and DX2 show higher catalytic activity than other three catalysts based on related phenol conversions and octyl acetates yields.

SCHEME 3: Acid-catalyzed reactions of acetic acid as a model carboxylic acid with 1-octene.

Bio-oil contains a large number of primary and secondary aliphatic hydroxyl groups from cellulose and hemicellulose pyrolysis. Table 7 pro­vides the phenol conversions and butyl octyl ethers yields of 1-octene re­actions with phenol/water/1-butanol mixtures over Dowex50WX2, SSA, Amberlyst15 and Cs2.5/K10, respectively. Except for the reactions men­tioned above, butyl octyl ethers were generated by either 1-butanol etheri­fication with octanols or 1-butanol addition across octenes (Scheme 4). SSA gave the highest phenol conversion (69.2%), illustrating it had the highest catalytic activity. Cs25/K10, which gave a phenol conversion of only 6.9%, lost almost all its activity during the reaction.

Multifunctional compounds such as hydroxyacetone or hydroxyetha — nal in bio-oil can oligomerize and polymerize. Aldol condensation reac­tions accelerate bio-oil aging [7]. Table 8 summarizes the effect of the presence of hydroxyacetone upon 1-octene reactions with phenol/water over Cs25/K10 and SSA catalysts. A good phenol conversion (64.0%) was obtained over SSA. Cs25/K10 was deactivated by hydroxyacetone based on the greatly reduced phenol conversion (2.2%). No hydroxyac­etone was detected after reaction. However, small amounts of 3-methyl — 2-hydroxycyclopent — 2-enone were detected. To further study this issue, neat 1-hydroxyacetone was heated at 100 °C for 1h over 30% Cs25/K10 and the products were identified by GC-MS (Table 9) [22]. Propionic acid (43.7%), hydroxyacetone dimers (24.8%) and 2-hydroxy-3-methylcyclo- pent-2-enone (9.7%) formed, together with about 1.8% of an unknown species. Three carbon a-hydroxycarbonyl species such as 1-hydroxyac- etone (acetol) can undergo enolization and dimerization to yield the struc­tures illustrated in Scheme 5. The keto form of 1-hydroxyacetone exists in equilibrium with its enol, enediol and aldehydo forms. In both neat and concentrated solutions, 1-hydroxyacetone can dimerize generating the cy­clic structures (a) and (b). Decomposition of (b) produces propionic acid. Meanwhile, intermolecular aldol condensation reactions of 1-hydroxyac — etone and subsequent serial of dehydration reactions occurred generat­ing 3-methyl-2-hydroxycyclopent-2-enone (c) and its isomers. As a con­sequence of dimerization and aldol condensation, the 1-hydroxyacetone monomers are expected to diminish with time during upgrading.

“Reaction conditions: (120 °C,3h, catalyst, 0.15 g; 1-octene (10 mmol); phenol (10 mmol); water (10mmol); hydroxyacetone (1.4 mmol) or 2-hydroxymethylfuran (1.0 mmol) or D-glucose (0.6mmol)); bUnkown compound; cAnhydrosugar: 1,6-Anhydro-3,4-dideoxy-fi — D-manno-hexapyranose; d GC area% of involved compounds versus the sum of the GC area% of all products remained after the reaction.

Furan derivatives such as 2-hydroxymethylfuran and hydroxymethyl- furfural present in bio-oil can polymerize easily and give tar in the pres­ence of acid [26]. Table 8 shows the effect of 2-hydroxymethylfuran ad­dition on the 1-octene reaction with phenol/water over Cs25/K10 and SSA catalysts. SSA shows a good catalytic activity giving a phenol conversion of 64.7%. However, the Cs25/K10 surfaces coked seriously, which greatly decreased its catalytic activity and reduced phenol conversion (4.5%). Po­lymerization of 2-hydroxymethylfuran occurred under acidic conditions forming coke or tar on the catalysts [27]. This has been further confirmed by the reaction products identified from GC-MS analysis (Table 8) of the reaction where neat 2-hydroxymethylfuran (2 g) was heated at 100 °C for or 1h over 30% Cs25/K10. In addition to the unreacted 2-hydroxymethyl — furan, difurffuryl ether (9.4%), 5-furffuryl-furfuryl alcohol (4.9%) and difu — ran-2-ylmethane (4.8%), together with about 2.6% other compounds were detected in the products [22]. Rapid polymerization of 2-hydroxymeth — ylfuran occurred at 120 °C via acid-catalyzed electrophilic condensation with the accompanying loss of formaldehyde [18], generating difurfuryl ether, 5-furfuryl-furfuryl alcohol, difuran-2-ylmethane, etc. (Scheme 6).

A large number of compounds with hydroxyl groups, particularly, an- hydro monosaccharides such as levoglucosan, were derived from pyrolysis of cellulose and hemicellulose components of the pine wood feed during bio-oil production [5]. Anhydro monosaccharides readily hydrate back to monosaccharides when heated with acid and water. Therefore, the effects

of D-glucose addition on 1-octene reactions with phenol/water over Cs25/ K10, DX2, A15 and SSA catalysts were studied at 120 °C for 3 h (Table 8). SSA gives a higher activity than DX2, A15 and Cs25/K10. Trace amounts of octyl formates were formed over A15 and DX2 by esterification of for­mic acid by octanols. The formic acid was generated by the known acid — catalyzed conversions pathways of D-glucose (Scheme 7) [2,27]. 5-Hy — droxymethylfurfural (HMF) was formed by D-glucose dehydration. Then, subsequent HMF hydration to its hemiacetal occurred in acidic media, followed by rehydration, ring-opening, loss of both water and formic acid to form levulinic acid. Also, small amounts of 1,6-anhydro-3,4-dideoxy — P-D-mannohexapyranose, generated by dehydration of D-glucose, were detected over SSA and Cs25/K10.

In addition to the reactions discussed above occurring between 1-oc- tene with model bio-oil components, some additional olefin reactions took place. Table 10 shows the product distributions of individual olefin re­actions conducted using equimolar amounts of olefin (1,7-octadiene, cy­clohexene, 1-octene, or 2,4,4-trimethylpentene)/phenol/water over SSA at 120 °C for 3 h. Skeletal isomerization reactions of all olefins occurred except with cyclohexene. Intramolecular diene cyclizations occurred for 1,7-octadiene, individually.

No oligomerization or cracking of either 1-octene or 1,7-octadiene oc­curred. However, 1-octene isomers were detected in the reactions where neat phenol and 1-octene operated at 100 °C over Cs25/K10 or A15 cata­lysts. 2,4,4-Trimethylpentene oligomerized to C16 olefins and cracked into C4 olefins readily at these conditions. Cracking of 2,4,4-trimethylpen — tene mainly produced isobutene. Cyclohexene’s dimer 1-cyclohexyl-1-cy­clohexene was found. Reoligomerization of isobutene gave C8, C12, and C16 olefins. Hydration reactions to form alcohols occurred for all olefins, followed by etherification of these resulting alcohols generating ethers and water (Scheme 8). Thus, the choice of olefin structure will play a role in the product distribution observed, but all the olefins help drive the upgrad­ing process to remove water and promote esterification, acetal formation, generate ethers and both O — and C-alkylate phenols.

“Reaction conditions: (SSA, 0.15 g; olefin: phenol: water (mmol) = 10:10:10; bPhenol conversions; cGC area% of involved compounds versus the sum of the GC area% of all products remained after the reaction.

“Reaction conditions: (solid catalyst, 0.15 g; time: 3 h; 1-octene: phenol: water: 1-butanol (mmol) = 10:10:11.6:3.4; b GC area% of involved compounds versus the sum of the GC area% of all products remained after the reaction.

In addition, intermolecular dehydration of 1-butanol occurred produc­ing dibutyl ether with both Dowex50WX2 and SSA (Table 11). Also, trac­es of butyl phenyl ether were formed. Small amounts of t-butyl phenols

(0.9%) were detected when 2-butanol was used as replacement of 1-buta­nol over SSA. Under acid catalyzed conditions, 2-butanol was protonated, and then dehydrated generating secondary carbocations. Some isomeriza­tion to tertiary carbocations must then occur. Phenol added to these C4 cations generating O-t-butylated phenol, followed by isomerization of the t-butyl phenyl ether to the thermodynamic C-t-butylated phenol products along with some bis-alkylated phenols formation (Scheme 9). Also, both acetic acid esterification and acetaldehyde acetalation reactions with 1-bu­tanol occurred, generating butyl acetate and 1,1-dibutoxyethane (acetal), respectively, when acetic acid and acetaldehyde were present.

Table 12 summarizes the product compositions of the model bio-oil (phenol/water/acetic acid/acetaldehyde/hydroxyacetone/D-glucose/2-hy- droxymethylfuran mixtures) reactions with 1-octene/1-butanol over all five acid catalysts at 120 °C for 3 h [22].

In addition to alkylated phenols (both O-octylated and C-octylated), octanols, dibutyl ether and dioctyl ether, butyl acetate and various octyl acetates, 1-octene oligomers and isomers mentioned above, 1,1-dibutoxy­ethane and butyl levulinate were formed over all the catalysts. 1,1-Dib — utoxyethane was formed by acetal formation between acetaldehyde and

1- butanol (Scheme 9). Levulinic acid reacted with 1-butanol forming bu­tyl levulinate. Levulinic acid was derived from the acid-catalyzed conver­sions of both D-glucose (Scheme 7) and 2-hydroxymethylfuran (Scheme 10), which are both known pathways. Levulinic acid has been obtained by dehydration of hexoses to 5-hydroxymethylfurfural (HMF) and its sub­sequent hydration in acidic media [2,28]. Acid-catalyzed conversion of 2-hydroxymethylfuran to levulinic acid in aqueous solutions also has been recently reported [29].

“Reaction conditions: (catalyst, 0.15 g; 1-octene: 1-butanol: phenol: water: acetic acid: acetaldehyde: hydroxyacetone: D-glucose: 2-hydroxymethylfuran (g) = 1.35: 0.15: 0.94: 0.15: 0.15: 0.12: 0.12: 0.15: 0.15; time: 3 h; nd: not detected); these results were first noted in the preliminary communication of this work found in reference 23; bC8, C16 and C24 olefins and their hydrates.

Butyl levulinate shows the extent of levulinic acid formation. Table 13 demonstrates that it originates from both D-glucose and 2-hydroxy — methylfuran. Increasing butyl levulinate formation was observed when the amount of 2-hydroxymethylfuran and D-glucose were increased in the model bio-oil [22]. Trace amounts of 3-methyl-2-hydroxycyclopent — 2-enone were generated from hydroxyacetone over Cs25/K10, A15 and DX2 catalysts. Among the new products formed, 1-octene oligomers were the most abundant components over Cs25/K10 or A36. However, phenol alkylates became the most abundant products for the other three catalysts. Trace amount of 2-(2-furylmethyl) furan, formed by furfuryl alcohol po­lymerization, was detected over the Cs25/K10 catalyst. No 2-(2-furylmeth — yl) furan was detected over other catalysts, because all the reactive furan products were consumed by these more acidic catalysts.