9.3.1 MATERIALS

The following reagents were used for the synthesis and characterization: Grubbs’ first generation catalyst (PCy3)Cl2Ru = CH-Ph, pluronic P123 (PEO20PPO70PEO20), 1-butanol, bromine, 3-chloro-1-propanethiol (98%), magnesium turnings (98%), sodium chloride, potassium thiocyanate, ammo­nium iron sulfate dodecahydrate, nitric acid (65%), iron (III) chloride (98%; anhydrous), toluene (anhydrous), acetonitrile, sulfuric acid (H2SO4), silver nitrate were obtained from Sigma-Aldrich (Bornem, Belgium). Acetone (>99.5%) was acquired from VWR (Belgium). Tetrahydrofurane (rotidry) and hydrochloric acid (37%; p. a.) were purchased at Carl Roth (Karlsruhe, Germany). Vinyltriethoxysilane (97%) was acquired from ABCR (Karl­sruhe, Germany). The following reagents were used for the catalytic reac­tion: acetic acid (puriss. p. a., ACS reagens, >99.8%, GC/T), glycerol (puriss. p. a., ACS reagens, anhydrous, dist., >99.5%) were purchased from Fluka (Bornem, Belgium). 1-Propanol, 1-heptanol (98%) and o-xylene (98%) were obtained from Sigma Aldrich. All chemicals were used as received.

Hydrogen partial pressure affects significantly the hydrotreating reactions as well as the catalyst deactivation. The catalyst deactivation rate is inverse proportional to the hydrogen partial pressure and to hydrogen feed-rate. However high hydrogen partial pressures correspond to high operation­al costs, which rise even higher for high olefinic feedstocks that exhibit higher hydrogen consumption due to the saturation reactions. Therefore hydrogen partial pressure should be balanced with the catalyst activity and catalyst life expectancy in order to optimize the overall process.

Table 1 summarizes the main characteristics of the different technologies analyzed in the present perspective paper for the conversion of biomass into liquid hydrocarbon fuels. Various routes have been compared in terms of important parameters (e. g., pretreatment required, external chemicals and hydrogen requirements, reaction conditions, cleaning/separation steps, number of reactors, use of precious metal catalysts) involved in the processing from the initial lignocellulosic biomass to the final liquid hy­drocarbon fuel. Additionally, technologies are compared in terms of the overall yield of LHF for each route, calculated according to recent data available in literature. The ability to use the entire organic matter in lig — nocellulose represents the main advantage of thermal routes (i. e., BTL and pyrolysis + upgrading) versus aqueous-phase approaches which, as indicated in Section 4.3, only can process the sugar fraction of lignocel — lulose (typically 60-80% depending on the source [13]). This constraint in aqueous-phase routes has important consequences for the overall process: (i) it limits the final yield to LHF and (ii) it negatively affects the econom­ics since additional reactors for pretreatment/hydrolysis steps are required to solubilize the sugar feedstocks (Table 1). On the other hand, aqueous — phase processes are carried out at milder conditions compared to thermal routes which allows for better control of the chemistry and, with it, higher selectivities to targeted hydrocarbon fuels. This control over the chemistry in aqueous phase technologies has important implications on the cleaning/ separation steps as well. Thus, unlike BTL and pyrolysis which produce intermediate fractions with high degrees of impurities that require deep cleaning and conditioning before the upgrading process, aqueous-phase routes achieve high-purity organic streams that spontaneously separate from water, with no further cleaning/ conditioning steps required. The higher number of reactors employed in the upgrading process for aque­ous-phase routes versus thermal routes is another important difference be­tween both approaches. In some cases (for example reforming/reduction of sugars, HMF and GVL platforms), the use of additional upgrading reac­tors is justified by the production of a final liquid hydrocarbon fuel with well-defined characteristics of molecular weight and structure that could be directly used for gasoline, jet fuel and diesel applications (see Fig. 7-9). This control of the final hydrocarbon fuel is more difficult to achieve by BTL and pyrolysis, which produce a mixture of hydrocarbons with broad molecular weight distributions and low control of the final chemical struc­ture. Consequently, thermal routes might need additional refining reactors to produce fuel-grade compounds.

As indicated in previous sections, two parameters are important to as­sess the economic feasibility of aqueous-phase catalytic routes: the num­ber of reactors and the use of external hydrogen. Thus, glycerol reforming, with only 2 reactors (reforming and F-T) and no hydrogen requirements (Table 1), would represent an interesting route. However, the final hydro­carbon yield (0.011 g of LHF per g of dry biomass), negatively affected by the low content of oils in the biomass source (20% in soybeans137), is low compared with other aqueous routes. With respect to these two pa­rameters, reforming/reduction of sugars over Pt-Re is a promising route since no external hydrogen is required (hydrogen required for reduction of sugars and hydrogenation of the ketone formed by aldol-condensation or ketonization is internally supplied in sufficient amounts by aqueous-phase reforming of a fraction of the sugar, Fig. 8), and only 4 reactors (hydroly­sis, reforming, C-C coupling and dehydration/hydrogenation) are needed to produce gasoline, diesel and jet fuel components from lignocellulosic biomass with an overall yield comparable to that of BTL (0.21 g of LHF per g of dry biomass). The main drawback of this approach is the high cost of the Pt-Re (10 wt%) reforming catalysts. The recently developed GVL platform to produce butene oligomers offers an attractive alternative with minimum external hydrogen utilization (required only during the final al — kene hydrogenation step) and no precious metal catalysts, which should give this process a promising economic assessment. The HMF platform route can achieve good yields to LHF (with a maximum yield close to 0.3) at the expense of needing external chemicals such as acetone and organic solvents (typically produced from fossil fuels like petroleum) and moderate amounts of hydrogen to carry out APD/H. The GVL C9 route offers versatility to produce gasoline and C9-C27 diesel components with acceptable yields, but it would require multiple reactors to transform bio­mass into the final hydrocarbon fuel depending on the upgrading process used. Finally, we note that taking into consideration all the parameters as a whole, pyrolysis coupled with upgrading processes appears to be a promising route to convert lignocellulose into LHF with high yields and low complexity. While advances have been made recently in the pyroly­sis step, challenges for this route are currently focused on the upgrading process, with particular emphasis on two crucial aspects: (i) designing strategies for the reduction of hydrogen consumption during HDO and (ii) development of hydrothermally stable catalysts (preferably without pre­cious metals) with high resistance to sulfur and alkaline impurities typi­cally present in bio-oils.

4.5 CONCLUSIONS

The production of liquid transportation fuels from renewable sources such as biomass is a promising route that can help to reduce our depen­dence on fossil fuels and to mitigate global warming effects. Ethanol, the most abundantly produced biofuel at the present time, suffers from low energy-density and compatibility issues with the existing transportation infrastructure, which is based on petroleum-derived liquid hydrocarbons. The current limitations of ethanol as a fuel can be overcome by develop­ing cost-effective technologies that allow conversion of non-edible lig — nocellulosic biomass into liquid hydrocarbon fuels chemically identical to those currently used in the transportation sector. In this respect, sev­eral promising routes are currently being developed worldwide, includ­ing gasification of biomass to syngas coupled with F-T synthesis (BTL), pyrolysis integrated with bio-oils upgrading processes, and aqueous-phase catalytic processing of biomass-derived sugars and derivatives. BTL and pyrolysis-upgrading are thermochemical routes that allow utilization of all the organic matter in lignocellulose, and aqueousphase processing is a catalytic route designed to operate with water-soluble sugars (and plat­form chemicals derived from them). While aqueous-phase routes require that the lignocellulosic biomass be subjected to pretreatment/hydrolysis steps, these routes offer the opportunity to selectively carry out a variety of reactions to achieve the deep chemical transformations and C-C cou­pling reactions required when converting sugars into liquid hydrocarbons. 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 in the present paper.

Among the porous systems for CO2 separation, both microporous (carbon, silica and zeolite membranes) and modified mesoporous membranes have been reported [63-64].

Zeolites are hydrous crystalline aluminosilicates that exhibit an in­tracrystalline microporous structure as a result of the particular three-di­mensional arrangement of their TO4 tetrahedral units (T = Si or Al) [65]. Zeolite membranes are commonly prepared as thin films grown on porous alumina supports via hydrothermal synthesis and dry gel conversion meth­ods [66]. Zeolite membranes of different structures have been developed to separate CO2 from other gases via molecular sieving [67-69]. For exam­ple, membranes prepared with the 12-member ring faujasite (FAU)-type zeolite show high separation factors of 20-100 for binary gas mixtures of CO2/N2 [69]. In the same sense, T zeolite membranes exhibited very high selectivity, of about 400, for CO2/CH4 and 104 for CO2/N2. The high selec­tivity of CO2/CH4 exhibited by T zeolites is due to the small pore size of about 0.41 nm, which is similar in size to the CH4 molecule but larger than CO2 [69]. Table 1 shows the kinetic diameter of various molecules that are present in CO2 containing gas mixtures such as flue and natural gas [70].

Deca-dodecasil 3R (DDR) (0.36 nm x 0.44 nm), and pseudo-zeolite materials like silicoaluminophosphate (SAPO)-34 (0.38 nm) also show
high CO2/CH4 selectivities due to narrow molecular sieving, which con­trols molecular transport into this material [69, 71-73]. For example, Tomita et al. [74] obtained a CO2/CH4 separation factor of 220 and CO2 permeance values of 7 x 10-8 mol m-2 s-1 Pa-1 at 28 °C on a DDR membrane [75].

As discussed, one of the most important factors controlling permeation through microporous membranes is the restriction imposed by the mo­lecular size of the permeant. However, the transport mechanism in micro­porous systems is more complex than just size exclusion and the perme­ation and selectivity properties are also affected by competitive adsorption among perment species that produce differences in mobility [76].

Thus, the diffusion mechanism for gas permeation through micropo­rous membranes can be characterized by two modes: one controlled by adsorption and a second one where diffusion dominates [63]. In the case of adsorption-controlled mode with permeating gases having strong af­finity with the membrane, a gas permeation flux equation is obtained by assuming steady-state single gas permeation, a constant diffusivity and a single gas adsorption described by a Langmuir-type adsorption isotherm, as in Eq. (5).

J = фqsDcL1 + bPf1 + bPp or J = фqsDcL1 — 0p1 + 0f

where J is the permeation flux, ф is a geometric correction factor that in­volves both membrane porosity and tortuosity, Dc is the corrected dif — fusivity of the permeating species, L is the membrane thickness, Pf and Pp represent the feed and permeate pressure respectively and 0f and 0p represent the relative occupancies.

Furthermore, if the adsorption isotherm of the permeating gas is linear (1 >> bP), then flux permeation is described by Eq. (6).

F = фqsDcLDcLK

where F is the permeance and K = qsb is the adsorption equilibrium con­stant. Therefore, from Eq. (5) it can be concluded that permeance is deter­
mined by both diflusivity (Dc) and adsorption (K). Based on the above, an interesting option to enhance membrane properties is to intercalate zeolite membranes with alkaline and alkaline-earth cations. Zeolite intercalation can enhance the separation between CO2 and other molecules such as N2 by promoting preferential CO2 adsorption [63, 77]. It is well known that zeolites show affinity for polar molecules, like CO2, due to the strong in­teraction of their quadrupole moment with the electric field of the zeolite framework. In this sense, the adsorption properties of zeolites can be en­hanced by the inclusion of exchangeable cations within the cavities of zeolites where the adsorbent-adsorbate interactions are influenced by the basicity and electric field of the adsorbent cavities [78-80]. Lara-Medina et al. [77] carried out separation studies of CO2 and N2 with a silicalite-1 zeolite membrane prepared via hydrothermal synthesis and subsequently modified by using lithium solutions in order to promote preferential CO2 adsorption and diffusion. CO2/N2 separation factor increases from 1.46 up to 6 at 25 psi and 400 °C after lithium modification. An et al. [79] studied a series of membranes prepared starting from natural Clinoptilo — lite zeolite rocks. Disk membranes were obtained by cutting and polish­ing of the original minerals, which were subsequently chemically treated with aqueous solutions containing Li, Na, Sr or Ba ions. Ionic exchanged membranes showed better permeation properties due to the presence of the extra framework cations.

Although zeolite membranes offer certain advantages in comparison with polymer membranes, such as chemical stability, the main issues are related to the selectivity decrease as a function of the permeation tem­perature. This is explained in terms of the contribution of the adsorption to the separation, which decreases sharply as temperature increases. At high temperature, physical adsorption becomes negligible and perme­ation is mainly controlled by diffusion [63, 76]. Additionally, due to the fact that CO2 and N2 molecules have similar sizes (Table 1), the dif­ference in diffusivity is not a strong controlling factor in determining selectivity.

Modified y-Al2O3 mesoporous membranes have been also reported as a means for CO2 separation [64]. Transport mechanisms in porous mem­branes have the contribution of different regimes. An overview of the dif­ferent mechanisms is given in Table 2.

Depending on the particular system, permeability of a membrane can involve several transport mechanisms that take place simultaneously. Considering no membrane defects and pore sizes in the range of 2.5-5 nm, y-Al2O3 based membranes theoretically have two transport regimes: Knudsen diffusion and surface diffusion. Eq. (7) describes the perme­ability of a membrane by taking into consideration the Knudsen and surface diffusion.

F = 2epr3RTL8RTnM0.5 + 2epDsrA0NavdxsdP

where r is the mean pore radius, p is a shape factor, R is the universal gas constant, T is the temperature, P is the mean pressure, M is molar mass of the gas, Ao is the surface area occupied by a molecule, Ds is the surface diffusion coefficient, Nav is Avogadro’s constant and Xs is the percentage of occupied surface compared with a monolayer [81].

For the cases when Knudsen diffusion dominates, selectivity can be correlated to the molecular weights of the permeating gases by the so called Graham’s law of diffusion, which establishes that the transport rate of any gas is inversely proportional to the square root of its molecular
weight. The CO2/N2 separation factor considering pure Knudsen diffusion is given by Eq. (8) and has a value of just 0.8. Therefore, Eq. (8) clearly shows that separation via Knudsen is limited for systems where species are of similar molecular weight.

aCO2N2 = MCO2MN2

Based on the aforesaid, CO2/N2 separation factor can be better en­hanced by promoting the surface diffusion mechanism (second term on the right hand side of Eq. (7)). Surface diffusion involves the adsorption of gas molecules on the surface of the pore and subsequent diffusion of the adsorbed species along the surface by a concentration gradient. Then separation properties of a membrane can be improved by generating such an interaction between one component of the feed gas mixture with the membrane; one option being via a chemical modification.

Cho et al [81] prepared a series of thin (2-5 pm thickness) y-Al2O3 and CaO — or SiO2-modified y-Al2O3 membranes for CO2 separation at temperatures between 25 and 400 °C. Impregnation of membranes with SiO2 or alkaline CaO was done in order to improve the CO2/N2 selectivity by promoting adsorption between CO2 gas molecules and the membrane pore wall. Although this kind of chemical modification of the membrane surface and the pore walls is able to activate the surface diffusion mecha­nism, an interesting behavior was observed. The CO2/N2 separation fac­tor increased from 1.0 to 1.38 at 25 °C after modification of the y-Al2O3 with SiO2. On the other hand, CaO impregnated membranes showed a separation factor of 0.98, which is even lower than that of the unmodified y-Al2O3. The same behavior has been reported by Uhlhorn et al. [82-83]. They reported MgO modified y-Al2O3 membranes which did not show significant enhancement in the permeation and selectivity properties as a result of the modification process. This fact was explained in terms of the surface diffusion mechanisms. As discussed, it is expected that physico­chemical modifications of the membrane can enhance preferential adsorp­tion of the gas species in the feed. Impregnations with alkaline oxide such as calcium oxide or magnesia on the y-alumina surface give more strong
base sites than those promoted by silica. Therefore, it promotes a strong bonding of CO2 on the alumina surface, causing CO2 molecules to lose mobility, resulting in a smaller contribution of surface diffusion to the total transport mechanism.

There is another kind of membrane where alkaline and alkaline-earth ceramic oxides have been used for the fabrication of CO2 permselective membranes. In these cases ceramic materials were chosen because of their well-known properties of physisorption of CO2 at low and intermediated temperatures.

Kusakabe et al. [84] prepared both pure and modified BaTiO3 CO2 permselective membranes via the alkoxide based sol-gel method; im­pregnation and calcination at 600 °C. In order to establish the effects of CO2 partial pressure, temperature and influence of the secondary oxide presence (CuO, MgO or La2O) on the CO2 adsorption properties of the membranes, pure and modified barium titanate powders were first evalu­ated by thermogravimetry and chromatography techniques. Dynamic CO2 absorption was evaluated by applying the impulse response method, wherein the BaTiO3 powder was packed in a separation column. The re­sults suggested that the CO2 molecules adsorbed on the BaTiO3 powder are mobile at temperatures about 500 °C. Therefore, this membrane ex­hibits CO2 permeation due to surface diffusion mechanism. Even though the prepared membranes showed selectivity, the Knudsen diffusion still has an important contribution to the gas transport due to the presence of membrane defects. The maximum separation factor of CO2/N2 through the membranes was estimated as 1.2. Therefore, further improvement of the permeation properties of this kind of membrane requires obtaining pinhole-free membranes.

Based on the same criteria, Nomura et al. [85] prepared Li4SiO4-based thin membranes on porous alumina supports. Membranes were obtained by the thermal treatment of different silica containing porous materials (Silicalite-1 and mesoporous silica) impregnated with lithium compounds. The authors called this method solid conversion. The use of different silica porous sources was proposed in order to enhance the reaction rate of Si and Li on the porous support at relatively low temperature, avoiding the reaction between the Li and alumina support itself. In the case of Sili- calite-1 (MFI zeolite), a zeolite thin film was first prepared on the support by following the dry gel conversion technique. Then, the prepared Sili — calite-1 layer was impregnated via dipping into a slurry containing lithium and silica fumed reactants (Li:Si = 4:1) and subsequently into a Li2CO3- K2CO3 slurry. The membrane was finally calcined at 600 °C for 2 h. It is believed that carbonate melts to fill the cracks and the pinholes of the Li4SiO4 formed membrane. A similar procedure of coating and calcination was carried out to prepare high quality membranes starting from mesopo — rous silica sources with pore sizes of 1.8-12.8 nm. Precursors react to form a Li4SiO4 membrane of 2-5 pm thickness that exhibits an N2 permeance of 1.8 x 10-9 mol m-2 s-1 Pa-1 at 400 °C. This suggests there are no big defects after impregnation of the membrane with the binary mixture of Li2CO3- K2CO3carbonate. Due to the fact that the membrane operates in a rich CO2 atmosphere, carbonates do not decompose even at temperatures of 600 °C. The maximum CO2/N2 permeance ratio was 0.85. The separation factor was higher than that for the Knudsen diffusion. Therefore, it can be conclude that Li4SiO4 layer was selective to CO2 over N2 at high tempera­ture of 600 °C.

Nomura [86] reported a two -stage approach for the preparation of Li — 4SiO4-CO2 selective membranes that involves the fabrication of a support­ed Li4SiO4 membrane and its subsequent modification by using a chemi­cal vapor deposition (CVD) method. First, for the preparation of a thin Li4SiO4 membrane the so called solid conversion method described before was used, which is based on the reaction between a porous silica source and a lithium containing solution coated on a porous alumina membrane support. Although the formed membranes showed certain selectivity due to the preferential adsorption of CO2 over N2, the presence of pinholes and cracks caused low separation factors. Therefore, the membrane defects were fixed by using the counter diffusion CVD method to form a silica coating that fills the gaps between the lithium orthosilicate particles that make up the membrane. N2 permeance was reduced about three orders of magnitude after CVD modification. Nitrogen permeance before and after the CVD treatment was 3.4 x 10-6 mol m-2 s-1 Pa-1 and 1.2 x 10-9 mol m-2 s-1 Pa-1 respectively. In the same sense, the CO2/N2 permeance rate increased from 0.7 to 1.2 at 600 °C. Some issues related with this system are the chemical and structural stability of the membranes observed during the permeation tests at elevated temperature. The membranes were broken

when permeation tests were carried out at temperatures higher than 700 °C, with the consequent decrease in the CO2/N2 selectivity. The aforesaid is the result of the CO2 chemisorption on the membrane. Lithium ortho­silicate reacts with CO2 to form lithium carbonate and lithium metasilicate (Li2SiO3) as products, as indicated by Eq. (9).

Thermodynamically, this reaction is prone to occur at temperatures be­tween room temperature and about 700 °C. However, experimentally it has been observed that reaction kinetics sharply increase above 550 °C. At these temperatures, the formation of carbonates involves an important change in volume that ends in the membrane’s rupture.

Therefore, one of the issues related to the development of this kind of inorganic membrane is the thermochemical stability. Due to reactivity of alkaline and alkaline-earth ceramic oxides with CO2 to form carbonates, not only preferential adsorption of CO2 molecules over N2 occurs, but CO2 chemisorption and reaction.. Therefore, it is mandatory to establish the operational temperature within a range where CO2 selective adsorption on the membrane layer promotes the separation process without reaction.

Catalytic hydrotreating of liquid biomass is continuously gaining ground as the most effective technology for liquid biomass conversion to both ground — and air-transportation fuels. The UOP company of Honeywell, via the technology it has developed for catalytic hydrotreating of liquid biomass (Figure 11), has announced imminent collaboration with oil and airline companies such as Petrochina, Air China and Boeing for the dem­onstration of the sustainable air-transport in China. This initiative will lead a strategic collaboration between the National Energy Agency of china with the Commerce and Development Agency of USA leading to the de­velopment of the new biofuels market in China.

In the EU airline companies collaborate with universities, research centers and biofuels companies in order to confront their extensive con­tribution to CO2 emissions. Since 2008 most airline companies promote the use of biofuels in selected flights as shown in Table 7 [62]. As it is obvious most pilot flights have taken place with Hydrotreated Renewable Jet (HRJ), which is kerosene/jet produced via catalytic hydrotreatment of liquid biomass. Moreover, Lufthansa has also completed a 6-month ex­ploration program of employing HRJ in a 50/50 mixture with fossil kero­sene in one of the 4 cylinders of a plane employed for the flight between Hamburg-Frankfurt-Hamburg with excellent results [63].

Besides the future applications for air-transportation, the automotive industry is also exhibiting increased interest for the broad use of biofuels resulting from catalytic hydrotreatment of liquid biomass. In fact these paraffinic biofuels can be employed in higher than 7%v/v blending ratio (which is the maximum limit for FAME) as they exhibit high cetane num­ber and have significant oxidation stability [64]

The highest interest is exhibited by oil companies around the catalytic hydrotreatment of liquid biomass technology for the production of biofu­els and particularly to its application to oil from micro-algae. ExxonMobil has invested 600M$ in the Synthetic Genomics company of the pioneer scientist Craig Ventner aiming to research of converting micro-algae to biofuels with minimal cost. BP has also invested 10M$ for collabora­tion with Martek for the production of biofuels from micro-algae for air-, train-, ground — and marine transportation applications.

2.2 CONCLUSION

Catalytic hydrotreatment of liquid biomass is the only proven technol­ogy that can overcome its limitations as a feedstock for fuel production (low H/C ratio, high oxygen and water content). Even though it has re­cently started to be investigated as an alternative technology for biofuels production, it fastly gains ground due to the encouraging experimental results and successful pilot/demo and industrial applications. Catalytic hydrotreatment of liquid biomass leads to a wide range of new alterative fuels including bio-naphtha, bio-jet and biodiesel, are paraffinic in nature and as a result exhibiting high heating values, increased oxidation stability and negligible acidity and corrosivity. As a result it is not over-optimistic to claim that this technology will broaden the biofuels market into scales capable to actually mitigate the climate change problems.

At first, a reference experiment was carried on the downdraft stove in order to determine the emissions, temperature profiles and pressure conditions during the operation of the stove in an unmodified state. This reference test is vital in the context of evaluating the effect of different modifications and changes in the stove which will be done in upcoming experiments. In Figure 2, the temperature profiles of different sections of the stove have been depicted. For every burning cycle, the stove was operated for the first 30 s in “up-draft” mode. After that, it was operated in downdraft (Twinfire mode) for the next 29.5 minutes. The average temperature in the grate was calculated to be around 750°C whereas, the temperature in the walls of the lower combustion chamber, where catalysts are planned to be installed in future experiments, was found to be ca. 650°C. In Figure 3, the timedepen — dent behavior of CO, VOC (Org.-C / THC) and aromatics (sum) has been depicted. These concentrations are recorded for four burning cycles of the reference test (Table 1).

Incorporation of a second metal into palladium catalysts can improve both alcohol selox stability and selectivity. Typical promoters such as Ag, Bi, Pb and Sn [157, 193-196], enhance oxidation performance towards chal­lenging substrates such as propylene glycol [197] as well as allylic and benzylic alcohols. Wenkin et al. [194] reported glucose oxidation to glu­conates was increased by a factor of 20 over Pd-Bi/C catalysts (Bi/Pds = 0.1) versus Pd/C counterparts. In situ XAS and attenuated total reflection infrared spectroscopy (ATR-IR) suggested that Bi residing at the catalyst surface protects palladium from deactivation by either over-oxidation (a hypothesis since disproved [166, 167, 169]) or site-blocking by aromatic solvents [153]. Prati et al. [200] first reported significant rate enhance­ments and resistance to deactivation phenomena in the liquid phase selox of d-sorbitol to gluconic/gulonic acids upon addition of Au to Pd/C and Pt/C materials [198], subsequently extended to polyol and long chain ali­phatic alcohols [199]. A strong synergy between Pd and Au centres was also demonstrated by Hutchings et al., wherein Au-Pd alloy nanoparticles supported on titania exhibited increased reactivity towards a diverse range of primary, allylic and benzylic alkyl alcohols compared to monometal­lic palladium analogues. The versatility of Au-Pd catalysts has also been shown in selox of saturated hydrocarbons [201], ethylene glycol [202], glycerol [203] and methanol [204], wherein high selectivity and resistance to on-stream deactivation is noted.

The effect of Au-Pd composition has been extensively studied for bimetallic nanoparticles stabilised by PVP surfactants [205]. An opti­mal Au:Pd composition of 1:3 was identified for 3 nm particles towards the aqueous phase aerobic selox of benzyl alcohol, 1-butanol, 2-butanol, 2-buten-1-ol and 1,4-butanediol; in each case the bimetallic catalysts were superior to palladium alone. Mertens et al. [206] examined similar systems utilising 1.9 nm nanoparticles, wherein an optimal Au content of around 80 % was determined for benzyl alcohol selox. The synergic interaction between Au and Pd therefore appears interdependent on nanoparticle size. It is well-known that the catalytic activity of Au nanoparticles increases dramatically <2 nm [207], hence it is interesting to systematically com­pare phase separated and alloyed catalysts. The author’s group prepared titania-supported Au shell (5-layer)-Pd core (20 nm) bimetallic nanoparti­cles for the liquid phase selox of crotyl alcohol and systematically studied the evolution of their bulk and surface properties as a function of thermal processing by in situ XPS, DRIFTS, EXAFS, XRD and ex-situ HRTEM. Limited Au/Pd alloying occurred below 300 °C in the absence of particle sintering [208]. Higher temperatures induced bulk and surface alloying, with concomitant sintering and surface roughening. Migration of Pd atoms from the core to the surface dramatically enhanced activity and selectiv­ity, with the most active and selective surface alloy containing 40 atom % Au (Fig. 18). This discovery was rationalised in terms of complementary temperature-programmed mass spectometric studies of crotyl alcohol and reactively formed intermediates over Au/Pd(111) model single crystal cat­alysts which reveal that gold-palladium alloys promote desorption of the desired crotonaldehyde selox product while co-adsorbed oxygen adatoms actually suppress aldehyde combustion. In contrast, the combustion of propene, the undesired secondary product of crotonaldehyde decarbonyl — ation, is enhanced by co-adsorbed oxygen [160].

Scott et al. prepared the inverse Au core-Pd shell nanoparticles and explored the catalytic cycle for alcohol selox to assess their associated stability [205, 209-212]. In situ Pd-K and Pd-LIII edge XAS of a Au nanoparticle/Pd(II) salt solution were undertaken to discriminate two pos­sible reaction mechanisms. No evidence was found that crotyl alcohol oxidation was accompanied by Pd2+ reduction onto Au nanoparticles, re­sulting in the formation of a metallic Pd shell (with oxygen subsequently regenerating electron-deficient palladium), and therefore proposed P-H elimination as the favoured pathway. Scott and co-workers proposed that the Au core prevents the re-oxidation of surface Pd0 atoms; no Pd-O and Pd-Cl contributions were observed by EXAFS.

In summary, the selective oxidation of complex alcohol substrates can be accomplished through Pd-mediated heterogeneous catalysis with high turnover and product selectivity. Application of in situ and operan — do techniques, such as X-ray and IR spectroscopies, has elucidated the mechanism of alcohol oxidative dehydrogenation and competing aldehyde decarbonylation. Surface PdO has been identified as the active catalytic species, and deactivation the result of reduction to metallic palladium and concomitant self-poisoning by strongly bound CO and carbonaceous residues. Breakthroughs in analytical tools and synthetic approaches to engineering nanoporous supports and shape/size controlled nanoparticles have delivered significant progress towards improved atom and energy efficiency and catalyst stability, however, next generation palladium selox catalysts necessitate improved synthetic protocols to create higher densi­ties of ultra-dispersed Pd2+ centres with superior resistance to on-stream reduction under atmospheric oxygen.

JUAN CARLOS SERRANO-RUIZ and JAMES A. DUMESIC

4.1 INTRODUCTION

Society has reached high levels of development during the last century. This progress, however, has been achieved at the expense of extensive consumption of natural resources, such as petroleum, natural gas and coal. These fossil fuel resources took millions of years to be formed, and they are currently being consumed at a rate that is orders of magnitude higher than their natural regeneration cycle, making them non-renewable sources of energy. The most recent data available for world energy consumption indicate that society still remains highly dependent on fossil fuels at the present time. For example in 2008, fossil fuels supplied 85% of the total energy consumed in the US, [1] and almost 80% of the energy produced in the European Union. [2] These fossil fuel resources are used to provide energy for various sectors of society (i. e., residential, commercial, indus-

Catalytic Routes for the Conversion of Biomass into Liquid Hydrocarbon Transportation Fuels. © Serrano-Ruiz JC and Dumesic JA. Energy & Environmental Science 4,83 (2011), DOI: 10.1039/ c0ee00436g. Reproduced from Energy & Environmental Science with permission from The Royal Society of Chemistry.

trial, transportation and electrical power), among which the transporta­tion sector is the largest and fastest growing energy sector, responsible for almost one third of the total energy consumed in the world. Moreover, a large fraction of the energy for the transportation sector (96%) is currently derived from petroleum. [3]

Three important issues are associated with the large-scale utilization of fossil fuels: availability, global warming and uneven geographic dis­tribution of reserves. Fossil fuels are finite and, as indicated above, their current consumption rate is higher than their corresponding regeneration rate, leading inevitably to depletion. Projections for the near future indi­cate that world energy consumption will increase by 35% over the next 20 years to meet the growing demand of industrialized countries and the rapid development of emerging economies, [4] and world demand for petroleum will raise by 30%, reaching 111 millions of barrels per day in 2035. [5] Taking into account these forecasts and current data of proven reserves, it has been estimated that oil, natural gas and coal will be depleted within the next 40, 60 and 120 years, respectively. [6] In the case of petroleum, many researchers predict a more dramatic situation and estimate that the global production of oil will reach a maximum in the year 2020 and decay thereafter. [7]

Global warming is, possibly, the most dramatic and known collateral effect produced by the massive utilization of fossil fuels. [8] Fossil fuels are transformed into energy by means of combustion reactions, leading to net emissions of CO2, a strong greenhouse gas, into the atmosphere. Ac­cordingly, the extraction of fossil fuels for energy production has allowed a large part of the carbon stored in the earth for millions of years to be released in just a few decades.

Fossil fuels reserves are not equally distributed around the world. The Middle-East countries control the 60% of the oil reserves and the 41% of natural gas supplies, and only three countries (US, China and Russia) ac­count for 60% of the world recoverable coal reserves. [4] This situation can lead to economic instabilities, requires the transportation of fossil fuel resources over long distances, and can cause political and security prob­lems worldwide.

The issues outlined above, inherently associated with fossil fuels, sug­gest that society requires new sources of energy to ensure progress and protect the environment for future generations. These new sources of en­ergy should: (i) have the potential to effectively replace fossil fuels in the current energy production system and (ii) be renewable, well distributed around the world, and not contribute to the accumulation of greenhouse gases into the atmosphere. In this respect, natural resources such as solar energy, wind, hydroelectric power, geothermal activity, and biomass meet these requirements. Unlike fossil fuels, they are abundant and allow the development of zero-carbon or carbonneutral technologies, thus contrib­uting to mitigation of global warming effects. Substitution of fossil fuel — based technologies for those derived from renewable sources is currently spurred by various governments, [9,10] and it will be done progressively and selectively. Thus, while solar, wind, hydroelectric, and geothermal have been proposed as excellent alternatives to coal and natural gas for heat and electricity production in stationary power applications, [11,12] biomass is the only sustainable source of organic carbon currently avail­able on earth, [13] and it is considered to be an ideal substitute for petro­leum in the production of fuels, chemicals and carbon-based materials. [14,15] However, when designing strategies for potential replacement of crude oil by biomass, it is important to note that the petrochemical industry currently consumes three quarters of the crude oil to cover the demand for liquid hydrocarbon fuels of the transportation sector, whereas only a small fraction of the petroleum is utilized in the synthesis of industrial chemicals and other derivatives. [16] Consequently, an effective implementation of biomass in the current energy system will necessarily involve the develop­ment of new technologies for the large-scale production of biofuels.

At the present time, two biomass-derived fuels (so-called first genera­tion of biofuels) have been successfully implemented in the transporta­tion sector: biodiesel (a mixture of long-chain alkyl esters produced by transesterifi cation of vegetable oils with methanol) and ethanol (produced by bacterial fermentation of corn and sugar cane-derived sugars). The pen­etration of these liquid biofuels in the transportation sector is still very weak, and in 2005 they represented only 2% of the total transportation energy. [3] However, the important environmental and economic benefits derived from their large-scale utilization will stimulate society to progres­sively increase reliance on biofuels. Thus, according to projections by the International Energy Agency, the world biofuel production will increase from the current level of 1.9 million of barrels per day (mbd) in 2010 to 5.9 mbd by 2030, which represents 6.3% of the world conventional fuels production. [4] Unlike petroleum-based fuels, liquid biofuels are consid­ered carbon neutral since CO2 produced during fuel combustion is con­sumed by subsequent biomass regrowth. [17] Furthermore, recent stud­ies indicate that the use of liquid biofuels produced domestically would strengthen economies by reducing the dependence of foreign oil and by creating new well-paid jobs in different sectors such as agricultural, forest management and oil industries. [18]

The diastereoisomerically pure E-isomer of the ethenylene precursor was synthesized according to a procedure described by our research group [43,45]. An amount of 0.0535 g of the Grubbs’ first generation catalyst and 42.95 mL of vinyltriethoxysilane (VTES) were mixed together under inert atmosphere. The mixture was stirred for 1 h at room temperature and subsequently refluxed. After 3 h, a distillation was performed to remove remaining VTES. Afterwards, the colorless E-1,2-bis(triethoxysilyl)eth — ene (E-BTSE) was distilled off under vacuum (~0.1 Pa).

Liquid hourly space velocity (LHSV) is defined as the ratio of the liquid mass feed-rate (gr/h) over the catalyst mass (gr) and as a result is expressed in hr-1. In fact the inverse of LHSV is proportional to the residence time of the liquid feed in the reactor. In essence the higher the liquid hourly space velocity, the less time is available for the contact of the feed molecules of the reaction mixture with the catalyst, thus the less the conversion. How­ever, maintaining large LHSV imposes a faster degradation of the catalyst therefore in industrial applications the LHSV is maintained in as high val­ues as it is practically possible.