The preceding observation that surface oxygen is not only critical for the removal of hydrogen adatoms but also to suppress decarbonylation of selox products over metallic palladium is in excellent agreement with an in situ ATR-IR study of cinnamyl alcohol selox over Pd/Al2O3 [148].

In related earlier investigations employing aqueous electrochemical protocols, the same researchers postulated that oxidative dehydrogena­tion of alcohols requires PGM catalysts in a reduced state, hypothesis­ing that ‘over-oxidation’ was responsible for deactivation of palladium selox catalysts [69]. A subsequent operando X-ray absorption spectros­copy (XAS) study by Grunwaldt et al. [150], bearing remarkable simi­larity to an earlier study to the author of this review [149], evidenced in situ reduction of oxidised palladium in an as-prepared Pd/Al2O3 catalyst during cinnamyl alcohol oxidation within a continuous flow fixed-bed reactor. Unfortunately the reaction kinetics were not measured in paral­lel to explore the impact of palladium reduction, however, a follow-up study of 1-phenylethanol selox employing the same reactor configura­tion (and oxygen-deficient conditions) evidenced a strong interplay be­tween selox conversion/selectivity and palladium oxidation state [151].

It was concluded that metallic Pd was the catalytically active species, an assertion re-affirmed in subsequent in situ ATR-IR/XAS measurements of benzyl [152-154] and cinnamyl alcohol [155] selox in toluene and under supercritical CO2, respectively, wherein the C=O stretching intensity was assumed to track alcohol conversion. It is interesting to note that the in­troduction of oxygen to the reactant feed in these infrared studies dramati­cally improved alcohol conversion/aldehyde production (Fig. 12), which was attributed to hydrogen abstraction from the catalyst surface [156, 157] rather than to a change in palladium oxidation state. In contrast to their liquid phase experiments, high pressure XANES and EXAFS measure­ments of Pd/Al2O3 catalysed benzyl alcohol selox under supercritical CO2 led Grunwaldt and Baiker to conclude that maximum activity arose from particles mainly oxidised in the surface/shelfedge [48].

FIGURE 12: Impact of oxygen on the selective oxidation of (top left) cinnamyl alcohol; (bottom left) 1-phenylethanol; and (right) 2-octanol. Adapted from references [148, 151, 154] with permission from Elsevier

In a parallel research programme, the author’s group systematically characterised the physicochemical properties of palladium nanoparticles as a function of size over non-reducible supports to quantify structure — function relations in allylic alcohol selox [133-137, 158, 159]. The com­bination of XPS and XAS measurements revealed that freshly prepared alumina [134, 137] and silica [135, 158] supported nanoparticles are prone to oxidation as their diameter falls below ~4 nm, with the fraction of PdO proportional to the support surface area and interconnectivity.

Complementary kinetic analyses uncovered a direct correlation between the surface PdO content and activity/TOFs towards cinnamyl and crotyl alcohol selox [134, 137]. Operando liquid phase XAS of Pd/C and Pd/ Al2O3-SBA-15 catalysts during cinnamyl alcohol selox evidenced in situ reduction of PdO (Fig. 13), however, by virtue of simultaneously measur­ing the rate of alcohol selox, Lee et al. were able to prove that this oxide ^ metal structural transition was accompanied by coincident deactivation. Together these findings strongly implicate a (surface) PdO active phase, consistent with surface science predictions that metallic palladium favours aldehyde decarbonylation and consequent self-poisoning by CO and or­ganic residues [143, 160], akin to that reported during fatty acid decarbox­ylation over Pd/MCF [161].

To conclusively establish whether oxide or metal is responsible for alcohol selox catalysed by dispersed palladium nanoparticles, a multi­dimensional spectroscopic investigation of vapour phase crotyl alcohol selox was undertaken (since XAS is an averaging technique a complete understanding of catalyst operation requires multiple analytical techniques [162-164]). Synchronous, time-resolved DRIFTS/MS/XAS measure­ments of supported and colloidal palladium were performed in a bespoke environmental cell [165] to simultaneously interrogate adsorbates on the catalyst surface, Pd oxidation state and reactivity under transient condi­tions in the absence of competitive solvent effects [166, 167]. Under mild reaction temperatures, palladium nanoparticles were partially oxidised, and unperturbed by exposure to sequential alcohol or oxygen pulses (Fig. 14). Crotonaldehyde formed immediately upon contact of crotyl alcohol with the oxide surface, but only desorbed upon oxygen co-adsorption. Higher reaction temperatures induced PdO reduction in response to crotyl alcohol exposure, mirroring that observed during liquid phase selox, how­ever, this reduction could be fully reversed by subsequent oxygen expo­sure. Such reactant-induced restructuring was exhibited by all palladium nanoparticles, but the magnitude was inversely proportional to particle size [168]. These dynamic measurements decoupled the relative reactivity of palladium oxide from metal revealing that PdO favoured crotyl alco­hol selox to crotonaldehyde and crotonic acid, whereas metallic palladium drove secondary decarbonylation to propene and CO in accordance with surface science predictions [143].

FIGURE 14: (Left) Cartoon of operando DRIFTS/MS/XAS reaction cell and resulting temperature dependent behaviour of Pd oxidation state and associated reactivity towards crotyl alcohol oxidation over a Pd/meso-Al2O3 catalyst—only selective oxidation over surface PdO occurs at 80 °C, whereas crotonaldehyde decarbonylation and combustion dominate over Pd metal at 250 °C; (top right) relationship between Pd oxidation derived in situ and crotyl alcohol conversion; (bottom right) summary of reaction-induced redox processes in Pd-catalysed crotyl alcohol selox. Adapted with permission from references [166, 168]. Copyright 2011 and 2012 American Chemical Society

Recent ambient pressure XPS investigations of crotyl alcohol/O2 gas mixtures over metallic and oxidised Pd(111) single crystal surfaces con­firmed that only two-dimensional Pd5O4 and three-dimensional PdOx surfaces were capable of crotonaldehyde production (Fig. 15) [169]. However, even under oxygen-rich conditions, on-stream reduction of the Pd5O4 monolayer oxide occurred >70 °C accompanied by surface poisoning by hydrocarbon residues. In contrast, PdOx multilayers were capable of sustained catalytic turnover of crotyl alcohol to crotonalde — hyde, conclusively proving surface palladium oxide as the active phase in allylic alcohol selox.

FIGURE 16: Comparative activity of Pd nanoparticles dispersed over amorphous, 2D non­interconnected SBA-15 and 3D interconnected SBA-16 and KIT-6 mesoporous silicas in the selective aerobic oxidation of crotyl alcohol. Adapted from reference [135]. Copyright 2011 American Chemical Society

Wood contains a wide variety of components that are extractable with vari­ous organic solvents or water. Non-polar and semi-polar solvents (hexane, dichloromethane, diethyl ether, MTBE etc.) extract lipophilic oleoresin and fat components, while polar solvents (acetone, ethanol, water, etc.) extract hydrophilic phenolics, sugars, starch and inorganic salts. Acetone and ethanol extract also lipophilic extractives.

A classification of wood extractives is given in Figure 21, while the an­alytical procedure for extractives is outlined in Figure 22. Group analysis

of fatty acids, resin acids, triglycerides, lignans and sterols can be done us­ing a short column GC (5-7 m/0.53 mm capillary column with 0.15 pm film thickness), or by HP-SEC (Figure 23) as well as thin layer chromatography.

The analysis of individual compounds can be done by GC on a longer column (20-30 m/0.20-0.32 mm capillary columns) and reverse phase HPLC, while the identification of compounds can be performed by GC — MS, LC-MS, and NMR of isolated substrates. In case of a poor separation between compounds, the following parameters could be modified: tem­perature gradient, column polarity, type of derivative used in derivatiza — tion. As seen in Figure 24 a better separation, for example, between abietic acid and tri-unsaturated C20 fatty acid, is achieved with somewhat higher ramping. The HP-1 column usually follows a boiling point order, however,

columns with different polarity could also be used (Figure 25) to allow better separation.

As previously mentioned, derivatization of fatty and resin acids is needed for accurate quantitative analysis. Although methylation is a com­monly used method, silylation can sometimes afford better separation (Figure 26). In addition, for some GC columns peak-tailing is more severe for methyl esters than for silyl esters.

The catalytic ability of the sulfonic acid functionalized PMO material has been explored for an esterification reaction, i. e., the glycerol acetylation reaction (Figure 4). The activity of EP-(CH2)3-SO3H is compared with a commercially available catalyst Amberlyst-15 and moreover the catalysts’ reusability is explored.

In this study the esterification of glycerol is probed due to its economic importance. Glycerol is an important by-product of first generation bio­diesel and is produced in a relative large quantity [50]. This overproduc­tion of glycerol can be used in order to develop second generation biodies­el which uses glycerol as a raw product. As carboxylic acid, acetic acid is probed as shown in the general reaction (Figure 4). Three products may in principle be obtained from this reaction: glycerol monoacetate (MAG),
glycerol diacetate (DAG) and glycerol triacetate (TAG). However, experi­mentally, only MAG (~94%) and DAG (~6%) are formed using the spe­cific catalytic conditions described in the experimental part [51].

The catalytic activity of EP-(CH2)3-SO3H for the esterification of ace­tic acid with glycerol is presented in Figure 5 where the total acetylation yield is shown as a function of time. The total acetylation yield is defined according to the equation below:

Yield (%) = ([P]/[HAC]0) (Ohac/V x 100

where [P]t and [HAc]0 represent the product and acetic acid concentra­tion at a certain reaction time and at t = 0, respectively. Furthermore, uHAc and up represent the stoichiometric coefficients of the acetic acid and the ester formed, i. e., 1 for mono-substituted, 2 for di-substituted and 3 for fully substituted products, respectively. Also, as acetic acid contains acid protons which can induce a self-catalyzed process, the reaction in absence of any solid catalyst was monitored. Corresponding data are shown in Figure 5. It is clear that the sulfonated PMO possesses a significant cata­lytic activity with a yield of almost 80% for this reaction after ~300 min; whereas the blank test (without any solid catalyst involved) yielded only ~50% of esters after ~300 min. The conversion of Amberlyst-15 is shown in the same figure. Comparing the two materials clearly shows that EP — (CH2)3-SO3H exhibits a similar catalytic activity as Amberlyst-15, which is a well-performing catalyst in this type of reaction.

FIGURE 5: The total acetylation yield for the catalytic reaction with EP-(CH2)3-SO3H and Amberlyst-15. Also the blank reaction is represented for clarity. A catalyst loading of 0.25 g per 40 mL of glycerol was used. The lines are intended as visual aids only.

Total acetylation yield / %

FIGURE 7: Recyclability experiment for EP-(CH2)3-SO3H: a comparison between the catalytic activity of the pristine material and the third catalytic run. The blank reaction is presented for clarity. The lines are intended as visual aids only.

FIGURE 8: The total acetylation yield for the EP-(CH2)3-SO3H, the second and third run. After 60 min (represented by the vertical line) the liquid is separated from the catalyst and the catalytic activity of the liquid phase of run 2 and 3 is further followed in function of time (open square and triangle). The blank reaction is presented for clarity. The dotted lines are intended as visual aids only.

Furthermore, the recyclability of the sulfonic acid containing PMO material is studied for three consecutive runs. First, the initial rate of the catalytic reaction is studied for each run by focusing on the first hour of the acetylation (Figure 6). These experiments are all performed in the same catalytic set-up as the standard catalytic experiment. The solid is filtered after 1 hour and re-used without any further treatment in the subsequent run with a fresh reaction medium (run 2); this being performed again for two additional consecutive runs (runs 3 and 4). As one can see from the figure, the material still possesses catalytic activity for the acetylation af­ter four runs. However, recycling of the material in the consecutive runs results in a slight decrease of the initial reaction rate.

After the last catalytic run, i. e., run 4, the total acetylation yield is mon­itored for approximately 10 h in order to compare it with the acetylation yield of the pristine sulfonated PMO material, EP-(CH2)3-SO3H (Figure 7). Although a decrease in the initial reaction rate was observed as already mentioned, at the third consecutive run, the material still reaches the equi­librium after approximately 5 h and finally results in the same acetylation level as the fresh pristine material.

Moreover, additional tests are performed to evaluate the actual heteroge­neous character of the observed catalytic activity. Therefore, the solids are removed from the liquid after 1 h along the first and second runs, and the corresponding recovered solutions are kept under the catalytic conditions to follow the occurrence of a further evolution of the total acetylation yield without solid catalyst in the system anymore. As can be seen from Figure 8, it is clear that the acetylation occurs much slower, i. e., in the range of the blank, when the catalysts are removed of the reaction media, than when the catalyst is maintained in the reactor for the whole test duration.

As it was mentioned earlier, the choice of catalyst and operating param­eters affect the reactions that take place within the hydroprocessing reac­tor. The key operating parameters of hydroprocessing include the reactor temperature, hydrogen partial pressure, liquid hourly space velocity and hydrogen feed-rate.

2.2.3.1 TEMPERATURE

Most catalytic hydrotreating and hydrocracking reactors operate between 290-450°C. The temperature range is selected according the type of cata­lyst and feedstock type to be processed. In the first stages of the catalyst life (after its loading in the reactor) the temperature is normally kept low as the catalyst activity is already very high. However as time progresses and the catalyst deactivates and cokes, the temperature is gradually in­creased to overcome the loss of catalyst activity and to maintain the de­sired product yield and quality.

Levulinic acid (4-oxopentanoic acid) is an important biomass derivative that can be obtained by acid hydrolysis of lignocellulosic wastes, such as paper mill sludge, urban waste paper, and agricultural residues, through the Biofine process. [123] Levulinic acid has been recently selected by the US Department of Energy (DOE) as one of the top 15 carbohydrate — derived chemicals in view of its potential to serve as a building block for the development of biorefinery processes. [91] Thus, taking advantage of its dual functionality (i. e., a ketone and an acid group), a number of use­ful chemicals can be synthesized from levulinic acid including methyl — tetrahydrofuran (MTHF) (a gasoline additive) and d-aminolevulinic acid (DALA) (a biodegradable pesticide). [124]

Recently, our group has developed a series of catalytic approaches to convert aqueous solutions of levulinic acid into liquid hydrocarbon trans­portation fuels of different classes (Fig. 9). The catalytic pathways involve oxygen removal by dehydration/hydrogenation (in the form of water) and decarboxylation (in the form of CO2) reactions, combined with C-C cou­pling processes such as ketonization, isomerization, and oligomerization that are required to increase the molecular weight and to adjust the struc­ture of the final hydrocarbon product. As a first step, aqueous levulinic acid is hydrogenated to water-soluble GVL, which is the key intermedi­ate for the production of hydrocarbon fuels. This hydrogenation step can be achieved with high yields by operating at low temperatures (e. g., 423 K) over non-acidic catalysts (e. g., Ru/C) to avoid formation of angelica lactone, a known coke precursor [125] which is produced by dehydration over acidic sites at higher temperatures (e. g., 573-623 K). [119] Interest­ingly, because equimolar amounts of formic acid (a hydrogen donor) are coproduced along with levulinic acid in the C6-sugars dehydration pro­cess, this hydrogenation step could be potentially carried out without uti­lizing hydrogen from an external source, and several groups have already explored this possibility. [126,127] This route is promising in that GVL has applications as a gasoline additive, [128] and as a precursor to poly­mers [129] and fine chemicals. [130]

Aqueous solutions of GVL can be upgraded to liquid hydrocarbon fu­els by following two main pathways: the C9 route and the C4 route (Fig. 9). In the former route, GVL is converted to 5-nonanone over a water-stable multifunctional Pd/Nb2O5 catalyst. In this process, GVL is first transformed into hydrophobic pentanoic acid by means of ring-opening (on acid sites) and hydrogenation reactions (on metal sites) at moderate temperatures and pressures. Pentanoic acid is subsequently ketonized to 5- nonanone, and reaction conditions can be adjusted to allow this transformation to take place on the same Pd/Nb2O5 reactor with a maximum of 70% carbon yield. [119] Nonanone yield can be increased to almost 90% by using a dual-cat­alyst approach with Pd/Nb2O5 + Ce0 5Zr0 5O2 in a reactor with two different temperature zones, which allows for optimum control of reactivity. [131] 5-Nonanone, which is obtained in a high purity organic stream that spon­taneously separates from water, is subsequently transformed into its cor­responding alcohol that serves as a platform molecule for the production of hydrocarbon fuels for gasoline and diesel applications. For example, the C9 alcohol can be processed (through hydrogenation/dehydration cycles) over a bifunctional metal-acid catalyst such as Pt/Nb2O5 [110] into linear n-nonane, with excellent cetane number and lubricity to be used as a diesel blender agent. Alternatively, the functionality of 5-nonanol can be utilized to upgrade the alcohol to gasoline and diesel components. In particular, 5-nonanol can be dehydrated and isomerized in a single step over an USY

FIGURE 9: Catalytic routes for the conversion of levulinic acid (LA) and g-valerolactone (GVL) into liquid hydrocarbon transportation fuels. Blue colour indicates water-soluble compounds, yellow symbolizes hydrophobic compounds, and green refers to liquid hydrocarbon fuels.

zeolite catalyst to produce a mixture of branched C9 alkenes with the ap­propriate molecular weight and structure for use in gasoline after hydro­genation to the corresponding alkanes. [131] Additionally, 5-nonanol can be converted into a C9-alkene stream (by means of dehydration reactions) which can be subsequently oligomerized over an acid catalyst such as Am — berlyst 70 to achieve good yields of C18 alkanes (after hydrogenation) for diesel applications. [132]

Recently, a promising route to upgrade aqueous solutions of GVL into jet fuels through the formation of C4 alkenes has been developed by Bond et al. [133] (Fig. 9). The process is based on a dual reactor system. In the first catalytic reactor the GVL feed undergoes decarboxylation at elevated pres­sures (e. g., 36 bars) over a silica/alumina catalyst, producing a gas stream composed of butene isomers and CO2. In a second reactor connected in se­ries, the gaseous butene stream is passed over an acidic catalyst (H-ZSM5, Amberlyst 70) that achieves oligomerization of butene monomers, yielding

TABLE 1: Summary of the different technologies for the conversion of biomass into liquid hydrocarbon transportation fuels

TABLE 1: Cont.

a [] indicates that hydrolysis and dehydration can be carried out in the same reactor, b 3 reactors if WGS syngas conditioning is required. c Depending on the upgrading process required. d According to ref. 134. e Calculated as: [0.50-0.70 yield of bio-oil from lignocellulose in pyrolysis135] <?> [0.65yield of LHF in HDO,136 or 0.30yield of aromaticsLHF in zeolite upgrading79].f Calculated as: [0.20 content of oil in soybeans137] <?> [0.1 glycerol in biodiesel process] <?> [0.552yield of alkanes104]. g Calculated as: [0.80-0.60 sugar content of lignocellulose] <?> [0.96yield of hydrolysis enzymatic] <?> [0.53-0.31 yield isomerization glucose to fructose138,139] <?> [0.69-0.58 yield of LHF from fructoseU0]. h Calculated as: [0.80-0.60 sugar content of lignocellulose] <?> [0.96 yield of hydrolysis enzymatic] <?> [0.52 yield of organic C116] <?> [0.57 yield of C7+ ketones116]. i Yield to n-nonane (diesel blender). Calculated as: [0.80-0.60 sugar content of lignocellulose] <?> [0.96 yield of hydrolysis enzymatic] <?> [0.45 yield to levulinic acid of biofine process] <?> [0.96 yield of levulinic acid to GVL119] <?> [0.80 yield of 5-nonanone from GVL119] <?> [1.00] hydrogenation of 5-nonanone to n-nonane. j Calculated as: [0.80-0.60 sugar content of lignocellulose] <?> [0.96 yield of hydrolysis enzymatic] <?> [0.45 yield to levulinic acid of biofine process] <?> [0.96 yield of levulinic acid to GVL119] <?> [0.78 yield of C8+ alkenes133] <?> [1.00] hydrogenation to final alkane product.

a distribution of alkenes centred at C12. While CO2 does not affect the oligo­merization process other than by dilution, water inhibits the acidic oligo­merization catalyst (especially Amberlyst), and it has to be removed prior to the second reactor by using a gas-liquid separator operating at 36 bars and 373-398 K. The final optimized yield to C8+ alkenes reaches 75% when silica/alumina and Amberlyst 70 are used.

Since GVL can be potentially produced from levulinic acid with no external hydrogen requirements, this technology allows the production of liquid hydrocarbon alkanes from lignocellulose with minimal utilization of hydrogen (i. e., hydrogen is only used during the final alkene hydro­genation step). Furthermore, the process is potentially cost-competitive with petroleum-derived technologies, since only two reactors are required, operating in series and using non-precious metalcatalysts. Finally, a CO2 gas stream is produced with high purity and at high pressures, thereby permitting effective utilization of sequestration or capture technologies to mitigate greenhouse gas emissions.

Membrane-based processes, related to gas separation and purification, have achieved an important level of development for a variety of industrial applications [60]. Therefore, the use of separation membranes is one of the promising technologies for reducing the emissions of greenhouse gases such as CO2. The term membrane is defined as a permselective barrier between two phases, the feed or upstream and permeate or downstream side [61]. This permselective barrier has the property to control the rate of transport of different species from the upstream to the downstream side, causing the concentration or purification of one of the species present in the feed gas mixture.

Membrane-based processes offer the advantage of large scale applica­tion to separate CO2 from a gas mixture. Figure 2 schematizes the pro­cess where concentrated CO2 is selectively separated from flue gas that is mainly composed of nitrogen and carbon dioxide along with other gases such as water vapor, SOx, NOx and methane. Subsequent to the membrane process, concentrated CO2 obtained at the permeate side can be disposed or used as raw material for the synthesis of several chemicals such as fuel and value-added products [62].

Of course, the rate of transport or permeation properties of a particular gas through a given membrane depend on the nature of the permeant gas, as well as the physical and chemical properties of the membrane.

Inorganic membranes are more thermally and chemically stable and have better mechanical properties than organic polymer membranes; ce­ramic membranes offer both the advantage of large scale application and potential for pre — and post-combustion CO2 separation applications, where membranes systems would be operating at elevated temperatures of 300­1000 °C [63].

CO2

Gas

Inorganic ceramic membranes can be classified as porous and nonpo­rous or dense. These differ from each other not only in their structures but also in the mechanism of permeation. In porous membranes, the transport of species is explained with the pore-flow model, in which permeants are transported by pressure-driven convective flow through the pore network. Separation occurs because one of the permeants is excluded (molecular filtration or sieving) from the pores in the membrane and remains in the re­tentate while the other permeants move towards the downstream side. On the other hand, in nonporous membranes, separation occurs by solution — diffusion, in which permeants dissolve in the membrane material and then diffuse through the bulk membrane by a concentration gradient [60].

As catalytic hydrotreating of liquid biomass has given promising results, the industrial world has given enough confidence to apply it in pilot and industrial scale. The NesteOil Corporation has developed the NExBTL technology for converting vegetable oil (primarily palm oil) into a renew­able diesel also known as “green” diesel (Figure 9). Based on this technol­ogy the first catalytic hydrotreatment of vegetable oils unit was construct­ed in Finland in 2007, within the existing Poorvo refinery of NesteOil, with a capacity of 170 kton/hr. The primary feedstock is palm oil, while it can also process rapeseed oil and even waste cooking oil. The same company has constructed a second unit within the same refinery while it has also planned to construct two new units, one in Singapore and one in Rotterdam, with the capacity of 800 kton/yr each.

The catalytic hydrotreatment technology of 100% waste cooking oil for biodiesel production was developed in the Centre for Research and Technology Hellas (CERTH) in Thessaloniki, Greece [21-24] and later demonstrated via the BIOFUELS-2G project [59], which was co-fund­ed by the European Program LIFE”. In this project WCO was collected from associated restaurants and the produced 2nd generation bio-diesel, to be called “white diesel” was employed. For the demonstration of the new technology, 2 tons of “white diesel’ were produced via catalytic hy­drotreatment of WCO based on the large-scale pilot units available in CERTH. The production process simplified diagram is given in Figure 10. The new fuel will be applied to a garbage truck in a 50-50 mixture with conventional diesel in August 2012, aiming to promote the new technol­ogy as it exhibits overall yields exceeding 92% v/v.

In the USA the Dynamic Fuels company [60] has constructed in Baton Rouge a catalytic hydrotreating unit dedicated to oils and animal fats with 285 Mlit capacity. The unit employs the Syntroleum technology based on Fischer-Tropsch for the production of synthetic 2nd generation Biodiesel while it also produces bio-naphtha and bio-LPG. The Bio-Synfining tech­nology of Syntroleum converts the triglycerides of fats and oils into n — and iso-paraffins via catalytic hydrogenation, thermal cracking and isomeriza­tion as it is applied in the Fischer — Tropsch wax upgrading to renewable diesel (R-2) and renewable jet (R-8) fuel.

FIGURE 11: Vegetable oil and animal fats conversion technology to renewable fuels of UOP [61]

In this work, two different synthesis routes have been developed in order to coat aluminium oxide foams with a mixed metal oxide active phase. These two respective techniques cannot be yet described in detail because of ongoing patent approval.

6.2 RESULTS AND DISCUSSION

Anchoring Pd nanoparticles onto support structures offers an effective means to tune their physicochemical characteristics and prevent on-stream deactivation e. g. by sintering. Supports employing porous architectures, acid/base character and/or surface redox chemistry e. g. strong metal sup­port interaction (SMSI), afford further opportunities to influence cata­lyst reactivity [170-173]. Mesoporous silicas are widely used to disperse metal nanoparticles [135, 136, 171, 174, 175]. The transition from low surface area, amorphous silica (200 m2g-1) to two-dimensional non-inter­connected pore channels (SBA-15) [73] and three-dimensional intercon­nected porous frameworks (SBA-16, KIT-6) [73, 176, 177] improved the dispersion of Pd nanoparticles and hence degree of surface oxidation and thus activity in allylic alcohol selox (Fig. 16), but had little impact on the mass transport of small alcohols to/from the active site. [135, 136] The high thermal and chemical stability of such mesoporous silica [178, 179] makes such supports well-suited to commercialisation. Pd nanoparticles confined within such mesoporous silicas demonstrate good selectivity in crotyl and cinnamyl alcohol selox to their respective aldehydes (>70 %), and excellent TOFs of 7,000 and 5,000 h-1 for the respective alcohols. Similar activities are reported for secondary and tertiary allylic alcohols, highlighting the versatility of silica supported Pd nanoparticles [51, 135, 136, 180-182]. Incorporation of macropores into SBA-15 via dual hard/ soft templating to form a hierarchically ordered macroporous-mesoporous Pd/SBA-15 was recently shown to promote the catalytic selox of sterically challenging sesquiterpenoid substrates such as farnesol and phytol via (1) stabilising PdO nanoparticles and (2) dramatically improving in-pore dif­fusion and access to active sites [158].

The benefits of mesostructured supports are not limited to silica, with ultra-low loadings of palladium impregnated onto a surfactant-templated mesoporous alumina (350 m2 g-1) generating atomically dispersed Pd2+ centres [137]. Such single-site catalysts were 10 times more active in cro — tonaldehyde and cinnamaldehyde production than comparable materials employing conventional (100 m2 g-1) y-alumina, owing to the preferen­tial genesis of higher concentrations of electron-deficient palladium [134, 137], due to either pinning at cation vacancies or metal ^ support charge transfer [183]. These Pd/meso-Al2O3 catalysts exhibited similar TOFs to their silica counterparts (7,080 and 4,400 h-1 for crotyl and cinnamyl al­cohol selox, respectively) [137], consistent with a common active site and reaction mechanism (Fig. 17).

Mesoporous titania and ceria have also attracted interest as novel cata­lyst supports. The oxygen storage capacity of ceria-derived materials is of particular interest due to their facile Ce3+^Ce4+ redox chemistry [173, 184-188]. Sacrificial reduction of the ceria supports by reactively formed hydrogen liberated during the oxidative dehydrogenation of alcohols could mitigate in situ reduction of oxidised palladium, and hence maintain selox activity and catalyst lifetime, with Ce4+ sites regenerated by dissociatively adsorbed gas phase oxygen [187, 189, 190]. Due to its high density, con­ventional nanocrystalline cerias possess meagre surface areas (typically ~5 m2g-1), hence Pd/CeO2 typically exhibit poor selox behaviour due to their resultant low nanoparticle dispersions which favour (self-poisoning) metallic Pd [189, 191, 192].

This chapter 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.

The analytical procedure depends very much on the objective of a par­ticular study as well as the available resources in terms of instruments, time, costs and human skills.

The main hurdles on the way toward the development of a reliable analytical method for a particular application are associated with a lack of time to check methods described in literature, a certain trust in already published procedures, even if they are far from being perfect, as well as a pressure from granting agencies/sponsors to get “real” catalytic results rather than means to develop or check analytical methods. In the latter case there is certainly more glory in developing new methods compared to just checking the old ones.

Finally, we should stress that no single method works perfectly for all kinds of samples. Moreover, dubious methods are sometimes presented in literature, which means that the results are not reliable. It can thus be emphasized once more that improving analytical methods, not only in the particular case of the catalytic transformation of biomass, but in general improves the quality of science.