Hierarchical porous catalysts tailored for biodiesel production

There is a pressing need for sustainable transportation fuels to combat both climate change and dwindling fossil fuel reserves. Biodiesel, synthesised from non-food plant (e.g., Jatropha curcas) or algal crops is one possible solution, but its energy efficient production requires design of new solid catalysts optimized for the bulky triglyceride and fatty acid feedstocks. Here we report on the synthesis of hierarchical macroporous-mesoporous silica and alumina architectures, and their subsequent functionalization by propylsulfonic acid groups or alkaline earth oxides to generate novel solid acid and base catalysts. These materials possess high surface areas and well-defined, interconnected macro-mesopore networks with respective narrow pore size distributions tuneable around 300 nm and 5 nm. Their high conductivity and improved mass transport characteristics enhance activity towards transesterification of bulky tricaprylin and palmitic acid esterification, over mesoporous analogues. This opens the way to the wider application of hierarchical catalysts in biofuel synthesis and biomass conversion.

Spatially orthogonal chemical functionalization of a hierarchical pore network for catalytic cascade reactions

The chemical functionality within porous architectures dictates their performance as heterogeneous catalysts; however, synthetic routes to control the spatial distribution of individual functions within porous solids are limited. Here we report the fabrication of spatially orthogonal bifunctional porous catalysts, through the stepwise template removal and chemical functionalization of an interconnected silica framework. Selective removal of polystyrene nanosphere templates from a lyotropic liquid crystal-templated silica sol–gel matrix, followed by extraction of the liquid crystal template, affords a hierarchical macroporous–mesoporous architecture. Decoupling of the individual template extractions allows independent functionalization of macropore and mesopore networks on the basis of chemical and/or size specificity. Spatial compartmentalization of, and directed molecular transport between, chemical functionalities affords control over the reaction sequence in catalytic cascades; herein illustrated by the Pd/Pt-catalysed oxidation of cinnamyl alcohol to cinnamic acid. We anticipate that our methodology will prompt further design of multifunctional materials comprising spatially compartmentalized functions.

In situ synthesis and catalytic activity in CO oxidation of metal nanoparticles supported on porous nanocrystalline silicon

Reactive surface of mesoporous nanocrystalline silicon was used to synthesise noble metal nanoparticles via in situ reduction of the precursor salt solutions. The synthetic methodology for metal nanoparticle formation was systematically developed, and reaction conditions of metal salts reduction were optimised to prepare nanoparticles of controlled size distribution in the order 5–10 nm inside the mesoporous silicon template. CO oxidation was used as a test reaction for the synthesised Pt/porous silicon catalysts. Sharp reaction light-off was observed at about 120 °C on the optimised catalysts. The catalysts were shown to be stable in the extended steady-state runs and in the catalysts re-use experiments. Metal nanoparticles were shown to be stable to sintering at elevated temperatures up to 1000 °C. However, after thermal treatment on air, Pt nanoparticles were covered by a SiOx layer and were less active in CO oxidation.

Designer catalysts for biodiesel synthesis

The combination of dwindling oil reserves and growing concerns over carbon dioxide emissions and associated climate change is driving the urgent development of clean, sustainable energy supplies. Biodiesel is a non-toxic and biodegradable fuel, with the potential for closed CO2 cycles and thus vastly reduced carbon footprints compared with petroleum. However, current manufacturing routes employing soluble catalysts are very energy inefficient, with their removal necessitating an energy intensive separation to purify biodiesel, which in turn produces copious amounts of contaminated aqueous waste. The introduction of non-food based feedstocks and technical advances in heterogeneous catalyst and reactor design are required to ensure that biodiesel remains a key player in the renewable energy sector for the 21st century. Here we report on the development of tuneable solid acid and bases for biodiesel synthesis, which offer several process advantages by eliminating the quenching step and allowing operation in a continuous reactor. Significant progress has been made towards developing tuneable solid base catalysts for biodiesel synthesis, including Li/CaO [1], Mg-Al hydrotalcites [2] and calcined dolomite [3] which exhibit excellent activity for triglyceride transesterification. However, the effects of solid base strength on catalytic activity in biodiesel synthesis remains poorly understood, hampering material optimisation and commercial exploitation. To improve our understanding of factors influencing solid base catalysts for biodiesel synthesis, we have applied a simple spectroscopic method for the quantitative determination of surface basicity which is independent of adsorption probes. Such measurements reveal how the morphology and basicity of MgO nanocrystals correlate with their biodiesel synthesis activity [4]. While diverse solid acids and bases have been investigated for TAG transesterification, the micro and mesoporous nature of catalyst systems investigated to date are not optimal for the diffusion of bulky and viscous C16-C18 TAGs typical of plant oils. The final part of this presentation will address the benefits of designing porous networks comprising interconnected hierarchical macroporous and mesoporous channels (Figure 1) to enhance mass-transport properties of viscous plant oils during biodiesel synthesis [5]. References: [1] R.S. Watkins, A.F. Lee, K. Wilson, Green Chem., 2004, 6, 335. [2]D.G. Cantrell, L.J. Gillie, A.F. Lee and K. Wilson, Appl. Catal. A, 2005, 287,183. [3] C. Hardacre, A.F. Lee, J.M. Montero, L. Shellard, K.Wilson, Green Chem., 2008, 10, 654. [4] J.M. Montero, P.L. Gai, K. Wilson, A.F. Lee, Green Chem., 2009, 11, 265. [5] J. Dhainaut, J.-P. Dacquin, A.F. Lee, K. Wilson, Green Chem., 2010, 12, 296.

Progress in the development of mesoporous solid acid and base catalysts for converting carbohydrates into platform chemicals

This chapter provides a general overview of recent studies on catalytic conversion of fructose, glucose, and cellulose to platform chemicals over porous solid acid and base catalysts, including zeolites, ion-exchange resins, heteropoly acids, as well as structured carbon, silica, and metal oxide materials. Attention is focused on the dehydration of glucose and fructose to HMF, isomerization of glucose to fructose, hydrolysis of cellulose to sugar, and glycosidation of cellulose to alkyl glucosides. The correlation of porous structure, surface properties, and the strength or types of acid or base with the catalyst activity in these reactions is discussed in detail in this chapter.

Comparison of the effect of pre-treatment and catalysts on liquid quality from fast pyrolysis of biomass

The overall objective of this work was to compare the effect of pre-treatment and catalysts on the quality of liquid products from fast pyrolysis of biomass. This study investigated the upgrading of bio-oil in terms of its quality as a bio-fuel and/or source of chemicals. Bio-oil used directly as a biofuel for heat or power needs to be improved particularly in terms of temperature sensitivity, oxygen content, chemical instability, solid content, and heating values. Chemicals produced from bio-oil need to be able to meet product specifications for market acceptability. There were two main objectives in this research. The first was to examine the influence of pre-treatment of biomass on the fast pyrolysis process and liquid quality. The relationship between the method of pre-treatment of biomass feedstock to fast pyrolysis oil quality was studied. The thermal decomposition behaviour of untreated and pretreated feedstocks was studied by using a TGA (thermogravimetric analysis) and a Py-GC/MS (pyroprobe-gas chromatography/mass spectrometry). Laboratory scale reactors (100g/h, 300g/h, 1kg/h) were used to process untreated and pretreated feedstocks by fast pyrolysis. The second objective was to study the influence of numerous catalysts on fast pyrolysis liquids from wheat straw. The first step applied analytical pyrolysis (Py-GC/MS) to determine which catalysts had an effect on fast pyrolysis liquid, in order to select catalysts for further laboratory fast pyrolysis. The effect of activation, temperature, and biomass pre-treatment on catalysts were also investigated. Laboratory experiments were also conducted using the existing 300g/h fluidised bed reactor system with a secondary catalytic fixed bed reactor. The screening of catalysts showed that CoMo was a highly active catalyst, which particularly reduced the higher molecular weight products of fast pyrolysis. From these screening tests, CoMo catalyst was selected for larger scale laboratory experiments. With reference to the effect of pre-treatment work on fast pyrolysis process, a significant effect occurred on the thermal decomposition of biomass, as well as the pyrolysis products composition, and the proportion of key components in bio-oil. Torrefaction proved to have a mild influence on pyrolysis products, when compared to aquathermolysis and steam pre-treatment.

On the thermal stabilization of carbon-supported SiO2 catalysts by phosphorus:evaluation in the oxidative dehydrogenation of ethylbenzene to styrene and a comparison with relevant catalysts

A strategy to enhance the thermal stability of C/SiO2 hybrids for the O2-based oxidative dehydrogenation of ethylbenzene to styrene (ST) by P addition is proposed. The preparation consists of the polymerization of furfuryl alcohol (FA) on a mesoporous precipitated SiO2. The polymerization is catalyzed by oxalic acid (OA) at 160 °C (FA:OA = 250). Phosphorous was added as H3PO4 after the polymerization and before the pyrolysis that was carried out at 700 °C and will extend the overall activation procedure. Estimation of the apparent activation energies reveals that P enhances the thermal stability under air oxidation, which is a good indication for the ODH tests. Catalytic tests show that the P/C/SiO2 hybrids are readily active, selective and indeed stable in the applied reactions conditions for 60 h time on stream. Coke build-up during the reaction attributed to the P-based acidity is substantial, leading to a reduction of the surface area and pore volume. The comparison with a conventional MWCNT evidences that the P/C/SiO2 hybrids are more active and selective at high temperatures (450–475 °C) while the difference becomes negligible at lower temperature. However, the comparison with reference P/SiO2 counterparts shows a very similar yield than the hybrids but more selective to ST. The benefit of the P/C/SiO2 hybrid is the lack of stabilization period, which is observed for the P/SiO2 to create an active coke overlayer. For long term operation, P/SiO2 appears to be a better choice in terms of selectivity, which is crucial for commercialization.

Hopping transport on a fractal: ac conductivity of porous silicon

We have measured the frequency dependence of the conductivity and the dielectric constant of various samples of porous Si in the regime 1 Hz-100 kHz at different temperatures. The conductivity data exhibit a strong frequency dependence. When normalized to the dc conductivity, our data obey a universal scaling law, with a well-defined crossover, in which the real part of the conductivity sigma' changes from an sqrt(omega) dependence to being proportional to omega. We explain this in terms of activated hopping in a fractal network. The low-frequency regime is governed by the fractal properties of porous Si, whereas the high-frequency dispersion comes from a broad distribution of activation energies. Calculations using the effective-medium approximation for activated hopping on a percolating lattice give fair agreement with the data.

Fast pyrolysis of cassava rhizome in the presence of catalysts

Cassava rhizome was catalytically pyrolysed at 500 °C using analytical pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) in order to investigate the effect of catalysts on bio-oil properties. The catalysts studied were zeolite ZSM-5, two aluminosilicate mesoporous materials Al-MCM-41 and Al-MSU-F, and a proprietary commercial catalyst alumina-stabilised ceria MI-575. The influence of catalysts on pyrolysis products was observed through the yields of aromatic hydrocarbons, phenols, lignin-derived compounds, carbonyls, methanol and acetic acid. Results showed that all the catalysts produced aromatic hydrocarbons and reduced oxygenated lignin derivatives, thus indicating an improvement of bio-oil heating value and viscosity. Among the catalysts, ZSM-5 was the most active to all the changes in pyrolysis products. In addition, all the catalysts with the exception of MI-575 enhanced the formation of acetic acid. This is clearly a disadvantage with respect to the level of pH in the liquid bio-fuel.

Synthesis and characterisation of pillared clay catalysts for hydro-cracking of heavy liquid fuels

A range of chromia pillared montmorillonite and tin oxide pillared laponite clay catalysts, as well as new pillared clay materials such as cerium and europium oxide pillared montmorillonites were synthesised. Methods included both conventional ion exchange techniques and microwave enhanced methods to improve performance and/or reduce preparation time. These catalytic materials were characterised in detail both before and after use in order to study the effect of the preparation parameters (starting material, preparation method, pillaring species, hydroxyl to metal ratio etc.) and the hydro cracking procedure on their properties. This led to a better understanding of the nature of their structure and catalytic operation. These catalysts were evaluated with regards to their performance in hydrocracking coal derived liquids in a conventional microbomb reactor (carried out at Imperial College). Nearly all catalysts displayed better conversions when reused. The chromia pillared montmorillonite CM3 and the tin oxide pillared laponite SL2a showed the best "conversions". The intercalation of chromium in the form of chromia (Cr203) in the interlayer clearly increased conversion. This was attributed to the redox activity of the chromia pillar. However, this increase was not proportional to the increase in chromium content or basal spacing. In the case of tin oxide pillared laponite, the catalytic activity might have been a result of better access to the acid sites due to the delaminated nature of laponite, whose activity was promoted by the presence of tin oxide. The manipulation of the structural properties of the catalysts via pillaring did not seem to have any effect on the catalysts' activity. This was probably due to the collapse of the pillars under hydrocracking conditions as indicated by the similar basal spacing of the catalysts after use. However, the type of the pillaring species had a significant effect on conversion. Whereas pillaring with chromium and tin oxides increased the conversion exhibited by the parent clays, pillaring with cerium and europium oxides appeared to have a detrimental effect. The relatively good performance of the parent clays was attributed to their acid sites, coupled with their macropores which are able to accommodate the very high molecular mass of coal derived liquids. A microwave reactor operating at moderate conditions was modified for hydro cracking coal derived liquids and tested with the conventional catalyst NiMo on alumina. It was thought that microwave irradiation could enable conversion to occur at milder conditions than those conventionally used, coupled with a more effective use of hydrogen. The latter could lead to lower operating costs making the process cost effective. However, in practice excessive coke deposition took place leading to negative total conversion. This was probably due to a very low hydrogen pressure, unable to have any hydro cracking effect even under microwave irradiation. The decomposition of bio-oil under microwave irradiation was studied, aiming to identify the extent to which the properties of bio-oil change as a function of time, temperature, mode of heating, presence of char and catalyst. This information would be helpful not only for upgrading bio-oil to transport fuels, but also for any potential fuel application. During this study the rate constants of bio-oil's decomposition were calculated assuming first order kinetics.

Ruthenium complexes as potential aqueous romp catalysts

The research described herein relates to studies into the Aqueous Ring-Opening Metathesis Polymerisation (ROMP) of bicyclic monomers using ruthenium complex catalysts. Two monomers were synthesised for the purpose of these studies, namely exo, exo-7-oxabicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid (7-oxanorbornenedicarboxylic acid) and exo, exo-bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid (norbornene dicarboxylic acid). A number of ruthenium complexes were synthesised, amongst them a novel complex containing the water soluble phosphine ligand trist(hydroxymethyl)phosphine P(CH2OH)3. Its synthesis and characterisation are described and its physical properties compared and contrasted to analogous compounds of platinum and palladium. Its peculiar properties are ascribed to a trans-placement of the phosphine ligands. Dilatometry was investigated as a technique for the acquisition of kinetic data from aqueous metathesis reactions. For the attempted polymerisation of 7-oxanorbonenedicarboxylic acid the results are explained in terms of a reverse Diels-Alder reaction of the monomer. The reaction between Ru(CO)Cl2(H2O) and 7-oxanorbonenedicarboxylic acid was monitored using UV/Vis spectrometry and kinetic data retrieved. The data are explained in terms of a two stage reaction consisting of consecutive first order processes.The reaction between 7-oxanorbornenedicarboxylic acid and Ru(CO)Cl2(H2O) or Ru(P(CH2OH)3)3Cl2 was found to produce fumaric acid as one of the major products. This reaction is previously unreported in the literature and a mechanism is proposed.

The use of enzyme catalysts in organic media for the synthesis of biodegradable polyesters

The enzyme catalysed polytransesterification of diesters with diols was investigated under various conditions. The most consistent results were obtained using crude porcine pancreatic lipase (PPL) suspended in anhydrous diethyl ether. Addition of molecular sieve to the above system gave higher molecular weight products. The PPL catalysed reaction of bis(2,2,2-trichlorethyl) adipate and glutarate with butane-1,4-diol in anhydrous ether with and without molecular sieve was investigated over a range of times from 8 to 240 hours. The 72 hour adipate reaction with molecular sieve gave the highest molecular weight polymer (Mn 6,500 and Mw 9,400). The glutarate gave the maximum molecular weight polyester after 24 hours (Mn 5,700 and Mw 9,500). Occasionally the glutarate reaction produced very high molecular weight polyester-enzyme complexes. Toluene generally gave lower molecular weight products than diethyl ether. Dichloromethane and tetrahydrofuran gave mainly dimers and trimers. Alternative enzyme and diol systems were also investigated. These yielded no polymeric products. The molecular weights of the polyesters were determined by 1H NMR end-group analysis and by GPC. The molecular weights determined by NMR were on average about twice as great as those determined by GPC. The synthesis of the following diesters is described: i)Bis(2,2,2-trichloroethyl) succinate, glutarate, adipate, trans-3-hexenedioate, and trans-3,4-epoxyadipate. ii) Diphenyl glutarate and adipate.iii)Bis(2,2,2-fluoroethyl) glutarate and trans-3-hexendioate.iv) Divinyl glutarate. v) N,N'Glutaryl dicyclohexanone oxime.The polytransesterification of all the above esters with diols was investigated. The easily synthesised bis(2,2,2-trichloroethyl) glutarate and adipate gave the best results and the work was concentrated on these two esters.

The steady performance of a porous and compliant aerostatic thrust bearing

Recent developments in aerostatic thrust bearings have included: (a) the porous aerostatic thrust bearing containing a porous pad and (b) the inherently compensated compliant surface aerostatic thrust bearing containing a thin elastomer layer. Both these developments have been reported to improve the bearing load capacity compared to conventional aerostatic thrust bearings with rigid surfaces. This development is carried one stage further in a porous and compliant aerostatic thrust bearing incorporating both a porous pad and an opposing compliant surface. The thin elastomer layer forming the compliant surface is bonded to a rigid backing and is of a soft rubber like material. Such a bearing is studied experimentally and theoretically under steady state operating conditions. A mathematical model is presented to predict the bearing performance. In this model is a simplified solution to the elasticity equations for deflections of the compliant surface. Account is also taken of deflections in the porous pad due to the pressure difference across its thickness. The lubrication equations for flow in the porous pad and bearing clearance are solved by numerical finite difference methods. An iteration procedure is used to couple deflections of the compliant surface and porous pad with solutions to the lubrication equations. Comparisons between experimental results and theoretically predicted bearing performance are in good agreement. However these results show that the porous and compliant aerostatic thrust bearing performance is lower than that of a porous aerostatic thrust bearing with a rigid surface in place of the compliant surface. This discovery is accounted to the recess formed in the bearing clearance by deflections of the compliant surface and its effect on flow through the porous pad.