Carbon nanotube-supported metal catalysts for NOx reduction using hydrocarbon reductants. Part 1:catalyst preparation, characterization and NOx reduction characteristics

A dual catalyst system for the Selective Catalytic Reduction of NOx with hydrocarbons (HC-SCR), including distinct low and high temperature formulations, is proposed as a means to abate NOx emissions from diesel engines. Given that satisfactory high temperature HC-SCR catalysts are already available, this work focuses on the development of an improved low temperature formulation. Pt supported on multiwalled carbon nantubes (MWCNTs) was found to exhibit superior NOx reduction activity in comparison with Pt/Al2O3, while the MWCNT support displayed a higher resistance to oxidation than activated carbon. Refluxing the MWCNT support in a 1:1 mixture of H2SO4 and HNO3 prior to the metal deposition step proved to be beneficial for the metal dispersion and the NOx reduction performance of the resulting catalysts. This support effect is ascribed to the increased Brønsted acidity of the acid-treated MWCNTs, which in turn enhances the partial oxidation of the hydrocarbon reductant. Further improvements in the HC-SCR performance of MWCNT-based formulations were achieved using a 3:1 Pt–Rh alloy as the supported phase.

Designing heterogeneous catalysts for biodiesel synthesis

Concerns over dwindling oil reserves, carbon dioxide emissions from fossil fuel sources and associated climate change is driving the urgent need for clean, renewable energy supplies. The conversion of triglycerides to biodiesel via catalytic transesterification remains an energetically efficient and attractive means to generate transportation fuel1. However, current biodiesel manufacturing routes employing soluble alkali based catalysts are very energy inefficient producing copious amounts of contaminated water waste during fuel purification. Technical advances in catalyst and reactor design and introduction of non-food based feedstocks are thus required to ensure that biodiesel remains a key player in the renewable energy sector for the 21st century. This presentation will give an overview of some recent developments in the design of solid acid and base catalysts for biodiesel synthesis. A particular focus will be on the benefits of designing materials with interconnected hierarchical macro-mesoporous networks to enhance mass-transport of viscous plant oils during reaction.

Glycerol valorization:dehydration to acrolein over silica-supported niobia catalysts

The catalytic dehydration of glycerol to acrolein is investigated over silica-supported niobia catalysts in a continuous fixed-bed gas-phase reactor. Various supported niobia catalysts are prepared and characterized using surface analysis and spectroscopic methods (XRD, UV-Vis, XPS, N2 adsorption), as well as with ammonia adsorption microcalorimetry. Good results are obtained with initial glycerol conversions of over 70% and with 50-70% selectivity to acrolein. We investigate the influence of changing the catalyst acid strength by varying the niobia content and catalyst calcination temperature. Glycerol conversion and acrolein selectivity depend on the surface acid strength. Catalyst deactivation by coking is also observed, but simple oxidative treatment in air restores the activity of the catalysts completely. © The Author(s) 2010.

Hierarchical nanoporous solid base catalysts for biofuels

Novel macroporous solid bases have been developed as alternative clean technologies to existing commercial homogeneous catalysts for the production of biodiesel from triglycerides; the latter suffer process disadvantages including complex separation and associated saponification and engine corrosion, and are unsuitable for continuous operation. To this end, tuneable macroporous MgAl hydrotalcites have been prepared by an alkali-free route and characterised by TGA, XRD, SEM and XPS. The macropore architecture improves diffusion of bulky triglyceride molecules to the active base sites, increasing activity. Lamellar and macroporous hydrotalcites will be compared for the transesterification of both model and plant oil feedstocks, and structure-reactivity relations identified.

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.

New solid base catalysts for biodiesel synthesis

The application of heterogeneous catalysts for the manufacture of renewable biodiesel fuels offers an exciting, alternative clean chemical technology to current energy intensive processes employing soluble base catalysts. We recently synthesised tuneable MgO nanocrystals as efficent solid base catalysts for biodiesel synthesis, and have developed a simple X-ray spectroscopic method to quantitatively determine surface basicity, thereby providing a rapid screening tool for predicting the reactivity of new solid base catalysts. Promotion of these MgO nanocrystals through Cs doping dramatically enhances biodiesel production rates due to the formaion of a mixed Cs Mg(CO ) phase. These MgO derived nanocatalysts permit energy efficent, continuous processing of diverse, sustainable oil feedstocks in flow reactors.

Heterogeneous catalysts for biodiesel production

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 non-toxic and biodegradable, with the potential for closed CO2 cycles and thus vastly reduced carbon footprints compared with petroleum fuels. However, current manufacturing routes employing soluble catalysts are very energy inefficient and produce copious amounts of contaminated water waste. This review highlights the significant progress made in recent years towards developing solid acid and base catalysts for biodiesel synthesis. Issues to be addressed in the future are also discussed including the introduction of non-edible oil feedstocks, as well as technical advances in catalyst and reactor design to ensure that biodiesel remains a key player in the renewable energy sector for the 21st century.

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.

Cs-doped H4SiW12O40 catalysts for biodiesel applications

Cs exchanged silicotungstic acid catalysts of general formula CsxH4−xSiW12O40 (x = 0.8–4) have been synthesised and characterised by a range of techniques including elemental analysis, N2 gas adsorption, XRD, XPS and NH3 flow calorimetry. Cs substitution promotes recrystallisation of the parent H4SiW12O40 polyoxometallate to the Cs4 salt, via a stable intermediate phase formed at compositions between Cs0.8–2.8. This recrystallisation is accompanied by a pronounced rise and subsequent fall in porosity, with a maximum mesopore volume obtained for materials containing 2.8 Cs atoms per Keggin unit. Calorimetry reveals all CsxH4−xSiW12O40 are strong acids, with ΔHθads(NH3) ranging from −142 to 116 kJ mol−1 with increasing Cs content, consistently weaker than their phosphotungstic analogues. CsxH4−xSiW12O40 materials are active catalysts for both C4 and C8 triglyceride transesterification, and palmitic acid esterification with methanol. For loadings ≤0.8 Cs per Keggin, (trans)esterification activity arises from homogeneous contributions. However, higher degrees of substitution result in entirely heterogeneous catalysis, with rates proportional to the density of accessible acid sites present within mesopores.

Optimising catalytic properties of supported sulfonic acid catalysts

Siliceous mesoporous molecular sieves (SBA-15) have been functionalised with propylsulfonic acid groups by both co-condensing 3-mercaptopropyltrimethoxysilane with the solid at the synthesis (sol-gel) stage and by grafting the same compound to pre-prepared SBA-15, followed, in both cases, by oxidation to sulfonic acid. The acidic and catalytic properties of the supported sulfonic acids prepared in the two ways have been compared, using ammonia adsorption calorimetry and the benzylation reaction between benzyl alcohol and toluene. Using a combination of X-ray photoelectron spectroscopy and other analytical techniques, the level of functionalisation and the extent of subsequent oxidation of tethered thiol to sulfonic acid, both in the bulk and close to the surface of SBA-15 particles, have been assessed. The research shows that the co-condensing route leads to higher levels of functionalisation than the grafting route. The extent of oxidation of added thiol to acid groups is similar using the two routes, about 70% near the surface and only 50% in the bulk. Comparison is made with polymer supported sulfonic acid catalysts, Amberlysts 15 and 35, and Nafion. Nafion shows the highest acid strength and the highest specific catalytic activity of all materials studied. Amongst the other materials, average acid strengths are broadly similar but there appears to be a relationship between the concentration of acid sites on the catalysts and their specific activity in the benzylation reaction. A model is proposed to explain this, in which clustering of sulfonic acid groups, even to a small extent, leads to disproportionately enhanced catalytic activity. © 2009 Elsevier B.V. All rights reserved.

Phenol methylation over nanoparticulate Co2FeO4 inverse spinel catalysts:the effect of morphology on catalytic performance

A series of CoFe2O4 nanoparticles have been prepared via co-precipitation and controlled thermal sintering, with tunable diameters spanning 7–50 nm. XRD confirms that the inverse spinel structure is adopted by all samples, while XPS shows their surface compositions depend on calcination temperature and associated particle size. Small (<20 nm) particles expose Fe3+ enriched surfaces, whereas larger (∼50 nm) particles formed at higher temperatures possess Co:Fe surface compositions close to the expected 1:2 bulk ratio. A model is proposed in which smaller crystallites expose predominately (1 1 1) facets, preferentially terminated in tetrahedral Fe3+ surface sites, while sintering favours (1 1 0) and (1 0 0) facets and Co:Fe surface compositions closer to the bulk inverse spinel phase. All materials were active towards the gas-phase methylation of phenol to o-cresol at temperatures as low as 300 °C. Under these conditions, materials calcined at 450 and 750 °C exhibit o-cresol selectivities of ∼90% and 80%, respectively. Increasing either particle size or reaction temperature promotes methanol decomposition and the evolution of gaseous reductants (principally CO and H2), which may play a role in CoFe2O4 reduction and the concomitant respective dehydroxylation of phenol to benzene. The degree of methanol decomposition, and consequent H2 or CO evolution, appears to correlate with surface Co2+ content: larger CoFe2O4 nanoparticles have more Co rich surfaces and are more active towards methanol decomposition than their smaller counterparts. Reduction of the inverse spinel surface thus switches catalysis from the regio- and chemo-selective methylation of phenol to o-cresol, towards methanol decomposition and phenol dehydroxylation to benzene. At 300 °C sub-20 nm CoFe2O4 nanoparticles are less active for methanol decomposition and become less susceptible to reduction than their 50 nm counterparts, favouring a high selectivity towards methylation.

Sol-gel sulfonic acid silicas as catalysts

Silica-supported sulfonic acids are a class of solid Brønsted acid catalysts that generally comprise organo-sulfonic acid groups tethered to silica surfaces. Methodologies to prepare organically modified silica have been widely developed in separation science and the techniques for their preparation are well documented. The application of this chemistry to prepare pure Brønsted sulfonic acid functionalized mesoporous silicas has stimulated significant research effort in this area, since these materials are interesting alternatives to commercially available sulfonated polymer resins, such as Amberlyst–15 and Nafion-H (sulfonated polystyrene and perfluorinated sulfonic acid resins respectively), which suffer from low surface areas and thermal stability. This chapter presents an overview of the preparation of mesostructured silica supported sulfonic acids, their catalytic applications and reviews the approaches taken to tune catalyst performance in organic transformations.

In-situ XPS study on the reducibility of Pd-promoted Cu/CeO2 catalysts for the oxygen-assisted water-gas-shift reaction

Cu/CeO2, Pd/CeO2, and CuPd/CeO2 catalysts were prepared and their reduction followed by in-situ XPS in order to explore promoter and support interactions in a bimetallic CuPd/CeO2 catalyst effective for the oxygen-assisted water-gas-shift (OWGS) reaction. Mutual interactions between Cu, Pd, and CeO2 components all affect the reduction process. Addition of only 1 wt% Pd to 30 wt% Cu/CeO2 greatly enhances the reducibility of both dispersed CuO and ceria support. In-vacuo reduction (inside XPS chamber) up to 400 °C results in a continuous growth of metallic copper and Ce3+ surface species, although higher temperatures results in support reoxidation. Supported copper in turn destabilizes metallic palladium metal with respect to PdO, this mutual perturbation indicating a strong intimate interaction between the Cu–Pd components. Despite its lower intrinsic reactivity towards OWGS, palladium addition at only 1 wt% loading significantly improved CO conversion in OWGS reaction over a monometallic 30 wt% Cu/CeO2 catalysts, possibly by helping to maintain Cu in a reduced state during reaction.

Evaluation of the activity and stability of alkali-doped metal oxide catalysts for application to an intensified method of biodiesel production

A series of alkali-doped metal oxide catalysts were prepared and evaluated for activity in the transesterification of rapeseed oil to biodiesel. Of those evaluated, LiNO3/CaO, NaNO3/CaO, KNO3/CaO and LiNO3/MgO exhibited >90% conversion in a standard 3 h test. There was a clear correlation between base strength and activity. These catalysts appeared to be promising candidates to replace conventional homogeneous catalysts for biodiesel production as the reaction times are low enough to be practical in continuous processes and the preparations are neither prohibitively difficult nor costly. However, metal leaching from the catalyst was detected, and this resulted in some homogeneous activity. This would have to be resolved before these catalysts would be viable for large-scale biodiesel production facilities.