Development of gold and copper catalysts for sustainable chemical processing

Supported Au and Cu catalysts have been developed for clean/sustainable production of value fine chemicals (including alcohols, ketone, amines and imines) from selective reduction (of benzaldehyde, nitrobenzene and furfural) and coupled (dehydrogenation-hydrogenation) reaction in the continuous gas phase operation. Critical catalyst physicochemical properties are characterised by applying a range of techniques and correlated to the catalytic response. The role of support, Au particle size and electronic character in determining catalytic activity and selectivity in the hydrogenation of benzaldehyde and nitrobenzene over oxide supported nano-scale (2-8 nm) Au has been established. Hydrogenation (turnover frequency) TOF increases with decreasing Au size (from 8 to 4 nm) with measurably lower TOF over Au < 3 nm. Repulsion of –C=O and –NO2 functionalities with respect to Auδ+ and strong binding to surface oxygen vacancies have been found to lower hydrogenation rates. Promotional effect of water via catalytic dissociation has been found to enhance the selective benzaldehyde hydrogenation rate. Two catalytic routes for imine (N-benzylideneimine) synthesis in continuous gas phase operation have been established. Reductive coupling of benzaldehyde with nitrobenzene (using external hydrogen) over supported Au generated up to 99% selectivity to the target imine. Coupling of benzyl alcohol dehydrogenation with nitrobenzene hydrogenation (in the absence of external hydrogen supply) over Au/TiO2 + Cu/SiO2 mixture produced imine with full hydrogen utilisation. Incorporation of Au/TiO2 to Cu/SiO2 created a synergy between Cu and Au and enhanced catalyst stability. A tandem dehydrogenation/amination/reduction process has been developed for high throughout production of benzylamine in continuous gas phase operation over Cu/SiO2 and Au/TiO2. A synergy between Cu/SiO2 and Au/TiO2 serves to promote benzylamine formation with 81% yield achieved through an optimization of process parameters. Coupling of 2-butanol dehydrogenation with nitrobenzene hydrogenation over Cu/SiO2 in the absence of an external H2 supply delivered exclusive production of both 2-butanone and aniline at full conversion. Hydrogen utilisation efficiency was appreciably greater (by a factor of up to 50) in the coupled system relative to conventional stand-alone hydrogenation using pressurised hydrogen. Selective conversion of biomass-derived furfural to furfuryl alcohol over supported Au catalysts has been established. A series of approaches (e.g. promotional effect of water via catalytic dissociation, increased spillover hydrogen with addition of oxide support and coupling strategy) directed at increasing the surface availability of reactive hydrogen were adopted to enhance furfuryl alcohol production and hydrogen utilisation. Continuous production of γ-butyrolactone has been established in both stand-alone hydrogenation of succinic acid (using external H2) and reaction coupling with formic acid decomposition (as a source of H2) over Cu/SiO2. Pd/SiO2 and Ni/SiO2 promoted propanoic acid formation at higher reaction rates. The results presented in this thesis establish feasible catalytic routes to high value alcohols, imines and amines where critical process optimisation is demonstrated in terms of catalyst composition/surface structure and reaction conditions

Selective production of ethylene via continuous oxidative dehydrogenation of ethane in (Dy2O3/MgO)-(Li-K)Cl composite membrane reactor

Highly selective production of ethylene via continuous oxidative dehydrogenation of ethane has been established in the oxygen conductive (Dy2O3/MgO)-(Li-K)Cl composite membrane reactor. The disk-shape membrane was fabricated by pellet-pressing and characterised in terms of nitrogen physisorption, XRD and SEM. Specific surface area and pore volume decreases with increasing chlorides content with invariable values ≥10.6% chlorides content. SEM analysis reveals exposure of chlorides particles on the membrane surface. The presence of alkali chlorides promoted conversion of ethane selective to ethylene. The membrane reaction exhibited higher selectivity of ethylene than the fixed bed reaction. Reaction over 10.6% chlorides loaded membrane delivered highest ethylene selectivity (98%) and largest specific ethylene formation rate. Extended contact time and higher temperature served to further enhance ethylene yield

Effect of support redox character on catalytic performance in the gas phase hydrogenation of benzaldehyde and nitrobenzene over supported gold

The authors are grateful to Dr. N. Perret for her involvement in this work. EPSRC support for free access to the TEM facility at the University of St. Andrews and financial support to Dr. M. Li and Dr. X. Wang through the Overseas Research Students Award Scheme (ORSAS) are also acknowledged.Peer reviewedPostprin

Tuning the catalytic performance of Ni-catalysed dry reforming of methane and carbon deposition via Ni-CeO2-x interaction

The role of tuning metal-support interaction in determining the catalytic activity and carbon formation in dry reforming of methane to syngas was examined over CeO2 supported Ni nanoparticles. The catalysts pre- and post- reaction were subjected to characterisation in terms of N2 physisorption, TPR, XRD, TEM, XPS and TGA-DTG. Reduction of Ni/CeO2 in H2 in the temperature range (773–973 K) generated a strong bonding between Ni and CeO2 that inhibited Ni particle sintering (8.7–9.4 nm). High-temperature (≥873 K) reduction induced decoration/encapsulation of Ni nanoparticles by a thin layer of reduced ceria support with partial coverage of Ni surface. The decoration/encapsulation effect strongly influences the catalytic properties of Ni, which enables to tune the catalytic activity of Ni/CeO2 and carbon deposition in dry reforming of methane

Coupled reforming of methane to syngas (2H 2 -CO) over Mg-Al oxide supported Ni catalyst

We report bi-reforming and coupled reforming of methane with carbon dioxide, steam and/or oxygen to produce syngas over Ni supported on Mg-Al mixed oxide. The catalyst has been characterised in terms of specific surface area, TPR, XRD, TGA-DTG, SEM and TPO-MS analysis. Ni/Mg-Al mixed oxide exhibited Ni particle size range (11–30 nm) with a mean of 20.7 nm. Syngas H2/CO = 2.0, suitable for methanol/Fischer-Tropsch fuel synthesis, has been achieved for both reactions (T = 1048 K, P = 1 atm). The impact of process parameters including temperature, feeding concentration and GHSV on conversion and H2/CO ratio has been demonstrated. The Ni catalyst suffered temporal activity decline in bi-reforming that can be linked to formation of carbon whiskers encapsulated Ni particles resulting in a loss of active sites. Coupled reforming delivered higher CH4 conversion and enhanced stability, but lower CO2 conversion than bi-reforming under similar conditions. The enhanced stability in coupled reforming can be attributed to lower carbon deposition on Ni particles due to combustion of carbon by oxygen

Metal-oxide interaction enhanced CO2 activation in methanation over ceria supported nickel nanocrystallites

The role of metal-support interaction in determining CO2 activation and conversion for methane production was examined over (CeO2, TiO2 and SiO2) supported Ni nanoparticles. Hexagonal Ni nanocrystallites on CeO2 with strong metal-oxide interaction selectively produced CH4 at (up to forty-fold) higher turnover frequencies (TOFs) than that recorded over (TiO2 and SiO2) supported Ni nanoparticles. A stronger adsorption of CO2 and H2 was identified for Ni/CeO2 using temporal analysis of products (TAP). Decoration/encapsulation of Ni nanoparticles by a thin layer of reduced ceria can decrease the catalytic capacity for CO2 activation/conversion. An initial loss of activity in the long-term stability evaluation over Ni/CeO2 can be linked to a reconstruction of hexagonal Ni nanocrystallites to quasi-spherical particles

Enhanced production of benzyl alcohol in the gas phase continuous hydrogenation of benzaldehyde over Au/Al2O3

Exclusive hydrogenation of benzaldehyde to benzyl alcohol in gas phase continuous operation (393–413 K, 1 atm) was achieved over Au/Al2O3, Au/TiO2 and Au/ZrO2. Synthesis of Au/Al2O3 by deposition–precipitation generated a narrower distribution (2–8 nm) of smaller (mean = 4.3 nm) Au particles relative to impregnation (1–21 nm, mean = 7.9 nm) with increased H2 uptake under reaction conditions and higher benzaldehyde turnover. Switching reactant carrier from ethanol to water resulted in a significant enhancement of selective hydrogenation rate over Au/Al2O3 with 100% benzyl alcohol yield, attributed to increased available reactive hydrogen. This response extends to reaction over Au/TiO2 and Au/ZrO2

Harnessing the selective catalytic action of supported gold in hydrogenation applications

Gold has untapped potential in terms of selectivity in the reduction of targeted chemical functions and substituents. In this chapter, the selective action of supported gold in the hydrogenation of R-NO2, R–CH=O and R–C≡CH is examined, with an analysis of the pertinent literature. Hydrogenation activity requires the formation of gold particles at the nanoscale where the support is critical in determining ultimate catalytic performance. The crucial catalyst structural and surface properties required to achieve enhanced hydrogenation are discussed. The chapter examines in turn the chemoselective hydrogenation of chloronitrobenzene, dinitrobenzene, nitrobenzonitrile, nitrocyclohexane, benzaldehyde, nitrobenzaldehyde, phenylacetylene and furfural. Catalytic gold use in hydrogenolysis is also considered, focusing on hydrodechlorination as a progressive approach to the transformation and recycle of toxic chloro-compounds. The catalytic response is related to possible thermodynamic constraints with an examination of process variables, notably temperature, contact time and H2 partial pressure. Process sustainability is evaluated in terms of mode of operation/productivity, solvent usage, the application of bimetallic catalysts, hydrogen utilisation and the viability of dehydrogenation–hydrogenation coupling. The chapter ends with an assessment of the current state-of-the-art and a consideration of possible future research directions