Heterogeneous Catalysts for Sustainable Chemistry

Abstract

In recent years there has been considerable interest in ‘green’ chemistry in order to reduceour dependence on oil and finite natural resources. One industrial processes that currentlyrelies on oil is the production of 1,3-butadiene (1,3-BD). It is possible to convert ethanol, asustainable source, in to 1,3-BD. Previously reported catalysts for this process do notpossess the desirable conversion or selectivity for 1,3-BD to be able to compete with thepetroleum based process. A number of heterogeneous catalysts have been prepared andtested on a fixed-bed, continous-flow reactor. The reaction products were analysed by gaschromatography and 1H NMR. Catalysts were initially based upon the naturally occurringmineral sepiolite, which was impregnated with a range of divalent metal ions, howeverresults were poor due to structural degradation of the materials during the calcinationprocess, confirmed by powder X-ray diffraction. Greater success was achieved with a rangeof bi- and tri-metallic catalysts where a combination of zinc, zirconium, and copper aresupported on silica. 1,3-BD selectivity and conversions were shown to be related to the porediameter of the silica. Silica with a larger pore diameter was shown by 29Si solid-state NMRto possess fewer Brønsted acidic sites on the catalyst. Reduction of these sites suppressedthe production of ethylene from the alcohol feed. A trimetallic catalyst of Cu/Zn/Zr on 150 Åpore diameter silica, containing 1 wt% of each metal, gave the most impressive results (44 %conversion, 67 % 1,3-BD selectivity over a 3-hour run) with a purely ethanolic feed. The yieldof 1,3-BD for bimetallic Zn/Zr catalysts can be improved by the addition of acetaldehyde, anintermediate in the conversion process, to the ethanol feed. The trimetallic catalyst did notshow any considerable improvement with added acetaldehyde, as copper is a well knownalcohol dehydrogenation catalyst, implying the catalyst struggles elsewhere.Biodiesel production is another area of sustainable chemistry that has been investigated.Biodiesel is produced from the transesterification of oils with methanol. A series of Schiffbaseligands were produced from various cyclic aldehydes and anilines. Eight Zn(II) Schiffbasecomplexes have been characterised by single crystal X-ray diffraction, and shown to beactive as homogenous catalysts for the transesterification of vegetable oil. Aluminium, titanium, and zinc silsesquioxane complexes have also been synthesised andfully characterised by single crystal X-ray diffraction for use as soluble models for silicasupported heterogeneous catalysts. Two distinct Zn(II)-silsesquioxane complexes wereproduced, with only one previously reported in the literature.

Date of Award	31 Dec 2010
Original language	English
Awarding Institution	University of Bath
Supervisor	Matthew Jones (Supervisor)