ABSTRACT
Centre of Environmental Research, University of Limerick, Limerick, Ireland
giacomo.grasso@ul.ie
Abstract A catalytic system Mo-V-Nb-W supported on alumina was prepared by impregnation and investigated for the selective oxidation of propane. The effects of the variation of each metal and of the catalyst preparation were analysed. The results show that Mo and V species supported on alumina can lead to catalysts with high selectivity to propene and reasonable selectivity to acrolein. The presence of Nb and W seems to have little effect. The catalyst can be affected by the method of impregnation. Introduction As consequence of an increased demand from the market for propene and C3 oxygenated products over the last fifteen years, the oxidative dehydrogenation (ODH) and the selective oxidation of propane have become reactions attracting much research into both possible catalyst formulations and reaction mechanisms. Previous work [1] from our laboratory has shown that a combination of Mo, V and other oxides supported on a low-area alumina calcined at 1150ºC (α-alumina) have interesting properties as catalysts for the ODH of propane; when water vapour was added to the reactant stream, they also gave significant yields of acrolein, particularly for a formulation containing the oxides of Mo, V, Nb and W. In order to gain a better understanding of this system and to optimise the methods of catalyst composition and preparation, a series of libraries of such catalysts have been prepared and tested, using both traditional and high throughput screening techniques. This paper summarises some of the results obtained with catalysts made using manual methods which illustrate the effects of catalyst composition as well as of preparation method on the properties of the final catalyst. The paper also emphasises the importance of the testing conditions used in determining the measured properties. Experimental Section Catalyst preparation. One library of catalysts, indicated below with the name Library 1, was prepared by wet impregnation to have a range of concentrations
of Mo, V, Nb and W on α-alumina. Table 1 shows the compositions of the standard material (S) and of a series containing variable quantities of Mo and V. Equivalent series with variable Nb and W contents were prepared but are not shown. These samples were prepared according to a ‘standard’ wet impregnation method as follows: the alumina support precursor (Alcan AA400) was first calcined at 1150ºC; a sample of 2g of this was then treated sequentially by aqueous solutions of the salts of each metal (ammonium molybdate, AnalaR; ammonium metavanadate, AnalaR; niobium oxalate, CBMM; ammonium metatungstate hydrate, Aldrich), the amount of each salt added corresponding to the desired final catalyst composition (assuming that all the salts added are subsequently taken up by the support). After leaving the system to equilibrate in an excess of solution overnight (about 15h), the samples were then dried using a rotary evaporator at 80ºC and were then finally calcined for 6h at 650ºC. The actual amount of metal incorporated for each sample was not analysed, it being assumed that all was deposited (see above); hence, if this assumption is not fully justified, the data presented show only self-consistent trends, giving an indication of the range of compositions over which the best performance can be obtained by this catalytic system.
