Exploitation of synergy in oxidation catalysis with bimetallic aluminophosphate catalysts

Abstract

A series of novel bimetallic aluminophosphate catalysts were prepared with the simultaneous isomorphous substitution of cobalt (III) and manganese (III) for aluminium (III) and titanium (IV) replacing phosphorus (V). By substituting simultaneously for both Al (III) and P (V) in the ordered microporous aluminophosphate AlPO-5, the potential for catalytic application as either a solidacid or redox catalyst is produced. The bimetallic hydrothermal synthesis produced a phase pure and highly crystalline microporous catalyst which displayed properties not seen in monometallic AlPO-5 materials. These catalysts were fully characterized by physical, diffraction, spectroscopic and electrochemical techniques including powder X-ray diffraction, diffuse reflectance UV/Visible spectroscopy, cyclic voltammetry, extended X-ray absorption fine structure, X-ray absorption near edge spectra, scanning electron microscopy and surface/elemental (BET and ICP) methods. A significant change was observed when two metals were combined in the same structural framework, leading to an enhancement in the catalyst performance over the monometallic analogues. EXAFS indicated a change in the relative bond lengths, thought to be as a result of compensation in the structural distortion, similarly UV/Visible spectroscopy highlighted a change in the tetrahedral environment around the titanium centre when in combination with a redox metal. These catalysts were tested for the ammoximation of cyclohexanone using benign reagents to produce cyclohexanone oxime, a vital precursor in the production of nylon-6. Currently produced through hazardous processes, the ammoximation of cyclohexanone with metal substituted catalysts provides a more environmentally favourable solution. The use of a redox metal such as cobalt (III) in combination with titanium (IV) was shown to facilitate higher catalytic conversions compared with the monometallic AlPO-5 analogues while maintaining the high oxime selectivity. Similarly the epoxidation of cyclohexene with the solid oxidant acetylperoxyborate (APB) indicated a significantly higher conversion with the CoIIITiIVAlPO-5 catalyst. The substitution of two metals to replace both aluminium and phosphorus sites clearly influences and modifies the nature of the active site and provides an enhanced catalytic performance for ammoximation and epoxidation reactions. This synergy exerted through bimetallic substitution provides an important alternative approach to academic and industrial application of metal substitution within heterogeneous catalysis

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ORCID for Robert Raja: orcid.org/0000-0002-4161-7053

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