Catalytic synergy and structure-property correlations in multimetallic aluminophosphate molecular sieves

Abstract

Traditionally, oxidation catalysis has used stoichiometric oxidants, often producing toxic and non-recyclable metal waste. Engineering catalyst active sites in heterogeneous catalysts which are capable of harnessing molecular oxygen from the air is greatly advantageous as it is benign, produces water as the by-product, is easy to handle and is cheap and readily available. This work examines the simultaneous isomorphous substitution of two transition metal centres into the inorganic framework positions of aluminophosphate (AlPO) structures. The two metal centres have a profound electronic influence on each other, displaying a synergistic effect to facilitate enhanced conversion and selectivity towards the desired product. For example in the ammoximation of cyclohexanone, when cobalt and titanium were simultaneously substituted into the AFI (CoTiAlPO-5) framework greater activity was achieved than for the monometallic analogues, or a physical mixture of the two. Probing the transition metal active sites using DR UV/Visible spectroscopy and EXAFS showed that the titanium site displayed greater tetrahedral character when present in the same framework with cobalt, while the cobalt site showed a greater proportion of cycling between Co(II)/C (III) compared to previously reported estimates for monometallic CoAlPO-5. Establishing the role of each metal in the CoTiAlPO-5 system and contextualising this as either inherent structural synergy (as is suggested by the trends in the spectroscopic data), as mechanistic synergy (where the presence of the two different transition metals facilitates an alternative reaction pathway) or a combination of the two, has allowed structure-property correlations to be deduced. The ratio of the two transition metals substituted into the AFI framework were varied and screened in a model epoxidation reaction, showing that the system was most effective with a lower (as small as 1 atom %) loading of the (oxophilic) titanium centre – highlighting the importance of site isolation – while the cobalt active centre was most effective in slightly higher loadings (of up to 6 atom %).

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

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