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

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 %)

A HYSCORE investigation of bimetallic titanium–vanadium microporous catalysts: elucidating the nature of the active sites

Vanadium and titanium bimetallic AlPO-5 molecular sieves have been synthesized and characterized by means of Electron Spin Echo detected EPR and Hyperfine Sublevel Correlation (HYSCORE) spectroscopy. Direct evidence for framework substitution of redox-active Ti ions and VO2+ units at Al sites is provided through the detection of large P-31 hyperfine couplings

Toward Understanding the Catalytic Synergy in the Design of Bimetallic Molecular Sieves for Selective Aerobic Oxidations

Structure–property correlations and mechanistic implications are important in the design of single-site catalysts for the activation of molecular oxygen. In this study we rationalize trends in catalytic synergy to elucidate the nature of the active site through structural and spectroscopic correlations. In particular, the redox behavior and coordination geometry in isomorphously substituted, bimetallic VTiAlPO-5 catalysts are investigated with a view to specifically engineering and enhancing their reactivity and selectivity in aerobic oxidations. By using a combination of HYSCORE EPR and <i>in situ</i> FTIR studies, we show that the well-defined and isolated oxophilic tetrahedral titanium centers coupled with redox-active VO²⁺ ions at proximal framework positions provide the loci for the activation of oxidant that leads to a concomitant increase in catalytic activity compared to analogous monometallic systems

Toward understanding the catalytic synergy in the design of bimetallic molecular sieves for selective aerobic oxidations

Structure–property correlations and mechanistic implications are important in the design of single-site catalysts for the activation of molecular oxygen. In this study we rationalize trends in catalytic synergy to elucidate the nature of the active site through structural and spectroscopic correlations. In particular, the redox behavior and coordination geometry in isomorphously substituted, bimetallic VTiAlPO-5 catalysts are investigated with a view to specifically engineering and enhancing their reactivity and selectivity in aerobic oxidations. By using a combination of HYSCORE EPR and in situ FTIR studies, we show that the well-defined and isolated oxophilic tetrahedral titanium centers coupled with redox-active VO2+ ions at proximal framework positions provide the loci for the activation of oxidant that leads to a concomitant increase in catalytic activity compared to analogous monometallic systems