Catalysis with Dispersed Cation-Exchanged Polyoxometalates

Abstract

Heterogeneous catalysts are complex materials that present challenges in understanding their physical and chemical properties as well as improving their performance in chemical processes. The pursuit of catalysts with well-defined active sites that are stable under practical reaction conditions may lead to the improved fundamental understanding of catalyst function at the molecular level and enable the development of catalysts tailored for a specific reaction. Polyoxometalates (POMs) supported on a high surface area support are model transition-metal oxide catalysts possessing well-defined, isolated, and tunable catalytic sites that are active for both selective oxidation and acid catalysis. POM acid and redox properties may be systematically varied by the replacement of a portion of POM protons with other cations, such as copper or sodium.

The model catalyst system phosphomolybdic acid (H3PMo12O40) supported on fumed silica was used to evaluate the effects of cation exchange on POM properties and reactivity, for both oxidation and acid-catalyzed reactions. The as-prepared catalysts were characterized by a variety of techniques to confirm that the POMs were intact and well-dispersed on the silica surface and to quantify changes in POM reducibility and acid properties with cation addition. The interaction of POMs and the cations was evident by the variation of many POM properties with the incremental addition of counter-cations.

The effect of cation addition on the POM catalytic activity was evaluated using parallel pathways of methanol dehydration to dimethyl ether and oxidation to formaldehyde and its derivatives as probe reactions. The catalytic activity of POMs for both the dehydration and oxidation of methanol decreased dramatically as cations (Na, Mg, Cu, or Al) replaced protons. The measured oxidation and dehydration rates over Na, Mg, and Cu exchanged POMs were remarkably similar, and the differences in the cation charge had little impact. Al-exchanged POMs tended to have a higher quantity of acid sites and higher TOF at comparable extents of cation exchange compared to the other cations. The perturbation of POM acid sites, via the introduction of counter-cations, was determined to be responsible for the decrease in the rates for both dehydration and oxidation of methanol. Although cation exchange was demonstrated to alter the POM reducibility, this had no observable effect on the oxidation activity of the cation-exchanged catalysts. Rather, the quantity of acid sites per POM was the primary reactivity descriptor for both the dehydration and oxidation of methanol over POMs, perhaps due to the presence of a proton-mediated intermediate in both the methanol dehydration and oxidation pathways over POMs.

While cation addition does not result in the production of a more active catalyst, there are many exciting applications for a model catalyst with a well-defined and tunable structure. For example, activity coefficients were developed to describe the non-ideal behavior of POM acid sites with cation addition. The acid sites of a heterogeneous catalyst were described using activity coefficients for the first time, providing a framework for the treatment of ionic environments in heterogeneous systems using the rigorous thermodynamic formalisms usually applied to solutions.

This work demonstrates the merits of investigating catalysts that retain a well-defined structure under practical reaction conditions to elucidate the underlying mechanisms behind their catalytic functions and to evaluate the effects of the composition and structure of the catalyst active sites on the function of the catalyst.