Active sites, kinetics, and inhibiting species in the catalytic dehydration of methanol over MIL-100(Cr)

Abstract

Metal organic framework (MOF) materials, especially those within the MIL category, have received extensive attention as acid and redox catalysts, with methanol dehydration commonly used as a probe reaction for deciphering acid site properties, and defect sites being invoked as active sites. In situ titrations employing pyridinic titrants, in combination with steady state and transient infrared spectroscopy measurements conducted under reaction conditions suggest that Brønsted acid-mediated dehydration of methanol can occur even in the absence of modulator-derived defect sites proposed previously to play a key role in dehydration catalysis. Steady state rate features, kinetic isotope effects, as well as steady state and transient infrared data can be rationalized using a dissociative scheme comprised of rate determining methanol–methanol dimer decomposition steps. Water inhibits dehydration rates under the conditions explored here, and its effects can be accounted for by invoking the presence of methanol–water dimers that assume greater significance at higher water to methanol ratios. Results and interpretations presented here obviate the need for proposing complex defect structures the formation of which is facilitated by the use of modulators, provide insights into reaction mechanisms prevailing over MOF surfaces, and suggest the need for caution prior to proposing MOF-mediated Lewis acid catalysis, especially under reaction conditions that produce water as a product.