Harnessing inactive molybdenum species in ill-defined catalyst to work in propylene metathesis by propane pretreatment

Rising imbalance in olefins demand and searching for more selective way of olefins production leads to increase importance of the on-purpose technologies. A special role plays olefin metathesis that (due to its reversibility) can be used as a switching reaction for compensation imbalance between produced olefins in the large-scale petrochemical processes. In this work a simple and effective procedure for generation of active in olefin metathesis sites over molybdenum oxide catalysts containing between 1 to 20 wt% of MoO 3 prepared by a simple incipient wetness impregnation of silica has been investigated. Compared with inert gas pretreatment a significant improvement (1–2 orders of magnitude) in propylene metathesis activity at 50 °C was achieved after pretreatment in propane between 550 to 650 °C. Physicochemical studies of the catalysts before and after the processes, in-situ UV–Vis analysis during activation, and temperature-programmed surface reaction studies allowed the identification of two distinct stages (clearly visible above 550 °C) during catalyst heating in a propane stream. In the first stage, partial reduction of Mo(VI) occurs simultaneously with the formation of propane reduction products, and as the reduction progresses, propylene is formed through propane dehydrogenation. The second stage, propylene aromatization, takes place on a deeply reduced catalyst. The metathesis activity increases significantly during the reduction stage, reaching its maximum value precisely when the aromatization stage begins. Possible pathways of Mo active sites generation at high temperature in the presence of propane were proposed based on temperature-programmed surface reactions and DFT calculations. • Integration of dehydrogenation with metathesis for flexible production of olefins • Simplify way for significant enhancing ill-defined Mo-based catalysts activity • Carbene active sites generation from Mo reduced species in propylene presence • 1–2 order magnitude metathesis activity achieved by propane pretreatment >550 °C