Ordered mesoporous MCM-41 silicon oxynitride solid base materials with high nitrogen content: synthesis, characterisation and catalytic evaluation

Abstract

We present here the synthesis, physicochemical characterisation and catalytic evaluation of mesoporous silicon oxynitride materials. The solid base catalytic materials were prepared by subjecting pure silica MCM-41 mesoporous molecular sieves to nitridation, i.e., treatment with ammonia at high temperature. The mesoporous MCM-41 silicon oxynitride materials are well ordered and possess high surface area and pore volume. The nitrogen content was controlled within the range 8.6 to 25.1 wt% by simply changing the nitridation temperatures (between 850 and 1150 °C) or the time allowed for nitridation (from 20 to 80 hours). The nitrogen is incorporated onto the MCM-41 framework in the form of various NH_x species, including terminal NH₂ and bridging NH groups as well as adsorbed ammonium ions. The oxynitride materials were found to exhibit considerable catalytic activity for the Knoevenagel condensation reaction. The catalytic activity was correlated to the nitrogen content and was therefore highest for materials nitrided at high (1150 °C) temperature. In all cases, the nitrided materials were found to be generally stable with respect to storage and they suffered no significant structural degradation over a period of a few months. Oxynitride materials with the highest nitrogen content (i.e., those nitrided at temperatures ≥1050 °C) were found to be more susceptible to loss of surface area and pore volume after storage under ambient conditions for 6 months. The ease of preparation and control of the nitrogen content of mesoporous oxynitride materials and their stability makes them attractive as alternative solid base catalysts especially for large molecule transformations.