Time- and space-resolved study of the methanol to hydrocarbons (MTH) reaction - influence of zeolite topology on axial deactivation patterns

Zeolites representing seven different topologies were subjected to life-time assessment studies as methanol to hydrocarbons (MTH) catalysts at 400 degrees C, P(MeOH) = 13 kPa and P(tot) = 100 kPa. The following topologies were studied: ZSM-22 (TON), ZSM-23 (MTT), IM-5 (IMF), ITQ-13 (ITH), ZSM-5 (MFI), mordenite (MOR) and beta (BEA). Two experimental approaches were used. In the first approach, each catalyst was tested at three different contact times, all giving 100% initial conversion. The life-time before conversion decreased to 50% at each contact time was measured and used to calculate critical contact times (i.e. the contact time needed to launch the autocatalytic MTH reaction) and deactivation rates. It was found that the critical contact time is strongly correlated with pore size: the smaller the pore size, the longer the critical contact time. The second experimental approach consisted of testing the catalysts in a double tube reactor with 100% initial conversion, and quenching the reaction after 4 consecutive times on stream, representing full, partial, and zero conversion. After quenching, the catalyst bed was divided into four segments, which were individually characterised for coke content (temperature-programmed oxidation) and specific surface area (N-2 adsorption). The axial deactivation pattern was found to depend on pore size. With increasing pore size, the main source of coke formation changed from methanol conversion (1D 10-ring structures), to partly methanol, partly product conversion (3D 10-ring structures) and finally mainly product conversion (3D 12-ring structure). As a result, the methanol conversion capacity changed little with contact time for ZSM-5, while it increased with increasing contact time for the catalysts with smaller pore sizes, and decreased with increasing contact time for pore sizes larger than ZSM-5.