Catalytic properties and deactivation behavior of H-MCM-22 in the conversion of methanol to hydrocarbons

The catalytic properties of H-MCM-22 zeolites with Si/Al ratios in the range from 13.5 to 47 and their dealuminated samples were investigated in the conversion of methanol to hydrocarbons. The fresh and spent samples were characterized with XRD, NH3-TPD, pyridine-adsorption infrared spectroscopy, Al-27 MAS NMR spectroscopy, TG-DTA, GC-MS, diffuse reflectance UV-vis, Raman spectroscopy techniques and nitrogen and p-xylene adsorption experiments. Although a decrease in protonic acid sites, as expected, increased the selectivity to light olefins such as propene and butene, the sample with a Si/Al ratio of 25-37 showed higher catalytic stability. It is worth noting that the external surface acid sites have a pronounced effect on the catalytic performance. Selective removal of these acid sites by dealumination with oxalic acid markedly increased the catalytic stability. This is because the acid sites in the supercages and external side pockets quickly induced formation of deposited coke species, which blocked the sinusoidal channels and caused the deactivation of the catalyst.