Enhanced Catalyst Performance of Zeolite SSZ-13 in the Methanol to Olefin Reaction after Neutron Irradiation

Abstract

Postsynthesis treatments are frequently used as a means of improving the performance of zeolitic catalysts. This study is concerned with the postsynthesis modification of zeolite H-SSZ-13 by neutron irradiation. An enhanced catalytic activity in the methanol to olefin (MTO) reaction is observed after neutron irradiation of the zeolite H-SSZ-13 catalyst. The magnitude of the neutron irradiation effect depends on the irradiation time and the Si/Al ratio of the parent material, as the highest increase in catalytic activity was observed for the irradiated high silica H-SSZ-13. FT-IR measurements with CO as a probe molecule revealed no significant change in the nature of the acid sites, and the acid strength of the catalytically important Brønsted acid sites was maintained after irradiation. Nitrogen physisorption measurements indicated a decrease in specific surface area and pore volume after neutron irradiation, whereas XRD confirmed that the crystallinity was maintained. The water content was found to be lower in the irradiated material, indicating an altered environment within the zeolite voids after irradiation. EPR measurements clearly showed the formation of radicals in consequence of the irradiation procedure. These radicals have been found to be stable in air, in daylight, and after heating to the MTO reaction temperature, and they were still detectable two years after irradiation. We suggest that the radical species consist of peroxy radicals (Si−O−O*) and nonbridging oxygen hole centers (NBOHC: ≡Si−O*) that are formed due to irradiation-induced bond rupture, and we further suggest that radiation-induced structural defects are responsible for the altered catalytic properties of zeolite H-SSZ-13.