Conversion of ethylene into propylene with the siliceous SSZ-13 zeolite prepared without an organic structure-directing agent

Abstract

The SSZ-13 zeolite was synthesized via seed-assisted method without an organic structure-directing agent (OSDA) from sodium silicate, sodium aluminate, and seed crystals using a wide range of silica/alumina ratios (SARs) in the precursor gels. The SAR of the SSZ-13 was quite high (10) when the SAR of the precursor gel was 40, which was the optimum ratio for high crystallinity. The zeolite was steamed at a high temperature to further increase the SAR via dealumination. The crystal structure of SSZ-13 was stable during steaming because of its high SAR, quite different from another SSZ-13 zeolite obtained from the conversion of zeolite Y in K+ ion in the absence of an OSDA. Steaming at 800 °C and further washing with acid led not only to an increase in the SAR up to 21, but also to a steady increase in porosity. However, the concentration of acid sites, especially strong ones, decreased steadily with increasing steaming time, and octahedral aluminum was removed successfully after adequate steaming. Both pristine and steamed SSZ-13 zeolites were applied to the ethylene-to-propylene (ETP) reaction. With increased steaming time, both the stability of the ethylene conversion and the maximum propylene yield increased. Compared to conventional SSZ-13 zeolites synthesized using expensive OSDAs, adequately steamed SSZ-13 zeolites were competitive for catalyzing the ETP. The exceptional performance of the SSZ-13 zeolite obtained in this study may be understood in terms of its adequate SAR and the decreased number of acid sites (especially strong acid sites), which might be active for hydrogen transfer and ethylene oligomerization. Therefore, it could be suggested that the SSZ-13 zeolite synthesized without an OSDA can be effective in the ETP reaction when steam-treated under suitable conditions.