Application Of SSZ-13 Molecular Sieve In MTO Reaction

Methanol-to-Olefins (MTO), which enabled the transformation of coal or natural gas to olefins via methanol synthesis, had centralized the world’s attention for its potentiality in taking the place of petroleum to produce light olefins. SAPO-34 molecular sieve had turned out to be a superior catalyst for the MTO reaction. This was due to the combination of a moderate acid strength and the steric restrictions imposed by the small pore structure of SAPO-34. But, a large amount of phosphorus in the wasted SAPO-34 could result in potential phosphorus pollution problems. SSZ-13 zeolite was the aluminosilicate homologue of SAPO-34 with CHA topology and had the same shape selectivity as SAPO-34 in the MTO reaction. It had been proposed to be a very promising candidate as the catalyst for MTO reaction.H-SSZ-13 with high Si/Al and low Si/Al were prepared hydrothermally with N,N,N-trimethyladamantammonium hydroxide (TMAdaOH) as the templating agent. The samples were characterized by XRD, SEM and NH3-TPD. The catalytic peformance of H-SSZ-13 in the MTO reaction was investigated. The MTO results showed that the selectivity to propane was high in the MTO process. In order to increase the light olefins selectivity, the SSZ-13 zeolite was modified and the MTO reaction conditions were optimized. The method of carbon deposition covering was used to modify the high Si/Al zeolite. Compared with the fresh SSZ-13, the decrease of the selectivity to propane on the modified catalyst in the beginning of the MTO reaction was up to 15.7%, the increase of the average selectivity to light olefins could be 5.8% and the increase of the time with 100% methanol conversion could be 5 minutes. The method of metal modification was used to modify the low Si/Al zeolite. The Mn/Mg-SSZ-13 zeolite showed improved light olefins selectivity and enhanced life. The light olefins selectivity could increase 14.4% and the methanol capacity could increase 2.19g. It indicated that the increasing of temperature, space velocity, diluent water, could obviously reduce propane selectivity and improve the light olefins selectivity. The best catalytic performance with average light olefins selectivity of 86.93% and methanol capacity of 5.28 g was observed when the conditions on the MTO reaction over H-SSZ-13 were with the reaction temperature of 400℃, Weight Hourly Space Velocity (WHSV) of 20 h-1，m(CH3OH)/m(H2O)=50/50.