Preparation Of Co-based ZSM-5 Catalyst And Performance Of Acid Dehydrogenation To Ethane

Ethylene is an important component of the petrochemical industry and is used to produce a variety of chemical products,but the current production capacity cannot meet normal consumer demand.So ethane dehydrogenation to ethylene technology has a very important role.It has been found that Lewis-Br(?)nsted acid pair has better dehydrogenation activity than single Lewis acid.Currently,studies on ethane dehydrogenation catalysts only focus on single Lewis acid site,and the ethane dehydrogenation catalytic effect is not good.Therefore,constructed Br(?)nsted-Lewis acid double-site synergism to improve the catalytic performance of ethane dehydrogenation catalyst.The acid concentration of ZSM-5 zeolite is related to the concentration of aluminum in the crystal lattice.In order to study the specific embodiment of the action of Br(?)nsted-Lewis acid double-site synergism in the dehydrogenation of ethane,we investigated the effects of Br(?)nsted acid and Lewis acid on the catalytic performance of ethane dehydrogenation by dealuminizing the catalyst,changing the Si/Al ratio and cation type of the zeolite.Firstly,the dealuminization of ZSM-5 molecular sieve was carried out.It has been reported that dealuminization can well regulate the acidity of molecular sieve and the relationship between B acid and L acid.After dealuminization,Al will be removed from the skeleton,so the amount of B acid will be reduced accordingly.The results of OH-IR characterization showed that the peak corresponding to the "T" site would appear after dealuminization.When the load increases and no additional "T" sites enter,Co will form corresponding Co oxide species on the surface of the molecular sieve.Therefore,the activity increased to a certain extent when the loading capacity was low,but when the loading capacity was high,there was little difference before and after dealuminization,and the optimal catalyst conversion rate of ethane was 40% and selectivity of ethylene remained at 88% after dealuminization.The by-product methane mainly comes from the cracking products of cobalt metal from the reduction of large size cobalt oxide obtained by impregnation.In order to solve the side reaction caused by easily reduced cobalt oxide obtained by impregnation method,Co@Na ZSM-5 catalysts with different Si/Al ratios were prepared by in-situ hydrothermal synthesis.Ethylenediaminetetraacetic acid was used as cationic ligand to prevent Co ion precipitation in the hydrothermal process,and a series of1Co@Na ZSM-5 catalysts with different Si/Al ratios were prepared.It was found that the reaction activity decreased with the increase of Si/Al ratio.Among them,1Co@Na ZSM-5-50 had the best activity,with a conversion rate of ethane is 40%,and the selectivity of ethylene is greatly improved to 98%.But when the Si/Al ratio increases to 300,there is almost no activity.This indicates that the active component is not only the L-acid produced after the introduction of Co,but also almost no activity when the amount of Bacid is very low.It is proved that the dehydrogenation of ethane to ethylene is a combination of B acid and L acid.In order to further improve the conversion of ethane dehydrogenation catalyst,the synthesized 1Co@ZSM-5-50 molecular sieve was ion exchanged with sodium chloride and ammonium chloride solutions for different times.It was found that ammonium chloride had the best activity after a single exchange,with a conversion rate of 46% for ethane and a selectivity of 90% for ethylene,followed by the coexistence of the original synthesis of Na and H,and finally,after the exchange of H and Na for three times,the activity decreased sharply.Therefore,only when the ratio of B acid and L acid is appropriate,there will be a good catalytic activity.