Study On The Reaction Pathway Of C₄⁼/C₅⁼ Catalytic Cracking To C₂⁼/C₃⁼

Catalytic cracking of butene/pentene and methanol to olefins are important means of producing ethylene/propylene.The current research focuses on how to increase the ethylene and propylene yields of the reaction process.At present,the strategy is mainly to adjust the acid strength of the zeolite and optimize the reaction process conditions,but the effect is still far from the theoretical value.The paper aims at the yield of ethylene and propylene.Firstly,the mechanism of the catalytic cracking of butene/pentene and its existing problems were analyzed systematically and the optimized reaction mechanism of butene was proposed.The source of the hydrogen transfer reaction and the factors affecting the ethylene/propylene yield in the catalytic cracking of butene/pentene were further investigated.Secondly,through the further understanding of the MTO daul-cycle mechanism and the discussion of the range of product P/E changes,MTP catalysts for strengthening propylene formation were developed.The main results obtained are as follows:1.The reaction mechanism of butene was analyzed systematically.Combined with the results of catalytic cracking of butene/pentene,it was found that the yield of pentene and propylene in the butene cracking product did not change with the change of molecular sieve acid strength.The reason for the analysis is that the dimerization of the cracking product pentene in the butene cracking process directly produces the hydrogen transfer precursor isobutylene.The precursor is retained in a large amount in the weakly acidic zeolite to reduce the effective conversion of butene cracking(butene of all systems is considered to be a reactant),further reducing the yield of ethylene and propylene;The precursor reacts in a strong acidic zeolite to form isobutane,thereby reducing the selectivity and yield of ethylene and propylene.Therefore,the cracking mode of the cracking product pentene is the core of the butene dimerization cracking reaction,and based on this,an optimized butene cracking reaction path is proposed.The rationality of the optimized butene cracking reaction pathway was further verified by the catalytic cracking process of 1-pentene.2.Efficient catalysts were designed based on the optimized butene cracking route:De-TS-Z-H~+with weaker acid strength than Z-1 is introduced by physically mixing molecular sieves,thereby achieving the purpose of retaining the cracking product pentene as much as possible in the dimerization of butene.Then,using FER molecular sieve as catalyst,the pentene single molecule is cleaved to produce ethylene and propylene,which improves the yield of ethylene and propylene in butene cracking.3.Based on the special catalytic effect of De-TS-Z-H~+in the catalytic cracking of butene/pentene,Combined with the analysis and understanding of the double-cycle mechanism of the MTO process,a high-efficiency MTP catalyst for strengthening propylene formation was developed:De-TS-H~+.The preliminary reaction results showed that the P/E of the product reached 11.15,the propylene selectivity increased to 65.63%,and the selectivity of non-olefin by-products was as low as 1.63%.