Synergistic Regulation Of ZSM-5 Pore Structure And Acidity For Preparation Of High Performance MTP Catalysts

Propylene as an important petrochemical raw material is used to prepare a variety of important downstream products.The development of methanol-to-propylene process with methanol as raw material can promote the efficient and clean utilization of coal,increase the proportion of non-oil-based propylene production,and reduce the dependence on oil imports,which is in line with China’s current energy situation,industrial structure adjustment and development plan and national economic security requirements.At present,the methanol to propylene（MTP）reaction in fixed bed reactor has the problems of product distribution regulation and catalyst stability.Through in-depth study and understanding of the catalytic mechanism,the performance of MTP catalyst can be further improved,and the understanding of the preparation and catalytic mechanism of heterogeneous catalysts is also beneficial.The low propylene selectivity and poor catalyst stability in MTP are mainly due to the strong acidity of ZSM-5 catalyst,which is beneficial to the formation of ethylene and aromatic hydrocarbons and the inhibition of the diffusion of carbon deposition precursors by microporous channels of the catalyst.The introduction of mesopores or macropores into the zeolite by alkali treatment can effectively improve the diffusion performance of the catalyst and adjust the acid properties,which is an effective means to improve the catalytic performance.Based on this method,a series of samples with different pore structures and acid properties were prepared by adjusting the ion distribution in alkaline solution.The catalyst system was characterized and the MTP reaction performance was tested,and the influence of catalyst performance on product distribution and carbon deposition rate was investigated.The main research results are as follows:The borosilicate MFI zeolite with good crystallinity was prepared by hydrothermal synthesis method,and its parent does not have MTP reaction activity.Al ions were added into the alkaline solution for post-processing,which can simultaneously realize the preparation of mesoporous by alkali desilication and the introduction of active center by deboron and aluminum supplement.The specific surface area,pore structure and acidity of the samples can be adjusted by adjusting the alkali concentration.When Na OH was treated alone,the mesoporous pore volume and strong acid content gradually increased with the increase of alkali concentration.The acid content of the sample treated with 0.10 mol/L Na OH was appropriate,and an appropriate amount of mesoporous was introduced,which had the highest propylene selectivity and the lowest carbon deposition rate.Long-term operation experiments were carried out on the catalyst BHZ-0.10A,which could run stably for 600 h under the condition of methanol space velocity=3 h^-1,and the propylene selectivity remained stable.The coke deposition rate of the deactivated sample was about 40%,and the carbon capacity was good.The hierarchical porous molecular sieves with different pore structure and acid properties can be prepared by the combined treatment of organic alkali and inorganic alkali.When the OH-concentration is constant,the sample obtained by the combined treatment has lower Si/Al,Si/B,total acid amount and weak acid amount than the sample treated by Na OH alone,and the propylene selectivity of the product in MTP reaction is higher.The molecular volume of TPA⁺is large and cannot enter the micropores of zeolite,which can protect the zeolite skeleton from excessive etching to a certain extent,and promote the removal of B atoms and inhibit the deposition of Al atoms.TPAOH single treatment cannot introduce mesoporous structure,but can remove most of B atoms in borosilicate zeolite.