Synthesis Of MCM-22 Family Zeolites And Their Catalytic Performance In Alkylation Of Benzene With Isopropanol

Cumene is one kind of important organic industrial chemicals to synthesize phenol and acetone.Since phenol has a widespread application in many fields,including plastics, synthetic fibers,medicines,agricultural chemicals and dyes,the demand of cumene has also increased rapidly.Cumene is industrially produced through the alkylation of benzene with isopropanol.In recent years,the market of acetone has been saturated and the surplus acetone has become a backlog of byproducts.Hence,an effective route to utilize the backlog of acetone is desired,and in a view of market,a circulation process of acetone has been come forward.In this process,the acetone,which is produced by oxidation of cumene,is firstly hydrogenated into isopropanol,and then an alkylation of benzene with isopropanol is directly conducted.In this sense,the combined route is organically formed between the synthesis of cumene and the circulation of acetone, which is very essential to adjust the proportion of phenol and acetone in the market.Currently,the family of MCM-22 zeolties has show an outstanding catalytic performance in various process of catalysis.And the alkylation of benzene with ispropanol over MCM-22 zeolites is initially carried out in recent years.The relationship between the acid properties and the catalytic performance of MCM-22 should be systematically studied,and how to increase the acid sites on the external surface of MCM-22 and to further increase the positive effect of catalytic process related to macormoleculars have become one of most important questions to solve.The synthesis of MCM-22 zeoltes,including MCM-22,MCM-49,,MCM-56,MCM-36and ITQ-2 and their use in the alkylation of benzene with isopropanol has been described here.The relationship between the zeolitic structures and the catalytic performance is investigated in detail.MCM-36 zeolites have been synthesized with larger amount of acid sites on the external surface by using different precursors of zeolites,which have show an improved catalytic performance in the title reaction. Compared with conventional route,this kind of synthesis gives relatively low cost because only one swelling agent is used in our case.Various technologies have been used to characterize the composition,structure and acid properties on the surface.And a comparison with other kinds of zeolites has also conducted in this reaction.Firstly, MCM-22 has been investigated in the alkylation of benzene under different catalytic conditions.It is found that MCM-22 shows relatively high activity and selectivity at 483K～503K with a space velocity of 2h^-1～6h^-1 and as the increase in the ratio of benzene/alcohol,the conversion of benzene is decreased accompanied with the increase of the selectivity of cumene.Compared with other kinds of zeolites,it is found that MCM-22 gives a similarity in the catalytic performance with Beta.Both the activities and selectivities can be described in order as:MCM-36＞MCM-56＞MCM-49＞Beta＞MCM-22＞ITQ-2.Both MCM-49 and MCM-22 have similar crystal structures (multi-layered crystals),while MCM-49 has relatively high content of Al,thus higher concentration of Br(o|¨)nsted acid sites,and shows higher catalytic activity and selectivity than MCM-22.A similar structure is owned by MCM-56 and ITQ-2,both of which give a single layered crystal cell.ITQ-2 displays a decrease in the order of crystal structures and in crystal size after the treatment by swelling method and ultrasonic process. However,MCM-56 gives higher number of Br(o|¨)nsted acid sites than ITQ-2,thus showing a higher catalytic activity.Therefore,we conclude that the content of Al and Br(o|¨)nsted acid sites should be detrimental for the catalytic performance of these zeolitic materials with similar structures.The relatively high catalytic performance of MCM-36(although owning less content of Al and Br(o|¨)nsted acid sites) should be due to its large external surface and special mesopores.Since a good catalytic performance has been shown by MCM-36,the external and internal surface passivation has been carried out to investigate the structural and acidic properties of MCM-36 compared with MCM-22 and Beta.The results showed that the Br(o|¨)nsted acid sites are mainly located on the internal surface of H-MCM-22,while mainly located on the external surface of H-MCM-36.Both H-MCM-22 and H-MCM-36 zeolites are active for the alkylation of benzene with isopropanol.Comparing with H-MCM-22,H-MCM-36 showed higher activity and selectivity of cumene,which would be due to the fact that H-MCM-36 possesses more accessible Br(o|¨)nsted acidic sites than H-MCM-22.The more accessible acid should be arisen from the column-braced meosopores owned by MCM-36.In other words,the Br(o|¨)nsted acid sites located on the external surface of zeolites should be the main active center for the alkylation of benzene with isopropanol.Under a relatively low space velocity,MCM-22 and MCM-36 give a closely catalytic performance.Compared with Beta,both of them show relatively high stability under high space velocity.This result suggests that the external acid sites owned by MCM-22 and MCM-36 could promote the diffusion,where the propylene by dehydration of isopropanol could react with benzene to form cumene and suppress the polymerization of propylene.Hence,the relatively high stability of MCM-36 should be mainly ascribed to its mesoporous structures present for the easy diffusion of reagents and products.Further increasing the catalytic performance of MCM-36 for this reaction is conduct by a new route to increase the external surface and external Br(o|¨)nsted acidic sites on them. In this route,the zeolitic precursor of MCM-56(denoted as MCM-56(P)) has been used to synthesize MCM-36(denoted as MCM-36(56)).Under relatively high space velocity, MCM-36(56) and MCM-22(36)) give a relatively high catalytic stability,which should be due to the fact that the Br(o|¨)nsted acidic sites at the external surface are hard to deactivate and easy to promote such catalytic process. It is known that both CTAB and TPAOH are used as the swelling agents to synthesize MCM-36 by a conventional method,without a doubt,which causes some problems like high cost and low efficiency.In our present case,by using CTAB as the only swelling agent and controlling its pH value,MCM-36 with relatively high purity and high crystallinity has been synthesized from the zeolitic precursors of MCM-22(P)or MCM-56(P)in a short time.This method has effectively decreased the cost of the synthetic process.When PH value of the swelling agent is kept at 14,the obtained zeolite show a large number of mesopores and big external surface,and show high catalytic activity,seletivity and stability.