Preparation And Catalytic Performance Of Highly Dispersed Metal Molecular Sieves

Benzyl alcohol is an important chemical intermediate,which can be used for the preparation of benzyl ester or ether.It is widely used in industry and in our life.It can be used for the preservative for ointments,plasticizer and the solvent for the paint.At present,there are many ways to produce benzyl alcohol.Benzaldehyde hydrogenation is a environmentally-friendly and energy-saving preparation method.The active components of catalysts for hydrogenation of benzaldehyde are palladium,platinum and copper,and palladium has high selectivity.MCM-41 is a kind of mesoporous molecular sieve with hexagonal structure.The pore channel is uniform and the specific surface area is large which can reach more than 1000 m2/g.The size of the pore size can be controlled well.Because of the pore wall made of amorphous SiO2,the thermal stability and hydrothermal stability is poor.The Si-O-Si framework has less acidic sites and low catalytic activity,which greatly limits its application in catalysis.At present,the main way to solve this problem is to introduce metal heteroatoms.The content of metal heteroatoms,the existence state of metal,the introducted method,and the species of heteroatom directly determine the properties of the molecular sieve,such as adsorption and catalytic activity.Based on this,the introduction of metal heteroatoms has great significance in improving the catalytic activity of molecular sieves and expanding the application fields.It is found that in the process of synthesis,two or more kinds of metal heteroatoms introduced into the molecular sieve at the same time has better catalytic activity than the molecular sieves with one kind of metal heteroatoms.In this paper,MCM-41 is used as carrier.Through the lattice location method,metal palladium and nickel are introduced into mesoporous molecular sieves,and the mesoporous molecular sieves are prepared with highly dispersed metal.In the early stage of synthesis,the synthesis conditions of molecular sieves were investigated.The samples were characterized by low angle XRD.Through compare the regularity of samples,and the optimum synthesis conditions were screened.Then prepare mesoporous molecular sieves with different metal contents.The samples were analyzed by XRD,HR-TEM,XRF,FT-IR and so on.In order to verify its catalytic activity,the molecular sieve was applied to the hydrogenation of benzaldehyde to benzyl alcohol.The catalytic conditions were investigated to obtain better catalytic effect.The main research results of this paper is as follows:(1)Palladium was introduced into the molecular sieve skeleton by the lattice location method.Then the metal ions in the skeleton were reduced in suit by hydrogen so that the reduced metal nanoparticles are highly dispersed on the internal surface of the molecular sieve.The samples were characterized and the results show that the prepared Pd-MCM-41 has a regular hexagonal channel.The pore size distribution is uniform(1.9-3.1 nm),the specific surface area is large(489?701 m2/g),and the metal palladium nanoparticles are well-dispersed in the inner surface of the molecular sieve channel.Under the conditions of 100 ? and 0.2 MPa,Pd-MCM-41 has good catalytic activity for the hydrogenation of benzaldehyde.The conversion rate reaches 60%and the selectivity is up to 100%under mild experimental conditions.The stability of the catalyst is better,and the conversion rate is reduced by 13%after 6 cycles.(2)Using lattice location method,the metal palladium and nickel were introduced into the molecular sieve framework,and a series of highly dispersed Pd/Ni-MCM-41 were prepared which the content of nickel was changed and the content of palladium was unchanged.The samples have uniform pore diameter,mainly concentrated in the 1.9-3.1 nm,large specific surface area(444-621 m2/g).The existence state of palladium is different from nickel in MCM-41.Palladium nanoparticles disperse on the inner surface of the channel,while nickel instead of silicon into the skeleton.Both of the metal are highly dispersed.Under the conditions of 100 ? and 0.2 MPa,the synthesized catalyst has better catalytic activity to benzaldehyde.Compared with the catalytic activity of the ono metal catalyst,the decrease of palladium and the increase of nickel increases the conversion rate,and the selectivity of it also reaches 100%.The catalytic activity is still good after 4 cycles.(3)In the process of synthesis of Pd/Ni-MCM-41,polyglucose was added into the reaction system and a series of Pd/Ni-MCM-41 catalysts modified by polydextrose were prepared though changing its dosage.Polydextrose changed the pore structure of MCM-41 and increased the conversion rate of benzaldehyde.At 100 ? and 0.2 MP.a,the conversion of benzaldehyde are all over 50%.