Controlled Synthesis And Performance Study Of Ammonium Phosphomolybdate And Tin Phosphate Based On Liquid Phase Friedel-Crafts Alkylation Reactio

Friedel-Crafts alkylations are very important reactions in organic chemistry,and the products are key industrial intermediates in the production of petrochemicals,pharmaceuticals,fine chemicals,and polymers.And they are typical acid-catalyzed reactions.The catalysts used in these reactions are often homogeneous catalysts,which is not in line with the development of green catalytic process.Phosphates solid acids are excellent catalysts to replace traditional liquid acids,but the phosphates prepared by general method have some disadvantages such as weak acidity,low acid sites and instability,which limit their industrial development.In this paper,ammonium phosphomolybdate and tin phosphate catalysts with good stability and excellent catalytic activity were prepared.And their catalytic activities for alkylations were investigated.Furthermore,the acid properties of the catalysts and the mechanism of the alkylation reactions were proposed.Firstly,hydrothermal method was used to prepare ammonium phosphomolybdate catalysts.The structure and acidity of the catalysts were modified by the addition of Pluronic F127.The catalysts were characterized by XRD,TG,NH₃-TPD,DRIFT and FT-IR,and the effect of F127 on the thermal decomposition behavior of ammonium phosphomolybdate was investigated.It was found that F127 contributed to the assembly of heteropoly anions with counter-ions in a hydrothermal process via the interaction between heteropoly anions and the protonated EO blocks of F127.Ammonium ions and the hydronium ions generated by protonation could be uniformly distributed around heteropoly anions.The removal of F127 made the decomposition and oxidation of ammonium ions occur at lower temperatures before the decomposition of heteropoly anions.Protons were formed due to the decomposition of ammonium ions,thereby leading to the formation of acidic ammonium phosphomolybdate solid solution.Under an optimum amount of F127,the obtained catalyst showed excellent catalytic performance in the alkylation of toluene with benzyl alcohol with an activity of 164.9 mmol·g^-1 h^-1.More importantly,this catalyst was found to have good stability and recyclability in liquid-phase reactions.Secondly,the synthesis of mesoporous tin phosphate was explored,and the effects of Sn/P ratio on its pore structure and acidic site were studied.The catalytic properties for the alkylation reaction of o-xylene with benzyl chloride were also investigated.It was showed that the mesoporous tin phosphate particles synthesized by using F127 as structure-directing agent consist of nanospheres with about 7 nm in diameter,resulting in mesoporous structure with the pore size of 3-8 nm.The characterizations of NH₃-TPD and DRIFT showed that the Sn/P ratio had a significant effect on the Lewis acid sites of the catalyst surface.With the increase of Sn/P ratio,the amount of Lewis acid sites increased and then decreased,reaching a maximum at a Sn/P ratio of1.The results of the alkylation reaction showed that the conversion of benzyl chloride increased significantly with the amount of Lewis acid sites.The catalyst was reused five times with only a slight decrease in reaction conversion and no significant loss of catalyst acid sites,showing good prospect for industrial application.Finally,Molybdenum was introduced into the structure of tin phosphate to further modify the acidity of mesoporous tin phosphate.And the effect of molybdenum doping on the acidity of the catalysts as well as their recycling performance in the alkylation reaction was investigated.XRD,TG,BET,DRIFT and FT-IR were used to study the changes of the used catalyst.The reaction mechanism of the alkylation of toluene with benzyl chloride and the properties of the acid sites in the catalyst were explored.Results showed that the deactivation of the catalyst was mainly caused by the difficulty in desorption of reaction products due to strong acid strength.While the introduction of molybdenum could reduce the surface acid strength of mesoporous tin phosphate and increase Lewis acid sites,which was conducive to the desorption of reaction products,and improved the recycling ability of the catalyst with high catalytic performance.