The Study On Factors Of Carbon Deposition And Modification On ZSM-5 Catalyst As Alkylation Of Benzene With Methanol

The benzene-methanol alkylation reaction can improve the current situation of overcapacity of benzene and methanol,and produce toluene and xylene with high economic value.ZSM-5 molecular sieve comes frequently carbon deactivation when catalyzing the alkylation of benzene-methanol.Therefore,based on the experimental conditions and molding conditions,the influence of carbon on the molecular sieve carbon content is discussed,and the optimal ratio is determined.Metal oxide modified molecular sieves was investigated for their physicochemical properties,alkylation activity,and carbon deposition to determine the optimum modified molecular sieve.This paper investigated the effect of the ratio of benzene and methanol on the molecular sieve carbon.It is found that methanol is preferentially adsorbed at the strong acid sites of the molecular sieve and it will react spontaneously,resulting in graphite-type carbon deposition,while benzene is almost inactive when adsorbs on molecular sieves.Reducing the methanol content of the raw material ratio will slow down the carbon deposition rate of the molecular sieve,but the alkylation activity is poor when the methanol content is too low.It was considered that the ratio of n?CH₃OH?:n?C₆H₆?=1:1.2 is the best.On the other hand,the silica-alumina ratio of the molecular sieve has a great influence on the carbon deposition amount.When the silica-alumina ratio of the molecular sieve is high,the acid strength is low,the acid density is small,the carbon deposition amount is low,and the stability is good,so from the viewpoint of carbon deposition,high silica to alumina ratio molecular sieves should be chosen.The deactivation of molecular sieve carbon is related to the amount of acid and the amount of mesopores.In this paper,ZSM-5 molecular sieves modified with the high silicon to aluminum ratio of nitric acid were used to modify ZSM-5 molecular sieve from the two points of nitric acid treatment concentration and treatment temperature.The results showed that when the concentration of nitric acid is increased,it will interact with the surface and the silica-alumina species in the pores,resulting in smooth surface of the molecular sieve,decreased crystallinity,increased acidity,and increased mesoporous pore volume.However,molecular sieve treated with too high concentration of nitric acid will lead to etching.The acid center is reduced and the alkylation activity is poor.Considering that the 3 mol/L nitric acid treatment is best,the corresponding catalyst has a benzene conversion rate of 15%higher than the original powder;when the molecular sieve is treated with nitric acid in the autoclave,The nitric acid penetrates from the surface to the pores,so that the lamellar structure of the molecular sieve is exposed,and the mesoporous pore volume is increased.When the treatment temperature is 110°C,the mesoporous pore volume is three times that of the original powder,and the path of the product diffusing out of the pore is short,and high alkylation activityIn this paper,the influence of the additives used in the molding on the carbon deposition of the catalyst was investigated.It was found that the pseudo-boehmite would cover the L acid sites of the molecular sieve,which led to the increase of B/L acid and the decrease of the acid amount,while the active sites of benzene and methanol are mainly B acid.Therefore,the higher the content of boehmite,the higher the carbon content,but the lower the boehmite content is,the lower the molding strength is;the dilute nitric acid solution enhances the force of the molecular sieve and the pseudo-boehmite,and improves the molding effect.However,the excessive amount of dilute nitric acid used will make the acid content of the molecular sieve high,resulting in high carbon deposition;the sesbania powder has almost no influences on the acid amount and carbon deposition of the ZSM-5 molecular sieve.From the perspective of carbon deposition,the optimal ratio is molecular sieve:pseudo-boehmite:sesbania powder=1:0.25:0.375 mass ratio,molecular sieve and nitric acid solid-liquid ratio of1.3:1.ZSM-5 molecular sieves are modified by equal volume impregnation method with different kinds of metal nitrates.In the range investigated,catalyst modified by the strontium nitrate has the lowest acid amount,high alkylation activity,low carbon deposition and stable operation.The properties are good,and the optimum amount of activity is achieved when the amount of lanthanum nitrate added is n?Sr?/n?Al?=5%.ZSM-5 molecular sieve powder treated with 3 mol/L nitric acid and 0.5 mol/L nitric acid solution for 6 h at constant temperature of 110°C was extruded,and its activity and stability were investigated at high space velocity.The activities of the two shaped molecular sieves are high.In the original powder,the molecular sieve modified in the autoclave has low acidity,high alkylation activity and good stability.The high-temperature nitric acid-modified shaped ZSM-5 molecular sieve was further modified by supporting cerium oxide.The composite modified molecular sieve maintains good activity and stability during high-altitude operation,so the composite modification was carried out at a space velocity of 15 h^-1.The molecular sieves were evaluated for stability,which maintained a high conversion rate of 46%in the first 50 h,but are inactivation at 75 h due to a large blockage of the micropores.