Preparation And Catalytic Properties Of Catalysts For Shape-Selective Alkylation Of Toluene With Methanol

Para-Xylene is an important essential organic material for the petrochemical industry, and it is mainly used to produce terephthalic acid (PTA) and terephthalates (DMT). Toluene (T) alkylation with methanol (M) to produce para-Xylene with high concentration is a new technologic route to increase para-Xylene. The influence factors on activity, para-selectivity and stability of catalysts are investigated from aspects such as the type of zeolites and binders, modification, reaction conditions and so on. The effect of pore structure and acid sites on the selectivity and coking of catalysts is analyzed in this work.The physicochemical property and the catalytic performance for toluene alkylation with methanol of microscale HZSM-5, nanoscale HZSM-5 and HMCM-22 are studied. The results indicate that HMCM-22 can result in further alkylation of toluene more easily comparing to HZSM-5 zeolites. The catalytic performance of modified MCM-22 and nanoscale ZSM-5 catalysts for toluene alkylation with methanol is compared. It is found that nanoscale ZSM-5 is easier to get high para-selectivity. Therefore, nanoscale ZSM-5 is a better parent zeolite for toluene alkylation with methanol.Nanoscale HZSM-5 is bounded with binders such as Al₂O₃, SiO₂, Al₂O₃/RE₂O₃ and SiO₂/RE₂O₃, respectively, and the modification of bounded nanoscale HZSM-5 catalysts is studied. The catalysts bounded with different binders all show good stability during alkylation of toluene with methanol. The nanoscale HZSM-5 catalysts bounded with Al₂O₃/RE₂O₃ as binder show better stability when the para-selective is high after modification with oxides. As the content of Al₂O₃ is higher, the coking of the catalyst is faster and the stability of the catalyst is lower. The nanoscale HZSM-5 catalysts which are bounded with 10-15 wt% Al₂O₃ and 10 wt% RE₂O₃ have better general properties. It can be concluded from the results of modification with oxides that the para-selectivity of nanoscale ZSM-5 can be improved with the decrease of acid strength and the ratio of Bronsted acid sites to Lewis acid sites. The catalysts impregnated with 3 wt% SiO₂, 5-10 wt% P₂O₅ and 3 wt% MgO show better catalytic properties for toluene alkylation with methanol.For the nanoscale ZSM-5 zeolite studied in this work, the total amount of acid sites decreases significantly and strong acid sites almost totally disappear with the hydrothermal treatment temperature higher than 500℃. High temperature hydrothermally dealuminated nanoscale ZSM-5 zeolite catalyst results in high catalytic activity for the alkylation of toluene with methanol and improves para-selectivity especially when the catalyst is treated at 700℃, but the catalytic activity is very low for alkylation of ethylbenzene with ethanol. It can be suggested that only a small part of the acid with acid strength +2.27≤H₀≤+4.8 seems to contribute to the alkylation of toluene with methanol. The crystal size, appropriate acid strength and right B/L acid site ratio of the steamed nanoscale ZSM-5 may be responsible for achieving the maximal values of both toluene conversion and para-xylene selectivity in toluene alkylation with methanol.The coking and catalytic properties of modified ZSM-5 zeolite catalysts by introducing water into the reactor as cofeed along with the toluene/methanol feed during the alkylation reaction are studied. When appropriate water is introduced during toluene alkylation with methanol, all the toluene conversion, para-xylene selectivity and stability are increased; coking is restrained and the age of catalyst is prolonged; the coking speed and coke quantity are decreased with increased H2O/M. But both the toluene conversion and para-xylene selectivity are lower than that in conditions without water introducing.For nanoscale ZSM-5 catalysts bounded with Al₂O₃/RE₂O₃ (15 wt% Al₂O₃ and 10 wt% RE₂O₃) as binder, after modification with 3 wt% SiO₂, 5 wt% P₂O₅ and 3 wt% MgO, the toluene conversion can be maintained between 20-35% (average is 30.70%) for 500 hours on-stream with para-xylene selectivity higher than 83% (average is 85.92%) during toluene alkylation with methanol at atmospheric pressure, 460℃, molar ratio of Toluene/Methanol=2, 2 h^-1 (WHSV of the mixture of toluene and methanol), molar ratio of H₂/(Toluene and Methanol)=8 mol/mol and molar ratio of H₂O/(Toluene and Methanol)=8 mol/mol. The properties of the regeneration and magnified preparation of this catalyst are favorable. Therefore, it is showed potential in application in industry.