Methanol To Propylene Over B-Al-ZSM-5Catalysts

Propylene is a basic organic chemical material, is mainly used to synthesize chemical products such as polypropylene, epoxypropane, and so on. In our country, resource of coal is abundant, and conversion methanol from coal to propylene (MTP) has broad industrialization prospects. The current catalysts for the MTP reaction in fixed bed reactor are mainly ZSM-5zeolites. Studies suggest that incorporation of boron into ZSM-5framework can adjust the acid strength effectively, which motivates higher selectivity of propylene and longer catalyst life during the MTP reaction. We used B-Al-ZSM-5as the catalyst in a fixed bed reactor and studied the effect of silicon to alumina ratio, silicon to boron ratio, crystal size, scale up synthesis, and metal modification on the MTP reaction performance. We further investigated the effects of reaction temperature and catalyst regeneration conditions towards optimizing a catalytic process for the MTP reaction.We first investigated the impact of silicon to aluminum/boron ratio on MTP performance. Increasing the silicon to aluminum ratio caused the strong acid site amount to decrease while the weak acid site amount increased. Increasing of the silicon to boron ratio did not impact the amount of strong acid sites but decreased the amount of weak acid sites. Fixed bed catalyst evaluation results show that the appropriate acid proportions can guarantee catalyst activity, and can improve the propylene selectivity significantly. As the silica to alumina ratio increased, the selectivity of propylene increased from37%to over44%. As the ratio of silicon to boron increased, the propylene selectivity decreased.MTP catalyst stability and selectivity to propylene were significantly improved over nanoscale B-Al-ZSM-5zeolite compared with reaction over the microscale B-Al-ZSM-5zeolite. The catalyst with smaller crystal size has shorter pore diffusion paths and more pore onfices, allowing propylene to spread out more easily, avoiding the occurrence of secondary reactions, and reducing the amount of carbon deposition on the catalyst.MTP reactivity studies were performed using catalyst synthesized in a scaled-up process within a20L autoclave. Changing the synthesis conditions lead to differences in catalyst aging behavior. The catalyst from the scaled-up synthesis, after aging, produced more alkanes, C5and C6+(including C6) in product than fresh catalyst, causing the selectivity to ethylene and propylene to significantly decrease. Catalysts synthesized with different template to silica ratios, TPABr/SiO2=0.3or TPABr/SiO2=0.6, showed no difference in aging behavior during MTP. We used2wt.%La2O3,2wt.%CeO2and2wt.%Y2O3to modify the catalyst synthesized without aging and with TPABr/SiO2=0.6. The catalyst modified with CeO2was conducive to MTP reaction; catalysts modified with La2O3or Y2O3are not conducive to MTP reactions. Higher reaction temperature, over the specific range explored, favored for the generation of propylene.A catalyst synthesized under aging conditions at80℃, TPABr/SiO2=0.3was subjected to a long reaction period evaluation. The selectivity of propylene decreased gradually; after reaction for300h the conversion of methanol began to decline significantly, with conversion down to90%after reaction for380h. After the catalyst was regenerated at600℃and700℃, the selectivity of propylene reached the initial level of the fresh catalyst and the decline of selectivity of propylene slowed down significantly.