Fabrication Of Highly Mesoporous ZSM-5 And Its Catalytic Performance In MTH Reaction

To meet the increasing demand of liquid fuel and basic chemicals including olefins and aromatics in China,petroleum plays an important role in the energy supply chains and chemical industry.With the gradually decreasing storage of petroleum and the frequent fluctuation of oil price,it is urgent to find a new green and effective technique to synthesize liquid fuel and hydroncarbons,which is also a significant option to optimize the China's energy structure and reduce the dependency of petroleum resources.Methanol to hydrocarbons reaction?MTH?is the process of the conversion of methanol to alkanes,alkenes and aromatics over acidic zeolite catalysts.ZSM-5 has been widely used in MTH reaction due to its tunable acidity,high selectivity and good thermal stability.Unfortunately,the limited diffusion properties of micropores in ZSM-5 is a key reason for the inactivation.Because methanol converts into coke precursors over the acid sites on the micropores and the coke precursors are constrained in micropores to further react to form coke.Therefore,the improvement of diffusion properties is a key fact to modify the catalyst activity.Alkaline treatment has been wildly used and proven to be an effective and simple method to fabricate mesopores.However,traditional alkaline treatment always causes the low mesoporosity because of variable effects of many factors.In this work,both the micro-scale and nano scale ZSM-5 were treated by mixed alkaline solution to improve the mesoporosity.The structure-activity relationship between mesoporosity and catalystic performance was also investigated.Firstly,based on commercial micro-scale ZSM-5,the introduction of TPAOH realized the fabrication of high mesoporosity with even mesopore distribution and centered pore size.Combined with the characterization results,the mesopore formation mechanism and the effect of external surface area on catalytic lifetime were determined.Then,the nano-scale ZSM-5 was treated by mixed alkaline solution of NaOH and Al?NO₃?₃.The introduction of Al³⁺ inhibited the formation of hollow crystals and facilitated the fabrication of mesopores in bulk body of ZSM-5.Meanwhile,the acidity was increased with the increasing concentration of Al³⁺ in the alkaline solution.Coupled with the MTH catalytic performance,the effect of external surface area loss and acidity on the catalytic lifetime and products distribution was inverstigated.The main conclusions in this work are made as follows:?1?The introduction of TPA⁺ facilitated the formation of large number of even distributed mesopores with centered pore size in body of micro-scale ZSM-5 crystals.With the increase of TPA⁺/Na⁺ ration,the external surface area of treated samples increased to the maximum of 210 m2·g-1 and then decreased.Based on the characterization results of ICP-AES,XPS and 29 Si NMR,the adsorption sites of TPA⁺ on ZSM-5 were mainly located on the Si?4Si,0Al?sites.The co-effect of protective desilication from TPA⁺ and preferential desilication near Na⁺ on the surface accounted for the formation of ZSM-5 wih high mesoporosity.Contributed to the enlarged external surface area,the TPAOHaided desilicated micro-scale ZSM-5 possessed long MTH lifetime of 102 h,nearly twice as that of ZSM-5 treated by NaOH solution.?2?The introduction of Al?NO₃?₃ inhibited the formation of hollow nanocrystals and facilitated the mesopore fabrication on the nano-scale ZSM-5,which was benefit to the high mesoporosity fabrication.Compared with the traditional NaOH alkaline treatment,the external surface area of nano-scale ZSM-5 treated with low concentration Al³⁺ alkaline solution was further increased to 144 m2·g-1.With the increasing Al³⁺ concentration in the alkaline solution,the external surface area began to decrease.Meanwhile,the acid amount of Al-aided treated ZSM-5 increased with the increasing Al³⁺ concentration in alkaline solution as well as the acid strength.Combined with the MTH catalytic test,the external surface area loss determined the catalytic lifetime.High acid amount and strong acid strength accelerated the coke formation and contributed to the high selectivity to aromatics.Besides,the ZSM-5 catalyst with large amount of acid sites distributing on the surface restricted the secondary reactions of light hydrocarbon products,leading to the higher selectivity to i-paraffins than that to aromatics.