Effect Of Hydrothermal Treatment With Sodium Hydroxide Solution On Framework Ti Local Environment Of TS-1 Zeolite And Gas-phase Epoxidation Of Propylene And H₂O₂

The gas-phase epoxidation of propylene and hydrogen peroxide to propylene oxide(G-HPPO)in the absence of methanol solvent is an attractive way to overcome the shortcomings of the well-known HPPO process.However,the G-HPPO route has to face the severe challenge of H2O2 self-decomposition which results in poor utility of H2O2.Inspired by the initial exploratory work following studies were conducted.First,a systematic investigation was made to find out whether the catalytic performance of a large crystal TS-1 zeolite could be significantly improved in the G-HPPO reaction by the hydrothermal modification with NaOH solution.In this section,emphases were put on the effects of the modification time and conditions,and the influence of different alkali metal hydroxides.Results showed that,the appropriate modification of the large crystal TS-1 zeolite with dilute NaOH solution remarkably slowed down the self-decomposition of H202 during the G-HPPO reaction.For example,when a TS-1 zeolite with average crystal size of 1×2×6?m was hydrothermally modified by the NaOH solution at optimized condition,it showed 63.8%H2O2 utility at the conditions of 110? and 4.69/1 propylene to H2O2 molar ratio.This value is exceeded the reported value in the literature,and is about 2.2 times higher than that on the parent TS-1 zeolite.The hydroxide solutions of other alkali metals,namely Li,K,Rb and Cs,have very similar hydrothermal modification effect on improving the catalytic performance of the large crystal TS-1 zeolite.The hydrothermal modificationwith NaOH solution was found to be a process of limited desilicication of zeolitic framework.It was accompanied by the introduction of considerable amount of sodium ions into the modified TS-1 zeolite.The pronounced change of the local environment of framework Ti was observed by the remarkable blue shift of the infrared absorption band related to framework Ti.Departure from this,tetrapropylammonium(TP+A)ion containing NaOH solution was employed to modify the large crystal TS-1 zeolite with the purpose of enhancing the zeolite recovery yield.In this regards,focuses were given to the effects of the hydrothermal treating time and the concentrations of TPA ion and NaOH.Special concern was also given to the applicability of the hydrothermal modification to the nanosized TS-1 zeolite when TPA+ ion containing NaOH solution was used.Results indicated that the appropriate modification of large crystal TS-1 zeolite with TPA+ ion containing NaOH solution could remarkably slow down the self-decomposition of H2O2 during the G-HPPO reaction.But the introduction of TPA+ion into the NaOH modification solution(0.05?0.15 mol TPA+ ion/L)lead to a dissolution-recrystalization process.Consequently,the recovery yield of the modified zeolite was notably increased.Meanwhile,the applicability problem encountered by the nano-sized TS-1 zeolite when NaOH solution was used alone was also overcomed.Besides,the introduction of considerable amount of sodium ions into the modified zeolite and the remarkable change of local environment of the framework Ti in the modified zeolite were all achieved in the initial stage of the hydrothermal modification or in even ealier room temperature pretreating stage,no matter the presence/absence of TPA+ions.However,significant elevation of the G-HPPO reaction performance of the modified TS-1 zeolite occurred in the later period of the hydrothermal modification.This phenomenon simply indicated the importance of the hydrothermal treating step and the hydrothermal treating conditions for the modification of TS-1 zeolite.In addition,the scaleup experiment was conducted by the hydrothermal modification of the large crystal TS-1 zeolite with NaOH solution in 1 L and 2 L autoclaves(three times each),the regulation of the sodium ion content in the modified TS-1 zeolite was achieved by sodium nitrate solution impregnation(sodium ion increase)and the ammonium ion-exchange(sodium ion reduction).The preliminary theoretical calculation study was also performed on the TS-1 zeolite hydrothermally modified by NaOH solution.Results showed that,the catalyst TS-1-NaOH-0.10(Na/Ti molar ratio=0.94),which was prepared by impregnating the NaOH solution scaleup hydrothermally modified large crystal TS-1 zeolite(Na/Ti molar ratio=0.68)with NaNO3 solution,exhibited 79.3%H2O2 utility together with 16.9%propylene conversion and 97.5%PO selectivity in the G-HPPO reaction at the conditions of 110? and 4.69/1 propylene to H2O2 molar ratio.It was suggested that the sodium ions in the modified TS-1 zeolite played a key role in regulating the local environment of the framework Ti and elevating the G-HPPO reaction performance.The preliminary theoretical study provided enlightening results which were in line with the experiments.