Synthesis Of ZSM-5@Silicalite-1and Its Application In Conversion Of Methanol To Propylene

ZSM-5is one of the most effective catalysts for the Methanol to Propylene Reaction (MTP), exhibiting high propylene selectivity and long catalyst life-span. However, coke deactivation occurs rapidly due to a high concentration of acid centers on the outer surface. Fortunately, coating ZSM-5by Silicalite-1is an effective means of extending the catalyst life-span because Silicalite-1blocks external zeolite acid sites and does not offer any acid catalyst activity itself. In this paper, we first studied the feasibility of coating a zeolite with the MFI topology with more zeolite of the same topology to obtain a MFI@MFI structure. Coating was performed by a hydrothermal method in a TPAOH system. Followed this initial work, we finely designed and synthesized catalysts (ZSM-5@S-1, ZSM-5, S-1@ZSM-5) with different distributions of aluminum atoms. The three catalysts were successively applied in the methanol to propylene reaction to study the effect of aluminum distribution on catalyst performance.In the MFI@MFI coating feasibility experiments, hollow silicalite-1zeolite (HolS-1) was used as probe seeds, followed by growing a Silicalite-1layer on the outer surface. Experimental results show that HolS-1has an obvious cavity structure. During HolS-1@S-1coating experiments, when HolS-1seeds were added in an amount of3%, the shell precursor grew dependently. With the amount of HolS-1increased, the proportion of the shell precursor growth around the seed decreased; meanwhile, TEM images show the products partly of solid crystal and partly of hollow crystal. Continually increasing the amount of excess seeds by50%, the shell precursor completely grew around HolS-1, and the independent nucleation of the shell precursor is inhibited completely; meanwhile, TEM images show the products all with a hollow structure, and the wall thickness varied.Three catalysts were used for the MTP reaction, each having a similar Si/Al ratio but a different surface distribution of aluminum. As the outer surface Si/Al ratio increased, the amount of acid sites on outer surface decreased. During the catalytic reaction of methanol to propylene, the life-span of the catalyst increased as the amount of acid sites decreased on the outer surface. The life-span of S-1@ZSM-5is only44h, whereas ZSM-5@S-1extended to139h. During the reaction, methanol first diffused into the central pore, followed by generating an active organic intermediate species or further diffusing outside the channel. A portion of the organic intermediates diffusing out of the channel pore would further react on outer surface acid sites. Due to the lack of channel restrictions, undesired isomerization or graphitization reactions become more likely. ZSM-5@S-1, compared to ZSM-5or S-1@ZSM-5, has a lower amount of acid sites on the outer surface, and the improvement in life-span is due to the reduced number of external acid sites reducing the occurrence of side reactions.