Regulation Of Acidic Properties Of ZSM-5 Zeolites And Their Catalytic Performance For Methanol-to-propylene

Propylene is an important chemical raw materials and widely used in production of synthetic plastics,fibers,rubbers and other fine chemicals?e.g.,polypropylene,and acrylic?.Driven by the rapidly growing propylene derivatives market,the demand for propylene has increased year by year.At present,propylene is mainly produced by the steam cracking of heavy oil and catalytic cracking of naphtha.However,due to the fluctuation of petroleum market in the world and the resource endowment of shortage of petroleum and nature gas and richness of coal in China,the development of non-petroleum routes to produce propylene is very important.Combined with the mature Coal-to-Methanol?CTM?process,Methanol-to-Propylene?MTP?has attracted increased attention in recent years,which could achieve the two targets of the clean utilization of coal and the increase production of propylene simultaneously.Due to the unique pore structure and adjustable acid sites,ZSM-5 zeolite has been the preferred catalyst for the MTP reaction.However,because of the diffusion limitation caused by the single microporous structure,the undesirable side reactions caused by excessive acid density and the influence of specific location of acid sites on MTP reaction pathways in ZSM-5,there exist some problems in industrial application,such as low propylene selectivity and short catalytic lifetime.Therefore,to improve the MTP catalytic performance of ZSM-5 catalyst,the rational design of materials meso/microporous structures and acid sites is desired.Based on the newly developed steam assisted crystallization?SAC?procedure for synthesis of hierarchical micro/mesoporous zeolites,this research focuses on the regulation of acid sites?distribution,type?in high-silica ZSM-5 by heteroatom incorporation and its effect on the MTP catalytic performance.The main results are as follows:?1?Regulation of framework Al distribution of high-silica?Si/Al=200?hierarchically structured ZSM-5 zeolites by boron-incorporation.The effect of B-doping contents and deboronation process on materials crystallinity,pore structure,particle morphology,acidic properties and MTP catalytic performance were investigated through XRD,N₂ sorption isotherms,ICP,SEM,TEM,FT-IR and solid²⁷Al and ²⁹Si NMR characterization.Results confirm that all synthesized B_x-HSZ samples with various B doping content had similar structure integrity,micro/mesoporous structure,particle morphology and strong acidity.The difference of materials catalytic performance on 1-hexene cracking reaction demonstrate that the framework Al of ZSM-5 zeolites was enriched in the narrow straight/sinusoidal channels when the B/Al molar ratio of reached 7 or 9.Compared with sample B₀-HSZ and B₃-HSZ which had framework Al concentrated in the wide channel intersections,B₇-HSZ and B₉-HSZ samples depicted the improved propylene selectivity of 49.3%and 48.9%and longer catalytic lifetime(t₉₀)of 88 h and 91 h under the identical MTP reaction conditions of WHSV of 2.8 g g^-1 h^-1 and reaction temperature of 450?.All these results confirm the competitive doping effect between boron and aluminum species during synthesis of MFI-type zeolites,its effectiveness on tuning the framework Al distribution of high-silica ZSM-5 and consequently the promotion of MTP catalytic performance.?2?Regulation of B&L acid sites of ZSM-5 zeolites by?Al,Ti?-codoping.This research was focused on the effect of Ti-incorporation on ZSM-5 zeolite crystallinity,pore structure,particle morphology,acid properties and MTP catalytic performance.Results showed that both Al and Ti heteroatoms had been successfully incorporated in the framework of synthesized ZSM-5 and present in tetrahedral-coordinated framework species.On the other hand,because of the difference on surface charge and hydrophilicity/hydrophobicity caused by the incorporation of Al and Ti atoms,synthesized Ti₀-HSZ sample consisted of sub-micro particles with rough surface,while sample Ti₂-HSZ and Ti₄-HSZ were composed of sub-micro and nano-sized particles simultaneously.And due to the intercrystal mesopores caused by the stacking of nanoparticles,the mesoporous volume of Ti₂-HSZ and Ti₄-HSZ slightly increased.More importantly,the Ti incorporation led to the introduction of more Lewis acid sites in synthesized materials and increase of weak acid density.The MTP catalytic performance demonstrated that sample Ti₄-HSZ has longest catalytic lifetime(t₉₀)of70 h among three synthesized catalysts,under the WHSV of 2.8 g g^-1 h^-1 and reaction temperature of 450?,which was mainly benefited from by the appropriate B/L ratio caused by the Ti-incorporation in ZSM-5.