Methanol To Propylene Over ZSM-5 Zeolites

Methanol to propylene is a new technology to replace the traditional way for propylene production.It not only relieves our dependence on petroleum resources,but also takes the advantages of abundant coal resources in China.Currently,catalysts modification has been widely studied.ZSM-5 zeolites with unique three-dimensional pore structure and suitable acidity have been always used for the methanol to propylene reaction over the fixed bed technology.And the porosity and acidity of the catalyst are the most important factors for the catalytic performance of MTP reaction.Thus,in this paper,the influence of catalysts pore structure and acidity on MTP reaction were investigated with reaction temperature of 480 oC,methanol WHSV of 3 h-1 and water to methanol molar ratio of 1: 1.Following by the main researches:ZSM-5 zeolites with the same Si/Al ratio but different crystal size were synthesized by controlling the amount of seed.The catalytic performance of these samples was tested in MTP reaction.The results showed that the diffusion path shorted with the crystal size reduced,thus catalysts lifetime increased due to faster diffusion of carbon precursor.Besides,propylene selectivity increased as crystal size decreased,which was attributed to reducing aromatic-based reactions.,and lower methane selectivity was received for small crystal size.Moreover,mesoporous were introduced to further enhance catalyst diffusion.ZSM-5 treated by NaOH had high lifetime of 200 h and higher propylene selectivity of 50%,but lower methane selectivity.However,enclosed mesoporous were introduced into ZSM-5 by 0.3 mol/L TPAOH treatment at 170 oC that showed no significant improvement of catalytic performance compared with parent ZSM-5.ZSM-5 zeolites with similar crystal size but different Si/Al were synthesized by controlling the amount of adding aluminum.All of the catalysts were evaluated in MTP reaction,catalysts lifespan enhanced by increasing Si/Al ratio.Besides,higher propylene selectivity but lower methane selectivity was obtained over catalysts with higher Si/Al ratio.In addition,Al-zoned ZSM-5 with Si/Al of 40 was synthesized.Oxalic acid washing selectively removed Al at the external surface of uncalcined ZSM-5,which showed two times longer lifetime as parent due to less carbon deposition on external surface.By combination analysis of the influence of different crystal size and Si/Al ratio to methane selectivity,it can be concluded that lower methane selectivity was received for catalysts with lower acidity but quick diffusion rate,besides,a stable methane selectivity during reaction was received.We can infer that methane comes from hydrogen transfer reaction between methanol and carbon precursor.Furthermore,it also can be found that methane selectivity kept close relationship with catalysts stability.Different concentration of NH₄ F modified ZSM-5 catalysts showed less framework defects and different amount of disturbed tetrahedral Al,and NH₃-TPD,OH-IR and 27 Al MAS NMR proved less acidity of those distorted tetrahedral Al.All of the catalysts were evaluated in MTP reaction.The 0.078 mol/L NH₄ F modified ZSM-5 had less defects and suitable acidity,the lifespan increased from 10 h to 135 h compared with parent,besides it also showed significant enhancement of propylene selectivity from 25% to 50%.