The Synthesis Of Small Size SAPO-34 And Its Catalytic Methanol To Olefins

In recent years,the demand for light olefins,such as ethylene and propylene,are increasing year by year due to the rapid development of industry.Methanol-to-olefins（MTO）process is a successful non-petrochemical process to produce light olefins,which uses natural gas,coal and biomass as raw materials.SAPO-34 zeolite with CHA topological structure shows good catalytic performance in MTO reaction because of its regular pore structure,excellent shape selectivity and suitable acidity.However,coke is usually formed due to the diffusion limitation of the SAPO-34 8-ring pore structure（3.8×3.8?）,which leads to deactivation of catalyst and reduction of catalyst service life.Therefore,speeding up the mass transfer rate and improving its catalytic performance in the MTO reaction by reducing the crystal size of SAPO-34 zeolite,which is great significance.To save time and economic cost in synthesis of SAPO-34 zeolites,the effects of hydrothermal time on structure and catalytic MTO reaction of SAPO-34 zeolites were investigated in detail.SAPO-34-xh series zeolites（x=3,4,8,12,24 and 48）were obtained by changing the synthesis time at 210 ^oC using the hydrothermal method.The obtained catalysts were characterized by XRD,SEM,ICP,TG-DTA,N₂adsorption-desorption,²⁹Si and ²⁷Al MAS NMR and NH₃-TPD techniques.Among these samples,SAPO-34-4h shows much longer lifetime（518 min）and higher light olefin selectivity（82.0%）in the MTO reaction.This can be attributed to the unique Si distribution and low acidity.Moreover,the SAPO-34-4h sample can still maintain good MTO reaction stability without complicated processing.This facile synthesis procedure may be beneficial to large scale industrial catalyst production because of its short synthesis time.In order to obtain the controllable acidity small sized SAPO-34 zeolite suitable for MTO reaction,the effects of reducing hydrothermal temperature and adding glycine of different proportion as the p H stabilizer on acidity and catalytic MTO reaction of SAPO-34 zeolites were investigated.SAPO-34-M and SAPO-34-T series zeolites were obtained by changing the amount of glycine added at 160 ^oC using the hydrothermal method,morpholine and tetra-ethylammoniumhydroxide were used as templates,respectively.The obtained catalysts were tested by XRD,SEM and NH₃-TPD.Among these samples,SAPO-34-T4 sample with tetra-ethylammoniumhydroxide as the template showed longer lifetime（384 min）in the MTO reaction.Controllable acidity small sized SAPO-34 zeolites were synthesized by the method by reducing the hydrothermal temperature and adding glycine in different proportion.