Effect Of Additives On Synthesis Of SAPO-34 And MTO Catalytic Performance

Silicoaluminophosphate（SAPO-n）molecular sieves are new generation ofmicroporous molecular sieve widely used in various chemical and petrochemical processes.Among them,SAPO-34 has relatively mild acidity,excellent thermal stability and hydrothermal stability,shape-selective catalytic properties of small pores,and has attracted widespread attention for its excellent catalytic performance in methanol-to-olefins（MTO）process.But SAPO-34 usually requires dozens of hours（>24 hours）of crystallization time in the traditional hydrothermal synthesis method.As a result of more particle aggregation,the particle sizes exceeds 1μm,the particle size distribution is uneven.Moreover,SAPO-34 is a typical microporous material,with only a single microporous structure,which affects the stability in the MTO process.In this work,the above problems are solved by adding additives to the precursor synthesis gel.The addition of crystallization promoters（acetylacetone,citric acid）enables the ultrafast synthesis of SAPO-34 under conventional hydrothermal synthesis conditions.The crystallization time of SAPO-34 is significantly shortened to less than 2hours.The time required is equivalent to that through the special heating intensification methods,and the yield SAPO-34 is over 75%.Through kinetic analysis,ultrafast synthesis is based on the chelation of crystallization promoters（especially citric acid,the organic acid chelator with multi carboxyl groups）and aluminum ions.After chelating citric acid with aluminum ions,the nanoclusters and metastable phases（semi-solid）substances formed in the c precursor synthesis gel.The substances can significantly increase the pre-exponential factor（A）and promote crystallization through a unique mechanism.In addition,the combined use of citric acid and seed can reduce the particle sizes of SAPO-34 to 0.5-1μm,effectively narrowing the particle size distribution.The addition of the structure promoter polyhexamethylene biguanide hydrochloride（PHMB）can control the hierarchal structure of SAPO-34.Moreover,PHMB can control the distribution of silicon element,the obtained SAPO-34 demonstrated high ratio Q⁰state Si specie,low surface Si enrichment.Under MTO catalytic reaction conditions(the temperature of 723 K and the methanol flux of 30 h^-1),the hierarchal SAPO-34synthesized with adding CA/seed/PHMB demonstrated superior stability.The stable time longer than 28 mins at conversion of methanol higher than 90%,and the conversion of methanol is still 99%in 21 minutes.The selectivity of ethylene and propylene is 78.6%and the selectivity of ethylene,propylene and butylene is 90.6%.In this work,ultrafast hydrothermal synthesis of SAPO-34 and its structural control are achieved through chemical routes.Based on the existing hydrothermal synthesis technology to promote crystallization through chemical methods.The selected additives（citric acid,polyhexamethylene biguanide hydrochloride）are low-cost,non-toxic,degradable,and environmentally friendly.Compared with the special heating intensification methods,it is simple,efficient,environmentally friendly,and cost controllable.There is no need to add or modify the original equipment.There is no bottleneck in industrial amplification,which is of great significance.