Synthesis of nano-sized SAPO-34 using a facile micron-meter seed processing method and their enhanced performance in methanol-to-olefin reactions

Abstract

Seed-assisted synthesis of nano-SAPOs usually requires a costly nano-sized seed, and the micron-sized seed used should be ball-milled or treated under reflux to broke them into nano-particles. In this study, nano-sized SAPO-34 was prepared using a micron-meter SAPO-34-M seed that was treated facilely with morpholine (MOR) within a Teflon-lined stainless steel autoclave to obtain a seed solution. As MOR was also the template for SAPO-34 synthesis, no extra template was added to the precursor gel. The effects of MOR treatment time on seed properties and the subsequent SAPO-34 fabrication were characterized via XRD, SEM, N₂ adsorption–desorption, ²⁹Si NMR and NH₃-TPD techniques. The effect of the seed amount on the products was also investigated. There existed optimum seed treatment time and seed addition amount for synthesizing uniform nano-sized SAPO-34 with the characteristics of larger mesopore volume, lower Si contents and acid density and weaker acid strength than conventional SAPO-34. Moreover, heterologous seed-assisted syntheses of SAPO-34 were carried out, and it is found that micron-sized heterologous seeds (AFI, RHO, LEV and AFX) treated with MOR can also deliver nano-sized SAPO-34, although AFI and RHO share no common composite building units (CBUs) with CHA, suggesting that seed selection in this synthesis strategy is not confined to a specific topology. The obtained nano-sized SAPO-34 catalysts exhibited about a 2.7-fold prolonged lifetime and nearly 5% increased selectivity for ethylene and propylene compared to their conventional micron-sized counterparts in methanol-to-olefin (MTO) reactions.