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.
Abstract Two different seed crystals (K–CHA and nano-scaled SAPO-34) were used to promote the rapid interzeolite conversion of USY and fumed silica to pure SSZ-13 zeolite phase via a grinding route. The results show that both seeds can shorten the crystallization time of SSZ-13 zeolite to 24h, which is much shorter than the syntheses in the absence of seeds. The phase purity, morphology, textural parameters, elemental compositions and acid properties of two typical H–SSZ-13 samples with seeds were analyzed by XRD, SEM, N 2 physisorption, EDS and NH 3 –TPD techniques in detail, and compared with the sample without seeds and a reference sample. The results indicate that the introduction of seeds can effectively modify the acid properties of H–SSZ-13, especially their medium strong acid sites. The methanol-to-olefins (MTO) reaction was employed as a probe to evaluate the catalytic stability of above four H–SSZ-13 catalysts. The results reveal that the catalytic lifetime of the two catalysts using seeds is much longer than that of the one without seeds and the reference sample H–SSZ-13–RS. It was speculated that the appropriate concentration of medium strong acid sites may play a pivotal role in prolonging their catalytic lifetime. Here our findings may throw light on the rational design of long-lifetime H-SSZ-13 catalysts in the MTO reaction.
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