An Al-sol gelation strategy for the synthesis of robust hierarchical SAPO-34 with enhanced ethylene selectivity

It's significance in enhancing the ethylene selectivity of MTO, however the partial regeneration and metal modification methods would reduce the lifetime of SAPO-34, and it's necessary to develop a method to simultaneously improve the ethylene selectivity and catalytic stability. Herein, an industrial aluminum sol gelation strategy was reported, the influences of the pH value (2.8–4.0), gelation temperature (40–80 °C), and Al species were detailed studied. The SAPO-34 crystals can partially be eroded by the Al gel, and intra-crystalline mesopores (5.2–6.4 nm) can be created. However, aluminum species in the sol can be adsorbed, and the nAl:(nP + nSi) of the catalyst increased from 0.86 to 0.96–1.32. Comparatively, the erosion and adsorption of Al species were more notable under higher pH value and gelation temperature conditions. Furthermore, the gelation can prevent the hydration of the crystals, but the isolated silicon species can be reassembled to silicon islands, and the strong acid centers increased from 0.45 mmol NH3/g to 0.54–0.69 mmol NH3/g. Although more coke can be brought about by the increased strong acids, the intra-crystalline diffusion limitation can be released by the mesopores, and the catalytic stability of SAPO-34 can be better retained. More importantly, the geometric effect has been intensified by the increased coke, and a significant increase (1.6–4.7%) in the initial ethylene selectivity and a slightly decrease (0.1–1.4%) in the selectivity of propylene and butenes was achieved, leading to enhanced selectivity of ethylene and propylene (SEP, 86.6%), light olefins selectivity (SLO, 94.6%), and ratio of ethylene to propylene (C2=:C3=, 1.44), especially under higher gelation temperature. The gelation strategy is efficient in enhancing the ethylene selectivity and improving the catalytic stability of SAPO-34, and it can be applied in the preparation of industrial MTO catalyst.