Design Synthesis of Low-Silica SAPO-34 Nanocrystals by Constructing Isomorphous Core–Shell Structure: An Effective Catalyst for Improving Catalytic Performances in Methanol-to-Olefins Reaction

It is well known that low-silica SAPO-34, with an extra porosity (meso- and/or macropores) system, affords excellent catalytic performance in the methanol-to-olefins (MTO) reaction, while the direct synthesis of low-silica SAPO-34 with a hierarchical structure is difficult to achieve, principally because the crystal impurities are usually formed under a low silica content in a gel precursor. Herein, low-silica SAPO-34 nanocrystals were successfully fabricated for the first time by constructing an isomorphous core–shell structure in an epitaxial growth manner. In which, low-silica, ultrasmall nanosquare-shaped SAPO-34 crystals with the same growth orientation along the (100) crystal plane compactly grow on the monocrystal SAPO-34 cores. Crucially, the external surface acid properties of the core SAPO-34 with the Si-rich outer layer are effectively modified by the low-silica SAPO-34 shell. Furthermore, the growth process and Si-substitution mechanism of the shell zeolite were comprehensively investigated. It was found that with the prolonged crystallization time, more and more coordinated Si(4Al) and Si(3Al) structures via two substitution mechanisms (SM2 and SM3) are generated in the nanocrystalline SAPO-34 shell, which endow moderate acidity of the core–shell SAPO-34. Compared to the uncoated SAPO-34, the core–shell SAPO-34 performs a longer lifespan and a higher average selectivity of light olefins (ethylene plus propylene) when applied to the MTO reaction, which is attributed to the positive effects of the luxuriant interstitial pores offering a fast diffusion channel and the moderate acid density depressing the hydrogen transfer reaction of light olefins. This work provides new insights into the fabrication of low-silica SAPO-34 nanocrystals, which are based on the rational design of the isomorphous core–shell zeolite.