In Situ Synthesis of SAPO-34 Grown onto Fully Calcined Kaolin Microspheres and Its Catalytic Properties for the MTO Reaction

Rapid deactivation and short lifetime are challenges for SAPO-34 catalysts for the reaction of methanol to olefins (MTO). To overcome these drawbacks, SAPO-34 on fully calcined kaolin microspheres (CKMs) was synthesized in situ by a hydrothermal method with tetraethylammonium hydroxide (TEAOH) and triethylamine (TEA) as templates. To facilitate the growth of SAPO-34, the CKMs were first pretreated by 2-14 wt % NaOH aqueous solution, and the effects of the NaOH concentration on the physicochemical properties of CKMs and the SAPO-34/kaolin microspheres (SCKMs) were characterized by means of XRD, XPS, SEM, low-temperature N-2 adsorption, and NH3-TPD. The results showed that NaOH pretreatment has obvious effects on the chemical compositions and morphologies of the CKMs and, thus, the physicochemical properties of the SAPO-34 grown on them. Moreover, the textural and catalytic properties of the SCKMs were found to depend strongly on the concentration of NaOH solution, and the optimal NaOH concentration was determined to be 4 wt %. For the MTO reaction catalyzed by the prepared SCKM catalyst, 100% methanol conversion, 89.8% light olefins selectivity, and a 964-min lifetime (the reaction time during which the yield of C-2-C-4 olefins exceeded 70%) were obtained at 450 degrees C, which are much better than the values for free SAPO-34 and SAPO-34 supported on metakaolin microspheres.