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MTO Synthesis and Characterization of ZnAPO-34 and SAPO-34: Effect of Zn on the Acidity and Catalytic Activity in the MTO Reaction

SAPO-34 and ZnAPO-34 materials (Zn incorporated by isomorphic substitution in AlPO-34 material) were synthesized by hydrothermal synthesis using triethylamine (TEA) as the structure directing agent (SDA). The structure presented by both materials is isomorphic to the chabazite zeolite (CHA). However, they have different properties such as textural properties (Z34 and S34 presented a surface area of 485 and 603 m/g, respectively), different crystal sizes and acid properties. The physicochemical properties of the zeotypes were studied using XRD (X-ray diffraction), N2 adsorption-desorption, temperature programmed desorption of NH3 and SEM (Scanning Electron Microscopy). The catalytic performance of these catalysts was studied in the MTO reaction at 400 °C and atmospheric pressure using a WHSV of 2.12 h in a fixed bed reactor. The incorporation of Zn had an important effect on acidity, generating a higher density of acid sites, increasing selectivity to light olefins. It was observed that when the crystal size decreases (ZnAPO-34), 100 % mol methanol conversion is obtained at short reaction times. The ZnAPO-34 material had a smaller crystal size (0.5 μm) and selectivity for olefins of 78 mol %. On the other hand, the SAPO-34 catalyst showed a larger crystal size (1.5 μm) and lower selectivity to olefins (72 mol %). The Z34 catalyst was compared with a previously reported MeAPSO-36 material, the latter was selective for the formation of aromatic compounds and lower selectivity to olefins (35 % mol) due to the presence of larger channels and lower density of acid sites.