Morphology control of SAPO-34 by microwave synthesis and their performance in the methanol to olefins reaction

Abstract

SAPO-34 molecular sieves with different crystal morphologies were synthesized under microwave conditions. The type of organic templates, crystallization time and temperature were optimized to control the synthesis. These SAPO-34 samples were characterized by XRD, SEM, EDX, TG, nitrogen adsorption–desorption, NH₃-TPD and solid state MAS NMR techniques. It was found that the crystallization conditions greatly affected the crystal morphology, textural data, Si incorporation and acidity of SAPO-34. Pure SAPO-34 with uniform sphere particles of 20 nm was obtained after crystallization at 165 °C for 0.75 h under microwave conditions using TEAOH as the template. Raising the temperature to 220 °C and prolonging the crystallization time to 2 h led to the formation of nano sheet-like SAPO-34 with a BET surface area of 593 m²/g. NH₃-TPD profiles showed that low crystallization temperature and short crystallization time resulted in fewer weak acid sites and strong acid sites, while higher crystallization temperature and longer crystallization time increased the amount of both acid sites, which may benefit the methanol to olefins (MTO) reaction. The nano sheet-like SAPO-34 exhibited 100 % methanol conversion for 380 min during the MTO process, an ethylene selectivity of 51.77 % and a C ⁼₂ –C ⁼₄ selectivity of 90.20 %. The result was much better than that of the sample with a larger cubic morphology of 1.5–2.5 μm synthesized under hydrothermal conditions, on which the 100 % methanol conversion could only be kept for 212 min as well as an ethylene selectivity of 49.84 % and a C ⁼₂ –C ⁼₄ selectivity of 86.81 %. The unique morphology of the nano sheet-like SAPO-34 decreased its coke formation rate, which might be responsible for its improved catalytic performance.