Synthesis, characterization and catalytic performance of SAPO‐34 molecular sieves for methanol‐to‐olefin (MTO) reaction

Abstract

The methanol-to-olefin (MTO) process is important because it offers an alternative to the conventional oil route for the conversion of natural gas or coal to light olefins. The most promising catalysts for the MTO reaction are solid acids with a zeolite structure, where reactants and products such as ethylene and propylene may freely diffuse through active catalysts, but products with larger kinetic diameters are trapped within the cages. In this work, SAPO-34 molecular sieves were synthesized by hydrothermal crystallization, using triethylamine (TEA) as the template. These samples were characterized by X-ray diffraction, scanning electron microscopy, thermo-gravimetric analysis-differential scanning calorimetry, energy dispersive spectroscopy, Fourier transform-infrared (FT-IR) and N 2 adsorption-desorption. The effects of template concentration and crystallization time on the physicochemical properties and catalytic performance were investigated. The crystallinity and morphology of SAPO-34 were influenced by the concentration of the template. Pure SAPO-34 was obtained when the TEA/Al 2 O 3 molar ratio was higher than 2.0. With increased crystallization time, the Si content in SAPO-34 crystals increased, which influenced the surface acidity. FT-IR spectra indicated that all samples were dominated by Lewis acid sites and the sample crystallized for 11 h exhibited the lowest number of Lewis acid sites. The catalytic performance of SAPO-34 was tested with the MTO reaction. High selectivity to olefins (C 2 H 4 + C 3 H 6 ) was obtained over the catalyst synthesized with a TEA/Al 2 O 3 ratio of 3.0 and crystallization time of 11 h. At the reaction temperature of 450• C, the methanol conversion approached 100% and the yield of C 2 -C 4 olefins was more than 80%. Hence, SAPO-34 molecular sieves were shown to be excellent catalysts for the MTO reaction. 

Different techniques and their effective parameters in nano SAPO-34 synthesis: A review

“…Because the particle aggregation takes place rapidly after nucleation, controlling the appropriate crystallization time is the most important factor in obtaining SAPO-34 with small crystals. Also, as a result of high crystallinity at longer crystallization span, the surface area is significantly increased and this result is in agreement with the data obtained by other researchers which showed that the surface area increases gradually with enhancing the crystallinity of SAPO-34 [34].…”

“…There are various works which studied effects of crystallization time [32][33][34][35][36][37][38]. They confirm that with increasing crystallization period, crystals grow larger.…”

“…SAPO-5 structure is obtained at low concentration of morpholine (2.0 M compositions) and pure SAPO-34 is formed by increasing its concentration to 4.0. Also, in samples which are synthesized with DEA, by reducing the template content SAPO-34 is transformed to AlPO 4 structure which is called "Cristobalite" and "Augelite", so it can be concluded that SAPO-34 framework which synthesized with low DEA concentration is unstable [34]. Finally, for the TEA, X-ray diffraction of samples shows that the main phase of final product is SAPO-5 structure which shows TEA is a template for synthesizing of SAPO-5 [26,52,53].…”

Computational fluid dynamics simulation of system hydrodynamics in a riser of stage circulating fluidized bed reactor

“…However, the MTO process is profitable only when the price of petroleum is high. To maximize the chemical reaction conversion, several industrial catalysts have been investigated …”

Facile synthesis of SAPO-34 with small crystal size for conversion of methanol to olefins

“…Hydrothermal method, as the conventional strategy, has been applied in the preparation of SAPO-34 by numerous researchers [4][5][6][7][8][9][10][11][12][13][14][15]. However, separation between solvents and solid products is required by filtration or centrifugation, and partial organic SDAs are inevitably preserved in the liquid phase, resulting in the wastewater pollutants and downstream purification system installation [16].…”