Abstract
SAPO-34 molecular sieves were synthesized under hydrothermal conditions using different combinations of tetraethyl ammonium hydroxide (TEAOH)/morpholine (Mor)/triethylamine (TEA) as templates, with different silicon:aluminum ratios. The physicochemical properties of the synthesized SAPO-34 were characterized using XRD, SEM, N2 adsorption–desorption, XRF, TG, NH3-TPD, FT-IR, and 29Si MAS NMR analyses. According to the SEM and the N2 adsorption–desorption of the catalysts produced by the ternary template exhibited a larger surface area and a smaller crystal size than those produced by the single or binary templates. The FT-IR analysis indicated the increased acidity of the catalyst prepared by the ternary template. A high activity and selectivity to olefins (C2= + C3=) and an optimal silicon to aluminum ratio of 0.4 were obtained from the catalyst synthesized with the ternary template. At the reaction temperature of 450 °C, the methanol conversion approached 100% and the ethylene–propylene selectivity and the lifetime of the catalyst reached maximums of 89.15% and 690 min, respectively.
Highlights
The methanol-to-olefin (MTO) process has attracted much attention as the most hopeful way to convert coal and natural gas to chemicals via methanol [1]
SAPO-34 molecular sieves were synthesized under hydrothermal conditions and synthesis conditions for the different SAPO-34 materials were showed in Materials and Methods part
The negative ion of the skeleton was generated at the same time as the formation of the silicon island structure [28,30], so the more templating agents used to balance the skeleton charge and the greater the relative content of the Si structure distribution in the silicon island structure, the stronger the acidity exhibited by the molecular sieve was, which is consistent with the results presented in Tables 4 and 5
Summary
The methanol-to-olefin (MTO) process has attracted much attention as the most hopeful way to convert coal and natural gas to chemicals via methanol [1]. Various molecular sieves have been used to catalyze this reaction effectively Among those molecular sieves, SAPO-34, with its CHA structure, small pore openings (about 3.8 Å), and gentle acidity [2,3] leads to high ethylene and propylene yields at high methanol conversions in the MTO reaction and could be the most promising catalyst for this process [4]. In order to solve the rapid deactivation problem, various synthetic strategies have been developed in order to inhibit the coke deposition, such as decreasing the crystal size, modifying the acidity, and building hierarchical structures with the introduction of mesopores or macropores. Small sized SAPO-34 nanocrystals have attracted widespread attention
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