Date on Master's Thesis/Doctoral Dissertation

12-2013

Document Type

Master's Thesis

Degree Name

M.S.

Department

Chemical Engineering

Committee Chair

Carreon, Moises A.

Author's Keywords

Catalyst; Microwave; SAPO-56

Subject

Carbon dioxide--Industrial applications; Carbonates

Abstract

The effective utilization of CO2 as a renewable raw material for the production of useful chemicals is an area of great interest. In particular, the catalytic conversion of CO2 into cyclic carbonates, which are useful chemical intermediates employed for the production of plastics and organic solvents, represents an attractive route for the efficient use of carbon dioxide. Microporous crystals, including zeolites and metal organic frameworks (MOFs), and mesoporous ordered oxides possess many desirable properties, which make them appealing for cycloaddition reactions. In general, these porous materials display chemical and thermal stability, moderate to high CO2 uptakes, an open porous structure for improved mass transfer, accessible pore volumes, acid sites which are known as active sites for cycloaddition reactions, high surface areas. SAPOs (silicoaluminophosphates), a particular type of small pore molecular sieves, have received considerable interest because of their applications in separations, catalysis, and adsorption. Their unique functional properties are associated with their chemical and thermal stability, unique shape selectivity, molecular sieving properties, ordered microporous crystalline structure, and surface properties. SAPO-56 is a crystalline microporous silicoaluminophosphate in which silicon substitutes for some of the phosphorous and aluminum atoms in the structural framework. The AFX topology of SAPO-56 is characterized by a three dimensional structure with pore cages arranged in interconnected networks, with window (pore size) sizes of ~3.4×3.6 Å. Due to its pore size similar to the kinetic diameter of several relevant gas molecules such as CO2, CH4, O2, N2 as well as due to relatively high CO2 uptakes, SAPO-56 may find potential applications for CO2 conversion to useful chemicals. A conventional hydrothermal synthesis approach used to synthesize SAPO-56 requires typically long synthesis times (days) and relatively high hydrothermal temperatures (200 °C). Microwave heating offers several advantages over conventional heating, such as fast crystallization, phase selectivity, narrow particle size distribution, abundant nucleation, morphology and size control and rapid and uniform heating. Herein we present the synthesis of SAPO-56 crystals via microwave heating. The resultant crystals displayed high catalytic activity in the synthesis of chloropropene carbonate from CO2 and epichlorohydrin. The Microwave as-synthesized SAPO-56 displayed crystal size as ~3-4 µm, while the crystal size hydrothermal as-synthesized SAPO-56 is ~50 µm. When 3-4 µm crystals were used, the yield to chloropropene carbonate was 84.8%, whereas the yield to the carbonate was only 42.2% when crystals of about 50 µm were used. The enhanced catalytic activity of SAPO-56 crystals was related to their high CO2 adsorption capacity, small crystal size, and the presence of acid sites. In addition, silica nanospheres present in the surface of the smaller SAPO-56 crystals may display a role as specific surface sites for the cycloaddition reaction. For this particular reaction, SAPO-56 seems to be robust catalytic phase because it can be recycled without loss in the catalytic activity.

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