Highly Selective H2 Separation Zeolite Membranes for Coal Gasification Membrane Reactor Applications
Zeolite membranes are thermally, chemically, and mechanically stable. They also have tunable molecular sieving and catalytic ability. These unique properties make zeolite membrane an excellent candidate for use in catalytic membrane reactor applications related to coal conversion and gasification, which need high temperature and high pressure range separation in chemically challenging environment where existing technologies are inefficient or unable to operate. Small pore, good quality, and thin zeolite membranes are needed for highly selective H{sub 2} separation from other light gases (CO{sub 2}, CH{sub 4}, CO). However, zeolite membranes have not been successful for H{sub 2} separation from light gases because the zeolite pores are either too big or the membranes have a large number of defects. The objective of this study is to develop zeolite membranes that are more suitable for H{sub 2} separation. In an effort to tune the size of zeolite pores and/or to decrease the number of defects, medium-pore zeolite B-ZSM-5 (MFI) membranes were synthesized and silylated. Silylation on B-ZSM-5 crystals reduced MFI-zeolite pore volume, but had little effect on CO{sub 2} and CH{sub 4} adsorption. Silylation on B-ZSM-5 membranes increased H{sub 2} selectivity both in single component and in mixtures with CO{sub 2}CO{sub 2}, CH{sub 4}, or N2. Single gas and binary mixtures of H{sub 2}/CO{sub 2} and H{sub 2}/CH{sub 4} were separated through silylated B-ZSM-5 membranes at feed pressures up to 1.7 MPa and temperatures up to 773 K. For one BZSM-5 membrane after silylation, the H2/CO{sub 2} separation selectivity at 473 K increased from 1.4 to 37, whereas the H{sub 2}/CH{sub 4} separation selectivity increased from 1.6 to 33. Hydrogen permeance through a silylated B-ZSM-5 membrane was activated, but the CO{sub 2} and CH4 permeances decreased slightly with temperature in both single gas and in mixtures. Therefore, the H{sub 2} permeance and H{sub 2}/CO{sub 2} and H{sup 2} /CH{sub 4} separation selectivities increased with temperature. At 673 K, the H2 permeance was 1.0x10-7 molxm-2xs-1xPa-1, and the H{sub 2}/CO{sub 2} separation selectivity was 47. Above 673 K, the silylated membrane catalyzed reverse water gas shift reaction and still separated H{sub 2} with high selectivity; and it was thermally stable. However, silylation decreased H{sub 2} permeance more than one order of magnitude. The H{sub 2} separation performance of the silylated B-ZSM-5 membranes depended on the initial membrane quality and acidity, as well as the silane precursors. Increasing the membrane feed pressure also increased the H{sub 2} flux and the H{sub 2} mole fraction in the permeate stream for both mixtures. Another approach used in this study is optimizing the synthesis of small-pore SAPO-34 (CHA) membranes and/or modifying SAPO-34 membranes by silylation or ion exchange. For SAPO-34 membranes, strong CO{sub 2} adsorption inhibited H{sub 2} adsorption and decreased H2 permeances, especially at low temperatures. At 253 K, CO{sub 2}/H{sub 2} separation selectivities of a SAPO-34 membrane were greater than 100 with CO{sub 2} permeances of about 3 x 10-8 mol m-2 s-1 Pa-1. The high reverse-selectivity of the SAPO-34 membranes can minimize H{sub 2} recompression because H{sub 2} remained in the retentate stream at a higher pressure. The CO{sub 2}/H{sub 2} separation selectivity exhibited a maximum with CO{sub 2} feed concentration possibly caused by a maximum in the CO{sub 2}/H{sub 2} sorption selectivity with increased CO{sub 2} partial pressure. The SAPO-34 membrane separated H{sub 2} from CH{sub 4} because CH{sub 4} is close to the SAPO-34 pore size so its diffusivity is much lower than the H{sup 2} diffusivity. The H{sub 2}/CH{sub 4} separation selectivity was almost independent of temperature, pressure, and feed composition. Silylation on SAPO-34 membranes increased H{sup 2}/CH{sub 4} and CO{sub 2}/CH{sub 4} selectivities but did not increase H{sub 2}/CO{sub 2} and H{sub 2}/N{sub 2} selectivities because silylation only blocked defects in SAPO-34 membranes. Hydrogen cations in SAPO-34 membranes were exchanged with Li{sup +}, Na{sup +}, K{sup +}, NH4{sup +}, and Cu{sup 2+} cations in non-aqueous solutions in an effort to improve their gas separation performance. H-SAPO-34 crystals were exchanged under the same conditions and their adsorption properties were measured. Ion exchange with Cu{sup 2+} increased CO{sub 2} adsorption strength, whereas Li{sup +} exchange decreased the SAPO-34 pore volume and saturation loadings of CO{sub 2} and CH{sub 4}. Ideal and separation selectivities for H{sub 2}/CH{sub 4} mixtures increased less than 18%, whereas ideal and separation selectivities for CO{sub 2}/CH{sub 4} mixtures increased up to 60% due to ion exchange of zeolite membranes. Gas permeances decreased upon ion exchange, and the decrease was larger for large cations, apparently due to steric hindrance. Multiple exchanges did not (ABSTRACT TRUNCATED)
- Research Organization:
- University Of Colorado
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- FG26-02NT41536
- OSTI ID:
- 908744
- Country of Publication:
- United States
- Language:
- English
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