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 H2 separation from other light gases (CO2, CH4, CO). However, current zeolite membranes have either too big zeolite pores or a large number of defects and have not been successful for H2 separation from light gases. The objective of this study is to develop zeolite membranes that are more suitable for H2 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 CO2 and CH4 adsorption. Silylation on B-ZSM-5 membranes increased H2 selectivity both in single component and in mixtures with CO2, CH4, or N2. Single gas and binary mixtures of H2/CO2 and H2/CH4 were permeatedmore » through silylated B-ZSM-5 membranes at feed pressures up to 1.7 MPa and temperatures up to 773 K. For one B-ZSM-5 membrane after silylation, the H2/CO2 separation selectivity at 473 K increased from 1.4 to 37, whereas the H2/CH4 separation selectivity increased from 1.6 to 33. Hydrogen permeance through a silylated BZSM-5 membrane was activated with activation energy of {approx}10 kJ/mol, but the CO2 and CH4 permeances decreased slightly with temperature in both single gas and in mixtures. Therefore, the H2 permeance and H2/CO2 and H2/CH4 separation selectivities increased with temperature. At 673 K, the H2 permeance was 1.0x10-7 mol{center_dot}m-2{center_dot}s-1{center_dot}Pa-1, and the H2/CO2 separation selectivity was 47. Above 673 K, the silylated membrane catalyzed reverse water gas shift reaction and still separated H2 with high selectivity; and it was thermally stable. However, silylation decreased H2 permeance more than one order of magnitude. Increasing the membrane feed pressure increased the H2 flux and the H2 mole fraction in the permeate stream for both H2/CO2 and H2/CH4 mixtures. The H2 separation performance of the silylated B-ZSM-5 membranes depended on the initial membrane quality and acidity, as well as the silane precursors. 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 CO2 adsorption inhibited H2 adsorption and decreased H2 permeances, especially at low temperatures. At 253 K, CO2/H2 separation selectivities of a SAPO-34 membrane were greater than 100 with CO2 permeances of about 3 x 10-8 mol{center_dot}m-2{center_dot}s-1{center_dot}Pa-1. The high reverse-selectivity of the SAPO-34 membranes can minimize H2 recompression because H2 remained in the retentate stream at a higher pressure. The CO2/H2 separation selectivity exhibited a maximum with CO2 feed concentration possibly caused by a maximum in the CO2/H2 sorption selectivity with increased CO2 partial pressure. The SAPO-34 membrane separated H2 from CH4 because CH4 is close to the SAPO-34 pore size so its diffusivity (ABSTRACT TRUNCATED)« less