1. Field of the Invention
The present invention relates generally to inorganic membranes that are permeable to small gas molecules. More particularly, the present invention relates to permeable membranes deposited on porous substrates, with or without an intermediate porous layer, that exhibit both a high hydrogen permeance and a high hydrogen permselectivity.
2. Description of the Related Art
Permeable materials are those through which gases or liquids may pass. Membranes are one type of permeable material and are composed of thin sheets of natural or synthetic material. Frequently, membranes exhibit different permeances—i.e., permeation rates—for different chemical species. In this regard, permselectivity is the preferred permeation of one chemical species through a membrane with respect to another chemical species. Permselectivity of the desired permeate with respect to another chemical species is calculated as the ratio of the permeance of the desired permeate to the permeance of the other chemical species.
Permselective membranes are promising in a variety of applications including gas separation, electrodialysis, metal recovery, pervaporation and battery separators. Recently, interest has developed in using permselective membranes in so-called membrane reactors, which allow the simultaneous production and selective removal of products. One regime in which permselective membranes are particularly promising is that of equilibrium-limited reactions. In such reactions, yields are reduced by reaction reversibility. Preferential removal of one or more of the reaction products effectively shifts the equilibrium—or, stated differently, decreases the rate of the reverse reaction—thereby overcoming thermodynamic limitations.
One example of an equilibrium limited reaction is the methane dry-reforming reaction [1]:CH4+CO22CO+2H2(ΔH°298=247 kJ·mole−1)  [1]. This reaction provides a pathway to convert carbon dioxide, a problematic greenhouse gas, and methane, a plentiful natural resource, into synthesis gas—i.e., a mixture of hydrogen and carbon monoxide. Synthesis gas is an industrially important feedstock that is used in the preparation of ethylene glycol, acetic acid, ethylene, fuels and several other commercially important chemicals. Unfortunately, the conversion of methane and carbon dioxide to synthesis gas is limited by the reversability of the reaction—i.e., the ability of hydrogen and carbon monoxide to regenerate methane and carbon dioxide. The yield can be improved, however, by selectively removing one or both of the products as they are formed. Doing so mitigates the extent of the reverse reaction.
Other examples of equilibrium-limited reactions that produce hydrogen gas are the decomposition of hydrogen sulfide [2] and ammonia [3]:H2SS(s)+H2  [2]2NH3N2+3H2  [3]. Hydrogen sulfide and ammonia are frequent and undesirable byproducts of numerous chemical reactions. Thus, reactions [2] and [3] offer an abatement technique for reducing the levels of these compounds. Like the methane dry-reforming reaction, the products of these reactions can be favored by removing hydrogen as it is produced. In short, hydrogen permselective membranes offer the potential to overcome several equilibrium-limited reactions in commercially useful ways.
Organic polymer-based membranes have been demonstrated in a variety of commercially-viable processes such as filtration, microfiltration, ultrafiltration and reverse osmosis. Although permselective polymer-based membranes exist, inherent limitations preclude their use in many applications. Polymeric membranes cannot be used at high temperatures and pressures: typical polymeric membranes cannot withstand temperatures in excess of 150° C. or pressure differentials in excess of several atmospheres. Consequently, these membranes have limited utility in applications such as high temperature membrane reactors and high pressure gas purifiers. For example, the methane dry-reforming reaction, even under catalytic conditions, usually entails temperatures of approximately 600° C. or more. In addition, polymeric membranes frequently cannot be cleaned with strong acids, bases and oxidizing agents because of their chemical reactivity.
Inorganic membranes have attracted much attention in the past decade because of their chemical, thermal and mechanical stability. The robustness of inorganic membranes compared to their polymeric counterparts permits their use in harsh environments such as chemical reactors. Thus, inorganic membranes offer the possibility of surmounting thermodynamic equilibrium limitations through the simultaneous formation and removal of reaction products, even in harsh environments. To be useful in this capacity, the membrane should exhibit a high permeability with respect to a reaction product while maintaining a low permeability to the reactants. In short, a suitable membrane for a membrane reactor should provide both high selectivity for a particular permeate—i.e., a high permselectivity—and a high permeability for that permeate.
The mechanism of separation in membranes limits their permselectivity. For example, the separation of gaseous species through Vycor™ glass membranes has been shown to proceed through Knudsen diffusion, a mechanism based upon molecular diffusion through the pores that decreases with increasing temperature. Because Knudsen diffusion is proportional to the inverse square root of the molecular weight of a species, the maximum selectivity obtainable is severely limited. For example, under Knudsen diffusion, the best selectivity that can be achieved for hydrogen (molecular weight 2) with respect to the molecules methane (molecular weight 16), carbon monoxide (molecular weight 28) and carbon dioxide (molecular weight 44) ranges from 2.8 to 4.9. This limitation can be overcome using inorganic membranes that operate outside the limitations of Knudsen diffusion and that exhibit better selectivities.
Deposition chemistries and techniques can profoundly affect membrane selectivity and permeability. Unfortunately, increasing permselectivity is frequently accomplished only at the expense of permeation rates: highly permselective membranes generally offer unacceptably low permeation rates for the desired permeate. A suitable membrane for commercial processes should offer both high permselectivity and permeability with respect to the desired permeate.
Several techniques, such as sol-gel processing and chemical vapor deposition (CVD), exist for depositing inorganic films. Sol-gel processing has been shown to provide higher permeability than CVD methods. Unfortunately, the sol-gel method suffers from a lack of reproducibility that makes it unattractive from a commercial perspective. CVD is a well-known method for depositing thin films and offers highly uniform and reproducible film deposition. CVD has been employed in the semiconductor industry for depositing layers of conducting and insulating materials during wafer processing. Consequently, reproducible and accurate techniques for the deposition of thin CVD films are well known. Although CVD offers numerous advantages such as highly selective and reproducible films, it requires substantial capital investment. Perhaps more importantly, acceptable permselectivities in prior art CVD membranes have come at the cost of unacceptably high losses in permeability.
Among the most promising inorganic membrane materials is silica. Several publications and patents have reported the preparation of silica-based membranes for the separation of hydrogen at high temperature. These are summarized in a recent publication, A. K. Prabhu and S. T. Oyama. J. Membr. Sci. 2000, 176, 233, which is hereby incorporated herein by reference. Silica membranes can be engineered to exhibit permselectivity to hydrogen. However, hydrogen permeances have been unacceptably low for commercial processes.
Furthermore, conventional silica membranes typically suffer from significant susceptibility to moisture at high temperatures and drastic losses in permeability over short time frames have been reported. This loss in permeability has been attributed to the removal of hydroxyl moieties from Si—OH groups and the concomitant formation of Si—O—Si bonds that close pores channels, thereby decreasing permeance. This phenomenon has been termed densification.
U.S. Pat. No. 5,453,298 (“the '298 patent”) discloses the deposition of silica membranes from various silicon precursors, including silicon halides (e.g., SiCl4), chlorinated silanes (e.g., SiH3Cl) and chlorinated siloxanes (e.g., Cl3SiOSiCl2OSiCl3) onto porous Vycor™ glass or alumina substrates. In each case, the silica product was formed through the reaction of the silicon precursor with moisture and/or oxygen. The '298 patent discloses that gas phase reactions between the silicon precursor and moisture result in particles that can adhere to the tube wall, decreasing permeability and inducing thermomechanical stresses that can result in membrane cracks and failure. To address this problem, the '298 patent teaches the use of the “alternating flow deposition” method in which the silica film was deposited by exposing the porous substrate to the silicon precursor, evacuating the system so as to remove all silicon precursor except that already adsorbed on the Vycor™ surface, and then admitting water vapor to react with the adsorbed silicon precursor. The '298 patent also notes that the “opposing reactants deposition” technique, in which one reactant is admitted on one side of the porous substrate and the other reactant is admitted on the other side of the porous substrate with reactions occurring inside the pores, causes undesirably thick deposits in the pores of the substrate. The '298 patent reports a silica membrane having a hydrogen permeance of approximately 0.20 cm3 (STP)/min·atm·cm2, or 1.5×10−8 mol/m2·s·Pa, with a H21N2 permselectivity of about 3000. According to the '298 patent, the hydrogen permeance drops “considerably” upon exposure to high temperatures, especially when moisture is present.
In Yamaguchi et al., Phys. Chem. Chem. Phys., 2000, 2, 4465-4469, researchers at the University of Tokyo have reported a CVD silica membrane deposited on an α-alumina porous tubular substrate with an intermediate γ-alumina layer deposited by sol-gel chemistry. The CVD membrane was prepared from tetraethyl orthosilicate (TEOS) and ozone (O3). The reaction employed opposing reactants deposition in which TEOS was admitted to the outer “shell” side of the porous tubular substrate while O3 in oxygen was admitted to the inner “tube” side of the porous tubular substrate. The deposition was performed at between 175° C. and 300° C. using a silicon precursor concentration of between 0.4 and 2.1 mol/m3 (STP). Although permeation data was reported for several gases, no permeation data was reported for H2.
Recently, one of us presented a new membrane, called Nanosil, with 100 percent permselectivity for hydrogen with respect to CH4, CO, CO2 and H2O in PCT Patent Application PCT/U500/02075 (2000), which is hereby incorporated herein by reference. The Nanosil membrane was prepared by CVD of TEOS onto a porous Vycor™ glass substrate at high temperature in the absence of oxygen or steam. The Nanosil membrane demonstrates marked resistance to moisture: the membrane lost only four percent of its permeability after exposure to ten percent moisture in argon at 873° K for 100 hours. Furthermore, the membrane exhibited tremendous selectivity for hydrogen gas. However, the hydrogen permeance for the membrane was approximately 10−8 mol/m2·s·Pa, which is less than that desirable for commercial applications.
Thus, currently available inorganic permselective membranes exhibit undesirably low permeances and are frequently susceptible to moisture. There is substantial interest in an inorganic membrane having a high permselectivity and permeability for hydrogen while exhibiting minimal susceptibility to moisture damage at high temperatures.