Preparation and reactive applications of nanoporous

Received 1 March 2004 Available online 19 August 2004
Abstract
SiC nanoporous membranes have been prepared by chemical-vapor deposition/chemical-vapor infiltration of two different precursors, namely tri-isopropylsilane (TPS) and 1,3-disilabutane (DSB). Both precursors produce nanoporous membranes. The TPS-derived membranes are hydrothermally stable, while the DSB membranes produced, so far, are not. SiC nanoporous membranes have also been prepared by conventional dip-coating techniques using a novel pre-ceramic polymeric precursor allyl-hydridopolycarbosilane (AHPCS), a partially allyl-substituted hydridopolycarbosilane (HPCS). These membranes are stable in air-treatment at 450 ◦C, but have, so far, also proven unstable to high-temperature steam. The nanoporous hydrogen selective SiC membranes show promise for application in membrane reactors for the water gas shift and steam reforming reactions. ᭧ 2004 Elsevier Ltd. All rights reserved.
Richard J. Cioraa, Babak Fayyazb, Paul K.T. Liua, Varaporn Suwanmethanondb, Reyes Malladab, Muhammad Sahimib, Theodore T. Tsotsisb,∗
aMedia and Process Technology Inc, 1155 William Pitt Way, Pittsburgh, PA 15238, USA bDepartment of Chemical Engineering, University of Southern California, 925 Bloom Walk, HED 216 Los Angeles, CA 90089-1211,J. Ciora et al. / Chemical Engineering Science 59 (2004) 4957 – 4965
Membrane reactors (MR) have been previously applied to these two reactions (Sanchez and Tsotsis, 2002) utilizing mostly metal (Pd, Pt, and their alloys) membranes. Using such noble metal membranes is dictated by the fact that other available membranes cannot withstand the high temperatures required (e.g., polymeric membranes), are sensitive to steam (e.g., SiO2 membranes), or are not permselective enough towards H2 (e.g., Al2O3 or zeolite membranes) under the prevailing high temperature, pressure, and steam conditions. Noble metal membranes perform well (Sanchez and Tsotsis, 2002); however, they are costly, are susceptible to deactivation from coke and sulfur impurities, and must be handled with great care to avoid embrittlement and mechanical failure. SiC membranes show, as a result, good potential for SMR and WGS MR applications. The performance of our group’s SiC membranes in high-temperature steam environments, relevant to the aforementioned two reactive applications, is discussed in this paper. Using the experimentally determined transport characteristics of one of the SiC membranes, we have modeled its performance in a SMR MR, in order to demonstrate its potential for this application.
Chemical Engineering Science 59 (2004) 4957 – 4965
/locate/ces
Preparation and reactive applications of nanoporous silicon carbide membranes
∗ Corresponding author. Tel.: +1-213-740-2069; fax: +1-213-740-8053. E-mail address: tsotsis@ (T.T. Tsotsis).
0009-2509/$ - see front matter ᭧ 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2004.07.015
Keywords: Silicon carbide; Membranes; Catalytic membrane reactor; Steam reforming
1. Introduction
Membranes find industrial applications in the separation of gas and liquid mixtures. Polymeric membranes are generally utilized in applications that occur at low temperatures and unreactive environments. Several applications exist, however, for which polymeric membranes may not be appropriate because their use is limited to temperatures below 150 ◦C. As a result, in recent years there has been a considerable effort to produce high-temperature ceramic (alumina, silica, zeolite), and metal (Pd, Pt, Ag, and their alloys) membranes, with improved properties. They have, so far, all proven unsatisfactory in terms of performance and/or material stability, however, in high-temperature applications, particularly in the presence of steam. A new membrane, which shows potential to overcome some of these difficulties,
is made of SiC (Suwanmethanond et al., 2000). SiC is a promising material, having high fracture toughness, good thermal shock resistance, and being capable of withstanding high temperatures and corrosive environments (Somiya and Intomata, 1991).
In this paper we present the results of our research on the preparation of SiC nanoporous membranes. Two different approaches have been utilized. One involves the use of chemical-vapor deposition (CVD)/chemical-vapor infiltration (CVI) techniques. We have utilized two different precursors, and have succeeded in preparing defect-free nanoporous membranes. The second approach involves the pyrolysis of pre-ceramic polymeric precursors. In contrast with prior efforts, a polymeric precursor that is capable of producing pure SiC membranes was selected. We envision the SiC membranes to eventually be utilized in reactive applications with the water gas shift (WGS) and methane steam reforming (SMR) reactions, where the membrane has to function in the presence of high-temperature steam.