Chemical Engineering Science 64 (2009) 1585--1590 Contents lists available at ScienceDirect Chemical Engineering Science journal homepage: www.elsevier.com/locate/ces The preparation and characterization of hydrotalcite micromembranes Tae Wook Kim, Muhammad Sahimi, Theodore T. Tsotsis ∗ The Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1211, USA ARTICLE INFO ABSTRACT Article history: Received 8 September 2008 Received in revised form 27 November 2008 Accepted 17 December 2008 Available online 6 January 2009 Keywords: Hydrotalcite Micromembrane Gas separation Carbon dioxide The focus in this study is on preparing hydrotalcite (HT) thin micromembranes and on understanding their transport characteristics. HT micromembranes were prepared on silicon wafers and on perforated stainless steel discs, using two different types of HT colloidal suspensions, and were characterized by various surface techniques, as well as with single-gas permeation tests. Electron microscopy indicates that the HT films are compact and dense, and well-adhering on the substrates. For the Si-based membranes, the presence of an intermediate -Al 2 O 3 layer seemed to improve the characteristics of the deposited HT films. The study of the permeation properties of the HT micromembranes indicated ideal separation factors that exceeded the corresponding Knudsen values. Some of these membranes were also permse- lective towards CO 2 . These HT micromembranes show good promise for application in sensor devices and in membrane microreactors. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction The term MEMS (Micro Electro Mechanical System) is used to de- scribe miniaturized mechanical machinery, such as motors, turbines, pumps, and actuators used in micro-systems (Madou, 2002). MEMS fabrication techniques also find use in the design of microchemical systems (MCS), particularly membrane microreactors (MMR), which are devices that combine reaction and separation into a single unit. MMR have been fabricated, for example, using silicon (Si) wafers as micro-fuel processors to function as an integral component of minia- ture, portable methanol fuel cells (Jensen, 1999; Bravo et al., 2004; Goerke et al., 2004). The fabrication of micromembranes has been studied by several investigators who investigated the preparation of zeolite and Pd membranes on Si wafers (Wan et al., 2001; Wilhite et al., 2004; Pattekar and Kothare, 2004). Santamaria and coworkers (Mateo et al., 2004, 2007, 2008), for example, prepared silicalite-1 micromembranes on laser-perforated stainless steel (SS) sheets and tested them for propane and N 2 gas separation. Yeung and cowork- ers (Wan et al., 2001, 2005; Leung and Yeung, 2004) prepared MFI zeolite membranes on Si microchannels, and also tested them for gas separation. The synthesis of hydrotalcite (HT) thin films and membranes is also attracting recent interest, due to their unique separation prop- erties. The HT are layered double-hydroxide (LDH) clays, with their ∗ Corresponding author. Tel.: +1 213 740 2069; fax: +1 213 740 8053. E-mail address: tsotsis@usc.edu (T.T. Tsotsis). 0009-2509/$ - see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2008.12.029 interlayer spaces containing various exchangeable anions. The chem- ical structure of an anionic HT clay, for example, is [M (II) 1-x M (III) x (OH) 2 ] x+ [A m- x/m ] · nH 2 O, where M (II) is a divalent metal cation (Mg, Mn, Fe, Co, Ni, Cu, Zn, Ga), M (III) a trivalent metal cation (Al, Cr, Mn, Fe, Co, Ni, and La), and A m- represents an m-valence interlayer anion (Drits et al., 1987; Fletcher et al., 2002). The HT have a number of existing and potential sci- entific and technological uses, for example, as membranes and thin film chemical sensors, ion-exchange materials, catalysts, antacids, optical devices, and electrodes (Kim et al., 2008a; Cai et al., 1994). HT inorganic membranes, for example, show good promise for CO 2 separation and capture from flue-gas, since they are stable in high- temperature steam, and in oxidative environments typically encoun- tered in power-plants. This is in contrast with some of the more conventional inorganic membranes that are not capable of function- ing in such environments (e.g., carbon molecular sieve membranes in oxidative environments, and zeolite and silica membranes in the presence of high-temperature steam). The HT membranes also show promise for application in mem- brane reactors for the production of hydrogen through the steam reforming or the water gas shift reactions. The advantage of the HT over the Pd membranes is that they are permselective towards CO 2 , and the H 2 product is, therefore, delivered at high pressures, thus requiring no re-pressurization for further use. The ability to prepare high quality HT micromembranes will make possible their use in MMR, and as components of miniaturized fuel cells (Fletcher et al., 2002). The preparation and characterization of the HT micromem- branes are, therefore, the focus of the present paper.