Assembled Surface-Anisotropic Colloids as a Template for a Multistage Catalytic Membrane Reactor Jung Hun (Kevin) Song and Ilona Kretzschmar* Department of Chemical Engineering, The City College of New York, Steinman Hall, 140th Street & Convent Avenue, New York, New York 10031 ABSTRACT A polymeric catalytic membrane reactor (CMR) is fabricated using alternating assemblies of surface-anisotropic (sa-) and plain (p-) polystyrene (PS) colloids as a template. We report the preparation of TiO 2 sa-PS colloids by physical vapor deposition of titanium onto a colloidal monolayer in an oxygen-rich environment and employ the modified colloids as a means to deliver the TiO 2 catalyst to the CMR pores. sa-PS and p-PS colloids are assembled into alternating cylindrical sections inside a microcapillary followed by infiltration and curing of a liquid polymer precursor in the interstitial space of the assembly. Subsequent organic solvent treatment results in a cylindrical porous CMR with embedded TiO 2 caps. TiO 2 cap embedment, composition and surface morphology, surface pore structure, and cross-sectional integrity are analyzed using variable-pressure scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. KEYWORDS: colloidal template • porous materials • fibers • polystyrene • multisectional • reactive separation • multistage • catalyst • titanium dioxide • titanium • physical vapor deposition • Janus particle INTRODUCTION P rocess intensification (PI) promises new modes of enhancement in reaction engineering by drastically improving the technique and operation of reactive separation (1). Novel and innovative technology provides a pathway for process improvement by reducing the equip- ment volume, processing time, energy consumption, and ultimately operational costs (2). A notable PI is a catalytic membrane reactor (CMR), where the unit operations of separation and reaction are combined into one spacial and temporal unit. CMRs offer numerous advantages in unit operations including (i) a chemical equilibrium shift brought on by the continuous removal of a product from a reaction mixture to enhance the yield, (ii) decreased product inhibi- tion and increased overall reaction rate by the continuous removal of a product, (iii) reduced side reactions through lower operating temperatures, and (iv) facilitated separation of the reactants and products through membrane separation (3, 4). Polymeric CMRs, unlike metallic or inorganic CMRs, are being widely explored for various processes because of their economic efficiency and tailorable properties (3, 5). Despite their advantageous process enhancement, polymeric CMRs are limited by the operating conditions; i.e., most often they are inoperable at an operating temperature above 250 °C. However, even with this limitation, polymeric CMRs have numerous applications in volatile organic compound deg- radation, hydrogenation, and other low-temperature cata- lytic reactive operations (6). The application, performance, and efficiency of a poly- meric CMR depend on the distribution of the selected catalyst and the porous structure of the CMR (7). These properties may be addressed and tailored by employing polymeric colloids in the fabrication of a CMR. For example, three-dimensional colloidal assemblies have been widely applied to template and synthesize ordered porous materials (8). On the other hand, surface-anisotropic modification of colloids has gained immense interest because of the wide range of modifications available (9, 10). Combining colloidal templating and surface-anisotropic modification techniques yields a surface-anisotropic colloidal delivery and template system (sa-CDTS) for CMR synthesis, where the catalyst material distribution and porous structure are precisely controlled. One of the many functional catalyst materials available that may be utilized in a CMR is titanium (Ti) and its oxides (Ti x O y ) (3, 11). Among multiple oxidation states of Ti x O y (x ) 1, 2; y ) 1-3), TiO 2 is the principle component of all oxides (12). TiO 2 has been reported to form on the surface of bulk Ti within milliseconds of exposure to air (13). With prolonged exposure to oxygen and increased transport of Ti from the suboxide layer to the surface, a natural 10 nm thin TiO 2 layer is formed (12-15). In addition to the natu- rally forming TiO 2 , many processes including thermal plasma (16), electrochemical (14), and sputter deposition (17) as well as electric-arc physical vapor deposition (PVD) (18) have been developed to induce the formation and deposition of TiO 2 films. Further, TiO 2 is extensively researched for its high photocatalytic activity at low temperature and economic * Corresponding author. E-mail: kretzschmar@ccny.cuny.edu. Received for review April 28, 2009 and accepted June 24, 2009 DOI: 10.1021/am900286k © 2009 American Chemical Society ARTICLE www.acsami.org VOL. 1 • NO. 8 • 1747–1754 • 2009 1747 Published on Web 07/15/2009 Downloaded by COLUMBIA UNIV on August 26, 2009 | http://pubs.acs.org Publication Date (Web): July 15, 2009 | doi: 10.1021/am900286k