Layered Clay Aerogels by a Freeze-Drying Process for a Platinum- Supported Catalyst Tomohiko Okada,* Taku Kato, Takeharu Yamaguchi, Toshio Sakai, and Shozi Mishima Department of Chemistry and Material Engineering, Faculty of Engineering, Shinshu University, Wakasato 4-17-1, Nagano 380-8553, Japan * S Supporting Information ABSTRACT: We prepared a clay aerogel by freeze drying an aqueous viscous suspension of a synthetic saponite (a layered clay mineral of the smectite family). The nanostructure and texture of the resulting aerogel varied with the freezing conditions. The freeze drying resulted in microporous aerogels with well-ordered (stacked) silicate layers. A fine texture was observed after freeze drying of a starting suspension at a lower concentration. Freezing in liquid propane yielded a well-defined continuous network texture. Toluene in vapor phase was oxidatively decomposed in air flow over aerogels on which platinum was deposited by an impregnation wetness method; the decomposition was probably affected by the size distribution of micropores. 1. INTRODUCTION Microporous and mesoporous solids have been investigated extensively as adsorbents and catalysts with molecular selectivity. To achieve precise molecular selectivity in adsorption/separation and catalysis, the shape and size of the pores in these solids are precisely controlled. Layered solids have been used as “scaffolds” to create nanospaces by the intercalation of guest species into the two-dimensional expandable interlayer spaces. 1-5 Among the layered solids, smectites (swellable 2:1-type layered silicates consisting of a negatively charged silicate layer (ca. 1.0 nm) and charge- compensating interlayer cations) have been investigated the most widely owing to their cation exchange ability, large surface area, and chemical and thermal stability. 6-8 The potentials for synthesis 9-15 and surface modification 16-23 give smectites and their resulting clay hybrids structural and compositional versatility. Therefore, smectites have been used widely, for example, as adsorbents of organic environmental contami- nants 1,17,18,21-23 and to support functional species, including catalytically and photoactive ones 1,3,8,18,22 in applications where the nature of the interlayer functionality and spatial density is important in designing functional nanostructures. Controllable morphology (i.e., particle size and shape) of smectites has also been important in optimizing the perform- ance of functional clay-based hybrids for practical and industrial applications. Whereas fine particles with a broad distribution of sizes are generally obtained, 9 it is possible to process smectites through the bottom-up self-assembly of silicate layers into noncordierite clay-based honeycomb monoliths; 24 they can also be processed into optically transparent films by casting suspensions on a substrate 25-28 and through alternate adsorption of a cationic polymer and anionic sheets of exfoliated clay. 29-34 Freeze drying of clay suspensions is another way of processing silicate layers as a “scaffold” into an aerogel with meso- or macroporosity. 35-37 Use of a highly stable aqueous suspension of smectites helps to give a homogeneous texture to the aerogel. When smectite particles disperse in water, delamination often occurs, partly leading to the formation of thixotropic gels. 8 The texture of freeze-dried aerogels is considered to be due to the presence of oligomeric layer aggregates arranged in a house-of-cards-like configuration. 36-38 Surface functionality has been modified by using pillaring agents to make interlayer micropores 38-40 and by hybridization with organic polymers. 41-43 The freeze-drying process often yields aerogels that have a unique texture and possess macropores; they have useful functions such as enhanced adsorption of organic molecules 44 and improved catalytic performance. 38,40 Because of a reduction in bulk density, aerogels with such a texture are usually tolerant to heat, liquid impregnation, and compressive stress. Thus, hierarchical hybridization by nanostructural design and nanosheet process- ing is expected to help in the development of useful functions for these materials. Here, we examine the catalytic oxidative decomposition of toluene in vapor phase after platinum immobilization by using an impregnation wetness method to further the industrial application of aerogels by freeze drying of an aqueous gel of a synthetic saponite (a clay mineral of the smectite family). We also discuss the formation of the aerogels and how both nanostructure and texture are affected by the preparation conditions (concentration of suspension and freezing rate). 2. EXPERIMENTAL SECTION 2.1. Materials. Synthetic saponite (Sumecton SA, JCSS- 3501, supplied by Kunimine Ind. Co., synthesized by a hydrothermal reaction) is a reference clay sample of the Clay Science Society of Japan and was used as received. Chemical composition of Sumecton SA is (Na 0.49 Mg 0.14 ) 0.77+ - [(Mg 5.97 Al 0.03 ) oct (Si 7.20 Al 0.80 ) tet O 20 (OH) 4 ] 0.77 - . 45 Tetraammineplatinum(II) nitrate, toluene, and ethanol were Received: May 24, 2013 Revised: July 23, 2013 Accepted: August 1, 2013 Published: August 1, 2013 Article pubs.acs.org/IECR © 2013 American Chemical Society 12018 dx.doi.org/10.1021/ie4015827 | Ind. Eng. Chem. Res. 2013, 52, 12018-12024