Tailored Mesoporous Silicas: From Confinement Effects to Catalysis
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Tailored Mesoporous Silicas: From Confinement Effects to Catalysis A. C. Buchanan, III and Michelle K. Kidder Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6197, U.S.A. Email: [email protected] ABSTRACT Ordered mesoporous silicas continue to find widespread use as supports for diverse applications such as catalysis, separations, and sensors. They provide a versatile platform for these studies because of their high surface area and the ability to control pore size, topology, and surface properties over wide ranges. Furthermore, there is a diverse array of synthetic methodologies for tailoring the pore surface with organic, organometallic, and inorganic functional groups. In this paper, we will discuss two examples of tailored mesoporous silicas and the resultant impact on chemical reactivity. First, we explore the impact of pore confinement on the thermochemical reactivity of phenethyl phenyl ether (PhCH2CH2OPh, PPE), which is a model of the dominant ȕ-aryl ether linkage present in lignin derived from woody biomass. The influence of PPE surface immobilization, grafting density, silica pore diameter, and presence of a second surface-grafted inert “spacer” molecule on the product selectivity has been examined. We will show that the product selectivity can be substantially altered compared with the inherent gas-phase selectivity. Second, we have recently initiated an investigation of mesoporous silica supported, heterobimetallic oxide materials for photocatalytic conversion of carbon dioxide. Through surface organometallic chemistry, isolated M-O-M’ species can be generated on mesoporous silicas that, upon irradiation, form metal to metal charge transfer bands capable of converting CO2 into CO. Initial results from studies of Ti(IV)-O-Sn(II) on SBA-15 will be presented. INTRODUCTION There is emerging interest in understanding the effects of confinement of organic molecules in nanoporous solids, including ordered mesoporous metal oxides (pore diameters of ca. 2 – 50 nm), on their chemical properties and reactivity [1-5]. Possible new applications include the use of these solids as nanoreactors for organic synthesis including solid phase synthesis [4,6], for drug and gene delivery [7,8], and for design of asymmetric catalysts for the synthesis of chiral organic molecules [9]. Our research has been focusing on the effects of pore confinement, pore size, and surface properties on the pyrolysis rates and product selectivities for organic molecules confined in mesoporous silicas by covalent tethers [10-14]. Additional insights into the molecular dynamics under pore confinement conditions have come from fluorescence spectroscopy with fluorescent tags [15], quasi-elastic neutron scattering (QENS) experiments [16], and molecular dynamics simulations [10,13]. Typical organic grafting methods involve the reaction of the surface silanol groups (ŁSi– OH) with silane coupling agents such as silyl chlorides (R3Si–Cl), silyl alkoxides (R3Si–OR), and disilazanes (R3Si–NH–SiR3), which resul
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