Zeolite Confined Ti(OH) 4 Nanoparticles in Highly Active and Selective Oxidation Catalyst
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Zeolite Confined Ti(OH)4 Nanoparticles in Highly Active and Selective Oxidation Catalyst Istvan Halasz • Jian-Jie Liang • George Fitzgerald
Received: 9 March 2011 / Accepted: 9 May 2011 / Published online: 25 May 2011 Ó Springer Science+Business Media, LLC 2011
Abstract Some highly active, hydrophilic, MFI structured titanium silicate catalysts are characterized by an intense UV absorption centered in the 213–233 nm (43000–47000 cm-1) range. The assignment of this band to a specific molecular structure has remained unclear unlike the well published \213 nm charge transfer excitations between the 2p lattice oxygen orbitals and the 3d orbitals of isomorphously inserted, isolated, tetrahedral Tiatoms in TS-1 and the [233 nm UV bands of higher coordinated Ti-atoms at lattice defects or in variously agglomerated extra lattice TixOy(OH)z species. To decipher the structural origin of this intermediate UV band, we used Time-Dependent Density Functional Theory (TD-DFT) to compute the UV spectra of various model compositions and show in this paper that only one model, in which isolated tetrahedral Ti(OH)4 molecules reside in the zeolite channels without any covalent bond to the framework, fits the experimental UV spectrum. Hence these confined tetrahedral Ti(OH)4 nanoparticles may account for the enhanced activity and selectivity of some MFI type titanium silicate catalysts. Keywords Titanium silicate MFI TS-1 Selective oxidation Catalyst UV spectroscopy DFT model Nano Titanium hydroxide
I. Halasz (&) PQ Corporation, R&D Center, Conshohocken, PA 19428, USA e-mail: [email protected] J.-J. Liang G. Fitzgerald Accelrys Inc., 10188 Telesis Ct., San Diego, CA 92121, USA
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1 Introduction A substantial variety of micro- and mesoporous titanium silicates have been explored as potential catalysts for numerous environmentally benign, ‘‘green’’ oxidation processes [1–3]. Still, the versatility of the first commercially successful MFI structured catalyst, TS-1, stands out from this family. A large number of in-depth studies lead to the widely accepted and emphasized view that the unique catalytic activity and selectivity of TS-1 is associated with its strong hydrophobicity and its isolated, tetrahedral Ti4? ions which are isomorphously substituted into the MFI silicalite lattice [1–4]. A fingerprint for this structure is a single, sharp UV band at \213 nm ([47000 cm-1) and the total absence of any absorption at [235 nm (\42500 cm-1) which would mark higher coordinated Ti4? ions covalently attached to the zeolite lattice and/or residing in extra-lattice sites as TixOy(OH)z species agglomerated at various degrees, that all impede the catalytic activity and selectivity of TS-1 [4–6]. It has been repeatedly noted, however, that for certain H2O2 based oxidation processes some hydrophilic MFIstructured titanium silicate catalysts can be even more active and selective than TS-1 even without applying organic solvents [7–14]. The most intense UV absorption of such catalysts was found in the 213–233 nm (42500–47000 c
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