One-pot incorporation of titanium catalytic sites into mesoporous true liquid crystal templated (TLCT) silica Maria E. Raimondi, a Leonardo Marchese,* b Enrica Gianotti, b Thomas Maschmeyer, c John M. Seddon* a and Salvatore Coluccia b a Department of Chemistry, Imperial College, London, UK SW7 2AY. E-mail: j.seddon@ic.ac.uk b Dipartimento di Chimica IFM, Universit` a di Torino, Via P. Giuria 7, I-10125 Torino, Italy c Davy Faraday Research Laboratory, The Royal Institution of G.B., 21 Albemarle Street, London, UK WX1 4BS Received (in Bath, UK) 20th August 1998, Accepted 26th November 1998 A one-pot synthesis procedure is described for the produc- tion of catalytically active titanium-incorporated mesopor- ous silica; the nature and accessibility of Ti(iv) sites derived from two different Ti sources was elucidated by means of catalytic tests, UV–VIS and photoluminescence spectros- copy. Titanosilicates are of great interest for various catalytic applications. 1 TS-1, a microporous titanium silicate containing catalytically active tetrahedral Ti(iv) sites, was first synthesised by Taramasso et al. 2 and was found to catalyse a wide range of low-temperature oxidations with hydrogen peroxide. In 1994, Corma et al. 3 and Pannavaia and coworkers 4 published their work on introducing titanium into the silica framework of MCM-41, and since then a considerable number of papers have been written dealing with various aspects of Ti-incorporated MCM-41, reviewed by Maschmeyer. 5 These materials can be prepared in principally two ways: (i) Grafting titanium species to the mesopore surfaces via a post-synthetic procedure 6 and (ii) substituting Ti into the silica framework by adding a titanium alkoxide precursor to the MCM-41 synthesis gel. 3,4 Here, we describe a novel ‘one-pot’ method for the synthesis of titanium-incorporated mesoporous silica via a true liquid crystal templating (TLCT) process. 7 The advantage of this method over those previously described for Ti-MCM–41 is that no post-synthetic, and potentially damaging, treatment of the mesoporous silicates is required, and the disordered side products often associated with classical MCM-41 formation are avoided. Two types of titanium precursor were used for comparison: titanium cyclopentadienyl dichloride (Aldrich) and Tilcom TAA (Tioxide Specialities Ltd). Tilcom TAA consists of a 75 wt% solution of diisopropoxy bis(pentane-2,4-dio- nato)titanium(iv), or titanium acetylacetonate, in isopropanol. The titanium content of Tilcom TAA is 9.9 wt%. The precursors were dissolved directly in the synthesis gel consisting of a 1 : 1 : 2.1 weight ratio of octaethylene glycol monododecyl ether (C 12 EO 8 , Fluka), 10 22 M HCl and tetramethoxysilane (TMOS, Aldrich), respectively. For 1.5 g of surfactant template, 0.06 g of titanocene dichloride or 150 ml of Tilcom TAA were added. The hexagonal mesoporous product formed overnight, under constant vacuum (to remove the methanol TMOS hydrolysis by-product), at room temperature. The deep orange colour of the titanocene-doped as-synthesised material suggested that the titanocene complex remained unhydrolysed throughout the synthesis (hydrolysis products would have been light orange/ yellow). It is also supposed that owing to the hydrophobic nature of the cyclopentadienyl ligands, the transition metal was located primarily in the central hydrocarbon regions of the polyoxyethylene (POE) cylindrical micelles (Scheme 1), i.e. on the internal surfaces of the mesopores once the materials were calcined. The POE micelles therefore protected the complex from the aqueous regions of the synthesis mixture, pointing to a potentially general method for mesopore functionalisation. We propose that due to the high solubility of Tilcom TAA in the aqueous regions of the liquid crystalline synthesis gel, a large proportion of the titanium sites in the final calcined products were buried in the silica framework (Scheme 1). Calcination of the materials at 500 °C produced silicate structures consisting of hexagonal arrays of long channels of ca. 25 Å diameter (as determined by TEM and N 2 adsorption measurements with BJH calculation). The hexagonal symmetry of the materials was confirmed by polarising microscopy (characteristic optical texture under crossed polarisers) and small-angle X-ray diffraction. After calcination at 500 °C, the hexagonal structure remained intact (seen by X-ray diffraction), but the location of the titanium species remained to be determined. The Ti contents in the calcined materials were calculated from the initial gel compositions, assuming complete hydrolysis and condensation to SiO 2 , and complete removal of the organic template by calcination. Assuming all of the titanium species remained implanted in the mesoporous struc- tures after calcination, the titanium metal loading for the products were 0.92 wt% for the titanocene-doped material (Ti[CP]) and 1.18 wt% for the Tilcom TAA-doped product (Ti[ACAC]). The amount of titanocene dichloride lost during the calcination process by evaporation/sublimation is not actually known; this would of course affect the Ti content in the calcined material. Catalytic tests (data not shown) on Ti[CP] showed that it was both active in the epoxidation of octene using tert-butylhy- droperoxide (TBHP) as the source of oxygen, 8 and in the peroxidative bromination of phenol red (phenolsulfonephtha- lein) to tetrabromophenol blue (3A,3B,5A,5B-tetrabromophe- nolsufonephthalein). 9 No catalytic tests were done on Ti[A- CAC], only on the calcined Ti[CP], as this appeared to be the most promising material from the spectroscopic studies. The epoxidation reaction was carried out at 80 °C under an inert argon atmosphere, and the reaction mixture consisted of 0.15 mol oct-1-ene, 0.0075 mol mesitylene, 0.0075 mol TBHP and 0.265 g Ti[CP] catalyst. Samples were taken from the reaction mixture at regular intervals and the reaction products were quantified by GC analysis. Octene was in large excess and acted as the solvent as well as reactant, mesitylene was the standard Scheme 1 Representation of the assumed locations of the titanium precursors, Ti acetyl acetonate and titanocene dichloride within the hexagonal liquid crystal phase. Chem. Commun., 1999, 87–88 87