Heterogeneity of Framework Ti(IV) in Ti−Silicalite as Revealed by the Adsorption of NH3. Combined Calorimetric and Spectroscopic Study

Abstract

The room-temperature adsorption of NH3 on Ti−silicalite (TS-1) was studied by means of IR and XANES−EXAFS spectroscopies and by microcalorimetry. The propensity of framework tetrahedral Ti(IV) heteroatoms to expand their coordination shell upon adsorption of NH3 was monitored by the perturbation induced on the Ti-sensitive framework stretching band (960 cm-1) and on the preedge peak at 4967 eV in XANES spectra. In both cases a modification of the local geometry of titanium sites upon adsorption of NH3 as an additional ligand was deduced. The amount of NH3 specifically adsorbed on the Ti(IV) sites was estimated by comparing volumetric data obtained for TS-1 with those obtained for a Ti-free silicalite taken as reference material. At pNH3 = 50 Torr, the number of NH3 molecules adsorbed per Ti atom was found to be close to 2, indicating that virtually all Ti atoms are involved in the interaction and have completed their 6-fold coordination shell. The molar heats of adsorption on the Ti(IV) sites were estimated following the same procedure. It was found that the molar heat of NH3 adsorption on titanium is higher (qm = 95−83 kJ/mol) than that measured for the silicalite matrix (qm = 66−58 kJ/mol). The interaction with ammonia was found to be essentially reversible at room temperature but for a significant amount of adsorbed species irreversibly held on titanium sites. The zero-coverage heat of adsorption was quite high (q ≈ 200 kJ/mol) not only for the first run of adsorption involving both irreversible and reversible interaction but also for the second run involving only the reversible component (q ≈ 130 kJ/mol). The evolution of the heat of adsorption with coverage was typical of heterogeneous surfaces; this is due not only to the presence of sites active toward ammonia on the silica matrix but also to the heterogeneous distribution of Ti(IV) sites, suggesting that a considerable number of framework sites (among the 12 available in the orthorhombic MFI framework) are occupied in a nearly equidistributed way. EXAFS measurements allowed the elongation of the Ti−O bond upon interaction with NH3 (0.05 ± 0.03 Å), the Ti−N distance (1.93 ± 0.03 Å), and the average number of adsorbed NH3 molecules per Ti site (1.9 ± 0.3) to be estimated. The results were in good agreement with the volumetric−calorimetric data. The effect of the pretreatment of TS-1 samples with ammonium acetate aqueous solution was also investigated by comparing the spectroscopic and the quantitative calorimetric results for both untreated and treated TS-1 samples.