Heterogeneity of framework Ti(IV) in Ti-silicalite as revealed by the adsorption of NH3. Combined calorimetric and spectroscopic study

room-temperature adsorption of NH3 on Ti-silicalite (TS-I) was studied by m eans of IR and XANES-EXAFS spectroscopies and by microcalorimetry. The prop ensity of framework tetrahedral Ti(TV) heteroatoms to expand their coordina tion shell upon adsorption of NH3 was monitored by the perturbation induced on the Ti-sensitive framework stretching band (960 cm(-1)) and on the pree dge peak at 4967 eV in XANES spectra. In both cases a modification of the l ocal geometry of titanium sites upon adsorption of NH3 as an additional lig and was deduced. The amount of NH3 specifically adsorbed on the Ti(TV) site s was estimated by comparing volumetric data obtained for TS-1 with those o btained for a Ti-free silicalite taken as reference material. At p(NH3) = 5 0 Torr, the number of NH3 molecules adsorbed per Ti atom was found to be cl ose to 2, indicating that virtually all Ti atoms are involved in the intera ction and have completed their B-fold coordination shell. The molar heats o f adsorption on the Ti(IV) sites were estimated following the same procedur e. It was found that the molar heat of NH3 adsorption on titanium is higher (q(m) = 95-83 kJ/mol) than that measured for the silicalite matrix (q(m) = 66-58 kJ/mol). The interaction with ammonia was found to be essentially re versible at room temperature but for a significant amount of adsorbed speci es irreversibly held on titanium sites. The zero-coverage heat of adsorptio n was quite high (q approximate to 200 kJ/mol) not only for the first run o f adsorption involving both irreversible and reversible interaction but als o for the second run involving only the reversible component (q approximate to 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 heterogene ous 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 th e elongation of the Ti-O bond upon interaction with NH3 (0.05 +/- 0.03 Angs trom), the Ti-N distance (1.93 +/- 0.03 Angstrom), and the average number o f adsorbed NH3 molecules per Ti site (1.9 +/- 0.3) to be estimated. The res ults were in good agreement with the volumetric-calorimetric data. The effe ct of the pretreatment of TS-1 samples with ammonium acetate aqueous soluti on was also investigated by comparing the spectroscopic and the quantitativ e calorimetric results for both untreated and treated TS-1 samples.