(153d) Insights into the Molecular Structure of Sulfated Mixed Metal Oxide Catalysts Via Vibrational Spectroscopy

Environmental concerns regarding the use of highly concentrated liquid acids, especially in the selective transformation of hydrocarbons, provide a strong incentive towards the design and development of solid acid catalytic materials as environmental friendly alternatives. Solid “super” acid catalysts could be considered the acidic materials that are stronger than the acid strength of 100% H₂SO₄. Those catalytic materials are extensively used in oil and petrochemical industries in order to replace highly toxic and non-reusable liquid acids towards economically and environmentally friendly processes. Although aluminosilicates have received considerable attention as solid acid catalysts, many other catalytic materials have been reported to show enhanced acid strength such as sulfated metal and mixed metal oxides (SO₄^2-/ZrO₂, SO₄^2-/TiO₂, SO₄^2-/TiO₂-SiO₂, SO₄^2-/ZrO₂-SiO₂).

There is limited understanding regarding the molecular structure of sulfate species developed on zirconia and tin oxides. To that extent, this talk will be focused mainly on Tin-Zirconium sulfated mixed metal oxides. A variety of catalysts have been prepared using co-precipitation/wet impregnation technique spanning different a) Sn:Zr molar ratios as well as b) sulfuric acid concentrations. Utilizing Raman and FTIR spectroscopies under hydrated and/or dehydrated conditions, the sulfate species developed depending on the composition of the mixed metal oxide are studied. The relative distribution of bidentate, tridentate and polymeric sulfate species is a strong function of the Sn:Zr molar ratio but not of the initial H₂SO₄ concentration. We also observe a strong dependence between the calcination temperature and the surface sulfate species are deposited on the mixed metal oxide. This in turn underscores that the sequence steps on which the sulfation applied controls not only the crystalline structure of the mixed metal oxide but also the relative distribution of acid functionalities. The catalytic evaluation of the aforementioned materials on the alkylation of phenol will be also discussed with respect to the intrinsic reactivity of the surface sulfate species.