Acidity and Hydrophobicity of TS-1

This report investigates the causes of the unique properties and reactivity of the titanium silicalite TS-1, a commercial zeolite containing 1% Ti by weight. The combined analysis of calorimetric and adsorption data, cal−ad, for the reaction of TS-1 with pyridine shows the absence of the strong acid site found in HZSM-5 and the presence of only 0.07 mmol g-1 of a −15.1 kcal mol-1 hydrogen-bonding site. Titrations with 2,6-lutidine indicate the same number of hydrogen-bonding sites with a lower enthalpy (−9.8 kcal mol-1). The magnitude of these enthalpies indicate that the acceptor sites are hydrogen-bonding sites that are comparable in acidity to the strongest sites on silica gel. The small amounts of these sites, 0.07 mmol g-1 compared to 0.8 mmol g-1 in silica gel and 0.5 mmol g-1 for the hydrogen-bonding sites of HZSM-5, account for the lower affinity of TS-1 for water. The absence of strong acidity prevents epoxide ring opening and explains the utility of TS-1 in epoxidation. Comparison of the results from the multiple equilibrium analysis of gas-phase adsorption isotherms for CO and CH4 by TS-1, HZSM-5, and silica gel indicates that the surface of TS-1 is not more polarizable than the other solids. Thus, the porosity and small number of hydrogen-bonding-sites enables TS-1 to adsorb hydrocarbons in the presence of water.