Acidity of Sulfonic and Phosphonic Acid-Functionalized SBA-15 under Almost Water-Free Conditions

The hydrogen bond interaction of pyridine with sulfonic and phosphonic acid moieties at the surface of SBA-15 ordered mesoporous silica has been studied by a combination of solid-state NMR techniques. The composition of the materials is characterized by 29 Si MAS NMR, the residual water content is inspected by 1 H MAS NMR, and the hydrogen bond interactions are characterized by 15 N CPMAS NMR at 130 K using pyridine- 15 N as a probe molecule. It is shown that (i) all acid moieties at the surface are accessible for pyridine; (ii) each sulfonic acid moiety interacts with one pyridine molecule; (iii) each phosphonic acid moiety can interact simultaneously with two pyridine molecules; (iv) for both materials the interaction of the acid moieties with the base results in proton transfer to pyridine. The observed proton-donating ability of the acid moieties depends on the presence of residual water. In contrast to nonfunctionalized SBA-15, the sulfonic acid-functionalized SBA-15 material contains about six water molecules per acid moiety after drying at 420 K in high vacuum. From the 15 N chemical shift of pyridine in the hydrogen-bonded complex, it is estimated that the proton-donating ability of the acidic functional groups solvated by such small water clusters is equivalent to that of acids in water exhibiting a p K a of about 0.6 and 1.3, respectively, for the sulfonic and phosphonic acid moieties. The O···H and H···N distances in the hydrogen bond of the pyridine complex are r OH ≈ 1.69 Å and r HN ≈ 1.05 Å for sulfonic acid, as compared to r OH ≈ 1.53 Å and r HN ≈ 1.09 Å for phosphonic acid.