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

Abstract

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 29Si MAS NMR, the residual water content is inspected by 1H MAS NMR, and the hydrogen bond interactions are characterized by 15N CPMAS NMR at 130 K using pyridine-15N 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 15N 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 pKa 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 rOH ≈ 1.69 Å and rHN ≈ 1.05 Å for sulfonic acid, as compared to rOH ≈ 1.53 Å and rHN ≈ 1.09 Å for phosphonic acid.