Sorption of Nitrogen Bases and XPS Study of Mesoporous Solid Acid SBA-15

Mesoporous SBA-15 material (Melero, J. A.; Stucky, G. D.; van Grieken, R.; Morales, G. J. Mater. Chem. 2002, 12, 1664) contains a high concentration of ⋮Si(CH₂)₃SO₃H functionalities accessible to ion exchange and adsorption of nitrogen bases. High-resolution XPS was used to determine stoichiometric equivalences between the −SO₃^- groups and NH₄⁺ cations exchanged for the −SO₃H protons as well as between the −SO₃H groups and adsorbed pyridine and ethylenediamine. XPS core-level shifts (CLS) of S2p photoemission to lower BEs identify the −SO₃H groups as sites for cation exchange and donor hydrogen bonding to the adsorbed nitrogen bases. The N1s CLS to higher BEs identify the nitrogen atoms of pyridine and one of the two N atoms of ethylenediamine as acceptors of the hydrogen bond. The CLS are smaller than those previously observed in Nafion-H and sulfated zirconia (SZ) (Johansson, M.; Klier, K. Topics Catal. 1997, 4, 99). An all-electron, nonrelativistic DFT theory yields core-level orbital energies where shifts correlate well with the observed XPS CLS. As a result of this combined study, the propylsulfonic groups of the SBA-15 material are identified as weaker acids than those of Nafion-H and SZ, albeit ones with high surface concentration and excellent ion-exchange capacity.