Published: December 19, 2011 r2011 American Chemical Society 1658 dx.doi.org/10.1021/jp206017j | J. Phys. Chem. C 2012, 116, 1658–1669 ARTICLE pubs.acs.org/JPCC Probing the Nature and Local Structure of Phosphonic Acid Groups Functionalized in Mesoporous Silica SBA-15 Yu-Chi Pan, † Hui-Hsu Gavin Tsai, † Jyh-Chiang Jiang, ‡ Chia-Chun Kao, † Tsai-Lung Sung, † Po-Jui Chiu, † Diganta Saikia, † Jen-Hsuan Chang, † and Hsien-Ming Kao* ,† † Department of Chemistry, National Central University, Chung-Li, Taiwan 32054, R.O.C. ‡ Department of Chemical Engineering, National Taiwan University of Science and Technology, #43, Sec. 4, Keelung Rd., Taipei, 106, Taiwan, R.O.C. b S Supporting Information ’ INTRODUCTION Since the pure mesoporous silica materials have no intrinsic catalytic activity, extensive efforts have been devoted to function- alize them with appropriate organic functional groups in order to tune their surface properties for designated applications in various research fields. 1À10 The properties of organic-functionalized mesoporous silica materials strongly depend on the nature, loading, and framework distribution of organic functional groups and their spatial proximities. To ensure a better control of the loading and homogeneous distribution of the organic groups, the materials are often prepared by the one-pot direct-synthesis route, namely, via cocondensation of tetraethoxysilane (TEOS) and an organosilane containing the desired organic functionality. Of particular interest is that mesoporous silica materials func- tionalized with a variety of acidic functional groups, such as sulfonic, 9 carboxylic, 11À13 and phosphonic acid groups, 14À17 have been synthesized and applied in a number of acid-catalyzed reactions. Moreover, these acidic organic functionalized materi- als also exhibit high enough proton conductivity to make them promising candidates as electrolytes for fuel cell systems. 18À20 The organic functional groups that incorporated in mesopo- rous silica materials via one-pot synthesis route are often assumed to be uniformly distributed without direct spectroscopic evidence. It is essential to gain detailed insights into the local structure and framework distribution of the organic groups and their interac- tions with the pore surface via suitable characterization tech- niques. To address this issue, solid-state NMR methods can make significant contribution. For example, we recently demonstrated Received: June 26, 2011 Revised: December 19, 2011 ABSTRACT: Well-ordered mesoporous silicas SBA-15 functionalized with variable contents of phosphonic acid groups (up to 25 mol % based on silica) have successfully synthesized via cocondensation of tetraethoxysilane (TEOS) and diethylphosphatoethyltriethoxysilane (PETES) using triblock copolymer Pluronic P123 as the structure-directing agent under acidic conditions. The status and local structures of the phosphonic functional groups are investigated by extensive multinuclear solid-state NMR studies. Solid-state 13 C and 31 P NMR results reveal that phosphonic ester moieties are obtained for the as-synthesized samples and for the samples subjected to template removal by concentrated H 2 SO 4 . The generation of phosphonic acid groups can be accomplished by dealkylation reaction via treating the template- extracted samples with concentrated HCl. Two distinct local environments for the phosphorus sites of phosphonic acid groups have been observed at 32 and 22 ppm in the 31 P magic angle spinning (MAS) NMR spectra. The relative ratio between these two species is not sensitive to the loading of phosphonic acid groups incorporated, but it strongly depends on the moisture present in the materials. The PO 3 H 2 groups forming the hydrogen bonds with the nearby Q 3 SiÀOH are the major species responsible for the 22 ppm peak based on the results of 1 H f 31 P f 29 Si double cross-polarization NMR experiments and density functional theory calculations (DFT). Of particular interest is that 29 Si{ 31 P} rotational echo double resonance (REDOR) NMR experiments are utilized to measure 31 PÀ 29 Si distances between the phosphorus site in the functional groups and the silicon sites in the silica framework. A 29 SiÀ 31 P distance of 5.0 Å is obtained for the phosphorus site in the functional groups to the silicon site of the Q 3 species for the as-synthesized sample. A reasonable fitting to the REDOR data for the acidified sample can also be achievable by assuming the presence of different structural units, whose 31 PÀ 29 Si distance information is referred from the DFT results. The combination of REDOR and 1 H f 31 P f 29 Si double cross-polarization NMR measurements and the DFT calculations allow one to gain deeper insights into the local environments of the organic groups functionalized in mesoporous silica materials.