A Novel and Versatile Grafting Procedure: Toward the Highest Possible Sulfonation Degree of Mesoporous Silica

Abstract

A unique method for the postsynthetic functionalization of ordered mesoporous silica materials is described. The functionalization is done via a chemical vapor deposition‐type method and leads to the quantitative reaction of isolated surface silanol groups of mesoporous silica materials with organosilane molecules. The result is a highly sulfonated material by anchorage of 3‐mercaptopropyltrimethoxysilane molecules and subsequent oxidation of the thiol groups to sulfonic acid groups. Ordered mesoporous KIT‐6 and SBA‐15 are chosen as model systems. The sulfonated material exhibits a functionalization degree of 1.05 resp. 1.32 molecules nm−2, which is on the maximum of the theoretical achievable functionalization density resulting in high ion exchange capacities of 1.22 and 1.12 mmol g−1. The versatility and mildness of this method opens up functionalization possibilities for a wide range of other materials, which could be sulfonated or otherwise modified with silanes, and tailored for the further application in catalysis or as solid‐state proton conductors.

Sulfonic Acid-Functionalized Inorganic Materials as Efficient Catalysts in Various Applications: A Minireview

“…For the synthesis of an efficient acid catalyst an important factor to take in consideration is the sulfonic loading of the silica and mesoporous materials. According to Timm et al [13], the principal drawbacks for a correct sulfonation load that may occur are represented by the collapse or the synthesis of non-ordered structure when the cocondensation method in a template-assisted synthesis is carried out. In the case of grafting procedure, the low functionalization loading despite the use of alkoxysilane in huge excess means it is an inefficient, uneconomic and environmental unfriendly process.…”

“…A powerful tool for the characterization of functionalized silica is 13 C and 29 Si NMR which permits analysis of the evolution of the chemical environment around silicon atoms introduced by addition of active components. The 29 Si NMR of bare silica presents features to be attributed to silicon without hydroxy groups Si-(O-Si) 4 , silicon atoms bearing one hydroxy group (Si-O) 3 -Si-OH and silicon atoms bearing two hydroxy groups (Si-O) 2 -Si-(OH) 2 called germinal silanols.…”

“…In addition to the changes in proportion between these peaks, when functionalized, a new set of peaks in the range {−45;−80} ppm appears giving evidence for the covalent incorporation of the functional groups. The peaks correspond to three different environments for the siloxanes groups: T 1 , T 2 , and T 3 assigned respectively to [T m = RSi(OSi) m (OMe) 3-m ] [11,13,[81][82][83][84][85]. Khrisna et al [86], for example, attribute the presence of more T 3 species in RSO 3 H-MCM-41 compared to RSO 3 H-SBA-15 to a higher number of Q 3 in the bare MCM-41 precursor indicating a more efficient condensation.…”

“…In the case of functionalized alumina by Maldonado et al [79], which used three different synthesis procedures, it was found that even though the prevalent signal was, in each case, T 2 , each procedure produced a different proportion between T 1, T 2 and T 3 signals (see Figure 16). Alternatively, the 13 C NMR allows us to follow the functionalization by looking at the different types of carbon. According to the literature [79,85,88,89] the signal at 13-15 ppm is related to carbon directly linked to silicon and the sign at 50 ppm may be related to carbon linked to sulfonic groups (see Figure 17).…”

“…Figure 17. 13 CNMR spectra of different extracted SBA-15 mesoporous silicas: (A) conventional SBA; (B) thiol-functionalized SBA, (C) sulfonic-functionalized SBA-15 obtained through the directsynthesis obtained through the direct-synthesis approach; (D) sulfonic-functionalized SBA-15 obtained through the direct-synthesis approach. Reprinted with permission from ref.…”