Design of Heterogeneous Catalysts via Multiple Active Site Positioning in Organic−Inorganic Hybrid Materials

Abstract

Catalytic materials bearing multiple sulfonic acid functional groups and positioned at varying distances from one another on the surface of mesoporous solids are prepared to explore the effects that the spatial arrangement of active sites have on catalytic activity and selectivity. A series of organosiloxane precursors containing either disulfide or sulfonate ester functionalities (synthons of the eventual sulfonic acid groups) are synthesized. From these molecular precursors, a variety of organic-inorganic hybrid, mesostructured SBA-15 silica materials are prepared using a postsynthetic grafting procedure that leads to disulfide and sulfonate ester modified silicas:. By subsequent chemical derivatization of the grafted species, thiol and sulfonic acid modified silicas are obtained. The materials are characterized by a variety of spectroscopic ( 13 C and 29 Si CP MAS NMR, X-ray diffraction) and quantitative (TGA/DTA, elemental analysis, acid capacity titration) techniques. In all cases, the organic fragment of the precursor molecule is grafted onto the solid without measurable decomposition, and the precursors are, in general, attached to the surface of the mesoporous oxide by multiple siloxane bridges. The disulfide species 2‚SBA and 3‚SBA are reduced to the corresponding thiols 7‚SBA and 8‚SBA, respectively, and 4‚SBA and 6‚ SBA are transformed to the aryl sulfonic acids 11‚SBA and 12‚SBA, respectively. 7‚SBA and 8‚SBA differ only in terms of the level of control of the spatial arrangement of the thiol groups. Both 7‚SBA and 8‚SBA are further modified by oxidation with hydrogen peroxide to produce the alkyl sulfonic acid modified materials 9‚SBA and 10‚SBA, respectively. The performances of the sulfonic acid containing SBA-15 silica materials (with the exception of 12‚SBA) are tested as catalysts for the condensation reaction of phenol and acetone to bisphenol A. The alkyl sulfonic acid modified material 10‚SBA derived from the cleavage and oxidation of the dipropyl disulfide modified material 3‚SBA is more active than not only its monosite analogue 9‚ SBA, but also the presumably stronger acid aryl sulfonic acid material 11‚SBA. It appears that a cooperative effect between two proximal functional groups may be operating in this reaction.

Cooperative catalysis by silica-supported organic functional groups

“…SBA-15 materials functionalized with sulfonic acid sites grouped into pairs have been investigated by Dufaud and Davis 41 and Mbaraka and Shanks. 42 These materials were obtained by incorporating a disulfide bridging group onto the surface, followed by reduction of the disulfide to pairs of thiols and subsequent oxidation with peroxide to generate the acid pairs.…”

Simultaneous Sampling of Vapor and Particle-Phase Carcinogenic Polycyclic Aromatic Hydrocarbons on Functionalized Glass Fiber Filters

“…Such a weight loss associated to a calorimetric peak in the range 150-300°C can be associated to the functionalization of the silica surface (Bugrayev et al, 2008Cataldo et al, 2008). An organic carbon content of 6% can be estimated using the thermogravimetric analysis (Dufaud et al 2003;Cousiniè et al, 2007). It can be also observed that the signal associated to desorption of water (below 100°C) is lower for the functionalized filter: this phenomenon can also be attributed to the functionalization, which lead to a less hygroscopic surface.…”

Investigation of the catalytic activity of extracted and smoothly calcined arenesulfonic modified SBA-15 materials

“…Such hybrid solids are usually characterised by MAS 29 Si [8,10,[13][14][15]24,29] and 13 C NMR [8,10,[13][14][15]19,24,25,29,31]. To our knowledge, no 33 S NMR measurements were actually performed on such materials.…”

“…The direct introduction of a stronger function brought by the 2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane (CSPTMS) is also considered with a growing attention [11,12,[16][17][18][19][20][21][22]. Co-condensation methods [10][11][12][13][15][16][17][18][19][23][24][25][26][27][28][29] are preferred over grafting procedures [14,[30][31][32][33][34] due to the better distribution of the functions throughout the material. The removal of the organic surfactants, cetyltrimethylammonium halide and Pluronic P123, used as structure-directing agents (for MCM-41 [10,13,25,26,28,29,33,35] and SBA-15 [10][11][12]15,[17][18][19]27], respectively) is usually conducted via their extractions by...…”

“…The removal of the organic surfactants, cetyltrimethylammonium halide and Pluronic P123, used as structure-directing agents (for MCM-41 [10,13,25,26,28,29,33,35] and SBA-15 [10][11][12]15,[17][18][19]27], respectively) is usually conducted via their extractions by HCl/ethanol and ethanol, respectively. The materials thus prepared have been successfully tested in a large number of acid-catalysed reactions such as esterification [8,13,15,18,19,23,25,30,35,36], etherification [9,27], alkylation [10], condensation [23,25,[31][32][33] and different rearrangement processes [10,12,16,19,22].…”