A Detailed Model of Local Structure and Silanol Hydrogen Bonding of Silica Gel Surfaces

Abstract

A refined and generalized version of a previously suggested model of the silica surface, in which geminal silanols are situated on surface segments similar to (100)-type faces of the β-cristobalite structure and single silanols are situated on surface segments similar to corresponding (111)-type faces, is supported by extensive spectroscopic data. In this model single silanols on the same (111)-type surface segment cannot form hydrogen bonds with each other. Whether or not adjacent geminal silanols on the same (100)-type surface segment can form hydrogen bonds with each other depends on the relative orientation of their hydroxyl groups. When two surface segments of either (100)- or (111)-type intersect convexly, hydroxyl groups cannot participate in hydrogen bonding across the intersection; but when two surface segments intersect concavely, those silanols situated at the intersection can form hydrogen bonds with their counterparts across the intersection. All the hydrogen-bonding silanols in this generalized β-cristobalite model have a common feature: when any two silanols are hydrogen bonded to each other, the two silicon atoms containing them are also situated on the same (100)-type surface segment. This idealized structure of the surface of silica gel, which is clearly known from X-ray diffraction to be an amorphous material, may be distorted for various thermodynamic or kinetic reasons during its formation; therefore, a wide range of hydrogen-bonding strengths between two hydroxyls is likely on a real silica gel surface. The generalized β-cristobalite surface model can also explain the reversible dehydroxylation and rehydroxylation processes on silica surfaces. Both single and geminal silanols participating in hydrogen bonding are most easily dehydroxylated under evacuation at temperatures between 170 and 450 °C and form low-strain bicyclo[3.3.0]octasiloxane rings. The mode of dehydroxylation on a silica surface undergoes a transformation between 450 and 650 °C, yielding highly strained trisiloxane rings for dehydroxylation at T ≥ 650 °C.

The surface chemistry of amorphous silica. Zhuravlev model

“…The theoretical values were reported in literature for the concentration of OH groups on the surface of different types of silica [1,23,25,32,38,80,92,159,160,265,285]. These data show that at present b-cristobalite model of De Boer and Vleeskens is accepted as the correct one.…”

“…There is a qualitative agreement between our results and the data as reported by Papee [24], Gokcek and Boehm [80,81] and several other authors. The b-cristobalite models of dehydroxylation process as developed by Branda et al [265], Sindorf and Maciel [159,160] and Chuang and Maciel [285] support the evidence of a two-stage temperature dependence of the silanol number h OH .…”

“…These data show that at present b-cristobalite model of De Boer and Vleeskens is accepted as the correct one. Thus, for instance, Branda et al [265], Sindorf and Maciel [159,160] and Chuang and Maciel [285] when working out their models to describe the surface of amorphous silica, resorted to the concept developed by De Boer and Vleeskens, but in these models they also took into account the existence on the silica surface of a certain part of geminal silanols (besides the {111} face they considered the {100} face of b-cristobalite).…”

Preparation, purification, and characterization of aminopropyl-functionalized silica sol

“…The two broad bands at 3386 and 3260 cm -1 , and the band at 1583 cm -1 can be assigned to the N-H stretching vibration and to the NH 2 deformation mode of free amino groups, accordingly [32][33]. [35]. According to the DP experiment the extent of surface modification is less than 1mol%.…”

Theoretical identification of structural heterogeneities of divalent nickel active sites in NiMCM-41 nanoporous catalysts

“…As an approximation, the structure of cristobalite is normally taken as a good representative for the amorphous silica materials in terms of type and density of the surface hydroxyl groups [51,[53][54][55][56]. Many references [42,43,[57][58][59][60] have then applied this model to ordered silica mesoporous materials such as MCM-41.…”