/pdf/the-surface-chemistry-of-amorphous-silica-zhuravlev-model-4kfs2lx66q.pdf

The surface chemistry of amorphous silica. Zhuravlev model

High-resolution solid-state NMR techniques were used to investigate the surface structure of Cab-O-Sil fumed silica. 1H NMR results obtained from CRAMPS, MAS-only and relaxation studies reveal the existence of both hydrogen-bonded silanols and isolated silanols on the Cab-O-Sil surface. A systematic dehydration study of fumed silica was carried out, with results on the quantity of each type of silanol on the surface at various dehydration stages. 29Si CP-MAS experiments, including CP spin dynamics studies and various other relaxation studies, were employed to probe hydrogen bonding and the local structural environments of various hydroxyl groups of silica surfaces. 29Si CP-MAS experiments on water-treated and deuterium-exchanged Cab-O-Sil indicate the existence of interparticle silanols and internal silanols in fumed silica. 1H and 29Si NMR show that for fumed silica both isolated and hydrogen-bonded silanols are present on the surface of an untreated sample, in contrast to the case of silica gel, where a...

The Fumed Silica Surface: A Study by NMR

The catalytic conversion of m-cresol in the presence of H2 has been investigated on SiO2-supported Ni, Fe, and bimetallic Ni–Fe catalysts at 300 °C and atmospheric pressure. Over the monometallic Ni catalyst, the dominant product is 3-methylcyclohexanone while 3-methylcyclohexanol and toluene appear in smaller amounts, even at high conversions. By contrast, on Fe and Ni–Fe bimetallic catalysts, the dominant product is toluene while the hydrogenation products (3-methylcyclohexanone and 3-methylcyclohexanol) are practically negligible in the entire range of conversions. To explain these differences, we have proposed a deoxygenation path that starts with the tautomerization of m-cresol to an unstable ketone intermediate (3-methyl-3,5-cyclohexadienone). The fate of this intermediate is determined by the ability of the catalyst to either hydrogenate the carbonyl group or the ring. The former would mostly occur on Fe and Ni–Fe catalysts that contain an oxophilic metal (Fe), while the latter would occur on Ni, which has a higher affinity for the aromatic ring. Hydrogenation of the carbonyl group produces a very reactive unsaturated alcohol (3-methyl-3,5-cyclohexadienol), which can be easily dehydrated to toluene. This would explain the high selectivity of Fe and Ni–Fe to toluene. By contrast, hydrogenation of the ring would result in 3-methylcyclohexanone, which can be further hydrogenated to 3-methylcyclohexanol. On supports that contain acid sites, which are active for dehydration, the formation of toluene would occur via dehydration of the alcohol and subsequent dehydrogenation. On the catalysts investigated in this work, dehydration of the corresponding alcohol does not occur, so the only path to toluene is via hydrogenation of the carbonyl of the unstable ketone intermediate. In addition, to the products mentioned above, xylenol is also observed in significant yields, which indicate that transalkylation of m-cresol is another reaction path occurring on these catalysts.

Selective conversion of m-cresol to toluene over bimetallic Ni–Fe catalysts

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 generali

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

Reaction of Ta[CH2C(CH3)3]3[CHC(CH3)3] (1) with a silica dehydroxylated at 500 °C has been previously reported to produce a mixture of ⋮SiOTa[CH2C(CH3)3]2[CHC(CH3)3] (2a) and (⋮SiO)2Ta[CH2C(CH3)3][CHC(CH3)3] (2b). Treatment of these two surface organometallic complexes under 1 atm of hydrogen up to 200 °C leads to the formation of a surface tantalum(III) monohydride:  (⋮SiO)2TaIIIH (3) as a major product and surface Si−H groups. 3 has been characterized in the following way:  it reversibly exchanges with deuterium to give the corresponding [Ta]−D species; 3 reacts with CH3I to give methane; 3 reacts quantitatively with di(tert-butyl)ketone to form the corresponding tantalum di(tert-butyl)methoxide; 3 reacts with D2O to give a mixture of HD and D2 (2 ± 0.2 mol equiv per tantalum); and 3 activates the C−H bond of cycloalkanes (C5 to C8) at room temperature to form the corresponding surface tantalum(III) monoalkyls (⋮SiO)2Ta-alkyl, with liberation of 1 ± 0.1 equiv of hydrogen. The surface tantalum-alkyls are...

Synthesis, Characterization, and Reactivity, in the C−H Bond Activation of Cycloalkanes, of a Silica-Supported Tantalum(III) Monohydride Complex: (⋮SiO)2TaIII−H

Hydroxyls of silica powders

Multilayers of alternately adsorbing poly(allylamine) (PAH) and poly(acrylic acid) (PAA) of opposite charges on silica have been studied by the spin labeling technique, as a function of pH. The two polyelectrolytes have been labeled independently by a nitroxide free radical. Its electron paramagnetic resonance spectrum is mainly sensitive to the local Brownian motion and shows lines typical of two different environments, namely, loops protruding in solution with a fast motion and trains adsorbed on the solid with a hindered motion. These two parts have been evaluated for each of the polymer layers separately, and the thickness of the coatings has been described more precisely by characterizing the four contributions existing, for example, for a bilayer. Complexation is demonstrated by the loss of loops and tails belonging to the first polyelectrolyte. The overall picture emerging from the data is explained in terms of compensation of charges and entropy of confinement.

Formation of weak polyelectrolyte multilayers studied by spin labeling.

Poly(ethylene oxide) chains labeled at one end by a nitroxide free radical were adsorbed on silica. The electron paramagnetic resonance spectrum is mainly sensitive to the local Brownian motion and shows lines typical of two different environments, namely, loops and tails protruding in solution with a fast motion, and trains adsorbed on the solid with a hindered motion. When a solution of nonionic Triton surfactant with different concentrations is put in contact with the coated surface, the polymer segments are progressively removed from the surface and the chains extend into solution. This displacement is recorded as a function of temperature and concentration and follows a well-defined smooth path.

Displacement of poly(ethylene oxide) adsorbed on silica by a nonionic surfactant studied by spin labeling

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Papers

Hydroxyls of silica powders

Formation of weak polyelectrolyte multilayers studied by spin labeling.

Displacement of poly(ethylene oxide) adsorbed on silica by a nonionic surfactant studied by spin labeling