Vicinal Silanols Research Articles

Gas phase Beckmann rearrangement of cyclohexanone oxime to ɛ-caprolactam over mesoporous, microporous and amorphous Nb2O5/silica catalysts: A comparative study

The gas phase Beckmann rearrangement of cyclohexanone oxime to ɛ-caprolactam was investigated over niobium containing mesoporous (Nb-MCM-41, Nb-HMS and Nb-SBA-15), microporous (Nb-Beta zeolite) and amorphous (Nb impregnated on SiO2) catalysts. A relation between catalytic activities, structural and the acidic properties was derived. The catalysts Nb2O5/SiO2, Nb-MCM-41, Nb-SBA-15, and Nb-HMS resulted in higher cyclohexanone oxime conversion and caprolactam selectivity compared with Nb-Beta zeolite. Almost 100% cyclohexanone oxime conversion and up to 98% caprolactam selectivity were achieved over Nb2O5/SiO2 catalyst. Further, the catalytic activity results of Nb-MCM-41 are fairly similar to Nb2O5/SiO2. The high activity of both catalysts is related to vicinal silanol groups and silanol nests with weak acidity and negligible Brønsted acid sites. The physico-chemical properties of the investigated catalysts were characterized by various techniques including X-ray diffraction, N2 physisorption, Fourier transform infrared spectroscopy (FTIR), UV–vis diffuse reflectance, atomic emission spectroscopy (ICP-AES), and Raman spectroscopy. Catalyst acidity was measured by NH3-TPD and pyridine adsorbed FTIR spectroscopy. The structural characterization results revealed that Nb was incorporated into the mesoporous and microporous materials framework.

New insights into the surface functionalities and adsorption evolution of water molecules on silica gel surface: A study by second derivative near infrared spectroscopy

Surface functionalities and adsorption evolution of water molecules on silica gel samples were studied by second derivative near infrared spectroscopy. Four different silica gel samples with varying surface area were used in this experiment. Each of the dry samples was allowed to equilibrate with surrounding air and the near infrared spectra were accumulated at different time intervals using reflectance technique. The evolved spectra were analysed using second derivative technique to study the adsorption evolution of water molecules on the surface. The second derivative spectral profiles have revealed some interesting features about the surface functionalities and some events of adsorption on silica gel surface that have never been reported in the literature. The near infrared spectroscopic evidence is provided for the first time to show the presence of hydrogen bonded silanol groups on silica gel surface. The results and the events show that the adsorption of water molecules readily takes take place on the hydrogen bonded vicinal silanol groups and spread over the space and ends with the hydrogen bonding at free silanol groups. Water molecules then build over these molecules to form a network of hydrogen bonded water molecules. There is clear evidence that the presence of hydrogen bonded silanol groups is a necessary condition for the effective adsorption activities on silica gel surface. Furthermore, the study reveals that while mono and multi-layer water molecular adsorption takes place at certain active sites of the surface, a large portion of the surface is free from any adsorption activities.

Grafting of Lanthanide Complexes on Silica Surfaces: A Theoretical Investigation

Grafting catalysts on a surface leads to heterogeneous catalysts with well-defined active sites. However, the grafting mode of a lanthanum complex onto silica remains unknown. To shed light on this grafting reaction, different studies have been achieved in the framework of density functional theory. The silica substrate hydroxylated at 700 degrees C has been simulated both by molecular and periodic models. The created molecular models are in agreement with the rigidity of the ligand, the surface density of silanol groups, and the different spectroscopic data of a silica surface partially dehydroxylated at 700 degrees C. Two possible models of surface have henceforth been considered: the first one with one isolated silanol and the second one with two vicinal silanols linked by a siloxane bridge. The thermodynamics of a grafting reaction of lanthanum catalysts on these models has also been investigated. This reaction leads to thermodynamically stable structures that reveal different types of grafting: monografted, bigrafted, or bigrafted after breaking of a Si-O-Si bridge. Similarly to experimental approaches, coordination of triphenylphosphine oxide (O=PPh(3)) has also been considered as a probe of the grafting mode. A good agreement between the theoretical and the experimental spectroscopic values has systematically been found, but none of the grafting modes seem to be more relevant. Accordingly, it is necessary to consider in subsequent studies that all grafting modes coexist, increasing the difficulty to theoretically investigate multistep reactions.

Mode of Adsorption of (CH3)2Au(acac) onto Partially Dehydroxylated Silica

The deposition of volatile cis-(CH3)2Au(O,O′-acac) onto silica partially dehydroxylated at 400 °C leads to molecular dispersion of the organogold complex. X-ray absorption near-edge structure demonstrates that the gold retains its oxidation state upon binding. IR and 1H magic-angle spinning NMR spectra suggest that the molecular framework also remains intact. Computational models involving hydroxyl-terminated octasilsesquioxane cube clusters to represent silica predict the most energetically favorable interaction to be hydrogen-bonding between two surface hydroxyls and the oxygen donor atoms of the chelated acetylacetonate ligand. This mode of adsorption was confirmed by analysis of the IR and the Au LIII edge extended X-ray absorption fine structure. The surface hydroxyls most likely to participate in the attachment of the gold complex to silica are vicinal silanol pairs that are not involved in hydrogen bonding to other silanols.

Reaction of Trimethylchlorosilane in Spin-On Silicalite-1 Zeolite Film

We present a study on the hydrophobization of spin-on Silicalite-1 zeolite films through silylation with trimethylchlorosilane. Microporous and micro-mesoporous Silicalite-1 films were synthesized by spin coating of suspensions of Silicalite-1 nanozeolite crystallized for different times. Ellipsometric porosimetry with toluene and water adsorbates reveals that silylation decreases the porosity and makes the films hydrophobic. The decrease in porosity depends on the exposed surface area in the pores. Water contact angle measurements confirm the hydrophobicity. Fourier transform infrared spectroscopy reveals that the trimethylsilyl groups are chemisorbed selectively on isolated silanols and less on geminal and vicinal silanols due to steric limitations. Time-of-flight secondary-ion mass spectroscopy and in situ ellipsometry analysis of the reaction kinetics show that the silylation is a bulk process occurring in the absence of diffusion limitation. Electrical current leakage on films decreases upon silylation. Silylation with trimethylchlorosilane is shown to be an effective hydrophobization method for spin-on Silicalite-1 zeolite films.

Nature of ≡SiOCrO2Cl and (≡SiO)2CrO2Sites Prepared by Grafting CrO2Cl2onto Silica

The room-temperature reaction between chromyl chloride and Sylopol 952 silicas pretreated at 200, 450, and 800 °C was investigated using IR, XANES, and EXAFS spectroscopy, as well as by DFT modeling. On the silicas pretreated at 200 and 450 °C, the structurally uniform sites formed by the reaction with one surface hydroxyl group are described as ≡SiOCrO2Cl. Unreacted silanols persist on these silicas even in the presence of excess CrO2Cl2, and on the silica pretreated at 200 °C some participate in hydrogen bonding with the grafted monochlorochromate sites. On the silica pretreated at 800 °C, both ≡SiOCrO2Cl and (≡SiO)2CrO2 sites are formed. The latter are produced despite the absence of hydrogen-bonded hydroxyl pairs on the support. The origin of the chromate sites is proposed to be the reaction between CrO2Cl2 and hydroxyl-substituted siloxane 2-rings. These rings are likely formed at 800 °C by condensation between a pair of vicinal silanols in which one of the silanols is also a member of a geminal pair.

Influences of ethylenediamine treatment of Silicalite-1 on the catalytic vapor-phase Beckmann rearrangement of cyclohexanone oxime

Silicalite-1 was modified by organic base of ethylenediamine. The characterization results indicated that after alkaline treatment, the crystal structure of S-1 was not destructed or changed; the silanol nests were maintained, but the terminal silanol groups were eliminated. The modified S-1 exhibited better performance in the vapor-phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam, especially in terms of the stability. It is inferred that the silanol nests are the most suitable centers for the Beckmann rearrangement reaction, while the terminal and vicinal silanol groups might be responsible for the coke deposition.

Hygrothermal Degradation of (3-Glycidoxypropyl)trimethoxysilane Films Studied by Neutron and X-ray Reflectivity and Attenuated Total Reflection Infrared Spectroscopy

Thin films of organosilanes have great technological importance in the areas of adhesion promotion, durability, and corrosion resistance. However, it is well-known that water can degrade organosilane films, particularly at elevated temperatures. In this work, X-ray and neutron reflectivity (XR and NR) were combined with attenuated total reflection infrared (ATR-IR) spectroscopy to study the chemical and structural changes within thin films of (3-glycidoxypropyl)trimethoxysilane (GPS) after exposure for various periods of time to air saturated with either D2O or H2O at 80 degrees C. For NR and XR, ultrathin (approximately 100 A) films were prepared by spin-coating. Both D2O and H2O provide neutron scattering contrast with GPS. Variations in the neutron scattering length density (SLD) profiles (a function of mass density and atomic composition) with conditioning time were measured after drying the samples out and also swelled with H2O or D2O vapor at room temperature. For samples that were dried out prior to measurement, little or no change was observed for H2O conditioning up to 3.5 days, but large changes were observed after 30 days of conditioning. The range of conditioning time for this structural change was narrowed to between 4 and 10 days with XR. The SLD profiles indicated that the top portion of the GPS film was transformed into a thick low-density layer after conditioning, but the bottom portion showed little structural change. A previous NR study of as-prepared GPS films involving swelling with deuterated nitrobenzene showed that the central portion of the film has much lower cross-link density than the region nearest the substrate. The present data show that the central portion also swells to a much greater extent with water and hydrolyzes more rapidly. The chemical degradation mechanism was identified by IR as hydrolysis of siloxane bonds. For ATR-IR, GPS films were prepared by dip-coating, which resulted in a greater and more variable thickness than for the spin-coated samples. The IR spectra revealed an increase in vicinal silanol generation over the first 3 days of conditioning followed by geminal silanol generation. Thus, the structural change detected by NR and XR roughly coincided with the onset of geminal silanol generation. Finally, little change in the reflectivity data was observed for films conditioned with D2O at 80 degrees C for 1 month. This indicates that hydrolysis of Si-O-Si is much slower with D2O than with H2O.

29Si and 27Al NMR study of amorphous and paracrystalline opals from Australia

Four opal-AG (amorphous) and two opal-CT (paracrystalline) samples obtained from various regions in Australia were investigated with 29Si NMR and 27Al NMR. The proton cross-polarization 29Si NMR technique was used and the resulting spectra consisted of two main resonances: −102.0 and −111.2 ppm for opal-AG; and −102.5 and −112.2 ppm for opal-CT. These peaks were assigned to the Q 3(1OH) and Q 4 resonances, respectively. Using very short contact times, a third, very weak peak at −94 ppm was resolved in an opal-CT specimen, which was assigned to silicon in the Q 2(2OH) arrangement (i.e. a silicon with twin hydroxyl groups). It was found that the opal-CT samples contained a higher proportion of both geminal and vicinal silanol groups (Q 2 and Q 3) than the opal-AG samples. The geminal silanol groups present in opal-AG and opal-CT are not restricted to opal-AN. The full-width at half-maximum (FWHM) values were 9.5 ppm for the opal-AG samples, and 6.5 ppm for both opal-CT samples, a result which confirms that opal-CT has a higher degree of short-range structural order than opal-AG. The 27Al NMR spectra of the opals all showed a single resonance at +52 ppm, indicating that the aluminium exists in a tetrahedral arrangement incorporated within the opal structure.

Study of the Acidic Sites and Their Modifications in Mesoporous Silica Synthesized in Acidic Medium under Quiescent Conditions

The number and nature of silanol groups could be estimated in mesoporous silica prepared with cetyltrimethylammonium bromide (CTMBr) in acidic medium to obtain a material appropriate as a stationary chromatographic phase, using thermogravimetric analysis (TGA) and diffuse reflectance infrared spectroscopy (DRIFT). The temperature-programmed desorption of pyridine in the DRIFT cell was quantitatively correlated to the change in intensity of the two IR bands, 1595 and 1446 cm-1, assigned to pyridine interacting respectively with hydrogen-bonded and free silanol groups, after the determination of the absorptivities for the two bands. For calcined or solvent extracted samples, the silanol density (3−4 groups/nm2), is represented mainly by hydrogen-bonded silanol groups. A significant increase is obtained by rehydroxylation. Activation by heating above 200 °C promotes the condensation of the vicinal silanol groups.

Surface Organometallic Chemistry: Synthesis and X-ray Characterization of Novel Silanolate Surface Models [Re2(CO)8(μ-H)(μ-OSiR2R‘)] and of the First Models with Two Homo and Hetero Metal Carbonyl Fragments Linked to Vicinal or Geminal Silanols

Reaction of [Re2(CO)8(THF)2] with R2R‘SiOH (R = Et, Ph; R‘ = Et, Ph, OH, OSiPh2OH) gives the new molecular models [Re2(CO)8(μ-H)(μ-OSiR2R‘)], which mimic a surface [Re2(CO)8(μ-H)(μ-OSi⋮)] species anchored to isolated, geminal, or vicinal silanols. The bridging structure has been confirmed by the X-ray structure of [Re2(CO)8(μ-H)(μ-OSiPh2OH)]. Thermal treatment (70 °C) of [Re2(CO)8(μ-H)(μ-OH)] with excess Et3SiOH is another way to form [Re2(CO)8(μ-H)(μ-OSiEt3)], which can be reconverted into the hydroxo complex by addition of excess water at the same temperature. Both [Re2(CO)8(μ-H)(μ-OSiEt3)] and [Re2(CO)8(μ-H)(μ-OH)] are readily transformed into [Re2(CO)10] under 1 atm of CO at 150 °C. These molecular models, which are the only neutral carbonyl rhenium complexes bearing silanolate ligands reported up to now, constitute a new tool to clarify the first step of the surface chemistry of the photochemical interaction of [Re2(CO)10] with a rather inert support such as silica. Although the free silanol OH of bo...

Low‐Temperature Modification of Mesoporous MCM‐41 Material with Sublimated Aluminum Chloride in Vacuum.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Low-Temperature Modification of Mesoporous MCM-41 Material with Sublimated Aluminum Chloride in Vacuum

Treatment of siliceous MCM-41 with aluminum chloride was performed in a vacuum at a temperature of 403 K. The obtained AlCl3/MCM-41 material was characterized by multinuclear solid-state NMR spectroscopy and by application of probe molecules. The results of NMR spectroscopy indicate a reaction of aluminum chloride with vicinal silanol groups at the surface of conditioned MCM-41 and the formation of tetrahedrally coordinated aluminum species bound to chlorine and framework oxygen atoms. In the local structures of 3% of these aluminum atoms, strong Brønsted acid sites are formed causing a 1H MAS NMR signal at 6.0 ppm. According to 13C MAS NMR spectroscopy of acetone-2-13C adsorbed on AlCl3/MCM-41 (241 and 245 ppm), these Brønsted acid sites are characterized by an acid strength higher than that of bridging OH groups in H−ZSM-5 (223 ppm).

Novel High-Yield Method for the Synthesis of [Os3(CO)10(μ-H)(μ-OSiR2R‘)] Species and X-ray Characterization of [Os3(CO)10(μ-H)(μ-OSiPh2OSiPh2OH)] and [Os3(CO)10(μ-H)(μ-OSiPh2OH)], First Molecular Models of Osmium Surface Species Anchored to Vicinal and Geminal Surface Silanols

The easy replacement of the hydroxy ligand in [Os3(CO)10(μ-H)(μ-OH)] by siloxy ligands affords a high-yield procedure to model compounds of the type [Os3(CO)10(μ-H)(μ-OSiR2R‘)] (R = Et, Ph; R‘ = Et, Ph, OH, OSiPh2OH), a springboard to better understand the reactivity of osmium carbonyl species on the silica surface. The new clusters [Os3(CO)10(μ-H)(μ-OSiPh2OH)] and [Os3(CO)10(μ-H)(μ-OSiPh2OSiPh2OH)], characterized by X-ray crystallography, represent the first molecular models of silica-anchored [Os3(CO)10(μ-H)(μ-OSi⋮)] having a ligand that mimics surface geminal and vicinal silanols, respectively.

Surface chemistry and structure of silicon oxycarbide gels and glasses

In this study, the surface chemistry and structure of methyl-substituted silica gels and porous oxycarbide glasses were investigated. FTIR was used to measure the relative concentration of Si−CH3 and Si−OH as a function of the degree of methyl-substitution and the pyrolysis temperature. The gels and glasses were further heated, dehydrated or hydrated, in situ, within the FTIR spectrometer. In the temperature range of 800–850°C, high surface area oxycarbide glasses were created with no detectable surface hydroxyl groups. Oxycarbide glasses synthesized in argon at 700°C displayed a weak band for surface hydroxyl groups and reversible physisorption of water, while those synthesized at 850/900°C showed a complete absence of surface hydroxyl groups and the formation of vicinal silanols upon chemisorption of water. Isolated silanols were observed upon heat treatment in vacuum. Formation of aromatic carbon species was found to correlate with the decomposition of the methyl groups. The oxycarbide surface is quite stable to densification (presumably due to elemental carbon on the pore surfaces). In the absence of oxygen, porous silicon oxycarbide glass powders maintain surface areas >200 m2/g at 1200°C. However, oxidizing species in the atmosphere deplete the aromatic carbon species, and the glasses lose surface area.

Textural, structural and acid properties of a catalytically active mesoporous aluminosilicate MCM-41

As-prepared, calcined and catalytically tested MCM-41 materials have been comprehensively characterised by N2 adsorption/desorption at 77 K, 1H, 27Al and 29Si MAS NMR as well as by temperature-programmed ammonia desorption (TPAD) with in situ FTIR. 29Si MAS NMR spectra of MCM-41 closely resemble those of amorphous silica, suggesting a broad range of Si—O—Si bond angles in this material. The obtained results from 1H MAS NMR and a combination of FTIR and TPAD investigations indicate that the washed, template-free MCM-41 and its protonic form behave like a solid-state acid with a broad acid strength distribution. With regard to the presence of bridging hydroxy groups in the investigated MCM-41 materials, the findings of these two methods are not consistent. Therefore, it is assumed that the broad absorption between 3650 and 3400 cm–1 in the FTIR spectra is connected with hydrogen-bonded vicinal silanol pairs. A signal in the 27Al MAS NMR spectra at 40 ppm is identified as a separate resonance, assigned preferentially to an isolated extranetwork tetrahedral aluminium species, whereby the assignment to a pentahedrally coordinated network aluminium species is also considered. When calcined, these species are reinserted into network positions. Subsequent catalytic testing leads to partial recovery of the former resonance. The results of the catalytic testing imply that the weak Bronsted acid sites interact with neighbouring coordinatively unsaturated aluminium species and that synergistically stronger acid sites form.

Total characterization of the BCl3-modified silica surface by means of quantitative surface analysis

The reaction of boron trichloride (BCl3) with a well characterized silica surface has been studied using Fourier-transform IR photoacoustic spectroscopy (FTIR–PAS) and quantitative boron and chloride analyses. Isolated and vicinal silanol groups, in addition to surface siloxane bridges, react with BCl3, resulting in the formation of chemisorbed monodentate or didentate surface groups. The parameters influencing the relative occurrence of these surface species have been investigated. A quantitative description of the grafted groups and reacted silanols has been obtained as a function of the pretreatment temperature of the silica, the reaction temperature and the reaction time.

EXAFS characterization of the adsorption sites of nickel ammine and ethylenediamine complexes on a silica surface

The evolution of the first and the second coordination shells of Ni 2+ ions in a nickel ion exchanged silica is monitored by EXAFS at the nickel K edge in the early stages of a Ni/SiO 2 catalyst preparation. There is a strong pH and ligand effect on the nature of the adsorbed state of Ni 2+ ions. At pH 9 in the wet gel, the coordination sphere of Ni 2+ ions completed by ammonia or ethylenediamine is not modified by electrostatic adsorption on the support. Below pH 9, ammonia can be substituted by water molecules to form penta- or tetraaammine complexes which can be thus grafted to the surface via a ligand substitution involving surface oxygens. Under theseconditions, the majority of nickel ions are engaged in the formation of a nickel surface layer which has a silicate structure. After drying at 353 K and whatever the pH of the exchange solution, the silicate is always formed. Below pH 7, no substantial cation exchange occurs on silica. On the contrary, the tris ethylenediamine nickel complex which is very stable above pH 7, does not lead to the silicate layer formation. The shift from electrostatic adsorption toward grafting only occurs under drying at 353 K leading to isolated grafted complexes. In all cases, a distance of 3.25–3.31 Å between nickel and 2 silicon ions is consistent with a grafting to a pair of vicinal silanol groups.

Chromatographic characterization of surfaces

Abstract

Thermal treatment of silica and its influence on chromatographic selectivity

It is generally accepted that the retention of solutes in non-polar eluents with silica as a stationary phase depneds on the silanol concentration. By thermall treatment silanols can be removed at temperatures above 200°C. It can be shown by Fourier transform (FT)-IR spectroscopy that the vicinal silanols are removed and that number of isolated silanols (sharp absorption at 3740 cm −) increases. The decrease of vicinal silanols can be detected chromatographically by a coresponding decrease in the retention of hydroxyl group-containing solutes. The retention of solutes with hydrogen-bonding acceptors is not affected by this removal of vicinal silanols. Their retention depends solely on the surface concentration of isolated silanols. At temperatures above 600°C the concentration of isolated silanols also decreases. Consequently, the retention of these solutes decreases only on silicas treated at higher temperature. Rehydration of silica surface is completely reversible, if the thermal treatment does not exceed 400°C. It appears that during chemical modification with chlorosilanes only the isolated hydroxy groups react.