Kinetic and Binding Studies Reveal Cooperativity and Off-Cycle Competition for H-Bonding Catalysis with Silsesquioxane Silanols

The catalytic activity, kinetics, and quantification of H-bonding ability of incompletely condensed polyhedral oligomeric silsesquioxane (POSS) silanols are reported. POSS-triols, a homogeneous model for vicinal silica surface sites, exhibit enhanced H-bonding compared with other silanols and alcohols as quantified using a ³¹P NMR probe. Evaluation of a Friedel–Crafts addition reaction shows that phenyl-POSS-triol is active as an H-bond donor catalyst whereas other POSS silanols studied are not. An in-depth kinetic study (using RPKA and VTNA) highlights the concentration-dependent H-bonding behavior of POSS-triols, which is attributed to intermolecular association forming an off-cycle dimeric species. Binding constants provide additional support for reduced H-bond ability at higher concentrations, which is attributed to competitive association. POSS-triol self-association disrupts H-bond donor abilities relevant for catalysis by reducing the concentration of active monomeric catalyst.     

Carboxylic groups in mesoporous silica and ethane-bridged organosilica: effect of the surface on the reactivity

Proton-donor ability of carboxylic groups incorporated by co-condensation into SBA-15 and ethane-bridged periodic mesoporous organosilica (PMO) has been studied through IR spectroscopy by dosing ammonia, which forms reversibly COO(-) groups and NH(4)(+) ions. The related equilibrium constants, determined by elaboration of IR data, reveal a lower reactivity of -COOH groups at the surface of PMO than on SBA-15, when the two samples have been outgassed at the same temperature. This finding is interpreted in terms of different dielectric constants and intermolecular interactions engaged with the surface species. Carboxylic groups on ethane-bridged organosilica react with silanols upon thermal treatment at 473 K to form a mixed anhydride species Si-O-C(O)-, at variance with the same groups on SBA-15.     

On the characterization of surface VOx-species by XPS: layered oxidic systems as model catalysts

Summary The influence of the support on vanadia, immobilized on Au, SiO₂, Al₂O₃ and TiO₂, suitable for the selective catalytic reduction (SCR) of nitric oxide by ammonia has been investigated. The layered oxidic substrates, which are deposited onto gold coated silicon wafers, represent model systems for the surfaces of technical SCR catalysts, that are suitable for surface science investigations. The oxidation states of the surface vanadia species immobilized on these samples have been studied by XPS after well-defined sample treatment in both oxygen and hydrogen atmospheres at 573 K and 723 K. From the observed shifts of the V 2p_3/2 binding energies, and from peak deconvolution, vanadia layers on gold, silica and alumina are found to exhibit characteristics similar to those of bulk vanadia. This behavior is characterized by the relative stability against a reductive atmosphere at 573 K. Changes in the oxidation states of the surface vanadia species are prominent only after treatment at 723 K. Titania exhibits a unique support interaction with vanadia which allows the reduction of vanadia at temperatures as low as 573 K. This fact may contribute to the outstanding performance of VO_x/TiO₂ in the selective reduction of nitric oxide by ammonia in waste gases.     

Mesoporous benzene-silica hybrid materials with a different degree of order in the wall structure: an IR comparative study

Recent joint IR and computational work (Onida et al. J. Phys. Chem B 2005) has allowed a detailed characterization of the isolated silanols at the surface of highly ordered benzene-silica hybrid material. In the present paper, a similar characterization is provided for a less ordered sample. The comparison permits the assignment of IR features to the interaction of silanols either with one another or with benzene rings of the structure. The extent of structural imperfections appears to be limited, for example, no more than pairs of interacting silanols are found, readily healed by thermal treatment. Evidence is also provided that probe molecules with simultaneous H-acceptor and H-donor properties (benzene, methylacetylene) may interact with both the acidic proton in silanols and the electronic cloud in the framework aromatic rings.     

Evidence for the pairwise disposition of grafting sites on highly dehydroxylated silicas via their reactions with Ga(CH3)3

Molecules encountering silica interfaces interact primarily with the hydroxyl groups that terminate the bulk structure. When the nominal surface density is very low, these "silanols" are presumed to be isolated. Nevertheless, silicas that are highly dehydroxylated by pretreatment at 800 °C react with Ga(CH(3))(3) at room temperature to give primarily disilanolate-bridged digallium sites, [(CH(3))(2)Ga(μ-OSi≡)](2). The EXAFS at the Ga K-edge shows a prominent Ga-Ga scattering path, regardless of whether an excess or a limiting amount of Ga(CH(3))(3) is used. Some dimers are formed by the concerted reaction of Ga(CH(3))(3) with an "isolated" silanol and an adjacent siloxane bond. These grafting sites are proposed to be hydroxyl-substituted 2-rings, formed by condensation within a vicinal Q(2)-Q(3) pair. Other dimers are formed by reaction of Ga(CH(3))(3) with vicinal Q(3)-Q(3) pairs which have not condensed, even at 800 °C. In a computational model for the dimer sites, the O-O distance is <2.6 ?, which is far shorter than the calculated mean interhydroxyl separation for the thermally treated silicas (12.2 ?). This highly nonrandom distribution of surface silanols, in combination with the coupled reaction of "isolated" silanols and strained siloxane bonds, accounts for the preferential formation of grafted site pairs rather than isolated grafted sites when silica surfaces are chemically modified.     

Isolation of an [SNS]Pd(II) pincer with a water ladder and its Suzuki coupling activity in water

A water-compatible Pd(ii) pincer with a hybrid [SNS]-donor set, [L(1)PdCl]Cl.2H(2)O [L(1) = bis-(2-(i-butylsulfanyl)-ethyl)-amine] has been isolated and crystallographically characterized; its solid lattice at 223 K contains a ladder-like water polymer of tetramers and extensive H-bonding exists among the cation, anion and water cluster, and this water-soluble complex is active in Suzuki-Miyaura coupling of phenylboronic acid and selected aryl bromides in water at 75 degrees C.     

Luminescent monomeric and dimeric Ru(ii) acyclic carbene complexes as selective sensors for NH3/amine vapor and humidity

A new class of luminescent bis(bipyridyl) Ru(ii) pyridyl acyclic carbene complexes with environmentally-sensitive dimerization equilibrium have been developed. Owing to the involvement of the orbitals of the diaminocarbene ligand in the emissive excited state, the phosphorescence properties of these complexes are strongly affected by H-bonding interactions with various H-bonding donor/acceptor molecules. With the remarkable differences in the emission properties of the monomer, dimer, and H-bonded amine adducts together with the change of the dimerization equilibrium, these complexes can be used as luminescent gas sensors for humidity, ammonia, and amine vapors. With the responses to amines and humidity and the corresponding change in the luminescence properties, a proof-of-principle for binary optical data storage with a reversible concealment process has been described.

A new class of selective ammonia/amine vapor and humidity sensors have been developed from the luminescent bis(bipyridyl) Ru(ii) pyridyl acyclic carbene complexes with environmentally-sensitive dimerization equilibrium.     

A well-defined mesoporous amine silica surface via a selective treatment of SBA-15 with ammonia

2D double-quantum (1)H-(1)H NMR unambiguously shows that the "isolated" ≡Si-OH surface silanols of dehydroxylated SBA-15 are converted upon treatment with ammonia into single silylamine surface site ≡Si-NH(2). The "gem" di-silanols (=Si(OH)(2)) remain intact. Treatment using HMDS produces (=Si(OSiMe(3))(2)) but leaves ≡Si-NH(2) untouched. The resulting surface is hydrophobic and stable.     

无定形硅铝负载镍催化剂上乙烯齐聚反应的活性位点结构鉴定

镍(Ni)基催化剂在低碳烯烃聚合领域具有重要的地位,也是该领域研究的热点.自Johnson等报道(J.Am.Chem.Soc.,1995,117,6414–6415)二亚胺配体络合的Ni(Ⅱ)催化剂可有效降低烯烃聚合度,降低产物中非线性烯烃的选择性,甚至可以生成α-烯烃以来,掀起了Ni基催化剂在烯烃聚合领域的研究热潮.从均相到负载型多相Ni基催化剂,从载体类型到配体性质,从Ni纳米粒子的粒径调控到金属表面价态,关于Ni活性中心的研究工作一直存在争论.本课题组之前研究结果表明,曾明确了无定形硅铝(ASA)载体负载的Ni催化剂,经惰性气氛(N2)预处理得到的一价Ni是烯烃齐聚反应的主要活性中心(J.Chem.Soc.Chem.Commun.,1991,126–127).本文进一步深入研究了不同Al2O3含量的ASA载体上Ni活性位点的结构及其在乙烯齐聚反应中的活性.27Al NMR结果表明,催化剂中的铝存在三种配位方式,分别为AlⅣ、AlⅤ和AlⅥ,其中AlⅣ含量随Al2O3含量的增加而增加.载体中铝配位方式的不同,导致其表面金属负载的金属Ni活性位点所处的结构亦不同.原位FTIR-CO和H2-TPR实验结果表明,催化剂表面存在两种不同结构分布的Ni位点,分别是接枝在弱酸性硅醇上的Ni2+阳离子和Si?(OH)?Al桥式羟基离子交换位置的Ni2+阳离子.多数研究者认为,位于离子交换位置处孤立的Ni阳离子是反应的活性中心.然而,近期有研究者提出负载在酸性硅烷醇表面孤立的Ni2+阳离子为反应的活性中心物质.本文研究发现,随着Al2O3负载量的降低,处于离子交换位置处的Ni2+离子含量逐渐减少,而处于硅醇缺陷位点处的Ni2+离子含量则逐渐增多.原位FTIR-CO分析结果表明,处于硅醇缺陷位点处的Ni2+离子物种在惰性气氛中更易于转化为活性中心Ni+.相应的催化反应结果表明,相比于离子交换位置的Ni2+物种,具有与硅醇缺陷位点相连的Ni2+离子结构更有利于表现出更高的乙烯齐聚化活性.由此可知,处于硅醇缺陷位点的Ni2+物种是乙烯齐聚反应的活性中心的前驱体.本文进一步研究了硅醇缺陷位点处的Ni2+离子物种更易于转化为活性中心Ni+的原因.H2-TPR结果表明,相比于离子交换位置的Ni2+物种,处于硅醇缺陷位点的Ni2+物种与载体之间的相互作用力更弱.C2H4-TPD结果进一步表明,具有这种相对较弱的金属载体间作用力结构的催化剂对反应物C2H4分子的吸附作用力相对更强,吸附量也相对增多,因此其乙烯齐聚的催化性能更优.本研究结果对理解活性中心结构和合理设计催化剂提供参考.     

High-pressure effect on the dynamics of solvated peptides

The dynamics of peptides has a direct connection to how quickly proteins can alter their conformations. The speed of exploring the free energy landscape depend on many factors, including the physical parameters of the environment, such as pressure and temperature. We performed a series of molecular dynamics simulations to investigate the pressure-temperature effects on peptide dynamics, especially on the torsional angle and peptide-water hydrogen bonding (H-bonding) dynamics. Here, we show that the dynamics of the omega angle and the H-bonding dynamics between water and the peptide are affected by pressure. At high temperature (500 K), both the dynamics of the torsional angle ω and H-bonding slow down significantly with increasing pressure, interestingly, at approximately the same rate. However, at a lower temperature of 300 K, the observed trend on H-bonding dynamics as a function of pressure reverses, i.e., higher pressure speeds up H-bonding dynamics.     

The Catalysis of Vapor-Phase Beckmann Rearrangement for the Production of ε-Caprolactam

Recently, Sumitomo Chemical Co., Ltd. developed the vapor-phase Beckmann rearrangement process for the production of -caprolactam. In the process, cyclohexanone oxime is rearranged into -caprolactam using a zeolite as a catalyst instead of sulfuric acid. EniChem in Italy developed the ammoximation process that involves the direct production of cyclohexanone oxime without producing any ammonium sulfate. Sumitomo Chemical Co., Ltd. has commercialized the combined process of vapor-phase Beckmann rearrangement and ammoximation in 2003.In this paper, the authors focus on some aspects of the vapor-phase Beckmann rearrangement catalysis. A solid catalyst that is mainly composed of a high-silica MFI zeolite (Silicalite-1) has been developed for the vapor-phase Beckmann rearrangement. This catalyst does not possess acidity that can be detected by ammonia TPD. Methanol fed into the reactor with cyclohexanone oxime improves the yield of caprolactam. Methanol reacts with terminal silanols on the zeolite surface and converts them to methoxyl groups. The modification of the catalyst by methanol has an important role for the Beckmann rearrangement reaction.Nest silanols located just inside the pore mouth of the MFI zeolite are supposed to be the active sites of the catalyst. We propose that the coordination between the NOH group of cyclohexanone oxime molecule and the nest silanols through hydrogen bonding is responsible for the reaction. The reaction mechanism of Beckmann rearrangement under vapor-phase conditions is the same as in the liquid phase, namely, the alkyl group in anti-position against the hydroxyl group of the oxime migrates to the nitrogen atom's position.     

Annealing of silica to reduce the concentration of isolated silanols and peak tailing in reverse phase liquid chromatography

Non-porous, colloidal silica particles were annealed at three different temperatures, 800, 900 and 1050 °C. The adsorption of lysozyme, a probe of surface roughness, was consistent with progressively reduced surface roughness as temperature increased. The heat treated silica particles were rehydroxylated and then used to pack UHPLC columns. The cationic protein lysozyme was used to probe silanol activity, which exhibited progressively less tailing as the annealing temperature increased. FTIR spectroscopy confirmed that the abundance of isolated silanols on the surface was reduced by annealing at 900 °C or 1050 °C. FTIR also revealed that there was markedly increased hydrogen bonding of the isolated silanols to neighbors after rehydroxylation. These results combine to support the hypothesis that (a) isolated silanols on silica cause tailing in RP-LC and (b) nonplanar topography gives rise to isolated silanols.     

Adsorption, Migration and Reactions of Hydrocarbons on Zeolites Observed by Ftir Spectroscopy

The adsorption, migration and reactions of hydrocarbons on zeolites studied by FTIR spectroscopy are briefly reviewed. At low temperatures, alkyl-BAS (Brønsted acid sites) and -BAS complexes were first formed before protonation of olefins. For 1-butene, it underwent double bond migration (DBM) to form cis- and trans-2-butenes. With increasing temperature, oligomerization occurred and resulted in highly branched dimer, e.g., 3,4-dimethyl-3-hexene. Compared with 1-butene, isobutene can oligomerize at much lower temperature and formed dimerized alkoxy species, even on isolated silanols of zeolites. Upon adsorption of cyclic olefins such as 1-methylcyclopentene, alkenyl carbenium ions were formed at temperatures as low as 150 K. The adsorption of butenes on ferrierite indicates the existence of energy barriers for their intercalation into the pores. The H/D isotope exchange reaction of alkanes with acidic hydroxyl groups of zeolites will also be discussed.     

Selective Manganese‐Catalyzed Oxidation of Hydrosilanes to Silanols under Neutral Reaction Conditions

The first manganese‐catalyzed oxidation of organosilanes to silanols with H₂O₂ under neutral reaction conditions has been accomplished. A variety of organosilanes with alkyl, aryl, alknyl, and heterocyclic substituents were tolerated, as well as sterically hindered organosilanes. The oxidation appears to proceed by a concerted process involving a manganese hydroperoxide species. Featuring mild reaction conditions, fast oxidation, and no waste byproducts, the protocol allows a low‐cost, eco‐benign synthesis of both silanols and silanediols.     

Characterization of the silica surface

Aerosil samples, heat-treated and then silylated with various silanes at various temperatures have been characterized by adsorption of benzene at 293 K. Adsorption and desorption isotherms were plotted and the specific surface areas were determined using the BET method. Cross-plots of adsorption were made between the silylated and the unsilylated samples to reveal in a simple way the resulting changes in surface properties. Modification of aerosils using silanes was found to alter the adsorptive properties of aerosils for benzene to extents depending on the silane used as well as the concentration of residual surface silanols.     

Chemisorption of HSi(OH)3 on silica surfaces: an ab initio periodic study

The study of the grafting of trialkoxysilane R′Si(OR)₃ molecules on amorphous silica has been undertaken at the Hartree–Fock level using a biperiodic model for the surface. Different types of slab cut out from the model system Edingtonite (a tetragonal silica structure with five SiO₂ groups per unit cell) have been used to simulate isolated and interacting silanol sites at the amorphous silica surface, while only the simple case of HSi(OH)₃ has been considered for the interacting molecule. In a first step, for each type of surface the geometrical parameters have been optimised and the surface formation energy determined. The geometrical structure of the grafted molecule is compared with that of the isolated one. The geometrical strains of the surfaces with either isolated or interacting silanols are also compared before and after the grafting reactions. The calculated values of the chemisorption energies show that the grafting process is favored on isolated silanols only if correlation effects are included.     

On the survival of peptide cations after electron capture: Role of internal hydrogen bonding and microsolvation

Electron capture by both bare and microsolvated small peptide dications was investigated by colliding these ions with sodium vapor in an accelerator mass spectrometer to provide insight into processes that occur on the microsecond time frame. Survival of the intact peptide monocation after electron capture depends strongly on molecular size. For dipeptides, no intact reduced species were observed; the predominant ions correspond to loss of hydrogen and ammonia. In contrast, the intact reduced species was observed for larger peptides. Calculated structures indicate that the diprotonated dipeptide ions form largely extended structures with low probability of internal ionic hydrogen bonding (i.e., charge solvation) whereas internal ionic H-bonding occurs extensively for larger peptide dications. Solvation of the peptide ions with between one to seven methanol molecules reduces the total extent of H loss even for dipeptides where intact reduced species can survive more than a microsecond after electron capture. The yield of ions corresponding to cleavage of NCalpha bonds (c+ and z+* ions) does not depend strongly on peptide size but decreases with the extent of microsolvation for the dipeptide dications. H-bonding appears to play an important role for the survival of the intact reduced ions but less so for the formation of c+ and z+* ions. Our results indicate that electron capture predominantly occurs at the ammonium groups (at least 70 to 80%), and provides important new insights into the electron capture dissociation process.     

Interaction of glycine with isolated hydroxyl groups at the silica surface: first principles B3LYP periodic simulation

The adsorption of a glycine molecule on a model silica surface terminated by an isolated hydroxyl group has been studied ab initio using a double-zeta polarized Gaussian basis set, the hybrid B3LYP functional, and a full periodic treatment of the silica surface/glycine system. The hydroxylated silica surface has been simulated using either a 2D slab or a single polymer strand cut out from the (001) surface of an all-silica edingtonite. A number of B3LYP-optimized structures have been found by docking glycine on the silica surface exploiting all possible hydrogen bond patterns. Whereas glycine is generally adsorbed in its neutral form, two structures show glycine adsorbed as a zwitterion, the surface playing the role of a "solid solvent" whereas intrastrand hydrogen bond cooperativity stabilizes the zwitterions. The adsorbed zwitterionic structures are no longer formed at a lower glycine coverage as simulated by enlarging the unit cell so as to break intrastrand hydrogen bonds, showing the importance of H-bond cooperativity in stabilizing the zwitterionic forms. Each structure has been characterized by computing its harmonic vibrational spectrum at the Gamma point, which also allowed us to calculate the free energy of adsorption. The experimental infrared features of chemical-vapor-deposited glycine on a silica surface are in agreement with those computed for glycine adsorbed in its neutral form and engaging three hydrogen bonds with the surface silanols, two of them involving the C=O bond and one originating from the glycine OH group. The NH(2) group plays only a minor role as a weak hydrogen bond donor.     

Infrared spectra of RuTPP, RuCOTPP, and Ru(CO)2TPP isolated in solid argon

Infrared spectra of unstable species such as CO-free ruthenium tetraphenylporphyrin RuTPP and RuCOTPP (species with vacant coordination sites) isolated in solid argon at 8 K have been recorded. Selective deposition conditions allow the isolation of either RuTPP and RuCOTPP or RuCOTPP and Ru(CO)2TPP. This depends on the preparation conditions of the sample. A specific Ru-CO bending mode has been characterized at 590.1 cm(-1) for Ru(CO)2TPP. The behavior of each vibrational mode of RuTPP, RuCOTPP, and Ru(CO)2TPP has been analyzed. Modes such as gamma8 at 721.3 cm(-1) (out-of-plane stretching mode gamma(Cbeta-H)sym) and nu41 at 1342.8 cm(-1) (nuCalpha-N coupled with deltaCalpha-Cm) reflect the charge transfer in the porphyrin. Indeed, the addition of one or two CO ligands to RuTPP reduces the charge transfer between the metal center and the porphyrin, which appears as an increase in the frequency of the nu41 mode and in a decrease in that of the gamma8 mode.     

Sensitivity of ammonia interaction with single-walled carbon nanotube bundles to the presence of defect sites and functionalities

Ammonia adsorption on single-walled carbon nanotubes (SWNTs) was studied by means of infrared spectroscopy at both cryogenic (approximately 94 K) and room (approximately 300 K) temperatures. At 94 K, vacuum-annealed SWNTs showed no detectable ammonia uptake. However, the ammonia adsorption was found to be sensitive to the functionalities and defects on the nanotube surfaces. NH3 adsorption was detected on HNO3-treated nanotubes, characterized by significant functionalities and defects, prior to vacuum annealing. NH3 desorbed from those nanotubes above 140 K, indicating a weak adsorbate-nanotube interaction (approximately 30 kJ/mol). Exposure of annealed samples to ambient air, which possibly regenerated functionalities and defects on nanotube surfaces, restored partially the ammonia uptake capacity. No ammonia adsorption on SWNTs was observed by infrared spectroscopy at room temperature with up to 80 Torr dosing pressure. This work suggests the influence of functionalities and/or defect densities on the sensitivity of SWNT chemical gas sensors. Our theoretical studies on NH3 adsorption on pristine and defective tubes, as well as oxidized tubes, corroborate these findings.