The effects of hydration and dehydration of silica-supported vanadia have been investigated with the aim of understanding how these processes alter the structure of the dispersed vanadia. Samples containing either 9 or 12 wt % V2O5/SiO2 were examined by in situ Raman spectroscopy during hydration in 3 kPa water vapor at room temperature and during dehydration at temperatures between 298 and 773 K. The vanadia in freshly dehydrated 9 wt % V2O5/SiO2 is present exclusively in the form of monovanadate species. Monovanadate species are predominant in the 12 wt % V2O5/SiO2, but a small amount of V2O5 is present as well. Room-temperature hydration causes a progressive loss of the Raman band at 1043 cm-1, characteristic of isolated monovanadate species, and the gradual appearance of bands at 1021, 986, 895, 773, 706, 666, 512, 415, 325, 267, and 158 cm-1, characteristic of a hydrated vanadia gel. Dehydration at elevated temperatures decomposes the gel and partially restores the presence of isolated monovanadate species. V2O5 particles are also formed during dehydration. Repeated low-temperature hydration and high-temperature dehydration leads to an irreversible conversion of isolated monovanadate species into V2O5 particles. A mechanism by which this process occurs is proposed.
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Read moreDonation is not from an sp3 orbital: Decomposition analysis, based on absolutely localized molecular orbitals, provides an alternative and somewhat unconventional view of hydrogen bonding in the water dimer. A new description of the electron-donating orbital is uncovered: unlike sp3 lone pairs, a single donor–acceptor orbital pair forms, in which the donating orbital changes its orientation according to the relative positions of the two molecules.
Read moreLignocellulosic biomass is an attractive resource for producing transportation fuels, and consequently novel approaches are being sought for transforming the lignin and cellulosic constituents of biomass to fuels or fuel additives. Glucose, the monomer of cellulose, is a good starting material for exploring such chemistries. We report here the results of an investigation aimed at identifying catalysts for the dehydration of glucose to 5-hydroxymethylfurfural (HMF) dissolved in ionic liquids and the subsequent conversion of HMF to 2,5-dimethylfuran (DMF), a high-energy content product that could be used as a fuel or fuel additive. Heteropoly acids were found to be exceptionally active and selective catalysts for the dehydration of glucose. Nearly 100% yield of HMF could be achieved using 12-molybdophosphoric acid (12-MPA) in a solution of 1-ethyl-3-methylimidazolium chloride (EMIMCl) and acetonitrile. The addition of acetonitrile to EMIMCl suppressed the formation of humins from glucose. The high HMF selectivity achievable with heteropoly acid catalysts is ascribed to stabilization of 1,2-enediol and other intermediates involved in the dehydration of glucose and the avoidance of forming the 2,3-enediol intermediate leading to furylhydroxymethyl ketone (FHMK). Carbon-supported metals, and in particular Pd/C, were effective in promoting the hydrogenation of HMF dissolved in EMIMCl and acetonitrile to DMF. The following intermediates were observed in the hydrogenation of HMF to DMF: 5-methylfurfural (MF), 5-methylfurfyl alcohol (MFA), and 2,5-dihydroxymethylfuran (DHMF). The relative rate of formation and consumption of these compounds was explored by using each of them as a reactant in order to identify the reaction pathway from HMF to DMF. It was also observed that HMF produced via glucose dehydration could be converted to DMF without isolation, if the dehydration catalyst, 12 MPA, was replaced by the hydrogenation catalyst, Pd/C. This two-step catalytic approach provides the basis for completely converting glucose to HMF and further converting HMF to DMF.
Read moreA study of the influence of lanthana promotion on the hydrogenation of CO over Rh/SiO/sub 2/ has been conducted. Lanthana-promoted Rh/SiO/sub 2/ exhibits higher turnover frequencies for the synthesis of CH/sub 4/, C/sub 2/-C/sub 4/ hydrocarbons, CH/sub 3/OH, and C/sub 2/ oxygenates than unpromoted Rh/SiO/sub 2/. The turnover frequency for each product goes through a maximum with increasing lanthana addition. Lanthana promotion is also found to increase the selectivity for the formation of CH/sub 3/OH, C/sub 2/ oxygenates, and C/sub 2/-C/sub 4/ hydrocarbons and to decrease the selectivity for CH/sub 4/. The selectivity for C/sub 2/ oxygenates is optimized at low levels of lanthana addition. In situ infrared observations show that lanthana promotion blocks the chemisorption of CO onto surface Rh sites. Of particular interest is the observation of a band at 1725 cm/sup -1/. This feature is assigned to CO adsorbed on a Rh site immediately adjacent to an LaO/sub x/ island. The low frequency of this vibration is attributed to a weakening of the C-O bond caused by the interaction of lanthana cations with the oxygen end of the adsorbed CO. It is postulated that the interaction of the lanthana with the CO is responsible for the enhancedmore » rates of CO dissociation and the formation CH/sub 4/ and C/sub 2+/ products. Infrared spectroscopy also provides evidence for acyl, formate, and acetate groups on the surface of lanthana-promoted Rh/SiO/sub 2/. The possible role of these species in the synthesis of products is discussed. 26 references.« less
Read moreTheoretical methods are used to analyze the thermodynamics of ZSM-11 synthesis from amorphous silica and an aqueous solution of tetraalkylammonium hydroxide (TAAOH). The overall process is represented by the reaction 96SiO2(a) + n(TAA+/OH-/200H2O) = (nTAA+/Zn-) + 200nH2O. Both tetrapropylammonium (TPA+) and tetrabutylammonium (TBA+) cations are considered as the structure-directing agents, and calculations are performed for occlusion of either three or four TAA+ cations per unit cell of the zeolite. Both estimates of the change in internal energy and Gibbs free energy reveal that the synthesis of ZSM-11 should be favored by the occlusion of three TBA+ cations per unit cell, consistent with experimental observation. The present analysis also demonstrates the importance of energy and entropy changes associated with the dehydration of TAA+ and OH- ions and with the occlusion of TAA+ cations into the zeolite. The interactions of OH- anions with the zeolite framework to form defects in the form of siloxy (≡SiO-) groups are also considered.
Read moreThe rate of spin-surface crossing from the singlet to the triplet potential energy surface during methanol oxidation has been examined for classically spin-forbidden crossings. The Landau−Zener equation has been used to calculate the thermally-averaged spin transition probabilities for the nonadiabatic surface crossing reaction. Two active sites have been investigated: isolated vanadate species supported on silica (VOx/SiO2) and titania (VOx/TiO2). The results show that the rate of spin-surface crossing is much faster than the rate-limiting H-abstraction step on both active sites and is therefore not kinetically relevant.
Read moreThe oxidative dehydrogenation of alkanes (C2H6, C3H8, i-C4H10, and n-C4H10) was investigated on VOx supported on Al2O3. Rate constants for alkane dehydrogenation (k1), alkane combustion (k2), and alkene combustion (k3) were measured, and a model was developed to describe the effects of alkane composition on these rate constants. The proposed model accounts for the effects of the number of C−H bonds available for activation and the relative strengths of these bonds in both the reactant and the product molecules. The Brønsted−Evans−Polanyi (BEP) relationship is used to relate activation energies of secondary and tertiary C−H bonds to that of primary C−H bonds. The model gives a reasonable approximation of the relative order of alkane reactivity, expressed by k1 + k2, and the relative ranking of alkanes with respct to combustion versus oxidative dehydrogenation, expressed by k2/k1. The ratio of k2/k1 is described by the product of two components: one that depends on the nature, number, and relative strength of C−H bonds of surface alkoxides, and a second one that is independent of the alkoxide composition and structure but depends on the difference in the entropy of activation for COx precursor versus alkene formation. The model also explains the observed variation of k3 with alkene composition by considering two precursor states for alkenes. One is strongly bound through π-orbital interactions with Lewis acid centers, and the second weakly binds via H bonding and van der Waals interactions, similar to the binding of alkanes. As a result, the rate of alkene combustion depends strongly on the large heats of adsorption of alkenes and only slightly on the presence of weak allylic C−H bonds. The high rate of C2H4 combustion is thus a consequence of its high heat of adsorption.
Read moreADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPlasma-initiated polymerization of hexachlorocyclotriphosphazeneJames A. Klein, Alexis T. Bell, and David S. SoongCite this: Macromolecules 1987, 20, 4, 782–789Publication Date (Print):April 1, 1987Publication History Published online1 May 2002Published inissue 1 April 1987https://pubs.acs.org/doi/10.1021/ma00170a014https://doi.org/10.1021/ma00170a014research-articleACS PublicationsRequest reuse permissionsArticle Views160Altmetric-Citations12LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts
Read moreExperimental and theoretical studies were conducted to investigate the influence of anionic ligands (e.g., CF(3)COO(-), CH(3)SO(3)(-)) on the catalytic activity and selectivity of Rh(III) in the oxidative carbonylation of toluene to toluic acid. The catalyst activity was found to pass through a maximum as the pK(a) of the conjugate Brønsted acid decreases from 4.63 to -2.00, with the maximum activity occurring at pK(a) = 0.35, corresponding to CClF(2)COOH. The theoretical analysis showed that the strength of toluene coordination increases with decreasing basicity of the anion (i.e., decreasing pK(a) of the corresponding acid). In contrast, the activation barrier for C-H activation increases with decreasing ligand pK(a). The experimentally observed effect of anion composition on catalyst activity can be explained using calculations of the apparent rate coefficient for toluene activation based on density functional theory and transition state theory. The ratio of p- to m-toluic acid formed increases with decreasing acid pK(a) and passes through a maximum for pK(a) = 0, corresponding to CF(3)COOH. The effect of anion composition on the isomer selectivity is attributed to changes in the charge density on the Rh(III) cation, which in turn affect the distribution of charge on the para and meta carbon atoms of the aromatic ring.
Read moreAbstract Carbonylation gets a phase lift : The usual liquid‐phase, high‐pressure processes for carbonylating formaldehydes are avoided in a novel vapor‐phase reaction. Using an acid zeolite (Faujasite) at near‐atmospheric pressure dimethoxymethane (DMM; the dimethyl acetal of formaldehyde; see scheme) is carbonylated to produce methyl methoxyacetate (MMAc). This approach provides a new route to ethylene glycol under mild conditions. magnified image
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