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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Heterogeneous rhodium catalysts supported on SiO2 were modified with PPh3 for the gas-phase hydroformylation of propene to produce n- and isobutanal. High selectivity to aldehydes was achieved, with no propane or alcohols formed. Investigation of the effects of reaction temperature, reactant partial pressures, total pressure, and PPh3/Rh ratio suggested that the supported catalyst behaved similarly to the homogeneous catalyst. In particular, the supported catalyst showed similar activation energies and partial and total pressure dependences of the reaction rates to those observed in homogeneous, liquid-phase reactions. The first order dependence of the hydroformylation rate on the partial pressures of propene, CO, and H2 individually led to a cubic dependence of butanal formation on total pressure for equimolar reactant mixtures. High regioselectivity with a typical n/i ratio of 14 was achieved.
The mechanism and structural requirements for ethanol oxidation to acetaldehyde were examined on VOx domains supported on γ-Al2O3 at surface densities of 1.7−11.8 VOx/nm2. Raman and UV−visible spectra showed that VOx species evolve from monovanadate to polyvanadate structures with increasing surface density with only traces of crystalline V2O5. Oxidative dehydrogenation (ODH) of ethanol to acetaldehyde occurs at low temperatures (473−523 K) with high primary selectivities of CH3CHO (∼80%) on a catalyst with one theoretical polyvanadate monolayer. ODH turnover rates (per V-atom) increased with increasing VOx surface density for surface densities up to 7.2 V/nm2, indicating that polyvanadate domain surfaces are more reactive than monovanadate structures. Similar trends were evident for alkane ODH reactions that also involve kinetically relevant H-abstraction steps within reduction−oxidation catalytic sequences. Turnover rates ultimately decreased at higher surface densities because of the incipient formation of three-dimensional structures. VOx domains of intermediate size therefore provide a compromise between site reactivity and accessibility during ethanol ODH. The effects of O2 and C2H5OH pressures on ethanol ODH rates and the kinetic isotope effects for C2H5OD and C2D5OD confirmed the kinetic relevance of H-abstraction from ethoxide species formed in quasiequilibrated ethanol dissociation steps; taken together with in situ infrared spectra, these data also show that ethoxide species are present at near saturation coverages on fully oxidized VOx domains that undergo reduction−oxidation cycles during each ethanol oxidation turnover.
ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTRaman study of the preparation of silica-supported titania from titanium tetrachloride and hydrogen chlorideMark G. Reichmann and Alexis T. BellCite this: Langmuir 1987, 3, 1, 111–116Publication Date (Print):January 1, 1987Publication History Published online1 May 2002Published inissue 1 January 1987https://pubs.acs.org/doi/10.1021/la00073a020https://doi.org/10.1021/la00073a020research-articleACS PublicationsRequest reuse permissionsArticle Views207Altmetric-Citations40LEARN 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
Theoretical 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.
The 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.
The 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.
