The gas-phase carbonylation of dimethoxymethane (DMM) to form methyl methoxyacetate (MMAc) can be catalyzed by acid zeolites. This reaction is a critical step in the synthesis of monoethylene glycol (MEG), a widely used chemical, from synthesis gas. The mechanism of DMM carbonylation occurring on H−MFI and H−FAU zeolites has been investigated using density functional theory. We find that the reaction involves three steps: initiation via reaction of zeolite protons with DMM to form methoxymethoxy species, carbonylation of the resulting species, and subsequent methoxylation of the resulting acyl species. Both the carbonylation and methoxylation processes proceed via carbocationic transition states that are stabilized by the framework O atoms of the zeolite. The activation barriers for carbonylation are similar in both zeolites, but the barriers for methoxylation differ significantly. Energy decomposition analysis indicates that a combination of the pore size and of the flexibility of the reactive species gives rise to the differences in reactivity between the zeolites. The effect of basis set superposition was assessed using a 6-311++G(3df,3pd) basis set. This effect depends strongly on the gas-phase molecules involved but very weakly on the zeolite framework, and its estimate can be transferred from one zeolite to another to reduce the computational expense of such simulations.

The reduction and reoxidation of submonolayer coverages of TiO2 deposited onto MCM-48 were investigated. The deposited TiO2 was characterized by Raman and UV−visible spectroscopy. Raman spectra show that Ti atoms are bonded to the silica support by Ti−O−Si bonds and that crystalline TiO2 is not formed. The results of the Raman and UV−visible spectroscopy suggest that the dispersed TiO2 is present as two-dimensional oligomeric structures. Reduction in H2 at 923 K produces Ti3+ cations observable by EPR (g = 1.932), suggesting the formation of oxygen vacancies. The fraction of Ti that could be reduced increased with TiO2 surface concentration. This observation is attributed to the ease with which O atoms can be removed from the TiO2 overlayer as the size of the titania patches increases. The amount of oxygen removed during reduction was quantified by pulsed reoxidation. It was observed that the temperature required for complete reoxidation decreased with increasing surface coverage of the silica support by TiO2. This trend is explained with a proposed model of the reoxidation process, in which the rate limiting step is the migration of peroxide species through or between the deposited TiO2 patches. A linear correlation was established between the intensity of the EPR signal for Ti3+ and the amount of oxygen removed from TiO2/SiO2. This relationship was then used to determine the oxygen vacancy concentration present on the surface of TiO2/SiO2 after temperature-programmed oxidation of methanol.

While supported particles of metals and oxides exhibit changes in specific activity with particle size and shape, the identification of what is meant by an active site on such catalysts is very difficult because of the diversity of possible active site. As a result, it is usually difficult to identify the exact cause for changes in catalyst activity and selectivity when changes are made in catalyst composition and structure. By contrast, single-site catalysts are exceptionally good models systems for exploring the consequences of site composition and structure on the mechanism and kinetics of catalyzed reactions because such catalysts can be prepared in such a way that almost all sites are identical, or nearly identical. Example of single sites include metal cations present in either framework or extra-framework positions in zeolites, metal oxo species dispersed onto oxide supports, and supported metal atoms and small clusters. This talk will illustrate the characterization of such structures by means of XANES and EXAFS, aided by simulations of such data based on theoretical models. Methods for studying the progress of elementary processes involved in catalyzed reactions will be illustrated for the oxidation of methanol to formaldehyde on supported vanadate species and the oxidative carbonylation of methanol to dimethyl carbonate on Cu-exchanged zeolites. These studies will also show that the hypotheses of reaction mechanism and the influence of site and support composition can be understood from first principles through the application theoretical analysis.

Abstract The CO hydrogenation activity and selectivity of the fcc phases of Mo 2 C and Mo 2 N are found to be identical, whereas the activity of the Mo 2 C hcp phase is only half that of these catalysts but its olefin selectivity is higher.

A new method based on absolutely localized molecular orbitals (ALMOs) is proposed to measure the degree of intermolecular electron density delocalization (charge transfer) in molecular complexes. ALMO charge transfer analysis (CTA) enables separation of the forward and backward charge transfer components for each pair of molecules in the system. The key feature of ALMO CTA is that all charge transfer terms have corresponding well defined energetic effects that measure the contribution of the given term to the overall energetic stabilization of the system. To simplify analysis of charge transfer effects, the concept of chemically significant complementary occupied-virtual orbital pairs (COVPs) is introduced. COVPs provide a simple description of intermolecular electron transfer effects in terms of just a few localized orbitals. ALMO CTA is applied to understand fundamental aspects of donor-acceptor interactions in borane adducts, synergic bonding in classical and nonclassical metal carbonyls, and multiple intermolecular hydrogen bonds in a complex of isocyanuric acid and melamine. These examples show that the ALMO CTA results are generally consistent with the existing conceptual description of intermolecular bonding. The results also show that charge transfer and the energy lowering due to charge transfer are not proportional to each other, and some interesting differences emerge which are discussed. Additionally, according to ALMO CTA, the amount of electron density transferred between molecules is significantly smaller than charge transfer estimated from various population analysis methods.

Low temperatures and low pressures suffice for the sulfonation of methane with a suitable free-radical initiator and promoter [Eq. (1)]. Since the RhCl3 promoter can be recycled, and the initiator complex is stable and easy to handle, development of this reaction into an industrial process is promising. MSA=methanesulfonic acid.

X-ray diffraction and X-ray absorption and Raman spectroscopies were used to determine the structure of dispersed and crystalline structures in MoOx/ZrO2 catalysts useful in the oxidative dehydrogenation of alkanes. The MoOx surface density on ZrO2 was varied over a wide range (0.35−50 Mo/nm2) by changing the Mo content (1−44 wt % MoO3) and the treatment temperature (393−973 K). Raman spectra showed that MoOx/ZrO2 samples with low surface density (<5 Mo/nm2) treated at temperatures below 873 K initially contain isolated tetrahedral MoOx species; these species oligomerize to form two-dimensional structures with bridging MoOMo bonds as the surface density increased to values typical for a polymolybdate monolayer (∼5 Mo/nm2). An increase in surface density led to a shift in the ν(MoO) Raman band to higher frequencies and to changes in the near-edge X-ray absorption spectra. Both of these are consistent with the growth of these polymolybdate domains with increasing Mo surface density, as also suggested by the concurrent decrease in the UV−visible absorption energy. Thermal treatment at 973 K led to the dissociation of MoOMo bonds and to the formation of tetragonal−pyramidal OMoO4 species. For MoOx/ZrO2 samples with Mo surface densities greater than 5 Mo/nm2, MoO3 and Zr(MoO4)2 were detected by Raman and for larger crystallites also by X-ray diffraction. Treatment of these samples in air at 723 K led to the predominant formation of MoO3, while higher temperatures led to a solid-state reaction between MoO3 and ZrO2 to form Zr(MoO4)2. This structural evolution was confirmed by the evolution of pre-edge and near edge features in the X-ray absorption spectra of these high surface density samples. Zr(MoO4)2 contains Mo6+ cations in a distorted tetrahedral coordination with one oxygen bonded only to molybdenum and the other three shared by Zr and Mo atoms. The Raman bands observed for Zr(MoO4)2 at 750, 945, and 1003 cm-1 were assigned to νsym(OMoO), νasym(OMoO), and ν(MoO) vibrational modes, respectively, based on the analysis of the Raman bands observed after 18O2 exchange with lattice oxygen atoms. Bridging O atoms in MoOMo species exchanged with gas phase 18O2 more readily than terminal MoO species.