By means of 29Si NMR spectroscopy, it is established that the distribution of silicate anions in alkaline silicate solutions is a moderate function of base composition. At a fixed SiO2 concentration and silicate ratio, the proportion of Si present in oligomeric and cage-like structures increases in progressing from Li to Cs hydroxide. This trend is ascribed to cation-silicate anion pairing and to a higher selectivity for ion pairing by large silicate anions as cation size increases.

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.

The influence of dispersion on the interactions of H/sub 2/ and CO with Pd/SiO/sub 2/ has been investigated. The distribution of H/sub 2/ adstates changes with dispersion, due possibly to a change in the morphology of the Pd crystallites or in the coordination of adsorbed H atoms with the Pd atoms. The ratio of linearly held CO to bridge-bonded CO decreases with the dispersion, as does the ratio of Pd(100) to Pd(111) planes on the surfaces of the Pd crystallites. CO dissociation occurs preferentially from bridge sites and proceeds more readily with decreasing dispersion. Consistent with this, the turn-over frequency for methanation increases with decreasing dispersion. 34 references.

The coordination of divalent metal cations to ZSM-5 has been investigated using gradient-corrected density functional theory (DFT). Coordination at both isolated charge-exchange sites and pairs of charge-exchange sites was considered for Co2+, Cu2+, Fe2+, Ni2+, Pd2+, Pt2+, Ru2+, Rh2+, and Zn2+. Thermodynamic calculations of the stability of M2+ to reduction to M0 and demetalation to form MOx particles were also carried out. The results indicate that Cu2+, Co2+, Fe2+, and Ni2+ are coordinated preferentially to five-membered rings containing two Al atoms, which are located on the walls of the sinusoidal channels, whereas Pd2+, Pt2+, Ru2+, Rh2+, and Zn2+ are coordinated preferentially to six-membered rings located on the walls of the sinusoidal channels. Examination of the stability of dimer cations of the form [M-O-M]2+ shows that such structures are not generally stable to hydrolysis, with the possible exception of [Cu-O-Cu]2+. The findings of these calculations are in good general agreement with experimental results.

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.

Heterogeneous catalysts, used in industry for the production of fuels and chemicals, are microporous solids characterized by a high internal surface area. The catalyticly active sites may occur at the surface of the bulk solid or of small crystallites deposited on a porous support. An example of the former case would be a zeolite, and of the latter, a supported metal catalyst. Since the activity and selectivity of a catalyst are known to be a function of surface composition and structure, it is highly desirable to characterize catalyst surfaces with atomic scale resolution. Where the active phase is dispersed on a support, it is also important to know the dispersion of the deposited phase, as well as its structural and compositional uniformity, the latter characteristics being particularly important in the case of multicomponent catalysts. Knowledge of the pore size and shape is also important, since these can influence the transport of reactants and products through a catalyst and the dynamics of catalyst deactivation.

Abstract The results of TPD and IR spectroscopy show that H 2 adsorbs molecularly on reduced Mn 2+ sites.

A theoretical study was conducted to investigate the chemical nature of an unusual interaction observed between carbonyl and acetate ligands in the Rh(CO)2(CF3COO)3 complex. This interaction is intriguing because it is only nominally longer (0.1−0.2 Å) than a typical carbon−oxygen σ bond, yet is associated with only a modest (∼10 kcal/mol) energy lowering of the complex. A localized bonding molecular orbital that promotes the notion of charge sharing is present between the interacting ligands. Constrained geometry optimizations in tandem with Mulliken population analyses indicate that the interaction stems from the inability of Rh(III) with highly electron withdrawing ligands to back-donate properly into the carbonyl ligands. This produces a charge imbalance in the ligands, which sets the stage for nucleophilic attack by the acetate oxygen to the carbonyl carbon. This interaction causes a shift in the predicted values of both IR and 13C NMR signals, which are compared to experiment. For a full explanation of the 13C NMR shifts, two explicit solvent molecules were added to the model and found to induce interaction of both carbonyls with acetate ligands. The chosen density functional (B3LYP) and basis set were validated by comparing theoretically predicted structures and vibrational frequencies with experimentally determined values for several complexes.