The effects of the coordination environment and connectivity of Ti on the rate of n-butanal self-condensation over Ti-silica catalysts were investigated. Ti was introduced in two ways, either during the synthesis of mesoporous SBA-15 or via grafting onto amorphous silica with a disordered pore structure. The connectivity of Ti was then characterized by XANES, UV-vis, and Raman spectroscopy. For the lowest Ti loadings, the Ti is found to be predominantly in isolated monomeric species, irrespective of the manner of sample preparation, and as the Ti loading is increased, a progressively larger fraction of Ti is present in oligomeric species and anatase nanoparticles. The turnover frequency for butanal condensation decreased monotonically with increasing Ti loading, and the apparent activation energy increased from 60 kJ mol-1 for monomeric species to 120 kJ mol-1 for oligomeric species. A kinetic H/D isotope effect was observed over isolated titanol and Ti dimer catalysts suggesting that α-H abstraction is the rate-determining step. This conclusion is supported by theoretical analysis of the reaction mechanism. In agreement with experimental results, the calculated activation barrier for alkanal condensation over a Ti dimer is roughly two times greater than that over Ti-OH sites. The cause for this difference was explained by energy decomposition analysis of the enolate formation step which showed that there is a large energetic penalty for the substrate to distort over the Ti-O-Ti dimer than the Ti-OH monomer.

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

Efficient identification of transition states is important for understanding reaction mechanisms. Most transition state search algorithms require long computational times and a good estimate of the transition state structure in order to converge, particularly for complex reaction systems. The growing string method (GSM) [B. Peters et al., J. Chem. Phys. 120, 7877 (2004)] does not require an initial guess of the transition state; however, the calculation is still computationally intensive due to repeated calls to the quantum mechanics code. Recent modifications to the GSM [A. Goodrow et al., J. Chem. Phys. 129, 174109 (2008)] have reduced the total computational time for converging to a transition state by a factor of 2 to 3. In this work, three transition state-finding strategies have been developed to complement the speedup of the modified-GSM: (1) a hybrid strategy, (2) an energy-weighted strategy, and (3) a substring strategy. The hybrid strategy initiates the string calculation at a low level of theory (HF/STO-3G), which is then refined at a higher level of theory (B3LYP/6-31G∗). The energy-weighted strategy spaces points along the reaction pathway based on the energy at those points, leading to a higher density of points where the energy is highest and finer resolution of the transition state. The substring strategy is similar to the hybrid strategy, but only a portion of the low-level string is refined using a higher level of theory. These three strategies have been used with the modified-GSM and are compared in three reactions: alanine dipeptide isomerization, H-abstraction in methanol oxidation on VOx/SiO2 catalysts, and C–H bond activation in the oxidative carbonylation of toluene to p-toluic acid on Rh(CO)2(TFA)3 catalysts. In each of these examples, the substring strategy was proved most effective by obtaining a better estimate of the transition state structure and reducing the total computational time by a factor of 2 to 3 compared to the modified-GSM. The applicability of the substring strategy has been extended to three additional examples: cyclopropane rearrangement to propylene, isomerization of methylcyclopropane to four different stereoisomers, and the bimolecular Diels–Alder condensation of 1,3-butadiene and ethylene to cyclohexene. Thus, the substring strategy used in combination with the modified-GSM has been demonstrated to be an efficient transition state-finding strategy for a wide range of types of reactions.

Density functional theory has been used to investigate the mechanism and kinetics of the liquid-phase, oxidative carbonylation of toluene to p-toluic acid (C7H8 + CO + 1/2O2 → p-C7H6COOH + H2O) catalyzed by Rh(III) cations. In toluene solution containing trifluoroacetic acid and dissolved CO, Rh(III) is coordinated to three trifluoroacetate (TFA) anions and two CO molecules as Rh(CO)2(TFA)3. The oxidative carbonylation of toluene is initiated by the addition of toluene across one of the Rh−O bonds of Rh(CO)2(TFA)3 to form (C7H7)Rh(CO)2(TFAH)(TFA)2. The latter species undergoes isomerization and CO migration to produce (C7H7CO)Rh(CO)(TFAH)(TFA)2, which then coordinates another molecule of CO. The mixed anhydride of toluic and tirfluoroacetic acid, C7H7C(O)O(O)CCF3 and Rh(CO)3(TFA), are produced by reductive elimination from (C7H7CO)Rh(CO)2(TFAH)(TFA)2. Para-toluic acid is then formed by hydrolysis of C7H7C(O)O(O)CCF3. The proposed reaction mechanism explains many of the observations reported in our previous experimental work (Zakzeski, J. J.; Bell, A. T. J. Mol. Catal. A 2007, 276, 8) and, in particular, the effect of temperature on the ratio of p- to m-toluic acid, the effects of H2O and the partial pressure of CO on the loss of catalyst activity, and the effect of Rh concentration on the formation of a catalytically inactive Rh dimer species.

Acidic protons in zeolites are known to be mobile at elevated temperatures. In this study, density functional theory was used to identify the reaction pathways for proton migration in a model that represents the zeolite ZSM-5. In the absence of water, the acidic proton "hops" or migrates between two of the four O atoms surrounding an aluminum center with an activation barrier of 28 kcal/mol. During proton transfer, the O atoms stretch closer together in order to stabilize the transition state. This is revealed by a 13.4° decrease in the O−Al−O bond angle. Adsorbed water bridges the proton donor and acceptor sites, reducing the barrier height by 24 kcal/mol. Hence proton migration depends heavily on the local geometry and conditions of the zeolite. We show that experimentally undetectable amounts of water can greatly influence the measured rates and apparent activation barriers. We broaden the scope of our study to consider hydrogen exchange with other gas-phase species of the form RO−H (RO = CH3O, CH3CH2O) and R−H (R = H, CH3, C2H5, C3H7, C6H5). It is evident that to a first approximation the activation barrier increases with an increase in the polarizability of the species RO−H. For the chemical series R−H, the activation energy increases with the deprotonation energy of the interacting species R−H. We also calculate the overall reaction rate constants for proton hopping and hydrogen exchange.

Surface interactions of H2, CO and CO2 with the perovskite-type oxide LaMnO3 have been studied by temperature-programmed desorption (t.p.d.) and i.r. spectroscopy. A t.p.d. desorption peak of H2 at 355–360 K, which increases in intensity with increasing reduction temperature of the oxide (Tr), is assigned to molecular adsorption of H2 on reduced manganese sites (Mnn+, n≈ 2). CO adsorption yielded t.p.d. peaks of CO and CO2. A peak of CO at 473 K (for oxidized LaMnO3) associated with a carbonate group and peaks at 360–395 K, 540–550 K and 773–800 K (for reduced LaMnO3) associated with linear and bridged CO species adsorbed on Mnn+ ions were observed. A very wide CO2 desorption peak at 473 K and tail centred at 773 K (oxidized LaMnO3) are associated with monodentate and bidentate carbonates interacting with Mn3+. CO2 adsorption yielded t.p.d. peaks of CO2 at 345–385 K and at 540–665 K whose intensity decreased and increased, respectively, with Tr. These are associated with monodentate and bidentate carbonates, respectively, interacting with reduced sites of manganese or La3+. Detection of bands at ca. 2900 cm–1 in the i.r. spectrum obtained after CO + H2 adsorption, the appearance of new CO desorption features at 570 K and above 860 K, and the detection of a new H2 desorption peak at 770–785 K in the t.p.d. spectra obtained after CO–H2 or H2–CO adsorptions suggest decomposition of an oxygenated species formed by interaction of CO and H2 adsorbed on the same adsorption centre.

Direct liquid-phase sulfonation of methane to methanesulfonic acid (MSA) with SO2 has been achieved in triflic acid using K2S2O8 as the oxidant and a small amount of a Ca salt as the promoter. The effects of reaction conditions on the conversion of SO2 to MSA were examined. Included were the influence of solvent acidity, reaction duration, reaction temperature, amount of K2S2O8, and composition and amount of promoters.

The interactions of water with H−ZSM-5, Cu−ZSM-5, and Co−ZSM-5 have been investigated by density functional theory (DFT). Calculations were also performed to determine the thermodynamics of metal removal from the zeolite. For all three forms of ZSM-5, water adsorbs by direct interaction of the O atom with the cation and hydrogen bonding of one of the two H atoms with an oxygen atom in the zeolite framework. The magnitude of the energy of adsorption of one H2O molecule decreases in the order Cu−ZSM-5 (Cu as Cu+) > Co−ZSM-5 (Co as Co2+(OH)-) > H−ZSM-5 > Cu−ZSM-5 (Cu as Cu2+(OH)-). Adsorption of a second H2O molecule occurs with a smaller binding energy, which decreases in the order Cu−ZSM-5 (Cu as Cu+) > Cu−ZSM-5 (Cu as Cu2+(OH)-) > Co−ZSM-5 (Co as Co2+(OH)-) > H−ZSM-5. At 800 K, demetalation to form a gaseous metal hydroxide species is unfavorable thermodynamically but will occur spontaneously if the final product is a metal oxide. Demetalation of Cu is projected to occur much more readily than demetalation of Co, in good agreement with experimental observation.