Search

The microkinetic modelling methodology that we developed previously to describe Bronsted acid-catalyzed propene oligomerization on medium-pore MFI zeolites has been extended successfully to large-pore Beta zeolites. The extension of the model was supported by the identification of the key descriptors that account for the different topologies and acid strengths of the zeolite frameworks (physisorption enthalpies, stabilization enthalpies, frequency factors). The model is validated with experimental conversion and selectivity data measured in a plug-flow reactor on a commercial Beta zeolite over a range of operating conditions. Analysis of net reaction rates allowed identifying the preferred pathways that increase oligomerization selectivity toward C9 species with increasing propene pressure. The model was additionally used to investigate how the stabilization enthalpies of chemisorbed intermediates, an important catalyst descriptor, influenced the selectivity and surface coverage at iso-conversion. This analysis provides mechanistic insights into the propene oligomerization reaction network and its dependence on zeolite topology, and demonstrates how microkinetic models can describe catalyst behaviour and aid in catalyst and process optimization.

Density functional theory (DFT) calculations were used to study the mechanism of CO2 hydrolysis by Zn-(1,4,7,10-tetraazacyclododecane), also referred to as zinc–cyclen, and evaluate the associated thermodynamic and kinetic parameters. A microkinetic model was then built based on the kinetics and thermodynamics derived from first principles. The model describes the sigmoidal dependence of the observed reaction rate constant on pH, which is similar to behavior observed for CO2 hydrolysis catalysts in nature, i.e., human carbonic anhydrase. The inflection point is identical to the pKa value of the zinc–cyclen complex, in good agreement with previous results. The overall reaction rate constant was calculated to be 3055 M−1 s−1, which is in excellent agreement with the range of experimental values reported (2691.5–3300 M−1 s−1). Importantly, though, the microkinetic model quantifies how the reaction rate and the associated overall rate constant varies as a function of time, underscoring that observed overall rate constants are a function of the species’ concentrations used to extract them, which may not be the initial conditions. The decrease in initial reaction rate over 0–12 ms was ascribed to the decrease in the concentration of the catalytic form, Zn-OH−, which was primarily converted to Zn-HCO3−. Through calculating the ratio of the net rate to the forward rate of each elementary reaction, the rate-limiting step of the catalytic cycle was identified as the adsorption of CO2. To aid in the interpretation of the complex microkinetic model, an analytical rate expression was derived based on a simplified potential energy surface comprised of three major regions: the adsorption of CO2, the release of HCO3− via displacement by water, and the deprotonation of Zn-H2O. Analysis showed quantitatively how the observed reaction rate constant depends not only on how fast a CO2 molecule is captured but also on how deep the Zn-HCO3− sits.

Experiments and microkinetic modeling were used to investigate the reaction mechanism of cis-cyclooctene oxidation with H2O2 on heterogeneous manganese 1,4,7-trimethyl-1,4,7-triazacyclononane (Mn–tmtacn) catalysts. A mechanism based on literature reports and model discrimination was identified that captured experimental data well, including data at reaction conditions that were not used for parameter estimation. H2O2 activation on the heterogeneous catalytic complex was identified as the rate-determining step (RDS), and a simple analytical rate expression was derived using the RDS and the pseudo-steady-state approximation for all intermediates. Predicted reaction orders for cis-cyclooctene, water, H2O2, catalyst, and diol and epoxide products are also consistent with experimental observations and can be rationalized according to the derived rate expression. In addition, the ratio of productive to unproductive H2O2 use is analyzed, and catalyst deactivation is found to be an important step in the reaction mechanism that is highly sensitive to temperature.

Experimental results in the literature show that two metal-organic frameworks (MOFs) containing copper and cobalt nodes are active for hydroperoxide decomposition, which is an important reaction in auto-oxidation processes. Density functional theory (DFT) calculations reported here for these systems suggest that the metal sites in the interior of these MOFs are not the active sites for this type of reaction due to the steric effects of the adjacent linkers. This implies that the experimental catalysis observed may occur on the exterior surface of the MOF crystals. Additional calculations with a copper paddlewheel node show that, despite being able to form complexes with hydroperoxides, the metal sites in copper paddlewheels do not catalyze hydroperoxide decomposition. Preliminary calculations involving undercoordinated metal atoms as a model for metal sites on the MOF exterior crystal surface suggest that these sites could be catalytically active.

Experiments and microkinetic modeling were used to investigate the kinetics of styrene epoxidation catalyzed by (porphyrin)Mn using iodosylbenzene. While the kinetics follow the general form of Michaelis–Menten rate expressions as proposed in the literature, these simplified rate forms cannot capture all the details of the kinetics simultaneously, most notably catalyst deactivation. In contrast, a microkinetic model based on elementary steps, including deactivation via μ-oxo dimer formation and irreversible degradation, is able to capture experimental data over all reaction times and for different (porphyrin)Mn. Experimentally, we show that encapsulation of (porphyrin)Mn in a supramolecular cavity known as a molecular square significantly reduces catalyst deactivation, which is in agreement with previous experimental studies. Microkinetic modeling also captured the kinetics of this system. Net rate analysis revealed that production of epoxide was primarily due to encapsulated catalysts, and the model was able to quantify the difference in the concentration of deactivated catalyst with and without encapsulation.

The epoxidation of 1-hexene with titanium silicalite-1 (TS-1) catalyst was investigated to gain insight into the effect of the solvent on the observed reactivity. Three different solvents were examined: methanol, acetonitrile, and acetone. Kinetic data were obtained from batch reaction experiments, with an emphasis placed on gathering more accurate initial rates than those reported in the literature. The dependencies of the rates on the concentrations of 1-hexene, water, and hydrogen peroxide were determined. The adsorption behavior of 1-hexene in TS-1 in the three different solvents was determined independently by batch sorption experiments. Results showed that the solvent has a significant effect on the adsorption of 1-hexene, and hence on the reaction kinetics. Kinetic modeling incorporating experimental and simulated quaternary adsorption isotherms to describe quasi-equilibrated steps revealed that the differences in the observed reaction kinetics may be attributed mostly, but not entirely, to differences of the partitioning of 1-hexene between the bulk and intraporous phases among the three different solvents.

Atomic carbon is a key intermediate which interacts with the surface during hydrocarbon growth reactions over transition metal surfaces. However, experimental data are scarce, available only for carbon–metal binding energies on nickel (111) and (100) single crystal surfaces. Therefore, to deepen our understanding of the chemisorption of carbon and to quantify its role in the catalytic formation of hydrocarbons, we have calculated the binding energy of atomic carbon on Ni(111) and Co(0001) surfaces using density-functional theory within the generalized gradient approximation and the full-potential linear augmented planewave (FP-LAPW) method. The results presented are in excellent agreement with known experimental values and substantially expand the database of geometric and energetic parameters describing adsorption of carbon on nickel and cobalt surfaces as a function of surface coverage and the adsorption site. The surface coverage dependence of the binding energy will be discussed and is used to interpret the tendency of the different surfaces toward molecular weight growth and their intrinsic reactivities.