No abstract

Isolated Ni­(II) sites supported on zeolites and other porous materials transform in situ during alkene dimerization to form active Ni­(II)-alkyl centers, and their density influences the kinetic orders and mechanisms of deactivation of Ni-Beta zeolites during ethene dimerization (453 K). Ni-Beta containing high Ni site densities shows deactivation rates that are second-order in Ni, consistent with a dual-site deactivation mechanism involving the formation of unreactive Ni-alkyl-Ni intermediates, as confirmed by DFT calculations. Under the same reaction conditions, by contrast, Ni-Beta containing low Ni site densities shows deactivation rates that are first-order in Ni, consistent with a single-site deactivation mechanism reflecting inhibition by strongly bound intermediates derived from heavier alkene oligomers. On Ni-Beta containing low Ni site densities, cofeeding H2 along with ethene results in a higher number of Ni­(II)-alkyl intermediates formed at initial reaction times and a concomitant change to deactivation kinetics that become second-order in Ni. These findings reveal the strong influence of the density of active Ni­(II)-alkyl centers in porous supports, which depends both on material properties and reaction conditions that generate active centers in situ, on the kinetics and mechanisms of deactivation during alkene oligomerization.

Bifunctional catalysts are challenging to model because there are two active sites capable of unique intermediates and reaction types. Nevertheless, they are versatile catalysts because the relative number of both active sites can be tuned to alter rate and selectivity in response to variation in feed compositions. In this work, a microkinetic model of ethene oligomerization on a Ni−H-β zeolite catalyst was developed based on nickel and Brønsted acid reaction families, with kinetic parameters estimated using transition-state theory, Evans− Polanyi relationships, and thermodynamic data. Species lumping allowed for the formation of products of high molecular weight at high conversion to be captured in the model while avoiding network truncation effects. The reaction mechanism culminated in a complex model describing the formation of C2−C12 products that accurately predicted three published experimental investigations using Ni−H-β (10 unique experiments) up to about 30% conversion. The agreement between the experiment and model predictions demonstrates the model's broad applicability and robustness. Ni sites produce linear alkenes of even carbon number, while Brønsted acid sites catalyze further oligomerization, cracking, and isomerization to broaden the product distribution. The model was used to probe potential experimental conditions and catalyst properties, without extrapolation, allowing for a better understanding of the effect of common experimental parameters (space time, temperature, pressure, Ni wt %) on reaction flux and selectivity to desired products, demonstrating the model as a powerful tool in catalyst and process design.

Porous aluminosilicates such as zeolites are ubiquitous catalysts for the production of high-value and industrially relevant commodity chemicals, including the conversion of hydrocarbons, amines, alcohols, and others. Bimolecular reactions are an important subclass of reactions that can occur on Brønsted acid sites of a zeolite catalyst. Kinetic modeling of these systems at the process scale requires the interaction energetics of reactants and the active sites to be described accurately. It is generally known that adsorption is a coverage-dependent phenomenon, with lower heats of adsorption observed for molecules at higher coverage. However, few studies have systematically investigated the coadsorption of molecules on a single active site, specifically focusing on the strength of interaction of the second adsorbate after the initial adsorption step. In this work, we quantify the unimolecular and bimolecular adsorption energies of varying adsorbates, including paraffins, olefins, alcohols, amines, and noncondensible gases in the acidic and siliceous ZSM-5 frameworks. As a special case, olefin adsorption was examined for physisorption and chemisorption regimes, characterized by π-complex, framework alkoxide and carbenium ion adsorption, respectively. The effects of functional groups and molecular size were quantified, and correlations that relate the adsorption of the second adsorbate identity to that of the first adsorbate are provided.

Acid-catalyzed hydrocarbon transformations are essential for industrial processes, including oligomerization, cracking, alkylation, and aromatization. However, these chemistries are extremely complex, and computational (automatic) reaction network generation is required to capture these intricacies. The approach relies on the concept that underlying mechanisms for the transformations can be described by a limited number of reaction families applied to various species, with both gaseous and protonated intermediate species tracked. Detailed reaction networks can then be tailored to each industrially relevant process for better understanding or for application in kinetic modeling, which is demonstrated here. However, we show that these networks can grow very large (thousands of species) when they are bound by typical carbon number and rank criteria, and lumping strategies are required to decrease computational expense. For acid-catalyzed hydrocarbon transformations, we propose lumping isomers based on carbon number, branch number, and ion position to reach high carbon limits while maintaining the high resolution of species. Two case studies on propene oligomerization verified the lumping technique in matching a fully detailed model as well as experimental data.