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Recent work has reported the discovery of metal surface catalysts by employing a descriptor-based approach, establishing a correlation between a few well-defined properties of a material and its catalytic activity. This theoretical work aims for a similar approach in solid acid catalysis, focusing on the reaction between propene and methanol catalyzed by Brønsted acidic zeotype catalysts. Experimentally, the ammonia heat of adsorption is often used as a measure of the strength of acid sites. Using periodic DFT calculations, we show that this measure can be used to establish scaling relations for the energy of intermediates and transition states, effectively describing the reactivity of the acid site. This allows us to use microkinetic modeling to predict a quantitative relation between the ammonia heat of adsorption and the rate of propene methylation from first principles. We propose that this is the first step toward descriptor-based design of solid acid catalysts.

Ni-containing porous aluminosilicates are promising heterogeneous catalysts for oligomerization of ethene, but little is known about the catalytic cycle. In addition, it remains unclear why the aluminosilicates work without the alkyl aluminum cocatalyst needed in homogeneous catalysis. As the first of its kind, this work uses density functional theory (DFT) to identify the most probable mechanism of oligomerization and active site formation. The periodic DFT calculations employed the BEEF-vdW functional to consider both short-range interactions involved in bond formation and long-range interactions with the zeolite framework. The calculations targeted Ni-containing SSZ-24 zeolite as a representative catalyst and considered Ni+, Ni2+ ions, and neutral nickel atoms as active sites. We investigated the catalytic cycles of the metallacycle and Cossee–Arlman mechanisms that have been proposed in the literature, in addition to a new proton-transfer mechanism. Free energy profiles were derived at a typical experimental reaction temperature of 393 K and used to kinetically discriminate the mechanisms with the energetic span model. On the basis of the results, we predict the Cossee–Arlman mechanism known from homogeneous catalysts to prevail also in the zeolite catalyst. The calculated intrinsic enthalpy of activation of 77 kJ/mol for ethene dimerization agrees well with available experimental data. We further propose a mechanism for formation of the active nickel–alkyl species by reaction between ethene and isolated Ni2+ ions. The results hence provide a solid starting point for experimental investigations of the catalytic cycle, to validate our predictions and ultimately determine the atom-scale properties that control catalytic activity.

We recently proposed the ammonia heat of adsorption as a reactivity descriptor in solid acid catalysis, using it to predict the activity of zeotype catalysts in the propene–methanol reaction (J. Phys. Chem. Lett. 2014, 5, 1516–1521). Here we extend the approach to a series of alkene reactants, establishing transition state energy scaling relations for ethene and butenes. Using these relations in connection with microkinetic modeling, we predict a change in reaction pathway as a function of acid-site reactivity and alkene size. The results illustrate the potential of the descriptor-based approach to model acid-catalyzed reactions and efficiently screen for improved solid acid catalysts.

When zeolite catalysts are subjected to steam at high temperatures, a permanent loss of activity happens, because of the loss of aluminum from the framework. This dealumination is a complex process involving the hydrolysis of four Al–O bonds. This work addresses the dealumination from a theoretical point of view, modeling the kinetics in zeolite H-SSZ-13 to gain insights that can extend to other zeolites. We employ periodic density functional theory (DFT) to obtain free-energy profiles, and we solve a microkinetic model to derive the rates of dealumination. We argue that such modeling should consider water that has been physisorbed in the zeolite as the reference state and propose a scheme for deriving the free energy of this state. The results strongly suggest that the first of the four hydrolysis steps is insignificant for the kinetics of zeolite dealumination. Furthermore, the results indicate that, in H-SSZ-13, it is sufficient to include only the fourth hydrolysis step when estimating the rate of dealumination at temperatures above 700 K. These are key aspects to investigate in further work on the process, particularly when comparing different zeolite frameworks.

Complete body of DFT-MD results, results of Monte Carlo simulations, setup and results of microkinetic modeling, additional experimental details and results (PDF) CatMAP files (ZIP) ■ AUTHOR INFORMATION

Catalyst deactivation during the methanol-to-hydrocarbons (MTH) reaction was investigated using five different commercially prepared microporous catalysts, including Mordenite, ZSM-22, ZSM-5, zeolite Beta and SAPO-34. The reaction was carried out in a fixed bed reactor at a constant feed rate per gram of catalyst. Deactivated and partially deactivated catalysts were obtained at increasing reaction times. The whole of the catalyst beds were characterized using nitrogen adsorption, thermogravimetric analysis, a dissolution-extraction protocol, and UV-Raman spectroscopy, focusing primarily on methods suitable for the quantification of the coke. The results illustrate that topology is the dominant parameter that influences not only catalyst lifetime and product distribution, but also the nature of the species causing the deactivation. For all catalyst topologies, when the entire catalyst bed is examined together, the micropore volume and BET surface area decrease more rapidly than total coke from TGA increases at short reaction times. In the materials with the more restricted access to the internal voids, such as ZSM-22 and SAPO-34, the loss of activity is to a large extent due to species which are soluble in dichloromethane and give rise to distinct features in the Raman spectra. For the Mordenite and Beta catalysts, which have larger pores comprising three dimensional networks, and to some extent for the ZSM-5 catalyst employed, the accumulation of more coke species which are insoluble in dichloromethane, presumably on the external surface of the zeolite crystals, is observed. This is linked to the appearance of more pronounced D and G bands in the Raman spectra, indicative of extended carbon species.

The active site in ethene oligomerization catalyzed by Ni-zeolites is proposed to be a mobile Ni(II) complex, based on density functional theory-based molecular dynamics (DFT-MD) simulations corroborated by continuous-flow experiments on Ni-SSZ-24 zeolite. The results of the simulations at operating conditions show that ethene molecules reversibly mobilize the active site as they exchange with the zeolite as ligands on Ni during reaction. Microkinetic modeling was conducted on the basis of free-energy profiles derived with DFT-MD for oligomerization on these mobile [(ethene)2-Ni-alkyl]+ species. The model reproduces the experimentally observed high selectivity to dimerization and indicates that the mechanism is consistent with the observed second-order rate dependence on ethene pressure.