Toward the Atomic Scale Simulation of Intricate Acidic Aluminosilicate Catalysts

Abstract

Zeolites are nanoporous aluminosilicates with well-defined crystalline structures, considered key assets in heterogeneous catalysis, with a broad range of industrial applications. Computational investigations dealing with zeolite catalysts have been undertaken for decades with simple models of the bulk sites, known to be bridging Si IV −OH−Al IV groups, in the case where the compensation cation is a proton. Real zeolite catalysts used in practice are however finite size and intricate objects, with external surface sites and defects, among other sources of complexity. Amorphous silica−alumina may also be obtained as a consequence of zeolite post-treatments or synthesized on purpose to obtain acid sites that are milder than those of zeolites. In the present Perspective, some of the achievements in the field of the atomic-scale simulation of intricate aluminosilicate catalysts (zeolites, amorphous silica−aluminas) of industrial relevance are reviewed. Emphasis is put on the simulation of the mechanisms of post-treatments of zeolites, and on the special structure and reactivity of acid sites at external surfaces of zeolites and on amorphous silica−alumina, that is shown to differ from the properties of the bulk bridging sites. Moreover, directions for future investigations are proposed.

Effect of Impurities on the Initiation of the Methanol-to-Olefins Process: Kinetic Modeling Based on Ab Initio Rate Constants

“…Density functional theory (DFT) calculations are increasingly employed to gain insight into the reactivity of zeolites [16][17][18][19][20][21], for methanol dehydration [22][23][24][25] as well as subsequent steps in the MTO process involving the reaction of olefins and aromatics [26][27][28][29][30][31][32][33][34]. Kinetic models were developed to study initiation, step-wise methylation via surface methoxy species (SMS), concerted methylation, cracking and deactivation through coking revealing many features of the process [35][36][37][38][39].…”

Experimental and Theoretical Evidence for the Promotional Effect of Acid Sites on the Diffusion of Alkenes through Small‐Pore Zeolites

“…[17,29] Finally,T AP experiments are conducted over commercial, micron-sized H-SAPO-34 crystals [35] to probe other levels of complexities such as the prominent influence of surface barrier resistances or inhomogeneities in crystallite sizes. [36][37][38][39][40] By monitoring transient responses of probe molecules through an ensemble of crystallites,t he TAPm ethod is capable of capturing intrinsic adsorption and diffusion characteristics from the external surface into the outer layers of the SAPO-34 crystals in the limit of low pore occupancy.…”

Zeolites at the Molecular Level: What Can Be Learned from Molecular Modeling

“…When addressing the future of molecular modeling, some remarks should also be given regarding the computational determination of reactive free-energy surfaces, stressing the credibility of predictions and qualitative insights into heterogeneous catalysis [ 5 , 144 ]. For example, the density functional theory molecular dynamics (DFT-MD) provides an efficient framework for accurately computing several types of properties and spectra [ 145 , 146 ].…”

“…The reader interested in general aspects of zeolite chemistry and their applications is referred to the comprehensive review on state of the art and future challenges of zeolites as catalysts [ 1 ] and to topical reviews on specific applications of zeolites in oil refining [ 2 ], in sustainable chemistry [ 3 ], and in biotechnology and medicine [ 4 ]. In turn, a general introduction to recent advances in atomistic simulation methods, the development of material databases and the use of machine learning for the prediction of zeolite properties can be found in many excellent up-to-date reviews [ 5 , 6 , 7 , 8 , 9 ].…”