Molecular interactions of alcohols with zeolite BEA and MOR frameworks

Abstract

Zeolites can adsorb small organic molecules such as alcohols from a fermentation broth. Also in the zeolite-catalyzed conversion of alcohols to biofuels, biochemicals, or gasoline, adsorption is the first step. Several studies have investigated the adsorption of alcohols in different zeolites experimentally, but computational investigations in this field have mostly been restricted to zeolite MFI. In this study, the adsorption of C1-C4 alcohols in BEA and MOR was investigated using density functional theory (DFT). Calculated adsorption geometries and the corresponding energies of the designed cluster models were comparable to periodic calculations, and the adsorption energies were in the same range as the corresponding computational and experimental values reported in the literature for zeolite MFI. Thus, BEA and MOR may be good adsorption materials for alcohols in the field of downstream processing and catalysis. Aside from the DFT calculations, adsorption isotherms were determined experimentally in this study from aqueous solutions. For BEA, the adsorption of significant amounts of alcohol from aqueous solution was observed experimentally. In contrast, MOR was loaded with only a very small amount of alcohol. Although differences were found between the affinities obtained from gas-phase DFT calculations and those observed experimentally in aqueous solution, the computational data presented here represent molecular level information on the geometries and energies of C1-C4 alcohols adsorbed in zeolites BEA and MOR. This knowledge should prove very useful in the design of zeolite materials intended for use in adsorption and catalytic processes, as it allows adsorption behavior to be predicted via judiciously designed computational models.

Study of Confinement and Catalysis Effects of the Reaction of Methylation of Benzene by Methanol in H-Beta and H-ZSM-5 Zeolites by Topological Analysis of Electron Density

“…Methanol adsorption inside zeolites is a research topic on which several studies, both experimental and theoretical, have been reported in the literature. ,− In the papers mentioned above, attention was mainly focused on the interaction between the methanol and the zeolite acid site. The presence of two hydrogen bonds has been described, the first being medium to strong and the second being much weaker.…”

Molecular Views on Mechanisms of Brønsted Acid-Catalyzed Reactions in Zeolites

“…Cluster investigations, up to the MP2 level, conclude that methoxonium does not constitute the local energy minimum for methanol, consistently with NMR spectra recorded for the H-Rho zeolite . However, the shape of the potential energy surface was shown to depend on the level of theory, on the type of zeolite model, on the zeolite framework, on the position of the Brønsted acid site, and on the alcohol under consideration. − The proton affinity of the alcohol, itself a function of the degree of substitution of the alcohol, was shown to be a relevant parameter to quantify its propensity toward protonation by the zeolite . Methanol in ferrierite at 300 K, ethanol, and 1-propanol in H-ZSM-5 close to 500 K, were shown by AIMD to be easily protonated.…”

Influence of Confinement on Barriers for Alkoxide Formation in Acidic Zeolites

“…We computed the adsorption energy of MeOH for various frameworks and obtained binding energies ranging from À 91 kJ/mol (H-SAPO-34) to À 119 kJ/mol (H-MOR and H-ZSM-22, see Table 1) in good agreement with earlier theoretical studies (see Table S4 for an in-depth comparison with data available in the literature). [36][37][38][39][40][41] The reaction of adsorbed MeOH with the acid site forming an SMS and water is computed to have reaction barriers ranging from 126 kJ/mol (H-SAPO-34) to 150 kJ/mol (H-BEA) when referenced to the adsorbed MeOH. We calculated the analogous reaction of ethanol (EtOH) with the acid site producing a surface ethoxy species (SES) for the eight acidic zeotypes considered herein as shown in Figure 2.…”

“…This reaction has been investigated theoretically to a great extend [30,36–41] as it constitutes the first step in stepwise (or dissociative) methylation reactions (e. g. stepwise formation of dimethyl ether (DME) via MeOH). We computed the adsorption energy of MeOH for various frameworks and obtained binding energies ranging from −91 kJ/mol (H‐SAPO‐34) to −119 kJ/mol (H‐MOR and H‐ZSM‐22, see Table 1) in good agreement with earlier theoretical studies (see Table S4 for an in‐depth comparison with data available in the literature) [36–41] . The reaction of adsorbed MeOH with the acid site forming an SMS and water is computed to have reaction barriers ranging from 126 kJ/mol (H‐SAPO‐34) to 150 kJ/mol (H‐BEA) when referenced to the adsorbed MeOH.…”

“…As mentioned in our previous work for SMS formation in H‐SZM‐5, [19] and in a study by Hibbitts and coworkers for SMS‐formation with co‐adsorbed toluene, [35] the barriers for protonation/rotation of the alcohols are negligible compared to the barrier for methylation. This reaction has been investigated theoretically to a great extend [30,36–41] as it constitutes the first step in stepwise (or dissociative) methylation reactions (e. g. stepwise formation of dimethyl ether (DME) via MeOH). We computed the adsorption energy of MeOH for various frameworks and obtained binding energies ranging from −91 kJ/mol (H‐SAPO‐34) to −119 kJ/mol (H‐MOR and H‐ZSM‐22, see Table 1) in good agreement with earlier theoretical studies (see Table S4 for an in‐depth comparison with data available in the literature) [36–41] .…”