Theoretical Study of the Adsorption of Alkylamines in H-Mordenite: The Role of Noncovalent Interactions

Abstract

The adsorption of NH 3 and alkylamines (MeNH 2 , Me 2 NH, and Me 3 N) on H-mordenite was studied using density functional theory (DFT) and the ONIOM method. The adsorption of the probe molecules was investigated with a set of functionals (B3LYP, B3LYP-D3, CAM-B3LYP, LC-ωPBE, and ωB97X-D) properly chosen to verify the importance of noncovalent interaction on the adsorption process. The adsorption energy increases with the size of the amine and with the type of functional used, according to the trend B3LYP-D3, ωB97X-D > CAM-B3LYP, LC-ωPBE > B3LYP. The noncovalent contribution to the adsorption energy is higher for Me 3 N, reaching 92 kJ/mol when B3LYP-D3 is used and 70 kJ/mol for ωB97X-D. Although the adsorption energies for each amine span a wide range of values depending on the functional used, the geometries of equilibrium are quite similar. The adsorption energies obtained by full geometry optimization of the complexes do not differ appreciably from those of single-point calculations using the geometries optimized at the B3LYP:UFF level of calculation, suggesting that for these systems the single-point strategy may be a reasonable alternative to reduce the computational cost of including noncovalent corrections in DFT results. A comparison was also made with the experimental differential heats of adsorption. At low coverage the computed E ads at the T4 site corrected for the basis set superposition (BSSE) and zero-point vibration energy errors exhibit the same trend of the experimental ΔH ads with the degree of methylation observed by Lee et al. (J. Am. Chem. Soc. 1996, 118, 3262−3268) as opposed to the one by Chen et al. (J. Catal. 1994, 146, 257−267). Also, the results obtained at the B3LYP-D3:UFF level of calculation including BSSE corrections strongly suggest that the falloff observed on the ΔH ads x coverage diagram, normally attributed to site heterogeneity, may be due to a combination of two factors: the number of molecules adsorbed on a given site and the acid site local configuration. ■ INTRODUCTIONOne of the main uses of alkylamines is as additives for the synthesis of colloidal semiconductor nanocrystals. 1 Furthermore, it has been reported that nitrogen hybrid zeolites with MeNH 2 , Me 2 NH, Me 3 N, EtNH 2 , n-PrNH 2 , and n-ButylNH 2 increase the basicity of MFI-type zeolites by the incorporation of N atoms into the framework. 2 As the first step of this process involves the adsorption of the amine on the zeolite framework, it is necessary to determine how this process is affected by the structure of the alkylamines.Mordenite (MOR) is one of the most abundant zeolites in nature, 3 and its protonic form (H-MOR) has been used in chemical processes such as hydrocarbon isomerization, 4 nparaffin cracking, 5 and alkylation reactions 6 and in the selective catalytic reduction (SCR) of NO. 7 The quantitative adsorption of alkylamines in H-MOR has been investigated using the microcalorimetry technique. 8,9 In these experiments the zeolite is kept at constant temperature in the cell where the probe molecu...

New theoretical insight on the acid sites distribution, their local structures and acid strength of the SAPO‐11 molecular sieve

“…According to the calculated red shift values reported on Table , the acid strength of the OH groups in the SAPO 4 ‐11 is: SiOH < POH < Si(OH)Al. As expected, the interaction energies calculated with the ωB97X‐D functional are greater than those calculated with B3LYP in agreement with published works . This shows the importance of noncovalent interaction on the adsorption process.…”

“…In general, the interaction energies calculated with ωB97X‐D are lower (more negative) than to those obtained with B3LYP and closer to the experimental values. This has been attributed to the fact that noncovalent interactions play an important role in the process of adsorption . However, it must be taken into account that these interaction energies cannot reproduce the experimental values exactly since they do not include the thermal and vibrational corrections and furthermore, the limitation of the model (number of atoms at the high level) and the basis set size limits the accuracy of theoretical results.…”

Effect of Steam Treatment on the Properties of Mordenite and Its Catalytic Performance in a DME Carbonylation Reaction

“…The clear downward trend was due to the removal of framework Al and blockage of side pockets. Moreover, the peak position at the high-temperature region in the curve of H- x samples shifted to lower temperatures, indicating the loss of BAS in 8MR channels because the strength of BAS in the 8MR channel was stronger than that in the 12MR channel. , The peak temperature in the high-temperature region did not shift for N-673, N-773, and N-873 samples, confirming that the steam treatment has a slight influence on the strong acid sites of these samples. The Brønsted acid sites of N-673, N-773, and N-873 were 1.04, 1.18, and 1.07 mmol/g, respectively, which were higher than that of the parent H-MOR.…”