DFT Study of Nitrogen-Substituted FAU: Effects of Ion Exchange and Aluminum Content on Base Strength

Abstract

We have studied base strengths of nitrogen-substituted (nitrided) zeolites with faujasite (FAU) structure by calculating sorption energies of probe molecules (BF3 and BH3) using density functional theory with mixed basis sets applied to embedded clusters. BH3 was found to be a better probe of base strength because it does not introduce competing metal−fluorine interactions that obfuscate trends. In all cases, the base strengths of nitrided zeolites (denoted M−N−Y) were found to exceed those of the corresponding standard M−Y zeolites, where M = Li, Na, K, Rb, or Cs charge-compensating cations. We have found that for a particular Si:Al ratio, BH3 sorption energies vary in the order Li < Na < K ∼ Rb ∼ Cs. Sorption energy and hence base strength was found to decrease with increasing Si:Al ratio from 1 to 3 beyond which the base strength was found to increase again. The initial regime (1 < Si:Al < 3) is consistent with the prevailing understanding that the base strength increases with Al content, while the latter regime (Si:Al > 3) involves the surprising prediction that the base strength can be relatively high for the more stable, high-silica zeolites. In particular, we found the sorption energy in Na−N−Y (Si:Al = 11) to be nearly equal to that in (Si:Al = 1). Taken together, these results suggest that K−N−Y (Si:Al = 11) optimizes the balance of activity, stability, and cost.

Exploring the Sodium Cation Location and Aluminum Distribution in ZSM-5: A Systematic Study by the Extended ONIOM (XO) Method

“…This calls for help from other techniques such as UV–vis spectroscopy, − FTIR spectroscopy, − microcalorimetry, − NMR spectroscopy, electron spin resonance (ESR) spectroscopy, − extended X-ray absorption fine structure (EXAFS) spectroscopy, − and the computational methods. Indeed, sophisticated quantum chemical calculations have already provided much valuable information on the structural, electronic, and spectroscopic properties for metal cations in zeolites and have shed light on the structure–property relationship at the molecular level. − …”

Relative Rates of Krypton and Rubidium Release from Zeolite Getters

“…The high level of theory was DFT with either the local spin density approximation (LSDA, equivalent to the LDA for spin-paired systems) or the hybrid exchange–correlation functional B3LYP. − We used the 6-311G­(d,p) triple-ζ basis set , for all elements except rubidium; we used the SDD basis set − for rubidium and also ran a comparison using the SDD basis set for krypton as well. This method of using mixed basis sets was used previously by Sung et al for barium and Agarwal et al for rubidium and cesium in zeolites and is necessary for “heavy” elements beyond the third row of the periodic table for which more common basis sets are unavailable. The choice of B3LYP/6-311G­(d,p):UFF with the ONIOM method has been shown to yield reasonably accurate zeolitic geometries and vibrational frequencies, as well as proton jump rates. ,,, Geometry optimizations were performed with no symmetry constraint for both transition and initial/final states using the “quadratic coupling” method of Vreven et al , The Berny algorithm was used to optimize to transition states, starting from an initial guess with the krypton or rubidium atom in the center of the six-membered ring and other atoms near their crystallographic positions.…”

“…78−80 We used the 6-311G(d,p) triple-ζ basis set 81,82 for all elements except rubidium; we used the SDD basis set 83−85 for rubidium and also ran a comparison using the SDD basis set for krypton as well. This method of using mixed basis sets was used previously by Sung et al 86 for barium and Agarwal et al 87 for rubidium and cesium in zeolites and is necessary for "heavy" elements beyond the third row of the periodic table for which more common basis sets are unavailable. The choice of B3LYP/6-311G(d,p):UFF with the ONIOM method has been shown to yield reasonably accurate zeolitic geometries and vibrational frequencies, as well as proton jump rates.…”

Adsorption of parent nitrosamine on the nanocrystaline M-ZSM-5 zeolite: A density functional study

“…To balance the excess of charges when substitutions of silicon with aluminum occur, cations are present. [21][22][23][24][25] Cation zeolites can be converted to protonic zeolites; then, one obtains solid acid catalyst. 26 The ZSM-5 zeolite, characterized by a threedimensional pore system with straight and sinusoidal channels, possesses unique channel structure, thermal and hydrothermal stability and acidity.…”