Kinetic stability of nitrogen-substituted sites in HY and silicalite from first principles

Agarwal, Vishal; Huber, George W.; Conner, W. Curtis; Auerbach, Scott M.

doi:10.1016/j.jcat.2010.01.001

Cited by 8 publications

(3 citation statements)

References 33 publications

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“…Nitrided zeolites have been under study for over 40 years since the initial report by Kerr and Shipman in 1968 . Although recent work on nitrided zeolites has revealed interesting base catalytic activities, , the structures of nitrided zeolites and their active sites have remained only ambiguously identified. ,,,− By combining 29 Si solid-state NMR and quantum calculations of chemical shifts, we have shown that nitridation in high-silica H−Y (Si:Al = 15) is consistent with an intact zeolite framework and occurs first at Brønsted sites (Si−OH−Al) and subsequently at siliceous sites (Si−O−Si). ,, The story is qualitatively similar for nitridation of Na−Y (Si:Al = 2.4), although the diversity of nitrided sites in Na−Y is much higher because of the higher Al content. ,, Furthermore, through a combination of X-ray diffraction, 29 Si solid-state NMR, and high-resolution adsorption studies, we have recently reported an optimal synthesis approach for nitrided zeolites emphasizing the importance of high ammonia flow rates during heating and reacting steps. ,, Finally, density functional theory (DFT) calculations of the nitridation mechanisms in H−Y and silicalite zeolites have been used to predict that nitrided zeolites remain stable at high temperatures even at saturation water loadings . The question still remains how the Si:Al ratio and the nature of the charge-compensating cation impact strengths of basic sites in nitrided zeolites.…”

Section: Introductionmentioning

confidence: 88%

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

2010

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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.

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Section: Introductionmentioning

confidence: 88%

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

2010

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“…Microkinetic calculations are useful for generating testable predictions from complex, multistep reaction models. − For each reaction pathway, we seek to determine the apparent activation energy consistent with the temperature-dependent Gibbs free energies computed for reactants, products, intermediates, and transition states. As in our previous work, we invoke the pseudo-steady-state approximation, , assuming that the concentrations of stable adsorbates are constant in time to yield a well-defined kinetic system that does not require information from the adsorption isotherms of reactants/products. Each aldol pathway consists of three or four elementary steps, assuming concerted or stepwise dehydration, respectively (see eqs and ).…”

Section: Methodsmentioning

confidence: 99%

“…Below, we report our results on delta-cluster convergence of free energies of intermediates and transition states for acetone aldol condensation with formaldehyde, furfural, and HMF, considering both concerted and stepwise processes where relevant. To organize all this information into predictions testable by experiments, we perform microkinetic calculations − to produce overall apparent activation energies for each process considered. In the end, we find that the neighbor-list radius of 4 Å is sufficient to converge barriers with respect to delta-cluster size for all reaction steps considered, yielding results in agreement with periodic DFT for a fraction of the computational cost.…”

Section: Introductionmentioning

confidence: 99%

On the Rational Design of Zeolite Clusters for Converging Reaction Barriers: Quantum Study of Aldol Kinetics Confined in HZSM-5

et al. 2018

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We have performed density functional theory calculations to investigate the convergence of reaction barriers with respect to zeolite cluster size, for multistep reactions catalyzed in HZSM-5. We constructed cluster models of HZSM-5 using the delta-cluster approach reported previously by us. We then computed barriers for different reaction types to determine the cluster sizes and neighbor-list radii needed to fully treat zeolite confinement effects. In particular, we studied the acid-zeolite-catalyzed aldol reactions of acetone with formaldehyde, furfural, and hydroxymethyl-furfural, in three steps: keto/enol tautomerization of acetone, combination between each aldehyde and the enol, and aldol dehydration. We found that the delta-cluster radius of 4.0 Å consistently converges barriers with respect to cluster size, yielding complete treatments of confinement in HZSM-5 with clusters containing up to 99 atoms. For comparison, periodic density functional theory (DFT) on HZSM-5 includes 288 atoms, requiring 19 times more CPU time in head-to-head comparisons. Our converged acetone−formaldehyde dehydration barrier agrees quantitatively with a comparable barrier obtained with periodic DFT, showing that cluster calculations can converge properties at a fraction of the cost of periodic DFT. Interestingly, we found that the bulkier, furan-containing aldehydes exhibit faster reactivity because of charge delocalization from aromatic rings, which significantly speeds up aldol dehydration.

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Case Studies: Raman Spectroscopy

Tigiripalli,

Agarwal,

Deo

2023

Springer Handbooks

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Kinetic stability of nitrogen-substituted sites in HY and silicalite from first principles

Cited by 8 publications

References 33 publications

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

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

On the Rational Design of Zeolite Clusters for Converging Reaction Barriers: Quantum Study of Aldol Kinetics Confined in HZSM-5

Case Studies: Raman Spectroscopy

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