Brønsted acidity in zeolites measured by deprotonation energy

Abstract

Acid forms of zeolites have been used in industry for several decades but scaling the strength of their acid centers is still an unresolved and intensely debated issue. In this paper, the Brønsted acidity strength in aluminosilicates measured by their deprotonation energy (DPE) was investigated for FAU, CHA, IFR, MOR, FER, MFI, and TON zeolites by means of periodic and cluster calculations at the density functional theory (DFT) level. The main drawback of the periodic DFT is that it does not provide reliable absolute values due to spurious errors associated with the background charge introduced in anion energy calculations. To alleviate this problem, we employed a novel approach to cluster generation to obtain accurate values of DPE. The cluster models up to 150 T atoms for the most stable Brønsted acid sites were constructed on spheres of increasing diameter as an extension of Harrison’s approach to calculating Madelung constants. The averaging of DPE for clusters generated this way provides a robust estimate of DPE for investigated zeolites despite slow convergence with the cluster size. The accuracy of the cluster approach was further improved by a scaled electrostatic embedding scheme proposed in this work. The electrostatic embedding model yields the most reliable values with the average deprotonation energy of about 1245 ± 9 kJ·mol−1 for investigated acidic zeolites. The cluster calculations strongly indicate a correlation between the deprotonation energy and the zeolite framework density. The DPE results obtained with our electrostatic embedding model are highly consistent with the previously reported QM/MM and periodic calculations.

Cooperativity of silanol defect chemistry in zeolites

“…The DPEs are the energy differences between the deprotonated and protonated cluster models. [57][58][59][60] The calculated numbers are large compared to a typical bond energy (given as the energy for a homolytic bond scission), because the heterolytic reactions include charge separation, and this artificial Coulomb contribution is substantial. However, DPEs can be useful, if they are applied for a comparison of similar chemical moieties regarding their acidity differences.…”

Influence of ion mobility on the redox and catalytic properties of Cu ions in zeolites

“…[14][15][16] The acid strength of protons in zeolites depends on the zeolite topology, composition and proton location in specic crystallographic positions. 12,17,18 However, the relation of proton acid strength and concentration on the catalytic activity is not straightforward, 10 also in relation to proton mobility, 19 which can be mediated by interaction with reactants or adsorbed molecules, such as water 17,20,21 or ammonia. [22][23][24] Metal cations are stabilized in specic crystallographic positions in the channels and cages of zeolites, depending on the nature of the metal cation combined with the topology and chemical composition of the material.…”

“…[4][5][6][7] Another fascinating aspect of the inner structure of zeolites is that it can stabilize cations in geometries and even oxidation states which are not usually observed in solids, solution or homogeneous complexes. 8,9 Protons represent a particular and important class of cations stabilized in zeolites, conferring them a specic Brønsted acidity, [10][11][12][13] which is crucial for many applications. [14][15][16] The acid strength of protons in zeolites depends on the zeolite topology, composition and proton location in specic crystallographic positions.…”

“…14–16 The acid strength of protons in zeolites depends on the zeolite topology, composition and proton location in specific crystallographic positions. 12,17,18 However, the relation of proton acid strength and concentration on the catalytic activity is not straightforward, 10 also in relation to proton mobility, 19 which can be mediated by interaction with reactants or adsorbed molecules, such as water 17,20,21 or ammonia. 22–24…”

Computational Investigation of Site-Dependent Activation Barriers of Zeolite-Catalyzed Protolytic Cracking Reactions

“…They attributed the invariance of intrinsic barriers to similar acid-site strengths for Al-substituted Brønsted-acid sites across different regions . However, it should be noted that intrinsic acid-site strengths, as typically equated with their deprotonation energies, are hard to quantify unambiguously. − Theoretical calculations consistently found appreciable site and structure sensitivities, − the magnitude of which is reduced only after accounting for the varying T-site densities and the resulting differences in the dielectric constants of the materials . Gounder and Iglesia also concluded that intrinsic barriers remained similar across different alkane chain lengths, based upon the adsorption enthalpies measured at low temperatures − and extrapolated to the cracking temperatures. , The same conclusion was reached by Li et al Gorte and co-workers studied the endothermic cracking of n -hexane in H-ZSM-5, H-BEA, H-MOR, and USY zeolites and found that a single rate expression was able to describe the kinetics over a very wide range of pressures. , The temperature dependence of rate constants was again identified to the adsorption enthalpies of n -hexane. , Kadam et al studied the protolytic cracking reaction of propane and n -butane in the FER, TON, MFI, and CHA zeolites at temperatures ranging from 580 to 710 K and observed that intrinsic activation barriers are insensitive to the zeolite structure.…”