Comparing the Acidities of Microporous Aluminosilicate and Silico-Aluminophosphate Catalysts: A Combined Quantum Mechanics-Interatomic Potential Function Study

Sauer, Joachim; Schröder, K.-P.; Termath, Volker

doi:10.1135/cccc19981394

Cited by 47 publications

(36 citation statements)

References 30 publications

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“…For two chabazite catalysts of different composition (the aluminosilicate material H-SSZ-13 and the silico-aluminium-phosphate material H-SAPO-34) we find excellent agreement (within 4 kJ/mol) between the combined QM-Pot result and the full ab initio calculations that apply periodic boundary conditions. 53 This confirms that the combined QM-Pot method takes the long-range effects properly into account.…”

Section: Energies Of Deprotonation (Kj/mol) Calculated By Different Msupporting

confidence: 64%

ChemInform Abstract: Combining Quantum Mechanics and Interatomic Potential Functions in ab initio Studies of Extended Systems

Sauer

Sierka

2001

ChemInform

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ABSTRACT:The errors made when large chemical systems are replaced by small models are discussed: interrupted charge transfer, missing structure constraints, neglected long-range interactions. A combined quantum mechanics (QM)-interatomic potential function (Pot) approach is described. Characteristic features of the QM-Pot approach include: (1) periodic boundary conditions, (2) consistent definition of forces in the presence of link atoms that terminate the QM cluster, (3) interatomic potential functions parametrized on ab initio data and accounting for polarization effects, (4) use of reaction force fields (EVB potentials) in combination with QM methods for efficient localization of transition structures in large systems, (5) implementation as a loose coupling of existing QM and Pot engines. Comparison is made with some other hybrid QM/MM methods. Applications of the combined QM-Pot method for ab initio modeling of the structure and reactivity of zeolite catalysts are reviewed with both protons and transition metal cations as active species. Potential functions of the ion-pair shell-model type available for such studies are compiled. The reliability of the method is checked by comparison with periodic ab initio studies and by examining the convergence of the results with increasing size of the QM cluster. The problems tackled are: different types of Cu + sites in the CuZSM-5 catalyst and their properties, acidity differences between active sites in different zeolite framework structures (energies of deprotonation, NH 3 adsorption energies), and proton mobility in acidic zeolites. The combined QM-Pot approach made possible a full ab initio prediction of reaction rates for an elementary process on the surface of solid catalysts and of how these rates differ between different catalysts with the same active site.

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Section: Energies Of Deprotonation (Kj/mol) Calculated By Different Msupporting

confidence: 64%

ChemInform Abstract: Combining Quantum Mechanics and Interatomic Potential Functions in ab initio Studies of Extended Systems

Sauer

Sierka

2001

ChemInform

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“…The values presented for the materials in this work are in the range expected from zeolites, which are indistinguishable in acid strength as measured by this approach. SAPO materials are weaker in acid strength than zeolites [60][61][62], as a result the frequency shift observed on SAPO-34 is the smallest in this work. The BET surface areas agree well with previously reported values; however, the surface area for the commercial MTT catalyst is somewhat low [13,51,[63][64][65][66].…”

Section: Catalytic Testscontrasting

confidence: 43%

Morphology-induced shape selectivity in zeolite catalysis

Teketel

Lundegaard

Skistad

et al. 2015

Journal of Catalysis

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a b s t r a c tZeolites are crystalline aluminosilicates that are widely used as shape-selective catalysts in the refinery and petrochemical industries. By synthesizing a series of structurally related zeolite catalyst and applying them in the industrially relevant conversion of methanol to hydrocarbons, new and extended understanding of the concept of shape selectivity has been reached. We have analyzed the relationship between particle morphology and pore system for catalysts having both small-and medium-sized pores using microscopy and diffraction, as well as performing extensive standard characterization. Two instances of preferential exposure of certain crystal facets, favoring access to one pore system over the other, have been found. Zeolites ZSM-57 (MFS) and SUZ-4 (SZR) both have both 8-and 10-ring channels. MFS prefers a plate-like or nanosheet-type morphology, where the 10-ring channels are perpendicular to the plates/sheets. This leads to favorable diffusion properties and a pronounced deactivation resistance. For SZR, needle-like crystals with high aspect ratio are preferred. In this instance, the sides of the needles are covered by the 8-ring pore openings, whereas the 10-ring pore openings are only exposed at the ends of the needles. This leads to unexpected product selectivity, as the material behaves essentially as an 8-ring catalyst. We argue that it should be possible to tune the shape selectivity of zeolite catalysts having pore systems of different dimensions by controlling the crystal morphology.

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“…H-SSZ-24 is an aluminosilicate zeolite, while H-SAPO-5 is a silicoaluminophosphate. This difference in composition leads to a difference in Brønsted acid strength [33][34][35], with H-SAPO-5 containing weaker acid sites than H-SSZ-24 [32]. The AFI framework is composed of columns of twisted four-and sixrings, together forming one-dimensional twelve ring channels running parallel to the c-axis.…”

Section: Introductionmentioning

confidence: 99%

How zeolitic acid strength and composition alter the reactivity of alkenes and aromatics towards methanol

Erichsen

Wispelaere

Hemelsoet

et al. 2015

Journal of Catalysis

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a b s t r a c tThis work encompasses a combined experimental and theoretical assessment of how zeolitic acid strength and composition affects acid-catalysed methylation reactions. Overall, higher methylation rates were observed over the material with higher acid strength. Co-reactions of methanol with benzene at 250°C over the two isostructural AFI materials H-SSZ-24 and H-SAPO-5 revealed large differences in selectivity. While the stronger acidic H-SSZ-24 mainly produced toluene and polymethylbenzenes, high yields of C 4+ aliphatics were observed over H-SAPO-5. These results strongly suggest that alkene methylation was preferred over H-SAPO-5 even at very low conversion during methanol/benzene co-reactions. Furthermore, a comparison of benzene and propene methylation at 350-400°C revealed a significantly faster rate of benzene than propene methylation in H-SSZ-24, whereas the rates of benzene and propene methylation were similar in H-SAPO-5. The observed difference in reactivity of the two hydrocarbons in both catalysts could be understood by careful analysis of various molecular dynamics simulations of the co-adsorbed complexes. The probability to form protonated methanol was, as expected, higher in the more acidic material. However, in H-SSZ-24, the probability for methanol protonation was higher when co-adsorbed with benzene than when co-adsorbed with propene, while the same was not observed in H-SAPO-5. Furthermore, it was found that benzene and methanol are more likely to form a reactive co-adsorbed complex in H-SSZ-24 compared to propene and methanol, while the opposite was observed for H-SAPO-5. This work shows that molecular dynamics simulations provide insights into the adsorption behaviour of guest molecules in large pore AFI materials. The obtained insights correlate with the experimentally observed reactivities.

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Comparing the Acidities of Microporous Aluminosilicate and Silico-Aluminophosphate Catalysts: A Combined Quantum Mechanics-Interatomic Potential Function Study

Cited by 47 publications

References 30 publications

ChemInform Abstract: Combining Quantum Mechanics and Interatomic Potential Functions in ab initio Studies of Extended Systems

ChemInform Abstract: Combining Quantum Mechanics and Interatomic Potential Functions in ab initio Studies of Extended Systems

Morphology-induced shape selectivity in zeolite catalysis

How zeolitic acid strength and composition alter the reactivity of alkenes and aromatics towards methanol

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