Computational studies of water adsorption in zeolites

Zygmunt, Stan; Curtiss, Larry A.; Iton, Lennox E.

doi:10.1016/s0167-2991(06)81878-1

Cited by 4 publications

(6 citation statements)

References 18 publications

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“…For x ≤ 1 the spectra in the mid-IR (Figure b, full lines, each spectrum corresponds to a dose of water sufficient to consume 1 / 5 of the Brønsted sites, as directly measured from the pressure drop) are similar to those reported by Jentys et al 16b. Following Pelmenschikov et al .,16d Haase et al .,24a and Zygmunt et al ., these spectra can be explained in terms of water adsorbed in neutral (hydrogen-bonded) form. In particular (i) the two broad bands at 2890 and 2480 cm -1 (which grow at the expenses of the peak at 3613 cm -1 due to the ν(OH) mode of the unperturbed Brønsted site) both belong to the ν(OH···O) mode of the group a in structure VI , split into two parts by an Evans window centered at 2685 cm -1 (A, B diad) (Scheme ).…”

Section: Resultssupporting

confidence: 75%

FTIR Investigation of the Formation of Neutral and Ionic Hydrogen-Bonded Complexes by Interaction of H-ZSM-5 and H-Mordenite with CH₃CN and H₂O: Comparison with the H-NAFION Superacidic System

et al. 1996

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The IR spectra of increasing doses of CH3CN interacting with H-ZSM-5, H-MOR, and H-NAFION are investigated and compared. In all cases the formation of neutral hydrogen-bonded adducts is observed and complete vibrational assignment is given. The basic IR spectroscopy of these complexes is discussed in the framework of the Evans approach on the Fermi resonance between the narrow 2δ and 2γ levels with the continuum distribution of levels associated with the ν(OH···B) mode coupled with the low-frequency ν(O···B) vibrations and with the external (thermal) librations. At low dosages the interaction of H2O with H-ZSM-5 and H-MOR gives neutral hydrogen-bonded adducts. At higher dosages these species are transformed into H+(H2O) n (n average = 3−5) ionic species. This process can be only partially reversed by decreasing the relative pressure of H2O. At low dosage the concentration of ionic species is higher on H-MOR than on H-ZSM-5. The interaction of H2O with the superacidic H-NAFION membrane readily gives directly the H+(H2O) n products without formation of the neutral intermediates. The decrease of the relative pressure of H2O is not accompanied by a back-transfer of the proton to the membrane. This differentiates the H-ZSM-5 and H-MOR solid acids from the superacidic H-NAFION membrane. The comparison of the IR spectra obtained on H-ZSM-5 and H-MOR on one side and on H-NAFION on the other side allows a detailed assignment of the spectroscopic manifestations of the neutral and protonated species.

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Section: Resultssupporting

confidence: 75%

FTIR Investigation of the Formation of Neutral and Ionic Hydrogen-Bonded Complexes by Interaction of H-ZSM-5 and H-Mordenite with CH₃CN and H₂O: Comparison with the H-NAFION Superacidic System

et al. 1996

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“…In this paper we report on a study of the interaction of water with an acid site in H-ZSM-5 incorporating large cluster size, electron correlation, and local geometry optimization in a unified way. The calculations presented here extend our earlier study of the adsorption of H 2 O on a 2 T atom (T = Si, Al) cluster model of H-ZSM-5 and a preliminary report of some aspects of this work . In section II we describe the theoretical methods, and in section III we present results for the geometry and interaction energy of the neutral H 2 O adsorption structure, the relative energy of the ion-pair structure resulting from proton transfer, and the vibrational frequencies of the H 2 O adsorption structure.…”

Section: Introductionmentioning

confidence: 60%

Computational Studies of Water Adsorption in the Zeolite H-ZSM-5

et al. 1996

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Ab initio molecular orbital calculations using Hartree−Fock theory and Møller−Plesset perturbation theory have been used to study the interaction of H2O with the Brønsted acid site in the zeolite H-ZSM-5. Aluminosilicate clusters with up to 28 T atoms (T = Si, Al) were used as models for the zeolite framework. Full optimization of a 3 T atom cluster at the MP2/6-31G(d) level indicates that the “ion-pair” structure, Z-···HOH2 +, formed by proton transfer from the acid site of the zeolite (ZH) to the adsorbed H2O molecule, is a transition state, while the “neutral” adsorption structure, ZH···OH2, is a local energy minimum. Partial optimization of a larger 8 T cluster at the HF/6-31G(d) level also gave results suggesting that the ion-pair structure is a transition state. Calculations were carried out to obtain corrections for high levels of theory, zero-point energies, and larger cluster size. The resulting energy difference between the neutral and ion-pair structure is small (less than 5 kcal/mol and possibly close to zero). The interaction energy of ZH···OH2 is 13−14 kcal/mol, in agreement with experiment. We find that addition of a second H2O molecule to Z-···HOH2 + in the 3 T atom cluster stabilizes the ion-pair structure, Z-···H(OH2)2 +, making it a local energy minimum. Finally, calculated vibrational frequencies for a 3 T atom cluster are used to help interpret experimental IR absorption spectra.

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“…The carbonium mechanism, in particular, is seeing increased research interest, with new experiments − but particularly with computational chemistry methods. − Carbonium ions, or protonated alkanes, were first detected in mass spectrometry experiments and have been spectroscopically detected only in the gas phase. They are highly reactive ions that are difficult to study experimentally due to their short chemical lifetime, and theoretical chemistry has therefore become the most valuable tool for their investigation.…”

Section: Introductionmentioning

confidence: 99%

“…Proponium and butonium ions have been recently studied computationally by Mota and co-workers, ,, demonstrating multiple possible geometries for protonation, as well as providing useful energetics. Calculations on larger carbonium ions have been limited in accuracy and/or scope. ,,,− Several computational studies have investigated the catalytic carbonium initiation mechanism directly, by attempting to model actual catalytic events involving carbonium ion formation, ,,,− ,− and although these models all suffer from incomplete treatment of long-range effects, they have resulted in one intriguing suggestionthat carbonium ions in condensed phases are not intermediates but transition states . Three of the outstanding questions regarding the carbonium initiation mechanism are the following: why does the proton attack some alkanes but not others, where on an alkane does the catalytic proton attack, and how exactly do these carbonium ions produce carbenium ions?…”

Section: Introductionmentioning

confidence: 99%

Properties of C−C Bonds inn-Alkanes: Relevance to Cracking Mechanisms

Hunter

East

2002

J. Phys. Chem. A

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The slight variations among the proton affinities and bond strengths of the C-C bonds in straight-chain n-alkanes have been determined to 1 kcal mol -1 accuracy for the first time, using computational quantum chemistry. Four computational methods (B3LYP, MP2, CCSD(T), and G2) were used to study n-alkanes (up to C 20 H 42 with B3LYP), including computations on the related alkyl radicals, carbenium ions, and carbonium ions. The proton affinities of the C-C bonds vary from 142 to over 166 kcal mol -1 , are highest for the center C-C bond, and decrease monotonically toward the end bonds. Bond strength, unlike proton affinity, is very constant (88 kcal mol -1 ), except for the R and bonds (89 and 87 kcal mol -1 , respectively). For thermal cracking, the results suggest that the most favored initiation step is the breaking of the bond of the alkane to create an ethyl radical. For Bronsted-acid-catalyzed cracking of straight-chain paraffins, if the initiation mechanism is via carbonium ions, then the results indicate that the central C-C bonds of n-alkanes will be most attractive to the Bronsted proton. However, for direct protolysis (Bronsted-mediated fission) of an n-alkane via a carbonium intermediate, the net exothermicities do not strongly discern among the C-C bonds. Trends in molecular geometry and infrared spectra features are also presented, and a signature IR band is predicted for carbonium ions that should aid in their identification.

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Computational studies of water adsorption in zeolites

Cited by 4 publications

References 18 publications

FTIR Investigation of the Formation of Neutral and Ionic Hydrogen-Bonded Complexes by Interaction of H-ZSM-5 and H-Mordenite with CH₃CN and H₂O: Comparison with the H-NAFION Superacidic System

FTIR Investigation of the Formation of Neutral and Ionic Hydrogen-Bonded Complexes by Interaction of H-ZSM-5 and H-Mordenite with CH₃CN and H₂O: Comparison with the H-NAFION Superacidic System

Computational Studies of Water Adsorption in the Zeolite H-ZSM-5

Properties of C−C Bonds inn-Alkanes: Relevance to Cracking Mechanisms

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Computational studies of water adsorption in zeolites

Cited by 4 publications

References 18 publications

FTIR Investigation of the Formation of Neutral and Ionic Hydrogen-Bonded Complexes by Interaction of H-ZSM-5 and H-Mordenite with CH3CN and H2O: Comparison with the H-NAFION Superacidic System

FTIR Investigation of the Formation of Neutral and Ionic Hydrogen-Bonded Complexes by Interaction of H-ZSM-5 and H-Mordenite with CH3CN and H2O: Comparison with the H-NAFION Superacidic System

Computational Studies of Water Adsorption in the Zeolite H-ZSM-5

Properties of C−C Bonds inn-Alkanes: Relevance to Cracking Mechanisms

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Product

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FTIR Investigation of the Formation of Neutral and Ionic Hydrogen-Bonded Complexes by Interaction of H-ZSM-5 and H-Mordenite with CH₃CN and H₂O: Comparison with the H-NAFION Superacidic System

FTIR Investigation of the Formation of Neutral and Ionic Hydrogen-Bonded Complexes by Interaction of H-ZSM-5 and H-Mordenite with CH₃CN and H₂O: Comparison with the H-NAFION Superacidic System