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ABSTRACT: With regard to a general interest in methane utilization in a rational way the activation and transformation of methane on Ag-modified zeolite ZSM-5 (Ag/H-ZSM-5) have been studied with solid-state NMR. The activation of methane occurs by dissociation of the C−H bond on silver cations via the "carbenium" pathway: methane C−H bond cleavage results in the methoxy groups (O−CH 3) and possibly silver-hydride species (Ag−H). The formation of surface methoxy groups on Ag/H-ZSM-5 has been detected experimentally with 13 C CP/MAS NMR at 508− 623 K for the first time. A comparative analysis of the kinetics of the H/D exchange between methane and acid hydroxyl groups for H-ZSM-5 and Ag/H-ZSM-5 zeolites reveals a significant promoting effect of silver cations on the H/D exchange reaction and therefore on methane activation. This effect has been rationalized in terms of reversible methane dissociation on the surface of Ag/H-ZSM-5 zeolite and further involvement in the exchange of the methoxy groups and the silver-hydride species. Ethane represents the first intermediate product of methoxy group transformation. It is formed by the reaction of a methoxy group with methane. Further, dehydrogenation of ethane offers ethene, producing immediately π-complexes with Ag + cations, which are stable at temperature as high as 673 K. At 823 K π-complexes decompose and ethene undergoes oligomerization, cyclization, dehydrogenation, and aromatization to give benzene. In the presence of methane, ethene π-complexes decompose and become involved in oligomerization and aromatization reaction at lower temperature, already at 673 K. Methane is also involved in the reaction of coaromatization with ethene. This involvement occurs by the alkylation of aromatics, formed from ethene, with methane. Further demethanation of methylbenzenes in the presence of dihydrogen evolved at the stages of ethene transformation to aromatics produces benzene as the main reaction product.
The Journal of Physical Chemistry C 04/2013; 117(15):7690-7702. · 4.80 Impact Factor
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ABSTRACT: The interaction of olefins with metal oxides affords surface species which are considered to be the intermediates in olefin isomerization reactions. The nature and the structure of these intermediates after earlier characterization by IR spectroscopy still remain debatable. In this paper by using 13 C solid-state NMR spectroscopy we have characterized the surface intermediates formed from propene, n-butene, and isobutene on γ-Al 2 O 3 and α-Ga 2 O 3 , based on analysis of specific chemical shifts expected for similar organometallic or oxygenated compounds. NMR clearly shows that both allylic and alkoxy intermediates are simultaneously formed on two studied metal oxides. Propene affords isopropoxy and allylic intermediates on both oxides. Allyl formed on alumina is bound to the Al 3+ cations of metal oxide surface in a η 1 ,η 2 -like fashion, whereas allyl on α-Ga 2 O 3 is bound to the Ga 3+ cation exclusively in a η 1 -like fashion. n-Butene gives 2-butoxy species for both metal oxides, whereas allylic species (σ-allyl) was identified for this olefin only for γ-Al 2 O 3 . Adsorption of isobutene results to the formation of tert-butoxy and allylic species on both metal oxides. π-Allyl with η 3 -like fashion of allyl bonding to the oxide surface is formed in the case of γ-Al 2 O 3 , whereas σ-allyl is formed in the case α-Ga 2 O 3 . Both allylic and alkoxy species can be involved as intermediates in a double bond shift reaction of olefins on metal oxide surfaces.
The Journal of Physical Chemistry C 01/2012; 116(40-40):21430-21438. · 4.80 Impact Factor
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ABSTRACT: 1. INTRODUCTION Ga-and Zn-modified high silica zeolites are effective catalysts for light alkanes aromatization. 1À5 The enhanced activity of alkane conversion to aromatics is attributed to a bifunctional character of the catalyst active sites, 6,7 capable to perform dehydrogenation and cyclo-oligomerization steps. 2,5,8À11 The role of different active sites in the particular steps of alkane-to-aromatics transformation is debatable. It is conventionally ac-cepted that the metal sites perform alkane dehydrogention and Brønsted acid sites (BAS) are involved in oligomerization and cyclization steps. 12 Iglesia et al. 3,11,13,14 claimed that the activa-tion of alkane occurs on the BAS of the zeolite, while the metal ion serves as a "porthole" for removal of hydrogen adatoms as dihydrogen, thus providing an increase in aromatics selectivity. Other authors provided evidence for the synergy of metal species and acid sites in alkane aromatization. 7,15À17 It has been sug-gested that metal active sites and BAS, located in the vicinity to each other, perform the alkane molecule activation. 18À20 On the basis of the analysis of the kinetics of H/D exchange between BAS and C 1 Àn-C 4 alkanes we have recently shown an involve-ment of Zn species in alkane activation by Brønsted acid sites for Zn-modified zeolites ZSM-5 and BEA. 21À23 This was demon-strated by a dramatic increase of the rate of H/D exchange at modification of zeolite with Zn for C 1 Àn-C 4 alkanes 22 and an
The Journal of Physical Chemistry C 07/2011; 13(115-28):13877--13886. · 4.80 Impact Factor
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ABSTRACT: The conversion of propane (propane-1-13C and propane-2-13C) on Zn/H-BEA zeolite at 520−620 K has been studied by 1H and 13C (CP) MAS NMR. Propene adsorption complex with zinc sites (π-complex) and σ-allylzinc species as intermediates have been identified in the course of propane conversion to aromatics. The mechanism leading to the formation of methane and ethane, which are constituents of an undesirable route in propane conversion, has been examined by kinetic modeling of the expected reaction network based on in situ 1H MAS NMR kinetic measurements of the reaction performance. The pathways for propane aromatization and hydrogenolysis have been proposed. Hydrogenolysis of propane has been concluded to occur with the involvement of both Brønsted acid sites and Zn sites.
06/2010;
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ABSTRACT: Kinetics of hydrogen H/D exchange between Brønsted acid sites of pure acid-form and Zn- or Ga-modified zeolites beta (BEA) and deuterated hydrogen (D(2)) has been studied by (1)H MAS NMR spectroscopy in situ within the temperature range of 383-548 K. A remarkable increase of the rate of the H/D exchange has been found for Zn- and Ga-modified zeolites compared to the pure acid-form zeolite. The rate of exchange for Zn-modified zeolite is one order of magnitude higher compared to the rate for Ga-modified zeolite and two orders of magnitude larger compared to the pure acid-form zeolite. This promoting effect of metal on the rate of H/D exchange was rationalized by a preliminary dissociative adsorption of molecular hydrogen on metal oxide species or metal cations. The adsorbed hydrogen is further involved in the exchange with the acid OH groups located in vicinity of metal species. The role of different metal species in the possible mechanisms of the exchange with involvement of zeolite Brønsted acid sites and metal species is discussed.
Physical Chemistry Chemical Physics 05/2010; 12(19):5149-55. · 3.57 Impact Factor
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ABSTRACT: journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit:: Acid-form and Zn-and Ga-modified zeolite BEA Acidic silanol groups 1 H MAS NMR IR spectroscopy a b s t r a c t Acidic OH groups of acid-form and Zn and Ga-modified zeolite beta (BEA) have been characterized with 1 H MAS NMR and by IR spectroscopy of adsorbed carbon monoxide. It is demonstrated that OH groups, which exhibit a vibration band at 3740 cm À1 , reveal acidity, which is similar to that of the OH groups with the band at 3610 cm À1 according to the value of the low frequency shift of OH vibrations with adsorbed CO (Dm OH/CO = 300 cm À1). The IR band 3740 cm À1 corresponds to the signal at ca. 2.1 ppm in 1 H MAS NMR spectrum. The OH groups with signal 2.1 ppm are involved in H/D exchange with methane-d 4 similar to acidic OH groups with the signals 4.0–5.1 ppm. The signals at 2.1 ppm in 1 H MAS NMR and at 3740 cm À1 in IR are attributed to the strongly acidic silanol groups of the faulted structure of the zeolite. The silanols with the signals at 1.8 ppm and 3745 cm À1 are weakly acidic (Dm OH/CO = 85 cm À1) and are not involved in the H/D exchange. Loading of the zeolite with Zn affords a notable decrease of the concentration of strongly acidic SiOHAl groups, whereas the quantity of these groups does not decrease upon loading the zeolite with Ga.
Microporous and Mesoporous Materials 02/2010; · 3.29 Impact Factor
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ABSTRACT: Acidic OH groups of acid-form and Zn and Ga-modified zeolite beta (BEA) have been characterized with 1H MAS NMR and by IR spectroscopy of adsorbed carbon monoxide. It is demonstrated that OH groups, which exhibit a vibration band at 3740 cm−1, reveal acidity, which is similar to that of the OH groups with the band at 3610 cm−1 according to the value of the low frequency shift of OH vibrations with adsorbed CO (ΔνOH/CO = 300 cm−1). The IR band 3740 cm−1 corresponds to the signal at ca. 2.1 ppm in 1H MAS NMR spectrum. The OH groups with signal 2.1 ppm are involved in H/D exchange with methane-d4 similar to acidic OH groups with the signals 4.0–5.1 ppm. The signals at 2.1 ppm in 1H MAS NMR and at 3740 cm−1 in IR are attributed to the strongly acidic silanol groups of the faulted structure of the zeolite. The silanols with the signals at 1.8 ppm and 3745 cm−1 are weakly acidic (ΔνOH/CO = 85 cm−1) and are not involved in the H/D exchange. Loading of the zeolite with Zn affords a notable decrease of the concentration of strongly acidic SiOHAl groups, whereas the quantity of these groups does not decrease upon loading the zeolite with Ga.
Microporous and Mesoporous Materials 02/2010; · 3.29 Impact Factor
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ABSTRACT: Ethane conversion into aromatic hydrocarbons over Zn-modified zeolite BEA has been analyzed by high-temperature MAS NMR spectroscopy. Information about intermediates (Zn-ethyl species) and reaction products (mainly toluene and methane), which were formed under the conditions of a batch reactor, was obtained by (13)C MAS NMR. Kinetics of the reaction, which was monitored by (1)H MAS NMR in situ at the temperature of 573K, provided information about the reaction mechanism. Simulation of the experimental kinetics within the frames of the possible kinetic schemes of the reaction demonstrates that a large amount of methane evolved under ethane aromatization arises from the stage of direct ethane hydrogenolysis.
Solid State Nuclear Magnetic Resonance 02/2009; 35(2):113-9. · 1.71 Impact Factor
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ABSTRACT: journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit:
Catalysis Today 01/2009; 144:265-272. · 3.41 Impact Factor
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ABSTRACT: Herein, we analyze earlier obtained and new data about peculiarities of the H/D hydrogen exchange of small C(1)-n-C(4) alkanes on Zn-modified high-silica zeolites ZSM-5 and BEA in comparison with the exchange for corresponding purely acidic forms of these zeolites. This allows us to identify an evident promoting effect of Zn on the activation of C-H bonds of alkanes by zeolite Brønsted sites. The effect of Zn is demonstrated by observing the regioselectivity of the H/D exchange for propane and n-butane as well as by the increase in the rate and a decrease in the apparent activation energy of the exchange for all C(1)-n-C(4) alkanes upon modification of zeolites with Zn. The influence of Zn on alkane activation has been rationalized by dissociative adsorption of alkanes on Zn oxide species inside zeolite pores, which precedes the interaction of alkane with Brønsted acid sites.
ChemPhysChem 11/2008; 9(17):2559-63. · 3.41 Impact Factor
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ABSTRACT: The kinetics of hydrogen (H/D) exchange between Brønsted acid sites of zeolite H-ZSM-5 and deuterated n-butanes (n-butane-d10 and n-butane-1,1,1,4,4,4-d6) has been monitored by 1H magic-angle spinning (MAS) NMR spectroscopy in situ within the temperature range of 423−448 K. The initial part of the kinetics is defined mainly by the hydrogen exchange, whereas the final part is strongly influenced by the chemical transformation of the alkane. Analysis of the initial part has been performed on the basis of consecutive, parallel, and cyclic kinetic schemes of the H/D exchange. It has been found that both the methyl and methylene groups of n-butane are directly involved in the exchange with acidic SiOHAl groups of the zeolite. No intramolecular hydrogen exchange between the methyl and the methylene groups of the adsorbed n-butane has been detected. Similar rates of the direct exchange of either the methyl or methylene group with acidic SiOHAl groups and the apparent activation energy of 108 kJ mol−1 are rationalized in terms of the carbonium ion mechanism of the exchange with the involvement of a pentacoordinated carbon atom in a transition state.
07/2008;
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Angewandte Chemie International Edition 02/2008; 47(24):4559-62. · 13.45 Impact Factor
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ABSTRACT: Elsevier journal. The attached copy is furnished to the author for non-commercial research and education use, including for instruction at the author's institution, sharing with colleagues and providing to institution administration. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Abstract Hydrogen (H/D) exchange between Brønsted acid sites of both the acidic form of zeolite beta (H-BEA) and Zn-loaded zeolite beta (Zn/H-BEA) and small alkanes (methane and ethane) has been studied by monitoring the kinetics of the exchange in situ with 1 H MAS NMR spectroscopy within the temperature range of 433–563 K. On Zn/H-BEA, the exchange has been found to be more than two orders of magnitude faster compared to that on H-BEA. The decrease of reaction temperature and activation energy of the exchange on Zn/H-BEA (86–88 kJ mol −1) compared to the acidic form of zeolite H-BEA (138 kJ mol −1) has been rationalized by the promoting effect of zinc. We propose that the mechanism of the H/D exchange on Zn/H-BEA involves Zn-alkyl species as intermediates.
Journal of Catalysis 01/2008; 253:11--21. · 6.00 Impact Factor
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ABSTRACT: The dynamic behavior of deuterated analogues of linear alkanes, n-C6−n-C22, adsorbed in zeolite 5A has been studied by deuterium solid-state NMR (2H NMR). Temperature dependences of spin−lattice (T1) and spin−spin (T2) relaxation times of the deuterium located in the CD3 groups of the adsorbed n-alkanes were rationalized on the basis of a model derived for the motion of n-alkanes located in the pores of the zeolite. The model implies that the adsorbed molecules consist of two ensembles: diffusing (or stretched) and temporarily blocked from diffusion (or coiled). The possible intramolecular motions for the alkane chains were taken into account based on both the finite size of the zeolite cage and the allowable hydrocarbon chain conformations. The coiled molecules are involved in two modes of motion: isotropic reorientation and intramolecular conformational isomerization, whereas the stretched molecules are additionally involved in a diffusion process. Dynamics parameters for different modes of motion and a proportion of the blocked and stretched molecules were derived from the analysis of relaxation data. The estimated proportion of the diffusing molecules correlates with the alkanes diffusivities earlier obtained by neutron spin echo measurements.
02/2007;
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ABSTRACT: By using 13C MAS NMR spectroscopy (MAS = magic angle spinning), the conversion of selectively 13C-labeled n-butane on zeolite H-ZSM-5 at 430-470 K has been demonstrated to proceed through two pathways: 1) scrambling of the selective 13C-label in the n-butane molecule, and 2) oligomerization-cracking and conjunct polymerization. The latter processes (2) produce isobutane and propane simultaneously with alkyl-substituted cyclopentenyl cations and condensed aromatic compounds. In situ 13C MAS NMR and complementary ex situ GC-MS data provided evidence for a monomolecular mechanism of the 13C-label scrambling, whereas both isobutane and propane are formed through intermolecular pathways. According to 13C MAS NMR kinetic measurements, both pathways proceed with nearly the same activation energies (E(a) = 75 kJ mol(-1) for the scrambling and 71 kJ mol(-1) for isobutane and propane formation). This can be rationalized by considering the intermolecular hydride transfer between a primarily initiated carbenium ion and n-butane as being the rate-determining stage of the n-butane conversion on zeolite H-ZSM-5.
Chemistry 01/2006; 12(2):457-65. · 5.93 Impact Factor
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ABSTRACT: The kinetics of hydrogen (H/D) exchange between Brønsted acid sites of zeolite H-ZSM-5 and variously deuterated propanes (propane-d(8), propane-1,1,1,3,3,3-d(6), propane-2,2-d(2)) have been monitored in situ by (1)H MAS NMR spectroscopy within the temperature range of 503-556 K. The contribution of intramolecular hydrogen transfer to the H/D exchange in the adsorbed propane was estimated by monitoring the kinetics of (13)C-labeled carbon scrambling in propane-2-(13)C in situ with (13)C MAS NMR at 543-573 K. Possible mechanisms of the exchange have been verified on the basis of the analysis of the variation of protium concentration in both the methyl and the methylene groups of propane in dependence of the reaction time. The main route of the exchange consists of a direct exchange of the acidic OH groups of the zeolite with either the methyl groups or the methylene group presumably with a pentacoordinated carbonium ion intermediate. The assumption that the intramolecular H scrambling between the methyl groups and the methylene group of propane via carbenium-ion-type intermediates is the fastest process among the other possible routes does not account for the experimental kinetics of H/D exchange for propanes with different initial contents and locations of deuterium in a propane molecule. The rate constant (k(3)) for intramolecular H/D exchange between the methyl and the methylene groups is 4-5 times lower compared to those of the direct exchange of both the methyl (k(1)) and the methylene (k(2)) groups with Brønsted acid sites of the zeolite, the k(1) being ca. 1.5 times higher than k(2). At lower temperature (473 K), the exchange is slower, and the expected difference between k(1) and k(2) is more essential, k(1) = 3k(2). This accounts for earlier observed regioselectivity of the exchange for propane on H-ZSM-5 at 473 K. Faster direct exchange with the methyl groups compared to that with the methylene groups was attributed to a possible, more spatial accessibility of the methyl groups for the exchange. Similar activation energies for H and C scramblings with a 2 times more rapid rate of H scrambling was rationalization by the proceeding of these two processes through an isopropyl cation intermediate, as in classical carbenium ion chemistry.
The Journal of Physical Chemistry B 11/2005; 109(42):19748-57. · 3.70 Impact Factor
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ABSTRACT: The kinetics of hydrogen exchange between the molecules of propane-d 8 , propane-1,1,1,3,3,3-d 6 , propane-2,2-d 2 , and Brönsted acid sites of the zeolite H-ZSM-5 have been monitored in situ by 1 H MAS NMR spectroscopy in the temperature range of 230–280 • C. The intramolecular hydrogen transfer was estimated by in situ 13 C MAS NMR spectroscopy of the kinetics of 13 C-label scrambling in adsorbed propane-2-13 C. The hydrogen exchange was found to occur directly between methyl or methylene groups of the alkane and the zeolite acid sites via a pentacoordinated carbonium ion. The exchange with the methyl groups is faster than that with the methylene group. This accounts for the earlier observed regio-selectivity of the hydrogen exchange for propane on acidic zeolites (J. Am. Chem. Soc. 117 (1995) 1135). The intramolecular hydrogen transfer between methyl and methylene groups is one order of magnitude slower than the hydrogen exchange of the both groups with the zeolite acid sites.
Journal of Catalysis 01/2005; 235:221--228. · 6.00 Impact Factor
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ABSTRACT: In situ monitoring of n-butene conversion on H-ferrierite by 1 H, 2 H, and 13 C MAS NMR: kinetics of a double-bond-shift reaction, hydrogen exchange, and the 13 C-label scrambling Abstract Kinetics of a double-bond-shift reaction, hydrogen exchange, and 13 C-label scrambling were monitored in situ by 2 H, 1 H, and 13 C MAS NMR for n-but-1-ene adsorbed on the zeolite ferrierite under batch reactor conditions at 290–373 K. A double-bond-shift reaction, the fastest among the three reactions studied, can be monitored provided that 97% of Brønsted acid sites are substituted by Na cations. The activation energy for this reaction was found to be 9.8 kcal mol −1 . Hydrogen exchange with protons from the zeolite is observed for both methene and methyl groups of n-but-2-ene, formed from the initial n-but-1-ene. The terminal olefinic =CH 2 group of n-but-1-ene is involved in the exchange, providing the pathway for the exchange into the methyl group of the n-but-2-ene, mainly observed in the spectrum in accordance with thermodynamic equilibrium between n-but-1-ene and n-but-2-ene. This offers similar apparent activation energies of about 7 kcal mol −1 for the exchange into methene and methyl groups of n-but-2-ene. The 13 C-label scrambling in n-but-2-ene is indicative of sec-butyl cation formation from the olefin in the zeolite framework, which can be formed as a small quantity of transient species not detectable by NMR but providing the label scrambling. The apparent activation energy for the 13 C-label scrambling was found to be 21 ± 2 kcal mol −1 , which is three times higher compared with the activation energy for the label scrambling in sec-butyl cation in a superacidic solution. 2004 Published by Elsevier Inc.
Journal of Catalysis 01/2005; 229:243--251. · 6.00 Impact Factor
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ABSTRACT: locate/jcat n-Butane conversion on sulfated zirconia: the mechanism of isomerization and 13 C-label scrambling as studied by in situ 13 C MAS NMR and ex situ GC-MS Abstract Using 13 C MAS NMR, conversion of selectively 13 C-labeled n-butane on sulfated zirconia catalyst has been demonstrated to proceed initially via two parallel routes: scrambling of the selective 13 C label in the n-butane molecule and selective formation of isobutane. The combination of the results obtained by both in situ 13 C MAS NMR and ex situ GC-MS analysis provides evidence for the monomolecular mechanism of the 13 C-label scrambling, whereas isomerization into isobutane proceeds through a pure bimolecular mechanism. Further, the intermolecular mechanism of n-butane isomerization is complicated and turns into conjunct polymerization. Besides isobutane, conjunct polymerization gives also the products of butane disproportionation, propane and pentanes, as well as the stable cyclopentenyl cations; the latter may be in charge of catalyst deactivation.
Journal of Catalysis 01/2003; 220:233--239. · 6.00 Impact Factor
