Methane Activation on In-Modified ZSM‑5 Zeolite. H/D HydrogenExchange of the Alkane with Brønsted Acid SitesSergei S. Arzumanov,†,‡ Ilya B. Moroz,‡ Dieter Freude,∥ Jürgen Haase,∥ and Alexander G. Stepanov*,†,‡

†Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, Prospekt Akademika Lavrentieva 5, Novosibirsk630090, Russia‡Faculty of Natural Sciences, Department of Physical Chemistry, Novosibirsk State University, Pirogova Street 2, Novosibirsk 630090,Russia∥Universitaẗ Leipzig, Fakultaẗ für Physik und Geowissenschaften, Linneśtrasse 5, 04103 Leipzig, Germany

*S Supporting Information

ABSTRACT: In relation to clarifying the pathway of methaneactivation on In-modified zeolites, a comparative analysis ofkinetics of hydrogen (H/D) exchange between methane-d4and Brønsted acid sites (BAS) for both the pure acid formzeolite (H-ZSM-5) and In-modified zeolites (In+/H-ZSM-5and InO+/H-ZSM-5) has been performed. Monitoring of thekinetics has been carried out with 1H magic-angle spinningNMR spectroscopy in situ within the temperature range of453−568 K. While the rate of exchange on In+/H-ZSM-5 is 1order of magnitude larger than that on H-ZSM-5, the exchange occurs on InO+/H-ZSM-5 by 2 orders of magnitude faster thanthat on H-ZSM-5. Significant increase of the rate and decrease of the activation energy (Ea = 74 ± 6 kJ mol

−1) and thetemperature threshold (453 K) for the reaction of the exchange on InO+/H-ZSM-5 compared to the rate, activation energy, andtemperature threshold (543 K) for the reaction on In+/H-ZSM-5 (Ea = 127 ± 27 kJ mol

−1) and H-ZSM-5 (Ea = 118 ± 9 kJmol−1) have been rationalized in terms of involvement of both InO+ and BAS in activation of methane molecules on the zeolite.Some transient intermediate complex of methane with the zeolite InO+ species and BAS has been assumed to be formed withinthe zeolite pore. This complex is involved either in the reaction of H/D exchange with BAS of the zeolite or evolves further tooffer indium methyl species.

1. INTRODUCTION

Methane is the most naturally abundant but extremely inerthydrocarbon. Therefore, its involvement in chemical trans-formation is a challenge for modern chemistry. Numerousattempts have been made to overcome a chemical inertness ofmethane with the use of heterogeneous catalysts, includingmetal oxides1,2 and metal-modified zeolites.3,4 Methane hasbeen shown to be activated on In-modified zeolite ZSM-5 andreacted with ethylene to obtain propylene.5,6 Some conclusionson the mechanism of methane activation on In/H-ZSM-5 havebeen recently made.7 In particular, the nature of surfaceintermediates formed at methane interaction with indiumcationic species of the zeolite has been clarified.7 However, therole of Brønsted acid sites (BAS) of In/H-ZSM-5 zeolite inmethane activation and the formation of the surfaceintermediates remains unrevealed.The hydrogen (H/D) exchange reaction between the

Brønsted acid sites of solid catalysts and the alkane moleculesprecedes usually the alkane chemical transformation.8 Thisreaction is often used to characterize the activation of alkaneson the catalysts.9−25 The information on the features ofactivation of alkane molecules can be derived by monitoring insitu the H/D exchange reaction between alkane and BAS ofzeolite with 1H MAS NMR.19,26,27 The application of this

method to the study of H/D exchange on zeolites modifiedwith Zn or Ga27−30 allowed us to establish the role of the metalcenters and BAS in the activation of C1−C4 alkanes on thesezeolites.We have analyzed the kinetics of H/D exchange of methane

with Brønsted acid sites of zeolites H-ZSM- 5 modified withindium (In+/H-ZSM-5 and InO+/H-ZSM-5) and comparedthese kinetics with the kinetics on the pure acid form of zeoliteH-ZSM-5. This approach to the study of small alkane activationon heterogeneous catalysts allowed us to establish somepeculiarities of methane activation by Brønsted acid site ofzeolite ZSM-5 modified by indium.

2. EXPERIMENTAL SECTION2.1. Material Syntheses, Characterization, and Sample

Preparation. The hydrogen form of zeolite ZSM-5 (H-ZSM-5) used in this work for the synthesis of the In-form zeoliteswas prepared from industrially produced (Tricat Zeolites)ammonium form of ZSM-5 by calcination at 773 K for 5 h. In-modified zeolite (In/H-ZSM-5) was prepared by a reductive

Received: April 16, 2014Revised: June 5, 2014Published: June 5, 2014

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solid-state ion-exchange reaction between indium(III) oxideand H-ZSM-5 zeolite as described previously.7 Approximately30% substitution of Brønsted acid sites of parent H-ZSM-5 forindium species was reached by this procedure. 29Si magic-anglespinning (MAS) NMR analysis31 revealed a silicon-to-aluminum ratio of 17 for both the pure acid form and In-modified H-ZSM-5 zeolite samples. Indium content was 7.9 wt% in In/H-ZSM-5, which corresponded to indium loading of8.0 wt % in the sample. 27Al MAS NMR spectra of the hydratedH-ZSM-5 and In/H-ZSM-5 zeolites did not show any extraframework aluminum atoms in these samples. Further, theactivation of the In/H-ZSM-5 sample (100 mg) was performedin a glass tube under vacuum (10−3 Pa) at 673 K for 24 h.Afterward, two samples with activated In/H-ZSM-5 wereprepared. Reduction of In/H-ZSM-5 in hydrogen (760 mbar)at 773 K for 30 min and further evacuation at 673 K for 2 hresulted in the sample, designated as In+/H-ZSM-5. Thesecond sample, denoted as InO+/H-ZSM-5, was first reducedand evacuated using the method described for the previoussample. However, it was then subjected to oxygen treatment(760 mbar O2 at 623 K for 30 min) and evacuated at 673 K for2 h. According to X-ray photoelectron spectroscopy (XPS) and27Al, 29Si, and 1H MAS NMR data,7 thus prepared In+/H-ZSM-5 sample showed unit cell composition In01.3In

+3.0H1.7Al4.7

Alnonfr0.7Si90.6O192 and contained indium in the form of In+

cations, whereas InO+ cations represented the main form ofindium in InO+/H-ZSM-5 sample, which exhibited the unit cellcomposition (In2O3)0.65(InO

+)3.0H1.7Al4.7Alnonfr

0.7Si90.6O192.Methane-d4 (99% D) used in this work was purchased from

Aldrich Chemical Co., Inc. Industrially produced oxygen andhydrogen gases were used without further purification.The zeolite sample (100 mg; H-ZSM-5, In+/H-ZSM-5, or

InO+/H-ZSM-5) located in axially high-symmetrical glass tubeof 5.5 mm outer diameter and 10 mm length was loaded undervacuum with methane-d4 (300 μmol g

−1). Afterward, thesample located in the glass tube was sealed by flame. While itwas being sealed, the tube with the sample was kept in liquidnitrogen, preventing heating of the sample by the flame. Theglass tube with the zeolite sample ideally fit NMR zirconia rotor(7 mm outer diameter). This allowed rotation of the sealedtube with the rate of 3000 Hz, which provided both the analysisof the reaction products in situ and monitoring the kinetics ofH/D exchange with 1H MAS NMR. Samples in the sealed glasstubes were kept at room temperature prior to NMR analysis.2.2. NMR Measurements. NMR spectra were recorded at

9.4 T on a Bruker Avance 400 spectrometer equipped with abroadband double-resonance-MAS probe. 1H MAS NMRspectra were recorded by the Hahn-echo pulse sequence (π/2−τ−π−τ−acquisition), where τ equals one rotor period (333μs). The length of excitation π/2 pulse was 5.0 μs, and typically16 scans were accumulated with a 10 s delay. 27Al and 29Si MASNMR spectra were acquired under conditions similar to thosedescribed earlier.7 The sample temperature was controlled by aBruker BVT- 1000 variable-temperature unit. The calibration ofthe temperature (373−573 K) inside the rotor for kineticmeasurements was performed according to the protocoldescribed in refs 30 and 32.Prior to acquisition of the signal at the kinetic measurements

of the H/D exchange, the NMR probe with the sample waspreheated for 20 min at the temperature at which the H/Dexchange did not yet occur at notable rate. This temperaturewas 470 K for H-ZSM-5 and In+/H-ZSM-5 and 410 K forInO+/H-ZSM-5 zeolite samples. Then the temperature was

rapidly increased within 3−10 min by 40−100 K to the reactiontemperature and equilibrated for 4−5 min; then, the acquisitionof the NMR signal started.

3. RESULTS AND DISCUSSIONThe In-modified zeolite ZSM-5 (In/H-ZSM-5), prepared byreductive solid-state ion exchange between indium(III) oxideand H-ZSM-5 zeolite, contains indium species either in theform of In+ or InO+ cations. In/H-ZSM-5 treated withhydrogen at 773 K produces In+/H-ZSM-5 zeolite containingindium in the form of In+ cations. Treatment of In+/H-ZSM-5with oxygen at 623 K gives the zeolite InO+/H-ZSM-5, inwhich indium exists in the form of InO+ cations.7 Both In+/H-ZSM-5 and InO+/H-ZSM-5 zeolite samples were tested in thereaction of H/D hydrogen exchange with methane-d4 (CD4).Monitoring H/D exchange between the acid OH groups andCD4 with

1H MAS NMR in situ for In+/H-ZSM-5 shows anincrease of the intensity of proton signal of methane CHnD4−n(n = 1−4) at 0.1 ppm in the 1H MAS NMR spectrum of CD4adsorbed on the zeolite (Figure 1). Increase of the intensity of

the signal from methane with time is indicative of H/Dexchange reaction, which occurs by a transfer of protium fromthe acid OH group of the zeolite (BAS) to the deuteratedmethane and simultaneous reverse transfer of deuterium frommethane-d4 to the zeolite OH groups. Analysis of the kinetics ofH/D exchange (Figure 2) shows that exchange occurs in thetemperature range of 540−570 K for In+/H-ZSM-5, i.e., similarto the temperature range of the exchange reaction of methanewith BAS of H-ZSM-5 zeolite.29 The activation energies of thereactions are also similar (within experimental error): 127 ± 27and 118 ± 9 kJ mol−1 for In+/H-ZSM-5 and H-ZSM-5,respectively. However, the rate of the exchange for In+/H-ZSM-5 is almost 1 order of magnitude larger than that for theexchange of methane on the pure acid form zeolite H-ZSM-5(see Figure 3). This indicates that indium existing in the zeolitein the form of In+ cations accelerates the H/D exchangereaction.For InO+/H-ZSM-5 zeolite, the temperature threshold of the

reaction is 90 K lower than that for In+/H-ZSM-5 zeolite; the

Figure 1. Stack plot of 1H MAS NMR spectra at 568 K of methane-d4adsorbed on In+/H-ZSM-5 zeolite. The first spectrum (bottom) wasrecorded 6 min after and the last spectrum (top) 500 min after thetemperature was raised to 568 K. The time between subsequentspectra recording was 10 min.

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exchange occurs at 450−520 K. Figure 4 shows the evolution of1H MAS NMR spectrum of CD4 adsorbed on InO

+/H-ZSM-5with time. The intensity of the proton signal of CHnD4−nincreases for the first 250 min of the reaction at 455 K, but withlonger times the intensity of the signal decreases (Figures 4 and5). A volcano shape of the kinetic curve is related to twoprocesses occurring on the zeolite after CD4 adsorption. Thefirst process is the H/D exchange reaction of methane withBAS, and the second one is the chemical reaction of methanewith InO+ cations to afford indium methyl species CH3InO.The occurrence of the latter reaction has been demonstratedrecently.7 Although the conversion of methane into CH3InOspecies is notable, no signal from the CH3InO (−1.7 ppm)

7

was observed in the spectra (Figure 4). This might be due to anextremely large width of this signal.7 The kinetic curves for

methane on InO+/H-ZSM-5 (Figure 5) have been furtheranalyzed in terms of two parallel reactions depicted in Scheme1: the H/D exchange reaction by pathway 1 and the conversionto the indium methyl species CH3InO by pathway 2 (details ofkinetics scheme for deriving kinetic parameters are provided inSupporting Information). Analysis of the kinetics within aframe of Scheme 1 has shown that the rates of H/D exchange(described by rate constant ka) and methane transformation toCH3InO species (described by the rate constant kb) are similarat the studied temperature range. The activation energies forthese reactions, derived from Arrhenius plots (Figure 3), are ofthe similar values (74 kJ mol−1).The kinetic parameters derived show that while the H/D

exchange occurs on In+/H-ZSM-5 1 order of magnitude fasterthan that on H-ZSM-5, the rate of the exchange on InO+/H-ZSM-5 is 2 orders of magnitude larger than the rate on H-ZSM-5 (see Figure 3). The activation energy of the exchange

Figure 2. Experimental kinetics for H/D exchange of methane-d4 on In+/H-ZSM-5 zeolite. The rate constants shown nearby each kinetic curve were

calculated based on exchange model described in refs 19, 28, 30, and 33.

Figure 3. Arrhenius plots for the H/D exchange reaction of methane-d4 on H-ZSM-5 (Ea = 118 ± 9 kJ mol

−1; ∇) (the data are adoptedfrom ref 29), In+/H-ZSM-5 (Ea = 127 ± 27 kJ mol

−1; Δ), and InO+/H-ZSM-5 (Ea = 74 ± 6 kJ mol

−1; ○). Arrhenius plot for the reaction ofmethane conversion to indium methyl species CH3InO (Ea = 74 ± 8kJ mol−1; □).

Figure 4. Stack plot of 1H MAS NMR spectra at 455 K of methane-d4adsorbed on InO+/H-ZSM-5 zeolite. The first spectrum (bottom) wasrecorded 5 min after and the last spectrum (top) 1000 min after thetemperature was raised to 455 K. The time between subsequentspectra recording was 30 min.

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reaction is smaller by 50 kJ mol−1 for InO+/H-ZSM-5. Theseresults indicate that both In+ and InO+ cationic species facilitatethe H/D exchange reaction. However, the effect of InO+

species on H/D exchange reaction is more striking, providingan essential increase of the rate and decrease of apparentactivation energy and leading to the decrease of thetemperature threshold for reaction proceeding by almost 90 K.

One can suggest that the promoting effect of both In+ andInO+ cations on the reaction of the H/D exchange is related toinvolvement of the cationic species by forming some transientcomplex with methane, which can further participate in H/Dexchange with BAS of the In-modified ZSM-5 zeolites. Thestronger effect of InO+ species on the kinetic parameters of theexchange might indicate that the complex of methane withInO+ species is more effective in terms of activation of methanemolecules.The significant acceleration of H/D exchange, coupled with

the decreased activation energy on InO+/H-ZSM-5 comparedto that on In+/H-ZSM-5 zeolite, demonstrates that thereversible cleavage and formation of the C−H(D) bonds inmethane occurs with InO+ species easier than with In+ species.Moreover, InO+ not only transfers H and D atoms between themethane and BAS of the zeolite but also participates in thechemical reaction, as has been shown before:7

+ → ++ −CH InO ZO CH InO ZOH4 3 (1)

where ZO− denotes the zeolite framework.It is reasonable to assume that the interaction of methane

with InO+/H-ZSM-5 gives rise to the formation of sometransient complex I as shown in Scheme 1. Formation of thiscomplex implies that Al pairs should be located at appropriatedistance from each other. For Si/Al = 17, statistical evaluationof Al distribution in zeolite framework predicts that the zeolitesample used in this work might contain about 30% of Al−Si−Si−Al pairs,34 whereas recent experiments show that thequantity of such pairs can reach 50%.35 So the formation ofcomplex I with involvement of Al−Si−Si−Al pairs is quitepossible. This complex I might be further involved in pathway1, leading to the H/D exchange reaction with another acidicOH group of the zeolite. Or this complex might follow pathway2 to form indium-methyl species CH3InO. We believe that theoxygen-containing InO+ species facilitates these reactions betterthan the same processes occurring with involvement of In+

cations only.

Figure 5. Kinetics of H/D hydrogen exchange for methane-d4 on InO+/H-ZSM-5 zeolite. The rate constants ka and kb correspond to pathways 1 and

2 in Scheme 1

Scheme 1. Possible Structure for Transient Complex I ofMethane Interacting with BAS and InO+ Species of InO+/H-ZSM-5 Zeolite and Pathways of the Complex Evolutiontoward Either the H/D Exchange or the Transformation toIndium Methyl Species CH3−InOa

aH/D exchange of methane on InO+/H-ZSM-5 zeolite is described bythe rate constant ka, and concurrent methane conversion to CH3−InO species is described by the rate constant kb.

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Thus, our experiments demonstrate a synergistic actionbetween the exchange cations and the BAS for activatingmethane on In-modified zeolites. Presumably a similarmechanism is valid on other metal-modified zeolites, loadedfor example with cations of Zn, Ga, or Ag.28−30,36−40

4. CONCLUSIONWe have performed a comparative analysis of the kinetics ofhydrogen (H/D) exchange between methane-d4 and Brønstedacid sites for both the pure acid form zeolite (H-ZSM-5) andIn-modified zeolites (In+/H-ZSM-5 and InO+/H-ZSM-5) with1H MAS NMR spectroscopy in situ within the temperaturerange of 453−568 K. The analysis shows that the rate ofexchange is 1 order of magnitude larger on In+/H-ZSM-5 and 2orders of magnitude larger on InO+/H-ZSM-5 than that on H-ZSM-5. Besides dramatic increase of the rate, a notabledecrease of the activation energy (Ea = 74 ± 6 kJ mol

−1) andthe temperature threshold (453 K) for the reaction of theexchange on InO+/H-ZSM-5 compared to the rate, activationenergy (Ea = 127 ± 27 kJ mol

−1), and temperature threshold(543 K) for the reaction on In+/H-ZSM-5 and H-ZSM-5 hasbeen detected. The significant change of the kinetic parametersof the exchange for InO+/H-ZSM-5 is rationalized in terms ofinvolvement of both InO+ species and BAS in activation ofmethane molecules on this In-modified zeolite. Some transientintermediate complex of methane with the zeolite InO+ speciesand BAS is assumed to be formed within the void of the zeolitepore. This complex is either involved in the reaction of H/Dexchange with BAS of the zeolite or evolves further to offerindium methyl species.

■ ASSOCIATED CONTENT*S Supporting InformationDescription of kinetics scheme for deriving kinetic parameters.This material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Tel: +7 952 905 9559. Fax: +7 383 330 8056. E-mail:stepanov@catalysis.ru.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors are grateful to Dr. A.V. Toktarev for the synthesisof In-modified zeolite. This work was supported in part byRussian Foundation for Basic Research (Grant 14-03-00040).

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■ NOTE ADDED AFTER ASAP PUBLICATIONThis paper was published to the Web on 6/19/2014, with aminor error in the Introduction. This was fixed in the versionpublished on 6/23/2014.

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