Anton A. Gabrienko, Sergei S. Arzumanov, Alexander V. Toktarev, Dieter Freude, J urgen Haase, and Alexander G. Stepanov*, 1.

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1 pubs.acs.org/jpcc Hydrogen H/D Exchange and Activation of C 1 n-c 4 Alkanes on Ga-Modified Zeolite BEA Studied with 1 H Magic Angle Spinning Nuclear Magnetic Resonance in Situ Anton A. Gabrienko, Sergei S. Arzumanov, Alexander V. Toktarev, Dieter Freude, J urgen Haase, and Alexander G. Stepanov*, Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, Prospekt Akademika Lavrentieva 5, Novosibirsk , Russia Universit at Leipzig, Fakult at f ur Physik und Geowissenschaften, Linnestrasse 5, Leipzig, Germany bs Supporting Information ABSTRACT: Kinetics of H/D hydrogen exchange between C 1 n-c 4 alkanes and Brønsted acid sites (BAS) of both the pure acid-form zeolite BEA (H-BEA) and Ga-modified zeolite BEA (Ga/H-BEA) was monitored by 1 H MAS NMR spectroscopy in situ at K. Comparative analysis of the rates of the exchange for H-BEA and Ga/H-BEA zeolites reveals a remarkable increase of the rate by 1 2 orders of magnitude, decrease of activation energy, and an appearance of regioselectivity of the exchange into the methyl groups of C 3 -n-c 4 alkanes upon modification of H-BEA zeolite with gallium. These data identify an evident promoting effect of Ga on activation of alkane C H bonds by BAS. The effect has been rationalized by preliminary dissociative adsorption of alkanes on gallium(iii) oxide species inside zeolite pores to form gallium-alkyl species, which are further involved in the exchange with neighbor BAS. Involvement of both BAS and Ga species in alkane activation accounts for earlier suggested synergistic effect (Buckles, G., et al. Catal. Lett. 1991, 11, 89) of both BAS and gallium species in alkane activation and aromatization on Ga-modified high silica zeolites. 1. INTRODUCTION Ga- and Zn-modified high silica zeolites are effective catalysts for light alkanes aromatization. 1 5 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. 2,5,8 11 The role of different active sites in the particular steps of alkane-toaromatics transformation is debatable. It is conventionally accepted 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 activation 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. 7,15 17 It has been suggested that metal active sites and BAS, located in the vicinity to each other, perform the alkane molecule activation On the basis of the analysis of the kinetics of H/D exchange between BAS and C 1 n-c 4 alkanes we have recently shown an involvement of Zn species in alkane activation by Brønsted acid sites for Zn-modified zeolites ZSM-5 and BEA This was demonstrated by a dramatic increase of the rate of H/D exchange at modification of zeolite with Zn for C 1 n-c 4 alkanes 22 and an appearance of the regioselectivity of H/D exchange for C 3 -n-c 4 alkanes. 21,23 The promoting effect of Zn in acceleration of H/D exchange was rationalized by the formation of Zn-alkyl species at preliminary dissociative adsorption of alkane on ZnO or Zn 2+ species of the zeolite, which are further involved in the reaction of H/D exchange with Brønsted acid sites, located in the vicinity of BAS. Contrary to Zn-modified zeolites, no detailed experimental information is provided so far on peculiarities of alkane activation on Ga-modified zeolites, despite the conclusion on synergistic action of both Ga-species and BAS in aromatization reaction 7,15 17 and necessity to have these active sites in a closed vicinity to each other to perform an alkane activation. 18,20 Monitoring the reaction of hydrogen H/D exchange between acid sites of the zeolite and an alkane molecule by spectroscopic techniques could help to establish the mechanism of alkane activation on Ga-modified zeolites. Indeed, a hydrogen exchange between the acid sites of solid catalysts and alkane molecules, which precedes usually the chemical transformation of alkane on acid catalysts, 28 is often used to characterize the activation of Received: May 11, 2011 Revised: June 10, 2011 Published: June 13, 2011 r 2011 American Chemical Society dx.doi.org/ /jp204398r J. Phys. Chem. C 2011, 115,

2 Table 1. Characteristics of Zeolite BEA Samples concentration of OH groups, b μmol/g zeolite AlOHSi, sample Si/Al a 4.1, 5.2 ppm AlOH, 2.9 ppm SiOH acidic, 2.1 ppm SiOH, 1.8 ppm zeolite unit cell H-BEA Al oct 0.707[Na + ] H Al tetr Si O 128 Ga/H-BEA (5.6% Ga) [Ga 2 O 3 ] Al oct [Na + ] H Ga tetr Al tetr Si O 128 a Estimated on the basis of 29 Si MAS NMR spectra with accuracy ca. 6%. b Error in estimation of concentration is 10 17%. alkanes and acidity of the catalysts A combination of experimental H/D exchange kinetics data with theoretical analysis of the possible intermediates of the exchange reaction provides a valuable information on the mechanisms of the alkane molecule activation and the H/D exchange on solid acid catalysts In this paper we have analyzed the kinetics of H/D exchange for Ga-modified zeolite BEA by 1 H magic angle spinning nuclear magnetic resonance (MAS NMR) in situ. This allowed us to clarify the role of Brønsted acid sites and Ga-species in small alkane activation at the initial step of alkane conversion to aromatics. 2. EXPERIMENTAL SECTION 2.1. Materials Characterization and Samples Preparation. The acid form of zeolite beta (H-BEA-59, Si/Al = 18) was synthesized using tetra-ethyl-ammonium hydroxide as a template with subsequent calcination at 823 K in an air flow for 6 h. 41 The zeolite sample used in our experiments is characterized by average crystallites size of μm, which forms the rounded agglomerates of μm diameter. Ga-modified zeolite beta sample (Ga/H-BEA) was prepared by impregnation of the parent sample of zeolite H-BEA with a saturated solution of gallium nitrate, subsequent drying at 473 K for 14 h and further calcination at 673 K for 4 h in a flow of air. Ga/ H-BEA zeolite samples contained 5.6 wt % of Ga. A detailed composition of the zeolite unite cell for the used zeolite samples was established (Table 1), similar to procedures described earlier, 42,43 taking into account the chemical analyses, quantitative analyses of bridged SiOHAl groups with 1 H MAS NMR, the Al IV and Al VI states by 27 Al MAS NMR, Si/Al ratio in zeolite unit cell by 29 Si MAS NMR, 44 and qualitative analysis of the states of Al and Ga in these materials, on the basis of X-ray diffraction (XRD), 1 H{ 27 Al} spin echo double resonance (TRAPDOR), 45,46 71 Ga MAS NMR, UV vis diffuse reflectance, and IR spectroscopy. The 27 Al MAS NMR has shown that the zeolite samples contained the signals from octahedrally coordinated extra-framework aluminum species (ca. 22% of signal intensity) in vicinity of 0 ppm in addition to the signal at 54 ppm (ca. 78% of signal intensity) because of the tetrahedrally coordinated framework aluminum atoms (see Figure 1S of Supporting Information). The concentrations of the acidic OH groups were obtained by the analysis of the intensities of the corresponding signals in 1 H MAS NMR spectra of all zeolite samples by comparing their intensities with that of adsorbed methane as internal standard. This concentration of bridged SiOHAl groups has not exceeded the expected one, estimated on the basis of Si/Al ratio derived from 29 Si MAS NMR spectra (see Figure 2S of Supporting Information) of the zeolite samples. 44 No any gallium oxide bulk phase was revealed with XRD analysis for Ga/H-BEA. 71 Ga MAS NMR of this sample has shown the signal at 159 ppm from the Ga sites tetrahedrally coordinated by oxygen (Ga IV ) and the signal at 0 ppm from the Ga sites octahedrally coordinated by oxygen (Ga VI ) (see Figure 3S of Supporting Information). These data indicate that the signal at 0 ppm arises from the Ga VI in Ga 2 O 3 species, which is however not detected by XRD possibly due to its small dimension, while the signal at 159 ppm belongs to a tetrahedrally coordinated Ga species in the zeolite framework. 47,48 Methane-d 4 (99% D), ethane-d 6 (99% D), propane-d 8 (99% D), and n-butane-d 10 (99% D) were purchased from Aldrich Chemical Co. Inc. and used without further purification. The samples for 1 H MAS NMR H/D exchange kinetics measurements were prepared by heating 80 mg of the zeolite sample in the axially high symmetric glass tubes of 5.5 mm outer diameter. The samples were activated by an increase of the temperature from 300 to 673 K at the rate of 10 K h 1 under vacuum. Further, the samples were maintained at 673 K for 24 h under vacuum (less than 10 2 Pa). The loading was performed at room temperature with 1.15 molecules (ca. 300 μmol g 1 ) of alkane per unit cell, and each sample was then sealed off (length of the glass tube = 10 mm). This glass tube could be tightly inserted in 7 mm zirconia rotors. Before acquisition of the signal, the NMR probe with the sample was preheated for 20 min at the temperature at which the H/D exchange did not yet occur at notable rate. This temperature was 423 K for Ga/H-BEA and 500 K for H-BEA zeolite samples, respectively. Then the temperature was rapidly increased within 3 10 min by K to the reaction temperature, equilibrated for 1 2 min, and then the acquisition of NMR signal started. It should be noted here that chemical conversion of C 1 C 4 alkanes to Ga-alkyl species occurred under conditions of our experiment, however this conversion did not exceed 2 4%, so the H/D exchange was the main transformation of the alkanes NMR Measurements. NMR spectra were recorded at 9.4 T on a Bruker Avance 400 spectrometer equipped with broadband double-resonance-mas probe. Zirconia rotors (4 or 7 mm outer diameter) with the inserted sealed glass tube were spun at 3 10 khz by dried compressed air at K. 1 H MAS NMR spectra were recorded by the Hahn-echo pulse sequence (π/2 τ π τ acquisition), where τ equals to one rotor period ( μs). The excitation pulse length was 4.5 μs (π/2), and typically 6 24 scans were accumulated with a 4 60 s delay. In double-resonance 1 H{ 27 Al}TRAPDOR experiments, 45,46 Hahn-echo sequence was applied to the 1 H channel with irradiation of aluminum during the both τ periods. The 27 Al nutation frequency of the irradiation field was about 60 khz. 27 Al MAS NMR spectra were acquired with a short π/12 radio frequency pulse (0.6 μs), and about 1000 scans were accumulated with a 0.5 s recycle delay. 29 Si MAS NMR spectra were recorded with π/2 excitation pulse of 5.0 μs duration, s repetition time, and 1000 scans for signal accumulation. Both dx.doi.org/ /jp204398r J. Phys. Chem. C 2011, 115,

3 Figure 1. 1 H MAS NMR room temperature Hahn-echo spectra of zeolite H-BEA sample activated under vacuum at 670 K (a) without 27 Al irradiation and (b) with 27 Al irradiation (on resonance). The difference spectrum of (a) and (b) is shown in (c). (d) Decomposition of the experimental spectrum (a) into 5 separate lines. The spinning speed was set to 3 khz, and τ was equal to one rotor period (333 μs). The spectrum for the Ga/H-BEA zeolite is similar to H-BEA. Asterisks belong to spinning sidebands. 27 Al and 29 Si NMR spectra were recorded using 4 mm rotors and a spinning rate of 10 khz. The chemical shift was referenced to tetramethylsilane for 1 H and 29 Si NMR and to 0.1 M Al(NO 3 ) 3 solution for 27 Al NMR. 71 Ga MAS NMR studies were performed on Avance 750 spectrometer at resonance frequency of MHz with sample spinning rates of 25 khz (2 mm MAS NMR rotor). For obtaining the 71 Ga MAS NMR spectra, a single-pulse excitation of 0.5 μs, a repetition time of 0.5 s, and scans were used. The 71 Ga MAS NMR signals were referenced to a saturated solution of Ga(NO 3 ) 3. The sample temperature was controlled by the Bruker BVT variable-temperature unit. For kinetics measurements, the calibration of the temperature ( K) inside the rotor was performed with an accuracy of (2 K by using lead nitrate, located inside the rotor, as a 207 Pb MAS NMR chemical shift thermometer Kinetics Model: Exchange of Methane and Ethane. The kinetics of the H/D exchange was analyzed assuming a parallel scheme of H/D exchange. In this scheme, all the deuterium atoms in methane-d 4 (or ethane-d 6 ) exchange consecutively by the protium of the SiOHAl groups (the signals 4.1 and 5.2 ppm in 1 H MAS NMR spectrum, see Figure 1). In parallel, there is also the exchange between methane-d 4 (ethane-d 6 ) and acidic SiOH groups (SiOH acidic ). These SiOH acidic groups, characterized by the signal at 2.1 ppm (Figure 1), exhibit the acidity which is similar to that of SiOHAl groups. 42 AlOH groups with the signal 2.9 ppm were not considered to be involved in the exchange with C 1 n-c 4 alkanes. Reactions 1 8 in Table 1 were used for simulating the H/D exchange of methane-d 4. Reactions 1 4 describe a consecutive exchange with SiOHAl groups, while the reactions 5 8 describe a consecutive exchange with SiOH acidic groups. Kinetic equations for the groups SiODAl and SiOD acidic, built on the base of reactions 1 8 from Table 2, are d½siodalš ¼ dt 4 i ¼ 1 d½siod acidic Š ¼ dt 8 R i R j j ¼ 5 By assumption that F SiODAl and F SiOD are the mole fractions of D isotope in the SiOHAl and SiOH acidic groups, eq 1 transforms into the system of two differential equations (see Supporting Information for more details) df SiODAl dt df SiOD dt ¼ k SiOHAl ½CD n Š 0 ¼ k SiOH ½CD n Š 0 1 F SiODAl F D eq ð1þ ( ) + ½SiOH acidicš 0 ðf SiODAl F SiOD Þ n½cd n Š 0 ( ) 1 F SiOD + ½SiOHAlŠ 0ðF Feq D SiOD F SiODAl Þ n½cd n Š 0 where [CD n ] 0, [SiOHAl] 0 and [SiOH acidic ] 0 are the initial concentrations of alkane and respective OH groups of the zeolite (in μmol g 1 ), n is the number of hydrogen atoms in alkane (e.g., n = 4 for methane), and F D eq is the equilibrium mole fraction of D isotope among reaction participants. It is determined by the initial concentrations F D eq ¼ n½cd n Š 0 n½cd n Š 0 + ½SiOHAlŠ 0 + ½SiOH acidic Š 0 ð2þ ð3þ The Runge Kutta method (fourth order) 50 with the integration step adaptation was used to solve the system of differential eq 2, with the initial conditions F SiODAl = 0 and F SiOD = 0 being taken into account. The protium concentration in alkane [X] CHn and acidic groups were calculated by the eqs 4 ½XŠ CHn ¼½SiOHAlŠ 0 F SiODAl + ½SiOH acidic Š 0 F SiOD ½SiOHAlŠ ¼½SiOHAlŠ 0 ð1 F SiODAl Þ ½SiOH acidic Š¼½SiOH acidic Š 0 ð1 F SiOD Þ Therateconstantsk SiOHAl and k SiOH were determined by fitting the experimentally obtained kinetics with theoretically calculated ones by eq 4. Time-dependent mole fractions F SiODAl and F SiOD in eq 4 are the solutions of the system of differential eq 2. Accuracies of the rate constants were in the range of 10 20% as determined from the spread of the experimental points on the kinetic curves. Equations 2 4 were also used for modeling the exchange kinetics of ethane-d 6 (n = 6) with SiOHAl and SiOH acidic groups. Exchange of Propane and n-butane. Having established that k SiOHAl and k SiOH are similar in the case of exchange of methane or ethane (vide infra) with zeolites acid sites, we further assumed that k SiOHAl = k SiOH for the analysis of exchange of propane and n-butane. Since both the methyl CH 3 and the methylene CH 2 groups could be involved in exchange with different a rates, we have introduced two mole fractions, F H and F b H, describing the amount of protium isotope in the methyl and methylene groups, respectively. A possible intramolecular hydrogen transfer between the methyl and methylene groups, identified earlier for H-ZSM-5, 38,51,52 was not taken into account. Thus, assuming the acid sites with total initial concentration of [OH] 0 = [SiOHAl] 0 + [SiOH acidic ] 0 are involved in parallel exchange with the methyl and the methylene groups, the mole fractions F a H and ð4þ dx.doi.org/ /jp204398r J. Phys. Chem. C 2011, 115,

4 Table 2. Exchange Reactions Used for Simulating the Kinetics of the H/D Exchange for Methane on H-BEA and Ga/H-BEA Zeolites no. exchange reaction rate expression 1 CD 4 + SiOHAl / CD 3 H + SiODAl R 1 = k SiOHAl [CD 4 ][SiOHAl] ( 1 / 4 )k SiOHAl [CD 3 H][SiODAl] 2 CD 3 H + SiOHAl / CD 2 H 2 + SiODAl R 2 =( 3 / 4 )k SiOHAl [CD 3 H][SiOHAl] ( 1 / 2 )k SiOHAl [CD 2 H 2 ][SiODAl] 3 CD 2 H 2 + SiOHAl / CDH 3 + SiODAl R 3 =( 1 / 2 )k SiOHAl [CD 2 H 2 ][SiOHAl] ( 3 / 4 )k SiOHAl [CDH 3 ][SiODAl] 4 CDH 3 + SiOHAl / CH 4 + SiODAl R 4 =( 1 / 4 )k SiOHAl [CDH 3 ][SiOHAl] k SiOHAl [CH 4 ][SiODAl] 5 CD 4 + SiOH / CD 3 H + SiOD R 5 = k SiOH [CD 4 ][SiOH] ( 1 / 4 )k SiOH [CD 3 H][SiOD] 6 CD 3 H + SiOH / CD 2 H 2 + SiOD R 6 =( 3 / 4 )k SiOH [CD 3 H][SiOH] ( 1 / 2 )k SiOH [CD 2 H 2 ][SiOD] 7 CD 2 H 2 + SiOH / CDH 3 + SiOD R 7 =( 1 / 2 )k SiOH [CD 2 H 2 ][SiOH] ( 3 / 4 )k SiOH [CDH 3 ][SiOD] 8 CDH 3 + SiOH / CH 4 + SiOD R 8 =( 1 / 4 )k SiOH [CDH 3 ][SiOH] k SiOH [CH 4 ][SiOD] F b H were obtained via solution of the following system of differential equations (see Supporting Information for more details) ( ) dfh a ¼ k CH 3 ½OHŠ 0 1 Fa H dt a Feq H + b½rdðch 2Þ b Š 0 ðfh a ½OHŠ Fb H Þ 0 ( ) dfh b ¼ k CH 2 ½OHŠ 0 1 Fb H dt b Feq H + a½rdðch 3Þ a Š 0 ðfh b ½OHŠ Fa H Þ 0 ð5þ (CH where [RD 3 ) (CH a ] 0 and [RD 2 ) b ] 0 represent the initial concentrations of the methyl and the methylene groups (in μmol g 1 ), respectively; a and b are the total number of hydrogen atoms in the methyl groups and methylene groups of a hydrocarbon, respectively (i.e., a =6,b = 2 for propane and a =6,b = 4 for n-butane); F H eq is the equilibrium mole fraction of H isotope among reaction participants, which is determined from the initial concentrations Feq H ¼ ½OHŠ 0 ð6þ ½OHŠ 0 + a½rd ðch 3Þ a Š 0 + b½rd ðch 2Þ b Š 0 A solution for the system of differential eq 5 was found by Runge Kutta method, assuming F a H = 0 and F b H = 0 as the initial conditions. The rate constants k CH3 and k CH2 were evaluated by fitting the experimental kinetics for the methyl [H] CH3 and the methylene [H] CH2 groups of propane and n-butane with the simulated kinetic curves, which were calculated by eq 7 3. RESULTS ½HŠ CH3 ½HŠ CH2 ¼ a½rd ðch 3Þ a Š 0 FH a ¼ b½rd ðch 2Þ b Š 0 FH b ð7þ 3.1. Characteristics of Hydroxyl Groups of H-BEA and Ga/ H-BEA with 1 H MAS NMR. 1 H MAS NMR spectra of both H-BEA and Ga/H-BEA show three main signals (Figure 1). The signals at 4.1 and 5.2 ppm are due to the free bridged and Figure 2. Stack plot of the 1 H MAS NMR spectra at 503 K of methaned 4 adsorbed on Ga/H-BEA. The first spectrum (bottom) was recorded 5 min after the temperature was raised and equilibrated at 503 K and the last spectrum (top) after 500 min of the reaction duration. hydrogen-bonded bridged hydroxyls (SiOHAl), respectively, and the most intense signal at 1.8 ppm arises from SiOH groups at framework defects and external SiOH groups The intense signal of SiOH groups is a result of the faulted structure of the zeolite beta (BEA). 57,58 1 H{ 27 Al} spin echo double resonance (TRAPDOR 45,46 ) experiments display selectively (Figure 1c) the signals of OH groups, which are connected to Al. Three resonances at 5.2, 4.1, and 2.9 ppm for both H-BEA and Ga/H-BEA were identified. The signals at 5.2 and 4.1 are due to SiOHAl groups; the signal 2.9 ppm belongs to the extraframework AlOH species. 42 The SiOH groups with the signal at 2.1 ppm observed as a left-hand shoulder to the intense signal at 1.8 ppm are strongly acidic (SiOH acidic ) as follows from the value of the low frequency shift of this SiOH vibration with adsorbed CO (Δν OH/CO =300cm 1 ). 42 The protons of these SiOH groups are involved in the H/D exchange with alkanes, similar to protons of the bridged SiOHAl groups dx.doi.org/ /jp204398r J. Phys. Chem. C 2011, 115,

5 Figure 5. Arrhenius plots for the H/D exchange reaction of ethane-d 6 with SiOHAl groups: H-BEA (9)(E SiOHAl = 105 ( 6 kj/mol) and Ga/ H-BEA (b) (E SiOHAl = 106 ( 1 kj/mol). k SiOH and E SiOH parameters for H-BEA (Ga/H-BEA) are similar to those of k SiOHAl and E SiOHAl for H-BEA (Ga/H-BEA). Figure 3. Experimental and simulated (solid curves) kinetics of the H/ D exchange for methane-d 4 on H-BEA (a) and Ga/H-BEA (b) zeolites. Simulation was performed in accordance to parallel kinetic scheme of the H/D exchange between methane and SiOHAl and between methane and SiOH. The rate constants k SiOHAl and k SiOH, derived from kinetics simulation at K are presented in Figure 4. Figure 6. Arrhenius plots for the H/D exchange reaction of propane-d 8 with acidic OH (SiOHAl + SiOH acidic ) groups of zeolites H-BEA and Ga/H-BEA. H-BEA: (0) k CH2,(E CH2 = 120 ( 9 kj/mol); (9) k CH3, (E CH3 = 109 ( 8 kj/mol); Ga/H-BEA: (O) k CH2,(E CH2 =97( 2 kj/ mol); (b) k CH3,(E CH3 =90( 2 kj/mol). Figure 4. Arrhenius plots for the H/D exchange reaction of methane-d 4 : on H-BEA with SiOH (0)(E SiOH =142( 3kJ/mol)andwithSiOHAl (9)(E SiOHAl =136( 11 kj/mol); on Ga/H-BEA with SiOH (O)(E SiOH =103( 4kJ/mol)andwithSiOHAl(b) (E SiOHAl =101( 6kJ/mol) H/D Exchange of Methane-d 4 and Ethane-d 6 on H-BEA and Ga/H-BEA Zeolites. H/D exchange between methane-d 4 and hydroxyl groups of both zeolites H-BEA and Ga/H-BEA results in decrease of the intensity of the signals at 5.2, 4.1, and 2.1 ppm from acidic SiOHAl and SiOH groups and increase of intensity of the signal at 0.0 ppm from methane (CD m H 4 m (m =0 3)) (Figure 2). Analysis of the kinetics of the H/D exchange (Figure 3) shows that the rate constants k SiOHAl and k SiOH are similar for H-BEA. A slight difference between k SiOHAl and k SiOH (k H-BEA SiOHAl 1.2k H-BEA SiOH ) may arise from some difference in acidity of SiOH and SiOHAl groups of the zeolite. 42 The difference is also within an experimental error for the rate constants determination. Modification of the zeolite with Ga results to an increase of the rate of H/D exchange by 2 orders of magnitude, e.g., k Ga/H-BEA SiOHAl 145k H-BEA SiOHAl at 543 K. At the same time an activation energy of the exchange decreases from 136 to 101 kj/mol (see Figures 3 and 4). For ethane-d 6 we have not found any noticeable difference between k SiOHAl, E SiOHAl and k SiOH, E SiOH for both H-BEA and dx.doi.org/ /jp204398r J. Phys. Chem. C 2011, 115,

6 Figure 7. Stack plot of the 1 H MAS NMR spectra of propane-d 8 at 493 K and n-butane-d 10 at 468 K adsorbed on Ga/H-BEA. Regioselective exchange into the methyl groups is evident as follows from preferential growth of the signal 1.0 ppm due to transfer of protium from the acidic OH groups into the methyl groups. Figure 9. Arrhenius plots for the H/D exchange reaction of n-butaned 10 with acidic OH (SiOHAl + SiOH acidic ) groups of zeolites H-BEA and Ga/H-BEA. H-BEA: (3) k CH2,(E CH2 = 109 ( 5 kj/mol); (9) k CH3, (E CH3 = 108 ( 5 kj/mol); Ga/H-BEA: (O) k CH2,(E CH2 = 107 ( 2 kj/ mol); (b) k CH3,(E CH3 =96( 2 kj/mol). Figure 8. Experimental and simulated (solid curves) kinetics of the H/ D exchange for propane-d 8 (a) and n-butane-d 10 (b) on Ga/H-BEA zeolites. Simulation was performed in accordance to parallel kinetic scheme of the H/D exchange between acidic OH (SiOHAl + SiOH acidic ) groups and the methyl and the methylene groups of the alkanes. The rate constants k CH3 and k CH2, derived from kinetics simulation at K are presented in Figure 6. For n-butane the values k CH3 and k CH2 were derived without taking into account a possible isomerization of n-butane into isobutane, because this process is extremely slow at the temperature range used in this study of the H/D exchange. 52,60 Ga/H-BEA zeolites. However, modification of the zeolite with Ga gives rise to a notable increase of the rate of H/D exchange similar to a case with methane (k Ga/H-BEA SiOHAl > k H-BEA SiOHAl ) (Figure 5) H/D Exchange of Propane-d 8 and n-butane-d 10 on H-BEA and Ga/H-BEA Zeolites. Analysis of the kinetics of the H/ Dexchangeforpropane-d 8 shows that the rate of the exchange for the methyl groups is higher than that for the methylene groups approximately by two times (k CH3 2k CH2 ) for H-BEA at K, whereas the activation energy of the exchange is higher for the methylene groups (Figure 6). Similar kinetics parameters of the exchange were observed for propane exchange on zeolite H-ZSM-5. 38,51,59 Faster direct exchange of protons of acidic OH group with the methyl groups compared to that with the methylene groups was earlier rationalized for H-ZSM-5 zeolite by more spatial accessibility of the methyl groups for the exchange. 38 Modification of the zeolite with Ga does not result to a notable change of k CH2, whereas the rate k CH3 is increased by 1 order of magnitude (Figure 6). The activation energy of the exchange reaction into the methyl group is decreased. A remarkable difference between k CH2 and k CH3 for the Ga/H-BEA is displayed as preferential enrichment of the methyl groups of propane-d 8 with protium at a transfer of protium of acidic OH groups of the zeolite to the alkane molecule (see Figures 7 and 8). This gives rise to an evident regioselectivity of the exchange into the methyl groups at modification of the zeolite with Ga. H/D exchange reaction of n-butane on H-BEA zeolite shows similar values of k CH2 and k CH3 ; they are identical to those found dx.doi.org/ /jp204398r J. Phys. Chem. C 2011, 115,

7 earlier for the exchange on H-ZSM For Ga/H-BEA, k CH2 is similar to k CH2 of H-BEA, whereas k CH3 increases by 1 order of magnitude (Figure 9). Similar to the case with propane-d 8 a regioselectivity of the exchange, i.e., a preferential enrichment of the methyl groups with protium, is observed (see Figures 7 and 8). Thus, modification of zeolite BEA with Ga provides an essential effect on the kinetic parameters of the reaction of H/D hydrogen exchange between C 1 C 4 alkanes and Brønsted acid sites of the zeolite. The rate of H/D exchange increases at least by 1 order of magnitude for all C 1 C 4 alkanes, the activation energy of the exchange decreases. For C 3 and n-c 4 alkanes, a regioselectivity of the exchange (preferential involvement of the hydrogens of methyl groups into exchange) is observed. 4. DISCUSSION Kinetic parameters of the H/D hydrogen exchange reaction between C 1 n-c 4 alkanes and acidic OH groups of the zeolite H-BEA are in good correspondence with the parameters that were experimentally obtained earlier for the acidform zeolites 34,35,38,51,52,59,61 or were estimated theoretically, 29 31,33,36,39,40,62 provided that the exchange occurs in a concerted step involving a penta-coordinated carbon atom (carbonium ion) in a transition state. This allows us to conclude that the mechanism of the exchange of C 1 n-c 4 alkanes with H-BEA zeolite involves conventionally accepted penta-coordinated carbon atom in a transition state ,33 37,39,40,62 Involvement of the SiOH groups with the signal 2.1 ppm in the exchange with alkanes, which was earlier assigned to the intrazeolitic exchange between SiOHAl and SiOH groups 22 should be assigned to their direct exchange with alkane in parallel with SiOHAl groups, since a strong acidity of this SiOH group was established. 42 Similar values of the rate constants k SiOHAl and k SiOH confirm strong acidity of SiOH groups with the signal 2.1 ppm for H-BEA zeolite, established by IR spectroscopy of adsorbed CO. 42 For Ga/H-BEA, the kinetic parameters of the H/D exchange remarkably change for C 1 n-c 4 alkanes. The rate constants k SiOHAl are at least 1 order of magnitude higher than those for H-BEA. The activation energy of the exchange decreases. The exchange proceeds with a notable rate at temperature K lower than the temperature of exchange on H-BEA. The higher rate of the exchange, the lower apparent activation energy, the lower temperature range of this reaction, regioselectivity of the exchange for propane and n-butane, all these factors imply a promoting effect of Ga on the reaction of the H/D exchange. The acceleration of H/D exchange, coupled with the decreased activation energy for Ga-modified zeolite, demonstrates that the C H bonds in alkanes reversibly break and form more easily with intermediate involvement of Ga species. Also, the regioselectivity of H/D exchange for propane and n-butane is clearly due to the presence of the Ga species in the modified zeolite. Evidently, the mechanism of the exchange should be different from that usually accepted for pure acid-form zeolites ,62 This implies that small alkane activation on Ga-modified zeolite occurs by a pathway different from that on pure acid form zeolite. Promoting effect of gallium on the H/D exchange reaction between alkane and BAS of the zeolite implies that both Brønsted acid sites and metal sites are involved in small alkane activation. The promoting effect of Ga on the reaction of H/D exchange of acidic OH groups with alkane molecules (or on alkane activation by Brønsted acid sites) can be rationalized by the formation of the intermediate Ga-alkyl species at dissociative adsorption of alkane on Ga-modified zeolite and further involvement of this species in the exchange with acidic OH groups. Indeed, a small quantity of Ga-alkyl species was recently shown to be formed on interaction of C 1 C 3 alkanes with gallium(iii) oxide (R-Ga 2 O 3 ) or Ga-modified zeoite BEA. 63 The possible pathway of H/D exchange for C 1 n-c 4 alkanes with the involvement of Ga-alkyl species is outlined in Scheme 1 (Pathway 1). Dissociative adsorption of the deuterated alkane on gallium oxide species, which precedes H/D exchange, leads to Ga-alkyl species and Ga OD groups. Protonation of the Gaalkyl species by the vicinal acidic OH group of the zeolite with synchronous reverse transfer of the deuterium from the Ga OD to the zeolite acid site finally provides a transfer of protium from the SiOHAl group of the zeolite to the deuterated alkane molecule. This mechanism could offer the kinetic parameters of the exchange to be significantly different from those for H-BEA zeolite. The mechanism also rationalizes the involvement of only the methyl groups of C 3 and n-c 4 alkanes in the exchange. Only the hydrogens of the CH 2 group, attached to Ga directly, can be involved in the exchange. XRD analysis did not show the existence of any gallium oxide phase for Ga/H-BEA. There is no gallium in the form of Ga 3+ cations, which could substitute protons of the acid sites. GaO +12,64 66 or (Ga 2 O 2 ) +67 species, which could play the role of active sites in activation and dehydrogenation, were not detected. This allows us to conclude that gallium could exists in the form of highly dispersed Ga 2 O 3 species with a dimension of gallium oxide cluster that could be easily localized inside the channel system of the zeolite. These clusters of gallium oxide seem to be uniformly distributed inside the zeolite pores and could be located in vicinity of acidic OH groups. Promoting effect of Ga on the reaction of H/D exchange and small alkane activation is evidently performed in a manner similar to the effect of Zn on these processes as was earlier demonstrated However, the effect of Ga on the H/D exchange rate is less profound as compared to that of Zn. Enhancement of the H/D exchange rate occurs (e.g., for methane) by 2 orders of magnitude upon modification of the zeolite with Ga, whereas this rate increases by more than 3 orders of magnitude at modification of the zeolite with Zn 22,23 (see Figure 10). Similar tendency in influence of Ga and Zn is observed for the reaction of H/D exchange between molecular hydrogen and Bronsted acid sites of Zn- and Ga-modified zeolites BEA. 43 A more profound effect of Zn on H/D exchange reaction means that Zn-modified zeolites can be more efficient in alkane activation and therefore in the conversion of alkanes to aromatics. Indeed, Zn-modified zeolites exhibit a higher activity in alkane-to-aromatics conversion. However, stability of these catalysts is lower due to the zinc cations reduction and its volatility at the reaction temperature. Despite lower activity, Ga-modified zeolites is more widely used in practice for alkane aromatization. 5,68 A higher promoting effect of Zn on the H/D exchange reaction rate and therefore on alkane activation may be related with difference in stability of the intermediate Ga- and Zn-alkyl species, in their electronic properties, or in spatial constraints, arisen from the mutual disposal of Ga (Zn)-alkyl species and Brønsted acid sites in the zeolite pores 68 to realize the exchange reaction by Scheme 1. Further experimental and theoretical studies are required to clarify quantitatively a profound difference dx.doi.org/ /jp204398r J. Phys. Chem. C 2011, 115,

8 Scheme 1. Possible Mechanisms of H/D Hydrogen Exchange (Pathway 1) and Dehydrogenation (Pathway 2) of Alkanes on Ga- Modified Zeolite a a Pathway 1 can account for a change of the exchange kinetics parameters for C 1 n-c 4 alkanes and regioselectivity for C 3 n-c 4 alkanes with respect to exchange on zeolite H-BEA. Pathway 2 rationalizes an involvement of both BAS and Ga-species in dehydrogenation of C 2 n-c 4 alkanes. dehydrogenation. The intermediate or transition state, which involves the zeolite acid sites, alkane molecule, and Ga sites, leads to alkane dehydrogenation, presumably by the energetically most favorable way. Thus, the enhancement of alkane conversion toward the formation of aromatic molecules on Ga-modified zeolites could be achieved. So, the effect of Ga, which was earlier attributed either to the dehydrogenation ability of the loaded metal 4,71 or its ability to perform the recombinative desorption of H adatoms, 3,11,72 is demonstrated to be the direct influence of Ga on the activation of alkane C H bonds by the catalyst Brønsted acid sites. Figure 10. Arrhenius plots for the H/D exchange reaction of methaned 4 with BAS of zeolites, demonstrating a difference in effect of Ga and Zn on the rate of the exchange reaction: H-BEA (b)(e SiOHAl = 136 ( 11 kj/mol); Ga/H-BEA (9)(E SiOHAl = 101 ( 6 kj/mol); Zn/H-BEA (2) (E SiOHAl =86( 5 kj/mol). Data for Zn/H-BEA are adapted from refs 22 and 23. in promoting effect of Ga and Zn on the H/D exchange reaction and alkane activation by Ga and Zn. The data obtained are an experimental demonstration of the suggested earlier 7,15 18,69,70 synergistic action of Ga species and acid sites and the necessity to have metal sites and BAS in close proximity to each other 18,20 for alkane activation in aromatization reaction. It becomes clear from our data that the activation of C H bonds of small alkanes by zeolite Brønsted acid sites depends on Ga species, which form an intermediate with the alkane. One can assume that the interaction of the Ga-alkyl species with zeolite acid sites can provide not only the H/D exchange reaction easily but also a dehydrogenation step (pathway 2 of Scheme 1), yielding olefins for the subsequent aromatization steps. Pathway 2 accounts for the synergy of the acid and the Ga sites (as Ga 2 O 3 clusters) for the effective aromatization of small alkanes by Ga-modified zeolites. Synergistic action of Ga and BAS in enhancing aromatization ability 7,15 17 could be achieved by involvement of both the zeolite acid OH groups and the Ga-alkyl species, formed on Ga sites, in the process of alkane 5. CONCLUSION In situ 1 H MAS NMR monitoring of the kinetics of the H/D hydrogen exchange between C 1 n-c 4 alkanes and Brønsted acid sites of zeolites H-BEA and Ga/H-BEA at K allowed us to draw the following conclusions: Modification of the zeolite H-BEA with Ga gives rise to a remarkable effect on the kinetic parameters of the exchange between C 1 n-c 4 alkanes and a Brønsted acid sites (BAS) of the zeolite: the rate of the exchange increases by 1 2 orders of magnitude, activation energy decreases, and a regioselectivity of the exchange into the methyl groups of C 3 n-c 4 alkanes appears. These data display an evident promoting effect of Ga on activation of alkane C H bonds by BAS. The effect finds its rationalization by the change of the mechanism of H/D exchange between BAS and alkanes from conventionally accepted 29 to another one. On Gamodified zeolite, the mechanism implies a preliminary dissociative adsorption of alkanes on gallium(iii) oxide species located inside the zeolite pores to form gallium-alkyl species, which is further involved in the exchange with neighbor BAS. Involvement of both BAS and Ga species in alkane activation accounts for earlier suggested synergistic effect 15 of BAS and gallium species in alkane activation and aromatization on Ga- modified high silica zeolites. ASSOCIATED CONTENT b S Supporting Information. Description of the kinetics modeling procedure for the H/D exchange of C 2 n-c 4 alkanes on H-BEA and Ga/H-BEA zeolites, 27 Al, 29 Si, and 71 Ga MAS dx.doi.org/ /jp204398r J. Phys. Chem. C 2011, 115,

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