Relationships between the Hydrogenation and Dehydrogenation

Jan 11, 2016 - In the case of the propane dehydrogenation, the strong H2/Pd ... on the in situ flow MAS NMR equipment and examples of 1H MAS NMR spect...
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Relationships between the Hydrogenation and Dehydrogenation Properties of Rh‑, Ir‑, Pd‑, and Pt-Containing Zeolites Y Studied by In Situ MAS NMR Spectroscopy and Conventional Heterogeneous Catalysis Utz Obenaus, Felix Neher, Matthias Scheibe, Michael Dyballa, Swen Lang, and Michael Hunger* Institute of Chemical Technology, University of Stuttgart, 70550 Stuttgart, Germany S Supporting Information *

ABSTRACT: The intrinsic hydrogenation activities of homologous series of noble-metal-containing zeolites Y were studied by in situ solid-state NMR spectroscopy under semibatch conditions. For the hydrogenation of acrylonitrile, reaction rates in the sequence Pd/H,Na−Y > Rh/H,Na−Y > Pt/H,Na−Y > Ir/H,Na− Y were determined. The dehydrogenation of propane at these zeolites gave a sequence of the turnover frequencies of Ir/H,Na−Y > Rh/H,Na−Y > Pd/H,Na−Y, while Pt/H,Na−Y zeolites showed significantly higher activities. The temperature-programmed desorption of hydrogen (H2-TPD) was utilized for studying the strength of H2/metal interactions. The positions of the hightemperature peaks were arranged according to 2.8Pd/H,Na−Y (723 K) > 2.3Rh/H,Na−Y (713 K) > 4.7Ir/H,Na−Y (663 K). Comparison of these data indicates that strong H2/metal interactions are accompanied by a preferred formation of surface hydrogen atoms, which are the reason for the high hydrogenation activity of Pd/H,Na−Y zeolites compared with Rh/H,Na−Y and Ir/H,Na−Y zeolites. In the case of the propane dehydrogenation, the strong H2/Pd interactions in Pd/H,Na−Y zeolites hinder the desorption of the reaction product H2, explaining the lower dehydrogenation activity of these zeolites compared with Rh/H,Na−Y and Ir/H,Na−Y zeolites. For the high catalytic activities of the Pt/H,Na−Y zeolites, an effect of strongly chemisorbed hydrogen atoms inside the Pt clusters is discussed.

1. INTRODUCTION The heterogeneously catalyzed hydrogenation of hydrocarbons on metal catalysts plays an important role in petrochemistry and refining, such as for the elimination of alkynes in gas streams of alkenes, the purification of feedstocks for polymerization reactions from polyenes, which are poisoning the polymerization catalysts, or the selective conversion of doubleand triple-bond-containing organic compounds with and without functional groups into desired products.1−4 Similarly, metal catalysts are utilized also for industrial dehydrogenation and dehydroisomerization reactions, e.g. the production of propene from propane and of butene and butadienes from nbutane.5 Despite the high importance of transition and noble metals for heterogeneous catalysis, only very few systematic studies exist, which compare the hydrogenation and dehydrogenation activities of homologous series of solid catalysts containing different metal types. Corvaisier et al. investigated the hydrogenation of styrene to ethylbenzene on silica-supported metal catalysts for comparing the turnover frequencies (TOF) determined for a large series of different metals.6 Focusing on the metals also considered in the present work, the above© 2016 American Chemical Society

mentioned authors obtained a TOF sequence for the hydrogenation of styrene of Pd > Rh > Pt ≅ Ir. Studying the hydrogenation of ethene on silica-supported metals, Schuit et al. obtained reaction rates, r, according to an activity sequence of Rh > Pd > Pt > Ir.7 The main difference in these two sequences is the catalytic properties of palladium and rhodium. For the heats of the H2 adsorption at evaporated porous metal catalysts, Beeck et al. determined very similar values for rhodium and palladium (Rh ≅ Pd),8 which may indicate that these two noble metals lead to very similar H2/metal interactions, probably also under the conditions of hydrogenation/dehydrogenation reactions. The temperature-programmed desorption of hydrogen (H2TPD) was mainly utilized for the investigation of supported platinum catalysts.9,10 For platinum-containing zeolite Y, Anderson et al. observed broad high temperature peaks at 700−750 K with peak maxima, which shifted to the upper temperature limit for H2-TPD experiments upon starting the Received: November 20, 2015 Revised: January 11, 2016 Published: January 11, 2016 2284

DOI: 10.1021/acs.jpcc.5b11367 J. Phys. Chem. C 2016, 120, 2284−2291

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Table 1. Loadings of Acrylonitrile on the Noble-Metal-Containing Zeolites Y under Study, Velocity Rate Constants, k, and Reaction Rates, r, Determined by in Situ 1H MAS NMR Spectroscopy of the Hydrogenation of Acrylonitrile under Semibatch Conditions

a

catalyst

acrylonitrile per supercagea

0.4Pd/H,Na−Y 0.4Rh/H,Na−Y 0.8Pt/H,Na−Y 0.8Ir/H,Na−Y 2.8Pd/H,Na−Y 2.3Rh/H,Na−Y 4.5Pt/H,Na−Y 4.7Ir/H,Na−Y

0.81 0.68 0.65 0.52 0.86 0.71 0.82 0.81

k/s−1 1.1 ± 0.3 (3.4 ± 0.3) (1.1 ± 0.1) (6.7 ± 0.7) (7.6 ± 2.5) (7.0 ± 2.4) (5.1 ± 0.5) (7.6 ± 0.8)

acrylonitrilea/mmol × × × × × × ×

10−2 10−2 10−4 10−1 10−1 10−2 10−3

0.027 0.020 0.021 0.017 0.026 0.023 0.027 0.030

r/mmol s−1 (2.7 (6.8 (2.3 (1.1 (2.0 (1.6 (1.4 (2.3

± ± ± ± ± ± ± ±

1.0) 1.0) 0.4) 0.2) 1.0) 1.0) 0.2) 0.3)

× × × × × × × ×

10−2 10−4 10−4 10−5 10−2 10−2 10−3 10−4

Determined by 1H MAS NMR with an accuracy of ±5%.

(nSi/nAl = 2.7) from Degussa AG, Hanau, Germany, as described in ref 14. Briefly, the purified zeolite Na−Y was suspended in demineralized water containing calculated amounts of RhCl3·nH2O (99.99%, Alfa Aesar), [Ir(NH3)5Cl]Cl2 (99.95%, Sigma-Aldrich), [Pd(NH3)4]Cl2·nH2O (40.62 wt % Pd, ChemPur), and [Pt(NH3)4]Cl2·nH2O (55.66 wt % Pt, ChemPur) at 353 K for 12 h. Upon washing in demineralized water and drying at 353 K for 24 h, the sample materials were calcined at 573 K for 3 h under flowing (60 mL/min) synthetic air (20% O2, 80% N2). Higher calcination temperatures led to increasing agglomeration of the metal atoms, especially in the case of iridium- and palladium-loaded zeolites Y. The pressed and sieved powders were reduced in flowing hydrogen (100 mL/min) at 623 K for 2 h and subsequently sealed in glass tubes. At this reduction temperature, the catalysts were reduced, but definitely not damaged, and were suitable for further investigations. The obtained samples were assigned according to xNM/H,Na−Y with the noble metal (NM) content x given in weight percent (compare Supporting Information, Table S1, columns 1 and 2) as determined by optical emission spectroscopy with inductively coupled plasma (Varian Vista-MPX). Considering the different metal atom weights, two homologous series of noble-metal-loaded zeolites Y were prepared, with 0.4−0.6 and 2.9−3.4 noble metal atoms per unit cell of zeolite Y (compare Supporting Information, Table S1, column 3). All noble-metal-loaded zeolites Y under study were characterized by X-ray diffraction (Bruker D8 diffractometer) and 27Al and 29Si MAS NMR spectroscopy for evidencing their structural intactness (for details, see ref 14). Details on the noble metal dispersion, D, as determined by hydrogen chemisorption, and the densities of Brønsted acidic bridging OH groups (acOH) are given in the Supporting Information (Table S1, column 5). For the Rh-, Pd-, and Ptloaded zeolites Y, noble metal dispersions of 43−92% were determined, while the iridium-containing samples have dispersions of 121−130% (Supporting Information, Table S1, column 4). These different D values are mainly due to different stoichiometries of H2 adsorption at the various metal species. Hence, these dispersion values and the low metal contents (see Table S1, columns 2 and 3, of the Supporting Information) hint at well-distributed metal sites inside the cages of the zeolites Y under study, but the formation of metal clusters cannot be totally excluded. For studying the strength of the H2 adsorption at the metals under study, zeolites Y with high noble metal contents were characterized by temperature-programmed desorption of H2 (H2-TPD). For these studies, calcined and pressed samples

H2 desorption at high desorption temperature (e.g., 623 K) and vice versa.9 Miller et al., on the other hand, observed four different peaks at 423, 548, 673, and 883 K for the desorption of H2 from Pt-containing zeolite K-LTL.10 All these studies indicated that hydrogen is strongly adsorbed at platinum clusters in zeolites and, furthermore, that different adsorption/ chemisorption states should exist for hydrogen at this metal. In the past decade, methodical improvements of highresolution solid-state NMR spectroscopy allowed the application of in situ techniques also for the study of hydrogenation reactions.11−13 In 2006, Sundaramurthy et al. demonstrated an observation of the hydrogenation of toluene to methylcyclohexane on Pt/ZrO2−SO4 under flow conditions.11 By in situ MAS NMR spectroscopy, Henning et al. investigated the intrinsic reaction rates, r, for the hydrogenation of preloaded acrylonitrile on Pt- and Rh-containing zeolites Y under semibatch conditions and found that the reaction rate for a rhodium-containing zeolite Y catalyst is about 1 order of magnitude higher than that for the corresponding platinumcontaining zeolite.12 Interestingly, the hydrogenation of preloaded acrylonitrile under semibatch conditions allows the exclusion of an effect of the reactant diffusion and catalyst deactivation on the experimentally observed reaction rates since the reactant acrylonitrile is already situated inside the zeolite pores at the beginning of the semibatch experiment, i.e. before starting the hydrogen flow. In the present work, two homologous series of zeolites Y loaded with ca. 0.5 and 3 noble metal atoms (Rh, Ir, Pd, Pt) per unit cell were prepared and utilized for a systematic study of their acrylonitrile hydrogenation activity by in situ 1H MAS NMR spectroscopy under semibatch conditions. For comparing the obtained hydrogenation reaction rates with the turnover frequencies of the reversed reaction, the nonoxidative dehydrogenation of propane was investigated at the same zeolite catalysts via conventional heterogeneous catalysis under flow conditions. Furthermore, temperature-programmed desorption of hydrogen (H2-TPD) was utilized to gain an insight into the strength of the hydrogen adsorption on the metal species of the zeolite catalysts under study. Based on the results of these complementary methods, the effect of the H2/metal interaction on the hydrogenation and dehydrogenation activities of the noble-metal-containing zeolites Y under study is discussed.

2. EXPERIMENTAL SECTION 2.1. Preparation and Characterization of the Sample Materials. The noble-metal-loaded zeolites Y studied in the present work were obtained by modifying parent zeolite Na−Y 2285

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The Journal of Physical Chemistry C were heated with a rate of 5 K/min up to 633 K under flowing hydrogen (40 mL/min) for a duration of 2.5 h. Subsequently, the samples were cooled to 313 K in flowing nitrogen (40 mL/ min) and again heated with a rate of 10 K/min up to 873 K. The high-temperature H2 desorption peaks (HT) were found at temperatures arranged in the sequence 4.5Pt/H,Na−Y (848 and 738 K) > 2.8Pd/H,Na−Y (723 K) > 2.3Rh/H,Na−Y (713 K) > 4.7Ir/H,Na−Y (663 K) (see Supporting Information, Figure S1). 2.2. In Situ 1H MAS NMR Studies of the Hydrogenation of Acrylonitrile under Semibatch Conditions. Before the in situ 1H MAS NMR experiments, 55 ± 5 mg of catalyst powder was filled into 4 mm MAS NMR rotors inside a glovebox purged with dry nitrogen. Subsequently, the rotors containing the sample materials were inserted into 5 mm glass tubes, connected via vacuum valves with a vacuum line, and evacuated at 293 K for 12 h. At this vacuum line with an inner volume of ca. 80 mL, these catalysts were loaded with ca. 6 mbar of acrylonitrile (natural abundance of isotopes, Acros Organics, purity of 99.9%) and, subsequently, evacuated at 293 K for 10 min to remove the only physisorbed acrylonitrile. The acrylonitrile loadings (see Table 1, columns 2 and 4) were determined by quantitative 1H MAS NMR spectroscopy by comparing the signal intensities with that of an external intensity standard (dehydrated zeolite 35H,Na−Y). These 1H MAS NMR measurements were performed on a Bruker AVANCE III 400 WB spectrometer at the resonance frequency of 400.13 MHz with π/2 single pulse excitation, a repetition time of 20 s, and a sample spinning rate of 8 kHz. The in situ hydrogenation of acrylonitrile on noble-metalcontaining zeolites Y was investigated using a Bruker 4 mm MAS NMR probe, which was modified as described in the literature (see the Supporting Information, Scheme S1).13,15−17 Briefly, the acrylonitrile-loaded catalysts were pressed to a hollow cylinder inside the rotor using a special tool and inside a glovebox purged with dry nitrogen gas. Then, a glass tube with an outer diameter of 1 mm was inserted into a 4 mm MAS NMR rotor via a hole (diameter 1.2 mm) in the rotor cap. Through the 1 mm glass tube, flowing hydrogen (300 mL/min; calibrated via Definer DryCal) was injected into the spinning MAS NMR rotors for performing the hydrogenation of acrylonitrile under semibatch conditions at 298 K. Since the hydrogen gas was injected to the bottom of the rotor, the complete catalyst bed was in contact with this reactant. The in situ 1H MAS NMR spectra were recorded at the resonance frequency of 400.13 MHz and with π/2 single pulse excitation, but with a sample spinning rate of 4 kHz. The first 1H MAS NMR spectrum was recorded 4 s after starting the hydrogen flow, while further spectra were obtained with one scan in steps of 10 s. After finishing the in situ experiments, the 1H MAS NMR signal intensities were evaluated by using the abovementioned external intensity standard (dehydrated zeolite 35H,Na−Y) and simulating the spectra using the Bruker software WinFit. 2.3. Dehydrogenation of Propane. The sieve fraction with particle diameters of 0.2−0.3 mm was utilized for investigating the dehydrogenation of propane (Westfalen AG, Muenster, Germany, purity of 99.95%) on the calcined and reduced noble-metal-loaded zeolites Y under study. Around 200 mg of catalyst particles with a bed height of ca. 13 mm was filled into a quartz glass fixed-bed reactor (inner diameter of 6 mm). Before the dehydrogenation reaction was started, the catalyst was in situ reduced in flowing hydrogen (50 mL/min)

at 623 K for 3.5 h and at atmospheric pressure. Subsequently, the apparatus was purged with nitrogen. Then, the catalysts inside the fixed-bed reactor were heated to 828 K under flowing nitrogen. When the reaction temperature of 828 K was reached, the gas flow was changed to a propane/N2 mixture in a ratio of 2:1 with a weight hourly space velocity of WHSV = 3 h−1. The hydrocarbons in the product flow were determined by online gas chromatography (Hewlett-Packard, HP 6890 Series), equipped with a capillary Chrompack PoraPLOT Q column (length of 50 m, film thickness of 10 μm) from Agilent J & W GC columns and using a temperature program consisting of 3 min at 343 K, then heating with 10 K/min to 453 K, holding at 453 K for 16 min, and a cooling down to 343 K in 5 min. The first reaction mixture was analyzed 35 min after starting the propane/N2 flow and again every 35 min. Finally, the dehydrogenation experiments were stopped after a time on stream of 175 min. The turnover frequencies, TOF, given in Table 2, last column, were calculated via the propane Table 2. Conversion of Propane, XC3, Selectivity to Propene, SC3=, Yield to Propene, YC3=, and Turnover Frequencies, TOF, for the Dehydrogenation of Propane on the NobleMetal-Containing Zeolites Y under Study, Determined with WHSV = 3 h−1, at T = 828 K, TOS = 35 min, and under Continuous-Flow Conditions catalyst

XC3a/%

SC3=a/%

YC3=a/%

TOFb/s−1

0.4Pd/H,NaY 0.4Rh/H,NaY 0.8Pt/H,NaY 0.8Ir/H,NaY 2.8Pd/H,NaY 2.3Rh/H,NaY 4.5Pt/H,NaY 4.7Ir/H,NaY

2 2 18 6 3 3 33 8

49 73 69 63 62 80 51 57

1 1 12 4 2 2 17 5

1.03 1.13 8.39 2.68 0.20 0.20 2.71 0.67

Determined by online gas chromatography with an accuracy of ±2%. Determined using the propane conversion at TOS = 35 min with an accuracy of ±5%.

a b

conversions, XC3, determined at TOS = 35 min, and the total number of metal atoms introduced into the zeolites Y (see Table 1, column 2, and Table S1, column 3, in Supporting Information, respectively). Considering the high dispersion and low content of the metal species in the zeolites Y under study, it is assumed that each introduced metal atom can act as a hydrogenation/dehydrogenation site.

3. RESULTS AND DISCUSSION 3.1. Hydrogenation of Acrylonitrile on the NobleMetal-Containing Zeolites Y. As shown in our earlier work,12 the hydrogenation of a preloaded reactant under semibatch conditions is a suitable approach for hindering a rapid catalyst deactivation and for excluding an effect of the reactant and product diffusion on the experimentally determined reaction rates. Therefore, this method was also utilized in the present work; i.e., the hydrogenation of acrylonitrile was performed under semibatch conditions with externally loaded acrylonitrile on a homologous series of noblemetal-containing zeolites Y. After transferring the loaded catalyst into the NMR spectrometer and upon starting the sample spinning and stabilizing the temperature at 298 K, the continuous hydrogen flow was started. The kinetics of the hydrogenation reaction was determined in situ via the time2286

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The Journal of Physical Chemistry C dependent changes of the 1H MAS NMR signals of the CH2 (6.3 ± 0.1 ppm) and CH groups (5.8 ± 0.1 ppm) of acrylonitrile and the CH3 (1.2 ± 0.1 ppm) and CH2 groups (2.3 ± 0.1 ppm) of propionitrile. As an example, Figure 1 shows the stack plot of the in situ 1H MAS NMR spectra recorded during the hydrogenation of acrylonitrile on zeolite 4.7Ir/H,Na−Y at 298 K up to the reaction time of 120 s.

Figure 2. Negative logarithms of the intensity ratios I(t)/I0 of the 1H MAS NMR signals of acrylonitrile plotted as a function of the reaction time, t, for the hydrogenation of acrylonitrile on Pt-, Ir-, Pd-, and Rhcontaining zeolites Y with low noble metal contents.

3 show the plots of these −ln(I(t)/I0) values as a function of the reaction time for the Pt-, Ir-, Pd-, and Rh-containing

Figure 1. Stack plot of the in situ 1H MAS NMR spectra recorded during the hydrogenation of acrylonitrile on zeolite 4.7Ir/H,Na−Y under semibatch conditions at 298 K.

The spectra recorded during the acrylonitrile hydrogenation on noble-metal-containing zeolites Y are characterized by 1H MAS NMR signals with different line broadenings. These signal broadenings depend on the reactant loading levels (compare with Figure 3 in ref 12), but are also different for zeolites Y containing different metal types, such as Ir and Pd (see Figure S2 in the Supporting Information). Generally, lower reactant loadings lead to broader 1H MAS NMR signals, probably due to stronger homonuclear dipolar interactions caused by a lower molecular mobility of the reactant molecules inside the pores of these zeolite catalysts and vice versa. For the same low reactant loading levels, on the other hand, broader 1H MAS NMR signals were observed for Pd- and Pt-containing zeolites Y compared with Rh- and Ir-containing zeolites Y, which hint at a broadening effect of the Pd and Pt species. For investigating the reaction kinetics of the acrylonitrile hydrogenation, the intensities of the 1H MAS NMR signals at 5.8−to 6.3 ppm were evaluated as a function of time and in comparison with an external standard, recorded before the in situ experiments. As also demonstrated in our previous study of the hydrogenation of acrylonitrile on a platinum- and a rhodium-loaded zeolite Y,12 the 1H MAS NMR signals of the reactant acrylonitrile at 6.2 and 5.7 ppm decrease immediately after starting the hydrogen flow, while signals of propionitrile at 2.4 and 1.1 ppm appear and increase until the acrylonitrile hydrogenation under semibatch condition was finished. For the determination of the velocity rate constants, the time-dependent intensities I(t) of the 1H MAS NMR signals of acrylonitrile were evaluated by a simulation and separation of the recorded spectra. Subsequently, the negative logarithms of the intensities I(t), related to the intensities I0 of the 1H MAS NMR signals of acrylonitrile before starting the hydrogen flow, were plotted as a function of the reaction time t. Figures 2 and

Figure 3. Negative logarithms of the intensity ratios I(t)/I0 of the 1H MAS NMR signals of acrylonitrile plotted as a function of the reaction time, t, for the hydrogenation of acrylonitrile on Pt-, Ir-, Pd-, and Rhcontaining zeolites Y with high noble metal contents.

zeolites Y with low and high noble metal contents, respectively. The obtained velocity rate constants, k, determined by the slope of a linear regression (least-squares fitted) passing through the coordinate origin, are summarized in column 3 of Table 1. Using the acrylonitrile loadings given in column 4 of Table 1, the reaction rates, r, given in column 5, were calculated. The reaction rates for the acrylonitrile hydrogenation on zeolites 0.4Pd/H,Na−Y, 2.8Pd/H,Na−Y, and 2.3Rh/H,Na−Y are very high and near the limit of the time resolution of the in situ 1H MAS NMR method applied in the present work. The curves in Figures 2 and 3, and the corresponding velocity rate constants, k, and reaction rates, r, in Table 1 indicate that, independent of the noble metal content, the sequence of the hydrogenation activity of the zeolite catalysts under study is Pd/H,Na−Y > Rh/H,Na−Y > Pt/H,Na−Y > Ir/ 2287

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The Journal of Physical Chemistry C H,Na−Y. For zeolites Rh/H,Na−Y and Pt/H,Na−Y, this sequence is the same as that found in our previous in situ 1H MAS NMR study of the acrylonitrile hydrogenation.12 Considered in detail, however, the velocity rates and reaction rates obtained in ref 12 and in the present work differ, which may be due to the slightly different reaction temperature and the different equipment, i.e. the use of 7 mm and 4 mm in situ MAS NMR probes in ref 12 and in the present work, respectively, leading to different shapes of the catalyst bed and catalyst weights. For the selective hydrogenation of styrene on silicasupported noble metal catalysts, Corvaisier obtained a sequence of the turnover frequencies according to Pd > Rh > Pt ≅ Ir, if exclusively the metals used also in the present work are considered.6 Hence, there is a good agreement between the hydrogenation properties published in the above-mentioned work and in the present study, independent of the specific type of the catalyst system. However, it should be mentioned that also other activity sequences for hydrogenation activities of the noble metal under study were found, e.g. with a reversed sequence for Pd and Rh (i.e., Rh > Pd > Pt) for the hydrogenation of ethylene on noble metal films and on silicasupported noble metals.7,8 3.2. Dehydrogenation of Propane on the NobleMetal-Containing Zeolites Y. As already discussed in section 1, there are no systematic studies of the hydrogenation and dehydrogenation properties of homologous series of zeolites Y loaded with different types and noble metal contents. After studying the intrinsic hydrogenation properties of homologous series of noble-metal-containing zeolites Y, therefore, the dehydrogenation of propane was chosen for investigating the properties of the reversed reaction. For these catalytic experiments, no catalyst deactivation by the reactant propane should occur, but the reaction product propene may cause the formation of byproducts and aromatic coke compounds. However, these side reactions are slow enough and allowed the reproducible determination of the dehydrogenation activities of the noble-metal-containing zeolites Y under study, at least after a short time on stream (TOS). In Figures 4 and 5, the propane conversions on the homologous series of zeolites Y under study with low and high metal contents, respectively, are plotted as a function of time on stream. For all noble-metal-containing zeolites Y, the highest propane conversion was observed at the shortest TOS of 35 min. The most important reaction product at TOS = 35 min was propene with a selectivity of ca. 50−70%, which rapidly increased to 80% after TOS of 70−105 min. The most important byproduct at TOS = 35 min was methane with a selectivity of ca. 25−30% for all zeolite catalysts under study, which rapidly decreased in the same manner as the propene selectivity was increased. The formation of methane can be explained by the hydrocracking of the reaction product propene at Brønsted acidic bridging OH groups (acOH), which have densities of 0.12−0.27 acOH per unit cell (u.c.) for the zeolites Y with low metal contents and 2.10−4.24 acOH/u.c. for the zeolites Y with high metal contents (Supporting Information, Table S1, column 5). The rapid decrease of the methane formation hints to a rapid deactivation of the above-mentioned few Brønsted acidic bridging OH groups, so that propene became the dominating reaction product for large TOS values. In Table 2, the conversions, XC3, of propane, the selectivities, SC3=, to propene, the yields, YC3=, to propene, and the turnover frequencies (TOF) for the dehydrogenation of propane on the

Figure 4. Conversion of propane during the dehydrogenation reaction on Pt-, Ir-, Pd-, and Rh-containing zeolites Y with low noble metal contents. The reaction was performed under continuous-flow conditions with a propane/N2 mixture in the ratio of 2:1, with a weight hourly space velocity of WHSV = 3 h−1, and at the reaction temperature of 828 K.

Figure 5. Conversion of propane during the dehydrogenation reaction on Pt-, Ir-, Pd-, and Rh-containing zeolites Y with high noble metal contents. The reaction was performed under continuous-flow conditions with a propane/N2 mixture in the ratio of 2:1, with a weight hourly space velocity of WHSV = 3 h−1, and at the reaction temperature of 828 K.

noble-metal-containing zeolites Y under study with low and high metal contents are given. Comparison of the TOF values (Table 2, last column) leads to an activity sequence according to Ir/H,Na−Y > Rh/H,Na−Y > Pd/H,Na−Y, independent of the metal content. This activity sequence for the dehydrogenation of propane is the reverse of the activity sequence for the Ir-, Rh-, and Pd-containing zeolites Y in the hydrogenation of acrylonitrile. The 0.8Pt/H,Na−Y and 4.5Pt/H,Na−Y zeolites, on the other hand, show roughly 1 order of magnitude higher TOF values for the dehydrogenation of propane and, therefore, are outside the above-mentioned sequence. 3.3. Discussion of the Hydrogenation and Dehydrogenation Activities of the Noble-Metal-Containing Zeolites Y. The present work offers the possibility of the comparison of the hydrogenation and dehydrogenation 2288

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addition of a hydrogen absorbing compound, such as Zr2Fe,21 the corresponding reaction is enhanced. In the case that the hydrogen eliminating compound is the active site, however, the contrary effect should occur. The high dehydrogenation activities of the Pt/H,Na−Y zeolites under study agree very well with the catalytic properties of Pt-containing solid catalysts described in the literature.25−32 One reason for this property could be the incorporation of hydrogen atoms inside Pt clusters, such as described for Pt13 clusters.33,34 Probably, the HT peak occurring at 848 K in the H2-TPD curve of zeolite 4.5Pt/H,Na−Y hints at the presence of internal chemisorbed hydrogen species inside the Pt clusters. The partial hydrogen saturation of the Pt clusters caused by internal chemisorbed hydrogen species, on the other hand, may accelerate the desorption of H2 formed at the outer surface during the dehydrogenation reaction, which is an important step influencing the reaction kinetics.

activities of homologous series of Rh-, Ir-, Pd, and Pt-containing zeolites Y with two significantly different levels of metal contents. For both metal contents, the same numbers of noble metal atoms (NM) per unit cell (u.c.) were introduced, i.e. 0.4−0.6 NM/u.c. for the catalysts with low metal contents and 2.9−3.4 NM/u.c. for the catalysts with high metal contents (Supporting Information, Table S1, column 3). The dispersions, D, of the noble metal atoms, determined by hydrogen chemisorption (Supporting Information, Table S1, column 4), were found to have typical values for comparable zeolite catalysts in the literature.18−20 Hence, two homologous series of model catalysts were available for the studies in the present work. The hydrogenation activities of the noble-metal-containing zeolites were determined via a very recently introduced method,12 allowing the exclusion of effects of the reactant diffusion and catalyst deactivation on the obtained results. For both series (different metal contents) of noble-metal-loaded zeolites Y, the obtained reaction rates (Table 1, last column) are arranged according to Pd/H,Na−Y > Rh/H,Na−Y > Pt/ H,Na−Y > Ir/H,Na−Y, independent of the metal content. The TOF for the dehydrogenation of the noble-metal-loaded zeolites Y under study in Table 2, last column, have the sequence Ir/H,Na−Y > Rh/H,Na−Y > Pd/H,Na−Y, also independent of the metal content. In this connection, it must be mentioned that the dehydrogenation activities of the Pt/ H,Na−Y zeolites under study were found to be significantly higher (up to 1 order of magnitude) in comparison with those of the other noble-metal-loaded zeolites Y studied in the present work. As an additional method, the temperature-programmed desorption of hydrogen (H2-TPD) of the zeolites Y with higher metal content was investigated (see section 2.1). Interestingly, the positions of the high-temperature peaks (HT) in the H2-TPD curves of the noble-metal-loaded zeolites Y under study were found to be arranged in a sequence according to 4.5Pt/H,Na−Y (848 and 738 K) > 2.8Pd/H,Na− Y (723 K) > 2.3Rh/H,Na−Y (713 K) > 4.7Ir/H,Na−Y (663 K), which agrees well with the sequence of the hydrogenation activities, excluding the 4.5Pt/H,Na−Y zeolite. Generally, strong and weak binding of hydrogen at the metal species in the zeolites Y under study should have very different influences on the reaction kinetics of hydrogenation reactions, with H2 as a reactant, and dehydrogenation reactions, with H2 as a reaction product. Focusing at first on the discussion of the Pd-, Rh-, and Ir-containing zeolites Y, the H2-TPD results indicate that hydrogen is more strongly adsorbed at Pd species and similarly at Rh species, but much weaker at Ir species in zeolites Y. This property may lead to a preferred adsorption of the reactant H2 and a rapid formation of reactive surface hydrogen atoms at Pd and, similarly, at Rh species compared with Ir species in zeolites Y. This property could be the reason for the higher hydrogenation activity of Pd/H,Na−Y and Rh/ H,Na−Y zeolites compared with Ir/H,Na−Y zeolites. For the dehydrogenation of propane, the stronger adsorption of hydrogen at Pd species, and similarly at Rh species, compared with Ir species may slow down the desorption of the reaction product H2 from Pd and, similarly, from Rh species compared with Ir species. This hindered H2 desorption from Pd and Rh species could be the reason for their lower dehydrogenation activity compared with Ir species in zeolites Y. The strong effect of a hydrogen elimination on reactions which contain a dehydrogenation step is well accepted.21−24 In the case of an

4. CONCLUSIONS In the present work, a recently introduced in situ solid-state NMR technique was utilized for studying the intrinsic hydrogenation activity of noble-metal-containing zeolites Y allowing the exclusion of an influence of reactant diffusion and catalyst deactivation.12 This experimental approach is based on the in situ 1H MAS NMR investigation of the hydrogenation of preloaded acrylonitrile under semibatch conditions. The determined reaction rates for the acrylonitrile hydrogenation give an activity sequence of Pd/H,Na−Y > Rh/H,Na−Y > Pt/ H,Na−Y > Ir/H,Na−Y, independent of the metal content. For the dehydrogenation of propane on the noble-metal-containing zeolites Y, studied under continuous-flow-conditions, the experimentally obtained turnover frequencies correspond to an activity scale of Ir/H,Na−Y > Rh/H,Na−Y > Pd/H,Na−Y, in this case, excluding the most active Pt/H,Na−Y zeolites. For the zeolite catalysts with high metal contents, H2-TPD studies were performed and gave positions of the high-temperature peaks (HT) in the sequence of 2.8Pd/H,Na−Y (723 K) > 2.3Rh/H,Na−Y (713 K) > 4.7Ir/H,Na−Y (663 K). Based on these experimental data, possible relationships between the strength of the hydrogen adsorption at the noble metal species inside the zeolite catalysts under study and their hydrogenation and dehydrogenation properties were considered. The high temperatures of the HT peaks for the Pd- and Rh-containing zeolites Y indicate strong H2/metal interactions, which may cause a preferred H2 adsorption and formation of reactive surface hydrogen atoms, and, therefore, may explain the high hydrogenation activities of these zeolites compared with the Ir/H,Na−Y zeolite. In the case of the dehydrogenation reaction with H2 as a reaction product, the strong adsorption of H2 at Pd and similarly at Rh species in Pd/H,Na−Y and Rh/ H,Na−Y zeolites, on the other hand, hinder the desorption of this reaction product, which leads to a lower dehydrogenation activity of these types of zeolite catalysts compared with Ir/ H,Na−Y zeolites. The chemisorption of hydrogen atoms inside the Pt clusters may be the reason for the high dehydrogenation activity of the Pt/H,Na−Y zeolites. The partial saturation of the Pt clusters with internal hydrogen atoms should be accompanied by a rapid desorption of H2 formed as product molecules during the dehydrogenation reaction at the surface of the Pt clusters. 2289

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b11367. Additional information on the in situ flow MAS NMR equipment and examples of 1H MAS NMR spectra, temperature-programmed hydrogen desorption curves, sample assignments, and product selectivities of the dehydrogenation of propane on the different metalloaded zeolite catalysts (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by Deutsche Forschungsgemeinschaft and Baden-Württemberg Stiftung is gratefully acknowledged. Furthermore, we thank Jens Weitkamp for most valuable discussions.

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REFERENCES

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