Hydrogenation of Unsaturated Six-Membered Cyclic Hydrocarbons

Jun 2, 2016 - In the hydrogenation of 1,3-cyclohexadiene and 1,4-cyclohexadiene, all NMR signals of cyclohexene exhibited PHIP effects, implying migra...
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Hydrogenation of Unsaturated Six-Membered Cyclic Hydrocarbons Studied by the Parahydrogen-Induced Polarization Technique Dudari B. Burueva,†,‡ Oleg G. Salnikov,†,‡ Kirill V. Kovtunov,*,†,‡ Alexey S. Romanov,†,‡ Larisa M. Kovtunova,§,‡ Alexander K. Khudorozhkov,§,‡ Andrey V. Bukhtiyarov,§,‡ Igor P. Prosvirin,§,‡ Valerii I. Bukhtiyarov,§,‡ and Igor V. Koptyug†,‡ †

International Tomography Center, SB RAS, 3A Institutskaya St., 630090 Novosibirsk, Russia Novosibirsk State University, 2 Pirogova St., 630090 Novosibirsk, Russia § Boreskov Institute of Catalysis, SB RAS, 5 Acad. Lavrentiev Pr., 630090 Novosibirsk, Russia ‡

S Supporting Information *

ABSTRACT: Parahydrogen-induced polarization (PHIP) is an efficient technique for mechanistic investigations of homogeneous and heterogeneous catalytic hydrogenations. Herein, heterogeneous gas phase hydrogenation of six-membered cyclic hydrocarbons (benzene, toluene, cyclohexene, 1,3-cyclohexadiene and 1,4-cyclohexadiene) over Rh/TiO2, Pd/TiO2, and Pt/TiO2 catalysts was studied using PHIP. As expected, cyclohexene hydrogenation led to the formation of cyclohexane which because of its symmetry should not exhibit any PHIP effects. However, the presence of 13 C nuclei at natural abundance (1.1%) breaks molecular symmetry, resulting in the observation of 13C satellite signals exhibiting PHIP effects in the 1H NMR spectra. In experiments with cyclohexene, the reactant’s NMR signals were also polarized, demonstrating the possibility of cyclohexene dehydrogenation to 1,3-cyclohexadiene and subsequent hydrogenation to cyclohexene. In the hydrogenation of 1,3cyclohexadiene and 1,4-cyclohexadiene, all NMR signals of cyclohexene exhibited PHIP effects, implying migration of CC bonds in 1,4-cyclohexadiene and cyclohexene. At the same time, upon hydrogenation of benzene and toluene the reaction products were those with saturated cycles exclusively (cyclohexane and methylcyclohexane, respectively), and their NMR signals were not polarized. The absence of PHIP effects for arene hydrogenation can be explained by a difference in the reaction mechanism compared to cyclohexane and cyclohexadienes hydrogenations, along with the larger extent to which hydrogen atoms undergo migration on the catalyst surface facilitated by lower catalyst coverage with an adsorbed substrate in case of arenes.

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INTRODUCTION The removal of aromatic compounds from fuels via hydrogenation is a major step in the petroleum industry due to environmental pollution problems.1 Therefore, the design of efficient catalysts for the petroleum refining processes is highly important. In recent decades, a large number of studies have been devoted to the design and optimization of catalysts active in arene hydrogenation.1−4 Typical industrially used catalysts are based on inexpensive transition metals such as Ni, Co, Mo, and W.4 However, these catalysts are active at relatively high temperatures when thermodynamic equilibrium shifts toward dehydrogenation processes (arene hydrogenation is a strongly exothermic reaction).1 On the other hand, catalysts based on noble metals are generally more active and can be used at significantly lower reaction temperatures.1 A typical reaction for testing the activity of these catalysts is hydrogenation of benzene to cyclohexane. Understanding the detailed mechanism of benzene hydrogenation is a key to designing an efficient catalytic system for this reaction. Because of this, many kinetic,5−8 spectroscopic,9 and quantum chemical simulation studies10,11 were performed for the benzene hydrogenation reaction. However, there is still no clear picture for the mechanism of benzene hydrogenation. Available results indicate © 2016 American Chemical Society

that benzene hydrogenation follows the Horiuti−Polanyi mechanism,12 which assumes a sequential addition of six hydrogen atoms. Although 14 possible reaction paths can be proposed in this case,13 DFT calculations for Pt(111) indicate that the preferable pathway is an addition of three H atoms in positions 1, 3, and 5 of the benzene ring;10 nevertheless, most kinetic models for benzene hydrogenation are based on a sequential hydrogen addition via formation of cyclohexadienes and cyclohexene. At the same time, Kehoe et al. correlated their results for benzene hydrogenation over nickel catalyst with the Eley−Rideal mechanism;5 this model implies that benzene is adsorbed on the catalyst surface and reacts with hydrogen molecules residing in the gas phase. Therefore, the true mechanism of benzene hydrogenation is still unclear. In this work, we made an attempt to get insight into this mechanism with the use of the parahydrogen-induced polarization (PHIP) technique. PHIP effects can be observed by NMR in the case of pairwise addition of parahydrogen (p-H2) to an asymmetric unsaturated substrate molecule.14 Pairwise hydrogen addition Received: March 31, 2016 Revised: June 2, 2016 Published: June 2, 2016 13541

DOI: 10.1021/acs.jpcc.6b03267 J. Phys. Chem. C 2016, 120, 13541−13548

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hexadiene were used as references. The binding energy was calibrated using the position of the Ti 2p peak (BE = 458.8 eV) corresponding to Ti4+ in TiO2. It should be mentioned that the sample loading process was not longer than 1 min in order to minimize the contact of the sample with air. Spectral analysis and data processing were performed with XPS Peak 4.1 software.27 For quantitative analysis the integral intensities of the spectra (Ti 2p, C 1s, and Rh 3d) were corrected by their respective atomic sensitivity factors.28 Hydrogenation Reactions and NMR Experiments. Commercially available hydrogen, benzene, toluene, cyclohexene (Sigma-Aldrich, ≥ 99%), 1,3-cyclohexadiene (SigmaAldrich, 97%), and 1,4-cyclohexadiene (Sigma-Aldrich, 97%) were used as received. Rh/TiO2, Pt/TiO2, and Pd/TiO2 catalysts were used for heterogeneous hydrogenation experiments. The catalysts (30 mg) were placed at the bottom of a 10 mm NMR tube located inside a 300 MHz Bruker AV 300 NMR spectrometer and maintained at 130 °C. For PHIP experiments, hydrogen gas was enriched with parahydrogen up to 83−85% using a Bruker parahydrogen generator BPHG 90. The mixtures of p-H2 and a substrate (1:6.7 for benzene, 1:25.6 for toluene, 1:7.4 for cyclohexene, 1:12.5 for 1,3-cyclohexadiene, or 1:10.3 for 1,4-cyclohexadiene) were obtained by bubbling parahydrogen through a two-neck flask containing the liquid substrate (the corresponding substrate/p-H2 ratios given in the brackets above were calculated according to the vapor pressure data). The resultant gas mixtures were supplied through a 1/16 in. OD PTFE capillary to the NMR tube containing a catalyst, and the effluent gas was dumped into a fume hood. Therefore, hydrogenations occurred at the 7.1 T magnetic field of the NMR spectrometer (PASADENA experiment14) at atmospheric pressure. The gas flow rate was varied from 3.1 to 5.7 mL/s with the use of an Aalborg rotameter. 1H NMR spectra were acquired using a π/4 radiofrequency pulse, which maximizes the PASADENA signal.29 Spectral Simulations. All 1H NMR spectra simulations were based on the conventional spin density matrix calculations using a π/4 radiofrequency pulse as in the experiments. The NMR line width (full width at half-maximum) was set to 10 Hz to match that in the experimental spectra. The values of 1H−1H and 13C−1H J-coupling constants were taken from the literature.30−32 a. Cyclohexane. Because of computer memory restrictions, an equatorial 1H nucleus at the carbon atom farthest from parahydrogen-derived protons was not considered in spectral simulations. Because hydrogenation of CC bonds usually proceeds as a syn addition,33 the first parahydrogen-derived proton was placed in the equatorial position of the cyclohexane ring and the second one in the axial position. The PHIP spectrum was calculated as a sum of six NMR spectra with different positions of the 13C nucleus. The thermal equilibrium and the PHIP NMR spectra were combined using the formula

means that two H atoms from one p-H2 molecule are incorporated into the same molecule of reaction product or intermediate as a pair. PHIP effects result in a characteristic antiphase line shape and the significantly enhanced intensity of NMR signals.14 Thus, observation of PHIP patterns in NMR spectra is an indicator of pairwise hydrogen addition, whereas NMR signal enhancement provided by PHIP allows one to detect low-concentrated species such as minor products and reaction intermediates.15,16 Pairwise hydrogen addition is quite common in homogeneous hydrogenations catalyzed by transition metal complexes, and in the last three decades, it was extensively utilized for mechanistic and kinetic studies of these reactions.17 In contrast, for heterogeneous hydrogenations the typical catalysts are supported metal nanoparticles, which favor dissociative adsorption of H2.12,18 This should lead to randomization of H atoms on a metal surface and to an absence of pairwise hydrogen addition. However, contrary to these assumptions, it was demonstrated earlier that PHIP effects can be observed in heterogeneous hydrogenations over supported metal catalysts, meaning that some of the hydrogen molecules can be added to double or triple bonds of substrate molecules in a pairwise manner.19 These results have opened perspectives for the utilization of PHIP for mechanistic and kinetic studies of heterogeneous catalytic processes,20,21 such as hydrogenation of hydrocarbons (propene, propyne, 1,3butadiene, 1-butyne)20,22,23 or α,β-unsaturated carbonyl compounds,24 acetylene oligomerization,25 and hydrodesulfurization of thiophene.26 The three latter studies showed the unique abilities of PHIP to detect products of side reactions as well as to differentiate the possible reaction pathways. Herein, we applied the PHIP approach to study heterogeneous gas phase hydrogenation of unsaturated six-membered cyclic hydrocarbons such as benzene, toluene, cyclohexene, 1,3-cyclohexadiene, and 1,4-cyclohexadiene.

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MATERIALS AND METHODS Catalysts Preparation. The Rh/TiO2, Pt/TiO2, and Pd/ TiO2 catalysts with different metal loadings were prepared by wet impregnation. The TiO2 powder (Hombifine N, SBET = 107 m2/g) was calcined at 500 °C for 2 h prior to use. For the preparation of supported Pt, Pd, and Rh catalysts, aqueous solutions of Pt(II), Pd(II), and Rh(III) nitrates, respectively, were used for impregnation. The impregnation of TiO2 support with a precursor solution was carried out for 1 h at room temperature. The excess solvent was then evaporated and the obtained materials were dried in air at 120 °C for 4 h. The obtained M/TiO2 samples were reduced in a H2 flow at 330 °C for 3 h. X-ray Photoelectron Spectroscopy (XPS) Experiments. The XPS experiments were performed on a SPECS spectrometer equipped with a PHOIBOS-150 hemispherical energy analyzer and the X-ray source with double Al/Mg anode. In the present work, the Al Kα (hν = 1486.6 eV, 200 W) was used as a primary irradiation. The binding energy (BE) scale was precalibrated using positions of Au 4f7/2 (BE = 84.0 eV) and Cu 2p3/2 (BE = 932.67 eV) core level peaks. The samples in the form of small granules were loaded onto a stainless steel mesh spot-welded on the standard SPECS sample holder. Before XPS analysis, the 15 wt % Rh/TiO2 sample was impregnated with various neat substrates (benzene, cyclohexane, cyclohexene, 1,3-cyclohexadiene, or 1,4-cyclohexadiene). Nonimpregnated Rh/TiO2 catalyst and TiO2 support as well as TiO2 impregnated with benzene and 1,3-cyclo-

I = Ith + IPHIP·η ·α13C·φpairwise

where Ith is the thermal NMR spectrum, IPHIP is the sum of the six PHIP NMR spectra, η is the theoretical signal enhancement, α13C = 0.0111 is the natural abundance of 13C nuclei, and φpairwise is the contribution of pairwise hydrogen addition to the overall reaction mechanism. The quantity η was calculated using the formula29 13542

DOI: 10.1021/acs.jpcc.6b03267 J. Phys. Chem. C 2016, 120, 13541−13548

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according to literature data.4,35 As can be expected, no PHIP effects were detected for cyclohexane due to the magnetic equivalence of its protons. However, in the toluene hydrogenation over the same three catalysts the similar results were obtained, that is, the lone reaction product methylcyclohexane was detected without PHIP effects (Figure S2). In methylcyclohexane, equivalence of the ring hydrogen atoms is destroyed by the presence of the methyl group; therefore, the absence of PHIP effects may indicate that toluene hydrogenation is entirely nonpairwise and it is impossible to have two H atoms from the same p-H2 molecule in the product methylcyclohexane. The same explanation is likely applicable to benzene hydrogenation as well. This conclusion is in agreement with the fact that no polarization for reaction products in the hydrogenation of benzene and toluene with parahydrogen could be detected even over the 1 wt % Rh/TiO2 catalyst, which is usually the most active in terms of PHIP.36 To verify our preliminary assumptions about the nonpairwise nature of arene hydrogenation, cyclohexene, 1,3-cyclohexadiene, and 1,4-cyclohexadiene were hydrogenated over Rh/ TiO2, Pt/TiO2, and Pd/TiO2 catalysts with 1 wt % metal loading. As expected, all catalysts were found to be active in cyclohexene gas phase hydrogenation and produced cyclohexane (Figures S3 and 1a,b). Also dehydrogenation to

4χp − 1 3

where f = 20 800 is the signal enhancement at the 7.1 T magnetic field with 100% p-H2, and χp = 0.83 is the parahydrogen fraction in H2 gas used in the experiments. The value of φpairwise was manually adjusted to match the relative intensity of the thermal and PHIP signals observed in the experimental spectra. The estimated value of φpairwise was 0.014, which is in agreement with previous results on hydrogenation of propene and other hydrocarbons over supported metal catalysts.20 b. Cyclohexene. 1H PHIP NMR spectra were simulated for cyclohexene molecules with six different positions of the p-H2derived protons: (i) at C1−C2, (ii) at C2−C3 (in axial position at C3), (iii) at C2−C3 (in equatorial position at C3), (iv) at C3−C4 (in axial position at C3 and equatorial at C4), (v) at C3−C4 (in equatorial position at C3 and axial at C4), and (vi) at C4−C5 (in axial position at C4 and equatorial at C5). Carbon atoms are numbered according to the standard nomenclature of organic compounds. For cyclohexene produced by dehydrogenation of cyclohexane, the statistical mixture of molecules with different positions of p-H2-derived protons was considered; in this case, the probabilities of formation are 1/12 for cyclohexene with p-H2-derived protons at C1−C2 or C2−C3, 2/12 for cyclohexene with p-H2-derived protons at C3−C4 or C4−C5, and 3/12 for cyclohexene with only one or zero p-H2-derived protons, which corresponds to thermal equilibrium polarization. The thermal and the PHIP spectra were combined using the formula I = Ith +

∑ IPHIP,i·η·pi ·φpairwise i

where IPHIP,i are PHIP NMR spectra of cyclohexene molecules with different positions of p-H2-derived protons, and pi are the corresponding probabilities of their formation. The value of φpairwise was manually adjusted to match the shape of the NMR signal of cyclohexene CH group to that observed in the experimental spectrum, and pairwise hydrogen addition selectivity 1.4% was obtained. For cyclohexene produced by addition of p-H2 to 1,3cyclohexadiene, the same formula was used, but only cyclohexene molecules with p-H2-derived protons at C3−C4 were considered with 1/2 probability of formation (because the p-H2-derived protons can be in the axial position at C3 and equatorial at C4, or vice versa). The value of φpairwise was 2% to match the shape of the NMR signal of cyclohexene CH group to that observed in the experimental spectrum.

Figure 1. (a) Reaction scheme of cyclohexene hydrogenation; (b) 1H NMR spectrum acquired during cyclohexene gas phase hydrogenation with p-H2 at 130 °C over 1 wt % Rh/TiO2 catalyst. The spectrum was acquired with 32 signal accumulations while the gas was flowing (5.2 mL/s). (c) Inset showing the 0.0−3.0 ppm part of the experimental 1 H NMR spectrum shown in panel b, and (d) simulated 1H NMR spectrum of a mixture of thermally polarized and hyperpolarized 13Clabeled cyclohexanes.

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RESULTS AND DISCUSSION In this work, we utilized titania-supported catalysts, since they usually provide significantly higher PHIP levels than silica-, alumina-, or zirconia-supported catalysts.20,22,34 The Rh/TiO2, Pt/TiO2, and Pd/TiO2 catalysts were used in the gas phase hydrogenation of five six-membered cyclic hydrocarbons (benzene, toluene, cyclohexene, 1,3-cyclohexadiene, and 1,4cyclohexadiene) with parahydrogen. All catalysts were active in these reactions and gave the same range of products with each substrate. However, the Rh/TiO2 catalysts were the best among these three kinds of catalyst in terms of polarization intensity. In the case of benzene hydrogenation over 5 wt % Rh/TiO2, Pt/TiO2 and Pd/TiO2 catalysts, only cyclohexane was observed as a reaction product (Figure S1), which is quite common

benzene was detected, especially in the case of Pd/TiO2 (Figure S3). An interesting fact is that all NMR signals of cyclohexene, the reactant, exhibited pronounced PHIP effects (Figure 1b). Such observation of reactant polarization in heterogeneous hydrogenations was previously reported for propene hydrogenation over such supported metal catalysts as Rh/TiO2,37 Pt/TiO2, Ir/TiO2,22 immobilized Rh complexes38 and bulk CeO2.39 However, the nature of this “pairwise 13543

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The Journal of Physical Chemistry C replacement”22 of hydrogen is still debated. It is clear that this phenomenon is caused by hydrogen exchange taking place on heterogeneous catalysts, most likely including pairwise hydrogenation and dehydrogenation steps. But the question is what is the order of these steps, that is, is the alkene first hydrogenated to alkane and then dehydrogenated to alkene37 or is it first dehydrogenated to alkyne and then hydrogenated to alkene?22 Herein, we observed “pairwise replacement” like effect in cyclohexene hydrogenation. Cyclohexyne has never been isolated and was only observed at temperatures below −100 °C40 or as a ligand in some metal complexes.41 From this perspective, formation of cyclohexyne as a highly unstable intermediate in cyclohexene pairwise replacement process seems unfavorable. Moreover, the observation of PHIP effects for the CH2 groups of cyclohexene (during cyclohexene hydrogenation with parahydrogen) with much higher intensity than for the CH group (Figure 1b) cannot be explained within this mechanism because hydrogenation of cyclohexyne would put the two H atoms of p-H2 in the two newly formed CH groups. However, these arguments are inapplicable to the dehydrogenation of propene to propyne and are not relevant to the pairwise replacement mechanism proposed in ref 22. Therefore, another possibility is that the PHIP effects observed for cyclohexene result from the pairwise hydrogen addition to cyclohexene followed by dehydrogenation of cyclohexane. We assumed that dehydrogenation of cyclohexane is a statistical process, that is, there is an equal probability for syn elimination of each pair of vicinal H atoms. For this situation, the 1H NMR spectral simulations were performed (see Materials and Methods for details). It was found that in the experimental spectrum, the relative intensities of PHIP signals of cyclohexene CH2 groups compared with those of the CH group were higher than predicted by simulations (Figure S4). It should be noted that longitudinal relaxation times (T1) were 1.23 s for the CH group (labeled as 4a in Figure 1b), 0.58 s for the allylic CH2 group (labeled as 4b) and 0.69 s for the other CH2 group of cyclohexene (labeled as 4c). Therefore, if relaxation phenomena were accounted for in our spectral simulations, the relative polarization for CH2 groups of cyclohexene would be even lower, which is in disagreement with the experimental data. Moreover, if cyclohexane dehydrogenation is not a statistical process, the parahydrogen-derived protons in cyclohexene molecules should be located predominantly at CH and to lesser extent at allylic CH2 groups because the corresponding CH2 groups in cyclohexane are closer to the catalyst surface right after the first cyclohexene hydrogenation step. This should also lead to lower polarization of CH2 groups. Therefore, it is unlikely that pairwise replacement in cyclohexene occurs via formation of polarized cyclohexane and its subsequent dehydrogenation. The third possibility is dehydrogenation of cyclohexene to 1,3-cyclohexadiene with a subsequent addition of H2 to one of the CC bonds. In this case, parahydrogenderived protons end up in the CH2 groups of cyclohexene; therefore, PHIP effects indeed should be more pronounced for CH2 groups as was observed experimentally. The observation of the weak polarization for the NMR signal of the CH group can be explained by migration of CC bond in cyclohexene (see results for hydrogenation of 1,3-cyclohexadiene below). Another argument is the observation of a characteristic shape of NMR lines (stronger emission part for the line at 1.7 ppm than for the line at 2.0 ppm), which was reproduced by spectral simulations (Figure S4). It should be noted that in principle 1,3-cyclohexadiene can be formed by dehydrogenation of

cyclohexene to 1,4-cyclohexadiene with its subsequent isomerization to 1,3-cyclohexadiene. Another possibility is hydrogenation of 1,4-cyclohexadiene and subsequent CC bond migration in cyclohexene. Both of these pathways can lead to the same PHIP NMR spectra as in the case of dehydrogenation to 1,3-cyclohexadiene and, therefore, could not be differentiated. However, dehydrogenation of cyclohexene to 1,3cyclohexadiene looks more preferable due to its higher thermodynamic stability. Thus, our results indicate that the hyperpolarized cyclohexene is most likely formed from 1,3cyclohexadiene. Another interesting fact is that the pronounced PHIP NMR signal was observed at 1.3 ppm in the 1H NMR spectrum (Figure 1b). We assume that this signal corresponds to the hyperpolarized cyclohexane molecules containing 13C nuclei at natural abundance, which amounts to ca. 1.1%. Because of the presence of a 13C nucleus, the symmetry of the cyclohexane molecule is broken, and p-H2-derived protons of cyclohexane become magnetically nonequivalent, leading to the observation of PHIP effects. According to the 13C−1H J-coupling constant of ∼120 Hz, two satellite NMR signals should be observed at ca. ±60 Hz (ca. ±0.2 ppm at 1H frequency of 300 MHz) from the main signal of cyclohexane molecules without 13C nuclei. The signal at 1.3 ppm (labeled as 6 in Figure 1b) is indeed observed in the NMR spectra while the second signal at 1.7 ppm most likely overlaps with the signal of the CH2 group of cyclohexene (labeled as 4c in Figure 1b). To confirm our assignment, the spectral simulations were performed (see Materials and Methods for details). It was found that the presence of 13C nuclei indeed allows one to observe PHIP effects at exactly the same positions as in the experimental spectrum (Figure 1c). By adjusting the contribution of pairwise hydrogen addition to a reasonable value of 1.4%20 we were able to obtain the simulated NMR spectrum of the mixture of thermally polarized and hyperpolarized cyclohexanes, which is in a good agreement with the experimental spectrum (Figure 1d). Our hypothesis was confirmed by additional experiments on the hydrogenation of ethylene to ethane. Similar results were obtained, that is, two PASADENA signals were observed at ca. ±60 Hz (ca. ±0.2 ppm; ethane with a 13C label at natural abundance) from the main signal of unlabeled ethane (Figure S5). Next, gas phase hydrogenation of 1,3-cyclohexadiene and 1,4-cyclohexadiene was addressed. Both substrates were hydrogenated to cyclohexene and cyclohexane or dehydrogenated to benzene (Figures 2, 3, S6, and S7). Similar to cyclohexene hydrogenation, cyclohexane exhibited PHIP NMR signals due to the presence of 13C nuclei at natural abundance. Importantly, in both 1,3-cyclohexadiene and 1,4-cyclohexadiene hydrogenation PHIP effects were observed for all NMR signals of cyclohexene. However, the simple pairwise hydrogen addition to one of CC bonds of 1,3-cyclohexadiene should lead to the observation of polarization for NMR signals of cyclohexene CH2 groups only, and no polarization should be detected for CH groups. The reasonable explanation of the observation of PHIP signals for the CH groups is the migration of CC bond in the polarized cyclohexene formed by pairwise hydrogen addition to one of the CC bonds in 1,3cyclohexadiene. As for 1,4-cyclohexadiene, pairwise addition to its CC bond should lead to formation of cyclohexene with parahydrogen-derived protons at C4 and C5 atoms (corresponding NMR signals are labeled as 4c in Figure 3). According to the NMR spectra simulations, this molecule should exhibit 13544

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cyclohexene, and isomerization of 1,4-cyclohexadiene to 1,3cyclohexadiene is also possible. It was found that in hydrogenation of cyclohexene, 1,3cyclohexadiene, and 1,4-cyclohexadiene, pairwise hydrogen addition is possible, whereas in hydrogenation of benzene and toluene PHIP effects were not observed. One of the possible explanations is based on the hypothesis that hydrogenation of arenes and hydrogenation of alkenes and dienes follow different reaction mechanisms. For example, DFT calculations of benzene hydrogenation on Pt(111) predicted that the most preferable pathway is the step-by-step addition of H atoms in positions 1, 3, and 5 of the benzene cycle with the formation of 1,3,5-trihydrobenzene intermediate.10 In this case, there can be a substantial delay between the additions of the first and the second H atoms to two adjacent carbon atoms (e.g., between H additions in positions 1 and 2), and this delay can be sufficient for surface migration of the second parahydrogen-derived atom far away on the molecular length scale. So, the probability of pairwise hydrogen addition to an arene can be significantly lower than in the case of alkene or diene hydrogenation. The second possible explanation is based on the difference in adsorption strengths of arenes, dienes, and alkenes. The adsorbed substrates can hinder hydrogen migration, leading to higher contribution of pairwise hydrogen addition. The adsorption of six-membered cyclic hydrocarbons has been the subject of several theoretical and experimental studies.42−44 The heats of adsorption of these molecules on Pt(111) were estimated as 37−58 kJ/mol for cyclohexane,42,44 71−81 kJ/mol for cyclohexene,42,43 155 kJ/mol for 1,3-cyclohexadiene,42 142 kJ/mol for 1,4-cyclohexadiene45 and 117−125 kJ/mol for benzene.43,46 For a qualitative comparison of the strength of adsorption of six-membered cyclic hydrocarbons, XPS experiments were performed. Figure 4 presents Rh 3d and C 1s corelevel X-ray photoelectron spectra obtained for the Rh/TiO2 catalyst after treatment with different neat substrates. The Rh 3d5/2 peak at 307.4 eV corresponds to metallic Rh (Rh0) and the peak at 309.2 eV can be attributed to Rh2O3 (Rh3+).28,47,48 The binding energy of the C 1s peak (Figure 4d,e,f) is 284.8

Figure 2. (a) Reaction scheme of 1,3-cyclohexadiene hydrogenation; (b) 1H NMR spectrum acquired during 1,3-cyclohexadiene gas phase hydrogenation with p-H2 at 130 °C over 1 wt % Rh/TiO2 catalyst. The spectrum was acquired with 8 signal accumulations while the gas was flowing (5.2 mL/s).

Figure 3. (a) Reaction scheme of 1,4-cyclohexadiene hydrogenation; (b) 1H NMR spectrum acquired during 1,4-cyclohexadiene gas phase hydrogenation with p-H2 at 130 °C over 1 wt % Rh/TiO2 catalyst. The spectrum was acquired with 8 signal accumulations while the gas was flowing (5.2 mL/s).

PHIP effects, but the shape of its NMR lines should be completely different from those observed experimentally (Figure S8), which, on the other hand, resemble NMR lines of cyclohexene with p-H2 derived protons at C3 and C4 (Figure S4d). So, it is clear that migration of CC bond occurs in

Figure 4. Rh 3d (left panel) and C 1s (right panel) X-ray photoelectron spectra of (a),(d) Rh/TiO2 catalyst impregnated with 1,3-cyclohexadiene; (b),(e) Rh/TiO2 catalyst impregnated with benzene; (c),(f) Rh/TiO2 catalyst without any additives. 13545

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The Journal of Physical Chemistry C eV, which corresponds to the atoms of surface hydrocarbons.28,49 It is clear that the use of different substrates changed the concentration of carbon species on the surface dramatically. It should be mentioned that in Ti 2p photoelectron spectra only one state with a binding energy of 458.8 eV (corresponding to Ti4+) was observed independently of the catalyst treatment. The atomic ratios calculated from XPS data are presented in Table S9. The comparison of C/Rh and C/Ti ratios for the impregnated Rh/TiO2 catalyst calculated from qualitative C 1s XPS data may indicate that 1,3- and 1,4-cyclohexadienes and even cyclohexene adsorb much stronger than benzene. The intensities of C 1s peaks in Figure 4e,f, where Rh/TiO2 without any additives of carbon and previously impregnated with benzene was used, are similar and related to carbon contamination from other sources. This difference in adsorption strength can tentatively explain the mechanistic differences in terms of PHIP effects: adsorbed reactants (cyclohexadienes or cyclohexene) slow down hydrogen migration over the catalyst surface, leading to a higher contribution of pairwise hydrogen addition, which makes observation of PHIP effects possible. The proposed scheme of hydrogenation of six-membered cyclic hydrocarbons on Rh/TiO2, Pd/TiO2, and Pt/TiO2 catalysts is presented in Scheme 1. The following processes

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CONCLUSIONS

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ASSOCIATED CONTENT

In this paper, the heterogeneous gas phase hydrogenation of six-membered cyclic hydrocarbons (benzene, toluene, cyclohexene, 1,3-cyclohexadiene, and 1,4-cyclohexadiene) over Rh/ TiO2, Pd/TiO2, and Pt/TiO2 catalysts was investigated using the parahydrogen-induced polarization technique. In the hydrogenation of cyclohexene, the formation of cyclohexane was observed. Despite the magnetic equivalence of its protons, NMR signals of cyclohexane exhibited PHIP effects observed as two antiphase PASADENA-type satellite signals at ca. ±60 Hz from the main NMR signal of cyclohexane. This fact is explained by the presence of 13C nuclei at natural abundance which breaks the molecular symmetry, resulting in the observation of the satellite PHIP signals. The similar satellite signals were observed in the hydrogenation of ethylene to ethane. Also, in experiments with cyclohexene, the NMR signals of the reactant were polarized, demonstrating experimentally that hydrogen exchange can take place. Possible reaction pathways of this pairwise hydrogen replacement were analyzed using 1H NMR spectral simulations. It was concluded that the most probable pathway is the dehydrogenation of cyclohexene to 1,3-cyclohexadiene with subsequent hydrogen addition to form cyclohexene. These results also are consistent with the reported results for polarized substrate (propene) formation via hydrogenation−dehydrogenation mechanism of pairwise replacement.22 In the hydrogenation of both 1,3cyclohexadiene and 1,4-cyclohexadiene, cyclohexene and cyclohexane were formed. Notably, all NMR signals of cyclohexene exhibited PHIP effects, implying migration of CC bonds in these compounds. At the same time, in the hydrogenation of benzene and toluene, the only reaction products were cyclohexane and methylcyclohexane, respectively. Their NMR signals did not exhibit any PHIP effects, which is possible if the two added H atoms in the reaction product are chemically and magnetically equivalent as in case of unlabeled cyclohexane. However, there is no such equivalence in methylcyclohexane, whereas for cyclohexane, such equivalence is avoided in 13Ccontaining molecules owing to the natural abundance of the 13 C isotope. Indeed, the observation of PHIP effects for the 13C satellites in the 1H NMR spectrum of cyclohexane produced in the hydrogenation of cyclohexene was demonstrated above. Therefore, the absence of PHIP effects in the hydrogenation of benzene and toluene can be explained by the differences in the reaction mechanisms, which makes pairwise hydrogenation of aromatic compounds hardly possible. Another explanation is based on the fact that, according to XPS experiments, adsorption of cyclohexadienes and cyclohexene is stronger than that of benzene. Therefore, in the case of arenes, hydrogenation by random H atoms produced upon dissociative chemisorption of H2 can be easier, leading to an absence of any PHIP effects. Thus, new insights into the mechanisms of the hydrogenation of six-membered cyclic hydrocarbons were proposed based on the study of pairwise hydrogen addition.

Scheme 1. Proposed Scheme of the Hydrogenation of SixMembered Cyclic Hydrocarbons on Rh/TiO2, Pd/TiO2, and Pt/TiO2 Catalystsa

a

Solid arrows show reaction steps that were unambiguously confirmed by the experimental results of this work. Dashed arrows show other possible reactions which were neither confirmed nor disproved. Benzene hydrogenation to cyclohexane can proceed via two possible pathways: (i) stepwise hydrogenation with the formation of cyclohexadiene and cyclohexene intermediates or (ii) direct hydrogenation to cyclohexane.

were unambiguously confirmed by our experimental results: (i) hydrogenation of 1,3- and 1,4-cyclohexadienes and cyclohexene, (ii) dehydrogenation of 1,3- and 1,4-cyclohexadienes and cyclohexene, (iii) CC bond migration in 1,3- and 1,4cyclohexadienes and cyclohexene. As for benzene hydrogenation, it is still not clear whether it is hydrogenated step by step via formation of cyclohexadiene and cyclohexene or it is directly hydrogenated to cyclohexane. The absence of parahydrogen-induced polarization effects in benzene hydrogenation can be explained by peculiarities of the reaction mechanisms or by fast migration of dissociatively chemisorbed H atoms enabled by lower substrate coverage of the catalyst surface.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b03267. Additional figures and the table with atomic ratios calculated from XPS data. (PDF) 13546

DOI: 10.1021/acs.jpcc.6b03267 J. Phys. Chem. C 2016, 120, 13541−13548

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The Journal of Physical Chemistry C

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +7 383 3307926. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. (D.B.B. and O.G.S.) These authors contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

K.V.K., O.G.S., and A.S.R. thank RFBR (16-03-00407-a, 14-0393183-MCX-a, 14-03-00374-a), the Council on Grants of the President of the Russian Federation (MK-4498.2016.3), and the joint SB RASMoST grant for the support of construction of experimental setup and simulations of NMR spectra. I.V.K. and D.B.B. thank Russian Science Foundation (project # 14-1300445) for the support of hydrogenation experiments. V.I.B., I.P.P., A.V.B., A.K.K., and L.M.K. thank Russian Science Foundation (project # 14-23-00146) for the support of catalysts preparation and XPS analysis. The basic funding for ITC SB RAS team was provided by FASO Russia (project # 0333-2014-0001), which supported the NMR studies of the relaxation times of substrates and products under investigation.

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ABBREVIATIONS PHIP, parahydrogen-induced polarization; NMR, nuclear magnetic resonance; DFT, density functional theory; p-H2, parahydrogen; XPS, X-ray photoelectron spectroscopy; BE, binding energy; OD, outer diameter; PASADENA, parahydrogen and synthesis allow dramatically enhanced nuclear alignment

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REFERENCES

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