Study of Catalytic Sites on Ruthenium For Hydrogenation of N

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J. Phys. Chem. C 2010, 114, 9720–9730

Study of Catalytic Sites on Ruthenium For Hydrogenation of N-ethylcarbazole: Implications of Hydrogen Storage via Reversible Catalytic Hydrogenation Katarzyna Morawa Eblagon,† Kin Tam,‡ K. M. Kerry Yu,† Shu-Lei Zhao,§ Xue-Qing Gong,§ Heyong He,| Lin Ye,| Lu-Cun Wang,| Anibal J. Ramirez-Cuesta,⊥ and Shik Chi Tsang*,† Department of Chemistry, Wolfson Catalysis Centre, UniVersity of Oxford, South Parks Road, Oxford, OX1 3QR, U.K., AstraZeneca, Alderley Park, Macclesfield, Cheshire SK10 4TG, U.K., Laboratories for AdVanced Materials, Research Institute of Industrial Catalysis, East China UniVersity of Science and Technology, 130 Meilong Road, Shanghai 200237, People’s Republic of China, Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and InnoVatiVe Materials, Fudan UniVersity, Shanghai 200433, People’s Republic of China, and Rutherford Appleton Laboratory, TOSCA, ISIS Facility, Building R3, Room 1-42, Chilton, Didcot, Oxon, OX11 0QX, U.K. ReceiVed: September 7, 2009; ReVised Manuscript ReceiVed: April 6, 2010

Hydrogen storage is a significant challenge for the development and viability of hydrogen-powered vehicles. Storage of molecular hydrogen in nitrogen-substituted polyunsaturated aromatic organic molecules through reversible catalytic hydrogenation and dehydrogenation is a promising approach. The success of developing a catalytic hydrogen storage concept is highly dependent on finding an efficient catalyst; however, understanding how molecules interact with metal catalytic sites is, at present, rather limited. In this work, a combined experimental and theoretical study is conducted to identify efficient catalytic sites on metallic surfaces and to understand the reaction mechanism for the forward hydrogenation reaction. It is clearly revealed from experimentation that hydrogenation of N-ethylcarbazole, a typical nitrogen-substituted polyunsaturated aromatic organic molecule, is taking place in a stepwise manner over metal catalysts. Because of steric constraints at terrace sites, the kinetically stable pyrrole intermediate, formed by partial hydrogenation of N-ethylcarbazole, cannot be readsorbed once desorbed into solution. Therefore further hydrogenation occurs at the low coordinated sites where no similar steric hindrance is encountered. Thus, the mechanism for hydrogenation involves an unusual shuttling of partially hydrogenated intermediates from terrace sites to higher indexed sites via solution. First-principles calculations confirm that the pyrrole intermediate can strongly adsorb to various low coordination sites, typically steps on the vicinal (109) surface, while the adsorption is extremely weak on flat (001) terraces. This work is the first example of catalytic site analysis to account for observed activity, selectivity and recyclability of a typical metal catalyst for catalytic hydrogen storage, which could lead to rational design of superior materials. Introduction The fossil fuels that are today the primary energy source are diminishing very rapidly; moreover, production of carbon dioxide from their use is a significant contributor to global warming.1,2 Thus, replacing fossil fuels with clean, sustainable energy sources is of paramount importance.3 Hydrogen was found to be a promising energy carrier; however, its storage is a major technical barrier to widespread application.4 An extensive effort to develop solid hydrogen storage systems using inorganic metal hydrides, complex hydrides, activated charcoals, and advanced carbons is currently being made.4 DuPont and others have recently reported hydrogen-storage materials based on imidazolium ionic liquids.5 Unfortunately, up to the date, no candidate material has been identified possessing the right combination of the required characteristics and low cost for commercial vehicular application. It is partly because most materials that have sufficiently low enthalpies of hydrogen * To whom correspondence should be addressed. E-mail: edman.tsang@ chem.ox.ac.uk. † University of Oxford. ‡ AstraZeneca. § East China University of Science and Technology. | Fudan University. ⊥ TOSCA, ISIS Facility.

adsorption are unable to take up sufficient quantities of hydrogen under ambient conditions, while materials with excessive enthalpies require unsuitably high temperatures for hydrogen desorption. Using polyunsaturated organic compounds to store hydrogen is an attractive solution due to their potential high hydrogen weight capacities. Organic molecules that can undergo reversible hydrogenation and dehydrogenation reactions (e.g., cyclohexane, decalin) have been investigated by a number of research groups.6–12 One promising approach based on reversible hydrogenation using nitrogen substituted polyaromatic hydrocarbons, such as N-ethylcarbazole, as liquid carriers has recently been proposed by Air Products in their patent.13 The substitution of nitrogen in the polyaromatic rings tunes the enthalpies and facilitates reversible hydrogenation and dehydrogenation under mild conditions.14 It is claimed that gravimetric capacities of up to 7.2 wt % H2 and volumetric capacities of up to 69 g H2/L can be achieved based on reversible catalytic hydrogenation of a more highly unsaturated compound, the phenanthroline, over proprietary catalysts.13,14 It is therefore relevant to explore the potential opportunities and challenges of N-ethylcarbazole as a carrier for hydrogen storage system. The dehydrogenation of N-ethylcarbazole has already been extensively studied,13,15 however there are few indepth studies concerning catalytic hydrogenation of nitrogen-

10.1021/jp908640k  2010 American Chemical Society Published on Web 05/06/2010

Catalytic Sites on Ru For Hydrogenation of N-ethylcarbazole

J. Phys. Chem. C, Vol. 114, No. 21, 2010 9721

SCHEME 1: Stepwise Hydrogenation of N-Ethylcarbazole to Perhydro-N-ethylcarbazole over Noble Metal Catalysts (the Structures of the Starting Material, +4[H], and +8[H] Come from Experimental Assignments from NMR Experiments)a

a The reactants and intermediates are named as follows: starting material, N-ethylcarbazole; +4[H]- N-tetrahydroehylcarbazole; +6[H]N-hexahydroethylcarbazole; +8[H]- N-octahydroethylcarbazole; and +12[H]- N-perhydroethylcarbazole.

Figure 1. Comparison of catalytic performance (conversion and selectivity) for hydrogenation of N-ethylcarbazole to the sum of perhydro-N-ethylcarbazole isomers over different supported Ru catalysts quenched after 3 h at 130 °C and 70 bar H2.

containing polyaromatic systems. Only very recently, an investigation on hydrogenation ofN-ethylcarbazole using a commercially available ruthenium catalyst has been initiated, by Sotoodeh et al.15 In this study, the detailed structure-catalysis relationship in hydrogenation of N-ethylcarbazole has been elucidated. It is believed that the results may lead to rational design of required catalysts for reversible hydrogen storage using liquid organic hydrides (LOH) approach. Experimental Section Mild chemical reduction was used for preparing ruthenium metal supported on high surface area silica-alumina, alumina, and rutile supports. An appropriate amount of ruthenium chloride hydrate salt was dissolved in 10 mL of distilled water. This solution was mixed with the support. After 48 h of stirring the precursor mixture was reduced by adding dropwise solution of sodium borohydride (0.337 g in 10 mL of distilled water) at 90 °C. The resulting material was extensively washed with distilled water followed by acetone and dried under ambient conditions. Ruthenium black and 5% Ru/alumina were purchased from

Sigma-Aldrich and pretreated at 400 °C under hydrogen atmosphere. The hydrogenation of N-ethylcarbazole was conducted in 300 mL stainless steel Parr autoclave reactor using a magnetic stirrer. N-ethylcarbazole (3 g) was dissolved in 100 mL of cyclohexane and 0.15 g of catalyst was added. Pure liquid substrate would be used without solvent for practical hydrogen storage but at present the high hydrogen uptake rate was moderated with solvent inclusion for kinetic study and product monitoring. It was found that the presence of the solvent did not affect the selectivity of the reaction and the conversion was also not altered significantly. Slightly faster rates were observed in the presence of the solvent, probably due to mass transfer limitations present in the melt. The reaction was carried out at 130 °C under 70 bar of hydrogen pressure over a 20 h test period. Reaction mixtures during and after hydrogenation were analyzed by GC-MS (Agilent 6890GC-MS) equipped with a nonpolar capillary column (Agilent 19091s-433) and autosampler. The DRIFT spectra were recorded using Bruker Vector 22 FT-IR spectrometer equipped with Spectra-Tech Diffuse Reflectance Accessory and a high-temperature in situ cell with ZnSe windows. A KBr beam splitter was used with a DTGS detector. A 0.1 g catalyst was first prereduced at 300 °C for 4 h under atmospheric pressure by a 30 mL min-1 stream of 5% H2 and cooled to room temperature under He at the same flow rate. A stream of 5% CO in He gas was introduced for 15 min before it was flushed with argon (99.99%) at the same flow rate. IR spectra were then collected. The Brunauer-Emmet-Teller (BET) specific surface areas of the calcined catalysts were determined by adsorption-desorption of nitrogen at liquid nitrogen temperature using Micromeritics TriStar 3000 equipment. Sample degassing was carried out at 300 °C prior to acquiring the adsorption. CO chemisorption was employed for the evaluation of the active metal dispersions and metal surface areas of the selected catalyst samples. The measurements were performed using Coulter Omnisorp 360CX apparatus accurate

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TABLE 1: Physical Characterization of Ru Catalytic Systems catalyst

particle size (TEM) [nm]

BET area [m2/g]

metal area [m2/g]

dispersion [%]

commercial 5 wt % Ru on alumina 5 wt % Ru on silica-alumina 5 wt % Ru on rutile 5 wt % Ru on alumina commercial Ru black

9.08 2.37 2.11 1.61 6.30

83.59 483.31 191.65 152.15 22.20

3.91 9.82 11.30 10.96 4.78

16.69 41.89 48.21 46.75 1.01

TABLE 2: Relative Chemisorbed CO Peak Areas and Corresponding Area Ratios of Multi- to Bridging Modes CO adsorption modes catalyst commercial 5 wt % Ru on alumina 5 wt % Ru on silica-alumina 5 wt % Ru on rutile 5 wt % Ru on alumina

linear (2110-2130 cm1) multi- (2040-2100 cm-1) bridging (1975-2025 cm-1) multicarbonyl/bridging 0.09 0.29 0.27 0.23

to (5%. For the chemisorption procedure, the solid sample was prereduced at 300 °C for 1 h in pure hydrogen with a ramp rate of 10 °C/min and then cooled to room temperature before the CO chemisorption was conducted. The gas used for the measurements was 99.9% pure CO. The DFT plane-wave calculations were performed using the Vienna ab initio simulation package (VASP) with the projectoraugmented wave method (PAW).16,17 The exchange-correlation function was treated with the generalized gradient approximation (GGA) of the Perdew-Wang (PW91)16,18 functional. The cutoff energy was 350 eV. The Ru(001) and (109) surfaces were modeled with slab models. For Ru(001), a 5 × 5 super cell of four-layer slab was used with a 16 Å vacuum gap perpendicular to the surface. For Ru(109), a 1 × 5 super cell of six-layer slab was used with a 18 Å vacuum gap perpendicular to the surface. These surface models are big enough to exclude interatomic interaction between neighboring cells. The Brillouin zone was sampled with the (1 × 1 × 1) Monkhorst-Park mesh k-points. In the calculations, all the atoms except those at the bottom two layers of the slabs were allowed to relax, and the force threshold was 0.05 eV/Å. Results and Discussion Site Dependent Catalysis. A range of supported metal catalysts containing Ru for hydrogenation of N-ethylcarbazole were studied. It is interesting to find that the catalytic hydrogenation appeared to occur in stepwise manner. Among our reaction products analyzed using GC-MS, high quantities of +4[H] and +8[H] and trace levels of +6[H] were detected.

0.55 0.86 0.76 1.28

0.22 0.15 0.14 0.20

2.51 5.58 5.47 6.37

All of these compounds were previously observed also using GC-MS by Sotoodeh et al.15 The simplified version of the reaction pathway is shown in Scheme 1. Supported ruthenium catalysts produced via chemical reduction were found to show a higher activity for the hydrogenation of N-ethylcarbazole to N-perhydroethylcarbazole (+12[H]) than commercial catalysts under our reaction conditions (Figure 1). Different ruthenium catalysts were tested for the hydrogenation reactions, and their particle sizes, metal and total surface areas and dispersions are given in Table 1. It is surprising to find that there were large differences in both activity and selectivity toward the +12[H] products among the Ru catalysts with the same metal content. As it can be seen from Figure 1, the commercial 5 wt % Ru on alumina (pretreated at 400 °C) displays poor activity and selectivity while the same catalyst formulation synthesized with milder chemical reduction (sodium borohydride, as a reduction agent) at 90 °C gives excellent activity and selectivity. No evidence of catalyst modification by the inorganic residues was obtained. The other two supported Ru catalysts give moderate activities but the selectivity toward the +12[H] products is below 40%, indicating that the rates of these stepwise hydrogenation reactions were very low. Careful inspection revealed that the activity and selectivity were inversely related to the size of the metal particle. The smallest particle size gave both the highest activity and selectivity. The evolution of the substrate and product concentrations along the reaction time in the case of 5 wt % Ru on rutile is plotted in Figure 2a. For comparison, product distribution in

Figure 2. Substrate and product concentrations versus time in the hydrogenation of N-ethylcarbazole to perhydro-N-ethylcarbazole over (a) 5 wt % Ru/TiO2 (left) and (b) 5 wt % Ru/alumina (right) at 130 °C and 70 bar of H2.


Figure 3. Comparison of different CO adsorption modes on supported Ru catalysts. Three different adsorption species, namely linear (terminal) carbonyl (2110-2130 cm-1), multicarbonyl (2040-2100 cm-1), and bridging carbonyl (1975-2025 cm-1) are visible.

the system with 5 wt % Ru/alumina is shown in Figure 2b. It is interesting to note that “volcano” profiles of some hydrogenated intermediates, particularly the +4[H] and +8[H], are obtained, suggesting their continuous formation and further conversion during the batch hydrogenation. According to this profile, the +8[H] seems to be formed from +4[H] in a stepwise manner. It takes much longer time for the less active 5 wt % Ru on rutile catalyst to convert the +8[H] to the +12[H] products. On the other hand, +8[H], the pyrrole intermediate rapidly attenuates during the hydrogenation and no +4[H] is detected over the active 5 wt % Ru on alumina. As far as the catalytic hydrogen storage for practical use is concerned, an efficient catalyst must be capable of promoting both hydrogenation and dehydrogenation reactions. The rapid interconversion of various intermediate substances between the unsaturated and fully hydrogenated forms will lead to a maximum of hydrogen being stored and delivered at acceptable rates. The formation of kinetically stable intermediates may cause problems in the recyclability of the process. It is believed that the rate of desorption of these intermediates from surface to solution and their subsequent conversion on surface are related to the kinetic


Figure 5. Substrate and product concentrations versus time in the hydrogenation of N-ethylcarbazole to N-perhydroethylcarbazole over Ru black at 130 °C and 70 bar of H2.

stability of the molecules as well as hydrogenation mechanisms. For the stepwise hydrogenation, one would expect that Nethylcarbazole would be different regarding the adsorption enthalpy and geometry from the partial hydrogenated intermediates. This may account for desorption of the intermediates (for example +8[H]) and their readsorption to other metal sites for further conversion. To elucidate these results, it is important to identify different surface sites on the supported Ru particles. Thus, CO chemisorptions was employed to examine surface sites of the prereduced catalysts examining different CO adsorption modes. The relative peak areas were calculated from the IR data and the results are presented in Table 2. Figure 3 shows the bands representing adsorption of CO molecules on nanosized Ru metal at room temperature, where linear (terminal) carbonyl (2110-2130 cm-1), multicarbonyl (2040-2100 cm-1), and bridging carbonyl (1975-2025 cm-1) species are clearly visible. The wavenumber values are dependent on the specific support present due to varied metal-support interactions.19 It is well-known that the linear and multicarbonyl CO adsorbed species favorably occur at low coordination metal sites (corner, edge, etc), while the bridging CO species prefers flat and highly coordinated metal

Figure 4. HRTEM images of 5 wt % Ru on alumina showing many small nanoparticles (1.5-2.0 nm) (left) and a commercial Ru black showing large faceted Ru particles (10-20 nm, strongly sintered) (right).

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Figure 6. (a) Calculated structure of ethylcarbazole adsorbed on terrace of Ru(001); (b) energetically favorable desorption of +8[H] intermediate from Ru(001); (c) ethylcarbazole adsorbed on the stepped Ru(109) with two atomic height. E(a) ) 2.56 eV E(b) ) 0.32 eV, E(c) ) 1.61 eV.

Figure 7. (a) Calculated structure of one-atom-height step of Ru(109); (b) calculated structure of adsorbed +8[H] intermediate at the step edge. The arrow in black specifies the ethyl group.

sites.20,21 The observed low ratio of multicarbonyl to bridging CO species over the commercial Ru/alumina is in a good agreement with the fact that large metal particles (synthesized at high temperature) contain large proportion of terrace sites. Similarly, the 5 wt % Ru on alumina catalyst prepared under mild chemical reduction contains smaller metal particles having large number of low coordination metal sites, which leads to high ratio of multicarbonyl to bridging CO modes (see Table 2). The TEM image of the 5 wt % Ru on alumina in Figure 4 (left-hand side image) indeed suggests the existence of small and disordered metal particles. The low coordination metal sites could therefore account for the extremely high catalytic activity and the rapid conversion of the intermediates to the fully hydrogenated product. Such a catalyst containing “defective” particles with steps is capable of hydrogenating most substrate molecules to fully hydrogenated product within 30 min, reaching the theoretical 5.7 wt % hydrogen storage value. On the other hand, the faceted particles are rather inefficient in further hydrogenation of intermediate(s) like +8[H] hence accounting for the accumulation of this intermediate over those samples with lower multicarbonyl to linear ratios. For comparison, it is also interesting to investigate the role(s) of the terrace sites in this stepwise catalytic hydrogenation. We therefore studied the performance of commercial Ru black sample (manufactured at high temperatures), which is known to contain a large proportion of terrace sites in the hydrogenation reactions. Figure 4 (righthand side image) shows the TEM image of the faceted particles of the Ru black sample. Figure 5 shows that the Ru black catalyst predominated by faceted sites is able to hydrogenate the N-ethylcarbazole to the +12[H] products most likely via a stepwise pathway on the surface before desorption of the end product. However, it also gives a high concentration of the partially hydrogenated product, +8[H], in solution. The accumulation of +8[H] clearly indicates

that the catalyst is indeed inefficient with further hydrogenation of the soluble +8[H] species to +12[H] even in the presence of an excess of hydrogen. This is in line with what was proposed earlier, that large proportions of terrace sites relative to lower coordination sites (large particle size) are poor in adsorption/ readsorption of +8[H] intermediate for further hydrogenation. This point has not been previously addressed in the literature. Theoretical Consideration. Density functional theory (DFT) calculations are a powerful tool to estimate the adsorption enthalpy of molecules on metal and metal oxide surfaces.22,23 In this work, DFT calculations were used to investigate adsorption of N-ethylcarbazole and the N-octahydroethylcarbazole (+ 8[H]) intermediate on both terrace and step sites (low coordination sites) on ruthenium surface. In the calculations, slabs of the Ru(001) and Ru(109) were used to model flat terraces and surfaces containing low-coordinated step sites, respectively. Adsorption energies Ea were estimated by using the equation Ea ) -[Eslab+Molecule - (Eslab + EMolecule)],where the Eslab+Molecule denotes the total energy of slab with adsorbed molecules, and Eslab and EMolecule are the total energies of ruthenium slabs and the single molecules in gas phase, respectively. In Figure 6a,c, the calculated structures of N-ethylcarbazole adsorbed at flat Ru(001) as compared to step Ru(109) are shown. The calculated adsorption energy for the former is 2.56 eV, which indicates that the N-ethylcarbazole more strongly adsorbs on the flat terrace sites than on typical step sites (1.61 eV). It can be expected that the high adsorption energy is due to the planar-like polyaromatic rings favorably interacting with the “flat” surface. We also tested the effect of coadsorbed hydrogen atoms on the adsorption of N-ethylcarbazole by filling the neighbor surface hollow sites around the adsorbed rings with hydrogen atoms, and it was found that such treatment gives nearly an identical adsorption energy (∼2.6 eV) for Nethylcarbazole. This indicates that the coadsorbed hydrogen, if



Figure 8. (a,c,e) Calculated structures of two-atom-height steps of Ru(109). (b,d,f,g) Corresponding adsorption structures of +8[H] intermediate on the surfaces.

SCHEME 2: Chemical Structures of All Six Possible Stereoisomers of the N-Perhydroethylcarbazole That Are Theoretically Distinguishable by Means of NMRa

a Absolute configuration shown arbitrarily with partial numbering of carbon atoms used for signal assignments. From upper left: A, sym-4H (4aβ4bβ8aβ9aβ); B, sym-2H,2H (4aβ4bR8aR9aβ); C, sym-2H,2H (4aβ4bβ8aR9aR); D, sym-2H,2H (4aβ4bR8aβ9aR); E, assym-3H,1H (4aβ4bβ8aR9aβ); F, assym-3H,1H (4aβ4bR8aβ9aβ).26

involved in the following hydrogenation reactions, would not affect the adsorption of N-ethylcarbazole. However, the adsorption of the +8[H] intermediate (with the pyrrole ring) in the

hydrogenation process on terrace sites, as shown in Figure 6b, gives Ea as low as 0.32 eV with the ethyl group facing up and a neglectable Ea with the ethyl group facing down. As it can be

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seen from the figure, this kinetically stable partially hydrogenated product is no longer in planar geometry and the newly introduced H atoms prevent other active species of the molecule, such as nitrogen, from closely interacting with the surface. Moreover, when the ethyl group is pointing toward the flat surface, it brings extra geometric constraint which further reduces the adsorption strength. The weak interaction with surface substantially affects further hydrogenation of the “pyrrole” ring on terrace sites. The above calculation results indicate that a substantial proportion of the surface +8[H] intermediate is expected to desorb to solution, rather than to go on with reaction for the formation of N-perhydroethylcarbazole (+12[H]) products. The adsorption of the +8[H] soluble intermediate was investigated on steps of the ruthenium surface on Ru(109). It was experimentally evident that the steps at vicinal Ru(109) may have reconstructed from one-atomic-height to two-atomicheight under mild conditions.24,25 Therefore, extensive calculations of adsorption at various one- and two-atomic-height steps on the surface were performed. The results are shown in Figures 7 and 8. Figure 7a shows the calculated structure of the one-atomicheight step of Ru(109), and the adsorption of +8[H] intermediate at this step is presented in Figure 7b. The adsorbed +8[H] intermediate can be seen to form one N-Ru bond with the surface, and the adsorption energy is estimated to be 0.89 eV, much higher than that at the flat terrace (∼0.3 eV). In Figure 8, we illustrate the calculated structures of different two-atomicheight steps on Ru(109) and the corresponding adsorbed +8[H] intermediate. The estimated adsorption energies at these different step sites are similar, Ea(b) ) 1.02 eV, Ea(d) ) 1.09 eV, and Ea(f) ) 0.92 eV, respectively. It is worth mentioning that besides the adsorption of the +8[H] with the ethyl group facing up, as shown in Figures 7 and 8, the adsorbed +8[H] with ethyl group pointing toward the surface at these steps was calculated. The results show that the ethyl group stays at the corner areas without inducing repulsive interaction with nearby surface atoms. As a result, the calculated Ea are very close to the corresponding ones reported in the above. For example, as shown in Figure 8g, the adsorbed +8[H] with the ethyl-group-down configuration gives the adsorption energy of Ea(g) )1.06 eV, ∼0.1 eV higher than that in the ethyl-group-up case. The above calculation results clearly show that the +8[H] intermediate at various step sites exhibits much higher stabilities in different configurations as compared to that at flat terrace. This is largely because of the repulsive interactions between the introduced hydrogen as well as the ethyl group in the molecule prevent the +8[H] intermediate from getting close to the terrace surface. Those repulsive interactions no longer exist at the step or edge. Active species in the molecule, such as N and unsaturated C, are then able to bind with Ru atoms along the edge. As a result, it would be expected that the +8[H] intermediate formed and desorbed from terrace sites will then readsorb favorably on the step of Ru(109) for subsequent hydrogenation to the fully hydrogenated +12[H] product, assuming sufficient defective step sites are present. On the other hand, the Ru black containing mainly the terrace sites gives accumulation of +8[H] intermediate. This species migration mechanism accounts well for our observation of the product distribution. Isomer Analysis. To further verify the mechanism proposed above, the distribution of +12[H] N-perhydroethylcarbazole steroisomers was analyzed in detail. The GC-MS analysis of the reaction mixtures identified three stereoisomers of the

Eblagon et al. +12[H] products (A, E, F; see Scheme 2), which is different from the results of Sotoodeh et al.15 who reported a single stereoisomer. We have then characterized the structures of the reaction intermediates and products using advanced NMR techniques, as described in detail in our recent work.26 NMR spectra of the reaction mixture revealed the presence of four instead of three stereoisomers of N-perhydroethylcarbazole (A, B, E, F; see Scheme 2). This prime difference between the amounts of various stereoisomers observed using GC-MS and NMR techniques can be explained by the similar retention times of the stereoisomers (A and B) having almost identical physical properties.26 Further study of the N-perhydroethylcarbazole isomers using a variety of 1D and 2D NMR techniques was performed to elucidate the structures of these four stereoisomers. The NMR study of these steroisomers was published26 and is summarized in this section. In short, the assignment of the structures of the stereoisomers was made following a stepwise procedure, that is, (a) identification of carbon atoms not yet assigned to already known structures over the heteronuclear single quantum coherence, HSQC cross signals, (b) identification of carbon atoms near these protons over the heteronuclear multiple bond correlation, HMBC correlated spectra, and (c) additional assignments of the 1H spin systems over 1H,1HDQF-COSY (double quantum coherence correlation spectroscopy) experiments. Since fully hydrogenated products of hydrogenation of N-ethylcarbazole possess 4 stereocenters at 4a, 4b, 8a, and 9a (see Scheme 2 for numbering), a total of 16 stereoisomers of the N-perhydroethylcarbazoles is theoretically possible. However because of the mirror planes and C2 axes, the total number of the possible structures is reduced to 6, which are shown together with their defined stereoisomerism in Scheme 2 below. It should be noted that for symmetric structures A-D, a total of 8 carbon resonances are expected in 13C NMR spectra; however for asymmetric products E and F, a total of 14 carbon resonances should be present. Two reaction mixtures obtained with 5 wt % Ru on rutile and Ru black catalysts were analyzed with the aid of NMR techniques. In both of the reaction mixtures, four different stereoisomers of the fully hydrogenated N-perhydroethylcarbazole were observed. The analysis of the reaction mixture after 20 h of reaction with 5 wt % Ru on rutile catalyst, has shown in the aliphatic region of 13C NMR spectra, 14 carbon resonances with similar intensities. That suggested that the main products were the asymmetrical structures E and F and in the case of Ru black, the main product was the symmetrical structure A (isomer B was found to be in much lower concentration than isomer A at prolonged contact time). The same reactions mixtures that were analyzed by NMR experiments, were also tested with the aid of GC-MS to obtain the individual concentrations of each of the N-perhydroethylcarbazole stereoisomers assigned as shown in Figure 9. It is interesting to note that for hydrogenation of the +8[H] to +12[H], two molar equivalents of [H2] are added to the pyrrole ring to produce the fully hydrogenated product. Considering the adsorption configuration of the +8[H] intermediate on the terrace surface, it may be expected that the hydrogen atoms would be added on the same side of each double bond. As a result, the sym-4[H] isomer A of +12[H] should be formed from the concerted hydrogenation of adsorbed N-ethylcarbazole on flat terrace sites. However, once desorbed to the solution, the +8[H] pyrrole intermediate would prefer readsorbing on the low coordination sites (step sites) and then undergo further hydrogenation. As we have shown, the steric constraints are



Figure 9. Concentration versus time for the three N-perhydroethylcarbazole isomers (+12[H]) in the hydrogenation of N-ethylcarbazole over (a) 5 wt % Ru on rutile (left) and (b) Ru black (right) at 130 °C and 70 bar of H2.

enthalpy (the sum of electronic energies derived from geometry calculations and thermal energies and translational and vibrational contributions corrected using a scale factor of 0.989 previously employed27) was compared to the sum of enthalpies for the starting material (N-ethylcarbazole) together with the total enthalpy of the moles of hydrogen added to create defined intermediate, according to the formula given below in eq 1

HC14H13N + nHH2 f HC14H13+2nN

Figure 10. DFT calculated enthalpy differences between N-ethylcarbazole starting material and intermediates in the gas phase.

greatly relaxed on these sites and the +8[H] adsorption in different configurations give higher stabilities. Therefore, the molecule can be hydrogenated from both sides of the double bonds with respect to the pyrrole ring. Accordingly, we found that the ratio of sym-4[H]A/all +12[H] isomers observed over the 5 wt % Ru on rutile catalyst is around 1.2 at the beginning of the experiment and then decreases to 0.5. On the other hand, the Ru black with predominant terrace sites produces extremely high initial ratio of greater than 18. The lower % of E and F clearly implies the relative low proportion of low-coordination sites on this catalyst. In both cases, ratio of sym-4[H] A/E and F forms decreases gradually during the hydrogenation due to isomerization of A stereoisomer to the more thermodynamically stable asymmetrical structures. Theoretical Considerations-Quantum Mechanics Calculations. To determine the relative energetic of the above intermediates quantum mechanics calculations were performed using the B3LYP hybrid functional (that takes into account electron correlation) and a 6-311 + G(3df, 2p) basis set known as a large set which provided the electronic energy for the geometry optimized molecules. Vibrational frequencies were estimated using the same basis as for the geometry optimization,27 from which thermal energies corrections were obtained. The derived zero point energies were corrected for vibrational, rotational contributions at standard conditions. Thus, the thermodynamic parameters of products and intermediates involving transitional, rotational, and translational modes were therefore taken into account.28 Subsequently, the calculated total

(1)

where HC14H13N is the enthalpy of 9-ethylcarbazole, HH2 is the enthalpy of the hydrogen molecule and HC14H13+2nN is the enthalpy of intermediate products. The reported numbers in Figure 10 correspond to the differences of enthalpies of dissociation at a finite temperature of 130 °C (403 K and atmospheric pressure according to eq 1). The reported enthalpies include thermal, rotational, and translational contributions in the gas phase. A total of 96 different possible products were modeled. As it can be seen in Figure 10, the favored intermediates are N-ethyl-tetrahydrocarbazole +4[H], N-ethylhexahydrocarbazole +6[H], N-ethyloctahydrocarbazole +8[H], and five isomers of N-ethyl-perhydrocarbazole +12[H] (isomer D from Scheme 2 did not converge). The more negative the calculated values, the more stable the compounds are in the gas phase. It can be seen that there is an energy penalty for most of the isomers considered; there are no products with N-ethyldihydrocarbazole +2[H] that are more stable than the reactants. A general trend is that only a few of the calculated structures are energetically favorable under these conditions: the +8[H] isomers, the 9-ethyldodecahydrocarbazole +10[H] isomers, and +12[H] isomers, where these hydrogenated products appear to be more stable than the reactants. Table 3 summarizes the enthalpies of dissociation of the more stable compounds. It is interesting to note that the lowest enthalpy structure indeed corresponds to the asymmetrical fully hydrogenated product E rather than the symmetrical product A (see Table 3). Experimentally, we have not observed +6[H] in NMR analysis, probably because the concentration observed on GCMS was in a trace level. However, we have observed high concentrations of +8[H] with a selectivity of up to 35-40% using ruthenium black commercial catalyst. It can be seen from Figure 10 and Table 3 that this intermediate is very favorable to be created due to large negative enthalpy difference that is almost as low as the enthalpy of the fully hydrogenated stereoisomers. In fact the calculations suggest that some of the +8[H] compounds are more stable than the +10[H] ones (see Figure 10). Moreover, the structures having the lowest enthalpy difference agreed well with the structures assigned based on

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NMR experiments. The theoretical calculations also show that several different stereoisomers of intermediates can arise in the hydrogenation reaction. Therefore it can be expected that the reaction is likely to take different parallel pathways, which can influence the final stereochemistry of the fully loaded products. It is important to underline that the calculations were done for all the reactants and products in the gas phase and do not take into consideration the effects of the catalyst in the reaction or any influence of the solvent. Nonetheless, the general trends in the calculations appeared to be in agreement with the experiments. Kinetic Model. A kinetic model for the stepwise hydrogenation of the N-ethylcarbazlole molecule at the terrace and low coordination sites has also been developed in this work (see Scheme 3). In accordance with the above kinetic model, the differential equations for each reaction steps below were readily solved to obtain the initial concentration-time profiles of the species by using the fourth and fifth order Runge-Kutta formulas with the following boundary conditions imposed29

dP0 ) -(k1 + k2 + k3 + k6 + k9)P0 dt dP4 ) k2P0 - k5P4 dt dP8 ) k3P0 + k5P4 - (k4 + k8 + k7)P8 dt dP12_a ) k6P0 + k8P8 dt dP12_b ) k1P0 + k4P8 dt dP12_c ) k9P0 + k7P8 dt

TABLE 3: Enthalpies of Hydrogenation Including Thermal, Zero-Point Energy, Rotational and Translational Contributions in [kJ/mol] for the Formation of the Thermally More Favorable Compounds, Observed with the Aid of GC-MS, As a Function of the Hydrogen Loading at a Finite Temperature of 403 K (130°C) As Described in Equation 1a

(1) (2) (3) (4) (5) (6)

At time ) 0, [P0] ) 100 (mole %), concentrations of other species ) 0, The first rate constant, k0, was determined by a regression analysis the P0 concentration-time profile (derived from the conversion) against a first order integrated rate law as follows

[P0] ) [P0]t)0 exp(-k0(t + δ))

a Numbers in the square brackets refer to the carbon positions at which the hydrogen atoms were added (see provided figure below for numbering); the capital letters for isomers correspond to the structures showed in Scheme 2.

SCHEME 3: A 2-Site Reaction Kinetic Model

where [P0]t)0 represents the concentration of P0 at time zero () 100 mol %). δ represents the correction factor for any lag time due to experimental error, which was also determined from the regression analysis. Note that k0 represents the overall first order decomposition rate constant of the starting material, and is equal to the summation of k1, k2, k3, k6 and k9. A least-squares method was used to derive the unknown rate constants. A cost function, f, was defined to quantify the mismatch between the experimental and calculated concentration time profiles of the four species

f)

ns ([Pi]exp - [Pi]cal)2 1 n - nk i)1 [Pi]exp

∑

where n and nk represent, respectively, the number of experimental data points and the number of unknown rate constant. The symbol ns represents the number of species. The calculation renders to a nonlinear least-squares optimization of the unknown rate constants (depending on the model used) to minimize f. The rate constant k0 (as generated above) was treated as a constant in the optimization calculation. The SIMPLEX method was utilized for this purpose.30 All calculations were performed using Matlab (v7.5) in a Linux cluster.

Kinetic analysis of the initial substrate and product concentrations (including isomers) for three selected catalysts based on the above model is presented. Testing conditions were as previously stated with the exception of 5 wt % Ru/alumina where a smaller quantity of catalyst (0.1 g) was used to attenuate the reaction. From Figures 11 through to 13, the correlations between experiment and model for the substrate and product concentrations at different times over different catalysts are excellent. What is particularly noteworthy from the summary of the rate constants in Table 4 is the higher k1 value (the direct hydrogenation rate of the N-ethylcarbazole to A +12[H] product


J. Phys. Chem. C, Vol. 114, No. 21, 2010 9729 TABLE 4: Summary of the k Values Derived from the Kinetic Model

Figure 11. Substrate and product concentrations versus time for the hydrogenation of N-ethylcarbazole to perhydro-N-ethylcarbazole (including isomer analysis) over Ru black at 130 °C and 70 bar H2 (experiment) and fitting curves based on 2-site kinetic model.

Figure 12. Substrate and product concentrations versus time for the hydrogenation of N-ethylcarbazole to perhydro-N-ethylcarbazole (including isomer analysis) over Ru on silica-alumina at 130 °C and 70 bar H2 (experiment) and fitting curves based on 2-site kinetic model.

Figure 13. Substrate and product concentrations versus time for the hydrogenation of N-ethylcarbazole to N-perhydroethylcarbazole (including isomer analysis) over 5% Ru on Al2O3 at 130 °C and 70 bar H2 (experiment) and fitting curves based on 2-site kinetic model.

and lower values of k7 and k8 (indirect hydrogenation rates of +8[H] to E and F) over the lower surface area Ru black which is dominated with terrace metal sites. This agrees with the theoretical calculations results which showed that the terrace sites have higher rate for initial hydrogenation of N-ethylcarbazole to +8[H] due to higher adsorption energy and lower hydrogenation rate due to poorer adsorption of +8[H], as revealed in Figure 6. Implications of This Work Storage of hydrogen in nitrogen-substituted polyaromatic molecules is a promising direction. These aromatic molecules

conditions

0.1 g 5 wt % Ru on alumina

0.15 g 5 wt % Ru on silica-alumina

0.15 g Ru black

k1 (hr-1) k2 (hr-1) k3 (hr-1) k4 (hr-1) k5 (hr-1) k6 (hr-1) k7 (hr-1) k8 (hr-1) k9 (hr-1)

0.013 1.781 0.117 0.408 1.961 0.024 0.056 0.034 0.063

0.022 0.200 0.051 0.080 0.337 0.007 0.064 0.022 0.021

0.246 0.509 0.000 0.075 0.530 0.008 0.016 0.006 0.014

give high hydrogen uptake values due to their relatively low atomic masses. Moreover, these compounds are in the liquid form under ambient conditions, which makes their transportation and possible commercial implementation much easier as compared to the solid hydrogen carriers. For practical use, the catalyst required for this type of reversible hydrogen storage should be able to sustain many cycles of hydrogenation and dehydrogenation, transporting hydrogen at the maximum loading (high selectivity). We showed in this work that the steric constraints imposed by the terrace sites of the Ru catalyst surface on the +8[H] pyrrole intermediate make this catalyst inefficient in terms of activity and selectivity in this hydrogenation reaction. Moreover, this catalyst can possibly create recyclability problems in this storage system due to the production of stable intermediates that are difficult to be hydrogenated into Nperhydroethylcarbazole. Additionally, it has been shown that isolated metal sites do not provide strong adsorption for the polyaromatic substrate in comparison to terrace sites. Hence, proper combination of terrace sites and low coordination sites on small catalytic particles is required for efficient storage of hydrogen using N-ethylcarbazole as a liquid carrier. Thus, our chemically reduced Ru on alumina with many small disordered and defective Ru particles proved superior to the Ru black containing high ratio of flat surfaces in the conditions studied in the present work. Therefore, for the design of new catalysts the precise balance of sites is required to tailor activities (i.e., nanoparticles decorated with promoter atoms on high indexed sites). This work also points out the way to design new organic carriers by tailoring the energies of adsorption in order to favor a surface concerted hydrogenation preferably without desorption of the intermediates away from the catalyst surface into the solution. However, one of the most important, findings here is that to maintain the high activity and selectivity of the catalyst in the hydrogenation reaction of N-ethylcarbazole, extensive metal sintering should be avoided. The sintered catalyst can lead to the formation of faceted islands of catalyst (disruption of the balance of sites) during repeated loading and unloading of hydrogen into the molecule. In conclusion, it is clear that the successive hydrogenation or dehydrogenation processes can impose subtle changes in the structures and adsorption geometries of the partially hydrogenated intermediates. In the case of N-ethylcarbazole, the +8[H] pyrrole intermediate, once formed from the terrace site, is likely to desorb due to the weak adsorption enthalpy on the planar structure. Thus, this intermediate compound is unable to be further hydrogenated on the same flat surface and has to move to lower coordination sites, such as steps, for subsequent hydrogenation. The low coordination sites impose less steric hindrance to subsequent hydrogen additions, leading to hydrogen scrambling on the either sides of the molecules. Similarly, for reversible process, it is expected that dehydrogenation of these

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scrambled hydrogenated molecules on the terrace sites would prove difficult. Acknowledgment. We thank for the financial support from the EPSRC and ISIS, U.K., Mr. Daniel Rentsch from Empa, Switzerland for the two-dimensional NMR work support and discussion, Mr. Tom Chapman, Mr. Fernando Eblagon, and Dr. Wiliam Oduro for fruitful discussions and corrections. References and Notes (1) Jalowiecki-Duhamela, L.; Jalowiecki-Duhamela, J.; Carpentiera, E.; Payena, F. H. Int. J. Hydrogen Energy 2007, 32, 2593–2597. (2) Van den Berga, W. C.; Arean, C. O. Chem. Commun. 2008, 15, 668–681. (3) Satyapal, S.; Petrovic, J.; Read, C.; Thomas, G.; Ordaz, G. Catal. Today 2007, 120, 246–256. (4) Orimo, S.; Nakamori, Y.; Eliseo, J. R.; Zuttel, A.; Jensen, C. M. Chem. ReV. 2007, 107, 4111–4132. (5) Stracke, M. P.; Ebeling, G.; Catalun˜a, R.; Dupont, J. Energy Fuels 2007, 21, 1695–1698. (6) Biniwale, R. B.; Kariya, N.; Yamashiro, H.; Ichikawa, M. J. Phys. Chem. B. 2006, 110, 3189–3196. (7) Hodoshima, S.; Nagata, H.; Saito, Y. Appl. Catal., A 2005, 292, 90–96. (8) Ferreira-Aparicio, P.; Rodriguez-Ramos, I.; Guerrero-Ruiz, A. Chem. Commun. 2002, 18, 2082–2083. (9) Itoh, N.; Tamura, E.; Hara, S.; Takahashi, T.; Shono, A.; Satoh, K.; Namba, T. Catal. Today. 2003, 82, 119–125. (10) Taube, M.; Taube, P. AdV. Hyd. Energy 1981, 2, 1077–1085. (11) Santa Cruz Bustamante, G. V.; Swesi, Y.; Pitault, I.; Meille, V.; Heurtaux, F. International Hydrogen Energy Congress and Exhibition, 2005, IHEC 2005; Vol. 1. (12) Schwarz, D. E.; Cameron, T. M.; Hay, P. J.; Scott, B. L.; Tumas, W.; Thorn, D. L. Chem. Commun. 2005, 5919–5921.

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