Comparative Study of Catalytic Hydrogenation of 9-Ethylcarbazole for

Article pubs.acs.org/JPCC

Comparative Study of Catalytic Hydrogenation of 9-Ethylcarbazole for Hydrogen Storage over Noble Metal Surfaces K. M. Eblagon, K. Tam, K. M. K. Yu, and S. C. E. Tsang* Wolfson Catalysis, Department of Chemistry, University of Oxford, Oxford OX1 3QR, U.K.

ABSTRACT: The use of liquid organic hydrides (LOH) as a chemical hydrogen store to supply hydrogen gas for a polymer electrolyte membrane fuel cell (PEFC) is explored. In the present work, hydrogenation of 9-ethylcarbazole is particularly investigated in the liquid phase over different unsupported noble metal powders. The kinetics obtained from the hydrogenation of the substrate over these catalytic systems are modeled, and the derived fundamental rate constants are systematically compared. It is found that the differences in activity and product distribution of the reaction over different metal surfaces depend critically on the electronic structures of the metals. From the prospective application of 9-ethyl-carbazole, an effective catalyst should be able to convert the substrate to the fully hydrogenated cis product without forming any kinetically stable intermediates. Ruthenium is the most active catalyst among all the metals studied for this reaction. However, this catalyst suffers from relatively low selectivity with the accumulation of large quantities of partially hydrogenated intermediates due to weak adsorption and poor surface diffusion of the intermediates for further hydrogenation.

1. INTRODUCTION Aromatic compounds can reversibly take up a large amount of hydrogen in chemical forms, which may offer a potential solution for storing hydrogen as a lightweight energy carrier. For instance, hydrogen gas can be supplied from these carriers on demand for mobile applications as a result of a change in pressure or temperature within a vehicle or device system. Interestingly, the organic based liquid-phase hydrogen carriers when assembled with fuel cells have been recently shown to deliver substantially higher energy density packages than those of lithium ion batteries.1 The original concept developed by Air Products of using substituted aromatic compounds with multiunsaturated bond structures has been extensively studied.2−9 Particularly, the dehydrogenation of 9-perhydrocarbazole as a liquid organic hydride (LOH) is the widely studied reaction with the assumption that the reversed hydrogenation would take place in a chemical factory.8,10−15 Thus, there has been limited research effort devoted to the hydrogenation step.14−18 However, a deep understanding for both forward and reverse processes is equally important in the implementation of © 2012 American Chemical Society

this new technology. In addition, there is still a lack of process economies and technical assessments in the recycling of spent fuels at a large scale, particularly with regards to the logistics in transportation and possible additional purification processes due to accumulation of nonrechargeable fractions, etc. However, the closed loop charging and discharging of hydrogen gas in a liquid organic based carrier inside the tank at smaller but technically less challenging scale should be carefully considered. Along this line, the development of this hydrogen storage concept based on LOH will be dependent on the success in identifying a suitable catalyst component. An efficient catalyst for this application should be capable of promoting both hydrogenation and dehydrogenation of the liquid carrier, avoiding the formation of kinetically stable intermediate(s) that may cause problems with the reversibility of the process.17,18 Received: December 19, 2011 Revised: March 2, 2012 Published: March 7, 2012 7421

dx.doi.org/10.1021/jp212249g | J. Phys. Chem. C 2012, 116, 7421−7429

The Journal of Physical Chemistry C

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The specific surface of the catalyst was measured following a standard experimental procedure using not less than 100 mg of the sample. 2.2. Catalytic Activity Tests. The catalytic hydrogenation of 9-ethylcarbazole in cyclohexane over Ru, Pt, Pd, and Ni powders was performed in a stainless steel 300 mL Parr batch reactor equipped with a thermocouple and a pressure gauge. The reaction mixture was stirred using a PTFE encapsulated bar and a magnetic stirrer produced by Ika-Werke at the speed of 600 rpm/min, which was placed under the heating mantle of the autoclave. Relatively mild reaction conditions were used to allow a direct comparison of catalytic performance between different noble metals and nickel. In a typical experiment, 1 g of 9-ethylcarbazole was dissolved in 100 mL of cyclohexane followed by adding 0.2 g of a selected catalyst. The reactor was then sealed, flushed with hydrogen and heated to 130 °C. When the desired temperature was reached, 70 bar of pure hydrogen was charged into the reactor, and the reaction time was taken as zero. To obtain catalytic information such as the concentration versus time profile, small liquid samples were removed periodically from the reactor for composition analysis using GC−MS technique. After 5−7 h of the reaction, the autoclave was cooled down to room temperature using a water bath, the pressure was then released, and the resulting reaction mixture was chemically analyzed. Four measurements were carried out to ascertain the reproducibility of the hydrogenation over Ru black under the same reaction conditions. The results showed a deviation of less than 5% on the values obtained for both the conversion and selectivity in the series of experiments. 2.3. Analysis of the Reaction Products. The intermediates and products of the reversible hydrogenation of 9ethylcarbazole were detected and separated using a GC−MS system (Agilent 6890-5975E GC−MS) equipped with a nonpolar capillary column (Agilent 19091s-433), an autosampler, and an MSD-Triple Axis Detector HED-EM. Helium at a pressure of 1.1 bar with a flow rate of 124 mL min−1 was used as a carrier gas. To minimize the injection error of the autosampler, 0.5 mL of 1,2,4trimethylbenzene (TMS) solution (0.09 mol dm−3) in cyclohexane was used as an external standard. The catalytic activity was calculated according to eq 2, and the selectivity was obtained as shown in eq 3 below;

It is well-known that the electronic structure of the noble metal component has a major bearing on activity and selectivity in a catalytic reaction. Noble metals possess rather complicated electronic structures with itinerant electrons originating from the valence s shells of the metals and the more localized d band electrons that are moving across the Fermi level.19 As a general rule, the optimum catalysis has to be a compromise between two extremes: too strong of an adsorption of substrate leads to poisoning and too weak of an adsorption gives no reaction on the metal surface, as described by the Sabatier principle.20 The reversible catalytic hydrogenation of 9-ethylcarbazole over a noble metal catalyst can be described by eq 1. C14 H25N ↔ C14 H13N + 6H2

(1)

For the hydrogenation of this simple compound, at the first glance, at least six main products were previously identified.16 These include partially hydrogenated intermediates and fully hydrogenated stereoisomers of 9-ethyl-perhydrocarbazole.17 The catalytic formation of different intermediates and isomers can have a profound effect on the reversible dehydrogenation step since different compounds can give different adsorption modes on a specific catalytic surface, hence giving different reaction rates and profiles. For example, various degrees of kinetic stability of intermediates during the catalytic hydrogenation of 9-ethylcarbazole depending on the type of active sites on supported Ru metal were reported.17 The kinetic stable intermediate, 9-ethyl-octahydrocarbazole, was found to be very reluctant to be hydrogenated over a large Ru particle.17 Also, the course for a multipath hydrogenation or dehydrogenation reaction depends mainly on activation energy barriers, which in turn depend on the electronic properties of the type of metal and geometric sites distribution.21 In addition, the selectivity of the reaction can be strongly affected by catalytic site competition between different chemical species, which can vary in significant magnitude over different types of metals.22−24 Here, we report a comparative study of noble metals and nickel for the hydrogenation of 9-ethylcarbazole in order to see their influence on activity and selectivity of this reaction. In this article, the variations in fundamental reaction pathways and rate constants by different catalytic surfaces are particularly investigated and discussed.

2. EXPERIMENTAL SECTION 2.1. Materials and Catalyst Characterization. The commercial Ru, Pt, Pd, Ni (99.8% metal basis), and 65 wt % Ni/SiO2−Al2O3 were supplied by Sigma-Aldrich and used without pretreatment. Cyclohexane, ethanol, and acetone (all HPLC-grade) solvents were purchased from Fluka. Hydrogen gas (99.9%) was provided by Air-products. In the present work, the catalysts were characterized using a HRTEM JEOL 2010 with an accelerating voltage of 200 kV at a nominal magnification of 590 K. The preparation of the samples for the TEM examination required the dispersion of a few milligrams of the specimen in 5 mL of acetone or ethanol using an ultrasonic bath. Subsequently, a drop of this suspension was deposited on a 3 mm copper grid covered with a perforated carbon film. The solvent was then dried out. The TEM investigations were carried out on the solid metal particles dispersed across the holes of the carbon film on the grids. For the determination of particle size distribution, not less than 100 particles from different areas of the sample were measured, using the Scandium software from Olympus Soft Imaging Solutions.

catalytic activity =

9‐ethylcarbazole converted [mol] total metal content [g] × time [s] (2)

selectivity =

specific product [mol] × 100 sum of products [mol]

(3)

2.4. Hydrogenation Kinetics. The product concentration−time profiles were obtained experimentally. With reference to these profiles, a kinetic model of the catalytic hydrogenation reaction has been developed, and it is shown in Figure 1 (the structures of all reaction products were assigned by NMR).16,17 In the hydrogenation of 9-ethylcarbazole over typical ruthenium based catalysts, six different species were identified, namely, the substrate (Pl 0 [H]), 9-ethyl-tetrahydrocarbazole (Pl 4 [H]), 9-ethyl-octahydrocarbazole (Pl 8 [H]), and three fractions of stereoisomers of the fully saturated 9-ethylperhydrocarbazole (Pl 12 [H] A, B, C). In this work, the fitting of the obtained kinetic data in the above model for all of the hydrogenation reactions studied was carried out by taking the hydrogenation pathways shown in 7422



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dP12C = k 9P0 + k 7P8 + k11P12B dt

(9)

where P0 is the concentration of 9-ethylcarbazole, P4 is the concentration of 9-ethyl-tetrahydrocarbazole, P8 is the concentration of 9-ethyl-octahydrocarbazole, P12A is the concentration of 9-ethyl-perhydrocarbazole fraction A, P12B is the concentration of 9-ethyl-perhydrocarbazole fraction B, and P12C is the concentration of 9-ethyl-perhydrocarbazole fraction C (see Figure 2). Subsequently, eqs 4−9 were readily solved to obtain the product concentration−time profiles of each species by using the fourth and fifth order Runge−Kutta method.25 It is noted that the following boundary conditions were imposed: at the starting point (time = 0 s) the concentration of the substrate was assumed to be 100% ([P0] = 100 mol %), and concentrations of all other species were assumed to be zero. The first rate constant, k0, was determined by regression analysis of the P0 concentration− time profile (derived from the conversion) against a first order integrated rate law as shown below: [P0] = [P0]t = 0 exp( −k 0(t + δ)) Figure 1. Model describing the reaction pathways on catalytic hydrogenation of 9-ethycarbazole. The acronyms for intermediates and products are described in the text.

where [P0]t=0 represents the concentration of P0 at the time zero (=100 mol %), whereas δ represents a correction factor for any lag time due to the experimental error (determined from the regression analysis). Additionally, k0 is the overall first order decomposition rate constant of the starting material and is equal to the summation of k1, k2, k3, k6, and k9. The remaining rate constants were derived using a Nelder− Mead method (simplex method). Generally, this simplex method was used to test adjacent vertices of a feasible set (polytope) in sequence, so that at each new vertex, the objective function improves or stays unchanged.26 The initial parameters given in this work were time, concentrations of the species, and a set of starting values of the unknown rate constants. The mismatch between experimental and modeled values was calculated using a rootmean-square of the difference (rms).27 Thus, the unknown rate constants were solved iteratively to fulfill the conditions described by the reaction model and optimize a set of k values that would result in a minimum mismatch value by changing the k values. Because of the inherent limitations of the Nelder−Mead simplex method, the code developed forced the simplex to be restarted once a plateau in the minimized function was achieved. This operation reset the size of the simplex to its initial value, reducing

Figure 1 with the isomerization reactions shown in Figure 2b taken into account. The following differential equations were derived and are listed below (eqs 4-9). dP0 = −(k1 + k2 + k3 + k6 + k 9)P0 dt

(4)

dP4 = k2P0 − k5P4 dt

(5)

dP8 = k3P0 + k5P4 − (k4 + k8 + k 7)P8 dt

(6)

dP12A = k6P0 + k8P8 + k10P12B dt

(7)

dP12B = k1P0 + k4P8 − (k10 + k11)P12B dt

(8)

(10)

Figure 2. (a) Four possible stereoisomers of 9-ethyl-perhydrocarbazoles collected in the three separated fractions during product analysis. The two isomers in fraction B cannot be separated by GC−MS, but NMR can offer their qualitative differentiation;16 (b) interconversions (isomerization reaction) between these stereoisomers. 7423



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the possibilities of convergence to nonstationary points or local minima. The calculations were continued until the convergence criteria were satisfied, and the mismatch was not greater than a preset value. In case a local minimum was found that the resetting of the simplex could not overcome, the k values were changed accordingly to allow better optimization of the fit. All calculations were performed using Matlab (version 7.5).

3. RESULTS AND DISCUSSION 3.1. Physical Properties of the Catalysts. Unsupported noble metal powders were deliberately selected for the catalytic testing in the present work in order to elucidate the intrinsic properties of particular metals on this reaction. First, the use of unsupported metal powders can allow disentangling of the complex metal support interaction, which can cause a subtle change in the electronic properties of the metal nanoparticles.28 Moreover, the presence of the support may also create additional active sites for the reaction as reported in the hydrogenation of benzene.29 Second, it may allow the metal sites distribution to be kept constant within a given range since different metal sites on terraces and edge/corners of a nanoparticle can play an important part in determining catalytic activity and selectivity. Some important physical properties of commercial powders, namely, Ru black, Pt black, Pd black, and Ni catalysts, are shown in Table 1.

Figure 3. Top left, TEM image of ruthenium black; top right, Ni nanopowder; bottom left, 65 wt % Ni/SiO2−Al2O3; and bottom right, Pd black.

The specific reaction rate constants were estimated from these fittings and compared within the catalysts used. From the initial catalytic tests, it was apparent that the pure unsupported nickel catalyst showed no activity at all. This could be attributed to the combination of the relatively lower metal surface area and the catalyst agglomeration due to magnetic interaction with the (magnetic) stirrer used during the catalytic reaction. As a consequence, a commercially available nickel supported on silica− alumina (65 wt % Ni/SiO2−Al2O3) was used to maximize the dispersion of nickel in the reaction solution, allowing a systematic comparison of Ni with noble metals. 3.2.1. Initial Catalytic Activity and Selectivity. Before conducting the detailed product concentration−time profile analysis for all the intermediates and products over the four catalysts at longer reaction periods, the testing results of the first hour were compiled to compare the initial activity and product selectivity over these catalysts. As seen in Table 2, the initial catalytic activity of different unsupported metal catalysts in the hydrogenation of

Table 1. Physical Properties of the Catalysts Studied in the Present Work catalyst

particle size (nm) (SD)

BET specific surface area (m2/g)

Ru black Pd black Pt black Ni nanopowder 65 wt % Ni/SiO2−Al2O3

6.3 (2.7) 6.8 (2.0) 6.0 (3.2) 20.0 (7.1) 7.5 (1.8)

22 22 30 5 96

Also, a representative HRTEM micrograph for each catalyst is included in Figure 3. As it can be seen from these micrographs, most of the unsupported noble metal nanoparticles are strongly aggregated, giving similar average particle size and surface area (Table 1). As a result, no large variation in metal sites distribution is evident. However, the nickel nanopowder may contribute the largest variation in sites distribution since it gives a significantly bigger particle size associated with a lower BET surface area. 3.2. Hydrogenation Activity and Selectivity of Different Noble Metal Catalysts. It is well-known that the position of the metal in the Periodic Table can have a significant impact on its catalytic properties.30 It was anticipated that the reaction pathways and the type of intermediates might be different among the metals tested because of the variations in absorption strength of the substrate and intermediates on these surfaces. Thus, in order to assess the influence of the electronic properties of the metals, hydrogenation of 9-ethylcarbazole was conducted over Ru, Pt, Pd, or Ni unsupported catalysts. The information obtained may help to evaluate the hydrogen storage method using the reversible hydrogenation of liquid carriers. In all of the catalysts studied, the reaction kinetics were found to be of first order with respect to the aromatic substrate and zero order with respect to the hydrogen, which was used in great excess. The data results obtained from these experiments were fitted into the mathematical model shown in Figure 1.

Table 2. Catalytic Activity and Selectivity of Unsupported Ru, Pd, Pt, and Supported 65 wt % Ni/SiO2−Al2O3 Recorded after 1 h of Hydrogenation of 9-Ethylcarbazole selectivity (%)

catalyst Ru black Pd black Pt black 65 wt % Ni on SiO2−Al2O3

catalytic activity × 10−2 (mM ETC·gram of metal−1·s−1)

Pl 4[H]

Pl 6[H]

Pl 8[H]

sum Pl 12[H]

0.56 0.45 0.18 0.16

18 50 6 64

Pt > Ni. The initial selectivity obtained in the series of catalyst based on different metals is also listed in Table 2. It is clear that regardless of the type of metal used, the same intermediates and 7424



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products previously identified over Ru at the lower catalyst loading were also found.17 However, the obtained selectivities were significantly different over the different metal surfaces. 3.2.2. Product Concentration Time Profiling and Modeling. The curve fittings for reaction profiles obtained over the four catalysts at longer reaction times according to the model (see Figure 1) are presented in Figures 4 to 7. The curves

Figure 7. Time-dependent product distribution obtained over 65 wt % Ni/SiO2−Al2O3 catalyst in 7 h of the reaction at 130 °C and 70 bar hydrogen together with modeled values illustrated as solid lines.

As seen from the Table 3 and the Figures 4−7, the derived k0 value decreases in the order Ru > Pd > Pt > Ni matching the same order of the initial rates at the first hour of reaction at the high surface coverage of hydrogen. From the product distribution shown in Figures 4−7, it is interesting to note that 9-ethyl octahydrocarbazole (Pl 8[H]) was formed at significant amounts over Ru catalyst (Figure 4). This partial hydrogenated product was also detected in the reaction mixtures obtained over Pd (see Figure 5) and Ni (Figure 7) catalysts but in a much lower concentration. In contrast, over the Pt catalyst, this partial hydrogenated product was produced only in traces (Figure 6). By comparing the individual rate constants for Pl 8 [H] production and conversion (Table 3 and Figure 8), it is clearly noted that the rates of formation of Pl 8 [H] were not hugely different among the different metals used (although Ru gave considerably higher rates). However, the rates of Pl 8 [H] conversion varied greatly over the different surfaces. For example, in comparison of the two extreme cases of Pt and Ru, the conversion rate of Pl 8 [H] obtained over the Pt was more than 40 times than that of Ru. It is also interesting to note that Pd gave Pl 4[H] as the major partial hydrogenated product in solution instead of the Pl 8 [H], accounting for over 30% selectivity at the first hour of reaction. Comparing the calculated rate constants (see Table 3), the rate of production of Pl 4 [H] over the Pd catalyst was almost twice as high as the rate of its conversion into Pl 8 [H]. This means that the activation barrier for further conversion of Pl 4 [H] on the palladium surface must be high and therefore accounting for the Pl 4 [H] accumulation in the solution. On the other hand, Ru gave a small amount of Pl 4 [H] but for Pt and Ni, no significant amounts of any of these intermediates were observed. 3.3. Variations in the Composition of the Fully Saturated Products over Ru, Pt, Pd, and 65 wt % Ni/ SiO2−Al2O3. For the fully hydrogenated products, it is noted that four isomers of the 9-ethyl-pehydrocarbazole were found in the reaction mixtures, as described previously.16,17 The structures of these four isomers16 identified by means of GC−MS and NMR techniques are shown in Figure 2a. It should be noted that the cis and trans isomers in the fraction B shown in Figure 2a (collectively called Pl 12 [H] B) cannot be resolved by the GC−MS and could only be qualitatively identified by the NMR. It is because the precise ratio between the cis and trans isomers in this fraction B could not be deduced due

Figure 4. Time-dependent product distribution obtained over Ru black catalyst in 5 h of reaction at 130 °C and 70 bar of hydrogen together with modeled values illustrated as solid lines.

Figure 5. Time-dependent product distribution obtained over Pd black catalyst in 6.5 h of reaction at 130 °C and 70 bar hydrogen together with modeled values illustrated as solid lines.

Figure 6. Time-dependent product distribution obtained over Pt black catalyst in 6.5 h of reaction at 130 °C and 70 bar hydrogen together with modeled values illustrated as solid lines.

matching with experimental data points in general indicate a very good fit of the experimentally obtained values to the model. The rate constants of each elementary step that were used for fitting kinetic data are summarized in Table 3. 7425



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Table 3. Calculated Rate Constant Values in h−1 Used for Fitting the Experimental Data to the Reaction Model Showed in Figure 1 catalyst

k0

k1

k2

k3

k4

k5

k6

k7

k8

k9

Ru black Pd black Pt black 65 wt % Ni/SiO2−Al2O3

1.5 1.2 0.2 0.1

0.8 0.263 0.002 0.028

0.4 0.9 0 0

0.3 0 0.1 0

0.2 6.1 8.1 0.6

1.1 0.5 0.1 0.7

0.017 0.038 0.034 0.026

0.01 0.001 0.005 0.17

0.002 0.003 0 0.011

0.003 0 0.021 0

Figure 8. Comparison of rate constants (calculated k values from Table 3) for production and further conversion of Pl 8 [H] in hydrogenation of 9-ethylcarbazole over different metals together with k0 rate constants.

an adsorbed molecule31 on the noble metal or alloy surface. Our order of activity (both initial and at steady-state) obtained appears to follow well the relative positions of the d-band centers (in electron volts) of the unsupported metals from Fermi levels being 1.41 (Ru) < 1.83 (Pd) < 2.25 (Pt). It is noted that the hydrogenation rate k0 decreases as the d-band center increases. However, the 65 wt % Ni/SiO2−Al2O3 catalyst apparently deviates from this general pattern. It is because the d-band center for pure Ni is 1.29, which is expected to be the most active surface. This discrepancy can be attributed to the presence of support material, which attenuates greatly on the activity of the supported catalyst (both coverage and electronic modification). It is well accepted that the closer the d-band center is to the Femi level (toward zero), the higher chemisorption energy is obtained.20 The catalytic activity of the metal catalyst in the reaction for many instances follows the adsorption strength of the reactant on the metal surface. The adsorption is also related to the degree of filling of the antibonding band created by adsorbate metal d-states and the degree of the coupling matrix element (Vsd).32 In general, if the energy of antibonding states is below the Fermi level, these states are very likely to be occupied, leading to a weaker bond.33 In addition, when the new species is chemisorbed on the surface metal atoms, metal− metal bonds may be broken. This introduces another correlation parameter relating the energy of metal−metal bonds to the cohesive energy of the metal. It should be noted that the degree of filing of the d-bands increases to the right of the Periodic Table, whereas Vsd increases to the left of the Periodic Table. Cohesive energy increases down the periodic groups.33 Thus, Pt is expected to be the least catalytically active metal studied because it has the largest d-band center value, highest filling of the d-band, and the highest value of cohesive energy (5.85 eV versus 3.94 eV for Pd).33 It is noted that the binding of our substrate (Pl 0 [H]) on metal surface is likely to take place via the transfer of lone pair electrons from the

to the presence of impurities causing a noisy background in the NMR spectra. However, one attempt was successfully made over the reaction mixture produced from Ru black catalyst, which contained >70% of the cis isomer. Their concentration time profiles over Ru, Pd, Pt, and 65 wt % Ni/SiO2−Al2O3 are shown in Figure 9. Although Table 4 shows that the primary fully hydrogenated product of this reaction is the Pl 12 [H] B regardless of the nature of the catalyst used, it should be noted that Pl 12 [H] B is not the most thermodynamically stable product as it can be converted to Pl 12[H] A and Pl 12[H] C gradually over a long reaction time. Over the Ru catalyst (Figure 9A), the fraction Pl 12 [H] C is mainly produced by the stepwise hydrogenation of Pl 8 [H] and to a smaller extent by the direct formation from the substrate over the Ru catalyst (see Table 3), but this fraction is a minor product, and its concentration does not seem to increase along the reaction time. However, it is interesting to observe from Figure 9B that Pd catalyst gives initially the Pl 12 [H] B as a main product, which rapidly isomerizes to the Pl 12 [H] A. As seen from Table 5, the calculated k10 value for this conversion over Pd black is apparently higher than that of the Ru catalyst presumably due to the weaker chemisorption of this saturated molecule. Figure 9C,D shows that the 65 wt % Ni/ SiO2−Al2O3 and Pt black catalysts give a very similar distribution on the isomers of 9-ethyl-perhydrocarbazole. Additionally, the Pl 12 [H] C product is mainly produced directly from the substrate over the Pt black catalyst due to higher rates for hydrogen dissociation and diffusion (see later section), whereas over the 65 wt % Ni/SiO2−Al2O3, it is formed via the isomerization of Pl 12 [H] (support assists isomerization). It is interesting to note from the above results that different activities and product distributions were obtained over different metal surfaces for this reaction. In order to rationalize the observations, the d-band center model is invoked here. The shift in the d-band center derived from the density functional calculations can explain the change in chemisorption energy of 7426



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show poorer reaction rates for this reaction. Thus, the expected activity order predicted by the d-band center model indeed agrees very well with our experimental observations. In order to rationalize our product distributions over different metals, we should then consider a number of elementary processes. For a partial hydrogenated product to appear in solution from the catalyst surface, it must be initially formed from the substrate through surface adsorption followed by hydrogenation steps (Scheme 1) and then desorption of this Scheme 1. Hydrogenation of 9-Ethylcarbazole to 9-Ethylperhydrocarbazole over Studied Metal Catalysta

a

The Pl 6[H] intermediate was only observed on supported Ni and on Ru black (trace quantities).

particular partial hydrogenated product. We have previously conducted a DFT calculation on the adsorption energy of the substrate, 9-ethylcarbazole adsorbed on a flat Ru (001) surface.17 The calculated adsorption energy for the former is 2.56 eV, which indicates that the 9-ethylcarbazole strongly adsorbs on this terrace metal surface. We 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 9-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 9-ethylcarbazole. This indicates that the coadsorbed hydrogen, if involved in the following hydrogenation reactions, would not affect the adsorption of 9-ethylcarbazole.17 This fact supports our observation that the global rate of hydrogenation depends on adsorption strength of the substrate on a particular metal surface as the rate determining step. However, the stability of a partially hydrogenated product of a particular structure such as Pl 8[H] on a metal surface against desorption to solution phase will depend on its own adsorption strength. We showed that Pl 8[H] (with the pyrrole ring) in the hydrogenation process on the same terrace sites gives Ea as low as 0.32 eV with the ethyl group facing up and a neglectable Ea with the ethyl group facing down. 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 Pl 8[H]

Figure 9. Concentration versus time profiles for different isomers in 9ethyl-perhydrocarbazole, obtained over (A) Ru black, (B) Pd black, (C) Pt black, and (D) 65 wt % Ni/SiO2−Al2O3.

Table 4. Composition Profiling (in Percent) Containing Different Isomers of Saturated 9-Ethyl-perhydrocarbazole Quenched in the First Hour of the Reaction selectivity (%) catalyst

Pl 12 [H] A

Pl 12 [H] B

Pl 12 [H] C


4 13 27 39

94 85 56 55

2 2 17 6

Table 5. Calculated Rate Constants (h−1) of Isomerisation Reactions (Refer to Figure 2) catalyst

k10

k11


0.02 0.09 0.1 0.004

0 0.02 0 0.6

unsaturated centers of the substrate to the d-band of the metal. As a result, the more electron rich metals (i.e., Pt) should also 7427



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were recorded over the Ru catalyst. The activity of the catalyst was found to depend strongly on the electronic structure of this metal, being the electron poorest metal with the lowest d-band center position of the three elements. Also, the type of metal used was found to critically affect the relative concentrations of the partially hydrogenated intermediates and products. The Pl 8[H] was found to be kinetically stable over the Ru catalyst. However, Pl 4[H] was identified as a kinetically stable intermediate over Pd black, whereas Pt black did not give any significant quantity of these intermediates. Thus, the reaction selectivity to a particular partially hydrogenated product is thought to relate to the relative rates for its formation and conversion (depend on surface hydrogen diffusion) and the kinetic stability for its surface adsorption versus desorption to solution, etc. In addition, the composition of the fully hydrogenated products was also found to depend on the type of metal used. Isomerization reactions were appreciably more pronounced in the case of 65 wt % Ni/SiO2−Al2O3 (isomerization assisted by the support) and Pt black catalysts (weakest adsorption of the isomers). From the perspective application of 9-ethylcarbazole for hydrogen storage, an effective catalyst should be able to convert the substrate into the fully hydrogenated cis product (Pl 12[H] B) reversibly without the formation of any stable intermediates or isomers. It is clear that ruthenium is the most active metal for this reaction. However, it suffers from relatively low selectivity with accumulation of large quantities of the Pl 8 [H] intermediate in solution due to the poor adsorption and weak hydrogen diffusion for the further conversion of this intermediate on the surface. Thus, further work should be done in the direction of modifying the electronic structure of a ruthenium-based catalytic system in order to optimize the kinetics of the fundamental surface processes. Theoretically, this may be achieved by decreasing of the Ru d-band center relative to the Fermi level toward the value of the d-band of Rh. This can be done by putting the Ru on another transition metal surface such as Pd or Ag by choosing an electron-donating support such as polar MgO, ZnO, Fe2O3, etc.

intermediate is expected to desorb to solution, rather than to go straight on with reaction for the formation of 9-ethyl-perhydrocarbazole (Pl 12[H]) products. This analysis also matches very well with our experimental observations. However, one would expect that the order of selectivity for Pl 8[H] should be inversely proportional to the order of catalytic activity as the adsorption strength of this partial hydrogenated product should follow the d-band center position, but we observed the most active Ru (the lowest d-band center position) actually gave the highest selectivity to Pl 8[H] than the other metals. In order to rationalize this observation, we should also consider the elementary routes for the further conversion of Pl 8[H] on the surface to other products before this species is desorbed to the solution due to the predicted weak chemisorption. In order to have efficient stepwise hydrogenation catalysis on the surface, both the dissociation of H2 molecule and particularly H surface diffusion need to have a low kinetic barrier. It has been reported from a systematic ab initio DFT investigation of H2 dissociation and subsequent atomic H diffusion on transition metal surfaces.34 The calculations showed that the elements such as Ni and Ru (see Figure 8) on the left of the Periodic Table, though offering a low barrier, bind H atoms very strongly to the surface, therefore hindering diffusion. Conversely, the elements on the right of the Periodic Table do not bind H; however, they do not reduce on the dissociation barrier either. Thus, Pd and Pt were reported to provide the low activation barriers for both processes. It is interesting to note (Figure 8) that the rates for the conversion of Pl 8[H] to further hydrogenated products on the Pt catalyst surface evaluated by our model (Table 3) are indeed the highest. Under this condition, the Pl 8[H] partial hydrogenated product will be quickly hydrogenated to other products on the Pt surface before it has a chance to get into the solution phase. Similarly, the rates for formation and conversion and the relative stability of the Pl 4 [H] species on the surface could also account for the observed selectivities to this partially hydrogenated product over different metal catalysts. For the reversible catalytic 9-ethylcarbazole hydrogenation, the product distribution is of importance, especially from the point of view of its applications in hydrogen storage. We show that the rate for the conversion of Pl 8 [H] to the fully hydrogenated product is very slow over ruthenium. Thus, this metal surface is not ideal for fast catalytic hydrogen storage and delivery as the intermediate product would accumulate and hinder the overall rate. Similarly, it was reported in the literature that the cis isomer of decalin has a much higher rate for hydrogen production than the trans decalin due to the facile dehydrogenation of the cis isomer by the flat surface.35,36 It is anticipated that the rates for cis isomer hydrogenation and dehydrogenation are higher than those of trans in our system. To conclude, in the view of these findings, an efficient catalyst should have a high hydrogenation/dehydrogenation activity to switch between the 9-ethylcarbazole and 9-ethyl-perhydrocarbazole (cis-Pl 12 [B]) on the surface reversibly in a fast manner without forming any kinetically stable intermediates or isomer products.

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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 We are thankful for the financial support from the EPSRC and ISIS, U.K. Dr Anibal J. Ramirez-Cuesta and Mr Fernando Eblagon are kindly acknowledged for their fruitful discussions and help with data analysis.

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REFERENCES

(1) Alhumaidan, F.; Cresswell, D.; Garforth, A. Energy Fuel 2011, 113, 4217−4234 (see also www.airproducts.com; publication number 352-10-001-GLB). (2) Von Wild, J.; Friedrich, T.; Cooper, A.; Toseland, B.; Muraro, G.; Tegrotenhuis, W.; Wang, Y.; Humble, P.; Karim, A. Liquid Organic Hydrogen Carriers (LOHC): An auspicious alternative to conventional hydrogen storage technologies. Presented at World Hydrogen Energy Conference, Essen, Germany, 2010. (3) Makowski, P.; Thomas, A.; Kuhn, P.; Goettmann, F. Energy Environ. Sci. 2009, 2, 480−490. (4) Wechsler, D.; Cui, Y.; Dean, D.; Davis, B.; Jessop, P. G. J. Am. Chem. Soc. 2008, 130, 17195−17203.

4. CONCLUSIONS The hydrogenation of 9-ethylcarbazole for reversible hydrogen storage was systematically studied over unsupported Ru, Pt, and Pd catalysts and compared with a supported Ni catalyst. The experimental concentration versus time profiles obtained over these catalysts can be rationalized by our kinetic model, which is developed for this application. The highest reaction rates 7428



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(5) Kariya, N.; Fukuoka, A.; Ichikawa, M. Appl. Catal., A 2002, 233, 91−102. (6) Schildhauer, T.; Newson, E.; Müller, S. J. Catal. 2001, 198, 355− 358. (7) Pez, G.; Scott, A.; Cooper, A.; Cheng, H. (Air Products and Chemicals, Inc.) Hydrogen Storage by Reversible Hydrogenation of PiConjugated Substrates. U.S. Patent 20050002857, January 6, 1995. (8) Pez, G.; Scott, A.; Cooper, A.; Cheng, H. (Air Products and Chemicals, Inc.) Hydrogen Storage by Reversible Hydrogenation of PiConjugated Substrates. U.S. Patent 7,101,530, May 9, 2006. (9) Pez, G.; Scott, A.; Cooper, A.; Cheng, H.; Wilhelm, F.; Abdourazak, A. (Air Products and Chemicals, Inc.) Hydrogen Storage by Reversible Hydrogenation of Pi-Conjugated Substrates. U.S. Patent 7,351,395, April 11, 2008. (10) Zhaohui, A.; Tonks, I.; Belli, J.; Jensen, C. M. J. Organomet. Chem. 2009, 694, 2854−2857. (11) Sootodeh, F.; Zhao, L.; Smith, K. J. Ind. Eng. Chem. Res. 2010, 49, 1018−1026. (12) Sootodeh, F.; Smith, K. J. J. Catal. 2011, 279, 36−47. (13) Sobota, M.; Nikiforidis, I.; Amende, M.; Sanmartin Zanon, B.; Staudt, T.; Hofert, O.; Papp, C.; Hieringer, W.; Laurin, M.; Assenbaum, D.; Wasserscheid, P.; Steinruck, H. P.; Gorling, A.; Libuda, J. Chem.Eur. J. 2011, 17, 11542−11552. (14) Sotoodeh, F.; Zhao, L.; Smith, K. J. Appl. Catal., A 2009, 362, 155−162. (15) Sotoodeh, F. Hydrogenation and Dehydrogenation Kinetics and Catalysts for New Hydrogen Storage Liquids. Ph.D. Thesis, University of British Columbia, 2011. (16) Morawa Eblagon, K.; Rentsch, D.; Friedrichs, O.; Remhof, A.; Zuettel, A.; Ramirez-Cuesta, A. J.; Tsang, S. C. Int. J. Hydrogen Energy 2010, 35, 11609−11621. (17) Morawa Eblagon, K.; Tam, K.; Kerry Yu, K. M.; Zhao, S.-L.; Gong, X.-Q.; He, H.; Ye, L.; Wang, L.-C.; Ramirez-Cuesta, A. J.; Tsang, S. C. J. Phys. Chem. C 2010, 114, 9720−9730. (18) Ye, X.; Xu, G.; Xu, G. J. Alloys Compd. 2010, 509, 152−156. (19) Duan, A.; Gao, J.; Xu, C.; Wang, D.; Zhao, Z.; Dou, T.; Chung, K. H. Mol. Simul. 2007, 33, 353−359. (20) Hammer, B. Top. Catal. 2006, 37, 3−16. (21) Okazaki, H.; Soeda, M.; Ikefuji, Y.; Tamura, R.; Mochida, I. Bull. Chem. Soc. Jpn. 1989, 62, 3622−3627. (22) Dokjampa, S.; Rirksomboon, T.; Osuwan, S.; Jongpatiwut, S.; Resasco, D. E. Catal. Today 2007, 123, 218−223. (23) Murzin, D.; Smeds, S.; Salmi, T. React. Kinet. Catal. Lett. 1974, 60, 57−64. (24) Inoue, Y.; Herrmann, J. M.; Schmidt, H.; Burwell, R. L. Jr.; Butt, J. B.; Cohen., J. B. J. Catal. 1978, 53, 401−413. (25) Borgwardt, K. H. The Simplex Method. A Probabilistic Analysis; Springer-Verlag: Berlin, Germany, 1987. (26) Huber-Carol, C.; Balakrishnan, N.; Nikulin, N. S.; Mesbah, N.; Goodness-of-Fit Test and Model Validity; Birkhauser: Boston, MA, 2002. (27) Kolda, T. G.; Lewis, R. M.; Torczon, V. SIREV 2003, 45, 385− 482. (28) Wiliams, M. F.; Fonte, B.; Woltz, C.; Jentys, A.; van Veen, J. A. R.; Lercher, J. A. J. Catal. 2007, 251, 485−496. (29) Chou, P.; Vannice, M. A. J. Catal. 1987, 107, 127−139. (30) Kasztelan, S. Appl. Catal., A 1992, 83, L1−L5. (31) Ruban, A.; Hammer, B.; Stoltze, P.; Skriver, H. L.; Nørskov, J. K. J. Mol. Catal. A: Chem. 1997, 115, 421−429. (32) Christensen, C. H.; Norskov, J. K. J. Chem. Phys. 2008, 128, 1−8. (33) Hammer, B.; Norskov, J. K. Nature 1995, 376, 238−240. (34) Pozzo, M.; Alfè, D. Int. J. Hydrogen Energy 2009, 34, 1922− 1930. (35) Hiyoshi, N.; Mine, E.; Rode, C. V.; Sato, O.; Shirai, M. Appl. Catal. 2006, 310, 194−198. (36) Hiyoshi, N.; Mine, E.; Rede, C. V.; Sato, O.; Shirai, M. Chem. Lett. 2006, 35, 188−189.

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Comparative Study of Catalytic Hydrogenation of 9-Ethylcarbazole for

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