Biphenyl hydrogenation over supported transition metal catalysts under supercritical carbon dioxide solvent (original) (raw)

Biphenyl hydrogenation over supported transition metal catalysts under supercritical carbon dioxide solvent

Norihito Hiyoshi a { }^{\text {a }}, Chandrashekhar V. Rode b { }^{\text {b }}, Osamu Sato a { }^{\text {a }}, Masayuki Shirai a, { }^{\text {a, }}
a{ }^{a} Supercritical Fluid Research Center, National Institute of Advanced Industrial Science and Technology, 4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551, Japan
b{ }^{\mathrm{b}} Homogeneous Catalysis Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India

Received 25 November 2004; received in revised form 19 March 2005; accepted 11 April 2005
Available online 31 May 2005

Abstract

Catalytic hydrogenation of biphenyl to bicyclohexyl, an organic hydrogen storage medium, was examined over supported transition metal catalysts in supercritical carbon dioxide solvent. The yield of bicyclohexyl was almost 100%100 \% over the charcoal-supported rhodium ( Rh/C\mathrm{Rh} / \mathrm{C} ) and ruthenium ( Ru/C\mathrm{Ru} / \mathrm{C} ) catalysts at the temperature of 323 K , which was much lower than that required for biphenyl hydrogenation in organic solvents ( 573 K ). The initial activity was higher over the Rh/C\mathrm{Rh} / \mathrm{C} catalyst, while the initial selectivity to bicyclohexyl was higher over the Ru/C\mathrm{Ru} / \mathrm{C} catalyst. The conversion of biphenyl increased with increase in hydrogen and carbon dioxide pressures, while the selectivity to bicyclohexyl was independent of hydrogen and carbon dioxide pressures over both catalysts. © 2005 Elsevier B.V. All rights reserved.

Keywords: Supercritical carbon dioxide; Hydrogenation; Biphenyl; Charcoal-supported rhodium catalyst; Charcoal-supported ruthenium catalyst; Bicyclohexyl; Hydrogen storage

1. Introduction

Hydrogen promises to be a clean energy source without any emission of greenhouse gases; however, its storage is a key process for its utilization. Cyclic saturated hydrocarbons, such as bicyclohexyl, decalin, methylcyclohexane and cyclohexane are proposed as compact and light hydrogen storage media for fuel cells [1-5]. Hydrogen can be obtained by dehydrogenation of the corresponding cyclic hydrocarbons to aromatic compounds. It is reported that the catalytic dehydrogenation rate of bicyclohexyl is the highest among the cyclic hydrocarbons [2], indicating that bicyclohexyl is the most promising organic hydrogen storage medium. Liquid phase hydrogenation of biphenyl over metal catalysts [6,7] has been reported. However, difficulty in the separation of bicyclohexyl from organic solvents becomes a critical issue in the liquid phase hydrogenation process.

[1]Supercritical carbon dioxide (Tc=304.2 K\left(T_{\mathrm{c}}=304.2 \mathrm{~K}\right. and Pc=7.4MPaP_{\mathrm{c}}=7.4 \mathrm{MPa} ) can be made miscible with light gases, hydrocarbon and aromatics by a proper choice of pressure and temperature. Hydrogenations with solid catalysts with supercritical carbon dioxide have several advantages: (i) higher reaction rates due to increased solubility of hydrogen and unsaturated compounds in supercritical fluid, thereby eliminating mass transfer resistance and (ii) easy separation of catalysts and products [8-10]. In this paper, we report the catalytic behavior of charcoal-supported rhodium ( Rh/C\mathrm{Rh} / \mathrm{C} ) and ruthenium ( Ru/C\mathrm{Ru} / \mathrm{C} ) catalysts at low temperature for biphenyl hydrogenation in supercritical carbon dioxide medium.

2. Experimental

Commercially available catalysts were used in this study: viz. 5wt.%5 \mathrm{wt} . \% charcoal-supported rhodium, platinum ( Pt/C\mathrm{Pt} / \mathrm{C} ), palladium (Pd/C), ruthenium and 5wt.%γ5 \mathrm{wt} . \% \quad \gamma-aluminasupported rhodium (Rh/Al2O3)\left(\mathrm{Rh} / \mathrm{Al}_{2} \mathrm{O}_{3}\right), platinum (Pt/Al2O3)\left(\mathrm{Pt} / \mathrm{Al}_{2} \mathrm{O}_{3}\right), palla-

1. a Corresponding author. Tel.: +81222375219 ; fax: +81222375224 . E-mail address: m.shirai@aist.go.jp (M. Shirai). ↩︎

dium (Pd/Al2O3)\left(\mathrm{Pd} / \mathrm{Al}_{2} \mathrm{O}_{3}\right), and ruthenium (Ru/Al2O3)\left(\mathrm{Ru} / \mathrm{Al}_{2} \mathrm{O}_{3}\right) from Wako Pure Chemical Ind. Ltd., Japan. The dispersion values of metal particles on the supports were determined by a hydrogen adsorption method [10]. All catalysts were used without further reduction for the hydrogenation of biphenyl. Also, a 5wt.%5 \mathrm{wt} . \% charcoal-supported nickel catalyst ( Ni/C\mathrm{Ni} / \mathrm{C} ) was prepared with an impregnation method using nickel chloride and charcoal (Wako Pure Chemical Ind. Ltd., Japan); following reduction at 573 K for 2 h , it was examined. The weighed amounts of catalyst (typically 0.02 g ) and biphenyl (typically 2.3 mmol ) were placed in a stainless-steel high pressure reactor ( 50 ml capacity) and the reactor was flushed three times with carbon dioxide. After the required temperature ( 323 K ) was attained with a hot air circulating oven, first hydrogen and then carbon dioxide were introduced into the reactor to the desired pressure levels. After the reaction period, the reactor was cooled down rapidly with an ice bath, the pressure was released slowly and the contents were discharged to separate the catalyst by simple filtration. The unreacted biphenyl and products were recovered with acetone, which showed a material balance of more than 95%95 \%. The quantitative analysis was conducted with GC-FID (HP-6890) with a DB-WAX capillary column.

3. Results and discussion

Table 1 shows the activities of various supported catalysts (expressed as TON) for hydrogenation of biphenyl under 6 MPa of hydrogen and 10 MPa of carbon dioxide at 323 K . Both rhodium and ruthenium supported on carbon were found to be highly active catalysts for the ring hydrogenation of biphenyl [11]. As can be seen from Table 1, the dispersion of rhodium and ruthenium metal particles on alumina was higher than that on charcoal; however, turnover numbers for alumina-supported rhodium and ruthenium catalysts were lower than those for charcoal-supported ones. The Rh/Al2O3\mathrm{Rh} / \mathrm{Al}_{2} \mathrm{O}_{3} catalyst was the most active among the alumina-supported catalysts under supercritical carbon dioxide; however, the turnover number for the Rh/Al2O3\mathrm{Rh} / \mathrm{Al}_{2} \mathrm{O}_{3} catalyst was much lower
(about one tenth) than that for the Rh/C\mathrm{Rh} / \mathrm{C} catalyst. Although further systematic investigation is needed about particle size and support effects, charcoal-supported catalysts were found to be more efficient for the ring hydrogenation of phenol also in a supercritical carbon dioxide medium [9]. Koussathana et al. reported that an alumina-supported platinum was the most active catalyst for liquid phase hydrogenation of biphenyl in nn-hexadecane solvent at 573 K [6]. In contrast to this, in our work alumina-supported catalysts, particularly Pt/Al2O3\mathrm{Pt} / \mathrm{Al}_{2} \mathrm{O}_{3}, showed the least (TON=8)(\mathrm{TON}=8) hydrogenation activity, while a different metal function (i.e. rhodium) was found to be highly active (TON=1490)(\mathrm{TON}=1490) for ring hydrogenation at a much lower temperature ( 323 K ) in supercritical carbon dioxide medium. This could be due to the fact that platinum could not activate the substrate at lower reaction temperature and/or in presence of supercritical carbon dioxide medium. Supported nickel catalysts are generally effective for hydrogenation of unsaturated compounds [12]; however, Ni/C\mathrm{Ni} / \mathrm{C} catalyst did not show any hydrogenation activity for biphenyl under supercritical carbon dioxide conditions.

For all the catalysts studied in this work, formation of cyclohexylbenzene and bicyclohexyl was observed by GC-MS and GC analysis. Bicyclohexyl is a preferred hydrogenation product as a hydrogen storage medium because its hydrogen content ( 7.3wt.%7.3 \mathrm{wt} . \% ) is larger than that of cyclohexylbenzene ( 3.8wt.%3.8 \mathrm{wt} . \% ). As can be seen from Table 1, both Rh/C\mathrm{Rh} / \mathrm{C} and Ru/C\mathrm{Ru} / \mathrm{C} catalysts also showed higher selectivity to bicyclohexyl in comparison with other catalysts studied in this work. It is also noteworthy that supported ruthenium catalysts were more selective to bicyclohexyl than supported rhodium catalysts (Table 1).

In order to compare the activities of Rh/C\mathrm{Rh} / \mathrm{C} catalyst in organic solvents, we also carried out some hydrogenation experiments separately, the results are presented in Table 2. For this purpose, nn-heptane and methanol were used as solvents at 323 K . It was found that conversion values of biphenyl as well as in methanol obtained in nn-heptane were substantially lower than those obtained in supercritical carbon dioxide, though the nature of nn-heptane (non-polar) was considered similar to that of supercritical carbon

Table 1
Catalyst screening for the hydrogenation of biphenyl a{ }^{a}

[1]

1. a { }^{\text {a }} Temperature 323 K ; reaction time 30 min ; catalyst 0.02 g ; hydrogen pressure 6 MPa ; initial biphenyl 2.3 mmol ; carbon dioxide pressure 10 MPa .
   b{ }^{\mathrm{b}} TON == moles of biphenyl reacted/moles of surface metal atoms. ↩︎

Table 2
The hydrogenation of biphenyl over Rh/C\mathrm{Rh} / \mathrm{C} catalyst in different solvents a{ }^{a}

a{ }^{a} Temperature 323 K ; reaction time 15 min ; catalyst 0.02 g ; hydrogen pressure 3 MPa ; initial biphenyl 2.3 mmol .
dioxide. We also observed that the catalyst activity under non-supercritical conditions ( 0.1 MPa of carbon dioxide) was lower than that under supercritical carbon dioxide ( 20 MPa of carbon dioxide; Table 2). This clearly demonstrates that low temperature hydrogenation of biphenyl catalyzed by Rh/C\mathrm{Rh} / \mathrm{C} in a supercritical carbon dioxide medium gave the highest TON and the highest yield of the desired bicyclohexyl product.

Fig. 1(a) and (b) show the reaction profiles for the hydrogenation of biphenyl over Rh/C\mathrm{Rh} / \mathrm{C} and Ru/C\mathrm{Ru} / \mathrm{C} catalysts, respectively, under supercritical carbon dioxide conditions.

Fig. 1. The hydrogenation of biphenyl under hydrogen pressure 3 MPa , carbon dioxide pressure 15 MPa and initial biphenyl 2.3 mmol at 323 K for 0.02 g of (a) Rh/C\mathrm{Rh} / \mathrm{C} and (b) Ru/C\mathrm{Ru} / \mathrm{C} : ( □\square ) Biphenyl; ( Δ\mathbf{\Delta} ) cyclohexylbenzene; (✓)(\checkmark) bicyclohexyl.

From the beginning of the reaction, both cyclohexylbenzene and bicyclohexyl were obtained over both the catalysts. In the case of the Ru/C\mathrm{Ru} / \mathrm{C} catalyst (Fig. 1(b)), higher selectivity to bicyclohexyl was observed than that for the Rh/C\mathrm{Rh} / \mathrm{C} catalyst. After more than 80%80 \% of initial biphenyl was hydrogenated, the amount of cyclohexylbenzene decreased rapidly and that of bicyclohexyl increased for both the catalyst systems and finally, the yield of bicyclohexyl was almost 100%100 \% after 110 and 140 min for the Rh/C\mathrm{Rh} / \mathrm{C} and Ru/C\mathrm{Ru} / \mathrm{C} catalysts, respectively. Thus, bicyclohexyl was formed by both the direct hydrogenation of biphenyl and the consecutive hydrogenation via cyclohexylbenzene. It is probable that the adsorption of biphenyl on rhodium and ruthenium surfaces is stronger than that of cyclohexylbenzene; the hydrogenation of cyclohexylbenzene thereby proceeds only after most of the biphenyl is consumed in the reaction system.

Fig. 2(a) shows the dependence of the conversion of biphenyl on hydrogen pressure for the Rh/C\mathrm{Rh} / \mathrm{C} and Ru/C\mathrm{Ru} / \mathrm{C} catalysts. The conversion of biphenyl increased by 2−2.52-2.5 times with increase in hydrogen pressure from 3 to 9 MPa

Fig. 2. (a) The effect of hydrogen pressure on the conversion of biphenyl at reaction time of 15 min for Rh/C(□)\mathrm{Rh} / \mathrm{C}(\square) and Ru/C(□)\mathrm{Ru} / \mathrm{C}(\mathbf{\square}), and (b) the selectivity to bicyclohexyl at hydrogen pressure 3MPa(◯),6MPa(△),9MPa(∇)3 \mathrm{MPa}(\bigcirc), 6 \mathrm{MPa}(\triangle), 9 \mathrm{MPa}(\nabla) for Rh/C\mathrm{Rh} / \mathrm{C} and 3MPa(∼),6MPa(Δ),9MPa(∇)3 \mathrm{MPa}(\boldsymbol{\sim}), 6 \mathrm{MPa}(\mathbf{\Delta}), 9 \mathrm{MPa}(\mathbf{\nabla}) for Ru/C\mathrm{Ru} / \mathrm{C}. Carbon dioxide pressure 15 MPa ; initial biphenyl 2.3 mmol ; catalyst weight 0.02 g ; reaction temperature 323 K .

for the Rh/C\mathrm{Rh} / \mathrm{C} and Ru/C\mathrm{Ru} / \mathrm{C} catalysts, respectively. The surface hydrogen atoms would increase with increase in hydrogen pressure, leading to the higher catalyst activity. Similar firstorder dependence was observed in the hydrogenation of an aromatic compound under supercritical carbon dioxide [9]. Fig. 2(b) shows that the selectivity to bicyclohexyl increased with increase in the conversion of biphenyl for the Rh/C\mathrm{Rh} / \mathrm{C} and Ru/C\mathrm{Ru} / \mathrm{C} catalysts; however, the conversion-selectivity curves were not influenced by the hydrogen pressure in the range of 3−9MPa3-9 \mathrm{MPa} studied in this work indicating that the hydrogen pressure did not influence the selectivity to bicyclohexyl.

Fig. 3 shows the effect of initial biphenyl concentration on conversion and selectivity under supercritical carbon dioxide conditions. We visually confirmed with a view cell that 2.3 mmol of biphenyl was completely dissolved in carbon dioxide under supercritical conditions ( 15 MPa , 323 K ) in a 50 ml capacity reactor. The ratio of initial concentration of biphenyl to the weight of the catalysts was kept constant in these experiments. The initial concentration of biphenyl influenced neither the conversion of biphenyl nor the selectivity to bicyclohexyl for both the catalysts

Fig. 3. The effect of amount of initial biphenyl on (a) the conversion of biphenyl and (b) the selectivity to bicyclohexyl for Rh/C(□)\mathrm{Rh} / \mathrm{C}(\square) and Ru/C(□)\mathrm{Ru} / \mathrm{C}(\mathbf{\square}). Hydrogen pressure 3 MPa ; carbon dioxide pressure 15 MPa ; reaction time 30 min ; reaction temperature 323 K . The ratio of initial biphenyl to catalyst was constant (0.12 mol/g)(0.12 \mathrm{~mol} / \mathrm{g}).

Fig. 4. The effect of carbon dioxide pressure on (a) the conversion of biphenyl and (b) the selectivity to bicyclohexyl for Rh/C(□)\mathrm{Rh} / \mathrm{C}(\square) and Ru/C(□)\mathrm{Ru} / \mathrm{C}(\mathbf{\square}). Hydrogen pressure 3 MPa ; initial biphenyl 2.3 mmol ; catalyst weight 0.02 g ; reaction time 30 min ; reaction temperature 323 K .
studied in this work. The kinetic order with respect to hydrogen and biphenyl appeared to be almost first order and zero order, respectively, in supercritical carbon dioxide medium.

Fig. 4 shows the effect of carbon dioxide pressure on conversion and selectivity for biphenyl hydrogenation at 3 MPa of hydrogen pressure over the Rh/C\mathrm{Rh} / \mathrm{C} and Ru/C\mathrm{Ru} / \mathrm{C} catalysts. The conversion of biphenyl initially increased linearly with increase in carbon dioxide pressure up to 15 MPa ; beyond this value it remained constant. The selectivity to bicyclohexyl was almost constant regardless of variation in carbon dioxide pressure in the range of 5 25 MPa . The enhanced activity with increase in carbon dioxide pressure was also observed for hydrogenation of phenol and isophorone; it can be attributed to the increased solubility of organic molecules at higher carbon dioxide pressure [9,10][9,10]. In another experiment using a view cell, it was observed that the solubility of biphenyl increased with increase in carbon dioxide pressure from 5 to 15 MPa ; that the initial amount of biphenyl ( 2.3 mmol ) was found to be completely dissolved under 12−13MPa12-13 \mathrm{MPa} of carbon dioxide pressure at 323 K and hydrogen pressure of 3 MPa . The

increased solubility of the reactant in supercritical carbon dioxide leads to enhanced mass transfer of reactant molecules from bulk phase to the active metal surface in carbon matrix. However, in the case of hydrogenation of biphenyl under supercritical carbon dioxide, the mass transfer of biphenyl is not the rate-determining step, because the rate of biphenyl hydrogenation was independent of the concentration of biphenyl. The reasons for higher catalytic activity under higher carbon dioxide pressure are: (i) the solubility of products increases with increase in carbon dioxide pressure, which would enhance removal of products from the active sites to carbon dioxide phase, and/or (ii) the rate of reaction on metal surface is influenced by carbon dioxide, which alters concentrations of adsorbed species or the electronic state of metal particle.

4. Conclusion

Hydrogenation of biphenyl to bicyclohexyl with >99%>99 \% yield could be achieved in supercritical carbon dioxide medium using rhodium on carbon catalyst at 323 K . A systematic comparative study showed that Rh/C\mathrm{Rh} / \mathrm{C} gave the highest activity (TON =1490=1490 ) among all the transition metals screened for biphenyl hydrogenation. It is also important to note that the most active metal function for biphenyl hydrogenation in supercritical carbon dioxide medium was found to be rhodium (at 323 K ); that was different from the results reported for such hydrogenations
in organic solvents (platinum, at 573 K ). The enhanced catalyst activity in supercritical medium could be due to the increased product solubility in supercritical carbon dioxide and/or different adsorption characteristics of reactants on the metal surface with modified electronic states due to interaction with carbon dioxide.

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