Selective Hydrogenation of Single Benzene Ring in Biphenyl Catalyzed by Skeletal Ni (2009)

The nickel lining on a Raney day: Highly selective catalytic hydrogenation of biphenyl (BP) to cyclohexylbenzene (CHB) was achieved by using skeletal Ni prepared from rapidly quenched Ni–Al alloys ribbons, affording 100 % conversion and 99.4 % selectivity in a one-pot reaction. Selective catalytic hydrogenation of unsaturated compounds has been attracting worldwide attention for decades.1 Many interesting results have been reported on various substrates with different catalyst systems, such as chemoselective hydrogenation of nitrobenzene on supported Au,2 partial hydrogenation of dinitrobenzene to nitroanilines on Ru/C,3 and selective hydrogenation of p-phenylphenol to p-cyclohexylphenol on Pd/C.4 However, very limited knowledge has been accumulated on the selective catalytic hydrogenation of a single benzene ring in a molecule containing two or more equal phenyl groups. Cyclohexylbenzene (CHB) is a highly valued chemical, which is typically applied as a synthetic intermediate5 or an electrolyte additive for overcharge protection in lithium ion batteries.6 In principle, CHB can be prepared by using several methods such as alkylation of benzene with cyclohexene,7 hydroalkylation of benzene,8 cross-coupling of alkyl halides with aryl Grignard reagents,9 and hydrodechlorination of chlorinated biphenyl.10 Among these methods, the alkylation technique shows advantageous characteristics and has been applied to industry; nevertheless the CHB yield is still poor . It is important to explore an efficient and environmentally benign approach for CHB production. Selective hydrogenation of biphenyl (BP) is known to be a superior technique. Selective hydrogenation of a compound containing two benzene rings was first reported in 1949 by H. Smith et al,11 in which Pd/C, Ru/C, Pt/C and Rh/C were studied. Later in 1968, a patent claimed the selective hydrogenation of BP to CHB using supported Pd, Pt, Ru and Rh as catalysts.12 Then, the hydrogenation activity of sulfided mixed oxide of Co and Mo on Al2O3 was studied.13 The BP hydrogenation was also performed under different conditions, for example in dilute aqueous alkaline solution14 or in supercritical carbon dioxide.15 Although these previous efforts have been attractive in terms of catalysis, the low atom-economic efficiency needed to be improved. Herein, we present a novel and highly efficient technology for the extremely selective catalytic hydrogenation of BP to CHB over skeletal Ni (S-Ni or Raney Ni). The most active skeletal Ni catalyst was prepared from rapidly quenched Ni–Al alloys with Mo as a promoting additive. Excellent 100 % BP conversion and 99.4 % CHB selectivity were obtained in a one-pot reaction. The kinetic equation for the hydrogenation of BP to CHB was also established. Theoretically, hydrogenation of BP is a typical consecutive reaction containing at least two steps (Scheme 1). CHB is the intermediate and bicyclohexyl (BCH) is the final product. Cyclohexenylbenzene may exist as another intermediate, but it was unstable and therefore not detected, which was confirmed by our experimental results at the early stage of this study. Reaction pathways for the hydrogenation of BP. The experimental details are described in the Supporting Information. Table 1 shows the hydrogenation results of the catalytic performance of five typical catalysts. It is clear that both Pd/C and Pt/C do not have enough activity, whereas Ru/C is relatively active but the selectivity to CHB is only about 66.8 %. In contrast, common skeletal Ni is much more active and selective than the supported noble metals and gave a maximum CHB yield of 99.1 %. Furthermore, the quenched skeletal Ni (QS-Ni) had the highest activity and selectivity with CHB yield being up to 99.4 % under lower temperature and pressure. The result was reproducible. By using a 100 times volume-enlarged autoclave, the same activity and selectivity were obtained. The QS-Ni catalyst was stable and retained good activity and selectivity even after running repeatedly. The same high yield was obtained for each run with only a slight increase of reaction time. To maintain a similar reaction time, addition of less than 1 wt. % fresh QS-Ni was necessary when the catalyst was recycled. Cat. T [K] P [MPa] t [h] Conv. [%] Sel. [%] Pt/C 433 3.0 8 1.2 91.2 Pd/C 393 3.0 8 35.6 98.9 Ru/C 343 1.0 4 86.2 66.8 S-Ni 348 1.8 5 100 99.1 QS-Ni 343 1.0 4 100 99.4 Figure 1 shows the XRD diffraction peaks of the QS-Ni. Typical Ni(111), Ni(200) and Ni(220) peaks were detected, indicating the structural characteristics of skeletal Ni. It has been reported that the skeletal Ni prepared by the rapid quenching technique exhibited higher activity than that obtained by the more conventional method.16, 17 The surface of the catalyst was concluded to be rich in low-coordination sites and defects, which are essential for adsorption and activation of the reactants.17 By using Scherrer’s equation, the average Ni crystalline size of QS-Ni was estimated to be 5 nm, smaller than the conventional S-Ni (about 6 nm). XRD patterns of QS-Ni catalyst. The concentration of BP, CHB, and BCH during the reaction process was monitored by gas chromatography (GC) and the relationships of component concentration versus reaction time are shown in Figure 2. It is clear that the hydrogenation of BP occurs consecutively with CHB as a stable intermediate and BCH as the final product. The hydrogenation rate of BP to CHB is much higher than that from CHB to BCH. Therefore it is theoretically possible to stop the reaction at the intermediate step with CHB as the main product. The concentration of BP in the reaction solution decreased almost linearly after the reaction started. BP was completely converted into CHB in 4 h with the average dissipation rate being 2.17×10−1 mol L−1 h−1 and then the hydrogen uptake rate suddenly became very slow. During the next 4 h, the concentration of CHB changed from 0.862 mol L−1 to 0.84 mol L−1. The average hydrogenation rate of CHB to BCH is 5.5×10−3 mol L−1 h−1, indicating that it is 40 times slower than the rate from BP to CHB. In principle, this rate difference is big enough to terminate the reaction following one benzene ring hydrogenation. Based on the GC analysis, 100 % BP conversion and 99.4 % CHB selectivity were achieved, and the other 0.6 % was the completely hydrogenated product BCH. No other byproducts were detected under our experimental conditions. The concentration-time plots for the hydrogenation of BP in THF. Reaction conditions: 343 K, 1 MPa, =0.867 mol L−1, V=23 mL, 1.125 g QS-Ni catalyst. The concentration-time plots of BP hydrogenation at different temperatures over QS-Ni (1.0 MPa, 19.48 mmol BP, 15 mL THF, 0.376 g net Ni calculated in its alloy powder form). The Arrhenius plot for BP hydrogenation. It is believed that the excellent catalytic performance of the QS-Ni is due to the intrinsic structural properties of the skeletal Ni. In addition to the catalyst itself, the distinct difference of the molecular structures between BP and CHB also contributes to the unique selectivity. Theoretical and experimental studies into the stereo-effect and adsorption state of BP and CHB on different catalyst surfaces are currently in progress. In summary, a highly efficient and environmentally benign technique for the selective hydrogenation of BP to CHB has been developed. Skeletal Ni prepared from rapidly quenched Ni–Al alloy was the most active and selective catalyst. The hydrogenation of BP occurs through two consecutive steps with a 40 times increase in the reaction rate. CHB can be produced with 99.4 % yield. This unique result will not only contribute to applied chemistry as a practical technique, but also promote the fundamental understanding of selective hydrogenation of other compounds containing two or more equal benzene rings and their derivatives. Analysis: Samples were quantitatively analyzed by GC (Agilent 6890, FID detector at 523 K, Tinj=553 K) with 30 m HP-5 capillary column from 393–453 K at a rate of 5 K min−1 by using a calibrated area normalization method. We thank the National High Technology Research and development Program of China (863 Program), Grant No. 2007AA03Z345. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. 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