277627 Alkene Hydrogenation and Isomerisation Over Rh Catalysts: Evidence of Sub-Surface Hydrogen
Alkene hydrogenation and isomerisation over Rh catalysts:
evidence of sub-surface hydrogen.
Lorna C. Begley, Kirsty J. Kakanskas, Andrew Monaghan and S. David Jackson*
Centre for Catalysis Research, WestCHEM, Department of Chemistry, University of Glasgow, Glasgow, G12 8QQ, Scotland
The hydrogenation of alkenes to alkanes is an area of catalysis that has been active for over 100 years and yet our understanding is not complete. Studies have shown that although the hydrogenation of ethene is structure insensitive, higher homologues such as propene  and pentene  do show structure sensitivity. Along with hydrogenation comes isomerisation and in some very elegant work Zaera and co-workers [3 and references therein] investigated alkene isomerisation and showed over Pt that the shape of the crystallite, and hence the crystal face, had a significant effect on trans-cis and cis-trans isomerisation such that the rate of each reaction was different depending on the starting isomer. It is clear that these apparently simple reactions are much more complex and sensitive to surface structure than was once thought. In this study we have examined the hydrogenation and isomerisation of alkenyl aromatics, namely allylbenzene, cis-b-methyl styrene and trans-b-methyl styrene over a Rh/silica catalyst .
The catalyst used throughout the study was a 2.5% w/w Rh/SiO2 (Rh dispersion 60 %, metal crystallite size 1.8 nm, support surface area 488 m2g-1). Davison Catalysts supplied the silica support, while the active catalyst was prepared by Johnson Matthey by incipient-wetness using aqueous rhodium chloride salts. The catalyst was dried and reduced in flowing H2/N2. All reactants were used without further purification. The reaction was carried out in a 0.5l Buchi stirred autoclave with a hydrogen-on-demand delivery system. Around 0.05 g of catalyst was added to 330 ml of degassed solvent, 2-propanol. Reduction of the catalyst was performed in situ at 313 K. For allylbenzene (AB), cis-b-methylstyrene (CBMS) and trans-b-methylstyrene (TBMS), 1.5 ml (AB 11.3 mmoles, TBMS and CBMS 11.6 mmoles) was injected into an unstirred solution, followed by 20 ml of degassed 2-propanol (IPA) to ensure that all the reactant was washed into the reactor. The vessel was pressurised with H2 to 1 barg. Following this the stirrer was set to a speed of 1000 rpm and samples taken at regular intervals. Liquid samples were analysed by GC using an FID detector and a 50 m CP-Al2O3/Na2SO4 column. Standard checks were undertaken to confirm that the system was not under mass transport control.
Results and Discussion
The three isomers were hydrogenated to phenyl propane (PP) at 313 K and 1 barg. When we examine the first order rate constants for the hydrogenation of the individual isomers, we find that CBMS has the fastest rate and we obtain a ratio of rates of CBMS:AB:TBMS of 4.2:2.8:1. This behaviour has been observed with alkenyl aromatics over Pd , where a similar ratio of CBMS:AB:TBMS of 3.8:2.2:1 was obtained. Comparable results were also found with pentene hydrogenation over Pd , where the cis-isomer was also found to be the most reactive followed by 1-pentene while the trans-isomer was the least reactive.
When CBMS and TBMS are co-hydrogenated, the first order rate constant for each isomer is approximately halved from that observed when each was hydrogenated singly. This is typical for a competitive reaction where the increased concentration of reactants reduces the number of sites available to each. On the contrary, when the pairing is AB/CBMS or AB/TBMS the AB reactivity is unaffected while the reactivity of the other isomer is decreased. This is, at first sight, a surprising result and we see it taken to the extreme when, with all three isomers present, TBMS does not hydrogenate to any significant extent (figure 1).
Figure 1. Comparison of first-order rate constants.
The rate of AB hydrogenation is barely affected in any of the competitive reactions, whereas the activities of both CBMS and TBMS are drastically reduced. Analysing the AB/CBMS reaction in detail reveals that AB hydrogenates more rapidly to phenyl propane, while CBMS isomerises to TBMS until all the AB has been hydrogenated, only then does CBMS hydrogenate to phenyl propane. A similar behaviour is seen with the AB/TBMS pairing. Thus AB inhibits hydrogenation but not isomerisation of CBMS and TBMS. This difference between hydrogenation and isomerisation has been observed previously over Pd  but not over Rh. In the Pd case it was found that over single crystal Pd(111) no hydrogenation took place with a pentene/hydrogen mix yet hydrogenation occurred over small Pd particles under identical conditions. The reason behind this difference lies in the availability of “sub-surface” hydrogen. Schauermann et al.  showed that for hydrogenation over palladium, fast diffusion of hydrogen into these sub-surface sites was required, however if this was inhibited, then only isomerisation was observed. The existence of a sub-surface state was first tentatively assigned by Nieuwenhuys et al.  who studied hydrogen adsorption on rhodium. Using hydrogen/deuterium mixtures coupled to TDS and HREELS, Winkler et al.  revealed the presence of sub-surface hydrogen on Rh(100). Therefore rhodium can accommodate sub-surface hydrogen in a manner similar to palladium. Over palladium it was also shown that rapid diffusion into the sub-surface occurred via modified edge and corner sites , coupled to this isomerisation/hydrogenation of internal alkenes has been shown to occur on terrace and plane faces of Pd crystallites, while isomerisation/hydrogenation of terminal alkenes has been shown to occur on edges and corners [2, 11, 12]. Therefore in the Rh system, if the adsorbed terminal alkene inhibited the fast diffusion of hydrogen to the sub-surface by reacting with it then we would expect isomerisation of the internal alkene but not hydrogenation – as is indeed observed. This suggests that alkene hydrogenation and isomerisation over rhodium is behaving in an analogous manner to palladium.
The co-hydrogenation of the three isomers behaves in a similar manner: AB is hydrogenated to phenyl propane, while CBMS is isomerised to TBMS. When the system is run with low concentrations a very clear demarcation of the hydrogenation activity is observed. AB is rapidly hydrogenated while CBMS is preferentially hydrogenated before TBMS, which does not hydrogenate until all the AB and over 60 % of the CBMS are hydrogenated (figure 2).
Figure 2. Conversion (%) of AB, CBMS and TBMS during co-reaction.
1. R.L. Burwell Jr, Langmuir, 1986, 2, 2-11; P.O. Otero-Schipper, W.A. Wachter, J.B. Butt, J.B. Cohen, R.L. Burwell Jr, J. Catal., 1977, 50, 494-507
2. A.M. Doyle, S.K. Shaikhutdinov, and H-J. Freund, Angew. Chem. Int. Ed. 2005, 44, 629-631.
3. F. Zaera, Acc. Chem. Res., 2009, 42, 1152-1160.
4. L.C. Begley, K.J. Kakanskas, A. Monaghan, S.D. Jackson Catalysis Science & Technology, 2012, Advance Article DOI: 10.1039/C2CY20105D
5. L.C. Begley, S. David Jackson, unpublished results.
6. A.S. Canning, S.D. Jackson, A. Monaghan, T. Wright, Catalysis Today, 2006, 116, 22-29.
7. A.M. Doyle Sh.K. Shaikhutdinov, H.-J. Freund, J. Catal., 2004, 223, 444-453
8. W. Ludwig, A. Savara, K.-H. Dostert, S. Schauermann, J. Catal., 2011, 284, 148-156
9. V.V. Gorodetskii, B.E. Nieuwenhuys, W.M.H. Sachtler, G.K. Boreskov, Surf. Sci., 1981, 108, 225-234
10. G. Pauer, A. Eichler, M. Sock, M. G. Ramsey, F. Netzer, A. Winkler, J. Chem. Phys., 2003, 119, 5253-5266
11. J.A. Anderson, J. Mellor, R.P.K. Wells, J. Catal., 2009, 261, 208-216
12. P.E. Garcia, A.S. Lynch, A. Monaghan, S.D. Jackson, Catalysis Today, 2011, 164, 548-551