(699d) 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

* david.jackson@glasgow.ac.uk

Introduction.

            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 [1] and pentene [2] 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 [4]. 

Experimental

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 [5], 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 [6], 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 [7]
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. [8] 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. [9] who studied
hydrogen adsorption on rhodium.  Using hydrogen/deuterium mixtures coupled to
TDS and HREELS, Winkler et al. [10] 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 [8],
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.

References.

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S.D. Jackson Catalysis Science & Technology, 2012, Advance Article
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