The preparation of silica entrapped homogeneous hydrogenation catalysts by conventional and ionic liquid mediated sol-gel routes

Article abstract of DOI:10.1016/j.jorganchem.2005.03.038

[RhCl(PPh₃)₃] has been entrapped inside silica matrices by two methods: a conventional sol-gel synthesis in ethanol/water and a new route performed in an ionic liquid. The activity of these heterogenised catalysts has been tested for the hydrogenation of styrene. The catalyst prepared in an ionic liquid was found to be more active and have low Rh leaching.

Full text of DOI:10.1016/j.jorganchem.2005.03.038

Journal of Organometallic Chemistry 690 (2005) 3518–3521
www.elsevier.com/locate/jorganchem
Note
The preparation of silica entrapped homogeneous
hydrogenation catalysts by conventional and ionic liquid
mediated sol–gel routes
Steven J. Craythorne, Alan R. Crozier, Fabio Lorenzini,
*
*
Andrew C. Marr , Patricia C. Marr
CenTACat, Centre for the Theory and Application of Catalysis, The Queens University of Belfast, The David Keir Building, Stranmillis Road,
Belfast, Northern Ireland, BT9 5AG, United Kingdom
Received 14 January 2005; received in revised form 21 March 2005; accepted 21 March 2005
Available online 10 May 2005
Abstract
[RhCl(PPh3)3] has been entrapped inside silica matrices by two methods: a conventional sol–gel synthesis in ethanol/water and a
new route performed in an ionic liquid. The activity of these heterogenised catalysts has been tested for the hydrogenation of sty-
rene. The catalyst prepared in an ionic liquid was found to be more active and have low Rh leaching.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Heterogenised homogeneous catalysis; Sol–gel; Ionic liquids; Hydrogenation
1. Background
Hydrogenation is of interest to chemists involved in
heterogenising homogeneous catalysts as it is an exam-
ple of a reaction for which heterogeneous and homoge-
neous catalysts are very widely applied in industry.
[RhCl(PPh3)3] (1), often referred to as WilkinsonÕs cata-
lyst, is certainly one of the most famous homogeneous
hydrogenation catalysts [4]. 1 will hydrogenate alkenes
under a wide variety of conditions; in this study, we
use it under conditions that emphasise the difference in
selectivity between [RhCl(PPh3)3] and a heterogeneous
catalyst Rh on carbon.
Styrene is an interesting substrate for initial catalyst
testing as it provides two potential sites for hydrogena-
tion: the terminal alkene and the aromatic ring. In gen-
eral 1 shows complete selectivity towards alkene
hydrogenation to yield ethyl benzene as the sole prod-
uct. In contrast, the heterogenous catalyst Rhodium
on carbon is capable of hydrogenating the ring and
selectivity is not absolute. The entrapment of catalysts
for the hydrogenation of styrene to ethyl benzene [5],
The perfect catalyst would combine the selectivity of
homogeneous catalysts with the ease of separation of
their heterogeneous counterparts. One solution to the
synthesis of such catalysts is the entrapment of a homo-
geneous catalyst within a porous matrix by the sol–gel
method [1].
Ionic liquids have been show to act as excellent tem-
plates for porous amorphous oxides [2]. The application
of ionic liquids to the preparation of highly active heter-
ogeneous hydrogenation and Heck catalysts has been
demonstrated by Marr and co-workers [3]. In this com-
munication, we report the use of an ionic liquid in the
synthesis of a heterogenised homogeneous catalyst.
*
Corresponding authors. Tel: +44 28 90 97 4442; fax: +44 28 90 97
4687.
E-mail addresses: a.marr@qub.ac.uk (A.C. Marr), pcmarr@tiscali.
co.uk, p.marr@qub.ac.uk (P.C. Marr).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.03.038

S.J. Craythorne et al. / Journal of Organometallic Chemistry 690 (2005) 3518–3521
3519
Table 1
Catalyst activity and selectivity
and the hydrogenation of an arene ring [6] have been re-
ported previously. Here, we record the facile prepara-
tion of a heterogenised homogeneous catalysts based
on [RhCl(PPh3)3] (1) in air by two methods, including
the first preparation of such a catalyst in an ionic liquid,
and contrast the stability and activity of the two solid
catalysts.
Catalyst
Conv (%) Ethyl
Ethyl-cyclo- Ethyl-cyclo-
hexene (%)
benzene (%) hexane (%)
(1)
(2) 1st run
100
100
100
88*
92*
100
0
0
6*
3*
0
6*
5*
0
(2) 2nd run 100
(3) 1st run
(3) 2nd run
(4) 1st run
(4) 2nd run
62
77
99
98
100
100
100
0
0
0
0
0
0
2. Materials preparation
*
The calculation of the listed percentages is based on the relative
areas.
Two methods of entrapment were used in this study.
The entrapment of a homogeneous catalyst in a conven-
tional sol–gel derived glass and the entrapment in an
aerogel prepared by an ionic liquid route.
In the case of the traditional sol–gel derived glass,
alcohol and water are the common solvents used to elicit
glass formation. Catalysts to be entrapped are usually
required to be soluble in the alcohol, water or the silica
precursor. The method described here adds the catalyst
in a minimum amount of a volatile solvent, followed by
fast gelation facilitated by the addition of benzylamine,
(a basic catalyst for the sol–gel reactions). Preparation
of the aerogel entrapped catalyst is performed in a sim-
ilar manner. The catalyst is solubilised in a minimum
amount of a volatile solvent and then added to the ionic
liquid. The silica precursor is then added and gelation is
catalysed by the addition of an acid; again gelation is
fast. Both sol–gel derived materials are washed with
dichloromethane and dried. The resulting silica en-
trapped catalysts were obtained as pale yellow powders.
Any catalyst decomposition is typically accompanied by
a darkening of the solid. Using other methods of cata-
lyst synthesis we have observed extensive decolouration,
when these solids were applied as catalysts in the hydro-
genation of styrene some hydrogenation of the aromatic
ring was observed.
cyclohexene and ethyl cyclohexane. A direct comparison
of the homogeneous and the commercial heterogeneous
systems (1 and 2) is possible as the quantity of rhodium
added was kept the same in each experiment (7.6 · 10À6
mol). The complete selectivity observed for the en-
trapped catalysts suggests that the entrapment of the
molecular hydrogenation catalyst in 3 and 4 has been
completely successful, this is further supported by the
continuance of selectivity for the second catalytic run.
Longer reaction times (2 h) led to full conversion and to-
tal selectivity. Further catalyst recycle is detailed in sup-
plementary data.
Table 1 allows a comparison of the percentage
conversion for the entrapped catalysts 3 and 4. The
catalyst entrapped in silica prepared by conventional
sol–gel synthesis yields a catalyst that converts styrene
to ethyl benzene 62% and 77% in the first and second
runs, respectively. This change in conversion may sug-
gest a change in catalyst character during activity. The
performance of the aerogel catalyst prepared in an
ionic liquid is more promising; styrene is close to full
conversion after 30 min (99% and 98% converted)
and activity is almost identical for the two consecutive
catalytic runs. These trends are continued when the
number of runs is extended to 10 (see Supplementary
data).
3. Catalyst testing
Four catalysts were tested for their activity in the
hydrogenation of styrene: homogeneous [RhCl(PPh3)3]
(1), 5% Rh on carbon (Aldrich) (2), silica-entrapped 1
prepared in ethanol/water (3) and silica-entrapped 1 pre-
pared in an ionic liquid (4). The results are given in Ta-
ble 1. In each catalytic experiment conditions were
identical. The solid catalysts (2–4), were reused by
removing the product solution and injecting a fresh sub-
strate solution.
Table 1 illustrates the differences in selectivity for the
molecular and entrapped catalysts (1, 3 and 4) and the
metallic catalyst (2). All catalytic systems derived from
[RhCl(PPh3)3] (1, 3 and 4) gave 100% selectivity, pro-
ducing ethyl benzene as the sole product. 5% Rh on car-
bon (2) yielded a mixture of ethyl benzene, 1-ethyl
Table 2 summarises ICP results on the metal loading
of the entrapped catalysts and rhodium analysis of the
product solutions resulting from catalyst testing. First
we should note that the concentration of rhodium is 4
times higher in catalyst 3, this is reflected in the turnover
frequency for the catalytic runs. We conclude that the
[RhCl(PPh3)3] entrapped in an ionic-liquid derived aero-
gel (4) is 5 times more active than the [RhCl(PPh3)3]
entrapped in silica prepared by a more conventional
sol–gel route (3). Catalyst leaching rates are also much
lower for the aerogel system. Whilst 2.8% of the
rhodium leached from catalyst 3 in the first catalyst
run and 0.3% in the second; less than 0.3% of the
rhodium leached from 4 in the first run and no leaching
was detected from the second run.

3520
S.J. Craythorne et al. / Journal of Organometallic Chemistry 690 (2005) 3518–3521
Table 2
Summary of ICP OES results and turnover numbers for entrapped catalysts
Catalyst
Rh conc. in entrapped
catalyst/(wt%)
Wt. entrapped
catalyst added/(g)
Rh leaching/
(mol · 10À8
Moles of Rh in
catalytic run/(10À7
Moles of styrene
converted/(10À3
)
Turnover
frequency/sÀ1
*
)
)
(3) 1st run
(3) 2nd run
(4) 1st run
(4) 2nd run
0.668
0.161
0.0150
0.0158
2.76
0.31
9.74
9.46
2.47
2.46
5.41
6.72
8.64
8.55
3
4
0.08
<0.05
19
19
* By ICP/OES.
4. Conclusions
The gel was covered and left to age for one week. After
one week the monolith was removed and the ionic liquid
was extracted by heating under reflux in dichlorometh-
ane. The product was a pale yellow monolith of frozen
smoke. The sample was ground into a powder and left
to dry before catalyst testing.
We have reported the successful entrapment of the
hydrogenation catalyst [RhCl(PPh3)3] by conventional
and ionic liquid-mediated sol–gel syntheses. The syn-
thetic procedures for catalyst synthesis are simple and
conducted in air; and the catalysts are active, selective
and reusable. Initial results suggest that that the ionic li-
quid route provides a catalyst of higher activity and
stability.
Further research is ongoing into the structure of the
catalysts, the kinetics of reactions, the hydrogenation
of alternative substrates, the entrapment of other cata-
lysts and the assessment of other ionic liquids as solvents
for aerogel synthesis.
5.2. Catalyst testing
5.2.1. [RhCl(PPh3)3] (1), was prepared by a literature
procedure [7]
The catalyst (0.0070 g 1, 0.0156 g 2, 0.0150 g 3,
0.0158 g 4) was weighed out and flushed with N2,
CH2Cl2 (10 cm3) and styrene (1 cm3, 8.73 mmol) were
added. The autoclave was pressurised to 20 bar with
H2, stirred (1200 rpm) and heated to 100 ꢁC for
30 min. Liquid products were analysed by GC/MS (Per-
kin–Elmer Turbo Mass fitted with a Restek RTX5MX
column). Conversion was calculated by peak area rela-
tive to internal standard tetrahydropyran.
5. Experimental procedures
5.1. Catalyst preparations
5.1.1. Preparation of glass entrapped catalyst (3)
Tetraethoxysilane (TEOS) 2.42 g (0.0107 mol) was
added to Ethanol 1.73 g (0.03604 mol) and stirred for
20 min. Four drops of distilled water was added under
5.2.2. Digesting the entrapped catalysts for ICP/OES
analysis
A mixture of conc. HCl (11.5 M, 2.5 cm3) and conc.
HNO3 (16 M, 2.5 cm3) was heated and added to the
sample to be digested (0.01 g). The solution was stirred
for 5 min then water was added to make the volume
up to 100 cm3. Once cooled a 10 cm3 aliquot was sub-
mitted for analysis on a ThermoElemental IRIS Duo.
continuous
stirring.
[RhCl(PPh3)3]
0.0189 g
(2.04 · 10À5 mol) was dissolved in the minimum quan-
tity of dichloromethane (4 drops). This catalyst solution
was added to the mixture and stirred for 60 s. Four
drops of benzylamine were added and stirring was con-
tinued until the gel point was reached. The sample was
left to age for one week. After which time the sample
was ground into a powder and extracted by soxhlet
extraction with dichloromethane. The resultant pale yel-
low powder was recovered and left to dry before catalyst
testing.
5.2.3. Preparing product solution for ICP/OES analysis
The sample was dried under reduced pressure to
leave the solid residues. A preheated mixture of conc.
HCl (11.5 M, 0.25 cm3) and conc. HNO3 (16 M,
0.25 cm3) was added and the solution stirred to dis-
solve the residue. Water was added to make the vol-
ume up to 10 cm3. Once cooled the solution was
submitted for analysis.
5.1.2. Preparation of aerogel entrapped catalyst (4)
[RhCl(PPh3)3] 0.0037 g (4.00 · 10À6 mol) was dis-
solved in the minimum quantity of dichloromethane (4
drops), this catalyst solution was added to 3-methyl imi-
dazolium bistrifluoromethanesulfonimide [Bmim][NTF2]
1 cm3 (0.00525 mol). TEOS, 0.919 g (0.00441 mol) was
added and the mixture was stirred for 20 min. To this
was added methanoic acid 2 cm3, 2.44 g, (0.0530 mol)
whilst stirring. The gelation time was less than 30 min.
Acknowledgements
We thank the McClay Trust and CenTACat (QUB)
for studentships (F.L. and S.J.C.); ASEP (QUB) and
QUESTOR (QUB) for analytical expertise; Johnson

S.J. Craythorne et al. / Journal of Organometallic Chemistry 690 (2005) 3518–3521
3521
C.J. Adams, A.E. Bradley, K.R. Seddon, Aust. J. Chem. 54 (2001)
679;
Matthey PLC for loan of precious metals, and QUILL
(QUB) for providing the ionic liquid.
M. Antonietti, D. Kuang, B. Smarsly, Y. Zhou, Angew. Chem., Int.
Ed. 43 (2004) 4988.
´
[3] K. Anderson, S. Cortin˜as Fernandez, C. Hardacre, P.C. Marr,
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Appendix A. Supplementary data
[4] J.F. Young, J.A. Osborn, F.H. Jardine, G. Wilkinson, Chem.
Commun. (1965) 131;
Supplementary data associated with this article can
be found, in the online version at doi:10.1016/
j.jorganchem.2005.03.038.
F.H. Jardine, J.A. Osborn, G. Wilkinson, J.F. Young, Chem. Ind.
(1965) 560;
J.A. Osborn, F.H. Jardine, J.F. Young, G. Wilkinson, J. Chem.
Soc. A (1966) 1711.
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