The hydrogenation of para-toluidine over rhodium/silica: The effect of metal particle size and support texture

Article abstract of DOI:10.1016/j.jcat.2006.05.019

The hydrogenation of para-toluidine was studied over a series of rhodium/silica catalysts. The reaction exhibits an antipathetic particle size effect, suggesting that plane face surface atoms, such as C³₉ sites, are the active site for ring hydrogenation. The reaction is zero-order in p-toluidine and first-order in hydrogen, with the mechanism of formation of the cis and trans isomers likely through surface immine and enamine intermediates. The activation energy for the trans isomer (62 ± 4 kJ mol^-1) was systematically greater than that of the cis isomer (51 ± 6 kJ mol^-1). The support was also shown to play a part, adsorbing trans-4-MCYA in the early stages of the reaction, resulting in a varying cis:trans ratio. The effect of pore size was also studied; a dramatic decrease in activity was observed at pore diameters <64 A.

Full text of DOI:10.1016/j.jcat.2006.05.019

Journal of Catalysis 241 (2006) 417–425
www.elsevier.com/locate/jcat
The hydrogenation of para-toluidine over rhodium/silica: The effect of
metal particle size and support texture
∗
Kenneth T. Hindle, S. David Jackson , Diane Stirling, Geoff Webb
WestCHEM, Department of Chemistry, The University of Glasgow, Glasgow G12 8QQ, Scotland
Received 13 March 2006; revised 16 May 2006; accepted 21 May 2006
Available online 21 June 2006
Abstract
The hydrogenation of para-toluidine was studied over a series of rhodium/silica catalysts. The reaction exhibits an antipathetic particle size ef-
3
9
fect, suggesting that plane face surface atoms, such as C sites, are the active site for ring hydrogenation. The reaction is zero-order in p-toluidine
and first-order in hydrogen, with the mechanism of formation of the cis and trans isomers likely through surface immine and enamine intermedi-
−
1
−1
ates. The activation energy for the trans isomer (62 ± 4 kJmol ) was systematically greater than that of the cis isomer (51 ± 6 kJmol ). The
support was also shown to play a part, adsorbing trans-4-MCYA in the early stages of the reaction, resulting in a varying cis:trans ratio. The effect
of pore size was also studied; a dramatic decrease in activity was observed at pore diameters <64 Å.
©
2006 Elsevier Inc. All rights reserved.
Keywords: Hydrogenation; Para-toluidine; Rhodium/silica; Particle size effect; Kinetics; Mechanism
1
. Introduction
ing agents, and as intermediates in the pharmaceutical industry.
This study investigates the hydrogenation of p-toluidine as an
exemplar reaction for the fine chemicals industry. p-Toluidine
was chosen for three reasons: (i) the aromatic ring can be hydro-
The fine chemical industry relies heavily on traditional or-
ganic synthesis, in which catalysed reactions are often per-
formed with homogeneous catalysts. Today, however, hetero-
geneous catalysts are beginning to be introduced to the fine
chemical industry because of their many advantages, including
separation, recovery, ease of handling and recycling, and stabil-
ity [1]. Another important advantage of heterogeneous catalysis
in fine chemical synthesis is the benefits to the environment.
The quantity of byproducts produced in a reaction is given
by the E-factor (kg of byproduct/kg of product). E-factors of
genated, partially or fully; (ii) the –NH group can be cleaved or
2
can remain available for further reactions; and (iii) the methyl
group adds cis/trans isomerism to the hydrogenated product.
The hydrogenation of toluidine has been rarely documented
in the literature. Friefelder et al. [3] found that an alkyl sub-
stituent on the aromatic ring has very little effect on aniline
hydrogenation when ruthenium dioxide is used as the catalyst.
Ranade et al. [4] investigated the hydrogenation of o-toluidine
using (S)-proline as a chiral auxiliary with the aim of diastero-
selective hydrogenation.
<
1–5 are usual for bulk chemical processes, with low values of
0
.1 for some petrochemical processes. In the fine chemicals in-
Aniline hydrogenation has been more widely covered in
the literature, although not to a great extent. Metal oxide
catalysts have been reported for aromatic amine hydrogena-
tion [5]; however, supported platinum, rhodium, ruthenium,
and palladium, as well as cobalt and nickel, are usually the
catalysts of choice [6–13]. Products reported from aniline hy-
drogenation include cyclohexylamine, dicyclohexylamine, N-
phenylcyclohexylamine, diphenylamine, ammonia, benzene,
cyclohexane, cyclohexanol, and cyclohexanone. The catalyst
dustry, values in the region of 5–50 are obtained [2]. The use of
catalytic instead of stoichiometric processes reduces these val-
ues.
Alicyclic amines are important for use in pesticides, plasti-
cizers, explosives, inhibitors of metal corrosion, and sweeten-
*
Corresponding author.
E-mail address: sdj@chem.gla.ac.uk (S.D. Jackson).
0021-9517/$ – see front matter © 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2006.05.019

418
K.T. Hindle et al. / Journal of Catalysis 241 (2006) 417–425
and the reaction conditions used determine product forma-
tion and selectivity. Narayanan et al. [9,10] proposed a re-
action scheme for aniline hydrogenation over Ni/Al2O3 and
Co/Al2O3 catalysts in vapour-phase reactions. Ni/Al2O3 was
more active than Co/Al2O3, with the major product being N-
phenylcyclohexylamine, whereas dicyclohexylamine and cy-
clohexylamine were also produced. The mechanism of aniline
2.2. Hydrogenation experiments
3
The hydrogenation reaction took place in a 500-cm Buchi
autoclave stirred tank reactor equipped with an oil heating
jacket and a magnetically driven stirrer. A Buchi press-flow gas
controller controlled the reactor pressure and the flow of H2 or
N2 and measured the H2 consumed in the reaction. Typically,
2 g of catalyst was added with 300 ml of 2,2,4-trimethylpentane
to the reactor vessel, and an in situ reduction was performed by
hydrogenation over Rh/Al O is thought to be different than
2
3
that by Ni or Co supported on alumina. Work performed with
a Rh/Al2O3 catalyst in vapour-phase hydrogenation gave cy-
clohexylamine and dicyclohexylamine as the two major prod-
ucts [11]; N-phenylcyclohexylamine and cyclohexane were
also produced in minor amounts. It was thought that cyclo-
hexane could be formed by the formation and subsequent hy-
drogenation of benzene. A comparison of supported Ni and
supported Ru catalysts in liquid-phase hydrogenation showed
that cyclohexylamine selectivity increased by approximately
3
−1
sparging H2 (280 cm min ) through this mixture for 30 min
while stirring at 800 rpm. During this time, the vessel was
heated to the reaction temperature. Then 2.1 g (0.0196 mol)
of p-toluidine (Sigma–Aldrich, 99.7%) was added to 50 ml of
2
,2,4-trimethylpentane (Lancaster Synthesis, 99%) and heated
gently to 323 K, until all of the p-toluidine was dissolved. Im-
mediately after the reduction process, this solution was injected
into the reactor vessel and stirred at 800 rpm for 5 s to allow
mixing. The stirrer was turned off and the vessel was purged
6
0% in a water solvent and by 15–20% in organic solvent
when changing from Ni to Ru [12]. The Ru/Al O gave cy-
3
−1
2
3
with N2 twice at a flow rate of 280 cm min before being
pressurised with N2 to 1 barg. A 2.5-ml sample was obtained.
The vessel was depressurised before being purged with H2
clohexylamine selectivity in the region of 90%. Other products
produced were dicyclohexylamine, cyclohexanol, and cyclo-
hexanone. Hydrogenations with a Rh/Al O catalyst in a water
2
3
3
−1
twice at a flow rate of 280 cm min , then pressurised with H2
to 2 barg. Once the vessel was pressurised, the stirrer was set to
solvent [13] gave cyclohexylamine as the main product with
selectivities of 64–79%; the other products were mainly dicy-
clohexylamine and cyclohexanol.
This short overview highlights the differences in product se-
lectivity depending on the catalyst and reaction conditions used.
The following presents the work from the liquid-phase hydro-
1
000 rpm, corresponding to t = 0. The reaction was monitored
by 2.5-ml gas chromatography (GC) samples obtained at regu-
lar time intervals and by H2 uptake. All gases were ꢀ99.995%
purity (BOC).
genation of p-toluidine over a series of Ru/SiO catalysts, with
particular emphasis on reaction kinetics and product selectivity.
2
2
.3. Analysis
GC was carried out on the liquid samples using a Varian
400 device equipped with an 8200 CX autosampler, a DB-5
2
. Experimental
3
WCOT GC column, and a flame ionisation detector. The cis
and trans isomers of 4-methylcyclohexylamine were identified
by cross-referencing their GC peaks with H nuclear mag-
2
.1. Catalysts
1
The catalysts used throughout the study were 2.5% w/w
Rh/SiO . Davison Catalysts supplied and characterised the
netic resonance (NMR) using a Bruker Spectrospin 400 Ultra-
shield device. This was done using a standard of cis + trans
4-methylcyclohexylamine (Sigma–Aldrich, 97%) by identify-
2
powder silica supports, which had a range of pore and particle
sizes (Table 1). The active catalysts were prepared by John-
son Matthey by the incipient-wetness method to fill the pore
volume using aqueous rhodium chloride salts. The catalysts
were dried and reduced in flowing H /N . Johnson Matthey per-
1
ing which isomer was present in the largest amount by H NMR
1
(the trans and cis isomers are identifiable by H NMR), and
comparing this with the corresponding gas chromatogram.
p-Toluidine conversion (χp-tol) and 4-methylcyclohexyl-
amine selectivity (SMCYA) were calculated using the correla-
tions:
2
2
formed the catalyst characterisation. The physical properties of
the active metal are also given in Table 1. The reproducibility
2
−1
.
of the rhodium surface area was ± 0.5 m g
Table 1
Catalyst and support characterisation data
Catalyst
Rhodium SA
Dispersion
(%, ± 4)
Average metal crystallite
size diameter (nm)
Surface area
Pore volume
Average pore
diameter (Å)
Average particle
size (µm)
2
−1
2
−1
−1
(m g
)
(m g
)
(ml g
)
Rh43/Si1a
Rh41/Si1b
Rh32/Si1c
Rh50/Si1d
Rh37/Si2
Rh94/Si3
Rh60/Si4
Rh71/Si5
4.7
4.5
3.5
5.5
4.1
10.3
6.6
7.8
43
41
32
50
37
94
60
71
2.6
2.7
3.5
2.2
3.0
1.2
1.8
1.6
321
321
321
321
348
344
488
594
1.06
1.06
1.06
1.06
1.21
0.96
0.78
0.35
132
132
132
132
139
111
64
24.0
49.9
96.3
241.0
10.2
9.5
10.0
10.7
24

K.T. Hindle et al. / Journal of Catalysis 241 (2006) 417–425
419
[
p-tol]0 − [p-tol]t
whether gas–liquid mass transfer was controlling, the reactor
stirrer speed was varied between 600 and 1400 rpm. The result-
ing H2 consumption rate was almost identical for each reaction,
χp-tol =
× 100
[
p-tol]0
and
−
1
−1
with values of 0.555 ± 0.030 mmolmin
g , indicating that
[MCYA]t
SMCYA =
× 100,
gas–liquid mass transfer did not control the system. A study
with double the catalyst mass gave a first-order dependency
indicating the absence of liquid–solid mass transfer control. In-
ternal mass transfer or pore diffusion limitations were studied
by crushing catalyst Rh50/Si1d, which had an average parti-
cle size of 241 µm, to a particle size of <125 µm. The results
thus obtained gave identical rates, signifying no pore diffusion
limitations. Based on the observations presented above the re-
actions were assumed to be under kinetic control at reactor
stirrer speeds >600 rpm and with catalysts with average par-
ticle sizes of ꢁ241 µm combined with average pore diameters
of ꢀ13.2 nm (i.e., catalysts Rh43/Si1a, Rh41/Si1b, Rh32/Si1c,
Rh50/Si1d, and Rh37/Si2).
[
p-tol]0 − [p-tol]t
where [p-tol]t and [MCYA]t represent p-toluidine and 4-meth-
ylcyclohexylamine concentration at time t, respectively. The
metal crystallite size was calculated from metal surface area and
metal loading data with the assumption of hemispherical active
−
20
2
rhodium sites and an atomic cross section of 7.52 × 10
m
atom [14].
3
. Results
3
.1. Reaction products
The typical reaction profile for the stirred tank reaction
All catalysts had average particle sizes of ꢁ241 µm; how-
ever, catalysts based on silicas-3, 4, and 5 had average pore
diameters of <13.2 nm (Table 1). Tests with these catalysts are
addressed later.
showed a nonlinear decrease in p-toluidine concentration and a
nonlinear increase in 4-methylcyclohexylamine concentration,
as shown in Fig. 1. As p-toluidine conversion increased, 4-
methylcyclohexylamine selectivity increased, typically reach-
ing a selectivity of 60–70%. The cis isomer was always pro-
duced in greater yields, with the observed cis/trans ratio de-
creasing slightly as the reaction proceeded (Fig. 1). The ratio
settled at a value of around 1.5.
3
.3. 4-Methylcyclohexylamine as substrate
4-Methylcyclohexylamine was used as the substrate to ex-
amine side-product formation. No side products were produced,
indicating bis(4-methylcyclohexyl)amine was not formed
through the coupling of two 4-methylcyclohexylamine mole-
cules. There was a slight drop in 4-methylcyclohexylamine con-
centration; however, because no other species were detected,
this was determined to be due to adsorption on catalyst (Fig. 2).
Concomitant with the drop in 4-methylcyclohexylamine con-
centration was a slightly increased 4-methylcyclohexylamine
cis/trans ratio, indicating that the trans isomer was preferen-
tially adsorbed on the catalyst over the cis isomer.
4
-MCYA selectivity was never 100%, due to the formation
of various byproducts, mainly bis(4-methylcyclohexyl)amine.
Because bis(4-methylcyclohexyl)amine was commercially un-
available, no quantification of this product was performed; how-
ever, its presence was confirmed by GC-mass spectroscopy
(
MS). Ammonia was a byproduct of this dimer formation and
was presumed to be produced in an equal ratio to the dimer.
Trace amounts of 4-methylcyclohexanol and 4-methylcyclo-
hexanone were also detected during the reaction.
3
.2. Mass transfer limitations
3.4. Influence of p-toluidine concentration
A series of tests were performed to examine whether mass
transfer effects were influencing the kinetic data. To study
An investigation of p-toluidine concentration on the reac-
tion rate was carried out at p-toluidine concentrations of 0.028,
Fig. 1. Typical reaction profile of p-toluidine hydrogenation reaction (F) p-
toluidine concentration, (1) 4-methylcyclohexylamine concentration, and (Q)
Fig. 2. Data from reaction using 4-methylcyclohexylamine as substrate (2) 4-
methylcyclohexylamine concentration, (Q) observed 4-methylcyclohexylamine
cis/trans ratio, and (- - -) 4-methylcyclohexylamine cis/trans ratio added.
4-methylcyclohexylamine cis/trans ratio.

420
K.T. Hindle et al. / Journal of Catalysis 241 (2006) 417–425
−1
0
.056, and 0.112 mol L using catalyst Rh60/Si4. The effects
on 4-methylcyclohexylamine selectivity and the cis/trans ratio
were also investigated. p-Toluidine concentration was shown
to have no role in determining the initial reaction rate, indica-
tive of a reaction of zero order with respect to p-toluidine.
4
-Methylcyclohexylamine selectivity and cis/trans ratio were
equally unaffected.
3
.5. Influence of H2 partial pressure
Different hydrogen partial pressures (1.51, 3.01, and 5.01
bar) were tested using catalyst Rh37/Si2 to examine the influ-
ence on the reaction rate. A simple power law model, rate =
x
y
k(PH ) [p-tol] , where k is the rate constant, x is the re-
Fig. 3. Effect of metal crystallite size on TOF for catalysts based on silica-1.
2
action order with respect to hydrogen, and y is the reac-
tion order with respect to p-toluidine (zero in this case), was
used. A value for x of 1.18 ± 0.22 was obtained. As with p-
toluidine concentration, hydrogen partial pressure did not influ-
ence 4-methylcyclohexylamine selectivity or cis/trans ratio.
3
.6. Metal crystallite size dependency
By using catalysts based on silica-1, which had identical sur-
face areas and pore structures, metal crystallite size dependency
could be investigated without interference from pore diameter
effects. Because the catalysts had different metal surface areas,
the turnover frequencies (TOFs), as opposed to reaction rates,
were compared. The TOFs were calculated by
Fig. 4. 4-Methylcyclohexylamine cis/trans ratio with respect to p-toluidine
conversion (F) Rh43/Si1a, (Q) Rh41/Si1b, (1) Rh32/Si1c, and (") Rh50/
Si1d.
nH reacted
2
TOF =
,
nRh
creased cis/trans ratio was observed with decreasing particle
size (Fig. 4).
where nH reacted is the number of moles of H2 reacted per
minute per gram of catalyst and nRh is the number of moles
of active rhodium per gram of catalyst.
2
3
.8. Effect of catalyst pore size
A plot of average metal crystallite size with respect to TOF
revealed a linear increase in TOF with increasing metal crys-
Catalyst pore size was investigated by comparing the results
2
tallite size (Fig. 3). Regression analysis gave an R value of
obtained with catalysts prepared from silicas-2–5. Once again,
metal crystallite sizes were normalised to 1 nm to decouple their
effect on catalyst activity. Pore size had a marked effect on the
reaction with catalyst Rh71/Si5, which had the smallest pore
0
.9989. However, a plot of metal crystallite size versus ob-
served cis/trans ratio revealed no relationship.
3
.7. Effect of catalyst particle size
−1
diameter (2.4 nm), giving a TOF of 0.99 min . Comparing
the TOF values for catalysts based on silicas-2–4 (1.91, 1.96,
−
1
Once the metal crystallite size effect was known and could
and 2.09 min , respectively) clearly shows that increasing the
pore diameter to >6.4 nm resulted in only minor increases in
TOF, indicating that pore diffusion limitations were negligible
in catalysts with pore diameters >6.4 nm. Examination of TOF
versus metal crystallite size again revealed a linear increase for
catalysts with no pore diffusion limitations; however, catalyst
Rh71/Si5 gave a TOF below that of the linear trend, due to the
effect of pore diffusion.
4-Methylcyclohexylamine selectivity was unaffected by cat-
alyst pore size; however, there were changes in the observed
cis/trans ratios. Fig. 5 demonstrates that the cis/trans ratio in-
creased with smaller pore size. In particular, catalyst Rh71/Si5,
which had the smallest pore diameter, gave a significant in-
crease compared with the other catalysts.
be compensated for, catalyst particle size could be investigated
more thoroughly. Catalysts based on silica-1 were tested, with
the observed reaction rates normalised to a metal crystallite
size diameter of 1 nm. As expected, the silica particle size
−
1
did not alter the TOF (1.81 ± 0.05 min ), confirming the ab-
sence of pore diffusion limitations. The pore diffusion test with
catalyst Rh50/Si1d and a crushed form revealed that although
p-toluidine conversion and 4-methylcyclohexylamine selectiv-
ity were unaltered, the observed cis/trans ratio at p-toluidine
conversions <100% were different, such that the smaller par-
ticle sizes gave a higher cis/trans ratio. Catalyst Rh50/Si1d
was also compared with the other catalysts based on silica-1
(
identical support except for silica particle size); again, an in-

K.T. Hindle et al. / Journal of Catalysis 241 (2006) 417–425
421
Fig. 5. 4-Methylcyclohexylamine cis/trans ratio with respect to p-toluidine
conversion (") Rh37/Si2, (Q) Rh94/Si3, (F) Rh60/Si4, and (2) Rh71/Si5.
Fig. 6. 4-Methylcyclohexylamine cis/trans ratio with respect to p-toluidine
conversion for Rh41/Si1b at (F) 313 K, (1) 328 K, (Q) 338 K, and (×) 353 K.
Table 2
Activation energies calculated from cis-4-methylcyclohexylamine formation
and trans-4-methylcyclohexylamine formation
−
1
−1
Catalyst
Ea (kJ mol ) (cis)
Ea (kJ mol ) (trans)
Rh43/Si1a
Rh41/Si1b
Rh32/Si1c
Rh50/Si1d
Rh37/Si2
Rh94/Si3
Rh60/Si4
Rh71/Si5
54
47
44
41
59
57
53
73
63
66
62
58
58
71
55
90
3
.9. Temperature dependency
Fig. 7. H uptake profiles for p-toluidine hydrogenation with (F) fresh cata-
2
Experiments were carried out with varying reaction tem-
peratures in the range 313–353 K. Arrhenius plots for cis-
-methylcyclohexylamine formation and trans-4-methylcyclo-
hexylamine formation were used to calculate the respective
apparent activation energies (Table 2). Activation energies cal-
culated from the formation of cis and trans isomers of 4-meth-
ylcyclohexylamine revealed higher values for trans over cis.
lyst, (1) first recycle of catalyst, and (Q) second recycle of catalyst.
4
p-toluidine conversions >80% (Fig. 8). The first and second re-
cycle produced similar cis/trans ratios, with the values from the
second recycle slightly lower.
4. Discussion
4
-Methylcyclohexylamine selectivity did not change with
reaction temperature; however, as the activation energies sug-
gest, the cis/trans ratio was altered. The lower reaction tem-
peratures gave higher cis/trans ratios for all catalysts. Fig. 6
displays this trend for catalyst Rh41/Si1b.
4
.1. Product formation
There is no useful literature on the p-toluidine hydrogena-
tion reaction; therefore, analogous reactions were examined
to reveal useful information concerning product formation.
The main product, 4-methylcyclohexylamine, was expected be-
cause the equivalent species, cyclohexylamine, was formed dur-
ing aniline hydrogenation over precious metals [7,8,11–13].
Obviously, aniline hydrogenation could not be used to un-
derstand cis and trans 4-methylcyclohexylamine formation;
therefore, the literature of the next most comparable reaction,
alkylphenol hydrogenation, was examined. The mechanism of
formation of 4-tert-butylcyclohexanol from 4-tert-butylphenol
is via a cyclohexanone intermediate, which is in tautomeric
equilibrium with its enol form [15–17]. For p-toluidine hy-
drogenation, it is plausible that the hydrogenation proceeded
via enamine/imine intermediate species, which were in tau-
3
.10. Catalyst recycling
Three reactions were performed with reuse of catalyst
Rh60/Si4 to investigate whether catalyst deactivation was oc-
curring (Fig. 7). Deactivation did occur as the reaction rate
decreased with each catalyst recycle. The largest decrease in
rate occurred between using the fresh catalyst and its first recy-
cle, with only a slight decrease evident on the second recycle.
The 4-methylcyclohexylamine cis/trans ratio changed be-
tween the fresh catalyst and the first recycle. At low p-toluidine
conversion, the cis/trans ratio was higher with the fresh cata-
lyst than with recycled catalyst, the values becoming closer at

422
K.T. Hindle et al. / Journal of Catalysis 241 (2006) 417–425
Fig. 9. Conformations of trans and cis isomers of 4-methylcyclohexylamine.
cally favoured over cis-4-methylcyclohexylamine; however, the
trans isomer is formed via an enamine intermediate, which,
when two hydrogen atoms are bonded to the nitrogen atom, is
thermodynamically less stable than the imine equivalent [22].
Therefore, the thermodynamically less stable cis isomer is pro-
duced from the thermodynamically more stable intermediate.
Thus the cis/trans ratio may be determined by the stability, and
hence potentially the concentration, of the intermediates. The
literature on 4-tert-butylphenol hydrogenation revealed similar-
ities. Reactions in iso-propanol over a 2% Rh/C catalyst yielded
more cis isomer than trans, with selectivity toward cis-4-tert-
butylcyclohexanol at 80% [23]. A separate study [15] over 5%
Rh/C produced cis/trans ratios between 1.7 and 5.9, depend-
ing on the solvent used. In the same study, support effects on
the cis/trans ratio were examined, and it was found that the
Fig. 8. 4-Methylcyclohexylamine cis/trans ratio with respect to p-toluidine
conversion using (F) fresh catalyst, (1) first recycle of catalyst, and (Q) sec-
ond recycle of catalyst.
use of Al O or graphite as the support reduced the ratio to
2
3
between 0.9 and 1.5 for all solvents tested. No study over a
Rh/SiO2 catalyst could be found; however, with alternative sup-
ports the cis isomer was the favoured product. Reactions using
base metals and nonsupported precious metal catalysts gave
different results. Over a Ni/Cr2O3 catalyst [24], the thermo-
dynamically favourable trans-4-tert-butylcyclohexanol isomer
was produced in a greater yield than the cis isomer, giving a
cis/trans ratio of 2.3. This finding is similar to that obtained
over a Rh2O3 catalyst [16], where again the trans isomer was
the favoured product.
The major byproduct, bis(4-methylcyclohexyl)amine, was
also expected as its equivalent species, dicyclohexylamine
was also formed during aniline hydrogenation reactions over
similar catalysts [7,8,11–13]. Coupling of a partially hydro-
genated imine intermediate and fully hydrogenated 4-methyl-
cyclohexylamine species has been suggested [8] as the route
to form dicyclohexylamine in aniline hydrogenation reactions
over precious metals. The reaction using 4-methylcyclohexyl-
amine as the reactant produced no bis(4-methylcyclohexyl)-
amine, confirming that an intermediate species plays a part
in dimer formation. Surface acidity is likely to play a role in
bis(4-methylcyclohexyl)amine formation [6]. Although the ex-
act role of the surface acidity is unclear, Ref. [6] shows that
in the presence of basic species, dimer formation is hindered.
During aniline hydrogenation, the yield of dicyclohexylamine
is diminished by adding alkali additives or by using a strongly
basic lanthana support [6]. The presence of alkali additives or
the use of a basic support is thought to weaken the adsorption
of cyclohexylamine, resulting in less side reactions. Mailybaev
et al. [12] also showed that the addition of alkali additives
decreased byproduct formation during aniline hydrogenation.
However, the suggested reason for this was that dicyclohexy-
lamine is formed on the acidic sites of the catalysts; the alkali
additives neutralise these acidic sites, causing less byprod-
Scheme 1. Pathway to form cis and trans isomers of 4-methylcyclohexylamine
(4-MCYA) through the hydrogenation of p-toluidine.
tomeric equilibrium; the cis isomer produced via the imine
intermediate, and the trans isomer produced via the enamine
intermediate (Scheme 1). No enamine or imine intermediate
was detected in the system at any time, whereas the ketone
intermediate has been detected in the substituted phenol hydro-
genation over Rh/C [15]. However, this was not unexpected,
because imines are known to be highly reactive toward hy-
drogenation [18], and the C=C bond in an enamine is easily
hydrogenated to form an amine [19]. Both cis and trans iso-
mers have two possible conformations [20,21]; however, they
exist almost entirely in conformations shown in Fig. 9. The
cis isomer was always produced in greater yields than the
trans isomer, resulting in a cis/trans ratios >1. The thermo-
dynamic considerations to the resultant cis/trans ratio are con-
tradictory. Trans-4-methylcyclohexylamine is thermodynami-

K.T. Hindle et al. / Journal of Catalysis 241 (2006) 417–425
423
uct formation. Greenfield [8] demonstrated that the addition
of aqueous ammonia to the reaction mixture decreased dicy-
clohexylamine formation during aniline hydrogenation over a
Ru/C catalyst.
The minor species produced in the reaction may be the re-
sult of trace water in the solvent. 4-Methylcyclohexanone can
be formed from the reaction of water with an enamine inter-
mediate, whereas 4-methylcyclohexanol can be formed by hy-
drogenation of 4-methylcyclohexanone. The presence of water
produces cyclohexanol in aniline hydrogenation [8,12], and the
mechanism of formation through the enamine intermediate was
suggested by Greenfield [8].
Certain reaction conditions and catalyst physical character-
istics altered the rate of product formation and selectivity. The
reaction with respect to p-toluidine was zero-order; hence p-
toluidine concentration had no role in determining the initial re-
action rate. The hydrogenation of 4-tert-butylphenol over Rh/C
was also shown to be zero-order in 4-tert-butylphenol [25]. The
cis:trans ratio of 4-methylcyclohexylamine was also unaffected
by changes in p-toluidine concentration.
Fig. 10. Effect of metal crystallite size on TOF for catalysts based on silica-1
and Rh94/Si3.
played is corroborated by the literature on aromatic ring hydro-
genation, because the activity of benzene hydrogenation over
Rh/Al O is low for very small Rh crystals [27]. Moreover, an-
2
3
other study of benzene hydrogenation over Rh/Al O authors
2
3
concluded that with catalysts with dispersions between 20 and
90%, there was a decrease in turnover number as dispersion in-
creased (i.e., smaller metal crystals) [28]. Benzene hydrogena-
tion over Pt/Al O also displayed decreased TOF with increas-
Increased H pressure resulted in an increase in reaction rate.
2
The reaction order with respect to H2 concentration was calcu-
lated to be 1.18 ± 0.22. This disagrees with literature available
on aniline hydrogenation, where it was shown the reaction was
zero order in H2 [13]; however, this result was gained by per-
forming gas–phase reactions at high pressure. Hydrogenation
of benzene, and its derivatives, at high pressures are known to
be zero-order in both H2 and aromatic species, due to com-
plete coverage of the catalytic surface by both reactants [17].
At low pressures, the reaction order in H2 can be greater than
zero-order [16,17]. Work performed on 4-tert-butylphenol hy-
drogenation over Rh/C at pressures between 0.2 and 1.0 MPa,
was also shown to be positive order with respect to H2 partial
pressure [25]. Due to the positive order in H2 during p-toluidine
hydrogenation, the amount of H2 adsorbed on the catalytic sur-
face is proportional to the reaction rate. Consequently, either H2
adsorbing on the catalyst or the addition of hydrogen to either
p-toluidine or a partially hydrogenated intermediate is the rate-
limiting step. H2 pressure did not alter the product distribution;
therefore, as with p-toluidine concentration, the cis:trans iso-
mer ratio of the product was unaltered by varying H2 pressure.
Studies on metal crystallite size revealed antipathetic behav-
iour or a negative particle size effect (Fig. 3) over the range of
crystallite sizes and temperatures (313–353 K) tested; that is,
a larger crystallite size produced a higher TOF. Fig. 10 shows
the relationship between TOF and metal crystallite size with
catalyst Rh94/Si3 data added. This catalyst has a different sil-
ica pore size, but it is outwith the diffusional regime. The data
from this catalyst extend the relationship down to around 1 nm.
Rhodium crystals have a fcc closed packed arrangement; as a
fcc crystal increases in size, the number of face surface atoms
2
3
ing dispersion [29]. Other studies of toluene hydrogenation over
platinum and iridium catalysts also revealed similarities to the
current study [30,31]. 4-Methylcyclohexylamine selectivity and
cis/trans ratio were not affected by changes in metal crystallite
size.
Catalyst pore size affected the TOF such that below a thresh-
old pore diameter, the TOF was perturbed. This threshold pore
diameter was <6.4 nm. Pore diameters above this threshold
showed only very slight differences in TOF. The perturbation
of the rate was due to a diffusion limitation through the cat-
alyst pores. This limitation was a “Knudsen-type” diffusion
limitation. Knudsen diffusion is usually described for gas-phase
molecules where the mean free path between molecules is
larger than the pore diameter of the catalyst; therefore, diffu-
sion of the molecule is interrupted by collisions with the pore
wall. In the present case, the reactant molecules diffuse through
a liquid, and consequently collisions with the solvent molecule
will occur before collisions with the pore walls. But because the
pore diameter in catalyst Rh71/Si5 was so narrow (2.4 nm), we
propose that some collisions with the pore walls occurred, caus-
ing the diffusion limitation; hence the term “Knudsen-type”
diffusion. A more quantitative way of examining this diffu-
sion limitation is by examining the diffusivity, D, as diffusion
is proportional to the diffusivity of the species. Diffusivity in
liquids can be up to 10,000 lower than for gas at atmospheric
pressure [32]. The diffusivity in catalyst pores is called the “ef-
fective diffusivity,” D , and is related to the diffusivity by the
e
following equation [33]:
3
9
Dφσ
(
C sites) increases at the expense of the edge (C ) and cor-
9
7
D =
e
,
9,10
τ
ner (C4 ) sites [26]. This suggests the hydrogenation reaction
took place on the plane face surface as opposed to edge and
corner sites. There are no analogous studies of metal crystallite
size effects on p-toluidine or substituted aniline hydrogena-
tions in the literature. However, the antipathetic behaviour dis-
where φ is the porosity of the catalyst, defined by the ratio of
volume of void space to total volume of catalyst, σ is the con-
striction factor, which accounts for the variation in the cross-
sectional area of the pores, and τ is the tortuosity, defined by

424
K.T. Hindle et al. / Journal of Catalysis 241 (2006) 417–425
Table 3
product until a constant ratio was achieved. The catalyst pore
diameter played a role in determining the observed cis/trans
ratio. The smaller pore diameters gave higher cis/trans ratios
at equal p-toluidine conversion. Catalyst Rh71/Si5, which had
the smallest pore diameter, had the largest support surface area
and the longest pore length per gram of catalyst. By compar-
ing catalysts Rh37/Si2 and Rh71/Si5 (the two extreme cases),
the increased cis/trans ratio with decreasing pore diameter can
be rationalised in two nonexclusive ways. Because the surface
area of catalyst Rh71/Si5 was larger, more trans isomer than
cis isomer may be adsorbed on the support. Moreover, because
catalysts Rh37/Si2 and Rh71/Si5 had almost the same particle
size, the increase in pore length is analogous to an increase in
the length of a chromatographic column, which results in in-
creased separation of species.
It is likely that p-toluidine also adsorbed on the support in
catalyst Rh71/Si5, contributing to the diffusion limitation ob-
served. But because aromatic amines are less basic than nonaro-
matic amines, p-toluidine would have less affinity for the acidic
sites than 4-methylcyclohexylamine and so would be displaced
by product.
Catalyst particle size also affected the cis/trans ratio, with
smaller particles demonstrating an increased cis/trans ratio.
This phenomenon was again due to the greater affinity of the
trans isomer to adsorb on the support. All of the catalysts had
the same pore structure, pore length per gram of catalyst, and in-
ternal support surface area. The only difference was the external
surface area of the catalysts, with the smaller catalyst parti-
cle sizes having larger external surface areas. (The surface area
measurements were taken from a single batch of support before
they were crushed to their corresponding sizes.) As more exter-
nal surface area was exposed with the smaller particle sizes, the
more trans adsorption with respect to cis occurred, resulting in
slight increased observed cis/trans ratios.
Reaction temperature changed the cis/trans ratio produced in
the reaction. Decreasing the reaction temperature increased the
cis/trans ratio. This is in keeping with the activation energies
calculated from the rate of formation of each isomer. Overall,
the activation energies calculated from trans isomer formation
were larger than those calculated from cis isomer formation,
reinforcing the proposed mechanism that the cis and trans iso-
mers are produced from different intermediates—an imine and
enamine, respectively.
Catalyst deactivation was evident on catalyst recycling. The
most significant drop in catalyst activity was between the reac-
tion using fresh catalyst sample and the reaction with the first
reuse of this sample. The second reuse of the catalyst produced
a slight drop in activity, indicating that deactivation was an
ongoing process that increasingly deactivated the catalyst. Ref-
erences [6–8,13] states that both aniline and cyclohexylamine
act as inhibitors in aniline hydrogenation. The basic lone pair
on the nitrogen atom forms a strong bond to the active metal;
therefore, it is likely that 4-methylcyclohexylamine inhib-
ited p-toluidine hydrogenation, with contributions from bis(4-
methylcyclohexyl)amine and ammonia. The catalyst recycling
experiments produced evidence corroborating the chromatog-
raphy effect, producing different cis/trans ratios. On the second
Variation of total pore length per gram of catalyst with average pre diameter for
each catalyst
Total pore length
Average pore
diameter (Å)
Catalyst
−
1
(m g
)
7740
7740
7740
7740
7969
9814
132
132
132
132
139
111
64
Rh43/Si1a
Rh41/Si1b
Rh32/Si1c
Rh50/Si1d
Rh37/Si2
Rh94/Si3
Rh60/Si4
Rh71/Si5
24,308
80,263
24
the actual distance that a molecule travels between two points
divided by the shortest distance between two points. First, ex-
amining Table 1 reveals that pore volume per gram of catalyst
decreased as the average pore diameter decreased, and hence
the porosity, φ, decreased. Therefore, as the catalyst diameter
decreased, the effective diffusivity decreased. Second, applying
the equation [32]
SA2
L = 4
πV
,
where L is the total catalyst pore length, SA is the support sur-
face area, and V is the support pore volume, demonstrates that
the pore length per gram of catalyst increases as average pore
diameter decreases (Table 3). Because the particle sizes of cat-
alysts based on silicas-2–5 were almost identical, the tortuosity
of the catalysts increased with increasing pore length per gram
of catalyst; thus the effective diffusivity decreased as catalyst
pore diameter decreased. Ren et al. [34] showed that tortu-
osity increased with decreasing pore diameter. The combined
influence of these two factors means an overall decrease in the
effective diffusivity with decreasing catalyst pore diameter. Be-
low the threshold diameter of 6.4 nm, diffusivity impinged to
such an extent to produce diffusion limitations.
The observed cis/trans ratio changed during all reactions,
decreasing over time toward a fixed value. For a simultane-
ous reaction, if the two competing reactions are of the same
kinetic order (as they are in this case), then pore structure
does not affect product selectivity [32]. Therefore, the change
in ratio was due not to a change in selectivity (i.e., enam-
ine/imine ratio), but rather to the trans isomer having a greater
affinity for the support than the cis isomer. The silica sup-
port in this study was acidic; therefore, the basic cis and trans
amines will have an affinity for the silica surface. The greater
affinity of the trans isomer was proved by the reaction using
4
-methylcyclohexylamine as the substrate, which displayed a
slightly increased cis/trans ratio due to the preferential adsorp-
tion of the trans isomer on the catalyst support. This chromato-
graphic effect of the silica produced a differential in the rate
of transport of the cis and trans isomers through the pores.
Consequently, the cis/trans ratios seen during the initial stages
of the reaction represents not the production of cis and trans
isomers at the catalytic site, but rather a modified ratio due
to preferential adsorption of the trans isomer. The observed
cis/trans ratios decreased as the support became saturated with

K.T. Hindle et al. / Journal of Catalysis 241 (2006) 417–425
425
and third recycles, no sharp decrease in cis/trans ratio occurred,
as was observed with the fresh catalyst. Because the used cat-
alysts have already been exposed to 4-methylcyclohexylamine
from the previous reaction(s), the acidic sites have been neu-
tralised to such an extent that any chromatographic effect is
diminished.
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[
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. Conclusion
We have shown that the hydrogenation of para-toluidine
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over Rh/silica exhibits an antipathetic particle size effect, sug-
[
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Morrell (Eds.), Catalysis of Organic Reactions, Dekker, New York, 2002,
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3
9
gesting that plane face surface atoms, such as C sites, are the
active site for ring hydrogenation. The reaction is zero order in
p-toluidine and first order in hydrogen, with the likely mecha-
nism of formation of the cis and trans isomers through surface
imine and enamine intermediates. The support was also shown
to play a part, adsorbing trans-4-MCYA in the early stages of
the reaction and resulting in a varying cis:trans ratio. The effect
of pore size was also studied; a dramatic decrease in activity
was observed at pore diameter <6.4 nm.
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