Stereochemistry of hydrodenitrogenation: The mechanism of elimination of the amino group from cyclohexylamines over sulfided Ni-Mo/γ-Al2O3 catalysts

Article abstract of DOI:10.1006/jcat.2001.3218

HDS and HDN are among the most significant catalytic processes in the petroleum industry, because during these processes sulfur and nitrogen are removed in the form of H₂S and ammonia from oil fractions. The HDN of cyclohexylamine and of the diastereomers of 2-methylcyclohexylamine and 2,6-dimethylcyclohexylamine was studied at 200°-350°C and 50 bar over a sulfided Ni-Mo/γ-Al₂O₃ catalyst. The rate of HDN of alkyl-substituted cyclohexylamines over sulfided Ni-Mo catalysts depended on the number of β hydrogen atoms and on their stereochemical relation to the amino group. Isomerization of olefinic products and the amines prevented meaningful mechanistic studies at 350°C. The cis diastereomers reacted faster than the trans diastereomers, because they allowed for an anti geometric relationship in the chair conformation between the amino group and a hydrogen atom on a β carbon atom. The syn elimination occurred to a considerable extent at higher temperatures in molecules that were unable to undergo anti elimination. The activation energy of anti elimination was lower than that of syn elimination, and the activation energy of anti elimination involving a hydrogen atom attached to a tertiary β carbon atom was lower than that involving a hydrogen atom attached to secondary β carbon atom.

Full text of DOI:10.1006/jcat.2001.3218

Journal of Catalysis 200, 389–399 (2001)
doi:10.1006/jcat.2001.3218, available online at http://www.idealibrary.com on
Stereochemistry of Hydrodenitrogenation: The Mechanism
of Elimination of the Amino Group from Cyclohexylamines
over Sulfided Ni–Mo/ -Al2O3 Catalysts
Fabio Rota,Vidyadhar S. Ranade,1 and Roel Prins2
Laboratory for Technical Chemistry, Swiss Federal Institute of Technology (ETH), 8092 Zurich, Switzerland
Received January 26, 2001; revised March 12, 2001; accepted March 12, 2001; published online May 15, 2001
as benzothiophene and dibenzothiophene, and quinoline,
indole, acridine, and carbazole, respectively (1). Removal
of nitrogen from heteroaromatic compounds with sulfided
Ni–Mo catalysts occurs mainly after hydrogenation of these
compounds. Therefore, industrial HDN essentially consists
of two types of reactions: hydrogenation followed by ni-
trogen removal. Both reactions occur on the same catalyst,
although it is not known if they occur on the same catalytic
site.
Studying the stereochemistry of these reactions should
provide important information about the mechanism of the
reaction on the surface of the catalyst as well as about the
structure of the catalytic site. Sulfided catalysts were em-
ployed in the past for hydrogenation reactions of organic
molecules (2), but they were largely superseded by sup-
ported metal catalysts, which are active under much milder
reaction conditions. Consequently, the stereochemistry of
hydrogenation over catalysts sulfided catalysts has hardly
been studied (3, 4). We have even lessinformation about the
stereochemistry of the elimination reaction. There are very
few stereochemical investigations of elimination reactions
over heterogeneous catalysts, a notable exception being the
elimination of the hydroxyl group over acidic alumina (5–
10). We report the very first investigation of the stereo-
chemistry of elimination of substituted cyclohexylamines
over sulfided catalysts.
The hydrodenitrogenation of cyclohexylamine and of the dia-
stereomers of 2-methylcyclohexylamine and 2,6-dimethylcyclo-
hexylamine was studied at 200 to 350 C and 50 bar pressure over a
sulfided Ni–Mo/ -Al2O3 catalyst. The removal of the amino group
from cyclohexylamines occurs primarily by means of a elimina-
tion mechanism. Elimination of a hydrogen atom attached to a
tertiary carbon atom is faster than that of a hydrogen atom at-
tached to a secondary carbon atom. The rate of elimination also
depended on the stereochemical configuration of the amino group
with respect to the hydrogen atoms. Elimination was more rapid
when the configuration of the amino group was anti periplanar to
the hydrogen atom (in a chair conformation) than when it was syn
periplanar (in a boat conformation). This is consistent with an E2
elimination mechanism. The relative elimination rates of all the di-
astereomers of the substituted cyclohexylamines were rationalized
in terms of the stereochemical relation between the amino group
and a hydrogen atom, the number and type of hydrogen atoms,
and the energy required to attain the required chair/boat confor-
mation(s). In contrast to previous proposals for anti elimination of
H2O from alcohols, it was shown that an anti elimination mecha-
nism is possible on the surface of heterogeneous catalysts if SH or
c
OH groups protrude from the catalyst surface.
2001 Academic Press
Key Words: hydrodenitrogenation; Ni–Mo/ -Al2O3; cyclohexy-
lamines; elimination, E2; stereochemistry.
1. INTRODUCTION
Several authors have studied networks of HDN reactions
Hydrodesulfurization (HDS) and hydrodenitrogenation with different model compounds containing nitrogen. o-
(HDN) are among the most important catalytic processes Alkylanilines are formed as intermediates in the HDN of
in the petroleum industry, because during these processes a majority of nitrogen-containing molecules in oil. Investi-
sulfur and nitrogen are removed in the form of hydrogen gation of the HDN network of o-toluidine as a model com-
sulfide and ammonia from oil fractions. Sulfided Ni–Mo pound is being studied in our laboratory (11–13). The HDN
or Co–Mo supported on -alumina is typically employed network of a simple model compound such as o-toluidine is
as a catalyst. Compounds containing sulfur or nitrogen in quite complex, as indicated in Scheme 1. The aromatic C–N
oil fractions are mainly polycyclic aromatic molecules, such bond is considerably stronger than the aliphatic C–N bond,
and hence the direct conversion of o-toluidine to toluene
takes place only to a minor extent under the typical process
1 Present address: Unilever Research Vlaardingen, 3130 AC Vlaardin-
conditions employed for HDN (350 C and 50 bar) (11).
gen, The Netherlands.
2 To whom correspondence should be addressed. Fax: +41 1 6321162.
E-mail: prins@tech.chem.ethz.ch.
Thus, 2-methylcyclohexylamine is an important intermedi-
ate in the HDN network of o-toluidine. Although there are
389
0021-9517/01 $35.00
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2001 by Academic Press
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390
ROTA, RANADE, AND PRINS
the bed was monitored with a thermocouple inside the re-
actor. The active Ni–Mo catalyst (0.05 g) was diluted with
8 g SiC to achieve plug-flow conditions in the fixed-bed re-
actor and was held in place by glass wool. The catalyst was
sulfided in situ with a mixture of 10% H2S in H2 at 370 C
and 10 bar for 4 h. The reactant feed was prepared by dis-
solving the amine in octane; heptane was added as an inter-
nal standard. Octane was used as a solvent to simulate the
presence of hydrocarbons during HDN, as in actual refin-
eryoperations. After sulfidation ofthe catalyst, the pressure
was increased to 50 bar; the liquid reactant was vaporized
in a preheater at 200 to 300 C and was fed concurrently
with the gases to the reactor by means of a high-pressure
syringe pump (flow rate, 0.02–0.25 ml min 1). The catalyst
was maintained in the sulfided state throughout the reac-
tion by allowing H2S and H2 to flow over the catalyst bed.
Weight time was defined as = wc/nfeed, where wc denotes
the catalyst weight and nfeed the total molar flow to the re-
actor. The weight time ( ) was changed by varying the flow
rates of the liquid and the gaseous reactants while keeping
their ratio constant. Reactions were performed at 0.2 bar
H2S partial pressure. The flow rate of H2 was adapted using
mass flow controllers so as to keep the H2 partial pressure
at 48 bar. The total pressure was maintained at 50 bar us-
ing a backpressure regulator. Reactions were conducted at
four different reaction temperatures ranging from 200 to
350 C. Samples were collected every 45 min at the reac-
tor outlet using an automated valve maintained at 300 C
and at the same pressure as the reactor. The reaction prod-
ucts were injected from the valve into a gas chromatograph
equipped with a 30-m DB-5 fused-silica capillary column
(J & W Scientific, 0.32-mm i.d., 0.25- m film thickness) for
quantitative analyses. Detection was made with a flame ion-
ization detector as well as with a pulsed flame photometric
detector, which is especially sensitive to compounds con-
taining nitrogen and sulfur. The reaction was stable after
3 to 4 h following a change in temperature or in the flow
rate. This entailed the use of about 10 g of amine per investi-
gation at one single temperature. Very little deactivation of
the catalyst occurred even after prolonged operation of the
reactor. The mass balance was between 95 and 100% for
all reactions. The activity of the reactor without the active
catalyst (but filled with alumina support and SiC, and sul-
fided as the catalyst) was only about 20 to 30% that with the
catalysts. This and the fact that sulfided Ni–Mo on carbon
catalysts had a similar activity for elimination as NiMo/ -
Al2O3 catalysts (16, 17) indicate that the Ni–Mo sulfide and
not the alumina enables the NH3 elimination.
SCHEME 1
two pathways from 2-methylcyclohexylamine to methylcy-
clohexane, the ultimate product of the HDN of o-toluidine,
the path involving direct scission of the aliphatic C–N bond,
plays a minor role (5–10% ) (11). The crucial step, which in-
volvesnitrogen removal, takesplace mainlyvia elimination.
Ni–Mo or Co–Mo catalysts are typically sulfided before and
during their use in HDN reactions. Sulfidation enhances
the C–N bond cleavage of the catalyst but decreases hydro-
genation (14, 15). Thus, although 2-methylcyclohexylamine
is an important intermediate in the HDN network of o-
toluidine, only traces of it are detectable, because the hy-
drogenation of the aromatic ring is the rate-limiting step.
It is clear that the stereochemistry of elimination can-
not be investigated starting from o-toluidine, because 2-
methylcyclohexylamine is observed in very small amounts.
Therefore, we synthesized and then studied the HDN of the
cisand transisomersof2-methylcyclohexylamine. The reac-
tivity of 2-methylcyclohexylamine was also compared with
the reactivity of cyclohexylamine and of the diastereomers
of 2,6-dimethylcyclohexylamines to provide additional sup-
port for our conclusions.
2. EXPERIMENTAL
Preparation of Catalyst and HDN Experiments
The Ni–Mo/ -Al2O3 catalyst used in this work contained
8 wt% Mo and 3 wt% Ni and was prepared by successive in-
cipient wetness impregnation of -Al2O3 (CONDEA, pore
volume 0.5 cm3 g 1, specific area 230 m2 g 1) with an aque-
ous solution of (NH4)6Mo7O24 4H2O (Aldrich) followed
by an aqueous solution of Ni(NO3)2 6H2O (Aldrich).
After each impregnation step, the catalyst was dried in air
at ambient temperature for 4 h and then dried in an oven
at 120 C for 15 h. The catalyst was subsequently calcined at
500 C for 4 h. It was crushed and sieved to a particle size be-
low 60 m to avoid diffusion effects on product distribution
and conversion (11).
Synthesis of 2-Methylcyclohexylamine and
2,6-Dimethylcyclohexylamine Diastereomers
Reactions were conducted in a continuous mode in a
fixed-bed reactor with a hastelloy tube (15-mm i.d., 320 mm
The trans diastereomer of 2-methylcyclohexylamine was
long) that was heated by an oven. The temperature inside obtained by repeated crystallization of the hydrogen

STEREOCHEMISTRY OF HYDRODENITROGENATION
391
chloride salt of a commercially available mixture of the cis
and trans diasteromers (1 : 4, Fluka). The purity of the trans
diastereomer was 95 to 98% ; it contained a small amount
of water (up to 0.5 wt% ) as determined by Karl–Fischer
titration. The cis diastereomer of 2-methylcyclohexylamine
was prepared according to two methods. The first method
involved a transamination procedure starting from 2-
methylcyclohexanone and 1-phenylethylamine (18). The
second method involved hydrogenation of o-toluidine over
a noble metal catalyst followed by separation of the
cis diastereomer from the by-products by crystallization.
The purity of the cis diastereomer was 92 to 96% . 2,6-
Dimethylcyclohexylamines were synthesized from 2,6-di-
methylaniline (BASF) by hydrogenation over a ruthenium
catalyst. The product was separated into two fractions,
namely, a fraction consisting of crystals of the cis,cis di-
astereomer and a fraction containing a 1 : 1 mixture of
cis,cis and cis,trans diastereomers together with a small
amount ofthe trans,transdiastereomer. Both fractionswere
used in HDN studies after purification. The purity of the
cis,cis diastereomer was 98% , while that of the mixture of
the cis,cis and the cis,trans diastereomers was 97% . The
crystals of pure cis-2-methylcyclohexylamine and cis,cis-
2,6-dimethylcyclohexylamine were insoluble in octane, but
were soluble after adding small amounts of water (up to
0.7 wt% ). Additional details on the synthesis, purification,
and solubilization of 2-methylcyclohexylamines and 2,6-
dimethylcyclohexylamines are given in the Appendix. The
HDN of pure cyclohexylamine (Fluka) produced a substan-
tial amount of the secondary dicyclohexylamine, which re-
duced the activity of the catalyst. The HDN reactions were
therefore conducted after adding small amounts of water
(up to 1.5 wt% ), since water was found to effectively sup-
press the formation of this by-product. Satterfield et al. re-
ported a higher rate of HDN reactions for sulfided Ni–Mo
catalysts due to water (19, 20). The concentration of wa-
ter in our experiments, however, was much lower than that
reported by Satterfield et al. as being necessary for a sig-
nificant effect on the reaction rates. We did not observe an
increase in the rate of the HDN of cyclohexylamine after
addingvariousamountsofwater, rangingfrom 0to 1.5wt% ,
to the cyclohexylamine in the feed.
FIG. 1. Product distribution as a function of weight time in the
HDN of cis-2-methylcyclohexylamine (c-MCHA) at 350 C. Left-hand
scale for the c-MCHA educt; right-hand scale for the products
methylcyclohexene (MCHE), methylcyclohexane (MCH), and trans-2-
methylcyclohexylamine (t-MCHA).
hydrogenation to methylcyclohexane. The former pathway
plays only a minor role under the present experimental con-
ditions. The latter pathway leads to 1-methylcyclohexene
and 3-methylcyclohexene, the relative amounts of which
depend on the precise mechanism of elimination. The pres-
ence of amine inhibits the hydrogenation of the methylcy-
clohexenes, and, therefore, only a small amount of methyl-
cyclohexane is formed as long as a substantial amount of
amine is present.
The conversion of the almost pure cis and trans diastere-
omers of 2-methylcyclohexylamine with increasing weight
time is shown in Figs. 1 and 2. It is clear that the cis di-
astereomer is consumed faster than the trans diastereomer.
Figure 1 reveals that the cis diastereomer isomerizes to the
trans diastereomer during HDN, and that the concentration
3. RESULTS
HDN of 2-Methylcyclohexylamines at 350 C
Our initial experiments to determine the stereochem-
ical effects during HDN were conducted at 50 bar and
350 C, conditions typically used in industry. As shown in
Scheme 1 and discussed in the Introduction, the removal of
the amino group from 2-methylcyclohexylamine can occur
by two pathways, namely, the direct conversion of the amine
to methylcyclohexane and the elimination of the amino
FIG. 2. Product distribution as a function of weight time in the HDN
oftrans-2-methylcyclohexylamine (t-MCHA) at 350 C. Left-hand scale for
the t-MCHA educt; right-hand scale for the products methylcyclohexene
(MCHE), methylcyclohexane (MCH), and cis-2-methylcyclohexylamine
group to give methylcyclohexenes and their subsequent (c-MCHA).

392
ROTA, RANADE, AND PRINS
of the trans diastereomer goes through a maximum. In con-
trast, Fig. 2 reveals that the concentration of the cis di-
astereomer does not increase significantly. Therefore, ei-
ther the isomerization of the trans diastereomer takes place
only to a small extent, or the cis diastereomer is formed
from the trans diastereomer and is consumed at an equal
rate. Two olefinic products, 1-methylcyclohexene and 3-
methylcyclohexene, were obtained directly from the cis and
trans diastereomers on elimination of the amino group. The
evolution of these products during HDN of the cis and
trans diastereomers is also shown in Figs. 1 and 2, respec-
tively. The selectivity between 1-methylcyclohexene and 3-
methylcyclohexene is about 68 : 32 for the cis diastereomer
and hardly changes with weight time. For the trans diastere-
FIG. 3. Formation of 1-methylcyclohexene (1-MCHE) by isomeriza-
omer, this selectivity is about 61 : 39 at the beginning and tion of 20 kPa 3-methylcyclohexene as a function of weight time at various
temperatures.
slowly reaches the value obtained for the cis diastereomer
with increasing weight time. Substantial amounts of methyl-
cyclohexane are obtained at higher weight times because
of the low concentration of the diastereomeric amines at
higher conversion.
HDN of Substituted Cyclohexylamines at Temperatures
Lower than 350 C
The above results indicate that there is considerable
isomerization not only of the cis and trans diastereomers
of 2-methylcyclohexylamine, but also of the two olefinic
products 1-methylcyclohexene and 3-methylcyclohexene.
To elucidate the HDN mechanism the reaction tempera-
ture must be lowered. Conversion of the cis diastereomer
of 2-methylcyclohexylamine as a function of weight time
at 295 C is plotted in Fig. 4. Figure 4 shows that the con-
centration of the trans diastereomer does not change with
weight time, whereas Fig. 1 shows that it does, indicating
that isomerization was considerably retarded at 295 C. The
concentration of methylcyclohexane is considerably lower
in Fig. 4 than in Fig. 1, even at higher weight times. The
ratio of 1-methylcyclohexene and 3-methylcyclohexene is
approximately 80 : 20 (Fig. 4) and does not change with
To determine whether the olefins isomerize at
350 C, we conducted experiments with a mixture of
3-methylcyclohexene and cyclohexylamine. Cyclohexy-
lamine (without water) was added to simulate the presence
of 2-methylcyclohexylamine during the reaction. The HDN
of cyclohexylamine produces cyclohexene, which does
not interfere with the analysis of the methylcyclohexenes.
Furthermore, cyclohexylamine retards the isomerization
of methylcyclohexene products by deactivating the acid
sites of the catalyst as well as inhibiting further hydro-
genation to methylcyclohexane. Three experiments were
conducted in this way at three temperatures (350, 295, and
250 C). We did not conduct similar investigations using
1-methylcyclohexene as the reactant feed, because it is the
more stable isomer (Saytzeff’s rule) and because isomer-
ization of methylcyclohexenes, if it proceeds at all, should
lead to the formation of 1-methylcyclohexene from 3-
methylcyclohexene. The formation of 1-methylcyclohexene
from 3-methylcyclohexene at three reaction temperatures
is shown in Fig. 3. At 350 C, there is rapid isomerization of
3-methylcyclohexene, and the molar ratio of the two olefins
approaches a value close to that obtained in the HDN of
cis- and trans-2-methylcyclohexylamine. Isomerization is
retarded to a significant extent at 295 C, whereas at 250 C
it is almost completely suppressed. In an experiment at
350 C, in which a 66 : 34 mixture of 1-methylcyclohexene
and 3-methylcyclohexene was used instead of pure 3-
methylcyclohexylamine, this ratio remained unchanged
with increasing space time, indicating that this is the
thermodynamic equilibrium value at 350 C. The selectivity
between 1-methylcyclohexene and 3-methylcyclohexene,
obtained in the HDN of the cis and trans diastereomers
of 2-methylcyclohexylamine at 350 C, is thus very close to
the equilibrium value.
FIG. 4. Product distribution as a function of weight time in the
HDN of cis-2-methylcyclohexylamine (c-MCHA) at 295 C. Left-hand
scale for the c-MCHA educt; right-hand scale for the products
methylcyclohexene (MCHE), methylcyclohexane (MCH), and trans-2-
methylcyclohexylamine (t-MCHA).

STEREOCHEMISTRY OF HYDRODENITROGENATION
393
FIG. 7. Product distribution of dimethylcyclohexene (DMCHE),
dimethylcyclohexane (DMCH), and isomers of cis,cis-2,6-
FIG. 5. Conversion of cis-2-methylcyclohexylamine as a function of
weight time at different reaction temperatures.
methylcyclohexylamine ( c,t- + t,t-DMCHA) as a function of weight time
in the HDN of c,c-DMCHA at 295 C.
weight time. The reaction rate of the cis diastereomer is
clearly lower at 295 C (Fig. 4) than at 350 C (Fig. 1). This
is due not only to the slower elimination, but also to the
slower isomerization of the cis diastereomer to the trans di-
astereomer. Figure 5 presents a comparison of the reaction
rates of the cis diastereomer at different temperatures. Be-
cause of the extensive isomerization of the cis diastereomer
at 350 C, the sum of the partial pressures of the cis and trans
diastereomers is plotted at this temperature. The reaction
rate decreases as the temperature decreases, as would be
expected from an Arrhenius-type relation between tem-
perature and reaction rate.
The HDN of the trans diastereomer at 295 C gives a
plot (not shown) similar to that presented in Fig. 2 ex-
cept that the reaction rate is lower. The selectivity be-
tween 1-methylcyclohexene and 3-methylcyclohexene is
about 67 : 33 and changes little with weight time. The
conversions of the cis and the trans diastereomers of 2-
methylcyclohexylamine with weight time at 295 C are
shown in Fig. 6. It is seen that the cis diastereomer reacts
about 1.5 times faster than the trans diastereomer. In ad-
dition to the HDN of 2-methylcyclohexylamine, the HDN
of cyclohexylamine and 2,6-dimethylcyclohexylamine was
also studied. The HDN of cyclohexylamine may result
in only one olefinic product, namely, cyclohexene. Sim-
ilarly, the HDN of 2,6-dimethylcyclohexylamine gives
only 1,3-dimethylcyclohexene. Figure 7 shows the evo-
lution of the products in the HDN of cis,cis-2,6-
dimethylcyclohexylamine at 295 C. Isomerization of the
cis,cis diastereomer to the cis,trans and trans,trans di-
astereomers occurs to a small extent. A small amount
of 1,3-dimethylcyclohexane is formed by the hydrogena-
tion of 1,3-dimethylcyclohexene. The conversion of cyclo-
hexylamine and cis,cis-2,6-dimethylcyclohexylamine with
weight time is also included in Fig. 6. Cyclohexylamine
reacts slower, while cis,cis-2,6-dimethylcyclohexylamine
reacts faster than the cis and trans diastereomers of
2-methylcyclohexylamine. In the HDN of a 1 : 1 mix-
ture of the cis,cis and cis,trans diastereomers of 2,6-
dimethylcyclohexylamine (not shown), the reaction rate of
the cis,trans diastereomer was about 0.75 times the reaction
rate of the cis,cis diastereomer.
Comparison of Rates of Formation and Distribution
of Olefinic Products in the HDN of
Substituted Cyclohexylamines
Formation of 1-methylcyclohexene from 2-methylcycl-
ohexylamine involves the elimination of the amino group
and of a hydrogen atom on the tertiary carbon atom.
This reaction is comparable to the reaction involved in
the elimination of 2,6-dimethylcyclohexylamine to 1,3-
dimethylcyclohexene. Hence, Table 1 presents a compar-
ison of the formation of 1-methylcyclohexene from the cis
and the trans diastereomers of 2-methylcyclohexylamine
with the formation of 1,3-dimethylcyclohexene from
cis,cis-2,6-dimethylcyclohexylamine at 295 C. The partial
FIG. 6. Conversion of cyclohexylamine (CHA), cis and trans diastere-
omers of 2-methylcyclohexylamine (c- and t-MCHA), and cis,cis-2,6-
dimethylcyclohexylamine (c,c-DMCHA) as a function of weight time at
295 C.

394
ROTA, RANADE, AND PRINS
TABLE 1
Rates of Elimination (in kPa mol g 1 cat min 1) of the Amino Group and a Hydrogen Atom
in Substituted Cyclohexylamines at Different Reaction Temperatures
Elimination with H on a tert carbon atom
Elimination with H on a sec carbon atom
Mainly anti
anti
anti
syn
c,c-DMCHA
→ DMCHE
c-MCHA
→ 1-MCHE
t-MCHA
→ 1-MCHE
c-MCHA
→ 3-MCHE
t-MCHA
→ 3-MCHE
T C
CHA → CHE
295
250
200
2.38
—
—
1.30
1.02
0.64
0.65
0.19
—
0.70
0.45
0.34
0.33
0.24
0.15
0.31
0.09
—
pressures of all olefinic products increased linearly with in-
creasing weight time. Because of this zero order, the rate
constants were determined directly from the rates of forma-
tion. The rate of formation of 1,3-dimethylcyclohexene is
about double the rate of formation of 1-methylcyclohexene
from cis-2-methylcyclohexylamine, which is in turn appro-
ximately twice as fast as the formation of 1-methyl-
cyclohexene from trans-2-methylcyclohexylamine. Forma-
tion of 3-methylcyclohexene from either the cis or the trans
diastereomer involves elimination of the amino group to-
4. DISCUSSION
The HDN of aromatic nitrogen heterocycles proceeds
over sulfided Ni–Mo catalysts mainly after complete satu-
ration of the and carbon atoms (21). The HDN activity
of the resulting aliphatic or alicyclic compounds is strongly
dependent on the number of hydrogen atoms on carbon
atoms in position relative to the carbon bearing the amino
group (22). This suggests that the primary mechanism of re-
moval of the amino group is elimination. Only the HDN
of amines that do not have hydrogen atoms proceeds via
a substitution mechanism (primarily SN2), for example, in
neopentylamine and benzylamine (23, 24). A substitution
mechanism also occurs to a lesser extent in molecules with
hydrogen atoms. By a substitution mechanism we mean
either direct substitution of the amino group by hydrogen
or a two-step process involving substitution by a sulfhydryl
group and then by hydrogen. Only 5 to 10% of the final
product in our reactions was obtained via these mecha-
nisms. This leaves only two mechanisms for the elimination
reactions, namely, E1 and E2. In the E1 mechanism, the
first step in the removal of the amino group is rate control-
ling, and the second step in the removal of a hydrogen
should have no influence on the overall reaction rate. If
this E1 mechanism were to occur, then we would expect
almost no differences between the reaction rates of the dif-
ferent diastereomers of 2-methylcyclohexylamine and 2,6-
dimethylcyclohexylamine. Furthermore, the E1 reaction of
these amines involves the formation of a secondary carbe-
nium ion, which should isomerize significantly to a tertiary
carbenium ion in the case of alkyl-substituted cyclohexy-
lamines. This should lead to rearrangement of the carbon
skeleton to give products like ethylcyclopentane from the 2-
methylcyclohexyl carbenium ion. Such products were, how-
ever, observed only to a very small extent ( 2% ), indicating
that the primary mechanism under the present experimen-
tal conditions is most likely E2.
gether with a hydrogen atom on a secondary
carbon
atom. This is comparable to the process that takes place
in the elimination reaction of cyclohexylamine to cyclo-
hexene. Table 1 shows a comparison of the formation of
3-methylcyclohexene from the cis and trans diastereomers
of 2-methylcyclohexylamine with the formation of cyclo-
hexene from cyclohexylamine at 295 C. The rates of forma-
tion of 3-methylcyclohexene from the cis and trans diastere-
omers of 2-methylcyclohexylamine are about the same and
are about half the rate of formation of cyclohexene formed
from cyclohexylamine.
Table 1 also presents the rates of the formation of olefinic
products in the HDN of cyclohexylamine and the cis and
trans diastereomers of 2-methylcyclohexylamine at 250 and
200 C. Because of problems with the mass balance, we
did not obtain reliable results for the HDN of cis,cis-2,6-
dimethylcyclohexylamine below 295 C. The rates of for-
mation of 1-methylcyclohexene and 3-methylcyclohexene
from trans-2-methylcyclohexylamine decrease with tem-
perature at a faster rate than the formation from the cis di-
astereomer. At 250 C, only about 10% of the trans diastere-
omer reacted at the highest weight time used and, hence,
we did not conduct the reaction of the trans diastereomer
at 200 C. The rate of formation of cyclohexene from cyclo-
hexylamine is approximately twice as fast as the formation
of 3-methylcyclohexene from cis-2-methylcyclohexylamine
at 250 and 200 C. The data presented in Table 1 show
that the dependency of the reaction rate on tempera-
ture is greater for the trans diastereomer than for the cis
diastereomer.
An E2 mechanism involves the single-step removal of
the amino group together with a hydrogen atom on the
carbon atom. In single-phase reactions, the E2 mechanism

STEREOCHEMISTRY OF HYDRODENITROGENATION
395
has a first-order dependence on the concentration of the
base and the substrate. This implies that if elimination takes
place by an E2 mechanism under these conditions, then
the reaction should have a first-order dependence on the
amine concentration on the surface and in the base (which
is widely assumed to be the sulfhydryl group attached to
a surface molybdenum atom). It is very difficult to change
the surface concentration of basic species on a surface and
keep other properties of a heterogeneous catalyst the same.
It is not easy to monitor the dependence of the reaction rate
on the surface concentration of the amine. This is because
amines adsorb very strongly on the surface of the sulfide
catalyst and, therefore, almost completely cover the cata-
lyst surface, even if their concentration in the gas phase is
very low. Thus, a zero-order dependence of the reaction
rate on the concentration of amine in the gas phase is ob-
served. This is advantageous, because a zero-order depen-
dence simplifies the calculation of reaction rates for paral-
lel reactions. The plots show the zero-order dependence of
the reaction rate on the concentration of amines in the gas
phase; namely, the concentration of the amines decreases
linearly with weight time (e.g., Fig. 6), and the concentration
of the olefinic products increases linearly with weight time.
Only toward the end of the reaction do the plots become
nonlinear because of competitive adsorption of ammonia,
a product of the elimination reaction (see Fig. 5). Thus, it
is not possible to determine the reaction mechanism from
the dependence of the reaction rate on the concentration
of the base and the catalyst.
SCHEME 2
most stable conformation of cyclohexylamine, the amino
Stereochemistry is a very effective tool for determining group is found in an equatorial position and cannot be elim-
the existence of an E2 mechanism. The isomerization of inated. For elimination to occur, the chair has to flip over so
diastereomers at 350 C, however, did not result in the de- that the amino group is in the axial position. Anti elimina-
termination of the mechanism, and hence the reaction tem- tion can now take place in either direction to cyclohexene.
perature was lowered. E2 elimination takes place prefer- Cis-2-methylcyclohexylamine is already in the preferred
ably when the hydrogen atom and the amino group to be conformation for elimination, i.e., with the amino group in
eliminated are in either a syn periplanar or an anti peripla- the axial position and the methyl group in the equatorial po-
nar configuration (25–27). In anti elimination, the hydrogen sition. Once again, elimination can take place on either side,
atom and the amino group are on opposite sides of the C–C but it iseasier to eliminate the hydrogen atom on the tertiary
bond, while in syn elimination the hydrogen atom and the carbon atom bearing the methyl group. The formation of
amino group are on the same side of the C–C bond. The anti 1-methylcyclohexene is also favored because of its higher
periplanar configuration is preferable, because it minimizes stability (Saytzeff’s rule) (25). Thus, the formation of 1-
steric interaction with substituents on the adjacent carbon methylcyclohexene from cis-2-methylcyclohexylamine oc-
atoms involved in the elimination process. Anti elimination curs more often than the formation of 3-methylcyclohexene
is even more favorable in reactions of substituted cyclohex- at all reaction temperatures (cf. Table 1). Surprisingly, the
anes, because this configuration can be easily attained in its product distribution is hardly affected by temperature (be-
more stable chair conformation. Syn elimination with ideal low 300 C) and the selectivity between the two products is
geometry, i.e., periplanar, is impossible in the chair con- always close to 80 : 20. As mentioned above, the process of
formation whereas it is possible in the less stable boat or elimination of a hydrogen atom from the secondary car-
boat-like conformation. Scheme 2 presents the elimination bon atom of cis-2-methylcyclohexylamine is comparable to
of the amino group from cyclohexylamine, the cis and trans the elimination of a hydrogen atom from the secondary
diastereomers of 2-methylcyclohexylamine, and cis,cis-2,6-
carbon atom of cyclohexylamine. This is illustrated very
dimethylcyclohexylamine. Only the hydrogen atoms in- well by our experiments, because the rate of formation of
volved in the elimination reaction are indicated. The most cyclohexene is approximately double the rate of forma-
stable conformers of the amines are shown on the left. In the tion of 3-methylcyclohexene at all reaction temperatures

396
ROTA, RANADE, AND PRINS
investigated (cf. Table 1). Although additional activation tionship of the amino group with the hydrogen atom on ter-
energy is necessary to flip the more stable chair conformer tiary carbon atomsin the case ofthe cis,transdiastereomer
of cyclohexylamine to its less stable chair conformer for of 2,6-dimethylcyclohexylamine is similar to that of the cis-
elimination, this energy is insufficient to retard the reac- 2-methylcyclohexylamine on the one hand and to that of
tion rate, probably because of the small size of the amino the trans-2-methylcyclohexylamine on the other. Thus, the
group.
cis,trans-2,6-dimethylcyclohexylamine reaction rate should
In the most favorable conformation of the cis,cis-2,6- be about the same as the sum of the rates of formation
dimethylcyclohexylamine, the amino group is in an ax- of 1-methylcyclohexene from the cis and trans diastere-
ial position, whereas the two methyl groups are in equa- omers of 2-methylcyclohexylamine. Accordingly, the reac-
torial positions. The stereochemical configuration is thus tion rate of the cis,trans diastereomer is about 0.75 times
ideal for anti elimination in either direction. There are now the reaction rate of the cis,cis diastereomer or 1.5 times the
two hydrogen atoms on a tertiary carbon atom which rate of formation of 1-methylcyclohexene from the cis di-
can be eliminated with the amino group compared with astereomer. The rate of formation of 1-methylcyclohexene
only one in the case of cis-2-methylcyclohexene. In agree- from trans-2-methylcyclohexylamine decreases much faster
ment with these stereochemical situations, the rate of for- with decreasing temperature than the corresponding rate
mation of 1,3-dimethylcyclohexene is approximately twice from the cis diastereomer (Table 1), as expected because a
as high as the rate of formation of 1-methylcyclohexene higher activation energy is necessary to attain the required
at 295 C (cf. Table 1). The fact that elimination of a boat/boat-like form for syn elimination (Scheme 2). How-
hydrogen atom on a tertiary
carbon atom is easier ever, since the hydrogen atom involved in the elimination
than that of a hydrogen atom on a secondary carbon is attached to a tertiary carbon atom, apparently its elim-
atom is supported by the observation that the rate of ination is still easier than the anti elimination involving the
elimination of cis,cis-2,6-dimethylcyclohexylamine (2.38) hydrogen atom on the secondary carbon atom.
is higher than the sum of the two elimination reaction
To the best of our knowledge, a comparable investiga-
rates of cis-2-methylcyclohexylamine (1.63), which in turn tion of elimination reactions over heterogeneous catalysts
is higher than the elimination rate of cyclohexylamine (0.7) with a quantitative comparison between the reaction rates
(cf. Table 1).
of different molecules and at different temperatures has not
In the most stable conformation of the trans-2- been made. Considerable work has been done on the dehy-
methylcyclohexylamine, both amino and methyl groups are dration of alcohols over alumina catalysts (e.g., Pines et al.,
in equatorial positions. As in the case of cyclohexylamine, Kno¨zinger et al., and Kochloefl et al.) (5–8, 28, 29). Pines
the chair must flip over so that both groups are in an axial and Manassen illustrated the prevalence of anti elimina-
position; only then can the elimination of the amino group tion in the dehydration of menthol and neo-menthol (5).
together with a hydrogen atom on the secondary carbon The stereochemical relation between the hydroxy group
atom occur. The results of experiments at 295 C indicate and the hydrogen atom on the tertiary carbon atom in
that the rate of formation of 3-methylcyclohexene from the menthol and neo-menthol is similar to the corresponding
trans diastereomer is about the same as that from the cis di- relation between the amino group and hydrogen atom in
astereomer (cf. Table 1). At a lower reaction temperature, the cis and trans diastereomers of 2-methylcyclohexylamine
however, the rate of the trans diastereomer is much lower respectively. It is surprising that they report that only a
than that ofthe cis diastereomer. A higher activation energy very small amount of neo-menthol reacted by syn elimi-
isapparentlynecessaryfor the transdiastereomer to flip, be- nation. Bera´nek et al. (28) studied the dehydration of sev-
cause in this process two groups must attain the unfavorable eral cyclohexanols and found that the cis diastereomers of
axial positions. This higher activation energy results in a sig- 2-alkylcyclohexanols always reacted much faster than the
nificant change in the reaction rate, in contrast to the case of trans diastereomers at 200 C. In line with their observa-
cyclohexylamine. Anti elimination of the hydrogen atom on tions, our investigations show that the cis diastereomers
the tertiary carbon atom together with the amino group of alkyl-substituted cyclohexylamines react faster than the
of the trans-2-methylcyclohexylamine is impossible, but syn corresponding trans diastereomers. They, however, did not
elimination can take place if a boat or a boat-like conforma- report the composition of the olefinic products in their reac-
tion is attained. This process is clearly much less favorable tions and, hence, no parallels can be drawn with our results
than a similar anti elimination from a chair conformation. in this respect. Bauer and Thomke investigated the mech-
Accordingly, the rate of formation of 1-methylcyclohexene anism of elimination of the methoxy group of erythro-2-
from the trans diastereomer is about half the rate of the methoxybutane-3-D over heterogeneous alumina catalysts
corresponding rate from the cis diastereomer at 295 C (cf. (30). After taking into account the kinetic isotope effects,
Table 1). This result agrees well with the results obtained for they found that the elimination reaction proceeds by the
the HDN ofthe mixture ofcis,cisand cis,transdiastereomers syn pathway (30% ) and by the anti pathway (about 70% )
of 2,6-dimethylcyclohexylamine. The stereochemical rela- at 220 C. This agrees qualitatively with our results because,

STEREOCHEMISTRY OF HYDRODENITROGENATION
397
as in their investigations, we found significant syn elimina-
tion for molecules for which comparable anti elimination
was not possible.
Pines and co-workers already discussed how difficult it is
to explain how anti E2 elimination can take place on the sur-
face of a heterogeneous catalyst (5, 8). The hydrogen atom
and the leaving group, although oriented in opposite di-
rections, must be coordinated simultaneously to the catal-
yst surface for the reaction to occur. It is much easier to
explain a syn E2 elimination, because the hydrogen atom
and the leaving group are on the same side and can co-
ordinate easily with the catalytic surface. Pines suggested
that anti E2 elimination takes place in crevices where op-
posite walls coordinate with the hydrogen atom and the
leaving group. It is obvious that this explanation takes for
granted that only a small part of the catalyst is active since
only a few pores have molecular dimensions. Also, the ex-
planation lacks generality. Kno¨zinger et al. tried to solve
this problem by suggesting that a rocking vibration of the
FIG. 8. Pictorial presentation of syn elimination of trans-2-methyl-
cyclohexylamine (left) and anti elimination of cis-2-methylcyclohexy-
lamine (right) on an MoS2 surface.
whole molecule may take place (9). The resulting inclina- sulfhydryl groups are found under typical reaction condi-
tion of the molecule should then enable the hydrogen tions. Furthermore, it had been assumed that the C, N, and
to reach the basic site. Closer inspection of the geometry Mo atoms are colinear (9). The dehydration of alcohols on
involved in the coordination of molecules with the surface alumina is easily explained in an analogous way, because
reveals, however, that anti elimination is indeed possible an acidic alumina catalyst has protruding hydroxyl groups
over catalyst surfaces, even if the reaction does not take attached to the surface, thus providing the necessary geom-
place on opposite walls or is not aided by rocking vibra- etry for anti elimination. In Fig. 8 we show only the edge of
tions.
Acidic sites and basic sites are necessary for elimination. replace a Mo atom as proposed by Raybaud et al. (33).
On an MoS2 surface, the sulfhydryl groups may behave as Our mechanism also explains why elimination prefer-
MoS2, but in case of nickel decoration, a nickel atom could
basic sites, while the molybdenum atoms can act as Lewis entially leads to cis olefins in the removal of water from
acid sites. These sites coordinate with the hydrogen atom butanol-2 (9). Instead of assuming the anti position in
on a
carbon atom and the amino group, respectively. the transition state, as in liquid phase elimination, the
Figure 8 shows how anti and syn elimination can occur from methyl groups preferably take up the gauche position dur-
cis- and trans-2-methylcyclohexylamine on an MoS2 sur- ing elimination on a solid surface because then both methyl
face. The basic difference between these and former models groups can be turned away from the surface (cf. Fig. 8).
is that the MoS2 edge surface is not considered to be a single This argument has already been proposed by Kno¨zinger
row of Mo and S atoms. Recent DFT calculations demon- et al. (9). Their model does not, however, explain the
¯
strate that, in the absence of H2S, the (1010) edge surface preferential formation of 1-methylcyclohexene rather than
of MoS2 contains Mo atoms that are bonded to four sulfur 3-methylcyclohexene from cis-2-methylcyclohexylamine.
atoms (31–33). In the presence of some H2S, however, ad- Since they assumed a flat catalyst surface and a C –O bond
ditional sulfur atoms become bonded to these surface Mo vertical to the catalyst surface, the steric hindrance between
atoms. These sulfur atoms may be singly bonded (on top po- the methyl group and the catalyst surface does not enable
sition, as in Fig. 8) or doubly bonded (between two surface elimination to occur in their model.
Mo atoms). In either case, the surface is not flat, because
sulfur atoms protrude from the Mo plane.
The stereochemical results of the elimination of NH3
from cis- and trans-2-methylcyclohexylamine also enable
The occurrence of syn elimination over such an MoS2 us to discard a mechanism that explained HDN by de-
surface is easy to explain, because the amino group and hydrogenation of an amine to an imine, followed by
the hydrogen atom can point toward the surface (Fig. 8A). reaction with water to a ketone and hydrogenation to
Since the sulfhydryl group protrudes from the surface, anti an alcohol, with final elimination of water to an olefin
elimination can easily occur as well, as illustrated by the (Scheme 3) (34). The metal sulfide would be responsible for
docking of the cis diastereomer (Fig. 8B). Previous reports the (de)hydrogenation and elimination, and water would
on elimination reactions did not explain anti E2 elimina- act as a cocatalyst. The intermediate formation of a ke-
tions on catalyst surfaces, because the surface was assumed tone would, however, lead to a mixture of cis- and trans-2-
to be flat. This is not the case with MoS2 where protruding methylcyclohexanol.

398
ROTA, RANADE, AND PRINS
was evaporated in a rotary evaporator. The isolated prod-
uct was then distilled under vacuum (bp. 73 C) to yield the
trans diastereomer with 95 to 98% purity, the main impurity
being the cis diastereomer.
Cis-2-methylcyclohexylamine was synthesized by two
methods. The first method involved transamination of 1-
phenylethylamine to 2-methylcyclohexanone, as reported
by Knupp and Frahm (18). The chloride salt of cis 2-
methylcyclohexylamine obtained at the end of the proce-
dure was neutralized with NaOH and treated further as
for the trans-2-methylcyclohexylamine. The cis diastere-
omer was obtained with about 98% purity as a colorless
liquid after vacuum distillation (bp. 74 C). In the second
method, 120 g of o-toluidine was hydrogenated over a Rh/C
(Aldrich) catalyst (substrate-to-metal weight ratio = 400)
in glacial acetic acid at 50 C and 100 bar hydrogen pressure
in an autoclave for 2 days. In addition to cis- and trans-2-
methylcyclohexylamine, significant amounts of secondary
amines (approximately 40% by weight), resulting from the
coupling of the primary amine products, were obtained. It
was not possible to separate the individual amines by dis-
tillation because of azeotrope formation. Therefore, after
filtration of the catalyst and removal of solvent, the product
was separated into three fractions by vacuum distillation.
The fraction that boiled at 118 to 121 C was isolated as
a thick colorless oil (approximately 130 g). Two hundred
grams of ethyl acetate was added to this fraction and the
solution was stirred slowly at room temperature overnight.
White microcrystals of cis-2-methylcyclohexylamine (40 g)
were obtained. The crystals were filtered and washed with
ethyl acetate and then recrystallized using ethyl acetate to
increase the purity if necessary. The purity of these crys-
tals was 92 to 96% , the main impurity being the trans
diastereomer. The cis diastereomer synthesized accord-
ing to the second method was, however, insoluble in oc-
tane, the solvent used for dissolving the reactant feed. It
was found that traces of water (up to 0.8% by weight)
stabilized/dissolved the cis diastereomer synthesized ac-
cording to the first method in octane. Hence, the cis
diastereomer obtained according to the second method
was neutralized by hydrochloric acid and the free amine
isolated via treatment with NaOH and extraction, as re-
ported for the trans diastereomer. The resulting cis diastere-
omer was then obtained ( 20 g) as a colorless liquid with
92% purity after vacuum distillation (bp. 74 C).
SCHEME 3
5. CONCLUSION
We demonstrated the influence of stereochemical factors
in HDN reactions over sulfided catalysts and reported
one of the very few detailed mechanistic investigations
of elimination reactions over heterogeneous catalysts. We
have shown that the rate of HDN of alkyl-substituted cy-
clohexylamines over sulfided Ni–Mo catalysts depends not
only on the number of hydrogen atoms, but also on their
stereochemical relation to the amino group. Isomerization
of olefinic products and the amines prevented meaning-
ful mechanistic investigations at 350 C, a temperature
typically employed in HDN. Our results were obtained
at lower reaction temperatures and can be explained by
the E2 elimination mechanism which, together with its
conformation requirements, accounts for the quantitative
relationships between the rates of the HDN of various
amines and the rates of formation of different olefinic
products. The cis diastereomers react faster than the trans
diastereomers, because they allow for an anti geometric
relationship in the chair conformation between the amino
group and a hydrogen atom on a carbon atom. The syn
elimination takes place to a considerable extent at higher
temperatures in molecules that are unable to undergo
anti elimination. The activation energy of anti elimination
is lower than that of syn elimination, and the activation
energy of anti elimination involving a hydrogen atom
attached to a tertiary carbon atom is lower than that
involving a hydrogen atom attached to secondary carbon
atom. Finally, we demonstrated the possibility of anti
as well as syn elimination on the surface of the catalyst
without assuming the geometry of the catalytic pores.
APPENDIX: SUPPLEMENTARY INFORMATION
Synthesis of cis- and trans-2-methylcyclohexylamine
A commercially available mixture of cis- and trans-
2-methylcyclohexylamine (cis : trans 78 : 22, Fluka) was
cooled with an ice-water bath and neutralized with
Synthesis of 2,6-Dimethylcyclohexylamine Diastereomers
2,6-Dimethylcyclohexylamines occur in three diastere-
hydrochloric acid to give the chloride salt. Repeated omeric forms, cis,cis, cis,trans, and trans,trans. The configu-
crystallization of the salt mixture in water or in a water– ration of the diastereomers was identified by referring to
1
ethanol mixture gave crystals of the chloride salt of trans- the H NMR data published by Feltkamp (35). A mix-
2-methylcyclohexylamine of increasing purity. Sufficiently ture of the three isomers was synthesized by hydrogena-
pure crystals (95–98% ) were treated with NaOH in water, tion of 100 g 2,6-xylidene (a gift from BASF AG) in an
and the free amine was extracted with diethyl ether. The ethanol–acetic acid mixture (2 : 1 weight ratio) over Ru/C
ether extracts were dried with sodium sulfate, and the ether (Aldrich, substrate-to-metal weight ratio = 400) at 80 C

STEREOCHEMISTRY OF HYDRODENITROGENATION
399
and 100 bar hydrogen pressure. After the hydrogenation
reaction, a solid mass was obtained as the product after
cooling the autoclave to room temperature. The products
could be dissolved again when more ethanol was added.
The products consisted mainly of the cis,cis and cis,trans
diastereomers with a small amount of trans,trans diastere-
omer. After the catalyst was filtered off, ethanol was re-
moved in a rotary evaporator and the remaining acetic acid
in the resulting liquid was neutralized with NaOH after dis-
solving in water. The insoluble amine diastereomers formed
a separate organic layer consisting of crystals and liquid.
Separation of the organic layer was facilitated by the ad-
dition of ethyl acetate. The crystals, although insoluble in
ethyl acetate, collected preferentially in the organic layer.
The aqueous layer was extracted twice with ethyl acetate,
3. Shabtai, J., Nag, N. K., and Massoth, F. E., in “Proceedings, 9th Inter-
national Congress on Catalysis, Calgary, 1988” (M. J. Phillips and M.
Ternan, Eds.), Vol. 1, p. 1. Chem. Inst. of Canada, Calgary, 1988.
4. Fischer, L., Harle´, V., and Kasztelan, S., Stud. Surf. Sci. Catal. 127, 261
(1999).
5. Pines, H., and Manassen, J., Adv. Catal. 16, 49 (1966).
6. Kno¨zinger, H., Angew. Chem. Int. Ed. Engl. 7, 791 (1968).
7. Kno¨zinger, H., and Scheglila, A., J. Catal. 17, 252 (1970).
8. Pines, H., and Haag, W. D., J. Am. Chem. Soc. 83, 2847 (1961).
9. Kno¨zinger, H., Bu¨hl, H., and Kochloefl, K., J. Catal. 22, 57
(1972).
10. Kno¨zinger, H., and Ress, E., Z. Phys. Chem. NF 54, 136
(1967).
11. Rota, F., and Prins, R., Stud. Surf. Sci. Catal. 127, 319 (1999).
12. Rota, F., and Prins, R., J. Mol. Catal. A 162, 359 (2000).
13. Rota, F., and Prins, R., Top. Catal. 11, 327 (2000).
14. Yang, S. H., and Satterfield, C. N., J. Catal. 81, 168 (1983).
15. Olalde, A., and Perot, G., Appl. Catal. 13, 373 (1985).
and all the ethyl acetate fractions were collected. The crys- 16. Eijsbouts, S., Sudhakar, C., de Beer, V. H. J., and Prins, R., J. Catal.
127, 605 (1991).
tals were made up primarily of the cis,cis diastereomer and
were removed by filtration from ethyl acetate. The ethyl ac-
etate from the filtrate was evaporated in a rotary evapora-
17. Hillerova, E., Vit, Z., Zdrazil, M., Shkuropat, S. A., Bogdanets, E. N.,
and Startsev, A. N., Appl. Catal. 67, 231 (1991).
18. Knupp, G., and Frahm, A. W., Chem. Ber. 117, 2076 (1984).
tor. Vacuum distillation of the liquid extract after removal
of the solvent yielded 42 g of an azeotropic mixture (bp.
88–90 C) of 5% trans,trans diastereomer, 45% cis,cis, and
50% cis,trans diastereomer. A solid residue was left after
distillation and consisted mainly of crystals of the cis,cis di-
astereomer. These crystals were collected and purified by
recrystallization from a diisopropyl ether–ethanol mixture.
The cis,cis diastereomer of 2,6-dimethylcyclohexylamine
obtained according to this method presented a problem.
Like the cis diastereomer of 2-methylcyclohexylamine syn-
thesized by the second method, it was insoluble in octane.
19. Satterfield, C. N., Smith, C. M., and Ingalls, M., Ind. Eng. Chem. Pro-
cess Des. Dev. 24, 1000 (1985).
20. Satterfield, C. N., and Yang, S. H., J. Catal. 81, 335 (1983).
21. Nelson, N., and Levy, R. B., J. Catal. 58, 485 (1979).
22. Portefaix, J. L., Cattenot, M., Guerriche, M., Thivolle-Cazat, J., and
Breysse, M., Catal. Today 10, 473 (1991).
23. Vivier, L., Dominguez, V., Pe´rot, G., and Kasztelan, S., J. Mol. Catal.
67, 267 (1991).
24. Cattenot, M., Portefaix, J.-L., Afonso, J., Breysse, M., Lacroix, M., and
Perot, G., J. Catal. 173, 366 (1998).
25. March, J., “Advanced Organic Chemistry.” Wiley, New York, 1992.
26. Sicher, J., Angew. Chem. Int. Ed. 11, 200 (1972).
27. Bartsch, R. A., and Za´vada, J., Chem. Rev. 80, 453 (1980).
To make it soluble, traces of water were sufficient. Cis- 28. Bera´nek, L., Kraus, M., Kochloefl, K., and Bazant, V., Collect. Czech.
Chem. Commun. 25, 2513 (1960).
29. Kochloefl, K., Kraus, M., Chin-Shen, C., Bera´nek, L., and Bazant, V.,
Collect. Czech. Chem. Commun. 27, 1199 (1962).
30. Bauer, R., and Thomke, K., J. Mol. Catal. 79, 311 (1993).
2-methylcyclohexylamine became soluble after converting
the amine to its chloride salt and back again to the free
amine using NaOH. The free amine was extracted with di-
ethyl ether and was obtained as a colorless liquid ( 22 g)
after removing the ether in a rotary evaporator; it was pu-
rified (97% ) by vacuum distillation (bp 84 C).
31. Byskov, L. S., Nørskov, J. K., Clausen, B. S., and Topsøe, H., J. Catal.
187, 109 (1999).
32. Raybaud, P., Hafner, J., Kresse, G., Kasztelan, S., and Toulhoat, H.,
J. Catal. 189, 129 (2000).
33. Raybaud, H., Hafner, J., Kresse, G., Kasztelan, S., and Toulhoat, H.,
J. Catal. 190, 128 (2000).
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