Mechanisms of the hydrodenitrogenation of alkylamines with secondary and tertiary α-carbon atoms on sulfided NiMo/Al2O3

Article abstract of DOI:10.1016/j.jcat.2003.12.013

The HDN of alkylamines with secondary and tertiary α-carbon atoms (2-pentylamine, 3-methyl-2-butylamine, 3,3-dimethyl-2-butylamine, 2-methylcyclohexylamine, 2-methyl-2-butylamine) and benzylamine and the HDS of corresponding alkanethiols were studied over sulfided NiMo/Al₂O₃. Alkanethiols and dialkylamines were primary products in the HDS of the amines with secondary products, formed from elimination and hydrogenolysis of the alkanethiols, as confirmed by the similar alkenes/alkane ratios in the HDN of the alkylamines and HDS of the corresponding alkanethiols. 2-Methyl-2-butylamine and benzylamine reacted much faster than the amines with secondary α-carbon atoms. Methylbutenes and methylbutane were the primary products of 2-methyl-2-butylamine, and toluene was the primary product of benzylamine. This and the different methylbutenes/methylbutane ratios in the HDS of 2-methyl-2-butylamine and HDS of 2-methyl-2-butanethiol indicated that 2-methyl-2-butylamine, with a tertiary α-carbon atom, and the activated benzylamine reacted by means of an E1 mechanism. The substitution of the NH₂ group by H₂S led to an alkanethiol and NH₃ and, thus, to total denitrogenation. Substitution by an amine led to a dialkylamine and NH₃ and to 50% nitrogen removal. High partial pressures of H₂S and alkylamine increased the rate of transformation of alkylamine to alkanethiol and thus, of denitrogenation. However, the rate of sulfur removal from the alkanethiol decreased.

Full text of DOI:10.1016/j.jcat.2003.12.013

Journal of Catalysis 222 (2004) 532–544
www.elsevier.com/locate/jcat
Mechanisms of the hydrodenitrogenation of alkylamines with secondary
and tertiary α-carbon atoms on sulfided NiMo/Al2O3
Y. Zhao and R. Prins ∗
Institute for Chemical and Bioengineering, Swiss Federal Institute of Technology (ETH), 8093 Zurich, Switzerland
Received 19 September 2003; revised 18 December 2003; accepted 18 December 2003
Abstract
The hydrodenitrogenation (HDN) of alkylamines with secondary and tertiary α-carbon atoms (2-pentylamine, 3-methyl-2-butylamine,
3,3-dimethyl-2-butylamine, 2-methylcyclohexylamine, 2-methyl-2-butylamine) and benzylamine and the hydrodesulfurization (HDS) of cor-
responding alkanethiols were studied over sulfided NiMo/Al O . Alkanethiols and dialkylamines were primary products in the HDN of the
2
3
amines with secondary α-carbon atoms, formed by substitution of the amine group by H S or an alkylamine. Alkanes and alkenes were
2
secondary products, formed from elimination and hydrogenolysis of the alkanethiols, as confirmed by the similar alkenes/alkane ratios in the
HDN of the alkylamines and HDS of the corresponding alkanethiols. 2-Methyl-2-butylamine and benzylamine reacted much faster than the
amines with secondary α-carbon atoms. Methylbutenes and methylbutane were the primary products of 2-methyl-2-butylamine, and toluene
was the primary product of benzylamine. This and the different methylbutenes/methylbutane ratios in the HDN of 2-methyl-2-butylamine
and HDS of 2-methyl-2-butanethiol indicate that 2-methyl-2-butylamine, with a tertiary α-carbon atom, and the activated benzylamine react
by means of an E1 mechanism.
2003 Elsevier Inc. All rights reserved.
Keywords: 2-Pentylamine; 3-Methyl-2-butylamine; 3,3-Dimethyl-2-butylamine; 2-Methyl-2-butylamine; 2-Methylcyclohexylamine; Benzylamine; HDN;
Hydrotreating catalysts
1. Introduction
formation of pentenes was negligible at short weight time
and that the major product was dipentylamine [8]. At higher
weight times pentenes and pentanethiol were observed and
the production of the pentenes was ascribed mainly to the
elimination of pentylamine from the dipentylamine and,
in part, to the elimination of H2S from pentanethiol. We
showed that, over a sulfided NiMo/Al2O3 catalyst at 300
to 340 ◦C and elevated pressure (3 MPa), n-hexylamine, di-
n-hexylamine, and tri-n-hexylamine react predominantly by
nucleophilic substitution of the amines by H2S and not by
elimination of NH3 [9]. The resulting hexanethiol reacts very
fast to hexenes as well as to hexane. The very low selectiv-
ity of the hexenes at low weight time demonstrates that the
hexenes are secondary not primary products in the HDN of
n-hexylamines. As a consequence, the hexene/hexane ra-
tio in the HDN product mixture is determined by the HDS
reaction, as demonstrated by the similar alkenes/alkane ra-
tios in the simultaneous HDN of hexylamine and HDS of
pentanethiol [9].
Nitrogen atoms are removed from hetero-aromatic com-
pounds by hydrogenation of the aromatic ring, which con-
tains the nitrogen atom, and breaking of the resulting
aliphatic C–N bonds to form a hydrocarbon molecule and
ammonia [1–5]. Hydrogenation is not required for the re-
moval of a sulfur atom from an aromatic ring that contains a
sulfur atom, as in (di)benzothiophene, because the relatively
weak C–S bond can be broken by hydrogenolysis [6,7]. The
presence of a large amount of alkenes and a minor amount
of alkanes in the reaction product of HDN suggests that
aliphatic C–N bond breaking occurs mainly by elimination
of NH3 [1,3]. Nucleophilic substitution of the alkylamine
by H2S, followed by C–S bond hydrogenolysis, explains
the presence of alkanes [1,4]. Cattenot et al. showed, how-
ever, that in the HDN of the linear n-pentylamine over
unsupported MoS2 at 275 ◦C and atmospheric pressure the
*
The nucleophilic substitution by a good nucleophile such
as SH− is aided by the accessibility of the α-carbon atom in
Corresponding author.
E-mail address: prins@tech.chem.ethz.ch (R. Prins).
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2003.12.013
2003 Elsevier Inc. All rights reserved.

Y. Zhao, R. Prins / Journal of Catalysis 222 (2004) 532–544
533
linear alkylamines. The accessibility of the α-carbon atom
decreases with substitution and, at the same time, the num-
ber of β-hydrogen atoms increases. Thus, amines with sec-
ondary and tertiary α-carbon atoms may react differently as
linear alkylamines [8]. Therefore, we report the results of the
organic part of our mechanistic investigation of the removal
of ammonia from such alkylamines in this work. In future
work we will report on the inorganic aspects, the catalytic
sites, and the mode(s) of adsorption.
respect to the amine, while their ratio was kept constant. The
accuracy in the measured conversion was 2% (relative).
2-Pentylamine (Lancaster), 2-pentanethiol (Aldrich), 3-
methyl-2-butylamine (Aldrich, 98%), 3-methyl-2-butane-
thiol (ABCR, 100%), 3,3-dimethyl-2-butylamine (ABCR,
100%), 2-methylcyclohexylamine(Fluka), 2-methyl-2-butyl-
amine (Aldrich, 98%), 2-methyl-2-butanethiol (TCI, Japan,
99%), benzylamine (Fluka, purum), α-toluenethiol (Fluka,
purum), and cyclohexane (Fluka, puriss.) were all used as
purchased in the HDN tests. 2-Methyl-3-pentylamine was
synthesized from 2-methylpentanone-3 by reaction with
hydroxylamine and reduction of the resulting oxime with
LiAlH4 in ether. The product was purified by destillation.
The gases used were hydrogen (PanGas 4.0) and a mixture
of 10% H2S in H2 (Messer Griesheim 3.0).
2. Experimental
HDN and HDS experiments were carried out in a mi-
croflow reactor with an NiMo/Al2O3 catalyst prepared by
sequential pore-volume impregnation of γ -Al2O3, as de-
scribed elsewhere [9,10]. The catalyst was sulfided in situ
and used immediately, without exposing it to nitrogen or air.
The total pressure in the HDN and HDS experiments was
3 MPa and the partial pressure of the alkylamines was 5 kPa.
A higher alkylamine pressure would not lead to a greater
amount of the product (which would be more easily de-
tectable at very low weight time) as long as the reaction has
an order above zero. At 5 kPa alkylamine the order of the
HDN reactions at 300 ◦C was about 0.5 in the alkylamine
pressure. The HDS of alkanethiols was studied, because they
are intermediates in the HDN of alkylamines. They were
studied at 5 kPa, always in the presence of 5 kPa hexylamine.
Although the presence of the alkylamine substantially de-
creased the rates of the HDS reactions of the alkanethiols,
the rates of HDS were still much faster than the rates of HDN
of the alkylamines. The experiments were carried out at 270,
300, and 340 ◦C with a partial pressure of H2S of 10 or 100
kPa. When the partial pressure of the reactant, was charged,
the solvent flow was adapted to keep the partial pressure of
hydrogen constant. The weight time was defined as the ra-
tio between the catalyst weight (wcat) and the total molar
flow fed to the reactor (nfeed) (1 (g min)/mol = 0.68 × 10−3
(g h)/l). As the influence of weight time on the product dis-
tribution was studied, the weight time was changed by vary-
ing the flow rates of the liquid and the gaseous reactants with
3. Results
3.1. 2-Pentylamine and 2-pentanethiol
The conversion of 2-pentylamine at 300 ◦C in the pres-
ence of 10 kPa H2S was 12% at τ = 0.9 (g min)/mol
and 53% at 8.9 (g min)/mol (Fig. 1). The conversion in-
creased slightly when the H2S pressure was increased from
10 to 100 kPa but increased considerably with an increase
in temperature from 300 to 340 ◦C. The products were 2-
pentanethiol, di-(2-pentyl)amine, 1-pentene, 2-pentene, and
pentane (Fig. 2). The selectivity of 2-pentanethiol increased
with decreasing weight time, showing that 2-pentanethiol
is a primary product (of the nucleophilic substitution of
2-pentylamine with H2S). Di-(2-pentyl)amine, the dispro-
portionation product of two molecules of 2-pentylamine,
behaved as a primary product as well. The selectivity of
2-pentanethiol increased and that of di-(2-pentyl)amine de-
creased with increasing H2S pressure.
The selectivity of the pentenes (i.e., the sum of the two
pentene isomers) as a function of weight time was different
at 300 ◦C than at 340 ◦C. At 300 ◦C the selectivity extrap-
olates to zero with decreasing weight time, which indicates
that pentenes are a secondary product. On the other hand, at
340 ◦C the pentenes behaved like a primary product because
◦
◦
Fig. 1. Conversions of 2-pentylamine (2-PA) and 3-methyl-2-butylamine (3M2BA) at 300 C (Q and P) and 340 C (2 and 1), and 10 kPa (drawn line) and
100 kPa H S (dashed line).
2

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Y. Zhao, R. Prins / Journal of Catalysis 222 (2004) 532–544
◦
◦
Fig. 2. Product selectivities in the HDN of 2-pentylamine (2-PA) at 300 C (Q and P) and 340 C (2 and 1), and 10 kPa (drawn line) and 100 kPa H S
2
(dashed line).
◦
◦
Fig. 3. Conversions in the HDS of 2-pentanethiol (2-PT) and 3-methyl-2-butanethiol (3M2BT) in the presence of hexylamine at 300 C (Q and P) and 340 C
(2 and 1), and 10 kPa (drawn line) and 100 kPa H S (dashed line).
2
their selectivity extrapolates to a nonzero value at τ = 0. At
both temperatures, the selectivity of pentane decreased with
decreasing weight time to a value close to zero.
trans-2-pentene were constant. This difference between the
pentenes must be due to the easier hydrogenation of the ter-
minal 1-pentene. The pentenes/pentane ratio, obtained from
the HDS of 2-pentanethiol, was similar to that obtained in
the HDN of 2-pentylamine (Fig. 4).
The conversion of 2-pentanethiol in the presence of
10 kPa H2S and 5 kPa hexylamine was much larger than
that of the corresponding 2-pentylamine. Thus, at τ =
0.9 (g min)/mol the conversion of the thiol was 83% at
300 ◦C and 95% at 340 ◦C (Fig. 3), while that of the amine
was 12% at 300 ◦C and 42% at 340 ◦C (Fig. 1). The con-
version of 2-pentanethiol decreased strongly when the H2S
pressure was increased from 10 to 100 kPa. The main prod-
ucts at 300 ◦C and 10 kPa H2S were 1-pentene (21%),
2-pentene (52%), and pentane (27%) at τ = 0.9 (g min)/mol
(not shown). All three molecules were primary products. The
selectivity of 1-pentene decreased and that of pentane in-
creased with weight time, while the selectivities of cis- and
3.2. 3-Methyl-2-butylamine and 3-methyl-2-butanethiol
The conversion of 3-methyl-2-butylamine was slightly
lower than that of 2-pentylamine (Fig. 1). It increased
weakly with increasing H2S pressure and increased sub-
stantially with increasing temperature. The main products
were 3-methyl-2-butanethiol, di-(3-methyl-2-butyl)amine,
2-methyl-2-butene, 2-methyl-1-butene, 3-methyl-1-butene,
and 2-methylbutane. The selectivities of 3-methyl-2-bu-
tanethiol and di-(3-methyl-2-butyl)amine increased with

Y. Zhao, R. Prins / Journal of Catalysis 222 (2004) 532–544
535
◦
Fig. 4. Pentenes/pentane ratio in the HDN of 2-pentylamine (2-PA) and HDS of 2-pentanethiol (2-PT) in the presence of hexylamine at 300 C (Q and P) and
◦
340 C (1), and 10 kPa (drawn line) and 100 kPa H S (dashed line).
2
◦
◦
Fig. 5. Product selectivities in the HDN of 3-methyl-2-butylamine at 300 C (Q and P) and 340 C (2 and 1), and 10 kPa (drawn line) and 100 kPa H S
2
(dashed line).
decreasing weight time (Fig. 5). This shows that they
are primary products of the nucleophilic substitution of
3-methyl-2-butylamine with H2S and with another mole-
cule of 3-methyl-2-butylamine,respectively. As expected for
a molecule with two chiral atoms (3-methyl-2-butylamine
has one chiral atom), the gas chromatogram of di-(3-meth-
yl-2-butyl)amine showed two peaks of equal intensity and
only a small difference in retention time; the correspond-

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Y. Zhao, R. Prins / Journal of Catalysis 222 (2004) 532–544
Fig. 6. Methylbutenes/methylbutane ratio in the HDN of 3-methyl-2-butylamine (3M2BA) and HDS of 3-methyl-2-butanethiol (3M2BT) in the presence of
◦
◦
hexylamine at 300 C (Q and P) and 340 C (2 and 1), and 10 kPa (drawn line) and 100 kPa H S (dashed line).
2
◦
◦
Fig. 7. Conversions of 3,3-dimethyl-2-butylamine (3,3DMBA) and 2-methylcyclohexylamine (2-MCHA) at 300 C (Q and P) and 340 C (2 and 1), and
10 kPa (drawn line) and 100 kPa H S (dashed line).
2
ing products had equal mass spectra. One GC peak is due to
the (R,R) and (S,S) isomers, the other to the meso (R,S)
isomer. The selectivity of di-(3-methyl-2-butyl)amine de-
creased with increasing H2S pressure, while the reverse was
true for the selectivity of 3-methyl-2-butanethiol.
As in the HDN of its isomer 2-pentylamine (Section 3.1),
the selectivities of the alkene products in the HDN of 3-
methyl-2-butylamine differed at 300 and 340 ◦C. At 300 ◦C
the methylbutene selectivities were low at τ = 0 and in-
creased with weight time, while at 340 ◦C and τ = 0 they
were higher and decreased with weight time. At both temper-
atures, all three methylbutenes behaved as primary products
(Fig. 5) as the selectivities extrapolated to nonzero with de-
creasing weight time to zero.
To determine whether a methyl-shift rearrangement oc-
curred during the HDN of 3-methyl-2-butylamine, the HDN
of 2-methyl-3-pentylamine was measured. The selectiv-
ity of the rearrangement products 3-methyl-2-pentene and
3-methylpentane was below 1%. The main products (di-
(2-methyl-3-pentyl)amine, 2-methyl-1-pentene, 2-methyl-2-
pentene, 2-methyl-3-pentene, 2-methylpentane, and 2-meth-
yl-3-pentanethiol) corresponded to those in the HDN of
3-methyl-2-butylamine and were present in similar amounts
(Fig. 5). In addition, a constant selectivity of 2% was ob-
served for 2-methyl-4-pentene.
3.3. 3,3-Dimethyl-2-butylamine
The conversion of 3,3-dimethyl-2-butylamine was even
lower than that of 3-methyl-2-butylamine:only 2% at 300 ◦C
and 10 kPa H2S at τ = 0.9 (g min)/mol but 24% at τ =
8.9 (g min)/mol. The conversion increased strongly with in-
creasing temperature and increased slightly with increasing
H2S pressure (Fig. 7). 3,3-Dimethyl-2-butanethiol formed
and behaved as a primary product, because its selectivity in-
creased with decreasing weight time (Fig. 8). We did not ob-
serve a disproportionation product, probably because of the
combined steric hindrance of the tertiary butyl and methyl
groups attached to the same carbon atom as the NH2 group.
In addition to the normal products (3,3-dimethyl-1-but-
ene and 2,2-dimethylbutane), rearranged products (2,3-di-
methylbutane, 2,3-dimethyl-2-butene, and 2,3-dimethyl-1-
butene) also formed. The sum of the selectivities of these
The conversion of 3-methyl-2-butanethiolin the presence
of 10 kPa H2S and 5 kPa hexylamine was much larger than
that of the corresponding 3-methyl-2-butylamine: The con-
version of the thiol was 74% at τ = 0.9 (g min)/mol at
300 ◦C and 100% at 340 ◦C (Fig. 3), while that of the cor-
responding amine was 7% at 300 ◦C and 29% at 340 ◦C
(Fig. 1). The main products at 300 ◦C and 10 kPa H2S
were 3-methyl-1-butene (17%), 2-methyl-2-butene (49%),
and 2-methylbutane (29%) at τ = 0.9 (g min)/mol, while the
selectivity of 2-methyl-1-butene was 5% (not shown). The
methylbutenes/methylbutane ratio obtained from the HDS
of 3-methyl-2-butanethiolin the presence of hexylamine was
slightly lower than that obtained from the HDN of 3-methyl-
2-butylamine at low weight time but was similar at high
weight time (Fig. 6).

Y. Zhao, R. Prins / Journal of Catalysis 222 (2004) 532–544
537
◦
◦
Fig. 8. Product selectivities in the HDN of 3,3-dimethyl-2-butylamine at 300 C (Q and P) and 340 C (2 and 1), and 10 kPa (drawn line) and 100 kPa H S
2
(dashed line).
rearranged products was 43% at 300 ◦C and 63% at 340 ◦C
and 10 kPa H2S at τ = 0.9 (g min)/mol. As in the case
of 2-pentylamine and 3-methyl-2-butylamine, the selectiv-
ity of 3,3-dimethyl-1-butene in the HDN of 3,3-dimethyl-
2-butylamine as a function of weight time was different at
300 ◦C than at 340 ◦C. At 300 ◦C the 3,3-dimethyl-1-butene
selectivity went through a maximum, while at 340 ◦C it de-
creased continuously with increasing weight time.
nethiol, and dialkylamine products in the HDN of 3-methyl-
2-butylamine, respectively (Fig. 5).
3.5. 2-Methyl-2-butylamine and 2-methyl-2-butanethiol
The HDN of 2-methyl-2-butylamine occurred fast; the
conversion was already 7% at 270 ◦C and 10 kPa H2S at
low weight time (0.9 (g min)/mol) and reached 90% at
τ = 8.9 (g min)/mol (Fig. 10). The main products were
2-methyl-2-butene, 2-methyl-1-butene, and 2-methylbutane
(Fig. 11). The nonzero selectivities of these products at
τ = 0 showed that they behaved as primary products.
The selectivity of 2-methyl-1-butene decreased and that
of 2-methylbutane increased with increasing weight time
due to isomerization and hydrogenation. A separate exper-
iment of 2-methyl-1-butene in the presence of hexylamine
at 270 ◦C, 3 MPa, and 10 kPa H2S showed 30% conver-
sion to 2-methyl-2-butene and 10% conversion to 2-methyl-
butane at τ = 10 (g min)/mol. The selectivity of 3-methyl-
1-butene, the isomerization product of 2-methyl-2-butene,
was less than 1% over the whole range of weight times.
A small amount of 2-methyl-2-butanethiol was observed
3.4. 2-Methylcyclohexylamine
2-Methylcyclohexylamine has the same molecular struc-
ture as 3-methyl-2-butylamine, with the amine group at-
tached to a secondary α-carbon atom and a methyl group
on the neighboring β-carbon atom. The conversion of
2-methylcyclohexylamine was lower than that of 3-meth-
yl-2-butylamine, both at 300 ◦C and at 340 ◦C, and H2S
had a positive influence on the conversion of both amines
(cf. Figs. 1 and 7). The selectivities of the methylcy-
clohexenes, methylcyclohexane, 2-methylcyclohexanethiol,
and di-(2-methylcyclohexyl)amine products (Fig. 9) were
similar to those of the respective alkenes, alkane, alka-

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Y. Zhao, R. Prins / Journal of Catalysis 222 (2004) 532–544
Fig. 9. Product selectivities of methylcyclohexenes (MCHE), methylcyclohexane (MCH), 2-methylcyclohexanethiol (MCHTHIOL), and di(2-methylcyclo-
◦
◦
hexyl)amine (Di-2-amine) in the HDN of 2-methylcyclohexylamine at 300 C (Q and P) and 340 C (2 and 1), and 10 kPa (drawn line) and 100 kPa H S
2
(dashed line).
◦
Fig. 10. Conversions in the HDN of 2-methyl-2-butylamine (2M2BA) and benzylamine (BA) at 270 C (2 and 1), and 10 kPa (drawn line) and 100 kPa H S
2
(dashed line).
that increased with increasing H2S pressure. Its selectiv-
ity was 1.5% at τ = 0.9 (g min)/mol and 100 kPa H2S.
Increasing the H2S pressure from 10 to 100 kPa hardly in-
fluenced the conversion and product selectivities of 2-meth-
yl-2-butylamine.
shown). The methylbutenes/methylbutane ratio, obtained in
the HDS of 2-methyl-2-butanethiol, was 3 at 270 ◦C and
10 kPa at τ = 0.9 (g min)/mol, which is very different from
the value of 16.5 obtained in the HDN of 2-methyl-2-butyl-
amine under the same conditions (Fig. 13).
The conversion of 2-methyl-2-butanethiol at 270 ◦C in
the presence of 10 kPa H2S and 5 kPa hexylamine was
much larger than that of the equivalent amine: It was al-
ready 66% at τ = 0.9 (g min)/mol (Fig. 12), while the
conversion of the corresponding amine was only 20%
(Fig. 10). The conversion of 2-methyl-2-butanethiol de-
creased with increasing H2S pressure. The main products
at τ = 0.9 (g min)/mol were 2-methyl-1-butene (35%),
2-methyl-2-butene (39%), and methylbutane (25%) (not
The conversion of benzylamine was lower than that of
2-methyl-2-butylamine at 270 ◦C (32% for benzylamine and
52% for 2-methyl-2-butylamine at τ = 3.4 (g min)/mol)
and was not influenced by the H2S pressure (Fig. 10). The
main product at 10 kPa H2S was toluene (> 99%); only
traces of α-toluenethiol (benzyl mercaptan), methylcyclo-
hexene, and methylcyclohexane formed. At 100 kPa H2S,
the α-toluenethiol selectivity was 3% at low weight time
and even smaller at high weight time. The conversion of

Y. Zhao, R. Prins / Journal of Catalysis 222 (2004) 532–544
539
◦
Fig. 11. Product selectivities in the HDN of 2-methyl-2-butylamine at 270 C (2 and 1), and 10 kPa (drawn line) and 100 kPa H S (dashed line).
2
Fig. 12. Conversion in the HDS of 2-methyl-2-butanenethiol (2M2BT) in
the presence of hexylamine at 270 C (2 and 1), and 10 kPa (drawn line)
and 100 kPa H S (dashed line).
2
Fig. 13. Methylbutenes/methylbutane ratio in the HDN of 2-meth-
yl-2-butylamine (2M2BA) and HDS of 2-methyl-2-butanethiol (2M2BT)
in the presence of hexylamine at 270 C (2 and 1), and 10 kPa (drawn
◦
◦
line) and 100 kPa H S (dashed line).
2
α-toluenethiol was 100% under all conditions at 270 ◦C,
even at the lowest weight time, and toluene was the only
product.
ination of NH3 from the alkylamines or of H2S from the
alkanethiols. The alkanes are formed by hydrogenolysis,
which is the rupture of the C–N or C–S bond and simultane-
ous hydrogenation. Dialkylamines, alkanethiols, and alkenes
can, in principle, form by acid–base catalysis as well as
metal-like catalysis, while alkanes can only form by metal-
catalyzed hydrogenolysis.
4. Discussion
4.1. HDN and HDS mechanisms
As shown above, the products of the HDN of alkylamines
are dialkylamines, alkanethiols, alkenes, and alkanes, while
alkenes and alkanes are the products of the HDS of alka-
nethiols. Dialkylamines and alkanethiols are formed by sub-
stitution of the NH2 group in alkylamines by an alkylamine
or H2S, respectively. The alkenes can be formed by elim-
4.1.1. Acid–base mechanisms
In acid–base catalyzed elimination [1,3] and nucleophilic
substitution [1,4] reactions of alkylamines, the amine group
reacts first with a proton or a Lewis acid in order to create a
better leaving group (in the subsequent schemes, only the re-
action with a proton is indicated). Then a concerted bimolec-

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Y. Zhao, R. Prins / Journal of Catalysis 222 (2004) 532–544
normally used for organic reactions, might increase the par-
ticipation of carbenium ions [11].
The E1 and E2 reactions of an alkylamine lead to an
alkene and the SN1 and SN2 reactions to an alkanethiol
or dialkylamine. The dialkylamine can undergo further nu-
cleophilic substitution to give an alkanethiol. As this and
other studies [9,12–15] have shown, alkanethiols react fast
to alkenes and alkanes. Acid–base-catalyzed elimination ex-
plains the formation of an alkene from a thiol by an E1 or E2
reaction. In both cases, the reaction rate and, thus, the con-
version might be positively influenced by the H2S pressure.
In the E1 reaction protonation of the SH group takes place
before the rate-determining breaking of the C–S bond. As a
consequence, the E1 reaction may be aided by H2S. In the
E2 reaction, a higher H2S pressure increases the concentra-
tion of the S2− or SH− base at the catalyst surface. However,
in the HDS reactions that we studied, H2S had a strong neg-
ative influence on the conversion of all n-alkanethiols, be it
primary, secondary, or tertiary alkanethiols (Table 1). This
must be due to the fact that H2S adsorbs rather strongly on
the catalyst surface and inhibits the adsorption of the alka-
nethiol.
Scheme 1. E2 and S 2 reactions of an alkylamine to an alkene and alka-
N
nethiol.
ular reaction takes place in the E2 elimination as well as SN2
nucleophilic substitution (Scheme 1), in which a base or nu-
cleophile reacts with the protonated amine and ammonia is
split off. In the E2 mechanism, the base subtracts a hydrogen
atom from the β-carbon atom of the alkylamine. That is why
this elimination can only occur when β-H atoms are present
and why it was proposed that the HDN of alkylamines occurs
faster when a greater number of β-H atoms is present [3]. In
the SN2 reaction, the base attacks the α-carbon atom of the
alkylamine. In classic nucleophilic substitution this attack
takes place from the backside of the molecule, at the side
opposite to the leaving amine group, with inversion of the
configuration at the α-carbon atom.
4.1.2. Metal-like mechanisms
Substitution and elimination of alkylamines can be cat-
alyzed not only by acids and bases, but also by metals.
Nucleophilic substitution can take place by a series of metal-
catalyzed reactions: dehydrogenation of an amine to an
imine, addition of H2S, elimination of NH3, and hydrogena-
tion of the resulting thioaldehyde to a thiol (Eq. 1) [16].
Analogous reaction schemes for homogeneous catalysts
have been proposed by Laine [17]:
In the E1 elimination and SN1 nucleophilic substitu-
tion mechanisms, ammonia splits off from the protonated
amine in the rate-limiting step and forms a carbenium ion
(Scheme 2). The carbenium ion quickly undergoes proton
abstraction to form an alkene (E1), or it reacts with a nu-
cleophile, such as H2S or another alkylamine, to form an
alkanethiol or dialkylamine, respectively (SN1). E1 and SN1
mechanisms are likely to occur only if relatively stable car-
benium ions (such as a benzyl, allyl, or tertiary trialkyl car-
benium ion) can be formed. Linear alkylamines will not react
by E1 and SN1 mechanisms, because they would lead to
an unstable primary carbenium ion. Secondary alkylcarbe-
nium ions are more stable than primary ions and may form
from alkylamines with a secondary α-carbon atom. They
will only form, however, if sufficiently strong acid sites are
available and this is not the case on the surface of metal sul-
fides. On the other hand, HDN reactions are carried out at
least 300 ◦C. Such temperatures, which are much higher than
R1R2CH–NH2 → R1R2C=NH → R1R2C(SH)–NH2,
R1R2C(SH)–NH2 → R1R2C=S → R1R2CH–SH.
(1)
Furthermore, the elimination of NH3 from an amine to form
an alkene can be metal-catalyzed [2,5]. In a first step, C–N
bond hydrogenolysis would take place. This is an easy re-
action on metal catalysts [18]; on supported platinum, for
example, it is already fast at around 150 ◦C [19]. The result-
ing alkyl fragment can lose a β-hydrogen atom and form an
alkene, or it can add a hydrogen atom to form an alkane.
Scheme 2. E1 and S 1 reactions of an alkylamine to an alkene and alkanethiol.
N

Y. Zhao, R. Prins / Journal of Catalysis 222 (2004) 532–544
541
Both reactions are well known in Fischer–Tropsch catalysis
Table 1
Conversions and effect of H S on conversion at time τ, and product selec-
2
by metals. The fast and reversible addition of the H atom
to the alkene and rupture from the alkyl fragment explain
the double-bond shift in the alkene. C–S bond breaking (hy-
drogenolysis) has also been demonstrated for the homoge-
neous Cp*2Mo2Co2S3(CO)4 cluster catalyst (Cp* stands for
pentamethylcyclopentadienyl) [20].
tivities at τ = 1 (g min)/mol
Reactant
=
T
τ
Conversion Effect C /C Selectivity (%)
n
n
◦
( C) ((g min)/
(%)
H S
2
RNHR
RSH
mol)
2M2BA
270
300
270
270
270
300
340
300
340
300
340
300
340
300
340
3
1
1
3
1
3
3
1
1
3
3
1
1
3
1
3
3
3
3
1
1
1
1
30
46
65
25
100
27
75
83
95
18
70
75
100
10
55
5
(+)
(+)
−−
0
16.5
16
3.0
0
0
2.9
0
0
0
0
2M2BT
BA
TT
4.2. HDN of amines with secondary α-carbon atoms
0
0
2-Pentanethiol and di-(2-pentyl)amine were primary
products and pentane and pentenes were secondary prod-
ucts in the HDN of 2-pentylamine at 300 ◦C (cf. Fig. 2). The
alkenes and alkane are formed from the corresponding alka-
nethiol, that is formed from the alkylamine
2PA
(+)
+
80
35
2
1
2PT
−−
2.9
3M2BA
3M2BT
2MCHA
+
+
3.5
3.5
2.5
3.2
1.0
3.5
1.5
4.1
0.9
1.3
2.0
2.0
0.8
0.8
45
10
9
3
−−
−−
(+)
(+)
+
RNH2 + H2S → RSH + NH3
and dialkylamine by substitution of the amine group by H2S
RNHR + H2S → RSH + RNH2. (3)
(2)
38
10
0
0
22
20
4
1
10
2
10
7
3,3DM2BA 300
340
HA [9]
35
8
+
The pentenes/pentane ratio in the HDN of 2-pentylamine
was very similar to that obtained in the HDS of 2-pentane-
thiol in the presence of hexylamine (Fig. 4). This demon-
strates that the branching ratio between alkenes and alkane
is determined by the thiol and that the thiol is an intermedi-
ate between alkylamine and hydrocarbons.
300
320
300
320
300
300
−
24
90
100
55
55
(−)
−−
HT
DHA [9]
THA [9]
14*
58
7
7
+
0, H S hardly influences conversion; (+), H S has a weak positive in-
2
2
At 340 ◦C, the pentenes behaved like a primary prod-
uct (Fig. 2) due to the fast formation of pentenes from
2-pentanethiol (Fig. 3). The decrease in the selectivity of
the alkenes and increase in the selectivity of the alkane with
weight time at 340 ◦C (Fig. 2) is due to hydrogenation of the
alkenes. At this elevated temperature the hydrogenation of
the alkenes is relatively fast because of the high conversion
(and thus decreased inhibition) of the alkylamine. The HDN
of 2-pentylamine is, thus, similar to that of n-hexylami-
ne [9]. This shows that amines with primary and secondary
α-carbon atoms do not undergo elimination to alkenes or hy-
drogenolysis to an alkane.
The reactions of 3-methyl-2-butylamine, 2-methylcyclo-
hexylamine, and 3,3-dimethyl-2-butylamine showed many
similarities (cf. Figs. 5, 8, 9). The conversions vary within
a factor of 3 in the order 3-methyl-2-butylamine > 2-meth-
ylcyclohexylamine > 3,3-dimethyl-2-butylamine (Table 1).
Alkanethiol and dialkylamine are primary products (non-
zero selectivities at τ = 0) and alkenes and alkane are pri-
mary as well as secondary products (increasing selectivity
with τ). The initial alkene selectivities at 300 ◦C and 100 kPa
H2S were about 22% for 3-methyl-2-butylamine, 8% for
2-methylcyclohexylamine, and 34% for 3,3-dimethyl-2-but-
ylamine. The nonzero selectivities of the alkenes at τ = 0
are due either to direct elimination of NH3 from the re-
spective alkylamine or to a relatively slow reaction of the
alkylamine to the corresponding alkanethiol followed by a
fast reaction of this thiol to the alkenes. The latter seems
more feasible, because it is hardly likely that 2-pentylamine
does not undergo elimination, but that the introduction of
fluence; +, H S has a positive influence; (−), H S has a weak negative
2
2
influence; −, H S has a negative influence; −−, H S has a strong negative
2
2
influence.
a methyl group on the neighboring β-carbon atom induces
such a change in the mechanism. The higher initial selectiv-
ities at 340 ◦C than at 300 ◦C are, thus, due to an even faster
formation of alkenes from the alkanethiol and the decrease
with weight time at 340 ◦C to a relatively fast hydrogenation
of the alkenes. This agrees with the fact that the HDS rates
of the alkanethiols decrease less than the conversions of the
corresponding alkylamines as a result of methyl substitution
(Table 1). Thus, the HDS of the intermediate is relatively
faster for the methyl-substituted thiol than for the unsubsti-
tuted thiol and the final secondary products tend to behave
like primary products.
Furthermore, the alkenes/alkane ratio, obtained in the
HDN of 3-methyl-2-butylamine, is about the same as that
obtained in the HDS of the corresponding 3-methyl-2-bu-
tanethiol (Fig. 6) and similar to the alkenes/alkane ratio
obtained in the HDN of 2-pentylamine (Fig. 4). These ra-
tios of the alkylamines and alkanethiols with a secondary
α-C atom are much lower than the ratio obtained for 2-meth-
yl-2-butylamine, an alkylamine with a tertiary α-C atom
(cf. Section 4.3). In Section 4.3 we will show that the lat-
ter alkylamine reacts by direct elimination of ammonia. The
alkenes/alkane ratios thus confirm that the alkylamines with
a secondary α-C atom do not react by elimination of ammo-
nia.

542
Y. Zhao, R. Prins / Journal of Catalysis 222 (2004) 532–544
Scheme 3. Reaction network of 3,3-dimethyl-2-butylamine.
The amines with secondary α-carbon atoms reacted faster
than the primary n-hexylamine [9] but much slower than
2-methyl-2-butylamine with a tertiary α-carbon atom and
the activated benzylamine (Section 3.5). 2-Pentylamine and
3,3-dimethyl-2-butylamine, both with secondary α-carbon
atoms, reacted four times faster at 300 ◦C than n-hexylami-
ne and neopentylamine [21], both with primary α-carbon
atoms, respectively (Table 1). This indicates that a pure
SN2 mechanism cannot be responsible for the reaction of 2-
pentylamine and 3,3-dimethyl-2-butylamine. In a pure SN2
mechanism the extra methyl group on the α-carbon atom
would hinder the approach of a nucleophile, and the rate of
reaction of the alkylamine with a secondary α-carbon atom
would be lower than that of the corresponding alkylamine
with a primary α-carbon atom [22]. The higher rates of the
amines with secondary α-carbon atoms are probably due to
a weakening of the C–N bond because of the higher ionic
character of the C–N bond for a secondary carbon atom. The
limit would be the dissociation of the amine group with the
formation of a secondary carbenium ion. This extreme situ-
ation did not occur, however, for the amines in this study. In
that case, an SN1 or E1 reaction would have taken place and
H2S would not have influenced the reaction rate.
The Wagner–Meerwein-type rearrangement of the car-
bon skeleton in the reaction of 3,3-dimethyl-2-butylamine
is ascribed to a nucleophilic substitution to 3,3-dimethyl-
2-butanethiol, followed (partly) by γ elimination aided
by the neighboring group effect of the methyl groups on
the β-carbon atom (Scheme 3), analogous to reactions of
branched alcohols [23]. As to be expected, the rearrange-
ment in the reaction of 2-methyl-3-pentylamine was much
less pronounced (< 1%).
Scheme 4. E2 and S 2 reactions of cis- and trans-2-methylcyclohexyl-
amine.
N
hexene and methylcyclohexenes, respectively, because cis-
2-methylcyclohexylamine(c-MCHA) reacted much faster to
1-methylcyclohexene than trans-2-methylcyclohexylamine
(t-MCHA) [24]. Elimination is assumed to occur with the
β-H atom in an antiperiplanar configuration relative to the
axial NH2 group. As the conformations in Scheme 4 show,
the elimination of c-MCHA should then be faster than that
of t-MCHA because of the presence of the hydrogen atom
on the tertiary carbon atom in the antiposition to the amine
group [24]. In t-MCHA the methyl group occupies this anti
position and cannot be eliminated. It was overlooked, how-
ever, that nucleophilic substitution of the amine group by an
SH group may also explain the faster reaction of c-MCHA.
As in the antiperiplanar E2 elimination, SN2 substitution
can only occur when the leaving group is in the axial po-
sition (Scheme 4). In that case, the methyl group in the
antiposition in t-MCHA hinders the approach of an SH nu-
cleophile from the backside of the carbon atom bearing the
amine group and, thus, the SN2 reaction should also be faster
The HDN of cyclohexylamine and 2-methylcyclohexy-
lamine and the HDS of the corresponding thiols were de-
scribed recently [15,24]. These compounds also have sec-
ondary α-carbon atoms. The removal of ammonia from cy-
clohexylamine and 2-methylcyclohexylamine was mainly
(60–70%) ascribed to an E2 elimination reaction to cyclo-

Y. Zhao, R. Prins / Journal of Catalysis 222 (2004) 532–544
543
for c-MCHA than for t-MCHA. Thus, our conclusion, that
2-methylcyclohexylamine reacts by substitution and not by
elimination, does not contradict the experimental results de-
scribed in Refs. [15,24].
reacts rather slowly in the presence of 100 kPa H2S. Thus, if
this thiol had been an intermediate in the HDN of 2-methyl-
2-butylamine, then a quantity larger than 1.5% would have
been observed.
Benzylamine cannot react by elimination, because it has
no β-hydrogen atoms. The high reaction rate and lack of an
effect of the H2S pressure suggest that benzylamine does not
react by an SN2 but by an SN1 reaction. Protonation of the
amine group and the removal of ammonia would lead to the
relatively stable benzyl carbenium ion. As indicated above
for the isopentyl carbenium ion, the benzyl carbenium ion
will move to a neighboring Mo or Ni atom or to a sulfur
atom. If it binds to a metal atom, an electron-transfer reac-
tion may take place with the formation of the benzyl radical,
which can be hydrogenated to toluene. If the carbenium ion
binds to a sulfur atom, adsorbed α-toluenethiol forms and
may react to toluene.
4.3. HDN of 2-methyl-2-butylamine and benzylamine
2-Methyl-2-butylamine and benzylamine reacted much
faster than the other amines studied. Whereas 2-pentylamine,
3-methyl-2-butylamine, 3,3-dimethyl-2-butylamine, and 2-
methylcyclohexylamine(all with secondary α-carbon atoms)
did not show appreciable conversion below 300 ◦C, 2-meth-
yl-2-butylamine, with a tertiary α-carbon atom, already
reached a conversion of 30% at τ = 3 (g min)/mol at
270 ◦C. The product distribution was also different. For
the most part, alkenes (2-methyl-1-butene and 2-methyl-2-
butene) and an alkane (methylbutane), but no di-(2-methyl-
2-butyl)amine, and only a trace of 2-methyl-2-butanethiol
were observed. This behavior indicates that 2-methyl-2-
butylamine, with a tertiary α-carbon atom, reacts by a dif-
ferent mechanism than the amines with secondary α-carbon
atoms. Furthermore, because H2S does not influence the re-
action rate, the most likely mechanisms are E1 elimination
and SN1 nucleophilic substitution.
If 2-methyl-2-butylamine were to react by a classic or-
ganic E1 or SN1 mechanism, then it would be protonated and
would react by ammonia removal to the tertiary isopentyl
carbenium ion (Scheme 2). In the E1 mechanism, this ion
would then react further to 2-methyl-1-butene and 2-methyl-
2-butene by proton removal and the formation of methyl-
butane would be unaccounted for. However, on the metal
sulfide surface, the 2-methyl-2-butylamine adsorbs with the
nitrogen lone pair on an Mo or Ni atom. After C–N bond
breaking, the isopentyl carbenium ion will either move to a
neighboring Mo or Ni atom or to a sulfur atom. If the carbe-
nium ion binds to a metal atom, an electron transfer reaction
may take place with the formation of the isopentyl radical.
As in Fischer–Tropsch chemistry on a metal surface [2], this
alkyl radical may react to an alkene by removal of a hy-
drogen atom, or it may add a hydrogen atom and become
an alkane. If the carbenium ion binds to a sulfur atom, then
adsorbed 2-methyl-2-butanethiol forms and the mechanism
changes to the SN1 type. 2-Methyl-2-butanethiolcan react to
2-methylbutenes as well as to methylbutane.
5. Conclusions
Our former [9] and present results show that alkylamines
with the NH2 group attached to a primary or secondary car-
bon atom react by substitution of the NH2 group by an SH or
amine group to form an alkanethiol or a dialkylamine. After
subsequent substitution by H2S the dialkylamine also reacts
to an alkanethiol. The alkanethiol finally reacts to an alkene
or alkane and H2S. Only an alkylamine with the NH2 group
attached to a tertiary or activated carbon atom reacts directly
to an alkene or alkane. The C–N bonds of alkylamines with
primary and secondary α-carbon atoms are too strong to be
easily broken. For such alkylamines elimination is, there-
fore, too difficult and they react by other mechanisms. The
stabilization of the tertiary or benzyl carbenium cation is
necessary to weaken the C–N enough for elimination to take
place.
The proposal by Portefaix et al. [3], that alkylamines react
by elimination and that the number of β-H atoms determines
their HDN rate, is thus incorrect. The fact that 2-methyl-2-
butylamine reacts much faster than n-pentylamine has noth-
ing to do with the four times larger number of β-H atoms but
has everything to do with the fact that the NH2 group of the
former amine is attached to a tertiary α-C atom and the NH2
group of the latter amine to a primary α-C atom. Even when
elimination occurs, as for 2-methyl-2-butylamine, the selec-
tivity for 2-methyl-2-butene is higher than that for 2-methyl-
1-butene, although there are three times more β-H atoms on
the terminal methyl groups than on the internal methylene
group. We checked that this is not due to a fast isomerization
of 2-methyl-1-butene to 2-methyl-2-butene. The higher se-
lectivity for 2-methyl-2-buteneis due to the fact that in an E1
mechanism the leaving group is gone before the proton. The
product is thus determined by thermodynamic factors and
Zaitsev’s rule applies: the double bond goes preferentially to
the most highly substituted carbon atom. We conclude there-
fore that the number of β-H atoms, as proposed by Portefaix
The methylbutenes/methylbutane ratio of the products
of the HDN of 2-methyl-2-butylamine was about five times
larger than that obtained in the HDS of the corresponding
2-methyl-2-butanethiol (Fig. 13). This demonstrates that the
methylbutenes/methylbutane ratio in the HDN is not de-
termined by the thiol and that 2-methyl-2-butylamine re-
acts by an E1 rather than an SN1 mechanism. In agree-
ment with this conclusion, only 0.3% thiol was observed
in the HDN of 2-methyl-2-butylamine in the presence of
10 kPa H2S at τ = 1 (g min)/mol; even in the presence of
100 kPa H2S, the initial selectivity of the thiol was only 1.5%
(Fig. 11). Fig. 12 demonstrates that 2-methyl-2-butanethiol

544
Y. Zhao, R. Prins / Journal of Catalysis 222 (2004) 532–544
et al., does not determine the HDN rate of the alkylamines.
In alkylamines with primary and secondary α-C atoms the
number of β-H atoms is of no importance, because substi-
tution rather than E2 elimination takes place. In alkylamines
with a tertiary α-C atom the reverse occurs: the hydrogen
atom is preferentially removed from the β-C atom with the
lowest number of β-H atoms!
Even though elimination of the NH2 group from an alk-
ylamine does not take place when it is attached to a primary
or secondary carbon atom, removal of nitrogen does take
place for such alkylamines. The substitution of the NH2
group by H2S leads to an alkanethiol and ammonia and, thus,
to total denitrogenation. Substitution by an amine leads to a
dialkylamine and ammonia and to 50% nitrogen removal.
The rest of the nitrogen is removed in the subsequent substi-
tution of the dialkylamine by H2S to an alkanethiol and the
original alkylamine. High partial pressures of H2S and alk-
ylamine increase the rate of transformation of alkylamine to
alkanethiol and, thus, of denitrogenation. At the same time,
however, the rate of sulfur removal from the alkanethiol de-
creases.
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