One-pot conversion of lysine to caprolactam over Ir/H-Beta catalysts

Article abstract of DOI:10.1039/c9gc00415g

Amino acid lysine could serve as an ideal bio-based feedstock for the synthesis of caprolactam (CPL), which is currently a petroleum-derived monomer. Herein, we report the one-pot conversion of l-lysine to CPL via hydrogenolysis over bifunctional metal supported catalysts. Among the various hydrogenation metals and different supports, the combination of Ir and HB zeolite gave the best performance. Under optimal conditions, a 30% yield of CPL from l-lysine and a 58% yield from the reaction intermediate α-dimethyl amino caprolactam (DMAC) were obtained over a 2Ir/HB-124 catalyst at 250 °C in an autoclave or fixed-bed reactor. The reaction solvent dramatically affected the reaction selectivity, and methanol was found to be the best due to its unique contribution towards the formation of α-dimethyl amino caprolactam (DMAC) as well as the following C-N breakage of the C-N(CH₃)₂ bond. The acid sites on the catalyst accelerate lactam formation, and the synergy between the acid sites and hydrogenation sites favours C-N bond hydrogenolysis to produce CPL. Besides the acidity, the large pore size of HB is able to accommodate big reaction intermediate molecules inside the pores further ensures the superior performance of Ir/HB. The reaction route was identified, i.e., l-lysine first undergoes cyclization and N-methylation to DMAC, and then C-N(CH₃)₂ bond hydrogenolysis to form CPL. The Ir/HB catalyst has reasonably good stability and high selectivity, making this one-pot conversion process a novel and environmentally benign way of producing CPL from easily available renewable feedstocks.

Full text of DOI:10.1039/c9gc00415g

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DOI: 10.1039/C9GC00415G
Graphical abstract
One-pot Conversion of Lysine to Caprolactam over Ir/H-Beta Catalysts
One-pot catalytic conversion of L-lysine to caprolactam over Ir/HBeta zeolites was
demonstrated. In methanol solvent, L-lysine is transformed to DMAC and then to CPL
via C-N bond hydrogenolysis by the synergistic catalysis of acid sites and hydrogenation
sites on the catalyst.

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DOI: 10.1039/C9GC00415G
Journal Name
ARTICLE
One-pot Conversion of Lysine to Caprolactam over Ir/H-Beta
Catalysts
a
a
a
a
a
a
a,b
Joby Sebastian , Mingyuan Zheng *, Yu Jiang , Yu Zhao , Hua Wang , Zhendong Song , Xinsheng Li ,
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
Jifeng Pang and Tao Zhang *
DOI: 10.1039/x0xx00000x
Amino acid lysine could serve as an ideal bio-based feedstock for the synthesis of caprolactam (CPL) which is currently a
petroleum-derived monomer. Herein, we report the one-pot conversion of L-lysine to CPL via hydrogenolysis over
bifunctional metal supported catalysts. Among various hydrogenation metals and different supports, the combination of Ir
and HB zeolite gave the best performance. Under optimal conditions, 30% yield of CPL from L-lysine and 58% yield from the
www.rsc.org/
o
reaction intermediate α-dimethyl amino caprolactam (DMAC) were obtained over 2Ir/HB-124 catalyst at 250 C in an
autoclave or fixed-bed reactors. Reaction solvent dramatically affected the reaction selectivity, and methanol was the best
one due to its unique contribution towards the formation of α-dimethyl amino caprolactam (DMAC) as well as the followed
3 2
C-N breakage of C-N(CH ) bond. The acid sites on the catalyst accelerate the lactam formation, and synergy between acid
sites and hydrogenation sites favours the C-N bond hydrogenolysis to produce CPL. Besides the acidity, the large pore size
of HB able to accommodate the big molecule of reaction intermediate inside the pores further ensures the superior
performance of Ir/HB. The reaction route was identified, i.e., L-lysine first undergoes cyclization and N-methylation to DMAC,
3 2
and then C-N(CH ) bond hydrogenolysis to form CPL. The Ir/HB catalyst has reasonably good stability and high selectivity,
making this one-pot conversion process be a novel and environmentally benign way for CPL production from easily available
renewable feedstocks.
valerolactone (GVL) as feedstock for CPL production. After
three-step cascade transformations, including GVL conversion
Introduction
Caprolactam (CPL) is an important monomer used for the
synthesis of the nylon-6 polymer. The global production of CPL-
with a current market price of around $2,000- 2,500 per metric
to methyl 3-and 4-pentenoates, then to pentamides and the
final hydroamidomethylation to form CPL in the presence of a
homogeneous noble metal catalyst, 38% yield of CPL was
obtained. There is still a long way for CPL production from
renewable feedstock in green and simple manners.
1
,2
tons- is anticipated to surpass 4.6 megatons by 2018.
Currently, CPL is a petroleum-based derivative, of which 90% is
produced via the cyclohexanone route and the remaining 10%
via the cyclohexane & nitrosyl chloride route (Beckman
rearrangement). These processes are multi-step and produce a
large amount of ammonium sulfate as the byproduct (1.8-5.0 kg
per 1 kg of CPL).1,3 A new synthetic route to CPL incorporating
renewability, eco-friendliness and economic viability is highly
desirable.
Lysine is the most abundant amino acid industrially
7
produced via the fermentation of glucose, with production up
to 2.5 million metric tons in 2016 (USD 3.6 billion) and growing
at a rate of about 7% annually. It would be a suitable feedstock
for CPL production in view of its relatively low price and
desirable functional groups of -NH
2
and -COOH contained in the
8
molecule (Scheme 1). Frost et al. investigated lysine conversion
to CPL by a two-step process. Lysine was first cyclized to α-
amino-ε-caprolactam (ACPL) and then deaminated to CPL (75%
yield) via stoichiometric chemical reaction with hydroxylamine-
O-sulphonic acid. To explore a low cost and less waste process,
In the past decade, great efforts have been exerted to
synthesize nitrogen-containing chemicals from renewable
4
5
biomass. In 2011, Buntara et al. proposed an alternate route
to CPL production from renewable hydroxymethylfurfural
(HMF), involving four steps with using of both heterogeneous
9
Frost et al. later studied the lysine hydrochloride conversion to
and homogeneous catalysts to give CPL at 77 mol% yield. In
6
2 2
CPL over 16wt%Pt-S/C catalyst. Under H and H S atmospheres,
2
014, Raoufmoghaddam et al. used biomass-derived γ-
1
5% yield of CPL was obtained in the single-step process. The
great challenge in the conversion of lysine to CPL still lies in how
to selectively break the C-N bond in lysine molecule via a simple,
low cost and environmentally benign process.
a. Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023,
P.R. China. E-mail: myzheng@dicp.ac.cn; taozhang@dicp.ac.cn.
b
University of Chinese Academy of Sciences, Beijing 100049, China.
†
Footnotes relating to the title and/or authors should appear here.
Scheme 1. Perceived reaction route for lysine to CPL in the absence of
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
solvents/catalysts.
O
CH3OH
H N
2
N
OH
+
+
N
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
H2
N
N
H
NH2
H
O
O
DMAC
CPL
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O
H2N
ARTICLE
H2
OH
+ NH3 + H2O
Journal Name
NH2
N
H
O
No.
Catalyst
Yield (mol%)
View Article Online
CDPOL I: 10.1039/C9GCD0M0A4C15G
Lysine
CPL
Herein, we report one-pot catalytic conversion of L-lysine to
CPL by using a multifunctional heterogeneous catalyst. Over the
Ir/HB-124 catalyst, acid sites catalyzed the cyclization of L-lysine
and then synergistically worked with hydrogenation sites to
break the C-N bond, giving CPL at a high yield. The reaction
pathway was identified by a series of conditional experiments.
Effects of solvent, zeolite type and reactors on the reaction
were investigated. The results in this work would provide an
inspiring understanding of C-N bond breakage via
hydrogenolysis and afford a green method for CPL production
from renewable feedstocks.
1
2
3
4
5
6
7
8
9
HB-124
1Ir/AC
5.7
6.2
54.8
8.8
2.2
HB-124 + 1Ir/AC
1Ir/HB-124
2Ir/HB-124
3Ir/HB-124
2Rh/HB-124
2Pt/HB-124
2Ru/HB-124
2Au/HB-124
2Pd/HB-124
4Re/HB-124
10Ni/HB-124
2Ir/Al O
13.6
15.8
23.0
23.9
16.4
12.0
8.3
6.9
5.8
6.9
6.1
<1
5.8
4.2
4.3
--
12.3
2.4
21.6
16.2
15.8
27.9
25.0
20.6
40.7
37.0
30.2
24.6
8.3
1.3
4.9
1.8
--
26.0
15.2
1
1
1
1
0
1
2
3
Results and discussion
Catalyst screening and identification: Our study focuses on
the following main aspects; a) production of CPL from L-lysine
over a stable heterogeneous catalyst in a green route, b)
identification of the reaction pathway from L-lysine to CPL, and
c) ascertaining the necessary conditions of C-N bond
hydrogenolysis leading to CPL formation.
14
15
16
2
3
MoS
La
Nb
2
+ 2Ir/AC
+ 1Ir/AC
2
O
3
1
1
1
2
7
8
9
0
2
O
5
+2Ir/AC
HB-124 + 2Ir/CeO
HB-124 + 2Ir/TiO
2
2
b
4Pt/TiO
2
Referring to the catalysts effective in the related reactions
studied previously, hypothetically, the one-pot catalytic Reaction conditions: L-lysine = 0.200g, catalysts = 0.100 g each, MeOH = 20 mL, PH2
o
a
=
30 bar, T = 240 C, t = 6 h. Conversions of L-lysine were all 100% after the reactions.
conversion of L-lysine to CPL necessitates a catalyst containing
b
o
DMAC = 0.050 g, 4Pt/TiO
2
= 0.025 g, MeOH = 20 mL, PH2 = 30 bar, T = 235 C, t = 2
bifunctional active sites, namely, acid sites for cyclic amide
h.
1
0
formation, and hydrogenation sites & acid sites for C-N bond
hydrogenolysis.11 The conventional catalysts used for
Table 1 summarizes the performances of various catalysts in
hydrodenitrogenation of petroleum-derived compounds are CPL production. HB-124 without the hydrogenation component
Co(Ni)Mo(W)/Al where the catalysts possess sufficient (Table 1, entry 1) largely gave α-dimethyl amino-ε-caprolactam
acidity and hydrogenation sites for the C-N bond (DMAC, 54.8% yield) but did not produce much CPL (5.7% yield).
2 3
O
1
1
hydrogenolysis. Additionally, zeolites bearing acid sites were This indicates that the Brönsted acid sites on HB-124 effectively
1
2
reported as good catalyst supports for hydrodenitrogenation.
catalyze the cyclization of L-lysine to its amide and the N-
In another study of the direct amidation of n-dodecanoic acid methylation of the α-amino group,14 but they are less active for
with aniline,13 zeolites H-Beta and HMFI showed better the C-N bond breakage. Over the hydrogenation catalyst 1Ir/AC
activities than that of metal oxides (Al
2 3
O , MoO
3 2 2
, ZrO , CeO ,
without the presence of HB-124 zeolite (Table 1, entry 2), only
etc.). Therefore, we preferred zeolite supported hydrogenation a very small amount of DMAC and CPL were produced.
catalysts to scrutinize the conversion of L-lysine to CPL. Zeolite Evidently, solely metal sites are ineffective for the C-N bond
H-Beta-124, denoted as HB-124 (Si/Al = 124), was chosen as the hydrogenolysis. Although the complete conversion of L-lysine
representative support for the preliminary studies.
was observed in these experiments, the reaction carbon
Various noble metals (Ir, Rh, Pt, Ru, Au, Pd, and Re) and non- balance was not very high, indicating that undesirable side
noble metal Ni were supported on the HB-124 to facilitate the reactions took place remarkably. Once HB-124 and 1Ir/AC were
C-N bond hydrogenolysis. For comparative purposes, metal used together in a physical mixture, 13.6% yield of CPL was
oxides such as TiO
were also tested as the catalyst supports. Undesirable side 3). Trimethylamine was also found to be the other byproduct15
products are largely expected in the L-lysine conversion owing contributing to the nitrogen balance. Supporting 1wt% of Ir on
to the multi-functionality of L-lysine (-COOH and NH ) and the HB-124 further improved the yields of CPL and DMAC to 15.8%
2 2 2 3
, CeO and Al O , and activated carbon (AC) obtained with a reduction of DMAC yield to 2.2% (Table 1, entry
,
2
multiple active sites on the catalysts. But unfortunately, these and 21.6%, respectively (Table 1, entry 4). Accordingly, the
side products cannot be identified in full by using GC-MS or catalysts simultaneously containing acid sites and
HPLC-MS methods. Therefore, we express the efficacy of hydrogenation sites are effective for CPL production from L-
catalysts mostly in terms of the CPL yield (based on mole of L- lysine. In addition, DMAC may serve as a reaction intermediate
lysine).
3 2
to form CPL via the C-N(CH ) hydrogenolysis, which is further
studied in the following section.
Table 1. The catalytic activity of various metal-supported catalysts in the one-pot
conversion of L-lysine to CPL.a
Increasing the Ir loading to 2% and 3% over HB-124 further
enhanced the CPL yield to ca. 23% with DMAC decreased
correspondingly (Table 1, entries 5, 6). As compared to the
previous report where 16wt%Pt-S/C catalyst was used in the
9
toxic H
2
S atmosphere to generate 15% of CPL, the present
catalyst system needs an only small amount of Ir and works
2
| J. Name., 2012, 00, 1-3
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under much more environmentally benign conditions to Table 2. Effect of zeolite framework and Si/Al ratio of HB on CPL yieldV. iew Article Online
DOI: 10.1039/C9GC00415G
produce CPL even at higher yields.
No.
Catalyst
Yield (mol%)
The performances of other noble metal and Ni catalysts
supported on HB zeolite are notably inferior to the Ir based
catalysts (Table 1, entries 7-13). Rh/HB-124 gave a slightly lower
yield of CPL than that of Ir/HB-124 (Table 1, entry 7), possibly
because these two noble metals belong to the same group in
the periodic table of the elements and have similar catalytic
properties. Accordingly, the hydrogenolysis of the C-N bond for
the CPL formation needs multiple catalytic functions, involving
C-N bond activation, hydrogenation, and synergy between
metal sites and acid sites. In the previous study of C-N bond
CPL
19.6
15.4
<1
23.0
10.6
12.1
4.1
DMAC
15.9
10.0
6.1
16.2
21.2
19.1
15.0
1
2
3
4
5
6
7
1Ir/HB-124a
1Ir/HY-10a
2Ir/ZSM-5 (42)a
2Ir/HB-124
2Ir/HB-80
2Ir/HB-52
2Ir/ZSM-5(42)
b
Reaction conditions: L-lysine = 0.200 g, catalysts = 0.100 g, MeOH = 20 mL, PH2 = 30
bar, T = 240 C, t = 6 h. Reaction time = 8 h. DMAC = 0.050 g, catalyst = 0.025 g.
o
a
b
9
hydrogenolysis of ACPL to CPL, Frost et al. found Pt as the best
among the noble metals and the performance decreased in an
Effects of zeolite framework: The effect of zeolite
order of Pt > Ru > Au > Rh > Pd > Ir > Re. However, in the present framework on CPL production was studied by taking zeolites
study, the C-N bond hydrogenolysis activity of metals followed with different framework topologies (Table 2). Large pore
1
6
a different trend as Ir > Rh > Pt > Ru > Au, Re, Ni, and Pd. The zeolites like HB-124 (0.76 x 0.64 nm, 0.55 x 0.55 nm) and HY-
difference might be related to the changes in reactants and 10 (0.74 x 0.74 nm)1 gave CPL in 19.6 and 15.4% yields
reaction environments (L-lysine and methanol solvent in this respectively, whereas medium pore zeolite ZSM-5 (0.56 x 0.53
6
1
6
study vs. APCL, tetrahydrofuran solvent, and H
Frost’s study).
2
S additive in nm, 0.55 x 0.51 nm) was totally inactive for DMAC and CPL
production (Table 2, entry 3). According to the molecular size,
Besides the metal sites, the support also showed significant linear L-lysine (molecular dimension = 0.89 x 0.42 nm <x, y>) can
effects on the catalytic performance. Al
is known to be the easily diffuse through the channels of all zeolites mentioned
conventional catalyst support for C-N bond hydrogenolysis,
2 3
O
1
1
above, but the formation of DMAC (the possible reaction
but herein, it showed incapability for CPL production from L- intermediate for CPL formation, molecular dimension = 0.77 x
lysine (Table 1, entry 14). This might be related to the relatively 0.62 nm) and its diffusion out of the pores would be difficult on
mild reaction conditions in the present study. Other Lewis acidic small pore zeolite ZSM-5. Even the simple cyclization of L-lysine
materials of MoS
catalysts (Table 1, entries 15-18) also gave low yields of CPL challenging inside the channels of ZSM-5. Camblor et al.
2 2 5 2 2 3
, Nb O , CeO and basic oxide La O
supported (without N-methylation) to ACPL (0.60 x 0.56 nm) would also be
1
7
1
3
(
~5%). Ali et al. reported Nb
among various metal oxides and zeolites for the reaction Beckmann rearrangement of the cyclododecanone oxime to the
between carboxylic acids and amines. In another study, TiO
corresponding lactams and found that the latter was inactive
Al
and AC supported Pt catalysts were reported to be very due to its small pore dimension. To further confirm the pore size
effective in the C-N(CH
hydrogenolysis of N,N- confinement effects, we used DMAC as the feedstock in the
2 5
O
as the best amidation catalyst compared the catalytic performances of HB and ZSM-5 in the
2
,
2 3
O
3 2
)
1
5
dimethylglutamic acid to dimethyl glutarate. However, these reaction (Table 2, entry 7). Over 2Ir/ZSM-5(42), 85.0% DMAC
four materials supported catalysts (Table 1 entries 2, 14, 17, 19) was converted but with a low yield of CPL (4.1%). Therefore, to
did not show the distinguished performance in comparison to achieve high performance in L-lysine to CPL conversion, besides
HB-124 supported catalysts in L-lysine conversion to CPL. In a the Brönsted acid sites, the pore size of zeolite being larger than
conditional experiment over 4Pt/TiO
contains C-N(CH
group in the molecule was used as the intermediates should be ensured.
feedstock, the CPL yield was only 2.4% with 84.8% DMAC
The effects of the Si/Al ratio of HB were investigated.
2
, where DMAC which the molecular dimensions of the reactant and reaction
3 2
)
converted (Table 1 entry 20). Accordingly, it can be concluded Compared to Si/Al ratio of 124, the ratios 52 and 82 gave
that strong Brönsted acidic sites and suitable hydrogenation notably lower yields of CPL and DMAC in the range of 11% and
sites are essentially required for the catalysts to produce CPL 20% respectively (Table 2, entries 4-6). The effects of Si/Al ratio
from L-lysine at a high yield.
on the reaction should be ascribed to the close relationship
between the Si/Al ratio and the acidity of zeolite.
Solvent effects on the reaction: Solvents usually play
important roles in tuning the reaction selectivity and
4
h,18
determining the products yields.
Several typical solvents
were tested in the reaction, including methanol, ethanol,
tetrahydrofuran (THF), 1,2-propanediol, γ-valerolactone and
water (Table S1, ESI†). They are found to be playing a crucial
role in the one-pot conversion of L-lysine to CPL. In contrast to
the high yield of CPL in methanol, in other organic solvents, no
any CPL product was detected. It is presumable that in the
This journal is © The Royal Society of Chemistry 20xx
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Journal Name
O
Table 3. Effect of reactant concentration on CPL yield.
View Article Online
a
b
c
H2N
OH
DOI: 10.1039/C9GC00415G
N
H
O
No. Concentration
Time (h)
Yield (mol%)
O
of L-lysine
CPL
DMAC
H2N
N
(wt%)
OH
NH2
N
N
H
1
2
3
4
5
1
1
0.5
0.5
0.25
3
4
2
4
1
23.4
28.5
29.6
29.1
23.5
18.2
5.9
18.5
5.6
H
O
O
N
H
O
25.7
Scheme 2. Reaction pathways to be identified for CPL from L-lysine over Ir/HB-124.
Reaction conditions: (L-lysine/[2Ir/HB-124]) = 0.5 (wt/wt), MeOH = 20 mL, T =
o
2
50 C, PH2 = 20 bar.
and reached the maximum at 20 bar (28.5%). The quantity of
catalyst in the reaction medium also showed influence on CPL
production. The ratio of catalyst to L-lysine at 0.5 is the best one
for CPL production (Figure S2, ESI†). In contrast, the DMAC yield
decreased continuously with increase in catalyst amount,
demonstrating that the parallel side reactions of DMAC escalate
with an increase in catalyst amount, and the conversion of
DMAC to CPL is not absolutely selective (vide infra).
In short, reaction optimization studies showed that higher
temperatures, short reaction time, low reactant concentration
and an optimal hydrogen pressure and catalyst amount are vital
for achieving a high yield of CPL in L-lysine conversion.
organic solvents, the cyclisation of L-lysine to its amide (ACPL)
can takes place. The major difference between reactions in
methanol and other organic solvents lies in the N-methylation
occurrence in the former case. Therefore, it seems that DAMC
formation via N-methylation is a crucial factor determining the
CPL formation.
To verify this conjecture, DMAC was used as the reactant
with THF and ethanol as the solvent. After 6 hours of reaction,
unexpectedly, the selectivity of CPL was as low as <1% (Table
S1, ESI†). Therefore, the best performance of methanol solvent
in L-lysine conversion to CPL should be attributed to its unique
contribution to the formation of DMAC as well as the followed
Identification of the reaction pathway: As shown in Scheme
2
, there may be three reaction routes for L-lysine to CPL. In
3 2
C-N bond breakage of C-N(CH ) group.
route a, L-lysine first undergoes C-N bond hydrogenolysis at the
α-carbon to form ε-aminocaproic acid which upon cyclization in
the second step produces CPL. In route b, DMAC is produced in
Water is also not a good solvent for L-lysine conversion to
CPL, not giving any CPL formation (Table S1, ESI†). In fact, even
with the presence of small amount of H
yield was notably decreased from the original 28.5% to 13.8%
Table S1, ESI†). The stability of CPL in MeOH-H O mixture (19.8
mL MeOH + 0.2 mL H O) and pure methanol solvent was tested
and compared. Larger degradation of CPL to byproducts was
observed in the MeOH-H O mixture (30% vs. 23%). Therefore,
the presence of H O in the reaction medium will not only inhibit
CPL formation but also lower the stability of CPL in the reaction
system. Considering that H O is a co-product of amide
formation and N-methylation in the reaction, adopting a more
suitable reactor, e.g. a fixed-bed reactor, to eliminate the side
reaction induced by water would be helpful for getting a higher
yield of CPL.
2
O (2.5 % v/v), the CPL
3 2
the first step and then undergoes α-C-N(CH ) hydrogenolysis to
form CPL in the second step. The third route to form CPL may
be cyclization and α-C-N hydrogenolysis occurring
simultaneously on the catalyst surface to produce CPL directly
(
2
2
(route c). However, according to the reaction result that a
2
notable yield of CPL was obtained when a physical mixture of
HB-124 and 1Ir/AC is used (Table 1, entry 3), the route c is very
unlikely to occur. Therefore, our further studies focused on the
identification of the existence of routes a and b in Scheme 2.
In order to corroborate the possibility of route a, we
performed the reaction in two steps (Scheme 3). In the first
step, 1Ir/AC was used as the catalyst which generated only small
amounts of DMAC (8.8%) and CPL (6.2%). After the reaction, the
catalyst was separated by filtration and the product mixture
was introduced again to the reactor along with HB-124. If any ε-
aminocaproic acid is formed in the first step and in case
undetected, the presence of HB-124 in the second step would
cyclize it to produce CPL. The yield of CPL at the end of this
second step reaction was only 3.8%, specifying that the catalytic
action of 1Ir/AC did not generate any potential intermediate for
CPL. This verifies that route a in Scheme 2 is the unlikely
reaction pathway to CPL when 2Ir/HB-124 or its physical
mixtures were used.
2
2
Reaction condition optimization: Various reaction
parameters were optimized to get enhanced catalytic
performance. From the results at different temperatures (Table
3
, entries 1, 2 vs. Table 2, entry 4), it can be found that an
increase in reaction temperature can increase the CPL yield with
the advantage of shortened reaction time. The effect of L-lysine
concentration on CPL yield was investigated (Table 3, entries 2,
4
, 5). A CPL yield of 29.6% was achieved for 0.5 wt% of L-lysine
conversion within 2 hours, in contrast to 4 hours required for 1
wt% L-lysine conversion at a similar CPL yield. A lower
concentration of L-lysine is beneficial for improving the CPL
yield after shorter reaction.
The effects of hydrogen pressure were depicted in
supporting information, Figure S1. The CPL yield showed a
volcanic relationship with H pressure in the range of 7-25 bar
2
4
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O
Table 4. Conversion of ACPL over 2Ir/HB-124 in different solvents. View Article Online
H2N
Step-1
N
DOI: 10.1039/C9GC00415G
OH
+
Catalyst-a
NH2
N
H
N
H
CH3OH
O
O
NH2
N
N
H2
Catalyst-b Step-2
N
N
N
H
H
O
H
O
O
DMAC
CPL
N
+
N
H
N
O
H
O
No.
Solvent
Time (h)
Yield (mol%)
CPL
34.3
32.6
--
DMAC
23.7
9.0
--
Scheme 3. Reaction route verification for L-lysine conversion to CPL by testing two-step
reaction performance over various catalysts.
1
2
3
4
MeOH
MeOH
THF
1
3
4
1
The existence of route b was also tested by performing the
conversion of L-lysine in two subsequent steps (Scheme 3). In
EtOH
--
--
the first step, solely HB-124 catalyst was used, and 54.8% yield Reaction conditions: ACPL.HCl = 0.050 g, 2Ir/HB-124 = 0.025 g, solvent = 20 mL, T
o
=
250 C, PH2 = 20 bar.
of DMAC and 5.7% yield of CPL were obtained. Then, the HB-
24 was removed and the product mixture was further
converted with the addition of 1Ir/AC. The result shows that the
1
DMAC yield was significantly reduced to 1.8% but not led to Table 5. Conversion of DMAC to CPL over different catalysts.
many formations of CPL (8.6%). The action of 1Ir/AC alone could
CH3OH
not convert DMAC to CPL. We further repeated the reaction
route b but with the use of 2Ir/HB-124 in the second step. In this
case, the DMAC yield was reduced from 58.8% to 3.2% and the
CPL yield was rose from 4.1% to 26.3% correspondingly after the
N
N
H2
N
N
H
H
O
O
DMAC
CPL
No.
Catalyst
Conversion
mol%)
CPL selectivity (mol%)
3 2
reaction. Evidently, the hydrogenolysis of α-C-N(CH ) bond in
(
DMAC to form CPL needs acid sites and hydrogenation sites on
the catalysts which work in a synergistic way. More importantly,
this experiment identifies the existence of route b in the
synthesis of CPL from L-lysine over 2Ir/HB-124 catalyst.
Verification of the reaction route b was further established
by taking the cyclized product of L-lysine, i.e., ACPL, as the
reactant. Over 2Ir/HB-124, CPL and DMAC were obtained at a
yield of 34.3% and 23.7%, respectively (Table 4, entry 1). If CPL
1
2
3
Blank
HB-124
3Ir/HB-124
66.8
59.3
89.2
0.0
24.0
46.2
Reaction conditions: DMAC from L-ACPL.HCl = 0.050 g, catalyst = 0.025 g, MeOH =
0 mL, T = 250 C, PH2 = 20 bar, t = 1 h.
o
2
Finally, we measured the performance of DMAC conversion
2
was formed through the C-NH bond hydrogenolysis of ACPL but to CPL over 3Ir/HB-124 (Table 5). CPL was obtained with 46.2%
not through the DMAC formation, a change in a solvent which selectivity (Table 5, entry 3), corroborating that the CPL
does not favour DMAC formation (i.e., N-methylation) is production from L-lysine and ACPL proceeds via DMAC
expected to produce at least some amount of CPL. However, formation. Therefore, the main pathway to CPL from L-lysine
when THF and ethanol were used as the solvents, no CPL was over the Ir/HB catalyst follows route b in Scheme 2. A summary
detected (Table 4, entries 3 and 4). Again, the route b in Scheme of the reaction routes of L-lysine to CPL over Ir/HB-124 is
2
can be confirmed and DMAC serves as a reaction intermediate provided in the supporting information (Figure S3, ESI†).
for the formation of CPL from L-lysine.
Catalyst stability: The catalyst stability was tested in the
As for the DMAC production, it may involve hydrogen conversions of L-lysine and DMAC to CPL. As shown in Figure 1,
borrowing amination of alcohols, in which C-O bond in in the three-cycle runs (L1, L2 and L3), 2Ir/HB-124 gave
methanol was transformed into a C-N bond. According to the consistent yields of CPL. Only a marginal decrease in CPL yield
studies summarized previously, the hydrogen borrowing from 29.6% in the first run to 26.8% in the third run is observed.
amination of alcohols resembles the reductive amination of When using DMAC as the feedstock, the CPL selectivity was
4
b,19
carbonyl compounds.
undergoes dehydrogenation to form formaldehyde and then increase (Figure 1, D1, D2 and D3). The slight decrease in CPL
reacts with NH of L-lysine to form imine, which is eventually yield in the catalyst stability test should be attributed to the
hydrogenated into DMAC without net consumption of mass loss during catalyst recycling and the small leaching of Ir
In the present case, methanol even slightly enhanced from 40.2% to 43% with cycle time
2
hydrogen.
(17 wt%). The catalyst system based on HB-124 and Ir is
reasonably stable and recyclable in the conversion of L-lysine or
DMAC to CPL.
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4
2
0
0
0
0
Conclusions
View Article Online
DOI: 10.1039/C9GC00415G
DMAC
CPL
The one-pot conversion of lysine to CPL is an inspiring yet
challenging reaction owing to the multi-functionality of the
reactant and the consecutive nature of the reaction. Our results
demonstrate that a bifunctional Ir/HB catalyst can produce CPL
in yields of 30% from L-lysine and 58% from DMAC, the highest
of hitherto reported, via catalytic hydrogenolysis at 240-250 C.
The reaction follows a sequential path, in which L-lysine is first
cyclized and α-N-methylated to DMAC in methanol solvent and
then undergoes C-N(CH
acid sites and hydrogenation sites on the catalyst are essentially
required for the α-C-N(CH hydrogenolysis of DMAC to form
CPL. The HB with large pore size accommodates the big
molecule of reaction intermediate inside the zeolite and
ensures the high activity. Solvents significantly affect the
reaction, and methanol is the best one due to its unique
contribution to the formation of DMAC and the followed
o
3 2
) bond hydrogenolysis to CPL. Both
3 2
)
L1
L2
L3
D1
D2
D3
Recycle runs
Figure 1. Recycle runs of 2Ir/HB-124 catalyst in L-lysine (L1, L2 and L3) and DMAC (D1,
D2 and D3) conversion to CPL. Reaction conditions: Table 3 for L-lysine (0.5 wt%) and
Table 5 for DMAC.
3 2
hydrogenolysis of C-N(CH ) bond. The Ir/HB catalyst has
reasonably good stability and high selectivity, making this
reaction process be a novel, environmentally benign and
promising method for CPL production from easily available
renewable feedstocks.
1
00
8
6
4
2
0
0
0
0
0
In terms of economic viability, one-pot conversion of lysine
is the simplest route to CPL, but it still lacks a high yield of CPL
and needs further improvement. The conversion of DMAC to
CPL seems to be an economically more promising approach, in
view of the advantage of high efficiency by using a fixed-bed
reactor and a higher yield of CPL. Moreover, DMAC can be
synthesized from lysine via a two-step process in an energy
efficient way and at near quantitative yields. This makes the
overall yield still rather high and endows the process with a
good viability.
CPL selectivity
Conversion
2
4
6
8
Time (h)
Figure 2. Continuous-flow conversion of DMAC to CPL. Reaction conditions: 2Ir/HB-124 Conflicts of interest
=
0.30 g, T = 250oC, PH2 = 65 bar, H2 flow = 50 mL/min, DMAC = 0.25wt% in MeOH
(WHSV = 100).
There are no conflicts to declare.
CPL yield enhancement using a fixed-bed reactor: As
mentioned above, the presence of H O co-product in the
2
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21376239, 21690081, 21690084,
methanol solvent will cause CPL yield to decrease. Also, CPL
undergoes further conversion with the reaction time extension.
A fixed-bed reactor might depress such negative behaviours and
be helpful for getting a higher yield of CPL. Therefore, we
further tested DMAC conversion in a fixed-bed reactor over
2
1776268, 21721004), "Transformational Technologies for
Clean Energy and Demonstration", Strategic Priority Research
Program of the Chinese Academy of Sciences, Grant No. XDA
2
Ir/HB-124. As shown in Figure 2, a high and stable selectivity
2
1060200. JS acknowledges DICP for outstanding postdoc
to CPL was obtained. The CPL yield reached 58.2% within two
hours of the reaction, largely maintained and levelled off at
fellowship.
4
8.3% after the eight-hour reaction. This indicates that the CPL
production from DMAC or L-lysine could be further improved to
higher selectivity by using suitable reactors under optimal
reaction conditions. Moreover, the Ir/HB catalyst is reasonably
stable during the reaction, making it attractive for the
application.
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