Nitrogen-Modified Activated Carbon Supported Cu(II)Cu(I)/NAC Catalysts for Gas–Solid Acetylene Dimerization

Article abstract of DOI:10.1007/s10562-021-03548-1

Improving dispersibility and stability of Cu(II)Cu(I)/activated carbon (AC) is a crucial aspect for enhancing its catalytic performance in the process of gas–solid acetylene dimerization. The Cu(II)Cu(I)/NAC-500 catalyst using nitrogen-modified AC (NAC) as a support, delivered excellent catalytic performance and stability vs undoped Cu(II)Cu(I)/AC at 100?°C and 120?h^?1 of C₂H₂ gas hourly space velocity. Under the optimal conditions, the Cu(II)Cu(I)/NAC-500 catalyst exhibited a stable catalytic performance during a 10?h test with 65% C₂H₂ conversion; and the selectivity to monovinylacetylene (MVA) reached 86%. The existence of nitrogen species can increase the interaction between copper and the support, and increase dispersion of the copper species on the support, which were benefit for the catalytic performance. Graphic Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-021-03548-1

Catalysis Letters
https://doi.org/10.1007/s10562-021-03548-1
Nitrogen‑Modifed Activated Carbon Supported Cu(II)Cu(I)/NAC
Catalysts for Gas–Solid Acetylene Dimerization
Congcong Li1,2 · Jianwei Xie1,2 · Jinli Zhang3 · Bin Dai2
Received: 12 October 2020 / Accepted: 24 January 2021
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC part of Springer Nature 2021
Abstract
Improving dispersibility and stability of Cu(II)Cu(I)/activated carbon (AC) is a crucial aspect for enhancing its catalytic
performance in the process of gas–solid acetylene dimerization. The Cu(II)Cu(I)/NAC-500 catalyst using nitrogen-modifed
AC (NAC) as a support, delivered excellent catalytic performance and stability vs undoped Cu(II)Cu(I)/AC at 100 °C and
120 h−1 of C2H2 gas hourly space velocity. Under the optimal conditions, the Cu(II)Cu(I)/NAC-500 catalyst exhibited a
stable catalytic performance during a 10 h test with 65% C2H2 conversion; and the selectivity to monovinylacetylene (MVA)
reached 86%. The existence of nitrogen species can increase the interaction between copper and the support, and increase
dispersion of the copper species on the support, which were beneft for the catalytic performance.
Graphic Abstract
Keywords Nitrogen-modifed · Urea · Gas–solid reaction · Acetylene dimerization · Cu(II)Cu(I)/AC · Activated carbon
* Jianwei Xie
cesxjw@foxmail.com
1 Introduction
* Bin Dai
db_tea@shzu.edu.cn
Monovinylacetylene (MVA) is produced as a starting mate-
1
rial for chloroprene rubber synthesis and can be used as a
raw material in producing other important chemical products
such as butanedione through hydration [1] and benzene via
polymerization with acetylene [2]. The representative cata-
lyst in acetylene dimerization is Nieuwland catalyst (NC),
which is comprised of CuCl and KCl or NH4Cl in acid solu-
tion [3]. Studies include the solvent efect [4], mechanism
[5], the NC structure [6], ligand modifcation [7, 8], kinetics
College of Chemistry and Bioengineering, Hunan
University of Science and Engineering, Yangzitang Road,
Yongzhou 425199, China
2
3
Key Laboratory for Green Processing of Chemical
Engineering of Xinjiang Bingtuan, School of Chemistry
and Chemical Engineering, Shihezi University,
Shihezi 832003, China
School of Chemical Engineering and Technology, Tianjin
University, Tianjin 300072, China
Vol.:(0123456789)
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C. Li et al.
[9], and metal additives on gas–liquid C2H2 dimerization
with NC [10, 11] have been reported. However, the short-
comings of these processes, such as low acetylene conver-
sion and MVA selectivity, more polymers produced and dif-
fcult to separate and recycle, still exist in gas–liquid C2H2
dimerization.
[22], and thus we wondered if N modifcation of AC can
also enhance the activity and dispersibility of the gas–solid
acetylene dimerization.
In this work, we incorporate nitrogen into the AC sup-
port via post-treatment method with urea to introduce nitro-
gen functional groups in AC that yield the corresponding
nitrogen-modifed activated carbon (NAC) supports. These
materials were heaved at diferent temperatures, and then
the gas–solid acetylene dimerization was studied over these
Cu(II)Cu(I)/NAC catalysts.
Recently, we have reported the synthesis of a Cu/AC cata-
lyst and evaluated it for gas–solid reaction for C2H2 dimeri-
zation [12]. Compared with the traditional C2H2 dimeriza-
tion reaction, the Cu/AC catalyst has better activity with
48.3% C2H2 conversion and 87.4% selectivity to MVA. This
prior work solved several drawbacks of traditional gas–liq-
uid NC-catalyzed C2H2 dimerization reactions. However,
the main reason for catalyst deactivation is the loss of cop-
per active species. In order to increase the catalyst’s activity
and overcome the loss of copper active species on Cu(I)/AC
catalysts for gas–solid C2H2 dimerization reactions, we then
prepared Cu(II)Cu(I)/AC with diferent Cu+/Cu2+ ratios and
analyzed the synergistic efects of both Cu2+ and Cu+ [13].
As expected, the results achieved the desired efect, which
showed that the addition of Cu(II) could inhibit Cu loss in
the catalysts. However, the dispersibility and stability of the
Cu(II)Cu(I)/AC require further improvements. For example,
the acetylene conversion of Cu(II)Cu(I)/AC decreases from
79 to 58% after running for 10 h.
2 Experimental
2.1 Preparation of N‑Doped AC Catalyst
A coconut-shell-activated carbon was used as the starting
material. This material was initially washed with 1 mol L−1
HCl at the temperature of 70 °C for 5 h, fltered, washed with
distilled water, and dried at 150 °C for 12 h (labeled AC).
A mixture of AC (5 g), glacial acetic acid (3 mL 37%),
and distilled water (20 mL) was stirred for 2 h. Urea (3 g)
was then added to the solution and stirred for another 2 h at
room temperature in the dark. The solution was evaporated
to dryness at 100 °C and calcined at 500 °C (heating rate:
5 °C min−1) under a N2 atmosphere for 1 h. The resulting
sample was labeled as NAC-500. Samples with other calci-
nation temperatures were named NAC-300 and NAC-700.
The preparation of the Cu(II)Cu(I)/NAC catalyst is shown
in Supplementary Fig. S1.
It is well known that the interactions between the sup-
port and catalyst play an important role in governing the
catalyst’s particle size. In general, nitrogen-modifed sup-
ports strategy are often used for this purpose. The litera-
ture reports that nitrogen introduced into the AC support
afects catalyst activity at least three aspects [14]. First, it
can increase catalyst particle dispersion and decrease cata-
lyst particle size during the nucleation and growth process
of catalyst nanoparticles. Then, the nitrogen atoms in the
AC supports are believed to provide improvement of the
nucleation rate of active species particles [15, 16] and higher
nucleation rates increase the surface defects [17]. Third,
increasing chemical binding between the support and cata-
lyst results in enhanced stability of the catalysts. The nitro-
gen functionalities act as chemical binding sites and increase
the bonding strength of the active ingredient and the carrier,
which, in turn, increases the catalytic activity by increasing
the dispersion of active species by providing resistance to
particle agglomeration [15].
EtNH2⋅HCl (3.83 g) was dissolved in mixed solvent
containing 1,4-dioxane (3 mL) and DMF (7 mL) at 80 °C
water bath. CuCl was then added to the solution with stir-
ring under N2 atmosphere. The NAC-500 was added into the
above mixture until the CuCl was completely dissolved. The
mixture was then stirred for 2 h, cooled to 25 °C and stirred
for another 12 h, fltered, and dried in vacuum oven at the
temperature of 80 °C for 12 h to produce Cu(I)/NAC-500.
The CuCl2 aqueous solution (3.24 g CuCl2 in 10 mL distilled
water) as an addition was added to the Cu(I)/NAC-500 pow-
der with stirring for 12 h at 25 °C. Subsequently, the mixture
was fltered and dried in vacuum oven for 12 h at 80 °C to
obtain the Cu(II)Cu(I)/NAC-500 catalyst with a Cu+/Cu2+
molar ratio of 1:0.3. The copper loading of all catalysts was
based on metallic copper.
Zhou et al. [18] reported Cu-based catalysts supported
on N-doped carbon nanotubes signifcantly improved the
electron conductivity ability of the carbon sheets. The N1s
spectra of N-doped activated carbon-containing materials
usually indicate the existence of multiple nitrogen-contain-
ing species [19]. These N species usually stabilize catalyst
nanoparticles [20] and generate favorable nucleation sites
for reaction [21]. Doping ways and/or preparation conditions
may afect the composition and distribution of N substances
2.2 Description of Analytical Methods and Catalytic
Tests
A fxed-bed micro reactor (i.d., 10 mm) was used to evalu-
ate the activity of the catalysts. N2 was used to pass into
the pipeline to clear away air before starting the reaction.
C2H2 (4.0 mL min−1) was pumped into the fxed-bed micro
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Nitrogen‑Modifed Activated Carbon Supported Cu(II)Cu(I)/NAC Catalysts for Gas–Solid…
reactor flled with 2 mL of catalyst to achieve a C2H2 gas
hourly space velocity (GHSV) of 120 h−1, and the reaction
temperature was increased to 100 °C. A gas chromatograph
(Shimadzu, GC-2014C) was used to analyze the composi-
tion of the reaction products. The evaluation criteria of the
catalytic performance was the acetylene conversion (X) and
selectivity to MVA (S). The following equation was used to
calculate the X and S:
of Cu(II)Cu(I)/AC decreases from 79 to 58% after running
for 10 h. The selectivity to MVA was 77% indicating that
Cu(II)Cu(I)/AC is rapidly deactivated under these reaction
conditions. This reveals that nitrogen-modifed AC supports
obviously afect the activity and stability of the catalysts.
3.2 Raman Spectroscopy
Raman spectroscopy characterized the ordered and disor-
dered structures of the prepared nitrogen-modifed AC. The
Raman spectra (Fig. 1) of AC, NAC-300, NAC-500, and
NAC-700 had two distinct peaks near 1590 and 1350 cm−1,
which are ascribed to the G-band and the D-band, respec-
tively [23]. The D band corresponds to the carbon disorder
and defective structures, and the G band refers to the gra-
phitic structure. The ID/IG ratios increases as the calcination
temperature increases. This result indicates that nitrogen-
modifed AC can increase the degree of disorder and the
number of surface defects [24]. Physical surface defects
often increase the nucleation rate [25], which increase the
catalyst’s particle dispersion.
(
) (
)
X = [ α2 + 2α3 + 2α4 + 3α5 ∕ α1 + α2 + 2α3 + 2α4 + 3α5 ] × 100%
(
)
S = [2α3∕ α2 + 2α3 + 2α4 + 3α5 ] × 100%
The α1, α2, α3, α4, and α5 are respectively represented vol-
ume fractions of acetylene, acetaldehyde, MVA, 2-chloro-
1,3-butadiene, and 1,5-hexadien-3-yne (DVA) in the gas
products.
2.3 Catalyst Characterization
X-ray diffraction (XRD) was tested by X-ray diffrac-
tometer (Bruker D8 ADVANCE) with a Cu Kα radiation
(0.1542 nm) in the 2θ scanning range 10°–90°. Transmis-
sion electron microscopy (TEM) images were carried out
a JEM2100F (FEI, Hillsboro, OR, USA) instrument. Tem-
perature-programmed reduction (TPR) tests were recorded a
Micromeritics ASAP 2720 instrument. The temperature was
increased from 25 to 900 °C with 80 mL min−1 10% H2/Ar.
The thermogravimetric analysis (TG) of the catalysts used
a NETZSCH STA 449F3 Jupiter instrument, and the tem-
perature increased from 25 to 900 °C in an air atmosphere.
The specifc surface areas of the samples were determined
by Brunauer–Emmett–Teller (BET) analysis. X-ray photo-
electron spectroscopy (XPS) were collected using a Kra-
tos AMICUS spectrometer (Shimadzu, JP). Raman spectra
was recorded using a Horiba Jobin Yvon LabRAM HR800
(633 nm laser excitation).
3.3 Dispersion and Sintering of Cu Species
Narrower size distributions and smaller Cu particles have
been observed on nitrogen-modifed AC even with rela-
tively high copper loadings suggesting that surface defects
can nucleate Cu particles more uniformly (Supplementary
Fig. S3). During catalyst nanoparticle deposition, nitrogen-
modifed AC can be modify the nucleation and growth [14].
This results in smaller catalyst particles and increased cata-
lyst particle dispersibility.
Supplementary Fig. S4 displays the obtained XRD of
AC, NAC-500, Cu(II)Cu(I)/AC, and Cu(II)Cu(I)/NAC-500.
XRD patterns show that there is no obvious Cu characteristic
3 Results and Discussion
3.1 Catalytic Performance of the Catalysts
for Acetylene Dimerization
Supplementary Fig. S2a, b shows the activity of catalysts
on NAC-300, NAC-500, and NAC-700. Supplementary Fig.
S2c, d presents the catalytic performance of the undoped
Cu(II)Cu(I)/AC and N-doped Cu(II)Cu(I)/NAC-500. As we
can see, Supplementary Fig. S2a, b shows that Cu(II)Cu(I)/
NAC-500 (with 14.17% of Cu in total measured by ICP-
AES) had the best performance among the tested catalysts
with a conversion that stabilized around 65%; the selectivity
to MVA was 86%. In comparison, the acetylene conversion
Fig. 1 Raman spectra of AC, NAC-300, NAC-500, and NAC-700
1 3

C. Li et al.
peaks in the catalysts except for amorphous difraction peaks
of activated carbon (JCPDS 75-1621) [26]. This is ascribed
to the high dispersibility of the Cu species or that content of
Cu is too low to be detected in XRD [27].
species in NAC-500 and NAC-700. The NAC-500 sample
has the highest amount of pyridine nitrogen (58.9%), which
indicates that the N species is dependent on the calcination
temperature [34]. Pyridinic nitrogen is the main nitrogen
species on NAC samples (Fig. 3c). Figure 3d showed the
XPS spectra (the Cu 2p data of Cu(II)Cu(I)/AC was from
our previous work [13]) of nitrogen-doping Cu(II)Cu(I)/
NAC-500 and undoped Cu(II)Cu(I)/AC catalyst. The peak at
934.2–934.6 eV is attributed to Cu2+, while 931.9–932.1 eV
is attributed to Cu+ species [35, 36], and the result shows
that nitrogen doping has little efect on the electronic struc-
ture of the active component.
The H2-TPR profles of undoped Cu(II)Cu(I)/AC and
N-doped Cu(II)Cu(I)/NAC-500 catalysts are shown in Fig. 2.
The Cu(II)Cu(I)/AC showed three peaks with lower tem-
perature peaks appearing at 361 and 456 °C, respectively;
a high temperature peak appeared at 784 °C (the H2-TPR
data of Cu(II)Cu(I)/AC was from our previous work [13]).
The reduction of Cu(II)Cu(I)/NAC-500 showed three peaks
with higher temperature peaks appearing at 379 and 496 °C,
respectively, and a high temperature peak appearing 806 °C.
The TCD signal at 700–850 °C may be the combustion gas
from activated carbon [28, 29]. The Cu(II)Cu(I)/NAC-500
catalyst showed reduction peaks at the highest temperature
indicating a relatively stronger interaction between active
components and carrier [30].
3.5 Coking Deposition During the Reaction
TG was used to study the carbon deposition on the catalysts
(Supplementary Fig. S6). Before 100 °C, the slight weight
loss is ascribed to the adsorbed water in the fresh Cu(II)
Cu(I)/AC and Cu(II)Cu(I)/NAC-500 catalysts. The obvious
weight loss at 100–200 °C is associated with the decomposi-
tion of residual solvent and cosolvent and urea in the fresh
catalyst. In the temperature range of 200–320 °C, the weight
losses of the used Cu(II)Cu(I)/AC and Cu(II)Cu(I)/NAC-
500 catalyst is 14.99 and 27.27%, respectively, while the
fresh Cu(II)Cu(I)/AC and Cu(II)Cu(I)/NAC-500 catalysts
have a loss of 23.10 and 21.89%, respectively. When the
temperature exceeds 320 °C, the weight decreases rapidly
due to the combustion of AC. From 200 to 320 °C, carbon
deposition with Cu(II)Cu(I)/AC and Cu(II)Cu(I)/NAC cata-
lysts were 8.11 and 5.38%, respectively (Table 1). This result
indicated that the carbon deposition of N-doped catalysts
can be improved.
3.4 Valence Changes of the Active Species
The XPS results detail the elemental compositions (Fig. 3a).
The peaks near 284.8, 399.5, and 531.7 eV are ascribed to
the binding energy of C1s, N1s, and O1s, respectively [31].
Supplementary Table S1 indicates the presence of nitrogen
species and nitrogen content of 3.0 wt% for Cu(II)Cu(I)/
NAC-500 sample. Figure 3a shows that nitrogen was suc-
cessfully doped into AC. There were three kinds of nitro-
gen, binding energies of pyridinic nitrogen, pyrrolic nitrogen
and quaternary nitrogen were at 398.6, 400.4, and 401.2 eV,
respectively (Fig. 3b) [32, 33]. Supplementary Fig. S5 shows
the relative contents of nitrogen species of the N-doped sam-
ples. The NAC-300 has the least pyridinic nitrogen (10.4%).
It is clear that the pyridinic nitrogen is the dominant nitrogen
3.6 Textural Properties of Catalysts
The BET data of AC, NAC-300, NAC-500 and NAC-700
had little change before and after the nitrogen modifcation
of AC (Supplementary Table S2). Therefore, the modif-
cation has only a small efect on the textural properties of
the support. When AC loaded CuCl2 and CuCl, the sur-
face areas (SBET) of support AC decreased from 1036.00
to 184.30 m2 g−1 which is likely due to the heavy load.
,
Compared to SBET and total pore volumes (V) in the fresh
catalysts, the SBET and V in used catalysts were smaller. This
is due to carbon deposition in the reaction process, which is
consistent with the TG data.
4 Conclusion
We demonstrated that the nitrogen-doped AC can enhance
the catalytic performance of Cu(II)Cu(I)/AC. The nitro-
gen-doped Cu(II)Cu(I)/NAC-500 catalysts with a nitrogen
Fig. 2 H2-TPR profles of undoped Cu(II)Cu(I)/AC and N-doped
Cu(II)Cu(I)/NAC-500 catalysts
1 3

Nitrogen‑Modifed Activated Carbon Supported Cu(II)Cu(I)/NAC Catalysts for Gas–Solid…
Fig. 3 XPS survey spectra a of AC, N-doped samples, and b N1s spectrum for the N-doped samples, c N1s spectrum Cu(II)Cu(I)/NAC-500 and
d Cu 2p in Cu(II)Cu(I)/NAC-500 and Cu(II)Cu(I)/AC catalyst
Acknowledgements This work was supported by the National Natural
Table 1 Amount of coke deposition on the Cu(II)Cu(I)/AC and
Science Foundation of China (Grant Nos. 21776179 and 21463021).
Cu(II)Cu(I)/NAC-500
Compliance with Ethical Standards
Samples
Amount of
coke deposition
(%)
Conflict of Interest There are no conficts of interest to declare.
Cu(II)Cu(I)/AC [13]
Cu(II)Cu(I)/NAC-500
8.11
5.38
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