Enhanced activity of hydrochlorination of acetylene using melamine-modified activated carbon supported gold catalyst

Article abstract of DOI:10.14233/ajchem.2013.15028

A series of modified activated carbon (MAC) was prepared by impregnating with a aqueous solution of melamine and heating under N2 flow at diffferent temperatures. The surface structure of the prepared MAC was characterized by BET, elemental analysis, SEM, FT-IR and XPS. The results of the single carrier activity test showed that MAC exhibited higher activity than activated carbon. Characterization results indicated that the enhanced carrier performance of MAC could be attributed to the presence of nitrogen-containing groups. Subsequently, the supported gold catalysts Au/activated carbon and Au/MAC-0.8-650 were prepared by impregnating with a H[AuCl4] 4H2O solution which was dissolved in aqua regia. Under the reaction conditions of feed volume ratio V (HCl)/V (C2H2) = 1.15, temperature 180 C, C2H2 hourly space velocity (GHSV) 360 h-1 and gold loading of 0.2 wt %, the acetylene conversion and vinyl chloride monomer selectivity of Au/MAC-0.8-650 were 95 and > 99.91 %.

Full text of DOI:10.14233/ajchem.2013.15028

Asian Journal of Chemistry; Vol. 25, No. 17 (2013), 9473-9477
http://dx.doi.org/10.14233/ajchem.2013.15028
Enhanced Activity of Hydrochlorination of Acetylene Using
Melamine-Modified Activated Carbon Supported Gold Catalyst
*
BIN DAI , NING MA, HAIYANG ZHANG and XUGEN WANG
School of Chemistry and Chemical Engineering, Shihezi University/Key Laboratory for Green Processing of Chemical Engineering of Xinjiang
Bingtuan, Shihezi 832003, Xinjiang Province, P.R. China
*Corresponding author: Tel: +86 993 2057277; E-mail: db_tea@shzu.cn; maningttgx@163.com
(Received: 26 December 2012;
Accepted: 1 October 2013)
AJC-14208
A series of modified activated carbon (MAC) was prepared by impregnating with a aqueous solution of melamine and heating under N2
flow at diffferent temperatures. The surface structure of the prepared MAC was characterized by BET, elemental analysis, SEM, FT-IR
and XPS. The results of the single carrier activity test showed that MAC exhibited higher activity than activated carbon. Characterization
results indicated that the enhanced carrier performance of MAC could be attributed to the presence of nitrogen-containing groups.
Subsequently, the supported gold catalystsAu/activated carbon andAu/MAC-0.8-650 were prepared by impregnating with a H[AuCl4]·4H2O
solution which was dissolved in aqua regia. Under the reaction conditions of feed volume ratio V (HCl)/V (C2H2) = 1.15, temperature 180
ºC, C2H2 hourly space velocity (GHSV) 360 h-1 and gold loading of 0.2 wt %, the acetylene conversion and vinyl chloride monomer
selectivity of Au/MAC-0.8-650 were 95 and > 99.91 %.
Key Words: Modified activated carbon, Melamine, Gold catalyst, Acetylene hydrochlorination.
of surface functional groups. In addition, further modification
of activated carbons can endow the carbon materials with
additional physicochemical functions that can improve their
performance in practical applications. Przepiórski13 found that
activated carbons with nitrogen-containing species have higher
adsorption capacities for phenol. Bandosz et al.21 reported that
the unique feature of materials modified with melamine is their
highly efficient adsorption for H2S removal. Zhao et al.23
reported that nitrogen-containing carbon materials show
excellent performance in gas storages and adsorption for
transition metal ions such as Fe(III), Au(III) and Cr(VI).
In a previous work, we prepared a family of supported
low and/or non-mercuric catalysts using various metals and
various supports. Zhang et al.2 prepared Au and Au-based
bimetallic catalysts. Catalysts with an Au loading of 1 wt %
exhibited satisfactory performance in hydrochlorination of
acetylene with 98 % acetylene conversion and 99.8 % selec-
tivity. This study was successful because of the development
of a low-cost conversion of C2H2 with high selectivity and
activity. The present study focuses on determining a method
that can be used to decrease Au content. Previous studies
revealed that increasing the nitrogen content in carbon enhances
the activity of a catalyst. Hence, this study hypothesizes that
Au catalysts supported on activated carbon modified by nitrogen-
containing species reduce Au content.
INTRODUCTION
The hydrochlorination of acetylene is the main industrial
process for producing vinyl chloride monomers (VCMs).
Earlier industrial production used mercuric chloride catalysts
to promote the reaction of hydrogen chloride and acetylene.
However, these catalysts can be short-lived and can lead to
severe environmental pollution problems because of their
higher toxicity and instability, with the loss of mercuric chloride
being the primary catalyst deactivation mechanism1,2. There-
fore, the development of new low and/or non-mercuric catalysts
for the hydrochlorination of acetylene is necessary. Many
studies on non-mercuric catalytic systems focused on precious
metal salts2-7, such as Au, Pt, Rh, Pd, etc. Hutchings et al.4-7
found that supported gold catalysts exhibit high activity and
stability. Thus, they are often used to promote the reaction of
hydrochlorination of acetylene.
Recently, carbon surface modification has drawn conside-
rable attention because of its capability to improve catalyst
performance. Activated carbons (ACs) are widely used in
catalysis and adsorption because of their chemical stability,
well-developed porous structure, large surface area and various
surface functional groups8-23. It is a clear fact that the activated
carbons surface can display acidic, basic and neutral charac-
teristics depending on surface modification and the presence

9474 Dai et al.
Asian J. Chem.
Currently, catalyst for the precious metal salts supported
approximately 100 ºC for 6 h and then analyzed with liquid
nitrogen adsorption at approximately 77 K. The elemental content
of the carbon samples was determined using an element analyzer
(VARIO ELIII, Germany). The surface morphology of the
samples was examined by scanning electron microscopy (SEM;
JSM-6490LV, Electronic Company, Japan) at accelerating voltage
of 10 kV. For fourier transform infrared spectroscopy (FTIR;
Nicolet AVATAR 360), the MAC samples were pressed into a
pellet mixed with finely divided KBr at a ratio of 1:100 and
then scanned from 4000-400 cm-1 at a resolution of 4 cm-1.
X-Ray photoelectron spectroscopic (XPS) measurements were
obtained using an Axis Ultra (Kratos Analytical Ltd., UK).
on activated carbons has been intensively investigated, but
relatively less work was reported related to modified carbons.
In the present study, commercial coconut shell activated carbon
was modified using melamine aqueous solution with different
impregnation mass ratios ranging from 0.1-1.2 and different
calcination temperatures ranging from 550-850 ºC. Modified
activated carbon (MAC) was used as a support forAu catalysts
and the activities of the catalysts for hydrochlorination of acety-
lene were measured in a fixed bed microreactor.
EXPERIMENTAL
Preparation of modified activated carbon: Prior to use,
activated carbon was washed with boiled deionized water and
then oven-dried at 110 ºC for 24 h. Surface modification of
the activated carbon was performed using an incipient wetness
impregnation technique with distilled water as a solvent. After
the activated carbon was added to a prepared melamine aqueous
solution, the mixture was stirred for 24 h at 85 ºC and then
dried for 24 h at 110 ºC. Calcination was performed in a tubular
furnace at different temperatures and a heating rate of 5 ºC
min-1 for 0.5 h under N2 flow to obtain MAC-x-y, where x
represents the impregnation mass ratio (melamine/activated
carbon) and y represents the calcination temperature.
RESULTS AND DISCUSSION
Effect ofAu catalysts on different carriers: In the same
condition, we studied the performance of the Au catalysts
supported on three different carriers. They are a new coconut
shell activated carbon (AC), pitch-based spherical activated
carbon2 (SAC), self-regulating mesoporous carbon (OMC).
The results are shown inTable-1. The catalysts’activity followed
the order:Au/SAC >Au/AC >Au/OMC. The catalysts’ activity
of Au/SAC is the best, but we chose a low-cost coconut shell
activated carbon as carrier in the next experiment.
TABLE-1
CATALYTIC PERFORMANCE OF GOLD CATALYSTS ON
DIFFERENT CARRIERS (C2H2 (GHSV): 360 h-1; T: 423 K; t = 10 h)
Preparation of catalyst: The supported (carbon-supported
and MAC-supported)Au catalyst (1 wt %Au/C) was prepared
using an incipient wetness impregnation technique with aqua
regia as a solvent. This method is currently the most effective
for catalyst preparation2-7. A similar preparation method was
used to prepare another Au catalyst. A solution of prepared
H[AuCl4]·4H2O in aqua regia was added dropwise to activated
carbon and MAC-0.8-650 under stirring, dipped for 10 h at
80 ºC, evaporated in a steam bath and then dried in an oven at
140 ºC for 18 h. The Au catalysts (0.2 wt %Au) were denoted
as Au/activated carbon and Au/MAC-0.8-650.
Catalyst
Au/AC
XC2H2 (%)
96.65
SC2H2 (%)
99.98
References
This work
2
Au/SAC
Au/OMC
99.90
99.97
70.36
99.89
This work
Carrier activity: The activity test conditions of the single
carrier were as follows: 1.15 feed volume ratio V (HCl)/V
(C2H2), 150 ºC temperature and 30 h-1 GHSV (C2H2).
The results of the activity tests on the fresh and melamine-
treated activated carbons are shown in Fig. 1. The calcination
temperature proved to be quite important in carrier activity.
Compared with the untreated activated carbon, each MAC
demonstrated enhanced activity of acetylene hydrochlorination
to VCM. The enhancement in C2H2 conversion depended on
the presence of nitrogen-containing groups. The introduction
of nitrogen into MAC by melamine impregnation and heating
treatment improved the performance of these carbons. However,
high temperatures could aggravate the decomposition of
melamine. Hence, the activities were in the following sequence:
activated carbon < MAC-0.5-550 < MAC-0.5-850 < MAC-
0.5-750 < MAC-0.5-650. This temperature was selected based
on the assumption that melamine decomposition occurs at 345-
360 ºC, the relatively low calcination temperature of 650 ºC
was likely not sufficient for the complete decomposition of
nitrogen-containing species in the presence of activated carbon.
The single carrier activity provided the best fit for the MAC
calcined at 650 ºC.
Activity measurement procedure: The activity test for
the hydrochlorination reaction of acetylene was performed in
a stainless steel-fixed bed microreactor with an internal
diameter of 10 mm. The pipeline was purged with nitrogen
before the reaction to remove water, air and residual gas in the
entire system. C2H2 (11.8 mL min-1) and HCl (11.1 mL min-1)
were fed through the filter to remove water and impurities
using calibrated mass flow controllers in a heated reactor
containing 2 mL of catalyst, giving a C2H2 hourly space
velocity (GHSV) of 360 h-1. The temperature of the reactor
was regulated using a CKW-1100 temperature controller
purchased from Chaoyang Automation Instrument Factory
(Beijing, China). The pressure of the reactants HCl and C2H2
ranged from 1.1 bar to 1.2 bar. This range was selected for
safety reasons and to test the catalyst under mild conditions.
The exit gas mixture was passed through a gas absorption bottle
containing NaOH solution and was analyzed using a Shimadzu
GC-2014C chromatograph.
Detection method: The specific surface area and pore
size distribution of the samples were analyzed using anASAP
2020C surface area and porosity analyzer (Micromeritics
Instrument Corporation, USA) that uses a nitrogen adsorption-
desorption method. The samples were initially outgassed at
As shown in Fig. 2, the addition of nitrogen species to
activated carbon caused the carrier to have a higher conversion
of C2H2 than activated carbon. This finding suggests that the
carrier activity and lifetime for hydrochlorination of acetylene
could still be increased. Moreover, the activity increased with

Vol. 25, No. 17 (2013)
Activity of Hydrochlorination of Acetylene Using Melamine-Modified Activated Carbon 9475
TABLE-2
PORE STRUCTURE PARAMETERS OF ACTIVATED
CARBON AND MODIFIED ACTIVATED CARBON
60
50
40
30
20
10
SBET
(m2/g)
Total pore volume
(cm3/g)
Average pore
diameter (nm)
Sample
Activated carbon
MAC-0.1-650
MAC-0.3-650
MAC-0.5-650
MAC-0.8-650
MAC-1.2-650
1219
1103
936
0.9751
0.9750
0.9750
0.9750
0.9749
0.9748
2.33
2.35
2.34
2.37
2.34
2.37
988
906
833
TABLE-3
ELEMENTAL ANALYSIS OF ACTIVATED CARBON
AND MODIFIED ACTIVATED CARBON
00
5
10
15
20
25
Sample
N (%)
0.37
1.26
5.29
3.93
3.24
2.12
3.21
6.65
8.48
C (%)
82.98
87.16
80.68
83.30
82.04
82.14
78.37
78.44
79.65
S (%)
0.26
0.21
0.15
0.14
0.12
0.09
0.06
0.11
0.10
H (%)
0.58
0.94
0.91
0.63
0.75
0.59
0.50
0.89
0.81
Time on stream (h)
Activated carbon
MAC-0.5-550
MAC-0.5-650
MAC-0.5-750
MAC-0.5-850
MAC-0.1-650
MAC-0.3-650
MAC-0.8-650
MAC-1.2-650
Fig. 1. Acetylene conversion by MAC with different calcination
temperature. Reaction conditions: temperature (T) = 150 ºC; C2H2
hourly space velocity (GHSV) = 30 h-1; feed volume ratio V (HCl)/
V (C2H2) = 1.15
60
50
40
30
20
10
rature increased significantly because of the presence of
melamine decomposition nitrogen-containing products. The
nitrogen content of MAC-1.2-650 reached up to 8.48 %.
SEM images of the samples (Fig. 3) show that no signifi-
cant differences exist in the surface morphology of activated
carbon (Fig. 3a) and MAC-0.1-650 (Fig. 3b), except for some
apparent pore narrowing in MAC. This result suggests that
they have similar surface structures. MAC-0.8-650 (Fig. 3d)
compared with MAC-0.1-650 had a visual impact in terms of
surface morphology. This finding may be attributed to the pore
blockage in the micropores caused by the formation of
melamine decomposition products during calcination. How-
ever, MAC-0.8-650 had wider pores than MAC-0.8 (Fig. 3c),
implying that the heat treatment partly removed some of the
melamine decomposition products lodged in the micropores.
00
5
10
15
20
25
Time on stream (h)
Fig. 2. Acetylene conversion by MAC with different impregnation mass
ratio. Reaction conditions: temperature (T) = 150 ºC; C2H2 hourly
space velocity (GHSV) = 30 h-1; feed volume ratioV (HCl)/V (C2H2)
= 1.15
increasing melamine content. MAC-0.8-650 exhibited a
satisfactory performance, with an almost 30 % higher C2H2
conversion than activated carbon within 24 h. Further increasing
the melamine content did not improve the activity. These results
indicate that the optimal impregnation mass ratio of MAC is 0.8.
Characterization of carrier: Surface modification using
nitrogen-containing species may result in changes in the pore
structure. The pore structure parameters of the activated carbon
and MAC are shown in Table-2. As expected, the Brunauer-
Emmett-Teller surface area of MAC significantly reduced from
1219 to 833 m2 g-1 after modification and decreased gradually
as the nitrogen species increased. These findings indicate that
the pores were blocked by the nitrogen products of melamine
partial decomposition.
Table-3 presents the elemental analysis of the samples.
The samples with a fixed amount of melamine but with different
calcination temperatures show that MAC-0.5-650 has a rela-
tively high nitrogen content (5.28 %), which is in accordance
with the activity test results. The elemental analysis (Table-3)
also shows that the nitrogen content of MAC with various
impregnation mass ratios but with a constant calcination tempe-
Fig. 3. SEM images of samples. (a) activated carbon, (b) MAC-0.1-650,
(c) MAC-0.8 and (d) MAC-0.8-650

9476 Dai et al.
Asian J. Chem.
Fig. 4 shows the FTIR spectra of untreated actived carbon
species and the band at 400.5 eV corresponds to pyrrole N22.
The presence of nitrogen-containing groups in MAC could
increase basicity, thus improving catalytic performance.
Catalytic activity and selectivity: Fig. 6 shows that the
Au/activated carbon (0.2 wt %Au) catalyst exhibited satisfactory
performance, with almost 88 % C2H2 conversion and > 99.98 %
VCM selectivity within 24 h. In comparison with the results
of Zhang et al.2. We choose a low cost activated carbon than
their pitch-based spherical activated carbon also had well C2H2
conversion with an Au loading form 1.0 to 0.2 wt %. However,
the addition of nitrogen-containing species to activated carbon
slightly decreased the induction period of hydrochlorination
and increased the conversion rate of the catalysts. The Au/
MAC-0.8-650 (0.2 wt % Au) catalyst exhibited a relatively
high activity, with 95 % C2H2 conversion and 99.91 % VCM
selectivity. Our experiment shows that the C2H2 conversion of
the Au/MAC-0.8-650 catalyst was higher than that of the un-
treatedAu/activated carbon catalyst. The results also confirmed
our hypothesis by the introduction of nitrogen-containing
species on activated carbon to reduce the Au content and then
improve the acetylene conversion rate is feasible.
and modified samples (AC, AC-0.1-650,AC-0.3-650,AC-0.5-
650, AC-0.8-650and AC-1.2-650). The spectra of the two
samples showed some marked similarities.A strong absorption
at 3850-3300 cm-1 was detected on the spectra of both samples,
which could be assigned to O-H stretching vibration9-11. The
band at ca. 2900 cm-1 probably corresponded to C-H stretching
vibration12. The bands at 1750-1630 and 1600-1450 cm-1 could
be assigned to C=O and C=C groups12. However, differences
in the spectra of the two samples can also be observed. Peaks
related to nitrogen-containing groups were observed in the
modified samples. Melamine treatment caused some changes
in the spectra of modified samples. A sharp bands at 3742 cm-1
related to N-H stretching vibrations was observed for all
modified samples.The spectra of MAC-0.8-650 showed a band
at 1540 cm-1, which may be related to C=N groups. In addition,
bands at 1158 and 1465 cm-1 may be associated with C-N
(amide) and N-H groups, respectively10-13. These results
indicate that melamine modification can produce new nitrogen
surface complexes and some basic groups.
100
90
80
70
Au/AC
Au/MAC-0.8-650
60
50
40
30
(a)
20
10
00
5
10
15
20
25
4000
3500
3000
2500
Wavenumber (cm–1)
Fig. 4. FTIR spectra for activated carbon and MAC sample
2000
1500
1000
.500
Time on stream (h)
100.0
99.5
99.0
98.5
98.0
Au/AC
Au/MAC-0.8-650
(b)
00
5
10
15
20
25
Time on stream (h)
Fig. 6. Catalytic performance of Au/activated carbon and Au/MAC
catalysts. Reaction conditions: temperature (T) = 180 ºC; C2H2
hourly space velocity (GHSV) = 360 h-1; Feed volume ratioV (HCl)/
V (C2H2) = 1.15
394 396 398 400 402 404 406 408 410 412 414
B.E. (eV)
Fig. 5. XPS spectra for MAC-0.8-650 sample
The deactivation mechanism of AuCl3 catalysts for
hydrochlorination of acetylene was studied by Zhang et al.3.
HCl and the AuCl3 dimer co-catalyze C2H2 and produce the
Fig. 5 shows the XPS N(1s) measurements for MAC-0.8-
650. The peak at 398.5 eV can be assigned to pyridine nitrogen

Vol. 25, No. 17 (2013)
Activity of Hydrochlorination of Acetylene Using Melamine-Modified Activated Carbon 9477
intermediate vinyl chloride. If HCl could not be adsorbed on
the Au site, the intermediate chlorovinyl will be difficult to
desorb from theAu catalyst. The catalytic activity of theAuCl3
catalyst would be reduced because the active Au sites are oc-
cupied. Thus, the efficient adsorption of HCl on the Au site is
necessary to retain the activity of the AuCl3 catalyst for
hydrochlorination of acetylene. Furthermore, the current ex-
periment shows that the presence of nitrogen-containing groups
in melamine-modified activated carbon increases the basicity
of MAC, thereby improving HCl adsorption. The modified
Au catalyst can efficiently adsorb HCl molecules rather than
C2H2, thus exhibiting satisfactory catalytic performance of
hydrochlorination of acetylene.
(No. 2010CB234605, 2012CB720302) and Program for
Changjiang Scholars and Innovative Research Team in
University (No. IRT1161).
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melamine at various calcination temperatures and with different
impregnation mass ratios. Characterization results from the
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ACKNOWLEDGEMENTS
The authors are grateful for their financial support from
State Key Development Program for Basic Research of China