Nitrogen-doped pitch-based spherical active carbon as a nonmetal catalyst for acetylene hydrochlorination

Article abstract of DOI:10.1002/cctc.201402018

A highly active nitrogen-doped pitch-based spherical activated carbon catalyst (PSAC-N) is synthesized through a nitrogen-doping treatment and used as a nonmetal catalyst for acetylene hydrochlorination. The conversion of acetylene on PSAC-N exceeds 68% and the selectivity of vinyl chloride is over 99% at a temperature of 250°C, an acetylene gas hourly space velocity of 120 h^-1, and a feed volume ratio V(HCl)/V(C₂H₂) of 1.15. DFT calculations performed with Gaussian 09 program package reveal that the active site of PSAC-N has a N-6v (quaternary nitrogen bonded between two 6-membered rings) structure. A seven benzene ring unit model is used in the DFT study. In addition, the reason for inactivation for PSAC-N catalysts is discussed. Of all the adsorption energies obtained, the adsorption capacity of hydrogen chloride on PSAC-N is the highest, which indicates strong ability for acetylene hydrochlorination. The reaction mechanism is determined, and the reaction energy of N-6v(7) calculated as 236.2 kJmol^-1.

Full text of DOI:10.1002/cctc.201402018

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DOI: 10.1002/cctc.201402018
Nitrogen-Doped Pitch-Based Spherical Active Carbon as
a Nonmetal Catalyst for Acetylene Hydrochlorination
[a, b]
[b]
[b]
[b]
Xugen Wang,
Bin Dai,* Yang Wang, and Feng Yu
A highly active nitrogen-doped pitch-based spherical activated
carbon catalyst (PSAC-N) is synthesized through a nitrogen-
doping treatment and used as a nonmetal catalyst for acety-
lene hydrochlorination. The conversion of acetylene on PSAC-N
exceeds 68% and the selectivity of vinyl chloride is over 99%
at a temperature of 2508C, an acetylene gas hourly space ve-
ternary nitrogen bonded between two 6-membered rings)
structure. A seven benzene ring unit model is used in the DFT
study. In addition, the reason for inactivation for PSAC-N cata-
lysts is discussed. Of all the adsorption energies obtained, the
adsorption capacity of hydrogen chloride on PSAC-N is the
highest, which indicates strong ability for acetylene hydro-
chlorination. The reaction mechanism is determined, and the
À1
locity of 120 h , and a feed volume ratio V(HCl)/V(C H ) of
2
2
À1
1.15. DFT calculations performed with Gaussian 09 program
reaction energy of N-6v(7) calculated as 236.2 kJmol .
package reveal that the active site of PSAC-N has a N-6v (qua-
Introduction
Polyvinylchloride (PVC), one of the most widely used plastics in
the world, has attracted significant research attention because
urgent necessity. Thus, the development of low cost, environ-
mentally benign, sustainable non-mercury catalysts, particularly
in response to the increasing needs of modern society and
emerging ecological concerns, is an important endeavor.
In our previous experiment (Table 1), activated carbon dem-
onstrated reaction activity for acetylene hydrochlorination.
Such a finding has prompted us to consider whether a nonme-
[
1]
of its diverse range of excellent properties. For it to replace
classical materials, such as wood, iron, copper, and rubber, PVC
must meet the requirements of various large-scale application
environments. The mercury catalyst (i.e., mercury chloride sup-
ported on carbon) plays an important role in producing vinyl
[2]
chloride monomers through acetylene hydrochlorination. Un-
fortunately, the mercury catalyst is highly toxic and causes seri-
3
ous environmental problems. To produce 10 kg of PVC, ap-
Table 1. Conversion of acetylene and selectivity of vinyl chloride with
[
a]
[
3,4]
various catalysts after 5 h.
proximately 0.12–0.20 kg of mercury is needed. Concerns re-
garding the use of mercury catalysts for acetylene hydrochlori-
nation have prompted scientists to continue the search for
Sample
X
A
[%]
SVCM [%]
coal-based AC
PSAC
coconut AC
13.24
16.77
4.65
0.32
0.52
0.29
99.35
99.35
99.46
–
–
–
[
5–7]
possible alternatives to these catalysts.
[
8,9]
[10,11]
Noble-metal catalysts (e.g., gold,
palladium,
and plati-
[
12]
num ) demonstrate stable catalytic activity and high acety-
lene conversion. However, these catalysts are significantly limit-
ed in application because of their low stability and short life,
and the use of such catalysts for acetylene hydrochlorination is
restricted. Moreover, noble-metal catalysts easily lead to
changes in valence state, catalyst aggregation, and deactiva-
SiO
Al
TiO
2
2
O
3
2
À1
[
1
a] Reaction conditions: T=423 K, GHSV(C
.15.
2 2 2 2
H )=36 h , V(HCl)/V(C H )=
[
13]
tion.
Although noble-metal catalysts can satisfy current
tal catalyst for acetylene hydrochlorination could be obtained.
We synthesized a highly active pitch-based spherical activated
carbon (PSAC) modified through a nitrogen-doping treatment
and evaluated its adsorption behavior by computational analy-
sis (i.e., a Gaussian model). The nitrogen-doped PSAC (PSAC-N)
demonstrated high catalytic activity and acetylene conversion.
Compared with mercury and noble-metal catalysts, PSAC-N ap-
pears to be a promising nonmetal catalyst. By eliminating the
effect of trace metals and pore structure, we confirmed that
the CÀN structure is the active site of PSAC-N. The CÀN optimi-
zation model was ascertained by computational analysis, and
the reaction mechanism of acetylene hydrochlorination with
the CÀN catalyst was studied by using DFT calculations per-
needs, research to meet the practical requirements remains an
[a] Dr. X. Wang
School of Chemical Engineering and Technology
Tianjin University
Tianjin 300072 (P.R. China)
[
b] Dr. X. Wang, Prof. B. Dai, Y. Wang, Dr. F. Yu
Key Laboratory for Green Processing of Chemical Engineering of Xinjiang
Bingtuan
School of Chemistry and Chemical Engineering
Shihezi University
Shihezi 832003 (P.R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402018.
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formed with the Gaussian 09 program package. PSAC-N dem-
onstrated superior performance, cost-effectiveness, and envi-
ronmental safety.
Results and Discussion
Catalytic behavior of carbon and oxide-based catalyst
support materials
As shown in Table 1, acetylene conversion was calculated and
estimated using carbon and oxide-based catalyst support ma-
terials [e.g., activated carbon (AC), SiO , Al O , and TiO ]. Acety-
2
2
3
2
lene conversion induced by AC is higher than that induced by
oxide-based catalyst supports. PSAC demonstrated a high cata-
lytic efficiency and acetylene conversion was up to 16.8%,
whereas all other oxide-based catalyst supports demonstrated
fairly low catalytic efficiency. Moreover, all the carbonic materi-
als, such as PSAC, demonstrated excellent selectivity toward
vinyl chloride, which could strongly improve acetylene hydro-
chlorination. Therefore, PSAC is used as a catalyst and not just
as a catalyst support.
Active site distribution of PSAC as a catalyst for acetylene
hydrochlorination
To determine the active site distribution of PSAC, nitrogen-
doped carbon (NC), trace elements, and the microstructure of
PSAC were investigated after PSAC activation. As shown in Fig-
ure 1a, nitrogen-doped PSAC (PSAC-N) demonstrated high
acetylene conversion. The acetylene conversion rate increased
with the increase in NC content. The as-obtained PSAC-N(0.8)
sample, which had a nitrogen content of 4.9% (Table 2), dem-
onstrated excellent acetylene conversion (66%). However,
PSAC with a nitrogen content of 0.7% demonstrated a low
level of acetylene conversion (16.8%). These results indicate
that the nitrogen content in PSAC and PSAC-N can improve
the acetylene hydrochlorination rate of the catalyst and dem-
onstrate high catalyst activity.
As shown in Figure 1b, acetylene conversion (from 54 to
Figure 1. Acetylene conversion of a) various samples at a temperature of
9
2%) on PSAC-N(0.8) is high [T=2508C; feed volume ratio
À1
1508C, a GHSV(C
2
H
2
) of 36 h , and a feed volume ratio V(HCl)/V(C
2
H
2
) of
V(HCl)/V(C H )=1.15] at various gas hourly space velocities
2
2
1.15; b) the PSAC-N(0.8) sample at a temperature of 2508C and a feed
À1
(
GHSVs), ranging from 30 to 150 h . To optimize catalyst activ-
volume ratio V(HCl)/V(C
2
H
2
) of 1.15; c) the PSAC-N(0.8) sample at a GHSV-
À1
À1
(C H ) of 120 h and a feed volume ratio V(HCl)/V(C H ) of 1.15.
ity, the GHSV(C H ) must be 120 h . The reaction temperature
2
2
2
2
2
2
of acetylene hydrochlorination with the PSAC-N(0.8) catalyst
was varied from 120 to 3008C. As shown in Figure 1c, the con-
version of acetylene on PSAC-N(0.8) increases with the increase
in temperature. If the temperature is 2508C, the increase in
acetylene conversion is slow. Thus, a suitable reaction tempera-
ture for the PSAC-N(0.8) catalyst is 2508C and the acetylene
conversion achieved was 68%. Thus, PSAC-N appears to be an
appropriate catalyst for acetylene hydrochlorination. Even after
Table 2. Elemental contents of the as-obtained PSAC and PSAC-N(X)
samples.
Sample
Elemental content [at %]
N
C
Others
PSAC
94.40
93.99
93.97
89.14
88.59
0.73
1.61
2.05
4.93
5.04
4.87
4.40
3.98
5.93
6.37
PSAC-N(0.3)
PSAC-N(0.5)
PSAC-N(0.8)
PSAC-N(1.0)
1
60 h (Figure S1), the acetylene conversion on the PSAC-N(0.8)
À1
catalyst remained 58% [T=2508C, GHSV(C H )=120 h ,
2
2
V(HCl)/V(C H )=1.15].
2
2
In addition to the NC content, we studied the microstructure
and trace elements of PSAC. The BET results (Table 3) revealed
that the specific surface area and the pore volume of coconut
AC were larger than those of PSAC; the corresponding XRD
patterns and SEM and TEM images are shown in Figures S2
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for inactivation should be acetylene polymerization, which
gives rise to carbon deposition and pore blocking.
Table 3. Pore structure parameters of AC, PSAC, and PSAC-N.
Sample
BETsurface
area [m g
Total pore volume Adsorption average
2
À1
3
À1
]
[m g
]
pore width [ꢁ]
Adsorption behavior of the catalysts with various NC
structures for acetylene hydrochlorination
AC
PSAC
PSAC-N(0.3)
PSAC-N(0.5)
PSAC-N(0.8)
PSAC-N(0.8)
1776.3
1097.8
1006.0
941.4
935.7
872.2
1.16
0.55
0.55
0.51
0.53
0.49
26.1
19.9
21.7
21.8
22.5
22.6
The XPS analysis of the as-obtained samples was performed to
examine the carbon and nitrogen states; and the results are
presented in Figure 2. The distinct peak at a binding energy of
(after reaction)
248.4 eV was observed in the high-resolution spectrum of C1s.
PSAC-N(1.0)
928.6
0.51
21.9
It is apparent that binding energies of N-5 (nitrogen atom
bonded in 5-membered rings) and N-Q (quaternary N bonded
among three 6-membered rings) species are determined to be
[14–16]
and S3, respectively. However, under the reaction conditions
400.2 and 404.9 eV, respectively.
Notably, N-6v (quaternary
À1
[
T=1508C, GHSV(C H )=30 h , V(HCl)/V(C H )=1.15], acety-
nitrogen bonded between two 6-membered rings) was ob-
served at 398.6 eV for PSAC, which suggests that N-6v should
be one of the active sites for acetylene hydrochlorination.
To confirm the active site of NC in PSAC, the structures of
various nitrogen species in AC was built by using the Gaussian
model, and the results are presented in Scheme 1. According
to the XPS results, the model for PSAC-N is only for N-5, N-Q,
N-6v, and N-6 (pyridinic nitrogen, nitrogen atoms bonded in 6-
membered rings) structures. The DFT model for N-6V(7) was
also build to verify the N-6v model. The adsorption energies
were calculated, and the results are summarized in Table S3.
The hydrogen chloride adsorptions onto all kinds of NC struc-
tures were stable owing to the low adsorption energies. The
acetylene adsorption energies onto N-6v and N-6 species are
much lower than those onto N-5 and N-Q species, which
means that acetylene was apt for adsorption onto N-6v and N-
2
2
2
2
lene conversion over PSAC was higher than that over AC,
which means that PSAC demonstrated higher catalytic activity
than did AC. The conversion of acetylene and selectivity of
vinyl chloride over PSAC were 16.8 and 99.3%, respectively.
This phenomenon was mainly attributed to NC in PSAC
(0.7 wt% of nitrogen).
The X-ray fluorescence technique was used to analyze trace
metal elements in the catalysts, and the results are summar-
ized in Table S1. The main contents of PSAC include Ca, Fe Al,
Ba, Zn, K, Cu, and Na elements. We designed an experiment in
which CaCl , FeCl , AlCl , BaCl , ZnCl , KCl, CuCl , and NaCl were
2
2
3
2
2
2
added at a certain mass ratio (i.e., 0.5%) to PSAC; the as-ob-
tained samples were labeled as PSAC-Ca, PSAC-Fe, PSAC-Al,
PSAC-Ba, PSAC-Zn, PSAC-K, PSAC-Cu, and PSAC-Na, respective-
ly. The addition of trace elements was not beneficial to improv-
ing acetylene conversion (Fig-
ure S4). X-ray photoelectron
spectroscopy (XPS) analysis re-
vealed that the contents of Cu
in AC and PSAC are similar,
which means that Cu species in
the AC catalyst are not the
active sites.
Inactivation of PSAC-N
catalysts
After the completion of the acet-
ylene hydrochlorination reaction,
the surface area of the PSAC-
N(0.8)
catalyst
decreased
(Table 3). The end gas was ab-
sorbed by the 1-methyl-2-pyrroli-
dinone solution and the solution
then subjected to GC–MS for
composition analysis. The possi-
ble byproducts are dichloro-
ethane, vinyl acetylene, and di-
vinyl acetylene (Figure S5 and
Table S2). The decrease in the
BET surface area after the reac-
Figure 2. XPS patterns of PSAC and PSAC-N(0.8): a) C1s for PSAC, b) N1s for PSAC, c) C1s for PSAC-N(0.8), and
tion indicated that the reason d) N1s for PSAC-N(0.8).
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Figure 3. TPD results of PSAC and PSAC-N(0.8) with HCl and C
TCD=Thermal conductivity detector.
2 2
H .
time. The adsorption results reveal that hydrogen chloride and
acetylene are adsorbed in the same site and hydrogen chloride
has a lower adsorption energy. The only possible mechanism
here for both N-6v and N-6v(7) is that first hydrogen chloride
is adsorbed and then acetylene, which forms co-ads1 and co-
ads1’. The hydrogen chloride–acetylene complex forms the co-
ordination structure co-ads1 (or co-ads1’) with N-6v (or N-
6v(7)) (Scheme 1), and the reaction occurs as shown in
Figure 3. The structure of co-ads1 (or co-ads1’) can convert
into co-ads2 (or co-ads2’) via transition states TS1
À1
À1
À1
(
41.0 kJmol ) (or TS1’: 23.4 kJmol ) and TS2 (166.0 kJmol )
À1
(
or TS1’: 236.2 kJmol ), which has a 6-membered ring struc-
ture. Finally, vinyl chloride desorbs from PSAC-N. The results
presented in Table S3 and Figure 4 revealed that the N-6v and
N-6 V(7) models follow the same route in adsorption and reac-
tion, which indicates that the calculation is applicable.
Scheme 1. a) Structures of N atoms in PSAC. b,c) Adsorption course of acety-
lene hydrochlorination: three benzene ring unit model N-6V (b) and seven
benzene ring unit model N-6V(7) (c). *: Cl, *: C, *: N, and *: H atoms.
Conclusions
A highly active nitrogen-doped pitch-based spherical activated
carbon catalyst (PSAC-N) is prepared through a nitrogen-
doping treatment. The optimal reaction conditions are as fol-
À1
6
species. Notably, the vinyl chloride adsorption energy is
higher than the acetylene adsorption energy onto N-6v (or N-
v(7)) whereas the vinyl chloride adsorption energy is lower
lows: T=2508C, gas hourly space velocity(C H )=120 h , and
feed volume ratio V(HCl)/V(C H )=1.15. Under these condi-
2
2
2
2
6
tions, the corresponding conversion of acetylene is more than
68%. The results reveal that PSAC-N could be a highly active
catalyst for acetylene hydrochlorination. The BET, X-ray fluores-
cence, X-ray photoelectron spectroscopy, GC–MS, and Gaussian
calculation results indicate that the nitrogen position has a dis-
tinct effect on the rate of acetylene hydrochlorination. The
analysis of products and the catalyst before and after the reac-
tion shows that the reason of inactivation is acetylene poly-
merization. Because hydrogen chloride and acetylene are ad-
sorbed in the same site and hydrogen chloride adsorption
energy is lower, first hydrogen chloride must be adsorbed and
then acetylene to form co-ads1. The reaction energy of acety-
lene hydrochlorination over PSAC-N (N-6V(7) structure) is
than the acetylene adsorption energy onto N-6 (or N-6v(7)).
The temperature-programmed desorption (TPD) results for the
PSAC and PSAC-N(0.8) catalysts with hydrogen chloride and
acetylene (Figure 3) suggested that the adsorption capacity in-
creased after nitrogen doping, and these results agree well
with those of the calculation model.
Theoretically, the reaction materials must be adsorbed
before the chemical reaction can proceed. To understand the
adsorption mechanism of hydrogen chloride and acetylene
over the PSAC catalyst for acetylene hydrochlorination, a sche-
matic illustration is proposed in Scheme 1. The three possible
processes are as follows: 1) hydrogen chloride is first adsorbed
onto PSAC and then on acetylene; 2) acetylene is first ad-
sorbed onto PSAC and then on hydrogen chloride; and 3) both
hydrogen chloride and acetylene are adsorbed at the same
À1
236.2 kJmol . All results from DFT calculations indicate that
acetylene conversion may be increased by increasing the
amount of N-6V structures on the PSAC-N catalyst.
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containing sodium hydroxide solution and then subjected to GC
[7,8]
(Shimadzu GC-2014C) for analysis.
All DFT calculations were performed with the Gaussian 09 program
[20]
package. No geometric constraints were assumed in the geome-
try optimization. The standard 6-311G+ +** basis set was used for
hydrogen, carbon, nitrogen, and chlorine atoms. Atomic charges
were calculated by using the Mulliken-type charges. The frequen-
cies of all the geometries were calculated at the same level to
identify the nature of the stationary points and obtain the zero-
point-energy corrections. All stationary points were characterized
as minima (no imaginary frequencies) or transition state (one imag-
[21,22]
inary frequency) by using Hessian calculations.
Intrinsic reac-
tion coordinate calculations were used to determine that each
transition state links the correct product with the reactant.
The specific surface areas and pore distribution of the catalysts
were analyzed with an ASAP 2020C surface area and porosity ana-
lyzer (Micromeritics Instrument Corporation, USA). Degassing was
performed for 6 h at 423 K and then the as-obtained samples ana-
lyzed by using liquid nitrogen adsorption–desorption at 77 K. The
elemental contents of the catalyst were measured with a Vario EL
III element analyzer (Germany). The XPS analysis was performed
with a Kratos AMICUS spectrometer equipped with a standard AlKa
X-ray source (300 W) and a pass energy analyzer (20 eV). The X-ray
fluorescence analysis was performed with a PANalytical’s Venus 200
spectrometer. TPD was also analyzed with an AutoChem 2720 in-
strument (Micromeritics Instrument Corporation, USA). The samples
were pretreated in hydrogen chloride or acetylene atmosphere at
the reaction temperature (1808C) for 4 h and then high-purity ni-
À1
trogen (50 mLmin ) was passed through the sample at 508C for
3
1
0 min. The sample was heated from 50 to 6508C (heating rate:
0 Kmin ) to collect the data.
À1
2 2
Figure 4. DFT-calculated surface energies for C H hydrochlorination:
a) three benzene ring unit model N-6V; b) seven benzene ring unit model
N-6V(7). *: Cl, *: C, *: N, and *: H atoms.
Acknowledgements
This work was supported by the National Basic Research Pro-
gram of China (973 Program, no. 2012CB720302) and the Pro-
gram for Changjiang Scholars and Innovative Research Team in
University (no. IRT1161).
Experimental Section
PSAC (20–40 mesh) was washed with dilute aqueous hydrogen
À1
chloride (1 mol·L ) at 708C for 5 h to remove Na, Fe, and Cu con-
taminants, which were poisons for the hydrochlorination reac-
Keywords: activated carbon
hydrochlorination · nitrogen · reaction mechanisms
· heterogeneous catalysis ·
[
8,17]
tion.
PSAC and melamine were fully mixed at weight ratios of
1
:x (x=0.3, 0.5, 0.8, 1.0), dried at 2008C for 10 h, and then sintered
[15,18]
at 9008C for 30 min.
Finally, the mixtures were boiled for 1 h
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À1
water and air from the system. Acetylene (10 mLmin ) and hydro-
À1
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2
À1
(
C H ) of 120 h . The pressure of the reactants was in the range of
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[
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.11–0.12 MPa; the pressure was selected both for safety reasons
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Received: February 6, 2014
Revised: March 4, 2014
2009, 47, 2032–2039.
Published online on July 23, 2014
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2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 2339 – 2344 2344