Catalytic properties of Pd/HY catalysts modified with NH4F for acetylene hydrochlorination

Article abstract of DOI:10.1016/j.catcom.2015.02.010

Ammonium fluoride (NH₄F) modified HY zeolite catalysts were prepared by ultrasonic-assisted impregnation and their efficient catalytic behavior in acetylene hydrochlorination reaction was investigated (the conversion of acetylene and the selectivity to vinyl chloride were all achieved above 99%). The results indicate that the presence of NH₄F in Pd/HY catalysts can partially enhance the surface acidity of catalysts, and weaken the occurrence of carbon deposition and the loss of Pd active component on the catalyst surface, thus improving the catalytic performance of catalysts.

Full text of DOI:10.1016/j.catcom.2015.02.010

Catalytic Properties of Pd/HY Catalysts modified with NH4F for Acetylene
Hydrochlorination
Lu Wang, Feng Wang, Jide Wang
PII:
DOI:
Reference:
S1566-7367(15)00069-2
doi: 10.1016/j.catcom.2015.02.010
CATCOM 4227
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
22 December 2014
6 February 2015
7 February 2015
Please cite this article as: Lu Wang, Feng Wang, Jide Wang, Catalytic Properties of
Pd/HY Catalysts modified with NH4F for Acetylene Hydrochlorination, Catalysis Com-
munications (2015), doi: 10.1016/j.catcom.2015.02.010
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ACCEPTED MANUSCRIPT
Catalytic Properties of Pd/HY Catalysts modified with NH F for
4
Acetylene Hydrochlorination
a
a
a
Lu Wang , Feng Wang * , Jide Wang
a
Key Laboratory of Oil and Gas Fine Chemicals, Ministry of Education
and Xinjiang Uyghur Autonomous Region, College of Chemistry and
Chemical Engineering, Xinjiang University, Urumqi, Xinjiang 830046,
People’s Republic of China.
*
Corresponding author. Tel.: +86-0991-8586079.
Fax: +86-0991-8582809.
E-mail: wangfeng62@126.com.
1

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Abstract
Ammonium fluoride (NH F) modified HY zeolite catalysts were
4
prepared by ultrasonic-assisted impregnation and their efficient catalytic
behavior in acetylene hydrochlorination reaction was investigated (the
conversion of acetylene and the selectivity to vinyl chloride were all
achieved above 99 %). The results indicate that the presence of the NH4F
to Pd/HY catalysts can partially enhance the surface acidity of catalysts,
and weaken the occurrence of carbon deposition and the loss of Pd active
component on the catalyst surface, thus improving the catalytic
performance of catalysts.
Keywords : Pd catalysis; HY zeolite; Ammonium fluoride; Acetylene
hydrochlorination;
2

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1
. Introduction
Vinyl chloride monomer (VCM) is the main monomer to
synthesized poly-vinyl chloride (PVC), which is one of the top five
engineering plastics widely used worldwide [1]. Moreover, the PVC
demand will be more than 40 million tons in 2016 [2] and about 70 % of
the total PVC production through the acetylene-based process in China
[
3]. However, the conventional mercury chloride catalyst used for
acetylene hydrochlorination is highly toxic and causes severe safety and
environmental problems [4]. Meanwhile, the short supply of mercury
resources also cannot be ignored. Therefore, exploring the non-mercury
catalysts in acetylene hydrochlorination reaction remains highly
imperative.
Many metal (including Au, Pd, Pt , Ir, Ru, Bi, Cu. etc.) chlorides
[
5-11], displaying the considerable activity in acetylene hydrochlorination
reaction, have been reported in the literature these years. The second
metallic compounds involving La, Bi, Ni, Co. etc. [12-15] have been
added in gold catalysts for improving the catalytic activity and inhibiting
the deactivation of catalysts. However, even though carbon-supported
non-mercury catalysts offer high initial activity, the reduction of metals,
the surface carbon deposition, the pulverization of solid catalyst, the loss
of active components remain the major reasons for the deactivation of
catalysts [5-14]. Hence, many other supports (SiO , TiO , Al O , CNTs.
2
2
2
3
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etc.) have attracted much attention for acetylene hydrochlorination
recently [15-17]. For example, studied Au-Bi/γ-Al O catalyst and
2
3
concluded that the Au-base catalyst supported on the γ-Al O has the
2
3
advantage of stronger mechanical strength and regeneration capacity over
the traditional Au-base catalyst supported on activity carbon [15]. This
was promoted us to consider whether a zeolite material used as the
support could be applied for acetylene hydrochlorination. In addition,
zeolites with their unique properties (including high surface area,
well-defined micro-porosity, high thermal stability and intrinsic acidity)
was used for cumene cracking [18-19] and oxidation carbonylation
[
20-21]. However, there was few literatures reported systematically the
application of Pd-based catalyst supported on zeolites in acetylene
hydrochlorination reaction.
In this paper, we reported a simple and efficient method for
preparation zeolite-supported the Pd/ NH F-HY catalysts, which can be
4
sufficiently applied for acetylene hydrochlorination. Characterization
results were indicated that Pd/HY catalysts modified NH F can enhance
4
the surface acidity of catalysts, and weaken the occurrence of coke
deposition and the loss of Pd active component on the catalyst surface.
2
2
Experimental
.1 Catalyst preparation
The initial sample of HY zeolites (purchased by Nankai University,
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Si/Al =9, 4.0 g) were added into the NH F aqueous solution under stirring,
4
so as to modulate the NH F solutions of various concentration (1.0 %,
4
5
.0 %, 10.0 %, 20.0 %). Having been at ambient temperature, the HY
zeolite was treated with the NH F solution with continuous stirring for 4
4
h and washed with distiller water to pH about 7. At last, water was
evaporated at 80 °C for 10 h and the NH F-HY support was obtained.
4
The Pd/ NH F-HY catalyst was prepared by ultrasonic-assisted
4
impregnation using PdCl aqueous solutions, as described our previous
2
work [22]. The loading of Pd was kept constant at 0.9 wt%. In terms of
the concentration of the NH F solutions, the obtained NH F-modified
4
4
Pd/HY catalysts were named Pd/ 1NH F-HY, Pd/ 5NH F-HY, Pd/
4
4
1
0NH F-HY, Pd/ 20NH F-HY, respectively.
4
4
2
.2 Catalyst Characterization
X-ray diffraction (XRD) data were acquired using a M18XHF22-SRA
diffractometer operating at 50 kV, 100 m A with Cu-K irradiation in the
scan range of 2 between 10 ° ~ 80 °. Scanning electron microscopy
(SEM) and energy dispersive X-ray (EDX) analyses were performed
using a LEO1450VP detector to determine the morphology and the
chemical composition of the samples. Palladium content was detected
using an inductively coupled plasma (ICP-6300) instrument.
Transmission electron microscopy (TEM) was estimated with a H-600-II
electron microscope from Hitachi Co. LTD at an accelerating voltage of
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2
00 KV. BET surface areas and pore analyses were measured by JW-BK
Brunauer-Emmett-Teller (BET) equipment（the samples were dried at
0 °C for 10 h）using nitrogen adsorption/desorption isotherm at -196 °C.
A thermogravimetric (TG) analysis was performed by a NETZSCH SAT
49F3 multifunctional thermal analyzer. Fourier transform infrared
8
4
spectrometer (FTIR) was used for the determination of functional groups
in samples (dispersed in potassium bromide and compressed into pellets),
using EQUINOX-55 ( Bruker Company, Germany) in transmittance (%)
−1
mode in the range 4000 ~ 400 cm . Temperature programmed
decomposition (TPD) was determined using a TP-5080 adsorption
instrument with ammonia over a temperature ramp of 0 ~ 900 °C, rate
-
1
-1
ramp of 10 °C min and flow of 100 mL min .
2
.3 Catalytic performance tests
The catalytic performance tests were evaluated in a fixed bed with
0-mm-diameter a quartz tube micro reactor operating just at normal
1
-
1
pressure. N flow (20 mL min ) via calibrated mass flow controllers
2
(
Sevenstar Huachuang electronics. Co. ltd, Beijing, China) in a heated
-
1
glass reactor and then hydrochloride at a flow rate of 20 mL min passed
through the reactor to activate the catalyst. After the reactor was heated to
-
1
-1
1
60 °C, hydrochloride (12.6 mL min ) and acetylene (10.1 mL min )
were fed through a mixing vessel containing catalyst (4.0 g), giving a
-
1
total gas hourly space velocity (GHSV) of 110 h . The exit gas mixture
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from the reactor was passed through an absorption bottle filled with
sodium hydroxide solution and then set into a gas chromatography (GC
2
010, Shimadzu) to analyzed the conversion of acetylene (X ) and the
A
selectivity of the selectivity of VCM (S ) immediately.
VC
3
Results and Discussion
Fig. 1 shows that the X-ray diffraction patterns (XRD) of the Pd/
NH F-HY and the Pd/ 5NH F-HY, which have a well crystallinity
1
4
4
similar to the HY zeolite. However, some changes of the peak had been
obviously found with the increase of the concentration of the NH4F
solutions. In the case of the Pd/ 10NH F-HY catalyst and the Pd/
4
2
0NH F-HY catalyst, the XRD peaks became broad and the intensity of
4
peaks were accordingly reduced, which indicated that some decrease in
crystallinity was noticeable. From a structural point of view,
dealumination of the HY zeolite with various concentration of the NH4F
solutions and the interaction between fluorine and aluminum might be
mentionable reasons [19]. Especially for the Pd/ 10NH F-HY catalyst, a
4
small amount of new crystal was observed close at 2θ = 15.9 °and 30.8 °,
which was strong signals of aluminum hydroxide fluoride AlF (OH)
1.96
1.04
connected with fluorination of NH F [23]. Apart from the diffraction
4
peaks of the support, no diffraction peaks of Pd species were detected in
the fresh and used catalysts, in spite of great efforts (greater Pd
concentration included) to detect them in the catalyst. This indicated that
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the Pd particles sizes were small [24] or the Pd active component was
highly dispersed on the catalyst surface [16-17].
BET results of HY supports are listed in Table 1. After modifying by
NH F, the BET surface areas of HY supports were reduced gradually as
4
the NH F doping increased. While the average pore diameters were
4
increased from 1.03 nm to 2.49 nm and the total pore volume changed a
little. The micromorphology of the HY and the 10NH F-HY supports are
4
shown in Fig. 2(a) and Fig. 2(b), indicating that the presence of NH4F
changed the surface morphology of the HY support. For the pure HY
zeolite, it consisted of typical octahedral structure with a relative smooth
surface morphology. While after the NH F treatment，the HY zeolite with
4
some etched surface and defects were obtained with bigger size about 3.5
~
4 μm. The TEM images also revealed better dispersed Pd species,
which was in good agreement with the XRD results, on the
Pd/10NH F-HY catalyst with diameters 4-12 nm in Fig. 2(c) and(d).
4
Element composition of catalysts are recorded by EDX and ICP in Table
2
. Large quantitative Si, Al, and O belonged to the HY support, and N and
F were well detected related with the NH F modification. The total Pd
4
content in the used catalyst is much smaller than that in the fresh. For the
Pd/HY catalyst, the loss ratio of Pd is 71.24% while for the
Pd/10NH F-HY catalyst the loss ratio is 49.34%. It is suggested that the
4
Pd catalyst modified with NH F has better catalytic performance owing to
4
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decreasing the loss of Pd active component. In addition, the relative C
content increased 9.86%, 8.40% for the used Pd/HY and Pd/ 10NH F-HY
4
catalysts respectively, which suggested that the carbon deposition was
produced during the reaction [15-17]. The decreasing of Al, N and F
content to some extent may be a considerable reason for the deactivation
of the Pd/NH F-HY catalysts.
4
The catalytic performance of Pd/HY and the Pd/ 10NH F-HY
4
catalysts are shown in Fig. 3. The untreated Pd/HY catalyst exhibited not
only severe deactivation but also low stability catalytic performance in
reaction, including the rapid deactivation and the decreased the acetylene
conversion from 98.67 % to 19.27 % after 160 min. It was suggested that
the HY zeolite modified with the NH F of 10 % was the best optional
4
support to enhance the catalytic activity of Pd-based catalysts for
acetylene hydrochlorination, with about 99.92 % acetylene conversion
and over 99 % selectivity to VCM about 400 min. This results indicated
the Pd/ 10NH F-HY catalysts displayed a slower deactivation rate than
4
the Pd/HY catalyst, which provided a basis for the development of a
Pd/HY catalyst for acetylene hydrochlorination.
Fig. 4(a) shows the FT-IR spectra of HY, the Pd/HY and the Pd/
1
0NH F-HY catalysts. Structural hydroxyl groups were the Si-OH group,
4
-
1
the bridging OH and the extra large cavity OH group at ~ 3740 cm , ~
-
1
-1
3
650 cm and ~ 3550 cm in the Y zeolite, respectively [18]. The spectra
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of the Pd/HY fluorinated NH F indicated a considerable decrease in the
4
-
1
number of the extra cage hydroxyl groups (~ 3550 cm ), which were the
most pronounced for the Pd/ 10NH F-HY catalysts, were effected more
4
-
1
-1
than the Si-OH (~ 3740 cm ) and the bridging OH groups (~ 3650 cm ).
The changes might be attributed to some aluminum was removed from
the zeolite during fluorination and some oxygen-containing and hydroxyl
groups in the zeolite were replaced by fluoride anions attached to the
zeolite lattice [19]. The asymmetric stretching of the T-O-T bridges
(where T are tetra-coordinated Si or Al atoms) [25] at about 1049 and 815
-
1
cm for the Pd/HY catalyst slightly shifted to higher wavenumber for the
-
1
-1
Pd/ 10NH F-HY catalyst (1049 ~ 1086 cm , 815 ~ 945 cm ,
4
respectively). Meanwhile, the asymmetric stretching of the T-O bridges at
-
1
about 455 cm towards higher wavenumber for the fluorinated samples.
It was well verified the dealumination of the zeolite in the NH F-
4
modified catalysts, which was consistent with what Lok reported [26].
From this perspective, we supposed that the occurrence of the inductive
of fluorine on the structural OH groups and the dealumination of the
zeolite might affect the surface acidity of zeolites, resulting in different
catalytic performance of the Pd/ 10NH F-HY catalyst.
4
The NH -TPD images in Fig. 4(b) can provide further evidence to
3
analyze the changes of acidic properties on the surface of the Pd/ 10
NH F-HY catalysts. There are three ammonia desorption peaks at ~
4
10

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1
00 °C, 473 °C and 810 °C observed the original HY zeolite, which
account for weak acid site, media strong acid site and strong acid site,
respectively. After fluorination with NH F, the NH desorption peak
4
3
intensity at ~ 100 °C and 350 - 500 °C, decreased and shifted to the
higher temperature. While the position of the high-temperature peak at
7
00 ~ 850 °C increased significantly and shifted to the lower temperature.
In other words, the Pd/HY catalyst with NH F modification decreased
4
both the strength and density of the HY weak and media strong acid sites,
but increased that of the HY strong acid sites. This results was different
from the literature reported [26]. Based on the catalyst performance
evaluation in Fig. 3 and FT-IR results in Fig. 4(a), we propose that the
dealumination of zeolites by NH F reagents effected the acidity of the
4
Pd/HY catalyst. Similarly, the strength and density of acid sites in the Pd/
1
0NH F-HY catalysts were also acid-affected because of the NH F
4
4
modification, which was the main reason that effected the catalytic
performance and enhanced the activity of the Pd/ 10NH4F-HY catalyst.
Fig. 5 displays the TGA curves of fresh and used Pd-based catalysts
to estimate the carbon deposition on the catalyst surface. The fresh and
used Pd-based catalysts have a visible loss of mass before 150 °C,
indicating minor water adsorption on the surface of catalysts. In the range
of 150-450 °C, the fresh Pd/HY (3.85%) and the fresh Pd /10NH F-HY
4
(4.37%) catalysts shows small weight loss. This is associated with the
11

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loss of frame water and the breakdown of active components linked the
Si-O-Si bond. When the temperature above 400 °C, the weight of used
Pd/HY (11.70%) and Pd /10NH F-HY HY (9.52%) catalysts decreased
4
rapidly due to oxidation of the carbon deposit. According to the literature
[
6,12-13], the actual amount of carbon deposited on the Pd /10NH F-HY
4
HY catalyst can be calculated as 5.15%, which is about 34.39% less than
that in the used Pd/HY catalyst (7.85%). This indicates that the
modification of the NH F to Pd/HY catalysts can reduce the formation of
4
the carbon deposit to some extent, thus improving the catalytic
performance of catalysts.
4
Conclusions
In summary, we demonstrate the Pd/ NH F-HY catalyst can be
4
effective to apply for acetylene hydrochlorination. A improved catalytic
performance is obtained via modification of the Pd/HY catalyst with a
proper concentration of the NH F solutions by ultrasonic-assisted
4
impregnation. The results were indicated that Pd/HY catalysts modified
NH F can enhance the surface acidity of catalysts, and weaken the
4
occurrence of coke deposition and the loss of Pd active component on the
catalyst surface, lead to improving the catalytic performance of catalysts.
12

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Acknowledgments
This work has been supported by National Natural Science
Foundation of China (Grant No. 21263025), Graduate Research and
Innovation Program of Xinjiang (Grant No. XJGRI2014023).
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Figure Captions
Fig. 1 XRD patterns of the HY zeolite, the Pd/HY catalyst and the Pd/
NH F-HY catalysts.
4
Fig. 2 Morphological characterization of the samples: the SEM images of
the HY zeolite (a) and the 10NH F-HY support (b); the TEM
4
images of the Pd/HY catalyst (c) and the Pd/ 10NH F-HY
4
catalyst(d).
Fig. 3 The C H conversion(X ) and the selectivity to VCM (S ) over
2
2
A
VC
Pd-based catalysts; Reaction condition: Temperature = 160 °C,
-1
feed volume ratio V :V
= 1.25, GHSV = 110 h .
HCl
C2H2
Fig. 4 FTIR (a) and NH -TPD images (b) of the HY zeolite and the
3
Pd-based catalysts.
Fig. 5 TGA curves of the fresh and used Pd-based catalysts.
Table Captions
Table 1 Specific structure parameters of HY supports.
Table 2 Element composition of the samples record by EDX, and the
total Pd content determined by ICP.
16

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Fig. 1
17

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Fig. 2
18

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Fig. 3
19

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Fig. 4
20

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Fig. 5
21

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Table 1 Specific structure parameters of HY supports.
2
−1
3
−1
Samples
HY
SBET (m g )
505.23
V(cm g )
0.11
D(nm)
1.03
1.10
1.12
1.43
2.49
1
5
NH F-HY
442.92
0.10
4
NH F-HY
435.67
0.10
4
1
2
0NH F-HY
345.68
0.12
4
0NH F-HY
117.28
0.12
4
SBET: surface area; V: total pore volume; D: average pore diameter.
22

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Table 2 Element composition of the samples record by EDX, and the
total Pd content determined by ICP.
Samples
C/
Si/
Al/
O/
Cl/
F/
N/
ωt%
0
ωt%
ωt%
ωt%
ωt% ωt%
ωt%
0
Fresh Pd/HY
Used Pd/HY
33.37 6.78
25.27 5.74
31.23 5.05
28.54 3.61
57.54 2.31
55.12 4.01
0
0
9.86
0
0
Fresh Pd/10NH F-HY
45.03 3.11 9.76
43.46 6.21 7.60
5.82
2.18
4
Used Pd/10NH F-HY 8.40
4
Total Pd (wt %)
Loss ratio of Pd
(%)
Catalysts
Pd/HY
Fresh
Used
0.591
0.679
0.170
0.344
71.24
49.34
Pd/10NH F -HY
4
23

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Graphical Abstract
24

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Highlights
●
The Pd/NH F-HY catalyst was first applied for acetylene
4
hydrochlorination.
●
●
The catalyst exhibited efficient catalytic activity in reaction.
Acetylene conversion and VCM selectivity were all achieved above
9
9%.
●
The presence of the NH F played a significant role for acetylene
4
hydrochlorination.
25