Non-mercury catalytic acetylene hydrochlorination over a NH4F-urea-modified Pd/HY catalyst for vinyl chloride monomer production

Article abstract of DOI:10.1039/c5nj03338a

A Pd/HY zeolite catalyst modified with ammonium fluoride and urea (Pd/NH₄F-urea-HY) was efficiently applied in an acetylene hydrochlorination reaction. It exhibited an enhanced catalytic performance compared to the untreated Pd/HY catalyst, which was attributed to the presence of ammonium fluoride and urea partly inhibiting carbon deposition and Pd²⁺ reduction.

Full text of DOI:10.1039/c5nj03338a

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Non-mercury catalytic acetylene hydrochlorination over the NH4F-
Urea-modified Pd/HY catalyst for vinyl chloride monomer
production
Received 00th Janu ary 20xx,
Accepted 00th January 20xx
Lu Wang,a Feng Wangb and Jide Wangc
DOI: 10.1039/x0xx00000x
www.rsc.org/
Pd/HY zeolite catalyst modified by ammonium fluoride and urea have been applied as catalyst supports for various catalytic
(Pd/NH4F-Urea-HY) was efficient applied in acetylene reactions because of their unique properties, such as high surface
hydrochlorination reaction and had an enhanced catalytic area, well-defined microporosity, high thermal stability and
performance compared to untreated Pd/HY catalyst, which was controlled density of acid sites6e. Compared with SiO2, TiO2
attributed to the presence of ammonium fluoride and urea partly supports4a in the acetylene hydrochlorination reaction, the initial
inhibited carbon deposition and Pd2+ reduction.
activity and selectivity of zeolites catalysts were higher but they
were still easily deactivated for the carbon deposition and the Pd
species loss5d. In our work, we have reported the Pd/HY catalyst
could be applied in acetylene hydrochlorination reaction and the
appropriate modification could affect the activities of catalysts, but
the changes of Pd states during the reaction were still not
discussed7. These results have promoted us to study the states of
Pd species in depths in the Pd/HY catalyst for acetylene
hydrochlorination.
Meanwhile, many nitrogen or phosphorus -doped Au/AC
catalysts have been reported recently and N,P-doped carbon
supports could activate acetylene and HCl molecules, which was
believed to stabilize metal components5a-5c. Recently, non-metallic
catalysts including carbon nitride have been developed as new
catalytic systems for acetylene hydrochlorination5b,6c. Despite the
significant improvements for this transformation, it remained a
great challenge to develop a new, active and stable catalytic system
for acetylene hydrochlorination. So in this work, an improved Pd/HY
zeolite catalyst modified byammonium fluoride and urea had been
synthesized by ultrasonic wave-assisted techniques, and its catalytic
activity for acetylene hydrochlorination had been studied compared
to untreated the Pd/HY catalyst. Finally, the effects of ammonium
fluoride and urea modification on the catalyst performance were
also investigated.
The catalytic performances of the Pd/HY and Pd/NH4F-Urea-HY
catalysts were shown in Fig. 1. The acetylene conversion of the
Pd/HY catalyst decreased from 98% to 19% after running for 2.5 h,
indicating that Pd/HY was rapidly deactivated under this reaction
condition. As a comparison, the Pd/NH4F-Urea-HY catalyst showed a
better stability (the acetylene conversion was over 99% in running
for 8 h and decreased 33% after 12 h), which was also much better
than single NH4F or urea modified Pd/HY catalysts (from Fig.S1).
These facts revealed that the Pd/NH4F-Urea-HY catalyst had much
slower deactivation rate than that of the Pd/HY catalyst, and both
Vinyl chloride monomer (VCM) is primarilymanufactured from
acetylene hydrochlorination by the mercuric chloride (HgCl 2)
catalysts, which are highly toxic and cause serious environmental
problems1. In addition, mercury resources are in short supply and
the application of mercuric chloride will be forbidden over the
world in the future2. Hence, it is imperative to explore efficient non-
mercuric catalysts for the acetylene hydrochlorination reaction.
At present, More than 30 non-mercuric metal chlorides
(including AuIII, PtII, PdII, RhIII, RuIII, BiIII, CuII, etc.) supported carbon
have been carried out for acetylene hydrochlorination and AuCl 3
may be the best for this process 3. Up to now, most researches have
demonstrated that metal chloride catalysts were often encountered
with rapid deactivation problems because that some metallic
cations were readily reduced to zero-valent metal by acetylene
during the reaction4-6. But for Pd-based catalysts, which also could
obtain the good catalytic activity in acetylene hydrochlorination,
the valence states of Pd species and their changes during the
reaction were still not clear5d,7. In addition, the weak interaction
between active species and supports was also resulted in loss of
activities of catalysts4a,4b,4e,4f. To address the problem, many other
(SiO2, TiO2, Al2O3, CNTs, etc.) supports have been investigated for
acetylene hydrochlorination in recent years besides activated
carbon5b,6a-6d,which is the most common support in reaction and
easily crushed and pulverized under reaction conditions5d. For
example, the Au -Bi/γ-Al 2O3 catalyst was reported and it had the
stronger mechanical strength and regeneration capacity than those
of the traditional Au-based/active carbon catalyst6d. These previous
work mentioned above inspired us to consider that the support
might playa notable role in the overall reaction. Nowadays, zeolites
a. Key Laborato ry of Oil  Gas Fine Chemicals Min istry of Education  Xinjia ng
Uyghur Autonomous Region, X injiang University, Urumqi 830046, X injiang , Ch ina
E-mail: wangfeng62@126.com; Fax: +86 0991 85828089; Tel: +86 0991 85828089
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the catalytic activity and stability of the Pd/ NH4F-Urea-HY catalyst Pd/NH4F-Urea-HY catalyst. It was likely that Pd2+ was reduced into
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0
DOI: 10.1039/C5NJ03338A
Pd under the reaction conditions, resulting in the deactivation of
were enhanced in the acetylene hydrochlorination reaction.
From XRD patterns (Fig. 2), the characteristic peaks at 6.3°, Pd-based catalysts. Comparing with the Pd/HY catalyst, the binding
15.9° and 24.0° were known to the reflection of the (111), (133)and energy of Pd 3d5/2 of the fresh Pd/NH4F-Urea-HY catalyst showed
(266) planes of Y-zeolite lattices8 and the NH4F-Urea-HY support the obvious negative shift (about 0.8 eV) and the interaction
kept few changes about the diffraction peaks. Moreover, the Pd/HY between Pd and NH4F-Urea-HY support was a considerable reason.
catalysts were better crystallized than the Pd/NH4F-Urea-HY In combination with the catalytic performance of catalysts (Fig.1), it
catalysts. Apart from the diffraction peaks of the zeolite matrix, no was reasonable to conclude that the enhanced catalytic
visible characteristic peaks of Pd species were identified in Pd-based performance in the Pd/NH4F-Urea-HY catalyst was attributed to the
catalysts, regardless of great efforts (adding greater Pd presence of nitrogen and fluorine (from Fig. S3), which affected the
concentration) were done, indicating that high particle interaction between Pd and NH4F-Urea-HY support, increased the
dispersion4b,5d or Pd species were supposed to be of a small particle electron density of Pd2+ and inhibited the Pd2+ reduction. Besides
size9. According to TEM results (from Fig. S2), It was found that the the surface characterization of XPSanalysis, the actual Pd content in
Pd dispersion was changed by NH4F and urea modification and the the fresh and used Pd -based catalysts were also measured byICP in
estimated size of detected Pd particles was about 5-20 nm in Table 1. It was obviously indicated that the loss ratio of Pd in the
Pd/NH4F-Urea-HY catalyst.
Pd/HY catalyst was 47.4% and that in the Pd/NH4F-Urea-HY catalyst
was 44.7% in the reaction process. It was suggested that Pd loss
remained one reason for catalyst deactivation and the Pd catalyst
with the NH4F-Urea-modified HY support could inhibit Pd loss.
Fig.1 The C2H2 conversion (a) and VCM selectivity (b) over Pd
based catalysts; Reaction condition: Temperature = 160 °C, feed
volume ratio VHCl:VC2H2= 1.25, C2H2 GHSV = 110 h-1.
Fig.3 XPS Pd 3d profiles of the Pd-based catalysts
Table 1 The relative content of Pd species in the fresh and used
catalysts determined by XPS, and the actual Pd content determined
byICP.
Catalysts
Pd0(area%)
Pd2+(area%)
Fresh Pd/HY
57.5
73.1
50.6
62.4
42.5
26.9
49.4
37.6
Used Pd/HY
Fig.2 The XRDimages of Pd-based catalysts.
The XPS region spectra of Pd 3d in the fresh and used Pd -based
catalysts were measured to further investigate the valence state
and relative amount of the active Pd species in Fig. 3. It should be
noted that all of the Pd (3d) signals had been divided into two
components responding to metallic Pd0 and Pd2+ species10. Curve
fitting were employed to analyze the ratio of Pd0 and Pd 2+ species
and the results were calculated in Table 1. According to the data,
the relative content of Pd 2+ was 42.5% presented in the fresh Pd/HY
catalyst,and the relative content of Pd 2+ was 49.4% observed in the
fresh Pd/NH4F-Urea-HY catalyst. However, the relative content of
Pd2+ was 26.9% for the used Pd/HY and 37.6% for the used
Fresh Pd/NH4F-Urea-HY
Used Pd/NH4F-Urea-HY
Total Pd (wt%)
Loss ratio of Pd
(%)
Catalysts
Fresh
Used
Pd/HY
0.95
1.23
0.50
0.68
47.4
44.7
Pd/NH4F-Urea-HY
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SEM results (from Fig. S4) indicated that the presence of NH4F profiles of Pd-based catalysts. From Fig. 5a, i t can beVoiebwseArrtivceledOtnhlinaet
and urea did not changed the surface morphology of the HY there was no significant band position shifDt,OIw: 1h0i.c1h03i9n/dCi5caNtJe0d33t3h8aAt
support and the agglomeration was occurred in used Pd NH4F-Urea- the zeolite framework remained intact after NH4F and Urea
HY catalyst. BET results (Table 2) showed that BET surface areas of modification. This result supported the earlier dissection on XRD.
the fresh Pd/HY and Pd/NH4F-Urea-HY catalysts were decreased Compared with the fresh Pd/HY catalyst, the intensities of the
and the average pore diameter of them were increased due to the hydroxyl groups (including Si-OH, Al-OH and H-bonded hydroxyl
blocking of pores of Pd addition. After reaction, a significant loss of groups) at 4000 to 3000 cm-1 were reduced in the fresh Pd/NH4F-
surface areas of Pd/HY (from 333 to 21 m2/g) and Pd/NH4F-Urea-HY Urea-HY catalyst, due to Pd cations interacting with hydroxyl groups
(from 255 to 39 m2/g) might be ascribed to carbon deposition4e,4f
which were certified by TGA results. It can be seen from Fig.4,
,
of NH4F-Urea-HY support11a-11e. After reaction, the band at 3450 cm-
was disappeared due to the formation of carbon deposition
1
neither the fresh nor used Pd-based catalysts had a visible loss of coating the effective acidic sites on the catalyst surface, resulting in
mass before 150 °C , indicating minor water adsorption on the decreasing the performance of catalyst. NH3-TPD images in Fig. 5b
surface of catalysts. In the range of 150-450 °C, the fresh Pd/HY could further provide evidence to analyze changes of acidic
showed a slow weight loss, reaching 3.85%. While the fresh properties on Pd-based catalysts. Two desorption peaks of
Pd/NH4F-Urea-HY had a significant weight loss (21.10%), which was ammonia at low (~100 °C ) and moderate temperatures (350~500 °C )
associated with the loss of frame water and the breakdown of were ascribed to weak acid sites and media strong acid sites,
active components linked the Si-O-Si bond. When the temperature belonged to the original HY zeolites 11d. After modification by NH4F
above 450 °C, the weight of used Pd/HY (11.70%) and Pd/NH4F- and Urea, the range of peak corresponds to weak acid sites at
Urea-HY (23.21%) catalysts decreased rapidly due to oxidation of around 100 °C from the surface hydroxyl groups shifted to the
carbon deposition. The amount of carbon deposition on the lower temperature, which were consistent with the FT-IR results
Pd/NH4F-Urea-HY catalyst could be calculated as 2.11%, which was discussed previously. Compared with fresh Pd/HY, the media strong
less than that in the used Pd/HY catalyst (7.85%)4a,4b,4f,5d. Combined acid sites slightlyincreased, shifting to the higher temperature, and
with BET and TGA results, it could be inferred the better catalytic induced a new acid site in the fresh Pd/NH4F-Urea-HY, which was a
performance of the Pd/NH4F-Urea-HY catalysts was attributed to considerable reason for improving the catalytic performance of
inhibiting the carbon deposition.
catalysts.
Table 2 Specific structure parameters ofsamples.
Samples
aSBET
bV(cm3 /g)
cD(nm)
(m2/g)
HY
505
296
333
21
0.11
0.28
0.23
0.07
0.20
0.06
1.03
2.57
1.24
2.92
2.28
2.30
NH4F-Urea/HY
Fresh Pd/HY
Used Pd/HY
Fresh Pd/NH4F-Urea-HY
Used Pd/NH4F-Urea-HY
255
39
Fig.5 FT-IR (a) and NH3-TPD (b) profiles of Pd-based catalysts.
a Surface area; b Total pore volume;c Average pore diameter.
In summary, we demonstrated the Pd/NH4F-Urea-HY catalyst
can be effective to apply for acetylene hydrochlorination. The
modification of NH4F and Urea in Pd/HY catalyst could significantly
enhance the catalytic performance, maintaining a good catalytic
performance with acetylene conversion (99 %) and a selectivity to
vinyl chloride (99%) for over 8 h. It was illustrated that the
enhanced catalytic performance was attributed to the NH4F and
Urea modification, which decreased the Pd loss, changed the
surface acidity of catalysts, prevented the Pd reduction and
inhibited the carbon deposition.
Experimental section
Fig.4 TG profiles of Pd-based catalysts.
The HY modified by ammonium fluoride and urea (denoted as
NH4F-Urea-HY) support was synthesized by the HY zeolites
( purchased by Nankai university, Si/Al =9) to a mixture of NH4F and
urea under stirring in 100 mL distilled water, so as to modulate a
Generally, the surface functional groups and acidic properties
on zeolites can affect the catalytic performance of zeolite -based
catalysts11a-11d. We compared the FT-IR (Fig.5a) and NH3-TPD (Fig.5b)
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2
3
4
Minamata
Convention
Agreed
mass ratio of NH4F/Urea keeping 2/1. After stirring 4 h at 298 K, the
mixture was filtered and washed thoroughly with distilled water
until the filtrate was neutral. Then it was dried at 353 K for 10 h. Pd-
based catalysts using the prepared NH4F-Urea-HY support or
original HY support were prepared by ultrasonic-assisted and
incipient wetness impregnation technique7. The Pd loading was 0.9
wt% and the obtained catalysts were denoted as Pd/HY and
Pd/NH4F-Urea-HY catalysts, respectively. The single NH4F or urea
modified Pd/HY catalysts was prepared by the same method as
shown above.
Activity tests of catalysts were evaluated in a fixed-bed quartz
tube reactor (i.d. 10 mm) at atmospheric pressure. A premixed
hydrogen chloride (99.99%) and acetylene (98%) stream with a
volume ratio of 1.25 were fed into a heated reaction containing 4 g
catalysts, keeping a C2H2 gas hourly space velocity (GHSV) of 110 h-1
and a reaction temperature of 160 °C . The exit gas mixture from the
reactor was passed through an aqueous NaOH solution to remove
untreated HCl before beingset into a gas chromatography (GC 2010,
Shimadzu) to analyzed the conversion of acetylene (XA) and the
selectivity of the selectivityof VCM (SVC) immediately.
Scanning electron microscopy (SEM) was performed using a
LEO1450VP detector and transmission electron microscopy (TEM)
was tested with H-600-II electron microscope from Hitachi Co. LTD
at an accelerating voltage of 200 KV. Palladium contents were
detected using an inductively coupled plasma (ICP-6300) instrument.
X-ray diffraction (XRD) data were acquired using a M18XHF22-SRA
diffractometer operating at 50 kV, 100 mA with Cu-K irradiation in
the scan range of 2 between 5°-80°. X-ray photoelectron
spectroscopy (XPS) data were executed by AXIS ULTRA
spectrometer (Kratos Analytical Ltd) and binding energies are
referred to the C1s line at 284.8 eV. Fourier Transform Infrared
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DOI: 10.1039/C5NJ03338A
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Acknowledgments
This work has been supported by National Natural Science
Foundation of China (Grant No. 21263025 and U1403293),
Graduate Research and Innovation Program of Xinjiang (Grant No.
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Graphical abstract:
Non-mercury catalytic acetylene hydrochlorination over the
NH4F-Urea-modified Pd/HY catalyst for vinyl chloride monomer
production
Lu Wanga, Feng Wanga* , Jide Wanga
aKey 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.
8 cm × 4 cm
The modification of NH4F and urea significantly enhances the stability of Pd/HY catalyst for
acetylene hydrochlorination.