Dehydrochlorination of 1,2-dichloroethane over Ba-modified Al2O3 catalysts

Article abstract of DOI:10.1039/c6ra08855d

Bimodal mesoporous alumina (Al₂O₃) was prepared using polyethyleneglycol (PEG 20,000) and cetyl trimethyl ammonium bromide as a template. The incorporation of Ba with various loadings was carried out by incipient wetness. Characterization was performed by XRD, N₂ sorption isotherms, and pyridine FTIR. Ba can be highly dispersed on Al₂O₃ covering the strong acid sites of Al₂O₃. In the catalytic dehydrochlorination of 1,2-dichloroethane (1,2-DCE), the Ba/Al₂O₃ catalysts present a high activity, of which Al₂O₃ is most active with 95% conversion at 325 °C, related to the more Lewis acidic Al³⁺ sites in a tetrahedral environment. 1,2-DCE adsorbs dissociatively on Lewis acid-base pair sites, forming chlorinated ethoxy species, which are supposed to be intermediate species for vinyl chloride (VC) production. At a temperature higher than 400 °C, the dehydrochlorination of VC occurs on the strong acid sites of Al₂O₃. Ba can promote greatly the selectivity for VC through a decrease in the strong acid sites. A high stable activity for dehydrochlorination and high selectivity for VC can be obtained over Ba/Al₂O₃ in the presence of oxygen.

Full text of DOI:10.1039/c6ra08855d

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Page 1 of 42
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DOI: 10.1039/C6RA08855D
Dehydrochlorination of 1,2-dichloroethane over Ba modified Al O3
2
catalysts
a
a
b
a
Shuxing Bai , Qiguang Dai *, Xinxin Chu , Xingyi Wang *,
Abstract
Bimodal mesoporous alumina (Al
and cetyl trimethyl ammonium bromide as template. The incorporation of Ba with various
loadings was carried out by incipient wetness. Characterization was performed by XRD, N
sorption isotherms, and pyridineꢀFTꢀIR. Ba can be highly dispersed on Al with covering
strong acid sites of Al . In the catalytic dehydrochlorination of 1,2ꢀdichloroethane (1,2ꢀDCE),
2 3
O ) was prepared using polyethyleneglycol (PEG 20000)
2
2
O
3
2
O
3
2
the Ba/Al O
3
2 3
catalysts present high activity, of which Al O is most active with 95% conversion at
o
3+
3
25 C, related to more Lewis acidic Al sites in a tetrahedral environment. 1,2ꢀDCE adsorbs
dissociatively on Lewis acidꢀbase pair sites, forming chlorinated ethoxy species, which is
supposed to be intermediate species for vinyl chloride (VC) production. At the temperature higher
o
than 400 C, the dehydrochlorination of VC occurs on strong acid sites of Al
2
O
3
. Ba can promote
greatly the selectivity for VC through decrease in strong acid sites. High stable activity for
dehydrochlorination and high selectivity for VC can be obtained over Ba/Al O in the presence of
2
3
oxygen.
Keywords: Dichloroethane, alumina, vinyl chloride, Lewis acidity, Ba, dehydrochlorination.
a
Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East China
University of Science and Technology, Shanghai 200237, PR China.
b
Key Laboratory of Nuclear Radiation and Nuclear Energy Technology, Shanghai Institute of
Applied Physics, Chinese Academy of Science, Shanghai 201800, PR China
.
Tel: +86 21 64253183; Fax: +86 21 64253372; E-mail: wangxy@ecust.edu.cn (X. Wang);
daiqg@ecust.edu.cn (Q. Dai).
1

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1
. Introduction
Vinyl chloride (VC) has been widely used in production of popular
homopolymeric and copolymeric plastic materials. The demand for VC as the basic
commodity increases greatly with the application of vinyl plastic materials. The
production of VC through chemical dehydrochlorination of l,2ꢀdichloroethane
(
l,2ꢀDCE) has been practiced on a large scale, of which thermal dehydrochlorination
of l,2ꢀDCE at 450–500 °C is industrially a main route with 50% conversion and 98–
9% selectivity for VC [1,2]. Catalytic dehydrochlorination presented high effectiveꢀ
9
ness in VC production. Generally, it was considered that the dehydrochlorination of
chloroalkanes was promoted by solid base and acid catalysts. Shalygin reported
l,2ꢀDCE dehydrochlorination over a series of silicate catalysts, such as Al
Ga /SiO , ZrO /SiO , BeO/SiO and Y /SiO , which were acidic solid materials
1]. There were a few reports on the interaction between chloroalkanes and Al
Mochida studied the dehydrochlorination of several chloroalkanes (including
,1,2ꢀtrichloroethane, 1,2ꢀdichloropropane and 1,1,2ꢀdiꢀchloropropane) over Al
base and acid catalysts under reductive reaction conditions [3]. Dehydrochlorination
reactions over dry Al were postulated to proceed through an E2ꢀconcerted
2 3 2
O /SiO ,
2
O
3
2
2
2
2
2
O
3
2
[
2 3
O .
1
2 3
O ,
2 3
O
mechanism, where the chlorine and hydrogen were eliminated almost simultaneously
with the basic and acid sites of Al O . FeijenꢀJeurissen [4] reported that the
2
3
2 3
mechanism of catalytic decomposition of 1,2ꢀDCE over γꢀAl O , and proposed that
the destruction of 1,2ꢀDCE occurs through dehydrochlorination to VC. Catalytic
cracking of EDC is also practiced industrially over silicates, metal promoted aluminas,
2

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or zeolites, but the advantage balance the drawback of needing catalyst regeneration.
The dehydrochlorination of chlorided linear paraffins to linear olefins in the
production of alkylbenzene sulfonate surfactants was performed catalytically in the
presence of silicaꢀaluminas or on metal packings of reactor columns acting as
catalysts. In this case, no information is readily available in the open literature
concerning catalyst stability.
Recently, transitional alumina has been among the most used materials in any
field of technologies, while details of their physicochemical properties are still a
matter of discussion and investigation [5]. The crystal structure of γꢀAl O was still a
2
3
matter of controversy, being a defective nonꢀstoichiometric spinel [6,7] or other cubic
or tetragonal structures with the occupancy of nonꢀspinel cationic sites [8ꢀ11]. Digne
et al. [12,13] reported DFT results that the two main orientations (under practical
operation conditions) of γꢀAl
2 3
O facets were <100> (17%) and <110> (70%). The
<
100> facets were fully dehydrated at 600 K, leading to the formation of
3+
2 3
coordinatively unsaturated (pentaꢀcoordinate) Al . As reported, the acidity of γꢀAl O
3
+
was related to coordination degree of Al species, of which, the strongest Lewis acid
sites were associated with very low coordination Al cations, such as triꢀ and tetraꢀ
3
+
coordinated Al . In the dehydration of ethanol, the most active sites were believed to
3+
be Lewis acidic Al sites in a tetrahedral environment located on the edges and
corners of the nanocrystals [14]. In this paper, the surface structure of Al
2
O
3
was
on
modified with dropping Ba, and the effect of the corresponding acidity of Al
2
O
3
the activity, selectivity and stability in the catalytic dehydrochlorination of l,2ꢀDCE
3

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were investigated. TPSR and in situ DRIFTS techniques were utilized to determine
reaction intermediates and thus explore a possible reaction mechanism.
2
. Experimental
2
.1. Catalysts Preparation
2 3
Bimodal mesoporous alumina (Al O ) was synthesized using polyethyleneꢀ
glycol (PEG 20, 000) and cetyl trimethyl ammonium bromide (CTAB) as template.
The typical procedure was described in Ref [15]. CTAB (1.38 g), PEG (2.25 g) and
aluminium isopropoxide (Al(OꢀiꢀPr)
3
5 g) were dissolved in ethanol aqueous solution
(63% V/V) with stirring vigorously, and then ammonia (19 ml) as a precipitation
o
agent was added and stirred for 30 min at 50 C. The produced suspension was aged
o
at 50 C for 24 h under static conditions. The obtained filter was washed with ethanol
o
o
and dried at 110 C overnight. Dried samples were calcined for 3 h at 550 C in air.
Commercial Al (Al ꢀC) was referred to as a reference.
A series of Ba/Al O catalysts with various Ba loadings were prepared by
2
O
3
2 3
O
2
3
conventional impregnation methods described elsewhere [9]. The support Al
2 3
O (2 g)
was impregnated with an aqueous solution (4 ml) (the content of barium nitrate is in a
−
1
o
range of 0.0048ꢀ0.095 g mL (0.05ꢀ10 wt%)) and then dried at 80 C for 12 h and
o
calcined in air at 550 C for 3 h. The obtained catalysts were noted as xBa/Al
2
O
3
where x presents the content of Ba wt%. Al O used in this work is the bimodal
2
3
mesoporous Al
2 3
O as unless stated otherwise.
2
.2. Catalyst characterization
4

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The powder Xꢀray diffraction patterns (XRD) of samples were recorded on a
Rigaku D/MaxꢀrC powder diffractometer using Cu Kα radiation (40 kV and 100 mA).
o
The diffractograms were recorded within the 2θ range of 10−80 with a 2θ step of
o
0
.01 and a time step of 10 s. The nitrogen adsorption and desorption isotherms were
measured at 77 K on a Micromeritics ASAP 2400 system operated in static
o
measurement mode. Samples were outgassed at 350 C for 4 h before the
measurement. The specific surface area was calculated by using the BET model. The
actual Ba contents were determined by inductively coupled plasma atomic emission
spectrometry (ICPꢀAES) using a Varian 710 spectrometer. Samples were dissolved by
using aqua regiaꢀhydrogen peroxide system to form a homogeneous solution. The
Xꢀray photoelectron spectroscopy (XPS) measurements were made on a VG
α
ESCALAB MK II spectrometer by using Mg K (1253.6 eV) radiation as the
excitation source. Charging of samples was corrected by setting the binding energy of
adventitious carbon (C1s) at 284.6 eV. The powder samples were pressed into
selfꢀsupporting disks, loaded into a subꢀchamber，which was evacuated for 4 h prior to
the measurements at 298 K. Temperature programmed surface reaction (TPSR)
measurement was carried out under the condition same as in catalytic activity tests.
First, the feed containing 1,000 ppm 1,2ꢀDCE and Ar balance flowed continuously
over the samples at 100 °C. After the adsorptionꢀdesorption reached an equilibrium,
the samples were heated from 100 °C to a specified temperature at the heating rate of
1
2
0 °C/min. The reactant and the products (such as DCE (m/z=98), CO (44), CO or
+
C
2
H
4
(28), CHO (29), Cl
2
2 2 2 3
(70), HCl (36), C H (26), C H Cl (62), C4 (41,
5

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+
CH
2 2
=CHꢀCH )) were analyzed onꢀline over a mass spectrometer apparatus (HIDEN,
QICꢀ20).
2
.3. Catalytic activity measurements
Catalytic dehydrochlorination was carried out at atmospheric pressure in a
continuous flow microꢀreactor (a quartz tube of 3 mm inner diameter). 75 mg catalyst
grain size, 40ꢀ60 mesh) was packed in the reactor bed. Before testing the activity and
(
selectivity, the transport effects were investigated to ensure that experimental results
were not significantly influenced by interphase transportation. Calculation of the
theoretical external transfer rate of reactants to the catalytic particles (at a typical
o
temperature 300 C) based on estimated massꢀtransfer coefficients gave a value
magnitude three orders greater than the measured reaction rates, indicating process
conditions were far from external diffusional limitations. The effects of external
massꢀtransfer resistances were experimentally evaluated by repeating a set of process
conditions whilst employing a different linear velocity. Results of these experiments
ꢀ1
indicated that conversion was not affected for linear velocity higher than 7 cm s ,
within the experimental error. Likewise, estimates of interphase temperature gradients
o
showed fluidꢀsolid differences of less than 1 C. The possibility of internal pore
diffusion was examined by measuring conversions at fixed conditions but varying
catalyst particle size. Results showed that pore diffusional resistance was absent for
particles less than 1 mm in diameter. Intraparticle massꢀtransfer resistances were
theoretically evaluated by computing effectiveness factors, which were calculated to
be greater than 0.98, indicating that intraparticle massꢀtransfer resistances were not
6

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significant. Finally, internal thermal gradients also proved to be negligible over the
range of conditions evaluated in this study. The feed flow through the reactor was set
at 40 ml/min (linear velocity of 9.4 cm/s) and the gas hourly space velocity (GHSV)
ꢀ1
was maintained at 30,000 h . Feed stream to the reactor was prepared by delivering
liquid 1,2ꢀDCE with a syringe pump into dry Ar and the injection position was
electrically heated to ensure complete evaporation of the liquid reaction feeds. The
temperature of the reactor was measured with a thermocouple located just at the
bottom of the microꢀreactor. The effluent gases were analyzed onꢀline at a given
temperature by using gas chromatographs (GC9790, FULI) equipped with a
KromatꢀKBꢀ5ꢀ30 m × 0.32 mm × 0.50 ꢁm capillary column and a flame ionization
detector (FID) for the quantitative analysis of the organic chlorinated reactant.
Catalytic activity was measured over the range 150−450 °C and the conversions were
calculated by subtracting the outlet concentration from the inlet concentration of the
reactant and dividing by the inlet concentration. These conversions were obtained at
different temperatures under steady state at each temperature. All the reactions were
repeated three times to assure reproducibility. Furthermore, carbon balances could be
as accurate as within 5%.
2
.4. In situ FTIR
In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)
experiments were conducted on a Nicolet 6700 FTIR fitted with a liquid nitrogen
cooled mercuryꢀcadmiumꢀtelluride detector (MCT). The DRIFTS cell (Harrick,
HVCꢀDRP) fitted with ZnSe windows was used as the reaction chamber that allowed
7

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o
samples to be heated to 550 C. All the spectra were obtained averaging 64 scans.
Considering the instrumental optics and the strong framework adsorption of alumina,
ꢀ1
the useful spectral range was: 4000ꢀ1100 cm . Prior to 1,2ꢀDCE adsorption
o
experiments, the samples were pretreated in Ar at 550 C for 2 h. Then the samples
o
were cooled down to 50 C in order to remove the contaminants. The spectra of the
o
samples (20ꢀ30 mg) were recorded from 100 to 400 C after 1,2ꢀDCE adsorption
following sweeping with Ar.
3
. Results and discussions
3
3
.1 Catalyst Characterization
.1.1 Physic properties
Fig. 1 shows N
distribution determined based on the adsorption branch using the BJH method. The
synthesized Al presents two continuous typeꢀIV curves with a H1ꢀtype hysteresis
2
sorption isotherms of samples with the insert of pore size
2
O
3
loop, which locates at relative pressure of 0.4ꢀ0.55 and 0.55ꢀ0.95, respectively. There
appear two kinds of pores with radios of 3.9 and 7.4 nm in the pore distribution curve,
2
.
respectively. BET area is estimated to be 330 m / g (Table 1). With the incorporation
of Ba into Al by 1~2 wt%, similar N sorption isotherms are observed. Further
2
O
3
2
increase in Ba loading makes two kinds of pores decrease to 3.7 and 5.0 nm,
2
respectively. BET areas of Ba/Al O samples are in a range of 310~334 m /g (Table
2
3
1
). Commercial Al
2
O
3
(Al
2
O
3
ꢀC) has a narrow pore size distribution centered at 6.1
2
nm with surface area of 117.9 m /g. XRD patterns of samples are presented in Fig. 2.
All of samples exhibit diffraction peaks at 19.4, 37.6, 45.8 and 67.0 °, ascribed to
8

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<
111>, <311>, <400> and <440> planes of γꢀAl
2
O
3
, respectively (JCPDS 10ꢀ0425).
Note that no diffraction peaks due to Ba species are detected for Ba/Al
O
2 3
samples
with ≤3 wt% Ba loading, implying that Ba species are highly dispersed on surface of
the Al O . With further increase in Ba loading up to 4 wt% or higher, there appear
2
3
new diffraction peaks at 23.9, 24.3, 33.7, 34.1, 34.6 and 42.0 °, which can be resulted
from BaCO crystalline phase (JCPDS Card No. 05ꢀ0378). The formation of BaCO
3
3
should be related to the interaction of BaO with CO
strong basicity of BaO.
2
during the calcinations due to
Fig. 3 shows HRTEM images of the synthesized Al O and 4Ba/Al O samples.
2
3
2
3
With the presence of Ba, no evident change in morphology is observed. The size of
observed Al particles is in a range of 3ꢀ7 nm (white circles in Fig. 2A). The small
Al particles were stacked together to form a large plane. The lattice spacing was
measured to be 0.14 and 0.20 nm, in good agreement with those of the <440> and
400> crystal planes of the standard γꢀAl O sample (JCPDS 10ꢀ0425). The
2
O
3
2
O
3
<
2
3
observation of electron diffraction rings in the selected area electron diffraction
SAED) patterns (insets of Fig. 3A) suggests the formation of a polycrystalline
structure. The images also show that the catalysts are homogeneous with the absence
of crystalline Ba oxide or BaCO phases. To further characterize the distribution of Ba
in 4Ba/Al O , the STEM mapping of Ba, Al and O was conducted. As shown in Fig.
(
3
2
3
S1, Ba species were uniformly and highly dispersed on the Al
shows the strong interaction between Ba and Al , as reported elsewhere [9,16,17].
A low BaO coverage of 2 wt% on γꢀAl monomeric BaO units were present almost
2 3
O surface. This result
2 3
O
2
O
3
9

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exclusively and these molecularly dispersed BaO units were concentrated on the (100)
faces of the alumina crystallites [9]. Density functional theory calculations predicted
that energetically most favorable BaO monomer and dimer units anchor to
3+
pentaꢀcoordinate Al sites on the (100) faces of γꢀAl O in such geometries that
2
3
2 3
maximize their interactions with the support surface [16]. In our case, Al O exposed
mainly <400>, which is really different from the above results [9,16]. BaO monomer
and dimer units should be formed because the aggregation of BaO can not be
observed on 4Ba/Al
2 3 2 3
O where Ba species is highly dispersed on Al O (confirmed by
HRTEM). Additionally, the crystalline particles of BaCO are not detected by
3
HRTEM, indicating that XRD diffraction from BaCO
from Al produced during calcinations.
.1.2 Pyridine adsorption
Pyridine, as a basic probe molecule, can be used for characterization of catalyst
surfaces, allowing also a definite determination of the existence of Lewis acidity. The
spectra recorded after adsorption of pyridine on Ba/Al samples are presented in
Fig. 4. On PyꢀFTꢀIR spectra, the typical features of Lewis bonded pyridine can be
3
may be due to a separate phase
2
O
3
3
2
O
3
ꢀ1
2 3
observed. Over Al O , three 8a components observed at 1574ꢀ92, 1612 and 1624 cm
reveal the existence of at least three different families of Lewis acid sites, as discussed
previously for transitional alumina [5,18ꢀ22]. The typical assignments for the first and
the last of these three components are to pyridine bonded to octahedral and tetrahedral
3
+
Al ions, respectively, both with a single coordinative unsaturation, thus being pentaꢀ
and triꢀ coordinated, respectively, before pyridine adsorption. The band in the middle
1
0

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3
+
can be resulted from pyridine species interacting either with Al ions in coordination
3+
five or to sites having Lewis acid strength slightly lower than triꢀcoordinated Al ions.
These sites may be triꢀcoordinated too, but with a nearest cation vacancy. The spectra
observed on the samples containing Ba are very similar, as also are most spectra
reported in the literature for pyridine adsorbed on aluminas [23]. As previously
reported, the pentaꢀcoordinate aluminum ions on γꢀAl
2 3
O were identified as the
preferential anchoring points for Ba [16] and Pt [17]. It should be noted that with an
ꢀ1
increase in Ba loading, the intensity of the band at 1624 cm decreases gradually and
ꢀ1
the 8a band at 1574ꢀ92 cm becomes weaker and narrower. This is possibly due to the
3+
coverage of Ba species on triꢀ and pentaꢀcoordinated Al ions, thus leading to a
decrease in pyridine molecule adsorbed on these sites. In the spectrum of commercial
2 3
Al O ꢀC, the features of pyridine adsorbed on Lewis sites are also evident. However,
ꢀ1
the band at 1612 cm due to pyridine species interacting either with Al ions in
coordination five becomes significantly strong, while the bands due to triꢀcoordinated
3
+
Al species, weak, indicating that the number of strong Lewis acid sites on Al
is smaller than Al and Ba/Al with low Ba loading.
.1.3 NH -TPD analyses
2 3 2 3 3
Fig. 5 shows NH ꢀTPD profiles of Al O and Ba/Al O samples. NH ꢀTPD
2 3
O ꢀC
2
O
3
2 3
O
3
3
3
o
profiles can be divided into two desorption temperature ranges of 150~300 C and
o
3
00~450 C, corresponding to ammonia desorption from weak and strong acidic sites,
respectively. The weak and strong acid amounts of sample with 1% Ba increase by
% and 23% compared with those of Al , respectively (in Table 1). As mentioned
8
2 3
O
11

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previously, the addition of Ba will create an interface between AlꢀO and BaꢀO species
which distorts the surface structure of γꢀAl , probably with electronic unbalance
2
O
3
due to the change in length of AlꢀO bonds, which can contribute the increase in acidity.
Further increase in Ba loading, however, both strong and weak acids decrease
significantly. Additionally, BaO is a base solid, and it is expected that the acid amount
decreases with the formation of BaO particles over Al
Ba loading of higher than 4%, the total acidity is inversely proportional to Ba content
Fig. 5 insert). On PyꢀFTIR spectra of all samples, there appear characteristic peaks of
2 3
O . Thus, for the samples with
(
ꢀ1
pyridine adsorbed on Lewis acid sites at ca. 1442, 1574ꢀ92, 1612 and 1624 cm (Fig.
3
+
2+
4
) [19]. Obviously, Al ions coordinated with different number and Ba ions for the
samples with low Ba content contribute to Lewis acid sites.
3
3
.2 Dehydrochlorination
.2.1 Activity test
The total conversions of 1,2ꢀDCE over Al O and Ba/Al O catalysts in dry feed
2
3
2
3
of 1000 ppm 1,2ꢀDCE and Ar balance as functions of temperature are shown in Fig. 6.
Al presents considerably high activity and the conversion of 1,2ꢀDCE increases
2
O
3
quickly with raising temperature. T95% (the temperature needed for 95% conversion of
o
1
,2ꢀDCE) is 325 C. Main products are composed of VC, aldehyde, ethyne and C
4
(butene or chloroꢀbutene) (Fig. 6), which are confirmed by TPSR (Fig. S2.). C4
appears only at high temperature with low selectivity (below 1%). The selectivity for
VC is promoted by raising temperature with a significant decrease in aldehyde content
o
and reaches 95% at 350 C. After that, the selectivity for VC decreases quickly, and is
1
2

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o
only 60% at 425 C. Generally, it is considered that the formation of VC is through
3+
the dehydrochlorination of 1,2ꢀDCE [4]. Cl atom of 1,2ꢀDCE adsorbs on the Al
2
ꢀ
2 3
acidic site and surface O species or hydroxyl group from the surface of γꢀAl O
nucleophlically attacks the carbon atom of 1,2ꢀDCE and the dehydrochlorination
occurs. The formation of aldehyde was considered to be the results of the reaction of
VC with surface hydroxyl group over acidic catalysts [24]. As the reaction proceeds
with the increase in temperature, the surface oxygen species become less and less, and
o
the formation of aldehyde becomes difficult. At the temperature of 375 C or higher,
VC can be further dehydrochlorinated into ethyne.
With incorporation of Ba, the conversion curve shifts to high temperature. T95% is
o
o
proportional to Ba loading (Table 1) and increases from 350 C for Al
2 3
O to 471 C
for 10Ba/Al , indicating that the activity is inhibited by Ba to some extent. Similar
2
O
3
product distribution over catalysts containing Ba is available. However, aldehyde
distribution become broader and higher with an increase in Ba loading and the
o
selectivity for aldehyde over 4Ba/Al
2
O
3
reaches 60% at 225 C. In all case, the higher
the 1,2ꢀDCE conversion, the higher the VC selectivity; the lower the DCE conversion,
the higher the aldehyde selectivity. At complete conversion, aldehyde is not formed at
all, suggesting that 1,2ꢀDCE partial pressure available is a key factor favoring
aldehyde (at low conversion, with more aldehyde available) or VC (at high
conversion). In fact, the dependence of the aldehyde formation is expected to have a
higher reaction order with respect to 1,2ꢀDCE than VC synthesis. On the other hand,
the presence of Ba can produce additional oxygen species as strong basic sites, and so
1
3

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be favorable for the formation of aldehyde through nucleophilic attack (see later).
Moreover, the formation of ethyne during increasing temperature decreases gradually
o
with Ba addition. At 425 C, the selectivity for ethyne decreases from 60% over Al
2
O
3
to 5% over 4Ba/Al O . The elimination of HCl from chloroalkanes is promoted by
2
3
solid base and acid catalysts. Generally, the reactivity of the chlorinated ethylenes
decreases with increasing chlorine content in the molecule. The rate determining step
probably does not involve breaking the C–Cl or C–H bonds. Bond et al. [25]
2 3
suggested that over a Pt/Al O catalyst, the rate determining step is the removal of
chlorine atom. Chintawar et al. extensively studied the oxidation of chlorinated
ethylenes [26]. They found a strong correlation between the adsorption capacity of the
molecule on the catalyst and the reactivity of the molecule. The elimination reactions
2 3
of the chlorocompounds over Al O in dry feed were postulated to proceed through an
E2ꢀconcerted mechanism where the chlorine and proton were eliminated almost
simultaneously by the acidic and basic sites of Al O . If the acidity of the proton was
2
3
weak enough, such as that in the case of VC, the removal of Cl atom on strong acid
sites was critical. At that time, the elimination may proceed via E1ꢀconcerted
mechanism. For Ba/Al
favorable for the activation of VC, even though the basicity of Ba/Al
strong. As expected, Al O ꢀC with less strong acidity shows a poor selectivity for
2
O
3
catalysts, the decrease in strong acid sites is really not
2
O
3
catalysts is
2
3
2 3
ethyne at high temperature. Al O ꢀC with less strong basic sites (basic oxygen species
as BaO possesses) presents lower selectivity for aldehyde.
In order to investigate the effect of acidity on catalysts on the kinetics of
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1
,2ꢀDCE dehydrochlorination, TOF based on the mole number of converted 1,2ꢀDCE
molecules per second per mole of acidic site on the surface of catalysts can be used
2 3 2 3
for comparing activity difference among the acidic sites of Al O and Ba/Al O . Fig. 6
o
insert shows the TOF at 300 C as a function of Ba loading. It can be seen that the
TOF decreases almost linearly with Ba loading. A few studies have been published on
2 3
the interaction between chloroethanes and Al O . A significant difference in TOF
indicates that a rate determining step probably does not involve breaking C–Cl bonds.
The removal of Cl species from the surface may be a slow step [27].
3
.2.2 The stability
Considering possible effects of Cl and carbon depositions on the surface of
catalysts, the stability tests in the feed of 1000 ppm 1,2ꢀDCE and Ar balance were
o
carried out at 400 C and the results are shown in Fig. 9. It can be seen that the
2 3 2 3
conversion on Al O and 4Ba/Al O decrease by 50% on stream within first 45 and 18
h, respectively. The used catalysts in stability test became black. EDS analyses
showed that there was 1.5~2.7% carbon deposition. This phenomenon implies that the
polymerization, fuse and carbonization can occur during reaction. Moreover, about
2
% Cl was detected by XPS and EDS analyses. As known, the removal of Cl species
deposited on Al can be considered to be rateꢀcontrolling step because of the strong
2
O
3
adsorption of Cl species [28]. The reaction of chloroꢀorganics with catalysts Al O3
2
was also described in the literature concerning the preparation of highly acidic
chlorinated alumina catalysts. Chlorination was successfully performed by the
reaction of chlorinated alkanes and alkenes containing at least two chlorine atoms.
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5

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Several chlorinated alkanes and alkenes can be considered to be less reactive in
chlorinating Al [28,29]. The chlorine adsorbed over alumina, as reported by
Muddada et al. [30,31], reduced the amount and the acidic strength of the Lewis sites,
2
O
3
3
+
saturating the coordinate vacancies of Al by the formation of AlꢀCl species. Higher
deactivation rate of 4Ba/Al can be ascribed to stronger adsorption of HCl on basic
2
O
3
sites. As reported in our previous work, the removal of Cl species from catalyst
surface can be promoted in the presence of water through providing hydrogen atoms
[
2 3
27]. In this work, the deactivation of Al O can not be inhibited by adding water at
o
3
50 C (Fig. S3), although water promotes the activity of the fresh catalysts at low
temperature to some extent (Fig. S4). XPS and EDS analyses showed that Cl
2 3 2 3
deposition on the surface of the used Al O and 4Ba/Al O on stream of wet feed at
o
3
50 C decreases by 50% or more. In the case of the reaction of dichloromethane with
2 3
water, the balance of a small amount of Cl adsorbed on Al O catalyst can be
o
maintained and the activity become constant at 300 C [27]. Obviously, the
o
deactivation of Al
deposition (Fig. S3). When Al
stability test, with the addition of 5% O
2
O
3
on wet stream at 350 C should be caused only by carbon
2
3 2 3
O and 4Ba/Al O became deactivated during the
2
(Fig. 9), the activity of the catalysts can
o
restore almost to the level obtained on the fresh catalysts at 400 C. TGꢀMS confirmed
that the removal of black carbon deposited on the surface of catalysts in the presence
o
of oxygen occurred at 430 C. In fact, the addition of oxygen, the activity of the
o
dehydrochlorination over Al
2
O
3
can not restore completely at 350 C (Fig. S5). Here,
the removal of Cl species as HCl from the surface of catalysts was promoted by water
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6

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produced from the oxidation of a small amount of 1,2ꢀDCE.
In the dry and oxygenꢀfree feed, the selectivity of VC increases gradually with
the deactivation of Al and reaches 96% at the conversion of 50%, due to the
2 3
O
decrease in strong acidic sites through carbon deposition. However, with the removal
of carbon from the strong acidic sites in the feed containing oxygen, the
dehydrochlorination of VC becomes significant, and the selectivity for VC finally
decreases to about 65%. This phenomenon indicates that the strong acidic sites on
which black carbon deposits are responsible for the second dehydrochlorination. It is
interesting to find that for 4Ba/Al O , the selectivity for VC maintains 90% during the
2
3
stability test (at least 80 h) and almost zero amount of ethyne was detected.
%Ba/Al in particular was tested in the feed containing 5% oxygen for another 80
4
2 3
O
o
h at 400 C with the conversion as high as 90ꢀ92% and the selectivity for VC as high
as 88ꢀ90%, respectively (SI). This result is really different from those obtained
previously, in which catalytic cracking at 300ꢀ400°C on pumice (SiO , Al O , alkalis)
2
2
3
2 2
or on charcoal, impregnated with BaCl or ZnCl has not found more widespread
application due to the limited life of the catalysts of about 10 h and VC can be further
oxychlorinated to ethyl trichloride and further transformed in the presence of oxygen
[
31]. To our best knowledge, the catalysts used for dehdrochlorination of chlorinated
linear paraffins were acidic catalysts, such as silicates and zeolites [1,3,32,33]. Similar
results were found in the dehydrochlorination of 1,2ꢀdichloropropane over silicaꢀ
alumina catalysts where 20% selectivity for allyl chloride and 10 h stability were
available [34]. The theoretic and experimental studies showed that the addition of Ba
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7

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2 3
species can cover strong acidic sites of Al O [16]. It provided an opportunity to
develop a new pathway of 1,2ꢀDCE dehdrochlorination over the catalysts with strong
basicity and weak acidity. The synergy of strong base and weak acid is critical to
insure good performance of 4Ba/Al O . Carbon balance can reach 95% or higher. CO
2
3
can be detected in effluent due to the oxidation of 1,2ꢀDCE. Inorganic Cl species
2
leave from the reactor as HCl but not as Cl . In fact, significant Deacon reaction (HCl
o
+
O
2
→ Cl
2
+ H
2
O) occurs at 700 C or higher over Al
2
O
3
[35].
In order to understand high selectivity for VC in the presence of oxygen, the
oxidation of VC over Al O and 4Ba/Al O was conducted. The activity of Al O and
2
3
2
3
2
3
4
2 3
Ba/Al O for VC oxidation is poor, and the conversion reaches 56% and 20% until
o
3
50 C (Fig. S6). Moreover, the addition of 1,2ꢀDCE can retard the conversion of VC,
indicating that 1,2ꢀDCE is more favorable for adsorption on active sites. High
selectivity for VC and stable activity over 4Ba/Al in the presence of oxygen
2
O
3
results from its high activity for dehydrochlorination of 1,2ꢀDCE and low activity for
the second dehydrochlorination and VC oxidation.
3
.2.3 The effect of space velocity
2 3
The effect of space velocities on DCE dehydrochlorination over Al O and
o
4
3
Ba/Al
2
O
3
was investigated at 275, 290 and 305 C within the space velocities of
ꢀ1
ꢀ1
0000ꢀ 90000 mL g h . The results (Fig. 10 and Fig. S7) show that the increase in
space velocities does not decrease the rate (based on the mol number of 1,2ꢀDCE
converted per second per square meter), not as expected. The highest reaction rate was
ꢀ
1
ꢀ1
ꢀ1 ꢀ1
obtained at 60,000 mL g h and the rates at 30000 and 90000 mL g h are almost
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equal. This anomalous datum is associated with the promotion of the removal of Cl
species produced during reaction by higher linear rate of feed in the reaction bed,
implying that at lower feed rate, HCl desorbed from the surface can go back to the
surface. Guido Busca observed this anomalous phenomenon in dehydration of ethanol
over Al
2
O
3
containing more chlorine impurities [14].
.3 In situ FTIR spectra
.3.1 Al
FTIR spectra collected at different temperatures during the treatment of the
synthesized Al O with the reaction feed of 1000 ppm 1,2ꢀDCE in Ar for 1 h after the
3
3
2 3
O
2
3
o
2 3
treatment in Ar at 550 C is shown in Fig. 11. The adsorption of 1,2ꢀDCE on Al O
o
has been studied between RT and 400 C. The spectra was recorded in the absence of
gas phase 1,2ꢀDCE unless stated otherwise. It can been seen that at RT, intense bands
−1
−1
at 2845 and 2945 cm and a weak band at 3040 cm (difficult to observe, due to a
low signal–noise ratio) are observed, indicating that several types of C–H bonds are
present. Raising temperature, the peaks shift to high wavenumbers, 2899 and 2966
−1
o
cm at 200 C, indicating the modification of chemical environment. In the section of
−1
hydroxyl group, spectra exhibited negative bands in the 3788ꢀ3637 cm range, which
usually are assigned to strong surface hydroxyl groups, along with a weak positive
−1
−1
band between 3630ꢀ3581 cm (centered at approximately 3608 cm ). These spectra
suggest that the Al surface hydroxyl groups interact with 1,2ꢀDCE molecules
2
O
3
during adsorption, leading to the formation of weaker hydrogenꢀbonded OHs [36,37].
o
After progressive heating in the range of 100–400 C, the negative bands in the
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−1
3
788ꢀ3637 cm range become strong with temperature in parallel to the increase in
the bands due to chlorinated ethoxy groups. Indeed, at RT, 1,2ꢀDCE can react with
surface hydroxyl groups. Vigué et al. [38] assumed that the reactivity of alumina
surfaces toward halogenated molecules highly suggested that the substitution of
surface OH groups by halogenide ions should be easier for the most basic OH groups,
corresponding to monocoordinated hydroxyl groups. Consequently, the disappearance
−1
of the bands in the 3788ꢀ3637 cm corresponds to the formation of AlꢀCl bonds. In
−1
−1
the frequency range between 1800 and 1000 cm , weak bands at 1160 and 1194 cm
corresponding to chlorinated ethoxy are observed within experimental temperature,
−1
which shift to higher values, compared with that for CH
39]. Previous investigations proposed that the first step in the catalytic oxidation of
chlorinated methane over Al catalysts was a nucleophilic substitution [40]. During
3 2
ꢀCH Oꢀ(1075 and 1116 cm )
[
2
O
3
the nucleophilic substitution, the chlorine atom was abstracted and replaced by
oxygen species, forming surface methoxy species and gas HCl [40]. It can be
expected that the formation of chlorinated ethoxy species on Al
2 3
O , as the first step of
dehydrochlorination, is a synergistic effect of nucleophilic attack by basic surface
3
+
hydroxyl group and the abstraction of Cl atom by acidic Al site. While the maxima
−1
−1
2 2
at 1462 cm (δas CꢀH of CClH ) and 1400 cm (δsym CClH ) are due to deformation
modes of the CClH group to which the CH scissoring mode is superimposed.
2
2
Compared with the case of CH
3
, these CꢀH bands shift to high values slightly,
probably due to substitution of Cl for hydrogen. It is interesting to note that, at low
o
temperature of a region from RT to 200 C, the absorption band centered at 1647ꢀ1665
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−1
cm is predominant, accompanied by the appearance of two broad strong bands
−1
centered at 1409 and 1310 cm . Three bands were attributed to a surface enolic
species, as similar those bands that were observed on Al during the adsorption of
CH CHO (not shown). One evidence is that the IR spectra of synꢀvinyl alcohol
2
O
3
3
(CH
2
=CHOH) in gas phase, show strong absorption band between 1644 and 1648
−1
−1
cm , which are accompanied by two bands at 1409ꢀ1412 and 1300ꢀ1326 cm
[
41,42]. The IR spectrum of adsorbed catechol on a TiO
2
colloid also gives a similar
−
1
band at 1620 cm [43]. Their common characteristic is an enolic structure. On the
−
1
other hand, a band at 1605 cm grows substantially with temperature on stream.
−
1
Dreoni et al. [44] assigned a strong band observed at 1598 cm during adsorption of
o
cyclohexanone on silica at 200 C to a C=C stretching vibration. Furthermore, these
−1
authors also assigned bands at 3075 and 3045 cm to unsaturated =CH– bond
−1
stretchings. Indeed, a similar band at 3040 cm is also observed in our case during
the adsorption of 1,2ꢀDCE and can be assigned to such a vibration mode. Additional
−
1
bands in the same region at 2939 and 2867 cm correspond to C=C and –CH– bonds
and are also the present in the spectra of adsorbed 1,2ꢀDCE. The presence of C=C and
–
1
CH– bonds further verifies the formation of the enolic form [44]. The bands at 1386,
−1
2
268 and 1605 cm (assigned to δCH , ρCH and γC=C of VC) become significant
o
until 250 C, where the conversion to VC in paralleled kinetic reaction is significant.
−1
At the same time, there appear new bands at 1680 and 1730 cm , ascribed to –C=O
group. Generally, an adsorbed acetaldehyde was corresponding to a coordination to a
3
+
Lewis acid, R–CH=OꢀAl [4]. In fact, ꢀCHO was detected in TPSR as M/Z=29, and
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in paralleled kinetic reaction, a significant amount of aldehyde in effluent can be
observed. Chintawar et al. observed for the adsorption of vinyl chloride on a
−1
chromium exchanged zeolite Y a band at 1678 cm and assigned this band to an
adsorbed aldehyde or ketone [40]. Zhou observed the formation of aldehyde during
o
the decomposition of 1,2ꢀDCE at 260~360 C [45]. VC can readily be protonated in
the presence of acid catalysts, like AlCl
stable reactive carbonium ion which then was attacked by a nucleophilic oxygen
species (basic site of alumina or adsorbed water), leading to the CH ꢀCHClꢀO species,
which would readily decompose to form acetaldehyde and leaving a chloride ion on
3
, in the presence of OH surface species into a
3
−
1
the surface. Moreover, there appear new bands at 1338~1356 and 1565 cm (assigned
−1
to carbonate bidentate), 1268, 1425 and 1538 cm (asymmetric stretching vibrationof
−1
carboxylates with the acetate type) [40,46ꢀ48], and at 1470 and 1356 cm
o
(
monodentate bonded carbonates) [49] at 250 C or higher, probably due to the
presence of surface oxygen species, such as basic hydroxyl group.
3
.3.2. 4Ba/Al
2 3
O
For 4Ba/Al
2
3
O , the bands of ascribed to 1,2ꢀDCE adsorption appear at 2880 and
−1
2
965 cm (several types C–H bond, Fig. 12), consistent with that observed on Al
2 3
O .
However, the band intensity seems much weak. Additionally, the band splits in
various range section can be observed, suggesting 1,2ꢀDCE adsorption on two
different types of sites associated with BaO and Al
2 3
O . At the same time, the bands
resulted from oxidized surface species as carbonate bidentate (appearing at
−
1
1
338~1356 and 1565 cm ), asymmetric stretching vibration of carboxylates with the
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−1
acetate type (appearing at 1268, 1425 and 1540 cm ) and monodentate bonded
−1
o
carbonates (appearing at 1473 and 1356 cm ) become much stronger at 200 C.
Oxidation products were seen in the absence of gasꢀphase oxygen, indicating
involvement of surface oxygen in this process. As reported, formaldehyde and formic
acid were detected on spectra during the adsorption of dichloroꢀmethane on Al
2 3
O
[
50]. In fact, their formation was resulted from the nucleophlical attacks by surface
hydroxyl group or oxygen species to dichloroꢀmethane to produce methoxy species
and the following disproportionation. Probably, these oxidized products in this work
are related to incorporation of hydroxyl group and basic oxygen species into the
adsorbed 1,2ꢀDCE molecules. A small amount of alkali metal was known to make
alumina basic [51]. The addition of Ba increases inevitably strong basic oxygen
species. As expected, there are more oxidized products on 4Ba/Al
noted that the band corresponding to carbonyl group of aldehyde is not observed on
Ba/Al O , while aldehyde was detected in effluent with a significant amount. Based
2 3
O . It should be
4
2
3
on the fact that 4Ba/Al
aldehyde produced mainly adsorbs on strong acid sites as R–CH=OꢀAl . The same
phenomenon is observed on Al ꢀC with less strong acid sites (Fig. S8), where the
2 3
O possesses less strong acid sites, it can be concluded that
3
+
2
O
3
carbonate bidentate, carboxylates with the acetate type and monodentate bonded
carbonates can not be observed within experimental temperature. These results imply
that the oxidation involving surface oxygen species on Al
relating to without strong basic sites or strong acid sites.
2 3
O ꢀC is weak, probably
Based on these experiments it was postulated that the first step in the interaction
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of 1,2ꢀDCE with Lewis acid sites occurs and the adsorbed 1,2ꢀDCE can be attacked
nucleophlically by oxygen to form chlorinated ethoxy intermediate through the
removal of chlorine atom. The formation of VC occurs through transfer of proton and
rearrangement of chlorinated ethoxy. The rearrangement of ethoxy intermediate was
considered to be an indispensable step in the dehydration of ethanol on pure Al
2 3
O
[
14]. At the same time, VC adsorbed on two sites, a pair of acidicꢀbasic sites can be
converted into carbonyl species which may or may not contain chlorine, and
stabilized by resonance (an enolic structure). And with the destruction of VC, the
formation of aldehyde was proposed. A following oxygen attack on the carbonyl
compound results in the formation of carboxylate and carbonate species [40]. On the
other hand, on strong acidic sites, chlorine atom of VC can be abstracted with the
transfer of proton, and ethyne formed. In order to obtain deep insights into this
2 3
reaction at a molecular level, a mechanism over Al O consisting of three elementary
steps is schematized (Fig. 13): (1) the formation of 1,2ꢀDCE adsorption complex
during the interaction of 1,2ꢀDCE with Lewis acid sites; (2) chlorine abstraction by
nucleophilic oxygen (surface hydroxyl groups) to form chlorinated ethoxy
intermediate; (3) dehydroxylation through rearrangement to form VC. Other side
reactions include: (1) the attack of basic oxygen species to VC; (2) the formation of
oxygenate species, such as aldehedy, carbonate bidentate, monodentate and partially
oxidized surface species such as enolic species, acetates, carboxylates of the acetate
type; (3) the formation of ethyne through the interaction of VC with strong Lewis acid
3
+
sites, such as tetrahedral Al ions at high temperature.
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4

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4
. Conclusion
Bimodal mesoporous Al
2
O
3
was prepared using polyethyleneglycol (PEG 20000)
catalysts with Ba
2 3
and cetyl trimethyl ammonium bromide as template. Ba/Al O
loading of 1~10 wt% obtained by incipient wetness were characterized by XRD, BET
and porosity measurements, and PyꢀFTꢀIR, and used in catalytic dehydrochlorination
2 3
of 1,2ꢀDCE. The results showed that Ba species was highly dispersed on Al O
3+
probably as monomeric or dimmeric BaO. The number of surface tetrahedral Al
ions decreases with Ba loading, which corresponds to the decrease in strong acid sites.
In the catalytic dehydrochlorination of 1,2ꢀDCE, Ba/Al O catalysts present high
2
3
o
2 3
activity, of which Al O is most active with 95% conversion at 325 C. The products
are composed of VC, aldehyde, ethyne and butene. The formation of aldehyde occurs
at low 1,2ꢀDCE conversion (at low temperature) and butane appears at higher
2 3
temperature with low selectivity (below 1%). For Al O , the selectivity for VC
o
reaches 95% at 350 C, However, at higher temperature, the selectivity decreases
o
quickly and is only 40% at 425 C, which is related to a significant formation of
ethyne. The addition of Ba really promotes the selectivity for VC at high temperature
through the decrease in strong acidic sites which are favorable for the second
2 3 2 3
dehydrochlorination. Al O and 4Ba/Al O deactivate heavily on the stream within 20
o
h at 400 C, due to the deposition of carbon and chlorine species. The presence of
oxygen with a small amount can effectively promote the stability, in which 90%
conversion and 90% selectivity for VC on 4Ba/Al
2 3
O , are available. In situ FTIR
showed that the first step in the interaction of 1,2ꢀDCE with Lewis acid sites occurs
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and the adsorbed 1,2ꢀDCE can be attacked nucleophlically by oxygen to form
chlorinated ethoxy intermediate through the removal of chlorine atom. The formation
of VC occurs through transfer of proton and rearrangement of chlorinated ethoxy.
Different types of partially oxidized products, including the enolic form of
aldehydeꢀtype intermediate, were observed because of the attack to VC by hydroxyl
2 3 2 3
or basic oxygen species existing on the surface of Al O or 4Ba/Al O catalysts.
Acknowledgments
This research was supported by Development Program for National Natural
Science Foundation of China (Nos. 21277047, 21477036 and 21307033).
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Table 1. Physicochemical properties of Al O and Ba/Al O with various Ba loadings
2
3
2
3
Total acidity
(mmol /g cat)
Activity
S
BET
ꢀ1
V
pore
ꢀ1
D
pore
a
o
Sample
( C)
2
3
(
m g ) (cm g ) (nm)
weak acid
0.149
strong acid
0.082
T
50
T
95
Al
2
O
3
330
301
321
334
304
118
0.434
0.366
3 / 7
3 / 6
3 / 6
2.7/5
2.5/ 5
6
308
315
329
338
392
386
355
360
381
404
472
441
1
2
4
1
Ba/Al
2
2
O
3
3
0.162
0.100
Ba/Al
O
0.383
0.173
0.070
Ba/Al O3
0.390
0.108
0.060
2
0Ba/Al
2
O
3
0.302
0.101
0.052
Al
2
O
3
ꢀC
0.253
0.088
0.043
a
Determination by NH
3
ꢀTPD tests.