Eco-compatible zeolite-catalysed continuous halogenation of aromatics

Article abstract of DOI:10.1039/c6gc00731g

A completely eco-compatible halogenation reaction of arenes has been developed allowing high conversions (>95%) of iodobenzene with nearly 100 kg iodobenzene converted per kg_cat in one day. Several solid acids, zeolites being the most promising, have been successfully tested in the chlorination reaction of iodobenzene by using trichloroisocyanuric acid (TCCA), a green chlorination agent. H-?BEA zeolites were found to be the most active catalysts for this model halogenation reaction. A strong structure-activity relationship could be established by thorough characterisation (SEM, BET, XRD, FTIR) of various synthetic zeolites. Indeed, nano-sized ?BEA zeolites and more specifically nanosponge-like ?BEA crystals exhibited the highest catalytic performance with a conversion up to 100% and a selectivity toward monochlorinated products up to 98%. Finally, the gained knowledge was applied to set-up an eco-compatible continuous flow halogenation process of different aromatics catalysed by H-?BEA zeolites.

Full text of DOI:10.1039/c6gc00731g

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Page 1 of 21
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DOI: 10.1039/C6GC00731G
EcoꢀCompatible ZeoliteꢀCatalysed Continuous Halogenation of
Aromatics
a
a
b,c
b,*
c
a
a,*
P. Losch, J. F. Kolb, A. Astafan, T. J. Daou, L. Pinard, P. Pale, B. Louis
a) Laboratoire de Synthèse Réactivité Organiques et Catalyse (LASYROC), UMR 7177,
Institut de Chimie, Université de Strasbourg, 1 rue Blaise Pascal, 67000 Strasbourg Cedex,
France. E-mail: blouis@unistra.fr
b) Université de Haute Alsace (UHA), CNRS, Equipe Matériaux à Porosité Contrôlée (MPC),
Institut de Science des Matériaux de Mulhouse (IS2M) UMR 7361, ENSCMu, Mulhouse
Cedex 68093, France. E-mail: jean.daou@uha.fr
c) Institut de Chimie des Milieux et Matériaux de Poitiers, UMR 7285 CNRS, 4 Rue Michel
Brunet, Bâtiment B27, 86073 Poitiers Cedex9 France.
Abstract
A completely ecoꢀcompatible halogenation reaction of arenes has been developed allowing
high conversions (> 95 %) of iodobenzene with nearly 100 kg iodobenzene converted per kgcat in one
day. Several solid acids, zeolites being the most promising, have been succesfully tested in the
chlorination reaction of iodobenzene by trichloroisocyanuric acid (TCCA), a green chlorination agent.
Hꢀ*BEA zeolites were found to be the most active catalysts for this model halogenation reaction. A
strong structureꢀactivity relationship could be established thanks to various synthetic zeolites and
corresponding thorough characterisation (SEM, BET, XRD, FTIR). Indeed, nanoꢀsized *BEA zeolites
and more specifically nanospongeꢀlike *BEA crystals exhibited the highest catalytic performance with
a conversion up to 100% and a selectivity toward monochlorinated products up to 98 %.
Finally, the gained knowledge was applied to setꢀup an ecoꢀcompatible continuous flow
halogenation process of different aromatics catalysed by Hꢀ*BEA zeolites.
Keywords: chlorination, BEA zeolite, acid catalysis, nanosponge, flow-chemistry.
1

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1
. Introduction
Halogenation of arenes leads to valuable starting molecules in fine chemistry as an entry
1
towards the synthesis of dyes, bioꢀactive compounds such as pesticides or pharmaceuticals. This
generally Lewis acid catalysed reaction is known since the 1950's and most commonly involves FeCl3
2
as a catalyst. Nowadays, it is implemented in industry at the megatonꢀscale and still facing an annual
3
growth demand of roughly 5 %. It is of paramount importance to underline the ongoing progress in
sustainable arene production complementing classical unsustainable resources, for instance biomassꢀ
4
derived dimethylfuran (DMF) and bioethanol to renewable xylene.
Unfortunately, industrial methods for the most commonly used aromatics halogenation
reactions usually produce mixtures of regioisomers, difficult to separate, thus raising the production
5
cost. Notably, the RaschigꢀHooker process has been industrially applied for a long time to produce
chlorobenzene and phenol in a continuous flow process with hydrochloric acid and in situ generation
6
of Cl
2
gas using a CuClꢀCuCl
2
mixed catalyst. Another drawback relies on the use of corrosive acid
catalysts such as Lewis acids (aluminum chloride or boron trifluoride), used in (super)stoichiometric
amounts and which are consumed during the course of the reaction. Likewise, strong Brønsted acid
catalysts such as sulphuric acid remain very corrosive and generate high amounts of salts as byꢀ
products.
Recently, the scientific community focused on developing more efficient and selective
7
processes for the halogenation of arenes. For instance, de Mattos et al. developed a methodology for
the halogenation of deactivated arenes using trichloroꢀ / trihaloisocyanuric acid (TC(Hal)CA) in a
8
superelectrophilic medium (Figure 1). TCCA is a stable and inexpensive solid, easily available in
pool supplies, commonly used as swimmingꢀpool disinfectant and bleaching agent. It is an efficient
chlorine source due to its highly electrophilic chlorine content and has already been used in numerous
,
9 10
chlorination reactions of diverse organic compounds as well as in oxidation reactions.
These early studies have been carried out in homogeneous superacidic conditions, while the
green chemistry principle of catalysis, encompassing both homogeneous and heterogeneous catalysis,
is becoming increasingly ubiquitous in organic chemistry. Heterogeneous catalysts are easily adopted
in gasꢀsolid or liquidꢀsolid flow processes; thereby they may allow to continuously produce highꢀvalue
1
1
added chemicals.
In heterogeneously catalysed reactions, the key solid acid catalysts in
1
2
petrochemistry, and increasingly in biomass reforming processes, remain zeolites. In addition, to
their threeꢀdimensional crystalline microporous network, zeolites are characterised by a strong acidity.
Those (often) synergetic properties led them to exhibit high conversions and excellent selectivities in
many targeted reactions either in petrochemical processes but in more complex reactions as well, for
,
13 14
instance biomass conversion.
Smith et al. highlighted the advantages and limitations of zeolites
1
5
uses in electrophilic aromatic substitutions, leading, in general, to higher para-selectivities.
2

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Our group recently transposed batch conditions for activated aromatics (toluene, anisole) to
gas phase semiꢀcontinuous flow operations for the chlorination of activated and deactivated arenes
1
6
(
toluene, chlorobenzene and nitrobenzene). An interesting conclusion of this work was the
correlation between the zeolite pore topologies and their ability to convert variously substituted
aromatics. Indeed, an optimal activity for the following couples has been reported: HꢀUSY ꢀ
16
nitrobenzene, HꢀZSMꢀ5 ꢀ chlorobenzene and HꢀZSMꢀ5 ꢀ toluene. Although this process already
avoids the use of toxic or volatile solvents and gaseous chlorine, we have to state the remaining
downsides consisting in a generally low productivity and relatively high reaction temperatures.
Furthermore, the sole fact that the operation was in a semiꢀcontinuous mode further hampers the
process viability. Nevertheless, these pioneering studies are pointing towards the way to design a truly
ecoꢀcompatible and competitive halogenation process (Figure 1, step 5).
N
N
R
2
CuHal
O
Hal
Hal
Hal
O
N
N
5
1
O
N
Hal
HHal
Hal
R
R
R
H-Zeolite
Continuous flow
CuHal / CuHal2
H-Zeolite
with
O
O
Hal
Hal
O
Cl
Cl
O
3
N
N
4
N
N
O
N
Cl
O
N
H SO in
HOAc
Batch
2
4
Hal
Semi-
Continuous
Flow
R
R
Figure 1. Different reaction pathways to produce halogenated aromatics 1) RaschigꢀHooker
6
7 8
,
process, 2) Sandmeyer reaction, 3) T(Hal)CA in superelectrophilic medium, 4) Semiꢀ
16
continuous chlorination reaction, 5) Heterogeneous continuous flow process presented
herein.
In the present work, we report the screening of heterogeneous catalysts for converting
iodobenzene into monochlorinated chloroiodobenzenes using trichloroisocyanuric acid (TCCA) as
chlorination agent. This reaction was chosen as a benchmark reaction for the optimisation of reaction
conditions. Nevertheless, its intrinsic interest is not negligible, because chloroiodobenzenes (High
1
7
value added chemicals cf. Table S1) are used in polymer chemistry, and in stepwise CꢀC coupling
1
8
reactions. However, the latter molecules are still produced with gaseous chlorine over AlCl
3
, FeCl
3
,
19 20
5
or SbCl catalysts.
Then, once optimised conditions could be set up towards an ecoꢀcompatible
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halogenation of variously substituted arenes in a batch reactor. Ultimately, we report a transposition
attempt to develop a continuous flow liquidꢀsolid process.
2
. Experimental
2.1 Catalyst preparation
Homogeneous catalysts, tested for comparison, were sulphuric acid (Aldrich, 98%) and
aluminum trichloride (Aldrich, > 99.0%) and were used as received.
Three different *BEA zeolites were synthesised and named hereafter *BEAꢀMC (MC) for
microcrystals, *BEAꢀNC (NC) for nanocrystals and *BEAꢀNS (NS) for nanosponges. The procedures
applied to obtain those materials are described in the S.I. and were recently reported by Astafan et
2
1
al.
The Hꢀforms of MC, NC and NS zeolites were obtained by double ion exchange of calcined
ꢀ1
zeolite with a 1M NH NO solution (liquid/solid ratio: 20 mL.g ) at 80°C for 1h and calcination in a
4
3
static oven at 550°C for 5h.
2.2 Catalyst characterisation
The crystallinity and purity of asꢀsynthesised materials were checked by powder Xꢀray
diffraction (XRD) on a PANalytical MPD X’Pert Pro diffractometer operating with Cu Kα radiation (λ
=
0.15418 nm) and equipped with an X’Celerator realꢀtime multiple strip detector (active length =
.122° 2θ). The powder pattern was collected at room temperature in the range 3 < 2θ < 50° with a 2θ
angle step of 0.017° and a step time of 220s. Small angle powder Xꢀray diffraction patterns in the 0.5ꢀ
0° region were recorded on a glass plate with a 0.02° step size (step time = 1s) and a variable slit
2
1
mode. The morphology, homogeneity and particle sizes of the samples were investigated with a
scanning electron microscope (SEM) (Philips XL30 FEG) and by transmission electron microscopy
(
TEM) using a Philips model CM200, under an acceleration voltage of 200 kV, with a pointꢀtoꢀpoint
resolution of 0.3 nm.
The Si/Al molar ratios were determined by XꢀRay Fluorescence spectrometry (Philips, Magic
X). The Al zeolite framework (AlFram) content was determined by infrared spectroscopy measurements
on a FTꢀIR Magna 550 Nicolet spectrometer. The position of the zeolite structural bands (450ꢀ1250
ꢀ1
ꢀ1
cm ) and especially that of the asymmetric stretch vibration at 1080ꢀ1200 cm allows estimating the
2
2
AlFram from the correlation given in literature. The TOT bands were determined using KBr wafers
containing 2 wt % of sample.
Homeꢀmade H/D isotope exchange technique was used to determine the number of Brønsted
2
3
acid sites present in the different solid acids.
4

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A Micromeritics 2420 ASAP was used for nitrogen sorption measurements. Prior to each
experiment, 50 mg of the calcined zeolite samples were outgassed to a residual pressure of less than
0
.8 Pa at 300°C for 15h. Nitrogen sorption measurements were performed at ꢀ196°C. Specific surface
areas were determined from the BrunauerꢀEmmetꢀTeller (BET) equation in the p/p range between
.05 and 0.25. The total pore volume was determined from the nitrogen adsorbed volume at p/p
.99 and the tꢀplot method was used to distinguish micropores from mesopores.
Infrared spectra (FTꢀIR) of pyridine (Py) adsorbed at 100°C were recorded on a Nicolet
0
0
0
0
=
ꢀ1
Magna 550 FTꢀIR spectrometer with a 2 cm optical resolution. After establishing a pressure of 1 Torr
at equilibrium, the cell was evacuated at 150°C to remove all physisorbed species. The amounts of
+
pyridine adsorbed on the Brønsted [PyH ] and Lewis [PyL] sites were determined from the integration
ꢀ1
24
of the band at 1545 and 1454 cm , using extinction coefficients previously determined.
The textural and acidic properties characterised by H/D isotopic exchange of commercially
available materials are listed in Table 1.
Table 1. Characterisation of the different commercial solid acid catalysts.
+
Entry
Catalyst
Si/Al
n(H )
S
BET
Pore
Crystal size [nm]
+
a
2
ꢀ1
[
mmolH /gcat
]
[m .g ]
425
size [Å]
1
2
3
4
5
6
7
ZSMꢀ5 (Zeolyst CBV2314)
ZSMꢀ5 (Zeolyst CBV5524)
MOR (Zeolyst )
13
25
10
3.6
2.2
6.5
ꢀ
1.48
0.86
1.90
3.80
5.37
2.38
0.32
5.5
5.5
7
< 100
< 100
425
550
1000 ꢀ 1500
500ꢀ700
500ꢀ800
2000
USY (Zeolyst)
750
12
12
12
ꢀ
Y (Aldrich)
710
EMT (UOP)
631
Cs
2
HPW12
O
40
123
1000ꢀ2000
100 ꢀ 400
b
8
*BEA (Zeochem)
14
1.07
620
6
a
b
Acid site densities are obtained by H/Dꢀisotope exchange ; Precise pore dimensions for *BEA micropores: 6.6
x 6.7 and 5.6 x 5.6 Å.
2.3 Catalytic testing
Classic batch conditions. Catalytic reactions were initially carried out in 5 mL
dichloroethane, to which iodobenzene (1 mmol, 0.112 mL, 1 equiv.), TCCA (0.34 mmol, 0.079 g, 0.34
equiv.) and the respective heterogeneous catalyst (100 mg) were added (isomass conditions were
chosen). The solution was stirred at 80°C and 500 rpm velocity (a yellow coloration was observed).
0
+X
The samples withdrawn for GC analysis were treated to neutralise residual TCCA, Cl ꢀ or Cl
species. Namely, 0.1 mL of the reaction mixture was filtered over a celite filled cartridge with 2 mL
CH Cl and vigorously stirred with 3 mL of a 10 wtꢀ% Na S O solution. From the resulting biphasic
2
2
2
2
3
2 4
solution, the organic layer was separated and dried over Na SO and finally injected in the gas
5

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chromatograph (GC II 5890 Hewlett Packard) equipped with a capillary column (PONA, 50 m) and a
flame ionisation detector (FID). A typical chromatogram and integration results are given in S.I.
(
Figures S1 and S2).
In principle, the chlorination of iodobenzene leads to monoꢀ, diꢀ or triꢀsubstituted aromatics.
Para- and orthoꢀsubstituted aromatics are favoured. The degree of conversion and the selectivity
toward the different products were calculated by taking into account the response factors from the
reagent iodobenzene and those from the products (monoꢀ, diꢀ and triꢀchlorinated aromatics) through
the use of an external standard (nꢀheptane). For the different reaction conditions, different substrates
and solvents were needed for the interpretation of GC results in order to determine conversions and
selectivities. They were realised by adding the substrate (1 mmol), dichloroethane (0.08 mL, as
internal standard) to 5 mL of solvent. 0.1 mL were extracted and mixed with 2 mL of EtOAc. The
solution was injected three times in the gas chromatograph.
Then, blank experiments were carried out without catalyst and with two homogeneous
catalysts : sulphuric acid and aluminum trichloride, tested under isoꢀsite conditions with respect to 100
+
mg HꢀZSMꢀ5 zeolyst CBV2314, ergo 0.148 mmol H or Al which corresponded to; 8 µL H
2
SO
4
and
2
0 mg AlCl . Besides, different solvents (5 mL) were tested in the benchmark reaction with
3
iodobenzene (1 mmol, 0.112 mL, 1 eq.), TCCA (0.34 mmol, 0.079 g, 0.34 equiv) and commercial
BEA zeolite (100 mg).
After replacing hazardous solvents (Table 4), the scope of *BEAꢀcatalysed halogenation
reactions was evaluated by testing different halogenating agents: Nꢀchlorosuccinimide (NCS, Aldrich,
*
99%), Nꢀbromosuccinimide (NBS, Aldrich, 99%) and Nꢀiodosuccinimide (NIS, Alfa Aesar, 99%).
Two different kinds of operations were applied as following:
a) Batch conditions (optimised). Reactions were carried out in 5 mL acetic acid, aromatic
substrate (1 mmol, 1 eq.), TCCA (0.34 mmol, 0.079 g, 0.34 equiv), dichloroethane (1 mmol, 0.08 mL)
as an internal standard for GC analysis and *BEA (100 mg) were added (isomass conditions). The
solution was stirred at 80°C and 500 rpm velocity (a pale yellow coloration was observed). The
0
+X
samples withdrawn for GC analysis were treated to neutralise residual TCCA, acetic acid, Cl ꢀ or Cl
species. Namely, 0.1 mL of the reaction mixture was filtered over celite filled cartridge with 2 mL
EtOAc and vigorously stirred with 3 mL of a 10 %wt. K CO solution. From the resulting biphasic
solution, the organic layer was separated and dried over Na SO and finally injected in the gas
ꢀ
2
3
2
4
chromatograph (GC II 5890 Hewlett Packard) equipped with a flame ionisation detector (FID).
b) Continuous flow. A glass Uꢀshaped tubular reactor (Figure 2) was used in a continuous
flow chlorination operation. The reactants TCCA (316 mg, 1.36 mmol), iodobenzene (0.448 mL, 4
mmol) and dichloroethane as an internal standard (0.32 mL, 4 mmol) were previously solubilised in
acetic acid and charged into a 20 mL syringe. The Uꢀshaped glass reactor was packed with 33 mg of
respective *BEA zeolite. Those loadings were chosen to set the same contact time corresponding to 12
consecutive runs under batch conditions.
6

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Recycling was tested; solid catalysts were reꢀengaged up to ten times after centrifugation and
2
washing three times with 5 mL H O/EtOH (50/50). No reꢀactivation was needed between consecutive
runs.
Figure 2. Continuousꢀflow synthesis of chloroarenes.
3
. Results discussion
Commercial catalysts were evaluated in the benchmark chlorination reaction of iodobenzene
in order to find an optimal heterogeneous catalyst. Those catalysts are characterised in Table 1
(
general screening).
Table 2. Screening of the catalysts. Data obtained after 1h of reaction at 80°C.
Entry
Catalyst
Conversion [%]
S(Monoꢀchlorinated S(paraꢀ
S(orthoꢀ
products) [%]
product) [%] product) [%]
1
2
3
4
5
6
7
8
9
ꢀ
2
0
0
0
AlCl
SO
3
28
80
20
11
21
72
66
49
4
84
88
68
71
89
92
92
86
84
97
46
44
42
53
50
49
52
49
45
58
38
44
26
18
39
43
40
37
39
39
H
2
4
HꢀZSMꢀ5 (13)
HꢀZSMꢀ5 (25)
HꢀMOR (Zeolyst)
HꢀUSY (Zeolyst)
HꢀY (Aldrich)
HꢀEMT (UOP)
1
0
1
Cs
Hꢀ*BEA
Zeochem)
2 40
HPW12O
1
71
(
Reaction conditions: 1 mmol iodobenzene, 0.33 mmol TCCA, 100 mg zeolite at 80°C, 1h, 5 mL DCE.
Conversions were calculated based on the disappearance of the reactant. Selectivities are determined after
corresponding calibration.
In the general screening, various acid site densities, pore sizes and topologies as well as crystal
sizes were compared (Table 1). *BEA zeolite (entry 11, Table 2) appeared as the most promising
7

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candidate, leading to a 71% iodobenzene conversion and extremely high 97% selectivity in
monochlorination products. Therefore, as it will be presented below, different kinds of *BEA zeolites
were prepared and compared for this reaction.
It is worthy to mention that in the absence of catalyst almost no reaction occurred (entry 1). In
contrast, homogeneous Lewis and Brønsted acids led to medium and high iodobenzene conversions
(
entries 2ꢀ3). Especially, sulphuric acid efficiently catalysed this chlorination reaction (80%
8
,9
conversion with a 88% selectivity). This result is in agreement with earlier studies. However, no
special regioselectivity can be observed as a 1 to 1 ortho- para-chloroiodobenzene mixture was
produced (entry 3). This result corresponds to the thermodynamical product distribution in the absence
of any zeolite related shape selectivity. Brønsted solid acids led to significant variations in conversions
and selectivities, which is obviously linked to their intrinsic properties: acid site density, pore
topology, crystal size and specific surface area (entries 4ꢀ9).
2
ꢀ1
2
Cs HPW12O40 heteropolyacid with a high SBET of 123 m .g remains almost inactive (entry 10)
indicating that a dispersed Brønsted acidity is not sufficient to perform the halogenation. Mediumꢀpore
sized ZSMꢀ5 zeolites led to conversions in the 11ꢀ20% range, proportional to the acid site density.
Interestingly, the selectivity in monochlorination products seems to be independent from the
conversion. Larger pore zeolites (entries 7ꢀ9) allowed to achieve higher conversions, i.e.; 72, 66 and
49 % for HꢀUSY, HꢀY and HꢀEMT, respectively. These results are in agreement with former studies
16a,c,e
which highlighted a better performance of USY and EMT zeolites for nitrobenzene chlorination.
Surprisingly, only 21% conversion could be achieved over HꢀMOR which may also indicate a fast
deactivation in 1D zeolite pores (entry 6).
Table 3. Characterisation of *BEA zeolites.
+
+
c
Entry
Catalyst
Si/Al
Si/Al
n(H ) [mmol H /gcatalyst
]
S
BET
V
µ
V
meso
Crystal
a
b
2
3
3
Global
Framework
[m /g]
[cm /g] [cm /g] size [nm]
Brønsted
Lewis
d
d
d
d
1
2
3
4
*BEA
13
ꢀ
0.50
0.51
560
0.24
0.24
0.23
0.30
ꢀ
100ꢀ400
40
*BEA NC 15
*BEA MC 14
*BEA NS 17
23
23
22
0.44
0.62
0.13
0.34
0.11
0.18
726
626
977
0.71
0.05
5000
< 50
0.74
a
b
ꢀ1
25
c
measured by XRF ; drawn from TꢀOꢀT band at 1080ꢀ1200 cm using the correlation given in
;
Acid sites
ꢀ
1
d
26
determined by integrating and normalizing Pyꢀabsorption bands at 1540 and 1455 cm ; Values taken from .
Interestingly, the highest selectivity towards targeted products was achieved over commercial
*BEA zeolite. This interesting result led us to focus on series of rationally designed *BEA zeolites
(
Table 3). According to the characterisation studies, those data show that the three different zeolites
obtained exhibit similar Si/Al ratios but significantly differ in crystal size. Consequently, major
8

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differences are observed in terms of textural properties: external surface areas (SBET ranging from 560
2
ꢀ1
to 977 m .g in entries 1 and 4 in Table 3), porosities which may ‘a priori’ impact the diffusion of the
reactants and products throughout the zeolite frame. It is important to note that the commercial zeolite
*
BEA is very similar to the NC sample which will be confirmed below by the characterisation but as
well in the catalytic behaviour.
Asꢀprepared *BEA zeolites: microꢀcrystal (MC), nanoꢀcrystal (NC) and nanoꢀsponge (NS
samples were thoroughly characterised by XRD, XRF, N adsorption, SEM, TEM and FTIR. Their
)
2
main features are summarized in Table 3 and Figures 3 and 4. Additional characterisation of these
materials can be found in the recent study dealing with their use in the ethanolꢀtoꢀhydrocarbons
21
reaction.
The XRD patterns of the three calcined samples are presented in Fig. 3. Whatever the
synthesis protocol, the only crystalline phase formed remains *BEA. For the NC and NS samples, the
reflections are both far less intense and broader than the intensities found in MC sample, probably due
to a smaller crystal size. For the NS sample, a broad peak in the lowꢀangle region of the XRD pattern
reveals a quite ordered mesoporosity (Fig.3d).
5
4
4
3
3
2
2
1
1
0000
5000
0000
5000
0000
5000
0000
5000
0000
20000
18000
16000
14000
12000
10000
8000
6000
4000
2000
0
a
b
(
311)
(
101)
(311)
(101)
(
106)
(008) (306)
(
116)
(106)
(008)
(
004)
(
004) (201)
(320)
(208)
209)
(611)
(306)
30
5
000
(424)
(320)
(209)
(
(611)
45
0
5
10
15
20
25
30
35
40
45
50
5
10
15
20
25
35
40
50
2Theta/CuKα (degree)
2Theta/CuKα (degree)
20000
18000
16000
14000
12000
10000
350000
d
c
300000
(
101)
250000
200000
8000
6000
4000
2000
0
150000
100000
50000
0
(
311)
(
004)
(
008) (306)
(
611)
5
10
15
20
25
30
35
40
45
50
0
1
2
3
4
5
2Theta/CuKα (degree)
2Theta/CuKα (degree)
Figure 3. XRD patterns of calcined a) micronꢀsized *BEA, b) nanoꢀsized *BEA and c) nanoꢀ
sponge *BEA, d) Low angle XRD pattern of calcined *BEA nanosponges.
SEM images for the three samples are given in Fig.4. SEM micrograph of MC crystals
prepared in the presence of TEAOH exhibits the characteristic truncated bipyramidal shape of beta
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2
7
zeolite with crystal sizes ranging from 6 to 10 µm. Pseudoꢀspherical crystals with an average size of
0 nm are observed for the NC sample. *BEA NS sample, synthesised from a polyꢀquaternary
ammonium surfactant (N4ꢀphe), exhibits a spongeꢀlike morphology with nanoꢀsized zeolite particles
4
(
Figure 4c). TEM micrographs demonstrated a somewhat ordered interꢀcrystallite mesoporosity for
2
1
NS sample as compared to the NC sample.
a
b
c
Figure 4. SEM micrographs of different *BEA zeolites a) MC, b) NC and c) NS.
Figure 5 presents the conversion of iodobenzene which seems to be related to the acid site
density, except for the investigated *BEAꢀzeolites. Indeed, the *BEA framework topology may
provide a peculiar spatial arrangment, where an optimum can be found. Indeed, this seems to be
achieved between a molecular diffusion inside the porous network and the acid site density. *BEA
zeolite displays two types of 12 memberedꢀring channels having 0.66 x 0.67 and 0.56 x 0.56 nm,
3
1
respectively. Iodobenzene exhibits a C2v symmetry with a CꢀI bond length of 0.21 nm. The size of
the aromatic ring fits well in the [001] channels (0.56 nm). Furthermore a nonꢀflat diffusion is also
possible at the channel intersections or in close to 0.7 nmꢀsized [100] channels for iodobenzene.
The studied reaction is an electrophilic aromatic substitution comparable to Friedel Crafts
2
8
acylations for which *BEA zeolites are reportedly very competitive catalysts.
Figure 6 then,
presents the o/pꢀratio of the benchmark reaction as a function of the crystal size of the different tested
zeolites, shows that *BEA zeolite's medium sized pores favour the production of paraꢀsubstituted
aromatics, large crystals, however, leading to long residence times, favour the production of the
thermodynamically favoured 1/1 p/oꢀmixture.
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Figure 5. The conversion of the benchmark reaction seems to be related to the acid site
density, except for the different *BEAꢀzeolites.
Figure 6. The o/pꢀratio as a function of the crystal size, large crystals exhibit long residence
times which favour the production of thermodynamically favoured 1/1 o/pꢀmixture.
It is therefore worthy to mention that this relatively high paraꢀselectivity can be
attributed to a special confinement of iodobenzene molecules within [100] and [001] *BEA
channels, thus inducing improved adsorption/desorption steps on fewer acid sites with respect
to FAU structure. Similar spatial confinement has already been reported for bromination
32
reactions in singleꢀwall carbon nanotubes having a diameter of 0.7 nm.
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Table 4. Catalytic data collected for *BEAꢀzeolites in the chlorination of iodobenzene.
Entry Catalyst
Conversion
S(monoꢀ
S(para)
[%]
58
S(ortho)
[%]
39
TOF [mol Iodobenzene conv.
a
[%]
chlorination) [%]
/ mol H+ * 1/h]
1
2
Hꢀ*BEA
Hꢀ*BEA
MC
71
98
97
15.2
11.6
72
50
47
3
4
Hꢀ*BEA NC
Hꢀ*BEA NS
68
86
98
97
57
59
41
38
15.5
66.2
reaction conditions: 1 mmol iodobenzene, 0.33 mmol TCCA, 100 mg zeolite at 80°C, 1h, 5 mL DCE.
Conversions were calculated based on the disappearance of the reactant. Selectivities were determined after
corresponding calibration. a: TOF were calculated after 60 min of reaction.
Table 4 shows the performances of these different *BEA zeolites. The design of *BEA
zeolites is indeed possible since one can observe an improvement in conversion after 60 min reaction
time (from 68 up to 86%). Likewise, a strong impact on TOF values could be evidenced i.e.; up to 9
times higher over the nanosponge beta zeolite compared with the commercial zeolite. The general
trend for these TOF values is the following: *BEA NS > NC = commercial *BEA > MC (Table 4)
which is a sound indication of a positive impact of hierarchical ordered porosity on the catalytic
properties of *BEA zeolites. This is supported by the fact that *BEA NC and NS zeolites exhibit high
21
external SBET values with respect to *BEA MC sample.
Figure 7. Reaction profile versus time of the batch chlorination of iodobenzene over different
asꢀsynthesised *BEA zeolites.
The change in crystal sizes further impacted the catalyst activity (Figure 7), where a
conversion limit of about 70% was achieved over MC sample, whereas nanosized samples further
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converted iodobenzene suggesting that mass transfer limitations could hamper the reaction paths in
*BEA MC zeolite and favour deactivation due to pore blocking.
After studying these different beta zeolites and evidencing the huge potential of tailorꢀmade
zeolites for improvement in this reaction if used at larger scale, our main goal now was to optimise the
catalytic reaction in order to setꢀup an ecoꢀcompatible process. Based on the results and
characterisation data, it is worthy to try to propose a ''greener'' halogenation process. In the following
part, the cheapest (commercial zeolite) was chosen for an optimisation of the reaction conditions:
solvent, scope of the reaction and lastly the development of a continuous process.
Table 5. Screening of solvents for the iodobenzene chlorination reaction.
Entry Solvent (pK
a
)
Conversion
%]
S(Monoꢀchlorinated
S(paraꢀ
S(orthoꢀ
[
products) [%]
product) [%]
product) [%]
1
2
3
4
5
6
7
8
9
1
1
1
1
1,2ꢀDCE
DMF
48
0
98
0
58
0
38
0
MeCN
Toluene
84
44
85
43
56
49
100
48
99
96
68
93
0
45
0
48
0
a,b
b
H
2
O (15.7)
71
23
54
97
83
9
39
15
25
51
47
4
31
7
b
2ꢀpropanol (16.5)
AcOEt
b
28
47
36
3
ꢀ
AcOH
c
0
AcOH
d
1
TFAOH (0.23)
HCOOH (3.77)
AcOH (4.76)
92
91
96
43
53
63
45
38
34
d
2
3
d
+
reaction conditions: 1 mmol iodobenzene, 0.33 mmol TCCA, 100 mg Hꢀ*BEA corresponding to 0.05 mmol H
5 molꢀ%) at 80°C, 30 min, 5 mL solvent. Conversions were calculated based on the disappearance of the
(
reactant. Selectivities are determined after corresponding calibration (Fig. S2).
a: toluene in excess as compared to iodobenzene is chlorinated, iodobenze degradates.
b: low monochlorination selectivities are due to both iodobenzene degradation for instance to form
chlorobenzene or diꢀ/triꢀchlorination.
c: blank experiment without Hꢀ*BEA.
d: same reaction conditions than in entry 9, but to compare the different carboxylic acids these tests have been
carried out at room temperature 25°C in order to evidence a link between the pKa of the solvent and the solventꢀ
catalyst activity.
A solvent screening, from apolar aprotic to polar and protic, was undertaken taking into
,
29 30
account GSK's expanded solvent selection guide.
Interestingly, acetic acid a sustainable and nonꢀ
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toxic solvent has proven to be the most promising for the reaction (Table 5). Indeed, it allows gaining
in catalytic activity while a very high selectivity towards monochlorinated products could be
maintained. As a weak Brønsted acid, acetic acid alone did not catalyse the reaction in the absence of a
strong heterogeneous Brønsted acid (Entry 9, Table 5). Several other interesting trends could be drawn
from this solvent screening. The reaction seems to perform better in protic polar solvents. In the case
of toluene (Entry 4), the aromatic solvent was chlorinated, while iodobenzene suffered from halogen
exchange forming chlorobenzene. The same phenomenon also occurred in 2ꢀpropanol and ethyl
acetate (entries 4ꢀ7). A further set of experiments carried out at room temperature with different
3 3
carboxylic acids (CF COOH, HCOOH and CH COOH entries 11ꢀ13) underlines the link between the
solvents pK and the activity of the considered solventꢀcatalyst system. Interestingly, at stronger
a
acidities the reactions seems to be catalysed by the solvent itself since the p/oꢀratio changes from 1.9
to 1.4 to 1 when comparing acetic acid to formic acid and to trifluoroacetic acid, respectively.
Figure 8. Proposed reaction mechanism leading to the observed byꢀproducts distribution
While chlorination reactions of aromatics with chlorine gas in apolar aprotic solvents tend to
involve radicals, in acetic acid with a Brønsted acid catalyst the ionic pathway is more plausible
(
Figure 8). Depending on the solvent and the zeolite used, we observed an undesired reaction leading
to the formation of chlorobenzene (up to 10% in selectivity). The earlier mentioned side reaction of
the halogen exchange may occur through the intermediate formation of iodobenzenedichloride, which
subsequently decomposes in contact with a zeolite to form chlorobenzene or orthoꢀsubstituted
chloroiodobenzene. Several tests have been performed to support this tentative explanation: when
iodobenzene was in contact with TCCA (with or without zeolite), one was to detect
1
13
iodobenzenedichloride by HꢀNMR and CꢀNMR (Fig.S3ꢀS6). In addition, a yellow coloration of the
reaction mixture could be observed suggesting the presence of PhICl and ICl. In the case of zeoliteꢀ
2
catalysed reaction, chlorobenzene could be detected by gas chromatography. In contrast, for the
chlorination of bromobenzene, which should not form bromobenzenedichloride intermediates, the
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halogen exchange reaction, thus the formation of chlorobenzene could not be observed. Furthermore
no coloration of the reaction mixture could be observed.
After finding optimal reaction conditions for our benchmark reaction, we steered our focus on
the extension of the reaction scope. Variously substituted aromatics were submitted to those optimal
conditions. For comparison, several other halogenating agents were also investigated (Table 6).
With TCCA, the monochlorination was achieved for all arenes, except with strongly
deactivated nitrobenzene (entry 6). As ‘a priori’ expected, a classical trend was observed with more
electronꢀrich aromatics led to higher conversions (Entries 5, 8 and 9). The unꢀfunctionalised and
cheapest aromatic benzene was quite successfully converted to the corresponding iodoꢀ, bromoꢀ or
chlorobenzenes (entries 1, 12 and 13). It is of utmost importance to note that these compounds could
be obtained very selectively (91ꢀ100%) in their monoꢀhalogenated forms and that non converted
aromatics may be reꢀcycled in a continuous flow set up at a larger scale. Rewardingly, the cheaper
TCCA is a better chlorinating agent both in terms of activity and selectivity than NCS (entries 1 vs 10
and 5 vs 9). Using NBS and NIS as brominating or iodinating agent the reaction also proceeds at room
temperature with low to modest conversion (entries 11ꢀ15).
Table 6. Scope of Hꢀ*BEAꢀcatalysed chlorination reaction.
Entry Reactant T [°C}
Halogenating Conversion S(Monoꢀ
S(paraꢀ
product)
[%]
S(orthoꢀ
product)
[%]
reactant
eqiv.)
[%]
chlorinated
(
products) [%]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
R= H
80
80
80
80
80
120
80
TCCA (0.33) 44
TCCA (0.33) 66
TCCA (0.33) 38
TCCA (0.33) 48
TCCA (0.33) 100
TCCA (0.33) < 5
TCCA (0.33) 61
TCCA (0.33) 89
91
ꢀ
ꢀ
R = Me
R = Cl
R = Br
R = I
98
73
58
52
59
ꢀ
25
40
48
39
ꢀ
100
100
83
R = NO
R = NH
2
2
100
84
37
79
45
ꢀ
59
21
35
ꢀ
R = OMe 80
100
80
R = I
80
80
20
80
20
80
20
NCS (1)
NCS (1)
NBS (1)
NBS (1)
NIS (1)
NIS (1)
56
26
18
40
37
86
< 5
0
1
2
3
4
5
R = H
R = H
R = H
R = H
R = H
R = H
2
100
76
ꢀ
ꢀ
ꢀ
ꢀ
100
89
ꢀ
ꢀ
ꢀ
ꢀ
I
2
(1)
100
ꢀ
ꢀ
+
reaction conditions: 1 mmol aromatic, 1 equiv. Halogen, 100 mg Hꢀ*BEA corresponding to 0.05 mmol H (5
molꢀ%) at 80°C, 30 min, 5 mL acetic acid. Conversions were calculated based on the disappearance of the
reactant. Selectivities are determined after corresponding calibration.
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Unfortunately, at higher temperatures the high selectivity towards monohalogenated products
decreases (entries 11 vs. 12 and 13 vs. 14), when compared to reactions carried out at room
temperature. The reaction with molecular iodine did not lead to significant degree of conversion (entry
15).
For large scale applications, selectivity plays a crucial role whereas conversion limitations can
be overcome by recycling unconverted reactant. This is of special interest when applied in a flow
process. Therefore, we performed a recyclability test for our system: commercial *BEA zeolite
exhibited a limited loss in activity after 10 reaction cycles, whereas *BEA NS zeolite deactivated
much faster (Figure 9). This might be due to its higher specific surface area, thus adsorbing more
water or other detrimental molecules which cannot be removed between consecutive runs. In addition,
the short crystal thickness of *BEA NS zeolite, combined with its low acidity, may limit the growth of
coke molecules as well as their accumulation inside micropores. In other reactions, however, in the
ethanolꢀtoꢀgasoline process for instance, the extreme downsizing of zeolite crystal led to an
2
1
unexpected pore blocking.
It is noteworthy that, after calcination the activity as well as the selectivity could be
completely reꢀestablished for both catalysts, reinforcing the earlier mentioned hypothesis for
deactivation.
Lastly, both promising nanoꢀsized *BEA zeolites were applied to a continuous flow setꢀup
(
Figure 9). Although *BEAꢀNS seems to deactivate more rapidly in consecutive batch reactions, it is
very tolerant and resistant to the flow setꢀup. Indeed, no different behaviour compared to the *BEAꢀ
NC sample could be noticed. The parameters of the continuous flow process were set to feed the same
iodobenzene quantity found after 12 consecutive runs under batch conditions. A molar hourly space
ꢀ1
velocity of 61 mmol(arene).(g(catalyst).h) was applied for this optimised reaction. Compared with
our early semiꢀcontinuous gasꢀsolid setꢀup exhibiting a molar hourly space velocity 1.7
ꢀ1 16b
mmol(arene).(g(catalyst).h) , this new continuous liquidꢀsolid process can produce more than 30
times faster a desired halogenated arene. Interestingly, both zeolites seem more resistant toward
deactivation under flow operations. Under these conditions, 100 kg iodobenzene may be converted
per kgcat in a single day.
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a
b
Figure 9. a) recycling experiment with commercial *BEAꢀzeolite: 30 min of reaction.
recycling with *BEAꢀNSꢀzeolite: 30 min of reaction and b) continuous flow chlorination of
iodobenzene corresponding to 12 runs under batch conditions.
Finally, a dependence between the solid acid pore structure, more precisely *BEA textural
properties, and activity / selectivity in chlorination reaction could be established. Further studies are
under progress to design a cost efficient flow process.
4
. Conclusion
Ecoꢀcompatible iodobenzene halogenation reaction by trichloroisocyanuric acid has been
developed, which allowed a conversion superior to 95%, with nearly 100 kg iodobenzene converted
per kgcat in a single day.
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Hꢀ*BEA zeolites were found the most performant catalysts among several solid acids and
zeolite structures. According to thorough characterisation (microscopy, H/D exchange, BET, XRD),
structureꢀactivity relationships could be established for various synthetic *BEA zeolites. Nanoꢀsized
*BEA zeolites and more specifically nanospongeꢀlike *BEA crystals exhibited the highest catalytic
ꢀ1
performances with a conversion up to 100%, corresponding to TOF values of 66 h and a selectivity
in monochlorinated products up to 98 %.
The main outcome of this study relies on the ability to setꢀup an ecoꢀcompatible continuous
flow halogenation process catalysed by zeolites (Figures S7 and S8).
Acknowledgements
PL would like to thank the National Research Fund Luxembourg (NRFL) for his PhDꢀ
grant (5898454).
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Green Chemistry
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DOI: 10.1039/C6GC00731G
Continuous iodobenzene chlorination reaction: experimental set-up and conversion - acid site
density dependence.