ChemComm
Communication
Fig. 3 Isomerisation of 1,4-DCB with AlCl
3
–LiCl (BR) (XAlCl3 = 0.67) and
3 3
Fig. 4 Isomerisation reaction of 1,4-BCB with AlCl –LiCl vs. AlCl –[EMIM]Cl
melt recycling (Rc) under various conditions: (A) substitution of leached
(both XAlCl3 = 0.67); 170 1C, 4 h, 20 mol% AlCl
3
.
AlCl
3
, no HCl-gas added, (B) recycling with HCl-gas added, no addition of
(C) recycling with added AlCl and HCl-gas [170 1C, 4 h, 250 g
, 10 mol% (7.21 mol) LiCl,
recycling with 1 bar HCl and/or the addition of 4.54 g AlCl ].
AlCl
3
3
(
1.7 mol) 1,4-DCB, 20 mol% (45.4 g) AlCl
3
reactivity can be derived from the fact that the inorganic
melt systems contain a higher concentration of acidic anions
compared to the organic ones.
3
For recycling, the resulting solid molten salt was re-melted
In conclusion we could demonstrate that alkali chloro-
and fresh 1,4-DCB was added. Two special precautions proved to aluminate melts – and in particular lithium chloroaluminate
be necessary to allow recycling with a constant yield of 1,3-DCB. melts – form highly active and recyclable catalyst phases for the
2
2
First, the AlCl
3
content of the organic product phase was technically relevant reaction of dihaloarene isomerisation.
analysed by ICP to determine the level of Al-leaching. This level Remarkable differences in reactivity have been found between
was found to be about 3% of the previously applied AlCl3. inorganic chloroaluminate systems and their imidazolium
Replacing this loss was found to be beneficial for the catalyst analogues at the same molar ratio of AlCl . It has been further
3
recycling. Secondly, it turned out that recycling is much more demonstrated that HCl partial pressure plays an important role
effective if after the product removal – a process that requires in the catalyst recycling, and thus obviously in the reactivity
the opening of the reactor under inert gas – 1 bar HCl-gas was of the isomerisation system. Replenishing AlCl losses and
3
added to the reaction system. Note that in an industrial applying 1 bar HCl during the reaction resulted in a fully
scenario HCl is always omnipresent around an arene chlorina- recyclable catalyst system for at least five dichlorobenzene
tion plant. With these two measures, the recycling of the acid isomerisation cycles.
catalyst phase worked indeed in an excellent manner with no
loss of activity over at least five recycling runs (Fig. 3C). Thus, in support and for permission to publish this research.
The authors thank LANXESS Deutschland GmbH for financial
2
1
comparison to the Olah system, the acidic lithium chloro-
aluminate system described here has three distinct advantages Notes and references
for DCB isomerisation: (a) drastic reduction of AlCl
limation; (b) straightforward product isolation; and (c) re-use
3
sub-
1
M. Earle, in Ionic Liquids in Synthesis, ed. T. Welton and P. Wasserscheid,
Wiley-VCH, Weinheim, 2008.
of more than 95 mol% of the applied AlCl
isomerisation reactions.
In order to evaluate the scope of the new catalytic system
for the isomerisation of other dihaloarene compounds, we
also studied the isomerisation of 1-bromo-4-chlorobenzene
3
in subsequent
2 S. J. Nara, J. R. Harjani and M. M. Salunkhe, J. Org. Chem., 2001, 66,
616–8620.
J. A. Boon, J. A. Levisky, J. L. Pflug and J. S. Wilkes, J. Org. Chem.,
986, 51, 480–483.
4 C. Meyer and P. Wasserscheid, Chem. Commun., 2010, 46,
625–7627.
J. Estager, J. D. Holbrey and M. Swadzba-Kwasny, Chem. Soc. Rev.,
014, 43, 847–886.
8
3
1
7
5
(BCB) (see the ESI† for details). It was found that the reaction
2
of the BCB substrate required less harsh reaction conditions
compared to the fully chlorinated substrate as expected from
6 A. A. Fannin, Jr., D. A. Floreani, L. A. King, J. S. Landers, B. J.
Piersma, D. J. Stech, R. L. Vaughn, J. S. Wilkes and J. L. Williams,
J. Phys. Chem., 1984, 88, 2614–2621.
2
1
literature reports.
7
8
9
J. Estager, A. A. Oliferenko, K. R. Seddon and M. Swadzba-Kwasny,
Dalton Trans., 2010, 39, 11375–11382.
R. A. Carpio, A. A. Fannin, Jr., F. C. Kibler, Jr., L. A. King and
H. A. Oye, J. Chem. Eng. Data, 1983, 28, 34–36.
R. J. Gale, B. Gilbert and R. A. Osteryoung, Inorg. Chem., 1978, 17,
2728–2729.
+
Interestingly, the acidic [EMIM] chloroaluminate melt exhi-
bits reasonable catalytic activity towards the isomerisation of
BCB still being much less reactive than its inorganic counter-
3
part (Fig. 4). While the AlCl –LiCl system reached 40% of the 1,3
1
1
0 S. J. Cyvin, P. Klaboe, E. Rytter and H. A. Øye, J. Chem. Phys., 1970,
2, 2776–2778.
1 G. Franzen, B. P. Gilbert, G. Pelzer and E. DePauw, Org. Mass
Spectrom., 1986, 21, 443–444.
isomer after less than 1 hour of reaction time, the same final
5
+
yield took 22 h with the [EMIM] melt of the same Al-content.
A reasonable explanation for this marked difference in
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 11705--11708 | 11707