Novel synthesis of isoprene from 3-methylbutan-2-one using phosphate catalysts

Article abstract of DOI:10.1039/a902072a

AlPO₄ and BPO₄ catalyse the conversion of 3-methylbutan-2-one to isoprene in high yields via 2-methylbut-2-en-1-ol as an intermediate.

Full text of DOI:10.1039/a902072a

Novel synthesis of isoprene from 3-methylbutan-2-one using phosphate
catalysts
a
a
a
b
Graham J. Hutchings,* † Ian D. Hudson, Donald Bethell ‡ and Don G. Timms
a
Leverhulme Centre for Innovative Catalysis, Department of Chemistry, University of Liverpool, Liverpool,
UK L69 3BX. E-mail: hutch@cf.ac.uk
Enichem Elastomers Ltd, Charleston Road, Hardley, Hythe, Southampton, UK SO4 6YY
b
Received (in Cambridge, UK) 16th March 1999, Accepted 18th June 1999
AlPO
-one to isoprene in high yields via 2-methylbut-2-en-1-ol as
an intermediate.
4
and BPO
4
catalyse the conversion of 3-methylbutan-
was reacted over the phosphate catalysts (0.3 g) using a standard
2
1
2
laboratory microreactor with nitrogen as a diluent (24 ml h ).
Products were analysed using GC and satisfactory mass
balances were obtained for all data presented. The results for
Isoprene is an important monomer for the stereoselective
polymerisation to 1,4-cis-polyisoprene which is used in the
manufacture of synthetic rubber. Isoprene is currently obtained
from naphtha cracking but in recent years there has been
continued interest in the identification of new synthetic routes.
One interesting possibility is the dehydration of 2-methylbut-
BPO
4
at 325 °C and AlPO
4
at 400 °C are shown in Fig. 1. AlPO
to observe a
4
required a higher reaction temperature than BPO
4
significant conversion level. Both catalysts convert
3-methylbutan-2-one to isoprene with high selectivity
(80–95%) and at reasonable conversion levels. The catalyst
performance was found to be very stable and, significantly, no
appreciable deactivation was observed.
anal which is available as a result of the commercialisation of
1
the low pressure hydroformylation of butene. BPO
4
has been
shown to be an effective catalyst2 for this reaction and can
–4
give 60–70% yield at high conversion, but deactivation can be
5
rapid. Recently, we have shown that BPO
4
can be readily
reactivated using a simple high temperature treatment, thereby
making these catalysts more suitable for industrial application.
However, together with isoprene, 3-methylbutan-2-one is
formed as a major by-product with both phosphate catalysts and
this, at present, limits the commercial applicability of the
process. Here we demonstrate that 3-methylbutan-2-one can be
readily converted to isoprene using BPO
catalysts. Furthermore, we demonstrate that isoprene and
-methylbutan-2-one formation from 2-methylbutanal are
4 4
and AlPO as
3
linked by a common intermediate, 2-methylbut-2-en-1-ol.
These results indicate that by recycling the 3-methylbutan-
2
-one by-product the overall yield of isoprene can be increased
by !10% for both BPO
4
and AlPO catalysts.
4
Boron phosphate (P+B = 1) was prepared by heating
phosphoric acid (93 ml, 85%) with boric acid (100 g) at 60 °C
for 1 h. Water (100 ml) was then added and the mixture was
refluxed for 5 h and then dried (110 °C, 16 h) and calcined
Fig. 1 Reaction of 3-methylbutan-2-one over (a) BPO
4
at 325 °C [(!)
conversion, (:) isoprene selectivity] and (b) AlPO
conversion, (5) isoprene selectivity].
4
at 400 °C [(-)
(
350 °C, 4 h). The product was confirmed to be the cristabolite
form of BPO by X-ray diffraction. Aluminium phosphate
Al+P = 1) was prepared by the slow addition of aqueous
ammonia (40 vol%, 5 °C), with continuous stirring, to an
aqueous solution containing equimolar quantities of AlCl and
4
(
H
H+
O
OH
OH
3
2
1
phosphoric acid (0.985 mol l , 5 °C). Aqueous ammonia
addition was continued until pH = 7.0 was attained. The white
precipitate was aged (18 h, 20 °C) and collected by filtration,
washed several times with propan-2-ol, dried (24 h, 120 °C) and
calcined (3 h, 800 °C). Powder X-ray diffraction showed that
the catalyst was a mixture of the cristabolite and tridymite
H+
–
H–
–H2O
H+
OH2
OH
2-methylbut-2-ene
OH2
2-methylbut-2-en-1-ol
OH
H2O
–H+
4
phases of AlPO . Both these solids were screened (200–250
mesh), and pelleted and sieved (600–1000 m) prior to use as
catalysts. The phosphates were investigated as catalysts for the
dehydration of 2-methylbutanal and isoprene was observed as
OH
OH
H+
5
the major product in agreement with our previous results. For
–
H+
4 4
both BPO and AlPO the only by-product was 3-methylbutan-
–
H2O
2
-one which was formed at ca. 18–20% yield. In a separate set
O
2
1
21
of reactions 3-methylbutan-2-one [0.67 ml (g catalyst)
h ]
†
9
‡
Present address: Department of Chemistry, Cardiff University, PO Box
12, Cardiff, UK CF1 3TB.
Present address: BNFL Ltd, Springfields Works, Salwick, Preston, UK
isoprene
3-methylbutan-2-one
Scheme 1 Proposed mechanism for the conversion of 2-methylbutanal and
PR4 0XJ.
3-methylbutan-2-one to isoprene.
Chem. Commun., 1999, 1489–1490
1489

Table 1 Conversion of 2-methylbut-2-en-1-ol over BPO a
Selectivityb
The observation that both 2-methylbutanal and 3-methylbut-
an-2-one could be converted to isoprene with these catalysts
prompted us to consider the reaction mechanism for these
catalytic processes. We believe that 2-methylbut-2-en-1-ol is a
central intermediate in the conversion of 2-methylbutanal into
both isoprene and 3-methylbutan-2-one (Scheme 1). To test this
proposal, 2-methylbut-2-en-1-ol was synthesised by reduction
of 2-methylbut-2-enal with sodium borohydride in MeOH.
4
T/°C
Conversion 2MBA
2MBE
I
1
3
10
00
87
99
12
25
49
47
39
10
a
(0.3 g), 2-methylbut-2-en-1-ol [0.67 ml (g catalyst) h21], N
21
BPO
24 ml min
I = isoprene.
4
2
2
-Methylbut-2-en-1-ol was then reacted [0.67 ml (g
21 b
(
.
2MBA = 2-methylbutanal; 2MBE = 2-methylbut-2-ene;
2
1
21
21
catalyst)
h ] in a nitrogen diluent (24 ml min ) over BPO
4
(0.3 g) at 110 and 300 °C. At these temperatures, 2-methylbut-
2
-ene was a major product (ca. 10–40%) but in addition
isoprene (37%), 2-methylbutanal (10%) and traces of 3-me-
thylbutan-2-one were also observed and the conversion in-
creased from 87% at 110 °C to 99% at 300 °C (Table 1). These
results support the proposal that 2-methylbut-2-en-1-ol is a key
intermediate in the conversion of 2-methylbutanal into both
isoprene and 3-methylbutan-2-one.
We thank EniChem Elastomers Ltd for financial support for
this work.
Notes and references
1 P. J. Davidson, R. R. Hignett and D. T. Thompson, in Catalysis, A
Specialist Periodical Report, ed. C. Kemball, Chemical Society, 1977,
vol. 1, p. 369.
This study indicates that the yield of isoprene from the
dehydration of 2-methylbutanal, using AlPO
catalysts, can be significantly enhanced if the major by-product,
-methylbutan-2-one, is reacted over the same phosphate
4 4
and BPO
2
3
4
5
H. Fischer and G. Schunchel, UK Pat., UK 1 385 348, 1975.
J. B. Moffat, Rev. Chem. Intermed., 1987, 8, 1.
J. B. Moffat and A. Schmidtmeyer, Appl. Catal., 1986, 28 , 161.
G. J. Hutchings, I. D. Hudson and D. G. Timms, J. Chem. Soc., Chem.
Commun., 1994, 2717.
3
catalyst under the same reaction conditions. It is anticipated that
this can be done via a recycle step and yield increases of ca. 10%
for BPO
5
4
(from 72 to 82 at 325 °C) and 16% for AlPO
4
(from
6 to 72% at 400 °C) can be expected.
Communication 9/02072A
1490
Chem. Commun., 1999, 1489–1490