One-step dehydration and isomerisation of n-butanol to iso-butene over zeolite catalysts

Article abstract of DOI:10.1039/c002240c

The efficient one-step conversion of n-butanol to iso-butene over zeolite catalysts by combined dehydration and isomerisation has been demonstrated. The medium pore-size unidirectional channel zeolites Theta-1 and ZSM-23 show high conversion and stable se

Full text of DOI:10.1039/c002240c

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One-step dehydration and isomerisation of n-butanol to iso-butene over
zeolite catalysts
Dazhi Zhang, Rashid Al-Hajri, Sami A. I. Barri and David Chadwick*
Received 2nd February 2010, Accepted 31st March 2010
First published as an Advance Article on the web 14th April 2010
DOI: 10.1039/c002240c
The efficient one-step conversion of n-butanol to iso-butene over
zeolite catalysts by combined dehydration and isomerisation has
been demonstrated. The medium pore-size unidirectional channel
zeolites Theta-1 and ZSM-23 show high conversion and stable
selectivity to iso-butene.
velocity (0.18 g cmÀ3 À1, GHSV = 54 hÀ1) was required to
h
produce around 31% iso-butene selectivity from n-butanol at
472 1C. Such low space velocities translate to a productivity of
iso-butene of o0.1 kg kg-catÀ1
industrially.
h
À1, which is not practical
Skeletal isomerisation of linear butenes has also been widely
studied.4 Skeletal isomerisation requires strong acid sites and
temperatures from 300 to 500 1C. The yield of iso-butene
is restricted by thermodynamic equilibrium. Zeolites with
10-ring unidirectional channels such as ferrierite, SAPO-11,
Theta-1/ZSM-22 and ZSM-23 have been used in this reaction
and show high selectivity to iso-butene.5 Three mechanisms
have been proposed which are monomolecular, pseudo-molecular
and bimolecular mechanisms.6 The monomolecular reaction
pathway is the most selective because it does not involve the
formation of higher hydrocarbon intermediates.
The production of iso-butene has attracted both industrial and
academic interest in the past decades, as it is widely used as
feedstock for a range of products including octane enhancer
additives, alkylation, polymers and rubbers. Since MTBE was
found to contaminate ground water, iso-butene has been used
to produce alternative octane enhancing additives for gasoline,
such as di-isobutene. Skeletal isomerisation of petroleum-
derived linear butenes has been the main route to iso-butene.
In view of environmental concerns over hydrocarbon-based
processes, production of iso-butene from a sustainable source
such as biomass is highly desirable.
Both dehydration and isomerisation occur over acid sites,
but these two reactions require different temperatures and acid
strengths. It is essential to find an optimum combination of
acidic catalyst and temperature to combine these two reactions
in a single step. In this study, three medium pore-size 10-ring
zeolites with unidirectional channels (Theta-1, ferrierite, and
ZSM-23) were used to carry out n-butanol direct conversion
to iso-butene. ZSM-5, which has 10-ring two-dimensional
intersecting channels, was also studied.w The catalysts were
Recent research shows that n-butanol is a potential product
from biomass fermentation on a large scale.1 n-Butanol can
also be produced from biomass indirectly by gasification
followed by mixed alcohol synthesis. Therefore, bio-butanol
could be a convenient feedstock for production of fuels and
chemicals in the near future. We report here a new process for
direct conversion of n-butanol to iso-butene in one step over
acidic zeolite catalysts (Scheme 1) by combined dehydration
and isomerisation. The selectivity to iso-butene (in the butenes)
reaches that expected from thermodynamic considerations,
giving a yield of about 30 wt% and a productivity exceeding
evaluated in
a continuous fixed-bed microflow reactor.
Following gas–liquid separation, liquid (organic and aqueous)
and gas products were analysed by GC-FID. Carbon balances
exceeded 90% in all cases. Fig. 1 gives the yields of the main
7.0 kg kg-catÀ1 hÀ1
.
Butanol dehydration has been reported extensively in
the literature,2 but few studies describe the conversion of
n-butanol to iso-butene.3 Butanol dehydration is a facile
reaction and takes place over many acidic catalysts at low
temperature. However, skeletal isomerisation cannot usually
occur at such conditions. It requires stronger acidity and
higher temperatures. Macho et al. studied dehydration of
various butanols and skeletal isomerisation over sulfated
g-alumina.3 The results showed that iso-butanol produced a
very high yield of iso-butene, while the linear isomers of
butanol produced a much lower yield. A very low space
Scheme 1
Fig. 1 Product yields from n-butanol conversion over Theta-1.
Department of Chemical Engineering, Imperial College London,
South Kensington Campus, London SW7 2AZ, UK.
E-mail: d.chadwick@imperial.ac.uk; Fax: +44 (0)20 7594 5637;
Tel: +44 (0)20 7594 5579
Reaction condition: n-butanol
: Ar (mol) = 1 : 1, GHSV
(n-butanol) = 5200 hÀ1. Yield of iso-butene (’); yield of n-butenes
(K); yield of propene (m); yield of pentenes (.).
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products of n-butanol conversion over Theta-1 as a function of
temperature. At all the investigated temperatures (300–500 1C),
n-butanol dehydrated completely and almost no oxygenates
were found in the products. At low temperature, around
300 1C, linear butenes were dominant in the product corres-
ponding to a total yield of 92.2 wt%. Only a small amount of
iso-butene (4.7 wt%) was found in the products. The yield of
iso-butene increased dramatically at higher temperatures
reaching over 30 wt% at 400 1C and above. The selectivity
to iso-butene (in the butenes) at 400 1C is close to the thermo-
dynamic value (0.468). Further increase of the temperature
results in a slight decrease in the yield of iso-butene and
corresponding increase in linear butenes. This effect is probably
due to equilibrium limitations since the thermodynamic yield
of iso-butene is known to decrease with temperature.7 The
results presented in Fig. 1 indicated that n-butanol dehydration
and skeletal isomerisation to iso-butene were well-combined at
400–500 1C over Theta-1 catalyst.
The reaction of n-butanol conversion over the different
zeolites was studied at 400 1C. The results are shown in
Table 1. The conversion of n-butanol was complete over all
the zeolites at the reaction conditions. In Table 1, the value of
i-C4Q/total C4Q is indicative of the activity for isomerisation.
For all the catalyst, the value of i-C4Q/total C4Q approximated
to that expected from thermodynamic equilibrium (0.468)
showing high isomerisation activity, but the overall selectivity
to iso-butene over ZSM-5 was dramatically different and the
reason for this is discussed below.
ZSM-5 produces high levels of side products such as
+
propene, pentane, C5 aliphatics, and aromatics. The two
dimensional channel network of ZSM-5 provides cage-like
intersections for the formation of the bulky intermediates. It
is known that increasing the aluminium content of ZSM-5
promotes a higher degree of aromatization, while the Si/Al
effect on the iso-butene selectivity is less pronounced.8 In this
study, ZSM-5 with high Si/Al ratio (280) was chosen particularly
to minimise the yield of liquid hydrocarbons and the hydrogen
transfer activity. Despite this, compared with the zeolites
having unidirectional channels, the ZSM-5 studied shows high
isomerisation activity, as evidenced by the high value of
i-C4Q/total C4Q (0.46), but a low yield of iso-butene, and
high yields of propene, alkanes, C5Q, C6+ (and of aromatics),
due to the dimerisation, cracking and aromatisation side
reactions.
The stability of the zeolite catalysts was also studied. The
results are shown in Fig. 2(a)–(d). Theta-1, ZSM-23 and
ZSM-5 showed good stability of iso-butene selectivity. The
yields of iso-butene and linear butenes over Theta-1 and
ZSM-23 increased slowly with time-on-stream, as the by-products
from hydrogen transfer, oligomerisation and cracking
decreased. The product distributions with time-on-stream over
Theta-1 and ZSM-23 are similar to those reported for linear
butene isomerisation over TON and MTT type catalysts.5 The
decrease of by-products is due to ‘‘coke’’ formation suppressing
side reactions more than butene skeletal isomerisation.9 The
presence of water as a product is probably advantageous in
this reaction as it limits coke formation and serves as a
reactants’ diluent.
As shown in Table 1, high iso-butene yield was achieved
over Theta-1, ZSM-23 and ferrierite at a time on stream of
25 min. Ferrierite shows the highest iso-butene yield (33.8 wt%)
although the value of i-C4Q/total C4Q(0.41) is not the highest.
Theta-1 shows higher activity for isomerisation (0.46) but the
total yield of C4 olefins is lower than ferrierite. ZSM-23 shows
a lower yield of iso-butene (28.2 wt%) compared to Theta-1
and ferrierite, but is very active in isomerisation (0.45). The
marginal differences in the iso-butene yields between Theta-1
and ZSM-23 on the one hand and ferrierite on the other are
due to the differences in the time-on-stream stability, which is
discussed further below. The activity for isomerisation relies
on the acidity of the zeolite. However the yield of iso-butene is
also determined by the channel structure of the zeolite. The
unidirectional medium pore structure of these zeolites promotes
the direct isomerisation of butenes via the mono-molecular
mechanism and suppresses the bimolecular mechanism that
leads to the formation of the butane dimers and their subsequent
In the case of ferrierite, the iso-butene yield decreased with
time-on-stream while the linear butenes yield increased. This
does not correspond to the observed pattern for conventional
linear butene isomerisation.10 Clearly, in the presence of the
product water from dehydration, the activity for skeletal
isomerisation over ferrierite decreases with time-on-stream.
The ferrierite with low Si/Al ratio (20) used here has more
affinity for water than Theta-1 and ZSM-23, which would
weaken the acidity and make ferrierite less effective catalytically
and may also lead to dealumination.
The hydrogen transfer activity of the zeolites, as indicated
by the propane and butane yields, Table 1, had the following
trend: ZSM-5 c Theta-1 > ZSM-23 > ferrierite. The higher
alkane yield of Theta-1 than ZSM-23 reflects the lower Si/Al
ratio. Ferrierite gave the lowest alkane yield although it also
had a low Si/Al ratio. Again, this is most probably caused by
the presence of water lowering the acidity of the acid sites and
cracking to propene and C5Q
.
Table 1 Products from n-butanol conversion over various zeolite catalystsa
Yields (wt%)
0
C3 + C4
0
+
i-C4Q
C4Q-1
t-C4Q-2
c-C4Q-2
C3Q
C5Q
C6
i-C4Q/total C4Q
Catalystb
Theta-1
Ferrierite
ZSM-23
ZSM-5
31.7
33.8
28.2
11.3
9.0
11.8
8.4
16.3
21.6
15.1
6.1
11.6
15.3
10.7
4.2
6.7
5.0
8.5
18.1
7.1
9.6
11.6
19.1
3.7
2.4
3.0
7.8
13.7
6.0
14.2
28.5
0.46
0.41
0.45
0.46
3.1
a
b
. SiO2/Al2O3 ratio: Theta-1 = 63,
Reaction conditions: 400 1C, time on steam = 25 min, n-butanol : Ar (mol) = 1 : 1, GHSV (n-butanol) = 5200 hÀ1
ferrierite = 20, ZSM-23 = 68, ZSM-5 = 280.
ꢀc
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We thank BP for financial support, Professor Martin Atkins
for advice, and Dr Laura Torrente Murciano for help with the
thermodynamic calculations.
Notes and references
w The H-form ferrierite and ZSM-5 were purchased from Zeolyst.
ZSM-23 and Theta-1 were synthesized according to ref. 11. The
as-synthesised samples were calcined at 550 1C, exchanged with
ammonium nitrate solution three times and calcined again at 550 1C
to convert to the H-form.
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Fig. 2 Product yields with time on stream from n-butanol conversion
over (a) Theta-1, (b) ZSM-23, (c) ferrierite and (d) ZSM-5.
Reaction conditions: 400 1C, n-butanol : Ar (mol) = 1 : 1, GHSV
(n-butanol) = 5200 hÀ1. Yield of iso-butene (’); yield of n-butenes
(K); yield of propene (m); yield of pentenes (.).
causing possible dealumination as discussed above. The high
hydrogen transfer activity of ZSM-5 resulted in relatively high
+
yield of C6+, Table 1, in addition to alkanes. Most of the C6
product was octenes from dimerisation of the butenes. The
yield of dimer products follows the same trend in terms of
structural and compositional properties of the zeolites as the
isomerisation and hydrogen transfer reactions.
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In the present study, we have demonstrated the possibility
of a new process to produce iso-butene directly from bio-derived
n-butanol. The zeolite family with medium pore size and
unidirectional channels (Theta-1, ZSM-23) showed high
conversion of n-butanol, high selectivity to iso-butene and
high stability. In contrast, ferrierite showed poor stability.
The zeolite with medium pore size and intersecting channels
(ZSM-5) showed low selectivity.
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4090 | Chem. Commun., 2010, 46, 4088–4090