Tetrabutylphosphonium Bromide Catalyzed Dehydration of Diols to Dienes and Its Application in the Biobased Production of Butadiene

Article abstract of DOI:10.1021/acscatal.7b01765

We report the use of the ionic liquid tetrabutylphosphonium bromide as a solvent and catalyst for dehydration of diols to conjugated dienes. This system combines stability, high reaction rates, and easy product separation. A reaction mechanism for the model compound 1,2-hexanediol is proposed and experimentally corroborated. This particular mechanism allows for the selective formation of conjugated dienes, in contrast with purely acidic catalysis. Next, the reaction is also performed on various other diols. As a first application, we assessed the biobased production of 1,3-butadiene. With 1,4-butanediol as the starting material, a 94% yield of butadiene was reached at 100% conversion.

Full text of DOI:10.1021/acscatal.7b01765

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Article
Tetrabutylphosphonium bromide catalyzed dehydration of diols to
dienes and its application in the bio-based production of butadiene
Maxime Stalpaert, Francisco Garcia Cirujano, and Dirk E. De Vos
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01765 • Publication Date (Web): 19 Jul 2017
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Tetrabutylphosphonium bromide catalyzed
dehydration of diols to dienes and its application in
the bioꢀbased production of butadiene
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Maxime Stalpaert, Francisco G. Cirujano and Dirk E. De Vos*
Centre for Surface Chemistry and Catalysis, Department of Microbial and Molecular Systems,
KU LeuvenꢀUniversity of Leuven, Leuven Chem&Tech, Celestijnenlaan 200F, Post Box 2461,
3001 Heverlee, Belgium.
ABSTRACT We report the use of the ionic liquid tetrabutylphosphonium bromide as a solvent
and catalyst for dehydration of diols to conjugated dienes. This system combines stability, high
reaction rates and easy product separation. A reaction mechanism for the model compound 1,2ꢀ
hexanediol is proposed and experimentally corroborated. This particular mechanism allows for
the selective formation of conjugated dienes, in contrast with purely acidic catalysis. Next, the
reaction is also performed on various other diols. As a first application, we assessed the bioꢀ
based production of 1,3ꢀbutadiene. Starting from 1,4ꢀbutanediol, a 94% yield of butadiene was
reached at 100% conversion.
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KEYWORDS ionic liquids, phosphonium bromide, diol dehydration, conjugated dienes, 1,4ꢀ
butanediol, butadiene
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Introduction
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Conjugated dienes are a valuable class of chemicals, due to their reactivity in important chemical
reactions such as the DielsꢀAlder reaction and various functionalization and polymerization
reactions. As a result, these compounds have tremendous application potential in the production
of both fine chemicals1–3 and polymers4,5. The most important diene from an industrial point of
view is 1,3ꢀbutadiene, which is mostly used for the production of polymers, such as styreneꢀ
butadiene rubber and polybutadiene.4 An interesting approach towards the synthesis of
conjugated dienes in general and 1,3ꢀbutadiene in particular is the dehydration of diols, some of
which can be obtained from renewable resources.6–8 However, in most cases conjugated dienes
are not the predominant products of diol dehydration due to side reactions such as dehydration to
aldehydes, cyclodehydration to cyclic ethers or rearrangements like the pinacol rearrangement.9
In past research, we have shown the activity of tetraalkylphosphonium chloride ionic liquids (IL)
as catalytic solvents in the dehydrochlorination of 1,2ꢀdichloroethane for the production of vinyl
chloride.10 In this reaction, the chloride anion functions as a base to catalyze the elimination
reaction. Since this reaction showed the activity of tetraalkylphosphonium halogenides as
defunctionalization catalysts, we investigated the use of these ionic liquids as dehydration
catalysts. Here we show that tetraalkylphosphonium halogenides, in particular
tetrabutylphosphonium bromide, are active catalytic solvents for the dehydration of diols,
selectively forming conjugated dienes.
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Experimental section
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Catalytic reaction. In a typical reaction 1.7 mmol of an IL was loaded into a glass vial. A
reaction mixture containing 0.5 mmol of substrate and 0.5 mmol of internal standards nꢀhexane
or 2,3ꢀdimethylbutane and nꢀtetradecane in 250 µL mesitylene was added. For substrates that do
not dissolve in mesitylene (for example butanediols), the substrate is weighed and a mixture
containing the internal standards in 250 µL mesitylene is added. Next, 0.022 mmol HBr and,
depending on the reaction, other additives were added. After addition of a magnetic stirring bar,
the vial was sealed under a N2 atmosphere using a cap with septum. The mixture was then heated
to 200 °C while stirring at 500 rpm. After reaction, the mixture was cooled down until it was safe
to handle, but the IL was still liquid. In order to quantitatively extract and analyze the products
and the internal standards, 1 mL of mesitylene was added through the septum using a syringe. A
picture of the phase separation is shown in figure S1. After a short time of stirring, the mixture
was cooled down further on ice. An aliquot of the mesitylene layer was removed with a syringe
for gas chromatographic (GC) analysis. This method allowed to quantify a compound as volatile
as 1,3ꢀbutadiene (with a boiling point of –4 °C); butadiene peaks are observed upon GC analysis
of the mesitylene phase if this compound is formed. Next, the cap was removed to add 1.5
equivalent of BSTFA per hydroxyl group in the substrate. The vial was sealed with a new cap
and the mixture was heated to 60 °C for 1 hour while stirring at 500 rpm. More BSTFA was
added in reactions with large amounts of water or acid. After derivatization, another aliquot was
taken for GC analysis.
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Product analysis and identification. Substrate conversion and product yields were determined
using GC analysis on a Shimadzu GCꢀ2010 with a 60 m CP SIL 5 CB column and an FID
detector. Samples taken after reaction were compared to a sample of the mixture before reaction.
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The peak areas of substrate and products were normalized by dividing them by the peak area of
an internal standard. For volatile compounds (boiling point < 100 °C was chosen as a limit), nꢀ
hexane or dimethylbutane were used as internal standard, to account for losses due to
evaporation. Less volatile compounds were quantified using nꢀtetradecane. To compare the peak
areas of different compounds, the effective carbon number (ECN) was used. This allowed to
calculate product yields. The yield of polar compounds was determined from the silylated
samples, since extraction of polar compounds from the IL proved unsuccessful. The calculated
conversion of the substrate never differed more than a few percent from the total calculated
yield, indicating a good mass balance. Unknown products were identified using gas
chromatography coupled to mass spectrometry (GCꢀMS). The identity of the products was
confirmed by comparing the retention time with that obtained for a commercial sample of the
compound. This was performed on an Agilent 6890N GC, equipped with a HPꢀ5 MS column,
coupled to a 5973N mass spectrometer. Representative gas chromatograms with annotated peaks
are shown in figure S2 and S3 for 1,2ꢀhexanediol and S4 and S5 for 1,4ꢀbutanediol.
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Results and discussion
To compare different tetraalkylphosphonium ILs in the same conditions, and since commercial
samples contain different levels of acidity, they were first neutralized using Bu4POH (see SI).
Then, 0.013 mol HBr (48% in water) per mol of ionic liquid was added. This amounts to 4.4
mol% compared to the diol substrate, so the relative molar amounts are diol : ionic liquid : HBr =
1 : 3.4 : 0.044. 1,2ꢀHexanediol (HDO) was chosen as a model substrate to study the reaction
system. Selective formation of dienes from vicinal diols is challenging as acid catalyzed
dehydration of 1,2ꢀalkanediols typically leads to the formation of aldehydes, through an enol
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intermediate.11,12 However, reaction in all tested ILs leads, rather unexpectedly, to predominant
formation of different conjugated hexadienes, in selectivities of at least 50% (Table 1). Next to
the formation of up to 62% hexadienes after 30 minutes at 200°C, the most important products
are 2ꢀhexanone and hexanal, their acetals with 1,2ꢀhexanediol and their aldol products. 2ꢀHexenꢀ
1ꢀol and 1ꢀhaloꢀ2ꢀhexanol (halo corresponding to the anion of the IL) were detected in yields
around 1%. For clarity, the reaction products have been divided into 5 groups: hexadienes (HD),
2ꢀhexanone and derived products (HON), hexanal and derived products (HAL), 2ꢀhexenꢀ1ꢀol
(HOL) and 1ꢀhaloꢀ2ꢀhexanols (XOH) (see SI). Next to these products, traces of 2,6ꢀdibutylꢀ1,4ꢀ
dioxane, the ether of two hexanediol molecules, were detected in all ionic liquids. Throughout
the reactions in this paper, this ether product is always detected in yields below 1% for reactions
that have not reached full conversion. After full conversion is reached, no ether is detected,
indicating the reversibility of etherification in the reaction conditions. Therefore, the role of the
ether product will not be further discussed.
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Table 1. Product yields in the dehydration of 1,2ꢀhexanediol.
Yield (%)
X
Solvent
Bu4PBr None
Acid HD HON HAL HOL XOH (%)
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1
0
<1
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Bu4PCl
Bu4PBr
Bu4PI
HBr
HBr
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2
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HBr
<1
0
ꢀ
Bu4NBr
Mes
HBr
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0
1
0
2
PTSA
1
ꢀ
9
ꢀ
ꢀ
11
Conditions: 0.5 mmol 1,2ꢀhexanediol, 1.7 mmol ionic liquid or 2 mL mesitylene, 0.022 mmol
acid, 200 °C, N2 atmosphere, 30 minutes. HD = hexadienes, HON = 2ꢀhexanone, HAL =
hexanal, HOL = 2ꢀhexenꢀ1ꢀol, XOH = 1ꢀhaloꢀ2ꢀhexanol, X = conversion, Mes = mesitylene,
PTSA = pꢀtoluenesulfonic acid.
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By using different ILs, the effect of cation and anion on the reaction was tested. Previously, we
thoroughly investigated the influence of the IL cation on basicity for the dehydrochlorination of
1,2ꢀdichloroethane.10 We concluded that the basicity and thus dehydrochlorination activity of the
IL depend on the strength of the interaction between cation and anion. The higher basicity of
phosphonium ILs compared to inorganic salts can be explained by the decreased electrostatic
interaction between cation and anion, caused by the bulkiness and dispersed charge of the
phosphonium cation.13,14 As a result, the anion is more free and its charge is less compensated,
which increases the basicity. Generally, amongst a broad variety of common ILs, the interaction
between cation and anion is weakest for tetraalkylphosphonium cations, resulting in the highest
basicity.15,16 In this research, the cation effect was only briefly investigated (table S2), leading to
the result that Bu4P+ is the optimal cation in this reaction, like in the dehydrochlorination of 1,2ꢀ
dichloroethane. Anion variation seems to have a larger effect on catalysis than cation variation
(Table 1). All tested ILs show a similar product distribution, but Bu4PBr strongly outperforms
the other ionic liquids. Therefore, this IL was selected for further experiments. To compare the
system with a purely acidic dehydration, the reaction was also performed in mesitylene as a
solvent with pꢀtoluenesulfonic acid as the catalyst. In these conditions, hexanal and its acetal are
almost exclusively formed from 1,2ꢀhexanediol, with a selectivity of 90%. As discussed before,
the selective formation of aldehydes and acetals in the acid catalyzed dehydration of 1,2ꢀ
alkanediols is well known.11,12 The entirely different product distribution in the presence of
phosphonium ILs clearly indicates a distinct reaction mechanism.
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In addition to the activity, also the stability of the catalyst is important. No breakdown of
tetrabutylphosphonium chloride (Bu4PCl) or bromide (Bu4PBr) was detected by 1H NMR of the
ionic liquid, nor were volatile decomposition products detected by GC. This was expected, since
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the longꢀterm stability of Bu4PCl at 240 °C in highly acidic conditions was reported before.10
However, reaction in tetrabutylphosphonium iodide (Bu4PI) leads to formation of small amounts
of tributylphosphine, tributylphosphine oxide and butenes and a dark red discoloration of the IL.
This can be caused by the reverse Menshutkin reaction, a typical decomposition pathway for
ammonium ILs.17 This reaction would lead to 1ꢀiodobutane and tributylphosphine. 1ꢀIodobutane
can be dehydroiodinated to form butenes, while tributylphosphine is very sensitive to oxidation,
even in the injection port of the GC. This decomposition was not investigated further, since it
does not occur in Bu4PBr, the most active catalyst by far. For comparison, the reaction was also
performed in Bu4NBr, resulting in severe decomposition of the ionic liquid to tributylamine, 1ꢀ
bromobutane and 1ꢀbutene, most likely due to the aforementioned Menshutkin reaction. The
decomposition temperature of ammonium ILs is known to be lower than for their phosphonium
analogs.17,18 For example, the thermal stability of a series of phosphonium and ammonium
dicyanamides has been compared and the decomposition temperature of the phosphonium ILs
was about 100 °C higher.19 Unsurprisingly, the hexadiene yield was very low for this catalyst,
highlighting the strong catalytic performance of the phosphonium ionic liquids, especially
Bu4PBr.
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As a means to elucidate the reaction mechanism, the time profile of the reaction in Bu4PBr was
investigated, both at 170 °C and 200 °C. At 200 °C, full conversion is already reached after 30
minutes (Figure 1 a). The selectivity towards HD, HON and HAL remains quite constant, around
62, 20 and 18% respectively. Interestingly, the hexadiene products are stable in this system; no
oligomerization products are detected and the hexadiene yield remains constant after reaching
full conversion. In the beginning of the reaction, trace amounts of 1ꢀbromoꢀ2ꢀhexanol (1B2H)
and 2ꢀhexenꢀ1ꢀol are formed, which might indicate that these are intermediate products. The
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reaction profile at 170 °C is more useful to understand the reaction network (Figure 1 b). Full
conversion is reached after 120 minutes at this temperature. The HD selectivity rises during
reaction, eventually reaching a similar selectivity at full conversion as for the reaction at 200 °C.
The HON and HAL selectivities are quite similar to the values reached at 200°C. The most
interesting observation regards the selectivities towards 2ꢀhexenꢀ1ꢀol and especially 1ꢀbromoꢀ2ꢀ
hexanol. After short reaction times at 170°C, these are higher than at 200 °C, and they gradually
decrease. This is an extra indication that these are intermediate products.
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Different reaction parameters were varied to investigate the effects on catalysis. First, increasing
the acid content increases the rate of the reaction (Figure 1 c). The selectivity towards HAL
seems to drop slightly, while the HON selectivity increases with about the same amount. The
selectivity towards HD remains quite stable, especially from 0.01 mmol HBr, or 2 mol%,
onward. Varying the type of acid shows the need for strong acids to obtain a high reaction rate
(Figure 1 d). Furthermore, the reactions with trifluoromethanesulfonic acid (TFSA), pꢀ
toluenesulfonic acid (PTSA) or H2SO4 as coꢀcatalyst demonstrate the compatibility of the
reaction system with strong acids other than HBr. For all these acids, similar results were
obtained. Furthermore, the system is also compatible with a strong solid acid catalyst, as
demonstrated by the use of Nafion. The use of HCl and Amberlyst 15 resulted in lower
hexadiene yields. For HCl this is most likely due to the high volatility of hydrochloric acid (28.3
kPa for 38% HCl in standard conditions) compared to the other acids. This results in a much
lower amount of protons in the ionic liquid than expected from the amount of added acid. Also
Amberlystꢀ15 resulted in lower yields than expected. However, this solid acid is not intended for
use above 120 °C, which can explain the lower yields. Next, increasing the temperature results
in a higher rate, with full conversion already achieved after 5 minutes at 220 °C (Figure 1 e).
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This amounts to an average turnꢀover frequency (TOF) of 2.1 hꢀ1 at full conversion. The
selectivities are again quite stable, reaching a slight optimum at 200 °C. Finally, addition of
water to the IL seems to have little effect either on conversion or yield (Figure 1 f). Besides the
variation of different reaction parameters, we also confirmed the excellent stability of the
catalytic system in recycling experiments (Figure S8).
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0
30 60 90 120
Time (min)
0
30 60 90 120
Time (min)
0,000
0,025
HBr (mmol)
0,050
Yield (%)
HD HON HAL X (%)
Acid
none
AA
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H3PO4 13
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120 min
30 min
HCl
HBr
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e
f
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d
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PTSA
TFSA
180
200
220
0
100
200
Temperature (oC)
Water added (mol%)
H2SO4 66
ALꢀ15 29
Nafion 64
Figure 1. Yields, selectivities and conversions in the dehydration of 1,2ꢀhexanediol: a) time
profile at 200°C, conditions: 0.5 mmol 1,2ꢀhexanediol, 1.7 mmol Bu4PBr, 0.022 mmol HBr, 200
°C, N2 atmosphere; b) time profile at 170°C; c) variation of acid content, 15 minutes; d) variation
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of acid type, 0.022 mmol acid, 30 minutes; e) variation of temperature f) variation of added
water content relative to 1,2ꢀhexanediol, 30 minutes; HD = hexadienes, HON = 2ꢀhexanone,
HAL = hexanal, X = conversion, AA = Acetic acid, PTSA = pꢀtoluenesulfonic acid, TFSA =
trifluoromethanesulfonic acid, ALꢀ15 = Amberlyst 15.
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Based on these experimental data, the mechanism in Scheme 1 is proposed. This mechanism
consists of acid and IL catalyzed substitutions with bromide followed by IL catalyzed
dehydrobrominations. In the first step, the bromide anion functions as a nucleophile, substituting
either the primary or the secondary hydroxyl group, leading to 1ꢀbromoꢀ2ꢀhexanol (1B2H), a
detected intermediate, and 2ꢀbromoꢀ1ꢀhexanol (2B1H) respectively. Since similar yields were
reached with strong acids other than HBr, such as H2SO4 and PTSA, it is likely that the bromide
of the IL acts as the nucleophile in this substitution reaction. The acid presumably serves only to
protonate the hydroxyl group, making it a better leaving group. In the next step bromide anions
can function as a base to catalyze dehydrobromination reactions. Thus, 2B1H can be
dehydrobrominated to either hexanal or 2ꢀhexenꢀ1ꢀol, which is a detected intermediate. The latter
can be dehydrated by bromide substitution followed by dehydrobromination (removing the
proton in the C4 position), but is also expected to be quite reactive in purely acid catalyzed
dehydration, since it is an allylic alcohol. The formed 1,3ꢀhexadiene isomerizes to form 2,4ꢀ
hexadienes, through bromohexene intermediates. On the other hand, 1B2H can either be
dehydrobrominated to form 2ꢀhexanone or the remaining secondary hydroxyl group can be
substituted by bromide to form 1,2ꢀdibromohexane, which eventually leads to hexadienes.
Another, purely acidꢀcatalyzed pathway from HDO directly to hexanal is also proposed (dotted
line in Scheme 1).
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Scheme 1. Proposed reaction mechanism for dehydration of 1,2ꢀhexanediol catalyzed by Bu4PBr
and an acid.
To the test the validity of this reaction scheme, different reactants and putative intermediates
were tested in the typical reaction conditions in 15 minute reactions at 200°C (Table 2). Entries 2
and 3 show that both primary and secondary alcohols can be dehydrated, with olefins as the main
product. Furthermore, the overall dehydration reaction is faster for a secondary alcohol than for a
primary alcohol. These results confirm the mechanism via a nucleophilic substitution step, since
1ꢀ and 2ꢀbromohexane are formed starting from 1ꢀ and 2ꢀhexanol respectively. No other
bromohexanes are detected. The elimination of HBr from the primary bromide proves more
difficult than from the secondary bromide, as is typical in elimination reactions.20 This can
explain why, in the dehydration of HDO, 1B2H is detected intermediately and 2B1H is not.
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Table 2. Product yields in the dehydration and/or dehydrobromination of different
5
6
reactants/putative intermediates.
7
8
Entry
1
Substrate
Products and yield (%)
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9
<1
10a
11a
Aldol products
Aldol products
Conditions: 0.5 mmol substrate, 1,7 mmol Bu4PBr, 0.022 mol HBr, 200 °C, N2 atmosphere, 15
minutes. a 200 mol% water added.
The reaction of 1ꢀhexene (entry 4) shows that double bond isomerization occurs. This
isomerization may proceed through a hydrobromination/dehydrobromination sequence, or
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simply through a carbenium intermediate. However, entry 5 shows that different 2ꢀ and 3ꢀ
bromohexanes are produced starting from 1ꢀbromohexane. This suggests a (de)hydrobromination
equilibrium, which lies mostly towards the olefins, but is pushed towards bromohexanes due to
the larger amount of HBr present compared to entries 2 to 4 (as HBr is produced in initial
dehydrobromination). The fact that this (de)hydrobromination equilibrium is operative, raises a
mechanistic question: the brominated products that are detected could be produced either by
nucleophilic substitution of alcohols or by dehydration of alcohols followed by
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hydrobromination. The first pathway is most likely, as it is corroborated by two experimental
findings. First, 2,2ꢀdimethylꢀ1ꢀpropanol (DMP, entry 6) yields 1ꢀbromoꢀ2,2ꢀdimethylpropane
(BDMP). As this product cannot be formed from DMP through dehydration followed by
hydrobromination, it must be formed through the nucleophilic substitution pathway. The yield of
BDMP is rather low, but this is most likely because DMP is a sterically hindered primary
alcohol, which is expected to react slowly in substitution reactions. Secondly, 1B2H is detected
as a reaction product, especially in short reactions at 170 °C. If this product is formed by
hydrobromination, it can only be formed from 1ꢀhexenꢀ2ꢀol. As this compound is the tautomer of
2ꢀhexanone, it should be possible to form 1B2H from 2ꢀhexanone in the reaction conditions. This
is however not the case (entry 10), suggesting 1B2H is most likely formed through substitution
of bromide into 1,2ꢀhexanediol, as in the proposed mechanism.
To further confirm the reaction mechanism, we examined the fate of different putative
intermediates in the typical reaction conditions. As expected, 1B2H (entry 7) leads to the
formation of hexadienes and 2ꢀhexanone, while 2B1H (entry 8) leads to hexadienes and hexanal.
A more elaborate discussion of these reactions is available in the SI. Next, 2ꢀhexenꢀ1ꢀol (entry 9)
is quickly dehydrated, forming hexadienes with 95% yield after 15 minutes. Finally, 2ꢀhexanone
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and hexanal (entries 10 and 11) are shown to be deadꢀend products, resulting only in their aldol
products (or acetals in the presence of HDO).
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This mechanism can also explain the most important observations from the variation of
parameters. First, only small amounts of bromide substituted products are generally detected.
This indicates that the nucleophilic substitution is most likely slower than the following
dehydrobromination; thus the nucleophilic substitution is the rate determining step. Typically, in
such reactions, the hydroxyl group needs to be protonated to make it a better leaving group. This
can explain the increase of reaction rate with increased acid content. Furthermore, increasing the
amount of acid slightly increases the 2ꢀhexanone selectivity, while decreasing the hexanal
selectivity. This might be because the primary substitution is more difficult and therefore is
accelerated more by the increased acid content. As the selectivity of 2B1H over 1B2H increases,
the HD yield is expected to drop (entries 8 and 9). However, the HD yield can remain stable, as
in the next step formation of 1,2ꢀdibromohexane from 2B1H is promoted at higher acid content.
Finally, the nucleophilic substitution seems to be irreversible in the reaction conditions, as other
alcohols than the starting compound are never formed. This explains why the addition of water
has almost no effect on yield or selectivity.
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Next to these experimental results, the reaction mechanism is also confirmed by literature. We
have noticed that this system shows some similarity with the one reported by Zhang et al.21 Their
system consists of a tetraalkylammonium bromide, combined with a polyoxometalate (POM) as
Brønsted acid and a Lewis acidic metal and is used for the dehydration of (primary) monoꢀ
alcohols. The Lewis acid is not required for dehydration, but serves to diminish the production of
ethers. They propose and experimentally confirm an acid catalyzed nucleophilic substitution of
the hydroxyl group of the alcohol by the bromide of the IL, followed by an IL catalyzed
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dehydrobromination. In the current work, however, the reaction scope is expanded to include
vicinal diols. Furthermore, the activity of our phosphoniumꢀbased system markedly surpasses
that of the ammoniumꢀbased system. This is evident from the turnꢀover frequencies. In the
optimized Zhang system a TOF of 0.005 hꢀ1 is reached for 1ꢀoctanol to octenes at a total octene
yield of 24%.21 In our system, we reach a TOF of 0.28 hꢀ1 for 1ꢀhexanol to hexenes at the same
total hexene yield. For secondary alcohols (2ꢀoctanol and 2ꢀhexanol respectively) the difference
is even larger, with 0.0055 hꢀ1 compared to 1.0 hꢀ1 at 64 and 86% yield respectively.
Furthermore, we have examined the reaction of 1,2ꢀhexanediol in the Zhang system. After 30
minutes, this leads a much lower hexadiene yield of 10%. 3% hexanal, 6% hexanone and 11% 1ꢀ
bromoꢀ2ꢀhexanol are formed as side products. In addition, small amounts of 1ꢀbromobutane and
tributylamine were detected, showing some breakdown of the ionic liquid even at a rather low
temperature.
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An explanation for the much higher activity in our system lies partly in the higher temperatures
that can be applied (200 °C versus 125 °C). As we have shown before, increasing the
temperature to 200 °C in a system employing Bu4NBr results in severe decomposition of the
ionic liquid. This might be the reason why increasing the temperature in the Zhang system to 140
°C, decreases the alkene yield.21 Thus, simply increasing the temperature would not bridge the
gap in activity. A more fundamental explanation for the increased activity is the higher
dispersion of the positive charge in the phosphonium cation compared to a similar ammonium
cation.22,23 For cations with short aliphatic chains, this decreases the electrostatic interaction
between cation and anion, thus increasing the basicity and nucleophilicity of the anion.15,24 This
would accelerate both the nucleophilic substitution and the dehydrobromination.
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Having developed and studied a stable, highly active dehydrating system for a model substrate,
we applied this system to the dehydration of other diols. Different diol patterns are expected to
have reactivities distinct from that of a 1,2ꢀdiol such as the model substrate. Therefore, we
investigated the fate of 1,3ꢀ, 1,4ꢀ and 2,3ꢀbutanediol (BDO), 2,4ꢀpentanediol (PDO), 1,6ꢀ
hexanediol and pinacol (2,3ꢀdimethylꢀ2,3ꢀbutanediol) in the reaction system (Table 3). Based on
these results, the diols can be divided into three groups. First, vicinal diols (entries 1 and 2) lead
to conjugated dienes and carbonyl compounds. Other diols (entries 3 to 6 and 8) do not form
significant amounts of carbonyl compounds, as expected from the proposed mechanism.
Secondly, diols with one sp3 carbon separating the hydroxyl groups (entries 3 and 4) lead only to
conjugated dienes and, in the case of 1,3ꢀBDO, intermediate products. Finally, as can be seen
from the reaction of 1,4ꢀBDO and 1,6ꢀHDO (entries 5 and 6), hydroxyls separated by two to four
sp3 carbons lead to the formation of cyclic ethers. This cyclodehydration is reversible, as shown
in the reaction of tetrahydrofuran (THF, entry 7).
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Table 3. Product yields in the dehydration of different diols and tetrahydrofuran.
Entry
Substrate
Products and yield (%)
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15
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7
Hexadienesa
Cyclic ethersb
26
<1
72
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1
1
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2
Conditions: 0.5 mmol substrate, 1.7 mmol Bu4PBr, 0.022 mmol HBr, 200 °C, N2 atmosphere,
a
b
15 minutes; Hexadienes (yield): 1,3 (< 1%), 2,4 (1%), 1,4 (1%), 1,5 (4%); Cyclic ethers
(yield): oxepane (24%), 2ꢀmethyltetrahydropyran (2%), 2ꢀethyltetrahydrofuran (< 1%).
Besides allowing to group different diols based on their structure, several of these results indicate
that formation of more substituted, thus more stable, alkenes is preferred. First, this can be
concluded from the higher dehydration rate for 2,4ꢀPDO compared to 1,3ꢀBDO. Next, this also
explains the slow dehydration of primary diols such as 1,4ꢀBDO and 1,6ꢀHDO. Furthermore, the
immediate product of 1,6ꢀHDO dehydration is 1,5ꢀhexadiene. However, 1,4ꢀhexadiene and
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conjugated dienes are also formed, which shows that isomerization to more stable dienes occurs.
Next, the high selectivity for 2ꢀbutanone in the dehydration of 2,3ꢀbutanediol (entry 2) is most
likely caused by preferential formation of the internal alkene 2ꢀbutenꢀ2ꢀol over the terminal
alkene 3ꢀbutenꢀ2ꢀol. 2ꢀButenꢀ2ꢀol then tautomerizes to butanone. In the reaction of pinacol (entry
8) formation of the internal double bond is avoided, because there is no proton available in the C2
or C3 position. However, the dehydration rate is still high, since the terminal alkenes that can be
formed are also disubstituted. This leads to a high selectivity for the conjugated diene. Only a
small amount of pinacolone is formed, most likely through acid catalyzed pinacol rearrangement.
This striking result highlights once more the distinct selectivity in this IL system, since in a
purely acid catalyzed reaction, pinacolone is typically the major product.9 Very recently, the
dehydration of pinacol derivatives to dienes using a solid acid catalyst in an imidazolium IL has
been reported.25 However, the activity of our system is again significantly higher (TOF 0.86 hꢀ1
compared to 0.022 hꢀ1 at similar conversions) for the same reasons as discussed before for the
Zhang system.
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As discussed before, the reaction of 1,4ꢀBDO results in the formation of 1,3ꢀbutadiene (BD),
with THF as the only side product. We decided to investigate this reaction more thoroughly,
since it presents an interesting step towards the bioꢀbased production of BD. Production of BD
from renewables, including butanediols, has been highlighted in a few recent reviews.6–8 1,4ꢀ
BDO can be produced by bacterial fermentation of sugar streams, including crude biomass
hydrolysates.26 Recently, the fermentative production of 1,4ꢀBDO has been patented by
Genomatica and licensed for commercial plants by BASF and Novamont, indicating greatly
increasing bioꢀbased availability of this compound in the near future.27,28 This could enable an
economically viable bioꢀbased production of BD from 1,4ꢀBDO. However, the dehydration of
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1,4ꢀBDO to BD through acidic catalysis is hampered by selective formation of THF.9 For
example, an old route for production of BD was the dehydration of 1,4ꢀBDO over a supported
H3PO4 and Na3PO4 catalyst bed.29 This leads to almost pure THF, so BD can only be formed
through lengthy recycling of THF. Some basic catalysts, such as CeO2, can avoid the formation
of THF and lead to a 90% BD selectivity at 275 °C, but only at low conversion (6%).30 Another
acid catalyzed side reaction is the cleavage of 3ꢀbutenꢀ1ꢀol, the first dehydration product of BD,
to propylene.31–33 Again, Ce based catalysts can suppress this reaction.31 To our knowledge, the
only reported example of a process directly leading to BD yields over 90% is the gas phase
dehydration of 1,4ꢀBDO over NiO and CoO on ZrO2.34 Very high yields can be achieved starting
from a 10 wt.% aqueous solution, but high temperatures (400 °C) are required and coke is
formed, leading to deactivation and necessitating frequent regeneration.
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In view of these problems, we examined the dehydration of 1,4ꢀBDO using the phosphonium IL
system. First, we tested the purely acidꢀcatalyzed reaction in mesitylene and Nꢀ
methylpyrrolidone. As expected, this leads to the formation of THF as the only product (Table
4), showing that the IL is necessary for the formation of significant amounts of BD. Next, to
increase the yield of BD, we examined the effect of higher temperature and acid concentrations.
The main products in all these reactions are BD and THF. A small amount of butanal is
produced, presumably through isomerization of 3ꢀbutenꢀ1ꢀol to 1ꢀbutenꢀ1ꢀol and rapid
tautomerization. 4ꢀbromoꢀ1ꢀbutene, 1ꢀbromoꢀ2ꢀbutene, 1,4ꢀdibromobutane, 4ꢀbromoꢀ1ꢀbutanol
and 3ꢀbutenꢀ1ꢀol are detected as side products with yields lower than 1%. Moreover, they react
away over time. Interestingly, although 3ꢀbutenꢀ1ꢀol is a reaction intermediate, no propylene is
detected. A possible explanation for this is the lower temperature applied in this system,
compared to the gas phase dehydrations of butadiene or 3ꢀbutenꢀ1ꢀol, which are typically
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performed at temperatures above 250 °C and often above 300 °C.31–33 Furthermore, this reaction
is acid catalyzed and only small amounts of acid are present in our system.
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Table 4. Product yields in the dehydration of 1,4ꢀbutanediol.
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Yield (%)
Time
(min) (°C) HBr
T
mmol
X
(%)
BD THF BA
15
15
200
0
<1
98
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53
72
66
40
31
25
5
<1
0
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78
200 0.022 0a
200 0.022 0b
200 0.022 27
220 0.022 31
220 0.022 58
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0
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<1 100
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220
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0.03
0.04
0.05
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74
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1
0.05 93c
/
<1
Conditions: 0.5 mmol 1,4ꢀbutanediol, 1.7 mmol Bu4PBr, N2 atmosphere. BD = 1,3ꢀButadiene,
THF = Tetrahydrofuran, BA = Butanal, X = conversion. a 2 mL mesitylene as solvent. b 2 mL Nꢀ
methylpyrrolidone as solvent. c THF as substrate.
Since cyclodehydration to THF is reversible, while dehydration to BD is not, higher rates for
both dehydration reactions lead to higher BD yield. As a result, a butadiene yield of 94% was
reached after 2 hours using 0.05 mmol HBr at 220°C. Keeping in mind the very strong tendency
for THF formation, this is, most likely, only possible with a highly active and selective catalyst.
To illustrate this, we have conducted the same experiment in the Zhang system. After 30
minutes, only tetrahydrofuran (95%) and 1ꢀbromoꢀ4ꢀbutanol (1%) are formed; no butadiene was
detected. A mechanism can be proposed inspired by the one for HDO dehydration (Scheme 2).
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Interestingly, all the intermediates in this reaction scheme are detected, providing further proof
of the validity of the mechanism. As THF is the only intermediate in this scheme that is not
analogous to the intermediates in the reaction of 1,2ꢀhexanediol, we performed the reaction in
optimized conditions using THF as the substrate. This leads to a comparably high BD yield of
93%, which is consistent with the proposed mechanism and demonstrates the use of THF as an
alternative substrate for the production of BD.
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Scheme 2. Proposed mechanism for the dehydration of 1,4ꢀbutanediol
Conclusions
In conclusion, a new catalytic system for dehydration of alcohols and polyols has been
developed, consisting of the ionic liquid tetrabutylphosphonium bromide and an acid. Its
advantages include thermal stability, easy product separation and greatly increased activity
compared to similar systems. Its effect on a vicinal diol has been investigated, demonstrating
selective formation of conjugated dienes, which is generally not achieved by acid catalyzed
dehydration. A mechanism relying on both the basic and nucleophilic properties of the bromide
anion has been proposed and confirmed experimentally. Furthermore, the reactivity of other diols
in this reaction system was also investigated. Also for some of these compounds, typical side
reactions can largely be avoided. Next, in light of renewed interest in bioꢀbased production of
butadiene, we focused on the dehydration of 1,4ꢀbutanediol. Using the reported system, 1,4ꢀ
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butanediol, which is available from waste fermentation, is converted to butadiene with 94% yield
in relatively mild conditions.
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ASSOCIATED CONTENT
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Supporting Information. Experimental detail, information on product grouping, ionic liquid
acidity and the effect of different cations on activity, discussion on the dehydration and
dehydrobromination of bromohydrins (PDF).
AUTHOR INFORMATION
Corresponding Author
*Eꢀmail: dirk.devos@kuleuven.be.
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
Funding Sources
M.S. thanks Fonds Wetenschappelijk Onderzoek (FWO) for his doctoral fellowship. D.D.V.
acknowledges IWT and FWO for research project funding, the Flemish government for longꢀ
term structural funding through Methusalem and Belspo (IAPꢀPAI 7/05) for financial support. F.
G. C. acknowledges MarieꢀCurie Individual Fellowship (MSCAꢀIF 750391ꢀSINMOF) program
for financial support
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENT
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M.S. thanks Fonds Wetenschappelijk Onderzoek (FWO) for his doctoral fellowship. D.D.V.
acknowledges IWT and FWO for research project funding, the Flemish government for longꢀ
term structural funding through Methusalem and Belspo (IAPꢀPAI 7/05) for financial support.
F.G.C. acknowledges the MarieꢀCurie Individual Fellowship (MSCAꢀIF 750391ꢀSINMOF) for
financial support.
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