A New Process for Maleic Anhydride Synthesis from a Renewable Building Block: The Gas-Phase Oxidehydration of Bio-1-butanol

Article abstract of DOI:10.1002/cssc.201500095

We investigated the synthesis of maleic anhydride by oxidehydration of a bio-alcohol, 1-butanol, as a possible alternative to the classical process of n-butane oxidation. A vanadyl pyrophosphate catalyst was used to explore the one-pot reaction, which involved two sequential steps: 1)1-butanol dehydration to 1-butene, catalysed by acid sites, and 2)the oxidation of butenes to maleic anhydride, catalysed by redox sites. A non-negligible amount of phthalic anhydride was also formed. The effect of different experimental parameters was investigated with chemically sourced 1-butanol, and the results were then confirmed by using genuinely bio-sourced 1-butanol. In the case of bio-1-butanol, however, the purity of the product remarkably affected the yield of maleic anhydride. It was found that the reaction mechanism includes the oxidation of butenes to crotonaldehyde and the oxidation of the latter to either furan or maleic acid, both of which are transformed to produce maleic anhydride. MA and PA together: Maleic anhydride (MA) can be produced by the one-pot oxidehydration of bio-1-butanol with a vanadyl pyrophosphate catalyst. The reaction mechanism includes the oxidation of butenes to crotonaldehyde and the oxidation of the latter to either furan or maleic acid, both of which are transformed to produce maleic anhydride.

Full text of DOI:10.1002/cssc.201500095

DOI: 10.1002/cssc.201500095
Full Papers
A New Process for Maleic Anhydride Synthesis from
a Renewable Building Block: The Gas-Phase
Oxidehydration of Bio-1-butanol
[
a]
[a]
[a]
[a]
Giulia Pavarelli, Juliana Velasquez Ochoa, Aurora Caldarelli, Francesco Puzzo,
[a, b]
[c]
Fabrizio Cavani,*
and Jean-Luc Dubois
We investigated the synthesis of maleic anhydride by oxidehy-
dration of a bio-alcohol, 1-butanol, as a possible alternative to
the classical process of n-butane oxidation. A vanadyl pyro-
phosphate catalyst was used to explore the one-pot reaction,
which involved two sequential steps: 1) 1-butanol dehydration
to 1-butene, catalysed by acid sites, and 2) the oxidation of bu-
tenes to maleic anhydride, catalysed by redox sites. A non-neg-
ligible amount of phthalic anhydride was also formed. The
effect of different experimental parameters was investigated
with chemically sourced 1-butanol, and the results were then
confirmed by using genuinely bio-sourced 1-butanol. In the
case of bio-1-butanol, however, the purity of the product re-
markably affected the yield of maleic anhydride. It was found
that the reaction mechanism includes the oxidation of butenes
to crotonaldehyde and the oxidation of the latter to either
furan or maleic acid, both of which are transformed to produce
maleic anhydride.
Introduction
The increased greenhouse gas emissions as a result of the use
of fossil fuels has made it necessary to seek renewable resour-
ces, such as biomass, to be used as feedstock for the chemical
The availability and relatively low cost of 1-butanol, if to be
used as fuel, make this compound an interesting bio-platform
molecule for the synthesis of chemicals. Alcohols generally
transform by either dehydrogenation to produce a carbonyl or
dehydration to produce an alkene; the dehydration and dehy-
drogenation routes compete in the gas phase in the absence
[1–18]
industry and as an energy source.
In this context, bio-1-bu-
tanol has the potential to play a significant role in a non-petro-
leum-based chemical and energy industry, for example, as
[19,20]
a transportation fuel.
1-Butanol can be produced by
of O up to T=5008C, whereas dehydrogenation predominates
2
[21]
[29]
means of various technologies, namely a) propylene hydro-
formylation to butyraldehyde and hydrogenation to the alco-
hol; b) from ethanol, with a multi-step process involving dehy-
drogenation to acetaldehyde, aldol condensation, dehydration
at higher temperatures. Over dehydrogenating catalysts, 1-
butanol transforms into two main products, butyraldehyde
[
30,31]
and 4-heptanone (ketonisation reaction),
and smaller
amounts of hydrocarbons, such as butene, heptene and 3-
methylheptane, as well as of other oxygenated compounds,
such as 4-heptanol, dibutyl ether and butyl butyrate. Butyral-
dehyde may undergo reactions such as Tishchenko dimerisa-
tion (to form butyl butyrate), aldolisation and ketonisa-
[22–28]
and hydrogenation of crotonaldehyde;
c) catalytic hydro-
genation of CO into higher alcohols; and d) biomass fermenta-
tion, from both five- and six-carbon sugars. The acetone/buta-
nol/ethanol (ABE) process was largely used during the first half
of the last century; today, there is a renewed interest in it, es-
pecially with the aim of producing bio-1-butanol as a biofuel.
Nowadays, various companies supply 1-butanol produced by
fermentation.
[
32–38]
tion;
however, deep dehydrogenation, aromatisation and,
lastly, carbonisation may also occur. Depending on the catalyst
and conditions used, other reactions may occur, such as either
decarbonylation to yield CO, H and C H or demethylation to
2
3
6
[
39]
yield propanal and a CH radical; the butenes formed by 1-
x
butanol dehydration may decompose to produce propylene
[
40]
[
a] Dr. G. Pavarelli, J. Velasquez Ochoa, Dr. A. Caldarelli, F. Puzzo,
Prof. F. Cavani
Dipartimento di Chimica Industriale “Toso Montanari”
Università di Bologna
Viale del Risorgimento, 4, 40136 Bologna (Italy)
E-mail: fabrizio.cavani@unibo.it
and a CHx radical.
In the presence of oxygen, 1-butanol
[39,41]
yields butyraldehyde by means of oxidehydrogenation.
Acid catalysts for 1-butanol dehydration into butenes in-
[
42–44]
[45,46]
clude heteropoly compounds,
AlPO4,
sulphated alumi-
[
47]
[48]
na,
hydroxyapatite,
Ta–montmorillonite and Ta/Si mixed
[
49,50]
[51]
[52,53]
[
b] Prof. F. Cavani
oxides,
silica–alumina, Al/P and Al/Ga/P oxynitrides.
CIRCC, Consorzio Interuniversitario Reattività Chimica e Catalisi
Research Unit of Bologna (Italy)
In this paper, we report on a study aimed at investigating
the possibility of transforming 1-butanol into maleic anhydride
[
c] Dr. J.-L. Dubois
Arkema, 420 Rue d’Estienne d’Orves, 92705 Colombes (France)
(
MA) by means of gas-phase oxidation. MA, an important inter-
mediate used for coatings and polymers, is currently produced
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201500095.
[
54–58]
by the oxidation of either n-butane or benzene,
but it
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[
54–61]
may also be produced from butenes and butadiene.
Indeed, in the past, butenes were also used industrially as raw
material for MA synthesis; however, their cost made this pro-
cess non-competitive. The interest in the production of MA
from renewable raw materials is also demonstrated by recent
papers in which 2,5-hydroxymethylfurfural or furfural is used as
a reactant for liquid-phase or gas-phase oxidation, even
though yields reported are not high enough for an industrial
exploitation, especially with consideration of the cost of the
[
62–64]
raw material used.
A bifunctional catalyst able to both de-
hydrate 1-butanol and oxidise the intermediately formed bu-
tenes into MA might make the direct transformation of the al-
cohol into MA feasible.
Figure 1. Catalytic behaviour of the VPP DuPont catalyst in chemical 1-buta-
nol oxidehydration as a function of temperature. Conditions: feed: 1 mol%
Results and Discussion
1-butanol, 20% O
2
, remainder N
2
; W (catalyst weight)/F (inlet gas flow rate
À1
at normal conditions)=1.33 gsmL . Symbols: 1-butanol conversion (^); se-
Preliminary thermal experiments performed without any cata-
lyst and either with an inert material (that is, corundum and
steatite) filling the reactor or with no inert material at all led us
to conclude that the best reactor configuration was without
any material filling the empty space above the catalytic bed
lectivity to maleic anhydride (&), 1-butene (~), 2-butenes (*), acetic
acid+acrylic acid (^), CO (~), CO
”).
(*), phthalic anhydride (&) and “lights”
2
(
ratio shown with increased temperature and the concomitant
formation of MA and PA suggest that the isomerisation of 1-
butene, the primary product of 1-butanol dehydration, into 2-
butenes is slower than the consecutive oxidation occurring
upon the olefins and that 2-butenes are more quickly oxidised
than 1-butene. On the other hand, the rapid interconversion
between the two diastereoisomers (trans-2-butene and cis-2-
butene) does not allow inference of whether only one of the
two or both react further. The butadiene selectivity recorded
(not shown in the figure) was very low, that is, less than 1%,
over the entire temperature range examined (for example, it
was 0.2% at T=3408C).
(
see the Supporting Information for a detailed description of
these experiments). Moreover, because of the important contri-
bution of gas-phase homogeneous reactions, we also found
that a catalyst for 1-butanol oxidative dehydration to MA work-
ing in an oxidising atmosphere has to be very efficient in 1-bu-
tanol dehydration to enhance further the rate of 1-butene for-
mation with respect to the alcohol thermal (oxi)dehydrogena-
tion and, thus, limit the parallel formation of butyraldehyde; in
fact, the latter is the precursor of several by-products identified
during thermal experiments, but it cannot be transformed into
MA. Furthermore, the catalyst must be very efficient in 1-
butene oxidation into MA. This led us to conclude that vanadyl
pyrophosphate (VPP), used industrially for n-butane oxidation
to MA, is the possible candidate catalyst for this reaction, be-
cause not only is VPP selective in 1-butene oxidation into
The formation of PA, with a maximum yield of 12% shown
at T=3408C, may occur through a Diels–Alder reaction be-
tween the intermediately formed butadiene and MA. In the
case of n-butane oxidation, the formation of PA becomes im-
portant only under the conditions of surface saturation as
a result of the coverage of active sites by olefinic intermedi-
ates, an event that only occurs under alkane-rich feed condi-
[
54]
MA,
but it is also characterised by the acidic properties
[54–58,65,66]
needed for alcohol dehydration.
In fact, 1-butanol has
been previously used in some studies as a possible reactant
for the purpose of demonstrating the polyfunctional character-
[
71,72]
tions with the VPP catalyst.
Under these circumstances,
[
67,68]
istics of VPP.
Recently, one patent also claimed the synthe-
the over-reduction of the V sites leads to the prevailing occur-
rence of bimolecular reactions, such as the cycloaddition of
MA and butadiene, with a minor contribution of olefin oxida-
tion to produce more MA. However, it is worth noting that, in
the case of n-pentane oxidation to MA and PA, the mechanism
proposed did not involve any Diels–Alder reaction between
pentadiene and MA but rather the oxidation of dialkylaromat-
ics, which are formed by olefin dimerisation and oxidative de-
[69]
sis of MA from 1-butanol with VPP.
Figure 1 reports the results obtained with the VPP catalyst at
various reaction temperatures. Under the conditions chosen,
the conversion of 1-butanol was complete over the entire
range of temperatures investigated; products obtained were
MA, light acids (acrylic and acetic acid), carbon oxides, phthalic
anhydride (PA) and minor amounts of other oxygenated com-
pounds, such as furan and formaldehyde. Butenes were
formed at T=3408C with a molar ratio between the two posi-
tional isomers, 2-butenes/1-butene, that was different from the
[
73–75]
hydrocyclisation.
Therefore, it cannot be ruled out that,
even in the case of 1-butanol oxidation, the formation of PA
may indeed occur by the oxidation of o-xylene that has been
formed by the oxidative dehydrocyclodimerisation of butenes.
The highest selectivity to MA was observed at T=3408C
(39%), with a value similar to that reported previously by Gu-
[
70]
equilibrium value; the latter is close to 3.7 at T=3308C,
whereas the experimental ratio was close to 2 at T=3008C
and to 1 at T=320 and 3408C. On the other hand, the ratio
between trans-2-butene and cis-2-butene was close to the
thermodynamic value (approximately 1.6 at T=3008C and 1.5
at T=3508C). The decrease in the 2-butenes/1-butene molar
[
67,68]
liants et al.
In the range T=300–3408C, the increase in MA
selectivity occurred with a concomitant decrease in selectivity
to butenes, whereas at higher temperatures, the MA selectivity
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decline was due to the prevailing formation of CO and CO2.
Acetic and acrylic acids also formed in relatively high amounts,
with a maximum overall selectivity close to 20% at T=3208C;
the two compounds were formed in similar amounts. Butyral-
dehyde selectivity was very low (for example, 0.1% at T=
show any further increase. This may be a result of the fact
that, under conditions in which the coverage of the VPP sur-
face by adsorbed species is small, because of the low amount
of residual butenes, the catalyst becomes very selective in the
oxidation of butenes into MA. In other words, the optimal con-
3
408C; not shown in the figure).
ditions for the VPP catalyst are those in which a high O /(1-bu-
2
These results indicate that, at low temperatures, the preva-
tanol+butenes) ratio makes it possible to maintain a clean
and selective surface, with a relatively abundant concentration
of “free” oxidising sites. These hypotheses will be confirmed by
the experiments described below. Another somewhat surpris-
lent reactions were 1) the transformation of 1-butanol into bu-
tenes, and 2) the oxidative cleavage of either the alcohol or
olefins into acetic acid and acrylic acid. If the temperature was
raised, the butenes were transformed into MA and PA, whereas
the light acids were oxidised into COx.
ing result is that the initial selectivity to CO and CO , that is,
2
the selectivity extrapolated to nil conversion, was almost zero.
This means that the parallel combustion of 1-butanol gives
a marginal contribution to the formation of these compounds,
at least at T=3408C, whereas the major contribution is derived
from the consecutive over-oxidation of butenes.
Figure 2. Catalytic behaviour of the VPP DuPont catalyst in chemical 1-buta-
nol oxidehydration as a function of the W/F ratio. Conditions: feed: 1 mol%
1
(
-butanol, 20% O
^); selectivity to maleic anhydride (&), 1-butene (~), 2-butenes (*), acetic
acid+acrylic acid (^), CO (~), CO
(*), phthalic anhydride (&) and “lights”
”).
2 2
, remainder N ; T=3408C. Symbols: 1-butanol conversion
2
(
Figure 3. Catalytic behaviour of the VPP DuPont catalyst in chemical 1-buta-
nol oxidehydration as a function of the O
mol% 1-butanol, O and N
2
molar fraction. Conditions: feed:
variable; T=3408C; W/F=1.33 gsmL . Sym-
À1
1
2
2
Figure 2 plots the effect of contact time on catalytic per-
bols: 1-butanol conversion (^); selectivity to maleic anhydride (&), 1-butene
~), 2-butenes (*), acetic acid+acrylic acid (^), CO (~), CO
(*), phthalic
anhydride (&) and “lights” (”).
formance at T=3408C. From these experiments, the kinetic re-
lationship between the butenes and both MA and PA is appar-
ent; in fact, the selectivity to both olefins declined as the con-
tact time was increased, whereas the selectivity to MA and PA
increased at the same time. The molar ratio between 2-bu-
tenes and 1-butene decreased from a value close to 2.2 (a
value that is still lower than the thermodynamic one) shown
for a very short contact time, to a value close to 1. This is a fur-
ther proof that the isomerisation of 1-butene to 2-butenes is
relatively slow at low temperatures, under conditions at which
the rate of the consecutive oxidation is negligible. Thereafter, if
either the temperature or the contact time is increased, the
consecutive oxidehydrogenation into MA occurs at a faster
rate on 2-butenes than on 1-butene.
(
2
Figure 3 shows the effect of the inlet oxygen molar fraction
on catalytic behaviour. Clearly, an increase in the O content
2
led to an increase in both MA and PA selectivity, whereas there
was almost no effect on selectivity to light acids. Selectivity to
CO and CO increased slightly in the O molar fraction range of
2
2
0.04–0.10 and then decreased until the O molar fraction was
2
equal to 0.20; under the latter conditions, the maximum selec-
tivity to MA and PA was recorded at 39 and 12%, respectively.
Lastly, both CO and CO increased again if the O molar frac-
2
2
tion was equal to 0.26. The trend of selectivity to CO and CO2
might be attributed to the fact that, under conditions at which
The selectivity to butadiene (not shown in the figure) also
decreased, but it was always lower than 1%, even for very
short contact times. It can also be seen that the conversion of
the butenes are the dominant products (for an O molar frac-
2
tion lower than 0.10), the catalyst surface is covered with the
adsorbed olefinic intermediates and combustion most likely in-
1
-butanol was less than 100% (but close to 98%) only for ex-
tremely short contact times. A surprising effect was that, in the
volves gas-phase O ; therefore, the butenes are not oxidised to
2
À1
W/F range between 0.6–1.3 gsmL , the only consecutive reac-
MA (or PA either) but are instead transformed into CO and
CO . At higher O inlet molar fractions, at which the butenes
tion was the transformation of butenes into MA, whereas the
2
2
formation of all other products, including CO and PA, did not
are transformed to a greater extent into subsequent com-
x
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5
+
pounds, the catalytic performance is governed by surface V
sites, which are more selective to partially oxidised compounds
and less selective to COx.
Therefore, there is a range in which the increased O partial
2
pressure shows a positive effect on both the MA and PA yields,
because of the decreased selectivity to both butenes (precur-
sors for MA and PA formation) and CO . It is important to note
x
that such an effect was not observed in n-butane oxidation. In
this case, in fact, the interaction between the alkane and the
VPP surface is very weak, and the saturation of the catalytically
active surface is shown only for very high n-butane partial
pressures (under the so-called hydrocarbon-rich feed). Other-
wise, the concentration of adsorbed olefins is very low, so
5
+
there is a large availability of oxidising V sites. In fact, the
Figure 4. Catalytic behaviour of the VPP DuPont catalyst in 1-butene oxida-
best selectivity to MA from n-butane was observed with the
tion as a function of temperature. Conditions: feed: 1 mol% 1-butene, 20%
[54,76]
À1
O
2
, remainder N
2
; W/F=1.33 gsmL . Symbols: 1-butene conversion (^); se-
lowest O /alkane molar ratio in the feed.
This could also
2
lectivity to maleic anhydride (&), 2-butenes (*), acetic acid+acrylic acid
^), CO (~), CO
(*), phthalic anhydride (&) and “lights” (”).
explain why, under normal conditions, PA forms in traces
(
2
during n-butane oxidation.
Instead, from 1-butene, the effect of O partial pressure was
2
similar to that observed with 1-butanol, with a maximum MA
Finally, we checked the reactivity of the catalyst in 1-butene
selectivity obtained for a very high O /hydrocarbon molar feed
oxidation. The results are reported in Figure 4. At low tempera-
ture, the prevailing products were 2-butenes (also, in this case,
both the cis and the trans isomers were formed, with a ratio
close to the thermodynamic one): their selectivity, however, de-
creased as the temperature was raised. The highest yield of
MA was 46%, which is slightly higher than that shown from 1-
butanol (39%); it is also worth noting that the reactivity of 1-
butene was slightly lower than that of 1-butanol. The other
products obtained were the same as those shown from 1-buta-
2
[
59]
ratio. In general, it may be concluded that, for a consecutive
reaction pattern that leads in the end to CO , with butenes
2
and MA/PA as intermediate products, the selectivity to these
latter compounds is a function of the fraction of oxidising sites
on the catalyst surface that are available under steady-state
conditions, and the latter is, in turn, a function of both the O /
2
[
59,66,77]
reactant feed ratio and the nature of the reactant itself.
Experiments performed with 2% 1-butanol and 20% O in the
2
feed confirmed the above-stated hypothesis (see Figure S3 and
the related discussion in the Supporting Information).
nol: PA, CO, CO , acetic acid and acrylic acid.
2
The water generated in 1-butanol dehydration to butenes
might affect the performance of the consecutive oxidation of
butenes during the one-pot reaction, so we also carried out
some experiments by co-feeding water in 1-butene oxidation.
We found that water had a very small effect on the catalytic
behaviour, as expected because the amount of water generat-
ed during olefin oxidation is greater than that generated
during 1-butanol dehydration. In the presence of 1 mol% co-
fed water, a slight increase (1–2%) of selectivity to both MA
and PA was observed, with a corresponding decline of selectiv-
ity to CO and CO2.
The data obtained highlight the fact that the butenes are
the intermediates in 1-butanol oxidehydration into MA. More-
over, the catalytic reaction is very selective in the first step of
alcohol dehydration to form the butenes (the initial selectivity
to butenes is very high, as shown in Figure 2), whereas the ki-
netically consecutive step of butene oxidation into MA shows
only a limited selectivity, because of several side reactions.
Another important result is that most of the by-products ob-
served in thermal experiments (Figure S1 and S2 in the Sup-
porting Information) are not formed during catalytic experi-
ments. This means either that they are burnt to CO or that
x
the catalytic transformation of 1-butanol into butenes and of
the latter into oxidised compounds prevails over thermal reac-
tions. Clearly, a contribution of the thermal dehydration of 1-
butanol to butene formation cannot be ruled out.
DRIFT spectroscopy study of the oxidehydration mechanism
of 1-butanol
To understand the interaction of the alcohol with the catalyst
surface and investigate the reaction mechanism using diffuse
reflectance infrared Fourier transform (DRIFT) spectroscoy, 1-
butanol was adsorbed over VPP at T=1408C; Figure S5 in the
Supporting Information shows the IR spectra obtained after al-
cohol adsorption with either He or air as the carrier
Figure S4 in the Supporting Information reports the results
obtained by carrying out the reaction under anaerobic condi-
tions, with feeding of 1% 1-butanol in N ; it is shown that the
2
catalyst was selective to butenes, with approximately 1% se-
lectivity to butyraldehyde as the only by-product. The molar
ratio between 2-butenes and 1-butene was close to the ther-
modynamic one; this confirms the hypothesis that, even
though the isomerisation of 1-butene to 2-butenes is relatively
À1
(8 mLmin ). In both cases, 1-butanol interacted strongly with
the OH groups present on the catalyst surface, as seen by the
À1
negative peaks at n˜ =3742 and 3661 cm . The positive peaks
À1
fast, the latter react even faster in the presence of O to pro-
at around n˜ =3250 and 2970–2880 cm correspond to the OH
2
duce MA.
and CÀH stretching of adsorbed butanol, respectively. In the
case of the oxidising atmosphere, the peaks at n˜ =1727, 1620
ChemSusChem 2015, 8, 2250 – 2259
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À1
and 1468 cm indicate the presence of oxidised species even
at this low temperature; these bands may be attributed to un-
saturated mono- or dicarboxylic acids (crotonic acid, isocroton-
[78]
ic acid, maleic acid, fumaric acid). As reported by Schrader
[
78,79]
and co-workers,
fumaric acid adsorbed on the VPP catalyst
at T=2008C exhibits an IR pattern similar to that of maleic
acid, and their adsorbed species form an identical surface com-
plex; both species transform into MA at temperatures higher
than 2508C.
These experiments confirm the strong interaction of the al-
cohol with the VPP surface and, thus, support the hypothesis
of the saturation of the surface.
When the adsorption was performed at T=3008C, differen-
ces among the spectra recorded under different conditions
were more evident (Figure S6 in the Supporting Information).
In the case of an inert atmosphere, the most important fea-
tures were the negative peaks at high frequency and broad
À1
bands at around n˜ =1630 and 973 cm . The former may corre-
spond to a C=C stretching of an olefin (butene) adsorbed on
the surface, whereas the latter is usually attributed to a V=O
[
80]
stretching.
Therefore, in this case, if no air is present, the catalyst is able
to dehydrate 1-butanol and produce butenes, but this alters
the catalyst surface. On the other hand, if air is present, olefins
are also formed (butenes and butadiene in this case) but are
further oxidised, as evidenced by both the sharp peak present
À1
À1
at n˜ =1778 cm and the shoulder at n˜ =1850 cm , which
[
81]
were attributed to the OÀC=O stretching of adsorbed MA.
À1
The bands at n˜ =1730 and 1616 cm indicate either the pres-
ence of species with a C=O and/or C=C bond, such as butyral-
dehyde, furan and alkenals, or some adsorbed unreacted ole-
fins. There were no changes in the low-frequency range; this
confirms that oxygen is necessary to keep the catalyst surface
species unaltered. As regards the OH interaction, in the oxidant
atmosphere, the bands were clearly less intense, and this
agrees with the fact (observed in reactivity tests) that oxygen
is needed to have fewer adsorbed molecular species, which
are more quickly oxidised because of the greater availability of
Figure 5. DRIFT spectra recorded after 1-butanol adsorption on VPP catalyst
at T=1408C and its transformation with temperature in inert (top) and oxi-
dising (bottom) environments.
À1
ly, the bands at around n˜ =1780 and 1850 cm were assigned
to adsorbed MA. Therefore, it may be concluded that, under
anaerobic conditions, the catalyst is able to dehydrate 1-buta-
nol and oxidise olefins until the unsaturated monocarboxylic
(crotonic and isocrotonic) or dicarboxylic (maleic and fumaric)
acids are formed; however, oxygen from the gas phase be-
comes necessary because it helps to keep the V centres oxi-
dised and also makes it possible for the intermediate acid to
dehydrate and desorb, with production of MA at the end.
Operando-mode experiments were also performed by feed-
ing 1-butanol over the VPP catalyst at T=3608C with and
without air (the results are presented in Figure S7 and S8 in
the Supporting Information). The results confirmed that MA
could be formed only if air was present and also confirmed the
5
+
V
sites.
Temperature-programmed desorption experiments were also
carried out. In this set of experiments, the adsorption was per-
formed at T=1408C and then stopped for monitoring of the
reactive transformations of the adsorbed species with tempera-
ture increase. As observed from the spectra (Figure 5), in the
case of the inert atmosphere, adsorbed 1-butanol evolved
mainly towards butadiene (broad band centred at n˜ =
À1
1
602 cm ), but then other olefins and carbonylic species also
À1
formed (C=C and C=O bands at around n˜ =1630–1724 cm );
bands previously attributed. In addition, with O in the feed,
2
this occurred concomitantly with an increase of the band at
the relative amounts of butenes were smaller (less accumula-
tion), whereas those of other intermediates, such as crotonal-
dehyde and furan, were greater.
À1
4+
n˜ =978 cm and, thus, confirms that vanadyl (V ) surface
species developed during the transformation of 1-butanol
under the inert atmosphere. The OH interaction was so strong
that it was maintained throughout the whole temperature pro-
gram (negative bands); in fact, a slightly decreased intensity
was seen only at T>3208C. On the other hand, if air was pres-
ent, oxidised carbonyl species were more readily formed,
which corresponds to further oxidation compounds; particular-
Reactivity of intermediates and reaction mechanism
The results of the reactivity and DRIFT spectroscopy experi-
ments suggest that crotonaldehyde may be a key intermediate
in the mechanism of 1-butanol oxidehydration to MA. There-
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volving crotonic acid as the intermediate in trans-crotonalde-
hyde oxidation with various types of V oxide catalysts was also
[
84]
[79]
formerly proposed by Church and Bitha. Xue and Schrader
suggested that, in n-butane oxidation over VPP, the main route
for MA formation occurs with crotonaldehyde as the key reac-
tion intermediate. It cannot be ruled out, however, as suggest-
[
81]
ed by Wenig and Schrader, that the formation of crotonalde-
hyde or crotonic acid is due to the oxidation of butadiene and
not of butene. Crotonaldehyde can also be oxidised to furan
[
85]
with high selectivity by using polyoxometalate catalysts.
Indeed, furan is another molecule that was proposed as an in-
termediate during C oxidation with the VPP catalyst. For ex-
4
[
67,68]
ample, Guliants et al.
reported that the oxidation of 2-
butene occurs through the formation of furan, which is then
oxidised to MA through the generation of a furan endoperox-
Figure 6. Catalytic behaviour of the VPP DuPont catalyst in crotonaldehyde
oxidation as a function of temperature. Conditions: feed: 0.3 mol% crotonal-
5+
ide produced by reaction with a V ÀOOC species. Through
À1
dehyde, 20% O
2
, remainder N
2
; W/F=1.33 gsmL . Symbols: crotonalde-
a dedicated experiment, we also confirmed that furan can be
oxidised with 59% yield to MA, by using 0.3% furan in air as
the inlet feed.
hyde conversion (^); selectivity to maleic anhydride (&), acetic acid+acrylic
acid (^), CO (~) and CO
(*).
2
Another set of experiments was carried out by using croton-
aldehyde as the reactant and varying the contact time (Fig-
ure S10 in the Supporting Information), to detect the possible
formation of furan as an intermediate during crotonaldehyde
oxidation to MA. Our results demonstrated that furan is indeed
formed directly from the aldehyde. However, it was also shown
that the C balance was poor, especially at low values of con-
tact time. We characterised the used catalyst after the experi-
ment carried out at a low contact time and recorded the IR
spectra by means of DRIFT spectroscopy. As shown in Fig-
ure S11 in the Supporting Information, the spectra highlighted
the presence of several compounds in the adsorbed state,
such as MA, unsaturated mono- and dicarboxylic acids, bu-
tenes and butadiene. The used catalyst after the reaction at
longer contact times, conditions at which the C balance was
improved, instead showed the presence of only minor
amounts of these adsorbed compounds. Therefore, at low con-
tact times, the catalyst retained adsorbed molecules that are
precursors for the formation of MA. When the contact time
was increased, the selectivity to MA also increased; this was
a result of both the oxidation of furan (the selectivity of which,
in fact, decreased) and of maleic acid, which was retained on
the catalyst surface at low contact times but desorbed as MA if
the contact time was increased, with concomitant improve-
ment of the C balance.
fore, we carried out experiments by feeding crotonaldehyde (a
mixture of cis and trans isomers in the molar ratio 1:20) and air
over the VPP catalyst; we used a very diluted feed of the alde-
hyde (0.2 mol% in air) to simulate the low concentration of
the adsorbed compound during 1-butanol oxidation. The re-
sults obtained are plotted in Figure 6; the yield of MA was
close to 50%, with by-products CO, CO , acetic acid, acrylic
2
acid and furan, although the latter was found only at T=260
and 2808C.
The rapid decline in selectivity seen at 3408C (which, con-
versely, is the temperature at which the best selectivity for MA
from 1-butanol was seen) may be ascribed to an enhanced
combustion rate of crotonaldehyde. Indeed, once again, the
oxidation properties of VPP may be strongly affected by the re-
actant-to-O ratio in the feed. With crotonaldehyde, which is
2
expected to show a weaker interaction with the catalyst sur-
face than 1-butanol, the use of a low crotonaldehyde-to-O2
feed ratio may lead to a catalyst over-oxidation at T=3408C,
which is responsible for the high selectivity to CO in the end.
2
This hypothesis is confirmed by the results reported in Fig-
ure S9 in the Supporting Information, which show the effect of
the inlet molar fraction of crotonaldehyde on MA selectivity at
T=3408C; it can be seen that an increased feed ratio led to
a remarkable increase in selectivity, with a yield of MA close to
that achieved from 1-butanol under similar reaction conditions
All of these experiments demonstrate that the mechanism
for MA formation most likely occurs with crotonaldehyde as
the key intermediate compound. The latter undergoes two
parallel reactions of transformation, one leading to furan and
the other to maleic (or fumaric) acid, possibly via crotonic (or
isocrotonic) acid. It is worth noting that cis-2-butene might be
oxidised to cis-crotonaldehyde, and the latter to either isocro-
tonic acid and maleic acid or furan (both precursors of MA),
whereas trans-2-butene might be transformed into trans-croto-
naldehyde, crotonic acid and finally fumaric acid; in order to
form MA by dehydration, fumaric acid should first be convert-
ed into maleic acid. However, the rapid isomerisation between
the cis and trans forms of 2-butene found experimentally and
(
that is, at a similar reactant-to-O feed ratio).
2
As for the hypothesis of crotonaldehyde as the key reaction
intermediate in C oxidation to MA, this possibility has already
4
been described in the literature. In fact, it has been reported
that crotonaldehyde can be oxidised to maleic acid by using
a catalyst based on V/Mo/O supported on pumice, with 46%
[
82]
[83]
yield. Cicmanec et al. reported that, with a similar catalyst,
crotonaldehyde follows two parallel pathways of oxidation, in
which the trans form is oxidised to crotonic acid and the latter
is then oxidised to MA, whereas the cis form is oxidised to
furan and the latter may then be oxidised to MA via 2-(3H)-fur-
anone and 2-(5H)-furanone intermediates. The mechanism in-
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ic content), with a water content
that was less than 1% in all
cases; the latter is expected to
have a negligible effect on the
reactivity behaviour, because the
amount of water generated
during the reaction was much
greater than that present in 1-
butanol. However, there were
differences in the amount and
type of impurities present, and
the following rank of impurity
Figure 7. Reaction scheme for 1-butanol oxidehydration to MA.
contents may be assumed (as
from a semi-quantitative assess-
the interconversion of maleic and fumaric acids reported in the
ment inferred from GC and GC-MS analysis): BB1>BB2>BB3.
It is also worth noting that the pH value of the aqueous solu-
tion obtained by dissolving BB (1 mL) in water (14 mL) was
slightly different for the three BB samples (Table S1 in the Sup-
porting Information). Lastly, we also searched for metallic im-
purities in the BB samples by means of detailed inductively
coupled plasma and X-ray fluorescence analysis, but no traces
of metallic impurities were found.
[78,79]
literature with the VPP catalyst,
make it irrelevant which
one of the two isomers is the precursor for MA formation.
Both furan and maleic acid are intermediates for MA forma-
tion, through oxidation or dehydration, respectively. The rela-
tive importance of the two parallel ways is probably a function
of the degree of surface availability of oxidising V sites and,
thus, of the degree of surface coverage by adsorbed species.
Another important difference is that furan desorbs easily from
the VPP surface, as demonstrated by DRIFT spectroscopy–MS
operando experiments, which showed no proof for the reten-
tion of furan at the adsorbed state, whereas furan was detect-
ed in a relatively large amount in the gas phase. Conversely,
maleic acid interacted strongly with the catalyst surface; in
order to desorb this compound, it was necessary to use condi-
tions that fostered the development of a clean and oxidised
surface.
Figure 7 summarises the proposed reaction network for the
transformation of 1-butanol to MA, as inferred by combining
our DRIFT spectroscopy and reactivity experimental results. The
mechanism involves the dehydration of 1-butanol to 1-butene,
the isomerisation to 2-butenes (with the cis and trans isomers
in equilibrium ratio) and the oxidation of the latter to crotonal-
dehyde. The latter can either cyclise to yield furan or be oxi-
dised until maleic acid is formed. Maleic acid desorbs in the
form of MA, whereas furan can desorb and be consecutively
re-adsorbed and further oxidised to MA. It is also important to
note that the endoperoxide compound formed by furan oxida-
tion can rearrange either into 4-oxocrotonic acid, which is also
Figure 8. Catalytic behaviour of the DuPont catalyst in the oxidehydration of
BB1; reaction conditions as in Figure 1. Symbols: 1-butanol conversion (^);
selectivity to maleic anhydride (&), 1-butene (~), 2-butenes (*), acetic
acid+acrylic acid (^), CO (~), CO (*), phthalic anhydride (&) and “lights”
2
(”).
[
68]
a precursor of maleic acid, or 5-hydroxy-2-(5H)-furanone. In
this way, the two parallel routes would share the same inter-
mediate, maleic acid, as also suggested by the DRIFT spectros-
copy experiments.
The catalytic behaviour shown by sample BB1 (the one con-
taining the greater amount of impurities) is reported in
Figure 8. A comparison with the behaviour shown by the CB
sample (Figure 1) highlights the following differences:
The reaction of bio-1-butanol
1
. With BB1, the maximum yield to MA shown was only 31%
at T=3408C, versus 39% for CB at T=3408C. On the other
hand, the selectivity to PA was higher than that shown with
CB; the PA yield was 25% at T=3208C and 17% at T=
3408C, whereas the PA yield was no higher than 12% with
CB. Therefore, the best overall selectivity to MA+PA, at
total 1-butanol conversion, was 51% with CB and 48% with
BB1.
After having established the effect of the main reaction param-
eters on the catalytic behaviour with a chemically sourced 1-
butanol (CB), we conducted experiments with three different
types of bio-1-butanol (BB), the main impurities of which, com-
pared with those of CB, are shown in Table S1 in the Support-
ing Information. It is important to note that all samples had
purity degrees higher than 99.5% (with reference to the organ-
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2
3
. With BB1, the selectivity to CO+CO was greater at low
maleic acid (or MA) with the intermediately formed butadiene,
with the assumption that PA forms by a Diels–Alder reaction,
or an increased rate of condensation and oxidehydrocyclisation
of butenes to the alkyl aromatic precursor of PA formation, if
the mechanism for PA formation occurs via the intermediate o-
2
temperatures but similar (or even slightly lower) at high
temperatures than the selectivity observed with CB.
. The selectivity to light acids and butenes was also lower
with BB1. Moreover, traces of other by-products were
found, such as lighter aldehydes, which probably formed
from small amounts of alcohols other than 1-butanol that
were contained as impurities in BB1.
[
73–75]
xylene.
Time-on-stream experiments (Figure S12 and S13 in the Sup-
porting Information) demonstrated that the difference in the
catalytic behaviour between the two reactants was most likely
related to some impurities present in BB1, which accumulated
on the catalyst surface and modified the surface properties
during the first 2–3 h of reaction time. In fact, the two reac-
tants gave a similar performance at the beginning of the ex-
periment (with the reaction conditions fixed at T=3408C).
However, in the case of the CB feed, the performance then
showed a very small variation, whereas the performance rapid-
ly changed in the case of BB1, with a decline in selectivity to
MA and a corresponding increased selectivity to PA; moreover,
the selectivity to light acids declined considerably, with a similar
increase of selectivity to CO and CO2.
Figure 9. Best MA and PA yields obtained with VPP catalyst feeding either
chemical 1-butanol (CB) or bio-1-butanol samples (BB). Reaction conditions
as in Figure 8; T=340–3608C.
The minor variations observed with CB, that is, a slight de-
cline of selectivity to MA and a corresponding increase of se-
lectivity to PA during the 6 h time-on-stream experiment, with
Lastly, Figure 9 compares the best MA and PA yields that
were achieved in all cases at total 1-butanol conversion and at
T=340–3608C with the various BB samples. It is apparent that
samples BB1 and BB2 both led to lower yields than CB, but the
catalytic behaviour shown with the BB3 sample (the purest
amongst the BB samples) was quite similar to that shown by
CB. The purification of BB1 (sample BB1-pur; see the Experi-
mental Section) led to a slightly increased yield to MA. Lastly,
the BB1-superpur sample, which had undergone a prolonged
treatment for the removal of heavier impurities and which con-
tained no trace of the carbazole compound, showed the same
behaviour as CB.
no variation of yields to both light acids and CO , suggests
x
that different types of poisoning may indeed occur, attributa-
ble to the different nature of the impurities, some of which are
also present in CB (see Table S1 in the Supporting Informa-
tion).
In regards to the nature of the impurities responsible for the
effect shown with BB1, it should be noted that most of the im-
purities had characteristics that were not much different from
those of the reaction products obtained during 1-butanol oxi-
dehydration. An important exception was a carbazole com-
pound, which was tentatively identified as 5H-naphtho[2,3-
C]carbazole-5-methyl. Carbazoles are antibiotics that can be re-
leased by microorganisms during the final period of the fer-
Conclusions
[
86]
mentation process. In order to confirm the role of this im-
purity, we carried out an experiment by adding 0.1 wt% carba-
zole (belonging to the same class of compounds as the impuri-
ty found in BB1) to CB; Figure S14 in the Supporting Informa-
tion compares the catalytic performance at T=320 and 3408C
for the reaction with CB (data taken from Figure 1) and with
the deliberately contaminated CB. It is shown that the same ef-
fects as those found during BB1 oxidehydration (Figure 8) were
also observed with the contaminated CB (with 1-butanol con-
version of 100% in all cases): a) a decrease of the yield to light
1-Butanol, both chemically sourced and bio-butanol, can be
oxidehydrated to maleic anhydride with a one-pot process, by
using a bifunctional catalyst based on vanadyl pyrophosphate.
The catalytic behaviour is primarily affected by the strong in-
teraction between the catalyst and the reactant; therefore,
a relatively high O -to-1-butanol inlet feed ratio is needed to
2
guarantee the availability of the surface density of oxidising
sites and avoid undesired side reactions that lead mainly to
the formation of CO and CO . Under optimised conditions, the
2
acids, with an increase of the yield to CO , and b) a decrease of
highest maleic anhydride yield of 39% was obtained, with
12% yield of phthalic anhydride.
x
the MA/PA selectivity ratio. The first effect can be attributed to
an interaction of the basic carbazole (adsorbed on the catalyst
surface) with the light acids formed, which may favour their
combustion; a similar effect might occur also with maleic acid,
which would contribute to the decrease of selectivity to MA.
The increase of PA selectivity is more difficult to explain; it
might be a result of either a more favoured reaction between
After the first step of alcohol dehydration to 1-butene, the
reaction mechanism includes isomerisation to 2-butenes and
the oxidation of the latter to crotonaldehyde, which is the key
intermediate of the reaction network. Crotonaldehyde is then
either oxidised to furan or to unsaturated mono- and dicarbox-
ylic acids. Both furan and maleic acid are precursors for maleic
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anhydride formation; however, furan desorbs into the gas
phase and is then consecutively adsorbed and oxidised, where-
as the unsaturated acids remain adsorbed on the catalyst sur-
face. Maleic acid finally desorbs after dehydration to maleic an-
hydride.
sorbed on the material. The sample was then cooled down to the
adsorption temperature (T=140, 300 or 3608C) and 1-butanol was
À1
fed at 0.6 mLmin until saturation was reached. Subsequently, He
was left to flow until the weakly adsorbed 1-butanol was evacuat-
ed. In the case of the temperature-programmed desorption experi-
À1
ments, the temperature was raised at 108Cmin up to T=4008C.
The impurities contained in bio-butanol affected the overall
process selectivity; the most impure sample showed a decrease
in the MA yield and an increase in the PA yield. However, a pu-
rification treatment made it possible to obtain yields of both
MA and PA that were similar to those achieved from chemically
sourced 1-butanol.
The IR apparatus used was a Bruker Vertex 70 with a Pike DiffusIR
cell attachment. Spectra were recorded by using a mercury–cadmi-
um–telluride detector after 128 scans and 2 cm resolution. The
mass spectrometer was an EcoSys-P from European Spectrometry
Systems.
À1
Acknowledgements
Experimental Section
The research leading to these results has received funding from
the European Union FP7 (FP7/2007-2013) under grant agreement
no. 241718 EuroBioRef.
Reactants
Different types of 1-butanol were used, both chemically sourced
(
CB, purchased from Aldrich) and bio-based, from different suppli-
ers (BB1, BB2 and BB3; see Table S1 in the Supporting Information).
The following method, aimed at lowering the impurity content in
BB1, was set up: bleaching earth (0.03 g) enriched with activated
charcoal was added to BB1 (10 g), the slurry was stirred for 5 h at
room temperature, then the solid was separated by centrifugation.
This “purified” BB is referred to as BB1-pur in Table S1 in the Sup-
porting Information. As can be seen, the treatment was only partly
successful, because the removal of the more undesired contami-
nants was not complete. The treatment was repeated for a longer
time; more specifically, BB1 (17 g) was treated with bleaching earth
enriched with activated charcoal (0.06 g, 0.35 wt%) at room tem-
perature for 24 h. The suspension was then centrifuged at
Keywords: alcohols · anhydrides · oxidation · renewable
resources · vanadium
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Received: January 17, 2015
Published online on June 12, 2015
ChemSusChem 2015, 8, 2250 – 2259
www.chemsuschem.org
2259
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