Highly Efficient Process for the Conversion of Glycerol to Acrylic Acid via Gas Phase Catalytic Oxidation of an Allyl Alcohol Intermediate

Article abstract of DOI:10.1021/acscatal.5b01843

The conversion of glycerol, the main byproduct of the biodiesel industry, to acrylic acid is one of the most attractive biomass to biochemical processes. It represents millions of tons of market demand. The previous tandem reaction protocol for the conversion of glycerol to acrylic acid via an acrolein intermediate always suffers from fast catalyst deactivation and thus cannot be commercialized. Herein, a novel two-step protocol for the conversion of glycerol to acrylic acid is presented: glycerol deoxydehydration (DODH) by formic acid to allyl alcohol, followed by oxidation to acrylic acid in the gas phase over Mo-V-W-O catalysts. In the second step, supported and unsupported Mo-V-W-O multiple-metal oxide catalysts were used for the transformation of allyl alcohol to acrylic acid and excellent activity as well as selectivity was achieved. Remarkably, the mesoporous silica-supported Mo-V-W-O catalysts showed superb stability on time stream under the optimal reaction conditions with an overall acrylic acid yield of 80%. This study provides a prominent catalytic process for the large scale production of acrylic acid from biorenewable glycerol.

Full text of DOI:10.1021/acscatal.5b01843

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Article
Highly Efficient Process for the Conversion of Glycerol to Acrylic
Acid via Gas-Phase Catalytic Oxidization of Allyl Alcohol Intermediate
Xiukai Li, and Yugen Zhang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01843 • Publication Date (Web): 23 Nov 2015
Downloaded from http://pubs.acs.org on November 25, 2015
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Highly Efficient Process for the Conversion of Glycerol to Acrylic Acid via
Gas-Phase Catalytic Oxidation of Allyl Alcohol Intermediate
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Xiukai Li, Yugen Zhang*
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Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos,
Singapore 138669, Singapore.
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ABSTRACT
The conversion of glycerol, the main byproduct of biodiesel industry, to acrylic acid is
one of the most attractive biomass to biochemical processes. It represents millions of
tons of market demand. The previous tandem reaction protocol for glycerol
conversion to acrylic acid via acrolein intermediate always suffers from fast catalyst
deactivation and thus cannot be commercialized. Herein, a novel twoꢀstep protocol for
the conversion of glycerol to acrylic acid is presented: glycerol deoxydehydration
(DODH) by formic acid to allyl alcohol, followed by oxidation to acrylic acid in gas
phase over MoꢀVꢀWꢀO catalysts. In the second step, supported and unsupported
MoꢀVꢀWꢀO multiple metal oxide catalysts were used for the transformation of allyl
alcohol to acrylic acid and excellent activity as well as selectivity was achieved.
Remarkably, the mesoporous silica supported MoꢀVꢀWꢀO catalysts showed superb
stability on time stream under the optimal reaction conditions with up to 80% overall
acrylic acid yield. This study provides a prominent catalytic process for the large scale
production of acrylic acid from bioꢀrenewable glycerol.
Keywords: glycerol, acrylic acid, allyl alcohol, gas phase reaction, heterogeneous
catalysis, deoxydehydration reaction, supported catalyst
INTRODUCTION
With the emphasis on renewable energy, increasing amount of biodiesel is
produced as an alternative fuel.1ꢀ3 It was expected that approximately 37 billion
gallons of biodiesel will be produced by 2016, and therefore approximately 4 billion
gallons of crude glycerol will be generated.4,5 It is imperative to find more uses for the
over supplied glycerol, which will be of great importance for the biodiesel economy.
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Potentially, the bioꢀgenerated glycerol can serve as a platform chemical for the
synthesis of valueꢀadded products.6ꢀ10 Among those, the most attractive one is acrylic
acid (AA), which is industrially produced from the gas phase oxidation of crude
oilꢀbased propene and is used primarily for the large volume production of
polyacrylates.11ꢀ13 The large market demand makes acrylic acid one of the most
sought after chemicals from biomass resources.
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The most known way to produce acrylic acid from glycerol is the twoꢀstep
tandem reaction (Scheme 1).14,15 Glycerol is first dehydrated to acrolein over an acid
catalyst, and then oxidized to acrylic acid in the second step. Since the oxidation of
acrolein to acrylic acid has already been well developed in industrial acrylic acid
process,16ꢀ18 dehydration of glycerol to acrolein in gas phase is the main emphasis of
current research.19ꢀ21 A wide variety of solid acid catalysts, such as metal oxides,22
zeolites,23,24 heteropoly acids,25,26 and phosphates,27,28 have been evaluated for gas
phase glycerol dehydration. Although up to 90% acrolein yield could be achieved
from glycerol dehydration, those acid catalysts suffered from fast deactivation due to
heavy coke deposition which was caused inevitably by the acidic nature of the
catalysts and the high reaction temperature. There have also been reports on direct
oxydehydration conversion of glycerol to acrylic acid over bifunctional catalysts.27ꢀ31
However, the selectivity to acrylic acid is normally lower than 50%, meanwhile
catalyst deactivation is still a severe problem.32 Various techniques such as coꢀfeeding
with oxygen,27,33 metal doping,25 and manipulating the catalyst texture34,35 have been
developed to reduce the coke formation, while the challenge still remains and
therefore limits its large scale practical application.
It is highly desired to create new processes for glycerol conversion to acrylic acid
with high efficiency and catalyst stability. Recently, it was reported that glycerol can
be converted to allyl alcohol by gas phase transfer hydrogenation,36,37 rhenium
complex catalyzed deoxydehydration (DODH) reaction,38ꢀ40 and formic acid mediated
nonꢀcatalytic DODH reaction.41 In view of the low efficiency in gas phase transfer
hydrogenation (25% product yield) and the high cost of the Re catalyst, formic acid
mediated DODH of glycerol to allyl alcohol is more attractive. It is noteworthy that
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formic acid is a cheap and green chemical that can be produced from biomass42ꢀ44 or
CO2.45,46 We envisaged that allyl alcohol could serve as an intermediate for acrylic
acid production from glycerol. As only oxidation catalyst is required for the
transformation of allyl alcohol to acrylic acid, the severe coke formation caused by
strong catalyst acidity as in the direct glycerol oxydehydration case can be avoided.
Herein, a new protocol for the conversion of glycerol to acrylic acid has been
developed (Scheme 1). Glycerol is deoxydehydrated (DODH) by formic acid to allyl
alcohol in a batch or continuous flow system (step 1). Subsequently, allyl alcohol is
oxidized to acrylic acid in a fixedꢀbed gasꢀphase system (step 2). Glycerol DODH to
allyl alcohol by formic acid (step 1) is known to happen in batch reaction,41 thus, in
this work, we have further studied some aspects closely related to the practical
applications, such as the reaction conditions, the tolerance to impurities, and the
feasibility of the continuous reaction. To our knowledge, allyl alcohol oxidation to
acrylic acid in gas phase (step 2) is a new reaction and has not been reported before.
Thus, in this work we have focused more on the second step reaction. MoꢀVꢀW based
multiple metal oxides were selected as the catalysts for the second step as they are
active for a variety of partial oxidation reactions including acrolein oxidation to
acrylic acid.11ꢀ13,18,47,48 The MoꢀVꢀWꢀO catalysts demonstrated high efficiency with an
overall acrylic acid yield up to 90%, and superb stability on time stream under the
optimal reaction conditions. The high efficiency, high selectivity, and high stability of
catalyst make this new system a great potential in large scale production.
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Scheme 1. Synthetic approaches for the conversion of glycerol to acrylic acid.
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EXPERIMENTAL SECTION
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All starting materials are commercially available and were used as received, unless
otherwise indicated. Formic acid (99%), glycerol (99%), ammonium monovanadate
(99%), and ammonium heptamolybdate (99%) were purchased from Merck.
Ammonium metatungstate (99%) was purchased from Fluka.
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Catalysts Preparation. The unsupported MoꢀVꢀWꢀO catalysts were prepared by
decomposing the ammonium salts of the metal precursors. Stoichiometric amounts of
ammonium monovanadate, ammonium heptamolybdate, and ammonium
metatungstate were dissolved in DI water and then evaporated to dryness. The
combination was calcined at 275 °C for 4 h in air and then at 325 °C for 4 h in Ar. The
powder sample was pressed into pellets and then crushed and sieved to 35 ~ 60 mesh
before activity evaluation. For the preparation of the supported catalyst, the aqueous
solution containing calculated amounts of vanadium, molybdenum, and tungsten
precursors were impregnated onto SBAꢀ15 under stirring at room temperature. The
material was dried at 100 °C overnight to remove solvent before calcination at 275 °C
for 4 h in air and then at 325 °C for 4 h in Ar.
Catalysts Characterization. Powder Xꢀray Diffraction (XRD) was conducted
using a Brucker automatic diffractometer (Brucker D8 discover GADDS) with
monochromatized CuꢀKα radiation (λ = 0.15406 nm) at a setting of 30 kV. The BET
surface areas and pore structures were measured by Micromeritics ASAP 2020. The
samples were degassed at 250 °C for 3 h before N2 adsorption. The TEM images were
obtained with a FEI Tecnai F20 transmission electron microscope.
Deoxydehydration of Glycerol to Allyl Alcohol. Glycerol (18.4 g, 0.2 mol) and
formic acid (16.5 g, 0.36 mol) were charged into a 50 ml flask. The flask was heated
by an alumina heating block mounted on a hot plate and the temperature was
controlled by a thermal couple. The reaction temperature was raised to 235 °C and the
distillation of product occurred immediately. The product was collected by
condensation over cooling water, giving 11.4 g of allyl alcohol (98% yield) together
with water and unreacted formic acid.
Deoxydehydration of Glycerol to Allyl Alcohol (Continuous). A mixture of
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glycerol and formic acid with molar ratio of 1:1.8 was charged into a 50 ml flask. The
flask was heated by an alumina heating block mounted on a hot plate and the
temperature was controlled by a thermal couple. The temperature of the flask was
raised to 235 °C quickly and then another portion of reactant mixture with the same
molar ratio of glycerol and formic acid was added dropwise into the reactor by a
funnel. The product was collected by condensation over cooling water, and the adding
rate was adjusted to keep a constant liquid volume in the reactor. Under the steady
state, 18.4 g of glycerol and 16.5 g of formic acid was added over about 2 hours, and
11.4 g of allyl alcohol (99% yield) was collected together with water and unreacted
formic acid.
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Gas Phase Oxidation of Allyl Alcohol to Acrylic Acid. Catalytic performance
for the partial oxidation of allyl alcohol to acrylic acid was evaluated in a quartz
fixedꢀbed microreactor (Φ = 6 mm) with a continuous reactant down flow. Typically,
0.2 g of catalyst (35 ~ 60 mesh) was charged into the reactor. The reactant (20wt%
allyl alcohol in water) was fed by syringe pump at a rate of 0.5 ml hꢀ1. The carrier gas
was 10%O2/He (flow rate 20 ml minꢀ1, GHSV 6700 h−1). Onꢀline gas chromatography
was used to analyze the outlet effluent after the system was stabilized for 1 h at each
temperature point.
Product Analysis. Analysis of the reactant and product in the deoxydehydration
of glycerol to allyl alcohol was carried out using HPLC (Agilent Technologies, 1200
series) with refractive index (RI) detector. Reactant and product were separated on an
Agilent HiꢀPlex H column (7.7 × 300 mm, 8 µm). The effluent was a solution of
H2SO4 (0.0001 M), flow rate 0.7 ml minꢀ1, 25 °C. The retention times were 12.1 min,
12.9 min, and 20.1 min for formic acid, glycerol, and allyl alcohol, respectively. The
analysis of reactant and products in the gas phase catalytic oxidation was carried out
using onꢀline GC system (Shimadzu GC2014) equipped with FID and TCD. A
capillary column (HP FFAP, 30 m × 0.32 mm, 0.25 µm) was used to separate the
organic components, and a packed carbon column (3 mm × 300 mm) was used to
separate CO and CO2.
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RESULTS AND DISCUSSION
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Allyl alcohol is also a highly valued chemical that is industrially manufactured
from propene. The synthesis of allyl alcohol from glycerol dehydration presents an
alternative choice for making this industrial chemical from a green feedstock. Formic
acid mediated DODH of glycerol is the most attractive method for the allyl alcohol
synthesis.41 Under the reaction conditions, one formic acid molecule can react with
the two vicinal hydroxyl groups of glycerol to form the cyclic carbonation
intermediate. With the assist of another formic acid molecule, the carbonation
intermediate decomposes to allyl alcohol, CO2, and H2O. In previous work, the
reaction was reported in batch reaction with pure glycerol as reactant. In this work, we
have further studied the reaction conditions, the tolerance to impurities, and the
feasibility of the continuous reaction. In a typical condition, glycerol was heated with
formic acid at 235 °C under ambient atmosphere, and allyl alcohol was collected by
condensation. The reaction is very fast and clean. The reaction rate depends on the
amount of formic acid added in the reaction (Figure S1). As the formic acid to
glycerol molar ratio is 1.8:1, 90% glycerol was converted (allyl alcohol yield 89%) in
2 hours. As the formic acid was added in 3 portions, the reaction was completed in 6
hours, and allyl alcohol was collected at 97% yield (Table 1, Entries 1 & 2). There
was no other side product collected besides water and unreacted formic acid. Moisture
and methanol are the main potential impurities in crude bioꢀglycerol; however, they
did not affect the current DODH reaction even at 20 wt% content (Table 1, Entries 3
& 4). A flow reaction system was also tested for this reaction (Figure S2; Table 1,
Entry 5). Under steady state, a mixture of glycerol (18.4 g, 0.2 mol) and formic acid
(16.5 g, 0.36 mol) was added continuously to a 50 ml flask (heated at 235 °C) in
around 2 hours, and 11.5 g of allyl alcohol (99% yield) was collected together with
5.6 g of unreacted formic acid. This experiment demonstrated the flow reaction
system for the production of allyl alcohol from glycerol with high productivity.
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Table 1. Deoxydehydrate (DODH) glycerol to allyl alcohol by formic acid (FA). a
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Entry
Glycerol feed
Conv. [%]e
Yield [%]
FA recovered [%]
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100%
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100%
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80wt% in H2O
80wt% in MeOH
100%
4b
5d
a
Reaction conditions: glycerol (18.4 g, 0.2 mol), formic acid (16.5 g, 0.36 mol),
235 °C, under N2. Formic acid was added in 3 portions (11.9, 2.3, and 2.3 g).
Reaction time: 6 h. Reaction carried under air. Continuous reaction. A mixture of
glycerol (18.4 g, 0.2 mol) and formic acid (16.5 g, 0.36 mol) was added continuously
to the reactor (Figure S2). Reaction time: 2 h. e Yield and conversion were determined
by HPLC.
b
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Mo, V, and W based metal oxides are active and selective for a variety of partial
oxidation reactions.18,47,48 They are also the basic components of the industrial acrylic
acid catalysts (acrolein to acrylic acid).11ꢀ13 It was recognized that Mo4VO14 and
Mo3VO11 are the active phases for acrolein oxidation to acrylic acid,49,50 and the
addition of W can stabilize the catalyst structure.51,52 Therefore, a series of metal
oxides Mo8V2WxOδ and Mo9V3WxOδ were fabricated and evaluated for allyl alcohol
oxidation. The XRD patterns (Figure 1) showed that all MoVW oxide catalysts were
well crystallized in the single hexagonal MoO3ꢀtype phase.16,52,53 All samples
exhibited small specific surface area values between 8 ~ 12 m2 gꢀ1 (Table S1). The
SEM images (Figure S3) depicted that the samples possessed an agglomeration of
irregular particles in micron size.
Mo9V3W2Oδ
Mo9V3WOδ
Mo9V3Oδ
Mo8V2W2Oδ
Mo8V2WOδ
Mo8V2Oδ
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20
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2θ / °
Figure 1. XRD patterns of Mo8V2WxOδ (x = 0, 1, 2) and Mo9V3WxOδ (x = 0, 1, 2)
catalysts.
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As shown in Figure 2, Mo8V2Oδ and Mo9V3Oδ catalysts are active, but acrolein is
dominant in the products when the reaction temperature is below 320 °C. Other
products are acrylic acid, acetaldehyde, acetic acid, and carbon oxides. When the
reaction temperature was increased, acrolein was consecutively oxidized to acrylic
acid; however the selectivity to acetic acid and the total oxidized products (CO, CO2)
was also increased. Thus, it is deduced that acrolein is the key intermediate in allyl
alcohol oxidation to acrylic acid in the present work. It has been proved that in
acrolein oxidation to acrylic acid, V2O5 is active but unselective, MoO3 is inactive,
and WO3 is inert. The V4+/V5+ species in the MoVW oxide catalysts are responsible
for the redox cycle and the high selectivity to acrolein, while the incorporation with
Mo stabilized the valence states of V and the addition of W improved the longꢀterm
stability of the catalysts.51,52 In the present work of allyl alcohol oxidation, the
addition of tungsten didn’t change the crystal structure of the catalysts but notably
improved the selectivity to acrylic acid. Over the Mo8V2WOδ and Mo9V3WOδ
catalysts, full allyl alcohol conversion and about 87% selectivity to acrylic acid were
achieved at reaction temperature of 340 °C. Adding more tungsten into the catalysts
leads to lower selectivity to acrylic acid but higher selectivity to the total oxidized
products. The Mo9V3WxOδ serial catalysts generally showed higher activities than
Mo8V2WxOδ serial. It may be due to the higher content of V in Mo9V3WxOδ catalysts
which have higher selectivity to the total oxidized products at higher reaction
temperatures. Our results indicated that tungsten does not only serve as a structural
promoter, but also significantly influences selectivity to acrylic acid in allyl alcohol
oxidation. Although W is not directly involved in the oxygen transfer process due to
the high oxygen binding strength, the incorporation of W might influence the
reducibility of the active V species, and consequently influence the selectivity to the
oxygenated products. It was found that in the presence of W, V species can be
reduced to the V2O3ꢀtype phase, while in the absence of W, V species can only be
reduced to VO2.51 Apparently, allyl alcohol oxidation to acrylic acid more relies on
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the composition of the MoVW oxide catalysts, and more research work is needed to
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give deeper insight into the structureꢀperformance relationships of this catalytic
system.
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A
100
80
60
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0
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ ꢀ
ꢀ
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ꢀ
Sel. Acr.
Sel. AA
Sel. AcOH
Sel. CO & CO2
Conv.
ꢀ
ꢀ
ꢀ
T/ °C =300 320 340 360 300 320 340 360
300 320 340 360
x = 0
x = 1
x = 2
B
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Sel. AA
Sel. AcOH
Sel. CO & CO2
Conv.
ꢀ
T/ °C =300 320 340 360 300 320 340 360
300 320 340 360
x = 0
x = 1
x = 2
Figure 2. (A) Allyl alcohol oxidation over serial Mo8V2WxOδ (x = 0, 1, 2) catalysts.
(B) Allyl alcohol oxidation over serial Mo9V3WxOδ (x = 0, 1, 2) catalysts. Reaction
conditions: catalyst, 200 mg, 35 ~ 60 mesh; feed, 20wt% allyl alcohol in H2O, 0.5 ml
hꢀ1; carrier gas, 10%O2/He, 20 ml minꢀ1. WHSV = 0.5 hꢀ1. Acr: acrolein. AA: acrylic
acid. AcOH: acetic acid. Data collected at 1 h time point.
Reaction conditions were further optimized based on the best performed
Mo8V2WOδ catalyst (Table S2). Space velocity with 0.5 hꢀ1 gave steadily good results.
The conversion of allyl alcohol generally increases with increasing oxygen content.
When 5% oxygen was used as the carrier gas (Table S3), 81% selectivity to acrylic
acid was achieved at 340 °C while the selectivity to the total oxidized products (CO
and CO2) is less than 1%. Increasing the oxygen content to 10% gave 87% yield of
acrylic acid at 340 °C. Notably, 82% yield of acrylic acid was also achieved when the
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more economic air (21%O2/N2) was used as oxidant. Generally, higher oxygen
content increases selectivity to over oxidized products, especially when reaction
temperature is high. Thus, the oxygen content around 10% is preferable for the
current oxidation reaction. Interestingly, acrylic acid yield can be further improved to
90% when feeding with 40wt% of allyl alcohol in water (Table S4). When the product
mixture (allyl alcohol/formic acid) of the first step glycerol DODH reaction was
directly used as feedstock, 76% acrylic acid yield was achieved. After removal of
formic acid from the mixture, the as synthesized allyl alcohol from glycerol gave 85%
yield of acrylic acid (Table S4). The temperature dependence of the reaction
performance over Mo8V2WOδ is shown in Figure 3. The allyl alcohol conversion
increased monotonously at elevated temperature in the 240 ~ 320 °C range, and full
conversion was achieved at the temperature above 320 °C. Acrolein is the key
intermediate to acrylic acid. The selectivity to acrolein decreased rapidly with the
temperature rise, indicating that acrolein was quickly oxidized at higher temperature.
In contrast, the selectivity to acrylic acid increased rapidly with the temperature rise,
reaching a maximum value of 87% at 340 °C. Further increase the temperature lead to
decreased selectivity to acrylic acid due to the over oxidation, as reflected by the
increasing amount of acetic acid, CO, and CO2. The selectivity to CO and CO2 are
lower than 10% in the whole temperature range. Acetaldehyde was observed as the
main side product at temperature below 320 °C, while the formation of acetaldehyde
was notably reduced as the temperature was increased. Significant amount of
acetaldehyde was also observed in the oxydehydration of glycerol.27,30,31,33 In the
current study, the selectivity of acetaldehyde is correlated to the selectivity of acrolein.
At higher temperatures, acrolein is quickly converted to acrylic acid, and concurrently,
acetaldehyde yield was apparently suppressed. These results indicated that allyl
alcohol oxidation over Mo8V2WOδ is temperature dependent and kinetically
controlled.
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SCO
Sothers
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T / °C
Figure 3. Temperature dependence of allyl alcohol oxidation over the Mo8V2WOδ
catalyst. Reaction conditions: catalyst, 200 mg, 35 ~ 60 mesh; feed, 20 wt% allyl
alcohol in H2O, 0.5 ml hꢀ1; carrier gas, 10%O2/He, 20 ml minꢀ1. WHSV = 0.5 hꢀ1. Act:
acetaldehyde. Acr: acrolein. AA: acrylic acid. AcOH: acetic acid. Data collected at 1 h
time point.
Generally, porous materials supported catalysts have advantages over the
unsupported ones, including larger surface area to volume ratio of active component
and controllable catalyst textures. Herein, mesoporous silicaꢀbased material
(SBAꢀ15)27 is adopted for the loading of Mo8V2WOδ. The large pores and high
surface area of SBAꢀ15 would lead to high capacity of loading catalysts and lower
limitation of internal mass transfer. The supported catalysts were prepared by
incipient wetness impregnation followed by calcination. The N2 adsorptionꢀdesorption
profiles of Mo8V2WOδ/SBAꢀ15 catalysts are illustrated in Figure 4, with the related
data listed in Table 2. As shown in Figures 4A & B, the mesopores of SBAꢀ15 (6.4 nm)
were apparently decreased as the loading of catalyst was increased. Concurrently,
smaller pores (3.2 to 3.5 nm, which were generated as the original pores were
partially occupied by catalyst) were observed. The average pore diameter and surface
area value (Table 2) decreased with increasing Mo8V2WOδ loading. The above
observation indicates that the mesopores of SBAꢀ15 were gradually occupied with
successive Mo8V2WOδ loading. This conclusion could also be verified from the
significant changes of pore volume and occupied pore volume of supported catalysts
(Table 2).
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Table 2. The BET surface area, porosity, and TOFs of catalysts.
Pore
Diameter
(nm)
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8
9
SBET
VPore
VOP
Catalyst
[m2 gꢀ1] [cm3 gꢀ1]
[cm3 gꢀ1]a
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20%Mo8V2WOδ/SBAꢀ15
SBAꢀ15
a Occupied pore volume (VOP) = [Pore volume of SBAꢀ15 × (1ꢀ catalyst loading%) +
Pore volume of Mo8V2WOδ × (catalyst loading%) – Pore volume].
A
500
Adsorption
Desorption
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300
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100
0
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20% loading
40% loading
60% loading
Mo8V2WOδ
0.0
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1.0
P / P0
B
0.04
0.03
0.02
0.01
0.00
SBA-15
20% loading
40% loading
60% loading
Mo8V2WOδ
0
20 40 60 80 100 120 140 160 180 200
Pore Width /
Å
Figure 4. (A) N2 adsorptionꢀdesorption isotherms and (B) pore size distribution of
Mo8V2WOδ/SBAꢀ15 catalysts.
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XRD patterns of Mo8V2WOδ/SBAꢀ15 catalysts exhibited much lower diffraction
intensities which probably owing to small particle size or low crystallinity (Figure S4).
For 20%Mo8V2WOδ/SBAꢀ15, there was no diffraction peak observed. This could be
due to the tiny Mo8V2WOδ entities (≤ 6 nm in diameter) dispersed into the mesopores
of SBAꢀ15, however, this does not exclude the possibility that the supported
Mo8V2WOδ entities are highly disordered or exist in amorphous phase.52 The
highꢀresolution transmission electron microscopy (HRTEM) images revealed that
there is no obvious Mo8V2WOδ particles on SBAꢀ15 of samples with low (20% and
40%) Mo8V2WOδ loading (Figures 5aꢀd). However, the selected area electron
diffraction analysis of 40%Mo8V2WOδ/SBAꢀ15 catalyst (Figure 5e) confirmed that
the Mo8V2WOδ components were indeed loaded onto SBAꢀ15. The image of the
60%Mo8V2WOδ/SBAꢀ15 catalyst (Figure 5f) showed that some Mo8V2WOδ particles
were attached on the external surface of SBAꢀ15 or segregated from the support. This
is also correlated to the N2 adsorptionꢀdesorption analysis (BET) data (Table 2). For
Mo8V2WOδ/SBAꢀ15 catalyst with 20% and 40% loadings, metal oxides occupied 0.09
and 0.17 cm3 pore volume of SBAꢀ15, suggesting that metal oxides were dispersed
into the pore of SBAꢀ15. While for 60%Mo8V2WOδ/SBAꢀ15, the metal oxides
occupied a pore volume of only 0.19 cm3. Based on these results, it is concluded that
the majority of Mo8V2WOδ components were dispersed inside the SBAꢀ15 channels
when the catalyst loading is low (≤ 40%). However, further increasing the catalyst
loading will result in Mo8V2WOδ components partially located on the external surface
of SBAꢀ15.
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Figure 5. TEM images of Mo8V2WOδ/SBAꢀ15 catalysts with different Mo8V2WOδ
loadings: (a) 20% loading; (b) & (c) 40% loading; (d) magnified from the circled area
shown in (b); (e) EDX spectrum for (d); (f) 60% loading.
The Mo8V2WOδ/SBAꢀ15 catalysts exhibited high activities similar to the
unsupported counterpart and full allyl alcohol conversion was achieved at above
300 °C (Figure 6). Catalysts with 60% and 40% Mo8V2WOδ loadings gave >82%
acrylic acid yields at reaction temperatures above 340 °C, which is comparable to the
unsupported Mo8V2WOδ. The 20%Mo8V2WOδ/SBAꢀ15 catalyst is less selective to
acrylic acid. Unlike in the unsupported case (Figure 3), the higher reaction
temperature does not affect the selectivity to acrylic acid significantly over the
supported catalysts. It is assumed that the highly dispersed metal oxides have strong
interactions with silica, and these interactions played a positive role in stabilizing the
Mo8V2WOδ active components, keeping the high activity and selectivity toward
acrylic acid. Figure 7 plots the specific activities (grams of allyl alcohol converted per
hour over one gram of Mo8V2WOδ catalyst) and the rate of acrylic acid formation at
340 °C. It can be seen that the catalytic activity and the rate of acrylic acid formation
can be achieved at 2.5 and 1.8 respectively over 20%Mo8V2WOδ/SBAꢀ15.
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ꢀ ꢀꢀ
ꢀ
ꢀ ꢀ ꢀꢀ
ꢀ ꢀ
ꢀ ꢀ
Sel. Acr.
Sel. AA
Conv.
ꢀ
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T/ °C = 320 340 360 380
320 340 360 380
320 340 360 380
x = 60
x = 40
x = 20
Figure 6. Allyl alcohol oxidation over x%Mo8V2WOδ/SBAꢀ15 catalysts. Reaction
conditions: catalyst (sum of active component and support) 200 mg, 35 ~ 60 mesh;
feed, 20wt% allyl alcohol in H2O, 0.5 ml hꢀ1; carrier gas, 10%O2/He, 20 ml minꢀ1.
WHSV = 0.5 hꢀ1. Acr: acrolein. AA: acrylic acid. Data collected at 1 h time point.
3.5
3.0
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2.0
1.5
1.0
0.5
0.0
Activity
AA yield
0
20
40
60
80
100
Mo8V2WOδ loading (%)
Figure 7. Specific activity (grams of allyl alcohol converted per hour over one gram
of Mo8V2WOδ catalyst) and acrylic acid formation rate (grams of AA produced per
hour over one gram of Mo8V2WOδ catalyst) at 340 °C.
In view of the higher specific activity of the supported catalysts,
40%Mo8V2WOδ/SBAꢀ15 was further tested at higher WHSV (Table S5). When
WHSV was increased from 0.5 hꢀ1 to 2.25 hꢀ1, above 90% allyl alcohol conversion
and over 80% selectivity to acrylic acid still can be achieved in the temperature range
of 260 ~ 340 °C. When the WHSV was further increased to 3.0 hꢀ1, the conversion of
allyl alcohol at 260 °C was 86.7%, and then the conversion decreased even with
temperature rise, indicating the catalyst was overloaded with the reactant at WHSV of
3.0 hꢀ1. Stability is one of the key parameters for a practical catalyst. Therefore, the
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catalytic performances of Mo8V2WOδ and 40%Mo8V2WOδ/SBAꢀ15 catalysts were
studied as a function of time on stream at WHSV of 0.5 hꢀ1 (Figure 8). The complete
conversion of allyl alcohol was kept for 90 ~ 100 h over Mo8V2WOδ and
40%Mo8V2WOδ/SBAꢀ15 catalysts. For the unsupported Mo8V2WOδ (Figure 8A), the
selectivity to acrylic acid decreased from 80% to 70% in the initial 30 h, and then
stabilized at around 70% till 90 h time on stream; meanwhile, the selectivity to acetic
acid increased from 8% to 20%. The 40%Mo8V2WOδ/SBAꢀ15 catalyst showed
excellent stability (Figure 8B). High selectivity to acrylic acid (> 80%) was well
maintained up to at least 100 h time on stream. Herein, the Mo8V2WOδ entities were
highly dispersed and confined inside the mesopores of SBAꢀ15. This structure
efficiently prevented the sintering and agglomeration of the tiny particles during the
long term reaction at high temperature and contributed to the high stability of the
catalyst. The used 40%Mo8V2WOδ/SBAꢀ15 catalyst was subjected to thermogravity
analysis (Figure S5) and the weight loss in the 300 ~ 600 °C range is less than 3%,
signifying insignificant coke deposition even after 100 h working. The TEM image
(Figure S6) of the used catalyst revealed that the mesoporous structure was also well
retained. When the 40%Mo8V2WOδ/SBAꢀ15 catalyst was tested at WHSV of 1.0 hꢀ1
at 340 °C (Figure S7), full conversion and around 80% selectivity to acrylic acid were
also maintained for over 60 hours. When the catalyst was tested at WHSV of 2.25 hꢀ1
at 260 °C (Figure 8C), the initial conversion was 94.5% and the selectivity to acrylic
acid and acrolein were 58.2% and 19.6%, respectively. However, the catalyst
deactivated rapidly after 5 h. A comparison on the total amounts of reactant converted
within the lifetime would be helpful to understand the stability of the catalyst. At
WHSV of 0.5 hꢀ1, the total conversion of the reactant was 10 g / (0.2 g of catalyst) in
100 h (still no deactivation observed); at the WHSV of 2.25 hꢀ1, the total conversion
of the reactant was around 2.1 g / (0.2 g of catalyst) in the initial 5 h (after that time
the catalyst started to deactivate). Thus, it is proposed that the observed catalyst
stability with respect to the WHSV might be related to the redox property of the
catalyst. The partial oxidation reactions over transition metal oxide catalysts generally
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determine the activity of the catalysts. At higher WHSV the supported Mo8V2WOδ
catalyst would be reduced much faster than the rate for catalyst reoxidation, and this
leads to the fast catalyst deactivation.
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Sel. AA
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15
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25
Time / h
Figure 8. Time on stream tests for allyl alcohol oxidation over (A) Mo8V2WOδ at
WHSV of 0.5 hꢀ1, (B) 40%Mo8V2WOδ/SBAꢀ15 at WHSV of 0.5 hꢀ1, and (C)
40%Mo8V2WOδ/SBAꢀ15 at WHSV of 2.25 hꢀ1. Reaction conditions: catalyst 200 mg,
35 ~ 60 mesh; feed for (A) and (B), 20 wt% allyl alcohol in H2O, rate 0.5 ml hꢀ1; feed
for (C), 60 wt% allyl alcohol in H2O, rate 0.75 ml hꢀ1. Carrier gas, 20 ml minꢀ1,
10%O2/He for (A) and (B), 21%O2/N2 for (C); reaction temperature, 340 °C for (A)
and (B), 260 °C for (C). AA: acrylic acid. AcOH: acetic acid. Acr: acrolein.
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CONCLUSIONS
In conclusion, we have demonstrated a highly efficient protocol for the
production of acrylic acid from glycerol. The process involves glycerol
deoxydehydration to allyl alcohol in liquid phase and allyl alcohol catalytic oxidation
to acrylic acid in gas phase with 90% overall yield of acrylic acid. The entire process
can be carried out in a continuous manner and demonstrated high efficiency and
selectivity. The SBAꢀ15 supported metal oxide (MoꢀVꢀWꢀO) catalysts have well
defined structure and exhibited good stability. The current study provides a new and
prominent method for the production of acrylic acid from bioꢀrenewable glycerol.
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ACKNOWLEDGEMENTS
This work was supported by the Institute of Bioengineering and Nanotechnology
(Biomedical Research Council, Agency for Science, Technology and Research
(A*STAR), Singapore), BiomassꢀtoꢀChemicals Program (Science and Engineering
Research Council, A*STAR, Singapore).
AUTHOR INFORMATION
Corresponding Author
Eꢀmail: ygzhang@ibn.aꢀstar.edu.sg. Fax: +65 6478 9084. Phone: +65 6824 7162
Notes
The authors declare no financial interest.
Supporting Information Available: including materials characterization and
additional experimental data. This material is available free of charge via the Internet
at http://pubs.acs.org.
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