Enhanced Acrylate Production from 2-Acetoxypropanoic Acid Esters

Article abstract of DOI:10.1021/acs.oprd.7b00047

Reaction conditions and reactor geometry for producing acrylates in high yield from lactic acid-derived 2-acetoxypropanoic acid (APA) esters are presented. An acrylate ester yield of 75% is achieved from methyl and benzyl APA esters at 550 °C in a fixed bed reactor filled with nonporous silica particles, carbon dioxide as a diluent gas, and acetic acid as a co-feed with the APA ester. The yield from methyl and benzyl APA esters is remarkably higher than from ethyl or butyl esters of APA, which have hydrogen atoms on the β-carbon of the ester functional group and thus can undergo alkene elimination, leading to reduced acrylate yield. Under optimum conditions, APA conversion to acrylates is stable over 30 h of continuous operation with little carbon deposition on the contact material.

Full text of DOI:10.1021/acs.oprd.7b00047

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Full Paper
Enhanced Acrylate Production from 2-Acetoxypropanoic Acid Esters
Iman Nezam, Lars Peereboom, and Dennis J. Miller
Org. Process Res. Dev., Just Accepted Manuscript • Publication Date (Web): 26 Apr 2017
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Enhanced Acrylate Production from 2-Acetoxypropanoic Acid Esters
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Iman Nezam, Lars Peereboom, and Dennis J. Miller*
Department of Chemical Engineering and Materials Science
Michigan State University
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East Lansing, Michigan 48824
*To whom correspondence should be addressed at: Department of Chemical Engineering and
Materials Science, Michigan State University, 428 S. Shaw Lane, East Lansing, Michigan 48824.
Email: millerd@egr.msu.edu; Voice: (517) 353ꢀ3928; Fax: (517) 432ꢀ1106.
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Table of Contents Graphic
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O
O
O
O
O
O
HO
+
R
O
R
acrylate ester
2-acetoxypropanoate ester
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Abstract
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Reaction conditions and reactor geometry for producing acrylates in high yield from lactic
acidꢀderived 2ꢀacetoxypropanoic acid (APA) esters are presented. An acrylate ester yield of 75%
is achieved from methyl and benzyl APA esters at 550˚C in a fixed bed reactor filled with nonꢀ
porous silica particles, carbon dioxide as a diluent gas, and acetic acid as a coꢀfeed with the APA
ester. The yield from methyl and benzyl APA esters is remarkably higher than from ethyl or
butyl esters of APA, which have hydrogen atoms on the β-carbon of the ester functional group
and thus can undergo alkene elimination that leads to reduced acrylate yield. At optimum
conditions, APA conversion to acrylates is stable over 30 hours of continuous operation with
little carbon deposition on the contact material.
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Keywords: Acrylate; Lactate; Acetoxypropanoate; Carbon dioxide
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1. Introduction
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Acrylic acid (2ꢀpropenoic acid, C3H4O2) is the simplest alkenoic acid. It can react with
itself or with other monomers to form polymers that have wide use as adhesives, polishes,
binders, coatings, paints, detergents, fibers, polyelectrolytes, flocculants, absorbents, and
dispersants. Acrylic acid is traditionally produced from propylene by a twoꢀstep gas phase
process which involves the catalytic oxidation of propylene to acrolein and further oxidation of
acrolein to acrylate in the presence of a catalyst1. In 2015, the acrylic acid market was 5.8 million
metric tons, with demand growing at 6.3% per year and price ranging from $1600ꢀ$2200 per ton
depending on grade2.
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Recently, renewable bioꢀbased routes to acrylic acid have received attention, both to
address resource and climate issues and to ensure a stable, inexpensive supply in light of volatile
petroleum markets. Among these routes is the dehydration of glycerol to acrolein3ꢀ5, which has
been demonstrated at 90% yield, followed by conventional oxidation to acrylic acid. The direct
conversion of glycerol to acrylic acid has also been carried out at 75% yield5, but in both routes
catalyst deactivation and sustaining selectivity are challenges.
Acrylic acid can be produced by the direct dehydration of lactic acid (2ꢀ
hydroxypropanoic acid) using either sulfate6ꢀ8 or phosphate9ꢀ12 salt catalysts. The highest yield of
68% was reported in one of the first studies6. Methyl lactate can also be directly dehydrated to
methyl acrylate;13ꢀ16 yields above 90%, have been claimed over molecular sieves,16 but most
studies with modified molecular sieves achieved yields no higher than 65% acrylate. Diluting
methyl lactate with methanol16 or water10ꢀ11gave modest improvement in acrylate yield. A recent
review17 gives an excellent summary of lactic acid and lactate ester dehydration pathways.
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Acrylic acid can also be readily prepared via direct dehydration of 3ꢀhydroxypropionic
acid produced by fermentation, with yields of acrylic acid up to 88% obtained using solid acid
catalysts.18ꢀ20 This route is the subject of current commercial development.21
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Lactic acid, lactate esters, or lactate salts can react with acetic anhydride22ꢀ23 or acetic
acid24ꢀ27 in the presence of catalyst to produce 2ꢀacetoxy propionic acid (APA), its esters, or its
salts in high yields. APA or its derivatives can then be subject to high temperature (400ꢀ550˚C)
to eliminate acetic acid and give the corresponding acrylate. Several results have been reported
with acrylate yields as high as 90% of theoretical,22ꢀ24 but yield invariably declines with timeꢀonꢀ
stream. While the use of acetic anhydride to form APA is prohibitively expensive, the ability to
make APA from acetic acid27 via reactive distillation makes acrylate production via this route
worthy of consideration. In this work, we present conditions that provide sustained high yields
of acrylate from lactic acidꢀderived APA esters.
2. Materials and Methods
2.1
Materials
Methyl (S)ꢀ(ꢀ)ꢀlactate (98%), ethyl (S)ꢀ(ꢀ)ꢀlactate (98%), butyl (S)ꢀ(ꢀ)ꢀlactate (97%),
benzyl (S)ꢀ(ꢀ)ꢀlactate (90%), isobutyl (R)ꢀ(+)ꢀlactate (97%) and acetic acid (99.7%) were
purchased from SigmaꢀAldrich and used as received. Methylꢀ2ꢀacetoxy propionate (MAPA)
(99%), ethylꢀ2ꢀacetoxy propionate (EAPA) (96%), butylꢀ2ꢀacetoxy propionate (BAPA) (98%),
benzylꢀ2ꢀacetoxy propionate (BeAPA) (96%), and isobutylꢀ2ꢀacetoxy propionate (IBAPA)
(99%) were produced by reaction of the corresponding lactate ester with excess acetic anhydride
under acidic conditions at 25˚C. The acetoxy ester was recovered by mixing the reaction solution
with diethyl ether and an aqueous solution of sodium bicarbonate in a separatory funnel. The
ether phase, which contains the APA ester, was separated and then washed again with an
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aqueous solution of sodium chloride. Finally, the APA ester was isolated and purified by
centrifuging and distilling off the ether under vacuum.
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Several materials were evaluated as contact materials for reaction. Nonꢀporous granular
quartz (SiO2, SigmaꢀAldrich) as the primary material was ground and sieved to 30/50 mesh size.
Additional contact materials included the porous silica Spherosil (XOA 400) and silicon carbide
(SiC).
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2.2
Reactor Configuration
Reactions were performed in a 0.5 inch ID x 40 cm length quartz tube reactor packed
with contact material and equipped with a 1/8” OD internal thermowell to measure and control
the reaction temperature. A schematic of the reactor system is given in Figure Sꢀ1 in the
Supplementary Information. Quartz (SiO2) was chosen for the reactor material because it has
been shown22ꢀ23 to minimize undesired reactant and product decomposition relative to other
materials of construction in APA conversion to acrylates. The reactor was placed in a tube
furnace and was sealed on each end with 316 SS Oꢀring fittings to facilitate reactant input and
product collection. Liquid reactants were fed to the top of the reactor through a 1/16” OD tube
that extended downward to approximately 10 cm above the contact material. Initially, diluent
gas was fed through the annulus surrounding the liquid feed tube; however, it was found that
better vaporization of the liquid feed took place, and higher acrylate yields were obtained, when
the liquid feed and diluent gas were both fed through the 1/16” tube. This arrangement was
therefore used in all experiments reported in this work.
Products were collected in two traps in series, one trap immersed in ice and a second
immersed in dry ice/acetone. Gas that passed through both traps was sampled periodically during
reaction using gas bags.
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Experiments were initiated by loading 1.0 g of contact material supported on quartz wool
into the reactor. The reactor was assembled in the tube furnace and diluent gas was passed
through the reactor (10ꢀ30 ml STP/min) while it was heated to reaction temperature. During
reaction, liquid feed was metered into the reactor using a PHD 2000 syringe pump at rates of 1.0
to 5.0 ml/hr. All experiments were conducted at atmospheric pressure.
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Product samples were collected from the ice trap (0˚C) after the first 60 minutes of
reaction and then at 90 minute intervals. Product collected in the dry ice trap was only analyzed
at the conclusion of the experiment. Liquid and gas samples were analyzed by gas
chromatography; details of the analytical methods and yield calculations are given in the
Supplementary Information.
2.3
Contact material characterization
Nitrogen adsorption at 78 K (ꢀ195˚C) in a Micromeritics ASAP 2010 instrument was
used to measure the BET surface area of contact materials. Materials were outgassed at 260˚C
for 24 h prior to adsorption measurements. Surface acidity and basicity were measured by
temperature programmed desorption (TPD) of NH3 and CO2, respectively, in a Micromeritics
Autochem 2910 instrument. For surface acidity, approximately 0.6 g of contact material was
thermally pretreated at 800˚C in 50 cm3/min of He (99.999%) for 60 minutes, cooled to 25˚C in
He, saturated at 25˚C with NH3 by flowing a mixture of 14.8 vol% NH3 in helium at 50 cm3/min
for 1 hr, and then purged with He (50 cm3/min) for 2 h to remove all physisorbed NH3.
Desorption was carried out by heating in He from ambient temperature to 600˚C at 10˚C/min and
holding for 30 min. The resulting NH3 peak area was quantified using an NH3 gas standard of
known composition. The identical procedure was used for measuring surface basic site density,
except that pure CO2 (99.8%) was used instead of the NH3/He blend.
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3. Results and discussion
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Scheme 1 describes the reaction pathways observed in experiments. Path (a) is acetic
acid elimination to form acrylates, or in another sense, elimination of APA as an alkene
(acrylate) from acetic acid. Path (b) is elimination of the alkyl ester group as an alkene that can
occur when there is hydrogen on the βꢀcarbon of the ester functionality. Both eliminations are
facilitated via formation of a cyclic sixꢀmember transition state involving hydrogen on the βꢀ
carbon.28 Complete product distributions and functional group balances (acetate (C2), lactate
(C3), and ester (C1 or C4)) were carried out for all reactions; results for key experiments,
denoted as R1ꢀR5 in the text and figures, are given in Table Sꢀ1 in Supplementary Information.
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Scheme 1. Reaction pathways for APA ester elimination reactions. Path (a): elimination of acetic
acid to form alkyl acrylate or acrylic acid; Path (b): elimination of alkene for ester R groups
containing hydrogen on βꢀcarbon; Path (c): decarbonylation of APA to acetaldehyde, acetic acid,
and CO.
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Two control experiments were conducted by feeding butyl acrylate and acetic acid at
550˚C and a liquid hourly space velocity (LHSV) of 1.9 kg solution/kg contact material/hr in
order to determine their stability at typical reaction conditions. The recovery of acetic acid from
the reactor after four hours of steady state operation was 94%, which is essentially complete
recovery within the uncertainty of the experiment. At the same conditions, butyl acrylate was
92% converted to acrylic acid with accompanying quantitative formation of 1ꢀbutene.
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3.1. Feed composition: CO2 as diluent gas and acetic acid as co-feed with APA
Adding acetic acid to butyl APA feed (Figure 1) modestly improves acrylate selectivity
to a maximum of 35% at 25 ꢀ 45 mol% acetic acid in the feed. Moderate quantities of acetic acid
may provide acidity to aid elimination or may interact with the contact material surface to limit
side reactions, but most likely acetic acid aids in dispersing and volatilizing the APA esters.
Acetic acid concentrations above 50 mol % lead to reduced acrylate yields; the large excess of
acetic acid may enhance unwanted decomposition reactions or thermodynamically limit the
extent of acetic acid elimination from APA. Subsequent experiments were carried out with 55
mol% butyl APA + 45 mol% acetic acid (80 wt% APA ester + 20 wt% acetic acid) as the liquid
feed.
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Mole Fraction Acetic Acid in Feed
Figure 1. The effect of acetic acid feed concentration on steady state acrylate selectivity from
butyl APA (R2 ꢀ 45 mol% acetic acid; R4 ꢀ 0% acetic acid); Reaction conditions: T = 550˚C;
quartz (SiO2) contact material; CO2 diluent gas 20 ml/min; LHSV = 1.7 ꢀ 2.4 kg feed/kg contact
material/hr.
Carbon dioxide and nitrogen were examined as diluent gases with butyl APA and with
benzyl APA as feed materials over quartz contact material. Conversion of both APA esters is
essentially complete at reaction conditions for both gases, but CO2 increases acrylate selectivity
relative to N2 (Figure 2). With butyl APA, the combined acrylate selectivity (butyl acrylate +
acrylic acid) with CO2 is approximately twice that with N2 as steady state is approached (300
minutes onꢀstream). Acrylate selectivity with N2 clearly declines with timeꢀonꢀstream, while it
remains essentially constant with CO2 as diluent. The effect is much less dramatic with benzyl
APA as the feed, but is significant during the rampꢀup to steady state.
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Figure 2. Total acrylate selectivity versus timeꢀonꢀstream with N2 or CO2 as diluent gas (20
ml/min) for benzyl and nꢀbutyl APA ester feed. Reaction conditions: T=550˚C; 80 wt% APA
ester / 20 wt% acetic acid feed; LHSV = 1.9 kg feed/kg contact material/hr. (▲) = benzyl APA,
CO2; (♦) = benzyl APA, N2; (○) = butyl APA, CO2 (R2); (□) = butyl APA, N2 (R1); (X) = butyl
APA, N2 repeated.
Carbon monoxide (CO) is present in the effluent gas stream with both N2 and CO2 as
diluent gases. If CO was only formed via decarbonylation of the lactate backbone, then on a
molar basis CO formation should be at most equal to that of acetaldehyde (because
decarboxylation of lactate can also occur). This is indeed the case for butyl APA conversion with
N2 (R1 in Table Sꢀ1). In contrast, CO formation with CO2 as diluent is eightꢀfold greater than
that of acetaldehyde. Given that carbon is deposited on the contact material during reaction by
partial decomposition of APA and other species, the large quantity of carbon monoxide produced
can be explained by the Boudouard reaction of CO2 with this deposited carbon (CO2 + C →
2CO).
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The deposited carbon is likely amorphous and porous in nature, and is thus an effective
“trap” for reactants and products in the system, causing them to remain in the high temperature
environment and further decompose. The presence of CO2 as diluent gas continually removes
these carbon deposits from the contact material, thus ensuring a short residence time for reactants
and products that minimizes decomposition and results in higher acrylate selectivity.
To further support the presence of the Boudouard reaction, the contact material was
removed from the reactor following experiments with N2 and CO2 as diluent gases, weighed, and
heated in air to 500˚C. The weight loss upon heating was three to four times higher for the
experiment with N2 as the diluent gas than with CO2 as the diluent, indicating greater carbon
deposition with N2 than with CO2. This result, along with increased pressure drop through the
reactor and reduction in acrylate selectivity after 300 minutes on stream with the N2 diluent,
support the hypothesis that CO2 maintains the integrity of the contact material surface by
removing deposited carbon during reaction.
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Finally, the composition of the effluent gas suggests that the reaction of CO2 with
deposited carbon is rapid and even approaches equilibrium. With butyl APA feed and CO2 as
diluent gas, the reactor effluent contains 10 mol% CO and 50 mol% CO2. These quantities of CO
and CO2 are close to the composition at P = 1 atm dictated by the equilibrium constant for the
Boudouard reaction at 550˚C (Ke ~ 0.02 = PCO2/PCO2).29
3.2. Acrylate yields from different APA esters
Methyl APA, benzyl APA, isoꢀbutyl APA, butyl APA, and ethyl APA were fed over the
quartz fixed bed at 550oC and LHSV = 1.9 kg feed/kg contact material/hr. Conversion of APA
ester ranged from 90% to nearly 100% for the different esters; total acrylate selectivity is given
in Figure 3 for each ester.
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Time On Stream (min)
Figure 3. Total acrylate selectivity vs. timeꢀonꢀstream for different APA ester feeds: (X) =
methyl (R3); (▲) = benzyl; (■) = isobutyl; (●) = nꢀbutyl (R2); (♦) = ethyl. Reaction conditions:
T = 550˚C; 80 wt% APA ester / 20 wt% acetic acid feed; CO2 diluent gas at 20 ml/min; LHSV =
1.9 kg feed/kg contact material/hr.
Figure 3 shows that those esters with hydrogen on the βꢀcarbon of the ester functionality
(ethyl, isobutyl, butyl) exhibit significantly lower selectivity to acrylates than those without
hydrogen on the βꢀcarbon (methyl, benzyl). Further, selectivity to acrylates declines as the
number of βꢀcarbon hydrogen atoms increases, and the acrylate product distribution shifts from
exclusively acrylate ester for methyl and benzyl APA feeds to a majority of free acrylic acid for
ester groups containing βꢀcarbon hydrogens. As shown in Scheme 1 (Path (b)), the ability of
ester groups containing βꢀcarbon hydrogens to eliminate as alkenes from alkyl acrylate leads to
free 2ꢀacetoxy propionic acid (APA) or free acrylic acid, which are significantly more reactive
toward undesired decomposition or polymerization than their ester counterparts. Free APA is not
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observed in the reactor effluent under any conditions in this study, even though nearly
stoichiometric quantities of alkene relative to feed are formed in some experiments. Thus, free
APA formed must quickly decompose at reaction conditions – partially to acrylic acid via the
desired acetic acid elimination, but also to undesired products via decarbonylation ((Path (c),
Scheme 1) and other decomposition reactions to form acetaldehyde, gases, free acetic acid, and
carbon on the contact material surface. Acrylic acid likewise must undergo polymerization or
decomposition that leads to reduced yields. The decomposition of free APA and free acrylic acid
in the reactor is further supported by low lactate (C3) group recovery for butyl APA reactions
(R1 and R2 in Table Sꢀ1) relative to methyl APA reactions (R3 in Table Sꢀ1), partially because
acetaldehyde (b.p. 20˚C) formed via decarbonylation escapes from collection traps and is lost in
the gas effluent stream.
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3.3. Reaction temperature
Figure 4 illustrates the dependence of butyl APA conversion, acrylic acid selectivity, and
butyl acrylate selectivity on reaction temperature from 420˚C to 690˚C over quartz contact
material. Conversion of butyl APA increases from 10% at 420˚C to nearly 100% at 550˚C,
indicating that Path (a) in Scheme 1 goes to completion. Selectivity to acrylic acid increases and
selectivity to butyl acrylate decreases above 500˚C, as butene elimination from butyl acrylate
(Path (b) in Scheme 1) becomes rapid. Total acrylate yield remains nearly constant at its
maximum value between 550˚C and 620˚C, suggesting that acrylic acid is relatively stable over
this temperature range and only conversion from butyl acrylate to acrylic acid occurs. Acrylic
acid decomposition predominates above 620˚C, such that no acrylic acid exits the reactor at the
highest temperature examined (690˚C). An estimate of the activation energy assuming butyl
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APA conversion is a simple first order reaction gives a value of 132 kJ/mole (R2 = 0.99) The
Arrhenius plot is provided as Figure Sꢀ2 in the Supplementary Information.
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400
450
500
550
600
650
700
Temperature (˚C)
Figure 4. Steady state butyl APA conversion (●), butyl acrylate selectivity (■), and acrylic acid
selectivity (▲) at steady state versus temperature (R5 – 490 C, R2 – 550 C). Reaction
conditions: 80 wt% butyl APA / 20 wt% acetic acid feed; CO2 diluent gas at 20 ml/min; LHSV =
1.9 kg feed/kg contact material/hr.
˚
˚
3.4. Different contact materials
Experiments were performed at 550˚C with 1.0 g of Spherosil (XOA 400) porous silica
and silicon carbide (SiC) as contact materials in the quartz tube reactor. Table 1 compares
acrylate selectivity from butyl APA with total surface area, surface acid site density, and surface
basic site density of each of the contact materials. At 550˚C, nearly complete butyl APA
conversion was observed for all three materials, and the highest acrylate selectivity was obtained
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for quartz and SiC. The porous silica contact material clearly does not perform well, most likely
because any feed material entering the pores is trapped and decomposes to carbon and gases. In
contrast, SiC and quartz are nonꢀporous and do not trap reactants or products, but instead provide
short contact time heat transfer surface area for volatilization and reaction of APA esters. Silicon
carbide and quartz have such low surface areas that their surface acidity and basicity were not
measurable.
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Table 1. Properties and experimental results for contact materials. Reaction conditions: T =
550˚C; 80 wt% butyl APA / 20 wt% acetic acid feed at 2.4 ml/hr; CO2 diluent gas at 20 ml/min;
LHSV = 1.9/hr.
Total
Surface Surface
Area Acidity
(m2/g) (µmol/g) (µmol/g)
Surface
Basicity Conversion
Butyl APA
Acrylate
Selectivity
%
Catalyst
%
SiO2 (35ꢀ50mesh) (R2)
SiC (50 mesh)
0.08
0.03
370
<10
<10
137
<10
<10
45
97.4
99.5
100
34.9
29.4
5.1
Spherosil (XOA400)
3.5 Space velocity
Butyl APA conversion and acrylate selectivity were measured at 550˚C with different
quantities of quartz (0.3 g, 1 g, and 3g) in the reactor that correspond to LHSV of 6.3, 1.9, and
0.65 kg feed/kg contact material/hr. Nearly complete conversion of butyl APA was observed
over this range of LHSV along with similar acrylate selectivity, an indication that even higher
space velocities should be suitable for APA ester conversion to acrylates at 550˚C.
3.6. Extended reaction
To demonstrate the improved stability of acrylate formation with CO2 as diluent gas and
inclusion of acetic acid as a coꢀfeed, an extended time experiment was carried out for 30 hours.
The results (Figure Sꢀ3 in Supplementary Information) shows that butyl APA conversion (98%)
and overall acrylate selectivity (32ꢀ35%) were stable over the 30 hr period of operation. The run
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was terminated at 30 hr because of depletion of feed material. Following reaction, the quartz
contact material was removed from the reactor and was found to have a quantity of carbon
deposited essentially the same as the typical 3ꢀ5 hour run. Overall material balance and backbone
fragment balances were excellent for this extended experiment.
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4. Conclusions
Feed composition and reaction conditions for enhanced acrylate production from APA
esters have been identified. Acrylate selectivity of 35% was achieved from butyl APA at 550˚C
and atmospheric pressure with 1.0 g of 35ꢀ50 mesh low surface area silica (quartz) as the
preferred contact material, 20 wt% acetic acid in the liquid feed, and CO2 as the diluent gas.
Experiments with methyl or benzyl APA ester feeds gave acrylate selectivity of 70+% at the
same conditions and are thus clearly preferred feed materials. Higher alkyl APA esters have
hydrogen atoms on the βꢀcarbon of their ester functionality that allows elimination of the ester
group as an alkene, liberating free acrylic acid and APA that decompose or polymerize to reduce
acrylate yield. Using CO2 as diluent gas increases acrylate formation from butyl APA and
reduces the quantity of deposited carbon on the contact material, thus maintaining reaction rate
over extended operation.
5. Acknowledgments
The authors wish to acknowledge partial support for this work from the U.S. Department
of Energy (Award no. DEꢀFG36ꢀ04GO14216) and the National Corn Growers Association for
financial support of this work.
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6. Supplementary Information
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A reactor schematic, the analytical method, and detailed product distribution of selected
experiments is provided in the Supplementary Information.
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