Tandem catalytic conversion of glycerol using solid catalysts followed by transesterification to produce alkyl lactate

Article abstract of DOI:10.1039/c5ra06285c

This study investigated alkyl lactate production from glycerol by tandem processes which included glycerol conversion to calcium lactate using solid catalysts and subsequent transesterification of calcium lactate to alkyl lactate using methanol/ethanol and carbon dioxide. The effect of reaction conditions on alkyl lactate production was systematically investigated. A central composite design (CCD) and response surface methodology were used to design the experiments and evaluate the optimum process conditions. At the optimum conditions, the methyl lactate yield reached 57 mol% with a glycerol conversion of 94 mol% using refined glycerol. A yield of approximately 42 mol% for ethyl lactate was obtained using refined glycerol at the optimum conditions. Similar glycerol conversion and alkyl lactate yields were obtained using crude glycerol, indicating that the impurities in the crude glycerol had no significant effects on alkyl lactate production. The examination of regenerated catalysts that had been reused twice showed no negative effects on glycerol conversion and methyl lactate production. Compared to the traditional lactic acid and alkyl lactate production, this two-step process for alkyl lactate production from glycerol is both economical and environmentally benign since no gypsum was produced and the solid catalysts can be regenerated and reused.

Full text of DOI:10.1039/c5ra06285c

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Page 1 of 26
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Tandem catalytic conversion of glycerol using solid catalysts followed by
transesterification to produce alkyl lactate
Shoujie Ren, X. Philip Ye*, and Paul D. Ayers
Department of Biosystems Engineering and Soil Science, The University of Tennessee
2506 E. J. Chapman Drive, Knoxville, TN 37996, U.S.A.
*Corresponding author: X. Philip Ye, Email: xye2@utk.edu
GRAPHICAL ABSTRACT
An environmentally benign process for alkyl lactate production from glycerol using solid
catalysts with alcohols and carbon dioxide
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Tandem catalytic conversion of glycerol using solid catalysts followed by
transesterification to produce alkyl lactate
Shoujie Ren, X. Philip Ye*, and Paul D. Ayers
Department of Biosystems Engineering and Soil Science, The University of Tennessee
2506 E. J. Chapman Drive, Knoxville, TN 37996, U.S.A.
*Corresponding author: X. Philip Ye, Email: xye2@utk.edu
ABSTRACT: This study investigated alkyl lactate production from glycerol by tandem
processes which included glycerol conversion to calcium lactate using solid catalysts and
subsequent transesterification of calcium lactate to alkyl lactate using methanol/ethanol and
carbon dioxide. The effect of reaction conditions on alkyl lactate production was systematically
investigated. A central composite design (CCD) and response surface methodology were used to
design the experiments and evaluate the optimum process conditions. At the optimum condition,
the methyl lactate yield reached 57 mol% with a glycerol conversion of 94 mol% using refined
glycerol. A yield of approximately 42 mol% for ethyl lactate was obtained using refined glycerol
at the optimum condition. Similar glycerol conversion and alkyl lactate yields were obtained
using crude glycerol, indicating that the impurities in the crude glycerol had no significant
effects on alkyl lactate production. The examination of regenerated catalysts that had been reused
twice showed no negative effects on glycerol conversion and methyl lactate production.
Compared to the traditional lactic acid and alkyl lactate production, this two-step process for
alkyl lactate production from glycerol is both economical and environmentally benign since no
gypsum was produced and the solid catalysts can be regenerated and reused.
KEYWORDS: Glycerol, Alkyl lactate, CaO, Methanol, CuO, Carbon dioxide
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1. Introduction
Biodiesel is a renewable fuel produced from the transesterification of vegetable oil and
animal fat. During biodiesel production, one mole glycerol is formed as a co-product with one
mole of triglyceride feed. With expanded biodiesel production, a large amount of glycerol is
produced and available in the market that resulted a significantly decrease of price of glycerol.
Due to its high availability and low price, glycerol has been selected as one of the top 12
platform chemicals from biomass.1
Lactic acid has been a commodity chemical used in food, chemical and pharmaceutical
industries traditionally. Alkyl lactate such as methyl lactate and ethyl lactate are derivatives of
lactic acid widely used as a green solvent, an additive and an intermediate chemical in the
biodegradable polymer industry.2 High purity methyl lactate is also used to produce highly pure
lactic acid, which has broad applications in dairy products and pharmaceutical products.3
Recently, lactic acid has been drawing great interest in producing biodegradable plastics.2 With
the extension of applications, the demand for lactic acid in the chemical industry has
significantly increased.
Lactic acid can be produced through both fermentation of carbohydrates and chemical
conversion starting from glycerol.1, 4-8 Lactic acid produced by fermentation needs to add
Ca(OH)2 or NaOH to maintain a neutral pH and precipitate the formed lactic acid. A large
amount of Ca(OH)2 is consumed in the process and this leads to large amounts of CaSO4
produced as a byproduct.9 Recent investigation of lactic acid production from glycerol under
hydrothermal conditions with homogeneous base catalysts such as NaOH and KOH showed the
product existed as a lactate form due to an instant reaction of lactic acid and base during the
production process.1, 5, 10 The high concentration of NaOH or KOH used in the process also
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causes severe corrosion—even to stainless steel reactors. It is worth noting that lactic acid
produced from the fermentation and chemical synthesis of carbohydrates or glycerol is of low
purity. To obtain a highly pure lactic acid, crude lactic acid needs to be esterified with methanol
or ethanol to form methyl or ethyl lactate and subsequent hydrolyzation of the methyl/ethyl
lactate.11-14 So, direct formation of alkyl lactate from glycerol is desirable when helping to reduce
the tedious process of lactic acid production in fermentation and in chemical synthesis and waste
generation.
Direct synthesis of alkyl lactate has been investigated using simple sugars as starting
materials with heterogeneous catalysts.15-18 Among these studies, West et al. reported
10
19
dihydroxyacetone and glyceraldehyde in the methanol solvent converting to methyl lactate.
This conversion utilized the zeolite H-USY as a catalyst. Final analysis reported over 90%
10
selectivity and yield for methyl lactate.15 Wang et al. also converted dihydroxyacetone and
19
glyceraldehyde
with alcohol to alkyl lactates using Brønsted acid with ion-exchanged
montmorillonite (Sn-Mont) as a catalyst and obtained 89–93% yield of alkyl lactates in the
conversion of DHA.17 DHA and GLA are C-3 sugars and have served as intermediate chemicals
in the hydrothermal conversion of glycerol to lactic acid with alkali.4, 7, 20 Yoon et al. reported
directly converting glycerol to alkyl lactate in published patent. But the catalysts they used are
the mixture of base such as CaO and MgO with NaOH or synthesized catalysts of
Na/NaOH/Al2O3, Na/MgO, and Sn/zeolite. The strong alkali used in the process or catalyst
synthesis are not desirable.21 Purushothaman et al. reported a one-pot conversion of glycerol to
methyl lactate in methanol and air but the precious golden are required as catalyst. 22
CaO as a solid-base catalyst has been investigated in biodiesel production.19, 23-25 In our
previous study, we investigated CaO as a solid-base catalyst for converting glycerol to racemic
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lactic acid with a 40.8 mol% yield of lactic acid and a glycerol conversion of 97.8 mol% thereby
achieving at the optimum condition.20 However, in this study lactic acid existed in the form of
calcium lactate salt at the end of the reactions and still required recovery and purification of
lactic acid and generated CaSO4 waste.
The objective of this study was to investigate alkyl lactate production from glycerol using a
tandem process. This developed process included glycerol conversion using solid catalysts (CaO
and CuO) to calcium lactate, and subsequent calcium lactate conversion to methyl/ethyl lactate
with CO2 and methanol/ethanol. Process conditions such as reaction temperature and time,
catalyst and methanol/ethanol loading in refined and crude glycerol were systematically studied.
Direct use of industrial crude glycerol was also investigated in order to investigate the effects of
impurities and the possibility of using crude glycerol as a starting material. The regeneration and
reusability of catalysts were also investigated in this study.
2. Materials and methods
2.1 Materials
Refined glycerol (99.5%) used as the starting material was first investigated in this study.
CaO and CuO were used as solid catalysts. CaO was calcined at 910°C in a muffle furnace for 3h
before used. Anhydrous methanol (99.99%) and ethanol (99.99%) were as solvents and reactants
in the process of this study. Lactic acid (90%), methyl lactate (>96%), ethyl lactate (>96%), and
propylene glycol (>99.5%) were used for product calibration of HPLC. All these chemicals,
refined glycerol, and catalysts were purchased from Thermo Fisher Scientific Inc. (Waltham,
MA, USA). Carbon dioxide in the form of dry ice used in this study was purchased from a local
supplier.
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2.2 Reactor
The reactions were performed in batch tube reactors made of stainless steel. The volume of
the tuber reactor was 2.72 mL with an outside diameter (OD) of 9.525 mm (0.375 inch), an
inside diameter (ID) of 7.036 mm (0.277 inch), and a length of 70 mm. The process of glycerol
conversion and alkyl lactate production were carried out in an oil bath.
2.3 Experimental procedure
The first-step of the process was glycerol conversion to calcium lactate using solid catalysts
of CaO and CuO. The second-step of the process was preparing calcium lactate to react with
alcohol and carbon dioxide to form alkyl lactate. After reaction, the mixture of products was
washed out by DI water and centrifuged. For testing the recyclability of catalysts, the solids from
centrifugation (mainly CaCO3, CuO and Cu) were calcined and reused in glycerol conversion.
To analyze the products, the pH of the collected supernatant was adjusted to around 3 by adding
diluted sulfuric acid (the concentration of diluted sulfuric acid is 10%). All unconverted calcium
lactate was converted back to lactic acid while sulfate salt precipitated in the solid form. If a
large amount of lactic acid is detected, it means low conversion of calcium lactate to
methyl/ethyl lactate. To the contrary, if a small amount of lactic acid is detected, it indicates that
there was a high conversion of calcium lactate to methyl/ethyl lactate.
2.3.1 Glycerol conversion to calcium lactate
In our previous research, the optimum condition for calcium lactate was at the molar ratio
CaO to glycerol of 1, the molar ratio CuO to glycerol of 0.2, and 33.3% water content of glycerol
26
at the reaction temperature of 230 °C. So the first-step process of calcium lactate formation
was performed at this condition. First, CaO, CuO, glycerol, and water were well mixed. About
0.5 g of well-mixed reactant was added to the tube. Before sealing the tube, argon gas was
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purged to repel the air from the tube. Then the tube reactor was immersed in a pretreated oil bath
at the desired temperature of 230 °C for 30 min. After reaching the reaction time, the reactor was
quenched in cold water immediately to stop any further reaction.
2.3.2 Calcium lactate conversion to methyl lactate
The quenched reactor from the first-step process was opened, and methanol and dry ice were
added in the second-step process in which the calcium lactate was converted to methyl lactate. In
this study, dry ice was the carbon dioxide source. The reaction was performed in the same oil
bath with desired temperature and time. During the reaction, the tubes were shaken by a shaker
(Wrist Action Model 75, Burrell Scientific, Inc.) to assure adequate mixing of reactants. After
the reaction, the reactor was quenched in cold water immediately to stop the reaction and the
reactant was washed out by DI water to prepare for analysis.
To optimize the process conditions of calcium lactate conversion and methyl lactate
production, a central composite design (CCD) was used. Three independent variables, reaction
temperature (X1, °C), reaction residence time (X2, min), and molar ratio of methanol to glycerol
(X3) were chosen along with one dependent variable of methyl lactate yield. For statistical
calculations, the variables Xi were coded as xi according to Equation 1 expressed as
푥ꢀ = (푋ꢀ − 푋0)/∆푋
(1)
where xi is dimensionless value of an independent variable, Xi is the real value of an independent
variable, X0 is the real value of the independent variable at the center point, and ∆X is the step
change. The central point of independent variables was set at 180 °C, 4 hrs, and 8 mole/mole of
methanol/glycerol and the step changes were chosen at 20 °C, 2 hrs and 4 mole/mole of
methanol/glycerol. A total of 18 experiments with 6 axial points (α=1.68) and 4 replications at
the center points in a 23-factorial CCD were employed for the optimization (Table 1). The second
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degree polynomial (Equation 2) was calculated to estimate the response of dependent variable.
Hence, the response is expressed as
2
푌 = 푏0 + 푏1푋1 + 푏2푋2 + 푏3푋3 + 푏11X1 + 푏22푋2 + +푏33푋2 + 푏12푋2푋1 + 푏13푋3푋1 +
ꢀ
2
3
푏23푋2푋3
(2)
where Yi represents the predicted response. X1, X2 and X3 are independent variables, b0, b1, b2, b3,
b11, b22, b33, b12, b13, and b23 are regression coefficients.
Table 1. Experimental design for optimizing the process condition and results
Reaction
temperature Residence
(°C)
Methanol:
glycerol
(mole/mole)
Unconverted
calcium
lactate (%)
Methyl
lactate yield
(%)
PG
yield
(%)
Carbon
closure (%)
ID#
time (hrs)
1
160
200
160
200
160
200
160
200
146
214
180
180
180
180
180
180
180
180
2
4
59.11
43.67
49.67
40.19
44.22
15.96
25.59
13.63
55.41
23.33
55.47
22.75
69.82
15.6
19.8
6.29
6.8
87.21
75.58
75.3
2
2
4
23.53
18.41
27.91
26.24
48.3
3
6
4
4.14
2.84
6.02
9.47
8.98
4.79
2.9
4
6
4
72.86
82.16
84.35
85.19
77.76
73.94
90.09
97.92
94.75
99.14
86.42
89.92
92.31
93.2
5
2
12
12
12
12
8
6
2
7
6
44.08
56.75
13.85
54.48
27.67
58.22
8.38
8
6
9
4
10
11
12
13
14
15
16
17
18
4
8
8.09
10.76
9.61
10.64
7.38
11.14
10.74
9.46
6.32
0.63
7.37
4
8
8
1.3
14.7
8
4
57.19
48.72
41.01
41.94
43.95
4
22.62
36.27
37.4
4
8
4
8
4
8
36.03
89.94
The statistical analysis of the model was performed by Design-Expert 8 software. And the
coefficient of determination (R2) and F test were used to determine the quality of fit of the
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second order equation. The effect of each independent variable and their interactions were
determined. F test was used to determine the model parameter’s significance (α = 0.05).
2.3.3 Calcium lactate conversion to ethyl lactate
Similar process with methyl lactate production was used for ethyl lactate production. The
experiments investigating ethyl lactate production with ethanol and carbon dioxide were
conducted at three different conditions based on results of methyl lactate and preliminary
experiments: 180 °C (reaction temperature), 4 hrs (reaction time) and 14.68 (molar ratio of
ethanol to glycerol); 200 °C, 6 hrs and 12; and 220 °C, 4 hrs and 12.
2.3.4 Catalyst regeneration and reuse
In our previous research, CaO was difficult to be regenerated and reused. To test the
regenerated CaO, we added sodium hydroxide to convert calcium lactate to Ca(OH)2 and sodium
lactate.20 However, this process is not practical for industrial process as the recovery of lactic
acid was still required. In this study, most calcium was converted to CaCO3 by adding carbon
dioxide that could be easily regenerated to CaO by simple calcination and then reused in the
process.
After reaction, the solution was centrifuged and the precipitates were collected to investigate
the catalyst regeneration and reusability. Due to the low amount of solids in each run, the solids
were collected from several runs and mixed together before regeneration. The mixed solids were
washed 3 times by DI water and then dried overnight in an oven at the temperature of 110 °C.
The dried solids were then calcined in the air at 910 °C for 3 hrs. The calcined solids were
ground to powder using mortar and pestle, and then passed through a 60 mesh sieve before they
were recycled or used again in the process. In this research, the CuO was not separated during
the process, so the calcined solids were a mixture of CuO and CaO. The calcined solids were
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weighed to determine how many catalysts can be recycled. We assumed that CuO could be
totally recycled. About 10–15% of CaO could not be recycled because there were small amount
of calcium lactate that was not converted to methyl lactate and CaCO3 in the second-step
reaction. However, we did not add fresh CaO to the regenerated catalyst.
The catalysts were regenerated and reused twice. Each time, regenerated catalysts were tested
for the glycerol conversion and methyl lactate production in the tandem process. The amount of
regenerated catalyst (mixture of CaO and CuO) used in the process was calculated based on the
molar ratio of CuO (in regenerated catalyst) to glycerol at 0.2. The reaction temperature and time
were same with the process using raw catalysts. The second-step process was conducted at a
reaction temperature of 180 °C, a reaction time of 4 hrs, and a molar ratio of methanol to
glycerol at 14.7:1 because at this condition, most calcium lactate can be converted to methyl
lactate according to results of previous condition optimization.
The spent catalyst and regenerated catalysts were characterized by Powder X-ray diffraction
(XRD). Powder X-ray diffraction (XRD) patterns were recorded on a Philips X’Pert PRO
PW3050 X-ray diffractometer using Cu K α radiation (0.154 nm) and a graphite generator. The
tube voltage and the current were at 45 kV and 40 mA, respectively. And scan range was 2º–80º
with the scan rate at 0.5º/min.
2.3.5 Sample analysis
The sample after reaction was washed out with DI water; then, centrifuged at a relative
centrifugal force (RCF) of 4070 g for 5 min. The clear liquid in the upper layer was collected and
the pH value was adjusted to around 3 with diluted H2SO4 in order to convert the unconverted
calcium lactate to lactic acid. After that the sample was fed through an ion-exchange column
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packed with DOWEX 50WX8-400 Resin (Sigma Aldrich). The samples after ion-exchange were
analyzed by HPLC equipped with SH 1011 column with the mobile phase of 0.5 mmol H2SO4.
The lactic acid, glycerol, and PG were identified by A Waters 410 RI detector (Waters
Chromatography Division, Millipore Corp., Milford, MA), and the methyl/ethyl lactate were
identified by a UV detector at 210 nm. External calibration was used to quantify each component
in the products. It is noteworthy that the purchased lactic acid usually contained a small amount
of lactic acid oligomers because of its high concentration. Therefore, before conducting
calibration using HPLC, the standard lactic acid was first diluted to a concentration of about 2%,
then hydrolyzed with 0.025 M H2SO4 at a temperature of 80 °C for 8 hrs.
Glycerol conversion, unconverted calcium lactate (equal to lactic acid determined by HPLC),
methyl/ethyl lactate yield and propylene glycol yield were calculated by the following equations:
푛
푓푒푒푑
−푛
푞푢푎푛푡푖푓푖푒푑
(
)
퐶ꢁ푙푦푐푒푟표푙 푚표푙% =
× 100%
(3)
푛
푓푒푒푑
where Cglycerol is glycerol conversion (mol%), nfeed is the moles of the starting glycerol, nquantified
is the remaining glycerol in the collected sample quantified by HPLC.
푛
푦푝푟표푑푢푐푡 푚표푙% = 푝푟표푑푢푐푡 × 100%
(4)
(
)
푛
푓푒푒푑
where yproduct is the yield of a specific product formed (methyl/ethyl lactate or propylene glycol,
mol%) or unconverted calcium lactate detected after reaction , nproduct is the moles of the specific
product (methyl lactate or propylene glycol) or unconverted calcium lactate detected after
reaction, and nfeed is the moles of the starting glycerol.
2.4 Crude glycerol conversion to methyl/ethyl lactate
Crude glycerol from a biodiesel plant, which contained 88.5 wt% of glycerol, 7.4 wt% of
water, 2.4 wt% soap, and 1.4 wt% of methanol was used to investigate the effects of impurities
on glycerol conversion and alkyl lactate production. According to the optimization for the
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second-step process in prior experiments, the reaction of methyl lactate was conducted at a
reaction temperature of 180 °C and 4 hrs, and a molar ratio of methanol to glycerol at 14.7:1.
The reaction of ethyl lactate was conducted at the reaction temperature of 220 °C and 6 hrs, and
molar ratio of ethyl to glycerol at 12.
3. Results and discussion
3.1 Response surface analysis
The details of reaction conditions for the second-step process and product yields are shown in
Table 1. In this tandem process, glycerol conversion was observed from 89.39 to 98.42%. The
propylene glycol yield was about 3% to 10% and varied according to the different process
conditions. However, the unconverted calcium lactate and methyl lactate yield greatly changed
with the process conditions. The unconverted calcium lactate ranged from 13.63 to 69.82 while
the methyl lactate yields ranged from 8.38 to 58.22%.
Using results of the experiment, a second order regression equation for methyl lactate yield
(Equation 5) was obtained with the function of reaction temperature (X1, °C), time (X2, min) and
molar ratio of methanol to glycerol ((X3, mole/mole); hence,
푌푀푒푡ℎ푦푙 푙푎푐푡푎푡푒 = −403.23 + 3.93푋1 + 2.95푋2 + 7.46푋3 − 0.0097푋12 − 0.27푋32
(5)
It was also found that the model terms X22, X12, X13, and X23 were insignificant when
Equation 2 was used to fit the data for methyl lactate yield. Equation 5 was obtained with its
significant terms (P-value < 0.0001) by using forward statistical analysis from Equation 2. The
ANOVA for the response surface model of methyl lactate yield is shown in Table 2.
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Table 2. The ANOVA for the response surface reduced quadratic model of methyl lactate
yielded (YMethyl lactate
)
Sum of
p-value
Source
Model
squares DF Mean square F value
Prob > F
3917.17
5
1
1
783.43
990.25
476.38
18.73
23.67
11.39
< 0.0001
0.0004
0.0055
X1: Reaction temperature (°C) 990.25
X2: Reaction time (hrs)
476.38
X3: Mole ratio of methanol to
glycerol
A2
C2
2061.94
200.89
254.35
1
1
1
2061.94
200.89
254.35
49.29
4.80
6.08
< 0.0001
0.0489
0.0297
Residual
Lack of Fit
Pure Error
Total
501.95 12 41.83
466.52
35.43
9
3
51.84
11.81
4.39
0.1253
4419.13 17
The correlation coefficients of determination (R-square) for Equations 5 was 0.89, implying
that the reduced quadratic model for methyl lactate accurately represented the experiment data
and the relationships among independent variable. Fig. 1 show the predicted methyl lactate using
Equations 5.
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Fig. 1. Comparison plots of predicted and actual methyl lactate using Equation 5
3.2 The effects of process conditions on methyl lactate yield
Methyl lactate yield significantly increased with the increase of reaction temperature, time,
and molar ratio of methanol to glycerol. Fig. 2A shows the effects of reaction time and
temperature at the constant molar ratio of methanol to glycerol of 8:1. At the constant molar ratio
of methanol to glycerol, methyl lactate yield increased with reaction temperature and time. The
maximum yield of methyl lactate was obtained 53.5% at the reaction temperature of 200 °C and
the reaction time of 6 hrs. Fig. 2B shows the effects of reaction temperature and molar ratio of
methanol to glycerol at the constant reaction time of 4 hrs. The methyl lactate yield increased
with the reaction temperature and molar ratio of methanol to glycerol; the maximum yield was
55.49% obtained at the reaction temperature of 200 °C with a methanol to glycerol molar ratio of
12:1. Fig. 2C shows the effects of reaction time and molar ratio of methanol to glycerol on the
methyl lactate yield at the constant reaction temperature of 180 °C. The methyl lactate yield also
increased with the increase of reaction time and molar ratio of methanol to glycerol and
maximum yield was 56.78% at the reaction time of 6 hrs and methanol to glycerol ratio of 12.
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These results suggested that the high reaction temperature, time, and molar ratio of methanol to
glycerol favor the methyl lactate production. From Fig. 2, we also found that the increasing rate
of methyl lactate yield trends to slow down when increasing the reaction temperature, time and
molar ratio of methanol to glycerol, implying that the formation of methyl lactate is close to
equilibrium at the higher reaction temperature, time, and molar ratio of methanol to glycerol.
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Fig. 2. Response surface profiles for the yield of methyl lactate. A is at the constant molar ratio
of methanol to glycerol of 8:1; B is at the constant reaction time of 4 hrs; C is at the constant
reaction temperature of 180 °C.
Barve et al. pointed out how the low concentration of calcium lactate in the methanol favors
the conversion of calcium lactate to methyl lactate, which gives the high yield of methyl lactate
obtained at the high molar ratio of methanol to glycerol.27 ANOVA analysis for the obtained
models for methyl lactate production in Tables 2 also suggests that the molar ratio of methanol to
glycerol has more effect on the methyl lactate yield than on the other two factors. Methyl lactate
shows great change with an increase of methanol to glycerol molar ratio at a constant reaction
temperature or time (Fig. 2). This figure suggest that the high molar ratio of methanol and
glycerol help to achieve the maximum methyl lactate yield. In this study the highest molar ratio
of methanol to glycerol investigated was at 14.73. And at this molar ratio of methanol to glycerol
with mild reaction temperature and time (180 °C and 4 hrs) the higher methyl lactate yield was
obtained at 57.19%. Therefore, the following studies on catalysts regeneration and reuse test was
conducted at this process condition.
3.3 Ethyl lactate production
Fig. 3 shows glycerol conversion, ethyl lactate yield, unconverted calcium lactate, and
propylene glycol yield at various temperatures in ethyl lactate production. The glycerol
conversion was about 88% to 96% which is similar with those observed in methyl lactate
production. The ethyl lactate yield showed an increasing trend with the second-step reaction
temperature. The ethyl lactate yield was 22.3% at a reaction temperature of 180 °C. When the
reaction temperature increased to 200 °C, the ethyl lactate yield greatly increased to 42%. By
further increasing the temperature to 220 °C, the ethyl lactate yield showed a slight increase. In
contrast, unconverted calcium lactate, which was 46.5%, was obtained at the reaction
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temperature of 180 °C. This indicates that most of the calcium lactate obtained from the first-step
reaction was not converted to ethyl lactate in the second-step reaction condition. When the
reaction temperature increased to 200 °C and 220 °C, the calcium lactate greatly decreased.
These results suggest that the high temperature favors the calcium lactate conversion to ethyl
lactate. The PG yield at this condition was about 8.11%, which closely compares to the yield of
PG in methyl lactate production.
100
90
180°C
200°C
220°C
80
70
60
50
40
30
20
10
0
Glycerol
Ethyl lactate
Unconverted
Propylene glycol
conversion
calcium lactate
Fig. 3. Comparison of glycerol conversion, ethyl lactate yield, unconverted calcium lactate,
propylene glycol yield at the different reaction temperature (second-step reaction) using refined
glycerol with 33.33% water (first-step reaction was conducted at the reaction temperature of 230
°C and 30 min. where CaO:glycerol=1mol/mol and CuO:glycerol=0.2mol/mol)
3.4 The effects of recycled catalysts on glycerol conversion and methyl lactate production
Fig. 4 shows the comparison of glycerol conversion, methyl lactate yield, unconverted
calcium lactate and propylene glycol yield using fresh catalysts and regenerated catalysts.
Regenerated catalysts showed a similarity of glycerol conversion with fresh catalysts. The
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regenerated catalyst had no negative effects on the glycerol conversion. The methyl lactate yield
obtained from glycerol conversion using regenerated catalysts were very close with the yield
from fresh catalyst. Both unconverted calcium lactate and propylene glycerol yields significantly
decreased when reused two times of catalysts. The total amount of methyl lactate and
unconverted calcium lactate obtained from the reused catalysts especially for 2nd use of the
catalyst was reduced greatly. This means that the regenerated catalyst had a noteworthy effect on
the calcium lactate selectivity in the first-step process. During the catalysts regeneration, we
found that only about 85% of fresh catalysts were regenerated. About 15% of fresh catalyst from
CaO interaction was maintained in the unconverted calcium lactate. Before the regenerated
catalysts were reused, we did not add fresh CaO in the process. That might limit the formation of
calcium lactate in the first-step process due to the reduced CaO in the regenerated catalyst. We
did not reduce the methanol loading in the second-step process which could have lowered the
concentration of calcium lactate and favor the calcium lactate conversion. In this study, a certain
amount of calcium lactate was not converted to methyl lactate. This unconverted calcium lactate
could be recycled in the process thereby helping to recycle the CaO catalyst.
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100
90
80
70
60
50
40
30
20
10
0
Fresh catalyst
1st time reusing
2nd time reusing
glycerol conversion
Methyl lactate
Unconverted
calcium lactate
Propylene glycol
Fig. 4. Glycerol conversion, methyl lactate yield, unconverted calcium lactate and propylene
glycol yield using regenerated catalysts.
The XRD pattern for the spent catalyst is shown in Fig. 5A. No peak associated with CaO
was observed on the XRD, suggesting that there was no CaO in the spent catalyst or it only
existed in an amorphous form. The peaks associated with CuO and Cu were detected, indicating
that CuO occurred with a partial reduction during the reaction. Peaks associated with CaCO3
were observed in the spent catalyst, which confirmed the transesterification of calcium lactate
with CO2 to form CaCO3. XRD patterns of regenerated catalysts were shown in Fig. 5B. Peaks
associated with CaO and CuO were observed in the 1st to 3rd regenerated catalysts. This result
confirmed that the regenerated catalysts are still composed of CaO and CuO. The peak intensity
of CaO and CuO during different times of regeneration was not reduced significantly, indicating
that the regenerated catalyst maintained a similar crystalline proportion during recycles.
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Fig. 5. XRD patterns of CaO, CuO and spent catalyst (A), and regenerated catalysts (B). The
peaks marked with “a” are the characteristic peaks of CaO; The peaks marked with “b” are the
characteristic peaks of CuO; The peaks marked with “*” are the characteristic peaks of CaCO3;
The peaks marked with “∆” are the characteristic peaks of Cu.
3.5 Methyl and ethyl lactate production from crude glycerol
The crude glycerol was used as starting material to produce methyl/ethyl lactate. Fig. 6
compares glycerol conversion with product yield between using refined glycerol and crude
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glycerol. Crude glycerol conversion and methyl lactate were similar to that of refined glycerol in
both methyl and ethyl lactate production. The unconverted calcium lactate in methyl lactate
production was slightly higher than that from refined glycerol. But the unconverted calcium
lactate in ethyl lactate was similar to that of refined glycerol. These results revealed that the
impurities in the crude glycerol had no significant effects on either the glycerol conversion or the
methyl/ethyl lactate production and the crude glycerol can be directly used as a feedstock in
methyl and ethyl lactate production.
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100
90
80
70
60
50
40
30
20
10
0
Refined glycerol
Crude glycerol
Glycerol conversion
Methyl lactate
Unconverted calcium Propylene glycol
lactate
100
90
80
70
60
50
40
30
20
10
0
Refined glycerol
Crude glycerol
Glycerol conversion
Ethyl lactate
Unconverted calcium Propylene glycol
lactate
Fig. 6. Comparison of glycerol conversion and product yield between refined glycerol and crude
glycerol for methyl and ethyl lactate production
3.6 Reaction mechanisms of methyl/ethyl lactate production
Glycerol conversion to methyl/ethyl lactate includes two steps. The first step is glycerol
conversion to calcium lactate using the catalysts of CaO and CuO. Multiple reactions occurred
during the glycerol conversion to calcium lactate. The reaction mechanism of this step has been
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proposed in our previous report.20 The initial reaction occurs between the H proton of the
hydroxyl group of glycerol and CaO to form glyceroxide anion and CaO hydrogen cation
(CaOH+). Glyceroxide anion and CaO hydrogen cation (CaOH+) further react to form
glyceraldehyde via dehydrogenation. In its basic condition, glyceraldehyde is converted to 2-
hydroxypropenal via dehydration and 2-hydroxypropenal further forms pyruvaldehyde via keto-
enol tautomerization. Pyruvaldehyde undergoes a benzilic acid rearrangement and finally forms
the calcium lactate with CaO. Hydrogen generated in the process can also react with 2-
hydroxypropenal and pyruvaldehyde to form propylene glycol especially when CuO catalyst
exists in the process.
The second step is calcium lactate reaction with CO2 and methanol or ethanol to form
methyl/ethyl lactate and calcium carbonate, which will precipitate out of the solvent. Barve et al.
investigated methyl lactate production from alkali metal salts of carboxylic acids using carbon
dioxide and alcohol.27 They proposed a reaction mechanism of esterification of calcium lactate
using CO2 and methanol made up of three steps: 1) the formation of carbonic acid by the reaction
of water with CO2, 2) the formation of lactic acid and CaCO3 by the reaction of calcium lactate
with carbonic acid, and 3) the esterification of lactic acid with methanol. However, in our
process, CaO used was excessive, which might also induce methanol to form methoxy anion,
which further reacts with CO2 to form intermediates.28 These intermediates might react with
calcium lactate to form methyl lactate and CaCO3. The possible reactions is shown in equations
6-9, and the proposed reaction pathway from glycerol to methyl lactate is shown in Fig. 7.
퐶퐻3푂퐻 + 퐶푎푂 → 퐶퐻3푂− + 퐶푎(푂퐻)+
퐶퐻3푂− + 퐶푂2 → 퐶퐻3푂퐶(푂)푂−
(6)
(7)
(8)
퐶푎(푂퐻)+ + 퐶퐻3푂퐻 → 퐶퐻3+ + 퐶푎푂 + 퐻2푂
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+
−
(
)
(
)
퐶퐻3 + 퐶퐻3푂퐶(푂)푂 + (퐶퐻3퐶퐻 푂퐻 퐶푂푂)2퐶푎 → 2퐶퐻3퐶퐻 푂퐻 퐶푂푂퐶퐻3 + 퐶푎퐶푂3 (9)
Fig. 7. Proposed reaction pathway from glycerol to methyl lactate using CaO and CuO as
catalysts with methanol and carbon dioxide
4. Conclusions
In this study, a two-step process for methyl lactate production from glycerol using solid
catalysts, methanol and carbon dioxide were developed, and the process conditions were
investigated. The reaction temperature, time and molar ratio of methanol to glycerol had
significant effects on the methyl lactate yields. The relatively high methyl lactate yield, which
was more than 55%, was obtained at the reaction temperature of 180-200 °C, reaction time of 4-7
hours and methanol to glycerol molar ratio of 8:1 to 14.7:1. The test for regenerated catalysts
suggests that the catalysts can be reused with little effect on the methyl lactate yield. The ethyl
lactate production was also investigated, and about a 42% yield was obtained using refined
glycerol at the optimum condition. The studies on crude glycerol conversion to methyl/ethyl
lactate revealed a similar methyl and an ethyl lactate yield when using refined glycerol, which
was achieved at the optimum condition. The impurities in the crude glycerol had no significant
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effects on alkyl lactate production. Compared with traditional methyl lactate production, this
two-step process for methyl and ethyl lactate production from glycerol is economic and
environmentally sustainable since no gypsum was produced and both CaO and CuO can be
regenerated and recycled.
Acknowledgements
This work was supported by the United Soybean Board and the U.S. Department of
Agriculture HATCH project No. TEN00428.
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