Catalytic dehydration of lactic acid to acrylic acid using calcium hydroxyapatite catalysts

Article abstract of DOI:10.1039/c3gc40144h

A series of calcium hydroxyapatite (HAP) catalysts were synthesised with a Ca/P ratio ranging from 1.3 to 1.89 by a co-precipitation method that involved changing the pH of the calcium and phosphorous precursors. The physicochemical characterization by XRD, SEM, BET surface area and CO₂ and NH ₃-TPD techniques confirmed the hydroxyapatite formation. These HAP catalysts were used for the vapour phase dehydration of lactic acid to acrylic acid. The HAP catalyst with a Ca/P ratio of 1.3 was found to be the most efficient catalyst among the synthesised series, which gave 100% conversion of lactic acid and 60% selectivity towards acrylic acid at 375 °C when a 50% (w/w) aqueous solution of lactic acid was used. The higher selectivity towards acrylic acid has been correlated to the increased acidity and reduced basicity of the HAP catalyst with a Ca/P ratio of 1.3 compared to the other HAP catalysts. The catalyst was found to be very stable and no deactivation was observed even after 300 h of reaction time. In situ FTIR studies were performed for understanding the mechanistic aspects and showed the formation of calcium lactate as an intermediate species during the dehydration of lactic acid to acrylic acid.

Full text of DOI:10.1039/c3gc40144h

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Catalytic dehydration of lactic acid to acrylic acid using
calcium hydroxyapatite catalysts
Cite this: Green Chem., 2013, 15, 1211
Vidhya C. Ghantani,a,b Samadhan T. Lomate,a Mohan K. Dongare†*a and
Shubhangi B. Umbarkar*a,b
A series of calcium hydroxyapatite (HAP) catalysts were synthesised with a Ca/P ratio ranging from 1.3 to
1.89 by a co-precipitation method that involved changing the pH of the calcium and phosphorous pre-
cursors. The physicochemical characterization by XRD, SEM, BET surface area and CO2 and NH3-TPD tech-
niques confirmed the hydroxyapatite formation. These HAP catalysts were used for the vapour phase
dehydration of lactic acid to acrylic acid. The HAP catalyst with a Ca/P ratio of 1.3 was found to be the
most efficient catalyst among the synthesised series, which gave 100% conversion of lactic acid and 60%
selectivity towards acrylic acid at 375 °C when a 50% (w/w) aqueous solution of lactic acid was used. The
higher selectivity towards acrylic acid has been correlated to the increased acidity and reduced basicity of
the HAP catalyst with a Ca/P ratio of 1.3 compared to the other HAP catalysts. The catalyst was found to
be very stable and no deactivation was observed even after 300 h of reaction time. In situ FTIR studies
were performed for understanding the mechanistic aspects and showed the formation of calcium lactate
as an intermediate species during the dehydration of lactic acid to acrylic acid.
Received 18th January 2013,
Accepted 27th February 2013
DOI: 10.1039/c3gc40144h
www.rsc.org/greenchem
C4–C7 ketones), which can be used for the production of high
energy-density liquid fuels (C4–C7 alcohols).
Introduction
Utilization of renewable feedstock (biomass) as a raw material
Acrylic acid, an important starting material for the polymer
for synthesis of fine chemicals is of the utmost importance as industry, can be produced by the dehydration of lactic acid. A
the world is facing dual problems of depleting fossil fuel major derivative of acrylic acid (2-propenoic acid) is used exclu-
resources and environmental concerns about current pro- sively to make acrylates and other derivatives. Glacial acrylic
cesses.1 The major advantages of using a biomass-derived acid is used in super absorbent polymers. Acrylic acid is conven-
feedstock is the high content of oxygenated groups (e.g., –OH, tionally produced from non renewable sources, such as propane
–CvO and –COOH) leading to high reactivity, low volatility and propylene. Hence, the production of acrylic acid from bio-
and high solubility in water. Hence, aqueous phase processing derived lactic acid, a renewable source, is highly desired.2
of the feedstock is also a major/added advantage.
The conversion of lactic acid and lactates to acrylic acid,
Lactic acid is one of the attractive renewable feedstocks acrylates and other products over silica and aluminium salts
obtained by biomass fermentation. It contains multiple reactive (sulfate, nitrate, phosphate, etc.) and mixtures of inorganic
functionalities and is considered as one of the most promising salts has been reported by Holmen.4 Various phosphate-based
building blocks for the future growth of a chemical industry catalysts, like sodium dihydrogen phosphate (NaH2PO4),
based on renewable raw materials. A number of value added Ca3(PO4)2 supported on silica, silica/alumina,5 aluminium
commodity chemicals, like acrylic acid, 2,3-pentanedione, phosphate6 and Na2HPO4 in supercritical water,7 have been
pyruvic acid, 1,2-propanediol and ethyl lactate, can be obtained used for lactic acid dehydration. NaY modified with potassium
from lactic acid.2 Ruiz and Dumesic3 have demonstrated the nitrate, lanthanum oxide and alkali phosphates, as well as
use of a Pt(0.1%)/Nb2O5 catalyst for the efficient conversion of various alkali earth metals (Ca, Mg, Sr, Ba, etc.), have also been
lactic acid to various valuable chemicals (propanoic acid and reported for lactic acid dehydration.8 Furthermore Miller et al.9
have investigated the activity of alkali metal salts supported on
low surface area silica–alumina for lactic acid dehydration.
The use of supercritical water for the conversion of lactic acid
aCatalysis Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road,
Pune-411008, India
to acrylic acid has been systematically studied in the literature
bAcademy of Scientific and Innovative Research, CSIR, Anusandhan Bhawan, New
and three primary reaction pathways were proposed in super-
Delhi-110 001, India. E-mail: sb.umbarkar@ncl.res.in; Fax: (+91)-2025902633/34
critical water: (i) decarbonylation to acetaldehyde, CO and
water, (ii) decarboxylation to acetaldehyde, CO2 and H2, (iii)
†Present address: Mojj Engineering Systems Ltd, 15-81/B, MIDC, Bhosari, Pune,
India – 411026.
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dehydration to acrylic acid.10,11 Acrylic acid has also been syn- pH, temperature and stirring time are known to influence the
thesized from lactic acid and acetic acid with a high consump- morphology, crystallinity and surface area of HAPs. In particu-
tion of acetic acid.12 Wadley and co-workers have shown, by lar, the pH of the solution is a very important parameter for
experimental design, that the highest yield and selectivity for controlling the Ca/P ratio in the final HAP catalyst. However,
2,3-pentanedione was favourable at low temperature, elevated after changing the pH (7, 10 and 12) the actual Ca/P ratio
pressure and long contact times, while the yield and selectivity obtained was estimated by EDAX analysis (Table 1). In this
to acrylic acid were most favourable at high temperature, low case the pH of the solution was gradually decreased from 12 to
pressure and short contact times when sodium nitrate sup- 7. At pH 12 catalyst HAP-1 was obtained with a Ca/P ratio of
ported on porous silica was used as catalysts.13 Lactic acid 1.85, whereas at pH 10 and 7, HAP-2 and HAP-3 were obtained
dehydration has been also reported using calcium sulphate with Ca/P ratios of 1.56 and 1.3, respectively. The catalysts
with cupric sulphate and phosphate as the promoter and CO2 were characterized by various physicochemical methods. The
as the carrier gas.14 Acrylic acid has also been prepared from powder X-ray diffraction (XRD) patterns of the synthesized cat-
3-hydroxypropanoic acid using metal oxides, such as Al2O3, alysts are shown in Fig. 1. The XRD pattern showed that the
SiO2 and TiO2.15
synthesized catalysts are crystalline in nature. The three most
Most of the previous studies illustrated that metal phos- intense peaks located at 2θ ≈ 31.7° (2 1 1)–100%, 32.95° (3 0
phates, mainly with alkali and alkaline earth metals, were 0)–62.8%, and 32.2° (1 1 2)–51.3% and other low intensity
effective catalysts for the dehydration of lactic acid to acrylic reflections are in good agreement with the reference data for
acid. However, previously a lower concentration of lactic acid Ca10(PO4)6(OH)2 [PDF 34-0010]. Although the crystallinity of all
(20–30% aqueous solution) has been used, which led to lower the catalysts was similar, a change in the crystallite size was
conversions as well as lower acrylic acid selectivity and in turn observed (Fig. 1),19 as evident from the broadening of the XRD
a lower per pass yield of acrylic acid. Mainly catalysts with peaks (the intense peak at 2Θ 32.9°). The smaller crystallite
mild acidity or mild basicity have been used for lactic acid size of HAP-1 may be due to the decrease in the crystal growth
dehydration in earlier work. Calcium hydroxyapatite (HAP) with the increase in the pH and was confirmed using Scher-
contains both weak acidic and weak basic sites in a single rer’s formula. The crystal size was calculated from Scherrer’s
crystal and hence it is expected to be an efficient catalyst for formula (Table 1) and it showed the gradual increase in the
dehydration.16 Our group has successfully prepared HAP pre- crystallite size for HAP-1 to HAP-3 from 107 to 171 nm, respecti-
viously and tested it for clinical applications in the human vely. Similar observations were also reported and explained in
body as a submerged tooth root implant.17 HAP has been used the literature, confirming that the synthesized powders were
as a catalyst for the dehydrogenation reaction of alcohols, as hydroxyapatite.16a The XRD spectra also contains a noisy back-
well as for the hydrogen transfer reactions of alcohols to ground which could be due to the glassy phosphate matrix.20
ketones.16 In stoichiometric HAP, an oxygen atom of the free
9
10CaðNO Þ ꢂ 4H O þ 6ðNH Þ HPO þ 8NH OH
=
3
2
2
4
2
4
4
–OH group is coordinated to three calcium cations, which are
connected to each other to form a triangle. In nonstoichio-
metric HAP, one –OH group is missing per cation, creating a
defect in order to maintain an overall charge balance, leading
#
ð3Þ
;
Ca10ðPO4Þ6OH2 þ 20NH4NO3 þ 20H2O
The synthesized catalysts were also characterized by FTIR
to a negative charge deficiency in a triangle. Such negative spectroscopy (Fig. 2). The FTIR spectra showed a broad band
charge deficiencies exhibit a Lewis type acidity eqn (1). When a at about 1000–1105 cm−1 indicating the formation of HAP.
water molecule replaces the missing –OH group, as in eqn (2), Bands at 1042 and 1094 cm−1 were observed due to the factor
the electron pair of the water oxygen becomes delocalized to group splitting of the γ3 fundamental vibrational mode of the
fill the orbitals of the cations in the triangles. This polarization tetrahedral PO43−. The bands observed at 960 cm−1 and at
leads to the generation of Brønsted acidity due to the protons 565 cm−1 correspond to the γ1 and γ4 symmetric P–O stretch-
3−
of the water molecules.18 We have made an effort to use these ing vibrations of the PO4
ion, respectively. The bands
properties of HAP for the vapour phase dehydration of lactic assigned to the stretching modes of the hydroxyl groups (–OH)
acid to acrylic acid and the results are reported herein.
in the HAP were observed in the spectra at 3570 cm−1 (strong)
and 632 cm−1. This confirms the formation of HAP.21
Ca10ꢀzðHPO4ÞzðPO4Þ6ꢀzðOHÞ2ꢀz
Ca10ꢀzðHPO4ÞzðPO4Þ6ꢀzðOHÞ2ꢀzðH2OÞz 0 < z ꢁ 1
ð1Þ
ð2Þ
The temperature programmed desorption (TPD) of NH3 and
CO2 was carried out to evaluate the total acidity and basicity of
the synthesized hydroxyapatite (Table 1). Phosphate groups in
HAP are responsible for the acidity of the catalyst, whereas the
Ca2+ ions are responsible for the basicity. Hence an increase in
the basicity and a decrease in the acidity was observed as
expected, with an increase in the Ca/P ratio, as shown in
Results and discussion
Synthesis and characterization
All the catalysts were prepared by a co-precipitation method Fig. 3.16a,18 With a decrease in the Ca/P ratio from 1.85 (HAP-1)
(Table 1). The precursor quantities corresponding to the to 1.3 (HAP-3), an increase in the concentration as well as the
required Ca/P ratio (1.5, 1.65 and 1.9) were taken for synthesis. strength of the acidity was observed. The surface area of all
The formation of HAP is represented in eqn (3). Factors like the catalysts determined using the BET method showed an
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Table 1 Textural characterization, acidity and basicity of the catalystsa
Catalyst
name
Ca/P molar
ratio
Crystal sizeb
(nm)
pH of
solution
Surface area
CO2 desorbed
NH3 desorbed
Selectivity toward
acrylic acid (%)
(m2 g−1
)
(μmol g−1
)
(μmol g−1
)
HAP-1
HAP-2
HAP-3
1.89
1.56
1.3
107
121
171
12
10
7
49
29
25
10.12
1.02
0.624
103
125
131
50
55
60
a Reaction conditions: lactic acid – 50% (w/w) in water, WHSV – 3 h−1, temperature – 375 °C, carrier gas – N2, carrier gas flow – 15 mL min−1
Lactic acid conversion 100%. b Crystal size calculated from Scherrer’s formula.
.
Fig. 4 SEM images of (a) HAP-1, (b) HAP-2 and (c) HAP-3.
increase in surface area with an increase in the pH due to a
decrease in crystal size (Table 1).
The catalyst morphology was studied using scanning elec-
tron microscopy (SEM) and the images are shown in Fig. 4.
HAP-1 and HAP-2 (with Ca/P ratios ≥1.56) were prepared at
higher pH and showed smaller particle sizes, whereas HAP-3,
with a Ca/P ratio of 1.3, prepared at a lower pH showed an
elongated rod-shaped highly crystalline material which is in
agreement with the XRD pattern and particle size calculated
using the Scherrer formula. The difference in shape may be
due to precipitation of tabular brushite (CaHPO4·2H2O) as a
precursor at low pH, which gradually transformed to a HAP
structure with an increase in pH.16a
Fig. 1 X-Ray diffraction patterns of HAP with different Ca/P ratios.
Catalytic activity
Influence of Ca/P ratio on the dehydration of lactic acid to
acrylic acid. The catalytic activities of the synthesized HAPs
were tested for the vapour phase dehydration of lactic acid
(Scheme 1) using a down flow reactor and the results are given
in Table 1. The reaction was carried out at 375 °C using a 50%
(w/w) aqueous solution of lactic acid and a WHSV of 3 h−1
using all the prepared catalysts. Surprisingly, 100% lactic acid
conversion was obtained with all the HAP catalysts, though a
difference in the acrylic acid selectivity was observed. With a
decrease in the Ca/P ratio an increase in the acrylic acid selec-
tivity was observed, with a maximum 60% selectivity for acrylic
acid when HAP-3 was used. This can be correlated to the
acidic/basic properties of the catalysts. With an increase in
the Ca/P ratio from 1.3 (HAP-3) to 1.85 (HAP-1) a decrease in
the acidity was observed. Hence, the high selectivity for acrylic
acid obtained with HAP-3 can be attributed to the higher
acidity, which is in agreement with the literature reports.22
The rate of dehydration can be correlated to the calcium
deficiency and increasing concentration of surface P–OH
groups.16c
Fig. 2 FTIR spectra of HAP-1 HAP-2, HAP-3.
Fig. 3 NH3 TPD profiles for HAP-1, HAP-2 and HAP-3.
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provides the optimum residence time for dehydration, leading
to minimum by-product formation.
Effect of lactic acid concentration
The effect of lactic acid concentration on reaction performance
was studied using HAP-3 as catalyst and the results are pre-
sented in Table 2 (entry 3). With an increase in the lactic acid
concentration from 50 to 80 wt% (wt/wt aqueous solution), a
decrease in lactic acid conversion from 100 to 70% was
observed; with a marginal decrease in acrylic acid selectivity
from 60 to 57%. The rate of lactic acid self-polymerization is
low at low lactic acid concentration and a better efficiency is
expected for the target reaction. The sum of the selectivities in
Table 2 are less than 100%, as the remaining selectivity is for
gaseous product (CO/CO2) formation as well as some escaped
acetaldehyde in gaseous form. No other unknown products
were observed in GC analysis.
The stability of the catalyst with time on stream was studied
at 375 °C over HAP-3 (WHSV = 3 h−1 and lactic acid concen-
tration = 50 wt%) for 300 h and there was no considerable
decrease in the lactic acid conversion and acrylic acid selectiv-
ity. After 300 h, the conversion remained constant (100%) with
a marginal decrease in acrylic acid selectivity from 60% to
57%. This proves the stability of hydroxyapatite catalysts under
the reaction conditions.
Scheme 1 General dehydration and decarbonylation scheme for lactic acid to
acrylic acid and acetaldehyde.
Effect of temperature
The influence of the reaction temperature on lactic acid dehy-
dration was studied in the range of 325–400 °C (Table 2, entry
1) using HAP-3, as it showed the maximum selectivity for
acrylic acid. When the reaction temperature was gradually
increased from 325 to 400 °C, the LA conversion increased cor-
respondingly from 60 to 100%, with a marginal increase in the
acrylic acid selectivity from 57 to 60% up to 375 °C. However,
the selectivity decreased to 54% with a further increase in the
temperature to 400 °C, with a corresponding increase in the
acetaldehyde selectivity (21.6%).
Effect of residence time on conversion and selectivity
The acrylic acid yield reported in the literature by lactic acid
dehydration varies between 40–56%, with the main use of
lower lactic acid concentrations (≤25% aqueous solution) in
the range of 300–400 °C. Whereas, the results obtained in the
present study, for lactic acid dehydration to acrylic acid, are
better than the previously reported studies in the literature
with 100% conversion and 60% acrylic acid selectivity.
Especially the use of high concentration of lactic acid (≥50%
aqueous solution) and very high WHSV as well as catalyst stab-
ility even up to 300 h are the major advantages of the present
studies.
The influence of the WHSV on reaction performance was
studied under optimized reaction conditions using HAP-3 as
the catalyst and the results are given in Table 2 (entry 2). The
lactic acid feed was varied from 12 to 20 mL h−1 (correspond-
ing to WHSV = 3–5 h−1 respectively) at 375 °C by keeping the
other reaction conditions constant. When WHSV was gradually
increased from 3 to 5 h−1, a decrease in lactic acid conversion
from 100 to 75% was observed, as expected, with a decrease in
the acrylic acid selectivity from 60 to 50%, indicating that the
WHSV of 3 h−1 with a corresponding contact time of 1.196 s
Zhang et al.23 has studied the different pathways for lactic
acid conversion to acrylic acid by dehydration, as well as decar-
bonylation and decarboxylation to acetaldehyde by DFT
studies using sodium polyphosphate as catalyst. In the case of
dehydration, the first step has been shown to be phosphate
formation by elimination of protons from the hydroxyl group
(–OH) of the lactic acid and the hydroxyl of phosphate group
(–POH) of the catalyst, which further led to the formation of
acrylic acid by decomposition of the phosphate ester formed
in the previous step. However, in the case of decarbonylation,
the first step is the formation of a phosphate ester via elimin-
ation of the carboxylic –OH (–COOH) group of lactic acid and
the proton of the –POH group of the catalyst. In the second
step, acetaldehyde and CO are formed via decomposition of
the phosphate ester. Though the formation of the phosphate
ester is the first step and the decomposition of the phosphate
ester is second step in dehydration, as well as decarbonylation,
in the case of dehydration, the ester formation involves the
hydroxyl group and decarbonylation involves the carboxylic
group of lactic acid. To check the possibility of calcium lactate
Table 2 Influence of reaction parameters on lactic acid dehydrationa
% Selectivity
Reaction
Entry parameters
Conv. Acrylic
Other
(%)
acid
Acetaldehyde productse
1
Temp.b (°C)
325 60
350 90
375 100
400 100
3 100
57
59
60
54
60
56
50
60
59
57
57
18.9
18
17.2
21.6
17.2
19.2
22
17.2
18.8
19.2
19.2
5.2
5
5.6
2.6
5.6
5.6
6
5.6
3.4
4.6
4.6
2
3
WHSVc (h−1
)
4
5
85
75
LA conc.d (%, w/w) 50 100
60 91
70 85
80 70
a Reaction conditions: catalyst – HAP-3, carrier gas – N2, carrier gas
flow – 15 mL min−1 b WHSV – 3 h−1, lactic acid conc. – 50% (w/w).
c Lactic acid conc. – 50% (w/w), temp – 375 °C. d Temp – 375 °C and
WHSV – 3 h−1 e Propanoic acid, 2,3-pentanedione.
.
.
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Scheme 2 Proposed mechanism for lactic acid dehydration using HAP.
Fig. 5 FTIR difference spectra of lactic acid adsorbed on HAP-3 at room temp-
erature and desorbed at (a) room temperature (b) 50 °C, (c) 100 °C, (d) 150 °C,
(e) 200 °C. FTIR spectrum of (f) calcium lactate (g) lactic acid from FTIR
database.
formation and, if the same mechanism holds true, for lactic
acid dehydration using HAP catalysts, lactic acid was adsorbed
on HAP-3 at room temperature and physisorbed lactic acid was
removed by drying the catalyst at room temperature for 8 h
and the FTIR spectrum was recorded. Further, the adsorbed
lactic acid was desorbed at various temperatures from
50–200 °C. Fig. 5 shows the subtraction spectra of adsorbed
lactic acid on HAP-3 and neat HAP-3. The spectrum was com-
pared with authentic lactic acid and calcium lactate from the
FTIR database. The spectrum clearly showed the formation of
calcium lactate. The IR band for the carboxylic –OH group of
lactic acid at 3373 cm−1 disappears and there is a considerable
shift in the carbonyl (CvO) frequency of the carboxylic group
from 1732 to 1604 cm−1, indicating strong interaction of the
carboxylic group with the calcium of HAP. The carbonyl
stretching frequency of calcium lactate is observed at
1578 cm−1. The presence of an additional carbonyl band at
1755 cm−1 matched well with the carbonyl frequency of free
lactic acid (1754 cm−1), suggesting that not all the lactic acid
has formed calcium lactate. This indicates that due to calcium
lactate formation, the carboxylic group of lactic acid is pro-
tected and hence decarbonylation is not possible as per the
mechanism suggested by Zhang et al. by DFT studies, leading
to very high selectivity for acrylic acid in the present case.
Miller et al.9 have also previously reported the formation of
alkali lactates by interaction of lactic acid with the alkali metal
of the catalysts when alkali metal salts supported on silica–
alumina was used for lactic acid dehydration and concluded
that the formation of alkali lactates was crucial for acrylic acid
formation. The part of lactic acid which does not form calcium
lactate on HAP may form acetaldehyde by the decarbonylation
route. Hence, the mechanism of lactic acid dehydration on
HAP can be explained as follows.
Scheme 3 Proposed mechanism for lactic acid decarbonylation using HAP.
decarboxylation to acetaldehyde. Dissociative adsorption of
lactic acid results in the formation of stable hydrogen phos-
phate groups which shows Brønsted acidity and forms phos-
phate ester with hydroxyl proton of lactic acid by dehydration.
In next step this phosphate ester further decomposes to form
acrylic acid. In HAP, the Ca–O distance is reported to be 0.239
and 0.240 nm due to this it can trap hydrogen. β-Tricalcium
phosphate (TCP) has PO4−3 ions, as in HAP; however, it cannot
take a nonstoichiometric form and trap hydrogen as HPO4
−2
,
hence TCP does not show good activity for lactic acid dehydra-
tion.16a However, in situ FTIR studies have shown the appear-
ance of an –OH group at 3566 cm−1 during the reaction, which
is assigned to the P–OH group. This clearly confirms the for-
mation of phosphate –OH, leading to the generation of in situ
Brønsted acidity.
During decarbonylation, dissociative adsorption of lactic acid
takes place on HAP (Scheme 3), where the hydroxyl group
becomes adsorbed on calcium, forming a C–O–Ca bond and the
hydroxyl proton gets abstracted by phosphate oxygen forming
HPO4−2, which further reacts with the carboxylic –OH group to
form the phosphate ester. This phosphate ester decomposed in
the next step to give acetaldehyde and carbon monoxide.
The lactic acid was dissociatively adsorbed on HAP as
lactate on the Ca site and the carboxylic proton on phosphate
oxygen (Scheme 2). Formation of lactate salt results in the pro-
tection of the carboxylic group avoiding decarbonylation and
Conclusions
Calcium hydroxyapatites (HAP) with various Ca/P ratios,
ranging from 1.3–1.85, could be synthesized by varying the pH
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of the reactants. The HAP with a Ca/P ratio of 1.3 has shown a Catalyst activity test
very high efficiency for vapour phase dehydration of lactic acid
Vapour phase dehydration of lactic acid to acrylic acid was
carried out using a quartz fixed bed down flow reactor
(id-11 mm) at atmospheric pressure. In the experiment, the
catalyst with a particle size 20–40 mesh was charged in the
middle section of the reactor with quartz wool packed in both
ends. Porcelain beads were placed above the catalyst bed in
order to vaporize the feed. Before catalytic evaluation, the cata-
lyst was pre-heated at the required reaction temperature for
30 min under a flow of nitrogen (15 mL min−1). The reaction
temperature was measured by a thermocouple inserted in the
catalyst bed. Lactic acid, of the required concentration (typi-
cally 50% w/w in water), was pumped into the pre-heating
zone of the reactor using a peristaltic pump and nitrogen gas
as the carrier. The reaction products were collected in a cold
trap at 8 °C. The reaction mixture was analyzed using gas
chromatography (Perkin Elimer) equipped with a FFAP capil-
lary column (50 M × 0.320 mM) and FID detector. The GC was
calibrated by external standard method. The amount of COx
(CO and CO2) was estimated based on the yield of acetaldehyde
and propanoic acid. The mass balance for all the reaction was
more than 95%.
to acrylic acid with 100% conversion and 60% selectivity for
acrylic acid at 375 °C and 50% lactic acid concentration. The
higher selectivity for acrylic acid may be correlated to the
synergistic effect of the higher acid site density with the lower
basic site density of HAP with a Ca/P ratio of 1.3. In situ FTIR
studies showed the formation of calcium lactate as an inter-
mediate species which favours the dehydration of lactic acid to
acrylic acid, avoiding decarbonylation to acetaldehyde.
Experimental section
Materials
Calcium nitrate, diammonium hydrogen phosphate and
ammonia solution were obtained from Thomas Baker chemi-
cals India Ltd. Lactic acid (89%) from our CSIR-National
Chemical Laboratory (CSIR-NCL) Pune pilot plant was
obtained from a sugarcane fermentation process (process
developed by CSIR-NCL).
Catalyst preparation method
Hydroxyapatite (HAP) was synthesized by a co-precipitation
method as reported by Dongare et al.17 In a typical synthesis,
the required amount of calcium nitrate tetrahydrate and di-
ammonium hydrogen phosphate were separately dissolved in
deionised water. Ammonia solution (25%) was added to both
the solutions until the required pH was achieved. The calcium
nitrate solution was added drop wise to the diammonium
hydrogen phosphate solution with constant stirring. A thick
white precipitate was obtained, which was filtered, washed
with a sufficient amount of water and dried at 120 °C for 16 h.
The oven dried solid was calcined in air in a muffle furnace at
600 °C for 4 h.
Acknowledgements
VCG acknowledges CSIR New Delhi for Junior Research Fellow-
ship. Mr Ketan Bhotkar is acknowledged for help in EDAX
analysis. The financial assistance from CSIR is acknowledged
for BLB project under XII FYP.
Notes and references
1 A. Corma, S. Iborra and A. Velty, Chem. Rev., 2007, 107,
2411–2502.
2 Y. Fan, C. Zhou and X. Zhu, Catal. Rev. Sci. Eng., 2009, 51,
293–324.
Catalyst characterization
The X-ray diffraction analysis was carried out using a Rigaku
X-ray diffractometer (Model DMAX IIIVC) with Cu-Kα radi-
ation. FTIR measurements were carried out using Shimadzu
8300 with KBr pellets in the range of 4000–400 cm−1 with
4 cm−1 resolution. The BET surface area was determined using
a NOVA 1200 Quanta chrome instrument. Prior to N2 adsorp-
tion, the sample was evacuated at 200 °C. Temperature pro-
grammed desorption of ammonia (NH3-TPD) and CO2 (CO2-
TPD) was carried out using a Micromeritics Autocue 2910. For
TPD measurements 0.1 g of the catalyst sample was used. The
sample was dehydrated at 600 °C for 1 h in a 50 mL min−1
helium flow and then cooled to 100 °C. Probe gas (5% NH3 in
helium for acidic sites and 10% CO2 in helium for basic sites)
was then adsorbed for 1 h in a 50 mL min−1 flow. The temp-
erature was raised to 750 °C at a rate of 10 °C min−1 in a
30 mL min−1 helium flow. To study the morphological appear-
ance of the samples, scanning electron microscopy (SEM) was
performed on a Leica Stereoscan-440 instrument equipped
with a Phoenix EDAX attachment operated at 20 kV.
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