Nonstoichiometric calcium pyrophosphate: A highly efficient and selective catalyst for dehydration of lactic acid to acrylic acid

Article abstract of DOI:10.1039/c4ra06429a

Calcium phosphate catalysts were prepared by co-precipitation method using calcium nitrate and mixtures of ammonium and different sodium phosphates as calcium and phosphate precursors, respectively. Depending on the phosphate precursor, the pH of the synthesis mixture changed during the catalyst precipitation. The catalyst characterisation by XRD and ICP revealed the formation of a calcium pyrophosphate structure with varying Ca/P ratio from 1.02 to 0.76 which could be correlated to the different pH of the synthesis solutions. Vapour phase dehydration of lactic acid to acrylic acid was carried out using these calcium pyrophosphate catalysts. Non-stoichiometric calcium pyrophosphate catalyst with Ca/P ratio 0.76 was found to be the most efficient catalyst among the synthesized series with 100% lactic acid conversion and 78% acrylic acid selectivity at 375 °C. The higher selectivity for acrylic acid has been correlated to the increased acidity and reduced basicity of non-stoichiometric calcium pyrophosphate compared to other stoichiometric pyrophosphates. In situ FTIR studies showed the formation of a higher amount of calcium lactate on non-stoichiometric compared to stoichiometric pyrophosphate leading to higher selectivity for acrylic acid. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra06429a

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Nonstoichiometric calcium pyrophosphate: a
highly efficient and selective catalyst for
dehydration of lactic acid to acrylic acid†
Cite this: RSC Adv., 2014, 4, 33319
ab
a
ab
Vidhya C. Ghantani, Mohan K. Dongare‡* and Shubhangi B. Umbarkar*
Calcium phosphate catalysts were prepared by co-precipitation method using calcium nitrate and mixtures
of ammonium and different sodium phosphates as calcium and phosphate precursors, respectively.
Depending on the phosphate precursor, the pH of the synthesis mixture changed during the catalyst
precipitation. The catalyst characterisation by XRD and ICP revealed the formation of a calcium
pyrophosphate structure with varying Ca/P ratio from 1.02 to 0.76 which could be correlated to the
different pH of the synthesis solutions. Vapour phase dehydration of lactic acid to acrylic acid was
carried out using these calcium pyrophosphate catalysts. Non-stoichiometric calcium pyrophosphate
catalyst with Ca/P ratio 0.76 was found to be the most efficient catalyst among the synthesized series
ꢀ
with 100% lactic acid conversion and 78% acrylic acid selectivity at 375 C. The higher selectivity for
Received 30th June 2014
Accepted 16th July 2014
acrylic acid has been correlated to the increased acidity and reduced basicity of non-stoichiometric
calcium pyrophosphate compared to other stoichiometric pyrophosphates. In situ FTIR studies showed
the formation of a higher amount of calcium lactate on non-stoichiometric compared to stoichiometric
pyrophosphate leading to higher selectivity for acrylic acid.
DOI: 10.1039/c4ra06429a
www.rsc.org/advances
etc. Dehydration of lactic acid to acrylic acid is being considered
as one of the attractive alternate routes for the manufacture of
Introduction
Depletion of fossil fuels and increasing pollution due to acrylic acid from renewable raw materials.
greenhouse gases emitted in the atmosphere has forced
Catalytic dehydration of lactic acid/lactates to acrylic acid/
researchers worldwide to develop clean technologies based on acrylates has been reported previously using inorganic salts
renewable raw materials for sustainable development. such as phosphates, silicates, carbonates, nitrates and
8–12
Sustainable production of fuels and chemicals is possible using sulphates as catalysts.
Catalysts having strong acidity mainly
lignocellulosic biomass as an attractive raw material. Lactic acid show decarbonylation/decarboxylation of lactic acid to acetal-
5
derived from biomass fermentation has been identied as one dehyde. Hence for zeolites, modiers such as potassium
of the high potential platform chemicals in the biomass nitrate, lanthanum oxide and alkali phosphates as well as
1,2
economy. Lactic acid is a very reactive molecule due to the various alkaline earth metals like Ca, Mg, Sr, Ba, etc. were used
presence of active functional groups, –OH and –COOH, which to change the surface acidity to achieve high efficiency for
13
can be used to convert lactic acid to many value added products selective dehydration of lactic acid to acrylic acid. Previously
3
4
such as acrylic acid, propionic acid, 2,3-pantanedione, acet- various phosphate based catalysts like sodium dihydrogen
5
6
7
aldehyde, pyruvic acid, 1,2 propane diol by different path- phosphate (NaH
2
PO
4
), Ca
3
(PO
4
)
2
supported on silica, silica/
ways. Acrylic acid, its amides and esters nd wide applications alumina, aluminium phosphate and Na
2
HPO in supercritical
4
14
in surface coatings, textiles, adhesives, paper treatment, water, were used for lactic acid dehydration. However some of
leather, bres, detergents, polymeric occulants, dispersants, the modiers leach out easily from the support during reaction
paints, adhesives, binders for leather, and in acrylate polymers, leading to catalyst deactivation. Hence water insoluble salts
with desirable acidity were employed in the dehydration of
lactic acid. Various alkali phosphates (ortho and pyro) such as
a
Catalysis Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, calcium, strontium and barium phosphates were used for lactic
15
Pune-411008, India
b
acid dehydration by Blanco et al., with maximum acrylic acid
selectivity of only 49% using Ba (PO . Peng and co-workers
Academy of Scientic and Innovative Research, CSIR, Anusandhan Bhawan, New
3
4 2
)
Delhi-110 001, India. E-mail: sb.umbarkar@ncl.res.in; Fax: +91-2025902633/34
investigated the effect of acidity of various metal sulfates on
†
1
‡
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra06429a
lactic acid dehydration, and BaSO showed 74% selectivity for
4
16
acrylic acid due to moderate acidity. When mixture of
Present address: Mojj Engineering Systems Ltd, 15-81/B, MIDC, Bhosari, Pune,
Ca
3
(PO
4
)
2
and Ca
2
P
2
O
7
(50/50 wt%) was tested for dehydration
India – 411026.
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of ethyl lactate, methyl lactate and lactic acid, the conversion
increased in the order ethyl lactate < methyl lactate < lactic acid,
Results and discussion
however the selectivity for acrylic acid was observed in the A series of calcium phosphate (CP) catalysts were prepared
17
reverse order. Matsuura and co-workers studied hydroxyapa- using calcium nitrate, diammonium hydrogen phosphate and
tite with different cations and anions for lactic acid dehydration additional phosphate precursors like NaH PO $2H O, Na H-
2
4
2
2
and results showed calcium and strontium hydroxyapatite with PO $2H O and Na PO $12H O to tune the Ca/P ratio by
4
2
3
4
2
moderate acidity to give highest acrylic acid yield. The same increasing the phosphate content. It is known that preparation
group studied addition of different bases like NaOH and of calcium phosphates is very sensitive to pH of the precursor
ammonia for adjusting the pH and formation of non-stoichio- solution and drastically affects the Ca/P ratio of the nal catalyst
metric calcium hydroxyapatite with incorporation of sodium composition. During the synthesis of catalysts using sodium
which resulted in 78% yield for acrylic acid using 38% lactic phosphates, the pH of Ca(NO ) and (NH ) HPO was adjusted
3
2
4 2
4
18
acid solution with very low WHSV. Similarly hydroxyapatite to 8. However due to difference in pH of the aqueous solution of
with Ca/P ratio of 1.62, calcined at 360 C has shown 71% acrylic sodium phosphate precursors, the pH of the mixed phosphate
acid selectivity with 70% lactic acid conversion. Normally solution was different (Table 1). Hence during addition of
different lactic acid concentrations ranging from 20 wt% to 40 calcium nitrate solution to phosphate mixture and ultimately
wt% were used for lactic acid dehydration with lower WHSV precipitation of calcium phosphate the pH of the mixture
ꢀ
19
leading to low space time yield.
gradually decreased reaching 5 aer complete addition of
Lactic acid dehydration requires catalysts with weak acidity calcium nitrate. Hence the change in the pH of the solution
and weak basicity, however ne tuning of surface acidity and during precipitation is expected to affect the catalyst structure,
basicity is very important for achieving maximum selectivity for Ca/P ratio and in turn the catalytic activity. The pH of the
acrylic acid. Recently we have reported lactic acid dehydration sodium precursors are given in Table 1. Hence for comparison
using series of hydroxyapatite with different Ca/P ratio as one catalyst was prepared without using any sodium phosphate
20
catalyst. The catalyst with minimum Ca/P ratio of 1.3 showed and denoted as CP. For this catalyst also the pH of the synthesis
maximum efficiency with 100% conversion and 60% acrylic acid mixture decreased from 8 to 5 aer complete precipitation.
selectivity along with the formation of acetaldehyde as the only
The bulk and surface Ca/P ratio was determined by ICP and
byproduct. The catalyst with Ca/P ratio 1.3 showed the required EDAX analysis and results are shown in Table 2. The Ca/P ratio
acid base balances to achieve acrylic acid selectivity of 60%. As for CP-1 and CP-2 were found to be 1.04 and 1.09 respectively
Ca/P ratio of hydroxyapatite was decreased from 1.67 (stoi- which is in accordance with the almost similar pH of the
chiometric) to 1.3, considerable improvement in acrylic acid combined phosphate solution. However for CP-3 the Ca/P ratio
selectivity was observed. Hence to further increase the acrylic decreased considerably to 0.76. The catalyst prepared without
acid selectivity, the calcium phosphate catalysts were prepared sodium phosphate showed higher Ca/P ratio of 1.27.
using modied synthetic procedure to decrease the Ca/P ratio
further using mixture of diammonium hydrogen phosphate predominantly HPO4 exists. HPO4 species has very high
At different pH different phosphate species exist, and at pH 8
2
ꢁ
21
2ꢁ
and sodium phosphates as phosphate precursor and without affinity for calcium leading to precipitation of CaHPO . The
4
maintaining constant pH throughout the catalyst preparation precipitation of calcium phosphate from calcium and phos-
and the results are reported herein.
phate solutions is a pH controlled process. At lower pH the
Table 1 pH of the phosphate solution during precipitation
Phosphate
precursor
pH of Na–
phosphate solution
pH of combined
phosphate solution
pH of solution aer
complete precipitation
Catalyst
CP-1
CP-2
CP-3
CP
NaH
2
PO
HPO
PO
4
$2H
$2H
2
2
O
O
O
4
8.5
12
—
7
7.5
10
—
5
5
5
5
Na
Na
—
2
4
3
4
$12H
2
Table 2 Textural characterization, acidity and basicity of calcium pyrophosphates
Catalyst
name
Surface2
CO
mmol g
2
desorbed,
NH
mmol g
3
desorbed,
Acid base
balance
Ca/P ratio
by ICP
Ca/P ratio
by EDAX
ꢁ1
ꢁ1
ꢁ1
area, m g
CP-1
CP-2
CP-3
CP
29
21
29
28
17.054
16.094
15.317
34.907
114
128
167
071
7
8
11
2
1.04
1.09
0.76
1.27
1.02
1.03
1.08
1.10
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solubility of calcium phosphate increased leading to the formation of stable b-pyrophosphate was observed in accor-
decrease in calcium incorporation, which in turn decreases the dance with literature reports. In Ca-pyrophosphate or monetite
22
Ca/P ratio. Addition of sodium phosphates led to increase in structure the theoretical Ca/P ratio is 1 which is matching with
phosphate concentration thus resulting in decrease in Ca/P Ca/P ratio in CP-1 and CP-2, however for CP-3 the ratio is
ratio. The additional phosphate alters the equilibrium between considerably low (0.76) indicating non-stoichiometric or Ca
3
ꢁ
2ꢁ
1ꢁ
PO
4
to HPO
4
to H
2
PO
4
which leads to the further decrease decient pyrophosphate structure. The only reported calcium
PO , mono-
in Ca/P ratio. It was reported that Ca/P ratio of the precipitate phosphate with lower Ca/P ratio of 0.5 are Ca(H
2
4 2
)
does not depend directly on the Ca/P ratio of initial reagents of calcium phosphate monohydrated (MCPM) or anhydrous
calcium and phosphate but depends on pH and temperature monocalcium phosphate (MCPA), which are water soluble and
during precipitation.
stable only in extremely acidic pH of 0–2. Hence in the present
The surface area of all the catalysts determined using the work we could precipitate the calcium phosphate with lower Ca/
2
ꢁ1
BET method was found to be in the range 21–29 m
shown in Table 2.
g
as P ratio in basic medium as well as retain the pyrophosphate
structure at high temperature with calcium deciency.
The change in pH during spontaneous precipitation of
The acid base properties of the catalysts were evaluated by
calcium phosphate from supersaturated solutions was studied temperatures programmed desorption (TPD) of NH
3
(Fig. S2†)
ꢀ
at 30 C, stirring speed 700 rpm and shown in Fig. S1.† It and CO
2
(Fig. S3†) and the results are shown in Table 2. As the
showed decrease in pH during the rst few minutes, followed Ca/P ratio decreased from CP, CP-1 to CP-3, there is decrease in
by an irregular and slow decrease and nally became stable basicity and increase in the acidity. Phosphate groups in cata-
2
+
aer 2 h.
lyst are responsible for acidity whereas Ca ions are responsible
The powder X-ray diffraction (XRD) patterns of synthesized for the basicity. Hence increase in the basicity and decrease in
catalysts are shown in Fig. 1. The XRD pattern showed the the acidity was observed as expected with increase in the Ca/P
20
crystalline nature for all the catalysts. The diffraction peaks at ratio. CP-3 catalyst showed the highest acidity. The basicity of
ꢀ
ꢁ1
2
6.7, 27.7, 28.9, 29.3, 30.69 and 32.37 match with the structure the reported CP catalyst was very weak (expressed in mmol g )
of b calcium pyrophosphate (JCPDS-33-0297). The intensity of and the TPD prole is shown in Fig. S3.†
ꢀ
the peak at 29.3 considerably decreased in CP-3 compared to
Catalyst structure was further conrmed by FTIR spectros-
remaining catalysts indicating structural distortion which may copy (Fig. 2). Typical pyrophosphate stretching and bending
ꢁ
1
be due to calcium deciency as observed by EDAX and ICP vibrations were observed at 1215–941 and 723 cm respec-
2
4
ꢁ1
analysis. However the overall XRD pattern retains the b-pyro- tively. Bands at 613–494 cm
are assigned to phosphate
25
phosphate structure. As the Ca/P ratio for CP-3 is considerably bending vibrations. Thus the b calcium pyrophosphate
low compared to pyrophosphate structure (Ca/P ¼ 1) the XRD of formation was conrmed by both XRD and FTIR studies. Like-
4
as synthesised catalyst was recorded (denoted as CP-3* in wise the formation of CaHPO in as synthesised CP-3 catalyst
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Fig. 1). The peaks at 26.38 , 30.15 , 32.52 , 35.82 , 49.26 , 53.02
matched with monetite structure of CaHPO (monetite JCPDS- band for HPO
4-0513) which transformed to pyrophosphate aer calcina- presence of monoclinic b-calcium pyrophosphate (Fig. 3). The
(CP-3*) was also conrmed by FTIR from the characteristic
1
ꢁ
ꢁ1
4
4
at 898 cm . Raman analysis showed the
0
23
ꢁ1
tions. Skinner has studied the conversion of CaHPO4 to Raman peaks at 1159, 1115, 1049, 736, 524, 356 and 355 cm
different forms of calcium pyrophosphates at different matched with the Raman peaks reported for monoclinic
temperatures and showed the formation of b calcium pyro- calcium pyrophosphate. The particle size of all the CP catalyst
26
ꢀ
phosphate to be at the temperature below 800 C. As all the was determined using SEM and the images are given in Fig. S4.†
ꢀ
catalysts in the present work were calcined at 600 C the
Fig. 1 The powder X-ray diffraction (XRD) patterns of synthesized Fig. 2 FTIR spectra of synthesized CP catalysts and as synthesized CP-
catalysts, CP-3*-as synthesised CP-3 catalyst.
3 catalyst.
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time, concentration on lactic acid dehydration was studied
using CP-3 catalyst and the results are given in Table 4.
Effect of temperature
The effect of temperature on lactic acid conversion and acrylic
acid selectivity was studied by varying the temperature from 325
ꢀ
to 400 C (Table 4, entry 1). Conversion of lactic acid increased
from 75 to 100% with increase in reaction temperature from 325
ꢀ
to 400 C with 50% lactic acid concentration and WHSV of
ꢁ
1
3
h . Initially increase in the acrylic acid selectivity was
observed from 32 to 74% with increase in the temperature from
ꢀ
3
4
25 to 375 C, however further increase in the temperature to
ꢀ
00 C led to decrease in the acrylic acid selectivity to 60% with
corresponding increase in the formation of acetaldehyde and
polylactates. At 325 C lowest lactic acid conversion and acrylic
Fig. 3 Raman spectra of synthesized CP catalysts.
ꢀ
acid selectivity was observed due to lactide formation.
However no considerable difference in particle size was
observed.
Thus the structure of all the synthesised catalysts was
conrmed to be b-calcium pyrophosphate.
Effect of residence time
The residence time of lactic acid on CP-3 was varied by
progressively changing the WHSV from 1 to 4.5 h . When
ꢁ
1
ꢁ
1
WHSV was increased from 1 to 3 h , the lactic acid conversion
remained at 100% however with further increase in WHSV to 4
Catalytic activity
ꢁ1
and 4.5 h , the conversion decreased to 90 and 85% respec-
tively due to decrease in residence time (Table 4, entry 2).
Whereas volcano type trend was obtained for acrylic acid
selectivity. The acrylic acid selectivity increased from 54% to
The synthesized calcium phosphate (CP) catalysts were tested
for vapour phase dehydration of lactic acid (Scheme 1) and the
results are shown in Table 3. The reaction was carried out with
20
ꢁ
1
the previously reported optimised reaction conditions i.e.
74% with increase in WHSV from 1 to 3 h
and then it
using 50% (w/w) aqueous solution of lactic acid with WHSV of
decreased to 68 and 65% with further increase in WHSV to 4
ꢁ
1
ꢀ
ꢁ
1
3
h
at 375 C. As expected 100% lactic acid conversion was
and 4.5 h respectively. As the residence time decreased there
was increase in selectivity, though with 100% conversion for
lactic acid. At very low residence time (WHSV-4, 4.5) there was
decrease in conversion of lactic acid as well as selectivity for
ꢀ
obtained with all CP catalysts at 375 C using 50% lactic acid
solution though with variable acrylic acid selectivity. CP-3
catalyst showed maximum selectivity of 74% for acrylic acid
compared to CP-2 (70%), CP-1 (68%) and CP (55%). As CP-3
catalyst showed highest selectivity for acrylic acid, the effect of
different reaction parameters such as temperature, residence
ꢁ
1
acrylic acid. WHSV-3 h provides optimum residence time for
maximum conversion as well as acrylic acid selectivity on CP-3
catalyst.
Effect of lactic acid concentration
The lactic acid concentration was gradually increased from 25
to 80 wt% (wt/wt aqueous solution) to study its effect especially
on acrylic acid selectivity (Table 4, entry 3). Complete lactic
acid conversion (100%) was obtained with 25 and 50% lactic
acid concentration however gradual increase in the lactic acid
concentration to 80% led to decrease in lactic acid conversion
to 70%. Maximum selectivity (78%) for acrylic acid was
Scheme 1 General schematic for lactic acid conversion to acrylic acid
and acetaldehyde.
a
Table 3 Dehydration of lactic acid to acrylic acid using calcium phosphate with different Ca/P ratio
Sl. no
Catalyst
Conv.
Acrylic acid selectivity
Acetaldehyde selectivity
Other products selectivity
1
2
3
4
CP
100
100
100
100
55
68
70
74
30
15
13
10
15
17
17
16
CP-1
CP-2
CP-3
a
ꢁ1
ꢁ1
ꢀ
Reaction conditions: carrier gas-N
2
, carrier gas ow-15 mL min , WHSV 3 h , lactic acid conc.-50% (w/w), temperature-375 C, other products-
2
propanoic acid, 2,3-pentanedione, CO, CO .
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a
Table 4 Effect of various parameters on dehydration of lactic acid to acrylic acid using CP-3 catalyst
%
acrylic
% acetaldehyde
selectivity
% other products
selectivity
e
Entry
1
Reaction parameters
% conv.
acid selectivity
b
ꢀ
f
Temp. ( C)
325
75
90
32
45
74
60
54
62
74
68
65
78
74
60
58
53
15
15
10
20
22
16
10
12
18
8
53
3
3
4
50
75
00
1
40
16
20
24
22
16
20
17
14
16
20
22
24
100
100
100
100
100
90
c
ꢁ1
2
3
WHSV (h )
2
3
4
4.5
85
d
Lactic acid conc. (w/w%)
25
100
100
96
80
70
5
6
7
8
0
0
0
0
10
20
20
23
a
ꢁ1
b
ꢁ1
c
2
Reaction conditions: catalyst-CP-3, carrier gas-N , carrier gas ow-15 mL min . WHSV-3 h , lactic acid conc.-50% (w/w). Lactic acid conc.-50%
e f
ꢀ
d
ꢀ
ꢁ1
(w/w), temp-375 C. Temp-375 C, WHSV-3 h . Other products-propanoic acid, 2,3-pentanedione, CO, CO and Lactide.
2
obtained when 25% lactic acid was used which steadily
decreased to 53% with increase in the lactic acid concentra- Ba (PO ) , Na HPO supported on NaY, calcium and stron-
tion to 80%. Acrylic acid yield of 78% is the highest yield tium hydroxyapatites as well as non-stoichiometric calcium
Recently many phosphate based catalysts such as
15
32
3
4 2
2
4
18a
18b
reported so far in the literature for lactic acid dehydration at hydroxyapatite modied with sodium
have been used for
ꢁ
1
very high WHSV (3 h ). With increase in the lactic acid lactic acid dehydration to acrylic acid. All the above mentioned
concentration residence time decreased which led to the phosphate based catalysts were operated in the temperature
ꢀ
decrease in Ca-lactate formation and consequently increase in range of 340–400 C with lactic acid concentration of 20–40%.
the decarbonylation leading to more formation of acetalde- Among the phosphate based catalysts Na HPO /NaY showed
2
4
hyde. It is worth mentioning that mass balance for all the maximum lactic acid conversion of 93.5% with 79.5% selectivity
33
above mentioned reaction was 85–90%. Catalyst stability of for acrylic acid. Zhang et al. have studied lactic acid dehy-
CP-3 was tested under optimised reaction conditions and the dration using NaNO /SBA-15 catalyst with almost complete
catalytic activity was stable upto 20 h. The acetaldehyde conversion, however with very poor (42%) acrylic acid selec-
3
formed in all the above mentioned reaction was predomi- tivity. When BaSO
nantly due to decarbonylation of lactic acid as conrmed by 400 C using 20% lactic acid solution, almost 100% lactic acid
GC analysis however formation of CO was also observed with conversion is reported with 66% selectivity for acrylic acid.
2
4
was used for lactic acid dehydration at
ꢀ
16
less selectivity indicating lesser extent of decarboxylation Lactic acid dehydration on different barium phosphates was
compared to decarbonylation. Decarboxylation of lactic acid is screened, however only barium pyrophosphate Ba P O has
2
2 7
associated with formation of hydrogen which may be respon- shown very high acrylic acid selectivity (76%) with 20 wt% lactic
sible for propionic acid formation by hydrogenation of acrylic acid concentration and with considerably lower WHSV (ow
ꢁ
1
34
acid. Alternate pathway for formation of propionic acid is rate 1 mL h ) compared to present work. Compared to
deoxygenation of lactic acid. However hydrogenation of acrylic previous reports on lactic acid dehydration to acrylic acid, the
acid is more feasible path for formation of propionic acid as results obtained in the present study are better with 100%
2
7
per previous studies. Deoxygenation of lactic acid has been conversion and 78% acrylic acid selectivity. Especially the use of
studied before in the literature. Previous studies have shown high concentration of lactic acid and very high WHSV leading to
deoxygenation of lactic acid in presence of acidic catalysts like high per pass yield are the major advantages of the present
2
8
5
hydriodic acid, supported heteropoly acids, supported catalysts.
4
a,b,17,29
13a
nitrates and phosphates,
platinum based catalyst under high hydrogen pressure.
as well as doped zeolites or
Though all the synthesised CP catalysts have shown b-
3
0
calcium pyrophosphate structure, only CP-3 catalyst has shown
Various homogenous catalysts (Pd, Pt or Ir complexes) also maximum selectivity for acrylic acid whereas CP has shown
have been used for deoxygenation of lactic acid in water at lowest acrylic acid selectivity. Hence the structure of all CP
2
7,31
31
lower pH.
However in the present work the catalyst is very catalysts was further studied by P-NMR spectroscopy (Fig. 4
weakly acidic as well as reaction conditions are totally different and S5†). The NMR spectra showed three isotropic peaks at 0.2
0
1
1
0
than typically required for deoxygenation of lactic acid. (Q ), ꢁ11.65 (Q ) and ꢁ13.51 (Q ) ppm. Q represents the
1
Hence the formation of propionic acid in present work would pyrophosphate species and Q represents the metaphosphate
be due to hydrogenation of acrylic acid and not by deoxygen- species (as shown in Fig. 4). The intensities of different
35
ation of lactic acid.
species in CP showed the concentration of pyrophosphate
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spectrum was recorded. Further, the adsorbed lactic acid was
ꢀ
desorbed at 50, 100 and 150 C. The difference spectra of CP and
ꢀ
CP-3 at 150 C are shown in Fig. 5. Calcium lactate formation on
both the catalysts CP and CP-3 was conrmed by carbonyl peak
ꢁ1
at 1600 cm whereas free lactic acid adsorbed on catalyst was
ꢁ1
conrmed by presence of carbonyl peak at 1748 cm . Carbonyl
peaks of calcium lactate and lactic acid were used as measure of
extent of calcium lactate formation which in turn can be
correlated to the selectivity for acrylic acid observed on partic-
ular catalyst. The ratio of area of the carbonyl peak at 1600 (for
ꢁ
1
calcium lactate) and at 1748 cm (for lactic acid) was calculated
for CP and CP-3. The ratio was considerably higher for CP-3
(0.583) compared to CP (0.350) indicating higher extent of
calcium lactate formation on CP-3 compared to CP which can be
directly correlated to higher selectivity for acrylic acid observed
on CP-3 compared to CP.
31
Fig. 4
P MAS NMR spectra (a) CP (b) CP-1, (c) CP-2 and (d) CP-3.
Though Ca/P ratio is less for CP-3 compared to CP the more
formation of calcium lactate is observed on CP-3 which may
indicate the more availability of calcium on the surface for
formation of calcium lactate in CP-3 which may be due to the
structural features of CP-3.
0
species (Q peak at 0.2 ppm) to be less than metaphosphate
1
species (Q peak at ꢁ11.65 and ꢁ13.51 ppm). In CP-3 the
concentration of pyrophosphate species was considerably
higher compared to metaphosphate species. The intensity of
pyrophosphate peak gradually increased from CP to CP-3 which
may be correlated to the acrylic acid selectivity which also
gradually increased from CP to CP-3. The Ca/P ratio changed
Conclusion
from 1.27 to 0.76 for CP and CP-3. More terminal P–OH groups Non stoichiometric calcium pyrophosphate catalyst prepared
are present in pyrophosphate species compared to meta- with Ca/P ratio 0.76 prepared by modifying synthetic procedure
phosphate species as shown in the structure in Fig. 4, which is using mixture of diammonium hydrogen phosphate and
in accordance with higher acidity of CP-3 compared to CP.
sodium phosphates as phosphate precursors. This catalyst has
As per the proposed mechanism in our previous work on shown very high efficiency for vapour phase dehydration of
lactic acid dehydration, calcium lactate formation is crucial for lactic acid to acrylic acid with 100% conversion and 74% acrylic
ꢀ
ꢁ1
acrylic acid selectivity. To check the formation of calcium acid selectivity at 375 C with WHSV of 3 h using 50% lactic
lactate species, in situ FTIR studies were carried out using acid concentration. The acrylic acid selectivity was further
20
previously reported experimental conditions on catalysts improved to 78% with 25% lactic acid concentration. The
which showed minimum (CP) and maximum (CP-3) selectivity higher selectivity for acrylic acid was attributed to acid base
for acrylic acid. Lactic acid was adsorbed on CP and CP-3 at balance at lower Ca/P ratio leading to formation of higher
room temperature and physisorbed lactic acid was removed by amount of calcium lactate as an intermediate compared to
drying the catalyst at room temperature for 8 h and the FTIR other stoichiometric calcium pyrophosphates as evidenced by
in situ FTIR studies.
Experimental section
Materials
Calcium nitrate, di-ammonium hydrogen phosphate, ammo-
nium hydroxide solution, sodium hydrogen phosphate (NaH2-
PO $2H O), disodium hydrogen phosphate (Na HPO $2H O),
4
2
2
4
2
and tri sodium phosphate (Na PO $12H O) were obtained from
3
4
2
Thomas Baker chemicals India Ltd. Lactic acid (89%) was
obtained from our CSIR-National Chemical Laboratory (CSIR-
NCL) Pune pilot plant which is obtained from sugarcane
fermentation process (process developed by CSIR-NCL).
Catalyst preparation method
Calcium phosphates catalysts using calcium nitrate, dia-
mmonium hydrogen phosphate and sodium phosphates were
prepared by precipitation method. Typical sodium phosphate
Fig. 5 FTIR spectrum of (a) lactic acid (b) calcium lactate from FTIR
database, FTIR difference spectra of lactic acid adsorbed (c) CP-3, (d)
CP at 150 C.
ꢀ
precursors like NaH
2
PO
4
$2H
2
O, Na
2
HPO
4
$2H
2
O
and
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Na
3
PO
4
$12H
2
O were used for the preparation and the nal line at 532 nm with 1–10 mW power. The spectrometer was
ꢁ
1
catalysts were denoted as CP-1, CP-2 and CP-3 respectively. In a calibrated using the Si line at 521 cm with a spectral resolu-
ꢁ1
typical synthesis of CP-1, calcium nitrate (36.95 g) and dia- tion of 3 cm . Morphological appearance of the samples using
mmonium hydrogen phosphate (13.8 g) were dissolved sepa- scanning electron microscopy (SEM) was performed on a Leica
rately in 125 mL deionised water each. Initially the pH of both Stereoscan-440 instrument equipped with a Phoenix EDAX
the solutions was adjusted to 8 by addition of ammonium attachment operated at 20 kV. P NMR analysis was carried out
hydroxide solution (5% aqueous). NaH PO $2H O (3.39 g) dis- using Bruker 300 MHz, with resonance frequency of
solved in 25 mL deionised water was added to the diammonium 121.49 MHz. The operating eld of 7.05 T, the spinning rates of
hydrogen phosphate solution. Calcium nitrate solution was 8 KHz, with 85% H PO as reference material and recycle delay
3
1
2
4
2
3
4
added drop wise to the above mixture of phosphate precursors of 5.0 s were used during analysis.
with constant stirring (700 rpm) at room temperature. As the
precipitation proceeds decrease in pH was observed. A thick
Catalyst activity test
white precipitate formed was aged for 12 h then ltered, washed
ꢀ
Vapour phase dehydration of lactic acid to acrylic acid was
with water and dried in an oven at 120 C for 12 h. The dried
2
0
ꢀ
carried out using procedure reported previously. In short
quartz xed bed down ow reactor (id-11 mm) was charged with
catalyst (particle size 20–40 mesh) in the middle section with
quartz wool packed in both the ends. Porcelain beads were
placed above the catalyst bed in order to preheat the feed.
Before catalytic evaluation, the catalyst was preheated at
required reaction temperature for 0.5 h under ow of nitrogen
catalyst was calcined at 600 C for 4 h. Catalysts CP-2 (Na
2
H-
PO
4
$2H
2
O 1.911 g) and CP-3 (Na
3
PO
4
$12H
2
O 2.7 g) were
prepared following the similar procedure as mentioned above
except the source of sodium phosphate. Calcium phosphate
catalyst without sodium phosphate was prepared for compar-
ison, with Ca/P ratio 1.5 and denoted as CP. For preparation of
CP calcium nitrate (38.9 g) and diammonium hydrogen phos-
phate (14.5 g) solutions were prepared separately in deionised
water (125 mL each). Initially the pH of both the solutions was
adjusted to 8 by adding ammonium hydroxide solution (5%
aqueous). Calcium nitrate solution was added drop wise to the
diammonium hydrogen phosphate solution with constant stir-
ring (700 rpm) at room temperature. As the precipitation
proceeds decrease in pH was observed. A thick white precipitate
formed was aged for 12 h then ltered, washed with water and
ꢁ
1
(
15 mL min ). The reaction temperature was measured by a
thermocouple inserted in the catalyst bed. Lactic acid of
required concentration (typically 50% w/w in water) was pum-
ped in to the preheating zone of reactor using peristaltic pump
and nitrogen gas as carrier. The reaction products were
ꢀ
collected in a cold trap at 8 C. The reaction mixture was ana-
lysed using gas chromatograph (Perkin Elmer) equipped with a
FFAP capillary column (50 M ꢂ 0.320 mM) and FID detector.
The GC was calibrated by external standard method. The
ꢀ
dried in an oven at 120 C for 12 h. The dried catalyst was
ꢀ
formation of CO and CO
and porapack Q column respectively using TCD detector. The
amount of CO (CO and CO ) was estimated based on the yield
2
was conrmed by GC, with molesieve
calcined at 600 C for 4 h.
x
2
Catalyst characterization
of acetaldehyde and propionic acid. The mass balance for all the
reaction was more than 90%.
The X-ray diffraction analysis was carried out using a Rigaku
X-ray diffractometer (Model DMAX IIIVC) with Cu-Ka radiation.
FTIR measurements were carried out using Shimadzu 8300 as
KBr pellet in the range of 4000–400 cm region with 4 cm
resolution averaged over 100 scans. BET surface area was
The in situ FTIR studies were carried out using previously
20
reported procedure. In a typical experiment lactic acid was
ꢁ1 adsorbed on CP and CP-3 catalyst at room temperature. The
physisorbed lactic acid was removed by drying the catalyst at
room temperature for 8 h and the FTIR spectrum was recorded.
The adsorbed lactic acid was desorbed at various temperatures
to detect the species formed in situ during the reaction. The
spectrum of neat catalyst was subtracted to get the spectrum of
adsorbed species.
ꢁ
1
determined using NOVA 1200 Quanta chrome instrument. Prior
ꢀ
to N
2
adsorption, the sample was evacuated at 250 C. To study
Ca/P ratio, scanning electron microscopy EDAX was performed
on a Leica Stereoscan-440 instrument equipped with a Phoenix
EDAX attachment operated at 20 kV, ICP analysis was carried
out on ICP AES SPECTRO ARCOS Germany FHS12 instrument.
Temperature programmed desorption of ammonia (NH -TPD)
3
Acknowledgements
and CO2 (CO -TPD) was carried out using a Micromeritics
2
VCG acknowledges CSIR New Delhi for Research Fellowship.
Autocue 2910. For TPD measurement 0.1 g of catalyst
ꢀ
sample was used. Sample was dehydrated at 500 C for 1 h in
ꢁ1
ꢀ
5
0 mL min helium ow and then cooled to 50 C. Probe gas
Notes and references
(
5% NH
3 2
in helium for acidic sites and 10% CO in helium for
ꢁ
1
basic sites) was then adsorbed for 1 h in 50 mL min ow. The
temperature was raised up to 600 C at a rate of 10 C min in
1 M. D. P. V. Wouwe, A. Dewaele, E. Makshina and B. F. Sels,
Energy Environ. Sci., 2013, 6, 1415–1442.
2 P. Maki-Arvela, I. L. Simakova, T. Salmi and D. Y. Murzin,
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3 T. J. Korstanje, H. Kleijn, J. T. B. H. Jastrzebski and
R. J. M. Klein Gebbink, Green Chem., 2013, 15, 982–988.
ꢀ
ꢀ
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ꢁ
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0 mL min helium ow. Raman spectra were recorded under
ambient conditions on a Lab RAM innity spectrometer
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(
and a frequency doubled Nd-YAG laser supplying the excitation
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