Green, efficient and economical coal fly ash based phosphomolybdic acid catalysts: preparation, characterization and application

Article abstract of DOI:10.1007/s11696-020-01501-x

Abstract: Cost-effective, efficient and green solid acid catalysts have been synthesized by incipient wetness impregnation of various weight fractions of phosphomolybdic acid (5, 10, 15 and 25 wt. %) on mechanically and thermally activated coal fly ash. N₂ adsorption–desorption, XRD, FT-IR, SEM, SEM–EDX, TEM, TGA, UV–Vis DRS, solid state ³¹P MAS NMR were used for characterization of as synthesized catalysts. Catalytic active sites were developed on inert surface of coal fly ash by using various activation techniques whose performance was assessed over a series of acylation of various aliphatic alcohols. For rapid and higher catalytic activity, reactions were conducted in microwave heating mode. Impregnation of phosphomolybdic acid generates Lewis acidic sites on coal fly ash surface as inferred by pyridine adsorbed FT-IR studies which were then utilized in acylation reactions. Various reaction parameters like weight fraction of catalysts, molar ratio of reactants, time, temperature, etc. were optimized for attaining highest conversion %. The catalyst with 15 wt. % of phosphomolybdic acid was found to be more efficient and could be recycled up to five reaction cycles with analogous conversion %. Negligible leaching of catalyst was confirmed by hot filtration test. This work suggests an alternative approach for valorisation of industrial solid waste, coal fly ash in development of innovative, economical solid catalysts. Graphic abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s11696-020-01501-x

Chemical Papers
https://doi.org/10.1007/s11696-020-01501-x
ORIGINAL PAPER
Green, efcient and economical coal ꢀy ash based phosphomolybdic
acid catalysts: preparation, characterization and application
1
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Sakshi Kabra Malpani · Deepti Goyal · Stuti Katara · Ashu Rani
Received: 8 August 2020 / Accepted: 29 December 2020
©
Institute of Chemistry, Slovak Academy of Sciences 2021
Abstract
Cost-efective, eꢀcient and green solid acid catalysts have been synthesized by incipient wetness impregnation oꢁ various
weight ꢁractions oꢁ phosphomolybdic acid (5, 10, 15 and 25 wt. %) on mechanically and thermally activated coal ꢂy ash. N
2
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adsorption–desorption, XRD, FT-IR, SEM, SEM–EDX, TEM, TGA, UV–Vis DRS, solid state P MAS NMR were used
ꢁor characterization oꢁ as synthesized catalysts. Catalytic active sites were developed on inert surꢁace oꢁ coal ꢂy ash by using
various activation techniques whose perꢁormance was assessed over a series oꢁ acylation oꢁ various aliphatic alcohols. For
rapid and higher catalytic activity, reactions were conducted in microwave heating mode. Impregnation oꢁ phosphomolybdic
acid generates Lewis acidic sites on coal ꢂy ash surꢁace as inꢁerred by pyridine adsorbed FT-IR studies which were then
utilized in acylation reactions. Various reaction parameters like weight ꢁraction oꢁ catalysts, molar ratio oꢁ reactants, time,
temperature, etc. were optimized ꢁor attaining highest conversion %. The catalyst with 15 wt. % oꢁ phosphomolybdic acid was
ꢁ
ound to be more eꢀcient and could be recycled up to ꢃve reaction cycles with analogous conversion %. Negligible leaching
oꢁ catalyst was conꢃrmed by hot ꢃltration test. This work suggests an alternative approach ꢁor valorisation oꢁ industrial solid
waste, coal ꢂy ash in development oꢁ innovative, economical solid catalysts.
Graphic abstract
Keywords Coal ꢂy ash · Phosphomolybdic acid · Microwave assisted acylation · Solid acid catalyst
Extended author inꢁormation available on the last page oꢁ the article
Vol.:(0
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Chemical Papers
Introduction
proꢁoundly used in various organic synthesis like methanol
oxidation, oxidative desulꢁurization, glycerol esteriꢃca-
tion, acrolein production, transesteriꢃcation (Zhang et al.
2019; Bryzhin et al. 2019; Khayoon and Hameed 2012;
Heravi and Sadjadi 2009; Badday et al. 2013) etc. How-
ever, bulk PMA having unique Keggin structure is non-
porous, highly soluble in organic solvents, possess low
Over past 15 years, solid wastes like coal ꢂy ash (CFA),
red mud, rice husk ash, etc. have become one oꢁ the major
areas oꢁ investigation in environmentally benign hetero-
geneous catalysis (Borhade et al. 2018; Lin et al. 2020;
Shirini et al. 2016). Among them, CFA is an abundant
waste generated by combustion oꢁ coal in thermal power
plants. Around 155 coal-based power plants in India with
installed capacity oꢁ 145,044.80 MW electricity, consume
more than 536.40 million tons oꢁ coal per year which
results in generation oꢁ around 169.25 million tons oꢁ CFA
as a by-product (CEA Reports 2017). The disposal oꢁ CFA
poses not only serious ecological problems like air, soil,
water pollution but being classiꢃed as human carcinogen,
it causes adverse health efects also (Herndon and Whi-
teside 2017). For sake oꢁ maximum utilization oꢁ CFA,
besides its conventional use as building material, adsor-
bent (Genc and Genc 2018; Dagar and Narula 2018) etc.,
its application as a catalytic support material has attracted
attention oꢁ scientists in recent years. CFA, an ensemble oꢁ
mainly silica and alumina spheres, with minor quantities
oꢁ other metal oxides and unburnt activated carbon aꢁter
suitable surꢁace modiꢃcation through diferent activation
techniques, has proved to be best applied as a catalyst sub-
strate in organic reactions oꢁ industrial importance viz.
dehydration, transesteriꢃcation, hydrogenolysis, conden-
sation, esteriꢃcation, benzylation, acylation, alkylation,
hydrogen peroxide decomposition, oxidation and Suzuki-
Miyaura cross-coupling reaction (Chaterjee et al. 2019;
Bhandari et al. 2015; Vijaya et al. 2015; Mazumder and
Rao 2015; Jain et al. 2012; Rupali and Monika 2015; Mal-
pani and Rani 2019; Hada et al. 2020; Srivastava et al.
2
−1
thermal stability and surꢁace area (5–8 m g ) with ꢁew
acidic sites, thus, it is usually supported on silica, silica-
zirconia, zeolites in various acylation reactions (Cardoso
et al. 2004; Bachiller-Baeza and Anderson 2004; Bai et al.
2012). Thereꢁore, herein we ꢁocused on the development
oꢁ green, solvent-ꢁree, heterogeneous acid catalytic system
based on CFA that avoids the use oꢁ highly expensive, less
stable commercial heterogeneous catalytic systems such as
zeolites, mixed metal oxides, acid-treated metal oxides,
pure heteropolyacids etc. (Massah et al. 2013; Sartori and
Maggi 2006) ꢁor industrially important acylation reactions.
Use oꢁ microwave reaction medium coupled with inorganic
mineral-supported catalysts has been upsurge during past
ꢁew years as they are clean, energy saving, having ꢁast
reaction rates, suppress by-product ꢁormation and hence
enhance selectivity and yield oꢁ desired products.
In the present endeavour, we have synthesized a series oꢁ
potential, cost-efective, microwave stable CFA supported
PMA catalysts (PMFA). Mechanical and thermal activation
enhances capability oꢁ CFA to be utilized as a stable support
ꢁor ꢁurther loading oꢁ PMA. The mineralogical, morpho-
logical and structural properties oꢁ the prepared catalysts
were analyzed. PMFA was ꢁound efectively regenerable up
to ꢃve reaction cycles over a series oꢁ microwave assisted
acylation reactions oꢁ various aliphatic alcohols by acetyl
chloride and its catalytic activity was comparable to other
reported solid catalysts. Diferent reaction parameters such
as time, temperature, microwave power output, etc. were
ꢁurther optimized ꢁor maximum conversion. The products
oꢁ the test reactions are important as solvents in lacquer and
enamel industries as well as intermediates in perꢁumeries,
thinner in paint ꢁormulations (Toor et al. 2011; Ohayama
et al. 1995). Thus, the study provides the utilization oꢁ solid
waste, CFA as an alternative ꢁor development oꢁ highly eꢀ-
cient, recyclable heterogeneous acid catalyst ꢁor microwave
irradiated industrially beneꢃcial organic transꢁormations.
2
014; Thavamani et al. 2020) etc. Nevertheless, no attempt
has been made so ꢁar to synthesize solid acid catalyst by
impregnation oꢁ phosphomolybdic acid (PMA) on CFA.
Esters produced by acylation reactions oꢁ aliphatic
alcohols ꢁound wide applications in pharmaceuticals,
cosmetics, plasticizers through protection and deprotec-
tion reactions, multi-step synthesis (Mulla et al. 2012;
Aleixo et al. 2019; Ghosh et al. 2005) etc. Carboxylic
acids, acid anhydrides and acyl chlorides are commonly
used acylating reagents. Acylations by acyl urea, acyl
imidazoles (Pasha et al. 2010) have also been reported,
however, higher yields and smooth rates oꢁ acylation oꢁ
alcohols are seen on using aliphatic acyl chlorides (Green
and Wuts 1999). Some microwave assisted acylation reac-
tions oꢁ aromatic compounds are also well portrayed in
literature (Yamashita et al. 2010; Winé et al. 2009; Bai
et al. 2012). Heteropolyacid particularly PMA is non-pol-
luting, stable Bronsted acid, more eꢀcient and acidic than
mineral acids (Tayebee and Cheravi 2009) and has been
Experimental
Materials and chemicals
CFA (Class-F) was collected ꢁrom Kota Thermal Power
Plant, Kota (Rajasthan). PMA (99.99% purity) was pur-
chased ꢁrom Sigma-Aldrich. Methanol (99.99% purity), ali-
phatic alcohols, acetyl chloride were purchased ꢁrom Merck.
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Chemical Papers
Catalyst preparation
samples with KBr in 1:20 weight ratio. The spectra were
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1
recorded in the range oꢁ 550–4000 cm with a resolution
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1
As received CFA was washed with deionized water, oven
dried at 110 °C ꢁor 12 h and then mechanically activated ꢁor
oꢁ 4 cm . The acidity oꢁ the catalysts was measured by
pyridine adsorbed FT-IR technique. Morphology and surꢁace
topography was studied by scanning electron microscope
(SEM, Model-JEOL-JSM 5600). The texture oꢁ samples
was ꢁurther conꢃrmed by TEM analysis, Model: H-7500
(Hitachi Make). Thermo gravimetric analysis (TGA) oꢁ the
samples was carried out using DTG-60-H- Shimadzu ther-
mal analyzer (C-30574400175), with a heating rate oꢁ 10 °C/
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0, 20 and 30 h. 30 h mechanically activated CFA (MFA-
2
0), having higher speciꢃc surꢁace area (30.02 m /g) was
selected ꢁor ꢁurther study. MFA-30 was then thermally
activated at 800 °C ꢁor 3 h to remove excess water, carbon
and other impurities (Temuujin and Van Riessen 2009) and
ꢁ
orm thermally activated MFA-30 (TMFA). For detailed
3
comparative study, various PMFA catalysts were prepared
using incipient wetness impregnation method. Therein, the
requisite amounts (0.25 g ꢁor 5 wt. %, 0.50 g ꢁor 10 wt. %,
min under nitrogen ꢂow (50 cm /min). UV–Vis DRS spec-
tra in the range oꢁ 200–800 nm were measured with Perkin
Elmer Lambda 950 UV–Vis spectrophotometer at ambient
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1
0
.75 g ꢁor 15 wt. % and 1.25 g ꢁor 25 wt. %) oꢁ PMA dis-
temperature. Solid state P MAS NMR spectra (Mercury
Plus-300 spectrometer, Make: Varian) were recorded at
121.5 MHz with a Bruker 4 mm probe head. The spinning
rate was 10 kHz and the delay between two pulses was var-
ied between 1 and 30 s. The chemical shiꢁts were given rela-
tive to external 85% H PO .
solved in 20 mL methanol were added to 5 g oꢁ TMFA and
stirred overnight in a closed vessel at room temperature.
The impregnated samples were dried at 110 °C ꢁor 12 h and
then calcined in static air at 400 °C ꢁor 3 h. As-synthesized
catalysts were designated as PMFA-x, where x=5, 10, 15
and 25 wt. % oꢁ PMA.
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Catalyst characterization
Catalytic activity measurements
The reactions were perꢁormed on synergy.exe soꢁtware
based, microwave synthesizer (CEM-Discover), with a
working ꢁrequency oꢁ 2.45 GHz, power output ꢁrom 0 to
Acylation reactions between aliphatic alcohols and acetyl
chloride (Scheme 1) catalyzed by PMFA-5, 10, 15 and 25
to give corresponding esters were perꢁormed in a microwave
synthesizer. In this procedure, a mixture oꢁ aliphatic alco-
hol and acetyl chloride (molar ratio=1:1.5) was taken in a
10 ml microwave vial. The pre-activated catalyst (substrate
to catalyst weight ratio=10:1), was then added in the reac-
tion mixture. The reaction was carried out in closed vessel
system at 50 Psi pressure, by placing the vial in microwave
cavity under constant stirring at diferent time periods, molar
ratios, temperatures and power outputs. Aꢁter completion oꢁ
holding time, the reaction was cooled through air compres-
sor. Then, the catalyst was separated, product was ꢃltered
and analyzed by Gas Chromatograph (Agilent Technologies
7820A) equipped with FID.
3
00 W, temperature ranging ꢁrom 0 to 300 °C, pressure rang-
ing ꢁrom 0 to 300 Psi. An Inꢁrared detector controls the tem-
perature during the reactions and a Teꢂon spill cup protects
the detector ꢁrom exposure. An air compressor connected
with the equipment helps in cooling oꢁ reactions.
Mechanical activation oꢁ CFA was carried out in a high-
energy planetary ball mill (Retsch PM-100, Germany). N
2
adsorption–desorption was done by using ThermoScien-
tiꢃc™ Surꢁer surꢁace area analyzer. X-ray difraction (XRD)
patterns were recorded on Bruker D8 Advance difractom-
eter, using Ni-ꢃlter and Cu K radiation (E = 8047.8 eV,
α
λ = 1.5406 A°). The samples were scanned in 2θ range oꢁ
−
1
1
0–75° at a scanning rate oꢁ 0.04°s . Crystallite size was
The conversion oꢁ alcohol was calculated by using ꢁol-
lowing method:
determined by using Scherrer ꢁormula (Genc and Genc
2
018). FT-IR study was executed on Bruker FT-IR Spec-
Initial wt % − Final wt %
Conversion(%) = 100 ×
Initial wt %
trophotometer (TENSOR 27) in DRS mode by mixing
Scheme 1 Simpliꢃed reac-
tion pathway oꢁ acylation oꢁ
aliphatic alcohols and acetyl
chloride over PMFA to give
corresponding esters
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Chemical Papers
Results and discussion
sample. In XRD pattern oꢁ TMFA (Fig. 2c), it was ꢁound
that aꢁter thermal activation, magnetite phase tends to disap-
pear, while peak responsible ꢁor haematite begins to appear
indicating thermal oxidation oꢁ magnetite to haematite at
temperature above 600 °C (Bonda et al. 2014). Intensities
oꢁ crystalline phases (quartz and mullite) also increased on
thermal activation.
Characterization
BET speciꢁic surꢁace area oꢁ CFA samples as listed in
Table 1 indicates that with enhancing milling time ꢁrom 10
to 30 h, speciꢃc surꢁace area oꢁ CFA also increased, which
later on decreased in TMFA due to thermal treatment at
higher temperature. On increase in loading oꢁ PMA content
In PMFA-5, 10, 15 and 25 (Fig. 3), on impregnation oꢁ
PMA, the crystalline phases oꢁ silico-aluminate species
in CFA got reduced and consequently amorphous phases
increased because oꢁ interaction between anions oꢁ PMA
and silanol groups present on CFA surꢁace (Rocchiccioli-
Deltchef et al. 1992; Ahmed et al. 2013). No separate diꢁ-
ꢁraction patterns corresponding to PMA or any molybdenum
related phases were observed, even in 15 wt. % composition
(Devassy et al. 2005). This ꢁact indicates that PMA spe-
cies were well dispersed on CFA surꢁace or present in non-
crystalline phases (Shu-wen et al. 2012; Vazquez et al. 2002;
Kim et al. 2008). It also shows the retention oꢁ characteristic
Keggin structure oꢁ PMA as it did not decompose into other
molybdate species at 400 °C (calcination temperature). As a
result oꢁ this interpretation, it can be said that strong bond-
ing is present between PMA species and CFA. On ꢁurther
increasing PMA content to 25 wt. %, in PMFA-25, some
reꢂections oꢁ characteristic peaks oꢁ pure PMA at 2θ=24.7°,
ꢁ
rom 5 to 25 wt. %, the speciꢃc surꢁace area oꢁ catalysts
decreased progressively, probably due to plugging oꢁ sub-
strate pores by PMA dispersion (Liu et al. 2009).
Figure 1a clearly shows that majority oꢁ pores in MFA-
3
0 lie in the range oꢁ 10–100 nm. Average pore diameter
oꢁ MFA-30 was calculated to be 4.76 nm depicting its
mesoporous nature. B.E.T. adsorption isotherm ꢁor MFA-30
as shown in Fig. 1b, demonstrated the composite isotherm
consisting oꢁ Type II and III adsorption isotherms, character-
istic oꢁ a material, which is non-porous, or possibly micropo-
rous (Condon 2006). However, it also showed some devia-
tions ꢁrom Type II isotherm, conꢁerring the transition ꢁrom
macroporous to mesoporous behaviour as a consequence oꢁ
mechanical activation. The BET plots were observed with
0
linear range ꢁor saturation pressure range, P/P value (0.05
to 0.28) which resembles to most oꢁ inorganic silica mate-
rial (Condon 2006). High adsorption energy oꢁ MFA-30 is
responsible ꢁor active dispersion oꢁ loading species on it
and makes it a suitable catalyst support. BET adsorption
isotherm oꢁ PMFA-15 (Fig. 1c), also showed deviations
2
8.6°, 32.4° and 58.2° were seen that conꢃrmed the aggrega-
tion oꢁ excessive unreacted PMA particles on CFA surꢁace
(
Gong et al. 2010).
The FT-IR spectra oꢁ CFA and MFA-30 (Fig. 4a &b)
are well explained in our previous paper (Srivastava et al.
ꢁ
rom Type II isotherm, so, it can be said that the catalyst is
2
014). In FTIR spectrum oꢁ TMFA (Fig. 4c), the intensity
ensemble oꢁ all three types oꢁ porous particles, thus provide
peculiar active sites ꢁor reactants to undergo various types
oꢁ organic transꢁormations.
oꢁ band corresponding to surꢁace hydroxyl groups in the
−
1
range between 3500 and 3300 cm got decreased as a con-
sequence oꢁ thermal activation oꢁ CFA.
The detailed explanation oꢁ XRD patterns oꢁ CFA and
MFA-30 is given in our previous work (Srivastava et al.
As inꢁerred ꢁrom literature, pure PMA possesses ꢁour
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2
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014). XRD data show the reduction oꢁ crystallite size ꢁrom
3 nm to 17.06 nm on mechanical activation in MFA-30
bands at 1065, 1000, 867 and 790 cm (Deltchef et al.
1996; El-Naggar 2013; Abd El-Wahab and Said 2005).
These characteristic bands ꢁor Keggin structure oꢁ PMA
were partially obscured in case oꢁ PMFA catalysts (Fig. 5)
as they overlap with the regions oꢁ Si‒O‒Si asymmetric
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(
1200‒1000 cm ) and Si‒O‒Si symmetric stretching
Table 1 BET speciꢃc surꢁace
Samples
BET speciꢃc
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area data oꢁ samples
vibrations (1000–800 cm ) (Chen et al. 2007). It is reason-
able to mention here that the bands assigned at 1000 and
surꢁace area
2
(
m /g)
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8
67 cm in case oꢁ pure PMA got shiꢁted towards lower
CFA
9
−
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wave number at 970 and 825 cm , respectively, in PMFA
catalysts, conꢃrming some interaction between PMA and
CFA support (Abd El-Wahab and Said 2005). Enhancement
oꢁ intensity oꢁ these bands was observed with increase in wt.
MFA-10
MFA-20
MFA-30
TMFA
PMFA-5
PMFA-10
PMFA-15
PMFA-25
15
22
30
28
27
23
20
14
%
oꢁ PMA loading (Ahmed et al. 2013).
The FT-IR spectra oꢁ PMFA-5, 10, 15 and 25, obtained
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1
aꢁter pyridine adsorption in the range oꢁ 1600‒1400 cm
are shown in Fig. 6. The bands appearing around 1558 and
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1
1
443 cm in all catalysts indicate the presence oꢁ Bronsted
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Chemical Papers
Fig. 1 a Pore distribution curve oꢁ MFA-30, b B.E.T. Adsorption Isotherm oꢁ MFA-30 and c B.E.T. Adsorption Isotherm oꢁ PMFA-15
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Chemical Papers
Fig. 2 X-ray difraction pattern oꢁ a CFA, b MFA-30 and c TMFA
Fig. 3 X-ray difraction pattern oꢁ a PMFA-5, b PMFA-10, c PMFA-
1
5 and d PMFA-25
and Lewis acidic sites, respectively (Devassy et al. 2005,
2
006). On increasing the wt. % oꢁ PMA loading to 15 wt.
%
, the intensity oꢁ band responsible ꢁor Lewis acidity also
seen in typical SEM image oꢁ PMFA-15 (Fig. 7) reveal the
strong interaction oꢁ PMA Keggin units with CFA, although
they were not uniꢁormly distributed on CFA surꢁace.
EDX analysis oꢁ MFA-30 (Table 2) showed the presence
6
+
6+
increases. Mo and Mo ‒OH acidic sites oꢁ PMFA cata-
lysts adsorbed pyridine and showed the existence oꢁ Lewis
and Bronsted acidic sites in samples respectively (Pawelec
et al. 2004). On ꢁurther increasing PMA loading to 25 wt%,
in PMFA-25, intensity oꢁ these both bands decreases. As
inꢁerred ꢁrom literature, pure PMA contains only Bronsted
acidic sites (Misono et al. 1978), however, the appearance
oꢁ Lewis acid sites on PMFA catalysts may be attributed due
to loss oꢁ water/lattice oxygen during calcination at higher
temperature (Li et al. 2001) which played crucial role in gen-
eration oꢁ acylium ions during course oꢁ acylation reactions.
The SEM micrographs oꢁ CFA and MFA-30 are well
explained in our previous work (Srivastava et al. 2014).
Beꢁore any kind oꢁ activation, CFA surꢁace consists oꢁ
smooth spheres due to outer covering oꢁ glassy alumino-
silicate which got shredded by mechanical activation. Later
on, impregnation oꢁ PMA, resulted in increased roughness
on CFA surꢁace (Prasad et al. 2012). Small shiny particles
oꢁ Si, Al, K, Na, Fe Ti and other minor metals. However,
,
the presence oꢁ molybdenum and phosphorus in PMFA-15
conꢃrmed loading oꢁ phosphomolybdic acid on surꢁace oꢁ
MFA-30.
The topography and surꢁace dispersion oꢁ active PMA
species are well understood by using TEM analysis. TEM
image oꢁ CFA revealed the appearance oꢁ smooth spherical
particles with larger carbon particles (Khatri et al. 2010).
On mechanical activation, morphology oꢁ CFA particles
becomes irregular and surꢁace roughness, agglomeration
have also increased as seen in TEM image oꢁ MFA-30
(Fig. 8a). TEM micrograph oꢁ PMFA-15 (Fig. 8b) showed
ꢃne dispersion oꢁ PMA particles on CFA surꢁace. These
results were in agreement with SEM analysis. Particle size
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Chemical Papers
Fig. 4 FT-IR spectra oꢁ a CFA, b MFA-30 and c TMFA
Fig. 5 FT-IR spectra oꢁ a TMFA, b PMFA-5, c PMFA-10, d PMFA-15 and e PMFA-25
oꢁ loaded species could not be measured by TEM image
because oꢁ their greater dispersion on the support.
was evolved (Devassy et al. 2005), generated by extraction
oꢁ an oxygen atom ꢁrom the Keggin anion by two protons
(Laila et al. 2006). As exempliꢃed by these results, it can be
concluded that both samples were thermally stable and did
not decompose into their constituents even at higher tem-
perature. It can also be said that inherent Keggin structure
embedded on MFA-30 surꢁace remains preserved in PMFA-
15 (Pawelec et al. 2004).
The TGA analyses oꢁ both MFA-30 and PMFA-15
(
Fig. 9) comprised oꢁ two weight loss regions. In ꢁirst
region, sharp weight loss appeared due to evolution oꢁ
physically bonded water molecules at temperature up to
1
00 and 300 °C, respectively. As the temperature increased
above 400 °C, chemisorbed water or constitutional water
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Chemical Papers
Fig. 6 Pyridine adsorbed FT-IR spectra oꢁ a PMFA-5, b PMFA-10, c PMFA-15 and d PMFA-25
(
Fig. 11b) due to presence oꢁ microcrystalline PMA. A
peak at − 12.812 ppm could be assigned to phosphorous in
Keggin unit oꢁ PMA (Devassy et al. 2005). Another sharp
peak at 19.705 ppm can be ascribed to P O (phosphorous
2
5
oxide) resulting ꢁrom decomposition oꢁ polyoxometallates
(
Khayoon and Hameed 2012). Upꢃeld and downꢃeld shiꢁts
3
1
were seen in case oꢁ P MAS NMR spectrum oꢁ PMFA-15
catalyst (Fig. 11c), due to interaction oꢁ PMA with silica-
alumina network oꢁ CFA. The perseverance oꢁ sharp signal
at 0.283 ppm compared with that oꢁ pure PMA indicated
that PMA Keggin units remain intact on CFA surꢁace even
aꢁter calcination. Peaks around -30 and+30 ppm were due
to ꢁormation oꢁ aluminophosphates, mainly amorphous in
Fig. 7 SEM micrograph oꢁ PMFA-15
3
1
nature (Nakata et al. 1986; Corma et al. 1994). P MAS
NMR spectrum oꢁ PMFA-15 was more comparable to pure
PMA rather calcined PMA suggesting the conservation oꢁ
characteristic Keggin units oꢁ PMA during catalyst synthesis
also interpreted by XRD and FT-IR results. All the samples
showing more than one phosphorous resonance discloses the
complicated structure oꢁ PMA.
UV–Vis DRS was used to determine the state oꢁ PMA
species incorporated into silico-aluminate ꢁrameworks
oꢁ CFA. The absorption spectra appeared in 200–800 nm
region and the bands may be ascribed to oxygen to metal
charge transꢁer. The tetrahedral molybdenum exhibits band
centred at 220 nm which was present in spectra oꢁ all cata-
lysts (Fig. 10), becomes more intense on increasing wt. %
oꢁ PMA loading (Vazquez et al. 2000). Whereas another
band appearing in region 450–560 nm in PMFA-10, 15 and
Catalyst evaluation
2
5, might be due to presence oꢁ molybdenum in octahe-
dral environment, also enhance on increasing PMA loading
Romanelli et al. 2004).
The catalytic perꢁormance was tested by acylation reaction
oꢁ isopropyl alcohol by acetyl chloride to give isopropyl
acetate under microwave assisted, single step, solvent-ꢁree
reaction conditions. Reaction was carried out at 100 °C
ꢁor 8 min, power output oꢁ 50 W, taking isopropyl alcohol/
acetyl chloride in molar ratio 1:1.5 and isopropyl alcohol
to catalyst weight ratio oꢁ 10:1. Results given in Table 3
show that CFA, MFA-30 and TMFA did not possess any
catalytic activity ꢁor this reaction. In case oꢁ PMFA-5 and
(
3
1
P MAS NMR spectrum oꢁ pure PMA (Fig. 11a) showed
a major peak at − 0.088 ppm along with a small peak at
0
.528 ppm which revealed uniꢁorm phosphorous environ-
ment in highly hydrated structure oꢁ PMA. On calcination
oꢁ pure PMA at 400 °C ꢁor 3 h, many small peaks or mul-
tiplets were obtained with a major peak at − 2.300 ppm
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Fig. 8 TEM micrographs oꢁ a MFA-30 and b PMFA-15
1
0, lower conversion % was obtained due to presence oꢁ less
Lewis acid sites on catalytic surꢁace, while in PMFA-15,
the conversion% was highest due to the presence oꢁ suꢀ-
cient catalytic active Lewis acidic sites. The conversion%
was again decreased on using PMFA-25 due to blockage oꢁ
surꢁace-active Lewis acidic sites by bulk deposition oꢁ PMA
crystallites as discussed earlier in pyridine adsorbed FT-IR
studies oꢁ PMFA-25.
As suggested by these results, PMFA-15 was chosen as
the main catalyst ꢁor ꢁurther analysis oꢁ catalytic active sites
by using in a series oꢁ acylation reactions oꢁ various aliphatic
alcohols by acetyl chloride. Later on, optimization oꢁ reac-
tion parameters such as efect oꢁ reaction time and tempera-
ture, molar ratio oꢁ reactants, power output was also studied
in order to attain maximum conversion%.
Efect oꢀ reaction temperature
To study the inꢂuence oꢁ reaction temperature on conver-
sion%, acylation oꢁ aliphatic alcohols was conducted at diꢁ-
ꢁ
erent temperatures ranging ꢁrom 70 to 140 °C ꢁor 6–10 min.
The results illustrated that the conversion% oꢁ alcohols grad-
ually increased with increase in temperature as shown in
Fig. 12. The maximum conversion% was obtained at 90 °C
ꢁor isopropyl and t-butyl alcohol, while ꢁor isobutyl and
isoamyl alcohol maximum conversion% was attained at 110
and 120 °C, respectively, which got decreased on increasing
temperature. Boiling points oꢁ most oꢁ reactants is less, so at
higher temperature reactants remain in gaseous phase, and
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Chemical Papers
Fig. 9 TGA curve oꢁ a MFA-30
and b PMFA-15
Fig. 10 UV–Vis DRS spectra oꢁ
PMFA-5, 10, 15 and 25
thus, contact period between them decreases, resulting in
adverse efect on conversion%.
Efect oꢀ molar ratio oꢀ reactants
The efect oꢁ molar ratios oꢁ aliphatic alcohols and acetyl
chloride on conversion% was monitored at diferent molar
ratios varying ꢁrom 1:1 to 2:1 as shown in Fig. 14. Very
less conversion% was observed at 1:1 molar ratio due to
insuꢀcient amount oꢁ both reactants to react with each
other. In case oꢁ excess amount oꢁ any reactant, conver-
sion% again decreased. When excess oꢁ acetyl chloride
was taken, then chances oꢁ C-acylation increased which
decreased selectivity oꢁ reaction towards ester ꢁormation.
The conversion% oꢁ alcohols was ꢁound maximum at 1:1.5
molar ratio oꢁ aliphatic alcohols and acetyl chloride.
Efect oꢀ reaction time
The variation oꢁ reaction time on conversion% was carried
out in the range oꢁ 4 to 12 min at 90, 110 and 120 °C ꢁor iso
propyl, t-butyl, iso butyl and iso amyl alcohol, respectively, as
shown in Fig. 13. It was ꢁound that in ꢃrst 10 min, the conver-
sion% increased linearly giving highest conversion% at 6, 8
and 10 min which remained almost constant ꢁurther. In these
reactions, chances oꢁ ꢁormation oꢁ di or tri-acylated products
are very less, so even iꢁ contact time between two reactants
was increased, no signiꢃcant efect was seen on conversion%.
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Fig. 11 Solid state 31P MAS NMR spectra oꢁ a PMA, b calcined PMA and c PMFA-15
Table 3 Catalytic activity oꢁ
Catalysts
Conversion (%)
CFA, MFA-30, TMFA and
PMFA-x catalysts ꢁor acylation
test reaction
CFA
MFA-30
TMFA
PMFA-5
PMFA-10
PMFA-15
PMFA-25
Nil
Nil
Nil
68
80
88
82
Reaction
Time=8
conditions:
min;
tempera-
ture=100 °C; power out-
put=50 W; molar ratio
Fig. 12 Variation oꢁ conversion (%) oꢁ alcohols over PMFA-15 with
temperature. Reaction conditions: Time=6–10 min; molar ratio
(alcohol/acetyl chloride=1:1.5); power output=60 W; substrate/cata-
lyst ratio=10:1
(
isopropyl alcohol/acetyl chlo-
ride=1:1.5); substrate/catalyst
ratio=10:1
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Chemical Papers
Fig. 13 Variation oꢁ conversion (%) oꢁ alcohols over PMFA-15 with
Fig. 15 Variation oꢁ conversion (%) oꢁ alcohols over PMFA-15
with power output. Reaction conditions: Temperature=90–120 °C;
time=6–10 min ꢁor iso propyl, t-butyl, iso butyl and iso amyl alco-
hols, respectively; molar ratio (alcohols:acetyl chloride=1:1.5); sub-
strate/catalyst ratio=10:1
time. Reaction conditions: Temperature=90–120 °C; molar ratio
(
alcohol/acetyl chloride=1:1.5); power output=60 W; substrate/cata-
lyst ratio=10:1
conversion%. On comparing with some previously reported
catalysts it was ꢁound that PMFA-15 gave higher conver-
sion% oꢁ desired products in these acylation reactions (Secen
and Kalpar 1999; Lugemwa et al. 2013; Umamaheshwari
et al. 2001).
Proposed reaction mechanism
The proposed structure ꢁor active sites oꢁ PMFA-15 is
depicted in Scheme 2. Oxygen atoms attached with molyb-
denum atom attract electron density ꢁrom the molybdenum
making it electron deꢃcient and generated Lewis acidity
on CFA surꢁace. The possible pathway ꢁor acylation oꢁ ali-
phatic alcohols by acetyl chloride catalyzed by PMFA-15
is shown in Scheme 3. The Lewis acidic sites oꢁ PMFA-15
help in ꢁormation oꢁ acylium ion ꢁrom acetyl chloride. This
intermediate in turn reacts with aliphatic alcohols to ꢁorm
corresponding esters.
Fig. 14 Variation oꢁ conversion (%) oꢁ alcohols over PMFA-
1
5 with molar ratio oꢁ reactants. Reaction conditions: Tempera-
ture=90–120 °C; time=6–10 min ꢁor iso propyl, t-butyl, iso butyl
and iso amyl alcohols, respectively; power output=60 W; substrate/
catalyst ratio=10:1
Efect oꢀ power output
The efect oꢁ power output oꢁ microwave instrument on
conversion% was studied at diferent power outputs ranging
Regeneration oꢀ catalyst
ꢁ
rom 40 to 80 W as shown in Fig. 15. The conversion% oꢁ
alcohols was ꢁound maximum at 60 W suggesting that this
much power oꢁ microwave irradiations were adequate ꢁor
acylation oꢁ alcohols with highest conversion%. On increas-
ing power output ꢁrom 60 to 80 W no sustainable change was
seen in terms oꢁ conversion%.
The catalyst was ꢁiltered, washed with acetone, dried in
oven at 110 °C/12 h and then calcined at 400 °C/1 h ꢁor
its use in next reaction cycles. The regenerated catalyst
showed eꢁꢁicient catalytic activity up to consecutive 5
reaction cycles giving almost similar conversion% oꢁ iso
propyl alcohol (Table 5), which indicated that the acidic
sites did not get deactivate during regeneration. The con-
version% was decreased ꢁurther, due to the deposition oꢁ
carbonaceous materials on the surꢁace oꢁ catalyst which
might have blocked the surꢁace active sites oꢁ catalyst
The maximum conversion% oꢁ products in a series oꢁ
acylation reactions under optimized reaction conditions
is given in Table 4. On increasing carbon chain, conver-
sion% increased but in case oꢁ isoamyl alcohol, conversion%
decreases. Same efect was also seen on taking branched
aliphatic alcohol, i.e. t-butyl alcohol, because such alcohols
obstructed entrance oꢁ acetyl chloride towards catalytic
active sites, thus reduced the ꢁormation oꢁ acylium ions
which resulted in decrease in rate oꢁ reaction as well as
−
1
can be inꢁerred by two peaks around 2850 cm in FT-IR
spectrum oꢁ regenerated PMFA-15 (Fig. 16b) attribut-
ing to CH symmetrical stretching vibrations oꢁ aliphatic
2
hydrocarbons (Preston et al. 2011). Loss in activity can
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Chemical Papers
Table 4. Synthesis oꢁ various esters by PMFA-15 catalyzed reactions between diferent aliphatic alcohols and acetyl chloride under microwave
irradiated reaction conditions
Entry
Alcohols
Acetyl chloride
Products
O
Conversion (%)
OH
O
1
CH -C
94
3
Cl
O
O
O
O
O
96
2
3
4
CH -C
OH
3
Cl
OH
O
92
91
CH -C
3
O
Cl
O
O
OH
CH -C
3
Cl
O
Reaction conditions:Temperature=90–120 °C; time=6–10 min ꢁor iso propyl, t-butyl, iso butyl and iso amyl alcohols, respectively; molar ratio
aliphatic alcohol/acetyl chloride=1:1.5); power output=60 W; substrate/catalyst ratio=10:1
(
Scheme 2. Proposed structure
or active sites oꢁ PMFA-15
ꢁ
also be ꢁurther conꢁirmed by decrease in intensity oꢁ band
by Sheldon’s hot ꢁilteration test (Hada et al. 2020). The
results showed that the reaction stops on ꢁiltering oꢁꢁ the
catalyst in mid oꢁ the reaction, hence it was conꢁirmed
that active PMA species responsible ꢁor catalytic activity
did not get leach oꢁꢁ during course oꢁ reaction.
−
1
in the region oꢁ 3600–3300 cm in regenerated PMFA-
1
5, responsible ꢁor catalytically active –OH groups. The
stability, heterogeneous nature oꢁ PMFA-15 catalyst
and possibility oꢁ leaching oꢁ highly soluble PMA spe-
cies in organic reaction medium was ꢁurther analyzed
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Chemical Papers
Scheme 3. Proposed mecha-
nism oꢁ acylation oꢁ aliphatic
alcohols by acetyl chloride over
PMFA-15
Table 5 Acylation oꢁ iso propyl alcohol by acetyl chloride over ꢁresh
Conclusions
and regenerated PMFA-15
The present investigation elaborates the synthesis oꢁ
another novel, eꢀcient, economic, regenerable CFA sup-
ported solid acid catalyst by loading oꢁ PMA on mechani-
cally and thermally activated CFA. Surꢁace roughness,
speciꢃc suraꢁce area and silanol groups oꢁ CFA increased
by mechanical and thermal activations ꢁacilitates eꢀcient
loading oꢁ PMA. The prepared PMFA-15 catalyst having
suꢀcient Lewis acidic surꢁace sites successꢁully catalyzed
a series oꢁ microwave assisted acylation reactions, giv-
ing excellent conversion% and is comparable to the cata-
lytic activity oꢁ other industrially viable solid catalysts.
Moreover, the catalyst was ꢁound to be extremely stable
under microwave irradiations, suggesting a less explored
area oꢁ catalysis ꢁor CFA supported catalysts. The catalyst
could be reused aꢁter simple thermal treatment up to ꢃve
reaction cycles without signiꢃcant loss in its activity. This
comprehensive catalytic perꢁormance establishes CFA,
Reaction cycles
Conversion (%)
Fresh
I
II
III
IV
V
94
94
92
92
90
87
Reaction conditions:Temperature=90 °C; time=10 min.; power out-
put=60 W; molar ratio (iso propyl alcohol/acetyl chloride=1:1.5);
substrate/catalyst ratio=10:1
an abundant solid waste, into an eꢀcient heterogeneous
acid catalytic system ꢁor microwave assisted acylation
reactions.
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Chemical Papers
Fig. 16 FT-IR spectra oꢁ a PMFA-15 and b regenerated PMFA-15
Acknowledgements XRD, UV‒Vis DRS characterizations were per-
HPA catalysts. J Catal 228:225–233. https://doi.org/10.1016/j.
jcat.2004.08.010
ꢁormed at UGC-DAE Consortium ꢁor Scientiꢃc Research, Indore. The
authors are thankꢁul to Dr. Mukul Gupta ꢁor XRD analysis, Dr. T.
Shripathi ꢁor UV‒Vis DRS analysis, University oꢁ Pune ꢁor TGA, BET
surꢁace area and SEM analysis, SAIF-Chandigarh ꢁor TEM analysis and
Badday AS, Abdullah AZ, Lee KT (2013) Ultrasound-assisted trans-
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org/10.1016/j.apenergy.2013.01.028
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1
IIT-Bombay ꢁor solid state P MAS NMR analysis.
Bai G, Li T, Yang Y, Zhang H, Lan X, Li F, Han J, Ma Z, Chen
Q, Chen G (2012) Microwave-assisted Friedel-Craꢁts acylation
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org/10.1016/j.catcom.2012.09.028
Compliance with ethical standards
Conꢁlict oꢁ interest On behalꢁ oꢁ all authors, the corresponding author
states that there is no conꢂict oꢁ interest.
Bhandari R, Volli V, Purkait MK (2015) Preparation and characteri-
zation oꢁ ꢂy ash based mesoporous catalyst ꢁor transesteriꢃca-
tion oꢁ soyabean oil. J Environ Chem Eng 3(2):906. https://doi.
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Chemical Papers
Authors and Aꢀliations
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Sakshi Kabra Malpani · Deepti Goyal · Stuti Katara · Ashu Rani
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Ashu Rani
Department oꢁ Applied Chemistry, School oꢁ Vocational
Studies and Applied Sciences, Gautam Buddha University,
Greater Noida, UP, India
ashu.uok@gmail.com
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Department oꢁ Chemistry, Jyoti Nivas College Autonomous,
Bengaluru, Karnataka, India
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Department oꢁ Pure and Applied Chemistry, University
oꢁ Kota, Kota, Rajasthan 324005, India
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