Manifestation of zeolitic pore characteristics of modified montmorillonite in oxazole synthesis by propargylation and cycloisomerization reactions

Article abstract of DOI:10.1016/j.apcata.2015.02.036

Zeolitic micropores are found to be developed on montmorillonite clay by treatment with phenoldisulphonic acid (PDSA). The micropores developed have the ability to enhance the cycloisomerization of the alkynylamide to form oxazole product. Pore characteristics by surface measurement techniques and acidity by pyridine adsorption using FT-IR were evaluated. Surface measurements showed significant increase in the surface area, acidity and the micropore volume but neither of them correlated well with the cycloisomerization activity. Variation in Br?nsted and Lewis acidity and pore volume by surface coverage using Br?nsted (B) and Lewis (L) acid compounds, caused variation in cycloisomerization activity. Best correlation with the amount of oxazole formed was observed when volume accessibility factor (VAF), a product of micropore volume and ratio of B to L acidity were considered. Based on this, a reaction scheme for cycloisomerization is proposed involving both B and L sites within the micropores.

Full text of DOI:10.1016/j.apcata.2015.02.036

Accepted Manuscript
Title: Manifestation of Zeolitic pore characteristics ofmodified
montmorillonite in oxazole synthesis by propargylation and
cycloisomerization reactions
Author: N.J. Venkatesha Y.S. Bhat B.S. Jai Prakash
PII:
S0926-860X(15)00134-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2015.02.036
APCATA 15278
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
4-2-2015
19-2-2015
21-2-2015
Please cite this article as: N.J. Venkatesha, Y.S. Bhat, B.S.J. Prakash, Manifestation
of Zeolitic pore characteristics ofmodified montmorillonite in oxazole synthesis by
propargylation and cycloisomerization reactions, Applied Catalysis A, General (2015),
http://dx.doi.org/10.1016/j.apcata.2015.02.036
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Highlights for review
Zeolitic pore formation in montmorillonite upon PDSA treatment.
Cycloisomerization of alkynylamide to oxazole is enhanced by zeolitic pore.
There is a good correlation between VAF and oxazole formed.
VAF is a term which is a product of acidity and micropore volume.
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Graphical Abstract:
2
Page 2 of 26

Manifestation of Zeolitic pore characteristics ofmodified montmorillonite in oxazole synthesis by
propargylation and cycloisomerization reactions
N.J Venkatesha, Y.S. Bhat*, B.S. Jai Prakash
Chemistry Research Centre, Bangalore Institute of Technology, K.R. Road,
Bangalore 560 004, India
*Corresponding author: E-mail-bhatys@yahoo.com,
Tel.: +91 80 26615865; fax: +91 80 26426796.
Abstract:
Zeolitic micropores are found to be developed on montmorillonite clay by treatment with
phenoldisulphonic acid (PDSA). The micropores developed have the ability to enhance the
cycloisomerization of the alkynylamide to form oxazole product. Pore characteristics by surface
measurement techniques and acidity by pyridine adsorption using FTIR were evaluated. Surface
measurements showed significant increase in the surface area, acidity and the micropore volume but
neither of them correlated well with the cycloisomerization activity. Variation in Brønsted and Lewis
acidity and pore volume by surface coverage using Brønsted (B) and Lewis (L) acid compounds,caused
variation in cycloisomerization activity. Best correlation with the amount of oxazole formed was
observed when volume accessibility factor (VAF), a product of micropore volume and ratio of B to L
aciditywere considered. Based on this, a reaction scheme for cycloisomerization is proposed involving
both B and L sites within themicropores.
Keywords: montmorillonite, phenoldisulfonic acid, propargylation, cycloisomerization, zeoliticpore.
1. Introduction:
Nucleophilic substitution and cyclization are two major reactions adopted in chemical industries[1-4].
Cycloisomerization reactions are important routes for the synthesis of heterocyclic compounds. These
3
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compounds being specialty chemicals find applications in pharmaceutical industry[4-6].Generally,
heterocyclic compounds are prepared by condensation of amines and carbonyl compounds such as acid,
ester, anhydrides and acid chloridesin the presence of homogeneous catalysts like, HCl, H2SO4, H3PO4,
TFA, p-TSA, BF3, acetic acid and pyridine in acetic acid[7-10].Metal complexes such as Cp*RuCl(μ2-
SMe)2RuCp*Cl]/AuCl3/NH4BF4[11] and Zn(OTf)2/ pRuPPh3(CH3CN)2PF6[12] have also been tried to
catalyze the cycloisomerization.
More powerful and useful transformations are possible when nucleophilic substitution and cyclization are
combined in a one-pot procedure. Propargylic substitution and subsequent cycloisomerization are good
examples of such transformations[1,13-16].
Solid acid catalysts are widely used in industrial practices for their easy handling, less cost, eco-friendly
and easy modification[5,17].In recent times, inorganic solid acid-catalyzed organic transformations are
gaining much attention due to the proven advantage of heterogeneous catalysts, like simplified product
isolation, mild reaction conditions, high selectivity, ease in recovery, reuse of the catalysts and reduction
in the generation of waste by products[18–20]. In this context, we have been investigating various
industrially important organic reactions aimed at adopting solvent-lesssingle step techniques and
replacing toxic and corrosive reagents with innocuous chemicals in presence of environmentally benign
solid acid catalysts[5].Over the past few years, the modifiedclaybased solid acids have been receiving
much attention, among other solid acids such as zeolites, heteropolyacids,ion exchange resins andmetal
oxides, due to their superior catalytic activity for hydrocarbon conversions[3, 21, 22].
Emphasis of preset work is on to modify the montmorillonite clay by treatment with phenoldisulfonic
acid (PDSA) and with an objective to study its performance as a bi-functional catalyst for propargylation
and subsequent cycloisomerization to form substituted oxazole product.
4
Page 4 of 26

2. Experimental
2.1Catalyst preparation
Montmorillonite clay obtained from Bhuj area, Gujarath was supplied to us by Ashapura Group of
Industries, India[23]. The parent clay was converted to homoionicby treatment with 1M NaCl solution,
and idealized structural formulaSi4[Al1.432Fe0.416Mg0.266] O10(OH)2Na(0.318)was evaluated using ICP
elemental analysis. This clay sample was used further for treatment with PDSA to improve the surface
area and acidity.
The modification was done by following reported procedure [23-25], 100ml of 1M PDSA solution was
taken in 250ml round bottom flask. In this,10g of montmorillonite clay was dispersed completely and the
mixture was subjected to microwave irradiation for 30min at 1100C by applying 800 watt energy.After
cooling, the mixture was centrifuged and the solid was repeatedly washed to completely remove PDSA,
dried at 1200C ground and then sieved with mesh size 0.075mm. Similarly, samples were also prepared
by treating with 0.1M and 0.5M PDSA. The treated samples were designated as 0.1PDSA, 0.5PDSA and
1PDSA.
The 1PDSA-clay was further loaded with 5-20 % PDSA by wet impregnation method[26].The procedure
briefly involved dispersing 5 g of the 1PDSA sample in 100 ml of water with required amount of PDSA
dissolved. The mixture was stirred using a magnetic stirrer overnight, water was evaporated on a hot plate
to get the PDSA-loaded solid catalyst samples.These were designated as 1PDSA(5%), 1PDSA(10%),
1PDSA(15%) and 1PDSA(20%).
2.2 Characterization
The prepared catalysts were characterized using pyridine-FT-IR, NH3-TPD, XRD, SEM and BET surface
area and Magic Angle Spin 27Al - NMR.
Acidity was measured by FT-IR spectroscopy using pyridine as probe molecule. All the samples were
activated by degassing at 1200C for 2 h and then saturated with pyridine. The catalyst samples were then
evacuated at 1150C for 30min to remove physisorbed pyridine. FT-IR spectra of the samples were then
5
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recorded in the range 1400–1600 cm-1 using IRAffinity-1 spectrophotometer having resolution of 4 cm-1
with 40 scans[7,23].
Surface area and porosity measurements were done using Quantachrome Nova-1000 surface analyzer
0
under liquid nitrogen temperature. N2 adsorption-desorption isotherm measurements at -193 C were
carried out in order to study the evolution of porosity and textural properties of the acid treated clay
samples. Pore size distribution was calculated from adsorption data by inferring pore geometry and using
deBoer t-plot and BJH method[9, 23].
Structural integrity of the catalyst samples after the reaction was checked by powder XRD. The data were
recorded by step scanning at 2= 0.0200 per step from 30to 120 on Shimadzu MAXima_X XRD-7000 X-
ray diffractometer with graphite monochromatized Cu Kα radiation (λ=1.5406 Ǻ)[2,3].
The NH3-TPD measurements were performed on the catalyst samples. In a typical experiment, 0.1 g of
catalyst was taken in a U-shaped, flow through, quartz sample tube. Prior to measurements, the catalyst
0
was pretreated inHe (30 cm3/min) at 150 C for 2 h. A mixture of NH3 in He (10%)was passed (30
0
cm3/min) at 60 C for 30 min. Then, the sample was subsequently flushed with He(30 cm3/min) at 60 0C
0
for 1 h.The TPD measurements were carried out in the range 100–7000C at a heating rate of 10 C/min.
Ammonia concentration in the effluent was monitored with a gold-plated, filament thermal conductivity
detector[24,27].
Morphology of parent, 1PDSA-clay and 1PDSA (5%) clay were recorded using JEOL JSM-6490
Scanning Electron Microscope (SEM) under high vacuum mode.
27Al-MAS NMR pattern were recorded using single pulse Jeol ECX-2 400MHz solid state magic angle
spin nuclear magnetic resonance spectrometer with 4mm rotor of spinning speed 104. The standard used
in the measurement was Aluminium nitrate.
The amount of interlayer aluminium was estimated by the method reported elsewhere [27]. It involved
stirring 2 g of clay sample with 0.1M HCl for 60min at 60 0C. Then the mixture was centrifuged and the
residue was repeatedly washed with deionized water. The centrifugate was used to estimate Al by
aluminon reagent method spectrophotometrically using Chemito UV-2100 Spectrophotometer[23,24, 28].
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2.3 Catalytic activity
The activity of catalyst samples was studied under microwave irradiation, using microwave lab station for
synthesis, “START-S” model from Milestone, Italy, having software which enables the control of
reaction mixture temperature with aid of infrared sensor by regulation of microwave power output in such
a way that, the reaction mixture was exactly in line with infrared sensor that monitors the temperature.
Variable power up to 1200W was applied by microprocessor-controlled single-magnetron system[2,3].
Ten mmol of Propargyl alcohol (PA) and ten mmol of amide were mixed with the catalyst in a microwave
reactor vessel with magnetic stirring bar. The vessel was kept in microwave reactor and an initial power
of 1200 W was applied for 1 minute to attain the reaction temperature of 1500C. The reaction
temperature was then maintained for 29 minutes to get maximum conversion. The reaction mixture was
then cooled for 10 minutes and extracted by stirring with 5 mL of ethanol for 5 minutes and filtered to
remove the catalyst. The ethanol extract containing the product and the unused reactants were analyzed
using Chemito GC-1000 gas chromatograph with TR-Wax capillary column of length 30m, 0.32mm
thickness and 0.55μm internal diameter with flame ionization detector[3,23]. The products were also
confirmed by GC-MS.
Catalyst characteristics and activity measurement were performed three times on each sample and the
mean of the measurements were used for calculating the standard deviation.
O
N
OH
+
Propargyl alcohol
H N
R
2
R
PDSA-clay
O
Oxazole
Amide
NH
Propargylation
Cycloisomerization
O
Alkynyl amide
Scheme 1 Reaction between PA and amide to form Oxazole in presence of PDSA-clay.
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Oxazole formation involves two steps, first step being condensation between propargyl alcohol and
acetamide(propargylation) forming alkynyl amide as primary product. In the second step, primary product
on contact with acidic sites gets cycloisomerized to form oxazole as secondary product.
3. Results and discussion:
3.1PDSA treatment of montmorillonite; zeolitic pores
Surface measurement values and other properties of the catalyst samples are given in Table 1.
Table.1 CEC, surface area, average pore diameter and micropore volume of unmodified, Al-clay and
PDSA treated clays.
SBET
Vm
Vtm
Clay samples
CEC
APD
Interlayer
Al (meq/g)
-
(meq/g)
(m2/g)
(Ǻ)
(cm3/g)
(cm3/g)
Untreated clay
0.83
27
50.3
0.011
0.067
Al-clay
0.1 PDSA
0.5 PDSA
1PDSA
0.83
0.79
0.75
0.71
27
50.3
0.011
0.067
0.074
0.132
0.14
0.83
0.49
0.62
0.70
109.14 44.13 0.026
196.09 33.34 0.064
276.44 26.04 0.203
*SBET-BET surface area, APD-average pore diameter, Vm - total pore volume, Vtm - micropore volume by t-plot method.
Untreated and Al- exchange clays had similar physico-chemical characteristics,a CEC of 0.83-0.85meq/g,
surface area of 28-32 m2/g, average pore diameter (50.27 Ǻ) and micropore volume (0.011 cm3/g) with
the exception in the interlayer Al content. On the other hand, PDSA treatment of montmorillonite clay
samples showed differences in physico-chemical characteristics such as (i) surface area, (ii) pore sizeand
pore volume (iii) CEC and (iv) interlayer Al and acidity in relation with both parent and ion-exchange
clay samples. It is reported that organic acids and chelating agents remove the structural Al, Fe and Mg
from the octahedral layer of the parent clay resulting in micropores onthe clay surface enhancing its
surface area[23,24]. The structural elements get dislodged by forming complex species which get
hydrolyzed on subsequent washing and release a part of the cations into the interlayer. The hydrated Al
and Fe ions, which displace the sodium ions in the interlayer act as acid sites, thus enhancing the
acidity[23,24]. Removal of structural elements would create voids in the structure resulting inincrease in
microporosity, micropore volume and surface area. It is known that when structural aluminium is
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removed, there is no reduction inCEC, while the removal of isomorphously replaced Fe and Mg causes a
reduction in CEC.
Fig. 1 (a) and (b) show the BJH curves and the t-plots for the untreated and PDSA treated clays at
different concentrations, respectively. The two plots reveal an increase in surface area, formation of
micropores and decrease in average pore diameter compared to untreated clay.
The 27Al MAS NMR spectra of the clay samples treated with PDSA of different concentrations. Decrease
in intensity of chemical shift value at 6ppm with respect to standard[31] indicates the removal of
octahedral aluminium from the treated clay samples as the concentration of PDSA increases from 0.1M to
1M.
14
12
10
8
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0.1PDSA
0.5PDSA
1PDSA
0.1PDSA
0.5PDSA
1PDSA
(b)
Untreated clay
Untreated
(a)
6
4
2
0
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75
Pore Diameter (A)
2
4
6
8
10
12
14
16
18
20
Statistical Thickness (Ao)
Fig 1(a) and (b) BJH curves and t-plots of untreated, 0.1, 0.5 and 1M PDSA treated montmorillonite clay.
3.1.1 Acidity: Acidity was measured by FTIR technique using pyridine as probe molecule. The results are
shown in Fig. 2. Treatment with increased amounts of PDSA exhibited an increase in the Brønsted (1545
cm-1) and Lewis acidity (1445 cm-1) of the clay samples. This is attributed to the removal of increased
amounts of structural Al from the octahedral layer, part of them replacing the interlayer Na ions [23,24].
Clay sample treated with 1 M PDSA had almost the same acidity as Al-clay.
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a
b
c
%T
d
1600 1580 1560 1540 1520 1500 1480 1460 1440 1420 1400
Wavenumber (cm-1)
Fig.2 FT-IR spectra of adsorbed pyridine on (a) Al-clay, (b) 1PDSA, (c) 0.5 PDSA and (d) 0.1 PDSA.
3.2 Propargylation of acetamide under microwave heating
The reaction conditions were optimized by varying parameters like reaction time, temperature, mole ratio
of reactants and catalyst amount for the propargylation and cycloisomerization of PA with acetamide
(AA). Maximum conversion was observed at mole ratio of PA:AA of 1:2 with 1PDSA-clay. Under the
experimental conditions, interphase and intraparticle heat and mass transfer as calculated using Koros-
Novak, Mears and Weisz- Prater criteria were negligible [31, 32].
3.2.1 Effect of catalyst amount.
In the absence of catalyst, no reaction was observed between PA and AA. Fig. 3 shows conversion of PA
to the products alkynyl amide and oxazole under the specified conditions when the catalyst amount was
varied from 0.1 to 1 g. The product formed increased with the increase in the catalyst quantity showing
equimolar concentrations of the products when the catalyst used was 0.6 to 0.7 g. With lower catalyst
amount, (<0.3 g), lower conversion of PA was observed; most of the conversion was towards the
condensation product (alkynylamide) with less yield of cyclized product (oxazole). Increase in the
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catalyst amount to 1 g showed an increase in PA conversion to 65% comprising of 40% oxazole and 25%
alkynyl amide. It could be clearly seen that the formation of the condensation product alkynyl amide is
favored at lower catalyst amounts and the cycloisomerized oxazole formation enhanced at higher catalyst
amounts.
70
% PA conversion
yield of Akynyl amide
60
Yield of Oxazole
50
d
l
e
i
y
/
n
40
o
i
s
r
e
v
30
n
o
C
%
20
10
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Catalyst amount (g)
Fig .3 Effect of catalyst amount on the propargylation and cycloisomerization of PA and AA
Reaction conditions: reaction time; 60 min, reaction temperature 150oC; reactants molar ratio PA:AA;
1:2.
3.2.2 Effect of temperature.
Fig.4 shows that temperature had a significant effect on the PA conversion which ranged from 11% to
0
0
70% when the temperature is raised from 80 C to 170 C. At lower temperature of 800C, 11% of PA
conversion was observed with 9% alkynyl amide, rest being oxazole. Further increase in temperature
showed increase of PA conversion with formation of equimolar amounts of the products at 1400C.
0
Beyond 140 C the amount of alkynyl amide decreased with proportionate increase in the oxazole. It
appears that the condensation is more selective to form alkynyl amide at lower temperature and
cyclisation of alkynyl amide to form oxazole predominates at higher temperatures.
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70
60
50
40
30
20
10
0
% PA conversion
yield of Akynyl amide
Yield of Oxazole
d
l
e
i
y
/
n
o
i
s
r
e
v
n
o
C
%
80
100
120
140
160
180
Temperature (oC)
Fig .4 Effect of temperature on the propargylation and cycloisomerizationof PA and AA.
Reaction conditions: catalyst amount; 1 g, reaction time; 60min, reactants molar ratio PA: AA; 1:2.
3.2.3 Effect of reaction time.
Reaction time as a function of conversion of PA under optimized reaction conditions are shown in Fig.5.
Initially, conversion of PA was observed to be 15%. As the reaction proceeds, the conversion of PA
gradually increased reaching a maximum of 65% at 60 min. Further increase in irradiation time, resulted
in no significant change in the conversion. At reaction time between 10 and 40 minutes,the reaction
mixture was found to contain more amount of alkynyl amide (28%) with 17% oxazole. Further increase in
reaction time, resulted in the increase in oxazole amount and at the end of 240 min,the yield of oxazole
was nearly 50%, rest being alkynyl amide. Apparently, cycloisomerization to form the oxazole product is
facilitated by the surface of the catalyst with progress in time.
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70
60
50
40
30
20
10
0
d
l
e
i
y
/
n
o
i
s
r
% PA conversion
yield of Akynyl amide
Yield of Oxazole
e
v
n
o
C
%
0
20 40 60 80 100 120 140 160 180 200 220 240 260
Reaction time (min)
Fig .5Effect of Reaction time on the propargylation and cycloisomerization of PA and AA
Reaction conditions: catalyst amount; 1 g, reaction temperature 150oC; reactants molar ratio PA:AA; 1:2.
3.3 Comparison of PDSA treated and Al-exchange catalyst samples
Reactions were conducted under the chosen conditions which yielded maximum amount of the Oxazole
0
product. This involved use of 1 g catalyst, 150 C, PA:AA mole ratio of 1:2, reaction time 60 minutes
under MW irradiation. Results are given in Table2.
Table2 Comparison of TPD-NH3 acidity, conversion of PA and yield of products with Al-clay, PDSA
treated, BEA zeolite and different PDSA loaded clay catalyst samples.
Catalyst
TPD-NH3
(µmol/g)
% PA
conversion Alkynyl
amide
%
% yield
of
Oxazole
Average
TOF
Vtm
SBET(m2/g
(cm3/g)
)
(min-1)
Al-Clay
250
78
46
30
36
18
10
12
0.011
0.026
0.066
0.256
27
0.1PDSA
109
0.5 PDSA
1 PDSA
196
236
500
228
248
262
232
44
65
70
67
70
60
48
20
25
15
21
20
28
36
24
40
55
46
50
32
12
0.064
0.203
0.305
0.104
0.064
0.019
0.004
0.256
0.282
0.432
0.365
0.348
0.154
0.10
196
276
552
213
146
106
70
BEA
1PDSA (5%)
1PDSA (10%)
1PDSA (15%)
1PDSA (20%)
Reaction conditions: catalyst amount; 1 g, reaction temperature 150oC; reactants molar ratio PA:AA; 1:2,
reaction time 60min. *SBET-BET surface area, Vtm-micropore volume by t-plot method.
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It could be seen from Table 2 that the catalytic activity for the conversion of PA increased with the
increase in concentration of PDSA used for treatment from 0.1 to 1M. A corresponding increase in the
alkynyl amide and the oxazole products was observed. This is attributed to the increase in the acidity of
the PDSA treated samples as measured by pyridine FTIR method (Fig 2). Al-clay although had
comparable acidity with 1PDSA, showed lower propargylation and cycloisomerization products. The
enhanced catalytic activity of 1PDSA having relatively large pore volumes and insignificant
cycloisomerized product in the case of Al-exchange clay having very low micropore volume could be
attributed to the combined effect of acidity and micropore volume. Further investigation was made to
relate the role of micropores and acidity with the amount of oxazole formed.
3.4 Maximization of the oxazole product:
To study the role of Brønsted and Lewis acid sites on the oxazole formation, we impregnated 5 % of
PDSA (Brønsted acid) and 5% each of ZnCl2 and FeCl3 (Lewis acids) on 10 g of 1PDSA-clay separately.
Results are presented in Table 3. The results show that the formation of oxazole decreased on loading
with the Lewis acids, ZnCl2 and FeCl3, while there was a substantial increase in the formation of oxazole
when loaded with 5 % PDSA. Apparently, the Lewis and Brønsted acid centers on the surface are playing
an important role in the formation of oxazole. Scanning electron micrographs(Fig.6) show morphological
changes after acid treatment in1PDSAclay and its partial surface coverage when loaded with 5% PDSA.
Increased loading beyond 15% PDSA on 1PDSA clay showed a decrease in the formation of
cycloisomerized product. This is attributed to enhanced coverage of the micropores upon loading with
more and more amounts of PDSA. This suggests an increase in Brønsted sites within the pores by the
added PDSA, which also accounts for reduction in the pore volume. Apparently, the cycloisomerization
requires the presence of both Brønsted and Lewis acids in the micropores formed. The effect of ratio of
Brønsted to Lewis acidity and micropore volume on the cycloisomerization was further investigated by
correlating the various parameters with the yield of oxazole. Combination of micropore volume and
acidity as explained by volume accessibility factor (VAF) was used [23]. Briefly, the term volume
accessibility factor is related to the product of micropore volume and acidity. VAF has been used to
14
Page 14 of 26

explain reactions involving Brønsted and Lewis acids that take place exclusively in micropores. In the
present study, cycloisomerization is observed to take place on the clay catalyst surfaces only when
micropores which facilitate access to the acid sites are generated. Treatment with organic acids, such as
PDSA displaces structural Al atoms in the clay matrix, thus creating micropores which provide access to
the acid sites. The factor VAF provides a measure of this accessibility for orientation of the molecules to
reach out to the acid sites. It thus determines the ease with which the transformation is brought out within
the micropores and aids the molecule to properly extend to reach out to the acid sites.XRD patterns of
1PDSA claysafter the reaction did not show any d(001) reflection change indicating there is no layer
swelling by intercalation of reactant or product molecules in the interlayer.
The results are given in Table3 which shows correlation results (r2 value) between mesopore volume, total
pore volume and volume accessibility factor and oxazole yield. Very poor correlation with oxazole yield
was observed for Brønsted (B), Lewis (L), (B + L) and (B/L). VAF values obtained by multiplying with
micropore volume showed slightly improved correlations for B, L and (B+L). Best correlation (r2 =
0.911) was seen for B/L. Both Al-exchange clay with low micropore volume and Lewis acid treated
1PDSA clays having relatively high micropore volumes showed lower oxazole formation. On the other
hand, 1PDSA loaded with 15 and 20% PDSA having high B/L values also exhibited decreased oxazole
formation. Neither an increase in Brønsted acidity nor in the Lewis acidity on the clay surface showed a
proportionate increase in the oxazole formation. Apparently, an appropriate combination of micropore
volume and B/L ratio facilitates maximum yield of oxazole. In the present study, 5 to 10 % loaded
1PDSA samples havea right combination of micropore volume and B/L ratio. The VAF(B/L) value of
0.15 and beyond show the best results under the conditions of the experiment.
15
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Fig.6 SEM images of (a) Untreated (b) 1 PDSA and (c) 1 PDSA (5%).
Table3 Correlation between volume accessibility factors and yields of oxazole for the different catalyst
samples
Vtm
Catalyst
% PA
conver
sion
% yield
of
Oxazole
B
L
B+L
B/L
Volume Accessibility Factor
(cm3/g) (µmol/g) (µmol/g)
(VAF)*
Pyridine FT-IR acidity
B
L
B+L
B/L
0.16
0.15
0.06
0.02
0.07
0.06
0.01
0.02
0.07
1PDSA (5%)
1PDSA (10%)
1PDSA (15%)
1PDSA (20%)
1PDSA (Zn-5%)
1PDSA (Fe-5%)
Al-clay
67
70
60
48
54
61
46
30
44
46
50
32
12
16
16
10
12
24
0.104
0.064
0.019
0.004
0.106
0.099
0.011
0.026
0.064
0.207
92
112
123
133
61
56
74
34
52
58
49
38
28
86
92
70
44
50
150
161
161
161
147
148
144
78
1.59
2.29
3.24
4.75
0.71
0.61
1.06
0.77
1.04
9.48 5.97 15.45
7.17 3.14 10.30
2.34 0.72
0.53 0.11
6.47 9.12 15.58
5.54 9.11 14.65
0.81 0.77
0.88 1.14
3.06
0.64
1.58
2.02
6.53
0.1PDSA
0.5PDSA
102
3.23
3.2
Correlation
factor, r2
0.096
0.014
0.077 0.002 0.61 0.03 0.252 0.911
*VAF = Micropore volume × Acidity (Example VAF(B) = 0.103 × 92 = 9.48.
B-Brønsted acidity, L-Lewis acidity.Vtm-micropore volume by t-plot method.
16
Page 16 of 26

Reaction conditions: catalyst amount, 1 g, reaction temperature 150oC; reactants molar ratio, PA:AA; 1:2;
reaction time, 60min.
Table4 Correlation between yield of oxazole formed with mesopore volume, total pore volume and
volume accessibility factorfor mesopore volume(VAF)meso
Catalyst
% PA
conversion
% yield
Volume Accessibility Factor for
Vmeso
Vm
(cm3/g)
(cm3/g) mesopore volume, (VAF)meso
of
Oxazole
Pyridine FT-IR acidity
B
L
B/L
0.28
0.31
0.44
0.55
0.11
0.10
0.07
0.07
0.17
B+L
22.50
21.74
21.90
18.84
22.76
23.68
9.65
1PDSA (5%)
1PDSA (10%)
1PDSA (15%)
1PDSA (20%)
1PDSA (Zn-5%)
1PDSA (Fe-5%)
Al-clay
67
70
60
48
54
61
46
30
44
46
50
32
12
16
16
10
12
24
0.150
0.135
0.136
0.117
0.155
0.160
0.067
0.094
0.132
0.186
0.253
0.199
0.155
0.121
0.261
0.259
0.078
0.121
0.196
0.216
13.8
15.12
16.73
15.56
9.45
8.96
4.96
3.19
6.86
8.70
6.61
5.17
3.28
13.33
14.72
4.69
4.14
6.60
0.1PDSA
7.33
13.46
0.193
0.5PDSA
Correlation factor, r2
0.240 0.001 0.029
Reaction conditions: catalyst amount; 1 g, reaction temperature 150oC; reactants molar ratio PA:AA; 1:2,
reaction time 60min.
*Vtm-micropore volume by t-plot method,Vmeso-mesopore volume.
Table4 shows the (VAF)meso values obtained by multiplying the different acidity values with mesopore
volumes. The results show a very poor correlation with the oxazole yield indicating insignificant role of
mesopores in the cycloisomerization reaction.
3.5 Zeolitic behavior of the modified clay samples: The transformation of alkynyl amide to oxazole is
facilitated by the Brønsted and Lewis acid sites accessed through micropores. To know whether such
transformation could be brought about in zeolites which are known to possess Brønsted and Lewis
acidities within the micropores, the propargylation reaction was studied on a sample of BEA(H-beta)
zeolite. The sample of zeolite having a relatively large micropore volume of 0.305 cm3 g-1 and a B/L ratio
of 2.39[22,33] and a VAF value of 0.72 showed an enhanced oxazole formation (Table 2). This confirms
the role played by zeolitic pores in the modified clay samples used in this study and also the applicability
of VAF to zeolitic systems.
17
Page 17 of 26

3.6 Plausible Reaction Mechanism
The reaction studied involves two steps (i) propargylation to form propargyl amide and (ii)
cycloisomerization of the amide to form the oxazole product. First step is favored by both L and B sites.
Other workers who have studied propargylation have also seen that the increase in either the B or L sites
favor the reaction [1]. Even in the system studied, clay treated with PDSA and also the Al-exchange clay
having both B and L sites show activity for propargylation. Mechanism of step 1 as shown in scheme 2
involves the protonation of the alcohol and nucleophilic attack of the protonated alcohol on the amide
group.
Cycloisomerization, on the other hand, requires both acid sites in such a way that an optimum ratio of B
to L is needed. Mechanism for cycloisomerization step therefore involves both the B and L sites in close
proximity. Such an availability of both B and L sites can happen only in a micropore.
The probability of the reactant molecules to position themselves on B and L sites as shown in scheme 2 is
greater within a pore. Enriching the number of one site over the other will only decrease the step 2. An
optimum ratio of B to L is thus required and is seen within the pores generated due to acid treatment. Al-
clay is not effective in cycloisomerization due to the fact that the requirement of B and L in close
proximity is not fulfilled. It appears that beta zeolite also has such pores with optimum B to L ratio in
close proximity.
Clay
OH2
H2
O
Al
H2
O
OH2
Bronsted
O
O
H
H
Bronsted
Clay
Al
+
2
OH2
OH2
H2
O
CH
+ H
OH
H2
O
NH
O
N
H2N
R
O
N
H
H
H
-
-
O
R
O
R
HC
R
O
Al
Al
Lewis
Lewis
Clay
Clay
H
N
N
H C
R
3
H C
R
O
2
O
Scheme 2.Possible reaction mechanism for the propargylation and cycloisomerization.
18
Page 18 of 26

4. Conclusion: Zeolitic micropores develop on the surface of montmorillonite clay by treatment with
organic acids. The micropores thus developed on the modified clay have the ability to enhance the
cycloisomerization of the intermediate alkynylamide to form valuable oxazole products. Micropores
provide an access to the acid centers for the intermediate product to undergo transformation to the final
product. Variation of Brønsted and Lewis acidity and pore volume on the modified clay surface showed
variation in the oxazole formed. The combinative role of micropore volume and acidity is expressed by a
factor called volume accessibility factor, VAF which is a product of micropore volume and acidity. Best
correlation for the oxazole formation was obtained with VAF (B/L) which indicates an optimal
combination of micropore volume and B/L ratio for higher rate of oxazole formation under the
experimental conditions studied. Thus manifestation of zeolitic pores on the surface of clays by organic
acid treatment can accelerate the transformation of reaction intermediates into value added products.
Furthermore, the VAF component of the micropore could possibly find application in zeolite catalysis.
Acknowledgement:
The authors would like to thank to the Principal and Members of Governing Council of Bangalore
Institute of Technology for the facilities provided and VTU for financial support. The authors also extend
their thanks to L.V Narendra NMRC IISc, Bengaluru for MAS NMR, Raghavendra RVCE for XRD and
PPISR for NH3-TPD measurments.
References :
[1] Y. Pan, F. Zheng, H. Lin, Z. Zhan, J.Org.Chem. 74 (2009) 3148-3151.
[2] S .Ramesh, B. S. JaiPrakash, Y. S. Bhat, Appl.Catal.A. 41 (2012) 157-162.
[3] N. J. Venkatesha, B.M. Chandrashekara, B.S. JaiPrakash, Y.S. Bhat, J.Mol.Catal.A. 392 (2014) 181-
187.
[4] M. Campanati, A. Vaccari, O. Piccolo, Catal.Today. 60 (2000) 289-295.
[5] B.M. Reddy, P.M. Sreekanth, P. Lakshmanan, J.Mol.Catal.A. 237 (2005) 93-100.
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[6]Y. J.Yang, Y. N.Yang, J. S.Jiang,Z. M.Feng,H. Y. Liu,X. D. Pan, P. C. Zhang, Chin.Chem.Lett. 21
(2010) 902-904.
[7] G. P.Romanelli, J. C.Autino, J. Org.Chem. 6 (2009) 359-366.
[8] N.Narender, K.S. K. Reddy, M.A. Kumar, C.N. Rohitha, S.J. Kulkarni. Catal.Let. 134 (2009) 175-
178.
[9] M. Miyashita, A. Yoshikoshi, P.A Grieco, J.Org.Chem. 42 (1977) 3772-3774.
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[11] M. D. Milton, Y. Inada, Y. Nishibayashi, S. Uemura, Chem. Com. 23 (2004) 2712-2713.
[12] M. P. Kumar, R. S. Liu, J.Org.Chem. 71 (2006) 4951-4955.
[13] E. Bustelo, P. H. Dixneuf, Adv.Synth.Catal. 349 (2007) 933-942.
[14] A. Arcadi, S. Cacchi, L.Cascia, G. Fabrizi, F. Marinelli, Org.Lett. 3 (2001) 2501-2504.
[15] V. Cadierno, J. Gimeno, N. Nebra,Adv.Synth. Catal. 349 (2007) 382-394.
[16] Z. Zhan, J. Yu, H. Liu, Y. Cui, R. Yang, W. Yang, J. Li, J.Org. Chem. 71 (2006) 8298-8301.
[17] K. Wilson, J.H. Clark, Pure.Appl.Chem. 72 (2000) 1313-1319.
[18] J.H. Clark, Acc.Chem. Res. 35 (2002) 791-797.
[19] A. Corma, Chem.Rev. 95 (1995) 559-614.
[20] T. Okuhara, Chem.Rev. 102 (2002) 3641-3666.
[21] P. Wipf, Y. Aoyama, T. E. Benedum, Org.Lett. 6 (2004) 3593-3595.
[22] R. J. Kalbasi, A. R. Massah, A. Shafiei, J.Mol.Catal.A. 335 (2011) 51-59.
[23] N. J Venkatesha, B. S Jai Prakash, Y. S Bhat, ‘Active Site Accessibility Aspect in Montmorillonite
for Ketone Yield in Ester Rearrangement’ Catal.Sci.Technol.(2014) DOI: 10.1039/C4CY01356E
[24] S. Ramesh, Y.S. Bhat, B.S. Jai Prakash, Clay. Miner. 47 (2012) 231-242.
[25] U. Flessner, D.J. Jones, J. Roziere, J. Zajac, L. Storaro, M. Lenarda, M. Pavan, A. J. Lopez, E.R
Castellon, M. Tro mbetta, G. Busca, J.Mol.Cat.A. 168 (2001) 247-256.
[26] A. Jha, A.C. Garade, M. Shirai, C.V. Rode. Appl.Clay.Sci. 74 (2013) 141-146.
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[27] E. B. Sandel, Colorimetric Determination of Traces of Metals, 3rd edition. Interscience Publishers,
New York, 1959.
[28] S. Selvaraj, B. V. Mohan, K. N. Krishna, B. S. Jai Prakash, Appl.Clay.Sci. 10 (1996) 439-450.
[29] S. Ramesh, B. S. Jai Prakash, Y. S. Bhat, Appl.Clay.Sci. 48 (2010) 159-163.
[30] I. Tkac, P. Kodamel, D. Muller, Clay.Miner. 29 (1994) 11-19.
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Figures:
14
12
10
8
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0.1PDSA
0.5PDSA
1PDSA
0.1PDSA
0.5PDSA
1PDSA
(b)
Untreated clay
Untreated
(a)
6
4
2
0
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75
Pore Diameter (A)
2
4
6
8
10
12
14
16
18
20
Statistical Thickness (Ao)
Fig 1(a) and (b) BJH curves and t-plots of untreated, 0.1, 0.5 and 1M PDSA treated montmorillonite clay.
21
Page 21 of 26

a
b
c
%T
d
1600 1580 1560 1540 1520 1500 1480 1460 1440 1420 1400
Wavenumber (cm-1)
Fig.2 FT-IR spectra of adsorbed pyridine on (a) Al-clay, (b) 1PDSA, (c) 0.5 PDSA and (d) 0.1 PDSA.
70
% PA conversion
yield of Akynyl amide
60
Yield of Oxazole
50
d
l
e
i
y
/
n
40
o
i
s
r
e
v
30
n
o
C
%
20
10
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Catalyst amount (g)
Fig .3 Effect of catalyst amount on the propargylation and cycloisomerization of PA and AA
Reaction conditions: reaction time; 60 min, reaction temperature 150oC; reactants molar ratio PA:AA;
1:2.
22
Page 22 of 26

70
60
50
40
30
20
10
0
% PA conversion
yield of Akynyl amide
Yield of Oxazole
d
l
e
i
y
/
n
o
i
s
r
e
v
n
o
C
%
80
100
120
140
160
180
Temperature (oC)
Fig .4 Effect of temperature on the propargylation and cycloisomerizationof PA and AA.
Reaction conditions: catalyst amount; 1 g, reaction time; 60min, reactants molar ratio PA: AA; 1:2.
70
60
d
l
e
50
i
y
/
n
o
i
40
s
r
% PA conversion
e
v
n
yield of Akynyl amide
o
30
C
Yield of Oxazole
%
20
10
0
0
20 40 60 80 100 120 140 160 180 200 220 240 260
Reaction time (min)
Fig .5 Effect of Reaction time on the propargylation and cycloisomerization of PA and AA
Reaction conditions: catalyst amount; 1 g, reaction temperature 150oC; reactants molar ratio PA:AA; 1:2.
23
Page 23 of 26

Fig.6 SEM images of (a) Untreated (b) 1 PDSA and (c) 1 PDSA (5%).
Tables:
Table.1 CEC, surface area, average pore diameter and micropore volume of unmodified, Al-clay and
PDSA treated clays.
SBET
Vm
Vtm
Clay samples
CEC
APD
Interlayer
Al (meq/g)
-
(meq/g)
(m2/g)
(Ǻ)
(cm3/g)
(cm3/g)
Untreated clay
0.83
27
50.3
0.011
0.067
Al-clay
0.1 PDSA
0.5 PDSA
1PDSA
0.83
0.79
0.75
0.71
27
50.3
0.011
0.067
0.074
0.132
0.14
0.83
0.49
0.62
0.70
109.14 44.13 0.026
196.09 33.34 0.064
276.44 26.04 0.203
*SBET-BET surface area, APD-average pore diameter, Vm-total pore volume, Vtm-micropore volume by t-plot method.
24
Page 24 of 26

Table2 Comparison of TPD-NH3 acidity, conversion of PA and yield of products with Al-clay, PDSA
treated, BEA zeolite and different PDSA amount loaded clay catalyst samples.
Catalyst
TPD-NH3
(µmol/g)
% PA
conversion Alkynyl
amide
%
% yield
of
Oxazole
Average
TOF
Vtm
SBET
(cm3/g)
(m2/g)
(min-1)
Al-Clay
250
78
46
30
36
18
10
12
0.011
0.026
0.066
0.256
27
0.1PDSA
109.14
0.5 PDSA
1 PDSA
196
236
500
228
248
262
232
44
65
70
67
70
60
48
20
25
15
21
20
28
36
24
40
55
46
50
32
12
0.064
0.203
0.305
0.104
0.064
0.019
0.004
0.256
0.282
0.432
0.365
0.348
0.154
0.10
196.09
276.44
552
BEA
1PDSA (5%)
1PDSA (10%)
1PDSA (15%)
1PDSA (20%)
213
146
106
70
Reaction conditions: catalyst amount; 1 g, reaction temperature 150oC; reactants molar ratio PA:AA; 1:2,
reaction time 60 min.*SBET-BET surface area, Vtm-micropore volume by t-plot method.
Table3 Correlation between volume accessibility factorsand yields of oxazole for the different catalyst
samples
Vtm
Catalyst
% PA
conver
sion
% yield
B
L
B+L
B/L
Volume Accessibility Factor
(cm3/g) (µmol/g) (µmol/g)
of
(VAF)*
Oxazole
Pyridine FT-IR acidity
B
L
B+L
B/L
0.16
0.15
0.06
0.02
0.07
0.06
0.01
0.02
0.07
1PDSA (5%)
1PDSA (10%)
1PDSA (15%)
1PDSA (20%)
1PDSA (Zn-5%)
1PDSA (Fe-5%)
Al-clay
67
70
60
48
54
61
46
30
44
46
50
32
12
16
16
10
12
24
0.104
0.064
0.019
0.004
0.106
0.099
0.011
0.026
0.064
0.207
92
112
123
133
61
56
74
34
52
58
49
38
28
86
92
70
44
50
150
161
161
161
147
148
144
78
1.59
2.29
3.24
4.75
0.71
0.61
1.06
0.77
1.04
9.48 5.97 15.45
7.17 3.14 10.30
2.34 0.72
0.53 0.11
6.47 9.12 15.58
5.54 9.11 14.65
0.81 0.77
0.88 1.14
3.06
0.64
1.58
2.02
6.53
0.1PDSA
0.5PDSA
102
3.23
3.2
Correlation
factor, r2
0.096
0.014
0.077 0.002 0.61 0.03 0.252 0.911
*VAF = Micropore volume × Acidity (Example VAF(B) = 0.103 × 92 = 9.48.
B-Brønsted acidity, L-Lewis acidity.Vtm-micropore volume by t-plot method.
Reaction conditions: catalyst amount, 1 g, reaction temperature 150oC; reactants molar ratio, PA:AA;
1:2; reaction time, 60 min.
25
Page 25 of 26

Table4 Correlation between yield of oxazole formed with mesopore volume, total pore volume and
volume accessibility factor for mesopore volume(VAF)meso
Catalyst
% PA
conversion
% yield
Volume Accessibility Factor for
Vmeso
Vm
(cm3/g)
(cm3/g) mesopore volume, (VAF)meso
of
Oxazole
Pyridine FT-IR acidity
B
L
B/L
0.28
0.31
0.44
0.55
0.11
0.10
0.07
0.07
0.17
B+L
22.50
21.74
21.90
18.84
22.76
23.68
9.65
1PDSA (5%)
1PDSA (10%)
1PDSA (15%)
1PDSA (20%)
1PDSA (Zn-5%)
1PDSA (Fe-5%)
Al-clay
67
70
60
48
54
61
46
30
44
46
50
32
12
16
16
10
12
24
0.150
0.135
0.136
0.117
0.155
0.160
0.067
0.094
0.132
0.186
0.253
0.199
0.155
0.121
0.261
0.259
0.078
0.121
0.196
0.216
13.8
15.12
16.73
15.56
9.45
8.96
4.96
3.19
6.86
8.70
6.61
5.17
3.28
13.33
14.72
4.69
4.14
6.60
0.1PDSA
7.33
13.46
0.5PDSA
Correlation factor, r2
0.240 0.001 0.029
0.193
Reaction conditions: catalyst amount; 1 g, reaction temperature 150oC; reactants molar ratio PA:AA; 1:2,
reaction time 60 min.
*Vtm-micropore volume by t-plot method,Vmeso-mesopore volume.
Scheme:
O
N
OH
+
Propargyl alcohol
H N
2
R
R
PDSA-clay
O
Oxazole
Amide
NH
Propargylation
Cycloisomerization
O
Alkynyl amide
Scheme 1 Reaction between PA and amide to form Oxazole in presence of PDSA-clay.
Clay
OH2
H2
O
Al
H2
O
OH2
Bronsted
O
O
H
H
Bronsted
Clay
Al
+
2
OH2
OH2
H2
O
CH
+ H
OH
H2
O
NH
O
N
H2N
R
O
N
H
H
H
-
-
O
R
O
R
HC
R
O
Al
Al
Lewis
Lewis
Clay
Clay
H
N
N
H C
R
3
H C
R
O
2
O
Scheme 2 Possible reaction mechanism for the propargylation and cycloisomerization.
26
Page 26 of 26