Superior activity and selectivity of multifunctional catalyst Pd-DTP@ZIF-8 in one pot synthesis of 3-phenyl propyl benzoate

Article abstract of DOI:10.1016/j.ica.2019.03.012

The catalytic efficiency of zeolitic imidazolate framework (ZIF-8) has been explored by making it multifunctional. Dual active sites were incorporated such as acid (dodecatungstophosphoric acid, DTP) and metal (Pd°) to prepare 5% Pd-DTP@ZIF-8. DTP was encapsulated inside the cage of ZIF-8 by in-situ and bottle around the ship approach whereas Pd was loaded ex-situ by simple wet impregnation method. The catalytic efficiency was tested for one pot synthesis of 3-phenyl propyl benzoate (3-PPB), a perfumery compound, from cinnamyl alcohol and benzoic anhydride. Trans-esterification of cinnamyl alcohol with benzoic anhydride gives cinnamyl benzoate which on further hydrogenation gives 3-PPB. Three different supports were screened such as ZIF-8, K10 and MCF out of which ZIF-8 showed the maximum activity because of its high surface area and smaller pore diameter. Further Pd, Ni and Cu metals were studied for selective hydrogenation of C[dbnd]C bond among which 5% Pd-DTP@ZIF-8 gave almost 98% conversion of cinnamyl benzoate to 3-PPB with 93% selectivity. Fresh and spent catalysts were characterized by various techniques. 5% Pd-DTP@ZIF-8 showed anti-leaching property with great thermal stability. The turn over frequency and turn over number of the catalyst was observed to be 0.058 s^?1 and 835, respectively. A kinetic model was developed with good fit using LHHW mechanism and the activation energy calculated as 17.45 kcal/mol for hydrogenation step. Thus, the reaction was found to be kinetically controlled. The entire process is green.

Full text of DOI:10.1016/j.ica.2019.03.012

Accepted Manuscript
Research paper
Superior activity and selectivity of multifunctional catalyst Pd-DTP@ZIF-8 in
one pot synthesis of 3-phenyl propyl benzoate
Radhika S. Malkar, Ganapati D. Yadav
PII:
S0020-1693(18)31811-5
https://doi.org/10.1016/j.ica.2019.03.012
ICA 18817
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
4 December 2018
13 February 2019
11 March 2019
Please cite this article as: R.S. Malkar, G.D. Yadav, Superior activity and selectivity of multifunctional catalyst Pd-
DTP@ZIF-8 in one pot synthesis of 3-phenyl propyl benzoate, Inorganica Chimica Acta (2019), doi: https://doi.org/
1
0.1016/j.ica.2019.03.012
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Superior activity and selectivity of multifunctional catalyst Pd-DTP@ZIF-8
in one pot synthesis of 3-phenyl propyl benzoate
Radhika S. Malkar; Ganapati D. Yadav*
Department of Chemical Engineering, Institute of Chemical Technology, Nathalal Parekh
Marg, Matunga, Mumbai-400 019, India
E-mail address: gdyadav@yahoo.com, gd.yadav@ictmumbai.edu.in
Tel.: +91-22-3361-1001, Fax: +91-22-3361-1020
*Author to whom correspondence should be addressed
1

ABSTRACT
The catalytic efficiency of zeolitic imidazolate framework (ZIF-8) has been explored by
making it multifunctional. Dual active sites were incorporated such as acid
(
dodecatungstophosphoric acid-DTP) and metal (Pd°) to prepare 5% Pd-DTP@ZIF-8. DTP
was encapsulated inside the cage of ZIF-8 by in-situ and bottle around the ship approach
whereas Pd was loaded ex-situ by simple wet impregnation method. The catalytic efficiency
was tested for one pot synthesis of 3-phenyl propyl benzoate (3-PPB), a perfumery compound,
from cinnamyl alcohol and benzoic anhydride. Trans-esterification of cinnamyl alcohol with
benzoic anhydride gives cinnamyl benzoate which on further hydrogenation gives 3-PPB.
Three different supports were screened such as ZIF-8, K10 and MCF out of which ZIF-8
showed maximum activity because of its high surface area and smaller pore diameter. Further
Pd, Ni and Cu metals were studied for selective hydrogenation of C=C bond among which 5%
Pd-DTP@ZIF-8 gave almost 98% conversion of cinnamyl benzoate to 3-PPB with 93%
selectivity. Fresh and spent catalysts were characterized by various techniques. 5% Pd-
DTP@ZIF-8 showed anti-leaching property with great thermal stability. The turn over
-1
frequency and turn over number of the catalyst was observed to be 0.058 s and 835,
respectively. Kinetic model was developed with good fit of LHHW model and activation
energy was calculated as 17.7 kcal/mol for hydrogenation step. Thus, the reaction was found
to be kinetically controlled.
Keywords: MOF; ZIF-8; hydrogenation; one pot synthesis; 3-phenyl propyl benzoate;
cinnamyl alcohol; benzoic anhydride
2

1
. Introduction
For exploring the catalytic activity of metal organic frameworks (MOF), to date various one
pot or cascade reactions have been carried out using MOF as a multifunctional catalyst. These
MOFs are just amalgamation of metal ion and organic linker with special interest areas
consisting of gas storage, catalysis, CO capture and separation [1–4]. The organic linker can
2
be a ditopic or polytopic carboxylates or similar molecule [5]. Combination of these linker with
metal unit leads to the formation of a robust crystalline structure with a great porosity. A wide
range of different combinations of these two components can be prepared. Therefore, till date,
more than 20,000 MOFs have been reported. MOF derived catalysts are now being widely
accepted in heterogeneous systems.
There are different kinds of MOF such as MIL-101, MIL-125, UiO-66, etc. used for the
synthesis of synergistic catalyst containing multiple active sites in a variety of combinations of
acid, base and metal [6–8]. Arnanz et al. have prepared Cu
3
(BTC) MOF based catalyst with
2
introduction of Pd and aminopyridine to make it multifunctional in tandem Sonogashira/click
reaction using 2-iodobenzylbromide, sodium azide and alkynes [9]. There is an another report
where base-metal functionalization of MOF has been applied in cascade reaction of aldol
condensation followed by hydrogenation [10]. Here, they have used Zr based MOF having
chiral NNN-pincer ligands and later modified by introducing metal to finally form (NNN)-M-
Zr-MOF (M=Rh, Ir) catalyst.
Zeolitic imidazolate framework (ZIF) is a class of MOF possessing zeolite like features
with great thermal, hydrothermal and chemical stability [11,12]. Its application in the catalysis
field has been explored by many scientists in different types of reactions like hydrogenation,
esterification, transesterification, oxidation, acylation, etc.[13–17]. Metal loaded or
incorporated ZIFs, reported for the reactions like hydrogenation and dehydrogenation, have
shown excellent yield and selectivity for the desired products [6,18,19]. Pd-ZIF-8 catalyst was
prepared by simple impregnation method and used for highly selective hydrogenation of
th
cinnamaldehyde to hydrocinnamaldehyde where the catalyst showed reusability up to 4 cycle
[
13].
Catalytic activity of ZIF-8 have earlier explored by many scientists such as, Wan et al.
(
(
2017) have developed a new hollow nanostructure with dual metal organic framework
HKUST-1 and ZIF-8) shell with embedded metal nanoparticles [20]. It was constructed using
two approaches as ship in a bottle and bottle around the ship. This kind of structure proved to
3

be giving well dispersion of metal nanoparticles and size selectivity. In another attempt Pd
nanoparticles were first loaded over ZnO microsphere afterwards fabrication of ZIF-8 was done
around Pd-ZnO core [21] (Pd-ZnO@ZIF8). Thus, prepared core-shell microspehere type of
catalyst showed size selectivity, stability and anti-poisoning quality during hydrogenation of
alkenes.
Cinnamyl alcohol and esters of cinnamyl alcohol always showed an importance in food,
flavour and perfumery industry [22]. 3-Phenyl propyl benzoate is a very sweet fragrance
molecule. It is also used as plasticizer in adhesives [23]. The synthesis of 3-phenyl propyl
benzoate (3-PPB) has been tried earlier using homogeneous as well as heterogeneous catalysts.
Ren et al. [24] have found out an effective method for the synthesis of esters using Cs
an oxygen source. The reaction was performed at 50 °C using 4-dimethylaminopyridine
DMAP) as a catalyst. There is another route followed by Sastraruji et al. [25] for 3-PPB
preparation by allylic oxidation of benzyl-(3-phenyl propyl) ether over DIB
TBHP/Mg(OAc) .4H O. The reaction achieved selectively benzoate in 95% yield. From the
literature, it was observed that majority of the reactions were performed using homogeneous
catalysts. Very few scientists have worked on solid catalysts such as ZnO [26] and Al [27]
2
CO
3
as
(
/
2
2
2
O
3
for the production of 3-PPB. Acid chlorides like benzoyl chlorides were preferred as reactant
in all the said cases instead of acid anhydride. Hence, such processes have become non-eco-
friendly.
In the present work we have prepared multifunctional ZIF-8 possessing acid as well as
metal site to synthesize 3-PPB from cinnamyl alcohol and benzoic anhydride in one pot system
(
Scheme 1). We have previously reported the synthesis of cinnamyl benzoate (CB) over novel
heteropolyacid encapsulated ZIF-8 (DTP@ZIF-8) by using benzoic anhydride as an limiting
agent [28]. Following are the four possible products can form after hydrogenation step (Scheme
2
); i) hydrogenation of C=C bond of CB produces 3PPB [a] which is an important perfumery
and flavour compound ii) hydrogenation of C=O bond of CB gives benzyl cinnamyl ether [b],
iii) complete hydrogenation of CB produces 3 phenyl propyl benzyl ether (3-PPBE) [d] and iv)
hydrogenation of C=O bond of 3PPB gives phenyl (3-phenylpropoxy)methanol [c].
In the current work, we have studied three different kinds of supports as mesocellular
foam (MCF), K10 and ZIF-8 for the synthesis of 3-PPB from cinnamyl alcohol and benzoic
anhydride. Large surface area, small pore diameter and moderate acidity of ZIF-8 helps in
selective synthesis of 3-PPB.
4

Scheme 1: One pot synthesis of 3-phenyl propyl benzoate from cinnamyl alcohol
Scheme 2: Possible hydrogenation products of cinnamyl benzoate and 3 PPB
2
. Experimental section
2
.1. Chemicals
Following chemicals were purchased from the sources given there, 1, 4 dioxane, zinc acetate,
n-decane (AR grade S.D. Fine Chem. Ltd, Mumbai, India), benzoic anhydride, cinnamyl
5

alcohol (98% Alfa Aesar, Mumbai), dodeca-tungstophosphoric acid (Thomas Baker, Mumbai),
methylimidazole (LobaChemie, Mumbai), Ni(NO .6H O, Cu(NO .3H O, PdCl (S.D.
2
3
)
2
2
3
)
2
2
2
Fine Chem. Ltd, Mumbai.); cesium chloride (Spectrochem, Mumbai) and K-10 clay (Aldrich,
USA). The chemicals were used without any further purification.
2
.2. Catalyst Preparation
Three different supports as K10, MCF and ZIF-8 were explored for the selective synthesis of
-PPB from cinnamyl alcohol and benzoic anhydride in one pot system. 5% Pd-Cs-DTP/K10
3
and 5% Pd-DTP-MCF were prepared by earlier reported method [29,30]. Heteropolyacid
encapsulated ZIF-8 was prepared by the earlier reported method [28]. Over which three
different metals like Pd, Ni and Cu were loaded by simple wet impregnation technique to study
their effect on hydrogenation of cinnamyl benzoate. Initially DTP@ZIF-8 was dispersed into
the 30 ml mixture of d/w and ethanol (1:1). PdCl
used as precursors for metals. In another beaker solution of these metal precursors were (2, 3,
wt %) prepared in a minimum amount of distilled water and drop-wise addition of it was
2
, Ni(NO
3
)
2
.6H
2
O and Cu(NO
3
)
2
.3H O were
2
5
made over the dispersed catalyst. The prepared solution was then stirred vigorously for
continuous 3 h at room temperature. Afterwards the slurry of catalyst was washed by using
d/w-ethanol mixture for 2-3 times and then dried in an oven for overnight. Ex-situ reduction of
metals were carried out by using hydrazine hydrate (2 ml).
2
.3 Characterization of catalyst
Fresh and reused catalyst was characterized by SEM, TEM, XRD, TGA, FTIR, BET surface
area and TPD (NH ). Details of these techniques have been provided in supplementary
3
information. As 5%Pd DTP@ZIF-8 was the best for one pot synthesis of 3PPB, its complete
characterization is being discussed in following sections.
2
.4 Experimental setup and analysis
6

One pot synthesis of 3PPB from cinnamyl alcohol and benzoic anhydride was carried out in a
00 ml batch autoclave reactor with temperature and pressure control unit and overhead stirrer
Amar autoclave, Mumbai). A known quantity of benzoic anhydride (0.025 mol) and cinnamyl
1
(
alcohol (0.005 mol) were taken into the reactor vessel with n-decane (300 µl) as an internal
standard. Optimised quantity of 5% Pd DTP@ZIF-8 was added to above reaction mixture.
Total volume of reaction mixture was made up to 32 ml with 1,4 dioxane as a solvent.
o
Temperature was raised to 120 C and continued for 2.5 h for complete conversion of cinnamyl
alcohol to cinnamyl benzoate and then the reactor was pressurized with molecular hydrogen
for second step to synthesize 3PPB. Periodic sampling was done and analyzed over GC with
TG-5 capillary column (30 m length and 0.25 mm diameter) and FID detector. 3PPB synthesis
was confirmed over GCMS (Thermo Scientific 1300), TG-5MS capillary column (30 m length
1
13
and 0.25 mm diameter) and H and C and values were compared with the reported data in the
literature. The turn over frequency and turn over number of catalyst was calculated based on
number of Pd atom. The detailed calculations have provided in supporting information.
3
3
3
. Results and discussion
.1. Catalyst characterization
.1.1. X-Ray diffraction
XRD of four samples as ZIF-8, DTP@ZIF-8, 5% Pd-DTP@ZIF-8 and reused 5% Pd-
DTP@ZIF-8 was carried out to check the crystallinity of the catalysts and also to verify it’s
rhombic dodeca-tetrahedron structure (Figure 1). All the samples showed characteristic peaks
at 2θ, 7.93°, 10.97°, 13.37°, 15.22°, 16.94°, 18.86°, 26.12°, 27.25° corresponding to (011),
(
002), (112), (022), (013), (222), (233) and (134) [31] which also proves that the structure is
stable even after the encapsulation of heteropolyacid and loading of Pd metal. 5% Pd-
DTP@ZIF-8 (fresh and spent) catalyst shows another set of peaks at 2θ, 40.22° and 45.42° that
0
can be attributed to (111) and (200) planes of Pd [32–34]. Here (111) diffraction pattern of Pd
7

metal shows its face centered cubic (fcc) phase [35]. Intensity of peaks in case of ZIF-8 is very
high as compared to rest of the catalyst. Also, absence of bulk peaks attributed to Keggin
o
structure of DTP (2θ: 10°, 14.7°, 21.2°, 25°, 35 , 38.3°, and 45.3°) concludes that it has properly
encapsulated inside the cage of ZIF-8 and not anymore on the surface of ZIF-8 [36].
3
.1.2. Surface area analysis and Ammonia- temperature programmed desorption (NH
3
-TPD)
Acidity of the prepared catalyst was measured by ammonia TPD in the range of 100-
o
4
8
00 . Acidity of different catalyts is in the following order; 5% Pd-Cs-DTP-K10 > DTP@ZIF-
> 5% Pd-DTP/MCF (Table 1). Zn metal ion and N atom of 2-methyl imidazole impart slight
2
acidity and basicity to ZIF-8, respectively [37]. It has a maximum surface area of 1306 m /g.
2
After DTP encapsulation and Pd loading surface area has reduced to 1052 m /g with pore size
3
of 1.1 nm with pore volume around 0.6 cm /g. 5% Pd Cs-DTP-MCF posses the largest pore
size as 10.1 nm therefore it will not help in the shape or size selectivity for the desired product.
As reported in our earlier work, large molecules of DTP will not leach out from the small pores
of the ZIF-8 and hence provide large catalytic activity.
Figure 2 explains the N
, 5% Pd-DTP@ZIF-8 and reused 5% Pd-DTP@ZIF-8. The steep increase in the adsorption
volume at low relative pressure proves that the structure is microporous in nature. It follows
type I adsorption isotherm. 5% Pd-DTP@ZIF-8 showed decreased N uptake volume as
2
adsorption desorption curves for three catalysts namely ZIF-
8
2
compared to pure ZIF-8 which is because of the encapsulation of DTP inside the ZIF-8 and
loaded Pd metal over it.
3
.1.3. Fourier transform infrared spectroscopy (FTIR)
Figure 3a and 3b represents FTIR images of pure ZIF-8, DTP@ZIF-8, 5% Pd-
DTP@ZIF-8 and reused 5% Pd-DTP@ZIF-8. The characteristic peaks for aliphatic and
aromatic imidazole ring stretching of ZIF-8 can be seen for every sample at 3066.43 and
8

-1
2
906.70 cm . Also, there are additional peaks for in-plane and out-of-plane bending of the
-
1
-1
imidazole ring in the area of 1350-900 cm . The sharp bands in the region of 1300-1450 cm
shows C=N absorption [38,39]. These are the common peaks from all the samples.
The IR spectra of b, c and d showed important peaks of heteropolyacids at 1080, 965,
-1
9
08 and 800.50 cm corresponding to (P-O), (W-O
t
), (W-O
c
-W), and (W-O -W), respectively
e
[
36,40]. It confirms the presence of heteropolyacid inside the ZIF-8. However no any
differences in the peaks were observed after Pd loading over DTP@ZIF-8. Thus this result
supports the conclusions of XRD that DTP is well entrapped inside the cage of ZIF-8. Reused
catalyst showed the very well retention of ZIF-8 structure along with the encapsulated DTP.
3
.1.4. Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM)
To identify the morphology of prepared catalyst, SEM analysis was carried out (Figure
). ZIF-8 possesses rhombic dodecahdral appearance with smooth outer surface [34] and which
4
is retained by the catalyst even after encapsulation of DTP. The smooth surface of rhombic
dodecahedral crystals became slightly rough after incorporation of Pd metal [18]. The particle
size of DTP@ZIF-8 and 5% Pd-DTP@ZIF-8 is around 50-100 nm. SEM image of reused 5%
rd
Pd-DTP@ZIF-8 (after 3 cycle) is also presented in Figure 4 and we can thus see the very well
retention of crystal structure of ZIF-8 with sharp edges. Thus catalyst is stable even after 3
cycles of recyclability. Transmission electron microscopic image (Figure 5) shows that the
dispersion of Pd is even with average particle size and lattice fringes 2.1-2.3 nm and 0.23 nm,
respectively [18].
3
.1.5. Thermal gravimetric analysis (TGA)
Thermal gravimetric analysis of all catalysts i.e ZIF-8, 5% Pd-DTP@ZIF-8 and reused
% Pd-DTP@ZIF-8 were studied for thermal stability of catalyst (Figure 6). The catalyst is
5
stable up to 450 °C and thereafter the structure start decomposing. Beyond 450 °C there was a
9

major weight loss, in which initially organic ligands from the structure starts decomposing and
rd
then all the framework gets destructed [15]. Reused catalyst after 3 cycle showed 9-10 %
weight loss because of the removal of adsorbed moieties from the reaction mixture. Further
continued stability up to 450 °C.
3
3
.2 Synthesis of 3-phenyl propyl benzoate
.2.1. Catalyst screening
Three catalysts were screened as 5% Pd-Cs-DTP/K10, 5% Pd-DTP/MCF and 5% Pd-
DTP@ZIF-8 for getting the higher conversion and selectivity towards 3 phenyl propyl benzoate
(
Figure 7). The maximum acidity of 5% Pd-Cs-DTP/K10, led to the highest conversion 85.4%
of cinnamyl benzoate but the selectivity for 3PPB was found to be only 79.2% and for 3-PPBE,
0.76% (Table 2). The lowest conversion of 73.5% was observed for 5% Pd-DTP/MCF as
2
compared with two other catalysts. As ZIF-8 possesses Lewis acidity because of Zn metal ion,
the experiment of trans-esterification (1st step) of cinnamyl alcohol with benzoic anhydride
was performed using ZIF-8. Only 30.5 % conversion of cinnamyl alcohol was obtained after
4
.5 h at 120 °C. The rate of formation of cinnamyl benzoate was very slow. Therefore, the
catalytic activity of ZIF-8 was found to be less as compared to DTP@ZIF-8. The highest
selectivity for 3-PPB was obtained over 5% Pd-DTP@ZIF-8 with 81.8 % conversion for
cinnamyl benzoate. Large surface area and moderate acidity of 5% Pd-DTP@ZIF-8 resulted
into good conversion and selectivity for the desired product. Hence it was optimized for one
pot synthesis of 3-PPB from cinnamyl alcohol and benzoic anhydride.
3
.2.2. Screening of metals for hydrogenation of cinnamyl benzoate
Pd, Ni and Cu were studied for hydrogenation of C=C of cinnamyl benzoate (Figure 8).
Catalytic activity for 5% Pd was found to be higher as compared to Ni and Cu. In case of Ni
loaded catalysts selectivity for 3-PPBE was observed to be higher as compared to 3-PPB (Table
10

3
). Maximum selectivity for 3-PPB was obtained with 5% Cu-DTP@ZIF-8 but the conversion
of cinnamyl benzoate was very less (55.84%). Pd on ZIF-8 has already proved its maximum
catalytic efficiency in selective hydrogenation of cinnamaldehyde to hydrocinnamaldehyde
(
HCAL) [13]. The ZIF-8 support has been proved best for hydrogenation of α, β unsaturated
aldehydes rather than any other MOF as a support. It gave almost 99.9% conversion of
cinnmaldehyde and 91% selectivity for HCAL. Therefore, with respect to better conversion
and selectivity for 3-PPB, 5% Pd-DTP@ZIF-8 was found to the best catalyst and hence chosen
for further reactions.
3
.2.3. Effect of Pd loading
Different amounts of Pd (2%, 3%, 5%, 7%) were loaded on DTP@ZIF-8 to see the
effect on hydrogenation of C=C bond of cinnamyl benzoate (Figure 9). With increase in Pd
loading, conversion of cinnamyl benzoate was increasing and it was reached to 91.56% for 7%
Pd loading (Table 4). The selectivity for 3-PPB was reduced up to 85.44%. Increased Pd
content led to the increase in the hydrogenation of C=O bond of α, β unsaturated aldehyde and
thus it is affecting selectivity of desired product. 2% Pd-DTP@ZIF-8 showed highest
selectivity but with minimum conversion for cinnamyl benzoate. Therefore 5% Pd DTP@ZIF-
8
was taken as an optimum Pd loading for this reaction.
3
.2.4. Effect of speed of agitation
To check the absence of external mass transfer resistance, reaction was conducted at
different agitation speed at 800, 900, 1000 and 1100 in the autoclave (Figure 10). As we go on
increasing the speed of agitation from 800 to 1000 rpm, conversion of cinnamyl benzoate was
observed to be increasing and after 1000 rpm there was insignificant difference in conversion.
Therefore 1000 rpm was considered as the optimum value for this reaction. As the catalyst is
11

microporous in nature, in previous report we have already provided a proof of absence of
external mass transfer resistance [28].
3
.2.5. Effect of catalyst loading
Effect of concentration of catalytic active sites was studied for conversion of cinnamyl
benzoate (Figure 11). With the increase in active sites i.e. catalyst loading the rate of
3
hydrogenation was increasing. At 0.004 and 0.005 g/cm the increase in the conversion was
almost negligible because of the availability of active sites were more than sufficient. Therefore
3
0
.004 g/cm catalyst loading was taken for further experiments. A plot of initial rate vs catalyst
loading was made and it is directly proportional to the catalytic active sites (Figure 12). The
initial rate of reaction of hydrogenation was increasing with increase in catalyst amount.
3
.2.6. Effect of mole ratio
Mole ratio of cinnamyl alcohol to benzoic anhydride was studied on conversion of
cinnamyl alcohol within specific time of 5h because the synthesis of 3-PPB is totally dependent
on the concentration of cinnamyl alcohol and benzoic anhydride (Figure 13). With the increase
in mole ratio the conversion of cinnamyl alcohol to cinnamyl benzoate was found to be
increasing. At 1:4 and 1:5 the conversion was almost 100% within 5h. At 1:2 and 1:3 the
conversion of cinnamyl alcohol was 65.24 and 89.21 % after 5h. Therefore, in the next step of
hydrogenation, remaining cinnamyl alcohol was also getting hydrogenated and therefore
selectivity for 3-PPB was decreasing up to 78.76 % and 89.69% respectively within (120 mins)
(
Table 5). Therefore at 1:4 mole ratio, after complete conversion of cinnamyl alcohol to
cinnamyl benzoate hydrogenation reaction gave 93.45% selectivity to 3-PPB and hence it was
chosen as an optimum value for one pot synthesis of 3-PPB.
3
.2.7. Effect of hydrogen pressure on conversion of CB to 3-PPB
12

The pressure of hydrogen was varied from 5 bar to 20 bar and which significantly
influenced the rate of hydrogenation (Figure 14). With the increase in hydrogen pressure the
rate of formation of 3-PPB was increasing i.e. the conversion of cinnamyl benzoate was
increasing. At lowest pressure of hydrogen i.e. at 5 bar the conversion was only 58%. When
hydrogen pressure was raised to 15 bar, the conversion was increased rapidly and reached to
1
00% after 4h. At 15 bar and 20 bar the conversion and selectivity for 3-PPB was almost similar
and hence 15 bar was selected as an optimum hydrogen pressure.
3
.2.8. Effect of temperature on hydrogenation of CB to 3-PPB and kinetic model
Temperature plays an important role in the conversion of reactant and for selectivity of
the desired product. Figure 15 showed that the conversion of CB increased with an increase in
o
o
the temperature from 100 to 130 C. At 100 C the conversion was only 67.59% and increased
o
sharply with temperature rise. At 120 and 130 C almost no reactant was left in the reaction
mixture and was get hydrogenated to 3-PPB. Above 120 °C, there was an insignificant change
in conversion and therefore taken as an optimum value for this reaction. At this point, the
selectivity for 3-PPB was 93.8%.
Based on above results a kinetic model was developed for hydrogenation of CB to 3-
PPB with the best fit of Langmuir-Hinshelwood-Hougen-Watson (LHHW) model. We have
earlier reported the kinetics of first step i.e. for esterification of benzoic anhydride and
cinnamyl alcohol with the Eley-Rideal best suited model. Considering CB and hydrogen gets
adsorbed on the vacant sites (S).
Chemisorption of CB (A) on site S can be shown as,
k
a


A S
AS


k '
a
The rate of reaction is given as,
r  k C C k ' C
AD
a
A
S
a
AS
13

CAS
KA
r  k (C C 
)
AD
a
A
S
Assuming the dissociative adsorption of hydrogen,
k
h
H  2S 2HS
2
k '
h
The rate of reaction for above can be written as,
2
C
2
HS
r  k (p C 
)
)
HD
h
H 2
S
Kh
2
C
2
HS
r  k (p C 
HD
h
H 2
S
Kh
The overall reaction can be written as,
k
1


AS  HS
FS  S


k '
1
Rate equation for reaction,
dCA

 k C C  k ' C C
1
AS HS
1
FS
S
dt
(
1)
Similarly, for product formed 3 phenyl propyl benzoate (F),
k
f


F  S
FS


k '
f
CFS
KF
r  k (C C 
)
FD
f
F
S
Assuming the adsorption and desorption steps to be very fast,
C  C C C C
T
AS
FS
HS
S
CT
C 
S
1
 K C  K C  K p
A
A
F
F
H
H
From equation no [1],
2
dCA C [k K C K p  k ' K C ]
T
1
A
A
H
H
1
F
F


2
dt
[1 K C  K C  K p ]
A
A
F
F
H
H
For initial rate of reaction, C
F
is almost negligible hence,
dCA
dt
[k K C K p ]w
1 A A H H


2
[1 K C  K p ]
A
A
H
H
14

At constant pressure,
dCA
dt
wk K C
1 A A


2
[1 K C ]
A
A
(
2)
Solving the equation no [2] in polymath 6.0 we have obtained the values of rate constants and
adsorption constants for hydrogenation of CB (Table 6),
From the Table 6, it can be seen that reaction rate constant is increasing while
adsorption constant of cinnamyl benzoate is decreasing with increase in temperature. The
Arrhenius plot (Figure 16) was made and activation energy was calculated as 17.7 kcal/mol.
Therefore, the reaction is kinetically controlled.
Table 7 shows the comparison of catalytic activity of different catalysts for the
synthesis of 3-PPB. In most of the studies, researchers have reported use of homogeneous
catalysts (entries 1-3) such as 4-dimethylaminopyridine (DMAP) [24], DIB
/
TBHP/Mg(OAc)
2
.4H
2
O [25] and TiCl
2
(OTf)
2
[41] using benzoyl chloride as an acylating
[27] (entries 4 and 5)
agent. Some of the heterogeneous catalysts such as ZnO [26] and Al
2
O
3
also been applied using the same acylating agent. However, making the use of such acylating
agent leads to the generation of HCl which can cause corrosion of reactor and generation of
hazardous waste. Though the yield obtained by such catalysts is above 90 %, some of the
reactions required lengthy and laborious synthesis time of 24 h. Oxidation of benzyl ether was
carried out over DIB /TBHP/Mg(OAc)
2
.4H O to obtain ester but the separation of product from
2
the solvent n-butyl butanoate was found to be difficult [25]. Hence they switched the solvent
to ethyl acetate. In contrast with all these reported catalysts, 5%-Pd-DTP@ZIF-8 has proved
its superior catalytic efficiency for the synthesis of 3-PPB using benzoic anhydride as a safer
benzoylating agent with reduced synthesis time.
15

3
.2.9. Reaction mechanism
% Pd-DTP@ZIF-8 consists of acidic sites as A and metal sites as M. Considering the
5
earlier report on the reaction mechanism of first step of trans-esterification it is assumed,
benzoic anhydride adsorbs on acidic sites of the catalyst and gets activated to carbonyl carbon.
-
-
O of hydroxyl group of cinnamyl alcohol attacks on the activated carbonyl group of the
benzoic anhydride over the surface of the catalyst to give cinnamyl benzoate as an intermediate.
Later, this cinnamyl benzoate adsorbs on another acid site for hydrogenation of C=C bond to
produce 3-phenyl propyl benzoate (Scheme 3).
Scheme 3: Reaction mechanism for one pot synthesis of 3-phenyl propyl benzoate
3
.2.10. Catalyst reusability and leaching
Stability of the catalyst was studied for the hydrogenation of CB to 3-PPB in one pot
system. For this, after the first reaction, the reaction mass was centrifuged to recover the
catalyst. Afterwards, it was refluxed with methanol for 3-4 times to get rid off the adsorbed
moieties from the reaction mixture. While performing these steps, 2-5% of the catalyst gets
16

lost. Therefore, catalyst weight was made up with the addition of fresh catalyst (2-5%) for the
next run. Each reaction showed a minor decrease in the conversion of cinnamyl benzoate. After
3rd run, the conversion of cinnamyl benzoate was 91.2% with 93.1% selectivity for 3-PPB
(
Figure 17). We have also applied the catalyst for the fourth and fifth cycle of recyclability, but
the conversion was dropped to 85% and then to 71%. Its probable reason we have already
reported in our earlier work that benzoic acid which was a co-product from the first reaction
start disintegrating the ZIF-8 structure and therefore, conversion started decreasing.
The stability of the catalyst is an important factor while performing catalytic reaction
not only for the leaching of the Pd and DTP but also for the selectivity of the desired product.
To check weather, the Pd nanoparticles has leached out into the liquid mixture, the hot filtration
test was carried out. For that the reaction was halted in a half way and solid catalyst was
separated by centrifugation. Afterwards the liquid supernatant was further catalyzed for
hydrogenation of cinnamyl benzoate. It was observed that the conversion practically remained
the same thereby confirming that there was no leaching of Pd in the reaction mixture. To further
assure the absence of Pd nanoparticles, the liquid supernatant was subjected for ICP-AES
analysis. It was observed that the leaching of Pd nanoparticles was below the detection level.
4
. Conclusion
In conclusion, multifunctional ZIF-8 was prepared by very simple and facile method
using bottle around the ship strategy with incorporation of acid as well as metal catalytic sites
5% Pd-DTP@ZIF-8). Its catalytic efficiency was tested for one pot synthesis of 3-phenyl
(
propyl benzoate from cinnamyl alcohol and benzoic anhydride in a single reactor. The fresh
and spent catalyst is thermally stable up to 450 °C and can be reused up to 3 cycles of reaction.
Separate kinetic model was developed for hydrogenation step with best fit of LHHW model.
Activation energy was calculated as 17.7 kcal/mol and thus reaction is kinetically controlled.
17

Acknowledgment
R. S. Malkar acknowledges University Grants Commission (UGC), India for the award of
fellowship in Green Technology. G. D. Yadav acknowledges support from R. T. Mody
Distinguished Professor Endowment and J. C. Bose National Fellowship of Department of
Science and Technology, Government of India.
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Figure Captions and Figure
Figure 1: XRD pattern of ZIF-8, DTP@ZIF-8, 5% Pd-DTP@ZIF-8 and reused 5% Pd-
DTP@ZIF-8
Figure 2: N adsorption desorption curve for ZIF-8, 5% Pd-DTP@ZIF8 and reused 5% Pd-
2
DTP@ZIF8
Figure 3: FTIR spectra of a) ZIF-8 pure, b) DTP@ZIF-8, c) 5% Pd-DTP@ZIF-8 and d) 5%
Pd-DTP@ZIF-8 reused
Figure 4: SEM analysis of ZIF-8 (a), DTP@ZIF-8 (b), 5% Pd-DTP@ZIF-8 (c) and reused
(
after 3 cycles) 5% Pd-DTP@ZIF-8
Figure 5: TEM of 5%-Pd-DTP@ZIF-8
Figure 6: TGA graph of ZIF-8 (A), 5% Pd-DTP@ZIF-8 (B) and reused 5% Pd-DTP@ZIF-8
(
C)
Figure 7: Conversion of cinnamyl benzoate over three catalyst
Figure 8: Conversion of cinnamyl benzoate over different metals supported on DTP@ZIF-8
Figure 9: Effect of Pd loading on catalytic activity: Catalyst 5% Pd DTP@ZIF-8, benzoic
anhydride 5.65 g (0.025 mol), cinnamyl alcohol 0.67 g (0.005 mol), solvent 1,4 dioxane, total
3
o
volume 32 ml, speed of agitation 1000 rpm, catalyst loading 0.004 g/cm , temperature 120 C,
pressure 10 bar.
H
2
Figure 10: Effect of speed of agitation: Catalyst 5% Pd DTP@ZIF-8, benzoic anhydride 5.65
g (0.025 mol), cinnamyl alcohol 0.67 g (0.005 mol), solvent 1,4 dioxane, total volume 32 ml,
3
o
catalyst loading 0.004 g/cm , temperature 120 C, H pressure 10 bar.
2
Figure 11: Effect of catalyst loading: Benzoic anhydride 5.65 g (0.025 mol), cinnamyl
alcohol 0.67 g (0.005 mol) (CA:BA mole ratio 1:5), catalyst 5% Pd-DTP@ZIF-8, solvent 1,4
o
dioxane, total volume 32 ml, speed of agitation 1000 rpm, temperature 120 C, H
bar, time 5h.
2
pressure 10
Figure 12: Plot of initial rate versus catalyst loading: Effect of catalyst loading: Benzoic
anhydride 5.65 g (0.025 mol), cinnamyl alcohol 0.67 g (0.005 mol) (CA:BA mole ratio 1:5),
23

catalyst 5% Pd-DTP@ZIF-8, solvent 1,4 dioxane, total volume 32 ml, speed of agitation 1000
o
rpm, temperature 120 C, H
2
pressure 10 bar, time 5h.
Figure 13: Effect of mole ratio: Catalyst 5% Pd-DTP@ZIF-8, solvent 1,4 dioxane, catalyst
3
o
loading 0.004 g/cm , total volume 32 ml, speed of agitation 1000 rpm, temperature 120 C, H
pressure 10 bar, time 5 h.
2
Figure 14: Effect of H pressure: Mole ratio of cinnamyl alcohol to benzoic anhydride 1:4,
2
3
catalyst 5% Pd-DTP@ZIF-8, solvent 1,4 dioxane, catalyst loading 0.004 g/cm , total volume
o
3
2 ml, speed of agitation 1000 rpm, temperature 120 C, time 4h.
Figure 15: Effect of temperature on hydrogenation of CB to 3 PPB: Mole ratio of cinnamyl
alcohol to benzoic anhydride 1:4, catalyst 5% Pd-DTP@ZIF-8, solvent 1,4 dioxane, catalyst
3
o
loading 0.004 g/cm , total volume 32 ml, speed of agitation 1000 rpm, temperature 120 C, H
pressure 15 bar, time 4h.
2
Figure 16: Arrhenius plot for hydrogenation of CB to 3PPB
Figure 17: Catalyst reusability Mole ratio of cinnamyl alcohol to benzoic anhydride 1:4,
3
catalyst 5% Pd-DTP@ZIF-8, solvent 1,4 dioxane, catalyst loading 0.004 g/cm , total volume
o
3
2 ml, speed of agitation 1000 rpm, temperature 120 C, H
2
pressure 15 bar, time 4h.
24

80000
70000
60000
50000
40000
30000
20000
10000
0
0
10
20
30
40
50
60
70
2Ɵ (Degree)
ZIF-8
% Pd DTP@ZIF-8
DTP@ZIF-8
Reused 5% Pd DTP@ZIF-8
5
Figure 1: XRD pattern of ZIF-8, DTP@ZIF-8, 5% Pd-DTP@ZIF-8 and reused 5% Pd-
DTP@ZIF-8
25

400
350
300
250
200
150
100
50
0
-
0.2
0
0.2
0.4
0.6
0.8
1
1.2
-
50
Relative pressure (P/Po)
ZIF-8
5% Pd-DTP@ZIF-8
Reused 5% Pd-DTP@ZIF-8
Figure 2: N
2
adsorption desorption curve for ZIF-8, 5% Pd-DTP@ZIF8 and reused 5% Pd-
DTP@ZIF8
26

Figure 3a: FTIR spectra of a) ZIF-8 pure and b) DTP@ZIF-8
27

Figure 3b: FTIR spectra of c) 5% Pd-DTP@ZIF-8 and d) 5% Pd-DTP@ZIF-8 reused
28

Figure 4: SEM analysis of ZIF-8 (a), DTP@ZIF-8 (b), 5% Pd-DTP@ZIF-8 (c) and reused
after 3 cycles) 5% Pd-DTP@ZIF-8 (d)
(
Figure 5: TEM of 5%-Pd-DTP@ZIF-8
29