From agriculture residue to catalyst support; A green and sustainable cellulose-based dip catalyst for C–C coupling and direct arylation

Article abstract of DOI:10.1016/j.carbpol.2019.115060

The core characteristics of a perfect catalyst include good activity, simple design, excellent stability, easy recovery from reaction mixture, recyclability, and have the provision for easy scale up. The ease in synthesis, recyclability and scale up makes the dip catalyst a major contender in this regard which possess most of the aforementioned characteristics. In this work, we report a dip catalyst made of cellulose, isolated from agriculture residue (sugarcane bagasse), in which biogenically synthesized palladium nanoparticles were dispersed. The prepared dip catalyst was characterized by FESEM, EDS, XRD, ATR-IR, TGA, TEM, XPS and ICP-OES analysis. Also, the activity of the dip catalyst was studied in the Suzuki-Miyaura cross-coupling reaction and was found to give excellent conversion with 15 recycles. Further, the activity of dip catalyst in C5-arylation of 2-substituted thiophenes was evaluated for which promising yields were obtained.

Full text of DOI:10.1016/j.carbpol.2019.115060

Accepted Manuscript
Title: From Agriculture Residue to Catalyst Support; A Green
and Sustainable Cellulose-Based Dip Catalyst for C-C
Coupling and Direct Arylation
Authors: Vishal Kandathil, Manjunatha Kempasiddaiah,
Sasidhar B. S., Siddappa A. Patil
PII:
DOI:
Article Number:
S0144-8617(19)30727-1
https://doi.org/10.1016/j.carbpol.2019.115060
115060
Reference:
CARP 115060
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Accepted date:
24 April 2019
16 June 2019
5 July 2019
Please cite this article as: Kandathil V, Kempasiddaiah M, B. S. S, Patil SA, From
Agriculture Residue to Catalyst Support; A Green and Sustainable Cellulose-Based
Dip Catalyst for C-C Coupling and Direct Arylation, Carbohydrate Polymers (2019),
https://doi.org/10.1016/j.carbpol.2019.115060
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From Agriculture Residue to Catalyst Support; A Green and
Sustainable Cellulose-Based Dip Catalyst for C-C Coupling
and Direct Arylation
Vishal Kandathila, Manjunatha Kempasiddaiaha, Sasidhar B. S.b, Siddappa A. Patila,*
aCentre for Nano and Material Sciences, Jain University, Jain Global Campus, Kanakapura,
Ramanagaram, Bangalore 562112, India.
bOrganic Chemistry Section, National Institute for Interdisciplinary Science and Technology
(CSIR), Trivandrum 695019, India.
Corresponding author: Dr. Siddappa A. Patil
Tel: +91 80 27577254
Fax: +91 80 27577211
E-mail: p.siddappa@jainuniversity.ac.in/patilsiddappa@gmail.com
Highlights
 Dip catalyst was designed and synthesized in an environmentally benign approach.
 Cellulose isolated from agriculture residue in a facile method.
 Pd NPs synthesized using biogenic method avoiding hazardous chemicals.
 Dip catalyst was efficient in catalyzing C-C coupling as well as direct arylation reactions.
 Easily recoverable and highly recyclable.
1

Abstract:
The core characteristics of a perfect catalyst include good activity, simple design, excellent
stability, easy recovery from reaction mixture, recyclability, and have the provision for easy scale
up. The ease in synthesis, recyclability and scale up makes the dip catalyst a major contender in
this regard which possess most of the aforementioned characteristics. In this work, we report a dip
catalyst made of cellulose, isolated from agriculture residue (sugarcane bagasse), in which
biogenically synthesized palladium nanoparticles were dispersed. The prepared dip catalyst was
characterized by FESEM, EDS, XRD, ATR-IR, TGA, TEM, XPS and ICP-OES analysis. Also,
the activity of the dip catalyst was studied in the Suzuki-Miyaura cross-coupling reaction and was
found to give excellent conversion with 15 recycles. Further, the activity of dip catalyst in C5-
arylation of 2-substituted thiophenes was evaluated for which promising yields were obtained.
Keywords: Bagasse; Cellulose; C-C coupling; C-H activation; Dip catalyst; Palladium
nanoparticles
1. Introduction
Metal nanoparticles (MNPs) find huge applications in diverse fields of science which
include electronics (Kumar et al., 2018), optics (Burda, Chen, Narayanan, & El-Sayed, 2005;
Kelly, Coronado, Zhao, & Schatz, 2003), drug delivery (Albrecht, Evans, & Raston, 2006), water
purification (Albertini, Ribeiro, Alves, Baleizão, & Farinha, 2018), energy (Ni et al., 2018; X. Xu
2

et al., 2018), catalysis (Liu & Corma, 2018; Rothfuss, Knofel, Roesky, & Barner-Kowollik, 2018)
and so on. The distinctive properties of MNPs which make them suitable for various applications
can be attributed to the large surface to volume ratio they have. In the catalysis field, diverse
research is underway to effectively utilize the noble MNPs for catalyzing different reactions and
for the convenient recovery of those MNPs for further recycles. Being nano in size, their recovery
is quite difficult from reaction mass which results in the incomplete recovery and also has the
consequences of MNPs aggregation (Hotze, Phenrat, & Lowry, 2010). The carbon-carbon, carbon-
heteroatom bond formation and C-H activation reactions are some important applications where
the astonishing properties of these MNPs can be implemented (Cheng, Zhang, Li, & Liu, 2014;
Chng, Erathodiyil, & Ying, 2013). Among the various MNPs available, palladium nanoparticles
appear to be one of the most important MNPs which is indispensable from catalyzing various
organic transformations of industrial and academic interest (Yin & Liebscher, 2007).
The immobilization of palladium nanoparticles on various supports has been studied
widely and much effort has been taken in this regard (Khalafi-Nezhad & Panahi, 2011; Shylesh,
Schunemann, & Thiel, 2010). Among the various kinds of supports which include metal oxides
(Jin et al., 2012; Okumura, Kobayashi, Tanaka, & Niwa, 2003), silica (Martínez et al., 2015; Yang
et al., 2012), polymers (Modak, Mondal, Sasidharan, & Bhaumik, 2011; Modak, Pramanik,
Inagaki, & Bhaumik, 2014) etc., biogenic and green supports are of high priority due to the
greenness and sustainability involved (Baran, Aciksoz, & Mentes, 2016; Baran, Inanan, & Mentes,
2016; Baran, Sargin, Kaya, & Mentes, 2016; Baran, Yılmaz Baran, & Menteş, 2018a, 2018b;
Khazaei, Khazaei, & Rahmati, 2015; Khazaei, Rahmati, Hekmatian, & Saeednia, 2013). Cellulose
is a good candidate in this regard as it falls in green and sustainable group of supports (Mtibe et
al., 2015; Yilmaz Baran, Baran, & Mentes, 2018). The natural polymer, cellulose, is available in
3

abundant in nature and can be isolated by various means (Xie et al., 2016; Zhao, Moser, Lindstrom,
Henriksson, & Li, 2017). The strong beta-(1-4) glycosidic linkage makes the cellulose strong
enough to withstand most of the reaction conditions which is applicable in various organic
transformations (Kaushik & Moores, 2016). Sugarcane bagasse, which is a rich source of cellulose
and an agriculture residue, is produced in large quantity as a byproduct from sugar mills, of which
some quantity goes to boiler industries and paper industries (Mandal & Chakrabarty, 2011). The
cellulose can be isolated from bagasse without much effort and can be utilized in production of
various value added products ranging from membranes to paper (Habibi, Lucia, & Rojas, 2010;
Teixeira et al., 2011). The biodegradability, abundance, low cost, and greenness are some of the
key advantages of cellulose which gives them an upper hand among other reported synthetic
supports (Habibi et al., 2010).
Amongst the various cross-coupling reactions, Suzuki-Miyaura cross-coupling still stands
lucrative because of the ease in performing this reaction, use of mild reaction conditions, high
yield, and wide range of functional group tolerance (Baran, 2018a, 2018b; Phan, Van Der Sluys,
& Jones, 2006). Even though there is a tremendous increase in research on new catalytic system
with high catalytic activity for Suzuki-Miyaura cross-coupling reactions, a very few are
commercialized (Blaser, Indolese, Naud, Nettekoven, & Schnyder, 2004). Some of the core
reasons behind the non-commercialization of the palladium based catalyst can be attributed to the
high cost in production, multi-step synthesis, stability of the catalyst, difficulty in the recovery of
the catalyst and recycling (Blaser et al., 2004). Direct arylation reaction (C5-arylation) of 2-
substituted thiophenes with aryl halides is another class of reaction which is very important in
organic chemistry since there is a wide range of products that can be synthesized by this strategy
ranging from active pharmaceutical ingredients to various agro products (Rossi, Bellina, Lessi, &
4

Manzini, 2014; Steinmetz et al., 2012). There is an urgent need in this field for further development
of catalysts which can catalyze the arylation under mild reaction conditions, which can be easily
recovered and recycled and also results in high yield. So in this regard “dip catalysts” are getting
more attention because of the ease of recovery and recycling associated with them (Ahmad, Kamal,
Khan, & Asiri, 2016; Hariprasad & Radhakrishnan, 2012; Mourya, Choudhary, Basak, Tripathi,
& Guin, 2018; Xiang, Chen, Liu, & Lu, 2018; Zheng, Polavarapu, Liz-Marzán, Pastoriza-Santos,
& Pérez-Juste, 2015).
In continuation of our strategy to work towards a green and sustainable approach in
catalysis (Kandathil, Dateer, Sasidhar, Patil, & Patil, 2018; Kandathil, Fahlman, Sasidhar, Patil, &
Patil, 2017; Kandathil, Koley, et al., 2018; Manjunatha et al., 2018; Vishal, Fahlman, Sasidhar,
Patil, & Patil, 2017), herein we report a new, efficient, economical, recyclable, and
environmentally benign palladium based dip catalyst. Here, cellulose pulp was separated from
agriculture residue, sugarcane bagasse, to which biogenically synthesized palladium nanoparticles
were incorporated and was made into strips. This dip catalyst was characterized by various
analytical techniques like ATR-IR, XRD, FESEM, EDS, TGA, TEM, XPS and ICP-OES and
studied for their activity in Suzuki-Miyaura cross-coupling reactions and C5-arylation of 2-
substituted thiophenes.
2. Experimental
2.1. Materials
Palladium nanoparticles were synthesized according to our previous report (Kandathil,
Dateer, et al., 2018). The sugarcane bagasse was collected from local juice shop in Bangalore,
India. Unless otherwise stated, all reactions were performed under aerobic conditions in oven-
5

dried glassware with magnetic stirring. Palladium acetate, aryl halides, 2-substituted thiophenes,
sodium carbonate, pivalic acid, phenylboronic acid, silver oxide and sodium sulphate were
purchased from Sigma-Aldrich and Avra chemical company and were used without further
purification. Heating was accomplished by either a heating mantle or silicone oil bath. Reactions
were monitored by thin-layer chromatography (TLC) performed on 0.25 mm Merck TLC silica
gel plates, using UV light as a visualizing agent. Purification of reaction products was carried out
by flash column chromatography using silica gel 60 (230-400 mesh). Yields refer to
chromatographically pure material. Concentration in vacuo refers to the removal of volatile solvent
using a rotary evaporator attached to a dry diaphragm pump (10-15 mm Hg), followed by pumping
to a constant weight with an oil pump (<300 mTorr).
2.2. Characterization
Attenuated total reflectance infrared spectra were recorded with a Bruker Alpha Eco-ATR
spectrometer. The elemental palladium content in dip catalyst was determined by Perkin Elmer
Optima 5300 DV inductively coupled plasma optical emission spectrometer. Transmission
electron microscope images were obtained using Jeol/JEM 2100 microscope. A JEOL JSM 7100F
field-emission scanning electron microscope (FESEM), fitted with energy dispersive X-ray
spectroscopy (EDS), was used to observe the morphology and elemental distributions of samples.
Thermogravimetric analysis was carried out using a thermal analyzer (TGA/DTA) (STA-2500,
NETZSCH, GERMANY) with a heating rate of 10.0 °C/min. X-ray powder diffraction patterns
were obtained using Rigaku X-ray diffraction Ultima-IV. X‐ ray photoelectron spectroscopy study
was performed using PHI 5000 Versa probe‐ Scanning ESCA Microprobe, Physical Electronics,
USA. 1H NMR spectra were recorded at 400 MHz, and are reported relative to CDCl3 (7.27) and
6

1
DMSO-d6 (2.49). H NMR coupling constants (J) are reported in Hertz (Hz) and multiplicities
are indicated as follows: s (singlet), d (doublet), t (triplet), and m (multiplet).
2.3. Preparation of palladium nanoparticles – cellulose pulp composite
The sugarcane bagasse was air died initially to remove the moisture content and then cut
into small pieces of ~1 cm length. The dried bagasse chunks were then taken in a beaker with 5%
sodium hydroxide solution and heated to 85 °C under mechanical stirring for 5-6 hours. After that
the pulp was separated by centrifugation and washed till neutral pH with distilled water to remove
the inorganic salts present.
The palladium nanoparticles were synthesized biogenically according to method reported
in our previous work (Kandathil, Dateer, et al., 2018). A suspension of biogenically synthesized
palladium nanoparticles in ethanol (1 M) was added to the pulp prepared as above and kept under
stirring for 4-5 hours. The pulp with palladium nanoparticles was then poured to a laboratory made
set up to get a sheet of palladium nanoparticles-cellulose strip which was then cut into required
size and length after drying at 40-45 °C for catalysis applications.
2.4. General procedure for Suzuki-Miyaura cross-coupling reaction
In a 10 mL glass vial equipped with a cap containing 5 mL of ethanol:water (1:1) mixture,
aryl halide (1 equiv), phenylboronic acid (1.1 equiv), K2CO3 (2.5 equiv) were added followed by
dipping of the dip catalyst into the reaction mixture which was then stirred magnetically at room
temperature for required time. The progress of the reaction was monitored by thin layer
chromatography (TLC). After reaction completion, the dip catalyst was simply removed from the
reaction mass and washed with ethanol (1 x 5 ml) and water (1 x 5 ml) and was reused without
purifying further. The product was extracted using dichloromethane (2 x 10 mL) and the combined
organic layer was subjected to water wash (2 x 10 mL) followed by drying of the organic layer
7

over Na2SO4. The dried organic layer was concentrated in vacuo, and the product was purified by
column chromatography using n-hexane and ethyl acetate as eluents to afford the corresponding
products in good to excellent yields. All the coupled products were known molecules and were
confirmed by comparing with our previous standards (Kandathil et al., 2017; Vishal et al., 2017).
2.5. General procedure for the C5-arylation of 2-substituted thiophenes
Aryl halide (1 equiv), 2-substituted thiophene (0.8 equiv), Na2CO3 (1.5 equiv), PivOH
(1.5equiv), Ag2O (0.75 equiv) were added to a 10 mL glass vial equipped with a cap containing 5
mL of DMF followed by dipping the dip catalyst in to the reaction mass. The reaction mass was
stirred at 120 °C for required time. TLC was employed for monitoring the reaction progress. The
dip catalyst was easily removed from the reaction mass following reaction completion and washed
with ethanol (5 ml) and water (5 ml) and was used in next cycle. The product was then extracted
with dichloromethane (2 x 10 ml). The combined organic layer was subjected to water wash (1 x
10 ml) and brine wash (1 x 10 ml). The final organic layer was dried over Na2SO4 and concentrated
in vacuo. The product was purified by column chromatography using n-hexane and ethyl acetate
as eluents to afford the corresponding products in good yields. All the coupled products were
1
known molecules and were confirmed by comparing the H NMR (see supplementary data for
details) with literature reports.
2.6. Procedure for the recovery of the dip catalyst
After the reaction completion, the dip catalyst was taken out effortlessly using forceps
followed by dipping the same in ethanol and water. The washed dip catalyst was then dried at 40-
45 °C and reused in further cycles without further purification.
8

3. Results and Discussion
3.1. Biogenic synthesis and characterization of palladium nanoparticles
The palladium nanoparticles were synthesized biogenically using black pepper extract as
per our previous report (Kandathil, Dateer, et al., 2018). The palladium nanoparticles were
characterized by ATR-IR, FESEM, EDS, UV-Visible spectrophotometry and XRD and were
found in accordance with the previous report (Kandathil, Dateer, et al., 2018).
3.2. Isolation of cellulose from sugarcane bagasse and casting of the dip catalyst
The sugarcane bagasse is a fibrous residue obtained as a byproduct in sugar industries after
extraction of sugar. The cellulose was isolated from sugarcane bagasse and the dip catalyst was
prepared step-wise as depicted in Fig. 1. Initially, the sugarcane bagasse collected was dried in
sunlight and cut into small pieces and were subjected to base hydrolysis for removing the lignin
and hemicelluloses. Lignin can be effectively removed by alkaline pretreatments which is even
cost effective when compared with other pretreatments which work at high temperature and
pressure (Rabelo, Fonseca, Andrade, Filho, & Costa, 2011). After delignification, the pulp
obtained was washed with water thoroughly up to neutral pH. The cellulose pulp was casted to
strips (cellulose strip) and was characterized by ATR-IR, FESEM, and XRD to find out the
chemical composition and morphology.
Bagasse Bagasse chuBnaskes hydrolysPisulp
Pulp-Pd NPDip catalyst
mixture
9

Fig. 1. Steps involved in the preparation of dip catalyst from sugarcane bagasse and
conceptualized representation of cellulose-palladium nanoparticles interaction.
The pulp obtained was then mixed with 1M dispersion of palladium nanoparticles in
ethanol and stirred for 4-5 hours at room temperature. During this period the palladium
nanoparticles will be stabilized and immobilized on cellulose support by the strong bonding
interactions of hydroxyl groups present in cellulose as shown in Fig. 1 (He, Kunitake, & Nakao,
2003; C. Xu, Cheng, Shen, & Liu, 2007). The Vander Waals force of attraction plays the major
role in interaction of palladium with hydroxyl groups of cellulose (Zheng, Kaefer, et al., 2015).
Also, being highly oxyphilic in nature, palladium encompasses strong attraction with the hydroxyl
groups present on the cellulose (Rosario et al., 2014).Thus the hydroxyl groups on cellulose may
act as the points of anchoring for the immobilization of palladium nanoparticles through
electrostatic interaction which also prevents the aggregation of the Pd NPs (He et al., 2003; C. Xu
et al., 2007).
3.3. Spectroscopic and microscopic characterization of dip catalyst
To begin with the structure of prepared cellulose strip as well as dip catalyst was evaluated
by ATR-IR (Fig. 2). The ATR-IR spectrum of cellulose strip (Fig. 2a) shows a broad band at ~
3335 cm-1 which is the characteristic peak due to O-H stretching. Also, the peak at ~ 1643 cm-1 is
10

due to the O-H overtone vibrations. The intense peak at 1029 cm-1 is due to the C-O-C skeletal
vibrations of pyranose ring. Additional peaks at 2854 cm-1, 1424 cm-1 and 1369 cm-1 are due to
the aliphatic C-H stretching, CH2 symmetric bending and C-H bending respectively. A small peak
at 896 cm-1 indicates the C-O-C, C-C-O and C-C-H deformation and stretching vibrations (Baran,
Baran, & Menteş, 2018; Baruah, Das, Hazarika, & Konwar, 2015). Thus ATR-IR spectrum
confirms the structure of cellulose extracted from sugar cane bagasse and also it shows that the
delignification process was effective due to the absence of any other peaks, for instance, peaks
which can be attributed to carbonyl groups and other aromatic stretching vibrations. From the
ATR-IR spectrum of dip catalyst (Fig. 2b), no new peaks can be observed when compared with
Fig. 2a, but a slight shift in the wavenumbers is observed for all the peaks which can be ascribed
to the bonding interactions between palladium nanoparticles and the cellulose (Baruah et al.,
2015).
Fig. 2. ATR-IR spectra of (a) cellulose strip and (b) dip catalyst.
11

The X-ray diffractograms recorded in the range of 10-80° for palladium nanoparticles,
cellulose strip and the dip catalyst are shown in Fig. 3. The peaks (Fig. 3a) at 2 value 40.0°, 46.5°
and 68.1° corresponds to the lattice planes (111), (200) and (220) respectively which confirms face
centered cubic crystal structure of palladium nanoparticles. The XRD pattern of cellulose (Fig. 3b)
show peaks at 2 value 15.8°, 22.4° and 35.2° corresponding to the (110), (200) and (004) planes
which represents cellulose-I structure (Gong, Li, Xu, Xiang, & Mo, 2017). To identify the
crystallinity of the isolated cellulose, crystallinity index (CrI) was determined using Segal’s
equation (Du et al., 2016; Segal, Creely, Martin Jr., & Conrad, 1959). The CrI for the cellulose
strip was found to be 55.4% where as for the dip catalyst it was 60.4%. The increase in crystallinity
in the case of dip catalyst can be attributed to the hydrolysis of hemicellulose and amorphous
regions of cellulose (Du et al., 2016). After the incorporation of palladium nanoparticles on
cellulose (Fig. 3c), there was no change in the peak position but the peak intensity of palladium
nanoparticles got reduced due to the high dispersion of palladium nanoparticles on cellulose.
The thermal stability of the dip catalyst was assessed using thermogravimetric analysis.
Fig. 4 depicts the thermogram for the dip catalyst in which the removal of moisture resulted in the
initial weight loss of ~5% up to a temperature of 110 °C. The second sharp weight loss was
observed from 225 °C to 370 °C which is due to the cellulose degradation. The higher thermal
stability of the dip catalyst can be attributed to the higher crystallinity of the cellulose support.
12

Fig. 3. X-ray diffraction pattern of (a) palladium nanoparticles, (b) cellulose strip and (c) dip
catalyst.
Fig. 4. TG curve for dip catalyst.
13

(b)
(a)
(c)
Fig. 5. FESEM images of (a) cellulose strip, (b) dip catalyst and (c) dip catalyst cross-section.
The morphological study was carried out using FESEM analysis (Fig. 5). The FESEM
images of the cellulose strip, dip catalyst and cross section of dip catalyst are shown in Fig. 5a, 5b
and 5c respectively. The cellulose strip showed a rough surface which may be the result of removal
of outer non-cellulosic layer like lignin and hemicelluloses during the base hydrolysis process. A
uniform distribution of palladium nanoparticles on cellulose support can be observed in the case
of dip catalyst from Fig. 5b which further underlines the dispersion of palladium nanoparticles on
cellulose as inferred from XRD results. A cross section image of dip catalyst was obtained for
finding out the thickness of the same and a thickness of ~ 42µm was observed which is shown in
Fig. 5c. TEM analysis of the dip catalyst in dispersion form shows the even distribution of the
palladium nanoparticles on cellulose support (Fig. 6) which further confirms the FESEM analysis
results.
14

Fig. 6. TEM images of the dip catalyst
The palladium content and composition of the dip catalyst was evaluated using EDS
analysis as shown in Fig. 7. The EDS spectrum confirms the presence of palladium along with
carbon and oxygen. The percentage of palladium from EDS analysis was found to be 4%. Also,
elemental mapping of the dip catalyst proved the uniform distribution of palladium nanoparticles
in dip catalyst as shown in Fig. 7. From survey XPS spectrum, the presence of palladium, carbon
and oxygen was confirmed a fresh (Fig. S1). Also, from XPS analysis, the presence of Pd(0) and
Pd(II) was revealed. The formation of PdO can be imputed to the surface oxidation of the Pd(0)
during the XPS sampling procedure (Celebi, Yurderi, Bulut, Kaya, & Zahmakiran, 2016). The
ICP-OES analysis was performed to check the exact palladium content of the dip catalyst and
found that the palladium content was 4.19 wt% which closely matches with the EDS analysis
result.
15

Fig. 7. EDS spectrum and elemental mapping of the dip catalyst.
3.4. Application of the dip catalyst in Suzuki-Miyaura cross-coupling reaction
After understanding the chemical composition, surface morphology and thermal behavior
of the dip catalyst, Suzuki-Miyaura cross-coupling reaction was selected for the initial catalytic
activity studies. The cross-coupling of iodobenzene with phenylboronic acid was chosen as a
model reaction for the optimization of reaction conditions with the dip catalyst. A series of
reactions were carried out to find out the optimum temperature, time, catalyst loading, solvent and
base for the Suzuki-Miyaura cross-coupling reaction (Table S1). Fig. 8 represents a typical reaction
set up used for the catalytic studies with dip catalyst.
The palladium weight percentage was calculated for the dip catalyst with dimension of 1.5
cm width and 3.0 cm length. The dimensions of the dip catalyst can be varied according to the
reaction conditions and reaction set up requirement. In our study, we have varied the starting
material loading and kept the dip catalyst dimensions constant to find out the optimum catalyst
loading. The optimum catalyst loading was found to be 0.15 mol% of palladium with respect to
aryl halide.
16

Fig. 8. Typical reaction setup with dip catalyst.
The dip catalyst was found stable in most of the solvent system at room temperature. A
mixture of ethanol:water (1:1) solvent system appeared to be the best solvent for Suzuki-Miyaura
cross-coupling with the dip catalyst with good stability. When ethanol facilitates the solubility of
organic substrates, the water helps to dissolve the base. Also, the palladium nanoparticles in the
interior layers of the dip catalyst are made more available for the reactants by the weakening effect
of the hydrogen bonding network of cellulose fiber matrix by the presence of water (Shen &
Gnanakaran, 2009). Among the different bases employed, potassium carbonate was the finest.
Strong bases like sodium hydroxide and potassium hydroxide resulted in fewer yields. In
ethanol:water system with potassium carbonate base, 30 minutes was sufficient for the reaction to
complete with highest conversion and yield at room temperature. Increase in time does not increase
the yield further and decrease in time resulted in lesser yield. Also, increase in conversion and
yield were negligible at higher temperatures when compared to that of at room temperature. Thus,
the reaction at room temperature for 30 minutes in presence of potassium carbonate base and 0.15
mol% of palladium in ethanol:water (1:1) mixture as solvent turned out to be the optimized
reaction condition for Suzuki-Miyaura cross-coupling using the dip catalyst to get an isolated yield
of 98 %.
17

After optimization of the reaction conditions, we proceeded to study the substrate scope in
Suzuki-Miyaura cross-coupling reaction to find out the applicability of the dip catalyst for a broad
range of aryl halides with different functional groups and arylboronic acid. The functional groups
involved were either electron withdrawing or electron donating in nature. Also, among aryl
halides, both aryl iodides and bromides were employed. Among aryl halides, iodides being easy
leaving group when compared with bromides reacted at a faster rate due to the faster oxidative
addition step during the reaction cycle of Suzuki-Miyaura cross-coupling. In addition, the aryl
halides with electron withdrawing groups were faster to give products when compared with those
aryl halides containing electron donating groups. Again, this can be attributed to the ease in
oxidative addition of aryl halide on to the catalyst because of the electron withdrawing effect of
the functional group present on the aryl halide. From the result obtained (Table 1) it can be
substantiated that the dip catalyst is very efficient in Suzuki-Miyaura cross-coupling with high
yield and excellent functional group tolerance and at the same time the yields obtained were good
to excellent when compared with other reported dip catalysts (Hariprasad & Radhakrishnan, 2012;
Xiang et al., 2018; Zheng, Kaefer, et al., 2015).
Table 1. Suzuki-Miyaura cross-coupling of different aryl halides with arylboronic acid using dip
catalysta
18

aReaction conditions: Aryl halide (1.0 equiv), phenylboronic acid(1.1 equiv), dip catalyst(0.15
mol% palladium with respect to aryl halide), K2CO3 (2.2 equiv) and EtOH:H2O (5 mL) in
air.Yields given are isolated yield after column chromatography; average of two runs.
19

To evaluate the applicability of the dip catalyst in drug molecule synthesis, a non-steroidal
anti-inflammatory drug, Felbinac (Kylmälä, Tois, Xu, & Franzén, 2009), was also synthesized via
Suzuki-Miyaura cross-coupling reaction (Entry 3p, Table 1). The optimized condition was well
suited for the synthesis of Felbinac with above 90 % yield within two hours of time at room
temperature. The use of green solvent system and mild base at room temperature with easily
recoverable dip catalyst for the synthesis of Felbinac is noteworthy.
3.5. Recyclability of the dip catalyst in Suzuki-Miyaura cross-coupling reaction
One of the key visions while designing and employing the dip catalyst was the ease of
recovery and recyclability which makes the dip catalyst exceptionally greener. In our case, after
the successful execution of the dip catalyst in Suzuki-Miyaura cross-coupling reaction, we
advanced with recyclability studies on the model reaction. The dip catalyst can be easily removed
from the reaction mass using forceps and washed as discussed in section 2.6. The recovered dip
catalyst was reused in new reaction cycle without further purification. There was no any change
in the conversion and yield up to 13 cycles which is depicted in Fig. S4, but there was a slight
decrease in the yield from 14th cycle onwards. Moreover, after 15th recycle, the edges of the dip
catalyst slowly started disintegration which can be attributed to the abrasion caused by the
magnetic stir bar.
The ATR-IR and FESEM analysis of the dip catalyst after 15th recycle was performed to
identify the state of the catalyst which is given in Fig. S2b and Fig. S3a respectively. From ATR-
IR analysis, the chemical composition was found unaltered. Even though the chemical composition
was unaltered, the FESEM image shows the transformation of palladium nanoparticles from
spherical to needle like morphology. The change in morphology adds to the decreased activity of
the dip catalyst (Hu & Liu, 2005).
20

3.6. Application of the dip catalyst in C5-arylation of 2-substituted thiophenes
Direct arylation reactions are of prime importance in organic chemistry field due to the
wide spread applications of these reactions in the synthesis of various drug molecules and
intermediates. After the successful application of the dip catalyst in Suzuki-Miyaura cross-
coupling reaction, we explored the feasibility of C5-arylation of 2-substituted thiophenes with dip
catalyst for the first time. The C5-arylation of thiophene-2-carboxaldehyde with 1-bromo-4-
nitrobenzene was selected as the model reaction for optimizing the reaction conditions. Table S2
shows the different reactions carried out during the optimization of C5-arylation of thiophene-2-
carboxaldehyde. A broad screening of solvents revealed that polar aprotic solvents like N,N-
dimethylformamide (DMF) and N,N-dimethylacetamide (DMA) were suitable for arylation with
dip catalyst. Even though the dip catalyst was stable in most of the solvent system, disintegration
was observed when DMA and water was used as solvents at higher temperatures and for our
surprise the dip catalyst was extremely stable in DMF even at elevated temperatures. At lower
temperature there was no any starting material conversion but at higher temperature the product
formation was observed. A temperature up to 120 °C was optimal for the maximum conversion
and good yield. Also, the optimal catalyst loading was found to be 0.25 mol% of palladium. An
increase in catalyst loading does not show any remarkable improvement in conversion and a
decrease in catalyst loading resulted in lower yields. The carbonate bases proved to be good enough
for arylation in our case and the choice of the base ended up in sodium carbonate as it is milder
and cost effective than potassium carbonate and cesium carbonate. The presence of silver salt as
oxidant and pivalic acid as an additive was proved to be essential for the reaction to proceed with
the dip catalyst. There was zero or less conversion in the absence of silver oxide and pivalic acid
during the arylation reaction. The Ag(I) is believed to abstract the halide ion from the Pd(II)
21

complex formed during oxidative addition thereby making the Pd(II) complex a more electrophilic
intermediate (Lebrasseur & Larrosa, 2008; Rene & Fagnou, 2010). The pivalate generated during
the reaction of pivalic acid with sodium carbonate is expected to play an important role in cleavage
of C-H bond (Lafrance & Fagnou, 2006; Pardo, Prendergast, Nolan, Ó Muimhneacháin, &
McGlacken, 2015). The methodology we present here for the direct arylation reaction is based on
this reasoning even though the precise role of additives is still ambiguous.
Subsequent to the optimization of the reaction conditions for arylation, we then proceeded
to explore the substrate scope for the arylation reactions under optimized conditions (Table 2). The
dip catalyst was successful in catalyzing C5-arylation of 2-substituted thiophenes with aryl halide
substrate containing both electron donating as well as electron withdrawing groups. In addition,
the dip catalyst was capable to generate comparable yield with both aryl iodides as well as aryl
bromides but with aryl bromides the reaction took longer time than their aryl iodide counterparts.
Table 2. Substrate scope for the C5-arylation reaction of 2-substituted thiophenes with aryl halides
in the presence of dip catalyst.
22

aReaction conditions: Aryl halide (1.0 equiv), 2-substituted thiophene (0.8 equiv), base (2.0 equiv),
Ag2O (0.75 equiv), PivOH (1.0equiv), dip catalyst (0.25 mol% of palladium with respect to aryl
halide) and DMF (5 mL) in air; Isolated yield after purification by column chromatography; average
of two runs.
3.7. Recyclability of the dip catalyst in C5-arylation of 2-substituted thiophenes
The recyclability of the dip catalyst was studied for C5-arylation of 2-substituted
thiophenes on the model reaction. Even though the activity of fresh dip catalyst was exemplary,
especially as this is the first ever dip catalyst applied in C-H activation reaction, the recyclability
was not that appreciable. There was a steady decrease in the yield from first recycle onwards as
shown in Fig. S5. The key reason for the decreased yield in arylation reaction can be attributed to
the deposition of silver salts on the strip there by blocking the active palladium sites. In order to
23

prove this, apart from ATR-IR and FESEM analysis, EDS analysis was also performed for the dip
catalyst after the third recycle which is given in Fig. S6. The EDS results confirm the presence of
Ag on the dip catalyst surface. The ATR-IR analysis of the dip catalyst after 3rd recycle (Fig. S2c)
revealed a similar IR spectrum as fresh catalyst but an extra peak was observed at 1473 cm-1 which
may be due to the aromatic reactants left out on the surface of the dip catalyst even after washing
while the FESEM analysis of the same (Fig. S3b) confirmed the agglomeration and deposition of
palladium as well as silver nanoparticles on the surface of the dip catalyst which was confirmed
by EDS as well.
3.8. Leaching study
In order to understand the heterogeneity of the dip catalyst and palladium leaching, we
performed the leaching study on the model Suzuki-Miyaura cross-coupling reaction under
optimized condition. After 15 minutes of reaction time, the dip catalyst was removed from the
reaction mass and the reaction was allowed to continue for further 3-4 hours. But the yield obtained
was 68% which shows that the reaction did not proceed after the removal of the dip catalyst. In
order to further confirm the leaching out of palladium from the dip catalyst during reaction, the
palladium content in the reaction mass from both applications were also analyzed by ICP-OES and
the palladium content in the reaction mass was found to be 2.1 ppm in the case of Suzuki-Miyaura
cross-coupling and 8.42 ppm in the case of C5-arylation of 2-substituted thiophenes. Also, the
palladium content of the dip catalyst after 15th recycle from Suzuki-Miyaura cross-coupling was
found to be 3.70 % w/w and that of the dip catalyst after 3rd recycle from C5-arylation of 2-
substituted thiophenes was found to be 3.86 % w/w from ICP-OES analysis, which was initially
4.12 % w/w. The removal of palladium from the surface of dip catalyst, while performing the
washings of the same, after reaction completion during each cycle can also account for the slight
24

decrease in the palladium content after recycles. From all these results it can be accomplished that
the electrostatic interaction of palladium nanoparticles with hydroxyl groups of cellulose is strong
enough for holding palladium nanoparticles during the course of reaction.
4. Conclusion
In this work, we were able to develop a green and sustainable protocol for the design of the
dip catalyst with cellulose as a support, which was isolated from the agriculture residue, and
biogenically synthesized palladium nanoparticles by following a simple and efficient as well as
economical method. The strong electrostatic interaction between the hydroxyl groups of the
cellulose and palladium nanoparticles leads to the immobilization of the nanoparticles on the
substrate. The dip catalyst proved to be efficient in Suzuki-Miyaura cross-coupling reaction and
the yields obtained were good to excellent when compared with other reported dip catalysts. Also,
the applicability of the dip catalyst was studied in the synthesis of a non-steroidal anti-
inflammatory drug, Felbinac, which resulted in excellent yield. The dip catalyst also proved to be
an excellent catalyst in C5-arylation of 2-substituted thiophenes. The ease of recovery from the
reaction mass and the increased number of recycles makes the dip catalyst very significant among
various heterogeneous catalysts. In addition, the dip catalyst can also be easily scaled up due to
the ease of synthesis and green protocol involved along with the choice of deployment of the dip
catalyst in the reaction set up. Moreover, the cellulose residue which is available in abundance in
nature can be isolated by following the simple protocol presented here and can be utilized
efficiently in the field of catalysis.
Conflicts of interest
There are no conflicts to declare.
25

Acknowledgements
The authors thank Nano Mission, Ministry of Science & Technology, Department of
Science & Technology, Government of India, for the financial support through the Grant
SR/NM/NS-20/2014.
Supplementary data
XPS spectra of the dip catalyst, ATR-IR spectra and FESEM images of the recovered dip
catalyst, optimization table for Suzuki-Miyaura cross-coupling and direct arylation reactions,
recyclability studies, the analysis data and 1H NMR spectra of cross-coupled products and direct
arylated products is available in the supplementary data.
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