Palladium nanoparticles immobilized on magnetic nanoparticles: An efficient semi-heterogeneous catalyst for carbonylation of aryl bromides

Article abstract of DOI:10.1016/j.catcom.2014.12.001

We describe a method for supporting palladium nanoparticles on magnetic nanoparticles modified with amino-functionalized dihydro-imidazolium groups. This catalytic system is very efficient in alkoxy- and amino-carbonylation reactions of aryl bromides. The catalyst can be easily recovered from the reaction mixture by applying an external magnetic field and reused for over five consecutive cycles without a significant decrease in its activity.

Full text of DOI:10.1016/j.catcom.2014.12.001

Catalysis Communications 61 (2015) 31–36
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Palladium nanoparticles immobilized on magnetic nanoparticles: An
efficient semi-heterogeneous catalyst for carbonylation of aryl bromides
⁎
Bishnu Dutta, Suheir Omar, Suzana Natour, Raed Abu-Reziq
Institute of Chemistry, Casali Center of Applied Chemistry, Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
a r t i c l e i n f o
a b s t r a c t
Article history:
We describe a method for supporting palladium nanoparticles on magnetic nanoparticles modified with amino-
functionalized dihydro-imidazolium groups. This catalytic system is very efficient in alkoxy- and amino-carbon-
ylation reactions of aryl bromides. The catalyst can be easily recovered from the reaction mixture by applying an
external magnetic field and reused for over five consecutive cycles without a significant decrease in its activity.
© 2014 Elsevier B.V. All rights reserved.
Received 10 September 2014
Received in revised form 10 November 2014
Accepted 2 December 2014
Available online 4 December 2014
Keywords:
Magnetic nanoparticles
Palladium nanoparticles
Nanocatalyst
Carbonylation
Aryl bromides
1. Introduction
developed. In this regard, silica [19], zeolite [20], carbon [21] and poly-
mer [22] supports were explored in the carbonylation reactions. Never-
Transition metal catalyzed carbonylation reactions, in particular,
alkoxy- and amino-carbonylation are considered as one of highly effi-
cient protocols for the synthesis of aryl carbonyl compounds [1–3].
These reactions have attracted numerous attentions and are widely ac-
knowledged as an essential tool in organic and industrial chemistry, in
which valuable building blocks for pharmaceutical, agrochemical and
biologically active compounds are synthesized [4–7]. In addition, car-
bonylation reactions are alternative methods for the traditional esterifi-
cation and amidation reactions since it endures an ample scope of
substrates [8–11]. Alkoxy- and amino-carbonylation reactions are direct
and constructive approaches for the synthesis of amides and esters via
coupling of aryl halides with an appropriate alcohols and amines. In
this context, various palladium based catalysts, such as PdCl2(PPh3)2/
PdBr2(PPh3)2 [12], PdCl2(PhCN)2 [13], Pd(OAc)2/PPh3 [14,15], and
Pd(dppp)Cl2 [16] have been utilized in the carbonylation reactions
under homogeneous conditions [17,18]. Although these catalytic sys-
tems exhibited high catalytic activity, their recovery from the reaction
mixture and reusability is challenging. Moreover, the removal of the
palladium and the toxic phosphine residues from the final product is
very complicated and uneconomic process, which makes the homoge-
neous catalysts inapplicable especially for large-scale production. To
overcome these inherent drawbacks, several methodologies and strate-
gies for heterogenization of phosphine free catalysts have been
theless, these heterogeneous catalytic systems still demand time
consuming and tedious procedures such as centrifugation and filtration
for their recovery. Hence, the development of alternative and easily sep-
arable heterogeneous catalysts while maintaining high accessibility and
catalytic activity is extremely desirable. Magnetic isolation based tech-
nique is considered as a convenient strategy which meets these require-
ments [23].
Magnetic nanoparticles (MNPs), with their intrinsic high surface
area, superparamagnetic properties, high stability and reactivity and
facile preparation have emerged as a robust heterogeneous catalyst sup-
ports which could be instantaneously isolated simply by applying an ex-
ternal magnetic field [24–27]. However, bare MNPs tend to agglomerate
within a short period of time. Avoiding this restriction could be achieved
by applying a surface coating and capping methods using organic/inor-
ganic polymers and small capping agents like silane reagents [28–34].
Apart from preventing the undesirable coagulations, various functional-
ities could be simultaneously introduced to the catalyst support.
To the best of our knowledge, magnetically retrievable palladium
based nanocatalyst has not been explored in the alkoxycarbonylation
and aminocarbonylation reactions. Herein, we report a facile synthesis
of phosphine free supported palladium nanoparticles on the surface of
MNPs modified with amino-functionalized ionic liquid (IL) groups as a
robust, efficient and easily recyclable semi-heterogeneous catalyst for
the alkoxy- and amino-carbonylation reactions of aryl bromides. These
ionic liquid groups are competent of tuning and enhancing the
dispersibility of the magnetic support and the immobilized catalyst in
various polar organic and aqueous solvents. In addition, the presence
⁎
Corresponding author.
E-mail address: Raed.Abu-Reziq@mail.huji.ac.il (R. Abu-Reziq).
http://dx.doi.org/10.1016/j.catcom.2014.12.001
1566-7367/© 2014 Elsevier B.V. All rights reserved.

32
B. Dutta et al. / Catalysis Communications 61 (2015) 31–36
Scheme 1. Synthesis of 3-(2-(1,3-dioxoisoindolin-2-yl)ethyl)-1-(3-(triethoxysilyl)propyl)-4,5-dihydro-1H-imidazol-3-ium bromide.
of the amine functionality highly contributes to the immobilization and
stabilization of the catalytically active palladium nanoparticles.
and 168.16. Elemental anal. for C22H34BrN3O5Si. Calculated: C 50.00,
H 6.48, N 7.95; Found: C 50.80, H 6.52, N 7.90.
2. Experimental
2.3. General procedure for the modification of magnetic nanoparticles with
the dihydro-imidazolium groups (MNP-Im-N)
2.1. Characterization
The magnetite nanoparticles were prepared according to Massart's
method [35]. 11.7 g FeCl3·6H2O and 4.4 g FeCl2·4H2O was dissolved
and mechanically stirred in 400 mL deionized water under inert atmo-
sphere. The mixture was heated at 85 °C followed by quick addition of
18 mL concentrated ammonia (25%) generating a black suspension of
magnetite nanoparticles. The mixture was heated for further 30 min,
and then it was allowed to cool to room temperature. The black magne-
tite nanoparticles were separated via external magnetic force and
washed 5 times with 200 mL deionized water. The magnetic nanoparti-
cles were suspended in 500 mL ethanol and sonicated for 60 min. The
resulted black suspension was mechanically stirred and a 100 mL etha-
nol solution containing the monomer Si-Im-N (5.29 g, 10 mmol) and
15 mL concentrated ammonia (25%) were added and the reaction mix-
ture was allowed to stir under nitrogen for 36 h at room temperature.
The resulted modified nanoparticles were magnetically separated and
washed with ethanol (4 × 50 mL). Finally, they were suspended in
300 mL of ethanol and sonicated for another 60 min.
Transmission electron microscope (TEM) and electron diffraction
spectroscopy (EDS) were performed with (S) TEM Tecnai F20 G2 (FEI
Company, USA) operated at 200 kV. The infrared spectra were recorded
at room temperature in transmission mode using a Perkin Elmer spec-
trometer 65 FTIR. Thermogravimetric analysis (TGA) was performed
on Mettler Toledo TG 50 analyzer. Measurements were carried out at
temperature range that extended from 25–950 °C at a heating rate of
10 °C/min under air. 1H NMR and 13C NMR spectra were recorded
with Bruker DRX-400 or 500 MHz instrument in CDCl3. Inductively
coupled plasma mass spectrometry (ICP-MS) measurements were per-
formed on 7500cx (Agilent company) using an external standard
calibration. Powder X-ray diffraction (XRD) measurements were per-
formed on the D8 Advance diffractometer (Bruker AXS, Karlsruhe,
Germany) with a goniometer radius 217.5 mm secondary graphite
mono-chromator, 2° Soller slits and 0.2 mm receiving slit. Low-
background quartz sample holders were carefully filled with the pow-
der samples. XRD patterns within the range 1° to 75° 2θ were recorded
at room temperature using CuKα radiation (λ = 1.5418 Å) with the fol-
lowing measurement conditions: tube voltage of 40 kV, tube current of
40 mA, step-scan mode with a step size of 0.02° 2θ and counting time of
1 s/step. The X-ray photoelectron spectroscopy (XPS) measurements
were performed using a Kratos Axis Ultra X-ray photoelectron spec-
trometer (Kratos Analytical Ltd., Manchester, UK) using an Al Kα mono-
chromatic radiation source (1486.7 eV) with 90° takeoff angle (normal
to analyzer).
2.4. Supporting palladium nanoparticles on the modified magnetic nano-
particles (MNP-Im-NH2-Pd)
A suspension of 100 mL of the modified magnetic nanoparticles in
ethanol was sonicated for 30 min followed by the addition of Na2PdCl4
(150 mg, 0.43 mmol) dissolved in 10 mL of deionized water. The reac-
tion mixture was mechanically stirred for 24 h, and then 6 mL hydrazine
hydrate was added. The reaction mixture was allowed to stir for further
5 h. The supported palladium nanoparticles were magnetically separat-
ed, washed with methanol (2 × 100 mL) and eventually suspended in
200 mL methanol. The loading of the palladium in the system is
0.16 mmol g−1 as determined by ICP–MS analysis.
2.2. Synthesis of 3-(2-(1,3-dioxoisoindolin-2-yl)ethyl)-1-(3-(triethoxysilyl)
propyl)-4,5-dihydro-1H-imidazol-3-ium bromide (Si-Im-N)
A mixture of N-(2-bromoethyl)phthalimide (5.06 g, 20 mmol) and
N-(3-triethoxysilylpropyl)-4,5-dihydroimidazole (5.5 g, 20 mmol) in
60 mL dry toluene was stirred under inert atmosphere at 120 °C for
48 h. The mixture was cooled down to room temperature, and the
solvent was removed under vacuum in a rotary evaporator resulting
in a brown viscous liquid. The product was washed with diethyl ether
(3 × 50 mL) to afford an off-white hygroscopic solid in 91% yield
(9.5 g). 1H NMR (400 MHz, CDCl3) δ (ppm): 0.54 (t, J = 8 Hz, 2H),1.2
(t, J = 4.4 Hz, 9H), 1.67–1.74 (m, 2H), 3.56 (t, J = 7.6 Hz, 2H), 3.79
(q, J = 7.2 Hz, 6H), 3.92–4.06 (m, 6H), 4.19 (t, J = 9.2 Hz, 2H), 7.73
(dd, J = 3.2, 2.8 Hz, 2H), 7.85 (dd, J = 3.2, 2.8 Hz, 2H), and 9.41
(s, 1H); 13C NMR (100 MHz, CDCl3) δ (ppm): 6.91, 18.29, 21.00, 35.22,
47.62, 48.39, 48.50, 50.52, 56.99, 58.50, 123.64, 131.67, 134.40, 158.63,
2.5. General procedure for the alkoxycarbonylation reaction of
bromoarenes
A 25 mL glass lined autoclave was charged with bromoarene
(4 mmol), triethylamine (8 mmol) and MNP-Im-NH2-Pd (0.02 mmol
Pd in 5 mL dry alcohol). The autoclave was sealed, purged three times
with carbon monoxide (CO), and pressurized to 500 psi with CO. The re-
action mixture was stirred at 130 °C for 24 h. The autoclave was cooled
to room temperature and CO was carefully released. The catalytic
system was magnetically separated and the solution decanted and
evaporated under vacuum in a rotary evaporator. The product was di-
luted with ether (20 mL), washed with aqueous hydrochloric acid (2
Scheme 2. Modification of magnetite nanoparticles with Si-Im-N.

B. Dutta et al. / Catalysis Communications 61 (2015) 31–36
33
Scheme 3. Preparation of palladium nanoparticles supported on magnetite nanoparticles.
× 10 mL, 1N), dried over anhydrous magnesium sulfate and evaporated
under reduced pressure. The products were purified by column chro-
matography on silica gel (ethylacetate:hexane 20:1 as an eluent sol-
vent) to afford the desired products.
drawbacks. It is noteworthy that in addition to the protection strategy,
the MNPs can be easily functionalized with various coupling agents or
functional groups. In our study, the MNPs were protected successfully
via facile covalent functionalization with the silane monomer 3-(2-(1,3-
dioxoisoindolin-2-yl)ethyl)-1-(3-(triethoxysilyl)propyl)-4,5-dihydro-
1H-imidazol-3-ium bromide. As depicted in Scheme 1, this silane mono-
mer was synthesized by reacting N-(3-triethoxysilylpropyl)-4,5-
dihydroimidazole with N-(2-bromoethyl)phthalimide in dry toluene
under inert atmosphere at 120 °C for 48 h. After cooling and washing,
the desired silane monomer was obtained as an off-white hygroscopic
solid in 90% yield.
The modification process of the MNPs was accomplished by reacting
the triethoxysilane groups of the Si-Im-N with the surface hydroxyl
groups of the MNPs via sol–gel condensation under basic conditions in
ethanol for 36 h (Scheme 2). While the bare magnetite nanoparticles
are not dispersible in organic solvents, the modified MNPs were highly
dispersible in water and polar organic solvents such as methanol and
ethanol.
The modified MNPs were utilized for supporting palladium nanopar-
ticles. Initially, Na2PdCl4 was adsorbed via ion exchange on the surface
of the MNPs by stirring the MNPs containing imidazolium units with
the palladium salt in ethanol for 24 h, followed by reducing the palladi-
um ions with hydrazine hydrate. Simultaneously; the phthalimide
groups were removed by reaction with the excess of hydrazine hydrate
to afford amine groups (Scheme 3). While the presence of the
imidazolium groups enhances the dispersibility of the resulted material
in polar organic solvents, the existence of amine groups provides coor-
dination sites that can strengthen the immobilization of the palladium
NPs on the magnetite surface. When bare MNPs were utilized as the
solid support, no palladium NPs formed on the surface of the MNPs as
confirmed by transmission electron microscope (TEM) and scanning
transmission electron microscope/energy dispersive X-ray spectrosco-
py (STEM/EDS) analysis (Supporting information, Fig. S1).
2.6. General procedure for the aminocarbonylation reaction of
bromoarenes
A 25 mL glass lined autoclave was charged with bromoarene
(4 mmol), amine (8 mmol), triethylamine (8 mmol) and MNP-Im-
NH2-Pd (0.02 mmol Pd in 5 mL dry toluene). The autoclave was sealed,
purged three times with carbon monoxide (CO), and pressurized with
CO to 500 psi. The reaction mixture was stirred at 130 °C for 24 h. The
autoclave was cooled to room temperature and CO was carefully re-
leased. The catalytic system was magnetically separated and the solu-
tion decanted evaporated under vacuum in a rotary evaporator. The
product was diluted with ether (15 mL), washed with water (3
× 5 mL), dried over anhydrous magnesium sulfate and evaporated
under reduced pressure. The products were purified by column chro-
matography on silica gel (ethylacetate:hexane 90:10 as an eluent sol-
vent) to afford the desired products.
3. Results and discussion
3.1. Synthesis of palladium nanoparticles supported on ionic liquid modified
magnetic nanoparticles
First, the MNPs were prepared via simple co-precipitation method of
iron salts, FeCl2 and FeCl3 in a basic aqueous solution at 85 °C according
to Massart's method [35]. The resulted particles were spherical in shape
with a size range of 5–20 nm. Nonetheless, an unavoidable obstacle as-
sociated with this size is that the particles are intrinsically unstable and
tend to agglomerate in order to reduce their surface energy. In addition,
bare MNPs are hardly dispersible in any medium. As a result, several
surface modifications and protection approaches of the bare MNPs
have been introduced in order to overcome the aforementioned
The MNP-Im-NH2-Pd catalyst was characterized by TEM (Fig. 1). The
TEM micrographs confirmed the formation of magnetite nanoparticles
with a size range of 5–20 nm in addition to the creation of palladium
50 nm
20 nm
Fig. 1. TEM micrographs of palladium supported on magnetic nanoparticles modified with amino-functionalized dihydro-imidazolium groups.

34
B. Dutta et al. / Catalysis Communications 61 (2015) 31–36
nanoparticles. STEM/EDS and X-ray photoelectron spectroscopy (XPS)
analyses also indicated the presence of the palladium nanoparticles on
the surface of the modified magnetite nanoparticles (Supporting infor-
mation, Fig. S2 and S3, respectively). In the XPS spectrum, the binding
energy of MNP-Im-NH2-Pd system exhibits two strong characteristic
peaks at 334.9 eV and 340.1 eV, which are ascribed to Pd 3d5/2 and Pd
3d3/2, respectively (Supporting information, Fig. S3).
The catalytic system was further analyzed by powder X-ray diffrac-
tion (XRD). The XRD pattern displayed the characteristic peaks of
MNPs at 2θ = 18.3, 30.1, 35.5, 43.3, 53.5, 57.1, 62.6, 71 and 74 with a
cubic unit cell (Fig. 2a). Additionally, it shows the characteristic peaks
of the palladium nanoparticles at 2θ = 40, 46.6 and 67.5 which match
well with the characteristic peaks of fcc palladium crystal structure
(Fig. 2b). These peaks are attributed to (100), (200) and (220) reflec-
tions, respectively. According to Scherrer equation the average crystal-
lite size of the palladium nanoparticles is 7.04 nm.
Further characterization was conducted by thermal gravimetric
analysis (TGA) to study the thermal decomposition of the bare MNPs,
MNP-Im-N and MNP-Im-NH2-Pd systems. The thermal decomposition
analysis was carried out in the temperature range of 25–950 °C under
air. This analysis showed an increase in the organic content after
functionalizing the MNPs with the silane monomer Si-Im-N with a
21% weight loss (Fig. 3c) compared to the bare magnetite nanoparticles
with weight loss of 3% (Fig. 3a), which belongs to solvent residues.
Whereas, the MNP-Im-NH2-Pd system exhibited weight loss of 18% in-
dicating a lower percentage of organic content due to the removal of the
phthalimide protecting group by hydrazine hydrate (Fig. 3b).
The removal of the phthalimide group was also confirmed by infra-
red analysis (Fig. 4). The disappearance of the C_O stretching vibration
peak at 1710 cm−1 and C–N (of the imide groups) peak at 1398 cm−1 in
MNP-Im-NH2-Pd (Fig. 4a), substantiates that the phthalimide groups
were disconnected successfully during the reduction of the palladium
salts with hydrazine hydrate.
Fig. 3. TGA curves of a) bare MNPs; b) MNP-Im-NH2-Pd and c) MNP-Im-N.
diverse alcohols. The results are presented in Table 1. Bromobenzene
reacted efficiently with various alcohols such as methanol, ethanol
and benzyl alcohol providing the corresponding products in high yields
(Table 1, entries 1, 5 and 6). The methoxycarbonylation of the substitut-
ed bromobenzene derivatives, 4-bromoanisole, 4-bromoacetophenone
and 4-bromotoluene afforded the proper ester in a yield of 90%, 92%
and 90% (Table 1, entries 2–4), respectively. These results show that
the presence of electron-donating and electron-withdrawing groups
has an insignificant effect on the catalytic efficiency.
Furthermore, the reactivity of the nanocatalyst MNP-Im-NH2-Pd
was investigated in the carbonylation reaction of bromobenzene with
phenol in the presence of Et3N and 500 psi CO at 130 °C (Table 1,
entry 7). This reaction was conducted in various solvents such as tolu-
ene, water, DMF and dioxane affording 85%, 0%, 50% and 60% yields, re-
spectively. Under the same reaction condition and with toluene as a
solvent, 4-bromoacetophenone reacted with phenol to provide the cor-
responding ester product in a yield of 86% (Table 1, entry 8). The
nanocatalyst MNP-Im-NH2-Pd was further applied in the amino-
carbonylation reaction of aryl bromides with various amines. First, a re-
action of bromobenzene and aniline was conducted with 0.5 mol%
MNP-Im-NH2-Pd in toluene under 500 psi CO at 130 °C for 24 h. 86%
of the desired amide product was obtained (Table 2, entry 1). However,
when other solvents such as DMF and water were used in the same re-
action, the yield was decreased to 75% and 70%, respectively.
3.2. Catalysis
The catalytic efficiency of the MNP-Im-NH2-Pd system was ap-
praised in two types of carbonylation reactions of aryl bromides:
Alkoxycarbonylation and aminocarbonylation. The alkoxycarbonylation
reactions were conducted with 0.5 mol% MNP-Im-NH2-Pd in dry meth-
anol at 130 °C under a constant CO pressure of 500 psi. Initially, the in-
fluence of the base type applied in these reactions was investigated
using bromobenzene and methanol as substrates. When inorganic
bases such as K2CO3 and Cs2CO3 were utilized, methyl benzoate was ob-
tained in a yield of 80% and 75%, respectively. However, the yield of the
desired product was increased to 90% when Et3N was employed.
The scope of the nanocatalyst MNP-Im-NH2-Pd in the alko-
xycarbonylation was further extended to a various aryl bromides and
Under the aforementioned reaction conditions, carbonylation
reaction of different aromatic and aliphatic amines with aryl bromides
was investigated (Table 2). The reaction of cyclohexylamine and
Fig. 2. XRD pattern of a) bare MNPs and b) MNP-Im-NH2-Pd.
Fig. 4. Transmission FTIR spectra of a) MNP-Im-NH2-Pd and b) MNP-Im-N.

B. Dutta et al. / Catalysis Communications 61 (2015) 31–36
35
Table 1
Table 2
Alkoxycarbonylation reactions of aryl bromides with various alcohols catalyzed by MNP-
Aminocarbonylation reactions of aryl bromides with various amines catalyzed by MNP-
Im-NH2-Pda.
Im-NH2-Pda.
aReaction conditions: 4 mmol aryl bromide, 8 mmol amine, 0.5 mol% MNP-Im-NH2-Pd,
8 mmol Et3N, 500 psi CO, 24 h at 130 °C, 10 mL toluene.
bIsolated yield.
aReaction conditions: 4 mmol aryl bromide, 5 mL alcohol, 0.5 mol% MNP-Im-NH2-Pd,
8 mmol Et3N, 500 psi CO, 24 h at 130 °C.
modified with amino-functionalized dihydro-imidazolium groups. The
modification of the magnetic nanoparticles with these groups was car-
ried out to enhance the dispersibility of the magnetic nanosupports in
polar organic solvents and stabilize the immobilized palladium nanopar-
ticles via coordination with the amine groups. The nanocatalyst devel-
oped in our study could be isolated easily by applying an external
magnetic field and recycled five times without a significant decrease in
its reactivity.
bIsolated yield.
c4 mmol phenol, 5 mL toluene.
benzylamine with bromobenzene afforded the desired amide
product in 84% and 80% yields, respectively (Table 2, entries 3 and 4),
whereas 4-bromoanisole with aniline provided an 80% yield (Table 2,
entry 2). Furthermore, substituted amines such as 4-methoxyaniline
and 4-methylaniline underwent aminocarbonylation reaction with
bromobenzene to afford 79% and 75% yields of the amide products
(Table 2, entries 5–6), respectively.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2014.12.001.
3.2.1. Recyclability of MNP-Im-NH2-Pd
Recovery and reuse of the heterogeneous catalysts is one of the
major concerns due to the perspectives of green chemistry. The recycla-
bility of the nanocatalyst MNP-Im-NH2-Pd was assessed in the
methoxycarbonylation reaction of bromobenzene with methanol. Due
to the magnetic properties of MNP-Im-NH2-Pd, the catalyst was easily
separated from the reaction mixture simply by applying an external
magnetic field and recycled over 5 times without observing any signifi-
cant loss in its catalytic activity (Fig. 5). In addition, ICP–MS analysis was
utilized to examine if there was a leaching of palladium species to the
reaction medium. This analysis revealed that 1.3 ppm of palladium ex-
ists in the solvent after methoxycarbonylation of bromobenzene. How-
ever, this result indicates that the leaching extent of the palladium
species from the magnetic nanosupport is negligible.
4. Conclusion
In this work, we have developed magnetically retrievable semi-
heterogeneous catalytic system for the alkoxycarbonylation and
aminocarbonylation reactions of aryl bromides. This catalytic system is
based on supporting palladium nanoparticles on magnetite nanoparticles
Fig. 5. Recyclability of MNP-Im-NH2-Pd nanocatalyst in the methoxycarbonylation of
bromobenzene.

36
B. Dutta et al. / Catalysis Communications 61 (2015) 31–36
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