Preparation and Application of Palladium Nanoparticle Impregnated Chloromethylated Polysulfone Matrix as an Efficient Catalytic Membrane for Oxidation of Alcohols

Article abstract of DOI:10.1515/zpch-2019-1382

The use of palladium nanoparticles embedded in a chloromethylated Polysulfone (CMPSf) matrix was developed for highly efficient oxidation of primary and secondary alcohols to corresponding aldehyde and ketone in organic solvent free condition. Pd (Π)/bis (2, 4-dihydroxybenzaldehyde) chelate chemically incorporated onto CMPSf was used to prepare beneficial catalytic membranes. Chemical structure and thermal properties of resulting membranes were characterized via FTIR, ¹HNMR, UV-vis, TGA and DSC techniques. Morphology and particle distribution throughout the catalytic membranes was elucidated using FE-SEM. An average particle size of Pd nanoparticles was estimated about 20 nm by XRD technique. ICP technique proved that no Pd particles were leached out of the membrane into the solutions; hence the as-prepared catalytic membranes could be used several times without significant loss in their activities. This is in good accordance with formation of chemical bond between Pd and polymer matrix.

Full text of DOI:10.1515/zpch-2019-1382

Z. Phys. Chem. 2021; 235(4): 483–496
Hossein Mahdavi* and Faeze Fayyazi
Preparation and Application of Palladium
Nanoparticle Impregnated Chloromethylated
Polysulfone Matrix as an Eflcient Catalytic
Membrane for Oxidation of Alcohols
https://doi.org/10.1515/zpch-2019-1382
Received January 25, 2019; accepted November 6, 2019
Abstract: The use of palladium nanoparticles embedded in a chloromethylated
Polysulfone (CMPSf) matrix was developed for highly efficient oxidation of pri-
mary and secondary alcohols to corresponding aldehyde and ketone in organic
solvent free condition. Pd (Π)/bis (2, 4-dihydroxybenzaldehyde) chelate chem-
ically incorporated onto CMPSf was used to prepare beneficial catalytic mem-
branes. Chemical structure and thermal properties of resulting membranes were
1
characterized via FTIR, HNMR, UV-vis, TGA and DSC techniques. Morphology
and particle distribution throughout the catalytic membranes was elucidated
using FE-SEM. An average particle size of Pd nanoparticles was estimated about
20 nm by XRD technique. ICP technique proved that no Pd particles were leached
out of the membrane into the solutions; hence the as-prepared catalytic mem-
branes could be used several times without significant loss in their activities. This
is in good accordance with formation of chemical bond between Pd and polymer
matrix.
*Corresponding author: Hossein Mahdavi, School of Chemistry, College of Science,
University of Tehran, P.O. Box 14155–6455, Tehran, Iran, Tel./Fax: +98–21–66495291,
e-mail: hmahdavi@khayam.ut.ac.ir
Faeze Fayyazi: School of Chemistry, College of Science, University of Tehran, P.O. Box
14155–6455, Tehran, Iran

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H. Mahdavi and F. Fayyazi
Keywords: catalytic membrane; oxidation of alcohols; Pd nanoparticles;
polysulfone.
1 Introduction
The oxidation of alcohols to their corresponding aldehydes or ketones is an
important reaction for both organic synthesis and industrial manufacture of
fine chemicals such as dyes, medicines, agrochemicals and perfumes [1–4].
Traditionally performing such transformations have been involved with the use
of stoichiometric or more than stoichiometric amounts of chromium (VI) salts or
other metal oxidants. These reagents generate high quantities of hazardous waste
of heavy metals and accomplish the oxidation reactions in solvents such as chlo-
rinated hydrocarbons, which make them environmentally inappropriate. From an
industrial perspective, heavy metal waste and solvent consumption led to the use
of alternative processes such as transition metal catalysts with more moderate
oxidants like H2O2, O2 and air [5, 6].
Among the transition metal catalysts, palladium catalysts are capable of car-
rying out many chemical reactions such as cross-coupling, hydrogenation and
oxidation reactions [7–14]. The common problem of the homogeneous cataly-
sis system is squiring the reactions with missing of valuable metal complexes
[6, 15, 16]. Many researchers have tried to solve this problem by offering various
catalytic systems. Using catalytic membranes which are prepared from the incor-
poration of catalysts into polymeric membrane was one of the solutions offered
by researchers. In this way the simultaneous product separation and reaction
accompany with maintaining the catalyst and its catalytic activity will be possible
[17–21].
Large diversity of polymeric materials with promising advantages such as
facile film forming, low cost, solvent resistance and selective transformation of
chemical species are suited for being used as catalytic membranes [22–26]. Among
the various polymeric membranes, polysulfone (PSf) due to its good mechan-
ical strength and stability at high temperatures has high popularity in mem-
brane industry [27, 28]. In spite of its merits, PSf membranes suffer from fouling
in filtration processes because of their intrinsic hydrophobic property [27–30].
Modification of PSf in order to increase its hydrophilicity can reduce this prob-
lem [31]. Furthermore, during chemical modification, it is possible to increase
the reactivity of the main chain by introducing new reactive functional groups
that permit further chemical reactions. Hence, a great deal of reports had been
focused on the PSf modification. One of these modifications is chloromethylation,
which can be performed under simple reaction conditions. This type of modifi-
cation creates functional groups and creates possibility of chemical attachment

Preparation and Application of Palladium Nanoparticle
485
between metal catalysts and polymer chains. Consequently, uniform distribu-
tion of the nano-palladium particles throughout the membrane takes place
[31–35].
In this work we attempt to prepare and characterize a modified PSf mem-
brane containing Pd chelate as catalyst and considering its catalytic activity for
the oxidation of alcohols.
2 Materials and methods
2.1 Materials
PSf (MW 28000), chlorotrimethylsilane were purchased from Sigma Aldrich.
2,4-dihydroxybenzaldehyde, methanol (MeOH, bp 65 °C, >99.99%), 1-butanol
(BuOH, bp 118 °C, >99.99%), 2-Propanol (PrOH, bp 97 °C, >99.7%), N-methyl-2-
pyrrolidone (NMP), paraformaldehyde and chloroform were all purchased from
Merck Company. All reagents were used without further purification except PSf,
which was kept in a vacuum oven for 24 h before use.
2.2 Chloromethylation of PSf
Preparation of Chlromethylated PSf (CMPSf) was carried out by following the same
method reported by Cozan and co-workers [35]. The polymer powder was kept in a
vacuum oven at 50 °C for 24 h in order to remove moisture before using. PSf (3 g,
6.72 mmol) dissolved in chloroform (125 mL) into a flask equipped with stirrer and
reflux condenser. Then paraformaldehyde (2 g, 67.2 mmol), trichloromethylsilane
(8.5 mL) and tin (IV) chloride (12 mL) were added to the solution. The mixture
was heated at 40–45 °C for 3 days. Then the solution mixture was poured into
methanol under stirring. Finally, the precipitate was filtered and washed with
methanol and finally dried in the vacuum at 40 °C for 18 h. The degree of PSf
chloromethylation was estimated using Mohr’s titration method and was to be
12 mmol/g.
2.3 Synthesis of Pd chelate
In this study palladium chelate was synthesized according to the
method of preparing Copper (II) bis(2,4-dihydroxylbenzaldehyde) [35]. 2,4-
dihydroxybenzaldehyde (27.6 g, 0.2 mol) was dissolved in of methanol (100 mL).
Then, Pd (II) acetate (0.3 g, 1 mmol) dissolved in of methanol (100 mL), and the
stock solution was added very slowly to the above solution and kept stirring for

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H. Mahdavi and F. Fayyazi
2 h, and finally allowed to cool in water bath at 0 °C. The resulting green solid
filtered and washed with methanol and dried under vacuum at 60 °C for 18 h.
2.4 Immobilization of palladium chelate on CMPSf
CMPSf was dissolved in NMP to make 20 wt% polymer solutions. Desired amount
of Pd chelate (10, 5, 2.5 mg) was added to solution of CMPSf (5 g) in NMP (2.5 mL).
Later the solutions treated for 12 min under ultrasonic irradiation. Then they were
kept under stirring for 5 h at room temperature and preserved under air tight
condition for preparation of the nanocomposite membranes.
2.5 Preparation of membrane
The prepared solutions from previous step were cast using a Doctor Blade with
thickness near of 200 µm. Then, the plate was immediately immersed in a dis-
tilled water bath for 1 min. The formed PSf-Pd membranes were washed with
deionized water carefully to remove the residual solvent completely. The chemi-
cal metal reduction was accomplished with immersing the membranes into 0.1 M
aqueous NaBH4 solution.
2.6 Membrane performance study
To measure water permeability of the membranes, they were placed into the mea-
suring cell where 25 mL of deionized water was poured into and the pressure of
1.5 bars was applied. The water flux was calculated according to the following
equation:
Permeate volume
Jw(Lm−2h−1) =
Membrane surface area × Filtration time
Membrane water uptake was determined by a simple method. Membrane
samples were immersed in water for 48 h and weighed (Ww), then dried in the
oven for 48 h at 80 °C and weighed again (Wd). Water uptake was calculated
according to following equation:
Water uptake % = W − W /W × 100
[(
)
]
w
w
d
Alcohols (5 mL) were fed in 4 cycles without washing between the steps.
Membranes with the effective area of 3.14 cm2 were placed in the home-made cell.
At this stage, the reusability of the catalytic membrane have been investi-
gated. The reusability feature was studied fifty times for the alcohol oxidation.

Preparation and Application of Palladium Nanoparticle
487
At the end of each 10 cycles, the catalytic membrane was easily removed from
the home-made cell and then washed with deionized water, and fresh alcohols
poured in the home-made reactor to repeat the alcohol oxidation, and the results
were collected.
Finally, we studied the leaching of PSf-Pd at the recycle experiments by deter-
mination of the Pd content in the extraction solution at the end of the reaction
following a procedure using ICP-MS method its detection limits was <0.001 ppb.
2.7 Membrane performance
The alcohol oxidation reaction was carried out using glass homemade reactor as
shown on Figure 1. The effective area of 3.14 cm2 was connected in the inner core
part of the reactor using connectors. The alcohol was fed through the part the pres-
sure gauge installed. The gas was imported the reagents inlet section and cause
both oxidation of alcohol and diffusion of the products from the inside to the out-
side of section. The temperature of the reactor was controlled through in-built
electrical coil and it was monitored using a thermocouple. Reaction was carried
out at a pressure of 1.5 bar and flux remained almost unchanged (1 mL/cm2 s).
2.8 Characterizations
Transmission ATR-FTIR measurements were performed in KBr pellets with
EQUINOX 55 Bruker FTIR specterometer. The 1HNMR spectra were recorded on a
Bruker Avance 500 MHz spectrometer using CDCl3 as the solvent and TMS as the
internal standard (a reference). The scanning electron microscope images were
taken with Hitachi S-4160 electron microscopy. Differential scanning calorime-
try (DSC) measurements were done using a Q 100790 Build 275 instrument DSC
Fig. 1: Homemade cataytic membrane system cell.

488
H. Mahdavi and F. Fayyazi
with a heating rate of 20 °C/min. The temperature range was between 25 and
250 °C. Thermo gravimetric analysis (TGA) was carried out using a Q 50 (TA,
USA) instrument over the temperature ranging 30 °C–800 °C at a heating rate of
10 °C/min under a nitrogen stream. X-ray measurements were performed with a
X-pert Philips Co Kα radiation (40 kV, 40 Ma, λ = 1.54051A0) instrument over a
2θ range from 20° to 100°. UV-vis absorption spectra were collected by UV Perkin
Elmer spectrometer. Gas chromatography (GC) analyses were done on Shimadzu
gas chromatograph instrument. A Varian Spectra A-400 atomic absorption spec-
trometer equipped with a deuterium background correction and an air–acetylene
burner was used for the analysis.
FTIR (EQUINOX 55 Bruker) spectroscopy was applied for verification of oxi-
dation of alcohols. The yields of the products were analyzed using GC Shimadzu
gas chromatograph instrument. Chemical analyses (ICP: Varian VISTA- MPX) was
carried out after oxidation reaction in order to estimate the amount of palladium
washed from the membrane. The film catalysts were characterized by DSC (Q
100790 Build 275), XRD (X-pertPhilips, pw 3040/60), SEM (Hitachi S-4160), and
UV-visible (Perkin Flmer) instruments.
3 Results and discussions
3.1 Analysis and evaluation of chlromethylated
polysulfone
Chloromethylationprocess of the polysulfone polymer was confirmed qualita-
tively by FTIR (Figure 2). The results indicated clearly the presence of methylene
Fig. 2: FTIR spectra of CMPSF.

Preparation and Application of Palladium Nanoparticle
489
Fig. 3: 1HNMR spectra of chloromethylated PSf.
groups along the polymer backbone after chloromethylationreaction carried out.
This can be observed at 2993 cm−1 in FTIR spectrum, which is related to the
stretching of the methylene (CH2) groups.
The contents of chlorine ion, was determined by potentiometric titration, with
0.05 N AgNO3 aqueous solutions. The degree of substitution for the modified PSf
was estimated 6%.
The percent of chloromethylation per repeat unit (DS, degree of substitution)
was determined from the integrated intensity of proton peak at 4.56 ppm in the
1HNMR spectrum [denoted in Figure 3 as (a)], relative to that of the reference peak
at 8.0 ppm, which represents the four meta protons on the phenyl ring closest to
the sulfonyl group [denoted in Figure 3 as (b)].
3.2 Thermal gravimetric analyses
The thermal stability of the polymer membranes was studied by thermal gravi-
metric analysis (TGA). The TGA curve Figure 4 shows a weight loss in the
temperature range from 243 to 290 °C, corresponding to the loss of water and sol-
vent molecules. The content of water and solvent was calculated about 19.7%.
The residual solvent could only be removed well above its boiling point when
the polymer is in the rubbery state. A weight loss about 11.46% starts at 374 °C
is attributed to the loss of chloromethyl groups. The third weight loss of CMPSf
sample is observed at 480.96 °C corresponding to the polymeric backbone degra-
dation. It was observed that the decomposition temperature of the polymer main
chain decreased by chlromethylation. These differences could be explained by a
reduction symmetry in the polysulfone structure due to the introduction of CH2Cl
groups renders it less regular and therefore, less stable.

490
H. Mahdavi and F. Fayyazi
Fig. 4: TGA diagram of chloromethylated PSF.
The degree of chloromethylation of PSf was estimated about 12%, 11.47% and
6% by 1HNMR, TGA and Mohr’s titration, respectively. As seen the value obtained
by titration is not in agreement with that of other two methods. This is attributed to
the fact that the all chloromethylene groups on PSf are not accessible and missing
of some chloromethylene groups in titration because of slow segmental motion of
PSf chains. Hence, the obtained titrated value will be deviated from the actual
amount.
3.3 Palladium chelate analyses
Formation of complex confirmed qualitatively by FTIR technique. Clearly, pres-
ence of the aldehyde group (CHO) was confirmed by absorption peak at 1660 cm−1
(Figure 5). In addition, UV-Vis spectra of Pd chelate shows two sharp peaks
(Figure 6) which are attributed to transition of the benzenoid ring Π → Π*
(282 nm) and oxygen non-bonding electron pairs n → Π* (340 nm).
3.4 CMPSf membrane containing palladium chelate
analyses
UV-vis spectroscopy was performed to verify the incorporation of modified PSf
with Pd chelate. Significant differences between the UV-vis spectra of the PSf and

Preparation and Application of Palladium Nanoparticle
491
PSf containing Pd chelate were observed, which are shown in Figure 7. An absorp-
tion peaks at 285 nm appeared in the spectra of the PSf, this peak is assigned
to the π → π* transition of the benzenoid ring. On the other hand, in the UV-
vis spectrum of PSf containing chelate of palladium, this peak was shifted to
265 nm indicating the hypsochromic effect due to the non-hybrid electron pairs of
oxygen.
Fig. 5: FTIR spectra of palladium chelate.
Fig. 6: UV-vis absorption spectra of palladium chelate.

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H. Mahdavi and F. Fayyazi
Fig. 7: UV-vis absorption spectra of PSf and CMPSf containing palladium chelate.
In order to calculate the amount of palladium chelate atomic absorption
analysis was used. The amount of palladium per gram of Polysulfone was to be
about 20.7 ppm. The X-ray diffraction pattern of the catalytic membrane powder
is shown in Figure 8.
The Bragg reflection peaks at 2Θ equals to 40° and 67° are attributed to diffrac-
tion from the planes of (1,1,1) and (2,2,0) Average Pd particle size of 20 nm was
estimated by the Debye Scherrer formula from XRD pattern.
Fig. 8: XRD pattern of PSf membrane containing 1% Pd chelate.

Preparation and Application of Palladium Nanoparticle
493
3.5 Differential scanning calorimetery study of prepared
PSf-Pd membrane
Differential scanning calorimeter provides useful information about polymer
thermal behavior and transitions. Chemical structure of polymer chains and
interactions among them has significant effect on Tg. Some of the factors could
decrease the glass transition temperature by reducing interaction between poly-
mer chains and increasing distance between chains. For example forming func-
tional groups on the polymer chain will cause decrease of Tg by making their
mobility easier. Considering that the glass transition temperature of polysulfone
is 188 °C, the Tg of polysulfone containing palladium nanoparticles was reduced
about 14 °C compared to the bare PSf (Figure 9). It is possible to consider that pal-
ladium chelated nano particles cause to create distance between chains, and this
makes movements of PSf chains easier. The decrease in Tg of PSf after blending
with Pd chelate means that there is a certain interaction or bonding between the
two materials.
3.6 Morphological studies
SEM micrographs were used to investigate surface morphology of catalytic mem-
branes. The SEM images of the three membranes were prepared with different
percents of Pd chelate are shown in Figure 10. No significant aggregations were
found. However, as the Pd chelate content increased, the aggregation of Pd chelate
slowed down.
Fig. 9: DCS diagram of chemically modified PSf membrane containing 1% Pd chelate.

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H. Mahdavi and F. Fayyazi
b
c
a
Fig. 10: SEM micrographs of catalytic membranes surface containing (a) 0.5%, (b) 1% and (c)
2% Pd chelate Catalytic oxidation of alcohols.
Tab. 1: Comparison data of the aliphatic alcohol oxidation.
Entry
Alcohol
Presence of pure oxygen (Yield %)
Presence of air (Yield %)
1
2
3
Methanol
2-Propanol
1-butanol
40
70
60
30
40
35
The poured alcohol in the home-made cell was transported through the mem-
branes containing 1% nanoparticles and transformed to the desired products
simultaneously without using bases or solvents. With this method, the obtained
products will have more purity, and since it could be separated during the reac-
tion. Reaction was carried out at a pressure of 1.5 bar and flux remained almost
unchanged (1 mL/cm2 s).
Aliphatic alcohols in four cycles passing through the membrane at 30 °C were
oxidized in the presence of pure oxygen and air (Table 1). The obtained yield in the
presence of pure oxygen is significantly higher than presence of air.
3.7 Reusability of catalytic membrane
The reusability of catalytic membrane is a very significant theme and makes them
useful for commercial applications. The prepared catalytic membrane showed
high stability of the catalyst activity in the oxidation reactions.
In the alcohol oxidation reaction, Pursuant to table data (Table 2) it is seen
that lower decrease in activity (7%) for catalytic membrane compared to 73%
decrease for PSf membrane after fifty cycles. The high yields of oxidation products
and almost constant activity over up to 50 reaction cycles, indicate the outstand-
ing reusability of the catalytic membrane in the alcohol oxidation reaction. On the
other hand, ICP analyses proved the absence of significant catalyst leaching into
the permeate solution.

Preparation and Application of Palladium Nanoparticle
495
Tab. 2: The reusability of catalytic membrane.
Number of cycles
10
20
30
40
50
Yield %
73
70
68
68
66
4 Conclusions
Well-dispersed PSf-Pd chelate nanocomposite was synthesized through a facile
method and then used as the catalytic membrane. In summary, we have
developed an efficient and simple procedure for selective oxidation of organic
compounds, using an eco-friendly catalytic system consisting of Pd chelate cat-
alyst. Most importantly, this catalytic membrane could be reused in further fifty
time’s reactions without any significant loss of catalytic activity and nanocom-
posite catalytic membrane has a great potential for industrial production and
practical application.
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