Novel iota carrageenan-based RhCl3 as an efficient and recyclable catalyst in Suzuki cross coupling

Article abstract of DOI:10.1016/j.mcat.2020.110841

RhCl₃ was heterogenized into renewable polysaccharide supports, and the effect of polysaccharide type on the new catalyst performances in a Suzuki cross-coupling reaction was studied. The conversion of the fresh iota carrageenan heterogeneous system (?-RhCl₃), was found to be the highest, whereas it was lower than the conversion of its homogeneous analogue. In addition, the ?-RhCl3 (catalyst loading of 6.5 % wt) was proven to be efficient heterogeneous catalyst that was easily recycled, whereas the conversion was increased in the first and second cycles. Scanning electronic microscopy (SEM) combining Energy dispersive X-ray spectrometry and Surface analysis by X-ray photoelectron spectroscopy were performed, confirming that RhCl₃ was embedded within the ?-carrageenan. The Fourier-transform infrared spectrometry of the heterogeneous ?-RhCl₃ catalyst was compared to that of the native polysaccharide, and no new bands were detected. Nonetheless, a comparison of SEM image of ?-carrageenan with and without RhCl₃, as well as rheological measurements of the aqueous solution of ? with and without RhCl₃, indicated the incorporation of the polysaccharide with the RhCl₃.

Full text of DOI:10.1016/j.mcat.2020.110841

MolecularCatalysis486(2020)110841
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Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Novel iota carrageenan-based RhCl3 as an efficient and recyclable catalyst in
Suzuki cross coupling
Sivan Leviev, Adi Wolfson*, Oshrat Levy-Ontman*
Department of Chemical Engineering, Sami Shamoon College of Engineering, Basel/Bialik Sts., Beer-Sheva, 8410001, Israel
A R T I C L E I N F O
A B S T R A C T
Keywords:
RhCl3 was heterogenized into renewable polysaccharide supports, and the effect of polysaccharide type on the
new catalyst performances in a Suzuki cross-coupling reaction was studied. The conversion of the fresh iota
carrageenan heterogeneous system (ἰ-RhCl3), was found to be the highest, whereas it was lower than the con-
version of its homogeneous analogue. In addition, the ἰ-RhCl3 (catalyst loading of 6.5 % wt) was proven to be
efficient heterogeneous catalyst that was easily recycled, whereas the conversion was increased in the first and
second cycles. Scanning electronic microscopy (SEM) combining Energy dispersive X-ray spectrometry and
Surface analysis by X-ray photoelectron spectroscopy were performed, confirming that RhCl3 was embedded
within the ἰ-carrageenan. The Fourier-transform infrared spectrometry of the heterogeneous ἰ-RhCl3 catalyst was
compared to that of the native polysaccharide, and no new bands were detected. Nonetheless, a comparison of
SEM image of ἰ-carrageenan with and without RhCl3, as well as rheological measurements of the aqueous so-
lution of ἰ with and without RhCl3, indicated the incorporation of the polysaccharide with the RhCl3.
Heterogeneous catalysts
Polysaccharides
Red algae
Suzuki cross coupling
1. Introduction
inorganic supports can be used for this purpose, such as oxides [9,10]
and polymers [11,12].
Transition metals have found numerous applications as catalysts,
either in their metallic form or as the center of metal complexes [1–6].
The main reason for their excellent performance is that they can be in a
variety of oxidation states, and easily interchange between these states,
thus lend electrons or withdraw electrons from different reagents. As
such, transition metals act by forming complexes with the reagent.
Although many research groups have focused on preparation of very
active and selective transition metal complexes (TMCs), only few in-
dustrial applications with TMCs are available. That is because homo-
geneous complexes suffer from several critical disadvantages, such as
high price, unique handling requirements, difficult to reuse and recycle,
and non-contentious mode operation. Therefore, heterogenization of
these complexes, thus combining the advantages of homogeneous and
heterogeneous catalysis, could translate the power of homogeneous
metal complex to industry [7,8].
Many TMCs and especially those of palladium were found to be
attractive catalysts for coupling reactions. Among all CeC coupling
reactions with a palladium catalyst, the Suzuki reaction of aromatic
halides with phenylboronic acid is of major importance in organic
synthesis and has drawn a lot of attention since it was first introduced
40 years ago [13–16]. The reaction can be catalyzed by different pal-
ladium metals, salts or complexes, and the addition of an organic or
inorganic base is necessary to neutralize the forming acid due to the
formal exchange of a hydrogen atom with the aryl group.
Besides recycling of the catalysts, a major problem with the Suzuki
homogeneous reactions is the formation of palladium metal, which is
very difficult to recover. Thereby, heterogeneous supported palladium
catalysts were also extensively studied especially on carbon support,
but in most cases the activity and selectivity were much lower than the
reaction with palladium salts or complexes and leaching of palladium
was observed [17]. Heterogenization of palladium complexes in order
to attain its recycling and recovery was also studied, using various
methods [17–20]. Finally, since the introduction of the first palladium
catalyst for the Suzuki reaction, many other metals were efficiently
employed for the reaction including nickel [21–23], iron [24,25],
copper [26] and rhodium [27–30]. Rhodium-based catalysts, which
In general, TMC heterogenization can be achieved via immobiliza-
tion of the TMC to a support or its entrapment or occlusion in a support
matrix [7,8]. The attachment can be accomplished via ionic or covalent
bonds or electrostatic interactions, whereas entrapment is realized by
proper matching between the TMC nature and the size of the support
pores as well as the polarity of the reaction solvent. Various organic and
⁎ Corresponding author.
E-mail addresses: adiw@sce.ac.il (A. Wolfson), oshrale@sce.ac.il (O. Levy-Ontman).
https://doi.org/10.1016/j.mcat.2020.110841
Received 14 December 2019; Received in revised form 10 February 2020; Accepted 13 February 2020
2468-8231/©2020ElsevierB.V.Allrightsreserved.

S. Leviev, et al.
MolecularCatalysis486(2020)110841
were found to be useful for the Suzuki reaction with halobenzens that
bare also aldehydes and nitriles on their aromatic ring, were also used
as nano- and micro-particles on different supports, yielding efficient
heterogeneous catalysts [31–36].
2.2. Heterogeneous catalyst preparation
A typical procedure for heterogeneous catalyst preparation was
performed as follows: First, 1 g of polysaccharide was dissolved in
100 ml DDW by heating the solution to 50 °C, and the mixture was then
stirred for several hours until a homogeneous solution was obtained.
Then 10 μmol RhCl3 was added to 3 ml DDW in a second vial, and
mixed by vortexing for homogenization. Next, it was poured together
with 3 ml of 1% wt/v of the polysacharide solution into a 15 ml poly-
propylene, sealed and mixed by vortexing for homogenization. The next
step was to deep-freeze the tube at -20 °C for 24 h, until all the liquid
was frozen. Then, the seal was removed and the tube was covered with
a paraffin sheet pierced by a disposable toothpick. The tube was then
lyophilized for 48 h. At the end of this process, the dried’ sponge-like
‘catalyst was cut into ∼ 1 cm × 1 cm pieces and added to the reaction
mixture.
We recently introduced a very easy and straightforward technique
to immobilize TMCs in polysaccharide matrices [37–39]. The hetero-
geneous catalysts were prepared by simply mixing an aqueous solution
of pallidum or rhodium salts with sodium triphenylphosphine trisulfo-
nate (TPPTS) as a ligand, e.g., Pd(OAc)2(TPPTS)2, PdCl2(TPPTS)2, RhCl
(TPPTS)3, with aqueous solution of polysaccharides, e.g., Carrageenan
types iota (ἰ), kappa (κ), lambda (λ) xanthan-gum (x), followed by deep-
freezing and lyophilization. The resultant xerogels, ‘sponge-like’ solid
catalyst systems, were characterized by new linkage between the sul-
fonic acid sodium salt on the TPPTS ligand and the hydroxyl groups on
the polysaccharide, as demonstrated by FTIR analysis, allowing catalyst
recycling without leaching of the complex [37–39].
Due to their renewable origin, biodegradability and low toxicity,
polysaccharides derived from natural sources, like cellulose, carragee-
nans and chitosan, have recently found a lot of applications in the
pharmaceutical, biomedical products, cosmetic, food, and building
blocks industries [40–44]. Various polysaccharides were also employed
as supports for metal catalysts [45–49] and in particular as palladium
[50–60] and rhodium [33] catalysts for Suzuki reaction, yielding het-
erogeneous and recyclable systems.
2.3. Reaction procedure
In a typical procedure, 10 μmol of RhCl3 (homogenous or hetero-
geneous in the form of xerogel) were added to a vial with 5 ml ethanol
together with 0.5 mmol halobenzene, 0.75 mmol phenylboronic acid
and 0.6 mmol Na2CO3. The mixture was placed in a preheated oil bath
at 60 °C and magnetically stirred for 24 h. At the end of the reaction, the
reaction mixture was cooled. For cases in which a heterogeneous cat-
alyst was used, the catalyst was removed by filtration. The organic
phase was then analyzed to determine the reaction conversion by gas
chromatography (GC) by using a HP-5 column.
As previously mentioned, rhodium and palladium complexes with
TPPTS as ligand were immobilized to various polysaccharides, via the
formation of a new sulfate ester, while simple palladium salts, i.e.,
PdCl2 and Pd(OAc)2, or palladium complex with triphenyl phosphine
(TPP), i.e., PdCl2(TPP)2 and Pd(OAc)2(TPP)2, were leached to the re-
action mixture [37,39]. In addition, it was found that addition of
polysaccharides to homogeneous reaction mixtures with palladium salts
[39] or complexes [37] increased the conversion rate. This enhance-
ment effect could be explained due to the hydrophilic nature of the
polysaccharides, i.e., hydroxyl groups on the polysaccharides supplied a
water like environment that accelerates the reaction [61,62].
In contrary to the reactions with palladium, addition of a poly-
saccharide to the homogenous reaction with rhodium chloride resulted
in a low conversion rate [39]. Hence, it was suggested that unlike
palladium, rhodium itself can interact with the functional groups on the
polymer backbone, and thus can be immobilized to the polysaccharide
even without a ligand. In accordance with these former findings, this
study focuses on elucidation of the rhodium-polysaccharides interaction
and evaluating its potential to be used as heterogeneous catalyst for the
Suzuki reaction.
2.4. Leaching tests
Leaching of the catalysts was tested as follows: (1) performing a
second reaction after the removal of the catalyst from the original re-
action mixture and the addition of the catalyst to a fresh reaction
mixture with the corresponding amounts of fresh substrates and sodium
carbonate, then checking the catalytic performance; (2) proceeding
with the reaction after the removal of the catalyst by running the re-
action mixture under similar conditions for an additional 24 h, to check
whether the conversion increases with time; and (3) by doing an in-
ductively-coupled plasma optical emission spectrometry (ICP-OES)
(Arcos, Spectro) analysis of the reaction medium after a 24 h reaction
time and the removal of the catalyst, to check for rhodium leftovers in
the solution.
In this study we report for the first time the heterogenization of
simple rhodium chloride (RhCl3), without any ligand, on various
polysaccharides derived from natural sources. The new heterogeneous
catalysts were used in the Suzuki cross-coupling of halobenzenes with
phenylboronic acid together with sodium carbonate in ethanol (Fig. 1).
2.5. Catalyst recycling
Catalysts were recycled by adding the recovered catalyst to a solu-
tion with similar amounts of fresh substrates and base and then running
the reaction mixture under similar reaction conditions for an additional
24 h.
2. Materials and methods
2.6. FTIR analysis
2.1. Reagents and polysaccharide supports
Lyophilized native ἰ and dried ἰ’ sponge-like ‘catalyst underwent
Fourier-transform infrared (FTIR) analysis by using a Nicolet 6700 FTIR
(Thermo Scientific, USA) spectrophotometer with an attenuated total
All chemicals (analytical grades) and polysaccharides were pur-
chased from Aldrich Israel.
Fig. 1. Suzuki cross-coupling of halobenzene
and phenylboronic acid.
2

S. Leviev, et al.
MolecularCatalysis486(2020)110841
reflectance (ATR) accessory outfitted with a diamond crystal. The re-
corded spectra were the means of 36 spectra taken in the
650–4000 cm−1 range with 0.5 cm−1 resolution and atmospheric cor-
rection applied at room temperature (25 °C).
coupling of phenylboronic acid or halobenzene, (i.e. Ullmann cou-
pling), reference reactions without the addition of RhCl3, and with or
without the presence of Na2CO3, were first tested and did not yield any
biphenyl. Moreover, it was previously reported that palladium con-
taminants in commercially available Na2CO3 were responsible for the
generation of biaryls [63], yet ICP-OES analysis of Na2CO3 solution did
not yield any trace of palladium. Performing the reaction in ethanol,
using RhCl3 as the catalyst yielded biphenyl (Table 1, entry 1). As ex-
pected, addition of water to the reaction mixture in ethanol tre-
mendously increased the activity (Table 1, entry 2), probably due to
better dissolution of the catalyst, sodium carbonate and phenylboronic
acid in the reaction mixture [37,60–62]. Yet, addition of aqueous so-
lution of ἰ to the reaction mixture, resulted in lower conversion
(Table 1, entry 3) than the reaction with water and without poly-
saccharide (Table 1, entry 2), in contrary to the addition of aqueous ἰ
solution to the Suzuki reactions when PdCl2 and PdCl2(TPPTS)2 were
used as catalysts [37,39]. Furthermore, addition of the polysaccharide
Guar Gum (G), which has some solubility in ethanol, to the reaction
mixture, without addition of water, also yielded lower conversion than
the homogeneous reaction in ethanol (Table 1, entry 4), again in con-
trary to the reaction with palladium catalysts [37]. Thus, it seems that
that rhodium could have the ability to coordinate or interact directly
with polysaccharides even without the addition of TPPTS ligand. At
last, as expected [37, 39], replacing iodobenzene by bromobenzene
yielded lower conversion (Table 1, entry 6), and using less active ar-
ylhalides, i.e. chlorobenzene or 2-bromoaniline, yielded even lower
conversion (Table 1, entry 7 and 8, respectively), yet proceeding the
reaction after 24 h yielded increased conversions rates.
2.7. SEM analysis
The dried ἰ’ sponge-like ‘catalyst was previously coated with gold.
Scanning electronic microscopy (SEM) was performed on a FEI Quanta
200 (ThermoFisher). Acceleration voltage was 25 kW.
2.8. Energy dispersive X-ray spectrometry (EDS)
Elemental analysis of the dried ἰ’ sponge-like ‘catalyst was per-
formed by SEM on a FEI Verios 460 l XHR (extreme high resolution,
Hillsboro, OR, USA) equipped with an energy-dispersive X-ray spec-
troscopy detector (EDS, Oxford Instruments).
2.9. Surface analysis by X-ray photoelectron spectroscopy (XPS)
XPS data were collected using an X-ray photoelectron spectrometer
ESCALAB 250 ultrahigh vacuum (1·10−9 bar) apparatus with an AlKα
X-ray source and a monochromator. The X-ray beam size was 500 μm
and survey spectra were recorded with pass energy (PE) 150eV, and
high energy resolution spectra were recorded with pass energy (PE)
20eV. To correct for charging effects, all spectra were calibrated re-
lative to a carbon C 1s peak positioned at 284.8 eV. Processing of the
XPS results was carried out using AVANTGE software.
A possible mechanistic pathway of the Suzuki cross-coupling with
the new catalyst, i.e., PSRhCl3 can be divided into four steps: (a) oxi-
dative addition of aromatic halides, R–X, to the catalyst yielding R-
PSRhCl3-X, (b) removal of the halide by sodium carbonate to form R-
PSRhCl3−CO3Na (c) transmetalation between the new rhodium species
and phenylboronic acid (R'-B(OH)2) to form the intermediate R-
PSRhCl3-R', and (d) reductive elimination, which releases the product,
R-R', to regenerate the catalyst. At last, though the common oxidation
state of rhodium is +3, like in RhCl3, it can also be in oxidation states
from 0 to +6. Thus, it seems that during the oxidative addition step at
the beginning of the catalytic cycle, the active species Rh(I) is oxidized
to Rh(III), and then regenerated back to Rh(I) during the reductive
elimination [64,65].
In the next stage the ability of ἰ to act as support for RhCl3 was
tested. At first, mixing aqueous solution of RhCl3 with aqueous solution
of ἰ followed by deep freezing and lyophilization yielded a 'sponge-like'
catalyst, designated ἰ-RhCl3 (Fig. 2A). After 24 h the ἰ-RhCl3 catalyst
yielded a conversion of 37 % (Table 1, entry 5), lower than that of the
homogeneous parent catalyst (Table 1, entry 1). Again, this is as op-
posed to the reactions with ἰ-PdCl2(TPPTS)2 and ἰ-Pd(OAc)2(TPPTS)2,
which yielded higher conversion than the homogeneous reactions with
PdCl2(TPPTS)2 and Pd(OAc)2(TPPTS)2 [37, 38].
2.10. Rheological measurements
The viscosity of 0.5 % w/v aqueous solution of ἰ, with and without
10 μmol of RhCl3 (that were prepared as described above), was de-
termined using HAAKE RotoVisco 1 (Thermo Scientific) equipped with
an extended temperature cell for temperature control using stainless
steel cone-and-plate (d =60 mm and θ = 0.5°). The samples were
measured at T =25 °C as a function of the shear rate in an upward
sweep from 1 to 1000 s−1. In addition Dynamic viscoelastic char-
acterization of the samples was also determined by the frequency de-
pendence (between 0.05−20 Hz) of the storage and loss moduli, G‘(ω)
and G“(ω), respectively, at T =25 °C. Measurements were carried out
using a rheometer TA AR2000 (TA instruments, New Castle Delaware,
USA) equipped with stainless-steel parallel plate (d =40 mm).
3. Results and discussion
The investigation began by performing homogeneous Suzuki cross
coupling of iodobenzene and phenylboronic acid (Fig. 1), using Na2CO3
as a base (Table 1). As biphenyl can be also synthesized via homo-
Next, the ability of ἰ-RhCl3 to be recycled and its reproducibility was
tested (Table 2, entry 1, Fig. 2). The new heterogeneous systems could
be easily separated from the reaction mixture and recycled three times,
whereas proceeding the reaction with the filtrated reaction mixture
after removal of the catalysts did not result in increase of the conver-
sion, thus indicating that the catalyst was not leaching. Moreover,
surprisingly the first and second recycling of the catalysts (Fig. 2B, 2C)
yielded higher conversion than the former reaction cycle (entry 1). In
addition, as can be seen in Fig. 2, the color of the catalyst became
darker from cycle to cycle. It can be attributed to the formation of
rhodium nanoparticles during the reaction, as was previously detected
when ἰ-PdCl2(TPPTS)2 was used as a catalyst under similar reaction
condition [37]. Yet, like in the ἰ-RhCl(TPPTS)3 system [39], analyzing i-
RhCl3 catalyst using TEM after reaction did not show any nanoparticles
(data not shown). Alternatively, the catalyst color might have been
changed in accordance to different coordination state of the metal due
Table 1
Comparison of homogeneous and heterogeneous reactions with rhodium based
catalystsa.
Entry Catalyst Substrate
Additive
Conversion (%)
1
2
3
4
5
6
7
8
RhCl3
RhCl3
RhCl3
RhCl3
ἰ-RhCl3
ἰ-RhCl3
ἰ-RhCl3
ἰ-RhCl3
Iodobenzene
Iodobenzene
Iodobenzene
Iodobenzene
Iodobenzene
Bromobenzene
Chlorobenzene
Non
50
87
49
8
37
17
6.2
6.1
2 mL distilled water
2 mL aqueous 1% wt/v ἰ
5 ml 1% wt/v G in ethanol
Non
Non
Non
b
b
b
b
2-bromoaniline Non
a
Reaction conditions: 0.5 mmol halobenzene, 0.75 mmol phenylboronic
acid, 10 μmol RhCl3, 0.6 mmol Na2CO3, 5 ml ethanol, 60 °C, 24 h.
b
Heterogeneous system.
3

S. Leviev, et al.
MolecularCatalysis486(2020)110841
Fig. 2. Representative pictures of ἰ-RhCl3 (A) lyophilized ἰ-RhCl3 (B) ἰ-RhCl3 in the reaction solution after the 1st recycling (C) ἰ-RhCl3 in the reaction solution after the
2nd recycling.
group was found in the polysaccharide backbone, like in the case of X
(entry 4), the RhCl3 also did not leach, probably due to coordination of
the metal center to the carboalkoxy group. Yet, the X-based catalyst was
partly dissolved in the reaction mixture, thus could not be recycled for
more than one time. The other polysaccharide-based RhCl3 catalysts
were either partly dissolved in the reaction mixture and/or showed
leaching of the RhCl3 (Table 2, entries 5–8). Moreover, when only hy-
droxyl groups were found on the polysaccharide the conversion was
very low (Table 2, entry 8), while when only hydroxyl and carboxylic
acid groups were found, the activity rate was higher but part of the
catalysts also leached (Table 2, entries 5–7).
In order to verify that the ἰ-based RhCl3 catalyst did not leach out
from the support, the reaction mixtures of the various cycles were run
for an additional 24 h under similar reaction conditions after removal of
the catalyst by filtration. In all the heterogeneous reactions the con-
versions of the reaction after removal of the catalyst and mixing for an
additional 24 h were not changed. Furthermore, ICP-OES detection of
the reaction mixtures after 24 h reaction and removal of the hetero-
gonous catalysts did not show any rhodium leftovers, as rhodium
content was below detection limit (< 1 ppm), revealing that the cata-
lyst were indeed heterogeneous.
As was proven from the leaching tests, that all the RhCl3 (0.002 g)
was loaded on the ἰ support (0.065 g), the calculated weight catalyst
loading percentage was 6.5 % of rhodium. Since the ἰ -based RhCl3
catalyst was the most efficient heterogeneous system, we chose to study
the nature of the interaction between the RhCl3 and ἰ. At first FTIR
analysis was employed (Fig. 3). As previously reported addition of
metal complexes of palladium or rhodium with TPPTS as ligand to ἰ
resulted in the formation of new sulfonic esters, by the interaction be-
tween the sulfonic acid on the ligand and the hydroxyls on the polymer,
hence demonstrating a new characteristic band at 1030 cm−1 [37–39].
As illustrated in Fig. 3, the FTIR spectra of native ἰ and ἰ-RhCl3 (Fig. 3A
and B, respectively) were similar, containing all the expected peaks
[68–71]. Although it might be that there is overlapping or low intensity
band/s in the ἰ-RhCl3 spectrum, it also could indicate that no new bond
was formed between the metal and the polysaccharide, thus implying
that it might be that the active groups on the polysaccharide were co-
ordinated to the metal.
Table 2
Performance comparison between different polysaccharide-based RhCl3 cata-
lysts in Suzuki reaction in ethanola.
Entry Polysaccharide Functional
Groups
Branched/ Conversion
Conversion
(%)
Linear
(%) 1st/2nd
/
3rd/4th
Leaching
cycle
testb
1
2
3
4
Iota (ἰ)
-OH,
-OSO3
-OH,
-OSO3
-OH,
-OSO3
-OH
Linear
37/45/56/
33
37
26
25
24
−
−
−
Kappa (κ)
Lambda (λ)
Linear
25/26/23/
18
Linear
25/27/17
Xanthan gum
(X)
Branched
24/30
-CH2OCOCH3
-COO−
-OH
5
6
7
Alginate (A)
Linear
24/18/7
27
29
32
28
2
-COO−
-OH
Karaya Gum
(K)
Tragacanth
Gum (T)
Branched
Branched
Branched
-COO−
-OH
16
-COO−
-OH
8
a
Guar Gum (G)
2/5
Reaction conditions: 0.5 mmol iodobenzene, 0.75 mmol phenylboronic
acid, 10 μmol RhCl3, 0.6 mmol Na2CO3, 5 ml ethanol, 60 °C, 24 h.
b
Proceeding with the 1st reaction after the removal of the catalyst by run-
ning the reaction mixture under similar conditions for an additional 24 h.
to its interaction with the polysaccharide. The change in the co-
ordination state of the metal can also explain the activation of the
catalyst during the cycles. Another explanation can be that the xerogel
swelling was changed during the cycles, allowing higher diffusion rates,
resulting in higher conversion rates.
To further investigate the effect of the polysaccharide on both the
RhCl3 leaching and recycling, RhCl3 was heterogenized using the same
procedure on different types of polysaccharides and employed in the
Suzuki reaction (Table 2).
From the results in Table 2, it is clear that the catalysts that were
supported on the polysaccharides with sulfate groups, i.e., ἰ, κ and λ,
were superior and could be recycled at least 2 times without leaching of
the RhCl3 to the reaction mixture (entries 1–3). Hence, it seems that the
RhCl3 coordinates to the sulfate group, yielding a new complex. Indeed
it was previously reported that rhodium can form complexes with dif-
ferent sulfur based molecules [66,67]. Furthermore, when an ester
To further investigate the heterogeneous catalyst structure, SEM
analysis was also performed. The SEM image of ἰ-RhCl3 was char-
acterized by a macro-porous structure (Fig. 4) which differed from the
laminar surface observed for the native ἰ, as illustrated previously in our
4

S. Leviev, et al.
MolecularCatalysis486(2020)110841
in the literature for the typical binding energy of Rh+3 [78–82]. These
findings demonstrate that the rhodium oxidation state was not changed
and kept its original form, which was derived from RhCl3, i.e +3. In
addition, the Cl2p spectrum consists of one doublet, corresponding to
binding energy of 198.72 (Fig. 5B), indicating a terminal Cl species
[80]. Finally, the observed atomic ratio of Rh:Cl in the heterogeneous
catalyst was around 3:1, as in the homogenous salt, as illustrated in
Table 3.
In order to better understand the specific interaction between the
RhCl3 and the ἰ, a physicochemical analysis was also employed.
Viscosity measurements of both aqueous ἰ solutions, with and without
RhCl3, were performed and are shown in Fig. 6. In general, both pre-
parations yielded typical non-Newtonian shear thinning behavior, in-
dicating that the shear stress reduces the viscosity. However, the ἰ-RhCl3
system yielded higher viscosity in comparison to the native ἰ solution.
This substantial increase in viscosity could be ascribed to screening of
intermolecular electrostatic repulsions by RhCl3, leading to a greater
degree of self-association [83].
The mechanical spectra of both aqueous solutions, i.e. the storage
(or elastic) modulus (G’) and viscous (or loss) modulus (G”), were also
measured as function of frequency (Fig. 7). It was found that in both
preparations, at low frequencies, the mechanical spectrum is similar to
that of dilute solutions (G” > G'), because there is sufficient time for the
chains’ relaxation. However, at high frequencies, there is no sufficient
time for recovery, and the solution responds as if it were a solid
(G' > G”). In addition, both prearations exhibited a crossover region,
while subjected to increasing frequencies, in which the storage modulus
crossed the loss (Fig. 7).
Up to 10.58 Hz the neat ἰ aquaous solution showed higher G” than
G', indicating that the solution is more viscous than elastic (Fig. 7A).
However, in the range above 2.3 Hz ἰ solution with RhCl3 yielded G’
values that exceeded those of the G” (Fig. 7B). Moreover, the G’ values
of the solution with RhCl3 were higher than those that were measured
with the neat ἰ solution. Hence, the ἰ aqueous solution with RhCl3
yielded an elastic gel-like structured system with more stable structure,
probably by promoting the increased association of the chain poly-
saccharide molecules. However, the moduli is still frequency depen-
dent, and G' values are less than 10 times larger than G”, indicating that
the desired developed gels were yet to be achieved.
Indeed, specific metal ions of different valency can form non- and
specific interactions with carrageenan solutions. These interactions can
cause aggregation of different carrageenan forms, leading to more vis-
cous systems, with stabilized gel structures [[83–88]. For example, an
addition of divalent Ca2+ ion to ἰ solution has a pronounced helix-
stabilizing effect in inducing gel formation [87,89]. The Ca-sensitivity
of ἰ is attributed to the joint effect of Ca2+ ions and water molecules in
mediating a strong interaction between the sulfate groups of neigh-
boring helices [87].
Fig. 3. Infrared spectra of native ἰ-polysaccharide (black curve) and ἰ-RhCl3
(light blue curve) (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article).
former publication [37,39] (Fig. 4). This difference indicates that the
RhCl3 supplement caused a change in the overall new structure and
implies that the RhCl3 was interacting with the polysaccharide. It was
previously reported that addition of various cations of different valence
to the anionic sulfate groups in ἰ-carrageenan aqueous solution yields
ionic forces and can produce sol-gel by the formation of a ‘double-helix’
structure [72–75]. In addition, the cations can also interact with the
hydroxyl groups on the carrageenan structure, as previously proposed
regarding different metal cations supported on carrageenans [76].
However, though, addition of PdCl2(TPPTS)2 to ἰ, i.e., ἰ-PdCl2(TPPTS)2,
was characterized by porous and ordered surfaces, when compared
with native ἰ [37], addition of RhCl3 to ἰ, i.e., ἰ-RhCl3, yielded a much
more porous and ordered system, revealing that the interactions of the
polymer with RhCl3 are different. Moreover, while PdCl2 leached out
from the ἰ-PdCl2 system, RhCl3 showed no leaching from ἰ-RhCl3, im-
plying that the rhodium is immobilized to the surface.
Elemental analysis of the ἰ-RhCl3 catalyst was also performed, using
SEM-EDS (Fig. 4). As expected, the analysis indicated that RhCl3 was
embedded within the ἰ, with molar elemental ratio of Cl:Rh 3:1 as de-
rived from the homogenous salt.
A surface-sensitive technique, XPS, was also applied to analyze the
surface composition of the ἰ-RhCl3 in comparison to the native ἰ. As seen
in Table 3, there was no significant difference between the atomic basic
elemental ratios and the peak of their binding energies in both samples.
In general, the C1 s XPS spectra for each sample could be decomposed
into three main contributions at different binding energies (BE), which
are associated with different types of elements that are linked to the
carbon atoms: aliphatic carbons, CeO and OeCeO, and R-SO3–. In
addition, the S2p3 could be composed into two different elements,
which are associated with the R-SO3– binding, depending on the R
structure [77]. Indeed, all these assignments are in accordance to the
polysaccharide structure. Therefore, the XPS analyses clearly demon-
strated that no new covalent bonds were formed between the poly-
saccharide and the rhodium chloride during the xerogel preparation.
As expected, the surface area of ἰ-RhCl3 contained rhodium, whereas
no rhodium was present in the native ἰ (Table 3). This is evidence that
the rhodium was successfully embedded into the polysaccharide matrix.
Moreover, the Rh3d doublet (Fig. 5A) represents a single rhodium state
with a Rh3d5/2 binding energy of 309.92 eV, which is in good agree-
ment with the values in the range of 309.7–310.3 eV that were reported
4. Conclusions
To conclude, RhCl3 was sucessfuly immobilized to ἰ in a very simple
and straightforward method, and was sucessfuly employed in a Suzuki
cross-coupling reaction. In addition, the salt did not leach out to the
reaction mixture and the catalyst was successfully recycled with an
increase in activity in first and the second recycles. Furthermore, the
characterization of the lyophilized ἰ-RhCl3 system showed that although
there is probably no new bond between the salt and the polymer, as
demonstrated by FTIR, the salt is embedded whithin the support.
Furthermore, SEM images show that addition of RhCl3 to ἰ yielded a
more porous and ordered structure in comparison to native ἰ, implying
on an interaction between the two. Additionally, XPS analysis showed a
typical binding energy of Rh+3 where the atomic ratio of Rh:Cl in the
heterogeneous catalyst was around 3:1. Finally, viscosity measurements
also revealed that RhCl3 interacts with the polysachride, as addition of
RhCl3 to aquaous ἰ solution reslted in increase of viscosity. Thus, it was
5

S. Leviev, et al.
MolecularCatalysis486(2020)110841
Fig. 4. Overall SEM-EDS analysis of various sections of RhCl3 supported on ἰ (A) Magnitude 40X (B) Magnitude 1000X.
Table 3
Summary of XPS data of native ἰ and the ἰ-RhCl3.
ἰ
ἰ-RhCl3
Element
Peak BE (eV)
Atomic (%)
Peak BE (eV)
Atomic (%)
C1 s
C1 s
C1 s
Total C
O1 s
S2p3
S2p3
Total S
Cl2p
284.77
286.21
287.66
27.25
39.72
33.03
54.69
39.53
53.26
46.74
5.78
–
284.82
286.37
287.59
32.04
43.74
24.22
53.25
39.67
46.17
53.83
5.92
532.86
168.75
170.38
532.66
168.78
169.96
–
–
198.72
309.92
1.52
0.58
Fig. 5. High resolution XPS spectra of ἰ-RhCl3 in the A) Rh3d region B) Cl2p
region.
Rh3d5
–
suggested that the rhodium salt was not entraped within the poly-
sacharide matrix but immobilized by ἰ-carrgennan coordination to
RhCl3 via the sulfate groups in their backbone.
CRediT authorship contribution statement
Sivan Leviev: Formal analysis. Adi Wolfson: Writing - original
draft, Supervision. Oshrat Levy-Ontman: Writing - original draft,
Supervision.
6

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MolecularCatalysis486(2020)110841
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Declaration of Competing Interest
There are no conflicts to declare.
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