Palladium nanoparticles supported on chitin-based nanomaterials as heterogeneous catalysts for the Heck coupling reaction

Article abstract of DOI:10.3762/BJOC.16.201

In this report, chitin and chitosan nanocrystals were used as biomass-based supports for Pd nanoparticles (NPs) used as a heterogeneous catalyst for the Heck coupling reaction. By using a one-pot fabrication method, a Pd salt precursor was directly reduced and deposited onto these nanocrystal catalysts. Characterization of these nanocomposites showed disperse Pd NPs on the surfaces of the chitinous nanocrystals. Heck coupling model reactions revealed full product yield in relatively benign conditions, outcompeting the use of other catalysts supported on biomass-based nanomaterials, including cellulose nanocrystals. These initial results show the potential for using chitinous nanomaterials as effective catalyst supports in cross-coupling reactions.

Full text of DOI:10.3762/BJOC.16.201

Palladium nanoparticles supported on chitin-based
nanomaterials as heterogeneous catalysts
for the Heck coupling reaction
Tony Jin1, Malickah Hicks1, Davis Kurdyla2, Sabahudin Hrapovic2, Edmond Lam*2
and Audrey Moores*1,3
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2020, 16, 2477–2483.
1Department of Chemistry, McGill University, Montreal, Quebec H3A
0B8, Canada, 2Aquatic and Crop Resource Development Research
Centre, National Research Council of Canada, Montreal, Quebec H4P
2R2, Canada and 3Department of Mining and Materials Engineering,
McGill University, Montreal, Quebec H3A 0E9, Canada
https://doi.org/10.3762/bjoc.16.201
Received: 12 August 2020
Accepted: 22 September 2020
Published: 07 October 2020
Email:
This article is part of the thematic issue "Green chemistry II".
Associate Editor: L. Vaccaro
Edmond Lam* - edmond.lam@cnrc-nrc.gc.ca; Audrey Moores* -
audrey.moores@mcgill.ca
* Corresponding author
© 2020 Jin et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
chitin; chitosan; Heck coupling; heterogeneous catalysis;
nanomaterial
Abstract
In this report, chitin and chitosan nanocrystals were used as biomass-based supports for Pd nanoparticles (NPs) used as a heterogen-
eous catalyst for the Heck coupling reaction. By using a one-pot fabrication method, a Pd salt precursor was directly reduced and
deposited onto these nanocrystal catalysts. Characterization of these nanocomposites showed disperse Pd NPs on the surfaces of the
chitinous nanocrystals. Heck coupling model reactions revealed full product yield in relatively benign conditions, outcompeting the
use of other catalysts supported on biomass-based nanomaterials, including cellulose nanocrystals. These initial results show the
potential for using chitinous nanomaterials as effective catalyst supports in cross-coupling reactions.
Introduction
Over the past decades, biomass-based nanomaterials have cluding drug delivery [3], food packaging [4], environmental
become a highly prevalent topic of research owing to their remediation [5], and catalysis [6]. With their high solubility and
sustainability, bioavailability, unique structural and morpholog- presence of functionalities such as hydroxy groups, sulfate half-
ical characteristics [1]. Particularly dominant in this field are esters, and carboxylates, CNCs are able to stabilize highly
cellulose nanocrystals (CNCs), which are rod-like nanocrystal- disperse metal nanoparticles (NPs), which can act as heterogen-
lites liberated from lignocellulosic biomass under acid hydroly- eous catalysts for a wide array of organic transformations [7-9].
sis conditions [2]. A spectrum of applications have been investi- Furthermore, the chiral nature of polysaccharides has also been
gated over the years for this sustainable bio-nanomaterial in- used as a tool for enantioselective catalysis such as carbonyl
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hydrogenations and amino acid hydrolysis, proving the unique nanoscale, which can allow for higher accessibility of their
ability of these biomass-based supports [10,11].
functionalities towards better stabilizing dispersed metal nano-
particle catalysts, along with increased solubility in aqueous
Chitin is another type of biomass feedstock that has attracted media. Very recently, we have shown that these bio-based
similar attention to cellulose. Found primarily in squid, insects, nanomaterials could stabilize highly disperse Au species on the
fungi, and the shells of crustaceans (shrimp, crab, and lobster), surface of these nanocrystals to create a highly active catalyst
chitin is the second-most abundant biopolymer after cellulose, for aromatic nitro reduction and aldehyde–amine–alkyne (A3)
with an annual availability of over 6 million tons from crus- coupling reactions [16]. Off this discovery, in this letter, we
tacean shell waste alone [12]. With shell waste being often further expand the scope of using both ChNCs and ChsNCs as a
discarded back into the sea or in landfills, it is imperative that catalyst support for Pd NPs to allow access towards other highly
downstream applications be developed such that environmental relevant C–C bond-forming reactions. A one-pot fabrication
concerns and disposal costs for this neglected resource are method is used to deposit Pd NPs directly onto both ChNCs and
reduced through the creation of bio-based sustainable technolo- ChsNCs, and the as-made heterogeneous catalysts were tested
gies [13]. In this manner, strategies for fabricating CNCs have with the Heck coupling reaction as a model for catalytic activi-
been adapted for chitin nanocrystals (ChNCs). From the ty.
seminal discovery of ChNCs by Marchessault in 1959 [14],
much work has been done to improve the monodispersity, mor- Findings
phology, and structure of this unique nanomaterial [15]. Very The fabrication of ChNC and ChsNCs was conducted using a
recently, we have reported the use of ammonium persulfate as a procedure previously reported by our group (Lam) (see Sup-
mild oxidizing agent to liberate the nanocrystallites existing porting Information File 1) [16]. ChNCs were treated with am-
within bulk chitin to yield ChNC with carboxylate functionali- monium persulfate (APS) for 16 h to form disperse ChNCs after
ties [16]. Moreover, deacetylation of ChNCs in alkaline condi- washing. ChsNCs were made by deacetylating ChNCs in the
tions, in the presence of NaBH4, led to chitosan nanocrystals presence of concentrated NaOH as well as a small amount of
(ChsNCs) with abundant amine groups.
NaBH4 (Scheme 1).
While groups are starting to investigate the usage of chitin and As seen through transmission electron microscopy (TEM) in
chitosan-based supports for heterogeneous catalysis, there are Figure 1, a relatively uniform distribution for both ChNCs and
still scarce investigations on using these biomaterials on the ChsNCs with individual rod-like nanocrystals was observed,
Scheme 1: Pathway for the formation of ChNC and subsequently ChsNCs from bulk chitin.
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Figure 1: TEM micrographs of (a) ChNCs and (b) ChsNCs. Both samples were stained and prepared on glow-discharged C-coated Cu TEM grids.
with an average length of 231 ± 38 nm for the ChNC and Figure S3) where broadening of the peaks was observed for
159 ± 34 nm for the ChsNC (Supporting Information File 1, ChsNC as amorphization of the internal ChNC structure
Figure S1). These measures were made from parts in the grid occurred during the deacetylation process. ChsNCs were readily
where the nanocrystals were well separated. Larger aggregates suspendable in aqueous media and formed a transparent solu-
of the individual nanocrystals were also observed in all samples. tion, owing to their positively charged amino functionality,
Glow-discharged carbon-coated TEM grids were used, along while ChNCs were less easily suspended. Zeta potential mea-
with uranyl acetate as a negative stain in order to provide higher surements of −24.6 mV for ChNCs and +36.8 mV for ChsNCs
contrast to the individual rods.
provided a rationale for these observations. With these differ-
ences between the two nanomaterials, we then explored how
they behaved as catalyst supports for Pd NPs.
Further characterization of the ChNCs and ChsNCs confirmed
the structural and chemical functional properties of these nano-
materials. We turned to Fourier transform infrared (FTIR) spec- A one-pot synthesis method was used to both deposit Pd salts
troscopy to access the degree of deacetylation (DDA) of the and reduce them into NPs onto the support material. First,
prepared materials (Supporting Information File 1, Figure S2). PdCl2 was mixed for 15 min with either ChNC or ChsNC in an
The ratio of primary amine over the sum of nitrogen-containing acidic aqueous medium to form a dark yellow mixture. This
functionalities can be derived through the measurement of the step facilitated coordination of Pd salts onto the support,
N–H bend and C–O stretch peaks absorbance, found at as evidenced when using CNC as support [7]. From their
1560 cm−1 and 1030 cm−1, respectively [17]. In general, bulk synthesis involving oxidative conditions, both ChNC and
chitin has a DDA of 0–20%, while chitosan has a DDA of ChsNC featured carboxylate functionalities on their surface
>80% [18]. The fabricated ChNCs had DDA values of 5–10%, which we expected to be good chelating functionalities for
while the ChsNCs had DDA values between 80–95%. A full Pd(II) (Scheme 1) [16]. For ChsNC, amines were unlikely to
spectral assignment for all of the FTIR peaks can be found in play any coordinating role, since they should be fully proto-
our recent report [16].
nated under acidic conditions. Then, the mixtures were subju-
gated to 4 bar H2 for 2 h at room temperature to reduce Pd(II)
This transformation of the acetamide functionality into an into metallic Pd NPs (Scheme 2), and the reaction mixture
amine one had drastic effects on the physicochemical proper- turned black. We also conducted a control study in the exact ex-
ties of the nanomaterials. Specifically, the deacetylation of perimental parameters were performed on PdCl2 and either
ChNC into ChsNC led to a decrease in crystallinity in the nano- ChNC or ChsNC, but with no H2 reductant. In this case, the
material. Indeed, this can be seen in the FTIR with the broad- solution color remained yellow, indicating that using either
ening of the N–H and O–H stretches from 3000–3500 cm−1. ChNC or ChsNC alone cannot fully reduce PdCl2. The
This was more notable in the powder X-ray diffraction (PXRD) resulting hybrid materials are noted PdNP@ChNC and
spectra of ChNCs and ChsNCs (Supporting Information File 1, PdNP@ChsNC, respectively.
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avoid any artefact in Pd imaging [19]. Dispersed “packets” of
Pd NPs were observed for both samples, with far more packets
observed for PdNP@ChNC (packet diameter of 42 ± 10 nm)
samples compared to PdNP@ChsNC (packet diameter of
24 ± 7 nm) samples for the same wt/wt loading of the PdCl2 salt
to ChNC/ChsNC (initially set to 1.6 wt %). It is also noted that
the packets found in PdNP@ChsNC were almost half as small
relative to PdNP@ChNC. At higher magnification, these
packets are seen to be extremely small Pd NPs agglomerated
together (Supporting Information File 1, Figure S5). A similar
packet formation was observed when the wt/wt loading of
PdCl2 was reduced by to 0.8 wt % to fabricate PdNP@ChNC
Scheme 2: Catalyst fabrication method for the deposition of Pd NPs
onto chitin (PdNP@ChNC) and chitosan (PdNP@ChsNC).
Dihydrogen was selected here because it is one of cleanest (Supporting Information File 1, Figure S6).
reductants in this context as it will limit the production of by-
products to chloride salts, by opposition to more classic X-ray photoelectron spectroscopy (XPS) was used to confirm
reducing agents such as NaBH4. Prior to characterization, the the oxidation state of Pd on both the support materials
non-dried samples were purified by dialysis. The zeta potential (Figure 3). The experimental XPS spectra were deconvoluted
measurements for PdNP@ChNC and PdNP@ChsNCs were and their match with thus obtained fitted data confirmed.
−13.9 and +57.9 mV, respectively. PdNP@ChsNCs were, Firstly, Pd on PdNP@ChNC was mainly Pd(0), with the
again, far more suspendable in aqueous solution as compared to Pd 3d5/2 peak residing at 335.1 eV, along with a small shoulder
PdNP@ChNCs. TEM micrographs of PdNP@ChNC at higher binding energy indicating the presence of Pd(II)
(Figure 2a) and PdNP@ChsNC (Figure 2b) confirmed com- (Figure 3a). In contrast, Pd primarily exists as Pd(II) on
plete immobilization of Pd NPs onto both the ChNC and PdNP@ChsNC (Figure 3b), which could be attributed to three
ChsNC, with energy dispersive X-ray (EDX) spectroscopy possible reasons: 1) the presence of PdCl2, 2) the oxidation of
confirming the presence of Pd (Supporting Information File 1, Pd NPs into PdO, and 3) the complexation of Pd(II) by
Figure S4). PdNP@ChNCs self-aggregated while drying during ChsNCs. To address the first point, a survey XPS scan showed
the TEM sample preparation procedure, despite the use of glow no Cl contribution to the overall atom distribution suggesting
discharged TEM grids. Conversely, the PdNP@ChsNCs were that no PdCl2 species were present in the nanocomposites. For
dispersed owing to the higher solubility of ChsNC. Both the second point, high-resolution XPS spectra on the O 1s scan
PdNP@ChNCs and PdNP@ChsNCs were imaged unstained to of both the PdNP@ChNC and PdNP@ChsNC samples show
Figure 2: TEM micrographs of (a) PdNP@ChNCs and (b) PdNP@ChsNCs. The samples were placed on glow discharged TEM grids, but unstained.
The images were taken purposefully with high contrast and large objective aperture to capture the nanocrystals.
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Pd NPs on ChsNC than ChNCs. This point is corroborated by
the very significant increase in zeta potential from +36.8 mV to
+57.9 mV from ChsNCs to PdNPs@ChsNCs, which can only
be explained by the integration at the nanocrystals surface of
positively charged species, namely Pd(II). This resistance to
reduction was surprising and in contradiction to what we ob-
served with deposition onto CNC of Pd in the presence of H2
[7], or Ag alone [20]. The striking difference between CNC on
one hand and ChNC/ChsNC on the other is the presence of
carboxylates on the latter. Carboxylates are expected to afford
much stronger coordination to Pd(II) than OH typically present
in CNC, and potentially prevent its full reduction.
Through FTIR (Supporting Information File 1, Figure S2) and
XRD (Supporting Information File 1, Figure S3) analysis, there
is little to no structural changes occurring in either the ChNC or
the ChsNC during catalyst fabrication. The lack of metallic Pd
peaks present in XRD is indicative of extreme broadening of the
reflections of very small Pd NPs within the packets found.
Figure 3: High-resolution X-ray photoelectron spectroscopy of the
Pd 3d region of (a) PdNP@ChNC and (b) PdNP@ChsNC. Deconvolu-
tion of the experimental peaks (grey line) of each spectrum with a fitted
(black line) leads to the Pd(0) (orange line) and Pd(II) (blue line)
doublets.
Heck coupling is a prominent reaction for arene alkenylation, as
the production of stilbene derivatives is highly relevant in areas
of research such as pharmaceuticals and materials technology
the exact same symmetric peak, similar to that of bare ChNC [21]. Furthermore, works in heterogeneous catalysis have
and ChsNC, indicating that no formation of a Pd–O bond is shown that the catalyst support plays a major role in the activi-
present (Supporting Information File 1, Figure S7). A more ty of transition-metal NPs such as Pd [7]. A model reaction was
accurate explanation is through the third point where Pd(II) is performed under the conditions listed in Table 1, at 90 °C for
present on ChsNC over ChNC, an observation further validated 24 h using only PdNP@ChNC at 1 mol % Pd relative to iodo-
by TEM micrographs that showed consistently fewer metallic benzene. Full product yield was achieved, and another replicate
Table 1: Heck coupling reaction optimization.a
entry
catalyst
time (h)
temperature (°C)
yieldb
1
2
3
4
5
6
7
8
PdNP@ChNC
24
24
6
90
70
90
90
90
90
90
90
100
<1
35
38
52
3
PdNP@ChNC
PdNP@ChNC
PdNP@ChNCc
PdNP@ChNC (0.8 wt % Pd)d
PdNP@ChsNC
ChNCe
24
24
24
24
24
0
PdCl2 and ChNC
43
aAll reactions listed used 0.2 mmol of iodobenzene and 0.24 mmol of styrene, and a Pd loading relative to iodobenzene of 1 mol %, unless otherwise
specified. The solvent was 1:1 acetonitrile/water. bYield was determined through GC–MS with hexamethylbenzene as an internal standard (Support-
ing Information File 1, Figure S8). cReaction done with 0.5 mol % Pd loading relative to iodobenzene. dReaction done with PdNP@ChNC using
0.8 wt % Pd relative to ChNC, as opposed to 1.6 wt % like the standard PdNP@ChNC. ePd loading is 0 mol %.
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was done for accuracy (Table 1, entry 1). However, a decrease organic solvents such as toluene. The comparison with recent
in the reaction temperature to 70 °C yielded virtually no prod- work in using chitin microspheres shows that chitin-based
uct (Table 1, entry 2). By keeping the system at 90 °C and supports are potentially valuable support materials, with full
shortening the time to 6 h, 35% product yield was already ob- product conversion in only 10 h, yet with mostly organic sol-
tained, indicating a promising reaction rate (Table 1, entry 3). If vents (4:1 DMF/H2O) [26].
the Pd loading was lowered to 0.5 mol %, a stark drop in the
yield was observed (Table 1, entry 4). In entry 5 (Table 1), Conclusion
PdNP@ChNC with a lower Pd wt % relative to ChNC was used ChNCs and ChsNCs were explored as sustainable supports for
which yielded a lower product yield of 52% despite retention of immobilizing Pd NPs to fabricate heterogeneous catalysts for
1 mol % loading relative to iodobenzene. The ChNC is poten- the Heck coupling reaction. Through TEM and XPS analysis,
tially hindering the ability for substrates to interact with the Pd metallic Pd NPs were formed and dispersed on the surface of
sites at such low wt % of Pd on the surface of the ChNC the supports, while FTIR and PXRD showed little to no struc-
support.
tural change to the biomaterials after metal deposition. Heck
coupling results demonstrate the importance of using ChNCs as
In contrast to PdNP@ChNC, PdNP@ChsNC showed little opposed to ChsNCs in order to control the redox chemistry of
product yield in the model reaction (Table 1, entry 6), which Pd, with full product yield in relatively mild conditions using
was surprising as our previous works showed that ChsNC was PdNP@ChNC.
the superior catalyst support for Au-catalyzed A3 coupling reac-
tions [16]. The XPS analysis of PdNP@ChsNC suggests that Pd
Supporting Information
is complexed to ChsNC in the +2 oxidation state (Figure 3).
These results align with our previous work where ChsNCs tend
Supporting Information features experimental procedures
to stabilize Au in the +1 oxidation state as opposed to metallic
depicting the materials used, the syntheses of ChNC and
Au. Since the Heck coupling primarily follows a classic oxida-
ChsNC, fabrication methods for PdNP@ChNC and
tive addition/reductive elimination pathway with Pd(0) being
PdNP@ChsNC, the standard reaction protocol for Heck
the active catalytic site in most cases [22], Pd(II) would be inac-
coupling, characterization information as well as additional
tive towards oxidative addition of the electrophilic iodobenzene,
characterization such as FTIR, PXRD, and supplemental
leading to no product formation. Importantly, even if mild
TEM images.
reducers were present to initiate the cycle and afford Pd(0), as is
Supporting Information File 1
Experimental part.
often the case in Pd cross coupling chemistry, the fact that 4 bar
H2 pressure was not able to reduce these species is a strong in-
dication of their stability against reduction. We also tested the
controls to show that the ChNC support alone was catalytically
inactive (Table 1, entry 7). We also demonstrated that direct
mixing of PdCl2 with ChNC had minor catalytic effect (Table 1,
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-16-201-S1.pdf]
entry 8), likely because the partial reduction taking place under Acknowledgements
these conditions was ineffective in affording the well-defined We thank the Facility for Electron Microscopy Research of
nanoparticles we synthesized as PdNPs@ChNC.
McGill University for help in data collection. We thank the
MC2 facility at McGill University for help in acquiring the
Comparisons within the literature were made with similar FTIR spectra. Specifically, we thank Dr. Hatem Titi from the
Pd NP-based systems (Table S1 in Supporting Information MC2 facility for help in acquiring PXRD spectra along with
File 1). Firstly, it can be seen that the PdNP@ChNC system fruitful scientific discussion.
outcompetes a similar system with Pd NP on CNCs, which led
to 75% yield in 24 h and 100 °C, albeit with lower Pd loading Funding
[7]. Further comparisons with Pd NPs on other supports such as We thank the Natural Science and Engineering Research
SiO2 as reported by Jadhav et al. also suggest our system has Council of Canada (NSERC) Discovery Grant and Discovery
higher catalytic activity in more benign conditions, with the Accelerator Supplement, the Canada Foundation for Innovation
Pd NP on SiO2 system yielding 92% stilbene product at 110 °C (CFI), the National Research Council (NRC) New Beginnings
and using dimethylformamide as the solvent [23]. Other exam- Initiative Ideation fund, the Fonds de Research du Quebec
ples using carbon-based supports such as carbon spheres [24] Nature et Technologie (FRQNT) - Centre du Chimie Verte et
and graphene oxide [25] also have formidable yields, yet with Catalyse (CCVC), the National Research Council Canada
either very high temperatures greater than 100 °C or using (NRC), and McGill University for their financial support.
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ORCID® iDs
Tony Jin - https://orcid.org/0000-0003-2339-7218
Edmond Lam - https://orcid.org/0000-0003-4343-4469
Audrey Moores - https://orcid.org/0000-0003-1259-913X
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doi:10.1039/c2gc16430b
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