Novel chitosan-based/montmorillonite/palladium hybrid microspheres as heterogeneous catalyst for Sonogashira reactions

Abstract

The objective of this study was to develop novel chitosan-based/montmorillonite/palladium (CS/MMT/Pd) hybrid microsphere catalysts with improved properties for use in Sonogashira reactions. Interactions between a chitosan matrix and a montmorillonite nanofiller were revealed by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and thermogravimetric (TG) analysis. The results confirmed the formation of the intercalation structure between CS macromolecules and MMT layers. X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HR-TEM) analysis results showed that the Pd species of different valencies were dispersed at the nano-level in both the CS matrix and the interlayers of MMT. CS/MMT/Pd hybrid microspheres were highly active for the Sonogashira reactions of aryl iodides and alkynes at a palladium catalyst loading of 0.3 mol%. They can be recycled 10 times without a significant decrease in coupling yields. It was concluded that introducing MMT into the CS matrix will effectively improve the thermal stability and Pd leaching-resistance of the hybrid microsphere catalysts. The results in this study demonstrated the great potential of such heterogeneous catalysts applied in Sonogashira reactions.

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Introduction

In organic synthesis, palladium-catalyzed C–C and C–X coupling reactions are among the most useful methodologies.¹ They are usually employed in a homogeneous system but cause difficulties in separation, recovery and reuse of palladium metal.² To avoid or at least mitigate these drawbacks, heterogeneous catalysis using highly active immobilized palladium catalysts has attracted more and more attention. The heterogeneous catalysts are often readily removed by simple filtration and recycled several times with no detectable metal leaching.^3–8 These catalysts are generally immobilized palladium species on supports such as activated carbon, alumina, silica, or polymers. Recently, more and more research has been carried out on environmentally-friendly and low cost natural polymers as solid supports.^9–12 Chitosan (CS), the N-deacetylation product of chitin, is the second most abundant natural biopolymer only to cellulose. It has been effectively used as a catalytic support for Pd, Cu, Ni, and so on.^13,14 The main advantages of CS as a catalyst support are plenty of polar functional groups (amine, carbonyl, carboxyl, amide) with good chelating ability for transition metals within the molecular backbone, and great convenience in preparation into different forms: flakes, gel beads, membranes, fibers and so on. However, several limiting factors of CS as supports should not be ignored: poor thermal stability, mechanical properties, and solvent-resistance. Therefore, CS needs to be modified either by chemical or physical methods to improve such properties.

Recently, chitosan-based nanocomposites have attracted more and more attention. The introduction of nanoscale fillers to a chitosan matrix, having at least one dimension in the nanoscale range (such as clay minerals, silica and nano-hydroxyapatite), can combine the properties of both organic and inorganic components, such as mechanical properties, thermal behavior, barrier properties, etc.^15–19 Among them, chitosan composites modified with montmorillonite (MMT, the most used natural clay) are of major interest.^20–22 MMT, a kind of natural layered silicate, consists of two fused silica tetrahedral sheets sandwiching an edge-shared octahedral sheet of either aluminum or magnesium hydroxide (general structure 2 : 1 type). The interlayer of MMT contains metal cations, mainly Na⁺, Ca²⁺, or Mg²⁺, which can be exchanged by other cations including polycationic polymers like CS in slightly acidic conditions. Moreover, the functional groups of CS can form hydrogen bonds with the Si–O–Si groups of a silicate multilayer and the silicate hydroxylated end groups of MMT. Consequently, strong interactions between CS and MMT can easily form, leading to significant improvements in the mechanical, thermal, antimicrobial, and corrosion properties of CS. However, to the best of our knowledge, most of the reported studies on CS/MMT nanocomposites are especially for absorbents, tissue engineering, pharmaceutical carriers, food packaging, etc.^23–27 Few works deal with CS-based/MMT nanocomposite materials as heterogeneous transition metal catalyst supports. In previous work, it was confirmed that a CS membrane catalyst directly crosslinked by Pd²⁺ cations was highly active and stable for Heck reactions.²⁸ Taking these antecedents into account, in this study, CS/MMT nanocomposite microspheres were prepared with a transition metal crosslinker (Pd²⁺) to form novel high-performance heterogeneous hybrid microsphere catalysts for coupling reactions. The effects of the MMT loading amount, intercalation structure, and Pd species dispersion on the morphology, thermal stability, and catalytic performances in Sonogashira-type coupling reactions of the hybrid microspheres have been investigated.

Results and discussion

In this study, FTIR analysis was applied to investigate the interactions between CS, MMT and Pd. As shown in Fig. 1f, the FTIR spectrum of CS shows broad bands at 3423 cm⁻¹ for the overlap of O–H and N–H stretching, 2923 and 2864 cm⁻¹ for aliphatic C–H stretching, 1665 cm⁻¹ for C=O stretching of residue amide, 1590 cm⁻¹ for N–H bending, 1425 and 1380 cm⁻¹ for C–H bending of methylene and methyl groups, and 1155 and 1026 cm⁻¹ for C–O stretching. In the MMT spectrum (Fig. 1a), it was shown that the vibration band at 3628 cm⁻¹ can be assigned to O–H stretching in the inner surface of MMT, 3445 cm⁻¹ to the interlayer and O–H stretching of H₂O, 1644 cm⁻¹ to O–H deformation of H₂O, 1037 cm⁻¹ to Si–O stretching, 907 cm⁻¹ to Al–OH vibration, 790 cm⁻¹ to Mg–OH vibration, and 521 cm⁻¹ and 467 cm⁻¹ to Si–O bending. From examination of Fig. 1b–d, the FTIR spectrum of the CS/MMT/Pd hybrid microspheres showed the combination of characteristic absorption peaks of CS and MMT, and the interactions between the components were reflected by the noticeable wavenumber shifts of the characteristic absorption peaks. The peaks at 1590 cm⁻¹ for N–H bending of –NH₂ groups in the starting CS were shifted to a higher wavenumber (1601 cm⁻¹) in the CS/MMT hybrid microspheres. Meanwhile, peaks at 3628 cm⁻¹ for O–H stretching in the starting MMT and 3423 cm⁻¹ for OH and/or NH₂ groups in the starting CS were all shifted to lower wavenumbers. These bands shifts are related to the electrostatic interactions between CS and MMT, confirming complexation between them. After immobilization of Pd species, the bands around the 3400 cm⁻¹ region were further shifted to a lower wavenumber, indicating a complexation reaction of Pd²⁺ with the –NH₂ and/or –OH functional groups of CS.

Fig. 1 FTIR curves of the microspheres: (a) MMT; (b) CS/MMT (50/50); (c) CS/MMT/Pd (50/50/1.2); (d) CS/MMT (75/25); (e) CS/MMT/Pd (75/25/1.2); (f) CS.

The structural phases in the hybrid microspheres have been investigated using XRD analysis. As shown in Fig. 2f, the pure CS XRD pattern had two broad peaks, one at 2θ = 10.37°, and the other at 2θ = 20.08°. The XRD pattern of MMT showed a characteristic reflection peak at 2θ = 7.06°, corresponding to a basal spacing d₀₀₁ of 1.26 nm. Incorporating MMT within the CS matrix led to a shift in the basal peak from 2θ = 7.06° to 6.19° (CS/MMT: 75/25, Fig. 2d) and 5.44° (CS/MMT: 50/50, Fig. 2b). The corresponding basal spacing, d₀₀₁, was 1.43 nm and 1.62 nm, respectively. This increase in d-spacing indicated the formation of the intercalated structure of CS/MMT hybrid microspheres. As proven by the FTIR analysis, amino and hydroxyl groups of CS could form hydrogen bonds with silicate hydroxylated end groups of MMT, leading to strong interactions between CS and MMT. Hydroxyl groups of CS could also form hydrogen bonds with Si–O–Si groups of the silicate multilayer of MMT. Moreover, NH₂ groups of CS have converted to NH₃⁺Ac⁻ in acidic media. Therefore, CS molecules can easily go inside the interlayers of MMT and form the intercalated structure through the cation exchange process. As shown in Fig. 2c and e, the addition of Pd²⁺ has little influence on the intercalation between CS and MMT. It means that the added Pd²⁺ cations mainly undergo complexation reactions with CS molecules and not with MMT.

Fig. 2 XRD patterns of the microspheres: (a) MMT; (b) CS/MMT (50/50); (c) CS/MMT/Pd (50/50/1.2); (d) CS/MMT (75/25); (e) CS/MMT/Pd (75/25/1.2); (f) CS.

The thermal stability of pure CS microspheres, MMT, and CS/MMT hybrid microspheres has been investigated using TG analysis under air atmosphere (as shown in Fig. 3). The TG analysis curve of pure CS microspheres showed a weight loss in three stages. The first stage (50–130 °C) was attributed to the loss of absorbed and bound water. The second stage (180–350 °C) was due to the destruction of the intermolecular interactions such as hydrogen bonding, crosslinking, destruction of the macromolecular backbone, carbonization, and thermal oxidation. The third stage (370–500 °C) was due to the carbonization and deep thermal oxidation. Pure MMT exhibited high thermal stability containing only one weight loss stage (50–110 °C) caused by the evaporation of the absorbed water. After incorporating MMT within the CS matrix, it was shown that the thermal stability of CS/MMT hybrid microspheres was obviously improved with an increasing amount of MMT. The nano-dispersed MMT in the CS matrix led to a significant delay in weight loss as compared with pure CS, especially at high temperatures (>300 °C). At 80% weight residue, the decomposition temperature of CS/MMT hybrid microspheres (277.9 °C) was 27.4 °C higher than pure CS microspheres (250.5 °C). At 60% weight residue, the decomposition temperature of CS/MMT hybrid microspheres (75/25: 361.3 °C; 50/50: 510.4°C) was yet even higher than pure CS microspheres (305.3 °C). The added MMT formed strong interactions with CS molecules and acted as a good thermal barrier for the CS matrix. Therefore, thermal stability of the CS matrix was improved effectively with addition of MMT. This is in favour of the improvement of the resistance to harsh reaction conditions when CS/MMT hybrid microspheres are used as catalyst support materials.

Fig. 3 TG analysis curves of the microsphere supports.

SEM is often used to study the morphology of polymeric composites. The SEM photos of the surface and cross-section of the hybrid microspheres with different magnification are shown in Fig. 4. For all the CS/MMT/Pd hybrid microspheres with different amounts of MMT, the interfaces between the MMT domain and the CS matrix had a rough structure, indicating good interfacial interaction between MMT and CS. MMT particles dispersed well within the CS matrix, with a size of about 5 μm. No agglomerates of larger sizes were observed with increasing MMT amount. These phenomenons indicated fairly good miscibility between CS and MMT.

Fig. 4 SEM observation of the CS/MMT/Pd hybrid microspheres with different magnification: (A) CS/MMT/Pd (75/25/1.2), surface, (×150); (B) CS/MMT/Pd (75/25/1.2), surface, (×2000); (C) CS/MMT/Pd (75/25/1.2), cross section, (×150); (D) CS/MMT/Pd (75/25/1.2), cross section, (×2000); (E) CS/MMT/Pd (50/50/1.2), surface, (×150); (F) CS/MMT/Pd (50/50/1.2), surface, (×2000); (G) CS/MMT/Pd (50/50/1.2), cross section, (×150); (H) CS/MMT/Pd (50/50/1.2), cross section, (×2000).

The nano-sized microstructure of the CS/MMT/Pd hybrid microspheres was further investigated with HR-TEM analysis. A previous study²⁸ has shown that Pd species immobilized on CS supports would undergo a valence transform cycle (Pd²⁺ to Pd⁰ to Pd²⁺) during the catalytic cycle as applied in coupling reactions. In the HR-TEM study, both fresh and activated CS/MMT/Pd hybrid microspheres (reduced with alcohol) have been investigated. As shown in Fig. 5, the HR-TEM image of fresh CS/MMT/Pd hybrid microspheres shows good dispersion of MMT with an intercalated structure (stacks of multilayers) in the CS matrix. Meanwhile, no individual separated Pd species were found for the fresh CS/MMT/Pd hybrid microspheres, suggesting that Pd²⁺ cations were dispersed at a molecular level. In the case of the activated CS/MMT/Pd hybrid microspheres, individual separated Pd⁰ nanoclusters about 5 nm in size were well dispersed both in the CS matrix and interlayers of MMT. The valence change of Pd can be confirmed by XPS analysis and the colour change of the CS/MMT/Pd hybrid microspheres. After activation, the CS/MMT/Pd hybrid microspheres showed a characteristic colour change from pale yellow to gray, meaning some Pd⁰ species were generated. As shown in Fig. 6, the Pd 3d_5/2 electron binding energy of the fresh CS/MMT/Pd hybrid microspheres was mainly at 337.8 eV, meaning that mainly Pd²⁺ species were in the materials. After activation, besides at 337.8 eV, Pd 3d_5/2 electron binding energy was also found at 334.6 eV, and the peak at 336.1 eV became stronger. These results indicated that both Pd²⁺ and Pd⁰ species coexist within the CS/MMT/Pd hybrid microspheres after activation with alcohol.

Fig. 5 HR-TEM observation of the microspheres: (A) CS/MMT/Pd (75/25/1.2), fresh; (B) CS/MMT/Pd (75/25/1.2), after activation.

Fig. 6 XPS analysis results of the microspheres: (A) CS/MMT/Pd (75/25/1.2), fresh; (B) CS/MMT/Pd (75/25/1.2), after activation.

Combining the characterization results of the prepared CS/MMT/Pd hybrid microspheres, the mechanism of the formation of interactions between CS, MMT, and Pd species has been proposed in Scheme 1. CS has good miscibility with MMT and can easily intercalate into the interlayers of MMT by cationic exchange process. Pd²⁺ cations interact strongly with functional groups of CS (mainly –NH₂ groups). Pd²⁺ cations can be reduced to Pd⁰ and form nanoclusters entrapped within the CS matrix and located between the interlayers of MMT.

Scheme 1 Phase dispersion diagram of the fresh and activated CS/MMT/Pd hybrid microspheres.

The Sonogashira reaction is known as the most powerful and straightforward method for the construction of the sp²–sp carbon–carbon bonds.^29–31 Similar activity for the reaction of iodobenzene with phenyl acetylene (as shown in Fig. 1S of the ESI†) was found between the CS/MMT/Pd (75/25/1.2) and CS/MMT/Pd (50/50/1.2) catalysts. Therefore, one catalyst (CS/MMT/Pd (75/25/1.2)) was chosen to assess the catalytic activities for Sonogashira coupling reactions of other aryl halides with alkynes. Table 1 shows that over 90% cross-coupling yield can be achieved for the reaction of iodobenzene with substituted phenyl acetylenes (entry 1–3) when the CS/MMT/Pd (75/25/1.2) catalyst loading is 0.3 mol%. Our catalyst loading is clearly much lower than recently reported Pd/PVA mat catalysts for the same reaction,³² indicating excellent catalytic activities of the CS/MMT/Pd hybrid microsphere catalyst for the Sonogashira-type cross-coupling reactions. Such good catalytic activity should be strongly related to the good dispersion of Pd species^33,34 (Pd⁰ of about 5 nm in size) in the composite microspheres. Smaller sizes of Pd⁰ species favoured higher catalytic performance for reactions.³³ The CS/MMT/Pd hybrid microsphere catalyst also works well for coupling other aromatic iodides with alkynes. High cross-coupling yields occurred with the substituted aromatic iodides either with an electron-donating group such as o-OCH₃ (entry 4), p-OCH₃ (entry 5), p-CH₃ (entry 6) or with an electron-withdrawing group such as o-Cl (entry 7), or p-F (entry 8). It is found that the substituted group’s steric effects on the reaction substrates slightly influence the yield. For example, the cross-coupling yield of entry 4 is clearly lower than that of entry 5, due to such steric effects of ortho substituted substrates (o-OCH₃) and para substituted substrates (p-OCH₃). The CS/MMT/Pd hybrid microsphere catalyst has low activity for bromobenzene with alkynes (entry 9), due to the greater strength of the C–Br bond than the C–I bond. Most other reported polymer supported Pd heterogeneous catalysts also have low activities for aryl bromides entering cross-coupling reactions with no addition of any ligands.^32,34–36

Table 1 Sonogashira cross-coupling of various aromatic halides with alkynes catalyzed by CS/MMT/Pd hybrid microspheres^a

Entry	Aromatic halide	Alkyne substrate	Cross-coupling yield^b (%)
a Reaction conditions: 1 mmol aromatic halide, 1.2 mmol alkyne, CS/MMT/Pd (75/25/1.2) hybrid microspheres (0.026 g, containing 3 μmol Pd), 3 mmol base (CH3COOK), in 3 + 0.2 ml (DMSO + ethylene glycol) solvent, at 110 °C for 5 h.b GC/MS yield.
1	C6H5I	C6H5CCH	94
2	C6H5I	4-CH3OC6H4CCH	90
3	C6H5I	4-CH3C6H4CCH	92
4	2-CH3OC6H4I	C6H5CCH	69
5	4-CH3OC6H4I	C6H5CCH	88
6	4-CH3C6H4I	C6H5CCH	85
7	2-ClC6H4I	C6H5CCH	78
8	4-FC6H4I	C6H5CCH	92
9	C6H5Br	C6H5CCH	Trace

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In this study, the prepared CS/MMT/Pd hybrid microsphere catalyst has a big diameter size (about 1.2 mm) and can be easily filtrated out from the reaction medium and reused for the following reaction runs. Sonogashira coupling of iodobenzene with phenyl acetylene was employed as the model reaction to assess the stability and reusability of the CS/MMT/Pd hybrid microsphere catalyst. As shown in Fig. 7, the novel catalyst can be recycled 10 times without a significant decrease in coupling yields, indicating higher stability than other reported heterogeneous polymer supported palladium catalysts for Sonogashira reactions, such as palladium-poly(3-aminoquinoline) hollow sphere composite (3 times),³⁴ and PdCl₂ on modified poly(styrene-co-maleic anhydride) (5 times).³⁵ The main reason for the higher stability of the prepared CS/MMT/Pd hybrid microsphere catalyst should be attributed to the following facts. Firstly, the CS/MMT/Pd hybrid microsphere catalyst has much higher mechanical strength and thermal stability than the palladium-poly(3-aminoquinoline) oligomer catalyst due to its much higher molecular weight of the matrix CS macromolecules. Secondly, the interaction of Pd species with S, N within thiazole groups, and –C=O groups in the case of the modified poly(styrene-co-maleic anhydride) supported Pd catalyst, is much weaker than that formed of Pd species with –NH₂ groups, and –OH groups within each repeat unit of the CS molecular backbone in the case of the CS/MMT/Pd hybrid microsphere catalyst. For comparison, with similar palladium catalyst loading amounts, the CS/MMT/Pd hybrid microsphere catalyst clearly has much better recyclability than a pure CS/Pd microsphere catalyst (6 times). Meanwhile, it was also found that the yield of the last 2 recycling runs in the case of the CS/MMT/Pd (50/50/1.2) hybrid microsphere catalyst is higher than that for the CS/MMT/Pd (75/25/1.2) hybrid microsphere catalyst. These results mean that the addition of MMT is greatly beneficial for improving the stability and reusability of the CS-based supported palladium catalysts.

Fig. 7 Dependence of the cross-coupling yield on the recycling runs of the microsphere catalysts as used in a model Sonogashira reaction: (A) CS/Pd (100/1.2) microspheres; (B) CS/MMT/Pd (75/25/1.2) hybrid microspheres; (C) CS/MMT/Pd (50/50/1.2) hybrid microspheres.

For heterogeneous catalysts, transition metal leaching is known as the main reason that causes declining catalytic activity with an increase in recycling times.³⁷ The Pd content of the freshly prepared as well as the recycled heterogeneous catalysts has been determined by means of ICP. As shown in Fig. 8, the Pd leaching percent is much lower in the CS/MMT/Pd hybrid microsphere catalyst than in the pure CS/Pd microsphere catalyst. It was also found that Pd leaching in the case of the CS/MMT/Pd (75/25/1.2) hybrid microsphere catalyst is slightly more serious than that for the CS/MMT/Pd (50/50/1.2) hybrid microsphere catalyst. The higher Pd leaching-resistance properties of the CS/MMT/Pd hybrid microsphere catalyst should be mainly attributed to the effective improvements in the mechanical and thermal stability of the composite microspheres after incorporation of MMT. Moreover, the formation of strong interactions involved in the CS molecules, MMT layers, and Pd species is also really favorable for reducing Pd leaching.

Fig. 8 Dependence of the Pd remaining percentage on the recycling runs of the microsphere catalysts as used in a model Sonogashira reaction: (A) CS/Pd (100/1.2) microspheres; (B) CS/MMT/Pd (75/25/1.2) hybrid microspheres; (C) CS/MMT/Pd (50/50/1.2) hybrid microspheres.

Conclusions

In summary, it has been demonstrated that CS combined with MMT produced a very good catalyst support for stabilizing palladium, decreasing the drawbacks of CS itself. CS molecules can easily intercalate into the interlayers of MMT by the cationic exchange process, leading to good miscibility of the CS matrix and MMT. Pd²⁺ cations acted as a metal crosslinker for CS molecules both in the matrix and interlayers of MMT. The reduced Pd⁰ nanoclusters, sized 5 nm, were entrapped tightly within the CS matrix and interlayers of MMT. Therefore, the thermal stability and Pd leaching-resistance of the CS/MMT/Pd hybrid microsphere catalyst was improved effectively, and a noticeable enhancement of the catalytic stability and activity was obtained. Both CS and MMT are natural, abundant, and cheap to obtain. The inexpensive and environmentally-friendly CS/MMT/Pd hybrid microsphere catalyst might be a good candidate for a heterogeneous palladium catalyst for organic reactions.

Acknowledgements

The authors acknowledge the financial support from the National Natural Science Foundation of China (grant number: 21203123 and 11475114), and the Academic Climbing Project for the youth academic leaders of Zhejiang Province, China (grant number: pd2013391).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra01902h

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