Synthesis and characterization of catalyst
The synthetic route to prepare Pd@Cell-EDTA catalyst was outlined in Scheme 1. In the presence of acetic anhydride, EDTA is dehydrated into EDTA dianhydride. Meanwhile, the microcrystalline cellulose is activated by 10% sodium hydroxide solution, converting its hydroxyl group into alkoxide to enhance its nucleophilicity. Thanks to the presence of abundant reactive hydroxyl groups in pre-activated cellulose, EDTA can be easily grafted onto the surface of microcrystalline cellulose successfully through the esterification reaction with EDTA dianhydride in the presence of pyridine in DMF. This reaction allows introducing carboxylic and amine functional groups that present high ability to form Pd complexes. Furtherly, Pd@Cell-EDTA catalyst was prepared by coordination of Pd2+ with Cell-EDTA. According to previous data (Busch et al. 1956), Pd(II) salts react with EDTA easily in water to form square complexes containing one ligand residue per Pd atom. EDTA is used as tetradentate ligand which coordinates with two nitrogen atoms and two carboxylic groups. Pd(0) was formed by the partly reduction of Pd (II) by the hydroxyl group of cellulose (Seyednejhad et al. 2019). The Pd content in fresh Pd@Cell-EDTA catalyst was determined by ICP-AES to be 0.11 mmol/g.
Then, various analytical techniques including XPS, FTIR, EDS, SEM, TEM, TGA, and DGT were performed to identify the components, structures, and morphology of Pd@Cell-EDTA catalyst.
XPS is used to confirm the components and oxidation state of elements present in the Pd@Cell-EDTA catalyst. Figure 1 showed the XPS survey scan spectra of the typical elements in the binding energy rang of 0-1400 eV and high-resolution spectra of N1s and Pd3d of catalyst. It was found that Pd, C, N and O elements were the predominate species in Fig. 1a. Two peaks centered at 400.75 and 399.63 eV in Fig. 1b corresponding N1s (Nolting et al. 2007), which can be ascribed to the N element in EDTA and indicate that EDTA has been successfully grafted onto cellulose chain. According to Fig. 1c, two valence states of Pd in the catalyst can be observed. XPS spectra indicated the presence of both Pd(II) and Pd(0) phases. The peak at 338.42eV corresponds to Pd (II) while 336.82eV corresponds to Pd (0) (Lin et al. 2017) which may be formed by the reduction of Pd (II) by the hydroxyl group of cellulose (Seyednejhad et al. 2019). Pd (0) can also be stabilized by EDTA owning to its vacant orbital, which effectively prevent Pd (0) from aggregating into palladium black and thus losing activity.
The FTIR of microcrystalline cellulose (curve a) and Pd@Cell-EDTA (curve b) in the range of 4000 − 500 cm− 1 are presented in Fig. 2. Compared with the spectra of microcrystalline cellulose, a new peak appeared at 1742 cm− 1 in the spectra of Pd@Cell-EDTA, which is corresponded to the carboxylic group in EDTA (d’Halluin et al. 2017), furtherly indicating that EDTA has been successfully grafted onto microcrystalline cellulose. Besides, the characteristic absorption peaks at 1431 cm− 1 and 1639 cm− 1 are respectively assigned to the symmetric and asymmetric stretching vibration of carboxylate groups (Lin et al. 2017; Zhang et al. 2016).
In order to have a further insight into the morphology of the catalyst, the SEM and TEM images of Pd@Cell-EDTA composites were depicted in Fig. 3. As can be seen in Fig. 3a-d, the catalyst was composed of micron-scale fibers with very rough surface, which provides a wide contact area between the reactants and catalyst. With more high magnification, it can be clearly observed in TEM images (Fig. 3e) that the catalyst was composed of quasi-spherical nanoparticles and the average diameter of these nanoparticles was about 30 nm as shown in histogram of nanoparticle sizes (Fig. 3f).
The distribution of Pd has important impact on the catalytic activity. To adequately analyse the elemental distribution of Pd@Cell-EDTA catalyst, SEM element mapping images are exhibited in Fig. 4. The elemental mappings conclusively evidence the existence of C, O, N, and Pd elements and their homogeneously dispersion throughout the catalyst.
EDS analysis shown in Fig. 5 was performed to determine the element content in Pd@Cell-EDTA catalyst. The results showed that the contents of each element in the catalyst were C: 53.74%, N: 3.44%, O: 42.65%, and Pd: 0.16%. From the content of carbon and nitrogen, the approximate ratio of cellulose to EDTA in the catalyst can be calculated. Set the amount of cellulose as nCell and the amount of EDTA as nEDTA, then there are:
C: 12×6×nCell +12×10×nEDTA = 53.74
N: 15×2×nEDTA = 3.44
It can be worked out that nCell = 0.55, nEDTA = 0.11, and nEDTA: nCell = 5:1, which indicating that about every five cellulose units can graft one EDTA molecule. Considering that each cellulose unit has three hydroxyl groups, the degree of substitution of EDTA between cellulose chain is 6.7%.
In order to test the thermodynamic stability of Pd@Cell-EDTA, TGA and corresponding DTG analyses of were performed and presented in Fig. 6. As can be seen in the curse of TGA (curse a), there is a slightly weight loss below 100 ℃ originated from the removal of physical adsorptive moisture. The catalyst exhibited a sharp loss in mass at nearly 250 ℃ in the curse of DTG (curse b), probably related to the decomposition of the cellulose macromolecule chain (Das et al. 2010). This result indicated that Pd@Cell-EDTA has a good thermal stability from room temperature to 250 ℃ and suitable for most organic catalytic reactions since they are usually carried out below 200 ℃. Moreover, due to the remaining of palladium, there is still a small weight of the residue at the higher temperature.
Application of Pd@Cell-EDTA in Suzuki reactions
To search the optimal reaction conditions for Suzuki reactions, iodobenzene (1.0 mmol) and phenylboronic acid (1.2 mmol) was selected as model substrates and various conditions were systematically explored (Supplementary materials, Table S1). The optimized reaction condition for the Suzuki model reaction was set at the usage of iodobenzene (1.0 mmol), phenylboronic acid (1.2 mmol), K2CO3 (2.0 mmol), and 0.05 mol % of Pd under 2 ml 78% EtOH at 78 ℃.
Obtaining the optimal reaction conditions, various iodobenzene and phenylboric acid were investigated to further examined the generality and limitation of this methodology. The results were generalized in Table 1. It can see that either electron-withdrawing or electron-donating groups of iodobenzene and phenylboric acid has no obvious effect on the reaction. Most of the substrates afforded high yields and only the reaction of 4-methyliodobenzene with 4-methoxyphenylboric acid gives a moderate yield of 77% (Table 1, entry 5). It indicates that this catalytic system has good universality and tolerance of various substrate. Additionally, it was worth noting that 2-iodothiophene can also obtain high yields (Table 1, entry 16), which meant this catalyst was compatible of a wide scope of substrates.
Application of Pd@Cell-EDTA in Sonogashira reactions
In order to furtherly explore the application universality of Pd@Cell-EDTA, Sonogashira reactions had been performed in the presence of Pd@Cell-EDTA catalytic system to prepare corresponding diphenylacetylene. The model reaction of iodobenzene (1.0 mmol) and phenylacetylene (1.2 mmol) was exerted to search of the optimal reaction conditions (Supplementary materials, Table S2). To conclude, the optimal condition for model reaction involved the usage of iodobenzene (1.0 mmol), phenylacetylene (1.2 mmol), K2CO3 (2.0 mmol), and 0.10 mol % of Pd of catalyst in 3 mL MeOH at 70 ℃. It is worth mentioning that there was no copper salt using as a co-catalyst in this catalytic system neither nitrogen protection device, indicating that Sonogashira reaction can also run smoothly without copper, which can effectively avoid copper-induced oxidative self-coupling of alkynes, i.e., the Glaser reaction (Sindhu, et al. 2014).
The generality of the Pd@Cell-EDTA catalyst for the Sonogashira reaction was explored via using structurally diverse iodobenzene and phenylacetylene with the optimized conditions, and the results are summarized in Table 2. Most of substrates bearing with electron-donating or electron-withdrawing groups produced the expected product in good to excellent yields. However, when iodobenzene bearing with a strong electron-donating groups, such as 4-methoxyiodobenzene as the reactant, the yield decreases significantly (Table 2, entries 11–13), which is in line with the reported literature that iodophenzene with electron-donating group has higher EHOMO (Highest Occupied Molecular Orbital energy), thus the intermediate after oxidation addition is more stable and difficult to conduct the subsequent reaction (an der Heiden et al. 2008). Fortunately, 1-octyne as the reactant also achieved in good yields, making enlarge the scope of substrates in this catalytic system.
Recyclability and heterogeneity test of Pd@Cell-EDTA in Suzuki reactions
For the purpose of practical application, the recyclability of the Pd@Cell-EDTA composites was evaluated by repetitive experiment on the Suzuki model reaction under the afore-mentioned optimal condition. The results shown in Fig. 7 demonstrated that the catalyst could be effectively recycled for more than 5 consecutive runs without significant dropping its catalytic activity.
To further investigated the homogeneity/heterogeneity of Pd@Cell-EDTA catalysts, hot filtration test was performed similarly using the Suzuki model reaction. First, the model reaction was carried out at 78 ℃ for 5 min, at that point the solid catalyst was filtered off. The left reaction mixture was allowed to react for another 25 min at reaction temperature. The product yields were obtained in 52% (5 min) and 65% (30 min), respectively. Only a slight increase in the product yield was observed from 52–65%, suggesting that the catalysis was characteristically heterogeneous in nature.
Explanation of the superior catalytic performance of Pd@Cell-EDTA on carbon-carbon cross-couplings
The highly catalytic activity of the synthesized Pd@Cell-EDTA catalyst towards carbon-carbon cross-couplings presumably due to the synergic effect of EDTA and hydroxyl groups of cellulose. The electron-donation of nitrogen and carboxylic groups of EDTA to Pd facilitates the electronic charge transfer, resulted in a highly negatively charged Pd center, which makes Pd more electron rich for facile oxidative addition of aryl halide, and hence significantly enhanced the catalytic activities for Suzuki, Heck, and Sonogashira reactions (Rai et al. 2016; Wang et al. 2017). It should be note that, to Suzuki reaction, the catalyst has superior catalytic performance with very low Pd loading (0.05 mol% of Pd). The activity of the catalyst can be attributed to the coordination of EDTA with Pd and the activation of phenylboronic acid via esterification of hydroxyl groups of cellulose (Zhai et al. 2015; Oshima et al. 1999). The reported works demonstrated the cyclic phenylboronate ester have better reactivity in Pd-catalyzed Suzuki reaction (Delaney et al. 2020; Nichele et al. 2009). The suggested synergic effect for Suzuki reaction depicted in Fig. 8.
Comparison of catalytic activity of Pd@Cell-EDTA in Suzuki reaction with other reported catalysts
To illustrate the advantages of Pd@Cell-EDTA catalyst, other previously reported Pd catalysts have been compared in Table 3 using the Suzuki model reaction as a reference. The comparative results demonstrate that Pd@Cell-EDTA has some outstanding merits over other solid Pd catalysts in terms of low catalyst loading, short reaction time and excellent yield.