Modification of Montmorillonite with Polyethylene Oxide and Its Use as Support for Pd0 Nanoparticle Catalysts

In this study, montmorillonite (MMT) was modified by intercalating polyethylene oxide (PEO) macromolecules between the interlayer spaces in an MMT-water suspension system. X-ray diffraction results revealed that the galleries of MMT were expanded significantly after intercalation of different loading of PEO. MMT/PEO 80/20 composite was chosen as the support platform for immobilization of Pd species in preparing novel heterogeneous catalysts. After immobilization of Pd species, the interlayer spacing of MMT/PEO (80/20) (1.52 nm) was further increased to 1.72 nm (Pd2+@MMT/PEO) and 1.73 nm (Pd0@MMT/PEO), confirming the well-immobilization of the Pd species in the interlayer spaces of PEO-modified MMT. High-resolution transmission electron microscopy (HR-TEM) observation results confirmed that Pd nanoparticles were confined inside the interlayer space of MMT and/or dispersed well on the outer surface of MMT. The conversion of Pd2+ to Pd0 species was evidenced by binding energy characterization with X-ray photo electron spectroscopy (XPS). The microstructure variation caused by the Pd immobilization was sensitively detected by positron annihilation lifetime spectroscopy (PALS) studies. The prepared Pd0@MMT/PEO (0.2/80/20) catalytic composite exhibits good thermal stability up to around 200 °C, and it showed high activities for Heck reactions between aryl iodides and butyl acrylates and could be recycled for five times. The correlations between the microstructure and properties of the Pd@MMT/PEO catalytic composites were discussed.


Introduction
The modification of montmorillonite (MMT) with organic polymers has attracted more and more interests for their excellent structural and/or functional performances [1][2][3]. Many water-soluble polymers, such as polyethylene glycol (PEG) [4], polyethylene oxide (PEO) [5], polyvinyl alcohol (PVA) [6], polyvinyl pyrrolidone (PVP) [7], etc., can be easily intercalated in MMT with the water solution intercalation method. In the MMT/polymer nanocomposites, an obvious increase in the interlayer spacing of MMT is frequently observed, indicating the intercalated polymer chains are well-confined in the narrow galleries between the MMT layers. Meanwhile, the lipophilicity of MMT will be significantly improved after the organic polymer chains intercalated. The prepared MMT/polymer hybrid composites are nowadays frequently applied in many fields, such as preparation of advanced nano-composites, drug delivery, and water treatment, etc. [8][9][10].

Sample Preparation
A total of 100 mL of 2 wt % of MMT suspension solution was prepared under magnetic stirring. Specific amounts of PEO were dispersed in 100 mL of deionized water to form homogeneous solution. The PEO solution and the MMT suspension solution were mixed and kept magnetically stirred in water bath at 55 • C for 12 h. The mass ratios of MMT/PEO were set as 100/0, 90/10, 80/20, 70/30, 60/40, and 50/50. 0.3 g of PdCl 2 was dissolved in 100 mL deionized water with the presence of excessive amounts of NaCl (about 2 g). Then, 2 mL of the fresh Pd 2+ solution was drop-wisely added into the above MMT/PEO (80/20) mixture and kept magnetically stirring for another 6 h. The Pd 2+ @MMT/PEO composite was separated by centrifugation and washed with deionized water until neutral (pH = 7). Afterwards, it was dried in an oven at 60 • C. The resultant Pd 2+ @MMT/PEO composite was reduced to Pd 0 @MMT/PEO with ethylene glycol at 80 • C. According to the ICP-AES determination results, the Pd content within the Pd@MMT/PEO catalytic composite was about 0.2 wt %.

Characterizations
The characterization methods were similar to those in our recent works [19][20][21]. The XRD analysis of MMT/PEO and Pd@MMT/PEO samples were performed with a PANalytical Empyrean X-ray diffraction system (conditions: 2θ from 3 • to 70 • , scanning rate of 2 • /min). The TGA and DSC curves of MMT/PEO and Pd@MMT/PEO samples were recorded with a Mettler Toledo TGA/DSC 2 STAR system (Zurich, Switzerland) (conditions: air atmosphere, from 30 to 700 • C, scanning rate of 20 • C/min). The XPS analysis of Pd@MMT/PEO sample was performed with a Thermo Fisher Scientific ESCALAB 250Xi X-ray photoelectron spectrometer. The Pd@MMT/PEO samples were embedded in epoxy resin and then microtomed for HRTEM observation, which was performed with a JEM-2100F HR-TEM (JEOL Ltd. Tokyo, Japan). The ICP determination of Pd@MMT/PEO samples were performed with a Leemann ICP-AES Prodigy XP inductively coupled plasma atomic emission spectrometry. The positron annihilation lifetime spectroscopy (PALS) analysis of MMT/PEO and Pd@MMT/PEO samples was performed with an EG & G ORTEC fast-slow system (conditions: time resolution of 198 ps). Before PALS measuments, the MMT/PEO and Pd@MMT/PEO samples were pressed into disks (diameter×thickness: 1 cm × 2 mm) using a 769YP-15A powder tableting machine (Shanghai Xinnuo Instrument Ltd., Shanghai, China). During lifetime spectra measurements, the positron source ( 22 NaCl, 16 µCi, deposited between two Kapton foils) was sandwiched between two pre-pressed Pd@MMT/PEO samples disks. The analysis of the positron annihilation spectra was performed with LT-9 (Lifetime-9) and MELT-4 (Maximum Entropy for Lifetime Analysis-4) programs.

Catalytic Test
In a 50 mL of round bottom flask reactor, a mixture of aromatic halide substrates (1 mmol), acrylates substrates (2 mmol), Pd 0 @MMT/PEO catalytic composite (3 µmol of Pd), CH 3 COOK base (3 mmol), and solvent (5 mL DMSO + 0.2 mL ethylene glycol) was magnetically stirred at 110 • C (oil bath heating) for 5 h. The coupling reaction progress was detected with both layer chromatography (TLC) method and gas chromatography-mass spectrometry (GC/MS) analysis (Agilent 6890N/5975 MSD GC/MS, Palo Alto, CA, USA). The coupling reaction yield is according to the GC/MS quantitative analysis results of the reaction mixture. All the coupling products' chemical structure was confirmed by the analysis results of both H 1 NMR and GC/MS, which was consistent with our recent works [21,29]. The recycling experiments of the Pd@MMT/PEO were performed as follows: firstly, filtration out the Pd@MMT/PEO from the reaction system; secondly, repeatedly washing of the filtrated Pd@MMT/PEO with ethanol for 3-5 times and drying; finally, putting the recycled Pd@MMT/PEO into the reaction mixture for use in next reaction run.  Figure 1 shows the XRD patterns of MMT/PEO and Pd@MMT/PEO and the basal spacing (d 001 ) value for the MMT estimated by Bragg's formula is summarized in Table 1. Meanwhile, when the thickness of the single layer of pure Na + -MMT (0.96 nm) is taken into account, the interlayer spacing of MMT can be estimated (also shown in Table 1). In the galleries between the MMT layers, the intercalated PEO chains would show different arrangements according to the interlayer spacing values and macromolecular configurations of PEO chains. For pure Na + -MMT, the d 001 is 1.25 nm, and its corresponding interlayer spacing is 0.29 nm. At the MMT/PEO ratios of 90/10, the d 001 and interlayer spacing increases to 1.44 nm and 0.48 nm, respectively. Some of previous works [23,30] showed that the intercalated PEO chains would be in helical conformation. However, it is worth noting that the size of PEO chains with a helical conformation is about 0.8 nm (>0.48 nm). It indicates that the helical conformation of PEO chains in the interlayer space of MMT is not to be the case. thickness of the single layer of pure Na + -MMT (0.96 nm) is taken into account, the interlayer spacing of MMT can be estimated (also shown in Table 1). In the galleries between the MMT layers, the intercalated PEO chains would show different arrangements according to the interlayer spacing values and macromolecular configurations of PEO chains. For pure Na + -MMT, the d001 is 1.25 nm, and its corresponding interlayer spacing is 0.29 nm. At the MMT/PEO ratios of 90/10, the d001 and interlayer spacing increases to 1.44 nm and 0.48 nm, respectively. Some of previous works [23,30] showed that the intercalated PEO chains would be in helical conformation. However, it is worth noting that the size of PEO chains with a helical conformation is about 0.    The microstructure of the Pd@MMT/PEO catalytic composite was further characterized with XPS and HR-TEM. As shown in Figure 2A, for Pd 2+ @MMT/PEO, the binding energy peaks are found at 337.5 eV (Pd3d 5/2 ) and 342.9 eV (Pd3d 3/2 ), confirming the presences of Pd 2+ species [32]. As shown in Figure 2B, for Pd 0 @MMT/PEO, the binding energy peaks shift to 335.6 eV (Pd3d 5/2 ) and 340.9 eV (Pd3d 3/2 ), respectively, confirming the presences of Pd 0 species [32]. Clearly, XPS characterization results supply a powerful evidence for the conversion of Pd 2+ to Pd 0 .  The microstructure of the Pd@MMT/PEO catalytic composite was further characterized with XPS and HR-TEM. As shown in Figure 2A, for Pd 2+ @MMT/PEO, the binding energy peaks are found at 337.5 eV (Pd3d5/2) and 342.9 eV (Pd3d3/2), confirming the presences of Pd 2+ species [32]. As shown in Figure 2B, for Pd 0 @MMT/PEO, the binding energy peaks shift to 335.6 eV (Pd3d5/2) and 340.9 eV (Pd3d3/2), respectively, confirming the presences of Pd 0 species [32]. Clearly, XPS characterization results supply a powerful evidence for the conversion of Pd 2+ to Pd 0 .

Results and Discussions
The sizes of the micro defects estimated with Equation 1 are listed in Table 2. The PALS result of pure MMT sample has been reported in our recent work [20], the o-Ps lifetime is 2.801 ns, andnd the mean micro defects size (l) of MMT can be calculated as 0.3443 nm. This value is  [33][34][35]. The size of the micro defects can be estimated from τ 3 with suitable models. It was evidenced that the modified Tao-Eldrup equation for cuboidal voids (Equation (1)) worked well for the MMT-based materials with layered structure [35][36][37][38]. Where, l refers to the mean width of cuboidal micro defects, and ∆l (0.17 nm) refers to the value of empirical parameter.
The sizes of the micro defects estimated with Equation (1) are listed in Table 2. The PALS result of pure MMT sample has been reported in our recent work [20], the o-Ps lifetime is 2.801 ns, and the mean micro defects size (l) of MMT can be calculated as 0.3443 nm. This value is bigger than that of MMT/PEO (80/20), 0.2901 nm. This is due to the fact that the interlayer space of MMT becomes more crowded after the PEO chains intercalate. Meanwhile, the effects of the Pd immobilization and reducing treatment on the microstructure have been sensitively detected. Though the interlayer space is expanded obviously after Pd 2+ immobilized as determined by XRD, the mean micro defects size (l) show a decrease from 0.2901 nm (MMT/PEO (80/20)) to 0.2792 nm (Pd 2+ @MMT/PEO (0.2/80/20)). By forming effective chelation with -OH groups, Pd 2+ cations play a role similar to cross-linking points for PEO chains, leading to a decrease in micro defects size within the intercalated PEO phase. Pd 0 species often has poorer chelation capability with polar groups than Pd 2+ species. Therefore, the micro defects size then undergoes an increase to 0.2886 nm for Pd 0 @MMT/PEO (0.2/80/20). Figure 4 shows the distribution of the longest lifetime (τ 3 ) and corresponding micro-defect's size (l) of the samples as fitted with MELT-4 program. Similar variation trend of the distribution range of the micro-defect's size is found. bigger than that of MMT/PEO (80/20), 0.2901 nm. This is due to the fact that the interlayer space of MMT becomes more crowded after the PEO chains intercalate. Meanwhile, the effects of the Pd immobilization and reducing treatment on the microstructure have been sensitively detected. Though the interlayer space is expanded obviously after Pd 2+ immobilized as determined by XRD, the mean micro defects size (l) show a decrease from 0.2901 nm (MMT/PEO (80/20)) to 0.2792 nm (Pd 2+ @MMT/PEO (0.2/80/20)). By forming effective chelation with -OH groups, Pd 2+ cations play a role similar to cross-linking points for PEO chains, leading to a decrease in micro defects size within the intercalated PEO phase. Pd 0 species often has poorer chelation capability with polar groups than Pd 2+ species. Therefore, the micro defects size then undergoes an increase to 0.2886 nm for Pd 0 @MMT/PEO (0.2/80/20). Figure 4 shows the distribution of the longest lifetime (τ3) and corresponding micro-defect's size (l) of the samples as fitted with MELT-4 program. Similar variation trend of the distribution range of the micro-defect's size is found.   respectively. This decrease should be due to the different aggregation states of PEO chains between pure PEO and MMT/PEO hybrid. Due to the high crystallization ability, most PEO chains tend to be regularly aggregated to form perfect crystals. As confirmed with DSC curves in Figure 5B, an endothermic peak of PEO crystal at 73 °C is observed. However, as confined in the narrow galleries of MMT, PEO chains have much lower probabilities in regular aggregation to form crystals. Therefore, no obvious endothermic peak has been detected for the MMT/PEO hybrids and Pd 0 @MMT/PEO (0.2/80/20) catalytic composite. Usually, the formation of perfect crystal of PEO chains is advantageous for higher thermal stability. As a result, PEO component in MMT/PEO  , respectively. This decrease should be due to the different aggregation states of PEO chains between pure PEO and MMT/PEO hybrid. Due to the high crystallization ability, most PEO chains tend to be regularly aggregated to form perfect crystals. As confirmed with DSC curves in Figure 5B, an endothermic peak of PEO crystal at 73 • C is observed. However, as confined in the narrow galleries of MMT, PEO chains have much lower probabilities in regular aggregation to form crystals. Therefore, no obvious endothermic peak has been detected for the MMT/PEO hybrids and Pd 0 @MMT/PEO (0.  Heck reactions between aryl halides and butyl acrylate were catalyzed with the prepared Pd 0 @MMT/PEO catalytic composite. As shown in Table 3, the Pd 0 @MMT/PEO catalytic composite shows high catalytic activity for the reaction between iodo benzene and n-butyl acrylate (entry 1, 91% yield). It still exhibits high catalytic activity for aryl iodides substituted with either an electron-donating group, such as p-CH3 (entry 2, 89% yield) and m-CH3O (entry 3, 84% yield), or an electron-absorbing group (such as p-F (entry 4, 88% yield), m-F (entry 5, 87% yield). Heck reaction between aryl iodides and t-butyl acrylate can be also effectively catalyzed with the prepared Pd 0 @MMT/PEO catalytic composite (entry 6-8). The Pd 0 @MMT/PEO catalytic composite show low catalytic activity for the reaction between bromo benzene and n-butyl acrylate (entry 9), which is mainly due to the much higher bonding strength of C-Br than C-I to break. Nevertheless, C-Br bond can be activated by substitution of strong electron-absorbing group such as m-COCH3 (entry 10). Clearly, the catalytic activity of Pd 0 @MMT/PEO is much higher than PEO-supported recyclable NC palladacycle catalysts [28]. And it is also comparable to recent other reported heterogeneous catalysts for Heck reactions [39,40]. After the reaction, the Pd 0 @MMT/PEO catalytic composite can be conveniently separated and recycled for the next run. As shown in Figure 6, the catalytic efficiency of the Pd 0 @MMT/PEO catalytic composite decrease gradually as the recycling times increase. Similarly, higher recyclability is observed for the Pd 0 @MMT/PEO catalytic composite as compared with PEO-supported recyclable NC palladacycle catalysts (can recycle 4 times with moderate yield) [28]. However, it is obviously lower than recent prepared Pd 0 @MMT/PVA or Pd 0 @MMT/PVP catalysts [19,20]. For PEO, the polar -OH groups are distributed in the ending of the chain rather than each repeating unit of the chain like PVA. Reasonably weaker chelation and quicker Pd leaching will occur in the case of Pd 0 @MMT/PEO.  Heck reactions between aryl halides and butyl acrylate were catalyzed with the prepared Pd 0 @MMT/PEO catalytic composite. As shown in Table 3, the Pd 0 @MMT/PEO catalytic composite shows high catalytic activity for the reaction between iodo benzene and n-butyl acrylate (entry 1, 91% yield). It still exhibits high catalytic activity for aryl iodides substituted with either an electron-donating group, such as p-CH 3 (entry 2, 89% yield) and m-CH 3 O (entry 3, 84% yield), or an electron-absorbing group (such as p-F (entry 4, 88% yield), m-F (entry 5, 87% yield). Heck reaction between aryl iodides and t-butyl acrylate can be also effectively catalyzed with the prepared Pd 0 @MMT/PEO catalytic composite (entry 6-8). The Pd 0 @MMT/PEO catalytic composite show low catalytic activity for the reaction between bromo benzene and n-butyl acrylate (entry 9), which is mainly due to the much higher bonding strength of C-Br than C-I to break. Nevertheless, C-Br bond can be activated by substitution of strong electron-absorbing group such as m-COCH 3 (entry 10). Clearly, the catalytic activity of Pd 0 @MMT/PEO is much higher than PEO-supported recyclable NC palladacycle catalysts [28]. And it is also comparable to recent other reported heterogeneous catalysts for Heck reactions [39,40]. After the reaction, the Pd 0 @MMT/PEO catalytic composite can be conveniently separated and recycled for the next run. As shown in Figure 6, the catalytic efficiency of the Pd 0 @MMT/PEO catalytic composite decrease gradually as the recycling times increase. Similarly, higher recyclability is observed for the Pd 0 @MMT/PEO catalytic composite as compared with PEO-supported recyclable NC palladacycle catalysts (can recycle 4 times with moderate yield) [28]. However, it is obviously lower than recent prepared Pd 0 @MMT/PVA or Pd 0 @MMT/PVP catalysts [19,20]. For PEO, the polar -OH groups are distributed in the ending of the chain rather than each repeating unit of the chain like PVA. Reasonably weaker chelation and quicker Pd leaching will occur in the case of Pd 0 @MMT/PEO. compared with PEO-supported recyclable NC palladacycle catalysts (can recycle 4 times with moderate yield) [28]. However, it is obviously lower than recent prepared Pd 0 @MMT/PVA or Pd 0 @MMT/PVP catalysts [19,20]. For PEO, the polar -OH groups are distributed in the ending of the chain rather than each repeating unit of the chain like PVA. Reasonably weaker chelation and quicker Pd leaching will occur in the case of Pd 0 @MMT/PEO.

Conclusions
In this study, PEO chains were successfully intercalated into interlayer spaces of Na + -MMT, which can be used as a novel support for Pd 0 nanoparticles. PEO chains and Pd species are well confined in the interlayer space of MMT, which is well elucidated by XRD, HR-TEM, XPS, TGA, and PALS. It was demonstrated that Pd 0 nanoparticles sized in 2-4 nm were successfully immobilized on MMT/PEO supports. The sub-nano level micro defects variation of MMT can be sensitively detected by PALS. After PEO intercalation and Pd immobilization, the micro defects undergo a slight decrease in size, which is mainly due to the fact that the interlayer space of MMT becomes more crowded. The prepared Pd 0 @MMT/PEO catalytic composite shows high catalytic activities for Heck reactions and can be recycled for five times. The lower recyclability of the Pd 0 @MMT/PEO than other reported Pd 0 @MMT/polar polymers catalytic composites should be mainly due to the weaker chelation of PEO with Pd. Nevertheless, the comprehensive catalytic performances of the Pd 0 @MMT/PEO are much improved as compared with Pd heterogeneous catalysts which are prepared by directly supporting of Pd species on PEO chains. This work supplies an alternative approach in the preparation of Pd heterogeneous catalysts with fairly good performances, and might have broad prospects in both experimental and industrial applications.

Conclusions
In this study, PEO chains were successfully intercalated into interlayer spaces of Na + -MMT, which can be used as a novel support for Pd 0 nanoparticles. PEO chains and Pd species are well confined in the interlayer space of MMT, which is well elucidated by XRD, HR-TEM, XPS, TGA, and PALS. It was demonstrated that Pd 0 nanoparticles sized in 2-4 nm were successfully immobilized on MMT/PEO supports. The sub-nano level micro defects variation of MMT can be sensitively detected by PALS. After PEO intercalation and Pd immobilization, the micro defects undergo a slight decrease in size, which is mainly due to the fact that the interlayer space of MMT becomes more crowded. The prepared Pd 0 @MMT/PEO catalytic composite shows high catalytic activities for Heck reactions and can be recycled for five times. The lower recyclability of the Pd 0 @MMT/PEO than other reported Pd 0 @MMT/polar polymers catalytic composites should be mainly due to the weaker chelation of PEO with Pd. Nevertheless, the comprehensive catalytic performances of the Pd 0 @MMT/PEO are much improved as compared with Pd heterogeneous catalysts which are prepared by directly supporting of Pd species on PEO chains. This work supplies an alternative approach in the preparation of Pd heterogeneous catalysts with fairly good performances, and might have broad prospects in both experimental and industrial applications.