General and Straightforward Synthetic Route to Phenolic Resin Gels

Nov 26, 2014 - Phenolic resin including phenol-formaldehyde resin (PFR) and resorcinol-formaldehyde resin (RFR) are one class of the most explored res...
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General and Straightforward Synthetic Route to Phenolic Resin Gels Templated by Chitosan Networks Zhi-Long Yu,† Zhen-Yu Wu,† Sen Xin,‡ Chan Qiao,† Zi-You Yu,† Huai-Ping Cong,‡ and Shu-Hong Yu*,† †

Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Collaborative Innovation Center of Suzhou Nano Science and Technology, Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China ‡ Anhui Key Laboratory of Controllable Chemical Reaction and Material Chemical Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, P. R. China S Supporting Information *

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phenol, are chosen to extremely reduce the cost. The polymerization reaction between phenol and formaldehyde is more controllable. Moreover, mechanical properties and microstructures of PFR gels could be controlled by the molar ratio of phenol to formaldehyde (P/F) and the quantity of chitosan, respectively. Most importantly, it is quite a general method not only appropriate to PFR, but also RFR, catechol-formaldehyde resin (CFR), hydroquinone-formaldehyde resin (HFR), orthoaminophenol-formaldehyde resin (o-AFR), m-aminophenolformaldehyde resin (m-AFR), and p-aminophenol-formaldehyde resin (p-AFR). Chitosan can dissolve well in acid environment and form three-dimension (3D) transparent and viscous gel, and it can be cross-linked by aldehydes, such as formaldehyde and glutaraldehyde.22,23 Also, chitosan can be usually used as an absorbent for phenols.24 Moreover, phenols and aldehydes can polymerize in an acidic environment. In this way, chitosan can serve as a 3D soft template, in which formaldehyde reacts with chitosan and phenol simultaneously. The rapidly generated PFR micromolecules gradually deposit on chitosan template, and finally the 3D PFR gels form (Scheme 1). During the hydrothermal process, chitosan gels are gradually dissolved and PFR gels inherit the 3D structures of the chitosan gels. The as-obtained PFR gels are named PFR-x, where x corresponds to the molar ratio of phenol to formaldehyde (P/F). After the drying process, different PFR aerogels can be easily obtained. Without special instructions, PFR aerogels are prepared by supercritical CO2 drying. The scanning electron microscopy (SEM) image and transmission electron microscopy (TEM) image (Figure 1a,b) show that the sample PFR-2.5 aerogel inherits the net-like framework structures of chitosan gel, which is similar to chitosan aerogel, and has many pores in the whole monolith.25,26 If dried directly by freeze-drying method, PFR-2.5 aerogel is a dense monolith in which the nanopores are blocked (Figure 1c). We cannot see the porous structure from the high magnification SEM image (inset of Figure 1c), which shows that organic molecules exist among the net-like structures. Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance (13C NMR) prove that the chitosan has reacted with PFR and the

henolic resin including phenol-formaldehyde resin (PFR) and resorcinol-formaldehyde resin (RFR) are one class of the most explored resins and promising candidates in thermal insulation material, energetic materials, energy storage, catalyst support, absorbent, photonic crystal, and drug delivery.1−7 These thermoset polymers can be easily obtained by polymerization of monomers, such as phenol, resorcinol, and formaldehyde. Due to the inexpensive raw materials, it is suitable for continuous largescale operations, which promises fine availabilities of phenolic resin. The synthesis of phenolic resin gel was first reported by Pekala et al. in 1989, which was performed by resorcinol and formaldehyde in the presence of base catalyst.8 Traditionally, it requires a long time (7 days) to prepare RFR gels, which is apparently unsuitable for industrial production. Besides, these RFR gels are always nonflexible brittle solids, and it is difficult to control their microstructures. What is more, the monomer resorcinol is not only more expensive but also difficult to preserve and control the reaction with formaldehyde as a result of the higher activity for the two hydroxyl groups attached to the benzene ring.9,10 Generally, the method is not appropriate to synthesize PFR and other phenol gels, such as aminophenol. There are also some new methods to synthesize PFR or RFR gels by acid catalyst or metal ion catalyst. However, these methods need either more than 5 days or irritative and toxic reagents such as acetonitrile and HNO3 or relative expensive graphene.11−13 To create more nanopores for higher surface areas, ionic liquids and colloidal particles, such as triblock copolymer Pluronic F127, SiO2, and metal oxide or hydroxide, were frequently used as template.14−20 However, these templated porous materials have some disadvantages such as low producibility, high production costs, and safety considerations which prevent their mass production. Some template materials are costly, and their eventual etching usually requires highly corrosive and toxic reagents such as NaOH and HF.21 Thus, the potential commercial applications of phenolic resin have been seriously suppressed. Considering these above attributes, it is urgent to develop a general straightforward, time-efficient, and economical production route to satisfy their applications. Herein, we report a novel straightforward acetic acid (HAc)catalyzed route to synthesize PFR gels with phenol, formaldehyde, and chitosan (CTS). CTS is used as soft template, and the typical multiday route is cut to a few hours. In this process, inexpensive and stable raw materials, including chitosan and © XXXX American Chemical Society

Received: November 1, 2014 Revised: November 20, 2014

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dx.doi.org/10.1021/cm504036u | Chem. Mater. XXXX, XXX, XXX−XXX

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Scheme 1. Schematic Illustration Showing the Synthesis of PFR Gels

Figure 1. Characterization of PFR gels and aerogels. (a, b) SEM and TEM images of PFR-2.5 aerogel. (c) SEM image of PFR-2.5 aerogel prepared by freeze-drying. The inset in (c) shows the SEM image with high magnification. (d) FT-IR spectra of PFR-2.5 aerogel, chitosan, and acetone extractant of PFR-2.5 wet gel. (e) 13C NMR spectra of PFR-2.5 aerogel and chitosan. (f) Chemical structures of PFR-2.5 and chitosan. (g) Densities of PFR-x aerogels prepared with different quantity of CTS. (h) Curves of stress and elastic modulus of the PFR gels at the set quantity of CTS of 0.45 g with different molar ratios of P/F from 0.5 to 3.5. (i) Large-scale synthesis of PFR-2.5 gel with 50, 100, and 1600 mL Teflon vessel, respectively.

final PFR gels contain a small amount of residue of chitosan. Chitosan and PFR-2.5 show a distinct amide I band and amide II band at 1650 and 1580 cm−1, respectively. N−H stretching and O−H stretching vibrations are characterized by a broad peak in the range of 3200−3500 cm−1 (Figure 1d).27 PFR-2.5 has strong NMR peaks at between 110 and 160 ppm, and the hydroxysubstituted carbons (C3) occur around 152 ppm (Figure 1e,f). The chemical shift of ∼130 ppm is assigned to substituted ortho or para carbons of phenolic rings.28 The weak peaks of the PFR2.5 at between 50 and 110 ppm are due to the residual chitosan.29

Additionally, PFR gels with similar net-like microstructures can be tuned by the quantity of chitosan (Supporting Information Figure S1). With increasing the chitosan addition quantity (0.2−0.45 g) in the specified volume (50 mL vessel), the densities of the organic PFR-x aerogels increase from 0.132 to 0.172 g cm−3 (Figure 1g). More soft template is added, more frameworks exist in the aerogel, and the corresponding density increases. However, PFR gels could be not obtained when CTS was less than 0.2 g because the soft template was too small to afford the whole gels. B

dx.doi.org/10.1021/cm504036u | Chem. Mater. XXXX, XXX, XXX−XXX

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Due to the cost-efficient raw materials, it is feasible and easy to directly scale up PFR gels from 50 to 1600 mL just by using a larger autoclave and without changing reactant concentrations or reaction time (Figure 1i). The formation mechanism can be described as follows. Before the hydrothermal treatment, phenol molecules disperse homogeneously in chitosan gel. Because of the rapid reaction of formaldehyde molecules and chitosan molecules, most formaldehyde molecules graft onto chitosan molecules or disperse around chitosan molecules, which results in the concentration of formaldehyde being higher than that of phenol (F/P > 1) in the region near the chitosan molecules. During the hydrothermal process, thermoset resol is synthesized and then cured to form the hydrophobic 3D PFR frameworks.30 The contrary is the case, in the far region among chitosan frameworks, phenol molecules are more than formaldehyde molecules (F/P < 1), which generate liner hydrophilic PFR micromolecules (thermoplastic novolak). With increasing the P/F, more novolak molecules generate, and PFR gels become more and more flexible. To verify the formation mechanism, the high concentration of PFR micromolecules was extracted by acetone (Supporting Information Figure S5a) and characterized by FTIR (Figure 1d). In the stretching region of the hydroxyl groups, two absorption bands are observed: one is a broad phenol−phenol hydrogen bonding band at 3365 cm−1 which is not seen in chitosan and PFR-2.5 and the other is a shoulder band due to free hydroxyl groups at about 3500 cm−1.31 As we all know, the thermoplastic novolak can be cured with curing agents, such as hexamethylenetetramine (HMT).32 Thus, we can also prove the novolak through this phenomenon. The orange extractant can be redissolved to form a homogeneous dark solution in acetone after being heated in 100 °C for 12 h without HMT (Supporting Information Figure S5b). However, they were cured to form yellow solid with 10 wt % HMT under the same condition (Supporting Information Figure S5c). There is rarely a report that the method developed by Pekala is suitable for other phenols, except cresol.33−35 The oneness of the conventional method has extremely hindered the developments and applications of phenolic resin gels. Herein, the novel synthetic method described above is quite general. We have expanded this method to other phenolic resin gels such as RFR, CFR, HFR, o-AFR, m-AFR, and p-AFR by simply changing solvents to fit the solubleness of different phenols or other acid catalyst (Figure 3a−f). It is worth mentioning that even the seldom-used catechol and hydroquinone can form gels with formaldehyde by this method (Figure 3b,c). All these resin gels with inherent colors and unique structures can be formed by hydrothermal method within 12 h. RFR have similar porous netlike structures with PFR which is different from the interconnected colloidal-like particles of RFR obtained by the Pekala method.8 The microstructures of CFR and HFR have the same features with many pores with different size. However, all the three AFR are layer structures with quite a few spheres attaching to both sides of the layer. Nitrogenous resin gels such as three kinds of AFR can be converted into N-doped porous carbon aerogels which have great potential applications in the future in energy conversion and storage such as oxygen reduction reaction, lithium-ion battery, and supercapacitor. In summary, a novel and general straightforward route has been developed to synthesize a family of phenolic resin gels, such as PFR, RFR, CFR, HFR, o-AFR, m-AFR, and p-AFR. In this developed both time-saving and energy-efficient method, chitosan serves as soft template and plays a key role in fabricating

Interestingly, in dramatic contrast to the brittle nature of conventional PFR and RFR gels, the obtained PFR gels exhibit different mechanical properties by controlling the molar ratio of phenol to formaldehyde. In our study, the conventional molar ratio of phenol to formaldehyde, which is usually set as 0.5, is impractical. The PFR-0.5 gel is stiff, brittle, and heterogeneous (Supporting Information Figure S2). Homogeneous and flexible PFR gels could only be obtained when the molar ratio of phenol to formaldehyde is above 2 (P/F > 2) (Figure 1h). PFR-2.5 (Figure 2a,b), PFR-3 (Figure 2c,d), and PFR-3.5 (Figure 2e,f)

Figure 2. Compressive stress−strain curves of PFR gel at different set strain ε and cyclic stress−strain curves at respective maximum strain. (a, b) PFR-2.5. (c, d) PFR-3. (e, f) PFR-3.5. The insets in (b, d) show the sequential photographs during the compression process. The inset in (f) shows the photograph of the flexible PFR-3.5 gel pressed with fingers.

can bear a compression strain as high as 30%, 40%, and 50%, respectively. The corresponding maximal stresses are measured to be about 26.4, 13.1, and 9.3 KPa, respectively. Thus, PFR gels are becoming more and more flexible with increasing the molar ratio of phenol to formaldehyde. PFR aerogels are obtained through a freeze-drying or supercritical CO2 drying process. Freeze-dried PFR-2.5 aerogels are hydrophilic and flexible and almost recover slowly to original volume after release of the compression in about 2 min (Supporting Information Figure S3a). Before the supercritical drying, acetone is used for solvent exchange and the colorless acetone turns orange (Supporting Information Figure S4), indicating that micromolecules exist in PFR gels and can be proven to be novolak by FTIR (blue curve in Figure 1d). Interestingly, the PFR aerogels dried under supercritical conditions are stiff, brittle, and hydrophobic, and they are easily broken with tweezers (Supporting Information Figure S3b). From the above results, it is demonstrated that the 3D PFR frameworks are hydrophobic cured PFR (resol), and the liner PFR micromolecules (novolak) endow the PFR gels flexibility and hydrophily. C

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(6) Khan, M. K.; Giese, M.; Yu, M.; Kelly, J. A.; Hamad, W. Y.; Maclachlan, M. J. Angew. Chem., Int. Ed. 2013, 52, 8921. (7) Fang, Y.; Zheng, G.; Yang, J.; Tang, H.; Zhang, Y.; Kong, B.; Lv, Y.; Xu, C.; Asiri, A. M.; Zi, J.; Zhang, F.; Zhao, D. Angew. Chem., Int. Ed. 2014, 53, 5366. (8) Pekala, R. W. J. Mater. Sci. 1989, 24, 3221. (9) Karousos, N. G.; Reddy, S. M. Analyst 2002, 127, 368. (10) Heijnen, C. G. M.; Haenen, G. R. M. M.; Minou Oostveen, R.; Stalpers, E. M.; Bast, A. Free Radical Res. 2002, 36, 575. (11) Mulik, S.; Sotiriou-Leventis, C.; Leventis, N. Chem. Mater. 2007, 19, 6138. (12) Brandt, R.; Petricevic, R.; Probstle, H.; Fricke, J. J. Porous Mater. 2003, 10, 171. (13) Wei, G.; Miao, Y. E.; Zhang, C.; Yang, Z.; Liu, Z.; Tjiu, W. W.; Liu, T. ACS Appl. Mater. Interfaces 2013, 5, 7584. (14) Yang, T.; Liu, J.; Zheng, Y.; Monteiro, M. J.; Qiao, S. Z. Chem. Eur. J. 2013, 19, 6942. (15) Fellinger, T.-P.; Thomas, A.; Yuan, J.; Antonietti, M. Adv. Mater. 2013, 25, 5838. (16) Guo, D.-C.; Mi, J.; Hao, G.-P.; Dong, W.; Xiong, G.; Li, W.-C.; Lu, A.-H. Energy Environ. Sci. 2013, 6, 652. (17) Feng, D.; Lv, Y.; Wu, Z.; Dou, Y.; Han, L.; Sun, Z.; Xia, Y.; Zheng, G.; Zhao, D. J. Am. Chem. Soc. 2011, 133, 15148. (18) Liu, R.; Shi, Y.; Wan, Y.; Meng, Y.; Zhang, F.; Gu, D.; Chen, Z.; Tu, B.; Zhao, D. J. Am. Chem. Soc. 2006, 128, 11652. (19) Wang, D. W.; Li, F.; Liu, M.; Lu, G. Q.; Cheng, H. M. Angew. Chem., Int. Ed. 2008, 47, 373. (20) Kubo, S.; White, R. J.; Yoshizawa, N.; Antonietti, M.; Titirici, M.M. Chem. Mater. 2011, 23, 4882. (21) Yan, J.; Wang, Q.; Wei, T.; Fan, Z. J. Adv. Energy Mater. 2014, 4, 1300816. (22) Rinki, K.; Dutta, P. K.; Hunt, A. J.; Clark, J. H.; Macquarrie, D. J. Macromol. Symp. 2009, 277, 36. (23) Monteiro, O. A. C., Jr; Airoldi, C. Int. J. Biol. Macromol. 1999, 26, 119. (24) Li, J.-M.; Meng, X.-G.; Hu, C.-W.; Du, J. Bioresour. Technol. 2009, 100, 1168. (25) El Kadib, A.; Bousmina, M. Chem.Eur. J. 2012, 18, 8264. (26) Primo, A.; Quignard, F. Chem. Commun. 2010, 46, 5593. (27) Kolhe, P.; Kannan, R. M. Biomacromolecules 2003, 4, 173. (28) Park, B.-D.; Riedl, B. J. Appl. Polym. Sci. 2000, 77, 1284. (29) Heux, L.; Brugnerotto, J.; res, J. D.; Versali, M.-F.; Rinaudo, M. Biomacromolecules 2000, 1, 746. (30) Young-Kyu, L.; Hyun-Joong, K.; Miriam, R.; Jonathan, S. Int. J. Adhes. Adhes. 2002, 22, 375. (31) Harri, K.; Janne, R.; Per, N.; Olli, I. Polymer 2001, 42, 9481. (32) Zhang, X.; Solomon, D. H. Macromolecules 1994, 27, 4919. (33) Zhu, Y.; Hu, H.; Li, W.-C.; Zhang, X. J. Power Sources 2006, 162, 738. (34) Li, W.; Reichenauer, G.; Fricke, J. Carbon 2002, 40, 2955. (35) Li, W.-C.; Lu, A.-H.; Guo, S.-C. Carbon 2001, 39, 1989.

Figure 3. Photographs (left) and SEM images (right) of other phenolic resin gels. (a) RFR; (b) CFR; (c) HFR; (d) o-AFR; (e) m-AFR; (f) pAFR. The insets in (d) and (f) show SEM images with high magnification.

the 3D gels. By finely tuning the synthesis parameters, PFR gels with different microstructures and densities can be successfully synthesized. PFR gels consist of hydrophobic cured resol frameworks and hydrophilic flexible novolak filler, resulting PFR gels exhibiting interesting mechanical properties. Due to the economical raw materials used in this efficient approach and also its scalable features, this straightforward and scalable route holds the promise for large-scale production of series of phenolic resin gels with great potential applications in various fields in the future.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details and additional information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.-H.Y. acknowledges the funding support from the National Basic Research Program of China (Grants 2010CB934700, 2013CB933900, 2014CB931800), the National Natural Science Foundation of China (Grants 21431006, 91022032, 91227103), and the Chinese Academy of Sciences (Grant KJZD-EW-M011).

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REFERENCES

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dx.doi.org/10.1021/cm504036u | Chem. Mater. XXXX, XXX, XXX−XXX