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Design of melamine sponge-based threedimensional porous materials towards applications Yi Feng, and Jianfeng Yao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01232 • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018
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Industrial & Engineering Chemistry Research
Design of melamine sponge-based three-dimensional porous materials towards applications Yi Feng and Jianfeng Yao* College of Chemical Engineering, Jiangsu Key Lab for the Chemistry & Utilization of Agricultural and Forest Biomass, Nanjing Forestry University, Nanjing, Jiangsu 210037, China. Tel:+862585427912, E-mail:
[email protected] ABSTRACT Due to the low density, high porosity, three-dimensional (3D) pore structure, high nitrogen content and excellent mechanical properties, melamine sponge (MS) has been widely used as the main building block to design and obtain 3D porous materials. Moreover, N-doped porous carbon foam can be formed via a simple carbonization of MS; thus making such carbon foam also an ideal carbon material either directly used or as support to load other materials. So far, many studies have been reported on the modification of MS, carbonized MS and MS-based composite materials in various applications including oil/water separation, water disinfection, adsorption, fire resistance, electrochemistry, strain/stress sensor, catalysis and so on. In this review, structure design of such MS-based materials and modification methods are summarized and discussed, and the obstacles and suggestions on how to design and synthesize such materials with better performance are provided, aiming to promote the wide application of MS in various fields.
1. Introduction Melamine
sponge
(MS)
is
a
foam-like
material
consisting
of
formaldehyde-melamine-sodium bisulfite copolymer. As a kind of low density material, it shows excellent sound absorption, flame retarding, thermal insulation, damp and hot stability, healthy, safety and good comprehensive properties.1-3 It has been widely used in the civil, building, transportation, aviation, military, daily electronic information and other fields, especially suitable for flame retarding, noise 1 ACS Paragon Plus Environment
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reduction and cleaner. MS is a new environmental material that has great prospects in the 21 century. MS has been commercialized mostly known as magic eraser. Table S1 gives the physical properties of commercial MS fabricated by Asian glory company (Hong Kong, China) and Figure 1 shows the optical view and SEM image of the pore structure in MS. MS has an interconnected network and smooth surface with a high porosity of over 99% and pore size of approximately 100 µm (Figure 1b). Due to the excellent properties of MS, it has been selected as support to synthesize various composite materials with 3D pore structure. Even after carbonization, the pore structure can be maintained with excellent mechanical properties.4 Therefore, both MS and carbonized MS can be used as ideal supports to synthesize 3D composite materials. Pristine MS, carbonized MS and MS-based composite materials can be used as adsorbents or membranes for dye removal, ions removal or oil/water separation. Moreover, MS has a high nitrogen (N) content and N-doped carbon can be formed via simple carbonization of MS. N-doped carbons usually exhibit good electrochemical performance and have been widely used as high-performance electrodes for supercapacitors (SCs), lithium ion batteries (LIBs) and oxygen reduction reactions (ORR).5-10 Thus carbonized MS and their composites have been synthesized and used as high-performance electrodes. Besides the above-mentioned applications, MS-based materials have also been used in catalysis, water disinfection, human motion detection, fire resistance, dental care and so on. In this review, modification methods on MS and applications are summarized and discussed, and suggestions on the performance improvements are provided, aiming to better use MS in various fields.
Figure 1. Optical view of commercial MS (a) and SEM image of MS (b). 2 ACS Paragon Plus Environment
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Industrial & Engineering Chemistry Research
2. Oil/water separation Pristine MS is hydrophilic and chemical modifications are needed for oil-water separation applications.3, 11-13 Basically, there are four ways to modify hydrophilic MS to hydrophobic sponge: 1) Direct carbonization of MS. Hydrophobic carbon foam would be obtained via direct carbonization of MS. After carbonization, the 3D porous structure can be maintained with good mechanical properties. In this regard, carbonization conditions (e.g. carbonization temperature, heating rate and residence time) usually played a key role in determining sorption capacities of carbonized MS. Stolz et al. studied the absorption capacities of pristine MS and MS carbonized at different temperatures14. Pristine MS showed almost similar absorption capacities for oil and water, indicating poor oil/water separation ability. The carbonized MS at 500 o
C or 600 oC absorbed 5-6 times of water less than pristine MS. However, when
further enhancing the carbonization temperature, carbonized MS absorbed much more water, resulting in the decrease of oil/water separation performance. 2) Gelation or polymerization of hydrophobic monomers onto the surface of MS. Introduce hydrophobic functional groups onto the surface of MS is an efficient way to modify MS with hydrophobic nature. In this method, monomers with hydrophobic functional groups were first wrapped onto MS surface followed by gelation or polymerization to form a
second layer closely coated onto MS.11,
15-16
Li et al.
used
polydimethylsiloxane (PDMS) as prepolymer, divinylbenzene (DVB) as monomer and 2,2-azoisobutyronitrile (AIBN) as initiator to coat MS.17 After coating, modified MS was hydrothermal treated at 100 oC for 20 h and hydrophobic MS was obtained. Hydrophobic modification of MS via gelation or polymerization of monomers usually involves the use of expensive monomers or toxic solvents, and sometimes hydrothermal treatment, UV irradiation or microwave treatment is needed.11,
16
Therefore, there is a need to avoid the use of expensive and toxic agents and to simplify the polymerization process. In our previous work, we reported a simple, low-cost and environmental-friendly method of hydrophobic modification of commercial melamine sponge (MS) via simply soaking MS in furfural alcohol 3 ACS Paragon Plus Environment
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followed by polymerization in acid solution.18 Results show that furfural alcohol not only reacts with MS but also polymerizes onto the surface of MS, leading to the hydrophobic nature of modified MS. 3) HCl treatment of MS. Wang et al. obtained protonated MS by treating MS in HCl solution.19 Such protonated MS has excellent antifouling properties and can be used as filter to continuously separate oil/water mixtures for up to 12 h with no increase in the oil content in filtrate, indicating good oil/water separation ability and long-term stability of such protonated MS. 4) Coating nanomaterials such as graphene oxide (GO) or metal organic frameworks (MOFs), onto the surface of MS.20-21 In this method, MS or carbonized MS was usually used as support to avoid the agglomeration of nanoparticles and facilitate to use and recover due to the bulk structure and high mechanical properties. Many studies used the GO or reduced graphene oxide (rGO) as modifying agent due to the hydrophobic nature of GO. There studies usually focused on the modifying methods (how to coat GO or rGO onto MS surface). Ji et al. dip-coated MS into GO solution followed by drying at 90 oC for 6 h and at 160 oC under vacuum to obtain the final product.20 Among these four ways of hydrophobic modification of MS, direct carbonization of MS is the easiest way but in this method, the BET surface area is usually low with sacrifice mechanical properties after carbonization. Gelation/polymerization method is an effective way to modify MS with hydrophobic nature but in most cases, expensive monomers or toxic agents were usually involved. HCl treatment is also an easy way, but acid is still used that is not environmental-friendly. By coating nanoparticles or GO, excellent hydrophobic nature is achieved but the long-term stability should be concerned considering that nanoparticles might peel off from MS. Despite the successful hydrophobic modification of MS, GO modified MS still have a low BET surface area. As a general trend, high BET surface area is an important factor to enhancing the absorption capacity of absorbents for oils. From this point of view, MOFs emerged as good coating agents and have been placed under the spotlight of scientific research. Zeolite imidazolate frameworks (ZIFs), a subclass of MOFs, consist of transition metal ions (Zn2+, Co2+) and imidazolate linkers that form 4 ACS Paragon Plus Environment
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Industrial & Engineering Chemistry Research
3D tetrahedral frameworks.22-27 Porous carbon can be obtained via simply carbonization of ZIFs and such carbons usually have very high BET surface areas. Therefore, ZIFs can be considered as ideal coating material onto MS to enhance the BET surface area of MS. Lei et al. used polydopamine (PDA) as coupling agent and in-situ grew ZIF-8 particles onto MS.28 In this case, PDA was used as linker due to the adhesive ability to coat onto MS followed by the in-situ growth of ZIF-8 particles (Figure 2). Such MS@ZIF-8 composite materials showed excellent absorption capacity ranging from 10 to 38 g/g for various oils, which is much higher than that of pure ZIF-8 (0.1-1.5 g/g). The enhanced oil/water separation performance of MS@ZIF-8 is mainly attributed to its high hydrophobicity, high porosity, low density, large surface area and hydrophobic nature. Table S2 summarizes the modification methods and sorption capacities of MS-based adsorbents.
Figure 2. Schematic diagram of ZIF-8 coated on MS. Reprinted with permission from ref. 28 .Copyright 2018, Royal Society of Chemistry.
Besides the hydrophobic modification of MS, MS with switchable wettability was also synthesized via coating MS with poly(N-isopropylacrylamode)(PNIPAAm)29: at low temperature (
