Modified macroporous P(2-hydroxyethyl methacrylate) P(HEMA) cryogel composites for H2 production from hydrolysis of NaBH4
Graphical abstract
Introduction
Recently, storage and delivery of pure molecular H2 has seen great interest due to its high energy density (142 MJ/kg) and cleaner nature in comparison to fossil fuels (47 MJ/kg) which also raise environmental concerns. Therefore, H2 is an ideal candidate as a clean and renewable energy carrier fuel to replace the current petroleum or gasoline in vehicular applications [1], [2], [3], [4]. However, the problems of providing safe storage and production from the available H2 systems are still a great challenge. Many researchers have focused on progressively developing gravimetric/volumetric H2 storage capacity, thermodynamic stability, kinetics of H2 desorption/generation, reversibility of storage and reduction of cost for practical use of H2 energy in daily life [5], [6], [7], [8]. H2 as potential energy carrier system can be produced from a wide range of renewable and non-renewable sources. Use of hydrogen-storage materials such as various borohydride compounds as fuel has shown high utilization efficiency. Among the various chemical hydrides NaBH4 is considered an attractive energy source, especially for mobile applications owing to its natural properties such as nonvolatility, nontoxicity, ready availability and so on [9], [10].
Schlesinger et al. reported two methods of releasing the H2 from the stabilized solution of NaBH4 [11]. The first one, lowering of the solution pH by addition of an acid to the medium, e.g., acid catalyzed hydrolysis reaction, can be carried out using various organic acids such as monocarboxylic acids (formic, acetic acids), di-carboxylic acids (oxalic, tartaric, malic, succinic acids), tri-carboxylic acid (citric acid), taurine and ascorbic acid according to the following reaction [12], [13].
The second method, releasing H2 from alkaline NaBH4 solution, includes certain concentrations of NaOH with an “effective metal nano catalyst” to control the reaction rate at mild conditions according to the following reaction [14], [15].
Highly active and powerful metal nanoparticles that are various sizes and shapes can be immobilized within supporting materials via in situ reduction method [16], [17]. To prevent aggregation of the metal nanoparticles various support materials, such as carbon, hydroxyapatite, clay, Ni foam, polymeric particles or hydrogels, have been used in H2 generation reactions [18], [19], [20], [21], [22], [23]. Nowadays, a novel class of polymeric hydrogels called cryogels were developed with a wide variety of applications ranging from drug delivery to removal of heavy metals [24], [25]. Cryogels are highly interconnected, superporous hydrophilic polymeric structures with pore sizes ranging from 1 to 100 μm in a highly crosslinked three-dimensional network. They can be synthesized through free radical cryopolymerization method where frozen water molecules are included below the crystallization temperatures to act as both solvent and porogen, and include other solute molecules such as initiator, crosslinking agent, catalyst and monomer molecules in the non-frozen liquid microphase where the polymerization occurs around the frozen ice-crystals [26], [27], [28]. After the freeze–thaw cycle, large pore structures with variable size and geometry form depending on the shape of the ice-crystals during polymerization [29], [30]. The highly porous structure of cryogels make them fast responsive systems against external stimuli such as pH, temperature and solute molecules, etc, and the choice of materials in catalytic H2 generation reactions.
Therefore, herein, we report the synthesis and catalytic application of superporous p(HEMA) cryogels and their metal composites. The prepared p(HEMA) cryogels were chemically modified to improve their metal absorbing properties from different environments by creating new chemical functional groups and active sites for catalytic application. Although several chemical modification methods were developed for catalytic hydrogen generation reactions, quaternization is found to be an effective method. It can be completed by alkylation or various cation-forming agents that provide metal salt absorbing ability from alcoholic solutions and then H2 generation via their corresponding metal nanoparticles from chemical hydride hydrolysis reaction [31], [32]. Recently, novel quaternization techniques were developed for a wide range of potential applications as drug delivery systems, antibacterial agents and other biomedical applications [33], [34]. Among these the use of 3-chloro-2-hydroxypropyl trimethyl ammonium chloride as quaternary ammonium group containing a novel modifying agent is practical as it only a one-step reaction. Therefore, we used this new approach for cationization of porous p(2-hydroxyethyl methacrylate) cryogels by using 3-chloro-2-hydroxypropyl trimethyl ammonium chloride in an alkaline medium. The modified Q-p(HEMA) cryogels were then used as template for Co and Ni nanoparticle preparation and then as superporous catalyst system in H2 generation from NaBH4 hydrolysis.
Section snippets
Materials
The monomer, 2-hydroxy ethyl methacrylate (HEMA) (99%, Sigma-Aldrich), the crosslinker, N,N′-Methylenebisacrylamide (MBA) (99%, Acros), the initiator system, potassium persulfate (KPS) (99%, Sigma-Aldrich), and the accelerator N,N,N′,N′-tetramethylethylenediamine (TEMED) (98% Acros) were used in cryogel preparation. A reagent, 3-chloro-2-hydroxypropyl trimethyl ammonium chloride (65%, Sigma-Aldrich) was used for modification of p(HEMA) cryogels. Cobalt (II) chloride hexahydrate (CoCl2⋅ 6H2O)
Characterization of super-porous Q-p(HEMA) cryogels and their metal composites
The chemical modification mechanism of p(HEMA) cryogels is shown in Fig. 1. In the quaternization reaction of p(HEMA), the alkaline solutions of p(HEMA) cryogels and QA were mixed. In the alkaline solution, a negative charge was generated on neutral p(HEMA) cryogels with NaOH solution, and in the QA solution a weak and reactive epoxy ring was formed upon treatment with NaOH solution of QA [32], [33]. Upon mixing these two solutions positive charge was generated and Q-p(HEMA) cryogels were
Conclusion
In this study, cryogel of p(HEMA) with large and interconnected pores was synthesized under freezing conditions and chemically modified with QA to generate positive charges that can be used to absorb metal salts from alcohol solution 45 fold more in comparison to nonquaternized form. P(HEMA) cryogel can only absorb 0.91 mg Co(II)/g cryogel whereas Q-p(HEMA) absorbed 45.2 mg Co(II)/g cryogel from ethanol. These metal salt-loaded cryogels were reduced to their metal nanoparticles in situ and then
Acknowledgments
This work is supported by the Scientific and Technological Research Council of Turkey (113T042).
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