Field of the Invention The present invention relates to a production method of hydrogel -metal composite which is prepared in order to be used in hydrogen production and various environmental practices, and prepared in the form of hydrogels comprising metal nanoparticles or metal nanoclusters. Background of the Invention

Because surface areas of metal catalysts, which are commonly used in catalytic applications, are very-high in proportion to their volumes they are generally preferred at nano-dimension in applications. Metals at nano-dimension provide superior properties in comparison to solid bulk phase. For example, gold (Au) does not display catalytic action in bulk form whereas it has a very-high catalytic activity at nano-dimension. A number of problems are experienced in preparing metal catalysts, which are prepared at nano-dimension, and using them as catalyst. The fact that metal particles or metal nanoclusters are unstable in solution medium, they are prone to aggregation (conglomeration), they lose their catalytic activities and precipitate easily are among problems confronted. Another important problem is that nano structures, which will be used as catalyst, particularly leave the solution medium or the product after they perform their functions.

Various methods were developed in order to make catalytic nanoparticles stable. Using surfactant as surface active agent or adsorption of stabilizer agents via electrostatic interaction onto the surface of metai nanoparticles are the some of these primary methods. Metal nanoparticles are made stable by employing these methods. However, due to the fact that new structures entered into the medium this causes high catalytic effects of the particles to be reduced and may lead no new problems. A support material is needed in order that metal particles to be used at nano-dimension are used as catalyst. Zeolites and silicates of different kinds are the leading foremost support materials used. Because most of the chemical reaction occurs in water; metal nanoparticles which are stable, dispersible in water and can be prepared without any stabilizer can present many superior properties. Hydrogels with these properties have significant advantages in preparing catalysts. Hydrogels are network structures which are formed by cross-linking of hydrophilic polymers. Upon the water enters the hydrogel network structure, dimensions of the hydrogel broadens and it may act as a reactor vessel depending on its dimension. In addition, hydrogels can be synthesized from hydrophilic polymer chains and they can be synthesized from monomers comprising functional group which can form charged ion in aqueous solution as well. Hydrogels comprising these functional chemical groups can uptake oppositely charged metal ions in themselves by incorporating them. Thus, the hydrogel network structure swollen in water become woven by metal ions. By reducing these metal ions via sodium borohydride reducer, their metal nanoparticles can be synthesized in hydrogel network structure. As a result, metal nanoparticles which are synthesized in hydrogel become stable because they cannot leave the hydrogel structure. These structures can serve as a reactor which contains the catalyst in itself for many aqueous reactions.

Various catalysts can be used in order to conduct reactions in hydrogen production. These catalysts are prepared by a variety of methods such as crushing, grinding. Metal nanoparticles or metal nanoclusters with high surface areas can also be prepared by using detergents (surfactants) or linear polymers. These prepared metal particles can be used for production of hydrogen from aqueous NaBH ₄ solutions.

The International patent document no. WO2010010123 discloses synthesis of composites and use thereof consisting of a polysaccharide matrix and metal nanoparticles. The method in the document discloses dispersion of metal nanoparticles homogeneously by putting these metal nanoparticles into neutral or charged biologically-based polymer (polysaccharide) and that the composite neutral or anionic polysaccharide forms gel by means of physical and chemical cross-linking.

The Japanese patent document no. JP2007100117 discloses preparation of metal nanoparticles by reducing metals such as Ru, Co, Rh, Ni, Pd, Pt, Os, Ag, etc. by surfactant surface active agents. It is stated that hydrogen can produced via hydrogenation of these metal nanoparticles generated in a reaction matrix.

Summary of the Invention

The objective of the present invention is to realize a production method of hydrogel-metal composite which can form long-lasting catalytic systems via catalysts embedded into hydrogel support material.

Another objective of the present invention is to realize a production method of hydrogel-metal composite which does not lose its activity fast.

A further objective of the present invention is to realize a production method of hydrogel-metal composite which can remove products, to be formed as a result of reaction in the presence of magnetic field, from medium easily.

Detailed Description of the Invention

"Preparation of hydrogel-metal composites" in order to fulfill the objectives of the present invention is illustrated in the accompanying figures, in which:

Figure 1 is the Cryo-SEM image of the 1% cross-linked acrylamido-2-methyl-l -propansulphonic acid) (p(AMPS)) hydrogel Figure 2 is the transmission electron microscope (TEM) image of the p(AMPS)-Ni hydrogel metal composite.

The inventive production method of hydrogel-metal composite comprises the steps of:

- preparation of hydrogels with chemical and physical structure which can contain metal ions in itself;

- putting the hydrogels into aqueous solution having metal ions;

- the hydrogels absorbing the metal ions from the metal ion solutions;

- in order to remove the metal ions which are not bind to the hydrogel and cannot be hold by the hydrogel can be removed putting the hydrogel into washing bath where there is solvent that will be able to remove the unbind metal ions from hydrogel structure;

- taking the hydrogels out of the washing bath;

- putting the metal ions absorbed hydrogel into reducing solution;

- reduction of metal ions, which exist within the structure of the hydrogel, via reducers inside the solution;

- obtaining metal nanoparticles and/or metal nanoclusters after reduction in the hydrogel structure;

- drying hydrogels which are taken out of the reducing solution and have metal nanoparticles and/or metal nanoclusters;

obtaining hydrogel-metal composite which is the final product after the drying process.

In the preferred embodiment of the invention, aqueous sodium borohydride (NaBH ₄ ) of 50 ml 0.05-0.5 M is used for reduction of metal ions which are absorbed by the hydrogel. In another embodiment of the invention, hydrogen gas, bases such as NaOH, KOH, acids such as citric acid and hydrazine hydrate are used in order to reduce metal ions which are absorbed by the hydrogel. In the preferred embodiment of the invention, synthesis of hydrogels comprises the steps of:

preparing a mixture which consists of monomer, cross-linker and UV initiator and/or a redox such as ammonium persulphate or free radical initiator;

- putting the mixture prepared into reactor;

- carrying out photo-polymerization process by application of UV radiation to the mixture, or polymerization and cross-linking by high-energy radiation such as gamma, electron-beam, X-rays or any polymerization technique;

passing the hydrogel through washing bath in order to eliminate impurities by removing the hydrogel formed after the photo-polymerization process; drying the hydrogel cleaned;

storing the hydrogel cleaned and dried in a closed container.

In another embodiment of the invention, polymers obtained as a consequence of subjecting natural polymers and/or linear polymers to radiation without using cross-linker are used as hydrogel. In a further embodiment of the invention, hydrogels are synthesized by polymerization techniques such as photopolymerization and redox polymerization where high-energy ionizing radiations such as gamma, electron-beams, X-rays, UV applied on monomer, monomer mixtures or aqueous and/or non-aqueous polymer solutions are used.

In synthesis of hydrogels; hydrogels are synthesized from monomers, which have different functional groups, in varying amounts and in the presence of different cross-linkers. Monomers or polymers which have functional groups such as sulfonyl, amine, carboxylic acid, phosphate, thiol with hydrophilic characteristic or comprise their salts or various forms can be used in synthesis of hydrogels. In synthesis of hydrogels; monomers comprising acid group such as acrylic acid (AAc), methacrylic acid (MAc), 2-acrylamido-2~methyl-l -propansuIphonic acid (AMPS), 2-acryIamido glycolic (AAGA) or monomers with basic characteristic such as N-vinyl imidazole (VIM), 4- vinyl pyridine (VP), allyl amine (AA) or monomers with acidic characteristic such as boric acid or monomer mixtures created by two or more of these monomers are used,

Hydrogels with network structure obtained by polymerization are polymers that have pore sizes ranging from micrometers to nanometers in order that the metal ions are absorbed. Changing the pore size varies by type and amount of the cross- linker used.

In the preferred embodiment of the invention, at least one compound selected from a group generated by ethylene glycol dimethacrylate (EGDMA), polyethylene glycol dimethylacrylate (PEGDA), N,N'-methylenebisacrylamide (MBA) and divinylbenzene (DVB) is used as cross-linker for hydrogel synthesis.

In the step of passing the hydrogel through washing bath; solvents -such as water, acetone, ethanol, dimethyl formamide (DMF), tetrahydrofurane (THF), toluene, chloroform, benzene- which are refreshed at regular intervals to remove impurities from the medium such that they will not damage the structure of the hydrogel are used.

In synthesis process of hydrogels, hydrogel cross-linked p(AMPS) in pipette with 5 mm radius is synthesized by photo-polymerization method using UV initiator. Aqueous solution is prepared from 50 wt% monomer by volume (2-acrylomide-2- methyl-l-propansulphonic acid- MPS). For solution, cross-linker (N, N'- methylene bis acrylamide - MBA) and initiator (2, 2'-azobis (2- methylpropionamidine) dihydrochloride) are solved in an amount corresponding to 0.5-2% of monomer and monomers inside distilled water and vortex mixture is carried out. In order to form hydrogel network structure, irradiation is carried out onto pipettes via photo reactor for 2 hours. In order to eliminate impurities such as monomer, cross-linker, initiator which did not incorporated in hydrogel after radiation; they are removed through washing procudure by replacing at wash water every 8 hour for 3 days. After the hydrogels are cleaned, they are dried in drying ovens at 40°C, and are kept in a closed container (vessels) in order to be used for generating metal nanoparticles.

In order that the hydrogels stored in the container absorb Ni (II) ions, they are kept in nickel chloride solution of 0.1 M 100 ml for 24 hours. Then, metal nanoparticles are formed by reducing Ni ions within the hydrogel by means of appropriate reducers such as NaBH ₄ . Hydrogels which have absorbed Ni ²⁺ ions are reduced at a constant mixing speed (200 rpm) by being put into NaBH solution of 0.1-0.5 M. Hydrogel comprising nickel particles is washed with distilled water by being changed in every 4 hours for at least 12 hours in order to eliminate impurities. The final black metal composite hydrogel is made ready to be used in production of hydrogel from sodium borohydride. p(A PS) hydrogel containing 0.2 g Ni nanoparticul is put into 50 ml of water at 30°C and it is mixed with 50 ml of 0.05-0.5 M aqueous sodium borohydride comprising 0-10% NaOH by weight. By means of hydrogel which has metal nanoparticles displaying catalytic activity as, hydrogen gas is generated from water reacting with sodium borohydride. Metal borohydrides such as LiBH ₄ , NaBH ₄ , KBH ₄ , Al(BH ₄ ) ₃ , Mg(BH ₄ ) ₂ and ammonia borane (I ₃ NBH ₃ ) are also used as hydrogen resource in production of hydrogen gas and hydrogen gas can be obtained from aqueous solutions of these hydrogen resources by means of the inventive hydrogel-metal catalysis.

With using the metal nanoparticle-hydrogel composites synthesized as catalyst, nanoparticle-hydrogel composites are used in production of hydrogen gas from metal hydrides and ammonia boranes. Reaction mechanisms occurring during application of the invention are given below:

M: Ag, Ru, Co, Ni, Rh, Pd, Os, Fe (II), Fe (III).

n: 1 , 2, 3, 4, 5, 6, 7.

In the reaction mechanism above; general representation of a hydrogel network, which is prepared by -SO ₃ H functional groups, keeping the metal ion and formation of metal nanoparticles as a consequence of reduction reaction occurring in the presence of sodium borohydride (NaBFLj) can be seen.

Whereas in the reaction mechanism above, more particularly; dry state of the p(AMPS) hydrogel synthesized and swollen state of the hydrogel after being treated with water are shown and in addition, formation of "p(AMPS) hydrogel- nickel composite" after absorbance of Ni ⁺² ions by this hydrogel in swollen state and reduction of these Ni ⁺² ions by NaBH ₄ reducer is shown.

Metal ions absorbed by hydrogel are provided from aqueous solution of at least one transition metal selected from a group consisting of Ag, Ru, Co, Ni, Rh, Pd, Os, Fe (II), Fe (III). Metal ions are bound inside the hydrogel structure as a result of interaction of the functional groups on the hydrogel with the metal ions.

With the composites which are obtained by preparation of metal nanoparticles as catalyst in hydrogels, great advantages are provided in production of hydrogen. Aqueous NaBH ₄ or H ₃ NBH ₃ to be catalyzed with catalysts embedded into hydrogel support materials are contacted without any limitation. Thus, long- lasting catalytic systems are obtained, In addition, for the reason that the reactions occur in the hydrogel structure, no additional reactor is needed and metal interactions that may be hazardous on environment and human health are prevented as well.

As the active centers of catalysts are placed in support materials causes a reaction with high yield and has a higher reaction rate can be ontained, by changing the amount of catalyst within hydrogel. In addition, the fact that the catalyst and its active centers are embedded into support material provides advantage in terms of catalyst poisoning and toxicity. Thus, life-span of the catalyst can be longer and the catalyst can be used repeatedly owing to flexible hydrogel network structure around it.

Experimental studies and analysis

Metal nanoparticle-hydrogel composites are grinded into small particles and they are placed into basic aqueous sodium borohydride which is 5% NaOH by weight in different concentrations and then mixed. Hydrogen is produced from this mixture at room temperature. Parameters such as NaBH ₄ concentration, amount of the catalyst, base concentration and temperature affect production rate of the hydrogen gas.

Production rate of the hydrogen increases as the temperature increases for the same reaction conditions. For example, in order to produce same of hydrogen gas approximately 50 min. is needed at 30°C whereas approximately 8 min. is sufficient at 70°C. (Graphic 1)

¾ Volume (mL)

10 20 30 40 50 60

Time (minute)

Graphic 1 : Change of hydrogen production rate in temperature

As the amount of NaBH ₄ increases, the total amount of the produced hyd; gas increases. (Graphic 2)

Graphic 2: The change in the amount of the hydrogen gas produced with the used amount of NaBH ₄ . It was studied under reaction conditions carried out using NaBH ₄ solution of 50 mM 50 mL at a mixing speed of 1500 rpm at 30°C using 0.2 g p(AMPS)-Ni composite comprising 24.4 mg Ni catalyst calculated based on TGA analysis, and it was observed that the amount of NaOH in the NaBH ₄ solution does not have much effect on production rate or the produced amount of the hydrogen gas after 2.5% by weight. Therefore, it was determined that amount of the NaOH should be at least 2.5% or more by weight. (Graphic 3)

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0 20 40 60 80

Time (minute)

Graphic 3: The volume of hydrogen gas production with time in the presence of different by weight %NaOH It is observed that production rate of hydrogen gas increases with the increase amount of catalyst under reaction conditions where NaOH amount is 5% by weight using 50 mL 50 itiM NaBH ₄ solution at a mixing speed of 1500 rpm at 30°C using p(AMPS)-Ni composite comprising Ni catalyst in varying amounts according to the value determined by TGA analysis under same reaction conditions. (Graphic 4)

20 40 60 80 100

Time (minute)

Graphic 4: The change in amount of hydrogen gas production of usingdifferent amounts of catalysts Even when the p(AMPS)-Ni catalyst is reused 5 times in a row, it gives 100% yield with over 75% activity. (Graphic 5)

Activity Conversion

% Conversion and Activity

1

Reuse Times

Graphic 5: The % conversion and activities of Poly(2-acrylamido-2-methyl-l - propansulphonic acid)-Nickel (p(AMPS)-Ni) hydrogel composites by as they are used for 5 times repeatedly for same reaction conditions

Although the p(AMPS)-Ni catalyst is stored in pure water for 1 month, conversion is provided for 100% and its activity is around 75%. (G raphic 6)

Activity Conversion

% Conversion and Activity

Start I Day l Week 2 Weeks 1 Month

Storage Time

Graphic 6: The graph of coversion and activities of p(AMPS)-Ni hydrogel composites in time upon they are kept in pure water for different periods of time To control size of metal nanoparticles, concentration of metal ion solution can be arranged as desired. Metal nanoparticles which will be formed in the hydrogel network structure can be synthesized in appropriate size and amount by analyzing TEM images. The size can be controlled by loading metal ion into the hydrogel. Loading amount of metal ion into hydrogel is directly proportionate to functional groups in the hydrogel structure. And this can be controlled by ratio of monomers to be used. p(AMPS) hydrogels has a porous structure and pore dimensions can be increased further by reducing the cross-linker amount. Thus, this enable that the reactants contact metal nanoparticles inside the hydrogel by passing through the pores easily and the reaction to occur fast. So, diffusion problem is eliminated. The fact that the hydrogels increase pore dimension by getting swollen in aqueous media and decreases the pore dimension by shrinking in non-aqueous media enables them to be used as a flexible material and thus provides great advantage in being used in production of hydrogen gas (Figure 1 ).

The TE image of Ni particles prepared in hydrogel materials is given in Figure 2.) p(AMPS)-Ni hydrogel metal composite image obtained via transmission electron microscope (TEM) is shown in Figure 2. Dark spots with approximately 100 nm dimension and in the form of sphere in the figure indicate metal nanoclusters/metal nanoparticles. Within the framework of these basic concepts, it is possible to develop a wide variety of embodiments of the inventive "production method of hydrogel-metal composite". The invention cannot be limited to the examples described herein; it is essentially according to the claims.