Energy and environmental usage of super porous poly(2-acrylamido-2-methyl- 1-propan sulfonic acid) cryogel support

Article abstract of DOI:10.1039/c4ra01386g

Superporous and nonporous poly(2-acrylamido-2-methyl-1-propansulfonic acid) [p(AMPS)] cryogels and hydrogels were prepared under freezing conditions (-18 °C) and at room temperature (25 °C), respectively. The swelling equilibrium values of the p(AMPS) cryogels were extremely fast, 3600 fold faster than those of conventional hydrogels. The p(AMPS) cryogels were further employed as highly effective supports for the in situ preparation of metal nanoparticles within superporous networks, by loading Co(ii) and Ni(ii) ions into the cryogel network from aqueous environments and reducing with NaBH ₄. The Co metal nanoparticle-containing cryogel composites demonstrated superior catalytic performances in comparison to nonporous p(AMPS) hydrogel composites for energy and environmental applications e.g., hydrogen production from the hydrolysis of sodium borohydride, and reduction of 4-nitrophenol to 4-aminophenol. The energy applications of cryogel-based p(AMPS)-Co metal composites, especially, were investigated in detail. The effect of various parameters on the rate of the hydrogen generation reaction, such as porosity, metal types, pH, the types of reaction water, temperature and reuse of catalyst, were examined for the p(AMPS)-Co cryogel composite materials. With the p(AMPS)-Co cryogel composite a very high hydrogen generation rate of 14 501 mL H₂ per min per g of Co was attained. This value is one of the best recorded values in comparison to the values obtained for other similar catalysts reported in the literature. p(AMPS)-Co composite cryogels were repeatedly used without significant loss of catalytic activity (82%) even after five repetitive uses for catalytic hydrolysis reactions with NaBH₄. Additionally, a very low activation energy for the p(AMPS)-Co cryogel composite systems was attained: E_a = 15.40 ± 0.3 kJ mol^-1.

Full text of DOI:10.1039/c4ra01386g

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PAPER
Energy and environmental usage of super porous
poly(2-acrylamido-2-methyl-1-propan sulfonic
acid) cryogel support
Cite this: RSC Adv., 2014, 4, 23886
ab
and Fahriye Sevenb
*
Nurettin Sahiner
Superporous and nonporous poly(2-acrylamido-2-methyl-1-propansulfonic acid) [p(AMPS)] cryogels and
hydrogels were prepared under freezing conditions (ꢀ18 ꢁC) and at room temperature (25 ꢁC),
respectively. The swelling equilibrium values of the p(AMPS) cryogels were extremely fast, 3600 fold
faster than those of conventional hydrogels. The p(AMPS) cryogels were further employed as highly
effective supports for the in situ preparation of metal nanoparticles within superporous networks, by
loading Co(II) and Ni(II) ions into the cryogel network from aqueous environments and reducing with
NaBH4. The Co metal nanoparticle-containing cryogel composites demonstrated superior catalytic
performances in comparison to nonporous p(AMPS) hydrogel composites for energy and environmental
applications e.g., hydrogen production from the hydrolysis of sodium borohydride, and reduction of 4-
nitrophenol to 4-aminophenol. The energy applications of cryogel-based p(AMPS)–Co metal
composites, especially, were investigated in detail. The effect of various parameters on the rate of the
hydrogen generation reaction, such as porosity, metal types, pH, the types of reaction water,
temperature and reuse of catalyst, were examined for the p(AMPS)–Co cryogel composite materials.
With the p(AMPS)–Co cryogel composite a very high hydrogen generation rate of 14 501 mL H2 per min
per g of Co was attained. This value is one of the best recorded values in comparison to the values
obtained for other similar catalysts reported in the literature. p(AMPS)–Co composite cryogels were
repeatedly used without significant loss of catalytic activity (82%) even after five repetitive uses for
catalytic hydrolysis reactions with NaBH4. Additionally, a very low activation energy for the p(AMPS)–Co
Received 17th February 2014
Accepted 14th April 2014
DOI: 10.1039/c4ra01386g
www.rsc.org/advances
cryogel composite systems was attained: Ea ¼ 15.40 ꢂ 0.3 kJ molꢀ1
.
derivative nitrophenols (4-NP) are a major concern because of
their high toxicity, carcinogenic nature, and low biodegrad-
ability, they accumulate in waste water as they are used in many
industries such as the production of anilines, paper, explosives,
pharmaceuticals, dyes, and therefore cause many problems.7,8
Due to its favorable properties such as having a selective strong
reducing capability, being a hydrogen generator and storage
material, and its usefulness in manufacturing pharmaceuticals,
and ne chemicals, NaBH4 is a preferred material attracting
great attention over the past decade and is one of the mostly
frequently used materials for various applications, as stated
earlier.9–11 NaBH4 can be used for hydrogen generation from the
hydrolysis reaction in the presence of a catalyst, and for the
reduction of toxic nitrophenols and their derivatives to envi-
ronmentally friendly amino phenols.12,13 In the use of metal
nanoparticles as catalysts, aggregation problems, and the
stabilization of metal nanoparticles (generally ꢃ5–100 nm) to
prevent early oxidation and poisoning, are the most cumber-
some problems and can be overcome by employing nanotech-
nology and polymer science.14,15 Recently, we have reported the
use of a smart hydrogel as a template for the preparation of
metal nanoparticles taking advantage of their hydrophilic 3-D
1. Introduction
In recent years, innovative research has been carried out with
regard to energy and environmental concerns.1,2 Energy is
essential to maintain all activities in daily life. However, the
production, transportation and use of energy, which is obtained
principally from fossil fuels, is the root of major adverse effects
on the environment and on health. Therefore, the generation of
renewable, clean and inexhaustible energy is of crucial signi-
cance.3,4 Many researchers have focused on hydrogen energy as
it is renewable, clean, safe, cheap and easily generated from the
hydrolysis of various storage materials such as chemical
hydrides.5,6 In addition to drawbacks in energy production, the
major concern for the environment and human health is the
existence of hazardous organic toxic chemicals. Amongst the
organic pollutants, halogenated organic compounds and their
aCanakkale Onsekiz Mart University, Faculty of Sciences and Arts, Chemistry
Department, 17100, Canakkale, Turkey. E-mail: sahiner71@gmail.com; Fax: +90-
2862181948; Tel: +90-2862180018-2041
bNanoscience and Technology Research and Application Center (NANORAC), Terzioglu
Campus, 17100, Canakkale, Turkey
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networks which have different numbers of functionalities prepared by a free radical polymerization technique at freezing
including –OH, –COOH, –NH2, –CONH2, and –SO3H and are and room temperatures, and used them for in situ metal
able to bind easily to metals without dissolving in aqueous nanoparticle preparation by loading Co and Ni ions from their
media They provide a range of environmentally responsive aqueous solutions and reduction. Finally, we also demonstrated
behaviors, such as bending, degrading, swelling, shrinking, and the use of p(AMPS)–M (M: Co, Ni) cryogel composites for H2
color changing against environmental factors such as pH, generation via NaBH4, and in an environmental application for
solvent, temperature, electric eld, magnetic eld, etc.16–18 the reduction of 4-NP to 4-AP. Various parameters affecting the
Despite the advantages of common hydrogels, the diffusion of H2 generation performance were evaluated and compared with
solvents, metal ions or any reactant into the hydrogel network the literature.
remains the foremost tiresome and time consuming problems
for the in situ preparation of nanoparticles within a hydrogel
network or for their use as reactants in catalytic applications. To
2. Materials and methods
2.1. Materials
improve their responsive properties, nano-, micro- and macro-
pores can be generated within hydrogel networks by various
techniques. One alternative solution is to use hydrogel networks
with macro- or superpores (of the order of a few tens of
micrometers) that can be generated within hydrogels by
preparing them under cryogenic conditions, namely as cryogels.
Recently much progress has been made in the cryogelation
method by preparing hydrogels under cryogenic conditions.19–22
The cryogelation method includes the use of a large amount of
pure water, nearly 90–95 wt% of the initial solution. The poly-
merization and crosslinking reaction can be carried out around
frozen water molecules (ice-crystals) under freezing conditions.
During the polymerization reaction, most of the water mole-
cules are converted to ice crystals, whereas bound water and
other solutes in the reaction medium, such as the initiator,
crosslinking agent, catalyst and monomer molecules, accumu-
late in a non-frozen liquid microphase.23–25 Cryogenic network
formation progresses via polymerization of the non-frozen
liquid microphase around the ice-crystals that act as a porogen.
Upon thawing the ice-crystals within the gel matrix, a super-
porous cryogel is formed that has large pores with variable size
and geometry depending on the shape of the ice-crystals.26,27
Furthermore, the natural characteristics of the cryogels, such as
the super pore size, pore wall thickness, elasticity, and
The monomer, 2-acrylamido-2-methyl-1-propanesulfonic acid
(AMPS, 50 wt%, Sigma-Aldrich), the crosslinker, N,N0-methyl-
enebisacrylamide (MBA, 99%, Across), the initiators, ammo-
nium persulfate (APS, 99%, Sigma-Aldrich) or potassium
persulfate (KPS, 99%, Sigma-Aldrich), and the accelerator,
N,N,N0,N0-tetramethylethylene diamine (TEMED, 98%, Across)
were used as received for the hydrogel and cryogel preparation.
Cobalt(II) chloride hexahydrate (CoCl2$6H2O, 99%, Sigma-
Aldrich) and nickel(II) chloride hexahydrate (NiCl2$6H2O, 98%,
Sigma-Aldrich) were used as metal ion sources for nanoparticle
preparation. Sodium borohydride (NaBH4, 98%, Merck) was
used as the reducing agent and as the H2 source in the hydro-
lysis reaction. 4-Nitrophenol (4-NP, 99%, Merck) was used as the
organic compound in the catalytic reduction reaction. Sodium
hydroxide (NaOH, 97%, Sigma-Aldrich), and hydrochloric acid
(HCl, 37%, Sigma-Aldrich) were used for the preparation of
acidic and basic solutions. Tap water, water from Sarı Çay
creek, and sea water were used as the reaction media for the
same hydrogen generation reactions. Distilled (DI) water of
18.3 MU cmꢀ1 was used for the experimental studies.
2.2. Synthesis of p(AMPS) hydrogels and cryogels
mechanical strength, can be tuned by adjusting the preparation Synthesis of p(AMPS) hydrogel was carried out via a free radical
conditions, and the composition of the solutions, such as the polymerization technique. Briey, 5 mL of AMPS, 0.002 g of
amount of water or organic co-solvents, cooling rate, tempera- MBA (0.1% mole ratio of monomer) and 5 mL of TEMED were
ture gradient, solute concentrations, precursor composition, mixed to obtain an homogenous solution, and 1 mL of APS
etc. Interconnected macropores within the three-dimensional initiator solution (1% mole ratio of monomer) was added by
cryogenic networks enable the retention and release of water or mixing. The obtained hydrogel precursor was placed in plastic
any solvent which is absorbed within the large pores by a straws of about 0.4 mm diameter. The reaction was completed
diffusion pathway via reversible squeeze–relaxation cycles within 15 min and the obtained cylindrical hydrogels were cut
under denite pressure.28–31 Due to their discernible properties, to 2–3 mm in length, and washed with DI water for 12 h by
most notably their fast responsiveness, structural exibility, placing in excess amounts of water and replacing the water
high mechanical resistance, large pores, and fast diffusion every 3 h.
pathways, cryogels can respond to external stimuli such as pH,
The preparation of p(AMPS) cryogel was performed via a
solvent, and ionic strength etc. much faster than conventional cryopolymerization technique. An ice bath was used to cool the
hydrogels.32,33 Moreover, these properties of cryogels make them AMPS precursors before cryopolymerization. Firstly, 0.78 mL of
ideal fast metal ion binding materials, and catalyst supports for AMPS aqueous solution (50 wt%) was added to 6.22 mL of DI
in situ metal nanoparticle preparation.16–18 Cryogels containing water in order to obtain a monomer ratio of 6 w/v% within the
metal composites can be used as catalysts for H2 generation for total solution volume of 8 mL. Then, 0.0354 g of MBA (10% mole
energy applications, and for the reduction of nitro compounds ratio of monomer), and 50 mL of TEMED were added into the
for environmental applications.
monomer solution and mixed by using a vortex mixer. Sepa-
Therefore, we report the use of poly(2-acrylamido-2-methyl-1- rately, 0.0054 g of KPS (1% mole ratio of monomer) was dis-
propansulfonic acid) [p(AMPS)] cryogels and hydrogels solved in 1 mL of DI water, and cooled under constant mixing in
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an ice-bath for 5 min. Finally, this redox initiator solution was temperature, for 24 h. Then, to remove the unattached metal
added to the cooled monomer mix, and then this mix was ions from the polymeric networks, metal ion-loaded polymeric
placed in plastic straws (about 8 mm in diameter). The cry- networks were washed with distilled water for 30 min. Aer
ogelation was achieved in a deep freezer at ꢀ18 ꢁC for 24 h via washing, the metal ion-loaded hydrogels and cryogels were
free-radical crosslinking polymerization. Finally, the obtained placed in 50 mL of 0.2 M NaBH4 solution for 3 h at a stirring rate
cryogels were thawed at room temperature, and cut into smaller of 500 rpm at room temperature for the reduction of metal ions
dimensions of 2–3 mm. The acquired cryogels were cleaned by in situ within the polymeric networks to their corresponding
washing with plenty of DI water to remove the unreacted species metal nanoparticles. Finally, the hydrogel or cryogel-M
such as the monomer, polymer, crosslinker and initiator from composites were washed with DI water again, and used for
the cryogenic matrices. The obtained cryogels were dried in an catalytic applications.
ꢁ
oven at 45 C for further experimental studies.
3. Results and discussion
2.3. Synthesis of metal nanoparticles within conventional
hydrogel and superporous p(AMPS) cryogel matrices
3.1. Characterization of nonporous, superporous polymeric
networks and their metal-containing composites
The preparation of metal nanoparticles inside the network
structure was carried out by metal loading from aqueous solu- The swelling characteristics, pore sizes and pore structures of
tions and in situ reduction. Briey, 100 mg of dried hydrogel and conventional hydrogel and novel cryogel networks were deter-
cryogel samples were placed in 100 mL of 500 ppm M(II) (M ¼ mined through swelling experiments, optical microscopy
Co or Ni) solutions at a constant mixing rate of 500 rpm, at room (Olympus bX53) and scanning electron microscopy (SEM, JEOL
Fig. 1 The polymerization mechanism of p(AMPS) cryogels (a), and visual demonstration of the swelling of 0.1% crosslinked p(AMPS) hydrogels
(b), and (c) 10% crosslinked p(AMPS) cryogels.
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2010) analysis. The metal ion absorption capacity and the metal eventually result in faster catalytic performance in their catalytic
nanoparticle contents of the hydrogel and macroporous cry- applications.
ogels were determined by atomic absorption spectroscopy (AAS,
Furthermore, the % porosity of cryogels was roughly calcu-
Thermo ICA 3500 AA SPECTRO) measurements. The size of the lated as 78.60% according to the following equation:
metal particles inside the bulk cryogel matrices was determined
% Porosity ¼ [(mwet cryogel ꢀ msqueezed cryogel)/mwet cryogel] ꢄ 100
via transmission electron microscopy (TEM, JEOL 2010) anal-
(3)
ysis. Finally, the thermal behaviour of the hydrogels, cryogels
and their various metal composites were investigated by ther-
mogravimetric analysis (TGA/DTA-SII 6300).
The interconnected and highly open structures of p(AMPS)
cryogels were examined through SEM analysis, and the optical
microscope images are illustrated in Fig. 2(a) through (f). The
pore structures of dried p(AMPS) cryogel samples were obtained
as a thin lm using a freeze-dryer under a vacuum. As can be
seen in Fig. 2(a)–(f), the pore size of the cryogels was more than
50 mm. Highly interconnected and open pores of cryogels
provide a faster contact time with the metal nanoparticles
embedded within them with reactants in the reaction medium.
It is obvious that the highly porous network and fast responsive
nature of cryogels make them indispensable materials in the
design of advanced template materials e.g., for metal nano-
particle preparation and for use as catalyst systems for catalytic
applications. Here, we prepared Co and Ni metal nanoparticles
as catalyst systems by using NaBH4 as the reducing agent within
p(AMPS) cryogels via an in situ reduction method. For that
purpose, Co(II) or Ni(II) metal ion-loaded 0.1 g p(AMPS) cryogels
were treated with 50 mL of 0.2 M NaBH4 solution at a mixing
rate of 500 rpm for 3 h. As clearly demonstrated in Fig. 3(a),
Co(II) ions inside the cryogel matrices were converted to Co
metal nanoparticles by NaBH4 treatment. Furthermore, the
TEM images of p(AMPS)–Co composites show that the size of
the Co metal particles are nanoscale, ranging from about few
To visualize the mechanism of cryopolymerization, a repre-
sentation of the cryopolymerization reaction of the non-frozen
liquid microphase occurring around the growing ice-crystals is
illustrated in Fig. 1(a). Firstly, the initiator activated by the
catalyst and the generated radicals reacts with the monomer
and crosslinker to produce branched oligomers and macromers
etc., followed by polymerization and the crosslinking reaction
around the ice-crystals, thus a frozen cryogel network is
obtained. Upon completion of polymerization and the cross-
linking reaction, and aer thawing, macrometer sized pores are
formed within the 3-D polymeric matrices in place of the ice-
crystals, as can be seen in the gure.
As the swelling behaviour is one of the most important
characteristics of hydrogels, the swelling behaviour of p(AMPS)
hydrogels and cryogels was investigated. Swelling studies of
hydrogels and cryogels were carried out by means of mass
increase by placing the dried and weighed polymeric networks
within 100 mL of DI water, removing them at certain time
intervals and determining their mass increase with time. The
maximum percent swelling (Smax %) of the hydrogels and cry-
ogels were calculated as about 28 000% and 500% according to
eqn (1) and (2). So, the high swelling degree of 0.1 mole %
crosslinked p(AMPS) hydrogel implies that it contains greater
amounts of water (ꢃ56 fold) than 10% crosslinked p(AMPS)
cryogel.
% S ¼ [(mwet hydrogel ꢀ mdry hydrogel)/mdry hydrogel] ꢄ 100 (1)
% S ¼ [(msqueezed cryogel ꢀ mdry cryogel)/mdry cryogel] ꢄ 100 (2)
As illustrated in Fig. 1(b) and (c), the time to reach Smax % for
hydrogels and cryogels is quite different with the hydrogels
reaching their Smax % at about 8 h, and the cryogels reaching
Smax % within 10 s. It is obvious that the swelling time for the
superporous cryogel is almost 3600 times shorter than the
corresponding conventional hydrogel, even though it has 100
fold more crosslinks with a lower swelling ratio. In fact, this
behaviour is due to differences in their preparation methods. As
cryogels have a high degree of crosslinker with a very large
interconnected porous structure and a low specic surface area
compared to their counterpart hydrogels, they swell quickly,
within seconds, with a small amount of water uptake in
comparison to normal hydrogels.34,35 The fast swelling property
of cryogels provides many advantages. For example, the main
advantages of the higher swelling rates, and faster swelling of
cryogels provide fast process times in cleaning of cryogels, and
loading of cryogels with different species such as metal ions
and their reduction to corresponding metal nanoparticles, and
Fig. 2 SEM images of p(AMPS) cryogels under (a) 50, (b) 150, (c) 370,
and (d) 400 magnification, and optical microscope images of p(AMPS)
cryogels under (e) 10ꢄ and (f) 20ꢄ magnifications.
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Fig. 4 (a) Metal ion absorption capacities of p(AMPS) hydrogels and
cryogels, and (b) TGA thermogram of p(AMPS) hydrogel, cryogel and
their metal nanoparticle-containing composites.
shorter than that of the hydrogel, the Co(II) ion absorption
capacity is much less than that of the hydrogels. This is
generally the case for cryogels because the specic surface area
is lower for the cryogels of p(AMPS) than that of the corre-
sponding hydrogels. The only downside of cryogels is their
lower specic surface area as has been reported in the litera-
ture.34,35 In addition to the comparison of their metal ion
Fig. 3 (a) The reduction of Co(II) ions to cobalt nanoparticles within absorption capacities, the metal nanoparticle contents were
cryogel matrices, and (b) TEM images of cobalt nanoparticles within
porous cryogel networks.
also determined via an acid dissolution method in which metal
nanoparticle-containing cryogels and hydrogels were treated
with concentrated HCl. Thus, 0.1 g of p(AMPS)–M (M: Co or Ni)
composites were treated with 50 mL of 5 M HCl solution for 12 h
tens of nm to about 50 nm as seen in Fig. 3(b). The TEM images three times repeatedly, and the elution solution was diluted at a
were obtained by grinding samples of p(AMPS)–Co composite ratio of 1/150 with pure water. The amount of metal ions within
suspension in acetone and a few drops of these composites were each polymeric matrix was measured by AAS. The amount of Co
placed on carbon coated Cu TEM grids. The TEM images were nanoparticles within 1 g of cryo-p(AMPS) was found to be
then acquired under an electron beam, accelerated by an 86.27 mg gꢀ1, whereas the amount of Co metal nanoparticles
applied 200 kV in vacuum. As demonstrated in TEM images, the inside 1 g of hydrogel–p(AMPS) was 124.88 mg gꢀ1. As can be
Co particles are almost homogeneously distributed throughout seen these values are consistent with their absorbed amounts.
the p(AMPS) cryogel matrices.
The metal contents of the cryogels and hydrogels are given in
The fast responsive behavior of porous p(AMPS) cryogels and Table 1. Additionally, the metal content of each polymeric
the corresponding hydrogels were compared in terms of Co(II) matrix was also conrmed via TGA as seen in Fig. 4(a) and Table
ion absorption times and amount as illustrated in Fig. 4(a). 1. The thermal behavior of the p(AMPS) cryogels, hydrogels and
About 1.0 g of p(AMPS) cryogel and hydrogels were immersed in their metal particle-containing composites were determined via
500 mL of 500 ppm CoCl2$6H2O solution at a mixing rate of TGA by heating about 2 mg of dried samples from 50 to 1000 ꢁC
500 rpm. It was found that 91.70 mg of Co(II) ions were absorbed at a heating rate of 10 ꢁC minꢀ1 under a nitrogen ow at 100 mL
by the cryogels at the end of 4 h, whereas 129.35 mg Co(II) ions minꢀ1. As can be seen, the thermal behavior of the p(AMPS)
were absorbed by the hydrogels at the end of 8 h. Although the cryogels and hydrogels seem very similar whereas their metal
Co(II) ion absorption time of the cryogels is about two fold nanoparticle-containing composites are different, depending
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Table 1 The amount of metal nanoparticles, TGA results, TOF (Turn Over Frequency) values and Hydrogen Generation Rates (HGR) for various
catalyst systemsd
Metal composites
aAmount of metal nanoparticles
Weight loss % by TGA (%) at 1000 ꢁC
bTOF
cHGR
Hydrogel p(AMPS)–Co
Cryogel p(AMPS)–Co
Cryogel p(AMPS)–Ni
3rd Loading p(AMPS)–Co
Magnetic p(AMPS)–Co
124.88 ꢂ 1
86.27 ꢂ 1.2
89.41 ꢂ 1.5
195.42 ꢂ 1
87.16 ꢂ 1.2
40.37
24.22
33.14
53.11
57.45
2.24 ꢂ 0.2
6.37 ꢂ 0.1
1.64 ꢂ 0.12
6.25 ꢂ 0.1
2.75 ꢂ 0.3
950.90 ꢂ 0.3
2697.91 ꢂ 0.5
699.11 ꢂ 0.2
2648.92 ꢂ 0.2
1171.54 ꢂ 0.1
a
The amount of metal nanoparticles within a cryogel/hydrogel as mg gꢀ1 determined by AAS measurements. b TOF (Total Turnover Frequencies)
[mol H2 per mol catalyst per min]. c HGR (Hydrogen Generation Rate) [mL of H2 per min per g of M]. d Reaction conditions: 50 mL of 50 mM NaBH4,
5% NaOH, 1000 rpm, 30 ꢁC.
on the metal types and their amounts. It can be inferred that the in Fig. 5(b), the inherently magnetic eld responsive behavior of
p(AMPS) cryogels and hydrogels start to degrade very sharply at a catalyst offers control over the reaction system by an externally
ꢁ
about 300 C, whereas the onset of sharp degradation of their applied magnetic eld. The preparation of ferrite metal
metal nanoparticle-containing composites occurs at about
350 ꢁC. The p(AMPS) cryogels and hydrogels and their
composites have different degradation temperatures as shown
ꢁ
in the TGA thermograms and this continues up to 1000 C as
illustrated in Fig. 4(b). The weight losses of the p(AMPS) cry-
ogels and their Co nanoparticle-containing composites were
22.58 and 17.65 wt%, whereas the weight losses of the p(AMPS)
hydrogels and their Co composites were 44.45 and 10.18% at
ꢁ
985 C. Consequently, due to the high crosslinking degree of
the cryogels, they have higher thermal stability than conven-
tional hydrogels. On the other hand, due to their low metal
nanoparticle content, the composites are less thermally stable
than the metal composites of the hydrogels. The thermal
resistances of other metal composites are given in Table 1, it
can be seen that the metal nanoparticle-embedded p(AMPS)
networks, whether hydrogel or cryogel, are thermally more
stable and have less sharp degradation temperature ranges
than their bare hydrogel or cryogel counterparts. Therefore, it
can be concluded from TGA analysis that the p(AMPS) cryogel
metal composites can be safely used at temperatures ranging
ꢁ
from room temperature up to 200 C without any signicant
weight loss. The higher thermal stability of the cryogels could
be due to the high amount of crosslinker used for the cryogels
than for the hydrogels as the crosslinking degree of the cry-
ogels is approximatelyone-hundred fold higher than those of
the hydrogels.
3.2. Energy and environmental applications of p(AMPS)–Co
metal composites
To demonstrate the potential use of p(AMPS) hydrogels and
cryogels in energy and environmental applications, H2 genera-
tion and reduction of 4-NP were carried out using NaBH4 as the
hydrogen source, and as a reducing agent using p(AMPS)–M
based materials as catalysts. The reaction mechanisms of both
the catalytic reactions are shown in Fig. 5(a). Interestingly, reduced 0.1 g p(AMPS)–Co (19.54 mg gꢀ1 nanoparticles) catalyst
magnetic ferrites can also be used for NaBH4 hydrolysis for H2
generation, as well as Co and Ni nanoparticles. Here, two types
of magnetic metal nanoparticles ferrite and Co containing
metal nanoparticles prepared within p(AMPS) cryogels were
used for H2 production from hydrolysis of NaBH4. As illustrated NaBH4, 30 ꢁC, mixing rate of 1000 rpm].
Fig. 5 (a) Reaction mechanisms for H2 generation from hydrolysis of
NaBH4, and reduction of 4-NP. (b) H2 generation from hydrolysis of
NaBH4 via ferrite-containing 0.1 g p(AMPS)–Co metal nanoparticles
(8.71 mg nanoparticles) and magnetic three times Co(II) loaded-
system under an externally applied magnetic field. [Reaction condi-
tions: 50 mL of 50 mM NaBH4, 5 wt% NaOH, 30 ꢁC, mixing rate of 1000
rpm.] (c) UV-Vis spectra of 4-NP reduction to 4-AP obtained using 0.1
g of p(AMPS)–Co cryogel composite catalyst systems (8.62 mg of Co
nanoparticles). [Reaction conditions: 50 mL of 0.01 M 4-NP, 0.28 M
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nanoparticles was done according to the literature,16 briey, 0.1
g of p(AMPS) cryogels were immersed within a metal ion solu-
tion containing a 1 : 2 mole ratio of Fe(II) (0.05 M) : Fe(III) (0.1 M)
mixture in 100 mL of aqueous solution for 12 h. Then, these
iron ion-loaded cryogels were washed for 1 h to remove
unbound metal ions from the cryogel matrices, and then placed
in 100 mL of 0.5 M sodium hydroxide solution for 5 h to convert
the iron ions into ferrite magnetic particles. Additionally,
magnetic p(AMPS) cryogels were also prepared with Co(II) ions
aer three loading and reduction cycles as mentioned in
Section 2.3. Aer the rst loading and reduction of Co(II) ions
into p(AMPS) cryogels, the prepared p(AMPS)–Co composites
were placed again in 500 mL of 500 ppm Co(II) ion solution for 5
h, and then reduced to metal nanoparticles with 50 mL of 50
mM NaBH4 solution for 3 h treatment, and the same procedure
was repeated three times to increase the Co metal nanoparticle
content of p(AMPS) cryogel with magnetic eld responsive
behavior. The use of magnetic metal nanoparticles is very
important to control the reaction e.g., the catalyst systems can
be removed from the reaction mixture and placed back into the
reaction medium to control the catalytic reactions. As illus-
trated in Fig. 5(b), H2 generation from the hydrolysis of NaBH4
using 0.1 g of ferrite-containing p(AMPS)–Co (containing 8.71
mg Co nanoparticles) was stopped at 12.5 min by removing the
catalyst system from the reaction medium, and continued at
22.5 min by placing it back into the reaction medium. The
reaction conditions throughout this investigation were 50 mL of
Fig. 6 (a) H2 generation from hydrolysis of 50 mL of 50 mM NaBH4 via
8.62 mg of Co nanoparticle-containing 0.1 g of cryogel p(AMPS)–Co
composite, and 0.069 g of hydrogel p(AMPS)–Co composite. [Reac-
tion conditions: 50 mL of 50 mM NaBH4, 5 wt% NaOH, 30 ꢁC, 1000
rpm.] (b) H2 generation from hydrolysis of NaBH4 via 0.1 g cryogel
ꢁ
50 mM NaBH4, 5 wt% NaOH, 30 C, and a mixing rate of 1000
rpm . The same H2 production control was done by using a 3rd
time Co(II) loaded and reduced p(AMPS)–Co composite cryogel p(AMPS)–Co (8.62 mg Co nanoparticles) and 0.1 g p(AMPS)–Ni
composites (8.94 mg Ni nanoparticles). [Reaction conditions: 50 mL
50 mM NaBH4, 5 wt% NaOH, 30 ꢁC, mixing rate of 1000 rpm].
catalyst system (containing 19.54 mg Co nanoparticles) at 2 and
4 min under the same conditions. The hydrolysis reaction was
completed within 30 min with a ferrite-containing p(AMPS)–Co
metal composite catalyst system whereas the hydrolysis reac-
cryogels as the catalyst support, and the reaction media for
tion was completed in only 6 min with a three times-loaded
catalytic applications is pertinent.
p(AMPS)–Co cryogel composite catalyst system. Therefore, the
inherently magnetic eld responsive p(AMPS)–Co cryogel cata-
lyst system provides a great advantage in controllable fast
catalysis for energy applications. Moreover, the environmental
3.3. The effect of porosity and metal types on NaBH4
hydrolysis
application of the p(AMPS) cryogel catalyst system was To determine the effect of porosity, the H2 production rates of
demonstrated by using 0.1 g of p(AMPS)–Co metal composite conventional hydrogel and superporous cryogel p(AMPS)–Co
for the reduction of 4-NP to 4-AP as illustrated in Fig. 5(c). The composites were compared under the same reaction conditions;
reduction of 4-NP was carried out using 50 mL of 0.01 M 4-NP for example, 0.1 g of cryogel p(AMPS)–Co composite (containing
by using rst time Co(II) loaded and reduced p(AMPS)–Co 8.62 mg of nanoparticles) and 0.069 g of hydrogel p(AMPS)–Co
cryogel composite catalyst system (containing 8.62 mg nano- composite (containing 8.62 mg of nanoparticles) were used for
ꢁ
particles) in the presence of 50 mL of 0.28 M NaBH4 at 30 C the hydrolysis of 50 mL of 50 mM NaBH4, 5 wt% NaOH content,
and a mixing rate of 1000 rpm. The reduction of 4-NP was at 30 ꢁC, and a mixing rate of 1000 rpm. The same amount (nearly
completed within 120 s by cryogel-based p(AMPS)–Co metal 250 mL) of hydrogen was produced from the hydrolysis reactions
composites. This time is relatively fast in comparison to the and the reaction was completed within about 10 min using cry-
reported values in the literature,13,16,36,40–44 suggesting that the ogel p(AMPS)–Co composite. The same reaction was completed
cryogel-based catalyst composite systems have great advan- within 30 min via the hydrogel p(AMPS)–Co composite as illus-
tages for faster catalytic performances. When considering trated in Fig. 6(a). The H2 production rate of the cryogel p(AMPS)–
these results and comparing with papers reported in the Co composite is three times faster than that of conventional
literature, the highly porous, large and open interconnected hydrogel-supported metal composites. At the same time, as
structures of the cryogels provide great benets in terms of fast shown in Table 1, the Hydrogen Generation Rate (HGR), and
reaction kinetics.16,40–44 Therefore, the choice of superporous Total Turnover Frequency (TOF) value of hydrogel p(AMPS)–Co
composites were calculated as about 950 mL of H2 per min per g
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of metal and 2.24 mol H2 per mol catalyst per min, whereas the
Different metal nanoparticles were prepared within cryogel
HGR and TOF values of cryogel p(AMPS)–Co composites were matrices, and their catalytic performances were compared for
calculated as 2697 mL of H2 per min per g of M and 6.37 mol H2 the hydrolysis of NaBH4 under the same reaction conditions as
per mol catalyst per min, respectively. These values are 2.839 shown in Fig. 6(b). As can be seen, H2 generation from the
times and 2.844 times higher for the cryogel catalyst system, hydrolysis of NaBH4 was completed in about 10 min by using
conrming the great benet of porosity. The porosity of the cry- 0.1 g of p(AMPS)–Co composite (containing 8.62 mg nano-
ogels enables easy and fast diffusion of the reactants and prod- particles) whereas the same reaction was completed in about 40
ucts in and out of the cryogel matrices and other species enabling min using the p(AMPS)–Ni composite (containing 8.94 mg of
faster kinetics with embedded metal nanoparticles inside the nanoparticles) under the same reaction conditions. The HGR of
pores. This phenomenon is the main reason for the catalytic the cryogel p(AMPS)–Co and p(AMPS)–Ni composites were
activity of the porous catalysts or cryogels and the most prom- calculated to be 2700 and 700 mL H2 per min per g of metal as
inent distinct feature of the catalyst.
given in Table 1. It is obvious that the HGR of the Co nano-
particle-containing p(AMPS) cryogels was nearly four-fold faster
Fig. 7 (a) The effect of pH and (b) the effect of water type on H2 generation reaction from hydrolysis of sodium borohydride via 0.1 g of cryogel
p(AMPS)–Co (8.62 mg of nanoparticles). [Reaction conditions: 50 mL 50 mM NaBH4, 5 wt% NaOH, 30 ꢁC, mixing rate of 1000 rpm].
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than that of the Ni nanoparticle-containing p(AMPS) cryogels, and TOF values were about 1288.99 ꢂ 1.3 mL H2 per min per g
as expected since the Co nanoparticle catalytic performance is of M and 3.03 ꢂ 0.5 mol H2 per mol catalyst per min for
better than that of Ni nanoparticles in accordance with the seawater in comparison to 2697.91 ꢂ 1 mL H2 per min per g of
literature.16,17,37–44
M and 6.37 ꢂ 0.4 mol H2 per mol catalyst per min for DI water as
illustrated in Table 2. As is very well known, the saltiness of sea
water is higher than that of other types of water as compared
with Sarı Çay water a creek water, tap water and DI water and
this prevents higher TOF and HGR values from being obtained.
In brief, due to the presence of different salts and some other
impurities within seawater, creek waters and tap waters, the
reduction in HGR and TOF valuables are plausible.
3.4. The effect of pH of the reaction medium and type of
water on NaBH4 hydrolysis
To determine the effect of pH of the medium on the hydrolysis
reaction of NaBH4 employing 0.1 g of p(AMPS)–Co metal
composite (containing 8.62 mg of nanoparticles), ve different
pH values were tested i.e., 4, 6, 8, 10 and 12 under the same
reaction conditions as shown in Fig. 7(a). To adjust the pH of
the reaction medium, 1 M HCl or 1 M NaOH were added into
the hydrolysis medium and the obtained solutions with
3.5. The effect of temperature on NaBH4 hydrolysis
different pHs were used for the hydrolysis reaction of NaBH4. The effect of temperature on the hydrolysis of NaBH4 was
Although the amount of produced H2 gas was not affected by studied using 0.1 g of p(AMPS)–Co composites at different
ꢁ
ꢁ
ꢁ
the change of medium pH, they all produced about 250 mL of temperatures between 30 C and 70 C with 10 C increments
H2, the HGR of p(AMPS)–Co composites were affected as can under the same reaction conditions: 50 mL of 50 mM NaBH4
be seen from the slope of Fig. 7(a). As can clearly be seen, as (containing 5 wt% NaOH), a mixing rate of 1000 rpm, using 0.1
the pH of the NaBH4 solution increased, the HGR also g of p(AMPS)–Co (containing 8.62 mg gꢀ1 Co). In addition, the
increased. The fastest hydrolysis reaction occurred at pH 12, hydrolysis reaction of NaBH4 was investigated under relatively
ꢁ
and it reduced with decreasing pH values. In other words, the colder conditions e.g., 0 C. This process was carried out in a
H2 generation reaction of NaBH4 was completed in about water-ice bath, and the temperature was kept constant by
10 min at pH 12, whereas the same reaction was completed in constantly adding ice around the reaction ask. As illustrated in
about 13 min at pH 4. Furthermore, the HGR and TOF values Fig. 8(a), although the amount of hydrogen produced remained
of the p(AMPS)–Co composite systems were calculated for the same, 250 mL of H2, for all the reaction temperatures, the
different pHs and it was found that they both increased as the time taken to produce the same amount of H2 dramatically
pH increased as given in Table 2. As the NABH4 hydrolysis is decreased with an increase in temperature. The hydrolysis
generally performed in basic media these results comply with reaction of NaBH4 was completed in about 2 min at 70 ꢁC,
the literature.16,40–44
whereas it was completed in about 77 min at 0 ꢁC, as shown in
The hydrolysis reaction of NaBH4 was also performed in four the inset of Fig. 8(a). The highest HGR and TOF valuesꢁwere
different types of water: pure water, tap water, Sarı Çay (local obtained for the hydrolysis reaction that occurred at 70 C as
creek) water, and seawater, via the use of 0.1 g of p(AMPS)–Co 14 501.16 ꢂ 1 mL H2 per min per g of m and 34.25 ꢂ 1.2 mol H2
composites (containing 8.62 mg nanoparticles) at the per mol catalyst per min as given Table 2. As an increase in
prescribed reaction conditions. As can be seen from Fig. 7(b), temperature increases the reaction rates, an increase of HGR
the HGR of the p(AMPS)–Co composites were strongly affected and TOF values with an increase in the hydrolysis of NaBH4 was
by the type of reaction water source. The fastest hydrolysis expected.40–44 The increase in reaction temperature leads to an
reaction, or highest HGR, was obtained in pure water whereas increase in the reaction rates due to the increase in the number
the slowest hydrolysis reaction, or the lowest HGR, was effective collisions with the reactants and catalyst. Therefore,
obtained in seawater. As can be seen from the gure, the H2 the reaction rate of hydrogen generation increases with an
generation reaction of NaBH4 was completed within about 10 increase in the reaction temperature. The kinetic parameters of
min in pure water, while the H2 generation reaction of NaBH4 the hydrolysis reaction of NaBH4 were calculated by using the
was completed in about 25 min in seawater. The calculated HGR very-well known Arrhenius (eqn (4)) and Eyring (eqn (5))
Table 2 The change in TOF and HGR values of cryogel p(AMPS)–Co metal composites (8.62 mg of nanoparticles) depending on different
reaction conditionsa
Change in TOF and HGR values with temperature, pH, reuse and the types of reaction water
T (ꢁC) TOF
HGR
pH TOF
HGR
Reuse TOF
HGR
Water
TOF
HGR
0 ꢂ 2
0.89 ꢂ 0.5
376.65 ꢂ 1.2
4
6
8
5.47 ꢂ 0.8 2320.18 ꢂ 1.5 1st
5.70 ꢂ 1.2 2416.86 ꢂ 1.2 2nd
6.37 ꢂ 0.5 2697.91 ꢂ 0.5 DI
6.32 ꢂ 0.7 2677.96 ꢂ 1.1 Tap
6.37 ꢂ 0.4 2697.91 ꢂ 1
4.04 ꢂ 0.7 1706.01 ꢂ 1.2
40 ꢂ 1 11.74 ꢂ 0.8
50 ꢂ 1 17.12 ꢂ 1
60 ꢂ 1 24.90 ꢂ 1
4974.66 ꢂ 1
7250.58 ꢂ 1.5
10 546.29 ꢂ 1
6.08 ꢂ 0.5 2577.98 ꢂ 1
3rd
5.87 ꢂ 0.5 2487.33 ꢂ 1.6 Sarı Çay 3.26 ꢂ 0.3 1381.06 ꢂ 1.5
10 6.22 ꢂ 1
2636.57 ꢂ 0.8 4th
5.47 ꢂ 1
5.26 ꢂ 1
2320.18 ꢂ 1.5 Sea
2230.95 ꢂ 1.2
3.03 ꢂ 0.5 1288.99 ꢂ 1.3
70 ꢂ 1 34.25 ꢂ 1.2 14 501.16 ꢂ 1
12 6.63 ꢂ 0.6 2807.58 ꢂ 1
5th
a
TOF (Total Turnover Frequencies) [mol H2 per mol catalyst per min]. HGR (Hydrogen Generation Rate) [mL H2 per min per g of M].
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Fig. 8 (a) The effect of temperature on the hydrolysis of NaBH4, (b), ln k versus 1/T (Arrhenius eqn.), and (c), ln(k/T) versus 1/T (Eyring eqn.).
[Reaction conditions: 50 mL of 50 mM NaBH4, 5 wt% NaOH, 1000 rpm, 8.62 mg of Co nanoparticles in 0.1 g of cryogel p(AMPS)–Co composite].
equations associated with the graphs ln k versus 1/T and ln(k/T)
versus 1/T graphs.38,39
ln(k/T) ¼ ln(kB/h) + (DS#/R) ꢀ (DH#/R)(1/T)
(5)
Here, k is the reaction rate constant and was calculated
according to a zero-order kinetic expression, Ea is the activation
ln k ¼ ln A ꢀ (Ea/RT)
(4)
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energy, T is the absolute temperature, kB is the Boltzmann checked that there is no leakage or elution of metal nano-
constant (1.381 ꢄ 10ꢀ23 J Kꢀ1), h is Planck's constant (6.626 ꢄ particles from the cryo- and hydrogels networks.16,40–44 The
10ꢀ34 J sꢀ1), DH is the activation enthalpy, DS is the entropy and reduction in the catalytic performance of hydrogel–metal
R is the gas constant (8.314 J Kꢀ1 molꢀ1). The kinetic parameters composites with repetitive use has been previously reported in
such as energy, enthalpy and entropy of cryogel p(AMPS)–Co the literature. For example, the reusability of p(VPA), p(AAm-co-
composite systems were Ea ¼ 15.40 ꢂ 0.3 kJ molꢀ1, DH ¼ 12.85 VSA) and p(SPM) have been previously reported as being about
ꢂ 0.2 kJ molꢀ1 and DS ¼ ꢀ153.05 ꢂ 0.5 J molꢀ1 Kꢀ1, respec- 15, 55, and 72%, respectively.18,43,44 Therefore, it can be said that
tively. The activation energy values reported here for cryogel although there is a reduction in the activity of cryogel p(AMPS)–
p(AMPS)–Co composite systems is one of the lowest in Co composite, it can be used successively with 100% conversion
comparison to conventional hydrogels [16, 17 etc.]. This value for H2 production from NaBH4.
proves that cryogel p(AMPS)–Co composite systems can be very
useful for catalytic applications.
4. Conclusions
3.6. The reusability of p(AMPS)–Co macroporous composites
Due to the appealing properties such as the excellent chemical
stability, improved response rates and diffusion of solutes, the
super porous structure, and mechanical stability of cryogels,
p(AMPS) cryogels were prepared and used as templates for the
in situ metal nanoparticle preparation of Co and Ni and the
resultant composites were used as catalyst media. Here, we have
The reusability of the porous p(AMPS)–Co composite catalyst
systems was investigated by using them consecutively in the
same hydrolysis reaction. In brief, 0.1 g of cryogel p(AMPS)–Co
composite (containing 8.62 mg nanoparticles) was used for H2
generation from NaBH4 hydrolysis under the usual reaction
conditions (50 mL of 50 mM NaBH4, 5 wt% NaOH content, at
successfully demonstrated the use of macroporous cryogel
p(AMPS)–Co composites in energy and environmental applica-
tions via H2 generation from the hydrolysis of NaBH4, and the
reduction of toxic 4-NP to 4-AP, respectively.
The following conclusions and inferences can be deduced
from this research:
ꢅ p(AMPS)-based superporous cryogels and conventional
hydrogels were used for the metal nanoparticle preparation of
Co and Ni, as catalysis media for 4-NP reduction and for H2
generation from the hydrolysis of NaBH4. It was found that Co
nanoparticle-containing cryogel p(AMPS) provided better cata-
lytic performances than its Ni nanoparticle-containing coun-
terparts. The catalytic performances of Co-containing cryogel
and conventional p(AMPS) hydrogel were compared for H2
generation from the hydrolysis of NaBH4 and the cryogel
ꢁ
30 C and a mixing rate of 1000 rpm). Aer the rst use, the
p(AMPS)–Co metal composites were removed from the reaction
medium, washed with plenty of water, and again placed into a
similar fresh reaction medium under the same conditions and
these steps were carried out ve consecutive times. As illus-
trated in Fig. 9, there is no change in the conversion of NaBH4
for H2 generation but its activity was reduced from 100% to 82%
at the end of the 5th use. The activity is calculated based on
theinitial HGR to the HGRs of each use. The HGR and TOF
values for the p(AMPS)–Co composite systems decreased to
2230 mL H2 per min per g of M and 5.26 mol H2 per mol catalyst
per min from 2697 mL H2 per min per g of M and 6.37 mol H2
per mol catalyst per min, respectively, at the end of 5th use as
can be seen in Table 2. The mass loss of catalyst during the
catalytic reaction was not observed as the amounts of metal ions
were determined before and aer the hydrolysis reactions.
However, due to the formation of NaBO2 and its accumulation
on the catalytic composite system, the activity decreased as we
p(AMPS)–Co
composite
exhibited
superior
catalytic
performance.
ꢅ Kinetic parameters such as energy, enthalpy and entropy of
superporous cryogel p(AMPS)–Co composite systems for H2
generation from the hydrolysis of NaBH4 were calculated as Ea ¼
15.40 ꢂ 0.3 kJ molꢀ1, DH ¼ 12.85 ꢂ 0.2 kJ molꢀ1 and DS ¼
ꢀ153.05 ꢂ 0.5 J molꢀ1 Kꢀ1. This activation energy is one of the
lowest Ea reported in the literature from similar studies.
ꢅ The superporous cryogel p(AMPS)–Co composite can also
be used in various environmental applications such as nitro
compound reduction and elimination of contamination by
dyes, pesticides etc.
ꢅ Here, it can be summarized that cryogels with a super-
porous structure can be used in many applications due to their
added properties compared to smart or intelligent hydrogels.
Acknowledgements
Fig. 9 The activity and the conversion ability of the cryogel p(AMPS)–
Co composite for repetitive NaBH4 hydrolysis. [Reaction conditions:
50 mL of 50 mM NaBH4, 5 wt% NaOH, 30 ꢁC, 1000 rpm, 8.62 mg of Co
nanoparticles in 0.1 g of cryogel].
This work was supported by the Scientic and Technological
Research Council of Turkey (113T042) and (110T649).
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