Application of isoconversional calculation procedure to non-isothermal kinetic study: III. Thermal decomposition of ammonium cobalt phosphate hydrate

Thermochimica Acta 543 (2012) 205–210

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Application of isoconversional calculation procedure to non-isothermal kinetic study: III. Thermal decomposition of ammonium cobalt phosphate hydrate Zhipeng Chen, Qian Chai, Sen Liao ∗ , Yu He, Yu Li, Xiahong Bo, Wenwei Wu, Bin Li School of Chemistry and Chemical Engineering, Guangxi University, Nanning, Guangxi, 530004, PR China

a r t i c l e

i n f o

Article history: Received 24 February 2012 Received in revised form 8 May 2012 Accepted 20 May 2012 Available online 30 May 2012 Keywords: Ammonium cobalt phosphate hydrate Non-isothermal kinetics Thermal decomposition Thermogravimetric analysis Solid-state reaction

a b s t r a c t The single phase NH4 CoPO4 ·H2 O with layered structure was prepared via solid-state reaction at 60 ◦ C. Based on the iterative isoconversional calculation procedure, the values of activation energy Ea associated with the first, second and third stages of the thermal decomposition of NH4 CoPO4 ·H2 O were obtained, which demonstrate that the three stages are all a single-step kinetic process and can be adequately described by a unique kinetic triplet. The most probable reaction mechanisms of the three stages were estimated by comparison between experimental plots and modeled plots. The values of pre-exponential factor A of the three stages were obtained on the basis of Ea and the reaction mechanisms. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Cobalt(II) phosphates and pyrophosphates as a kind of important inorganic multifunctional materials have many industrial and commercial applications and have been used for catalyst, fertilizers, magnetic devices, fire retardants in paints and plastics, and pigments which have good anticorrosion properties and are suitable for coating due to their open framework and diversity of structure types. From a structural point of view, the redox properties of Co(II) sites enhance the catalytic properties of porous materials, and Co(II)–Co(II) magnetic interactions can give rise to interesting magnetic properties; on the other hand, cobalt(II) can adopt a tetrahedral coordination environment identical to that exhibited for silicon and aluminum in the formation of zeolites and create zeolite-like Co(II) frameworks [1–10]. For instance, ammonium cobalt phosphate, which belongs to the series of cobalt(II) phosphates of general formula MCoPO4 (M = Na+ , K+ , NH4 + , Rb+ ), consists of purely tetrahedral Co2+ in structures that is the same as or highly related to aluminosilicate tridymite and zeolite ABW, and shows important porosity characteristics and additional magnetic properties [1]. The preparation of ammonium cobalt phosphate hydrate (NH4 CoPO4 ·H2 O) was reported by various researchers. Carling et al. [7] synthesized NH4 CoPO4 ·H2 O by precipitation from aqueous

∗ Corresponding author. Tel.: +86 771 3233718; fax: +86 771 3233718. E-mail addresses: [email protected], [email protected], [email protected] (S. Liao). 0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2012.05.027

solution and determined the crystal structures of ND4 CoPO4 ·D2 O obtained from NH4 CoPO4 ·H2 O. All these compounds were said to crystallize in the orthorhombic space group Pmn21 . Bramnik et al. [11] also prepared NH4 CoPO4 ·H2 O as a precipitate by adding (NH4 )2 HPO4 solution to an alkaline solution of CoCl2 . And it is well known that cobalt pyrophosphate can be prepared by decomposition reaction of the NH4 CoPO4 ·H2 O precursor [12]. Kinetic analysis of thermal decomposition process is important and has received considerable attention all along the modern history of thermal decomposition study [13–15]. Kinetic analysis can have either a practical or theoretical application. A major practical application is the prediction of process rates and material lifetime. The theoretical application is the interpretation of experimentally determined kinetic triplets which can mathematically describe the process and determine its thermodynamic properties. However, no kinetic study of thermal decomposition of ammonium cobalt phosphate hydrate has been reported in literature. The aim of this work is, therefore, to study the kinetics of the thermal decomposition of pure nanocrystalline NH4 CoPO4 ·H2 O prepared using a novel synthetic technique. Kinetic data were collected using simultaneous TG/DTA technique. Non-isothermal kinetics of the decomposition process was analyzed using a new modified method [16–20]. The values of Ea were obtained from an iterative procedure. The most probable mechanism function g(˛) of the thermal decomposition reaction was deduced by comparison between experimental plots and modeled plots. The pre-exponential factor A was calculated using Ea and g(˛). The kinetic (A, Ea and g(˛)) parameters of the thermal decomposition were discussed for the first time.

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2. Experiment 2.1. Preparation of ammonium cobalt phosphate hydrate and characterization The ammonium cobalt phosphate hydrate (NH4 CoPO4 ·H2 O) was prepared via solid-state reaction at low-heating temperature. This preparation technique has been developed in recent decades and is of simplicity, low cost, high output and little pollution [21–24]. Co(Ac)2 ·4H2 O was used as the source of cobalt (II) and (NH4 )3 PO4 ·3H2 O was used as the sources of phosphorus and ammonium. The synthesis procedure was as follows: (NH4 )3 PO4 ·3H2 O (30.0 mmol, 6.09 g), Co(Ac)2 ·4H2 O (20.0 mmol, 4.98 g), and surfactant polyethylene glycol-400 (PEG-400) (0.5 mL) were put in a mortar, and the mixture was fully ground for 30 min. The reaction mixture was sealed with preservative film and kept at room temperature for 3 h and then kept at 60 ◦ C for 3 h, washed with deionized water to remove soluble inorganic salts subsequently. The solid was then washed with a small amount of anhydrous ethanol and dried at 80 ◦ C for 3 h to give the single phase NH4 CoPO4 ·H2 O. Thermogravimetry (TG) and differential thermal analysis (DTA) measurements were made using a Netsch 40PC thermogravimetric analyzer. High purity nitrogen gas (99.999%) was used as protective atmosphere, flowing at 20 mL min−1 . The samples were loaded without pressing into an alumina crucible. (The results presented in this paper were calculated by the programs compiled by ourselves). X-ray powder diffraction (XRD) was performed at a scanning rate of 5◦ min−1 from 5◦ to 70◦ for 2 at room temperature using a Rigaku D/max 2500 V diffractometer equipped with a graphite monochromator and a Cu target. Fourier transform infrared (FT-IR) spectra was recorded on a Nicolet IS10 spectrometer in the wavenumbers range of 4000–500 cm−1 for samples made in KBr pellet form. The morphology of the product and its calcined residues were examined by S-3400 scanning electron microscopy (SEM) on samples mounted on an alumina slice and coated with Au. 3. Theoretical According to non-isothermal kinetic theory, kinetic equation of thermal analysis [13] can be expressed as follow:

 E  a

A d˛ = exp − RT dT ˇ

f (˛)

(1)

where ˛ is the extent of conversion, ˇ is the heating rate (K min−1 ), Ea is the apparent activation energy, A is the pre-exponential factor, R is the gas constant (8.314 J mol−1 K−1 ).

a new value of Ea2 for the activation energy from the plot of ln[ˇ/(h(x)T2 )] vs. 1/T; (iii) repeat step (ii), replacing Ea1 with Ea2 . When |Eai − Ea(i–1) | < 0.01 kJ mol−1 , the last value of Eai is considered to be the exact value of the activation energy of the reaction. These plots are model independent since the estimation of the activation energy does not require selection of particular kinetic model (type of g(˛) function). Therefore, the activation energy values obtained by this method are usually regarded as more reliable than those obtained by the Coats–Redfern method. 3.2. Determination of the most probable reaction models [13] 3.2.1. Determination parameters of compensation effect The compensation effect can be observed when a model-fitting method, such as the Tang equation (Eq. (4)) [26], is applied to a single heating rate run.

 ln

T˛ 1.894661

ln

AR Ea ˇ = ln − RT g(˛)Ea h(x)T 2

where h(x) is expressed by the fourth Senum and Yang approximation formulae [25]: h(x) =

x4 + 18x3 + 88x2 + 96x x4 + 20x3 + 120x2 + 240x + 120

(3)

where x = Ea /(RT). The iterative procedure is performed including the following steps: (i) assume h(x) = 1 to estimate the initial value of the activation energy Ea1 . The conventional isoconversional methods stop the calculation at this step; (ii) using Ea1 , calculate

ˇR

+ 3.63504095 − 1.894661 ln Ei

Ei RT˛



(4)

The different pairs of the Arrhenius parameters, ln Ai and Ei , which are yielded by substitution of different models gi (˛) [27] into the Tang equation and fitting it to experimental data, all demonstrate a strong correlation known as a compensation effect [13]: ln Ai = aEi + b

(5)

Different models gi (˛) [27] combined with the temperature T corresponding to conversions ˛ are put into Eq. (4), the slope (k) and intercept (B) are obtained from the plot of ln[gi (˛)/(T1.894661 )] vs. (1/T). First, the Ei and ln Ai can be estimated from the slope (k) and intercept (B). Then, the parameters of compensation effect (a and b) can be calculated from Eq. (5) with the Ei and ln Ai . 3.2.2. Determination ln A0 Eq. (6) is used to estimate ln A0 : ln A0 = aE0 + b

(6)

where a and b are obtained from Eq. (5), E0 is average value of the apparent activation energy Ea calculated from Eq. (2). 3.2.3. Determination of reaction models For constant heating rate conditions, integration of Eq. (1) leads to Eq. (7) [13].



(2)

 A E  i i

= ln

− 1.00145033

3.1. Calculation of the values of Ea by the iterative procedure The iterative procedure [16–19] is used to calculate the approximate value of Ea approaching to the exact value, the equations are expressed:



gi (˛)

g(˛) ≡ 0

˛

d˛ A = f (˛) ˇ

T exp

 −E  RT

dT

(7)

0

The temperature integral in Eq. (7) can be replaced with various approximations, h(x) as follows [19,25]: g(˛) =

AE h(x) exp(−x) 2 ˇR x

(8)

where A = A0 , E = E0 . A, E, ˇ and the temperature T corresponding to conversions ˛ are put into the right hand side of Eq. (8). And the experimental plot of g(˛) vs. ˛ is obtained. The analytical form of the reaction model (i.e., equation) can then be established by comparison between the experimental plot and the theoretical plots obtained from the g(˛) equations representing the reaction models [27,28] and finding the best matching theoretical plot.

Z. Chen et al. / Thermochimica Acta 543 (2012) 205–210

3.3. Calculation of pre-exponential factor A

2000

2 f  (˛ RTmax max )

exp

 E  0 RTmax

(9)

where g(˛)f (˛) = −1. In Eq. (9), the subscript max denotes the values related to the maximum of the differential kinetic curve obtained at a given heating rate.

1500

Intensity/a.u.

−ˇE0

NH4CoPO4 H2O

a

353 K

The pre-exponential factor A can be estimated from the following equation [13,29]: A=

207

1000

500

4. Results and discussion 4.1. Characterization results

0 10

20

30

40

50

60

70

o

2

NH4CoPO4 Co2P2O7

b

698 K

Intensity / a.u.

568 K

523 K

468 K 10

20

30

40

2

50

60

70

o

1000

Co2P2O7

c

973 K 800

Intensity/a.u.

Fig. 1 shows the XRD patterns of the product dried at 353 K and the samples resulted from calcination at different temperatures for 2 h. The strong intensity, smooth baseline and a diffraction pattern characteristic of the product (Fig. 1a) indicate that the product is well crystallized. All the diffraction peaks in the pattern are in agreement with that of orthorhombic NH4 CoPO4 ·H2 O, with space group Pmn21(31) and cell parameters: a = 0.555 nm, b = 0.885 nm, and c = 0.481 nm, and ˛ = ˇ =  = 90◦ (PDF card 74-2092). No diffraction peaks of impurities, such as Co3 (PO4 )2 ·4H2 O and NH4 CoPO4 , are observed, which indicates that the single phase NH4 CoPO4 ·H2 O is successfully synthesized by solid-state reaction at low-heating temperature. The strong diffraction peak at 10◦ for 2 is attributed to the layered structure of NH4 CoPO4 ·H2 O. Fig. 1b shows that all the diffraction peaks of the products calcined at 468 K and 523 K, respectively, can be indexed to that of NH4 CoPO4 (PDF card 220042), which is also of a layered structure. When the product was kept at 568 K for 2 h, the characteristic diffraction peaks of crystalline NH4 CoPO4 almost disappeared, which indicated that structure of the crystalline NH4 CoPO4 was destroyed. After calcination at 698 K for 2 h, all the peaks of NH4 CoPO4 disappeared. The material became poorly crystallized with some new but weak peaks belonging to Co2 P2 O7 emerged. When the sample was heated at 973 K for 2 h (Fig. 1c), a diffraction pattern of well-crystallized monoclinic Co2 P2 O7 was observed (space group P21/c(14), PDF card 79-0825), which is in sharp contrast to the pattern of the residue treated at 698 K. Fig. 2 shows the TG/DTA curves of the synthetic product at four different heating rates, respectively. As can be seen from Fig. 2a that the mass loss starts at about 380 K, ends at about 820 K. The observed mass loss in the TG curve (ˇ = 15 K min−1 ) is 23.50%, which is in good agreement with the theoretic 23.19% mass loss of the thermal decomposition of NH4 CoPO4 ·H2 O. It can be seen from the TG curves that the thermal decomposition of the synthetic product below 820 K occurs in three stages. There are two inflection points at about 525, 630 K in the TG curve (ˇ = 15 K min−1 ), respectively, which the two points can be used as end points of the first and second stages, respectively. Therefore, The first stage starts at about 380 K, ends at about 525 K, and is characterized by a strong endothermic DTA peak at about 485 K, which can be mainly attributed to the dehydration of NH4 CoPO4 ·H2 O and the formation of anhydrous NH4 CoPO4 . The second decomposition stage begins at about 525 K, and ends at about 630 K, which involves an endothermic process with the DTA peak at about 600 K, and can be attributed to the deamination of NH4 CoPO4 to form CoHPO4 . The third stage begins at about 630 K, and ends at about 820 K, which can be attributed to the condensation of CoHPO4 to give amorphous Co2 P2 O7 . The exothermic DTA peak at about 880 K can be ascribed to phase transition from amorphous Co2 P2 O7 to monoclinic phase Co2 P2 O7 . FT-IR spectra of the prepared and calcined samples are shown in Fig. 3. From Fig. 3a, the strong bands in the region of 1100–930 cm−1

600

400

200

0 10

20

30

40

2

50

60

70

o

Fig. 1. XRD patterns of the NH4 CoPO4 ·H2 O (a) and its calcined samples (b, c) at different temperatures for 2 h.

are attributed to the P–O stretching vibrations. The bands which appear at 559 and 622 cm−1 in the spectrum of the prepared samples can be ascribed to the OPO vibrations. The weak band observed at about 1666 cm−1 is assigned to the bending mode of the HOH of the water of crystallization, while the band at 3405 cm−1 is assigned to the stretching OH vibration of the water molecule. The band in the region of 3215–2769 cm−1 is due to the stretching vibration of NH4 + , while the bands at 1429 and 1465 cm−1 can be the bending

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Z. Chen et al. / Thermochimica Acta 543 (2012) 205–210

100

1.2

a -1

1.0

5 K min

-1

10 K min

15 K min -1 20 K min

0.6 0.4

90

0.2

b

0.0

5 K min

-1

-0.2

10 K min

-1

15 K min 20 K min

endo

85

80

DTA/ V/mg

TG/%

0.8

-1

95

-0.4

-1

-1

-0.6 -0.8

75 300

450

600

750

900

Temperature/K Fig. 2. TG/DTA curves of the NH4 CoPO4 ·H2 O at different heating rates: (a) TG, (b) DTA.

mode of NH4 + . The bands at 3405 and 1666 cm−1 disappear after calcining at 468 K (Fig. 3b), which can be explained by the dehydration of NH4 CoPO4 ·H2 O. Comparison of Fig. 3b and c indicates that the FT-IR spectra of the sample calcined at 468 K is almost the same as that of the sample calcined at 523 K, which is in good agreement with the XRD analysis. The intensity of bands at the region of 3215–2769 and 1470–1430 cm−1 disappear at 698 K, which indicates that NH4 CoPO4 ·H2 O eliminates its crystal water and ammonia at 698 K. The SEM micrographs of NH4 CoPO4 ·H2 O and its calcined product at 973 K are shown in Fig. 4. From Fig. 4a, it can be seen that the NH4 CoPO4 ·H2 O sample is composed of platelets. There is soft agglomeration phenomenon among the particles of NH4 CoPO4 ·H2 O, which is attributed to the strong absorption to each other existing among particles with layered structure. Fig. 4b shows that the calcination has changed the morphology of the decomposed product and the shape of the particles has become irregular.

Fig. 4. SEM image of the NH4 CoPO4 ·H2 O (a) and its calcined product at 973 K (b).

NH4 CoPO4 ·H2 O below 973 K can be suggested as following four stages:

4.2. Results of thermal decomposition kinetics The results from TG/DTA, XRD and FT-IR analyses of the product and its calcined products suggest that the thermal process of

NH4 CoPO4 ·H2 O(cr) → NH4 CoPO4 (cr) + H2 O(g) NH4 CoPO4 (cr) → CoHPO4 (am) + NH3 (g) CoHPO4 (am) → 0.5Co2 P2 O7 (am) + 0.5H2 O(g)

e

Transmittance / %

Co2 P2 O7 (am) → Co2 P2 O7 (cr)

d

The kinetics of the first three stages are analyzed as follows.

c

4.2.1. Calculation of Ea by the iterative procedure The kinetic data from the thermal decomposition were processed for the calculation of Ea using the isoconversional iterative method (Eq. (2)). Therefore, the values of Ea of the three stages of thermal decomposition of NH4 CoPO4 ·H2 O corresponding to different conversions ˛ (˛ = 0.01–0.999) are obtained, and shown in Fig. 5. If Ea values are independent of ˛, the decomposition process is dominated by a single reaction step [30,31]; on the contrary, a significant variation of Ea with ˛ should be interpreted in terms of multi-step mechanism [20,30,32]. It can be considered that the Ea values are independent of ˛ if the difference between the maximum and minimum values of Ea is less than 20% of the average Ea [13,33]. So, from Fig. 5, it is obvious that thermal decomposition of the three stages (˛ = 0.18–0.82 for the first stage, ˛ = 0.18–0.90 for the second

b a

4000

3500

3000

2500

2000

Wavenumbers /

1500

1000

cm-1

Fig. 3. FT-IR spectra of the NH4 CoPO4 ·H2 O (a) and the NH4 CoPO4 ·H2 O calcined at 468 K (b), 523 K (c), 698 K (d) and 973 K (e).

Z. Chen et al. / Thermochimica Acta 543 (2012) 205–210

260

(a)

209

1.0

240 220

0.8

The third stage

0.6

180

The second stage

g( )

Ea / kJ mol

-1

200

0.4

160 0.2

140 120

The first stage 0.0

100 0.1

80 0.0

0.2

0.4

0.6

0.8

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.8

0.9

1.0

Fig. 5. Dependence of Ea on ˛ obtained by Eq. (2) for the three stages of the thermal decomposition of NH4 CoPO4 ·H2 O.

(b)

2.2

2.2

n= -0.42 n= -0.43 n= -0.44 1.8 n= -0.46 O n= -0.48 1.6 + n= -0.5 2.0 *

2.0 1.8

1.4

g( )

1.6

g( )

stage, ˛ = 0.10–0.90 for the third stage) could be all considered as single-step reaction mechanism and can be adequately described by a unique kinetic triplet (activation energy, reaction mechanism and pre-exponential factor). And the average values of Ea of the first, second and third stage were determined to be 137.3, 199.3 and 212.5 kJ mol−1 , respectively.

1.2

1.2

1.0

1.0

0.8

0.8

0.75

0.80

0.85

0.90

0.6 0.4 0.2 0.0 0.2

(c) 100

100

90

90

80

80

70

70

60

60

g( )

0.3

*

g( )

4.2.2. Determination of the most probable reaction mechanism function Accurate determination of the reaction model can be accomplished [13] by using the aforementioned compensation effect process where Eq. (8) is used to generate experimental plots of g(˛) vs. ˛ for ˇ = 20 K min−1 of the three stages. And the resulting experimental plots do not demonstrate any significant variation with the other three heating rates. Then, the analytical forms of the reaction model (i.e., equation) for the three stages are established by comparing the above experimental plots with the theoretical plots obtained from 36 g(˛) equations [27,28] representing the reaction models, and shown in Fig. 6. From the results of the comparisons (Fig. 6), the most probable reaction mechanism functions for the three stages are determined roughly, and the best matching mechanism functions are Nos. 27, 3 and 5, respectively. The integral forms of reaction mechanism functions of the three stages are g(˛) = ˛2 for the first stage (No. 27), g(˛) = (1 − ˛)−1/2 − 1 for the second stage (No. 3) and g(˛) = (1 − ˛)−2 − 1 for the third stage (No. 5). The three equations can be reformatted as g(˛) = ˛n (No. 27), g(˛) = (1 − ˛)n − 1 (No. 3) and g(˛) = (1 − ˛)n − 1 (No. 5). As can be seen from Fig. 6a, the experimental curve and theoretical curve (No. 27) match well enough for the first stage and indicate that the n value of the equation is not needed to further correcting. However, the experimental curves and theoretical curves do not match so well enough for the second stage (n = −0.5) and third stage (n = −2). So, n values of the equations for the two stages are needed to further research and correcting. The theoretical curves for the second and third stages with different values n are also illustrated in Fig. 6b and c, respectively. From the results of the further comparisons, accurate n values of the most probable reaction mechanism functions for the two stages are: n = −0.43 for the second stage, and n = −1.88 for the third stage. So, the exact integral forms of reaction mechanism functions of the three stages of thermal decomposition of NH4 CoPO4 ·H2 O are as follow: (i) g(˛) = ˛2 for the first stage, (ii) g(˛) = (1 − ˛)−0.43 − 1 for the second stage, (iii) g(˛) = (1 − ˛)−1.88 − 1 for the third stage, which belong to (i) the mechanism of

1.4

50

+

0.4

0.5

0.6

0.7

n= -1.84 n= -1.88 n= -1.92 n= -2

50 40

40 30

30 20

20

0.78

0.80

0.82

0.84

0.86

0.88

0.90

10 0 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Fig. 6. Comparison between model results (dash line or different symbols) and experimental results for ˇ = 20 K min−1 (solid line) of the thermal decomposition of NH4 CoPO4 ·H2 O: (a) the first stage, (b) the second stage, (c) the third stage.

one-dimensional diffusion, (ii) the mechanism of chemical reaction and (iii) the mechanism of chemical reaction, respectively. 4.2.3. Calculation of pre-exponential factor A The pre-exponential factor A for the three stages of the thermal decomposition of NH4 CoPO4 ·H2 O were estimated with Eq.

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(9). Because there was no DTG peak of the third stage, so the temperature of ˛ = 0.8 of the third stage was put into Eq. (9) to calculate the corresponding pre-exponential factor A. The results show that the pre-exponential factor A for the three stages is 1.81 × 1012 s−1 for the first stage, 1.89 × 1015 s−1 for the second stage and 1.16 × 1014 s−1 for the third stage, respectively. 5. Conclusions The single phase NH4 CoPO4 ·H2 O with layered structure was successfully prepared using the novel solid-state reaction. The thermal process of NH4 CoPO4 ·H2 O involves dehydration, deamination, polycondensation and crystallization. The activation energies calculated for the first three stages indicate that the three stages are, respectively, a single-step kinetic process and can be adequately described by a unique kinetic triplet (Ea , A, g(˛)). The most probable mechanism for the first stage is one-dimensional diffusion, and for the second and third stages are all chemical reaction. Acknowledgements This study was financially supported by the National Natural Science Foundation of China (No. 21161002), the Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, Ministry of Education, Guangxi University (No. GXKFZ-02), the Guangxi Natural Scientific Foundation of China (Grant Nos. 0991108 and 0832111), and the Guangxi Science and Technology Agency Research Item of China (Grant No. 0895002–9). References [1] P.Y. Feng, X.H. Bu, S.H. Tolbert, G.D. Stucky, Syntheses and characterizations of chiral tetrahedral cobalt phosphates with zeolite ABW and related frameworks, J. Am. Chem. Soc. 119 (1997) 2497–2504. [2] D. Maspoch, D. Ruiz-Molina, J. Veciana, Old materials with new tricks: multifunctional open-framework materials, Chem. Soc. Rev. 36 (2007) 770–818. [3] T.F. Laura, T. Camino, R.G. José, G.G. Santiago, Synthesis and structural studies of ammonium-cobalt-nickel phosphates, NH4 [Co1 − x Nix PO4 ]·H2 O, Acta Cryst. A 66 (2010) 193–194. [4] J. Chen, S. Natarajan, J.M. Thomas, R.H. Jones, M.B. Hursthouse, A novel openframework cobalt phosphate containing a tetrahedrally coordinated cobalt(II) center: CoPO4 ·0.5C2 H10 N2 , Angew. Chem. Int. Ed. 33 (1994) 639–640. [5] S.S. Lin, H.S. Weng, Liquid-phase oxidation of cyclohexane using CoAPO-5 as the catalyst, Appl. Catal. A 105 (1993) 289–308. [6] D.A. Bruce, A.P. Wilkinson, M.G. White, J.A. Bertrand, The synthesis and characterization of an aluminophosphate with chiral layers; transCo(dien)2 ·Al3 P4 O16 ·3H2 O, J. Solid State Chem. 125 (1996) 228–233. [7] S.G. Carling, P. Day, D. Visser, Crystal and magnetic structures of layer transition metal phosphate hydrates, Inorg. Chem. 34 (1995) 3917–3927. [8] H. Onoda, K. Yokouchi, K. Kojima, H. Nariai, Addition of rare earth cation on formation and properties of various cobalt phosphates, Mater. Sci. Eng. B 116 (2005) 189–195. [9] O.F. Ikotun, W. Ouellette, F. Lloret, P.E. Kruger, M. Julve, R.P. Doyle, Synthesis, structural, thermal and magnetic characterization of a pyrophosphato-bridged cobalt(II) complex, Eur. J. Inorg. Chem. 2008 (2008) 2691–2697. [10] H. Wen, M.H. Cao, G.B. Sun, W.G. Xu, D. Wang, X.Q. Zhang, C.W. Hu, Hierarchical three-dimensional cobalt phosphate microarchitectures: largescale solvothermal synthesis, characterization, and magnetic and microwave absorption properties, J. Phys. Chem. C 112 (2008) 15948–15955.

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