Microporous and Mesoporous Materials 153 (2012) 115–123

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Microporous and Mesoporous Materials

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Synthesis of hureaulite by a reflux process at ambient temperature and pressure

Hui Yin, Fan Liu, Xiuhua Chen, Xionghan Feng, Wenfeng Tan, Guohong Qiu ⇑College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 8 June 2011Received in revised form 25 November 2011Accepted 30 November 2011Available online 8 December 2011

Keywords:Mineral materialsHureauliteRefluxX-ray diffractionRietveld refinement

1387-1811/$ - see front matter � 2011 Elsevier Inc. Adoi:10.1016/j.micromeso.2011.11.057

⇑ Corresponding author. Tel./fax: +86 (0)27 872802E-mail address: qiugh@mail.hzau.edu.cn (G. Qiu).

Hureaulite was first successfully synthesized by reflux reactions of Mn(H2PO4)2 and MnCl2/H3PO4/NaClOsolutions at 40–100 �C under atmospheric pressure. Powder X-ray diffraction (XRD), Rietveld structurerefinement, Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and scan-ning electron microscopy (SEM) were utilized to characterize the crystal structures and morphologies ofthe products. Cell parameters of the as-prepared hureaulites were compared with those formed byhydrothermal reaction. Hureaulites obtained from different reaction systems exhibited various morphol-ogies. Rod-like hureaulite was synthesized from a mixture of 30 mmol H3PO4 and 100 mL of 0.3 mol L�1

Mn(H2PO4)2 solution with or without NaClO solution to the system at 100 �C. However, hureaulite couldonly be formed from the MnCl2 and H3PO4 solutions by the addition of NaClO solution. The formationmechanism of hureaulite was investigated by refluxing 100 mL of 0.3 mol L�1 Mn(H2PO4)2 solution at40 �C. A mixture of hureaulite and MnHPO4�3H2O was formed, and the former was approximately59.24% and 86.11% by mass as the reflux reaction lasted for 12 and 24 h, respectively. Single-phasehureaulite was formed when the reaction was prolonged up to 48 h at 40 �C. Results of Rietveld structurerefinement showed that the lattice parameters were compressed in the c direction and b decreased, andthe crystal density increased as the reflux temperature was increased. The present work facilitates a thor-ough knowledge of the natural genesis of hureaulite, and the preparation and applications of hureaulitein the field of materials science and chemistry.

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1. Introduction

Hureaulite, Mn5(PO3(OH))2(PO4)2�4H2O, is one of the commonspecies in the family of hydrated first-transition-series phosphates.As microporous materials, the hureaulite structure consists of anopen three-dimensional network formed by octahedral pentamericentities that share vertices with the [PO4] and [PO3(OH)] tetrahedra.The crystal structure of hureaulite is illustrated in Fig. 1 based on itsspace group, cell volume and cell parameters as reported in Ref. [1].The unique structure of [PO3(OH)], which rarely occurs in other me-tal compounds, is similar to [PO4], except that an O2� in [PO4] is re-placed by OH� [2,3]. The phosphate framework can stabilize themetal ions with different oxidation states due to the relatively highcharge in the phosphate tetrahedra, which facilitates the formationof anionic frameworks with a high degree of mechanical, chemical,and thermal stabilities [4–6]. In addition to some current and poten-tial applications in the field of magnetics, optics, and electrochemis-try, hureaulite is widely used as an additive to obtain highlycorrosion-resistant surfaces in order to improve the wear resistanceof steels [3,7–9]. A dense, finely-structured phosphate layer firmlyattaches to the substrate when hureaulite is used to form

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71.

corrosion-resistant surfaces [8,10,11]. Recently, the synthesis andphysicochemical properties of hureaulite have garnered widespreadattention in mineralogy and materials science [5,12–14].

Hureaulite has been reported to be formed from the hydrother-mally altered triphylite–lithiophilite crystals in granite pegmatitesor from the hydrothermal reaction of Mn-rich phosphatic solutions[2]. Hydrothermal reactions and techniques have been widely usedto obtain single and doped hureaulite based on its formation pro-cess in nature [15]. Hureaulite was fabricated by hydrothermaltreatment of a (NH4)2[Mn3(P2O7)2(H2O)2] solution in an autoclaveat 160 �C for 7 d [16] and Fe(II)-doped hureaulite was preparedwhen the temperature was controlled above 95 �C [17]. Cobalt-containing hureaulite was formed when MnCl2, CoCl2 and H3PO4

solutions were mixed in an autoclave at 170 �C for 5 d as the pHwas adjusted to 4 by triethylamine [5]. These reactions are per-formed under high temperature and pressure or with some harshconditions, which are unfavorable in terms of energy consumptionand operating costs.

Refluxing reactions under atmospheric pressure work at rela-tively low temperatures with simpler operations, higher productiv-ity, lower cost, and can be easily controlled whenever necessary.Some hydrothermal reactions under high temperature and highpressure can also work in a refluxing process at atmosphericpressure. For example, serrabrancaite (MnPO4�H2O) has been

[MnO6]

[PO4]

O

H

Fig. 1. Schematic illustration of the crystal structure of hureaulite [1].

116 H. Yin et al. / Microporous and Mesoporous Materials 153 (2012) 115–123

synthesized under high temperature and pressure; however, thisphase was also prepared by a reflux reaction at atmospheric pres-sure [18]. The reflux process provides a more convenient methodto investigate the occurrence and transformation of transition-me-tal phosphate materials. In our previous work, hureaulite wasformed as a byproduct when serrabrancaite was prepared by a re-flux process [18]. These results suggest that a reflux method mightoffer a mild synthetic route to prepare hureaulite. However, the for-mation mechanism of hureaulite and key influencing factors arestill unclear. The synthesis of hureaulite simulating the formationprocess at near room temperature has not been reported. Althoughstructure refinements of natural hureaulite and its analogs haveshown different crystal characteristics [3], the effect of reactionconditions on the lattice parameters are not definite.

Here, hureaulite was prepared by reflux processes under atmo-spheric pressure. Various reaction systems were selected at differenttemperatures such as 40, 50, 60, 80, and 100 �C. The as-obtainedsamples were characterized by powder X-ray diffraction (XRD),Fourier-transform infrared spectroscopy (FTIR), scanning electronmicroscopy (SEM), and thermogravimetric analysis (TGA), and thecrystal structures and lattice parameters were analyzed by Rietveldrefinement. The transformation mechanism of hureaulite at nearroom temperature was also investigated. These results aid in theunderstanding of the formation process of hureaulite in a naturalenvironment and provide a feasible synthetic method for prepara-tion of hureaulite on a large scale, which facilitates applications inthe fields of materials science, chemistry, and mineralogy.

2. Experimental

2.1. Materials and reagents

Mn(H2PO4)2�2H2O (AR), MnCl2�4H2O (AR), H3PO4 solution (AR,mass concentration P85%), and NaClO solution (AR, available chlo-rine content P12%) were all purchased from China Reagent(Group), Shanghai Chemical Reagent Corporation. A constant-tem-perature oil bath equipped with a magnetic stirrer was supplied byGongyi City Yuhua Instrument Co. Ltd., China.

2.2. Synthesis of hureaulite

First, 30 mmol Mn(H2PO4)2�2H2O was weighed and dissolvedinto 100 mL of distilled water in a 500 mL triangular beaker con-nected to a condenser. Then, 30 mmol H3PO4 solution was added,and the mixture was heated and maintained at a constant refluxtemperature (30, 40, 50, 60, 80, and 100 �C) with continuous stir-ring. A certain amount of NaClO solution (0, 60 mmol, availablechlorine content P12%) was then added dropwise into the triangu-lar beaker at a constant rate. After refluxing for a given time at afixed temperature, the suspension was cooled to room tempera-ture. The precipitate was washed with repeated filtration until

the conductivity of the filtrate was less than 10.0 lS cm�1. Theproducts were then dried in an oven at 60 �C for 12 h.

Hundred milliliter of 0.3 mol L�1 MnCl2�4H2O solution was usedas a divalent manganese source and mixed with a certain amountof phosphoric acid. Then, 60 mmol of NaClO solution was added tothe system. The subsequent steps were the same as those forMn(H2PO4)2�2H2O.

Hydrothermal reaction was conducted to prepare hureaulite.15 mmol Mn(H2PO4)2�2H2O was dissolved to 50 mL of distilledwater, and then was transferred into a Teflon-lined stainless steelpressure vessel, sealed, and maintained at 170 �C for 12 h. Thereaction mixture was then cooled to room temperature andwashed by repeated filtration until the conductivity of the super-natant was less than 10.0 lS cm�1. Then the products were thendried in an oven at 60 �C for 12 h.

In order to simplify the description of experimental conditionsand procedures, the products obtained from different conditionswere summarized as Table 1.

2.3. Characterization

The crystal structures of the as-prepared samples were charac-terized by powder X-ray diffraction performed on a Bruker D8 Ad-vance diffractometer in a Bragg–Brentano geometry equipped witha LynxEye detector using Ni-filtered Cu Ka radiation(k = 0.15418 nm). The diffractometer was operated at a tube volt-age of 40 kV and a tube current of 40 mA with a scanning rate of10�/min at a step size of 0.02�. For the collection of high-resolutiondigital patterns for Rietveld structure refinements of hureaulite,primary soller slits, secondary soller slits, and divergence slits wereset as 4�, 4�, and 1 mm, respectively. A scan rate of 1�/min wasadopted to attain adequate diffraction intensity. Rietveld structurerefinement and quantitative analysis were performed using theprogram TOPAS. Fourier-transform infrared spectra of the productswere collected using a Nicolet Avatar 300 on powder samples dis-persed in KBr pellets in the range of 4000–400 cm�1 at a resolutionof 4 cm�1 for 32 scans. The morphologies of the products werecharacterized by scanning electron microscopy (SEM, JSM-6390LV). The thermal behavior of the obtained materials wasexamined by performing thermogravimetric analysis (TGA) on aNETZSCH TG 209 thermal analysis system by using about 10 mgpowder sample in a dynamic nitrogen atmosphere at a flow rateof 20 mL min�1 and a heating rate of 10 �C min�1.

3. Results and discussion

3.1. Synthesis of hureaulite with Mn(H2PO4)2�2H2O

Fig. 2a and b show the powder XRD patterns of the products ob-tained by refluxing Mn(H2PO4)2�2H2O solutions at 50 and 100 �Cfor 12 h, respectively. All patterns of the products obtained havereflections at the same positions, and well matched those ofhureaulite (JCPDS Card No. 86-1521), indicating that pure-phasehureaulite was synthesized. Hureaulite was formed during the re-flux process mainly based on the chemical equation as follows:

5MnðH2PO4Þ2ðaqÞ þ 4H2OðlÞ¢ Mn5ðPO3ðOHÞÞ2ðPO4Þ2 � 4H2OðsÞ þ 6H3PO4ðaqÞ ð1Þ

As reported, hureaulite could be formed by hydrothermal reac-tion at 170 �C, however, the XRD patterns were not given [5]. Inthis work, 50 mL of 0.3 mol L�1 Mn(H2PO4)2 solution was kept at170 �C for 12 h, and the powder XRD patterns of the as-obtainedproduct were shown in Fig. 2c. The XRD peak intensity remarkablyincreased, suggesting an increase in the degree of crystallinity withan increase in reaction temperature. The positions of reflections

Table 1Reaction conditions and product names.

Reaction conditions Product name

100 mL of 0.3 mol L�1 Mn(H2PO4)2, 50 �C, 12 h Sample 1100 mL of 0.3 mol L�1 Mn(H2PO4)2, 60 �C, 12 h Sample 2100 mL of 0.3 mol L�1 Mn(H2PO4)2, 80 �C, 12 h Sample 3100 mL of 0.3 mol L�1 Mn(H2PO4)2, 100 �C, 12 h Sample 450 mL of 0.3 mol L�1 Mn(H2PO4)2, 170 �C, 12 h Sample 5100 mL of 0.3 mol L�1 Mn(H2PO4)2, 30 mmol H3PO4 solution, 100 �C, 12 h Sample 6100 mL of 0.3 mol L�1 Mn(H2PO4)2, 60 mmol NaClO solution, 100 �C, 12 h Sample 7100 mL of 0.3 mol L�1 Mn(H2PO4)2, 30 mmol H3PO4 solution, 60 mmol NaClO solution, 100 �C, 12 h Sample 8100 mL of 0.3 mol L�1 MnCl2, 30 mmol H3PO4 solution, 60 mmol NaClO solution, 100 �C, 12 h Sample 9100 mL of 0.3 mol L�1 Mn(H2PO4)2, 40 �C, 12 h Sample 10100 mL of 0.3 mol L�1 Mn(H2PO4)2, 40 �C, 24 h Sample 11100 mL of 0.3 mol L�1 Mn(H2PO4)2, 40 �C, 48 h Sample 12

Note: Sample 5 was obtained by hydrothermal reaction, samples 1–4 and 6–12 were obtained by reflux at atmospheric pressure.

Fig. 2. XRD patterns of typical products: (a) sample 1, (b) sample 4, (c) sample 5, (d) sample 6, (e) sample 7, (f) sample 8, and (g) sample 9.

H. Yin et al. / Microporous and Mesoporous Materials 153 (2012) 115–123 117

matched well with those of standard hureaulite (JCPDS Card No.86-1521). However, the relative intensity of diffraction peaks ofthis product was different from that of others synthesized at lowertemperatures, such as Fig. 2a and b. These intensities of peaks at8.05 Å, 4.03 Å, 2.69 Å, and 2.02 Å, corresponding to (110), (220),(330), and (440) reflections, respectively, were very strong(Fig. 2c, Fig. S1), which suggested that high temperature and highpressure facilitated the growth of particles along some fixed direc-tions, and the corresponding crystal faces were thickened underhydrothermal conditions. The strongest diffraction peak at (110)plane further indicated that hydrothermal condition enhancedthe growth rate of hureaulite crystal along the c axis. Fig. S2ashows the SEM images of Sample 5, and these particles wereformed with well-defined arris and the sizes were about 150 lm,further suggesting that they likely grew along some directions.When Sample 5 was ground by hand in agate mortar for 30 min,particle sizes decreased to about 5 lm (Fig. S2b), and both peakpositions and relative intensity of powder XRD were matched wellwith those of standard diffraction patterns of hureaulite (Fig. S1b,JCPDS Card No. 86-1521). It was further proved that the purehureaulite was formed under this hydrothermal condition.

Fig. 3 exhibits the FTIR transmission spectra of hureaulite synthe-sized by refluxing and hydrothermal reactions. All the absorptionbands of the synthesized samples by reflux and hydrothermal reac-tions agreed with the standard data for hureaulite. The strongabsorption bands at 3446 cm�1 was attributed to the hydroxylstretching vibration [5,19]. Absorption peak at 1303 cm�1 was dueto the in-plane bending vibration of the P–O–H in [PO3(OH)]2�

groups. The absorption peaks at 1147, 1069, 1024, 976, and934 cm�1 were assigned to the P–O bond anti-symmetric stretchingvibration in the [PO4]3� groups. The bands at 752 and 705 cm�1 wereattributed to the symmetric stretching vibration of the P–O bond.The two peaks at 578 and 529 cm�1 were assigned to the anti-symmetric in-plane bending vibrations of the O–P–O in [PO4]3�

groups [5]. For the [MnO6] octahedra in hureaulite, the stretchingvibrations of Mn = Od, Mn–Ob–Mn, and Mn–Oc–Mn are positionedat 976, 893 and 705 cm�1, respectively. The structure consists of athree-dimensional network formed by octahedral pentameric enti-ties [Mn5O16(H2O)6] that share either vertices or edges with the[(PO4)3�] and [(HPO4)2�] tetrahedra. Three oxygen species coordi-nated to P and Mn atoms are double-coordinated oxygen (Ob), tri-ple-coordinated oxygen (Oc) and terminal oxygen, respectively.

A peak at 444 cm�1 for hureaulite obtained by hydrothermalreaction could be assigned to the in-plane bending vibrations ofO–P–O [20], which was not discernable for the product obtainedby refluxing at 100 �C (Fig. 3a). The absorption peak at 752 cm�1

for hureaulite synthesized by reflux at 100 �C became weak inthe spectrum of Sample 5 (Fig. 3b). The anti-symmetric stretchingvibration bands of P–O bond in the [PO4]3� groups and the stretch-ing vibrations of Mn = Od, Mn–Ob–Mn, and Mn–Oc–Mn in the[MnO6] octahedron became stronger for the product obtained byrefluxing 30 mmol Mn(H2PO4)2 at 100 �C for 12 h.

Fig. 4 shows the typical micromorphologies of hureaulites fab-ricated at different reaction conditions. The prismatic particles,approximately 10 lm in length, were obtained when 0.3 mol L�1

Mn(H2PO4)2�2H2O solutions were conducted for 12 h at 100 �C(Fig. 4a). The morphology was similar to those of the nanocrystal-line manganese dihydrogen phosphate dehydrate crystals pre-pared by Danvirutai et al. [21]. When hydrothermal reaction wasconducted under high temperature and pressure, the particle sizesof prismatic crystals remarkably increased with about 150 lm inlength (Fig. 4b and c), which agreed well with the powder XRD re-sult. Compared with the reported results, the average size of theas-prepared materials gradually increased with increased reactiontemperatures from 50 to 170 �C (some figures not shown) [22].

The thermal stability of hureaulites obtained in different condi-tions was also investigated (Fig. 5). The TGA profiles of hureaulitessynthesized by refluxing (Fig. 5a) and hydrothermal reaction

Fig. 3. FTIR spectra of hureaulites: (a) sample 4 and (b) sample 5.

118 H. Yin et al. / Microporous and Mesoporous Materials 153 (2012) 115–123

(Fig. 5b) exhibited a similar change trend from room temperature to800 �C. The removal of water molecules occurs in the25–460 �C range with three steps. The first one occurred in therange of 25–150 �C, corresponding to the loss of physically ad-sorbed water on the mineral surfaces. The second step occurredin the 150–300 �C range and the mass loss can be associated withthe elimination of a molecule of crystalline water. The last one tookplace in the 300–420 �C range and corresponded to the loss of thethree molecules of crystalline water. The weight loss for thehureaulites obtained at 170 and 100 �C were 12.79% and 13.34%in this temperature range from 25 to 460 �C, respectively. Finally,in the range of 460–510 �C, weight losses of 0.57% and 0.26% oc-curred for the above two hureaulites, respectively, which could beassociated to the transformation of the hydrogen–phosphate an-ions into the phosphate groups. These inorganic residues wouldbe stable up to 800 �C likely due to the formation of Mn2(P2O7)and Mn3(PO4)2 [5].

The additives of acid and oxidant, such as H3PO4 and NaClO,were used to participate in the reactions. 30 mmol H3PO4 solutionwas added to the above Mn(H2PO4)2�2H2O solution and then re-fluxed for 12 h at 60, 80, and 100 �C, respectively. Hureaulite couldonly be formed at 100 �C (Fig. 2d) with the yield of 3.4% according tothat 4.3719 g of hureaulite should be formed when Mn(H2PO4)2�2H2O was completely transformed into target product, and the par-ticles exhibited rod-like shapes with a high aspect ratio (Fig. 4d). Asshown in Eq. (1), a high concentration of H+ in the system promotedthe reverse reaction, which was unfavorable for the formation ofhureaulite. When 60 mmol NaClO solution was added to the abovereaction system at 100 �C for 12 h, hureaulite (Fig. 2f) was obtained,and the yield increased to 56.0%. The forward reaction (Eq. (1)) wasaccelerated likely due to the hydrolysis of NaClO as follows:

NaClOðaqÞ þH2OðlÞ¢ HClOðaqÞ þ NaOHðaqÞ ð2Þ

NaOHðaqÞ þH3PO4ðaqÞ¢ Na3PO4ðaqÞ þH2OðlÞ ð3Þ

When phosphoric acid was further added, the oxidation abilityof NaClO will be enhanced, and MnPO4�H2O was obtained insteadof hureaulite [18]. When 100 mL of 0.3 mol L�1 Mn(H2PO4)2�2H2Oand 60 mmol of NaClO solution were mixed and refluxed for12 h, uniform rodlike hureaulite (Fig. 2e) was obtained as illus-trated in Fig. 4e. The addition of NaClO solution facilitated the for-mation of hureaulite under the above conditions. These resultsagreed well with the fact that the stability of hureaulite wasstrongly affected by the physicochemical parameters of the naturalenvironment, such as the redox potential (Eh) and pH [23]. Hypo-chlorous acid and its related salts can be generated by many natu-ral pathways [24,25], and the results reported here also canimprove the understanding of the formation of hureaulite in thenatural environment.

3.2. Synthesis of hureaulite with MnCl2 and H3PO4

To elucidate the possibility that hureaulite can also be formedwith sufficient quantities of Mn2+ and H2PO�4 under the above con-dition, a mixture of 100 mL of 0.3 mol L�1 MnCl2 and 30 mmolH3PO4 solution was refluxed at 60, 80, and 100 �C for 12 h, respec-tively. Hureaulite was not obtained from such systems, which waslikely due to the side effects of concentrated acid according to reac-tion (Eq. (1)). Precipitation occurred when 60 mmol NaClO solutionwas added to the above systems. Fig. 2g shows the XRD patterns ofproducts, suggesting hureaulite crystals were formed withoutimpurities. These results further confirmed that NaClO reducedthe acidity and facilitated the formation of hureaulite. When60 mmol H3PO4 was added to 100 mL solution containing 30 mmolMnCl2 and 60 mmol NaClO, single-phase MnPO4�H2O was formed.The high acidity of the solutions enhanced the oxidation abilityof NaClO, and Mn2+ was oxidized to Mn(III) [18], which agreed wellwith those experiments involving the reflux reaction of theMn(H2PO4)2�2H2O solution. Fig. 4f shows the morphologies of syn-thesized hureaulite crystals, which have an average size of 1–2 lm.The particle sizes differed from those in Fig. 4a–e.

To a certain degree, product morphology reflects the crystalgrowth process. The same crystal species may show various exter-nal forms under different growth conditions. The crystal morphol-ogy was formed during the precipitation–dissolution processes,and controlled by both internal (structural) and external (growthor dissolution parameters) factors. This is the most useful informa-tion in deciding the growth or post growth history [26]. The mor-phology of the product is greatly influenced by the reactionconditions, including the concentration of starting materials, reac-tion temperatures, pH. Products with various morphologies can beobtained using different reactants [27,28]. For the precipitationreaction of MnSO4, NH4HCO3 and C2H5OH, a spherical MnCO3 pre-cursor was obtained. However, the addition of (NH4)2SO4 facili-tated the formation of cubic crystals [28]. Reaction temperatureis also an important factor in controlling morphology. For thesynthesis of spherical particles of mesoporous silica SBA-16, thediameter of the spherical particles can be controlled in the rangeof 0.5–8 lm by varying the reaction temperatures from 1 �C upto 40 �C [29]. The pH of the reaction solutions similarly influencedthe morphology of the product. Acidic conditions resulted in thediameter variations of the products, owing to the related crystalgrowth mechanism during the transformation of cryptomelanetype MnO2 [30]. In the present work, hureaulites with differentmorphologies were obtained by using different reactants and addi-tives (Fig. 4). The crystal size of hureaulite synthesized by hydro-thermal reaction at 170 �C (Fig. 4b and c) was 20 times largerthan that obtained by refluxing at 100 �C (Fig. 4a). When 30 mmolH3PO4 was dissolved to the reaction system, rod-like hureaulitewith a high aspect ratio was formed instead of prismatic crystals(Fig. 4d). This is likely due to that the precipitation–dissolutionequilibrium (Eq. (1)) was changed, and the nucleation process ofhureaulite was also affected. When 30 mmol MnCl2, 30 mmolH3PO4 and 60 mmol NaClO solutions were used in the reflux pro-cess at 100 �C for 12 h, uniform granular hureaulite was formed(Fig. 4f) suggesting the manganese (II) salt also affected the nucle-ation process, which was observed in the difference in the mor-phologies of product (Fig. 4a–e). The solubility of the precursor isalso likely to be an important factor in the controlling morpholo-gies of product. The nucleation mechanism of hureaulite is underinvestigation in our group.

3.3. Effect of temperature on the lattice parameters of hureaulite

The Rietveld structure refinement technique using X-ray pow-der diffraction data provides a valuable approach to the study of

Fig. 4. Typical SEM images of hureaulites: (a) sample 4, (b) and (c) sample 5, (d) sample 6, (e) sample 7, and (f) sample 9.

Fig. 5. TGA profiles of synthesized hureaulites: (a) sample 4 and (b) sample 5.

H. Yin et al. / Microporous and Mesoporous Materials 153 (2012) 115–123 119

mineral structures [31]. In the present work, Rietveld refinementwas performed using the program TOPAS based on the structuralmodel ICSD 82617. Full pattern fitting was processed by a funda-mental parameters approach (FPA). The background was modeledusing a 6th-order Chebychev polynomial. The zero error was cor-rected and the 83 independent parameters were refined, includingsample absorption, lattice constants, atom positions and preferredorientations. The difference between the experimental and calcu-lated data indicated satisfactory refinement results (Fig. 6) and also

confirmed that the obtained products were single phase hureaulite,which agreed well with the XRD and SEM images.

Rietveld structure refinement of the sample, which was ob-tained by refluxing 100 mL of 0.3 mol L�1 Mn(H2PO4)2 solution at100 �C for 12 h, was conducted as shown in Fig. 6d. The latticeparameters were calculated to be a = 17.6418 Å, b = 9.1479 Å,c = 9.5103 Å, and b = 96.5665�. The cell volume and crystal densitywere 1524.761 Å3 and 3.0106 g cm�3, respectively (Table 2).

Single-phase hureaulite was formed as the Mn(H2PO4)2 solutionwas refluxed for 12 h at 50, 60, 80, and 100 �C, respectively. Fig. 6and Table 2 show the XRD patterns and cell parameters of hureau-lite synthesized at different temperatures, respectively. The prod-ucts formed at different temperatures exhibited very similarcrystal characteristics. The lattice parameters were approximatelythe same, except that the values of c and b gradually decreasedwith an increase in reflux temperature. Similar trends were alsoobserved in the growth of other crystals possibly due to high tem-perature used to facilitate the growth of crystals with a shorterlength in the c direction [26]. Crystal density increased as cell vol-ume decreased. The crystal densities of the hureaulite were 3.0087,3.0095, 3.0097 and 3.0106 g cm�3 as prepared at 50, 60, 80, and100 �C, respectively. These densities obtained in the present workwere much smaller than that (q = 3.240 g cm�3) of the naturalhureaulite formed by hydrothermal reactions under a high temper-ature and high pressure [2]. As for the pure hureaulite synthesized

120 H. Yin et al. / Microporous and Mesoporous Materials 153 (2012) 115–123

in the present work, the crystal density correlated positively withthe reflux temperature, suggesting that high temperature facili-tated the growth of crystals with high density. Furthermore, manycations, particularly the first-row transition-metal ions (such asFe2+, Co2+, Zn2+, Cu2+, and Cd2+), exist in natural environmentsand can be incorporated into the structure of hureaulite to form fi-nite solid solutions. Hureaulite is known to allow the formation ofMn2+ in solid solutions containing Fe2+ with all concentrations [17],and the incorporation of Mn2+ significantly affects the crystal den-sity. The partial substitution of Co for Mn in the solid solution[Mn(Co)]5(PO3(OH))2(PO4)2�(H2O)4 induces a gradual increase inthe crystal density with an increase in cobalt content [5]. Crystaldensities calculated by the structure refinement technique couldbe used as a convenient diagnostic method for the formation con-ditions of hureaulite in the environment.

3.4. Formation process of hureaulite at ambient temperature

To verify that hureaulite can be formed at near room tempera-ture, hureaulite was synthesized by refluxing 100 mL of0.3 mol L�1 Mn(H2PO4)2�2H2O solution at 30 and 40 �C with a long-er time, respectively. Hureaulite was not formed at 30 �C; however,precipitate was produced when the reflux solution was heated andkept at 40 �C. The products obtained at different reaction timeswere weighed and characterized by powder XRD, SEM and FTIR.

Products were obtained with masses 0.0746, 0.1398, and0.4458 g as the reflux reaction lasted for 12, 24, and 48 h, respec-tively. Powder XRD analysis showed that the products obtainedby refluxing for 12 h (Fig. 7a) and 24 h (Fig. 7b) were a mixtureof hureaulite and MnHPO4 3H2O (JCPDS Card No. 25-0541). Sin-gle-phase hureaulite was obtained when the reaction time wasprolonged up to 48 h (Fig. 7c).

Fig. 6. Rietveld structure refinement results of hureaulite: (a) sample 1, (b) sample 2, (c)lines are the calculated profile. The difference between the observed and calculated pat

Fig. 8 shows the morphologies of the products obtained byrefluxing 100 mL of 0.3 mol L�1 Mn(H2PO4)2�2H2O solution at40 �C at different times. As illustrated in Fig. 8a, nonuniform parti-cles were obtained suggesting a mixture was formed when the re-flux reaction lasted for 12 h. Prismatic crystals formed when refluxwas prolonged to 24 h (Fig. 8b). The prismatic component of thesamples gradually increased as the reaction time was extended.Only prismatically-shaped particles were obtained after refluxingfor 48 h (Fig. 8c), and the particle sizes were much larger than thoseshown in Fig. 8b. These results indicated that pure hureaulite wasformed when Mn(H2PO4)2�2H2O solutions were refluxed for no lessthan 48 h at 40 �C.

The formation process of hureaulite was also elucidated by FTIRas presented in Fig. 9. Although both hureaulite and MnHPO4�3H2Ocontain [PO3(OH)]2� groups, the frequencies of P–O–H in-planebending vibrations in these groups are different. The P–O–H in-plane bending vibrations in the [PO3(OH)]2� group of hureaulite isat 1303 cm�1 [5], however, the vibration frequency is at1249 cm�1 for [PO3(OH)]2� group in MnHPO4�3H2O [32]. A sharppeak at 1249 cm�1 was observed in the spectrum of when0.3 mol L�1 Mn(H2PO4)2 was refluxed at 40 �C for 12 h (Fig. 9a). Thisabsorption peak gradually diminished as the reaction went on(Fig. 9b and c). A weak absorption peak at 1303 cm�1 was observedin Fig. 9a, indicating that the product obtained from the above solu-tion was a mixture of MnHPO4�3H2O and hureaulite. When the re-flux time was prolonged to 24 h, a peak at 1303 cm�1 was observed(Fig. 9b). Absorption peak at 1303 cm�1 and the absence of peak at1249 cm�1 suggested that a single phase of hureaulite was formedwhen the reactions was conducted for 48 h (Fig. 9c). A relativelysharper peak located at 1638 cm�1 in Fig. 9a was assigned to thein-plane bending vibration of H–O–H in water molecules in thestructure of MnHPO4�3H2O [32] compared with that of hureaulite

sample 3, and (d) sample 4. The blue crosses are the observed data, and the red solidterns is plotted below as light gray solid lines.

Table 2Cell parameters obtained by Rietveld refinement for hureaulite synthesized at different temperatures.

Product Sample 1 Sample 2 Sample 3 Sample 4

Reflux temperature (�C) 50 60 80 100Molecular weight (g mol�1) 728.65 728.65 728.65 728.65Crystallite system Monoclinic Monoclinic Monoclinic MonoclinicSpace group C12/c1 C12/c1 C12/c1 C12/c1a (Å) 17.6405 17.6407 17.6424 17.6418b (Å) 9.1489 9.1484 9.1486 9.1479c (Å) 9.5167 9.5142 9.5122 9.5103b (�) 96.6063 96.5897 96.5787 96.5665Z 4 4 4 4Cell volume (Å3) 1525.72 1525.30 1525.20 1524.76Crystal density (g cm�3) 3.0087 3.0095 3.0097 3.0106

R factors (%)Rexp 0.85 0.80 0.81 0.81Rwp 3.47 3.47 3.10 3.30Rp 2.31 2.25 2.18 2.21

Fig. 7. Powder XRD patterns of synthesized products: (a) sample 10, (b) sample 11,and (c) sample 12. Fig. 9. FTIR spectra of obtained products: (a) sample 10, (b) sample 11, and (c)

sample 12.

H. Yin et al. / Microporous and Mesoporous Materials 153 (2012) 115–123 121

due to that the former contains more water molecule per unitweight.

To further investigate the transformation process of MnHPO4�3H2O to hureaulite, two-phase Rietveld quantitative analyses wereconducted using the crystal-structure models of hureaulite (ICSD82617) and MnHPO4�3H2O (ICSD 65686). The fitting patterns wereplotted as shown in Fig. 10, and the mass percentage of the twophases in the intermediate products was calculated. The mixturewas composed of 59.24% hureaulite and 40.76% MnHPO4�3H2O bymass when the reflux reaction lasted for 12 h. The latter componentgradually transformed to hureaulite as the reaction proceeded. Thecontent of hureaulite was increased from 59.24% to 86.11% and100% as the reaction lasted for 24 and 48 h, respectively.

Based on the above results and discussion, Eq. (1) may occur bya two-step process as follows:

Fig. 8. SEM images of synthesized products: (a) s

MnðH2PO4Þ2ðaqÞ þ 3H2OðlÞ¢ MnHPO4 � 3H2OðsÞ þH3PO4ðaqÞ ð4Þ

5MnHPO4 � 3H2OðsÞ¢ Mn5ðPO3ðOHÞÞ2ðPO4Þ2 � 4H2OðsÞ þH3PO4ðaqÞ

þ 11H2OðlÞ ð5Þ

To further confirm the transformation processes in reactions(Eqs. (4) and (5)), the pH values of the reaction system in the initialand end stages were monitored. The pH value of the Mn(H2PO4)2

solution was approximately 2.77 at the initial stage, and decreasedto 2.69, 2.57, and 2.35 when the reflux reaction lasted for 12, 24,and 48 h, respectively. The slow decrease in pH suggested thatH3PO4 was gradually generated during the formation of thehureaulite. These results further confirmed the reaction processesproposed in reactions (Eqs. (4) and (5)).

ample 10, (b) sample 11, and (c) sample 12.

Fig. 10. Rietveld structure refinement results of synthesized products: (a) sample 10 and (b) sample 11. The observed profile (blue) and calculated profile (red) from two-phase fit and difference plot (gray) for the two-phase quantitative analysis of the products. (For interpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)

122 H. Yin et al. / Microporous and Mesoporous Materials 153 (2012) 115–123

4. Conclusions

Hureaulite has been successfully fabricated by the reflux reac-tion of 100 mL of 0.3 mol L�1 Mn(H2PO4)2 solution at 40–100 �C.All the synthesized samples showed non-uniform prismatic crystalmorphologies. Rod-like hureaulite could be synthesized when30 mmol H3PO4 was added to the reaction system at 100 �C. Theyield of hureaulite can be enhanced by adding a certain amountof NaClO solution. Hureaulite could not be formed by refluxingthe mixed MnCl2 and H3PO4 solutions under the same conditions,unless NaClO solution was added to the reaction system. The prod-uct morphologies were controlled by adjusting the divalent man-ganese source under the same conditions. Lattice parameters cand b decreased, accompanying an increase in crystal density whenthe reaction temperature was increased. Hureaulite was formed atnear room temperature and MnHPO4�3H2O worked as an interme-diate product. The present work further facilitated a thoroughknowledge of the genesis of hureaulite in the nature and simplifiedpreparation process of hureaulite materials.

Acknowledgements

The authors thank the National Natural Science Foundation ofChina (Grant numbers: 41171375, 20807019, and 40771102), theSpecialized Research Fund for the Doctoral Program of Higher Edu-cation of China (Grant number: 20070504053), and FundamentalResearch Funds for the Central Universities (Program number:2011PY015) for financial support. We owe our gratitude to Dr. Li-hong Qin and Jianbo Cao at Huazhong Agricultual University forkindly providing access to the SEM. The authors also gratefullyacknowledge Dr. Steven L. Suib, Dr. Frank Galasso, and researchassistant Homer C. Genuino at the University of Connecticut andthe anonymous reviewers for their valuable comments and sugges-tions to improve the quality of this paper.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.micromeso.2011.11.057.

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