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Hydrothermal sy

aSchool of Materials Science and Technology

100083, PR China. E-mail: mahw@cugb.edubBeijing National Laboratory for Condens

Chinese Academy of Sciences, Beijing, 10019

Cite this: RSC Adv., 2016, 6, 54503

Received 29th March 2016Accepted 18th May 2016

DOI: 10.1039/c6ra08080d

www.rsc.org/advances

This journal is © The Royal Society of C

nthesis of analcime andhydroxycancrinite from K-feldspar in Na2SiO3

solution: characterization and reaction mechanism

Jiangyan Yuan,a Jing Yang,a Hongwen Ma,*a Changjiang Liua and Chenglong Zhaob

Hydroxycancrinite and analcime were generated in the reaction process of K-feldspar and various

concentration solutions of Na2SiO3. The effect of the concentration of Na2SiO3 on the formation of the

product samples and the reaction mechanism were investigated. The X-ray diffraction (XRD) results

indicate that there exists hydroxycancrinite (Na8Al6Si6O24(OH)2$2H2O) with a hexagonal structure at

a Na2SiO3 concentration of 2.85–3.9 mol kg�1 and analcime (NaAlSi2O6$H2O) with a cubic structure at

2.34–2.85 mol kg�1 in the reaction of K-feldspar–Na2SiO3–H2O system. Rietveld refinements were

utilized to investigate the crystallographic parameters of hydroxycancrinite based on the XRD results,

showing that Na1 atoms existed in the form of [Na$H2O]+ cluster and water molecule (O8) are located in

the center of those six-membered rings and slightly displace from the plane of the six-membered ring,

as well as Na2 atoms and larger anionic group OH�(O7) are seated in the larger channel. Fourier

transformation infrared spectrometry (FTIR) results suggest that the existence of the –OH and water

molecules makes the absorption bands at 3569 and 3451 cm�1 broaden and overlap with each other as

the concentration of Na2SiO3 decreases. The sharp bands at 1634 cm�1 disappear gradually with the

decrease of Na2SiO3 concentration owing to the difference of product. It turns out that the reaction

process of K-feldspar and Na2SiO3 as well as the synthesis of analcime and hydroxycancrinite is an

interface coupled dissolution–reprecipitation process mainly in the presence of Na+, [Al(OH)4]�,

[H2SiO4]2� and SiO3

2� and followed by the repolymerization process of the silicate framework.

1. Introduction

Analcime is a crystalline microporous aluminosilicate mineral,the three-dimensional framework of which consists ofa network of pores and cavities built by the corner-sharing of[SiO4] and [AlO4].1,2 Analcime (NaAlSi2O6$H2O) is a scarce zeolitein nature with small irregular channels consisting of 4, 6 and 8membered distorted rings and some cavities occupied by Naions generating an octahedral-coordination.3 The uniquestructure makes analcime have wide applications in catalysis,ion exchange, ion adsorption reactions, and uoride wastewatertreatment.4,5

The synthesis of analcime has been reported in severalpapers and patents. Generally, analcime is synthesized usingchemical reagents sodium silicate and aluminate. Azizi et al.6

used effectively NaOH solution and rice husk ash as a silicasource for the synthesis of analcime. Analcime was synthesizedusing a hydrothermal method using chrysotile and rice huskash as the sources of silica in the report of Petkowicz et al.7 In

, China University of Geosciences, Beijing,

.cn

ed Matter Physics, Institute of Physics,

0, PR China

hemistry 2016

addition, there are many aluminosilicate minerals that could beused as silica, such as kaolin,8 coal y ash9 and perlite.10

As a feldspathoid mineral, cancrinite found in nature withan ideal chemical composition Na6Ca[AlSiO4]6CO3$2H2O isformed by the stacking of layers of six-membered rings in anABAB sequence and consists of 5-cages and larger twelve-membered channels similar to those of zeolite. In general, thechannels of cancrinite are easily blocked by inorganic guestanions which limits the application of the porous structure.11–13

Moreover, the removal of guest anions by annealing of the guestanion causes the destruction of the framework. However, OH�

and H2O molecules can be removed under mild conditions,which makes hydroxycancrinite a suitable material to utilize theporous structure as compared with cancrinite.14

K-feldspar is widely distributed in China as an importantpotassic mineral resource for relieving the shortage of solublepotassic salt if utilized efficiently.15,16 The stable structure of K-feldspar means it cannot be used in soil directly. The reporteddissolution methods of K-feldspar have more than 30 cate-gories, including limestone sintering,17,18molten salt leaching,19

inorganic acid dissolution, hydrothermal alkaline dissolu-tion,20,21 and so on. Hydrothermal alkaline digestion has greatpotential in industrial applications, and has several advantages,such as a lower reaction temperature, higher heat utilization

RSC Adv., 2016, 6, 54503–54509 | 54503

Fig. 1 Schematic illustration of the synthesis process of analcime andhydroxycancrinite.

Fig. 2 (a) XRD patterns of K-feldspar and the solid products formedunder various concentrations of Na2SiO3 (RC15, RC-N-1–RC-N-5,from bottom to top); the crystal structure of microcline (b) and anal-cime (c).

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rate and lower environmental impact. The hydrothermaldissolution of K-feldspar in Ca(OH)2, NaOH and KOH solutionshas been studied in our previous work.22–24

However, synthesis of analcime from K-feldspar and Na2SiO3

is seldom reported. Moreover, the insoluble potassium can beextracted from K-feldspar in the formation process of analcimeand hydroxycancrinite, which is very signicant for China owingto the rare potassium resource. In this work, analcime andhydroxycancrinite were synthesized in the hydrothermal disso-lution process of K-feldspar. The effect of the concentration ofNa2SiO3 on the formation of different products and the reactionmechanism were discussed. The products were characterized bymeans of X-ray diffraction (XRD), transmission electronmicroscopy (TEM), wet chemistry analysis, Fourier transforminfrared spectroscopy (FTIR), and scanning electronmicroscope(SEM).

2. Experimental2.1. Materials

The K-feldspar (RC15) sample used in this study was collectedfrom Rongcheng County of Shandong Province, China. Thechemical composition of the K-feldspar powder is shown inTable 1. The main chemical components of K-feldspar are SiO2

(64.80%) and Al2O3 (17.35%) with a K2O content of about15.31%. Sodium metasilicate nonahydrate (analytical reagentgrade) was supplied by Beijing Modern Eastern FinechemicalCo., Ltd. The deionized water produced in the local laboratorywas used in the experiments.

2.2. Experimental

In the hydrothermal experiments, 10 g of K-feldspar powder wasground, passed through a 200-mesh sieve, and mixed with 3.90,3.19, 2.85, 2.57, or 2.34 mol kg�1 solvent (expressed in molalityrepresenting the number of moles of solute divided by thekilograms of solvent) in a 100 mL Teon liner. Aer magneticstirring, the liner was placed in a steel hydrothermal autoclaveassembled in a temperature controlled homogenous reactor at250 �C for 5 h with a rotating speed of 10 rpm. The solidproducts (named with RC-N-1–RC-N-5) were separated from thesolution using ltration and then investigated using X-raydiffraction, scanning electron microscope, transmission elec-tron microscope and Fourier transform infrared spectroscopy.The elemental contents in all the aqueous samples and solidsamples were analyzed using wet chemical analysis. The sche-matic illustration of the reaction is displayed in Fig. 1.

2.3. Characterization

The XRD patterns of the K-feldspar powder and solid productsobtained under different conditions were recorded using

Table 1 Chemical composition of potassic feldspar (wB/%)

Sample SiO2 TiO2 Al2O3 Fe2O3 FeO Mg

RC15 64.8 0.00 17.35 0.078 0.29 0.1

54504 | RSC Adv., 2016, 6, 54503–54509

a SmartLab (Rigaku) X-ray diffractometer with Cu Ka radiation.The powder diffraction data were analyzed using a computersoware General Structure Analysis System (GSAS) package.25

Transmission electron microscopy using a JEM-2100 made inJapan was used to study the microstructure of the as-preparedsamples. The morphologies of the samples were examinedusing a Sirion 200 scanning electron microscope under EHT ¼10.0 kV. Fourier-transform infrared spectra of the samples werecollected using a Perkin Elmer 2000 in the 4000–400 cm�1

region using potassium bromide as the diluent and binder. TheSiO2 content in the products was determined using a poly-ethylene oxide dehydration method, the Al2O3 content wasdetermined using EDTA complexometric titration and the K2Oand Na2O content were determined using the ame photo-metric method.26

O CaO Na2O K2O P2O5 Loss Total

7 0.40 0.66 15.31 0.007 0.55 99.62

This journal is © The Royal Society of Chemistry 2016

Table 2 Chemical compositions of the solid products prepared at 250 �C under various concentrations

SamplesConcentrations(Na2SiO3)/mol kg�1 SiO2/wt% Al2O3/wt% Na2O/wt% K2O/wt%

RC15 — 64.8 17.35 0.66 15.31RC-N-1 3.90 45.16 26.44 16.60 2.93RC-N-2 3.19 48.02 23.06 14.44 4.33RC-N-3 2.85 46.27 22.40 12.66 4.68RC-N-4 2.57 54.22 23.43 11.00 4.48RC-N-5 2.34 55.25 22.35 11.00 4.32

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3. Results and discussion3.1. Synthesis of analcime and hydroxycancrinite

The X-ray diffraction patterns of the solid products with variousconcentrations of Na2SiO3 are presented in Fig. 2a. Thediffraction peaks of the samples can be well indexed intoanalcime (JCPDS 41-1478), hydroxycancrinite (JCPDS 46-1457),and microcline (JCPDS 19-0932), indicating that hydrox-ycancrinite (Na8Al6Si6O24(OH)2(H2O)2) with a triclinic structurewas formed in the Na2SiO3 concentration of 3.9–2.85 mol kg�1

(RC-N-1–RC-N-3) and analcime (NaAlSi2O6$H2O) with a hexag-onal structure was generated in the lower concentration of 2.37–2.57 mol kg�1 (RC-N-4–RC-N-5), which is similar to the disso-lution of quartz syenite in NaOH solution reported by Ma et al.27

The strongest peak of hydroxycancrinite at 27.47� is assigned toits (211) crystal plane, and that of analcime found at 25.9�

Fig. 3 (a) Experimental, calculated XRD patterns and their difference for thhydroxycancrinite (c) crystal structure of the hydroxycancrinite based on

This journal is © The Royal Society of Chemistry 2016

corresponds to the (400) diffraction plane. As-synthesizedanalcime can be used to exchange with K2CO3 and (NH4)2CO3

solution in order to gain leucite and ammonioleucite, which notonly improve the ability of retaining water, but also can increasethe N and K content of plants when added to soil.28

The chemical compositions of the solid products at 250 �Cunder various concentrations are listed in Table 2. When theconcentration of Na2SiO3 in solution was in the range of 3.90 to3.19 mol kg�1, the mass fraction of SiO2 in the solid productswas between 45.16 and 55.25% corresponding to the massfraction of SiO2 in the chemical formula of Na8Al6Si6O24(-OH)2(H2O)2 (37.19%) lower than that in NaAlSi2O6$H2O(54.54%). Moreover, the variation of Al2O3 was between 26.44and 22.35% and Na2O was between 16.60 and 11.00% which isconsistent with the mass fraction of Al2O3 and Na2O in thechemical formula of Na8Al6Si6O24(OH)2(H2O)2 (31.6% and

e Rietveld fits of hydroxycancrinite using the GSAS program; (b) TEM ofthe refinement result.

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Table 3 The refined positions of all the atoms and the latticeparameters of hydroxycancrinite

Sample: hydroxycancrinite

Lattice constants are: a ¼ b ¼ 12.7705(8)�A, c ¼ 5.1903(0)�A; a ¼ b ¼ 90�;g ¼ 120�; V ¼ 733.06(9) �A3

Atom x y z Occupancy

Na1 2/3 1/3 0.1246(5) 1.00Na2 0.1378(3) 0.2670(1) 0.2952(6) 1.00Al1 0.0762(5) 0.4196(2) 0.7503(6) 1.00Si1 0.3298(1) 0.4125(1) 1/4 1.00O1 0.2064(3) 0.4051(4) 0.6801(2) 1.00O2 0.1149(6) 0.5690(5) 0.7193(2) 1.00O3 0.0241(6) 0.3562(4) 0.0590(7) 1.00O4 0.3190(4) 0.3611(5) 0.0421(9) 1.00O5 0.0558(7) 0.1014(5) 0.6716(6) 0.17O6 0.0562(4) 0.1089(7) 0.9456(1) 0.17O7 0 0 0.6902(6) 0.33O8 0.6105(1) 0.3061(5) 0.6863(3) 0.33

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25.6%) higher than that in NaAlSi2O6$H2O (23.18% and14.09%). Compared to the RC15, the mass fraction of K2O inRC-N-1–RC-N-5 (2.93–4.68%) is far lower than that in RC15(15.31%), demonstrating that most K2O has been transferredfrom the solid to liquid.

Fig. 2b and c present the crystal structures of microcline andanalcime. The crystal structure of microcline is made up ofa three dimensional framework built by [AlO4] and [SiO4] tetra-hedrons, which share their oxygen atoms to form 4 membered-rings along the a axis. There exist larger cavities occupied bycation (K+) with a larger radius among the rings. As shown inFig. 2c, the crystal structure of analcime consists of 4, 6 and 8rings built by [SiO4] and [AlO4]. Na

+ surrounded by a distortedoctahedron is connected with four oxygen atoms and two watermolecules located in the continuous channels consisting of[AlO4] and [SiO4] tetrahedrons along the triply screw axis. Thestructure of hydroxycancrinite has been reported few times so far.

In order to understand the crystal structure of hydrox-ycancrinite, Rietveld structure renements of hydroxycancrinitewere performed based on the GSAS program using the powderXRD data. Therefore, hydroxycancrinite with a better

Fig. 4 FTIR spectra of K-feldspar and the solid products formed under vato top) (a) 400–2000 cm�1, (b) 1300–4000 cm�1.

54506 | RSC Adv., 2016, 6, 54503–54509

crystallinity was synthesized by increasing the concentration ofNa2SiO3$9H2O, the XRD Rietveld t pattern of which is dis-played in Fig. 3a and the nal rened unit cell parameters aresummarized in Table 3. It is obvious that all the Bragg diffrac-tion lines of hydroxycancrinite are assigned to the hexagonalsystem with a space group P63/m. In order to understand themicrostructure of this hydroxycancrinite, the TEM and crystalstructure are depicted in Fig. 3b and c. The TEM image dis-played in Fig. 3b conrms the rod structure of hydrox-ycancrinite. The crystal structure of hydroxycancrinite iscomposed of a three dimensional framework consisting of[AlO4] and [SiO4] connected alternately and sharing their corner-oxygen atoms to form 4, 6 and 12 membered rings. The 3-foldaxis runs through the center of these rings. The cages formed bythe six-membered rings are in parallel with the c axis. Watermolecule (O8) is disordered about the 3-fold axis in each cagebecause of the hydrogen bonding. One interstitial cation site,Na1 atoms with the form of a [Na$H2O]

+ cluster are located inthe cage on the 3-fold axis and are slightly displaced from theplane of the six-membered rings.29 The other interstitial cationsites, Na2, are situated in the larger channel containing largeanionic groups, such as OH�(O7), CO3

2�, SO42� and H2O.

The FTIR spectra of the solid products at 250 �C undervarious concentrations are given in Fig. 4a and b. Analcime hasFTIR wavenumbers of 449, 740, 1026, 1639, and 3620 cm�1

according to the report of Markovic et al.30 As shown in Fig. 4a,there are absorption bands of water molecules at 3569 and 3451cm�1 broadening and overlapping with each other as theconcentration of Na2SiO3 decreases, the reason of which is thatthere exists –OH in the structure of hydroxycancrinite (Na8Al6-Si6O24(OH)2$H2O) but not in analcime (NaAlSi2O6$2H2O). Thegradual disappearance of the sharp bands at 1634 cm�1 is alsoattributed to the impact of –OH. The band appearing at 1012cm�1 is associated with the T–O (T ¼ Al, Si) asymmetricstretching vibrations and is divided into two bands (1012 and966 cm�1) in the structure of hydroxycancrinite. The bands at728 and 438 cm�1 are owed to the existence of T–O (T ¼ Al, Si)symmetric stretching vibrations and the bending vibrationalcoupling of T–O–T (T ¼ Al, Si) as well as Na–O, respectively.Three stronger bands at 677, 625 and 568 cm�1 of hydrox-ycancrinite generated by the bending vibration of O–Si(Al)–O

rious concentrations of Na2SiO3 (RC15, RC-N-1–RC-N-5, from bottom

This journal is © The Royal Society of Chemistry 2016

Table 4 Ionic concentration in the filtrate of samples RC-N-1–RC-N-5

SamplesConcentration(Na2SiO3)/mol kg�1

SiO2 (molL�1)

Al3+ (mmolL�1)

Na+ (molL�1) K+ (mol L�1) hK (%)

RC-N-1 3.90 0.4000 10.1157 0.4839 0.0560 85.89RC-N-2 3.19 0.3833 9.3118 0.4839 0.0494 75.77RC-N-3 2.85 0.3767 7.5863 0.5097 0.0470 72.18RC-N-4 2.57 0.3633 7.2843 0.4839 0.0470 72.18RC-N-5 2.34 0.3550 7.0804 0.4839 0.0470 72.18

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grow weaker and weaker as the concentration of Na2SiO3

decreases. In addition, the bands at 538 cm�1 are matched withthe structure of microcline, indicating there still exists a littleamount of microcline in accordance with the description inFig. 2a.31

Fig. 5 SEM images of the K-feldspar and solid products formed underschematic illustration of the product changes in the reaction.

This journal is © The Royal Society of Chemistry 2016

3.2. Reaction mechanism

The results indicated that the crystal structure of K-feldspar wasdestroyed under the hydrothermal reaction in our experiment,analcime and hydroxycancrinite were formed owing to thevarious concentrations of Na2SiO3. The dissolution of feldsparunder hydrothermal conditions has been widely studied in

various concentrations of Na2SiO3 (RC15, RC-N-1–RC-N-5) (a–f); (g)

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recent years.32 Fu et al.33 reported that the dissolution of silicateminerals and the precipitation of secondary minerals are inte-grated processes in the hydrothermal alteration of rocks. Assamples obtained in this paper, the dissolution of K-feldspar aswell as the synthesis of analcime and hydroxycancrinite is anintegrated process and could not be separated from each other.Gautier et al.34 argued that the Al–O bond was more easilybroken than the Si–O bond during the dissolution of K-feldspar.It is agreed that the formation of secondary phases is aninterface coupled dissolution–reprecipitation process mainly inthe presence of a uid phase and followed by repolymerizationof the relict silica framework.35,36

Reaction of K-feldspar and Na2SiO3 can be described usingthe following chemical equations:

Na2SiO3$9H2O / 2Na+ + SiO32� + 9H2O (1)

4Na+ + 2SiO32� + H2O / Na2Si2O5Y + 2Na+ + 2OH� (2)

KAlSi3O8 + 6OH� + 2H2O / K+ + Al(OH)4� + 3H2SiO4

2� (3)

8Na+ + 6Al(OH)4� + 18H2SiO4

2� / Na8Al6-Si6O24(OH)2$2(H2O)Y + 12SiO3

2� + 10H2O (4)

Na+ + Al(OH)4� + 2H2SiO4

2� / NaAlSi2O6$H2OY + 4OH� +

2H2O (5)

By analyzing eqn (2), there exists another solid precipita-tion Na2Si2O5 not shown in Fig. 2a except for hydrox-ycancrinite and analcime, the reason for which may be thatNa2Si2O5 decomposes into Na2SiO3 and SiO2. Every ionicconcentration in the ltrate of samples RC-N-1–RC-N-5 is lis-ted in Table 4. The concentration of Al3+ decreased from10.1157 to 7.0804 mol L�1 while the concentration of Na2SiO3

increased, suggesting that the building of analcime neededmore Al than that of hydroxycancrinite. The morphology of theK-feldspar grains and the solid samples generated in varyingconcentrations of Na2SiO3 is investigated using SEM (Fig. 5a–e). As shown in Fig. 5a, the morphology of the K-feldspar istabular with no xed shapes (Fig. 5a). The solid products (RC-N-1–RC-N-3) obtained in the Na2SiO3 concentrations of 3.9–3.19 mol kg�1 show rods with a length of around 2 mm andthose obtained in the concentrations of 2.85–2.34 mol kg�1

present tetragonal trisoctahedrons. K-feldspar is dissolved inalkali solution and the ions of Na+, [Al(OH)4]

�, SiO32� as well

as H2SiO42� are precipitated to hydroxycancrinite (Fig. 5b–d)

with a uniform rod morphology. When the concentration ofNa2SiO3 is decreased to 2.57 mol kg�1, the concentration ofions is too small to build the structure of hydroxycancrinite, sothe tetragonal trisoctahedron analcime (e and f) witha uniform size is formed. Fig. 5g displays the schematicillustration of the reaction process. According to the analysisabove, we can conclude that the dissolution of K-feldsparas well as the formation of analcime and hydroxycancriniteis an interface coupled dissolution–reprecipitation processmainly in the presence of Na+, [Al(OH)4]

�, [H2SiO4]2� and

SiO32 and followed by a repolymerization process of the sili-

cate framework.

54508 | RSC Adv., 2016, 6, 54503–54509

4. Conclusion

In summary, the effect and mechanism of the concentrationof Na2SiO3 on the formation of analcime and hydrox-ycancrinite were studied in this paper. By the XRD results, itis shown that analcime and hydroxycancrinite were synthe-sized successively when the concentration of Na2SiO3

decreased. In the structure of hydroxycancrinite, [Na$H2O]+

clusters are formed by Na1 atoms and water molecules (O8)situated in the center of the six-membered rings, as well asNa2 atoms and larger anionic groups OH�(O7) are seated inthe larger channel according to the results of the Rietveldanalysis. In terms of the analytic results of FTIR, gradualdisappearing of sharp absorption bands in 1634 cm�1 isattributed to the vanishment of OH from Na8Al6Si6O24(-OH)2$2H2O to NaAlSi2O6$H2O. According to the analysis ofthe dissolution mechanism, the formation of analcime andhydroxycancrinite is an interface coupled dissolution–repre-cipitation process and a repolymerization process of thesilicate framework in the presence of Na+, [Al(OH)4]

�,[H2SiO4]

2� and SiO32�.

Acknowledgements

The present work was supported by the Fundamental ResearchFunds for the Central Universities (2652014017, 2652015371 &2652015422).

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