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Note sProgress of Crystallization of Zeolite Na-A From

Clay

P. C. BORTHAKUR, M. C. UPRETI, S. N. DUTTA & M. S. IYENGA~Regional Research Laboratory, Jorhat 785 006 I-

Received 17 October 1978; accepted 27 March 1979

The progress of crystallization of Na-A zeolite from clay hasbeen followed employing X-ray, IR spectroscopy ,DTA, DTGand TG, The onset of crystallization can be inferred from theappearance of high temperature exotherm in the DTA of theproducts formed during rhe course of the reaction or from theattaimnent of minimum value of either the Na20/H20 or Si021Al203 ratio in mother liquor. Absence of inflection other than theone at 120°C in the low temperature endotherm of the DTA curvesof the products or the attaimnent of constant Si02/Al203 ratioby the solids or mother liquor in the course of the reaction indicatescomplete crystallization. Increase in smoothness and sharpnessof the infrared bands around 3400, 1650 and 1100 cm-I and theshift of 1100 cm-I band to lower frequency (1000 cm-I) distinguisesqualitatively the final crystals from intermediate amorphousproducts.

ZHDANOVI and Flanigen" have reviwed zeolitecrystallization extensively. As the sequence

of reactions occuring during crystallization is com-plicated and not well understood, a great dealof empiricism pervades the synthesis of zeolites.Most of the work in the field of preparing syntheticzeolites and for interpretation of the kinetics andmechanism of zeolite nucleation have been carriedout with alumino silicate hydrogels prepared bymixing sodium aluminate with sodium silicate orsilica sol, The influence of different factors affectingzeolitization of clay has been investigated by Barrerand Mainwaring", essentially to deterimine the opti-mum conditions for the formation of various zeolites.In the present investigation, thermal, chemical andX-ray analysis and IR spectroscopy have been usedto investigate the progress of crystallization ofzeolite Na-A from clay.

A kaolinite type of clay occurring in Garo Hillsregion of North-Eastern India containing smallamount of anatase and quartz (less than 5 %) wasused as the starting meterial". Powdered clay (-100B.S. Sieve) after activation by heating at 700° C for2 hrs was allowed to react with sodium hydroxidesolution maintaining the appropriate conditionand composition for the synthesis of zeolite A 4.

Different mixtures of clay and sodium hydroxidesolution were heated around 100"C with constantstirring for periods ranging from 15 min to 6 hr.After each experiment the contents were filtered andthe filtrate analysed for Na20, Si02 and Al203 todetermine the Na20/H20 and Si02/A1203 ratios.

tPresent address : Mis. M. S. Iyengar & Associates,Post Box No. 2817, New Delhi 110060

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The solids were washed, dried at 100°C, equilibratedat 100% relative humidity and then subjected toX-ray, IR, thermal and chemical analysis.An alumino silicate gel (Si02/AI203 mole ratio about

2, ideal for formation of Na-A) was prepared byadding sodium aluminate solution (30 ml) con-taining 2.23 % AI203 to silica sol (25 ml) containing3.06 %Si02with constant stirring at room temperature.The resulting gel was filtered, dried at 100°C andequilibrated at 100 % relative humidity and thensubjected to X-ray, IR, thermal and chemical analysis.

The X-ray patterns were obtained with aPhillip's PW-1010 X-ray diffraction unit fitted withiron filtered Co K~ source. The chemical analysiscarried out following standard procedures. IRspectra, in the range 625-4000 em'? were recordedon Perkin-Elmer spectrophotometer model 237-Bin KBr. Differential thermal analysis (DTA), diff-erential thermogravimetry (DTG) and thermog-ravimetry (TG) of the samples were carried out sim-ultaneously upto 1000°C in a MOM derivatograph.

X-ray analysis of the products reveal that zeoliteNa-A is formed in small amounts only after the reac-tion has progressed for 3 hr. As the reaction proceedsfurther, the solid consists mainly of zeolite Na-Aand after 5 hr traces of zeolite Na-P is also foundto be present. This indicates that maximum con-version of the clay into zeolite Na-A takes place bet-ween 3 hr and 5 hr. Anatase and quartz present inthe clay, however, do not react with sodium hydroxide.

IR spectra (v max. in cm") of the activated clayshows a small band around 3400 assigned to v-OH,a broad band around 1100 due to Si-O and aweak band around 800 due to v-Si-O-Als. Distinctchanges in the nature of IR spectra of the reactionproducts are observed as the reaction between clayand sodium hydroxide proceeds. The '1-0H becomesbroad, increases in intensity and shifts to 3500. Anew band at 1650 due to absorbed water, which isnot shown by the activated clay, appears and becomesmore intense with time. The '1-Si-O at 1100 shiftstowards lower frequency region 1000 cm-' and chan-ges in its shape are also observed. The IR spectrumof aluminosilicate gel is somewhat intermediatebetween the 2 hr and 3 hr reaction product.

The 1100 band is assigned to combination stretchvibrations of Si04-AI04 tetrahedra in alumino-silicates. 6 It has been reported that the frequency ofthe band increases with increasing molecular weightof the silicate species and decreases with the sub-stitution of Al for Si7. It may be argued that theclay reacts with sodium hydroxide to form mixedaluminosilicates similar in nature to aluminoslicategel. With the progress of the reaction, deploymeriz-ation of silicate units in alumino silicate networktakes place causing the shift in the 1100 band to lowerfrequency (lOOOcm-I). The shape of the band remainsbroad and shallow upto 3 hr after which it becomessharp and smooth. After 4 hr of reaction, when the

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INDIAN J. CHEM., VOL. 18A, SEPTEMBER 1979

product is mostly Na-A, sharpness of the bands at3500 and 1000 increases, the number of shouldersdecreases and the spectrum resembles closely that of astandard zeolite Na-A.

DTA curves of an aluminosilicate gel and theproducts collected in the course of zeolite Na-Aformation from' clay, all exhibit low temperatureendotherm starting from room temperature to around400°C, followed by another endotherm around400-600°C (Fig. 1). These endothermic changesare associated with weight loss due to removal ofwater as observed by TG and DTG. The low tem-perature endotherm in the case of aluminosilicategel (curve VI, Fig. 1) is quite sharp than those exhi-bited by the intermediate products formed during thereaction of clay and sodium hydroxide solution.A number of inflections in the low temperature endo-therm starting from room temperature are exhibitedby the products after 1,2 and 3 hr of reaction, whereasthe products after 4 hr of reaction exhibit only oneinflection around 120·C (Fig. 1). It has been reportedearlier" that a particular zeolite exhibits perfectlysmooth endotherm for water loss with a small in-flection at 120°C due to the removal of loosely heldsurface moisture. Additional inflections in thischaracteristic water loss endotherm are due to thepresence of hydrated non-zeolitic material or mixtureof zeolites. The 1,2 and 3 hr reaction products areall amorphous to X-ray and DTA curves exhibit anumber of inflections in addition to that at 120°C.These solids, therefore, are mixtures of hydratednon-zeolitic materials. It could thus be concludedthat the absence of inflections other than the one at120°C in the low temperature endotherms of DTAof the reaction products indicate complete crystalli-zation to zeolite.

From the DTA curves (Fig. 1) it appears that thealuminosilicate gel (curve VI) and the 2-hr reactionproduct of clay and sodium hydroxide solution(curve II) do not exhibit any high temperature exo-thermo A ~ery small exotherm, characteristic of the

1 2 ~ A 5 6 7 8 9' 10.100"C

Fig. 1 - DT A curves of reaction products after 1 hr (I), 2 hr(II), 3 hr (III), 4 hr (IV) and 5 hr (V) [Curve VI of pure

aluminosilicate gel]

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,001

.....-'o~

o-010~

'"--...o~z

b. • •

• •Fig. 2 - Changes in Na.O/H.O ratio (curve a) and SiO.1

AlIOs ratio (curve b) of mother liquor with time'

starting clay is exhibited by the l-hr product (curve I)around 1000°C which disappears after 2-hr of reactiondue to complete dissolution of the clay. This is in agree-ment with the findings from IR data as discussedabove. However, the 3-hr product, exhibits smallexotherms around 850°C which become more promi-nent in the 4- and 5- hr reaction products. These hightemperature exotherms are due to' structural break-down of zeolite crystals and have been shown to beidentical to those exhibited by zeolite AS. Sincezeolite crystal formation has not taken place upto2 hr of reaction, the l-and 2- hr reaction productsdo not exhibit any exotherms. Therefore, theappearance of exotherm in the high temperatureregion around 800°C may be considered as an indic-ation of the begining of crystal formation.

A perusal of Fig. 2 indicates that Na20/H20ratio of the mother liquor decreases steadily upto3 hr and then becomes almost asymptotic. SimilarlySi02/AI20a ratio of the mother liquor gradually dropsuntil 2 hr and then becomes almost constant. TheSi02/AI20a ratio of the solids initially undergo changesand approach a constant value after 3 hr of reaction.As the system tends to be ordered from a disorderedstate with the commencement of crystallization,the Si02/AI20a ratio of the solid attains a constantvalue. This is reflected by X-ray, IR and DTAdata also.

The authors thank Mr. G.' C. Bhattacharya,Mr. A. A. Naqvi and Mr. B. D. Chattoraj for theX-ray results, IR spectra and DTA curves respectively.

References1. ZHDANOV, S. P., Adv. Chern. Ser., 101 (1971), 20.2. FLANIGEN, D. M., Adv. Chern. Ser., 121 (1973), 119.3. BARRER, R. M. & MAINWARING, D. E., J. Chern. Soc.

Dalton Trans., (1972), 1254, 1259.

4. BORTHAKUR, P. C, Synthesis of Zeolite to be used as mole-cular sieves from clay and study of their physicochemicalcharacteristics, Ph. D. Thesis, Gauthati University, 1976.

5. STUBICAN, V. & Roy, R., J. phys. Ch~II1., 65 (1961),1348.

6. FLANIGEN, D. M., Adv. Chem. Ser., 101 (1971), 20J.

7. BEARD, W. C., Adv. Chem. Ser., 121 (1973), 162.8. BoRTHAKUR, P. C, CHATTARAJ, B. D., DUTTA, S. N. &

IYENGAR, M. S., J. appl. Chem. Biotechnol., 23 (1972\,415.

Nature of Water in Phosphorus Hydroxylapatite &Arsenic Hydroxylapatite & Their Solid

Solutions

T. S. B. NARASARAJU*, B. K. ](APOOR & U. S. RA!

Department of Chemistry, Banaras Hindu University,Varanasi 221 005

Received 12 December 1978; accepted 1 March 1979

Tbermoanalytical, infrared and electron microscopic analysesof the samples of hydroxylapatites of phosphorus and arsenic anda series of their solid solutions precipitated from aqueous mediasuggest that the water present in them is exclusively due toadsorption. .

PHOSPHORUS hydroxylapatite[(PHA), Ca10(P04)6

(OH)2l undergoes a series of cationic and anionicexchange reactionsv" significant among the latterbeing the replacement of PO!- by AsO!- to form ar-senic hydroxylapatite [(AsHA), Ca10 (AS04)6 (OH)Jan isomorph of PHA. The mechanism of arsenicpoisoning" in human beings involves such an ex-change at the bone/body-fluid interface leading tothe formation of a solid solution of PHA and AsHA.In order to throw light on the influence of such anexchange on the principal bone processes, a series ofsolid solutions of PHA and AsHA over the entirecompositional range was prepared and charac-terized+". However, the ambiguity=!" associated withthe nature of water present in such samples promptedus to undertake the present investigation.

For TG work a convenient weight (about 0.5 g)of each sample washed with acetone and air-dried andsieved to ~OO(BSS) mesh was taken in a platinumcrucible (20 ml capacity) and heated at a convenientrate (5°/min) up to a maximum of 900°, the tempe-rature being measured with a chromel-alumel ther-mocouple. Differential thermal analysis up to900° was carried out separately on a few representativesamples using an equipment specially fabricated forthe purpose .consisting essentially of a potentiometrictechnique to detect the onset of a phase transfor-mation, ex-alumina being used as the reference sub-stance. The experimental details of IR and electronmicroscopic investigations are reported earlier"!".

Within the temperature range investigated, thedehydration is found to be rapid and uniform up toabout 500°, the weight of the samples remaining vir-

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tually constant on heating up to 900°. The TG curvesare characterized by, the absence of discontinuities.In addition, the maximum per cent mass loss associ-ated with PHA is found to be higher than that forAsHA while the solid solutions show an intermediatebehaviour. The TG curve of a precipitate free fromchemically bound water is characterized by the absenceof discontinuities-". The isobaric temperature-composition curves" of a solid containing non-essential water have been shown to be free' fromdiscontinuities. It is therefore suggested, basedon TG curves, that water associated with thesample is of the non-essential (adsorbed) type13,16,17.

These conclusions have been substantiated by theabsence of peaks in the DTA curves of the samples.IR spectra in nujol (vmax . in em= of a solid sol-ution of PHA and AsHA taken before and afterdehydration exhibited peaks characteristic'v'" ofPO!-(962 and 1040), AsO!-(S60) and OH- ions (3570)in addition to those of nujol. The peaks observed inthe unheated sample at 1670 and 3500-3100 due to thepresence ofwaterW20 disappear in the heated sample.The 'IOH overlapping with the water peak in the range3500-3100 in the unheated sample becomes more dis-tinct in the heated sample due to the elimination ofwater; A similar behaviour is exhibited by other sam-ples also. It could be shown that PHA gives outwater of constitution 21 (hydroxyl water) in accordancewith reaction (I).

Ca10 (PO.MOH)2-'?-2Ca3 (P04)2 + Ca2 P207+ 2CaO+ H20 ...(1)

The onset of reaction (I) can be detected by thepresence of the pyrophosphate beaks (1410, 1121and 917) in the IR as well as by a correspondingchange in the X-ray patterns. The absence of eitherof these indicates that the mass loss in the presentstudies is not due to the removal of such bound water.Thus, the IR studies coupled with the thermoana-lytical investigations show that the water associatedwith the sample is not of an essential (chemicallybound) type but is due to a mere trapping within thebulk of the sample. The total per cent mass loss inthe case of PHA is higher than that in AsHA whilethe solid solutions exhibit an intermediate behaviour.This conclusion may be supplemented by an approxi-mate assessement of the crystal dimensionsv'! of thesamples. The electron micrograph reveals that theaverage crystal dimensions of AsHA are higher thanthose of PHA, the solid solution exhibiting an inter-mediate behaviour .. The tetrahedral covalent radius'"of the As (I .IS A) being slightly. larger t.han that' ofP (1 .10 A) islikely to lead to a dilation in the crystaldimension Of the samples, the conditions of precipi-tation being the same. Such an increase in thecrystal dimensions leading to a decrease in the speci-fic surface area can be supposed to account for adecrease in the uptake of 'water by AsHA as com-pared to PHA.

Authors thank Prof. T. R. Anantharaman, Depart-ment of Metallurgical Engineering and Dr. S. P.Singh, Department of Ceramics Engineering, BanarasHindu University, for electron microscopic andthermoanalytical facilities respectively.

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