THE 1IECHANISM OF T H E DEHYDRATIOX OF ZEOLITES‘ W. 0 . MILLIGAN
ASD
HARRY B. WEISER
Department of Chemistry. The Rice Institute, Houston, Texas
Received June 22, 1957
In recent years calcium sulfate hemihydrate, or plaster of Paris, has been frequently referred to as a zeolite rather than as a definite hydrate. This point of view resulted froin the observations ( a ) that the dehydration isobar was continuous and ( b ) that there was little or no change in crystal structure on dehydration. The present investigators hare shown t h a t calcium sulfate hemihydrate in the form of submicroscopic crystals (15) prepared by dehydration of selenite, or of macrocrystals (14) prepared b y crystallization from a nitric acid solution, is a definite chemical hydrate, giving a stepwise dehydration isobar when care is taken to establish equilibrium at cach temperature, and showing characteristic differences in the x-radiogram before and aftcr dehydration. I n the light of these observationq, it I\ aii belierrd t h a t a similar dehydration and x-ray diffraction study of varioui zeolites might yield information of intercst conccrning the mechanism of their dehydration. The comprehen4ve reports of Hey (4) and of Hey and Bannister ( 5 ) make uniiecesqary more than a brief mention of how water is thought to be bound in zeolites. Hey (4) considers zeolites to be “a n-ell-definetlgroup, generally regarded as hydrated silicate.: of aluminuni and the alkalis or alkalinc mrths,” with the ratio O/(Al Si) = 2. I t is generally considered (12) that their unusual base exchange and dchyclration properties are the rwult of the nature of the crystal lattice, IT-hichconsists (1, 10) of SiO, and .1104anions in a framework containing chaiinelu large enough to hold cations and water molecules. Three general theories have been prop o ~ e dto explain the manner in which n ater is bound in the zeolite : ( a ) Solid solution theory. The original zeolite is assumed t o be a definite hydrate, but upon dehydration the resulting product forms a continuous seriw of solid solutions with the hydrate. Thiq solid solution theory is intended t o reconcile the continuous dehydration iwbar for a hydrate with thc phase rule. ( b ) “T’agabond water” theory. The water nmlwult+ are helieved t o be frce to m o w about in the lattice, and not to occupy definite positions in the
+
Presented a t the Fourteenth Colloid Symposium, held a t Minneapolis, l l i n n e s o h . J u n e 10-12. 1937.
1029 T H E JOC;RS.AL 01. PHYSICAL CHEMISTRY, VOL.
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unit cell. It is not clear frorn this theory whether or not the zeolites are to be regarded as true hydrates. ( e ) Adsorption theory. The water is held by adsorption forces within the capillary-like clinnnels of the crystal lattice. X-ray analysis has qhown (16) that n-ater or the constituents of water may be present in the lattice in three ways: ( a ) H,O groups arranged around another ion as in coordination compoundb like -41Cls.6HzO; ( b ) OH groups as in definite hydroxides; or (c) H20 groups held in lattice channels such as exist in the zeolites. To this enumeration there may be added (2, 6) OH0 bridges such as are found in diaspore, ,YH02 (2). It is generally conceded that thc changes in the x-ray patterns of zeolites before and after dehydration may be very small. Some writers cite this evidence to show that the lattice changes, and that the water molecules occupy a definite position within the unit cell. On the other hand, other writers use these same facts to shov hov non-essential the mater molecules are to the lattice. These changes, large or small, will be considered in further detail in later paragraphs. I t is the purpose of this paper to report the results of a dehydration and x-ray diffraction study of the following zeolites: scolecite, stilbite, heulandite, thomsonite, analcite, natrolite, mesolite, and chabazite. EXPERIMESTAL
Preparation of samples Selected samples of the various zeolites t o be studied were ground in an agate mortar t o approximately 200 mesh. The ground samples were exposed t o the water vapor in the atmosphere for several days, and therefore could be weighed without any special precautions to avoid loss or gain of water during the weighing.
Isobaric dehydration The dehydration isobars were obtained by methods already described (13, 15). Care was taken t o allow the establishment of equilibrium at each temperature point of the isobar. To establish some points, as long as 1200 hours were required. The isobaric dehydration data for the several samples are given in table I, and are shown graphically in figures 1 t o 8 inclusive.
X-ray examhutton Samples for x-ray diffraction analysis of each zeolite were taken a t various temperatures from separate portions which had been treated in the same way and a t the same time as the weighed samples for the dehydration isobars. -it all temperatures the samples were sealed off according to a method similar to one already described in detail (15). When the
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DEHTDRATIOS O F ZEOLITES
-.
.t
X
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W. 0. MILLIGAN A S D H A R R Y B . W E I S E R
zeolite was in equilibrium, as evidenced by t h e constancy of w i g h t a t a definite temperature and pressure, a small portion \vas placed, itill hot, in a preheated Llarli tube (a thin capillary tube of Liiideniann glass). The BIn1.1; tube w . 6 then placed beside t h e sample bottle and heated a t the same temperature for a t least twenty-four more hours in the event t h a t there had been some displacement in the equilibrium during the transfer; t h e tube was then carefully sealed off while still hot. X-ray diffraction patterns w r c obtained for the various samplw using Cu li, x-radiation from a Philips tube; the camera diameter was about 5 i mni. Diagrammatic representations of the results of the x-ray analyses are given in figures 1 to 8 inclusive.
SAMPLE 6 SAMPLE C
0 REHYDRATION A 2ndDEHYDRATION x DEHVDRATJON 0 DEHYDRATION
v?
0
200
300
400
TEMPERATURE DEGREES C
FIG. 1. Dehydration isobar and x-ray diffraction patterns for scolecltc DISCUSSION
Scoleczte The acccptcd composition oi scolecite is CaA12SisOlo.3H20, b u t there is no conclusive vvidrnce as t o whether or not this zeolite is a definite hydrate. Pauling (8) and Taylor, Meek, and Jackson (11) concludcd from x-ray data that the water is a constituent part of the 1attic.e. Rinne (9) obserl-cd alterations in the x-radiogram upon dehydration; on the other hand Kelly, Jenny, and Brown ( 7 ) obtained a smooth dehydration isobar. Thc iqobar hhown in figure 1 indicate, quite clearly that a stepnise curve is ol)tained for scolecite n-hen caw is taken to maintain a constant aqueous vapw premirc and sufficimt timc is alloivcd for the establishment of equilibrium at each point. T h e break< in the isobar fur the tri- and dih y d r a h of hcolecite are quitt’ ns h a r p as for most hydrates. The xradiograms, also sho~i-nin figure 1, lead to the same conclusion. The
DEHYDRATIOK OF ZEOLITES
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differences between the x-radiograms of the tri- and di-hydrates are est r e n d y small. If it is assumed that the crystal nierclly loses one molecule of water from the channels in the lattice, little alteration n-odd be expected to take placP either in the positions of the lines or iii their inteneiticq. I t should be rccalled that the changes in the s-radiogram-: of calcimn sulfatr liemihydrate before and d t c r dehydration are not marlicd, although there are characteristic c h a n p IT hich ran he e a d y dctrctcd. -1fter clehydratioii at 400"C'., tlic -c.i)lcc~itr~ lattice i q completely 111*olte11don-11
FIG.2. Dehydration isobar and x-ray diffraction patterns for stilbite
Stilbate The dehydration isobar (figlire 2) for stilbite shon 5 no indicatiori that this zeolite is a definite hydrate. The break in the r w v e is indicativr of the formation of a second modification of stilbite (similar to the two modifications of heulandite, previously rccognized). I t will be noted that the break in the c u r w is riot rever-ible. Upon heating stilbite t o around 100120°C , the x-radiogram changes quite markedly; rehydration does not again alter the x-ray diffraction pattern. After thc second modification of stilbite is formed, thc dehydration-rehydration curve is reversible. Heating to arouiid 400°C. breaks d0n.n the lattice.
He I ( 1a:ditf The dehydration isobars and x-ray diffraction p a t t m x for heulandite, given in figure 3, indicate that this zeolite exists in at least t h r w modifica-
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W . 0. XILLIGAK A S D HARRY B. KEISER
tion.. ‘L’hc, trailsition at 20U-22O”C. i.; n-ell knou 11 (4)!but the tran*itioii at 100-120°C. has not berii previously recognized, t o the knowlcdge of thc present investigators. The shape of the isobar could be explained by hydrate formation if the breaks had occurred at definite ratios of n-atcr to anhydrous mineral. K c a r e of the opinion that heulandite i q not a tl&nitc hydrate. Tlzomsomlc ‘L‘lic, isobars for thom.onitc given in figure 4 .ho\\- 110 indicatioii of 113’dratc formation, and the accompanying s-radiograms show only :I iliglit latticc contraction. The data indicate that thoiiisonite is not a dcfinitc. hydrate, hut that the n-ater i. held by adsorption forces \Tithin thr channcls
SAMPLE 5.
0
100
Y
@E>VDRkTICI.I
1
I
200
300
TEMPERATURE DEGREES
I
400
C
FIG.3. Dehydration i b o b n i a n d x-ray diffi,icti(~np:ittelns for hculnndlte
in t h e lattice. It should b e nicrltioned that Hey (5) obtained a contin~io~is isobar for the usual T-ariPtj- of thomsonitc, Init oli-crvcd a break in t h c isobar for a spccial wmplc. ‘Thc lircak n-as attributrd t o the formation o f a second modification of thonihonitP. Sin( P thc iample exainincd by 11.c g a w no lircak in ?hc iwhnr, niir reqiiltc throw 1x0 light on thiq qiicqtioii. -1tcnlcztc
.ln:ilcitc i i usually cmiwlcrcd to 3a;\1Si2O6 H20. Grmicr (3), I\ 110 obw\-cd a slight lattice -iiriiikagc and a 4ight alteration in inten4tiec after dc>hydration, wggest5 that the n-utcr I < (d “vagabond TT ater,” ( b ) contninccl in a larger lattire C O I ~ I ~ ~ F ‘ or X , ( e ) a(l+orbctl. His s u g arc apparently hased on thc €act that t h t n a t r r ha. I i t t l p iiifluencc on the
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DEHYDR-4TIOX O F ZEOLITES
lattice dimensions. The dehydration isobar given in figure 5 shon-s 110 indication of the formation of a hydrate. It is apparent that the analcite
SAClPLE 3
A
DEHYDRAT'OV
1
12 -
1
1
-
'\ \
I
O II=
$
TCM 1 1
,,7-
, I
12
g4 z
I
v)
3
6-
LfGE/.S SAMPLE A
0 DEHYDWTlOh 0 REYYDRAXON A Z o d DEHYORATlO\ x OErYCiRATlON
SAMPLE 8
8-
I
Trc. 5. Dehydration isobar and x-rny diffraction pattern> foi analcite
isobar is not completely reversible until after the first dehydration. If it should be argued that the original sample is a definite monohydrate, then the same evidence ~ o u l require d the sample upon rehydration likewise
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W. 0. MILLIGAN AND HARRY B. WEISER
t o 1 )e it drfinitc. hydratc having the approxiniatc composition Xa-41Si206.0.9HzO. Since thiy i* iniprobable, onr mu+t c.oncIiide either that analvitt i. not a true hydratc or that the. .triicturc i h hascd on a lattice much niorc complex than that of the other ztolite.. In the abccncc of coi~clu~ive evidtiiccl, t h t prtiviit author< prc>fcr t o ahqiiiiie that the water in analcite is held by adm-ption forws in thc. cl~nniieliwithin t h r lattice, and that the irreversibility is the r e d t of thr> heat trcatmrnt. If thc analcite had been heated to a higher tenip(ntiir(', q i i c ~ has 4O0-50O0c'., a niricli maller aniount of water would he taken up 011 rc,hydration. It waq observed that the lattice shrinks only a littlc on heating froin 30" t o 230°C. or even to 400"C., in gciioral agrcernciit with Griincr. Heating t o 2% mucli higher trmpcrature ~voiild1w exprrted t o 1)rcdi tlorvn the lattirc ( w ~ i plt~tciy.
Y\KD-L
3
r-l
A.
SAMPLE B
0 DTHVGRATION 0 REHYDRATION A Znd DEHVGRATION x DEHYDRATION
z
12
0
100
200
300
400
TEMPERATURE. DEGREES C
FIG.6. Dphyc1r:itioii isobar and x-ray diffrncticin p:itttms for iirttrolit,r Satldltc '1'11(> forinula oi nntrolitc, i:, iimally n ritteii Sa~A412Si101,, .2H&. In confirmation of thib, thr rrwlts of iqobaric dehydration and x-ray diffraction cwmiinatioli (figure 6) *how clearly that natrolite iq a dihydratt which de('omposesabove 160-180°C. to form a nionoliydratc, *table to around 250°C. The x-radiogram *bows :I slight but significant i.hange when the first n-~oleculrof water is loqt, and a pronounced change when the monohydrate break.: down.
N l e , E O l ? fe
Xesolite is thought to be a mixture or a solid solution of varying amounts of scolecite, CaAlzSi&,.3HzO, and natrolite, XazA12Si3010.2HzO (3, 20).
DEHYDRATION O F ZEOLITES
1037
X-radiograms of these three zeolites are very niuch alike for short exposiires, and shon- oiily siiiall tliffereiices on largcr exposures. Thc shape of the de!iydration isobar (figure i )is huch a5 might hc expwted froin a solid solutioii of ccolrcite and iiatrolitt. The first break n.ould eorrespoiitl t o thP dehydration of the scolecite trihytlrate aiid the natrolite dihydrate to forin thc reqiectivc lower liyclratcs. The srconci break, which is not so pronounced, v-oxicl be iiitcrliretcd as t!ie point of decomposition of the iuixtuit~ of loo-cr 1iytir:ites t o thc anhydrou. riiatt’rial. T l i ~i-radiograi:ls for saniples at 30°C‘. and c!chyc!rstcti to 230°C. ehoir a .;light alteration siiiiilnr to thow o b t a i n 4 n licii w i l ~ . i ~ i trihytlratc to drcoiilpo to thc clihydrat c‘. Hcating illP5olit? to 400°C’. break- down the 1attic.r caonipletely.
x
z 28 -
s
U G & VD SAMDLE A. 0 DEHgDRATION 0 RCHVDRATION A 2d DEHYDRATION
12 - SAMPLE 8. 0
X
100
DEHVDRATION
200 300 400 TEMPERATURE, DEGREES C.
FIG.7 . Dehydration isobar and x-ray diffraction patterns for mesolitp
Chabazitc T h e dehydration isobar for chabazite given in figure 8 is a typical desorption curve showing no indication that n hydrate is formed. ?‘herr is sonic evidence of hysteresis at temperatures around 100-150°C. The diagrams of the x-ray diffraction patterns, also given in figure 8, ~ h o \ \that t h r latticr shrinks slightly upon dehydration.
7’h deli ~ d r c r foin mecii a t i ism Thr results of isobaric dclipdration iridicatc that : ( a ) icoiecite itud natrolite arc definite hydrates; ( b ) iiitsolitt is not a hydratr but SL niixturr of solid solution of qcolecite aiid natrolitP; (c) stilbite, heiilaiidite, thonisonitc, analcite, aiid chabazitr arc not hydratea, h i t hold water by adsorption forccs within rhaiiiiels in the lattice; (d qtilbite aiid hrulanditv cxist in iiiore than oiic c.ry.;talline modification. l’heqc results do not dis-
1038
W . 0 . MILLIGAX .4SD H A R R Y B. FI’EISEI?
agree with the general view that tlic zeolitc structure consists of a franiework of A 1 0 4 and Si04 anions p o w s i n g clhannels large rlnough to contain sodium or c~alciiiiiications, watcr molecule~s,or small molwules of various organic liqiiids. It is apparent that the binding forcc‘: d i f h from zeolite t o zoolitc. Thus scolecite and natrolite hold the I\ atcr by definite chemical forces, as does calcium siilfatr~ Iicniihydratc. 11 licrcas other zeolites, such :is analcite, stilbite, hculanditr, niid chabazitr, h(J1d the watcr molecule< by adsorption forces. Sincr s w h zeolites as sco1r.de and natrolite give Stqvr iw dehydration isobars and ~ o n i csignificant ~ c>li:ingesin the x-ray patt ern‘: ~ i p o ndeliydration, thcrc ;;reins t o be no rea
