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Thermal Stability of the Stilbite-Type Framework: Crystal Structure of the Dehydrated Sodium/Ammonium Exchange Form

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Thermal Stability of the Stilbite-Type Framework: Crystal Structure of the Dehydrated Sodium/Ammonium Exchange Form American Mineralogist, Volume 6E, pages 414419, 1983 Thermal stability of the stilbite-type framework: crystal structure of the dehydratedsodium/ammonium exchange form Wrlrnreo J. Monrren Katholieke Universiteit Le uven Centrum voor Oppervlaktescheikundeen Colloildale Scheikunde de Croylaan 42 8-3030Leuven (Heverlee),Belgium Abstract O-T bridges between them occurs. The cations are located at the six-ring site and the flat eight-ring site, and residual water moleculesat the boat-shapedeight-ring. The framework Oistortionsare cation-induced and readily affect the cuboid polyhedron of the framework which is formed by joining adjacent structural units' Introduction protons, but also cations such as K+ and Rb* with a sufficiently low ionic potential (Passaglia, 1980) may Stilbite-type minerals are zeolites with a two-dimen- "stabilize" the stilbite-type framework. The K and Rb sional interconnected channel system. Ten-membered forms do not undergo appreciable contraction and their ringsof (Si, Al) Oatetrahedra (free diametet4.1 x 6.24, destruction occurs only at fusion, i.e., at about 10fi)"C. Meier and Olson, 1978)Iimit the pore size of the largest As the dehydrated Ca-form exhibits a smaller fraction channels. These intersect with smaller ch-annelswith of broken T-O-T bonds (Alberti et al', 1978)than the eight-ring apertures(free diameter 2.7 x 5.7A, Meier and dehydratedNa-form (Alberti et al.,1978), the number of Olson, 1978). The inner surface should therefore be cations might be an important parameter. A further readily accessibleto small molecules and these minerals reduction of the number of cations is attempted here in could have a potential use as molecular sieves or cata- the study of a dehydrated Na-H-stilbite form. It will be lysts. This would require an "activation" at elevated shown that the framework-contraction and the frame- temperatures to remove the adsorbed water molecules' work-destruction steps can be separated. Unfortunately, in the presenceof exchangeablecations, and the framework is de- stilbite contracts on heating Methods and results stroyed (Passaglia,1980). Contraction and destruction are related to the attractive force of the extra-framework Crystals of natural stilbite (STI) from the Faroe Islands cations. When a crystal of an isostructural phase, Na- (Denmark; Virginia Polytechnic Institute and State Uni- barrerite is heatedto 250'C (Alberti and Vezzalini,1978), versity Samplenumber B179)were exchangedin a lN 1: I the framework is very much distorted and T-O-T bonds NaCl:NHa Cl solution for five months. The exchange are broken. A similar behavior, but with a much smaller solution was renewed three times during this period' fraction of broken T-G-T bonds, occurs in dehydrated Electron microprobe analyseswere made on crystals of stellerite, the Ca variety of stilbite (Alberti et al., 1978). the same batch. (The analysis was made at the Depart- Jacobs et al., (1979)characterized the hydrogen form ment of GeophysicalSciences, The University of Chicago as the most stable. This was confirmed by a structural under the following conditions: solid-state detector, study of the mineral after ammonium exchange and Reed-Ware data reduction, beam current:I00 nA, no dehydration (Pearceet al.,1980). Only very minor differ- corrections for water or Na loss, totalwt'Vo between 80.2 enceswere found between the framework parametersof and 82.3). Calibration factors for the different elements the hydrated form (Slaughter, 1970;Galli, 1971) and those were determined using Anzoglass for Ca, Si, Na and Al' of the dehydrated hydrogen form. By a reduction of the and asbestos microcline for K. Taken strictly at face cation-framework interactions, which become progres- value, when calculatedto 72 oxygens with sufficient NHn sively pronounced as the water ligands are removed, a addedto balancethe charges,the unit cell of Na-NHaSTI collapse of the framework can be prevented. Not only contains before dehydration: 0003-0M)083/0304-04l 4$02. 00 MORTIER: STILBITE FRAMEWORK: DEHYDRATED SODIUMIAMMONIUM FORM 415 Table l. Positional parameters,population factors and anisotropic thermal parametersfor d-Na-H-STI Aton Equi- Popula- x y z 'l Bzz A Btz -13 -23 point tion I r(l) 8j 7.r3(3) o.99309(10) o. t9272(7) o.25505(t2' 327(tt) l40(4) 4s4(r5) lo(5) 272(to) t2(6) r (2) 8j 7. l e(3) 0,261l7(lo) o.30682(7) o,2s557(ll) 235(lo) r39(4) 430(15) l (s) 177(to) -l I (5) r (3) 8j8 0.2oooo(lo) o. o8843(6) 0.499s4( | 2) 465(f0) 98(3) 769(t5) -17(5) 389(lo) - 2(6) r (4) 8j8 o. l l36s(9) o.3l 659(6) o,5oo45(tl) 334(9> r l8(3) 609(13) -r2(s) 3o7(e) - 5(6) r (s) 494 o 0.2soo7(9) o 348(13) l9o(5) 497(t82' o 248(13) o o(r) 8j8 o.9804(3) o. t987(2) o. rors(3) loo(3) 27(t) roo(4) -r8(2) 80(3) -r4(2) o(2) 8j8 o. l 203(2) o,3o2r(2) o. lo5o(3) 42(2' 26(r) 95(4) - 4(r) l8(3) t2(2) o(3) 8j8 0. 06l 7(3) o.2668(2) o.35s7 (3) r I r (4) 27(r) 108(s) -r3(2) 73(4' -2t(2) o(4) 8j8 0.0693(3) 0.l r89(2) o.35oo(3) 83(3) 25(r) 100(4) r2(2) 4e(3) t5(2) 0(5) 8j8 o,2927(3) o.2525(2' o.3s3s(3) 80(3) 26(r) 106(s) 8(2) 36(3) 2r(2) 0(6) 8j8 0.2812(3) o.3790(2) 0.3s21(3) 88(3) 26(t) loo(4) - 3(2) 54(3) -r1(2) o(7) 8j8 o.3513(3) 0.3l 23(2) o. 2os4(3) 73(3) sl (2) l4o(s) -r2(2) 7o(4) -23(2) o(8) 8j8 0,3 182 (3) o.l l03(2) o.4996(3) 68(3) re(l) 143(5) - 6(r) 7l(3) - 2(2' o(9) 4i4 o.l87r(4) o o.so28(5) 83(5) rs(l) t44(7) o 76(5) o o(lo) 4h4 o o.3460(2) v2 s7(4' 25(t) t32(7' o 65(4) 0 Na(l) 8j l.4s(4) o.2323(r2) o.2780(7) -0.0339( 1 4) 6. I (4) ro.tr Na(2) 2d t.74(4' o tl2 tl2 518(28) 65(5) 7os(38) o 361(27' o (3) Na 4i. 2,78(4' o.3860(7 ) t/2 o.2702(8) 239(ll) 6s(3) 395(r7) o r98(ll) o r(l)' 8j 0.62(3) 0.0960( I 2) 0.2o39(8) 0.2918(14) 2.O(4)'.tr'. r(2)' 8j o.s3(3) 0.1936(13) o.2955(8) 0.2954(r5) 1.2(4)t"'r' ll estinated standard errors i.n parentheges refer to the la€t disit: :i:r anisotropic ther@l par@etere of T(l) to T(5) x lO5, otherslx iO4; t3r.:3 themal par&eters of Na(l), T(l)t and T(2)' isotro;ic : D/i2. (NHa)o76Na2 a5K6.15Caa s6Ale asSi26.52072. xH2O density at each of the T(l) and T(2) sites was split into separatepeaks, T(l) and T(l)', andT(2)and T(2)' respec- This compares with the original sample composition tively, and their population factors were allowed to vary (Pearceet al.,1980) of independently.The T(l) + T(l)' and T(2) + T(2)' popula- Na15K6 1Caa.1Ale 6Si26.aO72 . xH2O tions sum to7.75 and7.72respectively, instead of 8. This deficiency is significant and might be an indication of partial A singlecrystal of NaNHaSTI0.15 x 0.3 x 0.3 mm3in dealumination, as it frequenfly occurs during heat treatment was size selectedand mounted in a glass capillary. After of NHa-exchangedzeolites (McDaniel and Maher, 1976). : evacuation of the capillary at room temperature (P The residual electron density- at the completion of the x 0.013 10-6bar), the temperaturewas raisedto 383 K refinementdid not exceed|elA'wilh the exceptionof pgaks over a 3 hour period and maintained for 18 hours, near the centersof Na(2) and 2elA) and Na(l) (1.6elA). A subsequentlyraised to 603K over a2Vzhour period and previous anisotropicrefinement of Na(l), with its position after another 2 Vzhour period at 603 K, sealed off and fixed at the center of the six-ring, had not converged and cooled to room temperaturefor data collection. yielded very elongatedthermal ellipsoids.While a position As derived from Weissenbergphotographs, the system- displacedfrom the centerconverged ifrefined isotropically, atic absenceswere consistentwith the spacegroup C2lm. the residual electron density at the center of the peak might graphite-monochromatized Using MoKa radiation, 5158 also be an indication of a mixed occupancy. This latter reflections were measuredon a Syntex P21single crystal could also be true for the eight-ring site Na(2). Residual : diffractometer in the omega scan mode up to sin0/)t water molecules,K3* or Ca2* ions or even Al3* extracted yielding 0.65, 2521independent reflections ofwhich 1793 from the framework could also be locatedat thesesites (see were larger than 3o1after averagingand were considered discussion).Although the T(l) and T(2) sites are split, no as observed. The unit cell parameterswere obtained by indicationwas found for split oxygen sites(see also discus- least-squaresrefinement of 25 intense reflections having sion). 20 : - yieldin : : = 21" 33', g a 13.57l(4), b 18.264(2),6 Atomic scatteringfactors (International Tables X- : for 11.323(4)Aand B 126.96(2). Data reduction was per- ray Crystallography,vol. lil, 1968)for Na+, Ot-, Al2* formed using the X-ray 76 system (Stewart et al., 1976).
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