CRYSTAL STRUCTURE OF HYDRATED PARTIALLY AND COMPLETELY NHI.

EXCHANGED FORMS OF STILBITE

A. ALBERTI'. A. MARTUCCI

"

M. SACERDOTI'. S. QUARTIERIî, c. VEZZALINI Î, p

CIAMBELLI !. M. RAPACCIUOLO $

' Istituto di Mineralogia. Universitv of Ferrara, Fenara, Italy; alb@dns.unife.it; Fax: +39-532-

293760-

Dipartimento di Scienze della Tena, University of Modena. Modena, ItalyI Dipartimento di Ingegneria Chimica e Alimentare, University of Salemo, Fisciano (Sa), Italy

.{BSTRACTCrystal structure refinements of a hydrated partially NH1-exchanged stilbite and of a

hrdrated completely NHl-exchanged stilbite rvere carried out on X-ray diffraction data olsinglecn'stals. BottL NHa-exchanged phases shou a strong tendency iionr'the monoclinic real F27ms\mmetry tow:uds the Fmmm topological symmetry. Unit cell volume increases by about l%*hencompared with the untreated phase. An analogous behaviour has been reported in literatureaÌso lbr dehydrated NH4-exchanged stilbite and hydrated NHl-exchanged banerite. Residual.ations occupy the same sites as in the untreated phase. Many extraframework sites, all withpartial occupancy, have been located.

'fhe correspondence of many of these sites with those

\rccupied by extrafìamework ions in untreated stilbite suggests that, to some extent, a memory ofthe initial location of the extraframework atoms remains. A reasonable attribution of these sitestrr NHr and H2O molecules has been reported.

I\TRODUCTION

This work is part ofa project rvhose purpose is to localize Bronsted and Lcrvis acid sites

rn natural or synthetic zeoliîes by diffractometric methods (X-ray and neutrons, on single crystals

.ind powders). The aim ofthis project is to study not only H-zeolites. but also their precursors

,,htained by cationic exchange with NH r.

This paper is to study the crystal stnrcture of hydrated forms of stilbite, partially and

romplelely exchanged with NH4. Structural studies of the dehydrated Na,î{n exchanged form

.rnd of the dehydrated NHa-exchanged form of stilbite were carried out by Mortier [ ] and Pearce

-' t al . [2]. respecîively.

Stilbite-type minerals are common zeolites with a two-dimensional interconnected

;hannel system. Ten-membered rings of tetrahedra intersect smaller channels with eight-ring

rpertures. The channel system is readily accessible to small molecules. so that these minerals

234512lh Inlernational Zeolile Conference o |999 Malerials Research Societv

could have a potential use as molecular sieves or catalysîs. The fundamental unit is the 4-4-l-1,

which consists of four 5-membered rings of tetrahedra and two 4-rings. These units share a

tetrahedral vertex to form chains parallel to the c direction. These chains are joined laterally by

T-O-T bridges to form dense silicate layers parallel to (010).

The framework topology [STI] is orthorhombic Fmmm. This is also the real symmetry in

stellerite [3], which is the Ca member of stilbite type minerals, whereas in barrerite, the Na-

member. the real symmetry is orthorhombic Amma [4]. In stilbite, the (Ca, Na)-member, the real

symmetrv is monoclinic Fl2lm1. This non-standard space group is commonly used for stilbite to

facilitate comparison between the related structures: stellerite. barreriîe and stilbite. These

different space groups are therefore related to differences in chemistrv which result in different

distributions of the extraframework cations, which in tum impose rotation displacements within

the frameu'ork , in barrerite and stilbite, with consequent lowering of the symmetry [3,4].

EXPERIMENTAL

This studl' was canied out on single crystals of stilbite from Poona (lndia). Their NHo-

forms were prepared by exchanging natural zeoìiîe with aqueous solution olNH,,NO.. 1'ypically,

30 mg of zeol i te were exchanged with 150 ml of 0. I M NH.NOì solut ion at 70o for about 24

hours to obtain a partially exchanged stilbite (h-CaNHq-S from now on) and for 125 hours to

obtain a completely exchanged stilbite (h-Nru-S). The samples were then washed with about 2 lt

of bidistilled water, filtered, and dried ovemight at 120'C. Electron microprobe analyses made

on some crystals of the two batches showed a partial and an almost complete NHa exchange,

respectively.

The composition of NH4-exchanged forms of stilbite, determined on the same crystals

used for the X-ray data collection by an ARL-SEMQ electron microprobe (wavelength dispersive

mode), is reported in Table L Diffraction data were collected on a Siemens XP-l8RA four circle

diffraclometer using a rotating anode X-ra;'' generator (MoKo radiation) in the co-20 scan mode

(2'<e<35"). Atomic scattering factors tbr neutral atoms were used; curves obtained by

interpolating the Si and Al percentages from chemical analysis were used for T framework sites.

Water molecules and extraframework ion occupancies were refined using a combination of three-

dimensional electron density synthesis and full matrix least squares techniques. An absorption

corection was performed using the DIFABS method proposed by Walker and Stuart [5].

Experimental details for the data collection and structure refinements are given in Table L Table

Table I crvstal data. data collection, and refinemenr parameters for h-caNFla-S and h-NFlr-s

h-CaNtlr-S h-NFL.SCrystal composition

Space groupa (A)b (A)c (A)

Bv (43)Measured refl.lndependent refl.KC l t . w t t n . f o> JohoNo of parametersFinal R o/o

tr inal Rwr 7oNo ofelectrons ofextrafr . cat ionsFrom structure refi nementFrom chemical analysisNo of NFL and H2O moleculesFrom structure refinement

INFL)rr c(Nao TeKo raCa::q)(AIre ToSirr 2s)Oì41 42.3H20F2lmr 3.669(5)l 8.26e(5)17.892(3)8e.87(2)446893 l3505ó3 8012166.2I 3 . 6

46ó0

(NFJ1)rr o(Nao orKo o:Cao n:)(Alrq seSis: r : )Or44 36.6HrOF2/mt3.632(s)r 8 .23s(5)l 7.896(3)8e.56(2)4448706945873 5 7 82t6ó .014.4

l 9t 9

43.054.2F ronr chemical analvsis

50.9) Ò . 1

2 reports the coordinates of atoms. their occupancies and equivalent temperature fàctors;rnteratomic distances are given in Table 3.

DISCUSSION

Pearce et al.[2] and Mortier Il], who respectively studied a dehydrated NH.-exchanged stilbiteand a dehydrated Na,trlHo exchanged forms of stilbite from Faeroe Islands fbund that thesecxchanged forms maintain the original F2,tm symmetry, and show unit cell parameters

pafamerers (a=13.583 A. b :18 .156, c=18.007, B=89.68 . [2 ] ; a=13,57 t A .a- . f t .20q. c=18.095.

F=89.8ó' i l l ) . r 'ery similar ro rhose of rhe natual one (a=13.625 A, u=ta.zs:, c=|7.g05.

B=89. l0' [6]). This implies that the interaction of residual Na and NHo molecules with thefiamework is quite weak, and does not cause the collapse of the structure as, on the contrary,occurs in non-exchanged zeolites with stilbite tlpe-framework when dehydrated [7-9]. Inh1'drated NHo-exchanged forms of stilbites, as well as in hydrated NHo-exchanged banerite [10],the unit cell parameters are slightly larger than those ofthe natural ones, with an increase ofthe

unit cell volume of about l%.

Table 2 Atomic coordinales, occupancy, and thermal pararîeters for h-CaNHcS and h-NHcS.

h-CaNH,-S h.NH.-S

Alom xJ, y/b

S r l 0 1 8 5 3 ( l ) 0 1 0 7 5 ( l )

S r l l 0 ó 1 3 3 ( l ) 0 3 0 7 1 ( l )

S r - 1 0 1 9 8 8 ( l ) 0 0 8 9 : ( r )

S r J 0 n 1 2 ( l ) 0 I 6 2 ( l )

S r 5 0 2 5 0 0 ( 0 ) 0 2 5 1 5 ( l )

o r 0 1 1 5 0 ( t ) 0 1 0 5 8 ( r )

o ì r 0 6 8 0 2 ( 2 ) 0 1 0 1 8 ( r )

o . i 0 1 7 2 5 ( 2 ) 0 2 l r ó ( l )

( ) r l 0 ó 2 7 0 ( 2 ) 0 2 1 2 8 ( l )

0 1 0 . ì 5 ? 7 ( 2 ) 0 1 7 9 0 ( l )

o . 1 r 0 ó 1 4 8 ( 2 ) 0 3 8 1 5 ( l i

( )7 0 .1979(2) 0_1161( l )

( ) 8 0 , ì 1 5 9 ( ? ) 0 i l 1 ó ( l l

oe 0 .1917(2) 00000

( ) r 0 0 0 0 0 0 0 1 5 0 1 ( 2 ì

C a 0 1 9 7 1 ( 1 0 ) 0 0 0 0 ( )

N a l 0 7 1 5 ( l ì 0 0 6 ó ( 2 )

w l r 0 4 l l ( 2 ) 0 1 i l ( 2 )

w l p 0 6 0 4 ( 6 ) 0 1 0 0 ( 1 )

w 2 . 0 4 9 7 ( r ) 0 I ] 3 0 ( ó )

w ] . 0 1 2 1 i 1 ) 0 0 0 0 0

\ ! . ìp 05ó l ) ( - l ) 00000

\ \ 5 . 0 1 9 8 S ( r ) 0 5 0 0 0

\ À 6 ' 0 1 6 0 ( 2 ) 0 5 0 0 0

$ 6 p 0 6 l l ( l ) 0 5 0 0 0

w8' 0.5.15(l) 0 5000

w r o 0 5 r r ( l ) 0 0 0 0 0

\ \ ' l r 0 4 1 9 ( 9 ) 0 0 0 0 0

\ \ l l p 0 5 2 7 ( 9 ) 0 0 0 0 0

w l l 0 5 { l ( 2 ) 0 r 2 1 ( t )

w D 0 1 r 4 ( 3 ) 0 s 0 0 0

N l 0 1 1 0 ( r ) 0 0 7 s ( r )

N lp 0 óó4(2) 0 .067( l )

N 2 0 i 0 5 ( 4 ) 0 0 8 3 ( 4 )

N3 0297(3) 00ó2( r )

zJc Occ. tìeq or [Ì iso'

0 . 3 7 1 7 ( t t I 0 0 0 1 1 ? ( t )

0 1 7 5 ó ( r ) l 0 0 0 1 s 3 ( r )

0 { 9 8 7 ( l r I 0 0 0 1 5 5 ( r )

0 5 0 0 2 ( r ) l 0 o O l d r ( r )

0 2 5 0 0 ( 0 ) r 0 0 0 1 8 7 ( 2 )

0 1 0 3 1 ( l ) l 0 0 0 3 5 0 ( 2 )

0 1 9 9 1 ( t ) l 0 0 0 3 1 5 ( ì )

0 . r 2 4 2 ( r ) I 0 0 0 j r E ( 5 )

0 . 1 2 . 1 9 ( l ) ì 0 0 0 1 5 . 1 ( 5 )

0 4 2 6 8 ( r ) I 0 0 0 l s ó ( 6 )

0 4 2 t 7 ( t ) I 0 0 0 1 8 r ( 5 )

0 1 1 8 0 ( l ) r 0 0 0 2 q 6 ( 1 )

0 4 q q 5 1 t ) l 0 0 0 1 7 ( 5 )

0 5 0 0 6 ( 2 ) I 0 0 0 1 5 . 1 ( 5 1

0 50000 I 0 r:) 0:59{ I )

0 2 9 l l ( 3 ) 0 2 1 0 0 5 5 9 ( l l

0 28.1(2) 0 05 0 06818)

0 1 0 7 ( 2 ) 0 1 8 0 1 0 3 ( 8 )

0 . 1 1 6 ( 2 ) 0 l l 0 0 ? 7 1 8 )

0 1060(5) 0 13 0 061(2)

0 115(?) 0 l , l 0 l16(9)

0 1 0 t ( 2 ) 0 t 2 0 l i ( l )

0 ì826(6) 0 .15 0 r . )ó r ( : )

0 l 0 l ( 2 ) 0 : 8 0 l l 6 1 s )

0 l l 9 ( 2 ) i ) 2 6 0 0 ó 6 ( 5 )

0 l l7 (2 ) 0 l1 0 0ó0(7)

0 t24(2) 0 28 0 074(ó)

0 5 r3 (8) 0 12 0 .20( .1 )

0 5 2 8 ( 7 ) 0 l 1 0 1 6 ( 1 ,

0 1053(9) 0 19 0 04{ ( l )

0 328(2) 0 11 0 05 . r (7 )

0 1 1 , 1 ( l ) 0 4 0 0 1 0 0 ( . 1 )

0 3 . 1 2 ( l ) 0 1 3 0 0 9 8 ( 5 )

0270(3) 001 0 .064( r2)

0 291(2) 0 15 0 095(9)

\ /e t /b

0 - 1 8 5 0 ( l ) 0 1 0 7 ? ( l )

0 6 1 1 0 ( l ) 0 l 0 7 5 t l )

0 1 9 8 ó ( t ) 0 0 8 q 1 ( l )

0 l l l 5 ( l l 0 l l 5 ó ( l )

0 2 5 0 0 ( 0 ) 0 1 5 1 5 ( l )

0 i l 5 - l { 2 ì 0 , ì ( ) 6 ó { l )

l ) 6 1 8 l 1 l ) 0 l f ì : 0 t l )

0 ì 7 2 1 1 2 ) 0 l l l 6 l

0 6 2 7 5 1 2 ) 0 l l l r r I )

0 1 5 8 7 ( 2 ) 0 . 1 7 8 6 ( l I

0 ó - 1 - ì 7 ( 2 t 0 l l l e i I ,

0 { 9 7 2 ( 2 ) 0 - l l ó 6 ( 1 ,

0 l l 5 ' l ( l ì r r l l { l ( l l

0 r 9 r 1 ( 2 ) 0 5 0 { r )

0 5 0 0 0 0 l l q 6 i l j

0 199(l) 0 00ri0

0 7 1 6 i 2 ) 0 0 1 1 1 6 |

0 l 8 l ( 1 ) 0 0 9 ó ( l )

0 6 l l ( l ) 0 l í ) f ] { l r

0 J q 5 ( l ) 0 l ] . 1 7 ( 6 )

0 1 l l ( 6 ) 0 0 i f ! l

0 t t l ( 5 r ( ) | 0 1 ) 0

( l 1 9 7 1 3 ( ó ) 0 5 0 0 ( l

0 lq{(- ì r ( l 5001)

0 5ó0(l i 0 i tx) t)

0 5 1 ó ( { ) 0 5 0 r r 0

0 . 1 7 8 ( 5 ) 0 0 0 0 f )

0 i : ó ( 5 ) 0 0 0 ( f r

0 5 j 0 ( l ) i l I : 9 1 ( 9 )

0 l i ó ( 2 1 0 0 7 j ( I )

0 ó67(2) 0 070( I i

0 5 0 1 ( 4 ) t ì 0 7 2 r ? )

0 l 0 l ( 5 ) 0 0 ó ó ( l )

z./c Occ, Ueq or Uiso*

0 1 7 7 4 i l ) I 0 0 0 1 2 1 ( r )

0 1 7 5 ó ( l ) l 0 0 0 B t ( l )

0 4 9 8 6 0 ) l 0 0 0 l l l ( t )

0 5 0 0 5 ( l ) I 0 0 0 r 2 3 ( l )

0 1 5 0 0 ( 0 ) l 0 0 . 0 1 5 ó ( 2 )

0 r 0 { ( r r l 0 0 0 3 0 3 ( 4 )

0 1 9 9 2 ( t ) l 0 0 0 . l l j ( 5 i

{ ) . f2 . l l ( l ) I 0 00106(4)

i - r 1 l i 9 ( l ) l 0 0 0 1 1 1 ( 5 1

0 { 2 ó 5 { l ) l 0 0 0 1 8 1 ( 6 )

0 . 1 2 1 0 0 ì I 0 0 . 0 1 ó 2 ( 5 )

u1 .18 .1( l ) l0 00212t1)

0 { 9 9 2 ( l i l 0 0 0 2 q 8 ( . 1 )

0 . 1 9 7 9 ( 2 ) r 0 0 0 3 1 4 ( ó )

0 i 0 0 0 0 l 0 0 0 1 5 8 ( 5 )

r ) 2908(7) 0 07 0 0559( t )

0 . 1 0 5 ( l ) 0 0 4 0 0 9 t 2 )

( r l l l ( 2 ) 0 1 4 0 0 7 5 ( 9 )

0 l l 6 ( 3 ) 0 0 8 0 0 4 1 ì ( 9 )

0 J05t (6 i 0 l0 0 041(2)

0 4 0 6 ( l ) 0 l E 0 l { ( 2 )

r l . l l - l ( l ) 0 2 5 o l l ( t )

i l . ì 8 1 7 ( 5 ) 0 4 5 0 0 1 ó ( t )

0 l l5 ( l ) 0 18 0 061(7)

0 3 r 9 ( 2 ) 0 1 8 0 0 5 5 ( 7 )

0 l 1 l ( . 1 ) 0 1 6 0 0 6 9 ( e )

0 126( l ) 0 28 0 087(8)

0 1 { 0 ( 3 J 0 2 9 0 l 0 ( l )

0 108719) 0 .21 0 027( l ì

0 , t26(2) 0 .38 0 085(5)

0 l 3 r ( l ) 0 3 9 0 1 0 2 ( 5 )

0 1 6 2 ( r ) 0 l l 0 0 7 ( l )

0 3 0 2 ( , r ) 0 i l 0 0 ó ( l )

+sane no ta t ion as [13 ]

2348

' l able J Selecled bond distances for h-CaNl l , -S and h-NH,-S

h-CaNHr-S h -NH -S

T l - o l-ol-o4-o1

n]can

' l ' l l - o l l

-o3 I-o4 I-o7

Ùìean

T]-()4-o4 l-o8-o9

mean

l .63e(3 )| .63 l ( l )I .623(3 )1 6 3 2 ( - ì )1 . 6 3 I

r .614(3 )| .640(3 )r .648(3 )1 .663(3)1 .619

r .ó4s(3)l .653(3 )1 .662( r )L633(2)1 .648

T4-O3-o3 I-o8- o l 0

mean

T5 -O l [ x2 ]-O I I [x2]

mean

Ca-Wl [x2]-w3p-w I p[x2]-w8-w I 2[x2]- w r 0-w3-w2[x2]-wóp-wó- w l 3

l .626(3 )1 . 6 r 7 ( 3 )1 .624(3)1 643(2)| . 6 2 1

t . 6 3 7 ( 2 )1.623(2)1 .6 , j0

2 . 2 5 ( r )2 34(3)2 . 3 8 ( l )2 .31(3)2 .36(2)2 .3e(3))..43(3)2 .44( t )2.48(2)2 . s 7 ( 3 )2 .45(3)

T l -o l t .62 '7 (3)- o 3 1 . 6 r 6 ( 3 )- o 4 1 . 6 1 0 ( 3 )-o7 l ó20(2)

mcan 1 .6 | 8

T i l - O l r r . 6 2 9 ( 3 )-o3 r 1 .618(3)- o 4 r 1 . 6 5 r ( 3 )-o1 1 .6ó4(2)

mean 1 .645

T l -o .1 1 .615(1)-o4 l 1 .662(3)-o8 1 .662(3)-o9 1 .633(2)

mean 1 .650

T1-Ol r .63 l (3 )- O 3 I l 6 1 4 ( 3 )-o8 l .61 8( l )-o r0 1 612(2)

mean 1-626

I - 5 - O I [ x 2 ] 1 6 5 8 ( 2 )- O l l [ x 2 l I . 6 3 ] ( l )

mean L645

Ca-Wl [x2 ] 2 .40(5)- W 3 p 2 . 1 1 ( ó )-w lp [x2 ] l .a8(a)-w8 2 .47(4)- w l 2 [ x 2 ] 2 . 4 3 ( l )-v / 10 2 .43(1)-w3 2.34(7 |-w2[x2 ] 2 .47( ) )-W6p 2.39(4)-w6 2.52(41

It is known that stilbite-type minerals in their cationic exchanged forms can change their

real space group, moving towards higher symmetry. Barrerite in its Ca-exchanged form Il]and

in its hydrated NHo form [10] changes its real symmetry from Amma to Fmmm, whereas

stellerite in its Na-exchanged form remains Fmmm [2].

The study of hydrated NHo-exchanged forms of stilbite allows us to tackle many problems of

stilbite-type minerals, i. e.

1. does stilbite maintain its real symmeÍry F2lm even in its hydrated -partial or complete- NHo -

exchanged form?

2. does the location of extraframework sites in these exchanged phases have a "memory" of the

location in the original ones? In other words, does the distribution of exchangeable cations in

natural stilbite-t1'pe zeolites influence, or othenvise, the distribution of NHa and HzO

molecules in the NHq-exchanged lorm?

J. are there any remarkable differences in the extraframework ions between NHa-exchanged

barrerite and NHa-exchanged stilbites, considering that the almost complete cation NHa

exchange makes the extraframework content very similar in both these zeolites?

4. is the distribution ofresidual cations, ofNHo, and HrO recognizable in the exchanged phases?

5. is there evidence of N-H O bridges that could be precursors of Brsnsted acid sites in the

stilbite calcinated H-form?

ln answer to these questions, we can make the following observations:

L Our results confirm the space group F2lm for both h-CaNHr-S and h-NH4-S. However, the

crystal structures are strongl)'pseudosymmetric Fmmm. Many structural features support this

result:

a) the B angle, which is 89.24o for natural stilbite from Poona [6], becomes 89.87' in h-CaNHq-S

and 89.5ó' in h-Ntl.r-S.

b) the T5-T5-T5 angle (where the T5-T5 vector is the length of the 4-4-l-1 fundamental unit in

the c direct ion) is very close to 180'(179.3'both in h-CaNH,r-S and in h-NHa-S), whereas in

natural stilbite it is around 175". This angle must be 180" in orthorhombic Fmmm symmetry.

c) the Ca atom shifts in the [100] direction towards the orthorhombic pseudo-minor plane at

x-l/2. In fact, x:0.4975 in h-CaNHq-S and x=0.4986 in h-NH4-S, to be compared with

x:0.4840 in stilbite from Iceland [ 1].

d) in both h-CaNH4-S and h-NFL-S each extraframework site has its nearly symmetric site with

respect to the orthorhombic pseudo-mirror plane at x:112, whereas this pseudo-symmetry

does not occur in stilbite.

This result can be explained as follows: in stilbite there are fwo extraframework cation sites, one

on a minor plane, fully occupied by Ca (or by Ca and Na), the other partially occupied by Na;

electrostatic forces between them are strong enough to push the calcium out of the mirror plane,

so allowing only F2lm symmetry; in stellerite there is no sodium. and this site is partially

occupied by water. so calcium can easily remain on the minor plane, and the symmetry is

[.mmm. In hydrated NHa-exchanged stilbites both cation sites are weakly occupied, so that a

Fmmm symmetry is now possible. The Na site, however. differs markedly from its pseudo-

svmmetric - by minor plane - site, occupied by water. so that a slight deviation from the

ropological symmetry is justified. An analogous result was found for NH4-exchanged banerite

[0]: in this case the NFl-exchanged form changes its real symmetry from Amma to Fmmm.

I'his result can be explained considering that the cation site (C3 in the notation of [4]), which

inrposes rotaîional displacement within the framework around the screw ÍIxes parallel to a, is

enrpn in NHr-banerite, justifying the restoration of the topological symmetry.

A strong pseudo-symmetry has been found also for dehydrated Na^lH4 [1] and NHa-

trchanged forms [2] of stilbite from the Faeroe Islands. The p angle varies from 89.10' of

: .rrural st i lbi te [6] to 89.86'and 89.68" respectively. whereas the T5-T5-T5 angle is 179.9" and

I -Q.7o.

respectively. Therefore there is a clear tendency of the structures of both hydrated and

i.'hr drated NHa-exchanged forms of stilbite towards a Fmmm symmetry.

I In natural stilbite quite a large number ofextraframework sites have been located (see Figure

It. Two of these are cation sites, as described above. The other sites (V/1...W9 in the notat ion

.,i I j]), are attributed to waîer molecules. All these sites, with the exception of W4, have

i.een found in both h-CaNH,l-S and h-NH4-S, but are now all partially occupied. It is to be

xored that the W4 site is also empty in NFla-exchanged barrerite [10]. Moreover, all sites

:ound in stilbite have their pseudo-symmetric site in both hydrated NHa-exchanged stilbites,

:.:rd six new sites have been found. If we consider this good conespondence between the

.'\rraliamework sites found in natural and hydrated NHa-exchanged stilbites, we can conclude

::,ìr. lo some extent, a memory of the initial location of the extraframework atoms remalns.

2351

9:lb

Figurc L Projection along [00] of the crystal structure of h-NH.r-S. Large empty circles represent Ca

sitcs, small empty circles W sites. small l ight gral circles N sites, and nredium bright gray circles Na

si tes.

A comparison of extraframeu'ork sites in NH1-barrerite and NH.1-stilbites shows that there are

strong similarities between the two structures. In both cases, thc \\'4 site is empty, as pointed

out betbre, and new, weakly occupied sites have been located. There is normally a good

conespondence between the coordinates of the framework sites, whereas major differences

are presenl in their occupancies.

As said before, in both hydrated NH1-exchanged stilbites, extraframework sites have been

found in Ca and Na sites of untreated stilbite with. as expected, Iorv occupancies. The bond

distances and coordination indicate clearly that these sites ar€ occupied by the residual

extraframework cations. They account for 0.97 Ca atoms for h-NFIa-S and 1.90 Ca and 0.80

Na atoms for h-CaNH4-S, respectively. It is virtually impossible to distinguish between NH+

and H2O molecules, because all sites have partial occupancy and large distances (> 2.84)

fìom the tiamework oxygens. However. it is reasonable to assunte that the sites r.vith bond

dístances less than 2.5 A from Ca are occupied by water molecules. The quite similar

?352

occupancy of Ca and these sites could be seen as supporting this assumption. It is impossible

to determine the true coordination of Ca. owing to the large number of H2O sites at

coordination distance and the very short distances arnong many of them which prevent their

simultaneous occupancy. Following the above assumption, 16.1 NH4 molecules and 26.9 HzO

molecules were located in h-NH,r-S. and 19.5 NHa and 31.4 H2O molecules in h-CaNH4-S.

which reasonably compare with values given by the chemical aaalysis reported above- A low

value of water molecules found in the structure refinement when compared with that given by

the chemical analysis is usual in zeolites characterized by many partially occupied sites in the

channels, as occurs in NHa-exchanged forms of stilbite, and also in NHa-exchanged forms of

banerite [10].

5. From the structure refinement it was not possible to localize H sites bonded to N atoms. l'his

result was expect considering the lo*' occupancies of N sites (never greater than 40%). We

cold only hypothesize that oxygen atoms at short distance from N atoms and possible acceptor

of' hvdrogen in a N-H O bond could be the precursors of Bronsted acid sites in the calcinated

H-form of stilbite. Such bonds involve the oxygens of Si5 tetrahedron (Ol and Oll) and 07

r \ -O l l -3 .15 ,4 ; N3-Ol l :2 .96 : . N1p-Ol=3.06r N2-O7=2.S4) . O l and Ol l head toward the

channels parallel to [00] delimited by l0-membered ring of terrahedra, whereas 07 heads

tow'ard the 8-ring parallel to [001]; therelore this oxygens could become acid sites.

lrnfòrtunately in both dehydrated NaòlHr stilbite [] and NH4 stilbite [2] any attempt to

locate Brsnsted sites was unsuccessful.

\CKNOWLEDGEMENTS

I hc Ministero della Università e della Ricerca Scientifica e Tecnologica is thanked for the

:ìnancial support to the research program "Relations beMeen structure and properties in

i,lrinerals: analysis and applications". The Consiglio Nazionale delle Ricerche of Italy is

.rcknowledged for financial support. Thanks are due to "Centro Interdipafimentale Grandi

\rrumenti" of the Universin' of Modena for the experimental facilities.

REFERENCES CITED

I V/.J. Mofier, Am. Mineral. , 68.441 (1983).

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