NMR Studies of Water and Ammonia inside the Cubooctahedra of

25 NMR Studies of Water and Ammonia inside the Cubooctahedra of Different Zeolite Structures W. D. BASLER

Downloaded by FUDAN UNIV on January 13, 2017 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0034.ch025

Institute of Physical Chemistry, University of Hamburg, Laufgraben 24, 2000 Hamburg 13, W. Germany

Zeolites of faujasite-type (Figure 1) are alumos i l i c a t e s , composed of cubooctahedral units, which are joined by double six-rings, forming a diamond- l i k e structure. Thus two systems of i n t r a c r i s t a l l i n e voids and channels are formed: The f i r s t system consists of large cavities of 13 Ådiameter, which are joined by windows of 8 Å diameter. The second system consists of the interior of the cubooctahedra, joined by the double six-rings. On the other hand, a passage from the large cavities into the interior of the cubooctahedra is possible through oxygen six-rings. The cations, which are necessary to compensate the smaller charge of the Al-ions, are partly localized before or in these six-rings. At room temperature a large cavity can take up 28, a cubooctahedra 4 water molecules(1-5). In previous NMR-studies(6-12) the water molecules in the large cavities were studied, the water inside the cubooctahedra was omitted i n the discussions. Perhaps, this was caused by the observation of only one single uniform NMR-signal, at least at room temperature and in zeolites, where the amount of OH-groups could be neglected. However, recently a second NMRsignal was discovered and attributed to the water i n side the cubooctahedra, independently by two other groups and us(13-15). Here we want to report our NMR-studies dealing with a) proving that the observed second NMR-signal originates from water inside the cubooctahedra, b) the equilibrium properties of this water, c) the kinetics of the water molecules passing from the large cavities into the interior of the cubooctahedra and d) f i r s t results using ammonia as sorptiv. Zeolites of faujasite-type and related structures containing cubooctahedral units, with different S i / A l -ratios and Fe 3 + -content were prepared by Kacirek, 291

Resing and Wade; Magnetic Resonance in Colloid and Interface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


292

MAGNETIC

Figure 1.

Model of the crystal structure of faujasite

A Β

.

C D

ε

.

100με

Figure 2. Free induction decay of water protons in various faujasites, denoted as in Table I. Full loading, Τ = 295°K.


RESONANCE


25.

BASLER

Water and Ammonia in Zeolite Structures

293

using newly developed methods of z e o l i t e c r i s t a l l i s a t i o n Q 6 , 1 7 ) . Exchange of cations was performed as u s u a l l y . The z e o l i t e powder wasj-outgassed at 400 C u n t i l the pressure was l e s s 10 Torr. The s o r p t i o n was performed u s i n g the corresponding vapor, the amount was determined "by weight. The measurements by pulsed proton-NMR were done at 60 MHz, u s i n g a Bruker B-KR 322 spectrometer. Whereas the outgassed z e o l i t e s showed no s i g n a l a f t e r the deadtime of 9 us at a d e t e c t i o n l i m i t of 1-2 mg H20/g z e o l i t e , the observed f r e e i n d u c t i o n de cay a f t e r a 90 -pulse f o r water i n d i f f e r e n t z e o l i t e s of f a u j a s i t e - t y p e at room temperature and f u l l l o a ding (conditioned over saturated NH4C1-s οlution) i s shown i n Figure 2. Beside the w e l l known r e l a x a t i o n of the water i n the l a r g e c a v i t i e s , here named phase A, with r e l a x a t i o n times of s e v e r a l ms depending s t r o n g l y on Fe3+-content, there i s a f a s t decaying component, named phase B, with T2B=25 ps and T1B=100 ms.Table I shows that the i n t e n s i t y IB ( 13%, e x t r a p o l a t e d 17%) and transverse r e l a x a t i o n time T2B of t h i s component are independent of S i / A l - r a t i o and of Fe3+-concentration. This i s a f i r s t h i n t that phase Table I. Transverse(T2B) and longitudinal(T1Β) r e l a x a t i o n times of phase Β i n d i f f e r e n t z e o l i t e s of f a u iasite-type.T=295 K, f u l l loading. I n t e n s i t y IB1 ( a f t e r the dead-time) and IB2(extrapolated) of phase B. IB2 IB1 Zeolite S i / A l Fe3+ T2B T1B °/o US ppm ms °C 12 70 150 A Linde 13X 1.18 25 16 12 400 Ε Linde 13Y 2.5 17 23 110 Β s e l f made 1.22 17 27 100 13 3 2.34 C " " 17 13 7 23 210 14 18 2.74 D " " 90 25 210 A and Β correspond to the water molecules i n the l a r g e c a v i t i e s and i n the i n t e r i o r of the cuboocta hedra, f o r these s t r u c t u r a l c h a r a c t e r i s t i c s are the only q u a n t i t i e s common to a l l z e o l i t e s of Table I. The second h i n t i s the f a c t that the i n t e n s i t y IB gives w i t h i n the l i m i t s of error(l590 a number of 4 v/ater molecules per cubooctahedra, j u s t what i s known from X-ray-data(1J3). Whereas the quotient T1A/ T2A=2-3 shows t h a t the molecules of phase A are w e l l mobile and i n a l i q u i d - l i k e state, the corresponding T1B/T2B=4000 i n d i c a t e s molecules with s t r o n g l y r e s t r i c t e d motion. This a l s o favours the i d e n t i f i c a t i o n of phase Β with the i n t e r i o r of the cubooctahedra, where only r o t a t i o n a l tumbling motions are p o s s i b l e .


MAGNETIC


294

RESONANCE

Thus f o r a water molecule r o t a t i n g about one axis one obtains T2=16 jus, which can be compared with the ex perimental T2B=25 jus. By D20-dilution i t could be proved t h a t phase Β does not c o n s i s t of i s o l a t e d OH-groups, but that each proton has i t s neighbour-proton: Replacing 80>'· of the protons by deuterons increased the transverse r e l a x a t i o n time T2B about 25 and T1B about 2.5 times. There f o r e , phase Β relaxes predominantly b y H-H-interaction. T h i s H-H-interaction i s intramolecular, as T2B was found to be independent of the f i l l i n g f a c t o r , too: Table I I . Transverse(T2B) and longitudinal(T1Β) r e l a x a t i o n times of phase Β at d i f f e r e n t loading and D20d i l u t i o n . F a u j a s i t e of Si/Al=2.74, s e l f made, T=295 K. Loading mg/g Bo 120 250 355 60 H20/240 D20

T2B jus 25 25 25 25 600

T1B ms T50 140 110 210 500

Summarizing, a l l experimental r e s u l t s on water i n f a u j a s i t e s support the assumption that phase Β are the water molecules i n the i n t e r i o r of the cubooctahedra. Consequently, t h i s phase Β should be found i n other z e o l i t e s b u i l d up of cubooctahedral u n i t s , too. Therefore, we studied z e o l i t e A and ZK 4, where the cubooctahedra are arranged i n a p r i m i t i v e cubic l a t t i c e , and z e o l i t e s o d a l i t e , which c o n s i s t s only of cubooctahedra without large c a v i t i e s . The r e s u l t s i n Table I I I show, as expected, i n the case of z e o l i t e A and ZK 4 two-phase-behaviour with r e l a x a t i o n times and i n t e n s i t i e s close to the values found i n f a u j a s i t e s . S o d a l i t e , on the other hand, showed only one r e l a x a t i o n v/ith values c l o s e t o those o f phase B: Table I I I . I n t e n s i t y IB and r e l a x a t i o n times T1 and T2 of phases A and Β i n z e o l i t e s containing cubooctahedral u n i t s . F u l l loading, T=295 K. Zeolite

Si/Ai

T2B LIS

Linde 13X Linde 13Y NaA ZK 4 Sodalite

Ί·18 2.5 1 1.61 1

25 23 100 70 50

T1B IB ms % 75 i t 110 17 70 16 60 18 65 100

T2A T1A ms ms 5.6 Ô.7 1.7 2.5 2.5 20 1.2 4.0 -



25.

BASLER

Water and Ammonia in Zeolite Structures

295

As i t was proved that water i n s i d e the cuboocta hedra could be studied separately by the NMR-signal named phase B, we studied the d i s t r i b u t i o n of the wa t e r molecules betv/een the large c a v i t i e s and the i n t e r i o r of the cubooctahedra of a f a u j a s i t e of Si/Al= 2.74 f o r d i f f e r e n t loading and i t s temperature depen dence. Figure 3 shows that the i n t e r i o r of the cubo octahedra i s f i l l e d up predominantly: At low f i l l i n g n e a r l y h a l f of the water i s found i n s i d e the cuboocta hedra. This f r a c t i o n decreases with i n c r e a s i n g cover age, i n agreement with the r e s u l t s of P f e i f e r ( l 3 ) . This e q u i l i b r i u m of s o r p t i o n i s s h i f t e d by temperature i n favour of the large c a v i t i e s ( F i g u r e 4). This i n d i cates that the heat of sorption i n s i d e the cuboocta hedra i s s l i g h t l y greater(1 kcal/mol) than i n the large c a v i t i e s . As we found two-phase-behaviour even i n the l o n g i t u d i n a l r e l a x a t i o n , i t follows that the exchange rate betv/een phase A and Β must be at l e a s t i n the range of seconds(l9). To study t h i s r a t e , the sorp t i o n was allowed ΤΞο take place slowly(appox. 1 h) while the z e o l i t e was h e l d at 0 C by a water-ice bath to avoid any v/arming. Loaded by t h i s technique, the z e o l i t e showed only phase A and no phase B - s i g n a l immediately a f t e r sorption. Thus i t was p o s s i b l e to study the appearing of phase B. The approach to the s a t u r a t i o n value i s w i t h i n the l i m i t s of e r r o r expo n e n t i a l (Figure 5). We studied the grov/ing of phase Β f o r f a u j a s i t e s NaX(Si/Al=1.18) and NaY(Si/Al=2.36) and a coverage of 100 mg H20/g and at d i f f e r e n t temperatures. The time needed f o r growing of h a l f of phase Β v a r i e d between 1 month at room temperature and 1 hour at 80 C. No d i f f e r e n c e was observed betv/een NaX and NaY. The Arr h e n i u s - p l o t ( F i g u r e 6) gives an a c t i v a t i o n energy of 24 kcal/mol f o r t h i s process. To pass i n t o the i n t e r i o r of the cubooctahedra, the water molecules have to get through the oxygen s i x - r i n g s , where some of the cations are l o c a l i z e d ( S 2 - s i t e s ) . Removing these S2-cations should increase the rate of growing of phase B. This can be done by three ways : 1) High water loading increases the mobi l i t y of the c a t i o n s . 2) High S i / A l - r a t i o s reduce the number of c a t i o n s . When Si/Al>2.43 there are not enough cations to occupy a l l s i t e s . 3) Exchange of Na by cations of higher charge has the same r e s u l t . We have studied a l l these three i n f l u e n c e s and found an a c c e l e r a t i o n of the rate i n every case: 1)At a loading of 200 mg/g phase Β i s b u i l d up 5 times f a s t e r at room temperature compared with the loading


MAGNETIC

296


100

200 mgHjp/ΝαΥ

RESONANCE

300

Figure 3. Intensity I of phase Β for different load ing. Faujasite NaT, (Si/Al «= 2.74) Τ — 295°K. B

150 h

100 η

οο 50

Δ Δ

ο ί

°

-oooooo°°o° [ .J

Δ

.

,

I

I

200

Δ

I

ι

Δ

ι* 400

300

,

Figure 4. Intensities Ι and Ι ο/ phases A and Β and ίοίαΐ intensity I /or different temperatures. Zeolite NaT (Si/Al = 2.74) 120 mg Η,Ο/g. Α

Β

T

200my H^O/gNaX 80 °C

Figure 5. Evolution of phase B, showing expo nential approach to equilibrium value

V

»-

_i

/

ι

ι

2 TIME [h]



25.

BASLER

Water and Ammonia

in Zeolite

Structures

297

o f 100 mg/g, and the a c t i v a t i o n energy seems to be smaller(20 kcal/mol, Figure 7 ) . At f u l l loading, phase Β showed i t s e q u i l i b r i u m value immediately a f t e r the s o r p t i o n , which l a s t e d one hour. This i s i n good agreement with the observation of mobile Na-ions f o r higher loading(20). 2) Whereas i n NaX and NaY a l l S2-sites are occu pied, and we consequently f i n d the same rate, a f a s t e r r a t e i s expected f o r Si/Al>2.5. We studied a f a u j a s i t e with Si/Al=3.40 and found a h a l f time of 200 hours (Table 4 ) . 3) When Na1 + i s exchanged against La3+, the num ber of cations i s reduced by a f a c t o r 3. We prepared a f a u j a s i t e with Si/Al=2.93, where 80^ of the Na was replaced by La. As i t i s known that the La-ions are i n the double s i x - r i n g s , almost every passage from the large c a v i t y i n t o the cubooctahedra i s open now. Phase Β was f u l l y observed immediately a f t e r sorption, i n d i c a t i n g that the e q u i l i b r i u m was reached i n l e s s one hour. Further, tv/o-phase-behaviour begins t o vanish at 150-200 C and r a p i d exchange on the time-scale of NMR begins. The removing o f the Na-ions a t the S2s i t e s increased the r a t e of passage through the oxygen s i x - r i n g s by more than 1000 times. The same was found using Ca2+. Table IV. Time t1/2 f o r forming h a l f of phase B. T=295 K, loading 100 mg H20/g. Zeolite NaX NaY NaY LaY CaY

Si/Al 1.18 2.36 3.40 2.93 2.93

t1/2(h) 800 800 200 C1

NMR Studies of Water and Ammonia inside the Cubooctahedra of

Recommend Documents