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High Resolution Sorption Studies of Argon and Nitrogen on Large Crystals of Aluminophosphate AlPO4-5 AND ZEOLITE ZSM-5
H.G.Karge,J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
HIGH RESOLUTION SORPTION STUDIES OF ARGON AND NITROGEN ON LARGE CRYSTALS OF ALUMINOPHOSPHATE A1P04-5 AND ZEOLITE ZSM-5
u.
MULLER',
K.K. UNGER',
Y. GRILLET',
F
DONGFENG
PAN^,
A. MERSMANN'
. ROUQUEROL3, J. ROUQUEROL3
'Institut fur Anorganische Chemie und Analytische Chemie, P. 0. Box 3980, Johannes Gutenberg-Universitat, D-6500 Mainz, F.R.G. 21nstitut fur Verfahrenstechnik der Technischen Universitat, Lehrstuhl B, Arcisstr. 21, D-8000 Miinchen 2, F.R.G. 3Centre de Thermodynamique et de Microcalorimetrie du C.N.R.S., 26 Rue du 141e R.I.A., 13003 Marseille, France ABSTRACT High resolution adsorption (HRADS) with argon and nitrogen at 77 K in the pressure range of c p/po < 0.5 were performed on large crystals of zeolite ZSM-5 (180 pm) and aluminophosphate AlP04-5 (150 pm) using a novel volumetric device. Multi-step isotherms of both adsorptives on ZSM-5 could be observed for the first time. The adsorption followed by low temperature microcalorimetry resulted in distinct exothermic signals at the steps in the adsorption isotherms. Based on the results of atom-atom potential energy calculations (AAP) as well as independent model building it was shown that 24 'kinetic' adsorbate molecules can be filled into a ZSM-5 unit cell. Experimental results are reasonably interpreted assuming a primary filling of narrow channels and a secondary adsorption in the wider channel intersections. Localized adsorption is understood as a possible filling mechanism. In ZSM-5 and nitrogen as adsorbate there is evidence for a transition of fluid-like to solid-like adsorbate phase. AlPOq-5 behaves as a homogeneous sorbent with a micropore capacity of four molecules per unit cell for argon and nitrogen. For both adsorptives in the molecular sieves under investigation the initial isosteric heat of adsorption for nitrogen was found to give values comparable to the enthalpies of adsorption derived from the temperature dependence of experimentally determined HENRY constants. HENRY constants and the initial isosteric heat of adsorption are indicative of a stronger adsorption of nitrogen compared to argon which is thought to be due to additional interactions between the nitrogen quadrupole moment and the crystalline molecular sieve framework. INTRODUCTION Zeolite ZSM-5 and aluminophosphate AlP04-5 have attracted considerable interest as microporous model adsorbents. Both materials are crystalline molecular sieves with strictly regular pore systems, viz. intersecting straight and zigzag 10-membered ring channels (ultramicropores) connected by larger cavities (micropores) in ZSM-5 111, and unidimensional 12-membered ring tubes (micropores) in A1POq-5 12 1 . Based on novel synthesis concepts 13,41 the growth of large uniform crystals at high yields and with narrow particle size distribution has been achieved. Employing large monocrystals in sorption investigations, no erroneous contributions by interparticle vapour condensation and external surface area
626 effects to adsorption occur as on polycrystalline powders. Thus, the intrinsic sorptive properties can be reliably determined with high precision over a wide range of coverage 8 . Gravimetric sorption experiments with nitrogen at 77 K on large ZSM-5 crystals led to the discovery of a sharp hysteresis loop at low pressures of p/po = 0.15 15-71. Furthermore, in contrast to the BDDT-model 181 which predicts a LANGMUIR-type of isotherms for microporous adsorbents, both argon and nitrogen revealed distinct multi-step isotherms on large ZSM-5 crystals whereas no deviation from type-I isotherms was seen for AlP04-5. This study presents experimental results obtained by combining high resolution adsorption techniques (HRADS) with low temperature microcalorimetry. The calculated thermodynamic properties like HENRY constants, heats of adsorption and atom-atom approximation for adsorbate-adsorbent interactions strongly suggest localized adsorption on energetically enhanced crystallographic sites. In addition, a transition of the adsorbate to a denser phase at higher coverage is assumed to explain the sorption behaviour of nitrogen on large ZSM-5 crystals. EXPERIMENTAL Large crystals, 600 pm, of AlPO4-5 131 were prepared by optimization of a procedure described elsewhere 191. A sample consisting of 150 pm hexagonal rods was used in this study. Monocrystals of HZSM-5 (180 pm with Si/A1 = 1000) were synthesized according to a process described in the literature 141. The products were characterized by scanning electron microscopy SEM. x-ray diffraction, thermal analysis and electron microprobe analysis. Additionally the HZSM-5 crystals were analyzed by IR and 29Si-MAS-NMR spectroscopy. Adsorption isotherms with argon and nitrogen at 77 K were recorded on a novel dynamic volumetric device (Omnisorb 360, OMICRON Corp., U.S.A.). pressure of p/po =
Data acquisition started at a relative
and was continued up to p/p" = 0 . 5 , thus collecting se-
veral hundred quasi-equilibrium points of the adsorption isotherm. Nitrogen adsorption isotherms at 303 to 430 K were determined gravimetrically at a relative sensitivity of lo6 (ultramicrobalance 4433, Sartorius, F.R.G.)
. Continuous calori-
metric measurements were performed on a reversible isothermal microcalorimeter
of Tian-Calvet type (C.N.R.S. Thermodynamique et Microcalorimetrie, Marseille, France). A detailed description of the microcalorimeter is given in the literature 1101. Prior to all experiments samples calcined at 823 K were degassed for 12 hours at 473 K and at a vacuum of
mbar.
CALCULATIONS Calculations of the intermolecular interactions of argon adsorbed on ZSM-5 and AlP04-5 respectively, were performed using atom-atom approximation for the potential energy (AAP) and Lennard-Jones 6:12 potentials. Only contributions of the framework oxygen atoms were taken into account, based on determinations of the refined structures of ZSM-5 111,121 in space group Pnma and P6cc for
627 A1P04-5
121. V a l u e s f o r t h e p o l a r i z a b i l i t y of a r g o n (1.63.10-3
nm3) a n d n i t r o g e n
(0.88-10-3 nm3) a s w e l l a s t h e i r r a d i i were t a k e n from l i t e r a t u r e d a t a 131. Cov-
e r a g e s €I were c a l c u l a t e d from t h e e x p e r i m e n t a l i s o t h e r m s as m o l e c u l e s of a d s o r bate per
n i t c e l l ( m o l e c / u c ) , u s i n g STP v a l u e s f o r a r g o n (1.784.10-3
nitrogen
1.251.10-3
g/cm3).
g/cm3) a n d
D a t a d e r i v e d by g r a v i m e t r y were c o r r e c t e d f o r buoy-
ancy w i t h t h e s o r b a n t c r y s t a l l i n e d e n s i t i e s . I s o s t e r i c h e a t s o f a d s o r p t i o n , qST, a r e t a k e n from t h e e x p e r i m e n t a l h e a t f l u x V(dp/dt) qST =
4 by
t h e equation:
-4 (1)
dn/dt
w i t h d n / d t as c o n t i n u o u s a d s o r b a t e u p t a k e r a t e a n d a c o r r e c t i o n V ( d p / d t ) f o r t h e h e a t l o s s d u r i n g t h e e x p a n s i o n o f gas i n t o t h e s a m p l e b u l b . The i s o s t e r i c h e a t of a d s o r p t i o n , qST, c a n b e compared w i t h t h e v a l u e of t h e d i f f e r e n t i a l e n t h a l p y of a d s o r p t i o n , q d i f f s
( q d i f f = qST - R T ) . From t h e i n i t i a l s l o p e of t h e i s o t h e r m s
t h e HENRY c o n s t a n t s KH were c a l c u l a t e d a c c o r d i n g t o €I = K H
-
p. The h e a t of ad-
s o r p t i o n , -AH, was d e r i v e d from HENRY c o n s t a r i t s u s i n g KH = Kfi e x p (-4H/RT). RESIJLTS and DISCUSSION I s o t h e r m s a n d isosteric h e a t of a r g o n a n d n i t r o g e n on AlP04-5 Type-I i s o t h e r m s o f a r g o n a n d n i t r o g e n (see F i g . 1, shown f o r N2) on large A1P04-5 c r y s t a l s w e r e o b s e r v e d f o r b o t h gases a t 77 K. A p p l y i n g LANGMUIR-plots as w e l l as t h e method of DUBININ-RADUSHKEVICH, a m i c r o p o r e f i l l i n g c a p a c i t y of
3.9
0.2 molec/uc f o r n i t r o g e n a n d 3 . 8 2 0 . 2 molec/uc f o r a r g o n w a s c a l c u l a t e d .
Dense a d s o r b a t e p h a s e s f o r n i t r o g e n a n d a r g o n i n AlP04-5 were assumed. I s o s t e r i c h e a t of a d s o r p t i o n , qST, r e a c h e d a maximum a t a c o v e r a g e o f a b o u t 3.5 m o l e c / u c ( 1 4 . 5 kJ/mol N2 and 1 3 . 2 kJ/mol A r ) a n d d e c l i n e d when t h e s a t u r a t i o n c a p a c i t y was e x c e e d e d . W i t h i n t h e e x p e r i m e n t a l e r r o r t h e i n i t i a l i s o s t e r i c h e a t , qST, f o r ads o r p t i o n o f n i t r o g e n (13 f 0.6 kJ/mol a t 0 e O . 1 ) c a n b e compared t o t h e h e a t o f adsorption, -AH,
c a l c u l a t e d from t h e t e m p e r a t u r e d e p e n d e n c e o f t h e HENRY c o n s t a n t
KH ( 9 . 9 2 3.3 k J / m o l ) . A t 77 K t h e l i n e a r s l o p e o f t h e n ' i t r o g e n i s o t h e r m a t
p / p o c 10-5 y i e l d e d v a l u e s of KH = 13.1 molec/uc'mbar a n d KH = 1 4 . 2 molec/uc.mbar r e s p e c t i v e l y . The l a t t e r was c a l c u l a t e d u s i n g t h e v i r i a l e q u a t i o n of BARRER 1141. F i g . 1. High r e s o l u t i o n a d s o r p t i o n (HRADS) i s o t h e r m s a t 77 K of a r g o n a n d n i t r o g e n o n large c r y s t a l s of a l u m i n o p h o s p h a t e A1P04-5 ( 1 5 0 pm) a n d z e o l i t e ZSM-5 ( 1 8 0 bm a n d S i / A 1 = 1000).
628 Isotherms and isosteric heat of argon and nitrogen on ZSM-5 Multi-step isotherms were observed for nitrogen on ZSM-5 at 77 K. Steps in the adsorbate uptake were monitored at coverages of 20, 22, 24 and 30.5 molec/uc, respectively (see Figs. 1,2). Standard deviations amounted to 0.8 molec/uc in the uptake values, based upon statistical certainties of 95%. The initial isosteric heat of adsorption of nitrogen was calculated to 16.5 kJ/mol. The exothermic peaks of 6.4 kJ/mol clearly coincide with the observed steps in the isotherm as illustrated in Fig. 2. The exothermic contributions at coverages of 20 to 22 and 24 to 30.5 molec/uc respectively, were comparable to the sum of the heats of
condensation (5.56 kJ/mol) plus solidification (0.72 kJ/mol) being required for phase transitions in tridimensional bulk nitrogen. At low coverage (0e0.1) values of qST (16.8 2 0.2 kJ/mol) equal the enthalpy change derived from van't HOFF plots 1.7 kJ/mol). HENRY constants for nitrogen on ZSM-5 at 77 K were computed
(18.2 to KH
=
830 molec/uc'mbar, using the virial equation at p/po*
decreased to KH
=
this value was
594 molec/uc.mbar.
Fig. 2. Nitrogen isotherm and isosteric heat of adsorption on large ZSM-5 crystals at 77 K. The argon adsorption on ZSM-5 occurred in steps of 20 and 24 molec/uc (Fig. 1). At these steps in the isotherm an exothermic increase (1.7 kJ/mol) of the isosteric heat of adsorption, qST, (see Fig. 3)occurred; however, the heat curve as well as the isotherm were less pronounced compared to the N2/ZSM-5 system. Potential energy distribution in A1P04-5 and ZSM-5 Results of atom-atom approximation (AAP) calculations for argon atoms adsorbed on A1P04-5 are summarized in Fig. 4. Six minima of the potential energy were found above the evenly distributed TO4 6-rings in the channel walls. Model-building with localized 'kinetic' gas molecules (0.38 nm diameter) at these minima, indicated a preferred population of three argon atoms per radial TO4 six-ring
629 channel segment. This represents a micropore filling of 4 molec/uc and is in close agreement with the experimentally observed data from sorption isotherms and enthalpy curves.
1 -7
-
.. .
.
... .,
Fig. 3. Argon isotherm and isosteric heat of adsorption on large ZSM-5 crystals at 77 K.
From AAP-calculations and independent model-building 24 minima of the potential energy in an unit cell of ZSM-5 were registered. Eight minima are situated at the channel wall 6-rings in the straight tubes (see Fig. 6) and twelve in the zigzag channels (see Fig. 7). Additional four ones are located at the wider channel intersections, thus giving a total of 24 rninima/uc. Note that in the argon and nitrogen isotherms steps occur at fillings of 20 and 24 molec/uc, respectively.
Fig, 4. Potential energy distribution (AAP approximation) for argon across a channel in A1P04-5.
630 Sorptive properties of large AlP04-5 crystals Adsorption studies of microcrystalline AlPOq-5 have been described in the literature 115-181. Unusual type-V isotherms of the system H*O/AlP04-5 have been reported and were attributed to crystal hydrate formation 1161. Studies of STACH and coworkers I17 I at ambient temperatures identified A1P04-5 as energetically homogeneous sorbent. At low temperatures type-I isotherms f o r oxygen and nitrogen are reported by WILSON et a1 1151 and BOND et a1 1181, respectively. However, these isotherms showed 30 to 40% of the final uptake in a relative pressure range of 0.05
of preadsorption of n-hexane on AlP04-5, previously investigated 131, clearly showed that the observed hysteresis was not caused by micropores in AlPO
5. 4The comparatively low heats of adsorption of argon and nitrogen obtained in
this study (see Fig. 5) support the view that AlP04-5 exhibits a mildly hydrophilic sorbent character. At low pressures the micropore filling capacity for kinetic gas molecules (4molec/uc) is reasonably predicted by theoretical AAPcalculations and model-building.
0.1
AlP04-5 I77 K1
I¶ I -
P/OO
kd-1
0.4
I¶
0.a 10
0.1
¶
0.1
0.0
0
l
a
a
4
1
Fig. 5. Nitrogen isotherm and isosteric heat of adsorption of argon and nitrogen on large AlP04-5 crystals at 77 K. Sorptive properties of large ZSM-5 crystals Data for nitrogen adsorption on polycrystalline powders of ZSM-5 habe been already reported in the literature 119,201. In general, the overall sorption capacities agree fairly well with this study. However, most of these measurements collected only few isothermal points (5 to 20) and were not extended into the low pressure range p/poe
A geometrical model of JACOBS et al. 1201 pro-
poses a closest-possible packing of nitrogen molecules in the larger channel intersections of ZSM-5. In contrast to this model, theoretical calculations of gas-
631 solid interactions by EVERETT and POWL 1211, recently adapted to zeolites by DEROUANE I22 I , clearly indicate an enhancement of adsorption energy in smaller pores of molecular diameter (e.g. 0.55 nm channels in ZSM-5). According to DUBININ 123 I these channel 'ultramicropores' are primarily filled due to overlapping force fields from opposite walls of the narrow micropores. With increase of the pore size up to 2-3 molecular diameters (i.e. 0.8 to 1.0 nm for argon and nitrogen and close to the space available in a ZSM-5 channel intersection) the superimposed enhanced adsorption energy deciines, and cooperative secondary rnicropore filling might occur 1241.
Fig.' 6. Potential energy distribution for argon in a straight channel of ZSM-5 (arrows: channel minima; asterix: minima at the intersection).
I ,-,, ,o..B/..-ciIl
Fig. 7. Potential energy distribution for argon in a zigzag channel of ZSM-5 (arrows: channel minima; asterix: minima at the intersections). For argon and nitrogen AAP calculations and model-building yielded a channel population of 20 molec/uc which is in full agreement with the first step in the adsorption isotherms (see Fig. 1). Further filling of the four larger intersec-
tions with one argon atom in each site is attributed to the second plateau in the argon isotherm, giving a final coverage of 24 molec/uc. At this stage all channels and intersections are occupied by kinetic gas molecules. The exothermic increase (1.7 kJ/mol) during the intersection filling (20 up to 24 molec/uc) might be due to adsorbate-adsorbate interactions (e.g. the heat of fusion for bulk liquid argon amounts to 1.21 kJ/mol). As already shown (figs. 1-3) the situation is more complex for the nitrogen in ZSM-5. Although the kinetic molecular diameter of argon (0.35 nm) and nitrogen (0.38 nm) are quite similar, nitrogen as a diatomic molecule offers a considerable
quadrupole moment which permits additional adsorbate-adsorbent interactions. Hence, the steps in the nitrogen isotherms are more pronounced and the isosteric heat of adsorption is enhanced (1.8 kJ/mol) compared to argon. After initial uptake of 20 molecules in the channels (i.e. first step in the isotherm) a further coverage up to 22 molec/uc follows. The latter connects the channel adsorbate (2 nm) to the overall crystal adsorbate macroscopic phase (at least 30,000 nm,
as taken from the crystal dimension along the (100) or (010) axis). At this point, monitored as a small substep in the isotherm, the adsorbate phase loses significantly in kinetic energy and mobility, as is obvious from Fig. 2. At a coverage of 24 molec/uc, corresponding to a filling of all channels and intersections, the largest step in the isotherm up to 30.5 molec/uc takes place. This unusual behaviour is known to be correlated with a sharp type-A hysteresis loop between the adsorption and desorption branch of the isotherm 15,61. It has independently been observed even on smaller ZSM-5 crystals 171. The energy which is dissipated when tridimensional nitrogen is transformed from a gaseous to a solid phase is close to the value of 6.3 kJ/mol which has been experimentally observed during the densification of the fluid adsorbate in ZSM-5. The uptake ratio of 24/30.5 molec/uc before and after the inflexion of the isotherm at p/po
=
0.15 is identical with the ratio of the density of liquid (0.808g/cm3)
to solid nitrogen (1.027 g/cm3). Freezing of adsorbed 2D nitrogen at 77 K is a well documented phenomenon occurring on homogeneous graphite or boron nitride as adsorbents 110,251, and complex phase diagrams have been calculated 1261. Abrupt transitions of dense fluid-like to solid-like adsorbate in slit-shaped pores was theoretically predicted over a range of 1 - 2 up to 3 - 4 adsorbate layers 1271. Adapted to ZSM-5, this situation occurs at the completion of fluid filling
in the wider intersections, where
the 2D channel adsorbate is contacted with a
3D adsorbate phase. Furthermore this result can be explained by the 'site-andbond' network hypothesis of MAYAGOITIA, which assumes cooperative filling processes when half of the network sites (i.e. intersections in ZSM-5 at N
=
22
molec/uc) are filled 1281. Monte Carlo simulations of gases in narrow pores suggest that phase transitions and metastable states are formed which should show up
in experimental sorption isotherms as hysteresis effects 1291. This is con-
633 sistent with earlier observations on the existence of a hitherto unknown lowpressure hysteresis loop in the nitrogen ZSM-5 system 15-71. CONCLUSION The results of both high resolution adsorption and microcalorimetry at low temperatures provide clear evidence that the stepped isotherms of argon and nitrogen on large ZSM-5 crystals can be rationally explained by localized adsorptive molecules at the channel walls and the channel intersections. The findings are consistent with theoretical predictions. Heats of adsorption on A1P04-5 are lower compared to ZSM-5. Further investigations on unidimensional zeolitic materials (e.g. high silica ZSM-12) should show whether this behaviour is caused by different surface force fields of aluminophosphates and zeolites or if it is due to the different pore sizes of 10-membered and 12-membered micropore channels. At least nitrogen adsorbed at higher loadings in the ZSM-5 network should be considered as an immobile dense phase. Low temperature NMR and neutron diffraction experiments are in progress to further elucidate the adsorbate structure of gases in microporous adsorbents. ACKNOWLEDGEMENTS We are grateful to Dr. G. Engelhardt, Konstanz, and Dr. H.-G. Karge, Berlin,
for 29Si-MAS-NMR and IR-spectroscopic characterization of the ZSM-5 sample. This work is financially supported by Deutsche Forschungsgemeinschaft. REFERENCES 1 D.H. Olson, G.T. Kokotailo, S.L. Lawton, W.M. Meier J. Phys. Chem., 85 (1981) 2238-2243. 2 J.M. Bennett, J.P. Cohen, E.M. Flanigen, J.J. Pluth, J.V. Smith Am. Chem. SOC., Symp. Ser., 218 (1983) 109-118. 3 U. Muller, K.K. Unger, Z. Kristallogr., 182 (1988) 190-2. 4 U. Miiller, K.K. Unger, Zeolites 8 (1988) 154-156. 5 U. Miiller, K.K. Unger, Fortschritte Mineralogie, 64 (1986) Beiheft 1, 128. 6 U. Miiller, K.K. Unger, Stud. Surf. Sci. Catal., 39 (1988) 101-8. 7 P.J.M. Carrott, K.S.W. Sing, Chem. Ind., (1986) 786-7. 8 S. Brunauer, L.S. Deming, W.E. Deming, E. Teller, J. Amer. Chem. SOC., 62 (1940) 1723-32. 9 S.T. Wilson, B.M. Lok, E.M. Flanigen, US Pat. 4,385,994 (1981). 10 J. Rouquerol, S. Partyka, F. Rouquerol, J. Chem. SOC., Faraday Trans. I, 73 (1977) 306-314. 11 H. van Koningsveld, H. van Bekkum, J.C. Jansen Acta Cryst., B43 (1987) 127-132. 12 H. Lermer, J. Steffen, M. Drager, K.K. Unger Zeolites 5 (1985) 131-4. 13 A.V. Kiselev, A.A. Lopatkin, A.A. Shulga, Zeolites 5 (1985) 261-7. 14 R.M. Barrer, J.A. Davies, Proc. Roy. SOC. London, A320 (1970) 289. 15 S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan, E.M. Flanigen, Am. Chem. SOC., Symp. Ser. 218 (1983) 79-106. 16 U. Lohse, M. Noack, E. Jahn, Ads. Sci. Techn., 3 (1986) 19-24.
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HIGH RESOLUTION SORPTION STUDIES OF ARGON AND NITROGEN ON LARGE CRYSTALS OF ALUMINOPHOSPHATE A1P04-5 AND ZEOLITE ZSM-5
u.
MULLER',
K.K. UNGER',
Y. GRILLET',
F
DONGFENG
PAN^,
A. MERSMANN'
. ROUQUEROL3, J. ROUQUEROL3
'Institut fur Anorganische Chemie und Analytische Chemie, P. 0. Box 3980, Johannes Gutenberg-Universitat, D-6500 Mainz, F.R.G. 21nstitut fur Verfahrenstechnik der Technischen Universitat, Lehrstuhl B, Arcisstr. 21, D-8000 Miinchen 2, F.R.G. 3Centre de Thermodynamique et de Microcalorimetrie du C.N.R.S., 26 Rue du 141e R.I.A., 13003 Marseille, France ABSTRACT High resolution adsorption (HRADS) with argon and nitrogen at 77 K in the pressure range of c p/po < 0.5 were performed on large crystals of zeolite ZSM-5 (180 pm) and aluminophosphate AlP04-5 (150 pm) using a novel volumetric device. Multi-step isotherms of both adsorptives on ZSM-5 could be observed for the first time. The adsorption followed by low temperature microcalorimetry resulted in distinct exothermic signals at the steps in the adsorption isotherms. Based on the results of atom-atom potential energy calculations (AAP) as well as independent model building it was shown that 24 'kinetic' adsorbate molecules can be filled into a ZSM-5 unit cell. Experimental results are reasonably interpreted assuming a primary filling of narrow channels and a secondary adsorption in the wider channel intersections. Localized adsorption is understood as a possible filling mechanism. In ZSM-5 and nitrogen as adsorbate there is evidence for a transition of fluid-like to solid-like adsorbate phase. AlPOq-5 behaves as a homogeneous sorbent with a micropore capacity of four molecules per unit cell for argon and nitrogen. For both adsorptives in the molecular sieves under investigation the initial isosteric heat of adsorption for nitrogen was found to give values comparable to the enthalpies of adsorption derived from the temperature dependence of experimentally determined HENRY constants. HENRY constants and the initial isosteric heat of adsorption are indicative of a stronger adsorption of nitrogen compared to argon which is thought to be due to additional interactions between the nitrogen quadrupole moment and the crystalline molecular sieve framework. INTRODUCTION Zeolite ZSM-5 and aluminophosphate AlP04-5 have attracted considerable interest as microporous model adsorbents. Both materials are crystalline molecular sieves with strictly regular pore systems, viz. intersecting straight and zigzag 10-membered ring channels (ultramicropores) connected by larger cavities (micropores) in ZSM-5 111, and unidimensional 12-membered ring tubes (micropores) in A1POq-5 12 1 . Based on novel synthesis concepts 13,41 the growth of large uniform crystals at high yields and with narrow particle size distribution has been achieved. Employing large monocrystals in sorption investigations, no erroneous contributions by interparticle vapour condensation and external surface area
626 effects to adsorption occur as on polycrystalline powders. Thus, the intrinsic sorptive properties can be reliably determined with high precision over a wide range of coverage 8 . Gravimetric sorption experiments with nitrogen at 77 K on large ZSM-5 crystals led to the discovery of a sharp hysteresis loop at low pressures of p/po = 0.15 15-71. Furthermore, in contrast to the BDDT-model 181 which predicts a LANGMUIR-type of isotherms for microporous adsorbents, both argon and nitrogen revealed distinct multi-step isotherms on large ZSM-5 crystals whereas no deviation from type-I isotherms was seen for AlP04-5. This study presents experimental results obtained by combining high resolution adsorption techniques (HRADS) with low temperature microcalorimetry. The calculated thermodynamic properties like HENRY constants, heats of adsorption and atom-atom approximation for adsorbate-adsorbent interactions strongly suggest localized adsorption on energetically enhanced crystallographic sites. In addition, a transition of the adsorbate to a denser phase at higher coverage is assumed to explain the sorption behaviour of nitrogen on large ZSM-5 crystals. EXPERIMENTAL Large crystals, 600 pm, of AlPO4-5 131 were prepared by optimization of a procedure described elsewhere 191. A sample consisting of 150 pm hexagonal rods was used in this study. Monocrystals of HZSM-5 (180 pm with Si/A1 = 1000) were synthesized according to a process described in the literature 141. The products were characterized by scanning electron microscopy SEM. x-ray diffraction, thermal analysis and electron microprobe analysis. Additionally the HZSM-5 crystals were analyzed by IR and 29Si-MAS-NMR spectroscopy. Adsorption isotherms with argon and nitrogen at 77 K were recorded on a novel dynamic volumetric device (Omnisorb 360, OMICRON Corp., U.S.A.). pressure of p/po =
Data acquisition started at a relative
and was continued up to p/p" = 0 . 5 , thus collecting se-
veral hundred quasi-equilibrium points of the adsorption isotherm. Nitrogen adsorption isotherms at 303 to 430 K were determined gravimetrically at a relative sensitivity of lo6 (ultramicrobalance 4433, Sartorius, F.R.G.)
. Continuous calori-
metric measurements were performed on a reversible isothermal microcalorimeter
of Tian-Calvet type (C.N.R.S. Thermodynamique et Microcalorimetrie, Marseille, France). A detailed description of the microcalorimeter is given in the literature 1101. Prior to all experiments samples calcined at 823 K were degassed for 12 hours at 473 K and at a vacuum of
mbar.
CALCULATIONS Calculations of the intermolecular interactions of argon adsorbed on ZSM-5 and AlP04-5 respectively, were performed using atom-atom approximation for the potential energy (AAP) and Lennard-Jones 6:12 potentials. Only contributions of the framework oxygen atoms were taken into account, based on determinations of the refined structures of ZSM-5 111,121 in space group Pnma and P6cc for
627 A1P04-5
121. V a l u e s f o r t h e p o l a r i z a b i l i t y of a r g o n (1.63.10-3
nm3) a n d n i t r o g e n
(0.88-10-3 nm3) a s w e l l a s t h e i r r a d i i were t a k e n from l i t e r a t u r e d a t a 131. Cov-
e r a g e s €I were c a l c u l a t e d from t h e e x p e r i m e n t a l i s o t h e r m s as m o l e c u l e s of a d s o r bate per
n i t c e l l ( m o l e c / u c ) , u s i n g STP v a l u e s f o r a r g o n (1.784.10-3
nitrogen
1.251.10-3
g/cm3).
g/cm3) a n d
D a t a d e r i v e d by g r a v i m e t r y were c o r r e c t e d f o r buoy-
ancy w i t h t h e s o r b a n t c r y s t a l l i n e d e n s i t i e s . I s o s t e r i c h e a t s o f a d s o r p t i o n , qST, a r e t a k e n from t h e e x p e r i m e n t a l h e a t f l u x V(dp/dt) qST =
4 by
t h e equation:
-4 (1)
dn/dt
w i t h d n / d t as c o n t i n u o u s a d s o r b a t e u p t a k e r a t e a n d a c o r r e c t i o n V ( d p / d t ) f o r t h e h e a t l o s s d u r i n g t h e e x p a n s i o n o f gas i n t o t h e s a m p l e b u l b . The i s o s t e r i c h e a t of a d s o r p t i o n , qST, c a n b e compared w i t h t h e v a l u e of t h e d i f f e r e n t i a l e n t h a l p y of a d s o r p t i o n , q d i f f s
( q d i f f = qST - R T ) . From t h e i n i t i a l s l o p e of t h e i s o t h e r m s
t h e HENRY c o n s t a n t s KH were c a l c u l a t e d a c c o r d i n g t o €I = K H
-
p. The h e a t of ad-
s o r p t i o n , -AH, was d e r i v e d from HENRY c o n s t a r i t s u s i n g KH = Kfi e x p (-4H/RT). RESIJLTS and DISCUSSION I s o t h e r m s a n d isosteric h e a t of a r g o n a n d n i t r o g e n on AlP04-5 Type-I i s o t h e r m s o f a r g o n a n d n i t r o g e n (see F i g . 1, shown f o r N2) on large A1P04-5 c r y s t a l s w e r e o b s e r v e d f o r b o t h gases a t 77 K. A p p l y i n g LANGMUIR-plots as w e l l as t h e method of DUBININ-RADUSHKEVICH, a m i c r o p o r e f i l l i n g c a p a c i t y of
3.9
0.2 molec/uc f o r n i t r o g e n a n d 3 . 8 2 0 . 2 molec/uc f o r a r g o n w a s c a l c u l a t e d .
Dense a d s o r b a t e p h a s e s f o r n i t r o g e n a n d a r g o n i n AlP04-5 were assumed. I s o s t e r i c h e a t of a d s o r p t i o n , qST, r e a c h e d a maximum a t a c o v e r a g e o f a b o u t 3.5 m o l e c / u c ( 1 4 . 5 kJ/mol N2 and 1 3 . 2 kJ/mol A r ) a n d d e c l i n e d when t h e s a t u r a t i o n c a p a c i t y was e x c e e d e d . W i t h i n t h e e x p e r i m e n t a l e r r o r t h e i n i t i a l i s o s t e r i c h e a t , qST, f o r ads o r p t i o n o f n i t r o g e n (13 f 0.6 kJ/mol a t 0 e O . 1 ) c a n b e compared t o t h e h e a t o f adsorption, -AH,
c a l c u l a t e d from t h e t e m p e r a t u r e d e p e n d e n c e o f t h e HENRY c o n s t a n t
KH ( 9 . 9 2 3.3 k J / m o l ) . A t 77 K t h e l i n e a r s l o p e o f t h e n ' i t r o g e n i s o t h e r m a t
p / p o c 10-5 y i e l d e d v a l u e s of KH = 13.1 molec/uc'mbar a n d KH = 1 4 . 2 molec/uc.mbar r e s p e c t i v e l y . The l a t t e r was c a l c u l a t e d u s i n g t h e v i r i a l e q u a t i o n of BARRER 1141. F i g . 1. High r e s o l u t i o n a d s o r p t i o n (HRADS) i s o t h e r m s a t 77 K of a r g o n a n d n i t r o g e n o n large c r y s t a l s of a l u m i n o p h o s p h a t e A1P04-5 ( 1 5 0 pm) a n d z e o l i t e ZSM-5 ( 1 8 0 bm a n d S i / A 1 = 1000).
628 Isotherms and isosteric heat of argon and nitrogen on ZSM-5 Multi-step isotherms were observed for nitrogen on ZSM-5 at 77 K. Steps in the adsorbate uptake were monitored at coverages of 20, 22, 24 and 30.5 molec/uc, respectively (see Figs. 1,2). Standard deviations amounted to 0.8 molec/uc in the uptake values, based upon statistical certainties of 95%. The initial isosteric heat of adsorption of nitrogen was calculated to 16.5 kJ/mol. The exothermic peaks of 6.4 kJ/mol clearly coincide with the observed steps in the isotherm as illustrated in Fig. 2. The exothermic contributions at coverages of 20 to 22 and 24 to 30.5 molec/uc respectively, were comparable to the sum of the heats of
condensation (5.56 kJ/mol) plus solidification (0.72 kJ/mol) being required for phase transitions in tridimensional bulk nitrogen. At low coverage (0e0.1) values of qST (16.8 2 0.2 kJ/mol) equal the enthalpy change derived from van't HOFF plots 1.7 kJ/mol). HENRY constants for nitrogen on ZSM-5 at 77 K were computed
(18.2 to KH
=
830 molec/uc'mbar, using the virial equation at p/po*
decreased to KH
=
this value was
594 molec/uc.mbar.
Fig. 2. Nitrogen isotherm and isosteric heat of adsorption on large ZSM-5 crystals at 77 K. The argon adsorption on ZSM-5 occurred in steps of 20 and 24 molec/uc (Fig. 1). At these steps in the isotherm an exothermic increase (1.7 kJ/mol) of the isosteric heat of adsorption, qST, (see Fig. 3)occurred; however, the heat curve as well as the isotherm were less pronounced compared to the N2/ZSM-5 system. Potential energy distribution in A1P04-5 and ZSM-5 Results of atom-atom approximation (AAP) calculations for argon atoms adsorbed on A1P04-5 are summarized in Fig. 4. Six minima of the potential energy were found above the evenly distributed TO4 6-rings in the channel walls. Model-building with localized 'kinetic' gas molecules (0.38 nm diameter) at these minima, indicated a preferred population of three argon atoms per radial TO4 six-ring
629 channel segment. This represents a micropore filling of 4 molec/uc and is in close agreement with the experimentally observed data from sorption isotherms and enthalpy curves.
1 -7
-
.. .
.
... .,
Fig. 3. Argon isotherm and isosteric heat of adsorption on large ZSM-5 crystals at 77 K.
From AAP-calculations and independent model-building 24 minima of the potential energy in an unit cell of ZSM-5 were registered. Eight minima are situated at the channel wall 6-rings in the straight tubes (see Fig. 6) and twelve in the zigzag channels (see Fig. 7). Additional four ones are located at the wider channel intersections, thus giving a total of 24 rninima/uc. Note that in the argon and nitrogen isotherms steps occur at fillings of 20 and 24 molec/uc, respectively.
Fig, 4. Potential energy distribution (AAP approximation) for argon across a channel in A1P04-5.
630 Sorptive properties of large AlP04-5 crystals Adsorption studies of microcrystalline AlPOq-5 have been described in the literature 115-181. Unusual type-V isotherms of the system H*O/AlP04-5 have been reported and were attributed to crystal hydrate formation 1161. Studies of STACH and coworkers I17 I at ambient temperatures identified A1P04-5 as energetically homogeneous sorbent. At low temperatures type-I isotherms f o r oxygen and nitrogen are reported by WILSON et a1 1151 and BOND et a1 1181, respectively. However, these isotherms showed 30 to 40% of the final uptake in a relative pressure range of 0.05
of preadsorption of n-hexane on AlP04-5, previously investigated 131, clearly showed that the observed hysteresis was not caused by micropores in AlPO
5. 4The comparatively low heats of adsorption of argon and nitrogen obtained in
this study (see Fig. 5) support the view that AlP04-5 exhibits a mildly hydrophilic sorbent character. At low pressures the micropore filling capacity for kinetic gas molecules (4molec/uc) is reasonably predicted by theoretical AAPcalculations and model-building.
0.1
AlP04-5 I77 K1
I¶ I -
P/OO
kd-1
0.4
I¶
0.a 10
0.1
¶
0.1
0.0
0
l
a
a
4
1
Fig. 5. Nitrogen isotherm and isosteric heat of adsorption of argon and nitrogen on large AlP04-5 crystals at 77 K. Sorptive properties of large ZSM-5 crystals Data for nitrogen adsorption on polycrystalline powders of ZSM-5 habe been already reported in the literature 119,201. In general, the overall sorption capacities agree fairly well with this study. However, most of these measurements collected only few isothermal points (5 to 20) and were not extended into the low pressure range p/poe
A geometrical model of JACOBS et al. 1201 pro-
poses a closest-possible packing of nitrogen molecules in the larger channel intersections of ZSM-5. In contrast to this model, theoretical calculations of gas-
631 solid interactions by EVERETT and POWL 1211, recently adapted to zeolites by DEROUANE I22 I , clearly indicate an enhancement of adsorption energy in smaller pores of molecular diameter (e.g. 0.55 nm channels in ZSM-5). According to DUBININ 123 I these channel 'ultramicropores' are primarily filled due to overlapping force fields from opposite walls of the narrow micropores. With increase of the pore size up to 2-3 molecular diameters (i.e. 0.8 to 1.0 nm for argon and nitrogen and close to the space available in a ZSM-5 channel intersection) the superimposed enhanced adsorption energy deciines, and cooperative secondary rnicropore filling might occur 1241.
Fig.' 6. Potential energy distribution for argon in a straight channel of ZSM-5 (arrows: channel minima; asterix: minima at the intersection).
I ,-,, ,o..B/..-ciIl
Fig. 7. Potential energy distribution for argon in a zigzag channel of ZSM-5 (arrows: channel minima; asterix: minima at the intersections). For argon and nitrogen AAP calculations and model-building yielded a channel population of 20 molec/uc which is in full agreement with the first step in the adsorption isotherms (see Fig. 1). Further filling of the four larger intersec-
tions with one argon atom in each site is attributed to the second plateau in the argon isotherm, giving a final coverage of 24 molec/uc. At this stage all channels and intersections are occupied by kinetic gas molecules. The exothermic increase (1.7 kJ/mol) during the intersection filling (20 up to 24 molec/uc) might be due to adsorbate-adsorbate interactions (e.g. the heat of fusion for bulk liquid argon amounts to 1.21 kJ/mol). As already shown (figs. 1-3) the situation is more complex for the nitrogen in ZSM-5. Although the kinetic molecular diameter of argon (0.35 nm) and nitrogen (0.38 nm) are quite similar, nitrogen as a diatomic molecule offers a considerable
quadrupole moment which permits additional adsorbate-adsorbent interactions. Hence, the steps in the nitrogen isotherms are more pronounced and the isosteric heat of adsorption is enhanced (1.8 kJ/mol) compared to argon. After initial uptake of 20 molecules in the channels (i.e. first step in the isotherm) a further coverage up to 22 molec/uc follows. The latter connects the channel adsorbate (2 nm) to the overall crystal adsorbate macroscopic phase (at least 30,000 nm,
as taken from the crystal dimension along the (100) or (010) axis). At this point, monitored as a small substep in the isotherm, the adsorbate phase loses significantly in kinetic energy and mobility, as is obvious from Fig. 2. At a coverage of 24 molec/uc, corresponding to a filling of all channels and intersections, the largest step in the isotherm up to 30.5 molec/uc takes place. This unusual behaviour is known to be correlated with a sharp type-A hysteresis loop between the adsorption and desorption branch of the isotherm 15,61. It has independently been observed even on smaller ZSM-5 crystals 171. The energy which is dissipated when tridimensional nitrogen is transformed from a gaseous to a solid phase is close to the value of 6.3 kJ/mol which has been experimentally observed during the densification of the fluid adsorbate in ZSM-5. The uptake ratio of 24/30.5 molec/uc before and after the inflexion of the isotherm at p/po
=
0.15 is identical with the ratio of the density of liquid (0.808g/cm3)
to solid nitrogen (1.027 g/cm3). Freezing of adsorbed 2D nitrogen at 77 K is a well documented phenomenon occurring on homogeneous graphite or boron nitride as adsorbents 110,251, and complex phase diagrams have been calculated 1261. Abrupt transitions of dense fluid-like to solid-like adsorbate in slit-shaped pores was theoretically predicted over a range of 1 - 2 up to 3 - 4 adsorbate layers 1271. Adapted to ZSM-5, this situation occurs at the completion of fluid filling
in the wider intersections, where
the 2D channel adsorbate is contacted with a
3D adsorbate phase. Furthermore this result can be explained by the 'site-andbond' network hypothesis of MAYAGOITIA, which assumes cooperative filling processes when half of the network sites (i.e. intersections in ZSM-5 at N
=
22
molec/uc) are filled 1281. Monte Carlo simulations of gases in narrow pores suggest that phase transitions and metastable states are formed which should show up
in experimental sorption isotherms as hysteresis effects 1291. This is con-
633 sistent with earlier observations on the existence of a hitherto unknown lowpressure hysteresis loop in the nitrogen ZSM-5 system 15-71. CONCLUSION The results of both high resolution adsorption and microcalorimetry at low temperatures provide clear evidence that the stepped isotherms of argon and nitrogen on large ZSM-5 crystals can be rationally explained by localized adsorptive molecules at the channel walls and the channel intersections. The findings are consistent with theoretical predictions. Heats of adsorption on A1P04-5 are lower compared to ZSM-5. Further investigations on unidimensional zeolitic materials (e.g. high silica ZSM-12) should show whether this behaviour is caused by different surface force fields of aluminophosphates and zeolites or if it is due to the different pore sizes of 10-membered and 12-membered micropore channels. At least nitrogen adsorbed at higher loadings in the ZSM-5 network should be considered as an immobile dense phase. Low temperature NMR and neutron diffraction experiments are in progress to further elucidate the adsorbate structure of gases in microporous adsorbents. ACKNOWLEDGEMENTS We are grateful to Dr. G. Engelhardt, Konstanz, and Dr. H.-G. Karge, Berlin,
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