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Calorimetric Measurements of the Systems Zeolite-Ammonia and Zeolite-n-Heptane in a Range of Temperature from 0° to 300°C K.-H. SICHHART, P. KOLSCH, and W. SCHIRMER Department of Adsorption of the Central Institute of Physical Chemistry of the German Academy of Science, Berlin, East Germany
Using a conduction calorimeter (improved type TianCalvet), we measured the differential enthalpies of adsorption —ΔH (kcal/mole) of the system zeolite NaCaA-NH within a temperature range from 23° to 300°C. We observed a step-like dependence of the values of —ΔH from the degree of pore volume filling of the zeolite. This result leads to the assumption that certain structures are formed between the NH molecules and the cations present in the zeolite cages. A drop calorimeter was used to measure the heat capacities of the system NaCaA zeolite—n-heptane in the temperature range 25°-240°C. The heat capacity shows a high maximum at approximately 100°C, especially for very small values of pore volume filling. This behavior may be explained by cooperation between defect structures of the adsorbent and configuration effects of the hydrocarbon chain. 3
3
For the determination of thermodynamic adsorption parameters, we used a drop calorimeter to measure specific heats and a conduction calorimeter to measure enthalpies of adsorption. In general, 2 types of calorimeters are used for measurement of specific heats, the adiabatic heated and the drop calorimeter. We chose the latter type for our study of specific heats because we needed the enthalpy function for our thermochemical calculations. This is the in tegral of the specific heat up to the experimental temperature, and can not be measured directly by an adiabatically heated calorimeter. At higher temperatures the integration error will be considerable, so that 132 Flanigen and Sand; Molecular Sieve Zeolites-II Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
48.
siCHHART E T A L .
Calorimetric
133
Measurements
i t is better to use t h e d r o p m e t h o d w h i c h p r o v i d e s t h e e n t h a l p y differ ences d i r e c t l y . The
filled
s a m p l e container is heated to t e m p e r a t u r e T . T h e n the
s a m p l e is t h r o w n i n t o the c a l o r i m e t e r at t e m p e r a t u r e T . K
test, the e n t h a l p y difference Η -Η τ
Τκ
After a blind
of t h e i n v e s t i g a t e d substance c a n
be c a l c u l a t e d . T h e c a l o r i m e t e r w h i c h w e c o n s t r u c t e d has a p r e c i s i o n of 0 . 0 2 - 0 . 1 % . M e a s u r e m e n t s are possible b e t w e e n 2 5 ° a n d 1 0 0 0 ° C . T h e c a l i b r a t i o n c o n stant r e d u c e d to 25 ° C is 2533.08 ± of m =
0.25 J / g r d w i t h a s t a n d a r d d e v i a t i o n
1.02 J / g r d ( 6 ) .
W e b u i l t a c o n d u c t i o n c a l o r i m e t e r of the T i a n - C a l v e t t y p e to m e a sure the h e a t o f a d s o r p t i o n o f gases o n zeolites.
F i g u r e 1 shows t h e
c o n s t r u c t i o n of this calorimeter. T h e m e t a l b l o c k reaches a t e m p e r a t u r e constancy of 0 . 0 1 ° - 0 . 0 3 ° C .
A t a b o u t 300 ° C , w e o b t a i n the same values
w i t h a n a u t o m a t i c adjustment.
Figure 1.
T h e c a l o r i m e t e r attains a t i m e constancy
Calorimeter
construction
A: Ag vessel B: External boundary C: Al block D: Electric heater E: Al cylinder F: 5 Al jackets G: Glass wool isolation H: Air isotàion I: Asbestos isolation K: Ceramic props
Flanigen and Sand; Molecular Sieve Zeolites-II Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
134
M O L E C U L A R
of 83 seconds a n d a sensitivity of 2.12 χ is 0.4%
(S).
10"
4
SIEVE
ZEOLITES
II
c a l / s e c m m ; the p r e c i s i o n
T h e d i f f e r e n t i a l heats of a d s o r p t i o n of a m m o n i a o n z e o l i t e
of the t y p e N a C a A ( 6 6 %
i o n e x c h a n g e ) have b e e n m e a s u r e d at
23 ό
3 0 0 ° C b y this calorimeter. F i g u r e 2 shows the results of these studies f o r 2 3 ° , 1 0 0 ° , 2 0 0 ° , a n d 3 0 0 ° C . W e o b s e r v e d heats of a d s o r p t i o n , 2 2 - 2 5 k c a l / m o l e , at the l o w e s t values of a m m o n i a content a n d a decrease of this v a l u e to 2 0 - 2 1
kcal/
m o l e at one m o l e c u l e of a m m o n i a p e r large cage. T h e heat of a d s o r p t i o n r e m a i n s constant at this v a l u e u p to the a d s o r p t i o n of 3.5 m o l e c u l e s / c a g e . T h i s c a n b e seen most d i s t i n c t l y at a m e a s u r i n g t e m p e r a t u r e of 200 ° C . B e t w e e n 3.5 a n d 4 m o l e c u l e s / c a g e , the heat of a d s o r p t i o n decreases to 14-15
kcal/mole.
T h e heat curves at 2 3 ° a n d 100 ° C also s h o w these steps b u t i n this case the decrease of heat of a d s o r p t i o n occurs i n the r a n g e 2.5-5 cules/cage.
T h e measurements
mole
at 300 ° C s h o w n o steps, a n d the values
decrease almost l i n e a r l y f r o m 25 to 10 k c a l / m o l e . W e i n t e r p r e t these results b y a s s u m i n g that the h i g h values for the heats of a d s o r p t i o n p e r t a i n to s p e c i f i c a l l y active centers, s u c h as lattice holes ( 7 ) .
D i e l e c t r i c r e l a x a t i o n measurements (4)
a n d K M R studies
(2)
substantiate s u c h conclusions. T h e adjacent step r e p r e s e n t e d b y values of 2 0 - 2 1
k c a l / m o l e leads
to the a s s u m p t i o n that the first m o l e c u l e s of a m m o n i a are a d s o r b e d at the 4 C a
2 +
cations i n the cage.
T h i s is s h o w n b y measurements
of the
a d s o r p t i o n of a m m o n i a i n several N a X zeolites p a r t i a l l y c o n t a i n i n g t r a n -
Flanigen and Sand; Molecular Sieve Zeolites-II Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
48.
siCHHART E T A L .
Calorimetric
135
Measurements
s i t i o n m e t a l cations a n d b y studies of t h e d i e l e c t r i c r e l a x a t i o n of t h e sys tems H 0 - N a C a A a n d H 0 - C a A 2
2
(4).
A v a l u e of 23 k c a l / m o l e ( 7 ) f o r t h e i n t e r a c t i o n of a m m o n i a w i t h t h e Ca
2 +
c a t i o n w a s c a l c u l a t e d f r o m t h e e n e r g y of a d s o r p t i o n of a m m o n i a
o n zeolite N a C a A b y c o m m o n p o t e n t i a l m e t h o d s ( analogous to R e f . 1 ). I n R e f . 3 other authors f o u n d s e v e r a l ranges of a d s o r p t i o n w h i c h are a c c o u n t e d f o r b y interactions w i t h t h e cations. T h e i n t e r p r e t a t i o n i n o u r p a p e r ( 7 ) thus is c o n f i r m e d . I n t h e t r a n s i t i o n r a n g e of temperatures, n o extreme d r o p is o b s e r v e d . T h i s c a n b e e x p l a i n e d b y d i f f u s i o n a l i n h i b i t i o n of t h e m o l e c u l e s w h i c h are a d s o r b e d at these temperatures
(I).
A f u r t h e r step f o l l o w s . O u r
e x p l a n a t i o n f o r t h e steep slope of the heats of a d s o r p t i o n at 3 0 0 ° C i n this case is that t h e a d s o r b e d a m m o n i a m o l e c u l e s h a v e a h i g h m o b i l i t y a n d d o n o t o c c u p y fixed positions. T h e s e measurements h a v e b e e n car r i e d o u t w i t h a p r e c i s i o n of ± 2 % . D i r e c t measurements
of t h e heat capacities of the system
N a C a A - n - h e p t a n e have b e e n m a d e i n t h e t e m p e r a t u r e r a n g e
zeolite
25°-240°C
w i t h t h e d r o p c a l o r i m e t e r d e s c r i b e d above. I n F i g u r e 3, the heat c a p a c i t y is s h o w n as a f u n c t i o n of t e m p e r a t u r e at 3 coverages, w i t h c o r r e c t i o n f o r the heat of a d s o r p t i o n . T h e q u a n t i t y [JgrdHol]
20UH
1500
moo
- -CpP-HÊpian
40
Figure
3.
160
Heat capacity zeolite
of the system NaCaA
240 ΤΙΚ]
n-heptane-
Flanigen and Sand; Molecular Sieve Zeolites-II Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
136
MOLECULAR
SIEVE
ZEOLITES
of a d s o r b e d η-heptane, a, is s h o w n i n m m o l e / g r a m zeolite.
The
II
heat
c a p a c i t y goes t h r o u g h a m a x i m u m b e t w e e n 8 0 ° - 1 2 0 ° C , r i s i n g to values m u c h l a r g e r t h a n c a n be a t t r i b u t e d to the p u r e η-heptane. T h i s m a x i m u m i n the curves w a s e x p e c t e d after h a v i n g m e a s u r e d isosteres of the system zeolite N a C a A - n - h e p t a n e ( 5 ) .
T h e h e i g h t of the m a x i m u m decreases
with loading. T h i s effect c a n be e x p l a i n e d o n l y b y r e g a r d i n g the system z e o l i t e a d s o r p t as a w h o l e . C e r t a i n defect structures of the adsorbent a n d a d d i t i o n a l c o n f i g u r a t i o n effects of the c h a i n of the h y d r o c a r b o n m u s t c o i n c i d e to g i v e these results. W e are p r e p a r i n g a q u a n t i t a t i v e e s t i m a t i o n of this problem for publication. Literature (1) (2) (3) (4) (5) (6) (7) (8) RECEIVED
Cited
Barrer, R. M . , Gibbons, R . M . , Trans. Faraday Soc. 1963, 59, 2569. Bulow, M . , Kohler, D., Spangenberg, H . J., Z. Chem. 1969, 9, 317. Dubinin, M . M . , Isirikian, Α. Α., Sarachow, A . I., Serpinksi, V . V., Izv. Akad. Nauk, SSSR Ser. Khim. 1969, 2355. Lohse, U., Stach, H., Hollnagel, M., Schirmer, W., in press. Meinert, G., Thesis, Humboldt-University, Berlin, 1969. Muscheites, K., Thesis, Humboldt-University, Berlin, 1967. Schirmer, W . , Grossmann, Α., Fiedler, K., Sichhart, K . - H . , Bulow, M . , Chem. Tech., in press. Sichhart, K.-H., Thesis, TH Merseburg, 1967.
February 18, 1970.
Discussion L . V . C . Rees ( I m p e r i a l C o l l e g e , L o n d o n ) : W h e n zeolites h a v e i n c r e a s i n g amounts of p o l a r m o l e c u l e s s o r b e d i n t h e m , the cations s h o w a great increase i n m o b i l i t y w h e n certain l o w fillings are o b t a i n e d . Is i t not possible that the decrease i n the heat of s o r p t i o n of N H at l o w iso 8
t h e r m temperatures represents the p o i n t w h e r e the cations loosen t h e i r a t t a c h m e n t to the f r a m e w o r k ? T h i s w o u l d be an e n d o t h e r m i c m o v e m e n t of a p o s i t i v e c h a r g e f r o m the negative f r a m e w o r k . A t 3 0 0 ° C , the cations w o u l d h a v e sufficient t h e r m a l energy to mask this effect. W.
Schirmer: I s h o u l d suppose that the l o o s e n i n g process of the
cations w o u l d take p l a c e at h i g h e r coverages, w h e r e the decrease i n heat of a d s o r p t i o n is rather l o w . If this process takes p l a c e — w e took i t i n t o c o n s i d e r a t i o n , t o o — t h e n w e m u s t get, for a certain range of
coverage,
a n a p p r o x i m a t e l y h o r i z o n t a l p a r t of the A vs. θ c u r v e ( a l l the C a cations b e i n g e q u a l i n e n e r g y ) — a p h e n o m e n o n w h i c h w e d i d not observe u n t i l now.
Flanigen and Sand; Molecular Sieve Zeolites-II Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
48.
siCHHART E T A L .
Calorimetric
137
Measurements
N . G . Parsonage ( I m p e r i a l C o l l e g e , L o n d o n ) : If one integrates u n d e r the c u r v e f o r a =
0.0007 i n F i g u r e 3, one obtains — 6 0 k c a l / m o l e f o r the
excess heat. T h i s seems to b e m u c h too l a r g e for a process
concerned
w i t h p h y s i c a l s o r p t i o n a n d suggests that a c h e m i c a l process is i n v o l v e d . A t these temperatures
o n the sieve, a c h e m i c a l r e a c t i o n w o u l d
appear
to b e q u i t e possible. W o u l d y o u agree? W . Schirmer: A l t h o u g h w e d i d not yet finish o u r i n v e s t i g a t i o n of this t o p i c , w e s h o u l d l i k e to c o n c l u d e that there m u s t b e a c h e m i c a l reac t i o n , p e r h a p s b e t w e e n some O H g r o u p s a n d the h y d r o c a r b o n m o l e c u l e . W e s h a l l use other techniques ( i n f r a r e d , N M R ) to solve the p r o b l e m .
Flanigen and Sand; Molecular Sieve Zeolites-II Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
