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Steric Effects in Nitrogen Adsorption by Mordenite D. T. HAYHURST Cleveland State University, Department of Chemical Engineering, Cleveland, OH 44115 M. D. SEFCIK Monsanto Company, Corporate Research Department, St. Louis, MO 63166

Zeolites are well known for their a b i l i t y to adsorb a large variety of gases, including even highly v o l a t i l e nonpolar permanent gases (1, 2). Nitrogen is among the most strongly adsorbed non-polar gases; in fact, among some of the earliest observations on adsorptive separation of gaseous mixtures was the selectivity shown by several zeolites for nitrogen from a i r . Early work of Barrer (3), McKee (4) and Domine and Hay (5) showed that calcium A, calcium X, mordenite and several types of natural zeolites could be used to enrich a i r by a selective adsorption of nitrogen. Several pressure-swing-adsorption processes u t i l i z i n g zeolite adsorbents have been developed which yield a product containing up to 95% oxygen at rates of 20 tons per day (6,7). The selective adsorption of nitrogen over oxygen is not a true molecular sieving effect since nitrogen (3.64A) is larger than oxygen (3.46A). Thus, the increased a f f i n i t y of many zeolites for nitrogen can be ascribed to a greater heat of adsorption for nitrogen than for oxygen (8). The i n i t i a l heat of adsorption i n zeolites adsorbents can be partitioned into contributions from repulsive and dispersive forces, polarization energy and electrostatic interactions arising from dipole and electric quadrupole moment interactions with electric fields in the zeolite (9). At higher concentrations of adsorbates, interactions between adsorbates must also be considered (10). The physical constants of oxygen and nitrogen (11) suggest that the heat of adsorption due to repulsive, dispersive and polarization effects should be similar. Contributions to the heat of adsorption by the electric-quadrupole field-gradient interactions are expected to be greater for nitrogen whose quadrupole moment is three times larger than that of oxygen. Barrer (12) has estimated that the electrostatic energy contri-

0097-6156/83/0218-0333$06.00/0 © 1983 American Chemical Society Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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INTRAZEOLITE CHEMISTRY

b u t i o n to the i n i t i a l heat o f a d s o r p t i o n f o r n i t r o g e n on Na-mord e n i t e is 2.5 K c a l / m o l e and 3.4 K c a l / m o l e f o r a d s o r p t i o n on N a faujasite. I t is p o s s i b l e , however, that these r e s u l t s s e r i o u s l y underestimate the true i n t e r a c t i o n . The r e s u l t s o f B a r r e r were c a l c u l a t e d from experimental data obtained i n a r e g i o n where one may reasonably expect that adsorbed n i t r o g e n is r o t a t i n g i s o t r o p i c a l l y (13, 1 4 ) . The e l e c t r i c - q u a d r u p o l e f i e l d - g r a d i e n t energy 0J.Q is given by, ^ ^ F - Q ' l ?

Ocoe

1)

Θ-1)

2

where F is the f i e l d g r a d i e n t , Q the e l e c t r i c quadrupole moment, r is the s e p a r a t i o n between the quadrupole and charge and θ is the angle between the quadrupole a x i s and the f i e l d g r a d i e n t . If the molecule has some r o t a t i o n a l m o b i l i t y i n i t s a d s o r p t i o n s i t e , the e x p r e s s i o n (3cos^9-l) i n Equation 1 must be replaced by^ Q where is the average value of (3cos Θ-1) weighted o v e r a l l all a l l o w a b l e o r i e n t a t i o n s o f the quadrupole a x i s . For a n o n r o t a t i n g molecule o r i e n t e d so that 9=0, = (3cos θ - 1 ) = 2 and f o r an i s o t r o p i c a l l y r o t a t i n g molecule the v a l u e of Q is z e r o . F o r a n i s o t r o p i c a l l y r o t a t i n g molecules ( 3 c o s 9 - l ) can have v a l u e s between - 1 and 2 depending on the o r i e n t a t i o n o f the r o t a t i o n a x i s . I f n i t r o g e n is adsorbed i n t o s i t e s which reduce o r e l i m i n a t e i t s r o t a t i o n a l freedom and, i f the n i t r o g e n quadrupole is a l i g n e d w i t h the c a t i o n f i e l d g r a d i e n t , then a s i g n i f i c a n t i n c r e a s e i n the v a l u e o f 0-Q may be expected. T h i s was e x a c t l y the r e s u l t observed f o r CO2 adsorbed on s m a l l - p o r t mordenite (19). At low coverages, CO2 molecules were adsorbed i n t o the pore shaped s i d e pockets and h e l d w i t h l i t t l e r o t a t i o n a l freedom. Z e o l i t i c cat ions are l o c a t e d a t the end o f each of these s i d e pockets (16), so #F-Q is maximum and, consequently, the heat o f a d s o r p t i o n is high. I t is the o b j e c t i v e o f t h i s research to demonstrate that for c e r t a i n adsorption conditions, s i m i l a r side-pocket adsorption is s i g n i f i c a n t f o r n i t r o g e n l e a d i n g to high heats of a d s o r p t i o n . θ

θ

2

a

EXPERIMENTAL Materials The samples used i n t h i s study were both n a t u r a l and s y n t h e t i c v a r i e t i e s o f mordenite and a r e l i s t e d i n Table I . Each m a t e r i a l contained more than 95% pure z e o l i t e w i t h the remainder being amorphous m a t e r i a l as measured by x - r a y d i f f r a c t o m e t r y . Before being used i n experimentation, each sample was exchanged to the sodium form.

Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

20.

HAYHURST AND SEFCIK

Nitrogen Adsorption by Mordenite

335

TABLE I ZEOLITE SAMPLES USED Sample Zeolon 900 HB-33 (Si/Al=7.75) HB-33 dealuminated Alaskan Mordenite ZE-502 Zeoharb

Z e o l i t e NaA

a. b.

b

Source (or s u p p l i e r )

a

(Si/Al=126)

Norton Company Norton Company Norton Company Horn's Mountain Area S i t e #45 Osaka Oxygen Industries Ltd. Japan Union Carbide Corp.

Lot No. 40644 L o t No. 6EH L o t No. 6EH D . B . Hawkins(18)

Lot No. 6203738

T h i s sample was dealuminated using procedures o u t l i n e d by Chang (17). The Zeoharb is a n a t u r a l mordenite mined i n Japan. Deposit l o c a l i t y is unknown.

Ion exchange was performed by crushing and s i e v i n g the z e o l i t e to between 70-200 mesh ( T y l e r standard). Approximately 50 grams was placed i n t o an Erlenmeyer f l a s k , 250 ml of IN NaCl s o l u t i o n was added and the s l u r r y was a g i t a t e d f o r 16 hours. This procedure was repeated f o r a t o t a l of f i v e exchanges. A f t e r fhe f i n a l exchange, the s l u r r y was washed s e v e r a l times w i t h d i s t i l l e d water i n a Buchner f u n n e l and the f i l t e r cake was placed i n a 1 1 0 ° C d r y i n g oven o v e r n i g h t . The d r i e d cake was mulled gently before u s e . Each mordenite was t r e a t e d using the same procedure except f o r the HB-33. Since t h i s sample was received i n the unexchanged form, the sample was a c t i v a t e d i n vacuum and exposed to anhydrous ammonia at room temperature to i n s u r e each z e o l i t i c c a t i o n was ammonium. The sample was then sodium ex changed u s i n g the procedure o u t l i n e d above. Adsorption

Adsorption isotherms were determined on a Cahn 1000 vacuum micro-balance system, using a Leybold-Heraeus Turbotronic TMP 120 turbomolecular pump f o r evacuation and a v a r i a b l e - l e a k v a l v e f o r adsorbate gas i n j e c t i o n . Approximately O.3 grams of the sodium-exchanged z e o l i t e was placed i n t o a 1 cm. diameter hemispherical quartz sample pan w i t h i n the adsorption chamber of the balance and a c t i v a t e d . The a c t i v a t i o n was accomplished by heating the mordenite to 4 0 0 ° C - 2 C under a vacuum of < 5 χ 10~6 t o r r and maintaining these c o n d i t i o n s f o r


INTRAZEOLITE

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CHEMISTRY

16 hours. The a d s o r p t i o n run was i n i t i a t e d by a l l o w i n g the sample to c o o l to run temperature and i n j e c t i n g the adsorbate gas. Adsorption e q u i l i b r i u m was assumed when the sample weight v a r i e d l e s s than 1 χ 10 grams i n 30 minutes. The e q u i l i b r i u m n i t r o g e n c a p a c i t y was measured at pressures from O.2 to 700 mmHg using a 170 s e r i e s MKS Baratron high accuracy e l e c t r o n i c manometer. When the f i n a l e q u i l i b r i u m pressure was reached, the sample was r e a c t i v a t e d . Three r e p l i c a t e s of the Alaskan mordenite sample were run to i n s u r e experimental accuracy and r e p r o d u c i b i l i t y . N-15 NMR A n a l y s i s Nitrogen-15 nmr spectra were obtained at 9.12 and 20.72 MH on spectrometers using e x t e r n a l l ^ F time-share, f i e l d - f r e q u e n c y l o c k . D e t a i l s of the spectrometer m o d i f i c a t i o n s are published elsewhere (19). Samples of approximately one gram of sodium-exchanged Alaskan mordenite were placed i n 10 mm O. D . nmr tube f i t t e d w i t h o - r i n g s e a l s and degassed under vacuum w i t h a programmed temperature r i s e to 3 5 0 ° C . A f t e r c o o l i n g , the samples were allowed to e q u i l i b r a t e w i t h a known pressure of (Prochem L t d . ; 95% ^N) and s e a l e d . Spectra were o b t a i n e d o n samples e q u i l i b r a t e d with 10, 72, 206, 760 and 792 mmHg N cor responding to O.16, O.54, 1.02, 2.26 and 2.32 g N adsorbed/100 g mordenite. Z

2

2

RESULTS AND DISCUSSION Adsorption Isotherms The e q u i l i b r i u m a d s o r p t i o n c a p a c i t y f o r n i t r o g e n was determined f o r each of the samples t e s t e d . Isotherms were measured i n the range of O.2 to 700 mmHg adsorbate gas pressure and at 22 , 40 and 65 C. Due to the amount of data taken, (and to reduce the number of f i g u r e s presented) a d s o r p t i o n data were f i t t e d to F r e u n d l i c h isotherm model. T h i s model was chosen a f t e r t e s t i n g a p p l i c a b i l i t y of s e v e r a l models, i n c l u d i n g the Henry's Law, Langmuir and Temkin models. In each i n s t a n c e , the F r e u n d l i c h isotherm was found to provide a s i g n i f i c a n t l y b e t t e r agreement w i t h the obtained d a t a . The F r e u n d l i c h equation is an exponential r e l a t i o n of gas l o a d i n g w i t h adsorbate gas p r e s s u r e . The form of the equation used f o r curve f i t t i n g is given below: X = KP m

2)

1 / n

where X/m is the grams of n i t r o g e n adsorbed per 100 grams of anhydrous z e o l i t e , Κ and η are the F r e u n d l i c h constants and Ρ is the e q u i l i b r i u m adsorbate gas pressure i n mmHg. To determine the q u a l i t y of f i t f o r the a d s o r p t i o n model, a c o e f f i c i e n t of determination, r , was c a l c u l a t e d f o r each 2


20.


HAYHURST AND SEFCIK

337

isotherm. A v a l u e of τ equal to 1.00 i n d i c a t e s a p e r f e c t fit. The F r e u n d l i c h constants and the c o e f f i c i e n t of d e t e r mination f o r each z e o l i t e are l i s t e d i n Table I I . 1

TABLE II FREUNDLICH CONSTANTS AND COEFFICIENT OF DETERMINATION Sample Zeolon

900 II II

Zeolon HB33 It

Zeolon HB33 dealuminated II It

Alaskan Mordenite

ZE-502 Zeoharb II

tt

Z e o l i t e NaA

a.

T(C°) 20 43 67 26 40 64 26 40 64 22 41 64 22 40 65 23 42 65

3 Kxl0

a

l/n

J

32.7 11.6 8.85 8.46 4.07 2.82 7.51 4.77 1.32 39.6 26.7 9.70 12.2 4.79 2.68 12.1 1.58 O.90

a

O.65 O.73 O.68 O.76 O.84 O.83 O.54 O.57 O.74 O.61 O.60 O.70 O.70 O.80 O.80 O.56 O.88 O.88

2 r 1. 00 1. 00 1. 00 O. 98 99 1. 00 O. 91 O. 95 O. 95 1. 00 O. 99 O. 98 O. 99 O. 99 O. 99 O. 92 1. 00 O. 98

σ.

From Equation 2.

Heats of A d s o r p t i o n The i s o s t e r i c heats of a d s o r p t i o n , A H £ , were c a l culated as a f u n c t i o n of n i t r o g e n l o a d i n g f o r each of the samples t e s t e d . For each sample, the A H i was found to decrease l o g a r i t h m i c a l l y w i t h increased l o a d i n g and t h i s f u n c t i o n a l i t y is c o n s i s t e n t with the assumptions of the F r e u n d l i c h adsorption equation. As b e f o r e , the b r e v i t y of p r e s e n t a t i o n , the heat of a d s o r p t i o n data were curve f i t t e d to an equation of the form: SO

s o

A H

iso

β

a + bln(X/m)

3)

where a and b are constants and X/m is the l o a d i n g as expressed i n the isotherm equation. The equation constants f o r each of the samples tested are l i s t e d i n Table I I I . Two t y p i c a l heat of a d s o r p t i o n curves are presented i n F i g u r e 1. Using these heats of adsorption curves, the i n i t i a l heats AHiso, were determined f o r the f i v e mordenite samples. Values f o r the AHiso were derived by a l i n e a r e x t r a p o l a t i o n of


338

INTRAZEOLITE

Figure 1.

CHEMISTRY

Heat of adsorption for Alaskan mordenite ( , ) and for Zeolon 900 ( e).


20.

HAYHURST AND SEFCIK


339

data taken at low n i t r o g e n l o a d i n g s . The i n i t i a l heats are p l o t t e d as a f u n c t i o n of s i l i c o n to aluminum r a t i o i n F i g u r e 2 and are l i s t e d i n Table I I I . These i n i t i a l heats of adsorption are s i g n i f i c a n t as they i n d i c a t e the energies of i n t e r a c t i o n between the n i t r o g e n and the most e n e r g e t i c a l l y f a v o r a b l e a d s o r p t i o n s i t e s i n the mordenite s t r u c t u r e . At these low l e v e l s of n i t r o g e n a d s o r p t i o n , the i n t e r a c t i o n energy due to adsorbateadsorbate r e p u l s i o n are n e g l i g i b l e . TABLE

III

HEAT OF ADSORPTION CONSTANTS, INITIAL HEATS AND SILICON ALUMINUM RATIOS Constants a b

Sample Zeolon 900 Zeolon HB33 Zeolon HB33 (Si/Al=126) Alaskan Mord. ZE502 Zeoharb Z e o l i t e NaA Barrer (19) T a k a i s h i (13)

Coef. Determ.

Range(gN /100g) I n i t i a l Final 2

b

Si/Al

iso

6.41 4.44 3.36

-O.76 -O.22 -O.30

O.97 O.85 O.91

O.01 O.01 O.01

1.00 O.80 O.20

4.18 7.75 126

9.5 6.0 5.5

6.72 3.89 3.49

-O.82 -1.08 -O.11

O.91 O.91 O.82

O.01 O.01 O.01

1.30 O.75 O.25

3.93 4.68

9.3 9.3 6.5 7.0 6.6

—

4.75 5.05

a . From Equation 3. b . T h i s is the range of n i t r o g e n loadings (gN2/100 g mordenite) where t h i s c o r r e l a t i o n is v a l i d . c. Determined by i n d u c t i v e l y - c o u p l e d argon plasma o p t i c a l emission spectroscopy by Monsanto Company's P h y s i c a l Science Center. The i n i t i a l heats of adsorption (Figure 2) of n i t r o g e n on mordenite show a marked decrease as the s i l i c o n content of the z e o l i t e increases. The r e s u l t i n g tflf curve is not a con tinuous f u n c t i o n , but r a t h e r has an abrupt d i s c o n t i n u i t y at S i / A l of 4.70. The curve c o n s i s t s of two approximately h o r i z o n t a l l i n e s having Δ Η ι equal to 9.3 to 9.5 at S i / A l below 4.70 and A H i of 7.0 to 6.0 being obtained above the c r i t i c a l Si/Al ratio. These r e s u l t s i n d i c a t e d that the i n i t i a l n i t r o g e n a d s o r p t i o n i n mordenite takes place at two s i t e s e n e r g e t i c a l l y d i f f e r e n t by 2.5 Kcal/mole and the existance of the high energy s i t e s r e q u i r e that the S i / A l be l e s s than 4.70. Z e o l i t e pore diameters increase as aluminum is i s o m o r p h i c a l l y s u b s t i t u t e d f o r s i l i c o n i n the c r y s t a l s t r u c t u r e . Given the r e l a t i o n s h i p observed between the i n i t i a l heat of adsorption and the s i l i c o n to aluminum r a t i o , we suggest that a s t e r i c a l l y d i f f e r e n t adsorption s i t e becomes a v a i l a b l e f o r n i t r o g e n when the S i / A l is below the c r i t i c a l v a l u e of 4.70. so

8 0

s o



Figure 2. The initial heat of adsorption for various S i / A l .

(φ)

SUCON/ALUMINMJM This study and (•) literature values.

20.

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A N D SEFCIK

341

The a v a i l a b i l i t y of such a new s i t e would most l i k e l y be the consequence of the mordenite s t r u c t u r e undergoing a c r y s t a l l a t t i c e expansion, opening the 8-membered r i n g s i d e pocket f o r adsorption. In order to i n s u r e the v a l i d i t y of the r e s u l t s p r e sented, heats c a l c u l a t e d were found to be i d e n t i c a l to those measured f o r NaA at near-ambient-temperature by other i n v e s t i g a t o r s (21, 22). Nitrogen-15 NMR A n a l y s i s The nitrogen-15 nmr s p e c t r a (Figure 3) of sodium exchanged Alaskan and HB33 mordenites were analyzed f o r the chemical s h i f t anisotropy t e n s o r s . The chemical s h i f t a n i s o tropy tensors have been measured f o r s o l i d n i t r o g e n at 4.2K(23) and c a l c u l a t e d to be 603.28 ppm f o r the s t a t i c m o l e c u l e . The observed chemical s h i f t tensors can be used to c a l c u l a t e an o r i e n t a t i o n a l parameter ξ, where 4)

J i s the observed, m o t i o n a l l y averaged chemical s h i f t anisotropy axiàC^ ~ 2

5)

9

where θ depends on the angular amplitude o f the r o t a t i o n . The same motion which reduces the chemical s h i f t d i s p e r s i o n of the adsorbed n i t r o g e n a l s o reduces the f i e l d gradient quadrupole i n t e r a c t i o n energy. For a molecule which is undergoing r o t a t i o n we can combine Equations 1 and 5 to give

i
4 . 7 0 , the side-pocket dimensions are reduced to the p o i n t where n i t r o g e n can no longer enter these e n e r g e t i c a l l y favored s i t e s and adsorption can only take p l a c e i n the z e o l i t e ' s 12-membered-ring main channel. Side-pocket n i t r o g e n adsorption is t h e r e f o r e , considered to be unique to mordenites with S i / A l of l e s s than 4 . 7 0 .

LITERATURE CITED (1) (2) (3) (4) (5) (6) (7)

(8) (9) (10) (11)

Breck, D.W., "Zeolite Molecular Sieves", John Wiley and Sons, New York, New York (1974). Neddenriep, R.J., J. Colloid Interface S c i . , 28, 293 (1968). Barrer, R.M., Proc. Roy.Soc.,London, 167A (1938). McKee, D.W., U.S. Patent 3,140,932 (1964). Domine, D. and Hay, L., Molecular Sieves, Soc. Chem. Industries, London (1968). Breck, D.W., "Zeolite Molecular Sieves", John Wiley and Sons, New York, New York (1974). Mumpton, F.A., Mineralogy and Geology of Natural Zeolites, M.S.A. Short Course Notes, Southern Printing Company (1977). Breck, D.W., "Zeolite Molecular Sieves", John Wiley and Sons, New York, New York p. 645-659 (1974). Barrer, R.M., J. C o l l . and Interface S c i . , 21, 415 (1966) Kingston, G.L. and MacLeod,A.C.,Trans. Faraday Soc., 55, 1799 (1959). Breck, D.W., Zeolite Molecular Sieves", John Wiley and Sons, New York, New York p. 650 (1974).


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(12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22)

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CHEMISTRY

Barrer, R.M., J. C o l l . and Interface Sci., 21, 415 (1966) Takaishi, T., Yusa, A., Ogino, Y. and Ozawa, S., Proc. 2nd. I n t ' l . Conf. Solid Surfaces (1974). Takaishi, T., Yusa, A., Ogino, Y and Ozawa, S., J. Chem. Soc., Faraday Trans. I, 70, 671 (1974). Sefcik, M.D. and Yeun, H.K., Thermochimico Acta, 26, 297 (1978). Meier, W.M., Z. Krist., 115, 439 (1961). Chang, H.D., Ph.D. Thesis, Worcester Polytechnic Institute, Worcester, Massachusetts (1970). Hawkins, D.B., Alaska Geological and Geophysical Surveys Special Report 9, College, Alaska (1976). Sefcik, M.D., Schaefer, J. and Skejskal, E.O., ACS Symp. Series 40, 344 (1977). Barrer, R.M. and Peterson, D.L., Proc. Roy. Soc., 40, 95 (1964). Peterson, D., ACS Symp. Series 135, 118 (1980). Habgood, H.W., Can. J. Chem., 42, 2340 (1964).

RECEIVED November 16,

1982


Intrazeolite Chemistry - American Chemical Society

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