Adsorption Separation of Methylnaphthalene Isomers on X and Y Zeolites

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

ADSORPTION SEPARATION OF METHYLNAPHTHALENE ISOMERS ON X AND Y ZEOLITES 1 1 2 V. SOLINAS , R. MONACIl, E. ROMBI and M. MOREIDELL1 'Dipartimento d i Scienze Chimiche, 09124 C a g l i a r i ( I t a l y ) .

Universita d i Cagliari,

V i a Ospedale 72,

L

Dipartimento d i Ingegneria Chimica e M a t e r i a l i , U n i v e r s i t a d i C a g l i a r i , Piazza d'Armi, 09124 C a g l i a r i ( I t a l y ) . ABSTRACT The separation o f 1-methylnaphthalene (1-MN) and 2-methylnaphthalene (2-MN) was studied on X and Y z e o l i t e s exchanged w i t h a l k a l i n e and a l k a l i n e - e a r t h c a t i o n s , i n a l i q u i d phase a t 293 K. The t o t a l adsorbed amount depends on t h e volume o f t h e c a t i o n i n t h e z e o l i t e framework. The z e o l i t e s exchanged w i t h t h e l a r g e r c a t i o n s showed h i g h e r s e l e c t i v i t y . The system e x h i b i t s a non-ideal behaviour showing a s t r o n g dependence o f select i v i t y on composition. INTRODUCTION The z e o l i t e adsorbents a r e commercially used i n v a r i o u s s e p a r a t i o n processes, such as n- p a r a f f i n , and i n xylene isomers separation processes ( r e f . 1 ) . I n t h i s work we a r e seeking s u i t a b l e z e o l i t e s on which one o f two methylnaphthalene isomers i s s e l e c t i v e l y adsorbed i n t h e c o m p e t i t i v e a d s o r p t i o n o f t h e isomeric mixture, with t h e aim o f developing a separation process. The technique o f c o m p e t i t i v e adsorption i n l i q u i d phase u s i n g an i n e r t s o l v e n t has been r e p o r t e d by Namba e t a l . ( r e f . 2 ) . The separation o f 1-methylnaphthalene (l-MN) and 2-methylnaphthalene (2-MN) was s t u d i e d on X and Y z e o l i t e s exchanged w i t h a l k a l i n e and a l k a l i n e - e a r t h c a t i o n s . The molecular dimensions o f MN, as maximal c r o s s - s e c t i o n a l s i z e , were 0

6.2 A f o r 1MN and 5.8 A f o r 2MN ( r e f . 3). These c r i t i c a l dimensions o f aroma0

t i c molecules a r e comparable w i t h 7 A C a u j a s i t e nominal pore openings. EXPEHIMENT

Absorbents X-

and Y-zeolites

v a r i o u s l y exchanged were prepared from commercially

596

a v a i l a b l e powder (Nal3X and LZY-52 from Union Carbide). c a r r i e d out by t r e a t i n g 10 g o f the commercial

The exchange was

z e o l i t e w i t h 150 cm3 o f a

b o i l i n g aqueous s o l u t i o n o f a l k a l i n e o r a l k a l i n e - e a r t h c h l o r i d e (1M) f o r 2 h. The procedure was repeated f i v e times. The exchanged z e o l i t e was then washed, d r i e d a t 373 K f o r 3 h and calcined a t 773 K f o r 12 h. The

adsorption

runs were

performed on

zeolite

as

powder.

The main

c h a r a c t e r i s t i c s o f the z e o l i t e s employed are summarized i n Table 1. Adsorbates and solvents 1- and 2-methylnaphthalene and n-octane were obtained from a commercial source.

They

were

high-purity

reagents

and

were

used

without

further

purification. Methods A l l t h e runs were performed i n a closed vessel i n which 0.5 g o f z e o l i t e ,

degassed a t 773 K f o r 5 h, were contacted w i t h 2 m l o f the s o l u t i o n o f 1- and 2-methylnaphthalene i n a-octane.

The i n i t i a l concentration o f each isomer was

4 wt%. The amount o f each isomer adsorbed was obtained from t h e concentration o f each isomer i n l i q u i d phase, which was determined by gas chromatographic analysis. The e f f i c i e n c y o f t h e z e o l i t e s i n separating the methylnaphthalene isomers was measured by t h e s e l e c t i v i t y (S 1 defined by t h e f o l l o w i n g formula: 2MN/ 1MN

%/1

=

2-MN adsorbed on z e o l i t e 1-MN adsorbed on z e o l i t e

X

1-MN i n s o l u t i o n

2-MN i n s o l u t i o n

(11

RESULTS AND DISCUSSION I n t h e competitive adsorption o f methylnaphthalene isomers on X and Y zeolites,

the amount o f each isomer

adsorption time (Fig. 1 ) .

adsorbed

increased monotonously w i t h

597

Fig. 1. Amount o f methylnaphthalene isomers adsorbed ( r ) on KY vs. adsorption; ( A 1 1-MN, ( 0 I 2-MN.

time o f

The adsorption e q u i l i b r i u m was a t t a i n e d w i t h i n 24 h. The e q u i l i b r i u m was confirmed by the coincidence o f the amounts o f each isomer adsorbed a f t e r 24 and 48 h o f adsorption. Table 1 shows the t o t a l amount o f methylnaphthalene isomers and t h e select i v i t y (S J , determined a f t e r 24 h o f adsorption. 2/1 I t can be seen t h a t t h e t o t a l q u a n t i t y o f isomers adsorbed i s u s u a l l y higher i n the Y z e o l i t e s than i n the corresponding X z e o l i t e s . This may be due t o the higher value o f the S i / A l r a t i o o f t h e Y z e o l i t e s , and thus t o a smaller population o f c a t i o n i c s i t e s .

I

I

From a comparison o f the s e l e c t i v i t y values f o r t h e Me X and Me Y z e o l i t e s

I

( w i t h Me = L i , Na, K, Rb, CsJ i t can be seen t h a t they vary i n q u i t e a small range, between 1 and 2 and between 0.9 and 2.4,

respectively.

598

TABLE 1 Adsorption of methylnaphthalene isomers on X and Y exchanged zeolites. Adsorbent

% exchangeda

Li X b NaX KX RbX csx M9X CaX Bax Li Yc NaY KY

71

RbY CSY

a

CaY Bay

/

92 64 82 72 98 96

61

/ 89 73 75 76 91 78

Amount of methyl naphthalene isomers adsorbed (mg/g zeolite) 141.81 122.42 b3.40 69.07 81.98 173.32 167.93 50.71 207.66 206.91 143.5b 116.22 99.95 169.36 168.66 153.01

s2/ I

1.16 1.18 1.60 1.61 1.98 U.8b

1.28 2.38 0.93 0.94 2.23 2.30 2.37 0.73 0.74 2.45

Ion-exchange degree with respect to the original form; b13X powaer, Si/Al = 1.4; 'LZY-52 powder, Si/A1 = 2.4. Amount o f adsorbent = 0.5 g., amount of adsorbate = 2 ml of solution (4% W/W of both 1MN and 2-MN).

I1 I1 On examining the Me X and Me Y zeolites (with ME"

=

Mg, Ca, Ba) the

tollowing considerations can be made: the selectivity increases about threefold from Mg to Ba; it is affected by the acidity in the structure caused by the exchange of the bivalent cations; it is similar for BaX and BaY zeolites. I1 Particularly for Me Y, an inversion of the selectivity (S

2/ I

)

for the

more acid zeolites (Mg and Ca) can be observed, with values approaching 0.7. This inversion was confirmed when using a totally decationated HY zeolite, which presents a low adsorption capacity (50 mg/g) and a selectivity of S

2/ 1

=

u.5. The high selectivity found for the BaY zeolite may be due to the fact that the Ba introduced comes out of the channel wal

I

(ref. 2).

figure 2 (a and b) shows the amount of methylnaphthalene isomers adsorbed 3 versus the volume (r ) of cations in the zeolite structure. X- and Y- zeolites show a similar trend: the total adsorbed amount decreases generally, on increasing the volume of the cations. It can be noticed that the

599

amounts adsorbed on Mg-X, Ca-X versus L i - x , Na-X a r e d i f f e r e n t and i n c o n t r a s t w i t h t h e c o r r e s p o n d i n g s e r i e s on z e o l i t e Y . 200

250 1

I

r 160

150 2oo'.

\

Li

b

Na

-. Mg

120

100 '.

80

40

cs

Ba O*'

0

2 (r3) 3

1

4

o

5

~ ( ~ 33 )

1

F i g . 2 . Amount of methylnaphthalene isomers adsorbed ( t h e exchanged c a t i o n . ( a ) X - z e o l i t e s , ( b ) Y - z e o l i t e s .

r 1 vs. volume

4

5

(r3 1 o f

On t h e o t h e r hand, t h e s e l e c t i v i t y i n c r e a s e s on i n c r e a s i n g t h e volume o f t h e c a t i o n s i n F i g u r e 3 ( a and b ) . 2.5

2.5

2.0

2.0

s2/1 1.5

%/1 1.5

1 .o

1 .o

0.5

0.5

0

1

2 (r3) 3

F i g . 3. S e l e c t i v i t y (S2,,) x-zeol i tes, (b) Y-zeol it e s .

4

vs.

volume

4

Li

3

N

Mg

0

5

I--

b

( r 1 of

Ca

1

2 (r3) 3

4

t n e exchanged c a t i o n .

5 (a)

600

Selectivity can also be correlated with the Sanderson electronegativity of the framework in Figure 4 (a and b).

2.1

a

2*I %/1 2.1

1 .I

1.

0.

3

3.3

Sint

3.3

3.6

3.6

Sint

3.9

Fig. 4. Selectivity (S2/1 1 vs. intermediate Sanderson's electronegativity (S. 1 I nt (a) X-zeolites, (b) Y-zeolites.

It

can be seen that the selectivity increases when Sint decreases, i.e. when

the structure becomes more basic. There are two sets ot results, one for the Y and one for the X zeolites. For each of these types the correlations are very simi 1 ar. Unsaturated hydrocarbons, which have

II

electrons capable o t a strong in-

teraction with the surface, can cause changes in the surface field properties. The difference noted between the X and Y zeolites may be related to nonidentical charge densities (linked to the aluminium atom content) in the two materials, producing dissimilar fields. Barthomeuf (ref. 4) shows that the specific field of the cations has a

601 small i n f l u e n c e a t h i g h coverage. The o n l y changes o r i g i n a t e f r o m t h e d i f ference i n aluminium atom content, which m o d i f i e s t h e t o t a l number o f charges i n t h e framework and hence t h e z e o l i t e f i e l d .

Ihe n i g n e r t h e aluminium content,

t h e lower t h e i n t e r a c t i o n energy between c a t i o n s i t e and adsorbed molecule. I t i s known t h a t t h e e f f e c t o f t h e z e o l i t e f i e l d due t o A10- on a d s o r p t i o n

4 i s b e t t e r s t u d i e d a t h i g h s u r f a c e coverage, w h i l e t h e s p e c i f i c c a t i o n i n f l u e n c e i s b e t t e r s t u d i e d a t low coverage. The above f e a t u r e s reveal a complex v a r i a t i o n o f t h e separation f a c t o r , showing no obvious c o r r e l a t i o n w i t h t h e charge, volume and p o l a r i z i n g power o f t h e exchangeable c a t i o n s o r t h e d i p o l a r moments o f t h e aromatic molecules. A t h i g h loadings i n t h e l i q u i d phase, t h e s e l e c t i v i t y i s determined by t h e e t f e c t s o f sorbate-sorbate i n t e r a c t i o n s ( r e f . 5). I n order t o v e r i f y t h i s hypothesis, runs were c a r r i e d o u t w i t h d i f f e r e n t r e c i p r o c a l concentrations o f t h e two isomers. Table 2 shows t h e s e l e c t i v i t i e s obtained. The s e l e c t i v i t y changes s i g n i f i c a n t l y w i t h t h e c o n c e n t r a t i o n o f t h e

i n i t i a l s o l u t i o n , and t h i s f a c t confirms t h e n o n - i d e a l i t y o f t h e b i n a r y m i x t u r e o f methylnapnthalene isomers. Table 2 S e l e c t i v i t y and amount adsorbed on KY a t 293 K upon changing t h e c o n c e n t r a t i o n o f methylnaphthalene isomers 1-MN (%w/wJ 2

2-MN (%w/w) 6 5

3

4 6

4

2

s2,, .I .50

1.63 2.23 2.80

r

1

r. z

36.3 42.8 63.6 90.9

r

95.0 82.3 80.0

39.2

total 131.3 125.1 143.6 130.1

x1

x2

0.277 0.342 U.443 0.69Y

0.723 U.658 0.557 0.301

Adsorption E q u i l i b r i a I n order t o b e t t e r understand t h e e q u i l i b r i u m behaviour o f t h e system under examination,

a more d e t a i l e d a n a l y s i s o f t h e adsorption e q u i l i b r i u m isotherms

was developed i n t h e case o f z e o l i t e KY. The pure-component e q u i l i b r i u m d a t a a t 293 K are shown i n F i g u r e 5 t o r both 1- and 2-methylnaphthaiene, c o n c e n t r a t i o n i n t h e adsorbed phase b u l k l i q u i d pnase C.

i n terms o f

r as a t u n c t i o n o f c o n c e n t r a t i o n

I n t h e same t i g u r e ,

i n the

t h e experimental d a t a a r e compared

w i t h curves c a l c u l a t e d through t h e Langmui r equi 1ibrium model

r / r"

= KC/l+KC

(2)

602

1

6

0.05

0.01

0

0.10

1

c

0.18

Fig. 5. Experimental data o f concentration i n t h e adsorbed phase, 2-MN. concentration i n the bulk phase, C(mol/l); ( 0 ) 1-MN, ( A

r (mg/g), vs.

using t h e f o l lowing values o f the equi 1ib r i um parameters, as obtained through t h e usual 1east-squares-estimati on tecnni que , K (l/mol)

rm (mg/g)

1-MN

2-MN

37.2

86

168

166

The observation t h a t two pure components e x h i b i t almost i d e n t i c a l values f o r the amount adsorbed a t s a t u r a t i o n conditions, r

m

, seems t o i n d i c a t e t h e

p o s s i b i l i t y o f d e s c r i b i n g b i n a r y e q u i l i b r i u m data through t h e multicomponent Langmuir e q u i l i b r i u m model, which i n t h e case o f two adsorbable components reduces as f o l l o w s :

r

-

- ir

KiCi

1+K.C.+K2C2

,i=

1,2

(3)

1 1

Such a model would p r e d i c t t h e f o l l o w i n g composition-independent expression f o r the binary s e l e c t i v i t y ( 1 )

which i n the case under examination leads t o S

2/1

= 2.31.

However, t h i s conclusion does n o t agree w i t h experimental f i n d i n g s , which, as shown by t h e data summarized i n Table 2, s e l e c t i v i t y upon composition.

i n d i c a t e a strong dependence of

603 This behaviour i n d i c a t e s t h e presence o f s t r o n g i n t e r a c t i o n s among adsorbate molecules, which are r e s p o n s i b l e t o r t h e observed d e v i a t i o n s f r o m i d e a l aehaviour. Thus, t h e m o d e l l i n g o f b i n a r y e q u i l i b r i u m data r e q u i r e s e x p l i c i t accounting f o r d e v i a t i o n s from i d e a l i t y i n t h e adsorbed phase.

l o t n i s aim t h e t h e o r y o f

adsorption from r e a l s o l u t i o n s , as f i r s t developed by Myers and P r a u s n i t z ( r e f . 6 ) i n t h e c o n t e x t o f gaseous m i x t u r e adsorption and subsequently extended t o l i q u i d mixtures ( r e f s . 7, 81, o f t e r s t h e most convenient framework. A t e q u i l i b r i u m c o n d i t i o n s t h e f u g a c i t i e s o f each i - t h component i n t h e l i q -

u i d and i n t h e adsorbed phase ( i n d i c a t e d w i t h a prime symbol) a r e equal

-

-

f. = f 1

1

’.

w i t h o u t going i n t o t h e d e t a i l s o t t h e e v a l u a t i o n o f t h e above f u g a c i t i e s , and r e c a l l i n g t h a t t h e system under examination i s a t low pressure,

one can

w r i t e as t o l l o w s ( r e f . I ) : T O ’ ’ ( T ) Y1. X1. = foYa(T,O)y;x; i i where

Y.

1

(5)

i n d i c a t e s t h e a c t i v i t y c o e f f i c i e n t , foYa(T,o) t h e pure-component 1

f u g a c i t y i n t h e adsorbed phase a t t h e same temperature, immersion values as i n t h e b i n a r y mixture.

and

0

f r e e energy o f

When t h e e q u i l i b r i a o f t h e pure

components a r e described through t h e Langmuir model , t h e q u a n t i t y foYa(T,o) can 1

be r e a d i l y evaluated from t h e tiibbs isotherm. This leads t o t h e f o l l o w i n g r e l a t i onshi p:

x

i

= - Y;X;

K.

1

c.

J

K .x

J j -

(61

yj

where i d e a l behaviour f o r t h e l i q u i d phase i s assumed ( i . e . y

= 1). From i eq(6) t h e f o l l o w i n g new expression f o r t h e b i n a r y s e l e c t i v i t y can be obtained:

(7) which depends on t h e composition o f t h e adsorbed phase through t h e a c t i v i t y c o e t f i c i e n t s . Such a f u n c t i o n can be q u a n t i t a t i v e l y evaluated by t h e use o f a model f o r t h e excess f r e e energy o t t h e adsorbed phase. F o r example, w i t h t h e n i l d e b r a n d model ( r e f . 91 t h e f o l l o w i n g expressions are obtained:

604

where the parameter A

i n d i c a t i n g t h e i n t e r a c t i o n between the two adsorbed 12’ species, can be considered as an adjustable parameter and estimated by d i r e c t

comparison w i t h the experimental data. This i s achieved through the f o l l o w i n g l i n e a r i z a t i o n r e l a t i o n s h i p : ,2 2 = A l p (x, x; 1 I n Y;/Y;

-

(Y)

This leads t o t h e l i n e a r p l o t shown i n F i g . 6, when t h e experimental values o f Table 2, the Langmuir constant values estimated above and eq(7) are used. From F i g . 6 the value A12 = 143 i s obtained.

I I n Y;/Yi

0

-0.5 0

-0.5

( x i 2 - x i2)

t0.5

Fig. 6. C o r r e l a t i o n o f experimental data from equation (9). Tnus, i n conclusion, the binary s e l e c t i v i t y can be obtained by s u b s t i t u t i n g eq(8) i n t o eq(71, as f o l l o w s : 2 K2 exp(A12x; /r-1 = 2 K1 exp(A12x; /P)

-

(10)

The values obtained by t h i s r e l a t i o n s h i p are compared w i t h experimental data i n Figure 7. As f u r t h e r support f o r t h e conclusion t h a t non-ideal adsorbate-adsorbate interactions

are

dominant

in

tne

system

under

examination,

the

binary

e q u i l i b r i u m data may be r e p l o t t e d according t o the s t a t i s t i c a l approach r e p o r t ed by Ihm and Lee ( r e f . 10). I n p a r t i c u l a r , t h e f o l l o w i n g r e l a t i o n s h i p i s considered : - I n S2,1

=

KO

t

(1

- 2xi)cw/2kT

(I11

605

where K" i s a c o n s t a n t r e l a t e d t o t h e s u r f a c e t e n s i o n o f t n e p u r e component, c

is t h e number o f t h e n e a r e s t neighbour s i t e , w h i l e w r e f e r s t o t h e v a r i o u s i n v o l v e d m o l e c u l a r i n t e r a c t i o n e n e r g i e s as f o l l o w s :

w = U' 1-1

(12)

0.5

0

xi

I

F i g . 7. C o r r e l a t i o n o f e x p e r i m e n t a l d a t a f r o m e q u a t i o n ( 1 0 ) and t h e s u r f a c e coverage X ' can be c a l c u l a t e d f r o m t h e 2/ 1 1 s o r p t i o n data. The c o n s t a n t K O and cw/EkT can be o b t a i n e d f r o m t h e i n t e r c e p t The s e l e c t i v i t y S

and s l o p e o f t h e l i n e a r p l o t s o f - l n S

vs.(l-2xi). 2/ 1 The e s t i m a t e d v a l u e cw/2kT = 0.75, which i s s i g n i f i c a n t l y d i f f e r e n t f r o m

zero, c o n f i r m s t h a t t h e system e x h i b i t s a n o n - i d e a l behaviour, w i t h U i 2

+ Uilor

F u r t h e r s t u d i e s a r e i n p r o g r e s s along t h e s e l i n e s , w i t h d i f f e r e n t z e o l i t e s and temperatures, a d s o r p t i o n process.

i n o r d e r t o b e t t e r understand t h e n a t u r e o f a s e l e c t i v e However some h i g h s e l e c t i v i t y values,

determined i n t h e

p r e s e n t study, suggest c l e a r l y t h a t t h e s e p a r a t i o n of t h e s e two isomers i s p o s s i b Ie

.

ACKNOWLEDGEMENTS We w i s h t o thank Mr. A. R i v o l d i n i f r o m t h e I s t i t u t o d i G i a c i m e n t i M i n e r a r i , U n i v e r s i t i d i C a g l i a r i , f o r t h e atomic a d s o r p t i o n analyses on t h e z e o l i t e s .

606

REFERENCES 1 O.M. Ruthven, i n P r i n c i p l e s o f Adsorption and Adsorption Processes, John Wiley and Sons ( t d i t o r s ) , New r o r k 1984. z S. Namba, Y. Kanai, H. Shoj and T. Yashima, Z e o l i t e s , 4 (1984) 77. 3 D. Fraenkel, M. Cherniavsky, 8. I t t a h , M. Levy, J. Catal., 101 (1986) 273. 4 D. Barthomeuf, B.H. Ha, J. Chem. SOC. Farad. Irans. I , 7 (1913) 2158. 5 D.M. Ruthven, i n Olson and B i s i o ( E d i t o r s ) , Proceedings o f t h e s i x t h I n t e r n a t i o n a l Z e o l i t e s Conference, Butterworths, 1983, p. 31. 6 A.L. Myers, J.M. Prausnitz, A.I.Ch.E.J., 11 (1965) 121. 7 C. Minka, A.L. Myers, A.I.Ch.E.J., 19 (1973) 453. 8 O.G. Larionov, A.L. Myers, Chem. Eng. Sci., 26 (19/1) 1025. 9 J.H. Hildebrand, J.M. Prausnitz, R.L. Scott, i n r e g u l a r and Related Solut i o n ; Van Nostrand Rheinhold ( E d i t o r ) , New York, 1970. 10 S.K. Ihm, H.S. Lee, i n Murakami, L i j i m a , ward ( E d i t o r s ) , New Oevelopments i n Z e o l i t e s Science and technology, E l s e v i e r Amsterdam 1986, p. 571.