Liquid Diffusion and Adsorption of Aqueous Ethanol, Propanols, and

C h a p t e r 28

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Liquid Diffusion and Adsorption of Aqueous Ethanol, Propanols, and Butanols in Silicalite by HPLC Y. S. Lin and Yi Hua Ma Chemical Engineering Department, Worcester Polytechnic Worcester, M A 01609

Institute,

The diffusion and equilibrium adsorption of aqueous alcohols in silicalite crystals have been studied using a novel HPLC technique. With a nonlinear mathematical model, the adsorption isotherms and intracrystalline diffusivities have been determined at 10, 30, 50, 70°C for ethanol, i-propanol, i-butanol, and at 30°C for n-propanol and n-butanol. The liquid intracrystalline diffusivities are found to be in the range of 10 to 10 cm /s and decrease in the folowing order: n-butanol>n-propanol> ethanol >i-propanol> i-butanol. The adsorption equilibrium results determined by the present HPLC technique are compared with those measured by the conventional batch method and good agreement is found between the two methods. -9

-11

2

S i l i c a l i t e i s a microporous c r y s t a l l i n e s i l i c a molecular sieve with remarkable h y d r o p h o b i c p r o p e r t i e s (_1) and has been c o n s i d e r e d t o o f f e r p r a c t i c a l a p p l i c a t i o n s i n the c l e a n - u p o f water c o n t a m i n a t e d w i t h h y d r o c a r b o n s and the s e p a r a t i o n o f e t h a n o l from d i l u t e f e r m e n t a t i o n aqueous s o l u t i o n s (2, _3> it> · Many s t u d i e s have been r e p o r t e d on the p r o p e r t i e s o f a d s o r p t i o n and d i f f u s i o n o f gases i n s i l i c a l i t e ( e . g . , 6_, 7_ 8, 9_, 10) . However, d e s p i t e t h e many p o t e n t i a l a p p l i c a t i o n s o f s i l i c a l i t e as an a d s o r b e n t i n l i q u i d phase, s t u d i e s on l i q u i d phase d i f f u s i o n and a d s o r p t i o n i n s i l i c a l i t e a r e r a t h e r s c a r c e . f

E a r l y work on l i q u i d phase a d s o r p t i o n i n s i l i c a l i t e i n c l u d e s s t u d i e s by M i l e s t o n e and B i b b y (3) on t h e a d s o r p t i o n o f a l c o h o l s , N a r i t a e t a l . (11) on a d s o r p t i o n o f p h e n o l s , c r e s o l s and b e n z y l a l c o h o l , b o t h from aqueous s o l u t i o n , and Ma and L i n (12) on l i q u i d hydrocarbon a d s o r p t i o n . Due t o c o n s i d e r a b l e d i f f i c u l t y i n v o l v e d i n l i q u i d d i f f u s i o n measurements i n m o l e c u l a r s i e v e s , however, no l i q u i d d i f f u s i o n s t u d i e s on s i l i c a l i t e were r e p o r t e d u n t i l r e c e n t l y . Ma and L i n (13) a p p l i e d an HPLC t e c h n i q u e t o t h e measurements o f l i q u i d d i f f u s i o n and a d s o r p t i o n i n s i l i c a l i t e . The i n t r a c r y s t a l l i n e d i f f u s i v i t i e s f o r methanol, e t h a n o l , a c e t o n e from aqueous s o l u t i o n and a c e t o n e , t o l u e n e from c y c l o h e x a n e s o l u t i o n i n s i l i c a l i t e were r e p o r t e d t o be i n the range from 1 0 " ^ t o 1 0 ~ H cm /s. 2

0097-6156/88/0368-0452$06.00/0 © 1988 American Chemical Society


28. LIN AND MA

Aqueous Ethanol, Propanols, and Butanois

453

L i q u i d chromatography (LC) has been, i n the p a s t decade, i n c r e a s i n g l y used f o r the measurement of l i q u i d a d s o r p t i o n e q u i l i b r i u m (14, 15, 16, 17). But the a p p l i c a t i o n of the LC t e c h n i q u e f o r the measure ment of l i q u i d d i f f u s i o n i n m o l e c u l a r s i e v e s was r a t h e r l i m i t e d (18, 19). The r e c e n t l y developed LC t e c h n i q u e u s i n g a commercial HPLC system (13) , w i t h many advantages over the c o n v e n t i o n a l b a t c h t e c h n i q u e s , e n a b l e s us to d e t e r m i n e l i q u i d phase d i f f u s i o n and a d s o r p t i o n e q u i l i b r i u m i n m o l e c u l a r s i e v e c r y s t a l s i n a s i m p l e r , more a c c u r a t e and r a p i d way. The p r e s e n t study r e p o r t s the measurements of i n t r a c r y s t a l l i n e d i f f u s i o n and a d s o r p t i o n e q u i l i b r i u m f o r e t h a n o l , p r o p a n o l s and bu t a n o i s from aqueous s o l u t i o n i n s i l i c a l i t e u s i n g a m o d i f i e d HPLC t e c h nique. The unique f e a t u r e o f the p r e s e n t work i s the use o f a math e m a t i c a l model w i t h a n o n l i n e a r a d s o r p t i o n i s o t h e r m e q u a t i o n to o b t a i n the i n t r a c r y s t a l l i n e d i f f u s i v i t y and a d s o r p t i o n i s o t h e r m p a r a m e t e r s . The a d s o r p t i o n e q u i l i b r i u m d a t a f o r a l c o h o l s from aqueous s o l u t i o n i n s i l i c a l i t e measured by the c o n v e n t i o n a l b a t c h method are a l s o r e p o r t e d and compared w i t h the r e s u l t s measured by the HPLC t e c h n i q u e . EXPERIMENTAL The l i q u i d a d s o r p t i o n and d i f f u s i o n measurements were c a r r i e d out i n a BECKMAN HPLC system, which c o n s i s t s of one model 421 system c o n t r o l l e r , two model 110 s o l v e n t m e t e r i n g pumps, one s o l v e n t m i x e r and one model 210 sample i n j e c t o r w i t h a 20 μ ΐ sample l o o p . A H i t a c h i model 100-40 UV-Vis S p e c t r o m e t e r was used as the d e t e c t o r . To i n c r e a s e the p r e s s u r e i n the UV-VIS d e t e c t o r c e l l , a back p r e s s u r e r e g u l a t o r was c o n n e c t e d to the e f f l u e n t stream from the d e t e c t o r to a v o i d f o r m a t i o n of a i r b u b b l e s due to v a p o r i z a t i o n i n the d e t e c t o r c e l l . Figure 1 shows the s c h e m a t i c of the HPLC system used i n the e x p e r i m e n t s . I n p e r f o r m i n g e x p e r i m e n t s , the s i l i c a l i t e samples were f i r s t a c t i v a t e d a t 300°C o v e r n i g h t and then c o o l e d i n a d e s i c c a t o r to room temperature. A column 2.0(L)x0.20(ID) cm was packed w i t h the s i l i c a l i t e powder by a d r y - p a c k i n g method. A f t e r the column was packed, i t was n e c e s s a r y to s t a b i l i z e i t by f l o w i n g the c a r r i e r s o l v e n t (water i n the p r e s e n t s t u d y ) t h r o u g h the column i n the HPLC system over an extended p e r i o d of time. The d e t a i l s on the column p a c k i n g and s t a b i l i z a t i o n f o r the s i l i c a l i t e LC column were g i v e n by Ma and L i n ( 1 3 ) . In the measurements o f the a d s o r p t i o n e q u i l i b r i u m and i n t r a c r y s t a l l i n e d i f f u s i o n d a t a , the i n j e c t i o n sample l o o p was f i r s t f i l l e d w i t h a sample s o l u t i o n (water as s o l v e n t ) of a known s o r b a t e c o n c e n t r a t i o n by a s y r i n g e . The sample was then i n j e c t e d i n t o the column a f t e r a s t a b l e base l i n e i n the r e c o r d e r had been o b t a i n e d . For each a d s o r bate a t a g i v e n t e m p e r a t u r e , about 4 to 6 samples o f d i f f e r e n t a d s o r bate c o n c e n t r a t i o n ( C from about 0.015 to 0.06 g/ml) and a t d i f f e r e n t c a r r i e r f l o w r a t e (Q from 0.5 to 2.0 ml/min) were i n j e c t e d to g i v e the c o r r e s p o n d i n g r e s p o n s e peaks a t the o u t l e t of the column. The r e s p o n s e peaks were r e c o r d e d and then d i r e c t l y r e a d from the r e c o r d i n g c h a r t and i n p u t to a DEC-20 computer f o r f u r t h e r a n a l y s i s . F i g u r e 2 shows some r e c o r d e d r e s p o n s e peaks from the s i l i c a l i t e LC column f o r e t h a n o l , np r o p a n o l and n - b u t a n o l . The dead volume of the HPLC system was m i n i m i z e d u s i n g the con n e c t i n g t u b i n g of s m a l l e s t i n n e r d i a m e t e r (d =0.0254cm) and the f i t t i n g s w i t h a v e r y s m a l l dead volume. The dead volume of the system Q

t


454

PERSPECTIVES IN MOLECULAR SIEVE SCIENCE

Figure

1.

Schematic

T=30°C

F i g u r e 2.

Recorded

o f HPLC E x p e r i m e n t a l A p p a r a t u s .

Q=1-0 ML/MIN

Response Peaks from S i l i c a l i t e

LC Column.


28.

LIN AND MA

Aqueous Ethanol, Propanols, and Butanols

455

was d e t e r m i n e d (13) to be 0.022 ml. The dead volume e f f e c t on the f i r s t moment was c o r r e c t e d f o r a l l the measured r e s p o n s e peak d a t a . The e f f e c t of the dead volume on the second moment of the r e s p o n s e peaks was found t o be n e g l i g i b l e i n the p r e s e n t LC systems. The b a t c h measurement f o r the e q u i l i b r i u m a d s o r p t i o n of a l c o h o l s from aqueous s o l u t i o n i n s i l i c a l i t e was c o n d u c t e d i n a i r - t i g h t b o t t l e s which were immersed i n a t h e r m a l b a t h a t 30°C ( 1 2 ) . Amounts of a c t i v a t e d s i l i c a l i t e sample, s o l u t e ( a l c o h o l s ) and s o l v e n t (water) i n each b o t t l e were measured g r a v i m e t r i c a l l y . The b o t t l e s i n the t h e r m a l b a t h were i n t e r m i t t e n t l y shaken to h a s t e n the e q u i l i b r i u m . A f t e r the b o t t l e s had been kept i n the t h e r m a l b a t h f o r over 10 h r , the e q u i l i b r i u m s o l u t i o n was withdrawn and a n a l y z e d by gas chromatography. The p a c k i n g m a t e r i a l of the gas chromatography column was poropak Q (Waters A s s o c i a t e s , I n c . ) . D e t a i l s of the a d s o r p t i o n e q u i l i b r i u m measurement by the b a t c h method can be found i n Ma and L i n (12) The s i l i c a l i t e i n s p h e r i c a l c r y s t a l p a r t i c l e (powder) form was o b t a i n e d from Union C a r b i d e Corp. The r e s u l t s of p a r t i c l e s i z e d i s t r i b u t i o n a n a l y z e d by an E l e c t r o z o n e C e l l o s c o p e p a r t i c l e s i z e a n a l y zer showed v e r y narrow c r y s t a l s i z e d i s t r i b u t i o n ( i n the range from 1.5 to 3.5 m i c r o n s i n d i a m e t e r ) . The a p p r o x i m a t e l y s p h e r i c a l form and the s i z e of the c r y s t a l p a r t i c l e s were v e r i f i e d by s c a n n i n g e l e c tron micrographs. The s p e c i f i c a t i o n of the s i l i c a l i t e sample and some of the column parameters a r e g i v e n i n T a b l e I . A l l s o l v e n t s used were of HPLC grade. In h a n d l i n g water, s p e c i a l c a r e was t a k e n to p r e v e n t i t from b e i n g c h e m i c a l l y or b i o l o g i c a l l y c o n t a m i n a t e d . Table I

S p e c i f i c a t i o n of s i l i c a l i t e LC column parameters

and

Average c r y s t a l r a d i u s R ( y l ) 1.17 C r y s t a l pore volume v ( m l / g ) 0.19 Crystal porosity ε ρ 0.33 Column s i z e (cm) 2.0(L)xO.2(ID) Bed p o r o s i t y ε^ 0.46 p

MATHEMATICAL MODEL AND

ANALYSIS

The m a t h e m a t i c a l model f o r the mass t r a n s f e r o f an a d s o r b a t e i n the LC column packed w i t h the s i l i c a l i t e c r y s t a l p a r t i c l e s i s based on the assumptions of (1) a x i a l - d i s p e r s e d p l u g - f l o w f o r the m o b i l e phase w i t h a c o n s t a n t i n t e r s t i t i a l f l o w v e l o c i t y ; (2) F i c k i a n d i f f u s i o n i n the s i l i c a l i t e c r y s t a l pore w i t h an i n t r a c r y s t a l l i n e d i f f u s i v i t y independent of c o n c e n t r a t i o n and p r e s s u r e ; and (3) s p h e r i c a l s i l i c a l i t e c r y s t a l p a r t i c l e s with a uniform p a r t i c l e size d i s t r i b u t i o n . A d e t a i l e d d i s c u s s i o n of t h e s e assumptions can be found i n ( 1 3 ) . The d i f f e r e n t i a l mass b a l a n c e s over an element o f the LC column and s i l i c a l i t e c r y s t a l r e s u l t i n the f o l l o w i n g two p a r t i a l d i f f e r e n t i a l equations :

456

PERSPECTIVES IN MOLECULAR SIEVE SCIENCE

for

the mass t r a n s f e r

+ u 3C +

at

1

i n the m o b i l e

q = D-

£

~ b

az

phase:

az2

e

h

d)

w i t h the a d s o r p t i o n r a t e q e x p r e s s e d by:


•

for

ο

ac

R

a

(2)

r

r

the d i f f u s i o n

D

p

^

(

+

i n the c r y s t a l s :

2

3 2

^ r

r

= R

)

=

_ ^

£

3r

Ρ

3

(3)

t

where C and Cp a r e the c o n c e n t r a t i o n o f a d s o r b a t e i n the m o b i l e phase and i n the s i l i c a l i t e c r y s t a l p o r e , r e s p e c t i v e l y ; i s the a x i a l d i s p e r s i o n c o e f f i c i e n t ( i t s e s t i m a t i o n was g i v e n i n ( 1 3 ) ) . Dp i s the i n t r a c r y s t a l l i n e d i f f u s i v i t y based on the t o t a l a r e a of the c r y s t a l p a r t i c l e p e r p e n d i c u l a r t o the d i f f u s i o n d i r e c t i o n . Other symbols a r e i d e n t i f i e d i n the n o m e n c l a t u r e . For a column i n i t i a l l y f l o w e d w i t h a pure s o l v e n t , the i n i t i a l conditions are: for

the mobile

phase:

C (Z, t ) = 0

at

for C

the

t = 0

the s i l i c a l i t e

crystal:

(Z,r,t) = 0

at t = 0

p

(4)

(5)

The f o l l o w i n g e q u a t i o n s a r e used as the boundary m o b i l e phase a t the o u t l e t o f the column: _ 0

at

conditions f o r

Ζ = L

(6)

az and a t the i n l e t C = Γ + ^

o f t h e column:

^

u

at

Ζ ·= 0

where Γ i s the square p u l s e i n p u t Γ

= r C coG

(7)

az function:

t

Liquid Diffusion and Adsorption of Aqueous Ethanol, Propanols, and

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