Perspectives in Molecular Sieve Science - American Chemical Society

C h a p t e r 28

Perspectives in Molecular Sieve Science Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 03/13/16. For personal use only.

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

Perspectives in Molecular Sieve Science - American Chemical Society

Recommend Documents