Intracrystalline Diffusion of Linear Paraffins and Benzene in Silicalite Studied by the ZLC Method

P.A. Jacobs and R.A. van Santen (Editors), Zeolites: Facts, Figures, Future 0 1989 Elsevier Science Publishcrs B.V., Amsterdam - Printed in The Netherlands

897

INTRACRYSTALLINE DIFFUSION OF LINEAR PARAFFINS AND BENZENE IN SILICALITE STUDIED BY THE ZLC METHOD

MLADEN EIC and DOUGLAS M. RUTHVEN Department of Chemical Engineering, University of New Brunswick, Fredericton, N.B., Canada ABSTRACT The zero length column (ZLC) method has been applied to study diffusion of benzene and several linear paraffins in large crystals of silicalite. The ZLC diffusivity data are broadly consistent with recently reported membrane, piezometric and frequency response values but the values (for propane) are much smaller than the NMR self diffusivities. The trends of diffusiwith carbon number for 5A, 13X and silicalite are qualitatively similar, with the diffusivities for silicalite lying between the corresponding values for 5A and 13X. For carbon numbers greater than 5, activation energies for silicalite are essentially the same as for NaX. INTRODUCTION Diffusion of linear paraffins in 5A zeolite crystals is relatively slow and has been studied in detail by several different experimental methods, notably by direct measurement of sorption rates. In the more open lattices of zeolite X (or Y) (12 ring) and silicalite (10 ring) diffusion is much faster so

that, even in the largest available crystals (100-200pm), uptake rates are

too rapid to permit reliable intracrystalline diffusivity measurements except for higher molecular weight sorbates. In the recently developed zero length column (ZLC) technique''' 2, the effects of heat transfer and extracrystalline mass

transfer

resistances,

which

limit

conventional

sorption

rate

measurements, are greatly reduced, thus making it possible to measure faster diffusion processes. \

The validity of this method has been confirmed in a

series of experiments carried out with systems for which the diffusivities had been

previously

determined

from

sorption

rate

or

tracer

exchange

measurements.(1-3) These studies were carried out with 5A and 13X zeolite crystals.

In the present paper we report the results of our recent ZLC

studies with large crystals of silicalite (105~45pm). Some of the sorbates studied have been investigated by other experimental techniques, both macroscopic and NMR. Detailed comparisons may therefore be drawn between the diffusivity values obtained by the different methods diffusivities for 5A, 13X and silicalite.

and between

the

898 THEORETICAL The central feature of the ZLC set-up is a small 'column' consisting of essentially a monolayer of zeolite crystals sandwiched between two sinter discs which act as flow distributors.

The 'column' is maintained at constant

temperature in a thermostatic oven and connected through suitable switch valves to a gas supply system which allows the adsorbent to be equilibrated with a hydrocarbon stream of known composition and purged with an inert gas at a known constant flowrate.

Full details have been given elsewhere").

It is

important that the purge flowrate should be high enough to maintain a very low hydrocarbon concentration at the crystal surface in order to ensure that the process is kinetically controlled.

The zeolite crystals should preferably be

of uniform size to permit easy and unambiguous analysis of the desorption curves.

By solving the Fickian diffusion equation it may be shown that, for a system of this type with negligible external film resistance to mass transfer, the desorption curve is given by: exe(-BiDt/R2)

-2L c

[ p + L(L-l)]

n-1

0

where p is given by the roots of the equation: n pncotp + L - 1 - 0 n and L

-

L

For

2 rvR /3(1-e)KDz or L -+

0

eqn.

1

-$

m g e Flowrate Crystal Volume

approaches

the

*

limiting

2 KD solution

for

equilibrium

control:

which contains no information on the kinetics.

pn

0

-

For large values of L,

nn and eqn. 1 reduces to:

-+

-

2G

n-1

2 2 t

sXD(-ll

=

2

+ L(L-I)I

which in the long time region simplifies to:

'c -

(L-1)

exp(-w2Dt/R2)

To determine the diffusional time constant the experimental desorption curve is matched to the dimensionless theoretical curve calculated from the solution t o eqns 1 and 2 or to eqn 5 as appropriate.

899 If external film resistance is significant the analysis remains the same except that the value of L is modified: 1-3zKD 1 - a 2 . D VR 2 (e)+Sh Dm

- KD

[

3

x Crystal Volume

Purge Flowrate x R

Intracrystalline diffusivities may therefore be measured, even when there is some film resistance, provided that the film resistance is not dominant. In the long time region the slope of a plot of In(c/c

)

vs t should always

approach ~32lD/R 2 . EXPERIMENTAL Measurements were carried out with a sample of large silicalite crystals. SEM photographs revealed that the crystals are well formed near rectangular

prisms with average dimensions 105x45 pm giving a mean equivalent radius of 2 6 . 5 pm.

Prior to the measurement the sample was heated at 55O'C in air for a

period of 3 - 4 hours to oxidize the template and then transferred to the ZLC system. Equilibration with hydrocarbon was achieved using a standard bubbler and bypass

system

(operated in a

low temperature thermostat).

Blank

experiments were performed to eliminate the effect of any extracrystalline sorption but in a well designed system such effects are minimal and the corrections are insignificant, To investigate the significance of external film resistance, measurements were carried out over a range of velocity with both He and Ar as purge gases. Representative desorption curves for propane are shown in figure 1.

The

curves have the form predicted by the theoretical model and show the expected trend with gas velocity. For this system the curves for He and Ar are almost coincident showing that external resistance to mass transfer must be insignificant. With the higher paraffins some difference was observed in the curves for He and Ar indicating some contribution from film resistance. However, when the data were analyzed in accordance with eqns 1 and 2, consistent values of D/R 2 were obtained. Results and Discussion The diffusivity data are summarized in Table 1 and figures 2 and 3 , in

which relevant data from some other experimental studies are also included. The ZLC values all refer to very low sorbate concentrations, well within the Henry's

Law region.

Where

transport diffusivities obtained at higher

900

Fig. 1

-

ZLC desorption curves for propane in silicalite (r 26.5 pm) showing effect of flowrate and nature of purge gas. Note that for the same flow conditions the desorption rate with He or Ar is essentially the same.

2.2

Fig. 2

2.4

2.6 2.0 10'/T (K-')

3.0

3.2

3.4

Arrhenius plot showing temperature dependence of limiting diffusivity for linear paraffins and benzene in silicalite (yyystals. 0, this work (ZLC); 0 , H a y h m t and Paravar, (membrane) X, Rees et al. (frequency response) ; +, van den flyqin et al.(s) (sq:wfiy). NMR and Karger et al. self diffusivity data from Car0 et al. .

90 I concentration levels are compared, the comparison is based on the corrected diffusivities, calculated according to: D

- Do(dPnc/dPnq)

NMR

(7)

self diffusivity values are compared directly since, at: low sorbate (4) Do DZLC'

concentrations, one may expect D s

-

-

Benzene Our ZLC values for benzene agree well with the diffusivity values reported by van den Begin et a1.(5),

who used a rapid square wave adsorption/

desorption technique, and they are also close to the piezometric values obtained by Zikanova et a1.(6).

Zikanova's data show clearly the large

difference in diffusivity between silicalite and HZSM-5.

Still

lower

diffusivity values for some samples of HZSM-5 have been reported by Wu and Ma(8) and Beschmann et a1.")

who used gravimetric methods.

The diffusivity

of benzene in NaX(2) is about twenty times higher than in silicalite although the activation energies are almost identical (- 6.5 kcal/mole). Linear Paraffins The ZLC diffusivity data for propane and n-pentane in silicalite are in reasonably close agreement with the values derived by Paravar and Hayhurst from measurements with a single crystal membrane(7)

(see figures 2 and 3).

The data for propane are also in reasonably close agreement with the values obtained by Billow et al. from frequency response measurements(10) although, since the ZLC data refer to zero concentration, there is some uncertainty in this comparison as a result of the strong concentration dependence of the frequency response diffusivity values.

In more recent work using the rapid

square wave adsorption-desorption technique, van der Begin et al. obtained much

higher

diffusivities which

again

showed

a

strong

concentration

dependen~e'~). The diffusivity data obtained in these studies for n-hexane by both the frequency response and square wave methods are also shown in figure 2.

It is evident that for this sorbate (as for benzene) the diffusivities

obtained by both techniques are reasonably consistent and the values appear also to be consistent with the present ZLC values (for n-pentane and n-decane). ComDarison with NMR Self Diffusivities The NMR relaxation time for linear paraffin in silicalite are short and, as a result, self diffusion measurements are possible only for the lighter species (C1

-

C3).

902

I

'

15

10 Corbon Number

5

20

Variation of intracrystalline diffusivity (at 334 K) for 1 paraffins with carbon number. 0, ZLC data for 5A zmlitetvarA, ZLC data fo i1icalite;Apembrane data for silicali . 0 , ZLC data for NaXr3'; X, NMR self diffusivity data for NaXe3) X I O - ~ ) . Note that the NMR values are 100 times higher than the ZLC values although the trends with carbon number are identical.

Fig. 3

..

I

5A

12.

- -1

4t1

Fig. 4

I

I

I

3

5

7

I

I

I

9 11 13 Carbon Number

I

I

I

I

15

17

19

21

Variation of diffusional awvation energy with carbon number for 0 (ZLCtI3) 0 (gravimetric) . Silicalite: A ( Z L C ) . NaX: o ( Z L C ) X(NMR)

I

(3: ,

903 The corrected diffusivities for propane in silicalite, derived from the rapid square wave measurements are quite close to the NMR self diffusivities (11,12) although the data obtained for this sorbate by frequency response, ZLC and membrane methods") are much lower. The ZLC diffusivity for propane is about 300 times lower than the NMR value (figure 2 ) .

A

similarly large

discrepancy is observed for the for the linear paraffins in NaX although the qualitative trends shown by the ZLd3) and NKR ~alues''~) may be seen from figure 3.

are very similar, as

Variation of Diffusivitv with Carbon Number The variation of diffusvity (at 334K) and diffusional activation energy with carbon number is shown in figures 3 and 4 for SA, silicalite and 13X. For all three zeolites the trends are similar; the diffusivity decreases rapidly with increasing carbon number up to about C6 and thereafter the rate of change is much smaller. Indeed the ZLC data indicate that in silicalite there is very little difference in diffusivity between C10, C14 and C20- In all three zeolites the diffusional activation energy increases rapidly at low carbon number and then levels off.

For propane and pentane the diffusional

activation energies for silicalite lie between the values for 5A and 13X, as is to be expected from consideration of the relative pore sizes. However, for

carbon numbers greater than about 6 the diffusional activation energies for silicalite and NaX are almost identical and this is true for benzene as well as for the linear paraffins.

CONCLUSIONS The ZLC method has been successfully applied to study the diffusion of benzene and some linear paraffins in large crystals of silicalite. The resulting diffusivity values are consistent with the values obtained by other macroscopic methods but much smaller than the NMR self-diffusivities (for propane).

Diffusivities for linear paraffins in silicalite are intermediate

between the corresponding values for 5A and 13X zeolites, as may be expected from pore size considerations. However, while the activation energies for the lower homologs are immediate between the values for 5 A and 13X, for the higher homologs the activation energies for diffusion in NaX and silicalite are essentially the same.

904

$mmarv of ZLC D i f f u s i v i t v Data f o r L i n e a r Pa r a f f i n s and Benzene i n S i l i c a l i t e C r v s t a l s Sorbate

C3H8

"'gH12

"'1OH22

Temp.

Purge Flow

(Deg C)

( cm3 STP/min)

Note:

9 DxlO 2 -1 cm .s

30 30 50 75 75 (Ar)

20 30 20 10 10

3.0 4.4 5.5 7.0 6.0

74 82 120 160 165

50 50 75 100 100 100 100 (Ar) 165

60 120 30 15 30 60 60 30

4.0 8.6 15 10 22 48 10 63

14 17 24 38 38 39 37.5 106

100 100 150 200 200

20 60 60 60 120

12

70 100 150

60 60 60

21 57 312

70 100 150 200

60 60 60 60 60

20 10 53 105

50 75 100 100 150

4 4 4 2 4

66 7 105 91 46 87

100 (Ar)

Benzene

L

35 6.0 5.0 7.6

6.5

Purge gas He except where noted as A r .

1

E

(kcal/mole)

3 .1

4.6

5.0

11.2 14.5

::i

7.3

il%

6.3 10.2

!:::

0.49

1.35 5.5

)

)

1

4.6

4.5

6.5

905

c

NOTATION sorbate concentration in purge stream

c

sorbate concentration level at which sample was initially equilibrated

D

intracrystalline diffusivity

Dm

molecular diffusivity (gas phase)

Ds

self diffusivity (NMR)

Do

corrected diffusivity (eqn. 6)

E

diffusional activation energy

kc K

external film mass transfer coefficient dimensionless Henry's Law equilibrium constant (q

L

see eqn. 2

q R

adsorbed phase concentration (at equilibrium)

Kc)

crystal radius

t

time

T

temperature (K)

v

interstitial gas velocity

z

depth of ZLC bed see eqn. 2

3! ,

voidage of ZLC bed

E

Sh

-

=

kc (2R)/Dm (Sherwood Number)

REFERENCES 1. 2. 3.

4.

5. 6. 7. 8.

9.

10. 11. 12. 13.

M. Eic and D.M. Ruthven, Zeolites B, 40 (1988). M. Eic, M. Goddard, and D.M. Ruthven, Zeolites 8 , 261 (1988). M. Eic and D.M. Ruthven, Zeolites 4, (1988). D.M. Ruthven, "Principles of Adsorption and Adsorption Processes", p. 124, Wiley, New York (1984). N. van den Begin, L.V.C. Rees, J. Car0 and M. Bulow, Zeolites - in press. A . Zikanova, M. Bulow and H. Schlodder, Zeolites z, 115 (1987). D.T. Hayhurst and A.D. Paravar, Zeolites 8, 27 (1987). P. Wa, A. Debebe and Y.H. Ma, Zeolites 2 , 118 (1983). K. Beschmann, G.T. Kokotailo and L. Riekert, Chem. Eng. Process 2 ,223 (1987). M. Bulow. M. Schlodder. L.V.C. Rees and R.E. Richards. 7th Internat. Zeolite Conf , , Tokyo (1986), p. 579, Proceedings Koanshu 'Elsevier, Tokyo (1986). J. Caro, M. Bulow, W. Schirmer, J. Karger, W. Heink and H. Pfeifer, J . Chem. SOC. Faraday Trans. I 2541 (1985). J . Karger, H. Pfeifer, D. Freude, J. Caro, M. Bulow and G. Ohlmann, 7th Internat. Zeolite Conf., Tokyo (1986), p. 633 Proceedings, Kodanshu Elsevier, Tokyo (1986). J. Karger, H. Pfeifer, M. Rauscher and A. Walter, J. Chem. SOC. Faraday Trans. 1 7 6 , 717 (1980).

a,

ACKNOWLEDGEMENT The sample of large silicalite crystals was kindly provided by Dr. David Hayhurst of Cleveland State University. Funding for this study was provided throqh

a

research grant from the Imperial O i l Company of Canada.