Probing the shape selective properties of zeolites by catalytic hydrocarbon reactions

451

Catalysis Today, 3 (1988) 451-457 Elsevier Science Publishers B.V., Amsterdam -

PROBING THE SHAPE SELECTIVE

Printed in The Netherlands

PROPERTIES

OF ZEOLITES

BY CATALYTIC

HYDROCARBON

REACTIONS

JENS WEITKAMP

and STEFAN

University of Oldenburg, Ammerlaender Heerstrasse Germany

ERNST Department of Chemistry, Chemical Technology, 114-118, D-2900 Oldenburg, Federal Republic of

ABSTRACT For routine applications, the effective pore widths of zeolites or zeolitelike materials can be easily characterized by suitably selected catalytic reactions. This method makes use of shape Selective catalysis. Three hydrocarbon reactions for probing the pore width are comparatively discussed. They have in common that their results can be expressed in a single figure named Constraint Index (CI), Modified Constraint Index (CI*) and Spaciousness Index (SI). Recommendations for routine applications are given.

INTRODUCTION Catalytic effective

test reactions

pore widths

literature

for this purpose

that the shape selective pressed

quantitatively

The Constraint relative

determined

stmint Index

Index

cracking

will be discussed.

properties

tools for probing proposed

the

in the recent

These three tests have in common

of molecular

sieve catalysts

can be ex-

in a single figure. (CI),

introduced

rates of n-hexane

by Mobil

(ref. l), is based on the

(n-Hx) and 3-methylpentane

In contrast,

(3-M-Pn).

both the Modified

It is

Con-

(CI*) introduced by Jacobs et al. (ref. 2) and the Spaciousness

(ref. 3) are measured

The Modified

Constraint

of 2-methylnonane a low conversion

on bifunctional

Index is defined

and 5-methylnonane around

forms of the zeolite

of isobutane

preferentially

tane. Since the Spaciousness

isomerization

set of reaction

of n-decane

(n-De) at

Index is given by the

and n-butane

butylcyclohexane

Index is independent

at an almost arbitrary

catalyst.

as the ratio of the rates of formation

during

5 % (ref. 2). The Spaciousness

ratio of the rates of formation of a C,O-naphthene,

evaluated

and powerful

Three test reactions

on an acid form of the zeolite.

Index

(SI)

are valuable

of zeolites.

during

hydrocracking

(B-CHX) or pentylcyclopenof the conversion,

it can be

conditions.

In the present paper, the usefulness of the three indices will be comparatively discussed.

From the results,

recommendations

deduced.

0920-5861/88/$03.50

0 1988 Elsevier Science Publishers B.V.

for potential

users will be

452 EXPERIMENTAL The zeolites sources

employed

(zeolites

lites). The organic sis were removed

in this study were obtained

Y, L, mordenite) templates

tional forms of the zeolites The catalytic on-line

occluded

by calcination

reactions

sampling

were obtained

were performed

to the procedures

1-3). For the determination

in the zeolite

from commercial

syntheses

(all other zeo-

pores during

the synthe-

in air at 540°C for 16 h. The acid and bifunc-

and glc analysis

mined according

either

or by hydrothermal

by standard

ion exchange

in a fixed bed flow-type

of the products.

reactor with

CI, CI* and SI were deter-

given in more detail

of the Spaciousness

techniques.

in the literature

Index, butylcyclohexane

(refs. was

used as the probe molecule.

RESULTS AND DISCUSSION The Constraint Typical straint

Index

time-on-stream

curves

that, on the large pore zeolite than its unbranched both alkanes

100

obtained

Indices of three zeolites

at comparable

alkane

of the Con-

is cracked

is true in HZSM-5.

seen

much faster

In H-mordenite,

rates. From the data shown in Fig. I,

I

I

I

I

the determination

in Fig. 1. It is clearly

Y, the branched

isomer. The converse

are cracked

during

are depicted

HZSM - 5 80

I

I

-

LaY

---

H -Mordenit

I

.

n-Hx 60

f_

~

y

3-M-Pn

P--

n-Hx 5 *I-

TIME

ON

STREAM,

.=

1.

h

Fig. 1. Catalytic cracking of an equimolar mixture of n-hexane and 3-methylpentane in three zeolites at T s 300°C. All other conditions were chosen according to (ref. 1).

453 the following

Constraint

Indices are calculated:

nite and 4.6 for HZSM-5. Additional HZSM-22

experiments

at T = 370°C gave Constraint

According

to (ref. I), zeolites

as large, medium the Constraint

ever, the Constraint lite, while

deduced

dence of the Constraint

classifies

is introduced

Index of medium

(ref. 1). With HZSM-22,

Upon increasing

the temperature

to 5.2. Therefore, compared

if they were determined

of the method during

is evident

acid catalyzed

difficult

to find a "window"

fluence

occurs

trast,

during almost

Constraint

examples

in Fig. 1: Deactivation

of CI. Also,

that these deposits

due to the build-up reactions

takes place during in the presence

controlled.

Y that, in this regime,

branching

mechanism

thermodynamic

in zeolites. hydrocarbon

conversion

formed

It is very typical

are formed at

of 3-methylnonane

different

is due to shape selectivity probably

(ref. 4). Expectedly,

of the methylnonanes

distribution

a completely

the isomer dis-

and 5-methylnonane

cyclopropanes

in iso-

for the large pore

shown that this is a direct consequence

the distribution

ring zeolite,

de-

By con-

of hydrogen.

of the methylnonanes

2-methylnonane

via protonated

equilibrium

On Pd/HZSM-5, regime which membered

It has been

conversion,

can have an in-

of carbonaceous

rates which are about one half of the rates of formation

creasing

there is

of coke inside the

are shown in Fig. 2. At low conversions,

of n-decane

are kinetically

or 4-methylnonane.

drawback

Index

tributions

equal

One further

can be so severe that it becomes

all acid catalyzed

for the distributions

merization

zeolite

from 8.3

should only be

pore width.

very little or no deactivation

Typical

zeolites

is due to the deposition

on bifunctiond forms of the zeolites

The Modified

the CI decreased

for the smooth determination

It is well known that deactivation posits

from 510°C to

was found in our laboratory:

at the same temperature.

and it must be expected

on the effective

depen-

For a given HZSM-5 sam-

Indices of different

of alkanes

no doubt that the deactivation pores of mordenite,

dependency

temperature

was decreased

from the data for mordenite

cracking

How-

pore zeo-

ring zeolite.

from 360°C to 42O"C,

the Constraint

when com-

structures.

ZSM-12 as a medium

pore zeolites.

a similar

Qualitatively,

correctly

by the pronounced

ple, CI varied from 1.5 to 11, as the temperature 300°C

, respectively.

rank the five zeolites

it is a 12-membered

inaccuracy

5 12 and CI > 12 are classified

materials

from their crystallographic

Index erroneously

in reality

An additional

ISCI

with CI
Indices given above

at T = 380°C and

Indices of 3.0 and 7.4, respectively.

and small pore zeolitic

pared to the pore widths

0.2 for Lay, 1.0 for H-mordewith HZSM-12

of the with in-

shifts towards

the

(ref. 4). distribution effects

of the restricted

is observed

in the kinetic

inside the pores of the IOtransition

state shape selec-

0.27 Pd I HY

0.27 Pd I HZSM - 12

0.27 Pd / HZSM - 5

1

I

40

60

80 100

x,,

De

I

I

o-v-v-

$

10

K -

0 0

20

40

60

80

100

20

CONVERSION

20

10

60

80

100

, %

Fig. 2. Distributions of the methylnonanes formed by isomerization of n-decane in 0.27 Pd/HY. 0.27 Pd/HZSM-12 and 0.27 Pd/HZSM-5 (pW2 = 2.0 MPa,pn_De= 20 kPa1.

tivity type (refs. 5, 6). 2-Methylnonane is now highly favored over S-methylnonane (Fig. 2). An intermediate behavior is encountered on zeolite Pd/HZSM-12. For a quantitative evaluation of the effect, the ratio of the rates of formation of 2-methylnonaneand 5-methylnonaneat about 5 % conversion of n-decane is taken. Like the Constraint Index, this figure increases with decreasing pore width of the zeolite, hence the terms "Modified" or "Refined Constraint Index" (ref. 2). From the data in Fig. 2, the following Modified Constraint Indices CI* are calculated: 1.1 for Pd/HY, 2.3 for Pd/HZSM-12 and 4.2 for Pd/HZSM-5. They are in fairly good agreement with the values published recently (ref. 7) for the same types of zeolites loaded, however, with 1 wt.-% of Pt instead of 0.27 wt.-% of Pd. The Spaciousness Index The Spaciousness Indices of a variety of zeolites (a77 in the H+ form and loaded with 0.27 wt.-% of Pdf are indicated in Fig. 3. To cover a broad range of conversions and yields of hydrocracked products, the reaction temperature was varied between 200°C and 300°C at W/FD_CRx = 400 g= h/mol, according to the activity of the respective catalyst. For the zeolites with known crystallographic structures, SI is found to increase with increasing space inside the

455 25

I

I

I

I Y

0

8 Y 20 -

f l

0

E z -

Beta

V

V

fr 15 -

: W z UI

MORDENITE ‘0

OFFRETI

2 0

5

,\

TE

<

0

cl

\,

-

EU-1

aQ

0

20

YIELD

40

OF

60

CRACKED

60

PRODUCTS

Vcr,,

100

Y.

Fig. 3. The Spaciousness Indices of selected zeolites (for 0.27 Pd/HEU-1 and 0.27 Pd/HZSM-20: pH = 100 kPa, pB_CHx = 1.3 kPa; for all other zeolites: pH2 = 2.0 MPa, pB_CHx = 2! kPa).

pore system.

Furthermore,

of the yield of cracked zeolites

with unknown

Spaciousness According

it is clearly products

open channel

Indices give valuable

systems.

system comparable EU-1 belongs

and, hence of reaction

crystallographic

to their SI values,

structures

information

to the one of offretite.

It is one of the great advantages

suffice

of the Spaciousness

chromatographic

on their effective

According

techniques

the

pore widths: with very

has a narrowed

pore

to its Spaciousness

Index,

ring zeolites.

of the Spaciousness

of the ClO-naphthene.

will usually

For the

ZSM-20 and Beta are large pore zeolites

to the class of 12-membered

lytic experiment

temperature.

(EU-1, Beta, ZSM-201,

EU-1, on the other hand, obviously

vary with the conversion

measurement

seen from Fig. 3 that SI is independent

Index that it does not

As a consequence,

for its,determination.

Index is very easy.

can be employed

a single tata-

In addition,

In particular,

since only isobutane

the

simple gas-

and n-butane

have to be analyzed.

CONCLUSIONS

AND RECOMMENDATIONS

The Constraint Indices

Indices,

for a number

the Modified

of zeolites

broad range of numerical

Constraint

are compared

Indices and the Spaciousness

in Fig. 4. CI and CI* offer a

values for lo-membered

ring zeolites

whereas

the 12-

456

ZSM-‘2

ZSM_22

ZSMd

ZSM-20

Mordenite

Y

Offretite

I I,

I

Range

of

12 - MR - Zeolites

Range

of

IO - MR - Zeolites

I

I

c1*

I

15

I

I

IO

5 ZSM-5

CI

Range

I

I

of

0 Mordenite

ZSM-20

10 - MR- Zeolites

I

I

15

SI

I

1

I

10

5

0

Fig. 4. Constraint Indices (CI), Modified Constraint Indices (CI*) and Spaciousness Indices (SI) of selected zeolites after (refs. 1, 3, 7, 8) and this study.

membered

ring zeolites

true for SI which while

it is nearly

IO-membered

in a very narrow range. The converse

a very broad range for 12-membered

insensitive

to changes

the Constraint

Index suffers

For the routine characterization

we therefore

recomnend

and the Spaciousness it is recommended

the combined

in the region of

Index. If

within

2, the Modified

from too many shortcomings

of zeolites

application

to start with the determination information

the group of 12-membered Constraint

and zeolite-like

of the Modified

of the Spaciousness is available

ring zeolites.

(vide materials,

Constraint

the porous solid under investigation

it is larger than about 2, the desired material

of the pore width

is

ring zeolites

ring zeolites.

As a whole, supra).

are compressed

provides

Index

is unknown, Index. If

for ranking

the

If SI is smaller than

Index should be determined.

ACKNOWLEDGEMENTS Financial stiftung,

support

by Deutsche

Forschungsgemeinschaft,

and Fonds der Chemischen

Industrie

Max-Buchner-Forschungs-

is gratefully

acknowledged.

457 REFERENCES V.J. Frilette, W.O. Haag and R.M. Lago, J. Catal. 67 (1981) 218-222. J.A. Martens, M. Tielen, P.A. Jacobs and J. Weitkamo. . - Zeolites 4 (1984) 98-107. J. Weitkamp, S. Ernst and R. Kumar, Appl. Catal. 27 (1986) 207-210. J. Weitkamp, Ind. Eng. Chem., Prod. Res. Dev. 21 (1982) 550-558. P.A. Jacobs, J.A. Martens, J. Weitkamp and H.K. Bever, Farad. Discuss. Chem. _ Sot. 72 (1982) 353-369. J. Weitkamp, P.A. Jacobs and J.A. Martens, Appl. Catal. 8 (1983) 123-141. P.A. Jacobs and J.A. Martens, in V. Murakami, A. Iijima and J.W. Ward (Editors), Proc. 7th Intern. Zeolite Conf., Kodansha, Tokyo and Elsevier, Amsterdam, 1986, pp. 23-32. D.H. Olson, R.B. Calvert and E.W. Valyocsik, Europ. Patent Appl. 102 716, assigned to Mobil Oil Corp., 1984.