Zeolite catalysis and molecular vibrations

Zeolite catalysis and molecular vibrations

Catalysis Today,3 (1988)387-394 Elsevier Science PublishersB.V., Amsterdam-PrintedinThe ZEOLITE R. CATALYSIS AND MOLECULAR 387 Netherlands VIBRA...

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Catalysis Today,3 (1988)387-394 Elsevier Science PublishersB.V., Amsterdam-PrintedinThe

ZEOLITE

R.

CATALYSIS

AND MOLECULAR

387 Netherlands

VIBRATIONS

LARSSON

Group of Catalysis of Lund,

Research

Inorganic

,

P 0 Box 124, S - 221 00 Lund

Chemistry, f Sweden

University

f

ABSTRACT A model for catalysis based on selective energy transfer from catalyst to the reactant is applied to some reactions catalyzed by zeolites or zeolite-related systems. It is indicated that the vibration mode that brings hydrocarbons to react is most probably the skeletal vibrations around 1200 cm-l.

INTRODUCTION A molecule

in contact

with, or forming

phase

will be in thermal

equilibrium

inter

alia, that when the molecule

collision

) it will rapidly

situation

stands

For this energy

latter

Such considerations absolute

rates

molecule

basis

supply

energy

to the vibration

phase

of the model

modes

used

excess the reaction.

,however,

of

the

source of

for the dissipation

of the condensed

for catalysis

.

(ref. 1 1.

must have a steady

that more than compensates

molecule

for treatments

its state of activation

in the condensed

by

This

of an acquired

mode may trigger

such as, e.g., the RRKM theory

energy

basis

vibration

are the principal

In order to reach

t e.g.

energy.

to that of a free gaseous

case, the statistical

into the critical

This means,

has been excited

lose its excess

in contrast

part of, a condensed

with this phase.

phase.

of

This is the

in this discussion.

ZEOLITES The above molecule

view is specially

entrapped

any other molecules

in the cavity present

enough

to give, by collision, of the potentially the molecule

in the case of a

of a zeolite.

There are few if

which have a kinetic

energy

to excite

obvious

an

reactive

energy

additive

molecule.

energy

high

to the free

The source

must thus come from the zeolite

of energy structure

333 itself by the transfer of vibrational energy from the zeolite to the molecule. It has been argued elsewhere (ref. 2 ) that the role of a catalyst is, inter alia, to supply energy by vibrational resonance directly into that vibration mode of the reacting molecule that most closely represents that distortion of the molecule which is critically needed for the reaction to proceed. Furthermore (ref. 3 1 , it

has been argued that the substrate

-catalyst complex can be regarded as an analogue to the classical concept of a forced, damped oscillator. The damping ( = friction loss of energy ) can be thought of as being analogous to the dissipation of energy of the partly excited molecule as discussed above. Applying the basic rules of classical, mechanics to this model ( ref. 3 ) molecule,V,

a relation between the frequency of the

the frequency of the catalyst,

isocatalytic temperature, 8,

W,

and the

was obtained. This relation took

the form

8 = N!E 9'

2

V -w ___-~ VI

2

. ---------~

1

____-_

(1)

"0 4.~12 - arctg -_--2 2 I li -a,*)

It has also been argued (ref. 2 ) that if

v

=w,

a

state of resonance will cause the most effective transfer of energy and thus cause the most rapid reaction.

It also follows

from this concept, and is indeed observed ( ref. 4,5 ), that for a certain catalytic reaction effected by a series of similar but non-equal catalysts, the energy of activation will change in a stepwise manner from one catalyst to another. The statements above can now be illustrated by some data from literature which are relevant to zeolite catalysts. As will be seen, some degree of subjectivity is still left in the choice of the proper vibration mode of the reacting molecule. In some cases, however,this mode can be determined by the analysis of the stepwise change of the energy of activation. Let us start with such an example.

389

BOUDART'S

OBSERVATION

OF THE COMPENSATION

( ref. 6 ),

In an early paper existed

relationship

a

cracking

of a series

alumina-silica

(2),

quantities

lnA=bE

+

related

relate

that there for the

hydrocarbons

that the logarithm related

observed parameters

on an

( the "compensation

This relation

(In A) was linearly

(E). These

the kinetic

of closely

system.

was such, cf. eqn factor

betwen

EFFECT

Boudart

effect")

of the preexponential

to the energy

to the Arrhenius

of activation

equation,eqn

(3).

c

( 2 1

In k = In A - E/RT

(3)

b = l/R@

(4)

According temperature

( ref. 6 ), the isocatalytic

to Boudart

6 was found to be 950 K.

The data that were used,

however,

seem to be have been misquoted

original

investigation

a conference

discussion

from the authors

( ref. I,8 ) .The data given by Boudart ( ref. 9 ),

and Nicholson

As the present

with the data of

( ref. 7,8 ).

author

is completely

in the numbers,

this discrepancy

in

and used by him thereafter

( ref. 6 ), are shown in Table 1 together Franklin

of the

unaware

the original

of the origin of

data are

preferred.

All the compounds rate law. Others, In Table in other

sucessive

kcal/mole

are muliples

change

of activation

formed between

energies

the

that to a very good

of one and the same factor,

. This latter value corresponds

with a wave number

did not.

as has been used

. It is found that these

form a set of values

approximation

a first order

and n-hexane

is undertaken

has been

in the table

1 obeyed

n-pentane

on the stepwise

The difference

entries

differences

Table

2 the same procedure

dicussions

( ref. 4,5 ).

shown in

viz., n-butane,

to vibrational

of 3730 cm ml or more probably

10.7 energies

3 x 1244 cm-l.

390

TABLE

1

Activation

energies

Compound

for hydrocarbon (ref. 6 )

Ea

(kcal/ mole)

cracking

on alumina-silica.

( ref. 7,8 ) In

E,

A

(kcal/mole)

Propane

43

41.7

19.56

2-methylpropane

35.3

30.5*

14.85

2-methylbutane

31.3

20.5

7.34

2,2_dimethylpropane

23.0

40.0

18.20

Cyclohexane

18.5

18.5

5.99

*

A corrected

** Calculated

TABLE

value

( ref. 8 )

from original

data

(ref. 7,8 )

2

Representation

of the differences

the five different made

**

in another

column,

substances.

between

activation

energies

If the order of the compounds

way, there will appear

other numbers

Activation Ea

energy

Difference

Factor

AE

(kcal/mole)

(kcal/mole)

Propane

41.7

11.2

1

2-methylpropane

30.5

10.0

1

2-methylbutane

20.5

19.5

2

2,2_dimethylpropane

40.0

21.5

2

cyclohexane

18.5

23.2

2

8

Total

sum of AE

85.4

Mean

BE

10.7

is

in the last

e.g. 0.

Compound

for

391 The reason that the increment occurs in sets of three is not obvious, but it is of great interest to note that the wave number 1244 cm -l is of the right size

to correspond

to a skeletal

vibration mode of the hydrocarbons under discussion. to the empirical assignments of Bellamy

( ref. 10 ),

According a

(CH3)3-C-R group has IR absorption bands at 1250 f 5 cm-l

and at

1250 -1200 cm-l whereas a (CH3)2-C group has absorption bands at 1170 f 5 cm-l

and at 1170 - 1140 cm-l.

In A 20-

15-

lo-

E/kcal

I

I 2b

I 30



mol’l I 40

I

Fig. 1. The compensation effect of

hydrocarbon cracking ( ref.

7,8 ) over alumina-silica. Now let us look at the compensation effect in terms of these original data. In Fig 1 In A from Table 1 has been plotted against the experimental activation energies ( ref. 7,8 ). The only point falling somewhat off a straight line is the one for 2-methylpropane. The data for activity for this compound, however,

were taken from one source ( ref. 7 ), whereas the

value for E, was from another one ( ref. 8 ). This might give the reason for a slight discrepancy. Otherwise, the data refer to measurements at only a few ( reported ) temperatures and the errors

may therefore be not completely ignorable.

392

A least square fit to these five points give a line with the slope

b = 0. 57 mol/kcal. From eqn (4) this corresponds to an

isocatalytic temperature of 877 K . Application of formula (1) tells us that under almost resonance conditions this value corresponds to n = 1226 cm-l. The agreement with the value obtained from the E, increment ( 1226 compared to 1244 ) is very good. This indicates that the molecular vibration that determines the catalytic reaction is probably the skeletal vibrations indicated above ( for the free molecules or for the corresponding protonized species ) and that the vibration of the zeolite catalyst will also be close to this value. Inspection of the infrared spectra of Y-zeolites ( ref. 11 ) indicates

a substantial splitting of the asymmetric (Al,Si)-0

stretch, giving one component around 1200 cm-l , the major one still near 1000 cm-l.

Hence a good opportunity for resonance

between catalyst and reacting molecule could exist. One must note that the errors in the estimation of the frequencies quoted above must be around +50 cm-l. THE CORRFLATION OF DEROUANE Having now discussed a case with one zeolite catalyst in common for reaction with a series of slightly different substrates, let us now turn to the contrary ; the catalytic reaction of one and the same compound at a series of slightly different catalysts. Very recently, Derouane ( ref. 12 ) has suggested a correlation between the catalytic activity of a series of zeolites and their pore dimensions. Plotting the data of Kikuchi et al.( ref. 13 ) for pentane cracking

against the pore size of the zeolites used,

he obtained a typical "volcano -shaped" relation. The position of the maximum should, in Derouane's interpretation,

correspond to the optimum pore size and optimum

possibility for interaction between substrate and catalyst. From the results above, however,

it seems to be of interest to

investigate also the relation between catalytic activity proper vibrations of the zeolite systems. give some indication of

which

and the As the above results

vibrations might be active, the

asymmetric stretch of the various zeolites has been chosen. With

393

no knowledge of the experimental details ( ref. 13 )

of the original work

a coarse estimation of the Al/Si ratios has been

used and hence, from the tabulations of Flanigen, Khatami and Szymanski ( ref. 14 ),

the corresponding vibration frequencies.

They are used in Fig 2 where the activity of pentane cracking is plotted agianst the asymmetric stretch frequencies.A good correlation is obtained.

From this curve, one can interpret the

efficiency of the ZSM-5 type catalysts as originating from the fact that their vibration frequency is closest to the frequencies of C-H deformation or skeletal vibration modes of the molecule concerned. In want of better information, it is not possible to discuss a correlation in terms of the upper branch of the split band, the one being close to 1200 cm-l.

log(TOF) -1 -

Fig. 2.

The turnover frequency of pentane cracking versus the

wave number of the asymmetric stretching vibration of the zeolites.

CONCLUSION To sum up and conclude from the two

examples of relation-

ships between activity of zeolitic catalysts and molecular vibrations, one might observe offer a pool of energy from

that any zeolite system that can

vibrations close to that one needed

394

to activate

the reaction

of vibrational 1400 cm-l

energy

( ref. 15 ).

temperatures

so low

effective.

This observation

( ref. 16 )

efficiency

will be highly

Such a source

might be found in the NH4+ vibration

of ammonium

might

explain

- containing

(<27OoC ) that ammonia

around

the

zeolites

at

( ref.17 )

is not

desorbed.

REFERENCES 1.

R.P. Wayne,

in

Comprehensive

C.H. Bamford

Chemical

Elsevier,

Kinetics,

and C.F.H.

Kinetics.

Chem. Scripta,

2.

R. Larsson,

3.

R. Larsson,

Chem. Scripta

4.

R. Larsson,

J. Mol.

, 27

Catal.,

Chem. Eng. Progr.,

J. L. Franklin

8.

J. L. Franklin

of

3.

(1987) 371.

38

R. Larsson, M. Boudart,

),

12 (1977) 78:

Z. physik.

6.

( Editors

2, The Theory

1969, Ch.

5.

7.

Vol.

Amsterdam,

Tipper

Chemie

(1986) 71

),

(Leipzig 57

268

( 1987) 721.

(1961) 33.

and D. E. Nicholson,

J. Phys. Chem.,

60

J. Phys. Chem.,

61

(1956) 59. and D. E. Nicholson,

(1957) 814. 9.

Adv. Catal.,

M. Boudart,

Methuen

& Co, Ltd, London, and

11. K. Oinuma quoted

New York,

12. E. G. Derouane,

Gakkaishi,

H. Khatami

Zeolites,

D.C.,

1971 , p. 201. Infrared

John Wiley

16. J. C. Miale,

100

8:l

( 1967)

Sieves,

and Y. Morita,

and H. A. Szymanski,

Spectra & Sons,

Am.Chem.

Murakami,

Sekiyu

of Inorganic

Molecular Sot., Wash.

and Coordination

Inc., New York, London,

N. Y. Chen and P.B. Weisz,

J. Catal.,

278. 17. C. V. Hidalgo,

;

John Wiley

(1986) 541.

K. Shimomura

Adv. Chem. Ser. 101;

Sieve

Compounds,

Molecular

(1985) 210.

14. E. M. Flanigen,

15. K. Nakamoto,

Molecules,

1974.

J. Catal.,

28

of Complex

J. Toyo Univ.

Zeolite

H. Nakano,

13. E. Kikuchi,

Spectra

1962.

H. Hayashi,

from D.W. Breck,

& Sons,

9 (1957 ) 636.

The Infra-red

10. L. J. Bellamy,

H. Itoh, T. Hattori,

J. Catal.,

85 (1984) 362.

M. Niwa

and Y.

1963.

6 ( 1966)