Kinetics and mechanism of propan-2-OL decomposition on zinc oxide

Applied

Catulysis,

Elsa&?r

Science

25 (1986)

Publishers

121-128 B.V., Amsterdam

121 - Printed

KINETICS AND MECHANISM

OF PROPAN-Z-OL

K.C. WAUGH,

R.W.

Imperical

M. BOWKER,

Chemical

Runcorn,

England,

aDepartment

PETTS,

Industries

in The Netherlands

DECOMPOSITION

H.D. VANDERVELL

plc, New Science

ON ZINC OXIDE

and J. O'MALLEYa

Group,

P.O. BOX 11, The Heath,

U.K.

of Chemical

Engineering,

Imperial

College,

London,

U.K.

ABSTRACT Tempera ture programmed reaction and desorption studies have been used to deteron zinc oxide. They mine the k inetics and mechanism of propan-Z-01 decomposition have shown that a-hydrogen atom abstraction from the adsorbed iso-propoxy species is rate limiting on acetone formation, the overall temperature de endence of of acetone formation having an activation energy of 37 kcal mol- 1 . Abstraction the B-hydrogen atom which results in propene formation has a slightly higher activation energy. Selectivity in the decomposition either to acetone or propene however, we have also shown it to be a is therefore dependent on temperature, function of the defect state of the zinc oxide.

INTRODUCTION Selectivity regarded catalysts

genation

activity

activity

of isopropanol

for specifying

have been classified

or dehydration

only

in the decomposition

as a test reaction

according

having

a selectivity

kinetics

of isopropanol

we found a selectivity

having

previously

so obtained

adsorbed

showed

acetone

desorbing

12°C later.

This result adsorbed roughly

ethanol

at 40°C when

A rationale

different

programmed

in roughly

temperature

product

to ethylene

and that on the zinc dominated

polar

with

0

spectrum

amounts,

the

(468 K) and the

findings,

(ethene)

found

on zinc oxide was confined

0166-9834/86/$03.50

equal

from our previous

spectro-

spectrum,

having

was found

in

[4].

for this high selectivity

interact

the detailed

reaction

of 195°C

of

to propene

investigating

was that all of the adsorption

group could

dehydrogenation for dehydro-

[3]. The desorption

desorb

the dehydrational

and

product)

in the desorption

at 40°C

and propene

first with a peak maximum

90% selectivity

to us when

by temperature

the isopropanol

in itself was quite

(the dehydration

the same selectivity

of -50% to propene

that acetone

propene

to propene

surprise

decomposition

scopy,

toward

has been set as a standard

et al. [2] found

at 90°C and so it was a considerable

has long been

of oxide surfaces

to their propensity

[I]. Zinc oxide

10% at 150°C [I]. Tamaru

(propan-2-01)

the nature

face only the R-hydrogen

the surface

1986

Elsevier

in our earlier to the polar

oxygen

Science

anions.

Publishers

atom of the ethoxy

Abstraction

B.V.

results

face(s)

of the a-hydrogen

122 atom by the surface limited

by the

which

atom and the surface rationalise

oxygen

case

paper

on zinc

The main

to produce

grammed

were

reaction

decomposition temperature grammed

(90:;)

area 3 m ' g -') and in the to account

and so a subsidiary

purpose

of this

view of ethanol

and isopropanol

decom-

is to explore

the possibility

that the

however,

in selectivity

experiments

in which

oxide

the conversion

is monitored

Additionally

we shall

surface

in isopropanol

determine

after

continuously using

programmed

on zinc oxide

temperatures

by mass

of given

of temperature

conversions

pro-

in isopropanol

spectrometer

of the intermediates

reaction

at which

out temperature

and selectivity

the technique

the nature

the attainment

from the temperature

decomposition

[Z] was the different

To this end we have carried

performed.

is increased.

the zinc oxide

situation

[3] and Tamaru

on zinc

desorption

deriving

to ethanol

to the idea of two sites on the surface

a unified

of this paper

by ourselves

the experiments

(surface

between

oxide.

purpose

cause of the difference observed

to the alkene

to be

of the 8x-hydrogen

reactions.

unsatisfactory

is to attempt

was thought

centres

so far we have been unable

in selectivity

had to resort

is a rather

position

However,

anion.

in the aldehyde

of 1.5 A between

(50%) on the same zinc oxide

we have

for the dehydrogenational This

have resulted

distance

the large difference

and isopropanol latter

would

large minimum

as the pro-

extant

on

and selectivities

experiments.

P.G.

NitrousOxide and/or and/or and/or and/or and/or

CarbonMonoxide Methanol Hydrogen Carbon Dioxide Nitrogen

Column Manifold of Gases

Valve

f P.R.

FIGURE

Pressure regulator

P.G.

Pressure gauge

N.V.

Needle

KATH

Katharometer

S.V.

Pneumatic

flow switching

SPL.V.

Pneumatic

sample valve

valve detector

M.S.

Mass spectrometer

S.D.

Sinter disc replaced

1

programmed

Line diagram desorption

M.S.

valve

(Vacuum Generator’s by a jet separator

of the apparatus and reaction.

“Supavac”)

in some experiments

used for rate measurements

and temperature

123 EXPERIMENTAL The apparatus grammed

used for the steady

desorption

into a U-tube adsorption

and reaction

is shown

so that in S-:SA surface

chromatography

was connected

sinter

disc and the mass spectrometer which

reported

allowed

were

carried

was normally fell

ten masses

previously

out on the same sample heated

to 480°C

Generators

by gas

area sample

characteristics

and reaction

by a PET

with temperature/ (36.5 m* g-l)

(in terms of the desorption zinc oxide

experiments

of zinc oxide

Supervac)

by a Commodore

continuously

to those of AnalaR

desorption

pro-

[6]. The exit from the catalyst

(Vacuum

to be monitored

identical

programmed

could be made

itself was controlled

[5], the adsorption

of which were

temperature

1. The zinc oxide was loaded

(2.99 g) was the high surface

on earlier

The temperature

in Figure

to a mass spectrometer

time. The zinc oxide

peak maxima)

rate measurements,

area measurements

as described

column

computer

state

which,

(3 rn* g-l).

on isopropyl

alcohol

before each experiment,

in dry helium. After this treatment the surface 2 -1 2 -1 of 36.5 m g to one of 13.6 m g .

from its initial value

area

spectrometer rewonse

Mass (Arbitrary

UnItSI

2

4096 m/e=45 3584

CH3CWOH)CH3

I

2048-

44

’ Heahg

started

1024-

512.

CH3COCH3

I 50

FIGURE

RESULTS

2

I 50

Temperature

The cracking

, 69

, 90

, 1 111 132 152 TemperatureTC

programmed

patterns

reaction

with temperature

to distinguish

and desorption

a stream

are common

increase

in opposition

the products

The temperature by passing

of isopropanol

of the masses

and 40 for propene)

reaction

I 58

, 173

I

143

of propan-2-01

17’1‘l’l 213 233

253

273

on zinc oxide.

AND DISCUSSION

Since most movement

, 50

m/e=58

acetone

of two masses

spectra

of helium

are listed

and products

of Figures

reaction (25 cm 3

signal

in the temperature

in Table

1.

the coincident

(43 and 58 for acetone

to the m/e = 45 (isopropanol)

from the isopropanol

programmed

and propene

to the reactant

and 41 is used

programmed

2-4.

spectrum shown in Figure 2 was obtained .-I mln ) saturated in propan-Z-01 (5.6%

124 TABLE

1

Isopropanol m/e

Acetone

45

43

27

29

100

17

16

IO

43

58

27

42

100

27

8

7

m/e

Propene

m/e

40

41

31

39

42

59

7

6

6

4

4

26

29

39

38

41

6

4

4

2

2

8.0

41

39

42

27

40

38

100

74

70

38

29

20

Mass Spectrometer Response (Arbitrary units)

Temperature/“C

FIGURE 3

Temperature

programmed

desorption

oxide having raised the temperature then lowered

spectrum

of propan-2-01

to 300°C in propan-2-al/He

on zinc

(5.6% in He) and

it to 50°C rapidly under the gas.

propan-2-01 in He) over the zinc oxide at 50°C. temperature programming at 10°C -1 in the propan-2-al/He being begun only after breakthrough, i.e., after 14 equilibrium adsorption had occurred. Equilibrium coverage was 3.9 x IO molecules -2 -2 CIT which compares well with the value of 3.6 x lOI molecules cm reported

min

by Tamaru et al. [2]. At 40°C only 16% (5.8 x lOI in the uptake was chemisorbed,

desorbing

molecules

cmS2) of that adsorbed

from two states whose peak maxima were

5 100°C (383 K) and 172°C (445 K) (see Figure 4) corresponding to desorption -1 -1 activation energies of 25 kcal mol and 30 kcal mol respectively. Imnediately

the temperature

physically adsorbed 40 and 43 peaks

propan-2-01

programming occurred

was begun, desorption

of some of the

, evidenced by the increase in the 41,

(Figure 2). Indeed the mass spectral

signal at 5O"C, after break-

125

Massspectrometerresponse (Arbitrary units) 2048-

(173%)

(226°C)

1x36-

/

FIGURE

4

1

’

’

”

”

136 150164178192206

Temperature

programmed

desorption

the cracking

pattern

I

’

I

I

1

’

’

’

221235248263278 Temperature "C

spectrum

obtained

after adsorption

at 40°C.

constitutes

spectrometer

used,

At 140°C the onset from this point. the m/e

II

77 92 106122

on zinc oxide

through,

I

62

48

of propan-Z-of

the m/e = 18 peak deriving of reaction

in the quadrupole

from a 200 ppm Water

the m/e = 2 (hydrogen)

occurs,

At 157°C both the m/e = 43 and 58 peaks

= 43 peak

mass

impurity.

signal

increasing

increase, -1 and the rise of 36 kcal mol

having

(acetone)

a temperature dependent -1 in the temperature range 160-200°C. At worst this -1 for the dehydrogenation reaction (acetone of 37 kcal mol

m/e = 58 peak 37 kcal mol activation

energy

formation)

will

of propan-2-01 However,

be a lower which

will

solely

decomposition

which

surface

programmed

which

have been

or hydride

from the

range. This

produces

isa curious phenomenon.

a surface

to hydrogen

ions are released

iso-propoxy state of the

and adsorbed

to be an anion

following

but once this conversion

oxygen

defect to the

on to it by hydrogen

iso-propoxy

in the m/e = 40 signal

close

over the zinc oxide

from the bulk of the zinc oxide

atom of the adsorbed

and maximises

alone

have been thought

at first decreases

conversion,

water

the water

to migrate

is an increase

(The m/e = 41 signal

increases

state would

atoms

from the a-carbon

At 193°C there

41 Signal

and comes partly

atom from the adsorbed

(see below)

and decomposes

induced

when hydrogen

isopropanol

passing

in this temperature

formation

adsorbs

This new surface

scission

the major

The temperature dependent rate -1 showing that it does not

of isopropanol

reactions

was observed

prior to acetone

might

increases.

the m/e = 45 peak,

was only 30 kcal mol

that the loss of the a-hydrogen

zinc oxide species.

off scale up to 190°C.

(hydrogen)

to acetone

fractions

of the water.

no decomposition

species

as the conversion

to be negligible,

from the dehydrogenation

In temperature

It appears

appears

peak remaining

of the m/e = 2 signal derive

be decreasing

this conversion

propan-2-01

limit since the 43 and 58 peaks are cracking

species.

which maximises

at 230°C.

the m/e = 45 signal, has virtually

i.e.,

completed,

to the m/e = 40 signal).

This

the m/e =

is therefore

126 propene

formation

addition

and so the zinc oxide

to dehydrogenational

signal,

the water

ture is raised

further

Just before

activity.

signal minimises

seen by the decrease

is now showing

dehydrational

the maximum

activity

in the m/e = 40

and then rises to go off scale.

the dehydrogenating

in the hydrogen

character

(2) and acetone

in

As the tempera-

of the catalyst

decreases,

(43,58)

at between

signals

250 and 2EO"C. An important

point

to note from Figure

160 to 260°C the selectivity on zinc oxide

changes

continuously

not only on the different but also on the defect on temperature There

a-hydrogen

iso-propoxy

which

decomposition

temperature

energies

in a manner

itself

range and propene

which

depends

and dehydration

is both dependent

gas mixture.

that it is elimination

species

preceeds

which

of the a-hydrogen acetone

have shown quite

propan-2-01

is removed

to acetone

for dehydrogenation

state of the zinc oxide which

and the reacting

c71, using deuterated

with

activation

can be no dispute

the adsorbed

2 is that in the temperature

in isopropanol

formation.

in acetone

formation.

unambiguously

atom from Tamaru

et al.

that it is the

The reaction

involved

is:

H CH3-C-CH3

+ S

CH3C-CH3

+

0

0

Zn

Zn

where

S is a surface

in which

case a proton would

negatively abstract

charged a hydride

Our previous react with centres

explanation

decomposition

we presented polar face

be removed,

anions

propounded

to account

distance

could easily

the activation

can be found

that the activation greater

species

anion

is

energy

in Figure

of acetone, energy

2 where

which

atom.

atom

to a-hydrogen Evidence

to alkene

this can be into the model

on the zinc dominated

anion

of only -0.2 i

vibration.

in moving

It appears

from ethanol

abstraction

to a value

for this lowered

the rate of propene

abstraction

to

their

atom of the adsorbed

to a surface

at the heating

for s-hydrogen

than that for the a-hydrogen

selectivity

for in a rocking

atom.

between

Indeed,

adsorbed

group on the a-carbon

be unlikely

ionic radii

the a-hydrogen

of approach

lower than that of the d-hydrogen

energy

oxygen

which would

distance

for the higher

species

be accounted

to propan-2-01,

lowers

atoms would

the minimum

must be erroneous.

[3] for the ethoxy

a methyl

some 20°C after'that

a zinc cation

the zinc and oxygen

then that substituting

vation

because

on zinc oxide,

has a minimum

which

be a vicinal

the iso-propoxy

with

[33 that the a-hydrogen

oxygen

if we introduce

previously

species

could

but since

is probably

[the (0001) face] of zinc oxide,

- a distance

slightly

species

species.

the surface

seen to be so for,

This surface

the interaction

was too great,

in ethanol

alkoxy

species.

+ H(a)

formation

rates used would was about

1

actipeaks

suggest kcal mol -1

127

Lowering

the temperature

did not reproduce the temperature 43,41,

heating

whose

regime

the water at

This

a defect

state on surface

treatment,

the desorption

helium

in

different

after

and cooling

previously

observed found

having

by Krylov

dosed

earlier

in a totally

propoxy

species

acetone

is facile

species

is formed

must

spectrum

experiment shown

of Figure

in Figure

However,

it does differ

of the m/e = 41 signal slight

bump on

separate,

observed

Previously

before

from that state

but which

the baseline

quite

are

heating

3, and temperature

(m/e = 43 and 58) and propene

the former

at 4O"C,

programmed

C31 and the propene

to 480°C

[3], from which since the isoof the

isopropoxy

in the thermal results

in the

to that seen previously

to defect

state

at 173'C

~31,

investigated

respect,

the m/e = 41 signal

at 223°C

what we

formation.

programming

in one major

step

desorption

which was exactly

namely

appeared

4 and so is not included)

desorbing

of acetone

two peaks were

(m/e = 41, 1 the m/e = 40 which

follow

in Figure

and formation

from the adsorbed

in acetone

has returned

than previously

of 37 kcal mol -'. The

configuration

4. It is not dissimilar

that the zinc oxide

indicating

process

at 50°C after

(550 K) and

and since the desorption

of the a-hydrogen

of propan-2-01

quite

on to Zinc

is not rate limiting;

at room temperature

abstraction

bY temperature

being

is the rate limiting

spectrum

experimental

of the acetone

be the rate limiting

Readsorption desorption

different

that desorption

from Purely

at 277°C

in a temperature

on to the zinc oxide

at -80°C and 130°C in the desorption

we conclude

acetone

to desorption

on

by isoProPano1

50°C higher

energy

its desorption

181. However,

acetone

is about

cooling,

This cycle of events

the propan-Z-01

activation

that

During

3 obtained

maximise

in

Of the m/e = 58,

different

in propan-2-01

(acetone)

energies

as indicating

as has been suggested experiment

to a desorption

in the activation

be interpreted

regime.

in Figure

by dosing

(m/e = 40 and 41) 17°C later. This

corresponding

correspondence could

shown

gas

signals

minimid

adsorption

of the zinc oxide Wite spectrum

heating

from that obtained

the propene

Pattern.

PWViOUSlY

due to displacive

oxide at 40°C. The m/e = 43 and 58 peaks

observed,

cracking

to zero in this temperature

thermal

and acetone

to the values

at 244°C having

probably

is

decreases

Produced

programming

in the propene

in the propan-Z-01

seen

signal maximised

233°C.

conversion

from 3OO'C to 50°C under the reacting

increases

270 t0 17O"c, both decreasing

and 40 signals

however,

rapidly

the transient

before.

that the is only 29% only

desorption

as

(450 K) about 40"~ earlier (496 K), roughly

a

peaks

than

the same as

[3].

CONCLUSION These studies labelled

experiments of Tamaru

isopropanol,

abstraction

in conjunction

with our earlier

et al. L-71 on the reaction Tamaru

of the a-hydrogen

et al. showed

work

of isopropanol

that acetone

from the adsorbed

[3] extend

was formed

iso-propoxy

the original

on zinc oxide.

Using

by the

species,

but, since

128 the work was carried rate determining propoxy

out at one temperature

step

co-existed

in the reaction.

on the surface

the existence

of this strongly

they could

paper

interaction

of both temperature

activation

the energy limiting

energy

barrier

is that of s-hydrogen that defect

versus

used to probe isopropanol

temperature

abstraction

by the presence

dehydrational

which

these properties

dehydrogenation

oxide.

for

pathway

results gives

in acetone -1 which

programmed

reaction. relative

ethanol

resulting function

formation.

The

is probably to be rate

1 kcal mol -I)

formation;

rise to propene

group

of zinc oxide

(by about

in propene

group

The lower energy

which we have shown

is lowered

- using

zinc

resulting

energy

of an a-methyl

character

held

to be a complex

is 37 kcal mol

abstraction A higher

abstraction,

during

to r-hydrogen

abstraction

of the o-hydrogen for this process

formation.

iSOprOpOXy

with zinc oxide

state of the

state of the zinc oxide which

only transiently barrier

of isopropanol

to a-hydrogen

in acetone

strongly

held end tautomer.

and of the defect

pathway is the abstraction overall

a more

On the

that the iso-

[3] we found no evidence

In this paper we have shown the fate of the adsorbed from the initial

not Cement

they postulated

of the zinc oxide with

In our previous

- its end tautomer.

species

only,

Additionally

however,

formation

The activation

appears energy

to that of B-hydrogen

so that the dehydrogenational

is also a function dehydration

of the molecule

dominates

while

using

dominates.

REFERENCES 1 2 3 4 5 6 7 8

O.V. Krylov, "Catalysis by Nonmetals", Academic Press, New York and London, 1970, p.115. 0. Koga, T. Onishi and K. Tamaru, JCS Faraday 1, 76 (1980) 19. M. Bowker, R.W.Petts and K.C. Waugh, JCS Faraday Trans. 1, 81 (1985) 3073. M. Bowker, H. Houghton and K.C. Waugh, JCS Faraday Trans. 1, 78 (1982) 2573. M. Bowker, H. Houghton, K.C. Waugh, T. Giddings and M. Green, J. Catal., 84 (1983) 252. M. Bowker, J.N. Hyland, H.D. Vandervell and K.C. Waugh, 8th Int. Cong. on Catal., 2 (1984) 35. E. Akiba, M. Soma, T. Onishi and K. Tamaru, Z. Phys. Chem., 119 (1980) 103. O.V. Krylov, "Catalysis by Nonmetals", Academic Press, New York and London, 1970, p.131.