Catalysis by metal fluoride surfaces: the adsorption of [35s] sulphur tetrafluoride by caesium and thallium(i) fluorides and the reaction between c

Applied Cotalyeb, Elsevier Scientific

CATALYSIS

BY METAL FLUORIDE

BY CAESIUM

Department

of Chemistry,

aPermanent

address:

(Received

BY CAESIUM

BETWEEN

CHLORINE MONOFLUORIDE

FLUORIDE

WEBB and John M. WINFIELO

The University,

National

19 September

OF [35S] SULPHUR TETRAFLUORIDE

AND THE REACTION

CATALYSED

Geoffrey

- Printed in The Netherlands

THE ADSORPTION

FLUORIDES

TETRAFLUORIOE

George A. KOLTAa,

Amsterdam

SURFACES:

AND THALLIUM(I)

AND SULPHUR

257

2 (1982) 257-266 Publishing Company.

Research

Glasgow

Centre,

1981, accepted

612 8QQ, Scotland.

Dokki, Cairo,

11 December

Egypt.

1981)

ABSTRACT The reaction temperature

pentafluoride, involving

monofluoride

behaving

less marked. thallium(I) behaviour

tetrafluoride

with chlorine

of caesium

fluoride,

has been shown to be a true surface

[36Cl] chlorine

of chlorine compound

of sulphur

and in the presence

monofluoride by caesium

as a catalyst

fluoride

fluoride

is ascribed

to differences

at ambient

but retention

from experiments Retention

and results

of sulphur

tetrafluoride

between

in the

tetrafluoride

is

are both adsorbed

does not behave as a catalyst. in the bonding

chloride

tetrafluoride.

is significant

and sulphur

but the compound

and the metal fluoride

reaction

and [35S] sulphur

poison,

Chlorine monofluoride

monofluoride,

to give sulphur

sulphur

by

The different tetrafluoride

surface.

INTRODUCTION Ionic metal fluorides, used as catalysts and inorganic catalyse

fluorine

the addition

to give sulphur with chlorine

fluoroanion

postulate of Group

compounds

appeared

believed

monofluoride

that such reactions

proceed

order Cs >> Rb > K > Na 2 Li, which

organic

fluoride

tetrafluoride

will

at 293 K

of carbonyl

fluoride

at 253 K [3].

through

of the type SF5- and CF30-.

in exchange

are widely

involving

caesium

to sulphur

[2], and the reaction

to have been obtained

0166-9834/82/000O-O000/$02.75

and potassium,

reactions

to form chlorooxytrifluoromethane

intermediates

I metal fluorides

addition

[l]. Thus, for example,

pentafluoride

monofluoride

those of caesium

or oxidative

of chlorine

chloride

It is generally surface

particularly

in addition

the formation

Evidence

of

for this

from the observation that the activities 18 [4] or F2C0 [53 were in the

with S18F4

is also the order of catalytic

0 Elaevier Scientific

Publishing Company

activity

observed

258 in synthetic work. However, subsequent investigations of the exchange of S18F4 and 18 F2C0 with a variety of mono-, di- and trivalent fluorides [6] showed that the situation

was more complex.

fluorides

between exchange

SF4, this is not general structural

a correlation

and no correlation

adsorption.

ability

correlated

caesium

pretreatment

fluoride,

The studies described

for the retention

provide an interesting

in an attempt

surfaces

of caesium

were chosen since earlier

and informative

their structures

proven catalyst,

surface,

[7] that

contrast

are significantly

TlF is not catalytically

to clarify

further

of sulphur

tetrafluoride

and retention fluoride

studies

different

of 36C1F, 35SF4

and thallium

(I)

[63 had shown that they

in behaviour.

exchange with S18F and both appear to retain small amounts However,

by 85Kr

effect on its surface structure.

the chemisorption

pretreated

These fluorides

and either

Not unexpectedly,

of S18F4 by the fluoride

using the chlorofluorination

by studying

and SF536 Cl on variously

activity

was established.

below were undertaken

of the catalysis,

as the model reaction,

exchange

of ClF with

and this may be related to the observation

of CsF has a significant

the mechanism

of the reaction

with the B.E.T. surface area as measured

Evidence was obtained

particularly

exists for the alkali metal

between

type or any single cation property

the exchange

fluoride.

Although

with S18F4 and catalysis

'8F_

Both undergo

of SF4 following

[8] and, whereas

exchange.

CsF is a

active.

EXPERIMENTAL Experiments

in which non-radioactive

fluoride were used were performed with chlorine were stored

trifluoride

in stainless

in Monel metal vessels by pressure

(Matheson

Inc.)

fall using a Bourdon gauge

system being calibrated

monofluoride

tetra-

vessels

and gaseous

fluorides

samples

were stored

over predried NaF. Reactions were monitored (Heise

, *1 tort-), each section of the vacuum

before use to allow reaction

from pressure measurements.

and sulphur

Inc.) before use. The metal fluoride

steel pressure

(both Hoke

chlorine

in a Monel metal vacuum system which was passivate'd

stoichiometries

Gas uptake by the metal fluorides

to be determined

was also determined

from pressure measurements. Where appropriate, pentafluoride

by low temperature, measurements

reaction mixtures

were analysed

containing

by separating

trap-to-trap

ClF, SF4 and sulphur

them into their individual

distillation,

ClF being identified

[9], and SF4 and SFSCl by their infrared

weight determination Experiments

after transfer

involving

radioactivity fluoride

with two intercalibrated

to be determined

samples were placed

by vapour pressure

ClO,ll] and molecular

to a Pyrex vacuum system.

the use of redioactively

labelled

and SF536 Cl, were carried out in a Pyrex glass vacuum vessel equipped

spectra

chloride

components

directly,

Geiger-Muller as described

in the vessel

species,

36C1F, 35SF4

system incorporating detectors elsewhere

a reaction

to allow the surface [12]. The metal

in vacua via a side arm and were contained

in a Pyrex glass boat which could be positioned

under the Geiger-Muller

detectors.

259 A linear relationship the labelled determined

substrates;

was found between reaction

from these calibrations.

carefully

dried, no problems

Provided

with hydrolysis

gas uptake by the solid determined

were handled

that reagents

in an argon atmosphere

[‘I] by (a) heating

were encountered

the different

after grinding,

after each period, or (b) reaction

acetonitrile,

followed

Chlorine

decomposition

employed.

fluoride

(Alfa, 99 %)

Co., H20 < 10 ppm)

or were pretreated

as previously

periods of 24 h at 423 K

with hexafluoroacetone of any adduct formed,

(B.D.H. Ltd., 99.9 %) was dried by treatment

potassium

from H36C1

in dry or (c) heating

with P205 before use.

(100 uCi, The Radiochemical

permanganate.

It was purified

KMn04 and P205. ClF was prepared

Centre, Amersham)

and dried by treatment

and

with solid

Cl2 and ClF3 [13], typical reaction 36 being 423 - 543 K for 48 h, initial pressure 400 - 600 torr. ClF was

conditions prepared

were

for 3 h at 573 K or, for CsF at 773 K.

36C12 was prepared aqueous

techniques

Engineering

in vacua for two successive

by thermal

were

and the results for

measuring

glove box (Lintott

with grinding

in dry nitrogen

for each of

and the equipment

(B.D.H. Ltd., Optran Grade) and thallium(I)

They were either used as supplied, described

and radioactivity

in gas uptake experiments

in the Pyrex glass system and the metal vacuum

despite

system were in good agreement, Caesium fluoride

pressure

stoichiometries

in a similar manner

use. It contained this was removed 35

from equimolar

[14]. SF4 (PCR Inc.) was treated with dry NaF before

a small quantity via the formation 35S

SF::pC;":,Er~~:~~~e~r~~om

of SOF2 as an impurity and in some experiments and decomposition

of the SF4.BF3 adduct

and iodine pentafluoride

as described

[15].

elsewhere

[16].

31C12, SF4 and CsF [173.

RESULTS Interaction Admission samples

of ClF, SF,, and SF&Cl with metal fluorides of 500 torr (6.40 mmol) of chlorine

of caesium

fluoride,

rapid initial uptake,

followed

by a relatively

The total uptake of ClF, after a contact in Table

monofluoride

which had been variously

to 2 g (13.17 mmol)

pretreated,

slow process,

36ClF tracer experiments

fluoride

or by heating

samples which

had been pretreated

in vacua at 423 K for 48 h, showed that

at room temperature,

even on evacuating

Over the range 40 - 300 torr the total uptake of ClF, measured

after

for 2 h.

1.5 h, varied

with the initial ClF pressure.

The behaviour

of ClF towards

with caesium

to a smaller

fluoride,

samples except

of thallium(I)

fluoride

that, as shown in Table

extent and the effect of sample pretreatment

The interaction similar

1.

time of 1.5 h, by each sample was as shown

using caesium

the uptake of ClF was irreversible

observed

in a

1.

either with hexafluoroacetone

linearly

resulted

as shown in Figure

behaviour

of SF4 with caesium to that observed

fluoride

to that

1, the uptake occurs

is much less marked.

and thallium(I)

with ClF, although

was similar

fluoride

the quantities

showed

of SF4 involved

260

Time bin)

FIGURE

1

Uptake of ClF by CsF pretreated

as follows:

48 h; 3) (CF3)2C0 t MeCN; 4) N2, 573 K, 3 h;

TABLE

1) none; 2) vacua, 423 K,

5) N2, 773 K, 3 h.

1

Uptake of ClF or SF4 by CsF or TIFa Pretreatment

ClF uptake after

SF4 uptake after

SF4 uptake after ClF

1.5 h/mm01

1.5 h/mmol

pretreatment/mm01

CsF

TlF

1. None

2.7

1.3

2. Vacua, 432 K,

2.9

1.3

0.1

0.1

2.4

0.1

3.0

1.4

0.2

0.2

2.9

0.2

4. N2, 573 K, 3 h

0.8

1.2

0.2

0.2

2.9

0.1

5. N2, 773 K, 3 h

0.3

CsF

TlF

TlF

CsF 0.8

48 h 3. (CF3)2C0 + MeCN

ausing

0.1

0.4

ClF = SF4 = 500 torr = 6.40 mmol; MF = 2.0 g = 13.17 mm01

(CsF), 8.95 mmol

01~).

were considerably

smaller

than the corresponding

amounts

of ClF (see Table

1 and

Figure 2). Using [35S] labelled

SF4 very small uptakes,

were found using untreated on the untreated

thallium

both caesium fluoride

caesium fluoride.

and thallium

fluoride

In contrast fluoride,

not detectable

samples;

by pressure measurement, was detected

no uptake of SF

to the behaviour

of

46

ClF towards

most of the 35SF4 adsorbed

on either

261

OO I

30 I

60 8

I

90 , 1

I

Time (mid

FIGURE 2

Uptake of SF4 by CsF pretreated

&sorption 9

75

60 l= 2= 3* 4=

Admission

0

pretreated

caesium

fluoride

was removed

for caesium

fluoride

pretreated

of a further

the first adsorption

15

Time 6nin)

23*1 % of the initial uptake of 35 SF sample of

- desorption

being retained 45

with

(CF3)2C0 + MeCN

by evacuation with

resulted

fluoride

at room temperature + MeCN in Figure 3,

fluoride. sample,

in a saturation

in the first cycle

of this second sample gave a surface

(CF3)2C0

by caesium

SF4 to a caesium

cycle,

rate which was less than that obtained evacuation

30

1st. Adsorption 1st. Lksorption 2nd.Pdwption 2nd.t&znptii

5.8 mmol 35SF4 was used in each case.

or untreated

This is illustrated

1.

bin)

Uptake of 35 SF4 by CsF (13.17 mmol),

and its desorption.

pretreated

Time 45

Wsorption

FIGURE 3

as Figure

following

surface count

(see Figure 3). Subsequent

count rate identical

to that obtained

following

the first evacuation.

(I) fluoride Admission resulted

No evidence

of SF536 Cl to a pretreated

caesium

in a small uptake of radioactivity

completely

Reaction

removed by evacuation

warm to ambient

fluoride

temperature

[Z], has generally

However,

presented

ClF and the caesium fluoride being an intermediate

were allowed

and, in consequence,

caesium

fluoride

fluoride

and thallium

and thallium

with caesium fluoride

fluoride

ClF by SF4 occurred

fluoride,

pretreated

as shown in Table 1,

temperature

for 1.5 h. After

treated

fluoride

The adsorption

in the SF4 - CsF system.

fluoride

with ClF

with thallium

by SF4 could be carried out several

fluoride

for

which had been pretreated

with

is greater

for the (CF3)2C0 pretreated

of SF4 is characterised process,

behaviour

by an initial,

very similar

rapid uptake

to that observed

In the case of ClF, there is a rapid initial adsorption,

by a more complex

process

adsorption

by the caesium

constant.

pretreated

of SF4 corresponded

in vacua at 423 K for 48 h.

by a slower adsorption

and SF4 retained

by

of adsorbed

Table 2 shows the results obtained

the amount of gas adsorbed

as further

fluoride

of the thallium(I)

with ClF followed

using samples of caesium

(CF3)2C0 + MeCN or by heating

fluoride.

enhanced

uptake of SF4. No SF5C1 was observed

times using the same sample of catalyst.

As expected,

(I) fluoride.

no displacement

of SF4 to caesium

of

in this manner.

of caesium

six such reactions

a small quantity

but not with thallium

Pretreatment

had no effect upon the subsequent (I) fluoride

to the pretreated

in [36C1] surface count rate on admission

to the amount of SF5 36C1 formed.

effectively

ClF

the

which had been

the extent of uptake of SF4 was considerably

as shown by admission

with 36 ClF; the decrease

decreasing

between

of adsorbed

by examining

fluoride

of ClF, as shown in Table 1. However,

the preadsorption

followed

to

to occur by formation

the possibility

of SF5Cl was explored

to react with 6.40 mmol ClF at ambient

With caesium

followed

by condensing

the mixture

of this species with ClF.

of the gas phase ClF, 6.40 mmol of SF4 were admitted

SF5Cl was obtained

caesium

been proposed

reaction

performed

and allowing

After 1.5 h the gas phase was removed and analysed;

Reaction

this was

with ClF.

Samples of caesium

evacuation

temperature

above show that there is a strong interaction

in the formation

of SF4 towards

pretreated

However,

of CsF, typically

at low temperature

species and subsequent

the results

sample at ambient

of metal fluorides

of a Cs+SF5- surface

behaviour

of SF4 by the thallium

temperature.

of SF4 in the presence

SF4 and ClF on to caesium

fluoride

by the surface.

at ambient

of SF,, with ClF in the presence

Chlorofluorination

surface.

for any retention

samples was obtained.

in which the pressure

occurs. fluoride,

slightly

before

and the yield of SF5C1 at each step, are

After six cycles the compositions

to the mole ratios CsF:C1F:SF4,

increased

As shown in Table 2, the amounts of ClF

11.85:9.3:7.45

of the solids correspond

and 11.85:3.75:3.1

for hexafluoro-

263 TABLE 2 Repeated

addition

of ClF (359 torr : 4.6 mmol) then SF4 (359 torr f 4.6 mm011 to

CsF (11.85 mmol) at room temperature Step

CsF pretreatment

Gas

time

gas

yield

time

gas

yield

/h

retained

SF5Cl

/h

retained

SF5C1

2

3

4

5

6

acetone

ClF

1.5

1.9

1.5

0.8

1.5

1.4

0.15

2.0

0.7

ClF

2.0

1.8

0.05

2.0

0.65

0.05

SF4

1.5

1.15

0.1

2.0

0.8

0.2

ClF

2.0

1.8

0.05

2.0

0.8

0.1

SF4

1.5

1.0

0.1

2.0

0.8

0.2

ClF

2.0

1.7

0.35

2.0

1.0

0.1

sF4

18.0

3.4

0.25

18.0

1.1

0.15

ClF

2.0

1.8

0.15

2.0

1.0

0.4

SF4

2.0

1.15

0.1

2.0

0.7

0.1

ClF

2.0

1.95

0.2

2.0

1.1

0.2

SF4

2.0

1.0

0.15

2.0

0.6

0.1

pretreated

and vacuum

heated samples

these solids at room temperature,

333 K. Progressively

heating

in the production

Addition results

/mm01

SF4

by pumping

resulted

/mm01

/llmlol

/mm01 1

423 K, vacua, 48 h

(CF312C0 + MeCN

added

of a mixture

in different

with

were followed

by change

the SF5C1 separated of experiments

of SF4 and ClF to caesium from that described fluoride

+ MeCN, at ambient in pressure.

from unchanged

in which

but was evolved

slowly by pumping

at

of 1.65 and 0.9 mmol SF5Cl respectively.

behaviour

(CF3)2C0

SF5C1 was not evolved

the solids from 333 K to 393 K over a 12 h period

SF4 and ClF were added to caesium pretreated

respectively.

After

samples.

fluoride

at ambient

above. Appropriate

temperature mixtures

of

13.17 mmol in each case,

temperature

and the subsequent

1.5 h volatile

material

reactions

was removed,

ClF and SF4 and its yield determined.

The results

the ClF varies from 0.64 to 3.20 mmol while SF4 is constant

at 3.20 mm01 are shown in Figure 4. For a 1:l mole ratio the yield of SF5C1 is 77 % and very little ClF + SF4 is retained at lower mole ratios. approximately

retention

With ClF at 0.64 mmol the fraction

becomes

significant

of gas retained

15 % of the total initial gas. The yield of SF5C1 increases

when ClF > 3.20 am101 and the fraction is markedly

by CsF, however

of SF4 + ClF retained

by caesium

is slightly

fluoride

increased.

Similar effects

on the yield of SF5C1 and the fraction

of ClF + SF4 retained

264

FIGURE 4

Changes

pretreated

in % composition

for SF4 + ClF -f SF5Cl over CsF (13.17 mmol),

(CF3)2C0 + MeCN. SF4 = 3.20 mmol,

with

(ClF + SF4)(g) and (ClF + SF4)(retained)

by caesium

fluoride

ClF is constant

are observed

when SF4 is varied

at 3.20 mmol. However,

SF4 concentration

is significantly

fluoride

the fraction

The surface concentrations

at low

of SF4 and ClF in a I:1 mole ratio is added

at room temperature.

less than that in the absence

from 0.64 to 3.20 mmol while of ClF + SF4 retained

higher than at low ClF concentration.

No SF5Cl is formed when a mixture to thallium(I)

SF5Cl % of initial ClF,

% of total initial gas.

The quantity

of gas retained

is a little

of SF4. of ClF and SF4 under reaction

conditions

were examined

reaction of 36C1F + SF 4 or 36 ClF + SF4 over caesium ClF + 35SF4 mixtures. Some typical results obtained with 36 ClF in the absence of SF4 is fluoride are shown in Figure 5: the behaviour of by monitoring

the activity

of the surface during

included for comparison. The [36Cl] surface activity followed

accurately.

Evacuation

behaviour

is complete

was observed

at a rate too rapid to be

reaching

surface activities

occurred

according

the second order rate constants [35SF4], after correction

complete,

Table 3 shows that

for the removal of adsorbed

for permanently

retained

except

the surface count

in both the [36Cl] and [35S]

to a second order process.

derived

value after ~1.24 h

had no effect upon the final

The decrease

zero; see Figure.6.

a constant

with 35SF 4 + ClF reaction mixtures

1.25 h, when the reaction was effectively

rate was effectively

agreement.

increased

it decreased,

of the system after reaction

count rate. Similar that, after

initially

Thereafter

radioactivity

[36ClF] and

are in good

Variation

FIGURE 5

36

of

Cl surface

activity

with time (i) for 36C1F

with

+ SF4 (161 torr) over CsF pretreated

in vacua at 423 K, (iii) 36C1F (198 t.Orr)

Variation

FIGURE 6 35SF4

with

+ MeCN,

(161 torr)

t SF4 (168 torr) over CsF pretreated

over CsF pretreated

(CF3)2C0

(168 ton-1

(ii) for 36C1F

(CF3)2C0 + MeCN.

of the 35 S surface activity

(187 torr) over CsF pretreated

with

with time for ClF (185 torr) t

(CF3)2C0

+ MeCN.

DISCUSSION The results caesium reaction

presented

fluoride between

above show that the genarally

catalysed

chlorofluorination

Cs'SF5- and gaseous

accepted

of sulphur

chlorine

mechanism

tetrafluoride,

for the

involving

monofluoride;

SF4(g) t CsF(s) c' Cs+SF5-(s) Cs+SF5-

+ ClF(g) + SF5Cl(g)

is incorrect chlorine

+ CsF(s)

in that such a mechanism

monofluoride

The results pentafluoride

and the caesium

presented

in Table 3 indicate

is a true surface

and adsorbed

sulphur

monofluoride

to sulphur

photochemically

reaction

tetrafluoride

induced reactions

the reaction

between

surface.

that the formation

involving

An excited

tetrafluoride.

adsorbed

molecular

has been reported

of sulphur

chlorine

addition

chloride

monofluoride

of chlorine

for the thermal

and

[18].

Our results may best be interpreted reactions:

does not take into account fluoride

by considering

the following

equilibrium

TABLE 3 Second order rate constants

for the reactions

ClF + 35SF4 + 35SF,C1 Initial pressure of 36C1F

Second order rate

36C1F + SF4 -f SF536C1 and

261 CsF(s)

+ SF4(g) -f Cs+SF5-(s)

rather than via equilibrium Admission fluoride

of sulphur

(2).

tetrafluoride

to chlorine

led only to very small yields

to those obtained

when the reactants

were admitted

However,

there was a marked

fluoride

when the caesium

leading,

in the case of the hexafluoroacetone

composition reactions

increase

fluoride

monofluoride

of sulphur chloride

pretreated

pentafluoride,

simultaneously

in the extent of retention

surface was pretreated

(CSF),_,~(C~F),.,,(SF~),.

pretreated

Together

caesium relative

to the catalyst. of sulphur

with chlorine catalyst,

these observations

tetra-

monofluoride

to a solid with indicate

that

of the type

Cs+ClF2-(s)

+ CsF.SF4(ads,) + SF5Cl(g)

+ 2CsF(s)

and

(cStclF2--sF4)

where

+

sF5ci(g)

+ cSF(S)

(Cs 'C1F2- -SF4) represents

monofluoride suggest

pretreated

the uptake of sulphur

catalyst,

that the (CstC1F2- -SF4) entity exists

and which only decomposes elevated

their reaction

monofluoride,

with thallium(I) monofluoride

to sulphur

presumably

observations

retention

fluoride

chloride

as TltC1F2-,

of sulphur tetrafluoride between

difference

in the bonding

caesium

in the reactive

fluoride between

at

adsorbed

fluorine

atom. Reaction formulated

coordination

between

this species

as Cs+F-...ClF

of the sulphur of sulphur

may be an important

formulated

surface

to equation

in contrast

temperature.

fluoride

tetrafluoride

geometry

[213, therefore

adsorbed

or Cs+F- . ..FCl [20], is readily

to the thallium(I)

interaction.

(5) has no obvious

to a

and the metal

studies of Cs+SF5'

as having a (Cs')F-...SF4 and reactive,

previously

the reactive bridging

chlorine monoenvisaged

via

[63 that

cation via its lone pair

In this situation a surface reaction mechanism.

to

[6], no The

we ascribe substrate

spectroscopic

lone pair. We have suggested

tetrafluoride

although

of chlorine

sulphur tetrafluoride

state. Vibrational

that the anion has square pyramidal

this solid it would not

Retention

at ambient

and thallium(I)

the sulphur

state may be reasonably

participation

pentafluoride.

was observed

indicate

analogous

on the catalyst,

tetrafluoride,

was also observed,

adsorbed

fluoride,

We further

pentafluoride

show that whereas

and sulphur

at 423 K using ['*F]-labelled

difference

fluoride

chloride

by the chlorine extent.

temperature.

The results obtained

previous

as a stable species

slowly to CsF and sulphur

would adsorb both chlorine catalyse

tetrafluoride

cannot occur to any appreciable

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