Benzene pyrolysis and soot formation at high temperatures

International Journal ofMass Spectrometry and Ion Physics, 47 (1983) 63-66 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

BENZENE

AND

PYROLYSIS

SOOT

FORMATION

AT HIGH

TEMPERATURES

H. R. UDSETH', A. L. JOHNSON' , AND R. 0, SMITH' 1 Chemical Methods & Kinetics, Pacific Northwest USA ? 'Chemistry

Department,

University

63

Laboratory,

of California,

Richland,

Berkeley,

CA

WA

94720,

99352,

USA

ABSTRACT Benzene was pyrolized a3 high temperatures (up to 19OO'C) in a Knudsen cell and at pressures from 10' torr to 5 torr. The products are examined using a modulated beam mass spectrometer. Concentration profiles are given for all important gas phase products as a function of temperature at fixed flow into the cell. The results suggest that at the higher pressures the pyrolysis process is bimolecular involving the reaction of an excited benzene molecule with benzene at low temperatures and goes over to a more direct process above 1200 C. At low pressures soot and hydrogen are the dominant products.

INTRODUCTION The high

temperature

been

actively

studied

that

has gone

into

has been made,

many

for many

these

there

and contradictory encountered

chemistry

still

results

remain

competing

pathways

intermediates

and heterogeneous

stood,

it difficult

In

spite

pounds

of these

is high

liquefaction,

of the

systems amount

often

of work

or total

to illustrate

through

are

has

deal of progress

short

lived

important

that obscurity,

the difficulties

the high temperatures

propagated

used hard

open

up

to detect

and poorly

under-

of aromatic

com-

experiments.

interest

in the pyrolysis

in the production

processes

large

of uncertainty

serves

effects

due to its role

in pyrolysis

areas

This

to compare

and organic

and the great

area where

difficulties,

today

inorganic In spite

many

abound.

in this

making

years'.

investigations

in working

complex

of both

and

of synthetic

in the formation

fuels,

in

of soot.

METHOD The apparatus

and methods

used

in this work

have been

described

elsewhere'.

RESULTS The primary rapid

result

formation

experiment_

of

the pyrolysis

of a carbonaceous

The rate

layer

of formation

OOZO-7331/83/0000-0000/$03.00

0

1983

of benzene

at high

in the cell during

of the

layer

increases

EIsevier ScientificPublishing

temperatures the course with

is the of an

increasing

Company

pres-

64 sure and temperature can be viewed At higher processes ment

and in all cases

as occurring

benzene

pressures

are also

carried

diameter

seen.

pressed

products

Although

as a function

suggest

percent

we cannot pyrolysis

C6H4 or C6H5 the benzyne

species molecule

1 [left)

examine

all experiments

from

in a cell

layer.

bimolecular

the data

C6H6

for an experi-

having

products

be important,

a 0.4 mm

or less

products

other

largest

initial

intermediate

It is poss7'ble that a radical

nor

of temperature

for C6H6.

as a function

it is 1ikeIy

Figure

reaction

with

is

for higher

the products

the higher

choice

molecular

short

or decomposes

lived

lived

are equally products

be remembered

is then whether

C12H12

is

weight

It must

The question

high mass

are the

The obvious

is C12H12_

a short

mechanisms

that

product.

2 shows

not be seen.

seen for these

of temperature

pyrolysis.

if the molecule

it will

signal

our results

the phenyl

as a function

by its absence

are consistent

possible

likely

is exsignal.

our results.

of a bimolecular

intermediate.

in a wall collision, high mass

profiles

of benzene

experiments

species,

bearing

pyrolysis.

seen and conspicuous

in these

carbon

g,as phase

conditions.

but neither

with

profiles

benzene

of 12OU'C

C12H12

bearing

is the most

could

of the decomposition

an excited

species

major

The concentration

carbon

our experimental

products

of the three

of temperature.

under

Concentration

weight

At temperatures

The

that

a carbonaceous

resulting

all of the intermediate

Concentration

2 (right)

molecular

profiles

are consistent

C4H2 and C2H2 during

whether

with

of 1 torr

of the total

identify

that an activated

in benzene

that

we will

pressure

fast

covered

of products

As an example,

the concentration

as the mole

result

a number

out at an effective

1 shows

gas phase

Figure

cells

orifice.

Figure

Figure

is sufficiently

in reaction

rapidly the

intermediate

and

likely. at this temperature

is

The This is a likely product from a rapid dehydrogenation of C12H12C12H10* other higher mass products probably come from a further decomposition of the C12H10

fragment

ued decomposition

through

loss of H atoms

of these

fragments.

or molecules

or acetylene

and a contin-

65 The most benzene

likely

molecule

alternative

is the

to give C,*H,O

reaction

to occur

and phenyl radicals are present 3 of benzene . We, however, see,no phenyl ments

done with

radical that

is easily

detected

is is not important

Another C,*H,O duced

alternative

directly, would

Further equal

at 0.3 torr

C,2H10

evidence

pressure

for the existence

the onset

Figure 3 (above) ducts of benzene

Experi-

believe

in our experiments. with

benzene excited

exchange

to give C.,2H,0 pro-

weiqht

distributions for various conditions.

benzene.

is at 98O'C.

suggests

species

done with

In an experiment

also corresponds

candidate.

pro-

CsHs

by experiments

in experiments

temperatures

is the best

of an activated

exchange

temperature

in onset

species

Lower molecular pyrolysis.

Figure 4 (right) Carbon benzene pyrolysis under

pyrolysis

that the phenyl

We therefore

the highly

provided

of isotopic

this

and to isotopic CgHg

was

and non-deuterated

conditions

This agreement

and an activated

show

a

is known

for us to detect.

formation

of deuterated

formation

mixtures.

enough

in our products.

of benzyne that

with

high pressure

conditions

of C,2H,0

be a reaction

radical

This reaction

quantities.

in the production

for the C,2H,0

experimental

radicals

the same

doubtful

atom.

in wall-less

in significant

would

be stable

amounts

under

but it is very

indirect

responsible

same

chlorobenzene

of a phenyl

plus a hydrogen

Under

done these

to the onset

done with a common

of

D2-benzene intermediate,

66 Figure

3 shows

temperature

its

the

than

lower

the onset

decomposition

reacts

At temperatures

begin

lene.

step

signals lene,

for this

times

show

polymers

greater

phase

the

ducts

as a function

occur

pressures early

By 16OO'C

iments.

and

torr

pyrolysis

mediate

to acety-

The

wall

step

are

was

results

typical

used

an

of soot

was

used

pro-

of the

at about

formation

lower

1000°C. accounts done

pressure

begins

pro-

at the

same

the benzene

resulting

of reaction

but

at

exper-

of gas phase

with

4

products

for experiments

and the onset

the onset

typical

soot

of the

The formation

Figure

gas phase

begins

and

for 80% of the carbon

is clearly

dominated

direct

collision

The

conversion is highly been

is the excitation

may

pressures of benzene

unlikely.

unable

systems

simplest

by heterogeneous

processes

and at the lowest

We have

activating

they

is soot.

Gas phase

formation

pyrolized

results

bimolecular

in other

signal.

the higher

that

results

times.

4b and 4c show

not affect

it is stable

our C12H,o

residence

with

and other

4a shows

soot

completely

studies

benzene

residence

7200°C.

is involved4.

although

low

being

in shorter results

in an

by 16OO*C.

of soot

are seen.

soot,

significant

accounts

does

and although

a single

in all these

aperture

different

suggesting

In (c) a 1.6 mm aperture

This

are not necessary

cesses

but with

the walls

Figure

and show

above

and by 16OO'C

The formation

diacety-

Our

.

between

Figures

pyrolized.

experiment

and

decomposition

with 5

observed

has been

until

times.

incomplete

of acetylene

dependent

the longest

In (b) a 1 mm exit

completely

upon

deposition

before

benzene

is delayed

residence

in

the decomposi-

as an intermediate4.

to be pressure

of temperature.

used

of 5x10-*

temperature

they

reactivity

distribution

for 80% of the carbon.

ducts

produced

signal.

involving

due to its rapid

pressure

ratios

product

at 800°C,

the

a pressure

at the same

of carbon

shows higher

at a higher

acetylene

the C8H6

processes

of C6H4

occur

and triacetylene.

source

carbon

that

to the production

are probably

the polyacetylene

The non-gas

suggest

direct

is the production

conducted

showing

excellent

products

to produce

more

to contribute

These

and

benzene

1200°C,

species

diacetylene,

Experiments

products.

of C,2H10

with

above

tion of benzene A likely

mass

and

explanation of a C6H6

processes

aid the formation used,

no bimolecular

to carbon Certainly

to detect benzyne

in 'our

of soot

and free

hydrogen

an activated

the phenyl

inter-

radical

is too reactive

of our results

pro-

to give

is that

the

species.

REFERENCES P. R. C. 43: S. 5. H. ::

S. 0. W. H. F.

Kirk, L. E. Chambers, and H. N. Woebcke, Adv. Chem. Smith, J. Phys. Chem. 83 (1979) 1553. Taylor, Con. J. Chem. 35 (1957) 739. Bauch and C. F. Aten, Chem. Phys. 39 (1963) 1253. Calcote, Comb. Flame 42 (1981) 215.

131

(1974)

237.