Prospects for the direct conversion of light alkanes to petrochemicalfeedstocks and liquid fuels - a review

Applied Catalysis, 32 (1987) l-22 Elsevier Science Publishers B.V., Amsterdam -Printed

PROSPECTS AND

FOR

LIOUID

THE

FUELS

DIRECT

CONVERSION

OF

in The Netherlands

LIGHT

ALKANES

TO PETROCHEMICAL

FEEDSTOCKS

- A REVIEW

M S SCURRELL National

Institute

Industrial

for

Research,

(Received

Chemical

P-0.

3 February

Box

1987,

Engineering 395,

Research,

Pretoria

accepted

0001,

1 April

Council

South

for

Scientific

and

Africa.

1987)

ABSTRACT Recent progress in effecting the catalytic conversion of small alkanes, especially methane, into higher hydrocarbons such as alkenes or aromatics is reviewed. Thermodynamic and kinetic considerations are included. The options for conversion include selective oxidation to either oxygenates, such as methanol and formaldehyde or hydrocarbons, particularly C products, selective chlorination, alkane-alkene coupling and aromatization. T&e degree to which each of these options promises a practical route to the desired products is discussed in turn. INTRODUCTION This and

review

other

direct

summarizes

light

aikanes

successful

conversion

progress

to chemicals

Strong

routes.

recent

emphasis of which

in the

and

liquid

is placed presents

on

catalytic fuels

the

special

conversion

by direct

reactions

or

of methane

reasonably

of methane,

challenges

to the

the

catalyst

designer. Over Several

the

nitrous

metal

oxide

oxides

presence

few

of

[23-341.

The

fundamental

[13-221

of

or

exploration

of

aspects

of the

dight

these

and

higher

processes

has

alkenes

by the

been

reactions

to aromatic

paralleled

use

over

in the

aromatization

of methane

of alkanes

area.

selective

hydrocarbons

light

[23];

addition

in this

to C 2 hydrocarbons

and

metals

made

including

other

alkanes

the

activation

been

of methane

transition

[24-331;

have

proposed, or

oxidation

coupling

zeolites

been

alcohois

selective

superacids

advances

have

to aldehydes,

[lo-1.21; the

by acid

significant

approaches

[l-9],

catalysed

the

years

different

of alkanes

oxidation of

past

quite

systems

by developments

by organometallic

systems

[35-381.

In

view

diversity timely.

of the of

increasing

reaction

Other

available

[39,40].

oxidation

of methane

0166-9834/87/$03.50

routes

reviews

interest and

covering

A review

catalysts some

dealing

to methanol

being

has

of

shown

in this

examined

the

so far,

aspects

specifically recently

field, such

contained with

appeared

0 1987 Elsevier Science Publishers B.V.

the

owing

a review

in this

direct

in this

and

to

the

seems

article

are

catalytic

journal

[41]

and

in

2 another of

survey,

methane

dealing

properties

including

considerations

of

with

was made by Gesser

desirable

the

mainly

present

of

C2+ alkanes

little

under

for

alkane or

the

[43]

is

given

to

alkene

temperatures

are

conversions

recently

to

to

discussed

of

of

catalytic

it

is

space

in

dehydrogenation

recognized

feedstocks

the

hydrocarbons,

considerations

the

although

conditions, routes

has oxidation

Owing

activation.

attractive

non-catalytic

selective

no attention

non-oxidative

offers

Kung

a 7. [42].

catalysts of

review,

dehydrogenation

et

free-radical-mediated

that

LPG

[44,45].

THERMODYNAMIC CONSIDERATIONS Figure of

1 emphasizes

methane

to

that

higher

co-reactants

(Thermodynamic

et

Conversion

a7 [46].)

can be achieved required such

for

at

the

fairly 2)

low

or

more

only

and/or

oxidation

or

a feasible

removing

are

too

high.

C461. Some are

with

Figure

and alkenes,

Many other

ASPECTS OF THE KINETICS Kinetics reaction of

rates

methane

obvious selective,

is

to

are

exceptions yet

of

of

the

that

formation both

to

such of

the

feasible

(Figure

(or

use of

very

prevent

selective total

chlorine

such

partial

for

hydrogen

with

a carbon-carbon

methane that

at

is

or

of

of

methane

2 and 3 illustrate

to

of

provided

thermodynamically

the

oxygen

reaction

coupling

from

oxidation

variables

formation

hydrocarbons

as dioxygen

Figures

to

are

increased.

ethylene

demands

Stull

mixtures

higher

is

by partial

but

route

from

become

be formed

cases.

ethylene

the

or

taken

temperatures

alkane

can

a co-reactant

methane,

are

as ethane

ethane

process

a feasible

review

conversions

concomitant

conversion

an

bond,

and acetylene,

reaction

reactions

or

temperatures can

be envisaged

below.

consideration

concerned. of

from the

direct

a co-reactant

OF METHANE CONVERSION

a vital

any

such

reaction,

4 shows

presents

discussed

in to

the

of

Much lower

As an alternative

hydrogen

be considered.

methane

of the

controlled

over-chlorination.

hydrocarbon,

use

fact

methane

strictly

selectively

the

ethane

indeed

unsaturated

not

over of

very

“C.

size

that

in

conversion is

catalysts

as the

for

and acetylene-ethylene

1600

alkanes

by the but

favoured

direct

this

alkane-to-benzene

imply

3),

needed

involvement

acetylene ca

higher

favourable

(Figure

chlorination)

can

of

the

throughout

to

above

general

temperatures

chlorine

the

in

used

methane

considerations

thermodynamically that

of

temperatures

and

thermodynamically

without

data

conversion

as benzene,

Thermodynamic

high

hydrocarbons

a variety total

sufficiently

in

alkane

Experience

has

of

products

oxidation active,

or

conversions, shown

normally free-radical

catalysts

for

that

particularly the

proceeds

selective slowly,

chlorination. various

as far

reactions

as

conversion with

the

The design of

methane

of has

3

GCH,-

6CH4 -C,H,t

3CQK

-20

FlGURE

TABLE

1

free

energies

of

reaction

of

the

comparison)

conversion

of

alkanes

for

required

for

AGY

= 0,

(‘Cl

CH4

selected

to aromatics

alkenes

Temperature

alkane

9H2

’

1

(for

5H,

-

Standard

Thermodynamics and

/

?OOK

500K

n - C,H,t

to benzene 1075

to alkene 1350

(C2H4)

C2H6

575

774

(C2H41

C3H8

450

655

(C3Hs)

n-C6H14

320

575

(Z-MP=)

conversions

of

methane

4

+200

r

5 E 3 -400 \

OL 43 a

T/K

IO00

600

600

400

CH,+O*-CC+

2H,O

-

-600

CH4+ 20, -co*+

2&O

-800

FIGURE

2

Standard

presented much The

attention

real

and

and

has

not

CHx radicals

[48] methyl

that

are

radicals

magnesium actually catalysts 77 K [S].

oxide

at

released via

to the

gives

the

into

the

oxides,

[48,49].

methane

the

and

[47].

It has

high

gas phase.

Methyl

by surface

of many

be

[SO]

radicals ions

per

and

are

the

phase

that

formation

these

can be produced

is a

oxides, can

adsorbed

in

such

can occur

temperatures

too

that metal

involved

materials

and that

at

se

H/D exchange it seems

that

that

of methane.

recalled

metals

metals

proposed

O-

possible

is

in nature.

of a hydrogen

temperatures

oxygen

activation

it should

on

and

of activation

insulator

been

absence

it

that

Carbanions

especially

in the

abstraction

Yet

surfaces

[47,48],

intermediates

metal

question

is reversible

deuterium

methane

However,

impression

problem. on

adsorption

sufficiently

hydrogen

years.

given

of

oxide

many

been

adsorb

such

over

from

for

involving

easily

important

magnesium

reactions

surmountable

presence

reactions

and

too

dissociatively

in the

similar

recently

a readily

furthermore

of

challenge

literature

will

proceed

energies

a formidable

latest

methane

free

as alumina of

on

radicals

are

on Mo/SiD2 as low as

5

+5Yl--_-7 WI,+

Cl,-

T/K

1000

800

600

400

C, H, + 2HCI

-2oQL

FIGURE 3

In

Standard

summary,

free

activation

particularly

when solid

the

fragments

adsorbed

overall

The progress will

SELECTIVE

reactions

may be less catalysts to

OXIDATION

methane

that

involved

made with

now be examined

The school

is

of

react

of

are

between

a problem

involved.

to yield

the

methane

than

is often

The real desired

and chlorine

suggested,

challenge

product

at

lies

in causing

a reasonable

rate.

years

both

energies

of

in the

approaches

that

have been

has considered

and ethane

formed

[1,6]

over

by decomposition

formation

of

the

in some detail Mo/SiO2 of

[2,5].

CH30H and HCHO is

in

partial

Further severely

oxidation inhibited

of

selective

catalysts.

oxidation

partial

by the

the

recent

oxidation

The general

products

ethane.

oxidation

oxidation

presence

methanol

from

of

products water

[2,5].

of

picture

N20 (when used as a co-reactant)

formaldehyde from methane, or ethylene and acetaldehyde 02ions present are considered to lead to non-selective CO2 and Ii20

investigated

in more detail.

USING NITROUS OXIDE

Lunsford

[2,5]

O- ions

various

are

and Any products: such as Methyl

CO,

6 radicals CH4/D2

on Mo"/SiO2

exchange

are highly

on Mo(CO),-Al203

the preparation

technique

to CH30H

appear

to be somewhat

CH3CH0

and C2H4 being obtained

methane

Combined

molecule

s-l site-l

products

other

number

with

Overall,

rates of CH30H Under

these conditions

have

Selectivities

of 70% selectivity

[8] who

obtained

studied

for

with

the kinetics

Of tht

were ca 8.5 x 10

the selectivity

of 2.6%.

of

N20 on

conversion

The results

and HCHO formation

than COx was 61% at a conversion

and

nature

for ethane

with

to 57% Es].

a figure

at 7% conversion.

at 863 K.

been demonstrated

C521. falls

by Khan and Somorjai

turnover

on the detailed

and up to 78% at 3% methane the selectivity

slow

that the activation

dependent

system

higher with ethane,

have been verified

system.

is greatly

This has certainly

used.

At 6% conversion

been claimed.

It appears

(by 02) over an MO-V-O

selectivities

Mo"/SiO2,

In contrast,

has been shown to be relatively

of CH4 on MO catalysts

oxydehydrogenation

even at 77 K C5].

of 373 - 473 K CSll.

even at temperatures reactivity

reactive

Elsewhere

for CH3D formation from CH4/D2 reaction over Mo(CO16-A1203 -4 at ca 6 x 10 molecule 5-l site-' at 673 K [51].

-4

to the turnover

was found to

be similar

In a slightly zeolites

undergo

of oxygenates

further

conversion

only attainable Selectivity

at very

the use of 02 rather

hydrocarbons decompose

Very high

well-known propene.

This encouraging

conversion.

produced

products

to products are

formation

Reports

of ethane

on the selectivity

for formaldehyde

that

[4] have not

for higher

in particular

to

result

similar

at conversions

dehydrodimerization

suggests

that several

oxydehydrogenation

(with 02) of ethane

[55,56]

promise

of ca 2 - 3%

[64], a

and aromatization other mixed-oxide merit

study

of systems

in CH4

for the selective

to ethylene,

and acetic

acid

is also

[52].

In conclusion capable

10 - 20% [4,9].

of CH4 with N20 over 9i203-SnD2

Mixed t4o-"oxides have shown

oxydehydrogenation

selectivities

then

(ca 1 - 2%) of CH4 [4,9].

(up to 92%) for formaldehyde

for the oxidative

of effecting

intermediate

of CH4, which

to form CO and H2 [53].

for the reaction

catalyst

the

over

that such selectivities

in the range

from the tendency

zeolites

involved

The highest

conversions

Restrictions

have been reacted

oxidation

it appears

leads to slight

[9].

selectivities

have been reported

adopted

partial

is typically

than N20

may arise

mixtures

to hydrocarbons.

low overall

by others

on ZSM-5

The strategy

30 - 4D%, and

to aromatics

been confirmed

CH4/N20

via selective

than COx are about

capable

approach,

of the ZSM-5 type.

formation

other

different

it appears

of permitting than H-ZSM-5,

in a 2 to 3 no1 exploitation. the formation

excess)

higher

that the metal-oxide-based selectivities

although

Of 'Ox cannot

alkanes

the use of N20 as a co-reactant

is not particularly

One additional

in converting

catalysts

factor

attractive

favouring

be avoided

to non-COx (usually

present

for commercial

the use of oxides

completely,

are potentially

it is probably

is that,

even

if

more acceptable

7

to produce useful

CO

rather

than

C-containing

[2,5,43]

give

SELECTIVE Much

CO

rather

OXIDATION of

oxidations

presents

a

method

than

USING

recent

has

concentrated

using on

extent

severe

e.g.

commercial

sought

from

methane,

CI

units

such

as C2H6

and

Cl

species

reaction

was

selectivity shown

has

much

of a number

which

of

products,

of

and

to the

state. during

Tl,

Cd and

Sb

for

conversions

exhibited

1000

[12]

the

active.

deactivate

been

have

to effect

the Two

main per

alkanes

major

HCHO

he acknowledges

of

classes

on

the

such of

one

to

conclusion pass

by this

extension

Conversion

His

higher

higher

to butadiene

and

conversions

selective

to

of

although

other.

has

a higher claimed

of

forming

of

Bhasin

were

was

with

that

some

highest

bulk

that

a

sufficient

recent

work

has

the

oxide

thereby

the

and

The

part

in to

the

cycle,

metal

is released

place

one

this

of the

oxides

compounds

taking

for

operation

oxidize

created

to C2

reaction

using

in an attempt

airflow

cycle.

by

each

oxides

switched

the

CH4

a cyclic

of the

of

from groups

supported used

During

selectivities

lO%,

26 who

of oxygen part

units

several

periodically

the

"sink"

by

[lo]

products.

CH4-flow

4 and

such

considered

into

The

made

[SS].

been

appears

[12].

rate

untreated

in the of

203

oxygen

in a

of Mn,

Pb,

(ca 40 - 50%) at

Other

of

work

[SS], LiCl

in this

most

to yield

reaction

[58]

and

72%

with

selective

catalysts

are

for

materials

to

on Sm203

shows

these that

selectivities

an

results

as

Baseline

indicated

high

as

a C2

attributed data

for

not

material 93%

in a higher

improvement

to COx.

this

C2

in the ca

tendency

and

paper

the

PbO/A1203

to Sm203

[60],

oxidation

simultaneously

to steer

with the

is the

product

total Sm

ca 53%

However,

PbO/Si02

addition

and

to be able

of

drawback

than The

methane

reported,

rapidly

fraction in the

over

PbO

feed

Selectivities

activity

ethene/ethane

to

A further

fairly

been

oxidation

the

although

and

air

the

the

Supported

PbO/Si02

depression

[9].

'C.

have

direction.

most

dominates

as opposed

as CH30H

[41].

viable

desired

of between

hydrocarbons. desired

on

screening

to diffuse

fashion

Attempts

The

methane

selectivity

-

N20

of butane

[57],

such

C2H4

been

by Keller

oxidation

500

to

catalysts

CO2

problematical.)

units

problem

has

approaches.

controlled

ca

the

coupling

is considered

a higher

than

of methane

conversion

by Foster

be developed,

described

supplies

improve

reviewed

yet

C2

was

oxygen

the

oxydehydrogenation

is more

commercially

oxidative

reaction

ethane

giving to

converted

promise.

Concerning so-called

and

recently

system

H-ZSM-5

operation

to propane

C2

easily

molybdenum-based

rather

behaviour

product

and

is more

that

with

selective

an approach

hand,

former

dioxygen

the

the

challenge,

is an established

are

the

is clear whereas

CO2,

effort

large

less

It

DIOXYGEN

the

(To a

alkanes.

since

CO2,

compounds.

to a partial

selectivity

of

have

tc

8 ca 413% at

313% CH4 conversion

than

of

that

93%

Oxidative

with

oxygen

c2 product

A 123 selectivity

containing

reaction

at

ratio

from

tendency operating Li/MgO The Li/MgO

of

for

catalyst

of

formation

of

surface-generated

catalysts

and

methane

treated

conversions

of MgO, oxides

Furthermore, production net

clearly the

over

surface

of

and

the

oxides

has

in the Both

Like

the

Li/MgO,

conversion

reactions

Other LaA103

La203

of

are

products

been

simulated

(a total

initiation

presence

methanol

of

and

thought

of of

small

has

been

to

by

the

lanthanum

catalysts

appear

prepared

by a mist

decomposition

selective Li/MgO,

for Sm203

Selective in a coupled converted

C formation 2 and La203. oxidation manner.

to methanol

has

but

selectivity

of

who

studied

in

the

alkali-metal-

formation. favours

total

C2

of methyl

rate

these

isolation

radicals

The

COx

ethene

selectivity. radicals

constants

for

by dioxygen

at the

the

various

oxidation

products.

total

oxidation.

than

La203

itself:

is reported

[66]

to be

using

the

than

two separate

demonstrated

that

liquid

for

radical

to non-COx

to catalyse

active

quartz

catalyst

gas-phase

selectivity

useful

over

has been discussed

selective

and again

is more

in

peroxide

as partial

oxide

more

been attempted

a 92%

matrix

streams

and

hydrogen

technique

also

with

over

[63]

oxidation

overall the

has

methane EPR

in

of the

while.

NalCaO.

expense

[65],

to be even

[67]

behaviour

and

be a reasonably

methane,

Koenig

increase

of gas-phase

oxygen

detected

from

Thus

from

an

worth

Kolts

known

of

The

the

using

the

observed

with

seems

be

to

involved.

C2 hydrocarbons

of

of

can

leads

generally

formation

using

14)

were

tendency

the

which

increase

and

O- ions

These

ions,

the

plus

methane

to be involved.

[SO].

radicals

the

quantities

shown to

radicals

conversion

with

formaldehyde

methane

is controlled

overall

starts

reactions

The surface-assisted samples

improves that

-

CH4:02

a study

at

of

to the

Li/CaO

to methane

deal

of CH3

involvement

production

steam

a good

formation

certainly

KIMgO,

show

38% has been reported -1 was used, with

demonstrated

the

C611.

zirconia

sensitive

to

by Kimble

NalMgO, C2

of

sequence

radical

for

is provided

favour

ethane

been

a catalyst,

by

of

gas-phase

support

Li/MgO,

addition

reaction

gas-phase

CaO,

light

[ll]

of

Li '

and

[41,43],

has

as

were

O- ions 02

pressures

oxides

Further

[62].

to C2

behaviour

other

lower

mol mol

products

reactions

superatmospheric

ca 2:l

the

In

to non-COx

at

MgO

to operate

between

centres.

in CH4/02

deal

obtained.

selectivities

by surface

selectivity

pressure

of

is thought

catalysts

spectroscopy

C641.

and

been

a conversion

ratio

interaction

[Li+O-]

the

is a good

use of Ag/Bi203

by the

lithium-doped

ca 50% at

H abstraction

as a result

establishment

The

The

of

in the

Conversions

973 K.

used.

use

of

CH4 and 03

G-l4 through

formed

in the

that

pumped via yttria-stabilized ca 47% have

up to

figure

1691.

been achieved

electrochemically

developments

promise.

no1

being

earlier

has also

selectivities

Recent

feed

reported

coupling

a selectivity

[60],

PbO/A1203,

reaction

methane

phase

equally

can

using

as

systems be one

9 reaction and

system

a Pd-Ag

Fell'

stoichiometric

oxidation

alloy-on-graphite and

to Fe**

reoxidation

catalyst.

has

be

to

with

of CH4

as the

The

effected

Fe(S04)3

as

process

the

leads

in a separate

oxidant

to

reduction

reactor

of

Using

air. A series

of

illustrates of various

[68],

albeit

Sb203

[69],

claimed

use

Ge02

[70],

and

at

C2+

hydrocarbons

to

between

selectivity can

be

to

as high

required. The of

The

on

in the

products.

approach

over

titanate

In some

cases

methane

does

ethane

not

over

fall

selectivities

for

having

Help

ethene

of

an

in attempting

or alloys

metals

produced

The known.

CH4

with

VIA

ready

use

that

in the of

has

were of the

the

found,

are

The

been

with

on

course

Bhasin

[lo].

dehydrogenation

Alkanes The

Mn-P-O are

conversion

oxidative

conversions

thermodynamic

of

dehydrogenation

[78]. in the

to drive The

limitations

methods the

may

of

Optimum range

light

be provided

conversion

hydride-forming

dehydrogenation

37 - 64%

catalyst.

by

metals

alkane

by using "storing" would

---->

of

methane

CH3Cl

to chloromethanes

to monochloromethane

+

HCl

(1)

using

I

dichlorine

is achieved

the

be used

CHLOROMETHANES

conversion

in

selectivity

the

described

stream

increase

sodium-promoted

number.

claims.

products

ca 0.5.

able

[79].

and

been

are time

and

for

oxidatively.

recently

the

is of

by Keller

[76]

and

C2+

bed

patents

carbon

of oxidative

as a hydride

conversion

Cl2

the

a conventional

If selective

+

>90%

Li-Ti-0

is effected

scope

to overcome

selected

CONVERSION

the

Sri:::ratio

without

combination

employed granted

no change

within

dehydrogenation

dihydrogen

that been

[75],

the

metal

at atmospheric

increasing

of the

be

Sri02

all

is a concomitant

basis,

to

'C,

as

have

beds

can

reducible

Conversions

with

There

oxidation

and

out

- 1000 COx.

decline bed.

such

[73]

carried

500 and

has

Sn203 -P 2 0 6 catalysts

for

catalysts

with

and

oxide

dehydrogenation

alkenes

ca

to C7)

periodic

hydrocarbons

into

of

are

C74]

elements

fixed

oxidation

Oxides

Mn304

Mg)

On a cumulative

but

is similar

zinc

up

the

Company

converted

range

most,

of

[72],

Reactions

(typically

70 - 80%,

In203

Using

selective

stream.

(in particular

10% at the

non-COx

Phillips

periods

regard.

reduction

as

short [71],

[68-741,

Company

reactions.

methane,

Petroleum

c771.

of

2 and

Richfield

stoichiometric flowing

PbO

temperatures

progressive

Atlantic

and

earth

in this

comprise

due

oxides

by alkaline

pressure

the

coupled

relatively

to be useful

usually

to

of

metal

for

promoted

granted

the

reducible

achieved,

oxides

patents

further

is well

as follows:

in

10 a route to methanol

can be envisaged

in which

a second

step would

involve

hydrolysis:

CH3Cl

+

H*O

Alternatively,

the second

monochloromethane

Limited

has received

production

metal [SO].

or other

zeolites metals

option,

than C12.

C833 which

comprises

crystalline steps

CH4

+

HCl

and conversion

including

or

halides

over

of methanol

to

such as W03/A1203

a Deacon-type

l/202

to hydrocarbons

those

[Sl] or of hydrogen,

catalyst

especially

----->

CH3C1

catalyst

based on cupric

an Ii-EM-5

+

may be combined,

in an oxychlorination

a dual-function

in the use of this catalyst

+

cations,

can be introduced

For example,

aluminosilicate,

involved

but no trichloro-

of the methyl

can be converted

over catalysts

various

or

such as Pd/BaSO4.

led to mixtures

halides

such as

1821.

the chlorine

HCl rather

catalysts

the

and Nafion-H/TaFS,

catalysts

hydrolysis

methyl

by passage

containing

The steps of chlorination a further

of the

acid catalysts

has been observed,

Catalytic

approach,

lower alkenes

copper

conversion

SbFS/graphite,

oxide/hydroxide

different

supported

or palladium

Pt/A1203,

are formed.

and dimethylether

In a slightly

over

of dichloromethane

Y-A1203-supported

siliceous

the direct

Olah et al. [SO] have described

attention.

ZrOF3/A1203,

including

tetrachloro-methanes

highly

(21

involve

of methane

TaOF3/AT203,

catalysts

oredominantly

HCl

step could

monochlorination

FeOxCly/A1203, platinum

+

to C2+ hydrocarbons.

This approach se?ective

CH3OH

->

and as

step using

has been described chloride

preparation.

and a

The essential

are then:

H20

(3)

and:

CH3Cl

-->

hydrocarbons

The use of oxychlorination, step

(4) to be recycled

further

step to produce

+

rather

HCl

than chlorination,

and used directly C12.

the reolite

in the corrosive

application

of this approach.

(41

in step

It is not yet clear HCl + l-l20environment

enables

(3) without to what would

the HCl produced first

extent

requiring

deterioration

limit the practical

in a of

11

ALKANE-ALKENE COUPLING REACTIONS Figure

4 illustrates

acetylene

(equation

the 5)

thermodynamic

or alkenes

feasibility

(equations

of

6 and 7)

coupling

to yield

methane

propene

with

or an alkane

respectively.

CHXH

+

CH2=CH2

+

+

In the

case

at

the

CH3CH2CH3

methane-alkene is

2 summarizes

case

most,

500 K.

(6)

(7)

coupling,

increased

an equimolar

In the

ca 700 K at

(5)

‘n+lH2n+4

(7)

for

pressure.

>

-

-->

of or

Table

equilibrium

lower

CH4

(6)

pressure.

CH3CH=CH2

CH4

‘nH2n

reactions

at

CH4 ->

of

the

ethylene,

whereas

extent

for

of

mixture

reaction higher

(equation

conversion

in temperature

maximum percentage

CH4-alkene

For acetylene

the

by a decrease

of

alkane

obtained

as a function

should

be sought

alkenes

the

l),

temperature

the

allowed

in

and an increase at

of

temperature

at

temperatures

temperature

limit

limit

in

is

and below

ca 200 K

is much higher

ca 1000 K.

TABLE 2 Maximum molX alkane (initial equations

formed = 1:l)

mol ratio

at

equilibrium

for

as a function

of

reaction

temperature

of

CH4 and alkenes

and pressure

[after

6 and 73

mol% alkane

T=400

K

at

T=600

equilibrium

K

T= 800 K

0.1

0.5

2.0

0.1

0.5

2.0

0.1

0.5

2.0

MPa

MPa

MPa

MPa

MPa

MPa

MPa

MPa

MPa

100

100

loo

36

63

79

negl.

negl.

ca 0.1

54

77

87

1

ca 1

ca 3

negl.

negl.

negl.

ethylene propylene or

higher

alkene

Alkane-alkene catalyst, complexes.

namely

coupling superacids,

The work

reactions

have

sometimes

on superacids

is

been

referred

investigated to

as magic

much more extensive.

using

two main

acids, It

is

types

and metal stressed

that

of

12 these coupling

reactions

known alkylation 2-methylpropane

with

task of coupling

is required

superacids

superacid

be converted C2H6-C2H4

and Mayer

to propane

mixtures

later

However,

of light alkenes

yielded

methane

ca 1:3 to 1:50.

[85].

with

40 - 60% selectivity Reactions

since the

in a catalytic mixtures

could

Similarly

carried

out

is typically

at low levels of

large catalyst:reactant

use of such superacids

demands

that care

several

[20]: to react with

ratio

light

have a potential

using TaFS-HF.

The successful

for the alkene

is non-catalytic

were typically

these

the tendency

alkenes. reactions

are higher

that CH4-C2H4

whether

fal

with

(oligocondensationl

light alkanes

uncertain

points

and ethane

the same superacids

It is then clear that comparatively

and it remains

latter reactions

The products

The CH4:catalyst at ca 40 'C at a pressure of ca 1 MPa. -1 ca 1:l mol mol , and the alkene:CH4 ratio is maintained

are employed

The

(as yet) much more

hydrocarbons

[19] demonstrated

n-butane.

of

of the latter coupling

are low and the process

with about

of the well-

in this section.

or SbF5-FSO3H

at Exxon

[84].

on the

treatment

into higher

depleted.

the coupling

or butene

light alkanes,

directly

but yields

is reductively

Siskin

manner.

evident

example

by the alkylation

concentrates

a separate

such as SbF5-HF

and alkanes,

use to effect

which

the very

can be converted

as another

are exemplified

such as ethyleue

viewpoint,

as will become

Methane

alkenes

alkenes

in this review

From a mechanistic

using

which

reactions,

are not discussed difficult

should not be regarded

systems

ratios

are truly catalytic.

is exercised

regarding

itself by oligomerization

must be

countered; (bl

the alkylation

product

before

transfer

hydride

reduction (cl

hydride

Consideration

occurs;

transfer

of these

must be rapidly

removed

to the reactive

cation

from the reaction takes place,

medium

otherwise

alkene

and from the reactant

requirements

alkane

in turn

should

be relatively

leads to the respective

difficult.

conclusions

that:

ia)

Large alkane:alkene

(b)

a continuous

(cl

the most suitable

should

be employed;

The use of solid investigated m01-')

product

were also methane

superacids

reacted

a fixed bed of catalyst

in fixed-bed

TaF5 supported

flow reactors

on a fluoridated

has been claimed

(Table 3).

Methane-acetylene

catalyst.

reactions.

There Tracer

is no direct experiments

has been (ca 9:l mol

alumina,

to ethylene)

hydrocarbons

in these

are CH4 and C2H6.

At 70 'C, with a feed of CH4 and C2H4

over a similar

participates

for example,

for use as reactants

(with respect

of C2-C5

be used,

with,

as catalysts

comprising

per pass

consists

should

and

alkanes

by Olah [13,15].

over a catalyst

38% conversion

ratios

flow arrangement

[13].

mixtures

evidence

using

a The

that

[13C]_CH4

[I61

13

+80

+60

3

+20

0‘L 0 a

0 T/K

1000

-20

-40

FIGURE

4

Standard

hydrocarbons. those

show

for

methane

that

take

with

such

it is clear place.

The

that

rather of

class

sulphate-treated

the

distribution

coupling

propane

than

occurred.

(Table

not

and

methane

propene

and

unsaturated

is almost

coincidental

with

This

and

to that

found

A particular

to may

VC2H8' under

of

both

therefore

to exhibit solid

coupling

but

of

conducted

but conditions

methane

have

been

and

in

ethylene

produced

in a

manner.

methane

3),

formation been

conversion

referred

claimed

[867.

to the always

catalytic

catalytic

catalysts

is similar

catalysts

deactivation.

sustained

both

between

methane

leads

have

methane-ethylene of

reactions

for

['%I-C,H,

zirconia

consumption

alumina

of

line

alkenes.

labelled

Another

about

the

experiments

stoichiometric

bringing

energies

that

+ n-C*+

reaction

unfortunately which

free

Note

has

but

ethylene with

no definite difficulty

superacidity been

with has

the

was

investigated

mixed been

use

proof the

is exemplified

of

success

presented. the

could

by

as a catalyst [87]. The

Evidence

for for

product

TaFS-fluoridated be obtained

rather

rapid

that

catalyst

catalytic

15

In general,

the

essential

steps

The

mechanism

of aromatization

involve

dehydrogenation

dehydrocyclooligomerization, side

as

with

and

depicted

in Figure

oligomerization,

hydrogenolysis

cracking,

and

5 may

apply.

including hydrogen

transfer

as

reactions.

Small

small

alkanes,

products

are

aromatics,

alkenes

or

hydrogen

naphthenes

and

can

(as side

be used

products)

and

the

light

potential

alkanes

and

light

alkenes.

[alkene

oligomers

1

I l ---

riaromatics

FIGURE

5

Three

Simplified

scheme

of catalytic

types

for

1

production

conversion

dehydrogenation/aromatization

(al

of aromatics

have using

attracted

from

most

catalysts

based

a

light

alkane

attention: on

platinum

or

chromia-alumina; acid-catalysed

(bl

234-5

conversions

(cl

(a) and Catalysts which

aromatization,

achieved

(b),

under

operate

(a)

via

include

the

steps

steps

on

occurring

reforming

by medium-pore

be

hexane.

alkanes,

since

they

the

zeolites

of

the

traditionally

with (usually

to the

in carbon Such

are

the

used

catalyst

platinum

production

are

The not

inactive

model,

platinum

chlorided

number.

catalysts

relatively

the

bifunctional

support geared

of

types

given

in

catalysts.

described

associated

the

no change

normally

combinations

composite

widely

is typically with

using

including

dehydrogenation

alkanes

especially

type‘; and

are

coupled

alumina) of

from

reactant

able

oligomerization

convert

practice

of

straight-chain

would

readily

to

isomerization

The

for

the

catalysts hydrogenation-

with

[go].

aromatics

lightest

reforming in which

therefore lighter

step

which

is

16 necessarily

of

be aromatized The

They

involved.

dehydrogenation

light

using

conversion

acid

of

several

formed

as

intermediates

The and

aromatic

formation since

steps

The

attention

H-ZSM-5,

is that which

it has

noted

the by

key

the

[33]

that

selectivity

during

product

step

involves

at

may

be

500

way

This "C.

The

places

achieved.

or

the

can

Figure

be

the

shown

jointly

seem

zinc

hydrogen-

requires needing

an

higher even

initial

reactions

already

can

6 then in the

by UOP

and/or

gallium

used

for

BP,

as

[95,96].

samples

containing

conversion

about

18%

by using

being

and

some

[333.

a means

of

based

the

alone.

gallium.

H atoms

at

Deactivation

lost

Process

propane of

gallium

occurs

52 and

modified

Cyclar

use

the

are

converting on

lost

if the

and

of the The

containing

volatilization

zinc

of

BTX

alkane

known

zinc

ion

in propane

the

been

H atoms

maximum

considerably

Catalysts

has

implication

conversion

to be

reaction

46%

if

is a

formed,

conversion of

on the

dihydrogen

zinc

in the

is only

applies

as

the

butane

best

is used

by

situation and

redistribution

for

there

by a carbenium

found

that

over

alkanes

"gained"

[93],

specia'

the

'Cl,

and

A similar

figure

the

of

(ca 400

are

is about

longer

since

high

restriction

6 shows

needs

of propene

abstraction

It appears

C273

too

H-ZSM-5

improved

product

and

dihydrogen.

preferred

be

no

aromatization

of aromatics

figures

elements,

and

to be

temperatures

other

appear

to aromatics

incorporating

BTX

alkenes,

and

selectivity

is also

corresponding

selectivity

in Figure

aromatization developed

H-ZSM-5

(BTXI. light

so that

overall

over

basis)

the

situation

'C over

alkanes

aromatization

"internal"

corresponding

reaction

then

or

alkanes

of

during

is not

a severe

(XC efficiency

containing

alkane

hydride

but

Fortunately

370

and

light

sequence

result

The

zinc

can

been

such

xylene

smaller

as alkanes.

is methane,

Plus

has

as

cracking

aromatization

to aromatics

is ethane.

catalysts

about

intermediates

that

side-product

C963,

type

methanol

and

the

of

proportions

side-product

butane

be fed

alkanes

with

in the

the

"lost"

in the

in this

ZSM-5

light

on the

that

to an alkane.

ZSM-5

27%.

of

same

bearing

between

aromatization

selectivity

during

can

at

'C, with

the

from

[95]

conversion

maximum

The

which

(bl.

ZSM-5

toluene

aromatization

at a temperature

appear

aromatization

BTX

are

lost

of H atoms

and

over

significant

- 475

450

a direct

been

is converted

propane

under

the

including

benzene,

production

direct

hydrogen

methanol

the

alkenes

catalytic

[9I],

alkenes.

number

for

The

about

relationship

intermediates found

of

simultaneously

is followed

particularly

the

the

alkene

mentioned

oxygenates,

becomes

place

Alkenes

the

It has

quantitative since

the

of

since

reaction.

type

zeolites The

chiefly

by the

above

step

for

fate

various

are

[33,94].

temperatures.

described

the

over [9Z].

alkenes

takes

e.g.

dehydrogenation

on

of

is accompanied

temperatures, higher

from

products

aromatization

transfer

of

alkenes

corresponding

[93].

aromatization

the

to the

investigations

of

be used to effect

however,

catalysts

light

object

dimethylether

can,

alkanes

and

zeolites or gallium

the

of

17

Zn-ZSM-5

catalysts

laboratory, coking

used

to aromatize

may in part

is also

likely

be due to to

propane loss

of

[33,97]

zinc

or butane

[27],

[97],

although

as seen

deactivation

in the

by

be important.

-.-.-

90

o XYLENE

--

80

--

o TOLUENE

-

70 t 60

or EtBENZENE

0 BENZENE

50 40 30 20

a

IO 0 I

I

I

C2”6

C”4

NATURE

FIGURE 6 the

Aromatization

nature

of

the

aromatization

as the

alkane,

of

the

a decreasing

distribution

than

There

to

the

[27,33,97]

(Table

4).

of

the

hydrothermal that

steam

conversion alkanes,

measure

for

the

does not

Some attention part

the

The method of

impregnation)

of

synthesis

of of

summary of the

the

of

because

of

of

the

alkanes

the

zinc

have a critical

source

crystalline

such silicates

of

zinc silicate

may effect

aromatics

increases H M2 forming

such as pentane

data

H-ZSM-5,

or gallium effect

to a third of

sets over

so named

exemplified

in H-form.

and butane.

between

(983,

internal

ZSM-5 catalyst

propane

hydrocarbons

reforming

of medium-sized

of

during

light

I’M2 forming”

conventional

propane

lost

of

reactions.

ethane,

been given

the

H atoms

aromatization

feed

with

agreement

the

all

The selectivity

the

introducing

where

that

BTX as a function

for

hydride-transfer

the

from

of

conversion

seem to

has already framework,

treatment

content encountered

smaller

is a broad

groups

to

conversions.

hydrogen

restriction

is more suited rather

via

H-ZSM-5

process

by platinum/alumina-catalysed with

ensuring

by Chen and Yan in their

differentiate

PRODUCT

Maximum selectivity

side-product

application

has been described to

propane.

side-product

appear

The potential

in order

of

OF SIDE

(ion

on the

method of

and hexane

obtained

by different

Zn-ZSM-5

and Ga-ZSM-5

exchange

catalytic

or behaviour.

introduction,

or gallium

is present

[99].

It

removal

of,

namely

has been claimed for

as

during

example,

[99]

gallium

18 and would

from the framework, aromatics

lead to increased

selectivity

for the production

of

from alkanes.

TABLE 4 Conversion

of propane

Si:Al

Catalyst

over various

ZSM-5

catalysts

Temperature

ratio

BTX selectivity

'C

Reference

% C

H-ZSM-5

35:l

500

31

33

H-ZSM-5

19:l

500

10

97

Zn-ZSM-Sa

35:l

500

63

33

Zn-ZSM-Sb

19:l

500

46

97

Ga-ZSH-SC

19:l

500

40

97

ZIPZSM-Sd

33:l

510

43

27

Ga-ZSM-5e

33:l

537.8

48

27

a b

Zn introduced

by ion exchange

using

excess

Zn

2+

Zn introduced

by impregnation,

Zn content

= 1 wt%

' d

Ga introduced

by impregnation,

Ga content

= 1 wt%

Zn introduced

by impregnation,

Zn content

= 3 wt%

e

Ga introduced

by impregnation,

Ga content

= 0.5 wt%

The

introduction

of zinc or gallium The effect

in the BTX selectivity. impregnated 1971.

material

It appears

the increased

Attempts

whereas

on Zn-ZSM-5

Oligomerization/aromatization alkene

than the hydride

The behaviour the alkane conversions

used [97). of propane,

BTX selectivity feature

during

ratio

higher

Al content.

to have a higher

step, which

results

of small

and isobutane

increasing reactions

samples

the overall

have failed is It has

is rate[33].

order

dependence

in increased

on

ETX selectivity. depends

catalyst,

on

the

are 14, 31 and 56% respectively. (971, which

is a general

[33, 98, lOO]. to aromatics

used for the conversion

activity,

of

takes place.

step

(C3/C4) alkanes

conversion

ClOO] that the selectivity

in Ga-ZSM-5

ratio does affect

dehydrogenation

At an HHSV of 1.0 and with a Zn-ZSM-5

with

loadings

in the BTX selectivity

is in quasi-equilibrium

the aromatization

of these aromatization

the Si:Al

the step

n-butane

leads to an increase

of the feed alkane

the primary

steps appear

transfer

increases

It has been claimed

for the increase

dehydrogenation

that over H-ZSM-5

sample

with higher

to use indium for this purpose

that the main reason

rate at which

been suggested controlled,

[97].

to a base ZSM-5

is more pronounced

with higher

is

not affected

of pentane,

rates being

by

but this

associated

with

19

There not

is

general

result

judged

from

conversion

[lOOJ.

abilities

Pt or

of

and Okazumi

converting product

form

et

a7.

on reactivity

have

but

addition

of

the

that

Pt-ZSM-5

as a result

of

to an increase

in the

addition

C5+ selectivity

on the entirely

a large

decomposition

[102].

of

for

of

reactivity

part the

ZSM-5.

H-ZSM-5

of

were

n-butane

effect

in

the

intermediate reported

a ZSM-5 catalyst

of the

of

than

Pt-ZSM-5 to

For propane,

activity

properties

for

does

has been

the dehydrogenation

was more active

chromia-alumina

H-ZSM-5

This

in cracking

combine

results

to

ammonia [33],

constituted of

C5+ selectivity

was almost

of

gallium

catalyst.

aromatization

Similar

platinum.

or

the

and activity

with

The addition

zinc

of

adsorption C97]

CI and C2 products

leads

of

acidity

been made to

demonstrated

on the

chromia-alumina

heat

chromia-alumina

[loll.

in the

the

apparently

a composite

increase

the

in the

hydrocarbons

Attempts

propane,

oligomers

Engelen

to

[30]

spectrum,

alkene

of

methanol

that

decrease

measurements of

hydrocarbons

Inui

agreement

in any marked

by

to

with

an

of

was even greater,

but

the

effect

absent.

CONCLUSIONS The

literature

approaches systems the

to

discussed tackling

in which

conversion

the

of

promising,

although

contender

if

useful

be of

selective are

that

a plentiful

to

ethane,

this

alkane.

industrial

are

but

the

of

are

catalytic

enormous

ethene

With

such unsaturated

remain

methane,

the

on the it

as far with

feed

is

seems

use of use of

a dioxygen,

that

the

use of

and highly

hydrocarbons

can be discovered, required

in order

methane, remains.

the

many recent

challenge

of

attempts designing

have

shown promise

suitable

selective

but,

particularly

and active

catalysts

may be but

to

approach. In summary,

a

Conversions

as activity

to unsaturated catalysts

to

being

a strong

appears

be designed.

associated

coupling

and selective

limited

is already

will

based

more attractive

problems

Reaction

essentially

significance, to

different

alkanes.

oxydehydrogenation

to

routes

has yet

Methane

very

small

and aromatization

cracking

catalyst

of

a catalyst

steam

oxidative

more active source

fed

alkanes,

Although

remain.

several

is

For

chlorination

concerned,

atmospheres

contains conversions

processing

and selective

provided

course, this

would

active

selectivity corrosive

process.

is mandatory.

successful,

involving

alone

review direct

and higher

conventional

as a means of

co-reactant

suitably

alkane

propane

used as a commercial

in this

reasonably

for

of

exploit

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