Catalytic cracking of benzene on iron oxide-silica: catalyst activity and reaction mechanism

Applied Catrrlysis, 16 (1985) 103-121 Elsevier Science Publishers B.V., Amsterdam -Printed

CATALYTIC

CRACKING

OF BENZENE

103

in The Netherlands

ON IRON OXIDE-SILICA:

CATALYST

ACTIVITY

AND REACTION

MECHANISM

Satish

S. TAMHANKAR*,

Department

Katsumi

of Chemical

U.S.A. * To whom correspondence 100 Mountain

(Received

Avenue,

TSUCHIYA

Engineering,

should Murray

24 September

and James

8. RIGGS

West Virginia

be addressed

at The BOC Group

Hill, NJ 07974,

1984, accepted

University,

Morgantown,

Technical

WV 26506,

Center,

U.S.A.

7 January

1985)

ABSTRACT Catalytic cracking of benzene was investigated on an iron oxide-silica material, originally developed as a sorbent for high temperature H2S removal. Experiments were conducted in a packed-bed microreactor at 500-650°C in the presence of hydrogen. The catalyst, in the reduced form, showed high activity toward benzene cracking and selectivity to methane formation. Hydrogen was found to play a critical role in the overall reaction and in suppressing catalyst deactivation. Based on the experimental observations a reaction mechanism has been postulated, which includes a mechanism for catalyst deactivation.

INTRODUCTION Coal,

in general,

hydrocarbons. a result

contains

of incomplete

ing benzene,

toluene,

be eliminated

equipment

it is advantageous

cracking

gasification, phenols,

must be destroyed

ed, they can be achieved Raising

fore be the preferred

product

cracking

gaseous

by raising

gas without

benzene

coals,

ino of these the benzene

with downstream

in many applications

cooling,

the undesirable

like ethane,

by

methane, controll-

or by using a catalyst.

the use of a catalyst

would

there-

way.

were monitored.

and toluene.

polyaromatics

includ-

This can be achieved

the temperature

is uneconomical;

as

need to

are known to be kinetically

In a recent study [I] hot gases from a fixed-bed different

Since

of

(400-7OO"C),

hydrocarbons

and can interfere

compounds,

reactions

of a variety

are hydrocarbons,

or corrosion,

at the gas temperature.

either

the gas temperature

comprised

low temperatures

etc., These

carcinogenic

to use the gasifier

Since hydrocarbon

matter,

among the products

by way of fouling

them to low-molecular-weight

hydrogen.

volatile

at relatively

polyaromatics,

since they are highly

gas processing

hydrocarbons

30-40%

When coal is gasified

The major

In independent

and phenols compounds ring. These

studies

in steps,

results

species

[2,3] thermal

have been investigated.

occurs

coal gasifier,

hydrocarbon

the slowest

are consistent

cracking

The results

with

step being

operated

identified

kinetics

indicate

of

that crack-

the breakdown

the calculated

with

were

of

ring destabiliz-

104 ation energies first

for various

step, benzene

the overall results

aromatic

was chosen

problem

of hydrocarbon

of this study would

Catalytic or diesel

cracking

range

products.

is well

Moreover,

extensive

since hydrogen

literature

compositions

ion catalysts.

For example,

extensively

A typical

hydrocracking

sites for hydrogenation.

[S] conducted

by hydrogenating

catalyst

the hydrocracking steel wool

Gasifier contaminant

which

exit temperatures,

needs

hydrogen

to be removed.

catalysts,

an interesting

possibility

to investigate

the feasibility

iron oxide-silica on the cracking fundamental

material reaction.

catalysis

related

sulfide

is not the focus.

to catalyst

deactivation.

Results

ed separately

in a following

activity,

of kinetic

cheap

et al.

catalysts

synthesis

to achieve

material

catalyst

for and

study wherein

effects

important

this at the gasifier

was developed

discussion of using

about

at the U.S. hydrocracking

this material

The objective

of catalytic

cracking

of typical

This paper

of benzene

operating

primarily reaction

is

on this

conditions in nature;

describes mechanism

and a reaction

for both,

of this study

is, thus, more exploratory

selectivity,

experiments

keep the acid sites

(H2S) as another

is presented

The work described

is

catalyst

[7]. McMichael,

relatively

of hydrocarbons,

and nature

sites

formed

the only relevant

sorbent

and to examine

(or coke)

of acid sites for cracking

ammonia

[9]. In light of the preceding

of H2S and the cracking

carbon

are reported.

In efforts

an iron oxide-silica

of Energy

results

is perhaps of benzene

gases contain

Department

the removal

This

has been studied

is a key to prevent

the metal

iron-based

is attributed

are known as hydrogenat-

to produce

consists

gases,

to the

silica-alumina

to cyclohexane

cracking

tests to identify

of hydrocracking

product

of benzene

catalyst

and found

to be very active.

kinetics

metals

the coke precursors

screening

of benzene

different

gaseous

product

According

and their activity

of benzene

In general,

unharmful

in the gasifier

reactions,

controlled

in the gasoline

In the present context,

of hydrogen.

transition

cracking

Hence,

as a Although

that the

selectively

to produce

species

catalysts,

Similarly,

hydrogenation

side reaction.

clean and active

overall

[4,5].

[6]. In this case,

deactivation.

cracking

known as cracking

acidity

it is hoped

industry.

completely

is a major

facts, study.

investigations.

to compounds

in the presence

on petroleum

are well

an undesired

is complex,

the hydrocarbons

needs to be studied

for the present

for future

known in the petroleum

to the surface

and metal

cracking

form a basis

[Z]. In view of these

compound

of heavy hydrocarbons

the aim is to disintegrate

the cracking

compounds

as a model

experimental and catalyst

rate model

are discuss-

paper.

EXPERIMENTAL A twin-reactor with other number

system

components.

of experiments.

for comparison. horizintally

developed

is a quartz

inside an electric

is depicted

in Figure

can be used alternately,

Also, the two, packed

Each reactor

mounted

for this study

The two reactors

with different

tube, furnace

1.0 cm I.D.

1 along

enabling

materials,

more

were

used

x 50 cm length,

and equipped

with a K-type thermo-

105

Pressure Manometer

I I

1

-

I Flowmeters

106 TABLE

1

Experimental

conditions

and catalyst

properties

Temperature:

500-650°C

Pressure:

atmospheric

Gas composition:

Benzene

O-300

ppm; H2 O-30 ~01%;

N2

balance Catalyst

particle

Catalyst

composition:

100-1000

size:

urn

45 wt% Fe203

+ 55 wt% SiO2

(physical

mixture) After

36.4 wt% Fe + 63.6 wt% SiO2

reduction:

Surface

4.97 m*/g

area:

Pore volume:

0.203 cc/g

Porosity:

0.43

couple moving Different

inside a quartz

gases from cylinders

flowmeters

into a mixing

of the reactors through

held

as the preheating

valve.

section.

sampling

ports located

with a thermal

specifically

conductivity

conditions,

Scott Environmental

gen from Alltech

gases

Associates,

concentration

in each experiment.

A gas mixture Matheson

used as inerts.

1. Table

research

a

ionization

In addition,

used to withdraw

samples

gas chromatograph

equipped

chosen

based on typical

2 lists conditions for benzene

used in

and methane

9.88 and 100 ppm benzene

pressure

allowing

containing

The catalyst,

0.96,

The actual

manometer,

from cylinder

were

another

through

Inc., and 100 and 1000 ppm methane

Co., was used as the source

1.71 ~01% H2S in nitrogen, were

comprising

Inc.,

by the mercury

with a flame

of hydrocarbons.

in this work were in Table

Technology,

was monitored

1.0 cm long, was

to a gas chromatograph

The FID was calibrated

analysis.

to reactors

near the entrance

(TCD).

used

and are given

calibration

bed, nominally

of the reactors

detector

conditions

the gas chromatographic standard

downstream

portion

either

flow is maintained

The lines leading

is equipped

for the analysis

nitrogen

calibrated

through

wool plugs at both ends.

are connected

of H2, 02, CO and CO2 using

The experimental gasifier

and then through

reactor

The catalyst using quartz

to the reactor.

can then be flowed

A purge

valve.

The gas chromatograph

(FID),

for the analysis

purifiers

and the empty

lines from reactors

detector

from

These

in place at the tube-center

gas-sampling

using

switching

(reduced)

0.5 (reduced)

concentric

until the start of an experiment.

and insulated.

The outlet

(original);

The gas mixture

5.4 m*/g

(original)

(0.2 cm I.D.)

pass through

chamber.

using a 4-way

the reactor

are heated served

thermowell

(original);

in the gas-sampling

calculation

543 ppm benzene

of exact

in nitrogen

of the test gas. The other purity

gases

CO, H2 and N2. Crushed

in the form of extrudates

in air

in nitrovalve

benzene

from used were:

sand particles

of a blend of iron

107 TABLE

2

Typical

GC analysis

conditions

Type of compounds analyzed

Hydrocarbons

H2' CD, CO29 02

Type of detector

HP5710A

HP5730A

Column

5% sP11200/

FID

TCD

CTR I - 8700

1.75% BentonTemperatures Detector Injector

port

Column

200°C

250°C.

Room temp.

Room temp.

80°C

60°C

20 ml/min

80 ml/min

Gas flow rates Carrier

gas (N2)

30 ml/min

Hz Air

240 ml/min

oxide

(Fe20g)

Technology particle

and silica

Center. sizes.

In a typical purge,

The composition experiment

pre-reduced

then kept mixture

(SiO2),

were

and properties

in some cases

benzene

vapor

to the main reactor

A decline

deactivation.

checked

again

RESULTS

AND DISCUSSION

Preliminary cracking

to ascertain

experiments

reactions.

a bed of nonporous a catalytic

stable

concentration,

experiment.

of reaction

as functions temperature,

products

were

conversion

conditions

carried

sand particles

study of the iron oxide-silica were examined

valve.

analysis

in benzene

Instead of an empty

cracking

into the other

to achieve

catalyst.

Thus,

pretreatment

was also investigated

is

overall

of time indicatis

the experiment.

thermal

time

its activity

as a function

was studied

identical

to that

with in

on a more detailed

and its deactivation

conditions,

and the presence

and catalytic

cracking

then concentrated

operating

gas

concentrat-

feed gas composition

thermal

residence

were

of the critical

1.

and is

gas flow

benzene

used to calculate

throughout

reactor,

the mixed

as a function

out to delineate

Efforts

in Table

The desired

reactor,

The outlet

At the end of each experiment

were

are given

to 500°C with a nitrogen

an experiment,

using the 4-way

by gas chromatographic

conversions.

ed catalyst

To begin

different

of 50 ~01% H2 in N2 for s I$ h, and

is introduced

by the gas chromatograph.

Energy

to obtain

of the catalyst

bed is heated

in a stream

switched

ions determined

and sieved

until the start of an experiment.

analyzed

benzene

from DOE's Morgantown

crushed

the catalyst

in a flow of nitrogen

containing

was obtained

The extrudates

such as hydrogen

of CO and H2S. The nature of temperature-and

hydro-

Reaction temp. Total flow rate Bed material wt. Particle size

0

= 65O'C 3 . = 200 cm /mln

STP

= 1.0 g = 398 urn

Sand

l Sand

I

I

120

140

16(

Time, min Benzene conversion

FIGURE 2

with and without

iron oxide-silica

catalyst

and hydro-

gen.

gen concentration.

Catalyst

The results

are discussed

below.

activity

Catalyst hydrogen. without

activity

At 650°C

hydrogen.

oxide-silica

was first evaluated

benzene

catalyst

converted

100% in the presence samples

revealed

(Fe203)

is reduced

observed

weight

changes

other

hand, with hydrogen

100 minutes.

slightly

the catalyst,

preliminary

results

developed

catalyst

should

deactivation

iron appears result

establish

as a high temperature

of

after

hematite

of hydrogen,

phase of hydrowith the

the activity On the

the test period

due to coke deposition,

From these

results,

to be catalytically

in a high activity

that the iron oxide-silica H2S sorbent,

50 minutes.

high throughout

the reduced

which

most active;

material,

of is

form of

from the beginning.

is also very active

and

of the catalyst

in the presence

about

and

2, the iron

of hydrogen

analyses

the original

In the absence

remained

of hydrogen.

viz. metallic

therefore,

in the absence

(Fe304), while

to decline

the activity

This suggests

in Figure

iron (Fe). This was also consistent

stages.

and started

by the presence

reduction,

phase

to metallic

at various

increased

suppressed

40% benzene

of hydrogen

to the magnetite

first

as depicted

Mtlssbauer spectroscopic

in the absence

reduced

and in the presence

over a sand bed was only % 2-3%, with

conditions,

about

of hydrogen.

that

gen it is further

conversion

Under identical

in the absence

preThese

originally as a hydrocarbon

109

P

0

80

Cat. wt. (g)

Treatment (GC)

60

Vin

'H2,in (vol %)

(cm3/min STP

500 pre-reduced

1.0

200

10.0

500

1.0

200

9.9

0.3

333

11.6

none

500 pre-reduced

40

20

I

0

I

I

I

I

a 120

100

80

60 Time, min

40

1

I

I

1

in

I

FOR ALL RUNS Initial benzene concentration = 230 ppm Catalyst particle size = 298 ppm

-

FIGURE

3

Benzene

oxide-silica

cracking

conversion

catalyst,

making

In view of the above, catalyst Figure reached

and methane

yield with

fresh

and pre-reduced

iron

catalyst.

samples,

it more

in the presence

3, with the pre-reduced almost

attractive

experiments

instantaneously,

as a possible

were conducted

of Hz, and the results

catalyst

benzene

dual-function

with fresh

conversion

while with the untreated

material.

and pre-reduced

compared.

As shown

in

level of Q 100% was catalyst

the same level

110

t Hydrogen flow stopped

d

= 398 urn

TP 0 v. in 'B,in

= 500°C = 200 cm3/min STP = 230 ppm

W

cat (g) 1.0

10.0

0

0.3

11.4

i

Effects

and methane

of the presence

and absence

the benzene

90

of hydrogen

on benzene

conversion

yield.

the pre-reduced

As a result,

I

I

60 Time, min

was reached after a time corresponding with

(vol %)

A

30

FIGURE 4

'H2,in

sample,

the methane conversion

hydrogen

yield

to the reduction

period.

In the experiment

flow was abruptly

stopped

at s 45 minutes.

was strongly

level remained

affected,

unaltered.

Thus,

but, as seen from Figure once the catalyst

3,

is reduced,

111

80

60

= 0.10 g

Wcat 3

40

T

= 5oo”c

0 v. in

= 333 cm3/min STP

-

= 230 ppm

'B,in

YH2’in

20 >

Oh

0

l

25

0

17

0

15

0

10

(vol

!

I

I

I

I

I

L

I

I

20

40

60

80

100

120

140

160

180

4

200

Time, min

FIGURE

about

5

Effect

of hydrogen

the same conversion

that the benzene

cracking

concentration

level

on benzene

is maintained

step is not directly

conversion

with or without influenced

and methane

hydrogen.

This

by the presence

yield.

implies

of hydro-

gen.

Nature of reaction According

products

to the mechanisms

postulated

[Z] for the therma 1 hydrocracking

of

112 IO{ I-

I

I

I

1.I-) -

Methane Yield

I’

/

f

I -

= 0.10 g

W

cat T

/

I

I

Conversion

= 500°C = 333 cm3/min STP

i

$n

I

= 230 ppm

3

NOTE : Average values of conversion and yield between 90 and 150 min interval are plotted.

/

I I

20

0 0

FIGURE 6

Benzene

20 30 Hydrogen Concentration,

10

conversion

and methane

yield

40 vol

as functions

50

%

of hydrogen

concentrat-

ion,

benzene,

the possible

study methane chromatographic confirmed species

products

are: ethane,

was the only gaseous analysis.

analysis

To account

increase

samples were regenerated analyzed balance

with dilute

by an elemental could be closed

analyzer.

within

From the resultsmhown ly responsible was suddenly conversion

product

gas stream.

balance

In this

using gas was

No other

it is postulated

surfa-ce. This was consistent

and the products

used catalyst

Based on the amounts

were quantitatively

of CO/CO2

produced

the mass

2%. 3 and 4 it is obvious

formation.

stopped the methane

level remained

ion mechanism.

mass

of the -catalyst. Moreover,

air,

in Figures

for the methane

and carbon.

be identified

of the product

on the catalyst

in weight

biphenyl

was the only gaseous

for the complete

that coke (CnHm) forms and deposits with the observed

methane, that could

The fact that methane

by a mass spectrometric

was detected.

product

yield

unaltered.

In the experiments

was used. When this was increased

Thus,

sharply

in experiments dropped

This provides

so far a fixed the methane

that hydrogen

an important

concentration

yield

where

to zero, while

clue

is direct-

hydrogen

flow

the benzene for the react-

of IO ~01% hydrogen

significantly

increased.

As

113

= 10

40

STP

%

vol

60 Time, min

FIGURE

7

Methane

depicted yields

yield

in Figures

as a function

5 and 6, with

were respectively

10, 15, 17 and 25 ~01% hydrogen

20, 47, 55 and 76%. The corresponding

conversions

were

represented

by the level of benzene

concentration; function

64, 89, 91 and 94%. Note that the initial

however,

as a result

of time is affected

particularly

significant

~01%; beyond contribute

The other As shown

to form methane

in Figure

parameter

is qualitatively

consistent

hydrogenation.

correspond

Effects

Thus,

affecting

to yields

level as a

from

10 to 15

does not seem to intermediate

deactivation.

of methane

is the temperature.

the methane

equilibrium

concentrations

of 61, 42 and 25% respectively

5)

is seen to be

concentration

the higher

with the thermodynamic The methane

The effect

hydrogen

suppresses

benzene

(Figure

of the hydrogen

but reacts with

the yield

7, the lower the temperature

methane

by coke

in hydrogen

step of benzene

and thereby

steady-state

the conversion

concentration.

is insignificant.

the methane

activity

is independent

of deactivation,

the increase

to the cracking

important

conversion

by hydrogen

with

that the effect

directly

coke precursors

of temperature.

yield.

This

for the formation

plotted~in

of

the figure

at 501, 552 and 649°C.

of CO and HZ5

In view of the intended application ion control

in coal gasification,

of the present

in addition

study to hydrocarbons

to He it is necessary

emiss-

to investigate

0

Pre-reduced 232

A

9.9

'he-reduced and sulfided

Time, min 1500

1000

wcat = 0.30 g 500

T

= 5oo"c 0 V. = 333 cm3/min STP In

0 FIGURE 8

Effect

of H2S on the catalyst

the role of CO, the other major

product

suspected

presence

that the simultaneous

due to possible

Fischer-Tropsch

known for these

reactions.

in the absence Experiments

activity.

of coal gasification.

type reactions,

Hence,

However,

of CO and H2 may complicate

it was decided

since

iron-based

to examine

these

it was

the situation

catalysts reactions

are well first

of benzene. were carried

out with a gas mixture

containing

20 ~01% Hz, 15 ~01%

115 CO and balance reactors,

nitrogen.

This mixture

one containing

was simultaneously

the iron oxide-silica

sand. At 500°C and 1.05 atm pressure about

13%, and the concentration

Under

identical

conditions

ingly produced

2000 ppm methane.

Thus,

very effective

for hydrogenation

of

ion and formation The heavy

of higher

hydrocarbons

with the existing benzene

cracking

results

discussed

catalyst

in the presence

As mentioned

earlier,

as a sorbent

the simultaneous Experiments nitrogen.

presence

were carried The results

effect;

on sulfidation

decrease

of catalyst

sulfidation

material

removal

in Figure

the catalyst

becomes

correspond

of the catalyst.

In another

results

Catalyst

but sequential

weight

the conversion

reported

For this purpose

of benzene

of only about

conversions

sample sample

removal

the are

of H2S and

If the first part

for benzene

cracking,

work

the

is necessary

mass transfer

bed height

ion on the conversion

by adding

of benzene

does not seem to affect level between

(FB

as a function = 300-1500

inerts.

was examined.

to have

effects,

and

if the ratio Wcat/FB,in to be the suitable

into a maximum

bed height

it was necessary

to maintain

In view of this the effect As shown

the

in) were varied,

was found

this translates

5 mm. To have near plug flow conditions

minimum

to 100% were obtained.

it was necessary

gas flow rate

W cat/FB,in

flow rates,

close

paper,

and to examine

(X) was observed

benzene).

With the attainable

conversion

a pre-reduced

is possible.

and

and subsequent

with a pre-sulfided with

in

poisoning

The increase

to reduction

step using air. Further

in a separate

(Wcat) and the reactant

(g-catalyst.hr/g-mol

dilution

inactive.

of

aspect.

HZ, H2S and benzene

and the remaining

in a single

so far benzene

study,

lower conversions.

a certain

treatment

the effect

dilution

For the kinetic

range.

the

this problem.

In most experiments

catalyst

Results

Hence,

is an important

imply that simultaneous

of the bed is used for desulfurization

to explore

almost

experiment

for comparison.

bed can be regenerated

However,

in the use of this

that H2S has a strong

respectively

was very poor from the beginning. These

problems

the study of

in this study was originally

activity

8 reveal

included

entire

used

out with a feed gas containing

activity

is not feasible,

problem

at high temperatures.

of HZ'S on the catalyst

depicted

operational

activity.

activity

benzene

serious

deposit-

this experiment.

of both H2 and CO was not pursued.

the catalyst

for H$

during

exit lines causing

used here is

of carbon

In view of these complexities

above bring out a possible

due to its Fischer-Tropsch

developed

A large amount

were also noticed

and clogged

assembly.

gas was only 8 ppm.

56% of the CO and correspond-

the iron catalyst

as expected,

hydrocarbons

condensed

reactor

converted

CO to methane.

the two

containing

over the sand bed was

in the product

the iron catalyst

through

and the other

the CO conversion

of methane

passed

catalyst

in Figure

the level of conversion.

the two sets of data are obviously

of dilut-

9, the degree

The difference

in the

due to the difference

of

116

80

Weight (g) Cat. Sand --

'0

Flow Rate 3 (cm /min STP)

'H2,in (vol %)

Bed Heigl (cm)

A

0.30 0.50

350

10.3

0.9

0

0.30

350

10.3

0.4

A

0.10 0.90

333

10.0

1.0

l 0.10 0.40

333

11.6

0.7

20 I

0

40 4

60 1

80 I

100 I

120 I

140 I

160 I NO

Time, min 1000,

FIGURE 9

I

Benzene

in W cat/fb,in

I

I

conversion

I

I

and methane

yield

negligible

as functions

I

I

of catalyst

dilution.

(s 3 times).

Van den Bleek, et al. [lo] have reported conversion,

I

I

and have developed effect

of dilution.

a criterion Accordingly,

studies

on the effect

for obtaining

conditions

the permissible

degree

of dilution necessary

on for

of dilution,

i.e.

117 the fraction

of inerts

in conjunction lished.

with

in the bed is up to 0.85. Based on the experimental

this criterion,

a range for the degree

Thus, 3 fixed bed height was maintained,

while

of dilution

Wcat could

results

was estab-

be changed

independently.

Catalyst

deactivation

In the discussion observations. deposition,

to obtain

time experiments

activity

level decreased the conversion both cases

faster.

level stayed

the benzene

even more

1 h period

with

the conversion

noticed,

indicative

the carbon these with

pressure

changes.

of e-carbon cannot various

of this work, ability

Reaction

simply,

is necessary high catalyst

besides,

the conversion with

hydrogen

regeneration,

it dropped

similar

in

only slightly

trends.

were made.

in the conversion

During

the period

1.3 to 3.8 psig.

In the

surface

in pressure.

the complex

sufficiently

change

in the course

alters

and highly

reactive

The high density

The anomalous analysis

was

of reaction

its form and causes

and inactive.

A more detailed

level was

from 6 to 9 h

No pressure

that

is known to be unstable

to understand

under the

But then, without

observations

It is suspected

the increase

activity,

after

level.

followed

is much more stable

however.

again

a certain

from 80% to 60%, but was then slowly

on the catalyst

a-carbon

B-carbon

after

of hydrogen;

hydrogen

hydrogen

decrease

from

5 h period.

may have caused

stages

level dropped 6 h a linear

also increased

For example,

be explained

yield

some anomalous

deactivation.

the first

while

while with

of a sustained

(or coke) depositing

hydrogen,

without

mechanism.

a few long-

10. As before,

in the absence

to a higher

level. Methane

by about 4 h. Beyond

during

in Figure

5% in less than 2 h, while

jumped

hydrogen

regained

noticed

air, and was tested

As a result,

rapidly,

reaction

was stopped

high up to 9 h. Immediately

conversion

at the previous

In the experiment

the system

dilute

are depicted

from about 40% to about

it dropped

and stayed

with

from various

is due to coke

life and regenerability,

the reaction

level was lower

was much

inferred

is a step in the overall

In these,

The results

has been

that the deactivation

on catalyst

was regenerated

or the conversion

the deactivation

first

information

conditions.

hydrogen

deactivation

indications

were conducted.

the catalyst

identical

are obvious

and that the coke formation

In order

time,

so far catalyst

There

observations

of the catalyst

phenomena.

long catalyst

life and regeneral-

are established.

mechanism

Based on the results postulated

to describe

discussed the overall

hitherto, reaction

the following mechanism.

at

For the purpose

reaction

scheme

is

0

FIGURE

0

20

80

100

10

4

u

20

l

0’

'A

40

I

\

0’0-0-0

4 I

%

Catalystdeactivation

A’

FA

2 I

'A

60

I

A

'A

d

I‘(

+-A-i I I

I

I

l I

;\A

I I

I f , I L 1

100

Time, min

80

12 I

1

14 I

16 1

18

I 1O~E:Jj:in I;::'

a-

140

I

A -A-A-

V.

160

I

12

0 = 300 cm3/minSTP ln

Wcat = 1.0 g, T = 65O'C

A In the absenceof H2

1::’

*

0 In the presenceof H2(10.2VOX%)

\

A

120

\

P Regeneration , I \ 8

Time, hr 8 10 I I

and regeneration.

‘I

dON,Th

6 I

119

0ClBenzene

The

rate controlling

aromatic

c

to be the destabilization

This could

species.

For example,

level,

be justified

once the catalyst

of the presence

concentration

by the conversion affected.

fast,(C,,H,Ja

step is assumed

level was independent in hydrogen

had no effect Figure

Virk et al. [2], from their studies compounds,

arrived

Once the a-form function

is transformed

at similar

ive steps,

of the coke

Thus, methane

consistent

with

of this effect

energy

e-form,

which

the experimental

in the literature

[6], another

00

(represented decay were

to methane

is considered

slow

estimated

to

is obviously

step, the a-form

responsible

deactivation

of

the initial

electrons,

In a parallel

observations.

for

are highly

competit-

Also, there are strong

[11,123.

reaction

scheme

for hydrogenation would

h

0

H2 Fe cat.*

---+(CnHm)

,

of benzene

be

CH4,

.. ..

cyclohexane

Benzene

However,

possible

a change

hydrogasification

They consider

In view of the known fact that iron is a catalyst to cyclohexane

rate

and catalyst

of pi(r)

and catalyst

conversion

Furthermore,

reaction

yield

its conversion

concentration.

formation

the benzene

on thermal

conclusions.

is formed,

of the hydrogen

into the inactive

deactivation.

evidences

methane

ring

on the experimental

of hydrogen.

on the initial

5), although

of the benzene

based

was reduced,

or absence

step to be due to the large delocalization -1 be Q 40 kcal mol .

a strong

CH4

2

Activated form

to form the activated observations.

+H

bl0*

slow,

this route can be ruled out based on the following

experimental

observat-

ions: Even in the absence

(i)

of hydrogen,

a finite

benzene

conversion

(s 40%) took

place. (ii)

When hydrogen dropped

(iii)

was stopped

sharply,

Cyclohexane

abruptly

but the benzene

was not detected

in some experiments,

conversion

in the product

the methane

yield

level was unaltered. gas stream

in any of the

experiments. Thus,

although

the mechanism

with experimental

observations

As for the catalytic For example,

suggested

information

and iron, separately

and

mechanism,

here +s speculative,

with

some of the reported

a much more detailed

on the adsorption-desorption

and in a mixture,

it is consistent

would

literature.

study would

behavior

be very helpful.

be necessary.

of benzene

on silica

This was beyond

the

120 scope of this work. Silica,

by itself

ing benzene,

However,

and iron, a well

for the methanation functions,

conversion that silica

possibility

was much

lower-in with

are discussed provide

catalyst,

here briefly.

acidic

sites for crack-

may then be responsible'

is that the iron serves

an inert support.

consistently

is almost

could

known hydrogenation

is simply

conversion increased

mechanisms

of alumina),

step. The other

and silica

the benzene

two plausible

(in the absence

Two observations,

theabsence--of

the reduction

inert, and iron provides

viz.

both the (i) that

hydrogen ,and (ii) that the

of the fresh catalyst,

sites for cracking

suggest

as well as hydro-

genation.

SUMMARY

AND CONCLUSIONS

Cracking catalyst

the effects products

of benzene

was studied

(in concentrations

in a packed-bed

of operating

and catalyst

of 9 300 ppm) on an iron oxide-silica The objective

microreactor.

canditions

on catalyst

activity,

The important

deactivation.

findings

nature

was to investigate of reaction

of this study are listed

below. (1)

(2)

The iron oxide-silica

is an excellent

The catalyst

is most active

rapidly (3)

deactivated

Once the catalyst of the benzene

Reaction

gen catalyst Consistent according

with

to which

developed

for benzene

in its reduced

is reduced,

reaction.

form as metallic

of hydrogen

hydrogen

as a high temperature

cracking.

does not directly

However,

iron, and is

due to coke deposition.

it helps

influence

suppress

the rate

catalyst

deactiv-

with the coke.

of hydrogen

in the selective

originally

catalyst

in the absence

cracking

ation by reacting (4)

material,

H2S sorbent,

with

formation

deactivation

the primary

is largely

these observations the aromatic

coke

of methane.

(alpha) on this catalyst

As a result,

in the presence

results of hydro-

suppressed.

a reaction

mechanism

ring destabilization

has been postulated,

is the rate controlling

step.

ACKNOWLEDGEMENT This work was carried ment of Energy,

Morgantown

out under a contract Energy Technology

(DE-ATZI-79MC11284)

from the Depart-

Center.

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9

IO 11 12

N. Choudhary

and O.N. Saraf, Ind. Eng. Chem. Prod. Res. Dev., 14 (1975) 74. S.K. Gangwal, D.A. Green and F-0. Nixon, "Vapor Phase Cracking and Wet Oxidation as Potential Pollutant Control Techniques for Coal Gasification," Research Triangle Report TRI/1934/00-OIF, 1981. (a) E.C. Oldaker, A.M. Poston, Jr., and W.L. Farrior, Jr., 170th National Meeting, ACS, Div. Fuel Chem., 20(4) (1975) 227. (b) E.C. Oldaker and D.W. Gillmore, 172nd National Meeting ACS, Div. Fuel Chem., 21(4) (1976).79. C.M. Van Den Bleek, K. Van Der Wiele and P.J. Van Den Berg, Chem. Eng. Sci., 24 (1969) 681. A.I. LaCava, C.A. Bernard0 and D.L. Trimm, Carbon, 20 (1982) 219. J.G. McCarty and H. Wise, J. Catal., 57 (1979) 406.

W.J. McMichael,