Infrared Study of Carbon Deposits on Catalysts

51

T.Inui (Editor), Successful Design of Catalysts © 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

INFRARED STUDY OF CARBON DEPOSITS ON CATALYSTS

R. P. EISCHENS Zettlemoyer Center for Surface Studies and Department of Chemistry, Lehigh University, Bethlehem, PA 18015, USA

ABSTRACT The deposition of carbon on alumina and on catalysts, in which alumina was the support for platinum and rhenium, was followed by simultaneously weighing the total deposit and observing the infrared spectra. At total carbon deposit levels of 1 percent or less, produced by exposure to acetylene at 525 K or ethylene at 625 K, bands were observed near 1580 and 1470 cm- l. Isotopic labeling with oxygen-18 and deuterium shows that these bands are attributable to the asymmetric and symmetric stretching vibrations of a carbon-oxygen species similar to a carboxylate ion. This species involves about one-sixth of the total carbon in deposits on alumina. Platinum does not markedly affect the total carbon or the fraction that is carboxylate. Rhenium does not affect the total carbon but it significantly lowers the carbon-oxygen species. Pretreatment of desulfurization catalysts with ammonia reduces the total carbon by about one-half and almost eliminates carboxylate formation. Carboxylates are not found in carbon deposits on silica.

INTRODUCTION The deposition of carbon is a limiting factor in the important processing

reactions of the petroleum industry.

reducing conditions. of

hydrocarbon

This deposition occurs under

The consensus view is that these deposits

species

has

been

are

mixtures

which have a relatively low hydrogen-to-carbon ratio

with this ratio an inverse function of the maximum temperature deposit

hydrocarbon

heated.

This

to

which

the

view was supported by an infrared study in

which exposure of hydrogen faujasite to hexene-l at 535 K produced bands at 1580 and 1460 cm- l. These bands were attributed to the carbon-carbon stretchings in hydrogen deficient ring structures and carbon-hydrogen spectively

(ref. 1).

bendings,

re-

Similar bands and conclusions were obtained after expo-

sure of dehydroxylated

Y zeolites to butenes at 430 K (ref. 2) and

of Pt/CaY to hexene-l at 600 K (ref. 3).

exposure

Even though efforts were not made to

verify the assignments by isotopic labeling, it is reasonable to accept the conclusions that the 1580 and 1460 cm- l bands are predominantly, if not entirely, due to carbon-carbon and carbon-hydrogen vibrations in these zeolite experiments. In

a

study

of

the double bond isomerization of 2,3-dimethyl-l-butene on

52 alumina, it was found that the isomerization was poisoned by

an

aldehyde

or

ketone-like carbonyl species (ref. 4). This species was detected by a band at 1675 cm- l at 355 K. When the system was heated above 450 K, bands were observed at 1570 and 1470 cm- l. These bands were assigned to the asymmetric and symmetric bands of a carboxylate. Raman

study

duction of carboxylates on alumina, by established

Carboxylates were also

detected

of the isomerization of olefins over alumina (ref. 5). (refs.

6-8).

In

heating

general,

adsorbed

alcohols,

conversion,

is

provided

a

is

well

the proposed alcohol-to-carboxylate

mechanisms follow Creenler's concept that the second oxygen, for the to-carboxylate

in

The pro-

alcohol-

by bridging oxygens or hydroxyls of

alumina and is not due to adsorbed or gaseous oxygen. In the case of carboxylates from olefins, both oxygens appear to be provided by the alumina (ref. 4). This could be visualized as a chemisorbed aldehyde, the 1675 em- l species, being converted to a chemisorbed carboxylate. The

work

to

be

discussed

below focuses on the production of an oxygen-

containing species under the reducing conditions encountered in the carbonization of hydrocarbons and the question of whether these species may be significant in the deactivation of catalysts ..

RESULTS AND DISCUSSION Carbon was deposited by exposure to 10 torr of acetylene at 525 K or

by

exposure to 500 torr of ethylene at 620 K (ref. 10).

An apparatus was

used in which the catalyst sample, a pressed disc weighing about 100 suspended

from

a

Cahn

microbalance

so

the

(ref. 9) mg,

was

weight of the deposit and the

infrared spectra could be measured simultaneously (ref. 11). Figure 1 was observed after exposing Alon-C (a gamma-alumina, surface 94

m2/g)

to acetylene for 5 h at 525 K.

A similar spectrum is produced when

Degussa Aluminum-C (100 m2/g) is exposed to ethylene at 625 K. aluminas «0.5%» work

are

Both of

these

prepared by burning aluminum chloride and both contain chlorine

as an impurity.

The question of whether chlorine is a factor in

discussed here has not yet been studied. pretreatment

was

carried

the

Prior to the carbon deposition

of Figure I, the Alon-C was treated for 18 h in flowing This

area

hydrogen

at

625

K.

out to provide conditions similar to samples

containing supported metals which required reduction.

With pure alumina,

the

hydrogen pretreatment did not affect the results other than as a drying procedure to obtain reproducible levels of surface hydroxyls. Figure 1 shows bands near 1580 and 1470 cm- l which are asymmetric

and

symmetric

stretching

attributed

frequencies of a carboxylate.

bands are observed when acetic acid is chemisorbed in

alumina.

to

the

Similar

Acetic

acid

53

0.5

~

U

Z

<

= 0.3 = < ~

0

~

0.1

Fig. 1. Spectrum obtained after treating Alon-C with acetylene for 5 h at 525 K.

spectra

were

used

as a calibration to obtain the approximate number of car-

boxylate groups represented in Figure 1. quantify

the

The high frequency band was used

to

carboxylate

because, with acetic acid, there was a detectable carbon-hydrogen band near 1450 cm- l which could cause difficulty in measuring the 1470 cm- l band. Either the band area or the absorbance of the 1580 cm- l

band gave straight lines when plotted versus the acid.

There

weight

of

adsorbed

sorbed as carboxylate when the acid was added in small doses at room ture.

acetic

was no evidence of physically adsorbed acid that had not chemitempera-

An absorbance of 0.5, such as that in Figure 1, was produced by 0.7 mg Thus, the 1580 cm- l band of Figure 1 is produced by carboxyl-

of acetic acid.

ates containing about one-sixth of the total carbon atoms in the deposit. This calculation assumes that the 1580 cm- l band in Figure 1 is predominantly due to

the

asymmetric carboxylate band without a contribution from an underlying

aromatic carbon-carbon vibration. assumption

A second, and

perhaps

more

questionable,

is that the specific intensities of the bands are the same for the

deposited carboxylates and the carboxylates from acetic acid. In Figure 2, the absorbance of the 1580 cm- l band, left ordinate, total

weight

and

the

of deposit, right ordinate, are plotted as functions of time of

54 0.6~--------------------,

l.:l

1.0 "'"' CIl

0.4

8

U Z

'-"

-e o:l 0::

f-<

::c

0.6

0

o

1JJ

l.:l

o:l

-e 0.2

~

0.2

o

2

6

4

TIME

(hrs)

Fig. 2. Curve A: weight of deposit; Curve B: absorbance of 1580' cm- l band for A1203 and for Pt/A1203; Curve C: absorbance of 1580 cm- l band for Re/A1203 and for Pt-Re/A1203'

acetylene exposure of 525 K.

Curve A of

weight

samples in which the results are so similar that

for

four

different

they can all be represented by a single platinum/alumina,

rhenium/alumina,

and

Figure curve.

2

represents

These

samples

the are

platinum-rhenium/alumina.

deposit alumina, In

all

band

for

cases, the metal loadings are 3 wt% for each metal. Curve B of Figure 2 represents the absorbance of the alumina

and

for

platinum/alumina.

1580

cm- l

This shows that platinum does not affect

the formation of carboxylate on alumina.

Curve C represents

for rheniunm/alumina and for platinum-rhenium/alumina.

the

absorbance

Comparison of curves B

and C shows that rhenium lowers carboxylate formation. Addition of rhenium to platinum/alumina increases the resistance to deactivation

by

carbon (ref. 12) even though the carbon deposition is not signifi-

cantly lowered.

This indicates that some components in the carbon deposit are

more deleterious than others. The

parallel between increased tolerance to deactivation by carbon and the

decrease of carboxylates suggests that the role of rhenium may be to the

fraction

of

carboxylates.

decrease

This concept is worthy of consideration even

though there are too many uncontrolled variables to warrant a firm conclusion.

55 the

When

data

of Figure 2 were first available, the activity of rhenium for

the hydrogenation of carboxylic acids (ref. 13) was considered as for the decreased carboxylate production.

the

However, this special hydrogenation

activity cannot be the only factor since other metals, such as lead can

also

enhance

the

performance

production

has not been studied.

tin,

Their effect on

The catalysts, which were used

for Figure 2, had metal loadings about ten times higher than used cial reforming catalysts.

and

of platinum/alumina reforming catalysts.

These metals would not have a special hydrogenation activity. carboxylate

reason

in

commer-

Infrared spectra of commercial platinum/alumina and

platinum-rhenium/alumina catalysts, used under commercial conditions, did show significant differences.

not

These catalysts had carbon levels of 10-14 per-

cent and the carboxylate band region was

partially

obscured

by

hydrocarbon

bands. In

the above discussion of Figures 1 1470 cm- l have been assigned to carboxylates.

and

2, the bands near 1580 and

This assignment has been

based

on analogy with known carboxylate spectra including those produced by the chemisorption of acetic acid on alumina. The carboxylate better

match

than

other

carbon-oxygen

species,

assignment

provides

a

such as those produced by

adsorption of carbon dioxide.

Chemisorption of carbon dioxide on Alon-C proand 1420 cm- l at room temperature and at 1630 and 1450

duces bands at 1640 cm- l after heating to 675 K. the

carboxylate

species

Because of the

potential

relationship

and the role of rhenium in reforming catalyts, iso-

topic labeling with oxygen-18 and deuterium was used to verify the to

a

carbon-oxygen

species

(ref.

were

observed

when

assignment

Both bands were 20 cm- l lower when

10).

produced by exposure of oxygen-18/alumina shifts

between

to

ethylene

oxygen-18/acetic

acid

at

was

625

K.

Similar

chemisorbed on oxy-

gen-18/alumina. Adsorption of C2D4 on deuterated alumina did not produce the large shifts of about 300 cm- l expected for the substitution of deuterium for hydrogen in carbon-hydrogen bending vibrations. This shows that the 1470 cm- l band

is

not

due

to

The shoulder at 1450 cm- l observed

a carbon-hydrogen.

after chemisorption of acetic acid disappears after deuteration. labeling

experiments

The isotope prove that the bands near 1580 and 1470 cm- l are due to

carbon-oxygen vibrations such as those of a carboxylate. species

The structure of the

producing these bands is not completely known because it is not clear

to what the carbon is bonded in addition to the two oxygens. Figures 3 and 4 were obtained in a CoOjMo03/A1203

desulfurization

study

catalyst

of

the

with ammonia.

were produced by treating the catalyst with 10 torr of Prior 2 h.

effect

of

pretreating

The data in Figure 3 acetylene

at

525

K.

to carbon deposition, the catalyst was dried by evacuation at 575 K for In Figure 3 the weight of carbon deposit on a 100 mg sample

is

plotted

56

4.0 III

-B

3.0

/1 0

/1

I-<

~

0

/1

0 /1

-

0.4

/1-

-

0.3 u' Z E

0 /1

0

0

0

0

/1

o 2.0 -

/1

/1/1

-

::r:: ~

/1

0

/10

-- 0

1.0 0

I

I

I

1

2

3 TIME

I 4

I 5

~ ....

«

~c::>

-

0.2

-

0.1

I

C2"2 525 K

-

-I-<

::r::

o

2.0

-

-

0

0

0

~

~

/1 - -

--0

0

1.0 ,....

of

0.4

-

U.... 0.3 Z'

-

0.2

-

0.1

~

«B

= 0 .... =« ~c::>

0

0

th~

-

-.

B 3.0

(/)Itl

6

Fig. 3. The weight of carbon deposit, 0, and the absorbance of 1570 cm- l band, 1::>., as functions of time for the exposure CoO/Mo03/A1203 to 10 torr of acetylene at 525 K.

..

0 ....

( hrs )

10 torr

4.0

= =« 101

(/)Itl

0 A

A

1

l>

~

*2

3 TIME

10 torr

~

4

i 5

I 6

( hrs )

C2"2 525 K

Fig. 4. The weight of carbon deposit, 0, and absorbance, I::>. , as functions of time for the exposure of NH3 treated CoO/Mo03/A1203 to 10 torr of acetylene at 525 K.

57 the

on

left

ordinate

and the absorbance of the asymmetric carboxylate band

near 1570 cm- l is plotted on the right. shows

that

the

total

carbon

Comparison with Curve A of

deposition

Figure

2

is about four times larger on the

desulfurization catalyst.

The absorbance, after 6 h, is about the same as for

Curve

A new acetic acid calibration was not made for the

B

of

Figure

2.

desulfurization catalyst. bration

However, if it is assumed that the

previous

cali-

is valid, the fraction of carboxylate is one-fourth as large as found

with Alon-C.

Only about 4 percent of the carbon is in the carboxylate form.

Figure 4 was obtained after the catalyst had been exposed to excess ammonia and

then

evacuated at 525 K.

The retained ammonia was strongly chemisorbed.

It is seen that the ammonia treatment has almost completely eliminated carboxylate

formation

half.

Ammonia pretreatment of alumina decreases both total carbon and carbox-

ylate

without

and the total carbon deposited has been lowered by about one decreasing

the

fraction

of

carboxylate.

CoOjMo03/A1203 lowers both carbon and carboxylate by about the

Presulfiding

one

half.

Thus,

ammonia pretreatment, which has a beneficial effect on the performance of

the desulfurization catalyst, has a disproportionate effect

on

lowering

the

carboxylate fraction in the carbon deposit. Use

of

zeoli tic

samples

produced spectra in which bands in the 1580 and

1470 cm- l region are attributable to hydrogen

bendings

(refs. 1-3).

carbon-carbon

stretchings

and

carbon-

These assignments imply that there is a sig-

nificant difference between alumina and zeolites with respect to the formation of

carboxylates.

atoms.

An ideal zeolite structure does not have adjacent aluminum

Thus, the absence of carboxylates on zeolites could

silicon

be

explained

if

does not participate in the mechanism which produces carboxylates and

if adjacent aluminum atoms are necessary.

Figure 5 was obtained

by

exposure

of Cab-o-Sil to acetylene at 525 K. This figure is similar to Figure 2 in that the weight of the carbon deposit is plotted on the right ordinate. of

Comparison

Figure 5 with Curve A of Figure 2 shows that the carbon deposition on Cab-

0-8il is only 0.25 mg after 24 h, while it is 1.0 mg after After

24

h

there

6

is a barely detectable band at 1580 em-I.

formation of carboxylate on silica is consistent

with

the

h

Alon-C.

The negligible

observation

benzaldehyde is not converted to carboxylate on silica (ref. 14). that work it was also found that benzaldehyde is converted to heating

on

However, in

carboxylate

by

on a 25% alumina-silica cracking catalyst. This latter observation is

not consistent with the concept that adjacent aluminum atoms are necessary providing

that

sites

for

in

carboxylate formation unless the possibility of alumina

clusters in the cracking catalyst is accepted. There are two general mechanisms which

might

account

for

the

oxidative

58

0.3

0.3

'"-l

--e ClII

0.2

U

z

<

'-'

l:l'.

:: c

= = 0.1 <

!-

-

0

VJ

0.1

o

12

8

4

TIME

16

20

'"-l

~

24

( hrs )

Fig. 5. The weight of carbon deposit, A, and absorbance of the 1580 cm- l band, 0, as a function of time for the exposure of Cab-o-Si1 to 10 torr of acetylene at 525 K.

properties

of

alumina.

One mechanism postulates that the oxygen is an inte-

gral part of the alumina surface. maximum carboxylate

ited to about 10% of the surface. bulk carboxylate. carbon,

Thus, it is a

is

reduced.

chemisorbed

rather

which

According to this concept, hydrogen is

The

than

a

lost

and

initially is bonded to the hydroxyl, becomes bondpn to an

oxygen bridged between two aluminums. like

alumina

Greenler's mechanism illustrates the formation of carboxyl-

ate from alcohols (ref. 6). the

In effect, the

indicated by Curve B of Figure 2 is estimated to be lim-

This produces a structure

an ester in that the carbon is bonded to two oxygens.

ture then converts to the carboxylate ion.

The

production

which

looks

This ester strucof

carboxylates

from olefins would require the initial formation of a single oxygen species as postulated by Corado (ref. 4) on the basis of the observed 1675 cm,l band. The second type of mechanism visualizes an external source of oxygen. nism was postulated on the basis of adsorption of H2S and CS2 (ref.

This mecha15).

An

oxidized alumina was produced by heating in oxygen at 675 K. Adsorption of CS2 on oxidized alumina at room temperature produced a 2000 cm- l band which was identified

as

COS.

If

the

oxidized

alumina

subsequent adsorption of CS2 did not produce COS. the

oxidizing

properties

were reduced with hydrogen, These

results

imply

that

of alumina are due to adsorbed oxygen which can be

59

removed by hydrogen reduction even though it is not removed by

evacuation

at

675 K. The

studies

of carboxylate formation during carbon deposition by exposure

of catalysts to acetylene or ethylene does not add detail to mechanisms have

previously

appeared

in

the

literature.

question of whether alumina-oxygen or adsorbed oxygen is carboxylate

which

However, with respect to the the

oxygen, the alumina-oxygen concept is favored.

gen forms cos instantaneously at room temperature.

source

of

the

The adsorbed oxy-

Carboxylate

formation

is

slow at elevated temperatures and treatment with ethylene-hydrogen mixtures at

625 K gives rates of deposition and carboxylate obtained

with

ethylene

alone.

Moreover,

formation

similar

to

those

pretreatment of the samples with

hydrogen for 16 h at 623 K did not modify the production of carboxylates. ACKNOWLEDGMENT This paper was written

with

the

support

of

the

Division

Sciences, Office of Energy Research, U.S. Department of Energy.

of

Chemical

Assistance in

preparation of the manuscript was provided by M. Sawyers and J. Datka.

REFERENCES

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