n-Butane isomerization on solid superacids

Catalysis Today,5 (1989)493-502 Elsevier Science Publishers B.V., Amsterdam -

n-BUTANE

J.C.

ISOMERIZATION

Instituto

ON SOLID SUPERACIDS

LUY and J.M. PARERA

YORI, J.C.

de Investigaciones

Santiago

493 Printed in The Netherlands

en Catalisis

de1 Ester0 2654 - 3000 Santa

y Petroquimica

-INCAPE-

Fe, Argentina

ABSTRACT n-Butane isomerization reaction was studied on H-mordenite and ZrO2 treated with sulphate ion. Zr02/SO' was calcined at different temperatures ranging 773933 K; when calcining at 8t33 K, an optimum in catalytic activity was found. Both catalysts isomerize n-butane in different ways. On H-mordenite, disproportionation and cracking reactions occur simultaneously with isomerization. On the other hand, ZrO2/SOi is a very selective catalyst for iso-butane formation, being the other reactions less favored. Differences in acid strength distribution can be mentioned to explain different catalyst behaviours. Catalysts deactivate with time on stream due to coke formation; H-mordenite activity diminishes continuously, and ZrOp/SOz reaches a steady state.

INTRODUCTION A variety of solids acidic

is effective

halides and dual function

n-butane

on strong acidic

for alkane

catalysts

isomerization,

111. Tanabe and Hattori

solids such as SbFS-Ti02-Si02,

SbFS-Ti02

Si02-A1203

12). These solids which have an acid strength

100% H2SO4

(Ho = -11.9) are called superacids.

Hino and

Arata

anf Hf02 produces in catalytic reaction

activity

mechanism

ion abstraction. posterior

(3-71 found that sulphate a remarkable

On superacid

H-mordenite,

dimer is formed,

follows

occurs

rapidly.

from n-butane

but partially

Guisnet et al.

Fuentes and Gates

1111 studied

chloride/sulphonic and produces

0920~5861/89/$03.50

the limiting

inhibits

n-butane

propane

A

in the carbenium

disproportionation

two

possible.

catalyzed

by

occurs parallel

in equimolecular

0 1989 Elsevier Science Publishers B.V.

does not

On H-mordenite,

to make this reaction

and pentane

that the

in which a C8

step. H2 presence

Disproportionation

ion

iso-butane

demonstrated

process

coke formation.

necessary

acid resin.

occurs.

IS-101 isomerized

a disproportionation

acid sites are considered

the

by hydride

the iso-butane.

of H2, and the same authors

through

Zr02,

and, consequently,

In the case of n-butane,

ions generated

produced

being this formation

affect the activity

isomerization,

reactions.

to Fe203, Ti02,

acidity

an alkane can be transformed

in the presence

reaction mechanism

aluminum

carbenium

ion transfer

and isomerization

adjacent

for acid-catalyzed

involves

ion addition

in surface

isomerized and SbFS-

higher than that of

In the adsorbed state, skeletal re-arrangement

hydride

on H-mordenite

enhancement

being mainly

amounts.

to

494

The isomerization isomer,

iso-butane

material

of n-butane

(iso-C4) which,

for gasoline

The objective

improvement

property

a low value product

after dehydrogenation, (MTBE, alkylate,

and ZrO,/SO~

and stabilities,

into its

is an important

raw

etc.).

of this paper is to study n-C4 isomerization

solid acids, H-mordenite, selectivities,

(n-C,) transforms

over two strong

their activities,

in order to compare

and relate the differences

to some measurable

as acidity.

EXPERIMENTAL Materials The catalysts surface

were: H-mordenite Zeolon 900-H from Norton Co. with a specific 2 -1 g measured according to 1121, and ZrO,/SO~ prepared as

area of 515 m

follows:

Zr(OH)4

ammonium

hydroxide

overnight.

TABLE

zirconyl

by pouring

on a filter paper. Then, Zr(OH)4/SOi

solid.

temperatures

Properties

chloride

with aqueous

and drying the precipitate

ion was introduced

3 h at different

resulting

by hydrolyzing

at pH = 10, washing

The sulphate

gram of Zr(OH)4 during

was obtained

was calcined

in the range 773-933

of Zr02/SOi

samples

at 383 K

15 ml of 1 N H2S04 per in an air flow

K, being Zr02/SOi

are listed

in Table

the

1.

1

SO; ion content

Calcination temperature,

K

773 823 893 933

and surface

so,

Surface

wt%

(BET), m2 g-I

4.0 5.2 4.5 4.5

153 138 120 118

n-C4 was Matheson Catalytic

area of Zr02/SOi

99.5%.

area

H2, N2 and air were pure grade AGA.

test

n-C4 isomerization

was carried

reactor at 1 atm. A constant during each run. Before

out using an isothermal

surface

reaction,

in a flow of H2, and ZrO,/SO~

quartz

H-mordenite

calcined

was activated

in the range 773-933

but in a stream of air. Activity

and selectivity

feeding

On the other hand, catalyst

pure n-C4 (12 ml min-').

were performed

feeding

(12 ml min-')

as carrier

Reaction

products

plug-flow

area (50 m2) of each catalyst

K also during

of catalysts

not only pure n-C4 (12 ml min-'),

was loaded

at 773 K during

3 h

3 h,

were checked deactivation

runs

but also H2 (or N2)

gas.

were analyzed

chromatographically

on line by using a FID

495 and a 6 m long, l/8 in. O.D., 25% dimethylsulpholane Acid strength

cyclohexane.

of ZrO,/SO~

indicators.

was examined

The Hammett

temperature.

indicators

(pKa = -10.5).

placed

in an air flow during

in

3 h at

used were: anthraquinone

1-Cl-3-nitrobenzene

(pKa = -13.6).

measurements

Acidity

measurements

H-mordenite

SO: calcined

were carried

was pretreated

at various

NH3 was passed during

out by using a NH3 adsorption-desorption

at 773 K in a H2 flow during

temperatures

range from 523 to 823 K, was carried each temperature

Samples

at different

out with flowing

level, and the NH3 desorbed

and titrated

3 h, and Zr02/

in an air flow for 3 h. After cooling

1 h at room temperature.

dried N2 at 523 K for 3 h. NH3 desorption

solution

by a colour change method

is added to a powdered sample

samples were pretreated

(pKa = -8.3), p-nitrotoluene

method.

catalyst

The indicator

Previously,

each calcination

Acidity -

P column.

determination

The acid strength using Hammett

on Chromosorb

in N2,

were then purged with temperatures

-within the

N2 for a period of 2 h at

was collected

in a 0.1 N H2S04

with NaOH.

RESULTS AND DISCUSSION Activity

and selectivity

Catalytic

activity

was taken as the conversion

after 5 min from the beginning ZrO,/SO~

is plotted

temperatures. obtained optimum

against

When calcining

the reaction

temperature

temperature

Conversion

and ethane

Selectivities

obtained

a suitable

was

ZrO,/SO~

catalyst

in activity

of the calcination

with ZrO,/SO~

at a

temperature.

calcined

at 893 K are

being the other ones pentanes

(C,) are also formed,

are similar

calcination

activity

This value seems to be the

showed a maximum

is the main product,

(C,). Methane

amount.

products

products

for different

in catalytic

temperatures.

of 573 K, independently

to all the reaction

(C5) and propane negligible

temperature

in order to obtain

All the catalysts

shown in Figure 2. iso-C4

to reaction

In Figure 1 conversion to iso-C4 on

at 893 K, a maximum

in a wide range of reaction calcination

for n-C4 isomerization. reaction

of each run.

in the catalyst

but in a

calcined

at the

other temperatures. Figure 3 shows results obtained -below 573 K, reaction temperature,

iso-C4 formation

to C3 increases similar

behavior

is negligible

with H-mordenite.

It can be observed

does not proceed to a significant increases

reaching

a maximum

all over the range of temperatures to iso-C4 reaching

a maximum

up to 623 K, then increasing

extent. When

studied,

that

increasing

at 673 K. Conversion but C5 shows a

at 673 K, and then decreasing.Ci

with the increment

From Figures 2 and 3, it is clear that Zr02/SOi

in temperature.

is a more active

and selective

25

25

500

550 REACTION

Figure

600

500

650 K

TEMPERATURE,

catalyst

for iso-C4 formation

advantageous

properties

The schematic catalyzed

of reaction

deactivation

(cracking) of a highly

unstable

to an increase introduction

however,

products

formation. it would

ion. The increase

of the process

Pathway require

in severity

and

II the formation can be due

acid, or to the

temperature

is increased,

leads to a greater

formation

(C3 and CS). iso-C4 and CS formation

reactions

leading

lower

This is the case of H-mordenite

in the case of C5 the decrease

due to cracking

in the scheme

(C3 and CS formation),

as HCi.

when the severity

in reaction

conversion

and IV are significant,

in severity

to the use of a stronger

values to iso-C4 must be observed.

disproportionation

and also these

in n-alkanes

III,

velocity

with n-C4 because

carbenium

of an acid promoter

where an increase

a maximum;

primary

I,

An increase

of coke precursors

excluded

in temperature,

From this scheme, selectivity

-because

pathways

only pathways

in disproportionation

is virtually

at 893 K.

in Figure 4 Ill/. When using n-C4 as reactant

mild conditions,

leads to an increase

calcined

occur at lower temperatures.

representation

by acids is summarized

under relatively

on Zr02/SOi

from n-C4 than H-mordenite,

being the first one the most important.

probably

K

TEMPERATURE,

1. n-C4 conversion to iso-Cq as a function of reaction temperature on ZrO2z/SO4 calcined at different temperatures: A 773 K, m 823 K, l 893 K and 4933 K

Figure 2. n-C4 conversion to reaction products & iso-C4, ? CS, d C3 and E Cj

catalyst

650

600

550 REACTION

passes through

at high temperatures

to C, products.

of

is

497

0 //. 550

600

650

REACTION Figure 3. n-C4 conversion Figure 2

700

750

TEMPERATURE,

to reaction

products

CRACKING

K on H-mordenite.

Same symbols

as

PRODUCTS A

ISOMERS (iso- Cq)

cc;, II

I n-

ALKANE

t ----cCARBENIUM

ION---~ALKENES(BIJTENES)

In-C,)

ALKYLATES

m CYCLIC

5

OlSPROPORTlONATlON PRODUCTS (C, ,Cs)

ml

T PRODUCTS

(DEACTIVATING

(COKE

AGENTS

)

I

Figure 4. Schematic reaction pathways in alkane conversion acid sites. Taken from 1111.

catalyzed

by strong

498 On the contrary, products

formation

In general, strength

strength

conjugated

acid-catalyzed

From the results of

to use the indicators

When studying

n-hexane

cations,

because

can be considered

the authors

as that of "moderate"

chemisorbed

up to 673 K) and, on the contrary,

the range 773 to 873 K) cracking

a solid superacid.

solid. modified

1131 found that the selectivity

increased

in the number of acid sites defined

strength

acid sites increased

when the number of "strong" and

(i.e., sites that retain NH3 chemisorbed

reactions

were the predominant

STRONG

VERY

ones.

STRONG

I-

I

I

650 DESORPTION Figure 5. Acid strength distribution calcined at 893 (e)

to by

(i.e., sites that retain NH3

6 MEDIUM

into the

the acid strength

on a series of Pd/Y-zeolites

iso-hexanes

"very strong"

(pKa = -13.6)

method to determine

and Dadashev

with the increase

the acid

showed that all the samples were

it is a colored

isomerization

Mamedov

fulfils

better than H-mordenite.

of ZrO$SOi

the basic form of l-Cl-3 nitrobenzene

in the case of H-mordenite,

with different

reaction,

to assume that ZrO,/SO~

acid form. In this way, ZrO,/SOi

It is not possible

and cracking

in the solid acid

for iso-Cq formation

determination

of changing

disproportionation,

that there is an optimum

it is possible

requirements

Acid strength capable

for a certain

activity

iso-CA,

a maximum.

it is considered

required

catalytic

on ZrOZ/SOi, pass through

TEMPERAfURE,K for H-mordenite

(O),

and ZrO,.JSOi

in

499

Figure 5 shows results of acid strength activated

at 773 K and ZrO,/SO~

desorption

method.

In order to differentiate in the different indicated

for H-mordenite

at 893 K using the NH3 adsorption-

of ZrO,/SO~

showed a similar

the types of acidic

temperature

obtained

sites from which NH3 is desorbed

a conventional

range studies,

profile.

classification

1141

in the upper part of Figure 5 was adopted.

NH3 desorption temperature, presents

calcined

Other samples

distributions

from ZrO,/SO~

meanwhile

a larger concentration

than H-mordenite.

shows a decreasing

H-mordenite

displays

of acidic

On the contrary,

of acid sites in the whole

profile with desorption

a roughly

constant

sites with medium

H-mordenite

one. ZrO,/SO~

and strong acidity

shows a significant

range of desorption

temperatures,

concentration

with predominance

of very strong acidity. Considering

that acidic

for the isomerization disproportionation

sites with medium

reaction,

and cracking

reactions,

distribution)

obtained

selectivities

to iso-C4 formation

of medium

for Zr02/SOi

and strong acidity

and strong acidity

are responsible

and that those of very strong acidity acidity

and H-mordenite

profiles

would explain

on these catalysts.

are predominant

the different

On ZrO,/SO~,

acidic

and it is clear that this solid preferably

lower quantities

of C3, C5 and Ci. On the other hand, on H-mordenite

obtained

in similar

amounts,

extent when severity Catalyst

but disproportionation

(temperature)

under mild

products

are

in a greater

increases.

tests with H-mordenite

at 573 K (optimum

diminishes

stabilization

were carried

temperaturesaccording

Figures 6 and 7 show deactivation H-mordenite

tested,

increases

iso-C4 and

deactivation

Deactivation ZrO,/SOi

yields

iso-C4 and disproportionation

(lower temperatures),

sites

(and those of very strong acidity

are negligible),

conditions

catalyze

(acid strength

continuously

value. This behavior

curves

out at 673 K, and those with

to Figures 2 and 3).

for both catalysts.

whereas,

on Zr02/SOi

is similar

The activity

it reaches

for other calcination

of

a temperatures

as shown in Figure 8.

When using no carrier higher partial

pressure

gas, higher activity

differences

volumetric

a loss of SO; during

For this reason, between

Coke content

because

of the

of the reactant.

In the case of ZrO,/SO~, deactivation.

values are obtained

SOi contents

the run could be the cause of

were analyzed

after each run, but no

initial and final values were observed.

on the used catalysts

equipment.

Results obtained

was analyzed

by using a combustion

with both catalysts

after the 4 h run are

shown in Table 2. H-mordenite consisting

is a crystalline

of parallel

alumino-silicate

tubes with an elliptic

with a porous structure

section

of 6.95 and 5.81 i in

500

TABLE 2 Coke deposition

on H-mordenite

and ZrO,/SO~

Activation temperature,

Catalyst H-mordenite

ZrO,/SO~

(*)

(**)

Activation:

after 4 h on stream

Reaction temperature,

K

Carrier K

gas

Coke, wt%

773 773 773

673 673 673

H2 N2

5.40 3.41 3.85

893 893 893

573 573 573

H2 N2

0.60 0.10 0.31

(*) in H2 flow;

(**) in air flow

25

c

_k n

0

0

2

1

n

3

4

TIME,h

TlME,h

Figure 6. n-C4 conversion to iso-C4 on ZrO /SOi calcined at 893 K. Reaction temperature 573 K. Pure n-C4 (a f , n-C4 + H2 (I) and n-C4 + N2 (A) Figure 7. n-C4 conversion to iso-C4 on H-mordenite activated temperature 673 K. Same symbols as Figure 6

diameters behavior

1151. This particular in reactions

porous structure. mordenite,

occurring

porous structure on H-mordenite

Low coke contents

becoming

are enough

the inner surface

at 773 K. Reaction

allows a different

catalytic

or other solids with a wider to block mouth

inaccessible

to reactant

pores of Hmolecules.

In

501

0

0

2

I

4

3

TIME

5

, h

Figure 8. n-C4 conversion to iso-Cq on Zr02/SOz at a reaction temperature 573 K for different calcination temperatures. Same symbols and operational conditions as Figure 1

this way, catalyst

activity

diminishes

drastically.

It is clear from Table 2 that coke deposition ZrO,/SO~

than on H-mordenite.

is observed poisoned

levels were very much lower on

The small amount of coke is probably

the very strong acid sites, blocking

of

them. In this way, a decrease

produced

on

in activity

during the first 3 h. After this time, strong acid sites are

stopping

coke formation,

since then the catalyst

activity

remains

constant. On both catalysts, Differences comparable. difference

Since coke formation of 100 K in reaction

coke contents reaches

H2 presence

in coke deposition

were produced

a maximum

partially

inhibits

coke formation.

levels for both catalysts is strongly

conditions

are not completely

dependent

on temperature,

could affect

the results,

at temperatures

where the isomerization

a but these

activity

for each catalyst.

ACKNOWLEDGEMENTS The authors wish to thank L.M. Krasnogor analysis,

and to M. Mahieu

for acid strength

for acidity

measurements

determinations.

and sulphate

502

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9 10 11 12

:: 15

F.E. Condon, in P.H. Emmett (Editors), Catalysis, Vol. 6, Reinhold Publ. Corp., New York, 1958, p. 48. K. Tanabe and H. Hattori, Chem. Lett., (1976) 625. M. Hino and K. Arata, Chem. Lett., (1979) 1259. M. Hino and K. Arata, J.C.S. Chem. Commun., (1979) 1148. M. Hino and K. Arata, J. Amer. Chem. Sot., 101 (1979) 6439. M. Hino and K. Arata, J.C.S. Chem. Commun., (1980), 851. K. Arata and M. Hino, React. Kinet. Catal. Lett., 25 (1984) 143. C. Bearez, F. Chevalier and M. Guisnet, React. Kinet. Catal. Lett., 22 (1983) 405. M. Guisnet, F. Avendano, C. Bearez and F. Chevalier, J.C.S. Chem. Comm., (1985) 336. C. Bearez, F. Avendano, F. Chevalier and M. Guisnet, Bull. Sot. Chimique France, 3 (1985) 346. G.A. Fuentes and B.C. Gates, J. Catal., 76 (1982) 440. D.A. Weitz, J.C. Yori and S.M. Caula, Lat. Amer. J. Chem. Eng. Appl. Chem., 16 (1986) 263. S.E. Mamedov and B.A. Dadashev, Kinet. Catal., 26 (1985) 204. Y.G. Yergiazarov, B.N. Isayev, M.F. Savchits, L.L. Potapova and S.Y. Radkievich, Petrol. Chem. U.S.S.R., 17 (1978) 209. P.E. Eberly and C.N. Kinberlin, J. Catal., 22 (1971) 419.