Dehydration-dehydrogenation_of_2-propano.pdf

React. Kinet. Catal. Lett., Vol. 53, No. 2, 397-404

(1994)

RKCL2494 DEHYDRATION-DEHYDROGENATION OF 2-PROPANOL AS A MODEL REACTION FOR ACID-BASE CHARACTERIZATION OF CATALYSTS M.A. Aramendfa, V. Borau, C. Jim4nez, J.M. Marinas, A. Porras and F.J. Urbano Departamento de Qufmica Org~nica, Facultad de Ciencias, Universidad de C6rdoba, Avda San Alberto Magno s/n, E-14004 C6rdoba, Spain

Received Accepted

March 31, 1994 April 20, 1994

The relationship between the acid-base properties of various solids, as determined by a spectrophotometric procedure and the apparent dehydrogenation and dehydration rate constant

(KI)

(K2) of 2-propanol

have been studied by using a c0ntinuous-flow reactor.

INTRODUCTION The acidity and basicity of solid catalysts are two influential factors on their activity and selectivity, not only in typical acid-base reactions, but also in many others involving redox transformations. Acidic solids have been widely used as catalysts in various industrial processes including cracking, alkylation,

isomerization and a variety of organic

syntheses. On the other hand, basic and amphoteric solids have received comparatively much less attention even though they have aroused growing interest lately. There are a number of methods for characterizing acid and base sites, most of which were recently reviewed [1,2]. In broad terms, they can be classified as titration, spectroscopic and reaction test methods.

The determination of acid-base pro-

Akad4miai Kiad6, Budapest

ARAMEND~A et al.:

2-PROPANOL

perties of solid catalysts has been addressed by using a variety of model reactions

involving t-butanol

[4] or isopropyl alcohol

[3], methylbutynol

[5,6] as the starting substrate.

catalyst acidity is related to its ability to dehydrate propyl alcohol, whereas dehydrogenation

iso-

is generally accepted

to require the presence of both acidic and basic sites. authors have found correlations

The

Some

between the dehydrogenation-to-

dehydration rate ratio and the number of basic sites [6]. In this work we studied the potential correlation between the acidity and basicity of various

solid catalysts

(Mg02,

Zr02, an 80:20 w/w SiO2/AIPO 4 mixed system called PM2, and sepiolite,

a natural support)

as determined by a spectrophoto-

metric procedure previously developed by the authors the dehydrogenation 2-propanol

(k I) and dehydration constant

in a continuous-flow

range of 200-350

[7] and

(k2) of

reactor over the temperature

~

EXPERIMENTAL Catalysts M a g n e s i u m oxide was prepared 5870) by calcination ture to 600 ~

from Mg(OH) 2 (Merck ref.

in a ceramic crucible

at 4 ~

(from room tempera-

and then at 600 ~

for 2 h), after

which the solid was allowed to cool back to room temperature. The zirconium oxide was synthesized chloride

from zirconium oxy-

(Merck ref. 8917) by preparing a zirconium hydroxygel

[8] and calcining

it using the same temperature

program as for

magnesium oxide. Catalyst BM50 was obtained by suspending a mixture of 29.0 g of Mg(OH) 2 and 0.35 g of B203 in 200 mL of distilled water and immersing

it in an ultrasonic bath for 1 h. Then,

pension was dried in a stove at 120 ~

the sus-

for 2 h and calcined

similarly as the previous two catalysts.

The final Mg/B ratio

obtained was 50:1. The procedure

used to synthesize catalyst PM2 was de-

scribed in detail elsewhere Finally, 398

[9].

the sepiolite was supplied by Tolsa S.A. and

ARAMEND~A et al.: 2-PROPANOL drawn from the company's ores in Vallecas

(Madrid, Spain). The

chemical analysis of the parent natural sepiolite as follows:

62.0% SiO2, 23.9% MgO, 1.7% A1203,

(85 wt.%) was

0.5% Fe203,

0.5%

CaO, 0.6% K20 and 0.3% Na20. The weight loss from 293 to 1273 K was 10.5%. The oxides are parts of the clay structure

(none is

in free form). The material contains smectite, quartz and dolomite as major impurities.

The sepiolite was designated by PS400,

where the subscript denotes the calcination temperature used (in ~ Table 1 summarizes lysts tested,

the chemical composition of the cata-

the precursors used in their synthesis,

calcination temperature,

specific surface area

termined by means of a Micromeritics ty and basicity.

and their

(SBET) as de-

ASAP 200 instrument,

acidi-

The last two were determined by using a spec-

trophotometric procedure described elsewhere [7], which allows the amount of irreversibly adsorbed acidic or basic compound used as titrant for basic and acid sites, respectively, titrated.

The equilibrium monolayer coverage at 25 ~

~mol g-l) was determined

to be X m (in

from the Langmuir adsorption isotherm

and was taken as a measure of acidic and basic sites corresponding to the specific pK a of the base or acid used as titrant. Table 1 Chemical nature, nomenclature,

specific surface area,

acidity and basicity of the catalysts tested Catalyst

Precursor

Tca I (0C)

SBE T (m2g -I

Acidity (~mol g-l)

Basicity (~mol g-l)

PM2

AICI3.6H20

650

402

21.7

3.4

BM50

Mg(OH)2/B203

600

104

2.8

98.4

PS400

Sepiolite

400

121

8%9

7.5

MgO

Mg(OH) 2

600

119

2.0

172.7

ZrO 2

ZrOCI2.8H20

600

220

3.2

18.4

399

ARAMEND~A

et al.:

Experimental

2-PROPANOL

set-up

We used a c o n v e n t i o n a l working

at a t m o s p h e r i c

reactor

consisted

continuous-flow

pressure

temperature

out the e x p e r i m e n t s walls.

in a tubular electric

was c o n t r o l l e d

to w i t h i n

by means of a t h e r m o c o u p l e

mixing

and complete

with the catalyst bed. were t r a n s f e r r e d

The p r o d u c t s

emerging

to a Sensorlab VG mass

changes w i t h time.

The selected masses were

58

and 102

(acetone),

(di-isopropyl

Catalytic sional effects

al variable

41

(propene),

processes

values,

size.

18

processes,

al-

(water)

gas flow-rate

and acetone

kinetic

operation-

and carrier gas flow12 mL h -I and

120 mL min -I

(both were

control).

effects, fulfills

treatment

or exter-

the space v e l o c i t y

The feed flow-rate was

[101

the c o n v e r s i o n

of

the r e q u i r e m e n t

of

for first-order

viz. in ( Ii~

the absolute

and k the a p p a r e n t

appropriate

as regards

of d i f f u s i o n a l

to propene

where X denotes

that al-

(isopropyl

layer and internal

in order to avoid d i f f u s i o n a l

the B a s s e t t - H a b g o o d kinetic

45

2 (hydrogen),

by selecting

particularly

(nitrogen)

In the absence 2-propanol

from the reactor

alcohol).

from the b o u n d a r y

and particle

optimized

prior to contact

of 16 mass peaks and gave their

(controlled through the catalyst w e i g h t

the carrier

to its

The cata-

runs were c a r r i e d out in the absence of diffu-

nal mass transfer

400

attached

spectrometer

monitoring

cohol),

through-

in order to ensure

vaporization

lowed s i m u l t a n e o u s

tant.

~

two layers of glass wool and the rest

of the reactor was p a c k e d w i t h glass beads homogeneous

The

furnace.

•

Fresh c a t a l y s t was used in every experiment.

lyst bed was held b e t w e e n

rate)

temperature.

of a piece of Pyrex glass of ii0 m m length

and 12 m m ID that was placed The reactor

fixed bed reactor

and constant

) = kT

conversion,

dehydrogenation

(i)

T temperature

or d e h y d r a t i o n

(in K )

rate cons-

ARAMEND~A

RESULTS

1 shows

the variation

tion and dehydration of time

catalyst

PM2 at various

for isopropyl

(a), as well as several

As can be seen,

for a reaction

temperatures

the catalyst

so all subsequent

2 gives

time of 32 min

obtained

dehydrogenaalcohol

as a

profiles

for

(b).

of kinetic

in the pro-

parameters

were

(the first point plotted),

gas flow-rates

the apparent

tion rate constants

reaction

was deactivated

calculations

once the feed and carrier Table

of the apparent

rate constants

function

done

2-PROPANOL

AND DISCUSSION

Figure

cess,

et al.:

had stabilized.

dehydrogenation

with various

and dehydra-

catalysts

at differ-

ent temperatures. Table Apparent constants

dehydrogenation

(k I) and dehydration

(mol atm -I g-I s-l) various

T

2

for 2-propanol

(k~) rate

obtained

with

catalysts

Catalyst

(OK)

PM2 kI

BM50

PS400

MgO

k2

kI

k2

kI

k2 2.17

ZrO 2

kI

kI

k2

0.13

0.25

3.25

473

0.20

3.65

0.21

1.02

0.23

523

0.20

4.41

0.19

1.30

0.21

3.01

0.05

0.23

0.22

3.31

573

0.09

4.92

0.16

1.61

0.17

4.73

0.06

0.44

0.17

4.32

623

0.08

5.11

0.15

1.66

0.13

6.33

0.06

0.58

0.15

4.82

Table

3 shows the results

bove apparent density,

rate constant

and the temperature

obtained

values

0.ii

k2

by correlating

(fitted to the equation

+ BDac + CDba + E), as well as the corresponding all significances As expected,

and correlation

site

Y = AT +

percent

over-

coefficients.

there was good correlation

the acid site density,

the a-

to the acid and basic

between

k 2 and

but not with the base site density.

401

A R A M E N D ~ A et al.:

2-PROPANOL

i

i

i

i

Ca)

~3 r0

0

00

60

50

I

100 150 Time (rain)

I

200

I

250

I

(b) ,, o

250oC

v -

350 ~ C 400 ~ C

300oC

4o co _

k-

0

0

20

Fig.

402

i.

I

I

50

100

I

150 ~me (rain)

I

200

(a) Variation of the apparent dehydrogenation (k I) and dehydration (k 2) rate constant of isopropyl alcohol with time. (b) Deactivation profiles for catalyst PM2 at four different temperatures

ARAMEND~A et al.: 2-PROPANOL

Table 3 Overall significance

(Sover), regression coefficient

correlation coefficients dehydration determined density

for the apparent dehydrogenation

(k 2) rate constants from eq.

and the kl/k 2 ratio

(i), with temperature

(Dac) and basic site density

parentheses

(r) and (kI) and (R) as

(T), acidic site

(Dba). The figur@s in

are the corresponding partial significances

Y

A (x 104 )

kI

-7.9(21)

-0.5(50)

1.3

(91)

0.7

0.89

98

k2

i01.0(50)

38.9(99)

3.3

(12)

-4.1

0.96

99

-8.6 (24)

-1.9(30)

4.3

(95)

0.9

0.86

90

R

Also,

B (x 102 )

C (x 102 )

the percent dehydration

E (x i0)

r

S

over

increased with increasing tempe-

rature. As regards kl, it was influenced by both acid and base sites; however,

the latter had a positive effect, whereas

former influenced this constant adversely. calculated as the kl/k 2 ratio,

the

In relation to R,

this parameter was more signifi-

cantly influenced by basic sites than was kl; therefore,

R can

be used instead of k I to obtain a correlation with the catalyst basicity,

even though the resulting correlation coefficient

will be somewhat smaller. It is interesting to emphasize

the adverse influence of

the temperature on k I and R, which results in an increase in the percent dehydrogenation with decreasing The results provided by the catalysts to be used in hydrogen transfer reactions,

temperature. tested allow them one of our group's

goals in forthcoming work. Acknowledgements.

The authors gratefully acknowledge

financial

support from the Consejerfa de Educaci6n y Ciencia de la Junta de Andalucfa and the Spanish DGICyT for the realization of this work in the framework of Project PB92-0816. 403

ARAMEND~A

et al.:

2-PROPANOL

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