Demethylation of toluene by reaction with steam under pressure

DEMETHYLATION OF TOLUENE BY REACTION WITH STEAM UNDER PRESSURE* G. L. RABI~OVICH, G. ~ . M~SLYA~SKII, V. S. VOROB'EV, L. ~ . BIRYUKOVA

and I. I. IOFFE All-Union Scientific Research Institute for Petrochemical Processes, Leningrad

(Received 11 ~lay 1972) DEMETHYLATIOI~ of toluene with steam using an alumino-rhodium catalyst was studied at a temperature of 400 to 480 ° at atmospheric pressure [1, 2]. Benzene is formed by the reactions: C6HsCH3Jr-H20 -~C~He+ CO + 2Hz ,

(1)

C6H~CHs~- 2H~O -*Cell6 ÷ CO2~-3H~ .

(2)

Reaction (2) essentially reflects the overall reaction (1) and the conversion of CO CO-t-HIO ~CO2~H2.

(3)

I n addition to some demethylation, toluene is completely decomposed during reaction with steam C6HsCH3-F 14HsO-*7COz~ 18H~ ,

(4)

C6H,+ 12HsO -~ 6COl-l- 15HI •

(5)

The presence of small amounts of methane in gaseous products of toluene conversion (1-4 vol. °/o) suggests t h a t conjugated hydrodemethylation of toluene m a y take place C6H~CH3-FHi -* C6He+ CH,.

(6)

From a practical point of view toluene should be demethylated with steam at increased pressure. Results are given in this paper of investigating this reaction at a pressure of 5 and 10 arm. A modified alumino-rhodium (0.6~ Rh) catalyst was used which has higher selectivity in demethylation of toluene than the catalyst used previously [1, 2]. EXPERIMENTAL

Experiments were carried out in a continuous pilot plant intended for use at increased pressure [3]. Toluene and distilled water were pumped into the evaporator, from which the steam-gas mixture entered a reactor (internal * Neftekhimiya 13, No. 4, 518-522, 1973. 155

156

(;.

diameter catalyst

36 ram) provided was

used

with

4 0 c m a, a t a s p a c e

L.

with a space

velocity

I,ABINOX

The experiments

at a temperature

of 435:465

et al.

an electrically velocity

of 4 hr-'.

was maintained.

ICH

heated

aluminium

of toluene

A molar

were carried

feed

ratio

of

unit.

l to 2 hr-'

of water

: toluene

o u t at, a p r e s s u r e

of 5 and

a n d 4 9 5 °. T e m p e r a t u r e

8 0 c m :~

was measurc(l

of

and six

10 a r m

bv therm(,

I ) E M I ~ ' T I t Y L A T [ O N OF T O L U E N E "WITH STEAM[ A!P I N C R E A S E D P R E S S U R b ]

M o l a r r a t i o o f w a t e r : t o l u e n e :--6 ....... Number o f oxperiment

!. . . . .

~(

i Sp~ce velocity i of ' i t'°lu'enc fee(l, h r

~iT e l n p , w-

. Bmtzel~eyield,~nol.°~_)i

i ature. ! ~)(, i *:

G a s c o m p o s i t i o n , vol. o~)

in t e r m s of the

in t,e r m s i of the ]

t,ohu,m,

toluene',

passed through

converted

i H~

CH~

CO

I CO2 i i ]

:

C, -C.~ I i

Pl'(!SgIII'O 5 ~/~Ill

1 2 3

1 1.3 "2

4

4

5 (i 7 8 9 10

I 2 4 1-3 "2 4

11 12

1 1.3

13

'2

14 15 16 17 18 19 20 21 2'2

4 I 1.3 '2 4

435

3(i.2 30.S 2o.6 16.6

98.2

4(i5

54.5 37.3 30-5 76.3 5(.t.7 45.6

93.4 94.3 96.5 90.3 94.3 96.4

495

Pressure 435

465

1 1.3 2 4

495

32.8 28.8 ."94. . ') 15.8 53.2 46.~i 42.2 28.3 74.1 58.8 47.7 45.5

96.2 96.7 97.3

64-1 68-5 69.4 70.9 63.3 66.0 72.5 58.6 62.5 71.l

9.8 5.8 5.5 2.4 10 7.2 3-4 13.7 9.4 5.5

0.45 0.35 0-3 0.4 0.8 0.7 0.4 1.6 1.5 0.3

25.5 0-15 25.2 0.15 24.6 0-2 26.3 (! 25.6 0-3 25.8 0.2 23.6 ] 0-1 1 25.9 ! 0-2 26.2 i).4 23.1 0

64.2 62.6

10'2 10.2

0.3 0.3

68.2

6

O.1

71.1 57.3 60.8 64'5 69.8 51.9 55'4 57.9 66.8

5.5 16.2 13.1 8.9 6 19.5 16.6 14.5 8

0.3 0-5 0-5 0'5 0.4 1.2 1-3 0"8 0-8

25 26-7 25.6 23-1 25-6 25-4 25"8 23.7 26'6 26.1 26.6 24.3

10 a r m 94.9 .(t5.7 ('~ ,} A-l 98.0 92.0 9'2.1 95.6 95-5 88.1 88.6 89.6 93.6

0.3 0'2 O. I 0 0.5 0.3 0.3 0.1 0.9 0.6 0.4 0.1

couples arranged in a container along the axis of the reactor and i,1 the unit. A given experimental temperature was maintained at the inlet in the catalyst layer by the indications of the thermocouple arranged in the container of the reactor. Each experiment lasted 20-24 hr. Liquid and gaseous products were analysed chromatographically, as in previous studies [1, 2]. Experimental results are tabulated.

Demethylation of toluene

157

RESULTS AND DISCUSSION

I t follows from the Table t h a t toluene is demethylated at increased pressure and at atmospheric pressure. According to conditions, benzene yield in relation to toldene supplied varies between 15 and 76 mol.%, while yield in relation to toluene reacted (selectivity), between 88 and 98 mol. % (see Table). A variation of pressure between 5 and 10 arm has little effect on the rate of toluene conversion. Only with a degree of conversion higher than 60% (temperature 495 °) does an increase in pressure cause some reduction in the rate of toluene conversion (Fig. 1).

8o~-

11~'~I-

0

0.25

0.50

-~' 485°

0.75 1.00 I ,hr -~

Fic~. 1. Effect of temperature ancl rate of toluene flow on the degree of conversion at pressures of 5 and 10 arm: ©, lk, []--pressure 5 arm; Q, A, 1--pressure 10 arm. mole % /00 ~ ~.

I

I

I0

20

I

I

30 40 50 80 70 Degree of conversion, %

80

Fro. 2. Relation between the selectivity of demethylation of toluene and the degree of conversion: 1--5 arm; 2--10 arm. The selectivity of demethylation is determined to a first approximation by the degree of toluene conversion. An increase in the degree of conversion reduces selectivity in an almost linear manner (Fig. 2). To plot a curve, results of experiments were used which were obtained at different temperatures and rates of reagent flow. I t follows from a study of the curve t h a t the select i v i t y of demethylation of toluene is determined not only by the degree of conversion, but also by pressure. With the same degree of conversion selectivi t y at 5 arm is higher t h a n at 10 arm. Thus, with a 50% degree of ~oluene

158

G . L . RABI~OVI(.:g et (H.

conversion at a pressure of 10 and 5 arm selectivities of 92.5 and 95 mol.(~, were achieved. With a reduction in the degree of conversion, selectivity of" demethylation at 5 and l 0 arm increases and with extrapolation to a limiting low degree of conversion both curves intersect at a point equiwHent to ] 00°.', selectivity. The breakdown of toluene with steam (4) has no marked role whel~ using the modified catalyst recommended in this study. When examining the composition of gaseous reaction products the considerable methane content is significant and in some experiments reaches l(i .... 20 tool.% (see Table. experiments 19 and 20). ~Vhen the reaction was carried out using this catalyst at atmospheric pressure, the amount of methane in gaseous products did not exceed 4 mo].°'~,, as shown previous]y Ill]. rno/es

~..

3oo

2 0 6 2 '80

""-" ...2~-o

•~ 2 2 0 I

•

-

--

002 * CO

I i

£1 ;

o

2 10

20 30 flegpee o f c o n v e m / o n ,

OH~ %

~8

FIo. 3. Relation between the yield of gaseous products (for 100 moles toluene reacts,d) and the degree of conversion: 1--5 atm; 2--10 arm.

Figure 3 shows a variation of gaseous reaction product yields on the basis of 100 mole toluene reacted, according to the degree of conversion. For plotting the graph results of experiments carried out at a temperature of 435 ° and pressures of 5 and 10 arm were used. Extrapolation of curves to a limiting" low degree of conversion results in a ratio of stoichiometric coefficients of reaction products, which almost precisely corresponds to reaction (2). This fact is additional evidence of the view t h a t demethylation of toluene with steam is essentially the sole primary reaction observed under the conditions studied. Carbon monoxide content in gaseous products at 435 ° is below 0.5 tool. ~/o

D e m e ~ h y l a t i o n of toluene

159

while the CO : COz ratio is not higher than 0.02 even with short contact times (see Table, experiments 1-4 and 11-14). I t should be noted that CO content in the entire range of temperatures studied is much lower than on carrying out the reaction at atmospheric pressure [1]. The rate of conversion of CO to C02 (3) under pressure, apparently, increases markedly. On increasing pressure from 5 to 10 atm methane yield increases and hydrogen yield decreases (Fig. 3). An increase in overall pressure and accordingly, partial hydrogen pressure obviously accelerates the conjugated reactions of hydrodemethylation of toluene (5) and hydrogenolysis of the benzene ring (7, 8) Cell8 ~ 9 H z - ~ 6 C H 4 ,

(7)

CeHsCH3-}- 10H~ ~ 7CH4 •

(8)

The high activity of the alumino-rhodium catalyst in hydrodemethylation and hydrogenolysis of toluene has been indicated previously [2]. I t is very likely that a reduction in the selectivity of demethylation of toluene with an increase in the degree of conversion and increase in pressure is at least partly due to hydrogenolysis of the ring. The possibility of methane formation due to hydrogenation of CO cannot be ruled out either. During the interaction of toluene with steam using an alumino-rhodium catalyst at increased pressure, in addition to demethylation of toluene and benzene conversion with steam, conjugated reactions of hydrodemethylation and hydrogenolysis take place. The main conversions of toluene under these conditions may be shown by the system: .........

~.o

HzO

.........

H20

C6HsCH3 ----> C6I-h ---> H2, CO~,CO H2 Hz CsHsCH3 ~ C6I-h ---> CH4 ? H, ?

SUMMARY

1. During demethylation of toluene with steam with an alumina-rhodium catalyst at temperatures of 430-500 ° an increase in pressure to 10 arm increases the rate of conjugate reactions with hydrogen, particularly hydrodemethylation of toluene. 2. In the temperature range studied an increase in pressure from 5 to 10 arm has no marked effect on the rate of toluene conversion. 3. The selectivity of demethylation at constant pressure, in the first approximation, is determined by the degree of conversion. An increase in pressure from 5 to 10 atm causes a reduction in selectivity. When the process is carried out at a pressure of 5 arm, selectivity of demethylation is 95% with a degree of toluene conversion of 500/0.

160

(~. L. ['~ABINOVICH Ct al.

REFERENCES ]. L. M. TICEIGER, G. L. RABINOVICH a~d G. N. MASLYANSKII, Neft(~khimi~ 12. 29, 1972 2. G. L. RABINOVICH, G. N. M A S L Y A N S K I I a a d L. M. T R E I G E R , Kin(~tika i kataliz 12. 1567, 1971 3. N. R. B U R S I A N and G. N. MASLYANSKII, Khinl. prom-sC, No. 3. 166, 196]