Catalysis by sulfur-promoted superacidic zirconia: Condensation reactions of hydroquinone with aniline and substituted anilines

Chrmicul Enginrering Science, Vol. 44, No. Pnnied in Great Bntain

I I. pp. 2535

2544,

53.00+ 0.00 oW9 2509:89 i 1989 Pergamon Press plc

1989.

CATALYSIS BY SULFUR-PROMOTED ZIRCONIA: CONDENSATION HYDROQUINONE WITH ANILINE ANILINES P. S. KUMBHAR Department

of Chemical [Rrcriwd

Technology, 10 Octobrr

SUPERACIDIC REACTIONS OF AND SUBSTITUTED

and G. D. YADAV’

University

of Bombay,

Matunga,

1988, accepted for publication

Bombay

14 April

400019,

India

1989)

Abstract~Industrially

important condensation reactions of hydroquinone with aniline and substituted out with a reusable and easily separable heterogeneous catalyst, namely, sulfurpromoted superacidic zirconia. Detailed kinetic data for these reactions are reported. A comprehensive model accounting mass transfer and kinetic resistances was used to interpret the data. The reactions were found to be overall second-order (first-order with respect to each component). The activation energies for both steps of the reaction between hydroquinone and aniline, and for the first step ofthe reaction with o-toluidine were found to be 89.40, 60.31 and 46.94 kJ/mol, respectively. The order of reactivity for substituted anilines was anilines

were

p-toluidine,

carried

aniline 1 o-toluidine.

A MODE

INTRODUCTION

In recent years interest in sulfur-promoted

zirconia

as

a superacidic heterogeneous catalyst has greatly increased. Several reports have appeared in the literature dealing with its usefulness as a powerful catalyst for various diversified reactions (Hino and Arata, 1980, 1985; Chaudhari and Rajadhyaksha, 1987, 1988). It is known that the condensation reactions of hydroquinone with aniline and substituted anilines are carried out industrially for the production of N,N’diary-p-phenylenediamines, which are used as antioxidants in rubber. Conventionally these reactions are carried out by using non-reusable catalysts such as ptoluenesulfonic acid (PTSA), ZnCl,, AlCl, etc. under superatmospheric pressures (Table 1). However, in some multiproduct industries using the same battery of equipment for different products, these reactions are carried out at atmospheric pressure and high temperatures. Nevertheless no detailed kinetic studies are reported in the literature. There is also a dearth of information on the use of superacidic zirconia as a catalyst. Consequently, studies were undertaken on catalysis by superacidic zirconia in the condensation reactions of hydroquinone with aniline and methyl-substituted anilines. These reactions are a combination of series and parallel steps proceeding via an intermediate p-hydroxyphenyl-aryl amine. Since these reactions are solid-catalysed slurry type reactions, a series of resistances come into the picture and it becomes necessary to find out the controlling regime under operating conditions, as delineated in what follows:

+ Author

to whom correspondence

FOR SOLID-CATALYSED

A(l)+

ZB(l)

cata’ys’w

p products

(1, s or g)

(4

Transport of A from catalyst surface.

W

Transport of B from bulk liquid catalyst surface. Intraparticle diffusion of A and catalyst pores. Adsorption of the reactants on the the catalyst. Reaction between A and B at the face. Desorption of the products from surface to the bulk liquid.

Cc)

(4 Cc) (f)

(1)

bulk

liquid

phase

to

the

phase

to

the

B within active

sites of

catalyst the

the

sur-

catalyst

In the present case dropping the adjuvant (1) for both A and B and noting that it is a complex reaction, it can be written as

C+ZB*D+H. 2535

REACTIONS

where Z is the stoichiometric coefficient of B. In the present case two liquid (organic) phase reactants react on the surface of the catalyst to form liquid phase (organic and aqueous) products at the reaction temperatures chosen. A number of steps are involved in the scheme:

A+ZB-

should be addressed.

SLURRY

study the intrinsic kinetics of these reactions a model was proposed to discern the controlling mechanism which is described here. Some models for gas-liquid-solid systems known slurry are (Doraiswamy and Sharma, 1984). These type of reactions can be represented as To

k,,

C+H

(2)

(3)

P.S.

KLJMBHAR

andG.

D. YADAV

The intrinsic kinetics of the reaction can be further modelled, depending on what processes occur at the catalyst surface. Several models exist in the literature, of which two models are of immediate concern: (a) the Langmuir-Hinshelwood-Hougen-Watson (LHHW) model, and (b) the familiar power law model used in homogeneous reaction kinetics. Here, a power law model was considered for the surface reaction as was evident from the preliminary set of experiments. Consequently, the rate equations were written for the general reaction represented by eq. (1). They could also be written for the LHHW model. If intraparticle diffusion resistance is assumed to be predominant, the effectiveness factor would come into the picture. In the present case it is assumed that there are no pore diffusional limitations as the particle size chosen for the present study was very small. The particles were sieved through a lOO-mesh sieve and the maximum size corresponds to 160 pm, for which the calculated Thiele modulus (4) was 1.05 x lo- 3, which corresponds to an effectiveness factor (r]) of 1. This is further confirmed by the high values of the activation energy typical of a surface reaction controlled regime. If the concentrations of both A and B are nearly equal and the rates of transport of A as well as B are limiting, then steps (a), (b) and (e) are the only ones to be considered. Under certain conditions, even though is much smaller [B,]> [A,], [B,] # [A,], if D, than D,, then the solid-liquid mass transfer coefficients of A and B would be different, and are designated as k SL--A and ksL-B7 respectively. Furthermore, reaction (3) is assumed to be first-order with respect to both A and B. The concentration profiles for such a system are as shown in Fig. 1. The steps involved in the transport of A and B through the diffusion film next to the catalyst surface are as follows:

(I)

dtjiusion from R,

=

RB =

bulk liquid to catalyst

k+.a,(C4J

surface:

-C41)

(4)

ksL-Ba,(C~,l -C&l)

(5)

and (2) reuctiun

at the catalyst R,, =

surface:

k,a,C~,l

C&l.

(6)

If intraparticle diffusion is important eq. (6) would contain the effectiveness factor on the right-hand side. From eq. (4):

t~sl=C.4-k

TAa .

sl.

A I)

(7)

At the steady-state: R,

=

ZR,.

(8)

Catalysis by sulfur-promoted

Catalyst surface

Zk,,-,k,,-,a~CA,lCB,l 1

IB,l

--R,

IA,1 Liquid film Surrounding catalyst (A)

2537

superacidic zirconia

1

)

( Zks,--,a,C&l +ks,-,&4,1

+l

=o.

In this situation the reaction rate will be governed by the transport of either A or B or both A and B to the catalyst surface. The various cases are as follows:

surface

(1) [A,] = 0, [B,] is finite, and the rate is governed by the transfer of species A when

C&lksL--I 4 ZCklksL--8. This is comes

shown

in Fig. R,

(2)

(C)

[B,]

(El

Fig. 1. Concentration profiles for the reaction between two liquids reacting on the catalyst surface: (A) generalised case, (BHE) various subcases.

Equation

(14) (13)

of species

Z

C&l k,,-,

(15)

C41.

B when

*C&l

=

in Fig. l(C). Equation

(13) then becomes

k-,a,CBol.

= Z

(17)

(18)

CA.lkx--A = ZCKJks,-,. This is shown

in Fig. l(D).

Equation

(13) then becomes

Substituting vahres of [A,] rearranging the terms gives

a;C41* -RA(k,,o,ZCAol)+l

-C41)

(9)

[S,]

in eq. (6) and

+kwqLAJ

+h+LXW

This is a quadratic quantities. The different follows.

)

+I =o.

(11)

in R, in terms of known cases that are possible are as

4,

Surface reaction very rapid The surface reaction is very rapid when the following condition is satisfied:

(12) this condition,

eq. (11) becomes

a,

1

CA,1t&J

% i,,l,cA,7

(201

C&J. when

+

1

k-s

EB,l :

(21)

In this case the reaction rate will be equal to the rate of the surface reaction and mass transfer resistances will be unimportant. Also, [A,] and [B,] will be practically equal to [A,] and [II,] [Fig. l(E)]. Hence eq. (13) becomes 1

(a)

For

(19)

C4J2 = 0

(b) Surface reaction very slow The surface reaction is very slow

k, 1

- ks, aP

=.a R,=

(101

a . .w. B p and

(R,

1

1

=o.

Rearranging:

-W+sa,(CfCl

-C&I-_ZkRt

(16)

6 CAolk~--A.

Ri

R,

be-

(3) [A,] = 0, [B,] = 0, and the rate is controlled by the transfer of both A and B to the catalyst surface when

%

eq. (5) becomes

then

is finite, and the rate is controlled

by the transfer

R,

(D)

t(B).

= &-,a,

= 0, [A,]

This is shown

Therefore

(13)

--R‘4

k, up C&I

CXI

+l=O

>

-

R, = k,a, CA.1C&l.

(22)

Using eqs (12), (14), (16), (18) and (21) one can determine the regime of reaction. These equations require values of solid-liquid mass transfer coefficients for both the species and the surface reaction rate constant.

P. S. KUMBHAR

2538

and G. D. YADAV

By studying the effects of various parameters on the rate of reaction one can discern the controlling step. If the reaction is kinetically controlled the rate of reaction will not be affected by the speed of agitation while catalyst loading will have a pronounced effect on the rate of reaction. Hence, for the study of the true kinetics of the reaction under consideration, the effects of various parameters on the rate of reaction were studied. Also, various values appearing in the equations for the model were determined (k,a, experimentally and ks, theoretically) and used in the model equations to make the regime of operation sure.

EXPERiMENTAL

Chemicals Hydroquinone, aniline, o-toluidine and xylene of AR grade (99.0% pure) obtained from firms of repute were used. p-Toluidine of technical grade (98% pure) was used without further purification.

Catalysts The pure and superacidic zirconia were prepared by the method described by Hino and Arata (1980). ZrOCl, 8H,O was dissolved in deionised water followed by precipitation to zirconium hydroxide using aqueous NH, (25% solution of ammonia in water). The zirconium hydroxide was washed with deionised water till a neutral filtrate with no chloride was obtained (phenolphthalein and AgNO, tests), and was dried at 110°C for 24 h. The dried zirconium hydroxide was crushed below 100 mesh (average particle size 160 pm). Pure zirconia was obtained by calcining the dried hydroxide at 650°C in a Pyrex tube for 3 h, whereas superacidic zirconia was obtained by impregnating the dried zirconium hydroxide with sulfuric acid followed by calcination. The impregnation was performed by immersing the dried hydroxide in 1 N H,S04 in the ratio of 1 g zirconium hydroxide to followed by drying (1.5 h) and 15 cm3 of H,SO, Cal&nation at 650°C for 3 h. Both catalysts were kept in sealed air-tight tubes and used for the reactions after drying at 110°C for 1 h. The surface area and pore size distribution of these catalysts were measured by using a mercury porosimeter (Micromeritics-Autopore IT, Model 9220) and are summarized in Table 2. p-Toluenesulfonic acid monohydrate of AR grade was used.

Table

2. Characteristics

Experimental set-up A schematic diagram of the experimental set-up is shown in Fig. 2. The reactor consisted of a fiat-bottom glass vessel of 5 cm i.d. and 10 cm height with four glass baffles, a Dean and Stark apparatus, and a fourblade disc turbine impeller located at a height of 0.5 cm from the bottom of the vessel. The assembly was kept in a thermostatic oil bath (High-therm). Pressure reactions were carried out in a Parr autoclave of 2 1 volume, employed with a turbine stirrer. Experimental methodology Preliminary experiments were carried out by using hydroquinone (0.2 gmol), aniline (0.4 gmol) and catalyst (superacidic zirconia) loading of 8% (w/w) based on the total weight of reactants with variation in the xylene (5-l 5%) and temperature amount of (l&5-230°C). An induction period was observed for all the runs. It was observed that at a temperature of 185°C negligible reaction took place, while with increase in temperature the rates of reaction increased rapidly. Ten percent of xylene (based on total weight of reactants) was found as an optimum quantity for removal of water formed during the reaction. Also, at a temperature of 210°C and with a reaction time of 6 h a quantitative yield (60%) of p-hydroxydiphenylamine (p-OH DPA) was obtained, while a negligible amount of N,N’-diphenyl-p-phenylenediamine (N,N’-DPPA) was obtained.

Fig. 2. Experimental set-up: (1) reaction vessel, (2) stirrer, (3) sample point, (4) thermometer pocket, (5) baffles, (6) constant-temperature bath, (7) Dean paratus, (8) condenser.

of the catalysts

Pure zirconia Surface area (m’/g) Pore volume (ml/g) Average pore diameter

(pm)

21.36 0.5256 0.1298

used Sulfur-promoted zirconia 91.14 0.9298 0.0408

and

Stark

ap-

Catalysis by sulfur-promoted Considering imental

all these aspects,

strategy

the following

superacidic zirconia 200

was adopted.

2539

TemDerature 214 -

exper204

I

(‘33 -W

I

Reaction procedure The desired amount of aniline was first fed into the reactor. The temperature was then increased to 160°C when hydroquinone (solid) was added, followed by 10% (w/w) xylene. The catalyst was added when the temperature reached to lB8”C under agitation. The reaction temperature was kept constant after the induction period was over, which made it possible to study the reaction at higher temperatures (203 and 214°C). Samples (0.3 cm3) were withdrawn periodically and analysed on a gas-liquid chromatograph. ANALYSIS

Samples were analysed on a Chemito Gas Chromatograph by using a flame ionization detector and a Spectraphysics integrator. For the analysis a S.S. column (2 m x 3 mm) packed with 5% SE-52 on Chromosorb W was used. The analysis was done by using diphenylamine as an internal standard. RESULTS

AND

Reaction of aniline The reaction scheme First

Fig. 3. Product distribution for a typical experimental run (hydroquinone + aniline): mole ratio = I : 2, catalyst loading = 12% (w/w). stirrer speed = 15 rps, temperature = 214°C. (0)

(w/w).

It was found

(0)

N, N’-

that in the range of stirrer speeds

studied (60&1200 rpm) there was no effect of the speed of agitation on the rates of reaction. This shows the absence of the resitance associated with the liquid

DISCUSSION

can be represented

Hydroquinone, (0) p-hydroxydiphenylamine, diphenyl-p-phenylenediamine.

as:

step: Rl: -

( hydroqumone

)

A

Second

OEi+-HH~

( p

( amline ) +

B

+

hydroxydiphenylamine

)

-

step: R2:

( N. N’ - dlphenyl C

+

-

H

Product distribution. Figure 3 shows a typical product distribution of an experimental run. An inductron period of about 2 h was observed, after which the temperature was kept constant. From the product distribution it can be seen that after 6 h only the first step (R 1) has taken place quantitatively, while the rate of reaction of the second step (R2) was very slow, with less than 5% N,N’-DPPA. Hence, for kinetic interpretations, the two steps were studied separately. For the first step, the conversion of hydroquinone was followed as a function of time, and for the second step conversion of p-OH DPA. Ef/^ect of agitation. The effect of the speed of agitation was studied at the maximum temperature of operation

H,O

(214°C)

and

a catalyst

loading

of

12%

u

p - phenylenediaminc

+

)

Hz0

film next to the solid surface. To ascertain this, the conditions in the above-developed model were checked. The diffusivities of both species A and B were predicted by using the Wilke-Chang equation and the various parameters required for this were determined from various group contribution methods (Reid er al., 1977), and were found to be: DA-,,

214”~ = 8.256

D,-,,

zi4”c = 8.98 x 10m9 m*/s.

The solid-liquid B were determined

x 10mg m2/s

mass transfer coefficients by assuming

a Sherwood

for A and number,

2540

P. S. KUMBHARand G. D. YADAV

Sh, of 2: k,,_,

= 1.32 x lo--

m/s

k,,_,

= 1.23 x 10m4 m/s.

The value of k, u,~A,],[B,]~ was determined from the initial rate of reaction, that is

w “E 5 5 b

z

2.42.22.01.8-

-

1.4-

1.6-

Initial rate of reaction = rate of surface reaction just after induction period = 4.83 x 10m4 kmol/(m3 s)

For a particle size of 160 pm the value of ap is equal to 1509 m’/m3 and the initial concentrations of A and B just after the induction period are [&Ii

= [&Ii

Catalyst

loading

Ix

10m3 kg/m3

(liquid

volume)]

Fig. 4. Effectof catalyst loading on rate of reaction:temperature 196°C. stirrerspeed = 15 rps, mole ratio= 1 : 2.

= 5.4 kmol/m’

and [B.li = [&Ii

= 11.5 kmol/m3.

Substituting the above values: l/(k,[AJi[BJj)

= 3.125 x lo6

(A)

and l/(ksr._A[A,]i)+l~(k,L_B[B,]i)=2.109

x 103. (B)

From eqs (A) and (B) it is clear that they satisfy the condition given by eq. (21), that is

Hence, the reaction is surface reaction controlled. From the above, it is clear that in the range of stirrer speeds one can study the true kinetics of the reaction. Hence all further studies were carried out at a stirrer speed of 15 rps. EfSect of intraparticle resistance. For the present system the effect of intraparticle resistance was assessed by calculating the value of the effectiveness factor (q). The Thiele modulus (4) for a second-order reaction was calculated for a particle diameter of 160 pm and from the values of k,, a,,, [A,], [S,], D,, D, and pa which are determined above. The value of 4 was found to be 1.05 x lo- 3. Since 4 is very small, q is equal to 1. This means that intraparticle diffusion is absent. EfSect of catalyst loading. Figure 4 shows the effect of catalyst loading on the rate of reaction at a temperature of 196°C. The catalyst loadings based on the total weight of reactants were varied from 4 to 12% (w/w). From the plot it can be seen that the conversion of hydroquinone increases with an increase in the catalyst loading because of the increase in the total number of acid sites available. This also adds to the earlier interpretation that the reaction is a surfacekinetically-controlled one.

Time(h)

---c

Fig. 5. Effect of mole ratio of hydroquinone to aniline on conversion of hydroquinone: temperature= 196”C, catalyst

loading=12%(w/w),stirrerspeed=15rps.(0)1:2,(~)1:1, (cl) 2:l.

Efict of mole ratio of reactants. The effect of the variation of the mole ratio (1: 1, 1: 2, 2 : 1) of hydroquinone to aniline at a temperature of 196°C and at a catalyst loading of 12% (w/w) is shown in Fig. 5. The most striking feature of the plot is the decrease in the induction period with an increase in the mole ratio of hydroquinone to aniline, and finally an absence of the induction period for a mole ratio of 2: 1. This is due to the fact that when the hydroquinone to aniline ratio was 2 : 1 the rate of the temperature rise of the reaction mixture was much faster, and this resulted in a faster rate of desorption of water formed during the reaction. E&x of nitrogen bubbling. To reduce the induction period, it was thought that the use of an inert gas like nitrogen might be able to sweep away the water at a

Catalysis faster

rate. Hence,

a run was carried

by sulfur-promoted

out

superacidic

2541

zirconia

second-order kinetics [first-order with respect to each species (Figs 6 and 7)]. Hence, for the first step, the rate of consumption of

by bubbling

in the reaction mixture at a rate of 30 ml/min. However, it was found that there was not any

nitrogen

hydroquinone

decrease in the induction period, except that the conversions in the induction period were slightly increased. Also, the product colour was found to be improved from brown to gray in the presence of nitrogen, which is due to the prevention of oxidation of the reaction mixture at high temperatures due to atmospheric oxygen. It also suggests that the rate of water removal from the reaction mixture even in the absence nitrogen bubbling was much faster, to offset equilibrium.

is given

step, the rate of consumption

and, for the second OH

DPA

by

of p-

is given by

(25)

= k,, a,CBolCC,I.

E’ct of temperature. With an increase in temperature the conversions were found to increase rapidly. The values of the rate constants of both steps (k,, and k,,) were obtained from the slopes of plots of In [(2 - xA)/2( 1 - xA)] and x/( 1 - x) against time (Figs 6 and 7). and are summarised in Table 3. energy for both steps was deter‘-The activation mined from the data for the reactions carried out at

Kinetics of the reaction. The kinetic analysis was carried out using an integral method, for both steps (Rl and R2) separately. For both these steps the analysis was done for the data after the induction period, by subtracting the initial conversions that have occurred in that period. Similarly the time and initial concentrations were also normalised with respect to the conversions. For both these steps the plot of In [(2--x,)/2(1 - xa)] and x/(1 -.u)_ against time was found to be a straight line, which showed that these steps follow

r

0.18 0.16

0.14 t

0.6-

I O.OSt_

./

3. Effect

Fig.

of

7. Kinetic

diphenylamine

temperature on rate + aniline)

Temperature

Rate constant

(“0

(m”/kmol

constants

(k,) x lo9

s)(m3/m2)

Second

1.6 2.952 3.365 5.9165

196 205

0.607 0.793

212

1.102

4

5

6

(h) --t

plots for the reaction between p-hydroxyand aniline at various temperatures: (0) 196°C. (A) 205”C, (n) 212°C. (hydroquinone

Activation energy (kJ/gmol)

First step 188 196 203 214 step

3 Time

(hl -

Fig. 6. Kinetic plots for the reaction between hydrnquinone and aniline at various temperatures: (0) I88”C, (0) 196”C, (0) 203°C (A) 214°C. Table

2

1

0

Time

89.40

60.31

P. S. KUMBHAR

2542

and G. D. YADAV

different temperatures. The data for Ink, against the reciprocal of the absolute temperature were fitted by linear regression, from which the activation energy was determined (summarised in Table 3). The high values of activation energies again show that the reactions are kinetically controlled and occur at the surface of the catalyst.

obtained by Chaudhari and Rajadhyaksha (1987). This might be due to the fact that sulfur-promoted zirconia which is a heterogeneous catalyst has a very wide acid site distribution and the number of acid sites available for the reactants is limited. While for PTSA the strength is directly related to the number of H+ available due to its homogeneous nature.

Reusability of super-acidic zirconia. The reusability of the catalyst was studied by recycling the filtered catalyst thrice under otherwise identical sets of conditions. It was observed that the conversion of hydroquinone decreased rapidly from the first recycle to the third. The decrease in the conversion could be attributed to the blockage of pores by bulkier product molecules, which was evident from the surface area and the pore size distribution measurements made after the runs which show that there is very large decrease in the surface area and no pore size distribution. To solve this problem, after each run the catalyst was washed with acetone and dried at 110°C for 2 h. This catalyst gave practically the same conversion after each recycle. under identical sets of conditions.

Effect of position of substituent (methyl) group Figure 9 shows the plot of conversion of hydroquinone against time for the reactions of hydroquinone with aniline, o-toluidine and p-toluidine carried out under identical conditions. From the plot it can be seen that the conversion of hydroquinone increases in the following order: ptoluidine > aniline > o-toluidine. The conversions for o-toluidine are lower than aniline because of the steric hindrance of the methyl group which makes the -NH, group less accessible for the attack of *H group of hydroquinone. In the case of p-toluidine the methyl group at the para position makes the ring more active, and hence the attack of the hydroxyl group is faster, which results in higher rates of reaction of hydroquinone with p-toluidine. For the system hydroquinone and o-toluidine kinetic analysis was carried out for the first step (product p-hydroxy-o-toluyldiphenylamine) and was found overall to be second-order (first-order with respect to each component). The values of the rate constants at various temperatures and the activation energy are summarised in Table 4.

Comparison of PTSA, pure zirconia and superacidic zirconia. Figure 8 compares the behaviour of all these catalysts for the conversion of hydroquinone as a function of time. The experiments were carried out under identical sets of conditions. It is evident from the plot that PTSA gives slightly higher conversions than the superacidic zirconia, while pure zirconia gives less than one-third of those of superacidic zirconia. While the increased conversion for superacidic zirconia (H, < - 16) in comparison with pure zirconia (H, = 3) is due to the large increase in the acidity due to sulfuric

Effect of autogenous pressure Figure 10 shows the effect of autogenous pressure on the conversion of hydroquinone, for both aniline and o-toluidine. Under autogenous pressure the conversions were found to be much higher than those at

acid treatment as is shown by Tanabe (1985), the higher conversions for PTSA (H, = - 1.34), with a much lower acidity than superacidic zirconia, is intri-

atmospheric

guing

but

0

is in

agreement

0

1

2

with

3 Time(h)

the

4

earlier

5

pressure,

even at lower catalyst

loadings.

results

6

--)

Fig. X. Comparison ol PTSA, sulfur-promoted zirconia, and pure zirconia as catalysts for the conversion of hydrotemperature quinone: catalyst loading = 12% (w/w)* = 196°C. stirrer speed = 15 rps, mole ratio 1:2. (0) Pure zirconia, (0) sulfur-promoted zirconia. (A) PTSA.

Time

(h) -

Fig. 9. Effect of position of substituent (methyl) group on conversion

of

hydroquinone: temperature speed = 15 rps, catalyst o-Toluidine, (A) &line. (0)

ratio = 1 : 2, stirrer (w/w)_ (0)

= 188°C. mole loading = 12% p-toluidine.

Catalysis Table

4. Effect

by sulfur-promoted of

Temperature

zirconia

temperature on rate constant + o-toluidine: first step) Rate constant (m’/kmol

(“C)

(kR) x lo9

s) (m3/m2) 1.17 1.63 1.82 2.35

188 196 205 214

$00

superacidic

2543

(hydroquinone

Activation

energy

(k J/m4 46.94

Under autogenous pressure, the rates were much higher than those obtained under atmospheric pressure, even with reduced catalyst loading. The reactivity of the catalyst increased in the order p-toluidine > aniline > o-toluidine for their reactions with hydroquinone. Thus, the results show the promise of a novel superacidic catalyst, “sulfur-promoted zirconia”, for industrial exploitation.

r

Acknowledgement-One of us (PSK) thanks the U.G.C. for the award of a J.R.F. which enabled this work to be carried ‘0

1

2

3

4

5

6

7I

81

Time(h)-+

Fig. 10. Effect hydroquinone:

autogenous pressure on conversion of catalyst loading = 4% (w/w), mole ratio

of

NOTATION

C&l

= 1: 2, stirrerspeed = 12 rps, temperature= 250°C. (0) Hydroquinone + aniline,p = 5.06610.13 x 10’ Pa; ( X) hydroquinone+ o-toluidine, p = 3.04-7.1 x 105 Pa.

CAli

Also, no induction period is observed. The autogeneous pressure obviously leads to higher operating temperatures with a minimum reflux rate (i.e. most of the reactants are in the liquid phase). It is the temperature rise due to pressure effects that helps increase the rates of reaction.

C&Ii

CONCLUSlONS This study indicates that the condensation reactions of hydroquinone with aniline and o-toluidine can be carried out efficiently using catalytic amounts of superacidic zirconia under atmospheric as well as autogenous pressures. The activity of the superacidic zirconia catalyst was found to be comparable to that of PTSA and higher than that of pure zirconia. In terms of reusability superacidic zirconia performs better than PTSA, which is non-reusable. Hence, even though the cost of superacidic zirconia is much more compared to that of PTSA, because ofits reusability (it just requires washing with solvent and drying), noncorrosiveness and easy separability, it might prove advantageous. The condensation reactions of hydroquinone with aniline and o-toluidine were found to be surface reaction controlled under the range of conditions used, and were found to obey second-order kinetics (first-order with respect to each component). CES 44:11-I

CA,1

% C&l C&Ii C&l CBsli D”-B

Da-, kRI, km ksr,-,, ks,-, RAYRB x

moles of reactant fed stoichiometric coefficient

Z Greek

PP

concentration of A in bulk liquid phase, kmol/m3 initial concentration of A in bulk liquid phase, kmol/m3 concentration of A at solid (catalyst) surface, kmol/m3 initial concentration of A at solid (catalyst) surface, kmol/m3 solid-liquid interfacial area, m2/m3 (of liquid volume) concentration of B in bulk liquid phase, kmol/m3 initial concentration of B in bulk liquid phase, kmol/m3 concentration of B at solid-liquid interface, kmol/m3 initial concentration of B at solid-liquid interface, kmol/m3 diffusion coefficient of A in B, m’/s diffusion coefficient of B in A, m2/s surface reaction rate constants for first and second step, respectively, (m3/ kmol s)(m3/m2) solid-liquid mass transfer coefficient for A and B, respectively, m/s rate of reaction for A and B, respectively, kmol/(m3 s) fractional conversion, moles of reactant converted

letter

density of catalyst particle, kg/m3

2544

P. S. KUMBHAR amd G. D. YADAV REFERENCES

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