alternative processes for the production of styrene

CATALYSIS

A: GENERAL

E L S E V I E R Applied Catalysis A: General 133 (1995) 219-239

Review

Alternative processes for the production of styrene

F. Cavani *, F. Trifir6 Dipartimento di Chimica Industriale e dei Materiali, Universit~ di Bologna, Viale Risorgimento 4,

40136 Bologna, Italy

Received 26 June 1995; accepted 1 September 1995

Abstract

This short review compares different technologies for the synthes is o f s tyrene which are current ly

s tudied as al ternat ives to the industr ial dehydrogena t ion o f e thylbenzene: dehydrogena t ion of ethyl-

benzene fo l lowed by oxidat ion o f hydrogen, catalytic and s toichiometr ic oxidat ive dehydrogena t ion

o f e thylbenzene , and dehydrogena t ion in m e m b r a n e reactors. The advantages and drawbacks o f each

technology are i l lustrated and d iscussed , and the catalytic sys t ems employed are described.

Keywords: Ethylbenzene; Styrene; Dehydrogenation; Oxidative dehydrogenation; Membrane reactor

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 2. Dehydrogenation of etylbenzene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221

2.1. The chemistry of the reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 2.2. Catalyst compositions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 2.3. The role of steam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222 2.4. The role of promoters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223

3. Dehydrogenation of ethylbenzene and oxidation of hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 3.1. Catalyst and reactor configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224

3.1.1. Injection of oxygen with the feed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 3.1.2. Injection of oxygen in the effluent of the first dehydrogenation reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 3. 1.3. Injection of oxygen in the effluent of the dehydrogenation reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226

3.2. The SMART process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 4. Oxidative dehydrogenation of ethylbenzene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228

4.1. Catalytic oxidation with acid oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 4.1.1. The nature of 'active coke' . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230

4.2. Catalytic oxidation with redox oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232 4.2.1. Catalytic oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232 4.2.2. Non-catalytic oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233

* Corresponding author. Tel. ( + 39-51 ) 6443680, fax. ( + 39-51 ) 6443682, e-mail [email protected]

0926-860X/95/$29.00 © 1995 Elsevier Science B.V. All rights reserved S S D I O 9 2 6 - 8 6 0 X ( 9 5 ) 0 0 2 1 8-9

220 F. Cavani, F. Trifirb / Applied Catalysis A: General 133 (1995) 219-239

5. M e m b r a n e technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234

6. Conc lud ing r emarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237

A c k n o w l e d g e m e n t s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238

1. Introduction

Styrene nowadays is produced by two processes: (i) dehydrogenation of ethylbenzene and (ii) as a by-product in the epoxidation of propene with ethylbenzene hydroperoxide and Mo complex-based catalysts. The former process accounts for more than 90% of the worldwide capacity, which is approximately 13.10 6 t/year. The latter process is commercialized by ARCO Chemical (formerly Oxirane) and by Shell. Approximately 1.2. l 0 6 t /year are currently produced with this technology.

The problems encountered in ethylbenzene dehydrogenation, as well as the possible solutions, are similar to those met in the dehydrogenation of alkanes. Therefore a careful analysis of the alternative solutions, that have been proposed for the synthesis of styrene, can be useful to evidence the general problems encountered in the development of alternative technologies for the simple dehydrogenation and aimed toward the introduction of a double bond in a hydrocarbon molecule.

The main problems in the actual ethylbenzene dehydrogenation process are: • the need for a reactant recycle, owing to the low conversion achieved per pass

( due to thermodynamic limitations); • the need for high steam-to-hydrocarbon ratios; • the high endothermicity of the reaction (AH°298 = 28.1 kcal/mol) ; • a slight irreversible deactivation of the catalyst (the lifetime is usually about two

years). The following alternative techniques have been proposed to give a solution to

the above described problems: • oxidative dehydrogenation, in order to realize an exothermic reaction and shift

completely the equilibrium towards the product formation and to carry out the reaction at lower temperature.

• dehydrogenation, followed by oxidation of the hydrogen, in order to furnish the heat of reaction from the inside, and in some cases (according to the chosen solution) shift the reaction equilibrium.

• membrane catalysis, in order to shift the equilibrium and to carry out the reaction at lower temperature. In the first part of this review the main aspects of dehydrogenation technologies

will be described, and in the latter, the alternative technologies will be examined.

F. Cavani, F. Trifirb /Applied Catalysis A: General 133 (1995) 219-239 221

standard free energy, kcal/mol

200

100

-100

-200

A C8Hlo --> CeHe + H2 B CsH,o+H2 --> CrHe+CH4 C CeH,o --> CeHe+C2H4

~ D CsH,o+SH20-'> 8CO+13H2 ~ - ~ ~ E C.H,--> 8C+4H~

I I I I I I I

0 100 200 300 400 500 600 700

temperature, °C

20

10

'0

t -10

-20

80O

Fig. 1. Standard free energy, referred to as one mole of reactant, for the several reactions involved in the ethylbenzene dehydrogenation.

2. Dehydrogenation of ethylbenzene

2.1. The chemistry of the reaction

The following reactions are involved in the dehydrogenation of ethylbenzene; some of these reactions are thermodynamically favored at low temperature, while others are at higher temperatures.

Direct reactions: ethylbenzene ~ styrene+H2 (dehydrogenation) ethylbenzene ~ benzene + C2H 4 (cracking) ethylbenzene + Hz ~ toluene + CH4 (hydrogenolysis) ethylbenzene + H20 ~ CO + H2 (steam reforming)

Consecutiue reactions styrene ~ precursors of coke (oligomerization) precursors of coke + H20 ~ CO2 + H2 (water gas shift) precursors of coke ~ coke (dehydrogenation)

Fig. 1 shows the values of the standard free energy of reaction (referred to one mole of reactant) for the various reactions as a function of temperature.

A reaction network which can explain all the main products is shown in Scheme 1. Approaching equilibrium the selectivity to styrene decreases, not due to the occurrence of consecutive reactions but rather due to the fact that the formation of styrene is inhibited, and the rate of formation of the by-products via parallel routes becomes more relevant. In fact, at low conversion the selectivity to styrene approaches 100% with many different catalyst compositions.

2.2. Catalyst compositions

Only one family of catalysts has been proposed, which contain the same main elements and different promoters whose choice and amount give rise to catalysts

222 F. Cavani, F. Trifirb /Applied Catalysis A: General 133 (1995) 219-239

e t h y l b e n z e n e = ~ s t y rene = coke precursors

toluene benzene COx coke

Scheme 1. Reaction network in the dehydrogenation of ethylbenzene.

presenting different activity/selectivity patterns, range of optimal temperature of operation and of steam-to-hydrocarbon ratio.

The different types of catalysts compositions always contain Fe and K oxides and, in addition, one or more promoters. A typical range of compositions for the main elements and for promoters is the following: Fe203 45-77 wt.-% K20 10-27% Cr203 0-3% Ce203 0-5% MoO3 0-3% MgO 0-10% AlzO3 0-0.1% VzO5 0-2.5% CaO 0-2.5%

The surface area of the several compositions is about 2 m2/g, and the operative conditions are the following: temperature ranging from 540 to 650°C, pressure from subatmospheric to 2 atm and steam-to-hydrocarbon molar ratio from 4 to 20.0.

2.3. The role of steam

Steam is present in excess with respect to ethylbenzene in all commercial processes; evolution in the last years has been towards the decrease of the steam-to- hydrocarbon ratio to molar values lower than 6, essentially through modifications in catalyst compositions. In fact, excess steam is a penalty in energetic costs.

The overall effect of the increase of the steam-to-hydrocarbon ratio is to increase the activity, the selectivity at the same level of conversion (or the conversion at the same value of selectivity), the lifetime and stability of the catalyst.

These overall effects are consequences of the following positive and negative effects: • shift of the equilibrium towards higher conversions of ethylbenzene through a

decrease of the partial pressures of ethylbenzene and of hydrogen; • supply of the heat of reaction; overheated steam is mixed with cool, fresh feed

in order to reach the right inlet temperature; • decrease of the amount of coke or of coke precursors by steam-reforming reac-

tions (the reaction is catalyzed by K2CO3) [ 1,2] ; • consumption of ethylbenzene and of styrene by steam-reforming; • avoiding catalyst over-reduction and deactivation by controlling the valence state

of iron through the equilibrium shown in Scheme 2 (the reoxidation of Fe304 occurs only with oxygen [ 3 ] and not with water) ;


H20 02 FeO i ~ FesO, ~ ~ Fe203

H2 H2 Scheme 2. Reactions of the variation of the valence state of iron oxide in the reaction medium.

• increase of the rate of formation of the active phase, KFeO2, and minimization of the deactivation rate under reaction conditions [4] ;

• interaction of steam with K2CO 3 forming free KOH.

2.4. The role of promoters

Potassium is the main promoter of Fe203; it increases the activity by more than one order of magnitude, and also slightly increases the selectivity to styrene and the stability of the catalyst. It has been well established in the last years that the promotional role of potassium consists of the formation of a surface ternary compound, KFeOz, which constitutes the active phase [2-4]. Another further role played by potassium is the formation of well dispersed K2CO 3, which acts as catalyst for the gasification with steam of carbonaceous deposits [ 1,2].

KFeO2 is metastable at low temperature and in the presence of oxygen and water vapor; this is the reason why only recently in-situ characterization techniques allowed its formation under reaction conditions to be demonstrated unequivocally [3,4].

The role of the other promoters for the Fe /K /O system is to favor the formation of KFeO2 and stabilize it under reaction conditions at lower steam-to-hydrocarbon ratios. Cr and A1 are considered to be structural promoters as they can enter in the Fe 3 + compounds; MgO may act as a support not only for KFeO2 but also for K2CO 3

increasing the selectivity and also the stability of the catalyst [2]. Ce oxide increases the activity and Mo the selectivity; therefore the presence of

both promoters is suggested in an improved catalyst composition [ 5 ].

3. Dehydrogenation of ethylbenzene and oxidation of hydrogen

Dehydrogenation/oxidation processes are characterized by the injection of a gas containing oxygen either in the effluent or in the feed of a dehydrogenation reactor, in order to catalytically oxidize, either in part or totally, the co-produced or co-fed hydrogen. Scheme 3 exemplifies the process chemistry for the case of ethylbenzene dehydrogenation [6].

The claimed advantages of the hydrogen oxidation coupled to the hydrocarbon dehydrogenation are:

ethylbenzene ~ } styrene + hydrogen

hydrogen + 1/2 oxygen b water

ethylbenzene ~ benzene, toluene, methane, ethylene

Scheme 3. Process chemistry for the ethylbenzene dehydrogenation and hydrogen oxidation.


• the internal supply of the heat for the endothermic reaction, thus avoiding costly reheating supplied by superheated steam or with heat exchangers, and also minimizing cracking reactions by shortening inter-reactor lines;

• the shift of the dehydrogenation equilibrium by consuming hydrogen, so achieving higher product yields. Higher yields are obtained both by increasing the conversion and by increasing the selectivity when moving far from the equilibrium. The oxidation of hydrogen is realized by a suitable oxidation catalyst whose

properties must include: • to be selective only in the oxidation of hydrogen, thus avoiding the oxidation of

the reagent and of the product; • to be stable in the severe reaction conditions, i.e., at high temperature (550-

650°C) and in the presence of steam; • to be very active, in order to consume all oxygen at the end of the oxidation zone,

for safety reasons and for the stability of the catalyst in a downstream dehydrogenation reactor. This type of process can be applied with dehydrogenating catalysts which are

stable or not poisoned in the presence of steam; in fact this type of process is also referred to as 'steam oxidation dehydrogenation'. This is the reason why the application of this technology is claimed in the dehydrogenation of ethylbenzene using iron oxide-based catalysts in the presence of steam and in the dehydrogenation of paraffins only with either spinel-type supports or supports based on alumina doped with rare earth, which are particularly stable in the presence of steam.

Alternatively, for catalysts which are sensitive to water, the concentration of steam can be kept low by controlling the amount of added oxygen.

3.1. Catalyst and reactor configurations

Two types of catalyst compositions have been proposed: • dual catalysts, with different catalytic functions, each one performing one reac-

tion (ethylbenzene dehydrogenation and hydrogen oxidation); • single polyfunctional catalyst.

The choice of a unique catalyst for the two reactions may have some advantages in simplification of catalyst loading and unloading, or in application in moving or fluidized bed reactors.

For the two solutions different reactor configurations have been proposed:

3.1.1. Injection of oxygen with the feed In this case a dual bed is employed, either with a mixture of the two types of

catalysts, or with stratified layers; alternatively, the same catalyst is used for both reactions in one reactor.

This kind of technical solution is exemplified in the patent by Reitmeier et al. [ 7], who proposed for the dehydrogenation of ethylbenzene to styrene a dual


catalyst bed, with an oxidation catalyst based on Pt or Pd. The oxidation catalyst is inserted in the dehydrogenation reactor by mixing or layering the two catalysts, or by supporting the active metal on the conventional iron oxide-based dehydrogenation catalyst.

Similar solutions have also been claimed for the dehydrogenation of alkanes; Drehman and Walker [ 8 ] proposed a single polyfunctional catalyst with dehydrogenation and oxidation properties for the dehydrogenation of n-butane in the presence of steam. The proposed catalyst seems to be a traditional dehydrogenation one, based on tin and alkali-promoted Pt, supported on a spinel compound. In order to obtain higher selectivity to olefins and to minimize the oxidation of hydrocarbons, hydrogen is introduced in the feed.

In patents, assigned to UOP, Imai and Jan [ 9 ] described a polyfunctional catalyst for paraffin dehydrogenation, constituted of tin/alkali-doped, alumina-supported platinum. The authors found in pilot plant tests a selectivity of 95% for the H2 oxidation (the balance was made up of oxygen consumed in hydrocarbon oxidation). Besides alkali metals, other promoters were selected including scandium, yttrium, lanthanum and actinium [ 10]. This type of catalyst is resistant to severe hydrothermal conditions.

Font Freide et al. [ 11 ] described an oxidation catalyst based on cordierite- supported platinum, for the dehydrogenation of n-paraffins in the presence of oxygen. The reaction is carried out at temperatures close to 800°C, where dehydrogenation occurs thermally in the gas phase; the oxidation catalyst supplies the heat of reaction by oxidizing hydrogen.

Bricker et al. [ 12] claimed a process for the dehydrogenation of ethylbenzene with a dual catalyst. The oxidation catalyst is still a tin, alkali-doped platinum one, but special emphasis was given to the property of the support to resist the severe hydrothermal conditions. The alumina, after the impregnation with tin, is calcined at a higher temperature than that of dehydrogenation; the resulting support has an apparent bulk density of 0.2 ml/g, with more than 40% of its pore volume constituted of pores with a size larger than 150 nm.

3.1.2. Injection of oxygen in the effluent of the first dehydrogenation reactor In this case an oxidation reactor is inserted between two dehydrogenation reac-

tors, thus supplying the heat for the second dehydrogenation stage. In patents assigned to UOP, Imai [13,14] proposed a dual catalyst for the

dehydrogenation of ethylbenzene, with the oxidation catalyst in between two dehydrogenation stages, or together in the presence of steam. As an oxidation catalyst, the author claimed a metal of Group VIII, doped with elements of Groups Ia and IIa with an ionic radius higher than 0.13 nm, in particular either rubidium or cesium or barium. The properties of the suggested promoters are their low volatility in the reaction medium and an effect of stabilization induced on alumina, under severe hydrothermal conditions.


d e h y d r o g e n a t i o n reactors

J . . . . I . . . . . . . . . . . . . . . . . .

_ separation

, , ,

I I I z o n e

A ! I . . . . . i i i A I i

, - - ; s t e a m r ~ ' ~ - - ,

, super~ator l I ' I I ' ~ , , I Io~oas I I

steam I i i i i i ~ I | , t - - - . . 4 , I I = ? w a s t e h e a t I 1 - - ,

steam , ~ . . J , I I : exche--ers I I styrer~ • I I I l

alr (°xYg_e.n)_,L~. _ _ . . . . . . _ .

~ . _ , J c o n d e n s e t e compressor ethylbenzene + water vapor

Fig. 2. Simplified flow sheet for the UOP process for the dehydrogenation of eythylbenzene and oxidation of hydrogen (adapted from Ref. [ 151 ).

In other patents from UOP a process with a dual catalyst for the dehydrogenation of both ethylbenzene and paraffins is claimed, where the oxidation catalyst is not only inserted between two dehydrogenation reactors, but is also placed before the first reactor, where hydrogen added to the feed (or coming from the recycle) is oxidized in order to supply the heat to the first dehydrogenation stage [ 15 ]. Fig. 2 displays a scheme of the reactor configuration. The amount of the oxidation catalyst is about 30% of the dehydrogenation one and clearly operates with a higher space velocity. In the case the process is used for paraffin dehydrogenation, the percentage of water in the stream must not exceed 2% (achieved by controlling the amount of injected oxygen), in order to avoid the deactivation of the dehydrogenation catalyst. The claimed oxidation catalyst was Pt, doped with alkali or alkaline earth elements.

3.1.3. Injection o f oxygen in the effluent of the dehydrogenation reactor The effluent of the oxidation reaction is sent directly to the separation zone. In

this case the heat of reaction is supplied to the fresh feed through heat-exchangers and the problems related to contamination of the dehydrogenation catalyst with water and oxygen are overcome.

Herber and Thompson [ 16] described for both ethylbenzene and paraffin dehydrogenation a dual catalyst-based process. The effluent of the selective oxidation stage, which is located downstream the dehydrogenation reactor, is sent directly to the separation unit. Fig. 3 shows a simplified flow-sheet for the ethylbenzene dehydrogenation/oxidation process. The claimed oxidation catalyst is a metal of the Group VIII, with a promoter with ionic radius greater than 0.13 nm.

3.2. The SMART process

The SMART process for the synthesis of styrene combines the Lummus technology with the UOP concept for oxidative reheating by selective catalytic oxidation of hydrogen [6]. The flow sheet of the process is shown in Fig. 4; the oxidation catalyst is inserted in the second and third reactor. Fig. 5 reports the design of

F. Cavani, F. Trifirb / Applied Catalysis A: General 133 (1995) 219-239 227

d e h y d r o g e n e t i o n reac tor

r - . . . . . . . . . . . . ate_m__ _ _ - v

f rac t iona t ing zone

'-f,~G asas

' hyd rocarbons to i

, s ty rene recove ry i i i i i

= ox ida t i on , = wa te r oxygen .~ reac to r I c , ' . . . . "

- - t . . . . . i

i

e thy lbenzene

Fig. 3. Simplified flow sheet of the UOP process for the dehydrogenation of ethylbenzene and oxidation of hydrogen: injection of oxygen in the effluent of the dehydrogenation reactor (adapted from Ref. [ 16 ] ).

hyd rogen ox ida t ion

i

e thy lbenzene dehydrogenation

"~_ . . . . . . . . . . . . . . . . . . . . . . . . . . .

styrene ~ hyd rogen ox ida t ion . . . . . . . . . . . . . . . . . . . . . . . . .

i

h y d r o g e n 4.. . . . . . . . ~ . . . . . . . . . . , it P . . . .

t hyd rogen ox ida t ion I I i ' l i gh t ends ~ , , , • i i i i I i i i l

:' ' h--rl '"'-':' I _ separa t i on

i

e thy lbenzene

Fig. 4. Simplified flow sheet of the SMART process (adapted from Ref. [6] ).

feed

dehydrogenation catalyst

steam and air i

i

. , ~ u e n t

utlet

~ o x i d a t i o n cat.

~ d e h y d r o g e n e t . cat.

dehydrogenation catalyst and oxidation catalyst

Fig. 5. Reactor configuration in the SMART process (adapted from Ref. [6] ).

reactor configuration. The gas flows radially from the center, where the oxidation catalyst is inserted, while in the external part the conventional dehydrogenation

228 F. Cavani, F. Trifirb / Applied Catalysis A: General 133 (1995) 219-239

catalyst is placed. The two beds of catalysts are separated by screens. The removal of hydrogen formed in the first reactor shifts the equilibrium to the formation of higher yields of styrene in the second one. With three reactors, a conversion per pass higher than 80% can be obtained, at the same styrene selectivity as for the conventional process. It is interesting to observe that these values are similar to the best ones obtained in the oxidative dehydrogenation of the ethylbenzene with a carbon molecular sieve [17].

The SMART process can be used to revamp conventional dehydrogenation plants, in order to increase the capacity with low investment costs. An example has been given by Romatier et al. [6] ; the revamping of an existing plant producing 272 000 t/year of styrene has been increased in capacity to 400 000 tpa by adding two new reactors in a three-stage dehydrogenation process; the existing reactors are not modified, and the new reactors are added in series. The feed rate can be increased by 10%, in view of the increased ethylbenzene conversion, and correspondingly the reduced recycle stream.

4. Oxidative dehydrogenation of ethylbenzene

In the oxidative dehydrogenation of ethylbenzene the strong exothermic reaction is used:

C6Hs-C2H5 + 0.502 --") C6Hs-C2H3 + H20 A/~298 = - 29.7 kcal /mol

in order to reach the following objectives: • to obtain styrene at almost complete conversion in order to decrease the sepa-

ration costs (dehydrogenation processes operate at conversions lower than 65%) ; • to eliminate or strongly decrease the use of superheated steam (steam/hydro-

carbon = 5/1 in dehydrogenation) ; • to reach higher selectivity (90% in dehydrogenation), not only to minimize the

waste of ethylbenzene but also to simplify the removal of the heat of reaction and avoid total consumption of oxygen. The transformation of one mole of ethylbenzene to styrene requires 0.5 02 while the combustion to CO2 requires 13 02. Belomestnykh et al. [ 18 ] report a reaction pattern with most of the representative

by-products (Scheme 4). Side reactions occur by cracking and by oxygen insertion reactions on ethylbenzene and on styrene.

In order to minimize the oxygen insertion reactions and to eliminate the problems which arise in the presence of molecular oxygen (mixture flammability, run-away) three types of solutions have been proposed: • catalytic oxidation in the presence of molecular oxygen with oxides possessing

medium strength acidity and with nil or limited redox properties [ 19]; • oxidation with redox-type oxides in the absence of molecular oxygen [ 20]; • electrochemical oxidation [ 21 ].

F. Cavani, F. Trifirb /Applied Catalysis A: General 133 (1995) 219-239

ethylbenzene = styrene = styrene oxide

/ \ a-methylbenzyl alcohol acetophenone b benzaldehyde

~ benzene ~

carbOn oxides Scheme 4. Reaction pattern in oxidative dehydrogenation of ethylhenzene (from Ref. [ 18] ).

4.1. Catalytic oxidation with acid oxides

229

The best performances of mixed oxide-based catalysts investigated after 1981 are reported in Table 1. Previous references from patent and scientific literature are reported in the paper of Emig and Hofmann [22]. The different oxides are active in the temperature range 450 to 600°C, and operate with a ratio O2/EB 0.8- 2, a concentration of EB in the range 8-15% and a contact time in the range 0.2-4

gear/(gEB h ) .

The best results, among all the catalysts listed in Table 1, have been obtained with carbon molecular sieve AX21 (Anderson Dev. Co.), at much lower temperature (350°C), with 80% conversion and 90% selectivity [ 17]. However, these results were not confirmed in a recent paper by the Drago and Jurczyk [23]. Comparable performances are obtained by metal phosphates, but at much higher temperatures [24].

On the basis of the first suggestions of Alkhazov et al. [32], up to the recent short review of Vrieland and Menon [ 19], it seems well documented and proved that the active catalyst in ethylbenzene oxidative dehydrogenation is the 'active coke' which forms in the first hours of reaction and reaches a stationary amount on the surface of the oxides.

Table 1 Catalytic performance of various catalysts in the oxidative dehydrogenation of ethylbenzene to styrene

Catalyst T (°C) Conversion (%) Selectivity (%) Ref.

SnO2-P205 450 38 82 [ 25 ] Zr phosphate 450 55 86 [ 22 ] SIO2-A1203 450 62 71 [ 26] Clinoptilolite 475 59 85 [ 27 ] Na sodalite 475 44 85 127 ] Pr /Mo/AI/O 500 67 86 11281 Ce4 (P207) 3 550 71 89 [ 24 ] Al(PO3)3 530 72 91 1241 Ce phosphate 605 76 90 [ 24 ] Ge phosphate 540 54 64 [ 29 ] Zr/Sn phosphate 500 64 83 [ 30] Carbon molsieve CMS AX21 350 80 90 [ 17] Carbon molsieve Ambesorb 575 300 82 73 [31 ]


Therefore, the investigated oxides are not catalysts, but rather 'carriers of the active component'. The formation of the 'active coke' in both internal volume and on external surface of the different oxides explains the low differences in catalytic performance observed for the different classes of metal oxides and phosphates, as shown in Table 1, as well as the higher yield in styrene obtained at much lower temperature with molecular sieve carbon. With metal oxide carriers higher temperatures are most likely necessary to form and to maintain a certain stationary amount of 'active coke'.

In the case of 'oxides as carriers' usually conversions not higher than 70% have been reported. This limitation on conversion is essentially due to the fact that the conversion of oxygen reaches 100%. In fact, the oxygen-to-styrene ratio in most publications and patents is not higher than 1, with a preferred concentration of ethylbenzene of 10%. The choice of this composition, the best for achieving a good selectivity, requires a costly dilution of air with nitrogen or steam and also causes oxygen starvation at high conversion. However, it is the preferred composition due to safety reasons, in order to operate outside the flammability region. The lower and higher limits of ethylbenzene in air at 30°C (those for styrene are similar) are 1.0 and 6.7%, respectively; therefore, operation with 10% ethylbenzene in air would be unsafe, not at the inlet of the reactor but rather inside it, when the composition of the reacting mixture enters the flammability zone. The choice of 10% oxygen with about 10% ethylbenzene gives safe conditions in all sections of the plant layout.

In order to increase the conversion to values which are economically competitive with dehydrogenation an oxide carrier presenting higher selectivity must be developed.

Another factor which can contribute to the non-complete conversion obtained with the oxide carriers is the strong adsorption of styrene which negatively influ- ences the rate of transformation of ethylbenzene, as can be deduced from the rate equations developed by Schraut et al. [ 33 ]. A modification of the catalyst aimed at facilitating the styrene desorption can contribute to increase the yield to styrene.

4.1.1. The nature of 'active coke' The composition of coke varies in the first hours of reaction reaching similar C-

H-O compositions (for different carriers). It has been found [ 34] that the 'active coke' on the surface of Zr-phosphate, under stationary conditions, presents the following composition: C19H703. Other authors [ 35 ], when using as the carrier a boron-doped alumina, have found an active coke with composition C22.5H6.603 .

Echigoya et al. [26] found an higher activity and selectivity on mixed oxides carriers based on silica, where a coke with a H/C ratio less than 1 was formed.

C=O bonds have been identified by various authors [34,36], and radical-like species were also found [34,35,37]. According to Schraut et al. [34], the radical species can be attributed to the aroxyl groups shown in Scheme 5. By means of


O. Scheme 5. Aroxyl groups in 'active coke' deposited on acid oxides (from Ref. [33] ).

SIMS analysis it was found that the deposited coke is similar to anthraquinone [34,36].

The activity of coke in the oxydehydrogenation of ethylbenzene has been attributed to the redox couples which form on the edge of its graphitic structure, after interaction with oxygen. In the absence of molecular oxygen, 'active coke' [ 22,28 ] and carbon molecular sieves [17,31] are practically inactive and do not form hydrogen. A decrease of the oxygen-to-ethylbenzene ratio in feedstock leads to a decrease of ethylbenzene conversion and to an improvement of styrene selectivity [ 23 ]. It has been hypothesized that these redox couples consist of quinone/hydro- quinone and aroxyl/phenol groups. A representation of the mechanism proposed by Emig and Hofmann [22] is given in Scheme 6; styrene forms after a concerted mechanism of hydrogen abstraction while carbon oxides partially form from the combustion of coke.

The different selectivities found on different catalysts, notwithstanding the pos- session of comparable amounts of coke, has been attributed to the different surface properties of coke. The most selective coke is that one that is less oxidizable by oxygen, while the least selective coke is that one that is easily burnt to carbon oxides even in the presence of high amounts of ethylbenzene [ 38].

The carbon molecular sieve AX21 investigated by Grunewald and Drago [ 17] presents a surface area of approximately 3000 ~ g , with an oxygen content of

COx active coke ' ~ H204r'~ 1/2 02

Scheme 6. Mechanism ofethylbenzene oxidative dehydrogenation over 'active coke' (from Ref. [22] ).


Table 2 Performance of carbonaceous catalysts in the oxidative dehydrogenation of ethylbenzene [ 17,31 ]

Catalyst Surface area (mZ/g) T (°C) Conversion (%) Selectivity (%)

PPAN 1" 8 350 11.6 90.5 PPAN 2 50 350 22.4 91.5 PPAN 3 10 350 14.6 76 AC ~ 800 350 26.8 81.7 CMS AX21 c 3000 350 80 90 Ambesorb 563 ~ 540 300 68.3 88 Ambesorb 572 1060 300 74.7 89 Ambesorb 575 785 300 82.3 73 Ambesorb 348F 725 300 55.7 88

° Pyrolyzed acrylonitrile, Aldrich. b Activate carbon, Mallinckrodt. c Carbon Molecular Sieve AX21, Anderson Dev. Co. d Carbon Molecular Sieve, Supelco.

6%. It likely that the chemical nature of this oxygen is similar to the quinone-type proposed to form in active coke when this is supported on oxide carriers. The higher activity of molecular sieve can be attributed to its high surface area, but the surface area cannot be the only parameter. In fact the activity data reported in Table 2, for different types of carbonaceous materials [ 17,31 ], cannot be interpreted only on the basis of differences in surface area. This is also confirmed by the fact that the micropores blocking during the reaction, due to polymerization of styrene and thermal decomposition to coke, did not remarkably affect the catalyst activity [ 23 ]. Apparently, most of the reaction occurs in mesopores and in macropores, as well as in the external surface area.

The data on PPAN, AC and AX21 were collected after 20 h time-on-stream [17]. In particular, AX21 was stable after one week on stream. The data on Ambesorb [ 31 ] were collected after only 1 h of reaction; longer periods of reaction deactivated the catalyst very quickly.

4.2. Catalytic oxidation with redox oxides

Oxidation of ethylbenzene to styrene has been achieved with redox-type catalysts, based essentially on MgO/V205 and using two different reactor configurations: • catalytic oxidation with air in the presence of steam [ 18,39] ; • oxidation of ethylbenzene with oxygen from the catalyst and reoxidation of the

reduced catalyst with air [20].

4.2.1. Catalytic oxidation In catalytic oxidation a maximum in activity and selectivity has been observed

at low V205 concentration, as shown in Fig. 6 [ 18]. Belomestnykh et al. [ 18] obtained the best performance (72% conversion and 90% selectivity) with the 12%

F. Cavani, F. Trifirb /Applied Catalysis A: General 133 (1995) 219-239 233 ethylbenzene conversion, %

°° I 40 ~ " 2 0 -

0 I I | 1

0 20 40 60 80 100

vanadia content, wt.%

Fig. 6. Ethylbenzene conversion as a function of vanadia content in the catalyst ( f rom Ref. [ 18] ).

V205 and some unspecified dopant, at the following conditions: temperature 480°C, feed composition E B / O J H 2 0 / N 2 1 / 1/8/20 molar ratio.

Other authors [ 39] also found the highest value of activity and selectivity at the 9% V205, but these values remained unchanged up to 14% of vanadia content. The best performance was obtained at 520°C with a conversion of 65 % and a selectivity of 93.5%; residence time was 0.4 s and feed composition was EB/O2/H20/N2 1/ 1/8/20 (molar ratio).

The performances obtained with redox catalyst are very similar to those obtained with acid oxides. The only difference is the presence of an higher amount of steam when redox oxides are used, in order to suppress bulk combustion.

According to some authors [ 18,39] the active sites for the selective oxydehydrogenation are surface clusters of V 5+ and V 4÷ in octahedral coordination. At low vanadium concentration isolated vanadium species form, in tetrahedral and octahedral coordination, while when the vanadium content is increased the amount of vanadium in octahedral coordination increases, with formation of associated vanadium species, and the catalytic activity is correspondingly increased. For even higher concentrations of vanadium a Mg-vanadate phase forms and the activity drops. Hanuza et al. [39] suggested that the sites which abstract hydrogen from ethylbenzene are surface V 5 ÷ =O bonds, while V 4÷ species dissociate molecular oxygen.

4.2.2. Non-catalytic oxidation In non-catalytic oxidation [ 20] mixed oxides with composition ranging from 40

to 60 wt.-% MgO, from 20 to 40 wt.-% SiO2 and from 10 to 30 wt.-% V205 are used as stoichiometric oxidants of ethylbenzene. The preferred composition is 50 wt.-% MgO, 30 wt.-% SiO2 and 20 wt.-% VzOs. The preferred preparation is spray- drying of a MgO-SiO2 suspension followed by impregnation of ammonium metavanadate, preferably with a further amount of silica and calcination at temperatures between 500 and 800°C.


In pulse reactor tests an ethylbenzene conversion of 98.1% and a selectivity to styrene of 92% were obtained at 505°C. After one regeneration with oxygen a small decrease in activity was observed. Tests in fluidized-bed reactor, at a WHSV of 3.4 h - ' and at the temperature of 500°C, gave ethylbenzene conversion of 63% and selectivity of 83.8%. The regeneration was carried out with a stream containing 10% oxygen in nitrogen. The stripping was carried out with pure nitrogen. These results, achieved at comparable contact time, are worse than those obtained with acid oxides as carriers.

The high activity and selectivity obtained in pulse reactor evidences the high specificity in hydrogen abstraction of transition-elements redox couples. Very likely, the low performance obtained in flow conditions might be due to an unfa- vorable formation of carbonaceous deposits. This allows us to exclude coke as the active component in these catalysts.

5. Membrane technology

Membrane technology has been proposed to modify dehydrogenation and oxidative dehydrogenation processes along the following lines: • to separate hydrogen from the product stream of a dehydrogenation reaction

occurring on one side of the membrane and pure hydrogen is obtained on the other side of the membrane.

• to separate hydrogen from the product stream and to make it react on the other side of the membrane with an oxygen-containing gas.

• to let oxygen diffuse selectively through a membrane in order to make it oxidize either the hydrogen formed or directly the hydrocarbon, on the other side of the membrane. The advantages which are claimed to be achieved when dehydrogenation reac-

tions are carried out inside a reactor which contains a membrane are the following: • to shift the dehydrogenation equilibrium and increase the conversion to the

product with also the possibility to operate at lower temperature so as to decrease side reactions;

• to develop a process in autothermal regime by coupling the endothermic dehydrogenation reaction (which occurs on one side of the membrane) with an exothermic reaction on the other side (such as hydrogenation or hydrogen oxidation) ;

• to carry out an oxidative dehydrogenation without mixing the hydrocarbon and the oxygen, thus minimizing undesired oxygen insertion reactions and avoiding problems related to flammability of the mixtures and to run-away. Two types of reactor configuration have been proposed:


ethylbenzene sweep gas catalytic membrane

Cata ly t i c "~ : : ::~ M e m b r a n e

Reac to r permeate

feed side ~ , ~ ; , . ~ side

styrene ~, sweep gas~, hydrogen

direction across the membrane

ethylbenzene sweep gas

1 io..meob . . . . I ,ne, E

~'~'~;~ M e m b r a n e ::i~. .~i~ Cata ly t i c I;~;!iii ~ ; ~ 1 Reac to r

styrenel, sweep gas]. hydrogen "

Fig. 7. CMR and ICMR configurations.

• the reaction is carried out completely in a membrane reactor; • the membrane reactor is added downstream to a normal reactor, after the reaction

has approached equilibrium conversion (hybrid system); this solution may be considered as a retrofitting of existing dehydrogenation plants. Two types of membranes have been proposed:

• non-porous (dense) membranes: metals or alloys (Pd; Pd-Ag) [40] ; • porous membranes: Vycor glass, alumina, ceramics or SiC [41,42].

Porous membranes have been investigated more for dehydrogenation reactions than dense metallic membranes. The effective permeability for porous membranes is dependent on the pore size, porosity and tortuosity of membrane structure, on the molecular weight of reagent and products, and on the temperature of separation. The permeation occurs by Knudsen diffusion and depends on collisions between the molecule and the walls of the pores with diameters below 10 nm. The separation factor is inversely proportional to the square root of molecular weight, and therefore it is very low for C2-C4 paraffins/hydrogen mixtures, while it has a reasonable value of 7.3 for ethylbenzene/hydrogen.

Two types of catalyst configuration have been proposed: the catalytic membrane reactor (CMR) and the inert membrane catalytic reactor (IMCR) [43,44]. Fig. 7 shows a scheme of the CMR where the membrane is impregnated with the catalytic active components, and the IMCR configuration, where the pellets of catalyst fill the reactor in the feed side, and where the membrane constitutes the wall of the reactor.

Bitter [ 45 ] has investigated ceramic membrane reactors to separate the hydrogen from the dehydrogenated products. Fig. 8 shows the reactor configuration: the catalyst, represented by the shaded area, is in a first reactor zone, as well as in the membrane reactor and in a post-dehydrogenation reactor zone. The first reactor is necessary to obtain hydrogen and decrease the amount of hydrocarbon which can


second dehydrogenation first dehydrogenation reactor: multitubular third dehydrogenation reactor membrane reactor reactor

ceramic membrane permeable to hydrogen

\ dehydrog. c a ~

A i

. . . . . . i ethylbenzene

styrene, hydrogen

retentate outlet," _ I ethylbenzene, styrene

' - ' ¢; " 7 - - * [e~'rens I IE i I H2, ' . . . . . . . . . .

i ! r ! ; i

! i . . . . . . . . . . . . . . . . . . . . . . . . i r ? i !~i~ } : 2 i ) i }

~ : . S 7 : "~ j f p e r m e a t e outlet: l hydrogen , t \

v (ethylb.,s reoo,' . . . . . \ I dehydrog, dehydrog,

catalyst catalyst

Fig. 8. CMR: reactors configuration in the process for hydrocarbons dehydrogenation developed by Shell (adapted from Ref. [44] ).

permeate with hydrogen in the membrane reactor. Hydrogen, with small amounts of hydrocarbons, permeates through the membrane walls. The post-dehydrogenation reactor treats the hydrogen-rich permeate stream, in order to transform the reagents which have permeated together with hydrogen through membrane walls. The latter consist of a layer of aluminum oxide membrane (pore size 10 nm, thickness 4-10/zm) on a porous y-alumina support (pore size 5-10 nm, thickness 4 mm).

In ethylbenzene dehydrogenation, with a Fe/K/V/Li /Cr/O catalyst, at a temperature of 625°C and with a LHSV of 0.65 h-~, a conversion of 65% and a selectivity of 94% have been obtained, while without membrane the conversion is only the 50.7%. A ceramic membrane has been investigated by Wu and Liu [46] to dehydrogenate ethylbenzene and separate hydrogen in order to increase the yield in styrene. They proposed an hybrid system where a packed-bed reactor, filled with a Fe/K oxides-based catalyst, is followed by a membrane reactor also filled with a dehydrogenation catalyst. The hybrid solution has been proposed in order to avoid the permeation of ethylbenzene when the concentration of hydrogen is low.

Moser et al. [47] have investigated the dehydrogenation of ethylbenzene with an alumina membrane reactor, in both CMR and IMCR systems. In the latter configuration, a conventional dehydrogenation catalyst has been located inside the tube of porous alumina on the feed site. In the CMR configuration, the pores of the membrane were first filled with a solution containing the catalyst, in order to achieve a dispersion of the active components. The conversion of ethylbenzene was increased above the equilibrium value by 10%, with the IMCR, and by 20-23%, with the CMR configuration.

Oxygen-permeable membranes have been used in catalytic and electrocatalytic membrane reactors, for the dehydrogenation with oxygen, hydrogen oxidation and for the oxidative dehydrogenation of ethylbenzene. Michaels and Vayenas [21 ]

F. Cavani, F. Trifirb /Applied Catalysis A: General 133 (1995) 219-239

Table 3 A comparison of the best results reported for the different methods of styrene synthesis from ethylbenzene

237

Configuration Conversion (%) Selectivity (%) T (°C) Catalyst Status

Dehydrogenation 60-65 90 600-650 Fe/K/Cr/O industrial Dehydrogenation/H2 80 90 SMART technology commercial oxidation Oxidative dehydrogenation 76 90 605 Ce phosphate research Oxidative dehydrogenation 80 90 350 carbon molsieve research Oxidative dehydrogenation 72 90 480 V/Mg/O research Non-catalytic oxidation 98 92 505 V/Mg/Si/O research Membrane technique 65 94 625 Fe/K/V/Li/Cr/O research

studied the vapor phase electrochemical oxidative dehydrogenation of ethylbenzene to styrene on a polycrystalline Pt electrocatalyst in a stabilized ZrO2 electrochemical reactor. The dehydrogenation rate was found to be enhanced by a moderate current density.

Standard Oil [48,49] claims the use of solid multicomponent membranes that consist of a monophasic mixed metal oxide with a perovskite structure with both electron- and oxygen-conductive properties. The membrane is impervious to gases and physically separates two zones in the electrochemical reactor: in the first zone the oxygen-containing gas is passed and contacted with one surface of the membrane (the cathode), where molecular oxygen is reduced. The resulting ion is transported to the other surface of the membrane (the anode), where it meets the hydrocarbon to be dehydrogenated. Reaction between ionic oxygen and the hydrocarbon occurs at the surface and releases electrons which are transported to the cathode surface. A catalyst can be placed in the hydrocarbon zone, to catalyze the selective oxidation of the hydrogen formed. The process offers the advantage of overcoming the thermodynamic limitations of direct dehydrogenation, thus achieving high conversion and limiting the production of carbon oxides which are formed by direct contact between the oxygen and the hydrocarbon. Single-phase membranes with perovskite structure claimed in the patents are: LaCoOx, Lao.6Sro.4CoO x,

Lao.2Sro.8CoOx, YBa2Cu3Ox. These systems were reported to be effective in the synthesis of styrene from benzene/ethane and benzene/ethylene mixtures. The benzene conversion at 700°C was 11%, with a selectivity to styrene which approached 70%, the by-products being biphenyl, ethylbenzene and toluene.

6. Concluding remarks

Table 3 summarizes the best catalytic performances obtained with the different technologies; dehydrogenation is the only process widely used at a commercial level. The SMART technology is commercially available, but to our knowledge it has found .no industrial application till now. The other methods of synthesis are at a research level. Oxidative dehydrogenation gives better performances in terms of

238 F. Cavani, F. Trifir6 /Applied Catalysis A: General 133 (1995) 219-239

conversion and selectivity than the industrial dehydrogenation process. The best performance in terms of conversion/selectivity are claimed with the non-catalytic oxidation on redox oxides, where 92% of selectivity were obtained at 98% conversion. However, for these two alternative technologies the lifetime of catalysts has not yet been evaluated and therefore pilot plant studies must be carried out in order to obtain informations about the modifications of catalytic performances with time- on-stream and about the productivity achievable. It is also necessary to gain a more quantitative knowledge of all the by-products. In fact, even if present in traces, the nature and amount of by-products can determine the final success of these new technologies.

Acknowledgements

This work was sponsored by MURST (Ministero dell' Universit~ e della Ricerca Scientifica).

References

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alternative processes for the production of styrene

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