Catalytic Materials - American Chemical Society

14

Downloaded by NORTH CAROLINA STATE UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: April 5, 1984 | doi: 10.1021/bk-1984-0248.ch014

Structure-Selectivity Relationship in Xylene Isomerization and Selective Toluene Disproportionation D. H. OLSON and W. O. HAAG Mobil Research and Development Corporation, Princeton, ΝJ 08540

As a result of steric constraints imposed by the channel structure of ZSM-5, new or improved aromatics conversion processes have emerged. They show greater product selectivities and reaction paths that are shifted significantly from those obtained with constraint-free catalysts. In xylene isomerization, a high selectivity for iso merization versus disproportionation is shown to be related to zeolite structure rather than composition. The disproportionation of toluene to benzene and xylene can be directed to produce para-xylene in high selectivity by proper catalyst modification. The para-xylene selectivity can be quantitatively described in terms of three key catalyst properties, i.e., activity, crystal size, and diffusivity, supporting the diffusion model of para-selectivity. Intermediate pore z e o l i t e s t y p i f i e d by ZSM-5 (1) show unique shape-selectivities. This has led to the development and commercial use of several novel processes i n the petroleum and petrochemical industry (2-4). This paper describes the s e l e c t i v i t y characteristics of two d i f f e r e n t aromatics conversion processes: Xylene Isomerization and Selective Toluene Disproportionation (STDP). In these two reactions, two d i f f e r e n t principles (5,6) are responsible for t h e i r high s e l e c t i v i t y : a restricted transition state i n the f i r s t , and mass transfer limitation i n the second. Xylene Isomerization P r i o r to the introduction of ZSM-5-based xylene isomerization processes, most of the commercial units operated with a 0097-6156/84/0248-0275$09.25/0 © 1984 American Chemical Society

In Catalytic Materials: Relationship Between Structure and Reactivity; Whyte, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

276

CATALYTIC MATERIALS


dual-functional catalyst containing a hydrogenation component usually platinum - and an acid catalyst. With such catalysts, the isomerization occurs in the presence of hydrogen via hydrogenated intermediates, e.g.:

pt

|t

pt

It

ft

It

in addition to direct methyl migration around the aromatic ring. Commercial C -aromatic streams contain considerable amounts of ethylbenzene, 15-20% when derived from reformate, and 35-55% i n pyrolysis gasoline from ethylene crackers. In xylene isomerization with dual-functional catalysts, ethylbenzene i s converted in part to additional xylenes by the same mechanism as shown for xylene isomerization, e.g.: 8

C.,-C

7

paraffins + naphthenes

However, the susceptibility to hydrocracking of the non-aromatic intermediates leads to considerable formation of light gases and naphthenes that reduce the xylene yield and the hydrogen purity.


14. OLSON & HAAG

Xylene Isomerization & Toluene Disproportionation

277


Early attempts to utilize the high acid activity of faujasite zeolite catalysts for direct xylene isomerization suffered from low selectivity. Considerable improvement was obtained f i r s t by using a large pore zeolite (7) catalyst and subsequently in several process modifications that use ZSM-5 as catalyst (2). In the following we w i l l show how these selectivity differences can be related to structural differences of the various zeolites. The acid catalyzed isomerization of xylene

is accompanied by xylene disproportionation, e.g.:

C This side reaction leads to undesirable losses of xylenes. With REHY zeolite as catalyst, disproportionation occurs at a rate comparable to that of isomerization of m-xylene (8), e.g., 14% disproportionation at 16% isomerization. In fact, the product, trimethylbenzene, is postulated as an important intermediate in isomerization (8). By contrast, under the same conditions, ZSM-5 produces orders of magnitude less disproportionation product, as shown i n Figure 1. We have examined the rate constants for disproportionation and isomerization for a variety of zeolites, using a commercial-type feed containing 70% m-xylene and 30% o-xylene i n a fixed-bed flow reactor. The results, listed in Table I, show the exceptionally low disproportionation/isomerization selectivity of ZSM-5 relative to synthetic faujasite. Synthetic mordenite and ZSM-4 have intermediate selectivities. It has been suggested that the reason for this difference i s the different site density. According to this proposal, the large concentration of acid sites in synthetic faujasite (ca. 5 meq/g ) favors the bimolecular disproportionation reaction relative to the monomolecular isomerization. By contrast, ZSM-5 has a low acid site concentration, typically less than 0.5 meq/g.



CATALYTIC MATERIALS

Figure 1. Comparison of the relative disproportionation versus isomerization selectivities of HZSM-5 and synthetic faujasite (8). Feed: m-xylene. Temperature: 300°C.


14. OLSON & HAAG

Xylene isomerization & Toluene Disproportionation

279

Table I. Selectivity in Xylene Isomerization Feed: 70% m-/30% o-Xylene, 316°C Pressure: 28 bar Si0 /Al 0 2

5 15 7 70

HY


Synthetic Mordenite ZSM-4 ZSM-5

2

3

disproportionation *isomerization 0.050 0.014 0.010 0.001

This argument, however, i s unlikely on theoretical grounds. Both disproportionation and isomerization rates should depend linearly on the number of acid sites. Experimental findings have confirmed this. Also, the data in Table I show no correlation of the selectivity with Si02/Al 0 ratio. The best correlation of the observed isomerization selectivities was found in terms of the diameter of the intracrystalline cavity, determined from the known crystal structure (9) of these zeolites, as shown in Figure 2. While faujasite, mordenite and ZSM-4 a l l have 12-membered ring ports and hence should be similar in their diffusion properties, they differ considerably in the size of their largest intracrystalline cavity; both mordenite and ZSM-4 have essentially straight channels, whereas faujasite has a large cavity at the intersection of the three-dimensional channel system. The correlation between selectivity and intracrystalline free space can be readily accounted for in terms of the mechanisms of the reactions involved. The acid-catalyzed xylene isomerization occurs via 1,2-methyl shifts in protonated xylenes (Figure 3). A mechanism via two transalkylation steps as proposed for synthetic faujasite (8) can be ruled out in view of the strictly consecutive nature of the isomerization sequence ο m ρ and the low activity for disproportionation. Disproportionation involves a large diphenylmethane-type intermediate* (Figure 4). It is suggested that this intermediate can form readily in the large intracrystalline cavity (diameter •1.3 nm) of faujasite, but is sterically inhibited in the smaller pores of mordenite and ZSM-4 (d «0.8 nm) and especially of ZSM-5 (d ~0.6 nm). Thus, transition state selectivity rather than shape selective diffusion are responsible for the high xylene isomerization selectivity of ZSM-5. 2

3

*Methyl substituted diphenylmethanes are present in trace amounts in the reaction product with ZSM-5 catalyst, and in larger quantities with ZSM-4 catalyst.



CATALYTIC MATERIALS

F i g u r e 3.

A c i d c a t a l y z e d xylene i s o m e r i z a t i o n mechanism.


14.

OLSON & HAAG


281


Isomerization of Xylene Containing Ethylbenzene Production of p-xylene via p-xylene removal, i.e., by crystallization or adsorption, and re-equilibration of the para-depleted stream requires recycle operation. Ethylbenzene in the feed must therefore be converted to lower or higher boiling products during the xylene isomerization step, otherwise i t would build up in the recycle stream. With dual-functional catalysts, ethylbenzene i s converted partly to xylenes and i s partly hydrocracked. With mono-functional acid ZSM-5, ethylbenzene i s converted at low temperature via transalkylation, and at higher temperature via transalkylation and dealkylation. In both cases, benzene of nitration grade purity i s produced as a valuable by-product. We have determined the relative rate constants for the various transalkylation reactions with a variety of zeolite catalysts (10) at 250-280°C. As indicated in Figure 5, the reaction designations (Ε,Ε), (Ε,Χ), (X,E) and (X,X) are chosen to indicate f i r s t the donor, then the acceptor of a transferred alkyl group, where Ε » ethylbenzene and X = xylene. Thus, (Ε,Χ) signifies the transfer of an ethyl group from ethylbenzene to a xylene molecule, while in reaction (X,E) a methyl group from xylene i s transferred to ethylbenzene. The relative rate constants for these reactions were obtained from plots of the various transalkylation products vs conversion as illustrated in Figure 6, and using a kinetic model based on second-order mass action competition. d Diethylbenzene dt

~ Ε,Ε

d Ethylxylene ~~

"

at

k

E

E,x C 3tX]

etc.

The validity of the second-order model was verified with feeds containing varying ratios of ethylbenzene and xylene. The data are summarized in Table II. They have been normalized to k s ι for each zeolite catalyst. In general i t is seen that the'transfer of an ethyl group (E,E;E,X) occurs faster than that of a methyl group (X,E;X,X). This i s in agreement with the indicated mechanism for transalkylation (Figure 4) which involves a benzylic carbenium ion intermediate. In the case of methyl transfer, this i s a primary cation, Ph-CH"*", whereas during ethyl transfer i t i s a more stable secondary cation, Φ-ίΗ-ΟΗ , which i s easier to form. It i s also apparent, that ethylbenzene i s a better acceptor than xylene. We suggest that this i s largely a consequence of the larger steric requirement of the bulky diphenylmethane intermediate for alkyl transfer to xylene vs to ethylbenzene. xχ

2

3


CATALYTIC MATERIALS


282

F i g u r e 5. Transalkylation xylene system.

r e a c t i o n s i n the ethylbenzene-


14.

OLSON & HAAG

Table II.


283

Transalkylation Kinetics Over Various Zeolite Catalysts 250-280°C., 28 bar, WHSV = 2-20


Relative Rate Constants Reaction

ZSM-4

Mordenite

ZSM-5

Ε,Ε Ε,Χ Χ,Ε Χ,Χ

10.4 2.2 1.3 1.0

20.8 3.6 1.5 1.0

125.0 16.8 3.6 1.0

k

8.0

13.9

34.7

k

2.2

3.6

16.8

Ethyl vs Methyl Transfer k

E,E/ X,E

k

E,x/ X,X

Ethylbenzene vs Xylene k

k

4.7

5.8

7.4

k

k

1.3

1.5

3.6

E,E^ E,X

X,E/ X,X

The effect of different zeolite structures and pore systems is also reflected in the data of Table II. With the intermediate pore ZSM-5, xylene i s apparently much less reactive than ethylbenzene, both as an alkyl donor and acceptor, than i t i s with the large pore zeolites, ZSM-4 and synthetic mordenite. This may be partly the result of increased steric crowding in the transition state of transalkylation. Another contributory factor to the increased selectivity in ZSM-5 i s the higher diffusion rate of ethylbenzene vs m-/o-xylene in ZSM-5 and hence a higher steady state concentration ratio [EB]/[xyl] in the zeolite interior than in the outside phase. Diffusional restriction for xylenes vs ethylbenzene may also be indicated by the better selectivity of synthetic mordenite vs ZSM-4, since the former had a larger crystal size. In commercial xylene isomerization, i t i s desirable that the necessary ethylbenzene conversion i s accompanied by a minimum conversion (transalkylation) of xylenes, since the latter constitutes a downgrading to less valuable products. The ability of ZSM-5 to convert ethylbenzene via transalkylation in high selectivity, as shown in Table II, leads to high ultimate p-xylene yields in a commercial process. With a simulated commercial feed containing 85% m- and o-xylene and 15% ethylbenzene, we have obtained the data shown in Table III. I t is seen that for a given ethylbenzene conversion, the xylene loss



284

CATALYTIC MATERIALS

12.0

Xylene Transalkylation (percent)

Figure 6 . Transalkylation of an ethylbenzene-xylene feed over HZSM-4. TMB = trimethylbenzene, DMEB = dimethylethylbenzene, DEB = diethylbenzene, and ETol = ethyltoluene. Feed: 16% EB, 62% m-xylene, 22% o-xylene. Temperature: 282°C. Pressure: 2 9 bar. WHSV: 8 - 2 0 .


14. OLSON & HAAG


285

is quite small with ZSM-5, twice as large with synthetic mordenite, and four times larger with ZSM-4. For a faujasite-type catalyst, the xylene loss i s even greater. These data were obtained on a once-through basis. In commercial recycle operation, the absolute selectivity values can be further optimized by varying the recycle ratio.


Table ÏÏÏ. Selectivity in Xylene Isomerization Feed: 15% Ethylbenzene, 85% Xylene (63% m, 22% o) % Xyl TransaDcylated Catalyst

% EB Transalkylated

ZSM-4 .36 M o r d e n i t e . 1 9 Z S M 5 . 0 9 As mentioned earlier, at higher temperature the selective conversion of ethylbenzene i s further enhanced by opening an additional pathway, i.e., dealkylation, that yields increased amounts of benzene of high purity: C

H

C

H

C

6 5~ 2 5

H

+

6 6 CH =CH 2

2

I

> C-C H /Metal 2

The reaction i s rendered irreversible by hydrogenating the ethylene with a selective hydrogenation catalyst. Toluene Disproportionation (TDP) At temperatures above 450°C ZSM-5 i s a very effective catalyst for the disproportionation of toluene. A process has been developed and put into commercial practice (2), The thermodynamic equilibrium composition (11) i s listed in Figure 7. The product obtained with ZSM-5 contains less of the highly substituted aromatics, as a result of diffusion and transition-state inhibition, such that the process can be approximated by the equation: 2

Toluene

> Benzene + Xylene

The xylenes are produced in an equilibrium mixture containing 24% ρ-, 54% m-, and 22% o-xylene (11). This i s readily understandable. The transalkylation occurs via an electrophilic substitution of toluene by a benzyl cation. In the absence of steric constraints, p- and o-xylene are expected as predominant


286

CATALYTIC MATERIALS

primary products. For example, the benzylation of toluene with benzyl chloride produces 55% ρ-, 41% ο-, and 4% m-methyldiphenyl methane (12). Indeed, with rare earth X zeolite as catalyst, the xylene produced from toluene at low conversion (50% p-xylene (13). For this catalyst, we estimate the ratio X j / k 7000, i.e. kj/kjj i s much faster than i t i s for the synthetic faujasite catalyst. n

S

™\?P^i

v e

D

Toluene Pisproportionation (STDP)

It has been found that the disproportionation of toluene over ZSM-5 catalyst can be directed such that p-xylene i s the predominant xylene isomer (14-17). This reaction, designated STDP, i s one of several in which disubstituted aromatics rich in the para isomer are produced. Others are the alkylation of toluene with methanol to produce p-xylene (15,18 ) and with ethylene to produce p-ethyltoluene (19,20), as well as the aromatization of olefins (20), paraffins (20) and of methanol . ~~ As i s apparent from the previous discussion on toluene disproportionation,the observation of high p-selectivity in STDP requires a dramatic change in selectivity. First, the primary product must be directed to be highly para-selective. Secondly, the subsequent isomerization of the primary p-xylene product must be selectively inhibited: p-Xylene

+

Benzene

Toluene A

/ ^ m,o-Xylene



14.

OLSON & HAAG


287

Various ways to modify ZSM-5 catalyst in order to induce para-selectivity have been described. They include an increase in crystal size (15,17,20) and treatment of the zeolite with a variety of modifying agents such as compounds of phosphorus (15,18), magnesium (15), boron (16), silicon (21), antimony (20), and with coke (14,18). Possible explanations of how these modifications may account for the observed selectivity changes have been presented (17) and a mathematical theory has been developed (22). A general description of the effect of diffusion on selectivity in simple parallel reactions has been given by Weisz (23). In this paper we present a quantitative, correlative model for STDP based on simple, measured catalyst properties. We find that the degree of para-selectivity obtainable depends uniquely on the activity and diffusion characteristics of the catalyst, independent of how these properties are obtained. While we w i l l discuss these relationships with regard to STDP, the principles involved are generally applicable to those reactions over ZSM-5 where dialkylaromatic products are formed. Μ ο < 3

£ί

f

Q

r

S

T

D

P

The general characteristics of toluene disproportionation are summarized by the data presented in Figure 8. With standard HZSM-5 catalyst, as indicated by the lowest curve, the xylenes produced contain essentially an equilibrium concentration of the para isomer (24%) and exceed i t only slightly at low conversion. The other curves result from a variety of HZSM-5 catalysts modified in different ways and to different degrees. It i s apparent that a wide range of para-selectivities can be obtained. At increasing toluene conversions, the para-selectivity decreases for a l l catalysts. The reaction scheme to be considered i s shown in Figure 9. Toluene diffuses into the zeolite with a diffusivity D . I t undergoes disproportionation to benzene and either p-, m-, or o-xylene with a total rate constant k . The i n i t i a l product distribution
BZ + DIEB > BZ + C \

= 2

•

Xylene + Xylene — >

Toluene + TMB

' \

It has also been shown that the selectivity features of para-selective catalysts can be readily understood from an interplay of catalytic reaction with mass transfer. This interaction i s described by classical diffusion-reaction equations. Two catalyst properties, diffusion time and intrinsic activity, are sufficient to characterize the shape selectivity of a catalyst, both i t s primary product distribution and products at higher degrees of conversion. In the correlative model, the diffusion time used is that for o-xylene adsorption at


OLSON & HAAG



Coke on external surface.

No coke internally. Cut away view of model for coke selectivated

HZSM-5.

Ί

Π

Π View of surface on molecular scale.

Figure 17. Schematic model for coke selectivated HZSM-5.


303

304

CATALYTIC MATERIALS


120°C.; the intrinsic activity i s characterized by hexane cracking at 538°C. A quantitative model requires knowledge of the diffusivity under reaction conditions and of the intrinsic activities for toluene disproportionation and xylene isomerization. While these are not easily obtained, the methodology has been worked out for the case of paraffin and olefin cracking (5). So far, we have obtained an approximate value for the diffusivity, D, of o-xylene at operation conditions from the rate of sorptive o-xylene uptake at lower temperature and extrapolation to 482°C (Table V).

JTable V.

Diffusion Properties of Selected HZSM-5 Catalysts

Crystal Size (2r, μη) Modification 2 l20*C D (cm /s) 2 482°C a D (cm /s) (r

/D)

120

(r

( s )

2

/D

482

0.05 None

Catalytic Materials - American Chemical Society

Recommend Documents