OXIDATION OF CYCLOHEXANE ON ZEOLITES

CONTAINING TRANSITION METALS WITH A

VARIABLE DEGREE OF Na/M EXCHANGE

O. V. Al'tshuler and I. L. Tsitovskaya UDC 542.943.7: 547.592.12:661.183.6

The oxidation of cyclohexane on zeolites, containing cer ta in transi t ion metals (Cu, Ag, Ni, Pd), was studied in [1]. It was shown that the p rocess can proceed in two directions: toward profound oxidation, and toward oxidative dehydrogenation with the formation of benzene. The ratio of the react ion products was determined by the employed experimental conditions and the nature of the cation introduced into the zeolite. In the present paper the oxidation of cyclohexane was studied for cer ta in ionic fo rms of the Y type of zeo- lite, which were not studied in [1]. In addition, for CuY and FeY we studied the dependence of the total ac - tivity and the selectivity of the p roces s on the degree of Na/M exchange, and the behavior of zeolites, con- taining the ions of the transi t ion metals, was compared with the behavior of the s tar t ing NaY zeolite.

E X P E R I M E N T A L

Most of the experiments were run by a method that was close to that descr ibed in [1]. The 02 and N 2 mixture of variable composit ion was sa turated with C6H12 vapors by its passage through a bed of g ranu- lated A1203, impregnated with liquid CGH12. The C6Ht2 concentrat ion was 3.7-3.8 volume %. The feed rate of the gas mixture was 50 cm3/min. The catalyst weight was 300 rag, and the granule size was 0.1-0.5 ram; the t empera tu re in the catalyst bed was measured with a chromel - alumel thermocouple, which was inser ted into a thin-walled quartz well. The pulse method was used in the individual experiments . All of the components were analyzed chromatographica l ly [column packed with zeolite 5A (N 2, O 2, CO), s i l ica gel KSK (CO2), and poly(ethylene glycol adipate) deposited on diatomaceous br ick (C~H12 , QH6) ].

D I S C U S S I O N OF R E S U L T S

The performed experiments confirmed the substantial activity of zeolite CuY in the oxidation of C6H12 and the ability of this catalyst to guide the process toward oxidative dehydrogenation, which was established in [1]. The temperature dependence of the conversion of C6HIz to C6H r CO, and CO 2 for samples, 'which differed in the degree of replacement of Na + ions by C-a + ions, is sho~ra in Fig. i. The tendency for an in- crease in the yield of benzene with increase in the degree of exchange is clearly expressed. Together with this, the conversion of C6H12 to C~H~, referred to one Cu 2+ ion, is maximum at the least degree of ex-

change (10%), as can be seen from Fig. 2. If we judge by the total C6Hi2 conversion, then the starting so- dium zeolite has the greatest activity, but the participation of the reaction for the formation of CsH, in the total conversion is minimum in the case of NaY. Apparently, oxidation in the absence of the transition metals proceeds by a theoretically different mechanism, which is discussed in detail in [2]. When based on the amount of CO 2 that is formed at low temperatures, the zeolite with a low degree of exchange ap- proaches the s tar t ing NaY. With increase in the tempera ture , for all of the zeolites the yield of benzene passes through a maximum, corresponding to 350-400~ The reason for this tempera ture dependence are d iscussed below.

With the exception of CuY, of the other ionic fo rms of the zeolite, and specifically MnY, CoY, and FeY, tes ted in the present study, only the la t ter exhibits noticeable activity in the react ion for the formation

Institute of Chemical Phys ics , Academy of Sciences of the USSR, Moscow. Translated f rom Izves - t[ya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 825-829, April , 1974. Original ar t ic le sub- mit ted June 26, 1973.

�9 1974 Consultants Bureau, a division of Plenum Publishing Corporation, 227 IVest 17th Street, New York, N. Y. 1001t. No part of this publication may be reproduced, stored in a retrieual system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the publisher. ~ copy of this article is available from the publisher for $15.00.

789

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80

50

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0

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0

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0 0

80

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? I b z/O Z

~, x...,~-x"',k"x/-~x ZO ~ ~

80

50

qO

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x

/ A

25O

!

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Fig. 1. Oxidation of C6H12 on zeo- l i tes NaY and CuY with a variable degree of exchange: a) NaY; b) CuY-10; c) CuY-28; d) CuY-50; e) CuY-70. 1) C~H6; 2) CO2; 3) CO.

of C6H 6 by the oxidative dehydrogenation mechanism. Replacing the Na + ions by Mn 2+ and Co 2+ ions leads to a deactivation of the zeolite - to a substantial increase in the tempera ture of the s tar t of oxidation (from 180-250~ Beginning with ~300~ unstable tempera ture conditions and the formation of coke are observed, which are charac te r i s t i c for NaY; benzene is not de- tected in the reaction products up to ~400~ Spontaneous heat- ing up is accompanied by the intense formation of profound oxi- dation products. As a result , when a par t of the Na + ions is r e - placed by Mn 2+ and Co z+ ions the zeolite loses to a substantial degree its activity in profound oxidation, which is associated with Na +, and at the same time the replacing Co 2+ and Mn 2+ do not impart to the zeolite the ability to guide the oxidative de- hydrogenation reaction.

The studied zeolite FeY samples differed in the extent of Na+/Fe 3+ exchange and the degree of crystal l ini ty. The t e m - pera ture dependence of the eyclohexane conversion to benzene for two samples with a degree of exchange of 50 and 1270 is shown in Fig. 3. The same as with CuY, the rate of C6H 6 fo r - mation increases with increase in the exchange, while its value, calculated per iron ion, decrease (at 360~ the rat io of the specific ra tes of C6H 6 formation is 2.6 for FeY samples with 12 and 50% exchange). The CGH12 eonvers ten to C~H~ is also cha r - ac te r ized by a maximum at ~360~ for the i ron-containing zeo- lites.

It is possible to assume that at this tempera ture the formed benzene already begins to be oxidized to a noticeable degree. To verify this assumption we ran some experiments in which C6H 6 vapors , instead of C6H12 vapors , were fed into the reactor . The maximum benzene concentration in the s tar t ing mixture was equal to its maximum concentration on exit from the reactor in the corresponding experiment with eyelohexane. The results, obtained for zeolite FeY (50% exchange) (Fig. 4), show that the start of noticeable C6H 6 oxidation actually corresponds to the temperature at which a decrease in the conversion of C6H12 to C~H 6 begins. From this it follows that the formation of profound oxidation products at moderate temperatures and corresponding- ly low conversions proceeds due to eyclohexane oxidation, while at higher temperatures it also proceeds due to combustion of the formed benzene. The same conclusion was reached by the authors of [I], who studied the conversion of CGHI2 to CO 2 and C6H 6 as a function of the contact time on zeolite CuY.

When the O 2 concentrat ion is decreased the selectivity of the oxidation relative to benzene increases : for the FeY sample

with an ~10% exchange the selectivity in C~H G at an 02 concentrat ion of 96, 20, and 8% was respect ively 12, 20, and 40% {temperature 370~

Apparently, the ability to guide the cyclohexane oxidation toward oxidative dehydrogenation is largely inherent specifically to the isolated ions of the transit ion metals. Convincing evidence of this are the c o m - parat ive data for CuY and CuO, deposited on si l ica gel, which are given in [1] (the selectivity in C6H 6 is g rea te r in the case of the zeolite), and also a compar ison of the oxidation course on a FeY sample and on samples of zeolites with close weight amounts of iron, bu t which have lost their crysta l l ine s t ructure . The corresponding data are depicted in Fig. 5. As the resul t of amorphizat ton there occurs not only a reduc- tion in the total activity, which can be at tr ibuted to a contract ion of the specific surface a rea of the zeolites during their degradation, but also to a change in the direction of the react ion (see Fig. 5). In Table 1 is given the selectivity in benzene at different degrees of total cyclohexane conversion for the crystal l ine and amorphous s a m p l e s . F rom Table 1 it can be seen that the rat io in the ra tes of the two reaction directions in the case of the amorphous zeolite changes in favor of profound oxidation.

790

25 50 75 "~ 0 [ I I _ I l y.~n 35s ~ ~150"

Degree of exchange, % ~ ~ Fig. 2 Fig. 3

Fig. 2. Specific activity of copper in oxidative dehy- drogenation at a variable degree of Na+/Cu 2+ exchange (A = C6H~/Cu 2+, a rb i t r a ry units): 1) 360~ 2) 340~ 3) 320~

Fig. 3. Oxidative dehydrogenation of C6Ht2 to C~H 6 on zeolite FeY. Exchange of Na+/Fea+: 1) 50%; 2) 12%.

Apparently, at high (>400~ tempera tures the formation of C6H 6 by the oxidative dehydrogenation r e - action on zeoli tes is supplemented by the direct dehydrogenation of CGtt12. This is indicated by the data of the exper iments where the amount of 02 in the gas mixture was var ied (Fig. 6). The yield of C6H ~ as a function of the 02 concentrat ion is not the same for different tempera tures . Below 350~ a decrease in the 02 concentrat ion f rom a cer ta in value is accompanied by a decrease, and even by a complete termination, in the formation of benzene. At 390~ the C6H G yield is independent of the 02 concentration, while at even higher t empera tu res a decrease in the convers ion of C6Hi2 to C6H 6 is now observed with increase in the oxygen concentration. It is probable that in this region the contribution of direct dehydrogenation In the p roces s of benzene formation is a lready substantial, while excess 02 facil i tates its combustion. The ex- per iments , c a r r i e d out under pulse conditions, revealed that, the same as in the oxidation of CO, NH 3, C2H 4, and H 2 [3], oxidative dehydrogenation can also proceed due to the oxygen that is bound in the zeolite to the exchange iron ions. For zeolite FeY (50% exchange) under pulse conditions, at 360~ the yield of C6H 6 in the absence of 02 in the gas phase was 26%. After reduction of the surface with CO the yield of benzene dropped to 10%. The small residual activity can be attributed par t ia l ly as due to incomplete r e - duction of the surface, and part ial ly as due to direct dehydrogenation.

The experimental data, obtained for the Oa 2+ and Fe 3+ zeolites, a re in agreement with the resul ts of studying the oxidation of C6H12 on zeoli tes that contain vanadyl ions as the exchange cations [4]. Also in this case the presence of even a low concentrat ion of VO 2+ ions in the Na zeolite importantly changes its proper t ies , impart ing to the zeolite the ability to guide the oxidative dehydrogenation. This is definitely contradic tory to the concepts, according to which the cations during exchange pr imar i ly occupy the inacces - s i n e sites (St) in the zeolite lattice, with which is associa ted the frequently observed nonlinear dependence of the activity of zeolites on the degree of exchange [5].

In o rder to change the mechanism of the p roces s in the given react ion it is probably sufficient that a very small port ion of the introduced cations prove accessible . As was indicated above, MnY and CoY in their behavior during the oxidation of C6H12 are c loser to the s tar t ing NaY; the p resence of divalent: Mn and Co ions does not increase the activity of the zeolite relative to the formation of C6H ~, as was also ob- se rved in cer ta in other oxidation react ions [3]. It is possible to assume that the p r o g r e s s of oxidative

'~ 20

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Fig. 4. Formation of C~H 6 f rom C6H12 (a), and oxida- tion of C~H s to CO 2 (2) and CO (3). FeY-50.

TABLE 1. Selectivity (%) in Benzene as a Func- tion of the Degree of Cy- clohexane Conversion

] Crystalline[Amorphous ~ o n - I sample, 16"~mpl~3 rn versionl 4 //o g~ g

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i

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Fig. 5 Fig. 6

Fig. 5. Oxidation of C6H~2 on c rys ta l l ine and amorphous zeol i tes FeY: 1) c rys ta l l ine FeY, 43 mg Fe /g ; 2) amorphous FeY, 65 mg Fe/g ; 3) amorphous FeY, 33 mg Fe/g.

Fig. 6. Fo rma t ion of C6H 6 at a va r iab le amount of O 2 in the s t a r t - ing gas mix ture . FeY-12. Tempera tu r e , ~ 1) 300~ 2) 310~ 3) 320~ 4) 350~ 5) 390~ 60 400~

dehydrogenation is fac i l i ta ted by the p r e s e n c e of "labile" oxygen, a t tached to the exchange cations, in the zeolite, which was not detected in zeol i tes MnY and CoY, in con t ras t to zeol i tes Cu and Fe [3].

The re la t ion between the se lec t iv i ty in benzene and the amount of reac t ive oxygen in the zeoli te is a lso conf i rmed by the data given in [1]. A compar i son of the se lec t iv i ty of the p r o c e s s for var ious ionic f o r m s of the zeol i tes studied in [1] r evea l s that NiY is c h a r a c t e r i z e d by a smal l se lec t iv i ty value when c o m - p a r e d with CuY (respect ive ly 16 and 74%), while the se lec t iv i ty for CrY under the same conditions was 20%. As was shown in [3], when based on the amount of labile oxygen the zeol i tes fall into the order: CuY > FeY > CrY > NiY. However, the exis tence of a d i rec t re la t ion between the se lect ivi ty as r ega rd s oxida- t ive dehydrogenation and the "s tock" of labile oxygen can be a s s e r t e d re l iably only by expanding the gamut of inves t iga ted zeol i tes . In pa r t i cu la r , the p r e s e n c e of reac t ive oxygen in vanadyl zeol i tes (VONaY) and i ts par t ic ipa t ion in the oxidative dehydrogenation of cyclohexane r equ i re s special study.

The authors e x p r e s s the i r grat i tude to O. V. Krylov for his in te res t in the p re sen t work and for some valuable suggest ions, a n d t o c o - w o r k e r s o f the Academy of Sciences of the Ge rman Democra t ic Re - public E. Alsdorf , J. Burkhardt , K. H. Shnabel, and M. N. Zelinina for grac ious ly supplying the s amples of vanadyl zeol i tes .

C O N C L U S I O N S

1. In con t ras t to the oxidation of cyclohexane on zeoli te NaY, together with the fo rmat ion of profound oxidation products , the oxidation of C6H12 on zeol i tes CuY and FeY p roceeds with the oxidative dehydrogena- tion of the C6Hi2 to benzene.

2. The yie ld of benzene i n c r e a s e s with i nc rea se in the degree of Na + exchange by the ions of the t rans i t ion me ta l s , but the specif ic act ivi ty for this react ion (when based on one Fe 3+ or Cu 2+ ion) dec reases .

3. The se lec t iv i ty in benzene fo r c rys ta l l ine s amples of a luminos i l ica tes that contain the ions of the t rans i t ion me ta l s is higher than for the cor responding amorphous ca ta lys t s .

i.

2.

LITERATURE CITED

I. Mochida, T. Yitsumatsu, A. Kato, and T. Seiyama, Bull. Chem. Soc. Japan, 44, 2595 (1971). I. L. Tsitovskaya, O. V. Al'tshuler, and O. V. Krylov, Dokl. Akad. Nauk SSSR, 212, 1400 (1973).

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3.

4.

5.

S. Z. Roginskii, O. V. Al'tshuler, O. M. Vinogradova, V. A. Seleznev, and I. L. Tsitovskaya, Dokl. Akad. Nauk SSSR, 19___66, 872 (1971). E. Alsdorf, J. Burkhardt, K. H. Shnabel, M. N. Zelinina, O. V. Krylov, O. A. A l'tshuler, and I. L. Tsitovskaya, Kinetika i Kataliz (in press). O. V. Al'tshuler, I. L. Tsitovskaya, O. M. Vinogradgva, and V. A. Seleznev, Izv. Akad. Nauk SSSR, Ser. Khim., 2145 (1972).

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