Activity stability of a copper(II) oxide—zinc(II) oxide catalyst for oxidative dehydrogenation of cyclohexanol to cyclohexanone

Applied

Catalysis, 41 (1988)

53

53-63

Elsevier Science Publishers B.V., Amsterdam -

Printed in The Netherlands

Activity and Stability of a Copper(I1) OxideZinc(H) Oxide Catalyst for Oxidative Dehydrogenation of Cyclohexanol to Cyclohexanone YU-MING LIN and IKAI WANG* Department of Chemical Engineering, (Republic of China)

National

Tsing Hua University,

Hsinchu,

Taiwan

and CHUIN-TIH YEH Department China)

of Chemistry,

National

Tsing Hua University,

Hsinchu,

Taiwan (Republic

of

(Received 23 March 1987, accepted 10 February 1988)

ABSTRACT Oxidative dehydrogenation of cyclohexanol to cyclohexanone over a commercial CuO-ZnO catalyst was studied in a fixed-bed microreactor. Experimental results showed that the catalyst maintained constant activity at low oxygen : cyclohexanol mol ratios and its stability was dependent on the raction temperature. Both the activity and stability of the catalyst could be improved by using nitrous oxide instead of oxygen as the oxidant or by modifying the catalyst with palladium oxide or heteropoly acid (K3PMo1204,,.3H20). An analysis of the deposit on the spent catalyst showed the existence of oligomers of cyclohexanone. The accumulation of these higher molecular weight products on the catalyst is believed to be the major cause of the catalyst deactivation.

INTRODUCTION

In industry, copper catalysts have been used effectively for the dehydrogenation of cyclohexanol to cyclohexanone which is an intermediate in the production of caprolactam from cyclohexane [ 1,2]. However, this conventional dehydrogenation process suffers from two primary disadvantages: (i) the reaction is highly endothermic so that a costly shell and tube reactor must be used in order to supply the heat of reaction; (ii) the conversion of cyclohexanol is severely constrained by thermodynamic equilibrium because the reaction temperature must be limited to below 280°C in order to prevent sintering of the copper catalyst [ 3,4 1. If by adding a gaseous oxidant to the cyclohexanol feed stream, the direct dehydrogenation process could be changed into an oxidative dehydrogenation

0166-9834/88/$03.50

0 1988 Elsevier Science Publishers B.V.

54

process [ 51, there would be three significant advantages over the conventional dehydrogenation process [ 6,7] : (i) the maximum extent of the reaction could be raised, perhaps even exceed the equilibrium conversion of direct dehydrogenation, (ii) by properly adjusting the oxidant gas in the feed stream, and partially converting hydrogen to water, a change of thermal effect could be attained, thus an improvement in the conventional process could be achieved; (iii) the energy consumption could be reduced because in-situ burning of hydrogen would eliminate the need of supplying heat externally. The results of a study of the activity and stability of a commercial CuO-ZnO catalyst for oxidative dehydrogenation of cyclohexanol are presented in this paper. Also described are the operating conditions and the stable regimes of oxidative dehydrogenation. EXPERIMENTAL

Catalysts The copper-zinc catalyst used in this study was a commercially available CuO-ZnO catalyst comprising copper oxide plus zinc oxide above 95 wt.-% (with a ratio of copper:zinc near 1:2) and alumina less than 5%. The measured BET surface area of the catalyst was about 30 rn’*g-‘. The catalyst modifiers such as palladium nitrate and HPA (heteropoly acid, K3PMo12040.3H20 ) [ 81 were obtained from the Catalyst Research Centre of China Technical Consultants, Inc. (CRC/CTCI) and Union Chemical Laboratories of the Industrial Technology Research Institute (UCL/ITRI) respectively. The particle size of the catalyst chosen for the reaction was 12-20 mesh. Before the reaction, the catalyst was reduced at 220°C [4] for more than 44 h by a gas mixture containing hydrogen and nitrogen (1:6 mol ratio). Reaction system and procedure A continuous flow fixed bed microreactor was used to test the activity and stability of the catalyst under atmospheric pressure for both the oxidative dehydrogenation and direct dehydrogenation of cyclohexanol. A stainless reactor of 1.6 cm I.D. was used to carry out the experiments. The reaction was carried out under the following conditions: catalyst loading varied from 0.15 cm3 (ca. 0.21 g) to 6 cm3 (ca. 8.4 g), LHSV (liquid hourly space velocity) from 0.5 to 20 h-l, concentration of cyclohexanol from 0.007 mol*l-l to 0.03 mol*l-l (adjusted by diluting with nitrogen and/or air) and reaction temperature from 180 to 270 a C. Liquid cyclohexanol with diluting nitrogen and/or air was heated to the reaction temperature in the preheating zone just before entering the catalyst bed. To prevent a strong interaction between the oxygen and the fresh catalyst, the starting procedure for oxidative dehydrogenation was as follows.

55

Cyclohexanol diluted with nitrogen was first fed into the reactor and allowed to reach a steady state. Then air was slowly added while simultaneously nitrogen was cut back to reach a designed oxygen to cyclohexanol ratio. Product analysis The hydrocarbons in the liquid product were analysed by a Varian 3700 gas chromatograph, equipped with a flame ionization detector and a Shimadzu C-RlA data processor. The separating column was a 240 cmx3.1 mm O.D. stainless steel tube packed with 5% DDP/5% Benton 34 on 100/120 Supelcoport. The gas chromatograph was operated under the following conditions: oven temperature, 150°C isothermal; carrier gas flow-rate, 40 ml*min-’ nitrogen. The water in the liquid product was determined by a MKA-3 Karl-Fischer Moisture Content Meter. The gas phase product was analysed by another Varian 3700 gas chromatograph, with a thermal conductivity detector (TCD) and a Hewlett-Packard 3390 A integrator. A Molecular Sieve 5A column was used to analyse the nitrogen, hydrogen and oxygen gas mixture at an oven temperature of 30’ C and a Carbosieve column was used to analyse nitrogen, nitrous oxide and carbon dioxide at an oven temperature of 40 oC. Temperature-programmed reduction (TPR) Approximately 0.4 g catalyst was loaded in the sample cell and reduced by hydrogen-nitrogen mixture (lo/60 ml-min-‘) at 250°C for more than 5 h. The sample was then evacuated at 110°C for more than 5 h at 1 x 10e5 Torr (1.3 mPa). Then the catalyst was reoxidized by chemisorption of oxygen at a temperature of 30” C and an oxygen partial pressure of 20-560 Torr (2.6-74.5 kPa) or by decomposing of nitrous oxide at 90’ C and 200 Torr (26 kPa). The TPR analysis started from 30 to 300’ C at a heating rate of 10’ C *min- ’ by a gas mixture of hydrogen and helium (lo/30 ml.min-' ). The water produced during reduction was removed by a liquid nitrogen trap. The amount of hydrogen consumed during reduction was detected by the TCD and recorded by a HP 3390 A integrator. At the same time, the reduction temperature was recorded. RESULTS AND DISCUSSION

Euidence for oxidative dehydrogenation Oxidative dehydrogenation was demonstrated by comparing the data obtained with and without oxygen under identical experimental conditions. Table 1 shows that the conversion of cyclohexanol increased as the oxygen to cyclohexanol mol ratio was increased in the feed stream. It may be noted that

56 TABLE

1

Conversion of cyclohexanol and selectivity for cyclohexanone, cyclohexene, carbon dioxide and heavier products of direct and oxidative dehydrogenation of cyclohexanol at liquid hourly space velocity=0.5 hh’ and concentration of cyclohexanol in feed=0.02 mol*l-’ Reaction

temperature

(’C )

240 Oz : C,H,,O conversion selectivity* selectivity* selectivity* selectivity* products

mol ratio (wt.-%) of C&H,,0 (wt.-%) for C,H,,O (wt.-%) for C,H,, (wt.-%) for CO, (wt.-% ) for heavier

0.000” 67.43 99.18 0.23 0.58

200 0.070 70.48 99.08 0.20 0.72

0.135 73.74 98.99 0.40 0.15 0.45

*Selectivity: weights of cyclohexanol converted to a specific product carbon dioxide, etc.) per unit weight of cyclohexanol converted. **Thermodynamic equilibrium conversion of direct dehydrogenation 68.69 wt.-% at 240°C and 45.22 wt.-% at 2OO”C, respectively.

o.ooo** 42.39 99.65 0.35

0.060 45.09 99.84 0.14

(cyclohexanol, calculated

0.116 48.58 99.59 0.12 0.12 0.17

cyclohexene,

from ref. 9 were

the conversion in the presence of oxygen exceeded the equilibrium value of direct dehydrogenation. The selectivity for cyclohexanone was greater than 99% and the yield of carbon dioxide was far less than 1%. Table 1 was abstracted from a detailed material balance sheet. The error of the total material balance was within 2% and the error of oxygen balance was within 3%. From the hydrogen and water production we can estimate the conversion of cyclohexanol through direct dehydrogenation and oxidative dehydrogenation respectively. The sum of the above estimated conversion was only less than 6% in error with the measured conversion. The overall heat of reaction could also be calculated from hydrogen and water production and a change of heat of reaction could be expected by adding the appropriate amount of oxygen in the feed. Conditions for stable operation Fig. 1 shows the influence of mol ratio of oxygen to cyclohexanol on the oxidative dehydrogenation. Under the low oxygen to cyclohexanol mol ratios, the oxidative dehydrogenation performed very well as demonstrated in Table 1. However, two difficulties arose under the high oxygen to cyclohexanol mol ratio. Firstly, at low LHSV’s the selectivity to cyclohexanone dropped and that to cyclohexene and carbon dioxide increased along with the increase of oxygen contents in the feed, although simultaneously the conversion of cyclohexanol kept on increasing. Secondly, at high LHSV’s, conversion of cyclohexanol dropped very rapidly. A typical deactivation curve at a high LHSV and a high

57

0

0.5 O2 X6y20

10 mole ratio

OA-

__.

U

zuu Tine

on

Stream,

LOO min

Fig. 1. Effect of oxygen to cyclohexanol mol ratio on oxidative dehydrogenation of cyclohexanol. Each point was taken at a time-on-stream of 4 h. Reaction conditions: T, 240°C; Re, 0.37; cont., 0.02mol1~‘;LHSV(@)0.5h~‘,(~)5h~‘,(~)10h~’,(0)20h~‘. Fig. 2. Typical deactivation curve of oxidative dehydrogenation performed at high mol ratio of oxygen to cyclohexanol and high LHSV. Reaction conditions: LHSV, 5 h-‘; cyclohexanol cont., 2.24*10-* mol 1-l; oxygen:cyclohexanol ratio, 0.229; T, 240°C.

oxygen content is given in Fig. 2. The deactivated catalyst showed an apparent colour change (brown to yellow or green). This colour change resulted from the deposition of high boiling products. This deposit was extracted from the spent catalyst with cyclohexanol and analysed by mass spectrometry. The results show that the molecular weight of the deposit is in the range of 196 to 292, corresponding to that of cyclohexanone dimer and trimer [ 10-121. The deactivated catalyst can be regenerated by burning out the deposit with air at 400°C for 8 h and then reduced by hydrogen. However, the catalyst activity could not be regenerated if the deactivated catalyst was only treated by hydrogen at elevated temperatures. To investigate the possible effects of operation conditions, the onset of conversion instability as a function of LHSV, reaction temperature, Reynolds number, particle size and cyclohexanol concentration was studied. Among which the most important factors are LHSV and reaction temperatures. Fig. 3 shows that the LHSV or cyclohexanol had a major influence on the maximum mol ratio of oxygen to cyclohexanol. The higher the LHSV, the lower was the maximum mol ratio. Below the curve shown, a constant conversion could be held for an operating period of 4 h. Above the curve, the conversion decreased rapidly as the LHSV was greater than 5 (as shown in Fig. 2). Since the catalyst decay starts at the top of the bed and then progresses down to the bottom, at very low space velocity of ca. 0.5 h-l, the conversion drop may not be observed during the operating period even under high oxygen to

low select~vlty and quasi -stable region

Unstable region

stable with high

0

10

20

LHSV. h-l

Fig. 3. Relation of maximum mol ratio of oxygen to cyclohexanol with LHSV. The shaded area represents the stable with high selectivity region for oxidative dehydrogenation. Each point was taken at a time-on-stream of 4 h. Reaction conditions: T, 240°C; Re, 0.37; cont., 0.02 mol 1-l. 100

. . .

5 % E

0N

._

2 L % % f u”

0.25

i u” -&

.

50

‘i; Unstable

Region

0

B

k

_0~0-=----

I

01 160

6

A Stable = Region-

I 200 Reaction

250 Temp. ‘C

290

0 0

200 Ime

400

on stream, hr

Fig. 4. Relation of maximum mol ratio of oxygen to cyclohexanol with reaction temperature. The shaded area represents the stable with high selectivity region for oxidative dehydrogenation; LHSV, 4.93 h-‘. Fig. 5. Life test of CuO-ZnO catalyst for oxidative dehydrogenation. Reaction conditions: oxygen:cyclohexanol ratio, 0.12; Re, 0.37; LHSV, 1 hP’, cont., 0.025 mol I-‘.

T, 270” C;

cyclohexanol mol ratio. However, with the increase of cyclohexene and carbon dioxide the selectivity for cyclohexanone decreases (as shown in Fig. 1). Thus the truly stable region with high selectivity of cyclohexanone should be below the flat portion of the curve, i.e. oxygen to cyclohexanol mol ratio less than about 0.07, for all LHSV. The deactivation of the catalyst at high oxygen content in the feed may be interpreted as follows. When the oxygen concentration increased, the copper was oxidized to its high oxidation states as chemisorbed oxygen atoms can not

59

be removed by the hydrogen generated. Therefore, the catalyst provides some acid sites and promotes the production of cyclohexene and the condensation of cyclohexanone. It is known from the literature the activation energy for the oxidation of copper is less than 10 kJ/mol [ 131, and that for the reduction of copper oxide is 67 -+ 10 kJ/mol [ 141. Therefore, raising the reaction temperature will increase the reduction rate faster than the oxidation rate. The stability of the catalyst will be increased at higher tempeatures. This is provided by the results as shown in Fig. 4. According to the above discussion, a life test for 300 h was made as shown in Fig. 5. Concerning the other operating conditions such as Reynolds number, particle size and concentration of cyclohexanol, our results showed that as the Reynolds number was smaller than 0.7 and the particle size was greater than 0.3 mm there existed significant external and internal mass transfer resistance which resulted in a lower conversion. However the effect of the above operating conditions on the stability of oxidative dehydrogenation proved to be negligible. Promoter effects From the above results, it would be desirable if the maximum mol ratio of oxygen to cyclohexanol for stable operation could be raised. In order to raise this maximum value, a variety of oxides including alkali oxides, alkaline earth oxides, rare earth oxides, silver oxide, palladium oxide and heteropoly acid (HPA) were impregnated on the CuO-ZnO catalyst. Only palladium oxide and HPA had a positive effect (Fig. 6). The positive effect of palladium oxide is believed to be due to its modification of the reducibility of the catalyst [ 141. HPA,wt’l. 0.50 O

5 I

1IO

0.1 Pd. wt’/.

Fig. 6. Effect of modification of CuO-ZnO via impregnation tion conditions: T, 240°C; LHSV, 4.93 h-l. (0 ) HPA, (0)

of palladium oxide and HPA. Reacpalladium oxide.

61 s

= 23.3 torr

/f---F

(a)

(b) Fig. 8. TPR profiles of (a) CuO-ZnO catalyst without prereduction, and reoxidized oxygen atoms on the prereduced CuO-ZnO catalyst surface via (b) oxygen chemisorption at various oxygen pressures and (c) nitrous oxide decomposition at 200 Torr (26.6 kPa).

The TPR spectra of Fig. 8 indicate that there are three kinds of oxygen atom adsorbed on the surface of the copper crystallites. These oxygen atoms, may be removed by hydrogen at temperatures around 19O”C, 16O”C, and 120°C respectively. Since mild oxidation of supported copper by nitrous oxide may generate a structure of Cu,O on the surface [19], the TPR peak at 120°C is, therefore, assigned to the reduction of this structure. The oxidation of copper by oxygen is more severe. Copper with high oxidation states may be attained. From the TPR profile of Fig. 8a for which the copper (II) oxide structure had been characterized by X-ray diffraction and compared to literature TPR data [ 14,211, the peak at 19O”C, generated by extensive oxygen oxidation, should come from reduction of the copper(I1) oxide structure. According to the discussions in the Section Conditions for stable operation, it is believed that an extensively oxidized copper surface attracts cyclohexanone strongly and causes the observed polymerization and deactivation. Significance for industrial application From the discussion in the Section Conditions for stabEeoperation, it is noted that stable operation can be obtained as the mol ratio of oxygen to cyclohexanol is below 0.07. Moreover, an apparent change of heat of reaction could be observed if the oxygen was added to the raction mixture. For an industrial process, this is significant in two ways. Firstly, a less costly and energy saving adiabatic reactor may be used to carry out the conversion of cyclohexanol to

62

cyclohexanone via oxidative dehydrogenation compared to the conventional process. Secondly, the capacity of a shell and tube reactor operated in a conventional process may be enhanced by increasing LHSV of cyclohexanol. In the conventional process, a temperature drop will occur inside the reactor because of the high endothermicity of dehydrogenation. The higher the LHSV of cyclohexanol, the greater is the temperature drop. Thus the LHSV is restricted to 0.5 to 1.0 h-’ for conventional direct dehydrogenation. However with proper addition of oxygen, the temperature drop will decrease because of the change in the heat of reaction. Therefore, a higher LHSV can be used without the problem of temperature and conversion drop. In this case, the output of an existing reactor can be increased. CONCLUSION

From the experimental results we conclude that with the addition of oxygen or air, direct and oxidative dehydrogenation of cyclohexanol may be simultaneously carried out over CuO-ZnO catalyst. The stability of the catalyst can be maintained at low oxygen:cyclohexanol ratios. The heat of reaction will apparently change by adding the oxygen to the reaction mixture. These results are of significance to industrial operations. From the evidence of TPR analysis, it is believed that the oxidation state of the adsorbed oxygen atoms may affect the activity and stability of the CuOZnO catalyst. However, the primary cause of catalyst deactivation is thought to be deposition of high-molecular-weight oligomers of the product of cyclohexanone on the active sites. Better activity and stability are obtained at high operating temperatures. They can be further improved by using nitrous oxide rather than oxygen gas as oxidant in the feed stock or by modifying the catalyst with palladium oxide and HPA. ACKNOWLEDGEMENT

The authors wish to express their gratitude for the support of this work by the Chinese National Science Council. Also, to Dr. N.Y. Chen for his inspiring discussion and advice.

REFERENCES 1 2 3

H.C. Ries, A. Private Report by the Process Economics Program, Stanford Research Institute, Report No. 7-A, March, 1968, p. 87. Y.L. Wu, Ph.D. dissertation, National Taiwan University, 1982, p. 10. W.L. Yang, Master’s thesis, National Cheng Kung University, Taiwan, 1983, p. 8.

63 Imperial Chemical Industries, “Catalyst Handbook.” Springer-Verlag, Billing and Sons, Ltd., New York, 1970, p. 111. 5 V.K. Skarchenco, Int. Chem. Eng., 9 (1969) 1. 6 C.N. Satterfield, Heterogeneous Catalysis in Practice, McGraw-Hill, New York, 1980, p. 199. H.H. Hsing, Chemical Technology Workshop, Section F, Taiwan, 1983, p. 418. 8 T.T. Su, Ind. Technol. Mon., 121 (1984) 11. 9 H.A. Cubberley, and M.B. Mueller, J. Am. Chem. Sot., 63 (1947) 1535. 10 H.E. Swift and J.E. Bozik, J. Catal., 12 (1968) 5. 11 G. Gut and R. Jaeger, Chem. Eng. Sci., 37 (1982) 319. 12 A.T. Nielsen and W.J. Houliham, Organic Reactions, Vol. 16, Wiley, New York, 1968, p. 81. 13 M.W. Roberts and C.S. McKee, Chemistry of the Metal-Gas Interface, Oxford University Press, Oxford, 1978, p. 439. 14 N.W. Hurst, S.J. Gentry, and A. Jones, Catal. Rev.-Sci. Eng., 24 (1982) 233. 15 R.G. Herman, K. Klier, G.W. Simmon, B.P. Finn, J.B. Bulko and T.P. Koblynski, J. Catal., 56 (1979) 407. 16 G.C. Chinchen and KC. Waugh, J. Catal., 97 (1986) 280. 17 T.H. Fleisch and R.L. Mierille, J. Catal., 97 (1986) 284. 18 J. Cunningham, G.H. Al-Sayyed, J.A. Cronin, C. Healy and W. Hirschwald, Appl. Catal., 25 (1986) 129. 19 M.I. Chen, C.T. Cheng and C.T. Yeh, J. Catal., 95 (1985) 346. 20 J.J.F. Scholten, A.P. Pijpers and A.M.L. Hustings, Catal. Rev. Sci. Eng., 27 (1985) 151. 21 H.P. Wang, and C.T. Yeh, J. Chinese Chem. Sot., 30 (1983) 139. 4