The effects of Mo oxidation states on olefin metathesis

Jouml

of Mokxular

Catal&.s,

65 (1991)

15

15-28

The effects of MO oxidation states on olefin metathesis Baoau

Zhaug, Nengsheng

Department of Chewzist?y, Dalian (China.)

Dalian

Liu, Qingsong Univmit~

Lin and Dai Jin

of Technology,

IS8 Zhong Shari Rd.,

Abstract In this study the influences of MO oxidation states and its configuration on both metathesis reaction and its competing side reactions, in particular carbon deposition, are described. Various oxidation states, namely Mo(VI), MO(V) and MO(W), have been prepared by oxidation and reduction pretreatments of the silica-supported MO catalysts. Molecular identi&ations of these species were obtained via XPS and ESR techniques. A combination of catalyst pretreatments, promoter effects and induction phenomena, together with the propene conversion suggests that the distorted square pyramidal MO(V) species plays the decisive role in the metathesis function of the catalyst under study. Reduction pretreatments of the catalyst with Ha in the temperature range 430-450 “C produce an abrupt increase in carbon deposition along with a marked drop in propene conversion. The results, associated with XPS and ESR characterisation of the oxidation states, suggest that the MO(N) species is principally responsible for the severe dehydrogenation and thereby coking out of the reacting olefin. MO(W) species in crystalline MOO, has no metathesis activity. However, it exerts a reducing effect on the reacting olefin to form undesired products, Ha0 and carbonaceous deposits. Hence, overloading of MOO, seems to be detrimental.

Introduction

In olefin metathesis, the role of the oxidation states and configuration of the MO species in comparison with other metathesis catalysts is an area of fundamental interest. However, as is often the case in catalysis, it is still a subject of debate. A variety of the MO valencies, namely Mo(VI) [ 11, MO(V) [2, 31, Mo(lV) [4, 51, Mo(II1) [6, 71 and No(H) [6, 71, have been reported as the active centers for metathesis. Investigation is significantly complicated by methods of preparation, influence of the ligands, type of support applied etc. from system to system. Recently Kim et al. [8] claimed that MO(V) was not responsible for the active center in propene metathesis. In our preceding investigation [91, on the contrary, increasing the amount of distorted square pyramidal MO(V) species was found to result in increasing activity during catalyst induction. Nevertheless, this subject merits further clarification. Thomas et al. [lo] concluded that crystalline trioxide and surface compounds of MO coexisted in silica-supported molybdenum catalyst. Only the amounts of surface MOcompound and its facile reducibility were correlated with the activity of MoO,/SiOB. On increasing the metal content, the fraction of crystalline bulk oxide also increased. In this case, does it remain an

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16

inactive component or become an active MO species, inducing side reactions involving the reacting olefin? This will be clarified to some extent in the present paper. Olefin metathesis is usually accompanied by several competing parallel reactions. Among these, it suffers a great deal from carbonaceous deposition. So far in the literature, the effect of MO oxidation states in relevance to this deposition problem has not been fully addressed. The present study aims to correlate the different oxidation states of MO, i.e. Mo(VI), MO(V) and MO(W), as characterised by ESR and XPS techniques, with their iniluences on both the main metathesis reaction and those competing side reactions involving the reacting olefins such as reduction, dehydrogenation, carbon deposition etc. The discussions are based upon propene and l-hexene metathesis on silica-supported and unsupported MOO, catalysts.

Experimental

Catalyst preparation The support material used in this study was SiOa (40-60 mesh, 316 m2 g-l). The support was calcined at 500 “C for 4 h. After cooling, it was treated with 12% HCl solution overnight, washed with distilled water until pH=6-7, dried at 120 “C and calcined again for 2 h at 500 “C. Supported catalyst was prepared by impregnation of the Si02 with an aqueous solution of (NH,&Mo,O~~. 4H20 (AR grade). Details of the preparation have been described elsewhere [9]. The MoOB loading was determined by ICP model 2000 to be 10.2 wt.%. Catalyst pretreatment The catalyst sample (-200 mg), contained in a quartz tube reactor fitted with a 4-way stopcock [9], was inserted in a temperature controlled tubular furnace. An air stream was passed through the catalyst at 500 “C until its color changed from blue to light yellow. The oxidized sample was then subjected to various pretreatments respectively: (1) Hz: reduction at 400 “C for 0.5 h, (2) N2: purging at 450 “C for 2 h, (3) H,-N,: treatment (1) followed by 0.5 h N2 flow at 450 “C, or (4) HP: 0.5 h reduction in the temperature range 400-560 “C. Reactions and ESR Following the above pretreatments under appropriate conditions, the reactor was quenched in an ice-water bath. ESR (Model JES-FEIXG spectrometer) spectra of the samples were recorded at ambient temperature. Then propene (polymer grade) was admitted to the reactor. Products were analyzed at different time intervals by an on-line GC attached to a CR-1B microprocessor (Shimadzu). After reaction, the catalyst sample was once again quenched and characterised by ESR.

17

XPS Measurements were carried out by the State Research Laboratory of Catalysis in Dalian. The XPS spectra were obtained with a VG Escalab MKII spectrometer using Mg K, X-ray. Under an operating pressure of 5 x lo-’ mbar, C Is, 0 Is, MO 3d and Si 2p lines were recorded within the binding energy range 1000-O eV. The scale was calibrated with the Si 2p line (103.4 eV) as internal standard.

TGA Thermal gravimetric analysis of carbon-deposited samples was performed on a model PCT-1 balance (Beijing Optical Instruments). Carbon okterminution Two alternative methods of carbon determination were used for crosschecking results. One was from the spin signal of carbon calculated from ESR measurements using DPPH (diphenylpicrylhydrazyl, 2.9X lOi spins mg-‘) as standard in peak area weight determination. The other result was obtained directly from weight loss in TG analysis.

Results

and discussion

Pretreatment

eflects on metathesis /Ma(v)

It is well known that pretreatments with various gases under different operating conditions are extremely sensitive to metathesis catalyst activation. Thus, our results of Hz, Nz and air pretreatments of the MoOB/SiOP catalyst were compared and analysed as follows. First, pretreatments with the same reducing gas under variable conditions were examined. Figure 1 illustrates propene conversion vwsus time of reaction on MoOg/SiOP catalysts which were previously reduced by hydrogen at different temperatures. It is worth noting that after 15 min on stream of the reactant, the curve of the catalyst without Hz pretreatment (that with no temperature indication) divides the pretreatment curves into two groups. One group, that reduced under mild conditions, shows higher conversion, while the other, with severe thermal pretreatment+ exhibits lower activity. The ESR characterisations of the catalysts corresponding to various redox pretreatments are shown in Fig. 2 All of the spectra are recorded after 30 min reaction. In Fig. 3, some typical values of the g-tensors are presented. For instance, g=2.0012 (carbon), g1 = 1.951, gli = 1.863 (a distorted square pyramid, Moo(l)) and g1 = 1.942, gll = 1.892 (a distorted octahedral MO(V)(~)). The detailed assignments of these paramagnetic species have been published elsewhere [ 9 1. In comparison with Fig. 1, the correlations of propene conversion with the relative intensity of the MO(V) species are plotted as a function of reduction temperature (Fig. 4). As seen from the data in Fig. 4, the two curves are almost parallel. This fact is in accordance with the conclusion

18

Reduction temp. (%I

5

15

30

Time (mid Fig. 1. Propene conversion versus time of reaction on catalysts pretreated at various reduction

temperatures (“C).

we have drawn previously. In addition, a pronounced decrease of the curves occurs within the temperature range 430-450 “C. Some insights into the nature of the MO oxidation state may be gained in the following context. At this point,the question may be raised as to which of the two configurations, belonging to the same MO(V) oxidation state, is more important in metathesis reactivity? To answer this question, experiments and calculations to discriminate these two configurations were carried out. Figure 5 is the second differential form of Fig. 2 in which only the relative intensity of the two No(V) signals are shown. Obviously, the decrease in activity, as predicted from propene conversion, is accompanied by a change in MO(V), not only in amount but also in quality. Initially, MO(V)(~) > MO(V)(~). The activity of the catalyst is comparatively high. As the ratio of Mo(V)(l)/Mo(V)(2) drops within a certain temperature range, the activity falls off. On further exposure of the reacted catalyst to air for 3 h, the MO(V)(~) signal totally disappears. The MO(V) relative intensity drops from 18.5 to 10.3, while the MO(V)(~) signal is still detectable. This implies that the MO(V)(~) species is much more reactive than MO(V)(~). On examinin g in particular the data for the MO(V)(~) relative intensity with respect to propene conversion, a linear curve fitting is found to have a correlation coefficient of 0.948. All these findings are in agreement with our earlier conclusion [9]. Second, the pretreatments of different kinds of gases under the same conditions were compared. In Fig. 6, the effect of a sequence of gas pretreatments on catalyst activity is easily obtained, i.e. air < N2< Hz < H2+ Na. This sequential increase ln activity is found to be associated with the increasing MO(V) signal ln the same trend (Fig. 7). It should be also pointed out that

19

FMuction temp.

420 “c

433 “c

450°C

500°C

558°C

Fig. 2. ESR spectra of the catalysts with various temperature pretreatments and after 30 min reaction.

a configurational change from MO(V)(~) withg I = 1.942 (Fig. 7(c)) to MO(V)(~) with g, = 1.950 (Fig. 7(d)) is also observed under subsequent heating in Nz atmosphere. A possible explanation is that the attached Hz0 molecule (arising from hydrogen reduction of MoOa) in the distorted octahedral No(V) is removed in the nitrogen stream. Thereby the distorted square pyramidal MO(V), which enables enhancement of the activity, is formed. Moreover, to obtain further information regarding the role of MO(V)(~) in metathesis, it is interesting to review some of the results of propene metathesis over rare-earth-promoted MoO,/SiOz catalysts [ 111. Addition of rare earth oxides gives rise to an enhanced catalytic performance. For instance, an increase in Tb40, loading from 0.10 wt.% to 0.42 wt.% results in reducing the induction period by one half. Meanwhile, as shown in Fig. 8, the growth of Mom(l) species in Tb,07-Mo03/Si02 catalyst is much

Room Temp.

a)

Fig. 3. (a) ESR spectra of MO(V)(I),MO(V)(~) and carbon; (b) the 2nd differential form of

(a).

faster than the non-promoted one. Consequently, the promotion effect can be readily explained provided the MO(V)(~) species is considered the active sites in metathesis. Combining the observations from catalyst pretreatments, promoter effect and induction phenomena, it seems reasonable to conclude that the distorted square pyramidal MO(V) species is decisive for the catalytic function of metathesis under consideration. Its favorable geometric and electronic properties are in accordance with the generally accepted metallocarbene mechanism. The agreement between the results in this study and those previously measured supports this conclusion. Pretreatment JfocN)

eflects on &hydrogenation.

and carbon okposition/

Commercial metathesis processes suffer from carbonaceous deposits formed from the reacting olefins. Usually, process cycle lengths vary from a few hours to several days [ 121. Catalyst regeneration is needed to burn off accumulated coke. This is no doubt a key problem to be solved. In our earlier study of ESR on. catalyst induction, we observed an isotropic signal of g=2.0012 (see Fig. 9). It is assigned to a carbon free radical in the carbonaceous deposits. To trace the evolution of this signal is indeed meaningful. This wilI uhimately yield a better understanding of catalyst

21

i-

Y

1

400

450

I

500

,

550

Reduction temp. (“C)

F’ig. 4. Correlation between MO(V)signals and conversions as a function of reduction temperature.

deactivation, and the future design of commercial operations might benefit from this progress. As seen from Pig. 10, the carbon signal appears at the very beginning, together with the propene conversion. It increases smoothly at first and then dramatically when approaching a certain reduction temperature range, cu. 430-450 “C. The sharp profile of the curve leads to an abrupt drop in the propene conversion, Two questions might arise from this point: First, does the spin intensity of the carbon free radical represent the total amount of the carbonaceous deposits? Second, what is the cause of the steepness of the profile? To answer the first question, thermal gravimetric analysis on carbondeposited catalyst was conducted. Experiments were made under air flow on the sample after 30 min of reaction. Figure 11 is a typical TGA pattern. The weight loss before 167 “C is due to desorption. A slight gain in weight is observed in the region 167-258 “6, which is accompanied by a small exothermal peak. It can be seen that the higher the pretreatment reduction temperature, the sharper the curve. This gain is tentatively attributed to the reoxidation of the catalyst by air, i.e. recombination of owgen atoms back into the reduced Mo-oxide lattice. Weight loss between 258-548 “C, which is associated with the second exothermal peak, corresponds to the combustion of carbon deposits. Obviously, it varies with variously reduced states of the catalyst samples. This loss amounts to l-2.7% in weight with good reproducibility. Plots of the data thus obtained with those from the ESR measurement are shown in Pig. 12. It is clear that the carbon deposits determined by two

22

Conversion (5%)

Reduction temp. (“cl

MO(V)(I) MO(V)(Z)

4.68

6.84 420

7.27

3.05 0.65 558 Fig. 5. ESR spectra

of MO(V) in their second

differential

forms.

cl

0

5

Fig. 6. Conversion

15 Time (min)

30

vs. time plots of various pretreatments:

J (@) air, (A) Nz, (0) Hz, (Cl) Hz + Nz.

23

-

id

X250 (a)

x50

(d)

Fig. 7. ESR signals of the samples under various pretreatments: (a) 500 “C, air; (b) 450 “C, N,, 2 h; (c) 400 “C, Hz, 0.5 h; (d) 450 “C, Nz, 0.5 h after (c).

alternative methods are proportional to each other. This result supports the validity of applying spin parameters to indicate the amount of carbonaceous deposition. The data, however, are distributed into two linear groups with striking differences in spins. The spin numbers of the first group are -2.64.6 X lOI spins mg-’ (catalyst). This quantity is approximately an order of magnitude smaller than that of the second group, i.e. 1.3-1.6 x 1015 spins mg-’ (catalyst). Ultimate analysis of these two groups also shows different extents of dehydrogenation. The H/C ratios are - 1.8 and 1.3 respectively. For convenience of discussion, these two kinds of deposits are denoted as A and B hereinafter. It should be emphasized that the transition point of carbonaceous deposit from A to B is determined by the pretreatment reduction temperature. The transformation takes place in the range 430-450 “C. The color change of the sample from blue to black appears in the same region. This implies that there must be some corresponding change in MO oxidation states. In fact, it provides an inspiring clue to the second question regarding the cause of the abrupt decline in profile on a molecular basis of the valence state. Therefore, XI’S characterisation techniques were applied. Figure 13(a) and (b) ilhrstrate the spectra of two reduced samples at temperatures below (400

24

g

18

Ij6:::::;

fz

64220

Fig. 8. Relative MoOJSiO,.

40

intensity

I 60

80

100

Time of reaction (min) of MO(V)(~) versus

120

140

time of reaction

-1

on (a) Tb407.MoOS/Si02

(b)

X25 Mn marker

Fig. 9. ESR signal of carbon.

“C) and above (450 “C) the transition point. In Fig. 13(a), the binding energies of MO 3d3,2 and 3dSn are measured to be 236.1 and 233.0 eV respectively. Computer curve fitting of the MO 3d signals indicates only the existence of the MO(W) state. In Fig. 13(b), the presence of three MO species, Mo(VI), MO(V) and Mo(IV), is indicated from curve fittings with binding energies of 236.6 and 233.5; 234.6 and 231.6; 232.1 and 229.0 eV respectively. The atomic ratio Mo(VI):Mo(V):Mo(IV) = 1.O:1.6: 1.3. Hence, in answering the previous question, let us give a brief summary of the results leading to the steep profile. Before the step-change, the conditions of reducing pretreatment are relatively mild; the propene conversion is higher;

25

7,

c

-1.2 8

3

.l.O z E ._

g5 c

.o t 5

-0.8;

3

0

6

31 I.

,I

q\

400

450

‘-.

‘,

11. 500

550

Reduction temp. (73

10. Propene conversion and number of spins of carbon as a function of reduction temperature.

Fig.

Fig. 11. TG curves of differently pretreated catalysts after 0.5 h reaction: (a) temperature c-e; (b) TGA curve; (c, d, e, f, g, h, i) TG curves.

carbonaceous deposition is reduced; XPS records the existence of Mo(VI) exclusively while ESR detects a weak signal from MO(V). This is probably due to the bulk technique and higher sensitivity of the latter method.

26

wt. of carbon (lo2 mg)/catalyst Fig.

12. Correlation

of carbonaceous

(mg) deposits

with number

of

spinsof

carbon.

In contrast, the data after the step-change are as follows: The conditions of reducing pretreatment are more severe; the propene conversion dramatically decreases and levels off; carbon deposition experiences a rapid change not only in amount but also in quality (as characterised by 1015 spins per mg and C:H = 1:1.3). ESR measurements (Fig. 14) reveal that when the carbon signal is still developing to its full extent, the MO(V) signal shows a descending trend, probably due to subsequent reduction to the Mo(IV) state; the black color of the sample indicates the presence of lower valence states other than. MO(W). Comparison and combination of all these data lead us to conclude that the MO(W) state is likely basically responsible for the steep change of the profile. In other words, deep dehydrogenation and thereby carbonaceous deposition are preferentially taking place on Mo(IV) species of the system under study. Reduction /Mo(vI) 1-Hexene, instead of propene, is used in a model reaction to investigate the catalytic behavior of the unsupported MoOa. The crystalline trioxide (4060 mesh) is prepared from calcination of its ammonium salt (NH&Mo70a4. 4H20. It is then evacuated and purged with nitrogen several times alternately at 500 “C in situ. Reaction of 1-hexene starts at temperatures above 360 “C. The liquid products collected separate into two phases. The lower layer is identified as water. The upper organic layer, as determined by GC-MS, consists of unreacted 1-hexene and benzene. To shed some light on this phenomenon, the following experiments were performed. TG analysis of the reacted sample (dark grey in color) shows behavior similar to that in Fig. 11, although the carbonaceous deposit is easier to burn off and

0 -

Mo(lV1 fitting

Fig. 13. XPS spectra of MO 3d in samples pretreated with Hz at (a) 400 “C and (b) 450 “C.

reoxidation of the reduced No state proceeds more markedly. In another run involving active carbon as catalyst, we identify that 1-hexene undergoes dehydrocyclization to form benzene and hexane. Hence, it is most likely that the reaction of benzene, arising from the unsupported MoOa sample, is ca~~~c~y induced by the carbonaceous deposit. Under our observation, crystalline MOO, itself has no metathesis activity. This agrees with the conclusion made by Thomas et al, However, the Mo(VI) species in MoOa

28

6-

(a)

20

I

*

40

60

Time of reduction

(min)

Relative intensity of MO(V) before (a) and after (b) the reaction and ESR signal of carbon at various reduction times (450 “C). Fig.

14.

plays a reducing role towards the reacting olefin in producing Hz0 and carbonaceous deposits. Since these are deactivation factors of metathesis reactivity, excess loading of MoOa proves to be not useful. Acknowledgement We thank the State Education Commission of China for financial support. References 1 A. Ismayel-MiIanvoic, J. M. Basset, H. PraIiaud, M. Dufaux and L. de Mourgues, J. Cutal., 31 (1973) 408. 2 N. Giordano, M. Padovan, A. Vaghi, J. C. J. Bart and A. Castellan, J. Cutal., 38 (1975) 3 4 5 6 7 8

K. Tanaka, K. Miyahara and K.-I. Tanaka, Chem. L&t., (1980) 623. B. N. Kuznetsov, A. N. Startsev and Yu. I. Yermakov, J. Mol. Cutal., 8 (1980) 135. Y. Iwasawa, H. Kubo and H. Hamamura, J. Mol. Cutal., 28 (1985) 191. A. Brenner and R. C. BurweII, J. Cutal., 52 (1978) 364. A. Brenner, D. A. Hucul and S. J. Harwick, Iwg. Chem., 18 (1979) 1478. E. H. Kim, S. M. Krasnopol’skaya, B. V. Timaahkova and F. K. Shmidt, Neftekhim., 26 (1986) 52. 9 B. Zhang, Y. Li, Q. Lii and D. Jin, J. Mol. Catal., 46 (1988) 229. 10 R. Thomas, J. A. Moulijn, V. H. J. de Beer and J. Medema, J. Mol. Catal., 8 (1980) 161. 11 B. Zhang, Y. Sun, D. Jin, Q. Lin and L. Lu, Chinese J. Appl. Chem., 4 (1989) 1. 12 R. L. Banks, Appl. In&. Catal., 3 (1984) 215.