SiO2 catalyst and oxygen spillover

Spillover and Migration of Surface Species on Catalysts Can Li and Qin Xin, editors 9 1997 Elsevier Science B.V. All rights reserved.

481

M e t h a n e activation over Mn203-Na2WO4/SiO2 catalyst and o x y g e n spillover Zhi-cheng Jiang*, Hua Gong** and Shu-ben Li Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China

A redox mechanism is proposed for the selective oxidation of methane to methyl radical. According to this mechanism, methane activation takes place on the W 6+ sites, while activation of gas-phase oxygen occurs on the Mn 3+ sites. Owing to the oxygen spillover from Mn203 to Na2WO4 surface, the catalytic activity of oxidative coupling of methane to ethylene over Mn203-Na2WO4/SiO2 is increased more than 3 times that over Na2WO4/SiO2 catalyst.

1. I N T R O D U C T I O N Oxidative coupling of methane (OCM) to produce C2 and higher hydrocarbons has received worldwide attention as a potentially promising process for upgrading natural gas. Many catalyst systems have been reported to be effective for this reaction, while only a few of them can achieve C2 selectivity of at least 80% with CH4 conversion level >15% for long periods I~1 Among the catalyst systems published so far, a novel catalyst comprised of Na2WO4 and Mn203 supported on silica for OCM has been developed in this laboratory 121. This catalyst has shown significantly better catalytic performance(CH4 conversion 35% and C2 selectivity 68%) than that of the previously reported systems for OCM reaction. In the previously published articles, we have reported the results of a surface characterization study on Na2WO4/SiO2 catalyst. A surface reconstruction associated redox mechanism for selective oxidation of methane to methyl was proposed, which involves heterolytic dissociation of methane between W 6+ and Na § vacancy created at the high temperature of the reaction, and the lattice oxygen adjacent to the Na + vacancy is believed to be the active oxygen species responsible for methane activation. The role of Mn oxide in the Mn203-Na2WO4/SiO2 catalyst is considered to enhance exchange between gas-phase oxygen and lattice oxygen and promote lattice oxygen transport. I31'I41 In this work, the catalytic performance of OCM reaction over selected Mn203Na2WO4/SiO2 catalyst in conjunction with XANES, EXAFS, TPR, TPO and high temperature quenching EPR characterizations will be presented for the purpose of exploring the functions of Mn203 and Na2WO4 and the synergetic effect of these two essential components in the reaction of selective oxidation of methane over MnzO3-Na2WO4/SiO2 catalyst.

To whom correspondence should be addressed. Fax: (931)8417088 Present address: Shanghai Research Institute of Petrochemical Technology, SINOPEC, Shanghai 201208, China

482

2. EXPERIMENTAL Samples of the catalysts were prepared by mixture slurry method from stoichiometric solution of the salts and silica gel. After drying, the catalysts were calcined at 1023-1123K for 8 h. The details of the catalyst preparation were reported in reference [2]. Catalytic runs were made on a conventional fixed bed micro flow reactor under atmospheric pressure. The reactant and product mixture were analyzed by gas chromatography with a thermal conductivity detector, using a Porapak Q column for separation of CH4, CO2, C2H6 and C2H4 and a 5A molecular sieve column for separation of 02, Nz, CH4 and CO. The details of TPR, TPO, ESR, XPS, XANES and EXAFS measurements have been reported elsewhere [4].

3. RESULTS AND DISCUSSION 3.1. The catalytic performance of Mn203-Na2WO4/SiO2 catalyst Table 1 is the catalytic reaction results of oxidative coupling of methane over Mn2OaNa2WO4/SiO2 catalyst. The results show that, though the selectivity of C2 products in the OCM reaction over Na2WO4/SiO2 can achieve 74.5%, CH4 conversion is rather low(10.7%). In contrast, Mn203/SiO2 possesses higher conversion(16.4%) with poor selectivity (34.3%). Co-loading of Na2WO4 and Mn203 on silica support greatly enhanced both activity and selectivity. The conversion(36.8 %) plus selectivity(64.9%) is over 100%. This is very unusual and makes this catalyst as one of the best catalyst for OCM reaction reported so far f~l It may be interesting to note that the Na2WO4 component seems to be more critical for selectivity, whereas MnOx is essential for CH4 conversion. Table 1 The catal~ic performance of Na2WO4/SiO2, MnOx/SiO2 and Mn203-NazWO4/SiO2 catal~,sts Conv(%) Selec(%) Yield(%) Catalyst CH4 C2 C2 C2:/C2 ~ 90.1 0.5 SiO2 0.6 0 74.5 8.0 0.7 5wt%Na2WO4/SiO2 10.7 34.3 5.6 0.9 1.9wt%MnOx/SiO2 16.4 64.9 23.9 2.1 1.9wt%Mn-5wt%Na2WO4/SiO2 36.8 GHSV of CH4=36,000 ml.gi.h "!, T=800 *C, CH4:O2:N2=3:1:2.6

3.2. The redox behavior of Mn203-Na2WO4/SiO2 catalysts The TPR profiles of various catalysts are shown in Fig.1. The reduction peak at about 750 ~ in Fig. 1a corresponds to the reduction of Mn ions in Na+-MnOx/SiO2 catalyst. As shown in Fig.lb, W 6+ ions in Na2WO4/SiO2 catalyst start reduction at about 600 ~ and the peak attains its maximum near 900 oC. The steps in the TPR profile indicate that reduction of W 6+ ion may go through some transition states with the increase of temperature. The reduction

483 profile of Fig.lc for Mn203-Na2WO4/SiO2 catalyst consists of two peaks at about 600 ~ and >1000 ~ respectively. The reduction peak does not attain its maximum even when the temperature reaches 1000 ~ In conjunction with the results of XPS experiments, the two peaks at 600 ~ and >1000 ~ can be assigned to the reduction of Mn 3+ ions and W 6. ions in the catalyst, respectively. Moreover, the following EPR experiments will provide more proof for this assignment. It is interesting to note that only Mn 3+ ions were reduced, while W 6+ ions were not reduced in the MnzO3-Na2WO4/SiO2 catalyst at 800 ~ o

c

a

looo

8~0

7~0

5;0

~o

~L

250

I00

r ,,

Figure 1. TPR spectra for the catalyst samples a) 2.5 wt%Na+-3 wt%MnOx/SiO2 b) 5 wt% Na2WO4/SiO2 c) 3 wt%Mn203-5 wt% Na2WO4/SiO2

1000 9()0 860 760 660 560 460 3(]0 260 100 T/~ Figure 2. TPO spectra for the catalyst samples a) 2.5 wt%Na+-3 wt%MnOx/SiO2 b) 5 wt% Na2WO4/SiO2 c) 3 wt%Mn203-5 wt% Na2WO4/SiO2

Fig.2 represents the TPO profiles of various catalysts. No reoxidation peak was observed for Na+-MnOx/SiO2 catalyst(Fig.2a). Only one peak at about 600 ~ appeared in the course of Na2WO4/SiO2 catalyst reoxidation as seen from Fig.2b, but the TPO profile of MnOxNa2WO4/SiO2 catalyst (Fig.2c) shows two maxima at about 620 ~ and 700 ~ respectively. In general, the reaction temperature for oxidative coupling of methane is about 800 ~ If Mn203- Na2WO4/SiO2 catalyst was just heated to 800 ~ in the TPR experiment, that is, only the first peak at about 600 ~ appeared in the TPR profile, but the second reduction peak above 1000 ~ did not appear, and the TPO measurement was carried out, then only one peak at 700 ~ could be observed in the TPO profile for this case. The strong peak at about 620 ~ in Fig.2c could not be observed anymore. According to this result, the reoxidation process at about 620 ~ can be assigned to the reoxidation of reduced W ions in Mn203-Na2WO4/SiO2 catalyst and the peak at about 700 ~ may be caused by reoxidation of low valent Mn ions. It is clear from Fig.1 that the reoxidation of low valent Mn ions in Mn203- Na2WO4/SiO2

484 catalyst becomes much easier than that in Na+-MnOx/SiO2 catalyst, because it has been found in this work (Fig.2a) and previously reported by Zhang et al TMthat the reduced Mn ions in Na ~ -MnOx/SiO2 could not be reoxidized in 02 atmosphere at 800 ~ In brief, the above TPR and TPO experiments have demonstrated that the reduction temperature 0t" W 6+ ion in MnzO3-Na2WO4/SiO2 is higher than that in NazWO4/SiO2 catalyst, but the reducti~n temperature of Mn ion in the catalyst is lower than that in Na+-MnOx/Si()2, and the rcoxidation of low valent Mn ion in the reduced sample is easier than tha! in Na'MnOx/Si()2 . 3.3. EPR m e a s u r e m e n t

The Na2WO4/SiO2 catalyst was heated to 750 ~ in vacuum(10 -5 Torr) for an hour, then the sample was quenched in liquid N2 for EPR measurements. A paramagnetic signal with gfactor=2.0046 is clearly indicated in the EPR spectrum according to Fig.3a. A possiblc structure model for the reconstructed surface of the Na2WO4/SiO2 system, formed by the WO,s tetrahedral attaching to the SiO2 (cristobalite) surface, has been proposed by Jiang et al. TM. 'lhc WO4 tetrahcdral occupied the central three-fold sites on the unit of cristabalite(111) surface. A W atom is bonded with 3 Si atoms through 3 bridge oxygen, producing 3 W-O-Si bonds. There is onc terminal oxygen left in the upward direction. According to this structure model, the terminal bonding may be broken easily to form oxygen ion vacancics under thc condition o1" hcating in vacuum. Therefore, the paramagnetic signal ot" g-factor=2.0046 may be assigncd to the tbrmation of an oxygen ion vacancy. Linet al. 161 assigncd a highly symmetric pcak with g-v~luc=2.0()5 to an F-type center (oxygen ion vacancy) on Na'/('a() catalyst. Aftcr the liquid N, qucnching I'~PR measurement, the liquid N2 bottle was rcmoved and pure oxygcn was admitted into the sample tube when temperature was raised to 20 ~ "I'hc reaction went on Ior 20 minutes, thcn the residual 02 was removed from the sample tube and it was qucnchcd with liquid N2 again for EPR measurements. The EPR peak at g=2.0046 still remained in l:ig. 3b. I1 pure oxygcn wcre admitted when the catalyst sample was heated to near 100 ~ thc paramagnctic signal with g-factor=2.0046 disappeared completely (Fig.3c). It is suggcstcd that thc dcpletion of terminal oxygen of distorted WO4 tetrahedron could be replenishcd through the molecular oxygen activation on oxygen ion vacancy.The rate ot" the production ot" lhc latticc oxygcn is so fast that we did not detect any intermediate oxygen species.

""'--'-'-""--~~N~

L

40mT i

Figure 3. Liquid N2 quenching EPR Spectra of Na2WOdSiO2 Catalyst pretrcatcd with a) heatcd in high vacuum at 750 "C for 1 hr. b) exposed a) to pure 02 at 25 ~ c) exposed a) to pure 02 at 100 ~

485 Fig.4 shows the EPR spectra for Mn203-Na2WO4/SiO2 catalyst with various pretreatment. The catalyst was heated up at 750 ~ for an hour in vacuum (10 .5 Torr), and then quenched in liquid N2 for EPR measurement. A high intensity EPR peak at g-factor=2.01 was observed in Fig.4a. However, no paramagnetic signal was detected at 77K for the catalyst without heating in vacuum. With the consideration that the catalyst had been treated in vacuum for an hour, it is possible that Mn 3+ might have been reduced to Mn 2+, thus the intense paramagnetic signal with g-factor=2.01 could be assigned to the formation of Mn 2+ IT.Sj. In addition to the signal of Mn 2+, another signal was shown in Fig.4a with g-value of 2.002, and the peak intensity is very weak in comparison with the strong signal of Mn 2+. The stability of the signal in the gas-phase oxygen atmosphere was investigated with the following procedure. First, the bottle containing liquid N2 was removed after the EPR spectrum was recorded, and the ice and salt bath was used immediately to keep the reaction temperature below 0 ~ Second, pure oxygen was admitted into the sample tube and the reaction went on for 20 min. Then the residual gasphase oxygen was evacuated and the sample tube was quenched with liquid N2 for EPR measurements. Fig.4b displays the EPR spectra recorded under this condition. The paramagnetic signal of Mn 2+ still remained, but the peak at g-factor=2.002 disappeared even at the ice and salt bath temperature. Because W 6+ ion in the catalyst was not reduced at 750 ~ according to the TPR study of MnzO3-NazWO4/SiO2 catalyst, the EPR signal with gfactor=2.002 could also be assigned to the formation of the oxygen ion vacancy. It is interesting to note that in the case of Mn203-Na2WO4/SiO2 catalyst the gas-phase oxygen can be trapped on the oxygen ion vacancy and replenish the released lattice oxygen even at the ice and salt bath temperature.

lit

Figure 4. Liquid N2 quenching EPR spectra of Mn203-Na2WO4/SiO2 catalyst pretreated with a) heated in high vacuum at 750 ~ for 1 hr. b) exposed a) to pure 02 at ice and salt bath temperature(-10 ~ )

2o

~

"-----2"

3.4. The electron transfer between W and Mn ions in

Mn203 - Na2WO4/SiO2

catalyst

The fine structure of an absorption edge in XANES spectrum is directly related to the local, l-dependent density of final states. The position and the fine structure of an absorption

486

A

4.0

'

4.0 B

~

iLo

o.o

- 20

E

50

I.I -

- 20

50

1.1

0"0

Figure 5. Normalized WL3 XANES Spectra a) Na2WO4 ~ 2H20 b) 5 wt% Na2WO4/SiO2 c) 1.9 wt%Mn-2 wt%Na2WO4/SiO2 d) 1.9 wt%Mn-4 wt%Na2WO4/SiO2

D

u.u

50

-20

(E - E~)Icv

-

50

20

(E- ~ / c V

edge can be used as a "fingerprint" for the changes in the valence and in the local arrangement of the neighboring atoms around the absorber atom, in comparison with reference compounds. I91 Figure 5. displays the WL3 edges of reference compound Na2WO4 ~ 2H20, Na2WO4/SiO2 and Mn203-Na2WO4/SiO2 catalysts. The characteristics of the spectrum of Na2WO4 ~ 2H20 are the appearance of the strong absorption edge, whose shape is the same with that of the reference compounds NazWO4 reported in ref. [9]. The WL3 absorption edges, which are frequently called as "white line", are related to electronic transitions from 2p3/2 to empty d electronic states of an absorber atom. The tungsten 2p--+5d transitions are possible because of a mixing of tungsten d orbital with oxygen p orbital in WO4 groups, which have ideal tetrahedral symmetry 1~~ The shape and full width at half-maximum (fwhm) of the "white line" in the WL3 edge portray distinctions between tetrahedral and octahedral tungsten oxides. The resonance line is very sharp with a fwhm of 5.3 eV if the tungsten atom has a tetrahedral oxygen environment (Na2WO4). A uniformly broad white line with a fwhm of 8.0 eV is observed in the WL3 edge of WO3, whose tungsten atom has a distorted octahedral oxygen environment 191. The WL3 edge XANES of Na2WO4/SiO2 catalyst is shown in Fig.5b, the shape and fwhm of the "white line" are fitted with that of reference compounds Na2WO4 ~ 2H20. This illustrates that tungsten atom in the Na2WOdSiO2 catalyst has a tetrahedral oxygen environment. XANES for MnzO3-Na2WOdSiO2 catalyst are displayed in Fig.5c and Fig.5d. First, it indicates that, the tungsten atom in this Mn203-NazWO4/SiO2 catalyst still remains a tetrahedral oxygen environment, because a uniformly broad "white line" as shown in the WL3 edge of WO3 in ref. [9] was not observed on the catalyst. Fang et al. also considered that tungsten atom has a tetrahedral oxygen coordination in Mn203-Na2WO4/SiO2 catalyst based on their UH-RV studies Illl. However, the sharp WL3 absorption peak of NazWO4/SiO2 disappears in the XANES spectra of Mn2Oa-Na2WO4/SiO2 system. In addition, as discussed above, TPR and TPO experiments have shown the apparent differences of redox behavior of W ions and Mn ions in Mn2Oa-NaEWO4/SiO2, in comparison with that in Na2WO4/SiO2 and

487 Na+-MnOx/SiO2 catalysts. Furthermore, EPR measurements have demonstrated that the temperature for replenishment of the lattice oxygen ion vacancy in Mn2Oa-Na2WO4/SiO2 decreased for almost 100 ~ in comparison with that in Na2WO4/SiO2 catalyst. We consider that the above interesting phenomena brought about by the co-loading of MnOx and Na2WO4 on SiO2 support are originated from the electron transfer between W and Mn ions in Mn203-Na2WO4/SiO2 catalyst. It is very likely that, when electron of W2p orbit was excited by the X-ray irradiation, the electron did not transfer directly to the vacant d electronic states Of W atom, as occurred in Na2WO4/SiO2, but to the orbit of Mn ion rapidly. So the sharp resonance line of WL3 edge was not observed in XANES of Mn203Na2WO4/SiO2 catalyst. It has been proved that electron could transfer between two metal ions in the mixed oxide catalyst. Bismuth molybdate for the selective oxidation of olefins is a well known example [12'13'~41. According to the EXAFS data obtained on the Mn203-NazWO4/SiO2 catalyst, a new absorption band is displayed next to the coordination shell of lattice oxygen adjacent to W ions (Fig.6c). Simulation demonstrated that this new feature represents the Mn coordination shell adjacent to W ion, through bridge lattice oxygen. That is to say a bridge structure of W-O-Mn is formed in the MnzO3-NazWO4/SiO2 catalyst. Electron may transfer between W and Mn ions through bridge lattice oxygen, just like what occurs in Mo-Bi system. It is interesting to note that XRD analysis made by Lunsford group has demonstrated that MnWO4 was formed in the mixed Mn-Na2WO4/SiO2 system. [lsl With this interpretation of electron transfer between W and Mn ions, the alteration of the redox behavior of W and Mn ions in Mn203-Na2WO4/SiO2 system might be well understood as follows: W 6+ ions should be first reduced when reduction occurs in the system. However, the reduction of W 6+ ions only happens in a transient time, the reduced W ions could be 27.1

Figure 6. Radial distribution function around

-2.3 ~.3.5

tungsten in Na2WO4 ~ 2H20 and catalysts a) Na2WO4 9 2H20 b) 5wt%Na2WO4/SiO2 c) 1.9wt%Mn-4wt%Na2WO4/SiO2

-|-8 10.6

- 09 5.1

00

r (A)

488 reoxidized immediately by rapid electron transfer from the reduced W ions to Mn 3+ , and that makes the latter one be reduced to Mn 2+, which is the final state observed in Fig. 1c and Fig.4a for the reduction of Mn203-Na2WO4/SiO2 catalyst below 800 ~ Accompanied with the reduction of Mn 3+ ions to Mn 2*, lattice oxygen ion vacancies were formed, thus an EPR signal with g value of 2.002 was trapped in our experiments. However, the oxygen ion vacancy in Mn203-Na2WO4/SiO2 differs from that in Na2WO4/SiO2. It was formed adjacent to the Mn sites instead of to the W sites. Owing to the different microenvironment of the lattice oxygen ion vacancies, their g-value showed a slight difference.

3.5. Mechanism of methane activation and oxygen spillover It is well known that methane activation and gas-phase oxygen activation are the two important processes involved in OCM reaction. The OCM activity and selectivity of a catalyst are determined by these two processes. In the previously published works, a surface reconstruction associated redox mechanism for selective oxidation of methane to methyl radical has been proposed for Na2WO4/SiO2 catalyst I3'41. According to this mechanism, methane activation may start from adsorption on the surface of Na2WO4/SiO2 catalyst. Upon the attack by the surface lattice oxygen, a C-H bond in methane is heterolytically broken between the two sites, with H + at the Na + vacancy, and CH3 at the W 6+ ion site. H + is abstracted by the neighboring lattice oxygen of Na § vacancy to form OH-, which would combine with another H + and desorb as H20 from the surface. A methyl radical is produced from an adsorbed CH3- by transferring an electron to W 6+. The methyl radicals escape into the gas-phase for further coupling and W 6+ ions are reduced to W 5+. So, selective oxidation of methane to methyl radical over Na2WO4/SiO2 is accompanied by the reduction of the catalyst and formation of the lattice oxygen vacancies. An redox cycle would be completed by the activation of molecular oxygen from gas-phase and replenishment of the depleted lattice oxygen. Correlating the above discussed redox property of Mn203-Na2WO4/SiO2 catalyst and its OCM reaction performances, we propose that, a redox mechanism for selective oxidation of methane to methyl over this mixed oxide catalyst could be described as follows: 800~ Na +- O 2" - W 6+

)

E] - 02- - W 6+ + Na + ~ H +- 0 2 - - - - - W 6+- CH3"

[_]- 02- - W 6+ + C H 4

H +- 0 2.---- W 6+- C H 3

~. O H H20

W 5+

Mn 3+ + e Mn 2+ + 02-

), W 6+ + e

> Mn 2+ x Mn 3+

+ C H 3 ~ + W 5+

( 1)

(2) (3)

C2H6

(4) (5) (6)

489

The methane adsorption and activation to methyl radical might still occur on Na2WO4 surface and W 6+ ions should be first reduced upon methyl radical production. However, owing to the rapid electron transfer from the reduced W 5+ ion to Mn 3+, W 5+ would be reoxidized immediately and the final state observed in these mixed oxide system is the reduction of Mn 3§ to Mn 2+ (Fig.lc and Fig.4a). The electron would further transfer to the molecular oxygen adsorbed on the lattice oxygen vacancy adjacent to Mn 2+ ion, and, in this way, molecular oxygen activation and gas-phase/lattice oxygen exchange would occur on Mn203 surface. The oxygen spillover is in the opposite direction of the electron transfer, that is, lattice oxygen should transfer from Mn203 to Na2WO4 surface in the selective oxidation of methane to methyl radical. It is reasonable to imagine that the redox cycle established in Mn203Na2WO4/SiO2 catalyst would be more effective than that in Na2WO4/SiO2 catalyst because, for the component catalyst, both methane oxidation and molecular oxygen reduction could only take place alternatively on the NazWO4 surface, whereas the two activation processes could simultaneously occur on the NazWO4 and Mn203 surfaces, respectively, in the complex oxide system. In fact, according to the experimental results of the redox property studies described above, co-loading of NazWO4 and Mn203 on SiO2 support has a considerable synergistic effect of enhancing the gas-phase/lattice oxygen exchange and promoting lattice oxygen transport. As a result the W 6+ reduction could not be observed any more and the temperature for replenishment of the lattice oxygen vacancy and reoxidation of reduced Mn 2§ was decreased to as low as -10 ~ Owing to this synergistic effect, the methane conversion of the OCM reaction over Mn203-Na2WO4/SiO2 was increased to more than 3 times that over Na2WO4/SiO2 catalyst with only a slight decline of selectivity. Oxygen spillover in a mixed oxide system may significantly enhance the catalytic activity of an oxidative reaction 1161. Bithmuth molybdate catalyst for oxidative dehydrogenation of nbutene to butadiene and Sb203-SnO2 for oxidation of isobutene to methacrolein are the two well known examples. Oxidative coupling of methane over Mn203-Na2WO4/SiO2 may be another good model system for the study of this interesting phenomenon. Apparently, in order to gain a better understanding of the mechanism of selective oxidation of methane to methyl radical over Mn203-Na2WO4/SiO2 catalyst, further studies are needed to obtain more detailed information about 1). the structure of the active sites for methane and molecular oxygen activation; 2). the pathway of electron transfer between W and Mn ions; and 3). the diffusion of lattice oxygen in the mixed oxide system, with relation to methyl radical production.

ACKNOWLEDGMENT The authors gratefully acknowledge the National Natural Science Foundation of China (NSFC) for financial support of this work. We thank Prof. Yuan Kou for his help in XANES measurements and helpful discussion. We also thank Prof. Liang-bo Feng and Mr. Yan-lai Chu for the assistance in EPR, TPR and TPO measurements. Especial thanks are given to Prof. Hong-li Wang for his stimulating encouragement to this work.

490

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