Catalytic Oxidative Coupling of Methane Over Alkaline Earth Metal Substituted Perovskite Oxides

Catalytic Oxidative Coupling of Methane Over Alkaline Earth Metal Substituted Perovskite Oxides

A. Holmen et al. (Editors),Natural Gas Conuersion 0 1991 Eleevier Science Publishers B.V., Amsterdam Jilin mwmity, changctnn, 130023, P.R.- Departme...

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A. Holmen et al. (Editors),Natural Gas Conuersion 0 1991 Eleevier Science Publishers B.V., Amsterdam

Jilin mwmity, changctnn, 130023, P.R.-

Department Of -try,

ABsIwII)T

123

-

A b o f a l k a l v 'l e earth netal (Ng, Ca, Sa) substituted lead titanate perovskite oxides synthesized. rn S t z u C t u E and tfie catalytic prcperties of the catalysts f k ~ mettrane coupling &ion were also mawred. m r c a l c h -laced cnrides surface ccnposition, surface basicity and their carresponding catalytic activity for methane anversion and the yield of hyriroCElrbns~Correlrlted.

(hparing w i t h mgnesim, b a r h and calciun stawed better nodification function for lead titanate w h n a part of lead icns were substituted.

IrnrnION

Although the catalytic oxidative coupling of methane has attracted m y catalysis chemists in the past a few years as a potential approach for converting natural gas t o hydrocarbons,1'2 there still exist a t least two serious problems on the catalysis of the reaction, one is how t o prolong the work l i f e of a catalyst, the other is how t o develope catalysts possessing l o w activity (roethane conversion) and higher selectivity for C2 hydrocarbon f o m t i o n .

Due t o the structural feathers, high s t a b i l i t y of the perovskite type oxide muld be expected for catalytic conversion of methane t o give C2 hydrocarbons. Taking into consideration two aspects that the resistance of alkaline earth roetal oxide t o high temperature loss and the valance variation of the transition metal ions and the activity of such oxides i n oxidative coupling of methane, the present authors have a t t q t e d t o synthesize a series of perovskite type oxides containing lead and titanium i n which Pb ions were p a r t k l l y replaced by alkaline earth ions (such a s Mg, Ca and Pa). According to sane reported data, it is 3 known that both PbO and Ti02 are active f o r attracting hydrogen atan fm methane mlecule to form CHj radicals, by mrbining two of which i n gas phase ethane form. EXPERIMENTAL

Preparation of catalysts (1) BaxPbl-xTi03 (04xQ.3)

124 A stoichiawtric mixture of Pb(AcI2 and EJa(Ac)2 were dissolved in needeed m u n t of water under m i l d heating followed by droplet addition of the mixed acetate solution into a 6M m n i u m carbonate solution. The formed white precipitate was settled for 12 hr. After filtration and then dehydration at 473K. the solid material was calcined at 973K in a furnace for 1 hr. A portion of previously weighed Ti02 was mixed with the oxides of lead and Barium. T k mixed oxides were heated at 1273-14731 for 7 hr to give perovskite type catalysts. (2) Ca(Mg)xFbl-x Ti03 (oCxLo.3) Because no acetates of calcim and magnesium were available in our lab, they wre synthesized by dissolving the related oxides in acetic acid, the formed acetates of calcium and magnesium were then treated by a similar process as in the case of BaxFbl-xTi03 preparation. RESULTS

AND DISCUSSION

wilkccnpo siticm of the catalysts X!?D patterns of these catalysts indicated that the alkaline earth metal substituted lead titanates MxFbl-xTi03 (M denotes C a , Mg and l3a and OcxCo.3) have the same crystal structure and their diffraction angles (20) are in good agreemnt with that of standard PbTi03 In addition, no apparent changes in the

.

diffraction patterns were formed for the series of catalysts before and after reaction. In Fig. 1 cnly patterns of Ca-substituted WTi03 oxides were displayed. Based on these results one can confirm that the substitution of alkaline earth ions for part of lead ions in pbTi03 oxides was quite ideal.

CaO. 3pb0.7Ti03 I

1 Before reaction 2 After reaction

1 2

20

40 Fig. 1 XRD patterns

60

29

125 Catalytic properties determination Fig. 2 and Fig. 3 give the e f f e c t of substitution in WTi03 on the c a t a l y t i c a c t i v i t y and C2 s e l e c t i v i t y of mthane coupling.

we can see that appropriate amount of substitution m y result in an obvious enhancement of methane conversion, C2 s e l e c t i v i t y and the corresponding y i e l d (reaction carried out at 1073K over catalyst calcined a t 1473K). For instance, over Ba- or Ca- substituted sanples with the increase in the amount of substitution w i t h x ranged 0.3,methane conversion increased; on the o t k r hand, C2 sel e c t i v i t y curves went thmugh maxim. Of course, maxim also appeared for y i e l d of C2 products. However, i n case where lead ions were p a r t i a l l y substituted by Mg w i t h x ranged 0-0.3 both methane conversion and y i e l d of C2 products showed m i n i m . The d i f i c a t i o n function of Ba and Ca

in FbTi03 se6ns to be v e appa~ ~

rent. A higher y i e l d of C2 hydrocarbon of 17.6% with a mthane conversion of 38%

over Bao.3Pbo.7Ti03 a t 1073K reached. Such a better r e s u l t has never been reported for this reaction over t h i s type of catalyst.

a&.

c,sf?lEc.% L

701

%

c2 yield%

cH4 m. %

20 15

10

10

1.0 Fig. 2

0.8

0.6

0.4

0.2

Sekctivities of C2 and

CO of mthane coupling over alkaline-earth m i 0 3 oxides

metal substituted

5

1.0

0.8

0.6

Fb

0.4

0.2

0

Id.%

Fig. 3 Methane conversion and C2 yield of methane coupling over alkaline-earth mtal substituted PbTi03 oxides

126 Surface ccnposition on calcium replaced PbTiO oxides 3 As can be seen in Table, an obvious segregation of Ca ion on the surface of t h i s series of perovskite oxides occuned. Table

B u l k and surface c m p s i t i o n s of CaxPbl-xTiO

lpbO

oxides

Surface

BUU

-0.

3

.gTi03

Cao.iPb~.67Ti0i.aa

Cao. 2pbo. aTi03

CaO. 64pbO. 57Ti02. 9 1

caO. 3pb0. 7Ti03

Ca0.70Pb0. 51Ti02. 86

Based on the experimntal data of surface ccmpositions, the authors calcul a t e d the surface atanic ratio of the samples which provided s a m i n f o r m t i o n

aboot the appearance of surface oxygen vacancy and assumed that due t o the existence of such kind of anion,vacancy oxygen is easy to adsorb on this series

of catalysts.

The Ca/Ti and %/Ti ratios on these three c a t a l y s t s were measured and correlated t o their c a t a l y t i c properties f o r methane coupling as shown i n Fig. 4 and Fig. 5. C2 y i e l d %

Conv. %

3c

30

2c

20

1c

10

C

0.6

0.7

Conv. %

0.1

Pb/Ti

conversion and C2 y i e l d on surface Pb/Ti ratio of alkaline-earth

metal substituted PbTiO3 oxides

0.3

0.5

0.7

0.9

Ca/Ti

Fig. 5 Fig. 4 Dependence of methane

%yield%

Dependence of methane

conversion and C2 y i e l d on surface C a / T i r a t i o of alkaline-earth

metal substituted PbTiO 3 oxides

127

Fmn the figure, one can find that with the increase in Ca/Ti and with the decrease in %/Ti, methane conversion and yield of C2 pruducts increased; In addition, the surface Ca/Ti ratio were increased with the substitution arrrxlnt of Ca in bulk of lead titanates. This fact enables the authors to conclude that there exists certain correlation between the catalytic properties for mthane coupling and the surface basicity expressed as M/B (M: alkaline earth ion and B: titanium ion in this work). Characterizations of catalysts The bulk canposition of all the synthesized peravskite oxieds was determined by XRD technique. The experimental values of di and Ii/Io were ccnpared with the standard data. The specific surface areas of the catalysts were measured using lowtanperature air adsorption method. The surface basicity of the catalysts was detected by using C02 TPD technique described elsewhem. In this work, h v e r , only a series of calcium substituted lead titanates were tested for getting basicity infonration. 1 ml catalyst was loaded in a scmple tube and purified by flowing high-purity nitrogen and heated to 1073K at a speed of 16*/min, then a portion of C02 was admitted to contact the sarnple for 30 min. The C02 TPD spectra were recoded at a heating speed of 32'/min, the desorbed gases were collected in a liquid nitrogen trap and analysed using a gas-chmmtograph. The surface canpositions of catalysts were determined by using an ESCALABM K I I ?lectron-spectraneter. Mg Koc (W1253.6ev) was used as photon source and Cls (BE=285.Oev) was referred as a standard to calibrate the obtained binding energies. Measuranents of catalytic properties The catalyst pellets were suspended in a 9 m l id q u a r t z tube that was placed in a furnace. Reaction temperature was 1073K. The mixed reaction gases of 16 ml/min of methane, 8 ml/min of oxygen and 50 ml/min of nitrogen were introduced. 2 ml 20-60 mesh catalyst was used in each run. Reaction products were analyzed gas-chnmatographically using a FID cubbed with a parapk Q colum, The reaction temperature was controlled by a probe themuple embedded in the catalyst bed. Before the reaction mixture was admitted, the catalyst was activated at 1073K by flowing high purity nitrogen for 0.5 hr. The stability of the catalyst was monitored until a time-independent stable activity and selectivity reached. Due to extremely desired stability of this type of catalyst the reaction was sampled at 15 min intervals in mst experiments. The experimental data of C02 TPD measurements: In order to understand basicity dependence of catalytic activity of this

128

series of catalysts, the authors detected the calciun ion substituted m i 0 3 TH) technique. The obtained results are displayed in Fig. 6. me can find fnm this figure that with increase of Ca ion amxlnt up t o 0.3 mle, three desoxption peaks of C02 were appeared, the tarperatures of peaks corresponding t o the basicity of C02 adsorption centers were 473K. 773K and 1073K, respectivily; the variation trend of the basic center anrunt are i n coincidence with their catalytic a c t i v i t y f o r rethane coupling. with x 0.3 by C02

-0.3

-0.2

cao.l -0.0

I

100

300 Fig. 6

500

700

900

T

C02 TPD spectra of Ca Pb Ti03 x 1-x

arJcLusIcNs 1. XFD masuremnts indicated successful substitutions for part of Fb ions by alkaline-earth ions t o f o m single (Ba, Mg)xFbl-xTi03 (0 x 0.3) oxide phases Like unsubstituted FbTi03. 2 . m i 0 3 is an active catalyst f o r methane coupling, hmever, part substitution of it by alkaline-earth metal m y enhance the conversion and C2 selectivity. 3. Calcination terrperature may apparently effect the c a t a l y t i c properties of the substituted p e m k i t e oxides. 4. 'Ihe surface ccrrpositions of the substituted oxides are quite different fran their bulk carpositions. In addition, obvious segregation of alkaline earth ion ( i n case where ~a ions were used t o substitute part of Fb ions) segregated m the surface region. 5. Surface -/Ti and %/Ti ratios were d e t e m k e d and wrrelated to the conwrsion of methane and yields of fom& hydrocarbons. Wre perhaps e x i s t s a parallel relationship beheen the basicity m u n t and the catalytic properties as detected by C02 TFD technique.

129 PE33REKES

1. K. Otsuka and K. J h o , J. Catal., 100. 353 (1986) 2. H. Imai and T. Tagawa, J. Chem. Soc., Chan. Ccmrun., N0.l. 3.

52 (1986)

M. Y. Sinev, G. A. Vombeva and V. N. Korchak, K i n e t i c s and Catalysis

(Russia)Vo1.27(5), 1164 (1985) AcmmmIx;EMENT

Financial support of this work fnm China

Natural Science FUnd Camrittee

is gratefully ackncwledged. Wang Jiang and L i senzi are also thanked for t h e i r assistance i n the preparation of this m u s c r i p t .