J . Phys. Chem. 1990, 94, 68 13-68 19
6813
Modification of the Surface Pathways in Alkane Oxidation by Selective Doping of Brransted Acid Sites of Vanadyl Pyrophosphate G.Centi,* G. Golinelli, and G. Buscat Department of Industrial Chemistry and Materials, University of Bologna, Viale Risorgimento 4 , 1-40136 Bologna, Italy (Received: November 28, 1989)
The modification of Br~nstedPOH groups of (VO)2P207by selective doping with K in an anhydrous medium causes a considerable modification of the surface oxidation pathways in C4-and C5-alkaneoxidative transformation, with (i) a considerable decrease in the selective formation of maleic anhydride from n-butane and of maleic and phthalic anhydrides from n-pentane, (ii) an increase in the relative formation of phthalic anhydride with respect to maleic anhydride from n-pentane, and (iii) an increase in the formation of C-containing surface residues. It is suggested that these effects are due to an inhibition of the catalyzed transformation of furan-like intermediates to corresponding lactones and then to anhydrides in the presence of gaseous O2and to a change in the relative rates of 0-insertion and H-abstraction on these surface intermediates. The role of the dynamics of competitive surface transformations of adsorbed intermediates on the overall behavior of vanadyl pyrophosphate in Cq- and C5-alkane selective oxidation is also discussed.
Introduction The catalytic behavior of metal oxides in heterogeneous vapor-phase selective oxidation reactions is often described in terms of acid/base concepts and oxidation/reduction pr0perties.I The acid/base properties of the catalyst are usually considered important for the adsorption/desorption and activation of organic reactants/products and the redox properties of the catalyst for the selective insertion of oxygen into the reactant m01ecule.~~~ The two catalytic functions are not independent of each other. For example,' in the mechanism of propylene oxidation to acrolein, the a-hydrogen abstraction is thought to be composed of two operations: transfer of a proton from the olefin molecule to a surface oxide ion and an electron transfer from adsorbed allyl to the metal cation of the catalyst lattice. The former process may be considered as an acid/base reaction and the latter as a redox process. The electron transfer makes the allyl species positive and susceptible to nucleophilic attack by oxide ions. In this sense, the consecutive oxygen insertion step may also be viewed as an acid/base reaction, in which a lattice oxygen with basic character gives nucleophilic addition into the allyl intermediate. Bridging oxygens in metal-oxygen polyhedra are more basic than terminal oxygens, and recent evidence indicates that these sites are involved in the mechanism of oxygen in~ertion,~ in contrast to oxo or dioxo groups invoked previously for the step of oxygen i n ~ e r t i o n .Adsorbed ~~ oxygen forms activated by electron transfer from the solid (for example, the 02-and 0-speciesB9)are strongly electrophilic reactants. Electrophilic addition results in the formation of peroxo or epoxy complexes that are usually intermediates of the degradation of the carbon skeleton or may act as selective H-abstraction sites from alkane m o l e c ~ l e s Classical . ~ ~ ~ selective oxidation reactions of activated substrates, such as propylene oxidation to acrolein, o-xylene oxidation to phthalic anhydride, or acrolein oxidation to acrylic acid,I may be described by using these concepts. In the selective oxidation of alkanes (n-butane to maleic anhydride), on the contrary, the complex reaction mechanism cannot be rationalized only on those bases'*I2 as well as the peculiar properties of vanadyl pyrophosphate, a unique catalyst for this reaction.I2 A crucial step in the mechanism, in particular, is the 2,5 oxygen insertion on the adsorbed dihydrofuran or furan intermediates" in order to form corresponding lactones and then maleic anhdyride.'*I6 No clear indications are present in the literature about the nature of the oxygen active sites involved in this step that cannot be rationalizedI2on the basis of previously discussed mechanisms. I t is that on the surface of the most active plane, (020), of vanadyl pyrophosphate Br~nstedPOH groups are also ~~
*To whom correspondence should be addressed. 'Institute of Chemistry, College of Engineering, University of Genoa, Fiera del Mare, Genoa, Italy.
0022-3654/90/2094-68 13$02.50/0
present in the vicinity of the vanadium sites (see Figure I). These vanadium sites are thought to be the sites involved in the 1,4 oxygen addition to the intermediate butadiene in order to form dihydrofuran or furan.12,15~16 A recent kinetic studym on the effect of steam addition on the selectivity to maleic anhydride from butane indicates a beneficial role of water. The effect was interpreted as an increase in the concentration of surface hydroxyl groups, which causes site blocking of oxidized surface sites, but it is evident that the effect may be alternatively interpreted as the role of Bransted sites in the selective pathway of maleic anhydride formation. We have, therefore, explored the possibility of participation of the POH groups on the mechanism of maleic anhydride formation of vanadyl pyrophosphate, with particular reference to the modification of the surface pathways induced by the selective inhibition of the B r ~ n s t e dsites. A method of inhibition of their reactivity by specific reaction with K+ ions in anhydrous medium was adopted. This method, in contrast to the usual impregnation methods, allows the specific reaction with Bransted acid sites without affecting other surface properties.
Experimental Section ( V0)2P207Preparation. Vanadium pentoxide (50 g from (1) Haber, J . in Proceedings, 8th International Congress on Catalysis, Berlin; Dechema: Frankfurt A.M., BRD, 1984; Vol. I, p 85. (2) Ai, M. J. Catal. 1986, 100, 336. (3) Ai, M.; Ikawa, T. J . Catal. 1975, 40, 203. (4) Glaser, L. C.; Brazdil, J. C.; Hazle, M. A.; Mehicic, M.; Grasselli, R. K. J . Chem. SOC.,Faraday Trans. I 1985, 81, 2903. (5) Grasselli, R. K.; Burrington, J. D. Ado. Catal. 1981, 30, 133. (6) Burrington, J. D.; Kartisek, C. T.; Grasselli, R. K. J . Catal. 1980,63, 235. (7) Rappe, A. K.; Goddard, W. A. J . Am. Chem. SOC.1982, 104, 448. (8) Che, M.; Tench, A. J . Ado. Catal. 1982, 31, 78. (9) Che, M.; Tench, A. J . Ado. Catal. 1983, 32, I . (10) Ebner, J. R.; Cleaves, J. T. In Oxygen Complexes and Oxygen Activation by Transition Metals; Martell, A. E., Sawyer, D. T., Eds.; Plenum Press: New York, 1988; p 273. ( 1 1 ) Centi, G.; Trifirb, F.; Busca, G.; Ebner, J. R.; Cleaves, J. T. In Proceedings, 9th International Congress on Catalysis, Calgary; Phillips, M. J., Ternan, M., Eds.; The Chemical Institute of Canada: Ottawa, 1988; Vol. 4, p 1538. (12) Centi, G.; Trifirb, F.; Ebner, J. R.; Franchetti, V. M. Chem. Reo. 1988, 88, 55. (13) Busca, G.;Centi, G.J . Am. Chem. SOC.1989, 1 1 1 , 46. (14) Centi, G.;Trifirb, F.; Busca, G.;Ebner, J. R.; Cleaves, J. T. Faraday Discuss. Chem. SOC.1989, 87, 214. (1 5) Busca, G.; Centi, Trifirb, F. In Catalysr Deactivation 1987; Delmon, B., Froment, G. F., Eds.; Elsevier Science: Amsterdam, 1987; p 427. (16) Centi, G.;Trifirb, F. J . Mol. Catal. 1986, 35, 255. (17) Busca, G.; Centi, G.; Trifirb, F.; Lorenzelli, V. J . Phys. Chem. 1986, 90, 1337. (18) Ramstetter, A,; Baerns, M. J . Catal. 1988, 109, 303. (19) Puddock, S. J.; Rochester, C. H. J . Chem. SOC.,Faraday Trans. I 1986,82, 2773. (20) Arnold, E. W.; Sundaresan, S. Appl. Catal. 1988, 41, 225.
0 1990 American Chemical Society
6814
Centi et al.
The Journal of Physical Chemistry. Vol. 94, No. 17, 1990 100
select. MA, Yo
Conversion, Yo
* Sld613K
75 Q P
100
75
e o O H
Figure 1. Surface crystal structure of the (020) plane of (VO)2P207.
Merck) was stirred into 0.6 L of a technical-grade mixture (1 :2) of benzyl and isobutyl alcohols. Orthophosphoric acid (in such an amount as to give a final P/V ratio of 1.1) dissolved in 0.1 L of isobutyl alcohol was then added to form a slurry that gradually darkened upon heating to the reflux temperature (approximately 378 K). After several hours, the mixture was cooled with continued stirring. The light-green product was isolated by filtration, washed, and then dried overnight at 333 K. Both X-ray diffraction patterns and infrared examination indicated the solid to be [VOHP04]2-H20.2'-24The vanadyl hydrogen phosphate hemihydrate was then calcined at 673 K in a mixture of 1.5% butane/air until steady-state conversion of butane to maleic anhydride was well established. The reactor was then cooled; the solid recovered was shown2' to be pure (VO)2P207by chemical, infrared, and X-ray diffraction analyses. Further details on the characterization of the catalyst and its redox properties as well a catalytic behavior in the oxidation of n-butane to maleic anhydride have been previously r e p ~ r t e d . ~ ' * ~ ~ - ~ ~ The already activated catalyst sample was doped with potassium. Doping was carried out by incipient wet impregnation in an anhydrous medium (water-free ethanol) using calibrated amounts of potassium acetate (RPE, Merck). After impregnation, the catalysts were dried overnight in air at 393 K and further activated in a butane-air flow at 673 K for 3 h. Reference tests without potassium were made in order to verify the absence of negative effects from the impregnation procedure. Reactivity Measurements. Catalytic tests under steady-state conditions were made in a continuous down-flow fixed-bed integral apparatus.25 The flow reactor was charged with 4 g of sample with particle dimensions in the range 0.125-0.0250 mm. The composition of the reagent mixture was 1.6%hydrocarbon, 16% oxygen, and 82.4%helium. The total flow rate was 2.4 L h-'. The reactor was provided with an axial thermocouple sliding inside in order to control isothermicity (within 3-4 K) during catalytic runs. The absence of significant diffusional limitations (heat and mass transfer) to the rate of hydrocarbon depletion was verified experimentally by varying the feed rate at a constant W / F ratio or testing catalysts with different particle sizes. Additional calculations (thermal balance over the catalyst granules using the Colburn analogy, determination of the Biot number) indicated the isothermicity of the whole particle. The reactor assembly was interfaced between the section of the reagent mixture preparation and flow control and the section for reagent mixture analyses of the reagent composition and of the reaction products. The outlet stream from the reactor was kept at 493 K to prevent condensation of organic products that were (21) Busca, G.; Cavani, F.; Centi, G.; Trifirb, F. J . Catal. 1986, 99,400. (22) Johnson, J. W.; Johnston, D. C.; Jacobson, A. J.; Brody, J. F. J . Am. Chem. SOC.1984, 106, 8123. (23) Torardi, C. C.; Calabrese, J. C. Inorg. Chem. 1984, 23, 1308. (24) (a) Bordes, E.; Courtine, P.; Johnson, J. W. J . Solid State Chem. 1984,55,270. (b) Bordes, E.; Courtine, P. J . Chem. SOC.,Chem. Commun. 1985,294. (c) Leonowicz, M. E.; Johnson, J. W.; Brody, J. F.; Shannon, H. F.; Newsam, J. M. J . Solid State Chem. 1985, 56, 370. (25) Centi, G.; Fornasari, G.; Trifirb, F. Ind. Eng. Chem. Prod. Res. Den 1985, 24, 32. (26) Cavani, F.; Centi, G.; Riva, A.; Trifirb, F. Catal. Today 1987, I, 17. (27) Cavani, F.; Centi, G.; Trifirb, F. J . Chem. SOC.,Chem. Commun. 1985, 492.
50
50
25
25
0 0
0.5
1
0 1.5
K:P surface ratio Figure 2. Effect of the K/P surface ratio (see text) on the catalytic behavior in n-butane conversion to maleic anhydride of K-doped (VO)2P207(n-butane conversion at 613 and 653 K, selectivity to maleic anhydride at 613 K and at 20% butane conversion).
analyzed in the first gas chromatograph using a flame ionization detector. After cooling of the gas stream, oxygen, nitrogen, carbon monoxide, and carbon dioxide were analyzed in a second gas chromatograph using a thermal conductivity detector. A 3-m Porapak QS column was utilized in the first chromatograph; the oven temperature was programmed to rise from 353 to 503 K at 16 K min-'. The second chromatograph was operated with a Carbosieve-I1column, and the oven temperature was programmed to increase from room temperature to 503 at 32 K min-' after an initial 9 min under isothermal conditions. An additional column of 23%S P 1700 on Chromosorb P at 353 K was utilized for the separation of C4 and C5 hydrocarbons. Fourier-Transform Infrared Spectra (FT-IR). Infrared spectra (self-supporting disk technique) were recorded with a Nicolet MXl Fourier-transform spectrometer (resolution 1 cm-I), equipped with a conventional gas manipulation and evacuation line and IR cells, allowing measurements up to 870 K. The usual pretreatment of Torr. Adsorbate the samples was evacuation at 723 K at compounds were hyperpure products from Carlo Erba and SI0 (Milano, Italy). Self-supporting disks (about 10-20 mg/cm2) for infrared examination were prepared by pressing the samples at 20000 psi. Thermogravimetric Studies. Thermogravimetric curves were recorded on a Perkin-Elmer TGS-2 thermobalance apparatus using an air flow of 1.8 L h-' and a heating rate of 0.631 K min-'. The valence state of vanadium, after different steps in the reaction, was checked by chemical analysis according to the method already reported .21
Results Effect of K on the Catalytic Reactivity. The effect of doping vanadyl pyrophosphate with K on the catalytic behavior in nbutane oxidation to maleic anhydride is summarized in Figure 2. The K/P surface ratio is calculated assuming all the K to have been added on the surface and a surface concentration of P atoms of 6.43 X lo8 P surface atoms/m2 of surface area (1.07 X lo-' mol of P/m2) as can be e ~ t i m a t e d ~by ~ vthe ~ ~volume . ~ ~ occupied by a formula unit of vanadyl pyrophosphate and a surface area of the P is at the surface in the of 18 m2/g. Approximately catalyst. The assumption that all the K added is on the surface is reasonable because the temperatures of calcination and of catalytic tests were below that for the formation of interstitial solid solution of K in the vanadyl pyrophosphate, and no modification in the skeletal vibrations of vanadyl pyrophosphate was observed by FT-IR. Also X-ray diffraction analysis gave no evidence of changes in the structure. A strong deactivation effect of K both on the conversion and on the selectivity to maleic anhydride was found over the entire range of temperatures (Figure 2). A parallel increase in the (28) Centi, G.; Golinelli, G.; Trifirb, F. Appl. Catal. 1989, 48, 13. (29) Pepera, M. A.; Callahan, J. L.; Desmond, M. J.; Milberger, E. C.; Blum, P. R.; Bremer, N. J. J . Am. Chem. SOC.1985, 107, 4883.
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 6815
Surface Pathways in Alkane Oxidation loo
Sel. PA I Sel. MA.
Conversion, Yo
/l
/
50-
K:P surface ratio 160
u
-
540
580
700
660
620
Temperature, K
569
Figure 3. Effect of the K/P surface ratio of K-doped (VO)2P207on the n-pentane conversion as a function of reaction temperature. Select. MA, Yo
600
633
Temperature, K Figure 6. Change in the ratio of the selectivities to phthalic anhydride and to maleic anhydride as a function of reaction temperature and of K/P surface ratio.
50
-+ 0.160 25 Y )l
A
0
540
I
I
I
580
620
660
700
Temperature, K Figure 4. Effect of the K/P surface ratio of K-doped (VO)2P207on the selectivity to maleic anhydride from n-pentane as a function of reaction temperature. Select. PA, % -1
I
10
I
I
I
3400
v,c"
1
I
2800
Figure 7. FT-IRspectra in the OH region of (VO)2P207(a) and 0.98% K(V0)2P207(b) evacuated at 670 K for 10 min.
620
660
700
Temperature, K Figure 5. Effect of the K/P surface ratio of K-doped (VO)2P207on the selectivity to phthalic anhydride from n-pentane as a function of reaction temperature.
selectivity to carbon oxides was observed. Even the presence of small amounts of K on the surface (K/P = 0.004) may cause a drastic inhibition of the activity and of the selectivity to maleic anhydride. With further additions of potassium, the conversion and the selectivity to maleic anhydride decreased further. No changes in the structure were found by X-ray diffraction and FT-IR analysis of skeletal vibration after the catalytic test^.'^,^^ The effect of the doping with K on the behavior of the catalyst in n-pentane selective oxidation is shown in Figures 3-5. As already reported, two anhydrides are formed in n-pentane oxidation on vanadyl pyrophosphate, one with less carbon atoms than the starting C5alkane (C4: maleic anhydride) and one with a higher carbon atom number (c8: phthalic anhydride). No other compounds are observed in these reaction conditions besides carbon oxides and traces of benzoic acid. The formation of two reaction products, one of which involves the formation of surface template
C-C bond~,3O*~l is useful in order to obtain a better understanding of the nature of the modifications in the surface pathways induced by doping with K. The addition of K decreases the activity of the vanadyl pyrophosphate in a way analogous to that observed for butane oxidation. A more complex situation is observed regarding the effect of K on the selectivities to maleic and phthalic anhydrides. In general, the addition of potassium decreases the selectivities of both anhydrides, but the effect is more pronounced for maleic anhydride. The analysis of the ratios of the selectivities to phthalic and maleic anhydrides (Figure 6) as a function of the temperature and of K/P surface ratio clearly evidences that over the entire range of temperatures the doping with K induces an increase in the ratio of selectivitiesbetween phthalic and maleic anhydrides. Global selectivity, however, decreases with a parallel increase to that in carbon oxides. In conclusion, the effect of K doping on the reactivity of vanadyl pyrophosphate in C4-and C5-alkane oxidation is to decrease the reactivity in alkane transformation and the selective pathway of oxidation, but the change of product distribution in n-pentane oxidation indicates the occurrence of more complex surface effects as well. (30) Centi, G.; Lopez Nieto, J.; Pinelli, D.; Trifirb, F. I d . Eng. Chem. Res. 1989, 28, 400.
(31) Centi, G.; Lopez Nieto, J.; Pinelli, D.; Trifirb, F.; Ungarelli, F. In New Developments in Selective Oxidation; Centi, G., Trifirb, F., Eds.; Elsevier Science: Amsterdam, 1989; p 635.
Centi et al.
6816 The Journal of Physical Chemistry, Vol. 94, No. 17, 1990
N
0
d
N N
m
0
A
I
0'5
i
I
K:P
1.0
i
1.J
Figure 8. Change in the absorbance of the u(0H) band at 3660 cm-' as a function of K/P surface ratio in K-doped vanadyl pyrophosphate.
)O
2250 .i,cm-'
2
D
Figure 10. FT-IR spectra of pivalonitrile adsorbed at room temperature and evacuated at 320 K on (V0),P2O7(a) and 0.98% K(V0)2P207(b).
IO
Figure 9. FT-IR spectra of pyridine adsorbed at room temperature and evacuated at 450 K on (VO),P207 (a) and 0.988 K(VO),P20-, (b).
Effect of K on the Surface Acidity. The FT-IR spectra in the
OH stretching region of vanadyl pyrophosphate before and after doping with 0.98% by weight K are shown in Figure 7. Both samples were evacuated at 670 K for I O min in order to obtain complete desorption of molecular water, as deduced by the disappearance of the bands at 1620 cm-l [6(OH),]. A weak, sharp band is observed at 3660 cm-l in the pure vanadyl pyrophosphate (a). The band may be assigned to the OH stretching of free O H groups of P0H.I' In the K-doped sample (b), no evidence of the presence of free OH groups can be observed for the same evacuation conditions of sample a. Figure 8 shows that the change in intensity of the IR band of the free OH group (3660 cm-l) versus the amount of K added follows roughly the observed trend in the change of the catalytic behavior (Figures 2-5), suggesting a correlation between the two phenomena. A better characterization of the modifications of vanadyl pyrophosphate acidity by doping with K may be obtained from the adsorption of pyridine. The spectra recorded after adsorption of pyridine at room temperature and subsequent evacuation at 450 K on pure vanadyl pyrophosphate (a) and after doping with 0.98% by weight potassium (b) are reported in Figure 9. The bands a t 1610, 1578, 1490, and 1440 cm-l are typical of pyridine chemisorbed on Lewis acid sites (u(8a), u(8b), u( I9a). u( 19b).
respectively). As well-known, the extent of the shift of the u(8a) vibration ( 1 578 cm-' in the liquid) may be taken as a measure of the strength of the Lewis sites.32 The frequency we measured (1610 cm-') indicates that pyridine is coordinated on strong Lewis sites. The bands at 1640 and 1542 cm-' are typical of pyridinium cations (u(8a), u(8b) and U( 19b), respectively), whose detection confirms the presence of strong Bransted sites on the activated surface. In the sample doped with K, in the same conditions, the bands due to pyridinium cations are absent, confirming the absence of Bransted acidity. The band v(8a) is not significatively shifted in comparison to that detected after pyridine adsorption on the undoped sample, indicating that the Lewis acid sites are not affected by potassium. Two additional bands are observed in sample b, namely, at 1598 and 1440 cm-l. These two bands may be attributed33to pyridine chemisorbed on weak K+ Lewis acid sites. To obtain a more detailed evaluation of the strength of the Lewis acid sites, the adsorption of the weaker base pivalonitrile (tertbutylcyanamide) was also studied (Figure IO). Adsorbed pivalonitrile on pure vanadyl pyrophosphate shows v ( C N ) at 2290 cm-' in contrast to the liquid-phase value of 2236 cm-]. These data confirm the strong Lewis acidity of (V0)2P20,. Doping with K does not cause any shift in the frequency of the u ( C N ) band, indicating that the Lewis acid sites are not affected by this treatment. The additional presence of weaker Lewis acid sites (band at 2247 cm-') is found, confirming the presence of very weak sites that can be attributed to K+ ions.33 Effect of K on the Nature and Reactivity of Adsorbed Species. The activated (600 K) anaerobic adsorption of n-butane on evacuated (4 h at 620 K) vanadyl pyrophosphate gives rise to a relatively intense adsorption band in the region 1800-1 500 cm11.13-15,18334A main band is observed a t 1620 cm-', with shoulders near 1710-1720 and 1780 cm-' (Figure 1 1 , a). The cited bands, related to u(C=O) or v(C=C) vibrational modes of species arising from the reactive adsorption of butane, are also detected after adsorption of butadiene at lower temperature^'^-'^ and are thought to be due to oxygen-containing species arising from the reaction of butadiene (formed from butane by H-abstraction) with V 4 surface groups. The spectrum is very similar to that originating from the reactive adsorption (480 K) of 2,5(32) Boehm, H.-P.; Knoezinger, H. In Catalysis Science and Technology; Anderson, J . R., Boudart, M.. Eds.; Springer-Verlag: West Berlin, 1983; Vol. 4, p 39.
( 3 3 ) Busca, G.; Ramis, G. Appl. Sur/. Sci. 1986, 27, 114. (34) (a) Wenig, R. W.; Schrader, G. L. J . Phys. Chem. 1987, 9 / , 191 I . ( b ) Wenig. R. W.: Schrader, G. L. J . Phys. Cbem. 1987. 91, 5674
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 6817
Surface Pathways in Alkane Oxidation r,
I
I1
'
' I I
b
I
991 450
I
I
I
650
850
temp., K I
2000
1800
30
1600
1800
1600
Figure 12. Thermogravimetric curves (air, 0.631 K min-') for (VO)2P207 (a) and 0.1 1% K(V0)2P207(b).
.i, cmi'
K:P ('5) or C:P surface ratio
Figure 11. FT-IR spectra of the adsorbed species arising from n-butane adsorption (spectra a and b) in anaerobic conditions at 600 K on (VO)2P207(a) and successivelyheated in oxygen (20 Torr) at 570 K (b) and of adsorbed species arising from 1-butane adsorption on (VO)2P207 (spectra c and d) and on 0.98% K(V0)2P207(spectra e and f) in anaerobic conditions at 600 K (spectra c and e) and successively heated in oxygen (20 Torr) at 570 K (spectra d and f).
dihydrofuran on (V0)2P207.13A similar but more intense and resolved spectrum is found after reactive (600 K) anaerobic adsorption of but-1-ene on the surface of vanadyl pyrophosphate (Figure 11, c). If the samples put into contact in anaerobic conditions with the C4 hydrocarbons are later put into contact with oxygen (200 Torr) at the same temperature (600 K), the spectrum is modified and the bands at 1780 and 1850 cm-I increase considerably in intensity (Figure 11, b and d). These latter bands are typical of v ( C 4 ) modes of cyclic anhydride~,I~J**~~ indicating that maleic anhydride is formed in the presence of gaseous oxygen. A different situation is found in the vanadyl pyrophosphate doped with 0.98% K. After activated anaerobic interaction of but-1-ene with K(V0)2P207in the 1500-2000-~m-~region, a spectrum comparable with that observed for pure (VO)2P207is found (Figure 1 1, e); however the consecutive admission of O2 does not cause the appearance of the typical bands of maleic anhydride at 1780 and 1850 cm-I but rather only a decrease in the intensity of the spectrum (Figure 11, f). The formation of gas-phase C 0 2 is also detected. These results indicate that doping with K specifically inhibits the selective pathway of maleic anhydride formation even on already activated intermediates, which react with oxygen to form only carbon oxides. Effect of K on the Amount of Adsorbed Species. Data obtained by using a new transient reactor with quadrupole analysis of products11J2J4have already shown that C-containing residues are formed on the surface of (VO)2P207during the catalytic runs in the presence of gaseous oxygen. The amount of the surface C-containing residues may be determined by titration and analysis of the amount of C 0 2 that forms when pure oxygen is fed to the sample after catalytic rea~ti0n.I~ The formation of these surface residues lowers the rate constant of butane depletion in comparison with that observed for a pure clean surface.I2*I4 An alternative method for the determination of the amount of C-containing residues is the thermogravimetric analysis of the weight changes of the catalyst after catalytic tests when heated in the presence of gaseous 02.The (VO)2P207used in this investigation was first equilibrated in a flow of butane/air for a long time (800 h). After this treatment, which causes an increase in the ~electivity,'~ the redox properties of the catalyst are dramatically altered. VIv become very stable and even after calcination at 400 "C for 3 h, less than 1-2% is oxidized to Vv. Its
/I
/I
I
0.8
0.4
1
0 "
.P surface ratio
I
1.51E-4
I
I
9.91E-5
6.55E-5
,
surface ratio
k(600 K)*lE-5 Figure 13. Rate constant of first-order n-pentane depletion at 600 K as a function of K/P surface ratio (for clarity the values shown are 5 times higher than the actual values) and corresponding C / P surface ratio (see text) estimated on the basis of thermogravimetric data.
oxidation occurs at temperatures higher than those for the combustion of the C-containing residues, and thus, the thermogravimetric method can be used to determine the amount of these species. Since the addition of K on the surface of (VO)2P207causes a change in the product distribution in n-pentane oxidation, we analyzed the amount of these C-containing residues on the surface of the various K-doped vanadyl pyrophosphate samples after the catalytic tests for C5-alkane oxidation. The thermogravimetric curves obtained for the pure vanadyl pyrophosphate and for the sample doped with 0.1 1% K (K/P = 0.15) after the catalytic tests in n-pentane oxidation are compared in Figure 12. In the 450-550 K temperature range, there is a slight decrease in weight due to the presence of weakly adsorbed species. In the 600-700 K range, there is a further more significant decrease in weight, the amount of which is a function of the K:P ratio. Mass quadrupole analyses confirm the evolution of C 0 2 from the catalyst in these conditions. At temperatures higher than 800-850 K, the catalyst starts to be oxidized to VOP04, with a consequent weight increase. The results summarized in Figure 13 show that when the K/P surface ratio is increased from 0 to 0.15 the rate constant of first-order pentane depletion is cut in half ( k at 600 K) along with a parallel increase in the total amount of C-containing residues. The C / P surface ratio passes from 0.53 in the pure (VO)2P207 to 1.26 in the sample doped with 0.1 1% K (K/P = 0.15). The amount of C on the surface is estimated from the weight decrease of the second transition (between 600 and 700 K--see Figure 12), assuming to a first approximation that only C is present in the surface residues. A surface area of 18 m2/g was adopted for the calculation of these values.
6818 The Journal of Physical Chemistry, Vol. 94, No. 17, I990
Centi et al.
SCHEME I: Schematic Reaction Mechanisms of n-Butane and n-Pentane Selective Transformation to Anhydride on Vanadyl Pyrophosphate
h
H.a b s t r. /
P
U
C . c o n t a i ni ng r e s i d u e s
a alkane H-abstraction b allylic H-abstraction c I,4 dienic 0-insertion d allylic 0-insertion
0
Discussion Surface Concentration of POH Groups. The characterization of the surface reactivity and acidity of (VO)2P207samples doped with K suggests a correlation between the change in Bransted acid properties and catalytic behavior, as confirmed also by the parallel trend in the decrease of IR bands of free Bransted acid sites (Figure 8) and of catalytic behavior in n-butane (Figure 2) and n-pentane (Figures 3-5) oxidation as a function of K/P surface ratio. From these results, it is also possible to estimate that the surface concentration of Bransted acid sites is nearly equal to that of surface P groups (see Figure 8). This value is about twice that estimated on the basis of a mechanism of vanadyl pyrophosphate formation by thermal transformation from the [VOHPO4I2.H20 precursor phase.'7,22-24 The transformation involves the topotactic rearrangement of acid phosphate groups to form pyrophosphate groups. The precursor phase, vanadyl hydrogen phosphate hemihydrate, has a layered structure with preferential surface exposition of the [OIO] plane in the most active catalysts prepared in an organic medium.2i The solid-state rearrangement with dehydration to form the vanadyl pyrophosphate does not involve morphological and topological changes but only a rotation of some of the tetrahedral phosphate or octahedral vanadyl groups. This is confirmed also by the same degree of preferential exposition of the [020] plane of (VO),P20, starting from [ VOHPO4],.H2O samples prepared with different degrees of preferential exposition of the topological equivalent [OIO] plane.2i. According to this mechanism of transformation, only half of the phosphate groups of the surface layer will be involved in the linking of the various layers through pyrophosphate groups, and thus, an O H / P surface ratio of about 1:2 may be expected. The present data on the change in IR bands of Bransted acid sites as a function of K/P surface ratio suggests an O H / P ratio near 1:I. This value is twice that expected on the basis of a surface structure equivalent to that of the bulk (VO)2P207. However, it should be noted that XPS analyses by other research g r o ~ p s ~ ~ (35) Satsuma, A.; Hattori, A.; Furuta, A,; Miyamoto, A,; Hattori, T.; Murakami, Y. J. Phys. Chem. 1988, 92, 2275. (36) Yamazoe, N.; Morishige, H.; Teraoka, Y. In Successful Design of Catalysis: Inui, T., Ed.; Elsevier Science: Amsterdam, 1988; p 15.
as well our own results on the same sample used in this work37 indicate an enrichment in phosphorus of the surface layer. The estimated P/V surface ratio by XPS is in the range 1.5-2.0, and considering that the excess of surface phosphorus with respect to stoichiometric bulk P/V = 1:l ratio must increase the surface concentration of POH groups, the present estimated OH/P = 1:l ratio is in good agreement with the XPS results. Modijlcation of Surface Pathways by Doping with K . As discussed previously, all the present data agree in showing a specific correlation between inhibition of Bransted acid sites by doping with K and inhibition of the pathways of anhydride formation from n-butane and n-pentane on (V0)2P20,. However, in order to discuss in which specific stage the POH groups may be important, it is useful first to analyze the possible reaction pathways of formation of maleic and phthalic anhydride from the two C, and C5 alkanes. It should be emphasized that all mechanisms occur on the surface without desorption of any intermediate, and thus, the present reaction mechanisms are rather speculative. They are based on data derived in steady-state oxidation of the various possible intermediates"-16,30.3'as well as on transient reactor studies in conditions where desorption of possible intermediates is favored.' Nevertheless, the proposed reaction mechanism is useful in order to clarify the effect of doping with K on the surface pathways of reaction. Scheme I summarizes the proposed mechanisms of maleic anhydride formation from n-butane and of maleic anhydride and phthalic anhydride formation from n-pentane. As previously reported,'i~i7,38~39 n-butane is activated by a concerted 2H-abstraction to form corresponding 2-butene on a Lewis acid siteoxygen reactive couple. The butene formed interacts with nearly all V=O groups, forming an allylic intermediate that evolves to adsorbed butadiene. It is probable that the same mechanism also occurs on n-pentane with a consequent formation of pentadiene. However, at this stage, one principal difference distinguishes the two intermediates. Allylic hydrogen is present in pentadiene, which .is~not ~ present in butadiene. The high activity of vanadyl pyrophosphate in allylic H-abstraction is known.40 In butadiene, ~
~
~~~~
(37) Centi, G.;Trifirb, F. Unpublished results. (38) Busca, G.; Centi, G. J. Am. Chem. SOC.1989, 111, 46. (39) Centi. G.;TrifirB. F Card. Today 1988, 3, 151.
Surface Pathways in Alkane Oxidation
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 6819
further abstraction of H is difficult due to the absence of reactive hydrogen atoms and may give rise only to a side reaction, leading to C-containing resides by further H-abstraction and dimerization. In the analogous pentadiene, the further H-abstraction with the formation of cyclopentadiene is relatively easier. In butadiene, however, 1,4-oxygen insertion may occur with the formation of adsorbed 2,Sdihydrofuran or of furan by further H-abstraction. Consecutive oxygen insertion may form the corresponding lactones and finally maleic anhydride. An analogous mechanism may occur on the pentadiene intermediate with final formation also of maleic anhydride. The difficulty of oxygen insertion in cyclopentadiene and its tendency toward dimerization can justify its reaction with other adsorbed species to form intermediates such as those suggested tentatively in Scheme I. These intermediates then evolve to phthalic anhydride. A key stage in the reaction pathways of n-butane and n-pentane oxidations is thus the consecutive evolution of the dienic hydrocarbon. Inhibition of the oxygen insertion activity may favor the formation of parallel waste reactions, leading to C-containing residues that may completely inhibit the reactivity of vanadyl pyrophosphate and in the case of n-pentane oxidation also favor the probability of the surface pathway to phthalic anhydride as compared with that to maleic anhydride (Scheme I). FT-IR evidence (Figure 1 1 ) on the reactivity of adsorbed intermediates on vanadyl pyrophosphate may clarify in which stage the inhibition of Brmsted acid sites is important. The results suggest that the interaction of C4 hydrocarbon on K-doped (V0)2P207gives rise to the formation of an adsorbed intermediate originating from the reaction of butadiene with V=O surface groups. This intermediate corresponds to the adsorbed form of dihydrofuran reported in Scheme I. In the presence of gaseous oxygen, this adsorbed intermediate can easily evolve to the corresponding lactone and then to maleic anhydride on pure vanadyl pyrophosphate, whereas on K-doped (VO)2P207this stage is specifically inhibited. The role of POH groups seems therefore connected to the catalyzed formation of lactone and maleic anhydride in the presence of O2 from the adsorbed furan-like intermediate. This latter intermediate is strongly held on the surface ~~
~~
~~~~~
(40) Cavani, F.;Centi, G.,Manenti, I.; Trifirb, F. Ind. Eng. Chem. Prod. Res. Deu. 1985, 24, 221.
as shown in stopped flow desorption e~periments,'~J~ and it is thus reasonable to assume that the inhibition of its further transformation to maleic anhydride completely prevents the reactivity of (VO)zP207toward the formation of oxygen-containing products other than carbon oxides. On the other hand, it is expected that the inhibition of this stage of the mechanism may enhance the formation of carbon-containing surface residues; this is an agreement with the observed increase adsorbed species on K-doped ( VO)2P207(Figure 12). In the case of n-pentane, similar conclusions can be derived about the modification of the surface pathways by doping with K. In the case, however, we expected, in addition to the general decrease of the selectivity to anhydrides with increasing K/P ratio, a change in the ratio of the selectivities to phthalic anhydride and to maleic anhydride, since the relative probability of the reaction of H-abstraction (cyclopentadiene formation) with respect to that of 0-insertion (methyldihydrofuran formation) would be further favored (see Scheme I). Figure 6 confirms this change in the ratio of the selectivities to the two anhydrides. The correlation between the decrease in the specific rate of n-pentane depletion and the increase in the C / P and K/P surface ratios is summarized in Figure 13. In conclusion, the inhibition of the Bransted POH groups by K doping leads to a considerable modification in the surface oxidation pathways in C4- and C5-alkanetransformation on vanadyl pyrophosphate. The effect seems to be connected to an inhibition of the catalyzed transformation of furan-like intermediates to corresponding lactones and then to anhdyrides in the presence of 02.This inhibition favors the formation of strongly held C-containing residues on the surface that in turn cause a considerable decrease in the surface reactivity of (VO)2P207and thus also in the rate of alkane transformation due to site blocking. In the case of n-pentane, in addition to this effect, a change in the ratio of selectivities to phthalic anhydride and to maleic anhydride is observed, due to the suggested modification in the relative rates of 0-insertion and H-abstraction on the intermediate pentadiene with a consequent change in the relative probabilities of occurrence of the two surface pathways of selective n-pentane transformation. Registry No. (VO)2P207,58834-75-6; K, 7440-09-7; n-butane, 10697-8; n-pentane, 109-66-0.
