Faraday Discuss. Chem. SOC., 1989,87, 173-187 Acid-Base and Oxidation Catalysis on Heteropolysalts with Surface Acid Layers Katarzyna Bruckman, Jerzy Haber and Ewa M. Serwicka" Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, 30-239 Krakow, ul. Niezapominajek I, Poland Physicochemical and catalytic properties of heteropolyacids of the series H3+rlPV,,Mo,2-,,040 ( n = 0-3), both pure and supported on the potassium salt K3PMo,2040, have been investigated. Thin acid coats formed on such a support display modified properties and enhanced thermal stability. In particular, it is postulated that the change in the acidic properties of the supported acids is a consequence of their modified hydration ability resulting from the epitaxial relationship with the support.Results of catalytic experi- ments for the oxidation of acrolein, methanol and alkanes are presented and compared for both series of the catalysts. Possible mechanisms of all these processes are proposed on the basis of experimental data and quantum- chemical calculations. Relatively weak thermal stability of the heteropolyacids with the Keggin structure (fig. 1) represents a major drawback in their potential application in catalysis. Goodenough and co-worker~'-~ in a recent extensive study of acrolein oxidation over the KxH3-xPMo12040 (0 < x < 3) system found that the best catalytic performance could be obtained for x = 2.5. They established that at this composition the catalyst consisted of the H3PMo12040 acid phase stabilized in the form of an epitaxial, isostructural layer at the surface of the K3PMo12040 particles.The latter, in contrast to the acid phase, is a water-insoluble compound, crystallizing in a cubic lattice, thermally stable up to 1000 K. An interesting question of far-reaching implications is whether it is a general phenomenon that heteropolyacids with the Keggin structure may be supported on the appropriate heteropolysalts, thus becoming stabilized in the form which otherwise decomposes under conditions of catalytic reaction. Heteropolyacids derived from H3PMo12040 by substitution of one to three molybdenum atoms by vanadium appear, in view of their similarity with the parent acid4 particularly promising objects for such a study. Furthermore, the literature data indicate the advantageous properties of such compounds in the selective oxidation processe~.~-~ For this reason the research into their thermal stability and stabilization effects is of great potential significance.Since all water-insoluble salts of these acids with the alkali cations (K+, Cs+, Rb', NH;,) are isomorphous with K3PMo,2040, this compound was chosen to serve as a support for each member of the series of acids studied. Experimental Materials The H3+nPV,Mo12-n040~~H20 ( n = 0-3) heteropolyacids were prepared according to the method of Tsigdinos and Hallada.' The exact water content of the crystalline acids hydrates was found to be H3PMoI2O4,,.28 H20, H4PVMol 1040-32 H20, H5PV2M010040.3 1 H20 and H6PV3M0904,,-30 H20, as determined by TGA. These samples are hereafter referred to as H3, H4, H5 and H6.The K3PM~12040 support, abbreviated to K3, was prepared by the method described by Tsigdinos"' from stoichiometric quantities of 173174 Heteropolysalt-supported Heteropolyacids Fig. 1. Structure of the Keggin unit. H3PMo12040 and K2C03. Its B.E.T. surface area was 160 m2 g-'. On the assumption that one Keggin anion occupies 144 A', catalysts corresponding formally to one monolayer coverage with the acid component, denoted hereafter H3/ K3, H4/K3, H5/ K3, H6/K3, were prepared by impregnation of the support with the desired quantity of aqueous solution of the respective acid, as described previously." All catalysts, pure and supported, were subjected to thermal treatment at 623 K for 3 h. The B.E.T. surface areas were as follows: for the pure, calcined acids 1-3 m2 g-', for the supported samples 5-8 m2 g-', and for the support 70 m2 g-'.Techniques X-Ray Diffraction Powder X-ray diffraction data were obtained with a DRON 2 diffractometer, using Cu K, radiation. Th erma 1 Analysis Thermogravimetric analysis was carried out in static air in a Setaram Micro ATD M5 thermal analyser. Samples of ca. 15 mg were heated at 5 K min-' up to 873 K. Electron Microscopy Microstructural characterisation of the surface topography of the catalyst particles was carried out with a JEOL 100 CX instrument equipped with an ASID 4D high-resolution scanning accessory operating at 100 keV. Raman Spectroscopy Raman spectra were acquired on a DFS-24 spectrometer equipped with an ILA-120 ( A = 647 nm, linear polarisation) laser source.Electron Spin Resonance E.s.r. spectra were recorded at room temperature and at 77 K with an X-band SE/X (Technical University Wroclaw) spectrometer. DPPH and n.m.r.-marker were used for the determination of g factors.K . Bruckman, J. Haber and E, M. Serwicka Acidity Measurements 175 In the temperature-programmed desorption experiments a 100 mg sample was first outgassed at 573 K for 0.5 h at 5 x Torr.? Then pyridine (20 cm3) was introduced at 293 K and the adsorption carried out for 0.5 h. The sample was then outgassed at 423 K for 1 h and the t.p.d. experiment was carried out in the temperature range 423-723 K at a heating rate of 10 K min-'. Desorbing products were analysed with a mass spectrometer. In the i.r. experiments the sample pretreatment and pyridine adsorp- tion were carried out in a similar way and the intensity of the pyridinium ion band was taken as a measure of the Bronsted-acid site concentration.The spectra were recorded with an IR-20 spectrometer. Catalytic Testing Reactions were investigated under continuous-flow conditions in a standard glass tubular microreactor. Products were analysed chromatographically. Results Catalyst Characterization Scanning Electron Microscopy SEM analysis was undertaken to obtain a- better insight into the catalysts morphology. Plate 1 shows the SEM images of the H4, K3 and H4/K3 calcined catalysts. The picture for other pure and supported acids is identical with those presented here as an example. There is a clear difference between the crystal habit of the K3 support and the pure acid phase. The potassium salt is composed of small, well formed round or hexagonal crystallites, of ca.1 p m diameter, whereas the acid sample consists of agglomerates of irregular, cracked crystallites varying in size and habit. The surface topography of the supported acids closely resembles that of the K3 support. Also, the defect-free, clean single crystallites dominate. No separate irregular crystallites, characteristic of the unsupported acid phase, can be seen, although the amount of the acid deposit constitutes almost 30% of the catalyst weight. Laser Raman Spectroscopy In view of the relative insensitivity of the i.r. spectra of the primary Keggin unit to the type of the secondary structure and nature of the counter cation, Raman spectroscopy proved to be particularly useful for elucidation of these phen~rnena.l*-'~ Our Raman investigation of the pure and K,-supported heteropolyacids demonstrated that the acid deposit represents structurally a new quality, combining the properties of the acid and the upp port.'^ Fig.2 shows as an example the Raman spectrum of the K3 support, together with those of pure and supported calcined H5. The characteristic feature of the K3 spectrum is a split band in the 230-250 cm-' region. The bands observed in this range are caused by deformation vibrations both of the terminal M=O groups and of the entire framework, and are sensitive to the cationic environment of the Keggin unit . The other characteristic fragment of the spectrum falls in the range 900-1050 cm-' where the valence vibrations of the individual M=O groups, the PO, and the breathing modes of all 12 M=O groups are expected. The spectrum of the calcined acid differs from that of the support in both regions.In the 230-250 cm-' range the band at lower 12-14 t 1 Torr = 101 325/760 Pa.176 )r Y .- 2 Y C .- Heteropolysalt-supported Heteropolyacids 200 250 950 1000 wavenumber/ cm- ' Fig. 2. Laser Raman spectra of calcined (623 K) samples: (-) H,, (- - -) K3, (- - -) H5/K3. wavenumbers becomes more intense and the other appears only as a shoulder. In the 900-1050 cm-' region the absorption maximum is shifted towards 1010 cm-'. A similar shift was observed by and assigned to a dehydrated form of acid. The Raman spectrum of the supported acid layer in the low-frequency range resembles that of the acid, whereas the 900-1050 cm-' bands are similar to those observed for the K3 support.Diflerential Thermal Analysis This technique is particularly useful for determination of the thermal stability of the acids, since, following the final loss of constitutional water, an exothermic peak appears around 700 K, associated with the irreversible destruction of the Keggin Accord- ing to d.t.a. results the decomposition temperatures of pure acids are as follows: H3, 706 K; H4, 715 K; H5, 685 K; and H6, 669 K. For each member of the supported-acid series a distinct shift of the exothermic peak position towards higher temperatures is visible, i.e. H3/K3, 727 K; H4/K3, 733 K; H5/K3, 725 K; and H6/K3, 700 K. This phenomenon clearly indicates thermal stabilization of the acid layer deposited on the potassium salt.X - Ray Digraction The X-ray data confirm the stabilizing effect of the support on the heteropolyacid structure." Fig. 3 shows as an example the diffraction patterns of ( a ) pure support K3, ( b ) overheated H5, (c) overheated H5/K3 and ( d ) a mechanical mixture of overheated H5 and K3 in ratio as in H5/K3. The additional heat treatment of the previously calcined catalysts was performed for 1 h at 673 K, i.e. close to the decomposition conditions determined for the H5 acid from the d.t.a. measurements. The pure support gives a typical powder spectrum of the K3PMo12040 cubic lattice. Overheated H5 shows peaks due to MOO, and some other, as yet unidentified, product of the decomposition of theK .Bruckrnan, J. Haber and E. M. Serwicka 177 I T I 0 I ( c ) LO 30 20 10 2810 Fig. 3. Powder X-ray diffraction patterns of (a) K3, (b) H5 overheated at 673 K, (c) H5/K3 overheated at 673 K, ( d ) mechanical mixture of H5 and K, overheated at 673 K in ratio as for H5/K3. 0, Diffraction pattern of MOO,; x, diffraction pattern of unidentified phase. Keggin unit (anhydrous, undecomposed H5 is practically amorphous to X-ray). The overheated supported H, shows only the pattern characteristic for the cubic lattice of the support, in agreement with the idea of formation of the epitaxial, isostructural layer of the acid on top of the K3 particles. Finally, a mixture of the overheated H5 and K3 in a quantitative ratio corresponding to that of the supported HS/K3 sample shows both the peaks characteristic for the K3 phase and those found in decomposed H5.Infrared spectra corresponding to the X-ray diagrams" confirm that the primary Keggin unit of the acid phase supported on K3 becomes stabilized against thermal decomposition. Electron Spin Resonance It has been demonstrated that e.s.r. can be successfully employed to follow various stages of the heteropolycompound dehydration and/or d e s t r ~ c t i o n . ~ ~ ' ' - ' ~ We have shown that the e.s.r. spectra observed for the K,-supported H3 sample calcined for 5 h at 673 K are characteristic of the preserved, undecomposed Keggin anions, whereas in the pure H3 phase treated in the same way the e.s.r. signal of Mo5+ in the MOO, matrix appear^.'^"'^ In fig.4 similar evidence is presented for the H,-supported catalyst. Initial spectra of both samples are similar, with gl = 1.976, A, = 7.43 mT, gll = 1.927, All = 20.28 mT [fig. 4( a)] and are typical of a V4+ ion in a hydrated, undecomposed acid phase.'* Prolonged calcination of the pure H, acid results in a spectrum dominated by a signal with clearly178 Heteropolysalt-supported Heteropolyacids Fig. 4. E.s.r. spectra of ( a ) H, and/or H,/K3 samples, ( b ) H, overheated at 673 K, ( c ) H,/K3 overheated at 673 K. Spectra recorded at 77 K. resolved gll= 1.913 and All = 18.79 mT [fig. 4(6)], typical of the vanadium-containing products of irreversible destruction of the Keggin units.I8 E.s.r. of the supported sample treated this way shows mainly a signal with well distinguished parallel compounds gll= 1.931, All = 18.79 mT [fig.4(c)], characteristic of V4+ ions in the undecomposed, partially dehydrated acid phase.'' Acidity Measurements Experiments with pyridine adsorption demonstrated that the deposited layers, when compared to the bulk acids, show modified acid-base properties. Fig. 5 shows the general trends for acidity measured by the amount of pyridine retained in the catalyst after outgassing at 423 K and detected by i.r. in the form of the pyridinium ion.*' ForK . Bruckman, J. Haber and E. M. Serwicka H3/K3 HLIK3 Hs/K3 Hb/K3 K3 I 1 I I I 179 Fig. 5. Acidity of calcined samples as determined from intensity of the i.r. 1540 cm-' band of the adsorbed pyridine. both types of catalysts investigated (bulk and the supported) a fall in Brgnsted acidity is observed as the vanadium substitution increases.Simultaneously, the deposition on the K3 support results in a general increase of the acidity. No significant number of Lewis-acid centres could be detected after pyridine adsorption. Parallel experiments with the t.p.d. of pyridine trapped in the catalyst after outgassing at 423 K, allow one to infer the nature of the acidic centres observed. From bulk, calcined acids pyridine is desorbed with a number of overlapping peaks, indicating the presence of centres of different acid strength [fig. 6 ( a ) ] . Also in this experiment the trend of decreasing acidity with increasing number of vanadium atoms is visible, parallelled by a shift towards relatively weaker acid sites.The insert in fig. 6 ( a ) shows the desorption profile from the H6 sample calcined at a temperature 150 K lower than the standard treatment. This clearly shows that the occurrence of the strongest acid sites is associated with the amount of water retained by the bulk of the acid phase. The maxima of pyridine desorption peaks from the supported samples occur at the same temperature for all samples [fig. 6( b ) ] , indicating similar strength of the acid sites. Here also the total amount of desorbed pyridine decreases as the number of vanadium atoms in the supported acid increases. Catalytic Testing Met h a n ol Oxida t ion Our recent study2' of the oxidation of methanol over pure and supported catalysts showed that in the temperature range 523-573 K introduction of vanadium into the Keggin anion influences the selectivity pattern of both series in a similar way, whereas their activity changes in the opposite manner.Formaldehyde, the product of oxidative dehydrogenation, and dimethyl ether, the product of dehydration, appear as major products for both series. Fig. 7 and 8 show the catalytic performance of both series at 533 K. Substitution with vanadium shifts the product distribution towards formaldehydeHeteropolysalt-supported Heteropolyacids 423 523 623 723 T/ K Fig. 6. T.p.d. of pyridine from the calcined catalysts. (a) Pure acids: (-) H3, (- - -) H,, ( . . . . - ) H,, (--.-) H6. ( b ) Supported acids: (-) H3/K3, (---) H4/K3, ( - - . - - ) H,/K3, (- ' -) H6/ K3. K H3/K3 Ht,/K3 H5/K H6/K < 100 - - 80 - - I s - 2 60 - 1 A > c .- .- 5 40 - 20 - Fig.7. Catalytic activity in methanol oxidation at 533 K on 0, pure acids, 0, supported acids. Conditions: He : O2 : CH30H = 76.14: 16.2 : 7.65. PCH30H = 58.2 Torr.K . Bruckman, J. Haber and E. M. Serwicka 181 1 .o T 9. 0, 5 0.5 d rn I 1 I - 1 I 1 I . I H3 H& H5 H6 Fig. 8. The ratio of CHzO and CH30CH3 selectivities in CH30H oxidation at 533 K. 0, Pure acids, A supported acids. i Table 1. Activity and selectivity to maleic anhydride in pentane oxidation" activity/ selectivity to catalyst T/ K pmo1(C2H5) s-' gi:id phase maleic anhydride ( O/O ) 96.2 147.5 217.7 121.5 178.5 53 1 .O 607.5 46 68 63 38 43 47 66 C5H12 : O2 : He = 1.8 : 25 : 73.2. for both the pure and supported acids. Simultaneously the activity of the pure acids decreases, whereas that of the supported samples increases.Acrolein Oxidat ion The results of the oxidation of acrolein to acrylic acid conducted at 623 K have shown" that the maximum yield of acrylic acid on pure heteropolyacids is observed for the thermally most stable H4 sample, although all vanadium-containing catalysts are better than the unsubstituted sample. The supported catalysts generally perform better than their unsupported counterparts, and the optimum catalytic properties are obtained for the samples with the highest vanadium content. Alkane Oxidation Vanadium-substituted heteropolyacids were shown recently to be surprisingly selective in the catalytic oxidation of n-pentane to maleic anhydride.* K,-supported acids show similar properties.The catalytic performance of pure and supported H5 is summarized in table 1. The data show that the activity expressed per gram of the acid component182 Heteropolysalt-supported Heteropolyacids is distinctly higher for the supported sample. Also, as the temperature increases, the selectivity of the pure acid shows signs of deterioration, whereas on the supported catalyst a steady increase is observed. Maleic anhydride is the only selective product of pentane oxidation on the heteropoly- catalyst, while on the industrial pyrophosphate catalyst a mixture of the maleic and phthalic anhydrides is formed.8 Oxidation of n-butane is also known to give maleic anhydride on heteropolyacid catalysk6 In order to obtain some insight into the nature of the activation of alkane molecules on the surface of the Keggin unit, the oxidation of n-butane, n-pentane and n-hexane were investigated.In each case maleic anhydride was the only selective product, although the activity in the n-hexane oxidation was distinctly smaller. Discussion All techniques employed to characterize pure and supported heteropolyacids clearly prove that the intimate interaction between the members of the H3+,, PV,Mo,,-,,O,, series and the K3PMo,2040 support produces surface acid coats of a new quality. The phenomenon has a general character and is independent of the composition of the supported acid layer. The most significant effect is the observed thermal stabilization of the deposited material. Analysis of the Raman spectra gives an indication as to the structure of the surface layer in the supported catalyst.The absence of the band at ca. 1000 cm-’ typical of the ‘anhydrous’ acid phase indicates that a certain number of water of crystallization molecules are retained in the structure. On the other hand, the similarity of the bands in the 230-250 cm-’. region, sensitive to the cationic environment, to those found for the pure acids indicates that the surface layer consists of an acid hydrate rather than of a solid solution involving potassium cations. Therefore, in agreement with earlier findings,’ we propose that the stabilization effect is a consequence of the formation of an epitaxial acid layer derived from H3+, PV, Mo,,-,,O~~-X H20, isostructural with the cubic lattice of the K3PMo1,04, support particles. Electron-microscopic data’ showed that in the partially dehydrated H,PMo,,O,, a phase exists that is isostructural with the potassium salt of this acid.Brown et al.” found that the 12-tungstophosphoric acid hexahydrate is isomorphous with the insoluble cubic salts of various 12- heteropolyanions. They established that the cationic positions in such a hydrate are occupied by ( HSO2>+ diaquahydrogen ions, and the structure can readily accommodate the extra protons expected from the stoichiometry of related heteropolyacids. In view of this, it seems reasonable to visualise the surface layer in the K,-supported catalysts as hexahydrates of the respective acids. Although, on their own, such hexahydrates are unstable and difficult to obtain, the epitaxial relationship with the support provides convenient conditions for their stabilization.Measurements of the acidity revealed that the supported acid layer shows acid-base properties different from those of the pure acids. Before discussing the details, it is necessary to recall that numerous investigations of the state of protons in the heteropoly- compounds concluded that basically two types of protons can be distinguished; non- localized hydrated protons and non-hydrated less mobile protons.’ The smaller the degree of hydration, the smaller is the proton mobility, and, consequently, a lower acid strength of the constitutional protons may be expected. This is clearly visible when comparing the results of t.p.d. of pyridine from the H, acid samples calcined at various temperatures (fig.6). The sample that was pretreated at lower temperature, and therefore retained more water, shows the presence of a significant amount of strong-acid centres, whereas the sample calcined in more drastic conditions lacks such acidic sites almost completely. We may thus conclude that a correlation exists between the acidity of protons and their degree of hydration.K . Bruckman, J. Haber and E. M. Serwicka 183 The second important factor that has to be taken into account is the increasing charge of the Keggin anion as the degree of vanadium substitution increases and, as a result, the increasing total number of constitutional protons available. However, a quantitative analysis of the i.r. and t.p.d. data shows that acidity, measured by the amount of pyridine retained in the catalysts after desorption of weakly adsorbed pyridine at 423 K, shows a falling trend with an increasing number of vanadium atoms.Thus, the number of acid sites strong enough to be seen by pyridine under these conditions represents only a fraction of the constitutional protons present in each sample and decreases on gradual substitution with vanadium. According to t.p.d., strongly adsorbed pyridine detects 12% ( H3), 8% ( H4), 6% (H,) and 4% ( H6) of the constitutional protons in the bulk, calcined acids, whereas the respective numbers for the supported samples are 45% (H3/K3), 32% (H4/K3), 20% (H,/K,) and 13% (H6/K3). This signifies that the majority of protons present in the calcined samples constitute weak-acid centres, firmly bound to the Keggin anions, their amount increasing with number of vanadium atoms.For the series of pure, calcined acids a shift towards weaker acid centres as the substitution with vanadium increases is also visible within the range of acidic sites detected by the t.p.d. of pyridine [fig. 6(a)]. Such behaviour shows that the more vanadium there is, the lower is the degree of rehydration responsible for the appearance of the strong acidity in the calcined acids. In the supported samples [fig. 6(6)] the falling trend in acidity within the series is also visible, but the n u m i v of sites detected by pyridine represents a larger fraction of the total number of constitutional protons expected from the sample's stoichiometry. The maxima of the desorption peaks occur at the same temperature, indicating the same strength of the acid centres responsible and, therefore the similar degree of hydration of the protons involved.The lack in the supported samples of stronger acid sites, observed for instance in the H, and H4 members of the pure acid series, indicates that the supported layers cannot accommodate more than a certain, well defined number of water molecules. It seems reasonable to assume that this maximum number corresponds to the number characteristic for the cubic acid hexahydrates, i.e. that the most hydrated protons in these samples are present as diaquahydrogen ions. There is no such limitation for the dehydration of the deposited acid coat, therefore here also most of the constitu- tional protons represent weak-acid centres, in amounts increasing with the number of vanadium atoms.Catalysis and Mechanistic Studies Detailed characterization of the physical and chemical properties of the catalysts investigated is essential for an understanding of the observed trends in their catalytic behavior. For the mechanistic analysis, however, it is important to realise that all chemical reactions discussed above are governed by HOMO-LUMO interactions. There- fore, suitable quantum-chemical calculations should help in elucidating the respective reaction mechanisms. According to Taketa et al.,23 the HOMO of the (PMo,,0,J3- ion is composed mostly of the 2p lone-pair orbitals of the bridging oxygens, Ob. The LUMO is a mixture of the Mo 4d and Ob 2p orbitals and is antibonding with respect to the Mo-Ob bond.Methanol Oxidation We have used the reaction framework proposed by MoffatZ4 and extended by Farneth et for the quantum-chemical interpretation of the methanol t.p.d. data:?'184 Heteropolysalt-supported Heteropolyacids CH30HT.*'OKU * CHl...OKu+ HzO CH3+..*OKU+~KU -, CH,O+ U + . - . ~ K U . (4) Our MNDO calculations gave the following HOMO and LUMO for the organic species involved. (a) CH30H. *HOMO = -0.83 p,(O) + 0.28 p x ( c ) - 0.34 S ( H&) 'PLUMO= -0.68 p,(C)+O.52 s(HkH)-0.26 s(HgH)-0.26 ~ ( H $ ~ ) - 0 . 2 9 s(HOH). S ( H:H) The HOMO represents a bonding orbital localized mainly on the oxygen atom, while the LUMO is the first orbital of the antibonding s stem, associated mainly with the CH3 charge of 0.19. group. The calculated C-0 bond length is 1.39 K and the carbon atom carries positive (6) CH30Hl.The HOMO'S highest electron density is associated with the CH3 group, while the LUMO has comparable contributions from all structural elements but the CH3 group. The C-0 bond is elongated to 1.46 A and the carbon atom increases its positive charge to 0.23, as intuitively predicted by Highfield and M ~ f f a t . ~ ~ (c) CH30-. *HOMO = -0.73 p,( 0 ) - 0.43 pz( 0 ) -4- 0.21 S ( H&) - 0.43 S ( HgH) + 0.21 S ( 'PLUMO= -0.24p,(0)-0.68 p,(C)-0.39 p,(C)+O.23 S(H;~)-O.~~S(H:.,) + 0.23 S ( HcH). The HOMO carries the highest electron density at the oxygen atom, while the LUMO is localized predominantly on the CH3 group. After taking into account all conceivable HOMO-LUMO interactions the detailed mechanism of the methanol interaction with the Keggin anion, presented in fig.9, has been proposed. Formaldehyde appears as a result of nucleophilic attack of a bridging oxygen lone pair (Keggin's HOMO) on the C-H bond of the methoxy group (configur- ations 111 and IV). Dimethyl ether is formed in a competitive reaction between the methoxy group and a methanol molecule, according to eqn (3). Substitution with vanadium results in changes of the charge distribution within the Keggin anion. In particular, from the "0 n.m.r. data reported by Maksimovskaya et a1.,27 it is known that the electron density on the bridging oxygens of the Mo-0-Mo type systematically increases. This is equivalent to an increased capability of performing a nucleophilic attack on the C-H bond of the methoxy group shown in configuration 111 and therefore should enhance the reaction path leading to formaldehyde.This picture is fully consistent with the catalytic data, showing that substitution with vanadium shifts the spectrum of the methanol oxidation products towards formaldehyde (fig. 8).K . Bruckman, J. Haber and E. M. Serwicka H \ /H H C II I + H 'c=o t / H H I 0 - 'Mo Mo /'*\Mo/ It 0 It 0 It 0 185 + 2 e + 2 e s XI Fig. 9. Mechanism of interaction of methanol with the Keggin unit. In order to explain the trends in activity, it is necessary to bear in mind that the activity towards methanol conversion will depend on the ability to absorb methanol by the secondary structure of the heteropolyacid catalysts.' From the acidity measurements [fig. 6 ( a ) and (6)] it follows that the acidity of the calcined acids decreases as the number of vanadium atoms increases, as will their ability to absorb polar molecules such as methanol.Therefore, the diminishing trend in catalytic activity observed for the bulk acids reflects primarily their absorptive capacities. The situation is different for the supported acid layer. Here the secondary structure is similar throughout the series, and more rigid owing to the epitaxial relationship with the support. Thus, no significant differences in the absorptivity should be expected. Consequently, the activity will depend on the efficiency of the generation of methoxy groups, i.e. on the methanol transformation in steps I and I1 of fig. 9. In view of the high proton affinity of the methanol molecule, even weak Brprnsted-acid centres can ensure methanol protonation. Therefore, the growing activity of the supported series reflects the increasing number of weak-acid sites resulting from substitution of Mo by V atoms.186 Heteropolysalt-supported Heteropolyacids Acrolein Oxidation It has been demonstrated that in the catalytic oxidation of acrolein, substitution with vanadium has a beneficial effect, but the low thermal stability of pure acids is responsible for the poorer performance of the H5 and H, samples." The HOMO-LUMO interactions of the acrolein molecule with the 12-molybdophos- phate anion have been discussed in detail by Serwicka et al.3 Here it has been demon- strated that the selective insertion of oxygen into the unsaturated aldehyde molecule is initiated by nucleophilic attack of a bridging oxygen lone pair on the carbonyl carbon for which, according to the quantum-chemical calculations, the LUMO of acrolein has the largest coefficient.The increasing selectivity to acrolein observed on substitution with vanadium is associated, according to the argument presented above, with an increased ability of the bridging oxygens to perform nucleophilic attack. Alkane Oxidation It is a striking feature of the catalysts investigated that maleic anhydride is the only oxygenated product of all the alkane molecules studied, i e . butane, pentane and hexane. No traces of such possible intermediate products as butene or butadiene could be detected, making the mechanism of consecutive dehydrogenation improbable.Con- versely, this may be taken as a hint that a unique mechanism is operating, consisting of a concerted abstraction of two hydrogen atoms from carbons 1 and 4, and simultaneous linking of these carbon atoms by oxygen to form a cyclic tetrahydrofuran-like structure. There are several possible sites at the Keggin unit, composed of three neighbouring bridging oxygen atoms, where such an operation could be visualised. All these oxygen atoms contribute two lone pairs of electrons to the HOMO of the Keggin unit. The HOMO and the LUMO of butane and higher alkanes can be deduced from the data available for propane28 in view of the same type of carbon-orbital hybridisation as well as the similarity of the C-C and C-H bonds throughout the series of higher alkanes.Following this reasoning, the HOMO of the butane molecule would be the last of the bonding set, essentially associated with the (T C-C bonds, while the LUMO would be the first antibonding orbital of the 7r type, involving mainly the CH3 and CH2 groups. The reaction can be visualised as an attack of the lone pairs of electrons of two adjacent bridging oxygens of the Keggin unit on the LUMO of the hydrocarbon molecule at carbon atoms 1 and 4. As the LUMO is localized mainly on the C-H bonds and is antibonding, such an attack would result in abstraction of H atoms to form surface OH groups. Simultaneously, the electrons of the C-H bonds would shift to the LUMO of the Keggin unit, which involves Mo 4d and Ob 2p orbitals. This would render carbon atoms 1 and 4 positive and susceptible to the attack of the lone pairs of the third adjacent bridging oxygen, closing the five-membered ring into a tetrahydrofuran-like structure.In the case of pentane or hexane cleavage of the methyl or ethyl group is required. Conversion of pentane is greater than that of hexane because the cleavage at the a position is easier than at the p position. The postulated tetrahydrofuran-like structure would easily undergo further dehydrogenation and oxidation to maleic anhydride. Analysis of the relevant interatomic distances in alkane molecules and in the Keggin unit shows that the geometrical fit easily allows the appropriate orbital overlap in consecutive stages of the concerted reaction. References 1 J. B. Black, N. J. Clayden, P. L. Gai, J .D. Scott, E. M. Serwicka and J. B. Goodenough, J. Catal., 1987, 106, 1. 2 J. B. Black, J. D. Scott, E. M. Serwicka and J. B. Goodenough, J. Catal., 1987, 106, 16. 3 E. M. Serwicka, J. B. Black and J. B. Goodenough, J. Caraf., 1987, 106, 23.K . Bruckman, J. Haber and E. M. Serwicka 187 4 H-G. Jerschkewitz, E. Alsdorf, H. Fichtner, W. Hanke, K. Jancke and G. Ohlmann, Z. Anorg. Allg. 5 M. Akimoto, H. Ikeda, A. Okabe and E. Echigoya, J. Catal., 1984, 89, 196. 6 M. Ai, J. Catal., 1984, 85, 324. 7 M. Misono, Catal. Rev. Sci. Eng., 1987, 29, 269, and references therein. 8 G. Centi, J. Lopez Nieto, C. Iapalucci, K. Bruckman and E. M. Serwicka, Appl. Catal., 1989, 46, 197. 9 G. A. Tsigdinos and C. J. Hallada, Inorg. Chem., 1968, 7, 137. Chem., 1985, 526, 73. 10 G. A. Tsigdinos, Znd. Eng. Chem. Res. Dev., 1974, 13, 267. 1 1 K. Bruckman, J. Haber, E. Lalik and E. M. Serwicka, Catal. Lett., 1988, 1, 35. 12 C . Rocchiccioli-Deltcheff, R. Thouvenot and R. Franck, Spectrochim. Acta, Part A, 1976, 32, 587. 13 L. P. Kazanskii, Zzv. Akad. Nauk SSSR, Ser. Khim., 1975, 3, 502. 14 E. N. Yurchenko, J. Mol. Struct., 1980, 60, 325. 15 Chau Dieu Ai, P. Reich, E. Schreier, H-G. Jerschkewitz and G. Ohlmann, Z. Anorg. Allg. Chem., 1985, 16 K. Bruckman, J. Haber, E. M. Serwicka, E. N. Yurchenko and T. P. Lazarenko, Catal. Lett., to be 17 E. M. Serwicka, 2. Phys. Chem. N. F., 1987, 152, 105. 18 R. Fricke, H-G. Jerschkewitz and G. Ohlmann, J. Chem. SOC., Farada Trans. 1, 1986, 82, 3479, and 19 E. M. Serwicka, Z. Phys. Chem. N. F., in press. 20 E. M. Serwicka, K. Bruckman, J. Haber, E. A. Paukshtis and E. N. Yurchenko, Catal. Lett., to be published. 21 K. Bruckman, J. Haber, E. M. Serwicka and J-M. Tatibouet, J. Catal., to be published. 22 G. M. Brown, M-R. Noe-Spirlet, W. R. Busing and H. A. Levy, Acta Crystallogr., Sect. B, 1977,33, 1038. 23 H . Taketa, S. Katsuki, K. Eguchi, T. Seiyama and N. Yamazoe, J. Phys. Chem., 1986, 90, 2959. 24 J. G. Highfield and J. B. Moffat, J. Catal., 1985, 95, 108. 25 W. E. Farneth, R. H. Staley, P. J. Domaille and R. D. Farlee, J. Am. Chem. SOC., 1987, 109, 4018. 26 E. M. Serwicka, E. Broclawik, K. Bruckman and J. Haber, Catal. Lett., in press. 27 R. I. Maksimovskaya, M. A. Fedotov, V. A. Mastikhin, L. I. Kuznetsova and K. I. Matveev, Dokl. 28 W. L. Jorgensen and L. Salem, The Organic Chemist’s Book of Orbitals (Academic Press, New York, 526, 86. published. references therein. Akad. Nauk SSSR, 1978, 240, 117. 1973), p. 181. Paper 9/00124G; Received 4th January, 1989