J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1293-1299 Spectroscopic Characterization of Magnesium Vanadate Catalysts Part 2.t-FTIR Study of the Surface Properties of Pure and Mixed-phase Powders Gianguido Ramis, Guido Busca and Vincenzo Lorenzelli lstituto di Chimica, Facolta di lngegneria, Universita P.le Kennedy, 1-16129 Genova, Italy The surface chemistry of the three stable monophasic powders magnesium orthovanadate Mg3(V0,), , magne-sium pyrovanadate Mg,V,O, and magnesium metavanadate MgV,O,, as well as of an amorphous oxide cata- lyst with an Mg : V atomic ratio of 3 : 2, has been investigated by FTIR spectroscopy. The bulk structure of the amorphous catalyst has been characterized by FT-FIR and Raman spectroscopies. The nature of the surface hydroxy groups, as well as the adsorption of molecular probes such as pyridine, acetonitrile and CO, , has been investigated.The samples show weak Lewis acidity and no Br~nsted acidity. Moreover, they show significant but not extreme basic character. They weakly adsorb oxygenate compounds like alcohols and ketones and interact only at high temperatures with alkanes and alkenes. These materials, active as alkane oxy- dehydrogenation catalysts, are much less reactive than other vanadia-based selective oxidation catalysts, like V,O,-TiO, and (VO),P,O, , towards organic molecules. This is attributed to the basic environment generated by the MgO component in catalysts belonging to the MgGV,O, system, which causes a parallel decrease of the Lewis acidity and of the oxidizing power of the active vanadium ions.Vanadium-based mixed oxides play a key role in industrial selective oxidation catalysis. Among others, the industrial catalysts for the synthesis of maleic anhydride from benzene oxidation are V,05-MOO, or v205-Wo3 mixed oxides,' those for the oxidation of butane to maleic anhydride are V205-P205mixed oxides,2 and those employed for the syn- thesis of phthalic anhydride from o-xylene oxidation belong to the V205-TiO, ~ystem.'.~ Recently, the VMgO system (V205-MgO) has been pro- posed as promising for the oxidative dehydrogenation of light alkanes to the corresponding light alkene~,~ a very attractive process according to the increased availability of light alkanes from natural gas. This catalytic system was pre-viously applied to styrene synthesis from ethylbenzene oxida- tive dehydr~genation.~ Since then, several research groups have investigated this reaction and this catalytic system.According to Kung and co-workers the active phase of the catalyst is the orthovanadate Mg3(V04)2 the high selec- ,476,7 tivity to dehydrogenation products being attributed to the absence of V-0-V bridges.8 Sheshan et al. correlated the IR V-0 stretching band of orthovanadates with the cata- lytic a~tivity.~ In contrast to Kung and co-workers, Volta and co-workers".' concluded that the actual alkane dehy- drogenation catalyst is the pyrovanadate Mg2V207. However, Bhattacharyya12 obtained good catalytic per-formances working with an MgO-V205 catalyst whose only X-ray diffraction (XRD)-detectable crystal phase was MgO.Burch and Crabb' concluded that the homogeneous reac- tion contributes to enhance the yield in alkene, and that no clear correlation exists between catalytic properties and crys- talline phase. On the other hand, the catalysts commonly used are p~lyphasic,~~'~~'~~'~ and an analysis of their phase composition including eventual XRD-undetectable species (like monolayers or amorphous phases) is not easy. The present paper summarizes some FTIR spectroscopic results on the surface and adsorption properties of different powders belonging to the MgO-V205 system. The aim is to try to correlate the surface properties of catalysts belonging to the MgO-V205 system with those of other vanadia-based catalysts, like V205-TiO, and V205-P205, which are active for the production of oxygenates instead of oxy-t Part 1: Ref.14. dehydrogenation compounds predominant on some Mg vanadates. Experimental The preparation and characterization of the pure MgO-V205 phase catalysts has been reported pre-viously.' O,' ',' An amorphous catalyst with an Mg: V atomic ratio of 3 :2 (8 m2 g- ') has been prepared by impreg- nation of fresh Mg(OH), with ammonium metavanadate fol- lowed by calcination at 673 K. Pure V205 (18 m2 g-') was from Degussa (Hanau, Germany). The IR spectra were recorded by a Nicolet Magna 750 Fourier-transform instrument. The skeletal spectra in the region above 400 cm-have been recorded with KBr pressed discs and with a KBr beamsplitter, while those in the far-IR region (400-50 cm-') have been recorded using the powder deposited on polyethylene discs, and with a 'solid substrate' beamsplitter.The IR spectra of the surface species have been recorded using pressed discs of the pure powders, in a heatable/liquid-nitrogen-cooledcell connected to a conven- tional gas-handling system. Removal of previously adsorbed species was carried out by outgassing, generally at 773 K. The FT-Raman spectrum has been collected with a Brucker RFS 100 instrument (Nd-YAG laser). Results and Discussion The experiments described here concern four MgVO cata- lysts. Three of them are the pure crystal phases Mg3(V0J2, Mg2V207 and MgV20,, whose catalytic activity has been reported previously.lo The fourth powder, here denoted the Mg vanadate catalyst, with an Mg :V atomic ratio of 3 : 2, appears amorphous to XRD analysis, although after calcina- tion at 873 K it crystallizes into a mixture of the three Mg vanadates. To obtain information on its structure, we investi- gated its FTIR, FT-FIR and FT-Raman spectra. Structural Characterization of the Amorphous Catalyst through Vibrational Spectroscopies The FTIR and FT-FIR spectra of the Mg vanadate catalyst are compared with those of V205 and MgO in Fig. 1. They can be discussed taking into account those of the pure phases 1294 Mg,(VO,), , Mg,V,O, and MgV,O, , discussed pre-vio~sly.'~The pair of sharp bands at 430 and 370 cm-', the shoulder near 550 cm-' and the broad band at 665 cm-' suggest that a phase very similar to MgV,O, is very likely to be present.On the other hand, the features at 980, 960 cm-', and at 337 cm-' show the presence of species similar to monoclinic Mg,V,07. The band near 845 cm-' can be due to the superimposition of the strong bands observed in the region 900-800 cm-' in all phases. On the other hand, note that the strong band apparent at 940 an-' and the shoulder at lo00 cm-' do not seem to belong to any of the three phases we characterized previously. Comparison with the spectrum of crystalline V205, also reported in Fig. 1, allows us to exclude the presence of this phase. On the other hand, amorphous vanadia also shows a strong V=O stretching band near 1020 cm- 1,15*16 like the crystalline compound, so its presence is also excluded.This seems relevant in view of the negative effect assigned to this phase with respect to cata- lytic activity in alkane oxidative dehydrogenation.' However, it seems very likely that MgO (in an amorphous form) is present too, although no definite evidence can be obtained by comparison of the spectra. Identification of the species present in the sample can also be attempted by analysing the 'first overtone' region, where IR-active combinations fall, typical for the crystalline phases.', In this region, the spectrum of the Mg vanadate catalyst [Fig. 2(b)] shows a sharp doublet at 1935, 1908 cm-', which provides definite evidence for the presence of Mg2V,0, .14 A strong, rather symmetric band at 1690 cm-' with a weak shoulder near 1790 cm-', in the region of the overtones of the terminal VO, stretchings, is also analogous to that observed for the divanadate Mg,V,O, .The triplet at 1220, 1150 and 1100 cm-' confirms the existence of V-0-V and/or V,O-V bridges. However, while the bands near 1220 and 1100 cm-' could be assigned to both 0.66 0.64 0.62 { 0.60 (0 s: 0.58; P (o 0.56 0.54 I 0.524 / ","'I"',"',' , I . ,I. ,. ;"'"''l'''l''' wavenumber/cm-' J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 J 2400.0 2200.0 2000.0 1800.0 1600.0 1400.0 1200.0 1000.0 waven u rnber/crn-Fig. 2 FTIR spectra of the Mg vanadate catalyst outgassed at room temperature (a) and at 773 K (b), and after following adsorption of pyridine (c) Mg,V,O, and MgV,O,, that at 1150 cm- ' does not belong to the pure phases we characterized in the overtone region.14 The definite absence of the band at 1370 cm-' and of the triplet nature of the band in the 1700 cm-' region seem to rule out the presence of microcrystalline Mg3(V0,), .Again, the combination modes of V205 are definitely absent. The Raman spectrum of the catalyst (Fig. 3) also clearly contains features assignable to the Mg,V,O, phase like the bands at 952, 900 and 633 cm-'. However, the strong band at 982 cm-' does not belong to any Mg vanadate species. Moreover, the presence of MgV,06, as well as of amorphous or crystalline V205 (whose Raman spectra are reported in ref.15), seems to be excluded. So, the IR spectra in the skeletal and first overtone region and the Raman spectrum suggest that this amorphous cata- lyst consists mainly of Mg,V,O, , although other struc-tures that should contain relatively short V-0 bonds (the IR bands at lo00 and 940 cm-' and the Raman band at 982 cm-') and V-0-V or V,O-V bridges (combination near 1150 cm-') should also be present. The presence of V205 particles is definitely excluded. Surface Characterization of MgVO Powders by IR Spectroscopy of Adsorbed Probe Molecules In Fig. 2 and 4, the IR spectra of the pure powder pressed discs of the Mg vanadate amorphous catalyst and of Mg,(VO,), , respectively, are reported before activation, and after activation and pyridine adsorption.In the region where the pure powder pressed discs partially transmit IR radiation ' '600' ' ' "400' " ' ' 0.2 ' wavenumber/cm-FTIR and FT-FIR spectra of (a) the Mg vanadate catalyst Fig. 1 (b)V,O, and(c) MgO Fig. 3 wavenumber/crn-' Raman spectrum of the Mg vanadate catalyst J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 + I 2400.0 2200.0 2000.0 1800.0 1600.0 1400.0 1200.0 1000.0 wavenumber/cm-' Fig. 4 FTIR spectra of Mg,(VO,), outgassed at room temperature (a)and at 773 K (b), and after following adsorption of pyridine (c). (d) Subtraction spectrum: (a)-(b). (e) Subtraction spectrum: (c) -(b). (above lo00 cm-I), together with the IR-active combinations of the skeletal modes, bands due to adsorbed species can also be found.Before activation [Fig. 2(a)], a very strong absorption is superimposed on the lattice combinations in the case of the Mg vanadate catalyst and this obscures the region between 1600 and 1400 cm-'. A similar but much weaker absorption is also found for Mg,(VO,), [Fig. qa)]. The subtraction spectrum [Fig. qd)] is dominated by bands at 1520, 1440 and 1080 cm-'. The same bands are also observed after adsorp- tion of CO, on the activated samples. Consequently, they can be assigned to carbonate species. The intensity of these bands compared with those of the bulk and of lattice combinations indicates that these species are far more abundant on the polyphasic amorphous catalyst than on Mg,(VO,), ,possibly this is related to the difference in surface area.These species need outgassing at near 573 K to be desorbed. Similar species, but in much smaller amounts, are also observable on the meta- and pyro-vanadate powders, and not at all on V205. Surface carbonates resistant to outgassing are typi- cally observed on metal oxide surfaces with basic character. They arise from the adsorption of atmospheric CO, and sometimes from the preparation method if carbonates or organic compounds (later burnt during calcination) are employed. However, if the oxide does not possess basic char- acter, these species, if present, are destroyed by room-temperature outgassing. These data suggest that the surface basicity of the catalysts increases with nominal MgO content, as expected. In spite of the poor transmittance of the samples in the region above 3000 cm-', due to radiation scattering, we also looked at the surface hydroxy groups of Mg vanadates.The spectra have a significant noise, but in all cases allow detec- tion of two weak absorption bands, one of which is very sharp near 3700 cm-' and the other slightly broader near 3650 cm-'. This is shown in Fig. 5 for the metavanadate and orthovanadate samples. According to our previous studies on Mg-containing spinels,I7 the band near 3700 cm-' is very typical for MgOH surface groups. On the other hand, a band near 3650 cm- has been found frequently on vanadia-based catalysts like V20,-Ti0, ,la and can also be envisaged on V205.19 This band can be assigned to VOH groups, in spite of the absence of Brarnsted acidity of these groups on Mg vanadates (see below).According to these assignments, which should both be taken as tentative, in all three pure-phase Mg wavenurnber/cm -Fig. 5 FTIR spectra (OH-stretching region) of MgV,O, (left) and Mg,(VO,), (right) outgassed at room temperature (a) and at 573 K (b) vanadates both Mg and V sites are actually located at the surface, although we cannot have information on a possible enrichment of one of them at the surface. The spectrum of the amorphous Mg vanadate catalyst in the OH stretching region (Fig. 6) strongly differs from those of the crystalline phases because of the presence of a strong, broad and split band with maxima at 3615 and 3510 cm-', which resists outgassing at 773 K.Stable species responsible for these features are very unusual on metal oxide powders with prevalently ionic character, but can be found in the spectra of predominantly covalent oxides like bulk V,O, l9 and WO,.,' In the present case, these species, not being associated with crystalline phases, should be related to amorphous phases containing covalent V-0 bonds. In Fig. 2(c) and 4(c) the spectra of the Mg vanadate cata- lyst and of Mg,(VO,), after pyridine adsorption are reported. In Fig. 7 the spectra of pyridine adsorbed on the Mg vana- date catalyst as well as on Mg,(VO,),, Mg2V207, MgV,O, and V205 are compared, after subtraction of the spectra of the corresponding adsorbents. The spectrum of pyridine adsorbed on pure V205 shows the presence of bands assign-ed to pyridine molecules coordinated on Lewis acid sites, 4.0 3.9 3.8 3.7 Q) 3.6 me g 3.5 a 3.4 3.3 3.2 3.1 -4 3800 3600 3400 3200 3000 wavenumber/cm-Fig.6 FTIR spectra (OH-stretching region) of the Mg vanadate catalyst outgassed at 673 K (a)and 773 K(b) 0) Cm fV2 1700 ' 1500 1700 1500 1700 1500 1300 wavenumber/crn-' Fig. 7 FTIR spectra of pyridine adsorbed on V,O, (a), Mg,V,O, (b),Mg,V,O, (c) and Mg,(V04), (d) and the Mg vanadate catalyst (e).For each compound, the upper and lower spectrum are relative to outgassing at room temperature and at 373 K, respectively, after pyridine adsorption. The last spectrum is reported only for V,O,.characterized by the sharp 8a mode shifted to 1608 cm-' (with respect to the liquid-phase value of 1583 cm-') and the 19b mode shifted to 1447 cm-' (from 1439 cm-' in the liquid). However, the bands at 1640, 1635 and 1535 cm-' also show the presence of pyridinium ions, evidence of the Brernsted acidity of this surface." The copresence of Brernsted and Lewis sites is also typical of other vanadia-containing catalysts like V,O,-TiO, catalysts18 and V~o~-P20~ cata-lysts?' On all MgVO catalysts only molecularly chemisorbed pyridine species are detected, bands due to pyridinium ions not being observed at all. No bands are observed near 1630 and 1530 cm-', so no significant Brernsted acidity is present on the Mg vanadate surfaces.Moreover, the positions of the bands of chemisorbed pyridine are slightly but definitely shifted to lower wavenumbers, with respect to the position observed on vanadia. We observe the 8a mode at 1605 cm- ' and the 19b mode at 1444 cm-l in all four Mg vanadate powders, without any marked difference among them. This means that the Lewis acid sites detected on Mg vanadates are weaker than those of vanadia. Comparison with the data obtained on V,O,-TiO, catalysts" and on v,o,-P,o, catalysts2' shows that Mg vanadates are the weakest solid acids of this series, as far as both the Lewis and Brernsted acid sites are concerned. This can be associated to the structural modification of the vanadium centres in these catalysts as a result of the presence of the basic component MgO, as dis- cussed previou~ly.'~ Identification of the Lewis sites as V ions arises from com- parison of the spectra of pyridine adsorbed on V,O,, Mg vanadates and MgO.In the last case, pyridine is adsorbed very weakly, with the 8a mode weakly shifted to higher wave- numbers (1595 cm-'). On the other hand, note that the struc- ture of Mg,(VO,), with its nearly cubic close-packed array of oxygen atoms with Mg in octahedral coordination and vana- dium in tetrahedral coordination is, according to Krishna- makhari and Calve,,, closely related to that of a cation deficient inverse spinel. It can be viewed as consisting of deficient spinel slabs related by antiphase boundarie~.~, We can consequently compare the surface properties of Mg3(V04), with those of the inverse spinel MgFe20, and those of the normal spinels MgAl,O, and MgCr,O,, investi-gated previou~ly.'~ From comparison of the data arising from pyridine adsorbed on these compounds, as well as on a number of other spinel-type compounds, we concluded that pyridine is adsorbed very weakly on Mg ions in unsaturated octahedral coordination, with the 8a mode below 1600 cm- '. We can consequently assign to pyridine on Mgz+ a band J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 near 1595 cm-' which we observe after contact with pyri- dine, but which disappears by simple outgassing at room temperature (Fig. 7), although an alternative assignment to pyridine molecules physisorbed on OHs cannot be definitely ruled out.The stronger sites responsible for pyridine coordi- nation resistant to outgassing, with the 8a band at 1605 cm-' are identified as vanadium cations exposed at the surface. As we noted previ~usly'~ the spectra of Mg3(VO4), pressed discs after activation show a very weak, sharp band at 1962 cm-l. This weak band is evident in Fig. 3(b), and disappears when basic molecules, like pyridine, are adsorbed. Consequently, this sharp band is rather evident in the sub- traction spectra of adsorbed pyridine as a negative peak [Fig. qe)]. Although the transmittance of the catalyst is extremely low below lo00 cm-', we can envisage in the subtraction spectra of adsorbed pyridine a negative band also at 982 cm-'.We consequently propose that these bands are the first overtone (1962 cm-') and the fundamental (982 cm-') absorption band relative to a surface mode that is perturbed upon pyridine adsorption. This mode is very likely to be the V-0 stretching of a surface vanadate ion acting as Lewis acid site for pyridine coordination. As discussed previo~sly,'~ the V-0 stretchings of bulk vanadate ions in Mg3(V04), fall in the region below 915 cm- '. However, these bands refer to V-0 bonds whose oxygen atom also coordinates two or three Mg ions, and which are vibrationally coupled. When a vanadate ion is exposed at the surface, one or more V-0 bonds can be free from cation coordination and their V-0 bond order can increase accordingly. Consequently, they can produce a surface V-0 stretching mode with a higher fre- quency than the bulk modes. These modes have already been found by us for different V,O,-containing catalytic systems, like V,O,-TiO, , where they fall at 1035 cm-' (fundamental) and 2045 cm-' (first ando~ertone),'~,~~on V,O, (1038 cm-', fundamental"). However, we showed that basic doping causes a strong shift of these modes on V20,-TiO, down to 1013, 1002 cm-' for the fundamental mode on K-V,O,-TiO, and down to 990 cm-' on Cs-V,O,-TiO, .24 Although coupling effects could also have a role, the values observed here (982 cm-', funda-mental; 1962 cm- ',overtone) can be taken as evidence for a weaker V-0 bond order for the surface vanadate species on Mg orthovanadate with respect to V205 and V,O,-TiO,. This is likely to be associated with the presence of tetrahedral vanadate species on the surface of Mg orthovanadate as well as in the bulk, in contrast to the square or trigonal-pyramidal coordination thought to be taken by vanadium ions on the surfaces of both V,O,-TiO, and V205.Similarly, the shift to lower wavenumbers of the V-0 surface stretching mode on V,O,-TiO, by basic doping, already reported,24 can be evi- dence of a progressive change from pyramidal to tetrahedral coordination for surface vanadium, due to the increased basicity of the oxide ligands. Moreover, the elasticity of the coordination of vanadium, from coordination four to coordi- nation six, also shown by the bulk structure of Mg vana- dates,', provides evidence for the ability of this cation to act as a medium-strong Lewis acid site.In Fig. 8 the spectra of acetonitrile adsorbed on Mg3(V0,), at room temperature are reported. Acetonitrile has been used previously by us as a basic probe for the acid sites of V205 and V20,-based catalyst^.'^*^' However, this molecule is also a probe for basic and nucleophilic sites; it tends to release hydrogen atoms from the methyl group giving the [CH,-CN]- anion upon attack of surface basic ions25-2 7 or to undergo hydrolysis due to attack of the nucleophilic sites on the electrophilic nitrile carbon at~rn.~'-~~On Mg orthovanadate, coordination of acetoni- trile on the Lewis acid sites certainly occurs, and is J. CHEM. SOC.FARADAY TRANS., 1994, VOL. 90 0.08441 1 0.0610 ! I;, -0.0557 2400.0 2000.0 1600.0 1200.0 wavenumber/cm-' Fig. 8 FTIR spectra of the surface species arising from the adsorp- tion of acetonitrile on Mg,(VO,), at room temperature (a) and after successive outgassing at room temperaure (b) responsible for the formation of the relatively sharp bands at 2268, 2295 cm-', due to CN triple-bond stretching and the 6(CH,) + v(CC) combination, which interact via a Fermi res- onance. The positions of these components confirm that the Lewis acid sites of Mg,(VO,), are weaker than those of V205, where these bands are detected at 2296, 2324 cm- ',as well as those of the V,05-TiO, and (VO),P,O, catalysts." These bands are shifted to higher wavenumbers from the liquid-phase values (2254, 2292 cm-') the stronger are the Lewis sites with which the molecule interacts. However, a strong band is also observed at 2211 cm-', whose intensity grows by increasing the contact time of the surface with acetonitrile.This band is similar to that observed on basic oxides like Mg0,26 Zn02' and CeO, 27 in the region 2200-2100 cm-', attributed to the CN stretching of coordinated [CH,-CNI- anions. This band is observed near 2050 cm-' for 'free' [CH,-CN]-ions in solution.28 This species, which is not observed on V,O, and other vanadia-based catalysts we investigated, is due to the exis- tence of basic sites on the surface of Mg,(VO,), which are able to abstract a proton from the methyl group of acetonitri- le.However, on a very basic surface like MgO, acetonitrile also undergoes hydrolysis of the CN bond, with the forma- tion of strong bands in the region 1800-1000 em-' due to amide, carboxylate and similar species. This has been shown for the acetonitrile-MgO interaction by Koubowetz et ~1.,'~ and has been confirmed by us. Analysis of this spectral region shows that this occurs, if at all, to a very small extent on Mg,(V0,)2. A comparatively weak band near 1700 cm- ' can be found [Fig. 8(a)], indicative of partial hydrolysis of the CN group. This indicates that only very few sufficiently strong nucleophilic sites are present, and this strongly dis- tinguishes the surface chemistry of Mg,(VO,), from that of MgO. IR Study of the Adsorption and Transformation of C,and C, Organic Compounds To obtain further information on the relationships between the acid-base and redox surface properties of Mg vanadates, as well as on the mechanism of their catalytic activity, their interaction with C, and C, organic compounds (i.e.the ketones acetone and methyl ethyl ketone, the alcohols isopro- pyl alcohol and sec-butyl alcohol, the alkenes propene and but-1-ene, butadiene and finally the alkanes propane and butane) has also been investigated. The results of these experiments will be compared with those obtained on vanadia-titania, and vanadyl pyrophosphate catalysts. '9, In Fig. 9 the spectra of acetone and isopropyl alcohol adsorbed on Mg,(VO,), are reported. Acetone is adsorbed weakly on Mg3(V0,), [Fig.9(b)] : the weakness of this inter- action is shown (i) by the poor vibrational perturbation of the molecule during this interaction; (ii) by its nearly complete desorption by outgassing at room temperature; (iii) by its sta- bility towards chemical transformation during adsorption. The CO stretching of adsorbed acetone is found at 1712 cm-' and the C-C-C stretching mode at 1231 cm- '. The position of these bands is intermediate between those of the gaseous molecule [1734 cm-', v(C10); 1215 cm-', V(CCC)~~]and those of the species adsorbed on V,O,-TiO, (1682, 1248 cm-I 29) showing that the interaction on the Mg vanadate is much weaker than that on V,O,-TiO,. The spectrum of isopropyl alcohol adsorbed on Mg,(VO,), is instead definitely similar to that observed on V,0,-Ti0,29 as well as on other metal oxides like pure TiO, .33 The sharp bands at 1468, 1385, 1368 and 1330 cm-' (the last one weak), are due to CH deformations of the iso- propyl moiety, while the very strong band with components at 1165, 1140 and 1125 cm-' is due to C-0 stretching coupled with C-C-C stretchings, typical of isopropoxy 1.75 1.70 1.65 1.60 m5 1.55B$ 1.50 1.45 1.40 1.35 11 0' '1700' '160C' 'l'5oo-'lbO0' '1300' '1200' '1'100' ' .' wavenumber/cm-' Fig. 9 FTIR spectra of the surface species arising from the adsorp- tion of isopropyl alcohol (a)and acetone (b) on Mg,(VO,), 1.701 n 1800 1700 1600 1500 1400 1300 1200 1100 wavenumber/cm-' Fig. 10 FTIR spectra of the surface species arising from the adsorp- tion of sec-butyl alcohol on the Mg vanadate catalyst, and successive evacuation at room temperature (a), 373 K (b), 453 K (c) and 523 K (4 groups.The weak band at 1300 cm-' is due to the deforma- tion of the OH groups of coordinatively adsorbed undis- sociated alcohol, present in small amounts, and desorbed by prolonged outgassing. The isopropoxy groups resist outgass- ing at room temperature and are progressively destroyed by heating under evacuation, disappearing near 473 K. During outgassing only traces of adsorbed acetone produced by its oxidative dehydrogenation are observed. In Fig. 10 the spectra of sec-butyl alcohol on the Mg vana- date catalyst are reported.Also in this case the adsorption of the alcohol is almost completely dissociative, as shown by the almost complete absence of the OH deformation mode near 1280 cm-', typical of coordinated sec-butyl alcohol, and by the presence of the strong C-O/C-C modes near 1100 crr-'. The bands near 1460 and 1380 cm-' are assigned to the asymmetric and symmetric deformations of methyl groups, that near 1420 cm-' to the scissoring mode of the methylene group and that at 1335 cm-' is the deformation mode of the methyne group. The alcoholates are progres- sively destroyed with the appearance of small amounts of methyl ethylketone near 450 K, evidenced by the sharp band near 1700 cm-'. These results show that polar molecules like the alcohols do react with the surface of Mg vanadates, giving rise to dis- sociative adsorption, while further dehydrogenation can also occur.However, the reactivity of this surface is much weaker than that of ~anadia-titania~'.~' where the alcohols are oxi- dized to ketones at 373 K, and the ketones are very strongly bonded, further transform to carboxylate species and finally completely decompose. This is mainly due to the stronger Lewis acidity of this surface than that of Mg vanadates, which causes stronger adsorption and greater perturbation upon adsorption. This conclusion, according also to our previous studies on the potassium doping of vanadia-based catalyst^,^, suggests that the Lewis acidity and oxidizing activity of V ions are strongly correlated, both being associated with electron with- drawal.Thus the oxidizing power of V ions is decreased when these centres are placed in a basic environment and this results in a lower oxidizing activity and, finally, in a lower catalytic activity in oxidation reactions. The weaker activity of Mg vanadates with respect to other vanadia-based catalysts is even more striking if the reactivity towards alkenes is considered. On the Mg vanadate catalyst, a surface interaction with but-l-ene is observed only near 523 K, with the formation of carboxylate species, characterized by bands at 1590 and 1420 cm-', due to the asymmetric and symmetric -CO, stretchings. However, these adsorbed car- boxylate species disappear by further heating at 673 K, owing to their complete combustion and/or the desorption of partly oxidized fragments.The same bands observed after contact with but-l-ene are also found after contact with buta-1,3- diene but at significantly lower temperatures (473 K). Closely similar behaviour is observed for the other powders belong- ing to the MgVO system. This behaviour strongly contrasts with that observed for vanadia-titania where alkenes are adsorbed strongly at room temperature producing alkoxy groups which are, in turn, easily dehydrogenated and oxi- di~ed.~'?~'The different behaviour of MgVO catalysts is associated with the lack of any surface Brransted acidity, in contrast to vanadia-titania where the Brransted sites easily protonate the alkene with the formation of alkoxy groups at room temperature or even lower.The reactivity of the Mg vanadates is also very different from that of vanadyl pyropho~phate~' where furan-like species and maleic anhy- dride are observed by interaction of C, alkenes near 523 K. The interaction with propane and butane does not give detectable adsorbed species on the Mg vanadates, although J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 interaction occurs above ca. 623 K, according to the forma- tion of gas-phase carbon oxides and to the decrease of the transmittance of the sample probably due to reduction. This behaviour is justified by the fact that at the temperature at which this interaction occurs the resulting partially oxidized surface species are desorbed or are transformed further very rapidly. Conclusions The conclusions of the present paper can be summarized as follows: (i) A careful IR analysis in the skeletal and overtone region allows characterization of structurally polyphasic as well as microcrystalline MgO-V,O, catalysts.In particular, the presence of microcrystalline Mg2V20, and the absence of V20, on an XRD amorphous MgO-V,O, catalyst, have been established . (ii) MgO-V,O, samples are characterized by significant surface basicity, and tend to adsorb C02 in the form of car-bonate species. However, they are not as basic and nucleo- philic as MgO. (iii) All stable phases on MgO-V20, show weak surface Lewis acidity but a lack of any surface Brsnsted acidity, in contrast to pure vanadia and supported-vanadia catalysts. (iv) Weak Lewis acid sites are identified as coordinatively unsaturated surface V ions which are characterized on MgJVO,), by a surface V-0 bond much longer than the surface vanadyls on pure and supported vanadia.Therefore, they are probably essentially in a tetrahedral-like form, as in the bulk of these compounds, in contrast to the vanadylic form observed on supported-vanadia catalysts and vanadyl pyrophosphate. (v) On the surfaces of crystalline MgO-V,O, powders, OH groups bonded to both magnesium and vanadium are detect- able, showing that both cations are located at the surface, although surface enrichment cannot be excluded. For an amorphous powder, strongly bonded internal OHs are also evident.(vi) Mg vanadates show significant reactivity towards polar molecules such as alcohols, which are adsorbed in a disso- ciative way. (vii)Mg vanadates are very poorly reactive towards hydro- carbons like alkanes and alkenes. The weak interaction with alkenes, associated with the absence of Brsnsted acidity, can explain the ability of these powders to catalyse selectively the oxidative dehydrogenation of akanes, with limited alkene overoxidation. The role of the MgO component in these oxy-dehydrogenation catalysts seems to be essentially related to the lowering of the acidity (probably of both Brransted and Lewis type) and consequent- ly of the oxidizing ability of the vanadium oxide key com- ponent, which limits the successive transformation of the desired products (alkenes and butadiene) although it also strongly limits catalytic activity.This work has been supported by MURST (Rome). The authors thank D. 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