The nature of the W-containing species intercalated in sample MgAlW has been studied by different techniques and the results have been previously reported elsewhere[33] and are here summarized.
The PXRD diagram of sample MgAlW is shown in Fig. 1, together with that of the precursor nitrate LDH, Mg2Al–NO3, and those of the calcined materials. The diagram for the uncalcined solid is characteristic of a layered material with the hydrotalcite-like structure, with a basal spacing of 10 Å (from the position of the first basal maximum below 2θ = 10°) larger than that of the precursor nitrate sample (8.5 Å), indicating a swelling of the layers due to the nitrate/p-tungstate exchange. Due to the pH used during synthesis[24,25] and from the gallery height, we concluded[33] that the interlayer anion is W7O246–. Nevertheless, the d003 value for this sample is smaller than that reported by other authors[21–23] because of the strong interaction between the anion and the brucite-like layers, favoured by the synthesis temperature (100 °C) and the long contact time between the solution and the layered precursor used.[33] No diffraction line corresponding to the Mg2Al–NO3 LDH is recorded, indicating a total exchange under the experimental conditions used.
Sample MgAlW/500 shows a PXRD diagrams with no defined maximum, characteristic of mostly amorphous materials. Only after calcination at 700 °C do sharp peaks develop which become sharper when calcination is carried out at 800 °C (diagram not shown); these peaks have been ascribed to two different phases of MgWO4, one corresponding to wolframite (file 27-789 of JCPDS files[34]), where both tungsten and magnesium cations are octahedrally coordinated, and another similar phase formed upon dehydration of MgWO4·xH2O (file 19-776 of JCPDS files[34,35]), where tungsten ions are tetrahedrally coordinated and magnesium ions maintain an octahedral coordination. When the calcination temperature is increased to 1000 °C, only lines due to wolframite are recorded. The diagrams are similar to those previously recorded[36] after high temperature calcination of solids prepared by impregnation of MgO with aqueous solutions of p-tungstate salts.
The FT-IR spectrum of sample MgAlW, Fig. 2, show bands characteristic of p-tungstate, although some of them are shifted with respect to the positions reported for the free anion, because of the interaction with the layers. The characteristic bands are recorded between 1000 and 500 cm–1 at 950, 925 cm–1 (terminal ν(W–O)), 878 cm–1 (corner-sharing νas(W–O–W)), and 747, 670 cm–1 (edge-sharing νas(W–O–W)). Finally, the bands recorded below 600 cm–1 (close to 552, 447, and 391 cm–1) are due to traslational Al/Mg–OH modes of the brucite-like layers.[37] Calcination at 500 °C leads to cancellation of the bands characteristic of polytungstate, and a broad absorption at 851 cm–1 dominates the spectrum. When the calcination temperature is increased to 700 °C, Fig. 2, individual bands are recorded at 1004, 860, 801, 751, 681 cm–1, due to stretching W–O modes of the two phases of MgWO4 the presence of which has been concluded from the XRD diagram. The intensities of these bands increase when the calcination temperature is further increased.
Surface texture data for the layered sample, as well as for the nitrate precursor and the solids obtained upon calcination at 500 and 700 °C are given in Table 1. Layered double hydroxides with the hydrotalcite-like structure usually do not show micropores, as the interlayer space is not accessible to the N2 molecule. However, swelling of the interlayer because of the incorporation of large anions makes this space available to adsorption as micropores.[38]
SEM images of some of the compounds prepared are shown in Fig. 3. The images for uncalcined sample MgAlW show agglomerates of rather large hexagonal plate-like crystals with a diameter between 5–10 µm. SEM images for the calcined sample are also shown in Fig. 3. Most of the crystallites show an average size close to 5 µm after calcination at 500 °C and 700 °C. Even at 700 °C the crystallites of MgWO4 (the phase detected by PXRD) are not observed. However, when the sample is calcined at 800 °C the needle-like crystals, characteristic of MgWO4, are clearly observed.
As mentioned above, surface acidity was studied through FT-IR monitoring of adsorption of pyridine (py). After outgassing the sample in situ at 400 °C for 2 h, a pressure of 400 N m–2 was equilibrated with the sample for 30 min, and the spectrum was recorded after outgassing at room temperature, 100, 200, and 300 °C.
The spectra recorded after adsorption of py on sample MgAlW calcined at 500 or 700 °C are very similar; those for sample MgAlW/500 are shown in Fig. 5. Bands at 1608, 1577, 1492, and 1446 cm–1 are recorded after outgassing at room temperature. These bands correspond to modes 8a, 8b, 19a, and 19b, respectively, of py coordinated to surface Lewis acid sites,[39,40]i.e., coordinatively unsaturated metal cations. The intensities of the signals decrease when the outgassing temperature is increased, but are recorded even after outgassing at 300 °C, indicating that they are rather strong acid sites. No band is recorded which could be ascribed to adsorption of py on surface Brønsted acid sites.
Although in this study we have followed the reactive adsorption of ethanol on our solids, as acetaldehyde and acetic acid are expected oxidation products of ethanol, the adsorption of these species has been also monitored by FT-IR spectroscopy. The range reported extends in all cases from 1800 to 1000 cm–1, as the more representative bands of the adsorbed species are expected in this range.
Adsorption of acetic acidThe spectra recorded after contacting acetic acid at room temperature with samples MgAlW/500 and MgAlW/700 are very similar, with bands at 1740, 1718, 1665, 1584, 1457, 1429, 1350, and 1295 cm–1. The two bands recorded at the highest wavenumbers (1740 and 1718 cm–1), together with that at 1295 cm–1, can be ascribed to modes ν(CO) and ν(CO)/δ(C–OH), respectively, of weakly, molecularly adsorbed acetic acid (the bands are recorded at 1770 and 1264 cm–1 for gaseous acetic acid.[41]) These bands disappear after gentle outgassing of the sample, indicating that the interaction should be rather weak. The band at 1665 cm–1 can be ascribed to the ν(CO) of acetic acid less weakly adsorbed on basic surface sites; this band is only removed after outgassing at higher temperatures. The bands at 1593, 1457, 1429, and 1350 cm–1 are due to modes νas(COO), νs(COO) + δas(CH3) and δs(CH3) of acetate species bonded to surface Lewis acid sites, through a dissociative adsorption of the acid from the gaseous phase. These species are even strongly held on the surface, as the bands are recorded (although rather weak) after outgassing the sample at 300 °C.
Adsorption of acetaldehydeThe spectra recorded after adsorption of acetaldehyde at room temperature on samples MgAlW/500 (Fig. 6) and MgAlW/700 are rather similar. Bands at 1711, 1350, and 1130 cm–1 are due to modes ν(CO), δs(CH3), and ν(C–C)/ν(CH)/δ(CCO) of molecularly, non-dissociated adsorbed acetaldehyde,[42] coordinated through the oxygen atom of the carbonyl group to surface Lewis acid sites. The bands at 1593, 1457, 1429, and 1350 cm–1 are similar to those recorded after adsorption of acetic acid and are due to acetate species. These species can have been formed through oxidation of acetaldehyde or they can arise from a Cannizzaro disproportionation of acetaldehyde to acetate and ethoxy species, a process that has been previously reported to occur on other metal oxides.[43] Such a disproportionation would account for acetate species formed even at room temperature (spectrum a in Fig. 6), and also for the bands recorded at 1400–1385 and 1100 cm–1, similar to those observed after adsorption of ethanol on these samples (see below), which can be ascribed to δ(CH) and ν(CO) modes of the ethoxy group.
When the sample is heated at 100–200 °C without removal of the gas phase, bands in the same positions are recorded, although their relative intensities change: so, the intensity of the band due to weakly adsorbed acetaldehyde (1711 cm–1) decreases, while a new band at 1669 cm–1 develops; this band is due to acetaldehyde adsorbed on surface Lewis acid sites. A new band at 1643 cm–1 is also observed, which, together with the band at 1183 cm–1, can be ascribed to modes ν(CC) and ν(CO) of enolate species, CH2CHO–, or, alternatively, crotonaldehyde, as a result of aldol condensation of acetaldehyde.[44,45] Although these species are rather stable, they are observed only in the presence of undissociated acetaldehyde. In our case, as the cell has not been outgassed after adsorption of acetaldehyde at room temperature, their formation cannot be discarded as the temperature is raised; these bands are similar to others recorded after adsorption of acetone or acetaldehyde on different metal oxides.[46]
Only the bands due to ethoxy species disappear when the sample is heated at 300 °C, Fig. 6; the intensities of those due to molecularly adsorbed acetaldehyde, to crotonaldehyde or to enolate species decrease, while those ascribed to acetate species markedly increase. These bands due to adsorbed acetate species are the only bands recorded if the sample is outgassed at room temperature after heating at 200–300 °C.
The spectrum recorded after adsorption of ethanol at room temperature (and outgassing at the same temperature) on sample MgAlW/500 shows, Fig. 7a, in addition to those bands corresponding to ν(CH) and ν(OH) modes at high wavenumbers (not shown), bands between 1480 and 1387, 1272, 1092 cm–1, due to modes δ(CH) and ν(CO) of the ethoxy group, formed upon dissociative adsorption of ethanol on acid–basic surface sites of the solid; a band at 1630 cm–1, due to mode δ(HOH) of molecular water, is also recorded.
When the sample is heated at 100 °C without outgassing, the intensities of the bands mentioned above decrease, and new, although weak, bands are recorded at 1710 and 1640 cm–1 together with a very weak band at 1355 cm–1. These bands have been recorded also after adsorption of acetaldehyde (see above) and have been ascribed to molecularly adsorbed acetaldehyde and a monomeric enol species or crotonaldehyde. Probably, in this case they correspond to the former species, as the concentration of acetaldehyde should be rather low. When the sample is further heated at 200 °C the bands due to molecular acetaldehyde strengthen (especially the bands at 1710 and 1355 cm–1), and a new band develops at 1669 cm–1, which corresponds to acetaldehyde coordinated to surface Lewis acid sites; this band is also recorded after adsorption of acetaldehyde on this sample; simultaneously, the intensities of the bands due to ethoxy species decrease, but no band due to acetate species is recorded, even if the sample is heated at 300 °C. Bands due to acetaldehyde are removed after outgassing at 200–300 °C.
The spectra recorded after adsorption of ethanol on sample MgAlW/700 (Fig. 8) are different. Adsorption and outgassing at room temperature gives rise to bands exclusively due to ethoxy species, similar to those recorded after adsorption of the alcohol on sample MgAlW/500. When the temperature is raised to 100 or 200 °C, without simultaneous outgassing, the spectrum only shows minor changes. Only after heating at 300 °C, are the weak bands at 1752, 1732, and 1662 cm–1, due to the ν(CO) mode of undissociated acetaldehyde adsorbed on surface Lewis acid sites with different strengths recorded, together with bands at 1587, 1457, 1425, and 1350 cm–1, due to acetate species.