1
An FT-IR study of the adsorption and reactivity of ethanol on systems derived from Mg2Al–W7O246– layered double hydroxides

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

2
A FT-IR spectroscopy study on the surface reactivity of MgAl-paratungstate LDHs calcined at 500 and 700 °C, prepared by anion exchange from a Mg2Al-nitrate hydrotalcite and precursor salt (NH4)10H2W12O42, for ethanol dehydrogenation, is reported.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

3
Calcination at 300 °C of Mg2Al–W7O246– LDH leads to amorphous species, and crystallisation of MgWO4 is observed at 700 °C.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

4
The solids calcined at 500 and 700 °C exhibit surface Lewis acid sites and are selective to acelaldehyde formation during ethanol dehydrogenation or to oxidative dehydrogenation, especially in the sample calcined at 500 °C, where carboxylate species are not detected in the temperature range tested (room temperature to 300 °C).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

Introduction

5
Layered double hydroxides (LDHs), also known as hydrotalcites or anionic clays, are materials which can be easily synthesised by cheap methods.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

6
Its structure consists of Mg(OH)2 (brucite) layers, where a partial M2+/M3+ substitution has taken place, the positive charge in excess being balanced by anions (most often carbonate) located, together with water molecules, in the interlayer.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

7
The general formula is [M1–x2+Mx3+ (OH)2] [Ax/nn–] mH2O.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

8
The value of x usually ranges between 0.17 and 0.33.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

9
Probably, one of the most interesting properties of these materials is their ability to exchange topotactically the interlayer anions, thus permitting the synthesis of a large number of compounds.1–7

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

10
Many polyoxometalates (POMs) of transition metal cations (e.g., Mo, W, V) with different structures (Keggin, Dawson or Finke type) have been incorporated into the interlayer space of anionic clays.3,6–23

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

11
The main problem found when synthesising these materials is that the precise nature of POM is pH-dependent24,25 and they are usually unstable under the basic conditions provided by the solid material, so a careful control of synthesis methods and conditions (e.g., pH, temperature, contact time of the LDH precursor with POM, etc.) is required.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

12
Intercalation of this sort of anion, in addition to swelling (which increases the accessible surface area) permits immobilisation of the POM, and its controlled release in a polar medium.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

13
These POM-exchanged LDHs found applications as catalysts and molecular sieves.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

14
The effectiveness of catalysts constituted by transition metal oxides associated with divalent cations (magnesium decavanadates, pirovanadates and molybdates) in oxidative dehydrogenation processes is well known.26–30

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

15
Their catalytic performance depends on the structure, composition and synthesis route; it has been reported29 that solids obtained upon calcination of hydrotalcites containing intercalated decavanadate are more active in propane dehydrogenation than those containing pirovanadate.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

16
Tungsten has chemical similarities with molybdenum and vanadium; however, very few papers deal with the catalytic properties of tungstates prepared from tungsten-containing hydrotalcites precursors.21,23

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

17
In this paper we report a study on the reducibility and surface acid properties of the systems obtained upon calcination of paratungstate-containing LDHs at 500 and 700 °C, and their activity in ethanol dehydrogenation; this study has been carried out through FT-IR monitoring of the adsorption of different molecules, such as pyridine, ethanol, acetic acid and acetaldehyde.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj2

Experimental

Preparation of the samples

18
The sample Mg2Al-paratungstate LDH was prepared by anion exchange.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

19
The Mg2Al–NO3 LDH precursor, with a basal spacing of 8.5 Å, was prepared following the method previously described,31 once prepared it was maintained in suspension under a nitrogen atmosphere.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

20
A portion of this suspension (108 ml, containing 4 g of Mg2Al–NO3) was slowly added to a solution prepared by dissolving 5.7 g of ammonium para-tungstate (NH4)10H2W12O42·10H2O, APW (from Merck), in 200 ml of decarbonated water at 50 °C.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

21
The mixture, under a nitrogen atmosphere, was magnetically stirred at 100 °C for 30 min and the pH was maintained at 6.5 (by adding 0.1 M HNO3).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

22
The solid was washed several times with decarbonated water by centrifugation, filtered and dried at room temperature in a vacuum desiccator leading to sample MgAlW.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

23
The amount of APW was that required to balance the positive charge of the brucite-like layers, plus a 5% excess, as it has been reported that a large excess favours formation of impurities.32

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

24
The hydrotalcite obtained, MgAl-paratungstate, was calcined at 500 or 700 °C, for 3 h in air, leading to solids named MgAlW/500 and MgAlW/700, respectively.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

Characterisation

25
Elemental chemical analyses for Mg, Al, and W were carried out in Servicio General de Análisis Químico Aplicado (University of Salamanca, Spain) by atomic absorption in a Mark 2 ELL-240 instrument after dissolving the samples in nitric acid.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

26
Powder X-ray diffraction (PXRD) diagrams were collected on a Siemens D-500 instrument using Cu Kα radiation (λ = 1.54050 Å) and quartz as external standard.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

27
Fourier transform infrared spectra (FT-IR) were recorded using a Perkin-Elmer FT-1730 instrument from Perkin Elmer, using the KBr pellet technique; 100 scans were averaged to improve the signal-to-noise ratio, with a nominal resolution of 4 cm–1.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

28
The nitrogen adsorption–desorption isotherms were recorded at –196 °C in a Gemini instrument, from Micromeritics, on samples previously degassed at 130 °C for 2 h.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp5

29
The specific surface areas were calculated by the BET method.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp5

30
The t-plot method was used to determine the total (St) and microporous (Sm) surface areas.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp5

31
Temperature-programmed reduction (TPR) analysis was carried out in a Micromeritics TPR/TPD 2900 instrument at a heating rate of 10 °C min–1 and using a H2/Ar (5% vol) mixture as the reducing agent (60 ml min–1).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

32
Scanning electron microscopy (SEM) studies were performed using a JEOL JSM 5600 LV SEM instrument.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp7

33
Surface acidity monitoring was carried out through an FT-IR spectroscopy study of pyridine adsorption in a Perkin-Elmer 16PC spectrometer coupled to a high-vacuum Pyrex system, and using self-supported discs, degassed in situ in a special cell (made of Pyrex, but with CaF2 windows, transparent to the IR radiation in the required wavenumber range) at 400 °C for 2 h, previously to pyridine adsorption.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp8

34
The spectrometer is coupled to a PC, and commercial software was used to process the spectra.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp8

35
One hundred scans were taken to improve the signal-to-noise ratio, at a nominal resolution of 2 cm–1.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp8

36
The gas is admitted to the IR-adsorption cell at room temperature and, after 15 min equilibration, the gas phase is removed by outgassing at different temperatures (from room temperature to 400 °C) and the spectrum is recorded.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp8

37
This same technique and apparatus were used to monitor the adsorption of ethanol, acetic acid and acetaldehyde (expected reactions products).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp9

38
The spectra were generally recorded without prior outgassing the gas phase.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp9

Results

Characterisation

39
The nature of the W-containing species intercalated in sample MgAlW has been studied by different techniques and the results have been previously reported elsewhere33 and are here summarized.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj3

40
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.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

41
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.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

42
Due to the pH used during synthesis24,25 and from the gallery height, we concluded33 that the interlayer anion is W7O246–.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

43
Nevertheless, the d003 value for this sample is smaller than that reported by other authors21–23 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

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

44
No diffraction line corresponding to the Mg2Al–NO3 LDH is recorded, indicating a total exchange under the experimental conditions used.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

45
Sample MgAlW/500 shows a PXRD diagrams with no defined maximum, characteristic of mostly amorphous materials.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

46
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 files34), where both tungsten and magnesium cations are octahedrally coordinated, and another similar phase formed upon dehydration of MgWO4·xH2O (file 19-776 of JCPDS files34,35), where tungsten ions are tetrahedrally coordinated and magnesium ions maintain an octahedral coordination.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

47
When the calcination temperature is increased to 1000 °C, only lines due to wolframite are recorded.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

48
The diagrams are similar to those previously recorded36 after high temperature calcination of solids prepared by impregnation of MgO with aqueous solutions of p-tungstate salts.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

49
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.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

50
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)).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

51
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

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

52
Calcination at 500 °C leads to cancellation of the bands characteristic of polytungstate, and a broad absorption at 851 cm–1 dominates the spectrum.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs4

53
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.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

54
The intensities of these bands increase when the calcination temperature is further increased.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

55
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.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

56
Layered double hydroxides with the hydrotalcite-like structure usually do not show micropores, as the interlayer space is not accessible to the N2 molecule.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

57
However, swelling of the interlayer because of the incorporation of large anions makes this space available to adsorption as micropores.38

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

58
SEM images of some of the compounds prepared are shown in Fig. 3.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

59
The images for uncalcined sample MgAlW show agglomerates of rather large hexagonal plate-like crystals with a diameter between 5–10 µm.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

60
SEM images for the calcined sample are also shown in Fig. 3.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

61
Most of the crystallites show an average size close to 5 µm after calcination at 500 °C and 700 °C.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

62
Even at 700 °C the crystallites of MgWO4 (the phase detected by PXRD) are not observed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

63
However, when the sample is calcined at 800 °C the needle-like crystals, characteristic of MgWO4, are clearly observed.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

Reducibility

64
The TPR curves for samples MgAlW/500 and MgAlW/700 are shown in Fig. 4.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs9

65
The reduction maximum shifts to higher temperatures when the calcination temperature of the sample is increased.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs9

66
The reduction percentages measured, assuming a W6+ to W4+ reduction in the whole temperature range studied, are rather low, 30 and 10%, respectively, for samples MgAlW/500 and MgAlW/700.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

Surface acidity

67
As mentioned above, surface acidity was studied through FT-IR monitoring of adsorption of pyridine (py).

Type: Method | Advantage: None | Novelty: New | ConceptID: Met1

68
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.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met1

69
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.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs10

70
Bands at 1608, 1577, 1492, and 1446 cm–1 are recorded after outgassing at room temperature.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs10

71
These bands correspond to modes 8a, 8b, 19a, and 19b, respectively, of py coordinated to surface Lewis acid sites,39,40i.e., coordinatively unsaturated metal cations.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res11

72
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.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res11

73
No band is recorded which could be ascribed to adsorption of py on surface Brønsted acid sites.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

Reactivity

74
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.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met2

75
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.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met2

Adsorption of acetic acid

76
The 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.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

77
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(CO) 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)

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

78
These bands disappear after gentle outgassing of the sample, indicating that the interaction should be rather weak.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

79
The band at 1665 cm–1 can be ascribed to the ν(CO) of acetic acid less weakly adsorbed on basic surface sites; this band is only removed after outgassing at higher temperatures.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

80
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.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

81
These species are even strongly held on the surface, as the bands are recorded (although rather weak) after outgassing the sample at 300 °C.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

Adsorption of acetaldehyde

82
The spectra recorded after adsorption of acetaldehyde at room temperature on samples MgAlW/500 (Fig. 6) and MgAlW/700 are rather similar.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

83
Bands at 1711, 1350, and 1130 cm–1 are due to modes ν(CO), 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.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

84
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.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

85
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

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

86
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.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

87
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.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

88
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(CC) and ν(CO) of enolate species, CH2CHO–, or, alternatively, crotonaldehyde, as a result of aldol condensation of acetaldehyde.44,45

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con3

89
Although these species are rather stable, they are observed only in the presence of undissociated acetaldehyde.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res16

90
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

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con4

91
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.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res17

92
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.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs11

Adsorption of ethanol

93
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.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res18

94
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.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs12

95
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.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res19

96
Probably, in this case they correspond to the former species, as the concentration of acetaldehyde should be rather low.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res19

97
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.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res20

98
Bands due to acetaldehyde are removed after outgassing at 200–300 °C.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs13

99
The spectra recorded after adsorption of ethanol on sample MgAlW/700 (Fig. 8) are different.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res21

100
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.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res21

101
When the temperature is raised to 100 or 200 °C, without simultaneous outgassing, the spectrum only shows minor changes.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res22

102
Only after heating at 300 °C, are the weak bands at 1752, 1732, and 1662 cm–1, due to the ν(CO) 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.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res23

Discussion

103
Catalytic decomposition of alcohols to form carbonyl compounds on metal oxides may proceed through (i) simple dehydrogenation, where the presence of molecular oxygen is not essential, the reaction taking place on basic sites and the carbonyl compound is formed together with H2, and (ii) oxidative dehydrogenation, leading to formation of the carbonyl compound and water through a Mars–van Krevelen mechanism;47 in this case, red-ox sites are required, as well as O2 for reoxidation of the catalyst.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

104
Oxidative dehydrogenation of ethanol to acetaldehyde and/or acetic acid may take place on samples MgAlW/500 and MgAlW/700, where Lewis type acid sites (W6+ and Al3+) are present, as evidenced by pyridine adsorption, as well as basic sites (OH– and O2–), due to the presence of strongly basic Mg oxides.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res24

105
Such a surface basicity is evidenced by bands due to enolate species (CH2CHO–) or crotonaldehyde detected after acetaldehyde adsorption, formation of which takes place easily on basic sites; redox sites also exist (W6+ species).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res24

106
However, reducibility of tungsten in these solids (W6+ → W4+), is very low, especially in the sample calcined at 700 °C.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res24

107
The FT-IR spectra indicate that acetaldehyde formation takes place through intermediate ethoxy species, the formation of which has been observed after adsorption of ethanol on both samples studied.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res25

108
These alcoxy species are formed as a result of dissociative adsorption of the alcohol on the acid–base sites, giving rise to new hydroxyl groups or water molecules (Scheme 1); the band corresponding to mode δ(H2O) at 1630 cm–1 is only observed upon adsorption of ethanol on sample MgAlW/500.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res25

109
As already mentioned, evolution of ethoxy species to formaldehyde takes place through simple or oxidative dehydrogenation.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res25

110
As the reducibility of tungsten species is very low in sample MgAlW/700, it may be suggested that on this sample acetaldehyde is formed following the simple dehydrogenation route (Scheme 2), which is favoured in the presence of basic sites.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con5

111
However, in the case of sample MgAlW/500, in addition to this type of dehydrogenation an oxidative dehydrogenation (Scheme 3) is also possible, because of the larger reducibility of this sample, following a Mars–van Krevelen mechanism,47 where lattice oxygen acts as the oxidant species.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con6

112
Formation of acetaldehyde takes place at lower temperatures on sample MgAlW/500 than on sample MgAlW/700, and acetate species are not detected on the former sample.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res26

113
Consequently, sample MgAlW/500 may be a good selective catalyst for acetaldehyde formation, as this species desorbs without further oxidation.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con7

114
On the contrary, acetaldehyde is formed, together with stable acetate species, on sample MgAlW/700 at higher temperatures (ca. 300 °C) these acetate species can undergo decomposition to CO2, thus decreasing the selectivity to acetaldehyde.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res27

115
The catalytic behaviour of sample MgAlW/700 is similar to that previously observed48 for WOx/MgO samples prepared by impregnation.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con8

116
As acid–base and structural properties are very similar for both samples, the different selectivity for acetaldehyde formation can be related to the different dispersion of the tungsten-containing active phase (magnesium tungstates).

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con8

117
The PXRD pattern of sample MgAlW/500 does not evidence formation of crystalline phases, and only amorphous ones (more easily reduced) should be formed; however, the presence of MgWO4 crystalline species (less easily reduced) in sample MgAlW/700 only permits a simple dehydrogenation at higher temperatures.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con9

118
On the other hand, Cannizzaro disproportionation of acetaldehyde to ethoxy and acetate species has been observed on both samples only at room temperature or at 100 °C, in the presence of gaseous acetaldehyde.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con9