3.1 XRD analysis
XRD profiles of synthesized and anion-exchanged hydrotalcite samples and those treated at 773 K are shown in Figs. 1 and 2, respectively. That of purchased hydrotalcite (HT–Wako) is also shown. Characteristic diffraction peaks at 2θ = 11.3° and 22.8° observed in the profile of HT–Wako in Fig. 1 were assigned to diffractions from (003) and (006) facets of typical hydrotalcite (Powder Diffraction File (PDF) No. 14–191). The lattice constants of hydrotalcite in the PDF are reported as a = b = 0.3070 nm and c = 2.323 nm. The distance between hydrogen atoms in the two layers is 0.29 nm [29]. The interlayer space is constructed by intercalated H2O. The ionic radius of Cl− (0.167 nm) is small enough for intercalation into the space between layers. Diffraction peaks of (003) and (006) facets are expected to shift to lower angle positions by the exchange of Cl− with a larger anion.
All anion-exchanged HTs showed much lower peak intensities in their XRD profiles. The peak height of HT − Cl was, at most, about 20% of HT–Wako. The carbonate anion is the most suitable to form a double layered hydroxide structure. Peaks of the (003) and (006) facets in HT–Cl appeared at the same positions as those of HT − Wako. Sharp peaks at 31.9° and 45.6° were assigned to NaCl, due to incomplete washing of the sample with water.
The diffraction peak of the (003) facet of HT − B2O7 was in the same position as that of HT − Wako. The peak of the (006) facet had a shoulder on the lower angle side. This indicates that some Cl− was exchanged with B2O72−, which is larger than Cl−, resulting in expansion of the space between layers. A very similar XRD profile to HT-B4O7 was obtained from HT − PO4.
In HT − Mo7O24, HT − MoO4, and HT − WO3, a new peak appeared around 18.5° in addition to the peak at the original position. The peak of the (006) facet at 22.3° was moved to a lower angle position by intercalation of larger oxyanions. In these samples, only partial exchange of Cl− with the corresponding oxyanions took place. In HT − WO4 and HT − SO4 samples, peaks of (006) and (003) facets moved to lower angle positions and no peaks were observed at the original positions. The exchange of Cl− took place more efficiently in these two samples.
XRD profiles of heat-treated samples are shown in Fig. 2. All samples gave low intensity and wide diffraction peaks. As shown in Fig. 1, HT − Wako possessed high crystallinity. However, the low crystallinity of the thermally decomposed product of HT–Wako was the same as the other samples. Peaks in the heat-treated HT − Wako sample at 2θ = 43.2° and 62.7° were assigned to (200) and (220) facets of MgO (PDF No. 4–829), respectively. All anion-exchanged and heat-treated samples gave small and wide peaks around 2θ = 43.2° and 62.7°. It can be concluded that MgO–Al2O3 mixed oxides prepared by anion exchange and heat treatment had the MgO structure.
A large halo in the range from 20° to 40° was observed in all samples. The halo was the clearest in the XRD profile of the HT–SO4 sample. The halo peak is observed in solids that have lost their long-range crystalline order. Broad X-ray amorphous halos are observed in disordered nanocrystalline materials that have short-range order [30]. The angle range of the observed halo corresponded roughly to lattice spacing from 0.44 nm to 0.23 nm. The lattice constant of MgO is 0.4213 nm. The bond lengths of Mg–O and Mg–O–Mg are 0.21 nm and 0.42 nm, respectively. The observed halo could be the diffraction from a short-range order of Mg–O and Mg–O–Mg. The large halo peak in heat-treated samples indicated that heat-treated anion-exchanged samples had the MgO structure.
The small sharp peaks in the profile of HT − Mo7O24 at 23.4° and 26.5° were assigned to MgMoO4 (PDF No. 21 − 961). The meta-tungstate anion would be partly decomposed to ortho-tungstate. HT − Cl was converted into MgAl2O4 (PDF No. 21 − 1152) by heat treatment.
3.2 TG − DTA
Results of thermogravimetric and differential thermal analysis are summarized in Figs. 3 and 4, respectively. A continuous weight decrease without a clear stepwise weight decrease was observed in all samples except HT − Wako. A large two-step weight decrease was seen in the TG profile of HT–Wako. This is the typical TG profile of hydrotalcite (Mg6Al2(OH)16(CO3)·4H2O) containing CO32− as an anion in the interlayer space [27, 31, 32]. The decreasing weight in the lower temperature range was attributed to the desorption of H2O in the interlayer space, while that in the higher temperature range was the hydroxide decomposition to oxide and CO2 desorption.
As shown in Fig. 3, the clear stepwise decrease in sample weight nearly disappeared in prepared HTs. The TG profiles of anion-exchanged samples were flatter than that of HT − Wako. The decreasing weight with hydroxide decomposition was barely observed in HT–Cl and HT–WO3 samples. All samples showed a hydroxide decomposition peak at 670 K or higher as an endothermic negative peak (Fig. 4). This peak was not clear in HT–B2O7, HT–WO4, and HT–SO4 samples.
Endothermic peaks of water desorption from the interlayer space in anion-exchanged samples moved to a lower temperature range. These were separated into two parts in some samples (Fig. 4).
3.3 IR measurements
Infrared spectra of prepared and anion-exchanged HTs were measured to confirm anion exchange by the applied procedures. IR spectra of dried samples are shown in Fig. 5. Bands from water molecules intercalated in the interlayer space and adsorbed on the surface of samples were observed in all samples around 3500 cm− 1. The strong wide band from 2700 to 3600 cm− 1 was assigned to the stretching mode of the O–H bond. The band at 1632 cm− 1 observed in all samples was assigned to adsorption of H2O in the bending mode.
The band at 1372 cm− 1 in HT − Wako was from the absorption by CO32− [33]. This band disappeared in HT–Cl, demonstrating that HT–Cl had been successfully synthesized.
Broad bands appeared in the same position as carbonate in the spectrum of HT–B4O7. The bands between 1250 and 1500 cm− 1 were assigned to BO33− stretching modes [34]. The absorption band at 1074 cm− 1 in HT − PO4 was assigned to the P–O stretching mode [35].
HT − MoO4 and HT − Mo7O24 samples exhibited very similar IR profiles in the wavenumber region below 1300 cm− 1. MoO42− was assigned to the chemical species having a band at 850 cm− 1 [36]. The shoulder peak at 934 cm− 1 in HT–Mo7O24 was assigned to the band of Mo7O246−. MoO42− is the most stable species among poly-molybdate anions in neutral water solution [36]. In the HT–Mo7O24 sample, Mo7O246− would be partially hydrated before intercalation in the interlayer space of HT.
HT–WO3 and HT–WO4 had spectra similar to the molybdate samples. The band at 830 cm− 1 was assigned to WO42− [37]. In the case of tungstate, WO42− is the most stable in alkali solution. WO3 was converted into (NH4)2WO4 by dissolution in ammonia solution, which exchanged WO42− with Cl−. A sharp peak at 1110 cm− 1 with a shoulder on the higher wavenumber side and a broad band at around 618 cm− 1 observed in the HT − SO4 sample were assigned to absorption of the SO4 group [38].
As shown in Fig. 4, HT–Cl without carbonate was prepared successfully. Exchange of Cl− with anions was confirmed in all exchanged samples. As shown in the XRD results in Fig. 1, the expansion of the interlayer distance was observed in HT − SO4 and HT − WO4 samples. It can be concluded that anion exchange of Cl− with oxyanions was completed in these samples. X-ray diffraction peaks with a wide diffraction angle range, or the appearance of a shoulder on the lower angle side, indicated that anion exchange partially took place in other samples.
3.4 Comparison of ethanol decomposition activity and selectivity
Ethanol is converted into ethylene and diethyl ether by dehydration on acid sites or acid–base pair sites. Acetaldehyde is formed by dehydrogenation on base sites. Acetaldehyde formation by oxidative dehydrogenation accompanying reduction of chemical species on the catalyst surface is also considered. The acid–base properties of the solid catalyst reflect the product selectivity [28].
The thermally decomposed product of HT–Wako was mainly composed of MgO. It was expected that the catalyst prepared from HT − Wako would show acetaldehyde selectivity to some extent due to the formation of base sites.
An enhancement of acidity was reported in metal oxides treated with oxyanions. Sulfated zirconia, prepared by introducing sulfate ion to zirconium oxide or hydroxide surface and heat treatment, is a representative example [22–24]. Introducing molybdate, tungstate, borate, and phosphate anions to the zirconia surface for acid catalyst preparation has also been studied [22–24, 39]. Therefore, solid catalysts obtained by thermal decomposition of anion-exchanged HTs were expected to have acid sites on their surfaces.
Conversion and product yields from ethanol decomposition on tested samples are shown in Table 1. All data were taken 3 h after the start of the reaction. HT–SO4 and HT–B4O7 showed stable activity during the operating period, while a decrease in activity was observed in the other catalysts. The decrease in activity was particularly large in HT–Cl.
Table 1 Ethanol dehydration over anion exchanged and heat treated hydrotalcites at 593 K.
* Reaction temperature was 503 K.
Dehydration and dehydrogenation took place on thermally decomposed HT–Wako. Formation of base sites over the MgO surface and acid sites composed of aluminum dispersed in the MgO matrix was expected in the HT–Wako sample. In HT–Cl, catalytic activity largely decreased, and acetaldehyde was not formed. Aluminates containing Sr or Ba as cations were inactive for base catalyzed retro aldol reaction of diacetone alcohol [40, 41]. MgAl2O4 found in the thermal decomposed product of HT–Cl was presumed to be inactive for base reactions, which was strongly considered as the cause for the low activity of HT–Cl. Additionally, it was considered that the remaining chloride ion formed acid sites. MgO which contained carbonate prepared by a partial thermal decomposition of basic carbonate (Mg5(CO3)4(OH)2·4H2O) showed sufficient base activity for the retro aldol reaction [42]. Acid catalyst selectivity in heat–treated HT–Cl seems to be caused by the lack of carbonate in hydrotalcite, and chloride remaining in the heat-treated sample.
HT–B4O7, HT–PO4, HT–WO3, HT–WO4, and HT–SO4 showed higher activity and higher diethyl ether selectivity. No acetaldehyde was formed on these catalysts. Higher activity and higher ether selectivity can be attributed to acid site formation by oxyanions. The acid sites were formed by introducing oxyanions, which had an electron withdrawing effect, to several kinds of metal oxides [22–24]. There have been no reports on the formation of acid sites on MgO surfaces by introducing oxyanions. Here, acid sites would be formed by the interaction of oxyanions with alumina highly dispersed in the MgO matrix.
The catalytic activity of mixed oxides tested in this study was much lower in comparison with that of the conventional SiO2–Al2O3 acid catalyst. The limited electron withdrawing effect of oxyanions on acid site generation was attributed to the basic environment of the MgO matrix.
Ethylene and acetaldehyde were formed on HT–Mo7O24 and HT–MoO4. Molybdate anion generated acid sites by its electron withdrawing effect [22–24]. The introduction of molybdate anion would be contributed to the acid site formation, but not to the increase of the basicity. Acetaldehyde might be formed by oxidative dehydrogenation accompanying reduction of the molybdate anion, rather than simple dehydrogenation over base sites.