Synthesis of Solid Acid Catalysts of MgO–Al2O3 Mixed Oxide Containing Oxyanions by Thermal Decomposition of Anion-Exchanged Hydrotalcites

Solid acid catalysts of MgO–Al2O3 mixed oxides containing B4O72−, HPO42−, Mo7O246−, MoO42−, WO42−, and SO42− were synthesized by anion exchange with Cl− located in the space between anionic layers of hydrotalcite, followed by heat treatment at 773 K. The distance between the hydroxide layers was expanded by the intercalation of oxyanions larger than Cl−. The exchange of oxyanions in the interlayer space was confirmed by IR spectroscopy. Acid sites were generated on the obtained mixed oxides of MgO–Al2O3 by the electron withdrawing effect of the oxyanions. The acid catalyzed ethanol dehydration into ethylene and diethyl ether took place on the obtained catalysts. The effect of exchanged anions in the generation of acid sites was the largest in SO42−.


Introduction
Hydrotalcite (HT) is an anionic-layered double-hydroxide [1][2][3]. The formula of the most popular hydrotalcite, Mg-Al HT, is Mg 6 Al 2 (OH) 16 (CO 3 )·4H 2 O. Mg-Al HT consists of brucite-like layers of Mg(OH) 2 , with partial substitution of some Mg 2+ ions with Al 3+ ions. The excess positive charge originating from Al 3+ is balanced by various intercalated inorganic and organic anions located between the layers along with water molecules. It is well known that anions within the interlayer can be exchanged. The stability of anions between the layers increases with increasing electric charge of the anion and decreasing anion size as follows, CO 3 2− > SO 4 2− > OH − > F − > Cl − > Br − > NO 3 − > I − [4,5]. Conventionally, Mg-Al HT is synthesized by co-precipitation of an aqueous alkaline solution of Mg and Al salts [6]. Generally, an aqueous NaOH or KOH solution is used to adjust the pH and an aqueous Na 2 CO 3 or K 2 CO 3 solution is added as the carbonate source, which is intercalated in the space between layers [6]. The CO 3 2− anion within the interlayer is the most stable, making it difficult to exchange with other anions by simple anion exchange procedures. Mg-Al HT has been applied as a solid base catalyst [1,2,[7][8][9][10][11][12][13]. The mixed oxide obtained by thermal decomposition of HT exhibits much higher activity than the original HT [14][15][16]. The base strength of Mg-Al HT is from 13.3 to 16 on the pK a scale [17], which is much lower than that of thermally decomposed HT [18] and simple MgO [19]. The MgO base catalyst modified with Al 2 O 3 shows sufficient activity for a base catalyzed reaction. The basicity was not lowered by modification with Al 2 O 3 [20]. The lower basicity of MgO-Al 2 O 3 mixed oxide is considered to be due to the electron withdrawing effect of CO 3 2− . The acidity of several kinds of metal oxides can be increased by introduction of sulfate ions to the hydroxide or oxide surface, followed by heat treatment at elevated temperatures [22][23][24]. A representative example is the sulfation of ZrO 2 , where the acidity was largely increased by introducing sulfate to ZrO 2 followed by heat treatment. Tungstate, molybdate, and borate also increased the acidity of ZrO 2 [22][23][24].
As described above, anions located between layers are exchangeable. In this study, Mg-Al HT containing Cl − as the interlayer anion was prepared, and then Cl − was exchanged with borate, phosphate, molybdate, tungstate, and sulfate. The effect of anions incorporated in the bulk of MgO-Al 2 O 3 , prepared by the thermal decomposition of anion-exchanged Mg-Al HT, on the acidity of the solid surface was studied.

Catalyst Preparation
To purge CO 2 from the distilled water used in this study, water was boiled under N 2 bubbling before use, and continuous bubbling of N 2 was performed during sample preparation. The hydrotalcite containing Clas the intercalated anion was prepared by a coprecipitation method [25][26][27]. Magnesium chloride hexahydrate (0.75 mol) and AlCl 3 ·6H 2 O (0.25 mol) were dissolved in 1 L of distilled water in an Erlenmeyer flask, and NaOH solution (1.0 mol L -1 , 1 L) was added dropwise to the mixed solution. The solution was stirred for 2 h while heating at 333 K. The precipitate was filtered, then suspended in decarbonated water, and filtered again. The obtained sample was dried in an oven at 378 K. This sample is denoted as HT-Cl.
Chloride anion intercalated in hydrotalcite was exchanged with oxyanions. The salts used for anion exchange were Na 2 B 4 O 7 , HNa 2 PO 4 , (NH 4 ) 6 Mo 7 O 24 , Na 2 MoO 4 , WO 3 dissolved in aqueous NH 3 , Na 2 WO 4 and MgSO 4 . The salt (1.5 times mol against Al 3+ contained in used HT-Cl) was dissolved in the decarbonated water, and the desired amount of HT-Cl was added to the solution. The solution containing the HT-Cl suspension was stirred for 1 h at room temperature, and then filtered. The precipitate was suspended in decarbonated water for washing and filtered again. Finally, the sample was dried in an oven at 378 K for one day. Anion-exchanged samples with Na 2 B 4 O 7 , HNa 2 PO 4 , (NH 4 ) 6 Mo 7 O 24 , Na 2 MoO 4 , WO 3 dissolved in aqueous NH 3 , Na 2 WO 4 and MgSO 4 are denoted as HT-B 4 O 7 , HT-PO 4 , HT-Mo 7 O 24 , HT-MoO 4 , HT-WO 3 , HT-WO 4 , and HT-SO 4 , respectively. In addition to the prepared and anion-exchanged HT samples, a purchased hydrotalcite (Fujifilm Wako) denoted as HT-Wako was examined. All samples were heat treated at 773 K for 2 h in air before reaction.

Catalyst Characterization
Thermogravimetry and differential thermal analysis (TG-DTA; Rigaku TG 8120) were performed on the prepared HTs under N 2 before heat treatment to observe the thermal decomposition profiles. Powder X-ray diffraction (XRD) spectra of samples dried at 378 K and heat treated at 773 K were measured using Cu-Kα radiation (Rigaku Ultima IV). Infrared spectra were recorded on a Fourier transform spectrophotometer (JASCO FT/IR-4000), with the HTs compressed in KBr discs.

Ethanol Dehydration for Evaluation of Acidity
Ethanol dehydration into ethylene and diethyl ether, and dehydrogenation into acetaldehyde, were used as test reactions. The ethanol dehydration occurs over many acid and base catalysts. Formation of ethylene and ether is observed even on a weak acid catalyst [28]. Dehydrogenation of ethanol to acetaldehyde takes place on a solid base catalyst [28].
Ethanol conversion was carried out on the prepared catalysts in a fixed-bed flow reactor. The heat-treated catalyst (100 mg) was placed in a glass tube and pretreated at 773 K for 2 h in a He flow of 20 mL min -1 . The ethanol was supplied to the reactor as a saturated vapor by passing He (20 mL min -1 ) through an ethanol trap cooled at 273 K. The saturated vapor pressure of ethanol at 273 K is 1.5 kPa. The reaction was carried out at 593 K for 3 h. Addition to the prepared catalysts, conventional SiO 2 -Al 2 O 3 (JRC-SAH-1) and SiO 2 -MgO (JRC-SM-2) supplied by Catalysis Society of Japan were applied. The product was analyzed by gas chromatography.

XRD Analysis
XRD profiles of synthesized and anion-exchanged hydrotalcite samples and those treated at 773 K are shown in Figs No. . 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 H 2 O. 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−B 4 O 7 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 B 4 O 7 2− , which is larger than Cl − , resulting in expansion of the space between layers. A very similar XRD profile to HT-B 4 O 7 was obtained from HT−PO 4 .
In HT−Mo 7 O 24 , HT−MoO 4 , and HT−WO 3 , 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−WO 4 and HT−SO 4 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)  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-SO 4 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.

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 (Mg 6 Al 2 (OH) 16 (CO 3 )·4H 2 O) containing CO 3 2as an anion in the interlayer space [27,31,32]. The decreasing weight in the lower temperature range was attributed to the desorption of H 2 O in the interlayer space, while that in the higher temperature range was the hydroxide decomposition to oxide and CO 2 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-WO 3 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-B 4 O 7 , HT-WO 4 , and HT-SO 4 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).

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 H 2 O in the bending mode.
The band at 1372 cm −1 in HT−Wako was from the absorption by CO 3 2− [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-B 4 O 7 . The bands between 1250 and 1500 cm −1 were assigned to BO 3 3stretching modes [34]. The absorption band at 1074 cm −1 in HT−PO 4 was assigned to the P-O stretching mode [35].
HT−MoO 4 and HT−Mo 7 O 24 samples exhibited very similar IR profiles in the wavenumber region below 1300 cm −1 . MoO 4 2− was assigned to the chemical species having a band at 850 cm −1 [36]. The shoulder peak at 934 cm −1 in HT-Mo 7 O 24 was assigned to the band of Mo 7 O 24 6− . MoO 4 2− is the most stable species among poly-molybdate anions in neutral water solution [36]. In the HT-Mo 7 O 24 sample, Mo 7 O 24 6− would be partially hydrated before intercalation in the interlayer space of HT.
HT-WO 3 and HT-WO 4 had spectra similar to the molybdate samples. The band at 830 cm −1 was assigned to WO 4 2− [37]. In the case of tungstate, WO 4 2− is the most stable in alkali solution. WO 3 was converted into (NH 4 ) 2 WO 4 by dissolution in ammonia solution, which exchanged WO 4 2− 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−SO 4 sample were assigned to absorption of the SO 4 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−SO 4 and HT−WO 4 samples. It can be concluded that anion exchange of Cl − with oxyanions was completed in these samples. XRD 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.

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][23][24]. Introducing molybdate, tungstate, borate, and phosphate anions to the zirconia surface for acid catalyst preparation has also been studied [22][23][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-SO 4 and HT-B 4 O 7 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.
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]. MgAl 2 O 4 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 (Mg 5 (CO 3 ) 4 (OH) 2 ·4H 2 O) 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  4 , and HT-SO 4 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][23][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 in similar level with SiO 2 -MgO. It was much lower in comparison with that of the conventional SiO 2 -Al 2 O 3 acid catalyst. The similar dehydration activity was obtained on SiO 2 -Al 2 O 3 at the reaction temperature lowered by 90 K. The acid strength of SiO 2 -MgO is the range from + 1.5 to -3.0 in Hammett function scale [43]. The acid strength of prepared catalysts was expected to be around H 0 = 0. 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-Mo 7 O 24 and HT-MoO 4 . Molybdate anion generated acid sites by its electron withdrawing effect [22][23][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.

Conclusion
Oxyanions of B 4  MgO-Al 2 O 3 mixed oxides prepared by heat treatment of HT containing oxyanions at 773 K showed activity for acid catalyzed dehydration of ethanol to ethylene and diethyl ether. The effect of exchanged anions on the generation of acid sites was the largest in SO 4 2− . The acid sites were expected to be generated by the electron withdrawing effect of the oxyanions. The catalytic activity of MgO-Al 2 O 3 tested in this study was much lower in comparison with that of conventional acid catalysts such as SiO 2 -Al 2 O 3 . The effect of oxyanion electron withdrawing on acid site generation was expected to be limited due to the basic environment of the MgO matrix.