Synthesis of Ca-based Layered Double Hydroxide from Blast Furnace Slag and Its Catalytic Applications

Abstract

A Ca-based layered double hydroxide (LDH) was synthesized from blast furnace slag (BFS) via a facile and low-cost manufacture process. The synthesis was performed through a two-step method including (i) an acid-dissolution and (ii) an alkali-precipitation process using BFS as a sole metal source. The formation of Ca–Al LDH crystals occurred above pH 9, and under the optimal synthetic conditions (at 373 K and pH 11.5) a well-crystallized stoichiometric hydrocalumite (Ca:Al:Cl=2:1:1) incorporating the slag-derived metallic elements in its structure was obtained with a metal recovery rate of 85%. The impacts of synthesis pH and temperature on the material structures were investigated in detail. The catalytic properties of the thus synthesized LDH material was demonstrated on industrially important several chemical reactions including (1) oxidation of alkyl aromatics with O₂, (2) CO₂ fixation reaction and (3) biodiesel synthesis from vegetable oil. These results open up a new route to fabricate a low-cost LDH compound and its potential availability as an alternative solid base catalyst.

1. Introduction

Iron and steel manufacturing process produces a massive amount of slag as a by-product. It is classified into blast furnace slag (BFS) (generated in iron-making process) and steel slag (generated in refining process of crude iron to steel). Among those, BFS is a more high-volume byproduct; approximately 290 kg of BFS is produced per ton of pig iron in blast furnace, whereas approximately 110 kg of steel slag is generated to produce one ton of steel in steel converters. According to the Nippon Slag Association, annual production of BFS in 2013 in Japan is reported to be approximately 25 million tons.¹⁾ Currently, BFS produced in Japan has mostly been recycled in civil engineering work as hydraulic cement and concrete aggregate, rather than being deposited in landfills.^2,3) However, the production rate of iron-making slag is anticipated to soon outpace the consumption of the slags, and the environmental regulation against slag-deposition is recently being more and more tightened. In some developing countries, utilization rate of iron-making slag still remains below 10%, and the large unused portion has been deposited in areas adjacent to steel manufacturing plants, occupying a large amount of farmland and polluting the surrounding environment.⁴⁾ Therefore, a continuous development of advanced recycling processes of BFS is still a matter of worldwide concern.

The main components of BFS are CaO, SiO₂, Al₂O₃, and MgO, and slight amount of metallic elements such as Fe, Ti and Mn account for the remaining proportion.¹⁾ Among those, CaO and MgO are derived from limestone added as a calcic flux for stripping the oxygen in the iron-refinery process and other elements are derived from the original iron ore. Since BFS is especially rich in calcia, magnesia and alumina, it can be regarded as a low-cost and abundant source for preparing layered double hydroxide (LDH) compounds. LDH is an anionic clay mineral whose general formula is represented by M²⁺_xM³⁺_y(OH)_2(x+y)Aⁿ⁻_y/n·mH₂O (abbreviated as M²⁺–M³⁺–Aⁿ⁻), where M²⁺ and M³⁺ are a divalent and a trivalent metal ion, respectively, and Aⁿ⁻ is an intercalated anion for charge-compensation. Structurally considering, LDH compound consists of two dimensional hydroxide layers with positive charge and exchangeable anions together with water molecules arranged in the interlayer to form neutral materials. With such structural features, LDH shows (i) anion exchange ability in the interlayer, (ii) easy accommodation of metal cations in the hydroxide layer, and (iii) strong basicity. Therefore, LDH compounds, exemplified by Mg–Al hydrotalcite, have now been utilized for a wide variety of applications in environmental remediation, catalysis, pharmaceutical and biological industries.^5,6)

Recently, we reported a facile conversion process of BFS into a single-phase Ca-based LDH material, Ca–Al hydrocalumite, which contains the slag-derived metal cations (Ca²⁺, Mg²⁺, Al³⁺, Fe³⁺, and Mn³⁺ etc.) in its structure.⁷⁾ The close similarity of the slagLDH and Mg–Al LDH raises a question on the possibilities for application as a low-cost alternative to the conventional Mg–Al hydrotalcite catalyst. Synthesis of practically available, high-value materials from BFS would meet the strong demand of effective utilization of waste slags in iron and steel industry and also may contribute to chemical industries in economical and energy-saving ways.^8,9,10) In this paper, we describe the detailed synthesis of LDH compound from BFS and the impacts of synthetic conditions on the structures. The present work is also concerned with its catalytic applications. Among the number of catalytic applications we tested, several reactions including (1) oxidation of alkyl aromatics with O₂, (2) CO₂ fixation reaction and (3) biodiesel synthesis from vegetable oil, were selected to show the prominent catalytic properties of the slagLDH and their results were briefly summarized.

2. Experimental

The chemical composition of BFS used in this study is described in Table 1. The BFS mainly consists of CaO, SiO₂, Al₂O₃ and MgO with the molar ratio of approximately Ca/Al=2.5, and Fe₂O₃, TiO₂ and MnO account for the remaining small proportion, which is fairly common in BFS produced in Japanese iron-making plant. This inherent Ca/Al ratio is suitable for the synthesis of Ca–Al–Cl type LDH (hydrocalumite; Ca₂Al(OH)₆Cl·2H₂O), of which stoichiometric molar ratio is in principle defined to be Ca/Al=2. Prior to use, the raw BFS sample was ball-milled at 650 rpm for 10 min and screened using a 45 μm mesh to facilitate the following dissolution process.

Table 1. Chemical compositions of water-quenched blast furnace slag used in this study (mass%).

CaO	SiO2	Al2O3	MgO	Fe2O3	TiO2	MnO	Total
40.09	34.58	14.78	5.29	1.53	0.78	0.27	97.32a

^a  Other components may include sulphur, chloride and sodium.

The synthesis of slagLDH was carried out according to the literature we reported previously.⁷⁾ The synthetic procedure of slagLDH from BFS is schematically illustrated in Fig. 1. Typically, 10.0 g of the ball-milled BFS was dissolved in 200 mL of 3 mol/L HCl aqueous solution, and was stirred at 373 K for 2 h (step 1). The obtained slurry was separated into hydrated silica gel (SiO₂ content >92 mass% as a dried product) and leaching-solution containing other metallic ions by filtration and washing with deionized water (step 2). To this leaching solution, 2 mol/L NaOH aqueous solution was added dropwise and pH was adjusted to 11.5±0.1, followed by aging at 373 K for another 18 h (step 3). The suspension was filtered, washed with deionized water several times and dried at 373 K overnight to yield 6.2 g of pale brown-colored slagLDH (step 4).

Fig. 1.

Synthetic procedure of layered double hydroxide from blast furnace slag.

Crystalline phase identification was performed by using X-ray diffraction (XRD) measurement. XRD patterns were recorded using a Rigaku Ultima IV diffractometer with CuKα radiation (λ=1.54056 Å) operated at step size 0.02° (2θ) over the 2θ range 10–70°. Nitrogen adsorption–desorption isotherms were measured to understand the structures, which was performed at 77 K using BELSORP-max system (BEL Japan, Inc.). Prior to measurement, the samples were outgassed under vacuum at 473 K to eliminate physisorbed water molecules. The BET (Brunauer-Emmett-Teller) method was applied to calculate specific surface areas. The morphology of the samples were observed by a field emission scanning electron microscopy (FE-SEM) in a JEOL JSM-6500 equipped with an energy dispersive X-ray fluorescence spectrometer (EDX), by which the elemental analysis was conducted.

In this study, three different chemical reactions were tested using slagLDH as a solid catalyst. All catalytic reactions were performed in a quartz vessel equipped with a reflux condenser with magnetic stirring. The amounts of organic substrates and products were quantified by using a gas chromatograph (GC; Shimadzu GC-14B) with a flame ionization detector equipped with a capillary column. Typical reaction procedures are as follows.

Oxidation of alkylaromatics with molecular oxygen: Before catalytic use, catalyst was treated at 473 K for 12 h to eliminate the physisorbed water molecules. The pretreated catalyst (corresponding to 0.03 mmol of Mn) and alkylaromatics (30 mmol) were placed into the reactor and were magnetically stirred at 408 K for 24 h under a continuous flow of oxygen (5.0 cm³/min).

Cycloaddition reaction of epoxides with atmospheric CO₂: Prior to the reaction, the catalysts were pretreated at 673 K in air for 6 h. Into a reactor, the pretreated catalyst (0.5 g), epoxides (4 mmol) and N,N-dimethylformamide (DMF; 3 mL) were placed. The air in the reactor was replaced with 1 atm of CO₂ using a rubber balloon and the mixture was magnetically stirred at 373 K.

Biodiesel synthesis from vegetable oil: Biodiesel synthesis was performed by the transesterification of soybean oil (edible grade, acid value <0.1 mg-KOH/g) with methanol. Typically, the catalysts were pretreated at 1073 K in air for 6 h prior to the reaction. The pretreated catalyst (3.0 g) was added to the reaction mixture containing soybean oil (210 g, 0.24 mol) and anhydrous methanol (120 mL, 2.88 mol), and the mixture was magnetically stirred at 333 K with stirring rate of 1000 rpm.

3. Results

3.1. Synthesis of Layered Double Hydroxide from BFS

The HCl acid-leaching treatment of BFS (step 1) and the subsequent filtration (step 2) afforded 3.7 g of silica gel (as a dried product) from 10.0 g of BFS, which nearly corresponds to the SiO₂ fraction in BFS (34.58 mass%). This suggests that Si component is successfully separated by the acid-leaching process, and the resulting leaching solution may contain other metal cations, such as Ca²⁺, Al³⁺, Mg²⁺, Fe³⁺, Mn³⁺ and Ti⁴⁺, as well as sufficient amount of Cl⁻ anion. By subsequently adding 2.0 mol/L NaOH aqueous solution into the leaching solution, the precipitation of solid immediately took place.

Figure 2 shows XRD patterns of the solid products precipitated from the leaching solution at different pHs, together with that of pure Ca–Al–Cl LDH. Above the synthesis pH of 9, characteristic peaks attributable to the layered structure of LDH were observed (Figs. 2(b)–2(d)), whereas an ambiguous broad peak was observed below pH of 9 (Fig. 2(a)). The observed XRD patterns were identical to that of Ca–Al–Cl type LDH (so-called “hydrocalumite”).¹¹⁾ The basal spacing, which represents the interlayer distance of LDHs, was calculated from the positions of the (002) basal planes with the strongest XRD intensity. As listed in Table 2, the basal spacings of the synthesized LDHs were determined to be 7.80–7.91 Å, which agree well with that of pure Ca–Al–Cl LDH (7.87 Å). The peak intensities of basal planes are frequently used as indicators of the extent of the crystal growth along the stacking direction. As shown in Fig. 2, the peak intensity increased as the synthesis pH increased, indicating that well-crystallized LDH can be obtained at higher pH conditions. Further increase of synthesis pH afforded a LDH compound with a similar XRD pattern to that synthesized at pH 11.5, indicating the formation of LDH structure is almost completed at pH 11.5. Furthermore, no other dominant crystalline phases were observed except for those for Ca–Al–Cl LDH. These results demonstrate that well-crystallized single-phase Ca–Al–Cl LDH can be synthesized from BFS under the pH of approximately 11.5.

Fig. 2.

XRD patterns of slagLDH materials synthesized at 338 K under different pH ((a) 8.5, (b) 9.5, (c) 10.5 and (d) 11.5) and (e) pure Ca–Al–Cl LDH as a reference.

Table 2. Structural parameters and chemical compositions of slagLDH materials synthesized at different conditions.

Synthesis conditions		Yield of product a (g/slag-g)	Basal spacing b (Å)	SBET c (m2/g)	Compositions d
pH	Temp. (K)				Ca/Al	Cl/Al
8.5	338	0.29	–	232	0.03	0.13
9.5	338	0.40	7.80	39.1	1.05	0.46
10.5	338	0.51	7.90	24.3	1.45	0.58
11.5	338	0.61	7.91	25.7	1.49	0.60
11.5	303	0.52	7.86	23.5	1.63	0.67
11.5	373	0.62	7.86	21.1	2.03	1.01

^a  As dried products.

^b  Interlayer distance calculated from d₀₀₂.

^c  Surface area calculated by the BET (Brunauer-Emmett-Teller) method using N₂ adsorption data.

^d  Molar ratios determined by EDX.

Figure 3 displays N₂ adsorption-desorption isotherms of slagLDH materials synthesized at different pHs, together with that of pure Ca–Al–Cl LDH. A drastic change of N₂ adsorption isotherm was observed between the synthesis pH of 8.5 and 9.5; a type II isotherm (according to IUPAC classification) was observed at precipitation pH 8.5 (Fig. 3(a)), while typical type III isotherms were observed at pH above 9.5 (Figs. 3(b)–3(d)). The latter isotherm type is characteristic of nonporous solid, and is similar to that of pure Ca–Al–Cl LDH. Surface area of slagLDH synthesized at pH 8.5 was estimated to be 232 m²/g, while that of slagLDH synthesized at 9.5 was to be 39.1 m²/g (see Table 2). This change in material structure is due to the formation of Ca–Al–Cl LDH with dense crystalline phase at higher pH conditions, being consistent with the crystallographic change observed by XRD.

Fig. 3.

N₂ adsorption-desorption isotherms of slagLDH materials synthesized at 338 K under different pH ((a) 8.5, (b) 9.5, (c) 10.5 and (d) 11.5) and (e) pure Ca–Al–Cl LDH as a reference.

The compositional change of the slagLDHs during the precipitation process was tracked by elemental analysis using EDX (Fig. 4). At the synthesis pH 8.5, the precipitated solid was mainly composed of Al and Mg with the molar ratio of Mg/Al=0.35. Considering the crystalline structures of the solid, the product at this pH is assumed to be a low-crystalline Mg–Al type LDH. Precipitation of Ca²⁺ occurred above pH 9.5. By increasing pH from 8.5 to 11.5, the molar ratio of Ca/Al increased from 0.03 to 1.49 and that of Cl/Al, which corresponds to the number of exchangeable-anion sites, simultaneously increased from 0.13 to 0.60, proving an incorporation of both Ca²⁺ and Cl⁻ into the solid at higher pH (see Table 2). In accordance with the pH increase, the amount of solid recovered from the reaction solution also increased. This trend can be rationally explained in terms of solubility equilibrium. From the calculations based on the solubility product constant (K_sp) concerning the each metal ion extracted from BFS, precipitation pH was roughly estimated as follows; Fe³⁺(2.3)<Al³⁺(3.4)<Mg²⁺(9.4)<Ca²⁺(11.7). This order fairly corresponds to the precipitation behavior observed above, and also theoretically suggests that solution pH near 11.5 is needed to yield Ca-based LDH. In fact, precipitation of Ca-based LDH occurred at lower pH region (pH>9.5) than that expected from the solubility equilibrium calculation. This is due to the increased calcium hydroxide activity triggered by high solution temperature (338 K). It is also presumable that higher synthesis temperature provides a LDH containing a larger amount of Ca and with a higher product yield. Thus it is important to precisely control the synthesis pH and temperature for maximizing product yield. From the chemical analysis, not only the above four components but also other slag-derived components (Fe, Mn, and Ti) were detected. These minor cations are likely to be uniformly incorporated into the hydroxide layer of the LDH during the alkali-precipitation process. On the other hand, sulfur and sodium were undetectable from any products. These elements are likely to be eliminated during the acid-leaching and filtration/washing processes, respectively.

Fig. 4.

The change of chemical components in the product as a function of synthesis pH (synthesis temperature=338 K).

Next, the effects of synthesis temperature on the structure and chemical composition were assessed. Figure 5 shows XRD patterns of slagLDH materials synthesized at three different temperatures (303/338/373 K) at a fixed pH of 11.5. The sample synthesized at near room temperature (T=303 K) exhibited XRD patterns almost resembling those synthesized at 338 K. On the other hand, the sample synthesized at higher temperature (T=373 K) exhibited diffraction peaks with stronger peak intensities for (002) and (004) basal reflections than those synthesized at lower temperature, indicating an improved crystallinity of LDH compound. The change of synthesis temperature did not cause any noticeable differences in fundamental structural parameters, such as basal spacing and surface area (Table 2), but gave rise to compositional change. Along with the increase of synthesis temperature up to 373 K, Ca/Al and Cl/Al molar ratios reached 2.03 and 1.01, respectively, which are almost equivalent to the stoichiometric molar ratios of the ideal Ca–Al–Cl LDH. This result clearly suggests that higher synthesis temperature affords a Ca–Al–Cl LDH compound with better crystallinity and larger amount of exchangeable-anion sites. More importantly, in accordance with the synthesis temperature increase, the amount of solid recovered from the reaction solution also increased, and 6.2 g of slagLDH was finally yielded from 10.0 g of BFS at pH 11.5 and at 373 K. Based on the material balance evaluation, about 55 mass% of BFS was finally converted into LDH compound, which corresponds to a metal recovery rate of 56% (based on metals). This value seems quite low, however, total metal recovery rate would reach 85% if the separated hydrated silica is fully utilized. Since more than 92 mass% of the separated hydrated silica is composed of SiO₂, it can potentially be utilized as an alternative low-cost silica source for beneficial purposes, such as chemical feedstock for zeolite synthesis.

Fig. 5.

XRD patterns of slagLDH materials synthesized at (a) 303 K, (b) 338 K and (c) 373 K at fixed pH of 11.5.

The morphology and particle size of the slagLDH compound synthesized at optimized conditions (at 373 K and at pH 11.5) were observed by FE-SEM (Fig. 6). The FE-SEM image showed plate-like particle morphology with a primary particle size of 0.1–1.0 μm together with some aggregated grains. The plate-like morphology is typical of LDH compounds having stacked layered structures, also elucidating the successful synthesis of LDH from BFS. Another concern that needs to be addressed is the potential influence of compositional variation of BFS upon the quality of the slagLDH. BFS produced in Japanese iron-making plant does not show large variations in chemical composition; however, when BFS with a significantly small content of Ca is used as a raw material, another make-up Ca source such as lime needs to be added to produce a stoichiometric Ca–Al–Cl LDH, otherwise, poorly-crystallized calciumaluminate hydroxide is produced.

Fig. 6.

FE-SEM image of slagLDH synthesized at 373 K and at pH 11.5.

3.2. Catalytic Applications of Slag-made LDH

The Mg–Al LDH, a most commonly used LDH, is known to be a stable and recyclable solid base catalyst, and has been used in a wide variety of chemical reactions such as aldol condensation, Knoevenagel condensation, Michael addition and transesterification.⁵⁾ Kaneda et al. reported that Mg–Al LDH-derived oxides act as active catalysts for CO₂ addition reaction with epoxides to afford the corresponding five-membered cyclic carbonates.¹²⁾ Cyclic carbonates are commercially valuable compounds for the production of engineering plastics as well as for the synthesis of pharmaceuticals and fine chemicals. Furthermore, CO₂ in the atmosphere has long been considered as a leading contributor to global warming and climate change, and iron and steel industry is emitting 5–7% of total world CO₂ gas emissions due to (i) reduction of iron ore with coke in a blast furnace and (ii) decarbonisation of limestone (CaCO₃) and dolomite (MgCO₃) added as fluxing materials together with coke. The amount of CO₂ to produce every ton of steel is estimated to be about 2200 kg of CO₂ on a world average (1800 kg of CO₂ for many developed countries).^13,14) Therefore, chemical fixation of CO₂ into such industrially beneficial, value-added compounds would provide interesting insights for sustainable development of iron and steel industry.

The cycloaddition reaction of various epoxides with atmospheric pressure of CO₂ was first examined using slagLDH as a solid base catalyst alternative to Mg–Al LDH (Fig. 7). When epichlorohydrin was used as a substrate, slagLDH afforded the corresponding carbonate in 92.3% yield and with 93% selectivity after 12 h at 373 K (Eq. (1)). Styrene oxide reacted with CO₂ to form the corresponding carbonate in 90.0% yield after 24 h (Eq. (2)), and aliphatic epoxides such as 1,2-epoxyhexane and 1,2-epoxyoctane gave yields of 93.6 and 92.1% for hexane carbonate and octane carbonate, respectively, with good selectivities (Eqs. (4) and (5)). Neither the conventional Mg–Al LDH nor a pure Ca–Al LDH obtained from commercially available chemical reagents provided such high activities under the same reaction conditions. This superior activity of slagLDH is likely due to the stronger basicity derived from Ca atoms than that from Mg and the productive effect derived from the slag-derived transition metals imbedded within the LDH structure.^15,16) In addition, the slagLDH catalyst was at least four times reusable with retaining the activity and selectivity. Indeed, the large amount of CO₂ gas cannot be fixed/reduced solely by this chemical reaction, however, the proposed strategy would be one of the possible approaches that contribute to both the efficient fixation of CO₂ and the waste management problems that iron and steel industry is currently facing.

Fig. 7.

Cycloaddition reaction of epoxides with atmospheric CO₂ using slagLDH catalyst.

Catalytic properties of metal/metal-oxo species located in the hydroxide interlayer of LDHs in redox reactions have been widely investigated.^6,17) We found that trace amounts of slag-derived Mn atoms imbedded within the hydroxide layer of slagLDH work as catalytically active sites for the oxidation reaction of arkylaromatics with O₂. Chemical analysis determined that the concentration of Mn in the slagLDH is 0.32 mass%. With slagLDH corresponding to 0.1 mol% of Mn, several alkyl aromatics were successfully converted into the corresponding aromatic ketones using atmospheric pressure of molecular oxygen as the sole oxidant (Fig. 8).¹⁵⁾ For example, ethylbenzene was oxidized into acetophenone with 24.7% yield and with 98% selectivity after 6 h of reaction at 408 K even under solvent-free reaction conditions (Eq. (6)). Other alkyl aromatics such as diphenylmethane and butylbenzene were also chemoselectively transformed into benzophenone and butyrophenone with yields of 28.3% and 29.0%, respectively (Eqs. (7) and (8)). A pure Ca–Al LDH without Mn atoms was inactive for this reaction, and the use of raw BFS instead of slagLDH resulted in a poor conversion rate (6.6% after 6 h), demonstrating that the Mn-oxo species in the LDH structure is the active site for this reaction. As is the case presented above, the catalyst was easily separable from the reaction mixture by simple filtration and was at least three times reusable without appreciable loss of its catalytic activity and selectivity. Catalytic oxidation using molecular oxygen is attractive from an economical and environmental point of view, and the produced ketone compounds are important as synthetic precursors in pharmaceutical chemistry and other industries as well. Although conversion rate needs to be further improved by manipulating operation conditions, slagLDH can potentially be an efficient oxidation catalyst offering good chemoselectivities.

Fig. 8.

Oxidation of alkylaromatics with molecular oxygen using slagLDH catalyst.

Our extensive exploration has found that the slagLDH can be used as a solid base catalyst for biodiesel fuel production from vegetable oils. Since vegetable oils are plant-derived, renewable carbon sources, biodiesel fuels have been accepted as one of the promising alternatives to petroleum-derived fuels. Normally, biodiesels are produced via a transesterification of vegetable oils with alcohols (e.g. methanol or ethanol) using homogeneous base catalysts (e.g. NaOH or KOH). However, they are unrecyclable and suffer from separation problems and waste water generation. Therefore, development of solid base catalysts that are active and less soluble in alcohols, such as alkaline earth metal oxides (e.g. MgO, CaO), have been investigated.^18,19) When slagLDH was used as a catalyst in transesterification of soybean oil with methanol at 333 K, 95% of fatty acid methyl ester (FAME) was obtained after 9 h of reaction, yielding 180 g of FAME from 210 g of soybean oil (Fig. 9, Eq. (9)). Notably, slagLDH catalyst exhibited superior catalytic performance compared to the conventional bulk CaO catalysts owing to its strong basicity and unique CO₂/H₂O-tolerance property,²⁰⁾ while Mg–Al LDH and raw BFS were both almost inactive for this reaction. Although extensive studies on the diversity of physical properties of vegetable oils (density, viscosity and acidity etc.), catalyst/product separation problems and the quality of biodiesel oils are clearly needed, slagLDH can be an alternative solid base catalyst useful for scalable and cost-effective biodiesel production.

Fig. 9.

Biodiesel synthesis from soybean oil using slagLDH catalyst.

4. Conclusions

We have reported on the detailed synthesis of LDH compound using BFS as an abundant and cheap chemical source and its potential catalytic applications. A Ca–Al type LDH incorporating the slag-derived metallic elements in its structure was successfully synthesized through a two-step method including (i) an acid-dissolution and (ii) an alkali-precipitation process using BFS as a sole metal source. Analyses combining XRD, N₂ physisorption and chemical analysis revealed that a single-phase Ca–Al type LDH formed at pH>9, and well-crystallized and stoichiometric LDH can be obtained at pH 11.5 and 373 K with a maximum metal recovery rate of 85%. We also examined the catalytic activity of slagLDH on three different catalytic reactions. In all cases, slagLDH catalyst showed comparable or even superior catalytic performances compared to the pure LDH analogs owing to the strong basicity originating from Ca and the catalytic ability of the slag-derived elements (such as Mn). Although further improvements for lowering production cost and reduction of water discharges and energy consumption over the process are needed before substantiating this process, our synthesis pathway offers a new route to synthesize a low-cost LDH and would meet the strong demand for the effective utilization of waste BFS in iron and steel industry. It is expected that further applications to chemical reactions to produce value-added chemicals or reactions strongly linked with “Green Chemistry” will surely contribute to the establishment of sustainable chemical processes.

Acknowledgement

This work was financially supported by the ISIJ Research Promotion Grant (Tekkou Kenkyu Shinko Josei) from the Iron and Steel Institute of Japan, and the Grant-in-Aid for Challenging Exploratory Research (KAKENHI) of Japan Society for the Promotion of Science (No. 23656511). Y. K. acknowledges the financial support from Frontier Research Base for Global Young Researchers, Osaka University.

References

• 1)  The Japan Iron and Steel Federation and Nippon Slag Association: The Annual Report of Statistics of Iron and Steel Slag 2013, http://www.slg.jp, (accessed 2014/12/4).
• 2)   B.  Das,  S.  Prakash,  P. S. R.  Reddy and  V. N.  Misra: Resour. Conserv. Recycl., 50 (2007), 40.
• 3)   Y.  Huang,  R. N.  Bird and  O.  Heidrich: Resour. Conserv. Recycl., 52 (2007), 58.
• 4)   Y.  Lun,  M.  Zhou,  X.  Cai and  F.  Xu: J. Wuhan Univ. Technol. Mater. Sci. Ed., 23 (2008), 737.
• 5)   F.  Cavani,  F.  Trifirò and  A.  Vaccari: Catal. Today, 11 (1991), 173.
• 6)   K.  Kaneda,  K.  Yamaguchi,  K.  Mori,  T.  Mizugaki and  K.  Ebitani: Catal. Surv. Jpn., 4 (2000), 31.
• 7)   Y.  Kuwahara,  T.  Ohmichi,  T.  Kamegawa,  K.  Mori and  H.  Yamashita: J. Mater. Chem., 20 (2010), 5052.
• 8)   Y.  Kuwahara,  T.  Ohmichi,  T.  Kamegawa,  K.  Mori and  H.  Yamashita: Chem. Lett., 38 (2009), 626.
• 9)   Y.  Kuwahara,  T.  Ohmichi,  T.  Kamegawa,  K.  Mori and  H.  Yamashita: J. Mater. Chem., 19 (2009), 7263.
• 10)   Y.  Kuwahara,  S.  Tamagawa,  T.  Fujitani and  H.  Yamashita: J. Mater. Chem. A, 1 (2013), 7199.
• 11)   E.  López-Salinas,  M. E. L.  Serrano,  M. A. C.  Jácome and  I. S.  Secora: J. Porous Mater., 2 (1996), 291.
• 12)   K.  Yamaguchi,  K.  Ebitani,  T.  Yoshida,  H.  Yoshida and  K.  Kaneda: J. Am. Chem. Soc., 121 (1999), 4526.
• 13)   C.-Q.  Hu,  L.-Y.  Chen,  C.-X.  Zhang,  Y.-H.  Qi and  R.-Y.  Yin: J. Iron Steel Res. Int., 13 (2006), 38.
• 14)   C.  Xu and  D. Q.  Cang: J. Iron Steel Res. Int., 17 (2010), 1.
• 15)   Y.  Kuwahara,  K.  Tsuji,  T.  Ohmichi,  T.  Kamegawa,  K.  Mori and  H.  Yamashita: ChemSusChem, 5 (2012), 1523.
• 16)   Y.  Kuwahara and  H.  Yamashita: J. CO₂ Util., 1 (2013), 50.
• 17)   V.  Rives and  M. A.  Ulibarri: Coord. Chem. Rev., 181 (1999), 61.
• 18)   S.  Gryglewicz: Bioresour. Technol., 70 (1999), 249.
• 19)   M.  Di Serio,  R.  Tesser,  L.  Pengmei and  E.  Santacesaria: Energ. Fuel., 22 (2008), 207.
• 20)   Y.  Kuwahara,  K.  Tsuji,  T.  Ohmichi,  T.  Kamegawa,  K.  Mori and  H.  Yamashita: Catal. Sci. Technol., 2 (2012), 1842.