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On the synthesis of layered double hydroxides (LDHs) by reconstruction method based on the “memory effect”
Microporous and Mesoporous Materials xxx (2015) 1e3
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Correspondence
On the synthesis of layered double hydroxides (LDHs) by reconstruction method based on the “memory effect” a b s t r a c t Keywords: Layered double hydroxides (LDHs) Memory effect Direct synthesis
This letter summarizes the literature data on the synthesis of layered double hydroxides (LDHs) in order to demonstrate that the preparation by reconstruction method is not based on the memory effect but it is simply a direct synthesis of these attractive solids. © 2015 Elsevier Inc. All rights reserved.
1. Letter Layered double hydroxides (LDHs) are anionic clays, also known as hydrotalcites compounds. They are represented by the general II M III ðOHÞ xþ ðAn Þ formula ½M1x x 2 x=n $mH2 O were x falls in the range 0.25 x 0.33 for pure phases. The structure consists of brucitelike layers with a x positive charge due to the substitution of trivalent metal cations MIII for bivalent metal cations MII. These layers are balanced by an equivalent inter-layered negative charge (Aen)x/n where n is the negative charge of anion [1e5]. LDHs find numerous applications in different fields such as water decontamination, additives for organic polymers, catalysis, medicine and pharmacy [6e14]. The main methods to prepare such solids are: coprecipitation, anionic exchange and reconstruction [10,15,16]. To favor their crystallinity, hydrothermal or microwave treatments are also adopted [17e19]. The co-precipitation consists in the addition of a base into a solution of the MII and MIII metal cations containing the anion to be intercalated [20,21]. The reconstruction method avails oneself of the memory effect, i.e. the ability to recover the original layered structure when the mixed MII (MIII)O oxide, obtained by a mild calcination (500 C) of the LDH precursor, is immersed in a solution of the anion to be intercalated [22e24]. The recovery of the original layered structure by reconstruction method, based on the memory effect, appears unconvincing. Our letter summarizes the literature data on this topic in order to demonstrate that the synthesis by reconstruction method is not based on the memory effect but it is simply a direct synthesis of these attractive solids. In aqueous media Mg,Al-LDHs were, in fact, directly synthesized from mechanical mixtures of microcrystalline MgO and alumina xerogel [25]. Li,Al-LDH was also directly synthesized from alumina xerogel and dissolved LiOH$H2O [26], whereas Ca,Al-LDH was synthesized from mechanical mixture of freshly ignited CaO and alumina xerogel [27]. The synthesis of meixnerite, a Mg,Al-LDH
intercalated with OH anions, was also performed at room temperature by dry milling of a mixture of magnesium and aluminum hydroxides followed by wet milling of the resulting mixture with water [28]. Large Mg,Al-LDH crystallites were obtained upon hydrothermal treatment of mixed MgO and Al2O3 oxides in aqueous suspensions. To explain the LDH formation, a dissociation-deposition diffusion mechanism has been proposed which can be applied to general synthesis of LDH with different methods of synthesis, such as coprecipitation and reconstruction [29]. Such mechanism is in disagreement with the memory effect. Synthetic hydrocalumite, a Ca,Al-LDH, calcined at temperatures ranging from 500 to 900 C, formed crystalline mayenite (Ca12Al14O33) and lime (CaO). The lamellar structure of hydrocalumite was completely recovered in deionized water at room temperature [30]. The lime formed in the above range of temperature is very reactive with water so favoring the reconstruction of Ca,AlLDH by direct synthesis. The direct synthesis of these LDH solids appears as a consequence of a typical reaction between a strong basic oxide (MgO, CaO, Li2O) with an amphoteric oxide (Al2O3) in aqueous media. In addition, it is very difficult to refer to a memory effect for LDHs phases synthesized from mixed Mg(Al)O oxides precursors reported in Ref. [31]. Such precursors were obtained, in fact, by combustion method from mixtures containing Al and Mg nitrates, Na2CO3 and saccharose as fuel. Although these oxides were without the memory of LDH precursors, they re-crystallized to hydrotalcites. In the synthesis by reconstruction method, based on the memory effect, the mixed oxides precursors are formed upon the mild temperature of calcination of LDHs. Generally these precursors are characterized by very low crystallinity and mixing homogeneity at atomic level as confirmed by their very broad diffraction peaks attributed to MgO [31e33]. According to [34], the term reconstruction should be used only when the reaction of calcined LDH with the aqueous solution allows to recover both the structure and composition of ex-LDH.
http://dx.doi.org/10.1016/j.micromeso.2015.03.024 1387-1811/© 2015 Elsevier Inc. All rights reserved.
Please cite this article in press as: G. Mascolo, M.C. MascoloOn the synthesis of layered double hydroxides (LDHs) by reconstruction method based on the “memory effect”, Microporous and Mesoporous Materials (2015), http://dx.doi.org/10.1016/j.micromeso.2015.03.024
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Correspondence / Microporous and Mesoporous Materials xxx (2015) 1e3
The regeneration properties of LDH after calcination and successive rehydration is not absolutely reversible due to a change in the local Al structure from octahedral to tetrahedral sites [35]. With the repetitive treatment of calcinations of Mg,Al-LDHs, spinel (MgAl2O4) crystallizes with consequent and progressive reduction of the regeneration process [36]. Grinding after each calcination hinders crystalline spinel formation whereas enhances the quantity of reconstructed LDH. The grinding favors the mixing and consequently the entity of the reconstruction via direct synthesis [37]. The recovery of the (Cu,Zn),Al-LDH phase from the precursors, obtained by thermal decomposition of a (Cu,Zn),Al-LDH catalyst, was affected by the temperature of thermal treatment. Surprisingly, the recovery of the layered structure from the precursor thermally treated at 400 C was faster than that one treated at 200 C, whereas a negligible recovery was observed for the precursor thermally treated at 600 C. Such finding agree with the recovery of LDH by direct synthesis. The presence of crystalline phases together disordered (Cu,Zn),Al-LDH phase in the precursor treated at 200 C determines, in fact, a less reactive system with a consequent slow recovery of the LDH phase. The thermal treatment at 600 C determines a precursor characterized either by a change in the local Al structure from octahedral to tetrahedral sites [35] or by a partial crystallization of spinel with a consequent reduction of the reactivity. On the contrary, the precursor thermally treated at 400 C appears more reactive for the recovery of LDH phase because it contains Cu,Zn,Al-mixed oxides together amorphous phases. Analogous results were detected for Euþ3-incorporated Mg,Al-LDH products calcined at temperatures between 400 and 600 C which contain more reactive mixed oxides [38]. In addition the calcined Li,Al-LDH powder with a poorly crystalline mixed oxide solid solution exhibits faster rehydration than the powder calcined at higher temperature and containing more crystalline forms of Y-LiAlO2 and LiAl5O8 [39]. The mixed oxides derived from the calcination of Zn,Al and Zn,Ga,Al-LDHs only partially recovered the original LDH structure [40]. In this case, the high crystallinity of the resulting mixed oxides was a hindrance for the recovery of ex-LDHs. Different memory effect phenomena were observed for calcined Mg,Al-LDH, Mg,Fe-LDH and Zn,Al-LDH. The calcined Zn,Al-LDH demonstrated a poor reconstruction of the ex-LDH, whereas Mg,Al-LDH and Mg,Fe-LDH showed high recovery of the ex- LDHs [41]. Such results can be related to the poorly crystalline and very reactive Mg(Al)O and Mg(Fe)O in comparison to that of Zn(Al)O characterized by high crystallinity and poor reactivity. Well crystallized (Ni,Al)O mixed oxide obtained by calcinations of Ni,Al-LDH showed a poor reactivity on rehydration and a limited recovery of the layer structure [42]. The regenerated Mg,Al-LDH phases upon the rehydration of the mixed oxides displayed also significant changes in textural and morphological properties in comparison with their parent LDHs. The regenerated crystals showed, in fact, a high tendency to the agglomeration with formation of large conglomerates with a drastic reduction of the surface area [43]. The Mg/Al-, Mg,Zn/Al-, Mg/Al,Ga-hydrotalcite-like compounds also displayed upon reconstruction changes in both the composition and the morphology with drastic drop of the surface area. An explanation was based on a dissolution-recrystallization mechanism of the reconstruction in contrast with the widely accepted topotactical character of regeneration [44,45]. In addition, no reconstruction was observed when the calcined phase, obtained from [(Cu,Co,Zn)6Al2(OH)16$H2O] precursor, was treated with an ethanol solution of KOH. The presence of water appears essential for the LDH reconstruction and, consequently, a dissolution-recrystallization mechanism appears to be necessary [46]. Recovery of hydrotalcite by gasphase reconstruction of poorly crystalline Mg(Al)O oxide was hampered above 303 K. That was attributed to limited adsorption
of water above room temperature on the surface of the solid oxide [47]. The presence of liquid water appears essential for the reconstruction of the ex-LDH. Such behavior agrees with the direct synthesis of LDH. The incorporation of drug molecules into LDH performed by reconstruction route, the morphology of drug/LDH nanohybridis was fairly different from that of ex-LDH [48]. The memory effect was appealed to the reconstruction of LDH by hydration of calcined LDH in the presence of acid brown 14 dye. But different texture and structure resulted between ex-LDH and reconstructed one [36]. Such finding contrasts with the regeneration of LDH by the memory effect. The rehydration at room temperature of very reactive mixed oxides justify the formation by direct synthesis of nano-slabs-like crystals very different from the LDH precursor which was micrometer in size [38]. The mixed oxides precursors obtained by calcination of the LDHs phases at mild temperatures are poorly crystalline and without segregation of oxide phases. The low crystallinity and the extraordinary mixing homogeneity of these mixed oxides determine a high reactivity with a fast conversion into LDH phases with crystals very small in sizes compared to the LDH precursor. Such high reactivity explains the preparation of well dispersed Ni particles over LDH particles as egg shell-type catalysts [49]. Adopting, in fact, both smaller heating rate and lower calcination temperature of hydrotalcite precursor favored the formation of mixed oxide mostly amorphous and with a high specific surface area. The rehydration of this very reactive mixed oxide in the presence of aqueous solution of Ni nitrate determines a fast reaction synthesis of the ex-hydrotalcite. The increase of pH locally the surface of hydrotalcite particles determines probably the precipitation of Ni as hydroxide. The fast recovery of hydrotalcite by direct synthesis might explain the absence of Ni inside the hydrotalcite particles with consequent formation of egg shell-type Ni-loaded catalysts. Meixnerite prepared by coprecipitation method displayed smaller crystallite sizes than those obtained by the hydration of the mixed oxides derived from the thermal decomposition of hydrotalcite in N2 flow [50]. The different crystallinity of meixnerite can be attributed to the different reaction rates of synthesis. A faster reaction of synthesis during the hydration of mixed oxide must be expected in comparison to that one of the coprecipitation. The following remarks can be made about the mechanism of rehydration of the calcined LDHs: - The thermal treatment at low temperature of LDH precursor with the removal of interlayer water determines the formation of highly disordered phase. Its rehydration involves a slow reconstruction of layered structure with no changes in composition and morphology in comparison to the ex-LDH. In this case the so called memory effect appears to be clear. - When the LDH precursor is mildly calcined with a full decomposition, mixed oxides more or less crystalline result with an increase in the surface area due to the crystal shuttering and to the formation of a maze micro-cracks of the ex-LDH crystals. The rehydration of poor crystalline mixed oxides determines a fast regeneration of the layered phase characterized by a drastic change in the morphology and decrease in the surface area. Such decrease is related to the formation of highly compact aggregates of regenerated LDH. In this case the direct synthesis appears to be the true mechanism of recovery of the layered structure. - At high temperature of calcination, crystalline mixed oxide and partial changes in the local structure of MIII and/or MII cations result in comparison to the ex-LDH. A partial regeneration of the layered phase takes place which is characterized by different composition and morphology in comparison to the LDH
Please cite this article in press as: G. Mascolo, M.C. MascoloOn the synthesis of layered double hydroxides (LDHs) by reconstruction method based on the “memory effect”, Microporous and Mesoporous Materials (2015), http://dx.doi.org/10.1016/j.micromeso.2015.03.024
Correspondence / Microporous and Mesoporous Materials xxx (2015) 1e3
precursor. Also in this case the mechanism of the partial recovery of LDH precursor involves a direct synthesis. The synthesis of LDHs by reconstruction method based on the memory effect is now within the scientific slang as a consequence of the massive bibliography on this topic, but in our opinion the reconstruction method of LDH phases, ascribed to a memory effect, appears to be simply based on a direct synthesis.
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G. Mascolo*, M.C. Mascolo Laboratory of Materials, Department of Civil and Mechanical Engineering, University of Cassino and Southern Lazio, Via G. Di Biasio, 43, 03043 Cassino, FR, Italy *
Corresponding author. Tel.: þ39 07762993710; fax: þ39 07762993711. E-mail address: [email protected] (G. Mascolo). 5 December 2014 Available online xxx
Please cite this article in press as: G. Mascolo, M.C. MascoloOn the synthesis of layered double hydroxides (LDHs) by reconstruction method based on the “memory effect”, Microporous and Mesoporous Materials (2015), http://dx.doi.org/10.1016/j.micromeso.2015.03.024
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Correspondence
On the synthesis of layered double hydroxides (LDHs) by reconstruction method based on the “memory effect” a b s t r a c t Keywords: Layered double hydroxides (LDHs) Memory effect Direct synthesis
This letter summarizes the literature data on the synthesis of layered double hydroxides (LDHs) in order to demonstrate that the preparation by reconstruction method is not based on the memory effect but it is simply a direct synthesis of these attractive solids. © 2015 Elsevier Inc. All rights reserved.
1. Letter Layered double hydroxides (LDHs) are anionic clays, also known as hydrotalcites compounds. They are represented by the general II M III ðOHÞ xþ ðAn Þ formula ½M1x x 2 x=n $mH2 O were x falls in the range 0.25 x 0.33 for pure phases. The structure consists of brucitelike layers with a x positive charge due to the substitution of trivalent metal cations MIII for bivalent metal cations MII. These layers are balanced by an equivalent inter-layered negative charge (Aen)x/n where n is the negative charge of anion [1e5]. LDHs find numerous applications in different fields such as water decontamination, additives for organic polymers, catalysis, medicine and pharmacy [6e14]. The main methods to prepare such solids are: coprecipitation, anionic exchange and reconstruction [10,15,16]. To favor their crystallinity, hydrothermal or microwave treatments are also adopted [17e19]. The co-precipitation consists in the addition of a base into a solution of the MII and MIII metal cations containing the anion to be intercalated [20,21]. The reconstruction method avails oneself of the memory effect, i.e. the ability to recover the original layered structure when the mixed MII (MIII)O oxide, obtained by a mild calcination (500 C) of the LDH precursor, is immersed in a solution of the anion to be intercalated [22e24]. The recovery of the original layered structure by reconstruction method, based on the memory effect, appears unconvincing. Our letter summarizes the literature data on this topic in order to demonstrate that the synthesis by reconstruction method is not based on the memory effect but it is simply a direct synthesis of these attractive solids. In aqueous media Mg,Al-LDHs were, in fact, directly synthesized from mechanical mixtures of microcrystalline MgO and alumina xerogel [25]. Li,Al-LDH was also directly synthesized from alumina xerogel and dissolved LiOH$H2O [26], whereas Ca,Al-LDH was synthesized from mechanical mixture of freshly ignited CaO and alumina xerogel [27]. The synthesis of meixnerite, a Mg,Al-LDH
intercalated with OH anions, was also performed at room temperature by dry milling of a mixture of magnesium and aluminum hydroxides followed by wet milling of the resulting mixture with water [28]. Large Mg,Al-LDH crystallites were obtained upon hydrothermal treatment of mixed MgO and Al2O3 oxides in aqueous suspensions. To explain the LDH formation, a dissociation-deposition diffusion mechanism has been proposed which can be applied to general synthesis of LDH with different methods of synthesis, such as coprecipitation and reconstruction [29]. Such mechanism is in disagreement with the memory effect. Synthetic hydrocalumite, a Ca,Al-LDH, calcined at temperatures ranging from 500 to 900 C, formed crystalline mayenite (Ca12Al14O33) and lime (CaO). The lamellar structure of hydrocalumite was completely recovered in deionized water at room temperature [30]. The lime formed in the above range of temperature is very reactive with water so favoring the reconstruction of Ca,AlLDH by direct synthesis. The direct synthesis of these LDH solids appears as a consequence of a typical reaction between a strong basic oxide (MgO, CaO, Li2O) with an amphoteric oxide (Al2O3) in aqueous media. In addition, it is very difficult to refer to a memory effect for LDHs phases synthesized from mixed Mg(Al)O oxides precursors reported in Ref. [31]. Such precursors were obtained, in fact, by combustion method from mixtures containing Al and Mg nitrates, Na2CO3 and saccharose as fuel. Although these oxides were without the memory of LDH precursors, they re-crystallized to hydrotalcites. In the synthesis by reconstruction method, based on the memory effect, the mixed oxides precursors are formed upon the mild temperature of calcination of LDHs. Generally these precursors are characterized by very low crystallinity and mixing homogeneity at atomic level as confirmed by their very broad diffraction peaks attributed to MgO [31e33]. According to [34], the term reconstruction should be used only when the reaction of calcined LDH with the aqueous solution allows to recover both the structure and composition of ex-LDH.
http://dx.doi.org/10.1016/j.micromeso.2015.03.024 1387-1811/© 2015 Elsevier Inc. All rights reserved.
Please cite this article in press as: G. Mascolo, M.C. MascoloOn the synthesis of layered double hydroxides (LDHs) by reconstruction method based on the “memory effect”, Microporous and Mesoporous Materials (2015), http://dx.doi.org/10.1016/j.micromeso.2015.03.024
2
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The regeneration properties of LDH after calcination and successive rehydration is not absolutely reversible due to a change in the local Al structure from octahedral to tetrahedral sites [35]. With the repetitive treatment of calcinations of Mg,Al-LDHs, spinel (MgAl2O4) crystallizes with consequent and progressive reduction of the regeneration process [36]. Grinding after each calcination hinders crystalline spinel formation whereas enhances the quantity of reconstructed LDH. The grinding favors the mixing and consequently the entity of the reconstruction via direct synthesis [37]. The recovery of the (Cu,Zn),Al-LDH phase from the precursors, obtained by thermal decomposition of a (Cu,Zn),Al-LDH catalyst, was affected by the temperature of thermal treatment. Surprisingly, the recovery of the layered structure from the precursor thermally treated at 400 C was faster than that one treated at 200 C, whereas a negligible recovery was observed for the precursor thermally treated at 600 C. Such finding agree with the recovery of LDH by direct synthesis. The presence of crystalline phases together disordered (Cu,Zn),Al-LDH phase in the precursor treated at 200 C determines, in fact, a less reactive system with a consequent slow recovery of the LDH phase. The thermal treatment at 600 C determines a precursor characterized either by a change in the local Al structure from octahedral to tetrahedral sites [35] or by a partial crystallization of spinel with a consequent reduction of the reactivity. On the contrary, the precursor thermally treated at 400 C appears more reactive for the recovery of LDH phase because it contains Cu,Zn,Al-mixed oxides together amorphous phases. Analogous results were detected for Euþ3-incorporated Mg,Al-LDH products calcined at temperatures between 400 and 600 C which contain more reactive mixed oxides [38]. In addition the calcined Li,Al-LDH powder with a poorly crystalline mixed oxide solid solution exhibits faster rehydration than the powder calcined at higher temperature and containing more crystalline forms of Y-LiAlO2 and LiAl5O8 [39]. The mixed oxides derived from the calcination of Zn,Al and Zn,Ga,Al-LDHs only partially recovered the original LDH structure [40]. In this case, the high crystallinity of the resulting mixed oxides was a hindrance for the recovery of ex-LDHs. Different memory effect phenomena were observed for calcined Mg,Al-LDH, Mg,Fe-LDH and Zn,Al-LDH. The calcined Zn,Al-LDH demonstrated a poor reconstruction of the ex-LDH, whereas Mg,Al-LDH and Mg,Fe-LDH showed high recovery of the ex- LDHs [41]. Such results can be related to the poorly crystalline and very reactive Mg(Al)O and Mg(Fe)O in comparison to that of Zn(Al)O characterized by high crystallinity and poor reactivity. Well crystallized (Ni,Al)O mixed oxide obtained by calcinations of Ni,Al-LDH showed a poor reactivity on rehydration and a limited recovery of the layer structure [42]. The regenerated Mg,Al-LDH phases upon the rehydration of the mixed oxides displayed also significant changes in textural and morphological properties in comparison with their parent LDHs. The regenerated crystals showed, in fact, a high tendency to the agglomeration with formation of large conglomerates with a drastic reduction of the surface area [43]. The Mg/Al-, Mg,Zn/Al-, Mg/Al,Ga-hydrotalcite-like compounds also displayed upon reconstruction changes in both the composition and the morphology with drastic drop of the surface area. An explanation was based on a dissolution-recrystallization mechanism of the reconstruction in contrast with the widely accepted topotactical character of regeneration [44,45]. In addition, no reconstruction was observed when the calcined phase, obtained from [(Cu,Co,Zn)6Al2(OH)16$H2O] precursor, was treated with an ethanol solution of KOH. The presence of water appears essential for the LDH reconstruction and, consequently, a dissolution-recrystallization mechanism appears to be necessary [46]. Recovery of hydrotalcite by gasphase reconstruction of poorly crystalline Mg(Al)O oxide was hampered above 303 K. That was attributed to limited adsorption
of water above room temperature on the surface of the solid oxide [47]. The presence of liquid water appears essential for the reconstruction of the ex-LDH. Such behavior agrees with the direct synthesis of LDH. The incorporation of drug molecules into LDH performed by reconstruction route, the morphology of drug/LDH nanohybridis was fairly different from that of ex-LDH [48]. The memory effect was appealed to the reconstruction of LDH by hydration of calcined LDH in the presence of acid brown 14 dye. But different texture and structure resulted between ex-LDH and reconstructed one [36]. Such finding contrasts with the regeneration of LDH by the memory effect. The rehydration at room temperature of very reactive mixed oxides justify the formation by direct synthesis of nano-slabs-like crystals very different from the LDH precursor which was micrometer in size [38]. The mixed oxides precursors obtained by calcination of the LDHs phases at mild temperatures are poorly crystalline and without segregation of oxide phases. The low crystallinity and the extraordinary mixing homogeneity of these mixed oxides determine a high reactivity with a fast conversion into LDH phases with crystals very small in sizes compared to the LDH precursor. Such high reactivity explains the preparation of well dispersed Ni particles over LDH particles as egg shell-type catalysts [49]. Adopting, in fact, both smaller heating rate and lower calcination temperature of hydrotalcite precursor favored the formation of mixed oxide mostly amorphous and with a high specific surface area. The rehydration of this very reactive mixed oxide in the presence of aqueous solution of Ni nitrate determines a fast reaction synthesis of the ex-hydrotalcite. The increase of pH locally the surface of hydrotalcite particles determines probably the precipitation of Ni as hydroxide. The fast recovery of hydrotalcite by direct synthesis might explain the absence of Ni inside the hydrotalcite particles with consequent formation of egg shell-type Ni-loaded catalysts. Meixnerite prepared by coprecipitation method displayed smaller crystallite sizes than those obtained by the hydration of the mixed oxides derived from the thermal decomposition of hydrotalcite in N2 flow [50]. The different crystallinity of meixnerite can be attributed to the different reaction rates of synthesis. A faster reaction of synthesis during the hydration of mixed oxide must be expected in comparison to that one of the coprecipitation. The following remarks can be made about the mechanism of rehydration of the calcined LDHs: - The thermal treatment at low temperature of LDH precursor with the removal of interlayer water determines the formation of highly disordered phase. Its rehydration involves a slow reconstruction of layered structure with no changes in composition and morphology in comparison to the ex-LDH. In this case the so called memory effect appears to be clear. - When the LDH precursor is mildly calcined with a full decomposition, mixed oxides more or less crystalline result with an increase in the surface area due to the crystal shuttering and to the formation of a maze micro-cracks of the ex-LDH crystals. The rehydration of poor crystalline mixed oxides determines a fast regeneration of the layered phase characterized by a drastic change in the morphology and decrease in the surface area. Such decrease is related to the formation of highly compact aggregates of regenerated LDH. In this case the direct synthesis appears to be the true mechanism of recovery of the layered structure. - At high temperature of calcination, crystalline mixed oxide and partial changes in the local structure of MIII and/or MII cations result in comparison to the ex-LDH. A partial regeneration of the layered phase takes place which is characterized by different composition and morphology in comparison to the LDH
Please cite this article in press as: G. Mascolo, M.C. MascoloOn the synthesis of layered double hydroxides (LDHs) by reconstruction method based on the “memory effect”, Microporous and Mesoporous Materials (2015), http://dx.doi.org/10.1016/j.micromeso.2015.03.024
Correspondence / Microporous and Mesoporous Materials xxx (2015) 1e3
precursor. Also in this case the mechanism of the partial recovery of LDH precursor involves a direct synthesis. The synthesis of LDHs by reconstruction method based on the memory effect is now within the scientific slang as a consequence of the massive bibliography on this topic, but in our opinion the reconstruction method of LDH phases, ascribed to a memory effect, appears to be simply based on a direct synthesis.
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G. Mascolo*, M.C. Mascolo Laboratory of Materials, Department of Civil and Mechanical Engineering, University of Cassino and Southern Lazio, Via G. Di Biasio, 43, 03043 Cassino, FR, Italy *
Corresponding author. Tel.: þ39 07762993710; fax: þ39 07762993711. E-mail address: [email protected] (G. Mascolo). 5 December 2014 Available online xxx
Please cite this article in press as: G. Mascolo, M.C. MascoloOn the synthesis of layered double hydroxides (LDHs) by reconstruction method based on the “memory effect”, Microporous and Mesoporous Materials (2015), http://dx.doi.org/10.1016/j.micromeso.2015.03.024