Contents
1 Introduction
2 Structure
2.1 Composition
2.2 Delamination process
2.3 Calcination/memory effect/reconstruction
2.4 Pillared LDHs
3 Properties
3.1 Physical properties
3.2 Magnetic properties
4 Preparation methods
4.1 Co-precipitation method
4.2 Hydrothermal approach
4.3 Anion exchange process
4.4 Urea hydrolysis
4.5 Sol-gel method
5 Application
5.1 Adsorbents and anion exchangers
5.2 Catalysts, catalysts support and photocatalysts
5.3 Biological compatibility of LDHs
6 Future aspects
7 Conclusion
1 Introduction
Clays are commonly used minerals in Europe and Asia. They have been used as paper coatings, drilling muds, foundry moulds, adsorbents, catalysts, catalyst supports, ion exchangers, decolourizing agents, etc. Clays[1,2]are also important in agriculture, because many soils have large amount of clay minerals which determine soil properties such as water retention, fertility, structure and texture, etc.
In ancient times, clays were used as cosmetics, for treatment of many skin diseases(cicatrices), for therapeutic purposes(excipients), and to mark different symbols. Kaolinite and palygorskite had been used to protect gastric and intestinal mucous membrane, in the treatment of duodenal ulcers and as antidiarrhoeaics since prehistoric times. Clays remove water from faeces, resulting in more compact material because of their better adsorption capacity and high surface area[3,4].
The use of clays for cosmetic purposes is common in the world especially in the spas of Africa. Color of clays plays a key role in utilization, for example, cleansing of skin is performed by reddish clays, bacterial infection on skin is obviated by yellowish clays, bluish clay is effective in the prevention of acne, black and green colored clays are used for overall body nourishment and to control the amount of oil on skin, respectively[5,6,7].
But a limited number of clays(talc, kaolinite, palygorskite, sepiolite) are used because of technical and legal aspects, e.g. dissolution of Al is increased by ingestion of clay minerals which can cause serious issues[8]. Therefore, it is demanded to mention the intended utilization of clay minerals[9,10,11,12].
There are two well-known categories of clays, i.e. cationic clays and anionic clays. The former is extensively present in nature and consists of alumina silicate layers having negative charge while cations are present in interlayer region in order to balance the charge. Anionic clays are rare in nature, but simple and inexpensive to fabricate. Anionic clays contain layers of positively charged metal hydroxides with water molecules and anions in interlayer region. There are two building blocks of clay minerals tetrahedral Si(O/OH) and octahedral M(O/OH), M=(Al3+, Mg2+, Fe3+or Fe2+)[13,2]. Anionic clays are considered as natural or synthetic layered mixed hydroxides consisting of exchangeable anions in interstitial region. Depending upon composition, these compounds are given many names but generally used term is hydrotalcite type compounds and widely used name is layered double hydroxides[14,15].
Large scale researches have been conducted on hydrotalcite(Mg-Al hydroxycarbonate) because it is simple and economical to fabricate, therefore hydrotalcite name is given to LDHs. Feitknecht assumed a structure having intercalated hydroxide layers and named these compounds Doppelschichtstrukturen meaning double sheet structure. However, after many years single crystal X-ray diffraction analysis of these compounds rejected his assumptions. The XRD pattern showed that cations are located in layers while anions and water molecules in between the layers[16].
2 Structure
2.1 Composition
Chemical composition of layered double hydroxides is the exciting feature which defines their use in various applications and imparts phenomenal properties in these compounds. The general formula to describe the chemical composition is given as follow[17].
where M2+=divalent metal cation, M3+=trivalent metal cation, A=interlayer anion, n-=charge on anion, x and m are fraction constants, n=0 indicates that neutral layers are attracted by weak van der Waals forces and value of x lies in the range of 0.2~0.33. The structure of LDHs closely resembles brucite Mg(OH)2[18,2]. If value of x exceeds 0.33, large number of trivalent cations result in M(OH)3; and if x is below 0.2, divalent cations in brucite type sheets consisting of octahedra result in the precipitation of M(OH)2[19].
There is stacking of layers in LDH structure in which divalent and trivalent metals are present in the form of octahedral unit and anions between the layers. There exists divergence in the nature of intercalated anions and therefore there is narrow distribution in LDH composition. Trivalent metal ration decides charge density, i.e. x=M3+/(M2++M3+) and formula of representation is shrinked as [M2+-M3+-x]. M2+ and M3+ are generally taken from third and fourth period of periodic table, i.e. M2+: Mg, Mn, Fe, Co, Ni, Cu, Zn, M3+: Al, Mn, Fe, Co, Ni, Ga. Ionic radius ranges from 0.65~0.80 Å and 0.62~0.69 Å for divalent and trivalent cations, respectively, except for Al 0.50 Å[21].
There is weak bonding between intercalated anions and host structure in these layered compounds. A number of anionic species can be intercalated between the layers during synthesis of layered structure. Intercalated anions[23] can belong to inorganic anions such as halides(F-, Cl-), oxo anions($\text{CO}_{3}^{2-}$, $\text{NO}_{3}^{-}$, $\text{SO}_{4}^{2-}$),(Mo7O24)6-,(V10O28)6- and organic anions, i.e. carboxylate, phosphates, alkyl sulfates, etc.
2.2 Delamination process
Delamination or exfoliation is a process in which layers of LDHs are peeled off into single or discrete sheets. After delamination process nanosheets of thickness 1~5 nm are obtained and it has remained a difficult task because of electrostatic interactions among layers[24].
Delamination process is influenced by the exchange of water molecules with incoming anions and completion time, usually twelve hours are required for Zn-Al delamination. The delamination process is summarized in Fig.3. It has been noticed that dispersion of dodecylsulfate in different solvents such as methane, hexane and ethanol results in variable colloidal suspension whereas stable translucent solutions are resulted in case of higher alcohols such as pentanol and hexanol[25].
Polyacrylate was obtained from the monomers of acrylate which in turn are produced as a result of delamination process of Mg-Al layered double hydroxide[26]. Process of delamination was studied in different solvents such as butanol, toluene, CCl4, acrylates and formamide which were used as dispersant and dodecylsulfate as anionic surfactant[27]. The colloidal solutions of lactate containing LDHs delaminated in water were obtained[28]. Polymer layered double hydroxide nanocomposites were prepared from LDH sheets which were obtained by delamination of glycine-LDH in formamide[29].
2.3 Calcination/memory effect/reconstruction
Calcination is a process in which layered double hydroxides are treated at high temperatures and the layer stacking is disturbed as a result of this thermal treatment. Process of calcinations takes place in two steps:(1) water molecules are removed from surface shown in eq 1,(2) water molecules are removed from interlayer(dehydroxylation) followed by decomposition of layered structure eq 2.
(OH)2· ·mH2O→ (OH)2· (s)+mH2O(g)
(OH)2· (s)→ Ox(s)+mH2O(g)+CO2
Ox(s)→M2+O(s)+M2+ Ox
Intercalated anionic species decide the calcine temperature of LDH. After dehydroxylation, a mixed metal oxide is obtained with high specific surface area, improved basic properties, fine crystallites and stable towards thermal treatments. Mg-Al is studied widespread and MgO is obtained after calcinations. The reconstruction of original LDH structure is achieved by thermally treating calcined LDH with water or any other solvent[31,22,32].
It is noticed that Mg-Al LDH loses its structure upon calcinations and after immersing in distilled water regains its structure, but some disorder in structure is observed. The anion exchange capacity of LDHs increases after calcinations and therefore their adsorption capacities are also increased than the corresponding uncalcined LDHs. Adsorption efficiency[34] is directly proportional to the amount of calcined LDHs(Fig.5). Calcined LDHs have proved to be efficient adsorbents in removal of indigo carmine dye[35], acid orange 10[36], and phosphate[37].
Number of patents has been increasing regarding calcined layered double hydroxides accommodating large organic molecules[38], polyoxometalate[39], terephtalate TP, benzene carboxylate BC, benzene sulfonate BS and benzene phosphonate BP[40], sebacic acid[41], and pentoses[42]. Calcined LDHs have been proved to be efficient sorbents for brilliant blue R(BBR). The calcined layered double hydroxides are utilized to detach pollutants of relatively high concentration and uncalcined LDH for low concentrations as shown below. It is noticed that amount of dye sorbed increased rapidly within three hours and became constant after nine hours for calcined LDHs while for uncalcined LDHs the amount of dye sorbed showed rapid increase within initial four hours and then uptake is slowed down in eighteen hours as shown in Fig.6 and Fig.7[43].
Fig.6 Effect of contact time on the uptake of Brilliant Blue R(BBR) by layered double hydroxides(LDHs) and calcined LDHs(CLDHs) at different initial concentrations[43] |
2.4 Pillared LDHs
In the process of pillaring the layered structure is retained when an inorganic compound is modified into thermally stable meso and microporous material. There is a molecular distribution of layers in the interlayer region which makes these pillared compounds chemical and thermal stable. Pillared LDHs are extensively used for adsorptive and catalytic application because porosity and specific surface area of pillared materials are enhanced[44]. The main benefit of pillaring LDH is that interlayer space increases by intercalating large anions such as polyoxometalate(POMs), phthalocyanines, diphosphates[O3P-(C6H4)n-PO3]4-, [Fe(CN)6]3-, etc. Moreover, the catalytic activity of LDHs is enhanced as a result of pillar layer combination. Low M2+/M3+ ratio favors the synthesis of pillared layered double hydroxide in low pH region which is normally obtained by urea assisted co-precipitation method[45]. Polyoxometalates are widely studied pillaring agents consisting of multiple layered structure with space filling oxygen atoms and high charge densities. Polyoxometalates are acidic in nature and therefore impart acidic characteristics to layered double hydroxides[46]. Pillaring of LDH depends upon the swelling conditions of LDH and ion exchange ability. In the pillaring of LDH with POM leads to the partial dissolution of metals than their starting LDHs[47].
LDH swelling(limited to one or two layers) and their high layer charge density will not allow the penetration of organic reagents. However, anions are readily transferred to external edge surfaces for reaction with reagents adsorbed at those sites. Synthesis of Mg-Al layered double hydroxide has been studied pillared with metatungstate and found that by increasing the surface charge density of LDH the micropore diameter decreases[48]. Alkenes such as 2-hexene was selectively epoxidized by POM pillared LDH with H2O2 by having advantage that subsequent hydrolysis was suppressed to form diols compared to hydrotalcite free catalyst[49].
Layered double hydroxide pillared with decatungstate was used as catalyst in the epoxidation of alkenes with H2O2 to form peroxotungstate which assists epoxidation by transferring oxygen atom to alkenes. This type of catalytic oxidation is less favored for larger alkene than smaller alkenes[50]. Conversion of aliphatic and aromatic thiols into disulfides by Zn-Al layered double hydroxide intercalated with {MoO2[OOCC(S)(C6H5)2]2}2- anion was also studied[51]. A detailed mechanism is also reported for this type of conversion[52,53].
2R-S + 2LDH → 2R-S- + 2LDH-H+
2LDH-M2+ + O2 → 2LDH-M3+ + 2O2-
2R-S- + 2LDH-M3+ → 2R-S- + 2LDH-M2+
2R-S- → R-SS-R
O2- + 2LDH-H+ → 2LDH-H2O + 1/2 O2
Overall reaction is
2R-SH + 1/2 O2 → R-SS-R + H2O
3 Properties
3.1 Physical properties
Particle size, surface area, crystallinity, porosity and tensile strength are important physical characteristics that influence the applications of LDHs. These properties are dependent on methods of preparation used to synthesize these layered compounds. Scanning electron microscopy and transmission electron microscopy can be used to determine the morphology[45].
Crystallinity and pore size are important to describe catalytic and adsorptive properties of LDHs. Mass transfer process and regioselectivity are promoted by the uniformity of porous structure. Nitrogen adsorption process can be used to analyze porosity of a compound. Cajra et al studied textural properties of layered double hydroxide and concluded that pore size of LDHs lies in micro to mesoporous range. The microstructure is analyzed by both mesoporous and microporous characteristics[54].
Particle size is influenced by synthesis condition variables, i.e. time, temperature, pH and reaction stoichiometry. Small particle size and rough surface is obtained with variable pH, while larger particles are obtained in constant pH method showing sharp peak. SEM and TEM techniques can be used to determine the particle size[55].
Layered double hydroxides have high specific surface area of(100 ± 300) m2·g-1. High surface area facilitates guest host interactions. Larger crystallites synthesized by constant pH method exhibit lower surface area. Surface area of particle can be enhanced after calcination by transforming them into corresponding oxides or oxy hydroxides[56].
Tensile strength of these compounds decreases by increasing the concentration of LDH. However, a sharp decrease in elongation at break and steady increase in modulus was observed with increase in concentration of LDH. It is concluded that change tensile properties with concentration of LDH is due to the combined effect of compatibilizer and dispersed LDH particles[57].
3.2 Magnetic properties
The layers in LDHs are composed of metal cations with different oxidation states which can impart magentism in layers. The introduction of magnetic substrate into magnetic layers can lead to novel applications[58]. A new nano hybrid aspirin-LDH has been reported with magnetic properties and utilized efficiently in controlled drug delivery system[59]. Magnetic properties for Cu-Al LDH intercalated with carbonate and surfactants have been reported. Temperature dependent susceptibility plot χT vs T is shown in Fig. 8. At low temperature antiferromagnetic interactions are prominent while for CAS paramagnetic behavior is dominant because thermal agitation overcomes magnetic order[59].
In Co-Al and Ni-Al hydrotalcite like compounds Co-Co and Ni-Ni ferromagnetic interactions are responsible for magnetic behaviour. Magnetic properties of calcined products appeared to be different from corresponding precursors but in this case consistent magnetism was obtained[60].
There are two main parameters which control the magnetism in LDHs. The first one is metal centers make in-layer magnetic superexchange via OH bridges and the second one is intense dipole interaction between individual layers. Spin frustration is a consequence of competing magnetic interactions and structural deformation. Ni-Cr layered double hydroxide shows spin frustration where Cr3+ is coupled antiferromagnetically to Ni2+ and spin of incoming Cr3+ is frustrated because it wants to be coupled antiferromagnetically to nearby Ni2+ and Cr3+ spins simultaneously. Spin frustration is shown below[61].
4 Preparatory methods
Although LDHs are easy to prepare but pure hydrotalcite like compounds are not easy to obtain. There are some points which are still vague such as relative amount of intercalated ion, range of cations, stacking arrangement and cation ordering in the layers. For preparation of pure LDHs, it is necessary to select the correct ratio of cation and anion. Countless methods are present in literature regarding synthesis of layered double hydroxides, e.g. co-precipitation, hydrothermal, solvothermal, topochemical synthesis, etc. In this review, five mostly utilized approaches are discussed, including(1) co-precipitation,(2) hydrothermal approach,(3) anion exchange method,(4) urea hydrolysis,(5) sol-gel method[31,65].
| Mineral | Structural Formula | Intercalated ion | Cell Parameters(Å) | Space Group |
|---|---|---|---|---|
| Fougerite | Fe42+Fe23+(OH)12[CO3]·3H2O | OH-, Cl-, CO32- are possible | a=3.17~3.18 c=22.7~22.9 | R$\bar{3}$m |
| Meixnerite | [Mg6Al2(OH)16][(OH-)2.4H2O] | OH- | a=3.046 c=22.93 | R$\bar{3}$m |
| Zincowoodwardite | Zn1-xAlx(OH)2[SO4]x/2·nH2O x<0.5, n<3x/2 | SO42- | a=3.063 c=8.91 and a=3.065 c=25.45 | P$\bar{3}$m and R$\bar{3}$m |
| Hydrotalcite | [Mg6Al2(OH)16][(CO32-)·4H2O] | CO32- | a=6.13 c=46.15 | R$\bar{3}$m |
| Pyroaurite | [Mg6Fe3+(OH)16][(CO32-)·4H2O] | CO32- | a=6.19 c=46.54 | R$\bar{3}$m |
| Mössbauerite | Fe63+O4(OH)8[CO3]·3H2O | CO32- | a=3.07 c=22.25 | R$\bar{3}$m |
| Charmarite | Mn4Al2(OH)12[CO3]·3H2O | CO32- | a=10.98 c=15.10 | P6322 |
4.1 Co-precipitation method
Co-precipitation method is commonly used procedure for preparing LDHs because it produces large amount of material and is easy to handle at laboratory level. In this procedure aqueous solutions of divalent and trivalent metals are used as precursors which contain intercalating anions. An interesting feature of this method is that there exists diversity in anionic species to intercalate in OH- sheets. Multiple cations can be precipitated by taking control over pH and supersaturation conditions. pH should be higher or equal to one at which most soluble hydroxide is precipitated[31,65].
Layered double hydroxides are prepared at high and low supersaturation conditions. Precipitation at low supersaturation requires slow addition of divalent and trivalent metals salt solution and pH is maintained at selected value. This method is performed to obtain high crystallinity and to obtain careful control of charge density of hydroxide layers.
On the other hand, co-precipitation at high supersaturation results in less crystallinity because of continual change in pH. Thermal treatment performed following co-precipitation may help increase the crystallinity of amorphous or badly crystallized materials[23]. Zn-Al LDH intercalating various amino acids have been prepared and found out that co-precipitation is maximum at pH 6~8 as zwitterion and as anion at pH 9~10 for phenylalanine[66]. Composites of tetra anion of perylene dye(PBITS) have been synthesized via co-precipitation at low supersaturation at pH=7.9. The PXRD pattern indicated that basal reflections 001 shift from 7.9 Å to 19.2 Å for Mg-Al LDH and 8.6 Å to 14.8 Å for Ca-Al LDH as shown in Fig. 11.
Fig.13 The SEM micrographs of the(a) ZnAl-4,(b) CZnAl-4-300 C and(c) CZnAl-4-500 C sample. The arrows in the left figures indicate where the enlargements(in the right figures) were taken. The marked spots(in the right figures) indicate the part taken for the EDX measurements[69] |
4.2 Hydrothermal approach
Post synthesis treatments are utilized to control the properties of LDHs. When an organic species having less affinity to LDH is needed to intercalate, anion exchange reactions and co-precipitation techniques are ineffective in this case. Hence hydrothermal synthesis has proven efficient because this method ensures no competing anion in the interlayer other than the intended by making insoluble hydroxides as inorganic source[70]. Hydrothermal approach is utilized to control particle size and its distribution. Ca-Al LDH has been prepared by hydrothermal synthesis and concluded that hydrothermal temperature affects the crystal structure of final product. A disordered structure is obtained at 100 ℃ and ordered at 120 ℃. It has been found that the main layers of both structures were similar and difference lies in the arrangement of interlayer anions and water molecules[65]. Hydrothermal approach is usually carried out to improve crystallinity and is studied for Mg-Al LDHs. It is found that increase in LDH crystal size results in improved crystallinity of hydrothermally treated samples. By increasing the Mg/Al ratio crystal size of LDH is observed to decrease[71,72]. Hydrothermal method involves reactions of metal oxides, customarily in aqueous solution, with the anion of interest. A general reaction is given below[73].
(1-x) O +(x/2) O3 +(x/n) +(1 + x/2)H2O=[ (OH)2] +(x)
Hydrothermal treatment is post synthesis method and formation of MgAl LDHs from their oxides MgO has been reported. Three different viewpoints were presented for LDH formation:(1) during titration poorly crystallized LDHs were obtained,(2) Agglomerates first are amorphous then they form layered structure,(3) reconstruction of LDH structure from their calcined oxides[74]. Zhao et al successfully fabricated LDH nanorods via hydrothermal approach with excellent morphology at different pH. TEM images are shown in Fig. 15[75].
4.3 Anion exchange method
Anion exchange is the most effective technique to synthesize layered double hydroxide consisting of anions other than carbonates. LDHs have been synthesized containing variable anions by exchange of NO3-[31].Pillared materials consisting of bulky organic(sebacic acid) or inorganic(polyoxometalate, such as (V10O28)6- or(Mo7O24)6-) anions have been prepared. The order of intercalation is as follows:CO32-≫ SO42-≫ OH- ≫ F- ≫ Cl- ≫ Br- ≫ NO3-≫ I-[76,77]. Thermodynamic studies indicate that exchange of anion depends upon the interlayer arrangement of LDH[78]. Various anion exchange mechanisms of LDH have been reported. Anion exchange process has been fount efficient for the intercalation of organic anions[77,14,79]. The equation of anion exchange is shown as follows:
IA=Intercalated Anion
The general representation for anion exchange is shown in Fig.16.
4.4 Urea hydrolysis
Urea is placed in Brönsted bases with pKb=13.8 and highly soluble in water and by controlling the reaction temperature its hydrolysis rate can also be controlled. Urea hydrolysis first proceed via formation of ammonium cyanate(rate determining step) followed by formation of ammonium carbonate as fast step[22]:
CO(NH2)2 → NH4CNO
NH4CNO + 2H2O → 2NH4+ + CO32-
The urea method is an ideal way for the synthesis of hydrotalcite because both of the ions liberated by the urea hydrolysis, hydroxides and carbonates, are the main components of hydrotalcite. In typical synthesis salt solutions of divalent and trivalent metals are mixed with urea solution in a specific ratio and pH followed by hydrothermal treatment[81,82,83].
In order to control particle size distribution and growth, urea hydrolysis is carried out in water/EG medium in ration 1/4. Reaction temperature, amount of urea, M2+/M3+ molar ratios also play an important role in controlling particle growth[84]. Specific surface area and crystallinity are observed to increase by using urea[85,81,82].
Zn-Al LDH films have been successfully fabricated and applied as anticorrosion coatings, in optical devices and electrochemical applications. Utilizing urea or ammonia as a weak base leads to films with strong adhesion between the LDH and the substrate[81].
4.5 Sol-gel method
Sol-gel method has received attention of chemists due to its efficient route to induce remarkable properties into manmade materials[86,87,88].Zn-Al LDH films have been reported via sol-gel synthesis supported on paper, sponge and cloth and exhibited high adsorption capability and surface area[89]. Crystallinity of LDHs is affected by nature of precursor, time of aging, nature of acid used in hydrolysis in this process[90,91,92]. Ni-Al and Co-Al LDHs have been synthesized in poylol medium using acetate as precursor. Unlike co-precipitation, sol-gel method has the advantage of not necessitating pH or atmosphere control. It has been noticed that uniform pore size distribution and high specific surface area is obtained which contribute to catalytic properties of synthesized layered compounds[93]. Layered double hydroxides containing different combinations of Mg with Ga, In, and Al have been synthesized and observed that crystallinity is not much affected as in case of traditional methods. Figure 19 indicates that most of the pores fall in the range of meso size showing the increased surface area[94]. Sol-gel route is extended to synthesize multivalent LDHs and showed high thermal stability[95].
5 Application
5.1 Adsorbents and anion exchangers
High anion exchange capacities and large surface area of layered double hydroxide make them excellent adsorbent for various organic and inorganic anions. Multiple molecules can be inserted in interlayer region of LDHs due to the flexibility of layers which helps in decontamination of water and participates in high anion exchange capacity making them preferable over conventional anion exchange resins. Targeted entity to be adsorbed may be of organic(phenols, aromatic carboxylic acids, dyes, pesticides, etc.) and inorganic(PO42-, SO42-, NO3-, CrO42-, F-, Cl-, Br-, I-, etc.) nature and a number of researches have been conducted to study the adsorption phenomenon of diverse species acting as contaminants in soil and water[65,56]. The pictorial representation for the adsorption process is given in Fig.23.
Petrochemical and petroleum industries produce many organic pollutants which produce annoying smell and phenols are one of them. Even at low concentrations phenols are harmful to organisms, therefore their removal from water is necessary. The phenomenal adsorption properties of layered double hydroxides make it easy to eliminate phenols from wastewater[108]. The OH- group of layered double hydroxide reacts with nitrogen and oxygen of methyl orange dye and hence removes it from wastewater[109].
Table 2 A brief literature of layered double hydroxide with different metal combinations |
| Metals-anion | Preparation methods | Charac. techniques | ref |
|---|---|---|---|
| Ca-Al-NO3 | Co-precipitation | PXRD, FTIR | 96 |
| Co-Fe-OH Co-Fe-OH/CO32- (pyroaurite group) | Topochemical synthesis Co-precipitation method | XRD, SEM, TEM, AFM XRD, FTIR, Mossbauer spectroscopy, DTA | 97 98 |
| Co-La-CH3COO- | hydrogen peroxide catalyzed hydrolysis reaction | FTIR, PXRD, TGA, PL spectroscopy | 99 |
| Cu-Al- CO32- | Hydrothermal approach | FTIR, PXRD, TGA | 100 |
| Zn-Cr-Cl-/CO32-/NO3- | Co-precipitation | UV-VIS analysis, PXRD, FTIR | 101, 102 |
| Zn-Ti-NO3 Zn-Al-CO3 Zn-Al-CO32-/NO3- | Co-precipitation Co-precipitation Co-precipitation at low supersaturation | TEM, SEM, PXRD, XPS SEM, TGA, PXRD, TEM SEM, XRD, EDX, IR, TG/DTG | 103 56 104 |
| Ni-Fe-Cl | Topochemical synthesis | SEM, TEM, PXRD, XPS, FTIR | 105 |
| Mn-Al-CO32-/NO3-/SO42-/Cl- | Co-precipitation | PXRD, FTIR, DTA/TG, SEM | 106 |
| NiTi-CO32- | Co-precipitation at high supersaturation | PXRD, SEM, ICP-AES, FTIR | 68 |
| Mg-Al-CO32- | Hydrothermal approach Urea hydrolysis | SEM, XRD, TGA, FTIR XRD, FTIR, BET | 71, 72, 84, 83 |
| Zn-Al LDH films | Urea hydrolysis Sol-gel route | XRD, SEM, TEM XRD, SEM | 81 89 |
| Ni-Al/Mg-Al-CO32- | Sol-Gel method | XRD, SEM, TEM, HRTEM | 90 |
| Zn-Al LDH | Sol-gel method | XRD, TG-DSC | 91 |
| Mg-Al/Ga/In | Sol-gel method | FTIR, XRD, DRIFT | 94 |
| Ni-Co-Al/Mg-Ni-Al | Sol-gel method | XRD, TEM, TGA-DTA | 95 |
Pesticides(2,4-dinitrophenol, 2-methy-4,6-dinitrocresol) and herbicides(organophosphate and organophosphanate) were effectively adsorbed by considering memory effect and ion exchange ability of LDH[110]. Arsenic is considered to be carcinogenic and a major cause of pollutants. Researchers have devoted a lot of time to investigating that Mg-Al LDHs can act as efficient adsorbents for As and Se as well. Adsorption efficiency and selectivity of arsenate ion depend upon two factors, molar ration of Mg2+ and Al3+ and orientation of intercalating anion. It has been found that poorly crystalline layered hydroxides exhibit high adsorption capacity of As[111,112]. One of the hazardous pollutants of our ecosystem is 2,4-dichlorophenoxyacetic acid contaminating soil and water and therefore is needed to be removed by some economical and competent adsorbents, layered double hydroxide in this regard did not disappoint researchers and was efficiently exploited to remove this pollutant. As in the synthesized Mg-Al/NO3 LDH, nitrate ion shows less affinity to the LDH sheets and can be successfully replaced with contaminants[113].
Heavy metal cations, i.e. Ni2+, Co2+, Cu2+, Pb2+, Hg2+ and radioactive metals were eliminated by using chelate ligands intercalated LDH because of flexible interlamellar region and specific pH. The adsorption characteristics of metal cations were separately studied and concluded that metal cations were separated due to high bond energy and high sensitivity of OH of LDH[114,115]. Hydrotalcite mineral is a crucial alternative to Fe(OH)3 and therefore has ability to remove arsenic from aqueous solution[116,112].
Various uncalcined and calcined layered double hydroxides LDH of Mg-Al, Ni-Al and Zn-Cr have been used to investigate the adsorption of Cr4+ ion. Calcinations temperature, amount of M3+ in LDH precursor, pH of metal solution, calcinations time in removing dichromate ions by adsorption was determined. The aqueous solution of carbonate and chloride were used to release and adsorb dichromate ions on LDHs[117].
ZnCr-LDH was synthesized with different molar ratios of Zn and Cr and intercalated with nitrate anions in order to study uptake of fluoride. Chromium is one of the major pollutants of environment released from tannery and electroplating industries and therefore LDHs having Cr as trivalent cation are of great significance from environmental viewpoint[118].
Methylene blue, methyl orange and formaldehyde were removed successfully by using ZnCr and MgAl layered double hydroxides. Photocatalytic activity of ZnCr LDHs has been restricted because of low capability to separate electron hole pair, rapid charge recombination and band energy ranges from 2~3 eV. For the removal of formaldehyde, ZnCr LDH does not show photocatalytic activity[119].
A food dye sunset yellow FCF was removed from water by using CaAl LDH intercalated with nitrate and adsorption reached equilibrium in fifty minutes. It is observed that degree of dissociation of anionic dye and surface charge of adsorbents are affected by pH[120].
Surface area and pore volume were determined from the knowledge of degree of dispersion exhibited by these materials[121]. Layered double hydroxides have been used for CO2 capture at temperature from 200~400 ℃. CO2 adsorption ability of LDHs having nano spherical and sand rose morphology was compared[122]. Mixed metal oxides obtained after calcinations exhibit excellent basic properties and surface basicity of these oxides is determined by metal ratio. Adsorption of carbon dioxide for MgAl LDH was investigated at 200 ℃. It was observed that by increasing metal ratio from 1.5 to 3.5, CO2 capture capacity also increased then decreased when x is 4[123].
Methyl orange and Cr4+ are considered harmful pollutants for human beings as well as for aquatic animals. CoFe LDHs intercalated with nitrate ion were fabricated by conventional co-precipitation method at different molar ratios and proved efficient for the removal of these contaminants. It is concluded from structural analysis and adsorption isotherm that removal of these effluents take place in two steps:(1) adsorption from exterior surface,(2) exchange of anion from interlayer region[124]. Removal of dyes from wastewater is of significant consideration because dyes being colored pollutant cause environmental hazards. Phenomenal characteristics of LDHs such as large surface area, high anion exchange capacity and adaptable interstellar space make these compounds favorable to remove contaminants from aqueous systems. Adsorption efficiencies of calcined and uncalcined LDHs were compared and found that better removal efficiency is exhibited by calcined LDHs[125].
Nanocomposite of graphene and pure LDHs showed exponential increase in adsorption capacities. By using the process of delamination, adsorption capacities of LDHs were increased because of the particle size decreased. It has been investigated that adsorption capacities of MgAl LDHs were increased by 62% after addition of only 7% of graphene oxide[126].
5.2 Catalysts, catalyst support and photocatalysts
Flexible composition, synthesis ease(inexpensive or simple), eco-friendliness and high versatility makes LDH viable candidate in comparison to catalysts used conventionally. For determining LDHs base properties, comparison among zeolites and LDH Mg-Al as catalyst is made within condensation reaction. Condensation(Knoevenagel) is observed via using zeolites whereas with usage of LDH as catalyst reactions, such that Michael type addition and Claisen condensation can be observed. Within pKa range of 16.5 basic sites of material are shown[132]. Oxides of nitrogen and generation of soot from vehicles are considered as one main participant in human health and pollution. Catalyst derived from LDH Mg-Al has been utilized effectively in such respect which leads to NO oxidation into nitrogen dioxide(NO2) and for enhancing catalyst efficiency noble metal is used[133]. These catalysts are also used for studying basic and acidic behavior demonstrated via Prinetto et al. Various parameters of structure influence catalyst efficiency, i.e. trivalent and divalent cations ratio, interlamellar anions nature and conditions of preparation in turn effects characteristics of base and acid[134].
LDHs exciting feature is their catalyst utilization via reconstruction thus providing opportunity for their decoration into several anions. LDHs basic properties are induced via reconstruction which makes them efficient catalysts for use in different reactions being catalyzed, i.e. Claisen-schmidt and aldol condensation plus epoxidation. Catalysts supported with iron ease in ethyl-benzene degradation into styrene[135,136]. Excitation of ZnO and TiO2 via UV light are used widely as catalysts for reactive dyes degradation which requires many lamps of UV for purpose of photocatalysis. Pollutants degradation rate is not fast enough. Thus, the catalyst’s role is limited by these factors. An advantage is induced by LDH for their utilization as catalyst via LDH Mg-Al and ZnO nano-composite synthesis in Acid Red G degradation[137]. LDH Mg-Al is therefore known as active UV photocatalyst with no response towards visible light and utilized successfully in generation of hydrogen photocatalytically[138].
LDHs of Zn-Cu-Fe-Al-Mn with Al/Fe/Mn/Zn/Cu possessing atomic ratios variably within synthetic mixture prepared via co-precipitation under controlled pH and temperature conditions. Precursor’s decomposition generated mixed oxides in air at 500 ℃. Characterization was done using various techniques such that TG-DTA-MS, TG-DTA, TEM, FT-IR, ICP-ES, XRD, H2-TPR and O2-TPD. Main objective is investigation of Fe and Mn partial substitution effect in to Al-Zn-Cu MMO derived from precursor LDH on catalytic, chemical and physical properties. Results indicated that substitution of manganese and iron decreased LDHs structure stability and improvement of LDHs calcined redox properties. The phenol oxidation with wet H2O2 catalytic activity is proportional to metal ions surface content and main centers of Cu2+ linked to lattice oxygen surface and deep oxidation degree if it’s related mainly to surface bonded weak oxygen reactivity which depends upon transition metallic ion’s nature within structure. Phenol per-oxidation via ·OH surface radical with origin from H2O2 helps in favoring next oxidation deeply[139].
Techniques of photocatalysis using metallic semi-conductors i.e. SnO2, ZnO, CdS, SnO2 applied widely for organic pollutant’s degradation in aqueous solutions. LDHs Al/Zn with different ratios of cations prepared through co-precipitation used conventionally at cons. pH. Improvement in photocatalytic activity is observed upon sample’s calcinations at 500 ℃ owing to large ZnO phase amounts formation[104].
LDH hybrid materials of La/Cr/Zn synthesized with excellent photo-catalytic activity. TiO2 photo-catalysts can’t be activated via visible light owing to larger band gap which generates low efficiency of electronic-photo conversion. Photocatalysts based on LDH emerged as potential candidate for replacing titanium dioxide due to novel layer structure, band gaps which can be tunable, easy scaling up and good activity of photocatalysis for splitting of water and related reactions[140].
Nano-composites containing titanium have prepared via Ti-Ni LDH calcinations used as precursor. Evaluation of titanium based nano-composite possesses excellent activity of photocatalysis under irradiation of visible-UV. Methylene blue degradation is observed via such nano-composites. Photocatalysis process relies on hole/electron pairs generation via radiation of band gap giving rise to reactions i.e. redox with species absorbance upon catalysts surface. Thus, it is necessary that particles of nano-composites from LDH of Ti-Ni(calcined) show property of photocatalysis under irradiation of both ultra-voilet and visible owing to their high areas of surface and absorption properties(in Vis-UV)[141].
Attention is being paid to LDHs for their utilization ads photocatalytic material attributed to its specific microstructure, adjustable band gap(forbidden) and properties of recyclable. Bi-metallic oxide can be formed when calcinated under highest temperature with highest dispersion of second metal and doping into materials of LDH where such materials photocatalysis can be improved dramatically[142].
Catalysts(semiconducting/basic bi-functional) synthesized via simple eco-friendly method afterwards incorporation of cerium oxide is done for purpose of degradation photo-catalytically of chlorinated and simple phenolic compounds. Photocatalysis efficiency is attributed to effect like self-sensitizing which occurs while dye adsorption to surface of CeO2.Support of ceria on Mg-Fe-Al LDHs and Mg-Al which are calcined have studies as catalysts with potency of sulfur oxides removal because of their properties of redox/basic[143].
In current work, advantage of LDH materials memory effect, fabrication of doped LDH surface supported with TiO2 was described possessing highest dispersibility. Ti4+,Al3+, Cu2+, and Mg2+ possessing LDH prepared via co-precipitation[144].
LDH of Zn/Fe with various intercalation anions(carbonate, chloride) and anion in terms of photocatalysts fabricated via method of co-precipitation. Methyl violet decolourization plus malachite green was performed photocatalytically. Principally, photocatalyst photo absorption depends upon hole-electron pairs mobility which evaluated electrons probability and holes for reaching photocatalyst surface reaction sites[145]. Chemical composition and synthesis conditions decide basic properties of LDHs. MgAl-LDH after calcination at 623 K was converted to MgAlO which possess large number of basic site[16]. In materials based on LDH, activation treatments and composition chemically determines basic property. For maintaining basic function, whatever may be content of metals within samples of Mg(Al)O/Pd, introduction of palladium was done via impregnation of Pd acetylacetonate in toluene solution which is free of water[146].
Catalytically active oxides exhibiting acid base behavior are prepared from layered double hydroxides which have high specific surface area. An important reagent extensively used in paint industry and in dewaxing of mineral oil methyl isobutyl ketone is prepared by acetone hydrogenation over NiAlO catalyst. It is reported that hydrotalcite type compounds are useful to convert acetone into methyl isobutyl ketone and methyl isobutyl carbinol[147].
5.3 Biological compatibility of LDHs
Layered double hydroxides have been utilized in various biological applications due to their limber interlayer region. Different anions such as aspartate, glutamate and salicylate are inserted LDHs for drug support against stomach irradiations[148]. Mg-Al LDHs as biocompatible are widely studied in literature. DNA was intercalated into layered double hydroxides by following encoding, encrypting, decrypting and decoding and proved advantageous for rapid analysis and stability in encrypting[149].
Various layered double hydroxides(MgAl, MnAl, NiAl, ZnAl, and ZnCr) have been prepared by co-precipitation method intercalating amino acids using phenylalanine[150,151]. Polyoxometalate are common pillaring agents for LDHs and heptamolybdate- and decavanadate-pillared hydrotalcite-type clays have also been fabricated. Organic anions consisting of aromatic rings are best substitute of polyoxometalate because layers of LDHs are separated to large extent[152].
5.3.1 Drug delivery applications
LDH after intercalation of citrate anion are studied to be beneficial as antacid and for drug delivery applications because those anions possessing greater affinity to layers are more stable to acid attack[153]. Riopan and Rioplus(both provide relief to sour stomach) have been proven efficient adsorbents for indomethacin(non-steroidal anti-inflammatory drug). Artificial membranes can be affected by this sort of adsorption[154].
In a biocompatible layered hosts, a drug remain reserved and protected from oxygenated and photo reactions. Because of the excellent anion exchange characteristics of layered double hydroxides, a drug may release after commanding interstellar compounds. The drug release rate depends upon the deintercalation rate. Aspirin was intercalated in Mg-Al LDHs and observed that rate of decomposition of aspirin is increased in Mg-Al layered double hydroxides[155].
A non-essential amino acid named L-Tyrosine(4-hydroxyphenylalanine) is fabricated within human body from phenylalanine and is used in providing relief from stress plus in the treatment of dementia. Because layered double hydroxides are effective as a matrix for controlled release of drug delivery system. Therefore, NiAl, MgAl and ZnAl LDHs intercalated with L-Tyr were synthesized[156]. Mg-Al LDH intercalated with prednisone-cholate ion has been synthesized by co-precipitation method. In order to study the release kinetics of prednisone from LDH, four kinetic models(i.e. first-order equation, Higuchi equation, Bhaskas equation and Ritger-Peppas equation) were selected and it was concluded that Ritger-Peppas equation described this procedure satisfactorily based on a directing excel based solver[157].
5.3.2 Biosensors
In order to resolve environmental and health issues, electrochemical biosensors have attracted the attention of scientists. Adsorption of enzyme was carried out on Zn-Al-Cl LDH to prepare electrochemical biosensor for determination of phenol based on deactivation of polyphenol oxidase[158]. For the accurate measurement of urea in urine or blood sample to treat kidney and liver diseases, biosensors are used. These biosensors are based on enzyme urease which catalyzes urea into hydrogenocarbonate and ammonium as expressed in equation below. For this purpose Mg-Al LDH/urease biosensors were fabricated[159].
(NH2)2CO + 2H2O + H+ →2N + HC
Glucose is of great importance in food chemistry, clinical chemistry, environmental chemistry and biochemistry, therefore its estimation is necessary to determine. A composite of chitosan and NiAl LDH nanoflakes is used to determine glucose[160]. DNA-LDH nanohybrid was synthesized successfully after intercalation of DNA into Mg-Al LDH via anion exchange process. Layered double hydroxide after intercalation of enzymes has been utilized as biosensors[161]. Polypyrrole-LDH composite was prepared in order to construct glucose/oxygen biofuel cell[151]. Large DNA fragments were inserted into LDH layers vis co-precipitation method and has been utilized in biological applications to carry out small oligonucleotides, plasmid DNA encoding genes for therapeutic purposes[162,163].
6 Future Aspects
LDHs have attracted attention because of their potential applications in industry. LDH host matrices to store biologically important species is the area of intense research. Numerous bioactive entities comprise carboxylic acids and hence intercalated into LDHs. Isomeric guest ion mixture is also separated by LDHs recently, e.g. equimolar mixture of 1,2-,1,3- and 1,4-benzenedicarboxylate was reacted with LiAl2Cl and noted that 1,4-BDC showed 100% selectivity while other anions remain in solution[164].
Reconstruction of LDHs is an exceptional feature which increases the demand of these layered compounds. Important structural evolution and phase changes occur when LDHs are treated at higher temperatures. LDHs have been found to have potential adsorbent capacities for SOx but a limited number of researches have been reported so far. In future they will replace conventional adsorbents on industrial scale[165].
It has been investigated that calcined LDHs show higher binding ability with cement matrix than their uncalcined precursor. Chloride ions in cement cause phase transition by ion exchange or adsorption process and hence affect the binding of cement, therefore chloride ions need to be removed from cement. LDHs can be used to perform this action[166].
Various health and environmental issues are created due to the presence of NOx such as acid rain formation, regional haze and increasing concentration of ozone. Therefore concentration of NOx needs to be controlled for daily life as well as industrial processes. Number of researches are present in literature for NOx sensors based on metal oxides but these sensors require high temperature in order to have considerable sensitivity. LDHs solve this issue and provide a sensor at room temperature due to their large surface area and flexible interlayer region[167]. It is clear that in near future layered double hydroxides will be used on large scale because they possess remarkable properties which decide their applications in various fields.
7 Conclusion
In this present review the structural aspects, common preparation methods, properties and important applications of various layered double hydroxide are summarized briefly. The exciting feature of these lamellar compounds is their composition which enables them to be utilized in multiple fields. The primary factors that play a role in the size and crystallinity of LDH particles are the methods through which they synthesized. Due to their potentially attractive properties, these compounds have replaced many conventional adsorbents and catalysts. This review studies the adsorption and catalytic applications for various layered double hydroxide containing different metal cations. By increasing the particle size of adsorbent, rate of adsorption decreases and particle size increases by thermally treating layered double hydroxides[109]. Mobility, solubility of dyes and equilibrium capacity can be influenced by temperature. Surface charge of LDHs, dissociation of functional groups and solubility of some dyes are affected by pH of solution. Furthermore, fascinating properties are observed in these compounds by modifying them after introducing ligands. The main objective of modification aims to enlarge their adsorption properties.