Layered double hydroxides
Encyclopedia
Layered double hydroxides (LDH) comprise an unusual class of layered materials with positively charged layers and charge balancing anions located in the interlayer region. This is unusual in solid state chemistry: many more families of materials have negatively charged layers and cations in the interlayer spaces (e.g. kaolinite, Al2Si2O5(OH)4).
LDHs are commonly represented by the formula [Mz+1-xM3+x (OH)2]q+(Xn-)q/n·yH2O.
Most commonly, z = 2, and M2+ = Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+ or Zn2+; hence q = x. Pure phases have been shown to exist over the range 0.2 ≤ x ≤ 0.33. However, values of x have been reported in the range 0.1 ≤ x ≤ 0.5.
Also possible is z = 1, where M+ = Li+ and M3+ = Al3+. In this case q = 2x - 1. The latter family of materials can be described by the formula [LiAl2(OH)6]X∙yH2O (LiAl2-X)). X represents a generic anion and the value of y is normally found to be between 0.5 – 4.
LDHs may be formed with a wide variety of anions X (e.g. Cl, Br, and NO3).
The anions located in the interlayer regions can generally be easily replaced. A wide variety of anions may be incorporated, ranging from simple inorganic anions (e.g. CO32-) through organic anions (e.g. benzoate, succinate) to complex biomolecules, including DNA. This has led to an intense interest in the use of LDH intercalates for advanced applications. Drug molecules such as ibuprofen
may be intercalated; the resulting nanocomposite
s have potential for use in controlled release systems, which could reduce the frequency of doses of medication needed to treat a disorder. Further effort has been expended on the intercalation of agrochemicals, such as the chlorophenoxyacetates, and important organic synthon
s, such as terephthalate and nitrophenols. Agrochemical intercalates are of interest because of the potential to use LDHs to remove agrochemicals from polluted water, reducing the likelihood of eutrophication
.
LDHs have been shown to have remarkable shape selective intercalation properties. For instance, reacting LiAl2-Cl with a 50:50 mixture of terephthalate (1,4-benzenedicarboxylate) and phthalate (1,2-benzenedicarboxylate) results in intercalation of the 1,4-isomer with almost 100 % preference. This means that there is significant potential to use LDHs to separate isomeric mixtures. The selective intercalation of ions such as benzenedicarboxylates and nitrophenols has importance because these are produced in isomeric mixtures from crude oil residues, and it is often desirable to isolate a single form, for instance in the production of polymers.
Hydrotalcite
is a layered double hydroxide of general formula
Mg6Al2(CO3)(OH)16·4(H2O), whose name is derived from its resemblance with talc and its high water content. Hydrotalcite has been studied as potential getter
for iodide in order to scavenge the long-lived 129I
(T1/2 = 15.7 million years) (and also other fission product
s such as 79Se
(T1/2 = 295 000 years) and 99Tc
, (T1/2 = 211 000 years)) present in spent nuclear fuel to be disposed under oxidising conditions in volcanic tuff at the Yucca Mountain nuclear waste repository. Unfortunately carbonate easily replaces iodide in its interlayer. Another difficulty arising in the quest of an iodine getter for radioactive waste is the long-term stability of the sequestrant
that must survive over geological time scales.
LDHs are well known for their anion exchange
properties. The formation of LDHs can be identified easily by X-Ray Diffraction
studies. Sharp peaks indicates that the material obtained is ordered. The broad asymmetric peaks indicated that the material is disordered.
Hydrotalcite is also used as an antacid
.
LDHs are commonly represented by the formula [Mz+1-xM3+x (OH)2]q+(Xn-)q/n·yH2O.
Most commonly, z = 2, and M2+ = Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+ or Zn2+; hence q = x. Pure phases have been shown to exist over the range 0.2 ≤ x ≤ 0.33. However, values of x have been reported in the range 0.1 ≤ x ≤ 0.5.
Also possible is z = 1, where M+ = Li+ and M3+ = Al3+. In this case q = 2x - 1. The latter family of materials can be described by the formula [LiAl2(OH)6]X∙yH2O (LiAl2-X)). X represents a generic anion and the value of y is normally found to be between 0.5 – 4.
LDHs may be formed with a wide variety of anions X (e.g. Cl, Br, and NO3).
The anions located in the interlayer regions can generally be easily replaced. A wide variety of anions may be incorporated, ranging from simple inorganic anions (e.g. CO32-) through organic anions (e.g. benzoate, succinate) to complex biomolecules, including DNA. This has led to an intense interest in the use of LDH intercalates for advanced applications. Drug molecules such as ibuprofen
Ibuprofen
Ibuprofen is a nonsteroidal anti-inflammatory drug used for relief of symptoms of arthritis, fever, as an analgesic , especially where there is an inflammatory component, and dysmenorrhea....
may be intercalated; the resulting nanocomposite
Nanocomposite
A nanocomposite is as a multiphase solid material where one of the phases has one, two or three dimensions of less than 100 nanometers , or structures having nano-scale repeat distances between the different phases that make up the material...
s have potential for use in controlled release systems, which could reduce the frequency of doses of medication needed to treat a disorder. Further effort has been expended on the intercalation of agrochemicals, such as the chlorophenoxyacetates, and important organic synthon
Synthon
A synthon is a concept in retrosynthetic analysis. It is defined as a structural unit within a molecule which is related to a possible synthetic operation. The term was coined by E.J. Corey...
s, such as terephthalate and nitrophenols. Agrochemical intercalates are of interest because of the potential to use LDHs to remove agrochemicals from polluted water, reducing the likelihood of eutrophication
Eutrophication
Eutrophication or more precisely hypertrophication, is the movement of a body of water′s trophic status in the direction of increasing plant biomass, by the addition of artificial or natural substances, such as nitrates and phosphates, through fertilizers or sewage, to an aquatic system...
.
LDHs have been shown to have remarkable shape selective intercalation properties. For instance, reacting LiAl2-Cl with a 50:50 mixture of terephthalate (1,4-benzenedicarboxylate) and phthalate (1,2-benzenedicarboxylate) results in intercalation of the 1,4-isomer with almost 100 % preference. This means that there is significant potential to use LDHs to separate isomeric mixtures. The selective intercalation of ions such as benzenedicarboxylates and nitrophenols has importance because these are produced in isomeric mixtures from crude oil residues, and it is often desirable to isolate a single form, for instance in the production of polymers.
Hydrotalcite
Hydrotalcite
Hydrotalcite is a layered double hydroxide of general formula Hydrotalcite is a layered double hydroxide of general formula Hydrotalcite is a layered double hydroxide of general formula (Mg6Al2(CO3)(OH)16·4(H2O) whose name is derived from its resemblance with talc and its high water content. It is...
is a layered double hydroxide of general formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
Mg6Al2(CO3)(OH)16·4(H2O), whose name is derived from its resemblance with talc and its high water content. Hydrotalcite has been studied as potential getter
Getter
A getter is a deposit of reactive material that is deliberately placed inside a vacuum system, for the purpose of completing and maintaining the vacuum. When gas molecules strike the getter material, they combine with it chemically or by adsorption...
for iodide in order to scavenge the long-lived 129I
Iodine-129
Iodine-129 is long-lived radioisotope of iodine which occurs naturally, but also is of special interest in the monitoring and effects of man-made nuclear fission decay products, where it serves as both tracer and potential radiological contaminant....
(T1/2 = 15.7 million years) (and also other fission product
Fission product
Nuclear fission products are the atomic fragments left after a large atomic nucleus fissions. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons and a large release of energy in the form of heat , gamma rays and neutrinos. The...
s such as 79Se
Selenium-79
Selenium-79 is a radioisotope of selenium present in spent nuclear fuel and the wastes resulting from reprocessing this fuel. It is one of only 7 long-lived fission products. Its yield is low as it is near the lower end of the mass range for fission products...
(T1/2 = 295 000 years) and 99Tc
Technetium-99
Technetium-99 is an isotope of technetium which decays with a half-life of 211,000 years to stable ruthenium-99, emitting soft beta rays, but no gamma rays....
, (T1/2 = 211 000 years)) present in spent nuclear fuel to be disposed under oxidising conditions in volcanic tuff at the Yucca Mountain nuclear waste repository. Unfortunately carbonate easily replaces iodide in its interlayer. Another difficulty arising in the quest of an iodine getter for radioactive waste is the long-term stability of the sequestrant
Sequestrant
A sequestrant is a food additive whose role is to improve the quality and stability of the food products. Sequestrants form chelate complexes with polyvalent metal ions, especially copper, iron and nickel, which serve as catalysts in the oxidation of the fats in the food...
that must survive over geological time scales.
LDHs are well known for their anion exchange
Ion exchange
Ion exchange is an exchange of ions between two electrolytes or between an electrolyte solution and a complex. In most cases the term is used to denote the processes of purification, separation, and decontamination of aqueous and other ion-containing solutions with solid polymeric or mineralic 'ion...
properties. The formation of LDHs can be identified easily by X-Ray Diffraction
X-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
studies. Sharp peaks indicates that the material obtained is ordered. The broad asymmetric peaks indicated that the material is disordered.
Hydrotalcite is also used as an antacid
Antacid
An antacid is a substance which neutralizes stomach acidity.-Mechanism of action:Antacids perform a neutralization reaction, increasing the pH to reduce acidity in the stomach. When gastric hydrochloric acid reaches the nerves in the gastrointestinal mucosa, they signal pain to the central nervous...
.
See also
- FougeriteFougeriteFougerite is a recently discovered representative of clay minerals, chemically and structurally related to so-called green rust, hydrotalcite-like minerals and other layered double hydroxides. The formula of the mineral is complex due to structural variabilities, as is for green rust, thus...
, an iron-bearing LDH mineral similar to green rust. - HydrotalciteHydrotalciteHydrotalcite is a layered double hydroxide of general formula Hydrotalcite is a layered double hydroxide of general formula Hydrotalcite is a layered double hydroxide of general formula (Mg6Al2(CO3)(OH)16·4(H2O) whose name is derived from its resemblance with talc and its high water content. It is...
, a LDH mineral.