Layered Double Hydroxides And Their Application In The Hydrolysis Of Esters

Layered double hydroxides (LDHs) is formed by positively charged layers and interlayer components. The formula of LDHs can be generalized as [MxM′y(OH)2(x+y)]y+An-y/n·mH2O, where M is usually a bivalent metal cation, M′a trivalent metal cation, An- an interlayer anion with n negative charges, m is the number of co-crystallized water. The outstanding properties of LDHs are excellent selective anion-exchange capability and thermochemical behaviors, which have been applied in preparations of various material (including organic-inorganic composite), removals of hazardous anions, separations of organic geometric isomers, storage and controlled release of biomolecules, flame retarding materials, catalysts and catalyst supporters, and topochemical syntheses.In order to study the possible applications of anion exchange ability of layered double hydroxides to control chemical reactions, the alkaline hydrolysis of dimethyl terephthalate (DMT) in the presence of [LiAl2(OH)6]Cl has been investigated. The results show that (1) most of the interlayer Cl- of [LiAl2(OH)6]Cl is quickly replaced by OH- from the alkaline solution because LiAl2_host favors OH- more; (2) because of this substitution, the alkaline hydrolysis of dimethyl terephthalate (DMT) in the presence of LiAl2_Cl is much slower than the alkaline hydrolysis of DMT without [LiAl2(OH)6]Cl; (3) there is an anion exchange equilibrium among chloride, terephthalate (TA2-) and hydroxide anions, and this equilibrium make DMT hydrolyze quicker than the reaction between DMT and LiAl2_OH. Consequently, it is believed that the selective anion exchange chemistry of the LDH plays key roles of both storage and controlled release of active reactant, thus makes the reaction proceed in a controlled way; while the intercalation of the product anion hides the anionic product from other product and makes the reverse reaction proceed slower.In the process of recovery of terephthalic acid(TPA) by alkaline hydrolysis of Polyethylene terephthalate(PET), the purification of recycled TPA is very difficult because of its low solubility and vapor pressure. In this paper, the selective anion exchange ability of [Mg2Al(OH)6]NO3 is used to purify and separate the TA2- formed in the alkaline hydrolysis of PET under hydrothermal condition, then pure TPA is obtained through further treatment. The used LDH is treated by calcination-regeneration method to reconstruct [Mg2Al(OH)6]NO3, so LDH is recovered and reused.Some calcined-LDH(LDOs) can undergo rehydration when in contact with aqueous solution containing anions, regenerating the layered structure of LDH with these anions in the interlayer space, this process is known as the reconstruction process of LDOs. In this paper, based on the property of the reconstruction process, the reaction of PET with Mg2Al_LDO (calcined [Mg2Al(OH)6]2CO3) under hydrothermal condition is investigated, the result shows that PET hydrolyzed and Mg2Al_LDO is reconstructed to [Mg2Al(OH)6] 2TA, then pure TPA is obtained through further treatment. The used LDH can also be calcined to form Mg2Al_LDO, which can be reused.