Synthesis Of Layered Double Hydroxides Using An Atom-Economic Reaction And Relational Kinetic Study

In this paper,a green method for the synthesis of layered double hydroxides (LDHs) was proposed based on an atom-economic reaction in order to avoid unwanted salts and excessive washing/filtering.The large family of LDH materials is derived from Mg/Al layered double hydroxide carbonate(MgAl-CO₃-LDHs).For the synthesis of MgAl-CO₃-LDHs,the possibility of this atom-economic reaction was thermodynamically analyzed,which indicates that the reaction is feasible in a wide range of temperature and pressure.The kinetic feature and reaction mechanism was studied.The reaction with infinite refux at ambient pressure follows a diffusion-controlled mechanism with instantaneous nucleation and two-dimensional nuclei growth represented by the first order Avrami-Erofe'ev kinetic model g(a)=-1n(1-a),while at high temperature and pressure it obeys a three-dimensional phase boundary reaction controlled mechanism represented by g(a)=1-(1-a)^1/3.As a result,pure MgAl-CO₃-LDH with high crystallization and particle size of 1～2μm was synthesized at 180℃under 1.0 MPa maintained by carbon dioxide.In order to evaluate the possibility of this green method for the synthesys of LDHs with anions other than CO₃^2-,preparation of Mg/Al layered double hydroxide borate (MgAl-B-LDHs) was studied.It was experimentally proved that the as-proposed atom-economic reaction is applicable to the preparation of MgAl-B-LDHs.As a result, pure MgAl-B-LDH with high crystallization and particle size of ca.500 nm was successfully synthesized at 220℃.Crystal structure analysis by XRD and Elemental analysis by ICP indicates that the MgAl-B-LDHs is intercalated by B₃O₃(OH)₄^- anion with oblique orientation and less B(OH)₄^- anion. Thermal decomposition is a significant property of LDH materials.The thermal behavior of Ni/Al layered double hydroxide nitrate(NiAl-NO₃-LDH) was studied by thermogravimetric-differential thermal analysis coupled with mass spectrometry (TG/DTA-MS).For the first time,kinetics and mechanism of the dehydroxylation and denitration,which take place simultaneously almost in a same region of temperatures and can not be distinguished from each other by the conventional thermal analysis method(DSC,TG et al.),were separately achieved by only using MS data sets recorded at different heating rates.The dehytroxylation follows the two-dimensional diffusion controlled mechanism with instantaneous nucleation represented by the first order Avrami-Erofe'ev equation with the average activation energy of ca.129.0 kJ·mol^-1,while the denitration obeys a three-dimensional diffusion-controlled mechanism represented by the Zhuralev-Lesokin-Tempelman equation with the average activation energy of ca.137.2 kJ·mol^-1.For many of their applications,the LDH materials are needed to be fabricated into a robust macroscopic form,and,as an important example,LDH films are desired for the application of LDHs as solid base catalysts.By mens of kinetic analysis,the famation mechanism of LDH films and the catalytic mechanism of different LDH films have been investigated.It was found that the nucleation rate of the MgAl-LDH film on the sulfonated polystyrene substrate follows a second-order exponential law and is lower than the rate of nuclei growth.A possible formation mechanism of MgAl-LDH films was proposed in order to interpret the nucleation and growth of the oriented dense LDH films firmly attatched on the surface of the sulfonated polystyrene substrate.The steps involved include:(â…°) transportation of M²⁺ and M³⁺ to the surface of the sulfonated polystyrene,(â…±) adsorption and enrichment of M²⁺ and M³⁺ on the active sites on the surface,(â…²) nucleation and oriented growth of MgAl-LDH films on the phase boundary,in which nucleation is the rate-determining step.The kinetic feature of self-condensation of acetone over differently oriented MgAl-LDH films to give diacetone alcohol(DAA) was evaluated.It was found that the catalytic mechanism highly depends on the structure of MgAl-LDH film.The reaction over MgAl-LDH film with a low density,high dispersion and oblique orientation immobilized on the surface of muscovite mica substrate(RLDH/MM) follows an interface reaction controlled mechanism represented by a reversible reaction,in which the forward reaction is second order with respet to the concentration of acetone and the backward reaction is first order with respect to the concentration of DAA,while it over MgAl-LDH film with a high density and perpendicular orientation immobilized on the surface of anodic aluminum oxide substrate(RLDH/AAO) obeys a one-dimensional diffusion-controlled mechanism represented by kinetic model g(a)=a².At the same time,the catalytic activity of RLDH/MM is higher than that of RLDH/AOO.These performances result from that highly dispersed thin LDH film with an oblique orientation is superior to dense LDH film with perpendicular orientation in the diffusion of acetone and diacetone alcohol (DAA).