Study On Dehumidification Performance Of Rare-earth Doped Molecular Sieve

The control of humidity is an important factor for improving habitationenvironment, product quality and manufacturing technology. The rotarydehumidification systems were applied widely because of their distinct advantages ofstrong dehumidifying ability, high efficiency, environmental protection and energyconservation. The adsorptive materials composing the dehumidification wheel was thekey unit which determined the performance of the adsorptive dehumidification system.Molecular sieve adsorbent as adsorptive materials composing thedehumidification wheel has widespread application for the excellent adsorptiveperformance under high temperature and low humidity condition. However, thedesorption temperature of molecular sieve is up to 250℃,so it needs to consume themassive heat energy.The research goal is to reduce desorption activation energy or desorptiontemperature of molecular sieve while its original adsorption performance has noobvious effect. The research provides highly application value and directions on theory.In this paper, using Lanthanum salt,neodymium salt as modifying agents, the rare earthion-doped molecular sieves were prepared by impregnation under acid condition.Impregnation temperature, impregnation time, and the aqueous solution acidity on theadsorptive performance of modified molecular sieve were investigated The composition andstructure of modified adsorbent were characterized using FT-IR analysis, X-ray diffraction(XRD), X-ray photoelectron spectroscopy (XPS), and porous medium analyzer, and thedesorption performance were evaluated by thermogravimetry (TG) and temperatureprogrammed desorption (TPD). The possible modification mechanism was preliminarilyproposed.The result of the static adsorption showed that the adsorptive performance ofmolecular sieve had a litter drop with the adding of rare earth, the saturated adsorption ratio ofmolecular sieve, La-doped molecular sieve and Nd-doped molecular sieve were 24.45%,22.68%, 22.48% respectively. FT-IR spectra showed that the rare earth doping had no effect onthe composition of molecular sieve. XRD spectra indicated that the crystallinity of modifiedmolecular sieve had a little decline. XPS analysis showed that some parts of sodium in themolecular sieve were replaced by La or Nd. The pore structure analysis showed that Brunauer-Emmett-Teller(BET) surface area, the total pore volume of the modifiedmolecular sieve decreased, while the average pore diameter increased.Differential thermogravimetry (DTG) and TPD curves indicated that the desorptionactivated energy of rare earth-doped molecular sieve is lower than that of molecular sieve,which was approved by the reduction of the peak tempreture in the DTG curve at themaximum thermogravimetric rate and inTPD curve in vapor desorption.DTG curves showed that the peak tempretures of molecular sieve, La-doped molecularsieve and Nd-doped molecular sieve were 161.9℃, 113.9℃and 114.8℃respectively, andTPD curves showed that their peak tempretures were 269.34℃, 165.87℃and 157.01℃respectively. By liner fit of the peak tempretures under different heating rates, desorptionactivated energies of molecular sieve, La-doped molecular sieve and Nd-doped molecularsieve were 167.71kJ/mol, 163.60 kJ/mol and 158.74 kJ/mol respectively.