Dissertation > Excellent graduate degree dissertation topics show

A Modified Study on Lithium Ion-exchange of Low Silica X Molecular Sieve

Author: HengXiaoPing
Tutor: HuJiaWen
School: Hunan University
Course: Physical and chemical
Keywords: LiLSX molecular sieve Water ion-exchange Solid-state ion-exchange Characterization
CLC: TQ424
Type: Master's thesis
Year: 2013
Downloads: 0
Quote: 0
Read: Download Dissertation

Abstract

With the rapid development of the Pressure Swing Adsorption(PSA) technologyfor the separation of nitrogen and oxygen in recent years, excellent molecular sieveadsorbent is increasingly demanded. Presently, LiLSX prepared by Li+exchange withlow Si-Al ratio X molecular sieve is recognized as the most excellent adsorbent forPSA separation of N2and O2due to its high polarizability, high adsorption capacityfor nitrogen, large nitrogen/oxygen separation coefficient and easy desorption.However, with difficulty of Li+and its low utilization efficient, the production cost ofLiLSX is very high and makes the price of LiLSX expensive. Therefore, developinglow-cost technique for the preparation of LiLSX becomes increasingly important.The research aims of this thesis are to improve Li+ion-exchange degree andreduce the production cost. We use liquid(water) ion-exchange technique andsolid-state ion-exchange technique to obtain LiLSX. The main research contents andconclusions obtained are outlined as follows:(1) Preparation of LiLSX based on water ion-exchanged technique. First, theorthogonal experiments were designed to reveal the effect of the concentration oflithium chloride, ion-exchange temperature, ion-exchange time and solid-liquid ratioon Li+-exchange degree.Then the orthogonal experiment data were analyzed byextreme difference analysis and variance analytic method to find out the optimumcondition for Li+-exchange. For water Li+-exchange, the optimum ion-exchangecondition is listed as follows: LiCl1.0mol·dm3, ion-exchange time2hours,ion-exchange temperature90℃and solid-liquid ratio1:50. At optimum condition,when ion-exchange was repeated for3times, the ion-exchange degree of LiLSX canreach93.49%. In order to economize Li and because the initial sites of the NaLSX canbe relatively exchanged by Li+, the used LiCl concentration and solid-liquid ratiowere gradually increased for each ion-exchange time. It was found that theion-exchange degree of LiLSX can reach96%when the concentrations of lithiumchloride and solid/liquid ratio were set as0.4,0.8,1.0mol·dm3and1:20,1:30,1:40for the first, second and third ion-exchange time respectively. For all theion-exchange times, the ion-exchange temperature and ion-exchange time were set as90℃and2hours and in between each ion-exchange time, middle roasting process wasused to further improve the ion-exchange degree. Scanning electron microscope (SEM), X-ray diffraction (XRD), Fouriertransform infrared spectroscopy (FT-IR) and thermogravimetry-differential scanningcalorimetry (TG-DSC) were employed to characterize the apparent morphology,skeleton structure and stability of the samples before and after ion-exchange. SEMimages show that morphology of molecular sieve is not influenced after Li+ion-exchange. FT-IR also spectra show that the characteristic peaks of the frameworkof the molecular sieve are not changed upon Li+ion-exchange. XRD analyses indicatethat the intensity and position of the characteristic diffraction peaks are changed afterion-exchanged. Their positions shift to large angle along with increasing withintensity. TG-DSC curves indicate that the water adsorption capacity of the molecularsieve increase, while its stability reduce to some extent upon Li+ion-exchange.(2) Preparation of LiLSX based on solid-state ion-exchange technique.According to the dense monolayer dispersion model, the dispersion capacity of LiCldispersed on the NaLSX supports can be theoretically calculated. Then, the effect ofNaLSX and LiCl mass ratio, exchange temperature and exchange time on theion-exchange degree were studied. The results show that LiLSX can achieve88%Li+ion-exchange degree at optimized conditions of NaLSX and LiCl mass ratio1:0.8,exchange temperature550℃and exchange time10h.SEM, XRD, TG-DSC, FT-IR were employed to characterize the samples beforeand after exchange. SEM images show that the morphology of molecular sieve is notinfluenced after solid-state ion-exchange. FT-IR spectra show that the characteristicpeaks of the framework of molecular sieve are not affected after ion-exchange. XRDpatterns of the LiLSX samples clearly show crystalline peak of NaCl, which stronglyindicate that Li+was dispersed into the internal pores of the molecular sieve andexchanged Na+on the molecular sieve. As a result, sodium chloride was formed,which diffused out of the pores of the molecular sieve, condensed into crystallinephases and thus give XRD patterns for NaCl. TG-DSC curves indicate that thestability of LiLSX is reduced to some extent in comparison with the raw NaLSX.

Related Dissertations

CLC: > Industrial Technology > Chemical Industry > Reagents and the production of pure chemicals > Adsorbent
© 2012 www.DissertationTopic.Net Mobile