Methods for preparing LiCa-LSX molecular sieves with mixed cations and applications thereof

Abstract

The present invention relates to a method for preparing LiCa-LSX molecular sieves with mixed cations and applications thereof. The method comprises the following steps; preparing CaLSX molecular sieves; replacing all Na.sup.+ in NaLSX molecular sieves with Ca.sup.2+ by multiple times of exchange with a solution containing Ca.sup.2+; changing CaLSX molecular sieves to LiCa-LSX molecular sieves by exchange with Li.sup.+, wherein a solution containing Li.sup.+ is used, and the exchanged sample is directly filtered in vacuum without washing and dried at normal temperature; and activating and pre-treating the sample to obtain LiCa-LSX molecular sieves with mixed cations. The LiCa-LSX molecular sieves can be used as selective adsorbents for N.sub.2 and O.sub.2 in oxygen production processes by PSA/VPSA. In the present invention, the loss of Li.sup.+ caused by the replacement of Li.sup.+ with H.sup.+ because of hydrolysis is avoided, the cost is reduced and the treatment time is short.

Claims

1. A method for preparing LiCa-LSX molecular sieves with mixed cations, comprising the following steps: step 1 of preparing CaLSX molecular sieves: during the preparation of CaLSX molecular sieves, replacing all Na.sup.+ in NaLSX molecular sieves with Ca.sup.2+ by multiple times of exchange with a solution containing Ca.sup.2+ kept at room temperature, washing to remove Ca.sup.2+ and any Cl.sup. adhered on the surface, and drying the filter cake obtained after washing to obtain CaLSX molecular sieves; step 2 of changing CaLSX molecular sieves to LiCa-LSX molecular sieves by exchange with Li.sup.+, with the mole percentage of Li.sup.+ in the resulting LiCa-LSX molecular sieves within a range from 2% to 5%, wherein during the exchange with Li.sup.+ to produce LiCa-LSX molecular sieves, a solution containing Li.sup.+ is used; the CaLSX molecular sieves are added in the solution containing Li.sup.+ in such an amount that the amount of starting material of Li.sup.+ is 0.15-0.3 times of that of Ca.sup.2+ contained in CaLSX raw powder; the exchange lasts for 0.5-2 hours, at room temperature; and an exchanged sample is directly filtered in vacuum without washing, and dried at normal temperature; and step 3 of activating and pre-treating the sample: heating, at a rate of 5-15 C./min, the sample which has been dried at normal temperature to a maximum temperature of 330-350 C., degassing for 0.5-3 hours, and naturally cooling to room temperature at the end of degassing, to obtain LiCa-LSX molecular sieves with mixed cations.

2. The method for preparing LiCa-LSX molecular sieves with mixed cations according to claim 1, wherein the solution containing Li.sup.+ is LiCl solution.

3. The method for preparing LiCa-LSX molecular sieves with mixed cations according to claim 1, further comprising a step 4 of performing isotherm tests.

4. The method for preparing LiCa-LSX molecular sieves with mixed cations according to claim 1, wherein the NaLSX molecular sieves refer to raw powder of molecular sieves having a SiO.sub.2/Al.sub.2O.sub.3 ratio of 2.0.

5. Applications, as selective adsorbents for N.sub.2 and O.sub.2, of the LiCa-LSX molecular sieves with mixed cations according to claim 1 in oxygen production processes by PSA/VPSA.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows cells of faujasite and distribution of sites;

(2) FIG. 2 shows adsorption isotherms of four samples for N.sub.2 at 25 C. and 1 atm;

(3) FIG. 3 shows adsorption isotherms of four samples for N.sub.2 at 50 C./1 atm;

(4) FIG. 4 shows adsorption isotherms of four samples for N.sub.2 at 70 C./1 atm;

(5) FIG. 5 shows adsorption isotherms of four samples for O.sub.2 at 25 C. and 1 atm;

(6) FIG. 6 shows adsorption isotherms of four samples for O.sub.2 at 50 C. and 1 atm; and

(7) FIG. 7 shows adsorption isotherms of four samples for O.sub.2 at 70 C. and 1 atm.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

(8) The present patent will be further described below by embodiments with reference to the accompanying drawings. However, the protection scope of the present patent is not limited to the specific implementations.

Embodiment 1

(9) 1. Change of Na-LSX to Ca-LSX

(10) Tank-type exchanges were performed in a glass beaker, each exchange lasted for 6 hours, and the temperature of the exchange solution was 25 C. The specific process was as follows.

(11) 50 g of NaLSX raw powder (dry basis) was added in 88 mL of CaCl.sub.2 solution having a concentration of 1.0 M, and then stirred by a magnetic stirrer. Six times of exchange were performed, and each exchange lasted for 6 hours. The mixture was filtered by a vacuum filter flask and washed with 500 mL of deionized water, and the filter cake was dried in an oven at 25 C.

(12) 2. Exchange with Mixed Ions

(13) Tank-type exchanges were performed in a glass beaker, each exchange lasted for 1 hour, and the temperature of the exchange solution was 25 C. The specific process was as follows.

(14) 1.5 g of the prepared CaLSX raw powder was added in each of two glass beakers, for exchange with LiCl solution having a concentration of 0.1 M. The amount of starting material of Li.sup.+ in the beakers was controlled to be 0.15-0.3 times of that of Ca.sup.2+ in the CaLSX raw powder. The mixture was magnetically stirred for 1 hour. The temperature of the exchange solution was kept at 25 C. and it was not required to adjust the pH value of the solution. The exchanged solid-liquid mixture was poured into a Buchner funnel and filtered in vacuum without washing with clean water, and the filter cake was dried for 24 hours at 25 C. The chemical compositions of the exchanged raw powder of molecular sieves with mixed cations were detected by an inductive coupling plasma optical emission spectrometer (ICP-OES). The result of detection indicates that the exchange degree of Li.sup.+ is 2.5% and 4.2% (mole percentage in the dry basis), and the molecular sieves were called Li.sub.2.5Ca.sub.46.75-LSX and Li.sub.4.2Ca.sub.45.9-LSX, respectively.

(15) 3. Pre-Treatment of Samples

(16) The samples would be dehydrated before isotherm tests. The specific operation was as follows.

(17) In-situ degassing/dehydration was performed by a degassing and activating device equipped in a Micromeritics ASAP 3020 adsorption instrument. The two molecular sieves with mixed cations, i.e., Li.sub.2.5Ca.sub.46.75-LSX and Li.sub.4.2Ca.sub.45.9-LSX, were dehydrated for 1 hour at 350 C. The molecular sieves Li-LSX and Ca-LSX were treated for 8 hours at 375 C. and 350 C., respectively. The samples were treated at a temperature rising rate of 10 C./min, and then naturally cooled to the room temperature. The short pre-treatment lime of the mixed cations was advantageous for preventing migration of Li.sup.+ to lower sites (SI, SII).

(18) 4. Adsorption Isotherm Tests and Result Analysis

(19) The adsorption isotherm tests were conducted by the Micromeritics ASAP 3020 adsorption instrument at 25 C., 50 C., and 70 C., and at 1 atm. The test results are shown in FIGS. 2-7.