Charge reduction of zeolite Y through exchange with lanthanum ions: Intracrystalline oxide and hydroxide formation

Abstract

La 3+ exchanged zeolite Y (68% exchanged or 13 ions/u.c.) heated to temperatures between 150° and 750°C for 24 h results in irreversible lanthanum migration into the small cavities. The residual Na + cation exchange capacity (CEC) obtained after thorough Na +-exchange, decreases with pretreatment temperature from 2.7 (at 150°C) to 1.9 (at 750°C) meq g −1 backbone. The apparent lanthanum charge decreases to 2.5 (at 400°C). Irreversible lanthanum-oxide or oxihydroxide formation occurs upon heating beyond 200°C. The residual charge in the small cages, obtained from the difference between the Cs + and Na +CEC, decreases from 1.5 meq g −1 backbone (Na +CEC original Y = 4.7 meq g −1 backbone) to about 0.2 meq g −1 backbone (at Na +CEC of 3 meq g −1 backbone). La 3+ reexchange of the samples heated to 300°C and reheated to different temperatures results in a maximum irreversibly blocked lanthanum content of 16 atoms/u.c, with an apparent charge of 2 and corresponding to a full occupation of all cubooctahedra by a lanthanum oxi(hydroxi)de complex. The data demonstrate that lanthanum incorporation in the small cages offers the possibility of preparing zeolites of variable exchangeable charge which is almost exclusively located in the big cavities.