Thermal transformation of Ba-exchanged A and X zeolites into monoclinic celsian

A comparative study of the thermal transformations of Ba-exchanged zeolites A, X and LSX

“…1 The essential literature concerning the synthesis of celsian, which was reviewed in previous papers, 2,3 reports that the occurrence of several drawbacks makes it quite difficult and expensive. Among these drawbacks the high temperatures and/or long reaction times of thermal treatments, 2,3 the use of alkoxides 4 or the set up of complex reacting systems, 5 may be cited.…”

“…Moreover most of the reported procedures of synthesis of celsian lead to the crystallisation of the hexagonal polymorph of celsian (hexacelsian). Actually it is the first polymorph to nucleate on account of the simpler crystal structure of the high-symmetry modification presenting a lower kinetic barrier to nucleation, 6 even though it is stable at temperatures higher than 1590 C. 7 A long term study 2,3,8,9 concerning the thermal transformation of Ba-exchanged zeolites into monoclinic celsian was undertaken. This study started from the work of Subramanian and coworkers 10À13 who crystallised alkaline-earth and alkaline framework aluminosilicates, which belong to the class of the feldspars, by thermal treatment of cation exchanged zeolites at temperatures slightly higher than 1000 C for some hours.…”

“…Moreover it was found that 6 h at 1100 C or 22 h at 1300 C were sufficient to transform samples of Ba-exchanged zeolite A, containing 0.58 and 0.20 meq/g Na residual amount, respectively, into fully monoclinic celsian. 2,3 Nevertheless the reactivity of the system appears strongly affected even by parameters other than Na residual amount. 2,3 Actually Ba-exchanged zeolite X was found to totally convert into monoclinic celsian by thermal treatment at 1550 C for 24 h. This result was reported by Hoghooghi and coworkers, 14 and in ref.…”

“…2,3 Nevertheless the reactivity of the system appears strongly affected even by parameters other than Na residual amount. 2,3 Actually Ba-exchanged zeolite X was found to totally convert into monoclinic celsian by thermal treatment at 1550 C for 24 h. This result was reported by Hoghooghi and coworkers, 14 and in ref. 2 for Baexchanged zeolite X samples bearing 0.61 and 0.26 meq/g Na residual amount, respectively.…”

Formation process of Sr-celsian from precursor zeolites

“…When a zeolite is heated, the crystal structure collapses, resulting in the formation of an amorphous phase and then the recrystallization to some nonzeolitic species of which the framework density is higher than that of zeolites. Due to these characterizations, zeolites as the precursor in the formation of aluminosilicate compounds have the following many advantages: 12) (1) The distribution of Al, Si, M, and O atoms in zeolites is in a well-mixed state due to the Löewenstein rule; (2) The desired composition of the starting material easily prepared by selecting zeolite with the required Si/Al ratio and by exchanging the cation in zeolite to the required metal cation; (3) Zeolites are synthesized at low temperatures around 100°C, and are widely used for many industrial and technological applications, and hence low-cost materials.…”