Partially dealuminated heulandite produced by acidic REECl3 solution: A chemical and single-crystal X-ray study

Single crystals (0.1-0.5 mm) of natural heulandite from Nasik (India) were treated in 4 M NaCl solution at 423 K for 12 weeks, yielding almost fully N a-exchanged heulandite (composition: Na8.44Ca0.09K0.01[Al8.63Si27.37O72]. n H(2)O) This precursor phase was subsequently treated in a Teflon autoclave with 0.5 M rare-earth element (REE) solution (0.25 M ErCl3. 6H(2)O and 0.25 M LaCl3. 7H(2)O; pH of 2.8) for 15 weeks also at 423 K. Er and La in the z eolite were subsequently measured by inductively coupled plasma (ICP) mass spectrometry yielding only 656 ppm Er and 195 ppm La, whereas electron-micr oprobe (EMP) analyses indicated that the Na concentration decreased from or iginally 8.44 Na pfu to 0.25 Na pfu. The low REE content may be explained b y the relatively small free diameter of the channel windows and the large s ize of hydrated REE ions. The low Na concentration is caused by partial dea lumination of the tetrahedral framework where Si replaced some Al of the fr amework and Al migrated into the structural channel. Dealumination or more generally dissolution phenomena on the crystal surface occurring due to the acidic milieu in the exchange solution were observed as etch pits on scann ing-electron microscope (SEM) images. X-ray single-crystal data of REECl3-treated heulandite were collected at 10 0 and 293 K and at 100 K after partial dehydration at 323 and 378 K. Struct ure refinement using all data sets suggested the presence of low concentrat ions of octahedrally coordinated Al3+ that was dissolved from the framework and incorporated into the channels. T-O distances in the framework, correc ted for rotational disorder, are significantly shortened compared with the Na-exchanged precursor heulandite. This indicates that REECl3-treated heula ndite has a significantly lower Al concentration in the framework than the Na-exchanged precursor phase.