Site preference of exchanged alkylammonium ions in heulandite: single-crystal X-ray structure refinements

Single crystals (0.1-0.5 mm) of heulandite, Na0.96K0.09Ca3.54[Al8.62Si27.51O72] · n H2O from Nasik, India, placed in a Teflon autoclave filled with a 2-4 M NaCl solution for 13 weeks at 423 K, yielded Na-exchanged heulandite. CH33NH+-, C2H5NH3+-, n-C3H7NH3 +-, and (CH3)2NH2 +-exchanged heulandites were obtained by further treatment (four weeks at 423 K) with corresponding 1 M alkylammonium chloride solutions. Single-crystal X-ray structure refinements at 293 K of CH3NH3 +-, n-C3H7NH3 +-, and (CH3)2NH2 +-exchanged heulandites performed in space group C2/m yielded R1 values of 3.45%, 4.05%, and 3.99%, respectively. Due to partial Si,Al ordering in the framework C2H5NH3 +-exchanged heulandite was triclinic, space group C-1(R1 = 3.64%). Seven to eight disordered and partly occupied alkylammonium ions per formula unit could be located in the structural channels, except for propylammonium of which for steric and kinetic reasons only traces were incorporated. Infrared powder-spectra and energy-dispersive analyses with a scanning electron microscope confirmed the incorporation of alkylammonium ions. The nitrogen positions of the CH3NH3-, C2H5NH3-, and n-C3H7NH3-molecules corresponded well with the nitrogen positions of NH4 +-exchanged heulandite refined by Yang and Armbruster (1998), while the (CH3)2NH2-ions showed a different arrangement.