Synthetic aluminosilicate based geopolymers - Second generation geopolymer HPLC stationary phases

A survey of existing stationary phases classified by the United States Pharmacopeia reveals that 120 groups of chromatographic supports mostly utilize silica-silane chemistry, polymeric materials along with some niche metal oxides. In this work, the synthesis and characterization of transition-metal free geopolymers as a new class of stationary phases for hydrophilic interaction liquid chromatography and normal phase separations is reported. The geopolymers were synthesized by reaction of synthetic aluminosilicate with potassium silicate (fumed silica dissolved in KOH) in a water-in-oil emulsion. For comparative purposes of peak shapes, a geopolymer from natural metakaolin was also synthesized. The geopolymers were examined by X-ray diffraction, energy dispersive spectroscopy, laser diffraction, and N₂-adsorption isotherms. This two-step approach gives spherical microparticles with surface area and pore size comparable to silica phases (150 m²/g and 120 Å respectively). Both synthetic aluminosilicates based and natural metakaolin based geopolymers occupy a unique "spot" in the HILIC selectivity chart when compared to 35 HILIC phases. An additional promising feature of geopolymers is high pH and temperature stability which are used to tune selectivity for small polar analytes. High pH separations are shown with carboxylic acids. Geopolymers also show mixed mode behavior in retention with ion-exchange properties in purely aqueous mobile phases. The separation of derivatized sugars is demonstrated and compared with porous graphitic carbon (Hypercarb™) as another pH-stable stationary phase.