The synthesis and modification of aluminium phosphonates

Two synthetic approaches to the preparation of mixed aluminium phosphite-ph osphonate solids have been made. First, the hydrothermal reaction of alumin ium hydroxide (gibbsite) with mixtures of phosphorous and methylphosphonic acids under conditions that give microporous aluminium methylphosphonate-be ta (AlMePO-beta) when methylphosphonic acid alone is used, and secondly, th e reaction of AlMePO-beta with increasing amounts of molten phosphorous aci d. Under hydrothermal conditions there is no evidence that AlMePO-beta can be prepared with phosphite groups randomly replacing methylphosphonate grou ps. Rather, the products are dominated over the intermediate phosphite/phos phonate compositional range by a novel phase that is thought, on the basis of Al-27 and P-31 MAS NMR and FTIR spectroscopies, to contain differing amo unts of phosphite and methylphosphonate groups. Reaction of AlMePO-beta wit h levels of molten phosphorous acid at 40% or more of the methylphosphonate content gives mixtures of AlMePO-beta and a new phosphite phase, whereas r eaction using lower amounts of the molten acid leaves AlMePO-beta as the on ly X-ray visible phase. Extension of the melt method to the separate reacti on of gibbsite with methylphosphonic and phosphorous acids yields, respecti vely, single crystals of a new aluminium methylphosphonate [Al(O3PCH3)(HO3P CH3). H2O] and a known aluminium phosphite [Al-2(O3PH)(3). 4H(2)O], the str ucture of which had only been solved from powder diffraction data. Single c rystal diffractometry improved the accuracy with which the structural param eters of the phosphite are known and enabled structure solution of the new aluminium methylphosphonate [Pnma, a = 19.075(6) Angstrom, b = 5.117(2) Ang strom, c = 8.439(2) Angstrom], which is made up of layers that contain isol ated, octahedrally coordinated aluminium linked by methylphosphonate groups .