SYNTHESIS, CHARACTERIZATION, TRANSFORMATIONS, AND APPLICATION OF LANTHANUM ORGANOPHOSPHATES AND LANTHANUM PHOSPHATES PRODUCED FROM SINGLE SOURCE PRECURSORS

Abstract

The chemistry of alkyl phosphoric acids and their lanthanum salts were explored and was found to be a rich source of new chemical reactions and methods for materials synthesis. Lanthanum dialkyl phosphates, La(O2P(OR)2]3 (R=butyl and 2-ethylhexyl), were prepared by precipitation reactions of lanthanum nitrate with the corresponding sodium salts. Mixtures of lanthanum monoalkyl dialkyl phosphates, La[(O2P(OR)2]3-x(O3P(OR)]0.5x (R=butyl, x=1.44 and R=isopropyl, x=1), were prepared similarly using commercially available mixtures of the organic phosphoric acids. The latter compounds provide an avenue for the preparation of lanthanum phosphates with a variable lanthanum to phosphorus ratio. To further expand and control the La:P ratio, compounds with the formulas La[(O2P(OR)2]3-x(O2CR]x (R=isopropyl, x=1 and 2) were precipitated using mixtures of sodium di 2-ethylhexyl phosphate and 2-ethylhexanoic acid. In all case, the compounds were found to lose their organic content upon heating in air to temperatures around 300˚C. The lanthanum phosphates produced were amorphous except when the La:P ratio was close or equal to one in which instance nanocrystalline monazite (LaPO4) was formed. Upon heating to 900˚C, the 1La:1P and the 1La:3P produced crystalline LaPO4 and LaP3O9, respectively, while the compounds with intermediate La:P ratios phase-separated into mixtures of crystalline LaPO4 and LaP3O9. Thus, it may be concluded that lanthanum alkyl phosphate salts are highly successful single-source precursors for both the known (and useful) lanthanum phosphate phases and amorphous lanthanum phosphates that may be useful proton-conducting glasses. In this investigation, the LaP3O9 produced by pyrolysis of La(O2P(OR)2]3 (R= butyl) as well as CeP3O9 and AlP3O9 were shown to be good catalysts for hydrolysis of 3,5-diacetyl-1,4-dihydrolutidine. The structures of the lanthanide alkyl phosphates contain bridging or capping phosphate ions that would be expected to polarize the oxygen-alkyl bond so that the alkyl groups could be electrophilic allowing the compounds to perform as alkylating reagents. This hypothesis was tested by performing reactions with toluene, anisole, and phenol. It was found that such reactions did produce monoalkylated, dialkylated, and trialkylated aromatic rings. The reactions were not clean with a variety of other products being formed that demonstrated unusual reactions. Oxidative coupling of two toluene molecules and formation of butylbenzoate showed that the lanthanide alkyl phosphates were capable of acting as oxidants. Anisole was found to undergo trans-methylation to produce 2- methylanisole and phenol. Notably, Al[(O2P(OBu)2]1.56(O3P(OBu)]0.77 produced only products derived from oxidation of the butyl groups. Alkylation reactions of the parent alkyl phosphoric acids with aromatic systems was also explored and these were found to be reasonably good reagents for these reactions, producing monoalkylated and dialkylated reactions in good yield. The oxidizing ability observed for the metal salts suggested that the corresponding phosphoric acids could be useful reagents for conversion of aldehydes to carboxylic esters. This is indeed that case and the reaction also yields phosphorous acid or monoalkyl phosphites as the reduction product.