High-Temperature ceramic coatings with geopolymeric binders

High-temperature (HT) resistant coatings represent an updating subject of high industrial interest on account of their relevant applications (turbines, engines, aeronautic, ecc.). While many HT resistant products are known, not simple appears to satisfy the requirement of their high and stable adhesion on the support. The aim of this work was to develop novel HT resistant ceramic coatings based on silicon carbide and/or zirconium oxide, using geopolymeric resins as binders. Geopolymers show many advantages respect to organic polymers, first of all their high heat resistance and refractoriness. Moreover, they are fully inorganic, do not require organic solvents and are not off-gassing. During the geopolymerization step, the polymineral resin (alumino-silicate binders) is formed, acting as glue sticking together the unreacted Al-Si source materials and fillers (ceramic powders), forming the ceramic-geopolymer composite coatings. In order to optimize the geopolimeric binders, different raw materials have been tested (caolins, meta-kaolins and alumina/silica fine powders), while the alkali aqueous solution was KOH/K2SiO3, fixing the ratios SiO2/Al2O3 = 4 and SiO2/K2O = 2. Setting conditions, microstructural evolution as a function of the temperature and thermal evolution either in air or inert atmosphere were deeply investigated in order to set-up the best preparation conditions. HT resistant coatings were prepared by mixing the ceramic fillers (90 wt%) with geopolimeric binders, then applying the obtained mixture on ceramic substrates by brushing. After a first setting, coatings were stabilized by a thermal treatment in inert atmosphere at 1350?C and then the oxidation behaviour and adhesion level on the substrates were studied. A key role of new glass-ceramic phases formed during the thermal treatments has been evidenced

Influence of fine structural characteristics of VPO catalysts on the formation of maleic and phthalic anhydrides in the oxidation of n-pentane

We report on the influence of the stages of preparation of a vanadium phosphate on the selectivity to phthalic anhydride (PA) and maleic anhydride (MA) in n-pentane oxidation. The attention was mainly focused on the extent of structural defects observed in precursors and catalysts. Vanadium phosphate catalysts were obtained from precursors prepared by a two-step synthesis. In the first step VOPO4-mixed isobutanol-water intercalates, with the amount of isobutanol per VOPO4 molecule varying from 1.6 to 0.05, were prepared by precipitation from a solution containing vanadyl isobutoxide and H3PO4 and a carefully adjusted water content. In the second step the precursors were formed by reflux using two different procedures: (i) in an inert medium (n-octane) or (ii) in a reductive medium (isobutanol). Catalysts were obtained by treating the precursors under the reaction conditions for about 40 h. By such procedures VPO precursors and catalysts with bulk P/V atomic ratio equal to 1.05 and displaying widely different structural defects were obtained. Precursors and catalysts were characterised by elementary chemical analysis, carbon analysis, oxidation state of vanadium, BET, XRD, FTIR, and XPS. Long range and short range orders were considered. Results show that the parallel routes of the n-pentane oxidation into MA and PA require different structural features of the catalyst. The formation of phthalic anhydride demands an ordered structure while maleic anhydride could be formed on a highly defective VPO catalyst. It is suggested that this high structural order for PA formation is necessary to create the complex active structure to provide the concerted process of PA formation. (C) 1999 Academic Press

Role of homogeneous reactions in the control of the selectivity to maleic and phthalic anhydrides in the oxidation of n-pentane

Oxidation of n-pentane on vanadium phosphate catalysts was followed in a mixed heterogeneous-homogeneous regime and the effect of the modification of both reaction regimes on the ratio of phthalic (PA) to maleic anhydride (MA) was evaluated. The extent of the oxidation under homogeneous conditions was controlled either by varying the volume of the void section of the reactor or by adding methyl-ter-butylether (MTBE). Results show that the homogeneous reaction represses preferentially PA formation, increasing the MA/PA ratio. MTBE inhibits both the homogeneous and the heterogeneous reaction. Such behaviour is reversible, however recovery of the PA formation was substantially slower after stopping MTBE addition. It could be concluded that, in the study of the activation of n-pentane (and alkanes), the interaction of both (homogeneous and heterogeneous) processes must be taken into account