Sol-gel synthesis of alumina modified by phosphorus: a solid state NMR characterization study

A sol-gel method using aluminium tri(sec-butoxide) and orthophosphoric acid , H3PO4, as the Al and P sources respectively, 2-butanol as the solvent and 1,3-butanediol as the chelating agent has been extensively used to prepare P-alumina catalytic supports. The study was particularly focused on the in fluence of the step of phosphorus introduction during the sol-gel procedure , on the amount of incorporated phosphorus and on the temperatures of dryin g and calcination of the gels to obtain the final mixed oxides. In addition to the classical chemical composition determinations and measurements of s pecific surface area, characterisations were mainly performed by using soli d state Al-27 and P-31 MAS NMR. More precise determinations of the nature o f aluminium sites were also obtained by Al-27 MQMAS NMR. XRD was also used to a lesser extent. Poorly crystallised boehmite is present in the dried samples with aluminium mainly localised in octahedral sites whereas phosphorus is detected as mon omeric and polymeric phosphates whose proportions depend on the phosphorus content. For the highest P/Al ratio (P/Al=0.2) and when phosphorus is intro duced with the hydrolysis water, the NMR data reveal the presence of bridge d entities such as Al-tetra-O-P. After calcination at 500 degrees C, badly crystallised gamma-alumina is for med with octahedral, pentacoordinate and tetrahedral aluminium sites. For t he highest phosphorus loading, a new aluminium site corresponding to the pr esence of AlPO4 is observed. The values of the second order quadrupolar eff ect for each species depend on the preparation procedure and characterise t he degree of distortion of the aluminium sites. The drying temperature up to 200 degrees C does not modify the gel structur e whereas transformation of boehmite into alumina occurs above 350 degrees C and, for the highest phosphorus content, there is partial destruction of alumina to form aluminium phosphate when the temperature of calcination is increased. Such an increase also has a non-negligible influence on the spec ific surface area which, however, remains as high as 350 m(2) g(-1) after c alcination at 700 degrees C.