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

Abstract

A sol–gel method using aluminium tri(sec-butoxide) and orthophosphoric acid, H ₃ PO ₄ , 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 influence of the step of phosphorus introduction during the sol–gel procedure, on the amount of incorporated phosphorus and on the temperatures of drying and calcination of the gels to obtain the final mixed oxides. In addition to the classical chemical composition determinations and measurements of specific surface area, characterisations were mainly performed by using solid state ²⁷ Al and ³¹ P MAS NMR. More precise determinations of the nature of aluminium sites were also obtained by ²⁷ Al 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 monomeric and polymeric phosphates whose proportions depend on the phosphorus content. For the highest P/Al ratio (P/Al = 0.2) and when phosphorus is introduced with the hydrolysis water, the NMR data reveal the presence of bridged entities such as Al _tetra –O–P.After calcination at 500 °C, badly crystallised γ-alumina is formed with octahedral, pentacoordinate and tetrahedral aluminium sites. For the highest phosphorus loading, a new aluminium site corresponding to the presence of AlPO ₄ is observed. The values of the second order quadrupolar effect for each species depend on the preparation procedure and characterise the degree of distortion of the aluminium sites.The drying temperature up to 200 °C does not modify the gel structure whereas transformation of boehmite into alumina occurs above 350 °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 specific surface area which, however, remains as high as 350 m ²  g ^–1 after calcination at 700 °C.