Effect of phosphorus introduction strategy on the surface texture and structure of modified alumina

A gel method, which used aluminum nitrate and (NH4)2HPO4 as the Al and P sources respectively and ammonia as the precipitator, has been utilized to prepare P-alumina catalyst supports. The study was particularly focused on the influence of the step of phosphorus introduction during the gel procedure, the amount of incorporated phosphorus and the calcination temperature of alumina. The phosphorus introduction step markedly influenced the properties of P-doped alumina, such as pore structures, surface acidity and thermal stability. These differences attribute to not only the interaction between phosphorus species and alumina but also the location of phosphorus atoms. Different location of P atoms in alumina structure was disclosed by 27Al and 31P CP/MAS NMR spectra. NMR data reveals the phosphorus locates in the bulk of alumina with AlIV (tetrahedral aluminum ions)–O–P groups, when phosphorus is introduced with ammonia (G samples). For phosphorus depositing on the surface of alumina by impregnation (GI sample), the presence of AlVI (octahedral aluminum ions)–O–P on the surface is detected.