Isolating Al Surface Sites in Amorphous Silica-Alumina by Homogeneous Deposition of Al³⁺ on SiO₂ Nanoparticles

Well-defined amorphous silica-alumina (ASA) with a relatively low Al loading were synthesized by homogeneous deposition-precipitation of Al³⁺ on SiO₂ nanoparticles to understand the nature and formation of Brønsted acid sites (BAS). The amount of Al grafted relative to the silanol density was varied by variation of the size of SiO₂ nanoparticles, reflected by their surface areas between 90 and 380 m²·g^-1. Two sets of ASA were synthesized, one aiming at a SiOH/Al ratio of 3, corresponding to the maximum amount of BAS represented by Al³⁺ perturbation of SiOH groups, and the second one aimed at studying the impact of Al dispersion by using a constant Al loading (Si/Al ≈ 103). ²⁷Al MAS NMR spectroscopy confirmed that the first sample set only contained tetrahedral Al species. Calcination did not affect the Al coordination. CO IR spectroscopy revealed that the BAS concentration substantially varied in the 15-133 μmol·g^-1 range by varying the Al loading and the SiO₂ nanoparticle size. At equal Al loading, the BAS concentration increased from 15 to 46 μmol·g^-1 with increasing SiO₂ surface area. Less than 30% of all grafted Al sites gave rise to BAS, independent of the surface area and calcination temperature. The ASA samples were screened for their catalytic performance in pyrolytic cracking of ultrahigh molecular weight polyethylene in a thermogravimetric analysis apparatus. The performance in pyrolysis, as gauged by the temperature at which the weight loss rate was highest, increased with the Brønsted acidity. The cracking temperature decreased from 490 °C without a catalyst to 463 °C using the most acidic ASA. At equal Al loading, the pyrolysis temperature decreased with increasing surface area, indicating that, besides acidity, cracking also benefits from a higher surface area where the long polymer chains can adsorb. Compared to zeolite, ASA produced more liquid hydrocarbons and less coke.