Isolating Al Surface\nSites in Amorphous Silica–Alumina\nby Homogeneous Deposition of Al<sup>3+</sup> on SiO<sub>2</sub> Nanoparticles

Well-defined amorphous silica–alumina (ASA) with\na relatively\nlow Al loading were synthesized by homogeneous deposition-precipitation\nof Al<sup>3+</sup> on SiO<sub>2</sub> nanoparticles to understand\nthe nature and formation of Brønsted acid sites (BAS). The amount\nof Al grafted relative to the silanol density was varied by variation\nof the size of SiO<sub>2</sub> nanoparticles, reflected by their surface\nareas between 90 and 380 m<sup>2</sup>·g<sup>–1</sup>.\nTwo sets of ASA were synthesized, one aiming at a SiOH/Al ratio of\n3, corresponding to the maximum amount of BAS represented by Al<sup>3+</sup> perturbation of SiOH groups, and the second one aimed at\nstudying the impact of Al dispersion by using a constant Al loading\n(Si/Al ≈ 103). <sup>27</sup>Al MAS NMR spectroscopy confirmed\nthat the first sample set only contained tetrahedral Al species. Calcination\ndid not affect the Al coordination. CO IR spectroscopy revealed that\nthe BAS concentration substantially varied in the 15–133 μmol·g<sup>–1</sup> range by varying the Al loading and the SiO<sub>2</sub> nanoparticle size. At equal Al loading, the BAS concentration increased\nfrom 15 to 46 μmol·g<sup>–1</sup> with increasing\nSiO<sub>2</sub> surface area. Less than 30% of all grafted Al sites\ngave rise to BAS, independent of the surface area and calcination\ntemperature. The ASA samples were screened for their catalytic performance\nin pyrolytic cracking of ultrahigh molecular weight polyethylene in\na thermogravimetric analysis apparatus. The performance in pyrolysis,\nas gauged by the temperature at which the weight loss rate was highest,\nincreased with the Brønsted acidity. The cracking temperature\ndecreased from 490 °C without a catalyst to 463 °C using\nthe most acidic ASA. At equal Al loading, the pyrolysis temperature\ndecreased with increasing surface area, indicating that, besides acidity,\ncracking also benefits from a higher surface area where the long polymer\nchains can adsorb. Compared to zeolite, ASA produced more liquid hydrocarbons\nand less coke.