Nanoparticled nanoporous alumina

Abstract

Mesoporous alumina is a very interesting material with broad applicability as a catalyst and catalyst support for various catalytically-active phases, which are employed in industry in a high number of large-scale technological processes. However methods to obtain the mesoporous alumina with ordered pore distribution and high surface area with nanosize particles are still being investigated due to the high tendency of fine alumina particles to agglomerate. In this research, mesoporous alumina nanoparticle (MAN) was synthesized by 2 methods. First, the sol-gel method, using various organic templates, i.e. cationic, non ionic and anionic surfactants. Four different types of templates were used to synthesize this material i.e. cetyltrimethylammonium bromide (CTABr), dimethyldistearylammonium chloride (DDAC), Triton X-114 and stearic acid, with aluminum isopropoxide as the aluminum source. Study showed that the cationic template is a better surfactant to obtain uniform γ-phase MAN as surface porosity the sample with CTABr has the narrowest peak distribution (9.4 nm) and a relatively high surface area (280 m2/g). The second method employed easy precipitation of aluminium acetate without the organic template, by using aluminium salts as aluminium source and ammonium acetate as precipitation agents. The effect of duration of ultrasonic treatment during the synthesis was carried out on the gel from both methods. The BET surface area of the obtained MAN is directly proportional to the duration of the ultrasonic treatment. FESEM and TEM results proved that the MAN nanoparticles were successfully obtained with worm-like morphology and particles size in the nano-region (10 – 50 nm). Thirty minutes of ultrasonic treatment was chosen as the optimum duration because it displayed the highest surface area and the narrowest pore size distribution of MAN samples regardless of methods of syntheses. In term of surface area, MAN obtained by sol-gel method gave 2-fold higher than those obtained by precipitation method. The MAN with the narrowest pore size distribution and relatively high surface area was then modified with Brönsted acid precursors, namely sulfuric acid, niobic acid, p-toluenesulfonic acid and also p-toluenesulfonic coated with octadecyltrichlorosilane. MAN incorporated with the four Brönsted acid precursors aforementioned were evaluated in Friedel-Crafts alkylation of resorcinol with MTBE. MAN, which showed only Lewis acidity was inactive in the reaction. Catalytic activity of the MAN nanoparticles incorporated with all four of the Brönsted acid precursors managed to produce 4-tert-butylresorcinol and 4,6-di-tert-butylresorcinol as the major and minor products respectively. MAN incorporated with sulfuric acid produced the highest amount of both the mono butylated resorcinol and dibutylated resorcinol among all the other catalysts. The reusability testing also showed that MAN incorporated with sulfuric acid maintained its catalytic performances, albeit at a reduced rate. This indicates that sulfuric acid is the better Brönsted acid precursor as it creates stable and higher strength of acidity in MAN nanoparticles.