MoO2(acac)2-encapsulated in TS-1 zeolite catalyst for CO2 coupling with olefins to cyclic carbonates

1.1.2 Preparation of Mo Encapsulated in TS-1 Zeolite(Mo@TS) The Mo@TS zeolite was synthesized via a hydrothermal route [[i]]. Tetraethyl orthosilicate(TEOS), tetrabutyl titanate(TBOT), ammonium molybdate tetrahydrate, and tetrapropylammonium hydroxide(TPAOH) were used as the silicon source, titanium source, molybdenum source, and organic structure-directing agent, respectively, for the preparation of the TS-1 zeolite. Notably, the synthesis was conducted without any mineralizer. The specific synthesis procedure was as follows: A certain amount of TEOS and TBOT were thoroughly mixed and stirred continuously at room temperature for 1 hour. Subsequently, an aqueous solution of ammonium molybdate tetrahydrate was slowly added dropwise into the above mixture, followed by further mixing under vigorous stirring for 0.5 hours. Then, the resulting mixture was slowly added dropwise into an aqueous TPAOH solution to form a homogeneous gel. Finally, the resulting gel was stirred at 60 °C for 3 hours for aging, then transferred to a Teflon-lined autoclave for hydrothermal crystallization at 175 °C under rotation at 20 rpm for 24 hours. The molar composition of the initial gel was SiO2: TiO2: TPAOH: H2O: MoO3 = 1.0 : 0.04: 0.50: 33: x, where x represents the molar amount of MoO3 used. The Mo loading ratio was calculated based on the total amount of SiO2 and TiO2 in the initial gel. Values of x were set to 1.05×10⁻⁴, 1.32×10⁻⁴, 1.59×10⁻⁴, and 1.86×10⁻⁴ to prepare catalysts with different Mo contents. After crystallization, solid-liquid separation was performed, and the zeolite was thoroughly washed with distilled water until the filtrate reached neutrality. The zeolite samples were first dried overnight at 80 °C, and finally calcined in a muffle furnace at 550 °C for 8 hours to remove the organic template. A series of Mo@TS zeolites with different encapsulation contents were ultimately obtained, designated as xMo@TS (where x = 1.5, 2.5, 3, and 3.5). For comparison, a TS-1 zeolite without the addition of the Mo precursor was also prepared in this study. The synthesis steps, post-treatment, and calcination conditions for this reference sample were consistent with those of the xMo@TS series zeolites, and it was designated as TS-1. 1.1.3 Preparation of MoO2(acac)2 Encapsulated in TS-1 Catalyst(Moac@TS) A certain amount of xMo@TS zeolite (where x = 1.5, 2.5, 3, and 3.5) was dispersed in a 1.0 wt.% ammonia solution and stirred at room temperature for 5 minutes to form a homogeneous suspension. Then, a specific amount of acetylacetone (acac) was added dropwise into the suspension, followed by continued mixing for 20 minutes to ensure sufficient coordination between acac and the Mo species in the zeolite. Finally, a 0.5 wt.% nitric acid solution was added dropwise to adjust the pH of the suspension to neutral. After the suspension cooled, the solid sample was collected by high-speed centrifugation and thoroughly washed sequentially with deionized water and ethanol. The resulting solid zeolite catalyst was dried at 120 °C to obtain the encapsulated structured Moac@TS catalyst, designated as xMoac@TS, where x = 1.5, 2.5, 3, and 3.5.