Influence of preparation method on ZnO/Al2O3 composite: Physicochemical attributes and catalytic performance toward transesterification of waste cooking oil

Utilization of low-grade feedstocks, such as non-edible and waste oils, is considered one of the possible ways to achieve sustainability in biodiesel production and in this respect, zinc oxide (ZnO) is found to have significant important as a heterogeneous catalyst for biodiesel synthesis from high free fatty acid (FFA) oil due to its high activity, amphoteric property and nontoxicity. Meanwhile, there is a dearth of information on the impact of preparation method on catalytic activity of supported ZnO catalyst. Herein, physicochemical attributes of alumina supported zinc oxide (ZnO/Al 2 O 3 ) catalysts prepared by three different techniques (coprecipitation, hydrothermal and impregnation) were examined, and catalytic activities of the composites in transesterification of high FFA waste cooking oil (WCO) were investigated. The findings revealed a clear influence of preparation method on properties of the composite catalysts, including textural, thermal stability, crystallographic structure, active sites density, morphological-elemental composition, and molecular structure and crystallinity. As a result, ZnO/Al 2 O 3 composites exhibited different performances toward conversion of high FFA oil to fatty acid methyl esters (FAME) via transesterification process. The combined influence of catalyst type and reaction parameters on FAME content was studied using Taguchi optimization approach. FAME contents of 95.2%, 89.1% and 84.7% were respectively offered by ZnO/Al 2 O 3 composites prepared by impregnation, hydrothermal and coprecipitation methods at 65 °C for 4 h with 12:1 methanol/WCO molar ratio and 3.0 wt% catalyst dosage. The obtained findings suggested that surface and textural properties, rather than morphological and structural features, were the significant factors in determining performance of the composite catalysts. • ZnO/Al 2 O 3 composite was synthesized via coprecipitation, hydrothermal and impregnation methods. • The three prepared composites (ZAC, ZAH and ZAI) were tested as catalysts for FAME production from waste cooking oil. • FAME contents of 84.7%, 89.1% and 95.2% were offered by ZAC, ZAH and ZAI, respectively. • ZAI composite repeatedly used for 6 cycles and offered biodiesel that conformed to ASTM standard.

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