Immobilization of Candida Rugosa lipase on Ca/Kit-6 used as bifunctional biocatalysts for the transesterification of coconut oil to biodiesel

•Candida Rugosa lipase (CRL) was immobilized on Ca/Kit-6 via adsorption method.•CRL/0.5Ca/Kit-6 possessed bifunctional roles in coconut oil transesterification.•Coconut oil conversion and biodiesel yield reached 95 and 98% under optimal reaction condition.•Stepwise addition of methanol favored coconut oil transesterification.•Catalyst showed higher catalytic activity under basic than acidic condition. Immobilization of Candida Rugosa lipase (CRL) on Ca doped Kit-6 solid was investigated via the adsorption method. Effects of the experimental parameters, such as pH value, temperature, time, and Ca content on the adsorption of CRL on Ca/Kit-6 were studied. In the Ca doped Ca/Kit-6 materials, Ca2+ coexisted in two environments: Ca2+ in CaO nanoparticles and in Ca-O-Si in the framework of Kit-6. There existed three kind of oxygen species: (1) lattice oxygen in CaO and SiO2; (2) Oxygen in structural defects; and (3) non-stoichiometric surface oxygen in OH form. The 0.5Ca/Kit-6 showed the most CRL immobilization capacity and stability under the optimal adsorption condition, which was correlated with its greater amount of basicity resulting from its more non-stoichiometric surface oxygen and oxygen in structural defects as well as more Ca2+ incorporation in the Kit-6 framework. In the production of biodiesel via transesterification of coconut oil, the catalysts showed better catalytic performance under weak basic than acidic condition. Under the optimal reaction conditions (40 °C, pH 7.5, 200 rpm, oil to methanol ratio 1:3 with stepwise addition of methanol, and 0.1 g of CRL/0.5Ca/Kit-6), the coconut oil conversion and biodiesel yield catalyzed with CRL/0.5Ca/Kit-6 was 95 and 98%, respectively. 10wt% water in the reaction mixture promoted the coconut oil transesterification to biodiesel. The CRL/0.5Ca/Kit-6 catalyst possessed bifunctional roles of the enzymatic catalysis role from immobilized CRL and the basic catalysis role from CaO doping. The CRL adsorption modes, water influence and the reaction mechanism of coconut oil transesterification were discussed. [Display omitted] .