Optimization of the controllable crystal size of iron/zeolite nanocomposites using a Box–Behnken design and their catalytic activity

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Experimental conditions for the synthesis of an iron nanoparticle (NPs)–zeolite composite (hereinafter denoted as Fe/zeolite NPs) via sol–gel method were optimized using a Box–Behnken design to produce a high formic acid yield. The effects of various parameters, including weight ratio of starting materials (Fe and zeolite), volume of polyethylene glycol (PEG) as a surfactant, and calcination temperature, on controllable crystallite size, and the relationship between crystallite size and formic acid yield were studied. The crystal size, as the main parameter indicating formic acid yield, of Fe NPs was evaluated through polynomial regression. Results revealed that the optimum conditions for producing small Fe NPs based on the model were obtained at a weight ratio of Fe to zeolite of 62.5%, a PEG volume of 2 mL, and a calcination temperature of 500 °C. The experimental results (52.02 nm) versus the predicted results (58.30 nm) of the crystal size of Fe NPs under the optimum synthesis conditions were similar. Furthermore, 62.5% Fe/zeolite NPs with a crystal size of 52.02 nm produced the highest formic acid concentration from CO2 hydrogenation. Conversely, 100% Fe/zeolite NPs had a smaller crystal size but exhibited a remarkably lower reaction performance. This high ratio of Fe and zeolite contributed to the increased agglomeration of Fe particles. The zeolite surface became fully covered and subsequently reduced the reactant interaction on catalyst surfaces.