Unravelling the effect of impurities on the methanol-to-olefins process in waste-derived zeolites ZSM-5

One of the main challenges in the next decades for the chemical industry is the valorization of low-grade industrial waste streams into valuable materials, as their increasing amount can have a major environmental impact. For example, in copper mining industry, waste flotation tailings can be valorized into solid acid catalysts, such as zeolites. Numerous studies have shown that modification of zeolites by metal ions has a positive effect in their catalytic performance. However, the presence of impurities in wastederived materials is usually disregarded. In this work, the influence of impurities in waste-derived zeolites ZSM-5 on the physicochemical properties and the catalytic performance in methanol-tohydrocarbons (MTH) reaction was investigated. Flotation tailings from copper industry, mainly consist of SiO2 as quartz, next to Ca, Mg and Fe species. By altering the pH in the SiO2 precipitation step, the impurities content in the waste-derived zeolites ZSM-5 could be controlled. Chemical analysis confirmed the presence of Fe, Ca and Mg in low concentrations in the waste-derived samples. UV-Vis Diffuse reflectance spectroscopy (DRS) and electron paramagnetic resonance (EPR) showed the presence of different Fe species in the waste-derived samples in the form of isolated Fe3+, oligomeric FexOy species and some extended Fe2O3-like clusters. Temperature programmed desorption (TPD) of NH3 and infrared (IR) spectroscopy measurements after CO and pyridine adsorption were used in order to probe the effect of the cations present in the acidity of the waste-derived materials. It was found that the zeolite material with a higher amount of metal impurities contained less Bronsted acid sites and more Lewis acid sites. Catalytic testing showed that the metal impurities had a positive influence on the overall catalyst stability and the yields towards ethylene and propylene were respectively 8% and 6% up. Meanwhile, operando UV-Vis DRS showed the influence of impurities on the catalytic mechanism, as its increase led to the formation of less hydrocarbon pool species. This study not only show that waste can be transformed into value-added materials, namely a zeolite-based catalyst material, but also that the impurities present in the waste can further improve the catalytic performance of the solid catalyst. (C) 2021 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Automated summary

One of the main challenges for the chemical industry in the future is to convert low-grade industrial waste into valuable materials, such as waste flotation tailings into solid acid catalysts. This study found that impurities in waste-derived zeolites can affect their physicochemical properties and catalytic performance.