Sonochemical Synthesis of Silica-Supported Iron Oxide Nanostructures and Their Application as Catalysts in Fischer–Tropsch Synthesis

Abstract

The emphasis on climate change requires processes to be more efficient to minimize CO2 emissions, and nanostructured materials as catalysts could play a crucial role due to their high surface area per unit volume. Herein, we report the synthesis of silica microspheres (450–600 nm) using a modified Stober process, on which iron oxide clusters were deposited by sonolysis of iron pentacarbonyl to yield a nanostructured iron material (Si-Fe). A suite of spectroscopic techniques was used to characterize the synthesized materials. The BET surface area of freshly prepared Stober silica was 8.00 m2/g, and the Si-Fe material was 24.0 m2/g. Iron is commercially used as a Fischer–Tropsch (F–T) catalyst due to its low cost. However, catalyst attrition causes catalyst loss and lower product quality. In this study, the synthesized Si-Fe materials were evaluated for F–T synthesis to address these challenges. For comparison, two commercial materials, UCI (silica-supported micron-sized iron oxide) and BASF (unsupported nanosized iron oxide), were also evaluated. All three materials were first activated by pretreatment with either CO or synthesis gas (a mixture of CO and H2) for 24 h, then evaluated for quick screening in batch mode for F–T synthesis in a Parr batch reactor at three temperatures: 493 K, 513 K, and 533 K. The F–T data at 513 K showed that the CO-pretreated Si-Fe catalyst demonstrated lower CO2 (<0.5%), lower CH4 (<0.5%), and higher (>58%) C8–C20 selectivity (mol% C) to hydrocarbons, surpassing both reference catalysts. The temperature dependence data for Si-Fe: 17.4%, 58.3%, and 54.9% at 493 K, 513 K, and 533 K, respectively, showed that the hydrocarbon yield maximized at 513 K. The surface area increased to 27.9 m2/g for the CO-reduced Si-Fe catalyst after the F–T reaction at 513 K. The morphology and structural change of catalysts, before and after the F–T runs, were imaged. Of all the catalysts evaluated, the SEM–EDS data analysis showed the least carbon deposition on the CO-treated Si-Fe catalyst after the F–T reaction at 513 K and minimized CO2, a greenhouse gas. This could pave the way for selecting nanomaterials as F–T catalysts that effectively operate at lower temperatures and produce negligible CO2 by minimizing water-gas-shift (WGS) activity.