Direct hydrogenation of CO2 to aromatics via Fischer-Tropsch route over tandem K-Fe/Al2O3+H-ZSM-5 catalysts: Influence of zeolite properties

Abstract

Direct hydrogenation of CO2 to valuable aromatics using multifunctional catalysts is an attractive technology to produce low-carbon footprint chemicals. In this work, the relevant zeolite parameters driving the formation of total and BTEX aromatics from CO2 and H2 on tandem K-Fe/γ-Al2O3+H-ZSM-5 catalysts following the Fischer-Tropsch (FT)-mediated route were investigated. To this end, a set of H-ZSM-5 zeolites covering a wide range of physicochemical properties (density of Brønsted and Lewis acid sites, external acidity, crystallite size, and mesoporosity) was used, characterized by different techniques (ICP-OES, XRD, N2 physisorption, FTIR-pyridine, FTIR-2,6,-di-tert-butyl pyridine, XPS, 27Al MAS NMR, and electron microscopy), and evaluated in CO2 hydrogenation at 400 °C, 30 bar, H2/CO2 = 3.1, and GHSV of 4700 mL/(gFe-cat·h). At these conditions, high and stable CO2 conversions of 50 – 55% and low CO selectivity of ca. 10% were obtained for the Fe-based and tandem catalysts. The density and, to a certain extent, the strength of Brønsted acid sites were found the main parameters determining the selectivity of aromatics, reaching initial (TOS = 1.5 h) values of about 79% and 40% in liquid (C5+) and total hydrocarbons, respectively, for the catalyst based on the most acidic zeolite. Lewis acid sites associated to extraframework Al species (EFAL), by contrast, did not appear to play a relevant role in our conditions. Moreover, although a positive effect of the total amount and strength of Brønsted acid sites on the selectivity to the most valuable BTEX aromatics was inferred from our results, the external Brønsted acidity was the most influential factor in this case. Hence, a remarkable BTEX selectivity of 75% in aromatics was achieved for the zeolite previously submitted to a surface-passivation treatment by silylation.