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High Selectivity to Aromatics by a Mg and Na Co-modified Catalyst in Direct Conversion of Syngas.

ABSTRACT: The demand for aromatics, especially benzene, toluene, and xylene, has been increased in recent years as the crucial feedstocks of coatings and pharmaceutical industry. In this work, a modified Fischer–Tropsch synthesis (FTS) catalyst FeNaMg was fabricated via a sol-precipitation method and integrated with an HZSM-5 aromatization catalyst for the aromatics synthesis from syngas by a one-step process. Syngas was first converted to lower olefins as intermediates on the active component of the FeNaMg catalyst followed by aromatization on zeolite. Different characterization approaches, such as BET, XRD, XPS, hydrogen temperature-programmed reduction, temperature-programmed desorption of CO, TG, and SEM, revealed that Mg efficiently optimized physicochemical properties of the Fe-based catalyst by generating a MgFe2O4 spinel structure. Further investigation demonstrated that the MgFe2O4 spinel structure could increase the syngas adsorption area, facilitating the reduction and carburization of the Fe phase, while Mg decreased CO2 selectivity (31.26 to21%) by restraining the water–gas shift reaction and improved the utilization efficiency of carbon. At the same time, alkali metal Na changed the surface electronic environment of the FTS catalyst to enhance CO adsorption as an electronic promoter, which suppressed methane formation by restraining over hydrogenation. Therefore, the synergism that existed between Mg and Na during the reaction escalated the CO conversion and aromatics selectivity to 96.19 and 51.38%, respectively.

SUBMITTER: Yang S

PROVIDER: S-EPMC7254791 | biostudies-literature | 2020 May

REPOSITORIES: biostudies-literature

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Project description:The effect of the phase transformation of a FePO4 catalyst material from the tridymite-like (tdm) FePO4 to the α-domain (α-Fe3(P2O7)2) during the direct selective oxidation of methane to methanol was studied using oxidant species O2, H2O and N2O. The main reaction products were CH3OH, carbon dioxide and carbon monoxide, whereas formaldehyde was produced in rather minute amounts. Results showed that the single-step non-syngas activation of CH4 to oxygenate(s) on a solid FePO4 phase-specific catalyst was influenced by the nature of the oxidizer used for the CH4 turnover. Fresh and activated FePO4 powder samples and their modified physicochemical surface and bulk properties, which affected the conversion and selectivity in the partial oxidation (POX) mechanism of CH4, were investigated. Temperature-programmed re-oxidation (TPRO) profiles indicated that the type of moieties utilised in the procedures, determined the re-oxidizing pathway of the reduced multiphase FePO4 system. Mössbauer spectroscopy measurements along with X-ray diffraction (XRD) examination of neat, hydrogenated and spent catalytic compounds, demonstrated a variation of the phosphate into a mixture of crystallites, which depended on operating process conditions (for example time-on-stream). The Mössbauer spectra revealed the change of the initial ferric orthophosphate, FePO4 (tdm), to the divalent metal form, iron(ii) pyrophosphate (Fe2P2O7); thereafter, reactivity was governed by the interaction (strength) with individual oxidizing agents. The Fe3+ ↔ Fe2+ chemical redox cycle can play a key mechanistic role in tailored multistep design, while the advantage of iron-based heterogeneous catalysis primarily lies in being inexpensive and comprising non-critical raw resources. When compared to the other catalysts reported in the literature, the FePO4-tdm phase catalysts showed in this work exhibited a high activity towards methanol i.e., 12.3 × 10-3 μmolMeOH gcat h-1 using N2O as an oxidant. This catalyst also showed a high activity with O2 as an oxidant (5.3 × 10-3 μmolMeOH gcat h-1). Further investigations will include continuous reactor unit engineering optimisation.

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High Selectivity to Aromatics by a Mg and Na Co-modified Catalyst in Direct Conversion of Syngas.

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