Cost-effective transition metal catalysts derived from metal-organic frameworks (MOFs) for catalytic oxidation of toluene

Abstract

Toluene, as one of the volatile organic compounds (VOCs) known for their carcinogenic, toxic, and photochemically reactive properties, poses serious health risks to human beings and contributes to generating secondary pollutants that harm the environment. This led to its categorization as a priority pollutant in numerous countries. Consequently, controlling its emissions becomes imperative. Various methods have been developed for toluene abatement, and catalytic oxidation was found as a promising and efficient approach. This method facilitates the conversion of toluene into environmentally friendly products without introducing additional pollutants into the environment. Although noble metal catalysts are highly effective for toluene oxidation, their practical use is restricted due to high costs and limited availability. Transition metal catalysts present a promising and more affordable alternative. Non-noble metal oxides have been found to exhibit the highest catalytic activity for toluene oxidation especially when they are supported. Metal-organic frameworks (MOFs), distinguished by their ordered crystalline structure and high specific surface area, offer support advantages over conventional mesoporous materials like Zeolite and activated carbon (AC). Thus, MOFs have gained attention as versatile precursors for producing functional materials suitable for catalytic reactions.

CeO2 and MnO2 are promising transition metal-based catalysts for toluene oxidation due to their redox properties, high natural abundance, environmental friendliness, and non-toxic nature. However, pure CeO2 often fails to deliver optimal catalytic performance owing to its dense structure, and physicochemical properties; pure MnO2 catalyst struggles to meet the catalytic demands owing to its low specific surface area and poor thermal stability; and only a few studies have been reported on their modifications and improvements for their enhanced catalytic performance. Additionally, Iron-based materials are considered promising catalysts for VOCs degradation owing to their affordability and non-toxicity. Nevertheless, the restricted catalytic efficiency of conventional iron oxide catalysts poses a challenge to their broader use in industrial applications and few reports have been reported on their modifications. Thus, there is a need to design modified CeO2, MnO2, and FeOx catalysts to improve their catalytic efficiency in VOC oxidation and increase their stability and performance at lower temperatures, thereby overcoming their practical challenges in application. This research aims to develop novel transition metal-based catalysts derived from MOFs to enhance their catalytic performance through modifications using supporting or by forming bimetallic oxides, then investigate their SO2 and water resistance as well as stability for toluene degradation along with the reaction mechanism.

Herein, the CeO2 catalyst supported on MIL-101(Fe) metal-organic framework, MOF-derived FeOx catalysts combined with various metals to make bimetallic oxides, and MnCeOx bimetallic oxide catalysts supported on MIL-101(Fe) were synthesized via hydrothermal methods followed by calcination. To assess the impact of Ce on the catalyst performance of CeO2@MIL-101(Fe), CeO2 supported on MIL-101(Fe) with various Ce concentrations was synthesized and tested for toluene degradation. Besides, to study the promotional effect of metal on the catalytic activity of the synthesized FeOx derived from MOFs, four metals (Ce, Co, Cu, and Mn) were selected based on their properties and combined with FeOx derived from MIL-101(Fe), then tested for toluene degradation to tackle the aforementioned challenges and further improve the catalytic oxidation of toluene at lower temperatures. Furthermore, to assess the impact of Mn on the catalyst activity of bimetallic MnCeOx@MIL-101(Fe), MnCeOx supported on MIL-101(Fe) with various Mn concentrations, and a fixed amount of Ce was synthesized and tested for toluene degradation. To test the catalytic performance for all catalysts, toluene (1000 ppm), 20% O2 balanced with N2 were fed into the reactor with 50 mL/min of total flow rate, which corresponds to a gas hourly space velocity of 30,000 mL/(g.h). The as-synthesized catalysts were comprehensively characterized by different methods such as X-ray diffraction (XRD), thermogravimetry (TG), N2 adsorption-desorption (BET), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma emission spectrometer (ICP-OES), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and proton transfer reaction-mass spectrometry (PTR-MS). MIL-101(Fe) MOF has been selected for this study due to its exceptional stability, large surface area, and the presence of unsaturated metal sites, which can facilitate its interaction with active sites for toluene oxidation. Compared to MIL-101(Cr) mostly reported for toluene adsorption, MIL-101 (Fe) is a more environmentally friendly option due to the toxic nature of Cr.

In Chapter 4, a CeO2 catalyst supported on MIL-101(Fe) with various Ce content was prepared and evaluated for toluene oxidation. The results demonstrated that CeO2 was successfully supported on MIL-101(Fe). Among the synthesized materials, CeO2@MIL-101(Fe) with 6% Ce exhibited superior catalytic performance, achieving 90% toluene conversion (T90) at 239 °C. The catalysts were ranked in performance as follows: 6%Ce@MIL-101(Fe) > 9%Ce@MIL-101(Fe) > 12%Ce@MIL-101(Fe) > 15%Ce@MIL-101(Fe) > 3%Ce@MIL-101(Fe) > 1%Ce@MIL-101(Fe) > CeO2-D. Interestingly, all the supported CeO2 catalysts with various amounts of Ce exhibited higher catalytic performance compared to pure CeO2 and MIL-101(Fe), which shows the synergy between MIL-101(Fe) support and the metal oxide (CeO2). This indicated that supporting CeO2 on MOF could enhance the catalytic efficiency for toluene oxidation. Moreover, CeO2@MIL-101(Fe) revealed remarkable stability, maintaining its activity for over 60 hours. This catalyst also demonstrated high resistance to water vapor and sulfur dioxide (SO2), with full activity recovery after the removal of inhibitors. In situ DRIFTS and PTR-MS provided insights into the reaction mechanism, indicating that lattice oxygen plays a crucial role in toluene activation. The synergistic interaction between CeO2 and the MIL-101(Fe) support enhances redox properties and oxygen mobility, contributing to improved catalytic efficiency.

In Chapter 5, MOF-derived FeOx catalysts combined with various metals were investigated for toluene oxidation. The experimental findings revealed that the selected metals (Ce, Co, Cu, and Mn) were successfully combined with FeOx derived from MIL-101(Fe) to make bimetallic oxides. Among the tested catalysts, FeCeOx exhibited excellent catalytic performance, achieving a T90 at 226 °C, which is 100 °C lower than the monometallic FeOx. The performance ranking of the tested catalysts was as follows: FeCeOx (T90=226 °C) > FeCuOx (T90=276 °C) > FeCoOx (T90=292 °C) > FeMnOx (T90=312 °C) > FeOx (T90=326 °C). The catalytic performance of the tested catalysts was consistent with their specific surface area. Interestingly, all the catalysts holding the second metal exhibited the highest catalytic performance compared to non-modified FeOx. This indicated that incorporating a guest metal could enhance the catalytic efficiency of MOF-derived FeOx catalyst for toluene oxidation. The highest catalytic activity of FeCeOx is attributed to its high surface area, abundant lattice oxygen species, and synergistic effects between Ce and Fe. This catalyst showed excellent stability, maintaining 93% toluene conversion at 230 °C for over 60 hours; good reusability with consistent toluene conversion rates for over three successive reaction cycles, and high resistance to SO2 poisoning. Although H2O vapors temporarily inhibited the catalytic activity, the effect was reversible. In-situ DRIFTS and PTR-MS analyses revealed the crucial role of lattice oxygen in the toluene degradation. The synergistic interaction between Ce and Fe boosted redox properties and oxygen mobility, enhancing catalytic performance.

MnCeOx bimetallic oxide catalysts supported on MIL-101(Fe) with various Mn concentrations were evaluated for toluene degradation in Chapter 6. The results showed that MnCeOx bimetallic oxide was successfully supported on MIL-101(Fe). Mn5Ce6Ox@MIL-101(Fe) achieved the best performance with T90 of 210 °C, outperforming pure MnO2, pure CeO2, and CeO2 supported on MIL-101(Fe). This indicated that the incorporation of MnO2 in CeO2 supported on MOF significantly enhanced the catalytic efficiency for toluene oxidation. The superior catalytic performance of Mn5Ce6Ox@MIL-101(Fe) was attributed to its highest surface area, synergistic effects of lattice oxygen species, optimal Mn4+/Mn3+ and Ce4+/Ce3+ ratio, lower Fe3+ content, and synergistic effect between Mn, Ce, and MOF support. The synthesized catalyst showed excellent catalytic stability, maintaining around 94% toluene conversion at a temperature of 220 °C for over 60 hours under dry reaction conditions. Mn5Ce6Ox@MIL-101(Fe) demonstrated good catalytic reusability, maintaining consistent toluene conversion rates over three consecutive degradation cycles. The catalyst exhibited strong resistance to SO2 and H2O vapors, maintaining over 90% toluene conversion in the presence of 5% vol.% water and 50 ppm SO2. In-situ DRIFTS analysis demonstrated the important role of lattice oxygen in the toluene oxidation.

This study shows considerable significance for practical VOC oxidation technologies, providing a potential pathway to enhance air quality and mitigate health risks linked to VOC pollution. Developing efficient and stable catalysts for toluene oxidation supports broader environmental protection objectives and fosters sustainability by encouraging cleaner industrial practices.

Date of Award	13 Jul 2025
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Jun He (Supervisor), Yong Sun (Supervisor) & George Zheng Chen (Supervisor)