Oxidation Of Isobutane Research Articles

Operando Spectroscopy to Understand Dynamic Structural Changes of Solid Catalysts.

Solid materials like heterogeneous catalysts are highly dynamic and continuously tend to change when exposed to the reaction environment. To understand the catalyst system under true reaction conditions,operando spectroscopy is the key to unravel small changes, which can ultimately lead to a significant difference in catalytic activity and selectivity. This was also the topic of the 7th International Congress on Operando Spectroscopy in Switzerland in 2023. In this article, we discuss various examples to introduce and demonstrate the importance of this area, including examples from emission control for clean air (e.g. CO oxidation), oxidation catalysis in the chemical industry (e.g. oxidation of isobutene), future power-to-X processes (electrocatalysis, CO2 hydrogenation to methanol), and non-oxidative conversion of methane. All of these processes are equally relevant to the chemical industry. Complementary operando techniques such as X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and Raman spectroscopy were utilized to derive the ultimate structure of the catalyst. The variety of conditions requires distinctly different operando cells that can reach a temperature range of 400-1000 °C and pressures up to 40 bar. The best compromise for both the spectroscopy and the catalytic reaction is needed. As an outlook, we highlight emerging methods such as modulation-excitation spectroscopy (MES) or quick-extended X-ray absorption fine structure (QEXAFS) and X-ray photon in/out techniques, which can provide better sensitivity or extend X-ray based operando studies.

Comparison of Structure and Reactivity of Hydrothermally Prepared Bi−Mo−Co−Fe−O Catalysts in Selective Propylene and Isobutene Oxidation

AbstractThe elucidation of structure‐activity correlations in selective oxidation of propylene and isobutene over mixed metal oxides (MMO) is attractive for knowledge‐based catalyst design and process optimisation. Particularly, 4‐component Bi−Mo−Co−Fe−O catalysts need to be studied since their complex metal oxide phase mixture leads to higher activity and selectivity than 2‐component Bi−Mo−O. Hence, three Bi−Mo−Co−Fe‐oxides with different metal ratios were prepared by hydrothermal synthesis and compared during selective oxidation tests with propylene and isobutene. The active phases after several days on stream were investigated by synchrotron X‐ray diffraction (XRD) and Raman spectroscopy, while the structural evolution under reaction conditions was followed by operando Raman spectroscopy, synchrotron XRD, and multi‐edge X‐ray absorption spectroscopy. Similar structural transformations were observed during selective oxidation of propylene and isobutene, with similar influence on catalytic performance. A phase mixture of β‐CoMoO4/β‐Co0.7Fe0.3MoO4, γ‐Bi2MoO6, Fe3O4 and Bi3FeMo2O12 was observed, whereby high amounts of β‐CoMoO4/β‐Co0.7Fe0.3MoO4 increased selectivity to acrolein/methacrolein. In contrast, high amounts of γ‐Bi2MoO6 and Fe3O4 favoured total oxidation to CO and CO2. The simultaneous presence of β‐CoMoO4/β‐Co0.7Fe0.3MoO4, Bi3FeMo2O12 and Fe2O3 increased selectivity to methacrolein in isobutene oxidation, whereas no comparative increase in acrolein selectivity was observed in propylene oxidation. This suggests different key active phases in both reactions.

Facet-dependent catalysis of γ-Bi2MoO6 for selective oxidation of isobutene to methacrolein

γ-Bi2MoO6 nanosheets catalyzed the selective oxidation of isobutene to methacrolein with a conversion of 88% and a selectivity of 67% due to the preferential termination of the reactive molybdenum species on the {010} facets.

Role of Iron and Cobalt in 4-Component Bi–Mo–Co–Fe–O Catalysts for Selective Isobutene Oxidation Using Complementary Operando Techniques

Role of Iron and Cobalt in 4-Component Bi–Mo–Co–Fe–O Catalysts for Selective Isobutene Oxidation Using Complementary <i>Operando</i> Techniques

Photothermal catalytic selective oxidation of isobutane to methacrylic acid over keggin-type heteropolyacid

Thermal and photothermal catalytic selective oxidation of isobutane to methacrylic acid (MAA) are comparatively studied over a keggin-type Cs2.9Cu0.34V0.49PMo12O40 heteropolyacid acid. An introduction of light was observed to enhance both the i-C4H10 conversion and the MAA selectivity, and consequently the MAA formate rate, particularly at low temperatures. Characterization results show that oxidation of methacrolein (MAL) to MAA is the rate-limiting step while UV light illumination promotes the oxidation of σ-bonded MAL with OH groups to σ-bonded MAA on the catalyst surface. These results demonstrate a synergistic effect of thermal catalysis and photocatalysis in selective oxidation of isobutane to MAA, which suggests photothermal catalysis as a promising strategy to catalyze the selective oxidation of higher hydrocarbons at relative mild reaction conditions.

MoOx monolayers over TiO2 for the selective oxidation of isobutene

The morphology effect of TiO2 on the dispersion of molybdena species and the performance of the resulting MoOx/TiO2 catalysts in selective oxidation of isobutene to methacrolein were examined. Two-dimensional MoOx monolayers in Mo–O–Mo bonds were well dispersed on TiO2 nanosheets that were preferentially enclosed by the {001} facets. However, aggregated MoOx clusters enriched in terminal MoO bonds were formed over TiO2 nanospindles dominated by the {101} facets. The superior performance of the MoOx monolayers, regarding activity, selectivity and stability, was ascribed to the synergetic effect between MoOx monolayers and TiO2 (001) facets.

Direct Tertiary Butyl Alcohol Formation via Catalytic Liquid Phase Isobutane Oxidation with Molecular Oxygen

A safe one-pot catalytic process for directly producing tertiary butyl alcohol (TBA) from liquid-phase isobutane oxidation with oxygen is demonstrated. This is achieved by operating outside the vapor phase flammability envelope and catalytically decomposing tertiary butyl hydroperoxide (TBHP), the main product of homogeneous isobutane oxidation, at 125 °C and 35 bar. Among a dozen heterogeneous metal-based catalysts tested, amorphous MnO2 (AMO) was found to be roughly an order of magnitude more active than supported Pd, Co, Au, and Nb-based catalysts for TBHP decomposition to TBA. As a result, high TBA/TBHP (up to 20.4) selectivity was achieved during isobutane oxidation in the presence of the AMO catalyst. The yield of gas phase C1 products was <1% and only a minor amount of acetone was produced as a β-scission byproduct. The use of n-hydroxy phthalimide (NHPI) as an initiator was found to enhance the TBA yield. This process provides a possible low energy pathway for making isobutylene from isobutane, avoiding the energy-intensive dehydrogenation and isobutane/isobutylene separation steps.

An Advanced Characterization Toolbox for Selective Olefin Oxidation Catalysts

AbstractThe selective oxidation of propylene and isobutene form a central cornerstone in chemical industry, typically catalysed by bismuth molybdate based multicomponent systems. Over the past decades, a detailed knowledge has been gained on the mechanism by theoretical and experimental studies including in situ and operando characterization techniques, using simplified model catalysts. These have provided excellent insight, but in industrially applied selective oxidation typically multi‐component catalysts are used, leading to higher activity and selectivity. New concepts and characterization methods allow now studies of the more complex multicomponent catalysts covering the various length and time scales and potentially providing new insights into the dynamics and cooperation during catalysis. Starting with an overview of past concepts and milestones, we report on latest developments and future trends in the field of selective oxidation of lower olefins. Thereby we focus both on conventional spectroscopic and advanced characterisation methods and discuss the importance of integral and spatially‐resolved approaches. We conclude that fundamental understanding of such complex oxidation reactions requires combined and interdisciplinary research, now possible with the new characterization tools, data analysis schemes and reactor modelling approaches. This allows bridging the various complexity scales.

Catalytic behavior of Mo–Bi–Fe–Co–K–M–O (M=Ce, Gd, CeGd) catalysts for selective oxidation of isobutene

The further improvement of methacrolein (MAL) selectivity from isobutene (IB) oxidation is crucial and challenging. In this study, based on the typical Mo–Bi–Fe–Co–K–O mixed metal oxide, the rare earth element Gd-doped, Ce-doped and CeGd co-doped catalysts were prepared by co-precipitation strategy to increase the selectivity of MAL from 47.9% to 49.8%, 64.2% and 68.6%, respectively. In order to elucidate in-depth the promoting effect of Ce and/or Gd, various characterizations were utilized including X-ray diffraction patterns (XRD), Raman, X-ray fluorescence spectrometry (XRF), X-ray photoelectron spectroscopy (XPS), O2-temperature programmed desorption (O2-TPD), H2-temperature programmed reduction (H2-TPR), CO2-temperature programmed desorption (CO2-TPD), IB-temperature programmed desorption (i-C4-TPD) and in-situ IB-Fourier transform infrared spectroscopy (IB-FTIR). Both Ce and Gd finely regulate the bulk and surface structure of the catalyst, thus altering the redox ability, oxygen mobility and storage ability and basicity. Compared with Ce, Gd addition slightly regulates the variation of Co2+/Co3+ redox couples, greatly enhances the interaction among the components on the catalyst, thus only increases the content of surface oxygen species and has little effect on their mobility. While Ce-containing catalyst performs stronger oxygen storage and migration ability, thus leading to the overproduction of surface Odefect species, which are proposed to be the active sites for the production of MAL and COx. The CeGd co-doped catalyst possesses the proper content of surface Odefect species, thus exhibits much higher MAL selectivity. Moreover, the promoting mechanism of Ce and/or Gd over IB oxidation is proposed. Therefore, this work is helpful for understanding the influence of rare earth elements on the structure of mixed metal oxides and the olefin selective oxidation reaction.

Optimizing lattice oxygen mobility and acidity of heteropoly acid catalysts for oxidation of isobutane to methacrylic acid

In this paper, a series of heteropoly acids, (Cs2.2Cu0.26V0.38)xPMo12O40 (x = 1.3, 1.1, 1.0, 0.88), are prepared, characterized and examined as the catalysts for the oxidation of isobutane to MAA. Under the employed reaction condition, Cs2.23Cu0.26V0.38PMo12O40 exhibits an isobutane conversion of 20% and a MAA selectivity of 51% at 335 °C, giving a MAA yield of 0.68 mmolMAA·h−1·gcatalyst−1. Lattice oxygen mobility and acidity of (Cs2.2Cu0.26V0.38)xPMo12O40 catalysts play key roles. A fast migration rate and a moderate release rate of lattice oxygen are beneficial to the selective oxidation of isobutane to MAA. Cs2.2Cu0.26V0.38PMo12O40 exhibits strong acid sites facilitating MAA desorption and surface sites strongly binding MAA, causing MAA polymerization which blocks the active sites and decreases the long-term catalytic activity. These results demonstrate that the optimization of lattice oxygen mobility and surface acidity is an effective strategy to fabricate efficient heteropoly acid catalysts for the oxidation of isobutane to MAA.

Catalytic selective oxidation of isobutane in a decoupled redox-process

The catalytic selective oxidation of isobutane to methacrolein (MAC) and methacrylic acid (MAA) is known to proceed via a Mars-Van Krevelen mechanism. When the reaction is performed in a common reactor with co-feeding of the reactant and oxygen, reactive oxygen species are present on the catalyst surface whereby non-selective oxidation reactions occur. In this work, we studied the possibility to decouple the redox mechanism meaning to separate the reactant oxidation step and the catalyst re-oxidation step for the selective oxidation of isobutane (IBAN) to MAC and MAA. For this purpose, two reactor configurations were employed: a periodic reactor and a two-zone fluidized bed reactor (TZFBR). Whereas the periodic reactor allows the temporal separation of the two steps, the TZFBR enables the spatial separation.A Cs-based Keggin type heteropolycompound was employed as catalyst and characterized before and after reaction by XRD, Raman and XPS to better understand the structural and chemical changes during the periodic operation. The effect of the oxygen/IBAN concentration on the performance was studied. For the periodic reactor it was found that the conversion of isobutane and the formation of products decreased with the consumption of available oxygen species in the catalyst during a period. For the whole periodic tests, low concentration of oxygen injected into the reoxidation part of the cycles led to a high selectivity to the desired products (MAC+MAA).

Insight into the selective oxidation of isobutene to methacrolein over Ce-accelerated Mo-Bi-Fe-Co-K-O catalyst

Understanding the mechanism of multi-component metal oxide catalyst for the selective oxidation of isobutene to methacrolein has long been challenging for its complexity. Herein, the key role of Ce on the Mo-Bi-Fe-Co-K-O catalysts for accelerating the selectivity of methacrolein was revealed. It was suggested that the Ce addition could replace the Bi3+ and then generate defects which changed the transformation of electron and the migration mobility of ions: Ⅰ. the production of Ce3+/Ce4+ and Co2+/Co3+ redox couples regulated the catalytic redox ability; and Ⅱ. the interaction effect between Ce and Co enhanced the storage capacity and mobility of oxygen which could benefit the adsorption and activation ability for isobutene molecules. Interestingly, it was found that adding appropriate amount of Ce adjusted the molar ratio of the catalytically active lattice oxygen and defect oxygen species. Therefore, proper amount of Ce addition to the catalyst significantly accelerated the methacrolein selectivity from 47.9% to 70.6%.

Infrared spectroscopic signature of a hydroperoxyalkyl radical (•QOOH).

Infrared (IR) action spectroscopy is utilized to characterize a prototypical carbon-centered hydroperoxyalkyl radical (•QOOH) transiently formed in the oxidation of volatile organic compounds. The •QOOH radical formed in isobutane oxidation, 2-hydroperoxy-2-methylprop-1-yl, •CH2(CH3)2COOH, is generated in the laboratory by H-atom abstraction from tert-butyl hydroperoxide (TBHP). IR spectral features of jet-cooled and stabilized •QOOH radicals are observed from 2950 to 7050 cm-1 at energies that lie below and above the transition state barrier leading to OH radical and cyclic ether products. The observed •QOOH features include overtone OH and CH stretch transitions, combination bands involving OH or CH stretch and a lower frequency mode, and fundamental OH and CH stretch transitions. Most features arise from a single vibrational transition with band contours well simulated at a rotational temperature of 10 K. In each case, the OH products resulting from unimolecular decay of vibrationally activated •QOOH are detected by UV laser-induced fluorescence. Assignments of observed •QOOH IR transitions are guided by anharmonic frequencies computed using second order vibrational perturbation theory, a 2 + 1 model that focuses on the coupling of the OH stretch with two low-frequency torsions, as well as recently predicted statistical •QOOH unimolecular decay rates that include heavy-atom tunneling. Most of the observed vibrational transitions of •QOOH are readily distinguished from those of the TBHP precursor. The distinctive IR transitions of •QOOH, including the strong fundamental OH stretch, provide a general means for detection of •QOOH under controlled laboratory and real-world conditions.

Insight into the Selective Oxidation of Isobutene to Methacrolein Over Ce-Accelerated Mo-Bi-Fe-Co-K-O Catalyst

Insight into the Selective Oxidation of Isobutene to Methacrolein Over Ce-Accelerated Mo-Bi-Fe-Co-K-O Catalyst

Insight into the role of PEG on Mo-Bi based catalyst in isobutene selective oxidation to methacrolein

Mo-Bi based catalyst has been synthesized by PEG-assisted hydrothermal method, and tested in selective oxidation of isobutene to methacrolein (MAL). It is found that the catalysts assisted by PEG exhibited higher selectivity and yield of MAL in oxidation of isobutene. To investigate the role of PEG on Mo-Bi based catalyst, a series of MoBiFeCoCeCs mixed oxide catalysts were prepared by PEG-assisted hydrothermal method. By combining the characterization of Mo-Bi based catalyst obtained by using X-ray diffraction (XRD) patterns, N2 physisorption measurements, Fourier transform infrared spectroscopy (FT-IR), NH3 temperature-programmed desorption (NH3-TPD) measurements, X-ray photoelectron spectroscopy (XPS), X-ray fluorescence (XRF) spectroscopy and scanning electron microscope (SEM), it was verified that the assistance of PEG could change metal composition and states, inhibit agglomeration of particles and adjust acidity of catalyst. Our finding may suggest the relationship of selective oxidation of isobutene with Mo-Bi based catalysts.

Supported Rb- or Cs-containing HPA catalysts for the selective oxidation of isobutane

Silica-supported catalysts based on Keggin-type heteropolyacids (HPAs) containing rubidium or cesium as counter cations have been prepared by the impregnation method and evaluated in the selective oxidation of isobutane to methacrolein and methacrylic acid. The catalysts were characterized by various techniques such as XRD, N2 physisorption, TGA, Raman spectroscopy, H2-TPR, and NH3-TPD in order to study their thermal stability, structural, and textural properties, acidity and reducibility. It was evidenced that the reducibility of the Keggin type HPAs was improved by supporting the active phase on SiO2. A loading of 40 wt% was the optimum for the selective oxidation of isobutane (IBAN) to methacrylic acid (MAA). The selectivities to MAA and methacrolein (MAC) at given conversion were increased when Cs+ was used as counter cation compared to Rb+. The same trend was observed for mono- and di-vanado-substituted phosphomolybdic acid, whereby the performance followed the order: CsV1/SiO2 > RbV1/SiO2 > CsV2/SiO2 > RbV2/SiO2. The density of acid sites was correlated to the catalytic activity, which underlines the importance of the acid sites for alkane activation.

Selective Oxidation of Isobutane to Methacrylic Acid by Metal-Substituted Ammonium Salts of Molybdovanadophosphoric Acid

Selective Oxidation of Isobutane to Methacrylic Acid by Metal-Substituted Ammonium Salts of Molybdovanadophosphoric Acid

Watching a hydroperoxyalkyl radical (•QOOH) dissociate.

A prototypical hydroperoxyalkyl radical (•QOOH) intermediate, transiently formed in the oxidation of volatile organic compounds, was directly observed through its infrared fingerprint and energy-dependent unimolecular decay to hydroxyl radical and cyclic ether products. Direct time-domain measurements of •QOOH unimolecular dissociation rates over a wide range of energies were found to be in accord with those predicted theoretically using state-of-the-art electronic structure characterizations of the transition state barrier region. Unimolecular decay was enhanced by substantial heavy-atom tunneling involving O-O elongation and C-C-O angle contraction along the reaction pathway. Master equation modeling yielded a fully a priori prediction of the pressure-dependent thermal unimolecular dissociation rates for the •QOOH intermediate-again increased by heavy-atom tunneling-which are required for global models of atmospheric and combustion chemistry.

Spectral fingerprint of stabilized •QOOH

Chemical Kinetics Carbon-centered radicals containing the hydroperoxy group, commonly denoted as •QOOH, are elusive but are among the most critical intermediate species for kinetic modeling of hydrocarbon oxidation in various atmospheric and combustion processes. Their direct experimental observation is a long-standing challenge, with only one successful previous attempt. Using a combination of infrared activation spectroscopy and ultraviolet laser–induced fluorescence detection, Hansen et al. directly characterized the vibrational structure of a •QOOH intermediate in isobutane oxidation, collisionally stabilized and isolated, and followed its dissociative evolution under infrared activation with time and energy resolution. High-level electronic structure calculations revealed an important role of heavy-atom tunneling in this process. Science , abj0412, this issue p. [679][1] [1]: /lookup/doi/10.1126/science.abj0412

Selective Oxidation of Isobutane to Methacrylic Acid and Methacrolein: A Critical Review

Selective oxidation of isobutane to methacrolein (MAC) and methacrylic acid (MAA) has received great interest both in the chemical industry and in academic research. The advantages of this reaction originate not only from the low cost of the starting material and reduced process complexity, but also from limiting the use of toxic reactants and the production of wastes. Successive studies and reports have shown that heteropolycompounds (HPCs) with Keggin structure (under the form of partially neutralized acids with increased stability) can selectively convert isobutane to MAA and MAC due to their strong and tunable acidity and redox properties. This review hence aims to discuss the Keggin-type HPCs that have been used in recent years to catalyze the oxidation of isobutane to MAA and MAC, and to review alternative metal oxides with proper redox properties for the same reaction. In addition, the influence of the main reaction conditions will be discussed.