Structured fiber supports for ionic liquid-phase catalysis used in gas-phase continuous hydrogenation
Introduction
Supported ionic liquid-phase (SILP) catalysis has attracted increasing attention in chemical reaction engineering for applications in continuous-flow reactors [1], [2], [3], [4]. The SILP applies a homogeneous catalyst in a layer of ionic liquid (IL) that is confined on the surface of a solid support with high specific surface area [5]. Although the resulting material is a solid, the active species in the IL phase acting as a homogeneous catalyst preserves its high selectivity. The advantage of SILP catalysis is the reduced amount of IL needed and its multiple reuses, being economically and environmentally beneficial. The feasibility of the SILP catalysis has been demonstrated by several authors on granulated silica randomly packed [2], [5] in fixed-bed reactors [6], [7], [8]. Moreover, most of the reactions were liquid/solid, with only few reported on the SILP catalyst applied to gas-phase reactions [6], [7].
Catalytic beds with a regular catalyst arrangement (structured catalytic beds) present multiple advantages, including a low pressure drop during the fluid passage through the reactor and an even flow distribution, allowing a narrow residence time distribution (RTD) [9]. This property is very important for complex reactions with an intermediate as a target product. It allows for high selectivity, leading to process intensification and favorable environmental impact. The sintered metal fibers (SMFs) in the form of thin plates are used in this study as structured supports for the IL phase containing a homogeneous catalyst. The SMF plates consist of micrometer-size filaments sintered into a homogeneous 3-dimensional structure. They have a high porosity (up to 80–90%) and high permeability, leading to a low pressure drop through the reactor bed. To increase the specific surface area (SSA) of the SMFs and to attain a homogeneous coverage by IL, the SMFs were coated by a layer of carbon nanofibers (CNFs). The CNF/SMF support has high thermoconductivity, suppressing hot spot formation during exothermic hydrogenation reactions [10].
The feasibility of the SSILP catalysis for gas-phase reactions is tested in the hydrogenation of 1,3-cyclohexadiene to cyclohexene as a model reaction. The reaction was carried out in a continuous fixed-bed tubular reactor with structured catalytic bed containing a homogeneous Rh-based catalyst immobilized in IL confined on CNF/SMF. To elucidate the influence of the support on catalyst activity/selectivity, the SMFs were also coated by a thin zeolite (ZSM-5) film on which the IL phase containing Rh catalyst was deposited. Catalysts were characterized by SEM and XRD, and high-pressure 1H NMR and 1H{31P} NMR spectroscopy was used to provide insight into the nature of the active catalytic species.
Section snippets
Materials
SMFs (Bekipor ST 20AL3; Bekaert Fiber Technology, Belgium) made of Inconel 601 (alloy composition: Ni, 58–63%; Cr, 21–25%; Al, 1.4%) in the form of panel (elementary filament diameter, 8 μm; panel thickness, 0.49 mm, porosity, 81%, weight, 750 g/m2) and SMF-40 μm (Southwest Screens & Filters SA, Belgium) made of Fecralloy (alloy composition: Cr, 20%; Al, 4.75%; Y, 0.27%; other elements, ∼1–2%; Fe balance) in the form of a uniform pore panel (elementary filament diameter, 20 μm; panel thickness,
Catalyst characterization
The SMF plates consist of uniform metallic filaments sintered into a homogeneous 3-dimensional structure with porosity of up to 80–90% with high permeability. The fibrous matrix exhibits high mechanical strength combined with chemical and thermal stability. It also acts as a static micromixer to prevent gas channeling within the catalytic bed. Due to their small fiber diameter (∼2–20 μm), SMFs have a relatively large specific surface area, being suitable supports for micrometer-thickness films
Conclusions
Based on our findings, we can state the following conclusions of this study:
- 1.
An SSILP Rh-based catalyst was shown to be active in the gas-phase hydrogenation of 1,3-cyclohexadiene with a high selectivity to cyclohexene (>96%). SMF plates with high porosity were used as structured supports.
- 2.
Excess phosphine ligand and acid in the IL phase are required to maintain catalyst activity and selectivity.
- 3.
The catalytic species active in the studied reaction is suggested to be Rh(H)2Cl(PPh3)3, as follows
Acknowledgements
The authors thank Professor Simon Duckett (York, UK) for helpful discussions. Financial support was provided by the Swiss National Science Foundation.
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2014, CarbonCitation Excerpt :This allows, thus, a broad variety of arrangements and interactions of the IL within the support surface. There are some works dealing with SILP systems based on carbon materials using for example a carbon cloth [17,18], carbon nanofibers that cover sintered metal fibers [19], commercially available mesoporous and microporous carbon beads [20], commercial activated carbons [21] and carbon nanotubes [22–25]. Beyond the development of SILP catalytic systems, the adsorption of IL on carbon materials has potential uses in other application areas like EDLC (Electric Double-Layer Capacitors) or the removal of IL from aqueous effluents, in which activated carbon has been used [26].
Fiber based structured materials for catalytic applications
2014, Applied Catalysis A: GeneralCitation Excerpt :Such fiber sheets were often applied for catalytic combustion and therefore the structures were coated for instance with perovskites [27,28,45–49]. Moreover these structures were coated e.g. with carbon nanofibers [50,51], zeolites [42,43] and ionic liquid-phase [52]. But generally coating of this structures is difficult, because conventional coating procedures lead to pore blocking due to the small fibers and pores [53].