Nanostructured Catalysts for Sustainable Acetylene-Based Vinyl Chloride Production

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Author
Date
2021Type
- Doctoral Thesis
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Abstract
The growing demand for plastics coupled with shrinking oil reserves has revived interest in the production of vinyl chloride (VCM), monomer to polyvinyl chloride, from coal-derived acetylene. However, the present acetylene hydrochlorination process relies on toxic mercuric chloride-based catalysts, the use of which will be banned from 2022, highlighting the urgency to implement sustainable and economically viable alternatives. Since decades, the search for a suitable replacement has been guided by the linear correlation between activity and the standard electrode potential of metal chlorides, directing research efforts primary towards carbon-supported gold catalysts. Still, several practical challenges and fundamental questions remain unsolved. While Au single atoms exhibit high initial activity, they rapidly agglomerate into inactive particles due to their insufficient stability on carbon. Furthermore, the established performance descriptor cannot provide reliable guidelines for the design of superior catalytic architectures, as speciation effects and stability of the metal nanostructures are not considered. Finally, the key role of carbon in generating active metal-based catalysts compared to any other support is still not understood. In fact, a complex interplay exists between the metal site and the carbon support, exhibiting as such notable activity in acetylene hydrochlorination, particularly upon functionalization with heteroatoms. This thesis disentangles the interplay between the metal nanostructure and the carbon support in acetylene hydrochlorination and unravels fundamental understanding on the active sites through speciation-performance analyses, providing guidelines for the optimal design of metal-free and nanostructured metal-based catalysts. To reach this goal, a holistic approach, combining precise material synthesis, in-depth characterization, quantitative catalytic evaluation, kinetic and transient mechanistic analyses, and density functional theory studies is adopted. Firstly, the potential of nitrogen-doped carbons (NC) as metal-free hydrochlorination catalysts is explored. By decoupling structural, compositional, and porous properties, an interplay of two activity descriptors is identified: (i) a high content of pyrrolic-N functionalities, being responsible for the adsorption of the reactants, and (ii) good electrical conductivity,x likely influencing the surface diffusion of adsorbed species. With this understanding, the first metal-free catalyst is developed that rivals the initial activity of gold-based systems at elevated reaction temperatures (473 K and 573 K, for metal-based and metal-free catalysts, respectively). However, the active sites promote extensive coking, leading to micropore blockage and rapid catalyst deactivation (deactivation constant kD = -20 h−1). The introduction of structurally more stable meso- and macropores results into a ca. 50-fold reduced deactivation rate of hierarchical NC at half the initial activity level compared to their purely microporous counterparts. Building on the acquired knowledge to functionalize carbons, host design strategies are developed to control the nuclearity and coordination environment of gold, platinum, palladium, ruthenium, rhodium, and iridium-based catalysts. Following this approach, metal speciation and host effects can be disentangled, enabling the derivation of generalized quantitative performance descriptors for acetylene hydrochlorination. Distinct active-site nanostructures were identified: (i) MClx single atoms of Au and Pt, (ii) metal oxide nanoparticles of Ru, Rh, and Ir and, (iii) metallic nanoparticles of Pd. The energy of acetylene adsorption is identified as speciation sensitive activity descriptor. Further, also the selectivity with respect to the formation of coke is mainly determined by the acetylene-affinity of the metals (i.e., maximized over Pd nanoparticles) and the functionalization of the carbon support (i.e., maximized over NC). Besides coking, chlorination and metal nuclearity changes are relevant deactivation mechanisms, originating from an interplay of two stability descriptors: (i) the single atom-carbon host interaction and (ii) the affinity towards chlorine. Specifically, all nanostructures of Au and Pd suffer from agglomeration on N-free carbon, while being sufficiently stabilized on NC. Oxidic nanoparticles of Ru, Rh, and Ir undergo chlorination and redispersion into fully chlorinated inactive single atoms, regardless of the host functionalization. In the case of Ru/NC, this process can be inhibited through encapsulation into single-layer graphene shells. In combination with optimized oxygen co-feeding to reduce coking while preserving the protective layer, the nanostructured Ru catalyst can achieve comparable activity and stability to Au single-atoms on NC (kD = -1.3 h−1). On non-functionalized carbon, Pt single atoms are identified as the only metal nanostructure with intrinsic stability on O/C defects. This endows them with unparalleled durability in acetylene hydrochlorination (kD = -0.1 h−1), ultimately surpassing the space-time-xi yield of state-of-the-art Au- and Ru-based catalysts and qualifying them as a new promising candidate for sustainable vinyl chloride production. The discovery of stable carbon-supported Pt single atoms further allowed to systematically vary the porous properties and surface functionalization of carbon while preserving the metal speciation, shedding light on the intrinsic role of the support. A high acetylene adsorption capacity in HCl-rich atmosphere is identified as the central activity descriptor, which is finely controllable through the porosity of the carbon host. The rate of coking as the main deactivation mechanism is decreased by reducing the density of acidic surface groups. With the aim to combine the high initial activity of Au single atoms with the unprecedented stability of their Pt-based analogs, synergies in bimetallic catalysts are explored. Thereby, we reveal the potential of Pt chloride in aqueous solution to disperse large gold agglomerates (>70 nm) on carbon carriers into single atoms, a phenomenon of practical relevance beyond the field of acetylene hydrochlorination. Furthermore, the formed bimetallic single-atom catalyst exhibits improved resistance against agglomeration, indicating cooperativity effects between gold and platinum atoms and giving exciting future prospects for multimetallic single-atom catalysis. This thesis demonstrates how catalysis can be enhanced via precise nanoscale engineering, giving momentum to future developments in acetylene hydrochlorination and single-atom catalysis. Show more
Permanent link
https://doi.org/10.3929/ethz-b-000482841Publication status
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Publisher
ETH ZurichSubject
Heterogeneous catalysis; Single-atom catalysis; Carbon Material; hydrochlorinationOrganisational unit
03871 - Pérez-Ramírez, Javier / Pérez-Ramírez, Javier
Funding
ETH-40 17-1 - Design of acetylene hydrochlorination catalysts for sustainable PVC production (ETHZ)
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