Timm Schmidt

Sustainable chlorine recycling via catalysed HCl oxidation: from fundamentals to implementation

The heterogeneously catalysed oxidation of HCl to Cl2 comprises a sustainable route to recover chlorine from HCl-containing streams in the chemical industry. Conceived by Henry Deacon in 1868, this process has been rejuvenated in the last decade due to increased chlorine demand and the growing excess of by-product HCl from chlorination processes. This reaction suffered from many sterile attempts in the past two centuries to obtain sufficiently active and durable catalysts. Intense research efforts have culminated in the recent industrial implementation of RuO2-based catalysts for HCl oxidation. This paper reviews the new generation of technologies for chlorine recycling under the umbrella of Catalysis Engineering, that is, tackling the microlevel (catalyst design), mesolevel (reactor design), and macrolevel (process design). Key steps in the development are emphasised, including lab-scale catalyst screening, advanced catalyst characterisation, mechanistic and kinetic studies over model and real systems, strategies for large-scale catalyst production, mini-plant tests with a technical catalyst, and reactor design. Future perspectives, challenges, and needs in the field of catalysed Cl2 production are discussed. Scenarios motivating the choice between catalysed HCl oxidation and HCl electrolysis or their integration for optimal chlorine recycling technology are put forward.

Shaped RuO2/SnO2–Al2O3 Catalyst for Large-Scale Stable Cl2 Production by HCl Oxidation

The gas-phase catalytic oxidation of hydrogen chloride to chlorine (Deacon process, 4 HCl+O2

A delafossite-based copper catalyst for sustainable Cl2 production by HCl oxidation

2 Cl2+2 H2O) is an eco-efficient route to recover Cl2 from HCl-containing waste streams in the chemical industry. For a long time, the HCl oxidation process suffered from a lack of suitable catalysts, as common systems based on copper (the Deacon catalyst) and chromium (Mitsui–Toatsu) exhibited low activity and were prone to volatilization and, eventually, corrosion in the plant. Ruthenium-based catalysts were first introduced by Shell in the 1960s using SiO2 as the support. [3] The remarkable Deacon performance exhibited by this metal has led the way to a wider scope for industrialization of the hydrochloric acid oxidation process. Sumitomo Chemicals brought a TiO2 (rutile)-supported RuO2 catalyst to market, which was optimized for use in a fixed-bed tube bundle reactor. Bayer MaterialScience AG and Bayer Technology Services recently patented an alternative Rubased catalyst using a SnO2 (cassiterite) support optimized for application in a single adiabatic reactor cascade. Despite the benefits introduced, further improvements are needed to address another critical aspect for a robust HCl oxidation catalyst, which is the long-term stability of the RuO2 phase under Deacon conditions. The origin of the catalyst deactivation has not been extensively investigated, but pretreatments of the support to favor the epitaxial growth of RuO2 as a film on top of TiO2 (rutile) or SnO2 (cassiterite) due to lattice matching of both the active phase and the carrier have been reported as the main “trick” to attain improved catalytic properties. We have found that this tactic is not sufficient to avoid deactivation, at least in the case of the catalyst supported on SnO2. Herein we present a novel shaped Deacon catalyst with high potential for large-scale implementation due to its high activity and remarkable longevity in pilot test for 7000 h. In addition to the appropriate choice of active phase (RuO2) and carrier (cassiterite), a binder/stabilizer (g-Al2O3) has been introduced. The latter compound is shown to minimize agglomeration of the ruthenium phase under reaction conditions, thus perpetuating stable behavior. The three main components of the catalyst have been designed as follows. Concerning the active constituent, ruthenium remained the most suitable option due to its unrivalled activity in HCl oxidation at low temperatures with respect to other metals. The commercial SnO2-cassiterite powder was calcined at 1273 K prior to use to ensure the formation of the rutile structure also at a surface level and therefore to allow the epitaxial growth of RuO2 onto the support. The procedure is effective in short times, leading to a material with a surface area (SBET) equal to 9 m 2 g . A good coating of the support by the active phase not only aims at preventing structural alterations of the RuO2 phase, but also at protecting SnO2 from eventual chlorination under Deacon conditions and consequent volatilization as SnCl4. The catalytically active phase was deposited through impregnation of RuCl3 over a SnO2–Al2O3 composite shaped in 2 mm spherical pellets (see Experimental section). Low-temperature calcination (523 K) was employed for the latter step to minimize sintering of the ruthenium phase, thus achieving a higher dispersion. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX) of a pellet cross-section indicated that the active phase is uniformly distributed within the pellet (see the Supporting Information, Figure SI 1). The binder matrix included in the catalyst formulation was g-Al2O3 [particle size distribution (by TEM) = 5–20 nm, SBET = 200 m g ] , the amount of which corresponded to 10 wt. % in the final catalyst. The primary function of this component is to improve the textural properties of RuO2/SnO2 to allow shaping of the material in order to derive a technical catalyst. Most importantly, the intimate contact of the alumina binder with the RuO2-coated SnO2 grains inhibits agglomeration of the active phase under Deacon conditions, which is the main factor responsible for catalyst deactivation (see below). The corresponding alumina-free RuO2/SnO2 catalyst was synthesized in powder form according to the same method to serve as a ref-

Industrial RuO2‐Based Deacon Catalysts: Carrier Stabilization and Active Phase Content Optimization

A copper catalyst based on a delafossite precursor (CuAlO(2)) displays high activity and extraordinary lifetime in the gas-phase oxidation of HCl to Cl(2), representing a cost-effective alternative to RuO(2)-based catalysts for chlorine recycling.

Development of industrial catalysts for sustainable chlorine production.

RuO2/SnO2–Al2O3 has been recently reported as an industrial catalyst for Cl2 production through HCl oxidation. The stabilizing role of the alumina binder in the material, essential for its durable performance, is elucidated here. Al2O3 prevents chlorination of the SnO2 carrier under relevant reaction conditions, whereas, in its absence, SnO2 losses exceed 80 wt % in very short times owing to volatilization as SnCl4. Characterization by using X‐ray diffraction, temperature‐programmed reduction with hydrogen, and high‐resolution TEM indicates expansion of the cassiterite cell in the SnO2–Al2O3 composite with respect to pure SnO2, which suggests the insertion of certain Al species upon mechanochemical and thermal activation of the oxide mixture. 27Al magic‐angle spinning NMR and X‐ray photoelectron spectroscopy studies reveal that the pentahedrally coordinated Al3+ cations interact with SnO2, generating an electron‐depleted region near the surface of SnO2 particles. This induces some acidic character in cassiterite, which possibly makes it inert toward HCl. Besides this electronic effect, the presence of thin porous amorphous alumina films, partly covering the SnO2 surface, can offer additional geometric protection of the support. Mechanical mixing followed by calcination is essential to attain stabilization, and maximized effects are achieved with a high‐surface area alumina. Other oxides such as SiO2 are ineffective in preventing tin losses during HCl oxidation. The practical implications of these findings are very important. The metal loading (fourfold decrease) and, thus, the cost of the catalyst can be significantly lowered without compromising its long‐term stability.

Performance, structure, and mechanism of CeO2 in HCl oxidation to Cl2

The heterogeneously catalyzed gas-phase oxidation of HCl to Cl(2) offers an energy-efficient and eco- friendly route to recover chlorine from HCl-containing byproduct streams in the chemical industry. This process has attracted renewed interest in the last decade due to an increased chlorine demand and the growing excess of byproduct HCl from chlorination processes. Since its introduction (by Deacon in 1868) and till recent times, the industrialization of this reaction has been hindered by the lack of sufficiently active and durable materials. Recently, RuO(2)-based catalysts with outstanding activity and stability have been designed and they are being implemented for large-scale Cl(2) recycling. Herein, we review the main limiting features of traditional Cu-based catalysts and survey the key steps in the development of the new generation of industrial RuO(2)-based materials. As the expansion of this technology would benefit from cheaper, but comparably robust, alternatives to RuO(2)-based catalysts, a nov el CeO(2)-based catalyst which offers promising perspectives for application in this field has been introduced.