Amol P. Amrute

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-

CuCrO2 Delafossite: A Stable Copper Catalyst for Chlorine Production

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.

Halogen-Mediated Conversion of Hydrocarbons to Commodities

CuCrO2 Delafossite: A Stable Copper Catalyst for Chlorine Production With time comes wisdom : Since the implementation of CuCl2 for HCl oxidation by Deacon in 1868, the search for stable copper catalysts has been futile. Cuprous delafossite, CuCrO2 (see picture), is shown to have unprecedented stability against chlorination, allowing for durable Cl2 production with no metal loss. Based on this, a highly active CuCrO2-CeO2 composite was developed, a cost-effective Cl2 recovery method. Angewandte Chemie

Understanding CeO2 as a Deacon catalyst by probe molecule adsorption and in situ infrared characterisations

Halogen chemistry plays a central role in the industrial manufacture of various important chemicals, pharmaceuticals, and polymers. It involves the reaction of halogens or halides with hydrocarbons, leading to intermediate compounds which are readily converted to valuable commodities. These transformations, predominantly mediated by heterogeneous catalysts, have long been successfully applied in the production of polymers. Recent discoveries of abundant conventional and unconventional natural gas reserves have revitalized strong interest in these processes as the most cost-effective gas-to-liquid technologies. This review provides an in-depth analysis of the fundamental understanding and applied relevance of halogen chemistry in polymer industries (polyvinyl chloride, polyurethanes, and polycarbonates) and in the activation of light hydrocarbons. The reactions of particular interest include halogenation and oxyhalogenation of alkanes and alkenes, dehydrogenation of alkanes, conversion of alkyl halides, and oxidation of hydrogen halides, with emphasis on the catalyst, reactor, and process design. Perspectives on the challenges and directions for future development in this exciting field are provided.

Kinetic aspects and deactivation behaviour of chromia-based catalysts in hydrogen chloride oxidation

CeO(2) has been identified as an efficient catalyst for HCl oxidation in the temperature range of 623-723 K provided that the oxygen content in the feed mixture was sufficiently high to avoid bulk chlorination and thus deactivation. Here we characterise ceria in its fresh and post-reaction states by adsorption of CO(2), NH(3) and CO. Micro-calorimetry, FTIR and TPD experiments are complemented by DFT calculations, which assess adsorption energies and vibrational frequencies. The calculations were performed on the lowest energy surface, CeO(2)(111), with perfect termination and with various degrees of hydroxylation and/or chlorination. Both experiments and calculations suggest that the basic character of the ceria surface has been eliminated upon reaction in HCl oxidation, indicating that most of the basic lattice O sites are exchanged by chlorine and that the OH groups formed are rather acidic. The density and the strength of surface acidic functions increased significantly upon reaction. An in situ FTIR reaction cell has been designed and constructed to study the evolution of OH group density of the ceria surface during HCl oxidation. The effect of experimental variables, such as pO(2), pHCl and temperature, has been investigated. We found that the OH group density positively correlated with the reactivity in the pO(2) and temperature series, whereas negative correlation was observed when pHCl was varied. Implications of the above observations to the reaction mechanism are discussed.

Catalyst design for natural-gas upgrading through oxybromination chemistry

The gas-phase HCl oxidation was studied over bulk and supported Cr2O3-based catalysts by means of kinetic experiments in a fixed-bed reactor at ambient pressure and variable temperature, inlet O2/HCl ratio, and contact time. Cr2O3 exhibited high activity for Cl2 production. X-ray diffraction of the used catalysts detected no phase change. Temperature-programmed reduction with hydrogen and X-ray photoelectron spectroscopy showed that the surface of the fresh catalyst contains chromium species in oxidation state (III) as well as higher oxidation states (V and VI) while that of the used sample features the reduction of Cr5+/Cr6+ and comprises a little amount of chlorine. Coupling the catalytic and characterisation data over Cr2O3 with activity tests over CrO3, a redox cycle is proposed in which chromium species shift between Cr3+ and Cr5+ + Cr6+. The positive dependence of the HCl conversion on the inlet O2 concentration suggests that catalyst re-oxidation is rate limiting. SiO2 was identified as a better carrier for Cr2O3 than Al2O3 and TiO2-anatase, as it favours the formation of Cr2O3 nanoparticles rather than (unstable) isolated chromate species. However, all the supported catalysts suffered from severe deactivation due to substantial chromium loss. The deactivation mechanism is assigned to the in situ formation of the highly volatile CrO2Cl2 from Cr6+ species and CrO2(OH)2 from both Cr3+ and Cr6+ species. The deactivation rate can be reduced, though not suppressed, by applying a high O2 excess in the feed mixture, thus indicating that the deactivation route via the oxychloride might be predominant. The features observed represent critical reasons justifying the restricted industrial implementation of chromium-based catalysts for HCl oxidation.

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

Natural gas contains large volumes of light alkanes, and its abundant reserves make it an appealing feedstock for value-added chemicals and fuels. However, selectively activating the C-H bonds in these useful hydrocarbons is one of the greatest challenges in catalysis. Here we report an attractive oxybromination method for the one-step functionalization of methane under mild conditions that integrates gas-phase alkane bromination with heterogeneously catalysed HBr oxidation, a step that is usually executed separately. Catalyst-design strategies to provide optimal synergy between these two processes are discussed. Among many investigated material families, vanadium phosphate (VPO) is identified as the best oxybromination catalyst, as it provides selectivity for CH3Br up to 95% and stable operation for over 100 hours on stream. The outstanding performance of VPO is rationalized by its high activity in HBr oxidation and low propensity for methane and bromomethane oxidation. Data on the oxybromination of ethane and propane over VPO suggest that the reaction network for higher alkanes is more complex.

Depleted uranium catalysts for 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.

Superior activity of rutile-supported ruthenium nanoparticles for HCl oxidation

This study demonstrates depleted uranium as a remarkable heterogeneous catalyst for the oxidation of HCl to Cl2. This reaction comprises a sustainable approach to valorise byproduct HCl streams in the chemical industry. Bulk α-U3O8 showed an outstanding stability against chlorination, which is crucial for its durability in catalytic tests. UO2 and γ-UO3 transformed into α-U3O8 under reaction conditions. Uranium deposition on different carriers by dry impregnation concluded the superiority of zirconia as support. HAADF-STEM investigations revealed that the uranium oxide on the surface of this carrier is present in the form of a film-like nanostructure with a thickness ranging from a monolayer to 1 nm as well as atomic dispersion. The effect of variables (temperature, feed O2/HCl ratio, metal loading, and Cl2 co-feeding) on the performance of U3O8/ZrO2 has been studied. The HCl conversion over this catalyst increased with reaction time as a likely consequence of in situ re-dispersion of the original uranium phase into atomically dispersed UOx. As demonstrated by H2-TPR, the uranium in the generated UOx phase is more oxidised than in the original U3O8. Such a highly dispersed active phase is produced faster in the uncalcined sample. The extraordinary stable Cl2 production over U3O8/ZrO2 at 773 K for 100 h on stream indicates its potential for application in high-temperature HCl oxidation. Under these conditions, other known catalytic materials suffer from significant deactivation.