Metal-mediated reductive hydrodehalogenation of organic halides.

While experts in various fields discuss the potential of carbon capture and storage (CCS) technologies, the utilization of carbon dioxide as chemical feedstock is also attracting renewed and rapidly growing interest. These approaches do not compete; rather, they are complementary: CCS aims to capture and store huge quantities of carbon dioxide, while the chemical exploitation of carbon dioxide aims to generate value and develop better and more-efficient processes from a limited part of the waste stream. Provided that the overall carbon footprint for the carbon dioxide-based process chain is competitive with conventional chemical production and that the reaction with the carbon dioxide molecule is enabled by the use of appropriate catalysts, carbon dioxide can be a promising carbon source with practically unlimited availability for a range of industrially relevant products. In addition, it can be used as a versatile processing fluid based on its remarkable physicochemical properties.

Chemical Technologies for Exploiting and Recycling Carbon Dioxide into the Value Chain

[fac-Re(bpy) (CO)3Cl] (bpy = 2,2′-bipyridine) is an efficient homogeneous catalyst for the selective and sustained photochemical or electrochemical reduction of CO2 to CO. A quantum yield of 14% and a faradic efficiency of 98% were measured in the presence of excess Cl− ions. The photochemical process took place under visible-light irradiation and consumed a tertiary amine as electron donor. A formato-rhenium complex was isolated in the absence of excess Cl− ions. Substitution by Cl− ion generated free formate, but no CO was detected. Luminescence measurements showed that the tertiary amine quenches the metal-to-ligand charge-transfer excited state of the rhenium complex via a reductive mechanism, with a rate constant of 3.4 × 107M−1S−1. The 19e-complex [Re(bpy) (CO)3X]− produced either photochemically or electrochemically appears to be the active precursor in the CO-generation process. Detailed spectroscopic studies on 13C-enriched carbonyl-rhenium and formato-rhenium complexes derived from 13C-enriched CO2 were performed in order to confirm the origin of the products and to study the exchange of the ligands. A mechanism for the present CO2 photoreduction process is presented; it involves separate pathways for CO and formate generation, in which the [Re(bpy) (CO)3X] complex plays the role of both the photoactive and the catalytic center.

Photochemical and Electrochemical Reduction of Carbon Dioxide to Carbon Monoxide Mediated by (2,2′‐Bipyridine)tricarbonylchlororhenium(I) and Related Complexes as Homogeneous Catalysts

CO2 utilization: Developments in conversion processes

Aluminium and its alloys Aluminium corrosion Atmospheric corrosion of aluminium Corrosion in water The action of mineral products The action of organic products The effect of different environments appendices

Corrosion of Aluminium

Oxalic acid has been prepared on a preparative scale in an undivided cell with sacrificial anodes and an aprotic electrolyte. Optimum current efficiencies (>90%) have been obtained in cells with zinc anodes, stainless steel cathodes and acetonitrile with tetrabutylammonium perchlorate as the electrolyte. Micro-pilot experiments revealed that a continuous electrochemical cell can be set up and that the reaction product can be removed by filtration. A complete process is designed by a combination of the oxalic acid production with a zinc electrolysis. No unwanted by-products appear, which is recently of increasing importance. Economic consideration shows that the process may become interesting in the future.

The production of oxalic acid from CO2 and H2O

Developments in the chemical and petrochemical industries, advances in chemical engineering, the introduction of new products, advances in the technology of intermediates, and problems in energy conversion have created new corrosion problems, involving the failure of metallic materials under the influence of aggressive organic solvents. Failures of this type lead not only to a deterioration of the mechanical properties of structural material, but also to discoloration and unwanted changes of the solvent. Since organic solvents in most cases are poisonous, inflammable, and, in the presence of air, explosive, the demands on the corrosion resistance of the materials used are immense. Therefore, in discussing corrosion of metals in organic solvents problems arise which have no parallel in the field of corrosion in aqueous media.

Corrosion of Metals in Organic Solvents

Introduction Methods of Measurement Electrochemical Transport Processes Mass Transport Charge Transport Heat Transfer Electrochemical Reaction Engineering Electrochemical Thermodynamics and Electrode Kinetics Kinetics in Electrochemical Reactors Separation Systems In Electrochemical Cells Optimization and Factorial Design of Experiments Materials and Corrosion Experimental Modelling of Industrial Processes Appendix Definitions of Electrochemical and Engineering Terms Problems Results and Discussion of Experiments Index.

Principles of electrochemical engineering

A low-energy dechlorination technique has been applied to the detoxification of chlorinated hydrocarbons (CHCs) in aqueous solution. This alternative to biological or physico-chemical decomposition methods achieves reduction by means of metal particles or copper-activated metal particles (Mg, Fe, Al, Zn). The paper presents data from two pilot studies, covering fundamental process design aspects. Experimental investigations of pilot study 1 were conducted in a laboratory-scale fixed bed reactor. In weak acidic or neutral medium (pH-range 4-7) proper process application resulted in very rapid removal of hexachlorocyclohexane concentration ({gamma}-BHC, lindane) from model wastewaters with up to 99.3% efficiency. During the course of reaction the main products formed are chloride and benzene, the by-product being small amounts of chlorobenzene. After employing different metals in the dechlorination reaction zinc proved to be the optimum reducing agent since it achieved the best space-time yield. Pilot study 2 dealt with a couple of screening reactions of various CHCs with zinc as the reducing agent, which were performed in the batch mode. Even volatile CHCs (chloroform, 1,1,1-trichloroethane) could be completely dechlorinated by forming the corresponding hydrocarbons (methane, ethane and ethene respectively). Pseudo-first order rate constants (k,s{sup {minus}1}) were determined to assess substrate reactivity with respect to possiblemore » process applications. Process design aspects, advantages and limitations are discussed. 27 refs., 9 figs., 4 tabs.« less

Development of a wastewater treatment process: Reductive dehalogenation of chlorinated hydrocarbons by metals

In vielen Korrosionsfallen spielt der Antransport des angreifenden Agens oder der Abtransport der Korrosionsprodukte eine bedeutende Rolle. Nach einer Erorterung der fur den Stofftransport wichtigen Grundlagen werden eine Reihe von Untersuchungsmethoden besprochen. Es sind dies Versuche mit Hilfe der rotierenden Scheibe, dem durchstromten Rohr und dem rotierenden Zylinder. Auch wird gezeigt, das Dauertauchversuche durch naturliche Konvektion beeinflust sein konnen.



Am Beispiel der Korrosion von Nickel in Salzsaurelosungen bei definiertem Sauerstoffpartialdruck wird der Einflus der Stromung untersucht. Dabei kommt die Methode der rotierenden Scheibe in einer modifizierten Form zur Anwendung, die den speziellen Forderungen bei Korrosionsuntersuchungen Rechnung tragt. Die erhaltenen Ergebnisse lassen darauf schliesen, das die untersuchte Korrosionsreaktion sowohl vom Transport als auch von der Phasengrenzreaktion abhangt.



Im letzten Teil der Arbeit wird die Transportgeschwindigkeit bei den in der Praxis verwendeten Umlaufapparaturen einerseits mit der an rotierenden Scheiben und Zylindern andererseits verglichen. Das Ergebnis ist, das sich Umlaufapparaturen durch die in ihrem apparativen Aufbau wesentlich einfacheren rotierenden Scheiben und Zylinder ersetzen lassen.



Investigations on transport processes during corrosion



In many cases of corrosion, the transport of the corroding agent or the removal of the corrosion products play an important part. After a discussion of the basic principles governing the transport of matter, a number of investigation methods are outlined. They include tests with rotating discs, flow tubes, and rotating cylinders. It is also shown that long-term immersion tests may be influenced by natural convection.



The influence of the flow is examined on the strength of the example of the corrosion of nickel in hydrochloric acid solutions at a given partial pressure of oxygen. This investigation is carried out with a modified version of the rotating disc method, which takes into account the special requirements with corrosion tests. The results permit the conclusion that the investigated corrosion reaction depends on the transport as well as on the phase boundary reaction.



In the last part of the paper, the transport rate with the circulation equipment used in practice is compared with that encountered with rotating discs and cylinders. It was found that it is possible to replace the conventional equipment by rotating discs and cylinders where the apparatus is considerably simpler.

Ewald Heitz

Papers

The production of oxalic acid from CO2 and H2O

Corrosion of Metals in Organic Solvents

Principles of electrochemical engineering

Development of a wastewater treatment process: Reductive dehalogenation of chlorinated hydrocarbons by metals

Untersuchung von Transportvorgängen bei der Korrosion