Albert Roessler

State of the art technologies and new electrochemical methods for the reduction of vat dyes

Until now, in most industrial vat dyeing processes, vat dyes are reduced mainly using sodium dithionite. This process produces large amounts of sodium sulphate and sulphite as by-products which increase the costs for waste water treatment. Hence, many attempts are being made to replace the environmentally unfavourable sodium dithionite by ecologically more attractive alternatives, such as organic reducing agents or catalytic hydrogenation. In recent investigations to improve the biocompatibility of the vatting process even further, various electrochemical reducing methods have been described, such as indirect electrochemical reduction employing a redox mediator, direct electrochemical reduction of indigo via the indigo radical, electrocatalytic hydrogenation and direct electrochemical reduction of indigo itself on graphite. These methods offer tremendous environmental benefits, since they minimize the consumption of chemicals as well as effluent load. However, most of these electrochemical processes are still in the development stage. This article gives an overview of the processes most commonly used and the state of development of recent electrochemical innovations.

Direct electrochemical reduction of indigo

Reducing agents required in the dyeing process for vat and sulfur dyes cannot be recycled, and lead to problematic waste products. Therefore, modern economical and ecological requirements are not fulfilled. The application of direct electrochemical reduction of indigo as a novel route has been investigated by spectrophotometric and voltammetric experiments in laboratory cells. Experiments yield information about the reaction mechanism and the kinetics, and they show the possibility of this new route for production of water-soluble indigo, which offers tremendous environmental benefits.

Direct electrochemical reduction of indigo: process optimization and scale-up in a flow cell

Reducing agents required in the dyeing process for vat and sulfur dyes cannot be recycled, and lead to problematic waste products. Therefore, modern economical and ecological requirements are not fulfilled. The industrial feasibility of the direct electrochemical reduction of indigo as a novel method has been determined and a preliminary optimization of electrolytic conditions was performed using a laboratory-scale flow-cell system. The role of current density, pH, temperature and the rate of mass transport are discussed. The influence of particle size reduction by the application of ultrasound is critically considered.

Electrocatalytic hydrogenation of vat dyes

Increasing eco-efficiency of textile wet processes has become an important topic. Reducing agents required for the application of vat dyes (i.e. indigo) cannot be recycled, and lead to problematic waste products. Therefore, modern economical and ecological requirements are not fullfilled. The application of electrocatalytic hydrogenation of vat dyes at electrodes comprising a thin grid coated with a layer of nickel in which fine particles of Raney nickel are dispersed has been investigated by spectrophotometric and voltammetric experiments in laboratory cells. Experiments show the feasibility of this new route which offers tremendous environmental benefits and has a vast potential in textile dyeing processes, because it does not require any reducing agent.

Electrocatalytic Hydrogenation of Indigo Process Optimization and Scale-Up in a Flow Cell

The industrial feasibility of the electrocatalytic hydrogenation of indigo on Raney nickel electrodes as a suspension in an aqueous medium has been studied in a divided flow cell. The attempt was made to establish optimized conditions in the system, and a scale-up in indigo concentration to 10 g/L was achieved. Increasing pH can enhance the reduction rate and a maximum conversion has been found by optimizing current density and temperature. The reaction rate is clearly enhanced in the presence of ultrasonic waves and organic solvents. Methanol is among the best cosolvents and was used in concentrations of 10 vol %.With Raney nickel electrodes an optimized current efficiency of 13% could he reached at 95% conversion. This value was enhanced to 19% by using electrodes doped with platinum particles.

Electrochemical reduction of indigo in fixed and fluidized beds of graphite granules

Reducing agents required in the dyeing process for vat and sulfur dyes cannot be recycled and lead to problematic waste products. The electrochemical reduction of indigo on a fixed bed cathode consisting of graphite granules has been investigated by spectrophotometric experiments in laboratory cells. Experiments yield information about the kinetics and show the possibility of this process for production of water soluble leuco indigo, which offers environmental benefits. The influence of noble metals deposited on the granules and of different pretreatment methods of the graphite is demonstrated. In addition, the immobilization of quinoid molecules on the graphite surface has been investigated.

Wenn die Rührgeschwindigkeit die Produktverteilung bestimmt: Selektivität mischungsmaskierter Reaktionen

Zahlreiche chemische Reaktionen laufen so schnell ab, dass ein signifikanter Umsatz bereits auftritt, bevor Konzentrationsinhomogenitaten bis in den molekularen Bereich ausgeglichen werden konnen. Die Transformation findet dabei in Regionen statt, in denen lokal vollig andere Konzentrationen vorliegen, als makroskopisch angenommen wird. Infolge der beschrankten Mischrate der Edukte im Reaktor treten daher Anderungen in der Reaktionsgeschwindigkeit auf, die signifikanten Einfluss auf die Produkteverteilung haben konnen. Kenntnisse daruber ermoglichen eine optimale Nutzung von Rohstoffen durch gezieltere Konstruktion und Operation von Reaktoren (Selektivitatsmaximierung). Mischungsmaskierte Reaktionen haben aber auch das Potential, quasi als Mikroproben, Inhomogenitaten bis in den molekularen Bereich zu registrieren. Sie fordern damit das Verstandnis uber Mischprozesse und manchmal auch uber den Verlauf von organisch-chemischen Reaktionen, deren Mechanismen nicht im Einklang mit allgemeinen mechanistischen Prinzipien der Chemie zu stehen scheinen. Many chemical reactions are so fast relative to mixing that significant conversion takes place before homogenization down to the molecular scale is reached. In such cases, the bond-breaking and bond-forming events mainly occur in regions where concentrations gradients are still present. Consequently the rate of mixing can have a pronounced effect on the kinetics and the product distribution in competitive chemical reactions. This review gives an account of such mixing-sensitive single-phase liquid-liquid chemical reactions where all reactants and products are soluble, because a better understanding of the coupling between mixing and chemical reaction is needed to optimize the product distribution by choosing the appropriate reactor geometry and mixing conditions. Additionally these reactions have the potential to be employed as an exploratory microprobe to investigate the local flow characteristics or the mixing process at the molecular level and to evaluate the mechanism of chemical reactions.