Date of Award

2018

Publication Type

Master Thesis

Degree Name

M.Sc.

Department

Chemistry and Biochemistry

Keywords

Ball mill chemistry; Synthesis of azobenzene derivative

Supervisor

Eichhorn, Stephan

Rights

info:eu-repo/semantics/openAccess

Abstract

Objective of this thesis is the development of a green, low cost, and scalable synthesis of 3,3’,5,5’-azobenzene tetracarboxylic acid, which is the organic building block for PCN-250. PCN-250(Fe3) is an important metal-organic framework that has excellent stability and gas adsorption properties and has been commercialized by Framergy, a start-up out of Texas A&M. All reported syntheses of 3,3’,5,5’-azobenzene tetracarboxylic acid are based on a one-step reductive coupling of commercially available 5-nitroisophtalic acid in alkaline aqueous or alcoholic solutions with either zinc or D-glucose as reducing agent. The first part of this thesis focuses on the optimization of the reported procedures in solution but the highest obtained yield of 45% remained well below the reported yields of 70%. In order to further reduce the amount of used solvent and potentially increase the yield, the reactions described above were subsequently conducted mechanochemically in a laboratory-scale ball milling unit. Optimization of the milling time, size and number of milling balls, amount and type of reducing agent, and solvent content improved the conversion to >70% but the formation of hydrazo and N-oxide side-products could not be avoided. Unexpectedly, these mixtures of compounds were advantageous in the formation of the azo product under specific workup conditions. When the crude mixture was directly immersed in alkaline solution for a period of 48 hours under atmospheric conditions, the unreacted starting material and aniline, hydrazo and N-oxide side-products convert to 3,3’,5,5’-azobenzene tetracarboxylic acid, most likely by redox reactions between the side-products and with oxygen. These optimized conditions for the ball milling process and workup protocol furnish the desired 3,3’,5,5’-azobenzene tetracarboxylic acid in 90% yield and >95% purity by using a green solvent assisted ball milling method and crystallization from aqueous solution as the sole purification step.

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