Development of Catalyst on Polymer Support for Organic Compounds Destruction

Polyacrylonitrile (PAN) fibre, in addition to polyvinylalchohol and polyamide fibres, is one of the most often used polymer matrix for production of ion-exchange, chelating and redox fibres as well as catalysts (Ishtchenko V.V., 2000, Petrov S.V., 2000) or fibres with some specific properties, for example anesthetic fibres (Shkhundrich P. et al, 1995). An advantage of PAN as a polymer support for a variety of fibres with chemical activity is, first of all, in that the polymer has a highly reactive functional group (nitrile, – C ≡ N) which can be converted to ion-exchange and chelating group or cyclic structure by action of physical factors (e.g. temperature) as well as chemical reagents. Second, PAN fibre is in production on a big scale: fibres made from PAN and co-polymers (usually methylmethacrylate, itaconic acid, vinylsulphonate) have been produced since 1949 (Gheller B, 2002). Thus, in this work PAN fibre was chosen as a polymer support for a heterogeneous catalyst, and the work itself is actually a continuation of the research on PAN modification performed at St.-Petersburg State University of Technology and Design (SUTD) in 1970 – 2000 as well as collaborative work of SUTD and DMU in 1997 - 2000 (Ishtchenko V.V., 2000). In addition to advantages of PAN fibre as a catalyst support, an important feature of the developed heterogeneous catalyst is that it is used in a form of knitted mesh. The mesh is produced from PAN complex threads and polypropylene monothreads that introduce flexibility and incompressibility to the mesh. The mesh also has substantial heat and mass transfer surface and relatively low hydraulic resistance which accelerates oxidation of pollutants in the reactor of catalytic wastewater treatment. To simplify PAN modification, the procedure was also performed using mesh. This PhD research is based on a previous investigation (Ishtchenko V.V., 2000) on modification of polyacrylonitrile (PAN) fibre to produce a catalyst for the decomposition of organic pollutants, such as anthraquinone dyes and phenol. The previous work used a modification mixture consisting of hydrazine and hydroxylamine to convert the nitrile groups of PAN fibre with the resultant formation of carboxylate, amine and hydroxamic groups. These functional groups are able to ligate cations of multivalent metals (e.g. iron (III)) to the fibre which then allows the fibre to act as a catalyst. In this work toxic hydrazine was replaced with ethylenediamine or EDTA. Thus, modifications were performed using mixtures of either hydroxylamine hydrochloride and ethylenediamine or hydroxylamine hydrochloride and EDTA (disodium salt of EDTA was used). Initially modification was performed under the same conditions as in the work (Ishtchenko V.V., 2000), i.e. pH of modification solution was 9.5 and duration of the process was 120 minutes. Modification of PAN knitted mesh was performed in 2 steps, and in contrast with Ishtchenko’s three-stage modification alkaline treatment of the mesh was not necessary. Thus, first step of the modification was treatment of PAN mesh with aqueous solution of hydroxylamine hydrochloride and ethylenediamine (or Na2EDTA) at pH = 7.5 – 10.5 at T = 98-1030C. Second step of the modification was impregnation of the modified PAN mesh with a solution of ferric chloride at room temperature for 19-24 hours. Modification of PAN with hydroxylamine hydrochloride and Na2EDTA was performed in 3 steps with alkaline treatment as a second step for which boiling solutions of sodium hydroxide of concentrations 2.5%, 5% and 10% (w/w) were used. Duration of alkaline treatment was 30 seconds. In the preliminary experiments on assessment of catalytic activity of modified impregnated PAN it was found that the fibre decolorizes 10 g/L solutions of Acid Blue 45, but dye removal from the solution proceeded mostly due to homogeneous decomposition as iron was leaching from the fibre and possibly due to sorption of the dye onto the fibre. In addition to this, FTIR spectra of PAN fibre modified with hydroxylamine hydrochloride and Na2EDTA indicated very poor decrease in the intensity of the cyanide peak, i.e. very low conversion of – C ≡ N groups of PAN. FTIR spectrum of PAN modified with mixture of hydroxylamine hydrochloride and Na2EDTA and FTIR spectrum of PAN modified with hydroxylamine hydrochloride alone were almost identical. Therefore modification of PAN with hydroxylamine hydrochloride and Na2EDTA was discontinued Optimisation of the parameters of PAN modification with ethylenediamine and hydroxylamine was directed to produce catalytically active PAN with satisfactory mechanical properties. To develop optimal parameters of the catalyst production, modifications were performed with ethylenediamine and hydroxylamine hydrochloride aqueous solutions under varied conditions: with different concentrations of modification reagents (concentration of ethylenediamine varied from 5.8 g/L to 17 g/L and concentration of hydroxylamine hydrochloride varied from 14 to 42 g/L), pH of the modification solution (pH=7.5-10.5) and different duration of the modification (from 30 to 120 minutes). Concentration of Fe (III) in the impregnation solution was also optimised, in the initial experiments 50 g/L solution of Fe(III) was used but due to iron leaching from the fibre in further work iron concentration was lowered to 2.07 g/L. Thus, it is very important that iron is strongly ligated to the fibre and does not leach to the solution therefore strength of iron (III) ligation to the modified PAN fibre was assessed by exposure of the modified impregnated fibre to the solution of a strong chelating agent (EDTA). Iron leaching from the fibre was assessed both in acidic (pH=3 and pH=4.5) and alkaline media (pH=9-12). It was found that modification of PAN with mixture of 19 g/L hydroxylamine hydrochloride and 8 g/L ethylenediamine at pH = 7.5 results in the highest concentration of iron on the fibre (0.132 mmol/g) and also provides strongest ligation of the metal to the fibre. The resultant fibres were characterized by means of FTIR, FTIR-ATR spectroscopy and Scanning Electron Microscopy. FTIR-ATR spectra of modified non-impregnated PAN (obtained for PAN complex threads) showed that modification of PAN proceeds on the surface of the fibre that contacts with the reagents (hydroxylamine and ethylenediamine) and complete conversion of the nitrile group proceeds at pH=7.5. SEM/EDX studies of modified impregnated PAN fibre go together with FTIR-ATR data, i.e. EDX data showed that iron is distributed mostly on the surface of the fibre and depth of iron penetration is 5 micrometers. Catalytic activity was investigated by the decolourisation of aqueous solutions of anthraquinone dye Acid Blue 45, decomposition of hydrogen peroxide and oxidation of sodium sulphide using the following analytical techniques: colourimetry, titration and sulphide selective electrode, respectively. Very poor catalytic activity of the fibre was observed towards decomposition of all substrates, and it was found that decolourisation of Acid Blue 45 solutions proceeded due to dye sorption onto the fibre, most likely by binding to protonated amino groups. Thus, ion-exchange and sorption properties of the modified PAN fibre were also investigated with the quantitative determination of anion and cation exchange capacities and sorption capacity towards Methylene Blue of PAN fibres modified under varied conditions. Investigation of Acid Blue 45 sorption by the modified PAN fibres under both static and dynamic conditions is also presented.