Heterogeneous catalytic reactions of 2-chlorophenol

Professional Doctorate - Doctor of Philosophy (PhD)Polychlorinated dibenzo-p-dioxins and dibenzofuran (PCDD/Fs) constitute a major toxic component of environmental pollutants. It is well documented that formation of these compounds accelerates in the presence of transition metals especially copper and iron. It has also been established that chlorinated phenols and chlorinated benzenes play a vital role in their formation. Numerous studies have been reported regarding surface-mediated formation of PCDD/Fs from reactions of chlorinated phenols. However, fewer experimental studies have been performed on alumina and iron oxide as active sites, in spite of being the most abundant compounds in fly ash. In this thesis, we have investigated the catalytic decomposition of 2-chlorophenol (2-CPh) into toxic products including polychlorinated dibenzo-<i>p</i>-dioxin and polychlorinated dibenzofuran (PCDD/Fs, dioxins). The catalysts that have been investigated for 2-CPh decomposition consist of silica, alumina, silica-supported iron oxide and alumina-supported iron oxide. To gain a thorough and more comprehensive understanding of the PCDD/Fs formation we arranged a series of reactions using a bench tubular reactor, with the main focus on varying temperature and time on stream. The reaction products were identified and quantified with a gas chromatogram quadrupole time of flight mass spectrometer (GC-QTOF), micro gas chromatogram (µGC) and an ion chromatogram (IC). In addition, we identified the bonds formed on the surface of the catalysts due to the decomposition of 2-CPh with an <i>in-situ</i> Fourier transform infrared (<i>in-situ</i> FTIR) spectrometer. Additionally, X-ray photoelectron spectroscopy (XPS), temperature program desorption (TPD), NO gas adsorption by in-situ FTIR, CO chemisorption, catalyst surface area measurement and inductively coupled plasma optical emission spectrometry (ICP-OES) have been performed for catalyst characterisation. Initially, we studied 2-CPh decomposition on the surface of neat silica. We have identified chlorinated phenols as volatile organic compounds (VOCs) and detected chlorinated and non-chlorinated DD/Fs including dibenzo-<i>p</i>-dioxin (DD), 1- and 2-monochlorodibenzo-p-dioxin (1-, 2-MCDD), 1,6-, 1,9-, 1,3-dichlorodibenzo-p-dioxin (1,6-, 1,9-, 1,3-DCDD), 4-monochlorodibenzofuran (4-MCDF) and 4,6-dichlorodibenzofuran (4,6-DCDF). In this chapter we showed that silica has the ability for chlorination/dechlorination and the dioxin products are formed by self-condensation of chlorophenoxy radicals on the surface and coupling of chlorophenoxy radicals on the surface with chlorinated phenols, also we proved that even in the absence of transition metals, PCDD/Fs could form on the surface of neat silica. Oxidative decomposition of 2-CPh on the surface of silica-supported iron oxide revealed the production of phenol, benzene, chlorinated phenols and chlorinated benzenes. Silica-supported iron oxide has a greater ability for chlorination/dechlorination of dioxin condensation products and results in PCDD/F congeners including DD, mono-triCDDs and mono-triCDFs. To gain further understanding of the chlorination pattern by electrophilic substitution of primary dioxins products, we estimated Hirshfeld charge distributions and Fukui indices based on the GGA functional and BLYO basis set by DMOl3 code. The decomposition of 2-CPh on the surface of alumina-supported iron oxide compared to silica-supported iron oxide results in additional products such as dibenzofuran (DF) and 1-, 2-, 3-monochlorodibenzofuran (1-, 2-, 3-MCDF). These products may form either from the carbon deposited on the surface or be due to the higher dechlorination affinity of alumina compered to silica support. The intriguing differences can be seen by a greater decomposition of 2-CPh on the surface and the appearance of PCDD/Fs after an induction period compared to silica-supported iron oxide. Decomposition of 2-CPh to oxidative products is favoured for alumina-supported iron oxide. We showed by <i>in-situ</i> FTIR and XPS that a large amount of coke was deposited on the surface of alumina-supported iron oxide and the formation of acetate, formate and carboxylate is favoured to the formation of anionic chlorophenolate/chlorophenoxy radicals (dioxin precursors) at low pressures of 2-CPh. Dioxin precursors appeared only after all of the active sites of the catalyst were covered with coke. However, the dioxin precursors were formed immediately on the surface of silica-supported iron oxide at very low pressures (0.01 µbar) of 2-CPh. Further more, we showed by XPS analyses that iron (III) reduced to iron (II) for the used catalyses. The major differences between the alumina- and silica-supported iron oxide can be summarised as much higher acidity of alumina compered to silica, identifying larger particles of iron oxide on the silica compared to the alumina support and existence of isolated iron (Fe²⁺) species on the alumina support. We confirmed these results by ammonia TPD, CO chemisorption and NO <i>in-situ</i> FTIR. Finally, a zeolite with aluminosilicate structure was chosen to gain a fundamental understanding for formation of PCDD/Fs over a well defined Cu sitet. Additionally, due to presence of significant large amounts of HCl in incinerators and considering copper as the most effective transition metal for formation of PCDD/Fs, we discussed the adsorption of hydrogen chloride gas (HCl) on the surface of copper modified ZSM-5 by <i>in-situ</i> FTIR. The results indicated a significant change in vibrational frequency of HCl after adsorption at low pressure (0.1 µbar). Higher HCl pressure (1 mbar) cause interaction of HCl with BrØnsted and terminal SiOH group of the zeolite. To understand the big shift in HCl frequency at low pressure, we carried out two layer ONIOM methodology. We consider Universal Force Field (UFF) for the low level and density functional theory (DFT) at B3LYP/6-31G(d,p) level of theory. The theoretical calculation indicated the frequency reduction to the interaction of Cl atom with the mononuclear Cu⁺ site in copper modified ZSM-5 framework