Catalytic Oxidation of Phenol over Zeolite Based Cu/Y-5 Catalyst: Part 1: Catalyst Preparation and Characterization

Potreba za uklanjanjem organskih nečistoća iz industrijskih otpadnih voda uvjetovala je razvoj novih, djelotvornih tehnologija s posebnim naglaskom na uštedu sirovina i energije te ekonomičnost procesa. Katalitička oksidacija organskih zagađenja vodikovim peroksidom u vodenom mediju, poznata kao metoda CWPO (eng. Catalytic Wet Peroxide Oxidation), jedan je od postupaka koji ispunjava navedene zahtjeve. Upotrebom vodikova peroksida, kao oksidacijskog sredstva, i pogodnog heterogenog katalizatora moguće je proces provoditi pri atmosferskom tlaku i temperaturi ispod 383 K. Zeoliti su zbog svojih specifičnih značajki (selektivnost prema veličini i obliku molekula, termička i kemijska stabilnost, neškodljivost živom svijetu) izrazito pogodni katalizatori za uporabu u selektivnim oksidacijskim procesima. Stoga su u ovom radu proučavani aktivnost, selektivnost i stabilnost katalizatora Cu/Y-5 u reakciji oksidacije fenola vodikovim peroksidom. Katalizator je pripravljen ionskom izmjenom protoniranog oblika komercijalnog zeolita. Karakterizacija katalizatora obuhvaćala je rendgensku difrakciju (XRD), pretražnu elektronsku mikroskopiju (SEM) i elementnu analizu atomskim apsorpcijskim spektrometrom (AAS), te određivanje specifične površine i obujma pora (standardna BET-metoda). Reakcija je provođena u šaržnom Parrovu reaktoru pri atmosferskom tlaku i temperaturama od 323 do 353 K. Katalizator je pripravljen u praškastom obliku, a maseni udjel bakra u zeolitu iznosio je 3,46 %. Početna koncentracija fenola u reakcijskoj otopini bila je 0,01 mol dm−3, a vodikova peroksida od 0,01 do 0,10 mol dm−3. Dobiveni eksperimentalni podaci testirani su sljedećim kinetičkim modelima za oksidaciju fenola rPh = k1 cPh cHP i raspad vodikova peroksida rHP = k2 cHP, a kinetički parametri procijenjeni su Nelder-Meadovom metodom nelinearnog optimiranja. Rezultati provedenih istraživanja pokazali su da je pri ispitivanim reakcijskim uvjetima primjenom postupka CWPO uz katalizator Cu/Y-5 moguće iz početne reakcijske smjese u potpunosti ukloniti fenol uz istodobno 54,5 %-tno smanjenje sadržaja ukupnog organskog ugljika.The necessity to remove organic pollutants from the industrial wastewater streams has forced the development of new technologies that can produce better results in terms of pollutant removal and process efficiency in combination with low investment and operating costs. One of the new emerging processes with a potential to fulfil these demands is catalytic wet peroxide oxidation, commonly known as the CWPO process. The oxidative effect of the hydrogen peroxide is intensified by the addition of a heterogeneous catalyst that can reduce the operating conditions to atmospheric pressure and temperatures below 383 K. Zeolites, among others, are especially appealing as catalysts for selective oxidation processes due to their unique characteristics such as shape selectivity, thermal and chemical stability, and benign effect on nature and the living world. In this work, catalytic activity, selectivity and stability of Cu/Y-5 zeolite in phenol oxidation with hydrogen peroxide was examined. Catalyst samples were prepared by ion exchange method of the protonic form of commercial zeolite. The catalysts were characterized with powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and AAS elemental analysis, while the adsorption techniques were used for the measurement of the specific surface area. The catalytic tests were carried out in a stainless steel Parr reactor in batch operation mode at the atmospheric pressure and in the temperature range from 323 to 353 K. The catalyst was prepared in powdered form and the mass fraction of the active metal component on the zeolite was 3.46 %. The initial concentration of phenol solution was equal to 0.01 mol dm−3 and the concentration of hydrogen peroxide ranged from 0.01 to 0.10 mol dm−3. The obtained experimental data was tested to a proposed kinetic model for phenol oxidation r = k1 cF cVP and hydrogen peroxide decomposition rHP = k2 cHP. The kinetic parameters were estimated using the Nelder-Mead method of nonlinear regression. On the basis of the obtained results of characterization process and conducted catalytic tests, the following can be observed. Zeolite structure of the prepared catalyst was confirmed through powder X-ray diffraction, scanning electron microscopy and adsorption techniques. Their catalytic performance was monitored in terms of phenol and total organic carbon (TOC) conversions, hydrogen peroxide decomposition, by-product distribution and degree of copper leached into the aqueous solution. The obtained experimental results indicate that in the space of 180 minutes, the use of these catalysts allows almost total elimination of phenol and significant removal of total organic carbon content with the use of small amounts of catalyst (0.1 g dm–3) and substoichiometric level (71.4 %) of oxidant required for complete oxidation of organic pollutant. The main product among aromatics was catechol, followed by hydroquinone and benzoquinone, which exhibited the typical pattern for a series reaction scheme. The distribution of carboxylic acids was as follows: maleic, fumaric, acetic and oxalic acids. These low-molecular carboxylic acids and aromatic compounds were responsible for the TOC that remained after almost complete removal of phenol. Moreover, one of the most interesting options was to use CWPO as a pre-treatment prior to biological treatment, for simple organic acids that are highly biodegradable. During the reactions, destabilization of the catalyst was observed in terms of leaching of copper from zeolite into the reaction mixture, but the previous investigations of similar catalytic systems showed that the activity of the solid catalyst was not due to the homogeneous contribution of the copper leached from the catalyst, but was more likely due to the activity of the heterogeneous catalyst. Further investigations on the mechanism of catalyst destabilization and methods of stabilization are the subject of the following article in the series. The proposed kinetic model adequately fits the experimental data. The rate of phenol oxidation and hydrogen peroxide decomposition increases with the increase in reaction temperature and decrease in initial concentration of hydrogen peroxide. The rate of phenol oxidation is first order toward the concentration of phenol and first order toward the concentration of H2O2. The rate of H2O2 decomposition is first order toward the concentration of H2O2. Activation energy in phenol oxidation was 86.0 kJ mol−1 for phenol oxidation, and 107.6 kJ mol−1 for hydrogen peroxide decomposition, which is in accordance with previously published results

Catalytic Oxidation of Phenol over Zeolite Based Cu/Y-5 Catalyst: Part 2: Influence of Postsynthesis Thermal and Chemical

Katalitička oksidacija zagađivala organskog podrijetla vodikovim peroksidom u vodenom mediju, poznata kao metoda CWPO (eng. Catalytic Wet Peroxide Oxidation), jedan je od postupaka kojim je moguće postići smanjenje organskog opterećenja otpadnih voda u praksi. U ovom radu zeolit Y-5 izabran je za nosač u koji je kao katalitički aktivna tvar ugrađen bakar. U radu je istražen utjecaj reakcijskih parametara te postsintetske kemijske i termičke obrade katalizatora na njegove katalitičke značajke: aktivnost i stabilnost. Katalizator Cu/Y-5 pripravljen je ionskom izmjenom protoniranog oblika komercijalnog zeolita Y-5. Radi poboljšanja njegovih katalitičkih značajki provođena je postsintetska kemijska (ispiranje sa H2SO4) i termička obrada (kalciniranje). Karakterizacija katalizatora Cu/Y-5 obuhvaćala je rengdensku difrakcijsku analizu na praškastom uzorku (PXRD) i elementnu analizu na atomskom apsorpcijskom spektrometru (AAS) te određivanje specifične površine, obujma pora i raspodjele obujma pora (standardna metoda BET). Aktivnost i stabilnost pripravljenih katalizatora ispitana je u reakciji oksidacije fenola vodikovim peroksidom u vodenoj otopini. Maseni udjel bakra na zeolitu bio je 3,46 % prije postsintetske obrade, odnosno 3,97 % nakon postsintetske termičke obrade te 0,94 % nakon postsintetske kemijske obrade. Eksperimentalni podatci dobiveni provođenjem katalitičkih testova testirani su sljedećim kinetičkim modelima za oksidaciju fenola rPh = k1 cPh cHP i raspad vodikova peroksida rHP = k2 cHP. Za procjenu kinetičkih parametara primijenjena je Nelder-Meadova metoda nelinearnog optimiranja. Oba postupka postsintetske obrade značajno su poboljšala stabilnost pripravljenog Cu/Y-5 katalizatora, dok je istodobno njegova aktivnost ostala nepromijenjena ili je poboljšana.The most important and the most vulnerable part of the global ecosystem are surface waters. Tothis day, numerous scientific investigations have been conducted in to develop new technologies for most effective purification of wastewaters polluted with organic compounds such as phenol and its derivatives. Catalytic wet peroxide oxidation, known as the CWPO process, is one of the methods that can be used for the minimization of organic pollution in practice. With the use of a catalyst (homogeneous or heterogeneous), the process can be successfully operated under mild conditions with low energy consumption (atmospheric pressure and temperatures below 353 K). Zeolites modified with copper possess good catalytic properties when compared to the other types of heterogeneous catalysts tested in CWPO reaction. Based on the literature overview and actual trends in scientific research concerning development of new catalytic oxidation processes for treatment of wastewaters burdened with organic pollutants, Y-5 FAU type of zeolite was selected as catalyst support for copper cations. In this paper, second in a series, investigated was the influence of reaction parameters, synthesis and postsynthesis chemical and thermal treatment of the prepared catalyst on its catalytic properties. The catalyst was prepared by ion exchange method of the protonic form of commercial zeolite. In order to obtain the catalyst with optimum catalytic properties, chemical (H2SO4 wash) and thermal postsynthesis treatment (calcination) was conducted. The catalysts were characterized with powder X-ray diffraction (PXRD) and AAS elemental analysis, while the adsorption techniques were used for the measurement of the specific surface area. Activity and stability of such prepared catalysts was tested in catalytic wet peroxide oxidation of phenol in aqueous medium. The mass fraction of the active metal component on the zeolite was 3.46 %, 3.97 % and 0.94 % before and after postsynthesis thermal and chemical treatment, respectively. The postsynthesis treatment had a profound positive impact on the stability of both the catalytically active component and the support of the catalyst. At the same time, its activity in the CWPO process remained very good or had even improved. With the use of a copper bearing zeolite based catalyst, the complete removal of phenol is obtained even when substoichiometric quantity of oxidant is used. Bearing that in mind, future research will be oriented towards the application of these catalysts in the CWPO process with industrial grade effluents that contain phenol and other phenolic compounds. Good results on model phenolic wastewaters are indicative of its possibly successful integration into the existing industrial and municipal wastewater treatment facilities. CWPO process could be, for example, coupled with the biological treatment in order to reduce the toxicity of the phenolic effluent prior to aerobic/anaerobic digestion. In that case, some of the by-products of CWPO phenol oxidation, such as acetic acid, could be used as a substrate. The obtained experimental data was tested to a proposed kinetic model for phenol oxidation rPh = k1 cPh cHP and hydrogen peroxide decomposition rHP = k2 cHP. The kinetic parameters were estimated using the Nelder-Mead method of nonlinear regression. Good accordance between the experimental (dots) and theoretical data (lines) was obtained. The kinetic model used for hydrogen peroxide decomposition showed somewhat less compliance with the experimental data. This can be attributed to the fact that hydrogen peroxide present in the reactor participates not only in the oxidation of phenol molecules but in other reactions such as oxidation of intermediates, hydrogen peroxide dissociation, decomposition on the reactor wall, which contributes to its ineffective dispense in this reaction system