Synthesis and characterization of catalysts for the total oxidation of chlorinated volatile organic compounds

Abstract The harmful emissions of chlorinated volatile organic compounds (CVOCs) originate only from man-made sources. CVOCs are used in a variety of applications from pharmaceuticals production to decaffeination of coffee. Currently, CVOC emissions are limited by strict legislation. For these reasons, efficient CVOC abatement technologies are required. Catalytic oxidation is very promising option for this purpose, since catalysts can be tailored to each case to maximize the efficiency and minimize the formation of unwanted products, such as dioxins or Cl2. The goal of this thesis was to study the role of the physico-chemical properties of catalysts in dichloromethane (DCM) oxidation. To reach the aim, several catalytic materials were prepared and characterized, and their performance was tested in total oxidation of DCM. The catalytic materials used were powders of four single metal oxides (γ-Al2O3, TiO2, CeO2, MgO), three mixed oxides (Al2O3-xSiO2) washcoated on a cordierite monolith and four active phases (Pt, Cu, V, Mn). At first, support properties were studied. It was found that the DCM conversion and HCl production are dependent on support acidity when the studied single oxides are considered. The best DCM conversions and HCl yields were observed with the support having the highest total acidity (γ-Al2O3). Further, the quality of the by-products formed was dependent on the type of the acid sites present on the support surface. Secondly, the impregnation of the active compound was observed to improve the selectivity of the material. From the tested active phases, Pt presented the best performance, but also V2O5 and CuO showed almost equal performances. Especially CuO supported on γ-Al2O3, that had less formation of by-products and is less toxic than V-containing oxides, seems to be a promising alternative to Pt. Concerning stability, no deactivation was observed after 55h of testing of Pt/Al2O3. Furthermore, in the used reaction conditions, the formation of CuCl2 is not thermodynamically favoured. Finally, the good characteristics of the powder form catalysts were successfully transferred to the monolith. The performance of the Pt/90Al2O3-10SiO2 catalyst in DCM oxidation was improved when the channel density was increased due to an increase in geometric surface area and mechanical integrity factor, and a decrease in open fraction area and thermal integrity factor.Tiivistelmä Haitallisten kloorattujen orgaanisten yhdisteiden (CVOC) päästöt ovat ihmisten aiheuttamia. CVOC-yhdisteitä käytetään mm. liuottimina lääkeaineiden valmistuksessa ja kofeiinin poistossa. Nykyisin CVOC-päästöjä rajoitetaan tiukalla lainsäädännöllä. Näistä syistä tehokas CVOC-yhdisteiden käsittelymenetelmä on tarpeen. Katalyyttinen hapetus on hyvä vaihtoehto tähän tarkoitukseen, koska katalyytit voidaan räätälöidä niin, että puhdistuksen tehokkuus saadaan maksimoitua samalla kun ei-haluttujen tuotteiden, kuten dioksiinit ja kloorikaasu, muodostuminen voidaan minimoida. Tämän väitöskirjatyön tavoitteena oli selvittää katalyyttien fysikaalis-kemiallisten ominaisuuksien yhteyksiä dikloorimetaanin (DCM) hapetukseen. Tavoitteen saavuttamiseksi valmistettiin useita katalyyttejä, jotka karakterisoitiin ja testattiin DCM:n kokonaishapetuksessa. Työssä tutkittiin neljää jauhemaista metallioksidia (γ-Al2O3, TiO2, CeO2 ja MgO), kolmea metallioksidiseosta (Al2O3-xSiO2), jotka pinnoitettiin kordieriittimonoliitille, sekä neljää aktiivista ainetta: Pt, Cu, V and Mn. Aluksi työssä keskityttiin tukiaineiden ominaisuuksiin. Työn tulokset osoittivat, että DCM:n konversio ja HCl:n tuotanto ovat riippuvaisia tukiaineen happamuudesta. Paras tulos saavutettiin alumiinioksidilla, jolla oli korkein kokonaishappamuus. Lisäksi havaittiin, että sivutuotteiden laatu riippuu tukiaineen pinnalla olevien happopaikkojen tyypistä. Aktiivisen aineen impregnointi tukiaineeseen paransi materiaalin selektiivisyyttä. Tutkituista aineista Pt osoittautui parhaimmaksi, mutta myös V2O5 ja CuO olivat lähes yhtä hyviä. Erityisesti CuO-katalyytti, joka tuotti vähemmän sivutuotteita ja joka on materiaalina vähemmän haitallinen kuin V2O5, osoittautui lupaavaksi jalometallikatalyyttien korvaajaksi. Materiaalien stabiilisuuteen liittyen Pt/Al2O3-katalyytin toiminnassa ei havaittu muutoksia 55 tunnin testauksen jälkeen. Lisäksi CuCl2:n muodostuminen ei mallinnuksen mukaan ole termodynaamisesti todennäköistä tutkituissa reaktio-olosuhteissa. Jauhemaisen katalyytin hyvät ominaisuudet pystyttiin pinnoituksessa siirtämään monoliittirakenteiseen katalyyttiin. Pt/90Al2O3-10SiO2 -katalyytin aktiivisuus DCM:n hapetuksessa tehostui, kun monoliitin aukkoluku kasvoi aiheutuen suuremmasta geometrisestä pinta-alasta ja mekaanisesta eheystekijästä sekä pienemmästä avoimen pinnan osuudesta ja termisestä eheystekijästä

Steam activation of waste biomass:highly microporous carbon, optimization of bisphenol A, and diuron adsorption by response surface methodology

Abstract Highly microporous carbons were prepared from argan nut shell (ANS) using steam activation method. The carbons prepared (ANS@H2O-30, ANS@H2O-90, and ANS@H2O-120) were characterized using X-ray diffraction, scanning electron microscopy, Fourier-transform infrared, nitrogen adsorption, total X-ray fluorescence, and temperature-programmed desorption (TPD). The ANS@H2O-120 was found to have a high surface area of 2853 m²/g. The adsorption of bisphenol A and diuron on ANS@H2O-120 was investigated. The isotherm data were fitted using Langmuir and Freundlich models. Langmuir isotherm model presented the best fit to the experimental data suggesting micropore filling of ANS@H2O-120. The ANS@H2O-120 adsorbent demonstrated high monolayer adsorption capacity of 1408 and 1087 mg/g for bisphenol A and diuron, respectively. The efficiency of the adsorption was linked to the porous structure and to the availability of the surface adsorption sites on ANS@H2O-120. Response surface method was used to optimize the removal efficiency of bisphenol A and diuron on ANS@H2O-120 from aqueous solution

Structured carbon foam derived from waste biomass:application to endocrine disruptor adsorption

Abstract In this paper, a novel structured carbon foam has been prepared from argan nut shell (ANS) was developed and applied in bisphenol A (BPA) removal from water. The results showed that the prepared carbon foam remove 93% of BPA (60 mg/L). The BPA equilibrium data obeyed the Liu isotherm, displaying a maximum uptake capacity of 323.0 mg/g at 20 °C. The calculated free enthalpy change (∆H° = − 4.8 kJ/mol) indicated the existence of physical adsorption between BPA and carbon foam. Avrami kinetic model was able to explain the experimental results. From the regeneration tests, we conclude that the prepared carbon foam has a good potential to be used as an economic and efficient adsorbent for BPA removal from contaminated water. Based on these results and the fact that the developed structured carbon foam is very easy to separate from treated water, it can serve as an interesting material for real water treatment applications

Activity, selectivity, and stability of vanadium catalysts in formaldehyde production from emissionsof volatile organic compounds

Abstract In this study, activity, selectivity and stability of vanadium catalysts supported on zirconia, hafnia, and alumina were examined in the oxidation of methanethiol and methanol to formaldehyde. The 3 wt-% vanadia–alumina catalyst with low VOx surface density showed the highest activity in the formaldehyde production. However, during the stability test, this catalyst deactivated due to the change in the oxidation state of vanadium from V5+ to V4+, decrease the amount of surface vanadium species and the formation of sulphates on the material surface. Zirconia and hafnia supported catalysts with high VOx surface density demonstrated better stability in the reaction conditions, but their activity in the formaldehyde production was lower. One reason for the lower activity might be the formation of metal-mixed oxide phases between vanadia and the support, which could also explain the decreased sulphur deposition on zirconia and hafnia after vanadium impregnation

Catalytic wet air oxidation of high BPA concentration over iron-based catalyst supported on orthophosphate

Abstract The catalytic performance of Fe supported on nickel phosphate (NiP) was evaluated for the removal of bisphenol A (BPA) by catalytic wet air oxidation (CWAO) at 140 °C and 25 bar of pure oxygen pressure. The prepared NiP and Fe/NiP materials were fully characterized by XRD, N₂-physisorption, H₂-TPR, TEM, and ICP analysis. Iron (Fe/NiP) impregnation of NiP support enhanced the BPA removal efficiency from 37.0 to 99.6% when CWAO was performed. This catalyst was highly stable given the operating conditions of acidic medium, high temperature, and high pressure. The Fe/NiP catalyst showed an outstanding catalytic activity for oxidation of BPA, achieving almost complete removal of BPA in 180 min at a concentration of 300 mg/L, using 4 g/L of Fe/NiP. No iron leaching was detected after the CWAO of BPA. The stability of Fe/NiP was performed over three consecutive cycles, noting that BPA conversion was not affected and iron leaching was negligible. Therefore, this catalyst (Fe/NiP) could be considered as an innocuous and effective long-lasting catalyst for the oxidation of harmful organic molecules

Comparative study on the support properties in the total oxidation of dichloromethane over Pt catalysts

Abstract The aim of this work was to study the influence of the support oxide properties on the total oxidation of dichloromethane in moist conditions. The support materials γ-Al2O3, TiO2, CeO2 and MgO were synthesized by a sol-gel method followed by wet impregnation of Pt and characterized by different physico-chemical techniques. The conversion of DCM was higher than 90% at 500 °C over impregnated and non-impregnated Al2O3, TiO2 and CeO2, even at high GHSV. CO, CH3Cl and CH2O were the major by-products observed and their amounts decreased after Pt impregnation. The CH3Cl formation was higher when Lewis acid sites were present while the existence of Brønsted sites promoted the CH2O formation. The complete conversion of DCM was achieved at around 450 °C over the Al2O3 and Pt/Al2O3 and at 500 °C for Pt/TiO2. These two catalysts exhibited the highest total acidities among the materials tested. The activity of Pt/Al2O3 remained the same also after 55 h of testing, however, increase in Pt particle size and decrease in acidity were observed. Pt/CeO2 while being less active showed smallest amount of by-product formation during the whole temperature range used in light-off tests. This is most probably due to its easy reduction ability. The textural parameters of the supports did not appear to be the key parameters when considering the activity and selectivity of the catalysts

Total oxidation of dichloromethane over silica modified alumina catalysts washcoated on ceramic monoliths

Abstract Silica modified alumina was used in this study for coating of a cordierite monolith substrate with two different channel densities. The performance of the prepared monolith catalysts was evaluated in catalytic total oxidation of dichloromethane before and after Pt impregnation. The characteristics similar to the powder form catalysts were kept rather successfully after washcoating the monolith as evidenced by electron microscopy (FESEM) and N2 physisorption. A dichloromethane (DCM) conversion of higher than 80% at 500 °C was reached over all the catalysts with 200 cpsi. The maximum conversion was obtained with the catalyst containing 10 mol % of silica. The total amount of major byproducts (CO, CH3Cl and CH2O) were slightly decreased by increasing the silica loading, and remarkably after Pt impregnation. After impregnation of Pt, the HCl yields were increased for two samples with the higher loading of silica (10 and 15 mol %) and reached the maximum when silica loading was 10%. Even though Pt impregnation did not significantly affect the DCM conversion, it improved the selectivity. Comparison between the two substrates (200 and 600 cpsi) evidenced that the key parameters of the monolith influencing the DCM oxidation are low value of open fraction area, hydraulic diameter, thermal integrity factor and high value of mechanical integrity factor and geometric surface area

Catalytic abatement of dichloromethane over transition metal oxide catalysts:thermodynamic modelling and experimental studies

Abstract Dichloromethane (DCM) is a noxious chemical that is widely used in industry. The current work focuses on the catalytic abatement of DCM from industrial effluents to minimize its harmful effects to the environment and human wellbeing. Three transition metal oxide catalysts (V, Cu and Mn) supported on γ-Al₂O₃ were synthetized for total oxidation of DCM in presence of steam. Thermodynamic modelling was used to reveal information related to the stability of the used transition metal oxides in the abatement conditions. The results showed that with 10 wt-% CuO and 10 wt-% V₂O₅ containing catalysts 100% conversion of DCM together with 90% HCl yield and insignificant by-product formation can be achieved at temperature around 500 °C. According to modelling, V₂O₅ should be stable at the conditions of DCM oxidation, while CuO would be more stable at higher temperature level (decomposition of CuCl₂ starts at 300 °C). MnCl₂ remains stable until 800 °C, which leads to deactivation of MnO₂ catalyst. Presence of steam inhibits the poisoning of the materials by chlorine based on thermodynamic calculation. XRF analysis supports the results of thermodynamic modelling — used MnO₂ and CuO catalysts contain chlorine, which was not detected in case of V₂O₅/Al₂O₃. CuO/γ-Al₂O₃ seems to be a good alternative to noble metal catalysts for the total oxidation of dichloromethane when used in the presence of steam and the temperatures above 300 °C to minimize Cl-poisoning. The outcomes of this study showed that the prepared metal oxides are promising catalysts to minimize pollution caused by chlorinated volatile organic compounds