LITERATURE REVIEW
Overview of volatile organic compounds
Volatile Organic Compounds (VOCs) are organic chemicals that easily evaporate at typical indoor temperatures and pressures Their definitions can vary and are primarily based on their vapor pressure.
In the USA, volatile organic compounds (VOCs) are defined as organic substances with a vapor pressure exceeding 13.3 Pa at 25°C, as per ASTM D3960–90 standards The US Environmental Protection Agency (EPA) broadly categorizes VOCs as a diverse group of organic chemicals, including all carbon compounds except for carbon monoxide, carbon dioxide, carbonic acid, metallic carbides, carbonates, and ammonium carbonate, that engage in atmospheric photochemical reactions.
In the European Union, a volatile organic compound (VOC) is defined as any organic compound with an initial boiling point of 250°C or lower, measured at a standard atmospheric pressure of 101.3 kPa This classification aligns with the standards set by the World Health Organization (WHO).
Table 1.1 Definition of volatile organic compounds (VOCs)
Volatile Organic Compounds (VOCs) are categorized based on their structure into several groups, including halogenated compounds, aldehydes, aromatic compounds, polycyclic aromatic hydrocarbons (PAHs), alcohols, ketones, and miscellaneous VOCs Halogenated VOCs, such as chlorobenzene and dichloromethane, are used as solvents and cleaning agents in chemical processing industries, but they significantly contribute to ozone layer depletion and human cancer Aldehydes, prevalent in treated wood resins, cosmetics, and plastic adhesives, also harm the ozone layer and pose chronic toxicity risks Common aromatic compounds like toluene, benzene, and xylene are found in household and industrial products, leading to ozone layer damage, photochemical smog, and carcinogenic effects on human health PAHs, resulting from incomplete combustion, include substances like naphthalene and phenanthrene, which are associated with various cancers Alcohols and ketones, found in cosmetics and personal care products, can enhance aldehyde formation in the atmosphere, further jeopardizing health Lastly, miscellaneous VOCs, such as propylene and ethylene from petrochemical processes, contribute to significant photochemical ozone creation potential (POCP).
Benzene, toluene, ethylbenzene, and xylene (collectively known as BTEX) are prevalent volatile organic compounds (VOCs) widely used in various industries and everyday life Toluene, in particular, is a versatile solvent found in products such as paints, paint thinners, adhesives, and disinfectants, and is also a by-product of coal coke production While low to moderate inhalation of toluene can lead to symptoms like tiredness, confusion, and memory loss, these effects often subside once exposure ceases However, high-level inhalation in a short period can result in severe health risks, including unconsciousness and even death.
Toluene, a volatile organic compound (VOC), contributes to the formation of photochemical smog, which has several detrimental effects on health and the environment When toluene combines with hydrocarbons, it produces irritants that can cause eye discomfort Additionally, air pollutants disrupt the nitrogen cycle by inhibiting the breakdown of ground-level ozone, leading to reduced visibility and respiratory issues.
Overview of VOCs treatment technologies
Volatile Organic Compounds (VOCs) pose significant risks to both human health and the environment, making their control and treatment essential Various methods exist for managing VOCs, which can be categorized into two main groups, as illustrated in Fig 1.2.
Effective management of VOC emissions involves altering process equipment, changing raw materials, or modifying processes While this approach is the most efficient method for controlling emissions, its implementation can be challenging due to limitations related to equipment, materials, and technology.
(ii) Treatment technology: Many techniques have been applied to remove
VOCs, such as adsorption, condensation, membrane, biology, and oxidation methods.
Figure 1.2 VOCs emission control technologies.
Oxidation is a common method to treat fuel gas that contains VOCs in the industrial processes Basing on the productions, oxidation is divided into two types:
- Complete oxidation that includes only CO 2 and H 2 O in the productions (A.1).
- Incomplete oxidation that includes some other substances than CO 2 and
However, oxidation of VOCs needs specific activation energy to start the reaction The activation energy depends on how strong the chemical bonds between hydrogen (H), Carbon (C), and other possible atoms.
Oxidation processes can be categorized into thermal oxidation and catalytic oxidation based on the presence of catalysts Utilizing a catalyst significantly reduces the activation energy required for the reaction compared to processes without catalysts.
VOCs Air Fuel a Thermal oxidation technology for treatment of VOCs.
Fuel b Catalytic oxidation technology for treatment of VOCs.
Figure 1.3 Catalytic oxidation technology for treatment of VOCs.
The complete oxidation of volatile organic compounds (VOCs) typically requires higher temperatures than standard combustion; however, the use of catalysts can significantly lower the oxidation temperature This reduction is influenced by various factors, including the type of catalyst used, flow rate, VOC concentration, and the presence of other gases.
Table 1.2 The temperature required for complete oxidation of VOCs
Table 1.3 The required temperature for catalytic oxidation of VOCs
Catalytic oxidation offers several benefits; however, it also presents challenges, including high costs, complex synthesis processes, and susceptibility to deactivation by acid gases These drawbacks limit the technology's practical applications.
Certain microorganisms have the ability to transform volatile organic compounds (VOCs) into harmless byproducts, making this natural process useful for VOC removal through microbiological methods Popular biotechnologies employed for this purpose include biofiltration, biotrickling filters, bioscrubbers, and biomembranes.
Bio-filtration is a process that utilizes a filter containing immobilized microorganisms on a porous material to purify contaminated air This filter creates a conducive environment for the microorganisms by providing essential elements such as moisture, temperature, oxygen, nutrients, and optimal pH levels As the contaminated air stream flows through the filter, pollutants are captured and transferred to the bio-film present on the packing materials, effectively reducing airborne contaminants.
The bio-tricking filter is a highly favored biological oxidation technique, known for its stable operation, efficient removal rates, low capital costs, and superior pH control.
Bio-scrubbers: Bio-scrubber unit consists of two subunits, namely an absorption unit and a bioreactor unit At the absorption column, VOCs
The liquid phase of the waste will undergo treatment in a bioreactor, where volatile organic compounds (VOCs) will be decomposed Following the gaseous treatment, the resulting solution may be recycled.
Membrane bioreactors represent the most effective filtration technique due to their extensive gas-liquid interface This process facilitates the transfer of volatile organic compounds (VOCs) through a membrane, where they are degraded by a biofilm Key advantages of membrane bioreactors over traditional biological reactors include their selective permeability for specific pollutants and their ability to effectively degrade VOCs that have low solubility in water.
Biotechnology offers an effective solution for the removal of volatile organic compounds (VOCs) at low concentrations, boasting numerous benefits However, it faces challenges, including microbial control and the need for optimal living conditions The restricted use of biotechnology in VOC treatment systems is detailed in Table 1.4 [7].
Table 1.4 Performance evaluation of bioreactors for VOCs and odor control
Low conc of VOCs/ odors
This method involves the contact of exhausted gases with a liquid, leading to the dissolution of gases or their conversion into less toxic substances Its effectiveness relies on factors such as the gas-liquid interface surface area, contact time, absorbing concentration, and the reaction rate between the absorber and the gas Previous studies have demonstrated that certain solutions can efficiently absorb organic solvent vapors, as detailed in Table 1.5 [6].
Table 1.5 The absorption solutions can absorb the organic solvent vapor
Adsorption is a widely used technique for capturing volatile organic compounds (VOCs) due to its effectiveness in removing pollutants at low concentrations However, its efficiency diminishes at high contaminant levels, as it quickly reaches adsorption equilibrium A significant challenge in this process is the necessity for secondary treatment of VOCs Once the adsorbent becomes saturated, it requires either replacement or regeneration to maintain its effectiveness.
The regeneration of adsorbents involves removing the adsorbate from their surfaces for reuse, often necessitating significant adjustments in conditions like temperature, pressure, or the introduction of inert gases and other chemicals.
The condensation of volatile organic compounds (VOCs) involves converting VOCs from gas to liquid phase, a widely used industrial method This process can be achieved by either increasing the gas phase's pressure at a constant temperature or lowering the gas phase temperature while maintaining pressure Condensation takes place at the dew point, where the partial pressure of VOCs in the gas phase matches their vapor pressure The correlation between temperature and vapor pressure for various common VOCs is illustrated in Figure 1.4.
Figure 1.4 The relationship between temperature and vapor pressure of the most common VOCs.
Depending on the composition and concentration of VOCs in the exhaust gas, there are many cold agents used, such as water, saline solution, NH 3 solution,and chlorofluorocarbons.
Catalytic oxidation of VOCs
1.3.1 Mechanisms and kinetics of catalytic oxidation of VOCs
Various mechanisms have been proposed for the complete catalytic oxidation of volatile organic compounds (VOCs), with their effectiveness largely influenced by the catalyst's properties, including the active metal and support, as well as the specific nature of the VOCs These mechanisms can be broadly categorized into three main types: Langmuir-Hinshelwood, Eley-Rideal, and Mars-van Krevelen.
Figure 1.5 The mechanisms of VOCs oxidation over catalysts.
(source: https://www.researchgate.net/publication/312493700)
The Langmuir-Hinshelwood (L-H) mechanism posits that reactions take place between adsorbed volatile organic compounds (VOCs) and adsorbed oxygen on the catalyst's surface For this process to occur, both VOCs and oxygen must adhere to the catalyst, either on the same active sites in the single site L-H model or on distinct active sites in the dual site L-H model.
The Eleye Rideal (E-R) mechanism describes a reaction involving adsorbed species interacting with gas phase reactant molecules, where the key step is the reaction between an adsorbed molecule and a gas phase molecule.
The Mars-van Krevelen (MVK) model posits that the reaction involving volatile organic compounds (VOCs) occurs between the adsorbed VOCs and the lattice oxygen of the catalyst, rather than with oxygen present in the gas phase This model outlines a two-step process for the oxidation of VOCs, beginning with the reaction of the adsorbed VOCs with the oxygen within the catalyst.
The redox mechanism involves the reduction of metal oxide followed by its re-oxidization using gas phase oxygen from the feed This process is essential for kinetics modeling of hydrocarbon oxidation reactions over metal oxide catalysts.
1.3.2 Catalysts for oxidation of VOCs
Catalysts used for the oxidation of VOCs can be classified into three major groups [9]: (i) noble metals catalysts; (ii) non-noble metal oxide catalysts; and (iii) mixed-metal oxides catalysts.
Supported noble metals such as platinum (Pt), palladium (Pd), rhodium (Rh), and gold (Au) are highly effective catalysts for the removal of volatile organic compounds (VOCs) at temperatures below 200°C, achieving conversion rates exceeding 90% Among the various methods for loading noble metals onto support materials, wetness impregnation remains the most widely used technique.
Noble-metal-based catalysts, while effective, are costly and can lose their efficacy due to sintering or poisoning, often lacking sufficient selectivity on their own Their performance is influenced by factors such as the preparation method, type of precursor, choice of noble metal, and the variety and concentration of volatile organic compounds (VOCs) like alkanes, acetone, aromatic hydrocarbons, and alcohols.
Previous studies indicate that platinum (Pt) is the most effective catalyst for the oxidation of volatile organic compounds (VOCs) at low temperatures Research by Rui et al demonstrated that Pt supported on Al2O3 can oxidize toluene at 200 °C with a 95% conversion rate Similarly, Sedjame et al studied the oxidation of m-butanol using a Pt/Al2O3 catalyst, achieving a 95% conversion at 165 °C from an initial concentration of 1000 ppm Furthermore, the oxidation of butanol over a Pt/CeO2 catalyst showed a 30 °C reduction in oxidation temperature compared to Pt/Al2O3, attributed to Ce's ability to store and release lattice oxygen Additionally, Joung et al examined the loading of Pt on activated carbon, revealing effective oxidation of benzene, toluene, and ethylbenzene.
Xylene was completely oxidized at temperatures of 112°C, 109°C, 106°C, and 104°C Researchers have explored platinum (Pt) catalysts on aluminosilicate materials, with Uson et al studying Pt/SBA-15 for n-hexane oxidation and Zang et al investigating Pt/ZSM-5 for propane oxidation A summary of various studies on Pt catalysts supported by different materials is presented in Table 1.6.
Furthermore, Pd is an excellent catalyst for oxidizing the BTEX group [16 , 20
Wang et al and Huang et al have studied the oxidation of xylene using Pd catalysts, but their findings varied due to differences in support materials and loading techniques, such as the precipitation method used for Pd/Co3O4.
Studies indicate that both post-impregnation methods maintained a consistent temperature range of 249°C to 254°C Huang's research demonstrated that using Al₂O₃ as a support with the wetness impregnation method could lower the temperature to 145°C Additionally, the oxidation of toluene over Pd/supports has been documented in the works of Rooke et al and Bendahou et al.
[25] The temperature of 100% conversion was recorded at 190 o C, 220 o C and
400 o C over the support of Al 2 O 3 , SBA-15 and activated carbon, which was reported by Kim et al [26], Bendahou et al [25] and Bedia et al [27] respectively.
Recent studies indicate that gold exhibits promising performance in the oxidation of volatile organic compounds (VOCs), influenced by factors such as synthesis methods, support characteristics, and the shape and size of gold on supports For instance, research by Ali et al demonstrated that 50% of propane was oxidized at 360°C using a catalyst composed of Au/CeO2-ZrO2-TiO2, which was prepared through a deposition-precipitation method Additionally, the oxidation of toluene was observed over catalysts with varying Au loadings on different metal oxides, including CuO.
Fe 2 O 3 , La 2 O 3 , MgO and NiO were reported in study of Carabinerio et al [19] This result showed that Au/CuO was the highest activity followed by Au/NiO, Au/Fe 2 O 3 , Au/MgO and Au/La 2 O 3 in the order Also, the oxidation of toluene was studied by Liu et al [28], this research reported that Au/Co 3 O 4 can convert 90% toluene into harmless productions at 138 o C This catalyst also was used to oxidize benzene and xylene with 90% of conversion at 189 o C and 162 o C, respectively Several previous investigations on noble-metal catalysts on different supports have been summarized and reported in Tab.1.6.
Table 1.6 The noble metal catalysts for VOCs oxidation
Non-noble metal oxides, including transition and rare earth metals, are effective for VOC oxidation due to their advantages such as high active component dispersion, availability, long lifespan, tolerance to masking, regeneration capability, and cost-effectiveness Despite having lower activity than noble-metal catalysts, non-noble metal oxides are widely utilized in industrial applications for VOC oxidation The performance of these metal-oxide catalysts is significantly influenced by support materials and preparation methods, with porous materials being favored for their high surface area and large pores, which enhance metal dispersion and catalytic activity Commonly used metal-oxide catalysts in this field include copper oxide, manganese dioxide, iron oxide, nickel oxide, chromium oxide, and cobalt oxide.
Co 3 O 4 is the most common non-noble metal catalyst in VOCs oxidation [30-
Co3O4 exhibits high performance in oxidizing various volatile organic compounds (VOCs), including acetylene, propylene, propane, and the BTEX group Notably, toluene is identified as the VOC for which Co3O4 serves as the most efficient catalyst for oxidation, according to Jiang S's study The research highlights that Co3O4 supported on carbon nanotubes (CNT) can completely decompose toluene at a temperature of 257°C.
The summary of literature review
Volatile Organic Compounds (VOCs) in the atmosphere pose risks to human health and the environment, necessitating effective treatment methods Among various technologies, adsorption is widely used to capture VOCs, although it often addresses high-value VOCs in large quantities Recently, catalytic oxidation has emerged as an effective technique for treating VOCs at lower temperatures This study focuses on the oxidation process during the desorption of adsorbed VOCs, aiming to lower oxidation temperatures and economically decompose low-value VOCs in smaller amounts This innovative approach holds potential for secondary VOC treatment across diverse industrial sectors.
Research on single metallic oxides of copper and cobalt has demonstrated their remarkable effectiveness in the oxidation of volatile organic compounds (VOCs) However, the exploration of bimetallic oxides, specifically the combination of copper and cobalt, remains underrepresented in existing literature Given that bimetallic oxides are recognized as promising catalysts for VOCs oxidation, further investigation into their potential is warranted.
30 | P a g e study the bimetallic oxides of Co and Cu will be prepared to oxidize toluene at temperature below 400 o C.
The significance of supports in the oxidation of volatile organic compounds (VOCs) is well-established, with porous materials being preferred for their advantageous properties This study focuses on the use of activated carbon, silica gel, and MCM41 as catalysts for the oxidation of VOCs.
The preparation method significantly influences catalyst activity, with various techniques like wet impregnation, sol-gel, and redox methods being utilized Wet impregnation is favored for its ability to achieve high metallic oxide loading on porous carriers and its simplicity Alternatively, the solid-solid blending method, which involves mixing metallic salts with supports, shows promise for producing high-performance catalysts This study focuses on employing the solid-solid blending method and comparing its effectiveness to the traditional wet impregnation approach.
EXPERIMENT
Catalyst preparation
Two nitrate salts of copper and cobalt were employed as the precursors, which were supplied by Xilong Chemical Co., Ltd (China) and Sigma (Germany)
Table 2.1 Properties of chemicals using to prepare catalysts
Activated carbon from Tra Bac JSC, MCM41 from Sud Chemie (Germany), and silica gel from Sigma were selected as supports for catalyst synthesis The catalysts were synthesized using two methods: wet impregnation and solid-solid blending.
This study employs the wet impregnation method, successfully utilized in previous research by Nguyen Thi Lan for synthesizing cobalt oxide and by Nguyen Hoang Hao for copper oxide on activated carbon.
The preparation process involves diluting two nitrate salts, Co(NO3)2·6H2O and Cu(NO3)2·3H2O, with distilled water to create 0.2M solutions Additionally, the support material is thoroughly cleaned and heated in an oven to ensure optimal conditions for the subsequent steps.
The wet impregnation method involved heating a mixture at 120 °C for 24 hours, followed by stirring at 70 °C for 2 hours Afterward, the mixture was heated again at 120 °C for 24 hours The resulting solids were then calcined at 180 °C for 2 hours, labeled, and stored in sealed bottles A visual representation of this process is illustrated in Fig 2.1.
Co(NO 3 ) 2 6H 2 O Cu(NO 3 ) 2 3H 2 O Supports
30 rounds per minute at 70 o C for 2 hours
O 2 (Air) with flow rate of 2 ml/min
Figure 2.1 Procedure of wet impregnation method.
The list of catalysts, which were prepared by the wet impregnation method, are summarized in Tab 2.3.
Table 2.2 List of catalysts prepared by wet impregnation method
Two nitrate salts were combined and melted at 180 °C The support, preheated at 120 °C for 24 hours, was then mixed with the melted salts and heated at 180 °C for 5 minutes, a process repeated five times Following this, the solid was calcined at 450 °C for 2 hours with a heating rate of 2 °C/min Previous research has demonstrated that the solid-solid blending method effectively oxidizes organic compounds The solid-solid blending method is illustrated in Fig 2.2.
Melting at 180 o C to become liquid
Calcinations at 450 o C with rate of 2 o C/minute
Figure 2.2 Procedure of solid-solid blending method.
The list of catalysts, which were synthesized by solid-solid bleeding method, are summarized in Tab 2.3
Table 2.3 List of catalysts prepared by solid-solid bleeding method
Catalyst characterization
Thermal analysis encompasses a range of techniques that assess the properties of a sample over time or temperature under controlled conditions Simultaneous thermal analysis specifically involves the concurrent use of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) on the same sample within a single instrument, ensuring that the test conditions for both analyses are identical.
Table 2.4 Technique of thermal analysis
Differential thermal analysis (DTA) measures the temperature difference between a sample and an inert reference during a controlled temperature change, while thermogravimetric analysis (TGA) monitors changes in sample weight due to reactions, vaporization, or decomposition under similar temperature conditions.
In this thesis, TG-DSC spectra were obtained using a NETZSCH STA 449 F3 at the Inorganic Functional Materials division of the Leibniz Institute for Catalysis in Germany, to evaluate how temperature influences the heat resistance capabilities of the catalysts.
Physical adsorption of gas molecules on solid surfaces is a fundamental technique for measuring a material's specific surface area, grounded in the BET theory (Brunauer, Emmett, and Teller) This theory relies on several key assumptions that underpin its application in surface area analysis.
- The surface of the adsorbent is flat.
- The adsorption process terminates at multilayer coverage.
- All adsorption sites are energetically equivalent.
-There is no mutual interaction between the adsorbed molecules or atoms (no lateral interactions)
In the initial adsorbed layer, the heat of adsorption is considered to be greater than that in subsequent layers, equating the heat of adsorption to the latent heat of condensation of the adsorbed gas.
In this thesis, physical adsorptions of the catalysts were tested by the Gemini VII Micrometrics equipment, in Advanced Institute for Science and Technology, Hanoi, Vietnam.
X-ray diffraction is a common technique to determine the phrase and crystalline sizes utilizing lattice structural parameters.
X-ray diffraction is the elastic scattering of X-ray photon by atoms in a periodic lattice X-rays scattered by atoms in an ordered lattice interfere constructively in directions given by Bragg’s law:
The equation for X-ray diffraction is given by the formula \( n\lambda = 2d \sin \theta \), where \( \lambda \) represents the wavelength of the X-rays, \( d \) denotes the distance between two lattice planes, \( \theta \) is the angle between the incoming X-rays and the normal to the reflecting lattice plane, and \( n \) is the integer known as the order of reflection.
(https://www.doitpoms.ac.uk/tlplib/xray-diffraction/HTML5/bragg2.html)
It can easily calculate the size of particles from Scherrer formula given:
= where D p is the average crystallite size, β is line broadening in radians, θ is Bragg angle, and λ is X-Ray wavelength.
XRD patterns were primarily obtained using a D8 Advance Bruker diffractometer at the Faculty of Chemistry, Hanoi University of Science, Vietnam This device utilizes a copper (Cu) source emitting Cu K radiation with a wavelength of 0.154 nm and operates with a step scan rate of 0.03 degrees per second.
Scanning electron microscopy (SEM) is a type of electron microscopy that effectively measures the size and shape of supported particles This technique also provides valuable insights into the composition and internal structure of these particles.
Figure 2.7 Schematic diagram of the core components of an SEM microscope (https://www.scimed.co.uk/education/sem-scanning- electron-microscopy/)
Scanning Electron Microscopy (SEM) operates by directing a focused electron beam across a sample's surface, while measuring the emission of secondary or backscattered electrons based on the primary beam's position.
In this thesis, SEM images were captured by using JSM-7600F Schottky Field Emission Scanning Electron Microscope (Advanced Institute for Science and Technology, Hanoi, Vietnam)
2.2.5 Chemical and temperature programmed desorption
Isothermal chemisorption analyses are obtained by two chemisorption techniques: a) static volumetric chemisorption, and b) dynamic (flowing gas) chemisorption.
- The volumetric technique is convenient for obtaining a high-resolution measurement of the chemisorption isotherm from very low pressure to atmospheric pressure at essentially any temperature from near ambient to
Commercial applications of this technique typically rely on automation, especially at temperatures reaching 1000 °C Achieving a high-resolution isotherm necessitates numerous precise dosing steps to reach the equilibrium point, along with multiple pressure adjustments Without automation, this process would be both time-consuming and prone to errors.
The dynamic gas technique functions under ambient pressure, where small, precise injections of adsorptive quantities are delivered in pulses to the cleaned sample until saturation is achieved, a process referred to as 'pulse chemisorption'.
The chemisorption technique offers remarkable versatility, allowing for extensive information to be gathered about materials Previous discussions have highlighted some of its capabilities, while the following sections will delve into additional features To simplify the analysis, the upcoming examples focus solely on catalysts made of a single active species In contrast, mixed metal catalysts would necessitate more complex calculations, involving the summation of terms for each adsorbing species, weighted by their respective contributions.
Temperature programmed reaction methods are techniques that monitor chemical reactions as temperature increases linearly over time These methods, applicable to real catalysts and single crystals, are experimentally simple and cost-effective compared to other spectroscopic techniques While qualitative interpretation is straightforward, extracting quantitative reaction parameters like activation energies and pre-exponential factors from TP methods is complex The instrumentation for these investigations is relatively simple, with setups for temperature programmed desorption (TPD) studies illustrated in Fig 2.8.
Figure 2.8 Experimental for temperature programmed reduction, oxidation and desorption.
CO pulse analysis was utilized to assess metal dispersion, surface area, and activated particle diameter, conducted using the Autochem II 2920 at the School of Chemical Engineering, Hanoi University of Science and Technology.
Adsorption and catalytic activity measurement
2.3.1 Adsorption and nitrogen desorption measurement
The adsorption and desorption of the catalysts are evaluated in the micro- reactor systems, which is shown in Fig 2 9.
1 N 2 cylinder, 2 N 2 mass flow controller, 3 N 2 mass flow controller, 4 Toluene generator, 5 Reactor, 6 Oven, 7 Temperature controller, 8 Gas Chromatography with TCD detector, 9 Computer, V Valve.
Figure 2.9 Adsorption and desorption experiment systems.
Nitrogen was extracted from a cylinder and split into two streams: one directed into the reactor and the other routed through a toluene generator to produce waste gas containing toluene The flow of nitrogen was precisely measured and regulated using mass flow controllers (MFC), which could be shut off when the gas was not in use.
The experiment adsorption and desorption process were performed as following:
Step 1: Operate GC Thermo Focus (Italia) (8) with the factors:
Table 2.5 Operating factors of GC
Step 2 Toluene was loaded into generator (4).
Step 3 0.2gram catalyst was placed into reactor (5) with diameter of 1/8 inches and put into oven (6).
Step 4 The oven (6) was turned on and remained at 180 0 C during experiment, it was controlled by temperature controller (7).
Step 5 Open and set the MFC (2) with a flow of 9.5 ml/min, corresponding to initial toluene concentration of 9000 ppm
Step 6 V1, V6 and V7 were opened and others were closed to use N 2 flow (1) to clear the catalyst for 15 minutes
Step 7 V1, V6 and V7 were closed and others were opened to measure initial toluene concentration by GC.
Step 8 When the initial toluene concentration was stable, V5 was closed and others were opened The toluene concentration at outlet was measured regularly.
Step 9 When the outlet toluene was similar to the initial concentration, the adsorption experiment was stopped.
Desorption process: When adsorption process finished, the desorption was conducted as follow steps:
Step 1 Open and set MFC (2) with a flow of 9.5 ml/min
Step 2 V1, V6 and V7 were opened and others were closed, this allows to use N 2 flow (1) to desorb toluene from the catalyst The toluene concentration at outlet was measure for a period time by GC.
Step 3 When the outlet toluene was zero, the desorption was stopped.
For the adsorption process, the adsorption capacity was calculated by throughout curve as the equation:
The adsorption capacity (A Ad) is determined by the total flow rate (Q Ad) of the process, the inlet toluene concentration (C o Tol), and the outlet toluene concentration at time t (C i Tol,t) The equilibrium time (t e) and the weight of the catalyst (m C) also play crucial roles in this process.
For desorption process, the desorption amount was also calculated basing on throughout curve as the following equation
The desorption amount (A De) is calculated using the total flow rate of N2 (Q De) in milliliters per minute, the outlet toluene concentration (C i Tol,t) in parts per million at a specific time (t i), the desorption time (t d) measured in minutes, and the weight of the catalyst (m C) in grams.
2.3.2 Catalytic activity measurement for complete oxidation of toluene
To evaluate the toluene oxidation, two experiment techniques are applied in the study:
The toluene oxidation over catalyst in desorption process:
The adsorption technique effectively treats toluene but requires additional high-temperature processes like desorption and incineration to recycle the adsorbents Consequently, the adsorbed toluene is released during these steps.
To effectively remove adsorbed toluene, oxidation is essential, especially when recovery is not required Volatile organic compounds (VOCs) initially adsorb onto porous materials such as activated carbon, MCM-41, and silica gel Subsequently, these VOCs are desorbed through high temperatures and a gas flow For the removal of toluene in the desorbed flow, it is crucial that the flow contains sufficient oxygen, while the adsorbents are equipped with activated catalytic centers The combination of catalysts and oxygen facilitates the immediate oxidation of desorbed toluene, converting it into carbon dioxide and water.
This technology assesses the oxidized toluene over catalysts during the desorption of adsorbed toluene, utilizing an oxygen flow after prior toluene adsorption The experimental setup is illustrated in Fig 2.10.
1 O 2 cylinder, 2 N 2 cylinder, 3 O 2 mass flow controller, 4 N 2 mass flow controller, 5 Toluene generators, 6 Reactor, 7 Oven, 8 Temperature controller, 9. Gas Chromatography with TCD detector, 10 Computer, V Valves
Figure 2.10 The toluene adsorption – desorption oxidation experiment systems.
The experiment toluene oxidation over catalyst in desorption process were performed as followings:
Step 1: Operate GC Thermo Focus (Italia) (8) with the factors in Tab.2.5
Step 2 Toluene was loaded into generator (4).
Step 3 0.2gram catalyst was placed into reactor (5) with diameter of 1/8 inches and put into oven (6).
Step 4 The oven was turned on and remained at 180 0 C during experiment, it was controlled by temperature controller (7).
Step 5 Open and set MFC (4) with a flow of 9.5 ml/min, corresponding to initial toluene concentration of 9000 ppm
Step 6 V1, V6 and V7 were opened and others were closed to use N 2 flow (2) to clean the catalyst for 15 minutes
Step 7 V1, V6 and V7 were closed and others were opened to measure initial toluene concentration by GC.
Step 8 When the initial toluene concentration was stable, V5 was closed and others were opened The toluene concentration at outlet was measured regularly.
Step 9 When the outlet toluene was similar to initial concentration, the adsorption was stopped.
Oxidation during the desorption process: When adsorption process finished, the desorption by oxygen was conducted as follow steps:
Step 1 Open and set MFC (3) with a O 2 flow of 9.5 ml/min
Step 2 V1, V6 and V7 were opened and others were closed, this allows to use O 2 flow (1) to desorb prior adsorbed toluene from the catalyst The toluene concentration at outlet was measure for a period time by GC.
Step 3 When the outlet toluene was zero, the desorption by oxygen was stopped.
The varying amounts of toluene produced during desorption processes using nitrogen and oxygen were believed to undergo oxidation at the reaction temperature Consequently, the conversion of toluene was determined using a specific equation.
Where ɳ Tol it the toluene conversion (%), A N2 De is the toluene desorption amount by using nitrogen flow (g/g), and A O2 De the toluene desorption amount by using oxygen flow (g/g).
The rate of conversion from toluene to CO 2 can be calculated as:
2 where γ CO2 is the rate of conversion from toluene to CO 2 (%), Y CO2 is the amount of
The desorption process generates CO2 through oxidation, measured in grams per gram (g/g) The amount of toluene desorbed using nitrogen flow is represented as A N2 De (mol/g), while the amount desorbed using oxygen flow is denoted as A O2 De (mol/g).
Amount of CO 2 by oxidation in the desorption process can be calculated as:
The desorption process of CO2 by O2 can be quantified using the formula 22.4 × m, where Y CO2 represents the amount of CO2 desorbed (g/g), Q Ox indicates the total flow rate of O2 (ml/min), C i CO2,t refers to the outlet CO2 concentration at time t (ppm), m C denotes the weight of the catalyst (g), and t d signifies the desorption time by O2 (min).
The complete oxidation technique involves the direct oxidation of toluene using a catalyst and oxygen from the reactant flow The experimental setup for this process is illustrated in Fig 2.11.
1 O 2 cylinder, 2 O 2 mass flow controller, 3 Toluene generator, 4 Reactor, 5. Oven, 6 Temperature controller, 7 Gas Chromatography with TCD detector, 8. Computer, V Valves
Figure 2.11 The complete oxidation of toluene experiment systems.
The complete oxidation process was implemented as:
Step 1: Operate GC Thermo Focus (Italia) (8) with the factors in Tab.2.5
Step 2 Toluene was loaded into generator (3).
Step 3 0.2gram catalyst was placed into reactor (4) with diameter of 1/8 inches and put into oven (5).
Step 4 Open and set MFC (2) with a O 2 flow of 9.5 ml/min, corresponding to initial toluene concentration of 9000 ppm
Step 5 The oven was turned on with a program of temperature (Increasing from room temperature to 450 o C with the rate of 2.5 o C/min).
Step 6 V4 and V6 were closed others were opened to analyze initial toluene concentration.
Step 7 When the initial toluene concentration was stable, V5 was closed and others were opened during experiment.
In this case, toluene conversion was calculated as:
= where ɳ Tol is the toluene conversion (%), C o Tol,T is the inlet toluene concentration at temperature T (ppm), C i Tol,T is the outlet toluene concentration at temperature T
The rate of conversion from toluene to CO 2 was calculated as:
7×( where γ CO2 is the rate of conversion from toluene to CO 2 (%), C i CO2,T is the outlet
CO 2 concentration at temperature T (ppm), C o Tol,T is the inlet toluene concentration at temperature T (ppm), and C i Tol,T is the outlet toluene concentration at temperature
2.3.3 Catalytic activity measurement for complete oxidation of methane
The methane oxidation experiment, illustrated in Fig 2.12, involved the examination of 0.2 grams of catalyst across a temperature range of 150-450 °C, with a heating rate of 2.5 °C/min The total mixed flow rate was maintained at 75 ml/min, comprising a gas mixture of N2, O2, and CH4 in the ratio of 60.75:13.5:0.75, resulting in an initial methane concentration of 1000 ppm, controlled by mass flow controllers.
1 N 2 cylinder, 2 CH 4 cylinder, 3 O 2 cylinder, 4 N 2 mass flow controller, 5 CH 4 mass flow controller, 6 O 2 mass flow controller, 7 Reactor, 8 Oven, 9. Temperature controller, 10 Gas Chromatography with TCD detector, 11 Computer,
Figure 2.12 Total methane oxidation experiment systems.
In this case, the methane conversion was calculated as:
4 where ɳ CH4 is the methane conversion (%), C o CH4,T is the inlet methane concentration at temperature T (ppm), and C i CH4,T is the outlet methane concentration at temperature T (ppm).
RESULTS AND DISCUSSIONS
Characterizations of supports and catalysts
To ensure the stability of catalysts based on activated carbon (AC) in high-temperature environments, thermal analysis in static air is conducted This analysis measures the thermal instability of activated carbon and evaluates the decomposition risk of the catalysts Figure 3.1 illustrates the thermal analysis results for activated carbon alongside selected catalysts.
AC SS-AC3Cu7Co
SS-AC3Cu7Co b DSC curves
Figure 3.1 Thermal analysis in static air of catalyst on AC.
The mass of the AC sample remained stable when heated in static air from 50 to 270 °C, while a significant mass loss was observed in the catalysts starting at 200 °C Notably, at temperatures exceeding 200 °C, the weights of SS-AC3Cu7Co, SS-AC5Cu5Co, and WI-AC5Cu5Co decreased by approximately 15% This mass reduction at elevated temperatures, coupled with the endothermic effect indicated in the DSC curves, suggests that the samples undergo combustion due to the incineration of activated carbon at high temperatures.
The impregnation of copper (Cu) and cobalt (Co) oxides significantly impacts the heat resistance of activated carbon due to their high catalytic activity for oxidation Samples SS-AC3Cu7Co, SS-AC5Cu5Co, and WI-AC5Cu5Co exhibited a greater mass loss during heating in static air, suggesting these catalysts enhance the combustion of activated carbon (AC) Consequently, to prevent the incineration of AC, the processes of adsorption, desorption, and oxidation using these catalysts must be limited to temperatures below 200°C.
The physical adsorption characteristics of the supports—activated carbon (AC), MCM-41, and silica gel—are illustrated in Fig 3.2 The adsorption-desorption isotherms for activated carbon and MCM-41 are categorized as type IV, indicating pore sizes ranging from 2 to 50 nm In contrast, the isotherm for silica gel is classified as type VI, which corresponds to pore sizes greater than 50 nm, according to IUPAC standards.
Adsorption Desorption a Isotherm linear plot of activated carbon (AC)
Isot her m line ar plot of Sili ca gel
Adsorption Desorption c Isotherm linear plot of Silica gel
Figure 3.2 Isotherm linear plot of AC, silica gel and MCM-41
The findings align with the pore distribution results from BJH desorption (Fig 3.3) and are consistent with previously published data for activated carbon (AC), silica gel, and MCM-41.
P or e di st ri bu ti on of A C, sil ic a ge l an d M C M - 41
The BET surface, pore volumes and average pore sizes of these sorbents are presented in Tab 3.1.
Table 3.1 The Surface characteristics of AC, silica gel and MCM-41
Activated carbon (AC) and MCM-41 exhibit significantly larger surface areas, exceeding 1000 m²/g, compared to silica gel, which has a surface area of only 295 m²/g However, silica gel has the largest average pore size at 96.2 Å, followed by activated carbon at 39.43 Å and MCM-41.
The findings indicate that AC and MCM41 exhibit a greater capacity to adsorb toluene compared to silica gel, primarily due to the significantly larger surface area of AC and MCM41 Additionally, the pore size of silica gel is too large in relation to the kinetic diameter of toluene, which ranges from 6.7 to 8.7 Å.
The BET surface areas of catalysts based on activated carbon (AC) and silica gel were analyzed, revealing that loading metallic oxides onto AC significantly reduces its surface area by 40-58%, while only causing a minor reduction of over 10% in silica gel's surface area Furthermore, the decrease in surface area of AC catalysts is more pronounced with higher cobalt (Co) content, as demonstrated by the WI-AC3Cu7Co sample, which has a surface area of just 418 m²/g.
Table 3.2 The surface characteristics of catalysts on AC and silica gel
The results of surface areas, BJH pore sizes, and volumes of the catalysts on MCM41 are shown in Tab 3.3
Table 3.3 The surface characteristics of catalysts on MCM-41
The deposition of bimetallic oxides on MCM-41 surfaces significantly reduced the surface areas and porous volumes The SS-M5Cu5Co sample exhibited the highest surface area while maintaining the pore size distribution typical of MCM-41, followed by SS-M10Cu, SS-M3Cu7Co, and SS-M10Co This trend can be attributed to the larger particle size of single cobalt oxide compared to samples containing both Co and Cu, which resulted in a narrower surface of MCM-41 Notably, the combination of 5% Cu and 5% Co on MCM-41 led to the formation of smaller particles, thereby enhancing the surface area of the SS-M5Cu5Co catalyst.
The isotherm and pore distribution of MCM-41 catalysts are classified as type II, whereas MCM-41 itself falls under type IV This indicates a noticeable alteration in the surface characteristics of MCM-41 when bimetallic oxides of copper (Cu) and cobalt (Co) are introduced The incorporation of these bimetallic oxides onto MCM-41 is achieved through solid-solid blending and wet impregnation methods.
57 | P a g e methods has a significant effect on pore size of MCM-41, which are shown in Fig. 3.4.
SS-M7Cu3Co SS-M5Cu5Co SS-
SS-M10Co SS-M10Cu a Prepared by solid-solid blending
0 b Prepared by wet impregnation method
Figure 3.4 Pore distribution of catalyst on MCM41.
Loading bimetallic oxides onto MCM41 notably alters pore size, with the exception of SS-M5Cu5Co The introduction of these oxides obstructs the mesopores of MCM41, resulting in the formation of larger pores However, the effect on SS-M5Cu5Co is minimal, likely attributed to the smaller size of the 5Cu5Co particles, which will be further analyzed in the XRD results.
Wet impregnation enhances the incorporation of Cu-Co oxides into the pores of MCM-41, leading to a reduction in the surface area of the catalysts, as shown in Tab 3.3 This indicates that both the type of bimetallic oxides used and the synthesis methods significantly influence the surface area and pore size of the loaded samples.
The XRD patterns for the catalysts WI-AC5Cu5Co and AC, depicted in Fig 3.5, reveal the absence of metal oxide peaks This is attributed to AC being an amorphous solid, which results in a high baseline in the XRD pattern, alongside the low metallic content that is insufficient to produce a detectable reflected beam.
A: WI-AC5Cu5Co; B: Activated carbon
Figure 3.5 XRD patterns of catalysts on AC.
The XRD patterns for catalysts on silica gel, namely SS-S20Co, WI-S20Co and WI-S5Cu5Co were presented in Fig 3.6 There was the only structure of Co 3 O 4
(ICSD-01-078-1969), which was detected on the samples of WI-S20Co and SS- S20Co, while the structures of both Co 3 O 4 and CuO (ICSD-01-080-0076) were found in WI-S5Cu5Co.
A: SS-S20Co; B: WI-S20Co; C: WI-S5Cu5Co
Figure 3.6 XRD patterns of catalysts on silica gel.
The crystalline sizes of Co 3 O 4 and CuO were determined by Scherer equation as shown in Tab 3.4
Table 3.4 Crystalline size and phase of Cu-Co/Silica gel
Catalysts synthesized using the wet impregnation method yielded smaller Co3O4 particle sizes on the silica gel surface Additionally, an increase in cobalt (Co) content resulted in larger Co3O4 particle sizes, with samples containing 20% Co demonstrating greater particle sizes compared to those with 5% Co.
The XRD patterns of catalysts on MCM-41 base, prepared by solid-solid blending method and wet impregnation method, were shown in Fig 3.7 and Fig 3.8, respectively.
A: SS-M10Cu; B: SS-M10Co; C: SS-M3Cu7Co; D: SS-M5Cu5Co; E: SS-
Figure 3.7 XRD patterns of 10% catalysts on MCM-41 prepared by solid-solid blending method.
A: WI-M10Co; B: WI-M3Cu7Co; C: WI-M5Cu5Co
Figure 3.8 XRD patterns of 10% catalysts on MCM-41 prepared by wet impregnation method.
The crystalline sizes of the metal oxides were determined by Scherer equation which showed in Tab 3.5
Table 3.5 Crystalline sizes and phases of 10% Cu-Co on MCM-41
The XRD pattern of SS-M7Cu3Co reveals structures corresponding to CuO and Co3O4, with average crystalline sizes of 22.87 nm and 9.26 nm, respectively In contrast, the SS-M5Cu5Co and SS-M3Cu7Co catalysts only exhibit Co3O4 due to their lower copper content, resulting in average crystalline sizes of 8.71 nm and 10.24 nm This indicates that increasing copper content reduces the crystalline size of cobalt particles, suggesting that bimetallic oxides can enhance the dispersion of metal oxide sites for improved catalytic performance.
Total oxidation ability of the catalysts for methane
Methane is belonging to the alkane group, which is considered as the hardest completely oxidation, even high temperature because of their strong bonds.
Methane undergoes partial oxidation to yield methanol, aldehyde, or carbon monoxide at temperatures exceeding 400 °C, while its complete oxidation requires temperatures above 850 °C Consequently, assessing the oxidation capabilities of catalysts necessitates evaluating their effectiveness in achieving complete methane oxidation.
73 | P a g e oxidize the hardest compound – methane Thus, the synthesized catalysts were pre- examined by methane oxidation in this study.
Methane is challenging to adsorb on porous materials, prompting the use of temperature-programmed desorption (CH4-TPD) to evaluate the adsorption and oxidation capabilities of various catalysts.
SS-M5Cu5Co SS-M10Co SS-M10Cu
SS-M7Cu3Co SS-M3Cu7Co
Figure 3.16 CH 4 –TPD profiles of Cu-Co/MCM-41
CH 4 –TPD profiles of the catalysts are presented in Fig 3.16, there are two desorbed peaks at low temperature (about 300 o C), and high temperature (about 500-
At temperatures of 600 °C, the adsorption of CH4 was notably higher on SS-M10Cu compared to SS-M10Co across both low and high temperature ranges, likely due to the unique properties of CuO Catalysts with elevated CuO content, such as SS-M5Cu5Co and SS-M7Cu3Co, demonstrated superior CH4 adsorption capabilities, attributed to their increased number of active CuO centers This enhanced ability to adsorb CH4 is expected to correlate with improved catalytic activity for CH4 oxidation.
Table 3.11 CH 4 -TPD quantities of Cu-Co/MCM-41
The results of methane oxidation over the catalysts on silica gel base were introduced in Fig 3.17
Figure 3.17 Catalytic activity of Cu-Co/silica gel for the complete oxidation of methane
The experimental results indicated that methane conversion did not occur at temperatures below 200 °C, as previous studies have shown that silica gel lacks the capacity to absorb methane, resulting in low conversion rates at this temperature range Additionally, methane oxidation over the catalysts commenced at temperatures exceeding 200 °C.
The catalytic activities of the catalysts increased with temperature, peaking at 450 °C The order of effectiveness among the catalysts was as follows: SS-S20Co exhibited the highest activity, followed by WI-S20Co, WI-S3Cu7Co, SS-S5Cu5Co, WI-S5Cu5Co, and finally WI-S7Cu3Co.
The catalytic activity of bimetallic oxides on silica gel is influenced by both the cobalt content and the preparation method used Specifically, the solid-solid blending method yields a catalyst with higher activity compared to the wet impregnation method This difference in activity may be attributed to the solid-solid blending technique, which exposes more metallic oxides on the surface, whereas the wet impregnation method tends to trap metallic oxides within the pores.
The highest conversion rate of methane to CO2 reached 83% with the SS-S20Co catalyst In contrast, catalysts that included both Cu and Co on silica gel demonstrated significantly lower activity, with conversion rates falling below 30% This suggests that the presence of copper negatively impacts the catalytic activity.
The Co3O4 catalyst is essential for methane oxidation but fails to completely convert methane to CO2 at 450°C In contrast, catalysts supported on silica gel show significantly lower methane conversion rates compared to unsupported catalysts, with a 20% loading demonstrating notably higher activity than a 10% loading.
The results of methane oxidation over catalysts on MCM-41, as illustrated in Fig 3.18, indicate that increasing temperature enhances methane conversion rates At 450 °C, the highest conversions of 100% and 95% were achieved with WI-M10Co and SS-M10Co catalysts, surpassing those of unsupported catalysts This suggests that the fine dispersion of cobalt oxide on MCM-41 provides more active sites for the reaction Conversely, catalysts containing only copper displayed poor activity, which improved only when the copper content was significantly reduced to 5% in WI-M3Cu7Co and WI-M5Cu5Co.
77 | P a g e b Methane oxidation over Cu-Co/MCM-41 prepared by wet impregnation
Figure 3.18 Catalytic activity of Cu-Co/MCM-41 for the complete oxidation of methane
Figure 3.19 illustrates the comparison of catalyst activities prepared using various methods When the cobalt content exceeded 5%, methane conversion rates for samples prepared by the wet impregnation (WI) method surpassed those prepared by the solid-solid blending method, attributed to the larger particle size of the bimetallic oxides, as indicated in Table 3.5.
Figure 3.19 Comparison of methane oxidation with different preparations at 450 o C
Bimetallic oxide compounds exhibit greater oxidation activities compared to copper single compounds, achieving a 33% increase; however, their performance is slightly inferior to that of single cobalt oxide This indicates that copper's influence on methane conversion is minimal, primarily serving to disperse the catalytic particles.
The results of methane oxidation over unsupported catalysts, as depicted in Fig 3.20, indicate that the catalytic activity of bimetallic oxides without MCM-41 follows the order: 100Co > 70Cu30Co > 50Cu50Co.
The study indicates that while the catalytic activities of 100Co, 70Cu30Co, and 50Cu50Co are quite similar, 100Cu exhibits a significantly lower activity This suggests that the Co3O4 oxide catalyst possesses superior oxidation properties compared to CuO Additionally, the inclusion of CuO in bimetallic oxide catalysts contributes to a reduction in the particle size of Co3O4, which enhances the exposure of active sites for reactions Notably, Co3O4-based catalysts can achieve nearly 90% methane conversion at a temperature of 450°C.
Figure 3.20 Catalytic activity of unsupported Cu-Co catalysts for the complete oxidation of methane
The comparison of methane oxidation over the catalyst over supports and without supports was presented in Fig 3 21.
The study found that utilizing supports resulted in a reduction of methane conversion Conversely, an increase in methane conversion was observed with higher cobalt (Co) content in the Cu-Co/MCM-41 catalyst.
Figure 3.21 Comparison of methane oxidation on Cu-Co with and without supports at 450 o C
The study reveals that the cobalt-only catalyst demonstrates superior activity for methane oxidation compared to the copper catalyst, which shows the least effectiveness This finding contradicts previous results indicating that copper catalysts on MCM-41 can adsorb higher amounts of methane and oxygen than cobalt catalysts Therefore, the type of oxide used for complete oxidation plays a more crucial role than the reactant adsorption capacity in this scenario.
Toluene treatment
3.3.1 Toluene adsorption on catalysts/ sorbents
3.3.1.1 Toluene adsorption over Cu-Co/Activated carbon
The simulated isotherm for toluene on different component catalysts of Cu- Co/AC are shown in Fig 3.22
Figure 3.22 Toluene adsorption breakout curves on AC base.
The adsorption quantity of toluene was determined using the breakthrough curve of the isotherm, as outlined in Equation 2.5 and presented in Table 3.12 The findings on toluene adsorption align with the previously discussed characteristics of the catalysts, including their surface properties and particle sizes.
Table 3.12 Adsorption amount of toluene on Cu-Co/Activated carbon
The fresh activated carbon (AC) exhibits the highest adsorption capacity of 0.28 g/g due to its extensive surface area The presence of metallic oxides on the surface and within the pores of the AC, as illustrated in the SEM images, contributes to a reduction in the effective surface area The adsorption capacity follows the order: AC180 > AC3Cu7Co > WI-AC5Cu5Co > WI-AC7Cu3Co Notably, the incorporation of bimetallic oxide catalysts onto the AC support, such as WI-AC5Cu5Co and WI-AC3Cu7Co, does not significantly diminish the toluene adsorption capability of the material, indicating that the adsorption performance remains largely unaffected during the toluene adsorption period.
3.3.1.2 Toluene adsorption over Cu-Co/Silica gel
The outlet concentration of toluene over Cu-Co/Silica gel is illustrated in Fig 3.23, indicating that the limited adsorption capacity of toluene on these catalysts correlates with the surface characteristics of silica gel.
Figure 3.23 Toluene adsorption breakout curves on silica gel base
The toluene adsorption capacity of Cu-Co/Silica gel was found to be low, as indicated in Table 3.13, due to the macro porous nature of silica gel, which limits its ability to adsorb toluene This finding aligns with previous research on toluene adsorption using silica gel Consequently, the impregnation of bimetallic oxide onto silica gel is not an effective method for toluene treatment via adsorption.
Table 3.13 Adsorption amount of toluene on Cu-Co/Silica gel
3.3.1.3 Toluene adsorption over Cu-Co/MCM-41
The break curve of toluene adsorption on catalysts over MCM-41 base were presented in Fig 3.24 and the adsorption amount were shown in Tab 3.14.
Figure 3.24 Toluene adsorption breakout curves on MCM-41 base.
The impregnation of bimetallic oxides on MCM-41 support significantly reduces the surface area, leading to decreased toluene adsorption In contrast, the impregnation of single metallic oxide (CuO) results in an even greater reduction in adsorption due to larger particle sizes occupying the support's pores Additionally, the synthesis method affects adsorption capacity; for instance, the SS-M5Cu5Co sample demonstrates a higher adsorption amount compared to IW-M5Cu5Co, attributed to its larger surface area Notably, SS-M5Cu5Co's adsorption capacity is nearly equivalent to that of MCM-41, indicating its potential effectiveness in toluene adsorption applications.
Table 3.14 Adsorption amount of toluene on Cu-Co/MCM-41
The impact of surface area on toluene adsorption by various catalysts is illustrated in Fig 3.25, revealing that surface area alone does not determine adsorption capacity Despite high surface areas of AC and MCM-41, their toluene adsorption abilities are not significantly superior to those of bimetallic oxide catalysts on other supports, indicating that pore size also plays a crucial role Notably, the presence of bimetallic oxides does not diminish the toluene adsorption capacity of AC or MCM-41, with AC demonstrating the highest adsorption ability MCM-41 offers reasonable adsorption while allowing for higher desorption temperatures compared to AC, which is limited to 200 °C In contrast, catalysts supported on silica gel show considerably lower toluene adsorption, making this support less effective for toluene treatment.
Figure 3.25 Effect of surface’s area of supports on toluene adsorption amount.
3.3.2 Oxidation over catalysts in desorption process
3.3.2.1 Toluene oxidation over Cu-Co/Activated carbon in desorption process
The desorption process of Cu-Co/AC, illustrated in Fig 3.26, reveals that the initial concentration of toluene was significantly higher than the toluene inlet concentration, aligning with findings from previous studies [69, 70].
87 | P a g e a Toluene generation by nitrogen flow in desorption process b Toluene generation by oxygen flow in desorption process
Figure 3.26 Generated toluene concentrations from heat desorption over
Table 3.15 illustrates the desorption amounts of previously adsorbed toluene using various carrier gases A notable difference in toluene concentration was observed when nitrogen (Fig 3.26 a) was substituted with oxygen (Fig 3.26 b) This variation is attributed to the oxidation of generated toluene by bimetallic oxide catalysts present on activated carbon in the presence of oxygen, resulting in a decreased toluene concentration in the outlet flow.
Figure 3.27 Formed CO 2 from heat desorption by oxygen flow over Cu-Co/AC.
The outlet CO2 concentration during desorption with oxygen flow is illustrated in Fig 3.27 The CO2 yield was calculated using Eq 2.9 and subsequently compared to the theoretical oxidation of toluene, as detailed in Tab 3.16.
Table 3.15 Generated toluene by thermal desorption
Table 3.16 Evaluation of total toluene oxidation over the catalysts on AC
The findings indicate that the amount of toluene desorbed by nitrogen is significantly lower than the amount adsorbed, suggesting that toluene is not fully desorbed and a portion remains trapped within the pores of activated carbon (AC).
The catalysts demonstrate the ability to oxidize toluene at 180 °C; however, they do not achieve complete oxidation at this temperature While the presence of metallic oxides on activated carbon (AC) and oxygen in the flow significantly reduces toluene concentration compared to nitrogen desorption, high levels of toluene persist during the initial minutes of the outlet flow, and CO2 yield remains low This suggests that toluene is being oxidized into other organic compounds Notably, the WI-AC5Cu5Co catalyst achieves a 100% CO2 yield, indicating its strong activity for oxidation, which aligns with the findings for methane oxidation.
It is clear that toluene is completely decomposed into CO 2 over WI-
At 180°C, the AC5Cu5Co catalyst achieves a conversion rate for toluene that falls short of 100% due to the high initial concentration of toluene during thermal regeneration Following this, the WI-AC3Cu7Co and WI-AC7Cu3Co catalysts show promising results Bimetallic oxides of cobalt and copper demonstrate superior activation for the oxidation of volatile organic compounds (VOCs), making this catalyst a preferred choice for synthesizing over alternative supports for VOCs oxidation evaluation.
Previous studies indicate that adsorbed oxygen is released at temperatures below 400 °C for Co3O4 and below 200 °C for CuO, while the oxidation temperature is only 180 °C This suggests that lattice oxygen is not responsible for the oxidation of toluene Consequently, the toluene oxidation mechanism over these catalysts is categorized as either Langmuir-Hinshelwood (L-H) or Eley-Rideal (E-R), rather than the Mars-van Krevelen mechanism, which relies on high-temperature lattice oxygen oxidation Further research is needed to confirm the mechanism applicable to this reaction.
3.3.2.2 Toluene oxidation over Cu-Co/ /Silica gel in desorption process
Thermal desorption of toluene using Cu-Co/Silica gel with nitrogen and oxygen flows resulted in low adsorption levels, as illustrated in Figures 3.28 and 3.29 This is primarily due to the macro porous nature of silica gel, which limits the amount of toluene that can be adsorbed.
91 | P a g e adsorption capacity of toluene It is in agreement with the previous studies in the toluene adsorption of silica gel.
Figure 3.28 Toluene generation on Cu-Co/silica gel by N 2 in desorption
Figure 3.29 Toluene generation on Cu-Co/silica gel by O 2 in desorption
At 180 °C, the adsorption and desorption amounts of toluene on silica gel were compared, as detailed in Table 3.17 The results indicate that toluene cannot be effectively captured on the silica gel surface due to its large pore size, resulting in comparable amounts of adsorption and desorption.
Table 3.17 Toluene adsorption capacity of catalysts on Silica gel base