MINISTRY OF EDUCTION AND TRAINING HA NOI UNIVERSITY OF SCIENCE AND TECHNOLOGY Ngo Quoc Khanh LOW TEMPERATURE CATALYTIC OXIDATION OF VOLATILE ORGANIC COMPOUNDS VOCs OVER DOCTORAL DISSE
LITERATURE REVIEW
Overview of volatile organic compounds
VOCs are organic chemical compounds that could quickly evaporate under normal indoor atmospheric conditions of temperature and pressure The definitions of VOCs are unclearly and depend on their vapor pressure
In the USA, a simplified definition is that VOCs are organic compounds that have a vapor pressure of more than 13.3 Pa at 25 o C, according to ASTM test method D3960–90 (Standard Test Method for Volatile Content of Coatings) The
US Environmental Protection Agency (US EPA) has defined VOCs very broadly They are a large group of organic chemicals that include any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participates in atmospheric photochemical reactions
In the European Union, it is generally defined such “A VOC is any organic compound having an initial boiling point less than or equal to 250° C measured at a standard atmospheric pressure of 101.3 kPa” [1], or classified by WHO (World
Health Organization) [2] as shown in Tab 1.1:
Table 1.1 Definition of volatile organic compounds (VOCs)
No Classification Boiling point range, ( o C) Examples
1 Very volatile organic compounds (VVOCs)
< 0 to 50-100 Propane, Butane, Methyl chlorine
50-100 to 240-260 Formaldehyde, d-Limonene, toluene, acetone, ethanol (ethyl alcohol) 2-propanol (isopropyl alcohol), hexane
3 Semi volatile organic compounds (SVOCs)
Depending on their structure, VOCs are commonly classified into the following groups: Halogenated compounds, Aldehydes, Aromatic compounds, Polycyclic aromatic hydrocarbons, Alcohols, Ketones and Miscellaneous [3- 5] Halogenated VOCs, such as chlorobenzene, dichloromethane, etc., can use as solvents, cleaning agent which emit from in some chemical processing industry These types of VOCs have a significant impact on the destruction of the ozone layer and cause of human cancer; Aldehydes are main pollutants of treated wood resins, cosmetics, plastic adhesives, which can format of ozone layer and cause of chronic toxicity; Aromatic compounds are popular VOCs, toluene, benzene and xylene, they are found in both domestic and industry example petrochemicals, paint, medicine, and detergents They are considered as reasons for damaging to the ozone layer, produce photochemical smog, as well as caused for carcinogenic in human’s health; Polycyclic aromatic hydrocarbons (PAHs) are production of incomplete combustion processes, such as naphthalene, phenanthrene, pyrene, etc PAHs have been linked to skin, lung, bladder, liver, and stomach cancers; Alcohols and ketones are main components in cosmetics and personal care products, which can increase the formation of aldehydes in the atmosphere and damage human’s health; Miscellaneous VOCs, example propylene, ethylene and methyl tert-butyl ether emitted from petrochemical syntheses, which are the cause of large photochemical ozone creativity potential (POCP)
Among of these VOCs, groups of BTEX (benzene, toluene, ethylbenzene, and xylene) are the most common solvent which are used in both industry and human life Because of the regulations, toluene, used for paints, paint thinners, silicone sealants, many chemical reactants, rubber, printing ink, adhesives (glues), lacquers, leather tanners, and disinfectants, is becoming a popular solvent It is also a by-product of the production of coke from coal Inhalation of toluene in low to moderate levels can cause tiredness, confusion, weakness, drunken-type actions, memory loss, nausea, loss of appetite, hearing loss, and color vision loss Some of these symptoms usually disappear when exposure is stopped Inhaling high levels of toluene in a short time may cause light-headedness, nausea, or sleepiness, unconsciousness, and even death
In the atmosphere, toluene, as other VOCs, is one of the components which are the cause of photochemical smog (Fig 1.1) This smog has many adverse effects When combined with hydrocarbons, the chemicals contained within it form molecules that cause eye irritation Radicals in the air interfere with the nitrogen cycle by preventing the destruction of ground-level ozone Other effects include reduced visibility and respiratory ailments
Overview of VOCs treatment technologies
VOCs are dangerous to human health and the environment, so VOCs control and treatment are urgently necessary There are many available methods to control VOCs, which can classify into two groups, seen in Fig.1.2:
(i) Management: The control of VOCs emissions is achieved by modifying the process equipment, raw material, and/or change of process It is the most effective and efficient method, but its applicability is limited because of difficulties in 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 CO2 and H2O in the productions (A.1)
- Incomplete oxidation that includes some other substances than CO2 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
Depending on the exits of catalysts on reaction, the oxidation process can classify into the thermal oxidation (Fig.1.3 a) and the catalytic oxidation (Fig 1.3.b) By using a catalyst, the activation energy is much lower than the energy without catalysts a Thermal oxidation technology for treatment of VOCs
Exhaust gas heat exchanger Fuel
10 | P a g e b Catalytic oxidation technology for treatment of VOCs
Figure 1.3 Catalytic oxidation technology for treatment of VOCs
Generally, the complete oxidation of VOCs only occurs at a higher oxidation temperature than the combustion temperature (Tab.1.2) However, when using the catalyst, the oxidation temperature significantly reduces, and it also depends on many factors, such as the kind of catalyst, the flow rate, VOCs concentration, exist of other gases, etc (Tab.1.3) [6]
Table 1.2 The temperature required for complete oxidation of VOCs
No VOCs The temperature required, ( o C)
Exhaust gas heat exchanger Fuel
Table 1.3 The required temperature for catalytic oxidation of VOCs
The temperature requires over catalysts, ( o C)
Although, catalytic oxidation has many advantages, it has some disadvantages, such as expensive, complex synthesis and easy deactivation by acid gases This leads to limitation of application of this technology
In natural, some microorganisms can convert VOCs into other harmless productions, so that this phenomenon is applied for removing VOCs by microbiology media There are some popular biotechnologies such as Bio-filtration; Bio-tricking filter; Bio-scrubbers; Bio-membrane [7]
Bio-filtration: The bio-filtration process involves a filter, included immobilized microorganisms on a porous material This filter provides a hospitable environment in the form of moisture, temperature, oxygen, nutrients, and pH to the organisms When contaminated air stream passes through the filter, contaminants are transferred to the bio-film located on the packing materials
Bio-tricking filter: Among the various biological oxidation techniques, bio trickling filter is the most preferred technique due to their stable operation, high removal rates, low capital expenditure, and better pH control
Bio-scrubbers: Bio-scrubber unit consists of two subunits, namely an absorption unit and a bioreactor unit At the absorption column, VOCs
12 | P a g e transfer to the liquid phase, then this liquid will be treated at bioreactor, and VOCs will be decomposed After the gaseous treatment, the solution of the waste may be recycled back
Membrane Bioreactor: Membrane bioreactors are the best filtration technique since they provide a larger gas-liquid interface In this filtration process, the VOCs are transferred with the help of a membrane, where they are degraded with the help of biofilm Essential advantages of membrane over other biological reactors are only particular pollutants can pass through it and its capability to degrade those VOCs, which are poorly soluble in water
Biotechnology is a promising method to remove VOCs at low concentrations with many advantages, it is having some challenges such as micro bacterial control and living environment conditions The limited application of biotechnology in VOCs treatment systems is shown in Tab 1.4 [7]
Table 1.4 Performance evaluation of bioreactors for VOCs and odor control
Capital cost Op cost Bioprocess control
Low conc of VOCs/ odors
High conc of VOCs/ odors
Bio-filter High Low High Low Low Low Low Low Low
Bio-trickling filter High Low High Low Low Low Low Low Low
Bio-scrubber High High High Low High Very low Medium Medium High
Membrane reactor High High High High Need long term evaluation
High High Need long term evaluation
The principle of this method is that the exhausted gas contact with the liquid, then the gases are dissolved in the liquid or transformed into less toxic substances This method's effectiveness depends on the gas-liquid interface surface area, the contact time, the absorbing concentration, and the reaction rate between the absorber and the gas In previous studies, some solutions can absorb organic solvent vapors with high efficiency, as shown in Tab 1.5 [6]
Table 1.5 The absorption solutions can absorb the organic solvent vapor
No The pollutant The absorption solutions The equipment The efficiency,
VOCs Oil The hollow tower 95
3 Methyl ethyl ketone (MEK) Silicone oil 95
5 Toluene Silicone oil The packed tower 99
6 n-Decane The packed tower and water The strayed tower 70
Adsorption is the most popular method to capture VOCs with many advantages, which is very useful in removing pollutants at low and deficient concentrations However, with a high level of contaminants, adsorption is not an efficient process since it rapidly reaches the balance of adsorption The biggest challenge of adsorption is the need for VOCs secondary treatment When exhausted, the adsorbent must either be replaced or regenerated, which is the process of
15 | P a g e removing the adsorbate from the surface of the adsorbent, so that it can be re-used The regeneration process may require some significant changes in conditions such as temperature, pressure, inert gas, or other chemicals
Condensation of VOCs is the process that transfers VOCs from the gas phase into the liquid phase, which is also the common method used in the industry There are two ways to move VOCs from the gas phase into the liquid phase: Increasing the gas phase's pressure at a specific temperature or reducing the gas-phase temperature at constant pressure Condensation occurs in the dew point when the partial pressure of VOCs in the gas phase is equal to the vapor pressure of the VOCs The relationship between temperature and vapor pressure of some common VOCs is shown in Fig 1.4 [6]
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, NH3 solution, and chlorofluorocarbons.
Catalytic oxidation of VOCs
1.3.1 Mechanisms and kinetics of catalytic oxidation of VOCs
There are many proposed mechanisms for the complete catalytic oxidation of VOCs The validity of each mechanism strongly depends on the properties of the catalyst (active metal and the support) as well as on the nature of the VOCs However, they generally fall into three main categories: a Langmuir-Hinshelwood b Eleye Rideal c 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 assumes that the reaction occurs between the adsorbed VOCs and the adsorbed oxygen Therefore, both the VOCs and oxygen molecules (species) need to adsorb on the surface of the catalyst The VOCs and oxygen may adsorb on similar active sites (single site L-H model) or two different types of active sites (dual site L-H model)
According to the Eleye Rideal (E-R) mechanism, the reaction occurs between the adsorbed species and reactant molecules in the gas phase The controlling step is the reaction between an adsorbed molecule and a molecule in the gas phase
Mars-van Krevelen (MVK) model considers that the reaction occurs between the adsorbed VOCs and the lattice oxygen of the catalyst rather than the oxygen in the gas phase This model assumes that the oxidation of the VOCs takes place in two steps In the first step, the adsorbed VOCs react with oxygen in the catalyst,
17 | P a g e resulting in the reduction of the metal oxide In the second step, the reduced metal oxide is re-oxidized by the gas phase oxygen present in the feed As the catalyst is first reduced and then re-oxidized, it is also known as the redox mechanism This model has been widely used for kinetics modeling of oxidation reactions of hydrocarbons 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 (Pt, Pd, Rh, Au, etc.) are attractive as catalysts due to their high efficiency for the removal of VOCs at low temperatures ( 50nm) basing on the IUPAC classification
54 | P a g e a Isotherm linear plot of activated carbon (AC) b Isotherm linear plot of Silica gel
55 | P a g e c Isotherm linear plot of Silica gel
Figure 3.2 Isotherm linear plot of AC, silica gel and MCM-41
Besides, these results are in agreement with the results of pore distribution obtained from the BJH desorption (Fig 3.3) They also are matched to the previous published data for AC, silica gel and MCM-41
Figure 3.3 Pore distribution of AC, silica gel and MCM-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
The surfaces’ areas of AC and MCM-41 were quite large, above 1000 m 2 /g, while the surface of silica gel is much lower (295 m 2 /g), but the biggest average pore size is recorded on silica gel (96.2A o ), followed by AC (39.43A o ) and MCM-
41 (33.74A o ) From the results, it can be predicted that AC and MCM41 can adsorb toluene on their surfaces more than silica gel does since surface area of silica gel is much smaller while its pore size is too big compared to kinetic diameter of toluene (6.7-8.7A o [64])
The BET surfaces of catalysts based on AC and silica gel were described as Tab 3.2 It is showed that loading metallic oxides on AC is the main reason to reduce the AC’s surface by 40-58%, while that has insignificant effects on the surface of silica gel with over 10% of reduction of surface area The surface area of catalysts on AC decreased more when the composition of Co increased, thus, WI- AC3Cu7Co sample possessed a surface area of only 418 m 2 /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 surface areas of catalyst based MCM-41 were reduced significantly when bimetallic oxides were deposited on MCM41 surfaces It was also considered as the cause of reduced porous volumes The highest surface area was obtained for SS- M5Cu5Co the only sample can remain pore size distribution of MCM-41, followed by SS-M10Cu, SS-M3Cu7Co, and SS-M10Co It can be explained that the particle size (calculated from XRD data) of the single cobalt oxide was bigger than those of the samples containing both Co and Cu, leading to the narrow of MCM-41’s surface (Tab 3.5) The combination of 5%Cu and 5%Co on MCM-41 produced smaller particles (Tab 3.5) Therefore, a larger surface was recorded for the catalyst of SS- M5Cu5Co
The isotherm and pore distribution of catalysts based on MCM-41 can be attributed to type II, while the isotherm and pore distribution of MCM-41 can classify in to type IV Thus, there was some change in surface characteristics of MCM-41 when adding bimetallic oxides of Cu and Co The loading bimetallic oxides on MCM-41 by the solid-solid blending as well as the wet impregnation
58 | P a g e methods has a significant effect on pore size of MCM-41, which are shown in Fig 3.4 a Prepared by solid-solid blending b Prepared by wet impregnation method
Figure 3.4 Pore distribution of catalyst on MCM41
SS-M7Cu3Co SS-M5Cu5Co SS-M3Cu7Co
WI-M7Cu3Co WI-M5Cu5Co
WI-M3Cu7Co WI-M10Co
It could be seen that loading bimetallic oxides on MCM41 has significant effects on pore size, except SS-M5Cu5Co The loading of oxides on MCM41 blocked the mesoporous of MCM-41, then created pores with bigger sizes The influence is only minor with SS-M5Cu5Co, which may be due to the smaller sizes of 5Cu5Co particles and it will be discussed later in XRD results
Furthermore, the wet impregnation can drove more particles of Cu-Co oxides inside of pore of MCM-41 and fill it up by This was a reason to explain for the smaller surface area of catalysts, which were prepared by wet impregnation (seen in Tab.3.3) It is clear that the surface area and pore size of the loaded samples were impacted by not only bimetallic oxides but also the synthesized methods
The XRD patterns of the catalysts WI-AC5Cu5Co, and AC were shown in Fig.3.5 There is no peak of metal oxides in the patterns, because AC is an amorphous solid with high base line in XRD pattern, and the metallic content was so low that the reflected beam is not recorded
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 Co3O4
(ICSD-01-078-1969), which was detected on the samples of WI-S20Co and SS- S20Co, while the structures of both Co3O4 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 Co3O4 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
No Catalysts Crystalline sizes, (nm)
As the results, it was recorded that catalysts prepared by wet impregnation method, produced smaller particle sizes of Co3O4 on silica gel’s surface; and the increase of Co content led to the increase of Co3O4 sizes (the samples with 20% Co exhibited bigger particle size than that of the sample 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- M7Cu3Co
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
Fig 3.7 shows that the XRD pattern of SS-M7Cu3Co presented the structure corresponding to CuO and Co3O4 with average crystalline sizes of 22.87 nm and 9.26 nm, respectively There was the only structure of Co3O4, which was detected in the catalyst of SS-M5Cu5Co and SS-M3Cu7Co because of lower Cu content Because of different ratios of Co and Cu, the average crystalline sizes of Co3O4 in SS-M5Cu5Co and SS-M3Cu7Co are 8.71 nm and 10.24 nm, respectively Thus, adding more Cu tends to decrease the crystalline size of Co particles, which means that using bimetallic oxides may help to make a better dispersion of metal oxide sites
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 Generally, methane is partly oxidized to produce methanol, aldehyde, or CO at a temperature above 400 o C [67]; however, the complete oxidation of methane is performed at a temperature of over 850 o C [68] Therefore, to determine the oxidation ability of the catalysts, it will be useful to test their ability to completely
74 | P a g e oxidize the hardest compound – methane Thus, the synthesized catalysts were pre- examined by methane oxidation in this study
Methane is one of the difficult substances to adsorb over porous materials, so the temperature program desorption of methane (CH4-TPD) was conducted to exam the methane adsorption and oxidation ability of the catalysts
Figure 3.16 CH 4 –TPD profiles of Cu-Co/MCM-41
CH4 –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-
600 o C) The amount of CH4 adsorbed is indicated in Tab.3.11, the amount of CH4 adsorbed on SS-M10Cu was higher than SS-M10Co at both range of low temperature and high temperature may be due to the characteristics of CuO The catalysts with high content of CuO as SS-M5Cu5Co and SS-M7Cu3Co performed as the best materials in CH4 adsorption because of the higher number of CuO activate centers The better CH4 adsorption ability may result in better catalytic activity for the oxidation of CH4
SS-M5Cu5Co SS-M10Co SS-M10Cu
SS-M7Cu3Co SS-M3Cu7Co
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 experiment results showed that the conversion of methane was not occurred at below 200 o C According to previous researches, Silica gel does not have the ability to absorb methane leading to low methane conversion at this range of temperature Moreover, the oxidation of methane over the catalysts began at over
200 o C and increased when temperature rises to 450 o C The catalytic activities of the catalysts were organized in order: SS-S20Co> WI-S20Co> WI-S3Cu7Co > SS- S5Cu5Co > WI-S5Cu5Co > WI-S7Cu3Co
As a result, the catalytic activity depended on not only the content of Cobalt in bimetallic oxides on silica gel but also the preparation methods: wet impregnation method produced the catalyst with lower activity than that produced by solid-solid blending may be due to the fact that solid-solid blending method resulted in more metallic oxides exposed on the surface while the metallic oxides stay more inside the pores when produced by wet impregnation method
The highest conversion from methane into CO2 was 83% corresponding to SS-S20Co, while the catalysts containing both Cu-Co/Silica gel were quite low (under 30%), indicating that the presence of Cu decreases the activity as the activity
77 | P a g e of CuO catalyst is lower than that of Co3O4 catalysts Thus, Co3O4 plays a vital role in oxidation of methane, but it cannot deeply oxidize methane to CO2 at 450 o C However, compared to unsupported catalysts, the catalysts on silica gel exhibited much lower conversion of methane, the loading of 20% exhibited much higher activity than those of the loading of 10%
The results of methane oxidation over the catalysts on MCM-41 were introduced in Fig 3.18 The results showed that the increase in temperature led to the rise of methane conversion At 450 o C, the first and second highest methane conversions were recorded of 100% and 95% corresponding to WI-M10Co and SS- M10Co, which are even higher than unsupported catalysts This means that the dispersion of cobalt oxide on MCM-41 support is fine enough to expose more active sites for the reaction than the unsupported one The catalysts containing only Cu exhibited the worse activities; the activity was only improved when the Cu content decreases significantly to 5% (WI-M3Cu7Co and WI-M5Cu5Co) a Methane oxidation over Cu-Co/MCM-41 prepared by solid-solid blending
78 | 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
The comparison of activities of the catalyst prepared by different methods was presented in Fig 3.19, when the content of Co was over 5%, methane conversion of the samples, that were prepared by WI method, was higher than those prepared by the solid-solid blending method, because of larger particle of bimetallic oxides (seen in Tab 3.5)
Figure 3.19 Comparison of methane oxidation with different preparations at 450 o C
The bimetallic oxides compounds had higher oxidation activities than those of the Cu single compound (33%), but slightly lower than single cobalt oxide That means copper has a little effect on methane conversion except for the role of dispersing the catalytic particles
The results of methane oxidation over the unsupported catalysts were introduced in Fig 3.20 The results showed that catalytic activity of bimetallic oxides without MCM-41 can arrange as the order: 100Co> 70Cu30Co > 50Cu50Co
> 30Cu70Co > 100Cu However, the differences in activities of 100Co, 70Cu30Co 50Cu50Co, 30Cu70Co are very slight while the activity of 100Cu is significantly lower This result suggests that Co3O4 oxide catalyst exhibits the higher oxidation nature than that of CuO catalyst but the presence of CuO in the bimetallic oxide catalysts help to reduce particle size of the higher particle size Co3O4 leading to more exposed active sites for the reaction Activity of Co3O4 based catalysts may reach to almost 90% of methane conversion at 450 o 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 results showed that the use of supports leading to a decrease of the methane conversion On the other hand, the increase of methane conversion was recorded when an increase of Co content in Cu-Co/MCM-41
Figure 3.21 Comparison of methane oxidation on Cu-Co with and without supports at 450 o C
In general, it can be concluded that the catalyst containing only Co exhibited the highest activity for the oxidation of methane while the Cu catalyst exhibited the worse activity This observation seems opposite to the ability to adsorb CH4 and O2 of the samples as it was previously showed that Cu catalysts on MCM-41 exhibited higher CH4 and O2 amount than that of Co catalysts on MCM-41 Thus, the nature of the oxide for the complete oxidation is more important than the ability to adsorb the reactants, which dominated in this case However, when a reasonable amount of
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 amount was calculated basing on the breakthrough curve of isotherm toluene adsorption as Eq 2.5, which was shown in Tab 3.12 The results of toluene adsorption on the catalysts were in agreement with the catalysts’ surfaces and particles’ sizes in previous parts
Table 3.12 Adsorption amount of toluene on Cu-Co/Activated carbon
As the results, the fresh AC has the highest adsorption capacity of 0.28 (g/g) because of its large surface area (Tab 3.1) Moreover, the metallic oxides placed on the surface and pore of AC, as seen in SEM images (Fig 3.12), which leads to narrow the surface area of the support The amount of adsorption arranges as the order: AC180> AC3Cu7Co > WI-AC5Cu5Co > WI-AC7Cu3Co The results show that the impregnation of bimetallic oxide catalysts on AC support - WI-AC5Cu5Co, WI-AC3Cu7Co did not decrease significantly the toluene adsorption ability of the material, which means that the capacity of the adsorption period was not lightly influenced, therefore the impregnation of bimetallic oxide catalysts on AC support still ensure the sorbent to work well 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 was shown in Fig 3.23 the limited adsorption amount of toluene over these catalysts was recorded matching to the surface characteristic of silica gel
Figure 3.23 Toluene adsorption breakout curves on silica gel base
The toluene adsorption of Cu-Co/Silica gel was presented in Tab 3.13; the adsorbed amount was quite low because silica gel is a macro porous material leading to the low adsorption capacity of toluene It is in agreement with the previous studies in the toluene adsorption of silica gel Thus, the impregnation of bimetallic oxide on silica gel is not suitable for the treatment of toluene by adsorption method
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
It was also clear that the impregnation of bimetallic oxides on MCM-41 support was a cause of the reduction of supports’ surface areas that leads to the reduction of toluene adsorption amount The impregnation of single metallic oxide (CuO) even resulted in higher decrease of adsorption amount due to their bigger particle sizes on pores of the support as shown in section 3.1.3 Synthesis method also influenced on the adsorption amount Sample SS-M5Cu5Co exhibited higher adsorption amount than that of IW-M5Cu5Co since the former possessed higher surface area than the latter SS-M5Cu5Co has adsorption amount almost equal to that of MCM-41, showing that it may be well applied to adsorb toluene during the adsorption period
Table 3.14 Adsorption amount of toluene on Cu-Co/MCM-41
The effect of the surface’s area on the toluene adsorption amounts of catalysts on various sorbents was described in Fig 3.25 It can be seen that the surface area is not the only influenced factor to the adsorption ability since the samples with very high surface area (AC and MCM-41) don’t possess much higher adsorption ability than the bimetallic oxide catalysts on supports That is because the toluene adsorption ability may also depend on the pore size In general, it can be seen that the loading of bimetallic oxides did not decrease the toluene adsorption ability of AC or MCM-41 Catalysts on AC showed the highest toluene adsorption ability Catalysts on MCM-41 showed the reasonable toluene adsorption ability and have an advantage of non-limited temperature for desorption period as AC (for AC the temperature of the desorption period cannot excess 200 o C as shown in section 3.1) The catalysts on silica gel support exhibited much lower toluene adsorption ability Therefore, this support may not good for the treatment of toluene
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 are shown in Fig 3.26; it is clear that initial toluene concentration was quite high in comparison to toluene inlet concentration The similar results were reported in some previous studies [69, 70]
88 | 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
The desorption amounts of prior adsorbed toluene by different carrier gases were presented in Tab 3.15 There was a significant difference in generated toluene concentration when the flow of carrier of nitrogen (Fig 3.26 a) was replaced by oxygen (Fig 3.26 b) It is because in the presence of oxygen, bimetallic oxide catalysts on AC had oxidized generated toluene, leading to reduction of outlet toluene concentration in outlet flow
Figure 3.27 Formed CO 2 from heat desorption by oxygen flow over Cu-Co/AC
The outlet CO2 concentration in the case of desorption using oxygen flow showed in Fig 3.27, and the CO2 yield was calculated as Eq 2.9, then compared with theoretical oxidation of toluene as in Tab 3.16
Table 3.15 Generated toluene by thermal desorption
Toluene conversion compared to the desorption amount, (%)
Toluene conversion compared to the adsorption amount, (%)
Table 3.16 Evaluation of total toluene oxidation over the catalysts on AC
No Samples Theoretical CO 2 amount, (mmol/g)
The results showed that the toluene desorption amounts by nitrogen are much less than the toluene adsorption amount, meaning that toluene was not desorbed completely, and a small part of adsorbed toluene was remained inside the pore of
It can conclude that the catalysts have the ability to oxidize toluene at 180 o C; however, these catalysts do not oxidize toluene completely at 180 o C Although the presence of metallic oxides on AC and oxygen in the flow reduced emitted toluene concentration significantly compared to the desorption by nitrogen, high concentration of toluene has still seen during the first minutes of the outlet flow, and the CO2 yield is still low, indicating that toluene was still oxidized to other organic compounds Only in the case of WI-AC5Cu5Co, CO2 yield can reach to 100%, indicating that 5Cu5Co catalyst exhibited strong activity for the oxidation, which is in agreement with the results for the oxidation of methane
It is clear that toluene is completely decomposed into CO2 over WI- AC5Cu5Co at 180 o C, but conversion of toluene does not reach 100% because of high initial toluene concentration in thermal regeneration It is followed by WI- AC3Cu7Co and WI-AC7Cu3Co, in the order Also, the bimetallic oxides of cobalt and copper perform the best activated in the oxidation of VOCs, so that this catalyst will be used to synthesize over other supports to evaluation VOCs oxidation
According previous studies, the adsorbed oxygen was released at temperature below 400 o C for Co3O4 [65] and below 200 o C for CuO [66] The oxidation temperature, however, was only 180 o C Therefore, it can predict that the lattice oxygen was not the factor to oxidize toluene It can conclude the toluene oxidation mechanism over these catalysts is classified in to Langmuir-Hinshelwood (L-H) or Eleye Rideal (E-R) but not Mar Van Klevelen, which is based on the oxidation of lattice oxygen at high temperatures It needs to study more to confirm the mechanism which is match to this reaction
3.3.2.2 Toluene oxidation over Cu-Co/ /Silica gel in desorption process
The generated toluene by thermal desorption on Cu-Co/Silica gel with N2 - flow and O2 flow were presented in Fig 3.28, and Fig 3.29, the adsorbed amount was quite low because Silica gel is a macro porous material leading to the low
92 | 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
The comparison of the toluene adsorption amount and desorption amount was calculated and shown in Tab 3.17 It could be seen that at 180 o C, toluene cannot be captured on Silica gel surface due to the big pore size of silica gel, leading to a similar amount of toluene adsorption and desorption
Table 3.17 Toluene adsorption capacity of catalysts on Silica gel base