Nghiên cứu tổng hợp hệ xúc tác CuOCo3O4 trên một số chất mang để oxi hóa hơi dung môi hữu cơ dễ bay hơi (VOCs) ở nhiệt độ thấp.

Nghiên cứu tổng hợp hệ xúc tác CuOCo3O4 trên một số chất mang để oxi hóa hơi dung môi hữu cơ dễ bay hơi (VOCs) ở nhiệt độ thấp.Nghiên cứu tổng hợp hệ xúc tác CuOCo3O4 trên một số chất mang để oxi hóa hơi dung môi hữu cơ dễ bay hơi (VOCs) ở nhiệt độ thấp.Nghiên cứu tổng hợp hệ xúc tác CuOCo3O4 trên một số chất mang để oxi hóa hơi dung môi hữu cơ dễ bay hơi (VOCs) ở nhiệt độ thấp.Nghiên cứu tổng hợp hệ xúc tác CuOCo3O4 trên một số chất mang để oxi hóa hơi dung môi hữu cơ dễ bay hơi (VOCs) ở nhiệt độ thấp.Nghiên cứu tổng hợp hệ xúc tác CuOCo3O4 trên một số chất mang để oxi hóa hơi dung môi hữu cơ dễ bay hơi (VOCs) ở nhiệt độ thấp.

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 CATALYSTS OF CuO-Co3O4 ON SUPPORTS

DOCTORAL DISSERTATION OF ENVIRONMENAL ENGINEERING

Ha Noi – 2021

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 CATALYSTS OF CuO-Co3O4 ON SUPPORTS

Major: Environmental Engineering Code: 9520320

DOCTORAL DISSERTATION OF ENVIRONMENAL ENGINEERING

SUPERVIORS:

1 Assoc Prof Dr Vu Đuc Thao

2 Prof Dr Le Minh Thang

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ACKNOWLEDGEMENT

First of all, I would like to thank Prof Nguyen Huu Phu, who raises myinterest in catalysis Secondly, I would like to thank Associate Prof Dr Vu DucThao and Prof Dr Le Minh Thang, who are my supervisors, because of theirguidance, encouragement, and kindly help in the scientific works

Also, I would like to thank my colleagues at Vietnam National Institute ofOccupational Safety and Health (VNNIOSH), lectures in School of EnvironmentalScience and Technology (INEST) and School of Chemical Engineering (SCE), andall members in Laboratory of the Petrochemical Refining and Catalytic Materials(LPRCM), and Laboratory of Environmentally Friendly Material and Technologies,that I believe my work cannot be completed without their generous assistance

Moreover, I would like to thank Dr Sebastian Wohlrab and all staff inLIKAT for their friendly attitude and support, when I conducted the short-courseresearch in University of Rostock - Germany

Finally, I would like to give special thanks to my parents, my wife, and mybeloved daughters because of their faced difficulties, supports, encourage as well aslove

The financial supports of the Rohan Program – DAAD & BMZ, German, and the Project no 216/02/TLD (VNNIOSH) are acknowledged in this thesis

The study has been conducted at the School of Environmental Science andTechnology (INEST), School of Chemical Engineering (SCE), Hanoi University ofScience and Technology (HUST), Leibniz-Institute for Catalysis (LIKAT),University of Rostock (Germany) and Vietnam National Institute of OccupationalSafety and Health (VNNIOSH) The work has been completed under thesupervision of Associate Prof Dr Vu Duc Thao and Prof Dr Le Minh Thang

I assure that this is my research All the data and results in the thesis areentirely true, were agreed to use in this paper by the co-author This research has notbeen published by other authors than me

Ngo Quoc Khanh

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TABLE OF CONTENTS

ACKNOWLEDGEMENT i

COMMITMENT ii

TABLE OF CONTENTS iii

LIST OF TABLES vi

LIST OF FIGURES viii

LIST OF ACRONYM AND ABBREVIATIONS xi

INTRODUCTION 1

CHAPTER 1 LITERATURE REVIEW 5

1.1 Overview of volatile organic compounds 5

1.2 Overview of VOCs treatment technologies 7

1.2.1 Oxidation method 9

1.2.2 Biological method 11

1.2.3 Absorption method 14

1.2.4 Adsorption method 14

1.2.5 Condensation method 15

1.3 Catalytic oxidation of VOCs 16

1.3.1 Mechanisms and kinetics of catalytic oxidation of VOCs 16

1.3.2 Catalysts for oxidation of VOCs 17

1.3.2.1 Noble-metal based catalysts 17

1.3.2.2 Non-noble metal oxides 22

1.3.2.3 Non-noble mix metal oxides 26

1.3.3 Catalytic supports and preparation methods for VOCs oxidation 29

1.4 The summary of literature review 30

CHAPTER 2 EXPERIMENT 32

2.1 Catalyst preparation 32

2.1.1 Wet impregnation method 32

2.1.2 Solid-solid blending method 34

2.2 Catalyst characterization 36

2.2.1 Thermal analysis 36

2.2.2 Physical adsorption 37

2.2.3 X-ray diffraction 38

2.2.4 Scanning electron microscopy 39

2.2.5 Chemical and temperature programmed desorption 40

2.3 Adsorption and catalytic activity measurement 43

2.3.1 Adsorption and nitrogen desorption measurement 43

2.3.2 Catalytic activity measurement for complete oxidation of toluene 45

2.3.3 Catalytic activity measurement for complete oxidation of methane 50

CHAPTER 3 RESULTS AND DISCUSSIONS 52

3.1 Characterizations of supports and catalysts 52

3.1.1 Thermal analysis 52

3.1.2 Physisorption53 3.1.3 X-ray diffraction (XRD) 59

3.1.4 Scanning electron microscopy 66

3.1.5 Chemisorption 69

3.1.5.1 CO pulse 69

3.1.5.2 Oxygen temperature programed desorption (O2-TPD) 71

3.2 Total oxidation ability of the catalysts for methane 73

3.3 Toluene treatment 82

3.3.1 Toluene adsorption on catalysts/ sorbents 82

3.3.1.1 Toluene adsorption over Cu-Co/Activated carbon 82

3.3.1.2 Toluene adsorption over Cu-Co/Silica gel 83

3.3.2 Oxidation over catalysts in desorption process 87

3.3.2.1 Toluene oxidation over Cu-Co/Activated carbon in desorption process 87 3.3.2.2 Toluene oxidation over Cu-Co/ /Silica gel in desorption process 91

3.3.2.3 Toluene oxidation over Cu-Co/MCM-41 in desorption process 93

3.3.3 Toluene treatment by complete oxidation over catalysts 97

3.3.3.1 Complete oxidation of toluene on Cu-Co/Silica gel 97

3.3.3.2 Directed oxidation of toluene on Cu-Co/MCM-41 98

3.3.3.3 Directed oxidation of toluene on Cu-Co oxides 100

CONCLUSIONS 104

RECOMMENDATIONS 105

LIST OF PUBLICATIONS 106

REFERENCES 107

APPENDIX 116

LIST OF TABLES

Table 1.1 Definition of volatile organic compounds (VOCs) 5

Table 1.2 The temperature required for complete oxidation of VOCs 10

Table 1.3 The required temperature for catalytic oxidation of VOCs 11

Table 1.4 Performance evaluation of bioreactors for VOCs and odor control 13

Table 1.5 The absorption solutions can absorb the organic solvent vapor 14

Table 1.6 The noble metal catalysts for VOCs oxidation 19

Table 1.7 The non-noble metal oxide catalysts overview 24

Table 1.8 The mixed non-noble metal oxide catalysts overview 27

Table 2.1 Properties of chemicals using to prepare catalysts 32

Table 2.2 List of catalysts prepared by wet impregnation method 34

Table 2.3 List of catalysts prepared by solid-solid bleeding method 36

Table 2.4 Technique of thermal analysis 37

Table 2.5 Operating factors of GC 44

Table 3.1 The Surface characteristics of AC, silica gel and MCM-41 56

Table 3.2 The surface characteristics of catalysts on AC and silica gel 56

Table 3.3 The surface characteristics of catalysts on MCM-41 57

Table 3.4 Crystalline size and phase of Cu-Co/Silica gel 60

Table 3.5 Crystalline sizes and phases of 10% Cu-Co on MCM-41 62

Table 3.6 Crystalline sizes and phases of 20% Cu-Co on MCM-41 64

Table 3.7 Crystalline sizes of Cu-Co oxides 65

Table 3.8 Crystalline sizes of catalysts without supports 66

Table 3.9 Metal dispersion of catalysts 71

Table 3.10 O2 - TPD profile of catalysts 73

Table 3.11 CH4-TPD quantities of Cu-Co/MCM-41 75

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Table 3.13 Adsorption amount of toluene on Cu-Co/Silica gel 84

Table 3.14 Adsorption amount of toluene on Cu-Co/MCM-41 86

Table 3.15 Generated toluene by thermal desorption 90

Table 3.16 Evaluation of total toluene oxidation over the catalysts on AC 90

Table 3.17 Toluene adsorption capacity of catalysts on Silica gel base 93

Table 3.18 Evaluation of total toluene oxidation over the catalysts on silica gel 93

Table 3.19 Evaluation of total toluene oxidation over catalysts on MCM-41 95

Table 3.20 Comparison with other studies 103

LIST OF FIGURES

Figure 1.1 Photochemical smog formation 7

Figure 1.2 VOCs emission control technologies 8

Figure 1.3 Catalytic oxidation technology for treatment of VOCs 10

Figure 1.4 The relationship between temperature and vapor pressure of the most common VOCs 15

Figure 1.5 The mechanisms of VOCs oxidation over catalysts 16

Figure 2.1 Procedure of wet impregnation method 33

Figure 2.2 Procedure of solid-solid blending method 35

Figure 2.6 Bragg ‘s diffraction 38

Figure 2.7 Schematic diagram of the core components of an SEM microscope 39

Figure 2.8 Experimental for temperature programmed reduction, oxidation and desorption 41

Figure 2.9 Adsorption and desorption experiment systems 43

Figure 2.10 The toluene adsorption – desorption oxidation experiment systems 46

Figure 2.11 The complete oxidation of toluene experiment systems 49

Figure 2.12 Total methane oxidation experiment systems 51

Figure 3.1 Thermal analysis in static air of catalyst on AC 52

Figure 3.2 Isotherm linear plot of AC, silica gel and MCM-41 55

Figure 3.3 Pore distribution of AC, silica gel and MCM-41 55

Figure 3.4 Pore distribution of catalyst on MCM41 58

Figure 3.5 XRD patterns of catalysts on AC 59

Figure 3.6 XRD patterns of catalysts on silica gel 60

Figure 3.7 XRD patterns of 10% catalysts on MCM-41 prepared by solid-solid blending method 61 Figure 3.8 XRD patterns of 10% catalysts on MCM-41 prepared by wet

Figure 3.9 XRD patterns of 20% catalysts on MCM-41 63

Figure 3.10 XRD patterns of Cu-Co oxides 64

Figure 3.11 Effects of Co/Cu on Co3O4 crystalline sizes 65

Figure 3.12 SEM images of Cu-Co/AC 67

Figure 3.13 SEM images of Cu-Co/MCM-41 69

Figure 3.14 CO pulse adsorption profiles of catalysts 70

Figure 3.15 O2 –TPD profiles of Cu-Co/MCM-41 72

Figure 3.16 CH4 –TPD profiles of Cu-Co/MCM-41 74

Figure 3.17 Catalytic activity of Cu-Co/silica gel for the complete oxidation of methane 76

Figure 3.18 Catalytic activity of Cu-Co/MCM-41 for the complete oxidation of methane 78

Figure 3.19 Comparison of methane oxidation with different preparations at 450oC .79

Figure 3.20 Catalytic activity of unsupported Cu-Co catalysts for the complete oxidation of methane 80

Figure 3.21 Comparison of methane oxidation on Cu-Co with and without supports at 450oC 81

Figure 3.22 Toluene adsorption breakout curves on AC base 82

Figure 3.23 Toluene adsorption breakout curves on silica gel base 84

Figure 3.24 Toluene adsorption breakout curves on MCM-41 base 85

Figure 3.25 Effect of surface’s area of supports on toluene adsorption amount 87

Figure 3.26 Generated toluene concentrations from heat desorption over 88

Cu-Co/AC 88

Figure 3.27 Formed CO2 from heat desorption by oxygen flow over Cu-Co/AC 89

Figure 3.28 Toluene generation on Cu-Co/silica gel by N2 in desorption 92

Figure 3.29 Toluene generation on Cu-Co/silica gel by O2 in desorption 92

Figure 3.30 Toluene generation on Cu-Co/MCM-41 by N2 in desorption 94

Figure 3.31 Toluene generation on Cu-Co/MCM-41by O2 in desorption 94

Figure 3.32 Comparison of toluene thermal regeneration by N2 and O2 flows 96

Figure 3.33 Toluene conversion over catalysts on silica gel 97

Figure 3.34 CO2 yield over catalysts on silica gel 98

Figure 3.35 Toluene conversion over catalysts on MCM-41 99

Figure 3.36 CO2 yield over catalysts on MCM-41 99

Figure 3.37 Toluene conversion over bimetallic oxides 101

Figure 3.38 CO2 yield over bimetallic oxides 101

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LIST OF ACRONYM AND ABBREVIATIONS

AC : Activated carbon

ASTM : Standard Test Method

BET : Brunauer, Emmett and Teller

BJH : Barrett, Joyner, and Halenda

BMZ : Federal Ministry of Economic Cooperation and DevelopmentBTEX : Benzene, Toluene, Ethylbenzene and Xylene

CNT : Carbon nanotuber

DAAD : German Academic Exchange Service

DGA : Differential thermal analysis

E-R : Eleye Rideals

GC : Gas Chromatograph

GHSV : Gas hourly space velocity

HPLC : High-performance liquid chromatography

HUST : Hanoi University of Science and Technology

INEST : School of Environmental Science and Technology

L-H : Langmuir-Hinshelwood

LIKAT : Leibniz-Institute for Catalysis

LPRCM : Laboratory of the Petrochemical Refining and Catalytic

MaterialsMBRs : Membrane bioreactors

MCM-41 : Mobil Composition of Matter No 41

MFC : Mass flow controller

MVK : Mars-van Krevelen

PAH : Polycyclic Aromatic Hydrocarbon

PAN : Peroxyacetyl nitrate

PCB : Poly Chlorinated Biphenyl

POCP : Photochemical ozone creativity potential

QCVN 20:2009/

BTNMT

: National technical regulation on industrial emission of organicsubstances

RFR : Reverse flow reactor

SBA 15 : Santa Barbara Amorphous 15

SCE : School of Chemical Engineering

SEM : Scanning electron microscopy

SS : Solid-Solid blending

SVOCs : Semi volatile organic compounds

TCD : Thermal conductivity detector

TGA : Thermogravimetric analysis

TPD : Temperature programmed desorption

US EPA : United States of America Environment Protect Agency

VNNIOSH : Viet Nam National Institute of Occupational Safety and HealthVOCs : Volatile organic compounds

VVOCs : Very volatile organic compounds

WHO : World Health Organization

WHSV : Weight hourly space velocity

WI : Wet impregnation

XRD : X-ray Diffraction

ZSM-5 : Zeolite Socony Mobil 5

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INTRODUCTION

1 Necessity of the study

Volatile organic compounds (VOCs) are widespread pollutants that haveadverse effects on the environment and human health Therefore, they should beconverted into harmless substances before releasing into the atmosphere Manytechniques, such as absorption, adsorption, biotechnology, thermal oxidation,catalytic oxidation, membrane etc., have been studied and applied for VOCsremoval Generally, adsorption is the most common technology used in industrybecause of its advantages as high adsorption capacity, low temperature process.However, it has some disadvantages in the desorption process as it is not suitable toapply for VOCs with small amount and it releases VOCs so it is not suitable forunvalued VOCs which are not worth to recover Catalytic oxidation is a promisingand effective technique, which can apply for VOCs decomposition because of thehigh activation However, this is a high temperature process and waste energy.Therefore, the combination of adsorption and catalytic oxidation in the desorptionprocess is proposed to treat unvalued VOCs or polluted VOCs with small amount

Most of the catalysts for the oxidation that have been used in industry arenoble metals, but they are easy to be deactivated by sintering or poisoning Singlemetallic oxides on porous materials were also used, but their activities are not asstrong as noble metals Recently, the bimetallic oxides are promising solutions toreplace the noble metal catalysts, because they are not easy to be deactivated byacid gas, they are acceptable cost, and their activities are more durable than thesingle metallic oxides Moreover, the loading of bimetallic oxides on adsorbents areeasy by various available methodologies and its activities can be improved.However, the activation of bimetallic oxide catalysts depends on factors such as thecomponents of metal, precursors, preparation methods, porous materials, etc Forthose reasons, the researches on bimetallic oxide catalysts impregnated onabsorbents to oxidize VOCs emitted during the desorption process have beenfocused recently The application of this technique to treat VOCs may help toimprove the treatment processes of VOCs, which increase possibilities to apply thistechnique in the industry Thus, choosing the topic “Low temperature

catalytic

oxidation of volatile organic compounds (VOCs) over catalysts of CuO-Co 3 O 4 on supports” for this research is necessary for industry and life.

2 Objective of the study

The general objective of this study is to produce catalysts of bimetallicoxides (Cu-Co), on porous materials (activated carbon, silica gel, MCM-41) toadsorb and oxidize VOCs, which is represented by toluene, a popular VOCs withparticular properties of the BTEX group in many industrial factories, at lowtemperature

The other aims are to optimize components of Cu-Co on varioussupports/adsorbents, to characterize and to compare the activities of these catalysts,which are prepared by different methods (wet impregnation and solid-solid blendingmethods)

3 Content of the thesis

Firstly, literature review on previous studies will be investigated to select thecomponents of the catalysts for oxidizing VOCs, and the preparation methods of thecatalysts

The various components of bimetallic oxides (Cu, Co) on several adsorbents(Activated carbon, MCM-41, silica gel) were prepared by two methods (wetimpregnation and solid-solid blending), then the catalysts were characterized bythermogravimetric, physical adsorption, SEM, XRD, TPD-O2 and chemisorption

The catalytic activities of these catalysts were pre-tested for CH4 oxidation,the most stable organic compound, to ensure they can work with other VOCs

The adsorption capacities, desorption by N2 and O2, and catalytic activities ofthese catalysts were evaluated on a micro-reactor system to determine the catalystwith the strongest activity for VOCs oxidation

4 Methodologies of the study

Literature review: it is a general method to collect related data from previousresearches such as the component of the catalysts, the preparation methods, theoxidation temperature, experiment conditions, etc

Experimental methods: the catalysts were prepared by wet impregnation andsolid-solid blending methods, then characterized by various techniques such asthermogravimetric, physical adsorption, SEM, XRD and chemisorption Finally, theVOCs adsorption, desorption capacities, and oxidation performances of thesecatalysts were evaluated using a micro-reactor system.

Data analysis: the method is used to gather and determine the effect offactors, such as support, temperature, component of Cu and Co, etc., on catalyticactivities It is also used to estimate the role of each metal oxide in the catalyst

5 Scope of the study

Volatile organic compounds (VOCs): Toluene is chosen to exam since it is apopular VOCs represented for BTEX group with similar properties

Catalytic active components: Bimetallic oxides of Cu and Co with variousratios on activated carbon, silica gel and MCM-41 are studied

6 Scientific and practical meanings

The thesis can provide a scientific background to synthesize the catalyst of

Cu and Co bimetallic oxides to oxidize methane and toluene with high efficiency atlow temperatures Since methane is one of the most difficult compounds to beoxidized and toluene is an aromatic compound represented for BTEX group, acatalyst with high efficiency to oxidize them will be certainly possible to oxidizeother VOCs

The catalysts can work at low temperature, resulting in cost reduction Thepreparation method is simple and costless which can apply for industry to treatvolatile organic compounds

7 Novelty of the study

Successfully synthesize the catalyst of bimetallic oxides on several supports(Activated carbon, MCM-41, Silica gel) by applying the solid-solid blendingmethod

These catalysts oxidize methane and toluene with high catalytic activities atexperimental conditions leading to potential application for environmentalprotection.

The catalyst SS-M10Co is recorded as the highest catalytic activity withmethane with the conversion of 93,5% at 450oC The catalyst WI-AC5Cu5Co cancompletely oxidize toluene emitted in the desorption process at 180oC The catalystwith 3% cobalt and 7% copper on MCM-41 can completely treat toluene at 400oC

in the complete oxidation process

8 Structure of the thesis

This thesis includes three main parts: the first is the general information andthe treatment technologies for VOCs, then related literatures are reviewed; thesecond part – experimental describes the catalytic preparation, characteristics andactivation evaluations; the third part shows the results and discussion aboutcatalysts’ aspects and catalytic activity of the catalysts for the oxidation of VOCs;final is the general conclusions of the performed work

CHAPTER 1 LITERATURE REVIEW

1.1 Overview of volatile organic compounds

VOCs are organic chemical compounds that could quickly evaporate undernormal 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 thathave a vapor pressure of more than 13.3 Pa at 25oC, according to ASTM testmethod 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 orcarbonates, and ammonium carbonate, which participates in atmosphericphotochemical 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 (WorldHealth Organization) [2] as shown in Tab 1.1:

Table 1.1 Definition of volatile organic compounds (VOCs)

No Classification Boiling point

3 Semi volatile organic

compounds (SVOCs)

240-260 to 400

280-Pesticides, PCB

Depending on their structure, VOCs are commonly classified into thefollowing groups: Halogenated compounds, Aldehydes, Aromatic compounds,Polycyclic aromatic hydrocarbons, Alcohols, Ketones and Miscellaneous [3- 5].Halogenated VOCs, such as chlorobenzene, dichloromethane, etc., can use assolvents, cleaning agent which emit from in some chemical processing industry.These types of VOCs have a significant impact on the destruction of the ozone layerand cause of human cancer; Aldehydes are main pollutants of treated wood resins,cosmetics, plastic adhesives, which can format of ozone layer and cause of chronictoxicity; Aromatic compounds are popular VOCs, toluene, benzene and xylene, theyare 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 combustionprocesses, such as naphthalene, phenanthrene, pyrene, etc PAHs have been linked

to skin, lung, bladder, liver, and stomach cancers; Alcohols and ketones are maincomponents in cosmetics and personal care products, which can increase theformation of aldehydes in the atmosphere and damage human’s health;Miscellaneous VOCs, example propylene, ethylene and methyl tert-butyl etheremitted from petrochemical syntheses, which are the cause of large photochemicalozone 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 andhuman 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 tomoderate levels can cause tiredness, confusion, weakness, drunken-type actions,memory loss, nausea, loss of appetite, hearing loss, and color vision loss Some ofthese symptoms usually disappear when exposure is stopped Inhaling high levels oftoluene 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 whichare the cause of photochemical smog (Fig 1.1) This smog has many adverseeffects When combined with hydrocarbons, the chemicals contained within it formmolecules that cause eye irritation Radicals in the air interfere with the nitrogencycle by preventing the destruction of ground-level ozone Other effects includereduced visibility and respiratory ailments.

Figure 1.1 Photochemical smog formation (source:

https://energyeducation.ca/)

1.2 Overview of VOCs treatment technologies

VOCs are dangerous to human health and the environment, so VOCs controland treatment are urgently necessary There are many available methods to controlVOCs, 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 ofprocess It is the most effective and efficient method, but itsapplicability is limited because of difficulties in equipment, materialsand technology

(ii) Treatment technology: Many techniques have been applied to remove

VOCs, such as adsorption, condensation, membrane, biology, and

- Incomplete oxidation that includes some other substances than CO2 and

H2O in the productions (A.2)

𝑦

𝐶𝑥𝐻𝑦𝑂𝑧 + 𝑂2 → 𝐶𝑥𝐻𝑦𝑂𝑧 + 𝐶𝑂2 +

2 𝐻2𝑂

A.2However, 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 canclassify 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

Fuel

Stack

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 oxidationtemperature than the combustion temperature (Tab.1.2) However, when using thecatalyst, the oxidation temperature significantly reduces, and it also depends onmany 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

Table 1.3 The required temperature for catalytic oxidation of VOCs

In natural, some microorganisms can convert VOCs into other harmlessproductions, so that this phenomenon is applied for removing VOCs bymicrobiology 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, includedimmobilized microorganisms on a porous material This filter provides ahospitable environment in the form of moisture, temperature, oxygen,nutrients, and pH to the organisms When contaminated air stream passesthrough the filter, contaminants are transferred to the bio-film located on thepacking materials

Bio-tricking filter: Among the various biological oxidation techniques, biotrickling 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 anabsorption unit and a bioreactor unit At the absorption column, VOCs

transfer to the liquid phase, then this liquid will be treated at bioreactor, andVOCs will be decomposed After the gaseous treatment, the solution of thewaste may be recycled back.

Membrane Bioreactor: Membrane bioreactors are the best filtrationtechnique since they provide a larger gas-liquid interface In this filtrationprocess, the VOCs are transferred with the help of a membrane, where theyare degraded with the help of biofilm Essential advantages of membraneover other biological reactors are only particular pollutants can pass through

it and its capability to degrade those VOCs, which are poorly soluble inwater

Biotechnology is a promising method to remove VOCs at low concentrationswith many advantages, it is having some challenges such as micro bacterial controland living environment conditions The limited application of biotechnology inVOCs treatment systems is shown in Tab 1.4 [7]

Table 1.4 Performance evaluation of bioreactors for VOCs and odor control

Bioreactor Type

Treatment efficiency for

Pressure drops

Capital cost Op cost

Bioprocess control

Low conc of VOCs/

odors

High conc of VOCs/

odors

High soluble VOCs

water-Low water insoluble VOCs

Fluctuating feed conditions

Bio-scrubber High High High Low High Very low Medium Medium HighMembrane reactor High High High High Need long

termevaluation

Need longtermevaluation

High High Need long

termevaluation

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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, thecontact time, the absorbing concentration, and the reaction rate between theabsorber and the gas In previous studies, some solutions can absorb organic solventvapors 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, (%)

1 The mixture of

2 Toluene Silicone oil

Absorption through themembrane

100

3 Methyl ethyl

5 Toluene Silicone oil The packed tower 99

6 n-Decane The packed

tower and water The strayed tower 70

7 Toluene Mineral oil

Absorption through themembrane

99

Adsorption is the most popular method to capture VOCs with manyadvantages, which is very useful in removing pollutants at low and deficientconcentrations However, with a high level of contaminants, adsorption is not anefficient process since it rapidly reaches the balance of adsorption The biggestchallenge of adsorption is the need for VOCs secondary treatment When exhausted,the adsorbent must either be replaced or regenerated, which is the process of

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 phaseinto the liquid phase, which is also the common method used in the industry Thereare two ways to move VOCs from the gas phase into the liquid phase: Increasingthe gas phase's pressure at a specific temperature or reducing the gas-phasetemperature at constant pressure Condensation occurs in the dew point when thepartial pressure of VOCs in the gas phase is equal to the vapor pressure of theVOCs The relationship between temperature and vapor pressure of some commonVOCs 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 exhaustgas, there are many cold agents used, such as water, saline solution, NH3 solution,and chlorofluorocarbons.

1.3 Catalytic oxidation of VOCs

There are many proposed mechanisms for the complete catalytic oxidation ofVOCs The validity of each mechanism strongly depends on the properties of thecatalyst (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 reactionoccurs between the adsorbed VOCs and the adsorbed oxygen Therefore, both theVOCs 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) ortwo different types of active sites (dual site L-H model)

According to the Eleye Rideal (E-R) mechanism, the reaction occursbetween the adsorbed species and reactant molecules in the gas phase Thecontrolling step is the reaction between an adsorbed molecule and a molecule in thegas phase

Mars-van Krevelen (MVK) model considers that the reaction occurs betweenthe adsorbed VOCs and the lattice oxygen of the catalyst rather than the oxygen inthe 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,

resulting in the reduction of the metal oxide In the second step, the reduced metaloxide is re-oxidized by the gas phase oxygen present in the feed As the catalyst isfirst reduced and then re-oxidized, it is also known as the redox mechanism Thismodel has been widely used for kinetics modeling of oxidation reactions ofhydrocarbons over metal oxide catalysts.

Catalysts used for the oxidation of VOCs can be classified into three majorgroups [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 totheir high efficiency for the removal of VOCs at low temperatures (<200oC) withthe conversion over 90% [10- 15] There are many methods to load noble metal onsupport materials, but the wetness impregnation is the most common method is[10, 11, 15, 16] Noble-metal-based catalysts are expensive and can be deactivated

by sintering or poisoning, and alone they are not ordinarily selective enough [17].Performance of these catalysts depends on the preparation, precursor type, noblemetal, kind and concentration of the VOCs (alkane, acetone, aromatic hydrocarbon,alcohol, etc.) [12, 16, 18, 19]

According to previous studies, Pt was performed as the strongest activity inVOCs oxidation at low temperature [11, 12, 14, 20] According to the research ofRui et al., Pt was loaded on the support of Al2O3 by wetness impregnation method,which can oxidize toluene at 200oC with a conversion of 95% [11] Oxidation of m-butanol over the catalyst of Pt/Al2O3, which also prepared by the wetnessimpregnation method, was studied by Sedjame et al [12] The research concludedthat butanol with the initial concentration of 1000 ppm was decomposed at 165oCwith the conversion of 95% The oxidation of butanol over the catalyst of Pt/CeO2

was also investigated, and the results showed that the oxidation temperature reduced

by 30oC in comparison with Pt/Al2O3 It could be explained by the ability of Ce tostore and release lattice oxygen Loading of Pt on activated carbon was examined

by Joung et al [14], the results pointed out that benzene, toluene, ethylbenzene, and

xylene were oxidized entirely at a temperature of 112oC, 109oC, 106oC, and 104oC.Moreover, the catalyst of Pt on aluminosilicate materials are captured by someauthors For example, Uson et al [21] studied in the catalyst of Pt/SBA 15 tooxidize n-Hexane, while Zang et al [22] studied in the catalyst of Pt/ZSM 5 tooxidize propane Several previous investigations on catalysts of Pt on differentsupports have been summarized and reported in Tab.1.6.

Furthermore, Pd is an excellent catalyst for oxidizing the BTEX group[16, 20, 23, 24, 25] Wang et al [23, 24] and Huang et al [15] reported oxidation ofxylene over the Pd catalyst However, the results were different because of differentsupports and loading methods, such as Pd/Co3O4 prepared by precipitation method[23] and post impregnation [24] provided the same conversation of 90%temperature of 249oC and 254oC Moreover, temperature could reduce to 145oC inthe study of Huang [15], when this author used Al2O3 as support and preparedcatalyst by wetness impregnation method The oxidation of toluene overPd/supports was also reported in studies of Rooke et al [16] and Bendahou et al.[25] The temperature of 100% conversion was recorded at 190oC, 220oC and 400oCover the support of Al2O3, SBA-15 and activated carbon, which was reported byKim et al [26], Bendahou et al [25] and Bedia et al [27] respectively

Recently, some authors believe that gold can show good performance inVOCs oxidation depending on synthesized methods, characteristics’ supports, shapeand size of gold over supports [10, 18, 19, 28, 29] In study of Ali et al [12], theresults showed that 50% propane was oxidized at 360oC over catalyst of Au/CeO2-ZrO2-TiO2 prepared by deposition-precipitation method Besides, the oxidation oftoluene over the catalysts of Au loading on several metal oxides, such as CuO,

Fe2O3, La2O3, MgO and NiO were reported in study of Carabinerio et al [19] Thisresult showed that Au/CuO was the highest activity followed by Au/NiO, Au/Fe2O3,Au/MgO and Au/La2O3 in the order Also, the oxidation of toluene was studied byLiu et al [28], this research reported that Au/Co3O4 can convert 90% toluene intoharmless productions at 138oC This catalyst also was used to oxidize benzene andxylene with 90% of conversion at 189oC and 162oC, respectively Several previousinvestigations on noble-metal catalysts on different supports have been summarized

19 | P a g e

Table 1.6 The noble metal catalysts for VOCs oxidation

Catalyst Support Prepare method

Ref.

Temp., ( o C)

Conv., (%)

Au CeO2/Fe2O3

Aqueousimpregnationmethod

Benzene 42 g/m3 4000h-1 200 100 10

Pt Al3O2

Simpleultrasonic-aided incipient wetness

Au CeO2-ZrO2

-TiO2

Precipitation Propane 1,000 12,000h

impregnationmethod

Propylene 150 120 cm3/min 200 100 22

Pt Al2O3

Wetnessimpregnationmethod

Au MgO The double Ethyl acetate 466.7 60,000 h-1 290 100 28

Toluene 1,000 20,000 ml h−1 g−1 260 90 29

1.3.2.2 Non-noble metal oxides

Non-noble metal, transition and rare earth metal oxides show good activityand performance in the oxidation of VOCs with many advantages: greaterdispersion of the active component, availability, long lifetime, masking tolerance,capability of regeneration and low cost Although non-noble metal oxides haverelatively lower activity than the noble-metal catalysts, they are commonly used inindustry for the oxidation of VOCs due to their advantages Besides, supportmaterials and the preparation methods are crucial in determining the performance ofmetal-oxide catalysts Porous materials are widely used because of the high surfacearea, the large pores favor high metal dispersion and good catalytic activity in theoxidation of VOCs The most commonly used metal-oxide catalysts include copperoxide, manganese dioxide, iron oxide, nickel oxide, chromium oxide, and cobaltoxide, etc

Co3O4 is the most common non-noble metal catalyst in VOCs oxidation [30

-36] because of the presence of mobile oxygen, high concentration of electrophilicoxide species [37] Because of their performance, Co3O4 was applied to oxidizevarious kinds of VOCs, such as acetylene, propylene [30], propane [33] and BTEXgroup [35] Among these VOCs, toluene is considered that Co3O4 show the mostefficient catalyst for oxidation in the study of Jiang S [36] This study pointed outthat Co3O4 on CNT support can totally decompose toluene at 257oC < Co3O4/Beta(317oC) < Co3O4/ZSM5 (335oC) < Co3O4/SBA-15 (363oC) In the study of PhungThi Lan et al [38], they applied a new technique, namely adsorption-catalysis, tooxidize m-xylene on catalyst of Co3O4/activated carbon In this study, the resultsshowed that the catalyst can play two roles adsorption and oxidation Also, whenapplying adsorption-catalytic technique, the catalyst containing 5% Co/AC candeeply oxidize m-xylene at 180oC if the adsorption time was reduced Moreover,when direct oxidation technique was applied, the results showed that m-xyleneconversion at 180oC, 200oC, 220oC and 250oC were 23.12%, 28.69%, 42.10% and55.02% respectively

Moreover, copper oxide is also highly active catalyst for the total oxidation

of VOCs [32, 36, 39, 40, 45] The catalyst of CuO/alumina support shows the best

oxide (cobalt, iron and nickel oxides) [42] The catalyst CuO/ activated carbon wasstudied in the research of Nguyen Hoang Hao [43] The results showed that m-xylene was oxidized at 180oC in VOCs oxidation Also, this study’s results reportedthat catalyst of can deeply oxidize m-xylene at 160oC, 170oC, 180oC, 190oC and

200oC with conversion of 12.22%, 14.71%, 18.21%, 22.77% and 26.88%

Beside of Co3O4 and CuO, there are many transition and rare earth metalwhich use to study in VOCs oxidation, such as Mn, Ni, Cr, Ce… [35, 39, 41, 44, 46,

47] However, these kinds of catalysts are easily deactivated because of Cl species.Tab.1.7 shows some commonly used non-noble metal oxide catalysts for VOCsoxidation in the previous studies

Table 1.7 The non-noble metal oxide catalysts overview

Catalyst Support Prepare method

Conv., (%)

Co3O4 Clay The pulsed-spray

evaporation chemicalvapor deposition

CuO SiO2 Wet impregnation m-Xylene 2,000 2 l/min 400 100 37