Nghiên cứu tổng hợp hệ xúc tác cuo co3o4 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 DOCTORAL DISSE

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 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

Major: Environmental Engineering Code: 9520320

DOCTORAL DISSERTATION OF ENVIRONMENAL ENGINEERING

SUPERVIORS:

1 Assoc Prof Dr Vu Đuc Thao

2 Prof Dr Le Minh Thang

Ha Noi - 2021

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ACKNOWLEDGEMENT

First of all, I would like to thank Prof Nguyen Huu Phu, who raises my

interest in catalysis Secondly, I would like to thank Associate Prof Dr Vu Duc

Thao and Prof Dr Le Minh Thang, who are my supervisors, because of their

guidance, encouragement, and kindly help in the scientific works

Also, I would like to thank my colleagues at Vietnam National Institute of

Occupational Safety and Health (VNNIOSH), lectures in School of Environmental

Science and Technology (INEST) and School of Chemical Engineering (SCE), and

all 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 in

LIKAT for their friendly attitude and support, when I conducted the short-course

research in University of Rostock - Germany

Finally, I would like to give special thanks to my parents, my wife, and my

beloved daughters because of their faced difficulties, supports, encourage as well as

love

The financial supports of the Rohan Program – DAAD & BMZ, German,

and the Project no 216/02/TLD (VNNIOSH) are acknowledged in this thesis

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COMMITMENT

The study has been conducted at the School of Environmental Science and Technology (INEST), School of Chemical Engineering (SCE), Hanoi University of Science and Technology (HUST), Leibniz-Institute for Catalysis (LIKAT), University of Rostock (Germany) and Vietnam National Institute of Occupational Safety and Health (VNNIOSH) The work has been completed under the supervision 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 are entirely true, were agreed to use in this paper by the co-author This research has not been 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

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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 Physisorption 53

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

3.3.1.3 Toluene adsorption over Cu-Co/MCM-41 84

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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

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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

Table 3.12 Adsorption amount of toluene on Cu-Co/Activated carbon 83

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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

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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 impregnation method 61

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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

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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 Development BTEX : 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

Materials MBRs : 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 organic substances

RFR : Reverse flow reactor

SBA 15 : Santa Barbara Amorphous 15

SCE : School of Chemical Engineering

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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 Health VOCs : 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 have adverse effects on the environment and human health Therefore, they should be converted into harmless substances before releasing into the atmosphere Many techniques, such as absorption, adsorption, biotechnology, thermal oxidation, catalytic oxidation, membrane etc., have been studied and applied for VOCs removal Generally, adsorption is the most common technology used in industry because of its advantages as high adsorption capacity, low temperature process However, it has some disadvantages in the desorption process as it is not suitable to apply for VOCs with small amount and it releases VOCs so it is not suitable for unvalued VOCs which are not worth to recover Catalytic oxidation is a promising and effective technique, which can apply for VOCs decomposition because of the high activation However, this is a high temperature process and waste energy Therefore, the combination of adsorption and catalytic oxidation in the desorption process 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 are noble metals, but they are easy to be deactivated by sintering or poisoning Single metallic oxides on porous materials were also used, but their activities are not as strong as noble metals Recently, the bimetallic oxides are promising solutions to replace the noble metal catalysts, because they are not easy to be deactivated by acid gas, they are acceptable cost, and their activities are more durable than the single metallic oxides Moreover, the loading of bimetallic oxides on adsorbents are easy by various available methodologies and its activities can be improved However, the activation of bimetallic oxide catalysts depends on factors such as the components of metal, precursors, preparation methods, porous materials, etc For those reasons, the researches on bimetallic oxide catalysts impregnated on absorbents to oxidize VOCs emitted during the desorption process have been focused recently The application of this technique to treat VOCs may help to improve the treatment processes of VOCs, which increase possibilities to apply this technique in the industry Thus, choosing the topic “Low temperature catalytic

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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 bimetallic oxides (Cu-Co), on porous materials (activated carbon, silica gel, MCM-41) to adsorb and oxidize VOCs, which is represented by toluene, a popular VOCs with particular properties of the BTEX group in many industrial factories, at low temperature

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

3 Content of the thesis

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

The various components of bimetallic oxides (Cu, Co) on several adsorbents (Activated carbon, MCM-41, silica gel) were prepared by two methods (wet impregnation and solid-solid blending), then the catalysts were characterized by thermogravimetric, 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 of these catalysts were evaluated on a micro-reactor system to determine the catalyst with the strongest activity for VOCs oxidation

4 Methodologies of the study

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

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

Data analysis: the method is used to gather and determine the effect of factors, such as support, temperature, component of Cu and Co, etc., on catalytic activities 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 a popular VOCs represented for BTEX group with similar properties

Catalytic active components: Bimetallic oxides of Cu and Co with various ratios 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 at low temperatures Since methane is one of the most difficult compounds to be oxidized and toluene is an aromatic compound represented for BTEX group, a catalyst with high efficiency to oxidize them will be certainly possible to oxidize other VOCs

The catalysts can work at low temperature, resulting in cost reduction The preparation method is simple and costless which can apply for industry to treat volatile 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 blending method

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These catalysts oxidize methane and toluene with high catalytic activities at experimental conditions leading to potential application for environmental protection

The catalyst SS-M10Co is recorded as the highest catalytic activity with methane with the conversion of 93,5% at 450oC The catalyst WI-AC5Cu5Co can completely oxidize toluene emitted in the desorption process at 180oC The catalyst with 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 and the treatment technologies for VOCs, then related literatures are reviewed; the second part – experimental describes the catalytic preparation, characteristics and activation evaluations; the third part shows the results and discussion about catalysts’ aspects and catalytic activity of the catalysts for the oxidation of VOCs; final is the general conclusions of the performed work

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CHAPTER 1 LITERATURE REVIEW

1.1 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 25oC, 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

3 Semi volatile organic

compounds (SVOCs)

240-260 to

280-400

Pesticides, PCB

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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

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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

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 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

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Figure 1.2 VOCs emission control technologies

Volatile organic compounds (VOCs)

Recovery

Destroy

Membrane Condensation

Adsorption Absorption

Oxidation Biology

Incinerator Catalytic oxidation

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

Combustion chamber

Exhaust gas heat exchanger Fuel

VOCs

Air

Stack

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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

chamber

Exhaust gas heat exchanger Fuel

VOCs

Air

Stack

1.2.2 Biological method

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

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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]

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 High Membrane reactor High High High High Need long

term evaluation

Need long term evaluation

High High Need long

term evaluation

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1.2.3 Absorption method

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, (%)

1 The mixture of

2 Toluene Silicone oil

Absorption through the membrane

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 the membrane

99

1.2.4 Adsorption method

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

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

common VOCs

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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

1.3 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,

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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

1.3.2.1 Noble-metal based 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 (<200oC) with the conversion over 90% [10- 15] There are many methods to load noble metal on support 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, noble metal, 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 in VOCs oxidation at low temperature [11, 12, 14, 20] According to the research of Rui 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 wetness impregnation method, was studied by Sedjame et al [12] The research concluded that butanol with the initial concentration of 1000 ppm was decomposed at 165oC with 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 to store 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

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xylene were oxidized entirely at a temperature of 112oC, 109oC, 106oC, and 104oC Moreover, the catalyst of Pt on aluminosilicate materials are captured by some authors For example, Uson et al [21] studied in the catalyst of Pt/SBA 15 to oxidize n-Hexane, while Zang et al [22] studied in the catalyst of Pt/ZSM 5 to oxidize propane Several previous investigations on catalysts of Pt on different supports 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 of xylene over the Pd catalyst However, the results were different because of different supports 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 in the study of Huang [15], when this author used Al2O3 as support and prepared catalyst by wetness impregnation method The oxidation of toluene over Pd/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 400oC over the support of Al2O3, SBA-15 and activated carbon, which was reported by Kim et al [26], Bendahou et al [25] and Bedia et al [27] respectively

Recently, some authors believe that gold can show good performance in VOCs oxidation depending on synthesized methods, characteristics’ supports, shape and size of gold over supports [10, 18, 19, 28, 29] In study of Ali et al [12], the results showed that 50% propane was oxidized at 360oC over catalyst of Au/CeO2-ZrO2-TiO2 prepared by deposition-precipitation method Besides, the oxidation of toluene 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] This result 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 by Liu et al [28], this research reported that Au/Co3O4 can convert 90% toluene into harmless productions at 138oC This catalyst also was used to oxidize benzene and xylene with 90% of conversion at 189oC and 162oC, respectively Several previous investigations on noble-metal catalysts on different supports have been summarized and reported in Tab.1.6

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Table 1.6 The noble metal catalysts for VOCs oxidation

Catalyst Support Prepare method

VOCs Experiment conditions

Ref VOCs Conc.,

(ppm) GHSV/ WHSV

Temp., ( o C)

Conv., (%)

Au CeO2/Fe2O3

Aqueous impregnation method

Pt Al3O2

Simple ultrasonic-aided incipient wetness

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Au

CeO2-ZrO2-TiO2

Precipitation Propane 1,000 12,000h

impregnation method

Ethyl acetate 466.7 60,000 h-1 311 100

19

Propylene 150 120 cm3/min 200 100 22

Pt Al2O3

Wetness impregnation method

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

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1.3.2.2 Non-noble metal oxides

Non-noble metal, transition and rare earth metal oxides show good activity and performance in the oxidation of VOCs with many advantages: greater dispersion of the active component, availability, long lifetime, masking tolerance, capability of regeneration and low cost Although non-noble metal oxides have relatively lower activity than the noble-metal catalysts, they are commonly used in industry for the oxidation of VOCs due to their advantages Besides, support materials and the preparation methods are crucial in determining the performance of metal-oxide catalysts Porous materials are widely used because of the high surface area, the large pores favor high metal dispersion and good catalytic activity in the oxidation of VOCs The most commonly used metal-oxide catalysts include copper oxide, manganese dioxide, iron oxide, nickel oxide, chromium oxide, and cobalt oxide, 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 electrophilic oxide species [37] Because of their performance, Co3O4 was applied to oxidize various kinds of VOCs, such as acetylene, propylene [30], propane [33] and BTEX group [35] Among these VOCs, toluene is considered that Co3O4 show the most efficient catalyst for oxidation in the study of Jiang S [36] This study pointed out that Co3O4 on CNT support can totally decompose toluene at 257oC < Co3O4/Beta (317oC) < Co3O4/ZSM5 (335oC) < Co3O4/SBA-15 (363oC) In the study of Phung Thi Lan et al [38], they applied a new technique, namely adsorption-catalysis, to oxidize m-xylene on catalyst of Co3O4/activated carbon In this study, the results showed that the catalyst can play two roles adsorption and oxidation Also, when applying adsorption-catalytic technique, the catalyst containing 5% Co/AC can deeply oxidize m-xylene at 180oC if the adsorption time was reduced Moreover, when direct oxidation technique was applied, the results showed that m-xylene conversion at 180oC, 200oC, 220oC and 250oC were 23.12%, 28.69%, 42.10% and 55.02% 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 implementation in total toluene oxidation among group catalyst of single metal

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oxide (cobalt, iron and nickel oxides) [42] The catalyst CuO/ activated carbon was studied 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 reported that 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 metal which 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 VOCs oxidation in the previous studies

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Table 1.7 The non-noble metal oxide catalysts overview

Catalyst Support Prepare method

VOCs Experiment conditions

Ref VOCs Conc.,

(ppm)

GHSV/

WHSV

Temp., ( o C)

Conv., (%)

Co3O4 Clay The pulsed-spray

evaporation chemical vapor deposition

Co3O4 Al2O3 Wet impregnation

Co3O4 CNT Wet impregnation

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

CuO Al2O3 Conventional

impregnation method Toluene 1,000 21,000 h

CuO ZnO2 Wet impregnation

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1.3.2.3 Non-noble mix metal oxides

It is well acknowledged that mixed oxides usually exhibit higher activity than single oxides in most catalytic reactions, which is considered as the result of higher surface mobility of oxygen and/or activated species as well as electron transport through the lattice for their multiple energy levels of metals and abundant associated oxygen anions To oxidize toluene, some researches have been implemented by using mixed metallic oxides: Ce-Co, La-Co [49], Mn-Ce [50], Cu-

Ce [51], Mn-Co [52] Among these catalysts, Ce-Co showed the best performance

in toluene oxidation with the lowest temperature of 250oC, because of the role of Ce

in enhancing the activity of Co3O4 These combinations of metal oxides catalysts also showed a high activity in oxidation of benzene, formaldehyde, and chlorobenzene

Although bimetallic oxides exhibit high catalytic activities, there was limited researches on loading bimetallic oxides on porous materials In the study of Zhou et

al [53], they loaded 20% of the bimetal content of Co and Mn (Mn/Co=1.5) on activated carbon by wet impregnation methods The results showed that this catalyst can convert 100% of toluene at a temperature of 250oC In the other study of M Popovaa et al [54], the author prepared the catalysts by impregnating Cr and Cu over SiO2 and SBA-15 This study also showed that the optimal metal oxide content for highly active and selective SBA-15-supported catalysts was 3 wt.% chromium and 7 wt.% copper with 100% conversion at 360oC The study of Kim SC [42] in bimetal supported catalysts for total toluene oxidation was in the order: 5Cu–15Mn/Al > 5Co–15Mn/Al > 5Ni–15Mn/Al > 15Mn/Al > 5Fe–15Mn/Al The catalyst of 5Cu–15Mn/Al was recorded as the highest catalytic activity, which oxidized toluene at 320oC with a conversion of 100% Some mixed metal oxides catalysts are introduced in Table.1.8