Study on the hydroformylation of ethylene with co and co2 using supported ionic liquid phase silptio2 and nano autio2 (sio2) catalyst 2

STUDY ON SYNTHESIS AND CHARACTERIZATION OF SUPPORTED IONIC LIQUID PHASE AND GOLD BASED CATALYSTS FOR HYDROFORMYLATION OF ETHYLENE Nghiên cứu tổng hợp và đặc trưng của một số xúc tác dị

MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

Truong Duc Duc

STUDY ON THE HYDROFORMYLATION OF ETHYLENE WITH

SILP/TiO2 AND NANO Au/TiO2 (SiO2) CATALYST

THE CHEMICAL ENGINEERING DOCTORAL THESIS

Hanoi – 2024

STUDY ON SYNTHESIS AND CHARACTERIZATION OF

SUPPORTED IONIC LIQUID PHASE AND GOLD BASED

CATALYSTS FOR HYDROFORMYLATION OF ETHYLENE

Nghiên cứu tổng hợp và đặc trưng của một số xúc tác dị

thể cho phản ứng hydroformyl hóa etylen

Hanoi University of Science and Technology Institute of Postgraduate Education

PhD Student: Truong Duc Duc

Supervisor: Prof Dr Le Minh Thang

Co-Supervisor: Prof Evgenii Kondratenko

Hanoi - 2024

MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

Truong Duc Duc

STUDY ON THE HYDROFORMYLATION OF ETHYLENE WITH

SILP/TiO2 AND NANO Au/TiO2 (SiO2) CATALYST

Major: Chemical Engineering Code: 9520301

THE CHEMICAL ENGINEERING DOCTORAL THESIS

SCIENTIFIC SUPERVISOR:

1 Prof.Dr Le Minh Thang

Hanoi – 2024

Preface

This thesis constitutes the written part of my Ph.D project and is funded by the German Academic Exchange Service (DAAD, No 57315854) and the Federal Ministry for Economic Cooperation and Development (BMZ)

The project was performed in the first period May 2017 to June 2018 at Hanoi University of Science and Technology (HUST) as well as in the second period July

2018 until January 2020 at Leibniz Institute for Catalysis (LIKAT) within the framework of the ROHAN - Rostock - Hanoi DAAD SDG Graduate School on

“Catalysis as key towards sustainable resource management” (2016-2020) My supervisors on the project have been Prof.Dr Thang Le Minh, Department of Organic and Petrochemical Technology – SCLS, HUST, and Prof Dr Evgenii V Kondratenko, Department of Reaction Mechanisms, LIKAT

The research described in the thesis is mainly inherited from disciplinary scientific approachs for many years of SILP group, which has been led by Prof Thang Le Minh (HUST) Moreover, influences obtained from a successful research stayed with the group of Prof Evegenii Kondratenko (LIKAT) have also had important impacts on the thesis as well

The thesis summarizes the main results and discoveries I've obtained in the last three years as a Ph.D student Most of the results obtained in the project have already been reported in the conference contributions listed in the Appendix and/or

in the six papers enclosed in the thesis

COMMITTAL IN THE THESIS

I assure that my scientific results are righteous They haven’t been published in any scientific documents yet I have responsibilities for my protestation and my research results in the thesis

HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

PP PRESIDENT DIRECTOR, DEPARTMENT OF ACADEMIC AFFAIRS

ACKNOWLEDGEMENTS

I would like to express my deep gratitude to the great teacher Professor Le Minh Thang, who has always taught and guided me throughout the journey of my studies, the teacher who always encouraged me to “never give up” all the difficulties of my life, I appreciated and always be grateful to her from the bottom of my heart

I would also like to express my faithful thanks to Prof Dr Evgenii Kondratenko (Head of Department “Methods for Applied Catalysis”, LIKAT) for his precious instruction for me during my studies in Rostock, Germany His guidance clearly led

my research route and encouraged my scientific passion to complete this research

At LIKAT, I would further like to thank Dr Tatiana Otroshchenko for helping me

in catalyst characterization (TPR, TPO, GC setup), Anna Perechodjuk for recording XRD, ICP-OES, BET…To people in Prof Kondratenko’s research group,

my appreciation is given to Qingxin Yang, Nils Ortner, Dan Zhao for their kind supports

At HUST, I wish to thank Dr Do Van Hung for his guidance in the synthesis of TPPTS ligands Acknowledgment is also given to Tuan Doan who shared all spare parts as well as devices for preparation and testing of catalysts

Finally, the effort to complete this thesis is my appreciation and gratitude to my family, friends, and colleagues who have always appreciated and trusted me throughout this difficult journey

I express my thanks and immeasurable gratitude!

Hanoi, 9th November.2024

1.1.4 Recent trends in the heterogenization of catalyst in

1.2.4 Directly conversion of carbon dioxide, hydrogen and olefins

1.2.5 Influence of alkali metals on supported metal catalysts in

1.2.6 Modification of supported gold catalyst over SiO2 for

1.2.7 Effect of point of zero charge (PZC) of supports to the

2.1.2.1 Preparation of Au/TiO2 catalysts using different

2.1.2.2 Preparation of Au/SiO2 catalysts with different Au

2.1.2.3 Preparation of Au/SiO2 catalysts with different Au

NP sizes by changing concentration of HAuCl4 solutions and

2.1.2.4 Preparation of Au/SiO2 catalysts modified with different amounts of Cs promoter (0.5% - 2%) 49 2.1.2.5 Preparation of 1%Au loading amount catalysts with

2.1.2.6 Preparation of bimetallic catalysts based on 1AuSiO2

2.2.9 Inductively Coupled Plasma Optical Emission Spectroscopy

2.3.1 Micro-line reactor setup for testing catalytic activities of catalyst in

hydroformylation of ethylene with CO and H2 62 2.3.2 Micro-line reactor setup for direct conversion of CO2 to

3.2.4 EPR characterization 78

3.3 The catalytic activity of SILP samples in the hydroformylation

3.3.1 Influence of ionic liquid loading content on catalytic activities 83

3.4 Nano Au/TiO2 catalysts and their catalytic activities in

3.5 The catalytic activity of gold supported catalyst in direct

3.5.1 Comparison of catalytic activities of catalyst testing in

3.5.2 Effect of CO/H2 ratio to catalytic activities of catalyst over

3.5.3 Effect of temperature at the second reactor to catalytic

3.5.10 Impact of dopping transition metals to bimetallic catalysts

based on 1AuSiO2 matrix and their catalytic activities in

hydroformylation of ethylene with CO2 and H2

LIST OF ABBREVIATIONS

ICP-OES Inductively Coupled Plasma Optical Emission Spectrometry

MAS NMR Magic-Angle Spinning Nuclear Magnetic Resonance spectroscopy

LIST OF TABLES

Page

Table 2.2 Summary of synthesized Au/TiO2-based catalysts using

different methods

47

Table 2.4 Prepared Au/SiO2 catalysts modified with different amount of

Table 2.6 Summary of Bimetallic 2X-1Au/SiO2 catalysts with different

Table 3.2 0,5Au_TiO2 catalysts and their selected physicochemical

properties

72

Table 3.3 Prepared Au/SiO2 and Cs Au/SiO2 catalysts and their BET

surface area values (SBET) , Au average crystallite size (d Au NP) 74 Table 3.4 Gold supported catalysts over different supports and their

Table 3.6 The selectivites of oxo products (i-propanol, n-propanol and

both of them) depend on reaction temperatures over

LIST OF FIGURES

Page

Figure 1.3 Products derived from aldehydes manufactured by

hydroformylation

15

Figure 1.7 Three stages of the catalyst development for the

hydroformylation reaction

20

Figure 1.8 Coordinative anchoring of a metal complex to the support

surface, A is a mononuclear complex, B is a polynuclear complex, L is a

ligand

22

Figure 1.15 Mechanism of ethene hydroformylation over metal (M)

supported catalysts with proposed intermediates

29

Figure 1.16 Equilibrium distribution at different temperatures with

methanation feed (H2/CO2 = 4) at atmospheric pressure

35

Figure 1.18 Schematic illustration of three types of supported bimetallic

sites (a) The supported binuclear metal site, (b) supported bimetallic

nanocluster, and (c) supported bimetallic nanoparticle It should be noted

that currently there is no definite size boundary between “nanocluster” and

“nanoparticle” In our opinion, metal clusters whose particle sizes are ≤1 nm

(subnanometer clusters) could show distinct catalytic properties in

comparison to metal nanoparticles with sizes >1 nm because subnanometer

clusters show molecule-like electronic structures and nearly all the metal

atoms in the clusters are exposed to reactants In this review, we mainly

refer to “nanoclusters” as metal species with sizes ≤1 nm

39

Figure 2.3 Illustrates how diffraction of X-rays by crystal planes allows

one to derive lattice by using Bragg relation

54

Figure 2.6 (a) A dual – reactor concept diagram and (b) real design reactor

system

66

Figure 3.2 1H – MAS NMR spectrum of synthesized TPPTS-Cs3 ligand (a)

and TPPTS-Na3 ligand (b)

68

Figure 3.3 31P-MAS NMR spectrum of synthesized TPPTS-Cs3 ligand (a)

and TPPTS-Na3 ligand (b)

69

Figure 3.5 Isotherm and pore distribution of 0,2Rh-L/Rh=10,

IL=2.5%-TiO2 catalyst with TPPTS-Cs3 ligand

71

Figure 3.6 Pore distribution of 0,2Rh-L/Rh=10, IL=5%-TiO2 (a) and

0,2Rh-L/Rh, IL=10%-TiO2 (b) catalysts with TPPTS-Cs3 ligand

71

Figure 3.7 Isotherm and pore distribution of 0,2Rh-L/Rh=10,

IL=2.5%-TiO2 catalyst with TPPTS-Na3 ligand

72

Figure 3.8 Isotherm and pore distribution of SILP catalysts (a)

0,2Rh-L/Rh=10, IL=5%-TiO2 and (b) 0,2Rh-L/Rh, IL=10%-TiO2 with ligand

TPPTS-Na3

72

Figure 3.9 XRD patterns of reduced samples (by hydrogen at 250oC for 4

h) (a) Differently loaded Au/SiO2 and (b) differently loaded Cs doping on

Figure 3.11 Au4f XPS spectra of fresh and used catalysts Samples with

different Au loading content over SiO2 (a) and samples with Cs doping over

1AuSiO2 (b)

77

Figure 3.13 EPR spectra of fresh SILP-TiO2 with 5%wt (a) and 2.5% (b)

ionic liquid loading contents

79

Figure 3.14 EPR spectra of 0,5Au_TiO2 samples recorded at room

temperature

79

Figure 3.15 TEM images and the respective gold particle size distribution

of fresh (a) 1Au/SiO2 and (b) 2Cs1Au/SiO2

Figure 3.18 Comparison of catalytic activity of SILP catalysts with

different IL loading using ligand TPPTS-Na3 (a) and that of using ligand

TPPTS-Cs3 (b)

83

Figure 3.19 Catalytic activity of SILP catalysts on temperature using

ligand TPPTS-Na3 (a) and that of using ligand TPPTS-Cs3 (b)

85

Figure 3.20 EPR spectra of 0,2Rh-L/Rh=10-IL=2,5%-TiO2 catalysts

before (a) and after deactivation (b)

86

Figure 3.21 Catalytic acitivities of 0,5Au_TiO2 samples at different

temperatures

87

Figure 3.22 Activities over 1AuSiO2 sample in Single-reactor mode at

2MPa, contact time of 50 g.min.L-1, nominal feedstock of CO2/H2/C2H4/N2

= 1/1/1/1

90

Figure 3.23 Activities over 1AuSiO2 sample in dual-reactor mode at 2MPa,

contact time of 50 g.min.L-1, temperature of the first reactor was kept

constantly of 650oC, nominal feedstock of CO2/H2/C2H4/N2 = 1/1/1/1

91

Figure 3.24 Activities over 2AuSiO2 sample in single-reactor mode (a) and

dual-reactor mode (b) at 2MPa, contact time of 50 g.min.L-1, temperature of

the first reactor was kept constantly of 650oC, nominal feedstock of

CO2/H2/C2H4/N2 = 1/1/1/1

91

Figure 3.25 Effect of CO/H2 to selectivity of oxo products and yield of oxo

products over 1AuSiO2 (a)and 2AuSiO2 (b) samples in dual-reactor mode at

2MPa, contact time of 50 g.min.L-1 , nominal feedstock of CO2/H2/C2H4/N2

= 1/1/1/1

93

Figure 3.26 Effect of temperature in the second reactor to selectivity of oxo

products (S oxo), conversion of ethylene (X(C2H4)) and yield of oxo

products (Y oxo) over 1AuSiO2 and 2AuSiO2 samples in dual-reactor mode

at 2MPa, contact time of 50 g.min.L-1, temperature of the first reactor was

kept constantly of 650oC, nominal feedstock of CO2/H2/C2H4/N2 = 1/1/1/1

93

Figure 3.27 Conversion of ethylene (a) and selectivity of oxo products (b)

depend on Au loading over dual-reactor mode reaction at 2MPa, contact

time of 50 g.min.L-1, temperature of the first reactor was kept constantly of

650oC, nominal feedstock of CO2/H2/C2H4/N2 = 1/1/1/1

94

Figure 3.28 Effect of Au particle size on Conversion of C2H4 and

Selectivity to oxo products (a) and Selectivity to ethane and Yield of oxo

products (b) over 1AuSiO2 at 175oC Reaction conditions at 2MPa, contact

time of 50 g.min.L-1, temperature of the first reactor was kept constantly of

650oC, nominal feedstock of CO2/H2/C2H4/N2 = 1/1/1/1

95

Figure 3.29 The effect of temperature in the downstream-located reactor on

ethylene conversion [X(C2H4)], oxo-selectivity [S(oxo)], and oxo-yield

[Y(oxo)] over 1AuSiO2 (a) and 2AuSiO2 (b) Contact time in the

downstream-located reactor: 50 g min L−1, temperature of the

upstream-located reactor used for the RWGS reaction: 650°C, total pressure: 20 bar,

and nominal feed composition: CO2/H2/C2H4/N2 = 1/1/1/1

96

Figure 3.30 Catalytic activities versus Cs loading over 1AuSiO2 in

dual-reactor mode reaction at 2MPa, contact time of 50 g.min.L-1, temperature of

the first reactor was kept constantly of 650oC, nominal feedstock of

CO2/H2/C2H4/N2 = 1/1/1/1

97

Figure 3.31 Effect of temperature and CO/H2 ratio to product distribution

over 1AuSiO2 dual-reactor mode reaction at 2MPa, contact time of 50

g.min.L-1, temperature of the first reactor was kept constantly of 650oC,

nominal feedstock of CO2/H2/C2H4/N2 = 1/1/1/1

98

Figure 3.32 Effect of temperature and CO/H2 ratio to product distribution

over 2AuSiO2 dual-reactor mode reaction at 2MPa, contact time of 50

g.min.L-1 , temperature of the first reactor was kept constantly of 650oC,

nominal feedstock of CO2/H2/C2H4/N2 = 1/1/1/1

99

Figure 3.33 Effect of pressure on the conversion of C2H4 and the

Selectivity to oxo products over 1AuSiO2 dual-reactor mode reaction at

175oC, contact time of in the downstream-located reactor of 50 g.min.L-1,

temperature of the first reactor was kept constantly of 650oC, total pressure

changing from 1 to 20 bar, and nominal feedstock of CO2/H2/C2H4/N2 =

1/1/1/1

100

Figure 3.34 Selectivity and Yield of oxo products (a) and Conversion of

ethylene (b) over different supports with 1wt% of Au loading content

Reaction conditions: 2MPa, 175oC, contact time of 50 g.min.L-1 , nominal

feedstock CO2/H2/C2H4/N2 = 1/1/1/1

101

Figure 3.35 S(oxo) and X (C2H4) of 1AuSiO2 and SILP/TiO2 catalysts in

dual-reactor mode at 2MPa, contact time of 50 g.min.L-1 , temperature of the

first reactor was kept constantly of 650oC, nominal feedstock of

CO2/H2/C2H4/N2 = 1/1/1/1

102

Figure 3.36 S(oxo) and X(C2H4) of different dopping 2% of transition

metals on based 1AuSiO2 catalysts in dual-reactor mode at 2MPa, contact

time of 50 g.min.L-1 , temperature of the first reactor was kept constantly of

650oC, nominal feedstock of CO2/H2/C2H4/N2 = 1/1/1/1

103

Figure 3.37 S(oxo) and Y(oxo) of different dopping transition metals (Co,

Sr, Ce) contents on based 1AuSiO2 catalysts in dual-reactor mode at 2MPa,

contact time of 50 g.min.L-1 , temperature of the first reactor was kept

constantly of 650oC, nominal feedstock of CO2/H2/C2H4/N2 = 1/1/1/1

104

INTRODUCTION

Nowadays, fossil carbon-containing raw materials are still the major source of energy and the main feedstock for a wide range of commodity chemicals However, both energy generation and chemical production are typically accompanied by the formation of carbon dioxide (CO2), which is the number one greenhouse gas The scientific community generally agrees that the continuously increasing concentration of this gas is the main reason for an increase in the average temperature, and thus CO2 emissions contribute to global warming [1,2] Therefore, reducing CO2 release into the atmosphere is of enormous environmental interest

In this regard, there are two approaches dealed with reducing CO2 in the atmosphere: the first one is to develop the green chemistry with low carbon consumption and the second one is to use CO2 as an alternative carbon source for the future By means of catalysis, a value-adding transformation of CO2 into chemicals can be achieved, but because CO2 is thermodynamically very stable (∆is

= –393 kJ/mol), highly energetic molecules are essential to activate the C-O bonds The reduction of carbon dioxide with hydrogen to form carbon monoxide is called Reverse Water-Gas Shift reaction (RWGS) The obtained carbon monoxide can then be used in various other established reactions, which are mainly based on the

so called synthesis gas mixture of CO and H2 [3] For example, higher hydrocarbons and alcohols can be produced via the Hydroformylation process over rhodium or cobalt based catalysts [4] Moreover, technologies for CH3OH and CH4 synthesis are being now commercially available [5, 6] RWGS is expected to be a process that requires a large amount of energy to produce CO, while other processes are more energy-efficient Therefore, if building an RWGS process alone would not be economically beneficial, it would need to be combined with another chemical production process to produce a product of higher value, that compensating for the overall energy cost as well as reducing the cost of toxic CO storage, thus improving production efficiency Hydroformylation could be a candidate one since CO from RWGS process is used directly for the reaction without further supply A novel approach to immobilizing homogeneous catalysts on solid supports (called Supported Ionic Liquid Phase – SILP catalyst) has been emerged recently [7], in which the hydroformylation complex catalyst was distributed in an ionic liquid medium contained in pore system of a solid support such as SiO2, ZrO2, zeolites, MCM-41 etc [8, 9, 10, 11] It revealed excellent stability, reusability and even improved activity of hydroformylation catalyst Applying these novel catalysts, the classical homogeneous hydroformylation becomes heterogeneous with solid catalysts in fixed bed reactors They are currently far from industrial relevance, but academia strongly pushes these catalysts forward so that these technologies can become economically feasible in the future

Therefore, the objective that covers the whole of this research is to develop novel heterogeneous catalysts for the hydroformylation of ethylene with CO or CO2 There are two research approaches to this thesis:

(1) The first aspect focused on the preparation of Supported Ionic Liquid Phase (SILP) catalysts and supported catalysts possessing Au nanoparticles Supports like SiO2 and TiO2 will be applied for catalyst preparation Alkaline metal compounds could be used as dopants for the Au-containing catalysts in order to affect both catalyst ability for CO2 activation and ethylene hydroformylation with these both properties being important for catalyst activity and desired selectivity Afterward, all prepared catalysts had been tested in both reactions: hydroformylation of C2H4

with CO and H2 to propanal; and direct conversion of CO2 with C2H4 and H2 to propanol In the first stage, prepared SILP catalysts and Au-based catalysts were tested in a hydroformylation home-made reactor in Hanoi University of Science and Technology (HUST), Hanoi, Vietnam

(2) For the second work, the research focused on the preparation and applying SILP and Au/SiO2-based catalysts in a “dual-reactor” system designed by Leibniz Institute for Catalysis e.V (LIKAT) for direct conversion of CO2, H2 and C2H4 to propanol, propanal at Rostock, Germany We develop the new “dual – reactor” catalytic test system for improving the selectivity To this end, a first reactor is used

to convert CO2 to CO through the reverse water gas shift (RWGS) reaction at a high temperature, while the hydroformylation of ethylene takes place in a second reactor

at a lower temperature Catalytic tests on hydroformylation of ethylene with CO and

CO2 had been performed at laboratories with the purpose of elucidating reaction mechanism and to establish relationships between catalyst performance and physico-chemical properties For the latter works, catalysts had been investigated by different but complementary characterization methods like XRD, BET, FT-IR, NMR, XPS, EPR, ICP-OES, TEM… Most of the work has been done in LIKAT, Rostock, Germany

The following specific questions shall be answered:

 What kind of catalyst, i.e gold supported catalysts or SILP catalysts, is more effective for hydroformylation of ethylene?

 How can catalyst deactivation through the leaching of active metal sites from the ionic liquid media be prevented?

 What is the optimal Au loading for an efficient synthesis of oxygenates as well as to suppress activity for ethylene hydrogenation to ethane? What’s the role of

Au active sites in the hydroformylation of ethylene?

 What is the role of surface acidic properties and pore structure to catalytic performance? The different TiO2 and SiO2 supports could impact SILP catalytic activities?

 What kind of preferred methods to prepare Au nanoparticles?

 Is it possible to replace CO with CO2 in the hydroformylation of ethylene and to produce desired products with high activity and selectivity?

The thesis contains four chapters The first chapter summarizes the literature review about the hydroformylation process, synthesis, the structure, the catalytic properties of SILP catalysts and Au-based catalysts The second chapter introduces the basic principles of the physico-chemical methods used in the thesis, catalyst synthesis, and catalytic activity measurement The most important chapter (chapter 3) focused on the catalytic activity of hydroformylation of ethylene with CO and H2

using synthesized SILP/TiO2 catalysts and Au/TiO2 catalysts In particular, the novel Au/SiO2-based catalysts were investigated in direct conversion of CO2 with

H2 and C2H4 into propanol The outstanding results of catalytic activities have been the first time discussed in this chapter Furthermore, the new idea of the present study elucidating the potential of a dual-reactor approach for improving the yield of propanol plus propanal and the corresponding selectivity on ethylene basis also was presented The last chapter (chapter 4) summarizes the general conclusions of the thesis as well as some prospective results contributed by the research study

CHAPTER 1

LITERATURE REVIEW

Our current economy strongly depends on fossil based products, not only for fuels, but also for the chemical and goods industry Declining resources but simultaneously increasing energy and commodity demands are characteristic of the last and also coming decades (Figure 1.1) [12] However, technical options for mining as well as the natural reproduction of these fossil resources, like natural gas, crude oil, and coal are limited Furthermore, burning fossil fuel emitted massive amounts of CO2 in the atmosphere which is the main reason contributing to global warming, politicians started to take action against climate change; Therefore the European Commission has set up an Energy Roadmap 2050 [13], in which they urged to transform the current economy into a “low carbon economy” and reduce

“greenhouse emission to 80-95% below 1990 levels”

Figure 1.1 Total world energy consumption by energy source, 1990-2040 (reproduced from [12])

This can be achieved by either capture/storage or utilization of this gas as a reactant for producing fuels or/and useful chemicals in an environmentally friendly manner [14] This is particularly valid when CO2 is converted into long-time stable and usable products like polymers, dyes, or resins, which are currently produced from ethylene and propylene The latter building blocks are currently generated from fossil carbon-containing feedstock through strongly endothermic cracking reactions resulting in large CO2 emissions [15] As CO2 conversion into value-added alcohols and hydrocarbons requires H2, the production of the latter should also meet the requirements for environmental compatibility Water splitting powered through solar and/or wind energy is a suitable option [16] Propanal is an important chemical produced on a large scale over homogenous Co- or Rh-containing catalysts through hydroformylation of C2H4 using H2 and CO as

additional reactants This aldehyde can be further hydrogenated to propanol, which

is one of the basic chemicals used in daily life [17].Various heterogenous catalysts were also applied for gas-phase hydroformylation of ethylene to propanal [17,18] However, all hydroformylation processes suffer from the usage of toxic CO at high pressure as well as decreased activity with increasing time on stream because of the leaching of catalytically active species [19] These drawbacks can be overcome with the idea of using CO2 instead of CO and/or applying alternative catalysts In this regard, the group of Kondratenko developed supported Au-based catalysts for the direct conversion of CO2 with H2 and C2H4 into propanol with near to 100% selectivity to this alcohol with respect to CO2 [20] This could be a very prospective utilization of CO2 for a more sustainable liquid hydrocarbon manufacturing in future This chapter, hydroformylation of ethylene with using of CO or CO2 will discuss in detail The catalysts for the hydroformylation process focused on the two kinds of novel Supported Ionic Liquid Phase – SILP catalysts and gold-based catalysts

1.1 Hydroformylation in green chemisty

The development of green catalyst routes to synthesize commercially important chemicals is an environmentally and economically beneficial effort Green chemistry involves designing, developing and implementing chemical products and processes to reduce or eliminate the use and production of hazardous substances to human health and the environment It's an innovative, non-regulatory, economically-oriented approach to sustainability Green technology is getting considerable attention as awareness of environmental issues has increased The concept of environmentally friendly product and process design is expressed in the

12 “Green Chemical Principles” as follows [21]

1 Waste prevention instead of remediation

2 Atom efficiency

3 Use of less hazardous/toxic chemicals

4 Design safer chemicals and products

5 Use innocuous solvents and reaction conditions

6 Design energy efficient processes

7 Preferably renewable raw materials

8 Shorter synthesis route and avoid derivatization

9 Use catalyst instead of stoichiometric reagents

10 Design products for degradation after use

11 Real time analytical methodologies for pollution prevention

12 Inherently safer processes to minimize the potentials for accidents

Catalysis plays an important role in the production of diverse products, with applications in medicines, plastics, agricultural chemicals, perfumes, detergents, food, clothing, fuels, etc [22] In addition, it plays an important role in the ecological and environmental balance by providing cleaner alternatives to stoichiometric technologies The green catalyst process efficiently uses all raw

material atoms, removes waste and avoids the use of toxic and/or hazardous reagents and solvents in the manufacture and application of chemical products

Hydroformylation is an important commercial process for converting alkenes, carbon monoxide and hydrogen into aldehyde for further use in the production of various chemicals Industrial processes operate in a homogeneous medium Thus, the development of a solid catalyst will solve the problems associated with catalyst separation and thus contribute to the reduction of waste from these chemical processes

In this regard, the creation of new hydroformylation catalyst production is concerned as green chemistry

Hydroformylation is one of the oldest and largest homogeneously catalyzed reactions of olefins The reaction was first discovered in 1938 by Roelen [23] while working for Ruhrchemie in Germany Roelen investigated the effect of added olefins to cobalt catalysts and identified aldehydes as one of the oxygen containing components

H2 and CO can add across the double bond of olefins to form aldehydes in the presence

of a Co (or Rh) catalyst

The process is frequently referred to as the “Oxo” process, with Oxo being short for Oxonation, i.e the addition of oxygen to a molecule However, the term hydroformylation is descriptively more accurate and more useful in characterizing this type of reaction catalyzed by various transition metal complexes because during the reaction a hydrogen atom and a formyl group are added to the olefinic double bond

“normal” “branched”

Figure 1.2 Depiction of hydroformylation of olefins [21]

The relative amounts of normal- and branched-chain aldehydes produced depend on the identity of R and other constituents of the reaction mixture Normal-chain aldehydes, the more desirable products, usually are hydrogenated, affording straight-chain alcohols, or self-condensed, affording more complex aldehydes

Figure 1.3 Products derived from aldehydes manufactured by hydroformylation [24]

With a terminal alkene as substrate, the normal/branched ratio is an important parameter in the industrial hydroformylation process; generally rule, the better catalytic performance, the higher the ratio, although significant markets have developed for the branched aldehydes In addition to linear terminal olefins, a wide variety of different olefins have been successfully hydroformylated, e.g linear internal olefins, unsaturated alcohols, phenols, ethers, and amides [24]

1.1.1 Commercial demand of hydroformylation products

The oxo process or hydroformylation of olefins with synthesis gas is the principal route to C3-C15 aldehydes, which are converted to alcohols, acids, or other derivatives By far the most important oxo chemical is n-butyraldehyde, followed

by C6-C13 aldehydes for plasticizer alcohols, isobutyraldehyde, propionaldehyde, valeraldehyde, and C12-C18 aldehydes for detergent alcohols Propylene-derived n-butyraldehyde and isobutyraldehyde accounted for approximately 77% of the world consumption of oxo chemicals in 2021

Figure 1.4 World consumption of oxo chemicals in 2021 (Source: IHS Market) [25]

High consumption volumes for both of the alcohol derivatives of butyraldehyde, n-butanol and 2-ethylhexanol (2-EH) will continue in the near future, largely owing to increased consumption of both alcohols in acrylate esters, acetate esters, and plasticizers 2-EH and n-butanol continue to account for the majority of plasticizer alcohols consumption, combining for three-quarters of the global total The pie chart (Figure 1.4) shows world consumption of oxo chemicals: Asia, Europe, and North America are the largest markets for oxo chemicals, together accounting for 95% of world demand in 2021 Oxo chemicals demand in mainland China is expected to grow relatively well, albeit at a slower growth rate than in previous years Other Asian consumption, excluding mainland China and Japan, is also expected to grow well; India and Malaysia are the main growth markets in this region Demand for oxo chemicals in the United States is expected

n-to grow modestly during 2021–2026 Western European consumption of oxo chemicals is forecast to also grow modestly, as will Japanese consumption

By far, most oxo aldehydes are hydrogenated to alcohols The large majority of the world consumption of n-butyraldehyde is converted to 2-EH and n-butanol, while all of the detergent and C7-C13 plasticizer oxo aldehydes are converted to their corresponding alcohols Other oxo chemicals, including propionaldehyde, valeraldehyde, and isobutyraldehyde, have more varied applications As a result, demand for oxo chemicals is strongly dependent on demand for C4-C13 plasticizer alcohols Consumption of plasticizer alcohols, especially C7-C13 alcohols, depends greatly on demand for plasticizers and flexible PVC Growth in the world consumption of plasticizer alcohols for plasticizers is forecast at 3.2% annually during the next few years Solvent/coating applications are the largest end use for

C4-C5 alcohols; this includes direct solvent use and derivative solvent use, mainly as acetates, glycol ethers, and acrylates

The Oxo Alcohol Market Size was valued at USD 14.3 Billion in 2022 [26] The Oxo Alcohol market industry is projected to grow from USD 14.98 Billion in 2023

to USD 20.1 Billion by 2030, exhibiting a compound annual growth rate (CAGR) of 5.2% during the forecast period (2023 - 2030) The market for oxo-alcohols was chiefly driven by escalating demand from the plasticizers as well as the solvents industry Increasing demand for plasticizers from their end-user industries has been the key factor for driving the Oxo Alcohols Market during the forecast period Nearly 10 million tons per year of oxo products having carbon chain lengths of

C3-C17 are produced by hydroformylation and this amount is expected to rise nearby 5% during 2021 About 7.5 million tons, or nearly ¾ of the annual production, are made from propene The share of ethene and longer olefin corresponds to about 2 % and 25 %, respectively [25, 26] This has therefore established butanal production as being far the most important industrial hydroformylation process n–Butanol is a versatile intermediate for chemical industry It reacts with acids to yield esters and with oxides to yield glycol ethers n–Butanol is an intermediate chemical for the synthesis of esters like butyl acetate, butyl acrylate, butyl methacrylate, etc and these esters are used as solvents for coating Other applications of n–butanol are solvent, cleaning fluids, herbicides, dyes, printing inks, personal care products, pharmaceuticals, plasticizers, textiles and lube additives The global consumption of the butanol is shown in Figure 1.5

Figure 1.5 Global consumption of n–butanol and iso–butanol (wt %) [26]

1.1.2 Economic Aspects of Hydroformylation Reaction

The fast growing market for the oxo products is due to continuous growth of the hydroformylation processes and its advanced leaps in catalyst’s generations Figure 1.6 shows the growth in the production of oxo products around the world

Figure 1.6 Oxo Chemicals Market Size 2023-2030 (Growth and forecast) [26]

The oxo chemicals market is expected to be a moderately growing market The factors contributing to this growth include rising demands in automotive production, original equipment manufacturer and the construction sector, growing applications and demand for plasticizers enhances the demand for oxo chemicals and shifts the dynamics of the market among different product types The oxo chemicals market is highly dependent on the global economic conditions Reviving economies of Europe and North America and expanding economies of Asia Pacific are expected

to aid the market growth However, there are certain restraining factors to the growth of the market which include rising environmental concerns and stringent environmental regulations proposed for the protection of the environment [25, 26] 1.1.3 Catalysts for hydroformylation reaction

The catalysts for hydroformylation, typically, consist of a transitional metal atom (M), especially from the platinum metal group These convertible metal complexes interact with carbon monoxide and hydrogen to form the metal carbonyl hydride species, which is an active catalyst Generally, complexes containing carbonyl ligands are called unmodified catalysts However, the introduction of tailor–made ligand into center metals is called the modified catalysts

Three developmental stages for hydroformylation catalysts have been applied in industry [23, 24, 27] The first stage of hydroformylation was exclusively based on cobalt (Co) containing catalyst The catalytic active species for hydroformylation reaction was the cobalt carbonyl hydrides (HCo(CO)4) in the pressure range of 240–

300 bar at 150–200°C temperature Separation of products from the reaction mixture, severe reaction conditions and low activities of catalysts were the main

limitations of this stage’s processes The research efforts led phosphine replacing carbonyl complexes as an electron donating ligand and this emerged as a fundamental step in metal carbonyl catalyzed reaction, which gives scientists the ability to customize—making catalysts by modifying the electronic and steric properties of the ligand (HCo(CO)3PBu3 and HRh(CO)(PR3)3)

The second phase of the hydroformylation reaction was the combined development in modifying ligands and replacing cobalt with rhodium (Rh) metal It took a decade of research before the first commercial process based on rhodium catalyst was introduced in 1974, and this process was known as the Low-Pressure Oxo process (LPO) Compared to cobalt based processes, many advances were made in the second developmental stage of hydroformylation, especially with respect to material and energy utilization Thus, second stage of hydroformylation was concluded with development of more effective Rh–phosphine catalyst However, the industrial problems of first stage such as, separation of products from reaction mixture, catalyst recovery, loss of costly metals, use of corrosive solvents, etc continued in the second stage too

4 = Union Carbide process (LPO);

5= Ruhrchemie–Rhone–Poulene process; LHSV = Liquid hourly space velocity

The third stage can be called a breakthrough in hydroformylation process or phase catalysis (biphasic or liquid multi-phase systems hydroformylation), because

two-of finding a way two-of separating the catalyst and the reaction products under mild conditions that is ecologically as well as economically efficient The basic idea is to apply a water-soluble catalyst by modifying the ligand and thus transferring the hydroformylation process to the water phase With the help of such catalysts, separations of desired products (which dissolves in organic phase) from catalysts have become an easy task The idea of applying water–soluble Rh–complex as a catalyst for the hydroformylation of propylene and 1–butene was taken up and commercialized by Ruhrchemie AG [11, 23] The first plant was commissioned in

1984, just two years after its laboratory-scale development, followed by rapid further increases in capacity to more than 3.106 tons/year An additional unit to produce n–pentanal (n–valeraldehyde) from 1–butene has been brought on stream

in 1995 The developments of hydroformylation processes in different stages are shown in Table 1.1 and the catalysts used are presented in Figure 1.7

Figure 1.7 Three stages of the catalyst development for the hydroformylation reaction

[23]

Presently, most of the industrial plants are running successfully with rhodium and cobalt based catalysts Attempts had been made to compare the catalytic activity of group VIII and IX metals for hydroformylation of alkenes to understand the role of metal atom in hydroformylation reaction [28, 29] Ruthenium is attracting the attention of the researchers after rhodium and cobalt; Nevertheless, it is yet to move from laboratory to pilot plant scale The ligand plays a significant role in the hydroformylation reaction from the catalytic activity, selectivity and regio–selectivity point of view Phosphines and phosphite based monodentate and bidentate ligands are most commonly used and accepted ligands for the hydroformylation reaction [28, 30] Nitrogen containing ligands showed lower reaction rates than phosphines due to their stronger coordination to the metal centers A comparative study of Ph3R (where R= elements of main group V) were made for the hydroformylation of 1–dodecane and showed following order: Ph3P >

Ph3N > Ph3As > Ph3Sb > Ph3Bi In another study, the activity of the triphenylphosphine, triphenylarsine and triphenylantimony ligands were compared for hydroformylation of ethylene and 1–hexene using transition metal catalysts [30, 31] Today, most of work in the homogeneous catalysis for hydroformylation is focused on the developments of the bulky phosphorous/phosphite ligands, which include both monodentate, and more bulky bidentate ligands

1.1.4 Recent trends in the heterogenization of catalyst in hydroformylation reaction

Commercial processes are using triphenylphosphine modified Rh–complex [HRhCO(PPh3)3] as a catalyst for the hydroformylation of lower carbon chain length alkenes (C2 – C5) under milder reaction conditions (temperature of 60oC to

120oC and syngas pressures of 10 to 50 bar) [23] This catalyst is limited upto the hydroformylation of lower carbon chain alkenes such as, ethylene and propylene due to separation problems of Rh–complex from the product mixture after completion of the reaction Conventionally, homogeneous catalyst is separated from the product mixture by stripping the products in vacuum (vacuum distillation) The thermal stress caused by the vacuum distillation process decomposes the expensive metal complex which is used as a catalyst for hydroformylation reaction Most homogenous hydroformylation catalysts are thermally sensitive and decompose below 150°C This is the main reason, which limits the applicability of Rh–complex for hydroformylation of lower carbon chain length alkenes because in case of higher carbon chain length alkenes, decomposition of rhodium complex occurred during the separation of catalyst from higher boiling point product mixture As far as hydroformylation reaction is concerned, Rh–complexes as catalysts typically work under mild conditions (80–100°C, 20–40 atm), showing good activity & selectivity (95–99%) to the desired linear (n–) aldehyde

For the hydroformylation of higher alkenes, cobalt catalysts are widely used, because it usually requires drastic reaction conditions (200°C, 200–250 atm) and yield poor selectivity for linear aldehyde The cobalt catalyst is recycled after vacuum distillation of the product mixture by the “decobalting” procedure In the decobalting process, regeneration of cobalt catalyst after reaction is carried out by changing the oxidation state of cobalt either by hydrothermal treatment or by oxygen treatment in acidic medium Typically, cobalt is recovered in the form of cobalt formate or acetate by addition of the oxygen and formic or acetic acid [28] Although, cobalt catalyst is recycled for hydroformylation of alkenes, but still has drawbacks of higher temperature, pressure, longer reaction time and lower selectivity of the desired aldehyde as compared to rhodium based catalysts Solving the product separation problem for the rhodium catalyzed hydroformylation in an effective and economically acceptable way, would present a major step forward in homogeneous catalysis Therefore, development of a heterogeneous catalyst for hydroformylation in today’s research scenario can be broadly classified into two categories

+ In the first category, catalyst is anchored or supported on the surface of solid inorganic material, which is used as heterogeneous catalyst either in continuous reactor (fixed bed) or in the high pressure batch reactor (autoclave) This type of process is often referred to as heterogenization of homogeneous catalysts (as illustrated at Figure 1.8) The coordinatively anchored metal complex catalysts, where metal complexes are chemically bonded to the functional groups of the support, are the most promising option for immobilised catalysts The strong bonding of the complex to the support through the functional groups, and the possibility for modification of the support properties at the same time, are the

obvious advantages compared to other types of immobilised metal complex catalysts The most commonly used functional groups are phosphine ligands that are bonded via a methylene chain to an oxide surface or an organic macromolecule The supported metal catalysts are prepared by impregnating metal salts and oxides on the support followed by reduction, or by decomposition of organometallic compounds on the support For instance, active carbon, silica, alumina or zeolites can be used as supports onto which e.g the metal nitrates are impregnated Both rhodium and cobalt (in inorganic form), separately and together, in combination with other metals on various supports have been studied in the hydroformylation of ethene [33, 34] and higher alkenes [35, 36]

Figure 1.8 Coordinative anchoring of a metal complex to the support surface, A is a

mononuclear complex, B is a polynuclear complex, L is a ligand [29]

Figure 1.9 Hydroformylation reaction in biphasic medium [29]

+ Second category is the designing of a water–soluble ligand, which is insoluble

in the product mixture, is often referred to as biphasic systems One of the most important developments in 1980-2000 in the area of homogeneous catalysis is the successful development of water-stable as well as highly water-soluble catalyst systems and the consecutive introduction of the aqueous two-phase technology The reaction in biphasic system involves aqueous and organic phases (Figure 1.9).With the hydroformylation reaction in biphasic medium; reaction takes place at the interphase Catalyst is separated from the reaction mixture using phase separator There are some reports presented in the literature on the development of cobalt and ruthenium biphasic systems for hydroformylation of alkenes Application of biphasic catalyst is limited to hydroformylation of propene and butene due to lower solubility of higher carbon chain length alkenes in aqueous medium (water)

Main problem in the heterogenization of homogeneous complex is the breaking

of bonds between metal and ligand during the course of catalytic reaction and this is the cause of leaching of the active metal species responsible for the reaction This

“leaching” process leads to the loss of catalytic activity in the reusability experiments The leaching problems can be solved upto certain extent by anchoring the homogeneous complex using some tethering agent or encapsulation inside the pores of the solid support used

Homogeneous catalysts are highly active and selective, but they have a number

of disadvantages: difficulty in separating the catalyst from the reaction products, expensive metal losses, limited solubility and corrosion of catalyst solutions For rhodium catalysts, economic activity requires recovery at ppb level due to the high cost of rhodium Therefore, many efforts have been made to heterogenise homogeneous catalysts on a solid support in order to deal with over drawbacks The heterogenised catalysts can be divided into two groups: immobilised metal complex catalysts and supported metal catalysts [32] The immobilised metal complex catalysts can be further divided into supported and anchored metal complex catalysts, as summarised in Table 1.2 as below

Table 1.2 Classification of immobilised metal complex catalysts [32]

1.Supported metal complex catalysts

Catalysts containing a dispersed phase of complex on a support

Supported Liquid Phase Catalysts (SLPC)

Supported Aquesous Phase Catalysts (SAPC)

2 Anchored Metal complex catalysts (Chemical bonding)

Even though the heterogenisation of the homogeneous precursors often results in

a decrease in activity, it has also resulted in improved performance

In liquid-phase applications, leaching of the active metal into the liquid phase has prevented the commercial use of heterogenised catalysts In gas-phase

hydroformylation, the use of supported metal catalysts is more feasible, since the operating conditions are mild: the reaction can be carried out at low pressures (and below 150ºC) where the competing Fischer-Tropsch reaction ceases

With attempts to find new catalysts from scientists, recently the supported ionic liquid phase catalyst (SILP) has raised as a new concept [18, 37, 38, 39], in these SILP systems, a thin film of ionic liquid containing the homogeneous catalyst is immobilised on the surface of a high-area, porous support material, as depicted in Figure 1.10, SILP catalysts appear as solids, the active species dissolved in the liquid phase on the support, maintaining the attractive properties of ionic liquid homogeneous catalysts such as good dispersion of molecular reactant, and high activity It can be understood that the SILP concept combines the advantages of catalytic homogeneous process, and heterogeneous process technology SILP hydroformylation catalysis is an alternative way of performing immobilized hydroformylation catalysis

Figure 1.10 Illustration of supported ionic liquid phase catalyst [37]

Applicability of the Supported Ionic Liquid Phase (SILP) catalysts, prepared by impregnation of the partly dehydroxylated silica support with an anhydrous MeOH solution of ionic liquid [BMIM][n–C8H17OSO3] containing catalyst precursor [Rh(acac)(CO)2] and bisphosphine ligand, was used as a effective catalyst for hydroformylation of propylene in a fixed bed reactor [7, 37] Review articles for the applicability of supported ionic liquid as catalysts for hydroformylation of propylene, ethylene have recently appeared in the literature [38, 40]

Although SILP catalysts are attractive for applications in industry, they still suffer from some drawbacks like losing catalyst activity with time on stream due to leaching of active metal components As a matter of fact, rhodium complex cannot

be stable in long-term since the covalent bonds between ligands and active metal sites are broken by high temperature and/ or high pressure It raises a question whether it is possible to increase the long-term catalyst stability of SILP-based catalysts or to develop alternative-type heterogeneous catalysts suitable hydroformylation of ethylene

A remarkable recent discovery is the hydroformylation of ethylene reactions over metals (M) supported catalyst [41] Several approaches were tested for hydroformylation of small olefins, for example the immobilisation of rhodium carbonyl complexes on silica [42], platinum group metal phosphides on silica [43]

or silica-supported ruthenium catalyst [44] More easy to prepare are typical heterogeneous catalysts like Rh/Al2O3 [41, 45], Rh/SiO2 [46-47] or Rh/zeolite [48]

A severe problem of most of these catalysts is the domination of olefin hydrogenation over hydroformylation, especially for rhodium based ones The mechanism for this reaction was already described by Anna M Trzeciak in 1999 [49] It was found that the size of metal particles has a drastic influence on the chemoselectivity Thus edges and corners of small crystallites promote hydroformylation [50] For example, a particle size of 5 nm was optimum for Rh/SiO2 [47] Gold catalyzed hydroformylation has been only rarely explored Liu

et al [51] reported the liquid phase transformation of higher olefins over a cobalt oxide supported gold catalyst (Au/Co3O4) At temperatures of 100–140°C and pressures of 3–5 MPa, selectivities of 85% to the desired aldehydes were reached Internal insertion of CO, leading to branched products, was proved to be much slower than the terminal insertion, obtaining the linear aldehyde The authors claimed that the role of gold was just to activate the hydrogen, which spills over and reduces the cobalt in the support

Thus, if the hydroformyltion of ethylene reaction can be done by using SILP catalyst and/or gold supported catalysts (Au/TiO2) as heterogenous way, it will be a promising way to apply in industry scale The goal of this research was to develop a solid catalyst for the heterogeneous hydroformylation of ethylene Rhodium is the most active transition metal for hydroformylation and it was obvious choice for the catalytic metals in the preparation of the solid catalysts Otherwise, nano gold metal also be a novel catalyst worth investigating TiO2 was chosen as supports, because it

is an inert, stable and almost neutral support material widely applied in catalysis

1.1.5 Mechanism of hydroformylation reaction

In the early 1960s Heck and Breslow [52, 53] formulated the generally accepted hydroformylation cycle for cobalt catalysis that is also valid for unmodified rhodium catalysts The hydroformylation mechanism for phosphine-modified rhodium catalysts follows, with minor modifications, the Heck-Breslow cycle According to Wilkinson [54], two possible pathways are imaginable: the associative and the dissociative mechanisms (as depicted in Figure 1.11 and Figure 1.12) The associative route begins with coordination of the alkene to the dicarbonyl species (Figure 1.12, 1B) The dissociative route involves dissociation of one of the ligands (PPh3 or CO) and is similar to Heck-Breslow cycle After the initial steps, the following steps in the associative and dissociative mechanisms are similar; following alkene coordination, an alkyl species (Figure 1.12, 1D, Figure 1.11, 2D)

is formed Alkyl migration to CO leads to acyl formation (Figure 1.12, 1E, Figure 1.11, 2F) Hydrogen addition produces dihydrido acyl species (Figure 1.12, 1F, Figure 1.11, 2G) Finally, elimination of the aldehyde and addition of CO regenerates the active catalytic species HRh(CO) L

Figure 1.11 Dissociative mechanism for hydroformylation cycle [58]

The associative mechanism involves 20-electron intermediates and is often rejected on the grounds that Rh should form 16 or 18 electron complexes It is accepted today that Wilkinson’s dissociative mechanism is the likely kinetic path for hydroformylation [54] In order to avoid referring to two figures depicting mechanism as the subsequent, literature review discussion will refer to Figure 1.12

as it is much simpler to follow

Figure 1.12 Associative mechanism for hydroformylation [54]

The active species are 16-electron hydrides of the general formula HRh(CO)x(PPh3)3-x (x = 1, 2) formed by the dissociation of CO from the 18-electron carbonyl hydride The basic steps in the hydroformylation reaction after the initial formation of the hydrido metal carbonyl are: (1) dissociation of CO to form the unsaturated 16-electron species, (2) coordination of alkene, (3) formation of the alkylmetal carbonyl species, (4) coordination of CO, (5) insertion of CO to form the acylmetal carbonyl, (6) oxidative addition of hydrogen, and (7) cleavage of the acylmetal species by hydrogen to form the aldehyde and regeneration of the hydridometal carbonyl It is generally believed that the oxidative addition of hydrogen to the rhodium-acyl complex (step 6 in Figure 1.13) is the rate determining step Leeuwen [55] has proposed that, roughly speaking, in phosphine catalyst systems the migratory insertion of the alkene into Rh-H (step 3 in Figure 1.13) is the rate-determining step under standard industrial process conditions

Figure 1.13 Mechanism for ethylene hydroformylation, L=PPh3 [54,55]

The reaction mechanism on supported catalysts follows a similar mechanism Henrici- and Olivé [56] have suggested that the decisive difference between the homogeneous and the heterogeneous process is the availability of a free, mobile, very reactive hydrido-metal species in solution According to them, the last step (steps 6 and 7 in Figure 1.13), the transformation of the acyl-metal species to the aldehyde, proceeds through reaction with a second catalyst species in homogeneous media, but in heterogeneous media the oxidative addition of molecular hydrogen to

an acyl-metal species is the only means of formation of the aldehyde The hydrogenation of the acyl intermediate was identified as the rate determining step at 0.1 MPa on Rh/SiO2 [57]

In some studies, the CO insertion selectivity on supported unmodified metal catalysts, is related exclusively to the linearly adsorbed CO on isolated Rh sites [31], whereas other studies show that reaction rate and selectivity for hydroformylation increases in the presence of Rh+ sites Thus, the dispersion of the catalytic metal and the extent of reduction are the main factors determining the CO insertion activity, and thereby, the selectivity towards aldehyde formation According to Sachtler and Ichikawa [58], two types of active sites are responsible for aldehyde formation: isolated, partially oxidised metal crystallites for the migratory CO insertion into metal alkyl bonds, and fairly large metal ensembles for the dissociation of hydrogen Hedrick et al [59] noticed that on a Mn- Rh/SiO2

catalyst, spill-over hydrogen from the metal to the silica surface plays a role in the hydrogenation of the acyl intermediate Thus, the hydrogenation of ethyl species to form ethane and the hydrogenation of adsorbed acyl species to form propanal, are involved with two different types hydrogen: metal adsorbed hydrogen and hydrogen from Si-OH

Besides the main reaction (hydroformylation reaction), the hydrogenation, and aldol condensation are unexpected reactions By-products can be aldehyde isomers, low – reactive alkene isomers, alcohols, alkenes, and heavy ends (Figure 1.14) The formation of heavy ends constitutes the biggest proplem Heavy-ends accumulation can cause serious process problems such as Rh leaching, diluted ionic liquid (in case catalyst is SILP)

For the heterogeneously catalyzed hydroformylation reaction over metals supported catalyst, the mechanism as depicted in figure 1.15 It showed that primarily ethene adsorbs as ethyl species (1, C2H5(ad)) on the active metal Depending on the neighboring adsorbate (2), which can be either hydrogen or carbonyl, the subsequent product would be ethane or an acyl species (3,

C2H5CO(ad)), respectively The acyl species can then be hydrogenated to form propanal or after an additional hydrogenation step propanol [47] Besides, adsorbing ethene can also form an ethylidine species (S), which is described as spectator species only that does not participate in the reaction, but is slowly hydrogenated to ethane [60] Moreover, Williams et al [61] unraveled that adsorbed CO can inhibit the hydrogenation of ethene over a rhodium foil covered with titania overlayers For the reaction studied in this work, that would mean, those catalysts with a high ability to form in situ CO from CO2, hydrogenation could be suppressed Beside the generation of oxo–products, also higher hydrocarbons can be formed during reaction, in analogy to Fischer–Tropsch synthesis By means of isotopically labeled

13CO and unlabeled ethene, Jordan and Bell [62] studied the interaction of those molecules with hydrogen over a Ru/SiO2 catalyst It was observed that ethene can undergo three different reactions with hydrogen: (i) hydrogenolysis to form CH4, (ii) hydrogenation to ethane and (iii) homologation to C4+ hydrocarbons Rising partial pressure of CO enhanced hydroformylation and strongly suppressed hydrogenation of ethene It is worth mentioning that only 10% of the produced

Figure 1.14 The formation of

heavy products (by-product)[55]

Figure 1.15 Mechanism of ethene hydroformylation over metal (M) supported catalysts with proposed

intermediates [41]

methane stems from CO and nearly no labelled carbon can be found in higher hydrocarbons This means that C4–olefins are most probably formed by dimerization of C2H4 [61], while C5–products can be formed by a reaction of CH2

units, deriving from CO hydrogenation or partial hydrogenolysis of C2H4 with a

C4–olefin In case of propene hydroformylation, also regioselectivity is of major importance The ratio of normal (n) to iso-butanol formation is assumed to be dependent on the chosen metal, where Rh and Ni produce more n-alcohol and Pd more iso-alcohol [63] The ratio seems also scalable by adjusting the flow rate of reactants [50] For homogeneous catalysts, the regioselectivity is mainly tuned by adjusting the ligands on the metal center Then partial pressures of CO or hydrogen

do not play a role in the range of 8 to 15 bar [64] Controlling the selectivity by using zeolites as support was not successful, because the reaction was found to proceed only on the outer surface of the zeolite and not in the pores [48]

1.1.6 Relative research studies on SLIP catalysts

Researches on SILP catalysts appeared since 2003 and the number of publication in this area increased dramatically recently Since 20 years after the first application by Riisager et al (2003) [7, 18, 39, 65] an industrial application of the SILP-concept for gas-phase hydroformylation is currently investigated at a technical readiness level of 7 (project MACBETH, 2022 [66]) While over 2000 hours time

on stream could be achieved using a mixed C4-stream as feed (R Franke, H Hahn Evonik Elem., 2015) [67] a small but steady deactivation occurred during the experiment The main reason for this deactivation was found to be the accumulation

of high-boiling products that eventually block the pores of the support (Kaftan et al., 2015 [68]) The C5-aldehyde produced in the hydroformylation of but-1-ene undergoes a consecutive reaction catalyzed by the acidic silanol groups of the silica support to form the high boiling C10 aldol 2-propyl-2-heptenal (Catalyst Separation, Recovery and Recycling, 2006 [69]) in two steps Although the concentration of the silanol groups on the calcined amorphous silica commonly used as support is very low with 25 µmol g−1, a small but steady formation of the aldol resulted in a continuous pore filling (Schörner et al., 2022 [70]) To overcome this problem, Markus et al [71] studied hydroformylation of butene-1 and evaluated the influence of the IL on the formation and the accumulation of 2-propyl-2-heptenal in the part of his work He concluded that the substrate solubility in the IL was found to directly influence the activity A more detailed investigation of SILP-catalysts with the hydrophilic IL [C2C1im][C2OSO3] and the hydrophobic IL [C2C1im][NTf2] showed that the accumulation, activity, apparent activation energy and deactivation is lower for SILP-catalysts compared to the bare support and lowest for the hydrophilic IL [C2C1im][C2OSO3] These observations could be explained by interactions of the IL and the active silanol-groups, the so called

“silanol suppressing effect” The stronger the interaction of the IL and the active centers, the lower the activity and accumulation

Hydroformylation reactions are extremely sensitive to experimental conditions

As discussed detailed in the above sections, various equilibria exist will influence catalytic rates Concentrations of Rh, CO, H2, alkene and added phosphine ligand

are the main factors that control the catalytic activity, paralled to other conditions such as temperature – pressure [37] In addition, the concentrations of gaseous reactants can become controlled by mass transfer limitations and result in increased amounts of hydrogenation and isomerization products Unfortunately, experimental conditions reported in the literature for Rh-phosphine catalysts vary widely and make comparisons difficult

It is clear that ligands having different electronic properties will alter the electronic properties of an organometallic catalyst, therefore altering its activity within a certain process These properties frequently play a role in the kinetic aspects of a chemical transformation

Beside of electronic properties of the ligand, the steric properties of the ligand influences the selectivity of a catalytic process, though there are cases where this parameter also influences the kinetics of a reaction Obviously, it is difficult to separate the steric properties from the electronic paramete, as they are often closely related Significant research has been carried on the Rh-catalysed hydroformylation reaction since the discovery of the use of rhodium catalysts in this process The Rh-catalysed hydroformylation reaction has showed to be quite sensitive towards the stereo – electronic properties of ligands [37]

Phosphine ligands have shown great application in various catalysed reactions including hydroformylation and other carbonylation reactions [38] These ligands are often easy to synthesize TPPTS-Na3 is similar properties of tri-(m-sulfonatophenyl)-phosphine ligand (TPPTS) – the popular ligand that is applied in industrial scale TPPTS-Na3 belongs to the monodentate ligand group and is the most electron-poor ligand

In Vietnam, the support ionic liquid catalyst phase on SiO2 has been studied in Nguyen Ha Hanh’s PhD thesis [8] In her study, she found immobilization of water-soluble TPPTS-Cs3 – Rh and sulfoxantphos –Rh complexes in the [BMIM][n-C8OSO3]

on SiO2 gave a good hydroformylation performance of ethene The catalytic performance of SILP depends on IL loading content, the ratio of ligand to rhodium, rhodium loading contents, as well as the type of ligand She also concluded that the IL loadings of 20-30%V pore were the most active while the IL loading of 10%V pore resulted in the most stable catalyst The samples with the L/Rh ratios from 3-10 were found to have good activity That study also investigated activities of samples with Rh content from 0.2-1.0% and the results revealed as Rh content increase, activity increase but when Rh content reached to 1%, activity did not increase significantly while it will result in high price of the catalysts For long time stability, The SILP catalysts using both of ligands were stable under the reaction conditions at the temperature from 70-

110oC and exhibited high activity However, activities of the catalysts decreased along with the time when high reaction temperatures were applied, especially with the samples with high IL loadings, high Rh loadings and low L/Rh ratios, which were the most active ones The activities of these samples even did not recover after exposed to high temperatures This reason for deactivation was believed to regard to leaching of

Rh complex during the reaction In 2016, Do Van Hung in his PhD study in SILP [9],

he tried to change numerous supports as SiO2, Al2O3, and synthesized ZrO2, MCM-41, SBA-15…etc in order to investigate the effect of support on the activity of SILP catalysts However, only SiO2 was the best support for SILP, and ZrO2 was the worst support The effect of size and shape of pore structure was not clear

1.2 Hydroformylation of ethylene with CO2 as an alternative feedstock

1.2.1 Utilization of carbon dioxides as an alternative carbon source

Using CO2 as renewable reactant for replacement of fossil carbon-containing feedstock have been prospective since catalysis is a key which could help to solve this problem By means of catalysis, a value-adding transformation of CO2 into chemicals can be achieved, but because CO2 is thermodynamically very stable (∆H°

= –393 kJ/mol), high energetic molecules are essential to activate the C-O bonds Unsaturated compounds, ammonia or hydrogen are quite often used for that purpose However, only a few reactions are commercialized at the moment, e.g methane, methanol and formic acid synthesis, formation of cyclic carbonates and some polymers Another interesting approach is the conversion of CO2 into CO, which can be achieved via dry reforming of methane (DRM) (Eq 1.1) [72] or direct hydrogenation

The reduction of carbon dioxide with hydrogen to form carbon monoxide (Eq 1.2) is called reverse water-gas shift reaction (RWGS) The obtained carbon monoxide can then be used in various other established reactions, which are mainly based on the so called synthesis gas mixture of CO and H2 [73] For example, higher hydrocarbons and alcohols can be produced via Fischer-Tropsch process over iron or cobalt based catalysts [74] and the bulk chemical methanol with mixed copper/zinc/aluminum catalysts [75, 76] Both reactions are recognized as opportunities for a more sustainable liquid hydrocarbon manufacturing However, both reactions are currently far from industrial relevance, but academia strongly pushes these systems forward so that these technologies can become economically feasible in the future The most striking argument for RWGS [72] compared to DRM is, that it is energetically more favorable and in principle, but H2 source is more expensive in comparison to CH4 as natural gas However, if it can combine the

CO generated by the RWGS process with another process such as hydroformylation

to form more valuable chemical products, then this process really makes sense when

it can increase economic efficiency, reduce environmental damage by utilizing CO2, and especially reduce the risk of CO toxicity from storage

An important commodity for that purpose is polypropylene, a plastic from which manifold everyday objects can be made of [76] Currently polypropylene (PP), the world’s second most traded polymer after polyethylene, is mainly made from petroleum But alternative production routes are under research Beside

Novozymes/Braskem [77], the dehydration of sustainably produced 1–propanol to propene can also be a key step prior to its polymerization This novel “green” 1–propanol formation has the first time suggested by Dr Kondratenko at el [78] As

(1.1)(1.2)

depicted in Eq 1.3, educts for this reaction are carbon dioxide, hydrogen and ethylene

The innovative direct 1–propanol synthesis from CO2, H2 and C2H4 over alkali promoted gold catalysts was recently developed in the group of E.V Kondratenko [79-81] and exhibited interesting and promising results, which initiated further studies The Kondratenko’s group developed a multifunctional heterogenous catalyst for directly conversion of CO2 with H2 and C2H2 into propanol over gold supported catalysts From a mechanistic viewpoint, this synthesis route is a multiple-step process including the reverse water-gas shift reaction (RWGS) for in situ CO generation, consecutive hydroformylation of C2H4 with resulting CO and

H2 to propanal and finally the hydrogenation of propanal to propanol

RWGS reaction: CO2 + H2 ↔ CO + H2O (∆ H = 39 kJ mol-1) (i) Hydroformylation: CO+C2H4+H2→ C2H5CHO (∆ H = −130 kJ mol-1) (ii)

The advantage of this process is to use CO2 instead of CO which is considered as very toxic gas TiO2 (rutile and anatase phase) as well as SiO2 were tested in the previous experiments as supporting material for gold nanoparticles (NP) and alkali promoters While TiO2 based Au-catalysts show a higher CO2 conversion, SiO2

supported ones are superior in terms of 1–propanol selectivity, which can further be increased by addition of alkali promoters, but the precise role of the alkali dopants and the supporting material was not completely clear

1.2.2 Hydrogenation and reduction processes of carbon dioxides

The direct hydrogenation of CO2 can result in CO (Eq 1.2), CH4 (Eq 1.4), methanol (Eq 1.5) or other hydrocarbons, depending on the conditions, stoichiometry and catalysts used Beside the typical catalytic transformation with classical homogeneous or heterogeneous catalysts, also photo- or electrocatalytic reactions have been studied in the past [80] Photocatalytic processes have the advantage of using light (UV/vis) for driving the reaction forward, and avoiding any other energy source (e.g high temperature), which would decrease the sustainability

of the reaction Unfortunately, the so called “solar fuels”, which are often heterogeneously produced using solid catalysts such as semiconductors like TiO2, ZrO2, β– Ga2O3, or hydroxides of Zn were not obtained in satisfactory rates [82] Moreover, the majority of the studied photocatalysts did not perform well with natural solar light, but required UV irradiation with λ < 400 nm, which make them less attractive, because a separate light source is necessary

(1.3)

(1.4)(1.5)

Heterogeneous electrocatalytic CO2 reduction is also conducted with a big variety of catalytic systems, which can be classified in terms of utilized electrode material, electrolyte, and also the applied potential [83] Depending on the chosen condition, possible products of this so called artificial photosynthesis range from

CO over CH4 to methanol, formaldehyde or other chemicals Catalysts, which contain only gold are not able to stabilize the CO(ad) intermediate strong enough for further hydrogenation and are therefore more selective in CO production, compared for example with copper catalysts Kim et al [84] studied the behavior of bimetallic gold-copper NP with different transition metal ratios They found that, when comparing Au3Cu with AuCu and AuCu3 catalysts, the adsorption of the intermediate CO* is strongest at AuCu3 and thus the main product is methane,

effort was made to develop active materials and study the mechanisms, present reactor setups and electrode production facilities are not capable for upscaling That

is why electrocatalytic CO2 treatment is not yet industrially applied

Conventional heterogeneous catalysts are in short-term perspective the most promising solids for large-scale application However, to activate the thermodynamically very stable CO2 molecule, neither light nor electric current can

be used to deliver the mandatory energy Thus, an appropriate reaction partner is required, who carries the demanded energy Hydrogen is such a high energetic material and is already widely used for reduction reactions with CO2, both in homogeneous [85] and heterogeneous catalysis [72] Additionally, hydrogen has the advantage, that it could be in the future a sustainable reaction partner, if the production routes via water electrolysis or photo reduction become competitive compared to conventional fossil sources (coal gasification, methane steam - or naphtha reforming) [86] However, high pressures and temperatures are quite often inevitable for satisfactory yields and selectivities Thermodynamic calculations [87] for equilibrium composition in CO2 hydrogenation indicate a strong influence of the temperature, as illustrated in Figure 1.16 With a stoichiometric ratio of H2/CO2 = 4 one would expect that CH4 is the main product, but at temperatures higher than 500°C, CO becomes the predominant reaction product Additionally, Gao et al [87] calculated the CO2 conversion, CH4 selectivity as well as CH4 yield with varying pressures and H2/CO2 ratios Huang et al [88] reported the Pt/Ni ratio over SiO2

support was crucial in determining the CH4/CO selectivity, Pt/Ni ratio of 1/48 showed the highest CO2 conversion of 59% and CH4 selectivity of 96% at atmospheric pressure and 400oC When Pt/Ni ratio raised uper 1 then CO selectivity increases sharply, the CO becomes the main product Stefan et al [20] reported the maximal CO2 conversion of 9% over 1Au/TiO2 at 200oC (2MPa, H2/CO2 = 3/1) Generally, a significant enhancement of all values could be achieved at temperatures larger than 400°C For example, CO2 conversion is increased at 500°C from 70% to more than 90%, when the pressure is raised from 1 to 30 atm according to Le Chatelier's principle, since the volume is reduced during the reaction Regarding that, for accelerating the endothermic RWGS reaction, higher temperatures are beneficial

Figure 1.16 Equilibrium distribution at different temperatures with methanation feed

(H2/CO2 = 4) at atmospheric pressure Reproduced from [87]

In contrast, elevated temperatures are unfavorable for many supported catalyst, because the active metal (nano)-particles tend to sinter This is not optimal, since a large number of applications require small NPs (< 10 nm) Moreover, coke formation is also a problem in reactions containing carbonaceous reactants Especially, in reactions, where carbon monoxide is formed, the Boudouard reaction (Eq 1.6) [89] can occur as an undesired side-reaction at a wide temperature range and forms carbon deposits on the surface, which also leads to deactivation of the catalyst

In conclusion, the hydrogenation process of the CO2 in gas phase using H2 as the agent is the complex process with multiple reactions occurring at the same time producing multiple products at once such as CO, CH4, methanol as well as coke In order to be able to adjust the desired products, the use of highly selective catalysts has been critical to optimizing the reaction conditions, approach of high selective desired product (CO) while remaining long-term working time of the catalyst The next section will discuss in more detail catalyst studies for CO2 hydrogenation 1.2.3 Supported catalysts for CO2 hydrogenation

Heterogeneous catalysts for the RWGS reaction are typically based on Cu, Pt or

Rh These metals are most often supported on TiO2, SiO2 CeO2 or Al2O3, but also

on Nb2O5 or ZrO2 [90, 91] Since noble metals like Pt and Rh have a higher ability

to activate dihydrogen compared to Cu, a lower H2/CO2 ratio and lower metal loadings are preferable for high CO selectivity, otherwise methanation is favored In all cases, a high dispersion of the active metal enhances the activity, because this enlarges the metal-support interface, where hydrogen spillover can take place and special intermediates are formed The metal oxide supports also strongly influenced

on the activity and played an important role in the mechanism of the reaction A

(1.6)