University of Groningen Chemo-catalytic synthesis of biobased higher alcohols Xi, Xiaoying

Chemo-catalytic synthesis of biobased higher alcohols

Xi, Xiaoying

DOI:

10.33612/diss.133328930

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Xi, X. (2020). Chemo-catalytic synthesis of biobased higher alcohols. University of Groningen. https://doi.org/10.33612/diss.133328930

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1

General

Introduction

10

1.1. Introduction

For the past two centuries, fossil fuels including petroleum oil, coal and natural gas have been

the main resource for energy, commodity chemicals and materials.

1

_{However, the depletion}

of these non-renewable fossil resources together with a strong drive to limit greenhouse gas

emissions have led to an increasing effort in the development of sustainable and green fuels.

BP’s Energy Outlook 2019

2

_{reveals that the use of renewable energy has grown rapidly in the}

last decade and is expected to grow even faster in the coming decades, and as such its share

in the global power markets will increase substantially (

Figure 1). Here, renewable energy

includes wind, solar, geothermal, biomass, and biofuels.

Figure 1. Consumption and shares of primary energy since 1970. Reproduced from ref. 2.

Copyright 2019 BP.

Higher alcohols (HA), typically defined as alcohols possessing two or more carbon atoms, have

attracted considerable interest owing to their broad range of applications.

3

_{With respect to}

the biofuels sector, a well-known example is bioethanol from sugars using fermentative

approaches,

4

_{which has been commercialized in the last decades. Bioethanol has replaced}

lead

5

_{as an octane booster and it is used as a gasoline additive to raise the octane number in}

the USA, Brazil, PR China and some European countries.

6, 7

_{Adding ethanol to gasoline also}

allows the fuel to combust more completely due to the presence of bound oxygen, which

increases the combustion efficiency and reduces air pollution. In addition to the flexible-fuel

11

1

vehicles that can run on any proportion of gasoline and bioethanol,

neat0 ethanol vehicles

(ethanol only) were also developed by among others Brazilian car manufacturers.

8

However, the use of ethanol as a fuel (additive) has some issues such as its low energy density,

high vapor pressure and high water solubility. The latter causes corrosion in piping when using

ethanol-rich blends.

9-11

_{These disadvantages may be alleviated by using C3+ alcohols, which}

have superior fuel properties, such as higher energy density, lower volatility and better

solubility in hydrocarbons (HC), while at the same time possessing comparable octane

numbers as found for gasoline.

5, 12-14

_{Therefore, HA with a high C3+ alcohol content are highly}

promising biofuels. Additionally, HA are also widely employed as chemical feedstocks,

intermediates and solvents for the synthesis of plastics, surfactants, detergents and

pharmaceuticals.

15-18

Nowadays, HA, especially ethanol and butanol are mainly manufactured through the

fermentation of sugars from starch or sugarcane

19-22

_{and the fermentation of green syngas,}

23-25

_{a gas mixture predominantly composed of CO, H}

₂

_{and some CO}

₂

₍

_{Scheme 1). The high}

energy demand in the workup step (water removal by distillation) restricts the wide

industrialization of such fermentation technology.

22, 26, 27

_{Other synthetic methodology to}

prepare HA involves the hydration of the corresponding alkenes over solid acid catalysts.

28, 29

_{This route is unattractive for large-scale production because of the very low single-pass}

conversion (~ 5%) and the dependence on unsustainable petroleum oil.

Scheme 1. Existing technologies for higher alcohol synthesis and the main focus (red) of this

12

Therefore, in order to meet the increasing demand for HA, new catalytic technology needs to

be developed.

3

_{Among the promising ones, the catalytic synthesis of HA directly from syngas}

has obvious sustainability advantages since unconventional biomass or even CO

2

can be

utilized. Another interesting technology involves the coupling of commercially available

bioethanol from sugar fermentations

6, 7

_{to higher alcohols, especially 1-butanol, using the}

Guerbet reaction.

30

_{The thus formed 1-butanol can subsequently react with ethanol or itself}

to form other higher alcohols, such as 1-hexanol, 2-ethyl-1-butanol and 1-octanol. Catalytic

systems featuring high activity, selectivity and stability are required for future large-scale

processes.

This introduction chapter will illustrate the developments of higher alcohol synthesis (HAS)

from CO hydrogenation, CO

2

hydrogenation and the Guerbet coupling of ethanol at the

molecular level (catalyst and reaction mechanisms). The emphasis will be on catalytic

transformations using heterogeneous catalysts. In each part, the state-of-the-art catalyst

performance will be described, key challenges will be highlighted and prospective directions

for research will be provided.

1.2. Syngas conversions: catalytic hydrogenation of CO

1.2.1. General considerations

Syngas, a gas mixture consisting primarily of CO and H

2

,

can be generated by steam reforming,

partial oxidation or auto-thermal reforming of fossil resources (natural gas and shale gas) as

well as by the gasification of coal and biomass.

31-34

_{Syngas is a very versatile feed and may}

also be used for higher alcohol synthesis. Typical byproducts are hydrocarbons, methanol and

CO

2

, formed by Fisher-Tropsch type of reactions (FTS,

eq. 1), methanol synthesis (MS, eq. 2)

and the water-gas-shift reaction (WGS,

eq. 3), respectively.

nCO + (2n + 1)H

2⇋

C

n

H

2n + 2

+ nH

2

O (1)

CO + 2H

2⇋

CH

3

OH

(2)

CO + H

2

O

⇋

CO

2

+H

2

(3)

Higher alcohol synthesis is thermodynamically possible at reduced temperature because of

the high exothermicity of the main reactions.

Figure 2 gives the Gibbs free energy changes

13

1

temperatures.

35

_{Methane is the most favored thermodynamic product of syngas conversion}

and has a negative Gibbs free energy in the whole range of temperatures. So, an appropriate

catalyst that imposes a kinetic barrier to CH

4

formation needs to be designed for higher

alcohol synthesis. The ΔG for methanol synthesis increases with temperature and is zero at

approximately 150 °C, whereas for ethanol formation remains negative over a range of

temperatures. Hence, equilibrium constraints for syngas conversions to higher alcohols are

of less importance than to methanol.

36

Figure 2. Gibbs free energy changes for the conversion of syngas to methane, methanol and

ethanol. Reproduced from ref. 35. Copyright 2008 American Chemical Society.

1.2.2. Catalyst developments

Low HA selectivity is a major issue for the direct synthesis of HA from syngas. A wide variety

of heterogeneous catalysts have been investigated, which generally are classified into four

categories: molybdenum-based catalysts, modified FTS catalysts, modified MS catalysts and

rhodium-based catalysts.

3

_{The catalytic aspect on a molecular level for HAS in relation to FTS}

and MS is briefly explained below. For FTS, the so-called carbide mechanism is the most

generally accepted one.

37, 38

_{Here, CO undergoes dissociative adsorption and the adsorbed C}

species are hydrogenated to metal-alkyl groups, which in subsequent (coupling) steps lead to

the formation of hydrocarbons (

Scheme 2a). For MS,

39

_{CO is non-dissociatively adsorbed and}

gradually hydrogenated to methanol (

Scheme 2b). HAS requires both the metal-alkyl species

14

great importance for good catalyst performance. Two routes can be envisaged for HAS, viz. i)

non-dissociatively adsorbed CO reacts with metal-alkyl species, forming adsorbed CH

x

CHO,

which are subsequently hydrogenated to the corresponding alcohols and ii) a partially

hydrogenated CHO unit couples with metal-alkyl species giving ultimately alcohols by further

hydrogenations (

Scheme 2c). According to this mechanism, bi-functional catalysts are

required for HAS. In the following, the state of the art regarding such bifunctional catalysts

will be reported. The use of Rh-based catalysts is excluded as they are comparatively very

costly and known to mainly lead to ethanol formation.

3

Scheme 2. Simplified CO hydrogenation reactions occurring upon (a) FTS, (b) MS and (c) HAS.

Reproduced from ref. 3. Copyright 2017 Royal Society of Chemistry.

1.2.2.1. Mo-based catalysts

Different types of Mo-based catalysts have been evaluated for HAS, examples are MoS

2

, MoO

x

,

Mo

2

C, MoN and MoP.

40

Among them, MoS

2

-based catalysts are of particular interest due to

their promising performance, high water-gas-shift activity and high resistance to sulfur

poisoning, thus avoiding the need for both water separation and deep desulfurization units.

5, 35, 41

_{Moreover, they are suitable for processing syngas with low H}

₂

_{/CO ratios, less sensitive}

15

1

catalysts were particularly studied in this thesis for HAS from CO hydrogenation. Mo-based

catalysts are generally used at T = 260–380 °C, P = 5–15 MPa (typically around 10 MPa), gas

hour space velocity (GHSV) = 2000–10000 cm

3

_g

_cat-1

_h

-1

_{and a molar ratio of H}

₂

_{/CO of 1, which}

is consistent with the composition of syngas generated from coal or biomass gasification.

41, 42

Literature benchmarks for HAS from syngas over MoS

2

-based catalyst are given in

Table 1.

Here, the order of individual entries is based on HA yield, with highest yields at the top in the

Table. Literature sources providing alcohol selectivity on a CO

2

-free basis only were excluded

since this leads to an overestimation of the actual HA selectivity and thus does not enable a

fair comparison. For comparison, also some entries for MoO

2

systems are provided. The

highest HA yield reported so far is 13.4 %, obtained at a CO conversion of 21% (

Table 1, entry

1). Based on this Table, it appears that i) MoS

2

-based catalysts give better performance than

MoO

2

catalyst, ii) the use of supports seem to have a positive effect and iii) promotion by

other metals is essential.

Table 1. Literature benchmarks of HAS from syngas over Mo-based catalysts.

Entry Catalyst Ta (°C) Pb (MPa) GHSV c _H2/CO XCOd (%) SHAe (%) YHAf Ref. 1 K-CoRhMoSx/CNT 330 8.3 3600 cm3 gcat-1 h-1 1.25 21 64 13.4% 43 2 K-RhMoSx/CNT 320 9.7 3600 cm3 gcat-1 h-1 1 44.7 24.4 10.9% 44 3 NiMoS2 310 6 1044 h-1 _{2 33 31 10.2%} 45 4 K-CoMoSx/C-Al2O3 360 5 760 cm 3 gcat-1 h-1 1 18.2 54.9 10% 46 5 K-CoMoSx/CNT 320 8.3 3600 cm 3 gcat-1 h-1 1 39.7 23.2 9.2% 47 6 K-CoMoOx/AC 300 5 2400 h-1 _{2 26 30 7.8%} 48 7 KCoMoSx/AC 270 14.5 1879 h-1 _{1.1 13.5 36.5 4.9%} 49 8 K-NiMoOx/Ni-CNT 285 8 10000 cm 3 gcat-1 h-1 1 15 31 4.7% 50

16

9 K-NiMoSx/MMO 350 5 3200 cm 3 gcat-1 h-1 1 9.5 37.5 3.6% 51 10 K-MoSx/C/MMO 310 10.3 1497 cm 3 gcat-1 h-1 1 12 29.2 3.5% 52 a _{Reaction temperature;} b _{Reaction pressure;} c_{Gas hourly space velocity;} d _{CO conversion;} e _{HA selectivity;} f _HA yield.

1.2.2.1.1. Catalyst structure and mechanistic aspects

MoS

2

has a layered structure (

Figure 3), where each layer consists of a slab of Mo atoms

sandwiched between two slabs of S atoms. The layers are held together by van der Waals

forces with an interlayer spacing of around 6.5 Å for the 2H-MoS

2

crystal structure.

53

It is

worth noting that MoS

2

-based catalysts often have a low crystallinity, and exhibit a disordered

morphology.

54

_MoS

₂

_{is a well-known intercalation host where the host molecules/atoms are}

located in the van der Waals gap between the MoS

2

slabs. Typical examples are Li

x

MoS

2

and

K

x

MoS

2

.

55-57

Intercalation leads to an increase in the interlayer spacing. For instance,

Somoano et al.

56

_{measured a 35% increase in the spacing of K}

_x

_MoS

₂

_{in the absence of}

moisture, while Zak et al.

57

_{reported a 50% increase in the presence of water due to}

intercalation of water (one to two water molecules per K atom). The intercalation of H

2

O with

K can affect the activity of K-promoted MoS

2

(100) catalysts for the WGS reaction.

58

Figure 3. Crystalline structure of layered MoS

2

. Reproduced from ref. 53. Copyright 2011

17

1

The generally accepted physical model for MoS

2

catalysts is the so-called rim-edge model

(

Figure 4),

59

_{where the catalyst is described as a stack of discs.}

60

_{The top and bottom discs}

are associated with the rim sites. The discs sandwiched between the top and bottom discs are

associated with edge sites. The catalytically active sites are located at the edge planes of MoS

2

,

not at the ordered basal planes, and these active sites are associated with sulfur vacancies.

61, 62

_{Dorokhov et al.}

63

_{proposed that the active centers for mixed alcohol synthesis are located}

on the edges of two adjacent K-MoS

2

layers. The adsorbed methyl groups participate in chain

growth with other adsorbed species on adjacent layers. As such, multilayered MoS

2

with more

edge sites are favored for HAS. The amount of edge sites is a strong function of the synthesis

protocol used to make the MoS

2

catalysts.

Figure 4. Rim-edge model for MoS

2

catalysts. Reproduced from ref. 59. Copyright 2008

Taylor’s University.

MoS

2

for HAS is typically produced either through reduction and sulfidation of molybdenum

trioxide or thermal decomposition of the costlier ammonium tetrathiomolybdate (ATTM,

(NH

4

)

2

MoS

4

). The main disadvantage of the first route is incomplete reduction/sulfidation.

64, 65

_{Innovative methods such as the use of micro-emulsions and sol-gel based methods have}

been developed and were shown to give smaller MoS

2

particle sizes, a higher promoter

dispersion and thus improved catalytic performance when compared to traditional synthesis

methods.

45, 66

_{Recently, in the fields of solid state physics, chemistry and materials science, a}

wide range of MoS

2

-based materials with diverse nanostructures have been synthesized for

a wide range of possible applications.

67-69

_{However, not much attention is being given to the}

18

Typically, unpromoted MoS

2

produces CO

2

and hydrocarbons from syngas.

70

Density

Functional Theory (DFT) modeling for MoS

2

catalysts reveals that a reaction pathway involving

C1 species is favored, see

sequence (1).

71

_{Hydrocarbon formation on MoS}

₂

_{is always}

accompanied with the WGS reaction, as shown in

sequence (2).

72

CO → CHO → CH

2

O → CH

2

OH → CH

2

→ CH

3

→ CH

4

(1)

CO + H

2

O → CO + OH + H → CO + O + 2H → CO

2

+ H

2

(2)

As first revealed by the Dow Chemical Company

73

_{and Union Carbide Corporation}

74

_{in the}

late 1980s, a high selectivity to mixed alcohols can be achieved when MoS

2

is doped with

alkali metals (AM) such as Li, Na, K, Rb and Cs. Among them, K, Rb, Cs are preferred and K is

the most widely used, although there are some discrepancies in the ranking of these

promoters.

40, 75, 76

_{Koizumi et al.}

76

_{varied the amounts of K, Rb and Cs in MoS}

₂

_{and the highest}

HA yield was achieved at an AM/Mo molar ratio between 0.1 and 0.2 for all promoters. In

other research, K/Mo molar ratios in the range 0.1-0.7 have been applied.

77-83

The role of K is two-fold: i) to suppress activity for the formation of hydrocarbons and ii) to

promote alcohol formation. It is proposed that KMoS

2

or K

2

MoS

2

phases, formed by the

intercalation of K into the MoS

2

structure, are responsible for the higher selectivity to alcohols

when compared to MoS

2

alone.

84-87

A dual site model has been proposed for the conversion

of syngas to mixed alcohols over K-MoS

2

catalysts (

Scheme 3).

88, 89

It involves the formation

of CH

x

species by gradual hydrogenation of CO or generated C species on the MoS

2

phase. The

direct hydrogenation of CH

x

species leads to the formation of CH

4

. Non-dissociative CO

adsorption and alcohol formation take place on the KMoS

2

phase. CO insertion leads to

carbon chain growth, also because alkyl coupling reactions are unfavorable on MoS

2

catalyst.

90, 91

_{A good balance and contact between the KMoS}

₂

_{and MoS}

₂

_{phases are considered to be}

of high importance to promote HAS.

Due to the low ionization potential of the alkali metals, the presence of K also leads to

stabilization of intermediate metal-alkoxy species.

92

_{In this way, K promotion suppresses}

hydrogenation of metal-alkyl species to hydrocarbons and enhances the rate of CO insertion

in the metal-alkyl bond to form metal-acyl species, which are subsequently converted to

alcohols.

40, 92, 93

19

1

Scheme 3. Reaction pathway for CO hydrogenation over a K-MoS

2

catalyst.

However, K modified MoS

2

catalysts normally suffer from low activity, typically associated

with blockage of catalytically active sites.

3, 94

_{Alcohol yields are typically low as a consequence}

of the generally observed trade-off between conversion and selectivity. Besides, the alcohol

product distribution when using K-MoS

2

catalysts always follows the so-called ASF

(Anderson-Schulz-Flory) distribution (

eq. 4),

95

_{which means that the chain growth probability (α) is}

independent of chain length.

𝑆𝑆𝑛𝑛

𝑛𝑛

= 𝛼𝛼

𝑛𝑛

×

(1−𝛼𝛼)

𝑎𝑎

(4)

Here, 𝑆𝑆

𝑛𝑛

is the selectivity to alcohols with a carbon number of 𝑛𝑛, and 𝛼𝛼 is the chain growth

probability. As a consequence, the amounts of individual alcohols in HAS follows the order

methanol > ethanol > propanol. Typically, the content of C3+ alcohols is less than 30% (carbon

atom fraction) of the alcohols produced.

47, 77, 96-98

The issues related to the use of K promoted MoS

2

catalyst for HAS have led to the

development of group VIII metals (cobalt and nickel) promoted catalysts.

99-102

_Nickel

promoted K-MoS

2

catalysts have shown higher activities than the Co promoted ones, but also

give higher amounts of CH

4

.

79, 87, 97, 98, 103

Cobalt is the preferred promotor in patent literature

73, 104

_{and it has been widely employed to promote carbon chain growth. As such, the use of}

Co promoted MoS

2

catalysts yields higher HA selectivity, though ethanol is the major product.

Co promoted MoS

2

catalysts have been widely used in hydrodesulfurization (HDS) reactions.

105

_{The promotor effect of Co is attributed to the formation of a Co-Mo-S phase, as shown in}

Figure 5, which is formed by partial substitution of Mo atoms at the edge of MoS

2

slabs by Co

atoms.

106-109

_{The Co-Mo-S phase was also observed in K modified Co promoted MoS}

₂

_catalyst

20

catalyst was employed in CO hydrogenation reactions and reported to yield only low amounts

of C1-C4 alkanes and no alcohols.

110

_{It has thus been claimed that a synergistic interaction}

between a MoS

2

phase and a Co-Mo-S phase contributes to enhanced HA formation. At higher

Co loadings, the number of active Co-Mo-S species was shown to decrease due to the

formation of Co

9

S

8

species.

47, 75, 115, 116

Due to a low affinity for Mo species, Co

9

S

8

is stable

under typical reaction conditions and enhances HC production. Thus, we can conclude that i)

the active phase in a Co promoted MoS

2

catalysts is a Co-Mo-S phase,

47, 75, 110, 115-118

ii) there

appears to be an optimal Co/Mo molar ratio for HAS and iii) the exact mechanism to promote

carbon chain growth is still under debate. As such, there is a strong incentive to further

explore and understand catalyst performance at molecular level.

Figure 5. Model of an ideal CoMoS arrangement: (a) top view, (b) side view of the S-edge and

(c) side view of the M-edge; the red, blue and yellow spheres represent Co, Mo and S,

respectively. Reproduced from ref. 109. Copyright 2017 Royal Society of Chemistry.

Besides unsupported MoS

2

catalysts, a great variety of supports have been studied,

52, 100, 116, 119-122

_{such as Al}

₂

_O

₃

_{, CeO}

₂

_{, SiO}

₂

_{, mixed MgAl oxide (MMO), activated carbon (AC) and}

multi-walled carbon nanotubes (CNT) to improve catalyst performance, as already shown in

Table

1.

1.2.2.2. Modified FTS catalysts

Research on modified FTS catalysts has been primarily focused on Fe- and Co-based catalysts.

In comparison with Co, the WGS activity of Fe based catalysts is relatively high, leading to

higher CO

2

selectivities. An overview for HAS using such modified FTS catalysts is shown in

21

1

Table 2. Typical operating conditions are a temperature between 200 and 350 °C, pressure

between 2 and 7 MPa, GHSV between 3000 and 10000 h

-1

_{and a molar ratio of H}

₂

_{/CO between}

1 and 2.

The best results were obtained with Co and Fe catalysts promoted with Cu on supports like

CNT and SiO

2

. The highest HA yield was 28% for CuCoAl/CNT at a CO conversion of 45% (

Table

2, entry 1). However, deactivation at long time on stream was observed due to sintering,

123

dealloying

124

_{and volatilization}

125

_{of active phases. The corresponding FeCu-based catalysts}

are also prone to deactivation, mainly due to the transformation of active iron carbides to

inactive iron oxide species.

126

_{Further studies with FeCu- and CoCu-based catalysts involved}

the use of other promoters such as alkali metals (AM),

127-129

_La

₂

_O

₃

_,

130-132

_Mn

133, 134

_{and the}

use of supports like Al

2

O

3

,

135, 136

SiO

2

,

137

CNT,

138, 139

MMO

133, 140, 141

were also explored to

improve performance.

Table 2. Literature benchmarks of HAS from syngas over modified FTS catalysts.

Entry Catalysta Tb (°C) Pc (MPa) GHSV d _H2/CO XCOe (%) SHAf (%) YHAg Ref. 1 CuCoAl/CNT 230 3 3900 cm 3 gcat-1 h-1 2 45 62 28% 142 2 K-CuFe/SiO2 320 5 6000 h-1 _{2 56 49 27%} 143 3 CoCu/GE-LFOa _{300 3} 3900 cm3 gcat-1 h-1 2 50 51 26% 144 4 CuCoAl 250 3 3900 cm 3 gcat-1 h-1 2 52 45 23% 145 5 CoCuMn/AC 220 3 500 h-1 _{2 58 39 23%} 146 6 CoCu/CNT 300 5 7200 cm3 gcat-1 h-1 1 39 58 23% 147 7 CoCu/LaFeO3 300 3 3900 cm3 gcat-1 h-1 2 76 29 22% 148 8 CuFeMg/MMO 300 4 2000 h-1 _{2 57 37 21%} 141 9 CoCu/CNT 300 5 10000 cm 3 gcat-1 h-1 1 38 54 21% 149

22

10 CoCu/Al2O3-CFa _{220 3} 3900 cm 3

gcat-1 h-1 2 39 43 17% 150 a _{GE-LFO: graphene sheet-LaFeO3, CF: carbon fibres;} b _{Reaction temperature;} c _{Reaction pressure;} d_{Gas hourly} space velocity; e _{CO conversion;} f _{HA selectivity;} g _{HA yield.}

1.2.2.2.1. Mechanistic aspects

A number of studies have reported on the mechanistic aspects for HAS using modified FTS

catalysts and a dual-site mechanism is proposed for Cu promoted Fe and Co catalysts (

Scheme

4).

89

_{The Fe and Co sites catalyze CO dissociation whereas Cu sites are required to enable the}

molecular adsorption of CO.

3

_{It is postulated that intimate contact between highly dispersed}

Fe or Co and Cu species is key for attaining high HA selectivity.

Scheme 4. Proposed dual site mechanism for alcohols and hydrocarbons for Cu promoted

modified FTS catalysts. Reproduced from ref. 89. Copyright 2013 Elsevier.

Theoretical studies

151

_{indeed have revealed that CO dissociation occurs primarily on Fe-rich}

surfaces, leading to CH

x

formation, whereas Cu-rich surfaces are potential sites for

physiosorbed CO molecules. Experimental investigations

152-154

_{have shown that both a Cu}

0

and a χ-Fe

5

C

2

phase are formed after activation of the catalysts in syngas, which are proposed

to be the two active sites for HAS. The ability of iron carbide to dissociatively adsorb CO and

to promote carbon chain growth is well known in the field of FTS. As stated above, intimate

contact between the Cu and Fe phase is essential for HAS. This hypothesis is supported by

23

1

experiments showing that the selectivity to alcohols decreases with time on stream due to

phase separation of Cu and Fe species.

152

However, for CoCu-based catalysts consensus about the nature of active sites has not been

reached yet. Based on DFT calculations on CoCu catalysts (

Scheme 5),

155

_{H assisted CO}

dissociation occurs on Co sites leading to CH

x

O species that are subsequently converted to

CH

x

. In addition, CO activation leads to HCO formation on a Cu sites, which subsequently

diffuses to the Co sites to form alcohols. Consequently, Cu is essential to provide CHO species

and also reduces the barriers for CHO insertion towards the formation of alcohols (

Scheme

5).

Scheme 5. Mechanistic aspects for HA synthesis using Cu promoted Co catalysts. Reproduced

from ref. 155. Copyright 2015 American Chemical Society.

Other studies indicate that both metallic Co and Co

2

C sites

156-158

are required for alcohol

synthesis. According to experimental and theoretical studies,

156

_Co

₂

_{C enables molecular CO}

adsorption while Co provides sites for dissociative CO adsorption. The CO insertion step

preferentially takes place at the Co

2

C-Co interface, as shown in

Figure 6. In this alternative

mechanism, Cu is suggested to be a promoter, rather than being an active site, which changes

the Co

2

C/Co ratio in the active specie by affecting the reducibility of Co.

132

24

Figure 6. Schematic illustration of alcohol formation on the surface of a Co@Co

2

C catalyst.

Reproduced from ref. 158. Copyright 2015 American Chemical Society.

1.2.2.3. Modified methanol synthesis (MS) catalysts

Modified MS catalysts can be broadly classified into low-temperature Cu-based catalysts (i.e.,

Cu-ZnO-Al

2

O

3

) and high-temperature Cr-based catalysts (i.e., Zn-Cr mixed oxides).

35

To

prevent sintering, Cu-based catalysts are typically operated at T = 250–300 °C, P = 1–5 MPa,

GHSV = 1200–9600 h

-1

_{and molar ratio of H}

₂

_{/CO = 1–2.5. Cr-based catalysts have comparably}

high catalyst stability and these catalysts are normally used at T = 350-400 °C, P = 7–10 MPa,

GHSV = 3000 h

-1

_{and molar ratio of H}

₂

_{/CO = 2.3–2.6.}

The best modified MS catalysts with respect to HA yields are given in

Table 3. Of interest is

the observation that catalysts in this list are all low-temperature Cu-based catalysts. The best

performance is obtained over a CsCu catalyst supported on Ce

0.8

Zr

0.2

O

2

(

Table 3, entry 1),

which exhibits high selectivity for HA, and remarkably low selectivity for HC (8%) and CO

2

(6%).

159

_{The use of ZrO}

₂

_{as the support promoted CO conversion (}

_{Table 3, entry 1 vs 4), while the}

addition of Cs significantly shifted the alcohol distribution from methanol to HA (

Table 3,

entry 4 vs 6). The results of entry 5 and 10 (vs entry 4 and 6) demonstrate a higher activity of

catalysts on CeO

2

in comparison with ZnO. K-Cu-Zn-Al

2

O

3

is an old though effective catalyst

(

Table 3, entry 7), which resulted in high selectivity for alcohols, though with only 11% in the

25

1

Table 3. Literature benchmarks of HAS from syngas over modified MS catalysts.

Entry Catalyst T a (°C) Pb (MPa) GHSV c _H2/CO XCOd (%) SAlc.e (%) YHAg Ref. 1 CsCu/CeZrO2 300 3 2400 h-1 _{2 35 84 (46)}f _16% 159 2 KLaCu/ZrO2 360 10 3000 h-1 _{2.5 63 34 (20) 13%} 160 3 FeCuMn/ZnO 260 4 6000 h-1 _{2 45 38 (27) 12%} 161 4 CsCu/CeO2 300 3 2400 h-1 _{2 22 81 (40) 9%} 159 5 CsCu/ZnO 300 3 2400 h-1 _{2 17 78 (33) 6%} 159 6 Cu/CeO2 300 3 2400 h-1 _{2 24 83 (13) 3%} 159 7 K-Cu-Zn-Al2O3 300 4 4860 cm 3 gcat-1 h-1 2 22 86 (11) 2% 162 8 CuMn/ZrO2 300 8 8000 h-1 _{2 37 64 (5) 2%} 163 9 K-CuZnMn 320 4 4860 cm3 gcat-1 h-1 2 12 82 (12) 1% 162 10 Cu/ZnO 300 3 2400 h-1 _{2 17 78 (5) 1%} 159 a _{Reaction temperature;} b _{Reaction pressure;} c _{Gas hourly space velocity;} d _{CO conversion;} e _{Total alcohols} selectivity; f _{in brackets: HA selectivity;} g _{HA yield.}

1.2.2.3.1. Mechanistic aspects

For Cu-based catalysts, it is assumed that H-assisted CO dissociation to CHO species occurs on

Cu surfaces,

164, 165

_{similar as proposed for modified FTS catalysts. Subsequent CHO}

hydrogenation and C-O cleavage gives CH

x

species, which either couple with CO or CHO to

ultimately form HA. The generally observed high methanol selectivity over Cu-based catalyst

is due to the higher energy barrier for C-O cleavage of CH

x

O species (to form HA) compared

to hydrogenation (to form methanol). As such, promoters are required to favor this cleavage

step. In addition, it is postulated that CH

x

hydrogenation or coupling reactions are less favored

than CO or CHO insertion reactions,

166, 167

_{and this might explain the generally low HC}

selectivity observed for modified MS catalysts.

It was discovered already in the 1920s that alkali metal (AM) addition to Cu-Zn-Al

2

O

3

catalysts

26

Cu-Zn-Al

2

O

3

catalysts has been studied widely, and similar to Mo-based catalysts, K and Cs

seem to be favored.

169, 170

_{DFT calculations on CuZn and Cs-CuZn catalysts reveal that the}

presence of Cs favors the coupling of CH

x

O with CH

x

species.

164

As such, AM enhance carbon

chain growth, though too high amounts lead to a reduction in catalyst activity due to blockage

of active sites

171

_{. Transition metal promoters like Fe,}

161, 163, 172

_Co

173, 174

_{and Ni}

175

_{have also}

been applied to favor the dissociative adsorption of CO and enhance HA selectivity of

Cu-Zn-Al

2

O

3

catalysts. Additionally, catalyst supports like CeO

2

,

159, 169, 176

TiO

2

and AC

177

have been

investigated and shown to have positive effects on catalyst performance by improving the

interaction between Cu and promoters.

Though Cr-based catalysts are inferior with respect to HA yields compared to Cu-based ones

(

Table 3), Cr-based catalysts have shown high activity and high selectivity for iso-butanol.

178

However, the nature of the active sites for Cr-based catalysts for HAS is still under debate. As

shown in

Figure 7,

179

_{some researchers consider a non-stoichiometric Zn-Cr spinel phase as}

the actives species,

180, 181

_{while others propose that ZnO is active and catalyzes the formation}

of iso-butanol.

182, 183

_{Promotion by AM has also shown to increase HA selectivity.}

179

Figure 7. Direct synthesis of iso-butanol from syngas on a Cr/ZnO-based catalyst. Reproduced

27

1

1.2.3. Overall conclusions on catalyst systems for CO hydrogenation to HA

In summary, and based on

Table 1-3, HA yields from CO hydrogenation are highest when using

modified FTS catalysts, followed by Mo-based catalysts and modified MS catalysts. Typically,

MoS

2

-based catalysts suffer from high CO

2

selectivity, and a better balance and interaction

between the WGS and HAS active sites are crucial to lower the selectivity of this byproduct.

In order to obtain commercially attractive performance, improvement on the understanding

of the roles of the active centers followed by rational design of catalyst structures are required.

1.3. Catalytic hydrogenation of CO

2

to HA

1.3.1. General considerations

The ever increasing amount of CO

2

in the atmosphere leads to an increase in global mean

temperature and ocean acidification and thus threatens our global ecosystem.

184

_CO

₂

_from

industry can be captured by existing technologies,

185

_{but efficient techno-economically viable}

technologies to permanently store CO

2

need to be developed.

186, 187

CO

2

conversions using H

2

to fuels and valuable commodity chemicals are high on the global

research agenda. H

2

is generally derived from the steam reforming of natural gas (

eq. 4),

188

the gasification of biomass (

eq. 5)

189

_{or water splitting (}

_{eq. 6) using electricity.}

190

_Steam

reforming of methane is currently the main source of hydrogen. The byproduct CO can also

be used for hydrogen generation by using the WGS reaction (

eq. 3).

CH

4

+ H

2

O

⇋

CO + H

2

(4)

C

x

H

y

O

z

+ O

2

+ H

2

O

⇋

CO + H

2

+ CO

2

+ C

n

H

m

+ tar (5)

2H

2

O

⇋

O

2

+ 2H

2

(6)

The main products of CO

2

hydrogenation are CO from the reverse WGS reaction (

eq. 7),

hydrocarbons (

eq. 8) and alcohols (eq. 9).

CO

2

+ H

2⇋

CO + H

2

O (7)

nCO

2

+ (3n + 1)H

2⇋

C

n

H

2n + 2

+ 2nH

2

O (8)

28

Within the typically used temperature ranges, methane is the most favored thermodynamic

product. The optimal thermodynamic conditions for alcohol synthesis from CO

2

hydrogenation are a relatively low temperature and high pressure. The CO

2

conversion and

product selectivity for CO

2

hydrogenation in a ternary system of methanol, ethanol, and CO

according to a theoretical study by Stangeland et al., are shown in

Figure 8.

191

_CO

₂

_conversion

is thermodynamically favored at low temperature and high pressure, while the formation of

ethanol is greatly favored over methanol (< 0.2%) regardless of the reaction conditions.

Moreover, CO is hardly produced, particularly when the overall pressure is above 30 bar.

Figure 8. (a) CO

2

conversion and (b) ethanol and CO selectivity in CO

2

hydrogenation to a

product mixture of methanol, ethanol and CO. Dashed lines in the left figure represent the

chemical equilibrium predicted by gas-phase thermodynamics.

Reproduced from ref. 191.

Copyright 2018 American Chemical Society.

1.3.2. Catalyst developments

Research on the conversion of CO

2

in the presence of catalysts predominately involves

hydrogenation reactions to produce CO, hydrocarbons (CH

4

, light olefins and liquid fuels),

methanol and HA.

192-194

_{In addition, the electrochemical conversion of CO}

₂

_{to hydrocarbons,}

oxygenates or CO using both heterogeneous and homogeneous systems is also being

explored to a great extent at the moment.

195, 196

_{Chemo-catalytic conversions of CO}

₂

_using

hydrogen have been focusing so far mainly on methanol synthesis,

197, 198

_{and studies on HAS}

directly from CO

2

hydrogenation are limited. Since our research is focused on HAS, the

development and possible opportunities of CO

2

hydrogenation to HA are reviewed in the

29

1

The direct synthesis of HA from CO

2

can be considered as a combination of the reverse WGS

reaction and subsequent HAS from CO hydrogenation.

192

_{This way, a catalyst that is active}

for both reactions at the prevailing reaction conditions would also be suitable for the overall

reaction. Examples are MoS

2

-based and Fe-based catalysts, as well as Rh-based catalysts, the

latter based on an observation that syngas can be converted into ethanol over a Rh/SiO

2

catalyst.

199

_{Recent advances on the use of heterogeneous catalyst for CO}

₂

_{hydrogenation to}

HA in continuous reactor set-ups are summarized in

Table 4. Data for batch reactors were

excluded,

200-205

_{to enable a fair comparison. The highest yield for HA reported so far is 13%,}

which is stated to be associated with the inertness of CO

2

,

206

the high kinetic barrier for the

formation of C–C bonds

207

_{and the possibility for the formation of multiple side products.}

This highest yield was obtained using a Cu/ZnO

2

catalyst promoted with Fe and K. Typically, a

H

2

/CO

2

molar ratio of 3 is applied, in line with the stoichiometric ratio (

eq. 9) for HA synthesis.

Table 4. Literature benchmarks of HAS from CO

2

hydrogenation.

Entry Catalysta T b (°C) Pc (MPa) GHSVd H2/CO2 XCO2e (%) SHAf (%) YHAg Ref. 1 KFeCu/ZnO 300 6 5000 h-1 _{3 42 32 13%} 208 2 KGaPdCu/ZnAl + KCuFe/Ala 330 8 20000 h -1 _{3 47 17 8%} 209 3 FeRh/SiO2 260 5 6000 cm3 gcat-1 h-1 3 27 16 4% 210 4 KCuZnFe/ZrO2 320 3 3000 h-1 _{3 25 9 2%} 211 5 Co3O4 160 2 6000 cm3 gcat-1 h-1 3 13 17 2% 212 6 KCuZn + KFeCuCoa _{350 6 5000 h}-1 _{3 31 ~6 2%} 213 7 KCoMoSx 320 12 3000 cm 3 gcat-1 h-1 3 30 5 2% 214 8 LiRh/SiO2 240 5 6000 cm 3 gcat-1 h-1 3 7 16 1% 199 9 IrMo/SiO2 200 4.9 2000 h-1 _{2 12 6 1%} 215 10 NaIrCo/SiO2 220 2.1 2000 h-1 _{3 8 8 1%} 216

30

a _{KGaPdCu/ZnAl + KCuFe/Al: physical mixture, KCuZn + KFeCuCo: two-stage bed catalyst combination system;} b Reaction temperature; c _{Reaction pressure;} d_{Gas hourly space velocity;} e _{CO conversion;} f _{HA (carbon number n} ≥ 2) selectivity; g _{HA yield.}

1.3.2.1. Mechanistic aspects

Based on the available knowledge on HAS from syngas, MoS

2

-based catalysts seem to be a

good candidate for HAS from CO

2

hydrogenation. However, only 3 publications with

experimental studies for HAS from CO

2

hydrogenation over MoS

2

catalysts have been

published.

214, 217, 218

_{A KCoMoS}

_x

_{catalyst showed a HA selectivity of 5% at 30% CO}

₂

_conversion

(

Table 4, Entry 7). The reported CO selectivity over MoS

2

-based catalysts is always very high,

217, 218

_{which indicates that the CO formed through the reverse WGS reaction is not effectively}

converted to products. The activation of CO may be inhibited by competitive adsorption of

CO

2

on the catalysts surface. As such, MoS

2

catalyst with a high surface area and exposed

active sites is required to facilitate the simultaneous activation of CO

2

and intermediate CO

to improve the yield of HA.

FeCu-based catalysts (modified FTS catalyst) have also been investigated for the

transformation of CO

2

to ethanol, see

Table 4 for details (entries 1, 2, 4 and 6 ).

208, 209, 213, 219, 220

_{As discussed in section 2, ethanol is formed by the coupling of CH}

_x

_{* units with CO* (}

_Figure

9). It is generally believed that the iron oxide phases adsorb and activate CO

2

, while iron

carbides induce the formation of CH

x

* units, as found for FT synthesis.

221

Cu with the proper

valence state is involved in the non-dissociative adsorption of CO.

151

_{Although FeCu-based}

catalysts are the best so far regarding CO

2

hydrogenation to HAS (

Table 4, entry 1), catalyst

deactivation is observed due to the oxidation of iron carbides to iron oxides by CO

2

and H

2

O.

222, 223

It was reported that the addition of low amounts of Pd and Ga is beneficial to maintain the

optimal state of the active metals during the reaction,

224

_{which may have a positive effect on}

catalyst stability. AM promoters were also suggested to reduce the conversion of active iron

carbides to inactive oxides.

225, 226

_{Besides, doping of the catalyst with AM has been shown to}

suppress the formation of CH

4

and enhance the selectivity to ethanol.

209

For example,

Takagawa et al. reported that at 300 °C and 7 MPa, the addition of K

2

CO

3

to a CuFe/ZnO

catalyst increased the selectivity of ethanol from about 6% to 20%

227

_{. K was also suggested}

31

1

Figure 9. Proposed reaction mechanism for ethanol formation from CO

2

hydrogenation.

Reproduced from ref. 210. Copyright 1997 Elsevier.

Alternatively, Chen et al.

202

_{suggested that CO is the intermediate leading to hydrocarbon}

formation, while CO

2

is the primary source for CH

3

OH (

Figure 10)

based on a comparison of

the product distributions over M/Mo

2

C (M=Cu, Co, Fe) catalysts for CO

2

hydrogenation. As

such, HA starting from methanol by extending the carbon units seems an interesting option.

Methanol synthesis from CO

2

hydrogenation has been widely studied with Cu-ZnO-Al

2

O

3

or

In

2

O

3

-based catalysts. In

2

O

3

supported on ZrO

2

catalyst was reported to give a high activity,

high selectivity towards methanol, and a remarkable stability for 1000 h on stream under

industrially relevant conditions.

229

Figure 10. Proposed reaction pathways to produce alcohols and hydrocarbons from CO

2

hydrogenation. The solid arrows denote major pathways and the dashed arrows denote

minor pathways. Reproduced from ref. 202. Copyright 2016 Elsevier.

32

Inspired by the high performance of In

2

O

3

catalysts, Gao et al. prepared a bifunctional catalyst

composed of In

2

O

3

and zeolites, yielding a high selectivity to gasoline-range hydrocarbons.

230

The oxygen vacancies on the In

2

O

3

surfaces were proposed to activate CO

2

and hydrogen to

form methanol, whereas C−C coupling subsequently occurs inside the zeolite pores to

produce gasoline-range hydrocarbons.

1.4. Guerbet coupling of ethanol

1.4.1. General considerations

The rapid growth of the global ethanol production

231

_{and the limitations of using ethanol as}

a sustainable fuel, as discussed above, has inspired the search for more efficient routes to

upgrade ethanol to more valuable chemicals such as 1-butanol. 1-butanol has an energy

density closer to gasoline, a lower miscibility with water and can be blended with gasoline at

higher concentrations than ethanol.

232

_{As a result, the efficient conversion of ethanol to}

1-butanol offers a promising route to obtain alternative fuels as well as bio-based chemicals

from biomass.

The upgrading of ethanol to 1-butanol can be realized by Guerbet reaction,

233

_{which is}

generally agreed to proceed through four steps (

Scheme 6)

234

_{: 1) dehydrogenation of ethanol}

to form acetaldehyde; 2) aldol condensation of the resulting acetaldehyde; 3) dehydration to

form crotonaldehyde and 4) hydrogenation of the unsaturated condensation products to

1-butanol. Therefore, the reaction requires the employment of catalysts that possess proper

basic/acid sites and suitable dehydrogenation/hydrogenation properties at the same time.

234-237

Scheme 6. A commonly proposed mechanism for the Guerbet coupling of ethanol.

Reproduced from ref. 234. Copyright 2017 Elsevier.

1.4.2. Catalyst developments

Well known catalytic systems for the Guerbet reaction include MgO, MgAlO

x

, hydroxyapatites

33

1

batch and continuous set-ups.

237-241

_{Unpromoted MgO was initially used owing to its basic}

character and dehydrogenation/hydrogenation ability, though it is only active at high

temperatures (350–450 °C). To increase catalyst activity, other components have been added

to the catalyst formulation, examples are mixed Mg-Al oxides and hydroxyapatites (HAP).

These materials are very versatile, since the Mg/Al or Ca/P ratio can be adjusted to tune the

number and strength of acid and basic sites.

242

_{Among them, MgAlO}

_x

_{, obtained from the}

thermal decomposition of hydrotalcite (HT), has attracted much attention due to its high

surface area, tunable basicity, cation-exchange ability of the Brucite layer and structural

stability.

243

Table 5. Selected examples of heterogeneous catalysts for the Guerbet coupling of ethanol to

1-butanol.

Entry Catalysta _{Tem. (°C) Conv. (%) 1-butanol}

Operation Ref. 1 MgO 450 56 18 Continuous 244 2 MgO 400 60 6 Continuous 237 3 MgAlOx 350 35 14 Continuous 245 4 MgAlOx 300 44.1 19.7 Continuous 246 5 FeMgAlOx 350 50 10 Continuous 236 6 CuMgAlOx 200 4.1 1.6 Batch 247 7 PdMgAlOx 200 3.8 2.9 Batch 235 8 CuNi-MgAlOx 320 56 22 Batch 248 9 HAP 350 26 18 Continuous 249 10 HAP 330 17 11 Continuous 240 11 HAP 298 20 14 Continuous 250 12 HAP-CO3a 400 40 22.4 Continuous 251 13 Ni/Al2O3 230 41 19.5 Batch 252 14 Ni/Al2O3 250 27 22 Batch 253 15 Cu/CeO2 330 67 30 Continuous 254

a _{HAP-CO3: carbonate-containing hydroxyapatites.}

Hydrotalcite is a layered anionic clay denoted as [M

2+(1−x)

M

3+x

(OH)

2

]

x+

A

n−x/n

·mH

2

O, where M

2+

34

widely used hydrotalcite is of the Mg-Al type, i.e. Mg

6

Al

2

(OH)

16

CO

3

·mH

2

O. Mg and Al can be

substituted by transition metal atoms or lanthanide atoms with the formation of new

metal-O

2-

_{acid–base pairs.}

242

_{Within this context, the surface basic property of the}

hydrotalcite-derived catalysts can be tuned by varying the Mg/Al ratio.

238, 256, 257

_{In addition, metal species}

with dehydrogenation/hydrogenation ability can be introduced by the addition of metal

compounds during the synthesis of the hydrotalcite, thus allowing for cooperative action

between basic sites of the hydrotalcite surface and supported metal species.

243

Figure 11. Schematic representation of the structure of hydrotalcites. Reproduced from ref.

255. Copyright 2011 Elsevier.

Cu is typically used as a metal component in the catalyst formulation owing to its well-known

activity in alcohol dehydrogenation.

242

_{However, Cu is also known to catalyze the undesired}

formation of esters from aldehydes by the Tishchenko pathway (

Scheme 7).

258

_{For instance,}

Bravo-Suárez et al. reported that the incorporation of Cu in MgAlO

x

drastically increased the

conversion of methanol and ethanol coupling reactions with significant production of both

C3+ alcohols and C3+ esters.

259

_{Cheng et al. suggested that the formation of esters for the}

coupling reactions is likely related to the Cu species rather than the acidic/basic sites in

Cu-MgAlO

x

catalysts, since esters are not formed over MgAlO

x

catalysts.

260

Benito et al. studied

the reaction over Cu-MgAlO

x

catalysts with different Cu contents and found that the catalyst

35

1

Scheme 7. Reaction network for the Guerbet coupling of ethanol showing the main reaction

and side reactions. Reproduced from ref. 258. Copyright 2018 Royal Society of Chemistry.

Product selectivity may also be tuned by the introduction of additional metal dopants such as

Ni.

1

_{For the condensation of 1-octanol using KOH as a homogeneous base in a batch reactor,}

the bimetallic CuNi-MgAlO

x

catalyst displayed remarkable activity and selectivity to the

hydrogenated β-branched alcohols.

262

_{Ni was suggested to favor the hydrogenation of the}

final dehydrated aldol-condensation product. Sun et al. also found that the hydrotalcite

derived Cu-Ni-doped porous metal oxide (CuNi-PMO) showed the best catalytic performance

and good stability in ethanol conversion without the addition of a homogeneous base (

Table

5, Entry 8), while the catalyst containing only Cu is highly selective toward ethyl acetate.

248

1.5. Aim and scope of this thesis

As is shown in the literature overview above, there is a high interest to develop efficient

catalytic technology for the synthesis of HA, and particularly to develop catalyst systems that

lead to high HA selectivities at high syngas/CO

2

conversions. The overall objective of this

thesis was to develop novel and improved catalytic systems for biobased higher alcohols by

CO hydrogenation (Chapter 2 and 3), CO

2

hydrogenation (Chapter 4) and the Guerbet coupling

of bioethanol (Chapter 5). The catalytic systems presented in this thesis featured remarkably

improved performance in terms of activity, selectivity and stability, in comparison with the

state-of-the-art performance as reported in the literature. Moreover, relations between

catalyst structure and performance have been investigated based on detailed catalyst

characterization studies and in depth analyses of products. A detailed outline of the thesis

chapters is given below.

In

Chapter 2, experimental studies are reported on the use of modified MoS

2

catalyst for CO

36

were prepared and tested in a dedicated continuous set-up with the objective to achieve high

selectivity of alcohols in general and C3+ alcohols in particular. The catalysts were

characterized in detail to determine the morphology and structure. The product composition

was determined and based on this, a carbon chain growth mechanism for alcohols is proposed.

On the basis of the results obtained in Chapter 2, the addition of Co to the K-MoS

2

catalysts

was investigated to further enhance C3+ alcohol yields and the results are provided in

Chapter

3. For this purpose, a series of K modified Co promoted molybdenum sulfide catalysts with

different Co contents and a fixed K content were prepared, characterized in detail and tested

for the conversion of syngas to higher alcohols. The role of Co in the catalyst formulation will

be discussed in detail and a mechanistic proposal will be provided. In addition, for the best

catalyst regarding Co content, the effect of process conditions, such as temperature, pressure,

gas hourly space velocity and H

2

/CO ratio was explored experimentally. The results were

quantified using statistical approaches allowing determination of the optimal process

conditions for higher alcohol selectivity and yield.

The synthesis of higher alcohols from CO

2

hydrogenation is discussed in

Chapter 4. For this

purpose, a series of bimetallic FeIn catalysts on a Ce-ZrO

2

support (FeIn/Ce-ZrO

2

) with

different Fe/(Fe+In) molar ratios were prepared and tested in a dedicated continuous set-up.

The influence of the amount of K in the catalyst formulation, the catalyst pretreatment

atmosphere, the molar ratio of Fe/(Fe+In) and the calcination temperature were investigated

in detail to optimize higher alcohol yields. Catalyst characterization studies were performed

and the results were used to gain understandings of the synergetic effects of different active

sites for higher alcohol synthesis via CO

2

hydrogenation.

In

Chapter 5, experimental studies are reported for the use of monometallic Cu and Ni as well

as a bimetallic Cu-Ni porous metal oxides (PMO) catalyst for the Guerbet coupling of ethanol

to 1-butanol in a continuous flow reactor. Such catalysts have been tested in batch with good

results, however, detailed information on catalyst stability is absent. Ethanol conversion and

product distribution for the three catalysts at extended runtimes (up to 160 h) were recorded

to identify the best catalyst in terms of activity, selectivity and stability. In addition,

characterization studies were performed to obtain insights in changes in the catalyst structure

and morphology during runtime.

37

1

Chapter 6 provides concluding remarks and perspectives for future work on the catalytic

synthesis of biobased higher alcohols.

References

1. Alonso, D. M.; Wettstein, S. G.; Dumesic, J. A., Bimetallic catalysts for upgrading of biomass to fuels and chemicals. Chemical Society Reviews 2012, 41 (24), 8075-8098.

2. <bp-energy-outlook-2019.pdf>.

3. Luk, H. T.; Mondelli, C.; Ferré, D. C.; Stewart, J. A.; Pérez-Ramírez, J., Status and prospects in higher alcohols synthesis from syngas. Chemical Society Reviews 2017, 46 (5), 1358-1426.

4. Balat, M.; Balat, H., Recent trends in global production and utilization of bio-ethanol fuel.

Applied Energy 2009, 86 (11), 2273-2282.

5. Surisetty, V. R.; Dalai, A. K.; Kozinski, J., Alcohols as alternative fuels: An overview. Applied

Catalysis A: General 2011, 404 (1-2), 1-11.

6. Ribeiro, B. E., Beyond commonplace biofuels: Social aspects of ethanol. Energy Policy 2013, 57,

355-362.

7. Patzek, T. W., Thermodynamics of the Corn-Ethanol Biofuel Cycle. Critical Reviews in Plant

Sciences 2004, 23 (6), 519-567.

8. Rico, J. A. P., Programa de Biocombustíveis no Brasil e na Colômbia: uma análise da implantação, resultados e perspectivas. 2007.

9. Keller, J. L., Alcohols as motor fuel. Hydrocarbon Processing 1979, 58 (5), 127-138.

10. Nie, X.; Li, X.; Northwood, D. O., Corrosion Behavior of Metallic Materials in Ethanol-Gasoline Alternative Fuels. Materials Science Forum 2007, 546-549, 1093-1100.

11. Swana, J.; Yang, Y.; Behnam, M.; Thompson, R., An analysis of net energy production and feedstock availability for biobutanol and bioethanol. Bioresource technology 2011, 102 (2),

2112-2117.

12. Vancoillie, J.; Demuynck, J.; Sileghem, L.; Van De Ginste, M.; Verhelst, S., Comparison of the renewable transportation fuels, hydrogen and methanol formed from hydrogen, with gasoline– Engine efficiency study. International journal of hydrogen energy 2012, 37 (12), 9914-9924.

13. Christensen, E.; Yanowitz, J.; Ratcliff, M.; McCormick, R. L., Renewable oxygenate blending effects on gasoline properties. Energy & Fuels 2011, 25 (10), 4723-4733.

14. Herman, R. G., Advances in catalytic synthesis and utilization of higher alcohols. Catalysis Today

2000, 55 (3), 233-245.

15. Mascal, M., Chemicals from biobutanol: technologies and markets. Biofuels, Bioproducts and

Biorefining 2012, 6 (4), 483-493.

16. Inc., N. Petrochemical Market Dynamics Oxo Alcohols; 2015.

17. Noweck, K.; Grafahrend, W., Fatty Alcohols. Ullmann's Encyclopedia of Industrial Chemistry

2006.

18. Fatty Alcohol Market Size, Share & Trends Analysis By Product (C6-C10, C11-C14, C15-C22), By

Application (Soaps & Detergents, Personal Care, Lubricants, Amines), By Region, And Segment Forecasts, 2015 - 2022.

19. Devarapalli, M.; Atiyeh, H. K., A review of conversion processes for bioethanol production with a focus on syngas fermentation. Biofuel Research Journal 2015, 2 (3), 268-280.

20. Bengelsdorf, F. R.; Straub, M.; Dürre, P., Bacterial synthesis gas (syngas) fermentation.

Environmental Technology 2013, 34 (13-14), 1639-1651.

21. Lan, E. I.; Liao, J. C., Microbial synthesis of n-butanol, isobutanol, and other higher alcohols from diverse resources. Bioresource Technology 2013, 135, 339-349.

38

22. Bai, F. W.; Anderson, W. A.; Moo-Young, M., Ethanol fermentation technologies from sugar and starch feedstocks. Biotechnology Advances 2008, 26 (1), 89-105.

23. Younesi, H.; Najafpour, G.; Mohamed, A. R., Ethanol and acetate production from synthesis gas via fermentation processes using anaerobic bacterium, Clostridium ljungdahlii. Biochemical

Engineering Journal 2005, 27 (2), 110-119.

24. Worden, R. M.; Bredwell, M. D.; Grethlein, A. J., Engineering Issues in Synthesis-Gas Fermentations. In Fuels and Chemicals from Biomass, American Chemical Society: 1997; Vol. 666, pp 320-335.

25. Klasson, K. T.; Ackerson, M. D.; Clausen, E. C.; Gaddy, J. L., Bioconversion of synthesis gas into liquid or gaseous fuels. Enzyme and Microbial Technology 1992, 14 (8), 602-608.

26. Dürre, P., Fermentative Butanol Production. Annals of the New York Academy of Sciences 2008,

1125 (1), 353-362.

27. Bankar, S. B.; Survase, S. A.; Ojamo, H.; Granström, T., Biobutanol: the outlook of an academic and industrialist. RSC Advances 2013, 3 (47), 24734-24757.

28. Stanley, G. G., Hydroformylation (OXO) Catalysis. Kirk-Othmer Encyclopedia of Chemical

Technology 2017, 1-19.

29. Nakagawa, Y.; Tajima, N.; Hirao, K., A theoretical study of catalytic hydration reactions of ethylene. Journal of Computational Chemistry 2000, 21 (14), 1292-1304.

30. Ueda, W.; Ohshida, T.; Kuwabara, T.; Morikawa, Y., Condensation of alcohol over solid-base catalyst to form higher alcohols. Catalysis Letters 1992, 12 (1), 97-104.

31. Evans, G.; Smith, C., Biomass to liquids technology. 2012.

32. Sutton, D.; Kelleher, B.; Ross, J. R. H., Review of literature on catalysts for biomass gasification.

Fuel processing technology 2001, 73 (3), 155-173.

33. Pakhare, D.; Spivey, J., A review of dry (CO2) reforming of methane over noble metal catalysts.

Chemical Society Reviews 2014, 43 (22), 7813-7837.

34. LeValley, T. L.; Richard, A. R.; Fan, M., The progress in water gas shift and steam reforming hydrogen production technologies–a review. International Journal of Hydrogen Energy 2014, 39

(30), 16983-17000.

35. Subramani, V.; Gangwal, S. K., A Review of Recent Literature to Search for an Efficient Catalytic Process for the Conversion of Syngas to Ethanol. Energy & Fuels 2008, 22 (2), 814-839.

36. Mawson, S.; McCutchen, M. S.; Lim, P. K.; Roberts, G. W., Thermodynamics of higher alcohol synthesis. Energy & fuels 1993, 7 (2), 257-267.

37. Rofer-DePoorter, C. K., A comprehensive mechanism for the Fischer-Tropsch synthesis.

Chemical Reviews 1981, 81 (5), 447-474.

38. van Santen, R. A.; Markvoort, A. J.; Filot, I. A. W.; Ghouri, M. M.; Hensen, E. J. M., Mechanism and microkinetics of the Fischer–Tropsch reaction. Physical Chemistry Chemical Physics 2013,

15 (40), 17038-17063.

39. Klier, K., Methanol synthesis. In Advances in catalysis, Elsevier: 1982; Vol. 31, pp 243-313. 40. Zaman, S.; Smith, K. J., A review of molybdenum catalysts for synthesis gas conversion to

alcohols: catalysts, mechanisms and kinetics. Catalysis Reviews 2012, 54 (1), 41-132.

41. Tijmensen, M. J. A.; Faaij, A. P. C.; Hamelinck, C. N.; van Hardeveld, M. R. M., Exploration of the possibilities for production of Fischer Tropsch liquids and power via biomass gasification.

Biomass and Bioenergy 2002, 23 (2), 129-152.

42. Göransson, K.; Söderlind, U.; He, J.; Zhang, W., Review of syngas production via biomass DFBGs.

Renewable and Sustainable Energy Reviews 2011, 15 (1), 482-492.

43. Boahene, P. E.; Surisetty, V. R.; Sammynaiken, R.; Dalai, A. K., Higher alcohol synthesis using K-doped CoRhMoS2/MWCNT catalysts: influence of pelletization, particle size and incorporation

of binders. Topics in Catalysis 2014, 57 (6-9), 538-549.

44. Surisetty, V. R.; Dalai, A. K.; Kozinski, J., Effect of Rh promoter on MWCNT-supported alkali-modified MoS2 catalysts for higher alcohols synthesis from CO hydrogenation. Applied Catalysis