Butanol synthesis from propylene

CERTIFICATE

Scheme 4.2: Butanol synthesis from propylene

The steady rise in fuel consumption to meet the ever increasing energy demand has caused high emission of greenhouse gases.¹⁰ The combustion of 1 kg of butanol emits lesser amount of CO2

as well as generates equivalent amount of energy in comparison to gasoline as shown in Figure 4.1.¹¹ On the other hand, to obtain equivalent amount of energy, more amount of ethanol is required, which again leads to a higher amount of CO2 emission (Figure 4.1).¹² In fact, the market of butanol globally is estimated to be 2.8 million tons annually, which is around 5 billion USD as of 2023.^13-14

Figure 4.1: Comparison of CO2 emission from different fuels (gasoline, butanol, and ethanol) for an energy generation of 29 MJ.¹²

The main energy requirement globally comes from fossil fuels (around 78.4%), which is a major concern for the predicted energy misbalance in near future (Figure 4.2).^{11, 15} The release of CO2 from burning of fossil fuels contribute to the growing climate change which needs to be addressed by implementation of a new strategy.¹⁶ By using modern technology, people are shifting significantly towards alternative and renewable energy source. Use of biofuel as an TH-3049_166122006

Kanu Das, Ph.D Thesis, IIT Guwahati 137 energy source has gained considerable attention to researchers, apart from the focus on batteries, solar and wind energies.¹⁷

World energy consumption

by different sources 2.3%

19.3%

78.4%

Fossil fuel

(Oils, Coals, Natural gas) Nuclear power

All renewables energy

10.2%

52.85%

47.15%

Traditional biomass Modern

renewables ^9.1%

Figure 4.2: World energy consumption by different sources.¹² 4.1.1 History

The use of biofuel is an alternative strategy for energy source. In1912, Rudolf Diesel introduced the use of peanut oils as a fuel source to an engine.^{14-15, 18} Henry Ford and Nikolaus August Otto proved that the pure ethanol is able to run an engine with their respective motors.

However, in 2005, David Ramey drove a car using biofuel (butanol) instead of gasoline across the United States. Though it consumed about 9% higher butanol than gasoline, there was a reduction in CO, hydrocarbons and NOx (x = 0.5, 1 & 2) emission (http://www.butanol.com/, as of 31 August 2022).^14-15

Recently, the Navigant research predicted an increase in demand of energy (biofuel) worldwide from n-butanol in comparison to that from other valuable alcohols Figure 4.3.¹² Bio-butanol is also a valuable precursor to synthesis of numerous industrial scale products.¹⁹ In addition, use of bio-butanol reduces the emission of CO2 in comparison with the corresponding use of fossil fuel.²⁰

4.1.2 Ethanol vs butanol

The Biofuel can be easily produced from biomass which is biodegradable and has properties such as renewable and reproductive, unlike gasoline. There are large number of biofuels available such as ethanol, butanol and biodiesel.²¹ The biofuels exist in three states as solid, liquid and gas and among them those derived from solids (e.g. bio-coal, wood and charcoal) TH-3049_166122006

Kanu Das, Ph.D Thesis, IIT Guwahati 138 constitute as the primary fuels for producing energy (table 4.2). The secondary fuels (i.e. liquid and gas) are mainly used for transportation. The four-carbon containing n-butanol is largely produced (at a cost of 7.0–8.4 billion dollars per year) in industry as chemical, solvent and as supplement to biofuel.²²

Figure 4.3: Total biofuels production around the globe.¹²

Table 4.1: Comparison of physical and chemical properties of butanol, ethanol, and gasoline

n-Butanol has more advantages over other biofuels due to a higher energy density and relatively low self-ignition temperature. The other advantage is its low flammable nature, less volatility and low vapor pressure. Butanol can blend up to 85% with gasoline in an unmodified petrol engine compared to ethanol which reaches only up to 10%. Though ethanol has been

Properties Ethanol Butanol Gasoline

Energy Density (MJ/L) 21.2 29.2 32.5

Fusion point (C) -114 -89.3 -40

Boiling point(C) 78 117 27-225

Auto ignition temperature (C) 423 385 257

Density (g/mL) 0.79 0.81 0.75

Water solubility (mL 100 mL^-1) miscible 9.1 <.0.01

Oxygen content (% vol) 34.7 21.6 <2.7

Vapour pressure (kPa) 16.5 18.6 75

TH-3049_166122006

Kanu Das, Ph.D Thesis, IIT Guwahati 139 considered as a biofuel, it has several limitations like poor energy density (70% with respect to gasoline),2-3, 23-24 corrosive nature²⁵ and higher water absorptivity^{23, 26-27} and possesses challenges in its handling under state-of-art technologies. These limitations can be circumvented by the use of n-butanol, which not only has a higher energy density (86%), but also is non-corrosive while being immiscible in water (Table 4.1).^{15, 25}

Table 4.2: Comparison of various fuel in terms of energy production²⁸

Entry Fuel Specific energy (M J/kg) Energy density (M J/L)

1 Methanol -22.70 -17.80

2 Ethanol -29.70 -23.30

3 1-butanol -36.10 -29.10

4 1-hexanol -39.00 -31.70

5 1-octanol -40.70 -33.50

6 Gasoline -47.30 -33.80

7 Kerosene -46.20 -38.30

8 Diesel -44.80 -37.10

9 Coal (Anthracite) -27.00 -36.40

10 Coal (Lignite) -15.00 -12.00

11 Wood -15.00 -9.00

n-Butanol can be synthesized via two major pathways; i) aldol condensation of acetaldehyde to form crotonaldehyde followed by hydrogenation to form n-butanol,²⁹ ii) hydroformylation of propylene followed by hydrogenation of butyraldehyde to n-butanol.^30-33 However, acetaldehyde also has to be synthesized via Waker process, where similar type of hydroformylation reaction involves reaction of ethylene with CO.³³ While, acetaldehyde or propylene productions requires the use of fossil fuels along with a huge energy consumption for the cracking of hydrocarbons, CO formation involves a partial thermal oxidation of alkanes (from fossil fuel and coal) which again leads to an overall high energy process.³⁴ Hence, conventional methodology has limitations for the industrial production of n-butanol synthesis.

Various heterogeneous catalytic systems are known to upgrade ethanol to n-butanol. Majorly, two features of the catalyst are required for the alcohol coupling reactions. One feature is related to the acidity/basicity of the catalyst.³⁵ The other involves the ability of the catalyst to facilitate dehydrogenation of the alcohol at the considered reaction temperature.³⁶ It is believed that a catalyst is active only when the main active sites on the catalyst are basic sites.³⁷ Therefore, to generate an efficient catalyst and to obtain n-butanol in high yields, the TH-3049_166122006

Kanu Das, Ph.D Thesis, IIT Guwahati 140 composition of the catalyst and the associated basic sites density and strength are very important. Metal promoted zeolites,^38-41 hydroxyapatites,^42-46 and the metal oxide^47-51 systems are usually employed as catalytic materials for the reaction, yielding different activities and selectivities. The heterogeneous reaction mainly requires harsh conditions (>200 C) and often suffers from lack of selectivity.³⁶

4.1.3 Homogenous organometallic catalysts towards biofuel production

In this context, methods that upgrade ethanol to either n-butanol or higher analogues with higher energy density²⁴ are a subject of great interest. Not surprisingly, the Guerbet reaction^52-

54 that involves the transformation of bio-ethanol to bio-butanol with water as the sole byproduct has been widely explored.^55-58 Based on the hydrogen-borrowing (HB) strategy (Scheme 4.3), this reaction involves alcohol dehydrogenation into aldehyde, followed by aldol condensation and hydrogenation of the resulting higher ,-unsaturated aldehyde into the higher molecular weight alcohol product.