1.0 Introduction

It is evident that
petroleum has been a vital resource in human endeavour. It is the major source
of energy that anchored industries, transportation, agriculture as well as
domestic purposes such as cooking, heating, etc. However, the depletion of
petroleum reserves and environmental impact of fossil fuel have led to the
search for alternative and renewable sources of energy. The burning of large
amount of fossil fuel has increased the carbon dioxide (CO2) level
in the atmosphere causing global warming (Mussatto, 2016). Global warming is
one of the environmental challenges facing the world. It is caused by an
increase in level of greenhouse gases (GHG) in the atmosphere due to human
activities since the beginning of industrial revolution (Asmare and Gabbiye,
2014). In recent years, researchers all over the world have been trying to find
new alternative fuels that are sustainable, renewable and environmentally
friendly (Liquat et al., 2010).

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Nowadays, the
production of synthetic fuels from atmospheric carbon dioxide (CO2)
has attracted enormous interest for its significant roles in overcoming the
environmental impacts of CO2 emissions and reducing dependence on
fossil based fuels (Wei et al., 2017).
Many researchers have used different approach to convert
CO2 into valuable chemical products like jet fuel, diesel, gasoline,
acetic acid, formaldehyde, dimethyl ether (DME), aromatics and olefins via
Fischer-Tropsch synthesis and methanol synthesis (Balasubramanian et al., 2018). However, the low
reduction efficiency of some catalysts is the main barrier for their
application due to poor reducibility of CO2, leading to the formation
of methane and short chain hydrocarbons. These have continued to inspire more
investigation and research. It has also been demonstrated that removing the
excess CO2 in the atmosphere by conversion into useful fuel is the
key to solving problem associated with the use of fossil fuels.

 

2.0 Research Problem

Despite the great
deal of work done on CO2 conversion to fuel, direct conversion of CO2
to petrol range (C5-C11) and diesel range (C10-C20) hydrocarbons remain an unsolved issue because CO2
is in full oxidation state. Another challenge arises with low carbon to
hydrogen (C/H) ratio obtained during CO2 hydrogenation due to low
heat of CO2 adsorption on the catalyst surface. This favour the fast
hydrogenation of surface adsorbed intermediates leading to the formation of
methane and decrease in chain growth. Solution to these problems could lies in finding
efficient catalyst that could convert CO2 directly into petrol and
diesel.

 

3.0 Aim

The aim of this
research work is to investigate the feasibility of using Zeolite catalysts for
the direct conversion of CO2 into petrol range (C5-C11)
and diesel range (C10-C20) hydrocarbons. The research
would also provide an insight on how to mitigate the high CO2
emissions by the oil and gas industries and other sources.

4.0 Objectives and Hypothesis

v  To synthesize and
characterize zeolite catalysts.

v  To modify the
synthesized Zeolite catalyst with Mg, Ca, Na and K metals.

v  To transform CO2
into petrol range (C5-C11) and diesel range (C10-C20)
hydrocarbons using the modified and unmodified zeolite catalysts.

v  To investigate the
fuel properties and chemical compositions of the products obtained using
different instrumental methods of analyses.

v  To compare and
contrast the fuel properties and chemical compositions of the products and
those of the commercially available petrol and diesel.

5.0 Materials and Proposed Methodology

The aim of this
research work would be achieved if the proposed methodology is use to serve as
a pathway for all the synthesis and analyses.

5.1 Materials

Materials that would
be use in this research are: carbon dioxide (CO2), hydrogen (H2),
aluminum sulphate (Al2(SO4)3.18H2O),
sulphuric acid (H2SO4, 98%), sodium silicate (Na2SiO3)
and sodium hydroxide (NaOH), tetraethylammonium hydroxide (TEAOH), sodium
hydroxide (NaCl), potassium chloride (KCl), silica (SiO2), aluminate
(NaAlO2) and ammonium nitrate (NH4NO3) which are
to be purchased from a chemical store.

 

5.2 Catalyst Synthesis

5.2.1 H-ZSM-5 Zeolite Synthesis

First, solution A
as the source of alumina would be prepared. The solution would contain 26.7g of
aluminum sulphate (Al2(SO4)3.18H2O),
56g of 98% sulphuric acid (H2SO4), and 15 cm3
of distilled water. After that, solution B as the source of silica would be
prepared. The solution would contain 56g of sodium silicate and 56g of 40%
(w/v) Sodium Hydroxide (NaOH). Solution A and Solution B would be slowly mix
together. The mixture would then be homogenized using magnetic stirrer at a
speed of 1200 rpm for 5 minutes. The crystallization would be performed in
static conditions at 180 °C for 48 hours using stainless steel teflon-lined
autoclave in an air oven. The solid product would be recovered by filtration,
washed several times with deionized water until the pH of the decanted water is
7, and then would be dried overnight at 105 °C. Finally, the catalyst sample
would be calcined to remove the organic template in a muffle furnace under an
air flow at 530 °C for 12 hours at a heating rate of 3 °C/minutes. The ZSM-5
zeolite in hydrogen form (H-ZSM-5) would be obtained through ion-exchange with
aqueous NH4NO3 solution. The
loading of magnesium (Mg), calcium (Ca), Sodium (Na), and Potassium (K) would
be carried out by wet impregnation method, treating the zeolites in stirred
aqueous solutions of the corresponding nitrates (Yaripour et al., 2015; Veses et al.,
2016; Widayat and Anissa, 2016; Niu et al.,
2017).

 

5.2.2 H-BEA zeolites synthesis

First, solution A
as a source of silica would be prepared. The solution would contain 59.4g of
deionized water, 89.6g of tetraethylammonium hydroxide (TEAOH, 40%), 0.53g of
sodium chloride (NaCl) and 1.44g potassium chloride (KCl). Then, 29.54g of
silica (SiO2) would be added followed by stirring until a homogenous
mixture is obtained. After that, solution B as a source of alumina would be
prepared. The solution would contain 20 cm3 of deionized water,
0.33g sodium hydroxide (NaOH), and 1.79g of sodium aluminate (NaAlO2).
Solution A and Solution B would be slowly mix together. After that, the mixture
would be homogenized using magnetic stirrer at a speed of 1200 rpm for 10 min.
The crystallization would be performed in static conditions at 135 °C for 20
hours using stainless steel teflon-lined autoclave in an air oven. The solid
product obtained would then be washed several times with deionized water until
the pH of the decanted water is approximately 9, and then would be dried
overnight at 77 °C. The BEA zeolite in hydrogen form (H-BEA) would be obtained
through ion-exchange with aqueous NH4NO3 solution. The
loading of magnesium (Mg), calcium (Ca), Sodium (Na), and Potassium (K) would
be carried out by wet impregnation method, treating the zeolites in stirred
aqueous solutions of the corresponding nitrates (Mintova and Barrier, 2016;
Veses et al., 2016).

 

 

5.4 Catalyst Characterization

The X-ray
diffraction (XRD) pattern would be identified using a Bruker D8 Discover
diffractometer equipped with a Copper tube (l = 0.15418 nm), 40 kV voltage, 40
mA, and a VANTEC-500 2-D detector. XRD data would be analyzed with PCXRD
software. Scanning electron microscopy (SEM) imaging would be determined using
a JEOL JSM-7100F equipped with an Oxford Energy Dispersive X-Ray Spectroscopy.
The thermal stability of the catalyst would be determined by thermogravimetric
analysis (TGA) using a TG/DTA analyzer (PL-STA-1640).  The apparent surface areas would be
determined using the Brunauer-Emmett-Teller (BET) method in the range between
P/P0 0.02–0.10, while the mesopore size distribution would be
obtained using the Barrett-Joyner-Halenda (BJH) model applied to the adsorption
branch of the isotherm (Zhang et al.,
2017; Cheng et al., 2018)

 

5.3 Carbon Dioxide Hydrogenation and Product analysis

The CO2
hydrogenation reaction would be carried out in a fixed bed continuous flow
reactor (FBCFR). Exactly 1g of the catalyst (20–40 meshes) would be used. Prior
to reaction, the catalyst would be in-situ reduced at 350 °C for 8 hours in a
pure H2 flow at atmospheric pressure. After reduction, the reactor
would be cooled to 320 °C. Then the reactant gas mixture H2/CO2/N2
(containing 4 vol% N2 as the internal standard) would be fed
into the reactor, and the system would be pressured gradually to 3
MPa. All of the products from the reactor would be introduced in a
gaseous state and would be analysed with two online gas chromatographs (GC)
(VARIAN 3800). N2, CO, CO2 and CH4 would be
analysed using a GC system equipped with a TCD, a Hayesep C column
and a molecular sieve 13X column. The organic compounds including
hydrocarbons and oxygenates would be analysed using another GC
system equipped with a flame ionization detector (FID) and a PONA
(Paraffins, Olefins, Naphthenes and Aromatics) capillary column. The reaction would
be carried out under the conditions of H2/CO2 = 320 °C, 3
MPa and 4,000 ml h-1 gcat-1 (Wei et al., 2017).

CO2
conversion would be calculated by the equation below:

                       

 

where CO2
in and CO2 out represent the molar fraction of
CO2 at the inlet and outlet respectively.

 

6.0 Relevance of the Research to Nigerian Oil and Gas
Industry

For over two
centuries, utilization of fossil fuels such as coal, oil and natural gas as the
main source of energy has speed up the progress of human civilization, economic
and social development. However, burning large amount of fossil fuels has led
to the depletion of petroleum reserve and also gave rise to huge amounts of CO2
emissions, which brings about adverse climatic changes. Therefore, converting
excess atmospheric CO2 gas into valuable chemicals and fuels will
contributes significantly to mitigating CO2 emissions. If this
research is successful, it can be put onto a commercial scale to meet the
ever-increasing energy demand and thus enhances energy security in light of the
depletion of fossil fuel resources. Therefore, by adoption of Carbon capture and storage technologies, CO2
can serve as a possible future raw material for oil and gas industries in
Nigeria.

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