Proceedings 24th Saudi Japan Annual Symposium
Catalysts in Petroleum Refining & Petrochemicals
KFUPM Dhahran, Saudi Arabia Dec. 1-2, 2014
Direct Reaction of CO2 with C-H Bond Activation of Methane to
Produce Chemicals
Emad N. Al-Shafei1*, Sai P. Katikaneni1, Ki-Hyouk Choi1 and Rob Brown2
1 Research
and Development Center, Saudi Aramco, Saudi Arabia
2 The
University of Huddersfield, Huddersfield, UK
* contact [email protected]
Abstract:
Activation of C-H bond of methane and its direct reaction with CO2 to produce
chemicals will result in elimination of several sever reaction steps while improving
reaction efficiency. The study investigated the methane coupling on heterogeneous
catalyst and possible reaction with CO2 as whole molecule to produce carbolic acid.
The catalysts of binary and ternary oxides based on TiO2 were synthesized and
calcined at 700 oC. The methane coupling reaction was carried out at atmospheric
pressure using fixed bed reactor at varied temperatures between 300 to 700 oC. The
results showed that the formation of ethane and ethylene in the products were due to
methyl radicals dimerization. By introducing CO2 molecule, the most of methyl
radicals were consumed by direct reaction with CO2 and producing acetic acid
molecule instead of methyl dimerization. The solid oxide catalysts showed different
optimum temperature and catalytic activity for methyl coupling formation and
reactivity towards CO2 molecule. The gas ratios of methane and CO2 were studied and
results indicated that an increase of acetic acid yield by direct reaction route between
methyl radicals of methane with CO2.
Introduction
A very few papers have been published in the literature on the direct reaction of CO2
as a whole molecule with methane[1][2][3][4][5]and ethane[6] to form acetic acid and
studied catalysts such as Pt/Al2O3[1], Pd/carbon[1], Cu/Co, Pd/SiO2, and V2O5PdCl2/Al2O3[3][4][5]. The motivation of this work is to study the possible utilization
of greenhouse gases of CO2 and CH4 to raw materials and that would reduce or
eliminate several reaction steps.
Methane is the least reactive alkane which makes difficult for its activation and
utilization[7]. Several research groups have studied the methods for the direct
conversion of methane to valuable chemicals by methane oxidation coupling to
ethylene and ethane, methane oxyhalogenation and methane aromatization [8][9]. The
hydrogen atoms of methane gas are acidic and have a small positive charge (δ+) and
the carbon atom is basic with a small negative charge (δ-). This is in contrast to CO2
in which the carbon atom is slightly acidic (δ+) and the oxygen atoms are basic (δ-).
CO2 has closed shells according to the octet rule. The C atom is sp hybridized and the
Proceedings 24th Saudi Japan Annual Symposium
Catalysts in Petroleum Refining & Petrochemicals
KFUPM Dhahran, Saudi Arabia Dec. 1-2, 2014
CO2 molecule exhibits strong bonds [10]. Markovits et al. [11] described CO2 in term
of weakly basic oxygen atoms and an acidic carbon atom. A study of the interaction
between CO2 and some transition metal surfaces showed that the CO2 molecule
adsorption has several feasible modes of coordination [10][12].
Direct reaction of methane with CO2 may suggest a radical reaction mechanism in
which CO2 first reacts with CH3 on the catalyst surface [3][5]. In general, those initial
studies were showed very low yields of acetic acid. Furthermore, the direct reaction
between CO2 and CH4 on heterogeneous catalysts to form acetic acid has not been
explained in terms of a suitable reaction mechanism so far. Due to the
thermodynamic limitations of the conventional acetic acid formation reaction, a
methyl radical mechanism is the most possibly way to produce acetic acid. This idea
has been supported by DFT calculations which suggest that reaction between CO2 and
adsorbed CH3 radical is the most favorable reaction route [13].
In this work we report formation of methyl radical by activating C-H bond of methane
and its direct reaction with CO2 by the insertion as a whole molecule on newly
formulated catalysts based on ZrO2/TiO2 oxides.
Experiment
The catalyst was synthesized from zirconium nitrate oxide ZrO(NO3)2, titanium
chloride (TiCl4) and titanium (IV) oxide by the wet impregnation and co-precipitation
methods. The catalysts were calcined at 700 oC in static air. The texture properties of
the catalysts were analyzed by nitrogen adsorption at 77 K. In addition, powder X-ray
diffraction and NH3-TPD were utilized to determine catalyst characteristics. High
purity CO2 and methane were used in the study. The catalyst activity was tested by
using a fixed bed reactor equipped with quadruple mass spectrometer (QMS).
Discussion
The catalysts are based on using Zr/Ti oxide prepared by impregnation and coprecipitation methods. These catalysts are characterized by various spectroscopic
techniques and these studies indicated semi-round shapes by SEM method and with
nano particles in the range of <100 nm (as showed in Figure 1). The X-ray diffraction
pattern of Zr/Ti oxide showed presence of anatase phase of TiO2 and tetragonal with
monoclinic of ZrO2. The impregnation catalysts showed the N2 adsorption of pore
volume distribution was wider than narrow pore volume distribution of coprecipitation catalysts. The NH3-TPD was showed wide acidic catalytic surface and
was correlated to the reaction activities.
Proceedings 24th Saudi Japan Annual Symposium
Catalysts in Petroleum Refining & Petrochemicals
KFUPM Dhahran, Saudi Arabia Dec. 1-2, 2014
Figure 1. SEM image of 5% Zr/Ti oxide
product, ppm
Several studies discussed the unique composition of mixed oxide catalysts formed
between ZrO2 and TiO2[14]. The catalysts are reported to have better catalytic activity
for several CO2 reactions due to the acidic and basic properties of the Zr–O–Ti
catalytic system compared to the single oxides, TiO2 or ZrO2[14][15][16]. In this
study the catalytic activities of the ZrO2/TiO2 catalyst for methane coupling to form
ethane (no CO2) is presented in Figure 2 and equation 1 which was also reported by
other groups ae well [18],[19],[20]. This reaction requires C-H bond activation to
form methyl (CH3(ads)) on the catalyst surface. The fact that these catalysts can
promote radical formation may be relevant to reactions that involve CO2.
8000
7000
6000
5000
4000
3000
2000
1000
0
Ethane
600
650
700
Temperature oC
750
Figure 2. C-H bond activation of methane to formulate ethane without CO2 over Zr/Ti
oxide catalyst
4CH4  CH3-CH3 + CH2=CH2 +3H2
(eq. 1) [19]
Proceedings 24th Saudi Japan Annual Symposium
Catalysts in Petroleum Refining & Petrochemicals
KFUPM Dhahran, Saudi Arabia Dec. 1-2, 2014
By introducing the CO2 to methane, at 590 oC resulted in the formation of ethane and
ethylene combined formulated from activation of methane by generating methyl and
methylene radicals which was not detected without CO2 as shown in Figure 3. In
addition, the acetic acid was synthesized at same temperature of C-H bond activation.
The study showed the consumption of H2 generated from C-H bond breaking at same
temperature of acetic acid formed which is an indication of hydrogenation of methyl
radical with CO2. The selectivity of acetic acid was about 2.5 – 19% but low yield.
The investigation extended to study the reaction mechanism and improve the catalytic
reactivity for higher selectivity and yield. The tests showed a selectivity of 16% with
9% yield for acidic acid production.
6000
product, ppm
5000
4000
3000
Ethane
2000
Ethylene
Acetic acid
1000
0
500
590
625 650 700
Temperature oC
750
Figure 3. C-H bond activation of methane and direct reaction with CO2 to formulate
carbolic acid over Zr/Ti oxide catalyst
Conclusions
The C-H bond activation shows differences between the catalysts activity of Zr/Ti
oxide prepared by the impregnation and co-precipitation methods. The selectivity of
acetic acid synthesis is improved by gas ratio of CO2/methane. The study highlights
the important of radical mechanism and C-H bond breaking from methane prior direct
route reaction with CO2.
Acknowledgements
The authors would like to thank Saudi Aramco and R&DC management for their
support and permission to present the paper. The authors also thank Hameed Badairy,
Husinsyah Sitepu, Ahmed Asseel, Ahmed Almuhaimeed and Abed Harthi of R&D
Center for their analytical support.
Proceedings 24th Saudi Japan Annual Symposium
Catalysts in Petroleum Refining & Petrochemicals
KFUPM Dhahran, Saudi Arabia Dec. 1-2, 2014
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