575
TMCs are reported as catalyst in hydrogenation and hydro-treatment (oil-derived
feedstock) process. In addition, TMCs act as a suitable support to get high metal
dispersion catalyst. Also (001) surface of TMCs of Ti, Zr, Hf, Nb,Ta, and Mo has
been found appropriate for CO 2 adsorption and activation [97]. The TMCs of Ti and
Mo are reported methanol synthesis via CO 2 hydrogenation. The incorporation of
small amount of Cu and Au over TiC (001) causes charge polarization and makes
CO 2 activation easy and also enhances methanol selectivity. The apparent activation
energy for methanol synthesis decreases to 11.6 kcal/mol on Cu/TiC (001) from
25.4 kcal/mol on Cu (111). The TOFs for methanol production on Cu/TiC (001)
were 170–500 times higher than on Cu (111) [98]. The interfacial sites of Cu and Au
adsorbed atoms to TiC (001) were more active than bare Cu (111) and Au (111)
[99]. The metal-to-carbon ratio in metal carbide catalyst plays a crucial role in
selective hydrogenation of CO 2 to methanol. The CO 2 hydrogenation over β-Mo 2 C
(001) having metal-to-carbon ratio of 2 results to hydrogenation of formate to CH 4
through cleavage of both C–O bond rather than CH 3 OH. Further, the orthorhombic
β-MoC (001) with polar Mo-terminated surface promotes CO 2 activation by breakage of one C–O bond and then leads to CO formation, whereas for C-terminated
non-polar orthorhombic β-MoC (001) surface, CO 2 activation occurs but without
C–O bond cleavage [100]. Geng et al. [101] used protic ionic liquid
(N-butylimidazolium
molybdophosphate
and
N-butylimidazolium
ptoluenesulfonate) as a precursor and synthesized N,P,S co-doped C@nano-Mo 2 C
catalyst by direct carbonization. At 200 °C and 30 bar, CO 2 conversion of 19% and
MeOH selectivity of 85.4% were observed for N,P,S co-doped C@nano-Mo 2 C carburized at 1023 K. Similar synthesis approach was used by Wei et al. [102] for
synthesis of N,S co-doped C@Mo 2 C and reported CO 2 conversion of 16% and
MeOH selectivity of 90.5% with STY of methanol 0.405 g MeOH /g cat /h (30 bar,
220 °C, and 7500 mL/g/h).
3 Reactors
Conventionally, there are mainly two types of reactors available for gas-phase methanol synthesis: adiabatic and isothermal reactors (Fig. 6). Recently, for liquid phase
synthesis of methanol, slurry reactors are coming into role along with overcoming
the problems associated with gas-phase reactors. Adiabatic reactors comprise fixed
bed reactors in series with cooling medium connected in the downstream of each
reactor for removal of heat. Fixed bed reactor systems lead to low installation cost
and high production capacity. Due to no exchange of heat between system and surroundings (adiabatic nature), temperature rises as reaction approaches to equilibrium state, and this high temperature is responsible for low single per pass
conversion. Accordingly, high recycle ratio, more dilution of reagents, and more
amount of catalyst are used to improve the efficiency. Design of isothermal reactors
is similar to the heat exchanger in which water/gas is continuously circulated
through the mantle of the tube to maintain the temperature inside the reaction zone.
Perspectives in Carbon Oxides Conversion to Methanol/Dimethyl Ether: Distinctive…
TMCs are reported as catalyst in hydrogenation and hydro-treatment (oil-derived
feedstock) process. In addition, TMCs act as a suitable support to get high metal
dispersion catalyst. Also (001) surface of TMCs of Ti, Zr, Hf, Nb,Ta, and Mo has
been found appropriate for CO 2 adsorption and activation [97]. The TMCs of Ti and
Mo are reported methanol synthesis via CO 2 hydrogenation. The incorporation of
small amount of Cu and Au over TiC (001) causes charge polarization and makes
CO 2 activation easy and also enhances methanol selectivity. The apparent activation
energy for methanol synthesis decreases to 11.6 kcal/mol on Cu/TiC (001) from
25.4 kcal/mol on Cu (111). The TOFs for methanol production on Cu/TiC (001)
were 170–500 times higher than on Cu (111) [98]. The interfacial sites of Cu and Au
adsorbed atoms to TiC (001) were more active than bare Cu (111) and Au (111)
[99]. The metal-to-carbon ratio in metal carbide catalyst plays a crucial role in
selective hydrogenation of CO 2 to methanol. The CO 2 hydrogenation over β-Mo 2 C
(001) having metal-to-carbon ratio of 2 results to hydrogenation of formate to CH 4
through cleavage of both C–O bond rather than CH 3 OH. Further, the orthorhombic
β-MoC (001) with polar Mo-terminated surface promotes CO 2 activation by breakage of one C–O bond and then leads to CO formation, whereas for C-terminated
non-polar orthorhombic β-MoC (001) surface, CO 2 activation occurs but without
C–O bond cleavage [100]. Geng et al. [101] used protic ionic liquid
(N-butylimidazolium
molybdophosphate
and
N-butylimidazolium
ptoluenesulfonate) as a precursor and synthesized N,P,S co-doped C@nano-Mo 2 C
catalyst by direct carbonization. At 200 °C and 30 bar, CO 2 conversion of 19% and
MeOH selectivity of 85.4% were observed for N,P,S co-doped C@nano-Mo 2 C carburized at 1023 K. Similar synthesis approach was used by Wei et al. [102] for
synthesis of N,S co-doped C@Mo 2 C and reported CO 2 conversion of 16% and
MeOH selectivity of 90.5% with STY of methanol 0.405 g MeOH /g cat /h (30 bar,
220 °C, and 7500 mL/g/h).
3 Reactors
Conventionally, there are mainly two types of reactors available for gas-phase methanol synthesis: adiabatic and isothermal reactors (Fig. 6). Recently, for liquid phase
synthesis of methanol, slurry reactors are coming into role along with overcoming
the problems associated with gas-phase reactors. Adiabatic reactors comprise fixed
bed reactors in series with cooling medium connected in the downstream of each
reactor for removal of heat. Fixed bed reactor systems lead to low installation cost
and high production capacity. Due to no exchange of heat between system and surroundings (adiabatic nature), temperature rises as reaction approaches to equilibrium state, and this high temperature is responsible for low single per pass
conversion. Accordingly, high recycle ratio, more dilution of reagents, and more
amount of catalyst are used to improve the efficiency. Design of isothermal reactors
is similar to the heat exchanger in which water/gas is continuously circulated
through the mantle of the tube to maintain the temperature inside the reaction zone.
Perspectives in Carbon Oxides Conversion to Methanol/Dimethyl Ether: Distinctive…