566
synthesis and other for methanol dehydration. Coprecipitation of Cu and Zn salts
using sodium carbonate was reported to yield active catalysts. Solid acids such as
γ-Al 2 O 3 and HZSM-5 were used as they are or often modified by basic oxides in
order to optimize the acidity and to attain high selectivity for DME [34].
1.4.2 Promoters
Since methanol synthesis is a structural sensitive reaction, many associated problems come along in realistic operating conditions such as agglomeration of the
metal particles, sintering, rapid deactivation, etc. To handle these problems, the role
of promoters comes into existence. Many promoters can be used along with Cu and
ZnO to modify the methanol synthesis component, viz., divalent cations Mg
2+
,
Mn
2+
, and Ca
2+
; trivalent cations Al
3+
, Cr
3+
, La
3+
and Ga
3+
; and tetravalent cations
Ce
4+
and Zr
4+
. Alkali metals such as K and Cs can also be used to promote the catalyst. These promoters can be used to modify the physicochemical characteristics of
the catalyst like acidity, basicity, surface area, uniform dispersion of the active
metal, etc. Also, the synergy between the Cu and ZnO can be enhanced using different promoters.
Out of all these promoters, Al 2 O 3 is the most popular and industrially relevant
component that is used as a promoter for methanol synthesis reaction. It is a trivalent refractory oxide that can induce monovalent cationic defects in the catalytic
entity because of its dissimilar trivalent cationic charge (with Cu
2+
, Zn
2+
). Al 2 O 3
forms a spinel with ZnO known as zinc aluminate (ZnAl 2 O 4 ) which accelerates the
adsorption as well as activation of CO molecule. These defective sites may serve as
the sites for adsorption of the intermediate species produced during the reaction and
can be converted into the product.
1.4.3 Carbon Source
It is still debated intensively whether the carbon of methanol is derived from CO or
CO 2 and what is the reaction mechanism corridor through which the reaction propagates [35–39]. Copper based which are used for methanol synthesis are also active
for the water gas shift (WGS) and reverse water gas shift reactions [35] (RWGS).
CO H O CO H
WGS
+
→
+
(
)
2
2
2
CO H
CO H O
RWGS
2
2
2
+
→
+
(
)
Since both CO and CO 2 are present as reactants as well as products, they created
a perplexed inference about the major carbon source for methanol. At present, it is
acknowledged that methanol is mainly formed through CO 2 hydrogenation at the
typical operating industrial process [40, 41]. However, CO hydrogenation was
K. Tripathi et al.
synthesis and other for methanol dehydration. Coprecipitation of Cu and Zn salts
using sodium carbonate was reported to yield active catalysts. Solid acids such as
γ-Al 2 O 3 and HZSM-5 were used as they are or often modified by basic oxides in
order to optimize the acidity and to attain high selectivity for DME [34].
1.4.2 Promoters
Since methanol synthesis is a structural sensitive reaction, many associated problems come along in realistic operating conditions such as agglomeration of the
metal particles, sintering, rapid deactivation, etc. To handle these problems, the role
of promoters comes into existence. Many promoters can be used along with Cu and
ZnO to modify the methanol synthesis component, viz., divalent cations Mg
2+
,
Mn
2+
, and Ca
2+
; trivalent cations Al
3+
, Cr
3+
, La
3+
and Ga
3+
; and tetravalent cations
Ce
4+
and Zr
4+
. Alkali metals such as K and Cs can also be used to promote the catalyst. These promoters can be used to modify the physicochemical characteristics of
the catalyst like acidity, basicity, surface area, uniform dispersion of the active
metal, etc. Also, the synergy between the Cu and ZnO can be enhanced using different promoters.
Out of all these promoters, Al 2 O 3 is the most popular and industrially relevant
component that is used as a promoter for methanol synthesis reaction. It is a trivalent refractory oxide that can induce monovalent cationic defects in the catalytic
entity because of its dissimilar trivalent cationic charge (with Cu
2+
, Zn
2+
). Al 2 O 3
forms a spinel with ZnO known as zinc aluminate (ZnAl 2 O 4 ) which accelerates the
adsorption as well as activation of CO molecule. These defective sites may serve as
the sites for adsorption of the intermediate species produced during the reaction and
can be converted into the product.
1.4.3 Carbon Source
It is still debated intensively whether the carbon of methanol is derived from CO or
CO 2 and what is the reaction mechanism corridor through which the reaction propagates [35–39]. Copper based which are used for methanol synthesis are also active
for the water gas shift (WGS) and reverse water gas shift reactions [35] (RWGS).
CO H O CO H
WGS
+
→
+
(
)
2
2
2
CO H
CO H O
RWGS
2
2
2
+
→
+
(
)
Since both CO and CO 2 are present as reactants as well as products, they created
a perplexed inference about the major carbon source for methanol. At present, it is
acknowledged that methanol is mainly formed through CO 2 hydrogenation at the
typical operating industrial process [40, 41]. However, CO hydrogenation was
K. Tripathi et al.