Jackson S.D., Hargreaves J.S.J. Metal Oxide Catalysis
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In alkaline earth metal oxides, the (100) surface termination plane, which
exposes equal numbers of anions and cations, is prevalent and, as illustrated in
Figure 21.2 , it can be anticipated that an entire family of different co - ordination
sites, of different basicity, can be exhibited. Furthermore, it would be expected that
this would lead to a dependence upon crystallite morphology and/or particle size.
Figure 21.7 The decomposition characteristics of various alkaline earth
metal oxide precursors as a function of temperature (a) SrO
2
, (b) BaO
2
,
(c) MgCO
3
, (d) CaCO
3
, (e) SrCO
3
and (f) BaCO
3
. (Reproduced from [5]
with permission).
Figure 21.8 The evaluation of MgO base site strength as a
function of Mg(OH)
2
decomposition temperature.
(Reproduced from [5] with permission).
21.2 Catalysts and Catalytic Processes 831
832 21 Base Catalysis with Metal Oxides
The morphology of, for example, MgO is known to be a critical function of the
preparative procedure adopted [41, 42] . Figures 21.9 to 21.11 are transmission
electron microscopy ( TEM ) images of MgO samples prepared from a number of
different routes [41] .
Whilst all the samples display (100) - terminated crystallites, decomposition of
the basic carbonate yields higher index mean planes generated by the aggregation
of (100) - terminated microstructures as shown in Figure 21.12 [43] . The high -
resolution TEM ( HRTEM ) image shown in Figure 21.13 displays terraced struc-
tures on a smoke sample recalcined after prolonged storage in air [44] . Further
considerations to be made are that decomposition of Mg(OH)
2
at intermediate
temperatures can lead to the generation of metastable (111) - terminated crystal-
lites as shown in Figure 21.14 [45] , which are believed to be stabilized by surface
Figure 21.9 TEM of MgO prepared by burning magnesium in
air and collecting the ribbon residue. (Reproduced from [41]
with permission).
Figure 21.10 TEM of MgO prepared by thermal
decomposition of Mg(OH)
2
at 800 ° C. (Reproduced from [41]
with permission).
Figure 21.11 TEM of MgO prepared by thermal
decomposition of magnesium hydroxycarbonate.
(Reproduced from [41] with permission).
Figure 21.12 High index mean planes exhibited by MgO
prepared by the decomposition of magnesium basic
carbonate precursor. (Reproduced from [43] with permission).
hydroxyl groups. To our knowledge, the role of morphology and crystallite size
on basicity has been little studied in relation to base catalysis and it would be
anticipated that low co - ordination sites lead to higher base site strengths. However,
caution in making such comparisons is necessary, since some precursors, in the
case of MgO for example, can lead to high impurity levels in the resultant samples
[41] .
With the advent of synthetic procedures leading to the production of highly
dispersed metal oxides [46, 47] , access to a wide range of surface area is now pos-
sible (that is < 3 m
2
g
− 1
to > 300 m
2
g
− 1
for MgO) An alternative approach to the
generation, and stabilization, of high surface area MgO samples is the use of a
support, for example MgO/ γ - Al
2
O
3
[48] .
Addition of alkali metal ions can also be used to modify base strength/defect
structure as required, for example Reference [49] , although this is not always
21.2 Catalysts and Catalytic Processes 833
834 21 Base Catalysis with Metal Oxides
benefi cial in terms of catalytic performance [50] and it can lead to substantial loss
in surface area. The deposition of alkali metals themselves leads to the generation
of “ superbasicity ” , for example References [20, 51] . It has recently been reported
that CaO can be made superbasic by appropriate treatment with ammonium
carbonate [52] .
The reaction of methanol and acetonitrile to form acrylonitrile has been studied
over solid base catalysts derived from MgO [53] :
CH OH CH CN CH CHCN H H O
33 2 22
+→ ++=
Along with acrylonitrile ( ACN ), propionitrile ( PPN ) was also produced. Heating
to 400 ° C in nitrogen activated the catalysts. High surface area MgO produced the
best yields of acrylonitrile and propionitrile but favored the hydrogenated nitrile.
The low surface area MgO was considerably less effective at hydrogenation and
Figure 21.14 MgO (111) polar microfacet. (Reproduced from [45] with permission).
Figure 21.13 TEM of commercial MgO smoke following
calcination after prolonged storage in air. (Reproduced from
[44] with permission).
gave a high ACN : PPN ratio. The main role of the modifi ers (Cr, Mn) was to inhibit
hydrogenation of acrylonitrile to propionitrile; however, the Cr - promoted MgO
showed slightly higher specifi c activity than MgO, suggesting a promotional
effect.
Studies on the use of SrO as a base catalyst are much less common than those
using MgO. However, Mohri and coworkers [54] investigated the use of SrO for
the base - catalyzed isomerization of 1 - butene. The isomerization of alkenes by base
catalysts has been known for many years [55] and usually requires a strong base.
Mohri and coworkers found that activity was only detected after heating SrO in
vacuo to 650 ° C and that a temperature treatment of 1000 ° C was required to achieve
maximum activity. Selective poisoning revealed that both the Sr
2+
and the O
2 −
ions
had discrete roles with the trans - 2 - butene yield associated with the Sr
2+
, while the
cis - 2 - butene yield was associated with the O
2 −
.
21.2.3
Hydrotalcites
Hydrotalcites, or layered double hydroxide s ( LDH ), are a class of material related
to the mineral hydrotalcite which has the formula [Mg
6
Al
2
(OH)
16
]CO
3
· 4H
2
O. They
are layered double hydroxides with the general formula
MM OH A HO
xx
x
m
z
2
1
3
2
2
+
−
+
()
⋅
which have the trivalent and divalent cations contained within the octahedral sites
of positively charged hydroxide sheets, in a structure akin to that of brucite,
Mg(OH)
2
. The anions, A
m
, and water are contained between the layers of sheets
and their intercalation and removal can be monitored by the position of the (003)
refl ection in their powder diffraction patterns [56] . A schematic of the structure is
shown in Figure 21.15 [56] .
Hydrotalcites are of interest because of their ability to function as base catalysts,
for example References [57 – 61] . In a very topical application, IFP have developed
a biofuels process using Al, Zn and Ti mixed oxides [62] . The process is being
commercialized by Axens. Figueras and coworkers [63] ) have studied Mg - Al
Figure 21.15 Schematic of the hydrotalcite structure.
(Reproduced from [56] with permission).
21.2 Catalysts and Catalytic Processes 835
836 21 Base Catalysis with Metal Oxides
hydrotalcites for the transesterifi cation of methyl acetoacetate. The uncalcined
material was as expected inactive; however, calcined and OH - exchanged materials
were active with yields of 55 and 77% respectively. The best catalyst reported was
a tert - BuO - exchanged hydrotalcite which gave 98% yield in a shorter reaction time
and could be recycled. This Mg - Al - O
t
Bu hydrotalcite was also used for a range of
other reactions, which suggested a strong base although no measurement was
made.
In addition, substitution of variable oxidation state metal ions such as Co
2+
,
Fe
3+
and Cu
2+
generates materials with redox catalytic properties, for example
References [57, 64] . Hydrotalcites themselves have been used as basic supports for
metals, leading to bifunctional catalysts. An example of this is provided in the
study of de Jong and coworkers in which palladium – hydrotalcite combinations
were reported to be active for single - stage synthesis of MIBK from acetone and
hydrogen [65] .
Although hydrotalcites are relatively stable (up to circa 500 ° C), they are also of
potential application as precursors of mixed metal oxide catalysts, for example
Reference [66] . Dehydration – rehydration equilibria account for the switching
between hydrotalcites and mixed/supported metal oxides, which is sometimes
termed the “ memory effect ” [67 – 69] . Recent advances have seen attempts to prepare
highly dispersed LDH systems, such as those dispersed within mesoporous carbon
[70] . Owing to widespread interest in their application, hydrotalcite catalysts have
been the subject of a number of reviews, for example References [71 – 75] . Other
layered - based systems have also attracted attention for application in catalysis, for
example Reference [76] .
21.2.4
Rare Earth Oxides
Although comparisons are often drawn between the chemistry of alkaline earth
and rare earth elements, rare earth oxides appear to have been less well studied
than alkaline earth metal oxides as base catalysts. The activity of the rare earth
oxides and preparation variables, with particular attention being paid to activity
in catalytic pyrolysis, has been reviewed elsewhere [77] . Figure 21.5 shows the
differential heats of CO
2
adsorption for some rare earth oxides. Rare earth oxides
are generally tested as sesquioxides (M
2
O
3
), notable exceptions being CeO
2
, PrO
2
and Tb
4
O
7
, and arguably most attention has centred upon La
2
O
3
. The bifunctional
nature of these materials has been described in relation to Ho
2
O
3
[78] where
acid – base properties have been elucidated with a combination of temperature
programmed isopropanol decomposition and FTIR spectroscopic studies of pyri-
dine adsorption. Schuth and coworkers [79] have demonstrated that it is possible
to disperse high loadings of yttria on SBA - 15, which may open up new possibili-
ties for the application of rare earth oxides. Furthermore, the inclusion of rare
earth oxide components in other host matrices, such as yttria - stabilized zirconia,
results in materials with catalytic activity, for example References [80, 81] ,
although this has been ascribed to Lewis acidity. A comprehensive study [82]
detailing the infl uence of preparation route, calcination condition and atmo-
sphere (often neglected) on the basicity and catalytic activity of single rare earth
and alkaline earth oxides as well as mixed rare earth – alkaline earth oxides has
been published recently.
21.2.5
Basic Zeolites
The occlusion of highly dispersed oxides of alkali metals leads to zeolites possess-
ing pronounced base strengths. Whilst basicity can be generated by exchange of
alkali metal ions, for example Reference [83] , zeolites containing occluded highly
dispersed alkali metal oxide clusters, which possess higher base strength, have
attracted most attention, for example References [84 – 87] . The most widely studied
systems have been based upon cesium, where loading is typically achieved by
impregnation with cesium acetate followed by thermal decomposition to yield
cesium oxide clusters. However, variation of the general procedure can modify
the nature of the occluded species. For example, Doskocil and Mankidy [88] have
reported that it is possible to obtain occluded sodium metal within NaX by thermal
decomposition of loaded sodium azide. Sodium oxide clusters were produced via
the oxidation of the sodium clusters or, alternatively, loaded sodium acetate. Zhu
and coworkers [89] have reported that dispersion and decomposition of KNO
3
over
KL zeolite leads to superbasicity. In addition to alkali metal modifi cation, shape -
selective base catalysts generated by the nitridation of zeolites and aluminophos-
phates have been reported by Ernst and coworkers [90] .
21.2.6
Zirconia Superbases
Solid superbases, which have been defi ned as materials with base strengths,
H
−
≥ 26, represent a highly desirable class of solid materials. This is because they
offer the possibility of activation of relatively unreactive species under mild reac-
tion conditions.
Whilst the application of superbases is reasonably well established in organic
synthesis, for example References [91 – 93] , the fi eld of heterogeneous catalytic
superbases is less well developed. However, it is important to acknowledge that,
as stated earlier, Na/NaOH/Al
2
O
3
, which is applied by Sumitomo on an industrial
scale for the isomerization of 5 - vinyl - 2 - norbornene to 5 - ethylidene - 2 - norbornene,
is reported to have a base strength of H
−
> 37. Whilst a number of nitrides, have
been claimed to be superbasic, for example Reference [94] we restrict our attention
to oxide - based examples in this section. In the sections above, mention was made
of the possibility of preparing superbases by alkali metal modifi cation of alkaline
earth oxides. The possibility of the generation of superbase sites via the controlled
thermal decomposition of KNO
3
/ZrO
2
has attracted some attention. Wang and
coworkers [95, 96] reported that sites with a strength H
−
= 27 could be generated
in such systems providing attention was paid to the loadings and thermal
21.2 Catalysts and Catalytic Processes 837
838 21 Base Catalysis with Metal Oxides
decomposition temperatures. Samples were prepared by grinding KNO
3
and ZrO
2
together followed by calcination. Furthermore, catalytic activity for the isomeriza-
tion of cis - but - 2 - ene at 0 ° C was reported and the samples were reported to be coke
resistant in the high - temperature decomposition of isopropanol. It was argued that
vacant sites in the structure of ZrO
2
were crucial to the development of such high
base site strengths and this was used to explain the maximum activity observed at
14 wt% KNO
3
/ZrO
2
. In a study investigating the infl uence of zirconia and potas-
sium precursor compounds upon the activity of potassium - doped zirconia for
gas - phase and liquid - phase reactions by Kemnitz and coworkers [97] , demon-
strated that potassium leaching occurred for liquid - phase reactions. In addition,
their studies indicated that employing hydrous zirconia precursors led to materials
with a broader range of base sites with higher strength. Superbase sites have also
been reported to be exhibited by KOH - modifi ed ZrO
2
used to support Ru in
ammonia decomposition catalysts for hydrogen generation [98] . Related systems,
other than those employing ZrO
2
, have also been identifi ed as superbases, for
example those derived from KNO
3
supported on MgO - modifi ed SBA - 15 [99] .
Yamaguchi and coworkers [100] have reported that decomposition of carbonate
and nitrate precursors, such as CsCO
3
and KNO
3
, over alumina and zirconia sup-
ports can lead to the generation of catalysts with strong basicity. It was reported
that both silica and titania were ineffective as supports and the generation of basic-
ity was argued to be a consequence of the liberation of free oxygen species on
thermal decomposition of the carbonate and nitrate salts (yielding CO
2
and O
2 −
and NO
2
and O
−
respectively).
21.3
Outlook
The study of metal oxide - catalyzed base catalysis is becoming more widespread,
although it still lags far behind acid catalysis. In this chapter, we have outlined
some of the general considerations to be made in the characterization of base
catalysts, have briefl y described some of the common classes of base oxide materi-
als applied as catalysts and have described a selection of the more commercially
relevant reactions. Table 21.5 , which is not intended to be exhaustive, summarizes
some of the other base - catalyzed reactions that have been reported in the more
recent literature. Attention has been restricted to oxides, although other basic
materials such as oxynitrides, for example References [124, 125] , may offer advan-
tages, for example in terms of the ability to tune base site strength through control
of the level of nitrogen incorporation. A survey of the literature demonstrates the
vast scope of base - catalyzed reactions that have been studied with metal oxide
catalysts. However, it is clear that in many cases fundamental studies, such as the
elucidation of structure – activity relationships, are lacking. Ideally, the develop-
ment of such understanding will lead to knowledge facilitating the tailored design
of effective catalysts.
Table 21.5 Some examples of base - catalyzed reactions.
Reaction Catalyst(s) Ref.
Transesterifi cation CaO superbase [52]
Transesterifi cation of soybean oil CaO [101, 102]
Transesterifi cation of rapeseed oil Li/CaO, Na/CaO, K/CaO, Li/MgO [103]
Transesterifi cation of methyl
benzoate and dimethyl terephthalate
with ethylene glycol
Na/NaOH/ γ - Al
2
O
3
and Cs
x
O/ γ - Al
2
O
3
[38]
Transesterifi cation of glyceryl
tributyrate with methanol
MgAl hydrotalcites [56]
Transesterifi cation of ethyl acetate
with methanol
NaOH, KOH, LiOH supported on
mesoporous smectites
[104]
Transesterifi cation of soybean oil
ZrO
2
, ZnO or γ - Al
2
O
3
supported
alkaline earth oxides
[105]
Synthesis of phytosterol esters La
2
O
3
[106]
Self - condensation of propanol MgO [50]
Dimerization of ethanol MgO, CaO, BaO, Na/Al
2
O
3
, K/Al
2
O
3
,
Cs/Al
2
O
3
, Mg/SiO
2
[107]
Isopropanol conversion to MIBK CuMI(MII)O
x
[108]
Cyclohexanol conversion to
cyclohexanone/cyclohexene
CaO/TiO
2
, SrO/TiO
2
, BaO/TiO
2
[109]
Cyanoethylation of methanol BaO, MgO, KOH/Al
2
O
3
, KF/Al
2
O
3
[110]
Cyanoethylation of alcohols MgO, CaO, SrO, BaO, Mg(OH)
2
,
Ca(OH)
2
, Sr(OH)
2
· 8H
2
O,
Ba(OH)
2
· 8H
2
O, La
2
O
3
, KOH/Al
2
O
3
,
KF/Al
2
O
3
[111]
Synthesis of dimethyl carbonate from
propylene carbonate and methanol
MgO, CaO [112]
Synthesis of ethylene carbonate from
urea and ethylene glycol
CaO, La
2
O
3
, MgO, ZnO, ZrO
2
and
Al
2
O
3
[113]
Synthesis of propylene carbonate
from urea and propane - 1,2 - diol
CaO, MgO, ZnO, La
2
O
3
[114] review
21.3 Outlook
839
840 21 Base Catalysis with Metal Oxides
References
Reaction Catalyst(s) Ref.
Synthesis of 1,3 - dialkylureas from
ethylene carbonate and amine
CaO, MgO, ZnO, ZrO
2
, CH
3
ONa [115]
Synthesis of dimethyl carbonate from
propylene oxide, carbon dioxide and
methanol
NaOH, KOH, LiOH supported on
mesoporous smectites
[104]
Propylene – ethylene conversion to
pentenes and heptenes
MgO, K/MgO [116]
Aldol condensation of citral with
acetone
MgO, Li/MgO, Na/MgO, K/MgO,
Cs/MgO
[117]
Self - condensation of acetone MgO, Li/MgO, Na/MgO, K/MgO,
Cs/MgO
[118]
Knoevenagel condensation of
benzaldehyde with ethyl cyanoacetate
NaOH, KOH, LiOH supported on
mesoporous smectites
[104]
Methylation of catchecol Cs
2
O/TiO
2
, Cs
2
O/Al
2
O
3
, Cs
2
O/SiO
2
[119]
Benzaldehyde reduction MgO, CaO, BaO, SrO [120]
Ligand - free Heck reaction Pd/MgO, Pd/CaO, Pd/SrO, Pd/BaO [121]
Naphthenic acid decarboxylation MgO [122]
N - alkylation of aniline Cs
2
O [123]
Table 21.5 Continued
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