Chemistry Reference
In-Depth Information
been shown to promote Cannizzaro and Tischenko reactions [1, 2],
Michael, Wittig and Knoevenagel condensations [3, 4], transesterifica-
tion reactions [5-8], double-bond isomerizations [9], self- and cross-
condensation reactions [10-13], Henry reaction [14], alcohol coupling
[15-17], and H
2
transfer reactions [18]. However, the MgO basicity needed
for eciently promoting these reactions depend on the rate-limiting step
requirements. MgO can be synthesized in a variety of presentation
formats, including nanosheets [19], nanowires [20] and nanoparticles
[21], but its catalytic properties depend greatly on the preparation
method. Nevertheless, most of reports on the preparation of magnesia
deal with the effect of the synthesis method and conditions on the
MgO structural and physical properties [22-24]. Very few papers have
attempted to tailor the distribution, density, and strength of surface base
sites of MgO upon synthesis in order to design the catalyst surface to
reaction requirements [25-27]. More insight on the relationship between
the synthesis procedure with the generation and control of MgO surface
base sites is then required to improve the ecient use of this oxide in
catalysis applications.
Detailed characterization of MgO base sites is crucial to establish
correlations between the surface basic properties and the catalyst activity
and selectivity for a given reaction. The most common methods for
characterization of solid basicity are thermal programmed desorption
(TPD) and infrared spectroscopy (IR) of preadsorbed probe molecules,
and the use of test reactions. TPD studies provide information on the
density and strength of base sites while additional insight on the base
site nature is often obtained by IR characterization. Carbon dioxide has
been largely employed as a probe molecule for evaluating the solid
basicity by TPD and IR techniques [28-31] although other acid molecules
such as acetic acid have been also used [32]. On the other hand, the test
reactions most frequently used for characterizing the catalyst acid-base
properties are the decomposition of alcohols, in particular 2-propanol
[33-35], 2-butanol [36, 37] and 2-methyl-3-butyn-2-ol [38-40]. In the case
of 2-propanol, it is generally accepted that 2-propanol dehydration
to propylene occurs on solid acids containing Brønsted acid sites via an
E
1
mechanism while on amphoteric oxides with acid-base pair sites
propylene is obtained through a concerted E
2
mechanism [41]. On strong
basic catalysts, 2-propanol is dehydrogenated to acetone via an E
1cB
anionic mechanism [42]. Thus, the catalyst acid-base properties may be
related to the propylene/acetone selectivity ratio. In contrast, test
reactions have been used only in few cases for characterizing base site
strength distributions on solid bases. For example, in a previous work
[43], we proposed that on alkali-modified MgO catalysts 2-propanol
decomposition to acetone and propylene takes place via an E
1cB
mechanism in two parallel pathways sharing a common 2-propoxy
intermediate; in this mechanism, the intermediate-strength base sites
promote acetone formation, whereas high-strength base sites selectively
yield propylene. Nevertheless, several studies have shown that the use of
test reactions is not sensitive enough to establish a basicity scale of the
catalysts [44].
