In
this paper, the performance of cobalt-based catalysts was reported
for oxidation of glycerol to dicarboxylic acids such as tartronic
and oxalic acids. Cobalt catalysts supported on Mg3Al(OH)
y(CO3)
z
structures prepared by a two-step modified sol–gel method
showed 100% glycerol conversion with 64% and 24% selectivity toward
tartronic and oxalic acids under mild conditions (55–70 °C
and 0.1 MPa O2). Surface and bulk characterization by N2 adsorption/desorption, X-ray diffraction, and temperature-programmed
reduction reveals that the cobalt sites interacting with surface hydroxides
are catalytically more active than those incorporated into a framework
leading to selective glycerol oxidation to dicarboxylic acids in one
pot. On the basis of the experiments at different cobalt contents,
temperatures, and concentration–time profiles, possible reaction
pathways are discussed to explain the selectivity profile. Deactivation
of the catalyst under certain conditions has been discussed as a result
of loss of surface area due to structural changes.
Transesterification
of cyclic carbonates to dimethyl carbonate
using metal oxide (CaO, BaO and SrO) catalysts is reported with the
objective of understanding the pretreatment effect of methanol and
cyclic carbonates on catalytic performance. Stirred batch reactor
experiments reveal that with untreated CaO as catalyst, significant
induction time was observed. The induction time was eliminated upon
CaO pretreatment with methanol and the transesterification activity
increased from 11 to 947 h–1. In contrast, pretreatment
with PC resulted in a prolonged induction time and rate inhibition.
Further, although the methanol pretreatment effects were found to
be irreversible, those with PC were reversible upon methanol treatment.
Pretreatment of CaO with other cyclic carbonates including ethylene
carbonate (EC) and 1,2-butylene carbonate (BC) showed similar transesterification
trends as PC. Based on these experimental results and complementary
catalyst characterization results using SEM, CO2-TPD, XRD,
FT-IR, XANES and 13C NMR, a possible reaction mechanism
that involves methoxy species as the key intermediate is proposed.
In the last, recycle experiments were carried out verified that the
catalyst is stable during successive cycles of substrate addition.
These results provide new fundamental insights into transesterification
catalysis and guidance for rational catalyst design and activation.
A highly
active and selective heterogeneous catalyst consisting
of Fe–Mn double metal cyanide is reported for the transesterification
of cyclic carbonates with methanol. Fe–Mn double metal cyanide
complex with a Fe/Mn metal ratio of 8 was found to provide significantly
higher TOF and stability for transesterification of propylene carbonate
with methanol to dimethyl carbonate. The experimental results showed
that Mn content has a significant influence on the catalytic activity.
TEM and XRD analyses suggested that Fe–Mn complex represents
a cubic crystalline structure. XPS analysis showed that all Fe exists
in Fe2+ state. However, for the Mn element, 86.8% of Mn
exists in Mn2+ state with only 13.2% in Mn4+ state. Furthermore, FTIR and DRIFT UV–vis results verified
the formation of a new mixed-metal complex of ferrocyanide moiety
and Mn ions via bridging cyanide ligands. NH3-TPD results
showed that Fe–Mn double metal cyanide has a strong acidity,
which correlates to the high activity of Fe–Mn double metal
cyanide complex. The effects of catalyst loading, methanol/PC ratio,
temperature, and different cyclic carbonate substrates (ethylene carbonate,
propylene carbonate, and 1,2-butylene carbonate) on the initial reaction
rate as well as concentration–time profiles are reported. Unique
feature of this catalyst is also nonleaching characteristics during
reaction and higher TOF unlike the conventional catalysts such as
CaO. Compared with the results from Fe and Mn alone as catalysts,
Mn in this complex was proposed to be acting as active species, while
Fe acting as a metal-dispersing agent and a stabilizer of the cyano-bridged
complex and ensures a truly heterogeneous catalyst. On the basis of
this, a possible reaction mechanism was proposed. The results presented
in this work provide useful insights on the reaction mechanism of
transesterification using Fe–Mn double metal cyanide catalysts,
which may also be useful to guide rational design of improved catalysts
and process for transesterification of cyclic carbonates.
Conversion of glycerol to acrylic acid represents an emerging application in transforming waste biomass to valuable products in chemical industry. However, achieving almost perfect atomic efficiency and intrinsic zero-waste generation...
A kinetic model involving the activation sequence of reactants PC, methanol and an intermediate provides the best description of the experimental data with respect to reaction parameters over a wide range of conditions.
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