The Catalyst Review December 2019 - 16

EXPERIMENTAL
Beginning with a batch synthesis of a silicaScheme 1. Synthesis and immobilization of
Scheme 2. Synthesis and immobilization of
organocatalyst 6b in batch.
supported first-generation MacMillian
organocatalyst 6b under continuous-flow.
organocatalyst 6a (Scheme 1), they set
out to prepare and immobilize the same
organocatalyst (6b) in a continuous-flow
reactor (Scheme 2). This resulted in a 40%
reduction in reaction time (132 hours
versus 52.5 hours), as well as a 1.7 times
improvement in overall yield (18% in
batch versus 32% under continuous flow).
Morphological analysis of the commercial
3-amino-propyl-functionalized silica,
organocatalyst 6a, and organocatalytic 6b
revealed that the immobilization under
continuous-flow produces a material with a
similar morphology to the commercial silica,
while the material resulting from the batch process is very different-namely, that catalyst 6a presents a more irregular distribution
of particles than catalyst 6b-indicating that silica mechanical degradation occurs in the batch process due to magnetic stirring,
altering the matrix properties.
The performances of organocatalysts 6a and 6b were then evaluated by running the enantioselective Diels-Alder reactions with
cyclopentadiene and E-cinnamaldehyde in both batch and continuous-flow. The reaction using organo-catalyst 6a was able to
produce the products in yields from 60% to 70% with enantioselectivities from 84% to 89% for around 10 hours. In contrast, under
the same conditions, the reaction employing organocatalyst 6b was able to produce the products in yields from 73% to 97% for
72 hours with enantioselectivities ranging from 80% to 90% for 26 hours and 48% to 80% for further 45 hours demonstrating the
superiority of continuous processing. Source: de Oliveira PHR, da S. Santos BM, Leão RAC, et al. (2019). ChemCatChem, DOI: 10.1002/
cctc.201901129.
Surface Digging Effect Stabilizes Supported Ni Catalyst for Efficient Methane Oxidation...
Supported metal particles show widespread applications in such
processes as methane oxidation, selective hydrogenation, oxygen
reduction reaction, CO oxidation, and CO2 reduction. However, the
sintering of supported metal species is usually inevitable resulting in
deactivation, thus limiting their industrial use. Herein, the authors
report the discovery of a surface digging effect of supported Ni NPs
on amorphous N-doped carbon, during which the surface-loaded Ni
NPs etches and sinks into the underneath carbon support to prevent
sintering (Figure 1)-a process driven by the strong coordination
interaction between the surface Ni atoms and N-rich defects.
Numerous analytical techniques (TEM, HAADF, EDS mapping,
and XRD) were employed to verify these findings.

Figure 1. (a) Scheme illustrating the proposed formation mechanisms.
(b-e) HAADF images of Ni SAs and the reference materials. (f) HAADF
and the corresponding EDS elemental line scan, (g) AC HAADF-STEM,
(h) the magnified HAADF image from (d) and the corresponding 3D
intensity-profile images, (i) EDS mappings of Ni SAs.

In order to quantify the catalytic activity of the as-obtained Ni catalysts,
the authors carried out direct catalytic conversion of methane (CH4)
to methanol (CH3OH). Conventional catalysts including noble metal
Pd, Pt, Au, Ru, and Rh supported on oxides usually require high
temperatures (100-500 °C) or strong acid medium conditions to break
highly stable C-H bond (104 kcal/mol). Catalytic measurements show
that the as-prepared CNT@PNC/Ni SAs could directly catalyze CH4 to
CH3OH at a low temperature of 50 °C with hydrogen peroxide as the oxidant. Figure 2a exhibits the catalytic yield of the prepared
metal-free carbon (CNT@NC), Ni NPs, and Ni SAs samples with different Ni loading. Density functional theory (DFT) calculations were
then performed to gain insight into the structure-activity relationships resulted from the surface digging effect. Several different
possible defective NiN4-C structures were first constructed (Figure 2c) based on above HAADF and XAFS results. For comparison,
the (defective) N-doped carbon matrix (NC), Ni NPs and pure NiN4 were also considered (Figure 2c). For defective NC, similar to the
pure NC, the second step is still an endothermic progress, indicating defects/digging engineering has little effect on the activity of
16

The Catalyst Review											

December 2019



The Catalyst Review December 2019

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