The Catalyst Review July 2019 - 14

EXPERIMENTAL
A Polyoxometalate-Based Microfluidic Device for Liquid-Phase Oxidation of Glycerol...
Liquid-phase oxidation is an vital technique for the conversion of petroleum or biomass-based materials into higher value
chemicals as well as being important for the treatment of environmental pollutants. Polyoxometalates (POMs), which are multimetal-oxide cluster anions of W, Mo, or V in their highest oxidation states, are well-established as key catalysts for the selective
oxidation of organic substrates by O2, H2O2, or organic peroxides. However,
Scheme 1. Preparation of HPMo-MR(2) based on electrostatic
there are problems associated with this approach, including selectivity
assembly.
(e.g., overoxidation and competing low-selectivity autoxidation sidereactions), safety, and catalyst stability, leaching, and recovery. In addition,
green heterogeneous catalytic reactions in conventional batch or semibatch mode frequently have mass-transfer challenges involving multiple
phases (substrates, oxidants, and solid catalysts. Microfluidic systems or
continuous flow microreactors (MRs), on the other hand, are frequently more
environmentally benign, have higher mass- and heat-transfer efficiencies,
exhibit higher surface/volume ratios, and generate higher outputs at higher
levels of safety. Herein the authors describe a microfluidic catalytic system
POM-MR (Scheme 1) based on POMs for organic cascade reactions. The focus
of this study involves the H2O2 oxidation of glycerol-a highly efficient and
highly selective pathway for obtaining the desired product, lactic acid (LA).
Initially, the influence of the molar ratio of H2O2 to glycerol was evaluated
(Table 1) at values of 2:1, 4:1, 6:1, and 8:1. As expected, increasing the
proportion of H2O2 enhances the conversion with a maximum value of 77%
in the case of H3PMo12O40 (designated as HPMo-MR(2)) at the molar ratio
of 4:1. The maximum yield of LA (54%) is obtained at this molar ratio and
decreases at higher peroxide concentrations owing to overoxidation. The
products of overoxidation include acetic acid, formic acid, and CO2. The
performance (yield and rate) of HPMo producing LA increases dramatically
ongoing from batch mode to microreactor mode, whereby TOFLA (molar
yield of LA/mol X reaction time) increases from 200 h-1 to 20000 h-1. HPMoMR(2) was reused several times with very little loss in activity. Conversion
and selectivity decreased from 98% and 95% to 93% and 86%, respectively.
After each catalytic run, the catalytic MR was not treated before reuse. The
loss in activity is likely attributable to the adsorption of products during the
peroxidation reaction.

Table 1. Optimization of the peroxidation of glycerol under the
catalysis of HPMo-MR.[a]

The POM microfluidic reactor described by the authors is readily prepared, scalable, highly stable, reusable, and potentially
applicable to selective oxidation of other bio-wastes. Source: Tao M, Li Y, Zhang X, et al. (2019). ChemSusChem, DOI: 10.1002/
cssc.201901057.
Active Edge-Site-Rich Carbon Nanocatalysts with Enhanced Electron Transfer for Efficient
Electrochemical Hydrogen Peroxide Production...
Hydrogen peroxide is typically produced via the anthraquinone process, which is highly energy intensive and which produces
significant amounts of organic wastes. In addition, the use of precious-metal catalysts and high-pressure H2 in the hydrogenation
step results in significant production costs. Alternative production technologies have focused on electrochemical H2O2 generation
via the oxygen reduction reaction (ORR)-making use of precious-metal-based catalysts comprising engineered active sites, such
as bimetallic Pt-Hg and Pd-Hg nanoparticle and atomically dispersed Pt. As low-cost alternative electrocatalysts, metal-free carbon
materials have been recently received increasing attention, and graphitic carbon edges have been demonstrated to exhibit much
superior (electro)catalytic properties than those of basal carbons owing to the modified local electronic structure. Herein, the
authors demonstrated that a graphitic carbon nano-catalyst with exposed abundant edge sites could catalyze H2O2 production with a
circa 28 times higher mass activity than that of a basal plane-rich CNT.
Catalyst preparation consisted of the infiltration of an aromatic carbon precursor into a mesoporous silica with a hexagonal
mesostructure, graphitization, and removal of the silica template. The mesopores in the silica template acted as nanoreactors within
which confined graphitization generated nano-sized graphite, whereas its silica frameworks transformed into mesopores after the
removal of the template. The resulting graphitic ordered mesoporous carbon (GOMC) showed a CMK-3-like mesostructure with
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The Catalyst Review

July 2019

The Catalyst Review July 2019

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