The Catalyst Review May 2019 - 16

EXPERIMENTAL
hydrogenation catalysts. The hydrogenation proceeds through the formation
of formamide intermediates. After hydrogenation, the immobilized amines
were easily filtered and collected to be reused. The catalyst and methanol
were recovered from the filtrate. Covalently-attached (to polymer support
or silica) amine functionalities displayed the highest recycling potential
with almost no leaching under the hydrogenation reaction conditions.
Using polyethyleneimine grafted onto a solid-silica support, the catalyst
and amine were successfully recycled, and CO2 (either pure or from the air)
was efficiently captured and converted to CH3OH over multiple cycles.

Figure 1. SSAs used in this study. The numbers in parenthesis
represents the percentage of organic content.

Two parameters are of major importance to ensure the use of SSAs
through multiple cycles: their ability to assist CH3OH formation (for
high yield) and minimal leaching of amines from the support under
the hydrogenation conditions (desired for repeated use). Eight solidsupported amines were screened as part of three different preparation
methods (Figure 1). All SSAs were able to assist in the hydrogenation
of CO2 to CH3OH with varying degrees of success. The highest amount
of CH3OH was observed with physically-impregnated silica-supported
amines. However, NMR analysis also revealed the presence of some
leached amines in the solution. Generally, lower CH3OH yields were
observed for covalently grafted amines although the filtrate after
these reactions did not show any traces of amine in the solution. In situ
crosslinked-solid-supported amines provided higher methanol yields.
From the above catalyst and amine screening studies, C-1 was identified
as the best hydrogenation catalyst along with covalently-attached SSAs 4, 5,
and 6 as the most suitable candidates owing to the minimal leaching under
hydrogenation conditions (Table 1). Source: Kar S, Goeppert A and Surya
Prakash GK, (2019). ChemSusChem, 12: 1-7.
Selective Production of Aromatics Directly from Carbon Dioxide Hydrogenation...
Efforts to mitigate rising concentrations of CO2 in the
atmosphere have focused on conversion strategies to produce
commodity chemicals that can be used either as fuels or as
precursors in many industrial processes. A strong focus of such
activities has involved direct CO2 hydrogenation to aromatics.
Herein, the authors report the synthesis of a highly effective
composite catalyst containing the Na-modified spinel oxide
ZnFeOx, which provides two types of active sites (Fe3O4 and
Fe5C2) and shows high CO2 conversion and considerable olefin
selectivity, and a hierarchical nanocrystalline HZSM-5 zeolite for
selective conversion of CO2 into aromatics.

Figure 1. Characterizations of ZnFeOx-nNa catalysts with different Na contents.
XRD patterns for (a) calcined and (b) spent ZnFeOx-nNa catalysts. (c) CO2-TPD
spectra. (d) Fe 2p XPS of ZnFeOx-ZnFeOx-4.25Na treated under the reaction
atmosphere (H2/CO2/N2 molar ratio 73/24/3) at 320 °C, 3.0 MPa, and
4000 mL gcat −1 h−1. R-320 °C-50 h (blue line) suggests that the catalyst was
first reduced in H2 at 350 °C for 8 h and then treated under the reaction
atmosphere for 50 h at 320 °C.

ZnFeOx-nNa nanocatalysts (Fe/Zn = 3/1) with different residual
Na contents were prepared by a simple one-pot synthesis
method. The samples with various Na concentrations, which
are denoted by n wt. % with n = 20.00, 5.79, 4.25, 2.68, 0.50
wt. %, were obtained by controlling the amount of water in the
filtration step. As shown in the XRD patterns of calcined samples
(Figure 1a), ZnFeOx-nNa catalysts mainly consist of a cubic
ZnFe2O4 phase, whose crystallinity increases with decreasing
Na contents, suggesting a strong interaction between Fe and Zn
oxides. This catalyst displays high aromatics selectivity with high
CO2 conversion (∼40%), and the combined selectivity of CO and
CH4 is less than 20%. The outstanding yield of aromatics (up to
16

The Catalyst Review

May 2019

The Catalyst Review May 2019

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