The Catalyst Review June 2018 - 15

EXPERIMENTAL
Enhancement of Molybdenum/ZSM-5 Catalysts in Methane Aromatization by the Addition of Iron
Promoters and by Reduction/Carburization Pretreatment...
Efforts to achieve an economically viable process for converting methane into higher hydrocarbons have involved two distinct
approaches: indirect and direct methods. Indirect conversion involves two steps: conversion of methane into syngas via reforming or
by partial oxidation followed by use of the Fischer-Tropsch (F-T) process or methanol-to-hydrocarbons (MTH) catalysis. Although the
indirect route is operated on an industrial scale today, the cost and energy consumption are very high. Direct conversion is a one-step
reaction that converts methane directly into added-value products via oxidative coupling of methane (OCM) or dehydroaromatization
to aromatics. However, the presence of oxygen can further oxidize the hydrocarbons to CO2 and H2O decreasing the selectivity of
the hydrocarbons at high methane conversions. Herein, the authors make use of the promotional effects of Fe on molybdenum
oxides supported on HZSM-5 zeolite in the direct conversion of methane to aromatics under nonoxidative conditions employing
two different pretreatment conditions: under He flow and under CH4/H2 flow (termed "precarburization"). Employing Mo catalysts
that consist of 6 wt.% Mo supported on ZSM-5 they found that if catalysts were pretreated in He, the best catalytic behavior was
observed with 6 wt.% Mo loading. To test the influence of Fe addition, they modified the 6Mo/ZSM-5 catalyst with a 0.2 wt.% The
activity results (Figure 1) indicate that the addition of Fe to 6Mo/ZSM-5 improves the catalytic performance regardless of the type
of pretreatment that the catalysts were subjected to. In order to understand how the presence of Fe affects the reactivity of the
Mo sites, benzene selectivity versus CH4 conversion data was obtained for all catalysts (Figure 2). The results show that regardless of
the type of pretreatment received, the nature of the sites in all three catalysts are different, as different benzene selectivity values
are obtained for similar CH4 conversions. Since carbides are thought to be the active sites for methane aromatization, it is probable
that during precarburization, a higher quantity and better distribution of these phases are created before reaction, which leads to
enhanced catalytic properties. In the precarburized samples, it is observed that the catalyst with 0.2 wt.% Fe loading is the most
stable catalyst, which is consistent with its lower carbon content and higher surface area after reaction compared to the catalysts
with 0 and 1 wt.% Fe. Source: Sridhar A, Rahman M, and Khatib SJ (2018). ChemCatChem, DOI: 10.1002/cctc.201800002.
Figure 1. Benzene yield for catalysts 6Mo/ZSM-5, 6Mo-0.2Fe/
ZSM-5, and 6Mo-1Fe/ZSM-5. Open symbols correspond to
He-pretreated catalysts and closed symbols correspond to
precarburized catalysts.

Figure 2. Methane conversion versus benzene selectivity plot for
He-pre-treated and precarburized catalysts: 6Mo/ZSM-5, 6Mo0.2Fe/ZSM-5, and 6Mo-1Fe/ZSM-5. Open symbols correspond
to He-pretreated catalysts and closed symbols correspond to
precarburized catalysts.

Photocatalytic Hydrogen Production Coupled with Selective Benzylamine Oxidation over MOF
Composites...
Photocatalytic water splitting requires separation of the mixed H2 and O2 products and is often hampered by the sluggish O2producing half-reaction. To achieve efficient photocatalytic H2 production, most reports on water splitting have focused on this halfreaction by consuming holes with diverse sacrificial agents, such as CH3OH, triethanolamine, and triethylamine. On the other hand,
as an undesired by-product, O2 is a good electron acceptor that suppresses the H2-producing reduction half-reaction and generates
system-damaging reactive oxygen species (ROS) via uncontrolled O2 reduction. Moreover, O2 has limited commercial value and is
associated with costly separation from H2. In this context, replacing the sacrificial agent consumption or O2 production with organic
oxidation would avoid the pollution/waste and enable the synthesis of value-added organic chemicals.
The Catalyst Review

June 2018

The Catalyst Review June 2018

Table of Contents for the Digital Edition of The Catalyst Review June 2018