The Catalyst Review - May 2015 - 15

EXPERIMENTal
OH groups to generate redox sites in zeolites. Specifically, mesoporous ZSM-5 materials were synthesized and employed in various
selective oxidation test reactions under microwave irradiation-focusing on the oxidation of benzyl alcohol to benzaldehyde, the
epoxidation of cyclohexene, and the multistep conversion of N,N-dimethyl formamide (DMF) into products of interest.
The textural and surface acid properties of zeolite materials synthesized in this work are summarized in Table 1. After
characterization, they were then employed in various selective oxidation processes under microwave irradiation. The results are
summarized in Table 2.
Table 1. Chemical composition, textural parameters, and surface
properties of hierarchical zeolites used in this work. Microporous
Z40c and Z15c are compared to post-treated and fe-functionalised
materials.[33]

Table 2. Microwave-assisted selective oxidation of benzyl alcohol to
benzaldehyde using hierarchical zeolites.

These results prove the remarkable catalytic activities of mesoporous
Scheme 1. Proposed peroxide species from alOH terminal
Z40c-H and Z15c-H, even after several catalytic cycles, as compared to their
groups involved in selective benzyl alcohol oxidation.
parent microporous zeolites. Benzaldehyde was selectively obtained (90-
99%) with conversions between 35% to 45% of benzyl alcohol under mild
reaction conditions and very short reaction times. A proposed mechanism is
shown in Scheme 1. These catalysts were equally effective in bringing about
the epoxidation of cyclohexene to valuable products (40-60% conversion)
and exhibited an interesting catalytic activity in the dehydration of N,Ndimethylformamide to acetonitrile (25-30% conversion), representing
the first example of transition-metal free catalysts in this reaction. Source:
Ojeda M, Grau-Atienza A, Campos R, et al. (2015). ChemSusChem. online preview,
DOI: 10.1002/cssc.201500124.

Hydrogen Spillover in Encapsulated Metal Catalysts:
New Opportunities for Designing advanced
Hydroprocessing Catalysts...
Efforts to elucidate an understanding of hydrogen spillover − the phenomenon whereby activated
hydrogen atoms migrate from hydrogen-rich metal clusters to the surface of a catalyst support
where H2 dissociation is improbable − have been stymied by the absence of a well-defined
model catalyst. With conventional catalysts, metal clusters are believed to be located on an open
porous support surface allowing for free access of organic reactants, as well as H2, to the metal
surface (Scheme 1a). However, this view makes it difficult to distinguish between reactions at
the metal surface from those taking place at the surface of the support via hydrogen spillover.
Such ambiguity can be avoided by designing model catalysts where metal clusters are selectively
encapsulated in a dense support matrix so that only small H2 (0.29 nm) can access, but larger
organic reactants cannot (Scheme 1b). Herein, the authors provide an overview of recent progress
made with such encapsulated metal catalysts, proving the existence and catalytic functions of
hydrogen spillover and providing new insights into designing advanced hydro-processing catalysts
with enhanced chemoselectivity and catalyst lifetime. Since the scope of this Concept Paper is so
vast, only the authors' conclusions can be presented (below).
The Catalyst Review

Scheme 1. Representations of
the hydrogenation pathways
in a) conventional Pt catalysts
supported on open porous
supports, and b) Pt encapsulated
in dense support matrix that
allows selective diffusion of
hydrogen over organic reactants.

May 2015

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