The Catalyst Review June 2020 - 15

EXPERIMENTAL
A Stable Silanol Triad in the Zeolite Catalyst SSZ-70...
Zeolites, critical for use in acid catalysis and redox
Figure 2. DFT-optimized structures
Figure 1. a) Framework topology of zeolite SSZ-70,
chemistry, are microporous materials comprising
(PBE-D3/def2-SVP) of two cluster models
highlighting in different colors the cages, which
of the silanol triad in SSZ-70, a) cyclic, b)
are interconnected (green) or having silanol groups
a 3D TO4�2 network of tetrahedral framework
open triad; cyan H, red O, gray Si. Bond
(blue), b) framework topology of ITQ-1 with all cages
atoms (T= Si, Al, Ti, B, ...). Their unique
lengths in [Å]. Full cluster models with all
interconnected, c) silanol monad and triad model;
65 T atoms are shown in the Supporting
cyan H, red O. O atoms are omitted in models in (a)
reaction spaces in their micropores control
Information.
and (b).
reaction mechanisms by virtue of adsorption
effects. Although silanol defects contribute to
the hydrophilic properties, thus influencing
adsorption in these zeolite pores, they can
also be an integral part of the reaction center.
The nature of defect sites and their formation
in zeolites has been the subject of numerous
studies, especially with respect to so-called SiOH
tetrad nests, which are thought to arise from the
removal of a T atom. Herein, the authors make
use of double (or higher multi) quantum 1H MAS
NMR experiments (MAS = magic angle spinning)
to measure homonuclear dipolar interactions
within Zeolite SSZ-70 and directly probe the
spatial proximities of silanol groups, enabling
their cluster sizes to be determined within a distance range of typically 5 Å (to a maximum of 8 Å), (Figure 1).
Nests of three silanol groups located on the internal pore surface of calcined zeolite SSZ-70 were rigorously analyzed-revealing
close proximity to the structure-directing agent (N, N'-diisobutyl imidazolium cations), wherein defects are negatively charged
(silanol dyad plus one charged SiO siloxy group) for charge balance. It is inferred that ring strain prevents the condensation of silanol
groups upon calcination and removal of the structure-directing agent (SDA) to avoid energetically unfavorable three-rings. The
experimental finding that the three silanol groups in the nest yield distinct chemical shifts was further addressed via electronicstructure calculations using two structural motifs: a) a cyclic and b) an open triad with one OH-bond bridging another Si-O-Si oxygen
atom across an adjacent 5-ring. The optimized structures are shown in Figure 2. These studies have confirmed the existence of a
silanol defect site nest with 3 SiOH groups in calcined zeolite SSZ-70. This defect site is negatively charged in the as-made material
to balance the organic structure-directing agent. Silanol condensation is inhibited by the high strain that would take place in the
resulting 3-rings. In contrast, silanol condensation is not inhibited upon calcination of all-silica zeolite ITQ-1 or boron removal
from zeolite B-SSZ-70. These findings highlight that the stability and cluster size of silanol nests depends on their local framework
environment, and silanol triads or tetrads are not expected to be stable in general for other zeolites. Source: Schroeder C, MuckLichtenfeld C, Xu L, et al. (2020). Angew. Chem. Int. Ed., doi.org/10.1002/ange.202001364.
CO-Assisted Direct Methane Conversion into C1 and C2 Oxygenates over ZSM-5 Supported
Transition and Platinum Group Metal Catalysts Using Oxygen as an Oxidant...
Fischer-Tropsch synthesis (FTS) of C1 and C2 oxygenate via syngas has limited
utilization outside of large-scale processing due to its high operating temperature
(>1073 K). Alternatively, low-temperature direct partial oxidation of methane using
gaseous oxygen as an oxidant (i.e., CH4+1/2O2 →CH3OH) is a far more desirable
approach for replacing the current energy-intensive and large-scale industrial
process. Herein, the authors describe the CO-assisted methane conversion into C1 and
C2 oxygenates over ZSM-5 supported transition and platinum group metals (i.e., Fe,
Co, Ni, Cu, Ru, Rh, Pd, Ir, and Pt).

Figure 1. Product formation rates [molproductmolmetal-1h-1]
of CH3OH (light blue dot), HCOOH (blue diagonal
line), and CH3COOH (red filled) over ZSM-5 supported
transition and platinum group metal (Fe, Co, Ni, Cu, Ru,
Rh, Pd, Ir, Pt) catalysts (0.2 MPa O2, 0.5 MPa CO, 2.0
MPa CH4, 8 mL H2O, 40 mg catalyst, 423 K, 3 h).

During this study, transition metals including Cu were also tested as reference
catalysts especially since Cu has been well-studied in the CO-free chemical loop
system. All the tested metals demonstrated the formation of C1 and C2 oxygenates
under the presence of CO (Figure 1), and precious metals exhibited higher catalytic
performance than Cu. Whereas the selectivity toward each product changes along
with the metal atoms, the reaction was not specific for the Rh catalyst previously
The Catalyst Review 										

	

June 2020

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http://doi.org/10.1002/ange.202001364

The Catalyst Review June 2020

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