The
proximity of the oxide-zeolite bifunctional catalysts plays
a crucial role in syngas conversion to light olefins. However, its
underlying mechanism is not well understood and the optimal proximity
is yet to be identified. Herein, we take ZnCrO
x
-SAPO-34 and MnO
x
-SAPO-34 as
examples and show that the reaction benefits from the shortened proximity
with the granules decreasing to the micrometer size due to reduced
mass transport limitation. CO conversion reaches 60.0%, light olefin
selectivity 75.5%, and a space time yield of light olefins 0.24
g·gcat
–1·h–1 over ZnCrO
x
-SAPO-34. However, at nanoscale
proximity, an interaction may develop between different active sites
due to the migration of metal species in addition to intermediate
exchange, which could modify their properties significantly. For instance,
zinc species migrate to SAPO-34 and form Zn-OH preferably over Brønsted
acid sites under reaction conditions, which leads to a deteriorating
light olefin selectivity due to enhanced hydrogenation. This can be
alleviated over zeotypes containing less acid sites. By contrast,
MnO
x
does not migrate under reaction conditions,
and the light olefin selectivity exhibits a feature of “the
closer, the better” over MnO
x
-SAPO-34.
These findings are essential for further development of analogous
bifunctional catalysts.
With
the development of NMR methodology and technology during the
past decades, solid-state NMR (ssNMR) has become a particularly important
tool for investigating structure and dynamics at atomic scale in biological
systems, where the recoupling techniques play pivotal roles in modern
high-resolution MAS NMR. In this review, following a brief introduction
on the basic theory of recoupling in ssNMR, we highlight the recent
advances in dipolar and chemical shift anisotropy recoupling methods,
as well as their applications in structural determination and dynamical
characterization at multiple time scales (i.e., fast-, intermediate-,
and slow-motion). The performances of these prevalent recoupling techniques
are compared and discussed in multiple aspects, together with the
representative applications in biomolecules. Given the recent emerging
advances in NMR technology, new challenges for recoupling methodology
development and potential opportunities for biological systems are
also discussed.
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