A Heterogenized Molecular Catalyst for the Gas-Phase Cyclotrimerization of Acetylene to Benzene

TL;DR

This paper presents a new solid catalyst that efficiently converts acetylene to benzene under industrial conditions, offering a sustainable and improved method for benzene synthesis.

Contribution

The study introduces a heterogenized molecular catalyst based on niobium chloride immobilized on silica gel, achieving high selectivity and extended lifetime.

Findings

The NbClx–silica gel catalyst achieves up to 70% benzene selectivity with 30–70% acetylene conversion.

The catalyst maintains ≥90% acetylene conversion for up to 8.5 hours under industrial conditions.

Active catalytic species form in situ through reduction, similar to homogeneous systems.

Abstract

Facilitating the cyclotrimerization of acetylene to benzene
in
the gas-phase on the surface of solid catalysts has captivated researchers,
both theoretically and experimentally, for several decades. Coupled
with acetylene production from renewable feedstocks, this reaction
offers a promising pathway for the direct, low-temperature synthesis
of renewable benzene. Recognizing the physical limitations of typical
solid catalysts, which adsorb benzene too strongly, this study investigates
various high-valent early transition metal chloridespotent
molecular cyclotrimerization catalystsas solid catalysts in
this gas-phase conversion. Alongside catalytic assessment under industrially
relevant conditions, various analytical techniques such as 93Nb solid-state NMR spectroscopy, X-ray emission
spectroscopy, and quasi in situ X-ray photoelectron spectroscopy were
applied to reveal that the catalytic functioning proceeds through
the in situ formation of catalytically active reduced species as reported
for a homogeneous reaction environment. Investigating deactivation
phenomena, various catalyst precursors and reaction conditions led
to the design of an immobilized molecular catalyst via chemical grafting,
dispersion, and spatial isolation of active catalytic species on a
mesoporous silica gel support. The resulting NbCl
x
–silica gel catalyst exhibits an average benzene selectivity
of up to 70% at an acetylene conversion level between 30% and 70%
and a lifetime at ≥90% acetylene conversion of up to 8.5 h
under industrially relevant conditions (10 vol % C2H2, 3 bar, 180 °C), outperforming previously reported solid
catalysts by orders of magnitude.

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