(119c) Synthetic Routes to Chemical Building Blocks from Formaldehyde over Lewis Acidic Molecular Sieves

normal">Synthetic Routes to
Chemical Building Blocks from Formaldehyde over Lewis Acidic Molecular Sieves

Viktor
J. Cybulskis

normal">Biomedical and
Chemical Engineering, Syracuse University, Syracuse, NY 13244 USA

The
vast reserves of unconventional gas contained within shale formations present
an unprecedented opportunity to devise new, synthetic routes for sustainably
transforming light hydrocarbons, such as methane (CH₄), and their
derivatives into a diverse range of suitable energy carriers and high-value
chemical intermediates. Molecular sieves (zeolites, zeotypes, mesostructured
materials) are ideal candidates for this endeavor owing to their unique size
exclusion properties that, when coupled with catalytically active sites, enable
these porous solids to be tailored specifically for reactions of interest. The
use of suitable reactions to chemically probe the effects of active site
confinement, electronic structure, and cooperative interactions within the
surrounding pore environment on product selectivities and reaction rates will
facilitate catalyst development by understanding how these individual
physicochemical properties influence kinetic performance.

Here,
cross-aldol condensation reactions of formaldehyde (CH₂O) – a highly
reactive, yet non-enolizable alkanal derived from CH₄ – with simple,
enolizable substrates, such as glycolaldehyde (GLA), are used to examine how
the molecular sieve topology, pore aperture, active site function, and
solvating properties of intra-crystalline void spaces can preferentially open
new pathways for C-C bond formation. Catalytic testing performed at 120 11.0pt;font-family:Symbol;mso-ascii-font-family:" times new roman>°C
in ethanol over a series of Lewis acidic zeotypes (Sn-, Ti-, Zr-*BEA; Sn-, Ti-MFI; Ge-STW) of varying pore size and acid
strength shows that ethyl lactate (EL) can be produced directly from CH₂O and GLA in modest
yields (13%) after 24 h and that the larger cavities (12-membered ring, MR)
within *BEA zeotypes facilitate EL formation compared to those in the 10-MR channels
of MFI and STW. Results from¹³CH₂O isotopic labeling studies,
indicate that CH₂O undergoes cross-aldol condensation with GLA over
acid-base pairs in Si-O-M ensembles to produce " times new roman>1,3-dihydroxyacetone ( 11.0pt;font-family:" times new roman>DHA),
followed by subsequent esterification to EL. A six-fold enhancement in EL yield when DHA is
used as the starting substrate implies that the initial C-C coupling to form the
triose is the rate-limiting step in this reaction pathway while esterification
to EL is facile.