(350b) Mechanistic Requirements for the Removal of Oxygen Heteroatoms and Formation of Intra- and Inter-Molecular C=C Bond in Biomass Derived Oxygenates

Mechanistic Requirements for the Removal of Oxygen Heteroatoms
and Formation of Intra- and Inter-Molecular C=C Bond in Biomass Derived
Oxygenates

Fan Lin and Ya-Huei (Cathy)
Chin*

Department of Chemical Engineering and Applied
Chemistry, University of Toronto, Toronto,
Canada

  *cathy.chin@utoronto.ca

Small alkanals
from the effluent of biomass pyrolysis are valuable
precursors for the production of aliphatics and
aromatics as drop-in liquid fuels. Such catalytic transformations may occur
over acid [1], base [2], or acid-base bifunctional [3]
sites protected within solid matrixes to remove oxygen heteroatoms
and lengthen the carbon chain by creation of inter- and intra-molecular C=C
bonds, leading to diverse alkenes and aromatics previously undetected in
homogeneous reactions. The kinetic phenomena have been reported and widely
investigated, but details of catalytic pathways, rate dependencies, site
structures, and the factors that govern the reaction specificities have yet to
be established.

We report the catalytic
pathways and requirements for the initial C=C bond formation of alkanal (RCH₂CHO, R=CH₃, C₂H₅,
C₃H₇) in two competing routes of uni-molecular
deoxygenation and bi-molecular condensation reactions
catalyzed by Brønsted acid sites confined in microporous or mesoporous solid structures to different
extents (H-MFI, H-Y, H-MOR, WO_x/ZrO₂, SO₄/ZrO₂,
and H₄SiW₁₂O₄₀/SiO₂) without the use
of external hydrogen sources based on rate and isotopic assessments, transient
studies, acid site titrations, and temperature programmed techniques. Alkanal reactions on Brønsted acid sites form intra- or inter-molecular C=C bonds via
competitive unimolecular or bimolecular routes to
evolve alkenes or larger unsaturated alkenals, respectively, as
the primary products. These primary products undergo sequential reactions to
produce diverse oxygenates, aromatics (C₆-C₁₅), and
olefins (C₂-C₉).
Here, we establish the rate and selectivity trends across the family of alkanal homologues and solid matrixes with different
pore/cage dimensions used for containing the acid sites. Brønsted
acid sites confined within medium pore microporous
crystalline structures (H-MFI) catalyze the uni-molecular
deoxygenation reaction much more effectively than
sites confined within larger pore structures. In contrast, unconfined Brønsted acid sites (WO_x/ZrO₂,
SO₄/ZrO₂, H₄SiW₁₂O₄₀/SiO₂)
catalyze bi-molecular condensation reactions much more efficiently than the
confined structures. Within a specific solid structure, larger alkanals prefer to undergo the uni-molecular
deoxygenation reaction rather than bi-molecular
condensation reaction and thus selectively form alkenes over larger alkenals and aromatics.

The reactivity trends across
the various solid structures and reactant dimensions are interpreted using a thermochemical cycle formalism that connects the catalytic
rates to the activation enthalpy and entropy required to evolve the transition
states of the respective kinetically-relevant steps from the gas phase alkanal reactants. The inter-molecular C=C bond formation
involves larger and late aldol-like transition state,
and therefore the activation enthalpy reflects the combination of deprotonation enthalpy of the Brønsted
acid sites,
heat of alkanal condensation reaction, protonation enthalpy of the aldol
intermediate, and ion-pair interaction energy between the transition state
complex and the zeolitic pores. As the molecular dimension of alkanal
increases from propanal to pentanal,
the heat of condensation reaction remains essentially constant, while the protonation enthalpy of the aldol
intermediate increases by 30 kJ·mol^-1.
These thermochemical data indicate that the decrease
in the inter-molecular C=C bond formation rate with increasing alkanal size in the confined structures (e.g. H-MFI) must
be caused by the increase in steric constraint within
the pores, as required for the formation of the larger, bimolecular typed transition
state, with increasing reactant sizes. In contrast, the uni-molecular
deoxygenation reaction involves hydrogen transfer and
the formation of smaller, cationic transition state. The smaller transition
state ?senses? the confinements provided by the zeolitic
pores to a lesser extent than the larger, bimolecular transition state and, as
a result, the rates for intra-molecular C=C bond formation vary much less
sensitively with increasing molecular dimension.

The effects of transition
state confinement demonstrated here influence the relative rates and reaction
specificity during alkanal deoxygenation
reactions, and the connections between the extents of site and transition state
confinements established here allow for the tuning of reaction pathways to attain
specificity for bi-molecular condensation or uni-molecular
deoxygenation reactions, required for the selective formation
of aromatics and olefins via kinetic coupling of these reactions to the
alkylation steps.

References

[1] T.Q. Hoang, X. Zhu, T. Sooknoi, D. E. Resasco, R. G. Mallinson, J of Catal. 271 (2010) 201?208.

[2] K. K. Rao,
M. Gravelle, J. S. Valente, F. Figueras, J of Catal. 173 (1998) 115?121.

[3] M. J. Climent, A. Corma, H. Garcia, R. Cuil-Lopez, S. Iborra, V. Fornes,
J of Catal. 197 (2001) 385?393.