(10c) Mechanism of Solid Acid Catalyzed C-C Bond Formation and Oxygen Removal From Aldehydes

Mechanism of Solid
Acid Catalyzed C-C Bond Formation and Oxygen Removal from Aldehydes

Fan Lin and Ya-Huei (Cathy) Chin¹*

¹Department of Chemical Engineering and
Applied Chemistry, University of Toronto, Toronto, Canada.

  *cathy.chin@utoronto.ca

Small oxygenates from biomass
conversion could be catalytically transformed to commodity chemicals as
chemical precursors or as aliphatics and aromatics with molecular size suitable
as liquid energy carrier. Aldehydes and ketones with alpha hydrogen undergo
aldol-type condensations in acid and basic medium to lengthen the carbon chain by
forming intermolecular C-C bond and then eliminate the H₂O to eject
the oxygen heteroatom. Mechanism for condensation reaction in the homogenous
phase has been well established: the reaction proceeds via either enol or
enolate intermediates for acid or base catalyzed pathway, respectively. Similar
reactions also occur over solid surfaces with acid [1,2], basic [3], or acid-basic
bifunctional [4] sites over a wide temperature range in both the liquid and
vapor phases. Although the reaction mechanism on solid sites has also been
proposed and examined extensively, details in rate dependencies and site
requirements have not been rigorously established. On Brønsted-acid sites
contained within well-defined microporous MFI network, aldehyde reactions at
moderate temperatures (473-673 K) lead to sequential intra-molecular C-C bond
formation and, upon further dehydration, selectively form aromatic species [1]; similar
products could also be formed over these acid sites using methanol as the
reactant in a methanol-to-gasoline route [5]. This contribution provides a
kinetic analysis on the relative rates for the various paths that lead to
primary condensation products, aromatics, and to undesired formation of small
hydrocarbons and establishes the rate dependencies on the reactions of small
aldehydes (C2-C6) over H-MFI zeolites. Taking this further, we will also provide
a mechanistic synergy and compare the identity of the kinetically-relevant steps
and their kinetic parameters between the heterogeneous and homogeneous reactions.

Figure 1. (a)
Reaction network of propanal on H-MFI zeolites (Si/Al=11.5) (r₁-r₄:
rates at 473K, mol of C s^-1 g_cat^-1); First-order
rate constants of propanal and butanal condensation reactions over H-MFI
zeolites (Si/Al=11.5) at 473 K. (b) Temperature dependence of the rates of formation
for primary condensation product (2-methyl-2-pentenal; ■), 2,4-dimethyl-2,4-heptadienal (◇), 2,3,4,5-tetramethyl-2-cyclopentenal
(●), aromatics (▲), and light alkanes and alkenes C1-C3 (×) during propanal
reactions over H-MFI (Si/Al=11.5) (space time 1.25 h, propanal pressure 1.1 kPa).
First-order rate constants for propanal reaction (○) over H-MFI (Si/Al=11.5).

The proposed reaction network for propanal reactions
over H-MFI zeolites (Si/Al=11.5) is shown in Figure 1a; identity of the primary and
secondary products were confirmed from rate measurements by varying the
residence times. At low residence times, propanal reactions form almost
exclusively 2-methyl-2-pentenal (473 K) as the primary condensation product. Extrapolation
of the yields of light alkanes (C1-C3) and aromatics to zero residence time
gives an initial slope of zero, indicating that these products are formed from secondary
reactions, which involve sequential condensation of 2-methyl-2-pentenal with
propanal, followed by dehydration, isomerization, and dealkylation steps. Temperature
dependencies of the rates of formation for the primary condensation product (2-methyl-2-pentenal),
2,4-dimethyl-2,4-heptadienal, 2,3,4,5-tetramethyl-2-cyclopentenal, light alkanes and
alkenes (C1-C3), and aromatics are provided in Figure 1b. The rate ratios of
the secondary (r₂-r₄) reactions to the initial C-C bond
formation (r₁) determine the product distributions. Lower
temperatures lead predominantly to 2-methyl-2-pentenal, because sequential
intra-molecular C-C bond formation that forms the aromatics occurs at rates
much slower than the initial inter-molecular C-C bond formation. As the
temperature increases, the rate ratios increase because secondary reactions
occur more effectively relative to the primary reaction. Such increase reflects
that the intra-molecular condensation and dehydration steps proceed via
pathways with higher effective activation barriers than the initial
inter-molecular C-C bond formation step. Condensation reactions of larger
alkanals (e.g. self condensation of butanal) occur at higher rates on H-MFI, as
also observed for homogeneous system. We illustrated here the relative rates
for inter- and intra-molecular C-C bond formation and dehydration step,
catalyzed by Brønsted acid sites contained within MFI zeolites to produce
selectively the primary condensation products at low temperatures but form
predominantly oxygen deficient aromatics via the rapid intra-molecular C-C bond
formation and H₂O removal.

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