262334 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.
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 , or acid-basic bifunctional  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 ; similar products could also be formed over these acid sites using methanol as the reactant in a methanol-to-gasoline route . 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 gcat-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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