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.