(492f) Mechanistic Pathways and Requirements for Alkanal Deoxygenation on Solid Brønsted Acid Catalysts

Light oxygenate fraction from biomass pyrolysis is a valuable precursor for the production of aliphatics and aromatics via catalytic deoxygenation reactions. Deoxygenation involves condensation, ring-closure, alkylation-dealkylation, and H transfer steps, which may occur in sequence or in parallel on acid [1], base [2], or acid-base bifunctional [3] sites grafted within solid microporous or mesoporous structures. Previous studies have reported a strong dependence of product distributions on the type of solid matrixes and reaction conditions, but mechanistic details, catalytic requirements, and rate dependencies for the individual reaction paths have remained as the subject of active debate.   

In this contribution, we apply rate and isotopic analyses, acid site titrations, and temperature programmed techniques to elucidate the primary and secondary reaction pathways for the deoxygenation of alkanals (RCH₂CHO, R=CH₃, C₂H₅, C₃H₇) in the absence of an external H source on solid Brønsted acid sites confined to different extents (H-MFI, H-FAU, H₄SiW₁₂O₄₀/SiO₂). Catalytic deoxygenation of alkanals involves the (1) initial inter-molecular C=C bond formation via bi-molecular, aldol type condensation reaction, (2) initial intra-molecular C=C bond formation as a result of sequential H addition and dehydration reaction, (3) direct isomerization and dehydration reaction, (4) secondary ring closure step, and at high temperatures (> 573 K), (5) alkylation-dealkylation reactions [4]. These reactions lead to the formation of larger alkenal, alkene containing the same number of carbon atoms with the alkanal reactant, diene, methyl or ethyl substituted benzenes, and diverse aromatic species (C₆-C₁₅).

We report a distinct reactivity trend across the alkanal homologues and with varying degrees of site confinement. The inter-molecular C=C bond formation is bi-molecular in nature; the rate of this reaction on Brønsted acid sites dispersed on unconfined H₄SiW₁₂O₄₀ clusters is more effective for larger than smaller alkanals, because larger alkanals form more stable bi-molecular transition state, according to the prediction from a Born Haber thermochemical cycle constructed using gas phase thermochemical properties. In contrast, the same reactions occur more effective on smaller than larger alkanals on Brønsted acid sites confined within the H-FAU and H-MFI structures, because site confinements destabilize the energy of the bi-molecular transition state. Isomerization-dehydration or inter-molecular H transfer-dehydration steps that evolve the diene and olefin, respectively, are uni-molecular in nature; their relative rates vary with the ability of adsorbed alkanal to accept hydrogen from H donors, which are formed from secondary ring closure of the aldol condensation product and, upon dehydrogenation steps, release H during the inter-molecular H transfer step. As the extent of confinement increases in the order of H₄SiW₁₂O₄₀/SiO₂, H-FAU, H-MFI and the rates of bi-molecular C=C bond formation concomitantly decrease, the total fraction of diene and olefin concomitantly increases. Within the diene and olefin fraction, the selectivities towards olefin increase linearly with increasing H donor pressures, because the inter-molecular H transfer from the ring closure product is required for olefin formation. This requirement of inter-molecular H transfer is confirmed from a linear increase in the olefin formation rates with increasing concentration of aromatics, the products of dehydrogenation, and consistent with the increase in the rates by incorporating tetralin, an efficient H donor, into the feed mixtures.    

The catalytic effects of reactant size and acid site confinement demonstrated here allow the tuning of the relative rates and reaction specificity during alkanal deoxygenation reactions and thus the average molecular size of hydrocarbon and the relative abundances of aliphatics and aromatics in the reaction products.

Acknowledgement

We acknowledge Natural Sciences Council of Canada (NSERC) and Canadian Foundation for Innovation (CFI) for their financial supports. Fan Lin acknowledge Hatch for their support via a Hatch Graduate Scholarship for Sustainable Energy Research.

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.

[4] F. Lin and Y-H. Chin, J of Catal. 311 (2014) 244-256.