Understanding the Catalytic Ring Opening of Furfural on Iridium
Glen R. Jenness, Ke Xiong, Geun Ho Gu, Dionisios G. Vlachos, Jingguang Chen
Catalytic ring opening (CRO) is an effective route to produce long-chain hydrocarbons from aromatic molecules. Recently, the cleavage of CO bonds in biomass derived furanics (such as furfural and hydroxymethylfuran (HMF)) via CRO has been shown to be a promising route to the production of industrially relevant precursors, such as 1,5-pentanediol (1,5-PeD), that are used in the production of plastics, polyesters, and lubricants. By employing either iridium (Ir) or rhodium (Rh) catalysts, with rhenium oxide (ReOx) as a promoter, moderate to high yields of 1,5-PeD have been reported.1–3 However, there are several unanswered questions, including, but not limited to, the nature of interactions between the metal and furanic molecule, the role of the promoter, the degree of hydrogenation of the furanic prior to ring opening, and how potential changes in the side group affect the overall chemistry. By understanding these problems from a molecular perspective, we can elucidate the key interactions to drive the development of inexpensive catalysts for this process.
To this end, we combine density functional theory (DFT) and microkinetic modeling with experimental surface science techniques (temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS)) to probe the CRO of furfural. Despite DFT predicting the favorable formation of a 1,5-PeD precursor, the iridium catalyst does not form 1,5-PeD under ultra-high vacuum conditions, but rather undergoes reformation into CO and H2. Using a combination of microkinetic modeling and Brønsted–Evans–Polyani (BEP) relationships,4 we are able to show that the majority of the H2 undergoes desorption as the furan ring opens, resulting in the dominant pathway being the CO reforming pathway due to H2 being a limiting reactant. Furthermore, using a combination of DFT and HREELS, we are able to identify several partially hydrogenated furfural species, indicating that partially hydrogenated species may play a key role in the CRO process. Finally, by examining the electronic structure of the furan ring with the various side groups, we identify the key molecular orbital responsible for the stability of the various furanics.
(1) Nakagawa, Y. et al. Catal. Today 2012, 195, 136–143.
(2) Nakagawa, Y. et al. ACS Catal. 2013, 3, 2655–2668.
(3) Chia, M. et al. J. Am. Chem. Soc. 2011, 133, 12675–89.
(4) Wang, S. et al. ACS Catal. 2014, 4, 604–612.
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