Synergistic Contribution of the Acidic Metal Oxide–Metal Couple and Solvent Environment in the Selective Hydrogenolysis of Glycerol: A Combined Experimental and Computational Study Using ReO_x–Ir as the Catalyst

Abstract

Comprehensive mechanistic insights into the aqueous-phase hydrogenolysis of glycerol by the ReO x –Ir catalyst were obtained by combining density functional theory (DFT) calculations with batch reaction experiments and detailed characterization of the catalysts using X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier transform infrared techniques. The role and contribution of the aqueous acidic reaction medium were investigated using NMR relaxometry studies complemented with molecular dynamics and DFT calculations. At higher glycerol concentration, the enhanced competitive interaction of glycerol with the catalyst improved the conversion of glycerol. Sulfuric acid increased the concentration of glycerol within the pores of the catalyst and enhanced the propensity for dissociative adsorption of glycerol on the catalyst, explaining the promotional effect of acid during hydrogenolysis. Partially reduced and dispersed Brønsted acidic ReO x clusters on metallic Ir nanoparticles facilitated dissociative attachment of glycerol and preferential formation of the primary propoxide. The formation of the dominant product, 1,3-propanediol (1,3-PDO), results from the selective removal of the secondary hydroxyl of glycerol, with a comparatively low activation barrier of 123.3 kJ mol–1 in the solid Brønsted acid-catalyzed protonation–dehydration mechanism or 165.2 kJ mol–1 in the direct dehydroxylation mechanism. The formation of 1-propanol (1-PO) is likely to follow a successive dehydroxylation pathway in the early stages of the reaction. Although 1,3-PDO is less reactive than 1,2-propanediol (1,2-PDO), it preferentially adsorbs on the catalyst in a mixture containing glycerol to form 1-PO. The thermodynamically favorable pathway involving dehydrogenation, dehydroxylation, and hydrogenation elementary steps led to the dominant production of 1,2-PDO on pure Ir catalyst with a high C–O bond cleavage barrier of 207.4 kJ mol–1. The optimum ReO x –Ir catalyst with an Ir/Re ratio of 1 exploits the synergy of the sites of both the components. The detailed insights presented here would guide the rational selection of catalysts for the hydrogenolysis of polyols and the optimization of reaction parameters.

Valorization of Biomass‐derived Small Oxygenates: Kinetics, Mechanisms and Site Requirements of H2‐involved Hydrogenation and Deoxygenation Pathways over Heterogeneous Catalysts

“…The nature of the support material (sometimes the modifier oxide itself), the supplemental use of external acids or bases (soluble or solid) in the reaction medium, and the solvent serve as additional means to boost catalytic rates and/or adjust the selectivity pattern further, but their exact functions still await in‐depth molecular level descriptions in the presence or absence of water/solvent. With respect to the role of sulfuric acid (or, effectively, hydronium ions), which is consistently reported to enhance the rate of glycerol hydrogenolysis without significantly changing the product distribution, a most recent study established using NMR relaxometry that the acid primarily facilitates the presence of a higher concentration of glycerol within the silica pores than the bulk solution, explaining, in part, the rate enhancement; more importantly, this work used DFT calculations to show that the hydronium ions enhanced the propensity for the dissociative attachment of glycerol on the active catalyst components forming a primary propoxide (i. e., 2,3‐dihydroxypropoxide, the most stable configuration) but did not provide additional dehydration activities because of the associated high barrier compared to that on Re−OH−Re clusters …”

“…Reaction conditions (e. g., Pt‐WO x catalysts are less active than Ir‐ReO x catalysts and require higher temperatures to achieve comparable rates), the extent of activity promotion by M 2 oxide species, and reactivity patterns widely vary with the nature of M 1 and M 2 and the ratio of the two components . For instance, Ir‐ReO x /SiO 2 offered high selectivity to 1,3‐PDO from glycerol hydrogenolysis, whereas ReO x ‐promoted Rh‐ and Ru‐based catalysts still showed rather low 1,3‐PDO selectivities .…”

“…In general, however, the intriguing effects of M 1 and M 2 identity, their nuclearity, proximity and atom connectivity on the hydrogenolysis activity and bond scission propensity remain outstanding fundamental questions. With the help of computational tools, these aspects are beginning to be understood …”

“…The modifier M 2 is present often in a highly dispersed oxide form, with its surface hydroxylated and its cluster size in the (sub‐)nanometer range. There may be constraints on the size, identity and morphology of the partially reduced metal oxides for optimum performance; large clusters may not be able to capitalize on the synergy of the sites on M 1 and M 2 components, while extremely small clusters may not restrict attachment of glycerol in any configuration (losing selectivity) . The modifier can also form an alloy with the M 1 phase.…”

Recent Advances in Glycerol Catalytic Valorization: A Review

“…Special attention must be paid when oxophilic metals such as MoO x , CrO x , and WO x are incorporated in the support. In general, with a metallic phase in the presence of WO x as a dopant or modifier, the alternative reaction mechanism proposed in Figure 9 is allowed and the kinetic product 1,3-PDO can be found in the reaction media [114,133,[146][147][148][149]. According to Aihara et al [150], a relationship between WO x 's positive effect in a 1,3-PDO yield and the modifier loading can be established as a perimeter interface length dependence in a Pt/WO 3 /Al 2 O 3 catalyst.…”