Boron Nitride- and Carbon-Supported Iridium–Iron Catalysts for Synthesizing Mono-Alcohols from Biomass-Derived Vicinal Diols

Abstract

Hydrogenolysis of 1,2-diols to primary and secondary alcohols was investigated over nonoxide-supported Ir–FeOx catalysts using 1,2-butanediol (1,2-BuD) as a model substrate. Boron nitride (BN) has been found as an effective support for 1,2-BuD hydrogenolysis to secondary alcohol (2-butanol (2-BuOH)), similar to rutile TiO2 in our previous report. The addition of Fe species (Fe/Ir = 0.25 (optimal ratio)) to Ir/BN greatly improved the catalytic activity (23 times; 2-BuOH selectivity: ∼70%; highest yield: 64%). Meanwhile, some other supports including carbon supports gave primary alcohol (1-butanol (1-BuOH)) as the main product, which was less affected by the Fe addition or types of carbon supports. Activated carbon (C) provided the highest activity among the screened carbon-type supports. The 74% yield of 1-BuOH was obtained over Ir–FeOx/C. The selectivity pattern given by Ir–FeOx/BN much depends on its calcination temperature: a low calcination temperature (<673 K) led to the change in the selectivity pattern (∼70% of 2-BuOH to >70% of 1-BuOH). Ir–FeOx/BN was reusable for the 2-BuOH synthesis at least 4 times when the suitable calcination treatment (573 K, 1 h) was adopted for catalyst regeneration. In contrast, the addition of acid was necessary for regenerating the Ir–FeOx/C catalyst. Secondary alcohol with high selectivity (>60%) was achieved in related alcohol hydrogenolysis over Ir–FeOx/BN (Ir = 5 wt %, Fe/Ir = 0.25) such as 1,2-propanediol to 2-propanol, glycerol to 1,2-propanediol, and 1,3-butanediol to 2-BuOH. 2,3-Butanediol was formed in 32% yield from erythritol. Over Ir–FeOx/C or Ir/C, good yields of 1-alcohols were formed from 1,2-alkanediols, while reversible cyclization occurred for erythritol. Characterizations with X-ray diffraction, transmission electron microscopy–energy-dispersive X-ray spectroscopy, temperature-programmed reduction with H2, CO adsorption, diffuse reflectance infrared Fourier transform spectroscopy of adsorbed CO, X-ray absorption near edge structure, and extended X-ray absorption fine structure suggested that Ir–FeOx/C (Ir = 5 wt %, Fe/Ir = 0.25) consists of Ir particles (about 2 nm), highly distributed and isolated FeOx (valence: ∼2+), and residual acid. In Ir–FeOx/BN (Ir = 5 wt %, Fe/Ir = 0.25), Ir–Fe alloy (Fe0/(Fe0 + Ir0) = 20%) with a two-dimensional nanodendrite structure was formed. The production of primary alcohol over C-supported catalysts is via the route of dehydration + hydrogenation, where the residual acid catalyzes dehydration as the rate-determining step. The high selectivity to secondary alcohol over Ir–FeOx/BN was derived from two aspects: removal of acid (HCl) by calcination and generation of the active Ir–Fe alloy phase in the formation of secondary alcohol.