A Carbon Molecular Sieve-based Catalyst with Encapsulated Ruthenium Nanoparticles for Bio-oil Stabilization and Upgrading

Abstract

Pyrolysis oil derived from biomass (bio-oil) is regarded as a potential substitute for petroleum crude for producing environmentally friendly fuels of the future. However, pyrolysis oil upgrading still remains an issue due to its complex composition, low stability, high oxygen and water contents, and low hydrogen-to-carbon ratio. Although hydrogenation was proposed as a promising technology to improve properties of pyrolysis oil, attempts to synthesize a selective and active hydrogenation catalyst have so far been unsuccessful. A major challenge is in obtaining bio-oils with stable composition that can be processed further to fuels in biorefineries. This work proposes a novel design for bio-oil stabilization catalyst with molecular sieve properties. This material consists of ruthenium nanoparticles encapsulated in an ultramicroporous carbon framework much like "berries-in-a-muffin". The hypothesis is that the most reactive bio-oil molecules (aldehydes and ketones below 5 Å that cause oligomerization) will be able to enter the pores and be hydrogenated by the ruthenium catalyst to non-reactive molecules, while other bio-oil components will not be able to access the pores and participate in chemical reactions on active sites. The stabilized bio-oil would then be ready for further hydroprocessing to produce fuels. Multistep synthesis of a carbon molecular sieve containing ruthenium nanoparticles was successfully accomplished. Transmission electron microscopy revealed that metal nanoparticles are less than 3 nm in diameter and uniformly distributed within catalyst pellets. Carbon dioxide adsorption at 273 K coupled with nitrogen adsorption at 77 K indicated that carbon porous structure is made up of ultramicropores with the total pore volume of 0.18 cm3/g and surface area of 646 m2/g. 75% of the pore volume consists of pores less than 8 Å. Adsorption of probe molecules measured by means of a tapered element oscillating microbalance (TEOM) confirmed that the catalyst possesses molecular sieve properties acting as a 5Å-molecular sieve. Slit-like pores of the carbon framework are accessible to bio-oil model compounds with minimum dimensions of 3.4-4.1 Å, such as furfural, acetaldehyde, acetone, and anisole. Water molecules as well as molecules of cyclohexanone and tetrahydrofuran (minimum dimension of 5.3 Å) are unable to adsorb on catalyst pores effectively. Estimated polarizabilities of model compounds confirm that the observed adsorption behavior is explained solely by the molecular sieve effect and does not follow from differences in interaction of the probe molecules with the carbon support. The observed catalyst pore cutoff size of 5 Å is shown to correspond to an estimated molecular size distribution in corn cob-derived bio-oil, allowing desired molecular size selectivity. This work suggests potential applications of a developed molecular sieve-based catalytic system including selective hydrogenation of light aldehydes and ketones involved in bio-oil stability issue, and selective reforming of low molecular weight oxygenates in bio-oil yielding in situ hydrogen.