DOI: 10.1002/cctc.201100219 Unprecedented Oxygenate Selectivity in Aqueous-Phase Fischer–Tropsch Synthesis by Ruthenium Nanoparticles Xian-Yang Quek, [a] Yejun Guan, [a, b] Rutger A. van Santen, [a] and Emiel J. M. Hensen* [a] Global demand for clean fuel and rapid depletion of available oil reserves has led to renewed interest in Fischer–Tropsch syn- thesis (FTS). In FTS, syngas (a mixture of CO and H 2 ) is convert- ed into clean transportation fuels. Syngas can be obtained from fossil resources such as natural gas through steam re- forming or from coal and biorenewable resources through gasification. Several group 8 metals (Co, Fe, Ru, Rh) are useful Fischer–Tropsch catalysts. [1] FTS occurs through polymerization of CH x building blocks generated from syngas by CO dissocia- tion followed by C hydrogenation. The product composition can be described by the Anderson–Schulz–Flory distribution. [2] Besides paraffins that range from liquefied petroleum gas (LPG) to waxes, [3–5] FTS produces relatively small amounts of oxy- genates, such as alcohols, acids, esters, ketones, and aldehydes. [6] Tuning the selectivity of FTS to oxygenates would provide an al- ternative source of these impor- tant intermediate chemicals. [7] Commercially, FTS is performed in the gas or slurry phase. Re- cently, Xiao et al. demonstrated that FTS can be performed in the aqueous phase. [8] These au- thors found gasoline-range hy- drocarbons by using a Ru nano- particle catalyst. Subsequent studies reported predominantly hydrocarbons and a maximum of 25 % oxygenates for liquid-phase FTS by using Co [9] and Fe [9b] colloids. In industry, Fe and Co are the preferred FTS cata- lysts because of cost reasons. [1b, 10] Ru is the most active FTS catalyst and typically produces high-molecular-weight hydro- carbons without promoters. [11] Its activity strongly depends on the particle size. [8, 12] Herein, we show that Ru nanoparticle catalysts dispersed in water convert syngas at low reaction temperatures with high selectivity into a mixture of oxygenates in the C 5 –C 10 range. An unprecedented aldehyde selectivity up to 70 % is obtained for 2.2 nm Ru nanoparticles at 125 8C. The selectivity strongly de- pends on the reaction temperature. An anomalous chain growth behavior of oxygenates (increasing chain growth prob- ability a with temperature) is reported, which is discussed in terms of the competition between CO dissociation and chain termination. The present data provide strong indications that oxygenates and hydrocarbons are formed on different reaction sites of the Ru nanoparticles. Ru nanoparticles encapsulated in polyvinylpyrrolidone (PVP) with an average size of 2.2  0.3 nm (Figure 1 a) were prepared by reducing RuCl 3 ·n H 2 O with NaBH 4 in water (PVP/Ru ratio = 6). The catalytic performance of these nanoparticles was inves- tigated in the aqueous phase at a total pressure of 30 bar (1 bar = 100 kPa) synthesis gas (H 2 /CO ratio = 2) as a function of the reaction temperature (GC–MS; see Figure S1 in the Sup- porting Information). The turnover frequency (TOF) increases with the reaction temperature (Figure 1 b). The CO conversion is 0.7% at 125 8C and 12.8 % at 185 8C (see Figure S2 in the Supporting Information). The TOF of 0.55 h 1 at 185 8C is simi- lar to values (0.3–1.2 h 1 ) reported for a supported Ru catalyst in a slurry-phase reactor at 200 8C (see Figure S3 in the Sup- porting Information). [13] The product distribution strongly de- pends on temperature and is at strong variance with results of earlier work. [8] The aldehyde selectivity of approximately 70 % at 125 8C is exceptionally high. The alcohol/aldehyde ratio in- creases with temperature. Concomitantly, the total alkane/ alkene selectivity increases at the expense of the oxygenate se- lectivity. The methane selectivity (< 15 %) compares favorably to a supported Rh catalyst for alcohol synthesis used at 230 8C. [14c] A separate experiment was performed with heptanal Figure 1. PVP-stabilized Ru nanoparticles reduced by NaBH 4 : a) electron micrograph and particle size distribution, and b) FTS activity and product distribution (30 bar syngas; H 2 /CO = 2). [a] X.-Y. Quek, Dr. Y. Guan, Prof. Dr. R. A. van Santen, Prof. Dr. E. J. M. Hensen Laboratory of Inorganic Materials Chemistry Schuit Institute of Catalysis Eindhoven University of Technology P.O. Box 513, Eindhoven (The Netherlands) Fax: (+ 31) 40-2455054 E-mail : e.j.m.hensen@tue.nl [b] Dr. Y. Guan Current Address: Catalytic Processes & Materials Faculty of Science and Technology University of Twente P.O. Box 217, Enschede (The Netherlands) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201100219. ChemCatChem 2011, 3, 1735 – 1738  2011 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim 1735