Introduction
Multiwalled carbon nanotubes (simplified as CNTs in later text) have been drawing increasing attention [1-3] since their discovery [4]. This new form of carbon is structurally close to hollow graphite fiber, except that it has a much higher degree of structural perfection. This kind of nanotube-C possesses several unique features, such as highly graphitized tube-wall, nanosized channel and sp 2 -C-constructed surface. They also display exceptionally high mechanical strength, high thermal/electrical conductivity, medium to high specific surface areas, and excellent performance for adsorption and spillover of hydrogen, which render this kind of nanostructured carbon materials full of promise as a novel catalyst promoter. The catalytic studies conducted so far on carbon nanotube-based systems range from selective hydrogenation [5,6], hydrofomylation of alkenes [7], selective dehydrogenation [8], selective oxidation [9], ammonia synthesis [10], Fischer-Tropsch synthesis [11], to methanol synthesis [12,13] and higher alcohol synthesis (simplified as HAS in later text) [14-17], etc. They have shown promising results in terms of activity and selectivity.
The higher alcohols (C 2+ -alcohols) and dimethyl ether (DME) have been considered as the most important species among coalbased clean synthetic fuels and chemical feedstocks. These C 2+oxygenates have been confirmed to be a better and cleaner automobile fuel with high octane number, and lower emissions of NO x , ozone, CO, and aromatic vapors [18]. Recently, methyl tertbutyl ether (MTBE) as additive of oil-based fuel has been prohibited to be used in some countries or regions due to the new environmental regulation, which has greatly renewed the interest in hydrogenation of syngas to the C 2+ -oxygenates as gasoline blends.
HAS on the Cu-modified Fischer-Tropsch catalysts based on Co, Fe, or Ni has been extensively studied since the late 1970s. A number of pioneer work and excellent reviews [19][20][21][22][23][24] have been published on this subject. Progress in this field has considerably contributed to the growing understanding of the nature of these catalytic reaction systems. Nevertheless, the existing technology of HAS is still on a small scale. The single-pass-conversion of the feedsyngas and selectivity to C 2+ -alcohols are both relatively low. Under the typical reaction conditions, most systems produce methanol (e.g., over alkali-promoted MoS 2 catalysts) or hydrocarbons (e.g., over modified Fischer-Tropsch catalysts) as the main product instead of C 2+ -alcohols [23][24][25][26][27]. Development of catalyst with high efficiency and selectivity for HAS has been one of the key objectives for R&D efforts.