Novel Ni-Co-Mo-K Catalysts Supported on Multiwalled Carbon Nanotubes for Higher Alcohols Synthesis

CATEGORY: NANOTECH
Journal of Catalysts, Volume 2013, Article ID 942145, 7 pages, http://dx.doi.org/10.1155/2013/942145
Novel Ni-Co-Mo-K Catalysts Supported on Multiwalled Carbon Nanotubes for Higher Alcohols Synthesis
Venkateswara Rao Surisetty, 1 Janusz Kozinski, 1 and Ajay Kumar Dalai 2
vrsurisetty@gmail.com
1 Faculty of Science & Engineering, York University, 4700 Keele Street, Toronto, ON, Canada M3J 1P3
2 Catalysis and Chemical Reaction Engineering Laboratories, Department of Chemical Engineering, University of Saskatchewan, Saskatoon, SK, Canada S7N 5A9
Abstract
Alkali-promoted Ni-Co-Mo catalysts supported on multiwalled carbon nanotubes (MWCNTs) were prepared using 9wt% K, 4.5 wt% Co, and 15wt% Mo, whereas Ni content was varied from 0 to 6 wt%. The catalysts were extensively characterized and studied for higher alcohols synthesis from synthesis gas. Alkali-promoted trimetallic catalyst with 3 wt% Ni showed the highest total alcohols yield of 0.284 gm/(gm of cat./h), ethanol selectivity of 20%, and higher alcohols selectivity of 32% at 330∘C and 9.0MPa using gas hourly space velocity (GHSV) of 3.8m3 (STP)/kg of catalyst/h and H2 to CO molar ratio of 1.25.
Introduction
Ethanol has been used as an additive for reformulated gasoline as unleaded gasoline has become the standard, and short ether compounds (MTBE, ETBE, etc.) have been banned as gasoline octane continues to improve in North America. The catalytic conversion of syngas to ethanol, and other higher alcohols, is generally recognized as an interesting route for the production of clean fuels and petrochemical feedstocks from coal, natural gas, and hydrocarbon wastes via gasification. The catalysts for higher alcohol synthesis (HAS) are divided into two main groups based on the product distribution. Alkali-doped high-temperature ZnCrO-based and low-temperature Cu-based catalysts produce mainly methanol and higher branched alcohols. Methanol synthesis catalysts modified with Fischer-Tropsch (FT) elements and modified Mo-catalysts are the second group of HAS catalysts. These catalysts yield a series of linear primary alcohols and gaseous hydrocarbons both with Anderson-Schulz-Flory (ASF) carbon number distribution. Comparatively, molybdenum-sulfide-based catalysts showed a high proportion of higher alcohols at lower pressure and high temperature. MoS2-based catalysts can tolerate sulfur and coke-buildup as a result of higher alcohols synthesis. WhenMoS2 is promoted with K2CO3, the same performance of the catalysts is achieved at a significantly lower temperature.
The alkali-promoted MoS2 catalysts promoted with Co showed high activity to alcohols and can also produce alcohols with a variable ratio of methanol to higher alcohols by changing the operating conditions. The CO hydrogenation was studied over K/Co/Mo/A12O3 and K/Co/Mo/SiO2 catalysts and found that all three elements are necessary for higher activity. Hydrocarbons and alcohols were produced in approximately equal amounts over both the catalysts.Copromotiononalkali-modifiedMoS2 catalysts leads to the shrinking ofMoS2 species, while Co existsmainly in the formof Co-Mo-S phase at low Co loading and partly in a Co9S8-like structure at high Co loading. This structural modification leads to the enhanced C1 →C2 homologation step that improves the formation of ethanol as the dominant product.
Ni is known as Fischer-Tropsch element which has strong methanation tendency in CO hydrogenation reaction. The effect of Ni for the production of higher alcohols was studied using La-promoted Ni/K2CO3/MoS2 catalysts. The authors found that Ni enhanced not only the CO hydrogenation activity but also C2+OH selectivity. This enhancement effect might be related to the improvement structure morphology of Ni and unique surficial chemical environment of the alkali-promotedMoS2 catalyst.Moreover Hedrick et al.  concluded that nickel was unique in keeping high activity after sulfur treatment on theGroup VIII metals, which is advantageous for the H2S-contained feed gas. Chung and Pien  concluded that nickel showed an excellent ability for CO insertion, which has an important significance for higher alcohol synthesis.The formation ofNi- Mo-S phase is related to the electron donation fromNi toMo, decreasing theMo-S bond strength to an optimumrange, and thus significantly increasing the activity of the catalyst. In our previous research, we have studied the effects of different loadings of active metals (Mo), alkali (K) promoters, and metal promoters (Co and Rh) on higher alcohols synthesis from synthesis gas using K-promoted monometallic, bimetallic, and trimetallic MoS2 catalysts [8, 13, 18, 19]. The formation of hydrocarbons and methanol was significantly less over theK-promoted trimetallic catalyst compared to that of monometallic and bimetallic catalysts. It is important to prepare K-promoted trimetallic catalysts using inexpensive metals such as Ni, as explained in the literature, to replace Rh for higher alcohols synthesis from synthesis gas. Amultiwalled carbon nanotube(MWCNT) is a newform of carbon material which is drawing special attention as a catalyst support. MWCNTs provide a relatively inert support and high-temperature stability, which is similar to that of activated carbon (AC) support.MWCNTs exhibit well-defined hollow interiors and display exceptionally high mechanical strength, thermal stability, and electrical conductivity. Their unique characteristics, such as appropriate pore-size distribution, and nanosized channels, make them a promising support in CO hydrogenation reactions.
In our previous research, we have compared higher alcohols synthesis from synthesis gas using alkali-promoted trimetallic Co-Rh-Mo-sulfided catalysts that are supported on AC andMWCNTs. Itwas found that total alcohols space time yield (STY) and selectivity were higher on the MWCNTssupported catalyst compared to catalysts supported on activated carbon. These results proved that support pore-size influenced particle size distribution, dispersion, and extent of reduction and plays an important role in diffusing the reactant molecules to the catalytically active centers that are located inside the pores. The microporous structure of activated carbon-supported catalysts caused pore plugging due to the formation of coke and deactivation of the catalyst, which results in transport limitation in the reaction. In the present paper, a series ofMWCNTs-supported Nipromoted catalysts with 9 wt% K, 4.5 wt% Co, and 15wt%Mo are prepared by varying Ni content from 0 to 6wt%, and the catalytic performance for higher alcohols synthesis over these catalysts is investigated at optimum operating conditions.
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