(10 ppm sulfur, 8% aromatics, and 90% alkanes) produced syngas at greater than 98% fuel conversion. Maximum selectivities of H2 and CO observed were 70% and 80%, respectively, at an O/C ratio of 1.4 and 25 ms contact time.
5.5.2.3. Substitution into Oxide Structures One of the challenges for the hexaaluminate structure is the limitation to which metals can be substituted into the lattice. No reports of substituting larger metals like Rh into the hexaa-luminate structure were found. However, Rh and Ru have been successfully substituted into oxides such as perovskites and pyrochlores.
Haynes et al. [104,115,116] reported CPOX testing on n-tetradecane for Rh- and Ru-substituted pyrochlores with a great deal of success. These catalysts demonstrated high performance and stability in the presence of sulfur and aromatics.
Figures 5.37 and 5.38show the performance for the CPOX of n-tetradecane of a Ru- and a Rh-substituted pyrochlore, respectively [104,116]. Both catalysts successfully reform the fuel into H2and CO, even in the presence of sulfur and aromatic species. Further, the Rh-substituted pyrochlore exhibits stable perfor-mance even in the presence of a much higher
sulfur concentration of 1000 ppmw, compared to 50 ppmw for Ru.
gas. Typically, the CPOX of alcohols can be run at lower temperatures than the CPOX of alkanes since they are more easily activated. Ni, Pt, and Rh [68,117e120] have been shown to be good catalysts for CPOX of alcohols, although Cu, Zn, Pd, and Au can be more common [121e126], especially for methanol and ethanol.
The purpose of this section will be to provide some basic insight into the conversion of different alcohols and product distributions, rather than a comparison or detailed analysis of the various catalysts used.
Cao and Hohn [68] examined the CPOX of methanol over Pt/Al2O3. These studies were conducted from 500 C to 700 C and at an O/C ratio of 0.5e2.0. Formate was identified as an important intermediate species and is formed from surface methoxy groups, which come from the dissociative adsorption of meth-anol at the oxygen site by splitting the OeH bond. Indirect formate decomposition was iden-tified to be dominant for CO2production at high temperatures. Important trends in the data were that CO2 production increased as temperature decreased. This is likely due to an increase in the rate of CO desorption preventing it from being oxidized on the surface. Overall, both H2
and CO selectivities were higher at higher temperatures. Also, higher oxygen concentra-tions favored H2and HCOOH formation; CO2 also increased with increasing oxygen.
The CPOX of ethanol was studied by Liguras et al.[118]. Ni/La2O3catalysts were supported on cordierite monoliths, ceramic foams, and g-alumina pellets. The catalyst prepared by wash-coating on cordierite monoliths was deter-mined to be excellent for H2 production from ethanol. Although this catalyst had significant coke formation, it did not appear to affect the activity or selectivity. The catalyst supported on zirconiaealumina foam gave slightly better performance. The same group examined the use of Ru catalysts for the CPOX of ethanol [117]. These materials also utilized cordierite monoliths, ceramic foams, and g-alumina pellets. In contrast to the study on Ni catalysts, the Ru catalyst on the ceramic foams gave the best performance. This was attributed to the smaller pore sizes and higher tortuosity.
Other noble metal catalysts supported on alumina foams were studied for the CPOX of ethanol by Salge et al.[119]. The catalysts were Rh, Rh-Ce, Pt, Pd, and Rh-Ru. Ethanol was found to adsorb dissociatively as an ethoxide 0
10 20 30 40 50 60 70 80 90 100
0 50 100 150 200 250 300 350
Time on stream (min)
Yield (%)
CO
CO
CO H2
H2
H2
CO2
CO2
CO2
CH4 CH4
Pump malfunction
Sulfur removed 1000 ppmw
sulfur added
FIGURE 5.38 Yields for LSRhZ during 5 h CPOX experiment: GHSV¼ 50,000 cm3 gcat1 h1, O/C ¼ 1.2, 900C, and 0.23 MPa. Reprinted from Haynes et al. [104], Copyright (2009), with permission from Elsevier.
OXYGENATED HYDROCARBONS 117
species. To produce syngas, these species then decompose to carbon, oxygen, and hydrogen species. H2and CO are formed by surface reac-tion of these species. The Rh catalysts produced syngas by this mechanism, and the Rh-Ce cata-lyst was found to give the highest selectivity and stability. This was partially attributed to the red-ox properties of Ce. In contrast, it was demonstrated that the ethoxides formed on the surfaces of Pt and Pd leads to dehydrogenation
and acetaldehyde formation, yielding little syngas production.
A major contribution to the study of the CPOX of alcohols is the work by Wanat et al.
[120], who examined methanol, ethanol, 1-propanol, and 2-propanol. A significant amount of data was provided in this effort.
One set of results selected for this discussion are presented inFig. 5.39, which compares the reaction of all four alcohols over one catalyst
FIGURE 5.39 CPOX of different alcohols at varied C/O ratio with Rh-Ce catalyst on a foam monolith. Reprinted from Wanat et al.[120], Copyright (2005), with permission from Elsevier.
5. CATALYTIC PARTIAL OXIDATION
118
(Rh-Ce on a foam monolith). Methanol conver-sion was nearly complete until very carbon-rich conditions were reached. Ethanol and 1-propanol had much higher conversions than 2-propanol. The temperature trends were similar for all alcohols, decreasing with lower oxygen concentrations, although for methanol the temperature drop is more drastic. Both H2
and CO selectivities increased for methanol at higher oxygen concentrations, which can be correlated to the much lower reactor tempera-tures. Overall, 2-propanol had the lowest conversion, as well as the lowest H2 and CO selectivities.
Some of the major conclusions for the entire study, which examined other catalysts and reac-tion condireac-tions, state that Rh-Ce catalyst was the best for H2 and CO with little formation of higher products. Rh and Rh-Co catalysts produced less H2 and CO with more higher products. The trends were similar for all alco-hols, except for methanol, which produced no higher products. Methanol and ethanol produced mostly H2and CO; however, higher alcohols have the potential to produce larger products. The size and structure of the alcohol will influence product distributions signifi-cantly. The difference in performance between 1-propanol and 2-propanol is an excellent example of how the structure can change the overall conversion and selectivity.
5.6.2. Dimethyl Ether (DME)
The CPOX of DME, CH3OCH3, for H2 and CO production has several advantages. First, it has a high H/C ratio and energy density. DME is inert, non-carcinogenic, non-mutagenic, non-corrosive, and non-toxic [127e128].
Further, its production from syngas is more economical and thermodynamically favorable than methanol. DME would also require less costly infrastructure for transport and storage since it possesses similar physical properties as LPGs.
Zhang et al. [129-130] studied the CPOX of DME over Rh-, Pt-, and Ni-based catalysts at ambient pressure and 600e750 C with yields greater than 90% and little methane production.
The reaction mechanism is analogous to the indirect mechanism for methane CPOX. In fact, promoting initial combustion is critical to prevent DME decomposition to CH4, CO, and H2, which yields low H2 and CO production.
Therefore, it was demonstrated that a catalyst that suppresses DME decomposition and is active for reforming will promote high H2and CO yields. Different alkaline metal promoters were added to Rh and Pt catalysts and the optimal performance was obtained with Na-Rh/Al2O3. 6 wt%Ni/Al2O3 was used as the reforming catalyst.
Wang et al. [131] examined catalysts on a variety of supports including alumina, silica, magnesium oxide, yttrium-stabilized zirconia, samarium-doped ceria, and a LaGa-perovskite.
The activities for the CPOX of DME with the catalysts supported on Al2O3were found to be ranked as follows: Ni> Rh > Co >> Ru > Fe >
Pt>> Ag. The catalyst that was most active and selective for conversion of DME to syngas was Ni/LaGaO3.
5.6.3. Biodiesel
Bio-derived fuels have received increasing interest over the past few years in efforts to reduce net carbon emissions from energy usage. Since liquid fuels will remain the primary transportation energy source for the foreseeable future, biodiesel is an attractive fuel option. Biodiesel is prepared by the trans-esterification of vegetable oils or animal fat by reaction with an alcohol. The resulting product is a mono-alkyl ester with a long hydrocarbon chain often containing one or more unsaturated bonds. For more details on this fuel type, see Chapter 3.
Like other liquid hydrocarbons, biodiesel can be vaporized and reformed to produce H2and
OXYGENATED HYDROCARBONS 119
CO. Reforming of biodiesel does not possess the same challenges as conventional petro-diesel since it does not contain sulfur or aromatic species. Further, there is a reduction in coke formation due to the presence of oxygen in the fuel itself.
There are very few studies in literature that report on the CPOX of biodiesel. One study by Nguyen and Leclerc[132] examined the CPOX of methyl acetate as a model fuel for biodiesel.
These experiments were conducted on Rh and Rh-Ce catalysts. The most important findings of this study include that the primary products from the oxidation of methyl acetate are CO and H2O, rather than CO2 and H2O. Further, methyl acetate produced less syngas than bio-diesel reported in another study, which used a soy oil-derived fuel[133]. This was suggested to be related to the likelihood that biodiesel will have longer carbon chains attached to the methyl ester functional group, which may be more reactive than a single methyl group.
Nevertheless, since the methyl ester group in biodiesel will limit H2yield, the authors stated the need to develop catalysts that target the decomposition of methyl esters.
5.6.4. Summary of Oxygenated Hydrocarbons
The CPOX of different oxygenated hydrocar-bons were discussed with a number of base and noble metal catalyst. The differences in mecha-nism, product distribution, and operating conditions for different alcohols were identified.
The use of DME for syngas production was evaluated. The potential for the CPOX of biodie-sel fuels was also considered.