Enhanced Olefin Production from Renewable Aliphatic Feedstocks

Publication Date (Web): January 11, 2010. Copyright © 2010 American Chemical Society. * To whom correspondence should be addressed. E-mail: ...
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Ind. Eng. Chem. Res. 2010, 49, 1611–1624

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Enhanced Olefin Production from Renewable Aliphatic Feedstocks and Co-Fed Lignin Derivatives Using Experimental Surrogates by Millisecond Catalytic Partial Oxidation Bradon J. Dreyer,§ Paul J. Dauenhauer,† Raimund Horn,⊥ and Lanny D. Schmidt*,‡ Department of Chemical Engineering and Materials Science, UniVersity of Minnesota, 421 Washington AVenue SE, Minneapolis, Minnesota 55455

To investigate the effect of co-fed lignin derivatives on olefin production in the catalytic partial oxidation of aliphatic feedstocks, benzene was selected as a lignin surrogate and n-hexane was selected as a renewable oil surrogate. Aromatic benzene and aliphatic n-hexane, along with the corresponding 80:20 and 50:50 molar n-hexane/benzene mixtures, were partially oxidized in millisecond contact time reactors, varying the fuel to oxygen ratio (0.8 < C/O < 2.0), the catalyst (5 wt % Pt or Rh), the support (45 or 80 pores per linear inch R-Al2O3) while maintaining constant space time (GHSV)105 h-1). The experiments indicate that the addition of benzene likely results in competitive catalytic adsorption which reduces the catalytic oxidation of n-hexane and increases production of olefins by homogeneous cracking. Under optimal conditions, selectivity to ethylene and propylene from n-hexane was increased from ∼35% using pure n-hexane to ∼65% when using a 50:50 molar mixture of benzene and n-hexane. Results indicate that the addition of lignin-derived aromatic species should increase production of olefins from catalytically reformed renewable oils. 1. Introduction Olefins, such as ethylene and propylene, are the largest volume intermediates produced in the petrochemical industry.1-3 High yields of olefins (>70%) from aliphatic hydrocarbons such as ethane, pentane, hexane, decane, and hexadecane can be produced autothermally from the catalytic partial oxidation (CPO) of hydrocarbons at millisecond contact times.4-8 In these reactors, O2 is combined with the hydrocarbon feed, and some of the feed is partially oxidized to synthesis gas (H2 and CO), releasing heat. This heat provides thermal energy for the endothermic cracking of the remaining hydrocarbons to olefins. The millisecond contact times and absence of external heat input typically translate to more compact lower capital cost reactors, which is an economic benefit over conventional steam cracking reactors. This technology has been extended to renewable and waste product feedstocks. Catalytic partial oxidation of long-chain fatty acid methyl esters (biodiesel) demonstrated that up to 35% of a volatile renewable feedstock could be converted to C2-C5 olefins on Rh-based catalysts.9 An alternative technique (reactive flash volatilization) permitted catalytic processing of the nonvolatile parent molecule of biodiesel, triglyceride, to olefins by combining endothermic homogeneous pyrolysis with exothermic catalytic partial oxidation.10 Similar results were obtained from the feedstock low-density polyethylene.11 This paper explores the possibility for increasing the production of olefins from aliphatic renewable feedstocks by cofeeding low value components derived from lignins. As the second most abundant polymer on the planet after cellulose, lignin supply greatly exceeds its relatively few chemical uses.12 In general, lignins are complex heteropolymers derived mainly from three aromatic monomers of differing degree of methoxylation: * To whom correspondence should be addressed. E-mail: [email protected]. † Present address: University of Massachusetts. ‡ University of Minnesota. § Present address: The Dow Chemical Company. ⊥ Present address: Fritz Haber Institute of the Max Planck Society.

p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol.13 These components are arranged to form several substructures within the lignin macromolecule which can be liberated from lignocellulose chemically to form a low value byproduct (black liquor) or thermally by fast pyrolysis to form a fraction of biooils. Originating from the same parent polymer, these lignin degradation byproducts of little value consist largely of aromatic components.14 For example, the light pyrolysis products derived from hardwood lignin by fast pyrolysis have been termed ‘monolignols’ and include species such as phenol, 1,2-benzenediol (catechol), 2,6-dimethoxyphenol, and 2-propyl-1,2-benzenediol.15 By cofeeding lignin-derived species with aliphatic feedstocks, the heat necessary to drive endothermic cracking can be derived from lower value aromatic compounds. As shown in reaction 1, aromatic species undergoing catalytic combustion to carbon dioxide and steam generate significant heat. Liberated energy from combustion then drives endothermic cracking of alkane chains to ethylene, propylene, and higher olefins. By maintaining high temperature autothermal operation with co-fed aromatics, the potential for cracked olefin products from aliphatic feedstocks increases.

The addition of aromatics to the process significantly complicates reforming chemistry. Previous work has demonstrated that the catalytic reforming of fuel mixtures to synthesis gas (H2 and CO) and olefins in CPO reactors differs from single component aliphatic surrogates.6,16,17 Mixtures displayed higher operating temperatures than single component aliphatic com-

10.1021/ie9013452  2010 American Chemical Society Published on Web 01/11/2010

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Figure 1. Schematic of the experimental reactor. Hydrocarbon fuel is introduced into the reactor through a low-flow automotive fuel injector, vaporized, and mixed with air. The heat-shields prevent thermal radiation heat losses in the axial direction, and the reactor tube is wrapped in insulation to reduce radial heat losses.

pounds at corresponding inlet stoichiometries. Furthermore, when steam was added to the CPO reactor (autothermal steam reforming), the mixtures were more resistant to water-gas shift, producing less H2, CO2, olefins, and more CO than the reforming of the aliphatic surrogates without the addition of steam.17-20 xC6H6 + (1 - x)C6H14 + yO2 f CO2 + H2O + olefins (3) In addition to the complexity of mixed feedstocks, the ideal renewable aliphatic feedstock, triglyceride, and the ideal aromatic feedstock, cracked lignin species obtained from biooils by extraction, are too complex to provide a simple demonstration of the benefit of mixed feeds for olefin production. Therefore, this paper investigates the effect of the simplest aromatic compound, benzene (R1-R6dH), in the CPO of a simple aliphatic compound, n-hexane, in millisecond contact times as shown in eq 3. Considered catalysts are Rh or Pt supported on R-Al2O3 foam supports with small pores (80 PPI) or large pores (45 PPI) to vary the relative magnitudes of heterogeneous and homogeneous chemistry which will further reveal the relative reactivities of benzene and n-hexane. This information will provide insight in the engineering of future CPO processes for the production of olefins or synthesis gas from complex fuel mixtures that utilize actual biomass oils and lignin-derived cofeedstocks. 2. Materials and Methods 2.1. Reactor System. The reactor consisted of a 19 mm inner diameter (ID) quartz tube as shown in Figure 1 and described elsewhere.6 The fuel, n-hexane (HPLC-grade 95+% purity with ∼5% C6 saturated hydrocarbon isomers, Sigma Aldrich, referred to as “pure component” or “pure” within for clarity when compared to mixtures of benzene and n-hexane) and benzene (99+% purity, Sigma Aldrich, referred to as “pure component” or “pure” within for clarity when comparing to mixtures of benzene and n-hexane) were delivered into the reactor through an automotive fuel injector (Delphi) from a 5 psig (35 kPa

gauge) pressurized tank. High pressure cylinders of high purity gases (99.9+ %, Airgas) fed the system with O2 and N2, and the flow rates were adjusted to air stoichiometry using mass flow controllers (Brooks 5850i). The walls along the vaporization section were heated with Variac controlled resistive heating tape. The catalyst supports were 45 and 80 pores per linear inch (PPI), cylindrical ceramic foam monoliths (92% Al2O3, 8% SiO2, 16.5 mm outer diameter (OD), 10 mm length, Vesuvius HiTech Ceramics). The catalyst was positioned between two uncoated cylindrical 80 PPI ceramic foam monoliths. These two foams were used as heat-shields and mixers to reduce the axial radiation heat losses and promote reactant mixing upstream of the catalyst, respectively. To measure catalyst operating temperatures, a K-type thermocouple (Omega) was positioned on the backface of the cylindrical foam catalyst. Under steady-state operation, the upstream feed temperature was maintained at ∼150 °C. During operation, the reactor pressure was maintained near atmospheric conditions (1.1 atm), and a total flow rate of 4 standard liters per minute (SLPM at 25 °C and 1 atm or GHSV ≈ 1 × 105 h-1) was also maintained. This total flow rate corresponds to catalyst contact times of ∼7 ms assuming an entering reactant temperature of 800 °C and pressure of 1.1 atm. Typically, samples were taken after continuously operating for 30 min at a specified inlet composition and flow rate. The feed stream stoichiometry is reported as the carbon to oxygen ratio (C/O). C/O is defined as the number of moles of carbon atoms divided by the number of moles of oxygen atoms from O2 in the mixture. Previous experimentation of hydrocarbon in CPO reactors suggest that the maximum ethylene and propylene occur in the fuel-rich regime where insufficient O2 is supplied to partially oxidize the fuel, typically between C/O of 1.1 and 1.5.5-7,17,21,22 Therefore, C/O ratios between 0.8 and 2.0 were investigated to evaluate the olefin production from the fuels. Results were reported where no observable catalyst deactivation was observed during experimental operation. Results at each specified inlet composition and flow rate were reported as an average of at least two experiments. 2.2. Catalyst. 45 PPI and 80 PPI R-alumina (92% Al2O3, 8% SiO2) foam monoliths were used as catalyst supports, coated with ∼5 wt % Rh and ∼5 wt % (of monolith) Pt. The 80 PPI catalyst supports were also coated with ∼4 wt % (of monolith) γ-alumina wash-coat (Alfa Aesar) prior to adding the precious metal using the incipient wetness technique, as previously described.6,17,23,24,37,38 The pore diameters of the 45 PPI and 80 PPI wash-coated monoliths are ∼1.00 and ∼0.50 mm, respectively, which were observed through SEM micrographs of the foams. The density of the 45 and 80 PPI foams were similar, resulting in comparable metal loadings. From previous experimentation, the larger pore size, 45 PPI monoliths promote less surface and more homogeneous chemistry than compared to the 80 PPI wash-coated monoliths.3,5,7,17,37,38 These two monoliths were chosen primarily to investigate the effect of the fuel in a reactor designed to enhance heterogeneous synthesis gas or to enhance homogeneous cracking reactions. The washcoat also increased the dispersion of the metal on the catalyst surface, further increasing available catalytic surface on 80 PPI supports. Using the incipient wetness technique, the Rh metal was applied by dropping an aqueous solution of Rh(NO3)3 onto the ceramic foam monoliths, and Pt metal was applied by dropping an aqueous solution of H2PtCl6. The Rh-coated monoliths were then dried and placed in a furnace in the presence of air at 600 °C for 6 h, while the Pt-coated monoliths

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Figure 2. Catalytic partial oxidation of n-hexane and of benzene on ∼5 wt % Rh on ∼4 wt % γ-alumina wash-coat, 80 PPI R-alumina foam support at 4 SLPM (GHSV ∼ 105 h-1), and 1.1 atm. A pure benzene feed is presented in panels A and C, while pure n-hexane feed is presented in panels B and D. Operating back-face temperature (T, )) and fuel conversion (X, (O) C6H6; (∆) n-C6H14; (0) O2) are displayed. Selectivities (SC or SH) are defined as (C or H atoms in product)/(C or H atoms in converted fuel) and are presented for H2 (∆), H2O (0), CO (2), CO2 (O), and C2H4 and C3H6 ()). n-C6H14* (2) indicates conversion of C in n-hexane to oxidation products (CO and CO2).

were dried and placed in a furnace under flow of 10 mol % H2 in N2 at 500 °C for 6 h. 2.3. Product Analysis. Product gases for n-hexane and benzene were analyzed using a single-column gas chromatograph (HP 5890 Series II) equipped with a capillary column (J&W Scientific GASPRO, 0.32 mm id, 60 m length) and thermal conductivity detector. This column was capable of detecting both permanent gases and higher hydrocarbons. Conversion (X) is defined as the fraction of a reactant species that is consumed by reaction. Reaction products containing carbon are reported on a carbon atom basis using selectivities. Product selectivity (S) is defined as the number of carbon or hydrogen atoms in product i divided by the carbon or hydrogen atoms in the converted fuel. H2 and H2O are reported as selectivity on a hydrogen atom basis. When calculating n-hexane conversion and product selectivity from converted n-hexane, measured C6 isomers in the n-hexane were considered n-hexane. The oxygen atom balance was closed to determine the water molar flow rate. Applying this quantitative method, the carbon and hydrogen atom balances typically closed within (7%. As shown later, benzene does not produce large selectivities to ethylene and propylene under the specified reaction inlet conditions. Since benzene appears not to produce significant levels of these olefins, the production of these olefins is attributed to the linear alkane fuel n-hexane. Thus, the carbon atom selectivities of ethylene and propylene from a mixture of benzene and n-hexane are also reported on the number of carbon atoms that were reacted in the n-hexane only.

3. Results 3.1. Pure Components on 80 ppi Supported Rh. Figure 2 displays conversion and the catalyst back-face temperature for benzene (panel A), and n-hexane (panel B) on Rh coated, 80 PPI foam supports. The catalyst back-face temperature for the pure component benzene was ∼300 °C higher than the pure component n-hexane at corresponding C/O ratios, while the total conversion of benzene and n-hexane were similar. For both benzene and n-hexane, the O2 conversion was >98% for all C/O ratios. Figure 2 also shows the hydrogen atom selectivity for H2 and H2O and also displays the carbon atom selectivities for CO, CO2, and ethylene and propylene for benzene (panel C) and n-hexane (panel D). H2, H2O, CO, CO2, CH4 (not shown), ethylene, and propylene, 1- 3 carbon linear hydrocarbons (not shown), and 4-6 carbon linear olefins (not shown) were the major products observed from n-hexane. From benzene, H2, H2O, CO, and CO2 were the major products with traces ( 1.0, H2 from benzene was replaced with H2O, while CO remained constant, without significant olefin production. The maximum H2 and CO selectivities were typically observed at C/O ≈ 1.0, and were determined to be ∼85% H2 and ∼90% CO selectivities for both benzene and n-hexane.

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Figure 3. Catalytic partial oxidation of n-hexane and benzene mixtures on ∼5 wt % Rh on ∼4 wt % γ -alumina wash-coat, 80 PPI R-alumina foam support at 4 SLPM (GHSV ≈ 105 h-1) and 1.1 atm. A 80:20 mol % n-hexane/benzene feed (80 H:20 B) is presented in panels A and C, while a 50:50 mol % n-hexane/benzene feed (50 H:50 B) is presented in panels B and D. Operating back-face temperature (T,)) and fuel conversion (X, (∆) n-C6H14; (O) C6H6; (0) O2) are displayed. Selectivities (SC or SH) are defined as (C or H atoms in product)/(C or H atoms in converted fuel) and are presented for H2 (∆), H2O (0), CO (1), CO2 (O), and C2H4 and C3H6 ()). n-C6H14* (2) indicates conversion of C in n-hexane to oxidation products (CO and CO2). C2H4 and C3H6* ([) indicates selectivity from carbon atoms in n-hexane.

3.2. Mixtures on 80 ppi supported Rh. Figure 3 displays the operating catalyst back-face temperature and benzene and n-hexane conversions on Rh-coated 80 PPI foam supports for a 20 mol % benzene/80 mol % n-hexane mixture (panel A) and for a 50 mol % benzene/50 mols% n-hexane mixture (panel B). At corresponding C/O, high benzene concentrations in the mixture resulted in elevated back-face temperatures. For example, the temperature of n-hexane was 800 °C at C/O ) 1.2, while the temperatures at this same C/O ratio for the 20:80 and 50:50 mixture were 840 and 940 °C, respectively. Similar to the pure components, the O2 conversion was >98% for all C/O ratios. Additionally, the conversion of benzene and n-hexane in the mixtures did not reflect the single component mixture conversion at corresponding C/O. In the 20:80 mixture, the n-hexane conversion was similar to the single component n-hexane conversion at corresponding C/O ratios. However, the conversion of benzene in the 20:80 mixture was much higher than the conversion of pure benzene. Also, when comparing the reactivities of benzene and n-hexane in the 20:80 mixture, the conversion of benzene was higher than the conversion of n-hexane for C/O > 1.3. Benzene conversion remained at ∼85% for C/O > 1.1, while n-hexane conversion decreased from ∼85% at C/O ) 1.1 to ∼40% at C/O ) 1.9. In the 50:50 mixture, the n-hexane conversion was greater than the single component n-hexane conversion at corresponding C/O ratios. However, the conversion of benzene in the mixture was similar to the conversion of pure benzene. Also, when

comparing the reactivities of benzene and n-hexane in the mixture, the conversion of benzene was lower than the conversion of n-hexane for C/O > 1.3. Benzene conversion decreased from 70% at C/O ) 1.1 to 55% at C/O ) 1.9, while the n-hexane conversion decreased from 95% at C/O ) 1.1 to 60% at C/O ) 1.9. Figure 3 also shows the hydrogen atom selectivity for H2 and H2O and also displays the carbon atom selectivities for CO, CO2, and ethylene and propylene for 20:80 (panel C) and 50: 50 (panel D) mixtures. Generally for both mixtures, as the C/O ratio increased, olefins (primarily ethylene and propylene) were produced in place of synthesis gas. The optimum H2 and CO selectivities were typically observed at C/O ≈ 1.0: ∼80% H2 and ∼90% CO selectivities for both mixtures. When compared to the pure components, the mixtures showed an increase in ethylene and propylene in place of synthesis gas. The ethylene and propylene selectivities increased from ∼5% for pure n-hexane to ∼15% and ∼30% for the 20:80 and 50:50 mixtures, respectively. 3.3. Pure Components on 80 ppi Pt. Figure 4 displays conversion and the catalyst back-face temperature for benzene (panel A), and n-hexane (panel B) on Pt-coated, 80 PPI foam monolith supports. Similar to the Rh results, the catalyst backface temperature for the pure component benzene was ∼250 °C higher than the pure component n-hexane at corresponding C/O ratios. However, contrary to Rh, the total conversion of benzene was lower than n-hexane. Also, at corresponding C/O ratios, the backface temperature was ∼50 °C higher for Pt than

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Figure 4. Catalytic partial oxidation of n-hexane and of benzene on ∼5 wt % Pt on ∼4 wt % γ-alumina wash-coat, 80 PPI R-alumina foam support at 4 SLPM (GHSV ≈ 105 h-1) and 1.1 atm. A pure benzene feed is presented in panels A and C, while pure n-hexane is presented in panels B and D. Operating back-face temperature (T, )) and fuel conversion (X, (O) C6H6; (∆) n-C6H14; (0) O2) are displayed. Selectivities (SC or SH) are defined as (C or H atoms in product)/(C or H atoms in converted fuel) and are presented for H2 (∆), H2O (0), CO (1), CO2 (O), and C2H4 and C3H6 ()). n-C6H14* (2) indicates conversion of C in n-hexane to oxidation products (CO and CO2).

Rh. For both benzene and n-hexane, the O2 conversion was >98% for all C/O ratios. Figure 4 also shows the hydrogen atom selectivity for H2 and H2O and also displays the carbon atom selectivities to CO, CO2, and ethylene and propylene for benzene (panel C) and n-hexane (panel D). Similar to Rh, H2, H2O, CO, CO2, ethylene and propylene, 1-3 carbon linear hydrocarbons (not shown), and 4-6 carbon linear olefins (not shown) were the major products observed for n-hexane. For benzene, H2, H2O, CO, and CO2 were the major products with traces ( 1.0, H2 from benzene was replaced with H2O, while CO declined only slightly from ca. 95% to 85%. The optimum H2 and CO were typically observed at C/O ≈ 1.0 and are determined to be ∼80% H2 and ∼90% CO selectivities for both benzene and n-hexane, which were similar to results obtained on Rh. Olefins and H2O selectivities were greater on Pt than Rh catalysts. 3.4. Mixtures on 80 ppi Pt. Figure 5 displays the operating catalyst back-face temperature and benzene and n-hexane conversions in a 20 mol % benzene/80 mol % n-hexane mixture (panel A) and in a 50 mol % benzene/50 mol % n-hexane mixture (panel B). At corresponding C/O, increasing benzene concentrations in the mixture resulted in elevated back-face temperatures. For example, the temperature of n-hexane was 880 °C at C/O ) 1.2, while the temperature at this same C/O ratio for the 20:80 and 50:50 mixture were 880 and 950 °C,

respectively. Similar to the pure components, the O2 conversion was >98% for all C/O ratios. The conversion of benzene and n-hexane in the mixtures did not reflect the single component mixture conversions at corresponding C/O. In the 20/80 mixture, the n-hexane conversion and benzene conversions were greater than their single component conversions at corresponding C/O ratios. Also, when comparing the reactivities of benzene and n-hexane in this mixture, the benzene conversion was lower than the n-hexane conversion for all C/O ratios studied. Benzene conversion decreased from ∼95% at C/O ) 0.85 to ∼50% at C/O ) 1.9, while n-hexane conversion decreased from ∼99% at C/O ) 0.85 to ∼55% at C/O ) 1.9. This result is contrary to conversion trends observed for Rh-coated catalysts. In the 50:50 mixture, the n-hexane conversion was greater than the single component n-hexane conversion at corresponding C/O ratios, and the conversion of benzene in the mixture was similar to the conversion of pure benzene. Also, when comparing the reactivities of benzene and n-hexane in this mixture, the conversion of benzene was lower than the conversion of hexane for all C/O ratios studied. Benzene conversion decreased from ∼90% at C/O ) 0.85 to ∼40% at C/O ) 1.9, while n-hexane conversion decreased from ∼99% at C/O ) 0.85 to ∼75% at C/O ) 1.9. Figure 5 also shows the hydrogen atom selectivity for H2 and H2O and also displays the carbon atom selectivities for CO, CO2, and ethylene and propylene for 20:80 (panel C) and 50: 50 (panel D) mixtures. Generally for both mixtures, as the C/O ratio increased, olefins (primarily ethylene and propylene) were

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Figure 5. Catalytic partial oxidation of n-hexane and benzene mixtures on ∼5 wt % Pt on ∼4 wt % γ -alumina washcoat, 80 PPI R-alumina foam support at 4 SLPM (GHSV ≈ 105 h-1) and 1.1 atm. A 80:20 mol % n-hexane/benzene feed (80 H:20 B) is presented in panels A and C, while a 50:50 mol % n-hexane/benzene feed (50 H:50 B) is presented in panels B and D. Operating back-face temperature (T,)) and fuel conversion (X, (∆) n-C6H14; (O) C6H6; (0) O2) are displayed. Selectivities (SC or SH) are defined as (C or H atoms in product)/(C or H atoms in converted fuel) and are presented for H2 (∆), H2O (0), CO (1), CO2 (O), and C2H4 and C3H6 ()). n-C6H14* (2) indicates conversion of C in n-hexane to oxidation products (CO and CO2). C2H4 and C3H6* ([) indicates selectivity from carbon atoms in n-hexane.

produced in place of synthesis gas. For both mixtures, the optimum H2 and CO selectivities (∼80% H2 and ∼90% CO) were typically observed at C/O ≈ 0.85. When compared to the pure components, the mixture showed an increase in ethylene and propylene in place of synthesis gas. The ethylene and propylene selectivities increased from 20% for pure n-hexane to 25% and 27% for the 20:80 and 50:50 mixtures, respectively. On Pt, pure n-hexane produced higher levels of ethylene and propylene than observed on Rh. However, when comparing the mixtures, the levels of ethylene and propylene generated were similar. 3.5. Rh, 45 ppi Support. Figure 6 displays the operating catalyst back-face temperature and n-hexane and/or benzene conversions for pure n-hexane (Panel A), a 20 mol % benzene:80 mol % n-hexane mixture (Panel B), and a 50 mol % benzene:50 mol % n-hexane mixture (Panel C) over Rh coated 45 PPI foam supports. The CPO of pure benzene was also attempted on 45 PPI supports, however, several problems were encountered that prevented steady state operation. These problems included the formation of coke downstream, increased pressure drop through the reactor, and ignition of the fuel with unconverted oxygen downstream of the catalyst. Since steadystate operation was not achieved with pure benzene, no data regarding reforming pure benzene on 45 PPI supports is presented. At corresponding C/O, increasing benzene concentrations in the mixture results in elevated temperatures. For example, the temperature of n-hexane was 840 °C at C/O ) 1.2, while the

temperatures at this same C/O ratio for the 20:80 and 50:50 mixture were 900 and 920 °C, respectively. Furthermore, the conversion of n-hexane was greater in the mixture than when hexane was partially oxidized separately. The conversion of pure n-hexane for C/O ) 1.2 was 85%, while the conversion of n-hexane in the mixture was 98% for corresponding C/O. Benzene displayed a lower conversion than n-hexane in the mixture for all C/O. Benzene conversion remained ∼20 to ∼30% lower than n-hexane for all corresponding C/O ratios. For n-hexane, the O2 conversion was >97% for all C/O ratios. For the 20:80 mixture, the O2 conversion decreased from >98% at C/O ) 0.85 to >93% at C/O ) 1.9, while for the 50:50 mixture, the O2 conversion decreased from >98% at C/O ) 0.85 to >90% at C/O ) 1.9. Figure 6 shows the hydrogen atom selectivity for H2 and H2O and also displays the carbon atom selectivities for CO, CO2, and ethylene and propylene. H2, H2O, CO, CO2, ethylene and propylene, and 4-6 carbon olefins were the major products observed for n-hexane. On 45 PPI supports, synthesis gas selectivities were lower and the CO2, H2O, and olefin selectivities were higher than those observed 80 PPI supports. The optimum H2 and CO selectivities were typically observed at the lowest studied C/O ratio (0.85). As the benzene concentration increased in the mixture, the maximum H2 selectivity decreased from ∼55% H2 for pure n-hexane to ∼40% H2 for the 50:50 mixture. The maximum CO selectivities remained relatively constant at ∼75% for both mixtures and pure n-hexane. When the C/O ratio increased, olefins, primarily ethylene and propy-

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Figure 6. Catalytic partial oxidation of n-hexane and n-hexane-benzene mixtures on ∼5 wt % Rh on 45 PPI R-alumina foam support at 4 SLPM (GHSV ∼ 105 h-1) and 1.1 atm. A pure n-hexane feed is presented in panels A and D; a 80:20 mol % n-hexane/benzene feed (80 H:20 B) is presented in panels B and E, while a 50:50 mol % n-hexane/benzene feed (50 H:50 B) is presented in panels C and F. Operating back-face temperature (T,)) and fuel conversion (X, (∆) n-C6H14; (O) C6H6; (0) O2) are displayed. Selectivities (SC or SH) are defined as (C or H atoms in product)/(C or H atoms in converted fuel) and are presented for H2 (∆), H2O (0), CO (1), CO2 (O), and C2H4 and C3H6 ()). C2H4 and C3H6* ([) indicates selectivity from carbon atoms in n-hexane.

lene, were produced in place of synthesis gas for n-hexane. When compared to the pure component n-hexane, the mixture displayed similar selectivities of ethylene and propylene in place of synthesis gas. 3.6. Pt, 45 ppi Support. Figure 7 displays the operating catalyst back-face temperature and n-hexane and/or benzene conversions for pure n-hexane (panel A), a 20 mol % benzene/80 mol % n-hexane mixture (panel B), and a 50 mol % benzene/50 mol % n-hexane mixture (panel C) over Pt-coated 45 PPI ceramic foam supports. While Rh-coated catalysts were successfully operated with high benzene concentrations at all C/O ratios designed for this study, the addition of benzene into the feed greatly reduced the autothermal operability limits on Pt-coated 45 PPI foam supports. As the benzene concentration increased in the mixture from 0% to 50%, the fuel-rich limit in autothermal operability decreased from C/O ≈ 1.9 to C/O ≈ 1.0. With the 20:80 mixture, the reactor did not operate at steady-state autothermally above C/O ratios of ∼1.5, while with the 50:50 nhexane-benzene mixture, the reactor did not run at steady-state autothermally at C/O ratios >1.0. Above these C/O ratios, the reactor temperature gradually decreased to temperatures that were ∼200 °C greater than preheat temperatures, and the oxygen conversion decreased to below 50% conversion over a 90 min operation period. On Pt-coated 45 PPI foam supports, the pure n-hexane fuel conversion and C and H atom selectivities to combustion products and olefins were greater than those observed on Ptcoated 80 PPI supports. When benzene was added to the n-hexane, the fuel conversion of n-hexane remained relatively

constant, while the benzene fuel conversion was lower than Ptcoated 80 PPI supports. On Pt-coated 45 PPI supports, the C and H atom selectivities to synthesis gas, combustion products, and olefins were similar to the selectivities observed for pure n-hexane at corresponding C/O ratios where autothermal operation was achieved. For n-hexane and the mixtures where autothermal operation was achieved, the O2 conversion was >85% for all C/O ratios. 3.7. Olefins from n-Hexane. As mentioned previously, benzene does not appear to produce ethylene and propylene under these reactor conditions. Assuming that benzene does not produce ethylene and propylene, the ethylene and propylene selectivity can be attributed to the fuel n-hexane only. Applying this reasoning, Figures 3, 5, 6, and 7 display ethylene and propylene selectivity with respect to the entire fuel mixture and with respect to n-hexane. Considering that only the carbon from n-hexane can produce ethylene and propylene, the maximum ethylene and propylene selectivity from n-hexane over 80 PPI Rh supports are ∼20% and ∼50% for the 20:80 and 50:50 mixture, respectively, while the maximum is