Increasing olefins by H 2 and CH 4 addition to the catalytic partial oxidation of n-octane

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1 Applied Catalysis A: General 313 (2006) Increasing olefins by H 2 and CH 4 addition to the catalytic partial oxidation of n-octane G.J. Panuccio, L.D. Schmidt * Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN , USA Received 4 April 2006; received in revised form 30 June 2006; accepted 5 July 2006 Available online 9 August 2006 Abstract The catalytic partial oxidation (CPO) of n-octane over Rh and Pt-coated a-al 2 O 3 catalysts at approximately 5 ms contact times is examined with H 2 and CH 4 addition to the reactor feed. The aim is to preferentially oxidize H 2 or CH 4 and allow more octane to react in the gas phase to form high value chemicals such as ethylene and other olefins. The addition of H 2 increases olefin selectivities on both 80 and 45 ppi foams coated with Rh or Pt, and the highest increase in olefin selectivity occurs at H 2 /O 2 = 3/1 on Pt-coated 45 ppi catalysts. In this case, the selectivity to ethylene increases from a maximum of 38 to 51%, and total olefin selectivity increases from 75 to 83% without a decrease in octane conversion. The effect of adding CH 4 is heavily dependent on the catalyst metal and average pore size of the support structure. On Pt catalysts and 45 ppi Rh-coated catalysts, the addition of methane can increase olefin selectivities by several percent. However, on 80 ppi Rh-coated foams, adding methane actually suppresses the formation of olefins and increases selectivity to H 2 and CO. # 2006 Elsevier B.V. All rights reserved. Keywords: Catalytic partial oxidation; Olefins; Rhodium; Platinum; Octane; Sacrificial fuel 1. Introduction Olefins are the largest commodities in the chemicals industry with approximately 100 million tonnes of ethylene being produced annually [1,2]. The most widely used method for olefin synthesis is homogeneous steam cracking. This reaction is highly endothermic and limited by the rate of heat transfer from a fired furnace into reacting tubes, so residence times of s are required. Furthermore, the reactor must be shut down periodically because of coke formation, and the fired furnace generates harmful emissions like NO x. The process of catalytic partial oxidation (CPO) is an alternative method for the production of ethylene and other olefins. In CPO, a hydrocarbon feedstock is reacted with O 2 over a noble metal catalyst such as Rh or Pt to generate high selectivities of syngas (H 2 + CO) or olefins. For liquid hydrocarbon fuels, the process can be tuned to produce high selectivities to either syngas or olefins by changing the inlet stoichiometry, catalyst contact time, catalyst metal, and support structure pore * Corresponding author. Tel.: ; fax: address: schmi001@umn.edu (L.D. Schmidt). diameter [3,4]. The overall process is exothermic, so there is no need for furnaces that generate emissions, the residence time required is reduced to 10 ms, and the catalyst is robust with no decrease in activity observed over 50 h of operation. Approximately 50% of the ethylene made from steam cracking utilizes naphtha as a feedstock [2]. Naphthas are mixtures of n-alkanes, isoalkanes, olefins, naphthenes, and aromatics that have a boiling point range between 30 and 200 8C. The quality and composition of naphtha can vary over a wide range, but a typical feedstock might contain approximately 40 wt% n-paraffins and generate approximately 30 wt% yield to ethylene. We have recently shown that CPO can generate high yields to olefins for naphtha boiling point range n-alkanes [3,4] branched alkanes [3,5], and cyclic alkanes [5]. In this study, we will use n-octane (bp 126 8C) as a model naphtha feedstock. We have recently shown that the CPO of liquid alkanes like n-octane can be described through a coupling of heterogeneous surface reactions and homogeneous gas-phase reactions as sketched in Fig. 1 [3]. The basic premise is that octane reacts on the surface with adsorbed oxygen in the first few mm of the catalyst to form H 2,H 2 O, CO, CO 2, and heat. The heat created by these exothermic surface reactions is used in the gas-phase X/$ see front matter # 2006 Elsevier B.V. All rights reserved. doi: /j.apcata

2 64 G.J. Panuccio, L.D. Schmidt / Applied Catalysis A: General 313 (2006) Fig. 1. Schematic of the coupling between heterogeneous and homogeneous chemistry in CPO. Hydrogen and methane are added to the reactant stream so that they might preferentially react with O 2 on the surface and thereby increase the amount of octane that is converted through the gas-phase pathway to form olefins. to drive the O 2 -free homogeneous octane pyrolysis reactions that result in the formation of olefins and other smaller hydrocarbons. The fundamental goal in this study is to increase the selectivity to olefin species in the product stream by cofeeding another reacting species with octane that would preferentially consume the O 2 on the surface and leave more of the octane to react in the gas phase. Hydrogen and methane are chosen as possible sacrificial fuels for this work (we will examine the effect of aromatic addition in a later paper). Hydrogen is a by-product of the CPO reaction (in cases where olefins are the desired product) and therefore could be recycled to the inlet of the reactor and be oxidized. Methane is often flared during the refining process and so should be readily available for use as an additive to the CPO of heavy fuels. The combustion of both species is highly exothermic and should therefore be able to provide plenty of heat to drive the homogeneous cracking of the heavy alkane to form olefins (see Eqs. (1) and (2)). H 2 þ 1 2 O 2! H 2 O DH 0 R ¼ 242 kj=mol (1) CH 4 þ 2O 2! CO 2 þ 2H 2 O DH 0 R ¼ 802 kj=mol (2) In the presence of noble metal catalyst, methane can also react with water to form hydrogen and carbon monoxide in the endothermic steam reforming reaction as shown in Eq. (3). CH 4 þ H 2 O! CO þ 3H 2 DH 0 R ¼þ205 kj=mol (3) The balance between the extents of reaction in Eqs. (2) and (3) is critical in determining the efficiency of increasing olefin yields by adding methane to the CPO of n-octane for a particular catalyst. Previous research has focused on the effects of adding sacrificial fuels to small alkane feedstocks. The effect of H 2 addition to the millisecond contact time catalytic oxidative dehydrogenation (ODH) of ethane to ethylene has been investigated on Pt/a-Al 2 O 3 catalysts [6,7]. These results show that adding H 2 to the feed of ethane and O 2 can marginally increase ethylene selectivity, but the overall yield decreases because ethane conversion decreases. Similar experiments performed with propane over Pt/a-Al 2 O 3 catalysts show that ethylene and propylene selectivities can be increased, but that propane conversion decreases with increasing H 2 [8]. Hein and Jess [9] have shown that H 2 and CO addition to propane ODH on Pt/Ni catalysts can improve the propylene and total olefin selectivity, while other researchers have compared the effects of adding H 2 and CO to ethane ODH over Pt catalysts [10,11]. There is however, very little work relating to improving olefin yields in heavier hydrocarbons. O Connor et al. [12] showed that benzene and cyclohexene selectivities can be improved on Pt-Sn/a-Al 2 O 3 at low cyclohexane conversion with H 2 addition, and Liu et al. [13] successfully increased the selectivity of olefins from H 2 addition to hexane on Pt/SiO 2 catalysts at much lower throughputs than considered here. All of these studies examined the effects of addition of a sacrificial fuel at only a single hydrocarbon stoichiometry and catalyst configuration. In this paper, we will explore the effects of H 2 and CH 4 addition to the CPO of a naphtha-like n-octane feed as a function of octane stoichiometry, H 2 (or CH 4 ) stoichiometry, catalyst metal, and support pore size. The optimum operating conditions to maximize olefins will be defined and mechanisms of the reaction will be discussed. 2. Experimental The catalysts used in these experiments were a-al 2 O 3 ceramic foam monoliths that were impregnated with Rh or Pt metal. The monoliths were 17 mm diameter and 10 mm length cylinders with 80 or 45 pores per linear inch (ppi). The 80 ppi supports were coated with 5 wt% g-al 2 O 3 washcoat to further roughen the surface while the 45 ppi monoliths were prepared without a washcoat. By roughening the surface with a washcoat, the difference in the extent of heterogeneous chemistry on 80 ppi foams compared to 45 ppi foams was magnified [3,14]. The washcoat was applied by preparing a slurry of g-al 2 O 3 in water and adding it dropwise to the 80 ppi a-al 2 O 3 supports. They were then dried in air and calcined in an oven at 600 8C for 4 h. Rhodium catalysts were prepared by dropwise addition of an aqueous solution of Rh(NO 3 ) 3 to a foam monolith, drying in air, and calcining in an oven under air

3 G.J. Panuccio, L.D. Schmidt / Applied Catalysis A: General 313 (2006) for 6 h at 600 8C. Platinum catalysts were prepared by dropwise addition of H 2 PtCl 6 aqueous solution to a foam monolith, drying in air, and calcining in an oven under N 2 10% H 2 atmosphere for 6 h at 600 8C. This prevented a significant quantity of Pt from being lost in the form of PtCl 2 during the calcining process [15]. The final products were 80 ppi 5 wt% Rh (or Pt) 5 wt% g-al 2 O 3, and 45 ppi 5 wt% Rh (or Pt) catalysts. A detailed description and sketch of the experimental reactor setup has been provided elsewhere [4,16]. The catalytic foams were placed between two blank (no metal or washcoat) monoliths that acted as radiation heat shields. This assembly was wrapped in Fiberfrax cloth and placed in a quartz tube. A K-type thermocouple was placed between the back-face of the catalyst and the downstream heat shield so that the catalyst temperature may be recorded. A low-flow automotive fuel injector sprayed a conical dispersion of high purity (>99%) liquid n-octane on the walls at the top of the quartz tube which created a thin film of octane that was vaporized by heating the walls of the tube with a Variac-controlled resistive heating tape. The vaporizing fuel mixed with the other gases in the reactant stream (N 2,O 2, and H 2,orCH 4 ) which were introduced through a side port at the top of the tube via calibrated electronic mass flow controllers. Experiments were performed at atmospheric pressure and N 2 and O 2 were always fed to the reactor in air stoichiometry (N 2 /O 2 = 3.76/1). The amount of octane in the reactant stream was determined from the desired C/O ratio for the experiment. The C/O ratio is defined as the moles of carbon atoms (from octane) divided by the moles of oxygen atoms in the feed. By this definition, the stoichiometric feed composition for the partial oxidation reaction is at C/O = 1.0. The experimental octane C/O ratio was varied between 0.8 and 2.0. The stoichiometric H 2 /O 2 and CH 4 /O 2 ratios for combustion are 2.0 and 0.5, respectively (see Eqs. (1) and (2)). For every C/O, the H 2 /O 2 ratio was adjusted from 0.0 to 3.0 (hydrogen addition experiments) and CH 4 /O 2 was varied between 0.0 and 2.0 (methane addition experiments). The sum of the flow rates of N 2,O 2, and n-octane was 4 standard liters per minute (SLPM). Hydrogen (or methane) was added to the reacting mixture according to the desired H 2 /O 2 (or CH 4 /O 2 ) ratio while keeping all other flow rates the same. Therefore, the total flow rate in the reactor increased with H 2 (or CH 4 ) addition which resulted in a maximum decrease in residence time from 8 ms with no H 2 addition to 5 ms at H 2 /O 2 = 3/1 at an average catalyst temperature of 900 8C. Product gases were sampled with a Hamilton gas-tight syringe downstream of the catalyst and analyzed with an Fig. 2. n-octane conversion, net H 2 production, C 2 H 4 selectivity, and total olefin selectivity for H 2 addition to n-octane CPO on 80 ppi Rh w/washcoat catalysts. Conversion is nearly constant at all octane C/O ratios with H 2 addition. Ethylene and total olefin selectivities increase with increasing H 2 in the feed and the overall reaction is a net producer of H 2 at all C/O and H 2 /O 2 ratios studied on 80 ppi Rh coated catalysts.

4 66 G.J. Panuccio, L.D. Schmidt / Applied Catalysis A: General 313 (2006) HP5890 Gas Chromatography (GC) instrument fit with a 60 m GS-Gas Pro capillary column and a thermal conductivity detector (TCD). The GC oven was also fit with liquid N 2 cooling so that an initial oven temperature was set to 80 8C. This allowed for efficient separation of all species from H 2 to n- octane using one GC separation column in less than 20 min. Nitrogen was used as the internal standard to calculate the flow rates of all other species. When they are co-fed as reactants, both octane and methane can react to form CO, CO 2,H 2, and H 2 O. Therefore, the calculation of the selectivity of those species should include both octane and methane as fuels. The C-atom selectivity (S i ) of CO and CO 2 was calculated from the number of carbon atoms in species i (n i ), and the flow rate of species i (F i ) as in Eq. (4). n i F i S i ¼ n C8 H 18 F C8 H 18 ;In þ n CH4 F CH4 ;In ðn C8 H 18 F C8 H 18 ;Out þ n CH4 F CH4 ;OutÞ A similar equation was used to calculate the H-atom selectivities to H 2 and H 2 O (with n i as the number of hydrogen atoms in species i). The selectivities of all other species were calculated based only on the carbon in octane (Eq. (5)) because the amount of those species formed from methane was (4) insignificant compared to the amount formed from octane. n i F i S i ¼ (5) n C8 H 18 F C8 H 18 ;In n C8 H 18 F C8 H 18 ;Out All C-atom species selectivities (including CO and CO 2 ) were calculated according to Eq. (5) for the hydrogen addition experiments. 3. Results Each data point in the following figures is an average of three experiments performed on at least two similarly prepared catalysts. Data is plotted as a function of H 2 /O 2 (or CH 4 /O 2 ) gas ratio in the feed (x-axis) and octane C/O ratio (in series). In the following section, the differences between H 2 and CH 4 addition to the CPO of n-octane on Rh and Pt coated foam monoliths of different pore sizes is examined Rh on 80 ppi (with washcoat) catalyst Fig. 2 shows the n-octane conversion, net H 2 production, ethyelene selectivity, and total olefin selectivity that result from the addition of H 2 to the CPO of n-octane on 80 ppi 5% Rh 5% g-al 2 O 3 washcoat catalysts. Octane conversion remains nearly Fig. 3. n-octane conversion, H 2 selectivity, C 2 H 4 selectivity, and total olefin selectivity for the addition of CH 4 to n-octane CPO on 80 ppi Rh w/washcoat catalysts. The addition of methane gives very different results from those of H 2 addition on 80 ppi Rh catalysts. Increasing the amount of methane in the feed decreases octane conversion, increases H 2 (and CO) selectivity, and suppresses the formation of olefins at all octane C/O ratios studied.

5 G.J. Panuccio, L.D. Schmidt / Applied Catalysis A: General 313 (2006) constant with increasing H 2 in the feed at all C/O ratios. Conversion decreases from 100% at C/O = 0.8 to 70% at C/ O = 2.0, which is in agreement with previously observed results of n-octane CPO [3]. Ethylene and total olefin selectivities increase with increasing feed H 2 /O 2 at all octane C/O ratios. For example, at C/O = 2.0, the ethylene selectivity increases from 19% at H 2 /O 2 = 0.0 to 24% at H 2 /O 2 = 3.0 and the total olefin selectivity increases from 42% when no hydrogen is fed to the reactor to 49% at H 2 /O 2 = 3.0. The selectivity of olefins produced in this reaction follows as ethylene > propylene > n-butylene > i-butylene in the same relative distribution as previously reported [3]. The trends in selectivity with respect to H 2 (and CH 4 ) addition for higher molecular weight olefins are the same as those observed for ethylene. Methane and other low molecular weight paraffins are produced in similar quantities as previously reported [3] and are not shown for brevity. The top right panel in Fig. 2 compares the moles of hydrogen produced with the moles of hydrogen fed to the reactor on a per mole of oxygen basis. This particular catalyst configuration is a net producer of H 2 since all experimental points lie above the H 2,Out =H 2,In line. This shows that no outside hydrogen would need to be purchased to run this reaction in an industrial setting. The results from the addition of methane to the CPO of n- octane on 80 ppi 5% Rh 5% g-al 2 O 3 catalysts are summarized in Fig. 3. Adding methane to this reaction yields very different results than the addition of hydrogen. Now, the conversion of octane decreases with increasing methane in the feed for all octane C/O ratios. Also, the selectivity of H 2 (and CO: not shown) increases with CH 4 addition, while the formation of ethylene and other olefins is suppressed by the addition of CH 4 to the reacting mixture. Examination of the differences in the measured catalytic back-face temperature for H 2 addition and CH 4 addition experiments (Fig. 4) shows that temperatures increase slightly when H 2 is added and decrease significantly with the addition of CH 4. For example, at n-c 8 C/O = 1.0, the measured catalyst back-face temperature increases from 924 to 954 8C when H 2 is added and decreases from 924 to 803 8C with the addition of CH 4. These results indicate that methane and hydrogen behave very differently on 80 ppi Rh catalysts and will be discussed later Rh on 45 ppi (no washcoat) catalyst Fig. 5 shows the results of adding H 2 to the CPO of n-octane on 45 ppi 5% Rh (no washcoat) foams. Octane conversion increases from 86 to 94% at C/O = 1.3 and from 64 to 70% at C/ O = 2.0 when H 2 is added to the reacting mixture. The maximum ethylene selectivity increases with H 2 /O 2 from 34% at C/O = 1.0 and H 2 /O 2 = 0.0 to 46% at C/O = 1.0 and H 2 / O 2 = 3.0. Ethylene selectivities also increase for other octane C/ O ratios when H 2 is added. The maximum total olefin selectivity also increases from 60% at C/O = 2.0 and H 2 / O 2 = 0.0 to 75% at C/O = 2.0 and H 2 /O 2 = 3.0. The selectivity to olefins is significantly higher for the 45 ppi foam than the 80 ppi support both with and without H 2 addition. The top-right panel in Fig. 5 shows that the overall reaction is a net producer Fig. 4. Measured catalyst back-face temperature for H 2 (top) and CH 4 (bottom) addition to n-octane CPO on 80 ppi Rh w/washcoat catalysts. While catalyst back-face temperature increases slightly when H 2 is added to the feed, the temperature drops precipitously when CH 4 is added. This suggests that the methane is converted to H 2 and CO through endothermic steam reforming reactions. of H 2 for octane C/O = 0.8 and a net consumer of H 2 for C/ O 1.0. The results of adding CH 4 to the CPO of n-octane on 45 ppi 5% Rh (no washcoat) catalysts are summarized in Fig. 6. Octane conversion decreases with increasing methane in the feed, which follows the same trends observed for methane addition on 80 ppi catalysts. However, the selectivity of H 2 decreases for CH 4 /O at C/O = 0.8 and remains nearly constant for all CH 4 /O 2 ratios at octane C/O 1.0, which is not the same trends as observed on 80 ppi Rh coated catalysts. Furthermore, ethylene and total olefin selectivities increase with CH 4 addition (more so at C/O 1.0 than C/O 1.3). This result suggests that the extent of heterogeneous chemistry is very important when considering the effects of adding CH 4 to the CPO of liquid fuels on Rh-coated foams Pt on 80 ppi (with washcoat) catalyst Fig. 7 summarizes the results of adding H 2 to n-octane CPO on 80 ppi 5% Pt 5% g-al 2 O 3 catalysts. As with the reactions on 80 ppi Rh catalysts, the conversion of n-octane is nearly constant with increasing H 2 in the feed stream. It should be noted that the conversion of octane on 80 ppi Pt catalyst is

6 68 G.J. Panuccio, L.D. Schmidt / Applied Catalysis A: General 313 (2006) Fig. 5. n-octane conversion, net H 2 production, C 2 H 4 selectivity, and total olefin selectivity for H 2 addition to n-octane CPO on 45 ppi Rh no washcoat catalysts. Octane conversion increases slightly at C/O 1.3 with increasing H 2 in the feed. Ethylene and total olefin selectivities also increase with increasing H 2 for all octane C/O ratios studied. The overall reaction is a net consumer of H 2 for octane C/O 1.0. higher than the conversion on 80 ppi Rh catalysts. For example, the conversion of octane is 70% on 80 ppi Rh and 85% on 80 ppi Pt catalysts at octane C/O = 2.0. Ethylene selectivity increases with increasing H 2 /O 2 for all octane C/O. The global maximum for ethylene selectivity without hydrogen addition is 38% at C/O = 1.3. This maximum shifts to 45% at C/O = 2.0 when hydrogen is added in an H 2 /O 2 = 3.0 ratio. Total olefin selectivity also increases with increasing H 2. The effect is more pronounced for C/O < 1.3 where the total olefin selectivities increase from 5 to 24% at C/O = 0.8 and 23 to 40% at C/O = 1.0. Overall, the reaction results in a net production of hydrogen for C/O 1.3 and a net consumption of hydrogen for C/O = 2.0. The results from the addition of CH 4 to n-octane CPO on 80 ppi 5% Pt 5% g-al 2 O 3 washcoat are shown in Fig. 8. Asis the case with Rh catalysts, octane conversion decreases with increasing CH 4 /O 2 for all C/O ratios. While H 2 selectivity increases with increasing methane in the feed on 80 ppi Rh catalysts (Fig. 3), the selectivity to H 2 decreases with increasing CH 4 /O 2 on 80 ppi Pt catalysts. Ethylene selectivity increases with increasing methane for C/O 1.0 and remains constant for C/O 1.3. The total olefin selectivity increases for C/ O 1.3 and remains constant for C/O = 2.0. These trends in olefin selectivity are the opposite of those observed for 80 ppi Rh catalysts where olefin selectivities decrease with increasing methane for all octane C/O ratios Pt on 45 ppi (no washcoat) catalyst Finally, the results of H 2 and CH 4 addition to n-octane CPO are investigated on 45 ppi 5% Pt (no washcoat) catalysts. The results of adding H 2 to the reaction on this catalyst configuration are shown in Fig. 9. Octane conversion is independent of H 2 /O 2 ratio for C/O 1.3 and increases from 64% at C/O = 2.0 with no H 2 to 75% at H 2 /O 2 =1.0. Ethylene and total olefin selectivity increase when hydrogen is added to the reacting mixture. The maximum selectivity of ethylene when H 2 /O 2 = 0.0 is 38% at C/O = 1.3. With H 2 addition, the maximum ethylene selectivity increases to 51% at H 2 /O 2 = 3.0 and C/O = 1.0. There is a net consumption of hydrogen at all H 2 /O 2 and octane C/O ratios on 45 ppi Pt catalysts. Adding methane to n-octane CPO on 45 ppi Pt catalysts gives very different results than the addition of hydrogen. Methane addition to the reactant mixture on this particular catalyst configuration severely limits the range over which the reaction will operate under stable conditions (data not shown). For example, when any methane is added to the CPO of n- octane at C/O > 1.3, the reaction is no longer autothermal and it extinguishes itself. This is marked by a continuous decline in the measured catalyst back-face temperature with time until the temperature on the catalyst is the same as the temperature in the

7 G.J. Panuccio, L.D. Schmidt / Applied Catalysis A: General 313 (2006) Fig. 6. n-octane conversion, H 2 selectivity, C 2 H 4 selectivity, and total olefin selectivity for CH 4 addition to n-octane CPO on 45 ppi Rh no washcoat catalysts. Octane conversion decreases with increasing methane in the feed at all octane C/O ratios. H 2 selectivity remains constant with increasing feed methane for C/O 1.0. Ethylene and total olefin selectivities increase with methane addition for octane C/O 1.0. upstream mixing zone of the quartz tube (which indicates that there is no reaction taking place). The addition of methane at lower octane C/O ratios severely hinders the reactor performance as octane conversion and H 2 selectivity fall sharply with increasing CH 4 /O 2. The maximum ethylene selectivity increases from 38% at C/O = 1.3 and CH 4 /O 2 = 0.0 to 42% at C/O = 1.0 and CH 4 /O 2 = But, as conversion (and consequently reactor temperature) drops, ethylene selectivity also decreases. The total olefin selectivity increases for C/O 1.0 with the addition of methane, but remains constant for C/O = 1.3. The results described in this section show that the performance of the CPO reaction with the addition of a sacrificial fuel is very dependent on the catalyst metal, the catalyst support structure, and whether the additional feed component is hydrogen or methane. Table 1 shows the maximum ethylene and total olefin selectivities obtained with no co-feed, with H 2,orwithCH 4 co-feed for all four catalysts studied in these experiments. In general, co-feeding H 2 with n-octane results in a larger increase in olefin selectivities than CH 4 addition. The global maximum for ethylene (51%) and total olefin (83%) selectivity occurs on 45 ppi Pt catalysts with a co-feed of hydrogen at H 2 /O 2 =3/1. The implication of these results is discussed in the next section. 4. Discussion 4.1. Two-zone reaction model A recent computational study on the oxidative dehydrogenation of ethane with hydrogen addition over Pt-coated foam monoliths combines elementary step heterogeneous (20 reversible reactions) and homogeneous (440 reversible reactions) mechanisms [17]. This study shows that ethane ODH with H 2 addition can be described accurately by two sequential zones in the catalyst. In the first zone, hydrogen and ethane are consumed on the surface with O 2 which creates oxidation products and heat. This is followed by a zone where the gasphase dehydrogenation of ethane dominates and ethylene is formed. We have also previously shown from experiments and modeling that a two-zone reaction scheme can accurately describe the CPO of n-octane on Rh-coated foams [3]. Aswith ethane ODH, the oxidation reactions occur in the first few millimeters of the catalyst where adsorbed oxygen and fuel react on the surface to produce H 2,H 2 O, CO, CO 2, and heat. The heat is then used in endothermic gas-phase pyrolysis reactions of octane that is not consumed on the surface to produce olefins and other smaller hydrocarbons. The experiments with H 2 and CH 4 addition to n-octane CPO can also be

8 70 G.J. Panuccio, L.D. Schmidt / Applied Catalysis A: General 313 (2006) Fig. 7. n-octane conversion, net H 2 production, C 2 H 4 selectivity, and total olefin selectivity for H 2 addition to n-octane CPO on 80 ppi Pt w/washcoat catalysts. Octane conversion is nearly constant with H 2 addition at all C/O ratios studied. Ethylene and total olefin selectivities increase with increasing H 2 in the feed and the overall reaction is a net producer of H 2 for octane C/O 1.3 and a net consumer of H 2 for C/O > 1.3. qualitatively explained with this mechanism. Now, octane and sacrificial fuel can both react catalytically with oxygen in the first few millimeters of the catalyst to generate oxidation products and heat. The extents to which either octane or sacrificial fuel react with oxygen in a competitive manner is dependent on the catalyst metal, the surface area to volume ratio in the catalytic bed, and the relative reactivity of the preferential oxidant in relation to the heavy hydrocarbon fuel H 2 addition The selectivity of ethylene and other olefins resulting from the CPO of n-octane can be improved by adding H 2 to the reactor feed stream. While ethylene selectivity increases by 5% (65 70%) from reactions with ethane [6,7] and 6% with propane (32 38%) [8], the maximum ethylene selectivity increases by 12% (34 46%) on 45 ppi Rh catalysts and 13% (38 51%) on Pt 45 ppi catalysts for H 2 addition to octane CPO (see Table 1). The total olefin selectivity increases by 14% (61 75%) on 45 ppi Rh and 8% (75 83%) on 45 ppi Pt catalysts when hydrogen is added. Furthermore, experiments performed with H 2 addition to ethane and propane ODH show that the conversion of hydrocarbon decreases as H 2 /O 2 and ethylene selectivity increase [6 8]. The results of this current work show that conversion of octane does not change with increasing H 2 / O 2 on all the catalysts studied. This shows that the effect of adding H 2 to the catalytic partial oxidation process improves reactor performance (in terms of olefin yields) more efficiently for heavier fuels than for lighter hydrocarbons because olefin selectivity is increased more substantially and conversion is not decreased. ThereisnodropinconversionwithH 2 addition to n- octane because of the gas phase reactivity of the hydrocarbon fuel. Since some of the oxygen is reacting with hydrogen in the front zone of the catalyst, there is more hydrocarbon remaining to react in the gas-phase. At the same temperature, a heavy molecular weight hydrocarbon like octane is more likely to pyrolyze than a light paraffin like ethane or propane. Therefore, conversion of a light paraffin will decrease because the gas phase reactivity cannot compensate for the decrease in surface conversion. The heavier paraffin is more likely to react homogeneously and therefore has the same overall conversion. It should be noted that the maximum for ethylene selectivity and the maximum for total olefin selectivity do not occur at the same C/O ratio. For example, for reactions on 45 ppi Pt-coated foams, the maximum ethylene selectivity is found at octane C/ O = 1.0 and H 2 /O 2 = 3/1. However, the maximum olefin selectivity occurs at C/O = 2.0 and H 2 /O 2 = 3/1. So, the conditions that are chosen for reactor operation will be a

9 G.J. Panuccio, L.D. Schmidt / Applied Catalysis A: General 313 (2006) Fig. 8. n-octane conversion, H 2 selectivity, C 2 H 4 selectivity, and total olefin selectivity for CH 4 addition to n-octane CPO on 80 ppi Pt w/washcoat catalysts. Octane conversion and H 2 selectivity decrease with increasing methane in the feed. Ethylene and total olefin selectivities increase with increasing methane for octane C/ O 1.3 and remain constant for octane C/O = 2.0. function of the end goal of maximizing ethylene or total olefin yields. For all catalysts (excluding 80 ppi Rh w/wc), there is a net consumption of hydrogen at the conditions under which the selectivities of ethylene and other olefins are maximized (H 2 / O 2 = 3/1). This means that hydrogen must be purchased and cannot simply be recycled from the product stream. The increased operating cost from the hydrogen may not justify the improved yields observed in these conditions. However, adding hydrogen at H 2 /O 2 = 1/1 may be a viable option because olefin selectivities are increased by a few percent, but under nearly neutral hydrogen production conditions CH 4 addition The effect of methane addition is heavily dependent on catalyst metal and pore geometry. The addition of methane to the CPO of n-octane on 80 ppi Rh-coated foams causes the temperature in the reactor to decrease, the selectivity of H 2 and CO to increase, and the selectivity of ethylene and other olefins to decrease (Fig. 3). These results can be explained by the consumption of methane via the steam reforming reaction as in Eq. (3). This reaction explains the increase in H 2 and CO selectivity as well as the decrease in measured catalyst backface temperature since the overall reaction is endothermic. This reforming reaction is likely to take place in the catalyst after the O 2 has been consumed (i.e. in the second zone of the catalyst in Fig. 1) because the catalytic reforming reactions should be kinetically slower than the oxidation reactions. A decrease in reactor temperature precipitates a decrease in the kinetic rates of the pyrolysis reactions and the suppression of the formation of olefins. This decrease in olefins is not observed on 45 ppi Rh-coated foams. In fact, the selectivities of olefins increase and H 2 and CO decrease when methane is added on this catalyst (more so at octane C/O < 1.3; see Fig. 6). This is probably because the ratio of the catalyst surface area to the volume of the gas is not large enough for the extent of catalytic steam reforming reactions occurring in the downstream section to be as large as the extent of homogeneous reactions in the gas phase. Methane is still oxidized somewhat in the front of the catalyst but is not reformed in the downstream section. The increase in olefin selectivities is therefore due to the fact that some of the oxygen is consumed in reactions with methane instead of octane, and the kinetics of the pyrolysis reactions are not hindered because the gas temperature is not lowered by the endothermic catalytic reforming reactions in the downstream zone. The addition of methane to octane CPO on 80 ppi Pt catalysts causes the selectivity of ethylene and other olefins to increase while the selectivity of H 2 and CO decreases (see Fig. 8).

10 72 G.J. Panuccio, L.D. Schmidt / Applied Catalysis A: General 313 (2006) Fig. 9. n-octane conversion, net H 2 production, C 2 H 4 selectivity, and total olefin selectivity for H 2 addition to n-octane CPO on 45 ppi Pt no washcoat catalysts. Octane conversion is constant with increasing feed H 2 for C/O 1.3 and increases and then decreases for increasing H 2 at C/O = 2.0. Ethylene and total olefin selectivities increase with increasing H 2 in the feed while the overall reaction is a net consumer of H 2 for all octane C/O ratios studied. Previous studies that compare the partial oxidation of alkanes over Pt and Rh-coated monoliths have shown that Rh catalysts produce much higher selectivities to syngas than Pt catalysts and Pt catalysts produce higher selectivity to H 2 O, CO 2, and olefins than Rh [18 22]. This suggests that Pt is a better combustion catalyst and a poorer reforming catalyst than Rh. Therefore, some methane is oxidized instead of octane in the first few mm of the Pt bed but is not reformed in the downstream section of the catalyst. In the absence of the reforming reactions, the reactor temperature does not drop substantially and the pyrolysis reaction rates remain high enough to form significant quantities of olefins. Since some of the oxygen reacts with methane instead of octane, more hydrocarbon is left to react through gas-phase chemistry and olefin selectivity increases. The addition of methane to the catalytic partial oxidation of octane on 45 ppi Pt-coated catalysts severely limits the range over which the reactor will operate autothermally. The reaction quenches itself when the rate of exothermic surface reactions is not fast enough to keep the overall process autothermal. When methane is added, the partial pressure of oxygen decreases and the surface reaction rate decreases. This is not observed on 80 ppi Pt catalysts because the higher surface area to volume ratio increases the overall reaction rate. Experiments performed on the ignition behavior of different fuels in the CPO process have shown that the sticking coefficient (the statistical probability that a species will adsorb when it comes in contact with a surface) for methane should be times smaller than the sticking coefficient for n-octane [16]. Therefore, methane Table 1 Maximum ethylene and total olefin selectivities with no co-feed, with H 2, and with CH 4 co-feed Catalyst C 2 H 4 Sel. (%) Total Olefin Sel. (%) No Add w/h 2 w/ch 4 No Add w/h 2 w/ch 4 Pt 45 ppi 38(1.3, ) 51(1.0, 3) 42(1.0, 0.25) 75(2.0, ) 83(2.0, 3) 71(1.3, 0.5) Pt 80 ppi 37(1.3, ) 44(2.0, 3) 38(1.0, 2.0) 70(2.0, ) 73(2.0, 3) 74(2.0, 2.0) Rh 45 ppi 34(1.0, ) 46(1.0, 3) 37(1.0, 0.25) 61(2.0, ) 75(2.0, 3) 65(2.0, 0.25) Rh 80 ppi 20(2.0, ) 24(2.0, 3) 20(2.0, 0) 42(2.0, ) 49(2.0, 3) 42(2.0, 0) The values in parenthesis are the octane C/O and H 2 /O 2 (or CH 4 /O 2 ) ratios at which the maximum occurs.

11 G.J. Panuccio, L.D. Schmidt / Applied Catalysis A: General 313 (2006) causes the reaction to quench on the 45 ppi Pt foams because it acts as a heat load, decreases the oxygen partial pressure, and also does not stick to the surface and react. Combine these effects with a lower surface area to volume ratio and the overall heterogeneous reaction rate is not fast enough to keep the process autothermal Flames and explosions Experiments where hydrogen and oxygen are both fed as reactants are potentially dangerous because of the possibility of ignition or detonation. Conditions in the H 2 addition to n-octane CPO experiments are typically within the explosion limits of both hydrogen in air (18 59%) [23,24] and n-octane in air (1 7%) [25]. However, no flames or explosions are observed over the range of H 2 /O 2 and C/O ratios used in these experiments. Hydrogen addition experiments with lighter alkane fuels also do not result in homogeneous ignition upstream of the catalyst [6 8]. As with ethane and propane, the presence of octane in the feed must stabilize the mixture because the heavy hydrocarbon can successfully quench any radicals that may lead to propagation reactions. If this process was adopted for industrial production of olefins, pure oxygen will probably be used instead of air in order to eliminate the cost of separating N 2 from the product stream. The use of pure O 2 increases the flammability and explosive limits and makes reactor operation more dangerous. Extra care should be taken in designing a reactor that does not operate with N 2 dilution. 5. Conclusions The addition of a sacrificial fuel such as H 2 or CH 4 to the catalytic partial oxidation of naphtha-like fuels like n-octane can increase the yields of ethylene and other olefins by catalytically oxidizing some of the sacrificial fuel and allowing more n-octane to undergo homogeneous pyrolysis. Hydrogen addition gives the best performance by increasing olefin selectivities while keeping the octane conversion constant. The choice of catalyst metal and support pore size is flexible because 45 ppi Rh, 45 ppi Pt, and 80 ppi Pt catalysts all generate high selectivity to olefins when H 2 is co-fed with octane. However, this is accomplished at a net consumption of hydrogen, and the increased operational cost from purchasing hydrogen may not justify the increase in olefin yields. The performance of the CPO of n-octane with methane addition is very dependent on the catalyst metal and the pore size of the support structure. When 80 ppi Rh catalysts are used, the selectivity to olefins actually decreases while the selectivity to syngas increases. This is most likely due to methane steam reforming in the second zone of the catalyst bed. Methane addition to reactions on 45 ppi Rh and 80 ppi Pt catalysts slightly increases the selectivity to ethylene and other olefins. It may be worthwhile to add methane to a catalytic partial oxidation feed in order to increase olefin selectivities a few percent in chemical plants that have access to a methane waste stream that is ordinarily flared or is otherwise not utilized. Acknowledgement This work was partially supported by the United States Department of Energy Grant number DE-FG02-88ER References [1] K.M. Sundaram, M.M. Shreehan, E.F. Olszewski, Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, Inc., [2] H. Zimmermann, R. Walzl, Ullmann s Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA, [3] G.J. Panuccio, K.A. Williams, L.D. Schmidt, Chem. Eng. Sci. 61 (2006) [4] J.J. Krummenacher, K.N. West, L.D. Schmidt, J. Catal. 215 (2003) [5] R.P. O Connor, E.J. Klein, L.D. Schmidt, Catal. Lett. 70 (2001) [6] A.S. Bodke, D. Henning, L.D. Schmidt, S.S. Bharadwaj, J.J. Maj, J. Siddall, J. Catal. 191 (2000) [7] A. Bodke, D. Olschke, L.D. Schmidt, E. Ranzi, Science 285 (1999) [8] A. 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