Article pubs.acs.org/EF
Role of Potassium and Phosphorus in Catalytic Partial Oxidation in Short Contact Time Reactors Reetam Chakrabarti and Lanny D. Schmidt* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States ABSTRACT: Potassium and phosphorus represent inorganics commonly found in biomass and can significantly affect catalytic gasification of biomass. The alkaline and acidic nature of potassium and phosphorus, respectively, can introduce different reaction chemistries. In this paper, the effects of potassium and phosphorus on Rh catalysts for catalytic gasification have been studied at different fuel/oxygen ratios and temperatures. Catalytic partial oxidation (CPO) of ethanol, a biomass surrogate, was carried out over potassium, phosphorus, and monobasic potassium-phosphate-doped catalysts at loadings of 0.1, 1, 10, and 100% of Rh atoms over a range of fuel/oxygen ratios. In addition, time-on-stream studies were also carried out with methane by predosing inorganics on the catalyst (10% loading) and ethanol by introducing inorganics in the feed (0.05 mol %) to understand their effects on the stability of autothermal operation. With ethanol CPO, synthesis gas selectivities did not change significantly with potassium-doped catalysts, whereas they decreased with phosphorus-doped catalysts. Monobasic potassium phosphate showed synergistic effects of potassium and phosphorus; however, at higher temperatures, the effect of phosphorus was predominantly observed as a result of volatilization of potassium. Similarly, with time-on-stream studies with methane CPO, potassium volatilized at higher temperatures, while phosphorus retained its catalytic activity at higher temperatures. With the introduction of potassium and phosphorus in ethanol feed, steady-state autothermal operation was possible without any deactivation of the catalyst. In all experiments, phosphorus exhibited a stronger poisoning effect on rhodium for catalytic partial oxidation than potassium at similar concentrations.
■
INTRODUCTION Gasification of biomass to produce syngas represents an attractive route to upgrade biomass to carbon-based fuels and chemicals. Cellulose, hemicellulose, and lignin represent the prinicipal components of lignocellulosic biomass. In addition, different biomass sources contain different concentrations of inorganics, collectively referred to as ash, such as Si, Ca, Na, K, Ca, Mg, P, S, Cl, Fe, Cu, and Mn.1−3 These inorganics in biomass can change catalyst activity, reaction rates, and product distributions. Schmidt and co-workers have shown that lignocellulosic biomass containing small concentrations of inorganics and other biomass model compounds can be gasified autothermally over Rh-based catalysts to syngas with high yields in millisecond contact times at temperatures between 600 and 1000 °C.4−6 No tars or chars are formed in this process. For catalytic gasification of lignocellulosic biomass to be feasible, the effect of inorganics on catalyst activity should be minimal and the catalyst should be easily regenerable. At high temperatures in catalytic partial oxidation (CPO), certain inorganics may be volatile or may combine with other species in the reactor, such as CO, CO2, H2, H2O, or other inorganics, to form volatile compounds and leave the catalyst.7,8 In such a case, biomass feedstocks containing low concentrations of certain inorganics may be processed to syngas by catalytic gasification without significant catalyst deactivation. Potassium and phosphorus represent two of the major inorganic constituents of biomass. Potassium content in ash can vary from ∼4 to 50% and can compose up to 3% of the total dry weight of biomass.9,10 Potassium content is high in biomass feedstocks, such as straws and grasses. Phosphorus concentration in biomass ash, ∼0.1−8%, is lower than potassium concentration. Potassium and phosphorus can also enter © 2015 American Chemical Society
biomass sources, such as plants, through fertilizers. Because of the significant concentrations of phosphorus and potassium in biomass, understanding their effects on the catalyst is essential for CPO of lignocellulosic biomass to be feasible. In addition, potassium and phosphorus also have widely different electronic properties; potassium is electropositive, while phosphorus is electronegative. Most other biomass inorganics have electronegativities in between the two; thus studying the effects of potassium and phosphorus encompasses almost the entire range of interactions of common biomass inorganics with rhodium. In this paper, CPO of methane and ethanol is used to understand the effects of potassium and phosphorus on rhodium catalysts on reaction chemistries. Methane represents the simplest carbon species and has negligible homogeneous chemistry at 600−1000 °C, which are typical temperatures for CPO.11 CPO of methane also has an easily quantifiable product spectrum, consisting of CO, H2, CO2, and H2O. Hence, any change in chemistry or product distribution can be attributed to the change in activity introduced by the inorganics on rhodium catalysts. Ethanol represents a simple oxygenated molecule, and contains all of the bonds present in biomass: C−C, C−H, C− O, and O−H. In addition to producing syngas and combustion products, CPO of ethanol can also form other products, such as methane, and non-equilibrium products, such as ethylene, ethane, and acetaldehyde. Inorganics in biomass can also introduce acid or base chemistry, which are not typically Received: October 21, 2015 Revised: November 23, 2015 Published: November 24, 2015 8102
DOI: 10.1021/acs.energyfuels.5b02490 Energy Fuels 2015, 29, 8102−8109
Article
Energy & Fuels
number of atoms of carbon or hydrogen in a particular species to the total number of carbon or hydrogen atoms in the products; unconverted fuel was not counted in the product stream. The three types of experiments performed are listed below. Doping at Different Concentrations with Ethanol CPO. For the first set of experiments, each catalyst was aged with ethanol (C/O = 0.8) for 1 h. Baseline performance was measured at C/O = 0.8 and 1 to C/O = 1.75 (increments of 0.25). After the catalyst was aged at the lowest C/O ratio for each fuel for 1 h, baseline performance was measured from the lowest C/O (highest temperatures) to the highest C/O (lowest temperatures). Inorganics were then added to the catalyst through precursors (potassium acetate for potassium, phosphoric acid for phosphorus, and potassium phosphate monobasic to test for their combined effect) by the incipient wetness technique and then dried. Loadings are expressed in terms of the ratio of the number of atoms of potassium/phosphorus (or molecules of monobasic potassium phosphate) to the number of atoms of rhodium. Performance testing with the inorganic-doped catalyst was then carried out from the highest C/O ratio to the lowest C/O ratio, to minimize the effect of inorganics volatilizing at higher C/O ratios. After a particular doping experiment, if the range of differences of a particular quantity (conversion/selectivity) over a C/O range compared to baseline was greater than 10% (absolute), then the experiment was repeated (from the highest C/O ratio to the lowest C/O ratio) to test for thermal regeneration by volatilization at higher temperatures. As a result of volatilization, catalytic activity can be partially or completely restored at lower temperatures. Transient Studies with Methane CPO. To study transients involved during volatilization of potassium and phosphorus, a time-on-stream study was performed with methane CPO. Potassium (10%) and phosphorus (10%) were added to the rhodium catalyst (5 wt %) after baseline performance testing at C/O = 0.75 and 1.5. After doping, the catalyst was lit off with hydrogen and switched to methane at C/O = 0.75 and the performance was measured for ∼60 min. The C/O was then switched to 1.5 for ∼20 min, and the performance was measured, after which it was again switched back to 0.75. The cycle was repeated for a total of 6 h. Transient Studies with Ethanol CPO. The experiments described above involve predosing the catalyst with inorganics. To simulate more closely how inorganics in actual biomass reach the catalyst, a third set of experiments was performed by dissolving the inorganic in ethanol and performing CPO of inorganic-doped ethanol. Two 5 wt % Rh foams (semicircular cross-section from the top) were used as the catalyst and placed in the reactor. The concentration of inorganics in the feed was 0.05% mol of inorganic/mol of ethanol. The same ethanol flow rate of 1.2 mL/min was used at a C/O of 1, which corresponded to a baseline temperature of ∼800 °C, typical range during CPO.
observed in noble metal catalysis. These additional reactions promote or inhibit production of non-equilibrium products with ethanol CPO. In previous works, we have surveyed the effects of common biomass inorganics on CPO, such as Na, K, Ca, Mg, S, P, and Si, on rhodium catalysts.12,13 Here, the effects of potassium and phosphorus on rhodium catalysts are studied in detail by depositing them at different loadings and temperatures (inorganic concentration = 0.1, 1, 10, or 100% of Rh atoms) using ethanol CPO. In actual biomass, inorganics also exist in the combined state with one or more inorganics. To understand the synergistic effect of potassium and phosphorus, monobasic potassium phosphate was added at similar loadings. Time-on-stream studies were also performed with methane and ethanol to understand the transients involved during operation with inorganics and long-term stability of reactor operation.
■
EXPERIMENTAL SECTION
Reactor Setup. Experiments were carried out in a quartz tube (19 mm inner diameter). Gas flow rates to the reactor were controlled by mass flow controllers accurate to within ±2%. The catalyst consisted of a 45 pores per linear inch (ppi) α-alumina cylindrical monolith (17 mm inner diameter and 10 mm long), which was coated with 5 wt % Rh. Catalysts were prepared by the incipient wetness technique using rhodium nitrate as the rhodium precursor.14 Catalysts were then dried in a vacuum oven and calcined at 800 °C for 6 h. An uncoated 45 ppi α-Al2O3 monolith was used as a back heat shield. The entire assembly was wrapped in aluminosilicate cloth to prevent bypassing of gases and inserted into a quartz reactor tube. The reactor was wrapped with 1 in. thick ceramic fiber insulation to minimize heat losses. For methane CPO, another uncoated 45 ppi monolith was used as a front heat shield. For ethanol CPO, the experimental setup was similar to that reported by Rennard et al.15 Ethanol was fed in through a highperformance liquid chromatography (HPLC) pump. The reactor was lit off with hydrogen and then switched to either methane or ethanol. The carbon molar flow rate with methane and ethanol was approximately 0.04 mol/min, which is equivalent to a flow rate of 1 standard liters per minute (SLPM) for methane and 1.2 mL/min for ethanol. A nebulizer was used to create a fine mist of ethanol, which impinged upon the front face of the catalyst. A pyrex annulus was placed just above the tip of the nebulizer to prevent convective recirculation in the ethanol experiments. Argon, in place of nitrogen, was fed with oxygen at air stoichiometry (3.76). Argon acted as a diluent and as an internal standard for gas chromatography (GC) analysis. The C/O ratio is defined as the ratio of carbon atoms in fuel to the oxygen atoms in air; oxygen in ethanol was not counted in the denominator. Analysis. Products were analyzed and quantified using a HP 5890 series II gas chromatograph with a 60 m PLOT-Q column. Response factors of each compound were determined by calibrating with known concentrations of species with respect to argon as an internal standard. Hydrocarbon species were quantified on the flame ionization detector, while H2, CO, Ar, and CO2 were quantified with the thermal conductivity detector. Hydrogen and oxygen balances were closed on water, and an average of the two results was used to determine the water flow rate. Carbon, hydrogen, and oxygen balances typically closed within ±10%, and 95% confidence intervals were typically less than 3% for H2, CO, CO2, CH4, and C2H4 and less than 7% for ethanol, acetaldehyde, and water. The effluent was sampled after approximately 1 in. of the bottom edge of the catalyst. Design of Experiments. Three experiments were performed to study the interactions of inorganics with the rhodium-based catalyst: doping the catalyst at different levels for ethanol CPO, a transient study with methane by doping the inorganics on the catalyst, and a transient study with ethanol by introducing inorganics in the feed. The changes with respect to baseline values are expressed in terms of absolute values (final − initial = difference). Product selectivities were defined on a carbon or hydrogen (for H2 and H2O) basis as the
■
RESULTS Doping at Different Concentrations with Ethanol CPO. Negligible changes in product distributions and temperatures were observed at 0.1% loading of potassium, phosphorus, and monobasic potassium phosphate (≤5% change). Effect of Potassium. In the case of ethanol CPO with potassium-doped catalysts, autothermal operation was possible at all loadings from 0.1 to 100% and negligible changes in ethanol conversion and temperatures were observed (≤5%). CO selectivities were essentially unchanged from 0.1 to 10% loadings. At 100% loading, the CO selectivity decreased by ∼7% at all C/O ratios. Hydrogen selectivities increased by ∼7% at higher C/O ratios at loadings of 10 and 100%, indicating water-gas shift activity (eq 1). Since changes in conversions and selectivities were less than 10%, no regeneration was attempted. CO + H 2O ↔ CO2 + H 2 8103
ΔHr = −41 kJ/mol
(1)
DOI: 10.1021/acs.energyfuels.5b02490 Energy Fuels 2015, 29, 8102−8109
Article
Energy & Fuels
Figure 1. CO, H2, and C2H4 selectivities during ethanol CPO at 10 and 100% loading of phosphorus. White bars represent the performance of the undoped rhodium catalyst. Hatched bars indicate the performance of the phosphorus-doped catalyst from high C/O ratios to low C/O ratios. Gray bars represent data when the reactor was operated again with the doped catalyst from high C/O ratios to low C/O ratios to investigate for thermal regeneration.
Effect of Phosphorus. Autothermal operation was possible with all loadings of phosphorus from 0.1 to 100%. Ethanol conversion showed negligible changes at all loadings. The largest decrease in CO selectivities was observed at C/O = 1 at 10 and 100% loadings (panels A and B of Figure 1). The decrease in hydrogen selectivities compared to baseline values increased with temperatures (panels C and D of Figure 1). At 100% loading, it decreased by ∼10% at C/O = 1.75 and ∼30% at C/O = 0.8. Ethylene selectivites increased by ∼10% at 100%
loading at high C/O ratios, which decreased slightly upon regeneration (panels E and F of Figure 1). Changes in temperatures were generally less than 50 °C. No significant changes were observed upon regeneration at all loadings, indicating the presence of phosphorus on the catalyst, despite exposure to higher temperatures. Effect of Monobasic Potassium Phosphate. Similar to potassium and phosphorus, autothermal operation was possible with potassium phosphate (monobasic) at all loadings from 0.1 8104
DOI: 10.1021/acs.energyfuels.5b02490 Energy Fuels 2015, 29, 8102−8109
Article
Energy & Fuels
Figure 2. CO, H2, and C2H4 selectivities during ethanol CPO at 10 and 100% loading of KH2PO4. White bars represent the performance of the undoped rhodium catalyst. Hatched bars indicate the performance of the monobasic potassium-phosphate-doped catalyst from high C/O ratios to low C/O ratios. Gray bars represent data when the reactor was operated again with the doped catalyst from high C/O ratios to low C/O ratios to investigate for thermal regeneration.
(panels E and F of Figure 2). The ethylene selectivites at higher C/O ratios decreased by about 6% upon regeneration, unlike selectivites at lower C/O ratios (≤1.25). At loadings other than 100%, temperatures were within 50 °C of their baseline values. At 100% loading, they increased by ∼120 °C but then decreased at higher C/O ratios to within 50 °C of their original values. Thus, cumulative effects of potassium and phosphorus were observed with potassium phoshphate. The higher ethanol
to 100%. Ethanol conversions increased by about 10% from C/ O = 1.75 to 1.25 and decreased to their original values upon regeneration. Significant regeneration was observed in CO selectivities at higher C/O ratios (panels A and B of Figure 2). The decrease in hydrogen selectivities compared to baseline values was more at lower C/O ratios and was unaffected by regeneration by exposure to higher temperatures (panels C and D of Figure 2). At 100% loading, ethylene selectivities increased by ∼11% at high C/O ratios (1.75−1.25) and 4% at C/O = 0.8 8105
DOI: 10.1021/acs.energyfuels.5b02490 Energy Fuels 2015, 29, 8102−8109
Article
Energy & Fuels
absence of the catalytic activity of potassium as a result of potassium volatilization from the rhodium catalyst. Effect of Phosphorus. The temperatures and methane conversions during the 6 h performance testing period with 10% phosphorus are shown in Figure 4A. After lightoff, the temperature was about 1030 °C, after which it dropped to a steady value of 985 °C in ∼50 min. Methane conversion increased with time, reaching 79% after 50 min, lower than the baseline value of 88%. At this stage, hydrogen selectivity was 71%, as compared to the original value of 82% (Figure 4B). At C/O = 1.5, the temperature steadied at ∼835 °C (∼70 °C higher than the baseline) and the methane conversion was similar to undoped values. Hydrogen selectivity was much lower at 48% compared to a baseline of 68%. Subsequent cycling did not show any significant changes in temperatures, conversions, and selectivities at both C/O ratios of 0.75 and 1.5 compared to their values during the first cycle. Negligible changes in CO selectivities at both C/O ratios were observed during the 6 h study. The higher temperatures and lower methane conversions and hydrogen selectivities indicate inhibition of endothermic reforming activity by phosphorus, which, unlike potassium, is largely unaffected by maintaining higher temperatures. CO selectivities were unchanged after doping with phosphorus at both C/O ratios of 0.75 and 1.5. Transient Studies with Ethanol CPO. The effect of adding 0.05 mol % potassium and phosphorus in ethanol feed is described below. Effect of Potassium. Potassium maintained steady-state operation for the full 8 h doping period at a C/O ratio of 1, as shown in Figure 5A. Potassium increased the ethanol conversion from 89 to 100% (Figure 5A). Temperatures with potassium during doping were slightly higher than the baseline values by about 25 °C. Potassium decreased the carbon monoxide selectivities by ∼10% (65% before doping versus 55% after doping), while acetaldehyde selectivities increased from about 8 to 16% (Figure 5B). Hydrogen selectivities were similar to baseline values, and water selectivities also decreased from 40 to 33%. Effect of Phosphorus. Phosphorus similar to potassium maintained steady-state autothermal operation for the full 8 h (C/O = 1), as shown in Figure 6A. Doping with phosphorus resulted in increased temperatures of about 100 °C. Ethanol conversion also increased from 88 to about 98% (Figure 6A). The changes in CO2, H2, and C2H4 selectivities are shown in
conversions compared to baseline at high C/O ratios result from higher temperatures, thereby increasing reaction rates. Transient Studies with Methane CPO. To study the transients involved during volatilization of potassium and phosphorus, methane CPO of doped catalysts (1 atom of potassium or phosporus for every 10 atoms of rhodium) was performed by switching between C/O ratios of 0.75 (∼60 min) and 1.5 (∼20 min). The results with potassium and phosphorus are shown below. Effect of Potassium. The temperatures and methane conversion during the 6 h performance testing period with 10% potassium loading are shown in Figure 3. After lightoff on
Figure 3. CH4 conversions (■) and temperatures (solid lines) during methane catalytic partial oxidation for 6 h at C/O ratios of 0.75 and 1.5 at 10% loading of potassium. Dashed and dotted horizontal lines indicate undoped values of conversions and temperatures, respectively.
the doped catalyst with hydrogen, the temperature increased to ∼1050 °C with methane at C/O = 0.75 compared to the baseline value of 900 °C. At the end of 1 h, the temperature was ∼940 °C. Methane conversions also gradually increased during this initial period. After switching to C/O = 1.5, the temperatures and product distributions were close to undoped values. Upon switching back to C/O = 0.75, the temperature continued to drop, although at a slower rate than before, with methane conversions similar to baseline values. Temperatures steadied out after ∼100 min on stream, with the product distributions and temperatures at both C/O = 0.75 and 1.5 thereafter being similar to their original values, indicating the
Figure 4. (A) CH4 conversions (■) and temperatures (solid lines) during methane catalytic partial oxidation for 6 h at C/O ratios of 0.75 and 1.5 at 10% loading of phosphorus. Dashed and dotted horizontal lines indicate undoped values of conversions and temperatures, respectively. (B) H2 selectivities over 6 h at C/O of 0.75 and 1.5, Undoped values are shown by dashed horizontal lines. 8106
DOI: 10.1021/acs.energyfuels.5b02490 Energy Fuels 2015, 29, 8102−8109
Article
Energy & Fuels
Figure 5. (A) Ethanol conversions (■) and temperatures (solid lines) during catalytic partial oxidation with ethanol containing 0.05 mol % potassium. (B) CO (■) and CH3CHO (●) selectivities during the doping period. Arrows indicate the trends (increase/decrease) introduced upon doping.
Figure 6. (A) Ethanol conversions (■) and temperatures (solid lines) during catalytic partial oxidation with ethanol containing 0.05 mol % phosphorus. (B) CO2 (■), H2 (▲) and C2H4 (●) selectivities during the doping period. Arrows indicate the trends (increase/decrease) introduced upon doping.
feed, similar to actual biomass and at higher temperatures than that reported by Chakrabarti et al.13 These three experiments combined give insight into the mechanism of interactions of potassium and phosphorus with rhodium catalysts for CPO. Effect of Potassium. Effect on Lightoff. In the second set of experiments, potassium appeared to be a strong poison for methane CPO after lightoff with hydrogen compared to phosphorus. Lightoff occurred at higher temperatures with potassium-doped catalysts (∼600 °C) compared to ∼400 °C for phosporus-doped catalysts. It has been reported in the literature that potassium reduces the sticking probability of methane on metal surfaces, thereby making it more difficult for potassium-doped catalysts to lightoff.17−19 Lightoff temperatures for ethanol CPO with phosphorus and potassium-doped catalysts were lower but similar (∼350−400 °C), likely as a result of the higher reactivity of ethanol compared to methane. Effect on Chemistry. A decrease in CO selectivity was observed with ethanol CPO upon doping with potassium without any significant change in hydrogen selectivities. We have shown previously that potassium poisons reforming, which reduces CO and H2 selectivities.12 Increased temperatures during ethanol CPO with potassium doping confirm decreased endothermic reforming activity. Alkali metals have been shown to promote the water-gas shift reaction,20,21 reducing carbon monoxide and increasing hydrogen production. Due to their
Figure 6B. Phosphorus resulted in a strong decrease in hydrogen selectivities from 53 to about 19%. Carbon dioxide selectivities decreased from about 22 to 4%, while ethylene selectivities increased by from 2 to about 17%. Water selectivities increased from 35 to 57%, and acetaldehyde selectivities decreased from about 10 to 4% (not shown). Similar to the methane experiments, CO selectivities were essentially unchanged during the addition of phosphorus-doped ethanol.
■
DISCUSSION
The effects of potassium and phosphorus on rhodium catalysts have been studied by three sets of experiments. The first set of experiments by doping the rhodium catalyst at different inorganic concentrations for ethanol CPO compares the tolerance of rhodium catalysts to inorganics. Rhodium catalysts supported on α-Al2O3 foams have been shown to have dispersions ≤1%.16 Since significant changes were observed at loadings ≥10% (effectively 10 inorganic atoms for every atom of rhodium, considering rhodium dispersion of ∼1%), the results demonstrate strong resistance to poisoning of rhodiumbased catalysts. The second set of experiments involving timeon-stream studies of methane CPO help in understanding the volatility of potassium and phosphorus during CPO. The third set of experiments introduces potassium and phosphorus in the 8107
DOI: 10.1021/acs.energyfuels.5b02490 Energy Fuels 2015, 29, 8102−8109
Article
Energy & Fuels electropositive nature, alkali metals reduce the rhodium work function, thereby promoting CO dissociation, a key step for the water-gas shift reaction.22−24 Thus, because of a combination of decreased reforming and increased water-gas shift, CO selectivities are decreased, while hydrogen selectivities are unaffected. With ethanol CPO, increased acetaldehyde selectivities may be attributed to the alkaline nature of potassium.13,25 Thus, potassium manifests its effect even under conditions of CPO by interacting with the rhodium catalyst. Volatility of Potassium. With potassium, catalyst regeneration was possible at higher temperatures in all experiments. In methane experiments, at 1 and 10% loading, potassium results in reduced CO selectivities. However, at higher temperatures (low C/O ratios), partial catalyst regeneration was observed, which led to an increase in CO selectivities of almost 11% at C/ O of 2 with methane. Similar trends were observed at corresponding loadings of monobasic potassium phosphate, in that the effect of decreasing carbon monoxide selectivities is reduced at higher temperatures. However, since the monobasic potassium-phosphate-doped catalysts run hotter, complete regeneration of the catalyst was observed even at higher C/O ratios as a result of potassium volatilization. The time-onstream studies with methane CPO show the volatility of potassium at typical temperatures in CPO. Operation for less than 1 h at C/O = 0.75 resulted in complete regeneration of the catalyst activity at C/O = 1.5 as well. In the ethanol transient experiment, steady-state operation with potassium was obtained, indicating that the rate of potassium coming in through the feed is balanced by the rate at which it volatilizes. The potassium precursor used in all experiments is potassium acetate, which has been shown to decompose to potassium carbonate at 400−460 °C, and potassium carbonate decomposes at 750−1000 °C without leaving any residue as a result of volatilization of potassium.26 Effect of Phosphorus. Effect on Lightoff. With regard to lightoff, phosphorus did not appear to inhibit lightoff as did potassium. Phosphorus-doped catalysts lit off at approximately the same temperatures as undoped catalysts in the second set of experiments with methane CPO (∼400 °C). Effect on Chemistry. Phosphorus, unlike potassium, is electronegative and causes an increase in the rhodium work function, thereby reducing CO dissociation.22 Hence, negligible changes in CO selectivities are observed with phosphorus with methane CPO at all loadings and during transient experiments. A common trend in all phosphorus experiments was the decrease in hydrogen selectivities. This can be attributed to a combination of poisoning of the water-gas shift and reforming reactions by phosphorus on rhodium catalysts.12 Also, phosphorus increased ethylene selectivities in both the doped and transient experiments with ethanol as a result of its acidic nature. Since phosphorus is a strong reforming poison, there is increased homogeneous chemistry of ethanol, which has been shown to produce ethylene at temperatures typically observed during CPO.27 Thus, both homogeneous and phosphoruscatalyzed heterogeneous chemistries contribute to increased ethylene formation. In the third set of tranient experiments with phosphorus-doped ethanol, water selectivities increased as a result of a combination of the reduced water-gas shift, reduced steam reforming, and increased ethanol dehydration to ethylene. Volatility of Phosphorus. In the first set of experiments with ethanol CPO, no regeneration was observed for CO and H2
selectivities. Similarly, in the methane time-on-stream experiment, catalyst temperatures were stable at around 980 °C, which resulted in no observable regeneration, even after multiple temperature cyclings. This shows that phosphorus is much more strongly bound to the rhodium catalyst (possibly as rhodium phosphide Rh2P)12 and, therefore, less volatile than potassium, retaining its catalytic activity even after exposure to higher temperatures.
■
CONCLUSION The effects of potassium and phosphorus at different levels of doping on rhodium catalysts were studied on a 5 wt % Rh/αalumina catalyst using ethanol CPO. In addition, time-onstream experiments were also carried out with methane (predosing the catalyst with inorganics) and ethanol (introducing inorganics in the fed) CPO to evaluate the volatility of the inorganics. Potassium at low temperatures decreased CO selectivities, and its effect on the rhodium catalyst decreased at higher temperatures as a result of its volatile nature. Phosphorus at higher loadings showed a much more severe poisoning effect than potassium, which persisted even at high temperatures and strongly reduced hydrogen selectivities. Potassium phosphate (monobasic) showed cumulative poisoning effects, wherein the potassium component could be volatilized at higher temperatures, with only the catalytic effect of phosphorus being observed. Overall, rhodium-based catalysts at temperatures typically encountered during CPO showed strong resistance to poisoning by potassium and phosphorus, indicating that biomass feedstocks contaning up to moderate concentrations of these inorganics can be effectively processed by CPO to produce a high-selectivity syngas stream.
■
AUTHOR INFORMATION
Corresponding Author
*Telephone: +1-612-625-9391. Fax: +1-612-626-7246. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■ ■
ACKNOWLEDGMENTS The authors acknowledge funding from the Minnesota Corn Growers Association. REFERENCES
(1) Obernberger, I.; Biedermann, F.; Widmann, W.; Riedl, R. Concentrations of inorganic elements in biomass fuels and recovery in the different ash fractions. Biomass Bioenergy 1997, 12, 211−224. (2) Nutalapati, D.; Gupta, R.; Moghtaderi, B.; Wall, T. Assessing slagging and fouling during biomass combustion: A thermodynamic approach allowing for alkali/ash reactions. Fuel Process. Technol. 2007, 88, 1044−1052. (3) Di Blasi, C. Modeling chemical and physical processes of wood and biomass pyrolysis. Prog. Energy Combust. Sci. 2008, 34, 47−90. (4) Salge, J. R.; Dreyer, B. J.; Dauenhauer, P. J.; Schmidt, L. D. Renewable Hydrogen from Nonvolatile Fuels by Reactive Flash Volatilization. Science 2006, 314, 801−804. (5) Dauenhauer, P. J.; Dreyer, B. J.; Degenstein, N. J.; Schmidt, L. D. Millisecond Reforming of Solid Biomass for Sustainable Fuels. Angew. Chem., Int. Ed. 2007, 46, 5864−5867. (6) Colby, J. L.; Dauenhauer, P. J.; Schmidt, L. D. Millisecond autothermal steam reforming of cellulose for synthetic biofuels by reactive flash volatilization. Green Chem. 2008, 10, 773−783. (7) Dayton, D. C.; Jenkins, B. M.; Turn, S. Q.; Bakker, R. R.; Williams, R. B.; Belle-Oudry, D.; Hill, L. M. Release of Inorganic
8108
DOI: 10.1021/acs.energyfuels.5b02490 Energy Fuels 2015, 29, 8102−8109
Article
Energy & Fuels Constituents from Leached Biomass during Thermal Conversion. Energy Fuels 1999, 13, 860−870. (8) Baxter, L. L. Ash deposition during biomass and coal combustion: A mechanistic approach. Biomass Bioenergy 1993, 4, 85−102. (9) Miles, T. R.; Miles, T. R., Jr.; Baxter, L. L.; Bryers, R. W.; Jenkins, B. M.; Oden, L. L. Boiler deposits from firing biomass fuels. Biomass Bioenergy 1996, 10, 125−138. (10) Jenkins, B. M.; Baxter, L. L.; Miles, T. R., Jr.; Miles, T. R. Combustion properties of biomass. Fuel Process. Technol. 1998, 54, 17−46. (11) Bizzi, M.; Basini, L.; Saracco, G.; Specchia, V. Short contact time catalytic partial oxidation of methane: analysis of transport phenomena effects. Chem. Eng. J. 2002, 90, 97−106. (12) Chakrabarti, R.; Colby, J. L.; Schmidt, L. D. Effects of biomass inorganics on rhodium catalysts: I. Steam methane reforming. Appl. Catal., B 2011, 107, 88−94. (13) Chakrabarti, R.; Tupy, S. A.; Schmidt, L. D. Effects of biomass inorganics on rhodium catalysts: II. Ethanol Autothermal Reforming. Energy Fuels 2011, 25, 4763−4769. (14) Bodke, A.; Bharadwaj, S.; Schmidt, L. The Effect of Ceramic Supports on Partial Oxidation of Hydrocarbons over Noble Metal Coated Monoliths. J. Catal. 1998, 179, 138−149. (15) Rennard, D. C.; Kruger, J. S.; Schmidt, L. D. Autothermal Catalytic Partial Oxidation of Glycerol to Syngas and to Nonequilibrium Products. ChemSusChem 2009, 2, 89−98. (16) Colby, J. Catalytic Autothermal Reforming of Biomass to Synthesis Gas. Ph.D. Thesis, University of Minnesota, Minneapolis, MN, 2010. (17) Juan-Juan, J.; Román-Martínez, M.; Illán-Gómez, M. Effect of potassium content in the activity of K-promoted Ni/Al2O3 catalysts for the dry reforming of methane. Appl. Catal., A 2006, 301, 9−15. (18) Osaki, T.; Mori, T. Role of Potassium in Carbon-Free CO2 Reforming of Methane on K-Promoted Ni/Al2O3 Catalysts. J. Catal. 2001, 204, 89−97. (19) Bengaard, H.; Alstrup, I.; Chorkendorff, I.; Ullmann, S.; Rostrup-Nielsen, J.; Nørskov, J. Chemisorption of methane on Ni(100) and Ni(111) surfaces with preadsorbed potassium. J. Catal. 1999, 187, 238−244. (20) Panagiotopoulou, P.; Kondarides, D. I. Effects of alkali promotion of TiO2 on the chemisorptive properties and water-gas shift activity of supported noble metal catalysts. J. Catal. 2009, 267, 57−66. (21) Zhu, X.; Shen, M.; Lobban, L. L.; Mallinson, R. G. Structural effects of Na promotion for high water gas shift activity on Pt-Na/ TiO2. J. Catal. 2011, 278, 123−132. (22) Nørskov, J. K.; Holloway, S.; Lang, N. D. Microscopic model for the poisoning and promotion of adsorption rates by electronegative and electropositive atoms. Surf. Sci. 1984, 137, 65−78. (23) Barbier, J., Jr.; Duprez, D. Steam effects in three-way catalysis. Appl. Catal., B 1994, 4, 105−140. (24) Ratnasamy, C.; Wagner, J. P. Water Gas Shift Catalysis. Catal. Rev.: Sci. Eng. 2009, 51, 325−440. (25) Chimentao, R.; Herrera, J.; Kwak, J.; Medina, F.; Wang, Y.; Peden, C. Oxidation of ethanol to acetaldehyde over Na-promoted vanadium oxide catalysts. Appl. Catal., A 2007, 332, 263−272. (26) Afzal, M.; Ahmad, H.; Mahmood, F. Decomposition Kinetics of Metal Acetates. J. Chem. Soc. Pak. 1991, 13, 219−222. (27) Kruger, J. S.; Rennard, D. C.; Josephson, T. R.; Schmidt, L. D. Effect of Functional Groups on Autothermal Partial Oxidation of Biooil. Part 2: Role of Homogeneous and Support-Mediated Reactions. Energy Fuels 2011, 25, 3172−3185.
8109
DOI: 10.1021/acs.energyfuels.5b02490 Energy Fuels 2015, 29, 8102−8109
