Article pubs.acs.org/IECR
Autothermal Partial Oxidation of Glycerol to Syngas over Pt‑, LaMnO3‑, and Pt/LaMnO3‑Coated Monoliths Shih-Kang Liu and Yu-Chuan Lin* Department of Chemical Engineering and Materials Science, Yuan Ze University, 135 Yuan-Tung Road, Chungli, Taoyuan 32003, Taiwan S Supporting Information *
ABSTRACT: Glycerol and glycerol/water solution were converted to syngas over Pt-, LaMnO3-, and Pt/LaMnO3-coated monoliths under autothermal partial oxidation. XRD and SEM were used to characterize the phase compositions and surface morphologies of freshly prepared and postreaction catalysts. A continuous fixed-bed design, without an upstream vaporizer or external heat supply, was operated at millisecond contact times. Among the catalysts used in this study, Pt was the least reactive, and its activity was found to be reduced by sintering. Pt also tended to produce nonequilibrium products such as ethylene and acetaldehyde in fuel-rich environments. In contrast, LaMnO3 was found to favor glycerol combustion, producing fewer minor products and generating sufficient heat to sustain autothermal operations in fuel- and steam-rich regimes. Pt supported on LaMnO3 showed a synergistic effect of Pt sintering suppression and a broad H2/CO distribution in the resulting syngas. This effect can be correlated to the interaction between Pt and LaMnO3.
1. INTRODUCTION Biodiesel promises to ease fossil-fuel addiction. Biodiesel synthesis, through transesterification of triglycerides (e.g., vegetable oils and animal fats) and methanol, generates glycerol as a byproduct. Analysts predict that, in the coming decade, the global biodiesel market will grow to nearly twice its current size.1,2 This will inevitably produce surplus glycerol. By 2020, approximately 5.9 billion pounds of crude glycerol will be produced, more than triple the present supply.1 The glycerol oversupply crisis will not only cause environmental problems in waste treatment and storage, but also hamper the development of the biodiesel industry. This scenario can be prevented by expanding glycerol’s applications. Dehydration,3 hydrogenolysis,4 and selective oxidation5 are currently the primary methods of glycerol utilization.6 Another promising method is the transformation of glycerol into syngas and hydrogen. The former is the building block for chemicals, whereas the latter powers fuel-cell systems. Steam reforming (eq 1),7 aqueous-phase reforming,8,9 and partial oxidation (eq 2)10−12 transform glycerol into syngas and hydrogen. Among these methods, partial oxidation can be operated in a self-sustaining (i.e., autothermal) mode under ambient pressure.13 Partial oxidation converts glycerol with substoichiometric amounts of oxygen compared to glycerol combustion (eq 3). Sometimes, steam can be co-fed to improve the yield of syngas in autothermal partial oxidation (eq 4). This process not only saves energy, but also allows on-site waste glycerol abatement and methanol synthesis from syngas, providing a recycled biodiesel plant structural design.
C3H8O3 + C3H8O3 +
7 O2 → 3CO2 + 4H 2O 2
(3)
⎛3 ⎞ ⎛3 ⎞ ⎜ − x⎟O2 + ⎜ − y⎟H 2O ⎝2 ⎠ ⎝4 ⎠
→ [3 − (2x + y)]CO2 + (2x + y)CO +
⎛ 11 ⎞ ⎜ − y ⎟H 2 ⎝2 ⎠ (4)
10
Schmidt and co-workers pioneered partial oxidation of glycerol and aqueous glycerol solution using Pt- and Rh-based monoliths and found that syngas can be generated autothermally with proper insulation in short contact times. They recently employed a nebulizer injecting glycerol as small droplets into the catalytic zone with millisecond contact times11,12 and reported that a high glycerol feed rate together with the presence of O2 promotes oxidation, which yields sufficient heat to ignite the catalyst in autothermal mode. The ignited catalyst volatilizes and activates glycerol concurrently, allowing a design that does not require preheating. Because of high operating temperatures (above 800 °C), catalytic activity can be reduced by thermal sintering. Swami and Abraham14 and Douette et al.15 investigated the oxidative steam reforming of glycerol with Pd/Ni/Cu- and Ni-based catalysts. However, both of these studies required external heat to maintain isothermal operations, and char formation deactivated the catalysts. This study investigates the autothermal partial oxidation of glycerol and glycerol/water solution under adiabatic conditions using Pt-, LaMnO3-, and Pt/LaMnO3-coated monoliths as the catalysts. Platinum is more economically favorable than rhodium but tends to yield nonequilibrium products such as
C3H8O3 + x H 2O → (3 − x)CO + xCO2 + (4 + x)H 2 (1)
Received: Revised: Accepted: Published:
(3 − x) O2 → xCO + (3 − x)CO2 + 4H 2 C3H8O3 + 2 (2) © 2012 American Chemical Society
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Figure 1. Autothermal partial oxidation setup.
acetaldehyde in glycerol partial oxidation.11 LaMnO3 perovskite has been widely used in methanol partial oxidation16−18 and ethane oxidative dehydrogenation19−24 and exhibits great durability in oxidative environments. LaMnO3 is also active in hydrocarbon combustion because of its excess-oxygen nature caused by the presence of Mn4+ cations.19,24,25 For example, LaMnO3 was found to be more active than LaCoO3 and LaFeO3 in methanol combustion because of its greater oxygen availability and mobility.26 This might have a positive effect in converting undesired products in glycerol partial oxidation to syngas, allowing a process that does not require separation and recycling. The integration of Pt and LaMnO3 (Pt/LaMnO3) has already been demonstrated to be favorable for ethane oxidative dehydration, exhibiting high ethane conversion and durability.22 This approach promises high stability and low byproduct yields for syngas preparation. We consider this contribution to be the first attempt to investigate LaMnO3based catalysts in the autothermal partial oxidation of glycerol.
linear inch (ppi), and the catalyst measured 19 mm in diameter by 1 cm in length. The surface area of the monolith was 0.23 m2/g. The Pt catalyst was prepared by wet impregnation of the monolith with a 1 wt % dilute solution of H2PtCl6 in water. After being dried at 100 °C overnight, the sample was calcined in air at 700 °C for 3 h. The impregnation procedure was repeated several times until the Pt loading was approximately 1 wt %. The LaMnO3-coated monolith was synthesized using the active-phase deposition method.27 The monolith was first dipped repeatedly into a slurry consisting of γ-Al2O3 powder (10 wt %), nitric acid (65%), and pseudobohemite (10 wt %). After being coated with approximately 25 wt % of alumina, the monolith was calcined in air at 550 °C for 3 h. Subsequently, 5 wt % La2O3 was added to the alumina-washcoated monolith as a stabilizer by wet impregnation of an aqueous solution of La(NO3)3·6H2O. This was followed by drying at 120 °C overnight and calcination at 800 °C for 3 h in air. Finally, the LaMnO3 active phase (20 wt %) was wet-impregnated onto the monolith and calcined at 1000 °C for 3 h. The Pt/LaMnO3-loaded monolith was synthesized by Pt impregnation on a LaMnO3-coated monolith. The impregna-
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. A cordierite (50% SiO2, 34% Al2O3, and 16% MgO) honeycomb monolith was used as the support for each catalyst. The foam density was 20 pores per 16279
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balances of C, H, and O atoms, mostly within ±10%. Water was estimated based on the oxygen-atom balance. Reported data points were recorded in triplicate under steady-state conditions to provide a 95% confidence interval except for time-on-steam results. The glycerol conversion, product selectivity, and feed ratio are defined as follows
tion procedure and the Pt loading were the same as for the Ptcoated monolith. 2.2. Catalyst Characterization. X-ray diffraction (XRD) patterns were recorded using a Shimadzu Labx XRD-6000 instrument with Cu Kα radiation (0.15418 nm). Scans were taken at a scan rate of 4°/min in the 20−80° range (2θ). The voltage and current used were 40 kV and 30 mA, respectively. Surface morphologies and platinum particle sizes were surveyed by scanning electron microscopy (SEM, JEOL JSM-5600). The SEM accelerating voltage was 10 kV. The scanned areas were approximately 0.12 mm × 0.09 mm. The histogram of Pt particle size distribution was estimated based on 1000 particles per sample using SPIP software. Both as-synthesized and used catalysts were characterized by XRD and SEM. 2.3. Reactor Design and Product Analysis. Figure 1 shows the reactor setup for the autothermal partial oxidation of glycerol. It is similar to the design of Rennard et al.11,12 Prior to all trials, Pt and Pt/LaMnO3 were pretreated in a 10% H2/N2 stream at 350 °C for 1 h. Note that LaMnO3 can still maintain its oxidation state at 350 °C according to our earlier studies of H2 temperature-programmed reduction.16,17 Glycerol or glycerol/water solution entered the system through a syringe pump (KD Scientific, KDS-100) or high-performance liquid chromatography pump (Jasco, PU-2080) at room temperature. The glycerol feed rate was kept at 0.54 mL/min for both pure glycerol and glycerol/water mixtures. The liquid feed was blended with nitrogen (inlet pressure of 50 psi) and then entered a nebulizer (Burgener Research, SS-50). The nebulizer sprayed the feed in small droplets into the catalytic zone. The contact times were in the ranges 45−80 ms for Pt (1155−522 °C), 50−85 ms for LaMnO3 (1000−490 °C), and 59−77 ms for Pt/LaMnO3 (920−559 °C). Detailed contact time estimation, inlet compositions, and flow rates are listed in Appendix A (Supporting Information). These conditions were set to maintain steady-state autothermal operations. The catalytic zone was assembled from two active-phase-coated monoliths on top of a blank piece of foam. A K-type thermocouple was sandwiched between the two coated monoliths to record the reaction temperature under autothermal operations. Moreover, to investigate the effects of the length of the catalyst bed, glycerol partial oxidation over single Pt-, LaMnO3-, and Pt/LaMnO3-coated monoliths was investigated. This design does not require any heat supply or premixer; thermal insulation (alumina mat, Zircar ceramics) was placed around the catalytic zone after light-off. All trials were performed adiabatically. Part of the exhaust was condensed by a cold trap, and the remaining exhaust was vented to a fume hood. Noncondensable products were collected in gas bags. Trapped outflows were analyzed by gas chromatography (GC)−mass spectrometry (MS) (HP 5890 II instrument with a 5972 mass-selective detector and a DB-5MS capillary column, 60 m × 0.25 mm), and gaseous products were analyzed on a gas chromatograph equipped with a thermal conductivity detector, a flame ionization detector, and a methanizer (SRI-8610, molecular sieve 13X and silica gel columns). Identified compounds included nitrogen, oxygen, carbon oxides, hydrogen, methane, ethylene, propylene, acetaldehyde, and glycerol. Acrolein and hydroxyacetone were not detected. Oxygen was completely converted in every trial using two monoliths, whereas oxygen breakthrough was observed when a single monolith was used. Nitrogen served as the carrier gas and the internal standard for GC response factors. Molar effluents were quantified to close material
glycerol conversion (XG) moles of glycerol reacted = × 100% moles of glycerol injected carbon selectivity (SC) C atoms in species α = × 100% C atoms in converted glycerol hydrogen selectivity (SH) H atoms in species α = × 100% H atoms in converted glycerol feed carbon‐to‐oxygen ratio (C/O) C atoms in glycerol = O atoms in the feed O2
feed steam‐to‐carbon ratio (S/C) moles of water in the feed = C atoms in glycerol
3. RESULTS 3.1. Catalytic Performance of Two-Monolith Catalyst Bed. Figure 2 shows the glycerol conversion and temperature
Figure 2. (a) Glycerol conversion (XG) and (b) reaction temperature as functions of the S/C ratio over Pt monoliths. Error bars denote the 95% confidence interval of the data.
as functions of the S/C ratio of Pt. Pure partial oxidation (S/C = 0/3) showed the highest conversions of 99−92% for C/O ratios of 0.8−1.2. Temperatures were also highest under these conditions, ranging from 1155 to 763 °C. With increasing S/C ratio in the feed, both conversions and temperatures decreased significantly. In some cases, the system could not be operated autothermally (e.g., for C/O = 1.1 and 1.2 at S/C = 3/3 and 4/ 3). 16280
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Figure 3 displays hydrogen- and carbon-atom selectivities as functions of the S/C ratio over Pt. Data describing C/O = 0.9
Figure 4. (a) Glycerol conversion (XG) and (b) reaction temperature as functions of the S/C ratio over LaMnO3 monoliths. Error bars denote the 95% confidence interval of the data.
= 1.2, where the least amount of oxygen was fed, the system could still be operated autothermally. Figure 5 depicts hydrogen- and carbon-atom selectivities as functions of S/C ratio over LaMnO3 for varying C/O values.
Figure 3. Carbon- and hydrogen-atom selectivities as functions of the S/C ratio for autothermal partial oxidation of glycerol over Pt monoliths. Error bars denote the 95% confidence interval of the data.
and 1.1 are excluded for clarity, but they appear in Appendix B (Supporting Information). The conversions and temperatures corresponding to the lowest (0.8) and highest (1.2) C/O ratios were highest and lowest, respectively, compared to those at other C/O ratios. The selectivity to H2 showed a peak-shape trend, except for the curve at C/O = 1.2. The apex of each peak was located at S/C = 1/3. The maximum H2 selectivity, 122%, was obtained at C/O = 0.8 and S/C = 1/3. Carbon oxides accounted for more than 80% in all trials, with methane, ethylene, and propylene as side products. At C/O = 0.8, CO increased from 26% to 32% with increasing S/C ratio, whereas CO2 dropped from 74% to 63%. At higher C/O ratios, CO increased and CO2 decreased with increasing S/C ratio. Methane, ethylene, and propylene exhibited similar behaviors. At C/O = 0.8, these side products increased with S/C ratio. In contrast, the trends were reversed when C/O was increased from 0.8 to 1.2. The highest amount of these side species, including CH4, C2H4, C3H6, and C2H4O, accounted for approximately 20% of the carbon-atom selectivity at C/O = 1.2 and S/C = 0/3. Figure 4 shows glycerol conversions and temperatures as functions of the S/C ratio of the LaMnO3. For glycerol partial oxidation (S/C = 0/3), the glycerol conversions and temperatures were the highest. Glycerol was nearly completely converted from 1000 to 664 °C. With steam as the co-feed, both conversions and temperatures decreased. The lowest glycerol conversion and temperature were approximately 83% and 490 °C, respectively, at C/O = 1.2 and S/C = 4/3. At C/O
Figure 5. Carbon- and hydrogen-atom selectivities as functions of the S/C ratios for autothermal partial oxidation of glycerol over LaMnO3 monoliths. Error bars denote the 95% confidence interval of the data.
Appendix B of the Supporting Information presents a summary of data for all C/O ratios including 0.9 and 1.1. As for the Pt catalyst, the peak of H2 selectivity was observed for each C/O value at S/C = 1/3. Differences in H2 selectivity were insignificant at each S/C value compared to those for Pt, mostly less than 10%. The greatest H2 selectivity, approximately 79%, was obtained at C/O = 1.1 and S/C at 1/3. The CO level 16281
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decreased with decreasing C/O ratios, showing a peak over the S/C range. The highest amount of CO (approximately 49%) was produced at C/O = 1.2 and S/C = 1/3. CO2 selectivity seemed to correlate with the CO trend. The lowest CO2 selectivity corresponded to the condition where the highest CO was generated. LaMnO3 produced some minor products accounting for approximately 17% of the carbon-atom selectivity, especially at C/O = 1.2 in 1/3 < S/C < 3/3. Figure 6 shows glycerol conversions and temperatures as functions of the S/C ratio of Pt/LaMnO3. The conversion
Figure 6. (a) Glycerol conversion (XG) and (b) temperature as functions of the S/C ratio over Pt/LaMnO3 monoliths. Error bars denote the 95% confidence interval of the data.
Figure 7. Carbon- and hydrogen-atom selectivities as functions of the S/C ratios for autothermal partial oxidation of glycerol over Pt/ LaMnO3 monoliths. Error bars denote the 95% confidence interval of the data.
profiles were similar to those of the LaMnO3 except for those at low C/O values at S/C = 3/3 and 4/3, where slightly higher conversions (>85%) appeared. Compared to those for Pt and LaMnO3, the temperatures for this system were within a narrower interval for each C/O ratio. The highest temperature was 920 °C at C/O = 0.8 without steam as the co-feed, whereas the lowest temperature was 559 °C at C/O = 1.2 and S/C = 4/ 3. Figure 7 shows the hydrogen- and carbon-atom selectivities as functions of the S/C ratio over Pt/LaMnO3 for C/O ratios equal to 0.8, 1.0, and 1.2. Appendix B (Supporting Information) summarizes the data for C/O = 0.9 and 1.1 with varying S/C values. A peak was identified for hydrogen selectivity except in the case of C/O = 1.2, which decreased with increasing S/C ratios. Unlike Pt and LaMnO3, for this system, the highest H2 selectivity was located at S/C = 2/3 instead of 1/3. The greatest H2 selectivity was about 93% at C/ O = 0.8 and S/C = 2/3. The CO content showed an upward trend, whereas the CO2 level displayed a downward trend with increasing S/C ratio. The highest amounts of CO and CO2 were achieved at C/O = 1.2 and 0.8, respectively. The sum of CH4, C2H4, and C3H6 accounted for less than 10% of the carbon selectivity for all S/C values. At S/C = 2/3, these light hydrocarbons showed the highest selectivity, but it was no more than 10%. The selectivities of CH4, C2H4, and C3H6 over Pt/LaMnO3 were generally smaller than those over LaMnO3. At low S/C ratios, Pt/LaMnO3 produced less ethylene and propylene than Pt. 3.2. Catalytic Performance of a Single Pt, LaMnO3, or Pt/LaMnO3 Monolith. Appendix C (Supporting Information) includes the catalytic results for single Pt-, LaMnO3-, and Pt/ LaMnO3-coated monoliths. As in the aforementioned cases, the
glycerol conversion and temperature decreased with increasing C/O and S/C ratios for all catalysts (see Figures C1, C3, and C5 of the Supporting Information). However, the glycerol conversion and temperature became lower, and more points could not be operated autothermally than when two monoliths were used under the same conditions. Note that at S/C = 4/3, Pt could not achieve an autothermal mode even at the lowest C/O ratio (0.8). Pure partial oxidation (S/C = 0) displayed the highest glycerol conversions, ranging from 80% to 60% for Pt, from 97% to 63% for LaMnO3, and from 97% to 88% for Pt/ LaMnO3 with increasing C/O ratio. The measured temperatures were in the ranges of 615−476 °C for Pt, 662−491 °C for LaMnO3, and 635−523 °C for Pt/LaMnO3. Oxygen breakthrough was also identified (see Tables C1−C5 of the Supporting Information). The product distributions showed similar trends for all catalysts: H2 and CO increased whereas H2O and CO2 decreased with increasing S/C ratio. At C/O = 0.8, relatively high CO2 and H2O were achieved. The maximum amounts of CO2 (60% for Pt, 70% for LaMnO3, and 84% for Pt/LaMnO3) and H2O (83% for Pt, 85% for LaMnO3, and 94% for Pt/ LaMnO3) were obtained without co-feeding steam. Compared to the results for two monoliths, relatively low amounts of CH4, C2H4, and C3H6 were generated, whereas considerable C2H4O was produced, mostly greater than 10%. At high C/O and S/C ratios with low temperatures (
