Catalytic pyrolysis of amino acids: Comparison of aliphatic amino acid and cyclic amino acid

Energy Conversion and Management 112 (2016) 220–225

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Catalytic pyrolysis of amino acids: Comparison of aliphatic amino acid and cyclic amino acid Guangyi Liu a,b, Mark M. Wright c,⇑, Qingliang Zhao a,d,⇑, Robert C. Brown b,c, Kaige Wang b,c, Yuan Xue b,c a

School of Municipal & Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China Bioeconomy Institute, Iowa State University, Ames, IA 50011, United States c Department of Mechanical Engineering, Iowa State University, Ames, IA 50011, United States d State Key Laboratory of Urban Water Resources and Environments (SKLUWRE), School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China b

a r t i c l e

i n f o

Article history: Received 3 November 2015 Accepted 11 January 2016 Available online 23 January 2016 Keywords: Leucine Proline Catalytic pyrolysis Hydrocarbon Nitrogen Deoxygenation

a b s t r a c t Catalytic pyrolysis (CP) of protein-rich biomass such as microalgae is a promising approach to biofuel production. CP of amino acids can help understand the cracking of protein-rich biomass in the presence of zeolite catalysts. In this study, as representatives of aliphatic amino acid and cyclic amino acid, respectively, leucine and proline were pyrolyzed with ZSM-5 catalyst in a Tandem micro-furnace reactor coupled with a MS/FID/TCD. At 650 °C, leucine produced more hydrocarbons (aromatic hydrocarbons of 29.6%, olefins of 34.9% and alkanes of 8.1%) than proline (aromatic hydrocarbons of 25.3%, olefins of 14.0% and alkanes of 5.5%) because its relatively simpler amino structure readily detached as ammonia during CP. However, with an N-cyclic structure, proline produced large quantities of nitrogencontaining heterocyclic compounds that favored coke formation in CP. Accordingly, 28.2% of the nitrogen in proline was retained in the solid residue while most of the nitrogen in leucine was converted into ammonia leaving only 4.3% in the solid residue. In addition, though decarboxylation to carbon dioxide was favored in non-catalytic pyrolysis of leucine and proline, decarbonylation to carbon monoxide became the primary deoxygenation pathway in CP. These results indicate that the chemical structures of amino acids have significant effects on product distributions during CP and N-cyclic amino acid is less favored in CP for production of hydrocarbons and ammonia. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Pyrolysis has been considered as a promising method for producing bio-fuels and bio-chemicals to replace fossil fuels [1]. Most work to date has focused on lignocellulosic biomass as feedstock [2,3]. More recently, protein-rich biomass such as microalgae and other species have been investigated as feedstocks because of their potential for high biomass productivity compared to terrestrial plants [4]. Pyrolysis of biomass yields a viscous liquid known as bio-oil [1]. Bio-oil from protein-rich biomass has high heating value, desirable pH and low total acid number [5–7]. However, it contains many oxygenates [8], which contributes to its instability in storage and ⇑ Corresponding authors at: School of Municipal & Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China. Tel.: +86 451 86283017; fax: +86 451 86282100 (Q. Zhao). Tel.: +1 515 294 0913; fax: +1 515 294 8993 (M.M. Wright). E-mail addresses: [email protected] (M.M. Wright), [email protected] (Q. Zhao). http://dx.doi.org/10.1016/j.enconman.2016.01.024 0196-8904/Ó 2016 Elsevier Ltd. All rights reserved.

makes it incompatible with commercial infrastructure designed for hydrocarbons [9]. On the other hand, the high nitrogen content of protein-rich biomass compromises the usefulness of the resulting oil as fuel [10,11] due to environmental concerns such as photochemical smog and acid rain [12]. Much research [13–15] has emerged focusing on catalytic pyrolysis to improve the quality of pyrolysis products of proteinrich biomass. Among the catalysts reported, ZSM-5 is relatively inexpensive and robust and is often employed in catalytic pyrolysis studies for hydrotreating and deoxygenation. ThangalazhyGopakumar et al. [16] conducted catalytic pyrolysis of Chlorella vulgaris with ZSM-5 catalyst and confirmed that the negative attributes of algae bio-oil such as high content of oxygen and nitrogen could be removed by reacting the biomass in the presence of catalyst. Wang et al. [17] employed ZSM-5 catalyst to convert dried distillers grains with solubles (DDGS) in a Tandem microscale reactor system and reported significant yields of hydrocarbons (56.8 C %) and conversion of nitrogen in the biomass to ammonia (45.1 N %). Although these studies reveal that ZSM-5 catalyst is

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effective in removing oxygen and nitrogen from the products of pyrolysis, it is difficult to describe the underlying mechanisms due to the complex composition of biomass. Researchers employed corn protein [17], egg white [18], soybean protein [19] as model compounds of protein to investigate the pyrolysis process of protein-rich biomass but the results were not consistent. This is because protein has a very complex structure, and its amino acid composition varies greatly from one biomass species to another which plays a significant role in product distributions during protein pyrolysis [20]. Therefore, an investigation of the amino acids pyrolysis process is needed to help understand the cracking of protein-rich biomass [21]. Pyrolysis of amino acids has been widely reported in the last several decades. Haidar et al. [22] pyrolyzed leucine and proline to investigate gas emissions. They found that leucine yielded relatively high carbon monoxide while proline yielded negligible carbon monoxide but large amounts of carbon dioxide through decarboxylation. Li et al. [23] investigated the thermal behavior of leucine and observed large amount of primary amine compounds while Sharma [24] conducted pyrolysis of proline and obtained large amounts of nitrogen-containing heterocyclic compounds including pyrrole, indole and pyridine etc. These reports demonstrated that chemical structures can significantly affect the pyrolysis process of amino acids. However, to our knowledge, limited work has focused on catalytic pyrolysis of amino acids. In order to better understand the influence of chemical structures on catalytic pyrolysis of amino acids, we employed leucine and proline, in this paper, as representatives of aliphatic amino acid and cyclic amino acid, respectively. They were pyrolyzed in a Tandem micro-furnace reactor coupled with MS/FID/ TCD. Yields of aromatic hydrocarbons, olefins and alkanes from leucine and proline were investigated. Removal of oxygen and nitrogen in catalytic and non-catalytic pyrolysis were comparatively examined.

2. Materials and methods 2.1. Materials Leucine and proline were selected as representatives of aliphatic amino acids and cyclic amino acids, respectively. They were purchased from Sigma–Aldrich Co, USA with a purity of 99%. Characteristics of leucine and proline are shown in Table 1. Effective hydrogen index (EHI) was calculated as (H 2O 3N)/C, where H, O, N and C are the number of moles of hydrogen, oxygen, nitrogen and carbon in amino acid [25]. ZSM-5 (CBV2314) with a silica to alumina ratio of 23 was purchased from Zeolyst, USA. The received catalyst was calcined in a muffle furnace at 550 °C for 5 h. The calcined catalyst was mixed with amino acid at a catalyst-to-amino acid weight ratio of 20:1 until the mixture was homogeneous. Both catalyst and catalyst/ amino acid mixtures were stored in sealed glass vials in a

desiccator to minimize moisture adsorption prior to the pyrolysis experiments. 2.2. Catalytic pyrolysis experiments Catalytic pyrolysis experiments were conducted in a tandem micro-furnace pyrolyzer (Rx-3050 TR, Frontier Laboratories, Japan). The micro-furnace was temperature controlled within the range of 40–900 °C with an interface heater operating at temperatures of 100–400 °C that prevents undesired temperature drops of the pyrolysis vapors as they exit the furnace. For a typical run, approximately 5 mg of catalyst/amino acid mixture was pyrolyzed in the first furnace at the desired temperature while the temperature of the second furnace and the interface were held at 320 °C to prevent product condensation. The products were analyzed by a gas chromatograph (GC, 7890A, Agilent Technologies, USA) installed with a three-way splitter that directed the gas stream to three GC columns. The GC oven temperature was programmed for a 3-min hold at 40 °C then increased at a 10 °C/min rate to 250 °C and finally held constant for 6 min. The injector temperature was 250 °C and the total helium flow passing through the reactor was 90 ml/min. Two identical capillary columns, Phenomenex ZB 1701 (60 m, 0.250 mm and 0.250 lm film thickness) were used to analyze the condensable volatiles: one was connected to a mass spectrometer (MS) (5975C, Agilent Technologies, USA) for compound identification, and the other one was connected to a flame ionization detector (FID) for product quantification. After molecular identification of peaks with MS, standard samples dissolved in methanol at 5 different level points were prepared to calibrate FID for products quantification [14]. A Porous Layer Open Tubular (PLOT) column (60 m, 0.320 mm) (GSGasPro, Agilent, USA) was connected to a thermal conductivity detector (TCD) for measurement of non-condensable gas (NCG) products including CO, CO2, CH4, C2H4, C2H6, C3H6, C3H8, C4H8 and C4H10. A standard gas mixture (Praxair, USA) was used to calibrate the yield of NCG. Carbon and nitrogen content in the solid residue was quantified with an elemental analyzer (vario MICRO cube, Elementar, USA). For nitrogen-containing gas collection, the micro-furnace pyrolyzer was detached from the GC and connected through a tar trap to a NaOH (0.1 mol/L) solution for HCN, or H2SO4 (0.1 mol/L) solution for NH3, respectively [19]. After collection, NH+4 and CN were analyzed by DR 3900 Benchtop Spectrophotometer (HACH, USA). All product yields were reported as molar carbon/oxygen/ nitrogen yield, which was defined as the molar ratio of carbon/ oxygen/nitrogen in a specific product to total moles of carbon/oxygen/nitrogen in the feedstock. Selectivity for aromatic hydrocarbons/olefins/alkanes in this study was defined as the molar ratio of carbon in a specific aromatic hydrocarbon/olefin/ alkane to total moles of carbon in aromatic hydrocarbons/olefins/ alkanes products. All measurements were performed in duplicate to mitigate the impacts of experimental error.

Table 1 Characteristics of leucine and proline. Name

Molecular formula

Effective hydrogen index

Leucine

Chemical structure

C6H13NO2

1.0

Proline

C5H9NO2

0.4

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3. Results and discussion 3.1. Effect of temperature on carbon distributions 3.1.1. Aromatic hydrocarbon Leucine and proline were pyrolyzed in several tests over the temperature range of 450–750 °C with ZSM-5 catalyst-to-amino acid weight ratio at 20:1. The aromatic hydrocarbon yields are shown in Fig. 1. Leucine produced more aromatic hydrocarbons than proline. Temperature had limited effect on aromatic hydrocarbon yield of leucine. However, aromatic hydrocarbon yield of proline depended more on reaction temperature especially below 650 °C. At 450 °C, the carbon yield of aromatic hydrocarbons was only 10.4% but significantly increased to 25.3% at 650 °C. These results could be due to the different chemical structures of leucine and proline. Leucine has simple chemical structure including an amino and a carboxyl which would be readily removed as ammonia and carbon oxide, respectively [23]. Thereafter, the saturated hydrocarbon side chain detached from leucine would crack down to small alkanes and olefins and be subsequently transformed into aromatic hydrocarbons by ZSM-5 catalyst [26–28]. However, proline has an N-cyclic structure side chain and produced large amount of nitrogen-containing heterocyclic compounds which require more thermal energy to remove the nitrogen [24]; thus, higher temperature is required to increase aromatic hydrocarbons yield of proline. Although proline yielded fewer aromatic hydrocarbons at temperatures below 550 °C, aromatic hydrocarbon yields were similar for leucine and proline above 650 °C. This indicates that the differences in aromatic hydrocarbon yields caused by the different side chain structures of amino acids could be mitigated by operating at higher catalytic pyrolysis temperatures.

3.1.2. Residue formation Residue yields from leucine and proline pyrolyzed at 450 °C to 750 °C with ZSM-5 catalyst-to-amino acid ratio of 20:1 are shown in Fig. 2. Unlike aromatic hydrocarbons, residue formation from both leucine and proline were significantly affected by reaction temperature. The residue yields of leucine and proline were 14.8% and 61.3%, respectively, at 450 °C, drastically decreasing to 1.1% and 15.8% at 750 °C. This result is consistent with previous studies that reported high temperatures favored volatile release and decreased solid residue formation [29]. Although neither leucine nor proline produced char for pyrolysis temperatures greater than 450 °C [23,30], residue (char and coke) formation from

Aromac hydroCarbon Yield (C %)

35

Fig. 2. Effect of reaction temperature on yield of residue during catalytic pyrolysis of leucine and proline (catalyst-to-amino acid weight ratio of 20:1).

proline was much greater than from leucine indicating that coke formation under catalytic pyrolysis conditions were different for each. The N-cyclic structure of proline produced large quantities of nitrogen-containing heterocyclic compounds such as pyrrole derivatives, indole derivatives and quinoline derivatives [24] which are unsaturated compounds lacking hydrogen and favor coke formation over ZSM-5 catalyst [31] resulting in higher carbonaceous residue yields. Unlike proline, leucine has a saturated hydrocarbon side chain, which would decompose to small alkanes and olefins to enter ZSM-5 catalyst pores and be converted into aromatics [26–28], resulting in less carbonaceous residue. This phenomenon can also be understood in terms of the different Effective hydrogen index (EHI) for leucine (EHI = 1.0) and proline (EHI = 0.4) because feedstock with lower EHI tend to produce more residue during catalytic pyrolysis with ZSM-5 catalyst [31]. 3.1.3. Light hydrocarbons Light hydrocarbons produced during catalytic pyrolysis of leucine and proline were mainly olefins and alkanes which are shown in Figs. 3 and 4, respectively. Similar to aromatic hydrocarbons yield, olefin yield from leucine was relatively stable across the reaction temperatures investigated in this study. In the case of proline, higher temperatures favored olefin production. As discussed previously, the decomposition of proline into low molecular weight species required higher energy input. Thus, the reaction temperature had more effect on catalytic pyrolysis of proline than leucine. Finally, the olefin yield from leucine was twice that of proline because simple amino and carboxyl of leucine would be readily converted into ammonia and carbon oxide, respectively [23].

30 25 20 15

Leucine Proline

10 5 450

550

650

750

Reacon Temperature (°C) Fig. 1. Effect of reaction temperature on aromatic hydrocarbon yield for catalytic pyrolysis of leucine and proline (catalyst-to-amino acid weight ratio of 20:1).

Fig. 3. Effect of reaction temperature on olefin yield for catalytic pyrolysis of leucine and proline (catalyst-to-amino acid weight ratio of 20:1).

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Fig. 4. Effect of reaction temperature on alkane yield for catalytic pyrolysis of leucine and proline (catalyst-to-amino acid weight ratio of 20:1).

The detached saturated hydrocarbon side chain would facilitate the formation of light hydrocarbons. Compared to olefins, the yield of alkanes from both leucine and proline were relatively low which is consistent with other studies on catalytic pyrolysis of biomass. Previous studies reported that the main hydrocarbon products were aromatic hydrocarbons and olefins [17,32]. As was the case for aromatic hydrocarbons and olefins, the yield of alkanes from proline was more sensitive to reaction temperature than from leucine. For leucine, alkane yields were stable around 5% in the temperature range of 450 °C to 550 °C and increased to 9.8% at 750 °C. For proline, alkanes were undetectable below 550 °C but increased to 10.4% at 750 °C.

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consistent to the result in catalytic pyrolysis of methanol with ZSM-5 [31] that high selectivity of xylene and low selectivity of benzene were observed. However, compared to leucine, proline has an N-cyclic structure which favors formation of nitrogencontaining heterocyclic compounds [24] which are unsaturated compounds lacking hydrogen. These compounds would go through oligomerization to form coke over ZSM-5 catalyst [32]and hinder catalyst performance to encourage formation of poly-aromatic compounds [33]. This result is consistent with previous findings that found catalytic pyrolysis of proline produced more carbonaceous residue than leucine. Olefins produced during catalytic pyrolysis were categorized as ethylene, propene and butene. Their selectivities for catalytic pyrolysis of leucine and proline at 650 °C are shown in Fig. 6. Compared to proline, leucine had higher selectivity for butene and lower selectivity for ethylene. The saturated hydrocarbon side chain of leucine, after deoxygenation and denitrogenation, favored formation of C4 hydrocarbons such as butane and butane [34], resulting in relatively high selectivity for butene compared to ethylene. On the other hand, through denitrogenation, the N-cyclic structure of proline is able to decompose to relatively small molecules, resulting in higher selectivity for ethylene compared to butene. Alkanes were categorized as methane, ethane, propane and butane. Their selectivities for catalytic pyrolysis of leucine and proline at 650 °C are shown in Fig. 7. Similar to olefins, selectivity for alkanes were different for catalytic pyrolysis of leucine and proline. Proline favored formation of methane while leucine favored butane production. During the denitrogenation of proline, the N-cyclic

3.2. Hydrocarbon selectivity Aromatic hydrocarbons were categorized as benzene, toluene, xylene, C9, and C10+ compounds [15]. The selectivity for aromatic hydrocarbons at 650 °C are shown in Fig. 5. Although aromatic hydrocarbon yields from leucine and proline were similar, there was a statistically significant difference in their selectivities. Compared to leucine, proline showed higher selectivity for benzene and C10+ compounds and lower selectivity for xylene. The saturated hydrocarbon side chain detached from leucine is rich in hydrogen and favor of producing xylene rather than benzene which is

Fig. 6. Olefin selectivity for catalytic pyrolysis of leucine and proline at 650 °C (catalyst-to-amino acid weight ratio of 20:1).

Fig. 5. Aromatic hydrocarbon selectivity for catalytic pyrolysis of leucine and proline at 650 °C (catalyst-to-amino acid weight ratio of 20:1).

Fig. 7. Alkane selectivity for catalytic pyrolysis of leucine and proline at 650 °C (catalyst-to-amino acid weight ratio of 20:1).

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side chain broke apart and formed large quantities of small hydrocarbons such as methane and ethane. However, leucine has a relatively simple chemical structure making it amenable to deoxygenation and denitrogenation without major decomposition of the side chain [34]. Therefore, leucine favored production of relatively larger alkanes such as butane. 3.3. Oxides products During catalytic pyrolysis of leucine and proline, oxygen was removed through decarbonylation as carbon monoxide, decarboxylation as carbon dioxide, and dehydration as water. These reactions can occur in both non-catalytic pyrolysis and catalytic pyrolysis. Table 2 shows the oxides products of non-catalytic pyrolysis and catalytic pyrolysis of leucine and proline at 650 °C. Water yields were calculated by difference as 100-catalytic CO-catalytic CO2, assuming that oxygen was fully released as water and carbon oxides [32]. During non-catalytic pyrolysis of both leucine and proline, oxygen was released mostly as carbon dioxide indicating that decarboxylation was the predominant deoxygenation pathway. Jie [23] also observed this phenomenon during TGA tests of leucine and declared that decarboxylation was the main deoxygenation reaction during leucine pyrolysis. However, the deoxygenation of leucine and proline were quite different with respect to carbon monoxide. As shown in Table 2, the yields of carbon monoxide during noncatalytic pyrolysis of leucine and proline were 31.1% and 2.2%, respectively. These results were consistent with the different chemical structures of these two amino acids. During non-catalytic pyrolysis of leucine, diketopiperazines (DKP) were formed and eventually decomposed into isocyanic acid, which was the main precursor to carbon monoxide [22]. Compared to leucine, the nitrogen in proline was locked within a cyclic structure, isolating its carboxyl group from other functional groups. Therefore, decarboxylation predominated during non-catalytic pyrolysis of proline with negligible carbon monoxide produced from decarbonylation [22]. During catalytic pyrolysis of leucine and proline, deoxygenation behavior was markedly different. Large amounts of oxygen were released as carbon monoxide while the yield of carbon dioxide was even lower than that of non-catalytic pyrolysis. This phenomenon indicates that ZSM-5 significantly changed the deoxygenation pathway. The presence of strong acid sites in ZSM-5 catalyst could be responsible for this observation. During catalytic pyrolysis, amino acids existed in a Zwitterion form [35], and the oxygen atom with a free electron is attracted to the Brunsted acid sites of ZSM-5. This would hinder decarboxylation and promote decarbonylation. Although the oxides products of leucine and proline in non-catalytic pyrolysis were quite different, they became similar in catalytic pyrolysis. These results indicate that ZSM-5 catalyst mitigated differences in deoxygenation pathway of leucine and proline during catalytic pyrolysis. 3.4. Fate of nitrogen The fate of nitrogen during non-catalytic and catalytic pyrolysis of leucine and proline at 650 °C are shown in Table 3. No solid

Table 3 Distributions of nitrogen for non-catalytic and catalytic pyrolysis of leucine and proline at 650 °C (catalyst-to-amino acid weight ratio of 20:1). Sample

Leucine

Condition

Non-catalytic

Catalytic

Proline Non-catalytic

Catalytic

NH3 (%) HCN (%) Residue (%)

44.4 ± 2.73 2.6 ± 0.16 0.0 ± 0.00

81.7 ± 3.66 6.3 ± 0.03 4.3 ± 0.10

14.2 ± 1.65 2.3 ± 0.08 0.0 ± 0.00

59.7 ± 2.62 5.8 ± 0.14 28.2 ± 1.34

residue was detected from non-catalytic pyrolysis of leucine or proline. Nitrogen in amino acids were converted into nitrogencontaining organic compounds or released as ammonia and hydrogen cyanide among non-condensable gases. This was consistent with previous findings on fast pyrolysis of sludge [36] and coal [37]. During catalytic pyrolysis, ZSM-5 catalyst significantly enhanced denitrogenation, transforming most of the nitrogen into ammonia and leaving small quantities of nitrogen in the residue. Nitrogen distributions for catalytic pyrolysis of leucine and proline were different: proline yielded more nitrogen in the residue while leucine yielded more as ammonia. Amino in leucine was structurally simpler and more readily decomposed to ammonia than the N-cyclic structure of proline. The N-cyclic structure of proline favored formation of nitrogen-containing heterocyclic compounds that readily polymerized and dehydrated to nitrogen-containing residue. During catalytic pyrolysis, hydrogen cyanide production was also enhanced for both leucine and proline which is consistent to other researches on catalytic pyrolysis of microalgae [15] and dried distillers grains with solubles (DDGS) [17]. This indicates that proper devise should be added during or after catalytic pyrolysis of protein-rich biomass to remove the toxic compound from the product stream. 4. Conclusions In this paper, leucine and proline, as representatives of aliphatic amino acid and cyclic amino acid, respectively, were investigated during catalytic pyrolysis (CP) with ZSM-5 catalyst. During catalytic pyrolysis, both leucine and proline were primarily converted into aromatic hydrocarbons, olefins, and alkanes. Carbon yields were distinct for leucine and proline due to their different chemical structures: with its saturated hydrocarbon side chain, leucine produced more hydrocarbons than proline; with its N-cyclic structure, proline yielded more solid residue than leucine. ZSM-5 catalyst significantly altered the deoxygenation pathways for both leucine and proline. As the primary deoxygenation pathway in non-catalytic pyrolysis, decarboxylation was restricted in catalytic pyrolysis and oxygen was mainly removed as CO through decarbonylation. Most of the nitrogen in both leucine and proline was converted into ammonia during catalytic pyrolysis. However, the nitrogen distributions of leucine and proline were distinct due to the different forms of nitrogen in these two amino acids. The N-cyclic structure of proline produced considerably more nitrogen-containing solid residue than leucine while the simple amino structure of leucine yielded more ammonia than proline. These results indicate

Table 2 Yields of oxides from non-catalytic and catalytic pyrolysis of leucine and proline at 650 °C (catalyst-to-amino acid weight ratio of 20:1).

a

Sample

Leucine

Condition

Non-catalytic

Catalytic

Proline Non-catalytic

Catalytic

Carbon monoxide (%) Carbon dioxide (%) Water (%)

31.1 ± 0.66 51.7 ± 0.37 –

53.2 ± 0.92 9.5 ± 0.08 37.3 ± 1.00a

2.2 ± 0.03 67.1 ± 0.55 –

50.4 ± 1.19 13.5 ± 0.13 36.1 ± 1.32a

Water yields were calculated by difference as 100-catalytic CO-catalytic CO2, assuming that oxygen was fully released as water and carbon oxides.

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