Chemoselective Hydrodeoxygenation of Carboxylic Acids to

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Chemoselective Hydrodeoxygenation of Carboxylic Acids to Hydrocarbons over Nitrogen-doped Carbon-Alumina Hybrid-Supported Iron Catalysts Jiang Li, Junjie Zhang, Shuai Wang, Guangyue Xu, Hao Wang, and Dionisios G. Vlachos ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04967 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 5, 2019

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Chemoselective Hydrodeoxygenation of Carboxylic Acids to Hydrocarbons over Nitrogen-Doped Carbon-Alumina Hybrid-Supported Iron Catalysts Jiang Li,*,† Junjie Zhang, † Shuai Wang, † Guangyue Xu, ‡ Hao Wang, ‡ and Dionisios G. Vlachos, *,§

†State

Key Laboratory of Heavy Oil Processing, College of New Energy and Materials, China

University of Petroleum (Beijing), Beijing 102249, China ‡Anhui

Province Key Laboratory of Biomass Clean Energy, Department of Chemistry,

University of Science and Technology of China, Hefei 230026, China §Department

of Chemical and Biomolecular Engineering and Catalysis Center for Energy

Innovation, University of Delaware, 221 Academy St., Newark, Delaware19716, United States

ABSTRACT: The establishment of catalyst systems for chemoselective hydrodeoxygenation (HDO) of carboxylic acids to hydrocarbons, such as the HDO of long-chain fatty acids to alkanes, is important for biomass-to-biofuel conversion. As the most abundant and probably the cheapest transition-metal on the earth, iron is a promising non-noble alternative to precious metals for large-scale conversion of biomass. However, it usually suffers from unsatisfactory activity. In this work, nitrogen-doped carbon-alumina hybrid-supported iron (Fe-N-C@Al2O3) catalyst is

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established for chemoselective HDO of carboxylic acids to hydrocarbons. By using stearic acid HDO as the model reaction, n-octadecane and n-hepadecane are produced with yields of 91.9% and 6.0%, respectively. Triglycerides can also be converted into liquid alkanes with total molar yield of >92%. In addition, the iron catalyst can chemoselectively catalyze the HDO of carboxylic acid group in the presence of other functional groups such as an aromatic ring. This chemoselectivity is rarely seen before because the aromatic ring is usually easier to be hydrogenated than carrying out HDO of carboxylic acid group. The characterization results showed that both the formation of nitrogen-doped carbon-alumina hybrid and the iron-loading are important for the Lewis basicity of these catalysts, in order to adsorb the acid substrates. The addition of melamine as nitrogen precursor during pyrolysis eliminates undesired reactions between iron precursor and alumina support to form inactive hercynite phase, leading to the formation of Fe3C active phase for the hydrogenation of –COOH to –CH2OH and the hybrid of N-C and alumina for the HDO of -CH2OH to –CH3.

KEYWORDS: Biomass conversion, Hydrodeoxygenation, Carboxylic Acids, Iron, Nitrogendoping, Heterogeneous catalysis

1. INTRODUCTION Rapid depletion of fossil reserves, increasing energy demand and global warming concerns prompt the utilization of sustainable sources such as biomass to produce chemicals and fuels.1,2 Triglycerides, which are esters of long-chain fatty acids and glycerol, are one of the most important sustainable feedstocks for biofuel production. The first generation biodiesel, fatty acid methyl ester (FAME), is produced by transesterification of triglycerides with methanol with an


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annual manufacture of more than 20 million tons.3 However, its poor low-temperature flow property and unsatisfactory thermal and oxidation stability limit its application as a high-grade fuel in engines.4 The direct conversion of triglycerides into diesel-range alkanes (C15-C18), on the other hand, has attracted significant attention because the as-prepared alkane biodiesel has no oxygen, and its better low-temperature flow properties make it compatible with current engines. To date, there are two approaches for the production of alkanes from triglycerides via hydrodeoxygenation (HDO). The first involves the use of conventional sulfide CoMo and NiMo catalysts,5-7 but suffers from contamination of the alkane products by unavoidable sulfur catalyst leaching. The second employs sulfur-free supported metal catalysts. Lercher and coworkers reported two impressive studies about using Ni/HBeta8 and Ni/ZrO29 catalysts to catalyze the HDO of stearic acid to octadecane and n-heptadecane, respectively via two different reaction pathways. Since then, noble metals such as Pd,10 Pt,11 Ru,12,13 and Ir-ReOx,14 and non-noble metals such as Ni,15-19 Co,20,21 W,22 Mo,23,24 and Fe25 have been tested. In view of economic considerations and abundance, non-noble metal catalysts are attractive. Among them, Fe is very promising due to the anticipated large scale processing of biomass, and the facts that Fe is 3,00030,000 times more abundant and 20-150 times cheaper than other metals (figure 1). Currently, the only study using Fe as metal center for the HDO of fatty acids was reported by Slowing et al.25 Iron nanoparticles supported on mesoporous silica nanoparticles (Fe-MSN) afforded noctadecane and n-hepadecane with yields of 83% and 10%, respectively from oleic acid. However, the Fe-MSN catalyst was ineffective in catalyzing the conversion of triglycerides such as microalgal oil to liquid alkanes. 16% alcohols, 33% unsaturated hydrocarbons and 18% saturated hydrocarbons are obtained at 67% conversion. Thus, it is still desirable to develop an effective Fe-based catalyst system with improved HDO activity.


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Figure 1. Abundance and price of some representative non-noble transition metals. The former is reported in Ref.26 The prices of Fe, Co, Ni, Mo are quoted in London Metal Exchange (LME) on May 22, 2018. The price of W is its export price in China on May 22, 2018. Recently, nitrogen-doping has been reported to improve the catalytic activity of iron catalyst. For example, numerous Fe-N-C catalysts have exhibited catalytic performance comparable to commercial Pt/C catalyst towards electrochemical reactions such as the oxygen reduction reaction.27 Notably, Beller et al. reported some pioneer work using Fe-phen catalysts, which were prepared by the pyrolysis of iron-phenanthroline complexes on supports at 800 oC, towards organic synthesis such as nitroarenes reduction, oxidative dehydrogenation of Nheterocycles, and hydrogen-free reductive aminations.28-30 For biomass conversion, we recently found that a Fe-N-C catalyst is capable of carrying out catalytic transfer hydrogenation of furfural, the HDO of 5-hydroxymethylfurfural, and the reductive cleavage of lignin-derived ether linkage.31-33 Even though the exact nature of active sites of Fe-N-C catalyst is still not clear, Fe NPs encapsulated in a nitrogen-doped carbon shell is usually observed.28 It is well known that nanocarbon materials are ideal supports due to their unique physical and chemical properties,34-36 and the nitrogen dopant could introduce additional unpaired electrons to the conjugation of graphitic  systems to facilitate electron transfer and substrate adsorption.37 Catalyst activity can


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further be boosted by Fe/Fe3C species.38 It should be noted that the activity of nitrogen-doped iron catalysts greatly depends on the selection of precursors and the heat treatment process.

Scheme 1. Hydrodeoxygenation of carboxylic acids. The graphitic carbon nitride (g-C3N4) polymer, which can be easily prepared by simple thermal polymerization of melamine, cyanamide, dicyanamide, urea, and thiourea at 550 oC, has recently drawn great attention because of its appealing electronic band structure, peculiar physicochemical stability, and “earth-abundant” nature.39 The high N content (ca. 60%) and the as-formed “nitrogen pots” with six nitrogen lone-pair electrons create ideal sites for metal inclusion (such as Fe), forming active sites for chemical conversions. In this work, nitrogendoped carbon-alumina hybrid-supported iron (Fe-N-C@Al2O3) catalysts, prepared by simultaneous pyrolysis of iron acetylacetone (Fe(acac)3) and melamine onto alumina at temperatures ranged from 300 to 1100 oC, are demonstrated to be active towards selective HDO of carboxylic acids to hydrocarbons (Scheme 1). By using stearic acid HDO as a model reaction, the effect of pyrolysis temperature, support, nitrogen-doping, and iron loading are explored in detail. Under optimal reaction conditions, n-octadecane and n-hepadecane are produced with yields of 91.9% and 6.0%, respectively at quantitative conversion of stearic acid over Fe-N-


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C@Al2O3-900 catalyst. In addition, triglycerides such as cyperus esculentus oil and jatropha oil can also be converted into liquid alkanes with total weight yields (molar yields) of 78.8 wt% (92.1%) and 81.8 wt% (95.2%), respectively. The iron catalyst can chemoselectively catalyze the HDO of carboxylic acid group in the presence of other functional groups, such as an aromatic ring. This chemoselectivity is rarely seen because hydrogenation of the aromatic ring is usually easier than the HDO of carboxylic acid group.40-42 To our knowledge, the systematic study of chemoselective HDO of arylcarboxylic acid has not been reported yet. Only a few studies on the conversion of benzoic acid to toluene have been reported. Cu/Al2O3 and LiAlH4 with Ti catalysts afford toluene with yields of 40% and 61%, respectively.43,44 The ZnO catalyst generated toluene as by-product during the conversion of benzoic acid to benzaldehyde, suffering from high reaction temperature (>400 oC) and energy-consuming gasification of benzoic acid.45 Thus, our work provides the first comprehensive investigation of chemoselective conversion of carboxylic acid groups to methyl groups in the presence of other functional groups such as aromatic rings. The catalyst is characterized by various techniques. The addition of melamine as N precursor during pyrolysis is important for the improved HDO activity of iron catalysts. It eliminates undesired reactions between iron precursor and alumina support to form inactive hercynite phase, leading to the formation of Fe3C active phase for the HDO reaction especially the hydrogenation of –COOH to –CH2OH. The Lewis basicity of the catalyst favors adsorption of the acid substrates to form carboxylate intermediates, which is influenced by the formation of nitrogen-doped carbon-alumina hybrid and iron-loading. The formation of hybrid also increased the ratio of Bronsted acid/Lewis acid sites of Al2O3 to enhance its activity towards HDO of – CH2OH to –CH3. 2. EXPERIMENTAL SECTION


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2.1 List of chemicals Iron(III) acetyl acetone (98%), metal oxide supports such as α-Al2O3 (30 nm, hydrophilic, 99.9%), TiO2 (99%) and SiO2 (99.9%), dodecane (98%), decane (98%), myristic acid (98%), tetradecane (99%), n-octane (99.5%) and salicylic acid (99%) were purchased from Aladdin Reagent Co. Ltd. Iron(III) nitrate nonahydrate (98.5%), ethanol (99.5%) and benzoic acid (99%) were purchased from Sinopharm Chemical Reagent Co. Ltd. Urea (99%), toluene (99.5%), oxylene (98.5%), p-xylene (98%) were purchased from Guangfu Chemical Reagent Co. Ltd. Dicyandiamide (98%), 1,10-phenanthroline monohydrate (99%), melamine (98%), activated carbon (99%), octadecane (98%), 1-octadecene (90%), octadecanol (98%), octadecanal (95%), phenol (99.5%), octoic acid (98%), p-phthalic acid (99%), o-phthalic acid (99%), pchlorobenzoic acid (99%), suberic acid (99%), α,α,α-trifluoro-p-toluic acid (98%), 4-tertbutylbenzoic acid (99%), p-hydroxybenzoic acid (99%), p-anisic acid (99%), p-fluorobenzoic acid (98%), 3-phenylpropanoic acid (98%), 4-chlorotoluene (98%), 4-fluorotoluene (99%), 4methylbenzotrifluoride (98%), 4-tert-butyltoluene (95%), 4-methoxytoluene (98%), p-cresol (99%) and o-cresol (99%) were purchased from TCI. 2.2 Catalyst preparation The iron catalysts were prepared by simultaneous pyrolysis of iron and nitrogen precursors onto various supports under Ar at 300-1100 °C. The iron loading is 20 wt% unless otherwise specified. Iron precursors such as iron(III) acetylacetonate and iron(III) nitrate, nitrogen precursors such as melamine, dicyandiamide, urea, and 1,10-phenanthroline, supports such as Al2O3, TiO2 and SiO2 and activated carbon were examined in this work. Typical preparation is as follows: a certain amount of iron(III) acetylacetonate was firstly added to 100 mL ethanol at room temperature. After the precursor was completely dissolved, melamine was added into the


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solution. Then 1 g Al2O3 was added into the mixture and the solution was stirred at 60 °C for 16 h followed by evaporation at 40 °C to remove the ethanol solvent. The obtained solid was dried at 60°C overnight. Then the mixture was grounded into fine powder and pyrolyzed in a tubular furnace with an argon flow rate of 100 mL min-1. To prepare the best iron catalyst, the following furnace temperature program was used: 20 °C hold for 60 min, ramp 10 °C min-1 to 550 °C and hold for 3 h, then ramp 5 °C min-1 to 900 °C and hold for 2 h. To prepare Fe-N-C@Al2O3-900 catalyst with different iron loadings, the amount of melamine was changed accordingly to keep the Fe/N ratio. The iron catalysts prepared from different batches exhibited the same catalytic performance, demonstrating that our preparation method is highly reproducible. 2.3 Catalyst characterization The morphology of the catalysts was characterized by Hitachi SU8010 scanning electron microscopy (SEM, Japan) at 200 kV. TEM, HAADF-STEM, and the energy dispersive spectrometer (EDS) mapping were performed on a JEOL JEM-2100F high-resolution transmission electron microscope operated at 200 kV. N2 adsorption measurements were conducted using an ASAP2020M adsorption analyzer which reports the adsorption isotherm, specific surface area, and pore volume automatically. The surface areas of catalysts were calculated using the BET method in the range of relative pressures between 0.05-0.20. The pore size and distribution were calculated from the adsorption branch of the isotherms using the BJH method. XRD patterns were recorded on an X-ray diffractometer (TTR-III, Rigaku Corp., Japan) with Cu Kα radiation (λ=1.54056 Å). The data were recorded over 2θ ranges of 10-70°. XPS was obtained using an X-ray photoelectron spectroscopy (ESCALAB 250Xi, Thermo-VG Scientific, USA) with monochromatized Al Kα radiation (1486.92 eV). The nitrogen content was detected by elemental analysis (Eurovector EA 3000). The iron content was detected by ICP-AES


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(Optima 7000DV, PerkinElmer Inc.). For NH3 and CO2-TPD tests, approximately 100 mg sample was loaded in a quartz reactor and then heated at 500 °C under argon flow for 2 h. Then the adsorption of NH3 or CO2 was carried out at 40 oC for 1 h. Subsequently, the catalysts were flushed with argon at 40 °C for 1 h, and then heated to 700 or 800 °C with a heating ramp rate of 10 oC min-1. The desorbed NH3 or CO2 was measured by a gas chromatograph equipped with a thermal conductivity detector (TCD). Raman spectra were measured on a Raman spectrometer (Renishaw, λ= 532 nm). FTIR spectra were recorded on a Nicolet 8700 FTIR spectrometer in the wavenumber range from 400 to 4000 cm-1. To confirm the chemical adsorption of stearic acid on Fe-N-C@Al2O3-900 catalyst surface, the treated catalyst was prepared as in our previous studies.46 0.1 g of Fe-N-C@Al2O3-900 catalyst was added into 20 mL of a 0.05 M n-hexane solution of stearic acid, and the mixture was stirred at room temperature for 12 h. Then the solid was separated by centrifugation by washing it 15 times with n-hexane. The residue was dried at 80 °C overnight under N2. Pyridine adsorption IR (Py-IR) is used to explore the acid types. A 10 mg tableting sample was first pretreated at 400 °C under vacuum for 2 h. After cooling down to room temperature, pyridine was adsorbed for 10 min, and the temperature was increased to 200 °C to desorb for 0.5 h (1 × 10−3 Pa) and then increased to 350 °C to detect the medium and strong acid sites. 2.4 Experimental procedure The catalytic HDO of carboxylic acids was carried out in a 50mL stainless reactor purchased from Anhui Kemi Machinery Technology Co., Ltd. For a typical reaction, 0.5 mmol stearic acid, 100 mg heterogeneous catalyst, and 20 mL dodecane solvent were loaded into a quartz lining in the reactor. The reactor was purged by hydrogen for 3 times, and then purged with 4 MPa H2 at room temperature. The reaction was carried out at 320 °C for 8 h with a


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stirring speed of 800 rpm. After reaction, the gaseous phase was analyzed by gas chromatography (GC). A ShinCarbon ST 80/100 packed column (Restek) and a thermal conductivity detector (TCD) were used to determine the yields of H2, CO, CO2 and CH4. A Plot Q column and a flame ionization detector (FID) were used to determine the yields of gaseous hydrocarbons such as CH4, C2H6, and C3H8. The liquid products were collected, and 0.5 mmol eicosane was added into the solution as internal standard. The products were analyzed using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). GC-MS analysis was conducted by an Agilent 7890B Gas Chromatograph equipped with a HP-5MS 30 m × 0.25 mm × 0.25 μm capillary column (Agilent). The GC was directly interfaced to an Agilent 5977 mass selective detector (EI, 70 eV). Some typical GC oven temperature programs were listed as follows: (1) For stearic acid, 210 °C hold for 2 min, ramp 20 °C min-1 to 300 °C and hold for 2 min;(2) For other substrates, 40 °C hold for 2 min, ramp 10 °C min-1 to 150 °C, then ramp 20 °C min-1 to 300 °C and hold for 2 min. Representative GC spectra are shown in supporting information (figure S1 and S2). Some important experiments were performed at least twice to ensure reproducibility. The carbon loss can be attributed to the formation of undetected products in GC or coke formation. The conversion and yield for the HDO of carboxylic acids were calculated by mol% as follows:

(

Conversion = 1 ―

Yield =

)

𝑚𝑜𝑙𝑎𝑟 𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑐𝑎𝑟𝑏𝑜𝑥𝑦𝑙𝑖𝑐 𝑎𝑐𝑖𝑑𝑠 𝑎𝑓𝑡𝑒𝑟 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 × 100% 𝑚𝑜𝑙𝑎𝑟 𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑐𝑎𝑟𝑏𝑜𝑥𝑦𝑙𝑖𝑐 𝑎𝑐𝑖𝑑𝑠 before 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛

𝑚𝑜𝑙𝑎𝑟 𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑒𝑎𝑐ℎ 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑎𝑓𝑡𝑒𝑟 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 × 100% 𝑚𝑜𝑙𝑎𝑟 𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑐𝑎𝑟𝑏𝑜𝑥𝑦𝑙𝑖𝑐 𝑎𝑐𝑖𝑑𝑠 before 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛

Some carboxylic acids are insoluble in dodecane, so that additional procedures are required to determine the conversions in some cases in table 2. (1) For stearic acid, benzoic acid, 4-tertbutylbenzoic acid, and 3-phenylpropanoic acid (entries 1, 5, 9, and 13 in table 2), no additional


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procedures are required; (2) For p-fluorobenzoic acid, p-chlorobenzoic acid, and o-phthalic acid (entries 6, 7, and 14 in table 2), the production solution was diluted with THF to 100 mL to dissolve all the acid substrates; (3) For α,α,α-trifluoro-p-toluic acid, p-hydroxybenzoic acid, panisic acid, and p-phthalic acid (entries 8, 10, 11, and 15 in table 2), the acid substrates are extracted from product solution by N,N-dimethylformamide (DMF) after the determination of yields of alkane products, and phenylethyl alcohol is used as internal standard to determine the conversions; (4) For myristic acid (entry 2 in table 2), decane is used as the solvent instead of dodecane to avoid the interference of the determination of yields of alkane products by the solvent; (5) For octoic acid and suberic acid (entries 3 and 4 in table 2), a control experiment without addition of substrate is performed to determine the yields of alkanes derived from the cracking of the solvent; (6) For salicylic acid (entry 12 in table 2), the conversion is not determined because the retention time of substrate is overlapped with the solvent on a HP-5MS column. It cannot be detected on a DB-WAX column because of high boiling point and high polarity. The weight yield and total molar yields of liquid alkanes products for the HDO of plant oil were calculated as follows: Yield(wt%) =

Total yield(mol%) =

𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑒𝑎𝑐ℎ 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑎𝑓𝑡𝑒𝑟 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 ∗ 100% 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑐𝑦𝑝𝑒𝑟𝑢𝑠 𝑒𝑠𝑐𝑢𝑙𝑒𝑛𝑡𝑢𝑠 𝑜𝑖𝑙 before 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛

𝑡𝑜𝑡𝑎𝑙 𝑚𝑜𝑙𝑎𝑟 𝑎𝑚𝑜𝑢𝑛𝑡𝑠 𝑜𝑓 𝑙𝑖𝑞𝑢𝑖𝑑 𝑎𝑙𝑘𝑎𝑛𝑒𝑠 𝑎𝑓𝑡𝑒𝑟 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 ∗ 100% 𝑚𝑜𝑙𝑎𝑟 𝑎𝑚𝑜𝑢𝑛𝑡𝑠 𝑜𝑓 𝑐𝑎𝑟𝑏𝑜𝑥𝑦𝑙𝑖𝑐 𝑎𝑐𝑖𝑑 𝑔𝑟𝑜𝑢𝑝𝑠 before 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛

3. RESULTS AND DISCUSSION 3.1 Catalyst characterization


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Figure 2. TEM image (a), HAADF-STEM (b), Fe, N, C, O, Al, and combined elemental maps (c-h) of Fe-N-C@Al2O3-900 catalyst. TEM images of various Fe-N-C@Al2O3 catalysts are shown in the Supporting Information (figure S4-S8). For Fe-N-C@Al2O3-300 catalyst, Fe atoms are homogeneously dispersed on the alumina flake without the formation of nanoparticles (NPs), as confirmed by the EDX spectroscopy (figure S4). Raising the pyrolysis temperature to 550 oC led to the formation of a structure where Fe NPs were encapsulated by a carbon shell. The Fe NPs are confirmed by EDX spectroscopy (figure S5), and the average particle size is 28.63 nm. When the pyrolysis temperature was further raised to 700 oC, cavities appeared and the average particle size increased to 52.03 nm (figure S6). Surprisingly, it seems that only alumina flakes and amorphous carbon can be observed in Fe-N-C@Al2O3-900 and Fe-N-C@Al2O3-1100 catalysts (figure S7 and S8). It is hard to distinguish Fe NPs, alumina, and carbon flakes in the darker areas in the TEM images. To get a clearer realization of the morphology of Fe-N-C@Al2O3-900 catalyst, EDX


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elemental mapping was further performed (figure 2). The brightest particles in HAADF-STEM image are Fe NPs as confirmed by Fe elemental map. The carbon and alumina flake can easily be distinguished by C and Al elemental maps. In contrast, it is hard to distinguish them in the TEM images. N atoms are found in the carbon and alumina flake, confirming the formation of nitrogen-doped carbon and alumina. The Fe Nps are supported onto both carbon and alumina flakes.


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Figure 3. XRD patterns of various iron catalysts. (a) Fe-N-C@Al2O3 catalysts prepared at different pyrolysis temperatures and Fe-Al2O3-900 catalyst. (b) Iron catalysts prepared on different supports with and without nitrogen-doping. (c) Fe-N-C@Al2O3 catalysts with different iron loadings. The XRD patterns of iron catalysts prepared at different pyrolysis temperatures are shown in figure 3a. Only characteristic peaks of alumina support (corumdum, syn, PDF#10-0173) are found in the XRD pattern of Fe-N-C@Al2O3-300 catalyst. The metallic iron phase can be detected at a pyrolysis temperature of 550 oC. Its intensity significantly increases when the pyrolysis temperature is raised to 700 oC. Besides the metallic iron phase, weak peaks of Fe3C phase (PDF#89-7271) are detected in the XRD pattern of Fe-N-C@Al2O3-900 catalyst (see figure S10 for a clearer observation of XRD pattern in the 2 range of 40-50 o). The absence of peaks for FeNx species is probably due to the quite low contents of N (0.19 wt% vs 11.34 wt% of C, table S1) and homogeneous dispersion without the formation of crystalline phase. The peaks of Fe3C phase disappear when the pyrolysis temperature is further raised to 1100 oC. The XRD pattern of Fe-Al2O3-900 catalyst show that the iron precursor reacts with the alumina support to form hercynite phase in the absence of nitrogen precursor during the pyrolysis.


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ACS Catalysis

The XRD patterns of iron catalysts prepared using different supports with and without nitrogen-doping are shown in figure 3b. The iron precursor reacts with metal oxide supports such Al2O3, TiO2 and SiO2 to form hercynite, FeTiO3, and fayalite phases, respectively. The formation of these phases can be avoided by the addition of nitrogen precursor such as melamine during pyrolysis. Intense peaks of metallic iron phase can be found in the Fe-N-C@TiO2 and FeN-C@SiO2 catalysts. For the Fe-N-C@Al2O3-900 catalyst, peaks of Fe3C phase are also observed besides metallic iron phase. When activated carbon was used as support, intense peaks of metallic phase are always observed irrespective of whether the nitrogen precursor is added. In addition, extremely weak peaks of Fe3O4 species are formed without nitrogen precursor, and extremely weak peaks Fe3C phase are detected in the presence of nitrogen precursor. The XRD patterns of Fe-N-C@Al2O3 catalysts with different Fe loadings are shown in figure 3c. Hercynite is the only iron species at 3 and 5wt% Fe loading, and the color of the catalysts is light grey (figure S11). Metallic iron can be found in the XRD patterns at iron loadings above 10 wt%. The peaks of hercynite phase completely disappeared at iron loading of 20 wt%, and weak peaks of Fe3C phase can be detected. The color of the iron catalysts turn darker with increasing iron loading (figure S11).


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Figure 4. FT-IR spectra of (Fe)-N-C@Al2O3-900 catalysts with and without treating with stearic acid. It is well known that the pyrolysis of melamine at 550 oC leads to the formation of graphitic carbon nitride (g-C3N4). Since characteristic bands of g-C3N4 are found in the IR spectra of FeN-C@Al2O3 catalysts (figure S12), g-C3N4 was probably formed during the catalyst preparation.47 In addition, the interaction between (Fe)-N-C@Al2O3-900 catalysts and stearic acid is further revealed using FT-IR (figure 4). The characteristic peaks at 2954, 2918, 2850, and 1467 cm−1 correspond to asymmetric stretching vibration of C−H in −CH3, asymmetric stretching vibration of C−H in −CH2−, symmetric stretching vibration of C−H in −CH2−, and deformation vibration of C−H of alkyl chain in stearic acid. These peaks also exist in the FT-IR spectra of (Fe)-N-C@Al2O3-900 catalysts upon stearic acid adsorption, confirming its adsorption onto the support and the iron catalyst. This adsorption is chemsorption rather than physisorption because the characteristic peak of carboxyl group in stearic acid at 1705 cm−1 is missing in the FT-IR spectra of (Fe)-N-C@Al2O3-900 catalysts, and instead, a weak new peak at 1563 cm−1, which could be assigned to the formation of carboxylate,13 is observed. The carboxylate is a key intermediate in the hydrogenation of carboxylic acid to aldehyde during stearic acid HDO.9


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ACS Catalysis

Figure 5. CO2-TPD profiles of Al2O3, N-C@Al2O3-900, Fe-Al2O3-900, and Fe-N-C@Al2O3-900 catalysts. The Lewis acidity and basicity of Fe-N-C@Al2O3-900 catalyst were explored using NH3and CO2-TPD, respectively. The NH3-TPD profile (figure S13) indicates that the Fe-NC@Al2O3-900 catalyst has no Lewis acidity. By comparison, the two intense peaks at 100-300 and 550-800 oC in the CO2-TPD profile (figure 5) suggest that the catalyst possesses both weak and strong Lewis basicity. This basicity facilitates chemisorption of carboxylic acid, as corroborated by FI-IR in figure 4. A much weaker peak at 100-300 oC observed in the CO2-TPD profile of Fe-Al2O3-900 catalyst, and the shift of the peak for strong Lewis basicity to lower temperature, indicate that the formation of nitrogen-doped carbon-alumina hybrid leads to stronger Lewis basicity. The broad peak at 160-610 oC in the CO2-TPD profile of N-C@Al2O3900 catalyst indicates that it has medium Lewis basicity. Thus, the Lewis basicity of Fe-NC@Al2O3-900 catalyst is controlled by both the formation of the nitrogen-doped carbon-alumina hybrid and the iron-loading.

Figure 6. N 1s XPS spectra of Fe-N-C@Al2O3 catalysts prepared at different pyrolysis temperatures.


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The N 1s XPS spectra of Fe-N-C@Al2O3 catalysts are shown in figure 6 and S14. The N 1s XPS spectrum of Fe-N-C@Al2O3-300 was displayed separately (figure S14) because its N content (23.26 at%, detected by XPS) is much higher than that of other catalysts (no more than 5.2 at%), leading to much more intense N peaks (8.5-28 times higher than that of Fe-NC@Al2O3-550 and Fe-N-C@Al2O3-1100 catalysts, respectively). Other N 1s spectra will be hardly distinguished if they are co-displayed with the spectrum of Fe-N-C@Al2O3-300 catalyst. The N 1s spectra of N-C materials are usually fitted into four species: pyridinic N at 398.3398.6 eV, pyrrolic N at 400.5 eV, graphitic N at 401.2 eV, and oxidized N at 402-405 eV.48,49 In addition, a peak at 399.7 eV can also be observed in Fe-N-C@Al2O3 catalysts prepared under 550 oC, which is assigned to bridge N in g-C3N4 (tertiary N bonded to carbon atoms in the form of N–(C)3 or H–N–(C)2).47 The possible N species and corresponding contents are shown in table S2. The increase of pyrolysis temperature from 300 to 900 oC led to a significant decrease of the pyridinic N and bridge N and an increase in graphitic N. Oxidized N appeared when the pyrolysis temperature reached 1100 oC, but the signal to noise ratio is quite low because of extremely low N content (entry 8, table 1). The C 1s XPS spectra are shown in figure S16. Graphitic C is observed as the only C species for Fe-N-C@Al2O3 catalysts prepared at pyrolysis temperature above 550 oC. For Fe-NC@Al2O3-300 catalyst, the C peak at 288.17 eV was identified as sp2-bonded carbon (N–C=N),47 suggesting that graphitic carbon nitride is formed during the pyrolysis, consistent with FTIR results in figure S12.


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ACS Catalysis

Figure 7. Raman spectra of Fe-N-C-900 and some alumina-based catalysts. The Raman spectra of Fe-N-C-900 and some alumina-based catalysts indicated the presence of a nitrogen-doped carbon-alumina hybrid (figure 7). The two peaks at ca. 1360 and 1390 cm-1 are characteristic of -Al2O3.[50] The Raman spectra remained unchanged after single nitrogen-doping or Fe loading. By contrast, the simultaneous loading of Fe and N-C showed typical peaks of D and G bands of carbon. In addition, the characteristic peaks of -Al2O3 significantly changed because they overlap with the D band of carbon. Table 1. The nitrogen contents and N2 adsorption/desorption data of various catalysts. Entry

Catalysts

N contenta

BET surface area

Pore Volume

Pore Size

[m²/g]

[cm³/g]

[nm]

[wt%] 1

Al2O3

-

61.3

0.46

29.67

2

Fe-Al2O3-900

-

6.15

0.02

14.68

3

N-C@Al2O3-900

2.93

8.39

0.03

13.17

4

Fe-N-C@Al2O3-300

7.20

14.0

0.06

3.66

5

Fe-N-C@Al2O3-550

0.80

56.7

0.21

3.77


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6

Fe-N-C@Al2O3-700

0.35

54.6

0.22

3.74

7

Fe-N-C@Al2O3-900

0.19

46.9

0.19

3.76

8

Fe-N-C@Al2O3-1100

0.08

35.9

0.17

3.69

9

Fe-N-C-900

0.50

704

0.63

4.77

10

Fe-N-C@TiO2-900

2.10

31.7

0.08

9.61

11

Fe-N-C@SiO2-900

0.73

114

0.73

25.49

aDetermined

by elemental analysis.

The nitrogen content and N2 adsorption/desorption data of various catalysts are shown in table 1. The N content of Fe-N-C@Al2O3 catalysts continually decreased with increasing pyrolysis temperature. The alumina support has a BET surface area of 61.3 m2 g−1, a pore volume of 0.46 cm3 g−1, and a pore width of 29.67 nm (entry 1). Loading of Fe or N-C leads to significant decrease in BET surface area, pore volume, and pore size (entries 2 and 3). By comparison, the simultaneous loading of Fe and N-C at 900 oC afforded much higher BET surface area and pore volume (entry 7). In addition, the pore size was further decreased to 3.76 nm. The N2 sorption isotherm of Fe-N-C@Al2O3-900 catalyst (figure S17) can be classified as a type IV isotherm with a nitrogen uptake at low relative pressures and an H3 hysteresis loop around a relative pressure of 0.4–1.0 P/P0, suggesting that the catalyst possesses well-defined mesopores as confirmed by the pore distribution.51 The hysteresis loop could also be observed in the N2 sorption isotherm of Fe-N-C@Al2O3 catalysts prepared at other pyrolysis temperatures (figure S17). The BET surface area of iron catalyst prepared on carbon support was much higher than those prepared on metal oxides. 3.2 Catalyst screening


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ACS Catalysis

Figure 8. HDO of stearic acid over various iron catalysts. Reaction conditions: 0.5 mmol stearic acid, 20 mL solvent, 0.1 g catalyst, 4 MPa H2, T=320 oC, and t=8 h. n-C17=heptadecane. C18=octadecene + octadecane. C18-OH= 1-octadecanol. C17-CHO=1-octadecanal. Trace yields of octadecene are observed in most cases except for Fe-N-C@Al2O3-300 and 550 catalysts. The iron catalysts are first tested for the HDO of stearic acid to liquid alkanes as a model reaction (figure 8). A glass vial was used to exclude influence from the reactor body. The blank test showed that the HDO reaction is unlikely to occur without any catalyst. The Fe-Al2O3-900 catalyst exhibited poor HDO activity, affording only 1.9% yield of heptadecane (n-C17) as the major liquid alkane product with 12.3% yield of 1-octadecanal as the major by-product. This poor HDO activity indicated that the hercynite species, the only iron species in the XRD pattern of Fe-Al2O3-900 catalyst, formed by the reaction of iron precursor with the alumina support in


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the absence of nitrogen precursor during pyrolysis, are inactive towards the HDO reaction. In addition, the poor catalytic performance of N-C@Al2O3-900 and Fe-C@Al2O3-900 catalysts indicated that both iron and nitrogen are indispensable to reach satisfactory HDO activity. A prominent peak of metallic Fe can be found in the XRD pattern of Fe-C@Al2O3-900 catalyst, and the crystalline structure of alumina support significantly changed (figure S18). Specifically, strong peaks of alumina support (corundum, syn, PDF#10-0173) became much weaker, and new weak peaks of alumina (alumina, PDF#11-0517) appeared. The HDO activity of the iron catalysts is significantly improved upon nitrogen-doping. 1octadecanol was the major product over Fe-N-C@Al2O3-300 catalyst (30.1% yield). Meanwhile, octadecane (n-C18) and octadecene (C18=) were the major hydrocarbon products. The presence of octadecene suggested that n-C18 was probably generated by the dehydration of 1-octadecanol to octadecene followed by hydrogenation. In addition, the Fe-N-C@Al2O3-300 catalyst is unstable during reaction, leading to opaque production solution and unsatisfactory carbon balance (see figure S19 in the supporting information). When the pyrolysis temperature was raised to 550 oC, the yield of 1-octadecanol further increased to 60.8%, suggesting that the hydrogenation activity of the iron catalysts increased significantly. When the pyrolysis temperature was further raised, the yields of alkanes increased dramatically, and the highest selectivity of alkanes was achieved for the catalyst with a pyrolysis temperature of 900 oC. 91.9% yield of n-C18 and 6.0% yield of n-C17 are obtained with only 1.9% yield of cracking alkane products. No oxygenated products were detected (see the GC spectrum in figure S19). The alkane yields significantly decreased when the pyrolysis temperature was raised to 1100 oC, and the major by-product was the 1-octadecanol with a yield of 56.9% , suggesting that the HDO activity of the iron catalyst significantly decreased.


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ACS Catalysis

Iron catalyst with 1,10-phenanthroline as N precursor (Fe-phen-C@Al2O3-900) was also examined because 1,10-phenanthroline has been known as excellent N precursor for nitrogendoped iron catalysts.28-33 However, it showed inferior HDO activity, giving 23.2% yield of 1octadecanol as major by-product. By contrast, use of other g-C3N4 precursors such as dicyandiamide and urea as N precursor led to similar HDO activity as melamine. The nitrogen content of 1,10-phenanthroline is much lower than g-C3N4 precursors such as melamine (17.3 wt% vs 66.7 wt%), so a much higher dosage of 1,10-phenanthroline was used to keep the same Fe/N ratios in the precursors. Correspondingly, the C/Fe ratios in the precursors of Fe-phenC@Al2O3-900 catalyst are much higher than that of Fe-N-C@Al2O3-900 catalyst. This caused significant changes in the crystalline structure of alumina support, which is similar to FeC@Al2O3-900 catalyst (figure S18). The replacement of iron(III) acetylacetonate with ferric nitrate led to the formation of hercynite phase as the predominant iron phase (see XRD pattern in figure S20), thus resulting in poor HDO activity. The color of the catalyst also turned into light grey (figure S21). The undesired redox reaction between nitrate ion and N precursor melamine during pyrolysis may hamper the reaction between Fe and melamine to form FeNx and Fe/Fe3C species, so that hercynite phase is formed as the major iron species. Much inferior HDO catalytic performance was also observed if other supports such as activated carbon, TiO2, and SiO2 were used. As intense peaks of metallic iron phase were detected in the XRD pattern of Fe-NC@Al2O3 catalysts prepared above 550 oC, one may presume that the metallic iron phase is the active center to catalyze stearic acid HDO. To test this hypothesis, Fe-C-900 catalyst, in which metallic iron is the most predominant iron species, was used for the HDO reaction. However, 9.3% yield of n-C17 was obtained as the major product, and the yield of 1-octadecanol was only


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0.2%. The poor yield of hydrogenated product and the different product distribution suggested that the metallic iron is not the key active center for the hydrogenation reaction. By contrast, the Fe-N-C-900 afforded 45.1% yield of 1-octadecanol as the major product. As weak Fe3C phases are observed in the XRD pattern, the presence of Fe3C phase is probably indispensible for the enhanced hydrogenation activity. The XRD pattern of Fe-N-C@Al2O3-900 catalyst, which is the most effective iron catalyst towards the HDO reaction, also has similarly weak peaks of Fe3C species. Similar synergy has recently been reported, where Fe/Fe3C nanoparticles were found to boost the activity of a Fe-N-C catalyst.38 The role of Fe3C phase during hydrogenation of – COOH group is further elucidated in the stability test section. The iron loading plays also a key role on the HDO activity of the Fe-N-C@Al2O3-900 catalysts (figure 9). The Fe-N-C@Al2O3 catalysts with Fe loading less than 10 wt% afforded poor yields of HDO products, and 1-octadecanal was one of the major products. As stated above, the XRD patterns showed that hercynite is the only iron species in these iron catalysts, further confirming that hercynite is inactive towards the HDO reaction. When the iron loading increased from 5 wt% to 10 wt%, metallic iron species can be found in the XRD pattern, and the yield of 1octadecanol increased from 14% to 31.6%. The optimal iron loading is 20 wt%. Further increase of the iron loading to 30 wt% leads to more n-C17 and cracking products.


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ACS Catalysis

Figure 9. HDO of stearic acid over iron catalysts with different iron loadings. Reaction conditions: 0.5 mmol stearic acid, 20 mL solvent, 0.1 g catalyst, 4 MPa H2, T=320 oC, and t=8 h. 3.3 Effect of support The HDO activities of various iron catalysts with and without nitrogen-doping are further examined to explore the effect of support (figure 10). The Fe-N-C@Al2O3-900 and Fe-N-C-900 catalysts exhibit much higher HDO products than the Fe-Al2O3-900 and Fe-C-900 catalysts, respectively, which possess Fe/Fe3C species in their XRD patterns. By contrast, the effect of nitrogen-doping is not so remarkable for Fe-N-TiO2-900 and Fe-N-SiO2-900 catalysts, for which predominant peaks of metallic Fe are observed in the XRD patterns.

Figure 10. HDO of stearic acid over iron catalysts prepared using different supports with and without nitrogen-doping. Reaction conditions: 0.5 mmol stearic acid, 20 mL solvent, 0.1 g catalyst, 4 MPa H2, T=320 oC, and t=8 h.


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Figure 11. HDO of 1-octadecanol over various supports with and without nitrogen-doping. Reaction conditions: 0.5 mmol 1-octadecanol, 20 mL solvent, 0.1 g catalyst, 4 MPa H2, T=320 oC,

and t=8 h. Alumina is well-known to be an efficient catalyst for alcohol dehydration since 1797.[52] As

stated above and discussed later in the reaction pathway section (Section 3.4), the dehydration reaction could be implicated in the HDO of stearic acid to C18. Thus, one may presume that the alumina support plays an important role towards the dehydration of 1-octadecanol. To test this hypothesis, the HDO of 1-octadecanol over various supports is examined (figure 11). On alumina support, the yields are 2.5% to C17, 9.1% to C18, 24.6% to C18=, 5.5%

to

cracking

products, and 1.5% to C17-CHO at a conversion of 77.3%. Surprisingly, TiO2 exhibited much better catalytic performance, while SiO2 and C gave significantly inferior results. Nitrogendoping led to a positive effect on alumina, towards both dehydration of –OH and hydrogenation of C=C, but a negative effect on TiO2. Such a promotion effect by hybrid-formation was also reported by Su et al.[53] In addition, the addition of Fe significantly improves the catalytic performance of alumina-based catalyst, but leaves that of TiO2-based catalyst unchanged.


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ACS Catalysis

Figure 12. IR spectra of pyridine adsorbed on Al2O3 (a) and TiO2 (b). The acidity of Al2O3 and TiO2 was explored using Py-IR to elucidate their different activity towards HDO of 1-octadecanol (figure 12). The amounts of Brønsted (B) and Lewis (L) acid sites are listed in table S4. Lewis acid sites are predominant on both supports, and the ratio of B/L acids sites is higher on TiO2 (0.24) than on Al2O3 (0.05). Although the peaks for Brønsted acid sites (ca. 1540 and 1635 cm-1) are hard to distinguish on both supports, the peaks for B+L acid sites are more intense on TiO2. It is well known that Bronsted acid sites are important for dehydration, rationalizing in part why TiO2 exhibited a better catalytic performance than Al2O3 towards HDO of 1-octadecanol. After nitrogen-doping, the ratios of B/L sites greatly increased


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Page 28 of 45

from 0.05 to 0.17 on Al2O3, leading to better catalytic performance. By contrast, nitrogen-doping led to a slightly lower ratio of B/L sites and inferior activity on TiO2. 3.4 Reaction pathway

Figure 13. Time courses for the HDO of stearic acid (a) and 1-octadecanol (b). Reaction conditions: 0.5 mmol substrate, 20 mL solvent, 0.1 g Fe-N-C@Al2O3-900 catalyst, 4 MPa H2, T=320 oC. The time course for stearic acid conversion was recorded (figure 13a). 88.6% yield of 1octadecanol was obtained at 0.5 h, showing that 1-octadecanol is an important intermediate during the HDO of stearic acid. After 6 h, 1-octadecanol was completely consumed, and the


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ACS Catalysis

yield of n-C18 increased from 2.4% at 0.5 h to 92.2% at 6 h. No changes in the product distribution were observed when the reaction was prolonged to 8 h, confirming that the produced alkanes would not further react over the Fe-N-C@Al2O3 catalyst. 1-Octadecanal was detected initially (~1 h) with yields less than 1%, so that the minor product heptadecane was probably generated by the decarbonylation of 1-octadecanal. The time course for the HDO of 1-octadecanol (figure 13b) also showed the generation of trace yields of 1-octadecanal during the first 2 h, suggesting reversible transformation between 1-octadecanol and 1-octadecanal, which is well known for transition-metal catalyzed HDO of stearic acid. Trace octadecene (C18=) was obtained during the first 4 h, suggesting that octadecane was probably produced by the dehydration of 1-octadecanol followed by hydrogenation. HDO of oleic acid confirms that the iron catalysts are capable of catalyzing the hydrogenation of C=C bond, for which stearic acid is observed as a major intermediate. Another possible mechanism entails the reverse Mars-Van Krevelen mechanism during the HDO of 1-octadecanol to octadecane. This mechanism is implicated by the reactivity of benzoic acid over the Fe-NC@Al2O3 catalyst. Although there is no α-H next to the carboxylic acid group in benzoic acid, it is still converted to toluene with 97.8% yield (entry 5, table 2). As benzyl alcohol is one of the key intermediate molecules, its HDO should proceed via a reverse Mars-Van Krevelen kind of mechanism rather than dehydration followed by hydrogenation because of the lack of α-H. The reverse Mars–van Krevelen mechanism, which is usually involved in HDO over reducible metaloxides.25,54 can directly remove the O atom in the oxygenate substrate by C-O bond scission over an oxygen vacancy and H back donation from the OH, leaving the O in the oxygen vacancy. The aromatic ring does not participate in the reaction during the HDO of benzyl alcohol, similarly to a recent report, by one of us, on the highly selective HDO of m-cresol to toluene


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over a Pt-WOx/C catalyst.55 The oxophilic nature of W and the synergetic effect of Pt are essential for the performance of this catalyst. Since alumina is an irreducible oxide,56 oxygen vacancies can exist on reduced Fe species such as metallic Fe and Fe3C to carry out this chemistry. Similar deoxygenation reactions have been demonstrated in previous reports by one of us using Mo and W carbides catalysts.57,58 The HDO of stearic acid over Fe-N-C@Al2O3-900 catalyst under an inert atmosphere can further confirm this hypothesis, which affords 5.3% yield of heptadecane and 2.9% yield of 1-octadecanol as major products. To sum up, the reaction pathway for iron-catalyzed stearic acid to alkanes is shown in scheme 2.

Scheme 2. Possible reaction pathways for the HDO of stearic acid to liquid alkanes, such as heptadecane and octadecane, over Fe-N-C@Al2O3-900 catalyst. During the conversion of stearic acid, the initial production rate of 1-octadecanol (8.86 mmol g-1 h-1) is nearly three times higher than the initial production rate of 1-octadecane from 1octadecanol (3.03 mmol g-1 h-1). The time course of 1-octadecanol conversion (figure 13b) shows that the production rate of 1-octadecane from 1-octadecanol is 3.99 mmol g-1 h-1, which is slightly higher than that during stearic acid HDO. Thus, the conversion of –COOH group to – CH2OH group is faster than the deoxygenation of –CH2OH group to –CH3 group over the Fe-NC@Al2O3-900 catalyst. 3.5 stability test


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Figure 14. Stability test for Fe-N-C@Al2O3-900 catalyst. (a) HDO of stearic acid. (b) HDO of 1octadecanol. (c) HDO of stearic acid with 50 mg reused catalyst being replaced with fresh catalyst after each run. Reaction conditions: 0.5 mmol substrate, 20 mL solvent, 0.1 g Fe-NC@Al2O3-900 catalyst, 4 MPa H2, T=320 oC, and t=8 h. The stability test of Fe-N-C@Al2O3-900 catalyst is shown in figure 14. For HDO of stearic acid, the yield of octadecane decreased from 91.9% to 76.7% after the first reuse of the Fe-NC@Al2O3-900 catalyst, and then sharply decreased after the second recycle. The ICP-AES analysis showed that the iron content slightly decreased after recycling (table S5). Correspondingly, the nitrogen content slightly increased. Only metallic Fe can be observed in the XRD pattern of reused Fe-N-C@Al2O3-900 catalyst (figure S24), suggesting the destruction of Fe3C phases to metallic Fe during the HDO of stearic acid. In addition, the pore size of the iron catalyst (table S5) is found to be significantly increased from 3.76 to 12.04 nm with decreased pore volume (0.191 to 0.138 cm3/g), indicating that quite a few micro/mesopores are lost during the recycle. This pore loss is unfavorable for the exposure of active sites, thus leading to inferior catalytic activity. The catalyst deactivates more rapidly without the addition of stearic acid. Surprisingly, this solvothermal instability is not observed during HDO of 1-octadecanol,


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indicating that the active centers towards HDO of –CH2OH to –CH3 are retained during recycling. Combined with the results on the effect of support, it could be concluded that the Fe3C phase contributes mostly towards hydrogenation of –COOH to –CH2OH while the hybrid of N-C and alumina contributes mostly towards the HDO of -CH2OH to –CH3. The replacement of part of reused catalyst with fresh catalyst can afford satisfactory results but the complete recovery of catalytic activity of Fe-N-C@Al2O3-900 catalyst has not been achieved to date. Further future efforts in ongoing in our lab will be devoted to developing a more solvothermal stable Fe3C phase. 3.6 HDO of plant oil

Figure 15. Time course for the conversion of cyperus esculentus oil. Reaction conditions: 100 mg oil, 20 mL solvent, 0.15 g Fe-N-C@Al2O3-900 catalyst, 4 MPa H2, T=320 oC. HDO of triglycerides such as cyperus esculentus oil over Fe-N-C@Al2O3 catalyst was first explored (figure 15). The fatty acid composition of cyperus esculentus oil is listed in table S6. It consists of saturated C16 fatty acid (12.6 wt%) and C18 fatty acid (2.3 wt%), unsaturated C18 fatty acids (84.1 wt%), as well as some other saturated fatty acid (1 wt%). As mentioned above, the Fe-MSN catalyst reported by Slowing et al. is effective towards the HDO of fatty acids but ineffective towards the HDO of triglycerides.25 In contrast,


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triglycerides can be effectively converted into liquid alkanes over Fe-N-C@Al2O3 catalyst. Fatty alcohols are detected as major intermediates at first, and then are completely consumed after 8 h. The total weight yields of alkane products achieved 78.8 wt%, and the total molar yield of alkane products is 92.1% (the calculation method is given in experimental section 2.4). The gaseous phase (figure S2) consisted of methane (1.8 wt%), ethane (0.4 wt%), propane (1.3 wt%), CO (0.1 wt%), and CO2 (1.7 wt%). The presence of CO2 is probably due to the iron-catalyzed hightemperature water-gas shift (HT-WGS) reaction (water is generated by the HDO of plant oil) or reforming of glycerol.59,60 Another kind of plant oil, jatropha oil, was further used to examine the HDO activity of FeN-C@Al2O3-900 catalyst (figure S26). It contains more linoleic acids (18:2, 39.1 wt% versus 9.6 wt% in cyperus esculentus oil, table S6), which have two C=C groups. A yield of 81.8 wt% of total alkanes is obtained under identical reaction conditions, with the total molar yield of alkane products being 95.2%. These results demonstrated that our Fe catalysts held great potential to catalyze the HDO of plant oils. 3.7 Substrate scope


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Table 2. The HDO of various carboxylic acids over Fe-N-C@Al2O3-900 catalyst.a Entry

Substrate

T [oC]

t [h]

Conv. [%]

1

15

COOH

320

8

>99

2c

11

COOH

330

8

>99

3

5

COOH

350

8

>99

350

8

>99

280

8

>99

280

8

>99

4

HOOC

4

COOH

Yield [%]b

91.9

15

11

86.7

11

5

83.2

5

5

48.4

15

COOH 5

5

33

280

8

>99

5

14.3

F

COOH Cl

9.1

0.4

92.8 F

F

7

4.8

97.8

COOH 6

6

84

3.8

Cl

COOH 8

320

8

12.2

38.5 F3C

F3C

0.2 F3C

COOH 9

320 t

8

>99

Bu

>99 t

Bu

COOH 10d

280

8

9.6(20.2)

93.7

73.4(58.3) HO

HO

HO COOH 11

320

8

73.3

98.8 MeO

MeO OH

OH

7.5 HO OH

COOH 12

320

8

N. D.

31.6

56.2

320

8

>99

81

3.1

350

8

>99

57.6

30.6

350

8

50.9

34.6

2.7

COOH 13

COOH COOH 14

COOH 15 HOOC

a Reaction conditions: 0.5 mmol carboxylic acid substrate, n-dodecane (20 mL), 0.1 g Fe-N-C@Al2O3-900 catalyst, 4 MPa H2. N. D.= not determined. bGC yield. cn-decane was used as solvent. dThe value in parentheses correspond to reaction proceeded at 320 oC.


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HDO of various carboxylic acids was performed to demonstrate the versatility of the Fe-NC@Al2O3-900 catalyst. The aliphatic carboxylic acids with shorter alkane chains afforded slightly lower yields of alkane with the same carbon number (entries 2 and 3). For suberic acid, which has two carboxyl groups, octane, octene, and heptane are afforded with yields of 48.4%, 14.3%, and 33%, respectively. The higher yield of alkane with one carbon less indicated that decarbonylation is more preferable for dicarboxylic acid. HDO of aryl carboxylic acids was also investigated (entries 5-15). Quantitative conversion of benzoic acid to 97.8% yield of toluene is achieved at 280 oC. Accordingly, the –COOH group in the substrate could be selectively converted to –CH3 group without any hydrogenation of the aromatic ring. The excellent selectivity suggested that the Fe-N-C@Al2O3-900 catalyst may hold great potential for chemoselective HDO of aryl carboxylic acids. The time course (figure S27) suggested that benzyl alcohol and benzaldehyde are key intermediates during the HDO of benzoic acid. As we disucussed above in section 3.4, the deoxygenation of benzyl alcohol to toluene should proceed via a direct C-O bond scission. With different functional groups in the para-position (entries 6-9), the aryl carboxylic acids generally could be converted into corresponding alkanes with high to excellent yields except trifluoromethyl group, which is strong electron-withdrawing group. 4-hydroxy and 4-methoxy benzoic acids are explored as model compounds for the HDO of lignin oxidation products.61 The carboxylic acid group was converted into methyl group in the presence of -OMe group in the para-position. In contrast, phenol was obtained as major product from 4-hydroxybenzoic acid. This phenomenon could be attributed to the interactions between – COOH and –OH groups such as intermolecular hydrogen bonding. Our hypothesis can be further


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confirmed by the conversion of salicylic acid, which has intramolecular hydrogen bond. Phenol was obtained with higher yield (56.2%) than o-Cresol (31.6%). HDO of 3-phenylpropanoic acid afforded 81% yield of n-propylbenzene, indicating that the iron catalyst is also capable of converting aryl aliphatic carboxylic acid. Finally, the HDO of aryl dicarboxylic acids were investigated. O-phthalic acid is more reactive than p-phthalic acid, for which o-xylene and toluene are obtained with yields of 57.6% and 30.6%, respectively. 4. CONCLUSIONS Nitrogen-doped carbon-alumina hybrid-supported iron (Fe-N-C@Al2O3) catalysts, which are synthesized by simultaneous pyrolysis of iron acetylacetone (Fe(acac)3) and melamine onto alumina at 900 oC, are demonstrated to be active towards the HDO of carboxylic acids to hydrocarbons. The effect of pyrolysis temperature, support, nitrogen-doping, and iron loading are explored extensively by using stearic acid HDO as a model reaction. Under optimal reaction conditions, liquid alkanes, such as n-octadecane and n-hepadecane, are generated with yields of 91.9% and 6.0%, respectively at quantitative conversion of stearic acid. In addition, triglycerides such as cyperus esculentus oil and jatropha oil can also be converted into liquid alkanes with total weight yields (molar yields) of 78.8 wt% (92.1%) and 81.8 wt% (95.2%), respectively. The iron catalyst can chemoselectively catalyze the HDO of carboxylic acid group without any hydrogenation of an aromatic ring. Extensive characterization showed that the addition of melamine as N precursor during pyrolysis is important for the improved HDO activity of iron catalysts. It avoids undesired reactions between iron precursor and alumina support to form inactive hercynite phase during pyrolysis, leading to the formation of Fe3C phases as active sites for the HDO reactions especially hydrogenation of –COOH to –CH2OH. The Lewis basicity of the catalyst is influenced by both nitrogen-doping and iron-loading, and is important to adsorb


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acid substrates to form carboxylate intermediate. The formation of N-C-alumina hybrid increased the ratio of Bronsted acid/Lewis acid sites of Al2O3 to enhance its activity towards HDO of 1octadecanol. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional SEM, TEM, XPS, XRD patterns, ICP-AES, GC spectra, photographs, time course, and stability test results (PDF) AUTHOR INFORMATION Corresponding Author *J. Li. Email: [email protected]. Telephone: (+86)-010-80734008 ext. 8022. *D. G. Vlachos. Email: [email protected]. ORCID Jiang Li: 0000-0001-7132-5302 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21702227), Science Foundation of China University of Petroleum, Beijing (No. 2462014YJRC037). DGV acknowledges support from the Catalysis Center for Energy Innovation, an Energy Frontier


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Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award number DE-SC0001004. Dr. Jiang Li also thanks the support of the China Scholarship Council (CSC). REFERENCES (1) Huber, G. W.; Iborra, S.; Corma, A. Synthesis of Transportation Fuels from Biomass: Chemistry, Catalysts, and Engineering. Chem. Rev. 2006, 106, 4044-4098. (2) Corma, A.; Iborra, S.; Velty, A. Chemical Routes for the Transformation of Biomass into Chemicals. Chem. Rev. 2007, 107, 2411-2502. (3) Jamil, F.; Al-Haj, L.; Al-Muhtaseb, A. H.; Al-Hinai, M. A.; Baawain, M.; Rashid, U.; Ahmad, M. N. M. Current Scenario of Catalysts for Biodiesel Production: a Critical Review. Rev. Chem. Eng. 2018, 34, 267-297. (4) Zhao, C.; Bruck, T.; Lercher, J. A. Catalytic Deoxygenation of Microalgae Oil to Green Hydrocarbons. Green Chem. 2013, 15, 1720-1739. (5) Huber, G. W.; O’Connor, P.; Corma, A. Processing Biomass in Conventional Oil Refineries: Production of High Quality Diesel by Hydrotreating Vegetable Oils in Heavy Vacuum Oil Mixtures. Appl. Catal. A: Gen. 2007, 329, 120-129. (6) Kubicka, D.; Kaluza L. Deoxygenation of Vegetable Oils over Sulfided Ni, Mo and NiMo Catalysts. Appl. Catal. A: Gen. 2010, 372, 199-208. (7) Anand, M.; Sinha, A. K. Temperature-dependent Reaction Pathways for the Anomalous Hydrocracking of Triglycerides in the Presence of Sulfided Co-Mo-Catalyst. Bioresour. Technol. 2012, 126, 148-155. (8) Peng, B.; Yao, Y.; Zhao, C.; Lercher, J. A. Towards Quantitative Conversion of Microalgae Oil to Diesel-Range Alkanes with Bifunctional Catalysts. Angew. Chem. Int. Ed. 2012, 51, 20722075.


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(57) Yu, W.; Salciccioli, M.; Xiong, K.; Barteau, M. A.; Vlachos, D. G.; Chen, J. G. Theoretical and Experimental Studies of C-C versus C-O Bond Scission of Ethylene Glycol Reaction Pathways via Metal-Modified Molybdenum Carbides. ACS Catal. 2014, 4, 1409−1418. (58) Xiong, K.; Yu, W.; Vlachos, D. G.; Chen, J. G. Reaction Pathways of Biomass-Derived Oxygenates over Metals and Carbides: From Model Surfaces to Supported Catalysts. ChemCatChem 2015, 7, 1402-1421. (59) Zhu, M. H.; Wachs, I. E. Iron-Based Catalysts for the High-Temperature Water-Gas Shift (HTWGS) Reaction: A Review. ACS Catal. 2016, 6, 722−732. (60) Huber, G. W.; Shabaker, J. W.; Dumesic, J. A. Raney Ni-Sn Catalyst for H2 Production from Biomass-Derived Hydrocarbons. Science 2003, 300, 2075-2077. (61) Wang, M.; Liu, M.; Li, H.; Zhao. Z.; Zhang. X.; Wang, F. Dealkylation of Lignin to Phenol via Oxidation-Hydrogenation Strategy. ACS Catal. 2018, 8, 6837−6843.


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Chemoselective Hydrodeoxygenation of Carboxylic Acids to

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