High yield of Hydrocarbons from Catalytic Hydrodenitrogenation of

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High yield of Hydrocarbons from Catalytic Hydrodenitrogenation of Indole Under Hydrothermal Conditions Ligang Luo, Shaotong Liu, Chunze Liu, Yuanyuan Wang, and Liyi Dai Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02322 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 29, 2017

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High yield of hydrocarbons from catalytic hydrothermal hydrodenitrogenation of indole. 391x110mm (72 x 72 DPI)

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High yield of Hydrocarbons from Catalytic Hydrodenitrogenation of Indole Under Hydrothermal Conditions Ligang Luo, Shaotong Liu, Chunze Liu, Yuanyuan Wang* and Liyi Dai* Shanghai Key Laboratory of Green Chemistry and Green Process, Department of Chemistry, East China Normal University, No.500 Dongchuan Road, Shanghai 200241, People's Republic of China. Email:[email protected]; [email protected] Abstract: We report herein on a new method to achieve a high yield of hydrocarbons from hydrothermal catalytic hydrodenitrogenation of indole, which is higher than that from conventional pyrolysis methods. The main hydrocarbon products were aromatic hydrocarbons and alkanes, which is similar with fossil oils to be used as liquid fuels in the future. Catalyst screening experiments show that noble metal catalysts (e.g., 5 wt% Pt, Pd, or Ru) supported on porous solids (e.g., carbon, Al2O3) enhanced the conversion of indole to hydrocarbons under hydrothermal condition. Of those different catalysts, Pd/γ-Al2O3 shows the greatest influence on the yield of hydrocarbons, which we focus on the catalyst Pd/γ-Al2O3 in more details. Based on the Pd/γ-Al2O3 catalyst, the effects of time, temperature, and H2 pressure on the hydrocarbons were discussed. HDN of indole reaction at 450 °C, 0.015 g/cm3 water density, 5 MPa H2 and 50 wt% Pd/γ-Al2O3 loading led to a maximum yield (51 mol%) of hydrocarbons at 120 min. It proposes the mechanism to acquire hydrocarbons from hydrogenational indole denitrogation, which experiences two different pathways, as (1) indole is directly hydrodenigrated into hydrocarbons, and (2) intermediate oxygenated products from hydrolysis of partly hydrodenigrated indole, were hydrodeoxygenated to removal O to acquire hydrocarbons. The factors for deactivation of catalyst under hydrothermal condition are also discussed by the results from charactering the surface, bulk structure and microscopy experiments.


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Key

word:

Hydrothermal

Conditions;

Hydrocarbons;

Indole;

Hydrodenitrogenation Introduction The increasing in oil demand and consumption of fossil fuels across the globe has led to explore the renewable and alternatives sources of energy. For decreasing greenhouse gas emissions and improving local economies, biomass is an important renewable source for the production of fuels and energy [1, 2]. Currently, bio-oils produced from microalgae, as a biomass, have shown great potential due to its photosynthetic efficiencies, fast growth rate, and limited controversy versus food [3-4]. Unfortunately, microalgae grow with plenty water (about 80 wt% water), which impractically makes conventional technologies to dry. These factors motivate attention to the process of wet microalgae through hydrothermal liquefaction (HTL) to produce liquid fuels. Processing the wet microalgae at 300-350 °C using hydrothermal liquefaction has been studied extensively [3, 4]. Unlike crude oil used in industries, crude biofuels from HTL of algae contains quantities of oxygen, nitrogen and sulfur heteroatoms [4], which reduces the stability of biocrude and creates environmental problems such as production of SOx and NOx while they are combusted. Therefore, removalling heteroatoms were paramount for the production of upgraded liquid fuels suitable for the demand of the current energy infrastructure. Supercritical water (SCW) (>374 °C) had been demonstrated to be an excellent solvent and good ability to break the carbon-heteroatom bonds [5-6]. Therefore, many research pay attention to upgrading biocrude in SCW toward O, N and S removal from the crude bio-oil. Recently, Bai et al [5] compared the activity of catalysts on the catalytic processing of pretreated crude bio-oil produced from the hydrothermal liquefaction of Chlorella pyrenoidosa. Duan et al [6] also studied the hydrothermal liquefaction of microalgae using a variety


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of common catalysts with H2 at 400 °C for 60 min to decrease the O/C and N/C ratios, and increased the higher heating values (HHV) up to 44 MJ/kg. However, there were few studies on the pathways, kinetics, and mechanisms of catalytic hydrothermal treatment of actual microalge due to the complex of biocrude compounds. To study the model compounds with the same functional groups as the corresponding biomacromolecules can provide useful comprehension into the reaction of catalytic hydrothermal treatment of actual biomass [7]. Several previous researchers have used model compounds to understand how various components of microalgae react during hydrothermal liquefaction [8,9]. Thomas usedbutyric Acid to understand the phenomena hydrothermal decarboxylation of fatty acids [8]. We also studied the catalytic hydrothermal treatment of soy protein [9]. However, N-compounds as important components of crude bio-oils have not been extensively explored in the HTL environment. Nitrogen-containing heterocycles including pyrroles, indoles and pyridines were found not only in biocrude but also in fossil oils [2-4]. However, the conditions of conventional pyrolysis of nitrogen-containing heterocycles were severe with high temperature and high pressure hydrogen or H2S [10,11]. High efficiency hydrothermal reducing conversion of nitrogen heterocycles into hydrocarbons seems to be an optimal way to acquire upgrading liquid fuels. Pyrroles could be cleaved to give 1,4-diketones under hydrothermal conditions with high pressure and temperature [12]. Pyridines undergo various ring openings to form m-cresol and methylamine [12]. However, indoles were much more hydrothermally stable than other Nitrogen-containing compounds (pyrroles or pyridines), and the removal of nitrogen in indole was more difficult due to high strength of the carbon-nitrogen bond with unavailable donation delocalized electrons. The ring opening of indole occurred at 350 °C under hydrothermal condition, but no nitrogen removal was observed and only produced small quantities of aniline and 2-methylaniline [12]. Under 10 wt% H3PO4 solvent, indole showed nearly 60 wt% conversion at 350 °C. Without


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catalyst, the removal of N was only observed over 550 °C in supercritical water [12]. Guo et al investigated the gasification of indole under supercritical water, which indicated more than 80% conversion over 650 °C with catalyst [13]. Nevertheless, these works were not real removal of nitrogen atom from indole and acquired trace hydrocarbons. To the best of our knowledge, there has been no prior work dictated on catalytic hydrodenitrogenation of indole to acquire hydrocarbons. We employed various catalysts used in the literatures under hydrothermal condition and traditional HDN of indole with organic solvents or pyrolysis cracking as a guide for this study. Ru/C, Pt/C, Pd/C, Mo2C, activated carbon and Ni had effected the desirable changes with hydrothermal catalytic treatment. Owing to great advantages of alumina, such as a large specific surface area, good hydrothermal stability and abundant of commercial applications in the environmental protection and petroleum industries [14], we selected catalysts that noble metals (Pd, Pt) used Al2O3 as support. Additionally, the results were also compared to those obtained with typical hydrotreatment catalyst CoMo/γ-Al2O3 (sulfided). In this article, we reported on the effect of several different catalysts (activated carbon, 5 wt% Pt/C, 5 wt% Pd/C, 5%Rh/C, 5 wt% Ru/C, Raney NiCu, 5 wt% Pd/γ-Al2O3, 5 wt% Pt/γ-Al2O3, CoMo/γ-Al2O3 (sulfided), Mo2C, MoS2) on the yield of hydrocarbons and then focus on the most activity catalyst of Pd/γ-Al2O3. It was discussed the influence of reaction temperature (350, 400, 450 °C), holding time (30-180 min), the pressure of H2 (0-5MPa) and water density (0-0.15 gmL-1) on the yield of hydrocarbons. The mechanism of hydrocarbons from HDN of indole under hydrothermal condition is also proposed. We observed the catalyst of Pd/γ-Al2O3 lost activation after reused, and then we also studied the factors for catalyst deactivation during hydrothermal HDN of indole. 2 Experimental Sections 2.1Materials


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Indole (>99% purity) was obtained from Alfa Aesar. CoMo/γ-Al2O3 was purchased by Sinopharm Chemical Reagent. Other catalysts (Mo2C, MoS2, activated Carbon, Ru/C, Pt/C, Pd/C, Pt/γ-Al2O3, and Pd/γ-Al2O3) were obtained from Sigma-Aldrich. The chemicals used for catalyst synthesis (Palladous nitrate, γ-Al2O3) were purchased from Sinopharm Chemical Reagent. Distilled and deionized water was used throughout the experiments. All other solvents were from Sigma-Aldrich in > 95 wt% purity and used as received. Air, hydrogen, Nitrogen, Helium and argon were obtained from Gas Cylinder shanghai company. Standards gases (CO, CO2, CH4, and C2H6) were purchased from Shanghai baosteels Gas Company. The bath reactors were constructed from 316 stainless-steel Swagelok tube fittings with a 1/2 in port connector and cap and a 1/2 in. to 1/8 in. reducing union. Each reactor was fitted with a 9 in length of 1/8 o.d. stainless steel tubing, which was connected to a High Pressure Equipment Company shut-off valve (HiP part 15-12AF2). Each reactor had internal volumes of approximately 4.3 mL. Before used in reactions, the reactors were loaded with water and treated in 400 °C for 60 min to expose the reactor metal walls into the hydrothermal condition. Reactions for investigating the activity loss on the catalysts were performed in a continuous flow reactor system assembled from stainless steel tubing and Swagelok parts in Figure S1. The reaction was run continuously at this nominal steady state for 24 h. 2.2 Preparation of supported Pd catalysts The commercial γ-Al2O3 (surface area: 155 m2g-1) supported Pd catalysts were prepared by deposition-precipitation method. Pd species were introduced to the supports by wet impregnation with Pd(NO3)2 as precursor. In a typical preparation process of Pd/γ-Al2O3, 50 mL Pd(NO3)2 aqueous solution (Pd:1.0 mgmL-1) was added to 1 g γ-Al2O3 support and then stirred for 12 h or overnight. The catalyst was fully dried at 120 °C overnight and then subjected to calcination pretreatment. The catalysts were calcinated in air at 500 °C for


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4h, and submitted to be reduced in ﬂowing H2 atmosphere for two hours at 500 °C. 2.3 Catalyst Characterization X-ray diffraction (XRD) measurements were obtained on a Bruker D8 diffractometer using Cu Kα radiation at a scan rate (2θ) of 2° min-1 for composite samples. Surface area (BET) was calculated by BET method using a BEL SORP mini II analyzer at liquid N2 temperature. Transmitting electron microscopy (TEM) was obtained with a JEOL 2100 F instrument operating at accelerating voltage of 200 KV. Inductively-coupled plasma atomic emission spectroscopy (ICP-AES) measurements were obtained by the Thermo Scientific iCAP 6300 instrument. 2.4 Experimental procedure All reactors were loaded with indole, catalysts and water with the indole loading was 15 wt%. Each reactor contained enough water for the expanded liquid phase to occupy 95% of the reactor volume at the reaction temperature. Loaded and sealed reactors were placed in a fluidized sandbath preheated to design temperature (350, 400, and 450 °C). The reactors reached the sandbath temperature in about 5 min. The reactors then remained in the sandbath for an additional 30-240 min after which they were carried out from the sandbath, put into cool water, and stored at room temperature for at least 4 h to reach equilibrium prior to performing the gas-phase analysis. After analyzing the gaseous products, we added 9 mL of dichloromethane to the reactor and collected the remaining reactor contents. These samples were centrifuged to separate the products into solid, organic and aqueous liquid phases.

The

separated

organic

phases

were

injected

into a

gas

chromatograph for analysis. And the solid phases were dried for characteristical analysis. To assess experimental variability, duplicate independent runs were conducted under nominally identical conditions.


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2.5 Analysis The gas-phase products were analyzed with an Agilent Technologies model 6890N gas chromatograph (GC) equipped with a thermal conductivity detector (TCD) using Carbon-PLOT column. The Ar carrier gas was flowed through the column at 40 mLmin-1. The programmed temperature was kept at 40 °C for 10 min and then increased to 225 °C at 10 °C min-1 [8].Separation of the liquid-phase products was achieved by using an Agilent 6890 GC equipped with a HP-5 MS capillary column. The injection port temperature was 310 °C with a H2 flow rate of 40 mLmin-1, air flow rate of 30 mLmin-1, and makeup flow of N2 at 5 mLmin-1. The column was initially held at 40 °C for 4 min. The temperature was ramped to 300 °C at 4 °C min-1 and held isothermally for 10 minutes. The products were also identified by an Agilent 6890 Mass Spectrometric (MS) detector. The quantification of identified products was also carried out with an Agilent Technologies 7890 gas chromatograph with a flame ionization detector (GC-FID) at the same conditions as GC-MS.The quantification of ammonia in the aqueous phase was measured with a nitrogen ammonia reagent set (HACH). The treated solution was measured at 655 nm using a Thermo Scientific Model Genesys 20 spectrophotometer, and the ammonia concentration was determined by analysis of standards with known concentrations. Conversion of indole, yield of products, and yield of hydrocarbons to the liquid phase are defined as follows: Conversion = (1 −

Yield =

Ca )*100 mol % (1) Co

Ci *100 mol % (2) Co

∑C Hydrocarbons =

* i

Co

*100 mol % (3)


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where Ca, Co, Ci and C*i refer to the concentrations, in molL-1, of indole exiting the reactor, of indole entering the reactor, of any product and of any hydrocarbon product, respectively. 3 Results and Discussion 3.1 Control experimental results The gasification of indole would happen over 500 °C supercritical water [15]. Then, we need to determine the influence of thermal decomposition for indole was significant below 500 °C. In Figure 1, several control experiments were completed under hydrothermal condition. The first experiment was thermal cracking of indole without water at 450 °C. After 180 min, 96±1mol% remained unreacted. The second experiment for indole in a helium atmosphere was conducted at 450 °C hydrothermal condition. After 180 min at 450 °C, the conversion of indole was 4±2 mol%. At last, we added hydrogen into indole at 450 °C supercritical water without catalyst, which illustrated that remain indole were still more than 92 mol%. After 180 min, there are no aromatic compounds, which were the main products from HDN of indole. Overall, these results indicated that no detectable amounts of products were generated with thermal cracking or hydrolytic decomposition at 450 °C and 180 min, the most severe conditions examined in this investigation.


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Figure 1. The trend for conversion of indole at each condition.

3.2 Analysis products from GC-MS The information of liquid products formed from HDN of indole was discussed in this section. Figure S2 showed the total ion chromatogram of the liquid product samples collected from HDN of indole in hydrothermal condition 450 °C for 60min over Pd/γ-Al2O3. Table S1 shows tentative identities for several of the individual molecular components in the peak of Figure S2 that is amenable to GC analysis. These identities are tentative because they are based solely on matches with mass spectra stored in the GC-MS computer library. These products in Table S1 included liquid-phase nitrogenous products (aniline, 2-methylaniline, 2-ethylaniline and indoline), oxygenated products (phenol,

2-ethylcyclohexanone,

o-cresol,

2-ethylcyclohexanol,

and

2-ethylphenol), and hydrocarbon products (toluene, ethylcyclohexane, and ethylbenzene). Hydrocarbon products include aromatic hydrocarbon and alkane, which were the final products from HDN of indole. It also contains some oligomerization products at the process of indole HDN (such as naphthalene, biphenyl and carbazole). Biphenyl may be acquired by the reaction

through

a

phenyl

radical

and

a

benzene

molecule.

Nitrogen-containing polycyclic aromatic compounds (carbazole) are also appeared in the results, which have been reported as products from hydrothermal

treatment

of

algae

[17].

Obviously,

almost

new

nitrogen-containing aromatic compounds were aniline and its derivatives, which were believed from opening the pyrrolic ring. Anilines could experience two paths to convert to hydrocarbons under hydrothermal condition. One path was anilines directly denitrogenated into aromatic hydrocarbons. Another way was that anilines maybe firstly go through the hydrolysis path into alcohols under hydrothermal condition [18]. Then, many oxygenated products appeared in the reaction, which were produced by hydrolysis of aniline and its derivatives. With hydrogen, oxygenated products experienced the HDN process to


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produce hydrocarbons. From Figure S2, ethylbenzene was the most abundant product and toluene also had abundant peak area, while the peak areas of alkanes were less. These results indicated the pyrrlic ring or anlines were less stable than phenyl ring and easily crack with H2. 3.3 Screening the effect of different catalysts A number of heterogeneous catalysts were explored to detect the most effective one on hydrocarbons from hydrothermal HDN of indole. The influence of different materials with different reaction conditions was shown in Table 1. In each experiment, the initial mole amount of indole was 0.25 molL-1. Among the major HDN products, the yields of aromatic hydrocarbon and alkane provide a good measurement of catalytic activity for HDN of indole. Then, the sum of these molar yields hydrocarbons was used as the activities of several different catalysts. The mole yield of NH3 was also a direct measure of N removal from indole, which was also listed in Table 1. All of the tested materials displayed some activities for HDN. Without hydrogen added into the reaction, no hydrocarbons were observed even in the presence of Pd/γ-Al2O3 (Entry 2, Table 1). HCOOH was used as a different hydrogen resource to discuss. Though high conversion of indole was obtained, the hydrocarbons were not noted due to the O of HCOOH introduced in products (Entry15, Table 1). Activated carbon shows little catalytic activity for HDN, which is much less than other supported noble metals on activated carbon. Though the catalyst HZSM-5 (Entry 4, Table 1) has a better activity on the conversion of indole than that of activated carbon, it has low yield of HDN products. Among carbon-supported metal catalysts, Ru/C shows a good yield of NH3 for HDN, but with the lowest yield of hydrocarbons (Entry 6, Table 1). Ru/C shows an effective activity for gasification under hydrothermal condition [16]. Under this condition, gasification and catalytic HDN of indole were competing reaction. The catalyst Ru/C was favorble to be function of gasification under hydrothermal condition, which reduced the yields of hydrocarbons. MoS2,


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Mo2C and sulfide CoMo/γ-Al2O3, as traditional effective HDN catalysts, performed poorly for hydrothermal HDN of indole. Though Raney NiCu shows good activity on hydrothermal hydrodeoxygenation (HDO) of phenols [19], the activity of Raney NiCu on HDN of indole was not high. Of all the catalysts tested, it was noted that 5 wt% Pd/γ-Al2O3 consistently gave the best performance for the HDN of indole under hydrothermal condition (Entry 12, Table 1). Over 51 mol% of hydrocarbons yield was obtained, whereas no other catalyst produced yields exceeding 50%, which is higher than that from traditional pyrolysis method (25.4%) [20]. Then, we also detected the influence of the Pd/γ-Al2O3 catalyst loading. While the Pd/γ-Al2O3 catalyst loading increased from 10 to 60 wt%, indole conversion was increasing and got to 99 wt% at 50 wt% loading (Entries 16-20, Table 1). It was also clearly that the yield of NH3 had the similar trend with catalyst loading increased from 10 to 60 wt%. Nevertheless, the yield of hydrocarbons was independent with the change of catalyst loading. It seems that maximum yields of hydrocarbon products were performed with 50 wt% catalyst loading. Having identified Pd/γ-Al2O3 as the most active catalyst for hydrothermal HDN of indole, the numerous reaction products with Pd/γ-Al2O3 from different reaction conditions was discussed blew. Table 1 Results from hydrothermal HDN of indole (5 MPa H2) with different catalysts under different conditions.

Entry

Catalyst

1 2

None a Pd/γ-Al2O3 a activated carbon HZSM-5 Pd/C Pt/C Ru/C Raney NiCu Mo2C

3 4 5 6 7 8 9

Catalyst Indole NH3 Hydrocarbons loading conversion yield (mol%) (wt%) (mol%) (mol%) / 5.3 / / / 26 10 /

T (°C)

t (min)

450 450

180 180

450

180

50

18

5.4

/

450 450 450 450 450 450

180 120 120 120 120 120

50 50 50 50 50 50

46 73 81 89 86 84

6.3 39 43 45 36 41

0.9 14 16 18 19 28


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10 11 12 13 14 15 16 17 18 19 20 a

MoS2 CoMo/γ-Al2O3 Ni/γ-Al2O3 Pd/γ-Al2O3 Pt/γ-Al2O3 Pd/γ-Al2O3 b Pd/γ-Al2O3 Pd/γ-Al2O3 Pd/γ-Al2O3 Pd/γ-Al2O3 Pd/γ-Al2O3

450 450 450 450 450 450 450 450 450 450 450

120 120 120 120 120 120 120 120 120 120 120

50 50 50 50 50 50 10 20 30 40 60

78 89 95 99 99 95 59 82 89 93 99

48 43 64 75 69 43 23 36 41 56 82

30 25 32 51 41 28 9.8 14 25 31 42

Without high-pressure H2, b with HCOOH.

3.4 The influence of temperature and time The Pd/γ-Al2O3 catalyst provided the highest yields of hydrocarbons and conversion of indole. Therefore, we examined the Pd/γ-Al2O3 catalyst in more detail.

Figure 2 variation of product (■ Indole, ● Nitrogenous products, ▲ Oxygenated products, ▼ Hydrocarbons) yields from indole HDN over Pd/γ-Al2O3 at 350 (a), 400 (b) and 450 (c) °C.


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In Figure 2, it shows the influence of temperature (350, 400, and 450 °C) on the yields of products from hydrothermal treatment of indole over Pd/γ-Al2O3. At low temperature (350 °C), remain amount of indole decreased with the reaction time, while the yield of Nitrogenated and oxygenated products increased with the reaction time. Hydrocarbons appeared after 60 min at 350 °C and get less than 10 mol% for 180 min. Obviously, the residual indole disappeared at 150 min under 400 °C. The trends of nitrogenated and oxygenated products were nearly similar, which reached a maximum value and approximately decreased after 120 min and 90 min at 400 °C, respectively. Simultaneously, the yield of hydrocarbons increased into 42 mol% at the same condition. The observations suggested that the nitrogenated and oxygenated products were the intermediated products, which indicated indole firstly converted into nitrogenated and oxygenated products and then went thorough hydronitrogention

and

hdyroxygenation

into

hydrocarbons.

While

the

temperature reached into 450 °C, the conversion of indole need 90 min to be 99 mol%, which means the temperature has important effect on conversion of indole. The yield of nitrogenated products also has similar trend as those with 400 °C, which need less reaction time to get the maximum yields. Nevertheless, the oxygenated products increased to some extent after 150 min, which may cause by products reaction with water. It also shows that the yield of hydrocarbons gets a high yield 51 mol% after 120 min to reach a platform. However, it illustrated that the yield of hydrocarbons declined at 180 min, which means it may pass through gasification into gas products. 3.5 The influence of water density The influence of water on the different product yields was shown in Figure 3. All of these reactions the amounts of 5 wt% Pd/γ-Al2O3 (50 wt%), indole (0.25 molL-1) and hydrogen (5 MPa at 25 °C) were loaded, which means that all the reactors had the same concentrations of H2, indole, and catalysts. Nevertheless, due to the different water loadings, the pressures in reactors


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were different. While we added 0.05, 0.10 and 0.15 gmL-1 water into reactor, the estimated pressure was 3100, 4200 and 4900 psig, respectively.

Figure 3 Effect of water loading on the product distribution.

Compared with no water added, the yield of hydrocarbons was decreased nearly 3 mol% and oxygenated products increased more than 5 mol% while the water density at 0.05 gmL-1. This result indicated that water was involved in the catalytic HDN of indole to convert to oxygenated compounds. In the presence of water, as the water density increased, the yield of nitrogenated and oxygenated products decreases while the yield of hydrocarbons increases. The highest molar yield of hydrocarbons was acquired in 0.15 gmL-1 of water. In Figure 3, it shows that the yield of oxygenated products shows increased firstly and decreased with the increasing amount of water. It suggested that this phenomenon maybe a result from competitive reactions between hydrolysis and hydrogenation for nitrogenated and oxygenated compounds. 3.6 The influence of H2 loading The role of H2 has two functions under hydrothermal conditions, which one was to hydrogenate the unsaturated bonds and the other one was to open the ring and crack the subsequent products to be hydrocarbons. So we studied the effect of initial H2 amount on the product yields from indole HDN in 450 °C for 2 h with 0.25 molL-1 indole. Figure 4 shows that the initial amount of H2


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loaded into each reactor showed a large influence on product yields from indole. It was noted that the H2 loading showed strongly effects on the yield of NH3 and product distributions. The yield of NH3 increased from 5 to nearly 60 mol% with an increase of the initial H2 loading from 0 to 3.0 MPa. A further increase of H2 pressure has a little influence on the yield of NH3. At lower initial hydrogen pressure, the reaction products are largely nitrogenated products due to the less occurred of HDN. The final hydrocarbons increased from 10 mol% to 51mol% with the amount of H2 increasing to 5 MPa. The oxygenated products reached a maximum yield with 3 MPa H2 and, then decreased obviously. The reason may be that the hydrolysis was in the dominant reaction at low amount of H2 and hydrodeoxygenation take the dominant reaction.

Figure 4 Product yields from HDN indole with hydrogen at 0 to 5MPa at 450 °C, 120 min.

3.7 Gas products We also investigated the gas products, which indicated that H2, CO, CO2, CH4, and C2H6 were the main gaseous products. Table 2 listed the gas products from reaction, not involved hydrogen, which is not the product from reactions. The total gas yields were around 2 wt%. CO and CO2 may be formed through steam reforming and water-gas shift. CH4 and C2H6 were believed to be produced from decomposion of hydrocarbon products. Table 2 shows that the yields of light gases (C1-C2) generally increase with


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temperature due to increasing temperature favored the process of decomposion for hydrocarbons. Table 2 The gas products detected from HDN of indole (5MPa H2, 180 min). Total yield Temperature(°C)

CO2 (µmol)

CH4 (µmol)

C2H6 (µmol)

CO (µmol) (wt %)

350

84±7

5.5±0.8

0

3.3±0.3

1.3±0.2

400

89±4

14±3

3.5±0.5

11±2

1.5±0.3

450

120±5

20±2

5.2±0.4

13±3

2.6±0.2

3.8 Proposed reaction network Due to oxygenated compounds appeared in products, the reaction network of indole in hydrothermal condition was unsimilar to those previously proposed for traditional HDN conditions of indole [19]. Then, according to the above results, it summarizes the major reaction pathways outlined in Figure 5. The indole firstly was hydrodenigrated into nitrogenated products with opening the pyrrolic ring. Almost nitrogenated products are anilines, which could go thorough hydrolysis path into corresponding alcohols under hydrothermal condition [18]. Then, the forward conversion of nitrogenated products took two different

pathways,

as

hydrolysis

to

oxygenated

products,

and

hydrodenitrogenation to removal N to form hydrocarbons. Meanwhile, the oxygenated products also went through hydrodeoxygenation process to hydrocarbons. Then, it suggested that forming hydrocarbons from the hydrothermal process of hydrodenitrogenation of indole involved two different pathways, (1) indole was directly hydrodenigrated into hydrocarbons, and (2) intermediate oxygenated products from hydrolysis of partial hydrogenation of indole were hydrodeoxygenated to removal O to acquire hydrocarbons.


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Figure 5 Proposed reaction network for indole hydrodenitrogenation under hydrothermal condition. 3.9 The cycle of catalyst activity Recently, Pd/γ-Al2O3 was used as an efficient catalyst in hydrothermal condition, however, the reusability and stability of the catalyst was not referred in the literature. Then, we investigated the reusability and stability of Pd/γ-Al2O3 at high temperature water. Table 3 showed the conversion and hydrocarbon yield from six cycles of indole HDN over the same 5wt% Pd/γ-Al2O3. The conversion of indole was steady before 3 cycles, and then decreased to 71 mol% at the sixth cycle. It was clear that the decrease trend of hydrocarbon yields for each cycle in Table 3, especially rapidly declined from fourth cycle. This result indicated that the activity of Pd/γ-Al2O3 was deactivated. Table 3 The indole conversion and hydrocarbon yield from indole HDN with Pd/γ-Al2O3 for 6 cycles used (mol%).

Cycle times Entry 1

2

3

4

5

6

Conversion of indole

99

99

85

75

70

71

Selection of hydrocarbons

51

43

38

32

22

10

After exploring the reusability of Pd/γ-Al2O3, it was utilized the flow reactor to investigate the stability of this catalyst. In Figure 6, it shows the variation of conversion and hydrocarbon yield with time on-stream (TOS). It also shows that the maximum hydrocarbon yield of 38 mol% occurred around 180 min on stream. After reaching this maximum, the liquid hydrocarbon yield declined.


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Likewise, indole conversion decreased rapidly after 300 min on-stream. It is clear that the Pd/γ-Al2O3 catalyst deactivates rapidly after 300 min on-stream. The cause of the deactivation is unknown and beyond the experiment results of this work. Because the CO was observed in the gas products, we firstly suggested a possible cause that CO poisoned the catalyst. Literature also indicates that CO poisoned under hydrothermal condition [20], causing the loss of catalyst activity. However, added H2 was favourably contributed to the removal of adsorbed CO due to go through as HCO or COH pathway to form methane [21,22]. Then, due to H2 also contained in our reaction, it illustrated that CO was not the cause for catalytic activity loss in hydrothermal condition. Then, we should find the other cause for the loss activity of Pd/γ-Al2O3 under hydrothermal condition in the follow parts.

Figure 6 Conversion and hydrocarbon yield from HDN of indole over 50 wt% Pd/γ-Al2O3 (5wt% Pd). W/F=50 min, Tinlet=450 °C, Toutlet=486 °C, pressure=300 Bar, feed solution (atm):0.25 molL-1, balance DI H2O, H2 = 0.008 molmin-1, feed flow rate (ambient temperature) = 0.200 mLmin-1. (■Indole conversion, ▲Hydrocarbon yield)

3.10 Catalyst characterization The reaction results presented in section 3.9 indicate that Pd/Al2O3 catalyst lost activity after used in hydrothermal condition. This section


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elucidates the factor of the losing activity by discussing the results from surface, bulk structure and microscopy experiments.

Figure 7 X-ray diffraction patterns of (a) γ-AlOOH, (b) used Pd/γ-Al2O3, (c) fresh Pd/γ-Al2O3, and (d) α-Al2O3.

The XRD patterns of fresh and used Pd/γ-Al2O3 are shown in Figure 7. Typical diffraction peaks corresponding to γ-Al2O3 support are observed in all Pd/γ-Al2O3 samples. Besides, diffraction peak at 33.8° corresponding to PdO can be detected in used Pd/γ-Al2O3 (curve b in Figure 7). While for fresh Pd/γ-Al2O3, diffraction peaks at 40.1° and 46.5° corresponding to Pd0 are observed in addition to the diffraction peaks corresponding to γ-Al2O3 support (curve c in Figure 7). This result indicated that metallic Pd converted into PdO after HDN reaction, which means active center in catalyst declined. It was noted that Pd/γ-Al2O3 has typical diffraction peaks of γ-AlOOH and α-Al2O3 at 10°, 60° and 65°, respectively. Fujii group considered that the hydrolysis of γ-Al2O3 appeared to rapidly convert to γ-AlOOH [20], which is in consistent with our results. Though α-Al2O3 crystalline forms need more than 1200 °C from γ-Al2O3, γ-Al2O3 can be converted to α-Al2O3 in 10 hours or less time when it was in high pressure and supported with metals under hydrothermal condition [23,24]. The results of XRD patterns indicated that the oxygenation of active


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center Pd and crystalline transformation of γ-Al2O3 lead the loss activity of catalyst. Table 4 Physical properties of fresh and used Pd/γ-Al2O3 catalysts. Entry

BET surface area (m2g-1)

Pore volume (cm3g-1)

Average pore radius (nm)

156 83

0.29 0.11

18 6

98

0.15

10

Fresh Pd/γ-Al2O3 Used Pd/γ-Al2O3 Calcinated used Pd/γ-Al2O3

The BET surface area, pore volume, and average pore diameter of fresh, used, and calcinated after used Pd/γ-Al2O3 are given in Table 4. The total pore volume of used Pd/γ-Al2O3 was much less than that of fresh catalyst. Compared fresh and used catalysts, it indicated that the surface area, pore volume, average pore decrease from 156 to 83 m2g-1, 0.29 to 0.11 cm3g-1, and 18 to 6 nm, respectively. The used Pd/γ-Al2O3 catalysts were also calcinated at 450 °C for 4h for investigation. It was found the surface area of catalyst increased only from 83 to 98 m2g-1. Though γ-AlOOH could completely transfer to γ-Al2O3 around 400 °C [24], the surface area of used Pd/γ-Al2O3 were not recovery after calcinated, which maybe cause by the sintering of Pd particles.

Figure 8 TG/DTA curve for used Pd/γ-Al2O3, under N2 flow at 20 °Cmin-1.

TG/DTA is a favorable method to discuss the coke or adsorption over catalysts. It illustrates TG and DTA curve of the used Pd/γ-Al2O3 in Figure 8, which


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shows that there were at least three steps of weight loss from 50 to 900°C. In the first step (below 200°C), a drastic loss rate was acquired in the DTA curve around 100 °C, which was obviously belonging to the departure of water. Meanwhile, it may also be belong to the pyrolysis of the surface absorption or residual organics. In the next step (300- 500 °C), it was observed that a weight loss of almost 15% was occurred, which was probably due to removal of water during the formation of the oxide phase on the surface [23]. The peak between 300 and 400 °C at DTA curve also probably corresponds to coke formed at the surface of Pd. The endothermic peak between 400 and 500 °C was resulted from the calcinations of γ-AlOOH to crystallite γ-Al2O3 and also due to removal water during the phase transformation of γ-Al2O3 according to the dehydration equation (5). In the third step, there was a small weight loss appeared from 700 to 900 °C. It was resulted by the transformation of γ-Al2O3 into δ- or θ-Al2O3 [25]. Moreover, it also was attributed to portion pyrolysis of the organics copolymer due to the copolymer pyrolyzed at low temperature was not completely decomposed until above 700 °C was reached. 2 γ-AlOOH→γ-Al2O3+H2O (5)


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Figure 9 TEM image of Pd/γ-Al2O3 catalyst: fresh (a, b) and used (c,d).

Figure 9 shows TEM of fresh and used 5wt% Pd/γ-Al2O3 catalysts. Figure 9a and b show that good dispersion of the palladium particles in the porous alumina supports. The palladium clusters with average diameters ranging from 5 to 8 nm can be detected in Figure 9b, which suggested that palladium tends to form nano-sized particles on γ-Al2O3 support. After used in the reaction of indole HDN, the sizes of supported Pd clusters become bigger to around 15nm. This finding indicated that it appeared the sintering of Pd particles under hydrothermal condition, which was another cause of catalyst deactivation.

Figure 10 The effect of steam time on content of Al3+ and Pd2+ in the liquid phase.

In Figure 10, it shows that the liquid phase from flow reactor was tested in inductively coupled plasma atomic emission spectroscopy (ICP-AES). The content of Al3+ was the highest at 120 min then decreased, which means that catalyst need time to subject the hydrothermal condition. Though the content of Pd2+ in the liquid phase decreased with increasing time, the ion of Pd2+ appeared always in the liquid phase for the design steam time. ICP results


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indicated evidence for Pd loss of either catalyst during the reaction. The loss of active center Pd also has a negative effect on the activity of catalyst. Conclusions In this article, it was the first time to report on hydrothermal hydrodenitrogenation of indole over catalysts. Though all of the catalysts show active more or less, 5 wt% Pd/γ-Al2O3 performs more activity than any other catalysts tested in the hydrothermal HDN of indole. Performing the HDN reaction at 450 °C, 0.015 gcm-3 water density, 5 MPa H2 and 50 wt% Pd/γ-Al2O3 loading led to complete conversion of indole at 90 min and reach a maximum yield (51 mol%) of hydrocarbons at 120 min, respectively. It proposed the mechanism of forming hydrocarbons from hydrogenational denitrogation of indole, which needs two different pathways, as (1) indole directly hydrodenigrated into hydrocarbons, and (2) intermediate oxygenated products

from

hydrolysis

of

partial

hydrogenation

of

indole

were

hydrodeoxygenated to removal O. The reusability of Pd/γ-Al2O3 was also test, which was deactivated rapidly after 4 time cycles. It has explored several causes after examining and charactering the catalysts, which are oxygenation of Pd into PdO, crystal transformation appeared in γ-Al2O3 support, coke formed in the catalyst, sintering of Pd particles, and loss active center Pd into liquid phase. Acknowledgements This work was finally supported by the key project of China Postdoctoral Science Foundation (No. 2016M601601), Shanghai Science and Technology Committee (No. 14231200300) and Shanghai Key Laboratory of Green Chemistry and Chemical Processes. Reference [1] Katritzky, A. R.; Luxem, F. J.; Murugan, R.; Greenhill, J. V.; Siskin, M.

Energ. Fuel. 1992, 6, 450-455.


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[2] Neveux, N.; Yuen, A.; Jazrawi, C.; Magnusson, M.; Haynes, B. S.; Masters, A. F.; Montoya, A.; Paul, N. A.; Maschmeyer, T.; Nys, R. Bioresource

technol. 2014, 155, 334-341. [3] Leow, S.; Witter, J. R.; Vardon, D. R.; Sharma, B. K.; Guest, J. S.; Strathmann, T. J. Green Chem. 2015, 17(6), 3584-3599. [4] Brown, T.; Duan, P.; Savage, P. E. Energ. Fuel. 2010, 24, 3639-3646. [5] Bai, X.; Duan, P.; Xu, Y.; Zhang, A.; Savage, P. E. Energ. Fuel. 2014, 120, 141-149. [6] Duan, P.; Savage, P. E. Energ. Environ. Sci. 2011, 4, 1447-1456. [7] a) Duan, P.; Savage, P. E. App. Catal. B: Environ. 2011, 108–109, 54-60; b) Changi, S.; Zhu, M.; Savage, P. E. ChemSusChem 2012, 5(9), 1743-1757. [8] Yeh, T. M.; Hockstad, R. L.; Linic, S.; Savage, P. E. Fuel 2015, 156, 219-224. [9] a) Luo, L.; Dai, L.; Savage, P. E. Energ. Fuel. 2015, 29(5), 3208-3214; b) Luo, L.; Sheehan, J. D.; Dai, L.; Savage, P. E. ACS Sustan. Chem. Eng. 2016, 4(5), 2725-2733. [10] Miga, K.; Stanczyk, K.; Sayag, C.; Brodzki, D.; Mariadassou, G. D. J. Catal. 1999, 183, 63-68. [11] Aguilera, G. R.; Gupta, V. G.; Yang, S.; Kuznicki, S. M.; McCaffrey, W. C.

Energ. Fuel. 2014, 28(10), 6570-6578. [12] a) Katritzky, A. R.; Barcock, R. A.; Siskin, M.; Olmstead, W. N. Energ. Fuel. 1994, 34, 990-1001; b) Katritzky, A. R.; Lapucha, A. R.; Siskin, M. Energ.

Fuel. 1992, 6(4), 439-450; c) Katritzky, A. R.; Shipkova, P. A.; Allin, S. M.; Barcock, R. A.; Siskin, M.; Olmstead, W. N. Energ. Fuel. 1995, 9(4), 580-589. [13] Guo, Y.; Wang, S.; Yeh, T.; Savage, P. E. App. Catal. B: Environ. 2015, 166, 202-210. [14] Wang, C. ; Han, L.; Zhang, Q.; Li, Y.; Zhao, G.; Liu, Y.; Lu, Y. Green Chem. 2015, 17(7), 3762-3765.


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[15] Teri, G.; Luo, L.; Savage, P. E. Energ. Fuel. 2014, 28(12), 7501-7509. [16] Guo, Y.; Wang, S.; Huelsman, C. M.; Savage, P. E. Chem. Eng. J. 2014,

241, 327-335. [17] Sudasinghe, N.; Dungan, B.; Lammers, P.; Albrecht, K.; Elliott, D.; Hallen, R.; Schaub, T. Fuel 2014, 119, 47-56. [18] Qi, X.; Zhuang, Y.; Yuan, Y.; Gu, W. J. hazard. Mater. 2002, 90(1), 51-62. [19] Dickinson, J. G.; Savage, P. E. ACS Catal. 2014, 4(8), 2605-2615. [20] Bunch, A.; Zhang, L.; Karakas, G.; Ozkan, U. S. Appl. Catal. A: Gen. 2000,

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J. Catal. 2008, 255(1), 6-19. [23] Ito, S.; Umehara, N.; Takata, H.; Fujii, T. Solid State Ionics 2004,172, 403-406. [24] Ji, G.; Li, M.; Li, G.; Gao, G.; Zou, H.; Gan, S. Powder Technol. 2012,

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High yield of Hydrocarbons from Catalytic Hydrodenitrogenation of

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