Behavior of Cholesterol and Catalysts in Supercritical Water - Energy

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Behavior of cholesterol and catalysts in supercritical water Jennifer N. Jocz, and Phillip E. Savage Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00924 • Publication Date (Web): 30 Jun 2016 Downloaded from http://pubs.acs.org on July 12, 2016

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Behavior of cholesterol and catalysts in supercritical water Jennifer N. Jocz† and Phillip E. Savage⇤,†,‡ †Department of Chemical Engineering, University of Michigan, Ann Arbor ‡Department of Chemical Engineering, The Pennsylvania State University, University Park E-mail: [email protected] Phone: +1 (814) 867-5876. Fax: +1 (814) 865-7846 Abstract Cholesterol reacted over 5 wt% Pt/C, 5 wt% Pd/C, and HZSM-5 in supercritical water at 400o C. The major products with the Pt/C and Pd/C catalysts are cholesterolderived steroids, polynuclear aromatic compounds, aliphatic hydrocarbons, and gases (H2 , CH4 , C2 H6 ) resulting from hydrogenation, dehydrogenation, and cracking reactions. HZSM-5 favored initial dehydration of cholesterol to form cholesta-3,5-diene and then catalyzed further isomerization and cracking reactions. The presence of H2 had little or no effect except when catalyst was absent, in which case the added H2 led to higher cholestadiene yields and lower cholesterol conversion. Increasing the catalyst loading resulted in higher yields of lower molecular weight products but also significantly lower carbon recoveries. Characterization of the Pt/C and Pd/C catalysts showed carbon support gasification and particle growth after exposure to supercritical water.

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Introduction Algae biocrude is an energy-dense material produced from the hydrothermal liquefaction (HTL) of microalgae. Water at HTL conditions (T

200o C, P

1.6 MPa) has a significantly

higher solubility for organic compounds and a higher ion product than water at ambient temperature. Combined, these properties facilitate acid- and base-catalyzed reactions that break down lipids, carbohydrates, proteins, and other algal biomolecules into biocrude containing fatty acids, hydrocarbons, cholesterol and other steroids, indole derivatives, and phenols. 1,2 The biocrude is viscous and contains 10-20 wt% N, O, and S heteroatoms. These heteroatoms must be removed to prevent the formation of NOx and SOx upon combustion, to reduce the total acid number, and to increase the energy density. To improve the properties of biocrude while preserving the oil’s energy content, a catalytic treatment of the biocrude in the hydrothermal environment has been proposed. Different catalysts in supercritical water (SCW) have been shown to reduce heteroatom content in algae biocrude and lower viscosity and total acid number. 3 Above its critical point (374o C, 22.1 MPa) water has fewer hydrogen bonds, a low ion product, and a significantly lower dielectric constant. These properties enable organic compounds and permanent gases such as H2 to be completely miscible in a single fluid phase, thus eliminating interphase transport limitations. 4 The complexity of biocrude makes it difficult to discern the reaction pathways and networks when working with algae biocrude directly. Consequently, studies with model compounds have been undertaken to probe reaction pathways, kinetics, and catalyst stability. Model compounds studied include fatty acids, benzofuran, phenols, pyridine, quinoline, and dibenzothiophene. 3 Cholesterol, along with other steroids derived from cholesterol, accounts for approximately 5% of the identified products in crude algae bio-oil 1 but it is totally absent in catalytically upgraded bio-oils. Specifically, the studies that report an absence or reduction of cholesterol and cholesterol derivatives in the upgraded bio-oil used Pt/C (with and without added H2 ) in SCW at 400o C for 4 hours, 5 Pd/C and H2 in SCW at 400o C for 1-8 2


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hours, 6 and Pt/C, Mo2 C, and HZSM-5 in SCW at 430-530o C for 2-6 hours. 7 Determining the behavior of cholesterol and other biocrude molecules in the upgrading reaction could facilitate catalyst and process design for biocrude upgrading. Meredith et al examined the hydropyrolysis of cholesterol on a sulfided Mo catalyst 8 and a Pt catalyst 9 to selectively defunctionalize the biological compound as a preparative technique for carbon isotope analysis. The catalysts hydrogenated the majority of the cholesterol to cholestanes, with some cholestene and diasterene side products. Rushdi et al 10 reacted cholesterol in high temperature water (150-300o C) for 24 hours and found that the presence of natural sediment and montmorillonite slightly increased aromatization and decreased the presence of cholestanes. Hydrothermal (300-350o C) kinetics and reaction pathways for cholesterol with no catalyst were studied by Hietala and Savage 11 who found that cholesterol in subcritical water undergoes dehydration with first order kinetics to form cholesta-3,5-diene and then undergoes further isomerization and backbone rearrangement to form cholestenes, cholestadienes, cholestatrienes, and C26 isomers. Cholesterol with catalyst in SCW may react similarly to cyclohexanol due to the similarities in their molecular structures. Crittendon and Parsons 12 reacted cyclohexanol and cyclohexane derivatives in 375o C SCW with PtO2 and 10% Pt/C catalysts and with added acid and base to determine the influence of the catalysts on the functional group transformation. The Pt catalysts were necessary for the dehydrogenation reactions of cyclohexanol to cyclohexanone and the aromatization of cyclohexanone, cyclohexene, and cyclohexane. The presence of acid or base catalyzed the dehydration of cyclohexanol. There has been no study to date that has examined the heterogeneous catalytic treatment of cholesterol in SCW. The abundance of cholesterol in algal biocrude and the absence of information about its reactivity in SCW and with biocrude upgrading catalysts motivated this work. Pt/C, Pd/C, and HZSM-5 were chosen as the catalysts, as these materials were used in previous bio-oil upgrading studies.

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Materials and methods The reactions were conducted in batch with temperature, SCW density, and time held constant at 400o C, 0.17g/mL, and 60 minutes, respectively, while the catalyst material, catalyst loading, and gas loading were varied. Subsequent experiments with 5 wt% Pd/C examined additional reaction times of 10 and 20 minutes. The 316 stainless steel batch reactors were constructed from Swagelok tube fittings (3/8 in. port connector, cap, and 3/8 in. to 1/8 in. reducing union) and connected with 8 in. of Swagelok tubing (1/8 in. o.d.) to a two-way angle high pressure gas valve rated to 15,000 psi (High Pressure Equipment Company). The gas valve attachment allowed for the exchange of gases in the reactor headspace and the entire assembly had a total internal reactor volume of 2.32 mL. Prior to use, the reactors were loaded with 0.4g deionized water and heated to 400o C for 60 minutes to expose the reactor walls to the hydrothermal environment and allow the SCW to remove any residual material on the reactor walls. For catalyzed reactions, 40 mg cholesterol ( 99%, Sigma-Aldrich), 0.4 g deionized water (prepared in house), and either 10 mg (low loading) or 40 mg (high loading) of 5 wt% Pt on activated carbon (Sigma-Aldrich), 5 wt% Pd on activated carbon (Sigma-Aldrich), or HZSM-5 zeolite were loaded into the reactor. The Pt/C and Pd/C catalysts were used as received without any pretreatment. ZSM-5 (SiO2 /Al2 O3 =30, Zeolyst International) was obtained in ammonium form and calcined in air at 550o C for 240 min to increase the acidity and form HZSM-5 before use. 13 Once the reactors were loaded, we coupled the gas valve attachments and tightened the connection with a torque wrench to seal the reactors. The valves were connected to a gas manifold containing He and H2 cylinders (ultra high purity grade, Cryogenic Gases) and a vacuum pump. A schematic of the batch reactors and gas manifold was published previously by Duan and Savage. 5 The reactor valves were opened and the air in the reactor was removed with the vacuum pump. The headspace was repeatedly flushed with He to ensure complete air removal. We performed control experiments to determine the recovery of water, cholesterol, and catalyst after pulling 4


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vacuum on the reactors and exchanging the gas in the headspace. The cholesterol and catalysts, which are solid and insoluble in water at room temperature, were completely recovered and evaporative losses of the water were less than 3wt%. The reactors were then pressurized to 10 bar with He, which was used as an internal standard for quantifying gas phase reaction products. For reactions with added H2 , the reactors were pressurized to 3 bar with He and then further pressurized to 10 bar with H2 , resulting in a molar ratio of approx. 5:1 hydrogen to cholesterol. After pressurization, the reactor valves were closed to seal the contents and then the reactors were disconnected from the gas manifold. The loaded reactors were immersed in a pre-heated, fluidized sand bath (Techne IFB-51 with a Eurotherm 3216 PID controller) at 400o C for 60 minutes, then quenched in cold water and allowed to equilibrate at room temperature for at least 60 minutes. An Agilent 6890N gas chromatograph with a Carboxen 1000 packed column and a thermal conductivity detector (GC-TCD) was used to separate and analyze gaseous products using a procedure outlined previously. 1 The resulting peaks were identified and molar fractions determined by using external gas calibration standards purchased from Grace Davison that contained H2 , He, CO, CO2 , CH4 , C2 H2 , C2 H4 , and C2 H6 . We used the ideal gas law to calculate the amount of He loaded into the reactor, which, along with the GC analysis, allowed us to determine the molar quantities of the other gases in the mixture. The reactors were then opened and the contents collected by flushing the reactors with 8mL of a 1:1 (v/v) mixture of dichloromethane (Fisher Scientific, (Fisher Scientific,

99.9%) and methanol

99.9%), which served to homogenize the aqueous and organic phases.

The solution was centrifuged to separate out the solid catalyst, which was recovered for analysis. Two Agilent 6890N gas chromatographs equipped with HP-5 capillary columns separated the reaction products in the solution. A mass spectroscometric detector (GCMS) on one of the gas chromatographs was used to identify the compounds while a flame ionization detector (GC-FID) on the other GC was used to quantify the compounds. External standard solutions of cholesterol, methyl-naphthalene ( 99%, Sigma Aldrich), 5


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and n-heptane ( 99%, Sigma Aldrich) were used to generate GC-FID calibration curves for cholesterol derivatives, aromatics, and hydrocarbons, respectively. Product molar yields were calculated as moles of product formed divided by moles of cholesterol loaded into the reactor. The accuracy of this product recovery method was tested by loading cholesterol, water, and He into the reactor and then releasing the He and recovering the cholesterol in solution for analysis with the GC-FID. 93.4% of the original cholesterol was measured. The recovered catalyst was repeatedly rinsed with the dichloromethane/methanol mixture and then with acetone to remove any reaction products not chemically adsorbed on the surface. It was then dried in a vacuum oven at 70o C overnight before it was re-weighed and characterized with X-ray diffraction (XRD) and transmission electron microscopy (TEM). Control experiments with only cholesterol in water (with and without H2 ) were done for comparison with the catalyzed reactions. We also conducted control experiments with each catalyst in water (with and without added H2 ) without cholesterol to determine the effect of the SCW environment on the catalyst and to assess the influence of the catalyst on the amount of H2 present at reaction conditions. The catalyst control experiments with H2 added to the reactors showed that in the absence of cholesterol, the catalysts do not change the amount of H2 recovered from the system. The catalyst control experiments without added H2 prompted additional experiments with an activated carbon catalyst support (Vulcan XC72R, Cabot Corporation) and a Pd/C catalyst synthesized via incipient wetness impregnation of a Pd(NO3 )2 • xH2 O (Sigma Aldrich) solution on the activated carbon support. All reactions were performed at least in triplicate to determine experimental variability, which is reported herein as standard error.

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Results and discussion Table 1 gives the cholesterol conversions for each of the reaction conditions. The reactions of cholesterol in SCW at 400o C without added catalyst resulted in conversions of 91.6±8.4% without any added H2 and 70.6±9.0% with added H2 after 60 minutes. In comparison,

the hydrothermal treatment of cholesterol at 350o C for 10 minutes achieved 97.9% con-

version. 11 The higher conversion of cholesterol at the lower subcritical temperature and a shorter reaction time may be due to the higher ion product of water at those conditions. The higher ion product would facilitate acid-catalyzed reactions such as the dehydration reaction observed at subcritical conditions. The lower conversion with the addition of H2 is likely caused by a H2 -induced shift in the reaction equilibrium between cholesterol and cholest-4-en-3-one. This reaction is explored further in the subsequent section. Complete cholesterol conversion was realized for all the Pt/C catalyzed reactions and near complete conversion was achieved for the Pd/C and HZSM-5 reactions. This result is consistent with the disappearance of cholesterol during algal biocrude catalytic upgrading. 5–7 Table 1: Cholesterol conversion after reaction in SCW at 400o C for 60 minutes The initial cholesterol concentration was 0.26 mmol/L and rSCW was 0.17g/mL. Catalyst None Pt/C Pd/C HZSM-5

Catalyst Loading Ratio (mg catalyst:mg cholesterol) 1:4 1:1 1:4 1:1 1:4 1:1

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Cholesterol Conversion without H2 (%) 91.6±8.4 100 100 99.3±0.1 99.8±0.1 98.1±0.5 98.1±1.3


Cholesterol Conversion with H2 (%) 70.6±9.0 100 100 99.0±0.7 99.6±0.2 98.5±0.5 99.2±0.6

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Reaction products The GC-MS and GC-FID analysis of the reaction products resolved up to 150 compounds. Figure 1 compares the product spectrum from each catalyst. Due to the high number of reaction products, we considered in detail only those most abundant. The labeled peaks in Figure 1 correspond to the compounds that were identified by GC-MS. Table 2 lists the corresponding compound names and structures. The identified compounds were classified as either aliphatic hydrocarbons, aromatic compounds, or cholesterol-derived steroids. 1200 21

17

1000

15 16 16 1314

No Catalyst 1

3

20 19 19

800 Relative Signal Height (pA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1 3 600

Pt/C 2

7 6

10 8

11

9

14

12

19

16

18

19 21

20

1 400

16 14

Pd/C 11

15

12

200

1

HZSM-5

20

40

17

17

12

4 5

19

60

20

21 19

20

80

100

Retention Time (min)

Figure 1: GC-FID chromatograms of the products from reaction of cholesterol in SCW at 400o C for 60 minutes. The initial cholesterol concentration was 0.26 mmol/L, the catalyst loading ratio was 1:4 (mg catalyst:mg cholesterol), and rSCW was 0.17g/mL. Peak numbers correspond to the compounds in Table 2. The chromatogram from the HZSM-5 catalyzed reaction is unique in containing a large signal region between 50 and 70 minutes that corresponds to a poorly separated cluster of reaction products. While these products could not be definitively identified using the Wiley MS library match system, visual inspection of the mass spectra throughout this 8


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Table 2: Products from the catalytic treatment of cholesterol in SCW at 400o C and 0.17 g/mL for 60 minutes. Compound Class Aliphatic Hydrocarbons

Aromatic Compounds

Peak ID

Compound Name

1 2 3

2-methylheptane 3,6-dimethyloctane 4-methyldecane

4 5

phenanthrene

7, 8

1 & 2-methylphenanthrene

9

2,3-dimethylphenanthrene

11

Steroids

4a-methyl-1,2,3,4,4a,5,6,7octahydronaphthalene 5-methyl-1,2,3,4tetrahydronaphthalene

6

10

Chemical Structure

2(phenylmethyl)naphthalene 1,4-dimethyl-2 & 5-phenylnaphthalene

12

chrysene

18

13-H-dibenzo(a,h)fluorene

13

cholestadiene isomers

14

coprostane

15

cholest-4-ene

16

cholestane isomers

17

cholesta-3,5-diene

19

cholestan-3-one isomers

20

cholesterol

21

cholest-4-en-3-one

9


-

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region revealed similarities to cholest-4-ene, cholesta-3,5-diene, cholesta-7,14-diene and cholesta-5,24-dien-3b-ol acetate, suggesting that these poorly separated compounds are many different conformations of cholestenes, cholestadienes and possibly cholestatrienes. Some of these compounds may also contain alcohol or ketone functional groups. The presence of catalyst increased the total number of reaction products relative to the reaction without catalyst. The presence of catalyst also shifted the product spectrum to lighter compounds that eluted at shorter retention times, implying more extensive degradation of cholesterol occurred as a result of the catalyst. Of the compounds listed in Table 2, only cholestane was reported in the upgraded bio-oil. 5 That upgraded bio-oil had been treated for an additional 3 hours, however, during which time additional reactions may have altered the product distribution. Cholest-4-en-3-one, cholestan-3-one, isomers of cholestane, cholestene and cholestadiene, 2-methyl heptane, and 2-methyl phenanthrene were identified as the most abundant liquid phase cholesterol products across all the conditions studied and hydrogen, methane, and ethane were the most abundant gaseous products. Table 3 contains the molar yields of these compounds for each of the conditions studied. Overall, the yields of the liquid phase reaction products indicate that the addition of H2 to the reactors had a minor impact on the catalyzed reactions while the catalyst type and loading were the most significant factors in determining the reaction product distribution. The addition of H2 to the reaction system had little influence on the molar yields of the liquid phase products except for the uncatalyzed reaction. In this reaction, cholestadienes and cholest-4-en-3-one were present in molar yields of 12.5±1.1% and 22.1±1.4% without added H2 and 20.9±8.8% and 13.3±3.3% with added H2 , respectively. The relatively high molar yields of these compounds show that cholesterol dehydration and oxidation simultaneously occur in SCW without any heterogeneous catalyst. Figure 2 illustrates these reactions. Cholestadienes were the major products from the hydrothermal reaction of cholesterol at 300-350o C, and very little cholest-4-en-3-one was formed. 11 In SCW at 400o C 10


1:4

Pt/C


11

1:1

1:4

1:1

1:4

Gas Loading

He H2 He H2 He H2 He H2 He H2 He H2 He H2

Cholest-4-en-3-one

22.1±7.9 13.3±1.9 2.6±0.2 0.04±0.04 0.5±0.3 0.2±0.1 3.3±0.3 2.4±0.2 0.4±0.1 0.6±0.3 trace trace trace trace

Cholestan-3-one 1.0±0.6 0.4±0.03 10.2±1.4 4.8±1.5 1.1±0.7 1.5±0.8 5.7±0.6 13.4±2.0 1.2±0.4 4.4±0.7 0 0 0 0

Cholestadienes 12.5±0.6 20.9±5.1 0.12±0.02 0.2±0.03 trace trace 1.0±0.4 0.2±0.0 0.2±0.02 0.4±0.1 3.2±0.3 7.0±1.4 2.9±1.0 2.8±1.0

Cholestenes 0.3±0.1 0.7±0.2 0 0 0 0 4.2±0.6 2.9±0.3 1.9±0.2 2.0±0.4 0.2±0.02 0.4±0.1 0.2±0.02 trace

Cholestanes 2.4±0.5 2.7±0.6 4.4±0.7 4.3±1.3 0.8±0.5 1.2±0.6 16.4±1.6 16.2±0.6 6.4±0.4 8.0±0.5 0.4±0.1 0.6±0.1 0.5±0.1 0.2±0.04

2-Methyl Phenanthrene trace trace 2.2±0.1 1.5±0.2 2.3±0.1 2.5±0.2 0.1±0.01 0.1±0.01 0.2±0.01 0.1±0.01 trace trace trace trace

2-Methyl Heptane 0.8±0.2 0 7.9±0.9 10.3±0.8 10.6±0.4 10.5±0.6 7.4±0.5 6.2±0.4 17.5±1.1 12.2±2.0 2.4±0.1 2.9±0.7 4.1±0.7 3.3±0.2

Hydrogen 3.2±0.1 -71±18 168±42 125±8 260±36 32.8±71.1 90.7±2.6 -4.4±16.9 140±2 56.8±8.1 23.6±4.3 -134±58 48.1±15.6 -87.8±2.8

Carbon Dioxide 0 0.9±0.7 14.7±5.1 3.8±0.3 29.7±0.7 12.3±1.6 2.9±0.4 1.2±0.1 9.5±0.5 3.6±0.3 0 2.8±0.4 3.1±0.9 7.3±5.2

5.1±0.5 6.6±2.4 88.4±11.9 36.5±0.8 200±4 105±18 28.1±1.0 9.8±0.9 47.2±0.8 16.6±1.6 11.2±1.9 6.3±1.9 14.4±2.9 4.4±0.2

Ethane 0.4±0.5 0 9.9±0.9 4.3±0.4 26.7±1.2 14.9±2.6 3.7±0.4 1.1±0.1 3.7±0.2 0.9±0.1 2.3±0.8 2.2±2.7 3.1±0.6 1.2±0.1

91.2±8.3 93.1±11.9 52.0±6.5 57.8±5.2 31.4±4.6 32.8±2.2 65.5±1.2 69.4±2.1 43.1±2.9 43.2±3.5 62.2±1.7 74.2±1.2 56.5±6.5 39.0±4.9

Liquid Phase

0.2±0.1 0.3±0.1 4.2±1.0 1.7±0.1 9.4±0.4 5.0±1.5 1.3±0.1 0.4±0.1 2.0±0.1 0.7±0.1 0.6±0.2 0.6±0.4 0.8±0.2 0.4±0.1

91.5±8.3 93.4±11.9 56.3±6.6 59.5±5.2 40.8±4.7 37.8±2.6 66.8±1.2 69.9±2.1 45.1±2.9 43.9±3.5 62.8±1.8 74.8±2.8 57.3±6.5 39.4±4.9

Table 3: Molar yields (%) of the most abundant products and carbon recovery (%) from the catalytic treatment of cholesterol in SCW at 400o C for 60 minutes. The rSCW was 0.17g/mL and the initial cholesterol concentration was 0.26 mmol/L. Uncertainty is reported as standard error. Catalyst loading ratio is expressed as mg catalyst:mg cholesterol. Hydrogen yield is based on the net hydrogen production (formation-initial loading).

HZSM-5

Pd/C

-

Catalyst

1:1

Catalyst Loading Ratio

None

Methane

% Carbon Recovery

Gas Phase

% Molar Yield of Gas Products

Total

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 % Molar Yield of Liquid Products

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(without added H2 ), however, the molar yield of cholest-4-en-3-one is almost twice that of the cholestadienes. This result suggests that the oxidation reaction to form cholest-4-en-3one becomes more competitive with the dehydration reaction when the system is at higher temperatures and supercritical conditions where the ion product, density, and dielectric constant are lower than in sub-critical water. The addition of H2 to the reaction without any heterogeneous catalyst increased the yield of cholestadienes and decreased the yield of cholest-4-en-3-one. Since the oxidation of cholesterol to form cholest-4-en-3-one involves the removal of two hydrogen atoms, the added H2 in the system shifted the reaction equilibrium backwards to favor higher concentrations of cholesterol and lower yields of cholest-4-en-3-one. This H2 -driven reverse reaction is consistent with the lower cholesterol conversion for the non-catalyzed, SCW reaction with H2 compared to the conversion without H2 (Table 1). The higher concentration of cholesterol in the reactor as a result of the reduced oxidation rate allows more cholesterol to undergo dehydration and produce the higher yield of cholestadienes when H2 is added to the reactor.

Figure 2: Reaction pathways for cholesterol in SCW and with Pt/C, Pd/C, and HZSM-5 catalysts at 400o C. The catalyst type and loading strongly influenced the molar yields of the quantified liquid phase reaction products. The analysis of the product distributions served to elucidate a likely reaction pathway for cholesterol with each of the three catalysts. Figure 2 illustrates 12


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this cholesterol reaction network. The reaction pathways were constructed by determining the likely primary and secondary reaction products from cholesterol based on the molecular structures and the values of the product yields at each catalyst loading. The increase in the catalyst mass is analogous to a decrease in weight hourly space velocity. With the addition of catalyst, yields of both cholestadiene and cholest-4-en-3-one significantly decrease, implying that these species are primary products from uncatalyzed hydrothermal reactions but undergo further reactions on the catalysts to form secondary products. With the HZSM-5 catalyst, the cholestadiene yield is slightly higher than with the Pt/C and Pd/C catalysts, (this value is even higher with added H2 ), but the cholest-4en-3-one yield is essentially zero. This outcome may be because the Bronsted acid sites of the HZSM-5 accelerate the rate of the dehydration reaction, much like the acid-catalyzed dehydration of cyclohexanol, 12 resulting in faster cholestadiene formation at the expense of cholest-4-en-3-one. After formation, cholest-3,5-diene further reacts over the HZSM-5 via rearrangements, double bond migration, migration of methyl groups, and demethylation and dehydrogenation to produce the large cluster of inseparable cholestadiene derivatives observed on the GC-FID chromatograms (Figure 1). The Pt/C- and Pd/C-catalyzed reactions both produced cholestan-3-one and cholestane, but very little of these products formed hydrothermally without catalyst. Pt/C and Pd/C are active hydrogenation/dehydrogenation catalysts, so cholestan-3-one is likely produced via the hydrogenation of the carbon-carbon double bond in cholest-4-en-3-one. Additionally, the Pt/C and Pd/C catalysts further hydrogenate cholestan-3-one to form a cholestanol intermediate which is rapidly hydrogenated to cholestane. An alternative pathway for the formation of cholestane is by the Pt/C or Pd/C facilitated hydrogenation of cholestadiene to cholestene and then cholestane. The latter reaction for cholestane formation is consistent with the very low yields of cholestadienes with the Pt/C and Pd/C catalysts. Figure 2 shows both of these Pt/C- and Pd/C-catalyzed reaction pathways. As the catalyst loading is increased, both cholestan-3-one and cholestane yields decrease, 13


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implying that the addition of more active sites results in further reactions. The increasing yield of 2-methyl heptane with increasing catalyst loading for all three catalysts suggests that this product forms in a pathway occurring after cholesterol has already been converted into its many derivative steroid compounds. Cleavage of the hydrocarbon chain on the five member ring of cholesterol and its steroid derivatives would produce 2-methyl heptane. Table 3 shows that the Pd/C and Pt/C catalysts were more active than HZSM-5 for side chain cracking. The zeolite activity was limited to the formation of many cholestene, cholestadiene, and cholestatriene related structures. Figure 2 shows formation of 2-methyl heptane from cholestadiene or cholestane. The presence of condensed aromatic structures in the product spectrum suggests that after removal of the hydrocarbon chain, the remaining ring structure undergoes dehydrogenation and cracking to form the aromatic products listed in Table 2. The higher yields of aromatic compounds and lower yields of cholesterol derivatives for Pt/C relative to Pd/C (Figure 1 and Table 3) imply that the Pt/C catalyst has a much higher activity for cracking and aromatization reactions. One aromatic compound that resulted from the Pt/C facilitated decomposition reactions was 2-methyl phenanthrene, but very little was observed with Pd/C and HZSM-5. The hydrothermal reactions of cholesterol produced H2 , CO2 , CH4 , and C2 H6 as the sole gaseous products. The yields of gaseous products listed in Table 3 are functions of catalyst type and loading and also of H2 loading. The net H2 formed during hydrothermal treatment of cholesterol was calculated by subtracting the moles of H2 added to the reactor from the moles of H2 measured in the product gas after reaction. As we show in a later section, the catalysts alone in SCW produced both H2 and CO2 , so the yields for these species in Table 3 include contributions from the catalysts along with those from cholesterol. The amount of H2 produced by a catalyst alone was almost always less than the standard deviation in the measured yields with cholesterol, so we take the H2 yields in Table 3 as good estimates of the H2 yields from cholesterol. The amounts of CO2 from catalyst 14


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alone, on the other hand, were much larger than the standard deviations and entirely comparable to the yields observed in Table 3. Therefore, we take the data for CO2 in Table 3 to represent CO2 formation from the catalyst in SCW and not CO2 formation from cholesterol. In fact, if one subtracted the CO2 produced from catalyst alone from the CO2 yields in Table 3, most of the CO2 molar yields would be zero and none would exceed 6%. The highest H2 production occurred with the Pt/C catalyst, which is consistent with the dehydrogenation activity and high yields of aromatic products observed with this catalyst. The Pd/C catalyst was slightly less active for H2 formation and the HZSM-5 catalyst produced very little H2 . For the reactions with H2 added to the reactors, the uncatalyzed hydrothermal reactions were net consumers of H2 . This uptake of H2 is consistent with the previously noted shift in the products from cholest-4-en-3-one back to cholesterol. In the presence of Pt/C and added H2 , the net H2 yield first increases at the lower catalyst loading and then decreases to a value statistically indistinguishable from zero at the higher catalyst loading. This behavior is consistent with H2 formation via production of aromatic products and its subsequent consumption at higher catalyst loadings. At low Pd/C catalyst loadings, the H2 produced was about equal to the H2 consumed in the reaction, but at the high catalyst loading formation of H2 is observed. The HZSM-5 catalyst at both loadings consumed the H2 that was added to the reactor, resulting in the negative net H2 production. Methane (CH4 ) and ethane (C2 H6 ) appear in higher yields in the Pt/C and Pd/C catalyzed reactions. The CH4 likely evolved from the removal of methyl substituents from cholesterol and its derivatives. The C2 H6 likely results from cracking of the hydrocarbon poly-ring structure to produce C2 H6 and an aromatic compound, such as 2-methyl phenanthrene. The Pt/C catalyst was most active for production of the hydrocarbon gas and also the most active for cholesterol conversion to 2-methyl phenanthrene and other aromatic compounds, so the formation of these products are likely correlated. The Pd/C was significantly less active for the formation of gaseous hydrocarbons, which is consistent 15


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drothermal reactions with and without H2 but in absence of catalyst achieved essentially full carbon recovery in the liquid phase. The addition of Pt/C at the 1:4 catalyst:cholesterol loading reduced the liquid phase carbon recovery by almost half both with and without H2 . Increasing the Pt/C loading to a 1:1 ratio resulted in an even further reduction in liquid phase carbon recovery. This downward trend in liquid phase carbon recovery with the increasing catalyst loading also occured for the Pd/C and the HZSM-5 catalysts, although the recoveries with these catalysts are slightly higher than with the Pt/C. The amount of carbon recovered in the gas phase increases with increasing catalyst loading as the cholesterol is more quickly degraded into smaller compounds including CH4 and C2 H6 . In general, the presence of H2 in the reactor suppressed the formation of carbon-containing gas products and resulted in higher carbon recoveries in the liquid phase. One possible explanation for the lower carbon recoveries in the presence of catalyst is coking. Pt and Pd strongly adsorb cyclic hydrocarbons and polynuclear aromatic compounds and facilitate subsequent conversion into graphene and coke. 14–18 Since the Pt/C and Pd/C catalysts produced polynuclear aromatics, it is possible that only a fraction of the aromatic species desorbed back into solution and the remaining adsorbed species polymerized to form large carbon networks. Other possible explanations for the missing carbon include the formation of light hydrocarbon species that are masked by the large DCM/methanol solvent peak in the chromatogram and the formation of higher molecular weight products that would not elute from a GC.

Catalyst stability and characterization We characterized the HZSM-5 catalyst using XRD to observe any changes in crystallinity. Figure 6 shows that the peak intensity of the used HZSM-5 is less than the fresh catalyst and that there are no new peaks and thus no significant crystalline or structural changes occurring during reaction. This result is different than the XRD results reported by Mo and Savage, 13 who observed two new peaks after reacting HZSM-5 with palmitic acid in 19


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SCW at 400o C for 180 minutes. The peak identities were unknown but regeneration of the catalyst resulted in removal of the two new peaks, implying that HZSM-5 retained its overall structure. The shorter exposure time of the HZSM-5 to the SCW may be the reason

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that new peaks were not observed in the present work.

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Figure 6: X-ray diffraction spectra for fresh (after calcination) and used (after 60 minutes with cholesterol and helium in SCW at 400o C and 0.17 g/mL) HZSM-5. ��/� (��) ��/� (��)

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Figure 7: X-ray diffraction spectra for fresh (as purchased) and used (after 60 minutes with cholesterol and helium in SCW at 400o C and 0.17 g/mL) Pt/C and Pd/C. XRD of the supported-metal catalysts (Figure 7) revealed that the Pt and Pd particles on the fresh catalyst were too small for effective X-ray scattering. After exposure to the hydrothermal reaction environment, however, the spectra revealed broad but distinguishable Pt and Pd diffraction peaks. Narrowing of the diffraction peaks suggests some particle 20


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sintering. Using the Scherrer equation, we estimated the post-reaction crystal size to be about 5.5 nm and 6.3 nm for Pt and Pd, respectively. TEM confirmed that the Pt particles were initially between 3-5 nm and grew to 5-7 nm. This metal crystalline growth with a Pt/C catalyst was not observed in previous supercritical water reactions at 380o C 19 nor in subcritical water reactions at 350o C, 20 however a loss of metal dispersion was reported for Pt/C and Pd/C catalysts after hydrothermal deoxygenation of palmitic acid at 370o C for 60 minutes. 21 One possible explanation for this particle growth is an increase in particle mobility on the carbon surface due to the higher temperatures employed in the present work relative to the previous studies. It is also possible that the reactant plays a roll in the re-dispersion of the metal. We performed control experiments without cholesterol to assess the amount of gas formation from the catalysts alone in SCW. The results in Figure 8 show the HZSM-5 catalyst produced some H2 , likely from impurities left on the material during synthesis, and negligible amounts of CO2 . The carbon support in the Pt/C and Pd/C catalysts reacted with the SCW to produce H2 and CO2 . No other gases were detected for any of the catalysts. To decouple the hydrothermal stability of the carbon support from any metal-support-water interactions, we also tested pure activated carbon and a Pd/C catalyst synthesized with the same batch of activated carbon. The pure activated carbon in SCW produced some H2 but no CO2 . Previous work has shown that gasification of activated carbon in SCW at 600o C for 2-10 hours produces H2 , CO2 , CO and CH4 . 22 The authors determined that the formation of CH4 only occurs via pyrolysis and is dependent on the initial hydrogen content of the activated carbon. Low hydrogen content of the activated carbon used in the present work may explain why CH4 was not formed. The absence of CO and CO2 is likely due to the lower reaction temperature and the shorter batch time in the present work. With the addition of Pd particles to the activated carbon, however, the production of H2 was more than doubled and there was significant CO2 formation. This result suggests that the supported Pt or Pd metal is responsible for catalyzing 21


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deactivation over 24 hours of continuous butyric acid decarboxylation in subcritical water, but the authors found that the predominant deactivation mechanisms were poisoning, coking, and pore structure collapse of the carbon support. 20 ��/� (��)

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��

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Figure 9: X-ray diffraction spectra for pure activated carbon support and the synthesized Pd/C before and after exposure to SCW at 400o C (rSCW =0.17 g/mL) for 60 minutes.

Conclusions Without catalyst, SCW facilitated dehydration, reduction, and subsequent isomerization of cholesterol. The presence of catalyst resulted in further cholesterol conversion. The highest conversion was observed with Pt/C, followed by Pd/C and then HZSM-5. Both Pt/C and Pd/C formed cholesterol derivatives, hydrocarbon chains, poly-nuclear aromatics, CH4 , C2 H6 , and also H2 through a series of hydrogenation and dehydrogenation reactions. HZSM-5 readily dehydrated cholesterol to form cholesta-3,5-diene before facilitating further structural rearrangement and cracking of the hydrocarbon sidechain. The addition of H2 had little effect on the liquid phase product distribution in the Pt/C and Pd/C catalyzed reactions, however H2 suppressed gas product formation and shifted the product distribution of the hydrothermal and the HZSM-5 reactions to favor more cholestadienes and less cholest-4-en-3-one. While Pt/C and Pd/C decomposed cholesterol into smaller organic molecules that are more desirable for upgraded biocrude, the catalysts experienced metal particle growth 23


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and gasification of the carbon support in the presence of SCW. The instability of the carbon support in SCW and possible coking are potential mechanisms for catalyst deactivation.

Acknowledgement The authors gratefully acknowledge Thomas Yeh for sharing his expertise on gas chromatography analysis and catalysis, David Hietala for collaborations on developing the experimental methods employed and the MS identification of cholesterol derivatives, and Joseph Mims for conducting several of the batch reactions. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE 1256260.

References (1) Brown, T. M.; Duan, P.; Savage, P. E. Hydrothermal liquefaction and gasification of Nannochloropsis Sp. Energy & Fuels 2010, 24, 3639–3646. (2) Biller, P.; Ross, A. Potential yields and properties of oil from the hydrothermal liquefaction of microalgae with different biochemical content. Bioresource Technology 2011, 102, 215–25. (3) Yeh, T. M.; Dickinson, J. G.; Franck, A.; Linic, S.; Thompson, L. T.; Savage, P. E. Hydrothermal catalytic production of fuels and chemicals from aquatic biomass. Journal of Chemical Technology & Biotechnology 2013, 88, 13–24. (4) Savage, P. E. Heterogeneous catalysis in supercritical water. Catalysis Today 2000, 62, 167–173. (5) Duan, P.; Savage, P. E. Upgrading of crude algal bio-oil in supercritical water. Bioresource technology 2011, 102, 1899–906. 24


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(6) Duan, P.; Savage, P. E. Catalytic hydrotreatment of crude algal bio-oil in supercritical water. Applied Catalysis B: Environmental 2011, 104, 136–143. (7) Duan, P.; Savage, P. E. Catalytic treatment of crude algal bio-oil in supercritical water: optimization studies. Energy & Environmental Science 2011, 4, 1447. (8) Meredith, W.; Sun, C.-G.; Snape, C. E.; Sephton, M. a.; Love, G. D. The use of model compounds to investigate the release of covalently bound biomarkers via hydropyrolysis. Organic Geochemistry 2006, 37, 1705–1714. (9) Meredith, W.; Gomes, R. L.; Cooper, M.; Snape, C. E.; Sephton, M. A. Hydropyrolysis over a platinum catalyst as a preparative technique for the compound-specific carbon isotope ratio measurement of C 27 steroids y. Rapid Communications in Mass Spectrometry 2010, 24, 501–505. (10) Rushdi, A. I.; Ritter, G.; Grimalt, J. O.; Simoneit, B. R. Hydrous pyrolysis of cholesterol under various conditions. Organic Geochemistry 2003, 34, 799–812. (11) Hietala, D. C.; Savage, P. E. Reaction pathways and kinetics of cholesterol in hightemperature water. Chemical Engineering Journal 2015, 265, 129–137. (12) Crittendon, R. C.; Parsons, E. J. Transformations of cyclohexane derivatives in supercritical water. Organometallics 1994, 13, 2587–2591. (13) Mo, N.; Savage, P. E. Hydrothermal catalytic cracking of fatty acids with HZSM-5. ACS Sustainable Chemistry & Engineering 2014, 2, 88–94. (14) Myers, C. G.; Lang, W. H.; Welsz, P. B. Aging of platinum reforming catalysts. Industrial & Engineering Chemistry 1961, 53, 299–302. (15) Shephard, F. E.; Rooney, J. J. Reactions of some C9 aromatics on platinum-alumina catalysts. Journal of Catalysis 1964, 3, 129–144.

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(16) Rodriguez, N.; Anderson, P.; Wootsch, a.; Wild, U.; Schlögl, R.; Paál, Z. XPS, EM, and catalytic studies of the accumulation of carbon on Pt black. Journal of Catalysis 2001, 197, 365–377. (17) Kepinski, L. Carbon deposition in Pd / CeO2 catalyst : TEM study. Catalysis Today 1999, 50, 237–245. (18) Albers, P.; Pietsch, J.; Parker, S. F. Poisoning and deactivation of palladium catalysts. Journal of Molecular Catalysis A: Chemical 2001, 173, 275–286. (19) Dickinson, J. G.; Savage, P. E. Stability and activity of Pt and Ni catalysts for hydrodeoxygenation in supercritical water. Journal of Molecular Catalysis A: Chemical 2013, (20) Yeh, T.; Linic, S.; Savage, P. E. Deactivation of Pt catalysts during hydrothermal decarboxylation of butyric acid. ACS Sustainable Chemistry and Engineering 2014, 2, 2399–2406. (21) Fu, J.; Lu, X.; Savage, P. E. Catalytic hydrothermal deoxygenation of palmitic acid. Energy & Environmental Science 2010, 3, 311–317. (22) Matsumura, Y.; Xu, X.; Antal, M. Gasification characteristics of an activated carbon in supercritical water. Carbon 1997, 35, 819–824. (23) Chang, T.; Rodriguez, N.; Baker, R. Carbon deposition on supported platinum particles. Journal of Catalysis 1990, 123, 486–495.

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Behavior of Cholesterol and Catalysts in Supercritical Water - Energy

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