Steam Reforming of Acetic Acid over Co-Supported Catalysts

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Steam Reforming of Acetic Acid over Co-supported Catalysts: Coupling Ketonization for Greater Stability Stephen D. Davidson, Kurt A Spies, Donghai Mei, Libor Kovarik, Igor Kutnyakov, Xiaohong S Li, Vanessa Lebarbier Dagle, Karl O Albrecht, and Robert A Dagle ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02052 • Publication Date (Web): 25 Aug 2017 Downloaded from http://pubs.acs.org on August 29, 2017

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ACS Sustainable Chemistry & Engineering

Steam Reforming of Acetic Acid over Co-supported Catalysts: Coupling Ketonization for Greater Stability

Stephen D. Davidson, Kurt A. Spies, Donghai Mei, Libor Kovarik, Igor Kutnyakov, Xiaohong S. Li , Vanessa Lebarbier Dagle, Karl O. Albrecht, and Robert A. Dagle*

Energy and Environmental Directorate, Institute for Integrated Catalysis Pacific Northwest National Laboratory 902 Battelle Boulevard Richland, WA 99352, USA

*Corresponding author: [email protected]

ACS Paragon Plus Environment


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Abstract We report on the markedly improved stability of a novel 2-bed catalytic system, as compared to a conventional 1-bed steam reforming catalyst, for the production of H2 from acetic acid. The 2-bed catalytic system comprises of i) a basic oxide ketonization catalyst for the conversion of acetic acid to acetone, and a ii) Co-based steam reforming catalyst, both catalytic beds placed in sequence within the same unit operation.

Steam reforming catalysts are

particularly prone to catalytic deactivation when steam reforming acetic acid, used here as a model compound for the aqueous fraction of bio-oil. Catalysts comprising MgAl2O4, ZnO, CeO2, and activated carbon (AC) both with and without Co-addition were evaluated for conversion of acetic acid, and its ketonization product, acetone, in the presence of steam. It was found that over the bare oxide support only ketonization activity was observed and coke deposition was minimal. With addition of Co to the oxide support steam reforming activity was facilitated and coke deposition was significantly increased.

Acetone steam reforming over the same Co-

supported catalysts demonstrated more stable performance and with less coke deposition than with acetic acid feedstock. DFT analysis suggests that over Co surface CHxCOO species are more favorably formed from acetic acid versus acetone. These CHxCOO species are strongly bound to the Co catalyst surface and could explain the higher propensity for coke formation from acetic acid. Based on these findings, in order to enhance stability of the steam reforming catalyst a dual-bed (2-bed) catalyst system was implemented. Comparing the 2-bed and 1-bed (Co-supported catalyst only) systems under otherwise identical reaction conditions the 2-bed demonstrated significantly improved stability and coke deposition was decreased by a factor of 4.

Keywords: biomass, biofuel, steam reforming, ketonization, acetic acid, acetone, hydrogen, cobalt, ceria, hydrothermal liquefaction, pyrolysis


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Introduction Over the last several decades worldwide energy demand and the environmental impact of fossil fuels have driven numerous advances in renewable energy technology. Of these, biomass has the unique potential to replace fossil fuels for transportation fuel production. Biomass-derived liquid fuels can be produced from direct liquefaction technologies that include fast pyrolysis, catalytic fast pyrolysis, and hydrothermal liquefaction

1,2

. These conversion

processes typically generate biphasic product stream that consist of an organic phase and an aqueous phase 3. The organic fraction can be further hydrotreated to remove oxygen in order to produce liquid fuels, while the aqueous stream is currently considered as a waste stream

4,5

.

Depending on the severity and extent of the deconstruction within the liquefaction process the aqueous phase may contain a considerable amount of biogenic carbon. Thus, effective utilization of the aqueous phase oxygenates is highly desirable from an economic perspective 2,6

. Analysis of the aqueous phase of bio-oil has identified acetic acid as one of the most

consistent and abundant components, making it an excellent model compound 2,7. While steam reforming of natural gas is the most common means of hydrogen production steam reforming can also be applied to a wide range of compounds

8,9

. Numerous

bio-derived oxygenated compounds have been studied as feedstocks including acetic acid. The reaction for complete steam reforming of acetic acid is shown below in Equation (1). Precious metal based catalysts (e.g., Rh) have been reported to be active for steam reforming, however, they also can produce a significant amount of undesirable CH4

3,10

and are relatively expensive

11

. Ni and Co have demonstrated comparable activity relative to platinum group metals. Ni has

similarly shown a high selectivity to CH4; however, Co does not demonstrate this high CH4 selectivity

12,13

. In our prior study for ethylene glycol steam reforming we rationalized the cause

for the lower CH4 selectivity over Co catalysts relative to Ni and Rh catalysts via computational modeling

10

. We found that over Co there is a relatively higher barrier for CH3 hydrogenation



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exists. This barrier coupled with relatively facile water activation favors an alternative reaction path, whereby adsorbed CH species undergo hydroxylation thus forming HCOH intermediate (then forming CO and H2) instead of CH hydrogenation 10. CH3COOH + 2H2O → 2CO2 + 4H2

(1)

Steam reforming of oxygenates are more complex than for hydrocarbons due to the CO bonds that introduce a large number of side reactions. The high tendency of thermally unstable oxygenates to decompose forming carbonaceous deposits, particularly acetic acid, has been recognized as a barrier to commercial implementation

14

. Thus, more stable steam reforming

catalysts are required. Takanabe et al. found that for Pt supported on ZrO2, under acetic acid steam reforming conditions, the Pt sites quickly deactivated due to oligomerization reactions and only the ZrO2 support showed any activity following deactivation

15,16

. Lemonidou et al.

found that both high oxygen mobility and excess steam were required for stable catalyst performance of acetic acid steam reforming

14

. In addition, using isotopic labelling Lemonidou

also reported that acetic acid adsorbs on the Rh crystallites forming acetates (CH3COO*) which then decarboxylate and form surface methyl groups that are subsequently converted to CO, CO2, and H2 14. It is has been suggested that these formed acetates can lead to coke formation 17

. Relatively few studies have been done on the steam reforming of acetic acid with either Co

or Ni. Those that are available have shown similar trends to those observed for the steam reforming of other oxygenates over Co and Ni

18-20

. Zhang et al. performed a comparative study

of acetic acid steam reforming over Co and Ni supported on Al2O3 and La2O3 18. Here it was shown that not only is the active metal important, but the nature of the support greatly affects the activity of the catalyst, particularly at lower temperatures where acetic acid steam reforming over La2O3 supported catalysts was significantly enhanced (both in terms of catalyst activity and stability) compared to catalysts supported on Al2O3 and Ni supported on Al2O3 and ZnO

18

. Similarly, Goicoechea et al. studied Co

20

. Here again, the nature of the support was found to be


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critical, directing acetone ketonization and decomposition

20

. Both studies also demonstrated

the importance of steam to carbon ratio (S/C) in directing reaction selectivity towards CO2 over CO and mitigating coke formation 18,20. Wang et al. studied acetic acid steam reforming over coprecipitated Co-Fe catalysts

19

. Here it was demonstrated that Co alone is capable of steam

reforming acetic acid, however, the high S/C needed for stable catalyst performance indicated the importance of support material to adsorb and activate water 21. Both acetic acid and ethanol feedstocks have also been observed to produce acetone, either as a side product or as a reaction intermediate

14,16,22

. The reaction for ketonization of

acetic acid is shown in Equation (2). Acetic acid ketonization has been widely studied over various metal oxides

23,24

. Many of the same catalyst supports used for steam reforming also

facilitate ketonization reactions. For both steam reforming and ketonization catalysts surface basicity is often desired in order to avoid undesirable dehydration reactions. In addition, surface redox ability is beneficial for formation of hydroxyl groups and desorption of water

8,24

. The two

primary pathways for acetic acid conversion of Co-supported catalysts, steam reforming and ketonization, are illustrated in Scheme 1. 2CH3COOH → CH3COCH3 + H2O + CO2

(2)

O Steam Reforming OH

i CO2 + H2

+ H2O

Acetic Acid

ii Ketonization

iii

+ H2O O

Steam Reforming + CO2 + H2O

Acetone

Scheme 1. Primary reaction pathways for acetic acid conversion over Co-supported catalysts.



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In our prior study we reported on the steam reforming of a synthetic fast pyrolysisderived aqueous bio-oil over Ni, Rh, and Co based catalysts; here again, lower CH4 selectivity was observed with the Co based catalyst as well as more stable catalytic performance 3. We also demonstrated an optimal catalyst stability at 500 °C, as compared to the higher temperatures investigated (i.e., 700 °C and 800 °C), as a result of decreased carbon formation 3. Thus, a concept analogous to the petroleum industries’ use of a pre-reformer, operated at approximately 500 °C for steam reforming of the heavier naphtha components, could be applied in the case of steam reforming biomass-derived oxygenates 3. However, while carbon formation was reduced when operating at 500 °C it remained problematic. In this work we investigate the catalytic performance of Co metal supported on different reducible (e.g., CeO2) and non-reducible supports (e.g., MgAl2O4, ZnO, and AC) for the steam reforming of acetic acid and its ketonization product, acetone. A dual-bed (2-bed) system of ketonization followed by steam reforming was also evaluated, primarily explored as a potential means to suppress carbon formation. While adding excess steam or oxidant to the feedstock is one option for coke reduction it also adds significant operating cost. Thus, our aim is to develop a catalytic process whereby enabling acetic acid steam reforming without the requirement of excessive oxidant in the feed but still with minimal coke deposition.


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Experimental Methods Catalyst Preparation MgAl2O4 is a commercial Sasol Puralox 30/150 and was calcined under air at 500 °C for 3 hours prior to impregnation. CeO2 was made by mixing ethanol and ceria oxide nanoparticles stabilized in nitrate (NYACOL CeO2(NO3), particle size 10 – 20 nm), with biopolymer Pluronic F127, dried at 110 °C for 8 hours, and then calcined at 525 °C for 4 hours (5 °C/min ramp rate). Activated carbon (AC) was supplied from PICA (coconut shell derived, 60 – 100 mesh) and was used as received. The Co/CeO2, Co/MgAl2O4, and Co/AC catalysts were prepared by incipient wetness impregnation of their respective supports with solutions of cobalt nitrate, Co(NO3)2. After impregnation the catalysts were dried at 110 °C for 8 hours and then calcined under air at 500 °C for 3 hours; for the Co/AC catalyst calcination was performed under N2. The Co/ZnO was prepared by co-precipitation of Co(NO3)2 and Zn(NO3)2 aqueous solutions as described elsewhere

11

. Commercially-supplied ZnO typically have relatively low surface area (e.g., ~ 5

m2/g) and thus Co/ZnO was prepared using co-precipitation in order to obtain a higher surface area (i.e., 24 m2/g, as shown in Table 1) and to remain consistent with prior steam reforming studies 11. The nominal cobalt loadings were 15 wt.%. In this work the catalysts are referenced by the weight percent of Co and the support material used. For example, 15wt.% Co supported on ZnO is designated as 15%Co/ZnO. Catalyst Characterization BET surface area and BJH pore volume and pore size measurements were performed on a Micromeritics Tristar 3000 using N2 adsorption at 77 K. Between 0.1 and 0.3 g of sample was used for each measurement. Prior to adsorption measurements, samples were degassed under vacuum at 150 °C for 12 hrs. A Micromeritics Autochem 2920 was used to perform H2 temperature programmed reduction (H2-TPR) experiments and temperature programed oxidation (TPO) of spent catalyst.



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For H2-TPR, 0.05 g of sample was loaded and pretreated under 50 SCCM He at 120 °C for 120 min. The sample was then cooled to ambient temperature and the gas flow changed to 5%H2/Ar at 50 SCCM. Temperature was then ramped to 800 °C at 10 °C/min. For TPO, 0.10 g of spent catalyst with α-Al2O3 diluent was loaded and pretreated under 50 SCCM He at 120 °C for 120 min. The sample was then cooled to ambient temperature and the gas flow changed to 5% O2/He at 50 SCCM. Temperature was then ramped to 800 °C at 5 °C/min. For both H2-TPR and TPO analysis gases were monitored by TCD. X-Ray diffraction (XRD) measurements were performed on a Philip’s X’Pert XRD with a Cu-Kα source set to 50 kV and 40 mA. For fresh measurements, the samples were finely ground and packed into an amorphous glass holder. For reduced measurements samples were finely ground then reduced ex-situ under 50 SCCM 5% H2/Ar at 500 °C (15%Co/MgAl2O4 was only sample reduced at 825 °C) for 2 hrs. The samples were then cooled to ambient temperature and passivated under 1% O2/N2 overnight. The passivated samples were then loaded into an amorphous glass holder. XRD patterns were collected from 10 – 80 °2Θ at a rate of 2 °2Θ/min and a step size of 0.06 °2Θ/step. Peaks were analyzed and fit using the MDI Jade 9 software, a reference background to powder Si was used for the peak fitting process. Co0 particle size was calculated using the Scherrer equation based on the 44.3 °2Θ for 15%Co/ZnO and 15%Co/CeO2 samples and the 51.5 °2Θ for 15%Co/MgAl2O4. The S/TEM analysis was performed with a FEI Titan 80–300 microscope operated at 300 kV. The instrument is equipped with a CEOS GmbH double-hexapole aberration corrector for the probe-forming lens, which allows for imaging with 0.1 nm resolution in scanning transmission electron microscopy mode (STEM). The images were acquired with a high angle annular dark field (HAADF) detector with inner collection angle set to 52 mrad. Elemental Analysis was performed with Gatan’s Electron Energy Loss Spectrometer (EELS).


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Reactivity Measurements Steam reforming and ketonization reactions were performed in a ½ inch OD fixed-bed alumina reactor. Alumina reactors were used to minimize potential side reactions that might occur with, for example, stainless steel reactors. Catalyst loadings of 0.20 g of were used, except for acetic acid steam reforming over CeO2 based catalysts where 0.10 g of catalyst was used. The catalyst was diluted with α-Al2O3 (20:1 weight ratio of α-Al2O3 to catalyst) in order to minimize the effect of heat transfer guises as discusssed in our prior work

25

. Catalyst and α-

Al2O3 mixture were mixed and held in place with quartz wool. A K-type thermocouple was placed in the center of the loaded catalyst bed to measure catalyst bed temperature. Prior to testing ZnO, AC, and CeO2 supported catalysts were reduced at 500 °C for 8 hours under a 10% H2/N2 gas mixture. The MgAl2O4 supported catalysts were reduced at 850 °C. Carbon balance for all experiments was found to be 85% or higher. The catalysts were tested at atmospheric pressure and at a reaction temperature of 500 °C. Nitrogen gas (typically ~ 20 mol% in the feed) was introduced into the system by a Brooks Mass Flow Controller (5890 E series) to serve as the carrier gas and internal reference standard. The liquid reactant feed was introduced using a HPLC pump (Chrom Tech series 1500) through a 1/16th inch stainless steel line to a microchannel vaporizer set at 150 °C. For the evaluations with acetic acid a 32.3 wt.% acetic acid in water feed was used.

This

corresponds to a molar steam-to-carbon (S/C) ratio of 3.5. For the evaluations with acetone a 23.5 wt.% acetone in water feed was used. This corresponds to a molar steam-to-carbon (S/C) ratio of 3.5. The flowrate of dry gas products was measured by a digital flow meter (DryCal). The gas composition was determined by a four channel Agilent Micro GC equipped with MS-5A, PPU, alumina, and OV-1 columns and a TCD detector for each column. Liquid products were trapped in a one-liter condenser cooled by circulating ethylene glycol for post-run analysis by



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LC. The liquid product from the reforming experiments was injected with and without dilution into an Agilent 1100 LC system with a Aminex HPX-87H ion exclusion column, (Bio-Rad), 300 mm long, 7.8 mm inner diameter guard column. The column was eluted isocratically with 0.005 M sulfuric acid through a refractive index detector with an optical temperature of 35 °C. The amount of carbon deposited on the spent catalyst was measured by Vario EL Cube Elemental analyzer. Spent samples were weighed on a Mettler XP6 microbalance into tin foil boats for analysis. Conversion was calculated from simple mass balance on either acetic acid or acetone.

− (R Out Gas + R Out Liquid R X =  Fed R Fed 

)  ⋅ 100  

(3)

Here conversion is X, RFed is the calculated feed rate of reactant (i.e., acetic acid or acetone) based on mass change during run period and feed composition, ROut Gas is reactant detected on the Agilent Micro GC integrated over the run, and ROut Liquid is reactant detected in LC analysis. Selectivity was calculated on a carbon basis using the following equation:

 C  ⋅F SProduct =  Product Product  ⋅ 100  ∑C  Product ⋅ FProduct  

(4)

Here SProduct is selectivity to a product compound (e.g., CO2), CProduct is the number of carbons in the product molecule, and Fproduct is the molar flow rate of the product. H2 yield was calculated based on the stoichiometric H2 value from Equation (1) using the following equation:

FH2    ⋅ 100 YH2 =   4 ⋅ FAcetic Acid 


(5)

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Here FH2 is the outlet molar flow of H2 and FAcetic Acid is the molar feed rate of acetic acid. Computational Details All calculations were performed using spin-polarized density functional theory (DFT) within the generalized gradient approximation (GGA) as implemented in the Vienna ab initio simulation package (VASP)

26-28

. The core and valence electrons were represented by the

projector augmented wave (PAW) method

29,30

with a kinetic cutoff energy of 400 eV. The

exchange correlation functional was described by the Perdew−Burke−Ernzerhof (PBE) functional

31

. The Co(0001) plane has been reported to be representative of bulk Co surfaces

and hence Co(0001) was used to mimic the supported Co catalyst for acetic acid conversion in this work

32,33

.

The ground-state atomic geometries of clean and the adsorbed Co(0001) was obtained by minimizing the forces on each atom to below 0.03 eV/Å. A periodic p(2×2) supercell Co(0001) surface slab with four atomic layers was used in this work. During the geometric optimizations and the transition state searching processes, the adsorbate(s) and the metal atoms in the top two atomic layers were allowed to relax while the metal atoms in the bottom two atomic layers of the surface slab were fixed. A 15 Å vacuum layer was inserted between the Co(0001) surface slab in the z direction to avoid non-physical interaction artifacts between the periodic systems modeled. To ensure the accuracy of calculations, the effects of slab thickness (up to six atomic layers) and different Monkhorst-Pack (MP) mesh sampling were tested. A (3×3×1) MP sampling schedule was found to be accurate to reach the total energy convergence of

Steam Reforming of Acetic Acid over Co-Supported Catalysts

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