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Enhanced Hydrothermal Stability and Catalytic Activity of LaxZryOz Mixed Oxides for the Ketonization of Acetic Acid in the Aqueous Condensed Phase Juan A. Lopez-Ruiz, Alan R. Cooper, Guosheng Li, and Karl O Albrecht ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01071 • Publication Date (Web): 09 Aug 2017 Downloaded from http://pubs.acs.org on August 9, 2017

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Enhanced Hydrothermal Stability and Catalytic Activity of LaxZryOz Mixed Oxides for the Ketonization of Acetic Acid in the Aqueous Condensed Phase

Juan A. Lopez-Ruiz†, Alan R. Cooper†, Guosheng Li†, and Karl O. Albrecht†* †

Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA 99352, USA

Corresponding author Karl O. Albrecht Energy and Environment Directorate Pacific Northwest National Laboratory 902 Battelle Blvd., Richland, WA 99352, USA Tel.: +1509 371-6775 Fax: +1509 375-6422 E-mail address: [email protected]

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TOC

Abstract Common ketonization catalysts such as ZrO2, CeO2, and TiO2-based catalysts have been reported to lose surface area, undergo phase transformation, and lose catalytic activity when utilized in the condensed aqueous phase. In this work, we synthesized a series of LaxZryOz mixed-metal oxides with different La:Zr atomic ratios with the goal of enhancing the catalytic activity and stability for the ketonization of acetic acid in condensed aqueous media at 568 K. We synthesized a hydrothermally stable LaxZryOz mixed-metal oxide catalyst with ketonization activity 265 and 45 times more active than La2O3 and ZrO2, respectively. Catalyst characterization techniques suggest that the enhanced stability of the LaxZryOz catalysts is observed with the formation of a phase isomorphic with tetragonal ZrO2. DRIFTS spectroscopy measurements indicated the enhanced catalytic activity of LaxZryOz catalysts correlated with greater acetic acid surface population in the presence of H2O versus pure ZrO2.

Keywords: ketonization; deoxygenation; carboxylic acid; acetic acid; hydrothermal stability: acetone production; ZrO2; La2O3; LaxZryOz; mixed oxides.

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1. Introduction The conversion of terrestrial biomass into fuels and chemicals generates waste aqueous streams that may contain a high fraction of biogenic carbon. For example, the hydrothermal liquefaction (HTL) of biomass produces a byproduct aqueous stream which may contain between 27 and 50 % of the total carbon fed to the process.1-2 A large portion of this “wasted” carbon consists of short chain carboxylic acids such as acetic acid and propionic acid.3 Current state-of-the-art is utilization of these feed streams for anaerobic digestion, which creates low value medium BTU gas utilized for process heat. Recovery and transformation of the “wasted” aqueous carbon into fuels and chemicals has the potential to greatly improve the economic viability and increase the C yield to useful products of several biomass conversion technologies.4 A promising route for upgrading biomass-derived carboxylic acids is catalytic conversion into olefins, alcohols, or H2 using the ketonization reaction as an intermediate step (Scheme 1). Ketonization of carboxylic acids over mixed-metal oxides

5-18

as well as the hydrodeoxygenation of ketones via

hydrogenation-dehydration reactions over metal supported catalysts and proton zeolites

19-21

have been

extensively studied. However, the presence of H2O, especially condensed liquid water, has been reported to inhibit the reaction rates.11-12, 19, 21.

Scheme 1. Proposed route for catalytic upgrading of carboxylic acids into fuels and chemicals. Water is expected to constitute the majority of the aqueous byproduct stream from nearly all conversion processes, even increasing above 90 wt% in some cases.3 Because the organic acids will be dilute, catalytic upgrading in the condensed phase is preferred over the vapor phase in order to eliminate the need to supply a large amount of latent heat to vaporize the aqueous stream. Further, performing 3


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condensed phase ketonization may also facilitate economic separations after the reaction step because the ketone products boil at lower temperatures than the parent acids. For example, in contrast to acetic acid (Tboiling = 391 K), acetone boils at 329 K and readily distills from water. Whereas the catalytic upgrading of carboxylic acids maybe performed in the condensed phase, catalysts and catalytic supports studied to date are not stable under aqueous condensed hydrothermal reaction conditions. Common ketonization catalysts such as ZrO2-, CeO2-, and TiO2-based catalysts have been reported to lose surface area, undergo phase transformation, and lose catalytic activity upon exposure to condensed dry and aqueous acetic acid at temperatures higher than 473 K.11-12,

14-16

For

example, Snell et al. showed that the crystal structure of CeO2 catalysts is modified when exposed to dry acetic acid at reaction temperatures between about 423 and 500 K.14, 16 Pham et al. reported excellent acetic acid ketonization activity over a TiO2-based catalysts in the gas phase.11-12 However, when this reaction was carried out in condensed H2O instead of an condensed organic solvent, the catalyst deactivated in less than 6 h.12 Therefore, developing hydrothermally stable catalyst and catalytic supports is paramount for the catalytic upgrading of carboxylic acids present in aqueous streams. Lanthanum is widely used to enhance the thermal and hydrothermal stability of materials such as γ-alumina and zeolites.

22-28

For example, Chen et al. showed that the addition of La2O3 to γ-alumina

inhibits sintering and phase transformations under high temperature operation (873 – 1423 K) when lanthanum species were highly dispersed on alumina.23 Yang et al. showed that the hydrothermally stability of ZSM-5 zeolites was greatly improved when La was partially exchanged with Al sites.

28

Furthermore, Yamada et al. reported La2O3 had superior ketonization activity and stability compared to other rare earth oxides like CeO2 during the vapor-phase ketonization of acetic acid at 623 K. 18 In the course of investigating numerous catalysts for the conversion of acetic acid to acetone under hydrothermal conditions, we discovered mixed metal oxide catalysts with the nominal formulation of LaxZryOz exhibited surprisingly stable activity over hundreds-of-hours on stream. Given the need for hydrothermally stable catalysts that can operate under these challenging process conditions, we 4


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synthesized LaxZryOz mixed-metal oxide catalysts and tested the catalytic activity and stability of these catalysts under aqueous condensed phase operation. In particular, LaxZryOz catalysts with different La:Zr atomic ratios were systematically synthesized, characterized, and tested for the ketonization of acetic acid under aqueous condensed hydrothermal conditions in the presence of H2O in a continuous flow fixed-bed reactor at 9600 kPa and 568 K. A series of catalyst characterization techniques such as inductively coupled plasma-optical emission spectroscopy (ICP-OES), N2 physisorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were performed to elucidate reaction activity/structure relationships and monitor changes in catalyst physical structure.

2. Materials and Methods 2.1. Catalyst synthesis Catalysts were prepared by co-precipitation using zirconyl (II) nitrate (ZrO(NO3)2) (Sigma-Aldrich, 35 wt. % solution in dilute nitric acid, ≥99%) and lanthanum (III) nitrate hexahydrate (La(NO3)3·6H2O) (STREM, 99.99%) to obtain pure La2O3, ZrO2, and a wide range La2O3/ZrO2 mixed oxides with different La2O3 loadings (1, 5, 10, 15, and 20 wt% La2O3 loadings on ZrO2). The precursors were mixed with deionized (DI) H2O and stirred with a magnetic bar. The pH of the solution was increased and maintained at 9 with ammonium hydroxide (NH4OH) (Sigma). The pH of the solution was continuously monitored with a PICCOLO® pH tester equipped with a pH electrode HI1280. DI H2O was constantly added at 5 mL min-1 for 60 min to the solution to ensure stirrability of the slurry due to the formation of catalyst solids. After the pH was stabilized at 9, the solution was covered to avoid losses of H2O and NH4OH due to evaporation, and aged for 3 days at constant pH of 9. During the aging process, the pH of the solution was constantly monitored and NH4OH was added as (if) needed. The slurry was washed with warm DI H2O to separate the catalyst solids from unreacted NH4OH and salts formed during

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the co-precipitation step. The washing step was continued until the levels of dissolved solids in the washed DI H2O were < 50 ppm, which was measured with a total dissolved solid meter. The clean slurry was then dried at 365 K in oven for 3 days. The dried solids were sieved to particle size < 149 μm, pelletized to 2.5 g pellets of 2 in. diameter using 40,000 lb pressure, and calcined under flowing air at 723 K for 4 h at a heating and cooling rates of 5 K min-1. The pellets were then sieved using 60 and 100 mesh sieves to a final particle size between 149 to 250 μm, and washed with room temperature DI H2O to remove fine particles attached to the surface of the catalyst. The sized catalyst particles were oven-dried overnight at 363 K, cooled to room temperature, and stored in a desiccator under vacuum. 2.2. Catalyst characterization Powder X-ray diffraction (XRD) analysis from 2Ө = 20 to 80° was carried out on a Rigaku MiniFlex II X-Ray Generator with monochromatic Cu Kα-radiation (λ =1.54056 Å) using a step size of 0.05°. Surface area and pore size analysis were determined by N2 physisorption using a QUANTACHROME AUTOSORB 6-B gas sorption system at 77 K. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) was performed on a Perkin Elmer Optima 7300DV. Scanning electron microscope (SEM) imaging was performed on a JEOL 7001F FESEM with a Bruker XFlash 6|60 EDS detector. Scanning transmission electron microscope (STEM) was performed on aberration-corrected Titan 80-300™. X-ray photoelectron spectroscopy (XPS) was performed on a Physical Electronics Instruments (PHI) Quantum 2000 equipped with a focused monochromatic aluminum K X-ray beam. 2.3. Catalytic conversion of acetic acid The catalytic ketonization of acetic acid (Acros, 99.8%) was performed in a 3/8 in. outside diameter (OD) × 9 in. length × 0.035 in. wall thickness grade 2 titanium fixed bed tubular reactor. Titanium was used due to the aggressive nature of the reaction feed under hydrothermal conditions at 568 K. The reactor was loaded with 0.5 to 3.0 g of catalyst supported on carbon felt

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(Alfa Aesar 43200, 99.0%) which was used as inert, thermally stable reactor filler. The carbon felt was added on top of the catalyst bed in order to fill the tube. Fresh catalyst was used for each experiment. After loading the catalyst in the reactor, the system was purged for 30 min with 20 cm3 min-1 of N2 (Oxarc, 99%) at atmospheric pressure to remove air from the system. The N2 flow was controlled with a Brooks Mass Flow Controller and pressure was controlled with a Tescom backpressure regulator. The pressure was monitored via a pressure transducer at the inlet of the reactor, a digital pressure gauge at the outlet of the reactor, and a pressure transducer built into the head of the ISCO pump. The temperature was then increased to the desired reaction temperature, 568 K, using a Watlow Dual-7KRG-2300 and held overnight under 20 cm3 min-1 of N2. A 2 in. OD × 5 in. length × 0.75 in. wall thickness aluminum block was placed between the heater and the reactor body to minimize temperature gradients along the reactor body. The internal thermocouple, made of grade 2 titanium to avoid metal corrosion, was placed at the top of the catalyst bed. External thermocouples made of stainless steel were placed across the heater, aluminum block, and outside reactor wall to monitor temperature gradients. A schematic of the reaction system is shown in Figure 1. A description of the reactor sampling protocol is shown in Supporting Information. The feed was composed of 10 wt% acetic acid and 90 wt% deionized H2O, and was pumped in the reaction system using a syringe pump (Teledyne ISCO 100X) operating at flow rates between 0.030 and 0.060 cm3 min-1 and 9800 KPa. The reactor was operated in an upflow configuration and N2 was co-fed into the system at 20 cm3 min-1 to carry product gases out of the system. The reaction products were sampled every 24 h. The liquid phase was analyzed by high-performance liquid chromatography equipped with a Waters 2414 refractive index detector. A Bio-Rad Aminex HPX-87H ion exclusion column (300 mm × 7.8 mm) was used for analytes separation. Sulfuric acid (0.005 M) was used as eluent at a flow rate of 0.55 mL/min. The gas phase was constantly monitored by a DryCal® flow meter and removed from the condenser(s) by N2 that was co-fed with the feed. An online Agilent Micro GC 3000 A

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equipped with Mol Sieve and Plot U columns and a thermal conductivity detector was used for the detection of He, H2, O2, CO, CO2, N2, acetone, acetic acid, and light hydrocarbons. Control experiments were performed to determine background conversion and stability of the carbon felt at our typical reaction conditions (568 K, 96 bar, and 0.03 cm3 min-1 of liquid feed composed of 10 wt% acetic acid and 90 wt% H2O). No conversion of acetic acid was observed in the absence of catalyst. 2.4. Post-mortem catalyst characterization After reaction, the catalyst was cooled down to room temperature and dried under flowing N2 at room temperature. Spent catalyst samples were recovered from the reactor and stored in a vial for post-mortem characterization to determine the effect of reaction conditions on the catalyst structure. 2.5. Diffuse reflectance Fourier transform spectroscopy A Fourier transform infrared (FTIR) spectrometer (Bruker Vertex 70) coupled with a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) cell (equipped with ZnSe Windows) was used to measure surface coverage of acetic acid in the presence and absence of H2O. As it will be discussed below, all catalysts restructured quickly during the first few hours under aqueous condensed reaction conditions. Therefore the DRIFTS measurements were conducted only on ZrO2 and LaxZryOz catalysts exposed to hydrothermal H2O only for 24 h (9600 kPa and 568 K). La2O3 was not studied because it was not active under condensed hydrothermal reaction conditions. Catalyst samples were ground to fine powders and placed in the DRIFTS cell for analysis. The instrument was allowed to purge for 1 h in flowing UHP N2 (Arco, >99.9999%). Fresh catalyst samples were loaded to DRIFTS cell for each set of experiment. The catalyst samples were dehydrated before each experiment under a flow of 20 cm3 min-1 UHP N2 at 623 K for 1 h and cooled to room temperature. The catalyst samples were exposed to different gas 8


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environments by flowing 20 cm3 min-1 UHP N2 for 15 min through a bubbler containing different solutions, such as pure acetic acid (Sigma-Aldrich, glacial ≥99.85%), DI H2O, and 10 wt% acetic acid in DI H2O at room-temperature. A 30 min purge with UHP N2 flowing at 20 cm3 min-1 was used to eliminate gas-phase and weakly physisorbed species from the DRIFTS cell and catalyst surface prior to DRIFTS measurements, respectively. Then, the temperature programmed desorption (TPD) experiment was performed at a temperature range between 303 and 573 K by ramping the DRIFTS cell at 5oC/min. Spectra were collected at each temperature after allowing equilibration for 15 min. 2.6. Calculations of rates Under the reaction conditions operated in this study (568 K, 9800 kPa, 0.010 to 0.030 cm3 min-1 liquid feed rate with a composition of 10 wt% acetic acid in H2O), all the catalysts tested were 100% selective for the ketonization reaction. The only observed products were CO2 and acetone in the gas and liquid phases, respectively. The mass and mole balance were calculated using the weight of feed fed in the reactor divided by the weight of products collected after the reactor. For all the data reported here, the balances were between 98 and 100%. The specific rate of acetone production was calculated using the rate of formation of the products after 24 h of reaction, after which the reactor system was operating under steady-state conditions. Therefore, the specific rate of acetone production rate was calculated as shown in Eq. 1. Similarly, the areal rate of acetone production was calculated as shown in Eq.2. Equation 1

Specific Rate of Acetone Production (mmols g h )

=

moles of acetone in product x 1000 (mmol) sample collection time (h) x amount of catalyst (g)

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Equation 2

Areal Rate of Acetone Production (mmols m# h ) =

moles of acetone in product x 1000 (mmol) sample collection time (h) x amount of catalyst (g) x surface area of catalyst (m# g ) The catalyst stability was monitored using the normalized acetone production rate which was calculated as the ratio between the specific rate of acetone production as a function of time divided by initial specific rate of acetone production obtained after 24 h of reaction as shown in Eq. 3. We refer to stable catalytic activity when the normalized acetone production rate is between 95 and 105%. We refer to catalyst deactivation when the normalized acetone production rate decreases as a function of time.

Equation 3

Normalized Acetone Production Rate (%) Specific Rate of Acetone Production (mmols g h ) = x 100 Initial Specific Rate of Acetone Production (mmols g h )

Similarly to a previously work,29 the Weisz-Prater criterion was used to assess internal, ϕI, and external, ϕE, transport limitations in the aqueous condensed phase operation.30-32 Using the highest observed rates for the condensed phase operation, 1.16 mmolacetic

acid

gcat-1 h-1 (0.78

mmolacetone gcat-1 h-1), the Weiz-Prater criteria suggest a lack of internal and external mass transfer limitations with ϕI and ϕE of 0.013 and 0.0094, respectively. A discussion of the Weisz-Prater criterion can be found in Supporting Information.

3. Results and Discussion 3.1. Catalytic activity and stability under hydrothermal reaction conditions LaxZryOz catalysts with different La:Zr atomic ratios were synthesized using a coprecipitation method. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) was

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used to determine the La:Zr atomic ratio of the catalyst obtained after synthesis. Table 1 summarizes the ICP-OES results and compares the La:Zr atomic ratios of the catalysts. Table 1. Characterization of catalyst after synthesis (initial) and after reaction (final) with ICP-OES and N2 physisorption. PM = physical mixture (i.e., the catalyst synthesized by physically mixing La2O3 and ZrO2, rather than through co-precipitation). Target La2O3 Loading

Target La:Zr

ICP-OES-derived

Surface Area

(wt%)

Atomic Ratio

La:Zr Atomic Ratio

(m2 g-1)

Catalyst Name

Initial

Final

Initial

Final

0

0

0

0

84

33

ZrO2

5

0.040

0.035 ± 0.00

0.025 ± 0.00

126

66

0.035LaxZryOz

10

0.08

0.075 ± 0.00

0.039 ± 0.00

135

49

0.075LaxZryOz

15

0.13

0.10 ± 0.02

0.095 ± 0.00

144

82

0.10LaxZryOz

20

0.19

0.17 ± 0.01

0.12 ± 0.00

185

92

0.17LaxZryOz

PM20

0.19

0.14 ± 0.00

0.12 ± 0.00

80

30

PM0.14LaxZryOz

25

0.25

0.20 ± 0.02

0.13 ± 0.00

86

66

0.20LaxZryOz

30

0.32

0.23 ± 0.07

0.12 ± 0.00

82

63

0.23LaxZryOz

100

∞

∞

∞

29

20

La2O3

As seen in Table 1, there is about a 20% difference between the target and measured La:Zr atomic ratio. Intimate mixing of the La and Zr precipitates appeared to be more challenging at higher La ratios as evidenced by the observation of an increasing amount of fine particulates during synthesis method compared to lower La samples. Interestingly, the surface area of all the LaxZryOz mixed-metal oxide was higher than that of pure ZrO2 and La2O3, 84 and 29 m2 g-1 respectively, and increased as a function of La:Zr atomic ratio up to the ratio of 20 wt%. Because of the differences of initial surface area and La:Zr ratio among all the catalysts, the catalytic activity is reported as a function of mass and initial surface area of the catalyst, specific rate and areal real respectively. The catalytic activity and reaction conditions are summarized in Table 2. 11


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Table 2. Summary of reaction conditions. All the catalysts were tested at 9800 kPa and 568 K with a feed composed of 10 wt% acetic acid in H2O. The reactivity data shown here is after 24 and 144 h of reaction for the initial and final rates, respectively.

Liquid

Acetic Acid

Specific Acetone

Areal Acetone

Conversion

Production Rate

Production Rate

(mmol gcat-1 h-1)

(µmol m-2 h-1)

Flow Rate

WHSV

after 24 h

Catalyst Name

(cm3 min-1)

(h-1)

(%)

Initial

Final

Initial

Final

ZrO2

0.015

0.32

13

0.031

0.017

0.26

0.53

0.035LaxZryOz

0.030

1.9

13

0.18

0.18

1.4

3.6

0.075LaxZryOz

0.030

1.9

23

0.32

0.32

2.4

4.9

0.10LaxZryOz

0.030

3.8

21

0.57

0.59

4.0

7.2

0.17LaxZryOz

0.030

3.8

28

0.78

0.78

4.2

8.7

PM0.14LaxZryOz

0.030

1.9

6.4

0.088

0.080

1.1

2.7

0.20LaxZryOz

0.030

3.8

14

0.25

0.53

2.9

8.1

0.23LaxZryOz

0.030

3.8

12

0.23

0.42

2.7

6.7

La2O3

0.030

0.63

4.8

0.022

0.0003

0.75

0.15

The ketonization of acetic acid was carried out on a continuous flow fixed bed reactor at 9800 kPa and 568 K with a feed composed of 10 wt% acetic acid in H2O. Catalyst loading and liquid feed flow rates were adjusted to keep the conversion of acetic acid above HPLC detection limits and below 30% conversion. Under these reaction conditions, all the catalysts were 100% selective toward the ketonization of acetic acid. As shown in Table 2, the pure ZrO2 catalyst exhibited low activity of 0.031 mmol gcat-1 h-1 after 24 h of reaction. Pure La2O3, which was identified by Yamada et al. as a very active material for ketonization of acetic acid in the vapor phase,18 was the least active most likely because of the presence of H2O. Surprisingly, all the LaxZryOz mixed-metal oxide synthesized by the co-precipitation method displayed enhanced catalytic activity and stability compared to the pure-metal oxides (i.e., ZrO2 and La2O3). As the La:Zr atomic ratio increased from 0.035 to 0.17, we observed a progressive and substantial

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increase in catalytic activity. The LaxZryOz mixed-metal oxide with a La:Zr ratio of 0.17 had the maximum observed specific catalytic activity, which was greater by a factor of 40 and 360 compared to ZrO2 and La2O3, respectively. For comparison, a physically mixed (PM) catalyst was prepared with the same weight composition (i.e., 20 wt% La2O3/ZrO2) as the most active catalyst, (i.e., 0.17LaxZryOz). Table 2 reveals that the PM catalyst, PM0.14LaxZryOz was not as active as the co-precipitated catalyst, 0.17LaxZryOz, 0.088 and 0.78 mmol acetone gcat-1 h-1 respectively, but was more active than ZrO2 or La2O3 alone. This result suggests that the enhanced catalytic activity and stability of the LaxZryOz catalyst comes from the intimate mixing of the La and Zr atoms in the mixed-metal oxide. Further, the elevated activity of PM0.14LaxZryOz compared to ZrO2 suggests that some integration of La and Zr at the bulk interfaces may have taken place under reaction conditions, thereby creating catalytic sites with greater activity than would be expected from pure ZrO2 or La2O3. Interestingly, as the initial La:Zr atomic ratio increased beyond 0.17 for the co-precipitated catalysts, the catalytic activity progressively decreased. The addition of La beyond an initial La:Zr atomic ratio of 0.17 results in the creation of La-rich particles, as suggested by the ICP results shown in Table 1, that are not as catalytically active under condensed hydrothermal reaction conditions. As shown in Table 1, the initial surface area for the LaxZryOz catalyst is larger than ZrO2 and La2O3, for this reason, the catalytic activity was also calculated in terms of areal rate. Table 2 shows that enhancement in catalytic activity from ZrO2 to the LaxZryOz is not correlated with the initial surface area of the material. For example, the initial areal acetone production rate for 0.17LaxZryOz is 16 and 5.7 times higher that of ZrO2 and La2O3, respectively. Interestingly, the initial areal acetone production rate of 0.10 and 0.17LaxZyOz catalysts is very similar, 4.0 and 4.2 µmol m-2 h-1 respectively, which suggest that the catalytic activity of the LaxZryOz catalysts near the optimum initial La:Zr atomic ratio display catalytic activities correlated to the initial surface area.

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The stability of the catalysts also improved for the LaxZryOz mixed-metal oxides. As shown in Figure 2, the specific rates of acetone production for pure ZrO2 and La2O3 decreased as a function of time on stream. However, the rate was stable for the LaxZryOz catalysts with initial La:Zr atomic ratio between 0.035 and 0.17. Surprisingly, the catalytic activity improved as a function of time on stream for the LaxZryOz catalysts with initial La:Zr atomic rations higher than 0.17. For example, with a La:Zr atomic ratio of 0.20, the initial catalytic activity was 0.25 mmol acetone gcat-1 h-1; however, the catalytic activity stabilized by the end of the experiment at 0.56 mmol acetone gcat-1 h-1. We speculated that this increase in catalytic activity as a function of time on stream might be caused by a catalyst restructuring when exposed to hydrothermal reaction conditions. Therefore, the catalyst was further characterized by N2 physisorption and ICP-AOS after exposure the hydrothermal reaction conditions. As shown in Table 1, the surface area and La:Zr atomic ratio decreased for of all the catalysts. The fact that the catalytic activity of the LaxZryOz catalysts with initial La:Zr atomic ratios between 0.035 and 0.17 remained stable during the 144 h of reaction indicates that the catalyst restructuring takes place early on during the experiment. Interestingly, the La:Zr ratio of all LaxZryOz seems to change under hydrothermal reaction conditions. For example, whereas there was negligible change on La:Zr atomic ratio for 0.10 LaxZryOz, all the catalyst with initial La:Zr ratio higher than 0.17 stabilized at a final La:Zr ratio of 0.12. Further, the final areal acetone production rate for all the LaxZryOz catalysts with final La:Zr atomic ratio of 0.12 are very similar, indicating that any difference in specific acetone production rate is mostly due to the differences in surface area of each material. The decrease in La:Zr atomic ratio during reaction conditions suggests that the excess La present in the LaxZryOz catalyst may be solubilized and removed from the active mixed-metal oxide phase under hydrothermal reaction conditions. 3.2. Understanding catalyst stability under hydrothermal reaction conditions

14


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X-ray diffraction (XRD) patterns of the freshly prepared and calcined samples (noted with an F) are illustrated in Figure 3. In general, co-precipitated LaxZryOz catalysts are amorphous; however, some features of ZrO2 can be observed in samples with low La:Zr atomic ratios. In contrast, the physically mixed catalyst, PM0.14LaxZryOz, shows reflections observed in both the F-La2O3 and F-ZrO2. This confirms the intimate mixing of the La and Zr in the co-precipitated samples to form an amorphous catalyst precursor. To support our hypothesis of catalyst restructuring, XRD patterns of the catalysts were taken after exposure to condensed hydrothermal reaction conditions (noted with a C), i.e. 10 wt% acetic acid in H2O, 9600 kPa, and 568 K. The 0.10LaxZryOz sample was chosen as a representative of LaxZryOz catalyst because it started as an amorphous material and exhibited high catalytic activity and stability under condensed hydrothermal reaction conditions. Figure 4 shows the XRD patterns for ZrO2, 0.10LaxZryOz, and La2O3 before (F) and after exposure to condensed hydrothermal reaction conditions (C). Our investigation revealed that the bulk phase of ZrO2, LaxZryOz and La2O3 restructured under hydrothermal reaction conditions. Whereas F-ZrO2 displayed a mixed-phase between monoclinic (m-ZrO2) and tetragonal (t-ZrO2), it restructured to m-ZrO2 only after exposure to hydrothermal reaction conditions (C-ZrO2), as previously reported.33-34 0.10LaxZryOz was amorphous after synthesis, but restructured into La-stabilized tZrO2 after exposure to hydrothermal reaction conditions (C-0.10LaxZryOz). Whereas the stabilization of t-ZrO2 with La and Ce was recently reported by Wang et al. after hightemperature treatment at temperatures above 1073 K,

35

to the best of our knowledge, this is the

first time the stabilization of La-doped t-ZrO2 has been reported under condensed hydrothermal reaction conditions. F-La2O3 shows an A-type hexagonal (ah-La2O3);36 however, after condensed hydrothermal reaction conditions, C-La2O3 displayed reflections consistent with a lanthanum hydroxycarbonate polymorphic crystalline structure, p-Lax(CO3)y(OH)z.37 Whereas the formation of La(OH)3 during condensed hydrothermal reaction conditions has been previously reported by

15


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Ma et al.,38 the phase transformation of La2O3 from ah-La2O3

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36

to p-Lax(CO3)y(OH)z under

condensed hydrothermal reaction conditions has not, to the best of our knowledge, been shown before. We theorize that the formation of h-La(OH)3 and p-Lax(CO3)y(OH)z structures might be responsible for the low ketonization activity of La2O3 under condensed hydrothermal reaction conditions due to the fact that surface species (e.g., surface hydroxyls) on the oxy-carbide phase are relatively stable and not conducive to the absorption of acetic acid. Catalyst restructuring under condensed hydrothermal reaction conditions was investigated by exposing as synthesized catalysts (noted as F) to hydrothermal H2O without acetic acid at 9600 kPa and 568 K for 24 h (noted as H). The hydrothermally treated catalysts were valuated with XRD. As shown in Figure 3, the XRD profile of C- and H-ZrO2 were identical, indicating that condensed hydrothermal H2O already caused the phase transformation. Similarly, C- and H0.10LaxZryOz were also identical. Interestingly, C- and H-La2O3 were very different. H-La2O3 mainly showed features of hexagonal La(OH)3, h-La(OH)3, and hexagonal La(CO3)OH, hLa(CO3)OH, with PDF Card No.: 00-036-1481 and 00-62-00300 respectively. Because H-La2O3 was never exposed to acetic acid, the formation of bulk h-La(CO3)OH phase was surprising and we speculate that CO2 dissolved in the H2O or possibly CO2 absorption from air after exposure to room temperature conditions might be the source of the carbonate. Because ZrO2 and La2O3 catalysts were not active and stable under condensed hydrothermal reaction conditions, they were not further investigated and we focused our efforts in the phase transformation of 0.10LaxZryOz catalyst from amorphous to La-stabilized t-ZrO2. Figure 5 shows micrographs taken with a scanning electron microscope (SEM) of the fresh and spent 0.10LaxZryOz, F- and C-0.10LaxZryOz respectively, and reveals that there were no appreciable morphological differences between them. Elemental mapping was used to investigate potential phase segregation of La2O3 from ZrO2 in the C-0.10LaxZryOz catalyst. However, as shown in Figure 6, the analysis revealed the La and Zr (Figure 6C and D, respectively) remained 16


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very well dispersed across the cross-sectional area of a catalyst pellet even after hydrothermal

reaction conditions. Thus, the change in crystallinity is not associated with phase segregation in the bulk of the catalyst. Energy dispersive spectroscopy (EDS) results of the C-0.10LaxZryOz catalyst cross-sectional area revealed a La:Zr atomic ratio of 0.090, similar to the ICP-OES-measured value shown in Table 3. Because the change in crystallinity was not readily observed with SEM, a scanning transmission electron microscope (STEM) was used. Figure 7 shows STEM images of the 0.10LaxZryOz samples before (F) and after reaction (C). STEM measurements verified the change in crystallinity of the material due to exposure to hydrothermal reaction conditions. For example, the lack of crystal structure seen in Figure 7A suggests that the catalyst was amorphous before reaction, but crystallized after exposure to condensed hydrothermal reaction conditions, Figure 7B, which was not observed at the 10-100 µm length scale. It has been suggested in the literature that the enhanced catalytic activity of ketonization catalysts could be related to changes in oxidation state and/or formation of oxygen vacancies in the catalyst surface.11-12,

14-15, 39

Because XRD, SEM, and STEM characterization techniques

mostly represent bulk properties and the ketonization reaction is a surface reaction, X-ray photoelectron spectroscopy (XPS) of ZrO2, 0.10LaxZryOz, and La2O3 was performed to understand the surface of the catalysts. As shown in Table 3, the XPS-derived La:Zr atomic ratios for the fresh catalyst are in agreement with the ICP-OES- and EDS-derived values for the 0.10LaxZryOz catalyst. Further, the molar ratio of O/(Zr+La) increases for 0.10LaxZryOz with respect to F-ZrO2, suggesting that there were no apparent oxygen vacancies formed. However, the molar ratio of O/(Zr+La) decreases for 0.10LaxZryOz with respect to F-La2O3, suggesting different stoichiometry with respect to La. Further, when comparing the XPS spectra of Zr 3d edge of the catalyst after reaction (see Figure 8), no significant change was observed in oxidation state for Zr 3d edge between the pure ZrO2. All 0.10LaxZryOz samples exhibited similar binding energies for 17


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Zr in the 3d region. The XPS results suggest that the cause of enhanced catalytic activity is not due to a change in oxidation state nor generation of oxygen vacancies in the catalyst surface with respect to Zr. Furthermore, the change of the bulk crystallinity from amorphous to La-stabilized tZrO2 did not affect the oxidation state of ZrO2, which is in contrast to behavior previously reported by Snell et al. for CeO2 catalysts.14 When comparing the XPS spectra of La 3d edge for F-La2O3 and all the 0.10LaxZryOz samples in Figure 9, a change was observed in the binding energy for the second peak of the doublet in both the La 3d3/2 and La3d5/2 edges by ≈ 0.5 eV. Whereas the doublets observed in the La3d edge are a common feature of lanthanides that has been previously reported,40 the shift in the second peak of both doublets suggests a valence change of La. We hypothesize that this shift is mostly related to the change in crystal structure and composition of La2O3 from hexagonal ahLa2O3 to being integrated with ZrO2. Further, Table 2 suggests F-La2O3 has a higher O:(Zr+La) ratio than the stoichiometric 1.5, suggesting that whereas XRD indicates that the bulk of the FLa2O3 is ah-La2O3, the surface of F-La2O3 might be hydrated and have a combination of La(OH)3 as well as La(CO3)OH with O:(Zr+La) ratios of 3 and 4, respectively. Table 3. Measurements of surface and bulk atomic ratios using ICP-OES, EDS, and XPS.

Catalyst La2O3

Catalyst Name F-La2O3

Conditions As synthesized

ICP-OESDerived N.A.

EDS-Derived N.A.

XPS Derived N.A.

O: (Zr+La) Atomic Ratio 4.26 ± 0.22

ZrO2

F-ZrO2

As synthesized

N.A.

N.A.

N.A.

2.33 ± 0.05

F-0.10LaxZryOz

As synthesized

0.10 ± 0.02

0.092 ± 0.02

0.094 ± 0.000

2.49 ± 0.07

C-0.10LaxZryOz

After exposure to acetic acid and H2O

0.095 ± 0.00

0.090 ± 0.02

0.070 ± 0.005

2.41 ± 0.03

La: Zr Atomic Ratio

0.10LaxZryO

3.3. TPD-DRIFTS of ZrO2 and 0.10LaxZryOz catalysts DRIFTS experiments were conducted to investigate the surface populations of adsorbed acetate species with and without exogenous H2O present. DRIFTS experiments suggest that the 18


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surface coverage of acetic acid in the presence of H2O is higher for the LaxZryOz compared to ZrO2, which likely explains at least in part the higher catalytic activity observed with LaxZryOz. Wang and Iglesia recently studied the vapor phase ketonization of carboxylic acids over m-ZrO2 and t-ZrO2, and reported similar ketonization rates over both materials.17 Even though those experiments were carried in the absence of exogenous H2O, we speculate that the difference in catalytic activity observed in this study when comparing ZrO2 and LaxZryOz cannot be solely attributed to the differences in crystal structure and the presence of exogenous H2O is likely playing an inhibitory role in the form of competitive adsorption. As discussed in the previous section, the catalyst restructured after exposure to condensed hydrothermal reaction conditions so we concluded that it wouldn’t be representative of reaction conditions to perform the DRIFTS experiments on freshly synthesized (F) catalysts. Catalysts exposed to reaction conditions (C) restructure into their final hydrothermally stable form, however, it might have products and reagents absorbed to the surface which could interfere with the DRIFTS signal. Instead, we used 0.10LaxZryOz catalyst treated with hydrothermal H2O only (H) for the DRIFTS study as it has the same XRD and XPS pattern as C-0.10LaxZryOz. Hasan et al. previously studied the absorption of acetic acid on metal oxides and showed that the absorption features of importance occurred in the range of 1000 to 2000 cm-1.41 Therefore, we focused on this spectral region for this study. The DRIFTS spectra of H-ZrO2 and H0.10LaxZryOz are shown in Figure 10 in the presence and absence of acetic acid, solid and dashed lines respectively. After exposure to acetic acid (solid line) at 303 K, the surface of H-ZrO2 shows strong absorption signals at 1420, 1460, 1485, 1542, and 1560, and weak signals at 1320 and 1720 cm-1 (Figure 10A). All the signals are distinct from the bare ZrO2 surface (dashed line) and consistent with absorbed acetate species on metal oxides. 41 As reported by Hasan et al.,41-42 the absorptions at 1542-1560 and 1460-1485 cm−1 are assignable to antisymmetric and symmetric νCOO− vibrations of surface acetate species respectively, and those at 1420 and 1320 cm−1 can be 19


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assigned to δCH3 vibrations. The weak absorptions at 1760–1720, and 1248 cm−1 could be assigned to νC=O and δOH vibrations of hydrogen bonded acetic acid molecules in a monodentate configuration. As previously shown by Hasan et al.

41-42

the presence of surface

bidentate acetate species can be explained by the fact that ∆νCOO− (the difference between symmetric and antisymmetric νCOO− vibrations, 100 cm−1) is less than 160 cm−1. Therefore, the observed ∆νCOO− (80-100 cm-1) in Fig. 10A supports the presence of adsorbed bidentate acetate species, which is consistent with several computational studies suggesting bidentate adsorbed species are a key surface reaction intermediate.13,

17, 39, 43-45

During the TPD experiment, the

signals associated with acetic acid absorbed in monodentate and bidentate configurations disappear as temperature is elevated, suggesting desorption or reaction. For example, weak signals at 1248, 1380, 1720, and 1760 cm-1, as well as, the strong signals at 1380, 1420, and 1485 cm-1 disappeared. Interestingly, the strong signals at 1460 and 1542-1560 cm-1 remained. Whereas these signals could be related to the formation of surface acetone as reported by Zaki et al.42, the DRIFTS of acetone shown in Figure S1 suggest that they are not as the acetone features observed at 1215, 1370, and 1738 cm-1 present at 303 K disappear by 473 K. Therefore, we speculate that the 1460 and 1542-1560 cm-1 signals observed during the TPD-DRIFTS of acetic acid at 573 K indicate the presence of tightly bound unreactive bidentate acetate species on the ZrO2 surface, as previously suggested by Wang and Iglesia,17and are different from the bidentate acetate species postulated earlier as surface reactive intermediates. If the H-ZrO2 catalyst is exposed to 10 wt% acetic acid accompanied by H2O, the absorption signals associated with absorbed acetic acid greatly decreased even at 303 K (Figure 10B). This becomes more apparent when comparing the absorption signal of the catalyst surface exposed to 10 wt% acetic acid in H2O (solid line) to H2O only (dashed line) during the TPD experiment, Figure 10 B, as the signal of the surface exposed to 10 wt% acetic acid in H2O and H2O only are nearly identical by 473 K. These results suggest that the presence of H2O greatly decreases the surface coverage of acetic acid on H-ZrO2. 20


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When H-0.10LaxZryOz catalyst is exposed to acetic acid (Figure 10C), we observed evidences of absorption of surface acetate species similar to that of H-ZrO2 (Figure 10A). For example, the DRIFTS spectra shows strong acetic acid absorption signals at 1400, 1420, 1440, 1460, 1560, and 1573, and weak signals at 1278, 1337, and 1700 cm-1 signals. Interestingly, when comparing Figures 10A and C, H0.10LaxZryOz gives a greater signal at 1400 and 1420 cm-1 compared to H-ZrO2. Furthermore, ∆νCOO− is ≈120 cm−1 for H-0.10LaxZryOz compared to ≈100 cm−1 for H-ZrO2, which could indicate that the acetate species are bound slightly differently to the Zr atoms. We speculate that this difference in ∆νCOO− could be related to fact that H0.10LaxZryOz and H-ZrO2, La-stabilized t-ZrO2 and m-ZrO2 respectively, have different lattice parameters of the unit cell46 and active plane as evidenced by the XRD patterns showed in Figure 4. During the TPD experiment, the weak signals at 1278 and 1700 cm-1, and the strong signals at 1400 and 1420 cm-1 decreased as a function of temperature, however, the strong signals at 1440, 1460, 1560, and 1573 cm-1 remained similar, which could also be associated with unreactive surface acetate species. Figure S1 shows that the signals at 1440, 1460, 1560, and 1573 cm-1 are not associated with acetone, as the TPD-DRIFTS of acetone on H-0.1-LaxZryOz does not show any acetone features at temperatures above 473 K. When exposing H-0.10LaxZryOz to 10 wt% acetic acid in H2O in Figure 10D, it is seen that unlike H-ZrO2, there is still absorption of acetic acid in the surface as evidenced by the presence of the strong signals at 1400, 1420, 1440, 1560, and 1573 cm-1. However, the weak signals at 1278 and 1700 cm-1 have disappeared. The presence of surface acetate species becomes more obvious when comparing the DRIFTS profile of the surface exposed to H2O absorption (dashed line) with the surface exposed to 10 wt% acetic acid solution in H2O (solid line). This result suggests that 0.10LaxZryOz has a higher surface coverage of acetic acid with respect to ZrO2 in the presence of H2O, which may be responsible for the enhanced reaction rate observed on 0.10LaxZryOz compared to ZrO2. As the temperature increased during the TPD experiment, the strong signals at 1420 cm-1 slightly decreased, but the signals at 1400, 1440, 1560 and 1573 cm-1 21


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remained similar, indicating the presence of unreactive surface acetate species. Interestingly, whereas the signal at 1460 cm-1 is lower with respect to 1440 cm-1 in the presence of H2O (Figure 10D), they have very similar intensity in the absence of H2O (Figure 10C). This result could suggest that the presence of H2O might also inhibit the surface coverage of a specific surface species.

4. Conclusions In conclusion, our results show that a LaxZryOz mixed-metal oxide catalyst exhibits superior catalytic activity for the ketonization of acetic acid in the condensed aqueous phase at 568 K for more than 144 h on stream. Furthermore, the LaxZryOz catalyst morphology and activity are stable in the acidic condensed hydrothermal reaction conditions employed in this study after a brief transition period experienced at the start of the reaction. In contrast, La2O3 and ZrO2, were significantly less active and quickly lost activity under identical reaction conditions. To the best of our knowledge, this is the first report of a mixed-metal oxide catalyst with sustained ketonization activity over hundreds-of-hours employed in condensed hydrothermal H2O. Catalyst characterization using XRD, SEM, STEM, and XPS revealed that the LaxZryOz crystallized under hydrothermal reaction conditions in the presence of H2O within 24 h of being exposed to hot condensed water. The stable LaxZryOz phase is isomorphic with tZrO2, which is consistent with previous literature reports of rare-earth stabilized zirconias. DRIFTS measurements suggest that the higher activity observed on LaxZryOz catalyst is due to the higher surface coverage of acetic acid compared to the less active ZrO2 catalyst, which likely explains the higher reaction rates observed on LaxZryOz catalysts. The stability exhibited by this catalyst suggests that it may be suitable as a catalyst or catalyst support in other reaction systems which would benefit from a material that is stable under aggressive hydrothermal conditions.

5. Acknowledgement 22


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Pacific Northwest National Laboratory (PNNL) is operated by Battelle Memorial Institute for the U.S. Department of Energy (DOE) under Contract No. DE-AC05-76RL01830. This work was supported by the DOE Office of Energy Efficiency and Renewable Energy through the Bioenergy Technologies Office. A portion of the research was performed using the Environmental Molecular Sciences Laboratory, a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research. We acknowledge guidance with catalyst synthesis from John Frye at the PNNL. We also gratefully acknowledge the help of Nathan Canfield, Libor Kovarik, and Mark Engelhard at the PNNL for their help with the catalyst characterization.

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI:

•

Additional information regarding the reactor sampling protocol.

•

Additional information regarding the calculations of mass transfer limitations.

•

TPD-DRIFT of H-ZrO2 and H-0.10LaxZryOz during the absorption of pure acetone/N2 and 5 wt% acetone in H2O/N2 during the TPD experiment.

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Figures

Figure 1. Schematic of reaction system.

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Figure 2. Specific acetone production rate (A) and normalize acetone production rate (B) under condensed hydrothermal conditions (10 wt% acetic acid in H2O, 9800 kPa, and 568 K) as a function of time on stream where represents La2O3, 0.17LaxZryOz,

represents ZrO2,

represents 0.035LaxZryOz,

represents 0.20LaxZryOz, and

represents 0.10LaxZryOz,

represents

represents 0.23LaxZryOz. The lines are only meant to help the

reader navigate through the results and are not regressions.

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Figure 3. X-ray diffraction patterns of all catalysts after synthesis (F).

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Figure 4. XRD patterns of A) ZrO2, B) 0.10LaxZryOz, and C) La2O3 after exposure to different reaction conditions where a sample denoted with an (F) represents after synthesis, (H) represents after exposure to condensed hydrothermal reaction conditions with only H2O, and (C) represents exposure to condensed hydrothermal reaction conditions with 10 wt% acetic acid in H2O. The hydrothermal reaction conditions were 9800 kPa and 568 K. represents m-ZrO2,

represents t-ZrO2,

represents h- La(CO3)OH, and

represents h-La(OH)3.

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Figure 5. SEM images of F (A) and C (B) 0.10LaxZryOz used for the ketonization of acetic acid under condensed hydrothermal reaction conditions (10 wt% acetic acid in H2O, 9800 kPa, and 568 K)

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A

B

C

D

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Figure 6. Elemental mapping results of C-0.10LaxZryOz used for ketonization of acetic acid under condensed hydrothermal reaction conditions (10 wt% acetic acid in H2O, 9800 kPa, and 568 K). A) represents the crosssectional area of the catalyst particle imaged with SEM, whereas B), C), and D) represent the elemental maps of O, La, and Zr respectively.

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Figure 7. STEM images of 0.10LaxZryOz catalysts A) before and B) after exposure to condensed hydrothermal reaction conditions (10 wt% acetic acid in H2O, 9800 kPa, and 568 K)

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Figure 8. XPS results of the of Zr 3d region of La2O3, ZrO2, and 0.10 LaxZryOz catalysts. The catalysts were studied after different conditions where: F represents after synthesis, H represents after exposure to condensed hydrothermal H2O, and C represents after exposure to 10% acetic acid solution under condensed hydrothermal reaction conditions. The hydrothermal reaction conditions were 9800 kPa and 568 K.

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Figure 9. XPS results of La 3d region of La2O3, ZrO2, and 0.10 LaxZryOz catalysts. The catalysts were studied after different conditions where: F represents after synthesis, H represents after exposure to condensed hydrothermal H2O, and C represents after exposure to 10% acetic acid solution under condensed hydrothermal reaction conditions. The hydrothermal reaction conditions were 9800 kPa and 568 K.

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Figure 10. TPD-DRIFTS spectra of H-ZrO2 and H-0.10LaxZryOz during the absorption of acetic acid /N2 and 10 wt% acetic acid in H2O/N2 during the TPD experiment. A) represents the TPD-DRIFT spectra of H-ZrO2 during the absorption of pure acetic acid/N2, B) represents the TPD-DRIFT spectra of H-ZrO2 during the absorption of 10 wt% acetic acid in H2O/N2, C) represents the TPD-DRIFT spectra of H-0.10LaxZryOz during the absorption of pure acetic acid/N2, D) represents the TPD-DRIFT spectra of H-0.10LaxZryOz during the absorption of 10 wt% acetic acid in H2O/N2. Dashed lines represent the TPD-DRIFTS spectra of the catalyst surface under only N2 or H2O/N2.

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Enhanced Hydrothermal Stability and Catalytic ... - ACS Publications

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