Carbon Promoted ZrO2 Catalysts for Aqueous-Phase Ketonization of

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Carbon Promoted ZrO2 Catalysts for Aqueous-Phase Ketonization of Acetic Acid Kejing Wu,† Mingde Yang,† Weihua Pu,† Yulong Wu,*,†,‡ Yanchun Shi,† and Hu-sheng Hu† †

Institute of Nuclear and New Energy Technology and ‡Beijing Key Laboratory of Fine Ceramics, Tsinghua University, No. 30 Shuangqing Road, Haidian District, Beijing 100084, PR China

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S Supporting Information *

ABSTRACT: The present study reports aqueous-phase ketonization of acetic acid over different ZrO2/C catalysts and promotion of carbon in ketonization activity. Highly carbonized ZrO2 catalysts are synthesized via a sol−gel and Zrbased metal−organic framework carbonization method, which possess high acetone production of 62.09 and 34.13 mmol/ gZrO2, respectively. ZrO2 catalysts with little carbon are synthesized via hydrothermal precipitation and a hydrolysis method, which possess acetone production of 13.54 and 15.51 mmol/gZrO2, respectively. ZrO2 supported on carbon nanotubes (CNTs) exhibits increased acetone production with high CNT content but lower activity than carbonized ZrO2. Carbon release, accompanied by crystal phase transformation, leads to activity loss. Carbon species in carbonized catalysts significantly reduce crystallite size of t-ZrO2 and enhance surface properties, resulting in high activity. Small particle size also benefits aqueous-phase ketonization. Larger nanoparticles of supported ZrO2cover on CNT supports, and carbon and ZrO2 show much weaker interaction, resulting in lower activity than carbonized catalysts. For carbonized catalysts, acetic acid is enriched on the catalyst surface and water adsorption is weakened, enhancing ketonization reaction over active ZrO2 species. KEYWORDS: Carbon, Zirconia, Ketonization, Aqueous phase, Acetic acid

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INTRODUCTION Environmental concerns and energy supply insecurity caused by excessive dependence on fossil fuels has encouraged researchers seeking renewable and sustainable energies.1−3 As one of the most promising energy resources, algae possess several advantages, such as high growth rate, high photosynthesis efficiency, and no competition with agricultural lands.4,5 To utilize all organic components rather than single lipids,6 thermochemical conversion, especially hydrothermal liquefaction, is widely applied to efficiently produce bio-oils.7,8 Aqueous product is always accompanied by bio-oil because of the high moisture content of algae and other biomass,9 and few investigations are conducted on this aqueous product.10 Aqueous product is rich in organic species,11 which account for 35% of the total energy in original biomass.12 In algal aqueous product, nearly half of the organic species are acids, typically acetic acid.13 Meanwhile, acetic acid is also a major component (30.8−53%) in aqueous product converted from lignocellulose and crop biomass.11,14−16 Similarly, acetic acid is abundant in aqueous product converted via pyrolysis.9,17 So far, utilization of aqueous product is limited; for example, it is recovered as a cultivation nutrient18,19 or reformed/gasified to produce combustible gas.20,21 These utilization methods possess disadvantages such as low conversion efficiency and critical reaction conditions. Therefore, additional methods © 2017 American Chemical Society

should be adapted for conversion of these biobased organics, especially acetic acid, to significantly improve efficiency of using algae or other biomass as energy resources.22 Ketonization can suitably convert biobased acids into ketones. Subsequently, aldol condensation and hydrogenation can be used to obtain liquid alkanes from the obtained ketones.22 According to Huber’s results,3 aldol condensation and hydrogenation can be efficiently realized under mild conditions with reaction temperatures lower than 265 °C. Thus, ketonization is important for the acetic acid conversion pathway of ketonization−aldol-hydrogenation. To date, most ketonization reactions are performed in gas-phase systems.23−28 However, gas-phase ketonization is a poor choice for acetic acid in aqueous product because of unavoidable water vaporization, which significantly increases energy consumption (see the Supporting Information). Recently, condensed ketonization in toluene is realized at 150−300 °C.29−32 However, use of organic solvents will increase recovery costs and environmental concerns, and acetic acid extraction from water is also difficult and expensive. Consequently, aqueous-phase ketonization is a promising option to overcome the above-mentioned disadvanReceived: January 21, 2017 Revised: March 5, 2017 Published: March 8, 2017 3509

DOI: 10.1021/acssuschemeng.7b00226 ACS Sustainable Chem. Eng. 2017, 5, 3509−3516

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Figure 1. XRD patterns of differently synthesized catalysts (a), CNT supported ZrO2 (b), and ZrO2-CA-600C oxidized at different temperatures in air (c), where None means no oxidization: (◊) tetragonal ZrO2, (●) monoclinic ZrO2, and (∇) graphite structure of CNT.

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tages, because water is a green solvent and this method saves much energy when converting acid compounds directly in aqueous phase.17,22,23 According to the literature,23,24,29 metal oxides with high lattice energy, for example ZrO2, TiO2, and CeO2, are active and stable for gas- or organic-phase ketonization. Density functional theory (DFT) investigation confirms efficient surface catalysis of ZrO2 and TiO2 crystalline to convert acetic acid into acetone.33 For aqueous-phase ketonization, Pham et al.17 reports an Ru/TiO2/C catalyst system and concludes that water will compete adsorbing on catalysts and cause deactivation. Thus, ketonization in aqueous phase is relatively different from that in gas or organic phase. However, other metal oxides, particularly ZrO2, are seldom investigated in aqueous phase. Hence, this study investigates aqueous ketonization over different ZrO2 catalysts. Furthermore, Ru/TiO2/C catalyst achieves higher conversion (54.2%) than Ru/TiO2 catalyst (32.9%), indicating that carbon support benefits improvement of activity and stability.17 Carbon materials are widely used to adsorb organic acid (such as carboxylic acids, 2,4-dichlorophenoxy acetic acid, phenoxyalkanoic acid, and phenol) in aqueous phase;34−38 for acetic acid, 71% adsorption efficiency can be achieved over activated carbon modified with NaOH,34 indicating significant enrichment of acetic acid on carbon materials. Therefore, carbon materials possess potential to promote ketonization of acetic acid in aqueous phase. In this study, comparison of ZrO2 catalysts with and without carbon species is conducted, and differences of carbon species, such as carbonized carbon and carbon support, are also investigated. The present work investigates ketonization in aqueous phase over different ZrO2 based catalysts and discusses promotion of carbon materials in aqueous-phase ketonization activity. For these purposes, different ZrO2 catalysts with varying carbon components are synthesized and subsequently used for ketonization experiments. Various characterization methods are applied to analyze detailed information on ZrO2 and carbon species; such methods includs X-ray diffraction (XRD), thermogravimetric analysis (TGA), Raman, X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption of ammonia (NH3-TPD), scanning electron microscopy (SEM), and high-resolution transmission microscopy (HRTEM).

EXPERIMENTAL SECTION

Catalyst Preparation. Different ZrO2 catalysts are prepared through four methods, namely hydrothermal precipitation, hydrolysis of organic zirconium, carbonization of Zr-based MOF materials, and the sol−gel method with citric acid. Hydrothermal Precipitation. A 16.1 g portion of Zr(NO3)4·5H2O is dissolved in 150 mL deionized water, and subsequently, the solution is sealed into a 180 mL Teflon lining, keeping at 180 °C for 24 h. White precipitate is observed, followed by directly dried in air at 150 °C. Then, it is calcined at 600 °C under 100 mL/min N2 flow. The final white powder is marked as ZrO2-HT-600C. Hydrolysis of Organic Zirconium. A total of 30 mmol zirconium npropoxide is dissolved in 240 mmol n-propanol. Subsequently, 240 mmol deionized water is rapidly added into the mixture with vigorous stirring for 4 h at room temperature. The obtained white mixture is aged for 24 h, dried in air, and calcined at 600 °C under 100 mL/min N2 flow for 4 h. The final white powder is marked as ZrO2-Hy-600C. For ZrO2 supported on carbon nanotube (CNT), a certain amount of zirconium n-propoxide is added into 10 mL ethanol containing 0.9 g CNT with vigorous stirring. The mixture is slowly hydrolyzed in atmosphere for 24 h. The powder is dried at 105 °C in air and calcined at 600 °C under 100 mL/min N2 flow. The final catalyst is marked as 70%ZrO2-Hy-CNT-600C and 30%ZrO2-Hy-CNT-600C, where 70% and 30% denote weight percentages of ZrO2 in catalysts. Carbonization of Zr-Based MOF Materials. A total of 7.5 mmol ZrCl4 and 7.5 mmol 1,4-benzenedicarboxylic acid are dissolved in 150 mL N,N′-dimethylformamide (DMF) at room temperature under ultrasonic dispersion. The solution is sealed in a 180 mL Teflon lining and kept at 130 °C for 24 h. The obtained white solid is centrifuged, and washed thrice with ethanol. Then, the solid is dried at 60 °C and activated at 150 °C for 12 h. The obtained MOF material is characterized as typical UiO-66 (Figure S1). Finally, the white powder is carbonized at 600 °C in 100 mL/min N2 for 4 h, which is marked as ZrO2-UiO-600C. In addition, a mixture of DMF and carboxylic acid, formic acid (FA), or acetic acid (AA) is used as solvent. The moles of acid used is 100 times that of ZrCl4, and DMF is added to maintain a total solvent volume of 150 mL. The resulting catalyst is marked as ZrO2-UiO-FA-600C and ZrO2-UiO-AA-600C along with carboxylic acids, respectively. Sol−Gel Method with Citric Acid. A total of 40 mmol Zr(NO3)4· 5H2O and 160 mmol citric acid are dissolved in deionized water. The resulting solution is then vaporized at 90 °C under vigorous stirring to obtain a viscous liquid, which is atmosphere dried at 105 °C for 24 h. A porous and foamlike solid is obtained and further carbonized in 100 mL/min N2 flow at 600 °C for 4 h. The final dark solid, marked as ZrO2-CA-600C, is ground and sieved to 200 mesh. ZrO2-CA-600C catalysts are further oxidized at different temperature in 100 mL/min air for 4 h, and the obtained catalyst is marked as ZrO2-CA-600CtemperatureAir. For example, ZrO2-CA-600C catalyst oxidized at 150 °C in air is marked as ZrO2-CA-600C-150Air. 3510

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Figure 2. TGA analysis of differently synthesized catalysts (a) and ZrO2-CA-600C oxidized at different temperatures in air (b), where None means no oxidization. Raman shift of ZrO2-CA-600C oxidized at different temperatures in air (c): (◊) tetragonal ZrO2, (●) monoclinic ZrO2. D and G refer to graphite carbon.

Figure 3. XPS results of C 1s (a), O 1s (b), and Zr 3d (c) for different ZrO2 catalysts. Peaks are fitted to obtain minimum sum of squared residuals. Aqueous-Phase Ketonization Experiments. Acetic acid is used as model reactant, and a 50 mL steel batch reactor is used for ketonization experiments. Typically, 0.3 g catalyst and 30 mL aqueous solution of 2 mol/L acetic acid are added to the reactor. Residual air is purged by N2, and the reactor is sealed without initial pressurization and stirring. The reaction is performed at 340 °C for 12 h. At 340 °C, the saturated water and gas density is estimated to be 559 and 52 kg/ m3, and water volume should be 54 mL, which is larger than total reactor volume. Thus, the water is maintained as condensed phase with slight vapor phase, and the reaction occurs in liquid water. Under different stirring and pressure conditions, comparison of reactions indicates that no external diffusion limitation is observed in the experiments (Table S1). After cooling down to room temperature, liquid product is filtered for analysis, while the remaining catalysts are collected, washed with water, and dried at 105 °C. These recovered catalysts weigh over 90% of what the original catalysts did (0.3 g). Stability experiment uses the total recovered catalysts and is conducted under a similar ketonization condition. In the present study, reaction temperature is significantly higher than that in Pham’s research. However, this ketonization temperature is similar to reaction temperature of algae hydrothermal liquefaction (300−400 °C). Liquid product is analyzed by Agilent 6820 gas-phase chromatography using external standard (see the Supporting Information).

possesses a mixed t- and m-ZrO2 phase, of which the former is highly crystallized. ZrO2 supported on CNT shows similar crystal phase with ZrO2-Hy-600C, as shown in Figure 1b. A significant graphite phase (ICDD PDF No. 41-1487) of CNT is observed, and a higher amount of CNT enhances t-ZrO2 crystallization. With increasing calcination temperature in air from 150 to 450 °C (Figure 1c), tetragonal peaks of ZrO2-CA600C catalysts are strengthened and sharpened. Weak m-ZrO2 peaks are accompanied by highly crystallized t-ZrO2. Further increase in calcination temperature results in transformation of t-ZrO2 to m-ZrO2.40 Carbon species in catalysts are identified through TGA and Raman shift. As shown in Figure 2a and b, significant weight loss is observed for ZrO2-CA-600C, ZrO2-CA-600C-150Air, and ZrO2-UiO-600C, and slight weight loss occurs for ZrO2CA-600C catalysts oxidized at high temperatures (300−600 °C). Raman shift in Figure 2c shows notable graphite carbon peaks, namely G (in-plane vibration of sp2) and D (defects and disorder) peaks;41,42 these peaks disappear when catalysts are oxidized at high temperature. Therefore, a significant amount of graphite carbon exists in ZrO2-UiO-600C and ZrO2-CA600C,41 while ZrO2-Hy-600C and ZrO2-HT-600C possess little carbon species because of differences in synthesis methods. These carbon species are highly correlated in improvement of surface properties, such as acidity and basicity (Figure S2). Additionally, when ZrO2-CA-600C is oxidized over 450 °C, t- and m-ZrO2 peaks are observed,40,43 and the Raman shifts for ZrO2-CA-600C-450Air and ZrO2-CA-600C600Air indicate significant crystal transformation from t-ZrO2 to m-ZrO2, which is consistent with XRD results.

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RESULTS AND DISCUSSION Catalyst Characterization. Different characterization methods are applied to study in detail synthesized catalysts (see the Supporting Information). As shown in Figure 1a, only poorly crystallized tetragonal (t-) ZrO2 (ICDD PDF No. 501089) exists for ZrO2-CA-600C and ZrO2-UiO-600C, whereas single poorly crystallized monoclinic (m-) ZrO2 (ICDD PDF No. 37-1484) exists for ZrO2-HT-600C. ZrO2-Hy-600C 3511

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Figure 4. TEM and HRTEM of (a) ZrO2-CA-600C, (b) ZrO2-uIo-600C, and (c) 30%ZrO2-Hy-CNT-600C.

ZrO2-UiO-FA-600C, and ZrO2-UiO-AA-600C (Figure S5), morphological structure relies on cosolvent used with DMF. Different from ZrO2-UiO-600C with accumulated particles measuring 100 nm (Figure S6), ZrO2-UiO-FA-600C possesses nanosphere structure of about 50−100 nm,52 whereas ZrO2UiO-AA-600C is a typical octahedron measuring 400 nm.53 All MOF materials remain in their original shape after carbonization in N2 (Figure S7). TEM and HRTEM images of catalysts with high carbon content are shown in Figure 4. At scope of 50 nm for ZrO and ZrO2-UiO-600C (Figure 4 a1 and b1), difficulty arises from identifying ZrO2 and carbon species. Energy dispersive spectroscopy (EDS) analysis of ZrO2-CA-600C hardly exhibits differences in Zr and C elements distribution (Figure S8). However, ZrO2 nanoparticles in 30%ZrO2-Hy-CNT catalysts are clearly observed to almost completely cover on CNT support (Figure 4 c1). This difference indicates that carbonization promotes mixing of ZrO2 and carbon species, whereas supported ZrO2 particles only cover carbon surface. Significant lattice fringe of t-ZrO2 is observed for ZrO2-CA-600C with small ZrO2 crystallites less than 5 nm (Figure 4 a2). This small ZrO2 crystallites result in poorly crystallized t-ZrO2, as shown in Figure 1a. The space between crystallites comprise carbon species, as graphite carbon can be observed in Raman shifts (Figure 2c). Similar ZrO2 crystallites exist on ZrO2-UiO-600C catalysts (Figure 4 b2), but particle size is larger than that of ZrO2-CA-600C. Although lattice fringe is less remarkable because of the interference of CNT (Figure 4 c2), ZrO2 nanoparticles on 30%ZrO2-Hy-CNT are larger than ZrO2-CA and ZrO2-MOF. Crystallite size calculated from XRD results confirm the following size order: ZrO2-CA-600C < ZrO2-UiO600C < 30%ZrO2-Hy-CNT-600C (Table S2). At edges of ZrO2 crystallites, the C−Zr bond exists as ZrO2 is surrounded by graphite carbon, and smaller ZrO2 crystallite size results in more C−Zr bonds. Therefore, it can be assumed that ZrO2-CA-600C contains more C−Zr bond than ZrO2-UiO600C. This assumption explains the existence of C−Zr peaks in

Carbon species are further analyzed through XPS to identify different carbon atoms. As shown in Figure 3a, significant C 1s peaks are observed for ZrO2-UiO-600C and ZrO2-CA-600C at 288.6, 286.5, 284.4, and 282.9 eV; these values are attributed to CO, C−O, graphite, and C−Zr, respectively.39,41,44 The presence of CO and C−O species may be due to −COOH groups in 1,4-benzenedicarboxylic acid and citric acid. Graphite is the main carbon species common in TGA and Raman results. Despite the absence of ZrC species in XRD or Raman results, peaks at 282.9 eV are assigned to C−Zr species, because most C 1s lines of metal-C are lower than 284 eV45−47 and metal-C species may exist at the edge between ZrO2 and graphite. These C−Zr species will be further discussed in HRTEM results. By contrast, only hydrocarbon is significantly observed at 284.6 eV for ZrO2-HT-600C and ZrO2-Hy-600C, which is attributed to adsorbed organics on catalyst surface. For O 1s in Figure 3b, two peaks near 531.8 and 530.2 eV represent typical oxygen atoms in molecules (Oβ) and surface lattice (Oα), respectively.48 Oβ/Oα ratio of ZrO2-CA-600C and ZrO2-UiO-600C is significantly higher than that of ZrO2-HT-600C and ZrO2-Hy600C, confirming high CO and C−O peaks in Figure 3a. Zr 3d in Figure 3c shows well-known ZrO2 peaks near 184.4 and 182.0 eV for 3d3/2 and 3d5/2, respectively.49 The presence of C−Zr species is confirmed by slightly lower Zr 3d5/2 (182.06 eV) for ZrO2-CA-600C than that (182.15 eV) for ZrO2-UiO600C because binding energy of ZrC is lower than that of ZrO2.50 Binding energy shifts for ZrO2-Hy may result from the differences in XRD results. SEM results indicate that ZrO2-HT-600C, ZrO2-Hy-600C, and ZrO2-UiO-600C exhibit irregular bulk (Figure S3) containing much smaller particles (Figure S4), whereas ZrO2CA-600C appears as giant particles. According to TEM analysis (Figure S4), a long and folded ZrO2 nanowire with diameter of approximately 10 nm is observed for ZrO2-HT-600C, and ZrO2 particles measuring 100 nm accumulate to form bulk ZrO2UiO-600C.51 Although no significant differences are observed in TGA and XRD characterization among ZrO2-UiO-600C, 3512

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condition, and high reaction rate remains within 12 h reaction. Comparison of different catalysts in Table S4 confirms the high activity of ZrO2-CA-600C at shorter reaction time of 80 min. The above-mentioned observations can be proven by aqueous-phase ketonization of ZrO2-CA-600C catalysts oxidized at different temperatures in air. As shown in Figure 5b, with increase in oxidization temperature, acetone production decreases significantly. From 150 to 300 °C, sharp decrease in is related to significant carbon release, and from 300 to 450 °C, second sharp decrease can be attributed to strong t-ZrO2 crystallization (Figure 1c), because both XRD (Figure 1c) and Raman (Figure 2c) results show evolution of strengthened t-ZrO2 . This result confirms the relationship between ketonization activity and poorly t-ZrO2 phase. Additionally, lower activity of ZrO2-CA-600C-600Air than ZrO2-CA-600C450Air indicates advantage of t-ZrO2 over m-ZrO2 for aqueousphase ketonization, because of the crystal transformation of tto m-ZrO2 (Figures 1c and 2c). Size distribution is directly obtained from TEM (Figure S9), and the size agrees well with SEM results (Figure S4 and S6). As shown in Figure 5c, ZrO2-UiO-FA-600C possesses the smallest particle size, whereas ZrO2-UiO-AA-600C is the largest; acetone production shows that smaller size contributes to higher activity. Crystal phase and carbon content of different ZrO2-UiO-600C catalysts is relatively similar (Figure S5). Thus, size difference primarily causes diversity in activities. For ZrO2 supported on CNT (Figure 5d), with increasing CNT content, acetone yield increases first and then decreases; this phenomenon results from the increase in surface area attributed to CNT support and decrease in ZrO2 weight percentage. However, production on ZrO2 basis increases significantly, indicating improvement of carbon species. Production of 70%ZrO2-Hy-CNT-600C is smaller than ZrO2UiO-600C catalysts with similar carbon content, and 30%ZrO2Hy-CNT-600C is also smaller than ZrO2-CA-600C despite lower carbon content of ZrO2-CA-600C (Figure 2). Therefore, carbon species in carbonized catalysts promote aqueous-phase ketonization activity more significantly than carbon in supported ones. Role of Carbon. Raman shift in Figure 2c indicates proves graphite carbon existence, and XPS results in Figure 3a identify different carbon species, including graphite, C−Zr, C−O, and CO. For ZrO2-CA-600C and ZrO2-UiO-600C, carbon separates ZrO2 into small crystallites (Figure 4a and b) and prevents high crystallization of t-ZrO2 (Figure 1a). Increased lattice distance (Figure 4) confirms carbon effects on crystal behavior. Carbonized ZrO2 possesses higher ketonization activity over supported ZrO2 on CNT. HRTEM in Figure 4 c1 shows that individual ZrO2 nanoparticles are supported on CNT, whereas ZrO2 crystallites are highly dispersed in carbon species for ZrO2-CA-600C and ZrO2-UiO-600C (Figure 4 a and b). Therefore, stronger interaction of carbonized ZrO2 with carbon improves ketonization activity in aqueous phase. Production on ZrO2 basis against carbon content for different catalysts is shown in Figure S10. Higher carbon content is accompanied by high ketonization activity, similar to supported ZrO2 on CNT (Figure 5c), and carbonized catalysts possesses higher activity than supported catalysts. Ketonization stability of ZrO2-CA-600C and 30%ZrO2-HyCNT-600C is shown in Figure 6. Although both catalysts contain large amounts of carbon, activity decrease is quite different. Only 7.05% of original product is lost for ZrO2-CA600C, but 44.99% is lost for 30%ZrO2-Hy-CNT-600C.

C 1s XPS results (Figure 3a). Lattice distance of t-ZrO2 (011) measures 3.07, 3.01, and 2.88 Å for ZrO2-CA-600C, ZrO2-UiO600C, and 30%ZrO2-Hy-CNT-600C, respectively. As highly crystallized ZrO2 nanoparticles of 30%ZrO2-Hy-CNT-600C (Figure 1b) only cover carbon supports, increased lattice distance of ZrO2-CA-600C and ZrO2-UiO-600C can be attributed to carbon atoms embedded in the poorly crystallized t-ZrO2 phase. Ketonization Activity. Aqueous-phase ketonization activity of different catalysts is listed in Table 1. The catalysts with high Table 1. Aqueous-Phase Ketonization of Differently Synthesized Catalysts, 340 °C for 12 h with 30 mL 2 mol/L Acetic Acid As Feedstock and 0.3 g Catalyst catalysts ZrO2HT600C ZrO2Hy600C ZrO2UiO600C ZrO2CA600C

acid conversion (%)

productiona (mmol/ gcat)

productiona (mmol/ gZrO2)

production per surface Zrb (mmol/ μmol surface Zr)

13.27

13.32

13.54

0.03636

18.24

14.99

15.51

0.07634

25.30

23.72

34.13

0.1018

38.56

38.09

62.09

1.609

a

Production means the mole of acetone produced per gram catalyst (gcat) or gram ZrO2 (gZrO2) used. bSurface Zr refers to Zr atoms in surface layer of 1 nm.

carbon content (ZrO2-UiO-600C and ZrO2-CA-600C) possess significantly higher ketonization activity in aqueous phase. Poorly crystallized t-ZrO2 largely influences ketonization activity. Despite poorly crystallized m-ZrO2 in ZrO2-HT600C, the activity remains low and is even lower than that of ZrO2-Hy-600C with highly crystallized t-ZrO2. Thus, t-ZrO2 is more active than m-ZrO2. Although ZrO2-CA-600C possesses quite larger particle size (Figure S4), generally resulting in lower surface areas, ketonization activity of ZrO2-CA-600C is significantly high. These results indicate that carbon content and crystal phase are more important factors affecting aqueousphase ketonization activity. Acetone yield of most catalysts is close to acid conversion (Table S3), indicating high selectivity and mole balance for ketonization. Production values in Table 1 are used to measure mole amounts of acetone produced over normalized catalysts and ZrO2 (mmol/gcat. and mmol/gZrO2, respectively),17 and ketonization activities of catalysts with different carbon contents are compared more efficiently, as carbon species possess very low ketonization activity (1.29% ketone yield over 0.15 g CNT under similar condition). The advantage of ketonization activity over ZrO2-HT-600C and ZrO2-Hy-600C is more significant according to production (mmol/gZrO2) data than original acetone yield (Table S4), because of less amount of ZrO2 species and high carbon content in ZrO2-CA-600C and ZrO2-UiO-600C. In addition, surface Zr atoms are estimated in Table S5 according to the average particle size, and acetone production per surface Zr is calculated in Table 1. Obviously, ZrO2-CA-600C possesses remarkable higher activity for aqueous-phase ketonization, when comparing acetone production per surface Zr. At different reaction times (Figure 5a), ketone yield over ZrO2CA-600C is almost linear with time at the experimental 3513

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Figure 5. Ketonization over ZrO2-CA-600C at different times (a) and ketonization activity of ZrO2-CA-600C oxidized in air (b), different ZrO2 catalysts synthesized through carbonization of Zr-based MOF materials (c), and ZrO2 supported on CNT (d). (b inset) Carbon content from TGA results. (c inset) Particle size distribution measured directly from TEM results.

materials benefit enrichment of acids on catalyst surfaces.17 Carbon materials possess almost no ketonization activity,17 and experimental data shows that at similar conditions, acetone yield of 0.15 g CNT is only 1.29%. Therefore, ketonization activity is attributed to ZrO2 species, and carbon functions as promoter for aqueous-phase ketonization. For carbonized ZrO2, acetic acid is enriched on surface of carbonized catalysts and converted into acetone through ketonization at ZrO2 sites. Meanwhile, competition adsorption of water is weakened because of hydrophobicity advantage. By contrast, acid enrichment is prevented by supported larger-sized ZrO2 nanoparticles covering carbon species, and carbon species only function as supports. Competitive adsorption of water with acetic acid has a negative effect on ketonization.17 Thus, carbonized catalyst possesses much higher ketonization activity in aqueous phase.

Figure 6. Ketonization activity comparison of fresh and reused ZrO2CA-600C and 30%ZrO2-Hy-CNT-600C catalysts.

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CONCLUSION The present work investigates aqueous-phase ketonization of acetic acid over different ZrO2/C catalysts and discusses carbon species to promote activity. Carbon species in carbonized catalysts positively affect formation of poorly crystallized t-ZrO2 and enhance surface properties, resulting in higher activity than other catalysts with little carbon. Both carbon release and crystal phase transformation from t-ZrO2 to m-ZrO2 negatively affect ketonization activity. Size effect of similar carbonized catalysts is observed for ZrO2-MOF catalysts. According to XPS and HRTEM results, t-ZrO2 crystallites for carbonized catalysts

Differences in stability mainly depend on different carbon species, which result in varied surface properties. For example, contact angles of fresh ZrO2-CA-600C and 30%ZrO2-Hy-CNT600C in Figure S11 show relatively different hydrophobicity. Clearly, the surface of ZrO2-CA-600C is more hydrophobic than that of 30%ZrO2-Hy-CNT-600C, which indicates the weaker competition adsorption of water on ZrO2-CA-600C surface. Carbon materials (such as activated carbon) are widely used to adsorb organic acids from aqueous solution,34−38 and these 3514

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ACS Sustainable Chemistry & Engineering

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are highly dispersed in graphite carbon species, and smaller crystallite size enhances ketonization activity. By contrast, larger nanoparticles of supported catalysts cover CNT supports, and carbon and ZrO2 demonstrates weaker interaction, resulting in lower activity than carbonized catalysts. For carbonized catalysts, acetic acid is enriched on catalyst surface, and competition adsorption of water is weakened, benefiting ketonization reaction over active ZrO2 species.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00226. Energy consumption estimation, catalyst characterization methods, detailed calculation of acetone yield and acid conversion, additional tables and figures of catalyst characterization and activity. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 10 89796163 (Y.W.). ORCID

Yulong Wu: 0000-0003-0212-6689 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported financially by National Natural Science Foundation of China (No. 21376140, No. 21576155, and No. 21376136), Program for Changjiang Scholars and Innovative Research Team in University (No. IRT13026), and Research Project of Guangdong Provincial Department of Science and Technology Department (No. 2015B020215004).

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DOI: 10.1021/acssuschemeng.7b00226 ACS Sustainable Chem. Eng. 2017, 5, 3509−3516

Research Article

ACS Sustainable Chemistry & Engineering

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DOI: 10.1021/acssuschemeng.7b00226 ACS Sustainable Chem. Eng. 2017, 5, 3509−3516