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Catalysis Today

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Review

Composition-structure-function correlation of Ca/Zn/AlOx catalysts for theketonization of acetic acid

Huajuan Linga, Zichun Wanga,b,⁎, Leizhi Wanga, Catherine Stampflc, Dan Wangd, Jianfeng Chend,Jun Huanga,⁎

a Laboratory for Catalysis Engineering, School of Chemical and Biomolecular Engineering, The University of Sydney, New South Wales 2006, AustraliabDepartment of Engineering, Macquarie University, Sydney, New South Wales 2109, Australiac School of Physics, The University of Sydney, Sydney, New South Wales 2006, Australiad State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China

A R T I C L E I N F O

Keywords:KetonizationHeterogeneous catalysisComposition-structure-function correlationBio-oil upgradingCarboxylic acids

A B S T R A C T

Ketonization can efficiently convert carboxylic acids into ketones, promising in bio-oil upgrading. For economic-sustainable bio-refining, Ca/Zn/AlOx (CZA) metal oxides with various Ca/Zn/Al ratios have been prepared bythe low-cost and natural abundant metals and are applied in the process here. Mechanistic study on the keto-nization of acetic acid revealed the reaction pathways strongly depends on “composition-structure-function”correlation of the Ca/Zn/AlOx catalysts. The reaction is mainly performed on strong base sites (e.g. CaO andZnO) via thermal decomposition of carboxylates at high temperature (≥375 °C), but depends on acid-base pairs(e.g. amorphous calcium aluminates) significantly at low temperature (≤350 °C). CZA(331) (Ca/Zn/Al= 3/3/1) obtained the highest acetone yield of 97.1% at 425 °C hitherto and retained for over 100 h with simpleregeneration process. Adding major bio-oil model compounds (e.g. phenol) into the reaction mixture has minoreffect on the CZA catalysts. Therefore, current highly active and stable CZA catalysts are promising in boostingthe efficiency and economy of bio-oil upgrading process in future.

1. Introduction

The increasing demand for fossil fuels and environmental issuesdrives the rapid development of renewable biomass-derived fuels thatcan directly replace fossil fuels in the current infrastructure in a greenand economic way [1]. Conversion of lignocellulosic biomass is a majorroute to yield liquid bio-fuels with high yield (up to 80%) [2]. Derivedbio-oil contained furanics, phenolics, carboxylic acids (e.g. 10 wt%acetic acid in bio-oil produced from sawdust), water (30–40wt%) andother oxygenates [3–6]. The obtained bio-oil is instable and corrosive(pH of 2–3), with high viscosity and low heating value (16–19MJ/kg)[1,7]. It can be upgraded through hydrodeoxygenation or zeolite up-grading to acquire high quality biofuel for using as transportation fuels[8–11]. The former consumes a large amount of expensive hydrogen[12], while the latter results in poor hydrocarbon yields and high cokeformation [2], which limit their industrial applications.

Carboxylic acids is one of the most oxygen-abundant group in bio-oils [13], in the forms of acetic acid, butyric acid, formic acid and etc[14]. The ketonization reaction can efficiently convert carboxylic acids

into ketones via CeC coupling, which can significantly increase theenergy density and stability of the bio-oil along with raising the pH[15]. It is a clean and environmental friendly and solvent-free chemicalprocess [1,16,17]. Moreover, ketones are desired building blocks andare readily to be converted into long-chain hydrocarbons via aldolcondensation followed by hydrogenation [1] to increase the yield oftransportation fuels [18]. When coupled with bio-oil upgrading process,ketonization has attract great attention both economically and techni-cally.

Solid catalysts, including metal oxides and mixed metal oxides,hydrotalcites and zeolites, are widely studied on ketonization of car-boxylic acids [1,17]. Metal oxides (e.g. CeO2, TiO2, and ZrO2) is widelyaccepted as the most active catalysts for ketonization [1,19–21]. Theirhigh activity has been attributed to surface acid-base properties, whichallows carboxylic acid adsorbed adjacently for ketonization via ketene[19,22] or β-ketoacid [23,24] based reaction mechanisms. In ketoni-zation of acetic acid (AcOH), the most abundant carboxylic acid com-pound in bio-oils [10,25,26], supported CeO2 catalysts exhibit muchhigher activity (93.1% acetone yield over CeO2/1%K2O/Al2O3) and

https://doi.org/10.1016/j.cattod.2019.01.057Received 30 October 2018; Accepted 25 January 2019

⁎ Corresponding author at: Laboratory for Catalysis Engineering, School of Chemical and Biomolecular Engineering, The University of Sydney, New South Wales2006, Australia.

E-mail addresses: zichun.wang@sydney.edu.au (Z. Wang), jun.huang@sydney.edu.au (J. Huang).

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stability (97% conversion of AcOH for 96 h over CeO2/SiO2) [7] thanthose over hydrotalcites (86.5–89 % yield of acetone) [27,28] andzeolites (100% acetone selectivity with very low AcOH conversion)[29,30]. However, these amphoteric reducible metal oxides are ex-pensive and low abundant in nature. Thus, it is desired to develop novelmetal oxide catalysts with low cost but highly active and stable forketonization reaction.

Solid bases, including CaO, MgO and ZnO [31], are active catalystsfor ketonization of carboxylic acids [7,20,32]. The strong bases withlow lattice energy can transfer carboxylic acids into acetone throughthermal decomposition of carboxylates [33]. These bases supported onSiO2 provide a yield of acetone from acetic acid in a range of 33–59 %,much lower than 97% over CeO2/SiO2 [20]. Alumina is a widely uti-lized support material to promote the dispersion of metal particles. Itcan form metal aluminates in mixed metal oxides, e.g. calcium alumi-nates [34] and zinc aluminates [35,36]. These mixed metal oxides(CaO/Al2O3 and ZnO/Al2O3) are thermally stable after calcination at800 °C [34,37]. Moreover, alumina with strong Lewis acidity can pro-mote the adsorption of carboxylic acid on catalyst surface. In wellmixed metal oxides, it is probably to cooperate or facilitate the reactionwith basic sites (e.g. CaO) in the local structure to improve the catalyticactivity, and thus, promising for ketonization.

In this work, Ca/Zn/AlOx (CZA) composited metal oxides weresynthesized for ketonization of acetic acid. XRD pattern and microscopywere utilized to investigate the morphology of CZA catalysts and con-firm the presence of CaO, ZnO, ZnAl2O4 and amorphous alumina andcalcium aluminates. The surface basicity of these catalysts has beenstudied by TPD-CO2. Combined with the reaction results of ketonizationof acetic acid, two reaction mechanisms based on temperature depen-dence has been proposed. These CZA catalysts exhibit high activity andstability comparable to CeO2/SiO2 that has been reported as the bestcatalyst for this reaction [7,20] and are easily to be regenerated. Fi-nally, the stability of CZA catalysts was further investigated by furfuraland phenol addition to reaction mixture individually, which are themajor compounds in pyrolysis bio-oil [2,25,38].

2. Experimental

2.1. Catalyst preparation

Ca/Zn/AlOx catalysts with various atomic ratios were synthesizedby co-precipitation method. Zn(NO3)2·6H2O, Ca(NO3)2·6H2O, Al(NO3)3·9H2O were purchased from Sigma-Aldrich and used as pre-cursors. Precursors with desired amount of Ca/Zn/Al molar ratios of1:1:1, 2:2:1, 1:1:3, 3:3:1, 1:1:2, 2:1:2, 3:2:3 and 1:2:1, were prepared bydissolving certain amount of metal salts in deionized water. The pre-cursor mixture was precipitated with saturated Na2CO3 solution dropby drop under stirring, with keeping the pH of the suspension within7–9 at 60 °C. After co-precipitation, the resulting suspension was agedunder agitation for an hour and then filtered under vacuum. The filtercake was rinsed with deionized water, and then dried at 80 °C over-night. Finally, the solid products were calcined at 800 °C for 4 h in staticair with a heating rate of 1 °C/ min. The nomenclature of obtained Ca/Zn/AlOx catalysts is defined as CZA(XYZ), where X, Y and Z are themole ratios of Ca, Zn and Al, respectively.

2.2. Catalyst characterization

X-ray diffraction (XRD) patterns of Ca/Zn/AlOx catalysts were col-lected in the range of 10-70° with a SIEMENS D6000 e diffractometerquipped with Cu-Kα radiation (λ=0.154 nm, 35 kV, 40mA).

The specific surface areas of Ca/Zn/AlOx samples were determinedby N2 adsorption/desorption isotherms on an Autosorb IQ-C system. Anamount of 150mg of each sample was degassed at 150 °C for 12 h undervacuum before the measurements and then recorded at 77 K.

The scanning electron microscopy (SEM) coupled with Energy-

dispersive X-ray spectroscopy (EDXs) was used to investigate the sur-face morphology and the element distributions of CZA catalysts. Beforescanning, samples were coated with gold under vacuum. Coating in-tensity and time was 25mA and 150 s, respectively. Then the imageswere recorded on a FESEM, Zeiss Ultra+.

The phases and microstructure of Ca/Zn/AlOx samples were ob-tained by using a transmission electron microscopy (TEM) JEOL 2200FSwith sample, which were mounted on a carbon coated copper grid bydrying a droplet of a suspension of the ground sample in ethanol.

Temperature programmed desorption using CO2 as probe molecule(TPD-CO2) was used to determine the basicity of CZA catalysts understudy. Prior to TPD, sample (50mg) was loaded and degassed at 450 °Cfor 1 h under N2 flow to completely remove molecules adsorbed oncatalyst surface. The sample was saturated with CO2 at 50 °C for 15min,flushed with N2 for 15min, and then desorbed with CO2 (total flow of80ml/min) from 50 to 650 °C with a ramp of 10 °C/min. The desorptiongas were analyzed by a Mass Spectrometer connected to TPD system.

2.3. Ketonization of acetic acid

The catalytic performance of CZA catalysts were tested in a fixed-bed reactor. The catalyst (100mg) was pretreated under nitrogen flowof 30mL/min for 30min at 400 °C. Reactant of 50 wt% aqueous AcOHsolution was pumped into the reactor at a flow rate of 0.4mL/h usingN2 as a carrier gas (10mL/min). The reaction products were condensedin an ice-water trap and collected hourly for GC analysis. The compo-sition of the product mixtures were determined using a gas chromato-graphy equipped with a RTX-5MS capillary column (30m×0.25mm×0.25μm) and coupled with a mass spectrometer(Shimadzu GCMS-QP2010 Ultra), and the conversion of AcOH and theamount of reaction product(s) were quantified by Shimadzu GCequipped with a RTX-5 capillary column (30 m×0.32mm×3μm) andcoupled to a flame ionization detector. The conversion of AcOH (CAcOH)and yield of acetone (Yace) was calculated by:

CAcOH (%)= 100 × [(AcOH)0-(AcOH)] / (AcOH)0 (1)

Yace (%)=200 × (i) / (AcOH)0 (2)

where (AcOH)0 and (AcOH) are corresponding to the molar con-centrations of AcOH before and after the reaction, respectively, and (i)is the molar concentration of the product acetone.

The stability and tolerance of CZA catalysts were tested by addingfurfural or phenol, both are the major compounds in pyrolysis bio-oil.Furfural or phenol (10 wt%) was mixed with acetic acid (40 wt%) andwater (50 wt%) as the reaction feed. All other reaction conditions arethe same as described above. The spent CZA catalyst was recycledthrough in situ calcination in air at 425 °C for 1 h.

3. Results and discussions

3.1. Catalyst characterizations

The XRD patterns of Ca/Zn/AlOx mixed oxides catalysts with var-ious Ca/Zn/Al atomic ratios are displayed in Fig. 1. It shows that theCa/Zn/AlOx catalysts consist of four compounds as ZnO, ZnAl2O4, CaOand Ca(OH)2. No crystalline Al2O3 can be observed in all samples.CZA(121) (Fig. 1h) possessing the largest amount of Zn content exhibitsthe diffraction peaks at 31.5°, 34.2°, 36.0°, 47.2°, 56.3°, 62.5°, 66.2°,67.8°, and 69.0°, corresponding to the ZnO crystal faces of 100, 002,101, 102, 110, 103, 200, 112 and 201, respectively [39,40]. Reducingthe atomic fraction of Zn can lower the intensity of these diffractionpeaks. With the Zn fraction lower than 1/3 (Fig. 1a-d), the diffractionpeaks of ZnO become broad and their intensity significantly decreased.Instead, the diffraction peaks at 31.0°, 37.0°, 44.5°, 56.5° and 59.0°,corresponding to ZnAl2O4 having the crystal faces of 220, 311, 400, 422and 333/511 [41], were strongly enhanced (Fig. 1a–d). Increasing the

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intensity of ZnAl2O4 correlates well with reducing the Zn/Al ratio(CZA(113)>CZA(212)>CZA(112)>CZA(323)>CZA(111)). This iscaused by a higher concentration of Al that can promote the solid re-action between ZnO and alumina to generate ZnAl2O4 particles underhigh calcination temperature (e.g. 800 °C) [37]. The diffraction peaks ofCaO at 32.0°, 37.1°, 53.6°, 63.8° and 67.1°, corresponding to the crystalfaces of 111, 200, 220, 331 and 222 [42,43], were detected withCZA(331) (Fig. 1g) having the largest Ca fraction. When exposed in theair, partial of CaO can be rehydrated to Ca(OH)2, showing diffractionpeaks at 17.7° and 28.0°. No crystalline phase between Ca and Znspecies can be detected, similar as that reported previously that onlypure CaO and ZnO can be observed in CaZnO mixed metal oxides [39].

The particle size and specific surface area of the catalysts are sum-marized in Table 1. The specific surface area of the Ca/Zn/AlOx is in therange of 11.6-62.2 m2/g. When Ca content ≥ 33.3% (CZA(111)), arelative low specific surface area (≤ 36.7m2/g) was obtained. With theCa content ≤25% and the Al content ≥25%, the specific surface areasignificantly enhanced for over 58%, up to 58.0–62.2 m2/g. This hasbeen explained by the strongly increase of CaO particle size at high Cacontent. Clearly, alumina as support materials can significantly improvethe dispersion of Ca and Zn species. Increasing the Al content can result

in smaller particles size on surface, e.g. CZA(113) having the largest Alcontent obtained the smallest particle size of CaO, ZnO and ZnAl2O4

among all samples. CZA(121) obtained the largest surface area, which isprobably due to the formation of different morphology and structure.

The morphology of these mixed metal oxides was investigated byscanning electron microscope (SEM) as shown in Fig. 2. CZA(111)(Fig. 2a) is strongly agglomerated, possibly resulting in the largestparticles size and the lowest specific surface area among CZA catalysts(Table 1). For CZA(221) (Fig. 2b) and CZA (331) (Fig. 2d) with highconcentration of Ca and Zn, the spherical metal particle (CaO and ZnO)are well-dispersed and stabilized on AlOx surface [45] in the range of15–30 nm, which is consistent with those reported in Table 1. Theimages of CZA(113), (112), (212) and (323) (Image c, e, f and g) show acrosslinked surface, which is attributed to alumina, amorphous aluminaor aluminate compounds [46–48] formed at high Al concentration(≥37.5mol%). The large CaO particles (20 nm) and the small round-shaped ZnO particles (10 nm) has been identified [49,50], in line withthose reported in Table 1. The ZnAl2O4 particles, formed by ZnO andalumina at high temperature (e.g. 800 °C) [37], are agglomerated insidethe crosslinked structure of alumina [51], which are hardly to beidentified on surface. In Fig. 2h, CZA(121) with the highest Zn content(50mol%), exhibits a flake-like morphology, similarly to layereddouble hydroxides (LDHs) catalysts [52]. LDHs are often featured withlarge surface areas [53], and possibly, the flake-like structure affords aspecific surface area of 62.15m2/g for CZA (121), which is the largestamong all the CZA catalysts.

The elemental analysis of CZA catalysts were studied by SEM cou-pled with EDX, and the EDX mapping images of CZA(113) andCZA(331) were shown in Fig. 3. CZA(113) with the highest Al contentexhibits a very bright color in Fig. 3a, corresponding to an alumina-richphase. A well dispersion of Ca and Zn species on alumina surface wereobserved, but their relatively low content lead to dark and blurryimages (Fig. 3b and c). The clear images of Ca and Zn species (Fig. 3eand f) were observed with CZA(331) having higher Ca and Zn content,while the Al mapping image is blurred (Fig. 3d) due to the lowest Alcontent. No single crystalline alumina-rich phase can be detected,confirmed the observation in XRD patterns. In both CZA catalysts, theAl and Zn mapping images are very similar, which indicates a co-ex-istence on their distribution. This may be caused by the formation of asolid solution between ZnO and alumina, resulting in ZnAl2O4 underhigh temperature calcination (e.g. 800 °C) [37], well in line with theobservation in XRD and SEM images.

HRTEM images of CZA catalysts are shown in Fig. 4. Two differenttypes of crystals were observed in Fig. 4a, which are assigned to ZnO(18 nm) and CaO (40 nm), respectively, correlated well with XRDanalysis. Similarly in Fig. 4b and c, small crystals with size of 10˜15 nm

Fig. 1. XRD patterns of Ca/Zn/AlOx catalysts. (a) CZA(113), (b) CZA(112), (c)CZA(323), (d) CZA(212), (e) CZA(111), (f) CZA(221), (g) CZA(331) and (h)CZA(121).

Table 1Specific surface area and crystal size of Ca/Zn/AlOx catalysts.

Catalyst Specific surface area (m2/g) Particle size (nm)a

CaO ZnO ZnAl2O4

CZA(111) 11.6 48.2 26.5 27.6CZA(323) 19.1 31.7 16.7 24.3CZA(331) 21.4 28.4 22.8 23.6CZA(221) 23.9 26.3 17.2 17.7CZA(212) 36.6 29.1 12.6 13.3CZA(112) 58.0 20.7 11.0 16.2CZA(113) 61.4 12.3 8.5 7.6CZA(121)b 62.2 – – –

a Calculated from FWHM (full width at half maximum) of the most intensepeaks by Scherrer Equation: Dc = Kλ /(β cos θ), where K is the shape factor, λ isthe wavelength of x-ray, β is the line broadening in radians at FWHM, and θ isthe Bragg angle and Dc is the mean particle size [44].

b The determination of each particle size is difficult due to the strong overlapof these peaks.

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are assigned to ZnO, which is the major component in CZA(113) andCZA(331) by XRD analysis. CZA(331) shows an amorphous structurethat similar to amorphous Al2O3 as reported previously [54], which isdifferent from CZA(111) and CZA(113) with higher Al content. Thelatter two exhibit an amorphous structure of calcium aluminates, whichis stable under calcination at 800 °C and directly converted into cor-responding crystal at T > 883 °C [34]. As shown in Fig. 3b and c, the

image of Ca is obviously blur than that of Zn, which may confirm Caspecies are incorporated into the Al matrix and are hardly to be de-tected compared to Zn. Besides ZnO and CaO, a cross-direction struc-ture with an angel of ca. 40° was observed in CZA (121) (Fig. 4d), whichis assigned to ZnAl2O4 [55] with a size of 15 nm, correlates well withXRD analysis.

Based on XRD and microscopy analysis, the distribution of basic

Fig. 2. SEM results of Ca/Zn/AlOx catalysts. (a) CZA(111), (b) CZA(221), (c) CZA(113), (d) CZA(331), (e) CZA(112), (f) CZA(212), (g) CZA(323) and (h) CZA(121).The CaO and ZnO particles identified are marked in black and red circles in images c and e, respectively (For interpretation of the references to colour in this figurelegend, the reader is referred to the web version of this article).

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metal oxides (CaO and ZnO) on alumina and their mixed oxides withalumina have been studied. These metal oxides are well distributed onalumina surface in amorphous domain at Al content ≥37.5%, but formlarge crystals at higher Ca and/or Zn content. These basic metal centersare potential active sites for base-catalyzed ketonization of acetic acid.Therefore, the distribution and density of these surface basic site wereevaluated using TPD-CO2 (Fig. 5). Both CZA(331) and CZA(121) withhigher metal content exhibit CO2-desorption peaks in the temperatureranges of T<200 °C, 200 °C< T<400 °C, and T>400 °C, corre-sponding the existence of weak, medium and strong basic sites in thesecatalysts [39,56]. CaO and ZnO are metal oxides possessing mainly

strong basicity [31]. The well-dispersed and stabilized CaO and ZnO onalumina surface (e.g. those observed in Fig. 2d) are corresponding tothe peak at T>400 °C [39,56] associated with isolated O2− on surfaceas base sites [57]. The medium to weak base sites are due to aluminaand metal aluminates [57,58], e.g. only peaks at temperatureT<400 °C can be observed with TPD-CO2 of ZnAl2O4 and attributed tosurface Mn+–O2− pair [57], while surface hydroxyl groups may adsorbCO2 at low temperature and act as weak base sites as well [58,59]. Thehigher peak intensity demonstrates that CZA(331) has more base sitesthan CZA(121) at each temperature range. With increasing the Alcontent, the CZA(111) provides the highest density of base sites but

Fig. 3. EDX image of CZA(113) (a–c) and CZA(331) (d–f).

Fig. 4. TEM of Ca/Zn/AlOx catalysts. (a) CZA(111), (b) CZA(113), (c) CZA(331), (d) CZA(121).

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mainly in medium strength (Fig. 5c). Further increasing the Al contentwith lowest (Ca+Zn) content in CZA(113) (Fig. 5d), leads to thelowest density of surface base sites with medium strength among all theCZA catalysts. It is probably caused by a large extent of Ca and Znmixed with alumina to form metal aluminates, e.g. metal componentsare well dispersed on CZA(113) surface (Fig. 1a), providing mainlymoderate basicity.

3.2. Ketonization of acetic acid on Ca/Zn/AlOx

Strong basic metal, e.g. CaO, are active catalysts for base-catalyzedketonization of acetic acid [60]. Based on TPD-CO2, current Ca/Zn/AlOx catalysts exhibit weak to strong basicity, which are promising forketonization of acetic acid. Moreover, the crosslinked structure andwell-dispersed metal oxides were observed by SEM and EDX, withamorphous to crystalline morphology. It might enhance the catalyticactivity and stability of Ca/Zn/AlOx, and thus, tested in ketonization ofacetic acid under various temperature and water loading, with furfuralor phenol as additives.

3.2.1. Effect of reaction temperature and CZA compositionsKetonization of AcOH is often carried out under temperature be-

tween 300 °C and 450 °C [28]. At this range, both catalytic conversionand thermal pyrolysis of acetic acid can be carried out [16]. The pyr-olysis of acetic acid can be performed in two different ways [61]: AcOHcan be 1) decomposed into ethenone at a temperature of 400 °C; or 2)decomposed into carbon dioxide and methane at reaction temperaturefrom 460 to 600 °C. By using sand as a reference catalyst, the pyrolysisof AcOH (2˜3%) is negligible when T ≤ 375 °C, while at 400 °C<T<425 °C, the AcOH conversion enhanced up to 17%. Meanwhile, theselectivity to acetone increased from 18% to 90% with increasing Tfrom 300 °C to 425 °C.

The catalytic conversion of AcOH was performed between 300 and425 °C. The AcOH conversion and the selectivity to acetone over Ca/Zn/AlOx with various Ca/Zn/Al molar ratios and temperature rangewere summarized in Table 2. The conversion of AcOH over CZA cata-lysts was quite low (≤ 44%) at T ≤ 325 °C, which dramatically raisedto 70% with increasing the reaction temperature to 375 °C, and can befurther enhanced to 93–100 % at 425 °C. In the meantime, the se-lectivity to acetone remains 100% at T ≤ 375 °C for all CZA catalysts,but slightly decreased to 97–99 % over CZA(331), CZA(221) andCZA(111) at temperature of 400–425 °C. CZA(121) with the highest Zncontent, exhibited a highly crystalline LDHs-like structure that ismainly in ZnO domain. ZnO is a weaker base than CaO [50], whileZnAl2O4 possesses only weak acidity and negligible basicity, [36,62]and thus, not as active as CZA catalysts having higher Ca content for

ketonization of AcOH.CZA(331) exhibited the highest catalytic activity among all the

catalysts in all temperature range except 300 °C. An AcOH conversion of92% was obtained, much higher than those obtained with other CZAcatalysts (< 55%) at 350 °C. As revealed by TPD-CO2, this correlateswell with the strongest basicity obtained with CZA(331) having thehighest Ca and Zn contents. Similarly, CZA(221) and CZA(111) bothexhibit higher activity than other CZA catalysts having lower Ca and Zncontent at T ≥ 375 °C. Since no by-products anhydride or ethenone canbe detected, the catalytic conversion of AcOH to CO2 and methane [63]corresponds to lowering the selectivity to acetone can be expected athigh temperature (e.g.≥ 400 °C), which is carried out at 460 °C withoutcatalysts [61] and also observed here by using sand to replace CZAcatalysts.

One should be noted, that CZA(323) with similar (Ca+Zn) contentleads to a much lower acetone yield (74–93 %) than that obtained withCZA(111) (84–99 %) at T ≥ 375 °C. CZA(323) is in a large extent ofamorphous domain, which is obviously different from crystallineCZA(111) (see Fig. 1). As discussed in the microscopy investigations(Figs. 3 and 4), calcium aluminates are formed in the amorphous phase,and mainly provides moderate basicity as observed in the TPD−CO2

experiments, while the formation of ZnAl2O4 possesses only negligiblebasicity [36,62]. CZA(323) with more calcium aluminates and ZnAl2O4

is lack of basicity, thus afforded the lower activity than those obtainedwith CZA(111) at T ≥ 375 °C. However, CZA(323) exhibits higher ac-tivity than CZA(111) at T ≤ 350 °C. It was further confirmed byCZA(212), CZA(112) and CZA(113) with lower (Ca+ Zn) content butincreasing amorphous domain, resulting in a higher activity than highercrystalline CZA(221) and (111) at low temperature (≤350 °C). It clearlydemonstrates the yield of acetone from ketonization of AcOH stronglydepends on the catalyst basicity and reaction temperature, probably viatwo different reaction pathways over CZA catalysts.

3.2.2. Reaction mechanismOn alkali and alkali earth oxides, a widely accepted ketonization

mechanism is AcOH can be strongly adsorbed on low lattice energybases to form bulk acetates, followed by thermal treatment to releaseacetone and CO2 in the gas phase [33]. The thermal decomposition ofacetates can be significantly enhanced with increasing reaction tem-perature, but hampered at low temperature [64]. Here, both CaO andZnO, as well as ZnAl2O4 are oxides with low lattice energies [35], butthe latter is inactive in ketonization due to its negligible acidity andbasicity [36,62]. CZA catalysts (CZA(331), CZA(221) and CZA(111))with highly crystalline structure and high content of Ca and/or Zn ex-hibit much higher basicity than CZA in amorphous domain (e.g.CZA(113)), revealed by TPD-CO2 analysis (Fig. 5). Our reaction resultsshows the activity of CZA catalyst correlates well with their basicity andare highly active at high temperature (T ≥ 375 °C). An AcOH conver-sion of 70–100 % was obtained, which strongly reduced to 7–55 % at T≤ 350 °C (Table 2). It demonstrates the thermal decomposition ofacetates is one of the major routes for ketonization of AcOH over CZAcatalysts, particularly works at high temperature. It can be performedvia a di-molecular mechanism, as shown in Scheme 1, which has beenproposed as an acetate molecule attacks the neutral molecule, formingthe beta-keto acid, followed by decomposition [33,65].

At low temperature, the catalytic performance of the CZA catalystswith higher Al content provides higher AcOH conversion than thoseCZA catalysts in highly CaO and ZnO domain (Table 2). Clearly, analternative reaction pathway dominates ketonization at low tempera-ture. The temperature-controlled reaction pathway has been previouslyreported for Fe2O3 [24] and CeO2 [66]. For ketonization over iron oxide[24], the reaction is carried out via thermal decomposition of bulkacetate at high temperature (> 400 °C) over Fe3O4, while at low tem-perature, it proceeds via a catalytic reaction pathway over Fe2O3 as thecatalyst.

Ketonization of carboxylic acid in gas-phase based on acid-base pair

Fig. 5. CO2-TPD of Ca/Zn/AlOx catalysts. (a) CZA(331), (b) CZA(121), (c)CZA(111), (d) CZA(113).

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sites has been proposed for oxides with high lattice energies[1,16,22,63,67]. It requires basic sites for dissociative adsorption ofcarboxylic on surface, which can react with another carboxylic acid thatis stabilized and activated on neighboring Lewis acid sites. Amphotericreducible metal oxides (e.g. CeO2, and TiO2), providing both mediumstrength Lewis acidity and basicity [31]. are better catalysts than pureacidic or basic oxides for ketonization [1], since it can balance the acid-base properties for carboxylic acid adsorption and activation on surface.

Both pure CaO and ZnO have strong Lewis basicity with very weakacidity [31], which only provide very few acid-base pairs on surface[43]. The surface base sites, e.g. unsaturated oxygen (CaeO2−) andhydroxide (Ca−OH−) sites [43], are able to strongly adsorb AcOH toform bulk acetates at low temperature, which can be then decomposedat high temperature [33], as shown in Scheme 1. On the other hand,AcOH can be strongly adsorbed on Al2O3 having strong Lewis acid sites[31,33,68]. However, Al2O3 is nearly inactive for catalytic ketonizationof AcOH, since it consists of nearly no acid-base pair sites as revealed byFTIR investigation [68].

The catalytic performance of CZA catalysts increased in line withincreasing calcium aluminates formed in amorphous domain at lowtemperature, which has been observed in Table 2 combined with XRDpatterns. The EDX mapping images (Fig. 3b and c) shows Ca can beincorporated into the Al matrix, resulting in well-mixed CaO and Al2O3

oxides in amorphous calcium aluminates. It is possible to provide strongbase sites on CaO in spatial proximity to the Lewis acid sties on Al2O3 asshown in 1 in Scheme 2. Carboxylates can be formed on CaO throughAcOH dissociated on surface basic oxygen sites upon adsorption, andcarboxylates (CH3COO−) bind to two surface cations in a bidentate

bridging configuration 3 [69], which are energetically more favourable[70]. 3 can be enolized to form 4, which is more energetically fa-vourable than direct α-hydrogen (hydrogen in blue) abstraction[21,23]. The enol 4 then undergoes nucleophilic attack by anotherAcOH adsorbed on neighboring Lewis acid sites from Al2O3. The re-sulting β-ketoacid 6 can be further decomposed into acetone and CO2,and release the acid-base pair sites to the reaction cycle.

3.2.3. Effect of furans and phenol on the ketonization of acetic acidFurans and derivatives are one of the major compounds in the bio-

oil derived from hemicellulosic and cellulosic materials [2], whilephenol is a primary compound in bio-oil produced from lignocellulosicmaterials [71]. Therefore, 10 wt% furfural or phenol was added to theaqueous AcOH to evaluate the effect of these major bio-oil compoundson the ketonization of AcOH. CZA(331) and CZA(221) with the highestactivity were selected for this study as shown in Fig. 6.

The conversion of 50 wt% aqueous AcOH kept constantly at 100%and 99.4–99.7% over CZA(331) and CZA(221), respectively (Fig. 6(a)).Upon furfural addition (Fig. 6 (b)), the conversion of AcOH decreasedfrom 94 to 88%, while only slightly affected the acetone selectivity(from 97.1 to 98.5% to 95–96%) after 10 h reaction over both catalysts.A similar phenomenon was observed for ketonization of AcOH overCeO2/1%K2O/Al2O3, where the AcOH conversion decreased from 99 to64% and the selectivity to acetone decreased from 86.7 to 84.9% after12 h reaction [7]. This has been attributed to the humin-like by-productformed by furfural addition, which could deposit on the catalysts andblock the active sites, similar as that reported earlier [7].

To clarify the effect of furfural on ketonization of AcOH over

Table 2The conversion of acetic acid and selectivity to acetone on Ca/Zn/AlOx catalysts.a.

Catalyst Conversion of acetic acid (%) Selectivity to acetoneb (%)

300 °C 325 °C 350 °C 375 °C 400 °C 425 °C 300-375 °C 400 °C 425 °C

CZA(331) 14 44 92 93 99.5 100 100 99.0 97.1CZA(221) 7 25 47 85 99.6 99.4 100 99.5 98.5CZA(111) 10 29 38 84 98.9 99.0 100 100 98.9CZA(112) 8 31 42 82 97.3 98.9 100 100 100CZA(212) 20 27 51 81 96.1 96.3 100 100 100CZA(113) 13 31 39 71 90.4 94.0 100 100 100CZA(323) 16 34 55 74 87.7 93.0 100 100 100CZA(121) 19 25 52 70 87.1 94.5 100 100 100

a Conditions: 50 wt% aqueous AcOH solution was pumped into the reactor with catalyst (100mg) at a flow rate of 0.4 mL/h with N2 as carrier gas (10ml/min).b The selectivity of acetone is determined by analysis of liquid product.

Scheme 1. Proposed mechanism of catalytic ketonization over CZA catalysts at high temperature.

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CZA(221) and CZA(331), the catalytic conversion of furfural was in-vestigated. As shown in Table 3, the conversion of furfural was 38–43%after 2 h reaction and declined to 18–24% after 10 h reaction, while theselectivities to the resulting by-products based on fufural were re-mained nearly no change. At the same time, the selectivity to acetonewas decreased from 97.1 to 98.5% in aqueous AcOH to 91–92% byfurfural addition after 2 h reaction, and then increased to 95–96% after10 h reaction. It demonstrates the primary-generated acetone partici-pated into the side reaction with furfural, for example, forming trans-furfurylideneacetone as secondary product (Scheme 3) [18]. These by-products can rapidly poison surface active sites for the first 2 h reaction,and gradually for others with time.

Unlikely, with 10 wt% phenol addition (Fig. 6(c)), the conversion ofAcOH (99%) remained no change after 10 h reaction over CZA(331),while that over CZA(221) decreased from 99 to 94%. This indicatesphenol addition has minor impact on the catalytic activity of CZAcatalysts than the addition of furfural. Over both CZA(331) andCZA(221), no by-products associated to phenol conversion can be de-tected with an acetone selectivity of 100%. It demonstrates current CZAcatalysts are quite stable upon phenol addition.

3.2.4. Stability and reusability of CZA catalystsThe stability of CZA(331), the best performance one in this work,

was tested for a long-term run under 425 °C with aqueous AcOH. Theconversion of AcOH as a function of reaction time has been shown inFig. 7 (a). Under the conditions shown in Table 2, the AcOH conversionis 100% at 56 h, slightly decreased to 92% at 72 h, and stronglydropped to 80% at 96 h. The catalyst deactivation may be caused bycarbon deposition on active sites [7]. In another run (Fig. 7 (b)), theused CZA(331) was regenerated at 72 h when deactivation was ob-served. Clearly, the conversion of AcOH reached 100% after re-generation, which indicates CZA(331) is highly stable and the depositedcarbon can be completely removed under current regeneration condi-tions.

4. Conclusion

Ca/Zn/AlOx mixed oxides (CZA) with various Ca/Zn/Al ratios re-sulted in the formation of various fractions of nano-sized CaO, ZnO,

Scheme 2. Proposed mechanism of catalytic ketonization over CZA catalysts at low temperature.

Fig. 6. Conversion of acetic acid on CZA(331) and CZA(221) at 425 °C: (a)50 wt% aqueous acetic acid, (b) 10 wt% furfural added to (a), (c) with 10 wt%phenol added to (a). Reaction was carried out under 425 °C for 10 h.

Table 3Conversion of furfural and selectivity on Ca/Zn/AlOx catalysts.a.

Catalysts Conversion of furfural(%)

Selectivity (%)

TFFAb FAc 1-POHd

2 h 10h 2 h 10h 2 h 10h 2 h 10h

CZA(331) 43 18 75 78 11 9 14 13CZA(221) 38 24 78 80 10 7 12 13

a Reaction conditions: same as in Table 2, except using 10 wt% furfural wasadded to 50 wt% aqueous AcOH solution.

b AcOH Trans-furfurylideneacetone.c Furfural alcohol.d 1-pentanol.

Scheme 3. Proposed pathway for trans-furfurylideneacetone.

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ZnAl2O4, amorphous alumina and calcium aluminates in catalysts. CZAcatalysts with high Al content (> 1/3) provide mainly moderate basi-city, which strongly increased with increasing (Ca+Zn) content asrevealed byTDP-CO2 studies.

In the ketonization of acetic acid, all CZA catalysts are highly activefor AcOH conversion (93–100 %) at 425 °C with a selectivity of97.1–100 % to acetone as the desired product. The performance of CZAcatalysts strongly depends on their compositions and surface structure,and two reaction pathway are revealed as: 1) the strong base sites (e.g.CaO and ZnO) on CZA catalysts at high (Ca+Zn) content governs thereaction at T ≥ 375 °C through acetates thermally decomposed toacetone; and 2) surface acid-base pairs (e.g. CaO and Al2O3) promotethe formation of β-ketoacid as an intermediate, which is readily fordecomposition into acetone and CO2 at T ≤ 350 °C.

Among CZA catalysts, CZA(331) obtained the highest acetone yieldof 97.1% at 425 °C and exhibited excellent tolerance towards furfuraland phenol (major compounds in pyrolysis bio-oil) addition. These CZAcatalysts can perform the reaction for 56 h without any activity loss andare easily to be regenerated with full activity at 425 °C. This demon-strates CZA catalysts introduced here is very active, stable, high toler-ance and easily to be regenerated, which is promising for industrialketonization and bio-oil upgrading in future.

Acknowledgements

This work was supported by the Australian Research CouncilDiscovery Projects (DP150103842 and DP140102432) and DiscoveryEarly Career Researcher Award (DE190101618), the Faculty’s MCRScheme, Energy and Materials Clusters and the Early Career ResearchScheme and the Major Equipment Scheme from the University ofSydney. J.F. and D.W. acknowledge the funding provided by theNational Natural Science Foundation of China (21620102007).

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