Ketonization of Propionic Acid to 3-Pentanone over CexZr1âxO2

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Kinetics, Catalysis, and Reaction Engineering

Ketonization of Propionic Acid to 3-Pentanone over CexZr1xO2 Catalysts: The Importance of Acid-Base Balance Shuang Ding, Hua Wang, Jinyu Han, Xinli Zhu, and Qingfeng Ge Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04208 • Publication Date (Web): 27 Nov 2018 Downloaded from http://pubs.acs.org on December 5, 2018

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Industrial & Engineering Chemistry Research

Ketonization of Propionic Acid to 3-Pentanone over CexZr1-xO2 Catalysts: The Importance of Acid-Base Balance

Shuang Ding,† Hua Wang,† Jinyu Han,† Xinli Zhu,*,† Qingfeng Ge†,‡

†Collaborative Innovation Center of Chemical Science and Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China.

‡Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois 62901, United States.

*Corresponding

author:

E-mail：[email protected]

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ABSTRACT: Ketonization of biomass-derived propionic acid was investigated at 270350 oC over CexZr1-xO2 mixed oxides with varying Ce/Zr ratios. The acidity of CexZr1-xO2 increases as Zr content is increased. The density of the strong base sites is reduced with increasing Zr content, while the density of the medium-strength base sites is maximized on Ce0.5Zr0.5O2. Diffuse reflectance infrared spectroscopy of propionic acid adsorption indicated that increasing Zr content in CexZr1-xO2 favors monodentate adsorption over bidentate adsorption. The reaction rate varies with the composition of CexZr1-xO2. The intrinsic reaction rate can be correlated with the medium-strength acid/base ratio. The rate maximizes at a medium-strength acid/base ratio of ~1.07 on Ce0.1Zr0.9O2. This value is close to 1, indicating that the balanced medium-strength acid-base sites play a crucial role in the ketonization. The balanced acid-base property


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influences propionic acid adsorption, enolate formation and C-C coupling, and therefore affects ketonization rate.

1. INTRODUCTION As a renewable and carbon-neutral alternative source of energy, biomass has received growing attention. The use of biomass energy may provide a partial solution to the


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increasing demand for fossil fuels as well as the related environmental issues.1-3 Biomass is composed of cellulose, lignin and hemicellulose. Hemicellulose and cellulose are decomposed in fast pyrolysis to small oxygenated compounds, such as alcohols, acids, aldehydes, esters, ketones and others.1,4 Carboxylic acid is one of the most abundant components in bio-oil, which mainly stems from hemicellulose and cellulose. For example, there is 7-12 wt% acid in the bio-oil produced from hemicellulose and cellulose and 0.3-0.9 wt% in the bio-oil from lignin pyrolysis.5 Carboxylic acids make the bio-oil unstable and corrosive. Therefore, upgrading carboxylic acids to stable and non-corrosive products is of great importance for further processing bio-oil.

Ketonization is an important approach for carboxylic acid upgrading, since it converts two carboxylic acids to a ketone and releases CO2 and H2O. Consequently, this reaction removes oxygen and increases carbon chain length without consuming H2, leading to increased stability and reduced acidity of bio-oil.


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In recent years, ketonization of carboxylic acids on metal oxides have been widely investigated.6-20 Though a significant advance has been made, the reaction mechanism and the active site for ketonization of carboxylic acid are still under debate. Various types of sites, such as Lewis acid-base pairs21-23 and coordinatively unsaturated metal cations,24 in the metal oxide catalysts have been proposed to be the active sites for ketonization. These surface sites can be tuned by incorporation of another cation into a metal oxide.25,26 To improve the catalytic activity for ketonization, mixed oxide catalysts have been investigated. Nagashima et al.25 reported that addition of a second metal (Mg, Mn, Fe, Ni, Cu and Zr) into CeO2 improves the activity for ketonization of propionic acid, and increases the selectivity to 3-pentanone at 350 oC. Bayahia et al.26 showed that Zn-Cr mixed oxides (10:1) are more active than the parent oxides of ZnO and Cr2O3 in ketonization of acetic and propionic acids at 300–400 oC and ambient pressure.

CexZr1-xO2 mixed oxides are widely used in different reactions, and have been studied for ketonization of carboxylic acids.9,27-29 Liu et al. prepared CeO2 and ZrO2CeO2 (Ce/Zr = 1) using co-precipitation and tested them in vapor phase ketonization of


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acetic acid at 350 and 450 oC.27 Their results showed that the activity of ZrO2-CeO2 is lower than that of CeO2 at 350 oC, but the reverse is found at 450 oC. They suggested that the activation energy on ZrO2-CeO2 is higher than that on CeO2. Gaertner et al. studied ketonization of hexanoic acid over Ce0.5Zr0.5O2 catalyst at 448-623 K, and indicated that the rate of ketonization shifts from second order to zero order as the partial pressure of acid increases.28 Pulido et al. studied ketonization of decanoic acid on CeO2, ZrO2 and CeZrO4 (Ce/Zr = 1) prepared by a precipitation method.9 At 350 oC, conversion on ZrO2 is higher than that on CeO2 and CeZrO4. Note that the above investigations only studied the Ce/Zr of 1 in CexZr1-xO2 mixed oxide for ketonization. Shutilov et al. investigated the effects of composition and microstructure of supported 520 wt% CeO2/ZrO2 catalysts, prepared by the incipient wetness impregnation, in ketonization of pentanoic acid at 300-400 oC.29 10 wt% CeO2 supported on ZrO2 showed the highest activity at 355 oC. They suggested that all types of acid sites and lattice oxygen atoms participate in the formation of the reaction intermediate and the higher conversion of pentanoic acid is due to higher concentration of surface coordinatively unsaturated ions. It is evident that varying the Ce/Zr ratio in the mixed


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oxide of CexZr1-xO2 has not been systematically investigated for ketonization of carboxylic acids. In addition, the effects of acid-base property and the adsorption property of the CexZr1-xO2 on ketonization are unclear.

In this study, the CexZr1-xO2 catalysts with a wide range of Ce/Zr ratios were synthesized by a co-precipitation method, and tested for vapor phase ketonization of propionic acid at 270-350 oC. The CexZr1-xO2 catalysts were characterized by various techniques and correlated to the catalytic performance in ketonization. The results indicated that Ce0.1Zr0.9O2 has the highest activity for ketonization, which is due to its balanced acid-base property.

2. EXPERIMENTAL

2.1. Catalyst preparation. The CexZr1-xO2 mixed oxide catalysts were prepared by a co-precipitation method. Appropriate amounts of Ce(NO3)3·6H2O and Zr(NO3)4·5H2O with targeted Ce/Zr molar ratio were dissolved in deionized water. Then, ammonium hydroxide solution was slowly added to the solution under stirring, until pH value reached 8.5. The resulting precipitates were washed with deionized water, dried in an


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oven at 120 oC for 12 h, and finally calcined at 450 oC for 5 h with a heating rate of 5 oC/min.

Pure CeO2 and ZrO2 were also prepared using the same method for

comparison. CexZr1-xO2 catalysts with a nominal Ce/Zr molar ratio of 90:10, 75:25, 50:50, 25:75 and 10:90 were named as Ce0.9Zr0.1O2, Ce0.75Zr0.25O2, Ce0.5Zr0.5O2, Ce0.25Zr0.75O2 and Ce0.1Zr0.9O2, respectively.

2.2. Catalyst characterization. The specific surface area of the catalysts was measured by N2 adsorption using a Micrometrics Tristar 3000 at liquid nitrogen temperature. All samples were pretreated at 300 oC for 3 h under atmospheric pressure prior to analysis. The phase structure of the CexZr1-xO2 mixed oxides were investigated by powder X-ray diffraction (XRD) on a Rigaku D/Max-2500 V/Pc diffractometer, using a filtered CuKα radiation source (λ=1.54056 Å). The diffractometer was operated at 40 kV and 20 mA, with data collected in the 2θ range of 20-100 o at a scanning rate of 4 o/min. The Raman spectra were collected on a Renishaw Raman spectrometer using Ar+ laser (532 nm) as the light source.

X-ray photoelectron spectroscopy (XPS) study was performed on a Physical


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Electronics PHI 1600 system with an Al Kα X-ray (1486.6 eV) operated at 250 W and 15 kV. The chamber pressure was ~1.6×10-8 Pa. The binding energies (BE) were corrected by referencing C1s peak of adventitious carbon at 284.6 eV.

Temperature-programmed desorption of CO2 (CO2-TPD) and NH3 (NH3-TPD) were performed in a micro-reactor, equipped with a MKS Cirrus 200 mass spectrometer (MS).30 The catalyst sample (200 mg) was loaded into the quartz tube reactor, pretreated in flowing He at 400 oC for 30 min with a flow rate of 30 mL/min, and finally cooled to 30 oC or 50 oC. After pretreatment, the sample was treated with flowing 5% CO2/He (50 mL/min) at 30 oC or with flowing 5% NH3/He (50 mL/min) at 50 oC for 30 min, followed by purging with flowing He (30 mL/min) for 60 min to remove physical adsorbed CO2 or NH3. Finally, it was heated up to 600 or 800 oC at a rate of 10 oC/min. The desorbed species were monitored online by the MS. The used samples were characterized by temperature-programmed oxidation (TPO) in the same apparatus.

The diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) study of propionic acid (or 3-pentanone) adsorption was performed on a PerkinElmer Frontier


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spectrometer, equipped with a liquid nitrogen cooled mercury cadmium telluride (MCT) detector, a diffuse reflectance accessory and a reaction chamber (Harrick). 80 mg of the catalyst powder was loaded in the cup of the reaction chamber, and pretreated at 400 oC

for 30 min in flowing He (30 mL/min). The sample was then cooled to 40 oC with a

background spectrum recorded. The flow rate of He was increased to 80 mL/min. Propionic acid (or 3-pentanone) was injected into the He flow using a syringe pump (KDS100, kd scientific) at a speed of 0.03 mL/h. The resulting partial pressure of the organic feed was 0.2 kPa. The DRIFTS spectra were recorded at a resolution of 2 cm-1 with accumulation of 64 scans.

2.3. Catalytic activity. Ketonization of propionic acid was conducted in a 6 mm outer diameter fixed-bed quartz tube reactor at atmospheric pressure.31 The catalyst sample (40-150 mesh) was loaded between two layers of quartz wool. A K-type thermocouple was attached to the wall of the reactor to measure the catalyst bed temperature. The catalyst samples were pretreated in flowing Ar at 350 oC for 0.5 h prior to reaction. Gases were controlled by mass flow controllers, and propionic acid was fed using a


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syringe pump (KDS100, kd scientific) and was vaporized before entering the reactor. All lines were heated to avoid any condensation of reactants and products. The products were quantified online in an Agilent 7890B GC, equipped with an Innowax capillary column (60 m) and a flame ionization detector (FID). The conversion and yield were reported in molcarbon%.

3. RESULTS

3.1. Catalyst Characterization. The N2 adsorption-desorption isotherms (Figure S1) of all samples are of type IV, indicating the presence of mesopores due to stacking of crystallites. The hysteresis loops for CexZr1-xO2 (Ce/Zr 1, while tetragonal phase (space group P42/nmc) increases with increasing Zr content and becomes the dominant phase for Ce0.25Zr0.75O2 and Ce0.1Zr0.9O2. Compared to pure ZrO2, a small amount of Ce in Ce0.1Zr0.9O2 stabilizes the tetragonal phase while preventing the formation of the monoclinic phase. The result is consistent with previous studies32,34 that showed phase segregations take place in the intermediate composition of Ce and Zr. It should be noted that the peaks of fluorite cubic structure slightly shift to higher 2θ values as Zr content is increased in the mixed oxides. This result implies that the lattice shrinkage of the mixed oxides results from the substitution of the large Ce4+ (0.097 nm) by small Zr4+ (0.084 nm) in the unit cell.35 Table 1 shows that the lattice spacing of the CexZr1-xO2 decreased with the substitution of Zr for Ce. And, the crystallite size, calculated by the Scherrer equation, also decreased with the incorporation of Zr into CeO2.

The Raman spectra of CexZr1-xO2 catalysts are shown in Figure 2. A strong band centered at 461 cm-1 is observed in CeO2 and CexZr1-xO2 (Ce/Zr ≥ 1), which is assigned


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to the F2g Raman active mode of the fluorite structure (Ce-O-Ce).35,36 In addition, a weak band at ~600 cm-1 is also observed, which is attributed to the asymmetric vibration caused by the presence of oxygen vacancies. 35,36 For CexZr1-xO2 (Ce/Zr ≥ 1), the band centered at 461 cm-1 shifts to 465 cm-1 as Zr content increases, indicating that the length of M-O bond is decreased due to

Zr doping into the CeO2.35 While for

Ce0.5Zr0.5O2, Ce0.25Zr0.75O2 and Ce0.1Zr0.9O2 samples, vibration modes at 147, 260, 321, 462 and 628 cm-1 are observed, which are indicative of the presence of tetragonal phase.37,38 For pure ZrO2, Raman active vibration modes at 90, 179, 191, 223, 335, 348, 381, 476, 617 and 638 cm-1 ascribed to monoclinic ZrO2 are evident.37,38 The results are consistent with XRD, and are in good agreement with the previous studies.37,38

The surface composition and chemical structure of CexZr1-xO2 samples were characterized by XPS. The surface Ce/Zr ratios (Table 2) are in good agreement with the targeted values, indicating the homogeneous distribution of Ce and Zr in the mixed oxides. The XPS spectra of Ce 3d, Zr 3d and O 1s regions of CexZr1-xO2 are reported in


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Figure 3. The spectra of Ce 3d (Figure 3A) can be fitted into eight peaks (u, u′, u′′, u′′′, v, v′, v′′, and v′′′) due to the hybridization between the partially occupied 4f levels of Ce and the 2p states of O.39,40 These peaks are assigned to Ce 3d3/2 (u, u′, u′′, and u′′′) and 3d5/2 (v, v′, v′′, and v′′′), respectively.39,40 The u′ and v′ are related to Ce3+, while other peaks are attributed to Ce4+. The Zr 3d spectra (Figure 3B) show two peaks of Zr4+ 3d3/2 and 3d5/2, respectively, for all samples.39-41 The spectra of O 1s (Figure 3C) are fitted with two peaks, which are attributed to the lattice oxygen (O-I) and oxygen ions with unusual coordination (due to the presence of vicinal oxygen vacancy) or hydroxyl-like groups (O-II), respectively.40-44 Usually, the peak are ratio of u′′′ to the total Ce 3d is used to reflect the relative concentration of Ce4+.39 And, the concentration of Ce3+ can be calculated by the peak area ratio of u′ + v′ to the total Ce 3d.39,42 As summarized in Table 2, the relative Ce4+ concentration is reduced while the Ce3+ concentration is increased upon doping of Zr. The Ce3+ concentration is maximized at Ce0.5Zr0.5O2. On the other hand, the ratio of O-II/O-I increases upon doping of Zr, and maximizes at Ce0.5Zr0.5O2. These results indicate that the oxygen vacancy and coordinatively unsaturated metal cation of Ce3+ are formed in the CexZr1-xO2 upon


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doping of Zr. And the maximum is reached at Ce0.5Zr0.5O2 due to the maximum amount of solid solution.39

NH3-TPD and CO2-TPD were performed to probe the surface acid-base properties of the mixed metal oxides, with quantified results summarized in Table 3. As shown in Figure 4A, the desorption profile of NH3 shows broad (from 50 to 450 oC) and asymmetric features, which suggests the presence of acid sites of different strengths. The coordinatively unsaturated cations on the surface of metal oxide show acidity of Lewis type, whose strength is dependent on the coordination.45 Consistent with previous studies,37,46 curve fitting of the profile resulted in three distinct desorption peaks. The desorption temperature of NH3 adsorbed on weak acid site is below 200 oC, the desorption temperature of NH3 adsorbed on medium-strength acid site is between 200400 oC, and the desorption temperature of NH3 adsorbed on strong acid site is above 400 oC.

37,46

It is evident that the peaks of medium-strength acid sites increase in

intensity and shift to higher temperatures gradually with increasing Zr content in CexZr1-


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The result indicates that the medium-strength acid sites are mainly related to Zr

content in CexZr1-xO2.

The CO2 desorption profile in Figure 4B also shows broad (from 50 to 700 oC) and asymmetric features, indicating the presence of different base sites. The coordinatively unsaturated O2- ions on the surface act as base sites, whose basicity is related to the coordination circumstances. The desorption profile can be deconvoluted into three distinct peaks. The first peak centered at ~130 oC is associated with formation of bicarbonate on the weak base site of surface -OH groups.46-48 The second peak centered at ~240 oC is related to formation of bidentate carbonates on medium-strength base site of Mx+-O2- pairs.46-48 And the third peak > 400 oC is ascribed to formation of unidentate or polydentate carbonates on strong base site of low coordination O2- ions.4648

The desorption peak of strong base site originating from CeO2 is reduced in intensity

and shifts to lower temperatures as Zr content is increased. On the other hand, the desorption peaks of weak and medium-strength base sites increase in intensity with increasing Zr content in the mixed metal oxides to Ce0.5Zr0.5O2, and then decrease in


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intensity with further increasing Zr. The maximum density of base site in Ce0.5Zr0.5O2 may be related to the formation of maximum amount of solid solution in this sample, which leads to the increased mobility of O2- in this sample and thus increased number of site for CO2 adsorption.49 Note that the peak of medium-strength base site continues shifting to higher temperatures with increasing Zr content, implying its strength is increased.

As shown in Table 3, the densities of medium-strength and total acid sites increase with increasing Zr content, while the densities of both medium-strength and total base sites are maximized at Ce0.5Zr0.5O2.

The DRIFTS spectra of propionic acid adsorption on CexZr1-xO2 catalysts are shown in Figure 5A. Vapor phase propionic acid (on KBr) shows O-H stretching vibration (νO-H) at 3578 cm-1, C=O stretching vibration (νC=O) at 1722, 1775 and 1791 cm-1 and C-O stretching vibration (νC-O) at 1146 cm-1. Propionic acid adsorption on the catalysts shows the bands of C-H stretching vibration (νC-H) located at 2978, 2942 and 2880 cm-1, C-H bending vibration (δC-H) at 1378 and 1300 cm-1, and C-H in-plane


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rocking vibration (ρC-H) at 1080 cm-1. The negative peaks at 3600-3800 cm-1 are associated with the perturbation of surface hydroxyl groups of metal oxides by propionic acid adsorption.21,50 The adsorption of the carboxyl of propionic acid on the metal oxide may result in monodentate and bridging or chelating bidentate configurations (Scheme 1). Identification of the adsorption configuration can be made from the frequency difference (△ν = νas-νs) between the antisymmetric (νas) and symmetric (νs) stretching vibration of C-O in the OCO group.51,52 Generally, the △ν follows the order of monodentate > free ionic > bidentate. The reference ionic △ν of sodium propionate is 158 cm-1 (νas=1571 cm-1, νs =1413 cm-1), shown in Figure S2. Accordingly, the bands at 1200-1800 cm-1 are assigned to specific adsorption configurations and summarized in Table 4. On CeO2, both monodentate (1580, 1420 cm-1) and bridging bidentate (1540, 1473 cm-1) adsorption configurations are observable.53,54 On ZrO2, monodentate (1603, 1419 cm-1) and bridging bidentate (1526, 1480 cm-1) adsorption configurations23,55 are in slightly different frequencies. In addition to the monodenate and bridging bidentate adsorption configurations, a chelating bidentate adsorption on CexZr1-xO2 is present at 1566 and 1446 cm-1.55,56 It is evident that the monodentate νas band shifts to a higher


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wavenumber and its intensity is increased as the Zr content in the mixed oxide is increased. The monodentate νas band on Ce0.1Zr0.9O2 is at 1618 cm-1, indicating the weakest adsorption of monodentate configuration on Ce0.1Zr0.9O2. It is noted that the bridging bidentate νas band on Ce0.1Zr0.9O2 is located at similar wavenumbers to other Ce containing samples, while it is red shifted to a lower wavenumber of 1526 cm-1 on ZrO2, suggesting a stronger adsorption of bridging bidentate on ZrO2. The variations in adsorption modes and adsorption strengths are related to the fine structure and acidbase properties changes due to varying Ce/Zr ratios in CexZr1-xO2.

The adsorption of 3-pentanone, the product of ketonization, is also followed by DRIFTS. As shown in Figure 5B, vapor phase 3-pentanone (on KBr) shows the carbonyl vibration at 1731 cm-1. The vibration is red shifted to lower wavenumbers when 3pentanone is adsorbed on the metal oxide catalysts, due to the interaction between O atom of carbonyl and surface Lewis acid sites of metal cations in an η1 configuration (Scheme 1). A major band at 1710 cm-1 with a shoulder at 1665 cm-1 is present for CeO2. They can be ascribed to carbonyl adsorption on two kinds of Lewis acid sites


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(metal cations with different coordinations) on CeO2 surface.57 However, only one adsorption band at 1697 and 1693 cm-1 is observed for Ce0.1Zr0.9O2 and ZrO2, respectively, suggesting only one type of Lewis acid site is present. The red shift of the carbonyl vibration band and the increase in its intensity with increasing Zr content is consistent with the increase in acidity of the mixed oxides with increasing Zr content, resulting in more and stronger adsorption of 3-pentanone on the acid sites.

3.2. Ketonization of propionic acid on CexZr1-xO2. Conversion of propionic acid on CexZr1-xO2 catalysts in an integral reactor were compared at the same space time (W/F, defined as weight of catalyst/organic feed flow rate, gcatgfeed-1h (h)) of 0.05 h, as shown in Figure 6. Increasing temperature from 290 to 350 oC increases the conversion of propionic acid, with 3-pentanone being the dominant product with selectivity higher than 95% on all catalysts. The minor products include methylketene, propionic anhydride, and propanal, consistent with previous studies on CeO2 and ZrO2.14,22 In this temperature range, the activity of these catalysts follows the order of Ce0.1Zr0.9O2 > ZrO2 > Ce0.9Zr0.1O2 > Ce0.75Zr0.25O2 > Ce0.25Zr0.75O2 > CeO2 > Ce0.5Zr0.5O2. The low activity


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on Ce0.5Zr0.5O2 catalyst is broadly consistent with previous studies on ketonization of acetic acid and decanoic acid,9,27 which showed that Ce0.5Zr0.5O2 is less active than pure ZrO2. On the other hand, when the reaction temperature is raised to 450 oC, the activity of Ce0.5Zr0.5O2 is improved to a level that is similar to that of ZrO2 (data not shown). The result is in accordance with the literature report.27 The result suggests that the active site may be partially poisoned by the propionic acid or CO2 due to strong adsorption, since Ce0.5Zr0.5O2 shows the highest density of base sites.

The stabilities of CeO2, Ce0.1Zr0.9O2 and ZrO2 catalysts were tested at 330 oC with a similar initial conversion of 40-50% by varying W/F. As shown in Figure 7, all samples deactivate with time on stream and deactivation is more severe within the first 300 min. It is apparent that Ce0.1Zr0.9O2 and ZrO2 deactivate at a similar rate, while CeO2 deactivates more drastically. TPO was performed to estimate the coke deposited on spent samples after 1200 min reaction. As shown in Figure S3, a sharp CO2 production peak at 209 oC is present on CeO2, while a weak peak at 362 oC is present on ZrO2. Besides these two peaks, another peak at 288 oC is also present on Ce0.1Zr0.9O2. The


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results suggest that ceria promotes the combustion of coke. The amounts of deposited coke decrease from 2.92% gcarbon/gcatalyst on CeO2, to 1.49% on Ce0.1Zr0.9O2 and 1.21% on ZrO2. This trend is in good agreement with the stability of these samples, indicating that coke deposition is the main reason accounting for the deactivation. The result is also consistent with DRIFTS result, which shows strongly adsorbed bidentate carboxylate is more favorable on CeO2 while relatively weakly adsorbed monodentate carboxylate is more favorable on Ce0.1Zr0.9O2 and ZrO2. The regeneration of the catalysts was performed by calcination at 450 oC for 60 min in a 5% O2-He stream. The regenerated samples show the same initial activity and deactivation profile as the fresh samples (Figure 7), confirming coke is the major reason for deactivation and the catalysts can be easily regenerated.

The intrinsic reaction rate of propionic acid conversion over CexZr1-xO2 catalysts was measured under differential conditions at 330 oC with conversion lower than 15%. As shown in Figure 8, the mass based intrinsic rate is dependent on the Zr content in


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the catalysts. The reaction rate appears to have two maxima at Ce0.9Zr0.1O2 (49.8 mmol∙g-1∙min-1) and Ce0.1Zr0.9O2 (60.3 mmol∙g-1∙min-1), respectively.

The Arrhenius plots of propionic acid conversion on CexZr1-xO2 catalysts in the reaction temperature range of 290-350 oC are displayed in Figure 9. The apparent activation energy for ketonization on different catalysts varies slightly from 106 kJ/mol on Ce0.9Zr0.1O2 to 116.4 kJ/mol on ZrO2, consistent with the previous reports of 117 kJ/mol on ZrO2,7 132 kJ/mol on Ce0.5Zr0.5O2,28 78-161 kJ/mol on CeO2,58 110 kJ/mol on Fe2O316 and 124 kJ/mol on Zn-Cr oxide26 for ketonization of carboxylic acid. The similarity in activation energy on CexZr1-xO2 catalysts with varying Ce/Zr ratio suggests that ketonization of propionic acid on these catalysts follows a similar reaction mechanism.

The dependence of reaction rate on the partial pressure of propionic acid was further studied on the most active catalysts of ZrO2 and Ce0.1Zr0.9O2 at 270 oC under differential conditions with conversion < 18%. As shown in Figure 10A, the reaction rate is increased with increasing propionic acid partial pressure, and levels off at higher


23


Page 24 of 52

partial pressures (0.3 and 1.2 kPa for ZrO2 and Ce0.1Zr0.9O2, respectively), indicating the reaction becomes zero order at higher partial pressures due to saturation of active sites. The similar trend on both catalysts suggests that a similar reaction mechanism may be operative.23,28 The resulting reaction orders of propionic acid at low pressures (Figure 10B) are 0.38 and 0.55 on Ce0.1Zr0.9O2 and ZrO2, respectively. These orders deviate from a typical bimolecular reaction, suggesting the reaction is strongly influenced by the adsorption of propionic acid on the surface and the subsequent dissociation. Compared to the reaction order of 0.55 on ZrO2, the lower reaction order of 0.38 on Ce0.1Zr0.9O2 suggests a stronger adsorption of propionic acid and a higher coverage can be achieved on the Ce0.1Zr0.9O2 surface, which is supported by the DRIFTS.

3.3 Discussion. It is generally accepted that the ketonization of carboxylic acid proceeds surface reaction over oxides with higher lattice energies, such as TiO2, ZrO2, CeO2.59 Therefore, the crystal structure may influence the catalytic performance. The XRD and Raman results indicate that the crystal structure transforms from fluorite cubic for pure CeO2, to cubic and tetragonal for CexZr1-xO2 mixed oxides, and to tetragonal for


24

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Zr rich mixed oxides, and finally to monoclinic for pure ZrO2. As shown in Figure 8, the reaction rate shows two maxima at Ce0.9Zr0.1O2 (49.8 mmol∙g-1∙min-1) and Ce0.1Zr0.9O2 (60.3 mmol∙g-1∙min-1), which have cubic and tetragonal structure, respectively. Thus, the crystal structure may have complex effect on ketonization and no clear trend could be found.

On the other hand, the BET surface area of mixed oxides is higher than that of pure CeO2 due to Zr doping. As a result, the improved reaction rate may be attributed to the increased surface area. The intrinsic reaction rate based on surface area was therefore calculated and plotted as a function of Zr content. As shown in Figure 11, the area based intrinsic reaction rate is not a constant value, but is dependent on the Zr content. Thus increasing surface area is not the key factor that increased the activity.

The surface coordinatively unsaturated metal cations have been proposed to be the active sites for ketonization of carboxylate acid,24,60 and the oxygen vacancy play an important role in this reaction. The intrinsic reaction rate is plotted as a function of concentration of Ce3+ (Ce3+/Ce) (Figure S4A) and concentration of oxygen vacancy (O-


25


Page 26 of 52

II/O-I) (Figure S4B). It is evident that no positive correlation can be found between the reaction rate and either the Ce3+ or the oxygen vacancy. The results indicate that coordinatively unsaturated metal cation (Ce3+) and oxygen vacancy alone may not be the active site under current reaction condition.

In previous studies, acid sites, base sites, and acid-base pairs have been proposed to be the active sites for ketonization of carboxylic acids on metal oxides catalysts by following different reaction mechanisms.22,24,61-63 To determine which kind of site is the active site for ketonization of propionic acid on CexZr1-xO2 catalysts, we correlate the intrinsic reaction rates with different kinds of sites. When the intrinsic reaction rate was plotted as a function of densities of weak acid sites, medium-strength acid sites, and total acid sites (Figure S5), no clear correlation can be found, indicating that the acid sites alone may not be the active sites for ketonization. Similarly, no correlation can be found between the intrinsic reaction rate and the densities of weak base sites, mediumstrength base sites, strong base sites and total base sites (Figure S6), indicating that the base sites alone are not the active site. We then correlated the reaction rate with the


26

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ratio of different types of acid sites to medium-strength base sites, as shown in Figure S7 and Figure 12. The results indicate that the reaction rate is related to the mediumstrength acid/base ratio (Figure 12). The reaction rate is maximized at the ratio of medium-strength acid/base ratio of ~1.07 on Ce0.1Zr0.9O2 among all catalysts. This value is close to 1. Either more medium-strength acid sites (ratio > 1) or more mediumstrength base sites (ratio < 1) appear to have a negative effect on the reaction rate. The result indicates that balanced medium-strength acid/base property is important. The medium strength Lewis acid-base pairs (Mx+–O2-) have the balanced strength and density of acid and base sites, and therefore are the active sites for the ketonization of propionic acid on CexZr1-xO2 catalysts.

The DRIFT spectra of propionic acid adsorption (Figure 5A) shows that the monodentate and bridging bidentate carboxylate are the most abundant surface species on CeO2, CexZr1-xO2 and ZrO2, while chelating bidentate carboxylate is a minor species present on CexZr1-xO2. The formation of these surface species may be related to both the surface geometry and surface acid-base properties. In the previous studies,9,24,64 it


27


Page 28 of 52

was believed that the bridging bidentate carboxylate is the active adsorption mode to form enolate toward C-C coupling, due to it is the most stable adsorption configuration. However, Wang and Iglesia22,23 proposed that monodentate carboxylate is the precursor of enolate, while the bridging bidentate carboxylate is a spectator due to it has a higher barrier for α-H abstraction for the formation of enolate. Recently, Cai et al63 suggested that the bridging bidentate carboxylate is firstly transformed to monodentate carboxylate, and then α-H abstraction is occurred toward enolate formation and C-C coupling. Therefore, the higher monodentate/bidentate carboxylate ratio on Ce0.1Zr0.9O2 (Figure 5A) favors enolate formation, leading to higher reaction rate.

Based on literature work22-24,63 and current results, the proposed possible surface reaction of ketonization is displayed in Scheme 2. Propionic acid adsorbs on Lewis acid site (metal cation) through carbonyl group, followed by dehydrogenates α-H to a vicinal base site (O anion), forming active enolate. Then, the enolate species nucleophilically attacks neighboring propionic acid, resulting in the formation of β-keto carboxylate.9,2224,63

The following H2O elimination and decarboxylation of CO2 happen easily toward the


28

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formation of 3-pentanone.

Apparently, concerted functionalities of acid site (metal

cation) and base site (O anion) are required to catalyze ketonization. The balanced acid-base property (both strength and density) is important for propionic acid adsorption, formation of enolate, as well as C-C coupling, and therefore influences the ketonization reaction rate.

4. CONCLUSION CexZr1-xO2 catalysts were prepared by a co-precipitation method and tested in vapor phase ketonization of propionic acid at 270-350 oC. The acidity of the CexZr1-xO2 is increased as Zr content is increased. The density of the strong base sites is reduced with increasing Zr content, while the density of the medium-strength base sites is maximized on Ce0.5Zr0.5O2. DRIFTS study of propionic acid adsorption indicated that monodenate and bridging bidentate carboxylate are the most abundant surface intermediates. And increasing Zr content in CexZr1-xO2 favors monodentate adsorption over bidentate adsorption. The reaction rate shows two maxima on Ce0.9Zr0.1O2 (49.8 mmol∙g-1∙min-1) and Ce0.1Zr0.9O2 (60.3 mmol∙g-1∙min-1), which have cubic and tetragonal


29


Page 30 of 52

structure, respectively, suggesting that the crystal structure may have complex effect on ketonization. Meanwhile, the intrinsic reaction rate of ketonization is maximized on Ce0.1Zr0.9O2 with a medium-strength acid/base ratio of ~1.07, indicating that the medium-strength Lewis acid-base pairs with balanced acidic/basic property are the active sites for ketonization. Our results indicate that tuning the Ce/Zr ratio is a facile approach to control the activity of ketonization of carboxylic acids.

ASSOCIATED CONTENT

Supporting Information.

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

DOI: of

N2

adsorption-desorption

isotherms;

DRIFTS

of

CH3CH2COONa;

Temperature-programmed oxidation profiles of spent catalysts; Correlation of ketonization rate with concentration of Ce3+ and the atomic ratio of O-II/O-I; Correlation of ketonization rate with density of acid site; Correlation of ketonization rate with density


30

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of base site; Correlation of ketonization rate with the density ratio of acid/mediumstrength base site.

AUTHOR INFORMATION

Corresponding Author E-mail: [email protected]

ORCID

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21676194 and 21576204).

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Figures and Tables Table 1. Specific Surface Area, Crystallite Size, and Lattice Distance of CexZr1-xO2 Catalysts Catalyst

SBET (m2 g-1)

Crystallite size a (nm)

Lattice distance a (Å)

CeO2

49

13.8

3.129

Ce0.9Zr0.1O2

67

13.2

3.128

Ce0.75Zr0.25O2

58

12.4

3.125

Ce0.5Zr0.5O2

57

11.4 b (6.2 c)

3.112 b (3.002 c)

Ce0.25Zr0.75O2

66

8.1

2.994

Ce0.1Zr0.9O2

72

10.0

2.969

ZrO2

81

9.8

3.159

The calculation of crystallite size and lattice distance were carried out by using peaks of (111) (2θ = 28.5°, fluorite cubic structure) for CeO2, Ce0.9Zr0.1O2, Ce0.75Zr0.25O2, and Ce0.5Zr0.5O2; (101) (2θ = 29.7°, tetragonal phase) for Ce0.5Zr0.5O2, Ce0.25Zr0.75O2 and _ Ce0.1Zr0.9O2; ( 11) (2θ = 28.2°) for ZrO2 (monoclinic phase). b Calculated by (111) (2θ = 1 28.5°). c Calculated by (101) (2θ = 29.7°). a


39


† †

 † †

* CeO2

 †

g

Intensity (a.u.)

† m-ZrO2



 t-ZrO2

f e

*

d

*

*

*

c b a

20

40

60 2 (o)

80

100

Figure 1. X-ray diffraction patterns of CexZr1-xO2 catalysts. (a) CeO2, (b) Ce0.9Zr0.1O2, (c) Ce0.75Zr0.25O2, (d) Ce0.5Zr0.5O2, (e) Ce0.25Zr0.75O2, (f) Ce0.1Zr0.9O2, (g) ZrO2. ††

g†

Intensity (a.u.)

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

Page 40 of 52

f

†† †

† •

• •

† •

†† •

* CeO2 † m-ZrO2 • t-ZrO2

* e

d c b a 200

400 600 Shift (cm-1)

800

1000

Figure 2. Raman spectra of CexZr1-xO2 catalysts. (a) CeO2, (b) Ce0.9Zr0.1O2, (c) Ce0.75Zr0.25O2, (d) Ce0.5Zr0.5O2, (e) Ce0.25Zr0.75O2, (f) Ce0.1Zr0.9O2, (g) ZrO2.

Table 2. Atomic Content of Ce, Zr and O, Atomic Ratios as well as Distribution of Different Valence States of Ce Derived from XPS


40

Page 41 of 52

Surface composition a (mol.%) Catalyst Ce

Zr

O

Atomic ratio of Ce/Zr

Relative concentration of Ce4+ b (area %)

Atomic ratio Ce3+/Ce c

Atomic ratio of O-II/O-I

(%)

CeO2

19.1

0

50.7

-

12.41

22.6

0.47

Ce0.9Zr0.1O2

17.6

1.9

48.5

9.26

10.70

26.5

0.52

Ce0.5Zr0.5O2

10.1

10.6

51.2

0.95

10.56

27.4

0.63

Ce0.1Zr0.9O2

1.5

21.3

51.0

0.07

11.66

26.1

0.52

ZrO2

0

24.9

53.2

-

-

-

0.46

C is not reported. b is calculated by the area ratio of u′′′ to total Ce 3d. c is calculated by the area ratio of u′ + v′to total Ce 3d. a

u (A) Ce 3d u''' u'' u' v'''

d

Counts per second

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


v''

v'

v

e e d

b

c

d

a a

920

O-I O-II

3d3/2

×2

c

(C) O 1s

3d5/2

(B) Zr 3d

b

c b

910 900 890 880 189 Binding energy (eV)

×2 a 186 183 180 177536 534 532 530 528 526 524 Binding energy (eV) Binding energy (eV)

Figure 3. XPS spectra of (A) Ce 3d, (B) Zr 3d, and (C) O 1s. (a) CeO2, (b) Ce0.9Zr0.1O2, (c) Ce0.5Zr0.5O2, (d) Ce0.1Zr0.9O2, (e) ZrO2. Solid line, experimental data; short dash lines, curve fittings.

Table 3. Quantification of Acid Sites and Base Sites


41


Acid site a (µmol g-1) Catalyst

Weak

Mediu m

Page 42 of 52

Base site b (µmol g-1)

Total

Weak

Medium

Stron g

Total

CeO2

64

70

134

123

106

46

275

Ce0.9Zr0.1O2

62

89

151

168

113

43

323

48

112

160

175

148

45

368

67

142

209

192

217

18

427

82

147

229

169

200

14

383

Ce0.1Zr0.9O2

61

189

250

168

176

13

357

ZrO2

90

271

361

136

155

11

302

Ce0.75Zr0.25O 2

Ce0.5Zr0.5O2 Ce0.25Zr0.75O 2

a Derived

from NH3-TPD. b Derived from CO2-TPD.


42

Page 43 of 52

(A) weak medium 232 141 341

0.5

MS Signal (a.u.)

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


g

(B) weak medium strong 132 249 467

f

f

e

e

d

d

c

c 270 135

100

b

200

a

200 300 400 500 Temperature (oC)

1.0 g

b 121

228

510

a

600 100 200 300 400 500 600 700 800 Temperature (oC)

Figure 4. NH3-TPD profiles (A) and CO2-TPD profiles (B) of CexZr1-xO2 catalysts. (a) CeO2, (b) Ce0.9Zr0.1O2, (c) Ce0.75Zr0.25O2, (d) Ce0.5Zr0.5O2, (e) Ce0.25Zr0.75O2, (f) Ce0.1Zr0.9O2, (g) ZrO2. Solid line, experimental data; short dash lines, curve fittings.


43


Absorbance (a.u.)

1603

2978 2942 2880

(A) e

1480 1419 1300 1526 1378

1566

0.2 1080

1446

d 1580

c

1540

b 3578

1775 1791 1722

1473 1420

1251

1146

a 4000 3500 3000 1800 1600 1400 1200 1000 Wavenumber (cm-1) (B) Absorbance (a.u.)

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

Page 44 of 52

1693

d

1697

c

1710 1665

b a

0.2

1731

400035003000 1800 1600 1400 1200 1000 Wavenumber (cm-1)

Figure 5. DRIFTS of 0.2 kPa propionic acid (A) and 0.2 kPa 3-pentanone (B) adsorption on CexZr1-xO2 catalysts at 40 oC. (A), (a) propionic acid vapor, (b) CeO2, (c) Ce0.5Zr0.5O2, (d) Ce0.1Zr0.9O2, (e) ZrO2; (B), (a) 3-pentanone vapor, (b) CeO2, (c) Ce0.1Zr0.9O2, (d) ZrO2.


44

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


Scheme 1. Adsorption configuration of propionic acid and 3-pentanone on metal oxides.

Table 4. The Assignment of Stretching Vibration Frequencies of OCO of Propionic Acid on the CexZr1-xO2 Catalysts

Catalyst

CeO2 Ce0.5Zr0.5O 2

Ce0.1Zr0.9O 2

Monodentate (cm1)

Chelating bidentate (cm-1)

Bridging bidentate (cm1)

νas

νs

νas

νs

νas

νs

1580

1420

-

-

1540

1473

1603

1418

1566

1446

1543

1475

1618

1413

1570

1452

1541

1477


45


ZrO2

1603

1419

-

-

1526

1480

35 CeO2

Conversion of propionic acid (%)

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

Page 46 of 52

30

Ce0.9Zr0.1O2 Ce0.75Zr0.25O2

25

Ce0.5Zr0.5O2 Ce0.25Zr0.75O2

20

Ce0.1Zr0.9O2 ZrO2

15 10 5 0

290

300

310 320 330 Temperature (oC)

340

350

Figure 6. Conversion of propionic acid on CexZr1-xO2 catalysts as a function of temperature. Reaction conditions: Pacid = 3.9 kPa, Ptotal = 101.325 kPa, W/F = 0.05 h, Ar/propionic acid = 25, Time on stream is 30 min for each temperature.


46

Page 47 of 52

a

CeO2

Conversion of propionic acid (%)

60

a

Ce0.1Zr0.9O2

a

ZrO2

50

b

CeO2

b

Ce0.1Zr0.9O2

40

b

ZrO2

30 20 10 0

0

200

400

600 800 1000 Time on stream (min)

1200

Ketonization rate (mmol·min ·g

-1 cat

)

Figure 7. Effect of time on stream on conversion of propionic acid on CeO2, Ce0.1Zr0.9O2 and ZrO2. a Propionic acid reaction on fresh catalysts. b Propionic acid reaction on regenerated catalysts. Reaction conditions: T = 330 oC, Ptotal = 101.325 kPa, Pacid = 3.9 kPa, Ar/Propionic acid = 25, W/F = 0.2 h for CeO2, W/F = 0.15 h for Ce0.1Zr0.9O2, W/F = 0.17 h for ZrO2. 60

-1

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


50 40 30 20 0

20

40 60 80 Zr content (%)

100

Figure 8. Effect of Zr content of CexZr1-xO2 catalysts on the mass based intrinsic ketonization rate. Reaction conditions: T = 330 oC, Ptotal = 101.325 kPa, Pacid = 3.9 kPa, Ar/Propionic acid = 25, Time on stream is 30 min, the conversion is < 15% by adjusting the space time (W/F).


47


360 5

350

340

Temperature (oC) 330 320 310

300

290

280

ln [r (mmolmin-1g-1)]

4

3

CeO2:107.6 kJ/mol Ce0.9Zr0.1O2:106 kJ/mol Ce0.75Zr0.25O2:115.6 kJ/mol

2

Ce0.5Zr0.5O2:106.4 kJ/mol Ce0.25Zr0.75O2:114.3 kJ/mol Ce0.1Zr0.9O2:112.7 kJ/mol ZrO2:116.4 kJ/mol

1 1.580 1.605 1.631 1.658 1.686 1.715 1.745 1.776 1.808 1000/T (K-1)

Figure 9. Arrhenius plots of ketonization of propionic acid CexZr1-xO2 catalysts. Reaction conditions: T = 290-350 oC, Ptotal = 101.325 kPa, Pacid = 3.9 kPa, Ar/Propionic acid = 25, Time on stream is 30 min for each temperature, the conversion is < 15% by adjusting the space time (W/F).

7.5 (A) 6.0 4.5

2.1 (B)

Ce0.1Zr0.9O2

ZrO2

3.0

Ln [r(mmol·min-1·g-1)]

Ketonization rate (mmol·min-1·g-1)

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

Page 48 of 52

1.8

Ce0.1Zr0.9O2: rate ∝ (Pacid)0.03

Ce0.1Zr0.9O2: rate ∝ (Pacid)0.38

1.5 ZrO2: rate ∝ ( Pacid )-0.005

1.2 0.9

ZrO2: rate ∝ ( Pacid )0.55

1.5 0.0 0.3 0.6 0.9 1.2 1.5 1.8 Pacid (kPa)

0.6 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 Ln [Pacid (Pa)]

Figure 10. Effect of propionic acid pressure on ketonization rate on Ce0.1Zr0.9O2 and ZrO2 catalysts (A), Reaction order of ketonization on Ce0.1Zr0.9O2 and ZrO2 catalysts (B).


48

Page 49 of 52

Reaction conditions: T = 270 oC, Ptotal = 101.325 kPa, Time on stream is 30 min for each point, the conversion is < 18% by adjusting space time (W/F).

Ketonization rate (mmol·min-1·mcat-2)

1.0

0.8

0.6

0.4

0.2

0

20

40 60 Zr content (%)

80

100

Figure 11. Effect of Zr content on surface area based intrinsic ketonization rate on CexZr1-xO2 catalysts. Reaction conditions: T = 330 oC, Ptotal = 101.325 kPa, Pacid = 3.9 kPa, W/F = 0.05 h, Ar/Propionic acid = 25, Time on stream is 30 min. The ketonization rate was measured at propionic acid conversion < 15%.

Ce0.1Zr0.9O2

Ketonization rate (mmol·min-1·g-1)

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


60 ZrO2

50

40

30

Ce0.9Zr0.1O2

Ce0.75Zr0.25O2 Ce0.25Zr0.75O2 CeO2 Ce0.5Zr0.5O2

20 0.6

0.8 1.0 1.2 1.4 1.6 1.8 Ratio of medium-strength acid/base site (a.u.)


49


Page 50 of 52

Figure 12. Effect of medium-strength acid/base site ratio on the mass based intrinsic ketonization rate. Reaction conditions: T = 330 oC, Ptotal = 101.325 kPa, Pacid = 3.9 kPa, Ar/Propionic acid = 25, Time on stream is 30 min, the conversion is < 15% by adjusting the space time (W/F).


50

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


Acid site Base site

O

O

O

O M M M is Ce or Zr cation

M

CH3 HO

Monodentate adsorption H H

O

CH2

O CH3

CH3

C

C

C

O

M

O

O

M

Bridging bidentate adsorption

CH

H

H

O

O

C

O H

O

M

M

O

CH3 H

Enolate O

O

O

O

M

CH H

C

O

M

CH3

H3C

O

O

O

M

H2C

CH

C

O C-C Coupling H

M

O

M

H

C

O H

O O

M

O

CH3 O C

Product

H3C CH2 H2C C O

O

M

O

M

O O

M

O

H

H O

Scheme 2. Proposed surface reaction of propionic acid ketonization over CexZr1-xO2.


51


Page 52 of 52

Table of Contents (TOC) Graphic Biomass-der ived O R C OH Car boxylic acids O CexZr 1-xO 2 R' C OH Fast pyrolysis

Acid-base site

CO 2

O R C R' H 2O


52

Ketonization of Propionic Acid to 3-Pentanone over CexZr1âxO2

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