Cross-Aldol Condensation of Acetone and N-Butanol Into Aliphatic Ketones Over Supported Cu Catalysts on Ceria-Zirconia

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Article Cross-Aldol Condensation of Acetone and n-Butanol into Aliphatic Ketones over Supported Cu Catalysts on Ceria-Zirconia

Minseok Kim 1, Jongha Park 1, Hari Prasad Reddy Kannapu 1,2 and Young-Woong Suh 1,2,* ID

1 Department of Chemical Engineering, Hanyang University, Seoul 04763, Korea; [email protected] (M.K.); [email protected] (J.P.); [email protected] (H.P.R.K.) 2 Research Institute of Industrial Science, Hanyang University, Seoul 04763, Korea * Correspondence: [email protected]; Tel.: +82-2-2220-2329

Academic Editor: Christophe Len Received: 7 August 2017; Accepted: 23 August 2017; Published: 24 August 2017

Abstract: A long-chain hydrocarbon biofuel of jet fuel range can be produced via aldol condensation of fermented products such as acetone and alcohols over the catalysts containing both metallic sites for the dehydrogenation of alcohols and basic sites for the condensation reaction. However, an efficient catalyst system has not been studied widely yet the route is promising for biofuel production. In this work, Cu catalysts supported on ceria-zirconia (Cu/xCeZr) were prepared using coprecipitated CexZr1-xO2 supports with different Ce/Zr ratios for the cross-aldol condensation of acetone and n-butanol into mono- and di-alkylated aliphatic ketones, 2-heptanone and 6-undecanone. The acetone conversion and 6-undecanone selectivity increased with specific Cu surface area due to formation of the dehydrogenation product butyraldehyde at a higher concentration. The total yield of cross-aldol condensation products was strongly dependent on a combination of Cu sites and basic sites. This was confirmed by the results in the reaction between acetone and butyraldehyde over supported Cu catalysts that additionally examined the adsorbed acyl species on Cu surface taking part in the aldol condensation reaction. The best catalytic performance was achieved with Cu/0.8CeZr showing the largest Cu surface and the highest base site density among Cu/xCeZr catalysts. While the activity of Cu/0.8CeZ was comparable to or a little higher than that of Cu/MgO, the former catalyst was more stable than the latter suffering from the transformation of MgO to Mg(OH)2 by the reaction. Consequently, it is suggested that Cu supported on ceria-zirconia of ceria rich contains such a dual function that it can efficiently catalyze the cross-aldol condensation of acetone and n-butanol.

Keywords: copper; ceria-zirconia; dehydrogenation; cross-aldol condensation; basic sites

1. Introduction

Due to CO2 emission and diminishing reserves of fossil fuels, carbon-neutral fuel alternatives have been attracting much attention in recent decades. A typical example is biofuel produced either by direct conversion such as fermentation, gasification, pyrolysis and liquefaction [1] or by indirect conversion based upon syncretic chemistry (i.e., combination of biological and chemical transformations) [2,3]. A promising route for the latter-related process has been recently proposed such that acetone, ethanol and butanol produced via anaerobic fermentation can be transformed into long-chain hydrocarbons of jet fuel range [4–8]. There are two important steps in this conversion; the first step is alcohol dehydrogenation into aldehyde over metallic sites and the second is aldol condensation between acetone of enol/enolate type (adsorbed acetone over basic sites) and in-situ formed aldehyde, finally resulting in mono- and di-alkylated aliphatic ketones. In case of the condensation reaction between

Catalysts 2017, 7, 249; doi:10.3390/catal7090249 www.mdpi.com/journal/catalysts Catalysts 2017, 7, 249 2 of 14 Catalysts 2017, 7, 249 2 of 14 acetoneand 6-undecanone and n-butanol, (C11), the which desired are products formed arewhen 2-heptanone a molar stoichiometry (C7) and 6-undecanone of acetone/n (C11),-butanol which is 1:1 are and 1:2, respectively. formed when a molar stoichiometry of acetone/n-butanol is 1:1 and 1:2, respectively. This reaction requires two catalytic functions for the dehydrogenation of alcohol into aldehyde This reaction requires two catalytic functions for the dehydrogenation of alcohol into aldehyde and for the condensation of acetone with aldehyde. The detailed reaction pathway is depicted in and for the condensation of acetone with aldehyde. The detailed reaction pathway is depicted in Scheme 1 including possible side reactions relevant to the conversion of n-butanol explained later. Scheme1 including possible side reactions relevant to the conversion of n-butanol explained later. In the first report of Toste and coworkers [5], a noble metal-supported catalyst and basic K3PO4 were In the ﬁrst report of Toste and coworkers [5], a noble metal-supported catalyst and basic K3PO4 were used together. Further study revealed a high performance of MgO–Al2O3 hydrotalcite as a basic used together. Further study revealed a high performance of MgO–Al O hydrotalcite as a basic support onto which Pd, Ru, and Cu (the most active) were loaded [6]. Moreover,2 3 they demonstrated support onto which Pd, Ru, and Cu (the most active) were loaded [6]. Moreover, they demonstrated how Cu/hydrotalcite and Pd-Cu/hydrotalcite catalysts affect the aldol condensation reaction and howwhich Cu/hydrotalcite elementary step and is rate Pd-Cu/hydrotalcite determining [7,8]. catalystsA series of affect their thereports aldol are condensation very elegant and reaction deliver and whichhelpful elementary insight into step catalytically is rate determining active sites [ 7for,8]. th Ae seriescross-aldol of their condensation reports are of very acetone elegant and and alcohols. deliver helpfulHowever, insight their into catalyst catalytically development active has sites been for limited the cross-aldol to hydrotalcite condensation as a support of acetone material. and alcohols. However, their catalyst development has been limited to hydrotalcite as a support material.

SchemeScheme 1.1. Cross-aldolCross-aldol condensation of of acetone acetone and and nn-butanol.-butanol.

In this work, we focus on ceria-zirconia mixed oxide (CexZr1−xO2), since it proved to be effective In this work, we focus on ceria-zirconia mixed oxide (CexZr1−xO2), since it proved to be effective inin oxygenate oxygenate couplingcoupling reactionsreactions suchsuch asas ketonizationketonization and aldol condensation. Mallinson Mallinson et etal. al. reported that zirconia-rich CexZr1−xO2 catalysts (x < 0.5) favored aldol condensation and ceria-rich reported that zirconia-rich CexZr1−xO2 catalysts (x < 0.5) favored aldol condensation and ceria-rich ones (x > 0.5) formed more ketonization products in the condensation of propanal [9]. The reaction ones (x > 0.5) formed more ketonization products in the condensation of propanal [9]. The reaction preference occurred by shifting a balance in the quantity and strength of CexZr1−xO2’s Lewis preference occurred by shifting a balance in the quantity and strength of CexZr1−xO2’s Lewis acid-base acid-base sites pairs [10]. The Ce/Zr ratio of 1:1 was found to be optimum in catalyzing both sites pairs [10]. The Ce/Zr ratio of 1:1 was found to be optimum in catalyzing both ketonization ketonization and aldol condensation reactions, resulting from the best combination of each reaction and aldol condensation reactions, resulting from the best combination of each reaction activity as activity as well as catalyst stability. This composition of ceria-zirconia catalyst has been used in well as catalyst stability. This composition of ceria-zirconia catalyst has been used in upgrading upgrading light fractions of a simulated bio-oil [11], aldehyde [12], and carboxylic acids [13,14]. A light fractions of a simulated bio-oil [11], aldehyde [12], and carboxylic acids [13,14]. A catalyst catalyst consisting of Pd metal and Ce0.5Zr0.5O2 support was also demonstrated to be stable and consisting of Pd metal and Ce Zr O support was also demonstrated to be stable and effective in effective in the ketonization0.5 reaction0.5 2 [15–17]. However, these studies focused on the theself-condensation ketonization reaction reaction [15 (i.e.,–17]. condensation However, these betw studieseen the focused identical on reactant the self-condensation molecules), which reaction is (i.e.,unlike condensation the reaction between in this the work identical involving reactant a co molecules),uple of alcohol which dehydrogenation is unlike the reaction and cross-aldol in this work involvingcondensation a couple between of alcohol acetone dehydrogenation and in-situ form anded cross-aldol aldehyde. condensation On the other between hand, acetonethe aldol and in-situcondensation formed aldehyde.of acetone On with the otherfurfural hand, has the been aldol extensively condensation investigated of acetone within recent furfural years has but been extensivelyCexZr1−xO2 investigatedhas never been in used recent as years a support but Ce [18].xZr 1 −xO2 has never been used as a support [18].

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Thus, CexZr1−xO2 is used herein as a support for the cross-aldol condensation of acetone and n-butanol,Thus, Ce andxZr the1− xactivitiesO2 is used of Cu herein catalysts as a supported support foron Ce thexZr cross-aldol1−xO2 are examined, condensation which of is the acetone first andreportn-butanol, as far as and we theknow. activities Note that of Cu Cu catalysts metal is supportedselected for on alcohol CexZr dehydrogenation1−xO2 are examined, because which its ishigh the firstactivity report is already as far asexamined we know. [19]. Note In the that preparation Cu metal of is Ce selectedxZr1−xO for2, supports alcohol hereafter dehydrogenation referred becauseto as xCeZr, its high the activityx value representing is already examined the [Ce]/{[Ce]+[Zr]} [19]. In thevaries preparation from 0 (pure of CeZrOxZr2) 1to− x1O (pure2, supports CeO2). hereafterFor comparison, referred to basic as x CeZr,MgO theandx acidicvalue representingAl2O3 are used the as [Ce]/{[Ce]+[Zr]} a support for Cu varies loading. from 0Physical (pure ZrO and2) tochemical 1 (pure CeOcharacteristics2). For comparison, of all supports, basic calcined MgO and CuO acidic samples, Al2O3 andare usedreduced as aCu support catalysts for are Cu loading.characterized Physical by and X-ray chemical diffraction characteristics (XRD), of allBrunauer-Emmett-Teller supports, calcined CuO samples,(BET) method, and reduced H2 Cutemperature-programmed catalysts are characterized reduction by X-ray (H diffraction2-TPR), N2O (XRD), reactive Brunauer-Emmett-Teller frontal chromatography (BET) (N2O-RFC), method, 3 2 3 2 Hand2 temperature-programmed temperature-programmed reduction desorption (H2 -TPR),of NH N and2O reactiveCO (NH frontal-TPD chromatographyand CO -TPD). The (N2 O-RFC),acetone conversion and product selectivities are correlated with specific Cu surface area, acid/base site and temperature-programmed desorption of NH3 and CO2 (NH3-TPD and CO2-TPD). The acetone conversiondensities andor a productcombination selectivities thereof. are From correlated this appr withoach specific and the Cu reaction surface area, results acid/base in the condensation site densities orof a combinationacetone and thereof.butyraldehyde, From this we approach intend to and figure the out reaction key catalyst resultsin properties the condensation for the cross-aldol of acetone andcondensation butyraldehyde, of acetone we intend and to n figure-butanol. out keyFinally, catalyst when properties compared for with the cross-aldol Cu/MgO in condensation terms of the of acetoneactivity and andn-butanol. stability, Finally,Cu supported when compared on ceria-zirconia with Cu/MgO of ceria in rich terms is ofconsidered the activity to andbe catalytically stability, Cu supportedactive and on stable ceria-zirconia in the reaction. of ceria rich is considered to be catalytically active and stable in the reaction.

2.2. Results Results

2.1.2.1. Catalyst Catalyst Characterization Characterization Results Results ForFor a a series series of ofx CeZr,xCeZr, calcined calcined CuO/ CuO/xxCeZr,CeZr, andand reduced reduced Cu/Cu/xCeZr samples with different x values,values, powder powder XRD XRD patterns patterns are are presented presented in in Figure Figure1 .1. The The patterns patterns of of pure pure supports supports 1CeZr1CeZr andand 0CeZr0CeZr resemble resemble the the reflections reflections of of cubic cubic ceria ceria (JCPDS (JCPDS PDF#34-0394) PDF#34-0394) andand tetragonaltetragonal zirconiazirconia (JCPDS PDF#27-0997),PDF#27-0997), respectively. respectively. As As the thex value x value decreases decreases to 0.2, to the 0.2, reflections the reflections of xCeZr of x samplesCeZr samples are shifted are toshifted high angles, to high indicating angles, indicating the transition the from transition the cubic from fluorite the cubic to the fluorite tetragonal to the structure tetragonal [20], structure and they are[20], broadened and they due are to broadened small crystallite due to sizes. small The crystal latterlite finding sizes. isThe confirmed latter finding by the is specific confirmed BET surfaceby the specific BET surface area (SBET) summarized in Table 1. On the other hand, the symmetrical shape of area (SBET) summarized in Table1. On the other hand, the symmetrical shape of all reflections for xCeZrall reflections with x = 0.2,for 0.5,xCeZr and with 0.8 is x due = 0.2, to no0.5, peak and splitting0.8 is due by to the no presence peak splitting of two by phases, the presence which suggests of two thephases, formation which of suggests single ceria-zirconia the formation solid of single solution ceria- [21zirconia]. These solid observations solution [21]. are These similarly observations found in calcinedare similarly CuO/ xfoundCeZr andin calcined reduced CuO/ Cu/xCeZr samples,and reduced explaining Cu/xCeZr that samples, Cu loading explaining and subsequent that Cu calcination/reductionloading and subsequent rarely calcination/reduction change the structure rarely of ceria-zirconia change the structure support. of ceria-zirconia support.

FigureFigure 1. 1.Powder Powder X-ray X-ray diffraction diffraction patterns patterns of ofx CeZrxCeZr (black), (black), calcined calcined CuO/ CuO/xxCeZr (red), and reduced

Cu/Cu/xCeZrxCeZr (blue) (blue) samples. samples. The The reﬂections reflections of of CeO CeO22 (black(black bars),bars), ZrOZrO22 (gray(gray bars), CuO (red bars), and CuCu metal metal (blue (blue bars) bars) are are included included for for peak peak identiﬁcation. identification.

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Table 1. Physical and chemical properties of xCeZr, CuO/xCeZr, and Cu/xCeZr samples.

TablexCeZr 1. Physical andCuO chemical/xCeZr properties of xCeZr, CuO/xCeZr, andCu/x Cu/CeZrxCeZr samples. NCO2 3 NNH3 3 SBET (m2 g−1) SBET (m2 g−1) DCuO 1 (nm) SCu 2 (m2 gCu−1) DCu 2 (nm) dCu 2 (%) xCeZr CuO/xCeZr Cu/xCeZr (μmol g−1) (μmol g−1) 3 3 1CeZr 2 43−1 2 62− 1 1 53 2 15.12 −1 2 45 2 7.4 N321.7CO2 NNH3473.4 SBET (m g ) SBET (m g ) DCuO (nm) SCu (m gCu ) DCu (nm) dCu (%) −1 −1 0.8CeZr 104 84 35 24.3 35 11.9 (µmol 619.0 g ) (µmol 823.7 g ) 1CeZr0.5CeZr 43104 62 82 53 51 15.1 10.7 45 63 7.4 5.2 321.7 497.6 473.4 842.3 0.8CeZr0.2CeZr 10498 84 66 35 72 24.3 9.0 35 75 11.9 4.4 619.0 618.7 823.71183.2 0.5CeZr 104 82 51 10.7 63 5.2 497.6 842.3 0.2CeZr0CeZr 98102 66 57 72 78 9.0 5.2 75 131 4.4 2.5 618.7560.4 1183.2944.2 0CeZr1 Calculated102 using the 57 CuO(111) reflection 78 at 2θ 5.2= 38.8°; 2 Calculated 131 using 2.5 the results 560.4 of N2O-RFC 944.2 1 ◦ 2 experiments;Calculated using 3 Measured the CuO(111) by TPD reﬂection experiments at 2θ = 38.8 using; Calculated CO2 and usingNH3 for the N resultsCO2 and of NN2NH3O-RFC, respectively. experiments; 3 Measured by TPD experiments using CO2 and NH3 for NCO2 and NNH3, respectively.

InIn allall XRD XRD patterns patterns of of CuO/ CuO/xxCeZr,CeZr, thethe reﬂectionsreflections ofof CuOCuO correspondingcorresponding toto tenoritetenorite (JCPDS(JCPDS PDF#05-0661)PDF#05-0661) are are detected detected at 2atθ =2 35.5θ =◦ 35.5°and 38.8 and◦. When38.8°. calculatedWhen calculated using the using latter the CuO(111) latter reﬂectionCuO(111) reflection by the Scherrer equation, the average particle size of CuO (DCuO) is the smallest for by the Scherrer equation, the average particle size of CuO (DCuO) is the smallest for CuO/0.8CeZr and CuO/0.8CeZr and the largest for CuO/0CeZr (Table 1). Thus, H2-TPR measurement was conducted the largest for CuO/0CeZr (Table1). Thus, H 2-TPR measurement was conducted in order to study a reductionin order to characteristic study a reduction of CuO characteristic particles and of also CuO an particles interaction and between also an interaction metal oxide between and support. metal Inoxide Figure and2, CuO/0CeZrsupport. In exhibitsFigure 2, a broadCuO/0CeZr reduction exhibi peakts witha broad a maximum reduction at peak 485 K, with while a CuO/1CeZrmaximum at shows485 K, thewhile similar CuO/1CeZr peak at shows 494 K the together similar with peak a at small 494 humpK together at 417 with K. a The small broad hump reduction at 417 K. peak The correspondsbroad reduction to direct peak reduction corresponds of bulk to direct CuO crystallites reduction toof CuObulk becauseCuO crystallites the reduction to CuO of bulk because CuO the is generallyreduction observed of bulk atCuO ca. 520is generally K. In case observed of mixed ceria-zirconiaat ca. 520 K. solidIn case solutions of mixed (e.g., ceria-zirconia 0.2CeZr, 0.5CeZr, solid solutions (e.g., 0.2CeZr, 0.5CeZr, and 0.8CeZr in this work), this peak is splitted into two peaks and 0.8CeZr in this work), this peak is splitted into two peaks towards low temperatures (denoted towards low temperatures (denoted as Tr1 and Tr2 for low- and high-temperature peaks). From as Tr1 and Tr2 for low- and high-temperature peaks). From literature [22–25], the peaks observed at literature [22–25], the peaks observed at Tr1 and Tr2 are ascribed to the reduction of highly dispersed Tr1 and Tr2 are ascribed to the reduction of highly dispersed CuO species strongly interacting with CuO species strongly interacting with the support and the reduction of segregated CuO crystalline the support and the reduction of segregated CuO crystalline particles, respectively. The shift to low particles, respectively. The shift to low temperatures and the difference between Tr1 and Tr2 becomes temperatures and the difference between Tr1 and Tr2 becomes larger as the x value increases from larger as the x value increases from 0.2 to 0.8. This indicates that CuO particles supported on 0.8CeZr 0.2 to 0.8. This indicates that CuO particles supported on 0.8CeZr are the smallest, which is in good are the smallest, which is in good agreement with XRD results of CuO/xCeZr samples. agreement with XRD results of CuO/xCeZr samples.

FigureFigure 2. 2.H H2 2temperature-programmed temperature-programmed reduction reduction proﬁles profiles of of CuO/ CuO/xxCeZrCeZr samples.samples.

Furthermore,Furthermore, N N22O-RFCO-RFC experimentexperiment waswas conductedconducted toto evaluate evaluate a a Cu Cu dispersion dispersion ( d(CudCu),), aa speciﬁcspecific CuCu surface surface area area (S Cu(S),Cu and), and a size a ofsize metallic of metallic Cu particles Cu particles (DCu) for (D reducedCu) for Cu/reducedxCeZr Cu/ samplesxCeZr (Tablesamples1). As(Table the CeO1). As2 content the CeO increases2 content the increases Cu dispersion the Cu increasesdispersion from increases 2.5% forfrom Cu/0CeZr 2.5% for toCu/0CeZr 11.9% for to Cu/0.8CeZr11.9% for Cu/0.8CeZr and then droppedand then todropped 7.4% for to Cu/1CeZr. 7.4% for Cu/1CeZr. The opposite The opposite trend is foundtrend foris found Cu particle for Cu sizeparticle because size smallerbecause particles smaller particles typically typically show higher show fractions higher fractions of exposed of exposed metal atoms metal at atoms the surface. at the

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surface. Thus, the SCu value shows a volcano-type relationship against the Ce/(Ce + Zr) ratio with a Thus,maximum the SCu ofvalue 24.3 showsm2 gCu− a1 for volcano-type Cu/0.8CeZr. relationship The Cu dispersion against theis closely Ce/(Ce associated + Zr) ratio with with the a maximumextent of 2 −1 ofT 24.3r1 shift m observedgCu for in Cu/0.8CeZr. H2-TPR measurement; The Cu dispersion in other words, is closely the associated sample with with a higher the extent Cu dispersion of Tr1 shift observedshows a in lower H2-TPR Tr1. Particularly, measurement; the inlargest other shift words, of T ther1 is sampleobserved with for CuO/0.8CeZr a higher Cu dispersionwith the highest shows a lowerdCu. Therefore,Tr1. Particularly, the N2O-RFC the largestand H2 shift-TPR ofresultsTr1 is suggest observed that for the CuO/0.8CeZr interaction of supported with the highest CuO anddCu . Therefore,Cu species the with N2O-RFC ceria-zirconia and H 2surface-TPR results is stronger suggest for ceria-rich that the interactionCexZr1−xO2 supports. of supported This is CuO consistent and Cu specieswith the with report ceria-zirconia of Liu et al. surface for CuO/Ce is strongerxZr1−xO for2 catalysts ceria-rich in Ce NOxZr reduction1−xO2 supports. that the Thisceria-rich is consistent phase withcam the interact report more of Liu strongly et al. for with CuO/Ce copperxZr species1−xO2 catalyststhan the zirconia-rich in NO reduction phase that and the thereby ceria-rich disperse phase cammetallic interact copper more particles strongly effectively with copper [22]. species than the zirconia-rich phase and thereby disperse metallicNext, copper the particles densities effectively of basic [22 ].and acidic sites for reduced Cu/xCeZr samples on a per-gram-sampleNext, the densities basis of basicwere andmeasur acidiced sitesby TPD for reduced experiments Cu/x CeZrusing samples CO2 and on aNH per-gram-sample3 as a probe molecule, respectively. Figure 3a shows the desorption profiles of CO2 detected using a TCD basis were measured by TPD experiments using CO2 and NH3 as a probe molecule, respectively. detector. All samples possess weak adsorption sites in 320–500 K associated with surface hydroxyl Figure3a shows the desorption profiles of CO 2 detected using a TCD detector. All samples possess groups [26]. When a small amount of ZrO2 is mixed with ceria, the amount of CO2 desorbed above weak adsorption sites in 320–500 K associated with surface hydroxyl groups [26]. When a small amount 450 K is enhanced, meaning a greater number of medium and strong CO2 binding sites associated of ZrO is mixed with ceria, the amount of CO desorbed above 450 K is enhanced, meaning a greater 2 n+ 2− 22− with M –O pairs and low coordination O anions, respectivelyn+ [27].2− This also happens when a number of medium and strong CO2 binding sites associated with M –O pairs and low coordination 2small− amount of CeO2 is mixed with zirconia. Thus, the relationship between the base site density O anions, respectively [27]. This also happens when a small amount of CeO2 is mixed with zirconia. (NCO2) and the Ce/(Ce + Zr) ratio appears to be bimodal; the maximum NCO2 value is ca. 619.0 μmol Thus, the relationship between the base site density (NCO2) and the Ce/(Ce + Zr) ratio appears to be g−1 for Cu/0.8CeZr and Cu/0.2CeZr (Table 1). On the other hand, the acid site density (NNH3) of bimodal; the maximum N value is ca. 619.0 µmol g−1 for Cu/0.8CeZr and Cu/0.2CeZr (Table1). reduced Cu/xCeZr samplesCO2 was measured using NH3-TPD profiles shown in Figure 3b. When a On the other hand, the acid site density (N ) of reduced Cu/xCeZr samples was measured using small amount of CeO2 is mixed with zirconiaNH3 having Lewis acid sites generally, the total area for NH3 NHdesorption3-TPD profiles appears shown to be inincreased Figure3 b.a little. When The a smallfurther amount addition of of CeO CeO2 2is leads mixed to a with decrease zirconia in surface having Lewisacidity. acid This sites is confirmed generally, by the the total trend area of forNNH3 NH value3 desorption against the appears Ce/(Ce + to Zr) be ratio increased with a amaximum little. The furtherof 1183.2 addition μmol of g− CeO1 for2 Cu/0.2CeZr.leads to a decrease (Table 1). in However, surface acidity. it should This be is confirmednoted here bythat the the trend NNH3 of valueNNH3 −1 valuemeasured against in the this Ce/(Ce work is +a Zr)combination ratio with result a maximum of surface of Cu 1183.2 speciesµmol and g xCeZrfor Cu/0.2CeZr.support, because (Table acid1 ). However,sites can it be should generated be noted by heredeposition that the ofN metallicNH3 value Cu measured particles inonto this support work is amaterials combination [28,29]. result For of surfacePd/ceria-zirconia Cu species and catalysts,xCeZr surface support, acidity because was acid report sitesed canto be be associated generated primarily by deposition with ofzirconia metallic and Cu particlesnot with onto ceria support and Pd materials particles [[16].28,29 ]. For Pd/ceria-zirconia catalysts, surface acidity was reported to be associated primarily with zirconia and not with ceria and Pd particles [16].

(a) (b)

Figure 3. (a) NH3 and (b) CO2 temperature-programmed desorption profiles of Cu/xCeZr samples. Figure 3. (a) NH3 and (b) CO2 temperature-programmed desorption proﬁles of Cu/xCeZr samples.

2.2. Catalytic Performance in the Cross-Aldol Condensation of Acetone and n-Butanol 2.2. Catalytic Performance in the Cross-Aldol Condensation of Acetone and n-Butanol The activity and selectivity results in the cross-aldol condensation of acetone and butanol over Cu/ThexCeZr activity catalysts and with selectivity different results x values in the are cross-aldol shown in Figure condensation 4. The conversion of acetone of and acetone, butanol which over Cu/is xtheCeZr limiting catalysts reactant with in different this work,x values is very are similar shown as in38 Figure± 2.0%4 for. The Cu/1CeZr conversion and ofCu/0CeZr, acetone, and which it is the limiting reactant in this work, is very similar as 38 ± 2.0% for Cu/1CeZr and Cu/0CeZr, CatalystsCatalysts 20172017,, 77,, 249249 66 of of 14 14

increases with the x value from 0.2 to 0.8. This means that an improved catalytic activity is obtained andwhen it increasesmore ceria with is themixedx value with from zirconia. 0.2 to Thus, 0.8. This the meansacetone that conversion an improved shows catalytic a volcano-type activity is obtainedrelationship when against more the ceria Ce/(Ce is mixed + Zr) with ratio zirconia. with a Thus,maximum the acetone of 52.3% conversion over Cu/0.8CeZr. shows a volcano-typeMeanwhile, relationshipthe major product against thein all Ce/(Ce catalytic + Zr) runs ratio is with 2-hept a maximumanone produced of 52.3% by over the Cu/0.8CeZr. cross-aldol Meanwhile, condensation the majorbetween product acetone in all and catalytic n-butanol runs isin 2-heptanone an equimolar produced stoichiome by thetry. cross-aldol Among condensationthe tested catalysts, between acetoneCu/0.8CeZr and nshows-butanol the in lowest an equimolar selectivity stoichiometry. to 3-hepten-2-one Among the (2.6%; tested the catalysts, cross-aldol Cu/0.8CeZr condensation shows theproduct), lowest the selectivity lowest selectivity to 3-hepten-2-one to 2-heptanone (2.6%; (60.0%), the cross-aldol and the highest condensation selectivity product), to 6-undecanone the lowest selectivity(14.7%). Note to 2-heptanone here that (60.0%),2-heptanone and theis the highest hydrogenation selectivity to product 6-undecanone of 3-hepten-2-one (14.7%). Note and here it that is 2-heptanonealkylated to is6-undecanone the hydrogenation once again product by ofbutyraldehyde 3-hepten-2-one formed and it isby alkylated dehydrogenation to 6-undecanone of butanol. once againTherefore, by butyraldehyde it is understood formed that bya high dehydrogenation selectivity to 6-undecanone of butanol. Therefore, (C11) is itdue is understood to enhanced that catalytic a high selectivityperformance to 6-undecanonein both the cross-aldol (C11) is duecondensation to enhanced and catalytic dehydrogenation/hy performancedrogenation in both the cross-aldolreactions. condensationOn the other hand, and dehydrogenation/hydrogenation the production of butyl butyrate is reactions. observed Onover the all other tested hand, catalysts, the productionindicating ofthat butyl n-butanol butyrate reacts is observed with butyric over all acid tested formed catalysts, by disproportionation indicating that n-butanol (Tishchenko) reacts withreaction butyric of acidbutyraldehyde formed by due disproportionation to the acidic/basic (Tishchenko) characters of reaction support of materials butyraldehyde [30,31]. due to the acidic/basic characters of support materials [30,31].

FigureFigure 4.4.Acetone Acetone conversion conversion and productand product selectivities selectivities of Cu/xCeZr of Cu/ catalysts.xCeZr C7 catalysts. includes 3-hepten-2-one,C7 includes 2-heptanone,3-hepten-2-one, and 2-heptanone, 2-heptanol. and 2-heptanol.

3.3. Discussion

3.1.3.1. Key Properties of Cu/xCeZr Catalysts in the Cross-Aldol Condensation of Acetone and n-Butanol The activity and product selectivities are correlated with the specific Cu surface area (S ) and The activity and product selectivities are correlated with the specific Cu surface area (SCu) and acid/base site densities (N and N ) of Cu/xCeZr catalysts. In Figure5, the acetone conversion acid/base site densities (NCO2 and NNH3NH3) of Cu/xCeZr catalysts. In Figure 5, the acetone conversion andand thethe selectivityselectivity to majormajor products, such as 2-heptanone,2-heptanone, butyl butyrate and and 6-undecanone, 6-undecanone, are are plotted against the S value. The acetone conversion appears to be in a straight-line relationship with plotted against the CuSCu value. The acetone conversion appears to be in a straight-line relationship S . Since self-condensation products of acetone are rarely observed under the reaction condition withCu SCu. Since self-condensation products of acetone are rarely observed under the reaction employedcondition inemployed this work, in acetonethis work, is consumed acetone is only cons byumed the cross-aldol only by the condensation cross-aldol withcondensation butyraldehyde. with Thisbutyraldehyde. means that theThis conversion means that of the acetone conversion is efficient of acetone when is more efficient butyraldehyde when more (the butyraldehyde dehydrogenation (the productdehydrogenation of n-butanol) product is produced. of n-butanol) That is,is theproduced. acetone That conversion is, the dependsacetone conversion on available depends surface on Cu atomsavailable responsible surface Cu for theatoms dehydrogenation responsible for of then-butanol. dehydrogenation Therefore, ofSCu nis-butanol. believed Therefore, to be a principal SCu is contributorbelieved to tobe thea principal acetone conversion.contributor Thisto the is aceton supportede conversion. by the fact This that is thesupported other catalyst by the properties, fact that i.e.,the other the acid/base catalyst properties, site densities, i.e., the are acid/base not clearly site relateddensities, with are thenot acetoneclearly related conversion with the (Figure acetone S1). Forconversion confirmation (Figure of theS1). above For co discussion,nfirmation basicof the MgO above and discussion, acidic Al2 Obasic3 were MgO used and as acidic a support Al2O material3 were forused Cu as loading a support of 10 material wt % same for asCu Cu/ loadingxCeZr of catalysts. 10 wt % Thesame acetone as Cu/ conversionxCeZr catalysts. is 55.3% The and acetone 39.3% 2 −1 2 −1 overconversion Cu/MgO is 55.3% (SCu =and 27.6 39.3% m g overCu Cu/MgO) and Cu/Al (SCu2 O= 327.6(SCu m2= g 9.2Cu−1 m) andgCu Cu/Al), respectively.2O3 (SCu = 9.2 Asm2 clearlygCu−1),

Catalysts 2017, 7, 249 7 of 14 Catalysts 2017, 7, 249 7 of 14

respectively. As clearly noticed in Figure 5a, the activity results of both catalysts against SCu are close to a fitted line, revealing again that SCu is the important catalyst property in determining the acetone noticed in Figure5a, the activity results of both catalysts against S are close to a ﬁtted line, revealing conversion. Cu again that SCu is the important catalyst property in determining the acetone conversion.

(a) (b)

FigureFigure 5. (5.a )(a Acetone) Acetone conversion conversion and and ( b(b)) product product selectivitiesselectivities plotted against against the the specific speciﬁc Cu Cu surface surface area (SCu) of Cu/xCeZr (circles), Cu/MgO (squares), and Cu/Al2O3 (triangles) catalysts. C7 includes area (SCu) of Cu/xCeZr (circles), Cu/MgO (squares), and Cu/Al2O3 (triangles) catalysts. C7 includes 3-hepten-2-one, 2-heptanone, and 2-heptanol. 3-hepten-2-one, 2-heptanone, and 2-heptanol.

On the other hand, the selectivity to 6-undecanone tends to increase with SCu, whereas the On the other hand, the selectivity to 6-undecanone tends to increase with SCu, whereas the correlations between SCu and the selectivities to 2-heptanone and butyl butyrate are not good correlations between SCu and the selectivities to 2-heptanone and butyl butyrate are not good (Figure 5b). The former finding is consistent with the above explanation because the increased (Figure5b). The former finding is consistent with the above explanation because the increased production of 6-undecanone from 2-heptanone is possible by a higher concentration of production of 6-undecanone from 2-heptanone is possible by a higher concentration of butyraldehyde butyraldehyde achieved over more surface Cu atoms. This discussion is still appropriate even when achieved over more surface Cu atoms. This discussion is still appropriate even when the product the product selectivities over Cu/MgO and Cu/Al2O3 are considered together. However, since the selectivities over Cu/MgO and Cu/Al O are considered together. However, since the latter latter observation is rather difficult to understa2 3 nd, a correlation work is further made between NNH3 observationand the product is rather selectivities, difficult to as understand, shown in Figure a correlation 6. The remarkable work is further point made to be betweenaddressedN NH3is thatand the the productselectivity selectivities, to butyl as butyrate shown inincreases Figure6 .with The N remarkableNH3. This means point tothat be its addressed formation is thatis dependent the selectivity on to butylacidic butyratesites generally increases known with to NplayNH3 a. key This role means in the that esterification its formation reaction, is dependent which is supported on acidic by sites generallythe high known ester toselectivity play a key (39.1%) role in obtained the esterification over Cu/Al reaction,2O3 (N whichNH3 = 2053.9 is supported μmol byg−1). the Thus, high the ester −1 selectivityselectivity (39.1%) sum of obtained mono-alkylation over Cu/Al products2O3 (N NH3(3-hepten-2-one,= 2053.9 µmol 2-heptanone, g ). Thus, and the selectivity2-heptanol) sum and of mono-alkylationdi-alkylation product products (6-undecanon (3-hepten-2-one,e) decreases 2-heptanone, with increasing and 2-heptanol) NNH3. and di-alkylation product (6-undecanone)However, decreases we failed with to find increasing a good correlationNNH3. between NCO2 and the selectivities to the major productsHowever, (not we shown failed for to the find sake a good of brevity), correlation although between aldol NcondensationCO2 and the reactions selectivities generally to the take major productsplace over (not basic shown sites. for This the may sake suggest of brevity), that the although production aldol of condensation cross-aldol condensation reactions generally products takeis placedetermined over basic not sites. only This by basic may suggestsites but thatsurface the productionCu atoms of ofsupported cross-aldol Cu condensation catalysts. Thus, products the SCu is value is multiplied by the NCO2 value for Cu/xCeZr catalysts, followed by normalization based on the determined not only by basic sites but surface Cu atoms of supported Cu catalysts. Thus, the SCu value of Cu/MgO (NCO2 = 717.7 μmol g−1) showing the highest calculation result. When the yields of value is multiplied by the NCO2 value for Cu/xCeZr catalysts, followed by normalization based on the cross-aldol condensation products are plotted−1 against the normalized value (Ndual) in the range from value of Cu/MgO (NCO2 = 717.7 µmol g ) showing the highest calculation result. When the yields 0 to 1, relatively good correlations are obtained, as presented in Figure 7. As the Ndual value increases, of cross-aldol condensation products are plotted against the normalized value (Ndual) in the range the total yield of C7 and C11 increases along with the C7 yield, indicating that the increased from 0 to 1, relatively good correlations are obtained, as presented in Figure7. As the N value formation of cross-aldol condensation products is achieved possibly by a combination of Cudual sites increases, the total yield of C7 and C11 increases along with the C7 yield, indicating that the increased and basic sites. Although the similar increasing trend is observed for 2-heptanone, the other C7 formation of cross-aldol condensation products is achieved possibly by a combination of Cu sites and products such as 3-hepten-2-one and 2-heptanol show different changes as a function of Ndual basic(Figure sites. 7b). Although This is the obviously similar increasingdue to more trend av isailable observed surface for 2-heptanone,Cu atoms for the the other hydrogenation C7 products suchreaction. as 3-hepten-2-one In the case of and6-undecanone 2-heptanol (di-alkylation show different product), changes a correlation as a function is estimated of Ndual to(Figure be linear7b). This is obviously due to more available surface Cu atoms for the hydrogenation reaction. In the 2 case of 6-undecanone (di-alkylation product), a correlation is estimated to be linear with R = 0.9661, Catalysts 2017, 7, 249 8 of 14 CatalystsCatalysts2017 2017, 7,, 2497, 249 8 of8 14 of 14 with R2 = 0.9661, indicating that the yield of this product is a good measure in evaluating the dual withcatalytic R2 = 0.9661, performance indicating in the that cross- thealdol yield condensation of this product of acetone is a good and measure n-butanol. in evaluating the dual indicating that the yield of this product is a good measure in evaluating the dual catalytic performance catalytic performance in the cross-aldol condensation of acetone and n-butanol. in the cross-aldol condensation of acetone and n-butanol.

Figure 6. Product selectivities plotted against the acid site density (NNH3) of Cu/ xCeZr (circles) and

Cu/Al2O3 (triangles) catalysts. C7 includes 3-hepten-2-one, 2-heptanone, and 2-heptanol. FigureFigure 6. 6.Product Product selectivities selectivities plotted plottedagainst against thethe acidacid sitesite densitydensity (NNH3) )of of Cu/ Cu/xCeZrxCeZr (circles) (circles) and and Cu/AlCu/Al2O2O3 3(triangles) (triangles) catalysts.catalysts. C7 C7 includes includes 3-he 3-hepten-2-one,pten-2-one, 2-heptanone, 2-heptanone, and and 2-heptanol. 2-heptanol.

(a) (b)

FigureFigure 7. 7.(a ()a) Acetone Acetone(a) conversion and and (b ()b product) product selectivities selectivities plotted plotted against( againstb the) normalized the normalized value (Ndual) of Cu/xCeZr (circles) and Cu/MgO (squares) catalysts. C7 includes 3-hepten-2-one, value (Ndual) of Cu/xCeZr (circles) and Cu/MgO (squares) catalysts. C7 includes 3-hepten-2-one, Figure2-heptanone, 7. (a) Acetone and 2-heptanol. conversion and (b) product selectivities plotted against the normalized value 2-heptanone, and 2-heptanol. (Ndual) of Cu/xCeZr (circles) and Cu/MgO (squares) catalysts. C7 includes 3-hepten-2-one, 3.2.2-heptanone, Reaction Pathway and 2-heptanol. in the Cross-Aldol Condensation of Acetone and n-Butanol over Cu/xCeZr 3.2. Reaction Pathway in the Cross-Aldol Condensation of Acetone and n-Butanol over Cu/xCeZr The above analysis suggests that surface Cu sites and acid/base sites should be well balanced 3.2. Reaction Pathway in the Cross-Aldol Condensation of Acetone and n-Butanol over Cu/xCeZr forThe the above selective analysis production suggests of thatcross-aldol surface condensation Cu sites and products, acid/base wh sitesich is should consistent be well with balanced a recent for the selectivereportThe byabove production Toste analysis et al. of[8]. suggests cross-aldol First, surface that condensation surface Cu sites Cu issites products,inevitably and acid/base whichnecessary is sites consistent for should the dehydrogenation withbe well a recent balanced report of byfor Tosten -butanolthe etselective al. [into8]. First,productionbutyraldehyde. surface of Cucr oss-aldolThe sites next is inevitably condensationis the cross-aldol necessary products, condensation for wh theich dehydrogenation is over consistent basic siteswith of throughan -butanolrecent intoreportenolate butyraldehyde. by formationToste et Theal. from [8]. next acetone First, is the surface and cross-aldol subsequent Cu sites condensation isattack inevitably of the over electron-richnecessary basic sites for enolate throughthe dehydrogenation to the enolate electrophilic formation of fromn-butanolcarbonyl acetone intocarbon and butyraldehyde. subsequent of butyraldehyde. attack The ofnext The the is electron-richresulting the cross-aldol ketol enolate is condensationtransformed to the electrophilic toover 3-hepten-2-one basic carbonylsites through by carbon the ofenolate butyraldehyde.dehydration formation reaction The from resulting acetoneover acidic and ketol sitessubsequent is transformedand further attack to toof 2-heptanone 3-hepten-2-onethe electron-rich by the byenolate thehydrogenation dehydration to the electrophilic reaction reaction overcarbonyl acidic sitescarbon and of further butyraldehyde. to 2-heptanone The resulting by the hydrogenation ketol is transformed reaction to over 3-hepten-2-one Cu sites. Furthermore, by the 2-heptanonedehydration is reaction hydrogenated over acidic to 2-heptanol sites and orfurther it is alkylatedto 2-heptanone once againby the into hydrogenation 6-undecanone reaction via the above condensation. Catalysts 2017, 7, 249 9 of 14 Catalysts 2017, 7, 249 9 of 14 over Cu sites. Furthermore, 2-heptanone is hydrogenated to 2-heptanol or it is alkylated once again into 6-undecanone via the above condensation. TheThe above-mentioned above-mentioned reaction reaction pathway, pathway, reﬂected reflected in in Scheme Scheme1 ,1, has has been been wellwell documenteddocumented for for Cu/hydrotalciteCu/hydrotalcite catalysts catalysts [6–8 ].[6–8]. Since theSince signiﬁcant the sign differenceificant difference between hydrotalcitebetween hydrotalcite and ceria-zirconia and is presumedceria-zirconia to beis presumed a basic character, to be a basic we conductedcharacter, we the conducted condensation the condensation reaction of acetonereaction andof butyraldehydeacetone and overbutyraldehydexCeZr without over Cu.xCeZr Interestingly, without Cu. the Interestingly, acetone conversion the acetone is measured conversion to is be measured to be 4–6% for all support materials, including MgO and Al2O3. The similar conversion 4–6% for all support materials, including MgO and Al2O3. The similar conversion values (4–7%) arevalues obtained (4–7%) in the are acetone-butyraldehyde obtained in the acetone-bu condensationtyraldehyde reaction condensation over Cu/ reactionxCeZr, over Cu/MgO, Cu/xCeZr, and Cu/MgO, and Cu/Al2O3. The low acetone conversion values in these experiments suggest that the Cu/Al2O3. The low acetone conversion values in these experiments suggest that the adsorbed acyl adsorbed acyl species is formed from n-butanol by surface Cu particle and it, but not free aldehyde, species is formed from n-butanol by surface Cu particle and it, but not free aldehyde, then reacts then reacts with an acetone enolate supplied by acetone deprotonation over basic sites, yielding the with an acetone enolate supplied by acetone deprotonation over basic sites, yielding the cross-aldol cross-aldol condensation products. In this regard, the product distribution needs to be investigated condensation products. In this regard, the product distribution needs to be investigated because it because it depends on acidic/basic and metallic character of catalysts. Figure 8 shows the product depends on acidic/basic and metallic character of catalysts. Figure8 shows the product selectivities selectivities obtained by the acetone-butyraldehyde condensation reaction over supported Cu obtainedcatalysts. by the Unlike acetone-butyraldehyde the condensation reaction condensation between reaction acetone over and supported n-butanol, Cu the catalysts. detected Unlikeproducts the condensationare 2-ethyl-2-hexenal reaction between (aldol condensation acetone and productn-butanol, of butyraldehyde), the detected butyl products butyrate are (dehydrogenation 2-ethyl-2-hexenal (aldolproduct condensation of hemiacetal product offormed butyraldehyde), by the reaction butyl butyrate between (dehydrogenation n-butanol and productbutyraldehyde of hemiacetal [8]), formed3-hepten-2-one, by the reaction and between2-heptanone,n-butanol while there and butyraldehydeis no products by [8 ]),condensation 3-hepten-2-one, of acetone. and 2-heptanone, The former whiletwo there products is no are products formed by by condensation the condensation of acetone. of butanol The itself, former while two the products others are are the formed cross-aldol by the condensationcondensation of butanolproducts itself, (denoted while theas othersC7 in areFigure the cross-aldol8). All catalysts condensation exhibit productsthe selectivity (denoted to as C72-ethyl-2-hexenal in Figure8). All in catalysts the range exhibit from 50% the to selectivity 77%, except to 2-ethyl-2-hexenalCu/MgO in which C7 in theis the range majority. from Among 50% to 77%,Cu/ exceptxCeZr Cu/MgO catalysts, in Cu/0.8CeZr which C7 is shows the majority. the highest Among C7 selectivity Cu/xCeZr of catalysts, 40%. The Cu/0.8CeZr common features shows thein highestCu/MgO C7 selectivity and Cu/0.8Ce of 40%.Zr are The both common high S featuresCu and N inCO2 Cu/MgO values. This and possibly Cu/0.8CeZr explains are boththat some high SofCu andbutyraldehydeNCO2 values. This (a possiblystrong electrophile) explains that is some adsorb of butyraldehydeed onto Cu surface, (a strong followed electrophile) by isthe adsorbed aldol ontocondensation Cu surface, followedreaction with by the an aldolacetone condensation enolate over reaction basic sites. with This an acetoneexplanation enolate is consistent over basic to sites.the Thiscatalytic explanation performance is consistent discussed to the by catalytic the results performance in Figure 7. discussed by the results in Figure7.

FigureFigure 8. Product 8. Product selectivities selectivities obtained obtained in in the the reaction reaction of of acetone acetone with with butyraldehyde butyraldehyde overover Cu/xxCeZr,CeZr,

Cu/AlCu/Al2O32O, and3, and Cu/MgO Cu/MgO catalysts. catalysts.

3.3.3.3. Comparison Comparison of Cu/0.8CeZr of Cu/0.8CeZr and and Cu/MgO Cu/MgO Catalysts Catalysts From the activity results, Cu/MgO appears to be a better catalyst than Cu/0.8CeZr, the most From the activity results, Cu/MgO appears to be a better catalyst than Cu/0.8CeZr, the most active among the prepared Cu/xCeZr catalysts. Thus, the experiments as a function of reaction time active among the prepared Cu/xCeZr catalysts. Thus, the experiments as a function of reaction time were conducted for the two catalysts, as shown in Figure 9. For Cu/MgO the acetone conversion were conducted for the two catalysts, as shown in Figure9. For Cu/MgO the acetone conversion increases to 60% at about 2 h and further increase is not observed for a prolonged time. The similar increasestime-dependent to 60% at aboutvariation 2 h is and obtained further on increase the selectivities is not observed to butyl for butyrate a prolonged and 2-heptanol time. The similarbeing time-dependentstable at 26% variationand 10%, respectively. is obtained on However, the selectivities the selectivity to butyl to butyrate2-heptanone and decreases 2-heptanol to being30% at stable 24 h at 26% and 10%, respectively. However, the selectivity to 2-heptanone decreases to 30% at 24 h and the selectivity to 6-undecanone increases to 32%, which is understood because the former product is consumed for the formation of di-alkylation product. On the other hand, Cu/0.8CeZr shows the Catalysts 2017, 7, 249 10 of 14 Catalysts 2017, 7, 249 10 of 14

Catalysts 2017, 7, 249 10 of 14 andand thethe selectivityselectivity toto 6-undecanone6-undecanone increasesincreases toto 32%,32%, whichwhich isis understoodunderstood becausebecause thethe formerformer productproduct isis consumedconsumed forfor thethe formationformation ofof di-alkdi-alkylationylation product.product. OnOn thethe otherother hand,hand, Cu/0.8CeZrCu/0.8CeZr shows the gradual increase in the acetone conversion to 63% at 24 h. Although the change in product gradualshows increase the gradual in the increase acetone in conversion the acetone to conversion 63% at 24 to h. 63% Although at 24 h. the Although change the in productchange in selectivities product selectivities is similar to that of Cu/MgO, the selectivities to 2-heptanone and 6-undecanone are is similarselectivities to that is ofsimilar Cu/MgO, to that the of selectivities Cu/MgO, the to 2-heptanoneselectivities to and 2-heptanone 6-undecanone and 6-undecanone are measured are to be measuredmeasured toto bebe bothboth 39%39% atat 2424 hh overover Cu/0.8CeZr.Cu/0.8CeZr. Particularly,Particularly, thethe selectivityselectivity toto 6-undecanone6-undecanone bothincreases 39% at 24linearly h over with Cu/0.8CeZr. time, which Particularly, explains its the improved selectivity formation to 6-undecanone at a longer increases reaction linearlytime than with time,increases which explainslinearly with its improved time, which formation explains atits a improved longer reaction formation time at than a longer Cu/MgO. reaction Since time a than higher Cu/MgO.Cu/MgO. SinceSince aa higherhigher selectivityselectivity toto 2-heptanone2-heptanone andand 6-undecanone6-undecanone isis preferredpreferred inin thethe cross-aldolcross-aldol selectivity to 2-heptanone and 6-undecanone is preferred in the cross-aldol condensation of acetone condensationcondensation of of acetone acetone and and n n-butanol,-butanol, Cu/0.8CeZr Cu/0.8CeZr is is believed believed to to be be more more active active than than Cu/MgO. Cu/MgO. and n-butanol, Cu/0.8CeZr is believed to be more active than Cu/MgO.

((aa)) ((bb))

FigureFigureFigure 9. 9.9.Acetone AcetoneAcetone conversion conversionconversion andandand productproductproduct selectivitie selectivitiesselectivitiess plottedplotted plotted againstagainst against thethe the rereaction reactionaction timetime time overover over (a) Cu/MgO and (b) Cu/0.8CeZr catalysts. (a) Cu/MgO(a) Cu/MgO and and (b )(b Cu/0.8CeZr) Cu/0.8CeZr catalysts. catalysts.

Additionally,Additionally,Additionally, we we examinedwe examinedexamined the catalystthethe catalystcatalyst stability stabilitystability by comparing byby comparingcomparing XRD XRD patternsXRD patternspatterns of fresh ofof (i.e.,freshfresh reduced) (i.e.,(i.e., andreduced)reduced) spent Cu/MgO and and spent spent and Cu/MgO Cu/MgO Cu/0.8CeZr and and Cu/0.8CeZr Cu/0.8CeZr catalysts. catalyst Ascatalyst showns.s. As As in shown shown Figure in in 10 Figure Figure, the spent10, 10, the the Cu/0.8CeZr spent spent Cu/0.8CeZr Cu/0.8CeZr displays verydisplaysdisplays identical veryvery reflections identicalidentical to reflectionsreflections the fresh to one,to thethe indicating freshfresh one,one, that indicatingindicating it is stable thatthat initit is theis stablestable reaction. inin thethe When reaction.reaction. Cu/0.8CeZr WhenWhen Cu/0.8CeZr is repeatedly used for the 6-h reaction, the acetone conversion and product distribution is repeatedlyCu/0.8CeZr used is repeatedly for the 6-h used reaction, for the 6-h the reaction acetone, the conversion acetone conversion and product and distribution product distribution are lower are lower by 10% than the first-run result (not shown for brevity) but they are rather maintained in by 10%are lower than by the 10% first-run than the result first-run (not result shown (not for show brevity)n for butbrevity) they but are they rather are maintainedrather maintained in further in further repeated runs. However, the reflections of spent Cu/MgO are very different from those of the repeatedfurther runs. repeated However, runs. However, the reflections the reflections of spent of Cu/MgO spent Cu/MgO are very are differentvery different from from those those of the of the fresh fresh one. The remarkable change from MgO to Mg(OH)2 is due to water molecules formed by the one.fresh The one. remarkable The remarkable change change from MgO from to MgO Mg(OH) to Mg(OH)is due2 is to due water to water molecules molecules formed formed by theby the aldol aldol condensation reaction, which means that Cu/MgO2 is structurally unstable in the reaction. condensationaldol condensation reaction, reaction, which means which thatmeans Cu/MgO that Cu/MgO is structurally is structurally unstable unstable in the in reaction.the reaction.

Figure 10. Powder X-ray diffraction patterns of fresh (black) and spent (red) Cu/0.8CeZr and Cu/MgO

catalysts. The reﬂections of Mg(OH)2 (black bars), MgO (red bars), and Cu metal (blue bars) are included for peak identiﬁcation. Catalysts 2017, 7, 249 11 of 14

4. Experimental

4.1. Catalyst Preparation

CexZr1−xO2 supports, denoted as xCeZr, were prepared via coprecipitation, where the x value is 1, 0.8, 0.5, 0.2, and 0. For the preparation of 0.5CeZr support, an aqueous solution (1.2 M of 50 mL) containing 13.29 g Ce(NO3)2·6H2O (98%, Yakuri Pure Chemicals, Kyoto, Japan) and 7.08 g −1 ZrO(NO3)2·nH2O (Strem Chemicals, Newburyport, MA, USA) was added dropwise (15 mL min ) into NaOH solution (0.3 M, 600 mL) at 343 K under vigorous stirring. After complete addition, the resulting suspension was aged at 343 K for 24 h, filtered and washed repeatedly with deionized water. The so-obtained white cake was dried at 378 K overnight, followed by crushing and sieving to less than 200 µm. Finally, the calcination was carried out at 873 K (ramping rate 5 K min−1) for 3 h in air. The other xCeZr samples were prepared in the same manner except the weights of Ce(NO3)2·6H2O and ZrO(NO3)2·nH2O added. MgO was also prepared via precipitation of Mg(NO3)2·6H2O (2.4 M, 175 mL) with NaOH solution (0.24 M, 4200 mL), followed by calcination at 773 K (ramping rate 5 K −1 2 −1 min ) for 3 h in air. Al2O3 (BET surface area = 180 m g ) was purchased from Strem Chemicals (Newburyport, MA, USA) and used without pretreatment. Supported Cu catalysts of 10 wt % Cu loading were prepared by addition of aqueous Cu2+ solution (ca. 2 mL) containing 0.85 g Cu(NO3)2·3H2O (99%, Daejung Chemicals & Metals, Siheung, Korea) into the support material (2 g) until the solid surface was pretty wet. The impregnated sample was dried at 378 K overnight and calcined at 673 K (ramping rate 5 K min−1) for 3 h in air, resulting in CuO-loaded samples denoted as CuO/xCeZr, CuO/MgO, and CuO/Al2O3. The final catalysts, such as Cu/xCeZr, Cu/MgO, and Cu/Al2O3, were obtained by reduction of the calcined samples at 523 K 3 −1 for 3 h in a H2 flow (100 cm min ).

4.2. Catalyst Characterization Powder XRD analysis was performed with a Rigaku MINIFlex 600 (Rigaku Corp., Spring, TX, USA) using a Cu Kα radiation source at 40 kV and 15 mA. The specific BET surface area of the sample (ca. 0.2 g) was measured at 77 K using a Micromeritics 3Flex (Micromeritics Instrument Corp., Norcross, GA, USA) after pretreatment at 373 K for 1 h under vacuum. Experiments for temperature-programmed desorption of CO2 and NH3 were performed using a BELCAT-B (BEL Japan, Inc., Toyonaka, Japan) coupled with a BEL-Mass detector (BEL Japan, Inc., Toyonaka, Japan). Prior to CO2 adsorption, CuO-loaded sample (50 mg) was reduced at 523 K for 3 h 3 −1 with 10% H2/Ar (50 cm min ) and cooled to 308 K under He. After 5% CO2 in He was fed at 308 K for 30 min and purged with He for 2 h, CO2-TPD experiment was started by heating to 1123 K at a −1 3 −1 rate of 10 K min in a flow of He (30 cm min ). For NH3-TPD experiment, CuO-loaded sample (50 mg) was reduced in the same manner as for CO2-TPD and then exposed to 5% NH3/He at 308 K for 30 min. After purge with He for 2 h, NH3-TPD experiment was started by heating to 1123 K at a rate −1 3 −1 of 10 K min in a He flow (50 cm min ). For H2-TPR measurement conducted in a Micromeritics AutoChem 2910 (Micromeritics Instrument Corp., Norcross, GA, USA), the calcined sample (50 mg) −1 was pretreated at 363 K in He and heated to 673 K at a rate of 5 K min using 10% H2 in Ar (50 sccm). For measuring a specific Cu surface area, N2O-RFC experiment was conducted in a BELCAT-B (BEL Japan, Inc., Toyonaka, Japan). After reducing a sample CuO/xCeZr (ca. 50 mg) at 523 K for 3 −1 3 −1 3 h with 10% H2/Ar (30 cm min ) and cooling to 313 K under He, 1% N2O/He (5 cm min ) was introduced and the released nitrogen (m/z = 28) was measured by a MS detector. The calculation was based on the reaction stoichiometry between copper and oxygen (Cu/O = 2) and the copper surface density of 1.46 × 1019 Cu atom m−2.

4.3. Activity Test Catalytic experiments for the cross-aldol condensation of acetone and n-butanol were conducted in a stainless-steel batch reactor (total volume 150 cm3: Hanwoul Engineering, Gunpo, Korea) into Catalysts 2017, 7, 249 12 of 14 which the reduced Cu catalyst (0.5 g), acetone (194 mmol), and n-butanol or butyraldehyde (388 mmol) were charged. After N2 purge to remove oxygen and moisture inside, the reaction mixture was heated −1 to 513 K at a rate of 10 K min . The reaction was conducted at 513 K for 6 h under N2 where the reactor pressure was nearly constant at 25 bar. Note that the reaction condition is very similar to that in the report of Toste et al. [5]. The product sample was taken out, filtered through a ADVANTEC® syringe filter (0.45 µm diameter; Toyo Roshi Kaisha, Ltd., Tokyo, Japan) and mixed with an internal standard cyclohexane. This mixture was analyzed with a Younglin YL6100 gas chromatograph (Younglin Instrument, Anyang, Korea) equipped with a FID detector (Younglin Instrument, Anyang, Korea) and an HP-INNOWAX column (50 m, 0.2 mm, 0.4 µm; Agilent Technologies, Santa Clara, CA, USA). The acetone conversion was calculated by the molar difference between the initial charge and the remaining acetone. The quantitative selectivities to products such as 3-hepten-2-one, 2-heptanone, 2-heptanol, butyl butyrate, and 6-undecanone were calculated on the basis of the following equation: selectivity to product i [mol%] = (mole of product i)/(mole of all products in the reaction mixture) × 100%.

5. Conclusions Among Cu/xCeZr catalysts, Cu/0.8CeZr produces the cross-aldol condensation products (all C7 plus C11) in the largest amount. The enhanced catalytic performance in the reaction of acetone and n-butanol is a combination result of more surface Cu atoms and more base sites responsible for the alcohol dehydrogenation and aldol condensation reactions, respectively. This is conﬁrmed by the results obtained in the condensation of acetone and butyraldehyde over Cu/0.8CeZr and Cu/MgO. Since the conversion of acetone is less efﬁcient in the reaction with butyraldehyde, it is suggested that the dehydrogenation product of n-butanol exist in a form of acyl species adsorbed onto Cu surface, not free aldehyde, followed by undergoing the condensation reaction with an acetone enolate formed by acetone deprotonation over basic sites. Although Cu/MgO shows the comparable cross-aldol condensation activity to Cu/0.8CeZr, MgO is transformed into Mg(OH)2 by water formed by the condensation reaction. Consequently, Cu supported on ceria-zirconia of ceria rich is catalytically active and stable in the cross-aldol condensation of acetone and n-butanol.

Supplementary Materials: The following are available online at www.mdpi.com/2073-4344/7/9/249/s1, Figure S1: Correlation of acetone conversion with (a) NNH3 and (b) NCO2 of Cu/xCeZr (circles), Cu/MgO (square), and Cu/Al2O3 (triangle) catalysts. Acknowledgments: The authors acknowledge the financial support from the New & Renewable Energy Core Technology Program through the Korea Institute of Energy Technology Evaluation and Planning under the Ministry of Trade, Industry & Energy, Republic of Korea (KETEP-20143030091040), and from the Basic Science Research Program through the National Research Foundation of Korea under the Ministry of Education (NRF-2016R1A6A1A03013422). The former funding source covers the cost to publish in open access. Author Contributions: Y.-W.S. conceived and designed the experiments; M.K. and J.P. performed the experiments; M.K. and H.P.R.K. analyzed the data; M.K. and J.P. contributed reagents/materials/analysis tools; M.K., H.P.R.K. and Y.-W.S. wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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