Zr-Modified Zno for the Selective Oxidation of Cinnamaldehyde to Benzaldehyde

catalysts

Article Zr-Modiﬁed ZnO for the Selective Oxidation of Cinnamaldehyde to Benzaldehyde

Pengju Du 1, Tongming Su 1, Xuan Luo 1, Xinling Xie 1, Zuzeng Qin 1,* and Hongbing Ji 1,2 1 School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China 2 School of Chemistry, Sun Yat-sen University, Guangzhou 510275, China * Correspondence: [email protected]; Tel.: +86-771-323-3718

Received: 31 July 2019; Accepted: 21 August 2019; Published: 25 August 2019

Abstract: ZnO and Zr-modified ZnO were prepared using a precipitation method and used for the selective oxidation of cinnamaldehyde to benzaldehyde in the present study. The results showed that physicochemical properties of ZnO were significantly affected by the calcination temperature, and calcination of ZnO at 400 ◦C demonstrated the optimum catalytic activity for the selective oxidation of cinnamaldehyde to benzaldehyde. With 0.01 g ZnO calcined at 400 ◦C for 2 h as a catalyst, 8.0 g ethanol and 2.0 g cinnamaldehyde reacted at an oxygen pressure of 1.0 MPa and 70 ◦C for 60 min, resulting in benzaldehyde selectivity of 69.2% and cinnamaldehyde conversion of 16.1%. Zr was the optimal modifier for ZnO: when Zr-modified ZnO was used as the catalyst, benzaldehyde selectivity reached 86.2%, and cinnamaldehyde conversion was 17.6%. The X-ray diffractometer and N2 adsorption–desorption characterization indicated that doping with Zr could reduce the crystallite size of ZnO (101) and increase the specific surface area of the catalyst, which provided more active sites for the reaction. X-ray photoelectron spectrometer results showed that Zr-doping could exchange the electrons with ZnO and reduce the electron density in the outer layer of Zn, which would further affect benzaldehyde selectivity. The results of CO2 temperature-programmed desorption showed that Zr-modification enhanced the alkalinity of the catalyst surface, which caused the Zr–ZnO catalyst to exhibit higher catalytic activity.

Keywords: ZnO; cinnamaldehyde; oxidation; benzaldehyde; Zr-modiﬁed

1. Introduction Benzaldehyde is the second most frequently used perfume in the world and is widely used in foods, beverages, medicines, and cosmetics [1–3]. With the increasing demand for benzaldehyde, research regarding the production of benzaldehyde has gained increasing attention from researchers globally [4–8]. In industry, benzaldehyde is obtained by the gas phase oxidation of toluene and air or oxygen on V2O5, WO3, or MnO2 catalysts; by the chlorination of toluene to benzyl chloride under light irradiation, and subsequent hydrolyzation and oxidation; by the chlorination of toluene to dichloromethylbenzene and then hydrolyzation; and by the ozonation of styrene. On the other hand, natural benzaldehyde is mainly obtained by using amygdalin from bitter almond as the raw material, and the combined process of enzymatic hydrolysis, steam distillation, and caustic washing; 40% of the amount sold on the international market is obtained by this method, with the remaining 60% provided by the biotransformation of microorganisms or natural cinnamon oil conversion. The alkaline hydrolysis of cinnamon oil (whose main component is cinnamaldehyde) is a simple, easily controlled, and environmentally friendly method, and does not require a large amount of equipment; this method has gained increasing attention. In addition, natural benzaldehyde has also been prepared by an emulsiﬁer alkaline hydrolysis method [9,10], β-cyclodextrin catalysis [11,12], and biomimetic catalysis [13]. However, these methods are homogeneous processes, and isolating the

Catalysts 2019, 9, 716; doi:10.3390/catal9090716 www.mdpi.com/journal/catalysts Catalysts 2019, 9, x 716 FOR PEER REVIEW 22 of 12 homogeneous processes, and isolating the products from the alkaline solution is not only difficult, products from the alkaline solution is not only difficult, but also increases the production cost and but also increases the production cost and affects the purity of the benzaldehyde. affects the purity of the benzaldehyde. In the present study, noble metal catalysts were also used in a benzaldehyde preparation process In the present study, noble metal catalysts were also used in a benzaldehyde preparation [14,15]. Although the use of noble metals significantly increases the cost of the catalytic reaction, few process [14,15]. Although the use of noble metals significantly increases the cost of the catalytic reaction, reports to date have investigated the combination of oxygen and transition metal oxide catalysts in few reports to date have investigated the combination of oxygen and transition metal oxide catalysts in the catalytic oxidation of cinnamaldehyde. Transition metal oxide is a very common catalyst, the catalytic oxidation of cinnamaldehyde. Transition metal oxide is a very common catalyst, exhibiting exhibiting a good catalytic activity and a lower price [16,17], and has been widely used in the fields a good catalytic activity and a lower price [16,17], and has been widely used in the fields of pollutant of pollutant degradation, air purification, and heterogeneous catalytic processes [18,19]. Compared degradation, air purification, and heterogeneous catalytic processes [18,19]. Compared with TiO2, with TiO2, Al2O3, Fe2O3, CuO, and other transition metal oxides, ZnO exhibits higher chemical Al O , Fe O , CuO, and other transition metal oxides, ZnO exhibits higher chemical stability, better stability,2 3 better2 3 biological compatibility, and a larger specific surface area, which shows the potential biological compatibility, and a larger specific surface area, which shows the potential for use in the for use in the catalytic oxidation of cinnamaldehyde to benzaldehyde. Furthermore, some metals or catalytic oxidation of cinnamaldehyde to benzaldehyde. Furthermore, some metals or metal oxides, metal oxides, such as Fe [20], Cu [21–23], Ce [24], Ag [25], Al [26], SnO2 [27], and NiO [28,29], have such as Fe [20], Cu [21–23], Ce [24], Ag [25], Al [26], SnO [27], and NiO [28,29], have been used to been used to modify or promote ZnO, which significantly2 increased the catalytic oxidation activity modify or promote ZnO, which significantly increased the catalytic oxidation activity of ZnO. Therefore, of ZnO. Therefore, ZnO and metal-modified ZnO exhibit the potential for the selective oxidation of ZnO and metal-modified ZnO exhibit the potential for the selective oxidation of cinnamaldehyde cinnamaldehyde to benzaldehyde. to benzaldehyde. Based on previous studies of the catalytic ozonation of cinnamaldehyde [30,31], in the present Based on previous studies of the catalytic ozonation of cinnamaldehyde [30,31], in the present work, work, ZnO and a transition metal-modified ZnO catalyst were prepared and used to catalyze ZnO and a transition metal-modified ZnO catalyst were prepared and used to catalyze cinnamaldehyde cinnamaldehyde to benzaldehyde, using O2 as an oxidant. The effects of the metal modification on to benzaldehyde, using O as an oxidant. The effects of the metal modification on the catalyst structure the catalyst structure and2 activity of ZnO were analyzed by combining various characterization and activity of ZnO were analyzed by combining various characterization methods and experimental methods and experimental results. The mechanism of the catalytic reaction was studied by gas results. The mechanism of the catalytic reaction was studied by gas chromatography (GC), and gas chromatography (GC), and gas chromatography-mass spectrometry (GC-MS). chromatography-mass spectrometry (GC-MS).

2. Results Results and and Discussion Discussion

2.1. Catalytic Catalytic Oxidation Oxidation of of Cinnamaldehyde Cinnamaldehyde

The oxidations oxidations of of cinnamaldehyde cinnamaldehyde to to benzaldehyde benzaldehyde by by catalyzing catalyzing ZnO, ZnO, TiO TiO2,2, CuO, CuO, AlAl22OO33, and and Fe2OO3,3, andand with with no no catalytic oxidation, areare shownshown in in Figure Figure1A. 1A. According According to to Figure Figure1A, 1A, in the in the case case of no of nocatalytic catalytic oxidation, oxidation, the conversionthe conversion of cinnamaldehyde of cinnamaldehyde and theand selectivity the selectivity of benzaldehyde of benzaldehyde were 15.6%, were 15.6%,and 4.3%, and respectively. 4.3%, respectively. After adding After the adding catalysts the (Fecatalysts2O3, CuO, (Fe2 TiOO3, 2CuO,, Al2O TiO3, and2, Al ZnO),2O3, and the conversionZnO), the conversionincreased to increased 9.6%, 43.9%, to 9.6%, 48.1%, 43.9%, 59.9%, 48.1%, and 16.1%, 59.9%, respectively. and 16.1%, Atrespectively. the same time, At the the same benzaldehyde time, the benzaldehydeselectivity increased selectivity to 12.7%, increased 10.8%, to 25.7%,12.7%, 22.3%, 10.8%, and 25.7%, 69.2%, 22.3%, respectively. and 69.2%, The respectively. selectivity order The selectivityof the catalysts order wasof the CuO catalysts< Fe2O was3 < CuOAl2O <3 Fe< TiO2O3 2<

Figure 1.1. TheThe catalytic catalytic oxidation oxidation of cinnamaldehydeof cinnamaldehyde on metal on metal oxides oxides (A) and (A transition) and transition metal modiﬁed metal modifiedZnO (B). ReactionZnO (B). conditions:Reaction conditions: 0.01 g ZnO 0.01 (or g otherZnO (or catalysts) other catalysts) calcined calcined at 400 ◦C at for 400 2 h°C as for a catalyst,2 h as a catalyst,8.0 g ethanol 8.0 g and ethanol 2.0 g and cinnamaldehyde 2.0 g cinnamaldehyde reacted at reac an oxygented at an pressure oxygen of pressure 1.0 MPa of and 1.0 70 MPa◦C forand 60 70 min. °C for 60 min.

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Figure1 B1B shows shows the the catalytic catalytic activities activities of Zr–ZnO, of Zr–ZnO, La–ZnO, La–ZnO, Ce–ZnO, Ce–ZnO, Co–ZnO, Co–ZnO, and ZnO and catalysts ZnO withcatalysts a 1.0% with doping a 1.0% amount doping of Zr,amount La, Ce, of andZr, Co,La, respectively.Ce, and Co, Afterrespectively. Zr modification, After Zr benzaldehydemodification, selectivitybenzaldehyde reached selectivity the peak reached selectivity the ofpeak 86.2% selectivity after a 1.0-h of 86.2% reaction, after compared a 1.0-h toreaction, 69.2% benzaldehydecompared to selectivity69.2% benzaldehyde of ZnO; when selectivity ZnO modifiedof ZnO; when by La, ZnO Ce, modi andfied Co were by La, used Ce, and as catalysts, Co were cinnamaldehydeused as catalysts, conversioncinnamaldehyde was maintainedconversion atwas about maintained 16–17% for at the about five catalysts,16–17% andfor benzaldehydethe five catalysts, selectivity and increasedbenzaldehyde to 72.6%, selectivity 75.2%, andincreased 80.4%, respectively.to 72.6%, 75.2%, Therefore, and in80.4%, the following respectively. study, Therefore, Zr-modified in ZnO the wasfollowing further study, studied. Zr-modified ZnO was further studied.

2.2. Surface Acidity and Surface Alkalinity

The CO2 temperature programmed desorption (CO 2-TPD)-TPD) profiles proﬁles of of ZnO, La–ZnO, Ce–ZnO, Co–ZnO, andand Zr–ZnOZr–ZnO areare exhibitedexhibited inin Figure Figure2 ,2, and and these these samples samples basic basic strengths strengths are are listed listed in in Table Table1. In1. In summary, summary, a highera higher basicity basicity of of the the adsorption adsorption center center corresponded corresponded to to a a higher higher temperaturetemperature COCO22 desorption peak peak [31]; [31]; and and the the weak, weak, medium, medium, and and strong strong base base sites sites had had CO2 CO desorption2 desorption peaks peaks at 100– at 100–300300 °C, 350–550◦C, 350–550 °C, and◦C, >600 and >°C,600 respectively◦C, respectively [32]. Two [32]. obvious Two obvious CO2 desorption CO2 desorption peaks were peaks shown were shownon all samples’ on all samples’ curves, curves, at about at 440 about °C 440and◦ C640 and °C, 640 which◦C, whichwere the were medium the medium and the and strong the strong base sites, base sites,respectively. respectively.

Figure 2. CO2-TPD-TPD profiles proﬁles for transition metal-modified metal-modiﬁed ZnO.

Table 1. Basic strength of transition metal-modified ZnO from CO2-TPD profiles. Table 1. Basic strength of transition metal-modified ZnO from CO2-TPD profiles. Basic Strength a (µmol g 1) Catalyst Basic Strength a (μmol·g· −−1) Catalyst MediumMedium (350–550(350–550 ◦°C)C) Strong Strong (>600 (>600 °C)◦C) Total Total ZnOZnO 558558 103 103 637 637 La-ZnOLa-ZnO 563563 112 112 660 660 Ce-ZnOCe-ZnO 582582 123 123 685 685 Co-ZnOCo-ZnO 621621 127 127 704 704 Zr-ZnO 637 134 765 Zr-ZnO 637 134 765 a a The Thebasic basic strength strength was was obtained obtained by usingusing an an external external standard standard method. method. According to the basic strength of transition metal-modified ZnO in Table 1, the total basicity sites Accordingof ZnO, La–ZnO, to the basic Ce–ZnO, strength Co–ZnO, of transition and Zr–ZnO metal-modified were 637, ZnO704, in685, Table 660,1 ,and the total765 μ basicitymol·g−1, sitesrespectively, of ZnO, and La–ZnO, the total Ce–ZnO, basicity Co–ZnO, order was and Zr–ZnO Zr–ZnO > wereCo–ZnO 637, > 704, Ce–ZnO 685, 660,> La–ZnO and 765 > µZnO.mol gThe1, · − respectively,order was identical and the to total the increasing basicity order order was of Zr–ZnObenzaldehyde> Co–ZnO selectivity,> Ce–ZnO proving> La–ZnO there was> ZnO.a positive The ordercorrelation was identical in the catalyst to the basic increasing strength order and of its benzaldehyde oxidation activity. selectivity, In these proving catalysts, there Zr–ZnO was a positivehad the correlationhighest benzaldehyde in the catalyst selectivity basic strength of 86.2%. and its oxidation activity. In these catalysts, Zr–ZnO had the highest benzaldehyde selectivity of 86.2%. 2.3. XRD Analysis 2.3. XRD Analysis X-ray diffraction (XRD) was used to characterize the crystallinity and composition of ZnO, Zr– ZnO,X-ray La–ZnO, diffraction Co–ZnO, (XRD) and was Ce–ZnO used to(both characterize doping amounts the crystallinity were 1.0%) and compositioncalcined at 400 of °C ZnO, (see Zr–ZnO, Figure La–ZnO,3). From Co–ZnO,Figure 3, andfive Ce–ZnOZnO diffraction (both doping peaks amounts at 2θ = 31.7°, were 1.0%)37.4°, calcined34.4°, 36.2°, at 400 and◦C 47.5° (see Figure(Powder3). diffraction file (PDF) card NO. 36–1451) [33] were found in all samples. However, there were no

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Catalysts 2019, 9, x FOR PEER REVIEW 4 of 12 From Figure3, five ZnO di ffraction peaks at 2θ = 31.7◦, 37.4◦, 34.4◦, 36.2◦, and 47.5◦ (Powder diffraction obviousfile (PDF) diffraction card NO. peaks 36–1451) of the [ 33tran] weresition found metals in of allZr, samples.La, Co, or However,Ce in the patterns, there were which no could obvious be becausediffraction the peaks doping of theamount transition was outside metals of Zr,the La,dete Co,ction or range Ce in theof the patterns, instrument, which or could because be because Zr, La, Co,the dopingor Ce were amount distributed was outside in the of ZnO the detection catalyst in range an amorphous of the instrument, state [34,35]. or because Calculated Zr, La, by Co, the or ScherrerCe were distributedequation [36], in the crystallite ZnO catalyst sizes in of an the amorphous ZnO (101) state of the [34 Zr–ZnO,,35]. Calculated Co–ZnO, by Ce–ZnO, the Scherrer La– ZnO,equation and [ 36ZnO], the were crystallite 166.2, 178.4, sizes 185.1, of the 209.3, ZnO (101) and of220.8 the nm, Zr–ZnO, respectively. Co–ZnO, A Ce–ZnO,smaller crystallite La–ZnO, andsize indicatedZnO were lower 166.2, crystallinity 178.4, 185.1, [37,38], 209.3, and and 220.8 the nm,datarespectively. of the crystallite A smaller size of crystallite Zr–ZnOsize indicated indicated an increaselower crystallinity in pore volume [37,38], and and specific the data surface of the crystallitearea, which size would of Zr–ZnO be more indicated favorable an increase to the reaction in pore volume[39]. and specific surface area, which would be more favorable to the reaction [39].

Figure 3. XRD patterns of ZnO catalyst mo modiﬁeddified by transition metal.

2.4. N2 Adsorption–Desorption Analysis 2.4. N2 Adsorption–Desorption Analysis The surface area, pore size distribution and the textural properties of the ZnO catalyst modified by The surface area, pore size distribution and the textural properties of the ZnO catalyst modified transition metal were obtained using N2 adsorption–desorption and are shown in Figure4. According by transition metal were obtained using N2 adsorption–desorption and are shown in Figure 4. to Figure4A, the isotherms matched Type III isotherms on the basis of the International Union of Pure According to Figure 4A, the isotherms matched Type III isotherms on the basis of the International and Applied Chemistry (IUPAC) classification [40]. The adsorption capacity of N2 was proportional to Union of Pure and Applied Chemistry (IUPAC) classification [40]. The adsorption capacity of N2 was proportionalthe relative pressure to the relative in the range pressure of P/ Pin◦ the= 0–0.7, range which of P/P was° = the0–0.7, low-pressure which was region. the low-pressure This phenomenon region. indicated that the adsorption of N on the ZnO surface changed from single layer to multilayer [41]. This phenomenon indicated that the2 adsorption of N2 on the ZnO surface changed from single layer toIn themultilayer high-pressure [41]. In region the high-pressure (P/P◦ = 0.7–1.0), region an H3(P/ typeP° = hysteresis0.7–1.0), an loop H3was type produced hysteresis by loop capillary was producedcondensation by capillary [42], indicating condensation slit-shaped [42], indicating pores (Figure slit-shaped4A). From pores Figure (Figure4B, the 4A). pore From sizes Figure of the 4B, five the poresamples sizes were of the 50–100 five samples nm, and were the pore50–100 size nm, of Zr-ZnOand the pore was thesize smallest. of Zr-ZnO was the smallest.

Figure 4.4. NN22 adsorption–desorptionadsorption–desorption isotherm isotherm (A (A) )and and pore pore size size distribution distribution ( (BB)) of ZnO catalystcatalyst modifiedmodiﬁed by transition metal.

The speciﬁc surface area, average pore size, and pore volume of ZnO modiﬁed by transition The specific surface area, average pore size, and pore volume of ZnO modified by transition metal were calculated using the isothermal adsorption–desorption data and are shown in Table2. The metal were calculated using the isothermal adsorption–desorption data and are shown in Table 2. Brunauer–Emmett–Teller (BET) surface areas of Zr–ZnO, Co–ZnO, Ce–ZnO, La–ZnO, and ZnO were The Brunauer–Emmett–Teller (BET) surface areas of Zr–ZnO, Co–ZnO, Ce–ZnO, La–ZnO, and ZnO were 6.231, 5.831, 5.423, 4.651, and 4.023 m2·g−1, respectively. The same change in pore volume was also found. These results indicate that modification of the transition metals, Zr, Co, Ce, and La, can

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2 1 6.231, 5.831, 5.423, 4.651, and 4.023 m g− , respectively. The same change in pore volume was also Catalysts 2019, 9, x FOR PEER REVIEW · 5 of 12 found. These results indicate that modification of the transition metals, Zr, Co, Ce, and La, can increase the specific surface area and pore volume of the catalyst. Notably, the surface area of Zr–ZnO was the increase the specific surface area and pore volume of the catalyst. Notably, the surface area of Zr– largest. For industrial applications of the catalyst, increased specific surface area and pore volume ZnO was the largest. For industrial applications of the catalyst, increased specific surface area and indicate more active sites, which are conducive to the catalytic oxidation of cinnamaldehyde [43] and pore volume indicate more active sites, which are conducive to the catalytic oxidation of cinnamaldehydeexplain why Zr has [43] optimal and explain catalytic why activity. Zr has optimal catalytic activity.

Table 2. Textural properties of ZnO catalyst modiﬁed by transition metal. Table 2. Textural properties of ZnO catalyst modified by transition metal. 2 1 3 1 Catalysts BET Surface Area (m 2g −1 ) Average Pore Diameter (nm) Pore Volume3 (cm−1 g ) Catalysts BET Surface Area (m··g− ) Average Pore Diameter (nm) Pore Volume (cm ·g ) · − ZnOZnO 4.023 29.6 29.6 0.362 0.362 La-ZnOLa-ZnO 4.651 28.5 28.5 0.442 0.442 Ce-ZnOCe-ZnO 5.423 26.3 26.3 0.541 0.541 Co-ZnOCo-ZnO 5.831 22.1 22.1 0.632 0.632 Zr-ZnOZr-ZnO 6.231 20.4 20.4 0.724 0.724

2.5.2.5. XPS XPS Analysis Analysis X-rayX-ray photoelectron spectrumspectrum (XPS)(XPS) analysis analysis was was carried carried out out to to identify identify the the chemical chemical states states of theof thesurface surface elements elements of pure of pure ZnO ZnO and and Zr–ZnO; Zr–ZnO; the the results results are shownare shown in Figure in Figure5 and 5 Tableand Table3. 3.

FigureFigure 5. 5. XPSXPS profile proﬁle of of Zn Zn 2p 2p ( (AA),), O O 1s 1s ( (BB),), and and Zr Zr 3d 3d ( (CC)) in in the the ZnO ZnO and and Zr–ZnO Zr–ZnO catalysts. catalysts.

InIn ZnO, ZnO, the the peaks peaks centered centered at at approximately approximately 1020.79 1020.79 and and 1043.86 1043.86 eV eV correspond correspond to to Zn Zn 2 2pp 3/23/2 p 1 p andand Zn Zn 2 2p ½2 [[44],44], respectively. For the Zn 2 p spectrum of Zr–ZnO, the peaks centered at 1020.95 1020.95 and and 1044.04 eV are attributed attributed to Zn Zn 2 2pp3/23/2 and and Zn Zn 2 2pp1/2,1/2, respectively, respectively, indicating that Zn existed in the the form form 2+ ofof Zn Zn2+ onon the the surface surface of of the the catalyst. catalyst. In In ZnO ZnO and and Zr–ZnO, Zr–ZnO, the the peaks peaks centered centered at at approximately approximately 529.60 529.60 andand 531.18 531.18 eV eV correspond correspond to O O 1 1ss,, which which corresponded corresponded to to the the O O 1 1ss characteristiccharacteristic peaks peaks in in the the band band of of Zn–O–ZnZn–O–Zn and the oxygenoxygen inin thethe hydroxylhydroxyl group group on on the the catalysts catalysts surface, surface, respectively respectively [45 [45].]. In In the the O O 1s 1spectrums spectrum of of Zr–ZnO, Zr–ZnO, there there was was a thirda third peak, peak, centered centered at at 530.25 530.25 eV, eV, which which corresponded corresponded to to the the O O 1s 1characteristics characteristic peaks peaks in in the the band band of of O–Zr–O O–Zr–O [ 46[46].]. In In the the Zr Zr 33ddspectrum, spectrum, thethe peakspeaks centered at 181.80 andand 184.20 184.20 eV eV were were attributed attributed to to Zr Zr 3 3dd5/25/2 and and Zr Zr 3 3dd3/2,3/2, respectively, respectively, indicating that Zr existed existed in in ZnO ZnO 4+ inin the the form form of of Zr Zr4+ [47].[47]. FromFrom the the above above data, data, there there were were 0.02 0.02 and and 0.01 0.01 eV eV blueshifts at at 529.62 529.62 and and 531.19 531.19 eV eV of of O O 1 1ss inin thethe spectra spectra of of Zr–ZnO Zr–ZnO in in comparison comparison with with pure pure ZnO. ZnO. There There were were also also 0.16 0.16 and and 0.18 0.18 eV eVblueshifts blueshifts of Zr–ZnOof Zr–ZnO relative relative to tothe the Zn Zn 2p 2 characteristicp characteristic peaks peaks of of pure pure ZnO. ZnO. These These results results indicate indicate that that Zr Zr can exchangeexchange electrons electrons with ZnO ZnO and and reduce reduce the the electron electron density density of of the the Zn Zn outer outer layer, layer, resulting resulting in in minor minor changeschanges in in the the chemical chemical state state of of ZnO, ZnO, thereby thereby resulting resulting in in a a bluesh blueshiftift of of the the binding binding energy energy of of Zn Zn elementselements [48], [48], and and thus thus affecting aﬀecting the catalytic pe performancerformance of selective ox oxidationidation of of cinnamaldehyde toto benzaldehyde. benzaldehyde. Therefore, Therefore, the the Zr–ZnO Zr–ZnO catalyst catalyst exhi exhibitedbited high high activity activity in inthe the catalytic catalytic oxidation oxidation of cinnamaldehyde.of cinnamaldehyde.

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Table 3. The electron binding energy (BE) of the ZnO and Zr–ZnO catalysts. Table 3. The electron binding energy (BE) of the ZnO and Zr–ZnO catalysts. BE (eV) XPS Spectra Element ZnO BEZ (eV)r-ZnO XPS Spectra Element O 1s O2– 529.60ZnO 529.62 Zr-ZnO O2– 531.18 530.25 O 1s O2– 529.60 529.62 O2– 531.19 O2– 531.18 530.25 Zn 2p Zn2+(Zn 2p3/2) 1020.79 1020.95 O2– 531.19 2+ 2+Zn (Zn 2p1/2) 1043.86 1044.04 Zn 2p Zn (Zn 2p3/2) 1020.79 1020.95 Zr 3d 2+Zr4+(Zr 3d5/2) 181.80 Zn (Zn 2p1/2) 1043.86 1044.04 4+Zr4+(Zr 3d3/2) 184.20 Zr 3d Zr (Zr 3d5/2) 181.80 4+ Zr (Zr 3d3/2) 184.20 2.6. Deactivation and Regeneration of Zr–ZnO 2.6. DeactivationStability is one and of Regeneration the most important of Zr–ZnO features of a solid catalyst in the catalytic process. In the presentStability study, is the one Zr–ZnO of the most catalyst important was treated features with of a several solid catalyst different in therecovery catalytic methods process. after In the a catalyticpresent study, oxidation the Zr–ZnO round, catalystincluding: was (A) treated filtration with and several washing different with recovery methanol methods and ethanol after a catalyticseveral timesoxidation to remove round, the including: adsorbed (A) material filtration from and the washing Zr–ZnO with catalyst methanol surface, and and ethanol desiccation several at times 110 °C to for 12 h; (B) desiccation at 110 °C for 12 h and calcination at 400 °C for 2 h only; (C) no treatment. The remove the adsorbed material from the Zr–ZnO catalyst surface, and desiccation at 110 ◦C for 12 h; (B) catalytic stability of Zr–ZnO in cinnamaldehyde oxidation regenerated by the different methods is desiccation at 110 ◦C for 12 h and calcination at 400 ◦C for 2 h only; (C) no treatment. The catalytic shownstability in of Figure Zr–ZnO 6. in cinnamaldehyde oxidation regenerated by the different methods is shown in FigureIn6 Figure. 6A, cinnamaldehyde conversion decreased slightly from 16.2% to 11.5% after the first five cycles,In Figure indicating6A, cinnamaldehyde that the Zr–ZnO conversion catalyst st decreasedill had good slightly activity from for cinnamaldehyde 16.2% to 11.5% after conversion; the first however,five cycles, benzaldehyde indicating that selectivity the Zr–ZnO decreased catalyst signific still hadantly, good by activity almost for 16.9%, cinnamaldehyde from 69.2% conversion; to 52.3%. Usinghowever, method benzaldehyde B (Figure selectivity 6B), both decreased conversion significantly, and selectivity by almost was 16.9%, stable from in 69.2%the first to 52.3%. five cycles, Using indicatingmethod B (Figurethat calcination6B), both conversionwas able to and maintain selectivity the was catalytic stable activity. in the first Finally, five cycles, after indicatingdirectly using that catalystscalcination without was able any to treatment maintain (Figure the catalytic 6C), benzalde activity. Finally,hyde selectivity after directly decreased using from catalysts 69.2% without to 48.5%, any representingtreatment (Figure a nearly6C), 20.7% benzaldehyde decrease selectivity of benzaldehy decreasedde selectivity, from 69.2% indicating to 48.5%, that representing without treatment a nearly the20.7% catalyst decrease activity of benzaldehyde decreased rapidly. selectivity, However, indicating for the thatZr–ZnO without used treatment in four cycles the catalyst and treated activity by methoddecreased C, rapidly.activity However,increased for to thealmost Zr–ZnO that used of the in fourfresh cycles cataly andst level treated after by methodhigh-temperature C, activity calcination (at 400 °C for 2 h). Interestingly, the used catalyst showed a pale-yellow color before the increased to almost that of the fresh catalyst level after high-temperature calcination (at 400 ◦C for 2 h). treatment,Interestingly, which the usedindicated catalyst that showed the adsorption a pale-yellow of an organic color before substance the treatment, was the major which reason indicated for thatthe deactivationthe adsorption of ofthe an catalyst. organic substance was the major reason for the deactivation of the catalyst.

FigureFigure 6. RecyclabilityRecyclability data data of of the the Zr–ZnO Zr–ZnO catalyst catalyst for for cinnamaldehyde cinnamaldehyde oxidation oxidation (( ((AA)) filtration, ﬁltration, washing, and desiccation; ( B) desiccation desiccation and calcination; calcination; ( C) no no treatment). treatment). Reaction Reaction conditions: conditions: 0.01 0.01 gg Zr–ZnO, 8.0 g ethanolethanol andand 2.02.0 gg cinnamaldehydecinnamaldehyde reacted reacted at at an an oxygen oxygen pressure pressure of of 1.0 1.0 MPa MPa and and 70 ◦70C °Cfor for 60 60 min. min. Thermal analysis, i. e., thermal gravity- diﬀerential thermal gravity (TG-DTG) and FT-IR were Thermal analysis, i. e., thermal gravity- differential thermal gravity (TG-DTG) and FT-IR were used to investigate the reason for the decease of benzaldehyde selectivity. After the fourth cycle, the used to investigate the reason for the decease of benzaldehyde selectivity. After the fourth cycle, the Zr–ZnO catalyst was washed with methanol and ethanol, followed by analysis. The used and fresh Zr–ZnO catalyst was washed with methanol and ethanol, followed by analysis. The used and fresh Zr–ZnO catalyst data are compared in Figures7 and S2. Zr–ZnO catalyst data are compared in Figures 7 and S2.

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Figure 7. FT-IRFT-IR of calcined treated Zr–ZnO ( aa),), and and Zr–ZnO Zr–ZnO before before use use ( (b)) and and after after being being used used five ﬁve timestimes ( (c).).

After fourfour cycles,cycles, the the surface surface groups groups on on the the Zr–ZnO Zr–ZnO catalyst catalyst exhibited exhibited significant significant change, change, as shown as 1 shownin Figure in7 Figure: water 7: was water found was on found the Zr–ZnO on the Zr–ZnO surface duesurface to the due 3700–3050 to the 3700–3050 cm − broad cm− peak1 broad and peak the 1 1 1 –1 and1650 the cm −1650peak cm [−491 peak]. The [49]. peaks The at peaks 3000–2750 at 3000–2750 cm− , 1250–1000 cm−1, 1250–1000 cm− , and cm 800–600−1, and cm800–600were cm assigned–1 were to the function groups –CH, –CH , –C–O–C–, –C–O, and –C=O, of cinnamaldehyde, benzaldehyde or assigned to the function groups2 –CH, –CH2, –C–O–C–, –C–O, and –C=O, of cinnamaldehyde, benzaldehydeethyl benzoate, or respectively, ethyl benzoate, which respectively, were absorbed which on were the Zr–ZnOabsorbed surface on the [Zr–ZnO50–52]. Comparedsurface [50–52]. with Comparedthe FT-IR spectra with the of FT-IR the fresh spectra and of used the fresh Zr–ZnO, and used these Zr–ZnO, substances these might substances have covered might have some covered of the somecatalyst’s of the active catalyst’s sites andactive caused sites aand selectivity caused a reduction. selectivity reduction. Moreover, Figure S2 shows three weight loss stages at 179, 212, and 249 °C◦C for the used used Zr–ZnO Zr–ZnO catalyst, which which corresponded to the decomposition of benzaldehyde, ethyl benzoate, and benzoic acid, respectively. respectively. Overall, this proved that some organic organic components covered the surface of Zr–ZnO Zr–ZnO catalysts after four cycles, which could explain the decrease of the ZnO catalyst activity. 2.7. Proposed Reaction Mechanisms 2.7. Proposed Reaction Mechanisms To investigate the reaction mechanism, the reaction products were analyzed by GC-MS and GC, To investigate the reaction mechanism, the reaction products were analyzed by GC-MS and GC, and results of the intermediate species are shown in Table S1. The intermediate species included and results of the intermediate species are shown in Table S1. The intermediate species included benzaldehyde, ethyl benzoate, benzoic acid, and triethyl orthoformate, as well as benzoin ethyl ether. In benzaldehyde, ethyl benzoate, benzoic acid, and triethyl orthoformate, as well as benzoin ethyl ether. the present process, the catalytic oxidation of cinnamaldehyde would be in a pathway of oxygen–olefin In the present process, the catalytic oxidation of cinnamaldehyde would be in a pathway of oxygen– reaction in the liquid phase [53,54]. Radical production from oxygen–olefin reactions is either a slower olefin reaction in the liquid phase [53,54]. Radical production from oxygen–olefin reactions is either independent process or due to a very small yield of radicals “leaking” from the oxidation process. a slower independent process or due to a very small yield of radicals “leaking” from the oxidation In general, unsaturated oil oxidation causes the formation of significant amounts of other products, process. In general, unsaturated oil oxidation causes the formation of significant amounts of other which is a double bond type- and location-dependent process [47]. In the present study, cinnamaldehyde products, which is a double bond type- and location-dependent process [47]. In the present study, is a trans-styrene aldehyde in which the aldehyde group with the electron-withdrawing electron is cinnamaldehyde is a trans-styrene aldehyde in which the aldehyde group with the electron- on the C=C double bonds [55,56], therefore, according to the above discussion, the mechanism of the withdrawing electron is on the C=C double bonds [55,56], therefore, according to the above cinnamaldehyde oxidation as proposed is shown in Figure8. discussion, the mechanism of the cinnamaldehyde oxidation as proposed is shown in Figure 8. Firstly, the cinnamaldehyde is adsorbed on the Zr–ZnO catalyst surface, and, at the same time, the Firstly, the cinnamaldehyde is adsorbed on the Zr–ZnO catalyst surface, and, at the same time, oxygen attacks the C=C bond to yield the unstable oxide (II) (process A), which rapidly decomposes to the oxygen attacks the C=C bond to yield the unstable oxide (II) (process A), which rapidly produce the carbonyl compound (III, IV) and carbonyl oxide (VI, V) (shown as process B) [57]. If no decomposes to produce the carbonyl compound (III, IV) and carbonyl oxide (VI, V) (shown as process participating solvents exist, the carbonyl compounds (III) and (IV) [or carbonyl oxide (V) and (VI)] can B) [57]. If no participating solvents exist, the carbonyl compounds (III) and (IV) [or carbonyl oxide (V) recombine and form the 1, 2-trioxolanes (VII) (process C). Decomposition of the oxide, 1, 2-trioxolanes and (VI)] can recombine and form the 1, 2-trioxolanes (VII) (process C). Decomposition of the oxide, (VII), also yields benzoin ethyl ether and triethyl orthoformate (process G). However, ethyl benzoate is 1, 2-trioxolanes (VII), also yields benzoin ethyl ether and triethyl orthoformate (process G). However, produced by ethanol and amphoteric ion (V) (process E), leading to the decrease of the benzaldehyde ethyl benzoate is produced by ethanol and amphoteric ion (V) (process E), leading to the decrease of yield [58]. Furthermore, processes D and E are synchronized, which also indicates a competitive the benzaldehyde yield [58]. Furthermore, processes D and E are synchronized, which also indicates process. After using Zr–ZnO as the catalyst, the basic site, i.e., surface O2 , on Zr–ZnO promotes the a competitive process. After using Zr–ZnO as the catalyst, the basic site, i.e.,− surface O2−, on Zr–ZnO fragmentation of the double bond, and provides an electron pair for the reaction of process D, which promotes the fragmentation of the double bond, and provides an electron pair for the reaction of increases the benzaldehyde yield. Finally, the benzaldehyde is desorbed from the Zr–ZnO surface, process D, which increases the benzaldehyde yield. Finally, the benzaldehyde is desorbed from the Zr–ZnO surface, which is not surprising as the O2 also oxidized the aldehydes to carboxylic acids [58]. Furthermore, with the increase of the reaction time, benzoic acid can be oxidized by the excess benzaldehyde (process F).

Catalysts 2019, 9, 716 8 of 12

which is not surprising as the O2 also oxidized the aldehydes to carboxylic acids [58]. Furthermore, with the increase of the reaction time, benzoic acid can be oxidized by the excess benzaldehyde (process F). Catalysts 2019, 9, x FOR PEER REVIEW 8 of 12

2. FigureFigure 8.8. SuggestedSuggested oxidationoxidation mechanismmechanism ofof cinnamaldehydecinnamaldehyde withwith OO2.

3.3. Materials and Methods

3.1.3.1. Catalyst Preparation The ZnO catalysts were prepared via a precipitation method. Typically, 1.2 mol L 1 Na CO water The ZnO catalysts were prepared via a precipitation method. Typically, 1.2· − mol·L2 −1 Na3 2CO3 solution was added drop by drop to a 200 mL 0.4 mol L 1 Zn(NO ) water solution at 200 rpm until water solution was added drop by drop to a 200 mL 0.4· − mol·L−1 Zn(NO3 2 3)2 water solution at 200 rpm theuntil solution the solution pH value pH reachedvalue reached 9 and a white9 and precipitatea white precipitate was obtained. was Afterobtained. filtration, After the filtration, precipitate the was dried at 90 C for 12 h and calcined at 400 C for 2 h to have ZnO. Fe O , Al O , and CuO were precipitate was ◦dried at 90 °C for 12 h and calcined◦ at 400 °C for 2 h to have2 3 ZnO.2 Fe3 2O3, Al2O3, and preparedCuO were by prepared a similar by method. a similar method. ModificationModification treatment:treatment: nitrates of transition metals were usedused asas rawraw material.material. Five didifferentfferent nitratesnitrates werewere weighedweighed accordingaccording toto 0.5%,0.5%, 1%,1%, andand 2%2% ofof thethe molemole amountamount ofof eacheach transitiontransition metalmetal element of Zn and then added into the prepared 0.40 mol L 1 Zn(NO ) solution. The catalysts were element of Zn and then added into the prepared 0.40 mol·L· − −1 Zn(NO33)22 solution. The catalysts were Zr–ZnO,Zr–ZnO, Co–ZnO,Co–ZnO, Ce–ZnO,Ce–ZnO, andand La–ZnO.La–ZnO. 3.2. Characterization of the Catalysts 3.2. Characterization of the Catalysts X-ray diffraction (XRD) analysis was carried on a D8 Advance X-ray diffractometer (Bruker X-ray diffraction (XRD) analysis was carried on a D8 Advance X-ray diffractometer (Bruker Karlsruhe, Germany). A NOVA 2200e physical adsorption instrument (Quantachrome Corp., Boynton Karlsruhe, Germany). A NOVA 2200e physical adsorption instrument (Quantachrome Corp., Beach, USA) was used in N2 adsorption–desorption. The Brunauer–Emmett–Teller (BET) and Boynton Beach, USA) was used in N2 adsorption–desorption. The Brunauer–Emmett–Teller (BET) Barret–Joyner–Halenda (BJH) methods were used to calculate the specific surface areas and the pore and Barret–Joyner–Halenda (BJH) methods were used to calculate the specific surface areas and the size distributions. A Thermo ESCALAB 250X multifunction imaging electron spectrometer (Thermo pore size distributions. A Thermo ESCALAB 250X multifunction imaging electron spectrometer Fisher Scientific, Waltham, USA) was used to obtain the X-ray photoelectron spectrum (XPS) of (Thermo Fisher Scientific, Waltham, USA) was used to obtain the X-ray photoelectron spectrum (XPS) catalysts, using the Al Kα ray as the X-ray source. Thermal analysis (TG-DTA) was performed using an of catalysts, using the Al Kα ray as the X-ray source. Thermal analysis (TG-DTA) was performed STA-449-F5 thermal analyzer (Netzsch Co. Ltd., Deutschland, Germany). Fourier transform infrared using an STA-449-F5 thermal analyzer (Netzsch Co. Ltd., Deutschland, Germany). Fourier transform (FT-IR) spectra were measured on a Nexus 470 Fourier transform spectrometer (Thermo Nicolet, infrared (FT-IR) spectra were measured on a Nexus 470 Fourier transform spectrometer (Thermo Waltham, USA). Nicolet, Waltham, USA).

Catalysts 2019, 9, 716 9 of 12

3.3. Catalytic Oxidation of Cinnamaldehyde The catalytic oxidation of cinnamaldehyde to benzaldehyde was reacted in a high-pressure tank reactor, as shown in Figure S1. During the ZnO-catalyzed oxidation cinnamaldehyde experiments, 2.00 g cinnamaldehyde and 8.00 g ethanol were mixed together in the reactor (40 mm diameter, 75 mm tube length), and 0.01 g ZnO was added. The gas atmosphere in the reactor was replaced with oxygen 7 times; subsequently, the oxygen pressure was adjusted to 1.0 MPa, and the cinnamaldehyde mixture was maintained at 70 ◦C. When using other catalysts, the reaction conditions were the same as for using ZnO as the catalyst. After the reaction, the products were quantitatively analyzed by GC Smart (GC-2018) gas chromatography (Shimadzu Corporation, Suzhou, China), using dichlorobenzene as the internal standard. To identify the composition of the products, a GC7820-MS GC-MS (Agilent Technologies, Santa Clara, USA) was used.

4. Conclusions ZnO and metal-modified ZnO were prepared and the catalytic oxidation of cinnamaldehyde on ZnO-based catalysts was studied in this paper. Zr–ZnO calcined at 400 ◦C for 2 h appeared to exhibit better structural and textural properties for cinnamaldehyde oxidation. Under optimal reaction conditions, namely, 0.01 g Zr–ZnO catalyst, oxygen pressure of 1.0 MPa, and reaction at 70 ◦C for 1.0 h, cinnamaldehyde conversion and benzaldehyde yield reached 17.6% and 86.2%, respectively. The catalyst characterization results indicated that: doping with Zr could reduce the grain size of ZnO and increase the specific surface area of the catalyst, which provided more active sites for the reaction; Zr-doping could exchange electrons with ZnO and reduce the electron density in the outer layer of Zn, which would further affect benzaldehyde selectivity; and Zr-modified ZnO enhanced the alkalinity of the catalyst surface, which caused the Zr–ZnO catalyst to exhibit higher catalytic activity. Compared with other research, the present study provided higher benzaldehyde selectivity under significantly milder conditions.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/9/9/716/s1, Figure S1: Schematic diagram of the reaction equipment, Figure S2: TG-DTG curves of Zr-ZnO catalyst after being fourth used, Table S1: Intermediates detected during catalytic oxidation of cinnamaldehyde. Author Contributions: Data curation, P.D. and X.L.; Formal analysis, T.S.; Investigation, P.D. and X.X.; Methodology, X.X.; Supervision, Z.Q. and H.J.; Writing—review & editing, X.L. Funding: This research was funded by National Natural Science Foundation of China (21868005), Natural Science Foundation of Guangxi Province (2016GXNSFFA380015), and Special funding for ‘Guangxi Bagui Scholars’. Acknowledgments: This work was supported by National Natural Science Foundation of China (21868005), Natural Science Foundation of Guangxi Province (2016GXNSFFA380015), and Special funding for ‘Guangxi Bagui Scholars’. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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