Solvent-Free Selective Oxidation of Toluene with O2 Catalyzed by

catalysts Article

Solvent-Free Selective Oxidation of Toluene with O2 Catalyzed by Metal Cation Modified LDHs and Mixed Oxides Xiaoli Wang, Gongde Wu *, Hao Liu and Qibo Lin Received: 13 December 2015; Accepted: 30 December 2015; Published: 15 January 2016 Academic Editor: Stuart H. Taylor Department of Environment and Technology, Nanjing Institute of Technology, Nanjing 211167, China; [email protected] (X.W.); [email protected] (H.L.); [email protected] (Q.L.) * Correspondence: [email protected]; Tel./Fax: +86-25-8611-8960

Abstract: A series of metal cation modified layered-double hydroxides (LDHs) and mixed oxides were prepared and used to be the selective oxidation of toluene with O2 . The results revealed that the modified LDHs exhibited much higher catalytic performance than their parent LDH and the modified mixed oxides. Moreover, the metal cations were also found to play important roles in the catalytic performance and stabilities of modified catalysts. Under the optimal reaction conditions, the highest toluene conversion reached 8.7% with 97.5% of the selectivity to benzyldehyde; moreover, the catalytic performance remained after nine catalytic runs. In addition, the reaction probably involved a free-radical mechanism. Keywords: LDH; modified; selective oxidation; toluene; benzyldehyde

1. Introduction The selective oxidation of alkanes is described as ”the Holy Grail of organic chemistry”, and is accepted as a very challenging problem. Toluene, as the simplest member of alkyl aromatics, can be oxidized into benzyl alcohol, benzyldehyde, benzoic acid, benzyl benzoate, etc. These oxidation products were wildly used in dyes, solvents, perfumery, plasticizer, preservatives, flame retardant and other fields [1–4]. Recently, with the increasing of demand for chlorine-free benzyldehyde in perfumery and pharmaceutical industries, the traditional toluene chlorination hydrolysis had been unable to meet needs [3,4]. Thus, the selective oxidation of toluene side chain attracted much attention in academia and industry. However, due to the inertia of toluene to oxidation reaction and the presence of side reactions such as deep oxidation and ring-hydroxylation, the conversion of toluene and the selectivity to object product were often low, and the atomic economy was poor. Therefore, in recent years, much effort has been paid to explore the high efficient oxidation process for toluene. Among all the processes of selective oxidation of toluene, the liquid phase oxidation process with O2 , especially in the absence of organic solvent, was eye-catching due to the ”green” and mild reaction conditions as well as high selectivity to object product, and various catalytic systems have been reported [5–12]. Xu et al. used CuFe/γ-Al2 O3 to the selective oxidation of toluene with O2 , and the 7.4% of toluene conversion and 46.5% of selectivity to benzyldehy were obtained [5]. Mac Leod et al. reported that Mn (salen) could catalytically oxidize toluene at room temperature with acetonitrile as slolvent, the benzyldehyde selectivity could reach 98%, but the toluene conversion was only 5% [6]. Kesavan et al. found that Au-Pd/C showed effective catalytic activity to the selective oxidation of toluene under solvent-free conditions. The conversion of toluene reached 94.4%, but the main product was benzyl benzoate, and the selectivity to benzyldehyde was less than 15% [7]. Thus, there was still an urgent need to develop effective catalysts for selective oxidation of toluene to benzyldehyde.

Catalysts 2016, 6, 14; doi:10.3390/catal6010014

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Catalysts 2016, 6, 14

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there was still an urgent need to develop effective catalysts for selective oxidation of toluene to benzyldehyde. The solid solid Mg-Al Mg-Al hydrotalcites hydrotalcites (HT) (HT) and and their their calcined calcined products products of of mixed mixed oxides oxides were were often often The considered as as promising promising catalysts catalysts or or catalyst catalyst supports supports owing owing to to their their perfectly perfectly adjustable adjustable surface surface considered basicity, along along with with high high surface surface area area and and thermal thermal stability. stability. Some Some cation cation and and anion anion modified modified Mg-Al Mg-Al basicity, hydrotalcites or or mixed mixed oxides oxides had had been been prepared prepared for for aromatization, aromatization, condensation, condensation, cyanoethylation, cyanoethylation, hydrotalcites cyclohexene dehydration, dehydration,alcohol alcoholoxidation, oxidation, [13–17]. However, to best the best ofknowledge, our knowledge, cyclohexene etc.etc. [13–17]. However, to the of our there there are few reports the design ofmodified cation modified Mg-Al hydrotalcites and mixed for the are few reports on the on design of cation Mg-Al hydrotalcites and mixed oxides foroxides the selective selective oxidation toluene. Here, we the efficient solvent-free selective oxidation oxidation of toluene.of Here, we reported the reported efficient solvent-free selective oxidation of toluene with Oof2 toluene with O2 catalyzed by nine kinds transition cation, sevenmetal kindscatoin of rare-earth catalyzed by nine kinds of transition metal of cation, seven metal kinds of rare-earth and two metal kinds catoin and two cation kinds of noble metal cation modified layered-double (LDHs) as well as of noble metal modified layered-double hydroxides (LDHs) ashydroxides well as their corresponding their corresponding mixed oxides. rolesinofthe thecatalytic metal cations in the and catalytic performance and mixed oxides. The roles of the metalThe cations performance stabilities of modified stabilities of modified LDHs and their mixed oxides were discussed in detail. LDHs and their mixed oxides were discussed in detail. 2. Resultsand andDiscussion Discussion 2. Results

2.1. Characterization Characterization of of Catalysts Catalysts 2.1. The 18 18 kinds kinds of of metal metal modified modified LDH LDH obtained obtained and and 18 18 kinds kinds of of metal metal modified modified mixed mixed oxides oxides The were characterized by X-ray diffraction (XRD), elemental analysis, N 2 sorption and were characterized by X-ray diffraction (XRD), elemental analysis, N2 sorption and thermogravimetric 3+ thermogravimetric analysis (TGA) techniques. Considering the similarity of characterization chart, analysis (TGA) techniques. Considering the similarity of characterization chart, Ti /Mg3 Al-LDH and Ti3+/Mg 3Al-LDH and Ti/Mg3(Al)O were selected as the representative catalysts for metal cation Ti/Mg 3 (Al)O were selected as the representative catalysts for metal cation modified hydrotalcites and modified hydrotalcites and mixed oxides, respectively. mixed oxides, respectively. 3+/Mg3Al-HT showed typical layered material containing sharp and intense 3+ XRD patterns of Ti XRD patterns of Ti /Mg 3 Al-HT showed typical layered material containing sharp and intense lines at at low low 22 theta theta values values and and less less intense intense asymmetric asymmetric lines lines at at higher higher 22 theta theta angular angular values values (see (see lines Figure 1). Three sharp and asymmetrical peaks of (003), (006) and (110) planes appeared, suggesting Figure 1). Three sharp and asymmetrical peaks of (003), (006) and (110) planes appeared, suggesting the formation formation of of aa highly highly crystalline crystalline material material [17]. [17]. For For Ti/Mg Ti/Mg33(Al)O, 006 in in the (Al)O, the the planes planes of of 003 003 and and 006 ˝ ˝ hydrotalcites disappeared, while two peaks at about 43° and 63° due to the MgAl mixed oxides and hydrotalcites disappeared, while two peaks at about 43 and 63 due to the MgAl mixed oxides and two peaks peaks at at about about 25 25°˝ and two and 36° 36˝ related related to to TiO TiO22 were were detected. detected. This This indicated indicated that that the the hydrotalcite-like hydrotalcite-like structures were were destructed destructed and and new new phases phases of of mixed mixed oxides oxidesappeared appearedduring duringcalcination. calcination. structures

003

Intensity (a.u.)

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MgAl(O) MgAl(O)

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3+3+/Mg Figure1.1.XRD XRDpatterns patternsofof(a) (a)TiTi /Mg3Al-LDH; (b) Ti/Mg3(Al)O. Figure 3 Al-LDH; (b) Ti/Mg3 (Al)O.

Chemical analysis of metal cation modified LDHs showed almost identical molar ratios of Mg, Chemical analysis of metal cation modified LDHs showed almost identical molar ratios of Mg, Al Al and the third metal (see Tables 1 and 2). This indicated that the introduction of metal cations did and the third metal (see Tables 1 and 2). This indicated that the introduction of metal cations did not not change the elemental composition of their parent LDH. Simultaneously, no significant change in change the elemental composition of their parent LDH. Simultaneously, no significant change in the the molar ratio of Mg, Al and the third metal was found before and after the calcination, suggesting molar ratio of Mg, Al and the third metal was found before and after the calcination, suggesting that that no loss took place during calcination. no loss took place during calcination. TGA curves of Ti3+/Mg3Al-LDH were typically illustrated in Figure 2. Two major weight losses appeared at about 100 °C and 400 °C, respectively. The former could be attributed to the elimination


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TGA curves of Ti3+ /Mg3 Al-LDH were typically illustrated in Figure 2. Two major weight losses appeared at about 100 ˝ C and 400 ˝ C, respectively. The former could be attributed to the elimination of physically adsorbed and interlayer water, while the latter was related to the dehydroxylation of the layers. This indicated that Ti/Mg3 (Al)O could be successfully prepared by the complete decomposition of the Ti3+ /Mg3 Al-LDH at 450 ˝ C. Table 1. Catalytic performance of metal cation modified layered-double hydroxides (LDHs) in toluene oxidation a.

Entry

Catalysts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

blank Mg3 Al-LDH Ti3+ /Mg3 Al-LDH Ti3+ /Mg3 Al-LDH b Ti3+ /Mg3 Al-LDH c Ti3+ /Mg3 Al-LDH d Cr3+ /Mg3 Al-LDH Mn2+ /Mg3 Al-LDH Fe2+ /Mg3 Al-LDH Fe3+ /Mg3 Al-LDH Co2+ /Mg3 Al-LDH Ni2+ /Mg3 Al-LDH Cu2+ /Mg3 Al-LDH Zn2+ /Mg3 Al-LDH La3+ /Mg3 Al-LDH Nd3+ /Mg3 Al-LDH Sm3+ /Mg3 Al-LDH Gd3+ /Mg3 Al-LDH Dy3+ /Mg3 Al-LDH Ho3+ /Mg3 Al-LDH Yb3+ /Mg3 Al-LDH Au+ /Mg3 Al-LDH Au+ /Pd2+ /Mg3 Al-LDH a b e

Metal Cation Radius (Å)

Mg/Al/M Molar Ratio

SBET (m2 /g)

Toluene Con. (mol %)

--- e --0.76 ditto ditto ditto 0.69 0.80 0.76 0.64 0.74 0.72 0.69 0.74 1.06 0.99 0.96 0.94 0.91 0.89 0.86 1.37 1.37/0.86

----2.93/1.0/0.017 ditto ditto ditto 2.93/1.0/0.018 2.93/1.0/0.015 2.93/1.0/0.017 2.93/1.0/0.018 2.93/1.0/0.016 2.93/1.0/0.016 2.93/1.0/0.018 2.93/1.0/0.017 2.93/1.0/0.016 2.93/1.0/0.017 2.93/1.0/0.017 2.93/1.0/0.015 2.93/1.0/0.016 2.93/1.0/0.018 2.93/1.0/0.017 2.93/1.0/0.018 2.93/1.0/0.018

--92 87 ditto ditto ditto 85 87 86 86 83 85 87 84 86 86 85 82 87 86 88 86 86

0 1.1 8.7 1.2 1.6 0.7 6.8 2.8 1.6 5.2 2.8 3.9 3.2 2.3 6.5 5.2 4.7 3.8 4.0 5.0 4.4 9.2 9.5

Sel. (mol %) Benzyl Alcohol 0 4.2 2.5 3.2 5.6 10.2 3.8 6.1 5.3 6.5 4.3 7.8 8.5 5.4 3.5 5.3 5.7 5.2 5.1 4.9 5.3 3.2 4.0

Benzyldehyde

Benzoic Acid

0 93.6 97.5 87.5 83.0 76.3 95.2 91.8 93.5 92.0 93.3 91.4 91.5 92.4 95.2 92.7 92.5 94.0 93.5 94.2 93.6 96.5 95.0

0 2.2 0 9.3 11.4 13.5 1.0 2.1 1.2 1.5 1.4 0.8 0 2.2 1.3 2.0 1.8 0.8 1.4 0.9 1.1 0.3 1.0

Reaction conditions: catalyst 0.18 g; toluene 0.1 mol; Oxygen pressure1 MPa, temperature 150 ˝ C; time 8 h; Adding pyrocatechol (0.02 mol); c adding resorcinol (0.02 mol); d adding hydroquinone (0.02 mol); “---” means no detection.

Table 2. Catalytic performance of metal modified mixed oxides in toluene oxidation a .

Entry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Catalysts

Mg3 (Al)O Ti/Mg3 (Al)O Cr/Mg3 (Al)O Mn/Mg3 (Al)O Fe/Mg3 (Al)O Fe/Mg3 (Al)O Co/Mg3 (Al)O Ni/Mg3 (Al)O Cu/Mg3 (Al)O Zn/Mg3 (Al)O La/Mg3 (Al)O Nd/Mg3 (Al)O Sm/Mg3 (Al)O Gd/Mg3 (Al)O Dy/Mg3 (Al)O Ho/Mg3 (Al)O Yb/Mg3 (Al)O Au/Mg3 (Al)O Au/Pd/Mg3 (Al)O a b

Mg/Al/M Molar Ratio --- b 2.93/1.0/0.016 2.93/1.0/0.016 2.93/1.0/0.015 2.93/1.0/0.016 2.93/1.0/0.015 2.93/1.0/0.017 2.93/1.0/0.015 2.93/1.0/0.016 2.93/1.0/0.017 2.93/1.0/0.017 2.93/1.0/0.016 2.93/1.0/0.015 2.93/1.0/0.016 2.93/1.0/0.017 2.93/1.0/0.016 2.93/1.0/0.016 2.93/1.0/0.017 2.93/1.0/0.016

SBET (m2 /g1 )


210 205 207 204 205 205 207 206 206 205 204 206 205 205 206 206 204 203 205

0.5 3.5 3.7 2.0 0.8 2.9 1.7 1.0 1.3 0.9 3.2 2.5 2.7 2.1 2.0 2.2 2.9 5.5 5.0

Sel. (mol %) Benzyl Alcohol 10.0 11.1 11.5 12.5 11.2 13.2 12.0 10.6 9.2 11.5 12.8 11.8 9.6 9.5 8.9 10.2 8.9 9.8 10.3

Benzyldehyde

Benzoic Acid

87.5 86.2 85.3 82.7 84.0 80.8 82.5 87.5 86.2 85.5 83.5 82.0 84.4 86.0 88.2 87.5 88.0 86.7 85.5

Reaction conditions: catalyst 0.18 g; toluene 0.1 mol; Oxygen pressure1 MPa, temperature 150 ˝ C; time 8 h; “---” means no detection.

2.5 2.7 3.2 4.7 4.8 6.0 5.5 1.9 4.6 3.0 3.7 6.2 5.8 4.5 2.9 2.3 3.1 3.5 4.2

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Weight Weight loss loss (%) (%)

100 100

50 50 00

-0.05 -0.05

200 400 600 200 400 600 OO Temperature((C) C) Temperature

800 800

3+ 3+3+ Figure2. Thermogravimetricweight weightloss losscurve curveand andderivative derivativeplots plotsof Ti /Mg3Al-LDH. 3Al-LDH. Figure 2.2.Thermogravimetric weight loss curve and derivative plots ofofTi Ti /Mg Figure Thermogravimetric /Mg 3 Al-LDH. 3+3+ TheN N2 2adsorption-desorption adsorption-desorptionisotherms isothermsof ofTi Ti /Mg3Al-LDH 3Al-LDHand andTi/Mg Ti/Mg3(Al)O 3(Al)Oboth bothdisplayed displayed 3+ The /Mg The N 2 adsorption-desorption isotherms of Ti /Mg3 Al-LDH and Ti/Mg3 (Al)O both displayed type IV isotherms with clear hysteresis loops associated with capillary condensation (see 3). type IV isotherms with clear hysteresis loops associated with capillary condensation (see Figure type IV isotherms with clear hysteresis loops associated with capillary condensation (see Figure Figure3). 3). This indicated that the mesoporous structures formed, probably because of particle aggregation [18]. This indicated that the mesoporous structures formed, probably because of particle aggregation [18]. This indicated that the mesoporous structures formed, probably because of particle aggregation [18]. 3+3+ Moreover, compared compared to toTi Ti3+ /Mg3Al-LDH, 3Al-LDH, Ti/Mg Ti/Mg3(Al)O 3(Al)O showed showed aa much much higher higher surface surface area area (see (see Moreover, Ti /Mg Moreover, compared to /Mg 3 Al-LDH, Ti/Mg3 (Al)O showed a much higher surface area (see Tables 1 and 2), which might be associated with the evolving of a large amount of CO 2 from the Tables Tables 11 and and 2), 2), which which might might be be associated associated with with the the evolving evolving of of aa large large amount amount of of CO CO22 from from the the 3+ 3+ Ti3+/Mg /Mg 3Al-LDHduring calcination. Ti 3Al-LDH calcination. Ti /Mg Al-LDHduring during calcination.

3

700 700

3 Volume Volume adsorbed adsorbed (cm (cm3/g) /g)

600 600 500 500 400 400 300 300

(b) (b)

200 200 100 100 (a) (a)

00 0.0 0.0

0.2 0.2

0.4 0.4

0.6 0.6

0.8 0.8

1.0 1.0

Relativepressure pressure(P/P (P/P0)0) Relative 3+ 3+3+ Figure /Mg Al-LDH;(b) (b)Ti/Mg Ti/Mg (Al)O. Figure3.3.3.N adsorption-desorptionisotherms isothermsof (a)Ti Ti /Mg3Al-LDH; (b) Ti/Mg 33 (Al)O. 22 2adsorption-desorption 33Al-LDH; Figure NN adsorption-desorption isotherms ofof(a) (a) Ti /Mg 3(Al)O.

2.2.Catalytic CatalyticPerformance Performanceofof ofMetal MetalCation CationModified ModifiedLDHs LDHs 2.2. Catalytic Performance Metal Cation Modified LDHs 2.2. Withoutany anyorganic organic solvent, phase transfer catalyst oradditive, additive, theobtained obtained catalysts were Without solvent, phase transfer catalyst or additive, the obtained catalysts were used were in the Without any organic solvent, phase transfer catalyst or the catalysts used in the selective oxidation of toluene. Under the present reaction conditions, only three selective oxidation of toluene. Under the present reaction conditions, only three products—benzyl used in the selective oxidation of toluene. Under the present reaction conditions, only three products—benzyl alcohol, benzyldehyde andbenzoic benzoic acid—weredetected. detected. alcohol, benzyldehyde andbenzyldehyde benzoic acid—were detected. products—benzyl alcohol, and acid—were Overall,compared comparedto tothe theparent parentMg Mg33Al-LDH, 3Al-LDH, Al-LDH,all allmetal metalcation cationmodified modifiedLDHs LDHsshowed showedenhanced enhanced Overall, compared to the parent Mg all metal cation modified LDHs showed enhanced Overall, catalytic performance, indicating that it was feasible to tune the catalytic performance of LDHsby by catalyticperformance, performance,indicating indicatingthat thatititwas wasfeasible feasibleto totune tunethe the catalytic catalytic performance performanceof ofLDHs LDHs by catalytic introducingmetal metalcations. cations.Among Amongthe thenine ninekinds kindsof oftransition transitionmetal metalcation cationmodified modifiedLDHs, LDHs,trivalent trivalent introducing metal cations. Among the nine kinds of transition metal cation modified LDHs, trivalent introducing cation modified LDHs exhibited much higher catalytic performance than divalent cation modified cation modified LDHs exhibited much higher catalytic performance than divalent cation modified ones cation modified LDHs exhibited much higher catalytic performance than divalent cation modified onesEntries (seeEntries Entries 3–14 inTable Table 1). On theLDH LDHlaminates laminates withtrivalent trivalent metal cations, moresurface surface (see 3–14 3–14 in Table 1). On the LDH laminates with trivalent metalmetal cations, moremore surface –OH ones (see in 1). On the with cations, –OH were were decomposed decomposed to to OH OH− − to to balance balance the the excessive excessive positive positive charge charge [19,20]. [19,20]. This This led led to to the the –OH enhanced surface basicity and could afford the higher catalytic performance of trivalent cation enhanced surface basicity and could afford the higher catalytic performance of trivalent cation


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were decomposed to OH´ to balance the excessive positive charge [19,20]. This led to the enhanced surface basicity and could afford the higher catalytic performance of trivalent cation modified LDHs. For the divalent metal modified catalysts, when the metal cations were Ni2+ or Cu2+ with the similar ionic radius to Mg2+ , the obtained catalysts showed relatively high catalytic performance in the selective oxidation of toluene. This could be attributed to the substitution of Mg2+ in hydrotalcite lattice by Ni2+ or Cu2+ in the process of catalyst preparation, which induced the increased disorder on hydrotalcite laminates and made more catalytic activity “exposure”. In the present reaction conditions, Ti3+ /Mg3 Al-LDH exhibited best catalytic performance, probably because the introduction of Ti3+ with larger ionic radius to LDHs promoted the decomposition of surface –OH to OH´ due to its relatively weak binding capacity. Moreover, seven kinds of rare-earth metal cation modified LDHs were more active in the selective oxidation of toluene than most of the transition metal cation modified LDHs except Ti3+ /Mg3 Al-LDH, which could be related to their larger cation radius. Among them, La3+ /Mg3 Al-LDH exhibited the highest toluene conversion of 6.5%, with the selectivity of 95.2% to benzyldehyde. For comparison, two kinds of noble metal modified LDHs were also prepared for the toluene oxidation. + Au /Pd2+ /Mg3 Al-LDH exhibited much higher catalytic performance than Au+ /Mg3 Al-LDH, which could be attributed to the synergistic catalytic effect of Pd2+ and Au+ , similar phenomena had also been reported previously [7]. 2.3. Catalytic Performance of Metal Modified Mixed Oxides Compared to LDHs, the same metal modified mixed oxides showed lower catalytic performance. During calcination, as described in the TGA experiment, the dehydroxylation of layer –OH on hydrotalcite laminates took place, so the amount of surface B basic sites reduced. This indicated that B basic sites derived from the surface –OH played an important role in the catalytic performance of modified LDHs. In addition, compared to their parent Mg3 (Al)O, the metal modified mixed oxides showed slightly higher catalytic performance. Among all modified mixed oxides, noble metal modified mixed oxides exhibited highest catalytic performance, simultaneously, inexpensive Ti3+ , Cr3+ and La3+ modified mixed oxides were also effective in toluene oxidation. Moreover, the trivalent metal modified mixed oxides also exhibited much higher catalytic performance than divalent metal modified ones due to the relatively more surface B basic sites. 2.4. Stabilities of Catalysts On the basis of the above experiments, the following three metal cation modified LDHs, which exhibited high catalytic performance in toluene oxidation, were selected as the representative catalysts to investigate their stabilities. After the first catalytic run, the catalysts were separated from the reaction system, washed several times with water to remove any physisorbed molecules, and then dried overnight. The obtained used catalysts were further used in another nine catalytic runs. The results in Figure 4 revealed that the three catalysts exhibited high stabilities in the present reaction conditions. Particularly, Ti3+ /Mg3 Al-LDH was still effective until being reused nine times. Chemical analysis revealed that the recovered Ti catalyst after nine times held almost the same elemental composition (Mg/Al/M molar ratio = 2.93/1.0/0.016) as the fresh catalyst (Mg/Al/M molar ratio = 2.93/1.0/0.017), indicating that metal leaching was negligible.


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Reused times Figure 4. Reusability of (a) Ti3+/Mg3Al-LDH; (b) Au + /Pd 2+ /Mg3Al-LDH; (c) La3+/Mg3+ 3Al-LDH of (A) Figure 4. Reusability of(A) (a) Ti3+ /Mg3 Al-LDH; (b) Au+ /Pd2+ /Mg3 Al-LDH;(B) (c) La /Mg3 Al-LDH of Effect of reused times on the toluene conversion, (B) Eeffect of reused time on the benzyldehyde (A)Figure Effect of times on(a) the conversion; Eeffect reused(c) time the benzyldehyde 3+/Mg3Al-LDH; + /Pd 2+ /Mg3of 4. reused Reusability Titoluene Au(B) La3+on /Mg 3Al-LDH of (A) selectivity. Reaction of conditions: catalyst 0.18(b) g; toluene 0.1 Al-LDH; mol; Oxygen pressure1 MPa, selectivity. Reaction conditions: catalyst 0.18 g; toluene 0.1 mol; Oxygen pressure1 MPa, temperature Effect of reused on8 the temperature 150 times °C; time h. toluene conversion, (B) Eeffect of reused time on the benzyldehyde ˝ C; time 8 h. 150selectivity. Reaction conditions: catalyst 0.18 g; toluene 0.1 mol; Oxygen pressure1 MPa,

2.5.temperature Effect of Reaction 150 °C;Conditions time 8 h. on the Catalytic Performance of Catalyst

Figure 6).

Benzyldehyde Sel. (mol %) Toluene Con. Toluene (mol %)Con. (mol %) Benzyldehyde Sel. (mol %)

2.5. EffectThe of Reaction on the Catalytic Performance of Catalyst effect ofConditions reaction conditions on the catalytic performance of catalysts was also investigated 2.5. Effect of Reaction Conditions on the Catalytic Performance of Catalyst 3+/Mg3Al-LDH as the representative catalyst. The reaction used Ti temperature waswas first also investigated, The effect of reaction conditions on the catalytic performance of catalysts investigated and the results were shown in Figure 5. When the reaction was performed at less than 70 °C, no 3+ The effect of reaction conditions on the catalytic performance of catalysts was also investigated used Ti /Mg3 Al-LDH as the representative catalyst. The reaction temperature was first investigated, products were detected. With the increasing of temperature from 70 °C to 220 °C, the conversion of 3+/Mg 3Al-LDH as the representative catalyst. The reaction temperature was first investigated, andused the Ti results were shown in Figure 5. When the reaction was performed at less than 70 ˝ C, no toluene increased continuously. This could be attributed to the was increased contactat possibility between ˝C ˝less and the results were shown in Figure 5. When the reaction than 70 °C, no of products were detected. With the increasing of temperature from performed 70 reaction to 220 C, the conversion reactant molecules and surface active sites of catalyst with the increased temperature in multi of products were detected. With the of temperature from 70 °C tocontact 220 °C,possibility the conversion toluene increased Thisincreasing could attributed to the increased between phase catalyticcontinuously. system. Simultaneously, thebe selectivity to main product (benzyldehyde) was found to toluene increased continuously. This could be attributed to the increased contact possibility between reactant molecules and surface active sites of catalyst with the increased reaction temperature in multi increase firstly and then decrease, while the selectivity to over oxidation product (benzoic acid) reactant molecules and surface active sites of catalyst with the increased reaction temperature in multi phaseincreased catalyticcontinuously. system. Simultaneously, selectivity found This indicatedthe that a much to toomain high product reaction (benzyldehyde) temperature waswas more phase catalytic system. Simultaneously, the selectivity to main product (benzyldehyde) was found to conducive to over of benzyldehyde. optimumtoreaction temperature was 150 °C. The acid) to increase firstly andoxidation then decrease, while the The selectivity over oxidation product (benzoic increase firstly and then decrease, while the selectivity to over oxidation product (benzoic acid) effectcontinuously. of reaction timeThis wasindicated illustratedthat in Figure 6. Ittoo washigh found that thetemperature toluene oxidation reacted fast increased a much reaction was more conducive increased continuously. This indicated that almost a much too high reactionfortemperature was more ˝ in the present catalytic system, the reaction reached equilibrium 8 h, and the further to over oxidation of benzyldehyde. The optimum reaction temperature was 150 C. The effect of conducive to over oxidation of benzyldehyde. The optimum reaction temperature was 150 °C. increase in reaction time was more favorable to the over oxidation of the obtained benzyldehyde (seeThe reaction time was illustrated in Figurein6.Figure It was6.found thethat toluene oxidation reacted fast in the effect of reaction time was illustrated It wasthat found the toluene oxidation reacted fast Figure 6). present catalytic system, the reaction almost reached equilibrium for 8 h, and the further increase in the present catalytic system, the reaction almost reached equilibrium for 8 h, and the further in reaction time was more favorable to favorable the over oxidation the obtained (see Figure 6). 100more increase in reaction time was to the overofoxidation of thebenzyldehyde obtained benzyldehyde (see 80 B 60 40 100 20 80 B 0

60 40 10 20 8 A 0

6 4 102 80

(a) (b) (c)

(a)

(b) (c)

A

6 20 40 60 80 100 120 140 160 180 200 220 Reaction temperature (oC) 4 2 Figure 5. Effect of reaction 0 temperature on the catalytic performance of catalyst (a) benzyldehyde; (b) benzyl alcohol; (c) benzoic selectivity. Reaction 20 acid. 40 (A) 60 Toluene 80 100conversion 120 140 (B) 160Benzyldehyde 180 200 220 conditions: catalyst 0.18 g, toluene 0.1 mol,Reaction oxygentemperature pressure 1(oMPa, time 8 h. C)

Figure 5. Effect of reaction temperature on catalytic the catalytic performance of catalyst (a) benzyldehyde; (b) Figure 5. Effect of reaction temperature on the performance of catalyst (a) benzyldehyde; (b) benzyl benzyl alcohol; (c) benzoic acid. (A) Toluene conversion (B) Benzyldehyde selectivity. Reaction alcohol; (c) benzoic acid. (A) Toluene conversion (B) Benzyldehyde selectivity. Reaction conditions: catalyst conditions: catalyst 0.18 g, toluene 0.1 mol, oxygen pressure 1 MPa, time 8 h. 0.18 g, toluene 0.1 mol, oxygen pressure 1 MPa, time 8 h.


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Sel.%) (mol %) Benzyldehyde Sel. (mol Toluene Con.%) (mol %) Benzyldehyde Toluene Con. (mol


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100 80 B 100 60 80 B 40 60 20 40 0 20 0 14 12 A 10 14 8 A 12 6 10 48 26 04 0 2 0 0

(a) (a) (b) (c) (b) (c)

4

8

12

16

20

24

28

32

36

28

32

36

Reaction time (h) 4

8

12

16

20

24

Reaction time (h) of catalyst (a) benzyldehyde; (b) benzyl Figure 6. Effect of reaction time onon the catalytic Figure 6. Effect of reaction time the catalyticperformance performance of catalyst (a) benzyldehyde; (b) benzyl alcohol; (c) benzoic acid. (A)(A) Toluene conversion selectivity. Reaction conditions: alcohol; (c) benzoic acid. Toluene conversion (B) (B) Benzyldehyde Benzyldehyde selectivity. Reaction conditions: Figure 6. Effect of reaction time on the catalytic performance of catalyst (a) ˝benzyldehyde; (b) benzyl catalyst 0.18 g, toluene 0.1 mol, Oxygen pressure 1 MPa, temperature 150 °C. catalyst 0.18 g, toluene 0.1 mol, Oxygen pressure 1 MPa, temperature 150 C. alcohol; (c) benzoic acid. (A) Toluene conversion (B) Benzyldehyde selectivity. Reaction conditions: catalyst 0.18 g,oftoluene mol,an Oxygen pressure 1 MPa, temperature °C. The dosage O2 was0.1also important factor in the catalytic 150 performance of the catalyst (see

Sel.%) (mol %) Sel. (mol Toluene Con.%) (molBenzyldehyde %) Benzyldehyde Toluene Con. (mol

The dosage of O2 was also an important factor in the catalytic performance of the catalyst (see Figure 7). With the increase in the dosage of oxidant, toluene conversion and benzyldehyde of O2 was alsodosage an important factor toluene in the catalytic performance of the catalyst (see Figureselectivity 7). The Withdosage the increase of stage, oxidant, conversion and benzyldehyde selectivity both increasedinatthe the beginning and then reached the maximum value at O2 pressure Figure 7). at With the increasestage, in the dosage of oxidant, toluene conversion and benzyldehyde both increased the beginning and then reached the maximum value at O pressure of 1 MPa. 2 benzyldehyde of 1 MPa. The further improvement in the O2 pressure led to a significant decrease in selectivity both increased at the beginning stage, and then reached the maximum value at O 2 pressure The further improvement the Oincreased led to conversion. a significant decrease in selectivity, selectivity, along with in a slight toluene Meanwhile, thebenzyldehyde increased amount of 2 pressure of 1 MPa. The further improvement in the O2 pressure led to a significant decrease in benzyldehyde were detectedtoluene by GC. This indicated Meanwhile, that the O2 pressure of 1 MPaamount was optimum in the acid alongbenzoic with a acid slight increased conversion. the increased of benzoic selectivity, along with a slight increased toluene conversion. Meanwhile, the increased amount of catalytic system. were present detected by GC. This indicated that the O pressure of 1 MPa was optimum in the present benzoic acid were detected by GC. This indicated2 that the O2 pressure of 1 MPa was optimum in the catalytic system. present catalytic system. 100 80 100 60 80 40 60 20 40 0 20 120

B

(a)

B

(a) (b) (c) (b) (c)

10 A 8 12 6 10 A 48 26 04 20.0 0.2 0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.4

1.6

Oxygen presure (MPa) 0.4

0.6

0.8

1.0

1.2

Oxygen presure (MPa) Figure 7. Effect of O 2 pressure on the catalytic performance of catalyst (a) benzyldehyde; (b) benzyl alcohol; (c) benzoic acid. (A) Toluene conversion (B) Benzyldehyde selectivity. Reaction Figure 7. Effect of O2 of pressure on theon catalytic performance of catalyst (a) benzyldehyde; (b) benzyl Figure 7. Effect O 2 pressure the catalytic performance of catalyst (a) benzyldehyde; conditions: catalyst 0.18 g, toluene 0.1 mol, temperature 150 °C, time 8 h. (b) (c) benzyl alcohol; benzoic acid.conversion (A) Toluene(B) conversion (B) Benzyldehyde selectivity. alcohol; benzoic acid.(c)(A) Toluene Benzyldehyde selectivity. ReactionReaction conditions: ˝ C, time150 conditions: catalyst 0.18mol, g, toluene 0.1 mol, investigated, temperature timeresults 8 h. catalyst 0.18 g, toluene 0.1 temperature 150 8and h.°C, the The amount of catalyst was further were shown in Figure 8.

The toluene conversion was found to increase with the increased amount of catalyst, but the The amount of catalyst was further investigated, and the results were shown in Figure 8. increasing trends became gradually slow. There a turning forinthe selectivity to The catalyst was andwas the werepoint shown 8. but The the toluene Theamount toluene of conversion wasfurther foundinvestigated, to increase with theresults increased amount of Figure catalyst, benzyldehyde with the increasingwith of catalyst dosage. This could of be catalyst, related to but the increased diffusion conversion was trends found to increase increased amount increasing became gradually the slow. There was a turning point for the the increasing selectivity trends to limitation originated from the increased amount of catalyst. Thus, the excess catalyst had an adverse became gradually with slow.theThere wasofacatalyst turning pointThis for could the selectivity tothe benzyldehyde with the benzyldehyde increasing dosage. be related to increased diffusion effect on the selective oxidation of toluene. Through the above experiments, the best toluene increasing of catalyst dosage. This could amount be related to the increased diffusion limitation originated limitation originated from the increased of catalyst. Thus, the excess catalyst had an adverse effect on the selective oxidation of toluene. Through the above experiments, the best toluene from the increased amount of catalyst. Thus, the excess catalyst had an adverse effect on the selective

oxidation of toluene. Through the above experiments, the best toluene conversion of 8.7% with 97.5% of the selectivity to benzyldehyde was achieved when the reaction was run for 8 h at 150 ˝ C with O2 presure of 1 MPa.

Catalysts 2016, 6, 14 Catalysts 2016, 6, 14

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Benzyldehyde Sel.(mol Toluene Benzyldehyde Sel.(mol %) %) Toluene Con.Con. (mol(mol %) %)

conversion of 8.7% with 97.5% of the selectivity to benzyldehyde was achieved when the reaction conversion 97.5% the selectivity Catalysts 8 of 11 was run2016, for 6, 8of14 h8.7% at 150with °C with O2of presure of 1 MPa.to benzyldehyde was achieved when the reaction was run for 8 h at 150 °C with O2 presure of 1 MPa. 100 80 B 100 60 80 B 40 60 20 40 0 20 0

(a) (a) (b) (c) (b) (c)

12 10 12 A 8 10 A 68 46 24 02 0 0.03 0.06 0.03 0.06

0.09 0.12 0.15 0.18 0.21 amount0.15 of catalyst 0.09The 0.12 0.18(g) 0.21

0.24 0.24

0.27 0.27

The amount of catalyst (g) Figure 8. Effect of catalyst dosage on the catalytic performance of catalyst (a) benzyldehyde; Figure 8.8.Effect of of catalyst dosage on the performance of catalyst (a) benzyldehyde; (b) benzyl Figure Effect catalyst dosage on catalytic the catalytic performance of catalyst (a) benzyldehyde; (b) benzyl alcohol; (c) benzoic acid. (A) Toluene conversion (B) Benzyldehyde selectivity. Reaction alcohol; (c) benzoic acid. (A) Toluene conversion (B) Benzyldehyde selectivity. Reaction conditions: (b) benzyl toluene alcohol; benzoic acid. (A) Toluene (B)150 Benzyldehyde conditions: 0.1(c) mol, Oxygen pressure1 MPa,conversion temperature °C, time 8 h.selectivity. Reaction toluene 0.1 mol, Oxygen pressure1 MPa, temperature 150 ˝ C, time 150 8 h.°C, time 8 h. conditions: toluene 0.1 mol, Oxygen pressure1 MPa, temperature

2.6. Reaction Mechanism 2.6. 2.6. Reaction Reaction Mechanism Mechanism The mechanistic probe for the selective oxidation of toluene with O2 was attracting continuous The probe oxidation of O continuous The mechanistic mechanistic probe for for the the selective selective oxidation oftoluene toluenewith withmechanism. O22 was was attracting attracting continuous interests, and the prevailing accepted one was the free-radical In the present interests, and the prevailing accepted one was the free-radical mechanism. In the present investigation, interests, andin the prevailing onemodified was theLDH, free-radical mechanism. In the present investigation, the presence of accepted metal cation the inhibition experiments of radicals in thedesigned presenceinby ofthe metal cation modified LDH,modified theradical inhibition experiments of radicals wereofdesigned investigation, of metal cation LDH, the inhibition experiments radicals were thepresence use of three conventional scavengers (pyrocatechol, resorcinol and by the use of three conventional radical scavengers (pyrocatechol, resorcinol and hydroquinone). were designed by the use of three conventional radical scavengers (pyrocatechol, resorcinol and hydroquinone). A significant lowering of the toluene conversion was observed (see Table 1), further A significant of theoftoluene conversion was conversion observed Table 1), further confirming the hydroquinone). A significant lowering of the toluene was observed (see Table further confirming thelowering involvement free-radical mechanism. Based on (see the above experiments and1), previous involvement ofinvolvement free-radical Based on the above and previous research on confirming the of[1,4], free-radical mechanism. Based experiments on thewas above experiments and previous research on toluene oxidationmechanism. a proposed reaction mechanism shown in Scheme 1. Namely, toluene oxidation [1,4], a proposed reaction mechanism was shown in Scheme 1. Namely, toluene was firstly research on toluene oxidation [1,4], a proposed reaction mechanism was shown in Scheme 1. Namely, toluene was firstly converted into benzyl radical over catalyst. Then, an electron was transferred from converted into benzyl radical over catalyst. Then, an electron was transferred fromwas benzyl radical the tolueneradical was firstly converted into benzyl radical over catalyst. Then, an electron transferred benzyl to the catalyst, which led to the benzyl cation, and, simultaneously, the catalysttofrom was catalyst, which led to the benzyl cation, and, simultaneously, the catalyst was regenerated to the benzyl radical to the catalyst, which led to the benzyl cation, and, simultaneously, the catalyst was regenerated to the original form. The benzyl cation was further transformed to benzyl alcohol. The original form. The benzyl cation was further transformed to benzyl alcohol. Theof formation of benzyldehyde regenerated to the original form. The benzyl cation further transformed to benzyl benzyl alcohol.with The formation of benzyldehyde was originated from the was further combination the radical was originated the further combination ofpathway thethe benzyl with O2 . According the proposed formation of benzyldehyde was originatedthe from further combination of the benzyl radical with O 2. According tofrom the proposed mechanism, ofradical forming benzaldehyde andto benzyl alcohol mechanism, the pathway of forming benzaldehyde and benzyl alcohol was independent. This could O 2 . According to the proposed mechanism, the pathway of forming benzaldehyde and benzyl alcohol was independent. This could afford the increasing selectivity to both benzaldehyde and benzyl afford the increasing selectivity to boththe benzaldehyde benzyl to alcohol 6 h (see Figure 6). was independent. This could6).afford increasing and selectivity both before benzaldehyde and benzyl alcohol before 6 h (see Figure alcohol before 6 h (see Figure 6). Mz+/LDH

M(z-1)+/LDH + H+

z+

M(z-1)+/LDH + H+

M /LDH

CH3

CH2

O2

O2 CH2 (z-1)+ + M /LDH + H

CH3 M(z-1)+/LDH + H+

M(z-1)+/LDH + H+

CH2 + OHCH2 + OH-

CH2OH CH2OH

Mz+/LDH Mz+/LDH

M(z-1)+/LDH + H+ O2 O2 CH3

Mz+/LDH

CH3

Mz+/LDH CH2O2 CH2O2

CH2 CH2 CH2OOH CH2OOH

CHO CHO

+ H 2O

Scheme Scheme 1. 1. Possible Possible reaction reaction mechanism mechanism of of the the selective selective oxidation oxidation of of toluene. toluene. Scheme 1. Possible reaction mechanism of the selective oxidation of toluene.

+ H 2O


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3. Experimental Section 3.1. Catalyst Preparation Hydrotalcites containing CO3 2´ (Mg3 Al-LDH) was firstly prepared by a co-precipitation method and hydrothermal treatment as described in our previous report [21]. Then, the obtained Mg3 Al-HT was modified with a series of metal cations. Typically, 1.0 g Mg3 Al-HT was added to a 100 mL aqueous solution contained 0.0005 mol of metal chloride MClx ¨ nH2 O (M = Ti3+ , Cr3+ , Mn2+ , Fe2+ , Fe3+ , Co2+ , Ni2+ , Cu2+ , Zn2+ , La3+ , Nd3+ , Sm3+ , Gd3+ , Dy3+ , Ho3+ , Yb3+ , Au+ , Au+ /Pd2+ ) with vigorous stirring at 25 ˝ C for 1 h. The resulting slurry was filtered, and the residue was washed with deionized water until natural. The solid sample was further dried at 100 ˝ C in air for approximately 12 h. In this way, a series of metal cation modified hydrotalcites was prepared, and was denoted as Mz+ /Mg3 Al-LDH. Furthermore, the resulting hydrotalcites were calcined in an oven at 450 ˝ C for 8 h under a flowing stream of pure nitrogen. Then, the a series of metal modified Mg-Al mixed oxides (M/Mgx (Al)O) were also obtained. 3.2. Characterization Powder X-ray diffraction experiments of samples were carried out on a Rigaku Miniflex diffractometer (Rigak, Tokyo, Japan), equipped with an automatic slit and with Cu Kα radiation at 50 kV and 30 mA, the data was collected at a scanning speed of 2˝ per minute. The specific surface area and average pore diameter were measured by N2 adsorption-desorption method using a Micromeritics Tristar 3000 (Micromeritics, Norcross, GA, USA). The samples were outgassed at 80 ˝ C and 10´4 Pa overnight and then the adsorption-desorption isotherms were conducted by passing nitrogen into the samples, which were kept under liquid nitrogen temperature, specific surface area was calculated by BET. TGA was carried out on a Setaram TGA-92 thermal analyzer (Setaram, Lyon, France) connected to a PC via a TAC7/DX thermal controller (Setaram, Kansas, MO, USA). The samples were heated from 30 ˝ C to 800 ˝ C at 10 ˝ C/min under nitrogen. 3.3. Catalytic Test The oxidation of toluene was carried out in a 100 mL Teflon-lined and magnetically stirred stainless steel autoclave. In a typical experiment, a certain amount of catalyst and 0.1 mol toluene was stirred for about 10 min, and then O2 was introduced. The autoclave was heated to the required temperature within 5 min. After the reaction run for the desired time, the products were filtered out of the catalyst and then were analyzed using a gas chromatograph with a capillary 30 m HP-5 column and an FID detector (Agilent, Santa Clara, CA, USA). 4. Conclusions The metal cation modified LDHs and mixed oxides, especially the modified LDHs, were promising catalysts for the selective oxidation of toluene to benzyldehyde with O2 . Owing to the introduction of metal cations with excessive positive charge and larger ionic radius, more surface –OH on the LDH laminates were decomposed to OH´ , which led to more B basic sites and could afford their high catalytic performance. The highest toluene conversion reached 8.7% with 97.5% of the selectivity to benzyldehyde, when the reaction was run for 8 h at 150 ˝ C with O2 pressure of 1 MPa. Moreover, the catalysts could be reused nine times. We believe this work will be helpful for the design of high effective heterogeneous catalysts for toluene oxidation. Acknowledgments: The authors acknowledge the financial support from the National Natural Science Foundation of China (21003073, 21203093), the Natural Science Foundation of Jiangsu Province (BK20141388), the Qing Lan Project of Jiangsu Province, the Academic Talents Training Project of Nanjing Institute of Technology, and the College students practice innovation training program of Jiangsu Province. Author Contributions: W.G.D. conceived and designed the experiments; W.X.L. analyzed the data and wrote the paper; L.H. and L.Q.B. performed the experiments.


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Conflicts of Interest: The authors declare no conflict of interest.

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