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Decarbonylation of Lactic Acid to Acetaldehyde over Aluminum Sulfate Catalyst Zhanjie Zhai,† Xinli Li,† Congming Tang,*,† Jiansheng Peng,† Ning Jiang,‡ Wei Bai,‡ Hejun Gao,† and Yunwen Liao† †

Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, China West Normal University, Nanchong, Sichuan 637002, PR China ‡ Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu, Sichuan 610041, PR China S Supporting Information *

ABSTRACT: Decarbonylation of lactic acid to acetaldehyde over several solid catalysts was investigated. Among the tested catalysts, aluminum sulfate has an excellent activity. In order to further understand the main reason which influenced the catalytic activity, NH3-TPD was used to estimate the acidity of the catalyst. According to the total acid amount, aluminum sulfate has a moderate amount. Heteropolyacids have strong acidity which caused serious carbon deposition on the surface of catalysts, resulting in a rapid deactivation of catalysts. Besides, FT-IR, XRD, and SEM were also utilized to characterize the fresh catalysts and the used. As for the aluminum sulfate catalyst, an evident adsorption band occurs in 2970 cm−1, suggesting a formation of poly lactate on the surface of the catalyst, and led to deactivation of the catalyst. Other parameters such as reaction temperature, lactic acid concentration, and LHSV (liquid hourly space velocity) were also discussed. Inspiringly, at high LHSV, lactic acid was efficiently converted to acetaldehyde via a decarbonylation reaction. As for stability and the recovery of aluminum sulfate, deactivation of the catalyst belongs to temporary deactivation caused by poly lactate covering the active sites of the catalyst, and only at simple calcination under the air atmosphere, the catalyst may be compeletely regenerated. Under the optimal reaction conditions, conversion of lactic acid achieved 100%, and the selectivity of acetaldehyde achieved 92.1% at 380 °C over the aluminum sulfate catalyst. polylactic acid.37 Among biomass resources, the technologies on lactic acid synthesis have made much progress, and wider ranges of biomass materials such as cellulose,8 sugars,38 and sorbitol39 have also been used to produce LA. Although much work on dehydration of LA to acrylic acid has been reported, rare research on decarbonylation or decarboxylation of lactic acid to acetaldehyde has been reported and a chemical reaction equation is given in Scheme 1. Acetaldehyde was always reported as a byproduct in previous literature.40,41 Recent years, silica supported heteropolyacids have been used to catalyze the decarbonylation of LA to acetaldehyde, achieving 91% conversion of LA as well as 81− 83% yield of acetaldehyde.27 The formation of acetaldehyde from LA catalyzed by acid has reached a consensus. However, the influence of the acid strength and acid amount of the catalyst on the reaction performance is not well-known. In the present work, we described the metal sulfates and heteropolyacids catalyzed conversion of LA to acetaldehyde on the gas solid catalytic reaction at high temperature. Effects of types of solid acid, reaction temperature, LA concentration, liquid hourly space velocity of lactic acid, and catalyst stability on decarbonylation of LA were also discussed. Besides, FT-IR, XRD, and NH3-TPD were used to characterize catalysts in

1. INTRODUCTION Acetaldehyde is widely used in the synthesis of α,β-unsaturated aldehydes, 3,4-dihydropyrimidin-2(1H)-ones, acetic acid, acetic anhydride, ethyl acetate, pentaerythritol, crotolaldehyde, chloral hydrate, etc.1,2 An important industrial route is that acetaldehyde is produced from ethene via the Wacker process using PdCl2−CuCl2. Besides, acetaldehyde is also obtained through the catalytic hydration reaction of acetylene over the mercury salts. Owing to facing an increasing shortage of fossil resources, these routes will be substituted by others in the near future. In recent years, bulk chemicals have been obtained from a sustainable biomass route,3−8 and for instance, acrolein from glycerol,9−12 acrylic acid from glycerol13 or lactic acid (LA),14−18 and n-butanol from ethanol.19−25 To some degree, these investigations have further indicated that it is a promising way for production of bulk chemicals depending on biomass resources. Acetaldehyde is also obtained from a biomass route. A case in point is that acetaldehyde is produced through catalytic dehydrogenation of ethanol or partial oxidation of ethanol.26 Another case is that acetaldehyde is also obtained from decarbonylation of LA accompanied by a byproduct of carbon monoxide.27 However, carbon monoxide is easily separated from ejection of compact and can be utilized to synthesize carbonyl compounds such as acetic acid, acrylic acid, and other α,β-unsaturated acids via a carbonylation reaction of corresponding substrates.28−31 It is well-known that biolactic acid is a crucial platform molecule which can be converted to many chemicals such as acrylic acid,5,16,32 acetaldehyde,27 2,3pentanedione,33,34 propionic acid,35 pyruvic acid,36 and © 2014 American Chemical Society

Received: Revised: Accepted: Published: 10318

March 7, 2014 June 2, 2014 June 4, 2014 June 13, 2014 dx.doi.org/10.1021/ie500988q | Ind. Eng. Chem. Res. 2014, 53, 10318−10327

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Scheme 1. Decarbonylation or Decarboxylation of Lactic Acid to Acetaldehyde

catalyst (20−40 meshes, 0.377 mL) was placed in the middle of the reactor, and quartz wool was placed in both ends. Before catalytic evaluation the catalyst was pretreated at the required reaction temperature (e.g., 380 °C) for 1.0 h under high purity N2 (0.1 MPa, 1.0 mL/min). The feedstock (20 wt % solution of LA) was then pumped into the reactor (lactic acid aqueous solution flow rate, 1.0 mL/h) and driven through the catalyst bed by nitrogen. The contact time of reactant over the catalyst is about 0.5 s, and the contact time is calculated by references (see eq 1 in the Supporting Information).15,42 The liquid products were condensed using an ice−water bath and analyzed off-line using a SP-6890 gas chromatograph with a FFAP capillary column connected to a FID. Quantitative analysis of the products was carried out by the internal standard method using n-butanol as the internal standard material. GC-MS analyses of the samples were performed using Agilent 5973N Mass Selective Detector attachment. The reaction tail gas was analyzed using GC with a packed column of TDX-01 connected to TCD detector. The conversion of LA and the selectivity toward acetaldehyde or other byproducts were calculated as follows

order to understand the relationship between structure and activity.

2. EXPERIMENTAL SECTION 2.1. Materials. Lactic acid (analytic grade) was purchased from Chengdu Kelong Chemical Reagent Co. and was used for the conversion of lactic acid to acetaldehyde via decarbonylation or decarboxylation of lactic acid. Triple-distilled water was prepared in the laboratory and was used to dilute lactic acid for required concentration. Aluminum sulfate (Al2(SO4)3· 18H2O), anhydrous ferric sulfate, nickel sulfate (NiSO4· 6H2O), barium sulfate, silicontungstic acid, phosphomolybdic acid, acetaldehyde, acrylic acid, propionic acid, acetic acid, 2,3pentanedione, and n-butanol together with hydroquinone were obtained from Sinopharm Chemical Reagent Co., Ltd. Acrylic acid, propionic acid, acetic acid, 2,3-pentanedione, and acetaldehyde were used for gas chromatograph reference materials, and n-butanol was adopted as internal standard material. Hydroquinone (0.3 wt %) was used as a polymerization inhibitor. 2.2. Catalyst Characterization. Powder X-ray diffraction measurement was conducted on a Dmax/Ultima IV diffractometer operated at 40 kV and 20 mA with Cu-Ka radiation. The FTIR spectra of the catalysts were recorded in the range of 500−4000 cm−1 on a Nicolet 6700 spectrometer. The particle size and the morphology of the catalysts were examined using a scanning electron microscope (SEM, JSM-6510). Surface acidity of the catalyst was tested by NH3 temperatureprogrammed desorption (NH3-TPD). 2.3. Catalyst Evaluation. The decarbonylation of LA to acetaldehyde over the catalysts was carried out in a fixed-bed quartz reactor with a 4 mm inner diameter operated at atmospheric pressure, and the experimental setup is shown in Figure 1. Prior to activity evaluation, all catalysts were calcinated at 500 °C for 6 h except for heteropolyacids. The

n0 − n1 × 100, n0 np Selectivity/% = × 100 n0 − n1

Conversion/% =

where n0 is the molar quantity of lactic acid fed into reactor, n1 is the molar quantity of lactic acid in the effluent, and np is the molar quantity of lactic acid converted to acealdehyde or other byproducts such as propionic acid, acrylic acid, acetic acid, and 2,3-pentanedione.

3. RESULTS AND DISCUSSION 3.1. Characterization. 3.1.1. FT-IR and XRD. FT-IR and XRD were used to characterize the structure of the catalysts and further understand the reaction mechanism, and the results are given in Figure 2 and Figure 3, respectively. As for aluminum sulfate, we compared the FT-IR patterns of the fresh catalyst with the used catalyst. From Figure 2, 2a, a weak adsorption of 2970 cm−1 occurs in the used catalyst, indicating that a little part of lactic acid was polymerized to poly lactate,15 while the XRD patterns of aluminum sulfate catalysts are brought into correspondence with each other, as shown in Figure 3, 3a. This suggested that poly lactate highly dispersed on the surface of aluminum sulfate catalysts. At the same time, this also indicated that deactivation of the catalyst was partly attributed to covering the active sites by poly lactate. As for heteropolyacids, the FT-IR patterns and XRD patterns are shown in Figure 2, 2b and Figure 3, 3b, respectively. Compared to FT-IR patterns of the used aluminum sulfate catalyst, the adsorption band of 2970 cm−1 is not evident in used phosphomolybdic acid. This result shows that deactivation of the catalyst was attributed to carbon deposition caused by strong acidic sites and not by poly lactate. Besides, from XRD patterns of heteropolyacids, evident differences in diffraction peaks existed between the fresh catalyst and the used catalyst,

Figure 1. Experimental setup for evaluation of the decarbonylation of lactic acid to acetaldehyde. 10319

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Figure 2. IR spectra of catalysts (2a: B1, fresh aluminum sulfate, B2, used aluminum sulfate without calcination, B3, aluminum sulfate run three cycles without calcination; 2b: B11, fresh silicontungstic acid; B12, used silicontungstic acid; B21, phosphomolybdic acid; B22, used phosphomolybdic acid; 2c: B1, fresh ferric sulfate; B2, used ferric sulfate).

Figure 3. XRD of catalysts (3a: B1, fresh aluminum sulfate, B2, used aluminum sulfate without calcination, B3, aluminum sulfate run three cycles without calcination; 3b: B11, fresh silicontungstic acid; B12, used silicontungstic acid; B21, phosphomolybdic acid; B22, used phosphomolybdic acid; 3c: B1, fresh ferric sulfate; B2, used ferric sulfate).

showing that the structure of heteropolyacids was encountered to collapse by high temperature of 380 °C. It is clearly seen that diffraction peaks in XRD patterns of used heteropolyacids became rougher. This further demonstrated that serious carbon deposition occurred on the surface of heteropolyacid catalysts. Similarly, compared FT-IR spectra of fresh ferric sulfate with that of the used (shown in Figure 2, 2c) adsorption band of 2970 cm−1 does not also occur, indicating that deactivation of the catalyst was not caused by poly lactate. 3.1.2. NH3-TPD and BET. The acidity data of solid catalysts estimated by NH3-TPD are given in Table 1. Also, the patterns of NH3-TPD are shown in Figure 4. The acidity of catalysts was

excellently characterized by the method of NH3-TPD, except for nickel sulfate. As well-known to all, nickel salt has a strong coordination with NH3 to form nickel hexammonate salt. According to Figure 4, nickel ammine completely decomposed as the temperature was above 450 °C. From the data of acidity given in Table 1, acidity order is BaSO4 < Al2(SO4)3 < Fe2(SO4)3 < H4[SiW12O40] < H3[PMo12O40]. 10320

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Table 1. NH3-TPD Results of Catalysts acid amount/μmol/g catalyst

weak (100−200 °C)

medium (200−400 °C)

strong (400−600 °C)

total acid amount /μmol/g

BaSO4 NiSO4 Al2(SO4)3 Fe2(SO4)3 H4[SiW12O40] H3[PMo12O40]

9.7 2600.7 31.8 69.6 306.1 472.5

11.8 1719.1 10.9 289.1 114.7 1901.0

3.6 1858.8 45.0 157.0 1461.2 1555.5

25.1 6178.6 87.7 515.7 1882.0 3929.0

these catalysts, the difference of activity does not lie in the catalyst surface but other properties such as acidity. 3.1.3. SEM. Figure 5 shows the SEM images of aluminum sulfate catalysts. Compared to the SEM images between the fresh aluminum sulfate and the used, the appearances are brought into correspondence with each other, indicating that the aluminum sulfate catalyst has an excellent stability in the process of the catalytic reaction. 3.2. Decarbonylation of Lactic Acid into Acetaldehyde over Metal Sulfates and Heteropoly Acids. It is known that decarbonylation of lactic acid is catalyzed by acid.16,27 Compared to liquid acids, solid acids have some excellent properties such as conformity at high temperature reaction, easy separation, and low erosion. Therefore, several solid acids were used to catalyze the decarbonylation of lactic acid, and the results are shown in Table 3. The decarbonylation reaction of LA was carried on the fixed bed reactor at 380 °C under an atmosphere pressure. The conversion of LA was hardly influenced with a change of solid acid types except for silicontungstic acid and phosphomolybdic acid. It is mentioned that much coke and carbon covered the surface of silicontungstic acid and phosphomolybdic acid, resulting in a decrease of active sites. However, product selectivity was drastically influenced by types of solid acids. Acetaldehyde selectivity rises from 18.7% in BaSO 4 to 87.5% in H4[SiW12O40]. According to the results of NH3-TPD for these catalysts (seen in Table 1), the increased order based on the total acid amount is BaSO4 < Al2(SO4)3 < Fe2(SO4)3 < H4[SiW12O40] < H3[PMo12O40]. It is noted that the acidity of NiSO4 is weaker than that of Al2(SO4)3 in previous reports.43,44 As is well-known to all, Ni2+ favored to combine the ammonia, and furthermore, the specific area of NiSO4 is much larger than that of Al2(SO4)3 (seen in Table 2). Thus, a greater amount of desorbed ammonia for NiSO4 is achieved. When the total acid amount of the catalysts rose in the moderate acid amount range, especially for BaSO4 and Al2(SO4)3, decarbonylation selectivity toward acetaldehyde also increased. However, too much acid amount is not favored which has been demonstrated by heteropolyacids such as silicontungstic acid and phosphomolybdic acid and favored to deep cleavage of the C−C bond resulting in depositing carbon and formation of coke. This phenomena is evident in our experiment carried out over the heteropolyacids. In a previous report, 91% conversion of LA and 81% acetaldehyde yield were obtained over the CARiACT Q-15 supported silicontungstic acid.27 Due to a minor amount of silicontungstic acid on the support, the acid density of the catalyst is relatively lower, which is the main reason for obtaining the excellent result. Besides, as for phosphomolybdic acid and ferric sulfate, a great deal of propionic acid is formed through hydrogenation of LA or acrylic acid with hydrogen. Interestingly, at the same time, a great deal of acetic acid is also achieved. It is concluded that phosphomolybdic acid as well as

Figure 4. NH3-TPD patterns of catalysts (4a: B1, BaSO4; B2, NiSO4; B3, Al2(SO4)3; B4, Fe2(SO4)3; B5, H4[SiW12O40]; B6, H3[PMo12O40]; 4b: enlarged NH3-TPD of B3, Al2(SO4)3).

BET data of catalysts are shown in Table 2. It is clearly seen that the tested metal sulfates have low specific surface areas. For Table 2. BET Data of Catalysts catalyst

BET surface area (m2/g)

Al2(SO4)3 NiSO4 BaSO4 Fe2(SO4)3 H4[SiW12O40] H3[PMo12O40]

1.7 15.4 3.3 16.6 2.9 18.0 10321

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Figure 5. SEM images of Al2(SO4)3 catalysts (5a: fresh catalyst; 5b: used catalyst).

temperature is a crucial factor.36,46 The reaction temperature for decarbonylation is related to the catalyst. As for the strong acidity catalyst, the reaction temperature is lower. A case in point is that the CARiACT Q-15 supported silicontungstic acid catalyzed the decarbonylation of LA at only 275 °C.27 However, as the catalyst reduces acidity, it has to enhance the reaction temperature for decarbonylation reaction. Compared to the CARiACT Q-15 supported silicontungstic acid, the aluminum sulfate catalyst has weaker acidity, leading to a relatively high reaction temperature. From the results given in Table 4, the influence of temperature on reaction

Table 3. Catalytic Activity of Metal Sulfates and Heteropolyacids for Decarbonylation of Lactic Acid to Acetaldehydea sel [%]b catalyst

LA conv [%]

AD

PA

ACA

AA

PD

BaSO4 NiSO4 Al2(SO4)3 Fe2(SO4)3 H4[SiW12O40]c H3[PMo12O40]c

99.7 99.8 100 100 81.0 90.0

18.7 54.1 86.7 35.7 87.5 69.1

6.1 3.6 2.5 18.5 4.1 22.1

1.3 1.6 1.3 2.0 1.9 3.8

72.1 13.7 3.2 6.2 1.3 2.1

0.6 0.7 0.9 1.0 0.3 1.1

Conditions: calcination temperature 500 °C, reaction temperature 380 °C, catalyst: BaSO4, 0.69g, NiSO4, 0.47 g, Al2(SO4)3, 0.39g, Fe2(SO4)3, 0.40 g, H4[SiW12O40], 1.03 g, H3[PMo12O40], 0.62 g, particle size: 20−40 meshes, carrier gas N2: 1 mL/min, feed flow rate: 1 mL/h, LA feedstock: 20 wt % in water. bLA: lactic acid, AD: acetaldehyde, PA: propionic acid, ACA: acetic acid, AA: acrylic acid, PD: 2,3-pentanedione, cNo calcination.

Table 4. Effect of Reaction Temperaturea

ferric sulfate has a redox character, especially for the former.35,45 With further analysis of hydrogen sources, no external hydrogen was provided and hydrogenation reaction to form propionic acid occurred. Here, the hydrogen utilized in the hydrogenation reaction in situ occurred by decarboxylation of LA. In addition, we utilized gas chromatography connected to a packed column to analyze the tail gas. The tail gas from phosphomolybdic acid and ferric sulfate catalysts contains more CO2 than CO, while the result obtained from the aluminum sulfate catalyst is converse. Thus, we may conclude that as for phosphomolybdic acid and ferric sulfate catalysts, acetaldehyde is formed by the path of decarboxylation, and as for the aluminum sulfate catalyst, actaldehyde is formed by the path of decarbonylation of LA. Besides, reaction temperature also may be a key factor for product selectivities. The effect of other reaction temperatures on the reactivity was also investigated to choose a better catalyst for decarbonylation/decarboxylation of lactic acid to acetaldehyde, and the results were given in Table S2. At a reaction temperature of 300 °C, ferric sulfate displayed an excellent activity. However, the stability on ferric sulfate is very poor (seen in Figure S1). Considering the conversion of LA as well as the selectivity toward acetaldehyde and catalytic stability, the aluminum sulfate catalyst is a potential candidate due to moderate acid density and acid strength for decarbonylation of LA. 3.3. Effect of Reaction Temperature. According to the choice of solid acid catalysts, the aluminum sulfate catalyst is a potential candidate. As for high temperature gas−solid reaction,

a

a

sel [%]b reaction temp [°C]

LA conv [%]

300 350 380 420

77.5 95.0 100 100

AD 93.1 89.2 86.7 72.4

AA

PA

PD

ACA

2.3 3.0 3.2 3.5

0.8 2.1 2.5 15.6

1.0 1.1 0.9 1.0

1.3 1.4 1.3 5.4

Conditions: Al2(SO4)3 catalyst 0.39−0.40 g, calcination temperature 500 °C, particle size: 20−40 meshes, carrier gas N2: 1 mL/min, feed flow rate: 1 mL/h, LA feedstock: 20 wt % in water. bLA: lactic acid, AD: acetaldehyde, PA: propionic acid, ACA: acetic acid, AA: acrylic acid, PD: 2,3-pentanedione.

performance is evident. When the reaction temperature increases from 300 to 380 °C, the conversion of LA also increases from 77.5% to 100%, whereas conversion of LA hardly changed with a further increase of temperature. Product distributions are also drastically influenced by the reaction temperature. As for acetaldehyde, the selectivity decreases with an increase of the reaction temperature, whereas as for propionic acid and acetic acid the selectivities congruously increase with an increase of the reaction temperature. Especially at 420 °C, both propionic acid and acetic acid are formed drastically. A rapid increase of the former amount indicated that hydrogenation of LA or acrylic acid occurred. This suggested that a part of acetaldehyde is formed through decarboxylation of LA accompanied by formation of hydrogen at high reaction temperature. An exorbitant temperature which is utilized in the reaction probably shortens the lifetime of the aluminum sulfate catalyst.11 Thus, an appropriate temperature for the aluminum sulfate catalyst should be about 380 °C. 3.4. Effect of Lactic Acid Concentration. The effect of LA concentration on the reaction performance was also investigated over the aluminum sulfate at 380 °C, and the results are given in Table 5. Notably, the conversion of LA was 10322

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Table 5. Effect of Lactic Acid Concentrationa

selectivity decreases only a little with a further increase of LA LHSV. At elevated LA LHSV, the contact time of LA with the catalyst shortened,15,42 resulting in higher selectivity of acetaldehyde, whereas byproduct selectivities such as propionic acid and 2,3-pentadione decreased, indicating that compared to other side reactions, decarbonylation of LA to acetaldehyde is a faster reaction. Encouragingly, at high LHSV (⩽37.1 h−1), LA was almost converted as acetaldehyde selectivity retained a high value. The time space yield (YTS) of acetaldehyde was calculated according to the following equation15

sel [%]b LA concn [wt %]

LA conv [%]

AD

AA

PA

PD

ACA

10 15 20 30 40

100 100 100 100 100

78.5 82.7 86.7 88.6 87.7

4.5 5.0 3.2 4.1 3.7

8.7 6.1 2.5 1.8 1.0

1.2 1.0 0.9 1.4 1.3

3.1 2.7 1.3 1.5 1.6

a Conditions: Al2(SO4)3 catalyst 0.39−0.40 g, calcination temperature 500 °C, particle size: 20−40 meshes, carrier gas N2: 1 mL/min, feed flow rate: 1 mL/h. bLA: lactic acid, AD: acetaldehyde, PA: propionic acid, ACA: acetic acid, AA: acrylic acid, PD: 2,3-pentanedione.

YTS =

hardly influenced, whereas product selectivity was influenced with a change of LA concentration. As for acetaldehyde, the selectivity drastically increased from 78.5% to 88.6% when the LA concentration increased from 10% to 30%. However, the selectivity of acetaldehyde was hardly enhanced with a further increase of LA concentration, but for byproducts such as acrylic acid, 2,3-pentanedione, propionic acid, and acetic acid, the influence of LA concentration was evidently different. The selectivity of propionic acid decreased with an increase of LA concentration. According to this result, we can infer that the path for decarbonylation of LA to acetaldehyde increased with an increase of LA concentration, and the path for decarboxylation of LA to acetaldehyde decreased. Similarly to that of propionic acid, the selectivity of acetic acid was also decreased with an increase of LA concentration. Besides, the selectivities for acrylic acid and 2,3-pentanedione were slightly influenced. 3.5. Effect of Liquid Hourly Space Velocity (LHSV). It is known that liquid hourly space velocity (LHSV) is a vital factor for evaluation of solid catalysts. The influence of LA LHSV on reaction performance is shown in Table 6. The reactions were

MAD MCat*t

where MAD is the mass of produced acetaldehyde (g); MCat is the mass of the catalyst; and t is the reaction time (h). Plotted in Figure 6 is the YTS of acetaldehyde versus LA liquid hourly

Table 6. Effect of LA LHSVa sel [%]b −1

LA LHSV [h ]

LA conv [%]

AD

AA

PA

PD

ACA

1.3 2.7 4.1 7.3 12.2 20.1 31.2 37.1 42.4 53.5

100 100 100 100 100 100 100 97 89 85

85.7 86.7 91.0 91.6 92.1 88.7 88.7 88.9 88.3 88.9

2.8 3.2 3.0 2.8 2.3 2.3 2.1 2.0 2.3 3.0

4.8 2.5 1.7 1.5 1.2 0.8 0.6 0.7 0.7 0.8

2.2 0.9 1.2 0.6 0.4 0.2 0.2 0.2 0.3 0.1

2.1 1.3 1.4 1.2 1.2 1.1 1.0 0.9 1.0 1.2

Figure 6. Time space yield of acetaldehyde at different LA LHSV.

space velocity. From Figure 6, YTS of acetaldehyde rapidly increased with an increase of LA LHSV, but, as the LA LHSV is above 37.1 h−1, LA cannot be fully converted. A case in point is that the residue of LA accounted for 15% at LA LHSV of 53.5 h−1. According to the results that we obtained, the LHSV for decarbonylation or decarboxylation of LA to acetaldehyde is far higher than that of dehydration of LA to acrylic acid reported in previous reports.5,7,15,18,42,46−49 For example, as for NaY zeolite modified with Na2HPO4 for dehydration of LA to acrylic acid at 340 °C, the residue of LA increased from 16% to 26.2% with an increase of LA LHSV from 1.8 h−1 to 3.2 h−1.15 This suggested that the formation of acetaldehyde from LA is faster than that of acrylic acid. This is also the main reason why we always obtained the acetaldehyde as a main byproduct in a dehydration reaction of LA to acrylic acid. 3.6. Catalyst Stability and Recovery. The stability of the catalyst with time on stream was studied at 380 °C over aluminum sulfate (LHSV = 2.7 h−1 and LA concentration = 20 wt %). Figure 7 shows the stability of the catalyst with time on stream for three catalytic cycles. For every catalytic cycle, the catalyst started to regenerate at 500 °C for 6 h under the air

a

Conditions: Al2(SO4)3 catalyst 0.39−0.40 g, calcination temperature 500 °C, particle size: 20−40 meshes, carrier gas N2: 1 mL/min, LA feedstock: 20 wt % in water. bLA: lactic acid, AD: acetaldehyde, PA: propionic acid, ACA: acetic acid, AA: acrylic acid, PD: 2,3pentanedione.

conducted at 380 °C with a flow rate of LA changed from 0.5 to 20 mL/h (corresponding LHSV = 1.3−53.5 h−1). Other operation conditions such as the flow rate of carrier gas N2 (1 mL/min) and LA concentration (20 wt %) were retained. It is clearly seen from the results given in Table 6 that acetaldehyde selectivity slightly increases from 85.7% to 92.1% as LA LHSV increases from 1.3 to 12.2 h−1. However, acetaldehyde 10323

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Figure 7. Stability of the catalyst with time on stream. Conditions: Al2(SO4)3 catalyst 0.39−0.40 g, calcination temperature 500 °C, particle size: 20− 40 meshes, carrier gas N2: 1 mL/min, LA feedstock: 20 wt % in water, feed flow rate: 1 mL/h.

Scheme 2. Pathway of LA Converted to Chemicals

catalyst can be excellently regenerated, indicating that the carbon formed from the decomposition of LA and other oganic compounds or LA polymers covered the active sites of the catalyst, resulting in temporary deactivation of the catalyst. 3.6. Reaction Mechanism. Based on the results presented in the previous sections, LA as a platform molecule can be converted to many chemicals such as acetaldehyde, propionic acid, acetic acid, acrylic acid, 2,3-pentandione, polylactic acid, and ethanol, and the reaction scheme can be proposed in Scheme 2. The reaction mechanism on dehydration of LA to acrylic acid over several catalysts was described in the previous reports,16,17,49,50 whereas the reaction mechanism of decarbonylation or/and decarboxylation of LA to acetaldehyde is rare.

atmosphere when the conversion of LA decreased to 70%. For the fresh aluminum sulfate catalyst (Cycle 1 in Figure 7), the conversion of LA is roughly 100% within initial 10 h. The conversion of LA begins to decrease quickly with a further increase of time on stream. When the time on stream is about 54 h, the conversion of LA is near 70%. Similar to that of LA conversion, the selectivity of acetaldehyde also decreased with an increase of reaction time. Such a catalyst deactivation is thought to be a result of LA polymerization or carbon deposition on the catalyst. To verify this point, the deactivated catalyst was treated in an air atmosphere at 500 °C for 6 h, and the treated catalyst was tested in a second run. The results given in Cycle 2 of Figure 7 suggest complete recovery of LA conversion. By the means of the calcination method, the 10324

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Scheme 3. Proposed Mechanism of LA Converted to Acetaldehyde over the Aluminum Sulfate Catalyst

More recently, Ghantani et al.14 proposed the decarbonylation reaction mechanism of LA to form acetaldehyde over the calcium hydroxyapatite. Similarly, based on the results achieved over the aluminum sulfate catalyst, we proposed the possible reaction mechanism on formation of acetaldehyde from LA as followed in Scheme 3. In the first step, the aluminum sulfate catalyst reacted with a water molecule to form active species (C) via an intermediate (B). Next, dissociative adsorption of LA takes place on active species (C) where the esterification between HSO4− and the carboxylic −OH group occurs to form the sulfate ester (D). Subsequently, the C−O−Al bond forms via intermolecular dehydration (E). This sulfate ester with heptacyclic structure (E) is unstable and decomposed to give acetaldehyde as well as carbon monoxide and water. Here, hydrogen comes from two sources. On one hand, hydrogen comes from decarboxylation of LA. On the other hand, it comes from the water gas shift reaction. Propionic acid is formed via hydrogenation of LA or acrylic acid with in situ hydrogen from decarboxylation of LA or/and the water gas shift reaction. When the reaction temperature is enhanced, it favered the water gas shift reaction, resulting in formation of more hydrogen. Thus, the selectivity toward propionic acid increased with an increase of reaction temperature (seen in Table 4), whereas acrylic acid is produced by the dehydration of LA.

catalyst has an excellent activity. The stability of aluminum sulfate catalyst is near 50 h. When the catalyst with deactivation is calcinated at 500 °C under air atmosphere, it can be completely regenerated as the initial cycle. Under the optimal reaction conditions, 100% of lactic acid conversion as well as 92.1% of acetaldehyde selectivity was achieved at the reaction temperature of 380 °C over the aluminum sulfate catalyst.

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ASSOCIATED CONTENT

S Supporting Information *

Table S1, Table S2, Figure S1, and eq (1) calculated on contact time (s). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 817 2568081. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Scientific Research Fund of Sichuan Provincial Educational Department with project number of 14ZA0128, Scientific Research Fund of Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, Scientific Research Fund of China West Normal University with project number of 12B019 and the Nanchong Key Technology R&D Program of Sichuan, China with project number of 11A0051.

4. CONCLUSIONS Several metal sulfates and heteropolyacids were used to catalyze the conversion of lactic acid to acetaldehyde. Due to a moderate acidity of aluminum sulfate, it has an excellent catalytic performance for decarbonylation of lactic acid. Heteropolyacids have a rapid deactivation due to carbon deposition covering the active sites caused by strong acidity. At high LHSV of lactic acid (LHSV = 31.2 h−1), lactic acid can be completely converted, indicating that the aluminum sulfate

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