Sustainable Production of 2,3-Pentanedione ... - ACS Publications

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Sustainable Production of 2,3-Pentanedione: Catalytic Performance of Ba2P2O7 Doped with Cs for Vapor-Phase Condensation of Lactic Acid Xinli Li,†,‡ Yu Zhang,†,§ Zhi Chen,‡ Ping Cao,‡ Weixin Zou,∥ Congming Tang,*,‡,§ Lin Dong,∥ and Yu Wang⊥ ‡

School of Chemistry and Chemical Engineering, Chongqing University of Technology, Chongqing 40054, People’s Republic of China § Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, China West Normal University, Nanchong, Sichuan 637002, People’s Republic of China ∥ Jiangsu Key Laboratory of Vehicle Emissions Control, Center of Modern Analysis, Nanjing University, Nanjing 210093, People’s Republic of China ⊥ School of Chemistry and Life Science, Guizhou Education University, Guiyang 550018, People’s Republic of China S Supporting Information *

ABSTRACT: Sustainable production of 2,3-pentanedione from lactic acid over Ba2P2O7 doped with Cs was investigated. Varying Cs components and calcination temperature can efficiently tune the acid−base properties of catalysts. It was found that both base site density and acid site density had a marked impact on catalytic performance for condensation of lactic acid to 2,3-pentanedione. Enhanced base site density and acid site density favor the catalytic activity. Furthermore, base/acid molar ratio has also played an important role on catalytic performance. From the observations based on the effect of Cs doped amount and calcination temperature, an appropriate base/acid molar ratio is around 8. Low reaction temperature as well as low LA LHSV favors selective formation of 2,3-pentanedione from lactic acid over the catalyst of 1.0CsNO3/2.5Ba2P2O7 calcined at 600 °C. Encouragingly, the condensation reaction of lactic acid proceeds efficiently at around 28 h on stream, and 2,3pentanedione selectivity remains around 60%.

present.3,4 In the catalyzed condensation of LA to 2,3pentanedione, MxH3−xPO4 and MNO3 (M = Na+, K+, and Cs+; x = 1, 2, and 3) are commonly used as a precursor for active species, while silicate, carbon, and Si−Al compounds are used as a support in pioneering work.1,4,25−29 Unfortunately, these catalysts displayed a poor activity for catalytic condensation of LA to 2,3-pentanedione, accompanied by formation of acetaldehyde and acrylic acid via LA decarbonylation and LA dehydration, respectively. To improve the performance of the catalyst, two key factors must be clarified, which involve (1) a full understanding of the relationship between the acid−base property of the catalyst and catalytic activity to improve reactivity, including LA conversion, 2,3pentanedione selectivity, and area-specific catalytic rate; and (2) strengthening the interaction between active species and support to improve the stability of the catalyst. In the proposed reaction mechanism for formation of 2,3-pentanedione from LA, two important steps relate to intermediate dehydration and

1. INTRODUCTION Lactic acid (LA) is widely believed as an inexpensive, renewable resource-derived chemical feedstock, which can be converted to a variety of high-valuable chemicals such as 2,3-pentanedione,1−5 acrylic acid,6−12 propionic acid,13−15 acetaldehyde,16−20 and biodegradable polylactic acid polymers.21,22 Among these chemicals derived from LA, the highest valueadded product should belong to 2,3-pentanedione, most of which is used as a flavoring agent and a photoinitiator.23 In a recent report,24 it has also served as a precursor to react with phenol to produce novel bisphenols, which are assessed as plasticizers for polyethylene terephthalate (PET), showing promising properties. In a traditional synthetic method, 2,3pentanedione is prepared via oxidation of methyl propyl ketone in the presence of excess NaNO2 and diluted HCl.2 Obviously, in this route, the main drawbacks lie in (1) separation between product of 2,3-pentanedione and homogeneous catalysts consuming higher energy via traditional distillation and (2) bringing a great quantity of waste acid. For that reason, exploring a green synthetic path for production of 2,3pentanedione has become an important topic. It is believed that catalytic vapor-phase synthesis of 2,3pentanedione from LA is a green and potential route at © XXXX American Chemical Society

Received: August 30, 2017 Revised: November 11, 2017 Accepted: November 16, 2017

A

DOI: 10.1021/acs.iecr.7b03595 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research intermediate decarboxylation.27 As for the former, catalytic dehydration requires weak−medium acidic sites, while for the later, decarboxylation requires basic sites. Thus, both acidic sites and basic sites on the surface of catalyst must be required for catalytic condensation of LA to 2,3-pentanedione. In the present work, we developed a simple and facile method for preparation of Cs-doped Ba2P2O7 materials via reaction of Ba(NO3)2 with (NH4)2HPO4 in aqueous solution to form Ba2P2O7, followed by immersion in CsNO3 aqueous solution and high temperature calcination. The relation between the acid−base property of the catalysts characterized by NH3-TPD and CO2-TPD, and the activity was investigated in detail, and the stability was also reported.

Instrument. The sample (ca. 50−60 mg) is purged with dry Ar (50 mL/min, purity> 99.999 vol.%) at 400 °C for 1.0 h, followed by reducing the furnace temperature to room temperature and switching to a flow of 8 vol.% NH3/Ar or 10 vol.% CO2/Ar for 1 h to execute NH3 or CO2 adsorption. Next, the samples are swept by dry Ar (50 mL/min, purity> 99.999 vol.%) at 80 °C for 1.0 h to remove the dissociative NH3 or CO2 on the surface of samples. Then, NH3 or CO2 chemically adsorbed on the sample is desorbed in the range of 80−800 °C at a rate of 10 °C/min. 2.4. Catalyst Evaluation. The condensation of lactic acid to 2,3-pentanedione over the catalysts was carried out in a fixed-bed quartz tubular reactor with a 4 mm inner diameter operated at atmospheric pressure. The catalyst (ca. 0.20−0.40 g, particle size: 0.38−0.83 mm) was placed in the middle of the reactor, and quartz wool was placed at both ends. First, the catalyst was pretreated at the required reaction temperature (ca. 300 °C) for 1.5 h under N2 with high purity (0.1 MPa, 0.8 mL/ min). The feedstock (20 wt % aqueous solution of LA) was then pumped into the reactor (LA aqueous solution flow rate, 1.6 mL/h) and driven through the catalyst bed by nitrogen. The liquid products were condensed using ice−water bath and analyzed offline using a SP-6890 gas chromatograph (GC) 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. GCMS analyses of the samples were performed using an Agilent 5973N Mass Selective Detector attachment. The reaction tail gas was analyzed online using GC with a packed column of TDX-01 connected to TCD detector. The conversion of LA and the selectivity toward 2,3-pentanedione or other byproducts were calculated according to eqs 1 and 2. n − n1 Conversion (%) = 0 × 100 n0 (1)

2. EXPERIMENTAL SECTION 2.1. Materials. Lactic acid (85−90 wt %) is purchased from Chengdu Kelong Chemical Reagent Co. and is used for the condensation reaction of lactic acid to 2,3-pentanedione without further purification. Acetaldehyde, acrylic acid, propionic acid, acetic acid, 2,3-pentanedione, and n-butanol, together with hydroquinone are purchased from Sinopharm Chemical Reagent Co., Ltd. Acrylic acid, propionic acid, acetic acid, 2,3-pentanedione, and acetaldehyde are used for gas chromatograph reference materials, and n-butanol is utilized as internal standard material. Hydroquinone (0.3 wt %) is used as an inhibitor to prevent polymerization of LA. Barium nitrate (Ba(NO3)2), ammonium hydrogen phosphate ((NH4)2HPO4), and cesium nitrate (CsNO3) are obtained from Energy Chemical Company. 2.2. Preparation of Catalysts. The preparation of carrier support (Ba2P2O7) in this work was similar to that described previously.30 Under the condition of continuous stirring at ambient temperature, 15.68 g of Ba(NO3)2 was added to 300 mL of distilled water and fully dissolved to form a homogeneous solution for 30 min. Next, 6.34 g of (NH4)2HPO4 was slowly added to the resultant solution to form a white precipitate for 2 h. Subsequently, the white precipitate was rinsed at least three times using distilled water and dried at 120 °C in the air circulating oven for 6 h and further calcined at 500 °C for 3 h. The Cs doped Ba2P2O7 samples were prepared by wet immersion of the obtained Ba2P2O7 with an aqueous solution of 0.049−0.147 M CsNO3 (16 mL per gram Ba2P2O7) at ambient temperature for 6 h and, next, slowly evaporated at 80 °C to form white solid powders. Subsequently, these samples were further calcined at 500−700 °C for 3 h before use as a catalyst for condensation reaction of LA. The resultant samples were coded as xCsNO3/2.5Ba2P2O7 according to Ba2+/Cs+ molar ratio (2.5:x). 2.3. 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 morphologic features of the catalysts were determined by scanning electron microscopy (SEM, JSM-6510). The specific surface areas of catalysts were measured through nitrogen adsorption at 77 K using an Autosorb IQ instrument. Prior to adsorption, the samples were treated at 250 °C under vacuum for 6 h and the specific surface area was calculated according to the Brunauer−Emmett−Teller (BET) method. The surface acid and base properties of the samples are estimated by NH3TPD and CO2-TPD, respectively, on a Quantachrome

, Selectivity (%) =

np n0 − n1

× 100

(2)

where n0 is the molar quantity of LA fed into the reactor per hour, n1 is the molar quantity of LA in the effluent per hour, and np is the molar quantity of lactic acid converted to 2,3pentanedione or other byproducts such as acetaldehyde, propionic acid, acrylic acid, and acetic acid per hour. The area-specific catalytic rate is defined as previous references reported18,31,32 and is determined with eqs 3 and 4. LA consumption rate =

amount of LA consumed per hour in the reactor (mmol/h) surface area of catalyst in the reactor (m 2) (3)

PD formation rate amount of PD formed per hour in the reactor (mmol/h) = surface area of catalyst in the reactor (m 2) (4)

3. RESULTS AND DISCUSSION 3.1. Characterization. 3.1.1. Textural Property of Samples. The surface area, total pore volume, and pore size of the samples doped with varying Cs amounts calcined at 600 °C were summarized in Table 1. The bare Ba2P2O7 sample has B


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Industrial & Engineering Chemistry Research Table 1. BET Data of Samplesa Sample Ba2P2O7 0.5CsNO3/2.5Ba2P2O7 1.0CsNO3/2.5Ba2P2O7 1.25CsNO3/2.5Ba2P2O7 1.5CsNO3/2.5Ba2P2O7

SBET (m2/g) 1.2 1.1 0.8 0.8 0.9

Vol (cm3/g) 7.2 6.3 3.9 3.5 6.1

× × × × ×

−3

10 10−3 10−3 10−3 10−3

confirmed from the HRTEM image according to d value (interplanar spacing). A slight difference between the tested d value and standard value existed, indicating a strong interaction between CsNO3 and Ba2P2O7. In addition, the Cs 3d5/2 binding energy in the catalyst calcined at 600 °C decreased in comparison with that before calcination, further demonstrating a strong interaction. 3.1.3. XRD and FT-IR. The wide-angle X-ray diffraction (XRD) patterns of the samples doped with different amounts of Cs component were shown in Figure 2. With an increase of Cs

Pore sizeb (nm) 17.4 3.4 3.4 3.4 3.8

Catalyst calcined at 600 °C. bCalculated from desorption branch data on the Barrett−Joyner−Halenda (BJH) model.

a

a surface area of 1.2 m2/g, pore volume of 7.2 × 10−3 cm3/g, and pore size of 17.4 nm. The samples doped with Cs have lower specific surface area, fluctuating between 0.8 and 1.1 m2/ g. Table 2 showed the textural property of the sample Table 2. BET Data of Samplesa Calcination temp [°C]

SBET (m2/g)

500 600 700

1.3 1.2 0.8 0.6

Vol (cm3/g) 9.3 4.5 3.9 3.8

× × × ×

10−3 10−3 10−3 10−3

Pore sizeb (nm) 3.4 3.8 3.4 3.0

a

Sample, 1.0CsNO3/2.5Ba2P2O7. bCalculated from desorption branch data on the Barrett−Joyner−Halenda (BJH) model.

(1.0CsNO3/2.5Ba2P2O7) calcined at different temperatures. It is clearly seen that the surface area of the catalyst as well as its pore volume decreases with an increase of calcination temperature. 3.1.2. SEM/TEM/HRTEM, EDX, and XPS. Figure 1 showed the SEM/TEM/HRTEM images of sample (1.0CsNO3/ 2.5Ba2P2O7) and its corresponding elemental mapping, EDX, and XPS. The large particle distributes in the range of 2 μm−10 μm in sphere, in which small particle interludes. From the results shown in the mapping, Cs disperses homogeneously in the sample. Besides, CsNO3 and Ba2P2O7 species can also be

Figure 2. XRD of Ba2P2O7 doped with different Cs amounts at calcination of 600 °C.

component, the characteristic peak located at 2θ = 28.3° indexed to CsNO3 (JCPDS No. 09-0403) increases linearly while the characteristic peak located at 2θ = 22.6° indexed to Ba2P2O7 (JCPDS No. 83-0990) decreases, suggesting that the concentration of CsNO3 increases on the surface of Ba2P2O7. It

Figure 1. SEM image of sample 1.0CsNO3/2.5Ba2P2O7 calcined at 600 °C (A); its corresponding elemental mapping (B) (red squares, Ba; light green squares, Cs; purple squares, N; blue squares, O; dark green squares, P); its corresponding EDX spectra (C); its corresponding TEM image (D)/HRTEM image (E); and its corresponding XPS spectra before and after calcination at 600 °C (F). C


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by XRD measurement (Figure 2). The characteristic absorption bands at 1131 and 558 cm−1, respectively, ascribed to Ba2P2O7, evidently existed as the doped Cs component changed, being inconsistent with the results obtained by XRD (Figure 2). These observations suggested that the crystallinity of Ba2P2O7 decreased or the crystal face (111) at 2θ = 22.6° indexed to Ba2P2O7 was weakened with an increase of the doped Cs amount. From Figure S1B, the characteristic absorption band at 1388 cm−1 ascribed to CsNO3 drastically decreased with an increase of calcination temperature, indicating that not simply the physical mixture but strong interaction existed between CsNO3 and Ba2P2O7. 3.1.4. Acid−Base Property of Samples. NH3-TPD and CO2-TPD are efficient means for characterizing the acid−base property of catalytic materials.6,8,16,18 NH3-TPD and CO2-TPD profiles of samples were shown in Figures S2 and S3, and the acid−base property of these samples based on their corresponding NH3-TPD and CO2-TPD profiles was depicted in Figure 4. From Figure 4A, the bare Ba2P2O7 has low base site density (9.66 μmol/m2) and lower acid site density (0.365 μmol/m2), offering a high base/acid molar ratio of 26.5. As Cs is doped to Ba2P2O7, the acid−base property of sample has drastically changed. For example, the small Cs component (Ba2+/Cs+ = 2.5:0.5) was doped to Ba2P2O7 to obtain 0.5CsNO3/2.5Ba2P2O7 sample, and the base site density and acid site density increased to 928 μmol/m2 and 139 μmol/m2, respectively. As the Ba2+/Cs+ molar ratio is 2.5:1, the largest base site density (1740 μmol/m2) and larger acid site density (210 μmol/m2) were obtained over the sample 1.0CsNO3/ 2.5Ba2P2O7, offering an appropriate base/acid molar ratio of 8.3 (see in the section of catalytic activity, the effect of the Csdoped amount). With further increase of the Cs-doped amount, the base site density began to decrease. It is noted that the base/acid molar ratio for all the Cs doped samples fluctuates between 5.1 and 8.3 as the Cs component is doped. From Figure 4B, we can find that the uncalcined sample of 1.0CsNO3/2.5Ba2P2O7 has low base site density (587 μmol/ m2) and acid site density (64.9 μmol/m2). As the sample is calcined at high temperature, the base-acid site density increases largely. The largest base-acid site density (2120 μmol/m2 and 325 μmol/m2, respectively) was obtained as the calcination temperature was up to 500 °C. As the calcination temperature further increased, the base site density displayed a decreased trend. Here, it is emphasized that the optimal catalytic performance is not obtained at calcination temperature

is noted that the other characteristic diffraction peak at 31.5° only slightly decreases, and no new diffraction peaks occur although the strongest characteristic diffraction peak located at 2θ = 22.6° decreases drastically and disappears almost in the end. These results obtained at calcination of 600 °C, which is up to the decomposed temperature of CsNO3, showed that strong interaction between CsNO3 and support of Ba2P2O7 existed, and the crystal face (111) at 2θ = 22.6° indexed to Ba2P2O7 was weakened by the doped CsNO3. Figure 3 showed

Figure 3. XRD of 1.0CsNO3/2.5Ba2P2O7 doped with Cs (Ba2+:Cs+ = 2.5:1) at different calcination temperatures.

the XRD patterns of the samples at different calcination temperatures. The characteristic diffraction peak at 2θ = 22.6° indexed to support of Ba2P2O7 increased evidently at calcination temperature of 500 °C, in comparison with the uncalcined sample. But it began to decrease at 600 °C and disappeared almost as the calcination temperature further increased to 700 °C. It is also noteworthy that the characteristic diffraction peak at 2θ = 28.3° indexed to CsNO3 still existed at 700 °C, far higher than its decomposed temperature (600 °C), again demonstrating an occurrence of strong interaction between CsNO3 and support. In order to study the interaction of CsNO3 with support of Ba2P2O7, FT-IR spectra of samples were measured, and the results were shown in Figure S1. With an increase of the doped Cs amount, the characteristic absorption band at 1388 cm−1 ascribed to CsNO3 also increased (Figure S1A),29 agreeing with the result obtained

Figure 4. Acid−base property of xCsNO3/2.5Ba2P2O7 (x = 0, 0.5, 1.0, 1.25, and 1.5) calcined at 600 °C (A), and acid−base property of 1.0CsNO3/ 2.5Ba2P2O7 calcined at different temperatures (B). D


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Table 3. Comparison of Condensation Reactivity in the Conversion of Lactic Acid to 2,3-Pentanedione for Ba2P2O7 Doped with and without Cs, along with Other Condensation Catalysts from the Literaturea Area-specific catalytic rate (mmol·h−1·m−2)

Sel. [%]b Catalyst

LA conv. [%]

Ba2P2O7c 0.5CsNO3/2.5Ba2P2O7 1.0CsNO3/2.5Ba2P2O7 1.25CsNO3/2.5Ba2P2O7 1.5CsNO3/2.5Ba2P2O7 MNO3/SiO2d K/NaZSM-5e Na2HAsO4/Si-Al Na2HPO4/Si-Al NaNO3/SBA-15g

7.1 55.9 89.1 87.5 81.2 87.8 52.4 5.5 58.8

PD 57.1 58.3 58.1 58.2 46.9 48 25.3f 44 62.1

AD

AC

20.1 18.8 18.3 18.1 24.2

PA

2.5 2.0 2.0 2.2 12.2

5.7 4.4 4.3 4.5 3.2

AA

LA consumption

PD formation

0.5 8.7 14.4 13.6 9.9

12.5 14.2 14.1 13 12.1

5.0 8.4 7.9 5.7

18.4

ref. This work This work This work This work This work ref 2 ref 33 ref 26 ref 25 ref 3

Catalyst, 0.38 mL, 0.21−0.44 g; calcination temperature, 600 °C; particle size: 20−40 meshes; carrier gas: N2, 0.8 mL/min; LA feed flow rate, 1.6 mL/h; LA feedstock, 20 wt % in water; reaction temp., 300 °C; TOS, 1−9 h. bLA, lactic acid; PD, 2,3-pentanedione; AD, acetaldehyde; PA, propionic acid; AA, acrylic acid; AC, acetic acid. cCalcination temp, 500 °C. Carbon balance >98%. dM = Cs, K, Na, and Li. eReaction temp: 280 °C. f 2,3-Pentanedione yield. gReaction temp: 340 °C. a

Table 4. Effect of Calcination Temperature on Catalytic Performance for the Condensation Reaction of LAa Area-specific catalytic rate (mmol·h−1·m−2)

Sel. [%]b Calcination temp [°C] − 500 °C 600 °C 700 °C

LA conv. [%] 51.1 69.2 89.1 67.9

PD

AD

AC

PA

AA

LA consumption

PD formation

53.5 53.3 58.3 58.5

22.2 21.1 18.8 18.4

2.8 2.9 2.0 2.3

6.5 6.1 4.4 5.2

12.7 13.3 14.2 13.4

5.3 7.2 14.4 18.6

2.8 3.8 8.4 10.9

1.0CsNO3/2.5Ba2P2O7; particle size, 20−40 meshes; carrier gas N2, 0.8 mL/min; LA feed flow rate, 1.6 mL/h; LA feedstock, 20 wt % in water; reaction temp, 300 °C; TOS, 1−9 h. bLA, lactic acid; PD, 2,3-pentanedione; AD, acetaldehyde; PA, propionic acid; AA, acrylic acid; AC, acetic acid. “−”: uncalcination. Carbon balance >98%. All experiments were carried out in triplicate, and average values were displayed in the table. The standard deviations were less than 3%. a

feed flow rate were showed in Table 3. The bare Ba2P2O7 has a very low activity for condensation of LA to 2,3-pentanedione. For example, LA conversion is very low, only 7.1%, and 2,3pentanedione is detected in trace using GC. However, a small quantity of CsNO3 was doped into the structure of Ba2P2O7, and the activity drastically enhanced. For example, LA conversion and 2,3-pentanedione selectivity are 55.9% and 57.1%, respectively, over the 0.5CsNO3/2.5Ba2P2O7 catalyst. This indicates that CsNO3 is necessary for the catalyst derived from Ba2P2O7. With an increase of the doped Cs amount, the activity increases rapidly. A case in point is that LA conversion increases from 55.9% to 89.1% when Cs+/Ba2+ molar ratio increases from 0.5/2.5 to 1.0/2.5. Besides, 2,3-pentanedione selectivity also increases slightly. However, with further increase of the doped Cs amount, LA conversion slightly decreases. It is noted that 2,3-pentanedione selectivity almost remains constant. These results suggest that the condensation reaction of LA to 2,3-pentanedione dominates over the Ba2P2O7 catalyst doped with Cs, and the doped Cs amount can efficiently control the amount of active sites. In terms of acid−base catalysis, Cs component can easily tune the acid−base properties of catalyst to catalyze the condensation reaction of LA to 2,3-pentanedione efficiently. Area-specific catalytic rate is also influenced by the Cs-doped amount. When the Cs+/Ba2+ molar ratio is 1/2.5, LA consumption rate and 2,3pentanedione formation rate are up to the optimal values, 14.4 and 8.4 mmol·h−1·m−2, respectively. The typical results reported by other investigators were also listed in Table 3. Early, the activity over the other catalysts is very low. For

of 500 °C, but 600 °C (see in the section of catalytic activity, effect of the calcination temperature). This suggested that apart from base-acid site density, base/acid molar ratio also became a marked impact on catalytic activity. 3.2. Activity. 3.2.1. Effect of the Cs-Doped Amount. According to previous studies,2,3,25,26,33 MNO3 or MxH3−xPO4 salts (M = Cs, K, and Na; x = 1, 2, and 3) are believed to be an effective precursor for condensation of LA to 2,3-pentanedione. Due to low surface area and poor mechanical strength of the MNO3 or MxH3−xPO4 salts, they cannot be directly used as a catalyst and are usually loaded on the supports such as SiO2, SBA-15, and Si−Al compounds via a wet-immersion method. The reaction mechanism for condensation of LA to 2,3pentanedione studied by Tam et al.25,27 discloses that dehydration and decarboxylation are two crucial steps. The former requires acid catalysis, and the latter requires base catalysis. This indicates that the effect of acid−base chemistry on condensation of LA to 2,3-pentanedione should be explored to achieve a high catalytic performance. It is noted that Ba2P2O7 is an excellent catalyst for dehydration of LA to acrylic acid, indicating that appropriate acidic sites exist on its surface to catalyze dehydration of LA.30 Therefore, it is reasonable for us to consider Ba2P2O7 as a potential support for supporting MNO3 or MxH3−xPO4 salts. In most reports,1,2 CsNO3 or CsOH is believed as an excellent precursor for condensation of LA to 2,3-pentanedione. In the present work, a wet-immersion combining with an evaporation-driven method was used to prepare xCsNO3/2.5Ba2P2O7 catalyst, and results on catalytic performance at 300 °C and 1.6 mL/h of LA aqueous solution E


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Industrial & Engineering Chemistry Research Table 5. Effect of Reaction Temperature on Catalytic Performance for the Condensation Reaction of LAa

Area-specific catalytic rate (mmol·h−1·m−2)

Sel. [%]b Reaction temp [°C]

LA conv [%]

PD

AD

AC

PA

AA

LA consumption

PD formation

280 300 320

60.1 89.1 92.2

67.1 58.3 49.3

16.4 18.8 18.8

2.3 2.0 2.3

5.1 4.4 4.9

7.1 14.2 18.5

9.1 14.4 14.9

6.1 8.4 7.4

a 1.0CsNO3/2.5Ba2P2O7, 0.28 g; calcination temp., 600 °C; particle size, 20−40 meshes; carrier gas N2, 0.8 mL/min; LA feed flow rate, 1.6 mL/h; LA feedstoc,: 20 wt % in water; TOS, 1−9 h. bLA, lactic acid; PD, 2,3-pentanedione; AD, acetaldehyde; PA, propionic acid; AA, acrylic acid; AC, acetic acid. Carbon balance >98%. All experiments were carried out in triplicate, and average values were displayed in the table. The standard deviations were less than 3.5%.

Table 6. Effect of LA LHSV on Catalytic Performance for the Condensation Reaction of LAa Area-specific catalytic rate (mmol·h−1·m−2)

Sel. [%]b −1

LA LHSV (h )

LA conv [%]

PD

AD

AC

PA

AA

LA consumption

PD formation

3.0 4.3 5.9

91.2 89.1 79.9

59.9 58.3 57.5

20.3 18.8 19.2

1.9 2.0 2.3

4.3 4.4 5.2

11.4 14.2 13.7

10.2 14.4 17.9

6.1 8.4 10.3

a Catalyst, 1.0CsNO3/2.5Ba2P2O7, 0.28 g; calcination temperature, 600 °C; particle size, 20−40 meshes; LA feedstock, 20 wt % in water; n(LA +H2O)/n(N2) = 30:1; reaction temperature, 300 °C; TOS, 1−9 h. bLA, lactic acid; PD, 2,3-pentanedione; AD, acetaldehyde; PA, propionic acid; AA, acrylic acid; AC, acetic acid. Carbon balance >98%. All experiments were carried out in triplicates, and average values were displayed in Table 4. The standard deviations were less than 3.8%.

example, Gunter et al.25 reported that LA conversion is very low, only 5.5% and 2,3-pentanedione selectivity is acceptable, 44%. Recently, 2,3-pentanedione selectivity of 62.1% has been reported by Zhang et al., but the LA conversion is also low, only 58.8% when the reaction temperature is high, up to 340 °C. In comparison to the previous catalysts, this catalyst offered more excellent activity. In addition, tail gas was analyzed online using GC, and the results were shown in Figure S4. It is clearly seen that CO2 is detected as a major gas product, and CO is hardly detected. This fact demonstrates that decarboxylation reaction indeed takes place during the condensation of LA to 2,3-pentanedione. 3.2.2. Effect of Calcination Temperature. According to the choice of the Cs-doped amount shown in Table 3, the 1.0CsNO3/2.5Ba2P2O7 catalyst with Cs+/Ba2+ molar ratio = 1/ 2.5 displayed the most excellent activity. In this section, over the 1.0CsNO3/2.5Ba2P2O7 catalyst, the effect of calcination temperature on the catalytic performance of the condensation reaction of LA to 2,3-pentanedione was investigated, and the results were depicted in Table 4. Over the catalyst without calcination, LA conversion is low, only 51.1%, as depicted in the previous work;3 2,3-pentanedione selectivity with 53.5% is acceptable. When the catalyst was calcined at 500 °C, the LA conversion enhanced notably, up to 69.2% although the selectivity to 2,3-pentanedione remained constant. With further increase of calcination temperature, LA conversion also continuously increased to 89.1%, and the selectivity to 2,3pentanedione slightly increased to 58.3%. However, LA conversion decreased to 67.9% when the calcination temperature increased to 700 °C. These results indicated that balance of acid−base on the surface of catalyst characterized using base/acid molar ratio as well as active sites characterized by base site density and acid site density played an important role for catalyzing condensation of LA to 2,3-pentanedione. It is evidently seen that the optimal calcination temperature is 600 °C, at which more active sites exist as well as an appropriate base/acid molar ratio for condensation reaction of LA to 2,3pentanedione formation.

3.2.3. Effect of Reaction Temperature. Reaction temperature for condensation reaction of LA to 2,3-pentanedione is related to catalyst and reactor types. So far, there is not a report that used the reaction temperature lower than 200 °C for this reaction. Naturally, the gas−solid catalytic reaction proceeded in the fixed bed reactor was adopted by pioneering investigators, which facilitated separation between catalyst and product.6−8,25,34−36 Over the 1.0CsNO3/2.5Ba2P2O7 catalyst calcined at 600 °C, the effect of the reaction temperature on reaction performance was investigated, and the results were shown in Table 5. From the data on activity given in Table 5, the influence of temperature on reaction performance is evident. When the reaction temperature increases from 280 to 320 °C, LA consumption rate increases steadily from 9.1 to 14.9 mmol·h−1·m−2, whereas 2,3-pentanedione formation rate first increases and then decreases, indicating that side reactions proceed rapidly at higher reaction temperature. For example, acrylic acid selectivity drastically increased from 9.1% to 18.5% when the reaction temperature increased from 280 to 320 °C. As a result, LA conversion increases with an increase of reaction temperature; on the contrary, 2,3-pentanedione selectivity decreases. This result can be well explained by a previous kinetic study on catalytic conversion of LA since the activation energy for the main reaction is lower than the side reactions.2,28 Thus, an appropriate temperature for the 1.0CsNO3 / 2.5Ba2P2O7 catalyst should be about 300 °C in overall consideration. 3.2.4. Effect of LA LHSV. LHSV is generally used to evaluate the performance of heterogeneous catalyst.37−39 Table 6 showed the influence of LA LHSV on reaction performance over the 1.0CsNO3/2.5Ba2P2O7 catalyst calcined at 600 °C. The reaction was conducted at 300 °C with LA flow rate changed from 1.1 to 2.2 mL/h (corresponding LHSV = 3.0− 5.9 h−1). For LA conversion, it decreases slowly from 91.2% to 79.9% when LA LHSV increases from 1.1 h−1 to 2.2 h−1. But the area-specific catalytic rates such as LA consumption and 2,3-pentanedione formation increased fast. For example, LA consumption rate and 2,3-pentanedione formation rate were 10.2 mmol·h−1·m−2 and 6.1 mmol·h−1·m−2, respectively, at LA F


Article

Industrial & Engineering Chemistry Research solution LHSV = 3.0 h−1 while they attained 17.9 mmol·h−1· m−2 and 10.3 mmol·h−1·m−2, respectively, at LA solution LHSV = 5.9 h−1. It is also noted that 2,3-pentanedione selectivity decreased gradually from 59.9% to 57.5% with an enhanced LA LHSV, suggesting that it is not shortening contact time but lengthening contact time between LA and surface of catalyst, favoring formation of 2,3-pentanedione. However, the selectivity to acetic acid, propionic acid, and acrylic acid slightly increased with an enhanced LA LHSV, suggesting that shortening contact time favors the side reactions to form acetic acid, propionic acid, and acrylic acid. These observations agreed well with the results obtained in previous work.28 For that reason, choosing low LA LHSV favors selective formation of 2,3-pentanedione. The time space yield (YTS) of 2,3-pentanedione was calculated according to the following equation.17,40 YTS =

MPD MCat × t

Figure 6. Stability of the catalyst: catalyst, 1.0CsNO3/2.5Ba2P2O7; calcination temp., 600 °C; particle size, 20−40 meshes; carrier gas, N2; 0.8 mL/min; LA feed flow rate, 1.6 mL/h; LA feedstock, 20 wt % in water; reaction temp, 300 °C.

where MPD is the mass of produced 2,3-pentanedione (g); MCat is the mass of catalyst; and t is the reaction time (h). Plotted in Figure 5 is the YTS of 2,3-pentanedione versus LA liquid

reaction time while 2,3-pentanedione selectivity still remained. For example, LA conversion reduced to around 80% at 28 h on stream. By the simple calcination under the air at 600 °C for 10 h, the used catalyst regained catalytic activity (see cycle 2 in Figure 6), suggesting that the possible reason for the reduced catalytic activity can be ascribed to covering the active sites of the catalyst surface by carbon deposit or hard coke.32,43,46 In general, 1.0CsNO3/2.5Ba2P2O7 catalyst offered an excellent performance including high selectivity and excellent stability. 3.2.6. Reaction Mechanism. Based on the above experimental evidence, the possible reaction mechanism was proposed (shown in Scheme 1).5,25,27 First, the basic sites on the surface of catalyst attack H+ at −COOH to produce an enolate form (I) via the proton transfer. After deprotonation, the process undergoes ketonic decarboxylation of carboxylic acids via the beta-keto acid to give form (II).47−49 The direct evidence confirmed the decarboxylation process: a large number of CO2 was detected while only a trace of CO was detected in the online reaction tail gas analysis. Next, acidic sites attack the β-OH of form (II) to remove a molecular water and form another enolate (III).50 Form (III) rapidly isomerizes into product of 2,3-pentanedione.

Figure 5. YTS of 2,3-pentanedione versus LA liquid hourly space velocity.

hourly space velocity. The YTS of 2,3-pentanedione increased with LA LHSV and achieved 2.1 g·gCat−1·h−1 as LA LHSV increased to 5.9 h−1. 3.2.5. Stability. Long-term stability is a very important characteristic for application of a heterogeneous catalyst on an industrial scale.41−45 However, few works on the catalytic stability of catalyst for condensation of LA to 2,3-pentanedione have been reported so far. Due to this cause, the catalytic stability of 1.0CsNO3/2.5Ba2P2O7 was investigated at 300 °C and a LA feed flow rate of 1.6 mL/h (corresponding LA LHSV = 4.3 h−1), and the results were depicted in Figure 6. LA conversion increased slowly and gradually with an increase of time on stream. Similarly, 2,3-pentanedione selectivity also increased slowly. For example, LA conversion and 2,3pentanedione selectivity were 87% and 57.1%, respectively, at the initial 1 h on stream and, after 9 h on stream, attained 92% and 61.1%, respectively. These results indicated that catalytic active sites increased with an enhanced reaction time. However, LA conversion began to reduce slowly with further increase of

4. CONCLUSIONS The Cs doped Ba2P2O7 catalyst was facilely prepared via reaction of Ba(NO3)2 with (NH4)2HPO4 in aqueous solution to form Ba2P2O7, followed by immersion in CsNO3 aqueous solution and high temperature calcination. Both Cs-doped amount and high-temperature calcination can efficiently regulate the acid−base property of catalyst to selectively catalyze condensation of lactic acid to 2,3-pentanedione. It was found that decreasing reaction temperature and LA LHSV favored the selective formation of 2,3-pentanedione from lactic acid. The test of catalyst stability showed an excellent stability, and it can be reused again only by calcination in the air atmosphere at 600 °C for 10 h. G


Article

Industrial & Engineering Chemistry Research Scheme 1. Possible Mechanism of 2,3-Pentanedione Formation from Lactic Acid over Ba2P2O7 Doped with Cs

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Notes

ASSOCIATED CONTENT

The authors declare no competing financial interest.

S Supporting Information *

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b03595.

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ACKNOWLEDGMENTS This work was supported by Sichuan province science and technology department application foundation project (2017JY0188), the opening foundation of Jiangsu Key Laboratory of Vehicle Emissions Control (OVEC 032), and the National Natural Science Foundation of China (21464005).

FT-IR spectra, TPD profiles, and GC profile. (PDF)

AUTHOR INFORMATION

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Corresponding Author

*Tel.: +86 23 62563250. E-mail address: tcmtang2001@163. com (C. M. Tang).

REFERENCES

(1) Tam, M. S.; Jackson, J. E.; Miller, D. J. Effects of ammonium lactate on 2,3-pentanedione formation from lactic acid. Ind. Eng. Chem. Res. 1999, 38, 3873−3877. (2) Sun, L. W.; Li, X. L.; Tang, C. M. Comparative study on catalytic performance of the production of 2,3-pentanedione from lactic acid condensation over SiO2-supported alkali metal nitrates. Acta Phys.Chim. Sin. 2016, 32, 2327−2336.

ORCID

Congming Tang: 0000-0001-5199-8286 Lin Dong: 0000-0002-8393-6669 Author Contributions †

X.L. and Y.Z. contributed equally to this work. H


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