Highly Selective Production of Acrylic Acid from Glycerol via Two

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Research Article pubs.acs.org/journal/ascecg

Highly Selective Production of Acrylic Acid from Glycerol via Two Steps Using Au/CeO2 Catalysts Minsu Kim and Hyunjoo Lee* Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon 34141, Republic of Korea S Supporting Information *

ABSTRACT: Using biomass resources for chemical production can provide more sustainable development in the chemical industry. In this work, acrylic acid was selectively produced from glycerol via two steps: Glycerol was efficiently converted to allyl alcohol by formic acid-mediated deoxydehydration (DODH), and then, the obtained allyl alcohol was oxidized without purification into acrylic acid in a basic aqueous solution. The Au/CeO2 catalysts were used for the selective oxidation, and it worked well even in the presence of residual formic acid and impurities. A high yield of 87% was obtained for the production of acrylic acid from glycerol: 94.5% from glycerol to allyl alcohol and 92% from allyl alcohol to acrylic acid. The different shapes of CeO2 such as rods, octahedra, and cubes were used as supports to deposit the Au active phase. Au deposited on octahedral CeO2 presented the highest yield toward acrylic acid, and it was the most stable for the repeated oxidations. The effects of reaction time, temperature, O2 pressure, and allyl alcohol concentration were evaluated to maximize the yield toward acrylic acid. KEYWORDS: Glycerol, Acrylic acid, Allyl alcohol, Au/CeO2, Selective oxidation

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INTRODUCTION With increasing demand in renewable resources, biomassderived chemicals are being developed to replace petroleumbased chemicals.1−3 Acrylic acid is a monomer for poly(acrylic acid) (PAA), which has been widely used for diapers, hygiene products, coatings, adhesives, paints, and specialty resins.4 The acrylic acid has been produced from propylene, which is obtained from fossil fuels, through the pathway of propylene to acrolein to acrylic acid using Bi−Mo oxide or Bi−V oxide catalyst.5 The propylene can be replaced with glycerol, which is obtained from triglycerides such as vegetable oils. Various value-added C3 chemicals can be produced from glycerol.6,7 Among them, the production of acrylic acid is particularly promising due to the high demand for acrylic acid in the chemical industry.7,8 One-step conversion of glycerol to acrylic acid via gas-phase or liquid-phase reactions has been actively studied.9−12 However, these reactions produce acrylic acid with low yields below 60%, and the catalysts often show poor stability. Indirect pathways, in which glycerol turns into an intermediate such as acrolein or allyl alcohol and then the intermediate is converted to acrylic acid, have been suggested for efficient production of acrylic acid from glycerol.8 The pathway through the allyl alcohol intermediate is particularly interesting.13−16 Although the production of acrolein from glycerol using solid acid catalysts often suffers from severe catalyst coking,17 the formic acid-mediated deoxydehydration (DODH) efficiently produces allyl alcohol from glycerol in a continuous process with a very high yield of 99%.13,18 Recently, it was reported that allyl alcohol can turn into acrylic acid via gas-phase oxidation without catalyst deactivation.13,15 However, these works used © 2017 American Chemical Society

purified allyl alcohol for the production of acrylic acid; thus, the effect of residual formic acid remaining after the DODH process has been ignored. The boiling points are 97 °C for allyl alcohol and 100.8 °C for formic acid, respectively, so it would be difficult to separate allyl alcohol and formic acid by conventional distillation. Previously, we showed that the Au/CeO2 catalyst can oxidize allyl alcohol to acrylic acid at a mild condition of 50 °C with a maximum yield of 51.1%.14 In this work, the interaction between Au and CeO2 supports has been modulated by changing the shape of CeO2 such as rods, octahedra, and cubes to maximize the yield for acrylic acid. Controlling the shape of CeO2 supports has been reported for CO oxidation, the water− gas shift reaction, propane oxidation, etc.19−30 The shape of CeO2 affects the activity and stability differently depending on the kind of deposited metal and the specific target reaction. The reaction temperature, oxygen pressure, and concentration of allyl alcohol was further controlled, and then, the yield from allyl alcohol to acrylic acid could be enhanced significantly. More importantly, the solution obtained after the DODH was used for subsequent oxidation without purification.

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EXPERIMENTAL SECTION

Preparation of Shape-Controlled CeO2 Supports. The different shapes of CeO2 particles were obtained by a hydrothermal method.20,26,31 For rod-shaped CeO2 (CeO2-R), 1.31 g of Ce(NO3)3· 6H2O (Kanto Chemical, 99.99%) was added to 35 mL of 9 M NaOH Received: July 21, 2017 Revised: October 19, 2017 Published: October 31, 2017 11371

DOI: 10.1021/acssuschemeng.7b02457 ACS Sustainable Chem. Eng. 2017, 5, 11371−11376

Research Article

ACS Sustainable Chemistry & Engineering

min. Then, a 5% H2/Ar mixture was introduced at 400 °C for 30 min. The weight loss resulted from oxygen removal by H2 was measured. Elemental analysis was performed using an inductively coupled optical emission spectrometer (ICP-OES 720, Agilent) to measure the actual amount of metal in the catalysts.

solution and stirred for 1 h. The solution was transferred to a Teflonlined autoclave, then heated at 100 °C for 12 h. After being cooled to room temperature, the obtained solid was washed with deionized (DI) water and dried at 80 °C. The obtained solid was calcined at 550 °C for 4 h in air. For cube-shaped CeO2 (CeO2-C), 9 M NaOH solution was used, and the hydrothermal treatment was performed at 180 °C for 24 h. The other conditions were the same as those used for the rod-shaped CeO2. For octahedral CeO2 (CeO2-O), 0.69 g of Ce(NO3)3·6H2O was added to 32 mL of 0.1 M NaOH solution and stirred for 15 min. The solution was transferred to a Teflon-lined autoclave and heated at 180 °C for 24 h. The obtained solution was cooled to room temperature, washed with DI water, and then dried at 80 °C overnight. The dried solid was calcined at 400 °C for 4 h in air. Deposition of Metal on CeO2 Supports. Au/CeO2 catalysts were prepared by a deposition−precipitation method.14 First, 12 mg of HAuCl4·3H2O (Aldrich, 99.9%) was dissolved in 50 mL of DI water. The pH of the aqueous solution was adjusted to 9.5−10.5 by adding 0.2 M NaOH solution and stirred for 20 min. After the solution turned clear, 200 mg of the CeO2 support was added to the solution and dispersed by ultrasonication for 10 min. The solution was stirred at 70 °C for an additional 1 h. The resulting solution was cooled to room temperature, filtered with DI water, and dried at 80 °C overnight. For comparison, Pt and Cu were deposited on the CeO2 support by the same method using H2PtCl6·6H2O (Aldrich, 99.99%) and CuCl2 (Aldrich, 97%) at pH 9.0 and pH 7.0, respectively.27 Pd was deposited on the CeO2 support using PdCl2 (Aldrich, 99%) at pH 9.0 by adjusting the pH using Na2CO3.32 Oxidation of Glycerol to Acrylic Acid in Two Steps. First, formic acid-mediated deoxydehydration (DODH) was performed in a batch reactor.13,33 Glycerol (Aldrich, 99.5%) (0.2 mol) and formic acid (Junsei, 99%) (0.26 mol) were mixed in a 50 mL flask, and then, the temperature was increased to 210 °C. The formic acid (0.05 mol) was added additionally twice every 2 h. The overall reaction time was 6 h. The evaporated product was collected in a separate flask. The product including allyl alcohol, water, and unreacted formic acid was quantified using high performance liquid chromatography (HPLC, YL9100, Younglin) with a Hi-plex H column and a refractive index detector (RID) using a 5 mM H2SO4 aqueous solution as a carrier with a flow rate of 0.6 mL/min. Second, the oxidation of the obtained allyl alcohol was carried out in a Parr reactor (50 mL) without any purification. The allyl alcohol solution containing 1.8 mmol of allyl alcohol and the catalyst (the mole ratio of metal to allyl alcohol was 400) were mixed with 17.5 mL of 3 M NaOH solution. The reactor was pressurized at 10 bar of oxygen while stirring at 800 rpm for 3 h. The liquid-phase product was obtained after separating the catalyst by centrifugation and analyzed with HPLC using the same method mentioned above. To identify the liquid-phase products, the obtained solution was also analyzed by 1H NMR and 13C NMR analysis (Agilent 400 MHz 54 mm NMR DD2) with D2O (Aldrich, 99.9%) and 3-(trimethylsilyl)-1propanesulfornic acid sodium salt (Aldrich, 97%) as an internal reference. Characterizations. Transmission electron microscopy (TEM) images were obtained using a FEI Technai F30 ST microscope operated at 300 kV. The average size of the nanoparticles was estimated from TEM images. The electronic states of the catalysts were investigated by X-ray photoelectron spectroscopy (XPS, K-alpha, Thermo VG Scientific). The binding energies were estimated using the advantageous C 1s signal at 284.8 eV as a reference. The specific surface area was measured using a BET instrument (Tristar II 3020). Before the BET analysis, the sample was preheated at 150 °C under vacuum to remove physically adsorbed water. Temperatureprogrammed reduction (TPR) was performed using a BELCAT-B (BEL).20 The sample was preoxidized at 300 °C for 30 min under pure O2 flow, purged with Ar gas to remove physically adsorbed oxygen, and cooled to room temperature. The measurement was carried out under 5% H2/Ar flow increasing the temperature from room temperature to 300 °C. The oxygen storage capacity was estimated using thermogravimetric analysis (TGA, TG 209 F3, Netzsch).24 The sample was heated under Ar flow to 280 °C, kept at this temperature for 1 h, heated again to 400 °C, and kept at this temperature for 15

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RESULTS AND DISCUSSION Deoxydehydration of Glycerol to Allyl Alcohol. Acrylic acid can be produced from glycerol via a two-step pathway as shown in Scheme 1. The first step is the deoxydehydration (DODH) of glycerol producing allyl alcohol. The second step is the selective oxidation of allyl alcohol to acrylic acid. Scheme 1. Two-Step Process of Deoxydehydration and Subsequent Oxidation of Glycerol for Selective Production of Acrylic Acid

The DODH of glycerol has been conducted by Re catalysts34,35 or a formic acid-mediated method.13,18,33 When the DODH was performed with formic acid without catalysts (glycerol + HCOOH → allyl alcohol +2H2O + CO2), high activity and selectivity could be obtained in both continuous and batch reactors.13,33 The detailed mechanism for the DODH with formic acid has been previously studied.18,33 An additive such as triethyl orthoformate was also used to enhance the activity.18 In this work, we obtained allyl alcohol from glycerol using the DODH with formic acid in a batch reactor. When 0.2 mol of glycerol was treated with total 0.36 mol of formic acid at 210 °C for 6 h, allyl alcohol was obtained with a yield of 94.5% as shown in Table S1. Additionally, allyl formate was obtained with a yield of 5.5%. The unreacted formic acid was also observed; 43.5% of formic acid remained after the DODH reaction. The subsequent oxidation was performed using this product solution. Purification was not conducted; thus, the oxidation occurred in the presence of residual formic acid and allyl formate. Selective Oxidation of Glycerol-Derived Allyl Alcohol to Acrylic Acid. The glycerol-derived allyl alcohol was oxidized using various supported metal catalysts. Au, Pt, Cu, and Pd were deposited on CeO2 supports by a deposition− precipitation method at appropriate pH values. The prepared Pt, Cu, and Pd catalysts were inactive in the oxidation of allyl alcohol at 25 °C, and only the Au catalyst was active at such a low reaction temperature as shown in Table 1. The kind of support was also varied with ZrO2, TiO2, and CeO2, and it was found that ZrO2 or TiO2 produced the hydrated products of 3hydroxypropionic acid (3-HPA) or glyceric acid (GA) more. Different shapes of CeO2 supports were synthesized and used for the oxidation of allyl alcohol. The Au catalyst deposited on an octahedral CeO2 support (denoted as Au/CeO2-O in Table 1) showed the highest yield for acrylic acid production with a yield of 92.0%. In this case, a very high yield of 87% could be obtained from glycerol to acrylic acid. Figure S1 shows HPLC results of the product solution obtained after the oxidation. The products of acrylic acid, 3-HPA, and GA were observed in addition to formic acid. 13C and 1H NMR in Figure S2 also confirms the presence of acrylic acid, 3-HPA, and formic acid in the final solution. Any other significant amount of side product was not observed. The Au catalyst deposited on CeO2 rods 11372

DOI: 10.1021/acssuschemeng.7b02457 ACS Sustainable Chem. Eng. 2017, 5, 11371−11376

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ACS Sustainable Chemistry & Engineering Table 1. Oxidation of Glycerol-Derived Allyl Alcohol Using Various Catalystsa Yield (%) Catalyst

Conv. of allyl alcohol

ACA

3-HPA

GA

Others

Cu/CeO2 Pt/CeO2 Pd/CeO2 Au/ZrO2 Au/TiO2 Au/CeO2-R Au/CeO2-O Au/CeO2-C Au/CeO2-commercial

0.0 0.0 8.0 100 100 100 100 100 100

0.0 0.0 0.0 69.4 74.8 81.9 92.0 70.1 73.8

0.0 0.0 0.0 17.5 16.4 7.5 7.4 14.6 6.9

0.0 0.0 0.0 5.5 2.1 0.6 0.6 0.9 0.0

0.0 0.0 8.0 7.6 6.7 10.0 0.0 14.4 19.3

Reaction condition: 3 M NaOH, 17.5 mL; 25 °C; O2, 10 bar; 3 h; 1.8 mmol glycerol-derived allyl alcohol; and a mole ratio of allyl alcohol/ metal 400. ACA is an acronym for acrylic acid, 3-HPA for 3hydroxypropionic acid, and GA for glyceric acid.

a

(denoted as Au/CeO2-R) also showed a high yield of 81.9% for acrylic acid, whereas the Au catalyst on cubic CeO2 (denoted as Au/CeO2-C) presented a relatively a low yield of 70.1%. The effect of residual formic acid was evaluated by comparing the oxidation of pure ally alcohol on the Au/CeO2-O catalyst, as shown in Table S2. The activity and selectivity was almost the same in the absence or presence of formic acid. The Au/CeO2 catalysts worked properly for the selective oxidation even in the presence of residual formic acid and allyl formate. Although the boiling points of allyl alcohol and formic acid are very similar at 97 and 100.8 °C, respectively, the boiling point of acrylic acid is 141 °C. Thus, separating acrylic acid from formic acid would be easier. Effect of Shaped CeO2 Supports in Au/CeO2 Catalysts. It has been reported that shaped CeO2 particles can affect activity and selectivity for various heterogeneous reactions.23,24,29 Interaction between Au and CeO2 surfaces can be controlled by changing the surface structure of the CeO2 support.19−22,25,30 Rod, cubic, and octahedral shapes of CeO2 particles have been synthesized as shown in Figure S3, and then, Au was deposited on the CeO2 supports using a deposition−precipitation method. BET surface areas of bare CeO2 particles are 68, 19, 110, and 30 m2/g for rod, cubic, octahedral shapes, and commercial CeO2, respectively. The largest surface area of the octahedral shape would be beneficiary for Au deposition and facile oxygen transfer. Figure 1 shows TEM images of the prepared Au/CeO2 catalysts, and Table 2 shows the physicochemical properties of the catalysts. The sizes of Au nanoparticles were estimated from TEM images, and their histograms are shown in Figure S4. The sizes were similar at 3.1 nm for Au/CeO2-R, 2.2 nm for Au/CeO2-O, 2.5 nm for Au/CeO2-C, and 2.6 nm for Au/CeO2-commercial, respectively. The oxygen storage capacity (OSC) was measured using TGA as described in the Experimental Section, and Au/ CeO2-O showed the largest OSC. H2-TPR results in Figure S5 also showed that Au/CeO2-R or Au/CeO2-O has a larger reduction peak at a lower temperature than Au/CeO2-C or Au/ CeO2-commercial. The higher reducibility of Au/CeO2-R or Au/CeO2-O may be due to a stronger interaction between Au and ceria.26 The oxidation state of Au was estimated using XPS measurements as shown in Figure S6. Au/CeO2-R or Au/ CeO2-O has more oxidic Au than Au/CeO2-C and Au/CeO2commercial, being consistent with the tendency of reaction. We

Figure 1. TEM images of fresh (a) Au/CeO2-R, (b) Au/CeO2-O, (c) Au/CeO2-C catalysts, and (d) Au/CeO2-commercial catalysts.

previously showed that oxidic Au produced more acrylic acid with the CC bond preserved, while metallic Au produced more 3-hydroxypropionic acid.14 Au is more oxidic, and surface oxygen is more active on Au/CeO2-R or Au/CeO2-O, resulting in higher acrylic acid production. Figure 2 showed the repeated reaction results for the allyl alcohol oxidation over the Au/CeO2 catalysts. The catalysts were separated from the liquid products by centrifugation and used for the next reaction without any regeneration step. Although there is a chance that some catalyst might be lost during the separation step, the same amount of the reactants was added for the repeated reactions. For Au/CeO2-O, the initial yield for acrylic acid was 92%, and it decreased to 84.7% after the fifth reaction. For Au/CeO2-R, the initial yield for acrylic acid was 81.9%, and it decreased to 72.9%. For Au/ CeO2-C, the initial yield for acrylic acid was 70.1%, and it decreased to 46.2%. While Au/CeO2-O retained its high activity for acrylic acid production over the repeated reactions, the activity of Au/CeO2-C was degraded significantly. The Au/CeO2 catalysts obtained after fifth reaction were examined as shown in Table S3. In cases of Au/CeO2-R and Au/CeO2-C, the Au contents decreased, and the size of Au nanoparticles increased significantly. Figure S7 shows the size histograms of Au nanoparticles after the fifth reaction estimated from TEM images. The Au size increased from 3.0 to 4.9 nm for Au/CeO2-R, from 2.1 to 2.3 nm for Au/CeO2-O, from 2.3 to 4.8 nm for Au/CeO2-C, and from 2.6 to 4.3 nm for Au/ CeO2-commercial. Figure S8 shows XPS results of the Au/ CeO2 catalysts after the fifth reaction. The Au surface became more metallic compared to the fresh catalysts. The changes in the Au content and Au size were the least on Au/CeO2-O, possibly due to strong interaction between the Au and CeO2(111) surfaces.36 The large surface area of the octahedral CeO2 support and its strong interaction with Au enabled highly active and durable oxidation of allyl alcohol to acrylic acid. Effect of Reaction Conditions for Selective Oxidation. The reaction time, oxygen pressure, temperature, and 11373

DOI: 10.1021/acssuschemeng.7b02457 ACS Sustainable Chem. Eng. 2017, 5, 11371−11376

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ACS Sustainable Chemistry & Engineering Table 2. Physicochemical Properties of Fresh Au/CeO2 Catalysts

XPS Au 4f peak (%) Catalyst

Au (atom%)a

dAu (nm)b

OSC (μmol/g)c

Au0

Au+

Au3+

Au/CeO2-R Au/CeO2-O Au/CeO2‑-C Au/CeO2-commercial

2.35 2.27 0.92 2.16

3.1 2.2 2.5 2.6

137.5 162.5 118.8 116.4

59.6 61.0 82.6 75.6

23.3 20.5 10.2 16.7

17.0 18.5 7.2 7.7

a Determined by ICP-OES. bSize of Au nanoparticles was estimated from TEM images. cOxygen storage capacity was obtained from the modified TGA analysis.

reaction temperature of 70 °C produced 17.3% of 3-HPA and 9.3% of glyceric acid, whereas 25 °C produced only 7.4% of 3HPA and 0.6% of glyceric acid. The dilute reactant solution was better for the desired selective oxidation. When the concentration of allyl alcohol was higher than 0.1 M, the production of hydrated species and dimers increased. Estimating “green metrics” on various reaction conditions can be beneficial to find the condition which contributes to actual sustainability the most.37

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CONCLUSION Highly selective production of acrylic acid from glycerol was studied in this work. The glycerol was converted to allyl alcohol using deoxydehydration (DODH) by reacting with formic acid without any catalyst. Then, the allyl alcohol was converted to acrylic acid using Au/CeO2 catalysts with a maximum yield of 92%. The solution obtained after the DODH process was directly used for the oxidation without any separation. The residual formic acid and side products did not affect the catalytic reaction on Au/CeO2. The separation of allyl alcohol from formic acid would be difficult due to their very similar boiling points, but separating acrylic acid from formic acid would be easier. The overall yield of 87% was obtained from glycerol to acrylic acid. Different shapes of rod, octahedral, and cubic CeO2 supports were used, and the octahedral shape

Figure 2. Recyclability test results for the oxidation of glycerol-derived allyl alcohol over various Au/CeO2 catalysts.

concentration of the allyl alcohol were varied as shown in Figure 3, and the optimal reaction condition which maximizes the production of acrylic acid was searched. The yield for acrylic acid was the highest for 3 h of reaction time. When the time was longer, the acrylic acid was degraded with less yield. The oxygen pressure was the key factor to enhance the yield for acrylic acid. When the oxygen pressure increased using an autoclave reactor up to 10 bar, the production of hydrated chemicals such as 3-HPA was suppressed and the yield for acrylic acid increased significantly. The lower reaction temperature was advantageous to suppress the hydration as well. The

Figure 3. Changes in the conversion of allyl alcohol and the yield for acrylic acid for various (a) reaction time, (b) oxygen pressure, (c) reaction temperature, and (d) allyl alcohol concentration using the Au/CeO2-O catalyst. 3-HPA indicates 3-hydroxypropionic acid and 3-APA indicates 3allyloxypropanoic acid. The standard reaction condition was 3 h, 10 bar, 25 °C, and 0.1 M. When one condition was varied, the other conditions were the same as the standard condition. 11374

DOI: 10.1021/acssuschemeng.7b02457 ACS Sustainable Chem. Eng. 2017, 5, 11371−11376

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showed the highest yield for acrylic acid and was the most stable upon repeated oxidations up to the fifth recycle. The effect of reaction conditions such as time, temperature, oxygen pressure, and reactant concentration was investigated. Low reaction temperature, high oxygen pressure, and dilute allyl alcohol concentration were more advantageous to produce acrylic acid while suppressing further hydration. This work would provide an alternative route for the efficient production of acrylic acid not from fossil fuels but from more sustainable biomass resources.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02457. Additional data: Tables S1−S3 and Figures S1−S8. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +82-42-350-3922. ORCID

Hyunjoo Lee: 0000-0002-4538-9086 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by NRF2015R1A2A2A01004467 through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology.

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REFERENCES

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DOI: 10.1021/acssuschemeng.7b02457 ACS Sustainable Chem. Eng. 2017, 5, 11371−11376

Research Article

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DOI: 10.1021/acssuschemeng.7b02457 ACS Sustainable Chem. Eng. 2017, 5, 11371−11376