Meerwein–Ponndorf–Verley Reduction of Crotonaldehyde over

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Meerwein−Ponndorf−Verley Reduction of Crotonaldehyde over Supported Zirconium Oxide Catalysts Using Batch and Tubular Flow Reactors Atsushi Segawa,*,†,∥ Keita Taniya,‡ Yuichi Ichihashi,⊥ Satoru Nishiyama,⊥ Naohiro Yoshida,∥,§,∇ and Masaki Okamoto# †

Central Technical Research Laboratory, JXTG Nippon Oil & Energy Corporation, 8 Chidoricho, Naka-ku, Yokohama 231-0815, Japan ∥ Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Engineering and § Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan ‡ Organization for Advanced and Integrated Research and ⊥Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan # Department of Chemical Science and Engineering, School of Materials and Chemical Technology and ∇Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan ABSTRACT: The authors studied Meerwein−Ponndorf−Verley (MPV) reduction of crotonaldehyde using batch and tubular flow reactors. Various ZrO2/SiO2 catalysts prepared from commercially available carriers and precursors were subjected to activity testing using autoclave batch reactors. To determine the degree of Zr dispersion in the ZrO2/SiO2 catalysts, X-ray photoelectron spectroscopy (XPS) measurements and the benzaldehyde-ammonia titration (BAT) method were carried out. The results suggested a positive correlation between Zr dispersion and crotonaldehyde conversion. Durability tests using tubular flow reactors were performed with the most suitable catalyst, which was selected through batch reactions and catalyst characterizations. Almost no degradation of the catalytic activity was observed over 2,200 h in a liquid-phase reaction, while catalyst durability was short in a gas-phase reaction. It was surmised that in the liquid-phase, 2-propanol (hydrogen donor) in the feed had dissolved the reaction byproducts to purge from the catalyst. In addition, a continuous process flow design that includes 2-propanol regeneration was proposed for industrial production.

1. INTRODUCTION There have been numerous reports on producing unsaturated alcohols from the corresponding unsaturated aldehydes. Unsaturated alcohols are widely used in the production of pharmaceuticals, agrochemicals, and fragrances.1,2 As such, these reactions are the focus of much attention in industrial and academic circles. There have been many recent reports of high selectivity in the selective hydrogenation of unsaturated aldehydes with hydrogen to produce unsaturated alcohols.3−6 In most of these studies, the reactions have been performed using batch reactors with noble metal catalysts and needed to be carried out under high pressure. Meerwein−Ponndorf−Verley (MPV) reduction has been well-known since the 1920s7−10 as a means of only the carbonyl group reduction when CC bonds are present, despite the fact that hydrogenation of CC bonds is a more thermodynamically favorable reaction.1 MPV reduction requires alcohols as © XXXX American Chemical Society

hydrogen donors and can be carried out under mild reaction conditions. In MPV reduction, it is generally accepted that a metal center acts as a Lewis acid site to promote the formation of a six-membered ring transition state with aldehydes and alcohols to accomplish a hydride transfer from an alcohol to a carbonyl group, without the use of noble metal catalysts.11,12 MPV reduction over a variety of catalysts has been known. Zr beta-zeolite13 and boron alkoxides14 showed high selectivity for cinnamyl alcohol and 3-methyl-2-butenol, respectively. Radhakrishan et al. have reported 100% selectivity in the reduction of various aromatic aldehydes with isopropyl alcohol (IPA) using potassium phosphate as a solid base catalyst.15 Aramendı ́a Received: Revised: Accepted: Published: A

September 23, 2017 November 28, 2017 December 4, 2017 December 4, 2017 DOI: 10.1021/acs.iecr.7b03961 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Commercially available silicas sold under the name “CARiACT Q” (Spherical type, 0.85−1.7 mm, Q6, Q10, Q30, and Q50) were obtained from Fuji Silysia Chemical Ltd., Japan and used as catalyst supports. A commercially available Zr reagent, zirconyl nitrate dihydrate ZrO(NO3)2-2H2O (Wako Pure Chemicals Co Ltd., Japan), was used as a Zr precursor. Zr was loaded on the support by way of the incipient wetness impregnation method with an aqueous solution of zirconyl nitrate dihydrate. The samples so obtained were dried at 120 °C overnight and calcined at 220−650 °C for 2−5 h. The catalysts were used without any pretreatment. 2.2. Activity Test. Commercial crotonaldehyde (Wako Pure Chemicals Co Ltd., Japan, purity: 99%, boiling point (bp) 104 °C) and anhydrous IPA (Kanto Chemical Co., Inc., Japan, purity: 99.7%, bp 82 °C) were used without any purification. For the initial activity tests, 30 mL of stainless steel autoclave batch reactors was used. The liquid phase reactions were performed under a nitrogen atmosphere (approximately 0.5 MPa (gauge)) at 130 °C for 3 h with 500 mg of the ZrO2/SiO2 catalyst. The IPA/crotonaldehyde molar ratio was 2 except in 3.1.4, and the Zr molar ratio (Zr in the catalyst/ crotonaldehyde) was 0.02 (5 wt % Zr/SiO2). 1.13 mL (13.7 mmol) of crotonaldehyde and 2.10 mL (27.4 mmol) of IPA were used for 500 mg of 5 wt % Zr/SiO2. For durability tests in the liquid and gas phases, a tubular flow reactor loaded with either 15 or 50 mL of a catalyst was used. The IPA/crotonaldehyde molar ratio was 4−18 for the liquid phase and 4 for the gas phase. The liquid hourly space velocity (LHSV) of the reactant mixture was 0.5−2.5 h−1. The reaction was carried out at 115−130 °C and 1.0 MPa (gauge) in the liquid phase and was performed at 130−150 °C and ambient pressure in the gas phase. In the liquid phase, the feed was supplied in upflow to the reactor, and a pressure control valve was used to keep both the feed and the product in a liquid state while in downflow in the gas phase. The flow reactor was made of SUS 316 (500 mm long and 16 mm i. d.). There were three electric heaters around the reactor, while one was used for preheating the feed and the catalyst bed was heated by the other two. For example, the long-time test over 2,000 h in the liquid phase was done with 50 mL (33.0 g) of the catalyst (5 wt % Zr/SiO2) at a LHSV of 0.5 h−1 (25 mL/h = 20 g/h) and the molar ratio of 4. LHSV was defined by the following formula: LHSV = (volume of the mixture of IPA and crotonaldehyde at room temperature supplied in 1 h)/(volume of the catalyst) Product analysis was performed on an Agilent Technologies GC6850 gas chromatograph equipped with an Agilent 19091Z436E HP-1 methyl siloxane capillary column (60.0 m long, 0.25 mm i.d., 0.25 μm film) and a flame ionization detector (FID). A calibration plot for each product was constructed from commercially available standards. The reaction products were further confirmed by GC-MS. Crotonaldehyde conversion (mol %) and crotyl alcohol selectivity (mol %) were determined using the following formulas: Crotonaldehyde conversion = {1 − (remaining crotonaldehyde/crotonaldehyde charged in the batch reactor or fed to the tubular reactor)} × 100 crotyl alcohol selectivity = (crotyl alcohol yield/crotonaldehyde conversion) × 100 (crotyl alcohol yield = (crotyl alcohol produced/crotonaldehyde

et al. used a hydrotalcite-Mg/Al catalyst for MPV reduction to obtain 99% crotyl alcohol selectivity.16 ZrO2 has been reported as one of the best catalysts for MPV reduction. Minambres et al. have described 88% crotyl alcohol selectivity using ZrO2 catalysts modified with Al2O3, Ga2O3, and In2O3.17 Axpuac et al. also have reported liquid- and gasphase MPV reduction of crotonaldehyde with IPA using ZrO2 catalysts.2 Komanoya et al. revealed the importance of basic sites on the ZrO2 in MPV reduction of cyclohexanone with IPA.18 Our group has reported on the activity of Zr and Sn catalysts in MPV reduction,19−24 including the result of 63% crotyl alcohol yield using the ZrO2/SiO2 catalyst, and has concluded that Zr4+ on ZrO2 were active sites in the MPV reaction.25 Since ZrO2 catalysts show high activity and are inexpensive, application of ZrO2 is suitable for industrial production. In this study, IPA and crotonaldehyde were used as a hydrogen donor and an unsaturated aldehyde for MPV reduction, respectively (Figure 1). Secondary alcohols such as

Figure 1. Crotyl alcohol production via MPV reduction.

IPA are known to be better as hydrogen donors for MPV reduction than primary alcohols such as ethanol.25 A proposed six-membered ring transition state that involves IPA, the ZrO2 catalyst, and crotonaldehyde is shown in Figure 2.2,19 Since acetone is always produced as a coproduct when IPA is used, a system for regenerating IPA from acetone must be implemented in an industrial setting.

Figure 2. MPV reduction of crotonaldehyde with IPA via the proposed six-membered ring transition state.

Reports of high selectivity can be found in many papers, but little data on long-term durability has been published. In this study, our purpose was to determine the possibility of industrial production of unsaturated alcohols from corresponding unsaturated aldehydes using ZrO2/SiO2 catalysts to be prepared from commercially available and inexpensive precursors. We focused our efforts on the following: (1) determining a suitable ZrO2/SiO2 catalyst, (2) confirming catalyst durability, and (3) designing a process flow for industrial production. B

DOI: 10.1021/acs.iecr.7b03961 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research charged in the batch reactor or fed to the tubular reactor) × 100) 2.3. Catalyst Characterization. 2.3.1. XPS Measurement. X-ray photoelectron spectroscopy (XPS) spectra were measured with a PHI X-tool spectrometer (ULVAC-PHI, Inc.) at the Center for Supports to Research and Education Activities, Kobe University. A monochromated Al Kα ray (1486.6 eV) was used for excitation, and the X-ray tube was operated at 100 W. The Zr 3d binding energy was calibrated with the Si 2p peak of the silica support at 103.3 eV. Data processing was done using Multipak, which was developed by ULVAC-PHI. 2.3.2. Benzaldehyde-Ammonia Titration (BAT) Method.20,26 To determine the amount of active sites on the surface of the ZrO2, the benzaldehyde-ammonia titration (BAT) method was carried out by means of the pulse technique. A pulse experiment was performed using a conventional flow system. To perform benzaldehyde-ammonia titration, 20 mg of the catalysts was packed into a Pyrex glass tube of 4 mm i.d. and pretreated in an oven at 250 °C under flowing He for 1 h. Benzaldehyde (1 μL) was pulse-injected until no more benzaldehyde was adsorbed. The temperature was increased to 400 °C and held at 400 °C for 20 min, and then 2 mL of NH3 was injected to form benzonitrile. Pulses of ammonia were injected repeatedly until benzonitrile was no longer detected. The amount of benzonitrile obtained from the reaction between benzaldehyde and ammonia was estimated by a GCFID (model GC-8A, Shimadzu) equipped with a 2 m Silicon DC 550 column. The column temperature was 210 °C, and the line connecting the oven and GC was kept at 220 °C. The amount of benzoate anions adsorbed on the surface of the ZrO2 was calculated from the total amount of benzonitrile detected. 2.3.3. Thermogravimetric (TG) Analysis and Surface Area Measurement. The thermogravimetric- differential thermal analysis (TG-DTA) was conducted using Shimadzu DTG-60 under the following conditions: sample weight was ca. 20 mg, and the temperature was increased from the room temperature to 600 °C at 10 °C/min under air flow. The surface area, pore volume, and average pore diameter were determined by nitrogen adsorption (BELSORP mini, BEL Japan Inc.).

It was suggested that all of the catalysts had enough size for the reaction between crotonaldehyde and IPA, so the study of the relationship between the catalyst surface area and the reactivity would be important. Figure 3 illustrates the relationship between the surface area of 5 wt % Zr/SiO2 catalysts and catalytic performance for MPV

Figure 3. Effect of surface area on catalytic performance in MVP reduction of crotonaldehyde (batch reactor). Reaction conditions: temp 130 °C × 3 h, pressure 0.5 MPa (gauge), feed: IPA/ crotonaldehyde = 2 mol/mol.

reduction of crotonaldehyde. MPV reduction of crotonaldehyde to crotyl alcohol in the liquid phase was performed in autoclave batch reactors over 5 wt % Zr/SiO2 catalysts with different SiO2 supports and calcined at 500 °C for 5 h. It is seemed that MPV reduction of crotonaldehyde over pure silica does not proceed at all.25 The crotonaldehyde conversion increased as the catalyst surface area increased. The catalysts with surface areas higher than 270 m2/g (S-1 and S-2 in Table 1) showed the highest crotonaldehyde conversion (approximately 80%). This suggests that the dispersion of the Zr species is probably improved by the catalysts with high surface areas. The products were crotyl alcohol and heavy compounds with carbon numbers higher than four; crotyl alcohol selectivity stayed around 70%, regardless of the catalyst surface area. It was suggested that the SiO2 support itself had almost no effect on crotyl alcohol selectivity. In this study, the heavy compounds were obtained as byproducts. Besides crotyl alcohol, hardly any compounds with four carbons, e.g. 1-butanol, 1-butanal, 3-buten-2-ol, and 3buten-1-ol, were detected by GC analysis. The results of GCMS analysis, used to identify the byproducts of the reaction, showed that C8-unsaturated aldehydes were detected in the main byproducts. Its chemical formula was C8H10O. Ordomsky et al. have reported that acetaldehyde condensation proceeds over a Lewis acid-weak base from the Zr−O and Brønsted acid from the silanol groups in ZrO2/SiO2 catalysts.27 This suggests that the C8-unsaturated aldehydes were produced via aldol condensation of crotonaldehyde. The silica support labeled “S-1″ was ultimately selected as the most suitable support for MPV reduction of crotonaldehyde. 3.1.2. Effect of Zirconium Loading on Catalytic Performance. Figure 4 shows the effect of Zr loading on catalytic activity for MPV reduction of crotonaldehyde in autoclave batch reactors. MPV reduction in the liquid phase was performed over 1.0−9.5 wt % Zr/SiO2 (S-1) catalysts calcined at 500 °C. Crotonaldehyde conversion increased as the amount of Zr on the ZrO2/SiO2 catalysts increased up to 5 wt % and decreased

3. RESULTS AND DISCUSSION 3.1. MPV Reduction of Crotonaldehyde Using Batch Reactors. 3.1.1. Effect of Surface Area of the Zr/SiO2 Catalyst on Catalytic Performance. For screening ZrO2 catalysts, catalytic activity tests were performed using autoclave batch reactors. Their surface areas, pore volumes, and average pore diameters are summarized in Table 1. The surface area decreased from 341 m2/g of S-1 to 75 m2/g of S-4, while both the pore volume and the average pore diameter increased. In all the catalysts in Table 1, the pore volumes and the average pore diameters were more than 0.56 mL/g and 6.6 nm, respectively. Table 1. Typical Properties of 5 wt % Zr/SiO2 Catalystsa with Different SiO2 Supports entry

silica

surface area, m2/g

pore volume, mL/g

av pore diam, nm

S-l S-2 S-3 S-4

Q6 Q10 Q30 Q50

341 273 116 75

0.56 0.88 0.96 0.98

6.6 13 33 52

a

Calcined at 500 °C for 5 h. C

DOI: 10.1021/acs.iecr.7b03961 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 4. Effect of Zr loading on catalytic performance in MVP reduction of crotonaldehyde (batch reactor). Reaction conditions: temp 130 °C × 3 h, pressure 0.5 MPa (gauge), feed: IPA/ crotonaldehyde = 2 mol/mol.

once Zr loading exceeded 5 wt %. This suggested that the Zr species would agglomerate at Zr loadings higher than 5 wt %. Crotyl alcohol selectivity was approximately 70%, regardless of the Zr concentration. These results indicated that a Zr loading of 5 wt % was optimal for MPV reduction of crotonaldehyde. 3.1.3. Effect of Calcination Temperature on Catalytic Performance. The effect that the calcination temperature of Zr/SiO2 catalysts has on crotonaldehyde conversion and crotyl alcohol selectivity is shown in Figure 5. Three 5 wt % catalysts

Figure 6. Effect of calcination temperature on the XPS spectra of Zr supported on S-1.

assigned to Zr 3d5/2 and Zr 3d3/2 were shifted more toward the higher binding energy side than those of the bulk ZrO2 sample. The higher binding energy of Zr 3d suggested a higher positive charge on the Zr in the Si−O−Zr bonds, due to the fact that Zr has a lower electronegativity than Si.29,30 Sushkevich et al. have reported that Zr−O−Si bonds were formed as Zr was incorporated into a zeolite BEA framework, which results in Zr 3d having a higher bonding energy over Zr-BEA.30 Meanwhile, Sancho et al. have reported that the Zr 3d binding energy of Zr-doped mesoporous silica was higher than that of ZrO2, supporting the idea that Zr had been incorporated into the silica framework.32 The shift of the Zr 3d peaks toward the higher binding energy side in Figure 6 suggests the presence of Si−O−Zr bonds. The Zr 3d binding energy, estimated by XPS analysis, and the molar ratio of benzonitrile (BN) to Zr (BN/Zr), measured by way of benzaldehyde-ammonia titration (BAT),26 are summarized in Table 2. With the BAT method, the amount

Figure 5. Effect of calcination temperature on catalytic performance in MVP reduction of crotonaldehyde (batch reactor). Reaction conditions: temp 130 °C × 3 h, pressure 0.5 MPa (gauge), feed: IPA/crotonaldehyde = 2 mol/mol.

Table 2. Effect of Calcination Temperature of Zr Supported on S-1 XPS analysis a

supported on the S-1 silica were calcined at temperatures between 220 and 500 °C for 5 h. Another catalyst of the same type was calcined at 650 °C for 2 h. The Zr precursor, zirconyl nitrate dihydrate ZrO(NO3)2-2H2O, is known to decompose over 200 °C.28 As the calcination temperature increased from 220 to 500 °C, crotonaldehyde conversion decreased slightly from 87% to 81%. At calcination temperatures higher than 500 °C, crotonaldehyde conversion fell sharply. The Zr 3d X-ray photoelectron spectroscopy (XPS) spectra for Zr/SiO2 catalysts calcined at various temperatures are shown in Figure 6. Two peaks at 181.8 and 184.2 eV were observed for the ZrO2 sample and assigned to Zr 3d5/2 and Zr 3d3/2, respectively.29−31 For all the Zr/SiO2 catalysts, the peaks

Zr/S-1 Zr/S-1 Zr/S-1 Zr/S-1 ZrO2 a

650 500 350 220

°C °C °C °C

× × × ×

2 5 5 5

h h h h

BAT titration a

3d3/2, eV

3d5/2, eV

benzonitrile/Zr, mol/mol

185.0 185.0 185.2 185.4 184.2

182.6 182.7 182.8 183.2 181.8

0.26 0.40 0.37 0.36

Binding energy of Zr 3d.

of basic sites for the metal oxides on the surface of a catalyst is determined by measuring the amount of benzonitrile generated by the reaction between benzaldehyde absorbed on the basic sites and introduced ammonia. The amount of benzonitrile correlates to the amount of metal oxides on the surface of the catalyst.20 D

DOI: 10.1021/acs.iecr.7b03961 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research MPV reduction proceeds through the proton drawing by the basic site on ZrO2, subsequently the formation of alkoxides on Lewis acid sites (Zr4+, see Figure 2).18,19 Thus, our group insists that the amount of basic sites (−O− on ZrO2) will influence MPV reduction. The amount of basic sites on ZrO2 should be the same as that of the corresponding Lewis acid sites (Zr4+), working as active sites in this reaction. The BAT method has been known as an excellent way to measure the amount of basic sites on basic oxides like Al2O3, TiO2, SnO2, and ZrO2, while almost no benzaldehyde is adsorbed on acidic oxides like SiO2, V2O5, MoO3, and WO3.26,33−35 The molar ratio of benzonitrile to Zr in Table 2 is the value obtained by dividing the amount of benzonitrile determined by BAT by the amount of ZrO2 in the catalyst. The BN/Zr molar ratio indicates the degree of dispersion of ZrO2 in the catalyst. The BN/Zr ratios of the catalysts calcined at temperatures 500 °C or below were all around 0.40. It should be said that BN/Zr may have been underestimated for the catalysts calcined at temperatures below 400 °C, because the reaction between absorbed benzaldehyde and ammonia is performed at 400 °C in the BAT method. The actual dispersity of ZrO2 in the catalysts calcined at temperatures under 350 °C may have been higher than indicated by the BN/Zr estimated by the BAT method. The BN/Zr ratios estimated by the BAT method on the Zr/ SiO2 catalysts calcined at 650 and 500 °C were 0.26 and 0.40, respectively. There results suggest that calcination temperatures higher than 500 °C promote aggregation of the Zr species in Zr/SiO2 catalysts, resulting in a decrease in catalytic activity for MPV reduction. When the calcination temperature of the catalysts was increased from 220 to 500 °C, the binding energy of the Zr 3d3/2 peak in Table 2 decreased from 185.4 to 185.0 eV, and that of the Zr 3d5/2 peak decreased from 183.2 to 182.7 eV. This indicated an increase in Si−O−Zr bonds at the lower calcination temperature, suggesting improved ZrO2 dispersity in the catalyst and higher crotonaldehyde conversion. Meanwhile, crotyl alcohol selectivity (approximately 70%) did not vary significantly at calcination temperatures between 220 and 650 °C. This indicated that ZrO2 dispersity did not affect the selectivity for crotyl alcohol. These results suggest that the suitable calcination temperature for ZrO 2 /SiO 2 catalysts for MPV reduction of crotonaldehyde is between 220 and 500 °C. We decided on 500 °C as the calcination temperature for the catalyst for the durability test, because we anticipated that high temperatures would be required for regeneration of the catalyst during the test. 3.1.4. Effect of the IPA/Crotonaldehyde Ratio on Catalytic Performance. The relationship between the molar ratio of IPA to crotonaldehyde (IPA/crotonaldehyde) and catalytic performance for MVP reduction of crotonaldehyde in the liquid phase is shown in Figure 7. Autoclave batch reactions in the liquid phase were performed over a 5 wt % Zr/SiO2 (S-1) catalyst calcined at 500 °C. The molar mixing ratio (IPA/crotonaldehyde) was varied from between 2 and 10. As IPA/crotonaldehyde increased from 2 to 10, crotonaldehyde conversion decreased slightly from 90% to 80%. Meanwhile, crotyl alcohol selectivity increased from 73% to 90%. Figure 8 shows the effect of the ratio (IPA/ Crotonaldehyde) on the composition of the reaction mixture. Increasing the IPA led to lower yields of heavy compounds, suggesting that the condensation of crotonaldehyde had been

Figure 7. Effect of the IPA/crotonaldehyde ratio on catalytic performance in MVP reduction of crotonaldehyde (batch reactor). Reaction conditions: temp 130 °C × 3 h, pressure 0.5 MPa (gauge).

Figure 8. Effect of the IPA/crotonaldehyde ratio on the reaction mixture composition (batch reactor).

controlled and the reaction between IPA and crotonaldehyde had been promoted. These results show that crotyl alcohol selectivity can be controlled by adjusting the ratio of IPA in the feed. Zhu et al. have reported MPV reduction of croton aldehyde over zirconium beta and the result of 100% crotyl alcohol selectivity.13 Their experiments were done at a much higher molar ratio of IPA to aldehyde (about 64), leading to a higher selectivity to the alcohol. 3.2. MPV Reduction of Crotonaldehyde Using Tubular Flow Reactors. 3.2.1. Durability Test of the Catalyst in Liquid Phase. A few reports have been published on catalyst durability in MPV reduction with continuous fixed bed tubular flow reactors. MPV reduction of cyclohexanone (bp 155 °C) with the supported zirconium-based continuous-flow monolithic silica microreactors has been reported.36 The reaction temperature was 95 °C, and 2-butanol (bp 99 °C) was used as a hydrogen donor, suggesting this reaction would be under a liquid-phase reaction. As a result of the catalyst durability test, they showed 48 h of stable reactivity including 8 catalytic cycles with ethanol purge. Battilocchio et al. have described the results of MPV reduction on a variety of aldehydes and ketones with flow reactors.37 For the industrial production, a continuous flow reactor process is desirable. We examined the durability of the catalyst screened by the results of the batch reactors. In this study, tests of catalyst durability in MPV reduction of crotonaldehyde were carried out in both the liquid and gas phase. The results of a long-term catalyst durability test in the liquid phase are shown in Figure 9. The reaction was performed using a fixed bed tubular flow reactor over a 5 wt % Zr/SiO2 (S-1) catalyst (50 mL) calcined at 500 °C. A mixture of IPA and crotonaldehyde E

DOI: 10.1021/acs.iecr.7b03961 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 10. Durability test in the liquid phase, effect of the IPA/ crotonaldehyde ratio on catalytic performance (tubular flow reactor). Reaction conditions: A: temp 120 °C, LHSV = 2.5 h−1, feed: IPA/ crotonaldehyde = 18 mol/mol; B: temp 120 °C, LHSV = 2.0 h−1, feed: IPA/crotonaldehyde = 14 mol/mol; C: temp 120 °C, LHSV = 1.5 h−1, feed: IPA/crotonaldehyde = 14 mol/mol; D: temp 115 °C, LHSV = 1.5 h−1, feed: IPA/crotonaldehyde = 14 mol/mol; E: temp 125 °C, LHSV = 1.5 h−1, feed: IPA/crotonaldehyde = 14 mol/mol.

Figure 9. Durability test in the liquid phase (tubular flow reactor). Reaction conditions: temp 130 °C, pressure 1.0 MPa (gauge), LHSV = 0.5 h−1, feed: IPA/crotonaldehyde = 4 mol/mol.

(IPA/crotonaldehyde = 4 mol/mol) was fed at a LHSV of 0.5 h−1. The reaction temperature was 130 °C, and the pressure was set at 1.0 MPa (gauge) to keep the reaction in the liquid phase. For the first 2,200 h of the test, catalytic activity was very stable and showed no deterioration. The material balance, calculated from the weight of the feed and the products, stayed at nearly 100%. During this period, crotonaldehyde conversion and crotyl alcohol selectivity stayed at 97% and 80%, respectively. To the best of our knowledge, there is no report on the catalyst durability over 2,200 h in MPV reduction with conventional tubular flow reactors. The crotonaldehyde conversion was higher than that obtained in the reaction using the batch reactor as shown in Figure 7 (83% at IPA/crotonaldehyde = 4), because the contact time between the catalyst and the substrate in the flow reactor was longer than that in the batch reactor. GC analysis of the products of the liquid phase reaction revealed the presence of several unidentified heavy byproducts. Meanwhile, no products having four carbons were detected except for crotyl alcohol. The GC-MS results showed that the main heavy byproducts were C8-unsaturated aldehydes, thought to have been formed via the condensation of crotonaldehyde as described in 3.1.1. In the industrial production, it is important to improve crotyl alcohol selectivity because the raw material, crotonaldehyde, is expensive. To improve crotyl alcohol selectivity, it is necessary to limit the aldehyde condensation. It was shown that crotonaldehyde condensation could be effectively controlled by increasing the ratio of IPA to crotonaldehyde in the feed, as described in 3.1.4. Figure 10 shows the results of a durability test in which the feed contained higher proportions of IPA. The reaction was performed using a fixed bed, tubular flow reactor over a 5 wt % Zr/SiO2 (S-1) catalyst (15 mL) calcined at 500 °C. The reaction temperature was between 115 and 125 °C, and the pressure was 1 MPa, so as to keep the products in the liquid phase. The molar ratios of the feed (IPA/crotonaldehyde) were 14 and 18. The LHSV of the mixed feed was between 1.5 and 2.5 h−1. When the ratio was 18 (A in Figure 10), crotonaldehyde conversion and crotyl alcohol selectivity were 96% and 95%, respectively. Moreover, these levels were maintained over 300 h of test time with no deterioration. It was thus confirmed that increasing the amount of IPA improved selectivity in a longterm durability test involving a continuous reaction in a tubular flow reactor. The disadvantage of this method of using larger amounts of IPA is the cost. At 300 h, the mixing ratio of the feed was

decreased from 18 to 14, without replacing the catalyst. With a ratio of 14 (B in Figure 10), conversion and selectivity stayed around 96% and 94%, respectively. Crotyl alcohol selectivity decreased slightly due to the lower concentration of IPA in the feed. At 550 h, the LHSV of the mixed feed was reduced from 2.0 to 1.5 (C in Figure 10). Conversion and selectivity were 98% and 94%, respectively. Reducing the LHSV did not affect selectivity in this durability test, whereas conversion increased from 96% to 98%. With the feed mixing ratio of 14, it was found that 94% crotyl alcohol selectivity (B, C, and E in Figure 10) could be achieved with reaction temperatures of 120−125 °C, while crotonaldehyde conversion increased with increasing reaction temperature (C, D, and E in Figure 10) and decreased with increasing LHSV (B and C). It was found that increasing the ratio of IPA not only improved the selectivity but also maintained the catalyst durability. The results of this long-term durability test in a continuous reaction using a fixed bed tubular flow reactor showed that high crotonaldehyde conversion, high crotyl alcohol selectivity, and long-term durability could all be achieved simultaneously. 3.2.2. Durability Test of a Catalyst in Gas Phase. Since an ambient pressure reaction is preferred to a high pressure one in industrial production, MPV reduction of crotonaldehyde at the ambient pressure in gas phase was studied. In Figure 11, the performance of a ZrO2/SiO2 catalyst in MPV reduction of crotonaldehyde using a fixed bed, tubular flow reactor is shown as a function of time on stream. The reaction was performed over a 5 wt % Zr/SiO2 (S-1) catalyst (50 mL) calcined at 500 °C. Since the boiling points of IPA and crotonaldehyde are 82 and 104 °C, respectively, the reaction was carried out at temperatures between 130 and 150 °C, so the reactants and products would remain in the gas phase under ambient pressure. With the exception of the reaction pressure, the test conditions were the same as those described in 3.2.1. At the start, both crotonaldehyde conversion and crotyl alcohol selectivity were over 90%. The conversion rate dropped from 96% to 34% in just 18 h. After that, although the reaction temperature was increased to 140 °C at 18 h and 150 °C at 24 h, the conversion rate did not recover. Meanwhile, selectivity had decreased from an initial level of 90% to 75% after 44 h. The catalyst, which started out white in color, had turned reddish-orange after the reaction. This meant that byproducts had accumulated on the catalyst and suggested that the deterioration in catalytic activity was caused by a decrease in F

DOI: 10.1021/acs.iecr.7b03961 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 13. Process flow scheme that includes IPA regeneration.

Figure 11. Durability test in the gas phase (tubular flow reactor). Reaction conditions: temp 130−150 °C, LHSV = 0.5 h−1, feed: IPA/ crotonaldehyde = 4 mol/mol.

mixed with crotonaldehyde. The residues in acetone separation column A are transferred to distillation column B, and unreacted (excess) IPA and crotonaldehyde are removed. These are recycled to the main reactor without any treatment. In distillation column C, heavy byproducts are removed from the residues, and high-purity crotyl alcohol is generated. Meanwhile, concerning the direct hydrogenation of crotonaldehyde, many results of high conversion and selectivity have been reported.3−6,41,42 Most of studies, however, were performed using batch reactors with noble metal catalysts, and the long-term catalyst durability using flow reactors has not been known. These facts suggested that the MPV reduction system including IPA regeneration with hydrogen will be much more reliable than the direct hydrogenation of crotonaldehyde, and we believe this process will enable industrial production of crotyl alcohol.

the number of active sites, due to their being covered by the byproducts. In Figure 12, the result of TG-DTA analysis of the reddishorange catalyst after the reaction is shown. A weight loss of

4. CONCLUSIONS In this study, we investigated the possibility of industrial production of crotyl alcohol via MPV reduction. The catalyst, ZrO2/SiO2, was prepared from commercially available carriers and precursors and optimized through activity tests using batch reactors and catalyst characterizations. Long-term, stable production of crotyl alcohol was achieved in a liquid-phase reaction using tubular flow reactors. By properly controlling the IPA/crotonaldehyde ratio, continuous and stable production of crotyl alcohol from crotonaldehyde was shown to be possible in a liquid-phase reaction with high conversion and selectivity and without undue shortening of catalyst life. A continuous process flow design that includes IPA regeneration was proposed for industrial production.

Figure 12. TG-DTA of the reddish-orange catalyst after the reaction.

16.1% was observed at a temperature range from 150 to 600 °C, and the two exothermic peaks between 300 and 500 °C were possibly caused by the combustion of heavy byproducts on the catalyst. There was a big difference in the durability of the catalysts used in the gas-phase and liquid-phase reactions. The catalyst used in the liquid-phase reaction stayed white, and byproducts were not found to have accumulated on the catalyst. Since the byproducts on the catalyst were easily soluble in liquid IPA, it was surmised that, in the liquid-phase reaction, IPA in the feed had dissolved the reaction byproducts to effectively purge the catalyst and prevent the byproducts from accumulating on the surface. 3.3. New Process Flow Design in Liquid Phase. On the basis of the results of this study, we propose a new process flow in liquid phase for industrial production. Our process flow, which includes an IPA regeneration system, is shown in Figure 13. In this scheme, crotonaldehyde is mixed with IPA, and the mixture is preheated and supplied to the catalyst-packed main reactor. After the MPV reduction, acetone produced from the IPA used as a hydrogen donor is removed in distillation column A. This acetone is supplied to a hydrogenation reactor to regenerate IPA, and hydrogen is needed for the IPA regeneration. An IPA-acetone-hydrogen chemical heat pump system is well-known and has been almost established.38−40 It is likely that we will be able to convert the system into our setting. The regenerated IPA is used as a hydrogen donor again to be

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Atsushi Segawa: 0000-0001-9674-5338 Notes

The authors declare no competing financial interest.

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REFERENCES

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