energies
Article Liquid-Phase Hydrogenation of Maleic Acid over Pd/Al2O3 Catalysts Prepared via Deposition–Precipitation Method
Mi Yeon Byun 1,2 , Ji Sun Kim 3 , Jae Ho Baek 1, Dae-Won Park 2 and Man Sig Lee 1,* 1 Ulsan Regional Division, Korea Institute of Industrial Technology (KITECH), Ulsan 44413, Korea; [email protected] (M.Y.B.); [email protected] (J.H.B.) 2 Department of Polymer Science and Chemical Engineering, Pusan National University, Busan 46241, Korea; [email protected] 3 Department of Chemical & Biological Engineering, The University of British Columbia, Vancouver, BC V6T 1Z4, Canada; [email protected] * Correspondence: [email protected]; Tel.: +82-52-980-6630; Fax: +82-52-980-6639
Received: 14 November 2018; Accepted: 16 January 2019; Published: 17 January 2019
Abstract: Succinic acid (SA) is a valuable raw material obtained by hydrogenation of maleic acid (MA). The product selectivity of this reaction is highly dependent on the reaction conditions. This study therefore investigated the effect of the reaction temperature, hydrogen pressure, and reaction time on the liquid-phase hydrogenation of MA by a Pd/Al2O3 catalyst. Complete conversion of MA and ◦ 100% selectivity for SA were achieved at a temperature of 90 C, H2 pressure of 5 bar, and reaction time of 90 min. Fumaric acid (FA) was formed as an intermediate material by hydrogenation of MA under nonoptimal conditions. The impact of the percentage of Pd dispersion and phase of the Al2O3 support (γ, θ + α, and α) was also examined. The Pd/Al2O3 catalyst with 29.8% dispersion of Pd and γ phase of Al2O3 exhibited the best catalytic performance. Thus, catalytic activity depends not only on the amount of Pd dispersion but also on the physicochemical properties of Al2O3.
Keywords: hydrogenation; succinic acid; supported catalyst; Pd catalyst
1. Introduction The growth of the petrochemical industry has brought about environmental problems such as industrial waste, pollution, and global warming. Therefore, many researchers have been investigating sustainable and renewable resources. In this regard, succinic acid (SA) has attracted attention as an eco-friendly raw material for the production of biodegradable plastics and biosolvents. SA is widely used as an intermediate material in the production of high value products such as γ-butyrolactone, 1,4-butanediol, and tetrahydrofuran [1,2]. It is usually obtained by hydrogenation of maleic anhydride (MAN), although the product of this reaction varies depending on the reaction conditions. Several researches have investigated various catalysts for this reaction, such as Ru/C, Ni/HY–Al2O3, Pd/C, Pd/SiO2, Ni/TiO2 and Pd/Al2O3 [3–9]. The reaction pathways of hydrogenation of maleic acid (MA) are shown in Figure1. Kim et al. reported a yield of 99.97% SA using Pd/C as the catalyst and ◦ conducting the reaction under 1.0 MPa of H2 at 90 C for 150 min [3]. Torres et al. reported achieving 100% selectivity for succinic anhydride through the reaction catalyzed by a mesoporous Ni/TiO2 catalyst at low temperature [8]. Yuan et al. reported achieving MA conversion of 98% and succinic anhydride selectivity of 99% using Pd/Al2O3 catalyst under 1.0 MPa of H2 pressure and 1,4-dioxane as solvent [5]. Among the various catalysts, the Pd/Al2O3 catalyst has an advantage for the chemical industry due to the high thermal stability and high dispersion of Pd [10,11]. However, few studies have been performed on the optimization of reaction conditions for the hydrogenation of maleic acid (MA)
Energies 2019, 12, 284; doi:10.3390/en12020284 www.mdpi.com/journal/energies Energies 2018, 11, x FOR PEER REVIEW 2 of 8 Energies 2019, 12, 284 2 of 8 on the optimization of reaction conditions for the hydrogenation of maleic acid (MA) to SA over a to SA over a Pd/Al2O3 catalyst. In our previous study, we reported that the particle size distribution Pd/Al2O3 catalyst. In our previous study, we reported that the particle size distribution of Pd is of Pd is influenced by the physicochemical properties of Al O (specific surface area and surface influenced by the physicochemical properties of Al2O3 (specific 2surface3 area and surface functional groups)functional and groups)catalyst preparation and catalyst conditions preparation (pH, conditions solution temperature, (pH, solution and temperature, reduction agent) and [12,13]. reduction Herein,agent) we [12 ,investigated13]. Herein, the we effect investigated of reaction the temperature effect of reaction (T), H2 pressure temperature (PH2), and (T), reaction H2 pressure time ( tP) H2), onand the reaction liquid-phase time ( thydrogenation) on the liquid-phase of MA over hydrogenation a Pd/Al2O3 ofcatalyst. MA over The a catalytic Pd/Al2O activities3 catalyst. of ThePd/Al catalytic2O3 withactivities varying of percentage Pd/Al2O3 ofwith Pd dispersion varying percentage and Al2O3 phase of Pd ( dispersionγ, θ + α, and and α) were Al2O also3 phase compared. (γ, θ +Finally,α, and α) thewere reusability also compared. of the Pd/Al Finally,2O3 catalyst the reusability was assessed. of the Pd/Al2O3 catalyst was assessed.
Figure 1. Reaction pathways in hydrogenation of maleic acid in water. Figure 1. Reaction pathways in hydrogenation of maleic acid in water. 2. Experiment 2. Experiment 2.1. Catalyst Preparation 2.1. Catalyst Preparation The catalyst support, Al2O3 (≥99%, Alfa Aesar, γ phase, average particle size: 20 nm), was dried ◦ ◦ at 105TheC catalyst for 4 h support, and calcined Al2O3 at(≥99%, various Alfa temperatures Aesar, γ phase, (900, average 1100, andparticle 1150 size:C) 20 for nm), 4 h. was The dried Pd/Al at2 O3 105catalysts, °C for containing4 h and calcined 5 wt % at Pd, various were preparedtemperatures by the(900, deposition–precipitation 1100, and 1150 °C) for method.4 h. The ThePd/Al detailed2O3 catalysts,procedure containing has been 5described wt% Pd, were in our prepared previous by research the deposition–precipitation [12]. Briefly, Al2O3 wasmethod. dispersed The detailed in the Pd ◦ procedureprecursor has solution been atdescribed 60 C, and in our the pHprevious was adjustedresearch using[12]. Briefly, 0.25 M Al NaOH2O3 was solution. dispersed Reduction in the Pd of the precursorcatalyst was solution carried at out60 °C, in theand liquid the pH phase was usingadjusted formalin using solution0.25M NaOH (10 wt solution. %, Sigma Reduction Aldrich, of St. the Louis, catalystMO, USA). was Thecarried prepared out in Pd/Althe liquid2O3 catalystsphase using are denotedformalin assolution Pd/Al 2(10O3 wt%,(X)_pH SigmaY, where Aldrich).X is theThe heat preparedtreatment Pd/Al temperature2O3 catalysts of the are Al 2denotedO3 support, as Pd/Al and Y2Ois3 the(X)_pH pH.Y The, where physicochemical X is the heat properties treatment of the temperaturecatalysts are of summarized the Al2O3 support, in Table and1. Y is the pH. The physicochemical properties of the catalysts are summarized in Table 1.
Table 1. Summary of physicochemical properties of Pd/Al2O3 catalysts [12,13]. Energies 2019, 12, 284 3 of 8
Table 1. Summary of physicochemical properties of Pd/Al2O3 catalysts [12,13].
Pd/Al O Pd/Al O Pd/Al O Pd/Al O Pd/Al O Catalysts 2 3 2 3 2 3 2 3 2 3 (105)_pH 7.5 (900)_pH 7.5 (900)_pH 11.5 (1100)_pH 7.5 (1150)_pH 7.5 1 Al2O3 phase γ γ γ θ + α α 2 Al2O3 surface area 195 146 146 54 6 2 Al2O3 Pore volume 0.82 0.62 0.62 0.28 0.007 3 Al2O3 Acidity (mmol/g) 0.47 0.37 0.37 0.22 0.07 Pd dispersion (%) 4 20.6 29.8 13.1 11.0 2.9 1 2 3 X-ray diffraction, Brunauer–Emmett–Teller analysis of N2 adsorption–desorption, NH3 temperature programmed desorption, 4 CO chemisorption.
2.2. Hydrogenation of Maleic Acid Liquid-phase hydrogenation of maleic acid was conducted in a 100 mL stainless steel autoclave. MA (0.29 mol), 46 g distilled water, and 0.1 g Pd/Al2O3 were placed in the autoclave. The reactor was purged with nitrogen three times to remove air, and hydrogen was then used to purge out nitrogen. The sealed autoclave was pressurized to the desired pressure and heated to the desired temperature, and the reaction mixture was stirred at 700 rpm. The catalytic reaction was carried out for 15–90 min. The reaction products were analyzed by high-performance liquid chromatography (HPLC, Shimadzu Co. Model Prominence) equipped with a refractive index detector and Agilent Hi-Plex −1 H(7.7 mm × 300 mm × 8 µm). The mobile phase was 5 mM H2SO4 with a flow rate of 6 mL min . The MA, SA, fumaric acid (FA), and malic acid (MLA) standards (Sigma Aldrich) were analytical grade and used without purification. The reaction was conducted at varying temperature, hydrogen pressure, and reaction time. Conversion and selectivity were calculated as follows:
Initial mole of MA − final moleof MA Conversion (%) = × 100 (1) Initial mole of MA
mole of desired product Selectivity (%) = × 100 (2) Initial mole of MA − final moleof MA
3. Results and Discussion
3.1. Effect of Reaction Temperature
The effect of reaction temperature on MA hydrogenation over the Pd/Al2O3 (900)_pH7.5 catalyst was investigated within the range of 30–110 ◦C (Figure2). As the reaction temperature increased, the conversion of MA increased, while the selectivity for SA decreased from 100% to 85%. At temperatures above 90 ◦C, the selectivity for FA and MLA slightly increased. It has been reported that, at high temperature, MA is unstable and can isomerize to the more stable FA [14]. On the other hand, the isomer FA undergoes hydration to form MLA at high temperature. Thus, 90 ◦C is the optimal temperature for high conversion of MA and selectivity for SA. Energies 2018, 11, x FOR PEER REVIEW 4 of 8
EnergiesEnergies 20182019, 11, 12, x, 284FOR PEER REVIEW 4 4of of 8 8
Figure 2. Effect of reaction temperature on maleic acid (MA) hydrogenation over Pd/Al2O3 (900)_pH7.5Figure 2. Effect at P ofH2 reaction = 3 bar temperatureand t = 60 min. on maleic acid (MA) hydrogenation over Pd/Al2O3 (900)_pH7.5 Figure 2. Effect of reaction temperature on maleic acid (MA) hydrogenation over Pd/Al2O3 at PH2 = 3 bar and t = 60 min. (900)_pH7.5 at PH2 = 3 bar and t = 60 min. 3.2. Effect of H2 Pressure 3.2. Effect of H2 Pressure 3.2. Effect of H2 Pressure TheThe effect effect of of H H2 2pressurepressure on on MA MA hydrogenation over thethe Pd/AlPd/Al2OO33 (900)_pH7.5(900)_pH7.5 catalyst was investigated within the range of 1–15 bar (Figure 3). MA conversion and SA selectivity increased from investigatedThe effect within of H the2 pressure range of on 1–15 MA bar hydrogenation (Figure3). MA conversionover the Pd/Al and SA2O3 selectivity(900)_pH7.5 increased catalyst from was 30% to 100% and 53% to 100%, respectively, as the H2 pressure increased from 1 to 15 bar. Within the investigated30% to 100% within and 53% the torange 100%, of respectively,1–15 bar (Figure as the 3). H MA2 pressure conversion increased and SA from selectivity 1 to 15 bar. increased Within from the range of 1–5 bar, the selectivity for FA gradually decreased from 40% to 1.6%. As demonstrated above, 30%range to of100% 1–5 and bar, the53% selectivity to 100%, respectively, for FA gradually as the decreased H2 pressure from increased 40% to 1.6%. from As 1 demonstratedto 15 bar. Within above, the FA can be produced by isomerization of MA at high temperature. However, FA is predominantly rangeFA can of 1–5 be produced bar, the selectivity by isomerization for FA gradually of MA at decreased high temperature. from 40% to However, 1.6%. As FA demonstrated is predominantly above, produced at low H2 pressure, indicating that FA is an intermediate in the conversion of MA to SA. Kim FAproduced can be produced at low H2 bypressure, isomerization indicating of MA that at FA high is antemperature. intermediate However, in the conversion FA is predominantly of MA to et al. reported that the formation of FA is determined by the relative reaction rates of hydrogenation producedSA. Kim at et low al. H reported2 pressure, that indicating the formation that FA of is FA an isinterm determinedediate in bythe the conversion relative reactionof MA to ratesSA. Kim of and protonation [3]. Protonation occurs faster than hydrogenation at H2 pressures lower than 5 bar but ethydrogenation al. reported that and the protonation formation [of3]. FA Protonation is determin occursed by fasterthe relative than hydrogenationreaction rates of at hydrogenation H2 pressures slower at higher pressures. andlower protonation than 5 bar [3]. but Protonation slower at higher occurs pressures. faster than hydrogenation at H2 pressures lower than 5 bar but slower at higher pressures.
◦ Figure 3. Effect of H2 pressure on MA hydrogenation over Pd/Al2O3 (900)_pH7.5 at T = 90 C and Figure 3. Effect of H2 pressure on MA hydrogenation over Pd/Al2O3 (900)_pH7.5 at T = 90°C t = 60 min. and t = 60 min. Figure 3. Effect of H2 pressure on MA hydrogenation over Pd/Al2O3 (900)_pH7.5 at T = 90°C and t = 60 min. EnergiesEnergies 20182019, 11, 12, x, FOR 284 PEER REVIEW 5 of5 of8 8
3.3.3.3. Effect Effect of ofReaction Reaction Time Time The effect of reaction time on MA hydrogenation over the Pd/Al O (900)_pH7.5 catalyst was The effect of reaction time on MA hydrogenation over the Pd/Al22O3 (900)_pH7.5 catalyst was investigated within the range of 15–90 min at a fixed reaction temperature of 90 ◦C and H pressure of investigated within the range of 15–90 min at a fixed reaction temperature of 90 °C and H22 pressure of 5 5bar bar (Figure (Figure 4).4). The The conversion conversion of of MA MA dramatically dramatically increased increased up to 80%80% uponupon increasingincreasing thethereaction reaction timetime from from 15 15 to to 30 30 min. min. It It subsequently subsequently changed changed more more gradually gradually as as the reaction time waswas prolongedprolonged fromfrom 30 30 to to 90 90 min. min. The The selectivity selectivity for for FA FA initially initially increased increased between between 15 15 and and 30 30 min and then decreased asas the the reaction reaction time time increased increased from from 30 30 to to 90 90 min. min. Thus, Thus, 100% 100% MA MA conversion and SA selectivity werewere achieved at a temperature of 90 ◦C, H pressure of 5 bar, and reaction time of 90 min. achieved at a temperature of 90 °C, H2 pressure2 of 5 bar, and reaction time of 90 min.
◦ Figure 4. Effect of reaction time on MA hydrogenation over Pd/Al2O3 (900)_pH7.5 at T = 90 C and Figure 4. Effect of reaction time on MA hydrogenation over Pd/Al2O3 (900)_pH7.5 at T = 90°C PH2 = 5 bar. and PH2 = 5 bar. 3.4. Effect of Al2O3 Support Properties and Pd Dispersion 3.4. Effect of Al2O3 Support Properties and Pd Dispersion The effect of the properties of the Al2O3 support and dispersion of Pd on the catalytic activity for liquid-phase hydrogenation of MA were investigated under the following conditions: T = 90 ◦C, The effect of the properties of the Al2O3 support and dispersion of Pd on the catalytic activity for PH2 = 5 bar, t = 60 min, and stirring rate = 700 rpm. The physicochemical properties of the different liquid-phase hydrogenation of MA were investigated under the following conditions: T = 90 °C, PH2 = 5 Pd/Al2O3 catalysts are summarized in Table1. The order of decreasing catalytic activity was Pd/Al 2O3 bar, t = 60 min, and stirring rate = 700 rpm. The physicochemical properties of the different Pd/Al2O3 (900)_pH7.5 > Pd/Al2O3 (105)_pH7.5 > Pd/Al2O3 (900)_pH11.5 > Pd/Al2O3 (1100)_pH7.5 > Pd/Al2O3 catalysts are summarized in Table 1. The order of decreasing catalytic activity was Pd/Al2O3 (1150)_pH7.5 (Figure5). The conversion of MA and selectivity for SA increased with increasing (900)_pH7.5 > Pd/Al2O3 (105)_pH7.5 > Pd/Al2O3 (900)_pH11.5 > Pd/Al2O3 (1100)_pH7.5 > Pd/Al2O3 dispersion (i.e., decreasing particle size) of Pd, indicating that Pd was the more active species in the (1150)_pH7.5 (Figure 5). The conversion of MA and selectivity for SA increased with increasing hydrogenation of MA. The dispersion of Pd in the Pd/Al O (900)_pH11.5 and Pd/Al O (1100)_pH7.5 dispersion (i.e., decreasing particle size) of Pd, indicating2 that3 Pd was the more active2 3 species in the catalysts was similar at 13.1% and 11.0%, respectively. However, the catalysts exhibited significantly hydrogenation of MA. The dispersion of Pd in the Pd/Al2O3 (900)_pH11.5 and Pd/Al2O3 (1100)_pH7.5 different MA conversion and SA selectivity owing to the difference in the properties of the Al O catalysts was similar at 13.1% and 11.0%, respectively. However, the catalysts exhibited significantly2 3 support. It has been reported that the isomerization of MA to FA can occur without a catalyst and different MA conversion and SA selectivity owing to the difference in the properties of the Al2O3 is strongly affected by the concentration of maleic acid, the reaction temperature, and the pH of the support. It has been reported that the isomerization of MA to FA can occur without a catalyst and is solution [15]. Al O in the Pd/Al O (900)_pH11.5 catalyst was in the γ phase, which has a large strongly affected by2 3the concentration2 3 of maleic acid, the reaction temperature, and the pH of the specific surface area and strong acid sites, whereas in the Pd/Al2O3 (1100)_pH7.5 catalyst, it was in solution [15]. Al2O3 in the Pd/Al2O3 (900)_pH11.5 catalyst was in the γ phase, which has a large specific the θ + α phase, which has a small specific surface area and weak acid sites. The physicochemical surface area and strong acid sites, whereas in the Pd/Al2O3 (1100)_pH7.5 catalyst, it was in the θ + α properties of the Al O support affect the adsorption strength of the reactant and Pd nanoparticles on phase, which has a small2 3 specific surface area and weak acid sites. The physicochemical properties of the active sites. Al2O3 in the γ phase can provide a large number of active sites for the hydrogenation the Al2O3 support affect the adsorption strength of the reactant and Pd nanoparticles on the active sites. reaction and allows increased retention time on the catalyst surface, resulting in high conversion of MA Al2O3 in the γ phase can provide a large number of active sites for the hydrogenation reaction and and high selectivity for SA. In contrast, the low Pd dispersion, small specific surface area, and weak allows increased retention time on the catalyst surface, resulting in high conversion of MA and high acid sites in Pd/Al O catalysts with Al O in the θ + α or α phase can cause reduced retention time selectivity for SA. In 2contrast,3 the low Pd dispersion,2 3 small specific surface area, and weak acid sites in Energies 2018, 11, x FOR PEER REVIEW 6 of 8 Energies 2018, 11, x FOR PEER REVIEW 6 of 8 Energies 2019, 12, 284 6 of 8 Pd/Al2O3 catalysts with Al2O3 in the θ + α or α phase can cause reduced retention time on the catalyst surface,Pd/Al2O resulting3 catalysts in with low efficiencyAl2O3 in the for θ the + α hydrogen or α phaseation can reaction. cause reduced These results retention suggest time that on the a decrease catalyst insurface,on the the retention catalystresulting time surface, in lowof MA resultingefficiency and H in2 foron low thethe efficiency hydrogencatalyst forsurfaceation the hydrogenationreaction. leads to Thesea lower reaction.results reaction suggest These rate.results that a decrease suggest inthat the aretention decrease time in the of retention MA and time H2 on of the MA catalyst and H2 surfaceon the catalystleads to surface a lower leads reaction to a lowerrate. reaction rate.
Figure 5. Effect of Al2O3 phase and Pd dispersion on MA hydrogenation at T =◦ 90°C, PH2 = 5 Figure 5. Effect of Al2O3 phase and Pd dispersion on MA hydrogenation at T = 90 C, PH2 = 5 bar, bar,Figure and 5. t =Effect 60 min. of Al2O3 phase and Pd dispersion on MA hydrogenation at T = 90°C, PH2 = 5 and t = 60 min. bar, and t = 60 min.
3.5.3.5. Reusability Reusability of ofPd/Al Pd/Al2O23O (900)_pH3 (900)_pH 7.5 7.5 Catalyst Catalyst in inHydrogenation Hydrogenation of MA of MA 3.5. Reusability of Pd/Al2O3 (900)_pH 7.5 Catalyst in Hydrogenation of MA ReusabilityReusability and and stability stability are are important important factors factors in in the the chemical chemical industry. industry. The reusability of thethe Pd/Al O (900)_pH7.5 catalyst was therefore investigated. Liquid-phase hydrogenation of MA was Pd/AlReusability2O23 (900)_pH7.53 and stabilitycatalyst arewas important therefore factorsinvestigated. in the Liquid-phasechemical industry. hydrogenation The reusability of MA of was the carried out under optimized reaction conditions. After each reaction, the catalyst was separated from carriedPd/Al2 Oout3 (900)_pH7.5 under optimized catalyst reaction was therefore conditions. investigated. After each reaction,Liquid-phase the catalyst hydrogenation was separated of MA from was thecarriedthe reactant reactant out underand and washed washedoptimized with with reaction deionized deionized conditions. water. water. The TheAfte conversion conversionr each reaction, of of MA MA the and and catalyst selectivity was separatedfor SA did from notnot decrease significantly for seven cycles, indicating that the Pd/Al O (900)_pH7.5 catalyst was stable decreasethe reactant significantly and washed for withseven deionized cycles, indicating water. The that conversion the Pd/Al of22O MA33 (900)_pH7.5 and selectivity catalyst for wasSA did stable not during the liquid-phase hydrogenation of MA (Figure6). duringdecrease the significantly liquid-phase for hydrogenation seven cycles, of indicating MA (Figure that 6). the Pd/Al2O3 (900)_pH7.5 catalyst was stable during the liquid-phase hydrogenation of MA (Figure 6).
◦ Figure 6. Reusability of Pd/Al2O3 (900)_pH7.5 catalyst in MA hydrogenation at T = 90 C, PH2 = 5 bar, Figureand t =6. 90 Reusability min. of Pd/Al2O3 (900)_pH7.5 catalyst in MA hydrogenation at T = 90°C, PH2 =Figure 5 bar, and6. Reusability t = 90 min. of Pd/Al2O3 (900)_pH7.5 catalyst in MA hydrogenation at T = 90°C, PH2 4. Conclusions= 5 bar, and t = 90 min.
Liquid-phase hydrogenation of MA to SA with different Pd/Al2O3 catalysts was investigated, focusing mainly on the effect of two factors on catalytic activity and selectivity: reaction conditions Energies 2019, 12, 284 7 of 8 and physicochemical properties of the catalyst. The effect of the reaction conditions was investigated by varying the reaction temperature, H2 pressure, and reaction time. Depending on the reaction conditions, FA was formed as an intermediate or final product through isomerization of MA. Under ◦ the optimal reaction conditions of T = 90 C, PH2 = 5 bar, t = 90 min, 100% MA conversion and 100% SA selectivity were achieved using the Pd/Al2O3 (900)_pH7.5 catalyst. The effect of the Al2O3 phase and Pd dispersion were also investigated. Catalytic activity and selectivity were found to increase with Pd dispersion. The characteristics of the Al2O3 phase strongly affected the adsorption strength of the reactant and Pd nanoparticles on the active sites and, consequently, the catalytic activity.
Author Contributions: Investigation and writing original draft preparation, M.Y.B.; Investigation, J.S.K.; Formal analysis, J.H.B.; writing—review and editing, D.-W.P. and M.S.L.; supervision, M.S.L. Funding: This research was financially supported by DR AXION CO. (Project No. IR180054) and the Korea Institute of Industrial Technology (KITECH) (Project No. EE180054 and JA180001).
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