CHEMICAL ENGINEERING TRANSACTIONS VOL. 76, 2019 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Petar S. Varbanov, Timothy G. Walmsley, Jiří J. Klemeš, Panos Seferlis Copyright © 2019, AIDIC Servizi S.r.l. ISBN 978-88-95608-73-0; ISSN 2283-9216 Co-Production of 1,2-propandiol and Ethyl Lactate from the Conversion of Glycerol co-fed with Bio-Ethanol over Reduced Cu0.3Mg1.7Al LDH-derived Oxide Catalyst without External Hydrogen Jirayu Kuljirasetha,b, Sirirat Jitkarnkaa,b,* aThe Petroleum and Petrochemical College, Chulalongkorn University, Phayathai Rd. 254, 10330 Bangkok, Thailand bCenter of Excellence on Petrochemical and Materials Technology, Bangkok 10330, Thailand sirirat.j@chula.ac.th Usually, the hydrogenolysis of glycerol into 1,2-propandiol requires H2 supply in the acetol hydrogenation step to 1,2-propandiol. However, in-situ hydrogen can be generated from glycerol dehydrogenation and glycerol reforming using metal catalysts. Moreover, ethanol can be co-fed as a hydrogen donor for this reaction and produce ethyl lactate via esterification with lactic acid which is by-product from glycerol conversion. In this study, the promotion effect of Cu partially-substituted Mg1.7Al mixed oxide was investigated on the conversion of glycerol co-fed with bio-ethanol in a PARR-reactor at 180 oC for 4 h without hydrogen feeding. Then, the products were analyzed using GC-MS/TOF. The catalysts were characterized using BET, XRD, XPS, XRF, and TPDRO. As a result of the Mg2AlO based catalyst, the glycerol can be converted to 1,2-propaniol, acetone, and acetaldehyde without ethyl lactate production at this condition. The reduced Cu0.3Mg1.7Al LDH-derived catalyst significantly enhanced the selectivity of 1,2-propandiol and ethyl lactate to 47.3 and 22.8 %. 1,2-Propandiol and ethyl lactate as specialty chemicals were found to be co-produced from co-feeding glycerol and bio-ethanol using the reduced Cu0.3Mg1.7Al LDH-derived catalyst. 1. Introduction The biodiesel production generates a large amount of glycerol as a by-product that is inexpensive with a small market. Therefore, the conversion of glycerol into useful bio-based chemicals is an attractive alternative resource for glycerol utilization and value addition of glycerol. 1,2-Propanediol (1,2-PDO), widely used in chemicals, food, and pharmaceutical industries, is one of the products from the alternatives that utilize glycerol and increase the value of glycerol via hydrogenolysis (Delgado et al., 2013; Yun et al., 2014). Usually, the hydrogenolysis of glycerol requires H2 supply in the acetol hydrogenation step to form 1,2-propandiol. Therefore, the in-situ simultaneous generation of hydrogen from glycerol reforming using a hydrogen-donating solvent, such as ethanol, can overcome these problems (Xia et al., 2013; Yun et al., 2014). Moreover, ethanol can, in parallel, produce ethyl lactate as a specialty chemical via the esterification with lactic acid that is also a by- product from glycerol conversion. Ethyl lactate is an important chemical widely used in the food, pharmaceutical, and chemical industries as well. Based on the reaction mechanism, the acid-base properties of catalysts play a critical role in glycerol hydrogenolysis. The acid sites catalyze the dehydration of glycerol to acetol (Chiu et al., 2006), whereas basic sites catalyze the dehydrogenation of glycerol to glyceraldehyde that is subsequently dehydrated into pyruvaldehyde (Delgado et al., 2013). The Mg-Al mixed oxide catalysts derived from layered double hydroxides (LDHs) are therefore promising catalysts for the reactions because they possess specific acid-base properties and significantly-high surface area with mesoporosity (Kuljiraseth et al., 2019). In previous work, Cu metal catalysts have shown their superior performance in terms of the 1,2-PDO selectivity of the glycerol hydrogenolysis reaction due to the ability to catalyze the cleavage of C-O and C-C bonds in the presence of H2 (Yun et al., 2014). Cu has been employed to enhance the activity, selectivity, and hydrogen utilization in various DOI: 10.3303/CET1976018 Paper Received: 13/03/2019; Revised: 05/06/2019; Accepted: 16/06/2019 Please cite this article as: Kuljiraseth J., Jitkarnka S., 2019, Co-Production of 1,2-propandiol and Ethyl Lactate from the Conversion of Glycerol co-fed with Bio-Ethanol over Reduced Cu0.3Mg1.7Al LDH-derived Oxide Catalyst without External Hydrogen, Chemical Engineering Transactions, 76, 103-108 DOI:10.3303/CET1976018 103 Cu-based bimetallic catalysts, such as Cu-ZnO (Gao et al., 2016), Cu-Al2O3 (Cai et al., 2016), Cu-MgO (Balaraju et al., 2012), Cu-Ni-Al2O3 (Yun et al., 2014), and Cu-Mg-AlO (Xia et al., 2013). As for previous work, the Cu0.4/Mg6.28Al1.32O8.26 (Cu:Mg:Al = 0.30: 4.76: 1) catalyst performed efficiently in the selective conversion of glycerol to 1,2-PDO, using ethanol as the hydrogen donor under N2 at 200 oC; however, ethyl lactate has not found co-produced (Xia et al., 2013). Since some reactions such as esterification require the bi-functional acid- base sites of a catalyst (Kuljiraseth et al., 2019), the different Mg/Al ratios of MgAl-LDO can provide acid-base pairs with different quantities and strength, which can drive the conversion of a feed as efficiently as possible. In this work, a low Mg/Al ratio of 2 was expected to promote the high activity of glycerol conversion in the case due to its high acid and base density. Moreover, Cu was reported to be an active catalyst for producing H2 from glycerol reforming (Yun et al., 2014) and ethanol dehydrogenation (Sato et al., 2012). Therefore, the Mg2AlO partially substituted by Cu was used as a catalyst. The objective of this study was to investigate the promotion effect of Cu by a partial substitution in the Mg2Al derived-LDHs, and examine the potentials of the catalyst in the conversion of glycerol co-fed with bio-ethanol, targeting to the co-production of 1,2-PDO and ethyl lactate, in a PARR-reactor at 180 oC for 4 hours without hydrogen feeding. 2. Experimental 2.1 Catalyst preparation The Cu0.3Mg1.7Al-LDH catalyst, whose atomic ratio of (Mg+Cu)/Al was 2, was prepared by co-precipitation. An aqueous solution of Mg(NO3)2·6H2O (0.2975 mol), Cu(NO3)2·3H2O (0.0525), and Al(NO3)3·9H2O (0.175 mol) were mixed to make a homogeneous solution. Then, the aqueous solution was added into a Na2CO3 (0.35 mol) solution, and pH was controlled at about 10 using NaOH solution. The sample was next aged at room temperature for 16 h, filtered and washed several times by deionized water until pH 7, and dried in an air oven at 65 °C overnight. The Cu0.3Mg1.7Al-LDH was calcined at 500 oC in air, and reduced in 10%H2 (H2/He) at 500 oC for 2 h. The reduced catalyst was named as reduced Cu0.3Mg1.7AlO. For comparison purpose, Mg2Al-LDH was also prepared, according to the above procedure, in order to investigate the effect of partial substitution of Cu. Before also used as a catalyst, the Mg2Al-LDH was calcined at 500 oC in air, named calcined Mg2AlO. 2.2 Catalyst characterization The X-Ray diffraction (XRD) patterns of samples were obtained using a Bruker X-Ray diffractometer system (D8 Advance) with a CuK radiation (1.5405 Å). The measurement conditions were in the range of 2θ = 5º to 70º and the scan speed of 0.02º (2θ)/ 0.6 s. The Brunauer-Emmett-Teller (BET) technique was used to determine the specific surface area, pore diameter, and total pore volume of catalysts, using the surface area analyzer (Quantachrome, Autosorb-1MP). Before the measurements, the samples were degassed under vacuum at 250 °C for 16 h. The elemental concentrations of catalysts were determined using X-ray fluorescence (XRF), Model S8 Tiger. The Temperature Programmed Desorption/Reduction/Oxidation analyzer (TPDRO), BELCAT II, was employed to determine the acidity and basicity from the temperature desorption of NH3 and CO2, of the calcined catalysts. A sample was pretreated in He (50 cc/min) at 450 oC for 60 min. Pre-adsorption of 10 % NH3/He or 99.995 % CO2 at 100 ⁰C for 30 min was performed prior to the analysis. Next, the temperature was increased to 650 °C at 10 °C/min, and later held for 30 min with He (30 cc/min). The reducibility of the calcined catalysts was determined using the temperature programmed reduction (TPR) technique with a heating rate of 10 oC/min to 800 ⁰C under 5 % H2/N2. X-Ray Photoelectron Spectroscopy (XPS) was performed using the AXIS ULTRADLD to determine the oxidation state of the samples. The system was equipped with a monochromatic Al X-ray source and a hemisphere analyzer. All peaks were calibrated from referring to the C1s spectrum located at 284.8 eV. 2.3 Catalytic activity testing The catalytic activity of glycerol hydrogenolysis was tested in a batch-type PARR reactor. The composition of the reaction mixture was 36.02 g of glycerol and 9 g of bio-ethanol and a fixed loading of 1 g of catalyst. The catalytic activity was carried out with the stirring rate of 300 rpm at 180°C for 4 h under auto-pressure. After the reaction, the catalyst was removed from the reaction mixture using a centrifuge, and the liquid product was analyzed using a Pegasas LECO GC-TOF/MS. The column was an Rxi-PAH (60 m x 0.25 mmID and 0.10 μm film thicknesses). Initially, the column temperature was 40 oC, kept for 1 min, then increased temperature step- by-step by 5.5 oC/min to 55 oC, 1.5 oC/min to 90 oC, 4.0 oC/min to 168 oC, and 5.0 oC/min to 210 oC. 104 3. Results and discussion 3.1 Catalyst characterization The elemental composition and d-spacing of the LDHs were examined using XRF and XRD, respectively. In Figure 1A(a-b), the XRD patterns show the characteristic peaks of a hydrotalcite (PDF No. 00-014-0191) in both Mg2Al-LDH and Cu0.3Mg1.7Al-LDH, indicating that all the LDH samples were successfully synthesized. Based on the chemical composition and d-spacing shown in Table 1, the mol % Al of Cu0.3Mg1.7Al-LDH is similar with that of Mg2Al-LDH, but the % mol of Mg is lower. It indicates that Cu substitutes in the position of Mg in the LDH sheets, which slightly enlarges the d-spacing of LDH from 7.63 to 7.78 Å. After calcination at 500 ⁰C, the peaks of metal oxide (PDF No.00-001-1235) and spinels (MgAl2O4; PDF No.01- 075-4038 and CuAl2O4; PDF No.01-073-1958) appear in both calcined Mg2AlO and calcined CuMgAlO, confirming the transformation to mixed metal oxide phases, as shown in Figure 1A(c-d). From the Al 2p XPS spectra (Figure 2a), only a single peak of Al3+ atoms appears. Al3+ is present in the octahedral position (74.3 eV) (Rao et al., 2005), and its peak shifts slightly to lower binding energy upon the partial substitution of Cu. Likewise, the Mg 2p XPS spectra that indicate the shifts of the Mg 2p peak to lower binding energy. It can be noted that the partial substitution of Cu has been made successful because the interaction with Mg and Al can be observed from the shifts of XPS peaks. Additionally, the spectrum of Cu2p of calcined Cu0.3Mg1.7AlO (Figure 2(c)) presents the peak of Cu2+ (934.4 eV) (Jiang et al., 2013), confirming that Cu can partially substitute in the Mg2+ position. Table 1: Chemical composition and physical properties of LDHs Sample Mol %Mga Mol %Ala d-Spacingb Mg2Al-LDH 29.7 14.6 7.63 Cu0.3Mg1.7Al-LDH 24.5 14.0 7.78 a determined using XRF, b determined using XRD Figure 1: (A) XRD patterns of (a) Mg2Al-LDH, (b) Cu0.3Mg1.7Al-LDH, (c) calcined Mg2AlO, (d) calcined Cu0.3Mg1.7AlO, and (e) reduced Cu0.3Mg1.7AlO, and (B) TPR of calcined LDHs Figure 2: XPS spectra of (a) Mg 2p, (b) Al 2p, and (c) Cu2p3/2 of catalysts 105 The H2-TPR profiles of calcined Mg2AlO and Cu0.3Mg1.7AlO are shown in Figure 1B. It can be seen that there are mainly two H2-TPR peaks; namely, the reduction peak of CuO species at a lower temperature (200-300°C), and the reduction peak of CuAl2O4 at a higher temperature (500–700 °C) (Luo et al., 2005). After the reduction at 500 ⁰C for 2 h, a Cu metallic peak (2θ = 43.5o, PDF No.00-001-1241) is observed in the XRD pattern of reduced Cu0.3Mg1.7AlO (Figure1A(e)). Moreover, the Cu 2p XPS spectrum of reduced Cu0.3Mg1.7AlO (Figure 2c) shows a new peak at 932.7 eV, attributed to Cu0 (Jiang et al., 2013), while the peak of Cu2+ also appears (934.6 eV), confirming that the Cu in the catalyst was partially reduced at this condition. Table 2 presents the surface area, pore volume, and pore diameter of catalysts. Both calcined Mg2AlO and reduced Cu0.3Mg1.7AlO catalysts are thus mesoporous materials because their pore diameters are in the range of 133.4 - 152.5 Å. The partial substitution with Cu increases the surface area of 211.3 m2/g with the higher pore volume of 0.8056 Å, as compared with those of calcined Mg2AlO. In addition, the total acidity and basicity are also shown in Table 2. The calcined Mg2AlO catalyst presents both basic (0.575 mmol/g) and acidic (0.355 mmol/g) properties. Upon the partial substitution, Cu suppresses the density of basic sites because the Cu partially substitutes in the Mg2+ position that is the basic site. So, the partial substitution of Cu reduces the total basicity of the catalyst to 0.431 mmol/g, and consequently enhances the total acid density to 0.425 mmol/g. Table 2: Chemical and physical properties of catalysts Catalyst Surface Area (m2/g)a Pore Volume (cm3/g)a Pore Diameter (Å)a Acid Density (mmol/g)b Base Density (mmol/g)c Calcined Mg2AlO 165.2 0.5507 133.4 0.355 0.575 Reduced Cu0.3Mg1.7AlO 211.3 0.8056 152.5 0.425 0.431 a determined using BET, b determined using TPD-NH3, c determined using TPD-CO2 3.2 Catalytic activity The calcined Mg2AlO and reduced Cu0.3Mg1.7AlO catalysts were tested for their activity on the glycerol co-fed with bio-ethanol. The conversion of glycerol, ethanol, and selectivity of products after a reaction time of 4 h without H2 feed, are shown in Figure 3. The results show that the Mg2AlO catalyst can produce 20.2 % 1,2-PDO at a low glycerol conversion (18.9 %) without ethyl lactate production. The conversion of glycerol catalyzed by the acid-base of catalysts is summarized in Figure 4. The acid function of a catalyst catalyzes the dehydration of glycerol to acetol (Chiu et al., 2006), whereas the basic site catalyzes the dehydrogenation of glycerol to glyceraldehyde (Delgado et al., 2013). In this work, the Mg2AlO catalyst promotes ethanol dehydrogenation to acetaldehyde (19.6 % selectivity) with 23.3 % ethanol conversion (Figure 3a), as indicated in Scheme 1b. The hydrogen generated from ethanol dehydrogenation is simultaneously supplied to the hydrogenation of acetol to form 1,2-PDO. With the partial substitution with Cu, the reduced Cu0.3Mg1.7AlO catalyst can enhance significantly the glycerol conversion to 49.8 % (Figure 3a), and increase the 1,2-PDO and ethyl lactate selectivity to 28.8 % and 14 % (Figure 3(b2)), respectively. As shown in Figure 4a, the in-situ hydrogen formation is possible either from glycerol reforming or from dehydrogenation of glycerol to glyceraldehyde. The hydrogen also can be produced from the ethanol dehydrogenation pathway (Figure 4b), which can be considered from production of ethyl acetate (4.4 %) as a product. Ethanol is co-fed not only as a hydrogen donor for this reaction, but also to simultaneously produce ethyl lactate via esterification with lactic acid that is a by-product from glycerol conversion, which can be assured by the existence of lactic acid in the final product. It is observed in this work that lactic acid is produced from oxidation of 1,2-PDO on the Cu metal site. Therefore, the Cu catalyst plays important roles in the reaction pathways; namely, (1) promotes the hydrogen production via glycerol reforming, glycerol dehydrogenation, and ethanol dehydrogenation pathways, which is then supplied to the hydrogenation of acetol to form 1,2-PDO, (2) drives the oxidation of 1,2-PDO to lactic acid, and then (3) promotes the esterification of lactic acid with ethanol to ethyl lactate. The nature of substituting metal appears to be more influential to the product selectivity than the acid-base properties. In summary, the reduced Cu0.3Mg1.7AlO catalyst gives an apparently-higher yield of 1,2-PDO (15.0 %) and ethyl lactate (6.8 %), as shown in Figure 5. It is noted that the product from reduced Cu0.3Mg1.7AlO consists of complex mixtures of oxygenates, which may be difficult to separate them to every single individual compound. As a result, one should be aware that the separation process may be not economical. However, it is possible that we can separate them by their groups such as alcohol, ester, and so on, using distillation and solvent extraction (Xiu & Zeng, 2008), and then used them in some applications such as bio-additive. 106 Figure 3: (a) Conversion of glycerol and ethanol, and (b) selectivity of products of the catalysts Figure 4: Proposed reaction pathways of reduced Cu0.3Mg1.7AlO catalyst: (a) glycerol hydrogenolysis, and (b) ethanol dehydration, where A = Acid site (Chiu et al., 2006), B = Base site (Delgado et al., 2013), and Cu = Copper metal sites proposed in this work Figure 5: Yield of products 4. Conclusions Mg2Al-LDH and Cu0.3Mg1.7Al-LDH have been successfully synthesized in this work. They were next calcined and then reduced at 500 oC, and later found possessing both acidic-basic and metal sites. Subsequently, they were studied for their activity on the conversion of glycerol with bio-ethanol. It was found that the conversion of 107 glycerol was highly enhanced by the partial substitution of Cu in Mg1.7Al-LDH. Based on the selectivity, the Cu- substituted catalyst promoted the glycerol hydrogenolysis pathway, producing 15.0 % yield of 1,2-PDO, together with the esterification of lactic acid with bio-ethanol that resulted in 6.8 % yield of ethyl lactate. The Cu catalyst played important roles in the in-situ hydrogen formation, oxidation of 1,2-PDO, and esterification to ethyl lactate. In addition, it should be noted that, due to their complexation, it may be difficult to separate the mixture of oxygenate products, generated using reduced the Cu0.3Mg1.7AlO catalyst, to individual compounds; therefore, the separation may cause the entire process uneconomical. Generally, the investment of a conventional petrochemical plant has a break-even point around 20-30 years, whereas a bio-based petrochemical plant may possibly take more than 50 years. 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