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Wet Oxidation of Glycerol Into Fine Organic Acids: Catalyst Selection and Kinetic Evaluation

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Wet Oxidation of Glycerol Into Fine Organic Acids: Catalyst Selection and Kinetic Evaluation Brazilian Journal of Chemical ISSN 0104-6632 Printed in Brazil Engineering www.abeq.org.br/bjche Vol. 31, No. 04, pp. 913 - 923, October - December, 2014 dx.doi.org/10.1590/0104-6632.20140314s00002655 WET OXIDATION OF GLYCEROL INTO FINE ORGANIC ACIDS: CATALYST SELECTION AND KINETIC EVALUATION J. E. N. Brainer1*, D. C. S. Sales1, E. B. M. Medeiros2, N. M. Lima Filho1 and C. A. M. Abreu1 1Universidade Federal de Pernambuco, Laboratório de Processos Catalíticos, LPC, Departamento de Engenharia Química, Universidade Federal de Pernambuco, 50740-520, Recife - PE, Brazil. Phone: + 55 (81) 94088082, Fax: +55 (81) 2126-7289 *E-mail: [email protected] E-mail: [email protected], [email protected], [email protected] 2Universidade Federal da Paraíba, Centro de Tecnologia de Desenvolvimento Regional, Campus I, 58051-900, João Pessoa - PB, Brazil. (Submitted: April 12, 2013 ; Revised: November 5, 2013 ; Accepted: November 26, 2013) Abstract - The liquid phase oxidation of glycerol was performed producing fine organic acids. Catalysts based on Pt, Pd and Bi supported on activated carbon were employed to perform the conversion of glycerol into organic acids at 313 K, 323 K and 333 K, under atmospheric pressure (1.0 bar), in a mechanically agitated slurry reactor (MASR). The experimental results indicated glycerol conversions of 98% with production of glyceric, tartronic and glycolic acids, and dihydroxyacetone. A yield of glyceric acid of 69.8%, and a selectivity of this compound of 70.6% were reached after 4 h of operation. Surface mechanisms were proposed and rate equations were formulated to represent the kinetic behavior of the process. Selective formation of glyceric acid was observed, and the kinetic parameter values indicated the lowest activation energy (38.5 kJ/mol) for its production reaction step, and the highest value of the adsorption equilibrium constant of the reactant glycerol (10-4 dm3/mol). Keywords: Glycerol oxidation; Catalysts; Glyceric acid; Kinetics; Modeling. INTRODUCTION This increasing production of biodiesel has re- sulted in a price decline of crude glycerol, making Glycerol is an important renewable resource de- aqueous glycerol an attractive compound for the rived from biomass and is currently used in pharma- synthesis of fine chemicals (Behr et al., 2008). ceutical, food and cosmetics applications (Corma et A range of possible products can be derived from al., 2007; Luo et al., 2008; Ferretti et al., 2010; Lehr glycerol oxidation, such as dihydroxyacetone (DHA), et al., 2007; Guo et al., 2009). The conversion of hydroxypyruvic acid (HPA), glyoxalic acid (GOX), glycerol is of interest because it is a byproduct of oxalic acid (OXA), glyceraldehyde (GLA), glyceric biodiesel production from plant and animal oils acid (GCA), and tartronic acid (TTA) (Fordham et (Tullo, 2007). The rapid expansion of biodiesel pro- al., 1996). Glyceric acid and dihydroxyacetone are duction capacity around the world has caused a the main products obtained from glycerol oxidation major surplus of glycerol (McCoy, 2006). in the presence of palladium and platinum catalysts *To whom correspondence should be addressed 914 J. E. N. Brainer, D. C. S. Sales, E. B. M. Medeiros, N. M. Lima Filho and C. A. M. Abreu (Mallat and Baiker, 1995; Kimura et al., 1993; Kimura, to the need to incorporate metal in the form of anions 1993; Fordham et al., 1996). Tartronic acid and oxalic or cations during impregnation. acid are also formed by oxidation of glyceric acid The salts H2PtCl6·6H2O (>99%) and Bi(NO3)3.5H2O and dihydroxyacetone (Fordham et al., 1996; Worz, (>98%) were supplied by VETEC (Brazil) and PtCl2 2009). An increasing number of studies on oxidation (99.99%) and PdCl2(99.99%) were supplied by Acros of glycerol with oxygen in aqueous solution have Organics. Glyceric acid (GCA, 99.9%) and 1,3-dihy- been conducted with catalysts based on palladium droxyacetone dimer (DHA, 97%) were obtained from and platinum (Kimura et al., 1993; Kimura, 1993; Sigma-Aldrich Corporation (USA), tartronic acid Fordham et al., 1996). The addition of bismuth as a (TTA, 98%) from ALFA AESAR (USA), glycolic acid promoter incorporated to the palladium and platinum (GLYCA, 70 wt.% in water) and oxalic acid (OXA, catalysts had an influence on the catalyst selectivity 99.9%) from VETEC (Brazil).” (Alardin et al., 2001). The mechanism of glycerol The platinum catalyst was prepared by wet im- oxidation on theses catalysts has been shown to in- pregnation of the activated carbon with an acidic solu- 2- volve oxidative dehydrogenation. Thus, it was ob- tion of [PtC16] ions at 298 K for 24 h. The water was served that the presence of NaOH was important for evaporated at an increased temperature under vacuum. the initiation of the catalytic glycerol oxidation The catalyst was dried in air at 333 K for 12 h and then (Carrettin et al., 2001; Demirel et al., 2005; Ketchie reduced in a hydrogen atmosphere for 2 h at 533 K. et al., 2007; Ketchie et al., 2007). The Pd-Pt-Bi/C catalyst was prepared by wet co- The knowledge of a phenomenological kinetic impregnation of the support with acidic solutions of model that can correctly predict the transformation PtCl2, PdCl2 and Bi(NO3)3.5H2O. The activated carbon of glycerol will be useful for selection of operating (AC) was previously washed with distilled water, fil- conditions depending on the desired goal. Phenome- tered off and dried overnight at 333 K. The catalysts nological models to represent experimental results of were impregnated with salt solutions and dried under glycerol oxidation, in order to obtain selective or- vacuum. First, the salt precursor Bi(NO3)3.5H2O ganic acids, have been proposed, (Ketchie et al., deposited on the support was decomposed upon heat- 2007; Demirel et al., 2007). ing under a nitrogen stream at 723 K during 18 h. The purpose of this work was to develop the cata- Then, the catalysts were reduced under a hydrogen lytic oxidation of glycerol in alkaline media in the atmosphere. The system was heated with a ramp presence of platinum, palladium and bismuth cata- rate of 5 K/min until 533 K, with a hydrogen flow lysts supported on activated carbon with different of 60 cm3/min and then was kept under isothermal metal compositions. The selectivity of the process was conditions for 5 hours. evaluated in terms of the main products glyceric and tartronic acids. The experimental evidences allowed Catalysts Characterization the formulation of a reaction scheme based on ki- netic modeling according to the Langmuir-Hin- The X-ray diffraction (XRD) analyses of the cata- shelwood approach. lysts were performed using a Rigaku DMAX model 2400 X-ray diffractometer (Cu Kα radiation, 40 kV, 20 mA). Diffraction data was recorded using a con- EXPERIMENTAL tinuous scanning at 0.02º/s, step 0.02º. The crystal- line phases were identified by reference to the JCPDS Catalyst Preparation data file. The chemical composition of the samples was determined by X-ray fluorescence analysis The carbon-supported catalysts were prepared by (XRF), using a Rigaku spectrometer, model Rix wet impregnation, followed by calcination and re- 3100, controlled by software Rix 3100, with an X-ray duction. The choice of the method was due to indica- tube of Rh anode. The textural characteristics, spe- tions of oxidation activities for glycerol conversion cific surface area and pore volume (BJH method) (Verde et al., 2004; Gallezot, 1997). The catalysts were determined by N2 physisorption at 77 K in a were formulated based on the following composi- Micromeritics ASAP 2020. tions: Pt(3.0 wt.%)/C and Pd(0.2 wt.%)-Pt(1.0 wt.%)- Acid sites of the support were identified by the Bi(2.0 wt.%)/C. The activated carbon (Ref. C-119) results of the determination of superficial groups by used as support was from Carbomafra (Brazil). The Boehm titrations with NaHCO3, NaOH, Na2CO3 for pH was adjusted to values above or below the carboxylic, phenolic and lactonic groups, respec- isoelectric point of activated carbon, about 6.8, due tively (Boehm, 1994, 2002). Brazilian Journal of Chemical Engineering Wet Oxidation of Glycerol Into Fine Organic Acids: Catalyst Selection and Kinetic Evaluation 915 Oxidation Experiments acid, glycolic acid, oxalic acid) were done by com- parison with standard solutions. The glycerol oxidation experiments were per- formed in a borosilicate glass reactor of the mechani- cally agitated slurry reactor (MASR) type (Figure 1) RESULTS AND DISCUSSION equipped with a two-bladed turbine-type impeller mixer. Characterization of Catalysts The diffractograms of the trimetallic Pd(4.0 wt.%)- Pt(1.0 wt.%)-Bi(5.0 wt.%)/C commercial catalyst from 1. Reactor; Evonik and of the Pd(0.2 wt.%)-Pt(1.0 wt.%)-Bi(2.0 2. Air bubble; wt.%) catalyst prepared from inorganic ligands indi- 3. Discharge valve; cated the presence of metallic Pd(2θ=40.1). The XRD 4. heating jacket; 5. Stirrer; analysis showed the presence of an intermetallic 6. Thermometer; compound with the composition BiPd3(2θ=28.5), 7. Collector sample; metallic bismuth(2θ=24.6) and minor amounts of 8. Condenser; Bi O (2θ=30.3). Moreover, XRD characterization of 9. 10. 11. Feeder; 2 3 12. Flowmeter. the monometallic Pt/C and trimetallic Pd-Pt-Bi/C catalysts prepared confirmed the presence of metallic Pt(2θ = 39.8). The chemical composition of the constituent metal components determined by X-ray fluorescence (XRF) Figure 1: Scheme of the mechanically agitated slurry analyses was found to be Pd:Pt:Bi: 0.18:1.00:1.95 for reactor (MASR). the trimetallic catalyst prepared and Pd:Pt:Bi: 3.80: The system was operated in batch mode for the 1.00:4.70 for the commercial catalyst. The error as- solid and liquid phases under continuous gas flow sociated with the analysis was ±0.01.
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