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Ind. Eng. Chem. Res. 2009, 48, 1840–1846
Catalytic Hydrogenolysis of Glycerol to Propylene Glycol over Mixed Oxides Derived from a Hydrotalcite-Type Precursor Lekha Charan Meher,† Rajesh Gopinath,† S. N. Naik,‡ and Ajay K. Dalai*,† Catalysis and Chemical Reaction Engineering Laboratories, Department of Chemical Engineering, UniVersity of Saskatchewan, Saskatoon, Saskatchewan S7N 5A9, Canada, and Centre for Rural DeVelopment and Technology, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
Selective hydrogenolysis of glycerol to propylene glycol was performed using an environmentally friendly hydrotalcite-derived mixed-metal oxide catalyst. The Mg/Al, Zn/Al, Ni/Mg/Al, Ni/Co/Mg/Al, and Cu/Zn/Al mixed-metal oxide catalysts were prepared from their corresponding hydrotalcite precursors having M2+/M3+ compositions over the range of 0.5-3.0. The physicochemical properties of the catalysts were studied by X-ray diffraction (XRD), inductively coupled plasma mass spectrometry (ICP-MS), NH3 and CO2 temperatureprogrammed desorption (TPD), and nitrogen adsorption studies. The XRD patterns of pure hydrotalcites exhibited characteristics of hydrotalcite phases, while those of calcined hydrotalcites showed the formation of corresponding metal oxides. The ICP-MS analysis showed agreement between the calculated and actual metal compositions. The prepared catalysts were evaluated for the hydrogenolysis of glycerol to propylene glycol in a Parr reactor. The activity studies indicated a maximum glycerol conversion and selectivity toward propylene glycol in the case of Cu/Zn/Al mixed-metal oxide catalysts. Further, the reaction parameters were optimized with the most active Cu/Zn/Al catalyst, and it was found that at a catalyst concentration of 5% (w/w) of aqueous glycerol, a hydrogen pressure of 200 psig, and 80% glycerol dilution, a maximum glycerol conversion of 52% with 93-94% selectivity toward propylene glycol were obtained. 1. Introduction Biodiesel is a vegetable oil derived fatty acid methyl/ethyl ester and is emerging as a green fuel. Presently, biodiesel is a potential environmentally friendly diesel fuel substitute or extender and has attracted the attention of oleochemical and petrochemical industries for the production of fuel from renewable lipid resources. The global biodiesel market is estimated to reach 37 billion gallons by 2016, growing at an average annual rate of 42%.1 The fatty acid methyl or ethyl ester is prepared by transesterification of vegetable oil with methanol or ethanol, which involves the chemical modification of triglycerides to methyl/ethyl esters and glycerol. Glycerol is the major byproduct of vegetable oil transesterification, which accounts for 10 wt % of vegetable oil or the product biodiesel. Purified glycerol or glycerin is a fairly high-value commercial chemical, valued at $0.60-0.90/lb., that is primarily used in the manufacture of various foods, beverages, pharmaceuticals, cosmetics, and other personal care products. In the near future, a huge amount of crude glycerol is expected to be produced as a byproduct from biodiesel industries and may be of little value. Presently, the high cost of biodiesel restricts its commercialization and usage. The utilization of crude glycerol may partially compensate for the production costs of biodiesel, making it economically feasible as well as overcoming the disposal problem of the surplus crude glycerol from biodiesel industries. Since the late 1940s, glycerol has been synthesized from epichlorohydrin, a petrochemical precursor, to fulfill the worldwide demand of glycerol for its use in food and drink, pharmaceuticals, personal care, polymers, explosives, and precursors for industrial chemicals such as glycidol, 1,2propanediol, and 1,3-propanediol. Presently, synthetic glycerol * To whom correspondence should be addressed. Tel: (306) 9664771. Fax: (306) 966-4777. E-mail:
[email protected]. † University of Saskatchewan. ‡ Indian Institute of Technology Delhi.
plants are closing because the natural lipid-based glycerol supplements its demand. The global glycerol market in 2005 was 800 000 tonnes, and 50% of glycerol was supplied from biodiesel byproducts.2 The current demand of glycerol is 1 million tonnes.3 The oversupply of glycerol is driving down its price to its bottom level, because of which crude glycerol may be treated as a waste product. Attempts are being made by researchers to utilize crude glycerol for its value addition in order to partially recover the biodiesel production costs. Propylene glycol (1,2-propanediol) is one of the commodity chemicals and environmentally friendly alternatives to an ethylene glycol based toxic deicing/antifreeze agent. The commercial route of producing propylene glycol is the chlorohydrin process, which utilizes propylene oxide derived from petrochemical sources. The crude glycerol obtained as the biodiesel byproduct is a renewable starting material that may replace the petroleum-based propylene oxide for the synthesis of propylene glycol. Current literature deals with the hydrogenation of glycerol to propylene glycol that employs severe reaction conditions,4-7 complex catalytic systems,8 and environmentally toxic catalysts9,10 which results in lower conversion, yield, and selectivity, respectively.11-13 It is observed from the literature that catalyst plays a vital role in the hydrogenolysis of the glycerol reaction, which involves dehydration of glycerol followed by hydrogenation for the synthesis of 1,2-propanediol. Hydrotalcites, a family of anionic clays, are lamellar compounds of magnesium and aluminum hydroxides with interlayer spaces containing exchangeable anions. The total or partial substitution of Mg2+ by other cations produces materials with isomorphous structures named hydrotalcite-like compounds. The mixed-metal oxides derived from hydrotalcites after calcination at high temperatures have acidic-basic and redox properties depending on the nature of incorporated cations, thermal stability, and interdispersion of elements, making it widely applied for different reactions.14-16 The properties of the hydrotalcite-based materials can be tailored according to the reaction requirement
10.1021/ie8011424 CCC: $40.75 2009 American Chemical Society Published on Web 01/23/2009
Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009 1841
by modification with different metals of varying compositions. The bifunctional catalysts having acidic-basic and hydrogenation properties would be the ideal catalyst for the conversion of glycerol to propylene glycol. In the present study, we have attempted to use mixed-metal oxides derived from hydrotalcite-type precursors as catalysts to synthesize 1,2-propanediol from glycerol. The mixed-metal oxides containing Mg/Al, Zn/Al, Ni/Mg/Al, Ni/Co/Mg/Al, and Cu/Zn/Al were prepared from their corresponding hydrotalcite precursors, and the catalytic activities were evaluated toward the dehydration and hydrogenation of glycerol to produce 1,2propanediol. 2. Experimental Section 2.1. Reagents. The metal nitrate precursors, i.e., Cu(NO3)2 · 3H2O, Mg(NO3)2 · 6H2O, Ni(NO3)2 · 6H2O, Zn(NO3)2 · 6H2O, Co(NO3)2 · 6H2O, and Al(NO3)3 · 9H2O (purity >98%), were purchased from Sigma-Aldrich-USA (St. Louis, MO), NaOH (>99%) from EMD Chemicals (Darmstadt, Germany), and Na2CO3 (ASC grade) from BDH Chemicals (Canada). 1,2Propanediol (>99%), acetol (90%) from Sigma-Aldrich-USA and 1,3-propanediol (>99%) from Sigma-Aldrich-Germany were used as standard chemicals for gas chromatographic (GC) analysis of the glycerol hydrogenolysis products. Glycerol (99%) used for the study was procured from EMD Chemicals Inc. USA. The crude glycerol was obtained as a byproduct of the alkali transesterification of canola oil with methanol. 2.2. Catalyst Preparation. Hydrotalcite samples were prepared using coprecipitation by increasing the pH method, as described by Cavani et al.,16 in which an aqueous solution of sodium hydroxide and sodium carbonate (1 N) as the precipitating agent was mixed with an aqueous mixture of 1.5 M metal nitrate solutions M(NO3)x, until pH 10.0 for complete precipitation of hydrotalcites. Hydrotalcites prepared had metal compositions of Mg/Al, Zn/Al, Ni/Mg/Al, Ni/Co/Mg/Al, and Cu/Zn/Al having M2+/M3+ ratios in the range of 0.5-3. The precipitate was dried at 80 °C for 24 h, followed by washing with hot water at 80 °C until the filtrate was neutral to pH in order to ensure complete removal of the base. The washed precipitate was dried at 120 °C for 72 h to obtain the hydrotalcites. The mixed-metal oxides were obtained by calcination of the hydrotalcites at 450 °C for 5 h. 2.3. Catalyst Characterization. The metal composition of the catalysts was determined by inductively coupled plasma mass spectrometry (ICP-MS). The X-ray diffraction (XRD) studies of the hydrotalcites and mixed-metal oxides were conducted on a Rigaku diffractometer (Rigaku, Tokyo, Japan) with Cu KR radiation at 50 kV and 100 mA at a scanning angle (2θ) of 5-80° and at a scanning speed of 5°/min. The specific surface area, average pore volume, and pore size distribution were measured using a Micromeritics adsorption instrument (model ASAP 2000) at 78 K using liquid nitrogen. Prior to the analysis, the catalysts were evacuated at 200 °C in a vacuum of 5 × 10-4 atm to remove all adsorbed moisture from the catalyst surface. The basicity studies of the mixed-metal oxide catalysts were conducted with CO2 temperature-programmed desorption (TPD) in a CHEM BET-3000 (Quantachrome Instruments, Boynton Beach, FL) instrument. About 0.15 g of the powdered sample was taken inside a quartz U tube and degassed at 200 °C for 1 h in helium gas at a flow rate of 35 mL/min. After cooling to 80 °C, the gas was changed to 3% (v/v) of CO2 in nitrogen and treated at the same temperature for 1 h. The gas was switched over to helium at the same temperature and heated at 10 °C/min up to 600 °C. The desorbed
Scheme 1. Glycerol Hydrogenolysis to 1,2-Propanediol
CO2 was quantified using an online thermal conductivity detector. The acidity of the catalysts was studied by TPD using the above procedure with NH3 as the probe molecule instead of CO2. 2.4. Catalytic Hydrogenolysis of Glycerol. The hydrogenation of glycerol was carried out in a Parr reactor (Parr Instrument Co., Moline, IL) equipped with a temperature controller. Initially, the catalyst was reduced at 300 °C with bubbling hydrogen for 3 h. The reactor was cooled to 100 °C, and then the aqueous glycerol solution was immediately added without further delay. The glycerol hydrogenolysis reaction was conducted at a reaction temperature of 200 °C, a hydrogen pressure of 200 psig, and a stirring speed of 600 rpm. Conversion of glycerol is defined as the ratio of the number of moles of glycerol consumed in the reaction to the total moles of glycerol initially present. Selectivity is defined as the ratio of the number of moles of the product formation to that of the glycerol consumed in the reaction, taking into account the stoichiometric coefficient. Yield is defined as the number of moles of 1,2propanediol in the sample multiplied by the ratio of glycerol to 1,2-propanediol. 2.5. Analysis of Glycerol Hydrogenation Products. The GC analyses of glycerol, acetol, 1,2-propanediol, and 1,3propanediol were carried out on a HP 5890 series II gas chromatograph coupled to a flame ionization detector. Known amounts of glycerol, acetol, and 1,2-propanediol were prepared and diluted 10 times with ethanol prior to the analysis. The separation of acetol, 1,2-propanediol, 1,3-propanediol, and glycerol was performed on a Restek 10638 Stabilwax capillary column (30 m × 0.25 mm × 0.5 µm) with the oven isothermal at 180 °C. 3. Results and Discussion 3.1. Catalysts Screening. Miyazawa et al.12 proposed the combined mechanism of hydrogenolysis and degradation reactions for the synthesis of 1,2-propanediol, 1,3-propanediol, 1-propanol, ethylene glycol, ethanol, and methanol from glycerol. The ideal catalysts for the conversion of glycerol to 1,2propanediol should have higher glycerol dehydration activity to acetol and higher hydrogenation activity of acetol to 1,2propanediol. Recently, Dasari et al.10 reported the conversion of glycerol via a dehydration reaction to acetol (hydroxylacetone) followed by a hydrogenation reaction to selectively produce 1,2-propanediol. The reaction scheme of glycerol conversion to 1,2-propanediol is shown in Scheme 1. The catalyst for the dehydration of alcohol should have either strong Lewis or Brønsted acidic sites.17 Initially, in the present work, the glycerol dehydration/hydrogenation study was carried out using Mg/Al and Zn/Al mixed-metal oxide catalysts with metallic compositions M2+/M3+ ranging from 0.25 to 1, as listed in Table 1. The reactions were carried out with 80% aqueous glycerol, 5 wt % catalyst, and 200 psig hydrogen pressures at a reaction temperature of 200 °C. It was observed that the glycerol conversion was as low as 1% with the formation of traces of acetol, without the formation of hydrogenation products, i.e., 1,2-propanediol. The introduction of a reducing metal, either Ni, Cu, or Co, was expected to carry out the
1842 Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009 Table 1. Catalytic Activities of Mixed-Metal Oxides for Hydrogenation of Glycerol to 1,2-Propanediola catalyst (mixed-metal oxides)
M2+/M3+ atomic ratio
glycerol conversion (%)
Mg/Al Zn/Al Ni/Mg/Al Co/Ni/Mg/Al Cu/Zn/Al Cu/Zn/Al
0.5, 0.75, 1 0.5, 1.0 0.5 3.0 1.50 2.13
