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Hydrogenolysis of Glycerol to 1,2-Propanediol over Nano-Fibrous Ag-OMS-2 Catalysts Ganapati D. Yadav,* Payal A. Chandan, and Devendra P. Tekale Department of Chemical Engineering, Institute of Chemical Technology, Nathalal Parekh Marg, Matunga, Mumbai-40019, India ABSTRACT: Increasing demand for biodiesel production would lead to tremendous supply of glycerol as a coproduct, which is almost 10% by weight. Therefore, utilization of bioglycerol into valuable chemicals is timely to reduce the cost of biodiesel. Both 1,2and 1,3-propanediols are very useful chemicals which can be produced from glycerol. This paper describes a novel process for selective hydrogenolysis of glycerol to 1,2-propanediol over silver incorporated octahedral molecular sieve (OMS-2) catalyst. Also, the current work describes the use of silver as the catalyst for the first time for hydrogenolysis of glycerol. Both batch and continuous mode of operations have been studied to determine the stability of the catalyst. Different loadings (1030% w/w) of Ag were incorporated in OMS-2 by the precipitation method and used in the reaction. Thirty % w/w Ag-OMS-2 was the best catalyst. It was characterized by various techniques such as SEM, TGA, XRD, N2 adsorptiondesorption, EDAX, TPD-TPR, and FTIR. It is a nano fibrous crystalline material. This Article reports a comprehensive account of this catalytic process.The initial studies were carried out in a batch reactor to optimize the reaction parameters. High conversion of glycerol (about 6570%) with about 90% selectivity toward 1,2-propanediol was obtained within 8 h under hydrogen pressure. The time-on-stream (TOS) analysis studies were conducted in a fixed bed reactor up to 150 h, which demonstrated that the catalyst was stable and the process has a potential for commercial exploitation.
1. INTRODUCTION The production of biodiesel as fatty acid methyl esters (FAME) is expected to grow almost 7-fold from 3.8 MMTPA in 2005 to about 25 MMTPA in 2015.13 Biodiesel coproduces 10% w/w of glycerol,4 which must be converted into value added products to reduce the cost of biodiesel on par with petro-based diesel.5 Glycerol with three hydroxyl groups can be converted catalytically to a variety of bulk chemicals such as 1,2- and 1,3propanediols, acrolein/acrylic acid, epichlorohydrin and 3-hydroxypropionic acid as well as many other specialty chemicals. 1,2-Propanediol (1,2-PDO) is a major commodity chemical with a production of over 0.5 MMTPA with overall growth of 4%.6,7 1,2-PDO is manufactured from acrolein and propylene oxide, which are sensitive to international crude prices. A global demand for 1,2-PDO is estimated to rise to ∼11.5 MMTPA, because of its major application in unsaturated polyesters required in fiber glass-reinforced structures and surface coatings. 1,2-PDO also finds applications as antifreeze, coolant, solvent and extractant, deicing agent, and humectant in the pharmaceuticals, cosmetic, animal foods and tobacco industries; and several other sectors of economy such as food industry, petroleum production, sugar refining, papermaking, toiletries, liquid detergents, alkyl resins, printing inks, plasticizers, and hydraulic break fluids. Because of its much lower toxicity, 1,2-PDO is preferred over ethylene glycol.810 The commercial route to produce 1,2-PDO is by the hydration of propylene oxide derived from propylene either by chlorohydrin process or hydroperoxide process. Hydrogenolysis of aqueous glycerol with syngas at 300 atm and 200 °C using homogeneous rhodium complex [Rh(CO)2(acac)] and tungstenic acid leads to 1,3-PDO and 1,2-PDO with 20% and 23% yield, respectively.11 Similarly homogeneous palladium complexes and r 2011 American Chemical Society
methane sulfonic acid in a water-sulfolane mixture resulted in to 1-propanol, 1,2-PDO, and 1,3-PDO in a 47:22:31 ratio.12 Dehydroxylation of glycerol in sulfolane catalyzed by a homogeneous complex of ruthenium as catalyst at 52 atm and 110 °C resulting in to very low yields 1,2-/1,3-PDO is reported.13 The hydrogenolysis of glycerol with copperchromiumbased catalysts yielded only 1,2-PDO.14 Hydrogenolysis of glycerol under 30 MPa H2 at 260 °C over Raney Ni, Ru, Rh, and Ir catalysts yielded mainly methane, but in the presence of Raney Cu, 1,2-PDO was the main product.15 Furthermore, other catalysts such as Cu/C,16 CuPt, and CuRu bimetallic catalysts17 have been reported at 1.04.0 MPa and 493513 K. Hydrogenolysis of glycerol and other polyols over Ni/Re catalyst under 8.2 MPa H2 and 230 °C in 4 h led to 44% 1,2-PDO, 5% 1,3-PDO and 13% ethylene glycol.18 Glycerol hydrogenolysis on several heterogeneous catalysts under 8 MPa H2 pressure at 180 °C was investigated over various metals (Cu, Pd, Ni and Rh), supports (ZnO, C and Al2O3), solvents (H2O, sulfolane and dioxane), and additives (H2WO4) were tested to improve the reaction rate and selectivity.19 Glycerol hydrogenolysis to ethylene glycol and 1,2-PDO was reported with Rh/SiO2 catalysts including the effect of sulfur and temperature.2022 Hydrogenolysis of glycerol over carbon-supported Ru, Pt, and bimetallic PtRu and AuRu catalysts is also reported.23,24 The effects of NaOH and CaO addition on the reaction rates were used to help Special Issue: Nigam Issue Received: March 6, 2011 Accepted: July 15, 2011 Revised: July 3, 2011 Published: July 15, 2011 1549
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Industrial & Engineering Chemistry Research elucidate metal-catalyzed versus base-catalyzed routes in the mechanism of glycerol hydrogenolysis. Hydrogenolysis of glycerol in basic aqueous solution under mild reaction conditions (170 °C, 3 MPa) is studied using Ru/TiO2 as a catalyst.25,26 Lowpressure hydrogenolysis of glycerol to 1,2-PDO was studied using nickel, palladium, platinum, copper, and copper-chromite catalysts.27 Despite several reports, this potentially important reaction is limited to a laboratory scale production because of common drawbacks such as the use of high temperatures (∼ 200350 °C) and pressures (∼ 1030 MPa). Use of dilute solutions of glycerol reduces the average spacetime yield increasing the energy consumption of the process and in turn decreasing the process profitability. Also most of the reactions are studied in batch reactors. In an effort to overcome these drawbacks, our research focused on developing a process knowhow to perform the reaction at lower temperatures and pressures, while simultaneously achieving high selectivity toward 1,2-PDO. No publications or patents so far describe the use of octahedral molecular sieve doped with metal for this reaction.28 Octahedral molecular sieves (OMS) have been used for dehydration reactions. More specifically, OMS-2 having 2 2 tunnel structure (Figure 1) doped with active metal or combination of metals is not reported for the hydrogenolysis of glycerol to PDO. OMS and octahedral layer (OL) materials based on porous mixed-valent manganese
Figure 1. Structure of OMS-2.
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oxides are potentially attractive catalysts because of their unique properties, such as excellent semiconductivity and porosity. Catalytic properties of OMS and OL materials have been shown to be related to the redox cycling of various oxidations states of manganese such as Mn2+, Mn3+, and Mn4+. A new form of OMS-2 was reported by us in a catalyst UDCaT-3, which was used for oxidation, hydrogenation and hydroxylation reactions.28 Also, the current work describes the use of silver as the catalyst for the first time for hydrogenolysis of glycerol. We have carried out both batch and continuous mode of operations to determine stability of the catalyst. Different loadings of Ag on OMS-2 were incorporated by the precipitation method. This paper reports a comprehensive account of this catalytic process.
2. EXPERIMENTAL SECTION 2.1. Chemicals. All chemicals were procured from reputed firms and used without further purification. Glycerol was procured from Merck Ltd., India. Hydrogen (99.9% purity) was purchased from INOX in cylinders. Potassium permanganate, silver nitrate, 2-propanol, and nitric acid (70% w/v) were purchased from M/s s. d. Fine Chem Ltd., Mumbai, India. 2.2. Catalyst Synthesis. Ag-doped manganese oxide octahedral molecular sieves (OMS-2) was synthesized by precipitation method as follows. Manganese acetate (21.0 g) was dissolved in 67.5 cm3 distilled water; to which concentrated nitric acid was added. Silver nitrate (9.97 g) was dissolved in 50 cm3 distilled water and added to acidic manganese acetate solution at room temperature. KMnO4 (13.3 g) in 275 cm3 distilled water was added to the above solution dropwise at 70 °C. The resulting black precipitate was refluxed at 100 °C for 24 h under stirring. The precipitate was washed several times with distilled water to attain a neutral pH, filtered and dried at 120 °C for 12 h to obtain nanofabrics of 30% Ag-OMS-2. These catalysts have typical cryptomelane structure with a one-dimensional 2 2 channel structure (Figure 1).
Figure 2. Schematic representation of fixed bed reactor setup. 1550
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Scheme 1. Some Major Products of Hydrogenolysis of Glycerol Reported in Literature
Scheme 2. Mechanism of Montessier et al.30
2.3. Experimental Setup. 2.3.1. Batch Reactor. Batch experiments were carried out in a stainless-steel autoclave of 100 cm3 capacity (Amar equipments, Mumbai, India). The autoclave was equipped with a standard four blade pitched turbine impeller, temperature controller, speed controller and pressure indicator. 2.3.2. Fixed Bed Reactor. Continuous hydrogenolysis of glycerol was also carried out in a bench scale, high-pressure, fixed-bed reactor (FBR) supplied by M/s Geomechanique, France. A schematic of the reactor setup is shown in Figure 2. This reactor set up consisted of a stainless steel single tube of 0.34 m length and 1.5 102 m i.d. The reactor was heated by two tubular furnaces whose zones (TIC1 and TIC2) were independently controlled at the desired bed temperature. The reactor was provided with mass flow controllers, pressure indicator, controllers (PIC) and two thermocouples to measure the temperature at two different points. A storage tank was connected to the HPLC pump through a volumetric buret to measure the liquid flow rate. The pump had a maximum capacity of 3 104 m3/h under a pressure of 10 MPa. The gasliquid separator was connected to other end of the reactor through a condenser. Similar experimental setup was also used by Rode et al.29
2.4. Reaction Procedure. 2.4.1. Batch Experiments. For a typical experiment, the following procedure was used: 10 g of glycerol, 40 g of 2-propanol, 0.5 cm3 of n-undecane as an internal standard, and 0.1 g of calcium hydroxide (during some experiments) were mixed together and transferred to the autoclave. Catalyst (0.5 g) was then added. Initially the autoclave was purged with H2 and the pressure was raised to 30 atm with a speed of agitation of 1200 rpm. The temperature of the autoclave was allowed to reach 200 °C, at which the total pressure reached 50 atm which was maintained throughout as a dead end reactor. Then, an initial sample was withdrawn. Further samples were drawn at periodic intervals up to 8 h. 2.4.2. Continuous Experiments in FBR. The powdered catalyst was converted into a pellet form by adding 1% sorbitol and applying a pressure of 34 ton for each 1 g of pellet. Each pellet was of 1 102 m length cut into 4 pieces each having 2.5 103 m diameter. 28.0 g Catalyst was pelletized and filled in to the reactor. The sections of 7 cm above and 11 cm below the catalyst bed were packed with carborundum as an inert packing, thus providing a catalyst bed of 16 cm. The reactor assembly was set and the catalyst bed preheated at 200 °C for 2 h with continuous nitrogen flushing to remove adsorbed species from 1551
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Scheme 3. Reaction Products for Glycerol Hydrogenolysis over Ag-OMS-2 in Current Work
Scheme 4. Hydrogenolysis of Glycerol27
Table 1. Effect of Silver Loading in OMS-2 selectivity (%) no. silver loading (% w/w) conversion of glycerol (%) 1,2-PDO acetol 1
10
47.8
86.5
5.6
2 3
15 20
63.8 61.3
88.2 90.1
3.9 2.7
4
30
69.3
91.6
2.1
the catalyst surface. Thereafter, hydrogen pressure was maintained at 30 atm. Hydrogen flow rate was measured continuously and was maintained at 10 NL/h. 20% Glycerol solution was prepared in 2-propanol and used as a feed. The liquid feed was “switched on” after the reactor reached the operating pressure and kept at that value for 1 h to obtain a constant liquid flow rate. The feed flow was initially maintained at 0.5 cm3/min. Samples were analyzed periodically. 2.5. Method of Analysis. The samples were withdrawn periodically from the reactor. For the batch experiment, the decrease in the pressure while taking the samples was again maintained to the original value by allowing more hydrogen. Analysis was carried out by using GC (Chemito, GC 1000) equipped with stainless steel BP-20 capillary column (0.25 i.d., 30 m length). For continuous operation, the samples were analyzed by GC (Varian 3600) equipped with a flame ionization detector and a capillary column (HPFFAP 30 m, 0.53 mm, 1 μm). All
reaction products were confirmed by GC-MS (Clarus 500 Model, Perkin-Elmer) with BPX-5 capillary column (0.25 i.d., 30 m length). 2.6. Catalyst Characterization. 2.6.1. Scanning Electron Microscopy (SEM). Surface morphology of the catalyst sample was captured by SEM (SU 30 microscope, JEOL, Japan). The dried samples were mounted on specimen studs and sputter coated with a thin film of gold to prevent charring. 2.6.2. Temperature Programmed Desorption (TPD) and Temperature Programmed Reduction (TPR). TPD experiments were performed by using AutoChem 2910 instrument of Micromeritics, USA. Acidic and basic sites of the catalyst were determined with ammonia and carbon dioxide as probe molecules, respectively. A quantity of 30 mg of the catalyst was placed in a quartz tube and degassed up to 350 °C under the flow of nitrogen. Then ammonia or carbon dioxide (as per the required test) was passed for 30 min over the catalyst surface. Physisorbed test gas was removed by passing nitrogen at room temperature. Then TPD was conducted up to 300 °C, and the desorbed gas was detected by TCD. Redox nature of the catalyst was characterized by TPR analysis as follows. The sample was first oxidized up to 400 °C under a continuous flow of oxygen and helium gas mixture (5% O2, 95% He). A flow of nitrogen was passed to remove all the physisorbed and excess oxygen at room temperature. Then TPR was carried up to 350 °C. 2.6.3. X-ray Diffraction (XRD). The X-ray scattering measurements were performed by using a Philips PW 1729 powder diffractometer with Cu KR (1.54 A°). The scattered intensities 1552
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Figure 3. SEM Images of 30% w/w Ag-OMS-2 catalysts.
Table 2. TPD Data of 30% w/w Ag-OMS-2 30% w/w Ag-OMS-2
NH3 TPD
CO2 TPD
maximum temperature (°C)
142.1
100.5
volume (mL/g STP)
3.30
5.99
peak concentration (%)
0.03
0.04
Figure 4. NH3 TPD: Determination of acidic sites.
Figure 6. TPR of K-OMS-2 catalyst.
Figure 5. CO2 TPD: Determination of basic sites.
were collected from 2θ values of 10° to 60° by scanning at 0.025° steps with a counting time of 1 s at each step. 2.6.4. Fourier Transform Infrared Spectroscopy (FTIR). FTIR studies of the catalysts were conducted by using a Bruker IFS-66 single channel Fourier transform spectrophotometer. A thin pellet was prepared by mixing the catalyst with spectroscopic grade KBr. The pellet was subjected to a number of scans to record the spectra.
2.6.5. Pore Structure Analysis. Surface area measurements and pore size distribution analysis were done by nitrogen adsorption on Micromeritics ASAP 2010 instrument. BET measurements were carried out at 77 K, after pretreating the sample under high vacuum at 300 °C for 4 h. Surface area and pore volume were derived from N2-adsorptiondesorption isotherms using the conventional BET and BJH methods. 2.6.6. Thermo Gravimetric Analysis (TGA). TGA analysis for weight loss measurement was carried out using Shimadzu 200 °C instrument under nitrogen atmosphere to study thermal stability of the catalyst up to 500 °C. 2.6.7. Energy Dispersive Analysis of X-ray (EDAX). The chemical composition of the catalyst was determined by EDAX using KEVEX X-ray spectrometer JED-2300, JEOL, Japan. 2.6.8. Transmission Electron Microscopy (TEM). Micrographs of the catalyst were obtained using PHILIPS, TEM CM 200 instrument at 2.4 A° resolution. The samples were prepared in 2-propanol and dried on the specimen. 1553
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Table 3. Pore Structure of OMS-2 Based Catalysts BET surface catalyst
pore
area (m2/g) volume (cm3/g)
pore diameter (nm)
K-OMS-2 (reference material)
89.1
0.14
4.63
30% Ag-OMS-2 fresh
67.49
0.15
8.51
Figure 7. TPR of 30% Ag-OMS-2 catalyst.
Figure 10. AdsorptionDesorption Isotherm.
Figure 8. XRD of 30% Ag-OMS-2.
Figure 11. TGA of 30% w/w Ag OMS-2 catalyst. Figure 9. FT-IR of Ag-OMS-2 catalyst.
3. RESULTS AND DISCUSSION 3.1. Product Profile. The published literature shows a variety of products upon hydrogenolysis of glycerol as given in Scheme 1. The mechanism of formation of 1,2-PDO is given by Montassier et al.30 in Scheme 2, whereas Dasari et al.27 have performed hydrogenolysis of glycerol to propylene glycol using nickel, palladium, platinum, copper, and copper-chromite catalysts. At temperatures above 200 °C and hydrogen pressure of 13.6 atm, the selectivity to propylene glycol decreased due to excessive hydrogenolysis of the propylene glycol. In the current studies, the major products of glycerol hydrogenolysis were 1,2-PDO, acetol, and ethylene glycol, with minor
quantities of ethylene glycol as shown in Scheme 3. In the case of batch experiments, the reaction time played a major role on product distribution and thus during short reaction times, acetol was the major product whereas for longer reaction times, the selectivity to 1,2-PDO increased, its being a sequential reaction. Then after much longer times, the formation of ethylene glycol due to severe hydrogenation would occur. This was confirmed when continuous hydrogenolysis was carried out in FBR. A possible mechanism is given in Scheme 4. The amount of silver on OMS-2 had a pronounced effect on product distribution and its effect was studied systematically. 3.2. Effect of Silver Loading in OMS-2. Catalysts with different loadings of silver in OMS-2 were prepared and used in the hydrogenolysis reaction at 50 atm and 200 °C. With an 1554
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Scheme 5. Formation of 1,2-PDO and Acetol over 30% Ag-OMS-2
Figure 12. TEM image of Ag-OMS-2.
Table 4. EDAX Analysis of K-OMS-2 and Ag-OMS-2 Catalystsa mass % w/w 5% Agelement K-OMS-2 OMS-2
10% Ag- 15% Ag- 30% Ag- 30% Ag-OMS-2 + OMS-2
OMS-2
OMS-2
Ca(OH)2 18.81
O
29.1
33.4
33.6
34.4
17.55
K
4.2
3.8
2.6
1.5
0.0
0.0
66.0
58.4
55.0
50.8
52.67
48.47
Mn Ag
0.0
4.3
8.3
12.9
29.78
25.74
Ca
0.0
0.0
0.0
0.0
0.0
6.98
a
N.B. The reference material OMS-2 is K-OMS-2. Part of the data from Yadav and Mewada.31
increase in the amount of silver, the conversion of glycerol was increased (Table 1). That would mean the acetol formed in situ is consumed and converted to 1,2-PDO; 30% w/w silver loading was thus chosen for the further studies. The catalyst was fully characterized. 3.3. Catalyst Characterization. The progress of the reaction indicated that incorporation of metal inside the tunnel framework was responsible for higher activity of the catalyst. In OMS-2 based catalysts, manganese itself is a redox metal which can exist in +2 as well as +4 valence state. In the presence of hydrogen under the reaction conditions, manganese undergoes oxidation and reduction continuously. Hydrogenolysis of glycerol gave high selectivity for 1,2-PDO using these catalysts which was attributed to lower acidic sites on the catalyst surface and its redox nature. Stronger acidic sites will lead to other dehydration products of glycerol like acetol, acetaldehyde, formaldehyde, acrolein, etc. High catalytic activity was attributed to the synergistic effects of metal ions and manganese oxide. Figure 3 shows the SEM image of Ag-OMS-2 catalysts obtained by reflux method respectively at various magnifications. A smoother surface layer is clearly seen from the SEM micrographs for the 30% w/w Ag-OMS-2 catalyst. SEM images show that catalysts are nano crystalline. Fibrous structures are observed. TPD analysis of 30% w/w Ag-OMS-2 with ammonia and carbon dioxide as probe molecule confirms the existence of mild acidic (Figure 4) and basic sites (Figure 5). Table 2 lists the volume of gas desorbed and the maximum temperature of the peak for both the TPD experiments. K-OMS-2 is the reference material which is designated as OMS-2 in literature. For TPR
experiments, incorporation of silver into OMS-2 tunnel resulted in drastic decrease in reduction temperature from 280 °C of K-OMS-2 (Figure 6) to 145 °C for 30% w/w Ag-OMS-2 (Figure 7). This may be due to the strong interaction of silver with OMS-2 which also confirmed the incorporation of metals into the OMS structure. The XRD pattern demonstrated lower crystallinity, with no evidence of any other crystalline phases except pure cryptomelane phase of tunnel manganese oxides (Figure 8). The reflections and 2θ values obtained are characteristic of 2 2 tunnel structured manganese oxide phase. The set of (1 0 1), (0 0 2), (3 0 1), (2 1 1), (3 1 0), (1 1 4) and (6 0 0) reflections at 2θ = 28.7, 37.4, 41.9, 50.0, and 56.0 match the patterns of synthetic cryptomelane (KMn8O16, JCPDS 34168). The crystal domain size of cryptomelane phase derived from the XRD peaks broadening was 20 nm. FT-IR studies show a broad absorption between 3380 and 3420 cm1 (Figure 9) and its shoulders may be assigned to the asymmetric stretching of internal bonds of MnO. The absorbance peaks between wave numbers around 400 and 700 cm1 are typically attributed to MnO vibrations found in OMS-2 cryptomelane materials. These peaks are retained in the IR spectra, which provide strong evidence that OMS-2 structure is retained after incorporation of metal in the tunnel framework. AgO vibrations are observed at 1651 cm1. Table 3 lists the BET surface area, pore volume and pore diameter for 30% w/w Ag-OMS-2. A surface area 67.49 m2/g is obtained. The adsorptiondesorption isotherm is given in Figure 10. It is a type III isotherm, suggesting a porous structure. Such type of adsorption occurs in cases where heats of adsorption are lower than the adsorbate heat of liquefaction. Therefore as adsorption proceeds, additional layers of adsorption are facilitated because the adsorbate interaction with an adsorbed layer is greater than the interaction with the adsorbent surface. The BET surface area of OMS-2 was 89.1 m2/g. Thermogram of 30% w/w Ag-OMS-2 in nitrogen atmosphere shows initial weight loss below 120 °C because of removal of water present on the surface or physisorbed water molecules on the manganese oxide lattice structure (Figure 11). The loss in weight was very low and negligible. Therefore, it was concluded that the catalyst is stable for carrying out high temperature batch liquid phase reactions. Tunnel structure of the catalyst was not broken. Figure 12 shows TEM of 30% w/w Ag-OMS-2 catalyst. A fibrous nanorod shaped structure was observed. EDAX results are shown in Table 4. Absence of K ion showed complete 1555
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Scheme 6. Products of Severe Hydrogenolysis at Longer Residence Times over 30% Ag-OMS-2
Figure 13. Effect of Ca(OH)2 quantity on the conversion of glycerol. Reaction conditions: 20% w/w glycerol solution, 40 cm3 of 2-propanol, 0.5 cm3 of n-undecane, 0.5 g of 30% w/w Ag-OMS-2 catalyst, 50 atm hydrogen pressure, 200 °C.
replacement of K ions with metal ions with Ag inside the tunnel framework. All metal cations have variations on the theoretical and actual compositions. This is probably because different metal cation dopants have different physical and chemical properties that affect the content of substituted cations during synthesis. For AgOMS-2 catalyst, almost 30% Ag ions were retained in the catalyst. The size of Ag ion is favorable to fit into the 4.6 A° tunnel of OMS-2. 3.4. Reaction Mechanism and Product Distribution. Cryptomelane-type manganese oxide octahedral molecular sieves (OMS-2) belong to a group of manganese oxides having a 2 2 tunnel structure formed by edge and corner sharing of manganese oxide (MnO6) octahedra, resulting in 1-D channel (Figure 1). Positive cations, for instance, K+ and water molecules occupy the tunnels. The tunnel cations can be partially or fully ion-exchanged with other ions with appropriate sizes, such as Cu2 + , Co2+, and Ni2+. The metal ions in the tunnel not only balance
Figure 14. Effect of Ca(OH)2 quantity on selectivity.
the charge of the mixed Mn2+, Mn3+, and Mn4+ but can also be active sites for selective catalysis.32 OMS-2 (also referred to as K-OMS-2) structure has Mn ions with mixed valency and the catalyst itself is black in color. Data were collected continuously in the 2θ range of 575° at a scan rate of 1.0 deg/min and the phase identified using a JCPDS database card number 29-1020. All peaks could be indexed to pure cryptomelane phase and no other phases were present. The small size of peaks in the XRD image indicated the small crystallite size in the catalyst. TPD analysis, with ammonia and carbon dioxide as probe molecules, confirmed the existence of mild acidic and basic sites of Ag-OMS-2. Addition of 30% w/w Ag inside the catalyst structure decreases the strong acidic sites and makes it mildly acidic. We have observed that increasing the loading of silver ion from 5 to 15% w/w increases number of weak active sites.31 Two different peaks at lower temperature confirm existence of two different types of acidic sites, one each for Ag+ and Mn4+. 1556
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Figure 15. Effect of catalyst quantity on the conversion of glycerol. Reaction conditions: 20% glycerol solution, 40 cm3 of 2-propanol, 0.5 cm3 of n-undecane, 0.1 g of Ca(OH)2, 30% of Ag-OMS-2 catalyst, 50 atm hydrogen pressure, 200 °C.
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Figure 17. Effect of pressure on the conversion of glycerol. Reaction conditions: 20% w/w glycerol solution, 40 cm3 of 2-propanol, 0.5 cm3 of n-undecane, 0.1 g of Ca(OH)2, 0.5 g of 30% Ag-OMS-2 catalyst, 200 °C.
Figure 16. Plot of initial rate vs catalyst loading.
However, only one peak was seen in CO2-TPD profile. This confirms the redox nature of MnO2 and basic sites are provided by Mn4+. 30% w/w Ag-OMS-2 offered better yield of 1,2-PDO because of its unique properties. Glycerol molecule is more susceptible to the strength of a basic site where the dehydration occurs than to the accessibility of the basic sites. In OMS-2 based catalysts, the existence of manganese in a mixture of oxidation states (+2, +3, and +4) is known.32 In Ag-OMS-2 catalyst, manganese exists in mixed valency Mn3+ (Mn2O3) and Mn4+ (MnO2). In our case, Ag+ ions have been incorporated in the tunnel structure of OMS-2. These Ag+ ions get converted to Ag2+ ions. The reactions taking place in the catalyst in the presence of hydrogen could be depicted as follows: 2AgO f Ag 2 O þ
1 O2 2
1 O2 f 2MnO2 2 Mn3+ and Mn4+ have empty orbitals that act as Lewis acidic sites, which help in the formation of acetol via electron transfer Mn2 O3 þ
Figure 18. Effect of glycerol concentration on the conversion of glycerol. Reaction conditions: 0.5 cm3 of n-undecane, 0.1 g of Ca(OH)2, 0.5 g of 30% Ag-OMS-2 catalyst, 50 atm hydrogen pressure, 200 °C temperature.
mechanism. In the current studies the major products of glycerol hydrogenolysis were 1,2-PDO and acetol, with minor quantities of ethylene glycol, which suggest that a reaction mechanism given by Schemes 5 should hold. The first step involves dehydration of glycerol on basic sites. There is dehydration of glycerol to acetol. In the case of batch experiments, the reaction time played a major role on product distribution and thus during short reaction times, acetol was a major product whereas for longer reaction times, the selectivity to 1,2-PDO increased. When severe hydrogenation is conducted for a long time, consecutive reactions also take place as per the mechanism given by Scheme 6, and the formation of ethylene glycol takes place. This was confirmed when continuous hydrogenolysis was carried out in FBR at different space velocities. The progress of the reaction indicated that incorporation of metal inside the tunnel framework was responsible for higher 1557
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Figure 21. Arrhenius plot. Figure 19. Effect of temperature on the conversion of glycerol. Reaction conditions: 20% glycerol solution, 40 cm3 of 2-propanol, 0.5 cm3 of n-undecane, 0.1 g of Ca(OH)2, 0.5 g of 30% Ag-OMS-2 catalyst, 50 atm hydrogen pressure.
Figure 20. Effect of temperature on the selectivity.
activity of the catalyst. In the current case, the K+ ions are totally replaced by Ag+ which is the hydrogenation site (Table 4). In the presence of hydrogen under the reaction conditions, manganese undergoes reduction continuously. High catalytic activity was attributed to the synergistic effects of metal ions and manganese oxide. Hydrogenolysis of glycerol gave high selectivity for 1,2PDO using these catalysts which was attributed to mild acidic and basic sites on the catalyst surface and its redox nature. Stronger acidic sites will lead to other dehydration products of glycerol like acetaldehyde, formaldehyde, acrolein, etc. These products were not identified in the current case since no stronger acidic sites were available. 3.5. Optimization Studies in Batch Reactor. 3.5.1. Effect of Ca(OH)2. It has been reported that higher pH (alkaline) of the glycerol solution enhances the conversion.24,25 Therefore, a small quantity of Ca(OH)2 was added to the reaction mixture. It was observed that with the increase in Ca(OH)2 quantity, the conversion of glycerol increased marginally. Above 1% w/w of Ca(OH)2, conversion of glycerol was not much affected; hence
1% w/w was taken as the optimum and used for further experiments (Figure 13). The selectivity toward 1,2-PDO was also higher at 1% w/w Ca(OH)2 loading (Figure 14). 3.5.2. Effect of Catalyst Loading. The effect of catalyst loading was studied in the range of 0.2 g (2% w/w of glycerol) to 0.65 g (6.5% w/w of glycerol). The conversion was linear for catalyst loading up to 5% (Figure 15). This is due to the increase in the number of active sites in the catalyst (Figure 16). However, beyond a catalyst loading of 5% w/w, there was no increase in conversion suggesting that more number of active sites were available. Hence further experiments were carried out using 0.5 g catalyst. The selectivity toward 1,2-PDO remains almost 90% at all catalyst loadings. 3.5.3. Effect of Hydrogen Pressure. Effect of hydrogen partial pressure was studied in the range of 1040 atm (Figure 17). Autogeneous pressure was also generated at 200 °C because of the presence of 2-propanol. Therefore, total pressure of the reaction was varied in the range of 3060 atm. The solubility of hydrogen in the solution increases with increase in the pressure. A total pressure of 50 atm gave the maximum conversion of glycerol with high selectivity toward 1,2-PDO. Further increase in the pressure results in the decrease in the conversion of glycerol. This may be due to catalyst deactivation at harsh conditions. 3.5.4. Effect of Glycerol Concentration. Effect of glycerol concentration in 2-propanol was studied in the range of 1030% w/w. It was observed that with change in the glycerol concentration, the conversion of glycerol was greatly affected (Figure 18). The catalyst to glycerol mass ratio was maintained. Therefore, total number of active sites of catalyst decreases for 10% w/w solution which lowers the rate of reaction. For 30% w/w glycerol solution, higher viscosity lowers the rate of reaction. 20% w/w glycerol in 2-propanol was found to give the best results. The selectivity toward 1,2-PDO remains almost same (90%) at all glycerol concentrations . 3.5.5. Effect of Temperature. Effect of temperature on the hydrogenolysis of glycerol was studied from 180 to 210 °C (Figure 19). The total pressure was maintained at 50 atm. It was observed that as the temperature increases, the conversion of glycerol goes on increasing. However, above 200 °C, the selectivity toward 1,2-PDO decreases because it further gets converted to ethylene glycol and methane (Figure 20). Initial rates of reaction were calculated and used to make the Arrhenius plot from which apparent activation energy of 27.4 kcal/mol was 1558
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Table 5. Reusability of the Catalyst 30% w/w Ag-OMS catalysts
conversion of
selectivity to
(reactions studied in a batch reactor)
glycerol (%)
1,2-PDO (%)
fresh with Ca(OH)2
69.3
91.6
first reuse with Ca(OH)2
18.4
87.6
second reuse with Ca(OH)2
16.8
89.6
fresh without Ca(OH)2
59.9
88.9
first reuse without Ca(OH)2
40.3
91.6
Figure 23. Without Ca(OH)2.
Figure 22. With Ca(OH)2.
obtained (Figure 21). The high value of the observed activation energy also suggests that the reaction is intrinsically kinetic controlled. 3.5.6. Catalyst Reusability. The initial stabilization of the reactor with the temperature and flow rate may have resulted in the fluctuation of data obtained. If there is poisoning or C-formation, then the catalyst activity would turn zero after few hours. But loss of catalyst activity was not observed until 92 h. Reusability of 30% w/w Ag-OMS-2 catalyst (with and without Ca(OH)2) was studied. After the completion of the reaction, the reaction mixture was filtered and washed with 2-propanol and then with water to remove the adsorbed species from the catalyst surface. The catalyst was dried in oven at 120 °C for 12 h before its reuse. The experiments with used catalyst reaction in the absence of Ca(OH)2 showed good activity vis-a-vis those with Ca(OH)2 (Table 3). When reaction was carried out with Ca(OH)2, the conversion of glycerol drastically decreased from 60% to 16%. This is due to the adsorption of the Ca+ on the catalyst surface, which results in catalyst deactivation. The surface area and pore size distribution of the reused catalyst without Ca(OH)2 was determined by nitrogen BET. Structural changes were observed which were responsible for the decrease in the conversion of glycerol from 69.3% to 40.3% (Table 5). The selectivity to 1,2-PDO was almost the same. The TPR of all the three catalyst showed decrease in reduction temperature after doping the OMS-2 with metals. TPR of reused 30% w/w Ag-OMS-2 catalyst with Ca(OH)2 showed catalyst deactivation, which lowered the conversion substantially (Figure 22). EDAX analysis clearly shows increase in the
Figure 24. Time on stream data for conversion of glycerol.
deposition of calcium on the catalyst surface which blocks the active sites on subsequent uses reducing the catalyst activity (Table 4). TPR of 30% Ag-OMS-2 catalyst without Ca(OH)2 showed that catalyst activity was retained (Figure 23). Thus, it is concluded that the reuse of Ca(OH)2 was found to have marginal effect on conversion when the catalyst was freshly used. However, repeated use of catalyst with Ca(OH)2 is detrimental since it progressively gets retained on the catalyst surface thereby reducing the hydrogenolysis activity. 3.6. Continuous FBR. As mentioned above, 30% Ag-OMS-2 catalyst was found to be the best catalyst during screening in a batch operation; hence, its time on stream activity was also evaluated for the continuous hydrogenolysis of glycerol. The powdered catalyst was converted into pellet form by adding 1% sorbitol and applying pressure of 34 tons for each 1 g of pellet to make pellets of 1 102 m diameter and cut into 4 pieces each having 2.5 103 m diameter. 3.6.1. Time-On-Stream Stability. The best results were obtained at 200 °C and 50 atm hydrogen pressure in the batch reactor. Therefore, these optimum reaction conditions were used for the continuous system for initial optimization. The reaction was carried out at 200 °C for 92 h to study the stability of the catalyst (Figure 24). Conversion of glycerol was less in the case of fixed bed reactor as compared to that obtained in the batch 1559
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Figure 25. Selectivity profile. Figure 28. Time on stream data at 220 °C.
Figure 26. Effect of feed flow on the conversion of glycerol. Figure 29. Selectivity profile at 220 °C.
Figure 27. Effect of temperature on the conversion of glycerol.
reactor. Initially high conversion of glycerol was obtained, which was steady between 25 to 30%. Selectivity of 1,2-PDO was between 70 and 80% (Figure 25). Major byproduct of the process was acetol. Formation of ethylene glycol was not observed as that in the case of batch reactor. This also proves that the reaction mechanism proposed is correct since further hydrogenation
of acetol to 1,2-PDO is limiting as a sequential reaction and thereafter to ethylene glycol and methane is totally prevented for the residence time. 3.6.2. Effect of Feed Flow Rate. It was observed that the major byproduct is acetol, which is actually an intermediate of the reaction. This showed that the residence time of glycerol in the catalyst bed was insufficient to convert it fully to 1,2-PDO. Therefore, it was thought to provide more time to reaction solution over the catalyst bed by changing the feed flow rate. With the decrease in feed flow rate from 0.5 to 0.3 cm3/min, a drastic increase in the conversion was observed from 25% to 52% (Figure 26) and the selectivity toward 1,2-PDO was between 70% and 75%. Further, this was also an optimum time which did not lead to conversion of 1,2-PDO to ethylene glycol and methane. 3.6.3. Effect of Temperature. The catalyst was found to be highly stable at 200 °C for about 92 h; however, the conversion was lower. Hence, temperature of the reactor was increased while maintaining the pressure and flow conditions. It was observed that the catalyst was more active at 220 °C. The conversion increased from 25% to 60% at 220 °C (Figure 27). The reaction was further continued at 220 °C for 40 h (Figure 28). It was observed that the conversion was high up to 60%. Selectivity toward 1,2-PDO (above 65%) decreases marginally because it is 1560
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4. CONCLUSION A novel 30% w/w Ag-OMS-2 catalyst was developed for the selective hydrogenolysis of glycerol and fully characterized. It is a very efficient catalyst for the hydrogenolysis of glycerol to 1,2PDO at 200 °C and 50 atm, which are milder conditions in comparison with those reported earlier. It possesses distinctive competitive advantages over traditional process which use severe conditions of temperature and pressure. Activation or pretreatment for the OMS-2 based catalyst was not required and the reaction was conducted within short time duration of 8 h in the batch reactor. The use of Ca(OH)2 was found to have marginal effect on conversion when the catalyst was freshly used. However, repeated use of catalyst with Ca(OH)2 is detrimental since it progressively gets retained on the catalyst surface thereby reducing the hydrogenolysis activity. It was not required for the continuous FBR experiments. Catalyst activity was tested before and after the reaction using various techniques such as EDAX, TPR and surface area. The activity and stability of the catalyst was also examined in a continuous fixed bed reactor for 150 h giving a stable and acceptable conversion of glycerol. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected];
[email protected]. Tel: +91-22-3361-1001; Fax: +91-22-3361-1002/1020. Notes
The Institute of Chemical Technology was formerly the Mumbai University Institute of Technology and is now a separate university.
’ ACKNOWLEDGMENT This research was supported by CSIR-NMITLI program under the project on “Bioglycerol based chemicals”. G.D.Y. acknowledges the support from the R. T. Mody Distinguished Professor endowment of ICT and J. C. Bose National Fellowship of Department of Science and Technology, New Delhi (GOI). Thanks are also due to Director, National Chemical Laboratory, Pune and Dr C.V. Rode for their assistance in use of lab facilities. This paper is dedicated to Professor K.D. P. Nigam, a long time friend of GDY and an alumnus of ICT Mumbai, on the occasion of his birthday and as a salute to his contributions to RTD and Chemical Reaction Engineering. ’ REFERENCES (1) Tyson, K. S.; Bozell, J.; Wallace, R.; Petersen, E.; Moens, S. Biomass Oil Analysis: Research Needs and Recommendations; NREL/ TP-510-34796; Golden, Colorado, 2004; pp 7475. (2) Chiu, C. W.; Dasari, M. A.; Sutterlin, W. R.; Suppes, G. J. Removal of residual catalyst from simulated biodiesel’s crude glycerol for glycerol hydrogenolysis to propylene glycol. Ind. Eng. Chem. Res. 2006, 45 (2), 791–795. (3) Mu, Y.; Teng, H.; Zhang, D. J.; Wang, W.; Xiu, Z. L. Microbial production of 1,3-propanediol by Lebsiella pneumoniae using crude glycerol from biodiesel preparation. Biotechnol. Lett. 2006, 28 (21), 1755–1759.
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