Article pubs.acs.org/IECR
Highly Efficient Production of Acrylic Acid by Sequential Dehydration and Oxidation of Glycerol Rong Liu, Tiefeng Wang,* Dali Cai, and Yong Jin Beijing Key Laboratory of Green Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua University, Beijing 100084, People’s Republic of China ABSTRACT: The selective dehydration−oxidation of glycerol to acrylic acid is a very attractive approach for glycerol utilization. In this work, we demonstrated an efficient two-bed system for this process. The dehydration catalyst was Cs2.5H0.5PW12O40 supported on Nb2O5 (CsPW-Nb), and the oxidation catalyst was vanadium−molybdenum mixed oxides supported on silicon carbide (VMo−SiC). The experimental results showed that the optimum reaction temperature and oxygen ratio for these two catalysts were very similar. This made the two-bed system simple and efficient. Compared with the single-bed system, the twobed system with the dehydration catalyst and oxidation catalyst loaded separately was more favorable to avoid the overoxidation reaction of glycerol. A high yield of acrylic acid was achieved at optimized conditions in the two-bed system. The Brønsted acid sites on the dehydration catalysts were the active sites for acrolein formation. The byproducts produced on Lewis acid sites in the dehydration step and the water in the glycerol feed did not show negative effects on the acrolein oxidation reaction. Both the CsPW-Nb and VMo-SiC catalysts were stable for at least 70 h and had very good thermal stability at the coke burning temperature of 500 °C. tion of glycerol,10−14 but the best yield of acrylic acid is only 34%, because a significant amount of COx is produced on the redox sites by overoxidation of glycerol. This suggests that two different catalysts should be used to separately catalyze the dehydration and subsequent oxidation reactions to increase the yield of acrylic acid. Witsuthammakul and Sooknoi studied the glycerol oxidehydration with zeolites and V−Mo oxides in a sequential two-bed system.15 The optimum reaction conditions for the two catalysts were different in their study. Although the acrylic acid selectivity was increased to 85%, the yield was only 50%, because of the low conversion of acrolein. Dubois et al. studied dehydration of glycerol on heteropoly acid supported on ZrO2 and subsequent oxidation of acrolein on V−Mo oxides and obtained 74% yield of acrylic acid.16 However, the dehydration catalyst used in this work might have the problem of fast deactivation, because it was reported by Chai et al. that the ZrO2 supported 12-tungstophosphoric acid deactivated fast due to coke formation and showed only 70% conversion after 10 h.17 For dehydration−oxidation system, it is a key issue to develop efficient and stable catalytic system for dehydration of glycerol to acrolein and subsequent oxidation of acrolein to acrylic acid under similar optimum reaction conditions. For glycerol dehydration to acrolein, both the strength and type of the acid sites (Brønsted or Lewis) have significant influences on the catalytic performance.8 The addition of a proper amount of oxygen can enhance the selectivity to acrolein and inhibit the coke formation.18,19 For the oxidation of acrolein to acrylic acid, many types of catalysts have been studied, among which the
1. INTRODUCTION Among the biomass-derived raw materials, glycerol has received much attention, because of its continuous overproduction from the biodiesel production process and its capability of being a sustainable and biodegradable raw material to synthesize various chemicals.1,2 Today, nearly two-thirds of glycerol is produced as byproduct of biodiesel process, the annual production capacity of which increases from 1 million tons in 2000 to ∼10 million tons in 2010.3,4 With the fast development of the biodiesel industry, the price of glycerol has decreased from $800/t to $300/t. While there are many recent attempts to utilize glycerol by oxidation, hydrogenolysis, pyrolysis, transesterification, esterification, polymerization, and dehydration, one of the most attractive processes is catalytic dehydration of glycerol to acrolein.5−8 However, acrolein is toxic and very irritating to the skin and eyes. The use of high concentration acrolein is limited to bactericide. Acrolein is usually converted to acrylic acid, which is widely used in polymer dispersions, adhesives, fibers, plastics, and other chemical intermediates. Although many works have been carried out to study the dehydration of glycerol to acrolein over the solid acid catalysts,2,7−9 the study of direct conversion of glycerol to acrylic acid is very limited. From the economic point of view, a subsequent conversion of acrolein to acrylic acid immediately after its formation can avoid the problems of acrolein storage and transportation, and it can make the utilization of glycerol more competitive.3 However, it is difficult to design a single catalyst that possesses both the acid sites and redox sites for oxidehydration of glycerol to acrylic acid. Deleplanque et al.10 studied the glycerol oxidehydration to acrylic acid over mixed-oxide catalysts in the presence of oxygen, and obtained 28% selectivity to acrylic acid over Mo− V−Te−-Nb mixed oxides. The major organic byproducts were acetaldehyde and acetic acid in their study. Some researchers have studied the performance of single catalyst for oxidehydra© 2014 American Chemical Society
Received: Revised: Accepted: Published: 8667
October 1, 2013 April 25, 2014 May 7, 2014 May 7, 2014 dx.doi.org/10.1021/ie403270k | Ind. Eng. Chem. Res. 2014, 53, 8667−8674
Industrial & Engineering Chemistry Research
Article
2.2.2. Preparation of wCsPW-NbT. The catalysts of Cs2.5H0.5PW12O40 supported on Nb2O5 were denoted as wCsPW-Nb, where w was the CsPW loading (in wt %) and T was the calcination temperature. For the preparation of wCsPW-NbT, the support Nb2O5 was prepared first using the same procedure as in section 2.2.1, but without the calcination process. The components Cs2CO3 and H3PW12O40 were then introduced into Nb2O5 using the vacuum-assisted impregnation method. After being separated and dried, the Nb 2 O 5 precipitates were pretreated under a vacuum of better than 0.1 Pa to remove the impurities and trapped air in the porous structure. The obtained Nb2O5 was not calcined before impregnation with Cs2CO3 and H3PW12O40. A desired amount of Cs2CO3 was added to deionized water, and a Cs2CO3 solution (0.10 mol/L) was added to mix with the Nb2O5 powder under vacuum. The vacuum was then turned off and the system was kept overnight at atmospheric pressure and room temperature to let Cs2CO3 get into the micropores and mesopores of Nb2O5 by capillary force. Excess water was removed using a rotary evaporator at 40 °C and dried at 100 °C in air for 2 h. After this, H3PW12O40 was impregnated into the prepared Cs2CO3/Nb2O5 with the same procedure except that a H3PW12O40 solution (0.08 mol/L) was used. The powder after drying was heated in flowing air to 500 °C at 5 °C/min, calcined for 4 h, and then cooled to room temperature at 5 °C/ min. The samples were stored in vials in a desiccator until used. 2.2.3. Preparation of VMo-SiC. For the oxidation reaction, the vanadium−molybdenum oxides supported on the silicon carbide, denoted as VMo-SiC in this work, was synthesized by incipient wetness impregnation.21 First, 0.59 g of ammonium metavanadate (NH4VO3, Alfa Aesar) and 3 g of ammonium heptamolybdate ((NH4)6Mo7O24·4H2O, Alfa Aesar) precursors were dissolved in 11.3 mL of deionized water at 80 °C. A solution of 0.33 g of ammonium oxalato-niobate Nb(NH4)(C2O4)2NbO·nH2O (Aldrich, 99.99%) was then added to this solution while stirring. An aqueous solution of 0.7 g of copper sulfate (CuSO4, Alfa Aesar) dissolved in 1 mL of deionized water was added similarly. 0.3 g antimony trioxide (Sb2O3, Alfa Aesar) was added to the solution. The precursor solution was slowly dropped onto 8.5 g silicon carbide (SiC, 150 μm, Alfa Aesar). The sample was dried at 80 °C for 15 min, and was further dried at 100 °C overnight. Finally, the catalysts were calcined at 380 °C for 5 h in flowing air. 2.3. Catalyst Characterization. The acidic properties of the CsPW-Nb catalyst were characterized with Fourier transform infrared (FT-IR) spectroscopy, using pyridine as a probe molecule. The FT-IR spectrometer (Nicolet NEXUS670) had a heatable IR cell with a KBr window connected to the gas dosing-evacuating system. The powder sample was pressed to self-supporting wafers (10 mm in diameter, 30 mg). The sample was pretreated in flowing argon at 300 °C for 1 h and then cooled to room temperature. Pyridine was adsorbed for 1 h by directing an Ar stream through a pyridine-containing saturator. Physically adsorbed pyridine was flushed away by evacuating for 5 min at room temperature. Pyridine desorption was carried out at 200 °C under high vacuum conditions to accomplish complete desorption of physically adsorbed pyridine. The infrared spectra were recorded in transmission mode with 2 cm−1 resolution and 100 scans. A background correction was made by collecting the spectra without sample prior to the measurement.
vanadium−molybdenum mixed oxides give the best catalytic performance and have been commercialized for the partial oxidation of propylene.15,20−23 However, the use of vanadium− molybdenum mixed oxides in the conversion of glycerol to acrylic acid needs further experimental validation, because of the different reactant composition and operating conditions.8 In the conversion of glycerol to acrylic acid, the steam fraction in the reactant is high and some byproducts are formed in the dehydration step. These may have a negative effect on the oxidation of acrolein. Another issue is that the reaction conditions of the dehydration and subsequent oxidation should be similar, which is favorable to simplify the reactor structure and operation, and it enhances the process efficiency. However, the studies on the partial oxidation of propylene show that the conversion of acrolein to acrylic acid should be performed at 245−300 °C,23 which is lower than the temperature (300−320 °C) for glycerol dehydration.8,20,21 For the dehydration− oxidation of glycerol, a proper temperature should be used because the conversion of glycerol will be incomplete at low temperature and the decomposition of acrolein will be significant at high temperature. Oxygen is reactant in the acrolein oxidation reaction, and it has an effect of inhibiting coke formation and enhancing the selectivity to acrolein in the glycerol dehydration reaction, even though it is not a required reactant for acrolein formation. However, excessive oxygen leads to increased formation of oxyacid and even combustion of glycerol in the dehydration step.24 This work aimed to develop a glycerol-based process for acrylic acid as a viable and sustainable alternative to the conventional propylene-based process. The dehydration catalyst was Cs2.5H0.5PW12O40 supported on Nb2O5 (CsPWNb), and the oxidation catalyst was vanadium−molybdenum mixed oxides (VMo-SiC). The two catalysts had very similar optimum reaction temperature and oxygen ratio. This made the two-bed system simple and efficient. A detailed study on the influence of reaction conditions and dehydration byproduct was conducted. The effect of Brønsted acid sites on the reactions was discussed.
2. EXPERIMENTAL SECTION 2.1. Reagents. Glycerol (Alfa Aesar, 99.99%), acrolein (Xiya Chemical Industries Co., Ltd.), acrylic acid (Alfa Aesar, 99.99%), and ethylene glycol (Alfa Aesar, 99.99%) were used as purchased. Deionized water was prepared by deionized water machine (RFD240HA, Advantec Toyo Kaisha, Ltd.). Oxygen and nitrogen of high purity (>99.99%) were purchased from Beiwen China. 2.2. Catalyst Preparation. 2.2.1. Preparation of Nb2O5. Nb2O5 was prepared by adding 0.4 g/mL aqueous solution of ammonium oxalato-niobate Nb(NH4)(C2O4)2NbO·nH2O (Aldrich, 99.99%) to ethylene glycol to give a volume ratio of 1:2. The pH value of the solution was controlled to be 8−9 by adding 28% ammonia aqueous solution. The solution was kept in a water bath, heated to 65 °C while stirring, and maintained at this temperature for complete precipitation. After precipitation, the solution was cooled to room temperature, and the solid was separated by centrifugation and washed three times with deionized water. The solid obtained was dried at 80 °C in vacuum, calcined in air at 500 °C for 2 h with a heating rate of 5 °C/min, and then cooled to room temperature at 5 °C/min. The obtained Nb2O5 powder was used as catalyst to compare with the supported catalyst wCsPW-NbT. 8668
dx.doi.org/10.1021/ie403270k | Ind. Eng. Chem. Res. 2014, 53, 8667−8674
Industrial & Engineering Chemistry Research
Article
The XRD analysis was performed on an automated powder X-ray diffractometer (40 kV, 40 mA, Bruker, Model D8 Avance) using a Cu Kα radiation source and a nickel filter in the 2θ range of 1−90°. The coke amount on the spent catalysts was measured using thermogravimetric analysis (Netzsch, Model STA 409PC) at a heating rate of 20 °C/min under 50 mL/min O2. 2.4. Reaction Evaluation. The experiments of dehydration−oxidation of glycerol to acrylic acid were carried out under ambient pressure in a fixed-bed reactor (10 mm i.d.). The reactant was fed from the top of the reactor. In the twobed experiments, 0.5 g CsPW-Nb was loaded in the top bed as the dehydration catalyst, and 0.5 g VMo-SiC was loaded in the bottom bed as the oxidation catalyst. The catalysts were diluted with inert quartz particles at a constant ratio of 20, to avoid hot spots. The reactor tube was placed in an electrical furnace with an inner diameter matching the outer diameter of the reactor tube to give a good heat transfer and a long zone of uniform temperature. The particle size of the catalysts was in the range of 325−500 μm. A HPLC pump (Series 2,001−5 mL/min, SS, S. G. Seal Self Flush, Pulse 224 Damper) was used to feed an aqueous solution of 20 wt % glycerol or 12 wt % acrolein into the reactor. Dry nitrogen mixed with oxygen was used as carrier gas, and the flow rates were controlled by mass flow controllers. A thermocouple was inserted in the catalyst bed to measure the reaction temperature. The reaction temperature was controlled with a precision of ±1 °C. The reactor was purged with a N2 flow for 30 min at the reaction temperature before the reactant feed was introduced into the reactor. All reactants and products in the tubings were heated to at least 200 °C, to avoid undesired condensation. The feed was first evaporated by a heated bed of quartz beads before it entered the catalyst bed. The reaction products and unconverted glycerol were collected in a cold trap (−5 °C) at the reactor outlet. The mass balance was determined for each trap. The liquids collected were analyzed offline by a gas chromatograph (GC 7900, Techcomp, Ltd.) equipped with an FID and a TM-SuperWax column (Techcomp, Ltd., 60 m × 0.25 mm × 0.25 μm). The gaseous stream containing oxygen and carbon oxides was fed to an automatic sampling system with a TDX-01 60/80 column (Techcomp, Ltd.) and analyzed by TCD. For comparison, the glycerol dehydration reaction over CsPW-Nb and the acrolein oxidation reaction over VMo-SiC were first separately studied. In such experiments, only the CsPW-Nb or VMo-SiC catalyst was loaded in the reactor. To compare with the two-bed system, the single-bed system was also studied, in which the CsPW-Nb and VMo-SiC catalysts were physically mixed and loaded in one bed. The reaction was carried out for 10 h in most runs. The condensed products during the first 2 h of reaction were not used because they did not satisfy the mass balance. The glycerol conversion, product selectivity, and yield were calculated based on moles of carbon atoms as
product selectivity (%) moles of carbon in a product defined = × 100 moles of carbon in glycerol reacted
product yield = glycerol conversion × product selectivity
The main products found under these conditions were acrolein, acetaldehyde, acrylic acid, acetic acid and hydroxyacetone, and COx. An acceptable carbon balance (>90%) was achieved under optimum reaction conditions. However, a low carbon balance was obtained under anaerobic conditions, because of the significant formation of heavy compounds.
3. RESULTS AND DISCUSSION The sequential glycerol dehydration and oxidation was complicated, and the final yield of acrylic acid was influenced by both the catalyst properties and the reaction conditions.25−31 The effects of CsPW loading on the catalyst properties and reaction results were studied. To determine the optimum reaction conditions, the glycerol dehydration reaction over CsPW-Nb and the acrolein oxidation reaction over VMo-SiC were first separately studied. The dehydration−oxidation process of glycerol was then carried out in a two-bed system loaded with CsPW-Nb and VMo-SiC. 3.1. Effect of CsPW Loading on Catalytic Property and Performance. The XRD patterns of the CsPW-Nb500 samples with different CsPW loadings (5−40 wt %) are shown in Figure 1. The increase of CsPW loading from 5 wt %
Figure 1. XRD patterns of CsPW (denoted by solid circles, ●) and CsPW-Nb500 catalysts with different CsPW loadings: (A) CsPW, (B) 40CsPW-Nb500, (C) 20CsPW-Nb500, (D) 5CsPW-Nb500.
to 60 wt % was characterized in the XRD patterns by an increase in the intensity of the diffraction peaks associated with Keggin structure. The 40CsPW-Nb500 catalysts, as shown in Figure 1B, gave XRD peaks at 2θ = 10.2°, 20.6°, 25.3°, 29.4°, and 34.6°, which were the characteristic diffraction peaks of Keggin CsPW (Figure 1A). For 5CsPW-Nb500 (Figure 1D) and 20CsPW-Nb500 (Figure 1C), no peaks assigned to CsPW were detected, demonstrating that the CsPW species was welldispersed on the Nb2O5 support. The surface acidity of the dehydration catalysts were characterized by FT-IR of pyridine. According to the literature, the bands at 1535−1545 and 1445−1460 cm−1 are characteristic of Brønsted (PyH+) and Lewis (PyL) acid sites, respectively,32−34 and the bands 1480−1490 cm−1 belong to hydrogen-bonded pyridine (hb-Py).32,33 As shown in Figure 2, the CsPW-Nb catalysts showed pronounced Brønsted acidity at
glycerol conversion (%) moles of glycerol reacted = × 100 moles of glycerol in the feed 8669
dx.doi.org/10.1021/ie403270k | Ind. Eng. Chem. Res. 2014, 53, 8667−8674
Industrial & Engineering Chemistry Research
Article
Figure 2. Infrared (IR) spectra of pyridine adsorbed on 40CsPW-Nb (spectrum A), 20CsPW-Nb (spectrum B), 5CsPW-Nb (spectrum C), and Nb2O5 (spectrum D) at 200 °C.
Figure 4. Influence of reaction temperature on glycerol dehydration over 20CsPW-Nb. Conditions: 0.50 g 20CsPW-Nb, and other conditions are the same as in Figure 3.
1538 cm−1 (Figures 2A−C), whereas bare Nb2O5 only showed Lewis acidity at 1447 cm−1 (Figure 2D). Figure 2 also shows the ratio between the Brønsted and Lewis acid sites (B/L) on the catalysts and supports, which was defined as the ratio between the peak areas at 1540 and 1450 cm−1 in the FT-IR spectra. In general, the B/L value was significantly higher on supported CsPW catalysts than on bare Nb2O5 samples. The amount of Brønsted acid sites increased with the CsPW loading, but the highest value of B/L was obtained at CsPW loading of 20%, as shown in Figure 2B. Figure 3 shows the influence of the CsPW loading on the catalytic performance of CsPW-Nb for glycerol dehydration.
95.7% and the yield of acrolein increased from 34.8% to 79.5%. With a further increase of reaction temperature to 360 °C, the selectivity to the products in the acetol route and the C1 and C2 products by C−C bond breakage significantly increased. The major byproducts formed at 360 °C were acetol (8.9%), acetaldehyde (15.7%), acetic acid (8%), and COx (9.7%). Acetaldehyde was formed by decarbonylation of acetol, and was further oxidized to acetic acid. The COx species were formed by the reforming of organic compounds or decarbonylation of aldehydes. Meanwhile, the selectivity to acrolein and acrylic acid decreased to 21.2% and 3.3%, respectively. There were also small amounts of other products, such as allyl alcohol (5.7%), acetone (1.3%), propionic acid (1.6%), and undetectable glycerol oligomers. Based on these results, 300 °C was chosen as the optimum temperature for glycerol dehydration. 3.2.2. Influence of Oxygen/Glycerol Ratio. Several studies have shown that the addition of a proper amount of oxygen can inhibit the coke formation and enhance the selectivity to acrolein in the glycerol dehydration reaction.18,19,35 In the present work, the influence of oxygen was studied with the oxygen/glycerol molar ratio varied from 0 to 20.5 at constant carrier gas (N2 + O2) flow rate of 40 mL/min. The oxygen ratio used in the glycerol dehydration system was high because a large amount of steam (molar ratio: H2O/glycerol = 20.4) could have inhibited the glycerol burning. The results of glycerol conversion and production distribution are shown in Figure 5. As the oxygen/glycerol molar ratio increased from 0 to 12.3, the glycerol conversion increased from 89.0% to 95.7%,
Figure 3. Influence of CsPW loading conversion and selectivity. Conditions: 300 °C, 0.50 g catalyst, glycerol/O2/H2O/N2 = 1/12/2/ 68 (molar ratio), 20 wt % glycerol at 0.6 mL/h (0.24 h−1 glycerol WHSV), TOS of 10 h.
The yield of acrolein increased from 44.3% to 79.5% and the yield of acetol decreased from 13.3% to 3.9% when the CsPW loading was increased from 0 to 20%. Further increases in the CsPW loading to 40% led to a 10% decrease in the glycerol conversion and a 20% decrease in the yield of acrolein. The 20CsPW-Nb catalyst with the highest B/L value was the best dehydration catalyst and it was used in the following study. 3.2. Glycerol Dehydration Reaction. 3.2.1. Influence of Reaction Temperature. The influence of the reaction temperature on the glycerol conversion and product distribution was studied with 20CsPW-Nb at 260−360 °C. The results are shown in Figure 4. The reaction temperature had a greater influence on the selectivity to acrolein than on the glycerol conversion. When the reaction temperature increased from 260 °C to 300 °C, the glycerol conversion increased from 85.0% to
Figure 5. Influence of oxygen/glycerol ratio on glycerol dehydration over 20CsPW-Nb. Other conditions are the same as those described in Figure 3. 8670
dx.doi.org/10.1021/ie403270k | Ind. Eng. Chem. Res. 2014, 53, 8667−8674
Industrial & Engineering Chemistry Research
Article
and the acrolein yield increased from 48.5% to 79.5%. A further increase in the oxygen/glycerol ratio to 20.5 decreased the acrolein yield to 45.2%. The formation of acetaldehyde and propanal, and their oxidation to acetic acid and propionic acid, respectively, were favored at a higher oxygen/glycerol ratio. The coke amount decreased from 2.3 to 0.9 wt % when the oxygen/glycerol ratio increased from 0 to 20.5, indicated that the addition of oxygen could inhibit coke formation. 3.3. Acrolein Oxidation Reaction. 3.3.1. Influence of Reaction Temperature. The influence of the reaction temperature on acrolein oxidation over VMo-SiC was studied using acrolein as a reactant. The results are shown in Figure 6. The Figure 7. Influence of oxygen/acrolein ratio on acrolein oxidation over VMo−SiC. Other conditions are the same as those described in Figure 6.
and acrylic acid to COx, with a yield of 20% at an oxygen/ acrolein ratio of 20.8. 3.3.3. Influence of Dehydration Byproducts. Although the glycerol dehydration reaction had a complex reaction network, acetol was the main byproduct. The influence of the byproduced acetol on the oxidation reaction over VMo−SiC was studied by adding acetol to the feeding acrolein. The results are shown in Figure 8. These experiments also provided Figure 6. Influence of reaction temperature on acrolein oxidation over VMo-SiC. Conditions: 260−360 °C, 0.50 g VMo-SiC catalyst, 40 mL/ min carrier gas (N2 + O2, O2 = 6 mL/min), 12 wt % acrolein solution fed at 0.6 mL/h (0.12 h−1 acrolein WHSV), TOS of 10 h.
acrolein conversion increased with increasing reaction temperature, and complete conversion was obtained above 300 °C. However, the selectivity to acrylic acid began to decrease when the temperature was higher than 300 °C, because of increased selectivities to COx and acetic acid. According to the literature, the optimum reaction temperature for acrolein oxidation catalyzed by molybdenum oxide in the process of propylene to acrylic acid is 245−300 °C.23 In the present work, 300 °C was used to run the glycerol dehydration and acrolein oxidation reactions at the same temperature in the two-bed system. 3.3.2. Influence of Oxygen/Acrolein Ratio. The influence of the oxygen/acrolein molar ratio on the acrolein oxidation reaction was also studied using acrolein as reactant at constant carrier gas (N2 + O2) flow rate of 40 mL/min. As shown in Figure 7, the addition of oxygen had a significant effect on both the conversion of acrolein and the yield of acrylic acid. At an oxygen/acrolein ratio of 12.5, the complete conversion of acrolein and the maximum yield of acrylic acid (98%) were obtained. This oxygen/acrolein ratio was higher than that used in the conversion of propylene to acrylic acid, where a typical H2O/oxygen/acrolein molar ratio was 0.37/2/1,22,23,36 because of the large amount of steam in the reactant feed with H2O/ acrolein molar ratio of 23 in the present work. The high oxygen ratio was also reported in previous study for glycerol dehydration and oxidation to acrylic acid.15 Excessive oxidation of acrylic acid became more favorable at a higher oxygen ratio. As the oxygen/acrolein ratio increased from 12.5 to 20.8, the yield of acrylic acid decreased from 95.5% to 65.6%, and, accordingly, the yield of decomposition products (COx and acetic acid) increased. The decrease in the yield of acrylic acid was mainly attributed to decomposition oxidation of acrolein
Figure 8. Acrylic acid yield (based on acrolein) and the byproduct flow rate for oxidation of acetol/acrolein mixture (acetol/acrolein molar ratio from 0/1 to 1/1) over VMo−SiC. Other conditions are the same as those described in Figure 6.
information for a clearer picture of the reaction scheme of acetol under conditions favorable for the production of acrylic acid. The conversion of acrolein was complete and the yield of acrylic acid based on acrolein was above 95% at acetol/acrolein molar ratios from 0/1 to 1/1. This result was consistent with the previous experiments on acrolein oxidation without addition of acetol. The product flow rate of acetaldehyde and acetic acid increased linearly as the acetol/acrolein ratio increased, and the yield of acrylic acid based on acrolein did not change with the addition of acetol. These results indicated that the presence of acetol had no negative effect on the selectivity to acrylic acid in the acrolein oxidation reaction. 3.4. Dehydration−Oxidation Process. 3.4.1. Comparison between Single-Bed and Two-Bed Systems. The two-bed system of CsPW-Nb and VMo-SiC was compared with the single-bed system that contained physically mixed CsPW-Nb and VMo-SiC catalysts. In the single-bed system, the glycerol dehydration reaction over CsPW-Nb competed with the glycerol oxidation reaction over VMo-SiC, leading to a high yield of small oxygenates and a low yield of acrylic acid (25%), 8671
dx.doi.org/10.1021/ie403270k | Ind. Eng. Chem. Res. 2014, 53, 8667−8674
Industrial & Engineering Chemistry Research
Article
acrolein oxidation to acrylic acid showed the highest acrylic yield (95%) at 20 vol % O2 in N2 on vanadium−molybdenum oxides. As a result, the glycerol oxidehydration at 10 vol % O2 in N2 gave only 50% yield of acrylic acid, because of partial oxidation. This problem was overcome in the present work by using CsPW-Nb and VMo-SiC that needed similar oxygen ratios. 3.4.2. Catalyst Thermal Stability. Although both the CsPWNb and VMo-SiC catalysts were relatively stable within 70 h in the two-bed system, a regeneration process was still necessary for a long-term run. To confirm the thermal stability of the catalysts during the regeneration process, the reactant flow was switched to airflow of 70 mL/min after 70 h of reaction. The catalyst beds were then heated to 500 °C and maintained at this temperature for 3 h. After the regeneration step, the catalysts were cooled to the catalytic reaction temperature, and the reactant flow was reintroduced into the reactor. The glycerol conversion and acrylic acid yield before and after the regeneration process are shown in Figure 10. A similar initial
as shown in Figure 9. The VMo-SiC catalyst had a strong oxidation activity and was active for the C−C bond breaking of
Figure 9. Comparison of (a) the single-bed system and (b) two-bed system. Conditions: 300 °C, 0.50 g 20CsPW-Nb, 0.5 g VMo-SiC, 40 mL/min gas flow rate (O2 = 6 mL/min), 20 wt % glycerol solution fed at 0.6 mL/h (0.24 h−1 glycerol WHSV). Figure 10. Evolution of the acrylic acid yield as a function of reaction time at 300 °C before and after in situ regeneration at 500 °C for 3 h. Reaction conditions are the same as those described in Figure 9.
glycerol. The oxidation sites on VMo-SiC produced more C2 species (15% acetic acid and 10% acetaldehyde) by decomposition oxidation reaction in the single-bed system than in the two-bed system shown in Figure 9b. In addition, the oxidation of acrolein was incomplete in the single-bed system, which could be attributed to the competitive adsorption of acrolein and glycerol on the oxidation sites of VMo-SiC. In the two-bed system, the yield of acrylic acid was mainly controlled by the acrolein yield in the dehydration step. To improve the acrolein yield in the dehydration step, a dehydration catalyst CsPW-Nb was developed. It was efficient at a relatively high oxygen ratio so that the dehydration and oxidation catalysts had similar optimum reaction conditions. A high acrolein yield of 79.5% was obtained in the dehydration step and the final yield of acrylic acid was 75% based on glycerol. This result was better than the highest yield of acrylic acid reported in the literature.11−15 The highest acrylic acid yield was only 34% over a single catalyst like W−V−Nb-O with the acid sites of Nb for acrolein formation and the V species incorporated inside the WO3 structure for acrolein oxidation to acrylic acid.10,13 For two catalysts in a single-bed system, the best acrylic yield was 30% because the direct oxidation of glycerol to undesirable C2 oxygenates was promoted when glycerol was in contact with the oxidation catalyst.13,15 For the two-bed system, Witsuthammakul and Sooknoi15 used HZSM-5 and vanadium− molybdenum oxides as the dehydration and oxidation catalysts, respectively. However, the optimum oxygen ratio for these two catalysts was different. The glycerol dehydration on HZSM-5 showed 80% acrolein yield under anaerobic conditions, but the
induction period was observed during which the selectivity to acrylic acid increased with time. There were some new Brønsted acid sites generated from Lewis sites under high temperature steam during the induction period.37 As is shown in the proposed overall reaction pathways in Scheme 1, the glycerol is dehydrated to 3-hydroxypropionaldehyde in the first step and dehydrated to acrolein in the second step on Brønsted acid sites, which is then oxidized to acrylic acid. The yield of acrylic acid increased by the enhanced formation of acrolein on the newly formed Brønsted acid sites after the induction period. The activity and selectivity in the steady state were the same as those before the regeneration process, indicating that both the CsPW-Nb and VMo-SiC catalysts were thermally stable at the coke burning temperature of 500 °C. 3.5. Overall Reaction Pathways. The main liquid byproduct of the glycerol dehydration−oxidation process was acetic acid. According to the results of acrolein oxidation over VMo-SiC, more than 95% acrolein produced in the dehydration step was converted into acrylic acid. The results of acetol oxidation (Figure 8) confirmed that acetic acid and acetaldehyde were formed from acetol as a byproduct of glycerol dehydration. CsPW was a pure Brønsted acid catalyst with very little basic sites.38 The acetol yield was high on pure Nb2O5 and 5CsPW-Nb500 that had a large amount of Lewis acid sites (Figure 1 and Figure 3), indicating that acetol was favorably formed on the Lewis acid sites. 8672
dx.doi.org/10.1021/ie403270k | Ind. Eng. Chem. Res. 2014, 53, 8667−8674
Industrial & Engineering Chemistry Research
Article
Scheme 1. Proposed Overall Reaction Pathways for the Glycerol Dehydration and Subsequent Oxidation
burning at 500 °C. A scheme of reaction pathways was proposed by dividing the reactions into two dehydration routes: glycerol dehydrated to acrolein on Brønsted acid sites and that to acetol on Lewis acid sites. The produced acrolein and acetol were further oxidized on VMo-SiC to acrylic acid and acetic acid, respectively. The direct conversion of glycerol to acrylic acid is an important approach to utilize glycerol that is produced by the rapidly grown biodiesel industry. Although the dehydration of glycerol to acrolein has been intensively studies and good results have been obtained, it is difficult to industrialize because acrolein is toxic and unstable. This problem is solved in this work by coupling the dehydration of glycerol and subsequent oxidation of acrolein in a two-bed reactor. Our work provides a green and efficient route to produce acrylic acid from glycerol, which is a promise alternative and complement to the petroleum-based production of acrylic acid.
Based on the above discussion, the following reaction pathways were proposed, as shown in Scheme 1. Two dehydration routes exist on different acid sites of the CsPWNb catalyst. On the Brønsted acid sites, the dehydration reaction produces 3-hydroxypropionaldehyde via the first dehydration step, which is unstable and further converts to acrolein via the second dehydration reaction. The produced acrolein is oxidized to acrylic acid in the subsequent VMo-SiC catalyst bed. On the Lewis acid sites, the dehydration reaction produces acetol, which, in turn, is oxidized to acetaldehyde and acetic acid in the VMo-SiC catalyst bed. The total selectivity to the products in the acrolein route of dehydration on Brønsted acid sites (acrolein and acrylic acid) and that in the acetol route on Lewis acid sites (acetol, acetaldehyde, and acetic acid) were calculated for different catalysts. The Brønsted acid sites were enhanced when the CsPW loading increased from 5 wt % to 20 wt %. Accordingly, the selectivity in the acrolein route increased from 64.4% to 84.2%, and the selectivity in the acetol route decreased from 26.8% to 8.9%. On the CsPW-Nb catalysts, the migration of H+ and Cs+ formed an almost-uniform solid solution and the protons distributed widely on the surface,39,40 which was favorable to the protonation of glycerol and the acrolein route of dehydration. In contrast, the bare Nb2O5 catalyst had very low Brønsted acidity that was generated by the interaction of Lewis sites with steam, and had only 48.3% selectivity of the acrolein route. These results confirmed that the glycerol dehydration route highly depended on the type of acid sites. The key to obtain a high selectivity to acrylic acid was to make the dehydration on Brønsted acid sites dominant over the dehydration on Lewis acid sites.
■
AUTHOR INFORMATION
Corresponding Author
*Tel.: 86-10-62794132. Fax: 86-10-62772051. E-mail: wangtf@ tsinghua.edu.cn. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors gratefully acknowledge the financial supports by Supported by Program for New Century Excellent Talents in University (NCET-12-0297).
■
REFERENCES
(1) Zhou, C. H. C.; Beltramini, J. N.; Fan, Y. X.; Lu, G. Q. M. Chemoselective catalytic conversion of glycerol as a biorenewable source to valuable commodity chemicals. Chem. Soc. Rev. 2008, 37, 527. (2) Liu, L.; Ye, X. P.; Bozell, J. J. A comparative review of petroleumbased and bio-based acrolein production. ChemSusChem 2012, 5, 1162. (3) Akizuki, M.; Oshima, Y. Kinetics of glycerol dehydration with WO3/TiO2 in supercritical water. Ind. Eng. Chem. Res. 2012, 51, 12253. (4) Mane, R. B.; Rode, C. V. Continuous dehydration and hydrogenolysis of glycerol over non-chromium copper catalyst: Laboratory-scale process studies. Org. Process Res. Dev. 2012, 16, 1043. (5) Konaka, A.; Tago, T.; Yoshikawa, T.; Shitara, H.; Nakasaka, Y.; Masuda, T. Conversion of biodiesel-derived crude glycerol into useful chemicals over a zirconia−iron oxide catalyst. Ind. Eng. Chem. Res. 2013, 52, 15509.
4. CONCLUSIONS The conversion of glycerol to acrylic acid was studied using the CsPW-Nb and VMo-SiC catalysts in both the single-bed and two-bed systems. The effects of reaction temperature and oxygen ratio were studied for the dehydration and oxidation reactions. A high acrylic acid yield (75%) was obtained in the two-bed system, whereas the acrylic acid yield in the single-bed system was only 25%. The CsPW-Nb and VMo-SiC catalysts had similar optimum reaction temperatures and oxygen ratios. The byproduct acetol in the dehydration step and the large amount of water in the reactant had no negative effect on the acrolein oxidation reaction over VMo-SiC. No deactivation was observed with the CsPW-Nb and VMo-SiC catalysts in 70 h of reaction. Both catalysts can be completely regenerated by coke 8673
dx.doi.org/10.1021/ie403270k | Ind. Eng. Chem. Res. 2014, 53, 8667−8674
Industrial & Engineering Chemistry Research
Article
(6) Posada, J. A.; Rincon, L. E.; Cardona, C. A. Design and analysis of biorefineries based on raw glycerol: Addressing the glycerol problem. Bioresour. Technol. 2012, 111, 282. (7) Katryniok, B.; Kimura, H.; Skrzynska, E.; Girardon, J. S.; Fongarland, P.; Capron, M.; Ducoulombier, R.; Mimura, N.; Paul, S.; Dumeignil, F. Selective catalytic oxidation of glycerol: Perspectives for high value chemicals. Green Chem. 2011, 13, 1960. (8) Katryniok, B.; Paul, S.; Belliere-Baca, V.; Rey, P.; Dumeignil, F. Glycerol dehydration to acrolein in the context of new uses of glycerol. Green Chem. 2010, 12, 2079. (9) Martin, A.; Armbruster, U.; Atia, H. Recent developments in dehydration of glycerol toward acrolein over heteropolyacids. Eur. J. Lipid Sci. Technol. 2012, 114, 10. (10) Deleplanque, J.; Dubois, J. L.; Devaux, J. F.; Ueda, W. Production of acrolein and acrylic acid through dehydration and oxydehydration of glycerol with mixed oxide catalysts. Catal. Today 2010, 157, 351. (11) Dolores Soriano, M.; Concepcion, P.; Lopez Nieto, J. M.; Cavani, F.; Guidetti, S.; Trevisanut, C. Tungsten-vanadium mixed oxides for the oxidehydration of glycerol into acrylic acid. Green Chem. 2011, 13, 2954. (12) Chieregato, A.; Basile, F.; Concepción, P.; Guidetti, S.; Liosi, G.; Soriano, M. D.; Trevisanut, C.; Cavani, F.; Nieto, J. M. L.; Deleplanque, J.; Dubois, J. L.; Devaux, J. F.; Ueda, W. Glycerol oxidehydration into acrolein and acrylic acid over W-V-Nb-O bronzes with hexagonal structure. Catal. Today 2012, 197, 58. (13) Wang, F.; Xu, J.; Dubois, J. L.; Ueda, W. Catalytic oxidative dehydration of glycerol over a catalyst with iron oxide domains embedded in an iron orthovanadate phase. ChemSusChem 2010, 3, 1383. (14) Tichý, J.; Vachula, J.; Machek, J.; Allachverdova, N. C. Catalytic oxidation of acrolein to acrylic acid over a molybdenum−vanadium catalyst. React. Kinet. Catal. Lett. 1986, 31, 159. (15) Witsuthammakul, A.; Sooknoi, T. Direct conversion of glycerol to acrylic acid via integrated dehydration-oxidation bed system. Appl. Catal., A 2012, 413, 109. (16) Dubois, J.; Duquenne, C.; Hoelderich, W.; Holderich, W. Methord for producing acrylic acid form glycerol. U.S. Patent No. US7910771-B2, 2006. (17) Chai, S. H.; Wang, H. P.; Liang, Y.; Xu, B. Q. Sustainable production of acrolein: Gas-phase dehydration of glycerol over 12tungstophosphoric acid supported on ZrO2 and SiO2. Green Chem. 2008, 10, 1087. (18) Ulgen, A.; Hoelderich, W. Conversion of glycerol to acrolein in the presence of WO3/ZrO2 catalysts. Catal. Lett. 2009, 131, 122. (19) Ulgen, A.; Hoelderich, W. F. Conversion of glycerol to acrolein in the presence of WO3/TiO2 catalysts. Appl. Catal., A 2011, 400, 34. (20) Dubios, J. L. Membrane catalyst to synthesize acrylic acid from glycerol, comprises a first layer of a phase able to selectively convert acrolein into acrylic acid a second layer of an acid phase able to selectively dehydrate glycerol to acrolein. World Patent No. WO2008007002-A3, 2008. (21) Teshigahara, I.; Kinoshita, H. Methord for producing acrylic acid form glycerol. World Patent No. WO2006114506-A1, 2005. (22) Dieterle, M.; Mestl, G.; Jäger, J.; Uchida, Y.; Hibst, H.; Schlögl, R. Mixed molybdenum oxide based partial oxidation catalyst: 2. Combined X-ray diffraction, electron microscopy and Raman investigation of the phase stability of (MoVW)5O14-type oxides. J. Mol. Catal. A: Chem. 2001, 174, 169. (23) Ovsitser, O.; Uchida, Y.; Mestl, G.; Weinberg, G.; Blume, A.; Jäger, J.; Dieterle, M.; Hibst, H.; Schlögl, R. Molybdenum oxide based partial oxidation catalyst: Part 3. Structural changes of a MoVW mixed oxide catalyst during activation and relation to catalytic performance in acrolein oxidation. J. Mol. Catal. A: Chem. 2002, 185, 291. (24) De Oliveira, A. S.; Vasconcelos, S. J. S.; de Sousa, J. R.; de Sousa, F. F.; Filho, J. M.; Oliveira, A. C. Catalytic conversion of glycerol to acrolein over modified molecular sieves: Activity and deactivation studies. Chem. Eng. J. 2011, 168, 765.
(25) Cavani, F.; Guidetti, S.; Marinelli, L.; Piccinini, M.; Ghedini, E.; Signoretto, M. The control of selectivity in gas-phase glycerol dehydration to acrolein catalysed by sulfated zirconia. Appl. Catal., B 2010, 100, 197. (26) Kim, Y. T.; Jung, K. D.; Park, E. D. Gas-phase dehydration of glycerol over silica−alumina catalysts. Appl. Catal., B 2011, 107, 177. (27) Kim, Y. T.; Jung, K. D.; Park, E. D. A comparative study for gasphase dehydration of glycerol over H-zeolites. Appl. Catal., A 2011, 393, 275. (28) Chai, S. H.; Wang, H. P.; Liang, Y.; Xu, B. Q. Sustainable production of acrolein: Preparation and characterization of Zirconiasupported 12-tungstophosphoric acid catalyst for gas-phase dehydration of glycerol. Appl. Catal., A 2009, 353, 213. (29) Chai, S. H.; Wang, H. P.; Liang, Y.; Xu, B. Q. Sustainable production of acrolein: investigation of solid acid-base catalysts for gasphase dehydration of glycerol. Green Chem. 2007, 9, 1130. (30) Oliveira, L. C. A.; Portilho, M. F.; Silva, A. C.; Taroco, H. A.; Souza, P. P. Modified niobia as a bifunctional catalyst for simultaneous dehydration and oxidation of glycerol. Appl. Catal., B 2012, 117, 29. (31) Jekewitz, T.; Blickhan, N.; Endres, S.; Drochner, A.; Vogel, H. The influence of water on the selective oxidation of acrolein to acrylic acid on Mo/V/W-mixed oxides. Catal. Commun. 2012, 20, 25. (32) Corma, A. Inorganic solid acid and their use in acid-catalysed hydrocarbon reactions. Chem. Rev. 1995, 95, 559. (33) Emeis, C. A.; Kalevaru, V. N.; Benhmid, A.; Radnik, J.; Pohl, M. M.; Bentrup, U.; Martin, A. Determination of integrated molar extinction coefficients for infrared absorption bands of pyridine adsorbed on solid acid catalysts. J. Catal. 1993, 141, 347. (34) Kalevaru, V. N.; Benhmid, A.; Radnik, J.; Pohl, M. M.; Bentrup, U.; Martin, A. Marked influence of support on the catalytic performance of PdSb acetoxylation catalysts: Effects of Pd particle size, valence states, and acidity characteristics. J. Catal. 2007, 246, 399. (35) Kim, Y. T.; Jung, K. D.; Park, E. D. Gas-phase dehydration of glycerol over ZSM-5 catalysts. Microporous Mesoporous Mater. 2010, 131, 28. (36) Drochner, A.; Kampe, P.; Kunert, J.; Ott, J.; Vogel, H. Steady state isotopic transient kinetic analysis of the acrolein oxidation on Mo-V-W-mixed oxide catalysts. Appl. Catal., A 2005, 289, 74. (37) Chai, S. H.; Wang, H. P.; Liang, Y.; Xu, B. Q. Sustainable production of acrolein: Gas-phase dehydration of glycerol over Nb2O5 catalyst. Green Chem. 2007, 250, 342. (38) Alhanash, A.; Kozhevnikova, E. F.; Kozhevnikov, I. V. Gas-phase dehydration of glycerol to acrolein catalysed by caesium heteropoly salt. Appl. Catal., A 2010, 378, 11. (39) Okuhara, T.; Watanabe, H.; Nishimura, T.; Inumaru, K.; Misono, M. Microstructure of cesium hydrogen salts of 12tungstophosphoric acid relevant to novel acid catalysis. Chem. Mater. 2000, 12, 2230. (40) Kamiya, Y.; Okuhara, T.; Misono, M.; Miyaji, A.; Tsuji, K.; Nakajo, T. Catalytic chemistry of supported heteropolyacids and their applications as solid acids to industrial processes. Catal. Surv. Asia 2008, 12, 101.
8674
dx.doi.org/10.1021/ie403270k | Ind. Eng. Chem. Res. 2014, 53, 8667−8674
