Catalytic dehydration of lactic acid to acrylic acid over sulfate catalysts.
The acrylates, including acrylic acid and its alkyl esters, are of great industrial importance in the production of polymers and copolymers such as poly acrylic esters, super-absorber polymers, washing agents, adhesives, etc. (Brockinton et al., 1986). Nearly all of the acrylic acid produced at present is originated from petroleum chemicals by two-step gas-phase oxidation of propylene and the total output is more than three million tons per year worldwide. The rapid rising price of crude oil drives the researchers to find alternative raw materials and new processes for acrylic acid production. Lactic acid fermentation is a matured industrial process, its feedstock covers various biomass (Goto et al., 2004; Oh et al., 2005; Gao et al., 2006) and higher than 95 % atomic utilization ratio can be reached in industrial fermentation with glucose as feedstock. Furthermore, as the application of anaerobic lacto-bacteria (Bai et al., 2003), energy saving is achieved in this process. Lactic acid is capable to produce various chemicals such as acrylic acid, propanoic acid, acetaldehyde, 2,3-pentanedione (Gunter et al., 1994, 1995; Waldley et al., 1997; Varadarajan and Miller, 1999), as well as biodegradable poly lactic acid. All the products from lactic acid are industrial chemicals, and the products can be separated by matured industrial process. The primary lactic acid conversion pathways are shown in Figure 1.
Compared with the dehydration of lactic acid in liquid phase or supercritical water, yield of acrylic acid was higher in gas phase dehydration reaction (Odell et al., 1985; McCrackin and Lira, 1993). The vapour phase dehydration of lactic acid to produce acrylic acid was first reported over CaS[O.sub.4]/[Na.sub.2]S[O.sub.4] catalyst (Holmen, 1958), and the selectivity to acrylic acid of 68% was achieved at 400[degrees]C (Holmen, 1958). In 1988, 58% yield of acrylic acid at 350[degrees]C was reported catalyzed by using [Na.sub.2]HP[O.sub.4] on silica/ alumina with NaHC[O.sub.3] as a pH adjuster (Sawicki, 1988), and 61 % yield of acrylic acid from ammonium lactate versus 43 from lactic acid was observed at 340[degrees]C using AlP[O.sub.4] as catalyst treated with N[H.sub.3] (Paparizos et al., 1988). Walkup et al. (1991) studied the conversion of methyl lactate to methyl acrylate over CaS[O.sub.4]/[Ca.sub.3] [(P[O.sub.3]).sub.2] catalysts at 350[degrees]C, but the maximum total yield of acrylic acid and methyl acrylate was only 33.4%. Formation of the alpha-acetoxy ester of methyl lactate was suggested as a route for converting lactic acid to acrylates early in 1940s (Smith et al., 1942). The production of methyl acrylate via this route led to a rather high yield at 550[degrees]C using inert reactor packing such as pyres, quartz, and carbon, etc. (Fisher et al., 1944). However, the attractiveness of this pathway is unfortunately offset by the high cost of acylating agent.
[FIGURE 1 OMITTED]
Most of previous studies are in the patent literatures (Holmen, 1958; Paparizos et al., 1988; Sawicki, 1988; Walkup et al., 1991) illustrate that sulfate and phosphate, especially their salts of alkali metals and alkaline-earth metals are effective catalysts for converting lactic acid to acrylic acid. Copper was claimed to inhibit the polymerization of acrylic esters (Barnes, 1941); copper, and cupric salts were also efficient catalysts for the decomposition of esters into the corresponding acid and olefin (Bachman and Tanner, 1942); thus copper salts may take effect in the conversion of lactic acid and its derivatives to acrylates. In our previous work, we evaluated the effects of cupric sulfate and phosphate on the dehydration of methyl lactate to acrylates, and got a favourable catalysts combination (Zhang et al., 2008). Therefore, further research work should be performed over this catalysts combination with lactic acid as feedstock. In this work, experiments were carried out in detail to evaluate the combined effects of feed-temperature and catalyst on the acrylic acid yield. The reaction conditions are also optimized to reach high selectivity to desired products while inhibiting the formation of acetaldehyde and propanoic acid on the basis of the reaction mechanism.
Lactic acid (85 wt.%, analytic grade) and methyl lactate (99 wt. %, chemical grade) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, PR China) and diluted to different concentration prior to be used. The high purity nitrogen (99.999 %, analytic grade) and carbon dioxide (99.999 %, analytic grade) are used as carrier gas and diluent to aid in the vapourization of lactic acid. High purity acrylic acid, propanoic acid, acetaldehyde, and other chemicals were used as calibration standards. Hydroquinone and its esters (analytic grade) were added to the feedstock as the polymerization inhibitor.
[FIGURE 2 OMITTED]
Catalyst Preparation and Characterization
In this work, calcium sulfate was applied as main component in the catalyst. Other salts, such as cupric sulfate and phosphate salts ([Na.sub.2]HP[O.sub.4], K[H.sub.2]P[O.sub.4]), were added as promoters. In the preparation of catalysts, anhydrous calcium sulfate and cupric sulfate were intensively dry mixed and ground into fine powder, then mixed with water in which [Na.sub.2]HP[O.sub.4] and K[H.sub.2]P[O.sub.4] were dissolved. After mixing, a cement-like hard material was produced. The mass ratio of in (CaS[O.sub.4])/m(CuS[O.sub.4])/ m([Na.sub.2]HP[O.sub.4])/m(K[H.sub.2]P[O.sub.4]) is 150.0:13.8:2.5:1.2. About 2 h after mixing, the above-hardened materials were crushed, sieved, and calcined at a temperature slightly higher than the operating temperature. This serves to only partially dehydrated calcium sulfate and retain the desired crystalline structure. A Nicolet 560 FTIR spectrometer was used for transmission FTIR spectroscopy of catalysts. Spectra were collected in the middle region 400-4000 [cm.sup.-1] with a resolution of 4 [cm.sup.-1]. N[H.sub.3]-temperature programmed desorption (TPD) was performed on Belcat-B-82 Chemisorption Catalysts Analyzer to get the nature of acid sites of catalysts.
Apparatus and Experimental Procedures
All reactions were performed in a vertical, down-flow fixed-bed reactor. A diagram of the reactor system is given in Figure 2. The reactor body was a stainless steel tube (600 mm in length and 14 mm in inner diameter).
The high-temperature zone of the reactor is heated by a salt bath and the temperature is regulated by a programmable temperature controller with a control thermal couple. The two ends of the reactor pipe were filled with porcelain inert packing in order to preheat the feedstock on the top and prevent the catalyst from falling or plugging the outlet on the bottom. Liquid feed solutions were injected at the top of the reactor by syringe pump along with nitrogen carrier. Reactor effluent is introduced to a cold trap to collect condensable products.
A typical run of experiment includes the loading and activation of catalyst, stabilization of reaction, and sample collection. Each catalyst was typically tested under three to five sets of reaction conditions before the reactor was shut down, cleaned, and reloaded with fresh catalyst. All the experiments in this work were carried out at atmospheric pressure, 250-420[degrees]C, with a liquid flow rate 0.05-0.6 mL [min.sup.-1] and 20 g catalyst loaded in the fixed bed reactor.
Analysis of condensable products is performed by Agilent 6820 gas chromatograph equipped with a HP-innowax capillary column and a FID detector. An internal standard method is adopted in the quantitative analysis. Crude liquid reaction products were filtered with disposable syringe filters to remove minor amounts of impurities. 1 mL filteted liquid products were taken as an analytic sample and 20 [micro]L valeric acid was added to it as internal standard. Good reproducibility of the products analyses was achieved by injecting 0.4 [micro]L samples into the GC injection room. The major components analyzed in the liquid product include acetaldehyde, propanoic acid, acrylic acid, lactic acid, 2,3-pentanedione; minor products include acetic acid, ethanol, acetone, etc. The main gaseous products are CO and C[O.sub.2], which are produced almost quantitatively along with the formation of acetaldehyde according to the primary conversion pathways in Figure 1. In this study, molar yield is defined as mol. % of individual products obtained divided by mol. % of starting material. Selectivity is reported as mol. % of individual products obtained divided by mol. % of starting material consumed.
RESULTS AND DISCUSSION
Effect of Carrier Gas on Lactic Acid Dehydration
In this section, gas dehydration experiments were performed to evaluate the effect of carrier gas. In the dehydration process, 20 g of the catalyst (calcinated at 430[degrees]C for 3 h) was loaded and 26 wt. % lactic acid was used as feedstock; the flow rate of lactic acid solution and carrier were 0.1 and 20 mL [min.sup.-1], respectively. The molar yields versus reaction temperature for acrylic acid, propanoic acid, and acetaldehyde are shown in Figure 3. The 63.7% molar yield of acrylic acid, which was obtained at 330[degrees]C with C[O.sub.2] as carrier gas was much higher than 46.1 % with [N.sub.2] as carrier gas. Both [N.sub.2] and C[O.sub.2] can act as diluent to aid vapourization of feedstock and facilitate the transportation in its tubular reactor, thus inhibiting coke formation. We got higher acrylic acid yield with C[O.sub.2] as carrier gas, which may owe to its special role in the dehydration process. In the dehydration of lactic acid to acrylic acid, acetaldehyde was produced by decarboxylation and decarboxylation, accompanied by the C[O.sub.2] formation. The reaction pathways are shown in Figure 1. Compared with inert [N.sub.2], C[O.sub.2] is one of the products in the conversion of lactic acid to acetaldehyde. The enhancement in acrylic acid molar yield with C[O.sub.2] as carrier gas in the catalytic lactic acid dehydration process can be attributed to the fact that excessive C[O.sub.2] inhibited decarbonylation/ decarboxylation, thus improved the dehydration selectivity.
[FIGURE 3 OMITTED]
Effect of Catalyst Calcination Temperature on Lactic Acid Dehydration
It is well known that the acidity of the sulfate catalyst is affinitive to its calcination temperature. Sets of experiments were performed to investigate the effect of catalysts' calcinations temperature on the dehydration of lactic acid with C[O.sub.2] as carrier gas. The flow rate of 26 wt. % lactic acid solution and carrier gas were 0.1 and 20 mL [min.sup.-1], respectively. The correlations of molar yield versus reaction temperature for products at different calcination temperature are shown in Figures 4-6. FTIR spectra of catalyst at different calcinations temperatures are given in Figure 7. Spectra show sharp peaks at bands 680-595 [cm.sup.-1] and strong absorption at bands 1160-1050 [cm.sup.-1], which suggests the presence of S[O.sup.2-.sub.4]. Peaks at 3548, 3445, 1683, and 1630 [cm.sup.-1] show the presence of water (Simons, 1984). Above 300[degrees]C, dissociative water is driven off as evidenced by the disappearance of peaks at bands 3548 and 1683 [cm.sup.-1]. As a matter of fact, sulfates show strong solid acidity when part of their crystalline water are driven off from their hydrates during calcinations (Takeshita et al., 1965, 1973), so the extent of removal of hydrated water greatly affects the acid properties. N[H.sub.3]-TPD results of catalysts at different calcinations temperatures are listed in Table 1 and five desorption peaks were found by N[H.sub.3]-TPD characterization. The desorption peaks corresponding to different desorption temperature ranges stand for the intensity of acidic sites of catalysts. We can see from Table 1 that acid amounts of weak acid sites (corresponding to low desorption temperature) get decreased as catalysts' calcinations temperature elevated, while amounts of strong acid sites (corresponding to high desorption temperature) showed the opposite trend when the calcinations temperature was lower than 460[degrees]C. In this work, highest yields of acrylic acid are observed over the catalyst calcinated at 430[degrees]C, which can be attributed to the moderate solid acidity of catalysts. This can also be explained as that crystalline water of catalysts was partly removed, which produced reasonable acid amounts of weak acid sites and strong acid sites. As for propanoic acid and acetaldehyde, however, the catalysts calcinations temperatures have little effects on the correlations of molar yields versus reaction temperature. The trends can be observed in Figures 5 and 6.
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
Effect of Feed Concentration on Lactic Acid Dehydration
In this part, experiments were performed at 330[degrees]C to evaluate the effect of feed concentration on products yields; the results are listed in Table 2. The catalyst was calcinated at 430[degrees]C for 3 h; the flow rate of lactic acid and carrier C[O.sub.2] were 0.1 and 20 mL [min.sup.-1], respectively. A lower feed concentration is favourable for the formation of products, especially acrylic acid. The highest acrylic acid molar yield of 63.4% was obtained at 330[degrees]C with 25 wt. % lactic acid as feedstock, and the final concentration of acrylic acid in products aqueous solution can reach up to 12.9 wt. %. As for acetaldehyde and propanoic acid, the yield did not increase gradually as feed concentration increased. This trend is not similar to the correlation of yield versus reaction temperature in which yields increase steadily as the temperature elevated. It is generally thought that vapourization of lactic acid is usually not complete due to self-polymerization at reaction temperature. This can be responsible for the high yields of unknown products and lower yields of acetaldehyde, propanoic acid, and acrylic acid with >25 wt.% lactic acid as feed. Low molecular weight lactic acid polymers, which constitutes the unknown products are essentially nonvolatile and deposit on the surface of the catalyst, which lead to coke formation when polymers decompose on prolonged heating. Therefore, the feed volatility plays an important role in dehydration process; low feed concentration should be accepted in lactic acid dehydration. However, high yield of acrylic acid is also hard to gain at lower than 15 wt. % lactic acid, which may be owed to the inhibition effect of water in feedstock.
Effect of Contact Time on Lactic Acid Dehydration
Contact time is defined as the time in seconds required for a unit volume of gaseous reactants (including carrier gas) to traverse one unit volume of contact material at the experimental temperature and pressure, on the assumption that no volume change occurs. In this article, the volume of contact material is the volume of catalyst packed. The formula used to calculate contact time (Smith et al., 1942) is:
TC = 3600 x 273 x [V.sub.c]/22, 400([N.sub.R] + [N.sub.c]) x T
where TC is the time of contact (s), [V.sub.c] the contact material volume (mL), [N.sub.R] the moles of reagent passed per hour, [N.sub.c] the moles of carrier gas passed per hour, T the experimental temperature (K).
The correlations of products yields with contact times at different temperatures are shown in Tables 3-5. The flow rate of 26 wt. % lactic acid and carrier gas C[O.sub.2] were 0.05-0.6 and 20 mL [min.sup.-1], respectively. It can be observed that the highest acrylic acid yield of 63.7 % was achieved with a rather long contact time of 88 s at 330[degrees]C. Compared with the results at 330[degrees]C, the corresponding contact times to get highest acrylic acid yields at 360 and 400[degrees]C were 29 and 12 s, respectively, which were remarkably shorter.
The reason that we can not get rather high acrylic acid yield with long contact time at elevated reaction temperature can be due to the occurrence of side reactions from lactic acid. As contact time prolonged, decarbonylation/decarboxylation and reduction of lactic acid to form propanoic acid occur more easily, especially at elevated temperatures. This can be responsible for the steady increase in molar yield of propanoic acid and acetaldehyde as contact times prolong at all reaction temperatures listed. Other side reactions such as dimerization or self-polymerization of less volatile lactic acid are easy to occur, which leads to the decrease in acrylic acid yield. As for the high yields of unknown products gained in most cases, the dimerization and self-polymerization of lactic acid should be the direct reason. The nonvolatile polymers with low molecular weight will form coke on the catalyst surface when decomposed on prolonged heating. Carbon deposition on the catalyst was measured as the weight loss of dried residual catalysts during heating in air at 500[degrees]C for 12 h. Based on the analysis, the amount of coke deposited was as much as 1.3 wt. % of catalyst. In the dehydration process, lactic acid can be reduced by reductive materials to form propanoic acid. What needs to be emphasized is that the reductive materials include activated carbon support (Gunter et al., 1995), formed coke on catalysts surfaces or hydrogen originated during dehydration process. In Table 5, molar yield of acrylic acid from 26 wt. % lactic acid is only 0.7% at 400[degrees]C and 60 s contact time, while molar yield of propanoic can reach up to 25% at the same condition. As coke formation is easily to occur at high temperature, the reduction effect of coke should be listed as one of the important factors which account for the high yield of propanoic acid at 400[degrees]C. Therefore, the formed coke may be the most probable reductant origin in this work. In Table 5, the yield of acrylic acid decreased greatly as contact time prolonged and this can be explained as the occurrence of second-reaction of acrylic acid. The concomitant high yields of unknown products may be the direct evidence.
The catalysts' activity gets strengthened as the temperature elevated and the side reactions such as decarbonylation/ decarboxylation, second-reaction by polymerization and reduction are also easy to occur at the elevated temperature. Therefore, it is difficult to obtain high molar yield of acrylic acid with longer contact time at the elevated temperature. However, in Table 3 we obtained the highest yield of acrylic acid at 330[degrees]C with a rather long contact time, the side reactions are not catalyzed effectively at low temperature may be the key role.
Catalytic dehydration of lactic acid to acrylic acid, propanoic acid, and acetaldehyde over calcium sulfate catalyst with cupric sulfate and phosphate salts as promoters were carried out at 250-420[degrees]C range. The best molar yield of acrylic acid of 63.7% is gained at 330[degrees]C and 88 s contact time with carbon dioxide as carrier gas.
In this work, the optimum catalyst with moderate solid acidity was obtained by adding promoters and manipulating calcination temperatures. In order to inhibit the occurrence of side reaction, carbon dioxide was taken as carrier gas to improve the selectivity to acrylic acid. However, more direct evidences are still needed to evaluate the effect of carbon dioxide on catalyst acidity.
An understanding of the key factors, which are responsible for the catalytic activity is critical to the dehydration of lactic acid, and this is also the subject of our continuing efforts. In this study, we have concentrated on evaluating the effect of catalyst calcination temperature, carrier gas, feedstock as well as contact time on lactic acid conversion, and this will be the focus, aiming to improve the acrylate yield in the future studies.
This work was supported by National Basic Research Program of China (No. 2007CB707805 and 2004CCA05500).
Manuscript received 9 July 2007; revised manuscript received 17 June 2008; accepted for publication 6 August 2008.
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Jinfeng Zhang, Jianping Lin * and Peilin Cen
Institute of Bioengineering, College of Materials Science and Chemical Engineering, Zhejiang University, Hangzhou 310027, PR China
* Author to whom correspondence may be addressed. E-mail address: email@example.com
Table 1. Results of N[H.sub.3]-TPD at different calcination temperatures N[H.sub.3]-TPD desorption Surface acid amount of catalyst at temperature ([degrees]C) different calcination temperatures (mmol/g) 300 370 400 100-150 0.0601 0.0509 0.0158 150-240 0.2295 0.0949 0.0462 240-300 0.1832 0.0511 0.0247 300-550 0.8446 0.5470 0.3662 >550 0.6909 0.7290 0.8542 Total 2.008 1.4729 1.3071 N[H.sub.3]-TPD desorption Surface acid amount of catalyst at temperature ([degrees]C) different calcination temperatures (mmol/g) 430 460 100-150 0.0083 -- 150-240 0.0345 -- 240-300 0.0173 -- 300-550 0.2109 -- >550 0.9598 0.6774 Total 1.2308 0.6774 Table 2. Products yields from lactic acid over catalyst at 330[degrees]C Concentration of lactic acid (wt.%) 5 15 25 35 Acetaldehyde (mol.%) 23.4 29.4 19.6 20.8 Acetic acid (mol.%) 0 2.0 1.6 0.9 Propanoic acid (mol.%) 30.4 18.4 10.4 8.3 Acrylic acid (mol.%) 43.9 45.9 63.4 34.5 Unkown products (mol.%) 2.3 4.3 5 35.5 Final concentration of 1.8 6.2 12.9 8.1 acrylic acid (wt.%) Concentration of lactic acid (wt.%) 45 55 65 Acetaldehyde (mol.%) 21.5 24.9 25.6 Acetic acid (mol.%) 0.9 0.7 0.6 Propanoic acid (mol.%) 6.6 6.7 7.8 Acrylic acid (mol.%) 27.4 23.4 22.1 Unkown products (mol.%) 43.6 44.3 43.9 Final concentration of 8.4 9.1 10.2 acrylic acid (wt.%) Table 3. Products yields with different contact time at 330[degrees]C Contact time (s) 8 13 30 50 Acetaldehyde (mol.%) 4.8 6.8 10.2 13.8 Acetic acid (mol.%) 0.3 1.6 2.5 1.6 Propanoic acid (mol.%) 4.1 5 6.3 8.5 Acrylic acid (mol.%) 8.5 19.0 26.9 28.6 Unknown products (mol.%) 82.3 67.6 54.1 47.5 Contact time (s) 60 68 79 88 Acetaldehyde (mol.%) 15.2 15.8 16.2 16.8 Acetic acid (mol.%) 2.0 1.2 1.5 1.6 Propanoic acid (mol.%) 8.9 9.0 11.3 12.3 Acrylic acid (mol.%) 37.8 55.0 62 63.7 Unknown products (mol.%) 36.1 19.0 9.0 5.6 Table 4. Products yields with different contact time at 360[degrees]C Contact time (s) 6 13 15 29 Acetaldehyde (mol.%) 7.8 11.2 12.5 14.5 Acetic acid (mol.%) 0.3 1.4 1.7 1.9 Propanoic acid (mol.%) 6.3 7.5 8 10 Acrylic acid (mol.%) 13.2 30 33.7 41 Unknown products (mol.%) 72.4 49.9 44.1 32.6 Contact time (s) 64 82 95 Acetaldehyde (mol.%) 16.5 17.9 18.1 Acetic acid (mol.%) 2.0 1.9 2.2 Propanoic acid (mol.%) 11.8 13 15.7 Acrylic acid (mol.%) 31.8 25.1 22.1 Unknown products (mol.%) 37.9 42.1 41.9 Table 5. Products yields with different contact time at 400[degrees]C Contact time (s) 12 23 31 43 Acetaldehyde (mol.%) 13.5 14.9 15.6 16.8 Acetic acid (mol.%) 1.4 2.1 2.6 4.1 Propanoic acid (mol.%) 10.4 11.8 13.2 13.9 Acrylic acid (mol.%) 31.0 16.4 11.8 4.5 Unknown products (mol.%) 43.7 54.8 56.8 60.7 Contact time (s) 50 53 59 Acetaldehyde (mol.%) 18.5 20 22.5 Acetic acid (mol.%) 6.8 6.9 6.9 Propanoic acid (mol.%) 15.8 18 25 Acrylic acid (mol.%) 1.9 0.8 0.7 Unknown products (mol.%) 57 54.3 44.9