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Ind. Eng. Chem. Res. 2008, 47, 6878–6884
Reducing Byproduct Formation during Conversion of Glycerol to Propylene Glycol Chuang-Wei Chiu, Ali Tekeei, Joshua M. Ronco, Mona-Lisa Banks, and Galen J. Suppes* Department of Chemical Engineering, W2033 Lafferre Hall, UniVersity of Missouri, Columbia, Missouri 65211
During the conversion of glycerol to propylene glycol, highly selective conversion is necessary for commercial viability. The greatest strides in achieving high selectivity are attained with catalyst and temperature. For the conversion of glycerol to propylene glycol, these parameters can be optimized to achieve selectivities of greater than 80%. This paper is on the optimization of more-subtle parameters such as concentration, water content, pressure, isothermal operation, and residence time to achieve selectivities in excess of 90%. Data reveal that low concentrations are important to reduce by-product whose formation relies on second-order reaction mechanisms. Water is important to reduce dehydration reactions and indirectly helps to maintain more-isothermal operation. An optimal hydrogen partial pressure between 5 and 15 bar minimizes the cumulative amount of by-product that results from hydrocracking versus dehydration side-reactions. Introduction Petroleum is a diverse mixture of hydrocarbons, and chemical syntheses using petroleum feedstocks tend to be highly reliant on separation processes to compensate for relatively nonselective conversions. In contrast, many biomass feedstocks have moreuniform chemical compositions with respective opportunities to have more-selective conversions during chemical synthesis. Glycerol is an example of a chemical that can be isolated from vegetable oils at high purity with relatively minimal processing. Glycerol is a biomaterial of particular relevance due to the burgeoning biodiesel industry. For purposes of the present study, the conversion of glycerol to propylene glycol (PG) is achieved through a reactive intermediate (acetol). First, glycerol is dehydrated to form acetol, and then, this acetol is hydrogenated in a further reaction step to produce propylene glycol as illustrated by reaction scheme 1.
In the formation of propylene glycol, competing reactions may exist that consume glycerol and acetol as well as subsequent reactions that consume propylene glycol. For this reaction over copper chromite at 220 °C, the reaction of glycerol to propylene glycol achieves a high selectivity toward propylene glycol, while exhibiting little selectivity toward ethylene glycol and other unknown by-product at optimal conditions. Recently, the gas-phase hydrogenation of glycerol to propylene glycol has produced technology advances that are needed for commercial viability of this process.1,2 High yields of 1,2 propane diol have been achieved with the use of a copper chromite catalyst. The gas-phase environment is well-suited for use of packed-bed catalysts with catalyst lives of several months. The gas-phase process is different from prior art. The most common conventional route of production is through hydro* To whom correspondence should be addressed. Tel.: +1- 573-8840562. Fax: +1-573-884-4940. E-mail address:
[email protected].
genolysis of liquid-phase glycerol, sugars or sugar alcohols at high temperatures and pressures in the presence of a metal catalyst producing propylene glycol and other alcohols.3–5 The selectivities toward propylene glycol are compromised due to the increased number of reactions possible at these severe conditions. In a recent publication,6 the authors proposed the two-step process of eq 1 for a highly selective production of propylene glycol from glycerol via a reactive acetol intermediate. More in-depth reviews on literature related to the production of propylene glycol are available in works of Dasari, Chiu, and Rivera-Ramos.6–9 This two-step process has a first dehydration step that produces acetol in high yield. Acetol then reacts in high selectivity to propylene glycol. Acetol has usually been prepared by the reaction between bromoacetone and sodium or potassium formate or acetate, followed by hydrolysis of the ester with methyl alcohol. Treatment of glycerol10 or propylene glycol11 at 200-300 °C with a dehydrogenating catalyst leads to the formation of acetol, while the direct oxidation of acetone with Bayer-Villiger acetone-peroxide reagent furnishes acetol together with pyruvic acid. Acetol is extremely reactive as it contains both hydroxyl and carbonyl functional groups. Accordingly, acetol may undergo a variety of reactions including polymerization, condensation, dehydration, and oxidation reactions. This paper proposes hydrogcracking as a possible reaction mechanism for producing ethylene glycol from glycerol as an alternative to retro-aldol and decarboxylation mechanisms. The retro-aldol reaction is associated with highly alkaline aqueous environments,12 and so, the gas-phase reactions of this paper over copper-chromite catalyst are not expected to exhibit this mechanism. Decarboxylation reactions would not be significantly impacted by increased hydrogen concentrations, and so, decarboxylation reactions are not expected to be a predominant mechanism for cleavage reactions that increase with increasing hydrogen partial pressures. Less is reported on the hydrocracking of sugars than for hydrocarbons. Arena13 as well as Andrews and Klaeren14 report the heterogeneous hydrocracking, cleaving of carbon-carbon single bonds, of six carbon sugars like sorbitol to form glycerol. Andrews and Klaeren15 also report the homogeneous cleavage of fructose to form glycerol. Ying16 reported the production of
10.1021/ie800300a CCC: $40.75 2008 American Chemical Society Published on Web 08/05/2008
Ind. Eng. Chem. Res., Vol. 47, No. 18, 2008 6879 Table 1. Impact of Water on Reactiona glyc (wt %) 75% 50% 25% 75% 50% 100% 75% 50% 25% 75% 50% 25% 75% 50%
T (°C)
H2 (L/min)
Ac
PG
230 230 230 260 260 230 230 230 230 230 230 230 260 260
2-3 2-3 2-3 2-3 2-3 5 5 5 5 0 0 0 0 0
1145 844 624 1403 1305 927 826 680 515 1305 1048 792 1083 802
822 435 266 850 430 737 646 474 357 190 197 145 75 59
EG
total BP
product mass ratio
23 15 9 61 27 18 14 10 10 9 6 4 0 0
707 162 54 661 308 283 209 115 54 743 295 144 355 222
2.8 7.9 16.4 3.4 5.6 5.9 7.1 10.0 16.1 2.0 4.2 6.5 3.3 3.9
a Reactions include gas-phase flow through a 0.5 in. i.d. reactor packed with approximately 85 g of copper-chromite catalyst with glycerol (or acetol) feed rates of about 200 g/h. Abbreviations include glyc, H2, Ac, PG, EG, and PB for glycerol, hydrogen, acetol, propylene glycol, ethylene glycol, and by-products. The percent glycerol is the percent glycerol in the liquid feed with the remainder water. Reactions were at 1 bar of pressure, absolute. Values are GC peak area divided by 106. The product mass ratio is the sum of Ac and PG peak areas divided by the BP peak areas.
Figure 1. Reactor configuration for gas-phase packed-bed catalysis reactions. An evaporator precedes the reactor where the liquid reagent is evaporated while in contact with hydrogen. A condenser after the reactor collects liquid for GC analysis.
ethylene glycol from saccharide or aldtol. However, hydrocracking of glycerol, which is a three-carbon rather than a sixcarbon sugar, has not previously been reported. The present work focuses on the parameters and potential mechanisms that can lead to loss of the desired product (propylene glycol and acetol) through side-reactions that produce undesired by-product. To understand trends in by-product formation, the catalyst hydrogenation of ethylene glycol was studied. No prior art has been located on the hydrogenation of ethylene glycol. Experimental Section A packed-bed reactor was used to produce propylene glycol from glycerol by means of a heterogeneous catalytic vapor phase reaction. Prior to reaction a glycerol feed was evaporated in an evaporator, and after reaction a cooling coil condensed the products from a hydrogen discharge. Most of the data were obtained using the packed-bed reactor illustrated by Figure 1. A circulating oil bath maintains the reaction temperatureseither heating or cooling as necessary. A 650 g portion of prereduced copper-chromite catalyst (from Engelhard Corporation, Elyria, Ohio) was loaded in the reactor. The reactor was 0.75 in stainless steel Swagelock tubing and could accommodate up to 8 ft of packing. All reactions were performed in the vapor-phase packed-bed reactor with a glycerol feed rate of 100 g/h and a hydrogen flow rate of 5 L/min.
An alternative but similar system and operating conditions were used for the data of Table 1. That system is described in the footnote of Table 1. Since both the dehydration and hydrogenation reactions are highly exothermic, both the heating/cooling oil and the diameter of the reactor can significantly impact product selectivity. Thermocouples were placed six inches in the packed bed, axially, from both the entrance and exit sides of the packed bed. Propylene glycol and the seven most-concentrated by-product were studied over a range of reaction operating conditions. The seven by-product had GC retention times of 8.74, 8.78, 9.11, 9.15, 9.28, 9.32, and 9.405 min. Peak 9.11 is ethylene glycol (EG). The reactions were carried out at 1, 2, and 4 bar and a temperature range of 180-240 °C. Unless otherwise stated, the data are at 100% conversion of glycerol or at conditions near equilibrium for propylene glycol or acetol as the feed reagents. Conversion is defined as the mass of reagent that reacts divided by the mass initially present. Residence times shortly after 100% conversion were selected for operation because these are near-optimal conditions. Higher residence times lead to greater by-product formation. Unless otherwise stated, the reported data are averages of 2-3 data points at the same conditions. Analyses were performed in a Hewlett-Packard 6890 (Wilmington, DE) gas chromatograph equipped with a flame ionization detector. Hewlett-Packard Chemstation software was used to collect and analyze the data. A Restek Corp (Bellefonte, PA) MXT WAX 70624 GC column (30 m × 250 µm × 0.5 µm) was used for separation. For preparation of the GC samples, a solution of n-butanol with a known amount of internal standard was prepared a priori and used for analysis. The samples were prepared for analysis by adding 100 µL of product sample to 1000 µL of stock solution into a 2 mL glass vial. A 2 µL portion of the sample was injected into the column. The oven temperature program consisted of the following: start at 45 °C (0 min), ramp at 0.2 °C/min to 46 °C (0 min), ramp at 30 °C/min to 220 °C (2.5 min). Concentrations of unknown by-product were compared using calibration constants generated for propylene glycol
6880 Ind. Eng. Chem. Res., Vol. 47, No. 18, 2008 Table 2. Reactions at Conditions Similar to Those of Table 1 except with Acetol As the Reactor Feed Rather than Glycerola acetol (wt %)
T (°C)
H2 (L/min)
Ac
PG
100% 75% 50% 25%
230 230 230 230
5 5 5 5
1399 1209 905 966
2082 1554 1276 1305
EG
total BP
product mass ratio
0 0 0 0
913 581 365 322
3.8 4.8 6.0 7.0
a Feed is acetol in the indicated weight percent in water. Values are GC peak area divided by 106.
Figure 2. Impact of hydrogen and water concentration on ratio of desired products to sum of undesired by-product at 230 °C. The “product mass ratio” is the mass of acetol (Ac) and propylene glycol (PG) divided by the mass of all by-product (BP) except for ethylene glycol. Reactions were conducted at 1 bar of pressure, absolute.
relative to an internal standard (IS) added during condensate product preparation for GC analysis. Results and Discussion Table 1 summarizes reaction conditions and reaction conversions with the purpose of evaluating the impact of water and hydrogen on product quality. Higher product qualities have higher ratios of desired products divided by by-product. Both acetol and propylene glycol are desired products. In all cases, higher water contents provided improved ratios. In the conversion of glycerol to propylene glycol over copper chromite catalyst the optimal yields are attained at 200-220 °C. Many earlier investigations of copper chromite in this application at higher temperatures revealed very low yields due to over-reaction on the catalyst. The ratio of desired products (acetol and propylene glycol) to by-product in the absence of hydrogen reveals higher selectivities at lower temperatures. Interestingly, at zero hydrogen flow propylene glycol is formed. It is believed that the acetol is scavenging hydrogen from glycerol to produce propylene glycol. This would also explain (in part) the higher product mass ratios when hydrogen are present. An anomaly in the data set is the comparison of product ratios at 75% glycerol at 2-3 L/m of hydrogen. In this case, the byproduct ratio of desired products appears to be better at 260 °C (ratio of 3.4) than at 230 °C. This anomaly can be explained by experimental deviation, especially with respect to hydrogen flowsthe hydrogen flow meter was simply not very accurate at these low hydrogen flows (hence reported as a range of 2-3 L/min rather than a single value). In addition, at these conditions, the amount of by-product is quite high. Error also accumulates when attempting to integrate tens of small by-product peaks. Of course, conditions with these large amounts of by-product formation (ratios less than 10) are not of commercial interest. Figure 2 readily illustrates how water improves selectivity. Higher hydrogen flow rates also improve product selectivity. At lower glycerol feed concentrations, the impact of hydrogen flow is less. A systematic trend in the data is that more diluent leads to less by-product formation. This is true for water or hydrogen as a diluent. At higher temperatures more by-product forms. The hydrogenolysis process is highly exothermic, and so the sensible heat of either hydrogen or water will reduce exothermic temperature
increases. At higher water concentrations in the feed, the secondary purpose of hydrogen as a heat moderator becomes less important. It is also known that many of the by-product are oligomeric in nature. Oligomer-forming reactions are second-order, and so, dilution with water tends to decrease their formation. It is believed that water’s dilution and heat capacity contribute to reduced by-product formation. A third mechanism related to the benefits of water is on water’s ability to inhibit dehydration reactions when such dehydration reactions are equilibrium limited. In the case of acetol formation, the dehydration reaction is not equilibrium limited, and so, water does not adversely impact glycerol dehydration. However, the formation of some by-product may be equilibrium limited and water could inhibit their formation. At 25% water in glycerol, significant decreases in by-product formation are observed. For this reason, the data are believed to substantiate this third mechanism for by-product reduction. Reactions with acetol as the feed (rather than glycerol; see Table 2) add additional insight into the mechanism of by-product formation. Even at high hydrogen flow rates, when acetol is used as the feed, the generation of by-product increases relative to glycerol as the reaction feed. The primary difference between the trials in Tables 1 and 2 at hydrogen flow rates of 5 L/min is that acetol is present at higher concentrations for a longer time when it is used as a feed. High initial acetol concentrations can be particularly impacting for reactions that are second-order in acetol concentration. These data support a conclusion that by-products are formed primarily from acetol substrate. High average concentrations of acetol lead to more by-product formation. The two conditions that result in the highest average acetol concentration during reaction are when acetol is used as the feed (Table 2) and when no hydrogen is present (last set of data in Table 1). Both of these experimental sets resulted in the lowest ratio of desired products to by-product (lowest product mass ratios). When considering the en-ol isomer of acetol, acetol has three functional groups that can be highly reactive. Acetol’s high reactive functionality combined with high concentrations (as compared to glycerol form) result in increased levels of by-product formation. Under the reaction mechanism of eq 1, acetol is in equilibrium with propylene glycol. Higher hydrogen concentrations shift the equilibrium to propylene glycol and hence reduce the driving force for acetol to react. This is an additional mechanism through which hydrogen reduces by-product formation. Due to equilibrium limitations, acetol will not react to 100% conversion. Acetol rapidly established an equilibrium concentration.8 Ethylene Glycol Production and Subsequent Reaction. The data of Table 2 also reveal the absence of ethylene glycol formation when acetol is used as the feed. This indicates that ethylene glycol is produced directly from reaction with glycerol as opposed to reaction with acetol. This topic is discussed later in this text.
Ind. Eng. Chem. Res., Vol. 47, No. 18, 2008 6881 Table 3. Conversion and Yield of Major Products for Ethylene Glycol over a Copper-Chromite Catalysta conversion
yield
temperature (°C)
ethylene glycol
acetic acid
ethanol
acetone
200 230 260
10.2% 38.1% 51.7%
1.1% 6.9% 11.8%
0% 1.36% 2.80%
0% 0.64% 1.68%
a Average values over hydrogen flow rates of 0-7 L/h, 1 bar of pressure, 92 g catalyst loading, and 10-40% ethylene glycol in water with a feed of mixture at 160 g/h. The yields were substantially insensitive to ethylene glycol feed concentration and hydrogen flow rate. Yields are in moles of product formed divided by moles of ethylene glycol initially present.
Figure 3. Impact of hydrogen on ethylene glycol formation relative to formation of desired products.
Trends in ethylene glycol production are also evident in Table 1. These trends are best exemplified by the Figure 3 plot of ethylene glycol versus the desired products. When hydrogen is present in the feed more ethylene glycol is produced (opposite the trends of other by-product). This further elucidates that ethylene glycol is likely produced from a hydrocracking reaction with glycerol. While hydrogen has a minimal impact on the rate of dehydration of glycerol to acetol, a glycerol hydrocracking mechanism as illustrated by Scheme 2 would project that higher pressures (implicit with higher hydrogen partial pressures) would lead to increased rates of hydrocracking. At the conditions of these reactions (Table 1, 1 bar) the amount of ethylene glycol being produced is quite low (less than 1%). At higher hydrogen partial pressures (>100 bar with 200 bar being representative), others4,5 have reported ethylene glycol at 10% or higher yields. In view of these higher yields reported by others at higher pressures, it is concluded that there are limited advantages to operating at higher pressures.
Table 4. Summary of GC Peak Areas for the Six Most-Prominent Reaction by-products at Different Temperatures and Pressures for Reaction of Glycerola temp (°C) pres PG/IS 8.74/IS 8.78/IS 9.15/IS 9.28/IS 9.32/IS 9.40/IS PG/BP 220 239.5 220 238.5
1 bar 1 bar 2 bar 2 bar
4.37 2.50 5.27 3.13
0.018 0.061 0.018 0.025
0.030 0.180 0.029 0.127
0.077 0.354 0.060 0.198
0.029 0.186 0.023 0.091
0.037 0.104 0.036 0.052
0.038 0.104 0.035 0.052
19.2 2.5 26.3 5.8
220 4 bar 6.50 240.5 4 bar 2.45
0.013 0.010
0.011 0.070
0.051 0.125
0.020 0.054
0.025 0.036
0.025 0.036
44.6 7.4
a PB is propylene glycol, IS is internal standard, and BP is the sum of the six by-products.
Table 5. Summary of GC Peak Areas for the Six Most-Prominent Reaction Byproducts at Different Temperatures and Pressures for Reaction of Acetola temp (°C) pres PG/IS 8.74/IS 8.78/IS 9.15/IS 9.28/IS 9.32/IS 9.40/IS PG/BP 201 242 184 202 221 182 203 242
1 bar 1 bar 2 bar 2 bar 2 bar 4 bar 4 bar 4 bar
3.8 1.9 5.3 4.0 3.7 6.7 5.6 3.1
0.068 0.149 0.038 0.048 0.085 0.029 0.047 0.084
0.105 0.256 0.023 0.070 0.166 0.009 0.064 0.130
0.093 0.203 0.033 0.070 0.124 0.024 0.053 0.087
0.061 0.199 0.022 0.043 0.111 0.009 0.035 0.110
0.020 0.055 0.012 0.018 0.030 0.010 0.016 0.039
0.025 0.036 0.009 0.016 0.025 0.007 0.015 0.025
10.1 2.1 39.2 15.0 6.8 76.9 24.4 6.6
a PB is propylene glycol, IS is internal standard, and BP is the sum of the six by-products.
The data in Table 1 indicate that more ethylene glycol accumulates in the product at lower temperatures than at higher temperatures. At some conditions the ethylene glycol was below detectable limits. It is possible that at higher temperatures the ethylene glycol was formed and rapidly proceeded to react to other products including components that are considerably more volatile than ethylene glycol. To evaluate this possibility, ethylene glycol was evaluated as the reactant feed at 200, 230, and 260 °C. The conversion and yield of major products are summarized in Table 3. The data of Table 3 demonstrate that the rate of ethylene glycol reaction increases substantially over the temperature range from 200 to 260 °C. These data substantiate the hypothesis that higher ethylene glycol yields from glycerol hydrogenolysis at lower temperatures can be attributed to ethylene glycol concentrations being depleted more rapidly at higher temperatures. The ethylene glycol reacts to form products such as acetic acid, ethanol, and acetone. The GC MS analyses repeatedly indicated that acetone was present (typically >88% certainty) as indicated by the data of Table 3 even though the mechanism through
which 2-carbon ethylene glycol forms 3-carbon acetone is not straightforward. Over this same temperature range and space velocities of 1.5 h-1, acetol and propylene glycol rapidly achieve equilibrium. Their decomposition is typically less than 20% at the higher temperatures. Ethylene glycol appears to react much faster than propylene glycol at temperatures greater than about 220 °C. At 260 °C, approximately one-half of the ethylene glycol reacts at space velocities of 1.5 h-1. In view of this, the ethylene glycol concentrations at higher temperatures in Tables 1 and 2 are significantly less than the amount of ethylene glycol that was actually formed. Acetol’s Role in by-product Formation. Tables 4–6 summarize an extended set of conversion data including reactions at higher pressures and with analyses of the six most prominent by-products other than ethylene glycol. Glycerol, acetol, and propylene glycol were separately evaluated as reaction substrates. The by-products are reported in terms of GC areas over an internal standard added during preparation of the GC samples (GC calibration curves are not available). The formation of all of these six by-products exhibited similar trends. Higher temperatures promoted more by-product formation, and higher hydrogen pressures decreased by-product formation. The greatest
6882 Ind. Eng. Chem. Res., Vol. 47, No. 18, 2008
Figure 4. Effect of reaction temperature and pressure on propylene glycol production from glycerol.
Figure 6. Ethylene glycol formation versus propylene glycol production of the glycerol to propylene glycol reaction.
Figure 5. Effect of reaction temperature and pressure on ethylene glycol formation of the glycerol to propylene glycol reaction.
Figure 7. Effect of reaction temperature and pressure on propylene glycol production from acetol.
ratio of propylene glycol to by-product was obtained at the lowest temperature and highest pressure. When acetol was used as the reagent, lower reaction temperatures were possible due to the greater volatility of acetol. Table 6 compares the formation of by-product to the formation of acetol since the concentration of propylene glycol may be biased due to it being the reagent. Higher hydrogen pressures and lower temperatures suppress acetol formation due to equilibrium limitations and this suppression leads to trends in the Ac:BP (acetol to sum of by-product) being different then the PG:BP trends. Reaction of Glycerol to Propylene Glycol. Figure 4 presents the effect of temperature on production of propylene glycol from glycerol at different pressures. The results indicate that as the reaction temperature decreases from 240 to 220 °C there is an increase in the production of propylene glycol. Also, more propylene glycol was produced at higher system pressures. It was observed that the by-product 9.11 (ethylene glycol) (see Figures 5 and 6) is the only prominent by-product that follows the trend of propylene glycol productionsthe accumulation of ethylene glycol increases with decreasing temperature and increasing propylene glycol production. As previously discussed, higher ethylene glycol concentrations at lower temperatures is an artifact of the rapidly increasing rate at which ethylene glycol reacts to form other products at higher temperatures. The most probable mechanism for ethylene glycol formation is the hydrocracking of glycerol. On the basis of the hydrocracking reaction mechanism, the concentration of ethylene glycol would be expected to increase with increasing temperatures and increasing hydrogen pressures. The data is consistent with this mechanism only when observing that less ethylene glycol accumulates at higher temperatures because of the increasingly greater reactivity of ethylene glycol at higher temperatures.
Toward the optimization of this reaction it is desirable to identify the optimal pressure at which to operate the gas-phase hydrogenolysis of glycerol so as to minimize by-product formation. The optimal pressure is likely a function of the reaction temperature. For 220 °C and based on the data of Table 4, higher pressures reduced the amount of by-product (other than ethylene glycol). Extrapolating the data from 4 bar, the amount of by-product approach 1% at about 15 bar. Ethylene glycol’s yield proceeds from 10% at about 200 bar.4,5 On the basis of these data, the optimal pressure is near 15 bar. On the basis of the observed trends, there are diminishing returns for further increases in hydrogen pressure (above about 15 bar). Above 15 bar of hydrogen pressure, most of the byproduct have diminishingly small yields while ethylene glycol will tend to form in greater yields at higher pressures. It should be noted that the different x-axes in Figures 3 and 6 illustrate different trends. Higher hydrogen pressures increase the yield of ethylene glycol relative to the sum of acetol and propylene glycol. These same higher pressures shift the equilibrium form acetol to propylene glycol leading to a change in the slope for the curves of Figure 6. Reaction of Acetol to Propylene Glycol. The effect of temperature (180-240 °C) on the conversion of acetol to propylene glycol at three different pressures (1, 2, and 4 bar) is presented in Figure 7. This figure indicates that more propylene glycol is produced at lower reaction temperatures, and this behavior is evident at each of the three pressure levels. It was also observed that more propylene glycol is produced at higher pressures. The result of this reaction is similar to the reaction of glycerol to propylene glycol as presented in Figure 4 where glycerol is the reagent. Figure 8 reaffirms that at lower temperatures more ethylene glycol remains in the product mixsin this instance, the reaction is with acetol reagent. The impact of higher pressures favoring
Ind. Eng. Chem. Res., Vol. 47, No. 18, 2008 6883 Table 6. Summary of GC Peak Areas for the Six Most-Prominent Reaction Byproducts at Different Temperatures and Pressures for Reaction of Propylene Glycola temp (°C) pres Ac/IS 8.74/IS 8.78/IS 9.15/IS 9.28/IS 9.32/IS 9.4/IS Ac/BP 202 237 183 207 216 242 197 241
Figure 8. Effect of reaction temperature and pressure on ethylene glycol formation of the acetol to propylene glycol reaction.
more ethylene glycol formation is more pronounced in Figure 8 than in Figure 5. Also, ethylene glycol formation is greater with glycerol as a reagent than with acetol as a reagent. In many reactions (depending on specific conditions), no ethylene glycol is in the product mix when acetol or propylene glycol is the reagent. This can be explained by a mechanism where the prominent means of ethylene glycol formation is the hydrocracking of glycerol. Since glycerol is not present in the reaction system of Figure 8 (acetol is the reagent), the ethylene glycol must be formed by the hydrocracking of either acetol or propylene glycol. This alternative route to ethylene glycol formation has a slower rate. Furthermore, a simple hydrocracking of acetol will not form ethylene glycolsleaving the most probable alternative route of ethylene glycol formation being hydrocracking of propylene glycol as illustrated by reaction scheme 3.
If the alternative path (alternative to glycerol hydrocracking) to ethylene glycol formation is the hydrocracking of propylene glycol, higher propylene glycol concentrations at higher hydrogen pressures and lower temperatures would lead to a greater dependence on hydrogen pressure. This mechanism is supported by increasing selectivity to ethylene glycol at higher pressures (Figure 8). This hydrocracking mechanism has not been reported previously. The Figure 5 data does not reveal increased rates of ethylene glycol production (from glycerol as previously proposed), and this casts some doubt on the mechanism of scheme 2. However, others4,5 have reported ethylene glycol at >10 wt % yields at higher pressures versus the yields of Figure 5 near 0.5 wt %, and these trends would substantiate a hydrocracking mechanism. Conclusions The three prominent paths for the formation of undesired byproduct during the gas-phase reaction of glycerol to propylene glycol over copper chromite include: (1) scavenging of glycerol for hydrogen when hydrogen is absent in the feed mixture, (2) side-reactions of acetol to several by-products, and (3) hydrocracking of glycerol and propylene glycol to form ethylene glycol. The scavenging of glycerol is easy to resolve by the addition of even minor amounts of hydrogen in the feed. This is recommended even when hydrogenation reactions are not
1 bar 1 bar 2 bar 2 bar 2 bar 2 bar 4 bar 4 bar
1.62 2.62 0.79 1.20 1.34 1.69 0.46 0.72
0.034 0.101 0.008 0.026 0.027 0.100 0.009 0.058
0.022 0.071 0.001 0.018 0.028 0.072 0.002 0.039
0.107 0.196 0.034 0.082 0.136 0.194 0.041 0.186
0.060 0.108 0.017 0.044 0.070 0.101 0.019 0.092
0.093 0.136 0.028 0.079 0.094 0.122 0.035 0.096
0.143 0.180 0.078 0.117 0.120 0.139 0.073 0.106
3.54 3.31 4.76 3.27 2.81 2.32 2.56 1.25
a AC is acetol, IS is internal standard, and BP is the sum of the six by-products.
desired such as when high selectivities to dehydration products (like acetol) are desired. The best approaches to limit acetol side-reactions are consistent with minimizing the amount of acetol in the system. Lower temperatures and higher hydrogen pressures shift the equilibrium from acetol to propylene glycol and substantially reduce by-product resulting from acetol side-reactions. In addition, many of the acetol by-products are formed through second-order reaction mechanisms, and so, dilution of glycerol with water, hydrogen, or an inert material will rapidly reduce by-product formation albeit at the potentially higher operating expenses associated with using dilute reagent mixtures. Water is believed to have auxiliary benefits associated with moderating temperature increases (sensible heat capacity) and inhibiting some dehydration reactions. Ethylene glycol formation is due, primarily, to the hydrocracking of glycerol. Lower hydrogen pressures decrease this hydrocracking. At higher temperatures (>220 °C), the ethylene glycol readily reacts to other products such as acetic acid, ethanol, and acetone which leads to lower concentrations of ethylene glycol in the product mix. A less-prominent mechanism for ethylene glycol formation is the hydrocracking of propylene glycol. Lower hydrogen pressures are the best means to reduce product loss from hydrocracking. A hydrogen partial pressure between 5 and 15 bar provides an optimal balance of reduced hydrocracking losses and reduced acetol side-reaction losses. Acknowledgment Parts of this research were funding by the National Science Foundation, Renewable Alternatives, LLC, the Missouri Soybean Merchandising Council, and The University of Missouri. Their support is greatly appreciated. Support was also provided by the LS-MoAMP (Louis Stokes-Missouri Alliance for Minority Participation) Undergraduate Summer Research program. Literature Cited (1) Tuck, M. W.; Tilley S. N. Process. Patent application PCT/GB2006/ 050181. (2) Suppes, G. J.; Sutterlin, W. R. Method of producing lower alcohols from glycerol. Patent application PCT/US2006/042707. (3) Ludwig, S.; Manfred, E. Preparation of 1, 2 propanediol. US Patent 5,616,817, 1997. (4) Tessie, C. Production of propanediols. US Patent 4,642,394, 1987. (5) Haas, T.; Neher, A.; Arntz, D.; Klenk, H.; Girke, W. Process for the production of 1,2 and 1,3 propanediol. US Patent 5,426,249, 1995. (6) Dasari, M. A.; Kaitsimkul, P.; Sutterlin, W. R.; Suppes, G. J. Low pressure hydrogenolysis of glycerol to propylene glycol. Appl. Catal. A: Gen. 2005, 281, 225–231. (7) Dasari, M. A. Catalytic Conversion of Glycerol, Sugars and Sugar Alcohols to Value-Added Products. Ph.D. Dissertation, The University of MissourisColumbia, Department of Chemical Engineering, Columbia, MO, 2005.
6884 Ind. Eng. Chem. Res., Vol. 47, No. 18, 2008 (8) Chiu, C. Catalytic Conversion of Glycerol to Propylene Glycol: Synthesis and Technology Assessment. Ph.D. Dissertation, The University of MissourisColumbia, Department of Chemical Engineering, Columbia, MO, 2006. (9) Rivera-Ramos, L. Equilibrium Limitations and Selectivity on Conversion of Glycerol to Propylene Glycol. M.S. Thesis, The University of MissourisColumbia, Department of Chemical Engineering, Columbia, MO, 2006. (10) Holmes British Patent 428,462, C.A. 29,6908, 1935. (11) McName, R. W.; Blair, C. M. Process for the production of hydroxyl propanone. US Patent 2,143,383, 1939. (12) Zhouwen, X. A process for producing dihydroxy alcohol and polyol by cracking sorbitol. Patent WO2006092085, 2006. (13) Arena, B. J. Hydrocracking of polyols. US Patent 4,496,780, 1985.
(14) Andrews, M. A.; Klaeren, S. A. Hydrocracking of carbohydrates making glycerol, glycols and other polyols. US Patent 5,026,927, 1991. (15) Andrews, M. A.; Klaeren, S. A. Selective hydrocracking of monosaccharide carbon-carbon single bonds under mild conditions. Ruthenium hydride catalyzed formation of glycols. J. Am. Chem. Soc. 1989, 111, 4131–4133. (16) Ying, X. C. Method for producing ethylene glycol and lower polyol using hydrocracking. Patent Application CN1762938, 2006.
ReceiVed for reView February 20, 2008 ReVised manuscript receiVed May 29, 2008 Accepted June 6, 2008 IE800300A
