Article pubs.acs.org/EF
Conversion of Methanol and Glycerol into Gasoline via ZSM‑5 Catalysis Guanqun Luo and Armando G. McDonald* Department of Forest, Rangeland, and Fire Sciences, University of Idaho, Moscow, Idaho 83844-1132, United States ABSTRACT: A quartz fixed-bed, microreactor was successfully constructed for both the catalytic methanol-to-gasoline (MTG) and methanol-and-glycerol-to-gasoline (MGTG) processes. The reaction products were analyzed by gas chromatography−mass spectrometry (GC−MS) and high-performance liquid chromatography (HPLC). Process variables of temperature and reaction time were studied to determine the effects on conversion rates, product yield, and ZSM-5 catalyst lifetime for both systems. Moreover, the factor of glycerol additions (10, 25, and 40% in methanol) was also investigated for the MGTG process. The MTG and MGTG generated oil phase showed a similar composition, mainly methylbenzenes, to regular gasoline, and composition changed as the reaction proceeded to favor heavier aromatics. In the MTG process, the best catalytic performance was achieved at 425 °C, at which the product yield and catalyst lifetime were 11.0 wt % and 20 h, respectively. Generally, the methanol conversion rate and the total liquid and organic-phase yield rates decreased with the reaction time at each temperature. In addition to gasoline-range aromatics, some oxygenates were also detected in the extracted aqueous phase from the MGTG process. The best MGTG catalytic performance was achieved at 500 °C with 10% glycerol in methanol, at which the product yield and catalyst lifetime were 14.9 wt % and 8 h, respectively. The higher glycerol content disfavored the production of aromatics but favored oxygenates. With an increasing reaction time at all reaction conditions, methanol and glycerol conversion rates were ≥99%.
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INTRODUCTION The technology of converting methanol into gasoline was discovered and commercialized more than 3 decades ago. In the 1980s, the first commercial plant to produce gasoline from natural gas via methanol was constructed in New Zealand, with a capacity of 14 500 barrels per day (bpd). However, the methanol-to-gasoline (MTG) part of the operation was shut down in the 1990s because of the low fossil fuel costs at that time.1 Currently, the increasing consumption and limited reserves of crude oil, as well as the problem of CO2 emissions mainly caused by the usage of fossil fuels, have led to a growing interest in the production of non-fossil-based energy. Methanol can be made from biomass that is abundant, renewable, and globally available, via synthesis gas (syngas), and further converted into gasoline; therefore, the MTG process is receiving renewed attention today.2,3 Over the years, a variety of zeolites have been tested in the MTG process, including SAPO-34, HY, H-β, and ZSM-5. The lattermost catalyst, ZSM-5, is widely accepted to be the most effective and selective catalyst to produce high-quality gasoline, which is mainly attributed to its network structure.4,5 The performance of the MTG process via ZSM-5 can be influenced by several factors, such as temperature and pressure.6 A major problem of the MTG process is deactivation of the catalyst because of the deposition of the carbonaceous residue; thus, it is still a key area of research to improve the catalyst lifetime by optimizing the catalyst pretreatment method and/or reaction conditions. In addition to methanol, many other chemicals, such as higher alcohols, carbonyl compounds, organic acids, and glycerol, have also been used as reactants in MTG-like processes.6−9 Glycerol, with properties of non-toxicity, edibility, biosustainability, and biodegradability, is widely available at a low price.10,11 Traditionally, glycerol is either obtained as a byproduct © 2013 American Chemical Society
of saponification of fats to form soap, biodiesel production, or synthesized from propylene oxide.10,11 Glycerol can also be produced by fermentation of sugars, such as glucose and fructose.10 Currently, the production of glycerol is further increased as the production of biodiesel obtains growing interest. From an economic standpoint, converting glycerol into valueadded chemicals and fuels can not only stabilize the price of glycerol itself but also lower the costs of production of biodiesel. Recently, various catalytic processes, such as selective oxidation, hydrogenolysis, and dehydration, have been studied extensively to transform glycerol into more useful chemicals.12 For the conversion of glycerol into fuels, most research focuses on the gasification of glycerol to produce syngas that can be further converted into gasoline or diesel via Fischer−Tropsch synthesis (FTS).13,14 Nevertheless, very little research into the direct conversion of glycerol to gasoline-range hydrocarbons has been reported. Hoang et al.9 studied a variety of zeolites (i.e., HZSM-5, HY, Mordenite, and HZSM-22) for the conversion of glycerol at 300−400 °C and atmospheric pressure or 20 atm. The formation of alkyl aromatics was only observed over three-dimensional HY and HZSM-5 at a temperature of 400 °C. In comparison to HY, HZSM-5 showed a higher yield of alkyl aromatics because of its medium pore size. Unfortunately, the effect of a temperature higher than 400 °C on the product yield and selectivity has not been investigated. In addition, the yield of aromatics started to dramatically decrease only after 2 h on stream. The reacting compound with an effective H/C ratio below 2, such as glycerol with an effective H/C ratio of 0.67, was found to render the excessive deactivation of zeolite catalysts.15 Thus, adding methanol Received: October 3, 2013 Revised: December 6, 2013 Published: December 6, 2013 600
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and separated into oil and aqueous phases. MGTG products had two types, namely, single- and two-phase products. Two-phase MGTG products were also centrifuged and separated into oil and aqueous phases. For the aqueous phase separated from two-phase MGTG products and the single-phase MGTG products, H2O (1 mL) and CH2Cl2 (1 mL) were added to separate them into two phases for further analysis. The product yield of different phases was calculated according to the equation below.
that has an effective H/C ratio of 2 into glycerol could increase the combined H/C of the feed and then improve the activity of the catalyst. The effective H/C ratio is defined in the following equation:16 H H − 2O effective = C C
where H, C, and O are the number of H, C, and O atoms present in the specific compound. In addition, using the mixture of methanol and glycerol as feedstock for a MTG-like process may also reduce the costs for cleaning the crude glycerol from the transesterification process, because excessive methanol is usually used to improve the production of biodiesel. In the present work, an integral study on conversions of methanol and a mixture of methanol and glycerol into gasolinerange hydrocarbons has been carried out using a benchtop, fixed-bed reactor. The effects of a higher reacting temperature and reaction time on the performance (i.e., product yield, selectivity, and catalyst lifetime) of both processes were investigated, as was the influence of glycerol addition on the methanol-and-glycerol-to-gasoline (MGTG) process.
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yield (wt %) =
amount of a certain phase × 100% amount of reactants
The products in the oil and CH2Cl2-extracted phases were analyzed with gas chromatography−mass spectrometry (GC−MS, Focus-ISQ, ThermoScientific). Separation was achieved on an RTx-5 ms column (0.25 mm Ø × 30 m), with He as the carrier gas and a temperature program of 40 °C ramped to 200 °C at 5 °C/min. 1,2,4Trichlorobenzene was added (0.1 mg/mL, 1 mL) to the sample and used as an internal standard (IS). The eluted compounds were identified with authentic standards and by spectral matching with the 2008 National Institute of Standards and Technology (NIST) mass spectral library. A sample of regular gasoline was obtained from a Cenex gas station (January 2010; Moscow, ID). The aqueous phase was analyzed by high-performance liquid chromatography (HPLC, with a Waters model 501 pump, TSP model AS2000 autosampler, and ERC-5710 refractive index detector) equipped with a Rezex ROA-organic acid column (Phenomenex) at 60 °C on elution with aqueous H2SO4 (0.005 N) at 0.5 mL/min. Standard curves of methanol and glycerol were created, and conversion rates were then calculated on the basis of the following equation:
EXPERIMENTAL SECTION
The catalyst used in this study was CBV3024E ZSM-5, with a Si/Al ratio of 30, supplied by Zeolyst International. Methanol (HPLC grade, >99.9%) and glycerol (ACS grade, >99.5%) were supplied by Fisher Chemicals. The catalytic reactions were carried out in a quartz tube (10 mm Ø × 300 mm with a “0” quartz frit connected 180 mm from the top to support the catalyst), fixed-bed microreactor (Figure 1) operating at
conversion (%) = 1 −
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amount of methanol or glycerol (out) amount of methanol or glycerol (in)
× 100%
RESULTS AND DISCUSSION MTG Process. Methanol conversion via ZSM-5 was investigated in a custom-built quartz tube, fixed-bed microreactor.
Figure 1. Schematic diagram of the reactor for the MTG and MGTG processes: (1) N2 cylinder, (2) syringe pump, (3) syringe, (4) mass flow controller, (5) 10 mm diameter quartz tubular reactor, (6) furnace, (7) K-type thermocouple, (8) J-type thermocouple, (9) proportional−integral−derivative (PID) temperature controller, and (10) salt−ice/water trap. atmospheric pressure and various temperatures for the MTG (425, 450, 475, 500, and 525 °C) and MGTG (450, 500, and 550 °C) processes. The reactor was heated using a small tube furnace (Supelco) and regulated with a digital temperature controller. Fresh ZSM-5 (300 mg) catalyst was mixed with silica (300 mg) and placed on a quartz frit in the reactor tube. Methanol (0.05 mL/min) and the mixture of 10, 25 and 40% glycerol in methanol (0.03 mL/min) were introduced to the reactor via a syringe pump (NE-300, New Era, 25 mL syringe). Products were condensed in an impinger placed in a salt−ice bath (−20 °C), and samples were collected every 2 h for the MTG reactions and every 1 h for the MGTG reactions. The reactor was purged with N2 for 5 min after each run to collect any residual products. After sampling, MTG products were centrifuged (1000 rpm)
Figure 2. Methanol conversion rate and liquid product and oil-phase yields with the reaction time at different temperatures. 601
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Process variables of temperature and reaction time were studied to determine their effects on the product yield, catalyst lifetime, and methanol conversion rate (Figure 2). With increasing temperature, the yield of both the total liquid phase and the oil phase decreased, because the formation of gaseous products was favored at higher temperatures because of secondary cracking reactions.6 At each temperature, the yield of the total liquid and oil phase was 63.5− 65.5 and 8.9−11.0 wt %, respectively. Considering slightly different reaction conditions, the oil-phase yield from our experiment is in
Figure 5. Selectivity of gasoline-range aromatics with the reaction time in the oil phase at T = 500 °C.
Figure 3. Pictures of oil samples from MTG: (top) from left to right, reaction times of 2, 4, 6, 8, 10, 12, and 14 h (T = 450 °C); and (bottom) from left to right, temperatures of 425, 450, 475, 500, and 525 °C (reaction time = 2 h).
Figure 6. Pictures of (left) two-phase and (right) single-phase products from the MGTG process.
agreement with literature results ranging from 5 to 20 wt %.6,17 As the reaction time increased, the oil-phase yield decreased slowly and the total liquid yield slightly increased during the first 4 h and then gradually decreased. The induction period probably resulted in a relatively lower total liquid yield during the first 2 h of reaction. The catalyst lifetime, defined as the time that the catalyst is active to produce oil products, was found to be 20, 16, 14, 12, and 10 h, respectively, at temperatures of 425, 450, 475, 500, and 525 °C. These data were in accordance with results from Schulz and co-workers,18 showing that, at temperatures higher than 400 °C, the catalyst lifetime decreased with increasing temperature. The lifetime of the catalyst was a result of ZSM-5 deactivation because of coke formation. In terms of the methanol conversion rate, it was around 100% at the beginning of all reactions and decreased with the temperature and reaction time, which was in agreement with the trends of the product yield mentioned above and literature results.6,8,19 Coked ZSM-5 was active for dehydration of methanol to produce dimethyl ether (DME);8 thus, the methanol conversion rate was still 70−85% when no oil
Figure 4. Total ion chromatograms of (a) regular gasoline and (b) oil phase from the MTG progress (8−10 h period; T = 500 °C; the compounds shown above from left to right are benzene, toluene, m-xylene, p-xylene, o-xylene, 1,3,5-trimethylbenzene, 1,2,4-trimethylbenzen, 1,2,4,5-tetramethylbenzene, 1,2,4-trichlorobenzene, pentamethylbenzene, and hexamethylbenzene). 602
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gasoline. This specific range of aromatics exhibited in the oil phase mainly resulted from the unique network structure of ZSM-5. In terms of the selectivity of the gasoline-range aromatics with reaction time, it was similar at each temperature; the composition changed with the reaction time favoring heavier aromatics (Figure 5). In addition, toluene, p-xylene, and 1,2,4trimethylbenzene were the major components, which can be attributed to the relatively lower diffusivity of the other compounds resulting from steric constraints.4,6 Bjorgen et al.20 investigated the stability of retained organic compounds disappearing from zeolite channels at rates according to the following order: xylenes > trimethylbenzenes > tetramethylbenzenes > pentamethylbenzenes = hexamethylbenzenes. MGTG Process. The conversion of glycerol in methanol using the ZSM-5 catalyst was also studied. There were two types of liquid products from the MGTG process: (i) single-phase system and (ii) two-phase system (Figure 6). The composition of the oil phase separated from the two-phase product was primarily gasoline-range aromatics, which was similar to those derived from the MTG process. Samples collected under conditions of 500 °C and 25% glycerol addition were used as an example to show the selectivity of aromatic compounds in the oil phase (Figure 7). The selectivity was also similar to those of the MTG process; that is, toluene, p-xylene, and 1,2,4-trimethylbenzene were major components, and the selectivity changed with the reaction time favoring heavier compounds. CH2Cl2 was used for extracting the single-phase products and aqueous phase of two-phase products. The extracted products were analyzed by GC−MS. Figure 8 is a representative chromatogram of reaction products from 40% glycerol in the methanol sample collected between 3 and 4 h at 500 °C. In addition to gasoline-range aromatics, various oxygenates, including 3,3-dimethoxy-1-propene, 1-methoxy-2propanone, 3-methoxypropanal, vinylfuran, 2-cyclopenten-1-one, and 2-methyl-2-cyclopenten-1-one, were found in the products. The acidic ZSM-5 catalyst was active for dehydration of both methanol and glycerol. It has been proposed that DME is the major product from methanol dehydration over ZSM-5.21−23 For glycerol dehydration over acidic catalysts, propenal and acetol have been reported as primary products, depending upon which
Figure 7. Selectivity of gasoline-range aromatics with the reaction time in the oil phase at T = 500 °C, with 25% glycerol in methanol.
was being produced. The above results can be further supported by the color of oil products (Figure 3); the oil phase color tended to be darker with an increasing reaction time and temperature, indicating coking of the catalyst.17 The oil products were analyzed by GC−MS. The chromatograms of each oil sample were similar. Figure 4 is a representative chromatogram of a sample collected between 8 and 10 h at 500 °C. The oil products generated from the MTG process showed a similar composition, mainly methylbenzenes, to regular
Figure 8. Total ion chromatogram of CH2Cl2-extracted MGTG product (40% glycerol in methanol; T = 500 °C; reaction time = 3−4 h period; the compounds shown above from left to right are benzene, 3,3-dimethoxy-1-propene, 1-methoxy-2-propanone, 3-methoxypropanal, vinylfuran, toluene, 2-cyclopenten-1-one, m-xylene, p-xylene, o-xylene, 2-methyl-2-cyclopenten-1-one, 1,3,5-trimethylbenzene, 1,2,4-trimethylbenzene, 1,2,4,5tetramethylbenzene, 1,2,4-trichlorobenzene, and pentamethylbenzene). 603
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Figure 9. Conversion and product yield with the reaction time at (a) T = 450 °C, (b) T = 500 °C, and (c) T = 550 °C, with 25% glycerol in methanol.
Figure 10. Conversion and product yield with the reaction time at the conditions of (a) 10%, (b) 25%, and (c) 40% glycerol in methanol, with T = 500 °C.
hydroxyl group is first removed. Those oxygenates listed above may be generated by reforming of methanol, glycerol, and their dehydrated products. For instance, 1-methoxy-2-propanone could be formed by condensation of methanol and acetol with water as the byproduct. Effects of the reaction time, temperature, and glycerol addition on the conversion and product yield of the MGTG process were investigated. Figure 9 shows the conversion rates 24−26
and product yield at different temperatures. The conversion rates of both methanol and glycerol were ≥99% along the reaction time coordinate at different temperatures. As the temperature increased, the total liquid yield slightly decreased because of higher production of gaseous products at higher 604
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Notes
temperatures. Different temperatures almost had no effect on the CH2Cl2-extracted oxygenate yield that was up to 35.4 wt %. In terms of the oil-phase yield and catalyst lifetime, the best performance (11.7 wt % and 5 h) was achieved at 500 °C, because aromatization reactions were favored at higher temperatures, but at 550 °C, coke formation occurred. With 25% glycerol, the total liquid and oil-phase yields at different temperatures were up to 75.0−85.3 and 10.5−11.7 wt %, respectively. As the reaction time increased, total liquid and CH2Cl2-extracted oxygenate yields went through a maximum value and then gradually decreased. The relatively lower yield at the beginning of the reaction could be attributed to the formation of more volatile products. The decreasing yield after the maximum could result from the decrease in activation of the catalyst and/or interparticle impact. A gradual reduction of gasoline-range aromatics was also observed with an increasing reaction time, indicating that the sites for aromatization were gradually decreased because of coke formation. Figure 10 shows the conversion rates and product yield with different glycerol additions at 500 °C as a function of the reaction time. Methanol and glycerol conversion rates were ≥99% along the reaction time for all of the reactions. The trends of total liquid, CH2Cl2-extracted oxygenate, and oil-phase yields with the reaction time were similar to those illustrated in Figure 9. At a temperature of 500 °C, the addition of more glycerol disfavored the oil-phase production. The opening of the ZSM-5 channel is about 6 Å.5,27 In comparison to methanol (3.9 Å), glycerol has a larger molecular size of around 5 Å, which may block the pores more easily, resulting in a deactivation of ZSM-5. However, different glycerol additions had almost no influence on the total liquid yield. In addition, the yield of CH2Cl2-extracted oxygenate compounds increased with increasing glycerol additions. The maximum value of that increased from 25.6 to 39.8 wt % when glycerol addition increased from 25 to 40%.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from both the United States Department of Agriculture (USDA)−National Institute of Food and Agriculture (NIFA) Grant 2010-34158-20938 and the National Institute for Advanced Transportation Technology (NIATT) Grant DTRT07-G-0056 are gratefully acknowledged.
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CONCLUSION The catalytic performance of both the MTG and MGTG processes using ZSM-5 was successfully investigated using a quartz, fixed-bed microreactor. The MTG- and MGTG-generated products were comparable in compositions to regular gasoline. In addition to gasoline-range aromatics, some oxygenates were detected in the CH2Cl2-extracted phase from the MGTG process. In the MTG process, the oil-phase yield (11.0 wt %) and catalyst lifetime (20 h) were optimized at a temperature of 425 °C. As the retention time increased, the methanol conversion rate and oil-phase yield decreased, whereas the total liquid yield rate slightly increased during the first 4 h and then gradually decreased. In the MGTG process, the best catalytic performance was achieved at 500 °C with 10% glycerol in methanol, at which the oil-phase yield was up to 14.9 wt % and the catalyst lifetime was 8 h. With increasing temperature, the formation of gaseous products was slightly increased. The conversion rate of both methanol and glycerol remained over 99%, and the total liquid yield increased to a maximum and then gradually decreased with an increasing retention time. With increasing additions of glycerol in methanol, a lower oil-phase yield was obtained, while relatively more oxygenate compounds were produced. This work shows that glycerol, a byproduct from biodiesel production, could be used as a substrate for producing drop-in fuels; however, further work is required to increase the catalyst lifetime.
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NOMENCLATURE MTG = methanol-to-gasoline MGTG = methanol-and-glycerol-to-gasoline GC-MS = gas chromatography−mass spectrometry HPLC = high-performance liquid chromatography bpd = barrels per day syngas = synthesis gas FTS = Fischer−Tropsch synthesis IS = internal standard DME = dimethyl ether
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 605
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