Conversion of Methanol and Glycerol into Gasoline via ZSM‑5CatalysisGuanqun 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−massspectrometry (GC−MS) and high-performance liquid chromatography (HPLC). Process variables of temperature and reactiontime 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. TheMTG and MGTG generated oil phase showed a similar composition, mainly methylbenzenes, to regular gasoline, andcomposition changed as the reaction proceeded to favor heavier aromatics. In the MTG process, the best catalytic performancewas achieved at 425 °C, at which the product yield and catalyst lifetime were 11.0 wt % and 20 h, respectively. Generally, themethanol 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 MGTGprocess. The best MGTG catalytic performance was achieved at 500 °C with 10% glycerol in methanol, at which the product yieldand catalyst lifetime were 14.9 wt % and 8 h, respectively. The higher glycerol content disfavored the production of aromatics butfavored oxygenates. With an increasing reaction time at all reaction conditions, methanol and glycerol conversion rates were ≥99%.

■ INTRODUCTIONThe technology of converting methanol into gasoline wasdiscovered and commercialized more than 3 decades ago. In the1980s, the first commercial plant to produce gasoline fromnatural gas via methanol was constructed in New Zealand, witha capacity of 14 500 barrels per day (bpd). However, themethanol-to-gasoline (MTG) part of the operation was shutdown in the 1990s because of the low fossil fuel costs at thattime.1 Currently, the increasing consumption and limitedreserves of crude oil, as well as the problem of CO2 emissionsmainly caused by the usage of fossil fuels, have led to a growinginterest in the production of non-fossil-based energy. Methanolcan be made from biomass that is abundant, renewable, andglobally available, via synthesis gas (syngas), and further convertedinto gasoline; therefore, the MTG process is receiving renewedattention today.2,3

Over the years, a variety of zeolites have been tested in theMTG process, including SAPO-34, HY, H-β, and ZSM-5. Thelattermost catalyst, ZSM-5, is widely accepted to be the mosteffective and selective catalyst to produce high-quality gasoline,which is mainly attributed to its network structure.4,5 Theperformance of the MTG process via ZSM-5 can be influencedby several factors, such as temperature and pressure.6 A majorproblem of the MTG process is deactivation of the catalystbecause of the deposition of the carbonaceous residue; thus, itis still a key area of research to improve the catalyst lifetime byoptimizing the catalyst pretreatment method and/or reactionconditions. 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-likeprocesses.6−9

Glycerol, with properties of non-toxicity, edibility, biosustain-ability, and biodegradability, is widely available at a lowprice.10,11 Traditionally, glycerol is either obtained as a byproduct

of saponification of fats to form soap, biodiesel production, orsynthesized from propylene oxide.10,11 Glycerol can also beproduced by fermentation of sugars, such as glucose andfructose.10 Currently, the production of glycerol is furtherincreased as the production of biodiesel obtains growing interest.From an economic standpoint, converting glycerol into value-added chemicals and fuels can not only stabilize the price ofglycerol itself but also lower the costs of production of biodiesel.Recently, various catalytic processes, such as selective oxidation,hydrogenolysis, and dehydration, have been studied extensivelyto transform glycerol into more useful chemicals.12 For theconversion of glycerol into fuels, most research focuses on thegasification of glycerol to produce syngas that can be furtherconverted into gasoline or diesel via Fischer−Tropsch synthesis(FTS).13,14 Nevertheless, very little research into the directconversion of glycerol to gasoline-range hydrocarbons has beenreported. Hoang et al.9 studied a variety of zeolites (i.e., HZSM-5,HY, Mordenite, and HZSM-22) for the conversion of glycerol at300−400 °C and atmospheric pressure or 20 atm. The formationof alkyl aromatics was only observed over three-dimensional HYand HZSM-5 at a temperature of 400 °C. In comparison to HY,HZSM-5 showed a higher yield of alkyl aromatics because of itsmedium pore size. Unfortunately, the effect of a temperaturehigher than 400 °C on the product yield and selectivity has notbeen investigated. In addition, the yield of aromatics started todramatically decrease only after 2 h on stream. The reactingcompound with an effective H/C ratio below 2, such as glycerolwith an effective H/C ratio of 0.67, was found to render theexcessive deactivation of zeolite catalysts.15 Thus, adding methanol

Received: October 3, 2013Revised: December 6, 2013Published: December 6, 2013

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that has an effective H/C ratio of 2 into glycerol could increase thecombined H/C of the feed and then improve the activity of thecatalyst. The effective H/C ratio is defined in the followingequation:16

= −HC

H OC

effective2

where H, C, and O are the number of H, C, and O atoms presentin the specific compound. In addition, using the mixture ofmethanol and glycerol as feedstock for a MTG-like process mayalso reduce the costs for cleaning the crude glycerol from thetransesterification process, because excessive methanol is usuallyused 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 gasoline-range hydrocarbons has been carried out using a benchtop,fixed-bed reactor. The effects of a higher reacting temperatureand reaction time on the performance (i.e., product yield,selectivity, and catalyst lifetime) of both processes wereinvestigated, as was the influence of glycerol addition on themethanol-and-glycerol-to-gasoline (MGTG) process.

■ EXPERIMENTAL SECTIONThe catalyst used in this study was CBV3024E ZSM-5, with a Si/Al ratio of30, 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 tosupport the catalyst), fixed-bed microreactor (Figure 1) operating at

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. FreshZSM-5 (300 mg) catalyst was mixed with silica (300 mg) and placedon a quartz frit in the reactor tube. Methanol (0.05 mL/min) and themixture 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 asalt−ice bath (−20 °C), and samples were collected every 2 h for theMTG reactions and every 1 h for the MGTG reactions. The reactorwas purged with N2 for 5 min after each run to collect any residualproducts. After sampling, MTG products were centrifuged (1000 rpm)

and separated into oil and aqueous phases. MGTG products had twotypes, namely, single- and two-phase products. Two-phase MGTGproducts were also centrifuged and separated into oil and aqueousphases. For the aqueous phase separated from two-phase MGTGproducts and the single-phase MGTG products, H2O (1 mL) andCH2Cl2 (1 mL) were added to separate them into two phases forfurther analysis. The product yield of different phases was calculatedaccording to the equation below.

= ×yield (wt %)amount of a certain phase

amount of reactants100%

The products in the oil and CH2Cl2-extracted phases were analyzedwith 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 temperatureprogram of 40 °C ramped to 200 °C at 5 °C/min. 1,2,4-Trichlorobenzene was added (0.1 mg/mL, 1 mL) to the sample andused as an internal standard (IS). The eluted compounds wereidentified with authentic standards and by spectral matching with the2008 National Institute of Standards and Technology (NIST) massspectral library. A sample of regular gasoline was obtained from aCenex gas station (January 2010; Moscow, ID). The aqueous phasewas analyzed by high-performance liquid chromatography (HPLC,with a Waters model 501 pump, TSP model AS2000 autosampler, andERC-5710 refractive index detector) equipped with a RezexROA-organic acid column (Phenomenex) at 60 °C on elution withaqueous H2SO4 (0.005 N) at 0.5 mL/min. Standard curves ofmethanol and glycerol were created, and conversion rates were thencalculated on the basis of the following equation:

= −

×

conversion (%) 1amount of methanol or glycerol (out)amount of methanol or glycerol (in)

100%

■ RESULTS AND DISCUSSIONMTG 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 MGTGprocesses: (1) N2 cylinder, (2) syringe pump, (3) syringe, (4) massflow 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.

Figure 2. Methanol conversion rate and liquid product and oil-phaseyields with the reaction time at different temperatures.

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Process variables of temperature and reaction time were studied todetermine their effects on the product yield, catalyst lifetime, andmethanol 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 highertemperatures because of secondary cracking reactions.6 At eachtemperature, the yield of the total liquid and oil phase was 63.5−65.5 and 8.9−11.0 wt %, respectively. Considering slightly differentreaction conditions, the oil-phase yield from our experiment is in

agreement with literature results ranging from 5 to 20 wt %.6,17 Asthe reaction time increased, the oil-phase yield decreased slowlyand the total liquid yield slightly increased during the first 4 h andthen gradually decreased. The induction period probably resultedin a relatively lower total liquid yield during the first 2 h ofreaction. The catalyst lifetime, defined as the time that the catalystis active to produce oil products, was found to be 20, 16, 14, 12,and 10 h, respectively, at temperatures of 425, 450, 475, 500, and525 °C. These data were in accordance with results from Schulzand co-workers,18 showing that, at temperatures higher than 400 °C,the catalyst lifetime decreased with increasing temperature. Thelifetime of the catalyst was a result of ZSM-5 deactivationbecause of coke formation. In terms of the methanol conversionrate, it was around 100% at the beginning of all reactions anddecreased with the temperature and reaction time, which was inagreement with the trends of the product yield mentioned aboveand literature results.6,8,19 Coked ZSM-5 was active fordehydration of methanol to produce dimethyl ether (DME);8

thus, the methanol conversion rate was still 70−85% when no oil

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, and525 °C (reaction time = 2 h).

Figure 4. Total ion chromatograms of (a) regular gasoline and (b) oilphase from the MTG progress (8−10 h period; T = 500 °C; thecompounds shown above from left to right are benzene, toluene,m-xylene, p-xylene, o-xylene, 1,3,5-trimethylbenzene, 1,2,4-trimethyl-benzen, 1,2,4,5-tetramethylbenzene, 1,2,4-trichlorobenzene, pentam-ethylbenzene, and hexamethylbenzene).

Figure 5. Selectivity of gasoline-range aromatics with the reaction timein the oil phase at T = 500 °C.

Figure 6. Pictures of (left) two-phase and (right) single-phaseproducts from the MGTG process.

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was being produced. The above results can be further supportedby the color of oil products (Figure 3); the oil phase colortended to be darker with an increasing reaction time andtemperature, indicating coking of the catalyst.17

The oil products were analyzed by GC−MS. The chromato-grams of each oil sample were similar. Figure 4 is a representativechromatogram of a sample collected between 8 and 10 h at500 °C. The oil products generated from the MTG processshowed a similar composition, mainly methylbenzenes, to regular

gasoline. This specific range of aromatics exhibited in the oilphase mainly resulted from the unique network structure ofZSM-5. In terms of the selectivity of the gasoline-range aromaticswith reaction time, it was similar at each temperature; thecomposition changed with the reaction time favoring heavieraromatics (Figure 5). In addition, toluene, p-xylene, and 1,2,4-trimethylbenzene were the major components, which can beattributed to the relatively lower diffusivity of the othercompounds resulting from steric constraints.4,6 Bjorgen et al.20

investigated the stability of retained organic compoundsdisappearing from zeolite channels at rates according to thefollowing order: xylenes > trimethylbenzenes > tetramethylben-zenes > pentamethylbenzenes = hexamethylbenzenes.

MGTG Process. The conversion of glycerol in methanolusing the ZSM-5 catalyst was also studied. There were two typesof liquid products from the MGTG process: (i) single-phasesystem and (ii) two-phase system (Figure 6). The composition ofthe oil phase separated from the two-phase product was primarilygasoline-range aromatics, which was similar to those derived fromthe MTG process. Samples collected under conditions of 500 °Cand 25% glycerol addition were used as an example to show theselectivity 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 majorcomponents, and the selectivity changed with the reaction timefavoring heavier compounds. CH2Cl2 was used for extracting thesingle-phase products and aqueous phase of two-phase products.The extracted products were analyzed by GC−MS. Figure 8 isa representative chromatogram of reaction products from 40%glycerol in the methanol sample collected between 3 and 4 hat 500 °C. In addition to gasoline-range aromatics, variousoxygenates, including 3,3-dimethoxy-1-propene, 1-methoxy-2-propanone, 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 bothmethanol and glycerol. It has been proposed that DME is themajor product from methanol dehydration over ZSM-5.21−23 Forglycerol dehydration over acidic catalysts, propenal and acetolhave been reported as primary products, depending upon which

Figure 7. Selectivity of gasoline-range aromatics with the reaction timein the oil phase at T = 500 °C, with 25% glycerol in methanol.

Figure 8. Total ion chromatogram of CH2Cl2-extracted MGTG product (40% glycerol in methanol; T = 500 °C; reaction time = 3−4 h period; thecompounds 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, 1,2,4,5-tetramethylbenzene, 1,2,4-trichlorobenzene, and pentamethylbenzene).

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hydroxyl group is first removed.24−26 Those oxygenates listedabove may be generated by reforming of methanol, glycerol, andtheir dehydrated products. For instance, 1-methoxy-2-propanonecould be formed by condensation of methanol and acetol withwater as the byproduct.Effects of the reaction time, temperature, and glycerol

addition on the conversion and product yield of the MGTGprocess were investigated. Figure 9 shows the conversion rates

and product yield at different temperatures. The conversionrates of both methanol and glycerol were ≥99% along thereaction time coordinate at different temperatures. As thetemperature increased, the total liquid yield slightly decreasedbecause of higher production of gaseous products at higher

Figure 10. Conversion and product yield with the reaction time at theconditions of (a) 10%, (b) 25%, and (c) 40% glycerol in methanol,with T = 500 °C.

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 inmethanol.

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temperatures. Different temperatures almost had no effect onthe CH2Cl2-extracted oxygenate yield that was up to 35.4 wt %.In terms of the oil-phase yield and catalyst lifetime, the bestperformance (11.7 wt % and 5 h) was achieved at 500 °C,because aromatization reactions were favored at higher temper-atures, but at 550 °C, coke formation occurred. With 25% glycerol,the total liquid and oil-phase yields at different temperatures wereup to 75.0−85.3 and 10.5−11.7 wt %, respectively. As the reactiontime increased, total liquid and CH2Cl2-extracted oxygenate yieldswent through a maximum value and then gradually decreased. Therelatively lower yield at the beginning of the reaction couldbe attributed to the formation of more volatile products. Thedecreasing yield after the maximum could result from the decreasein activation of the catalyst and/or interparticle impact. A gradualreduction of gasoline-range aromatics was also observed with anincreasing reaction time, indicating that the sites for aromatizationwere gradually decreased because of coke formation. Figure 10shows the conversion rates and product yield with differentglycerol additions at 500 °C as a function of the reaction time.Methanol and glycerol conversion rates were ≥99% along thereaction time for all of the reactions. The trends of total liquid,CH2Cl2-extracted oxygenate, and oil-phase yields with the reactiontime were similar to those illustrated in Figure 9. At a temperatureof 500 °C, the addition of more glycerol disfavored the oil-phaseproduction. The opening of the ZSM-5 channel is about6 Å.5,27 In comparison to methanol (3.9 Å), glycerol has a largermolecular size of around 5 Å, which may block the pores moreeasily, resulting in a deactivation of ZSM-5. However, differentglycerol additions had almost no influence on the total liquid yield.In addition, the yield of CH2Cl2-extracted oxygenate compoundsincreased with increasing glycerol additions. The maximum valueof that increased from 25.6 to 39.8 wt % when glycerol additionincreased from 25 to 40%.

■ CONCLUSIONThe catalytic performance of both the MTG and MGTG processesusing ZSM-5 was successfully investigated using a quartz, fixed-bedmicroreactor. The MTG- and MGTG-generated products werecomparable in compositions to regular gasoline. In addition togasoline-range aromatics, some oxygenates were detected in theCH2Cl2-extracted phase from the MGTG process. In the MTGprocess, the oil-phase yield (11.0 wt %) and catalyst lifetime (20 h)were optimized at a temperature of 425 °C. As the retention timeincreased, the methanol conversion rate and oil-phase yielddecreased, whereas the total liquid yield rate slightly increasedduring the first 4 h and then gradually decreased. In the MGTGprocess, the best catalytic performance was achieved at 500 °C with10% glycerol in methanol, at which the oil-phase yield was up to14.9 wt % and the catalyst lifetime was 8 h. With increasingtemperature, the formation of gaseous products was slightlyincreased. The conversion rate of both methanol and glycerolremained over 99%, and the total liquid yield increased to amaximum and then gradually decreased with an increasingretention time. With increasing additions of glycerol in methanol,a lower oil-phase yield was obtained, while relatively moreoxygenate compounds were produced. This work shows thatglycerol, a byproduct from biodiesel production, could be used as asubstrate for producing drop-in fuels; however, further work isrequired to increase the catalyst lifetime.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: armandm@uidaho.edu.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

Financial support from both the United States Department ofAgriculture (USDA)−National Institute of Food and Agricul-ture (NIFA) Grant 2010-34158-20938 and the NationalInstitute for Advanced Transportation Technology (NIATT)Grant DTRT07-G-0056 are gratefully acknowledged.

■ NOMENCLATURE

MTG = methanol-to-gasolineMGTG = methanol-and-glycerol-to-gasolineGC-MS = gas chromatography−mass spectrometryHPLC = high-performance liquid chromatographybpd = barrels per daysyngas = synthesis gasFTS = Fischer−Tropsch synthesisIS = internal standardDME = dimethyl ether

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