Optimization of hydrothermal synthesis of H-ZSM-5 zeolite for dehydration of methanol to dimethyl et

Available online at www.sciencedirect.comJOURINLOFScienceDirect。NATURALGAS; CHEMISTRYELSEVIERJoumal of Narural Gas Chemistry 21(2012)344 -351www.elscvier.comvlocatejingcOptimization of hydrothermal synthesis of H ZSM-5 zeolite fordehydration of methanol to dimethyl ether using full factorial designSamaneh Hosseini，Majid Taghizadehl*，Ali Eliassi21. Chemical Engineering Deparment, Babol Universiy of Technolog, P. 0. Box 484, 4714871 167 Babo, Iran;2. Chemical Technologies Research Department, lrarian Research Organization for Science and Technology, Tehran, Iran[Manuserip rccived October 12, 2011; rvised December 29, 2011]AbstractH-ZSM-5 zeolite was synthesized by hydrothermal method. The efects of dfferent synthesis parameters, such as bydrothermnal crytallizatiotemperature (170 -190。C) and Si/AI molar ratio (100-150), on the catalytic performance of the dehydration of methanol to dimethyl ether(DME) over the synthesized H-ZSM-5 zeolite were studied. The catalysts were characterized by N2 adsorption-desorption, XRD, NH3-TPD,TGADTA, and SEM techniques. The full factorial design of experiments was applied to the synthesis of H-ZSM-5 zeolite and the efets ofsynthesis conditions and their interaction on the yield of DME as the response variable were determined. Analysis of variance showed thattwo variables and their interaction significantly affected the response. According to the experimental results, the optimized catalyst preparedal 170°C with the Si/Al molar ratio of 100 showed the best catalytic performance among the tested H-ZSM-5 zeolie.Key wordsfull factorial design; H-ZSM-5 synthesis; mecthanol dehydration; dimethyl ether1. Introduction3C0+ 3H2←+ CHzOCH3 + CO2(2)DME as an important chemical material and a potentialclean fuel substitute for LPG and diesel oil because of its lowFrom the literature, it can be concluded that catalytic de-NO; emission, near-zero smoke, and lower engine noise com-hydration of methanol takes place on different solid-acid cat-pared with those of traditional diesel fuels has attracted muchalysts such as y-alumina, modified-alumina with silica andmore attention in recent years. It is currently used as a sol-phosphorus, Al2O3-B2O3 and zeolites materials (chabazites,vent and propellant in various aerosol products. DME is amordenites, SAPOs, H-ZSM-5, H.Y..).. in a temperaturecolorless gas which boils at . -25 °C at ambient pressure and isrange of 250- 400。C and pressures up to 18 bar [10,11].easily liquefied under pressure. It is a chemically stable com-Zeolites are crystalline aluminosilicates with periodic ar-pound with properties similar to those of propane and butane.rangements of cages and channels, which find extensive in-Its toxicity is very low, which is less than that of methanol anddusrial use as catalysts, adsorbents, and ion exchangers. Thecomparable to that ofLPG [1-4].most important applications are found in the field of fluid cat-Two processes are used for DME production, indirect andalytic cracking (FCC), hydrocracking, isomerization, alkyla-direct processes. In indirect process, methanol is convertedtion and reforming reactions [12-14]. Many researches onto DME in a catalytic dehydration reactor over a solid-acidH-ZSM-5 zeolite have been conducted due to its particularcatalyst by the following reactionstructural and physical-chemical performance, shape selectiv-ity, stability and the flexibility in tailor -making of catalyst2CH3OH←+ CH3OCH3 + H20(1)for various reactions [15,16]. Among the zeolites, H-ZSM-In the second process (direct process) [5-9], a synthesis5 shows a good activity for processes such as dehydration ofgas (a mixture of H2 and CO gases) is used as the feed of themethanol and fluid catalytic cracking [1,17].process In this process, the synthesis gas is primarily con-” 中国煤化Fhed for synthesis ofverted to methanol and then it is followed by methanol dehy-H-ZSM工[14.18.19) and withdration to DME. The net reaction is as follows:differe|YHC N M H Gwith dfferenet sources' Corresponding author. Tel: +9-111-3234204; Fax: +8-111-3234201; E-mail: m taghizadehfr@yahoo.comCopright@2012. Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Al rights reserved.doi:10.1016/S1003- 9931160375-7Joumal of Naural Gas Chemistry Vol. 21 No. 32012345of materials [23-25]. The effects of synthesis conditions ondiameter were characterized using N2 adsorption desorptioncrystal morphology and reaction timevere studied by sometechniques at liquid nitrogen temperature (77 K) using NOVArescarchers [18.21.22]. Usually, the classical method of vary-2000 series instrument (Quantachrome, Boynton Beach, FL).ing one parameter at a time was applied to optimize synthesisBefore the adsorption desorption measurements, the samplesconditions; the nmajor defect of this method is that it could notwere degassed at 473K in a N2 flow for 16h to remove theconsider any possible interaction between the factors, and itmoisture and other adsorbents.Phase idenification was perforned by X ray diffactionalso required many experiments. A best solution is to perform(XRD) using a Cu Ka radiation source (入= 1.5406 A) and amultivariable examination using statistical design of experi-nickel filter in the 20 range of 0°_ 70°.ments [26- -28].Acidity of the samples was measured by temperature-The present work investigates the catalytic activity ofprogrammed desorption of ammonia (NH3-TPD) titration,different H-ZSM-5 zeolites for the methanol dehydration pro-using a PulseChemiSorb 2705 instrument (Micromeritics,cess using a fixed-bed flow reactor. H-ZSM-5-type ze0-Norcross, GA) with a conventional flow apparatus, whichlites have been prepared with different Si/A] molar ratiosequipped with an online thermal conductity detector (TCD).al different temperatures by a hydrothermal crystallizationIn a typical analysis, 0.25 g of the sample was degassed atmethod. The performance of prepared H ZSM-5 catalysts was573 K under a helium flow rate of 30 mL/min for 60 min atevaluated to develop a suitable catalyst for producing DMEa heating rate of 10 K/min. Then, the sample was cooled tofrom methanol A two-factor (Si/Al molar ratio and synthesis423 K and saturated with pure NH3 for 120 min. The sam-temperature) three-level full factorial design of experimentsple was then purged with a helium flow for 60 min to removehas been used to investigate simultaneously efects of indi-weakly and physically adsorbed NH3 on the surface of thevidual and interactional parameters on the performance of H-catalyst. For the next step, the sample was cooled to roomZSM-5 zeolite.temperature and then reheated at a rate of 10 K/min, under aflow of helium carrier gas (40 mL/min), from 303 to 1073 K.2. ExperimentalEventually, the amount of NH3 in the effluent was measuredusing TCD and recorded as a function of the temperature.The suitable temperature of the calcination process2.1. Materialsfor the dried samples was determined in a thermo gravi-metric analyzer using Diamond thermo gravimetric anal-The reactant materials were aluminum nitrate nanoysis/differential thermal analysis (TGADTA) instrumentshydrate (ANN; Al(NO3)3.9H2O， 98.5 wt%， Merck),(PerkinElmer. Waltham, MA). Samples were heated from 298a tetrapropylammonium hydroxide solution (TPAOH;to 973 at 5 K/min in flow of 75 ml /min of nitrogen until noC12H2gNO, 40% aqueous solution, Merck), and tetraethylweight loss could be detected. The coke content of the usedorthosilicate (TEOS, Si(OC2Hs)4, 98 wt%, Merck).catalyst was also determined by this method. Approximately5 mg of the catalyst was first heated from 293 to 423 K in2.2. Catalyst preparationa flow rate of 100 mL/min of nitrogen until no weight lossoccured. Then catalyst was heated at a rate of 2 K/min inA series of H-ZSM-5 catalysts (Z1 to Z9) were preparedzero-air flow (100 mL/min) from 293 to 973 K. In these con-by a hydrothermal crysallization method [17,29]. The alu-ditions, combustion of coke occurred in the temperature rangemina source and water were added to TPAOH, the mixtureof573- -973 K.The morphology of the individual crystals was observedwas stirred at room temperature for I5 min, and then TEOSby scanning electron microscopy (SEM) with TESCAN-was added dropwise. The final mixture was allowed to reactVEGA scanning electron microscope.under a strring rate of about 350 rpm at room temperature for3 h to hydrolyze TEOS completely. Finally, the obtained clear2.4. Catalytic activity testsolution was flld into Teflon-lined stainless steel autoclavesand crystallized by a thermal treatment under hydrothermalconditions for 72 h at a temperature between 170 to 190 °C.Schematic view of the experimental setup is shown in Fig-The molar composition of the oblained clear supersaturatedure 1. Pure methanol was pumped from a feed tank by the dos-solution was xAl2O3/60SiO2/21.4TPAOH/650H2O, in whiching pump (LMI Milton Roy series P-133). The influent wasτ was varied from 0.6 to 0.4. The solid product was washedconstantly kept at temperatures above 300。C, until methanolseveral times with distilled water, dried at 105。C for 12 h, andremained in the gas phase. The dehydration reaction was per-calcined in air at 650°C for 10h at a heating rate of 5。C/minformed in a fixed-bed stainless steel reactor (I.D.= 13 mm;to remove the residual water, ammonia, ammonium nitrate,L = 900 mm). All of activity tests were carried out at 300°C1 g of the catalyst wasand organic substances.loaded中国煤化工Jd weight hourly space2.3. CharacterizationvelociYH-and 26.07 h-', respec-tively.CNMHGorefuintwssettothe gas chromatograph (GC) for online analysis. The reactionTextural properties of the catalysts such as the Brunauer- performance results, including methanol conversion, DME se-Emmett-Teller (BET) surface area, pore volume, and porelectivity and yield, were subsequently calculated.346Samaneh Hosseini et al/ Joumal of Natural Gas Chemisty vol. 21 NO, 3 2012Temperature四indicatorCoolerDh向Elecarical 门Reactorindicator heaterGGascylinderO-MixerDrain- D < -@。VentPressureMethanolFlowindicaorLiquid productvesel .Dosing pumpFigure 1. Schematic of the experimental seup2.5. Full-factorial design of experiments merthodDME yield (%) =;Mole of DME produced x2-x100.The design of experiments (DOE) method has becomeMole of MEOH (initial amount)one of the most popular statistical techniques since 1990s(3and is widely employed in various industries and academicAnalysis of variance (ANOVA) based on the linear statis-fields, depending on the requirements of the situation [26- tical model was used to analyze the resuts. Vanious stistial28,30]. By means of this method, optimizing the diferent pro-data such as standard error of estimate, sum of squares of thecess variables is possible and number of required runs woulderrors, F-ratio, and P-value were examined.be reduced. There are three different types of experimen-TIable 1. Factors and tbeir levels used in full factorial designtal designs: full-factorial, fractional factorial, and TaguchiLevelsParametersorthogonal arrays. The fractional factorial method and the23150Taguchi orthogonal array exclude some of the factor-levelsMolar ratio of SUAI1012from the fll-factorial design to achieve an optimized combi-Temperature (C)18019nation with minimum time and computational cost comparedwith the full- factorial method. Although the fll-factorial de-Table 2. Expertmental layout using ftll factorial designsign requires a great number of experiments or calculations,Experiment No. Catalys name Molar ratio of SUAI Temperaturecit provides very accurate results on the interaction among theZ-1factors, so that the conclusions are highly credible and repro-Z-22-3ducible. Therefore it is economical for characterizing a com-4plicated process. The most important advantages are that not25only the effects of individual parameters but also the relativeimportance of these parameters concermed in a given process7can be obtained and that the interactional effects of two or-8more variables can also be known. This is not possible in thez.9traditional one-variable at-a-time experiment [31]. To deter-mine the effects of the reaction parameters and their interac-tions on the yield of DME and to find the optimum synthesis3. Results and discussionconditions, a two-factor (Si/Al molar ratio and synthesis tem-perature) three-level full factorial design of experiment has.I.中国煤化工been used. To evaluate experimental error, duplicate deter-minations were made for each of these experiments. TheseJYHC N M H Galcination process forfactors and their respective levels are summarized in Table 1.phase transformation and the creation of a stable phase wasTable 2 shows the experimental matrix. The response was thedetermined by the TGADTA method. The TGA/DTA curveyield of DME, which was defined and calculated as follows;of a dried Z-I sample is shown in Figure 2. According toJoumal of Naural Gas Chemisty Vol. 21 No, 32012347this figure, weight losses were observed at two temperaturethe synthesis temperature or silica/alumina ratio, pore volumeregions: below 573 K and between 573 and 873 K. Theseand average pore diameters as well as the BET surface area ofweight losses were atributed respectively to the desorptionthe catalysts will be decreased.of water, which is strongly incorporated in the matrix, andthe decomposition of an organic compound such as TPAOHand the conversion of hydroxide compounds to oxide form.Therefore, it is suitable to perform the calcination process atZ-9the temperature of about 923 K,E「10-8_-0z-79820含96E 94Z411.56%/-1_9088604.00.20.0.686Relative pressure (p/Po)84-1Figure 4. Nz adsorption-desorption isotherms of dfferent zeolite samples373 473 S73 67773873 973Temperature (K)Table 3. Crystal sizes and some characterization data of difTerentH-ZSM-5 catalysts at constant synthesis temperatureFigure 2. TGADTA cuve of the dried Z-I sampleCatalystSurface area Pore volume Pore diameter Crystal sizeXRD patterns of the five samples were compared in Fig-(m2/g)(cm/g)(m)(mm)ure 3. The XRD patterms show peaks in the 20 range of 23°2-14860.675.819.。-250 , which correspond to specific peaks of the H-ZSM-800.565.319.85 sample (JCPDS No. 42-24). So, these results showed2-7770.473.920.2the presence of a H-ZSM-5 crystal phase in different testedcatalysts. The average crystallite sizes were estimated usingTable 4. Crystal sizes and some characterization data of differentScherer equation, summarized in Tables 3 and 4.H-ZSM-5 catalysts at constant SiAl molar ratioSurface area Pore volume Pore diameter Crystal sizre(cm'/g)(nm)Z-8105.721.40.222.223.82yIn order to understand the distribution of the surface acid-ity and the strength of the acid sites, a systematic study ofNH3-TPD measurements were performed. The NH3-TPDprofiles of H-ZSM-5 samples are shown in Figure5. Sur-2-8face acidity of various catalysts from NH3-TPD has been col-lected in Tables 5 and 6. There are three desorption peaks inZTPD profles of catalysts with maxima in the range of92- 240,306070240- 430 and 430- 800 °C, which can be ascribed to the NH320/(° )desorbed from acid sites with low, medium and high strengths,respectively. The NH3-TPD results show that the total acidityFigure 3. XRD pttems of diferet calcined zeoliesof the catalysts increases with a decrease of Si/Al molar ratioThe N2 adsortion-desorption isotherms of the samples and synthesis temperature. Most of the researchers claimedare shown in Figure 4. As it can be seen, all the samples canthat中国煤化工: strength are responsisbe classifed as a type IV isotherm and show a mesoporous ble foE and strong acid sitesstructure.are resC N M H Grocarbons. Among thePhysical properties of the catalysts (specific surface area, prepared catalysts, Z1 showed the highest methanol conver-total pore volume, and average pore diameter) are also pre-sion and DME selectivity and thus was selected as the mostsented in Tables 3 and 4. The results show that by increasing suitable catalyst for the methanol dehydration process.348Samaneh Hosseini et al./ Joumnal of Natural Gas Chemistry Vol. 21 No. 3 2012Table 6. Results of NHy-TPD titration of diferent H-ZSM-5 catalysts600in methanol dehydration at constant SVAI molar ratioZ-1I 800Maximum desorption Amount of NH3Total acidity400Catalysttemperature('C)(mmolga)_(mmol/gary smple_I 600Z-71050.1890.5213900.0741 406400.258200 IZ-81000.1560.4372003050.037730.244z9900.1410.425.4于80310.042700.242400 FFigure 6 shows the SEM images of the Z-1, Z-4 and Z-7samples after calcination. From the micrograph, it is obviousthat the crystals of the prepared samples have good dispersionand the morphology of the crystals is regular. These resultsare in good agreement with the crystallite sizes estimated by-7180XRD data.I 400| 200子80子60SEMAcIOK Daes Dede VEGAIESCANSEH4:150N w 8.7261mm 9000」200Coltrisdy: 12明11 Ve HarRA/[Z4z.98001400 1000 2000 30000 40000 5000 60000 7000Time (s)SIN150w1726]m崇mCatindyt 120/11 Var HWvirRanlFigure 5. NHz-TPD profiles of synthesized zcoliesZ7Table 5. Results of NH-TPD titration of different H-ZSM-5 catalystsin methanol dehydration at constant synthesis temperatureMaximum desorpion Amount of NHztemperature (C)(mmol/gcat)___ (mmol/gdy sample)1100.2170.5633600.0996200.247中国煤化工Z-0.2060.5393400.087CNMHG710.246ow HCAISCAMrl64Figure 6. SEM images of three slected zeoliesJournal of Natural Gas Chemistry Vvol. 21 No. 320123493.2. Activity measurements100All catalyst samples are evaluated for methanol dehy-dration under the same operation conditions (T = 300°C,p= 1 atm, and WHSV = 26.07h-l) for 5 h. The reaction testresults are listed in Table 7. Based on the results of the char-acteristics and catalytic performance of zeolites, it can be con-0cluded that by increasing of Si/Al molar ratio or synthesistemperature, the catalytic activity and selectivity decreased.The results showed that samples with the highest proportionof weak/moderate acid sites exhibited the best catalytic per-formance under the test conditions used. With decreasing syn-20-thesis termperature, Si solubility decreased; thus the amount ofacidity increased.Table 7. Catalytic ativities of difterent H-ZSM-5 catalystsZ.] Z-2 Z-3 Z4 Z.5 Z-6 Z7 Z-8 Z-9ConversionDME sectivityDME YeldCatalystsCatalyst(%)Figure 7. Variation of the yield of DME over dfferent H-ZSM-5 catalysts atz189.99T = 300°C. p= 1 atm, and WHSV = 26.07 h-Z28999.9688.96Z38699.6985.73Z48799.8186.83Z.58399.7682.803.3. Analysis of variance of the experimental dataZ68099.6279.70z-799.6083.66The full factorial design was applied to establish the rela-Z-87978.59tionship between the response (yield of DME) and the factorsZ97499.2173.42(molar ratio of Si/Al and synthesis temperature). To furtherdetermine the significant main and interaction effects of fac-According to the results shown in Figure 7, the Z-1 sam-tors on the yield of DME, an analysis of variance (ANOVA)ple was relatively active and selective to DME formation andwas performed. The degrees of freedom (DF), sequentialthe yield of DME is maximal among the prepared catalysts.sums of squares (seq SS), adjusted mean of squares (adjThe stability of Z-1 catalyst was evaluated under operatingMS), P-value, the F-value, defined as the ratio of the respec-conditions (300°C, 1 atm, and WHSV of 26.07hrl) for 30htive mean square effect and the mean square error, and rel-on-stream, showing that the catalytic activity remained prac-ative percentage contribution among the factors, are showntically constant without any considerable deactivationin Table 8.Table 8. Analysis of variance (ANOVA) for factors and their interactionsFactorDFSeqSSAdj MSF-valueP-valucPercentage contribution (%)Molar ratio of SVAI295.637147.81859.1359.91Temperature139.18769.59327.8428.21 .Molar ratio of Si/Alxtemperature36.1349.0340.0257.32Error2.54.56Total493.45900Assuming a 95% confidence level, the calculated criticalthese factors as well as two factor interaction effects on YDMEF-values which determine the order of the major factor effectas the response parameters are significant. In other words, theand the interaction effect between factors are F0.05,2.9=4.26mean YDME values are different for different temperature andand Fo.05,4,9= 3.63, respectively [31]. As can be seen fromSi/Al molar ratio levels (Table 9). In addition, the first orderTable 8, the F-test values of all factors and interactions aremodel presented an adjusted square correlation coefficient R2considerably greater than the extracted F-value of Ref. [31](adj) of 97.5%, flting the statistical model quite well. In thiswith 95% confidence. This means that the variance of eachway, the yield of DME could be expressed using the followingfactor and their interactions are significant compared with theequation:variance of error and that all of them have an important effecton the response. P-value is the probability value that is used中国煤化.590915(/A4)(4)to determine the statistically significant effects in the model.MHCN M H Gs/AnxTIf the P-values are closer to zero, then the efect for the temis significant. All factors and their interactions have P-valuesThis equation describes how the experimental variablesless than 0.05 (see Table 8), which means that the effects ofand their interactions influence the DME production. Accord-350Samanch Hosseini et al./ Joural of Naural Gas Chemisty Vol 21 No. 3 2012ing to Equation 4, Si/Al ratio and synthesis temperature hada greater effect, but at lower SiAI molar ratio, these changesa positive effect on DME yield, while SiAl-temperature in-were not so significant.teraction term had a negative efect. The Equation 4 enablesus to predict the yield of DME as a function of Si/Al ratio andsynthesis temperature. Not only are the main effects modeled,90but also their interactions, which is the main advantage of the882k factorial design compared to the traditional approach.。 86Table 9. Least squares means for yield of DMEParanetersMeanMolar ratio of Si/AI10087.34+0.645512581.54+0.64551507.46+0.6455Temperature (°C)78 t85.55+0645518082.06+06455761025151780 119078.74+06455SiVAlxtemperatureMolar mtio of SiUAITemperature (C)100x17088.701.1180Figure 8. Main efct plot (data means) for the yicld of DME100x 18087.65+1.1 180100x19085.66+1.1180125x17084.82+1.1180 .125x 18081.07+1.1180125x19078.741.1180150x17083.13+1.1180150x 1807.45+1.1180B 85150x 19071.81+1.118080Meanwhile, the relative contribution percentage of eachfactor and interactions on YomE are mentioned in the last col-! 75 S/AIt Temperature (C)umn of Table 8, which shows that the Si/Al molar ratio, withrelative contribution percentage of 59.91% of the total effect,+180is the most significant factor on the yield ofDME as compared- 150-+19070 [to synthesis temperature and interaction terms.70190 100The main effect and an interaction plot are shown in Fig-Molar ratio of SiVAIures 8 and 9, respectively. Each data point in Figure 8 repre-Figure 9. Interaction plot (data means) for the yield of DMEsents the mean of the response variable (YDmE) for each factorlevel. Also, the dotted line represents a reference line at theoverall mean, which is 82.11% YDME. This plot indicates that4. Conclusionsthe synthesis temperature and Si/Al molar ratio have a positiveeffect on the yield of DME. A comparison of the slopes of thelines can be used for determination of the relative magnitudeIn this study, a series of H-ZSM-5 catalysts were preparedof the factor effects. Therefore, the importance of the factorsby the hydrothermal crystallization method using full facto-increase in the following order; Si/AI molar ratio> synthesis rial design. This method was used to investigate simultaneoustemperature. According to Figure 8, the factor levels indicateeffects of individual and interactional parameters on the per-the optimum conditions are as follow: Si/AI molar ratio= 100formance of prepared catalysts. The catalysts were used forand synthesis temperature = 170°C. This result is consistent the synthesis of DME through the dehydration of methanol.with P-value, and the relative percentage contribution of Si/AlThe structure and morphology of catalysts were studied bymolar ratio and synthesis temperature in the ANOVA table.X-ray diffraction (XRD) and BET. XRD patterms showed thatIt is well known that an interaction is present when the the extent and percentage of crystallinity of catalysts do notchange in the response mean from the low to high level of adiffer significantly. BET results showed that by increasingfactor depends upon the level of a second factor. The inter- the synthesis temperature or Si/Al molar ratio, pore volumeaction plot for Si/Alxtemperature presented in Figure 9 indi-anda中国煤化Ilysts will be dereedcates that decreasing rysallization temperature from 190°CNH3-'Emple with smaller cry8-to 170 °C increase the yield of DME by 11.32% (from71.81%talliteYHC N M H Gition of medium acidicto 83.13%) at Si/Al molar ratio of 150% and 3.03% (romsites and consequently a higher catalytic activity.85.67% to 88.7%) at Si/Al molar ratio of 100. This indicatesThe results of analysis of variance of the experiment datathat at higher Si/Al molar ratio, changes in temperature have indicated that the activity of the aforementioned catalyst haveJounal of Natual Gas Chemnisty Vol 21 No.32012been susatillyl afected by the Si/AI molar ratio and syn- [12] Ismail A A. Mohamed R M. Fouad 0 A, Ibrahim IA Cryst Resthesis temperature.Technol, 2006, 41: 145According to the experimental results, the catalyst pre-[13] Chen N Y, Degnan T R. 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