Conversion of Methanol Using Modified H-MOR Zeolite Catalysts

2011Chinese Journal of CatalysisVol. 32 No.3Article ID: 0253-9837(2011)03-0412-06DOI: 10.1016/81872-2067(10)60187-8Article: 412- 417Conversion of Methanol Using Modified H-MOR Zeolite CatalystsSameh M. K. ABOUL-FOTOUH+, Noha A. K. ABOUL-GHEIT2, Marwa M. I. HASSAN'Chemistry Department, Faculty of Education, Ain-Shams Universit, Roxy, Cairo, Egypt'Process Development Department, Egyptian Petroleum Research Institute, Cairo, EgyptAbstract: The conversion of methanol was carried out over various mordenite zeolite catalysts to evaluate their catalytic performance. Apost-preparation treatment of the H-MOR catalyst by halogenation with NH4Cl or NH4F and by hydrohalogenation with HCI or HF wascarried out and its effect on methanol conversion reactions at 100 -300 。C in a continuous flow reactor was investigated. The as-synthesizedH-mordenite (H-MOR) is generally more active during dimethyl ether (DME) production than the NH4-MOR and Na-MOR. Fluorinatedtreatment with HF or NH4F significantly improved the catalytic activity during methanol conversion and the formation of DME in compari-son to chlorinated treatments with HCI or NH4CI. This is principally atibuted to the higher Si/Al ratio and an increase in the number of acidsites and their strength. Halogenation treatment with the acids of both F or cr gave the highest conversion activity for DME productioncompared to halogenation treatments with the salts of the same halogens. Zeolite dealumination by the acids was more profound than that bythe halogen ion salts, which resulted in a decrease in the crystallinity and crystallite sizes of the zeolite.Key words: methanol; conversion; H-MOR; halogenation; hydrohalogenationCLC number: 0643Document code: AMethanol will possibly be one of the main fuels used incyclohexene over metal/Al2O3 with and without CI and Ffuture when the petroleum resources are depleted. Methanolions has been studied by Aboul-Fotouh and Aboul-Gheitis produced from synthesis gas (mixture of CO2 and H2),[23]. Introducing CI or F ions into aluminate alumina inwhich is formed by the steam reforming of natural gas, thedifferent ways causes Bronsted acid sites to appear and agasification of coal or from biomass. Methanol is abundantdrastic increase in both the skeletal isomerization and totaland, therefore, it has been used as a raw material for theconversion. Arena et al. [24] indicated that CI adsorbed onproduction of gasoline and olefins. It can be obtained fromthe surface of y-alumina results in promotion because of arenewable sources such as biomass. The catalytic conver-substantial change in the electronic properties of the outer-sion of methanol to gasoline and to olefins attracted themost alumina layer. Ultimately, these effects result in aattention of many researchers when the price of fuel startedweakening of the 0 H bond, which renders the proton moreto increase. Literature concerning the conversion of metha-acidic.nol to hydrocarbons over zeolite catalysts is extensiveDoping H-ZSM-5 with a low concentration of F fol-[1-12]. Its nature and the extent of the reaction depend onlowed by thermal activation enhances surface acidity by thethe acid site density and strength as well as the reactionformation of new Bronsted acid sites and the strengtheningconditions [13-16].of other acid sites on the parent zeolite [25- -27]. The aim ofMetal/metal oxide incorporation on to the surfaces ofour work is to evaluate different mordenite (MOR) zeoliteszeolites leads to an increase in methanol conversion and(H-MOR, NH4-MOR, and Na-MOR) and study the effect ofdimethyl ether (DME) yield [17-21]. If the metal is sup-doping H-MOR with 3.0 wt% HCI, HF, NHLCl, and NHFported on Al2O3 [22], the acid sites are of the weak Lewisto visualize this effect on methanol conversion in a flowtype where the acid strength is not strong enough to pro-reactor that operates in a flow of argon at temperatures ofmote methoxonium ion formation and hence the dehydra-100 300 °C.tion reaction cannot occur. However, the H-forms of thezeolites possess strong Bronsted acid and Lewis sites that1 Experimentalpromote methanol conversion to DME and olefins and theirreaction schemes are complicated. Nevertheless, treatment1.1 Preparation of the catalystsof cation-exchanged zeolites with HCI or HF leads to deca-tionation and partial dealumination. The hydroconversion of1.1.1 H-MOR中国煤化工MYHCNMH G*Corresponding author. Tel: +20-2-22757840; Fax: +20-2-22581243; E-mail: saboulfotouh@hotmail.comEnglish edition avilable online at ScienceDirect (tp://wwcc/ic.coc/scicnc/jouma/18722067.www.chxb.cn Sameh M. K. ABOUL-FOTOUH et al: Conversion of Methanol Using Modified H-MOR Zeolite Catalysts413The sodium ions in the Na-mordenite zeolite (Zeolon(Mettler TA-3000) using standard Al crucibles. Presorbed900-Na, kindly supplied by Norton Co., USA, in the form ofammonia was desorbed in the DSC cell using dry nitrogen1.59 mm extrudates) were exchanged five times using anas a purge gas at a flow rate of 30 cm /min. The heating rateNH4NO3 solution under reflux. A fresh solution was usedin the DSC runs was always 10 °C/min and the full-scale .each time and the reflux lasted for 8 h at 70 °C. The zeoliterange was 25 mW.was then separated, washed with distilled water until it wasfree of NO3 , dried at 110 °。C overnight, and then calcined in2 Results and discussionair at 550 °C for 3 h. The H-form (H-MOR) had a Si/Alratio of 6:1, a surface area of 470 m'/g, an effective pore2.1 Characterization of the catalystsdiameter of 0.67 nm, and a pore volume of 0.27 cm'/g.2.1.1 XRD patterns1.1.2 Chlorinated and fluorinated H-MOR catalystsFigure 1 shows that none of the main diffraction bands inSome of the batch zeolite prepared in section 1.1.1 was the XRD patterns obtained for the modified H-MOR show adoped with 3.0 wt% aqueous HCl, HF, NH4CI, or NH4F,measurable shift in their 20 positions compared to the stockdried and then calcined as mentioned above.H-MOR sample. However, the diffraction intensities of allthe bands decreased for the halogen-modified zeolites.1.2 Methanol conversion and product analysisMoreover, the mordenite samples treated by chlorinationwith NH,Cl or hydrochlorination with HCI gave higher dif-A silica glass flow tubular reactor system packed with 0.1fraction intensities than those treated by fluorination withg of the zeolite catalyst was used in all the conversion runs.NHF or hydrofluorination with HF and this can be attrib-The reactor was heated in an insulated wider silica tubeuted to a greater extent of zeolite dealumination by the Fjacket, which was kept to within +1。C. Argon gas was usedions than by the CI ions [22,27].as a carrier at a flow rate of 30 cm /min in all the runs. Thfeed (methanol) was introduced into the reactor by continu-ous evaporation using the argon flow that passed into aclosed jar and that was kept at a fixed temperature of 26 °C.The amount of methanol used was always 49.8 mmol/h. Thereaction temperatures investigated ranged from 100- -300。Cat 25 °C increments. The reaction effluent that passed fromNHF/H-MORthe reactor was injected into a Perkin-Elmer Autosystem XLN MM\ vihunMwwmtWHMORgas-chromatograph containing a 4 m column packed with10% squalane on 10% didecyl phthalate on chromosorpNH4CI/H-MORW-HP of 80 100 mesh. A flame ionization detector and theHJR-MORTurbochrom Navigator Program were used for analysis.1.3 Characterization of the catalystsH-MORX-ray diffraction (XRD) patterns of the catalysts wereobtained using a Brucker AXS D8 Advance. Patterms were10 2(304050607080run using a Ni filter and copper radiation (2 = 0.15404 nm)20(°)at 40 kV and 40 mA and at a scan rate of 0.02/min.Fig. 1. XRD patterms of the halogenated and hydrohalogenatedFor temperature-programmed desorption (TPD) of am-H-MOR catalysts.monia experiments, we applied the procedures reported byAboul-Gheit et al. [28] and Aboul-Fotouh [29] using differ-Figure 2 shows that treatment of H-MOR by halogenationential scanning calorimetry (DSC) to detect the desorptionor hydrohalogenation leads to a decrease in the degree ofresponse of presorbed ammonia from the catalysts. Ammo-crystallinity (crystallite size). The as-synthesized1 H-MORnia was primarily adsorbed on the catalyst in a silica tubewas found to H-ze among thefurnace. After evacuation at 1.33 x 103 Pa and upon heatingtreated samples中国煤化ICcoring to:at 500 °C and subsequent cooling under vacuum to 50 °CH-MOR > NHCYHc N M H GHF/H-MORammonia was introduced to the catalyst at a flow rate of 50> HF/H-MOR.cm/min. The samples were then assessed in a DSC unitThe stronger attack on the Al in the zeolite framework is414催化学报Chin. J. Catal, 2011,32: 412- 417120and, therefore, to the activity of DME production. This fig-ure shows that the number of acid sites decreases whereas100the acid site strength increases with the different treatments宣80(Table 1).谷60Table 1 HT-ammonia desorption enthalpy, HT-peak temperature, andSi/Al ratios for the zeolite catalystsHT-peak5 20Catalyst0H(J/g)Si/AI ratiotemperature (C)oH-MOR8.35306.MORHF/H-MOR82.25378.NH4F/H-MOR79.633HCI/H-MOR74.3357.3Fig. 2. Crystallite sizes from XRD of the H-MOR zeolite catalysts.NH.CVH-MOR70.531HT: High temperature.known to be followed by the migration of Si atoms to fillthe vacant Al positions. Therefore, the Si- 0- Al bonds are2.2 Conversion of methanolpartially changed to Si 0- Si bonds to stabilize the zeoliticstructure. Since Si- 0 bonds are shorter than Al- 0 bonds,The studied catalysts were prepared to investigate the ef-the zeolitic crystal units cells shrink while the channelsfect of different mordenite zeolite treatments by the above(pores) and the surrounding crystals become narrower. Fmentioned halogenation and hydrohalogenation processes.ions are more reactive than C1 because of their larger in-The conversion of methanol was investigated in a fixed-bedductive effect [30,31].reactor using an argon flow of 30 cm/min for a reactiontemperature range of 100- 300。C to cover all possible exo-2.1.2 Surface acidity of the catalyststhermic and endothermic reactions. The major products ofthe reaction under these experimental conditions were eth-TPD of the presorbed ammonia on the acid sites of the .ylene, propylene, and DME.catalysts was carried out using DSC with nitrogen as aninert purge gas. TPD profiles for the studied catalysts are2.2.1 Conversion of methanol with the H-, NH4, andgiven in Fig. 3. Each profle consists of two endothermicNa-MOR catalystspeaks, one of which is a low temperature peak (100- 300°C) that is not related to the activation of carbonium ionThe conversion of methanol was examined over theformation and, therefore, it cannot be correlated withH-MOR, NH4-MOR, and Na-MOR catalysts at 100- 300 °Cchanges in the catalytic activity during DME formation. Thein a flow reactor. Figure 4 shows the DME and hydrocarbonhigher temperature peak (400- 650 °C) that appears in theproducts that were produced by methanol conversion. Thethermogram is closely related to carbonium ion formationdata shows that the dehydration and alkylation products(DME and olefins, respectively) were obtained usingH-MOR and NH4-MOR whereas only the dehydration| H-MOR| HF/H-MORproduct (DME) is produced when using Na-MOR. This is| NHF/H-MORattributed to the absence of strong acid sites, which are ca-| HCVH-MORpable of promoting olefin formation. Campbell et al. [32]| NH,CI/H-MORused CH,OH-TPD for the characterization of the acid prop-erties of the surface zeolite (strength and nature of the acidsites) and concluded that methanol dehydration to DME andthe production of light hydrocarbons are related to the typeand strength of the acid sites. Bronsted acid sites with mod-erate to high strengths are responsible for the formation of100 200300400500600700 800between methand中国煤化工id sites do notTemperature (C)show catalytic aYHCNMHGfmethanoltoFig. 3. TPD of ammonia from the untreated and halogenatedDME, Bronsted and Lewis sites cooperate and the conver-H-MOR catalysts.sion is proportional to the acid strength.www.chxb.cn Sameh M. K. ABOUL-FOTOUH et al: Conversion of Methanol Using Modified H-MOR Zeolite Catalysts41550NH4F, or NH4Cl, were found to be efective (Fig. 5). More--■- H-MOR(a)over, fluorination and hydrofluorination were found to be40一一o- NH-MORmore effective than chlorination and hydrochlorination,-▲- Na-MOR宫30-respectively.告20■- H-MOR-●- HF/H-MORE 10. NHF/HMOR- HCIH-MORNH,CIH-MORb)。40一-o- NHL-MOR多30-目20s 10101500250300Temperature (CC)Fig.5. Total conversion of methanol using the halogenated and hy-1002030Temperature (C)drohalogenated H-MOR catalysts.Fig. 4. DME (a) and hydrocatbons (b) in the mordenite catalystproducts.Figure 6 shows that higher activities for methanol con-version to DME were obtained by F inclusion compared toFigure 4 shows the activities of the studied catalysts fromCI inclusion. Moreover, direct acid treatment with HF or100- -300 °C for the conversion of methanol to DME andHCl results in higher activities than that of the correspond-olefins, respectively. The activity decreases as follows:ing ammonium salt treatments because of direct halogenH-MOR > NH4-MOR > Na-MOR. This order is compatibleliberation in the former treatments. The superiority of Fwith the order of mordenite catalyst acidity [28]. From thetreatment over CI treatment is attributed to its higher elec-acid site desorption of NH3 in the Na-MOR and H-MORtronegativity [30,31]. DME production increases as a func-Aboul-fotouh [29] indicated that the acidity of H-MOR canbe attributed to the strong acid sites whereas the acidity ofa)Na-MOR is due to the weaker acid sites. On the other hand,Aboul-Gheit et al. [28] showed that the acidity ofNH4-MOR is lower than that of H-MOR since the acid sites0卜in H-MOR are strong Bronsted sites.一Y- H-MOR~2.2.2 Conversion of methanol using halogenated andHF/H-MORNHLF/H-MORhydrohalogenated H-MOR zeolite catalystsHCVH-MORNHCI/H-MORFluorination and chlorination using NHF and NH,Clb)|with halogen concentrations from 1.0 to 6.0 wt% has beencarried out in previous works [22,23,26,27] and a halogen60 F- H-MORcontent of 3.0 wt% was found to be optimum. Hence 3.0NHF/H-MORwt% F or CI was used in this study to prepare the halo-▼- HCVH-MORgenated H-MOR zeolite catalysts by NH4F and NH4Cl dop--<- NHCIH-MORing. Moreover, hydrohalogenated catalysts were also pre-pared by doping with 3.0 wt% HCI or 3.0 wt% HF solu-tions, respectively.The prepared halogenated and hydrohalogenated H-MOR中国煤化工-catalysts were examined for methanol conversion fromYHCNMHG100- -300 °C. The total methanol conversion activities of thefig. 6. DME (a) and hydrocarbons (b) in the product upon halo-studied H-MOR catalysts doped with 3.0 wt% HF, HCI,genated and hydrohalogenated HMOR catalysis.416崔化学报Chin. J. Catal, 2011, 32:412- -417tion of reaction temperature up to 200 °C beyond which-1DME decreases in the product as the temperature increasesfurther because of hydrocarbon formation (Fig. 6).-14The relatively higher activities of all the treated catalystscompared to the H-MOR catalyst are attributed to the higheracid site strengths of the catalysts (Fig. 3 and Table 1). Thenumber of acid sites is expressed by the desorption enthalpyof the pre-adsorbed NH3 from HF/H-MOR, HCI/H-MOR,Na-MORHCVH-MORand the H-MOR catalysts and amount to 82.2, 74.3, and-181 NH-MORNH4F/H-MOR98.3 J/g, respectively. The acid site strengths as expressed口H-MORHF/H-MORby the desorption temperature were found to be 537, 535,◆NH,C/HMORand 530 °C. Additionally, the desorption enthalpies of the2.2.02.8pre-adsorbed NH3 from NHF/H-MOR, NH4Cl/H-MOR,(10/T)/K- 'and H-MOR are 79.6, 70.5, and 98.3 J/g, respectively. Theacid site strengths as expressed by the desorption tempera-and untreated mordenite catalysts.ture were found to be 533, 531, and 530。C, respectively.These data indicate that the fluorination of mordenite in-reaction temperature.creases its acid strength, which is evident from the increasedHT-peak temperature determined during NH3 desorptionover the studied catalysts are given in Table 2. The E。values(acid site strength).determined for methanol dehydration to DME generallyFigures 1 and 2 show XRD spectra for the halogenationagree with the activities of the catalysts used. Na-MORand hydrohalogenation of the H-MOR catalysts. Evidently,showed lower activity particularly at lower temperaturesF and CI incorporation into the catalyst results in smallerand, therefore, the E。value for this catalyst is higher than(nanoscale) H-MOR crystallites, which increases catalystthat of the NH4-MOR and H-MOR catalysts since the latteractivities. The decrease in H-MOR crystallinity because ofare more acidic. The treatment of H-MOR with HF, HCl,the removal of some Al from the framework increases theNH4F, or NH4Cl results in a decrease in the activation en-hydrophobicity of the zeolite and its Si/A1 ratio (Table 1).ergy (E) of the former catalyst from 41.2 kJ/mol beforeCorma [33-35] found that the dealumination of zeolitestreatment to 21.9, 25.2, 31.4, and 35.6 kJ/mol, respectively,increases their hydrophobicity, which results in a higherafter treatment. This decrease in E。after halogenation orcapacity for hydrocarbon adsorption and a lower capacityfor water adsorption and both these factors are favorable forfrom halogen (F, CI1) inclusion. Moreover, these values in-an increase in activity. Since the dehydration reactions aredicate that fluorine is more activating than chlorine.equilibrium-limited and water is continuously removedfrom the catalyst surface because of its hydrophobic nature,Table 2 Activation energies for methanol conversion to DME basedthe catalytic activity of the dealuminated zeolite is greatlyon the studied catalystsenhanced.CatalystE:/(kJ/mo)H-MOR41.22.3 Activation energy of methanol conversion to DMENH4-MOR48.756.1The reaction rate constant was calculated for the differentreaction temperatures according to the second-order flowNH4F/H-MOR .31.4reactor equation (1):HCI/H-MOR25.2k= (F/W)[x/a(a -x)](1NHCVH-MOR35.6Where F is the rate of feed injection 49.8 mmol/h, k is the .reaction rate constant, W is the catalyst weight (0.1 g),x is3 Conclusionsthe mole fraction of methanol converted to DME, and a isthe initial concentration of methanol.Na-MOR was found to be the least active catalyst forThe E。values for the dehydration of methanol to DMEDME production中国煤化-1ordenite cata-using the studied zeolite catalysts were determined using thelysts. It was corin productionArrhenius equation (2) and the plots are given in Fig. 7. .because of theYHCNMHGheincreancrease ink= Aexp(- Eg/RT)(2methanol conversion and the formation of DME on hydro-Where A is the pre-exponential factor and T is the absolutehalogenated H-MOR compared to the halogenated catalystswww.chxb.cn Sameh M. K. ABOUL-FOTOUH et al: Conversion of Methanol Using Modified H-MOR Zeolite Catalysts417is atributed to the increase in acidity. The higher activity of15 Anwar A, Abdel-Ghaffar A, Aboul-Fotouh S, Ebeid F. Collectthe halogen treated H-MOR zeolite is attributed to an in-Czech Chem Commun, 1994, 59: 820crease in the Si/Al ratio and a decrease in the crystallinity of16 OhS H, Lee w Y. Korean J Chem Eng, 1992, 9: 3717 Zaidi H A, Pant K K. Catal Today, 2004, 96: 155the current zeolite catalysts. Hydrofluorination treatment18 Aboul-Fotouh S M K, Hassan M M I. Acta Chim Slov, 2010,was found to be consistently more effective than hydrochlo-57: 872rination treatment.9 Freeman D, Wells R P K, Hutchings G J. J Catal, 2002, 205:358References20 Kang M. J Mol Catal A, 2000, 160: 43721 Zhu z, Hartmann M, Kevan L. Chem Mater, 2000, 12: 27811 Marchi A J, Forment G F. Appl CatalA, 1993, 94: 9122 Aboul-Fotouh S M, Aboul-Gheit A K. Appl Catal A, 2001,? Mikkelson 0, Kolboe s. Microporous Mesoporous Mater,208: 551999, 29: 1733 Aboul-Fotouh S M, Aboul-Gheit A K. In: Proceedings of the3 A-arallah A M, EI-Nafaty U A, Aillahi M M. Appl CatalA,9th International Symposium on Heterogencous Catalysis,1997, 154:117Varna, Bulgaria, 2000. 1634 Stocker M. Microporous Mesoporous Mater, 1999, 29:324 Arena F, Frusterl F, Mondeller N, Giordano N. J Chem Soc,5 Freeman D, Wells R P K, Hutchings G J. J Catal, 2002, 205:Faraday Trans, 1992, 88: 3353355 Le Van Mao R, Le T S, Fairbaim M, Muntasar A, Xiao S, De-6 Dubois DR, Obrzut D L, Liu J, Thundimadathil J, Adek-nes G. Appl Catal A, 1999, 185:41 .kanattu P M, Guin J A, Punnose A, Seehra M S. Fuel Process26 Aboul-Gheit A K, Aboul-Fotouh S M, Abdel-Hamid S M,Technol, 2003, 83: 203Aboul-Gheit N K A. J Mol Catal A, 2005, 245: 1677 Chang C D, Silvestri AJ. J Catal, 1977, 47: 24927 Aboul-CGheit A K, Aboul-Fotouh S M, Abdel-Hamid s M,8 Bjorgen M, Kolboe s. Appl Catal A, 2002, 225: 285Aboul-Gheit N K A. Appl Catal A, 2006, 297: 1029 Inoue Y, Nakashiro K, Ono Y. Microporous Mater, 1995, 4:28 Aboul-Gheit A K, Abdel-Hamid S M, Ghoneim S A, Al-Owais379AA. Erdol Erdgas Kohle, 199, 115: 9010 Tsoncheva T, Dimitrova R. Appl Catal A, 2002, 225: 10129 Aboul-Fotouh s M. J Chin Chem Soc, 2003, 50: 115111 Yaragadda P, Lund C R F, Ruckenstein E. Appl Catal, 1989,30 Hirschler A E. J Catal, 1963, 2: 42854: 13931 Ghosh A K, Kydd R A. Catal Rev-Sci Eng, 1985, 27: 53912 Zaidi H A, Pant K K. In: Proceedings of the 53rd Canadian32 Campbell S M, Jiang X Z， Howe R F. MicroporousChemical Engineering Conference, Hamilton, Ont, 2003.32713 Alkhawandch A, Wu x, Anthony R G Catal Today, 2003, 84:33 Corma A. Chem Rev, 1995, 95: 55934 Corma A. Chem Rer, 1997, 97: 23734 Caizares P, de Lucas A, Dorado F, Duran A, Asencio I. Appl35 Corma A, Faraldos M, Martinez A, Mifsud A. J Catal, 1990,Catal A, 1998, 169: 137122: 230中国煤化工MHCNMH G