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• 

  Methanol conversion to dimethylether (DME) is an internationally up-to-date industrial achievement for the production of environmentally friendly motor fuels because it is composed of two carbons plus six hydrogens and one oxygen. Its preparation from methanol is a highly profitable reaction since methanol is an agricultural by-product of fermentation of waste plants.

  DME has been found to be an alternative diesel fuel because it has low NO_x emission,near-zero smoke amounts and less engine noise compared with traditional diesel fuels^{[1, 2]}. It can also be used to replace chlorofluorocarbons (CFCs) which destroy ozone layer of the atmosphere and used as an intermediate for producing many valuable chemicals such as lower olefins,methyl acetate,dimethyl sulfate and liquefied petroleum gas (LPG) alternative. It is also used in power generation and as an aerosol propellant,such as in hair spray and shaving cream,due to its liquefaction property^[3-7]. Hence,there is a growing demand to produce a large amount of DME to meet the global need.

  DME can be produced by methanol dehydration over a solid-acid catalyst or direct synthesis from syngas by employing a hybrid catalyst,comprising a methanol synthesis component and a solid-acid catalyst^[8]. Methanol dehydration to DME is a potentially important process and more favorable in the views of thermodynamics and economy^[9]. Commercially,γ-Al₂O₃ is used as the catalyst for this reaction. It has high surface area,excellent thermal stability,high mechanical resistance and catalytic activity for DME formation due to its surface acidity. Recently,many methods have been applied to synthesize alumina with a higher specific surface area and activity for DME synthesis^[10].

  Methanol to DME (MTD) dehydration over a solid acid catalyst in a fixed bed reactor was first reported by Mobil in 1965. Since then,many methanol dehydration catalysts have been examined^[11] including γ-Al₂O₃^[12-17],crystalline aluminosilicates^{[17, 18]},zeolites (ZSM-5)^[19],clays^[20] and phosphates such as aluminum phosphate^{[21, 22]}. However the most common catalysts used are γ-Al₂O₃ and zeolites.

  Chlorination and fluorination of Al₂O₃ and zeolites were carried out by the author^[23-25] to promote the catalytic acidity. The results of hydroconversion of cyclohexene over metal/Al₂O₃ with or without Cl^- and F^-ions were studied^[24]. Introducing Cl^- or F^- ions into aluminate aluminas in different ways causes Brnsted acid sites to appear and a drastic increase of both skeletal isomerization and total conversion. Also,the fluorination enhances the acidity and catalytic activity of H-MOR towards DME production^[23].

  Ultrasound irradiation can enhance and improve the catalytic performance of the catalyst. Application of power ultrasound has been shown to be instrumental in improving the rates,yields and product properties of a variety of processes in synthetic chemistry^{[26, 27]}. The effects of ultrasound have been investigated for different cases,including polymerization reactions and the syntheses of various amorphous and crystalline materials. Significant changes have been commonly observed in the processes and properties of the reaction products in the presence of ultrasound. The main purpose of using ultrasound in different chemical reactions has been to enhance the reaction rates,yields and selectivity to desired product.

  In the present work,the catalytic dehydration of methanol to DME has been studied using chlorinated or fluorinated H-MOR,HZSM-5 and HY zeolite catalysts prepared by the impregnation method with or without ultrasonic irradiation. The effects of ultrasonication and/or halogenation on the textural,acidic properties and catalytic activity of zeolite samples have been investigated.

  1   Experimental

  1.1   Preparation of the halogenated zeolite catalysts

  The chlorinated and fluorinated zeolite catalysts were prepared by stirring the catalysts for 1h at 25℃ in the impregnation of 1g zeolites (H-ZSM-5,H-MOR or H-Y) catalysts in 2cm³ of aqueous solution of ammonium chloride or ammonium fluoride containing the requisite quantity for 3.0% (by weight) NH₄Cl or NH₄F,respectively. The catalyst was dried at 110℃ overnight and calcined at 400℃ for 2h in an air flow.

  1.2   Preparation of sonicated halogenated zeolite catalysts

  0.3g of prepared halogenated zeolites was sonicated in methanol as a liquid carrier of 7.5cm³ for 60min using Ultrasonic Processor UP50H (Hielscher) with the titanium sonotrode S₃ having a tip diameter of 3mm and 50W/cm² power intensity (related to 100% amplitude setting). The irradiation with ultrasound was continued for 1h at 25℃,then centrifuged for 30min. The catalyst was dried at 110℃ overnight and calcined at 400℃ for 2h in an air flow.

  1.3   Hydroconversion reactor system and reaction product analysis

  A silica glass flow-type tubular reactor system loaded with 0.1g of the zeolite catalyst was used. The reactor was heated in an insulated wider silica tube jacket,thermostated to ± 1℃. Argon was used as a carrier gas at a flow rate of 30cm³/min in all runs. The methanol feed was introduced into the reactor via continuous evaporation applying argon flow passing into a closed jar thermostated at a fixed temperature of 26℃ ,whereby the quantity of methanol was always 4.98×10^-2 mol/h. The reaction runs were investigated at temperatures ranging between 100-300℃,with 25℃ increments. The reaction effluent was analyzed using a Perkin-Elmer Autosystem XL gas-chromatograph with a 4 m long column,packed with 10% squalane plus 10% didecyl phthalate supported on Chromosorb W-HP of 80-100 mesh. A flame ionization detector and a Totalchrom Navigator Programme computed were used.

  1.4   Characterization of the catalysts

  1.4.3   Scanning electron microscope (SEM)

  The SEM samples were mounted on aluminum slabs and sputter-coater with a thin gold layer of ~15mm thickness using an Edward sputter-coater. The samples were then examined in a scanning electron microscope model JSM-5410 with Electron probe micro-analyzer (JEOL) at 30kV.

  1.4.1   Temperature programmed desorption (TPD) of ammonia

  The TPD of presorbed ammonia on the acid sites of the zeolite supports was carried out in differential scanning calorimeter (DSC) using nitrogen as a purge gas according to the procedure adopted by the author^[28].

  1.4.2   X-ray diffraction patterns of the catalysts

  The X-ray diffraction patterns of the catalysts under study were carried out using a Phillips X,Pert Diffractometer PW 1390 at 40kV and 30mA with Ni filter and Cu Kα radiation. The XRD runs were carried out up to 2θ of 60°.

  2   Results and discussion

  2.1   Catalyst characterization

  2.2   Catalytic activity of the zeolites

  2.1.3   Surface properties

  Figure 6 shows the effect of chlorination and ultrasonication on the pore size distribution of H-MOR catalyst. The effect of chlorination before and after ultrasonication on the pore size distribution of H-MOR catalyst is insignificant. In these Figures 6(a)-6(c),the scale of dv(r) is almost the same and the rejoin proportion of pores fall in the pore radius range <1.5nm. Moreover,there is also 20% of the pores falling in the wider range 2-6nm. This may assume that the effect of chlorination and ultrasonication on the pore volume distribution has been mollified.

  Figure 6.  Effect of chlorination and ultrasonication on the pore size distribution in H-MOR catalysts

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  Table 2 shows surface area and total pore volume values of different halogenated zeolites. In Table 2 the surface areas of the samples become very close (H-MOR 350m²/g,Cl-HMOR 368m²/g. Also the total pore volume of the two samples become close (0.22 and 0.23cm³/g). It is to be signified that ultrasonication of chlorinated H-MOR acquires a surface area of 478 m²/g and a total pore volume of 0.44cm³/g.

  Table 2.  Surface area and total pore volume values of different halogenated zeolites

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  CatalystTotal pore volume v/(cm³·g^-1) Surface area A/(m²·g^-1)H-MOR0.22350Cl-HY (U)0.45468

  Cl-HMOR	0.23	368
  Cl-HMOR (U)	0.44	478
  H-ZSM-5	0.32	357
  F-HZSM-5	0.37	375
  F-HZSM-5(U)	0.33	346
  HY	0.38	435
  Cl-HY	0.42	464

  Table 2.  Surface area and total pore volume values of different halogenated zeolites

  Figure 7 shows the pore size distribution curves obtained for the synthesized H-ZSM-5 catalysts. It is evident in Figure 7 that the three curves give very similar pore distribution curves. Almost 95% of pores are falling in <1.5nm region,and the remaining pores are present as low intensity peak at ~2nm.

  Figure 7.  Effect of fluorination and ultrasonication on the pore size distribution in H-ZSM-5 catalysts

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  Table 2 shows the surface area and pore volume of H-ZSM-5 catalysts. It can be assumed that ultrasonication has caused a limited agglomeration of a portion of the F-HZSM-5 catalyst. Fluorination increased surface area from 357 to 375m²/g and increased total pore volume from 0.32 to 0.37cm³/g,which resulted in increasing the activity for methanol conversion to DME from 72.6% to 85.44% at 250℃. However,the crystal size seems to be unaffected. A further treatment with ultrasonication after fluorination appeared to have a deterioration effect,it decreased the surface area of HZSM-5(F) to 346m²/g and the pore volume to 0.33cm³/g and also decreased the catalytic activity.

  Figure 8 shows the pore size distribution curves obtained for the synthesized HY catalysts. It can be seen that chlorination and ultrasonication can caused a large change of the pore size distribution in HY catalysts.

  Figure 8.  Effect of chlorination and ultrasonication on the pore size distribution in HY catalysts

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  2.1.1   XRD

  Figure 1 shows all main diffraction bands for the modified H-MOR zeolites (Cl-HMOR,Cl-HMOR(U),F-HMOR and F-HMOR(U)),which do not include significant variation of their 2θ positions relative to the mother H-MOR sample. Moreover,the mordenite sample (Cl-HMOR) treated by chlorination using NH₄Cl gives stronger diffraction intensity. On the contrary,the sample (F-HMOR) treated by fluorination using NH₄F gives lower diffraction peaks. The treatment of halogenated H-MOR by ultrasonication leads to increasing the degree of crystallinity.

  Figure 1.  XRD patterns of halogenated H-MOR catalyst

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  Table 1 shows all the crystal sizes of zeolite catalysts. The crystal size of fluorinated H-MOR is smaller than that of chlorinated H-MOR,which indicates further that F^- ions exhibit a stronger attack on the framework aluminum causing a larger Al-leaching. Evidently,the removal of Al from the framework structure of the zeolite will be followed by migration of Si atoms to fill the vacant positions resulting by Al removal. A part of Si-O-Al bonds are partially changed to Si-O-Si bonds in order to stabilize the zeolitic structure. Since Si-O bond is shorter than the Al-O bond,the zeolitic crystal will suffer shrinkage of the unit cells and the channel (pore) formed by the surrounding crystals will become relatively narrow. The F^- ions are more aggressive than Cl^- ions because of the higher inductive effect of F^[29-32].

  It is noted that ultrasonicated chlorinated H-MOR sample gives higher diffraction intensities and lower crystal size than the chlorinated sample,but in the case of fluorinated sample the ultrasonicated fluorinated H-MOR has a slight decrease in crystal size than fluorinated H-MOR catalyst.

  Table 1.  Crystallite sizes of the phases present in XRD of zeolite catalysts

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  CatalystCrystal size d/nmHMOR42.8F-HZSM-5(U)21.2

  Cl-HMOR	64.0
  Cl-HMOR(U)	56.8
  F-HMOR	43.9
  F-HMOR (U)	42.4
  H-ZSM-5	32.7
  Cl-HZSM-5	38.3
  Cl-HZSM-5(U)	31.3
  F-HZSM-5	32.4

  Table 1.  Crystallite sizes of the phases present in XRD of zeolite catalysts

  Figure 2 shows all main diffraction bands for the modified H-ZSM-5 zeolite (Cl-HZSM-5,Cl-HZSM-5(U),F-HZSM-5 and F-HZSM-5(U)),do not show measurable shift of their 2θ position than the mother H-MOR sample (2θ= 23°,24°). The HZSM-5 samples treated by chlorination or fluorination show higher diffraction intensity than untreated HZSM-5 indicating slight dealumination of the zeolite support. The sonicated Cl-HZSM-5 sample has higher diffraction intensity compared with the unsonicated Cl-HZSM-5,however the diffraction intensity of sonicated F-HZSM-5 is lower than that of unsonicated F-HZSM-5.

  Figure 2.  XRD patterns of H-ZSM-5 catalysts

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  2.1.4   Surface acidity of H-MOR catalysts

  The TPD of presorbed ammonia on the acid sites of the zeolite supports was carried out in differential scanning calorimeter (DSC) using nitrogen as a purge gas according to the procedure adopted by Aboul-Fotouh^[28]. The thermograms obtained (Figure 9) for H-MOR samples show two peaks. A low-temperature peak (LT-Peak) corresponds to the ammonia desorption enthalpy (ΔH_d) from the weak acid sites of the catalyst,and a high temperature peak (HT-Peak) corresponds to the ammonia desorption enthalpy from the strong acid sites. The ΔH_d values are proportional to the number of acid sites,whereas the peak temperatures (t_max) are taken to correlate the acid sites strength of the catalysts. The higher the t_max value is,the stronger the acid site is in the catalyst (Table 3). Table 3 shows that the number of acid sites (ΔH,J/g) decreases slightly with chorination and more with ultrasonication,whereas the acidity strength (t_max) increases. This effect is of significant importance in the reaction mechanism of the current reaction (DME formation).

  Figure 9.  NH₃-TPD for H-MOR catalysts

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  Table 3.  Ammonia TPD of the H-MOR catalysts

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  CatalystAcid sites strength distribution∆H /(J·g^-1)^aLT-peak temperature t_min/℃HT-peak temperature t_max/℃^b H-MOR89.6199530Cl-HMOR (U)80.5210537

  Cl-HMOR	84.3	204	533
  a: proportional to acid sites number b: proportional to acid sites strength

  Table 3.  Ammonia TPD of the H-MOR catalysts

  2.1.2   SEM

  Figure 3 shows the SEM micrographs obtained for the halogenated and halogenated sonicated H-MOR. From this figure it is shown that ultrasonic irradiation decreases the size of particles.

  Figure 3.  SEM images of halogenated H-MOR catalysts

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  Figure 4 shows the SEM micrographs for the HZSM-5,F-HZSM-5 and F-HZSM-5(U) (ultrasonicated). F^- incorporation in the HZSM-5 catalyst causes the HZSM-5 crystallites to be smaller and hence the catalyst activity. From SEM of F-HZSM-5(U) catalyst it shows a creation of some agglomerates of the particles,and so its activity became somewhat lower than that of F-HZSM-5.

  Figure 4.  SEM images of fluorinated H-ZSM-5 catalysts

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  SEM micrographs of chlorinated HY catalysts are shown in Figure 5. The crystallites in the Cl-HY (U) sample exhibit the least agglomeration. However,the Cl-HY catalyst (Figure 5(b)) indicates that its crystallites suffer higher agglomeration than Cl-HY (U) but suffers less agglomeration than the parent zeolite HY. This can prove that the ultrasonication and physical aspect are more influencing on catalytic activity of Cl-HY(U).

  Figure 5.  SEM images of chlorinated HY catalysts

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  2.2.3   HY

  The results on methanol conversion activities of the H-Y zeolite catalysts are depicted in Figure 13. Evidently NH₄Cl or NH₄F doping enhances the conversion activities of H-Y zeolite,and the catalysts activities are in the order of Cl-HY > F-HY > HY.

  Figure 13.  Dimethyl ether (DME) in product of methanol conversion using chlorinated and fluorinated H-Y catalysts

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  Sonication treatment of halogenated H-Y sample (Figures 13(b) and 13(c)),shows that sonicated Cl-HY has higher activity than unsonicated Cl-HY due to the surface area and pore volume increases with ultrasonication (Table 2). On the contrary,sonication of the fluorinated HY has lower activity than the unsonicated sample,which may because that a fluorinated debris is formed and can cause pore diffusion limitation^[25].

  2.2.1   H-MOR

  In previous work^[33-35],fluorination and chlorination using NH₄F and NH₄Cl with different halogen concentrations (1.0% to 6.0% F or Cl) were carried out. A halogen content of 3.0% was found to be an optimum. Therefore this halogen content was used in the present work.

  The results of prepared halogenated and H-MOR catalysts on methanol conversion are shown in Figure 10. It has been found that fluorinated H-MOR acquires significantly higher activity than the chlorinated version (Figure 10(a)). Moreover,ultrasonication increases the activity of the chlorinated H-MOR but decreases the activity of fluorinated H-MOR catalyst (Figure 10(b),10(c)). These can be attributed to higher acidity of Cl-HMOR (U) than that of unsonicated Cl-HMOR (Table 3).

  Figure 10.  Dimethyl ether (DME) in product of methanol conversion using H-MOR catalysts

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  It can be assumed that chlorination increases the density of Cl^- on the surface via substitution of OH^- groups,since this halogen on the surface is more strongly acidic,hence enhancing the DME formation activity. Moreover,fluorination gives stronger acid sites on the H-MOR surface than chlorination by virtue of the higher electronegativity of F^- than Cl^-,so fluorinated H-MOR has higher activity than chlorinated H-MOR. Furthermore,ultrasonication can be assumed to cause some agglomeration of the catalytic particles in the presence of F^-,which does not occur in case of the chlorinatied H-MOR^[36].

  Table 2 shows that the ultrasonication of Cl-HMOR increases the surface area from 350 to 478m²/g whereas chlorination of H-MOR slightly increases the surface area to 368m²/g. Accordingly the ultrasonication is activated for DME formation. Chlorination followed by ultrasonication is preferred for higher catalytic reaction. The pore volume data decrease with chlorination but increase with ultrasonication which activates the catalytic activity.

  Figure 11 shows the selectivities of the current methanol dehydration reaction at 300℃ producing DME together with olefinic hydrocarbons (HC) using halogenated and halogenated sonicated H-MOR. The DME selectivity is enhanced by halogenation,and the selectivity of fluorinated H-MOR is higher than that of chlorinated H-MOR. On the other hand,the selectivity for olefins decreases by halogenation. The selectivities of sonicated and chlorinated H-MOR catalysts follows the order of H-MOR< Cl-HMOR < Cl-HMOR (U). For sonicated and fluorinated H-MOR catalysts the selectivities of DME formation using F-HMOR and F-HMOR (U) are nearly the same.

  Figure 11.  Dimethyl ether (DME) and hydrocarbon selectivity at 300℃ using H-MOR catalysts

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  2.2.2   H-ZSM-5

  The results on methanol conversion activities of the H-ZSM-5 zeolite catalysts are illustrated in Figure 12. The activity of the catalyst is increased as function of reaction temperature till reaching a maximum at 275℃,beyond which the activity remained unchanged. The activities of the catalysts are in the order of Cl-HZSM-5 > F-HZSM-5 > H-ZSM-5.

  Figure 12.  Dimethyl ether (DME) in product of methanol conversion using H-ZSM-5 catalysts

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  The ultrasonication is found to enhance the activity of the chlorinated catalyst (Figure 12(b)),whereas it decreases the activity of fluorinated catalyst (Figure 12(c)). Table 2 shows that the fluorination of HZSM-5 increases the surface area from 357 to 375m²/g,whereas fluorination flowed by ultrasonication of HZSM-5 decreases the surface area to 346m²/g. Also,the pore volume data increases with fluorination but decrease with ultrasonication due to a fluorinated debris is formed and can cause pore diffusion limitation and large agglomeration^[25] (Figure 4).

  Aboul-Gheit et al^[25] concluded from the TPD of presorbed ammonia from HCl and HF doped H-ZSM-5 zeolite that both acid treatments of zeolite increased its acid sites number and strength,however HCl increased acid sites number more than HF did,whereas,HF increased the acid sites strength more than HCl did. The larger number of acid sites in the HCl promoted catalyst than in HF promoted one can be considered the influencing factor for increasing its activity.

  Le Van Mao et al^[37] show that doping H-ZSM-5 with a low concentration of fluoride species enhances the surface acidity via: formation of new Brnsted acid sites,strengthening some acid sites of the parent zeolite. Moreover,Arena et al^[38] have indicated that Cl^- adsorbed on the surface of Al₂O₃ results in a significant change in the electronic properties of the outer layer of Al₂O₃ that decreases the basic Lewis sites,or inducing a stronger ‘inductive effect’ of Cl^- on the neighboring hydroxyl groups. This electronic effect weakens the O-H bond,rendering the proton more strongly acidic.

  3   Conclusions

  Ultrasonic irradiation during catalyst preparation affects the catalyst characters because intensive collisions occur between molecules.

  Ultrasonication and/or halogenation enhance the acidity of zeolite catalysts leading to higher production of DME.

  Ultrasonic irrdiation enhances the proformance of the halogenated zeolite catalysts to DME production.

  The chlorinated catalysts with or without ultrasonic irrdiation show significantly enhanced catalyst selectivity to DME formation.

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  Figure 1  XRD patterns of halogenated H-MOR catalyst

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  Figure 2  XRD patterns of H-ZSM-5 catalysts

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  Figure 3  SEM images of halogenated H-MOR catalysts

  (a): H-MOR; (b): Cl-HMOR; (c): Cl-HMOR (U); (d): F-HMOR; (e): F-HMOR (U)

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  Figure 4  SEM images of fluorinated H-ZSM-5 catalysts

  (a): H-ZSM-5; (b): F-HZSM-5; (c): F-HZSM-5 (U)

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  Figure 5  SEM images of chlorinated HY catalysts

  (a): HY; (b): Cl-HY; (c): Cl-HY(U)

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  Figure 6  Effect of chlorination and ultrasonication on the pore size distribution in H-MOR catalysts

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  Figure 7  Effect of fluorination and ultrasonication on the pore size distribution in H-ZSM-5 catalysts

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  Figure 8  Effect of chlorination and ultrasonication on the pore size distribution in HY catalysts

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  Figure 9  NH₃-TPD for H-MOR catalysts

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  Figure 10  Dimethyl ether (DME) in product of methanol conversion using H-MOR catalysts

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  Figure 11  Dimethyl ether (DME) and hydrocarbon selectivity at 300℃ using H-MOR catalysts

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  Figure 12  Dimethyl ether (DME) in product of methanol conversion using H-ZSM-5 catalysts

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  Figure 13  Dimethyl ether (DME) in product of methanol conversion using chlorinated and fluorinated H-Y catalysts

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  Table 1.  Crystallite sizes of the phases present in XRD of zeolite catalysts

  CatalystCrystal size d/nmHMOR42.8F-HZSM-5(U)21.2

  Cl-HMOR	64.0
  Cl-HMOR(U)	56.8
  F-HMOR	43.9
  F-HMOR (U)	42.4
  H-ZSM-5	32.7
  Cl-HZSM-5	38.3
  Cl-HZSM-5(U)	31.3
  F-HZSM-5	32.4

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  Table 2.  Surface area and total pore volume values of different halogenated zeolites

  CatalystTotal pore volume v/(cm³·g^-1) Surface area A/(m²·g^-1)H-MOR0.22350Cl-HY (U)0.45468

  Cl-HMOR	0.23	368
  Cl-HMOR (U)	0.44	478
  H-ZSM-5	0.32	357
  F-HZSM-5	0.37	375
  F-HZSM-5(U)	0.33	346
  HY	0.38	435
  Cl-HY	0.42	464

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  Table 3.  Ammonia TPD of the H-MOR catalysts

  CatalystAcid sites strength distribution∆H /(J·g^-1)^aLT-peak temperature t_min/℃HT-peak temperature t_max/℃^b H-MOR89.6199530Cl-HMOR (U)80.5210537

  Cl-HMOR	84.3	204	533
  a: proportional to acid sites number b: proportional to acid sites strength

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