Para-Xylene Maximization Part VIII: Promotion of H-ZSM-5 Zeolite by Pt and HF Doping for Use as Cata

催化学报2010Chinese Journal of CatalysisVol. 31 No. 10Article ID: 0253-9837(2010)10- 1209-08DOI: 10.1016/S1872-2067(10)60113-1Article: 1209 -1216Para- .Xylene MaximizationPart VIII: Promotion of H-ZSM-5 Zeolite by Pt and HF Doping forUse as Catalysts in Toluene Alkylation with MethanolAhmed K. ABOUL-GHEITl, Ateyya A. ABOUL-ENEIN', Ahmed E. AWADALLAH',Salwa A. GHONEIM', Eman A. EMAM2'Process Development Division, Egyptian Petroleum Research Institute, Nasr City, PO Box 9540, Cairo 11787, Egypt'Department of Petroleum Refining and Chemical Engineering, College of Petroleum and Mining Engineering,Suez Canal University, Suez, EgyptAbstract: Toluene was alkylated with methanol in a flow type reactor at temperatures between 300 and 500。C using H-ZSM-5 zeolite,0.2%Pt/H-ZSM-5 and hydroflorinated 0.2%PVH-ZSM-5 with HF concentrations of 1.0%, 2.0%, 3.0%, or 4.0%. Pt primarily enhancestoluene conversion, total xylenes production, and p-xylene relative to its thermodynamic equilibrium. As the concentration of HF increasesfrom 1.0% to 3.0%, the catalyst activity increases because of the increase in the number of acid sites and their strength. Addionally, thesurface arca and Pt dispersion also increases. An advantage of increased HF doping is that the formation of voluminous trimethylbenzene(TMB) byproducts is inhibited. However, at a HF concentration of 4.0%, Al and Si are partially leached and then deposited mostly in thewider catalytic pores. This was determined by evaluating the pore volume distribution and we determined that reactivity inhibition was ob-viously present and was due to diffusion restriction.CLC number: 0643Document code:AThere is considerable industrial demand for para-xyleneide groups and because fluorine is very electronegative, itbecause it is a raw material in the production of terephthalicpolarizes the framework, which increases the acidity andacid, which is a major component in the production ofreactivity of the surface. Because zeolites are inherentlypolyester fibers. Additionally, polyethylene terephthalateresins, vitamins, and other pharmaceuticals also requirein the literature on the effect of fluoride treatment on zeo-terephthalic acid. The methylation of toluene has been car-ites. However, it has been pointed out that fluoride treat-ried out over several acidic zeolites such as ZSM-5, mor-denite, SAPO-11, and Y zeolites. It has been found that thebility of some important siliceous zeolites such as ZSM-5.catalytic activity and selectivity differs greatly among theseBecker et al. [8] reported the modification of H-mordenitezeolites. Aboul-Gheit et al. [1-3] investigated the alkylationwith ammonium fluoride solutions and gaseous CHF3. Pat-of toluene with methanol using H-ZSM-5 zeolite and thisents exist about high-silica pentasil zeolites that exhibitreaction was promoted by a noble metal (Pt) to stabilize andenhanced Bronsted acidity and therefore improved catalyticactivate carbonium ion formation, which is necessary for theactivity after treatment with various fluorine containingalkylation reaction. However, modification of the acidity ofcompounds [9,10]. The hydrofluorination of mordenite hasH-ZSM-5 may afect the pore structure of the zeolite andbeen reported by Ghosh et al. [11] who found that aqueousthe co-reactions can be effectively restricted, and the selec-HF treatment of this zeolite results in the leaching of a por-tivity for para-xylene improved [4,5]. Zhao et al. [6] re-tion of aluminum under mild conditions. Silicon is leachedported that H-ZSM-5 zeolite loaded with strongly hydro-under more severe conditions from the zeolite structure asgenating metals (Pt, Pd, Co, Ni) exhibit excellent stability,well.high activity, and p-selectivity.In this work, 0.2%PVH-ZSM-5 doped with 0, 1%, 2%,The modification of H-ZSM-5 using fluoride treatment3%，and 4% HF was investigated as a catalyst for toluenehas been used to promote acid catalyzed reactions [7]. Thealkylation with qmethanol ina flow reactor at temperaturesfluoride ion is assumed to replace surface oxide or hydrox-of 300- -500 °C中国煤化工-ation.TYHCNMHGReceived date: 26 March 2010.*Corresponding author. Tel: +20-2-22729100; Fax: +20-2-22747433; E-mail: aboulgheit2000@hotmail.comEnglish edition available online at ScienceDirect (ttp://ww.sciecedirect.com/ science/jourmal/ 18722067).催化学报Chin. J. Catal, 2010, 31: 1209- -12161 Experimentalpercentage in producttotal xylenes in product.1.1 Catalyst preparation1.3 Catalyst characterizationSodium ZSM-5 zeolite (Si/Al = 25) in powder form wasThe surface properties (area and pore volume distribu-kindly supplied by Sid-Chemie AG, Germany. It was trans-ion) were determined by conventional BET nitrogen ad-formed to the ammonium form by cation exchange using asorption-desorption using a Quantachrome NovaWin2molar solution of NH4NO3 over five repetitions of 8 h each(Nova 3200, USA) apparatus. The pore volume ditributionusing fresh solutions of NH4NO3 at 70。C under reflux andrelationships were calculated by applying the DFT differen-stirring. The zeolite was then separated from the solution,tial method through plotting the pore volume vs. pore width,washed thoroughly with distilled water, dried overnight atas derived from the N2 adsorption-desorption isotherms at110 °C，and calcined at 530 。C for 4 h to obtain the-196 °C. The catalyst samples were primarily pretreated viaH-ZSM-5 zeolite.degassing for 18 h at 350 °C.A catalyst containing 0.2%Pt was prepared by impreg-The temperature-programmed desorption of ammonianating the H-ZSM-5 zeolite with an aqueous solution con-(TPDA) procedure using differential scanning calorimetrytaining the required amount of chloroplatinic acid(DSC) for detecting and evaluating the desorption of pre-(H2PtCl:6H2O). To this was added a small quantity of citricsorbed ammonia from the catalysts via programmed tem-acid to assist deep Pt penetration into the support [12,13].perature increase was carried out in an inert atmosphereThe catalyst was dried overnight at 110 。C, calcined in a[14,15]. Accordingly, Ammonia was adsorbed on the acidmufle furmace at 530 °C for 4 h in air and then reduced insites of the catalyst after previous heating in air flow at 500the catalytic reactor in a flow of dry hydrogen at 20 cm /min°C for 3 h in a silica tube furnace. After cooling to 50。C,ammonia gas flow of 50 cm/min was applied over theFour hydrofluorinated catalysts， H-ZSM-5 (1%HF),evacuated catalyst. The DSC measurements were then car-H-ZSM-5 (2%HF)， H-ZSM-5 (3%HF)， and H-ZSM-5ried out in a DSC-30 unit of the TA-3000 Mettler system(4%HF) were prepared by doping H-ZSM-5 with 1%, 2%,(Germany) using standard Al crucibles in a flow of 503%, and 4% HF, respectively, followed by drying overnightcm' /min oxygen-free nitrogen purge gas. The DSC condi-at 110。C and calcination at 450 °C for 4 h. These hydro-tions were as follows: temperature, 50- 600 °C; heating rate,fluorinated catalysts were impregnated with an aqueous5 °C/min; full scale range, 30 mW; plot, 10 cm; samplesolution of H2PtCl6:6H2O to which was added a small quan-mass; 10 mg.tity of citric acid as described above. The catalysts wereThe desorption enthalpy obtained corresponds to the acidthen washed, dried, calcined, and reduced as above to obtainsite density whereas the DSC peak temperature was used tothe 0.2%Pt/H-ZSM-5(HF) catalysts.compare the acid site strength. Two DSC endothermic peaksappear in each thermogram; the lower temperature peak1.2 Alkylation procedure and apparatuscorresponds to desorption of weak acid sites that are notinvolved in catalysis and the higher temperature peak cor-The catalytic runs were carried out at atmospheric pres-responds to desorption of the strong acid sites responsiblesure in a fixed-bed down-flow reactor. The alkylation runsfor catalyzing the reaction under study. The higher the de-were carried out at reaction temperatures between 300 andsorption peak temperature, the higher the acid site strength500 °C, a space velocity of 2.6 hi , and a continuous hydro-The dispersion of platinum in the support (metal fractiongen flow of 20 cm/min. One gram of the catalyst powderexposed) was determined by hydrogen chemisorption usingwas diluted with inert porcelain particles then placed in thea pulse technique based on that described by Freel [16]. A .reactor tube. A mixture of methanol and toluene (molar ratiospecific amount of calcined catalyst was heated in the2.5:1) was fed into the reactor and the product was cooled inchemisorption furnace at 500。C for 1 h in a flow of 50a condenser at 3 °C and then passed into a receiver. Thecm/min ultrapure hydrogen. The flow was then replacedliquid and gaseous products were analyzed by gas chroma-with oxygen-free nitrogen at 30 cm/min for 2 h at 500 °Ctography using a column packed with 5% Bentone 34 and(degassing). The furnace was shut off and the catalyst was5% diisodecylphthalate supported on Chromosorb Wcooled to room temperature. Hydrogen was then injected(Resteck Corp., USA).into the nitroger中国煤化工arance of hy-Percentage gaseous product = 100 - percentage liquiddrogen peaks eqne of injectedproduct. Toluene conversion= (T- Tr)/T; where T; is thepulses). HydrogefYHCNMH(ydrogen atomsweight of toluene feed to the reactor and To is the weight ofadsorbed per total metal atoms on the basis of 1:1unreacted toluene. Para-xylene selectivity = para-xylenestoichiometry [17].www.chxb.cnAhmed K. ABOUL-GHEIT et al: Para-Xylene Maximization-Part VII: Promotion of H-ZSM-5 Zeolite12112 Results and discussion65 t2.1 Toluene conversion60 FTraditionally, toluene alkylation is an acid catalyzed reac-tion and, therefore, the acid sites provided by the H-ZSM-50Fzeolite support in the current catalysts are necessary for5tcarbonium ion formation. However, the use of platinum asan active metal component dispersed on the catalytic sur-face results in alkylation enhancement because it assists the5r- 0.2%PVH-ZSM-5(4%HF)0.2%PVH-ZSM-5(3%HF)stabilization of the carbonium ion that forms on the acid.0.2%PVH-ZSM-5(2%HF)sites. This occurs because of a decrease in the rate of the0- 0.2%P/H-ZSM-5( 1%HF)backward decomposition to reform methanol and toluene.- - - 0.2%P/H-ZSM-5Alkylation is enhanced because of the activity of the plati-20 LUntreated H-ZSM-5num atoms which possess vacant electron d-orbitals.300350400450500Aboul-Gheit et al. [3] investigated the performance of aReaction temperature (°C)series of catalysts containing varying Pt content (0.1%,Fig.1. Toluene conversion during alkylation with methanol using0.2%, or 0.3%) loaded on H-ZSM-5 zeolite for toluene me-H-ZSM-5 and 0.2%PVH-ZSM-5 treated with HF.thylation with methanol. Para-xylene production andpara-xylene selectivity was found to increase with an in-catalyst can, therefore, be considered to be the most active.crease in the Pt content. Although diffusion restriction in theThe increase in acid concentration increased the acidcatalytic pores of these catalysts increases with an increasestrength and this effectively increased the activity towardsin the Pt content, the activation energy of toluene methyla-toluene conversion. Furthermore, this increase in acid con-tion increased and the activation entropy became less nega-centration assisted in increasing the dispersion of Pt in thetive. This indicates that actiation of the current reaction viazeolites (Table 1). On the contrary, a further increase in HFsuccessive Pt occlusion up to 0.3% over-compensated thedoping to 4% causes a significant decrease in activity at :diffusion restriction effect.higher temperatures. This can be atributed to the leachingHF doping on the zeolite surface is accomplished byof some structural aluminum and perhaps silicon in the zeo-modifying the acidity of the catalyst through increasing orlitic structure of H-ZSM-5. The leached materials are de-decreasing the number of acid sites and/or the acid strength.posited in the form of fluoro-alumino and fluoro-silico spe-The H-ZSM-5 zcolite is better than the other zeolites be-cies in the zeolite channels and this causes diffusion restric-cause of its channel dimension (~0.5 nm), which preferen-tion, which is an effect that appears at higher temperaturestially admits the diffusion and reaction of toluene with ad-since the difusion rate greatly decreases relative to the ratesorbed methanol. This enhances para-xylene selectively byof the chemical reaction at high temperatures so that theeliminating the production of ortho- and meta-xylenes be-diffusion process becomes rate controlling. With 4% HFcause they have larger diameters.acid treatment, the number of acid sites and the Pt disper-In this work, We determined the extent of H-ZSM-5 zeo-sion (Table 1) decreases and the surface area as well as porelite promotion with 0.2% Pt as well as promotion by 0.2%structure is altered.Pt and HF of 1.0%, 2.0%, 3.0%, or 4.0%. Figure 1 showsThe pore volume distribution curves for thuntreatedthat the total conversion of toluene increases by incorporat-H-ZSM-5 zeolite and the 0.2%P/H-ZSM-5 catalyst areing 0.2% Pt into the H-ZSM-5 zceolite to a significant extentshown in Fig.2(a). The larger pores have a pore width rangeat all reaction temperatures but, particularly, at higher tem-peratures between 400 and 500 。C. Doping theTable 1 Ammonia desoption cnthalpy (AHs), peak temperaure(T), Pt dispersion, and BET surface area (BEr) for the 0.2%Pt/H-0.2%PVH-ZSM-5 catalyst with 1% HF further increased theZSM-5 catalyst and its hydrofluorinated versionsactivity (total conversion) at higher temperatures (450- -500TPD of ammonia_ Pt dispersion Amer/。C). A further increase of HF to 2% resulted in a futherCatalystOHe/(Jg)_ TyPC(%)(m/g)increase in activity particularly in the lower temperature :105.1380345range (300- 350 °C). This change of temperature region0.2%PV/H-ZSM-5中国煤化工。338activation can be atributed to the heterogencous distribution0.2%P/H-ZSM-5(4of the active sites in the catalyst, i.e.. Pt sites and acid sites.0.2%PVH-ZSM-5(2HCNMHGo348However, a further increase in the acid to 3% HF resulted in0.2%P/H- ZSM-5(3%HF) 100.037872a significant increase in activity at all temperatures and this0.2%PV/H-ZSM-5(4%HF)___ 96.1381663221212催化学报Chin. J. Catal, 2010, 31: 1209- -12160.020(ab)0.016J 0.012- Untreated H-ZSM-5，- 0.2%Pt/H-ZSM-5(2%HF)---- 0.2%Pt/H-ZSM-5---- 0.2%Pt/H-ZSM-5(3%HF)...... 0.2%Pt/H ZSM-5(4%HF)0.0080.0040.0006Pore width (nm)Fig. 2. Pore volume distribution by the DFT differential method for different samples.of 2.2- -5.7 nm in the as-synthesized H-ZSM-5 zeolite andgion (0.3- 2.7 nm) have the highest values. This is evidentlythey are larger than those in the 0.2%PtH-ZSM-5 catalyst.attributed to the flling of most of the large pores by theThere is no significant difference between the volume dis-debris formed through the highly activated leaching of Altribution of the fine pores in the 0- 2.2 nm width range. Ob-and Si at the highest HF concentration (4%).viously, the larger pores of the unloaded zeolites are moreThe behavior of xylenes production (Fig. 3) shows someunoccupied.Figure 2(b), on the other hand, shows that the pore vol-ene conversion (Fig. 1) with respect to the modificationsume distribution curves obtained for the catalysts containingapplied in this work, i.e, the incorporation of 0.2%Pt and0.2%PtH-ZSM-5 treated with 2% and 3% HF exhibit simi-doping with HF from 1% to 4%.lar behavior and they match those shown in Fig. 2(a). Also,Apart from the main intended reaction, i.e, the alkylationthe 1% HF treated 0.2%PtH-ZSM-5 catalyst (not shown inof toluene to produce xylenes (ortho-, meta-, and para-xy-Fig. 2(b) to exclude overlap and complexity) gives more orless similar behavior to the 2% and 3% HF treated catalysts.trimethylbenzenes (TMB) and smaller products such asNevertheless, for the 0.2%Pt/H ZSM-5 catalyst treated withbenzene and hydrocarbon gases were obtained. Benzene4% HF significantly different behavior is evident. For thisformation, however, is insignificant at yields up to 3.9% atcatalyst, the large pores in the distribution curve (in the500 °C over the hydrofluorinated catalysts. Benzene isrange 2.7- 5.5 nm) have the lowest values among all the HFformed by toluene hydrodealkylation and methane is alsotreated catalysts (almost zero) whereas the finer pores re-produced (Eq. ()). Moreover, the disproportionation of twomolcules of toluene produces one benzene molecule and50 Cone xylene molecule (Eq. (2)- 0.2%Pt/H-ZSM-5(3%HF)HCH45 F-- - 0.2%Pt/H-ZSM-5(2%HF)- - 0.2%Pt/H-ZSM-5(1%HF)CH,OH+ H2O(1)- + 0.2%Pt/H-ZSM-5- Untreated H-ZSM-5-CH3H3200(2)30CH,3252.1.1 Side reactions during toluene methylation20 EIn this work, we used unloaded H-ZSM-5 as a catalyst00350400450and ethylbenzen中国煤化工1 0.2%PtH-Reaction temperature (C)ZSM-5 and itshylbenzene isFig. 3. Total xylenes in the products during the alkylation of tolueneobtained in traceMHCNMHGuenesareob-with methanol using H-ZSM-5 and 0.2%Pt/H-ZSM-5 treated with thetained over all catalysts in larger quantities than ethyl ben-HF.zene, which can be attributed to the initial formation of1213www.chxb.cnAhmed K. ABOUL-GHEIT et al: Para-Xylene Maximization-Part VIII Promotion ofH-ZSM-5 Zcoliteethylbenzene in significant quantities and then rapid alkyla-between ethylene and toluene will then produce ethyltolu-tion to ethytoluene. The side chain alkylation of toluene toene [21](Eq. (3)).-H2O .ethylbenzene is ruled out because it requires basic catalysts2CHzOH- CH2OCH3_ 20、CH2 =CH2 (3)[18].The gaseous product yields range between 3.0% 6.6% onA more undesired product is the heavy alkylated fractionH-ZSM-5 at temperatures from 300 -500。C and betweenformed by the further alkylation of xylenes in the product6.2% and 9.5%, respectively, on the 0.2%PtH-ZSM-5 cata-(TMBs). The yield of TMBs can be as high as ~16% at 400lyst and vary by士0.6% over the other 0.29%PVH-ZSM-5。C on the unloaded zcolite. These high yields shit to 450(HF) catalysts.。C when using the 0.29%PVH-ZSM-5 catalyst. TMBs areformed by excessive alkylation during xylene production on.2 Para-xylene selectivity of the catalystsall the catalysts studied without sgnificant diferences atlow temperatures(300- 350 °C). This indicates that thisFigure 5 shows that the 0.2%P/H-ZSM-5 catalyst has thereaction is not significantly dependent on the modificationslowest seletivity for para-xylene production among allwe aplied to H-ZSM-5 (Pt or P-HF combination) at lowercurrent catalysts. However, the unloaded catalysttemperatures. However, at higher temperatures (400 500(H-ZSM-5) has higher selectivity than the platinized cata-。C) the increase in HF concentration on shape selectivity islyst, which indicates that it has a faster para-xylene trans-more obvious during TMB production. In this temperatureformatin compared to the other xylenes. This behavior(Fig.4), the activities of the catalysts for TMB pro-shows that Pt does not act as a shapeduction dcrease as fllows: 0.2%P/H-ZSM-5 > 0.2%PV/H-para-xylene formation. This may also imply that the Pt inZSM-5(1%HF) > 0.2%PVH-ZSM-5(2%HF) > 0.2%PtH-this catalyst was mostly distributed over the extemal surfaceZSM-5(3%HF) > 0.2%PV/H-SM-5(4%HF). Evidently, theof the zeolite rather than being dispersed throughout theincrease in HF concentration results in more debris deposi-channels of the zeolite (intemnal surface)-. This is becausetion in the zeolitic channels and a greater restriction onhigh Pt dispersion in the channels should have increased theTMB formation is a consequence. This is a clear example ofshape sletivity of the 0.2%PVH-ZSM-5 catalyst and con-shape selective reactions in modifed zeolites.sequently the para-xylene selectivity should have increasedThe gascous products include hydrocracked C-Csbecause of the incorporation of Pt instead of having de-molecules. Ring opening reactions can take place at lowlevels in the lower temperature region since the benzeneThe benefit of an increase in para setivity by HF addi-ring can be hydrogenated at these temperatures to cyclopar-tion to 0.2%PVH-ZSM-5 is obvious by considering the cor-affin (exothermic reaction) and then they are opened andrelation between para-selectivity and temperature over thefurther hydrocracked. However, at much lower tempera-lower temperature range (Fig. 5). Here, para-xylene selec-tures, methanol undergoes a bimolecular dehydration pro-tivity follows the order 0.2%P/H-ZSM-5(1%HF)< 0.2%PVducing dimethylether which again undergoes unimolecularH-ZSM-5(2%HF) < 0.2%PV/H-ZSM-5(3%HF) < 0.2%PV/H-dchyration and this resuts in ethylene [19,20]. A reaction。0.2%P/H-ZSM-5(4%HF)24 f- o- 0.2%PVH-ZSM-5(4%HF)- 0.2%Pt/H- ZSM-5(3%HF)0.2%PVH-ZSM-5(3%HF)-十0.2%PV/H-ZSM-5(2%HF).0.2%PVH-ZSM-5(2%HF)- - 0.2%P/H-ZSM-5(1%HF)0.2%P/H-ZSM-5( 1%HF)◆- 0.2%PV/H-ZSM-5- o- 0.2%Pt/H-ZSM-5- o- Untreated H-ZSM-5_ . UntreatedH-ZSM-5宣34F6F0F1228-剪84F00400 .45050022L中国煤化工50Reaction temperature (C)MYHCNMHGFig.4. Trimethybenzenes in the product during toluene alkylationFig, s. Para-xylene slctivity during the alkylation of toluene withing H-ZSM-5 and 0.2%P/H-ZSM-5 treated with HF.with methanol using H-ZSM-5 and 0.2%PVH-ZSM-5 treated wih HF. methanol using1214催化学报Chin. J. Catal, 2010, 31: 1209- -1216ZSM-5(4%HF).the XIX。ratio for ortho-xylene is the highest for the otherMoreover, para-xylene selectivities depend on the reac-xylenes and ranges between 1.26 and 1.71 whereas thetion temperature; at higher temperatures, physical ratherpara-xylene ratio ranges between 1.18 and 1.28 and forthan chemical effects are accelerated when using the 2%meta-xylene, the ratio is as low as 0.57- 0.80; i.e, the orderHF, 3% HF, and 4% HF containing catalysts. Furthermore,of these ratios decreases as follows:the para-selectivity of 0.2%PtH-ZSM-5(4%HF) decreasesortho-xylene > para-xylene > meta-xylene(I)greatly at temperatures higher than 350。C and it is the leastThe thermodynamic equilibrium distribution of the threeselective at 500 °C. This is attributed to an attack onisomers of xylene at different temperatures in the rangeH-ZSM-5 because of the higher HF concentration, which300- -500 °C is given in Table 2 and these values are takencauses significant dealumination that seriously decreases thefrom Taylor et al. [22].proton acidity and in addition leads to the deposition ofexcessive fluoro-alumino and fluoro-silico species (debris)Table 2 Thermodynamic equilibrium values of para-, meta-, andin the channels of the zeolite as mentioned above. This isortho-xylenes at different temperaturesevident by the large drop of para-xylene selectivity fromProduct39.2% at 300 °C to 29.5% at 400°C over the 4%HF con-300°C 350°C 400°C_ 450 °C 500 °Ctaining catalyst.p-Xylene23.8823.73235523.3723.19m-Xylene53.6552.98 52.4251.9451.562.3 Para-xylene production relative to the_o-Xyene22.4723.29 24.03 24.6925.25thermodynamic equilibriumEvidently, more than half the thermodynamic equilibriumAll the catalysts under study show an increase inproduct is composed of meta-xylene whereas approximatelypara-xylene production relative to the thermodynamic equi-one quarter of this equilibrium is composed of the or-librium, i.e. XIX。(Fig. 6), where X is the percentage of thetho-isomer and the other quarter by the para-isomer. Theindividual xylenes in the total xylenes produced on a givenmost popular xylene isomer in the petrochemical industry iscatalyst and at a given temperature. X。is the thermodynamicpara-xylene whereas the least desired xylene isequilibrium value of the xylene isomer at the same tem- meta-xylene. Therefore, it is economically important toperature. Using unloaded H-ZSM-5 as a catalyst (Fig. 6),convert meta-xylene to para-xy lene via hydroisomerization.1.81.Untreated H ZSM-50.2%Pt/H-ZSM-50.2%PV/H-ZSM- 5( 1%HF)1.4(12)1.2-( (21)号1.0-(3) .0.8(2)0.60.40.2%PtH- ZSM-5(2%HF)0.2%PtH-ZSM-5(3%HF)0.2%PtH-ZSM-5(4%HF)(1)~ 1.0~ (20.8 t(3))+0.6-中国煤化工30035000 450500 300350 400 450 50YHCNMHG 500Temperature (C)~Fig. 6. XIX。 values at different reaction temperatures. (1) p-Xylene; (2) o-Xylene; (3) m-Xylene.www.chxb.cn Ahmed K. ABOUL-GHEIT et al: Para-Xylene Maximization- Part VII: Promotion of H-ZSM-5 Zeolite1215Wei [23] calculated the molecular diameter of the xylene-7.isomers and found that both ortho- and meta-xylenes have■0.2%Pt/H-ZSM-5(4%HF)almost equal diameters while para-xylene has a signifi--7.60.2%PVH-ZSM-5(3%HF)cantly smaller diameter.Accordingly, the order of the X1X。-7.80.2%PvH-ZSM-5(2%HF)0.2%Pt/H-ZSM-5( 1%HF)values in Fig. 6 is not in accordance with Wei's moleculart0.2%PtH-ZSM-5diameter values because:-8.0 1Untreated H-ZSM-5ortho-xylene≈meta-xylene > para-xylene(I)首-8.2 IThe order of xylene isomers according to their shape se-lectivity, ie., according to their molecular diameter favora--8.4-bility is as follows:para-xylene > ortho-xylene≈mea-xyleneHence, crrelation of (1) and (I1) shows that the unloaded-88H-ZSM-5 cannot be considered as a shape selective catalyst1..6.8(10*/T)/K 1to maximize para-xylene production since ortho-xylene ismore selectively formed than para- xylene.ig. 7. Arthenius plot for the alkylation of toluene with methanolUsing the 0.2%Pt H-ZSM-5 catalyst (Fig. 6), the efetusing the untreated and hydrofluorinated 0.2%Pt/H-ZSM-5 catalysts.of incorporating platinum in the zcolite is evident becausepara-xylene is selectively formed over the other xylenes.Applying absolute reaction rate theory, k is related to theThe XIX。ratio for para-xylene is 1.07-1.31, for or-activation enthalpy, sHr, and the activation entropy, AS astho-xylene is 0.84-1.36, and for meta-xylene is 0.71-1.01.in Eq. (5):The orth-/para-privilege over the meta-directing privilegek=ETsNrIRr.AS/R(5)is evident over the lower temperature region (< 400 °C).The hydrofluorinationf 0.2%P/H-ZSM_5 has a positiveThe actiation entropy (AS) values (Table 3) are similareffeet on improving the para-xylene selectivity relative to(-71.0 to -72.4e.u.) for the catalyst containinghe thermodynamic equilibrium and the increase in HF0.2%P/H-ZSM-5 and those doped with 1.0% to 3.0% HF.concentration from 1% to 4% results in an increased in theHowever, the unloaded and the highlydopedx1Xxe values particularly at lower temperatures (Fig. 6).(0.2%PV/H-ZSM-5(4%HF)) catalysts show higher negative .However, formation of the ortho- or meta-xylene isomersentropy changes (-74.7 to -75.8 e.u.) (Table 3). The ran-was mostly found to fall below the eqilibrium values anddomness for the H-ZSM-5 can be atributed to Al(OH), spe-ortho-xylene surpasses the meta-isomer particularly in thecie deposition on the internal surface of H-ZSM-5 after thelower temperature region. This behavior is in accordancesynthesis. However, for the 4% HF doped catalyst the ran-with the para-xylene shape selectvity. However with re-domness can be atributed to the debris deposited after A1spect to the two more voluminous isomers, which possessand Si leaching with higher concentrations of HE.equal diameters the ortho-directing reaction is evidentlymore mechanistically favorable compared to the meta-di-Table 3 Activation parameters for the alkyation of toluene withrecting one.methanol using the studied catalysts._CatalystExI0' (J/mol)sr/(e.u.)_2.4 Kinetics of toluene alkylation with methanolUnloaded H-ZSM-511.0-74.70.2%PVH-ZSM-17.1-72.00.2%PV/H-ZSM-5(1%HF)-71.0From the reaction data of total xylene in Fig. 3, the reac-0.2%P/H-ZSM-5(2%HF)15.4-72.4tion rate constant, k, was calculated according to the simple0.2%P/H-ZSM-5(3%HF)17.6-71.5first order flow reactor equation:.0.2%PV/H-ZSM-5(4%HF)11.9-75.8k=EInL _]= WHSVIr0(=(4)3600The E。values are in accordance with the AS* values sinceWhere F is the rate of feed injetion (cm/s), WHSV is thelower values were obtained using untreated and 4% HFweight hourly space velocity (h "), W is the catalyst weightcontaining zeolite indicating a higher diffusion limit for the(1 g) and x is the wt% of total xylenes in products.reaction.中国煤化工The activaion energy (E) for the alkylation reaction us-3 Conclusioing the current catalysts was calculated using the tradition-.TYHCNMHGally accepted Arrheniusuation and k values were obtainedThe promotion of H-ZSM-5 with 0.2% Pt and HF at(Fig. 7).concentrations between 1% and 3% is found to be efctive1216催化学报Chin. 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