Toluene Alkylation with Methanol to p-Xylene over Heteropoly Acids Supported by Clay

Available online at www.sciencedirect.comScienceDirectJoumal of NatunlGas CheristyJournal of Natural Gas Chemistry 16(2007)42-45SCIENCE PRESSww.serier.ocmlocatejngcArticleToluene Alkylation with Methanol to p-Xyleneover Heteropoly Acids Supported by ClayV. V. Bokadel,2*，S. S. Deshpande,R. Patil，S. Jain'，G. D. Yadav21. Catalysis Division, National Chermical Laboratory, Pune-411008, India;2. University Institute of Chemical Technology, Mumbai 19, India[Manuscript received August 14, 2006; revised October 23, 2006]Abstract: The alkylation of toluene with methanol for the selective formation of p-xylene was system-atically studied. Very few studies have been reported on the use of superacids such as heteropolyacidson cheap supports, such as clay. This article deals with the use of different heteropoly acids (HPAs)，viz, Dodeca- Tungstophosphoric acid [H3PO4:12WO3:xH2O] (TPA), Dodeca-Molybdo phosphoric acid am-monium salt bydrate [H12 Mo12NgOsoP+aq] (DMAA), Dodeca Molybdo Phosphoric acid (PMA) on clay(Montmorillonite, K- 10) and as such plain clay. This comparative study reveals that 20%PMA/Clay shows62% toluene conversion and 100% selectivity toward p xylene.Key words: toluene; methanol; p-xylene; heteropoly acid; clay1. Introductionallkylation with methanol, as well as coreactions, hasnot been clearly understood. This hinders the devel-Because of the sustained demand for individualopment of a commercial catalyst for toluene methy-C8 aromatic compounds, especially p-xylene and ethyllation. For these types of alkylation reactions, thebenzene, over the past three decades, better cata-zeolites are active at higher reaction temperature oflysts and processes have been investigated for large-623 K and above. The clays, heteropolyacids are ac-scale commercial plants. Although improvementstive catalysts at lower temperature of 473- 523 K thanhave been made to the traditional Fridel-Crafts tech-zeolites (>573 K). In the process of toluene alkylationnology based on aluminum chloride, certain inber-with methanol, the main coreactions are toluene dis-ent undesirable features are still associated with theirproportionation, the further alkylation of xylene withuse. Product streams are contaminated with corro-methanol, and the conversion of methanol to hydro-sive compounds, catalysts cannot be regenerated atcarbons and will occur at high temperatures. Tolueneplant sites, and disposal of spent catalyst wastes con-disproportionation to benzene and xylene may taketributes to growing pollution problems [1].place over zeolites with strong acidity at high temper-The selective production of p-xylene via tolueneatures. The low-temperature toluene methylation re-alkylation with methanol, i.e. methylation of toluene,action with clays and modified clays with heteropoly-is a promising alternative to the conventionalacids avoids the coreaction and byproducts formation.adsorption-separation of xylene isomers or to tolueneIn this study, the use of superacids such as het-disproportionation. In previous studies, methylationero中国煤化工_clay, is investigatedof toluene was carried out over several acidic zeolites,any and selectivity to-such as ZSM-5, Mordenite, Y, and SAPO-11 [2]. How~ war(:YHCNMHGdataontheefetofever, the effect of acidity of the catalyst on toluenedifferent heteropoly acids over plain clay were inter-* Corresponding author. Fax: +91-020 25902634; E mail: vv.bokade@ncl.res.inJournal of Natural Gas Chemistry Vol. 16 No.1 200713preted. The catalyst characterizations are also com-in Figure 1. The XRD analysis confirmed that allpared with experimental data.synthesized catalysts are well crystallized.2. Experimental2.1. Chemicals usedThe chemical reagents used in this study includeheteropoly acids, such as Dodeca Tungstophosphoricacid [H3PO4:12WO3:xH2O] (TPA),Dodeca-Molybdo phosphoric acid ammonium salt bydrate[H12Mo12N3O4oP+aq] (DMAA)， Dodeca-MolybdoPhosphoric acid (PMA), and Methanol and Clay(Montmorillonite, K-10) and were purchased fromM/s S.D. Fine Chemical, Mumbai. All reagentswere of analytical grade and used without further3070purification.20/(° )Figure 1. Powder XRD plots for plain clay (K-10) and2.2. Catalyst preparationHPA-loaded K-10(1) K-10, (2) DMAA/K-10, (3) TPA/K-10, (4) PMA/K-10The HPA on support was prepared using the in-cipient wetness technique. Typically, for 10 g scale2.3.2. Nitrogen adsorptioncatalyst, the preparation was as follows. Ten gramsof support such as Clay was used in combinationSpecific surface area was obtained from nitrogenwith 2.0 g of HPA (wt/wt), which corresponds toadsorption-desorption isotherms measured using an20% HPA loading. The weighed HPA was dissolvedSA 3100, USA analyzer. The adsorption reaction wasin methanol, which formed a homogeneous solution.carried out overnight at 77 K in the presence of resid-The methanaolic HPA solution was added slowly onual nitrogen. The surface area of all the preparedthe weighed supports, viz clay, with constant stirringsamples was calculated using the BET method. Thewith the help of glass rod. Inially, supports (Clay)measured surface areas for all the prepared sampleswere in powder form but on subsequent addition ofare shown in Table 1.methanolic HPA solution on them, they form a pasteTable 1. Nitrogen adsorption using BET methodor wet cake. The wet catalyst sample thus formedwas dried on a water bath for 1.5 h at 373 K. TheTo.CatalystSurface area (m2/g)sample was further dried in an oven for2 h at 373 KClay223in order to complete remove methanol and then the220%DMAA/Clay104dried catalyst sample was stored in a sealed bottle.20%TPA/Clay13520%PMA/Clay176Before use, the sample was dried in an oven for 1 h at373 K to remove the moisture.2.3.3.Temperature-programmed desorption2.3. Catalyst characterization(TPD) of ammonia2.3.1. XRDThe total amount of acidity present in the cat-alyst was estimated using temperature-programmedThe crystallinity and the phase purity ofdesorption (TPD) of NH3 using a Micromeritics Au-synthesized samples were analyzed by powdertoChem 2910 instrument. It was carried out by dehy-X-ray diffraction (XRD) patterns using X-raydratir中国煤化工’rleat 773 K in dry .diffractometer (Rigaku Miniflex, Japan) using Cu Kahelium for 0.5 h.radiation. XRD was carried out using the catalystThe tTYHCNMHGo398Kundertheand was scanned within the 20 range of 10°- 80°. Theflow of helium and then 0.5 ml NH3 pulses were sup-XRD pattern for all the prepared samples is shownplied to the samples until no more uptake of NHg was44V. V. Bokade eot al./ Journal of Natural Gas Chemistry Vol. 16 No.1 2007observed. NH3 was desorbed in He flow by increas-the concentration of acid sites after heteropoly aciding the temperature to 813 K, with a heating rateloading are summarized in Table 2. The loading ofof 10 K/min, and NH3 desorption was measured us- different heteropolyacids on plain clay (K-10) cata-ing a TCD detector. Details regarding the amount oflyst led to an increase in the concentration of aciddesorbed ammonia (total acidity) and the increase in sites by 10.3%, 18.4%, and 28.4%, respectively.Table 2. Total acidity of the samples investigated (TPD)Increase in concentration of acidNo.CatalystTotal acidity (mmol/g)sites after heteropoly acid loading (%)Clay (K- 10, Plain)0.139一20%DMAA/Clay0.24210.320%TPA/Clay0.32318.420%PMA/Clay0.42328.42.3.4. Catalyst test3. Results and discussionThe commercial as well as synthesized catalystThe alkylation of toluene with methanol as thesamples were studied for the vapor phase alkylation ofalkylating agent for the selective formation of xylenetoluene using methanol as the alkylating agent. Theisomers, especially p-xylene, is systemnatically stud-vapor phase reaction was carried out in a SS316 (20ied. The different HPA loading from 0 to 30% wasml volume) continuous downflow reactor (Figure notsynthesized, characterized, and evaluated. The opti-shown). The catalyst was activated at 523 K in themum data of only 20% HPA loaded clay is presentedpresence of nitrogen (50 ml/min) for 2 h, before theand compared.All the prepared catalyst samplesstart of the actual reaction. The reaction was initiatedwere observed to be well crystallized as far as X-rayby injecting the reaction mixture of toluene:methanoldiffraction patterns were concerned (Figure 1). Thein the molar ratio of 1:1.5, at 523 K, at weight hourlyactivity of HPA-supported clay for toluene methy-space velocity (WHSV) of 2.5 h-1 and atmospbericlation with methanol showed the following trend:pressure. Each reaction was carried out for a period20%PMA/C>20%TPA/C>20% DMAA/C>C (Clay-of 6 h. The sample was collected at 3, 5, and 6 h. TheK-10, plain). The results are shown in Table 3. Thestable data at 5 h are shown in Table 3 for compari-activities are well correlated with the XRD, surfacearea, and acidity measurement by ammonia TPD.son.The decrease in surface area with the deposition ofheteropoly acid may be due to the blocking of some2.3.5. Analysisof the surface area with HPA. The increase in aciditywith HPA-loaded samples (Table 2) favors the activ-The liquid sarmples were analyzed using Gas Chro-ity and selectivity to xylenes. The maximum toluenematography HP 5890 series II, using column Su-conversion and yield of 62% and p-xylene selectiv-plco, SPB-5M, with nitrogen as the carrier gas and aity of 100% was observed with 20%PMA/Clay sam-programmable temperature range of 473-548 K. Theple. This increased in activity and selectivity withproduct samples were confirmed by GC-MS.20%PMA/Clay sample may be due to increase in acid-Table 3. Comparison of catalysts for the alkylation of toluene with methanol to xyleneSelectivity (%)Conversion of toluene (%)Yield of p-xylene (%)m-xylenep-xyleneo-xyleneClay (K-10, Plain)6.374.425.61.750.431.162.86.131.67.17.642.43.0_6中国煤化工_62Reaction conditions: reaction temperature 523 K, WHSV 2.5 h~:MYHC N M H Gmethanol=1:1.5,time on stream 6 h, 5th h stable data are includedJournal of Natural Gas Chemistry Vol. 16 No.1200745ity by 28.4%, as compared with plain clay. The activ-vestigated and the activity, selectivity, and yield of P-ity was observed to be always more with HPA-loadedxylene are compared with plain clay. The HPA-loadedsamples as compared with plain clay sample. Amongsamples were observed to better compared with plainthe HPAs used, 20% PMA (Dodeca Molybdo Phos-clay, as far as activity, selectivity, and yields were con-phoric Acid) seems to be a better HPA compared withcerned. The 20% PMA/Clay sample was observed tothe other HPAs studied and these observations arebe the best among the samples studied. The toluenewell comparable with XRD, surface area, and acid-conversion, p-xylene yield of 62%, and p-xylene se-ty measurement by temperature-programmed des-lectivity of 100%, was observed with 20%PMA/Clay.orption (TPD) of ammonia.This increase in activity, selectivity, and yield of p-xylene with 20%PMA/Clay catalyst may be due to4. Conclusionsincrease in acidity by 28.4%, as compared with plainclay.The alkylation of toluene with methanol for theselective formation of p-xylene was systematicallyReferencesstudied at lower reaction temperature of 523 K ascompared to 623 K with zeolites. Very few stud-[1] Kaeding W W, Chu C, Young L B, Butter S A. Jour-ies have been reported on the use of heteropoly acidnal of Catalysis, 1981, 69: 392(HPA) for this type of reaction. The use of dfferent[2]ZhuZR,ChenQL,XieZK,YangWM,LiC.Mitypes of HPA on cheap support such as clay was in-croporous and Mesoporous Materials, 2006, 88: 16中国煤化工MYHCNMHG