Performance of HZSM-5 as Catalyst for Alkylation of Methyl naphthalene with Methanol in Supercritica

CHEM. rEs. CHINESE U.2005, 21(6), 680- -683Performance of HZSM-5 as Catalyst for Alkylation of Methyl-naphthalene with Methanol in Supercritical Phase*BAI Xue-feng'2**, WU Wei? and HU Hao-quan'1. School of Chemical Engineering , Dalian University of Technology, Dalian 116012, P. R. China;2. Heilongjiang Institute of Petrochemistry, Harbin 150040, P. R. China;3. School of Chemistry and Chemical Engineering , Heilongjiang University, Harbin 150001, P. R. ChinaReceived June 7, 2005The alkylation of methylnaphthalene( MN) with methanol in the presence of HZSM-5 is a pronising route for pro-ducing 2 ,6 dimetylnaphthalene(2 ,6-DMN) with a high selectivity. However, the conversion of MN is very low andthe catalyst will be deactivated mpidly with increasing time on stream. In this study, the effects of the reaction pressure on the reactivity, selectivity and life of the catalyst of alkylation of MN over HZSM-5 modifed by BaO were investigated. It was observed that with the enhancement of pressure, the conversion of MN increased, but the selectivityof 2 ,6-DMN kepl unchanged, which was about 40% - -42%. When the alkylation was carried out under a supercriti-cal condition, the conversion of MN was 3- 6 times higher and the life of catalyst was 25- 30 times longer than thoseat an ambient pressre. The thermograrimetric analyses of the deactivated catalysts at diferent reaction pressures in-dicate that the amount of coke deposited on the catalysts was about 10% to 12 % , and the coke-burning reactionsmainly took place in a temperature range from 720 to 860 K , and the apparent activation energies of the coke-burningcatalysts at0.1 MPa(10 h) and 7.6 MPa(108 h) were, respectively, 65. 90 and 84. 72 kJ/mol. It is concludedfrom the results that the superertical condition is advantageous to enhancing the conversion of alkylation and extrac-tion in silu, and to transporting those high molecular-weight poly aromatic compounds so as to extend the catalyst lifesuccessively.Keywords Methylnaphthalene , Alkylation, Shape selective catalysis , Supercritical phase , Coke-burningArticle ID 1005 9040(2005 ) 06-680-04Introductionor chemical reaction involves secondary reactions requi-In recent years, there has been worldwide interest ring separations to guarantee the purity of the interme-in the production of 2 ,6 naphthalene dicarboxylic aciddiates or the end product. Alkylation of methylnaphtha-as a monomer for the high performance polyester, poly-lene( MN) with methanol in the presence of HZSM-5 asethylene naphthalate( PEN) , which provide better heata catalyst can be used to produce 2 ,6-DMN with a highresistance and mechanical properties than polyethyleneselectivity and it is a very promising route. The mainterephthalate( PET) in applications, such as being theproblems in such a process are the low MN conversion，material of films，containers and molded parts， etc. .and the easy deactivation of the catalyst caused by car-2 , 6-DimethyInaphthalene (2, 6-DMN) is a desirablebon depositing on the catalyst surface. In general, thecatalyst life is only a few hoursfeedstock for the production of 2 ,6-naphthalene dicar-Superertical fluids possess special properties ,boxylic acid!".such as high density and high diffusivity, which make2, 6-DMN can be found in refinery streams andthem attractive as media for chemical reactions. Con-coal tar as the mixtures of DMN isomers in low concen-ducting chemical reactions in supercritical phases aftrations. However, the separation of 2, 6-DMN fromfords the opportunities to control the reaction environ-these DMN isomers is very difficult and expensive duement( solvent properties) by controlling the pressure,to their similar physical and chemical properties. Atto eliminate inter-phase transport limitations on reactionpresent there is an available process to selectively syn-rates, and to enhance the reaction conversion. Abovethesize 2 ,6-DMN through alkylation, cyclization, dehy-中国煤化工compounds with adrogenation and isomerization "The multi-stephigher:YHCNMHGcomatic coke precur-process is very costly obviously, in addition, each step* Supponted by the National Natural Science Foundation of China( No.20276011 & No. 20376012).* *To whom correspondence should be addressed. E mail: bxuefeng@ 163. netNo.6BAI Xue-feng et al.681In this paper, the performance of HZSM-5 as the be converted into a-MN through isomerization!to]. Forcatalyst for the alkylation of MN with methanol in a su-this reason, mixed MN(a mixture of a-MN and β-MNpereritical phase was investigated to explore the possi-with a molar ratio of 24: 75) was selected as feedstockbiity in increasing MN conversion and prolonging the in this work for its much lower price than pure B-MN.catalyst life by use of the characteristics of supercriticalThe conversion of MN in the following text does not in-fluids.clude the part of isomerization between a-MN and β-ExperimentalMN.1 Preparation of Zeolite CatalystFigs.1 and 2 show the conversion of MN and theIt has been reported that the selective neutraliza-selectivity of 2 ,6-DMN, resectively ，in the alkylationtion of acid sites on the extermal surface of HZSM-5of MN with methanol at different pressures.erysallites by means of modification with BaO through amechanochemical method results in the rise of 2, 6-DMN selectivityt6) ，so the catalyst used in this study is10HZSM-5 with a molar ratio of 38 of SiO2 to Al2O3 and itwas modified by BaO. The catalyst was prepared fromσξ: 6 MPaNaZSM-5 by a two-step ion exchange with a 1 mol/L一32 MPaNH, NO, aqueous solution at 80 C and calcination at550 C for 6 h between the two steps. The zeolite pow-20406080100120der was milled together with BaO(5%), pressed withReaction tioe/hbinders, erushed and sieved. The particles with a sizeFig.1 Effect of reaction pressure on theranging from 20 to 40 mesh were used for the alkylationconversion of MN.experiments.50厂ReagentsMN used for alkylation reaction was a mixture of45-a-MN and β-MN with a molar ratio of 24:75.1,3,5-Trimethylbenzene(1 ,3 ,5-TMB, 99. 5%) was the puri-世7.6 MPsfied product from the industrial grade product. Metha-nol was of analytic reagent purity.一0:1MPa.3 AlkylationThe alkylation of MN with methanol at ambient2040 60 80 100 120pressure and in a supereritical phase was carried out inReaction time/ha high-pressure flow-type apparatus with a fixed bed re-Fig.2 Eiftect of reaction pressure on theselectivity of 2,6-DMN.actor designed for temperatures up to 550 C and pres-sures up to 10 MPa. Prior to the experiments, the cata-From Fig. 1 it can be seen that at ambient pres-lyst was activated for4 h in the reactor at 500 C at am-sure, the MN conversion decreases from 6% to a valuebient pressure in flowing nitrogen. A liquid mixture ofless than 2% in 10 h. With the increase of the pres-MN, methanol and 1,3,5-TMB with a molar ratio ofsure, the MN conversion in the alkylation increases and1:0. 5:3 was pumped into the reactor after preheating.the stability of the catalyst activity is improved greatly.The alkylation of MN with methanol was carried out atAt a pressure of 7.6 MPa, the partial pressure of 1,3,460 C and at pressures between 0.1 and 7. 6 MPa.5-TMB will be over its critical pressure of3. 1 MPa andThe space velocity ( WHSV) was 0.5 h-' referred tothe reaction system will be in a supereritical state. IMN. The product samples were taken periodically fromthis case, the MN conversion will level off at aboutthe depressurized product stream and analyzed by GC .12% for 108 h.From Fig.2 it can be seen that the selectivity ofwith a flame ion detector by using a 20 m capillary col-umn, a liquid crystal as the stationary phase and H2 as2,6-中国煤化工. thermodynamic equi-the carrier gas.libriuThis indicates thatHZSIYHC N M H Gexellent catalyst forResults and Discussion1 Conversion of MN and Selectivity of 2 ,6-DMNgetting a high selectivity of 2, 6-DMN. The selectivityDuring the alkylation of B-MN with methanol inof2,6-DMN remains in a range of 40% - 42% withthe presence of HZSM-5, more than half of B-MN maythe increase of reaction time at different presures.682CHEM. RES. CHINESE U.Vol. 21The above mentioned results indicate that it is pos- very low as seen from the final activity at the end ofsible to improve the MN conversion and the catalyst life108 h. .in the catalytic alkylation of MN with methanol over100 NHZSM-5 while the high selectivity of 2 ,6-DMN is keptby using a supercritical fluid.5F2 Thermal Analysis of Used Catalysts--0.1 MPaTo understand the deactivation behavior of the cat--”- 3.2 MPaalysts, the coke-burning of the used catalysts obtained-v-7. 6 MPaat different pressures was carried oul with an atmos-pherie thermogravimetric apparatus ( Mettler Toledo300 400 500 600 700 800 900 1000 1100TGA/SDTA851*). Ina general test, about 30 mg of aT/Kused catalyst sample was placed in a crucible and com-Fig.3 The plots of weight loss vs. temperature of thebusted at a heating rate of 10 K/min from 300 K toIsed catalyst samples obtained at different1073 K in air with a flow rate of 30 mL/ min.pressures upon coke-burning.Figs. 3 and 4 show the weight loss and weight loss100厂rate( DTG) curves of the used catalyst samples and Ta-- 0.1 MPable 1 gives the characteristic parameters of the thermo-30--7.6MPagravimetric analysis. It can be seen that with the in-creases of reaction time and pressure， the amount of10叶coke formed on the catalysts at different pressures de-20-creases. The amount of coke( expressed as the percent-age increased in the catalyst mass at the end of therun) is about 11. 86% for the used catalyst atO.1 MPain 10 h. In contrast, at 7. 6 MPa, the amount of cokeFig.4 DTG curves of the used catalyst samples obtainedis only 10. 49% , and the catalyst deactivation rate isat different pressures upon coke-burning.Table 1 Coke-burning characteristic parameters of the used catalyst sanples at different pressuresForming conditions ofMax weight loss1st weightloss( % )2nd weigh-los( % )Temperature ofthe used catalyats(%)(Temperature rnge/K)(Temperature range/K)weight Loe Tate/K0.1MPa,10h11. 860.28(384- 568)10. 86(599- -933)7723.2 MPa, 35 h11. 240. 14(391- 576)10. 81(591- -956)7875.4 MPa, 64 h10. 930. 70(362- -545)10. 13(583- 964)8107.6 MPa, 108 h10. 49.0. 14(392-576)9.93(576- 971)The coke-burning activation energies of the usedlyst samples are given in Table 2.catalyst samples obtained at 0.1 MPa(10 h) and 7.6-12.0MPa(108 h) were calculated according to the Coats-- 12.5-Redfern method with the following equation'2] :-13. 0In[ -lnx/T2]=In R[1 -2RT/E]-一与-13.5where 1 represents the ratio of the not-burned coke旦-14.0f --0.1 MPa [or 10h-o-7.6MPafor108hweight at temperature T to the total coke weight; β re--14.5presents the rate of temperature rising; A is preexpo--15. 01.101.151.201.251.301.351.40nential factor; E is coke-burning activation energy; R108 r-1/K-1is the gas constant.Fig.5 shows the plot of ln[ - Inw/T'] us. 1/T .Fig.5 Plots ofn [ - lnw/T'] .1/T for coke-burningof the used catalyst samples.The coke-buming kinetic parameters of the used cata-中国煤化工Table 2 Coke-burning kinetic parametersForming conditions of the used catalystsCoke-burming activation energy/(kJMYHC N M H G Crelative cfrier0.1 MPafor 10h65. 92. 04 x10*-0. 9947.6 MPa for 108 h84. 71.73x105-0. 996It can be seen from Table 2 that although increas-ing the reaction time does not result in more cokeNo.6 .BAI Xue-feng et al.683formed under the supercritical condition than that in2] Sikkeng D. L., lambJ. D., Zaenger1. C, et al.，US Patens4950825, 1990the ambient pressure process , the coke-buming activa-[3 ] OzaweS., Takagawa M., Dnamase K.，US Patent 5396008.tion energy of the used catalyst samples is about 191995kJ/mol higher than that at 0.1 MPa(10 h). lt canNiemela M. , US Patent 6472576, 2002generally be believed that the higher the coke-burning[5 ] KomatsuL, Araki Y.，Namba S., 比al. . Sud. Surf. Sei.activation energy is, the higher the C to H ratio of cokeCatal, 1994, 84, 1821is, i.e.，the more coke possesses polymeric rings. It[6]， SongC. S.， Shen J P.， Lllwiz L. D.， W0 Paten!can be explained that a supercritical fluid has a better[7 ] Tomoyukil, Pus. B., KugaiJ. 1, Appl. Catal. A: General,ability of in situ extracting higher molecular weight pol-1996, 146, 285ymeric compounds than a gas-phase fluid does. This[ 8 ] Subrananiam B.. Appl. Catal. A: General, 2001, 212, 199has also been. confirmned by the weight loss curve in[9] BaiX. F., Xia Y. L.. Hu H. Q. J. Chem. Ind. & Eng.，2004, 55, 485Fig.3 that the coke in the used catalyst sample ob-[10] WeitkampJ，Neuber M. , Stud. Suf. Sei. Catal. , 1991, 60,tained at 0.1 MPa shows a relatively lower temperaturethan that obtained al 7. 6 MPa at the same weight loss. .[11] Shiroto Y., Aromaic, 1991, 43, 7[12] Coals A. D., Redemn J. P., Narture, 1964, 201, 68References[ 1 ] Yamamoto K., Yamamolo S.. Tanaka T, et al.，US Patent6894202, 2005中国煤化工MYHCNMHG