Indi an Journal of Chemi ca l T ec hn ology Vol. 9, July 2002, pp. 334-340 Articles Kinetics of toluene methylation over silica modified HZSM-S zeolites J Da s", P R Char", A B Hal geri "*, R K Mewada b & N Subra hmanyam" "Researc h Ce ntre, Indian Petrochemi ca ls Co rp ora tion Limited, Baroda 39 1 346, India "Chemical Engineering Department, Faculty of Tec hn ology and Engineerin g, The M .S. University of Baroda, Baroda 390 DO I. India Recei ved 30 May 2001: revised recei ved 15 Febmary 2002: accepled 21 March 2002 Kinetics of toluene alkylation with methanol over silica modified HZSM-S catalyst under experimental conditions that are close to envisaged industrial conditions are studied. Of the several possible kinetic models tested and compared, a simple power law model is found satisfactory in predicting toluene conversions and p-xylene selectivities. However, further studies with a comprehensive experimental program, is suggested for investigating the numerous side reactions and to account for a large number of by-products observed. Se l ec tiv e alk yl at ion of toluene with methanol is one of the most promising routes to produce para xyl ene. Thi s subject has been receiving considerable attention as can be asce rtained by th e number of patents be ing filed l - 5 and papers being published. The quint esse ntial factor to success is the design of a catalyst that gives commercial ly viable conversions and se lectivi ty. Thus, th e st udy of reac tion kineti cs ass umes importance in evaluating ca talysts and in op timi zing reaction cond iti ons. Zeolites, being economical and env ironmentally sa fe , are the so ught after catalysts for alkylations. By proper se lection of the type of zeo lit e and it s tr ea tment with sili ca to control pore mouth dimensions and pass ivati on of ex ternal ac id sites, it is po ss ible to obtain hi gh performanc e. The kinetic studi es help in improvin g th e catalyst des ign based on th e observed stability, toluene conve rsion, and product purity. Earlier lit erature on se l ec ti ve alkylation of tolu ene rel ates mainly to th e use of HZSM -S zeolite and it s modifications to regulate pore mouth openin g. Se l ec tiv e production of p-xylene with unmod ifi ed ZSM-S zeo lit es was studied by Yashima et 0/ 6 and Bh at 7 . Chen and Garewood 8 sugges ted th at the se l ec tivity observed in va ri ous reactions ca rri ed out ove r ZSM-S zeo lit es is not r es ulting from sharp mol ecu lar si ev ing but from diff erences in intra- cr ys talline mob ilit y due to steric hindrance. Similarly Chen et al. 9 obser ved enh anced formation of p-xy l ene in th e me th ylation of toluene on HZSM-S on ly in large r crystals or after "modifications" of zeo lit es with ph os ph oru s. Bhat ef al. lo and Besc humann and * For Corres po ndence : (E-mail: abhal ge ri @hotmail.com; Fa x: +9 1-265-27209 S) Riekert ll al so have report ed higher pa ra se l ectivity in larger crystal s. Wei 12 proposed a mathematical model representing th e kineti cs and different diffu siviti es of th e xy lene isomers in zeo lit e crystals. Kaedi ng et a l. 13 have suggested a mechanism for toluene meth yl at ion. A second approach reported by So telo ef 01. 1-1 and Uguina et 01.1 5 co nsiders tha t p-xylene IS , preferentiall y produced inside th e z o lit e cha nn els due to diffu sional constraints and tha t acid sit es located on the external sur face are mainly res ponsible for i so merization. In order to enhance the shape selectivity, various methods of mod ifi cation of th e zeoli te property were proposed by different authors. Kaeding el al. l :1 and Hibin o ef al. 16 exp lained the en han c .ment of {h e shape-se l ec tivity as a r es ult of th e narro wi ng or pore opening si ze. Yashima ef 0 1. 17 , Say ed ef 0 1. IS and Kim ef al. 19 explained higher shape se lectiv it y as a re sult of contro llin g the ac idit y. Wang ef al 20 and Da s et al 21 ex pl ai ned higher shape se lectivity In term s of inactivation of externa l surface Jcid sit e .. Kinetic studies of toluene met hylation over HZSM - 8 and HZSM -S were reported by Bhat ef al. 22 and Mantha ef al. 23 , re spectively. They proposed a kinetic model using the Lang muir-Hin shel woo d-H oughen- Wats on (LH HW) I, echanism. Similarly V ayss il ov ef a1 2 -1 proposed a power-law model where on ly toluene alkylati on and xy l ene isomeri za ti on were considered . These authors have consider ed both ex ternal and internal surface ac id sit .s of the zeo lit es crystals to ex plain th e obse r ved product distribution. However side re ar- ti ons such as methanol dehydration to a number of gaseous products have not bee n accounted for in all the above cases.
Transcript
Indian Journal of Chemica l Technology Vol. 9, July 2002, pp. 334-340
Articles
Kinetics of toluene methylation over silica modified HZSM-S zeolites
J Das", P R Char", A B Halgeri"*, R K Mewadab & N Subrahmanyam"
"Research Centre, Indian Petrochemica ls Corpora ti on Limited , Baroda 39 1 346, India
"Chemica l Engineering Department, Faculty of Technology and Engineering, The M.S. University of Baroda, Baroda 390 DO I. India
Received 30 May 2001: revised received 15 Febmary 2002: accepled 21 March 2002
Kinetics of toluene alkylation with methanol over silica modified HZSM-S catalyst under experimental conditions that are close to envisaged industrial conditions are studied. Of the several possible kinetic models tested and compared, a simple power law model is found satisfactory in predicting toluene conversions and p-xylene selectivities. However, further studies with a comprehensive experimental program, is suggested for investigating the numerous side reactions and to account for a large number of by-products observed.
Se lec tive alkylat ion of toluene with methanol is one of the most promising routes to produce para xy lene. Thi s subject has been receiving considerab le attent ion as can be ascertained by the number of patents being filed l
-5 and papers being publi shed. The quintessential
factor to success is the design of a catalyst that gives commercial ly viable convers ions and select i vi ty. Thus, the study of reacti on kinet ics assumes importance in evaluating catalysts and in optimi zing reaction cond itions. Zeolites, being economica l and environmentally sa fe, are the sought after cata lysts for alkylations. By proper selection of the type of zeo lite and its treatment with sili ca to con trol pore mouth dimensions and passivation of ex ternal acid sites, it is poss ible to obtain high performance. The kinet ic studi es help in improving the catalyst design based on the observed stability, to luene conversion , and product purity.
Earlier literature on selec ti ve alky lation of toluene relates mainl y to the use of HZSM -S zeolite and it s modifications to regulate pore mouth opening. Selec tive production of p-xylene with unmod ified ZSM-S zeo lites was studied by Yashima et 0 /
6 and Bhat7
. Chen and Garewood8 suggested that the se lec tivity observed in vari ous react ions carri ed out over ZSM-S zeolites is not resulting from sharp molecu lar siev ing but from differences in intracrys talline mobility due to steri c hindrance. Similarly Chen et al.
9 observed enhanced formation of p-xy lene in the meth y lation of toluene on HZSM-S on ly in larger crystal s or after " modifications" of zeo lites with phosphorus. Bhat ef a l .
lo and Beschumann and
* For Correspondence: (E-mail: abhalgeri @hotmail.com; Fax: +9 1-265-27209S)
Riekert ll also have reported higher para selectivity in larger crysta ls. Wei 12 proposed a mathematical model representing the kineti cs and different diffusiviti es of the xy lene isomers in zeolite crystals. Kaedi ng et a l. 13
have suggested a mechanism for toluene methy lat ion. A second approach reported by Sotelo ef 01. 1-1 and Uguina et 01.1 5 considers that p-xylene IS , preferent iall y produced inside the z olite channels due to diffusional constraints and that acid sites located on the external surface are mainly responsible fo r isomerization.
In order to enhance the shape selectivity, various methods of modification of the zeo li te property were proposed by different authors. Kaeding el al. l
:1 and Hibino ef al. 16 exp lained the enhanc .ment o f {he shape-se lectivity as a result of the narrowi ng or pore opening size. Yashima ef 0 1.
17, Sayed ef 0 1. IS and Kim
ef a l .19 explained higher shape selecti v ity as a result of
controlling the ac idity. Wang ef al20 and Das et al
2 1
explai ned higher shape se lectiv ity In terms of inact ivat ion of externa l sur face Jcid site . .
Kinetic studies of toluene methy lation over HZSM -8 and HZSM -S were reported by Bhat ef al.
22 and Mantha ef al.
23, respectively. They proposed a kinetic
model using the Langmuir-Hinshel wood-HoughenWatson (LH HW) I, echanism. Simi larly Vayss ilov ef a12
-1 proposed a power-law model where on ly toluene alkylat ion and xy lene isomerizati on were considered . These authors have considered both ex ternal and internal surface acid sit .s of the zeo lites crysta ls to ex plain the observed product distributi on. However side rear- ti ons such as methanol dehydration to a number of gaseous products have not been accounted for in all the above cases.
Das el al .: Kine li cs of to luene melhy lali o n over s ilica modified HZS M-S zeo liles Articles
Sotelo el al. 14 studied kinet ics of the toluene methylation with both unmodifi ed and mod ifi ed HZSM-5 zeo lites. After fittin g various models like LHHW, Eiley- Ridea l and power law model for main alkylati on reaction , they had chosen the power law model to exp lai n the product di stributi on obtained though Eiley-Rideal mode l fitted the data best. They considered the secondary reacti on of th e methanol and xy lenes in the reaction scheme.
Other important as pect of thi s reac ti on such as effect of coke depos iti on25
, ZS M-5 type catal yst covered wi th silicate sheIl 2(>, cata lys is by Ga(H)ZS M-527 and ge nerati on of shape selec ti vit/ s
have also been di scussed in literature. Present study is concerned mainly with the
performance of in-house developed catalyst at di fferent operat i ng condi ti ons, formu lation of a si mple kinetic model that adequately represents the main reactions, its testing and agreement of parameter values with earli er studi es. It considers the secondary reactions such as methanol dehydrati on, toluene di sproportionation, p-xy lene dealkylation and ethylation of toluene with ethyl ene.
Experimental Procedure Silica modifi ed ZSM-5 zeolite catal ys t used in thi s
study was prepared by the meth od explained in Das el al?). The reacti on was carried out at near atmospheri c pressures in a fixed bed continuous down-fl ow glass reactor having a di ameter of 2.54 cm. The catalyst was supported on a sintered glass ring. About 1-2 g of catalyst was charged and reactor space above the catalyst was fill ed with in ert glass helices. All chemical s used were of analytica l grade.
Prior to reac ti on, the catalys t was treated with a hydrogen stream at the des ired temperature. A mixture of toluene and methanol was fed using a sy ringe pump (Sage, Model 362) th rough vapouri zer to the reactor top. The products were collected from the bottom of the reactor after condensation wi th chi lled water. The gases were vented and the liquid product was analyzed using a HP 5890 Seri es- II gas chromatograph.
There was no significant deactivation during the durati on of ex periments, as can be seen from Fi g. I . Experiments were carri ed out at two different catalyst weights but with same w/I rati o to ascertain ex ternal mass transfer effects and to select proper range of feed fl ow rates. The results are shown in Fig . 2. Likewise ex periments were carried out with three different sizes of catalyst particles and the results
Fi g. 2- Eva luat io ll o f ex te rnal mass tra nsfe r res istance:
I = 450°C: H2/HC = 2: To l:MeO H = 4
showed almost identi cal conversions at same conditi ons indicating absence of macropore diffusional res istance.
Initially reactions were conducted to study the effects of different parameters In the ranges: temperature 350° to 475°C to luene to methanol mol rati o I: I to 12: I, and H2 to hydrocarbon mol ratio I: I to 8: I. Based on these observations, toluene to methanol ratio and H2 to hydrocarbon rati o were selected as 4 and 2 respecti vely and further ex periments were carried out at diffe rent \II/I ratios and repeated at tbree different temperatures, viz 400, 425 and 450°C.
Results and Discussion Attempts are made first to study the effects of
temperature, toluene to methanol ratio, hydrogen to hydrocarbon rati o and space veloci ty to select appropri ate conditions for obtaining des irable
335
Articles
performance. Vary ing temperatu re from 350° to 475°C shows that the toluene conversion increases with increase in temperature as presented in Fig. 3. At lower temperatures both toluene conversion and total xy lene selecti vity are low though para xy lene selecti vity is high. At hi gher temperatures, both toluene convers ion and xy lenes selecti vity increase with a slight reduction in para xy lene selecti vity. It was also noticed that further increase of temperature results in toluene disproportionation as was observed by the increas ing benzene fraction in liquid products. From this, a temperature range of 400-450°C appears to give reasonable conversions with acceptable p xy lene selecti vity. Fi g. 4 shows the effects of vary ing toluene to meth anol rati o from I to 12. At lower va lues of thi s ratio, total xy lenes selec ti vity is low and this probably indicates various side reacti ons of methanol. At higher toluene: methanol ratios, the ut ilizati on of methanol fo r alkylation increases and at around a value of 4, the xy lenes selecti vity
336
100 ,--------------------,
80 J(
o Tol conversion
J( p-xyl selectivity
o m-xyl selectivity
o p-xyl selectivity
II. Total xyl Selectivity 20
Temperature, C
Fig. 3- Effec t of reaction temperature: Tol:MeOH = 4; H2/HC = 2; WHSV=3.74 h·1
Fig. 4- Effect ofTol:M eOH Ratio : I = 25°C; H2/HC = 2; WHSV =3.74 h' l
15
Indian J. Chem. Techno!. , July 2002
approaches about 72% and remains constant thereafter. However the p-xy lene electivity drops slightly with increas ing toluene: meth anol ratio. Hence further decrease in methanol has adverse effects in terms of p-xy lene selec ti vity and toluene recycle load. The effect of varying hydrogen:hydrocarbon ratio is shown in Fig. 5. While increasi ng hydrogen concentrati on in feed reduces the to luene conversion sli ghtly, probably due to dilution, it has a marked influence on total xy lenes selecti vity and p-xy lene selecti vity both of which increase. For fu rther studies, a rati o of 2 has been chosen. It was observed that sti ll lower hydrogen concentrations in feed speed up the catalys t deac ti vation by coke build up. The resul ts of varying WHSV fro m 1-17 are show n in Fig. 6. Toluene conversion and total xy lenes selec ti vi ty decrease wi th increas ing WHS V where as p-xy lene selecti vity increases. No definite trend in the distribution of higher aromatics could be di scerned from the results. Similar results were observed at
Fig. 5- Effecl of H2/HC Ratio: WHSV=3.74 h' l ; I = 425°C; Tol:MeOH = 4
100
~ ____ ~r-----------~
H2/HC mole ratio
Fi g. 6- Effect of WHSV: I = 425°C; H2/HC =, 2; Tol: MeOH = 4
Das el 01.: Kinetics of toluene methy lation over sil ica mod if ied HZSM -5 zeo lites Articles
other temperatures, viz. 400 and 450°C. From the large number of chemi cal spec ies
detected in the product stream, it was obvious that many simultaneous reac tions were occurring. Here only the main reacti ons are considered to enable us predict total toluene conversion, conversion to xy lenes and para xy lene selecti vity. It is reported in the literature that p-xy lene is the primary product of methylation. As the surface acti ve sites are pass ivated in the present work , it is assumed that p-xy lene isomeri zation takes place within catalyst channels and the conversion to /Il- and o-i somers is controlled by the diffusiviti es which are low due to reduced pore opening size. Consequentl y, the rate constants obtained are masked by micropore mass transfer resistances . Similarly methanol reacts to produce a number of gaseous products but this reaction is approximated by the dehydration reaction. In addition, all isomers of ethyl toluene are grouped into one species and trimethyl benzenes and hi gher aromatics are ignored as they were present in traces. The reaction scheme considered is,
Toluene Alkylation: C7Hg + CH30H~p-CgH JO +H20 ... (1)
Isomerization to ortho: p-CgH ' 0 ~0-C8H '0 ... (6)
For the integral reactor with I :0.25 mole ratio of toluene and methanol feed, the moles of diffe rent species present at any point where toluene conversion is XI' and methanol conversion is XM and toluene conversion to any aromati c species P is Xp are expressed in terms of conversions as,
Using these relati ons, parti al pressures of the spec ies are expressed as below fo r hydrogen to hydrocarbon rat io of 2: I and at total pressure of 1 atmosphere fo r the ac tual ex perimental conditions.
PT = PM = Ppx = PMX = Pox = Po= P ET = PG.P = where A
Here XGP is defined as moles of gaseous product fo rmed per I mol of toluene and 0.25 mol methanol fed and A gives the total number of moles including hydrogen. The corresponding rate equations are: Rate of disappearance of toluene:
Tab le I - Results of optimi za tion programme for various models
Type of M ode l
L HHW model with methanol adsorption controlling
LH HW model with surface rcac tion cont ro lling
L HHW model with p -xy lene desorption controlling
Eiley- Rideal model with methanol adsorption cont ro lling
Eilcy-Ridea l model wi th surface reac ti on contro lling
Eilcy-Ridea l model with p-xy lene desorpti on cont ro ll ing
Power-Law model
Va lue of the objective functi on (a t temp.=425°C)
2. 111 930
2.2224 17
2.326689
0.707808
0.689453
0.752543
0.728438
A non-linear regress ion programme based on BoxComplex method with fourth order Runge-Kutta algorithm for solution of above differential equat ions was employed to esti mate the kinetic constan ts by minimisi ng the sum of squares of errors:
Tab le 2-Reacti on rate constants of the power-law model
Temperature, K 673.0 698 .0 723.0
Parameter. kl 0.8500 1200 1.4 125000 1.80120
k] 45.0 102000 70.0 10 10 84.0042
k 3 0.00240 10 0.00285 10 0.0056208
k4 0.00030 10 0.000330 0.000406106
where, Xn.lll . is the conversion of species n in the mIll experiment and Xn•1ll ca l is that predicted with given parameter values.
Results of thi s and other few models tri ed for data fi tting are shown in Table I. Though Eiley-Rideal mechani sm with surface reaction controlling fits the experimental data better, the simple stoichiometri c model is selected for further work for its simplicity . Tab le 2 presents the val ues of rate constants at different temperatures and the A rrheni us parameters are shown in Tab le 3.
The ac ti vati on energy obtained here for toluene methy lat ion matches we ll with that reported by the Sotelo e l al.l~, and is slightly less than 7 1.5 kJ /mol reponed by Wang el 012 0 and 79.8 kJ/mo l reported by Mantha el 01. 23 . Act i vati on energies obtained here for p-xy lene isomeri zation to lI1-xy lene and a-xy lene are higher than the corresponding val ues reported by Li el 0 1. ",0 and Mantha el al.D , using an unmodified HZSM -S cata lys t, but are lower than corresponding values reported by Uguina el 01.31 for M g, Si -modified ZSM-S. However, ac ti vati on energies of to luene alky lation, methanol dehydration and toluene disproporti onati on are only high enough to indicate the temperature sensitivity of a chemica l reaction. In isomeri zation and ethy l tol uene format ion reactions diffusion limitati on appears to be controlling the rates due to the con trolled size of pore open ings. The kinetic model was used to predict total toluene conversion and conversion to p-xy lene at di fferent WHSV and the results are shown in Fig. 7. Simi lar
338
k-, 0.020 10 18 0.0220060 0.0305632
k6 0.0 132646 0.0 150 100 0.02 13967
k7 0.00 1500 1 0.00 18340 0.00283826
Table 3- A rrhenius parameters
Rate constant Pre-ex poncntial Ac ti va tion energy, factor. /.:. " E. kl /mo!.
kl 47858.3076 60.9882
kl 40393 1.2624 50.7 137
/.:. ) 440.5835 68.2978
k4 0.02 19 24.0965
k, 7.9035 33.655 1
k6 12.2877 38.4456
k7 13.8988 5 1.339t:
60
0 To luene (exp.)
c 50 0 p-xylene (exp.) .2 -t>- Toluene (cal.) In .... 40 -e- p-xylene (cal.) II> > C 0 u 30 II> Cl .!!! 20 c -e II>
~ 0 u
~ .... II> 10 El a.. 0
0
0 10 20 30 40 50 60 70
W/F. g.h/gmol.
Fig. 7- Comparisoll of modc l pred iclions with experi menlal values
Das el 01.: Kineti cs of toluene methylation over silica mod ified HZS M-5 zeo lites Articles
50
c: 40 0 'iii ... II> > 30 c: 0 U
II> Cl 20 !S c: II> ~ II> 10 Il.
0
0
o Toluene(exp.)
x p-xylene(exp.) --&- Toluene (cal.) __ p-xylene (cal.)
2 4
H2/HC ratio ,
6
o x
8 10
Fig. 8- Comparison of model pred ictions with ex perimental va lucs
evaluation was made varying hydrogen to hydrocarbon ratio, the results of which are shown in Fig. 8. However prediction of methanol conversion is not sati sfactory as under the ex perimental conditi ons nearl y the entire quantity of methanol di appears in reaction and its estimation in product stream is difficult.
Conclusion Performance of a silica modified HZSM-5 catalyst
for alkylation of toluene with methanol has been studied using a fi xed bed reactor. The results show a steady performance under envisaged operating conditi ons with acceptable conversions and selec ti viti es. Effects of vari ous parameters are inves ti gated and a simple stoichiometric model representing the main reacti ons is proposed. The rate constants are evaluated and goodness of fit is tested. However, the ac tual process is complex involving several reactions and intrachannel mass transfer effects. It is also observed that though methanol conversion is complete, its utili zati on fo r alkylation is less. Further work is needed fo r more accurate representation of thi s process .
Nomenclature A
E F H1/HC rat io
k, ko P, Tol I MeOH ratio
",T ota l no. of moles present in the reaction mi xture based on I mole to luene fed.
'" Activation cnergy, kJ/mol. = Molar now rate, gmollhr. =H ydrogen to hydrocarbon ra tio.
gmol/gmo l. = reaction rate constant. i = I to 7. = Pre-ex ponential factor. = Parti al pressu re of co mponent i. atm. = Toluene to methanol rat io. gmol/g mol.
W WIF
= Weight of catalys t. gm. = Space time, gm.hr/gmol.
X'J =Conversion of cO lllponent i 111 f h
ex periment. X ij' cal. =Calculated conversion of component
for / ' ex periment. t = Space time va ri able g.hr/g mol.
Compounds B = Benzene E.T. = Ethyl Toluene G. P. = Gaseous product M = Methanol PX = Para-xylene OX = Orlho-xy lene MX = Mew-xy lene T = Toluene
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