Journal of Scientific & Industrial ResearchVo1.61, October 2002, pp 810-816

Comparative Performance of Ion-exchanged ZSM -5 andY-Zeolite Catalysts for Toluene Disproportionation Reaction

Parmesh K Chaudhari " Pradeep K Saini' and Shri Chand '*'Department of Chemical Engineering, Government Engineering College, Ujjain 456 010'Department of Chemical Engineering, Indian Institute of Technology, Roorkee 247667

Received: 01 February 2002; accepted: 03 June 2002

The toluene disproportionation reaction was carried out using ZSM-5 as well as Y-zeolite catalysts after ion exchange withactive metals Ni, Pb,and Cr. The reaction was carried out under atmospheric pressure. The effect of type of cation used for ionexchange, silica-alumina ratio, type of zeolite,and particle size were also studied. The per cent p-xylene yield (which is the mostdesirable product), was found to increase after ion-exchange with active cations. A maximum p-xylene yield of2.3 per cent wasobtained with Cr HZSM-5 (SAR-19) at 600C and I atmos pressure.

IntroductionToluene disproportionation is one of the

important techniques to convert less valuable toluene tohigher value benzene and xylenes. Among xylenes P:xylene is of greater significance for its use in polyesterfibres and as feedstock in many petrochemical industries.The selective production of p-substituted benzenederivatives by means of various modified zeolite catalystshas already been reported ". In particular, HZSM-5zeolite has been known to be of considerable interest,because its channel dimensions are favourable to theproduction of most of the aromatic hydrocarbons "'.

The commercial process involves the vapourphase disproportionatoin of toluene over various acidzeolites>'. It has been proposed that toluenedisproportionation yields benzene and a mixture ofxylenes as initial products inside the zeolite pores. Thisproduct subsequently diffuses out of the pore system withp-xylene diffusing faster than the other two isomers,owing to its smaller size. The 0- and m- components ofxylene undergo isomerization reaction within the zeolitecrystal. Enhanced p- selectivity has been further obtainedby the use of modifier agents, such as Si, B, P, and Mg'.The deposition of modifier agents by suitable techniques,such as chemical vapour deposition has been reportedto:

*Author for correspondenceE-mail: schanfch@iitr.erneLin; Fax: +91-1332-73560

(i) Partially block the pores, increase tortuosity of thepores, and thereby delay the exit of larger molecules,and (ii) Block the unselective active sites. Zeolitesmodified by ion-exchange, however, have hardly beenstudied towards its use as catalyst for thedisproportionation reaction.

Based on the fact that the ion exchange of azeolite may lead to: (i) Selectivity of the reactions basedon the nature of the cation, (ii) Blockage of the zeolitepore mouth in proportion to the size of the exchangedion, thereby leading to shape selectivity for the desiredproduct (p-xylene in the present case), and also (iii)Greater stability of the ion exchanged form, incomparison to the modified form'.

The present work, therefore, was undertaken tostudy the activities of HY and HZSM-5 zeolites afterexchange with certain transition metal cations for thedisproportionation reaction. The cations selected forexchange were Ni, Cr, and Pb.

Experimental ProcedureZSM-5 and Y zeolites having different silica-

alumina ratio were obtained from Indian Institute ofPetroleum, Dehradun. Na-zeolites were first convertedto Hszeolites before further exchange by a cation oftransition metal. 0.5 mole NH4NO) was mixed with 109zeolite powder and the mixture was subjected to ionexchange at 90'C for 17 h under total reflux conditions.The solution was filtered and NH/ exchanged zeolite was

CHAU DH ARI et al.: TOLUENE DISPROPORT IONATI ON REACTION 8 11

Nil~

Reg ulotor

Ni t rog er>-----

Gaseous

·~

I

Toluene

Condense r

- ·

Prehe ating s ~c. tion

Reactor calotysl b ed

Post hea l ing sectio n

sa mp\o co ll ec tion

Wate r circ u \ol i=:h _lh~-----,--J p um p

Coot'1n g water Tonk

Figure 1- Schematic diagram of ex perimental set up

dried at II 0 ·C for 12 h. The dried N H ,-zeolite powder was calcined at SOO·C for 5 h, where NH,-zeolite got transformed in to H-zeo li te . T he process was repeated thrice to assure almost complete exchange.

The H-zeolites were subsequentl y modified by exchang in g the H' ion by trans iti o n meta l ca ti ons, subsequentl y mod ified the H-zeolites. Required amount of exchangeable metal salt d issolved in di stilled water was mi xed with H-zeolite powder. The solution was heated at 50 ·C for I h under constant stirring, fo llowed by heating unde r tota l retlux conditi on for 17 h at 90C. The catalys t was fil tered, dried , and calcined at SOOC fo r 5 h and the re maining fi ltrate was used for atomi c absorpt ion spec trometer and IC P studies, and degree of exc ha nge (w t pe r ce nt of ca ti o n in zeo lite) was determined. The calcined catalyst was pelleti zed at 10 tJ em' press ure, crushed and sieved to get the des ired size (+1 2- 14 BSS) fo r use in the reactor.

T he cata lys t ac ti v it y tes ts fo r th e di sproporti onati on of to luene were carr ied out in a continuous dow n fl ow reactor, as shown in Figure I . Toluene was fed to the upper pre-heating zone of the reactor by a peri staltic pump, where it came in contac t wi th carrier (ni trogen) gas. After the pre-heating zone the toluene vapours and nitrogen gas were passed th rough the catalys t bed. T he reaction products from the reac tor

came out th rough the post-heating zone of the reactor, which were subsequentl y cooled and condensed. T he react ion produ cts were was a na lyzed by gas- li q ui d chromatograph to determine its compositi on.

The reactor used was made of sil ica glass ( 12mm id), which was kept in an electrically heated furnace. T he temperature of the furn ace was contro lled by a PlD te mperature contro lle r. For the experime ntal runs the reactor was filled with 2g of catalyst mi xed with an equal s ize (-14 +22 BSS) and a n equal amount of qu artz parti cles. The catalyst was acti vated at 500'C in a current of air for I h. The acti vities of the catalysts were measured at different temperatures and under atmospheric pressure. The parameters whi ch were maintained constant during the reac tion are :

Flow rate of to luene Flow rate of nitrogen Wt of catalyst I to luene feed rate (W IF,) Ni trogen/to luene feed ratio (molal)

Results and Discussion

0.4 mL /min 0.40 L /min 8.85 g hI mol of to luene 4.79

Catalyti c properties of a catalyst are function of its c harac te ri s ti cs . The tolue ne di spro port io nati on

.---._-_._--------_ ..

812 J SCT TNDRES VOL 61 OCTOBER 2002

::ll=::~F~~;,~~~--------·-1.25% PbHZSM·5(SAR·32)r-·HY (SAR·3)30:':-~5~"p_~.HY (SA~3_)__ ,

250 300 350 400 450

I

I

II .I

.--~--.---.~---J500 5~O 600 650 7()O

Figure 2-Effect of reaction temperature on toluene conversion

reaction is strongly dependent on the followingcharacteristics of the catalyst:

(i) Acidity and acid site distribution,(ii) Structure and crystallinity,(iii) Composition (silica alumina ratio,

percent ion exchange).

The zeolite samples were characterized by FTIRspectroscopy to estimate the relative strength of Bronstedacid sites (strong, weak, and medium) and Lewis acidsites. X-ray diffraction patterns were obtained to estimatephase formation and per cent crystallinity of the zeolitecrystals. SEM was done to obtain particle size and shape,whereas atomic absorption spectroscopy and inductivelycoupled plasma technique were used for elementalanalysis

The r R structural studies of zeolites were carriedout in r R region of wave number 400-4000 cm', Theband at wave numbers: (i) 3739, 3660, and 3609 cm 'were assigned to weak, medium and strong Bronstedacid sites, respectively, (ii) 1700 crn' to water band,(iii) 800 and 92 per cent, respectively, for Cr, Ni, andPbHZSM-5.

The scanning electron micrograph showed theshape, size, and morphology of zeolite crystallites. Thecrystallite size of the catalyst used varied from 12 to36/1. The shape and morphology of crystallite were seento vary from spherical to cubic shape and from plate typeto rod like structure. HZSM-5 crystallites could be seento have a near spherical shape with plate type structure,

whereas, PbHZSM-5 crystallite showed an irregular graincomposite. Cr and NiHZSM-5 could be seen to possessa plate type structure.

The atomic absorption spectrophotometer andinductively coupled plasma were used to estimate thecomposition of various elements in the zeolite samples.The ion exchanges with the HZSM-5 and HY werealways less then 100 per cent. The actual amount ofexchanged metal in the zeolite was less than the amountof metal used in the metal nitrate solution used forexchange. In each case, a 4 per cent metal solution wastaken for the ion exchange. The typical values of actualexchange have been mentioned against each catalyst,wherever they appear in the text.

The experimental results are presented asvariation of toluene conversion, per cent p-xylene inxylenes product, benzene to xylene ratio, and p-xyleneyield as a function of reaction temperature. Thecatalysts used for the studies include HZSM-5(SAR-19),HZSM-5 (SAR-32), iHZSM-5(SAR-32), PbHZSM-5(SAR-32), HY(SAR-3), and PbHY(SAR-3). The effectsof particle size on percentage toluene conversion andp-xylene yield were also studied using NiHZSM-5(SAR-32) catalyst. The effect of silica-alumina ratioon the performance of the catalyst for the toluenedisproportionation reaction was studied withCrHZSM-5.

Figure 2 presents toluene conversion as afunction of reaction temperature for various catalysts. It

CHAUDHARI e1 a/.: TOLUENE DISPROPORTIONATION REACTION 813

2.5 ,.-------· - - ----------

;?.

-+- HZSM-5 (SAR-19)

-- HZSM-5 (SAR-32)

--- 1.59% Ni HZSM-5 (SAR-32)

2 . - 1.25% Pb HZSM-5 (SAR-32)

--::- HY (SAR-3)

-- 3.95% Pb HY (SAR-3)

0 1.5 . §. ., -.; ·:;:,

"' c:

"' ~ 1 · a.

0.5 .

Temperature (°C)

Figure 3- Effect o f reac ti on te mperature on p-x ylcne yie ld

is observed that the toluene conversion starts at around 400C, which increases graduall y with the increase in temperature, i. e. up to 600"C, and therea fter starts decreasing. PbHY is an exception, where the maximum conversion is obtained at 500·C. The conversion in thi s case decreases sharply at hi gher temperatures. The maximum conversion obtained at 600C was around 35.5 per cent with HZSM-5 (SA R- 19), which was fo llowed by HY (34 per cent), HZSM-5 (SA R-32) ( 12.2 per cent) Ni HZSM-5 (SAR-32) ( I 0.2 per cent) and PbHZSM-5 (SA R-32) ( 10. 1 per cent ). The maximum per cent conversion obta ined w ith PbHY at 500"C was 33.5 per cent.

The reason for sudden drop in the acti vity of PbHY above 500C may be attributed to probable melting of Pb metal at higher temperature, leading to the blockage of the acti ve sites. PbHZSM -5 (SA R-32). however, had max imum conversion tak ing place at 600-c. The reason for this may be due to the lower amount of Pb present in the catalyst in compari son to PbHY.

The higher activities shown by HZSM-5 (SAR­I 9) and HY (SAR-3) in compari son to ZSM-5 (SAR-32) based catalysts, may be attributed to the higher alumina contents of the former catalysts, which , in turn, was

responsible for higher acidity of the catalysts. The decline in the acti vity of most of the catalysts above 600·C is explained on the basis of formati on and deposition of coke on the active centers.

Figure 3 depicts p-xylene yield as a functi on of temperature. p-xylene yield is defined as mol p-xy lene formed per mol toluene fed. It is a combined effect o f per cent toluene conversion as we ll as per cent p-xy lene in xy lene product. The relati ve proporti ons of the three isomers of xy lenes in the xy lenes product were found to shift from the thermodynamic equilibrium values by the use of ion-exchange zeolite catalysts. It is presumed that the exchanging cations having different ionic size control the effective pore mouth size differently. The results for selecti vity ofp-xylene in xy lenes product, using Y -zeolite and HZSM-5 ca talys ts show a va lue c loser to it s equilibrium value of24 per cent. However, the selecti vity is found to increase (to 28-30 per cent) with the add ition of Pb and Ni as exchanged cation on HZSM-5. The p­xy lene y ield, thus obtained was seen to be in order with the per cent toluene conversion. T he exception being Ni and Pb HZSM-5 catalys ts, where the p-xylene yield is found to be higher than its parent HZSM-5, inspire of the later giving higher toluene conversion.

814 J SCI IND RES VOL 61 OCTOBER 2002

16~----------------------- __ ----------------------------------------------~--~

14

12

~ 10.0oSg 8'f••>c:8 6

4

2

__ 0.604mm Ni HZSM-5 (SAR-3ill)--0.955 mm Ni HZSM-5 (SAR-32)__ 1.302mm Ni HZSM-5 (SAR-32)

o+-------~------~,_----~------~----~------~------~------~------~250 300 350 400 450 . 500 700550 600 650

Temperature (0C)

Figure 4--Effect of reaction temperature on conversion of toluene for

1.59per cent NiZSM-5 catalyst having different panicle size

1.4~----------------------------------------------------~------~

1.2

~"0oS 0.8:2"':;'••~ 0.6

~Q.

__ 0.604mm Ni HZSM -5(SAR-32)__ 0.955mm Ni HZSM-5 (SAR-32)__ 1.302mm Ni HZSM -5 (S",~32)

0.4

0.2

02150------~~~O~~~~3~5~O~=====4!OO========45=O~~~~5:0~O------55~0-------6~O-O----~-6-~50-------J700Temperature i°C)

Figure 5-Effect of reaction temperature on p-xylene yield for 1.59 per cent NiZSM-5Catalyst having different particle size

The reason for high p-xylene yield with Ni andPb HZSM-5 is attributed to pore blockage of the zeolitecatalysts resulting, in isomerization of larger size 0- andm-xylene into smaller size p-xylene component, insidethe zeolite pores. The best activity towards the p-xylene

yield was obtained with HZSM-5 (SAR-19), which wasin accordance with the higher per cent tolueneconversion obtained by it. The maximump-xylene yieldobtained was 2.13 per cent with HZSM-5 (SAR-19) at60crC. The p-xylene yield obtained by PbHY (l.76 per

CHAU DH ARI e £ a/.: TOLUENE DI SPROPORTION ATION REACTION 8 15

35 ~-------------------- -------------

~ 0 .s

30

25

l5 20 "§

Cl.> > c 0

~ 15 c Q)

:J

0 1-

10 .

-+- 3.34%Cr HZSM-5(SAR-19)

--- 3. 4%Cr HZSM-5(SAR -32)

· - • - 3.03% Cr HZSM-5 (SAR-43)

Temperature ( ° C)

Figure 6- effect of reacti on temperature on toluene conversion

cent) at 500'C, however, was almost comparable to the highest va lue or 1.79 per cent obtained by HZSM -5 (SA R- 19) at the same temperature. The p-xy lene y ield by PbHY decreases sharpl y above 500'C and reaches to one of the lowest va lues found w ith the other catal ysts.

Mass Balance of" the Che111ica/ Species (on e.ra111plc) Reacti on:

2 (T\J iuenc) ~

I 00 mol toluene with 30pcr cent conversion ~

1( 13 enzene) + I (X y lcncs: o-.m-.p-) + (carbon form at ion due to c rac kin g.

9- 10 per cent + escape of prod uct al ong w ith the carrier N . 8-9per cent) Per cent p -xy lene yie ld at 30per cent to luene( say):

(-) 10.0 mol xy lcnes ~ (-) 10.0 X 0.24 = (-) 2.4 mol p -xy lenc (= 2.4per cent. a va lue closer to that obtained experimentall y).

The resu Its on the effect of size of zeal i te catalyst particles on its performance have been shown in Figure 4 and 5. The percentage toluene con version was found to increase with temperature, giving a maximum at 60(}C for all the catalyst sizes . Three sizes, 0.604, 0.955, and 1.302 mm were used for the activity tests. The results

w ith the smallest size catalys t have shown the highest percentage toluene conversion. which was also reflected identi ca ll y in percentage p-xy lene yield . It may. thus, be observed that effect of particle size has I i ttle effec t on altering per cent p-xy lene in xy lenes product. The higher ac tivity towards per cent toluene conversion with smaller size catal ys t parti cle was attributed to the higher surface area of the catalys t per unit o f its weight.

Figure 6 and 7 present the effect of variati on in sili ca-a lumina rati o of ZSM-5 zeolite catal ys t on their performan ce towa rd s to lu ene di sproporti onati on reacti on. The per cent to luene conversion was found to be inverse ly proporti onal to silica-alumina rati o, which was in accordan ce with the results obtained by the prev ious investi gators. Hi gher al umina content was shown to be responsible for higher acidity or the ca talyst and, therefore, higher conversion. The p -xy lene yield depicted in Figure 7 also showed a similar order, with SA R-19 gi v ing the hi ghest y ield among th e o ther catalys ts. The max imum p-xy lene y ield obtained in th is case was 2.3 per cent, using CrHZSM -5 (SAR- 19) at 600·C A compari son of the magnitude of p -xy lene y ield by HZSM -5 (SA R-19) and CrHZSM-5 (SAR- 19) at 600'C (from Figure 3 and 7) shows an increase in the value due the ion exchange by Cr.

816

2

:: 1.5 0

.s '0 .. ·:;.

" c " ">. l 1

0.5

J SCIIND RES VOL 6 1 OCTOI3ER 2002

; 4 '/:Cr HZSM-~ (SAR~g-} -

1

- .4 % Cr HZSM-5 (SAR-32)

.03% C r HZS M-5 (SAR-43)

0~~===:_ -~-300 .350 400 450 500 -:----5~50 ___ _ 6_,0_0 _ __ 650

Temperature (0 C)

Figure 7-Effect o f reaction temperature on p-xy lene yield

700

Conclusions References

Following conclusion may be drawn from the present studi es:

•

•

•

•

Th e pe rformances of the catalys ts towa rds toluene conversion were in the o rder: HZS M-5 (SAR-19) > HY (SAR-3) > HZSM-5 (SAR-32) .

p -xylene yield was found to increase after the HZSM-5 (SAR-32) was exchanged with Cr, Ni or Pb cations. However maximum p-xylene yield was obtained with HZSM-5 (SAR-19), with a value of2.13 per cent at 600·C.

Smaller sizes of zeolite catalyst particles were favourable to higher to luene conversion as well as higher p-xylene yie ld .

The per cent to luene conversions as well asp­xylene yield increase with increase in alumina content of the zeo lites.

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