Fuel Processing Technology 90 (2009) 1226–1233

Contents lists available at ScienceDirect

Fuel Processing Technology

j ourna l homepage: www.e lsev ie r.com/ locate / fuproc

Effects of steaming-made changes in physicochemical properties of Y-zeolite oncracking of bulky 1,3,5-triisopropylbenzene and coke formation

A. Bazyari a, A.A. Khodadadi a,b, N. Hosseinpour a, Y. Mortazavi a,b,⁎a Catalysis and Nanostructured Materials Laboratory, School of Chemical Engineering, University of Tehran, P.O. Box 11155/4563, Tehran, Iranb Oil and Gas Center of Excellence, University of Tehran, P.O. Box 11155/4563, Tehran, Iran

⁎ Corresponding author. Catalysis and Nanostructureof Chemical Engineering, University of Tehran, P.O. Boxfax: +98 21 6696 7793.

E-mail address: mortazav@ut.ac.ir (Y. Mortazavi).

0378-3820/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.fuproc.2009.06.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 11 January 2009Received in revised form 31 May 2009Accepted 1 June 2009

Keywords:FCCUSY zeoliteCracking catalystPore sizeDiffusion limitationCoke

The effects of acidic properties and structural changes of Y zeolite, produced by steaming, on the zeolitecracking activity, coking tendency and distribution of various products during catalytic conversion of bulky1,3,5-triisopropylbenzene (TIPB) are reported. NaY zeolite with framework Si/Al ratio of 2.4 was synthesizedby a hydrothermal method and ammonium exchanged. The zeolite was dealuminated by a temperature-programmed steaming to form USY1 and USY2 zeolites with framework Si/Al ratio of 8.1 and 12.3respectively. The catalysts were characterized by XRD, XRF, SEM, AAS, NH3–TPD and N2 adsorption–desorption techniques. The samples were in-situ activated at 748 K and evaluated by TIPB cracking at 623 K.The coke content of the catalyst beds was estimated by TPO using an FT-IR gas cell. The results of activitymeasurements reveal that the dealuminated zeolites lead to lower cracking activity initially; while, theyexhibit higher activity at longer times. In addition, a slight modification of the window diameter of Y zeolite,as revealed by pore size distribution analyses, alters the diffusion limitation of the reactant and productsthrough the pores of the zeolite and significantly affects the adsorbent–adsorbate interactions. TPOexperiments show that compared to the precursor zeolite, lower amount of coke is formed on thedealuminated catalysts possessing lower density of acid sites. However, the coke formed on USY samples isheavier than that formed on its precursor Y zeolite. This may be attributed to the larger pores shaped in thedealuminated catalysts which in turn provide suitable places for coke formation and growth.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Fluid catalytic cracking (FCC) of hydrocarbons continues to remaina novel process in the petroleum refining industry for upgradingvacuum distillates and residues to valuable gasoline and diesel fuels[1–4]. Y or ultrastable Y (USY) zeolites are the active and the keycomponents of FCC catalysts which provide the major part of thesurface area and the active sites for catalytic cracking of hydrocarbons[1–6]. Their pore dimensions make them selective as to whichhydrocarbon molecules diffuse where they are exposed to strong acidsites and converted. Steaming of Y zeolite is commonly used as themost preferred dealumination method for preparation of USY zeolites[6–8]. Dealumination of Y zeolite improves its thermal and hydro-thermal stability, hydrophobicity and also modifies its hydrogentransfer activity. These improvements enhance the zeolite selectivityto lighter products and suppress the coke formation tendency of thecatalyst in the course of hydrocarbon cracking reactions. Thehydrothermal dealumination of the zeolite framework proceeds

d Materials Laboratory, School11155/4563, Tehran, Iran. Tel./

l rights reserved.

with hydrolysis of the zeolite Al–O–Si bonds and thereby slightlydestroys its crystal structure through shaping meso-pores [9–11] andleaving aluminum as different forms of extra-framework species(EFAl) in the zeolite structure [12,13]. It is generally accepted thatcracking activity of the zeolite is attributed to its Brønsted aciditywhich catalyzes hydrocarbon cracking reactions via carbenium ionchemistry [14,15]. These active sites are located both in the internalpores and also on the outer surface of the zeolite crystallites. However,majority of the active acid sites are located within the zeolite pores [3].The number and strength of acid sites present in the structure of Yzeolite are the key parameters for controlling the activity andselectivity of the zeolite in FCC reactions [16]. There is not a generalconsensus on the effect of EFAl species on cracking activity of thezeolite among the researchers in the field. Several authors reportednegative effects [17–19]; while, others found the positive [20–23] orsomewhat neutral effects [24] for EFAl species in hydrocarboncracking reactions.

In the regenerator of FCC units, Y-zeolite undergoes severehydrothermal conditions and consequently changes in its physico-chemical properties would bemade. On the other hand, the feedstocksof FCC units have become heavier in the recent years [1,2]. Since themajority of the active sites of the zeolite are within its pores, thecracking of heavier feedstocks tends to be more diffusion controlled.

1227A. Bazyari et al. / Fuel Processing Technology 90 (2009) 1226–1233

Therefore, it is of great scientific interest and commercial value tounderstand and compare performance of Y and USY zeolites incracking of the heavy feedstocks.

Catalyst deactivation via coking is of significant importance incatalytic cracking and numerous studies have been conducted to shedlight on this phenomenon [16,25]. The coke formation has unfavorableeffects on the catalyst activity by covering some parts of the acid sitesand/or partial blocking of the zeolite channels which limits theaccessibility of the reactant molecules to the inner active sites of thecatalyst [16,26,27]. In general higher acid site density significantlypromotes coke formation by hydrogen transfer reactions [1,2,26,27].Unsaturated hydrocarbons are strongly adsorbed on the acid sites ofthe catalyst and become increasingly hydrogen deficient, finallyforming coke via cyclization. Coke decreases the accessible surfacearea by covering the surface and coating the active acid sites of thecatalyst. Therefore, the catalyst pores are blocked with the cokebuildup and active acid sites become coated, resulting in catalystdeactivation via coking. [1,2]. Since FCC facilities utilize largequantities of Y or USY zeolites, small modifications in the deactivationbehavior of the catalyst are of potential interest and economicallyuseful.

The main aim of this work is to study the role of structural,particularly intracrystalline diffusion and acidic properties of the USYzeolites on their cracking activity, product yield and coke formationduring catalytic cracking of bulky 1,3,5-triisopropylbenzene (TIPB)probe molecule.

2. Experimental

2.1. Preparation of NH4NaY zeolite

NaY zeolite was prepared according to the recipe reported for Yzeolite synthesis in the literature [28]. The precursors used forsynthesis of the NaY zeolite are sodium silicate solution (Merck, extrapure), sodium hydroxide (Merck, pellets), sodium aluminate powder(Riedel-de Haёn, technical grade) and distilled water. Followingpreparation of the seed, it was aged at room temperature for 24 h, andthen mixed with the feedstock gel. Thus-prepared overall gel with amolar composition of 4.62Na2O:1Al2O3:10SiO2:180H2O was aged at298 K for 24 h followed by crystallization at 373 K for 6 h. Finally, thesample was washed thoroughly with distilled water to the pH of 7 anddried at 373 K overnight. NH4NaY zeolite with a sodium ion exchangedegree of 70%, as determined by atomic absorption spectroscopy(AAS) analyses, was obtained by exchanging the as synthesized NaYzeolite twice in an aqueous solution of 1 M NH4NO3 (salt fromMerck)at 353 K for 6 h under reflux and stirring conditions. The resultantNH4NaY zeolite was dried at 373 K overnight followed by crushing andsieving to 30–50 mesh size.

2.2. Preparation of USY zeolite

Hydrothermal treatment of the ammonium exchanged zeolite wasperformed in a flow system, composed of a quartz tube reactor with15 mm ID placed in a vertical tubular furnace. 1 g of the catalystsample was placed between two layers of quartz wool in the reactor.Distilled water was injected into the hot zone of the reactor by asyringe pump (Atom, Japan). About 20 cm height of the reactor,upstream of the catalyst bed, was filled with quartz chips of 30–40mesh size to vaporize the injected water and preheat the deal-umination feed stream. Thewater vaporswere dilutedwith an air flowto prepare a continuous stream of 90.0 vol.% steam in air as the feed forthe dealumination experiments. In this series of experiments, atemperature-programmed steaming was employed to prepare USYzeolites. The temperature of the zeolite bed was increased by 3 K/minfrom 298 K up to 873 or 973 K as the final dealumination temperature.The zeolite was steamed for 1 h at the final temperature of 873 and

973 K to prepare USY1 and USY2 samples, respectively. The injectionof water into the quartz bed was begun when the zeolite bedtemperature reached to at 473 K. After 1 h steaming at the finaldealumination temperature, the furnace power supply was turned offand the water injection continued until the zeolite bed temperaturewas dropped to 473 K. The flow of dry air was also stopped when thezeolite bed temperature reached to 373 K. Finally the USY zeoliteswere dried at 373 K overnight.

2.3. Catalyst characterization

The average particle size and morphology of the zeolite particleswere studied by SEM micrographs taken using a CamScan MV2300instrument.

Themicrostructure and the characteristics of the NH4HY, USY1 andUSY2, namely specific surface area (SSA) and pore size distribution(PSD), weremeasured by analyses of nitrogen isotherms collected by aMicromeritics ASAP 2010 apparatus. After degassing the sample(0.1 g) at 673 K for 4 h, the U-shape cell was dipped into liquidnitrogen at 77 K. The amount of N2 desorbed was measured by athermal conductivity detector (TCD). N2 adsorption–desorptionisotherms of the samples were measured at the liquid nitrogentemperature and almost total range of N2/He relative pressure.

The structural changes made by the hydrothermal treatments ofthe samples were evaluated by measuring X-ray diffraction ofNH4NaY, USY1 and USY2 catalysts. The X-ray diffraction data wascollected by a X'Pert Philips diffractometer (CuKα2 radiation,λ=1.54439 Å) in the 2θ range of 5–50° and at a scanning rate of1.5°/min. The relative crystallinity (RC) and unit cell parameters (UCS)of the samples were determined according to ASTM D3906 and ASTMD3942 standards, respectively. The (511), (440), (533) and (555)peaks were used for estimation of the RC and UCS of the samples. Theframework aluminum (FAl) content of the samples was obtained byapplying Sohn's correlation [29] on the estimated unit cell parameters.

The NH4NaY, USY1 and USY2 were ammonium exchanged for twoextra steps in order to remove the remaining sodium ions. Acidity ofNH4Y, USY1 and USY2 catalysts were determined by NH3–TPDtechnique using a Quantachrome CHEMBET-3000 apparatus. 0.15 gof the sample was loaded into the quartz cell of the equipment. Priorto ammonia adsorption, the temperature of the catalyst bed wasincreased by 10 K/min to 748 K under a helium flow, at which thesample was evacuated for 3 h. Adsorption of ammonia was thencarried out at 373 K by a 5.0 vol.% NH3/He gas mixture passed throughthe catalyst bed for 40 min. After flushing the physically-boundammonia at 373 K by a He stream for 1 h, the NH3–TPDwas performedat a constant heating rate of 10 K/min from 373 up to 973 K. Theamount of NH3 desorbed was measured by online monitoring of theeffluent gases with a TCD detector.

2.4. Cracking activity measurement

Catalytic cracking of TIPB on the catalysts was studied by anexperimental setup shown in Fig. 1. A quartz tube microreactor withan ID of 12 mm was used for the cracking experiments. The reactorwas loaded with 0.2 g of the catalyst particles (30–50 mesh size), inwhich the catalysts were in situ activated at 748 K under a 40 sccmflow of dry air for 3 h. Following the catalyst activation, air wasswitched to Argon (140 sccm, 99.999%), then TIPB (Merck, 96+%)wasinjected by the syringe pump into the 20 ml quartz chips placed abovethe catalyst bed to prepare a gas mixture of 5.0 vol.% TIPB/Ar as thefeed for the cracking experiments. The reactor effluents wereinstantaneously analyzed by an online gas chromatograph (GC)equipped with a flame ionization detector (FID) and a 125 cm lengthand 1/8 in. ID stainless steal tube column packed with Silicone Oil SE-30 (25%) on Chromosorb P. The temperature of the GC oven wasincreased by 20 K/min from 393 to 453 K, at which fixed for 9 min. In

Fig. 1. Schematic representation of the experimental setup used for the cracking tests.

Fig. 2. SEM micrograph of the microsized NaY zeolite prepared by the seeding method.

Table 1Textural and structural properties of the parent and dealuminated zeolites.

Sample Final steamingtemperature (K)

UCS (Å) FAl/u.c.a FAl XRDb

(mmol/g)(Si/Al)c RC

(%)eSSA(m2/g)

NH4NaY – 24.76 55.8 4.8 2.44d 100 748USY1 873 24.43 21.0 1.47 8.1 81 630USY2 973 24.37 14.4 0.85 12.3 68 602

a Calculated from UCS using published correlation in Ref. [29].b Al conc.=(%crystallinity/100)(FAl/u.c.)/(11.520 g/mmol) [31].c Framework Si/Al atomic ratio was calculated from Si/Al=(192-FAl)/FAl with the

assumptions that extra framework species contain no silicon but have aluminum atoms.d Determined from both XRF and XRD analyses.e NH4NaY is selected as the reference.

1228 A. Bazyari et al. / Fuel Processing Technology 90 (2009) 1226–1233

order to investigate the possibility of TIPB thermal cracking on thequartz chips placed ahead of the catalyst bed, a series of blankexperiments were done with only quartz chips loaded in the reactor.No TIPB thermal cracking was observed at 623 K, as the temperaturechosen for the catalytic cracking experiments. The data for TIPBthermal cracking has been presented in our previous report [2].

2.5. Temperature-programmed oxidation (TPO)

The amount and refractory properties of the coke formed on eachcatalyst was studied via TPO technique. 0.1 g of each catalyst coked inthe cracking experiment was loaded in a 5 mm ID quartz tube reactorin a tubular furnace. A downward flow of 2.0 vol.% O2/N2 (80 sccm)was passed through the coked catalyst bed as the TPO feed. Heating ofthe reactor was carried out in the furnacewith a ramp rate of 10 K/minfrom room temperature to 1173 K. Infrared spectra (IR) of CO2 and COevolved from the coke oxidation experiment were instantaneouslyrecorded on a Bruker Vector22 spectrometer equipped with a DTGSdetector and a gas cell with KBr windows. The spectrometer was usedin transmission mode with a resolution of 5 cm−1 and at the scanningspeed of 10. Peak areas in the ranges of 2400–2280 cm−1 and 2235–2030 cm−1 are used for CO2 and CO, respectively.

3. Results and discussion

3.1. Catalyst characterization results

The synthesis method described earlier resulted in NaY crystalswith an average particle size of ca. 0.5 µm in the form of irregularpolyhedra as shown in Fig. 2. Sombatchaisak et al. [30] studied theeffect of average particle size on the hydrothermal stability of Y zeolitecrystals. They found that an average particle size of 0.45 µm was theoptimum value which could retain a high percentage of its initialcrystallinity during hydrothermal treatment. It should be noted that0.5 µm is a common size of Y zeolite crystals used in FCC catalysts [2].

Table 1 summarizes the properties, estimated from XRD patterns(Fig. 3), and SSA of the starting NH4NaY and USY zeolites prepared bytemperature-programmed steaming. The results show a decline in thecrystallinity of USY1 and USY2 samples compared to that of NH4NaYzeolite. For the hydrothermal conditions used in the dealuminationexperiments, an increase in the final steaming temperature leads to alower crystallinity for the final sample. According to the widelyaccepted mechanism for hydrothermal dealumination [7], the loss ofcrystallinity could be attributed to vacancies left by aluminumremoval from the lattice which are not filled by silicon atoms

migrating in the zeolite structure. Parts of the Y zeolite crystals,where aluminum concentration is higher, suffer more structuredamage which could result in formation of mesopores by joining thevacancies formed during the steaming process. The higher thesteaming temperature, the more acid sites form initially andsubsequently the more dealumination occurs via the mechanismproposed for hydrothermal dealumination of Y zeolite [9–13].Comparing the SSA of the samples reported in Table 1, it confirmsthe decrease in crystallinity as a result of dealumination. Lower SSA forthe dealuminated samples may be ascribed to the partially collapsingof the zeolite crystalline structure leading to loss of small microporesand shaping super-micropores or mesopores in the structure. Also, theUCS decreased as a result of the dealumination. The zeolite UCSchanged from 24.76 Å for the precursor zeolite to 24.37 Å for USY2sample. It is generally accepted that the contraction of the unit cell isascribed to substitution of longer Al–O bonds (1.73 Å) with the shorterSi–O bonds (1.61 Å) [32] which results in larger pore apertures andcavities. A comparison between XRD diffractograms, presented inFig. 3, proves that with increasing the steaming temperature, eachspecific peak shifts to higher 2θ angles, in line with UCS contraction.According to the baseline of the XRD diffractograms, it is supposedthat NH4NaY is 100% crystalline and all of its Al and Si atoms arelocated in the zeolite framework. X-ray fluorescence (XRF) was usedto determine framework Si/Al atomic ratio of NH4NaY in order to finda reliable correlation for estimating framework Si/Al atomic ratio ofthe dealuminated samples from XRD data. Sohn's correlation, whichwas obtained based on NH4/Na, H, and NH4 exchanged forms of Yzeolite and by considering the extra-framework aluminum [33],showed the best agreement with our results.

Fig. 3. XRD patterns of (a) NH4NaY; (b) USY1; (c) USY2 samples.

1229A. Bazyari et al. / Fuel Processing Technology 90 (2009) 1226–1233

In the N2 isothermmeasurements, all three NH4NaY, USY1 and USY2zeolites showed typical type-I profiles in the Brunauer et al. classifica-tion, characteristic of microporous structures (Fig. 4). After steamingtreatments, USY zeolites exhibited hysteresis in their isotherm profiles.Two distinct steps can be seen in the isotherms. The first one, located atintermediate partial pressures (0.4bP/P0b0.5), is due to the capillarycondensation of N2 inside the intra-particle micro/meso pores. Thesecond step, at a high relative pressure (P/P0N0.85), corresponds to thefilling of the large inter-particle cage-like pores. As compared to theparent Y zeolite, it is observed that theUSY catalysts have a reduced totalpore volume and their N2 isotherms are paralleled after microporefilling, indicating little change in themicropore structure of theY zeolite.Inset of Fig. 4 illustrates the PSD obtained from desorption branch of thenitrogen isotherms based on BJH method (Inset is the intra-particle'sPSD but the distribution of inter-particle pores is not shown here). It canbe seen that for untreated zeolite there is one peak maxima related tointera-pore at 1.21 nm that is equal to supercage diameter of Y-zeoliteunit cell. After thedealuminationprocess, thepick shifts to higher valuesat 1.64 and 1.85 nm for USY1 and USY2, respectively. In addition, it isshown that for USY2whichendured themost sever steaming conditionsthere is one more peak maxima related to intera-pore at 2.71 nm. Theresults of N2 adsorption–desorption isotherms, consistent with XRDanalyses, reveal that the pore window and average pore diameter of thezeolite are enlarged during the dealumination process.

Na content of the samples was found negligible after four times ofthe ammonium exchange process, demonstrating the effectiveness ofthe method used for sodium removal from the zeolite structures.

NH3–TPD technique provides information on the amount andstrength of acid sites by using NH3 as a basic probe molecule. The peak

Fig. 4. N2 adsorption–desorption isotherm of (■) HY; (Δ) USY1; (◆) USY2 (inset, poresize distribution of the zeolites structures).

area under the NH3–TPD profile represents the amount of acid sites;while, the peak positions and shapes correspond to the strength of theacid sites. The NH3–TPD curves of the catalysts are presented in Fig. 5and the results are summarized in Table 2. The peaks in the NH3–TPDprofiles are classified and attributed to three types of acid sites withdifferent acid strengths, i.e., weak, medium and strong, according tothe temperature of their peak maxima. The first desorption peak at ca.473 K is assigned to the weak acid sites, whereas, the second peakbetween 569 and 611 K corresponds to moderate strength, and above648 K is set for the strong acid sites [34]. For the HY sample, there arethree peaks at 469, 569 and 648 K, while these peaks are shifted tohigher temperatures, i.e. 471, 600, 688, 886 K, for USY1 and 473, 611,733, 875 K for USY2 catalyst. The low intensity and broadness of theNH3–TPD profiles of USY1 and USY2 samples indicate that thehydrothermal dealumination process results in catalysts havinglower amounts yet stronger acid sites compared to the parent Yzeolite. It is generally accepted that acid strength is a function ofdegree of isolation of acid sites. The acid site strength is increasedwithdecreasing the number of Al atoms in next nearest neighbor (NNN)positions of Al atoms in the zeolite framework [32]. Furthermore,another postulate suggests the synergy between Brønsted acid sitesand the Lewis acidity of the extra framework aluminum (EFAl)species, formed during the hydrothermal dealumination, as analternative explanation for the formation of strong Brønsted acidsites in the USY catalysts [35,36]. Considering the frameworkaluminum (FAl) concentration, Table 1, and the total amount of NH3

desorbed from HY, Table 2, it is found that they have 1:1 proportion. Alittle deviation form 1:1 ratio, i.e. 1:1.06, may be resulted from self-steaming of Y zeolite during the in situ activation period prior to TPDexperiment. The self-steaming process can cause a slight decline inthe crystallinity of the zeolite. As compared to their FAl concentration,higher amounts of ammonia adsorbed on USY1 and USY2. This may beattributed to the Lewis acidity of extra-framework Al-containingspecies present in the structure of USY1 and USY2 samples.

3.2. 1,3,5-TIPB cracking activity and product yield

3.2.1. 1,3,5-TIPB conversionIn order to study the effect of diffusion on the activity and the

selectivity of the catalysts, cracking experiments of a bulky probemolecule, i.e. TIPB, were conducted. The kinetic diameter of TIPBmolecules is 9.4 Å, larger than the opening of the micropores of Yzeolite. Thus TIPB molecules have a limited access into the microporesof Y zeolite crystals.

Fig. 6 presents TIPB conversion on HY, USY1 and USY2 samples at623 K as a function of the catalysts time on stream. It is observed thatfor all the catalysts, catalytic activity is reduced with time on stream,decreasing from a relatively high value at the beginning to a residual

Fig. 5. NH3–TPD spectra of (a) HY; (b) USY1; (c) USY2 after 3 h activation at 475 °C, NH3

adsorption at 100 °C by 20 sccm of 5.0 vol.% ammonia in He, NH3 desorption at 10 °C/min.

Table 2Acidic properties of the catalysts determined by NH3–TPD technique.

Sample Acidity (mmol NH3/g-catalyst) (±0.06)

Total acid Weak acid Medium acid Strong acid

HY 4.50 1.45 (32%) 1.93 (43%) 1.12 (25%)USY1 1.92 0.62 (32%) 0.43 (22%) 0.87 (46%)USY2 1.23 0.64 (52%) 0.36 (29%) 0.23 (19%)

Fig. 7. Dependence of the 1,3,5-TIPB cracking activity of the samples on their latticealuminum content: (◆) HY; (▲) USY1; (■) USY2.

1230 A. Bazyari et al. / Fuel Processing Technology 90 (2009) 1226–1233

activity in a quasi-steady state at the end of the experiment. Similarresults for cumene cracking activity of Y zeolite have been reported byFleisch et al [37]. At 3 min, The TIPB cracking activities of the catalystsare decreased in the order of HYNUSY1NUSY2; while, at higher timeson stream, the USY samples are more active than the parent Y zeolitecatalyst. The untreated zeolite with about 56 framework Al/unit cell(FAl/u.c.) exhibits the highest initial activity but deactivates rapidlyvia coking. However lower deactivation rates are observed for thedealuminated USY zeolites possessing 21 and 14.4 FAl/u.c.

The conversion of TIPB, at 3 min time on stream, on the catalysts asa function of their FAl/u.c., i.e. the sources of Brønsted acidity, ispresented in Fig. 7. It is observed that activity increases linearly withFAl/u.c. in the range of 14.4–55.8 Al/u.c. This linear relationshipimplies that the turn over frequency (TOF) based on FAl is constant.Since the percentage of acid sites with specific strengths are differentin the structure of HY, USY1 and USY2 catalysts, it seems that thecracking of TIPB is readily proceed even on weak acid sites; inagreement with other reports in the literature [38]. TIPB with thekinetic diameter of 9.4 Å can diffuse seldom into the highly crystallineY zeolite structure where most of the acid sites are located. Thus theresults imply that pre-cracking of TIPB on the surface acid sites of thezeolite crystals and in turn diffusion of the products into the zeolitepore structure is the reason of the highest initial TIPB cracking activityof the HY sample. It should be noted that the size of pore openings of Yzeolite is about 7.4 Å. USY zeolites possess larger pore openings,mesopores, crevices and fissures on the external surfacewhich formedduring dealumination process. These features facilitate the diffusionand thereby cracking of TIPB molecules on the acid sites of the USYcatalysts. Consequently, the difference in the initial activity of thecatalysts is quite expected since cracking occurs on protonic sitesassociated with FAl atoms, and the more the FAl the more the initialactivity would be. On the other hand, the SSA of untreated catalyst ishigher than USY catalysts leading to a higher accessible surface, for theavailable hydrocarbon molecules, which is a central parameter for thecracking reactions.

The difference in the deactivation behavior of the samples can beexplained by the extent of coking of the catalysts. The Al-rich catalystlays downmore coke rather than the dealuminated zeolites as a result ofits higher acid site density. Cracking products, Figs. 8–12, with lowermolecular weights having lesser diffusion limitations are considered tobe themain sources of the coke formation. Thesemolecules readily formion radicals with acid sites of the catalyst, polymerize with other

Fig. 6. Conversion of 1,3,5-TIPB at 623 K versus time: (◆) HY; (▲) USY1; (■) USY2,Amount of catalyst=0.2 g, TiPB residence time of about 0.4 s.

unsaturated hydrocarbons and then dehydrogenate to aggregates ofcoke [1]. Coke, upon formation, transfers hydrogen to gas phase and/orits adjacent adsorbed components on the high density acid sites. Upondealumination, some parts of Y zeolite acid sites are removed from thestructure, resulting in lower acid site density of the USY samples ratherthan that of the Y zeolite catalyst. Hydrogen transfer reactions arebimolecular and involve components on adjacent acid sites. Thus, if sitedensity is reduced, the hydrogen transfer is considerably inhibited andless coke results [1,2]. This leads to lower deactivation rates for the USYcatalysts compared to the parent Y zeolite. Despite the initial times onstream, at which diffusion limitations do not have any significant role incatalytic activity of the untreated HY zeolite, it seems that, at longertimes, diffusion is the limiting factor. The cokedepositedprogressively inthe channels of the zeolite decreases the free spaces next to its acid sites,leading to a decrease in the effective channel size and disfavoringreactions involving large molecules [39].

3.2.2. Products yieldThe main products of TIPB cracking are propylene, benzene,

cumene, 1,3-DIPB and 1,4-DIPB presented in most of the reports oncatalytic cracking of TIPB [1,2,40,41]. The distribution of TIPB crackingproducts confirms the domination of the three-step series reactionssuggested by Mahgoub et al. [40], i.e. (a) dealkylation of TIPB to 1,3-DIPB, (b) dealkylation of 1,3-DIPB to cumene and (c) dealkylation ofcumene to benzene. In addition to these main reactions there are sidereactions such as isomerizatin of 1,3-DIPB to 1,4-DIPB and hydrogentransfer reactions.

Fig. 9 illustrates the 1,3-DIPB yield as a function of the catalyststime on stream. Considering Figs. 6 and 9, it is evident that the yield of1,3-DIPB is almost inversely proportional to TIPB conversion. DIPB isthe heaviest component produced during TIPB cracking. Therefore,higher yield of 1,3-DIPB is the characteristic of a catalyst having loweractivity for TIPB cracking to lighter products, i.e., cumene and if anybenzene. At the beginning of cracking, where the activity of the HYzeolite is high, the yield of 1,3-DIPB product is low due to theaccessibility of 1,3-DIPB to inner acid sites of the zeolite leading to

Fig. 8. Propylene yield at 623 K versus time: (◆) HY; (▲) USY1; (■) USY2, Amount ofcatalyst=0.2 g, TiPB residence time of about 0.4 s.

Fig. 9. 1,3-DIPB yield at 623 K versus time: (◆) HY; (▲) USY1; (■) USY2, Amount ofcatalyst=0.2 g, TiPB residence time of about 0.4 s.

Fig. 11. Benzene yield at 623 K versus time: (◆) HY; (▲) USY1; (■) USY2, Amount ofcatalyst=0.2 g, TiPB residence time of about 0.4 s.

1231A. Bazyari et al. / Fuel Processing Technology 90 (2009) 1226–1233

more chance for cracking to cumene. However, the yield of 1,3-DIPBincreases steadily due to the diffusion limitations and/or poisoning ofsurface acid sites occurred as a result of coking of the catalysts. For HYwith the smallest pore openings and highest acid site density, cokeformation affects the cracking of 1,3-DIPB more than the deal-uminated catalysts. In addition, the super-micropores, mesopores,crevices and fissures, formed during the dealumination process, makethe diffusion of TIPB and/or 1,3-DIPB into the pore structure of thecatalysts easier, resulting in lower yield of 1,3-DIPB on the deal-uminated catalysts as compared to that on the untreated HY zeolite.

Fig.10 shows the cumene yield versus the catalysts time on stream.At first minutes of TIPB cracking, the HY sample has the highest andlowest yield for cumene and 1,3-DIPB, respectively. At longer times,the USY2 sample shows the maximum yield of cumene with theconcurrent minimum yield for 1,3-DIPB. These phenomena can beexplained in analogy to what was mentioned for the progressivedeactivation of the catalysts via coking. Effects of the coke formationon the surface acid sites and diffusion of components into the catalystsare not significant at the first minutes of cracking, resulting in themaximum cumene yield on the HY sample possessing the highestdensity of acid sites. However, at longer times, the coke builds up onthe surface and diffusion of components become limited; ultimatelyleading to lower yield of cumene on the HY zeolite with smaller poredimensions compared to the USY catalysts.

Benzene, with the kinetic diameter of 4.9 Å, is produced in thethird step of dealkylation reactions proceeding in TIPB cracking. All ofthe catalysts exhibit low benzene yields which sharply decline withtime on stream, as shown in Fig. 11. The low yield of benzene may beascribed to the low cracking temperature of 623 K, as previouslyreported by Mahgoub et al. [40]. The HY catalyst shows the highestbenzene yield, concomitant with the acid sites of the catalyst. Thedifference between the benzene yields of the samples is diminishedwith the catalysts time on stream. This implies that the diffusionlimitations become significant as a result of coking of the catalysts.

Considering Figs. 9–11 collectively, it is observed that the intersectionof the curves representing the yield of 1,3-DIPB, cumene and benzeneon

Fig. 10. Cumene yield at 623 K versus time: (◆) HY; (▲) USY1; (■) USY2, Amount ofcatalyst=0.2 g, TiPB residence time of about 0.4 s.

HY, USY1 and USY2 shifts to longer times for the products with smallerkinetic diameters. The intersection of the catalysts yields is respectivelyat 7, 39, and longer than 50min for 1,3-DIPB, cumene and benzene. Theproduction of the components with larger kinetic diameters isaccompanied by penetration of their heavier precursors into the poresof the catalysts and departure of the products from the catalyst surfaceby diffusing out of the pores. For instance, the only precursor of 1,3-DIPBis TIPB with the largest kinetic diameter. Thus, the diffusion limitationaffects more rapidly the penetration of TIPB molecules into the pores ofHY zeolite rather than TIPB diffusion into the pores of USY samples. Inaddition, the diffusion of 1,3-DIPB, as a product of TIPB cracking, out ofthe catalyst pores is quickly affected by the blockage of the poresoccurred by the coke depositing on the catalysts. In TIPB crackingexperiments, three precursors for the production of benzene are TIPB,1,3-DIPB and cumene. Therefore, the effects of the diffusion limitationhave not a fast impact on the benzene yield since 1,3-DIPB and cumenehave smaller kinetic diameters compared toTIPB. Benzene itself has alsoa small kinetic diameter, thus upon formation, it easily leaves the porestructure of the catalysts.

The yield of 1,4-DIPB of the samples versus the catalysts time onstream is presented in Fig. 12. 1,4-DIPB has the lowest yield among theproducts of TIPB cracking. 1,4-DIPB is produced from isomerization of1,3-DIPB, as a side reaction; therefore, it is not a direct product of TIPBcracking [2]. Linear 1,4-DiPB isomer has a high diffusion rate throughpores of the catalysts and a more chance for cracking to cumene. Thisresults in the low yield of 1,4-DiPB on the catalyst samples.

The results presented for the products yields imply that in aspecified reaction condition, the extent of the reaction steps towardsformation of different products depends upon the catalyst structuralcharacteristics and composition.

3.3. Coke characterization

Coke has unfavorable effects on the catalyst activity and selectivityvia covering the acid sites and blocking the pores, which limit theaccessibility of the reactant molecules to the active sites of thecatalyst. It is worth noting that the color of the USY catalysts was dark

Fig. 12.1,4-DIPB yield as at 623 K versus time: (◆) HY; (▲) USY1; (■) USY2, Amount ofcatalyst=0.2 g, TiPB residence time of about 0.4 s.

Fig. 13. CO2 evolution during TPO of the coked catalysts in 2.0 vol.% O2/N2 feed by theheating rate of 10 K/min: (◆) HY; (Δ) USY1; (■) USY2.

Table 3Coke content and CO2 and CO evolution in TPO of the coked catalysts.

Sample HY USY1 USY2

Total CO evolution (μmole/g-catalyst) 4233.9 3335.6 3362.6Total CO2 evolution (μmole/g-catalyst) 7057.5 3532.4 2519.2Carbon content (wt.%) 13.55 8.24 7.06

1232 A. Bazyari et al. / Fuel Processing Technology 90 (2009) 1226–1233

brown after the cracking experiments, in contrast to the HY zeolitecatalyst which was black due to the excessive coke deposition.

CO2 and CO evolutions during TPO of the coked catalysts arerespectively presented in Figs. 13 and 14 and the results aresummarized in Table 3. Comparing the areas under the peaks for theHY, USY1 and USY2 samples, it is evident that the HY catalyst has thehighest amount of coke. The coke content of USY1 andUSY2 samples isrespectively diminished by about 39 and 48% compared to that of theHY catalyst. Thus, the dealumination of the catalysts results information of lower amounts of coke. Combustion of saturated cokecomponents occurs up to 823 K, beyond which highly aromatic orunsaturated coke compounds are oxidized [42]. It is well establishedthat olefinic and polyaromatic hydrocarbons have a high tendency toform coke agglomerates. The higher acid site density of the HY catalystcan explain its higher coke formation tendency. The USY2 samplewiththe lowest acid site density has the lowest amount of coke. In addition,CO evolution during TPO of coked USY1 and USY2 catalysts is about20% lower than that of the coked HY sample. Therefore, during theregeneration of the dealuminated samples with lesser coke contents,lower quantities of CO are released. CO is formed during cokecombustion in dense phase of the regenerator of FCC unites wherelean air is available. Subsequent oxidation of CO in dilute phase of theregenerators is a problem which can cause a significant temperaturerise and provide destructive effects on cyclones and the regeneratordownstream pipelines [1,2]. The temperature of the maxima in TPOprofiles of the HY, USY1 and USY2 samples is at 863, 893 and 903 Krespectively, arisen from the nature of the coke formed on thecatalysts. This indicates that increasing the degree of dealumination ofthe catalysts results in formation of lesser amount but heavier cokewith lower H/C ratio. Coke formation is a shape selective process andis significantly affected by the catalyst geometry. The mesopores andsuper-micropores shaped in the structure of the USY catalysts provideenough spaces for coke agglomerates to become increasingly hydro-gen deficient and form larger aggregates of coke via cyclizationreactions. Instead, in micropores of crystalline HY zeolite the space is

Fig. 14. CO evolution during TPO of the coked catalysts in 2.0 vol.% O2/N2 feed by theheating rate of 10 K/min: (◆) HY; (Δ) USY1; (■) USY2.

more confined. This slightly inhibits cyclization reactions to continueand thereby results in lighter coke compounds compared to that onthe USY samples.

4. Conclusions

The effects of structural changes of Y zeolite, produced by steaming,on the zeolite cracking activity, coking tendency and distribution ofvarious products during catalytic conversion of bulky 1,3,5-triisopro-pylbenzene (TIPB) were studied. Y zeolite surface acidity and porestructure play important roles in catalyst activity for cracking of bulkyhydrocarbon molecules. In the absence of diffusion limitations, i.e. atinitial period of cracking, a linear relationship between the crackingactivityand the numberof frameworkAl atomspresent in theunit cell ofthe zeolites is observed. Turn over frequency (TOF) of the parent HYzeolite and the dealuminated ultra stable Y (USY) zeolites is similar,suggesting thatweak acid sites are sufficient fordealkylation of TIPB. Theactivities of the USY catalysts for TIPB cracking decrease slowly withtime on streamwhile a higher deactivation pace is observed for the HYzeolite. It is the combination of acid site density and the zeolite porestructure which leads to the slower deactivation rate for the USYsamples. As a result of the coking of the catalysts, the yield ofdiisopropylbenzenes on the catalysts increases with time on stream atthe expense of lower yield for deep cracking products, i.e. cumene andbenzene. Furthermore, the dealumination of Y zeolite results information of lower amounts but heavier coke components on the USYcatalysts. The mesopores and super-micropores shaped in the structureof the USY catalysts provide enough spaces for coke agglomerates tofollow cyclization and form heavier aggregates of coke.

Acknowledgement

Partial financial support by the National Iranian Oil Refining &Products Distribution Company is acknowledged.

References

[1] N. Hosseinpour, A.A. Khodadadi, Y. Mortazavi, A. Bazyari, Nano-ceria–zirconiapromoter effects on enhanced coke combustion and oxidation of CO formed inregeneration of silica–alumina coked during cracking of triisopropylbenzene, Appl.Catal. A (2008), doi:10.1016/j.apcata.2008.10.051.

[2] N. Hosseinpour, Y. Mortazavi, A. Bazyari, A.A. Khodadadi, Synergetic effects ofY-zeolite and amorphous silica-alumina as main FCC catalyst components ontriisopropylbenzene cracking and coke formation, Fuel Process. Technol. (2008),doi:10.1016/j.fuproc.2008.08.013.

[3] G. Tonetto, J. Atias, H. de Lasa, FCC catalysts with different zeolite crystallite sizes:acidity, structural properties and reactivity, Appl. Catal. A 270 (2004) 9–25.

[4] H.H. Kung, B.A. Williams, S.M. Babitz, J.T. Miller, R.Q. Snurr, Towards understandingthe enhanced cracking activity of steamed Y zeolites, Catal. Today 52 (1999) 91–98.

[5] B.A. Williams, S.M. Babitz, J.T. Miller, R.Q. Snurr, H.H. Kung, The roles of acidstrength and pore diffusion in the enhanced cracking activity of steamed Yzeolites, Appl. Catal. A 177 (1999) 161–175.

[6] S. Al-Khattaf, The influence of Y-zeolite unit cell size on the performance of FCCcatalysts during gas oil catalytic cracking, Appl. Catal. A 231 (2002) 293–306.

[7] Q.L. Wang, G. Giannetto, M. Torrealba, G. Perot, C. Kappenstein, M. Guisnet,Dealumination of zeolites II. Kinetic study of the dealumination by hydrothermaltreatment of a NH4NaY zeolite, J. Catal. 130 (1991) 459–470.

[8] Z. Yan, D. Ma, J. Zhuang, X. Liu, X. Liu, X. Han, et al., On the acid-dealumination ofUSY zeolite: a solid state NMR investigation, J. Mol. Catal. A 194 (2003) 153–167.

[9] F. Hernández-Beltrán, J.C. Moreno-Mayorga, M.L. Guzmán-Castillo, J. Navarrete-Bolaños, M. González González, B.E. Handy, Dealumination–aging pattern ofREUSY zeolites contained in fluid cracking catalysts, Appl. Catal. A 240 (2003)41–51.

1233A. Bazyari et al. / Fuel Processing Technology 90 (2009) 1226–1233

[10] R. Dutartre, L.C. de Ménorval, F. Di Renzo, D. McQueen, F. Fajula, P. Schulz,Mesopore formation during steam dealumination of zeolites: influence of initialaluminum content and crystal size, Microporous Mater. 6 (1996) 311–320.

[11] A. Zukal, V. Patzelová, U. Lohse, Secondary porous structure of dealuminated Yzeolites, Zeolites 6 (1986) 133–136.

[12] K.U.Gore,A. Abraham, S.G.Hegde, R. Kumar, J.P. Amoureux, S.Ganapathy, 29Si and27AlMAS/3Q-MAS NMR studies of high silica USY zeolites, J. Phys. Chem. B 106 (2002)6115–6120.

[13] A. Samoson, E. Lippmaa, G. Engelhardt, U. Lohse, HG. Jerschkewitz, Quantitativehigh-resolution 27Al NMR: Tetrahedral non-framework aluminium in hydrother-mally treated zeolites, Chem. Phys. Lett. 134 (1987) 589–592.

[14] B.C. Gates, Catalytic chemistry, Wiley, New York, 1992.[15] R. Sadeghbeigi, Fluid catalytic cracking handbook, second ed., Gulf Professional

Publishing, Houston TX, 2000.[16] S.K. Sahoo, N. Viswanadham, N. Ray, J.K. Gupya, I.D. Singh, Studies on acidity,

activity and coke deactivation of ZSM-5 during n-heptane aromatization, Appl.Catal. 205 (2001) 1–10.

[17] E. Jacquinot, F. Raatz, A. Macedo, C. Marcilly, in: H.G. Karge, J. Weitkamp (Eds.),Zeolites as catalysts, sorbents and detergent builders: Studies in surface scienceand catalysis, vol. 46, Elsevier, Amsterdam, 1989, p. 115.

[18] E.H. van Broekhoven, S. Daamen, R.G. Smeink, H. Wijngaards, J. Nieman, in: P.A.Jacobs, R.A. van Santen (Eds.), Zeolites, facts, figures, future, Elsevier, Amsterdam,1989, p. 1291.

[19] V. Mavrodinova, V. Penchev, U. Lohse, T. Gross, Factors influencing the conversionsof alkylaromatic hydrocarbons on high-silica zeolites: part II. Presence ofextralattice Al, Zeolites 9 (1989) 203–207.

[20] A. Corma, in: P.A. Jacobs, R.A. van Santen (Eds.), Zeolites, facts, figures, future:Studies in surface science and catalysis, vol. 49, Elsevier, Amsterdam, 1989, p. 49.

[21] S.W. Addison, S. Cartlidge, D.A. Harding, G. McElhiney, Role of zeolite non-framework aluminum in catalytic cracking, Appl. Catal. 45 (1988) 307–323.

[22] R.J. Pellet, C. Scott Blackwell, J.A. Rabo, Catalytic cracking studies and character-ization of steamed Y and LZ-210 zeolites, J. Catal. 114 (1988) 71–89.

[23] L. Kubelkova, S. Beran, A. Malecka, V.M. Mastikhin, Acidity of modified Y zeolites:effect of nonskeletal Al, formed by hydrothermal treatment, dealumination withSiCl4, and cationic exchange with Al, Zeolites 9 (1989) 12–17.

[24] S.M. Babitz, M.A. Kuehne, H.H. Kung, J.T. Miller, Role of Lewis acidity in thedeactivation of USY zeolites during 2-Methypentane cracking, Ind. Eng. Chem. Res.36 (1997) 3027–3031.

[25] J. Scherzer, in: J.S. Magee, M.M. Mitchell (Eds.), Fluid catalytic cracking: Scienceand technology, Elsevier, Amsterdam, 1993.

[26] M.F. Reyniers, Y. Tang, G.B. Marin, Influence of coke formation on the conversion ofhydrocarbons: II. i-Butene on HY-zeolites, Appl. Catal. 202 (2000) 65–80.

[27] B. Paweewan, P.J. Barrie, L.F. Glaggen, Coking during ethene conversion onultrastable Y zeolite, Appl. Catal. A 167 (1998) 353–362.

[28] H. Robson, Verified synthesis of zeolitic materials, Elsevier, Amsterdam, 2001,pp. 156–158.

[29] J.R. Sohn, S.J. DeCanio, P.O. Fritz, J.H. Lunsford, Acid catalysis by dealuminatedzeolite Y. 2. The roles of aluminum, J. Phys. Chem. 90 (1986) 4847–4851.

[30] S. Sombatchaisak, P. Praserthdam, C. Chaisuk, J. Panpranot, An alternativecorrelation equation between particle size and structure stability of H-Y zeoliteunder hydrothermal treatment conditions, J. Ind. Eng. Chem. Res. 43 (2004)4066–4072.

[31] M.A. Kuehne, S.M. Babitz, H.H. Kung, J.T. Miller, Effect of framework Al content onHY acidity and cracking activity, Appl. Catal. A 166 (1998) 293–299.

[32] H.W. Kouwenhoven, B. de Kroes, in: H. van Bekkum, E.M. Flanigen, P.A. Jacobs, J.C.Jansen (Eds.), Introduction to zeolite science and practice: Studies in surfacescience and catalysis, vol. 137, Elsevier, Amsterdam, 2001, pp. 694–697.

[33] J.A. Kaduk, J. Faber, Crystal structure of zeolite Y as a function of ion exchange,Rigaku J. 12 (1995) 14–34.

[34] Q. Tan, X. Bao, T. Song, Y. Fan, G. Shi, B.S.C. Liu, et al., Synthesis, characterization,and catalytic properties of hydrothermally stable macro-meso-micro-porouscomposite materials synthesized via in situ assembly of preformed zeolite Ynanoclusters on kaolin, J. Catal. 251 (2007) 69–79.

[35] J.H. Lunsford, Surface interactions of NaY and decationated Y zeolites with nitricoxide as determined by electron paramagnetic resonance spectroscopy, J. Phys.Chem. 72 (1968) 4163–4168.

[36] P.D. Hopkins, Cracking activity of some synthetic zeolites and the nature of theactive sites, J. Catal. 12 (1968) 325–334.

[37] T.H. Fleisch, B.L. Meyers, G.J. Ray, J.B. Hall, C.L. Marshall, Hydrothermal deal-umination of faujasites, J. Catal. 99 (1986) 117–125.

[38] N. Katada, Y. Kageyama, K. Takahara, T. Kanai, H.A. Begum, M. Niwa, Acidicproperty of modified ultra stable Y zeolite: increase in catalytic activity for alkanecracking by treatment with ethylenediaminetetraacetic acid salt, J. Mol. Catal. A211 (2004) 119–130.

[39] C. Costa, J.M. Lopes, F. Lemos, F. Ramôa Ribeiro, Activity–acidity relationship inzeolite Y Part 1. Transformation of light olefins, J. Mol. Catal. A 144 (1999) 207–220.

[40] A. Mahgoub, S. Al-Khattaf, Catalytic Cracking of hydrocarbons in a riser simulator:the effect of catalyst accessibility and acidity, Energy Fuels 19 (2005) 329–338.

[41] M. Falco, E. Morgado, N. Amadeo, U. Sedran, Accessibility in alumina matrices ofFCC catalysts, Appl. Catal. A 315 (2006) 29–34.

[42] C. Li, T.C. Brown, Temperature-programmed oxidation of coke deposited by 1-octeneon cracking catalysts, Energy Fuels 13 (1999) 888–894.