1-s2.0-S0926860X14007698-main

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ARTICLE IN PRESSG ModelAPCATA-15156; No. of Pages 11Applied Catalysis A: General xxx (2015) xxxxxx

Contents lists available at ScienceDirect

Applied Catalysis A: General

jou rn al hom ep age: www.elsev ier .com/ locate /apcata

Catalytic cracking of n-hexane for producing propon MCM

Yong Wa ndoChemical Resou 226-8

a r t i c l

Article history:Received 14 SeReceived in reAccepted 10 DAvailable onlin

Keywords:Catalytic crackDealuminationLewis acid site

r proH-MC

propith am

propoun-22

andd a ca

1. Introduction

Light alkenes, in particular propylene, which are mainly sup-plied as a by-product of ethylene production through the thermalcracking ofin chemicalenergy intecially the Therefore, cing attentiocatalysts givmation of lopartly via tsical bimolecracking, thlower tempCO2 emissio

The cataane, heptancatalysts asformance omany repor

CorresponE-mail add

[email protected]

to allow discussion on the reaction mechanism and/or the effects ofstructure of zeolites [312]. However, there are only a few papersdealing with the catalysts working at high temperatures above873 K which is necessary in order to obtain high alkene yields.

http://dx.doi.o0926-860X/ this article in press as: Y. Wang, et al., Appl. Catal. A: Gen. (2015), http://dx.doi.org/10.1016/j.apcata.2014.12.018

naphtha, are acquiring more and more signicance industry. It is known that the thermal cracking is annsive process, where the product distribution, espe-ratio of propylene to ethylene, is hard to control.atalytic cracking of naphtha has been drawing increas-n [1]. The catalytic cracking of alkanes over acidic zeolitees a high propylene/ethylene ratio, since the transfor-ng-chain alkanes to short-chain alkenes occurs at leasthe carbenium ion/-scission mechanism, i.e. the clas-cular cracking [2]. Compared with the thermal steame catalytic cracking of naphtha which is carried out ateratures would consume 20% less energy and reducen by approximately 20% [1].lytic cracking of light alkanes, such as pentane, hex-e and octane, has been studied over various zeolite

a test reaction of naphtha cracking to clarify the per-f catalysts and the reaction mechanism [3]. There arets on cracking of alkanes at relatively low temperatures

ding authors. Fax: +81 45 924 5282.resses: [email protected] (T. Yokoi),es.titech.ac.jp (T. Tatsumi).

Among the catalysts examined for the catalytic cracking, ZSM-5zeolite has been recognized as a prime candidate because of its highthermal and hydrothermal stabilities and its considerable resis-tance to deactivation caused by coking as well as its strong acidity[1,13,14]. Yoshimura et al. [1] have carried out the cracking of lightnaphtha in the presence of steam at 923 K and found that the addi-tion of lanthanum (10 wt.%) to H-ZSM-5 (Si/Al = 100) enhanced theyield of ethylene and propylene up to 61 C-% and that the ethyl-ene/propylene ratio in the product was approximately 0.7, whichcorresponds to a higher proportion of propylene than the com-position obtained by using the current steam cracking. Moreover,further addition of phosphorus (2 wt.%) improved the stability.Recently, dealuminated MCM-68 catalyst with multidimensionalchannels of 10-MR and 12-MR has also been reported to exhibit ahigh propylene selectivity of 45 C-% [15]. However, the selectivityto propylene is uncertain when the conversion approaches 100%.

MCM-22, a kind of layered zeolite invented by Mobil in 1990[16], belongs to the MWW type zeolite and possesses two inde-pendent pore systems. One consists of two-dimensional sinusoidalchannels composed of slightly elliptical 10-MR, and the other ischaracteristic of 12-MR supercages accessible by 10-MR windows[17]. In addition, half supercages form surface pockets on the outercrystal surfaces. Because of its unique structure, MCM-22 zeolite

rg/10.1016/j.apcata.2014.12.0182014 Elsevier B.V. All rights reserved.-22 zeolites

ng, Toshiyuki Yokoi , Seitaro Namba, Junko N. Korces Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama

e i n f o

ptember 2014vised form 5 December 2014ecember 2014e xxx

ing of H-MCM-22

a b s t r a c t

The catalytic cracking of n-hexane fomodel reaction of naphtha cracking. direct or post-synthesis methods. Theratio. Dealumination of H-MCM-22 win the increase in the catalytic life andbe attributed to the decrease in the amresultant coke formation. Del-Al-MCMC-%) which was higher than H-ZSM-5conversion of 95%. Moreover, it showecatalyst.ylene

, Takashi Tatsumi

503, Japan

ducing propylene on MCM-22 catalysts was carried out as aM-22 catalysts with different Si/Al ratios were prepared byylene selectivity was improved with an increase in the Si/Almonium hexaurosilicate (AHFS) (Del-Al-MCM-22) resulted

ylene selectivity at a very high n-hexane conversion. This mayt of Lewis acid sites which accelerate the hydride transfer and

(Si/Al = 62) catalyst showed a high propylene selectivity (40 H-Beta catalysts with similar Si/Al ratios at a high n-hexanetalytic life comparable with H-ZSM-5 and longer than H-Beta

2014 Elsevier B.V. All rights reserved.

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ARTICLE IN PRESSG ModelAPCATA-15156; No. of Pages 112 Y. Wang et al. / Applied Catalysis A: General xxx (2015) xxxxxx

has been proved to be a shape-selective catalyst in the alkyla-tion [18,19], disproportionation and isomerization [2023], etc.Obviously, the acid properties (amount, location, type and strength)can also play important roles in these catalytic reactions. As weknown, thethe zeolite.

A numblite, concluof about 10lite [24,25]methods arPost-syntheSi/Al ratios sion of deboprecursor hamount andboth decreaobtaining hacid leachinand acid lea[20,2730],MCM-22 ar

MCM-22for FCC proas crackingless dry gasMCM-22 shratio and athat of the in n-heptan[33], who fodue to trapcages with ing occurs tthe supercasoidal chanimpossible Though thewas explaining the acid

In this swith differemethods ancracking ofto producecatalytic pediscussed.

2. Experim

2.1. Catalys

H-MCM-pared by twsynthesis mAl contentssynthesizeddirecting aaluminate (respectivelypH. DeboroMWW boroing in 6 M Hthe crystallof 1 SiO2:0.MWW-type

post-synthesis method. Del-B-MWW zeolite was used as a Si sourceand added into an aqueous solution of sodium aluminate and HMIunder stirring. The resulting mixture had a molar composition of1 SiO2:(0.0050.007) Al2O3:1 HMI:30H2O. The crystallization was

outauto, wawideor 10-form

1 M N73 Kst-sy-22

etermlumed frs so. Thebtainto 0.066 wt wit

Cata M H

talys

catatios on spdiffraaku)tion

instr HitacM). Ttler Tmmeed oollectra oted wt in acuasorp(1.3 kmoved at

talyt

cataw ret (18tivatactor

PureactorTo int wem 1.t.

reacraphn (50e this article in press as: Y. Wang, et al., Appl. Catal. A: Gen. (2015)

acid properties are closely related to the Si/Al ratio in

er of studies dealt with the synthesis of MCM-22 zeo-ding that only a very narrow range of the Si/Al ratio25 is suitable for the synthesis of pure MCM-22 zeo-. Besides the direct synthesis methods, post-synthesise also frequently used for preparing high silica zeolites.sis of highly crystalline MWW zeolite with variablein a wide range (12500) through the structural conver-ronated Si-MWW zeolite to lamellar Al-MWW zeoliteas been studied by Liu et al. [26]. They found that the

strength of Brnsted acid sites in H-MCM-22 zeolitesed with increasing Si/Al ratios. Another method forigh silica MCM-22 zeolites is dealumination includingg, hydrothermal treatment, combination of steamingching, ammonium hexauorosilicate (AHFS) treatment

etc. Obviously, the acid properties in dealuminatede dependent on the different delamination methods.

has also proved to be a good cracking zeolite additivecess [31,32]. Corma et al. [32] found that when used

additive, MCM-22 produced less total gases, as well ases, with a lower loss of gasoline than ZSM-5. Moreover,owed higher propylene/propane and butane/butane

good steam stability which was even higher thanZSM-5 zeolite. The catalytic performance of MCM-22e cracking have been investigated by Meloni et al.und that a very fast initial deactivation was occurred,ping of carbonaceous compounds (coke) in the largesmall apertures. They suggested that n-heptane crack-hrough the classical carbenium ion chain mechanism inges, however it occurs through protolysis in the sinu-nels, the bimolecular reaction of hydride transfer beingin the narrow space available near the protonic sites.

role of shape selectivity on the catalytic performanceed in the above paper, the role of acid properties includ-

amount, strength and type was seldom investigated.tudy, the catalytic performance of MCM-22 catalystsnt Si/Al ratios synthesized by direct or post-synthesisd treated with AHFS was investigated for the catalytic

n-hexane as a model reaction of naphtha cracking propylene. In addition, the relationship between therformance and the acid properties of the catalysts was

ental

t preparation

22 zeolite catalysts with different Si/Al ratios were pre-o methods according to literature [26,34]. (1) Directethod was used for the synthesis of zeolites with high

. The MCM-22 lamellar precursors were hydrothermally by using hexamethyleneimine (HMI) as a structure-gent (SDA). Fumed silica (Cab-o-sil M5) and sodium58.3% Al2O3, 32.3% Na2O) were used as Si and Al sources,, and sodium hydroxide was used to adjust the gelnated MWW (Del-B-MWW) obtained by calcination ofsilicate zeolite at 873 K for 20 h, followed by reux-NO3 at 393 K for 20 h, was used as seed to speed up

ization. The resulting mixture had a molar composition15 Na2O:(0.01250.025) Al2O3:0.9 HMI:45H2O. (2) The

precursors with low Al contents were synthesized by a

carriedunder lteredwith a 823 K fThe Nait withair at 7and poH-MCMratio d

Deaobtainaqueoufor 3 hwere o0.005 Si/Al = catalyserencewith 1

2.2. Ca

TheSi/Al raemissiX-ray III (RigadsorpJapan)with a(FE-SEa Metprograrecordwere cIR specsupporwas sewas evThe advapor then rerecord

2.3. Ca

Thebed ocatalysand acthe re(Wakothe re6 kPa. catalysied froamoun

Thematogcolum://dx.doi.org/10.1016/j.apcata.2014.12.018

in rotated (20 rpm) Teon-lined stainless autoclavesgenous pressure at 423 K for 5 days. The samples wereshed, and dried at 373 K to produce lamellar precursors

range of Si/Al ratios. Direct calcination of the samples at h resulted in the products with the 3D MWW structure.

MCM-22 was transformed into the H-form by treatingH4NO3 twice at 353 K for 2 h, followed by calcination in

for 2 h. The H-form MCM-22 zeolites prepared by directnthesis methods were named H-MCM-22 (n) and Post-

(n), respectively, in which n denotes the Si/Al molarined by ICP-AES analysis.

inated MCM-22 (Del-Al-MCM-22) catalysts wereom H-MCM-22 (Si/Al = 19) by the dealumination withlution of ammonium hexaurosilicate (AHFS) at 363 K

Del-Al-MCM-22 catalysts with different Al contentsed by varying the concentration of AHFS solution from5 M. For control, a small-sized H-ZSM-5 catalyst withas synthesized according to literature [13] and H-Betah Si/Al = 67 was obtained from H-Beta (Si/Al = 12) (Ref-lyst, The Catalysis Society of Japan) by dealuminationNO3 solution at 393 K for 1 h.

t characterizations

lysts were characterized by various techniques. Thewere determined by inductively coupled plasma-atomicectrometer (ICP-AES) on a Shimadzu ICPE-9000. Thection (XRD) patterns were recorded on a Rint-Ultima

diffractometry using a Cu K X-ray source. The N2was carried out at 77 K on a Belsorp-mini II (BELument. The crystal morphology and size were examinedhi S-5200 eld-emission scanning electron microscopehermogravimetric analyses (TGA) were carried out onGA/SDT apparatus in air atmosphere. Temperature-d desorption of ammonia (NH3-TPD) spectra weren a Multitrack TPD equipment (Japan BEL). IR spectrated on a Nicolet NEXUS-FTIR-670 spectrometer. Thef adsorbed pyridine were recorded as follows: a self-afer (9.6 mg cm2 in thickness and 2 cm in diameter)

a quartz IR cell, sealed with CaF2 windows, and thented at 723 K for 2 h before the pyridine adsorption.tion was carried out by exposing the wafer to pyridinePa) at 423 K for 0.5 h. The physisorbed pyridine wased by evacuation at 423 K for 1 h. The IR spectra were

423 K.

ic cracking

lytic cracking of n-hexane was carried out with a xed-actor under atmospheric pressure. Typically, 0.1 g of30 mesh) was put into a tubular reactor (6 mm in i. d.)ed in an air ow of 20 ml min1 at 923 K for 1 h. After

temperature was adjusted to the desired, n-hexane Chemicals Ind.) vapor diluted in helium was fed into. The initial partial pressure of n-hexane was set atvestigate the effect of contact time, the ratio of theight to the n-hexane ow rate (W/Fn-hexane) was var-6 to 64 g-cat h/mol-n-hexane by changing the catalyst

tion products were analyzed with an on-line gas chro- (Shimadzu, GC-14B) with an FID detector and a HP-AL/S

m 0.32 mm 8 m). The selectivities to the products

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ARTICLE IN PRESSG ModelAPCATA-15156; No. of Pages 11Y. Wang et al. / Applied Catalysis A: General xxx (2015) xxxxxx 3

1000 A 1000 B

Fig. 1. XRD pa d) and(d).

were calculwas calculaloss from 6contents of

3. Results

3.1. Physico

All of thMWW topono impurityAs shown inBET surfaceN2 adsorptiimages of scof H-MCM-ples were cin length anmorphologygels and thedifferent Alpatterns (Fiobserved inparent matessentially

Table 1 MCM-22 zedealuminatof the alum0.005 M AHthe Si/Al ratThe specicMCM-22 saan increasewith an incthe micropoexternal suAHFS treatmin a signicaber of agglohappen throneighboringa chemical

3 sh22 carol

and idiceak he act cor22 ze

of Alnto tect ocreaer he in FS trlite.SM-5cated

area (Si/e sizee of Al raimila3530252015105

d

c

b

Inte

nsity

/ cp

s

2Theta / degree

a

5

Inte

nsity

/ cp

s

tterns of H-MCM-22 samples (A) with Si/Al ration of 19 (a), 34 (b), 66 (c) and 100 (

ated based on the carbon numbers. The coke amountted by thermogravimetric analysis (TGA). The weight73 to 1073 K in each TG prole was dened as the

coke on the used catalyst.

and discussion

chemical characteristics of the zeolites

e H-MCM-22 zeolites synthesized in this study had thelogy and a relatively high crystallinity, and contained

of other phase as detected by XRD patterns (Fig. 1A). Table 1, the H-MCM-22 samples had reasonably high

areas in the range of 448540 m2 g1 as determined byon, which indicated that they were of good quality. Theanning electron micrographs (SEM) revealed that both22 (Si/Al = 19) and Post-H-MCM-22 (Si/Al = 100) sam-omposed of ake-like crystals, approximately 500 nmd 50 nm in thickness (Fig. 2a and b). Thus, the crystal

and size were not dependent on the Si/Al ratio in the synthesis method. The Del-Al-MCM-22 samples with

contents were further characterized by powder XRDg. 1B). After dealumination by AHFS, little change was

their diffraction patterns compared with those of theerial. Namely, the crystal structure of MCM-22 zeolite

Fig.MCM-these pl-peaknon-acthe h-pThus, tamounMCM-to thatrated ithe dirwas dethe othincreasthe AH22 zeo

H-Zas indisurfaceH-Betaaveragage sizthe Si/were s this article in press as: Y. Wang, et al., Appl. Catal. A: Gen. (2015), http

remained.lists the Si/Al ratios and textural properties of Del-Al-olite catalysts. It can be seen that AHFS is an effectiveion reagent for MCM-22 zeolite, and more than 40%inum atoms can be removed by the treatment withFS solution. With an increase in the AHFS concentration,io increased, which is consistent with the literature [30].

surface area and micropore pore volume of of Del-Al-mples are also listed in Table 1. It can be seen that with

in the Si/Al ratio of the Del-Al-MCM-22 samples, viz.reasing in the AHFS concentration, the surface area andre volume of the samples slightly decreased, while the

rface area was almost constant. On the other hand, theent, especially with high AHFS concentration, resultednt fragmentation of crystal morphology to form a num-merates (Fig. 2d). The formation of agglomerates couldugh binding of the surface hydroxyl groups (Si-OH) of

crystallites, which can be facilitated in the presence ofagent like AHFS [35].

3.2. Catalyt

3.2.1. EffectThe effe

ing of n-heThe initial the temperity to propybetween 72ture, appareinto aromatity to butenBTX (benzewas low coature; howthe reactionnium ions, 3530252015102Theta / degree

d

c

b

a

Del-Al-MCM-22 samples (B) with Si/Al ratio of 34 (b), 56 (c) and 62

ows the NH3-TPD proles of H-MCM-22 and Del-Al-talysts with different Al contents. As shown in Fig. 3, alles were composed of two desorption peaks so-calledh-peak. The l-peak corresponds to NH3 adsorbed onOH groups and on NH4+ by hydrogen bonding, whilecorresponds to NH3 adsorbed on true acid sites [36,37].id amount was calculated by the area of h-peak. The acidresponded to only 6377% of the total Al content in H-olites (Table 1). The low concentration of acid relative

are attributed to the fact that the portion of Al incorpo-he MCM-22 framework was not very high, regardless ofr post-synthesis methods. Obviously, the acid amountsed with the Si/Al ratio increased (Fig. 3A, Table 1). Onand, the acid amount was gradually decreased with anthe AHFS concentration (Fig. 3B, Table 1), which meanseatment is an effective dealumination method for MCM-

and H-Beta catalysts had relatively high crystallinities by XRD patterns (not shown) and reasonably high BETs (Table 1). As shown in Fig. 2, H-ZSM-5 (Si/Al = 66) and

Al = 67) were composed of cofn-like crystals with an of about 200 nm and sphere-like crystals with an aver-about 100 nm, respectively (Fig. 2e and f). In addition,tio and acid amount of H-ZSM-5 and H-Beta catalystsr to those of Del-Al-MCM-22 catalyst (Table 1).://dx.doi.org/10.1016/j.apcata.2014.12.018

ic cracking of n-hexane over H-MCM-22 catalysts

of reaction temperaturect of the reaction temperature on the catalytic crack-xane over H-MCM-22 (Si/Al = 19) is shown in Fig. 4.conversion of n-hexane increased with an increase inature, achieving nearly 100% at 923 K. The selectiv-lene was almost constant in the temperature ranging3 and 923 K. With an increase in the reaction tempera-ntly butenes once formed were gradually transformedic compounds, resulting in the decrease in the selectiv-es accompanied with the increase in the selectivity tone, toluene, and xylenes). The selectivity to ethylenempared with those to other alkenes at low temper-ever, the value increased steeply with an increase in

temperature. Ethylene is formed via primary carbe-regardless of whether the monomolecular mechanism

Please cit


Table 1Physicochemical properties of various catalysts.

Cat CAHFS/M Si/Ala Al contentb/mmol g1 Acid contentc/mmol g1 SBETd/m2 g1 S Exte/m2 g1 V microe/cm3 g1

H-MCM-22 (19) 19 0.84 0.53 517 92 0.17H-MCM-22 (34) 34 0.48 0.37 412 74 0.14Post-H-MCM-22 (66) 66 0.25 0.17 594 96 0.20Post-H-MCM-22 (100) 100 0.17 0.12 562 92 0.19

Del-Al-MCM-22 (34) 0.005 34 0.48 0.31 497 97 0.16Del-Al-MCM-22 (56) 0.02 56 0.29 0.23 466 112 0.14Del-Al-MCM-22 (62) 0.05 62 0.26 0.16 450 127 0.13

H-ZSM-5 (66) 66 0.25 0.19 445 24 0.18H-Beta (67) 67 0.25 0.18 587 201 0.15

a Molar ratio determined by ICP.b Calculated by ICP result.c Determined by NH3-TPD.d Measured by N2 adsorption at 77 K.e Calculated by t-plot method.

or the bimolecular mechanism operates [5,7] and therefore theapparent activation energy for ethylene formation should be high.Thus, the higher reaction temperature is of benet to the higherethylene yield. Moreover, the cracking via the monomolecularmechanism may be of more advantage in the ethylene forma-tion. The selectivities to propane and butanes decreased with an

increase in the reaction temperature, because the subsequent reac-tions of propane and butanes take place at high temperatures,resulting in the formation of alkenes, BTX and so on [38]. In addi-tion, the selectivities to methane and ethane, which form solely viathe monomolecular mechanism, are higher at the higher reactiontemperatures. These ndings clearly indicate that the cracking via

Fig. 2. SEM imH-Beta(Si/Al =e this article in press as: Y. Wang, et al., Appl. Catal. A: Gen. (2015), http

ages of H-MCM-22(Si/Al = 19) (a), Post-H-MCM-22(Si/Al = 100) (b), Del-Al-MCM-22(Si/A 67) (f) samples.://dx.doi.org/10.1016/j.apcata.2014.12.018

l = 34) (c), Del-Al-MCM-22(Si/Al = 62) (d), H-ZSM-5(Si/Al = 66) (e) and

Please cite


900800700600500400300

H-MCM-22(19) Del-Al-MCM-22(39) Del-Al-MCM-22(56) Del-Al-MCM-22(62)

Temperature / K

B

900800700600500400300

H-MCM-22(19) H-MCM-22(34) Post-HMCM-22(66) Post-HMCM-22(100)

Temperature / K

A

Fig. 3. NH3-TPD proles of H-MCM-22 (A) and Del-Al-MCM-22 (B) samples with different Si/Al ratios.

the monomolecular mechanism is more predominant at the hightemperature.

According to the product distributions at different reactiontemperatures, we conclude that higher reaction temperature isbenecial toet al. report[14].

3.2.2. EffectThe effe

gated. The weight to thchange of ccracking coincreased wat the W/Fnthe decreaspanied withBTX is formhand, primaat the highene would butanes an

Sel

. / C

-%

Fig. 4. Effect on H-MCM-22n-hexane = 64 g-c

ethylene selectivity [38]. Meanwhile, the selectivities to propaneand butanes decreased with an increase in the conversion, whilethose to methane and ethane simultaneously increased.

Effect widir Si/

on tifferel conhexaic crmal e thes, th

ln(

kc isnversspecounhe li n-hexane conversion and propylene production. Kuboed similar results in n-heptane cracking over H-ZSM-5

of contact timect of the contact time on the reaction was investi-contact time expressed as the ratio of the catalyste n-hexane ow rate (W/Fn-hexane) was changed by theatalyst weight to keep the contribution of the thermalnstant. As shown in Fig. 5, the conversion of n-hexaneith an increase in W/Fn-hexane, achieving nearly 100%

-hexane of 64 g-cat h/mol-n-hexane. Fig. 5 indicates thate in the selectivities to propylene and butenes is accom-

the increase in the selectivity to BTX, suggesting thated mainly from propylene and butenes. On the otherry carbenium ions would be relatively easy to produce

reaction temperature of 923 K [5,7]. Therefore, ethyl-be formed not only by n-hexane cracking but also byd/or propane cracking, resulting in an increase in the

50 100

3.2.3. It is

on theaciditywith dof the Athat n-catalytof therand thkinetic

kcW

F=

wherethe colyst, reacid amtures. T this article in press as: Y. Wang, et al., Appl. Catal. A: Gen. (2015), http

9509008508007507006500

10

20

30

40

Con

v./ %

Temperature / K

0

20

40

60

80

of reaction temperature on the catalytic cracking of n-hexane(Si/Al = 19). Reaction conditions: Cat., 0.1 g; Pn-hexane = 6 kPa; W/Fat h/mol-n-hexane; temp., 723923 K, TOS = 15 min.

Fig. 5. Effect 22(Si/Al = 19). temp., 923 K, o of Al contentely known that acidity of zeolites is largely dependentAl molar ratios. In order to elucidate the effect of thehe catalytic cracking of n-hexane, H-MCM-22 zeolitesnt Si/Al ratios were employed as catalysts. First, effecttent on catalytic activity was investigated. We considerne cracking at such high temperatures is the sum of theacking and the thermal cracking since the contributioncracking is not negligible. If both the catalytic crackingrmal cracking at high temperatures obey the rst-ordere following equation is derived;

11 x

) ln

(1

1 xp

)(1)

the rate constant for the catalytic cracking, x and xp areion for the n-hexane cracking with and without cata-tively [14]. Fig. 6 shows the dependence of kc on thet of H-MCM-22 catalysts at different reaction tempera-near relationship between log kc and log acid amount of

50 100://dx.doi.org/10.1016/j.apcata.2014.12.018

807060504030201000

10

20

30

40

Con

v. /

%

Sel

. / C

-%

W/Fn-hexane / g h mol-1

0

20

40

60

80

of contact time on the catalytic cracking of n-hexane on H-MCM-Reaction conditions: W/F n-hexane = 6.464 g-cat h/mol-n-hexane;thers see Fig. 4.

Please cit


0.0-0.2-0.4-0.6-0.8-1.0-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

logk

c

log (Acid amount)

c

b

a

Fig. 6. Dependence of kc on the acid amount of H-MCM-22 catalysts at differentreaction temperatures of 723 K (a), 823 K (b) and 923 K (c).

H-MCM-22 catalyst was observed at every reaction temperature. Asshown in Fig. 6, the slopes of the straight line observed in the logkc vs. log (Acid amount) plots are 1.8, 1.6, and 1.3 at 823, 873, and923 K, respectively. The kc is almost proportional to the acid sitedensity at 923 K; however, at lower temperatures the effect of theacid site density is enhanced with a decrease in reaction tempera-ture. Haag ecracking of acid site den-heptane cSi/Al ratios

Then, thwas investselectivitiesn-hexane cdecrease inties slightlydecreased. amount supcyclization-Thus the H-the higher hexane. Meand/or prop

Fig. 7. Relatiocatalysts at antemp., 923 K, T

100

Fig. 8. Changecatalysts withFig. 4.

923 K was in the decrincrease inhere).

ides for a

in -22

tivityhe H) gavas tha. 80

cont wi

of min

a lae amne hich

e trag, eted ted

win increase the diffusion limitation, leading to the high cokeion. The coke composition and the mode of deactivation int al. reported that the catalytic activity in the catalyticn-hexane at 811 K over H-ZSM-5 is proportional to thensity [3941]. A similar dependence of n-hexane andracking on the acid density over H-ZSM-5 with varioushas been reported by Post et al. and Kubo et al. [6,14].e effect of the Al content on products distributionigated. Fig. 7 shows the relationship between the

and the acid amount of H-MCM-22 catalysts at anonversion of ca. 80%. It can be seen that, with a

the acid amount, propylene and butenes selectivi- increased while ethylene and BTX selectivities slightlyThese ndings suggest that the decrease in the acidpressed the secondary reactions (hydride transfer andaromatization) of propylene and butenes to form BTX.MCM-22 catalysts with the higher Si/Al ratios showedpropylene selectivity in the catalytic cracking of n-anwhile, the subsequent cracking reaction of butanesane to form ethylene at high reaction temperature of

40

50

Besfactor changeH-MCMtial acratio. Tand 34whereonly chexanecatalysversionof 210tion ofof largsons. Osites, whydridcouplinbe relaconnec10-MRwouldformate this article in press as: Y. Wang, et al., Appl. Catal. A: Gen. (2015), http

0.60.50.40.30.20.10.00

10

20

30

BTX

Sel

. / C

-%

Acid amount / mmol g-1

nship between product selectivities and acid amount of H-MCM-22 n-hexane conversion of ca. 80%. Reaction conditions: Pn-hexane = 6 kPa;OS = 15 min.

n-heptane and establi22 (Si/Al = 1due to the sion of hydThese resucontents exAl contents

In additistability. Mlyst exhibitto large extHowever, incontents bymorphologsize on cata0

20

40

60

80

H-MCM-22(19) H-MCM-22(34) Post-H-MCM-22(66) Post-H-MCM-22(100)

120 150 180 210 2409060300TOS / min

Coke wt./%

9.56.52.21.4

Con

v. /

%

in n-hexane conversion with time on stream (TOS) for H-MCM-22 different Si/Al ratios. Reaction conditions: temp., 923 K, others see

reduced with a decrease in the acid amount, resultingease in the ethylene selectivity accompanied with the

the selectivties to butanes and propane (not shown

activity and selectivity, stability is another importantpplications of catalysts in industry. Fig. 8 shows then-hexane conversion with time on stream (TOS) for

catalysts with different Si/Al ratios. Obviously, the ini- and deactivation rate closely depended on the Si/Al-MCM-22 catalyst with higher Al contents (Si/Al = 19e nearly 100% n-hexane conversion at the initial stage,e catalyst with a lower Al content (Si/Al = 100) showed% conversion of n-hexane. A sharp decrease in the n-version with TOS was observed for the H-MCM-22th high Al contents. For example, the n-hexane con-H-MCM-22 (Si/Al = 19) decreased to ca. 60% at TOS. This deactivation was probably due to the deposi-rge amount of coke (95 mg/g-catalyst). The formationount of coke can be explained by two possible rea-reason may be related to the high amount of acid

would accelerate the secondary reactions includingnsfer, cyclization-aromatization and dehydrogenatingtc. and resultant coke formation. Another reason mayto the special structure of MCM-22 zeolite. The non-property between 12- and 10-MR channels and thedow openings accessible to the 12-MR supercages://dx.doi.org/10.1016/j.apcata.2014.12.018

cracking over MCM-22 zeolite had been investigatedshed by Meloni et al. [33]. In the case of Post-H-MCM-00) catalyst, the deactivation rate was relative slow,smaller amount of acid sites leading to the suppres-ride transfer and coke formation (14 mg/g-catalyst).lts indicate that H-MCM-22 catalysts with lower Alhibit a better stability than the catalysts with higher.on, the crystallite size might strongly affect the catalyticochizuki et al. reported the small-sized H-ZSM-5 cata-s a higher stability in n-hexane catalytic cracking, owingernal surface area and short diffusion path length [13].

our present study, MCM-22 catalysts with different Al direct or post-synthesis method had similar crystallitey and sizes (Fig. 2ad). Thus, the inuence of crystallitelytic stability could not be examined.

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120 150 180 210 24090603000

20

40

60

80

100

H-MCM-22(19) Del-Al-MCM-22(39) Del -Al-MCM-22(56) Del -Al-MCM-22(62)

Con

v. /

%

TOS / min

Coke wt. / %

9.56.8

2.23.4

Fig. 9. Change in n-hexane conversion with time on stream (TOS) for H-MCM-22(Si/Al = 19) and Del-Al-MCM-22 catalysts with different Si/Al ratios. Reactionconditions: temp., 923 K, others see Fig. 4.

3.3. Catalytic cracking of n-hexane over Del-Al-MCM-22 catalysts

From the results reported above, H-MCM-22 catalysts withlower Al contents prepared from the post-synthesis methodexhibited the higher propylene selectivity. However, the catalytic

stability was still not satisfactory. Thus, the catalytic performance ofseveral high-silica MCM-22 zeolites prepared by the dealuminationwas further investigated.

Fig. 9 shows the change in n-hexane conversion with TOSfor H-MCM-22 (Si/Al = 19) and Del-Al-MCM-22 catalysts with dif-ferent Si/Al ratios. The stability of Del-Al-MCM-22 catalysts wasgreatly improved by the dealumination, though the initial n-hexaneconversion slightly decreased. In the case of Del-Al-MCM-22(Si/Al = 62), the initial n-hexane conversion was about 95%, andonly slightly decreased to 88% at 210 min. This also may be dueto the smaller amount of acid sites in the Del-Al-MCM-22 catalyst,resulting in suppression of the hydride transfer and coke forma-tion (22 mg/g-catalyst). It is noted that the catalytic stabilities ofPost-H-MCM-22 (Si/Al = 66) and Del-Al-MCM-22 (Si/Al = 62) cata-lysts were different, though they had similar Si/Al ratios and acidamounts. This nding indicates that some other factor than the acidamount affects the catalytic stability, which is to be discussed inSection 3.5. In addition, the amount of coke on the Post-H-MCM-22(Si/Al = 66) and Del-Al-MCM-22 (Si/Al = 62) catalysts was nearly thesame, however the stabilities were different, indicating the differ-ent coke compositions and locations on the two catalysts. This maybe due to the difference in the type, strength and location of acidsites.

Fig. 10 shows the changes in selectivities to alkenes and BTXin the n-hexane conversion on H-MCM-22 (Si/Al = 19) and Del-Al-MCM-22 catalysts with different Si/Al ratios. As shown in Fig. 10,for H-MCM-22 (Si/Al = 19) catalyst, the selectivities to propylene

Fig. 10. Changratios. Reactio this article in press as: Y. Wang, et al., Appl. Catal. A: Gen. (2015), http

es in selectivities to C3= (A), C2= (B), C4= (C) and BTX (D) with n-hexane conversion on H-Mn conditions: see Fig. 5.://dx.doi.org/10.1016/j.apcata.2014.12.018

CM-22(Si/Al = 19) and Del-Al-MCM-22 catalysts with different Si/Al

Please cit


Fig. 11. Chang(Si/Al = 66), H-conditions: tem

and buteneversion, whselectivity the n-hexanCompared Al-MCM-22butenes andat high n-hlysts showen-hexane cof Del-Al-Malmost conof 95%. Theform BTX frbutanes andature of 923was decrea

3.4. Compawith H-ZSM

In the pr22 was alsowhich are tshows the 5, H-Beta aand acid am5 (Si/Al = 66nearly 100%during the This may bezeolite, whideactivatiotle lower inwas still gooto coke forto its acid sof H-ZSM-5high initial

coke formed thereon during the reaction for 210 min (107 mg/g-catalyst) was much higher than those on the other two catalysts,leading to a signicant deactivation. Despite the milder acidity ofH-Beta catalyst compared with H-MCM-22 and H-ZSM-5 catalysts

he po comide has y of

andf 72

cavn adve i-22

fn-l.

12 s-hexlystsectivreasecreaAl = 6nt (coreo

showtaly

of Ms indt is sic crn co

vestianc

show on

Withts, thhe dts we in n-hexane conversion with time on stream (TOS) for H-ZSM-5Beta (Si/Al = 67) and Del-Al-MCM-22 (Si/Al = 62) catalysts. Reactionp., 923 K, others see Fig. 4.

s were decreased along with increasing n-hexane con-ile those to ethylene and BTX were increased. The

to propylene was decreased from 41 to 30 C-% whene conversion was increased from 60% to nearly 100%.

with the parent H-MCM-22(Si/Al = 19) catalyst, Del- catalysts showed high selectivities to propylene and

simultaneously low selectivities to ethylene and BTXexane conversions. In addition, Del-Al-MCM-22 cata-d higher selectivities to butanes and propane at highonversions (not shown here). For example, in the caseCM-22(Si/Al = 62), the propylene selectivity was kept

stant (ca. 40 C-%) even at a high n-hexane conversionse ndings indicate that the secondary reactions toom propylene and butenes and the cracking reaction of/or propane to form ethylene at high reaction temper-

K were suppressed when the acid amount of catalyst

[42], tof coketheir wresult stabilitH-Betaature oand/or[31]. Ialso haof MCMthe co(Fig. 2)

Fig.with n22 catathe selan incBTX in22 (Si/consta95%. M95%, itBeta caabilityndingcatalyscatalytreactio

3.5. Inperform

As closelyalysts.catalysing in tcatalyse this article in press as: Y. Wang, et al., Appl. Catal. A: Gen. (2015), http

sed.

rison of the catalytic performance of Del-Al-MCM-22-5 and H-Beta catalysts

esent study, the catalytic performance of Del-Al-MCM- compared with that of H-ZSM-5 and H-Beta catalysts,he catalyst mostly used in FCC process [31,32]. Fig. 11change in n-hexane conversion with TOS for H-ZSM-nd Del-Al-MCM-22 catalysts with similar Si/Al ratiosounts. Among these catalysts investigated, the H-ZSM-) catalyst gave a high initial n-hexane conversion of

and the highest stability. The amount of coke formedreaction for 210 min was very low (29 mg/g-catalyst).

related to the tridirectional 10-MR channels in ZSM-5ch is benecial to the mass transfer, leading to very lown. The Del-Al-MCM-22 (Si/Al = 62) catalyst showed a lit-itial n-hexane conversion (95%); however the stabilityd. The resistivity of Del-Al-MCM-22 (22 mg/g-catalyst)

mation was as high as that of H-ZSM-5, probably duetrength and density of acid sites are similar to those

[36]. Although H-Beta (Si/Al = 67) catalyst showed a n-hexane conversion (nearly 100%), the amount of

lysts showelife than Hbe ascribedthe secondatransfer to 5 zeolite wbetter perfoethylene [4

It is welstrength anthe catalytishowed thetroscopy won the type

Fig. 13 sent Si/Al radesorbed astretching vtwo strongexternal siladdition, tware assigneframework://dx.doi.org/10.1016/j.apcata.2014.12.018

lymerization of olens and the successive formationponents in the 3-dimensional 12-MR micropores and

intersections may predominantly occur [15]. A similarbeen reported by Corma et al., who suggest that the

H-MCM-22 zeolite is intermediate between those of H-ZSM-5 in n-heptane cracking at the lower temper-3 K, mainly because MCM-22 zeolite has larger poresities than ZSM-5 and smaller ones than Beta zeolitedition, the crystallite size and morphology of zeolitenuenced on the mass transfer. The ake-like crystal

zeolite is benecial to the mass transfer, compared toike crystal ZSM-5 and sphere-like crystal Beta zeolite

hows the changes in selectivities to alkenes and BTXane conversion on H-ZSM-5, H-Beta and Del-Al-MCM-. As shown in Fig. 12, for H-ZSM-5 and H-Beta catalysts,ities to propylene and butenes steeply decreased with

in n-hexane conversion, while those to ethylene andsed drastically. However, in the case of Del-Al-MCM-2) catalyst, the propylene selectivity was kept almosta. 40 C-%), even at an n-hexane conversion as high asver, when the n-hexane conversion was higher thaned a lower BTX selectivity than either H-ZSM-5 or H-

sts, which could be due to the inferior hydride transferCM-22 catalyst at high n-hexane conversions [43]. Theseicate that the catalytic performance of Del-Al-MCM-22uperior to that of H-ZSM-5 and H-Beta catalysts in theacking of n-hexane to produce propylene under thesenditions.

gation of the relationship between catalytice and acidity of MCM-22 catalysts

n in Figs. 610, the catalytic performance dependedthe Si/Al ratio of H-MCM-22 and Del-Al-MCM-22 cat-

increasing Si/Al ratio of H-MCM-22 or Del-Al-MCM-22e acid amount was decreased (Fig. 3, Table 1), result-ecrease in the cracking activity of catalysts. H-MCM-22ith the higher Si/Al ratios and Del-Al-MCM-22 cata-d a higher propylene selectivity and longer catalytic-MCM-22 catalysts with high Al contents. This could

to the decrease in the acid amount, which suppressedry reaction of propylene and butenes and the hydrideform coke. Similarly, Zhu et al. also reported the ZSM-ith lower acid amount (higher Si/Al ratio) exhibitedrmance in butene catalytic cracking to propylene and4].l know that not only the acid amount but also the acidd type of acid site, Brnsted or Lewis acid site affectc performance. However, the results of NH3-TPD only

change of the acid amount. So, the pyridine-IR spec-as investigated to provide more detailed information

and amount of acid sites and their strength.hows the IR spectra of H-MCM-22 catalysts with differ-tios before (A) and after (B) pyridine was absorbed andt 423 K. As shown in Fig. 13A, in the region of hydroxylibration, the H-MCM-22 (Si/Al = 19) catalyst exhibited

bands at 3743 and 3615 cm1, which are attributed toanols and structural Si(Al)OH hydroxyls, respectively. Ino weak bands appeared at 3727 and 3663 cm1, whichd to internal silanols and hydroxyls related to extra-

Al, respectively [20,34]. The 3615 cm1 band gradually

Please cite


Fig. 12. Changcatalysts. Reac

Fig. 13. IR spe

decreased ia decrease Post-H-MCMcantly in in this article in press as: Y. Wang, et al., Appl. Catal. A: Gen. (2015), http

es in selectivities to C3= (A), C2= (B), C4= (C) and BTX (D) with n-hexane conversion on H-tion conditions: see Fig. 5.

11800

Abs

orba

nce

W

BSBAS

1.

1.

2.

2.

3200340036003800

Abs

orba

nce

Wavenumber / cm-1

a

b

c

d

0.1A

3727

36153500

37433663

ctra of H-MCM-22 catalysts with Si/Al ratio of 19 (a), 34 (b), 66 (c) and 100 (d) before (A)

n intensity with a decrease in Al content, indicatingin the amount of Brnsted acid sites. In the case of-22 catalysts, the 3727 cm1 band increased signi-

tensity and a broad band around 3500 cm1 attributed

to hydrogedecreased. more defecmethod.://dx.doi.org/10.1016/j.apcata.2014.12.018

ZSM-5 (Si/Al = 66), H-Beta (Si/Al = 67) and Del-Al-MCM-22 (Si/Al = 62)

140015001600700

c

avenumber / cm-1

0.1

LAS

d

a

b

/SLAS9

9

8

6

BAS1543 1454

and after (B) pyridine was absorbed and desorbed at 423 K for 1 h.

n bonded silanols appeared when the Al content wasThese facts indicate that Post-H-MCM-22 catalysts havet sites than the catalysts prepared by the direct synthesis

Please cit


Fig. 14. IR spe 9 (b)desorbed at 42

Then, thring-stretchas shown iattributed trespectively1543 and 1not only BrHowever, tharea of peaPost-H-MCMalysts; e.g., H-MCM-22tively. Thusof Lewis acdirect synthshown in Ficatalysts exMCM-22 cathe dealumdue to the thesized MColens to etransfer re[45].

Similarlyalysts withwas absorbally decreaswith a progthe intensitand Lewis aluminationthe framewAHFS treatmdealuminatmore readinot only thedealuminatminated M(Si/Al = 19) (Si/Al = 62) a

specttion he su

prois acion.

distn be

sho-22

M-22ing dthe ctes do 72ity of75% acreas

Del-creasctra of H-MCM-22 (Si/Al = 19) (a) and Del-Al-MCM-22 catalysts with Si/Al ratio of 33 K for 1 h.

e IR spectra of adsorbed pyridine in the range of pyridineing modes after desorption at 423 K was measured,n Fig. 13B. The bands at 1543 and 1454 cm1 wereo Brnsted acid sites (BAS) and Lewis acid sites (LAS),. With a decrease in Al content, the intensity of the454 cm1 band decreased simultaneously, indicatingnsted acid but also Lewis acid decreased in amount.e ratio of BAS/LAS, which was calculated by using theks ascribed to the Brnsted and Lewis acid sites, for-22 catalysts was lower than that for H-MCM-22 cat-

the BAS/LAS ratios for Post-H-MCM-22 (Si/Al = 100) and (Si/Al = 19) were calculated to be 1.9 and 2.6, respec-, the Post-H-MCM-22 catalysts have higher proportionid sites than the H-MCM-22 catalysts prepared by theesis method. Based on the catalytic performance resultsgs. 8 and 9, Section 3.2.3, although the Post-H-MCM-22hibited better stabilities than the direct synthesized H-talysts, their stabilities were not so high compared toinated MCM-22 catalysts. The possible reason may behigher proportion of Lewis acid sites in the post syn-

2.6, reproporlyst. Tlife andof Lewformat

Thesites caFig. 15H-MCMAl-MCfollowfor all acid si423 K tintensby ca. tion incase ofwas ine this article in press as: Y. Wang, et al., Appl. Catal. A: Gen. (2015), http

M-22 zeolites. Since the Lewis acid sites interact withnhance their oligomerization and subsequent hydrideactions, leading to the formation of coke products

, Fig. 14 shows the IR spectra of Del-Al-MCM-22 cat- different Si/Al ratios before (A) and after (B) pyridineed and desorbed at 423 K. The 3615 cm1 band gradu-ed in intensity; however the 3743 cm1 band increasedress of dealumination (Fig. 14A). Fig. 14B showed thaty of the 1543 and 1454 cm1 bands ascribed to Brnstedcid sites, respectively, decreased with a progress of dea-. These ndings imply that aluminum atoms not only inork but also of extra-framework are removed by theent. Moreover the BAS/LAS ratio increased after the

ion, which means that the Lewis acid sites are removedly than the Brnsted acid sites. Thus, it was deduced

acid amount but also the acid type was changed withion using AHFS treatment. The BAS/LAS ratios for dealu-CM-22 catalysts were higher than that for H-MCM-22catalyst; e.g., the BAS/LAS ratios for Del-Al-MCM-22nd H-MCM-22 (Si/Al = 19) were calculated to be 4.3 and

acid strengtwith that othe other hintensity ofdesorption ture was inascribed to tively, whithan that othe intensitunchangedPost-H-MCacid sites onDel-Al-MCMlower stabicatalyst in nalso conclube another of Del-Al-Mcant role inmuch heav, 56 (c) and 62 (d) before (A) and after (B) pyridine was absorbed and

ively. Thus, the Del-Al-MCM-22 catalysts have lowerof Lewis acid sites than the parent H-MCM-22 cata-periority of Del-Al-MCM-22 catalysts in the catalyticpylene selectivity could be due to their smaller amountid sites which accelerate the hydride transfer and coke

ribution of the acid strength of Brnsted and Lewis acid measured from the thermal desorption of pyridine [46].ws the IR spectra in the pyridine vibration region of

(Si/Al = 19) (A), Post-H-MCM-22 (Si/Al = 66) (B) and Del- (Si/Al = 62) (C) catalysts after pyridine adsorption andesorption at different temperatures. It is apparent thatatalysts, the intensity of the band ascribed to Brnstedecreased with desorption temperature increasing from3 K. For H-MCM-22 and Post-H-MCM-22 catalysts, the

the band ascribed to Brnsted acid sites was reducednd 90%, respectively when the temperature of desorp-ed from 423 K to 723 K. No band was observed in theAl-MCM-22 catalyst when the desorption temperatureed to 723 K. These facts demonstrate that the Brnsted://dx.doi.org/10.1016/j.apcata.2014.12.018

h of Del-Al-MCM-22 catalyst is relatively low comparedf parent H-MCM-22 and Post-H-MCM-22 catalysts. Onand, for H-MCM-22 and Del-Al-MCM-22 catalysts, the

band ascribed to Lewis acid sites also decreased withtemperature increased. When the desorption tempera-creased from 423 K to 723 K, the intensity of the bandLewis acid sites was reduced by ca. 10% and 50%, respec-ch suggests that the strength of Lewis acid is higherf Brnsted acid in these two catalysts. It is noted thaty of the band ascribed to Lewis acid sites was almost

with desorption temperature increased in the case ofM-22 (Si/Al = 66), indicating that the strength of Lewis

Post-H-MCM-22 is higher than that of H-MCM-22 and-22 catalysts. This could be a possible reason for the

lity of Post-H-MCM-22 compared with Del-Al-MCM-22-hexane cracking. According to the above ndings, it is

ded that the decrease in the strength of acid sites wouldreason for the higher propylene selectivity and stabilityCM-22 catalysts, since strong acid sites play a signi-

coke formation and coke formed on strong acid sites isier than that formed on weak acid sites [47].

Please cite


e

0.1

d

A

e

0.1

c

d

C

140

e

-1

0.1

d

B

Fig. 15. IR spe (Si/Aadsorption and

4. Conclus

H-MCM-post-synthealytic crackacid amountion of prowas still nothe Post-H-which accelalytic life awere improto the smalstrength of propylene s(Si/Al = 67) cversion of 9that of H-ZS

Acknowled

This wochemistry popment Org

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Catalytic cracking of n-hexane for producing propylene on MCM-22 zeolites1 Introduction2 Experimental2.1 Catalyst preparation2.2 Catalyst characterizations2.3 Catalytic cracking

3 Results and discussion3.1 Physicochemical characteristics of the zeolites3.2 Catalytic cracking of n-hexane over H-MCM-22 catalysts3.2.1 Effect of reaction temperature3.2.2 Effect of contact time3.2.3 Effect of Al content

3.3 Catalytic cracking of n-hexane over Del-Al-MCM-22 catalysts3.4 Comparison of the catalytic performance of Del-Al-MCM-22 with H-ZSM-5 and H-Beta catalysts3.5 Investigation of the relationship between catalytic performance and acidity of MCM-22 catalysts

4 ConclusionsAcknowledgementsReferences

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