physicochemical and catalytic properties of hzsm-5 zeolites dealuminated by the

8/13/2019 Physicochemical and Catalytic Properties of HZSM-5 Zeolites Dealuminated by The

1/6

Physicochemical and Catalytic Properties of HZSM-5 Zeolites Dealuminated by the

Treatment with Steam

J. Datka, S. Marschmeyer, T. Neubauer, J. Meusinger, H. Papp,* F.-W. Schu1tze, andI. Szpyt

Faculty of Chemistry and Mineralogy, Institute of Technical Chemistry, UniVersity of Leipzig,04103 Leipzig, Linnestrasse 3, Germany

ReceiVed: March 6, 1996X

We studied the physicochemical properties (by 27Al MAS NMR, XPS, and IR spectroscopy) and catalyticactivity (in n-heptane cracking) of a series of HZSM-5 zeolites treated at 770 K with water vapor at variouspartial pressures (0, 7, 13, 40, and 93 kPa). The treatment with steam removed Al from framework tetrahedralpositions (as evidenced by 27Al MAS NMR studies) and yielded a decrease in the amount of acidic Si-OH-Al groups. Quantitative IR studies of pyridine sorption showed that the amount of Si-OH-Al in parentand in dealuminated zeolites was very close to the amount of tetrahedral Al which remained in the framework.The Al atoms removed from the tetrahedral positions migrated (as given by XPS) to the surface. Only asmall amount of these removed Al created Lewis acid sites. IR studies suggested that the vacancies createdby removal of Al were filled with Si atoms migrating from other places in the zeolite. From IR studies ofthe desorption of ammonia it was concluded that mild dealumination (with 7 kPa of H2O) increased the

acidic strength of Si-OH-Al groups. The experiments of chlorobenzene sorption suggested that this increasewas in the first order due to removal (by steam treatment) of the less acidic hydroxyls. The more severedealumination decreased the acidic strength of Si-OH-Al groups. The results of catalytic tests ofn-heptanecracking agreed well with the IR results concerning acidity. The mild dealumination resulted in an increasein the catalytic activity which can be related to the increase in the acidic strength of Bronsted sites whichcompensate the decrease in the number of sites. The further decrease in the cracking activity (for moreseverely dealuminated zeolites) may be explained by the decrease in both concentration and acidic strengthof Bronsted sites. The cracking activity of our HZSM-5 zeolites was higher when hydrogen was used (insteadof nitrogen) as the carrier gas. This indicates that hydrogen transfer plays an important role in n-heptanecracking.

Introduction

The physicochemical and catalytic properties of dealuminatedzeolites are matter of growing scientific and commercial interest.This is due to the fact that dealumination results in thestabilization of the framework against mineral acids andtemperature,1 and also, it can enhance the activity in acid-catalyzed reactions of the materials.2-6 It concerns especiallythe zeolites dealuminated under mild conditions. There arenumerous publications dealing with these problems, and thereare a few new characterization methods (like EPR, XPS)involved,7,8 helping to explain the observed catalytic phenomena.There exists, therefore, a growing base of results and additionallysome models for interpretation of experimental results like thephenomena of very strong acid sites.24-29

Most of them (e.g., refs 3 and 9) focus on the point that the

observed enhancement of catalytic activity is due to a synergisticinteraction between Bronsted acidic sites present in the zeolite(bridging Si-OH-Al groups related to framework aluminum)and any type of Lewis acidic sites connected with coordinativelyunsaturated extraframework aluminum species. An increase inthe acidic strength of bridging hydroxyls observed under milddealumination conditions is an effect of an increase in polariza-tion of O-H due to interaction with such Al sites. On the other

hand, Biaglow et al.,10 who studied by microcalorimetry a set

of steamed HY zeolites, presented another point of view. Theydid not find superacidic hydroxyls and suggested that theenhanced catalytic activity of steamed zeolites was not due tohigh acidity. A similar interpretation was also proposed byZholobenko et al.,11,12 who suggested the polarization of aparaffin molecule by a Lewis acid site created by steaming isthe reason for the high catalytic activity of steamed zeolites.

The aim of our study was to investigate the status of Al inHZSM-5 zeolite dealuminated by steaming. The acidic proper-ties of steamed zeolites were compared with their catalyticactivity in n-heptane cracking.

We used 27Al MAS NMR spectroscopy, XPS, TPD, and IRspectroscopy to characterize our zeolite samples. 27Al MASNMR gave information about the amount of tetrahedral and

octahedral Al species, XPS (in combination with XRD) gaveinformation about the localization of Al removed from theframework by steaming, and quantitative IR studies providedinformation on the concentration of Bronsted and Lewis acidicsites, on the acidic strength of Bronsted sites (bridging hy-droxyls), and on the distribution of the acidic strength of acidichydroxyls. The results of catalytic tests (cracking ofn-heptanecarried out in nitrogen and in hydrogen) were correlated withthe data concerning acidity. We studied a series of HZSM-5zeolites steamed at 770 K with various vapor pressures of water.

Experimental Section

Preparation. The starting material was a HZSM-5 zeolite(Si/Al ) 15) synthesized template free by Chemie AG Bitterfeld

* To whom correspondence should be addressed. Faculty of Chemistry, Jagiellonian University, 30-060 Cracow, Ingar-

dena 3, Poland. Research Centre Julich (KFA), Institute of Energy Process Engineering,

P.O. Box 1913, 52425 Julich, Germany.X Abstract published in AdVance ACS Abstracts, July 1, 1996.

14451J. Phys. Chem. 1996, 100, 14451-14456

S0022-3654(96)00685-5 CCC: $12.00 1996 American Chemical Society


2/6

(Germany). Hydrothermal treatment was performed in a 2 mmdeep bed under flowing nitrogen. Nitrogen with different partialpressures of water, 0, 7, 13, 40, and 93 kPa (0, 50, 100, 300,and 700 Torr), was flowing over the zeolite at 770 K for 2.5 h.

NMR Studies. 27Al MAS NMR spectra were taken afterdehydration at 673 K and subsequent 40 h rehydration (oversaturated NH4Cl solution) of zeolite samples. The spectra wererecorded with a BRUKER MSL 500 spectrometer with aspinning rate of 4.5 kHz.

XRD Studies. Diffractograms of our ZSM-5 zeolites wererecorded with a HZG 4 from Fa. SEIFERT FPM diffractometerwith a graphite monochromator.

XPS Studies. The measurements were made with a LHS10 (MCD) system using Mg KRexcitation.

IR Studies. For IR studies, our zeolites were pressed intothin wafers (4-8 mg/cm2) and activated at 673 K in situ in theIR cell under vacuum (10-3 Torr) for 5 h. The spectra wererecorded with BRUKER 66v and BRUKER 48 PC spectrom-eters equipped with MCT detectors.

Catalytic Tests. The acidic cracking of n-heptane wascarried out in a quartz glass tube reactor at 623 K and a totalpressure of 2.5 MPa. The activation of the samples wasperformed in nitrogen 673 K for 3 h under normal pressureconditions. The partial pressure ofn-heptane was fixed at 0.33MPa. The weight of catalysts was constant for all experimentswith 0.03 g. Cracking products were analyzed by on-line GCwith a PLOT column.

Results

27Al MAS NMR Studies. The 27Al MAS NMR spectra offour of our HZSM-5 zeolites (parent as well as dealuminatedwith 7, 13, and 93 kPa) are presented in Figure 1. There is asharp line at 60 ppm due to tetrahedrally coordinated Alframework species. A broader line with much lower intensityis visible at 0 ppm. It is due to the octahedrally coordinated

Al species. While the signal at 60 ppm decreases distinctlywith dealumination, the signal at 0 ppm remains almost constant.

The decrease in intensity of the 60 ppm signal indicates adecrease in the amount of tetrahedral Al. The constant intensityof the 0 ppm signal due to octahedral species should beinterpreted in two ways. On one hand we can say that none(or a very low amount) of octahedral Al species are producedby the dealumination of the zeolite by water vapor. On theother hand, however, it cannot be excluded that extraframeworkAl creates bigger aggregates inside the channels or on theexternal surfaces.

The concentrations of both tetrahedral and octahedral Al werecalculated from the intensities of 60 and 0 ppm signals by usinga procedure described in detail in ref 13.

XPS Studies. XPS experiments were performed in order to

study the migration of Al atoms in zeolite crystallites. Figure

2 shows the recorded spectra the Al 2p peak. The observed

binding energies are practically constant. However, dealumi-

nation results in a strong increase in the intensity of the Al 2p

peak of surface Al species. Figure 3 shows Si/Al values on

the surface (obtained from XPS experiments) and in the bulk

(calculated from XRD results). The Si/Al values of the surface

were calculated from the intensities of Si 2p and Al 2p peaks

by using appropriate sensitivity factors (taken from ref 15), and

the Si/Al values concerning bulk from the XRD data (by using

a correlation of the unit cell parameter and Si/Al; ref 16).

According to the data presented in Figure 3, the steaming results

in a distinct enrichment of Al in the crystallite surface. In the

most dealuminated zeolite, the Si/Al ratio on the surface (about

8) is more than four times lower than in the bulk (about 35). It

Figure 1. 27Al MAS NMR spectra of HZSM-5 zeolites (parent,dealuminated with 7, 13, and 93 kPa H2O).

Figure 2. XPS spectra of HZSM-5 zeolites (parent, dealuminated with13 and 93 kPa H2O).

14452 J. Phys. Chem., Vol. 100, No. 34, 1996 Datka et al.


3/6

means that a great amount of Al removed from the frameworkmigrated to the surface.

IR Studies. Spectra of OH Groups. The spectra of OHgroups in HZSM-5 parent zeolite and zeolites dealuminated with7, 13, 40, and 93 kPa of water are presented in Figure 4. Thetreatment with steam results in a decrease in the Si-OH-Alband at 3610 cm-1 and in an increase in the Al-OH band at

3662 cm-1

. The silanol group bands at 3725 and 3742 cm-1

are practically uninfluenced by this treatment.Concentration of Bronsted and Lewis Acid Sites. The

concentrations of both Bronsted and Lewis acid sites weredetermined in quantitative studies of pyridine sorption. Smallportions of pyridine were sorbed at 420 K until the intensitiesof the pyridinium ion (PyH+) band at 1545 cm-1 and thepyridine bonded to Lewis acid sites (PyL) at 1450 cm-1 becameconstant. The concentrations of Bronsted and Lewis sites werecalculated from these maximal intensities and extinction coef-ficients of PyH+ and PyL bands. The extinction coefficient ofthe PyH+ band (1545 cm-1) was determined in experiments inwhich measured portions of pyridine were sorbed in an activatedparent HZSM-5 zeolite. The intensity of the 1545 cm-1 band

increased linearly with the amount of pyridine sorbed, and thevalue of the extinction coefficient (0.079 cm2/mol) wascalculated from the slope of this line. The extinction coefficientof the PyL band (1450 cm-1) was determined in experimentsin which known portions of pyridine were sorbed in a dehy-droxylated HY zeolite (activated at 1100 K). The intensity ofthe 1450 cm-1 band increased linearly with the amount ofpyridine, reacting with Lewis sites (the amount of pyridinesorbed minus the amount of pyridine reacting with Bronstedsitesscalculated from the intensity of PyH+ and the extinctioncoefficient of this band). The value of the extinction coefficientof the 1450 cm-1 PyL band was 0.269 cm2/mol. The valuesof the concentrations of both Bronsted and Lewis acid sites arepresented in Table 1. In all the zeolites, parent and dealuminated

ones, concentrations of Bronsted sites are very close to theamounts of tetrahedral Al determined in 27Al MAS NMRstudies. The concentration of Lewis sites increases upon thedealumination, but this increase is relatively low (from 0.2 to0.8 site/unit cell). The amount of Lewis sites created upondealumination is distinctly lower than the amount of Al removedfrom tetrahedral positions (4.0 Al/unit cell) in case of the mostdealuminated zeolite (93 kPa H2O).

Acidic Strength of Bronsted Sites. The acidic strength ofBronsted acid sites (Si-OH-Al groups) was studied in am-

monia thermodesorption experiments followed by IR spectros-copy. An excess of ammonia was first sorbed at 320 K inactivated zeolites, and the intensity of the band of ammoniumions at 1540 cm-1 (Ao) was measured. Ammonia was thendesorbed at 660 K for 30 min, and the spectrum was recordedat 320 K. The intensity of the 1450 cm-1 band of ammoniumions which remained on the surface after desorption (A660) wasmeasured, and the ratio A660/A0 (expressing the fraction ofammonium ions remaining at the surface after desorption) wastaken as the measure of the acidic strength of Bronsted sites.The A660/A0 values are presented in Figure 5. The maximalacidic strength of Bronsted sites is observed in the mildlydealuminated (7 kPa H2O) zeolite.

Distribution of the Acidic Strength of OH Groups. The

information about the distribution of the acidic strength of Si-OH-Al groups was obtained in experiments of chlorobenzenesorption. Two zeolites were studied: parent HZSM-5 andzeolite dealuminated with 7 kPa H2O (showing the highest acidstrength and the highest catalytic activity, vide infra). Such astudy was impossible for the more dealuminated zeolites (with40 and 93 kPa H2O) because of a very low total amount of OHgroups. The spectra of activated zeolites and zeolites withchlorobenzene sorbed and difference spectra are presented inFigure 6. Figure 6C shows expanded also the spectra of Si-OH-Al groups interacting (by hydrogen bonding) with chlo-robenzene. In the case of mildly dealuminated HZSM-5 zeolite(spectrum B), the band is narrower, thus suggesting a narrowerdistribution of the acidic strength. It should be noted that the

Figure 3. Si/Al values on the surface and in the bulk for parent anddealuminated HZSM-5 zeolites.

Figure 4. IR spectra of OH groups in parent (a) and dealuminatedHZSM-5 zeolites (b-e).

Figure 5. Acidic strength of Bronsted sites (A660/Ao) as function ofthe partial pressure of water.

TABLE 1: Characterization of Samples by NMR and IRSpectroscopy

27Al MAS NMRIR spectroscopy ofpyridine adsorption

vapor pressureof H2O used

(kPa)

tetrahedralAl species(sites/u.c.)a

octahedralAl species(sites/u.c.)

Bronsted acidsites

(sites/u.c.)

Lewis acidsites

(sites/u.c.)

0 (parent) 6.0 0.6 4.8 0.27 3.2 0.4 3.1 0.2

13 3.2 0.4 3.1 0.3

40 2.0 0.7 1.9 0.493 1.4 0.8 0.8 0.6

a Site/u.c. means site per unit cell of the zeolite.

Properties of HZSM-5 Zeolites J. Phys. Chem., Vol. 100, No. 34, 1996 14453


4/6

submaxima of the highest frequencies (of the lowest ) aremissing, suggesting that the less acidic hydroxyls are removedby dealumination.

Catalytic Tests. The catalytic test reaction of n-heptanecracking was performed with nitrogen and hydrogen as carriergases. The first point of measurement was after 2 min of timeon stream (TOS). Figure 7 shows the initial cracking rate (TOS2 min) as a function of the partial pressure of water duringdealumination. Under the given reaction conditions, the conver-sion ofn-heptane was lower than 5% and the rate was calculated

from the rates of formation of hydrocarbons with one to sixcarbon atoms.14 The isomerization reactions were lower than15% of the total conversion. A slight maximum of catalyticactivity in n-heptane cracking can be observed (Figure 7) inthe case of mildly dealuminated zeolite (7 kPa H 2O). Thereaction rate observed in case of the most dealuminated (93kPa H2O) zeolite was even lower than in the non-dealuminatedparent HZSM-5 zeolite. The same effects were observed whenhydrogen was used as the carrier gas. The rate of n-heptanecracking was higher in hydrogen than in nitrogen. In both

experiments, the main products of reaction were C3 and C4hydrocarbons. Saturated hydrocarbons dominated over unsatur-ated products after cracking in hydrogen (Table 2).

Discussion

The treatment of HZSM-5 zeolite with steam resulted inremoval of Al atoms from framework tetrahedral positions. Thiswas demonstrated by 27Al MAS NMR studies (Table 1). Theamount of framework Al decreased from 6 to 1.4 Al/u.c. uponthe treatment with 93 kPa H2O. The loss of framework Al wasaccompanied by the loss of acidic Si-OH-Al groups asevidenced by IR spectroscopy. The OH band at 3610 cm-1

decreased (Figure 4), and the quantitative IR studies of pyridine

sorption showed that the amount of acidic Si-

OH-

Al waspractically equal to the amount of framework tetrahedral Alwhich remained in the framework after steaming (Table 1).

While the treatment with steam removes Al atoms fromtetrahedral framework positions, 27Al MAS NMR studiesevidenced, however, that the amount of octahedral Al waspractically constant in all the zeolites studied (Figure 1 and Table1). It means that the removed Al atoms do not have octahedralcoordination. It is also possible that some Al forms largeragglomerates in the channels or on the surface. The quantitativeIR studies of sorbed pyridine showed (Table 1) that theconcentration of Lewis acidic sites formed in steamed zeoliteswas much lower than the amount of Al which left tetrahedralpositions. For the most dealuminated zeolite (93 kPa H2O),

the amount of Lewis acid sites formed was 0.6 sites/unit celland the amount of Al removed from tetrahedral positions was4.0 Al /unit cell. Therefore only a small part of the removedAl creates Lewis acid sites. IR studies showed also that theintensity of the Al-OH band (3662 cm-1) increased upondealumination (Figure 4), indicating that some Al atoms formedAl-OH species. The results of our XPS experiments (Figures2 and 3) have shown that most of Al removed from theframework by the treatment with steam migrated to the externalpart of the crystals (this was evidenced by the distinct decreasein Si/Al ratio near the surface).

The fact that the intensity of the Si-OH band (3742 cm-1)is practically uninfluenced by steaming (Figure 4) suggests thatnew silanol groups are not formed under our steaming condi-

tions. A previous 29Si MAS NMR and IR study17 showed thatin Y zeolite dealuminated by EDTA treatment, vacancies leftby Al atoms removed, were not filled by Si atoms and manynew Si-OH groups were formed. Cross-polarization29Si MASNMR experiments evidenced that these Si-OH groups wererelated to the vacancies. In the present study, of HZSM-5dealuminated by the treatment with steam, no new silanol groupsare formed (Figure 4), suggesting that the vacancies formed byAl-removing are filled by Si atoms migrating from other placesin the zeolite. As mentioned, dealumination with EDTA leavesvacancies not filled with Si atoms. This difference can beexplained by taking into account that, in our study, thedealumination (by treatment with steam) was done at a muchhigher temperature (770 K) than in the study with EDTA (370

Figure 6. Distribution of the acidic strength of OH groups in parent(A) and dealuminated with 7 kPa H2O (B) HZSM-5 zeolite: a, activatedzeolite; b, zeolite with chlorobenzene sorbed; c, difference spectra. (C)Spectra of OH groups in parent (a) and in dealuminated (b) zeolitenormalized to the same band area.

Figure 7. Catalytic activity (rate ofn-heptane cracking) as a functionof the partial pressure of water obtained with nitrogen and hydrogenas carrier gases.



5/6

K). At this high temperature, the mobility of framework atomsis higher and it results in filling the vacancies left by Al.

From the results of our IR studies of ammonia desorption, itcan be concluded that the dealumination by steaming yields ina variation of acidic strength of Si-OH-Al groups. Accordingto the data presented in Figure 5, the mild dealumination (7kPa H2O) results in an increase in the acidic strength of Si-OH-Al groups. Further dealumination dimishes the acidicstrength. Two explanations of the increase in the acidic strength

in mildly dealuminated zeolites can be considered. One of themassumes that this increase in acidity is due to the interaction ofSi-OH-Al with an electron acceptor Al site created bydealumination (as considered by some authors, e.g., ref 9, inthe case of faujasites dealuminated by steaming). Such aninterpretation which was probably true for faujasites is lessprobable in the case of HZSM-5 zeolites in which theconcentration of Al and Si-OH-Al groups is much lower thanin faujasites; therefore, the distances are longer and theelectrostatic interaction is much weaker. Our quantitative IRstudies have shown that the concentration of Lewis acidic sitesin our steamed zeolites is low (much lower than the concentra-tion of Si-OH-Al groups) and that most of the Al migrates tothe surface (XPS results). All these results suggest that the

increase in the acidic strength of OH groups in mildly dealu-minated HZSM-5 is probably not due to the interaction withelectron acceptor Al species. Therefore, another interpretationcan be taken into consideration. Our previous studiesevidenced18-20 that Si-OH-Al groups in HZSM-5 zeolite wereheterogeneous; five kinds of bridging hydroxyls of various acidicstrength were found. We suppose that the heterogeneity ofhydroxyl groups in HZSM-5 was due to the presence of Si-OH-Al of various bridge geometries (XRD studies of Olsonet al.21 found that the T-O-T bridge angle in the MFI structurevaries from 143to 175and T-O bond distances from 0.152to 0.167 nm). It may be assumed that the increase in the acidicstrength of OH groups in mildly dealuminated HZSM-5 is theresult of variation of the distribution of the OH acidic strength

upon dealumination. The distribution of the acidic strength wastherefore studied by following IR spectra of OH groupshydrogen-bonded to chlorobenzene molecules (Figure 6). It wasfound that in mildly dealuminated zeolite (7 kPa H2O) thedistribution of the acidic strength of Si-OH-Al is narrowerthan in the parent zeolite. The band of hydrogen-bonded OHis narrowed from the site of high frequencies, indicating thatthe hydroxyls of the lowest acidic strength (of the lowest )are missing. It may be suggested that the increase in the acidicstrength of OH groups in mildly dealuminated zeolite (evidencedin ammonia thermodesorption experiments, Figure 5) is due toremoval of the less acidic hydroxyls in the first order. It maybe also suggested that the Al atoms which form the less acidicSi-OH-Al hydroxyls are these, which are initially removed

(by the treatment with steam) from the framework. The factthat the distribution of the acidic strength of Si-OH-Al inmildly dealuminated zeolite is narrower than in the parent oneis another argument supporting a hypothesis that the increasein the acidic strength is not due to the interaction of Si-OH-Al groups with electron acceptor Al species. If such aninteraction would be the reason of the increase in the acidicstrength of Si-OH-Al, the IR band of OH interacting withchlorobenzene should be only shifted to lower frequencies

without visible variation of its half-width. The data presentedin Figure 5 indicate also that the acidic strength of Si-OH-Algroups decreases in more strongly dealuminated zeolites. Atthe present moment no resonable interpretation of this decreasecan be presented. It would be very interesting to study howstronger dealumination (at higher vapor pressures of water)influences the distribution of the acidic strength. Such a studyis, however, not possible because the concentration of Si-OH-Al hydroxyls is too low to obtain spectra of hydroxyls hydrogen-bonded to chlorobenzene with reasonable signal-to-noise ratios.

It should be noted that Auroux et al.,22 who studied acidityof HZSM-5 zeolites of various Al contents by microcalorimetry,reported a maximum of the acidic strength in zeolite with about4.5 Al/unit cell, which is close to the value 3.2 Al/unit cell in

our mildly steamed HZSM-5. According to Auroux, both theincrease and decrease in Al content from the value 4.5 Al/unitcell result in a lowering of the acidic strength. Similar effectswere also observed in our case (Figure 6).

The catalytic properties of our zeolites were studied in then-heptane cracking reaction. The data presented in Table 2show thatn-heptane cracking is a bimolecular reaction.14 Theproduct distribution did not depend significantly on the acidityof zeolites.

The results of our catalytic studies (Figure 7) agreed wellwith the data concerning zeolite acidity (Table 1 and Figure 7).The catalytic tests indicated that a maximum in catalytic activityis observed in mildly dealuminated zeolite (7 kPa H2O). Theincrease in the catalytic activity can be explained by considering

the increase in the acidic strength of Si-OH-Al groups (asevidenced by IR spectroscopy, Figure 6). This increase in theacidic strength compensates the decrease in the number of acidsites (from 4.8 to 3.1 H+/unit cell). The further dealuminationresults in the decrease in the catalytic activity that can beexplained by the decrease in both the concentration and acidicstrength of Si-OH-Al. Similar effects were observed byLukyanov,5 who reported also an increase in the catalytic activityof HZSM-5 inn-hexane cracking in mildly dealuminated zeoliteand a decrease in activity for more severly dealuminated zeolites.Our results and also the results of Lukyanov5 suggest that thereare defined conditions of zeolite dealumination which givezeolites of the optimum concentration and acidic strength ofactive sites.

TABLE 2: Product Distribution and Molar Ratio of Paraffins to Olefins for Two Different Carrier Gases (H2 and N2) afterCracking of n-Heptane over Dealuminated Zeolites

zeolite/partial pressure of water (kPa)

0 7 13 40 93

hydrocarbons (%) H2 N2 H2 N2 H2 N2 H2 N2 H2 N2

C1 2.41 0 1.77 0.16 2.11 0 1.91 0 2.02 0C2 1.28 0.65 1.30 0.82 1.37 0.85 1.38 0.52 1.35 0.78C3 26.18 28.32 26.67 28.75 27.14 29.96 27.20 25.86 24.50 30.44C4 32.01 34.28 33.37 36.35 32.08 35.57 34.12 33.27 33.09 35.68C5 17.13 15.75 18.13 17.81 17.02 16.75 18.37 16.55 18.13 16.28

C6 10.52 7.94 11.00 9.29 10.33 8.59 9.89 9.17 10.96 7.98iso-C7 10.47 13.06 7.75 6.81 9.96 8.30 7.12 14.63 9.96 8.84P/Oa 5.30 1.56 5.66 1.77 4.67 1.70 4.20 1.59 4.47 1.51

a Ratio of paraffins to olefins (mol/mol).

Properties of HZSM-5 Zeolites J. Phys. Chem., Vol. 100, No. 34, 1996 14455


6/6

According to the data presented in Figure 7, higher crackingactivity is observed if the reaction is carried out in hydrogen,instead of nitrogen. It suggests that hydrogen transfer plays animportant role in the cracking reaction drawn in previouspapers.14,23 Another piece of evidence on the role of hydrogentransfer in cracking is a considerably higher paraffins/olefinsratio in products (Table 2) when the reaction is carried out inhydrogen.

Acknowledgment. This study was supported by a grant

(0634/P3/94/07) of Komitet Badan Naukowych. The studiesat Leipzig University were supported by Deutsche For-schungsgemeinschaft (DFG) within the Sonderforschungsbereich294 Molekule in Wechselwirkung mit Grenzflachen at theUniversity of Leipzig. F.W.S. was supported by a grant of theHochschulerneuerungsprogramm (HEP) of the SachsischesMinisterium fur Wissenschaft und Kultur. Additionally wethank D. Freude for measurement of the 27Al MAS NMR spectraand helpful discussion.

References and Notes

(1) Scherzer, J.J. Am. Chem. Soc. Symp. Ser. 1984, 248, 157.(2) Brunner, E.; Ernst, H.; Freude, D.; Krause, C. B.; Prager, D.;

Reschetilowski, W.; Schwieger, W.; Bergk, K.-H. Zeolites 1989, 9, 282.(3) Lago, R. M.; Haag, W. O.; Mikowsky, R. J.; Oslon, D. H.; Hellring,

S. D.; Schmitt, K. D.; Kerr, G. T. In Proceedings of the 7th InternationalZeolite Conference; Murakami, Y., et al., Eds.; Kondansha: Tokyo, 1986;p 677.

(4) Topsoe, N. Y.; Joensen, F.; Derouane, E. G. J. Catal. 1988, 110,404.

(5) Lukyanov, D. B. Zeolites 1991, 11, 325.(6) Lukyanov, D. B. Zeolites 1994, 14, 233.(7) Witzel, F.; Karge, H. G.; Gutche, A. In Proceedings of 9th

International Zeolite Conference; von Balmoos, R., et al., Eds.;Butterworth-Heinemann: Boston, MA, 1993; p 283.

(8) Arribas, J.; Corma, A.; Fornes, V.; Melo, F.J. Catal. 1987, 108,135.

(9) Makarova, M. A.; Garforth, A.; Zholobenko, V. I.; Dwyer, J.; Earl,G. J.; Ravlence, D.Proceedings of the 10th International Zeolite Conference;Weitkamp, J., et al., Eds.; Elsevier: Amsterdam, 1994; p 365.

(10) Biaglow, A. I.; Gorte, R. J.; Kokotailo, G. T.; Parrillo, D. J. J.Catal. 1994, 148, 213.

(11) Zholobenko, V. L.; Kustov, L. M.; Borovkov, V. Yu.; Kazansky,V. B. Kinet. Catal. 1987, 28, 965.

(12) Zholobenko, V. L.; Kustov, L. M.; Kazansky, V. Y.; Loeffler, E.;Lohse, U.; Peuker, Ch.; Oehlmann, G. Zeolites 1990, 10, 304.

(13) Freude, D.; Ernst, H.; Wolf, I.Solid State Nucl. Magn. Reson.1994,3, 271.

(14) Meusinger, J.; Liers, J.; Moesch, A.; Reschetilowski, W.J. Catal.

1994, 148, 30.(15) Briggs, D.; Seah, M. P. Practical Surface Analysis; John Wiley

and Sons: Chichester, U.K., 1983; p 511.

(16) Kraak, P.; Schodel, R.; Goehl, I.; Brand, A.; Schwieger, W. Chem.Tech. 1992, 7, 8.

(17) Datka, J.; Kolodziejski, W.; Klinowski, J.; Sulikowski, B. Catal.Lett. 1993, 19, 159.

(18) Datka, J.; Boczar, M.; Rymarowicz, P. J. Catal. 1988, 114, 368.

(19) Datka, J.; Boczar, M. Zeolites 1991, 11, 397.

(20) Datka, J.; Boczar, M.; Gil, B. Langmuir 1993, 9, 2496.

(21) Olson D. M.; Kokotailo, G. T.; Lawson, S. L.J. Phys. Chem.1981,85, 2238.

(22) Auroux, A.; Gravelle, P. C.; Vedrine, J. C. Proceedings of 5thInternational Conference on Zeolites; Rees, L. V., et al., Eds.; Heyden:London, 1980; p 433.

(23) Meusinger, J.; Corma, A. J. Catal. 1995, 152, 189.

(24) Dwyer, J. In InnoV

ation in Zeolite Materials Science; Stud. Surf.Sci. Catal.; Grobet, P. J., et al., Eds.; Elsevier: Amsterdam, 1988; p 333.

(25) Fritz, P. O.; Lunsfort, J. H. J. Catal. 1989, 118, 85.

(26) Makarova, M. A.; Dwyer, J. J. Phys. Chem. 1993, 97, 6337.

(27) Garallon, G.; Corma, A.; Formes, V. Zeolites 1989, 9, 84.

(28) Chambellan, A.; Chevreau, T.; Khabtou, S.; Marzin, M.; Lavalley,J. C. Zeolites 1992, 12, 306.

(29) Sauer, J.; Schirmer, W. InInnoVation in Zeolite Materials Science;Stud. Surf. Sci. Catal.; Grobet, P. J., et al., Eds.; Elsevier: Amsterdam,1988; p 323.

JP960685I


physicochemical and catalytic properties of hzsm-5 zeolites dealuminated by the

Documents