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Role of Structure and Acidic Nature on CatalyticBehavior of Nickel Supported H-mordenite, H-ZSM-5,and alumina CatalystsM. S. Ghattas

a, H. M. Gobara

a, S. A. Henin

a, S. A. Hassan

b& F. H. Khali l

a

aCatalysis Department, Egyptian Petroleum Research Inst it ute, Nasr Cit y, Cairo, Egypt

bChemistry Department, Facult y of Science, Ain Shams Universit y, Abbasia, Cairo, Egypt

Available online: 24 Oct 2007

To cite this art icle: M. S. Ghattas, H. M. Gobara, S. A. Henin, S. A. Hassan & F. H. Khali l (2007): Role of St ruct ure and AcidNature on Catalyt ic Behavior of Nickel Support ed H-mordeni te, H-ZSM-5, and alumina Catalysts, Petroleum Science and

Technology, 25:10, 1279-1291

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Petroleum Science and Technology, 25:12791291, 2007Copyright Taylor & Francis Group, LLCISSN: 1091-6466 print/1532-2459 onlineDOI: 10.1080/10916460600803645

Role of Structure and Acidic Nature on Catalytic

Behavior of Nickel Supported H-mordenite,

H-ZSM-5, and alumina Catalysts

M. S. Ghattas, H. M. Gobara, and S. A. Henin

Catalysis Department, Egyptian Petroleum Research Institute,Nasr City, Cairo, Egypt

S. A. Hassan

Chemistry Department, Faculty of Science, Ain Shams University,Abbasia, Cairo, Egypt

F. H. Khalil

Catalysis Department, Egyptian Petroleum Research Institute,Nasr City, Cairo, Egypt

Abstract: Nickel metal was loaded in different percentages (7, 10, and 13% w/w)on different supports (H-mordenite, H-ZSM-5, and alumina). The prepared catalyst

samples were tested in cyclohexane conversion using microreactor pulse technique.Structure was followed up by XRD analysis. Chemisorption of tert-butylamine (TBA)was adopted for estimating the number of surface acid sites. It was found that allprepared samples displayed cracking activity, being mostly related to the fractionof acid sites remaining on the surface after coverage with supported Ni atoms. H-mordenite-supported samples exhibited mainly isomerization functionality by showinga larger portion of surface acid sites. H-ZSM-5-supported samples showed higherdehydrogenation activity. Agglomeration seemed to be responsible for lower activityof the sample of higher Ni content. The formed NiOOH phase was suggested to beresponsible for increased dehydrogenation activity on H-ZSM-5 samples and increasedcracking activity on alumina-supported samples of higher Ni content.

Keywords: alumina, acidity, cyclohexane conversion, H-mordenite, H-ZSM-5,metal-support interaction

Address correspondence to Maged Samir Ghattas, Petroleum Refining Division,Egyptian Petroleum Research Institute, Nasr City, Cairo 11727, Egypt. E-mail: [email protected]

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1280 M. S. Ghattas et al.

INTRODUCTION

Bifunctional catalysts with metallic and acidic functionality are used in nu-merous industrial processes in petroleum refining and in petrochemical indus-tries. Transition metal can catalyze reactions such as isomerization, (de)hydro-genation, and hydrogenolysis of hydrocarbons.

Catalytic hydrocracking is an essential process in a modern refinery forthe conversion of middle and heavy distillates and residuals into more valu-able products (Arroyo et al., 2000).

Zeolites play an important role in the catalysts used in hydrocarbonconversion because they improve catalytic activity, selectivity, or stability byimparting shape selectivity (Blauwhoff et al., 1999; Jensen et al., 2002; Habiband Dahlberg, 2002; Van Veen et al., 2002; Corma et al., 2002; Kramer et al.,2001; Daage et al., 2002).

It is well known that zeolites such as ZSM-5 (Gervasini, 1999; Iwamotoet al., 1988) and mordenite (Haag and Lago, 1983; Lzaro et al., 1991;Caizares et al., 1998) can be modified by incorporating metals such as Cu,Co, Ni, Mn, Ga, etc., in order to obtain catalysts for these industrial processes.

Nickel catalysts are considered cheap economic substitutes for platinumcatalysts toward conversion of hydrocarbons. They play different roles suchas cracking, isomerization, and (de)hydrogenation catalysts according to theirsupport and nickel percent loading.

The state of nickel on some supports, such as alumina and HY zeolite,can be strongly modified causing surprising changes of the catalytic prop-erties. For example, the well-known hydrogenolysis activity of nickel couldbe suppressed, whereas its dehydrogenating property increased and becamepredominant in some cases (Lanh et al., 1991; Hoang et al., 1994).

The acid-site, metal-site densities, and acid-strength distribution are im-portant parameters where their proper balance is critical in determining theactivity of these catalysts (Taylor and Petty, 1994; Walters et al., 1995). Thepresence of a metal on zeolite surfaces may modify the Brnsted centers(Minchev et al., 1994) and the metal state may be modified by the influenceof the support (Lanh et al., 1991).

This work was undertaken to study the effect of nickel-support inter-action in different samples composed of nickel with different percentageloadings (namely, 7, 10, and 13%) on the catalytic activity of the catalystsunder investigationthe support chosen being H-mordenite, H-ZSM-5, andalumina.

EXPERIMENTAL

Catalyst Preparation

H-Mordenite, H-ZSM-5, and Al2O3 were treated as follows:


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Role of Structure and Acidic Nature on Nickel Supported Catalysts 1281

(1) H-Mordenite (Si/Al ratio 10.8) was obtained by treatment of Na-

mordenite (School of Chemical Engineering, Seoul Natl University,Seoul) with 0.2 N NH4Cl solution with stirring for 72 hr at room temper-ature, then washed with distilled water til free from chloride ions. Theslurry was dried at 120C and calcined at 400C in a stream of air for4 hr.

(2) H-ZSM-5 was obtained by a treatment of Na-ZSM-5 (of high Si/Alratio D 80 supplied by the Institute of Organic Chemistry, Academy ofScience, Moscow), first by heating in a stream of air at 550C to excludeorganic matters, then applying the same procedure as described for Na-mordenite.

(3) Al2O3 (specific surface area D 230 m2 g1, pore volume D 0.72c.c. g1, and containing Na2O3 and Fe2O3 < 0.02 wt%) supplied by theInstitute of Catalysis (Novisipersk, Russia) and was calcined at 450C in

a stream of air for 6 hr.

The supported samples were prepared by impregnating the requiredamount of the support with nickel nitrate hexahydrate solution of the properconcentration. The obtained slurry was stirred vigorously for 15 min anddried at 110C for 16 hr, then calcined at 200C for 4 hr in a stream of air.To effect homogenity, the produced oxide was reduced using in a flow of H 2at 350C for 4 hr.

Catalyst Characterization

X-ray Diffraction (XRD) Analysis

The x-ray diffraction (XRD) analysis for all metal oxide catalyst sampleswas carried out using x-ray diffraction equipment model PW/1710 (PhilipsCompany, the Netherlands) using an Ni filter and Cu k-radiation ( D1:542 ) at 40 kV and 30 mA.

Acidity Determination

The chemisorption of t-butyl amine (TBA) was applied in order to estimatethe surface acidity (Andrs et al., 1990) of all catalyst samples at the ap-propriate temperature, i.e., far from its physical adsorption and before itscatalytic decomposion of TBA. A microreactor pulse technique was applied.

0.5 g of dry sample was preheated at 450

C for 2 hr in a stream of hydrogen.The adsorption of TBA was carried out under atmospheric pressure using aflow rate of 30 ml/min. T-butyl amine was injected in doses of 2 L to thesample cell in a stream of hydrogen as a carrier gas. The desorbsion peaksof TBA after complete neutralization of all surface acid sites were detectedusing computerized data acquisition.


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Catalytic Activity

The catalytic activities of the prepared catalysts were tested by cyclohexanedehydrogenation as a model reaction using a microcatalytic pulse technique.The microreactor was filled with 0.25 g of the tested catalysts. The prod-ucts were then directly passed through a chromatographic column, 200 cmin length and 0.6 cm in internal diameter packed with acid washed PWand (6080 mesh size) loaded with 15% by weight squalane adjusted andcontrolled at 40C. A flam ionization detector (FID) was used. The reactionswere carried out under atmospheric pressure in the temperature range 180C450C. Hydrogen flow rate and cyclohexane doses were kept constant at 50ml/min and 2 L, respectively. A computerized data acquisition was used forintegration and recording the effluents yield.

RESULTS AND DISCUSSION

X-ray Diffraction Analysis

Ni/Hmordenite (Ni/H-M) Catalysts

The XRD pattern of H-mordenite support is shown to be diminished uponloading with nickel in different percentages (Figure 1), retaining the character-istic lines of the different faces, although with less intensity. No characteristiclines of different Ni species could be detected.

Ni/H-ZSM-5 Catalysts

Figure 2 indicated that XRD patterns of the supported nickel samples differfrom the traditional one of H-ZSM-5, only in that a new peak at dD 6:9 characterizing most probably NiOOH (ASTM card No. 6-75). This phaseseems to be formed due to the interaction between NiO with the free OHpresent in the framework of the support. The intensity of this phase increasesby increasing Ni content especially in highly loaded samples (10 and 13%w/w Ni). No characteristic peaks of NiO phase could thus be detected.

One may suggest that a fraction of the formed nickel species can beimbedded in the channels or the cages of the support, being more pronouncedin the sample containing 7% Ni. This may confirm the possibility of inter-action of this phase with OH of the support and formation of NiOOH in the

samples of higher Ni content.Ni/alumina Catalysts

The obtained XRD pattern of the used alumina support (Figure 3) is char-acteristic for the alumina (ASTM card No. 10-425) with the indicated d-spacing. No sign of a presence of an NiO phase in the case of the supported


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Figure 1. XRD of nickel/H-M catalysts.

sample. One cannot exclude the possibility of formation of the strong in-teraction phase of nickel aluminate (NiAl2O4), d-spacing of which (2.43,2.01, 1.54, and 1.42 ) seem to overlap with these characteristic peaks ofalumina shown. Moreover, for the catalyst of the higher nickel loading(viz., 13% w/w), a small peak is observed with d-spacing D 6.7 , which

may be referred to the NiOOH phase starting to be formed at this high content.

Acidities of the Catalysts Under Investigation

The acidities expressed in mole TBA adsorbed per g catalyst, for differentsupports, and supported Ni samples are summarized in Table 1. These values


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Figure 2. XRD patterns of Ni/H-ZSM-5 catalysts.


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Figure 3. XRD of nickel/alumina catalysts.

were converted into the number of acid sites on the surface of 1-g catalystsamples.

It is clearly evident that the acidity of the pure support run in order,H-mordenite > H-ZSM-5 > alumina. Mordenite has the highest aciditymost probably due to textural structure where Si/Al ratio 10.8, whereasit is 80 in case of high silica H-ZSM-5, which consequently have less OH

groups.In all supported samples, acidity decreases gradually with the increasein Ni wt%. Acidity suffers a greater decrease compared to the original sup-port in the sample of 7% wt after which the decrease becomes monotonous.Considering the percentage of the total acid sites existing on each supportthat being covered by the supported Ni species, it appears that the highestcoverage with Ni species takes place in the order H-ZSM-5 > H-mordenite >


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Table 1. Acidity function of the investigated catalysts

Catalysts mole/g catal.No. acid

sites/g catal.% acid sites

covered by Ni

H-Mordenite (supp.) 536a 2:14 1020 7% Ni/H-Mordenite 263a 1:58 1020 26.210% Ni/H-Mordenite 236a 1:42 1020 33.613% Ni/H-Mordenite 177a 1:07 1020 50.0H-ZSM-5 (supp.) 214b 1:29 1020 7% Ni/H-ZSM-5 114b 6:84 1019 47.010% Ni/H-ZSM-5 108b 6:94 1019 46.013% Ni/H-ZSM-5 93b 5:58 1019 56.7-Al2O3 (supp.) 107c 6:41 1019 7% Ni/-Al2O3 93c 5:6 1019 12.6

10% Ni/-Al2O3 79c 4:76 1019 25.713% Ni/-Al2O3 63c 3:78 1019 41.0

aTBA adsorption at 200C.bTBA adsorption at 250C.cTBA adsorption at 160C.

alumina. This means that the remaining exposed acid sites are greater inH-mordenite-supported surface than on H-ZSM-5, which should reflect onthe isomerization activity. Moreover, the Brnsted acids are known to beconverted to Lewis acids at a higher temperature >350C (Ghattas, 1995)leading to higher cracking products with the original Lewis acid formed in

mordenite support.The fraction of nickel interacted with the support surface is larger onH-ZSM-5 than on H-mordenite and alumina (as evidenced from XRDanalysis). It should take into consideration that the pore structure of thesupport plays a role in the evolution of the acidity.

Blocking of micropores by Ni atoms may take place in H-mordenite andH-ZSM-5, whereas alumina have wider pores that permit penetration ofNi atoms inside.

Catalytic Activity of the Catalysts Under Investigation

Cyclohexane Conversion over Ni/H-mordenite

Figure 4 represents the catalytic conversion of cyclohexane over Ni/H-morden-ite catalysts (7, 10, and 13 w/w Ni). Over these catalysts, cyclohexane isomer-ization is more pronounced than its dehydrogenation, which seems to be dueto the presence of strong Brnsted acid sites being increased by increasingboth nickel-loading and reaction temperature up to 270C. At a temperature


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Figure 4. Catalytic conversion of cyclohexane over Ni/H-mordenite catalysts: (a) 7%Ni, (b) 10% Ni, and (c) 13% Ni.

more than 270C, the formation of other products probably, propane may be

favored according to thermodynamic feasibility.Cracking yield in mole percentage and selectivity increase with the in-crease in reaction temperature and nickel-loading. It achieves 100% at 360Con the catalyst sample containing 13% w/w Ni.

Isomerization reaction occurs at low temperatures (viz., 210C360C),while cracking is more favored at a higher temperature (viz., 360C480C).


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Figure 5. Catalytic conversion of cyclohexane over Ni/H-ZSM-5 catalysts: (a) 7%Ni, (b) 10% Ni, and (c) 13% Ni.

The two reactions seem to take place at different acid sitesBrnsted forisomerization and Lewis acid sites for cracking. The result in general run inharmony with those of acidity data.

Cyclohexane Conversion over Ni/H-ZSM-5

The results illustrated in Figure 5 indicated that both cracking and dehydro-genation of cyclohexane take place with no isomerization products. Referring


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to results of acidity, the cracking activity can be referred to the remaining

exposed acid sites (after covering with nickel), being less than those existingin H-mordenite supported samples. The dehydrogenation activity appears todecrease with the increase in nickel percentage loading. This can be under-stood in view of the narrower pore system of H-ZSM-5, where more fractionsof nickel atoms will exist on the surface in larger aggregates of lower dehy-drogenation activity, becoming more pronounced on the catalyst sample of13% Ni by weight. The absence of isomerization products can also be in-terpreted in view of the observation that the major fraction of surface nickeland probably NiOOH species formed (cf. XRD results) is site-interacted withthe support acid sites remaining only acid site exposed for the isomerizationreaction.

Cyclohexane Conversion over Ni/aluminaLow yield of cyclohexane dehydrogenation to benzene was observed overNi/alumina catalysts as shown in Figure 6. It exhibits higher crackingactivity, which increases by elevating the nickel-loading and reaction temper-ature. The much-less dehydrogenation activity may be referred to the less-exposed nickel species as a result of the formation of a strong interactionphase, namely, NiAl2O4 with the defective tetrahedral sittings of alumina.

The cracking activity seems to result from some remaining acid sitesof the alumina surface after being covered by nickel atoms. Moreover, theproduced NiOOH species, especially in the sample of high nickel content(viz., 13% w/w) may also be responsible for this cracking activity (refer toXRD results).

CONCLUSIONS

The following conclusions may be drawn. The three types of investigatedcatalysts, Ni/H-mordenite, Ni/H-ZSM-5, and Ni/Al2O3, were found to dis-play cracking especially in the high temperature range, 360C480C. Mostof the fraction of acid sites remained on H-mordenite > H-ZSM-5 afterbeing covered with supported nickel atoms. NiOOH phase formed in Al2O3-supported samples (especially of 13% Ni w/w) played an additionalrole in cracking activity. H-mordenite-supported samples were consideredas isomerization catalysts. The remaining acid site on the support surface

after being covered with Ni atoms appear operating factor in this reaction(especially the sample of 13% Ni w/w). H-ZSM-5-supported samples wereconsidered as dehydrogenation catalysts. The fraction of exposed Ni atomsexisting, mainly covering the extent of its acid sites, appear as operating fac-tors which associate in less active larger particles in the sample of higher Nicontent.


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Figure 6. Catalytic conversion of cyclohexane over Ni/alumina catalysts: (a) 7%Ni, (b) 10% Ni, and (c) 13% Ni.

REFERENCES

Andrs, R., Aguayo, T., Arandes, J. M., Olazar, M., and Bilbao, J. (1990).Ind. Eng. Chem. Res. 29:1621.

Arroyo, J. A. M., Martens, G. G., Froment, G. F., Marin, P. A., Jacob, P. A.,and Martens, J. A. (2000). Appl. Catal. A: General 192:9.


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Blauwhoff, P. M. M., Gosselink, J. W., Kieffer, E. P., Sie, S. T., Stork,

W. H. J., Weitkamp J., and Puppe, L. (1999). Catalysis and Zeolites.Berlin: Springer, p. 437.Caizares, P., de Lucas, A., Dorado, F., Durn, A., and Asencio, I. (1998).

Appl. Catal. A 169:137.Corma, A., Martinez, A., Guisnet, M., and Gilson, J.-P. (2002). Zeolites for

Cleaner Technologies. Imperial College Press, p. 29.Daage M., Guisnet, M., and Gilson, J.-P. (2002). Zeolites for Cleaner Tech-

nologies. Imperial College Press, p. 167.Ghattas, M. S. (1995). Preparation and study of some metal supported pe-

troleum refining catalysts. Ph.D. Thesis, Ain Shams University, Cairo,Egypt.

Gervasini, A. (1999). Appl. Catal. A 180:71.Haag, W. O., and Lago, R. M. (1983). U.S. Patent 4374396.

Habib, M. M., and Dahlberg, A. J. (2002). Hydrocarbon Eng. 7:45.Hoang, D. L., Berndt, H., Miessner, H., Schreier, E., Vlter, J., and Lieske,

H. (1994). Appl. Catal. A 114:295.Iwamoto, M., Yahiro, H., Tanada, K., and Inui, T. (1988). Successful Design

of Catalysis. Amsterdam: Elsevier, p. 219.Jensen, R. H., Guisnet, M., and Gilson, J. P. (2002). Zeolites for Cleaner

Technologies. Imperial College Press, p. 75.Kramer, D. C., Lok, B. K., and Krug, R. R. (2001). ASTM Special Techn.

Publ. STP 1407:25.Lanh, H. D, Khoai, N., Thoang, H. S., and Vlter, J. (1991). J. Catal. 129:58.Lzaro, J. J., Corma, A., and Frontela, J. M. (1991). U.S. Patent 5057471.Minchev, C., Zubkov, S. A., Valtchev, V., Minkov, V., Micheva, N., and

Kanazirev, V. (1994). Appl. Catal. 119:195.

Taylor, R. J., and Petty, R. H. (1994). Appl. Catal. 119:121.Van Veen, J. A. R., Guisnet, M., and Gilson, J.-P. (2002). Zeolites for Cleaner

Technologies. Imperial College Press, p. 131.Walters, W. J. J., van der Waerden, O. H., de Beer, V. H. J., and van Santen,

R. A. (1995). Ind. Eng. Chem. Res. 34:1166.

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