Transformation of methylcyclohexane on an FCC catalyst

Transformation of methylcyclohexane on an FCC catalyst

A.Rabeharitsara^I; H.S.CerqueiraII, ^*; P.Magnoux^I; M.Guisnet^I; A.F.Costa^II; E.F.Sousa-Aguiar^{II, III}

^IUniversité de Poitiers, UMR CNRS 6503. 40, Phone [phone], Fax [phone], Avenue du Recteur Pineau, 86022 Poitiers Cedex - France

^IIPETROBRAS, Centro de Pesquisas e Desenvolvimento Leopoldo A.M. de Mello (CENPES), Downstream Research and Development/FCC Technology, Ilha do Fundão, Phone: [phone], Fax [phone], Quadra 7, Cidade Universitária, CEP 21949-900, Rio de Janeiro - RJ, Brazil. Email: [e-mail] ^IIIEscola de Química, Universidade Federal do Rio de Janeiro, Centro de Tecnologia, Bloco E, Ilha do Fundão, Rio de Janeiro - RJ, Brazil

ABSTRACT

The transformation of methylcyclohexane at 723 K over on a USHY sample and on an FCC catalyst composed of 30% USHY and 70% matrix containing 25% Al₂O₃ was studied. With both samples, C₂-C₇ alkenes and alkanes, cyclopentane and methylcyclopentane (cracking products), dimethylcyclopentanes and ethylcyclopentane (isomers) and aromatics appeared as primary products. The activity and selectivity of fresh samples as well as the influence of coke deposits on porosity and selectivity are discussed.

Keywords: USHY zeolite, FCC catalyst, methylcyclohexane, coke, deactivation.

INTRODUCTION

Naphthenes are an important class of molecules in the fluid catalytic cracking (FCC) process. Little can be found in the open literature concerning their transformation on USHY zeolites (Corma and Agudo, 1981; Lin et al., 1989; Abbot, 1990; Sousa-Aguiar et al., 1996; Scofield et al., 1998; Cerqueira et al., 1999; Weitkamp et al., 2000; Cerqueira et al., 2001A) or FCC catalysts (Corma et al., 1991; Mostad et al., 1990; de la Puente et al., 1996).

In this work, the transformation of methylcyclohexane at 723 K on a USHY zeolite and on an FCC catalyst composed of 30% USHY and 70% matrix containing 25% Al₂O₃ was studied with the aim to determine the effect of the matrix on the activity and selectivity of fresh samples. Since both catalysts underwent deactivation, the nature of coke as well as its influence on porosity and selectivity is discussed.

EXPERIMENTAL METHODS

A NaY zeolite, synthesised according to a Toyo Soda patent (Toyo Soda Manufacturing Co. Ltd., 1984), was ion-exchanged with NH₄Cl, dried overnight at 393 K and then calcined at 773 K for 1 h, yielding USHY which was dealuminated with H₂SO₄ as described elsewhere (Sobrinho et al., 1995). The SiO₂-Al₂O₃ matrix was prepared in a pilot plant by a modified version of the Magee procedure (Magee and Blazer, 1976). The zeolite is referred to as USHY-A and the FCC catalyst (30% zeolite and 70% matrix) as PP-A22.

The physicochemical characteristics (crystallite size, pore volume and total acidity) of the samples are given in Table 1. PP-A22 has more mesopores (> 2 nm) and Lewis sites due to the presence of the matrix. It should be noted that the Brönsted acidity of PP-A22 per gram of zeolite (1140 µmol/g) is 2.1 times higher that the value obtained for USHY-A.

The pore volume of fresh samples was obtained by nitrogen adsorption at 77 K with the ASAP 2000 gas adsorption system (Micromeritics). The zeolite crystallite sizes were estimated by the CM120 scanning electron microscopy (Philips) images. The acidity of the fresh samples was estimated by adsorption of pyridine followed by infrared spectroscopy. The zeolites were pressed into thin wafers which were pretreated in vacuum (1.33´ 10^-4 Pa) at 473 K for 1 h prior to adsorption of the excess pyridine at 323 K; after 15 min physisorbed pyridine was removed by evacuation for 1 h at the same temperature. All the spectra were recorded at room temperature. In order to compare the acid strength of the protonic sites, a stepwise desorption of pyridine was conducted at 523, 623, 723 and 773 K. The residual Brönsted acidity, defined as the ratio between the number of Brönsted sites able to retain pyridine at a given temperature and at 423 K, indicates that the acid site distribution is similar for both catalysts (Figure 1).

Methylcyclohexane (Aldrich, 99% pure, percolated over SiO₂ before use) was transformed on the fresh catalyst samples in a fixed-bed reactor at 723 K, P_N2 = 9´10⁴ Pa, P_mcha = 1´10⁴ Pa at various contact times (t =1/WHSV) from 0.0065 to 0.65 h and times on stream (TOSs) from 1 min to 2 h. Before reaction, the catalyst samples were pretreated at 723 K under nitrogen flow for 2 h. Reaction products were analysed on-line by gas chromatography (GC) with a 50 m Plot Al₂O₃/KCl fused silica capillary column. The temperature program for the GC analysis was as follows: 353 K for 5 min with an increase in temperature (3 K/min) up to 423 K (10 min) followed by an increase in temperature (5 K/min) up to 473 K (25 min).

The coke content of the samples after reaction was measured by total burning at 1293 K in helium and oxygen with a Thermoquest NA2100 analyser. The pore volume of coked samples was measured in a Sartorius microbalance by nitrogen adsorption at 77 K. The method for recovering the coke from the coked zeolite had been previously described (Guisnet and Magnoux, 1989).

RESULTS AND DISCUSSION

The reaction products were divided into four families (Cerqueira et al., 2001A): olefins (C₂⁼-C₇⁼), paraffins (C₂-C₇ plus cyclopentane and methylcyclopentane), isomers (dimethylcyclopentane and ethylcyclopentane) and aromatics (benzene, toluene, xylenes and trimethylbenzenes). All these families are apparent primary products, but formation of aromatics is favoured at higher conversions. Non desorbed products ("coke") were also formed.

The initial conversion, defined as the conversion after 1 minute of reaction (TOS=1 min) was plotted versus contact time (actually 1/WHSV, the weight hourly space velocity) and initial activities of 450 and 230 mmol.g^-1.h^-1 were estimated for USHY-A and PP-A22, respectively (Figure 2). If one considers that PP-A22 contains only 30% USHY-A, the initial activity per gram of zeolite is equal to 766 mmol.g^-1.h^-1, a value 1.7 times higher than that obtained for USHY-A, indicating that the matrix plays a role in the transformation of methylcyclohexane according to the higher acidity per gram of zeolite found in PP-A22. Although the coke formation was faster on the USHY zeolite (Figure 3), when coke per gram of zeolite was calculated, its formation was faster on PP-A22 than on USHY, confirming the role played by the matrix. However, the residual activity calculated at a 7 wt% coke content per gram of zeolite was close to 0.6 for both catalysts, indicating that the effect of coke on deactivation of the catalyst is the same.

For the fresh samples distribution of the product over (after 1 min reaction) at iso-conversion was very similar for both catalysts (Figure 4). This indicates that although the matrix has some influence on activity, selectivity is governed by the zeolite. Also, on both catalysts, the paraffin yield was higher than the olefin yield (e.g. at 15% conversion, the olefin/paraffin molar ratio is close to 0.5), indicating that bimolecular hydrogen transfer reactions are very important and do not depend on the SiO₂-Al₂O₃ matrix.

The composition of coke on both samples was determined for different coke contents (hence TOS values) and for a t of 0.20 h (0.600 g of zeolite, 4 ml.h^-1 of methylcyclohexane) by the technique developed previously, i.e., dissolution of the zeolite in hydrofluoric acid and recovery of coke molecules in two fractions: one soluble in methylene chloride (CH₂Cl₂) and the other insoluble, hence composed of very polyaromatic compounds (Guisnet and Magnoux, 1989). Figure 5 shows that whichever the zeolite, the amount of insoluble coke per gram of zeolite is comparable. This indicates that the reaction scheme for coke formation is the same on both catalysts. In the case of the transformation of methylcyclohexane, the matrix does not yield additional high polyaromatic compounds that are insoluble in CH₂Cl₂.

The composition of the CH₂Cl₂-soluble fraction of coke was analysed by GC-MS and the three main families observed were methyl derivatives of pyrene (C_nH_2n-22) and of indenopyrene or dibenzochrysene (C_nH_2n-32) and bulkier aromatic compounds (alkylcoronenes or alkylbenzoindenopyrenes) with a general C_nH_2n-36 formula. All those components had already been found in the transformation of different reactants on the Y zeolites (Moljord et al., 1995; Henriques and Monteiro,1998; Cerqueira et al. 2001B) and are characteristic of its pore structure. These results indicate that for both catalysts the CH₂Cl₂-soluble fraction of coke is located inside the zeolite pores.

Residual pore volume (V/Vo) as a function of coke per gram of zeolite, obtained from microbalance experiments, is reported in Figure 6. At iso-coke per gram of zeolite the residual pore volume measured by nitrogen adsorption was affected more for USHY-A than for PP-A22. This result suggests that coke molecules, and particularly those of insoluble coke are not located in the same manner. On PP-A22, the higher polyaromatic compounds of insoluble coke are located either on the SiO₂-Al₂O₃ matrix or on the outer surface of zeolite crystallites, whereas on USHY-A, insoluble coke molecules are located on the external surface of zeolite crystallites and/or inside the zeolite pores.

Finally, since the product distribution at iso-conversion is very similar for fresh and for coked samples, in the present case coke deposits do not seem to influence the selectivity of the reactions involved. As an example, data on initial (TOS=1 min) and deactivated (TOS > 5 min) PP-A22 are presented in Figure 7.

CONCLUSIONS

The performances of a USHY zeolite and an FCC catalyst during the transformation of a model naphthenic compound (methylcyclohexane) at 723 K were compared. The initial activity per gram of zeolite on the FCC catalyst (PP-A22) was 1.7 times higher than the value obtained for the USHY zeolite, whereas the total acidity per gram of zeolite was 2.1 times higher over the PP-A22 catalyst. These results indicate that the SiO₂-Al₂O₃ matrix plays a role in the transformation of methylcyclohexane. On the other hand, the matrix has no influence on reaction selectivity.

In accordance with the higher level of activity (and acidity), the FCC catalyst yielded more coke per gram of zeolite. Nevertheless, the composition of coke was the same on both catalysts. The CH₂Cl₂ soluble coke molecules are characteristic of the Y zeolite pore structure and are located inside the zeolite pores of both catalysts. However, the effect of coke on the residual pore volume was not the same, indicating that on PP-A22, the higher polyaromatic compounds of insoluble coke are located either on the SiO₂-Al₂O₃ matrix or on the outer surface of zeolite crystallites, whereas on USHY-A, insoluble coke molecules are located on the external surface of zeolite crystallites and/or inside the zeolite pores. No effect of coke deposits on reaction selectivities was observed.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the financial support of CAPES foundation (Brazilian government) and FCC S.A..

Received: October 30, 2001

Accepted: January 20, 2003

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Publication Dates

• Publication in this collection
  25 June 2003
• Date of issue
  June 2003

History

• Received
  30 Oct 2001
• Accepted
  20 Jan 2003