RKCL4616. CATALYTIC PROPERTIES OF HZSM-12 ZEOLITE IN n-heptane CATALYTIC CRACKING

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1 Jointly published by React.Kinet.Catal.Lett. Akadémiai Kiadó, Budapest Vol. 84, No. 2, and Springer, Dordrecht (2005) RKCL4616 CATALYTIC PROPERTIES OF HZSM-12 ZEOLITE IN n-heptane CATALYTIC CRACKING Marcelo J. B. Souza a, Antonio O. S. Silva a, Anne M. Garrido Pedrosa b and Antonio S. Araujo b, * Federal University of Rio Grande do Norte, a Department of Chemical Engineering, b Department of Chemistry, CP 1662, , Natal, RN, Brazil Received July 13, 2004 In revised form September 27, 2004 Accepted October 6, 2004 Abstract The catalytic cracking of n-heptane was carried out using HZSM-12 zeolite (Si/Al = 40). The materials obtained were characterized by X-ray diffraction (DRX), BET surface area and atomic absorption (AA). The catalytic tests were accomplished at 350, 400 and 450ºC in a fixed bed continuous flow reactor coupled on-line to a gas chromatograph. The results showed that HZSM-12 was active in n-heptane catalytic cracking with the production of hydrocarbons in the range of C1 to C6 and an activation energy of 95.5 kj mol -1. Keywords: n-heptane, catalytic cracking, HZSM-12 zeolite, activation energy INTRODUCTION Catalytic cracking is a very important process of the refining industry. The main objective is to convert fractions of high to low molecular weight hydrocarbons. Many papers suggest that monomolecular [1, 2], bimolecular [3-5] and oligomeric [5, 6] reaction mechanisms participate in the cracking of pentane, hexane and other alkanes on the Brönsted acid sites of the zeolites [7]. Since the 1930's it was found that when heavy oil fractions are heated over clay type materials, cracking reactions occur, which lead to significant yields of lighter hydrocarbons. While the search was directed to suitable cracking catalysts *Corresponding author: asa-ufrn@usa.net /2005/US$ Akadémiai Kiadó, Budapest. All rights reserved.

2 288 SOUZA et al.: n-heptane catalysts based on natural clays, some companies concentrated on the development of synthetic catalysts. This resulted in the synthetic amorphous silica/alumina catalyst, which was commonly used until 1960, when it was slightly modified by incorporation of some crystalline zeolite materials. With the success of the fixed-bed process, the oil companies decided to develop a process using finely powdered catalyst, which led to the development of the Fluidized bed Catalytic Cracking (FCC) process, which has become the most important catalytic cracking process. The FCC catalyst consists of a multicomponent system containing a matrix material, usually catalytically active, and a co-catalyst of significantly higher intrinsic activity than the matrix, usually an acid zeolite. Several zeolites were used in the catalytic cracking of hydrocarbons, such as Y and ZSM-5. In this work was studied the potential of ZSM-12 zeolite in the catalytic cracking of n-heptane. The ZSM-12 zeolite is a synthetic molecular sieve with one-dimensional 12-membered-ring (12MR) channel system with pore openings of about 5.7x6.1 Å [8, 9]. This kind of channels is interesting for application in several reactions of industrial interest due to its good stability, resistance to deposition of hard coke and excellent performance in catalytic processes [10-12]. EXPERIMENTAL The NaZSM-12 zeolite was prepared by hydrothermal crystallization [13] in a Teflon-lined steel autoclave at 140 o C by 144 hours, under autogenous pressure. The synthesis gel was prepared from a mixture containing sodium hydroxide (Merck), pseudobohemite (Vista), silica gel (Merck) and methyltriethylammonium (MTEA, Merck) as structure template. The molar composition of synthesis gel was: 10(MTEA) 2 O : 10Na 2 O : 1.25Al 2 O 3 : 100SiO 2 : 2000H 2 O. The synthesis product was filtered, washed with distilled water and dried at 100 o C for 2 h. The sample was calcined at 500 o C in nitrogen flow for 6 h, and afterwards in dry air for 2 h, at the same temperature. The HZSM-12 was obtained by ion exchange from NaZSM-12 zeolite, with 1 M ammonium chloride solution, and subsequent calcination [14]. The HZSM-12 zeolite was characterized by atomic absorption (Varian AA-175), X-ray diffraction (Shimadzu XRD-6000) and BET surface area using nitrogen adsorption (Quanta Chrome NOVA 2000). The acid properties were investigated by n-butylamine desorption on TGA/SDTA-851 (Mettler) equipment. The catalytic tests were accomplished in a fixed bed continuous flow reactor [15-17] coupled on-line to a gas chromatograph CP3800 (Varian). The reaction products were analyzed using a fused silica capillary column with 60 m of length and 0.53 mm of internal diameter (5 µm of film thickness). The reactions were carried out under

3 SOUZA et al.: n-heptane 289 atmospheric pressure and catalytic bed temperature of 350, 400 and 450 o C. In each run were used 0.2 g of fresh catalyst and a hydrogen/n-heptane ratio of 8.2. The space velocity (WHSV) used was 15 h -1. RESULTS AND DISCUSSION XRD patterns for NaZSM-12 and HZSM-12 are shown in Fig. 1. The diffractograms showed an intense peak at ca º due to the (310) plane of WKH =60 ]HROLWH 2WKHU WZR PDLQ SHDNV ZHUH LGHQWLILHG ZLWK,3/ respective Miller indexes of 7.55º (101) and 8.95º (201). All the peaks were indexed as a monoclinic cell of C2/c space group, in agreement with a Mobil twelve (MTW) structure [9,13], which has one-dimensional 12-membered-ring pores. The chemical analysis showed that the materials present the following composition: Na 1.9 Al 1.9 Si 54.1 O 112 and H 1.7 Na 0.2 Al 1.8 Si 54.2 O 112 with Si/Al ratios of Intensity (a.u.) a) b) (degree) Fig. 1. XRD powder patterns of (a) NaZSM-12 and (b) HZSM-12 zeolites 28.5 and 30.1 and Na/Al 1 and 0.11, respectively to NaZSM-12 and HZSM-12. These results show that the HZSM-12 zeolite was obtained with ca. 88 % ion exchange degree. The surface acidity of the HZSM-12 was determined via n- butylamine desorption [14]. TG/DTG curves showed typically two larger events

4 290 SOUZA et al.: n-heptane of mass variation: one at o C attributed to elimination of n-butylamine adsorbed on the weak acid sites, and another at o C attributed to elimination of n-butylamine adsorbed on the medium and strong acid sites. The total acidity was ca mmol g -1. Through the BET surface area analyses using nitrogen adsorption, it was verified that the zeolitic materials present adsorption isotherms of type I according to the classification of Brunauer-Emmet-Teller [18], with typical characteristics of microporous solids. Table 1 shows the values of the total surface area, calculated by the BET method, external surface area, micropore area and micropore volume calculated by t-plot method [19]. The reduction in the micropore area can be attributed to the partial dealumination during the process of ion exchange and subsequent calcinations, which may generate the extra-framework aluminium species inside the pores and channels of the zeolite, decreasing the micropore area. In addition, a small increase in the Si/Al ratio was verified by chemical analysis of the zeolite. Table 1 Surface properties of the ZSM-12 based materials Sample Total area External area Micropore area Pore volume (m 2 g -1 ) a (m 2 g -1 ) b (m 2 g -1 ) b (cm 3 g -1 ) b NaZSM HZSM a = by BET method; b = by t-plot method Through gas chromatographic analyses the reaction products of n-heptane catalytic cracking on the HZSM-12 zeolite was identified. The products obtained were typically due to cracking and isomerization by acid zeolite. The main products were paraffins and isoparaffins in the range of C1 to C6 and in smaller proportions the respective olefins. The formation of aromatic compositions was not observed owing to the shape-selective properties of the ZSM-12 type zeolite. Figure 2 shows the variation of the conversion degree of a function of temperature and reaction time. With the progress of time, the reaction rate decreased exponentially, indicating catalyst deactivation, until the pseudo-steady state was reached in ca. 1 hour. The conversion values obtained after 105 min of reaction were 13.4; 19.1 and 36.1 % for the temperatures of 350, 400 and 450 o C, respectively.

5 SOUZA et al.: n-heptane Conversion (%) o C 400 o C 450 o C Time on stream (minutes) Fig. 2. Conversion degree of n-heptane cracking as a function of time and temperature over HZSM-2 zeolite o C C1 C2 C3 C4 C5 C6 450 o C Selectivity (%) Time on stream (minutes) Fig. 3. Selectivity of products of n-heptane cracking as a function of time and temperature over HZSM-12 zeolite

6 292 SOUZA et al.: n-heptane 5,0 4,5 Eapp = 95,5 kj mol -1 4,0 ln(k) (s -1 ) 3,5 3,0 2,5 2,0 1,35 1,40 1,45 1,50 1,55 1, T -1 (10 3 s -1 ) Fig. 4. Apparent activation energy of n-heptane cracking over HZSM-12 zeolite Figure 3 shows the selectivity to hydrocarbons in the range of C1 to C6 as function of time and temperature. It can be observed that initially large amount of methane formed but later it was reduced to ca. 1%. At 20 min of reaction, high selectivity for C3 (propane and propene) and C4 (mainly n-butane and isobutene) in the range between 30 to 45% were obtained at different temperatures. C2 (ethane and ethene) and C5 (n-pentane, 2-methyl-butane, isomers and some C5 olefins) were also obtained in the selectivity range between 5 and 10 %. The production of reasonable amounts of C6 (isomers and olefins) was observed of 2 to 9 %. The pseudo-first order kinetic constants were obtained by the method of the initial rates used for obtaining apparent activation energy from the Arrhenius equation (Fig. 4). The apparent activation energy was ca. 95 kj mol -1, in agreement with typical values in the literature [15] for the catalytic cracking of C7 hydrocarbons. CONCLUSIONS The structural analysis of the ZSM-12 zeolite showed that hydrothermal synthesis was effective to obtain the HZSM-12 zeolite with high crystallinity. The catalytic potential of the HZSM-12 zeolite was demonstrated in n-heptane catalytic cracking. HZSM-12 zeolite has a total acidity of ca mmol g -1 which is in close agreement with the literature for similar zeolites with high

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