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Effect of Zeolite Crystallite Size on Pt/KL Catalysts Used for the Aromatization of N-Octane

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Effect of Zeolite Crystallite Size on Pt/KL Catalysts Used for the Aromatization of N-Octane Chem. Eng. Comm., 194:946–961, 2007 Copyright # Taylor & Francis Group, LLC ISSN: 0098-6445 print/1563-5201 online DOI: 10.1080/00986440701232403 Effect of Zeolite Crystallite Size on Pt/KL Catalysts Used for the Aromatization of n-Octane SUPAK TRAKARNROEK,1 SOMLAK ITTISANRONNACHAI,1 SIRIPORN JONGPATIWUT,1 THIRASAK RIRKSOMBOON,1 SOMCHAI OSUWAN,1 AND DANIEL E. RESASCO2 1The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok, Thailand 2School of Chemical, Biological, and Materials Engineering, The University of Oklahoma, Norman, Oklahoma, USA The effects of varying zeolite crystallite size in n-octane aromatization over Pt=KL have been studied on a series of catalysts. Various KL zeolites were synthesized via microwave-hydrothermal treatment, which allows for good control of crystallite morphology. Zeolites with different crystallite sizes were prepared by varying aging time (17–24 h), amount of barium (0–445 ppm), and seeding (0–8 wt%). The results showed that higher aging time resulted in smaller zeolite crystallite size, whereas the addition of barium resulted in larger crystallite size. Moreover, the addition of seeding reduced the crystallite size from 1.47 to 0.94 lm. Pt supported on different zeolite catalysts (Pt=KL) was prepared by vapor phase impregnation (VPI). The fresh catalysts were characterized by DRIFTS of adsorbed CO and volumetric hydrogen chemisorption. The results indicated that Pt clusters are well dispersed inside the zeolite channel in all the catalysts prepared. The aromatization of n-octane was tested on the different catalysts at 500C and atmospheric pressure. It was found that the catalytic activity of all catalysts dropped rapidly after about 200 min on stream due to coke plugging inside the pore of the KL zeolite. It was also observed that less ethylbenzene (EB) and o-xylene (OX) were obtained as the con- version increased because both EB and OX are converted to smaller molecules such as benzene, toluene, etc., by secondary hydrogenolysis. Furthermore, the EB=OX ratio increases with zeolite crystallite size due to an enhanced preferential conversion of the larger OX molecules compared to the narrower EB as their path through the pores is restricted. Keywords Microwave-hydrothermal treatment technique; n-Octane aromatiza- tion; Zeolite Introduction The aromatization of n-alkane is an important reaction used to obtain high–value- added products from a naphtha feedstock that is abundant in refinery operations. This reaction can be used in many industrial applications and can be carried out with Address correspondence to Siriporn Jongpatiwut, The Petroleum and Petrochemical College, Chulalongkorn University, Soi Chula12, Phyathai Rd., Pathumwan, Bangkok 10330, Thailand. E-mail: [email protected] 946 n-Octane Aromatization 947 either bifunctional (acid-metal) or monofunctional (only-metal) catalysts. The advantage of using monofunctional catalysts is that they are not active for isomer- ization paths, which typically occur on the bifunctional catalysts and result in lower selectivity to aromatics (Meriaudeau and Naccache, 1997). Platinum supported on alkaline LTL zeolite (Pt=KL) is an efficient catalyst for the dehydrocyclization of n-hexane into benzene (Bernard, 1980; Hunges et al., 1986; Tamm et al., 1988), How- ever, for n-octane aromatization, Pt=KL catalysts are not as effective as for n-hexane aromatization. Although Pt=KL catalysts prepared by vapor phase impregnation (VPI) provide very high dispersion of Pt clusters that remain inside the channels of the zeolite (Jacobs et al., 1996, 2001), the selectivity for n-octane aromatization is still low and quickly drops after a few hours on stream (Jongpatiwu et al., 2003). The product distribution shows benzene and toluene as major aromatic pro- ducts, with small quantities of ethylbenzene (EB) and o-xylene (OX), which are the only two expected products from a direct six-membered ring closure. The pore size of the KL zeolite is 0.71 nm, that is, larger than the critical diameter of EB but smaller than that of OX, thus OX diffuses through the zeolite crystal much more slowly than EB. As a result, OX is preferentially converted to benzene and toluene before escap- ing from the pore of zeolite. In our previous study (Jongpatiwu et al., 2003) it was proposed that the pore length of the zeolite should have a great impact on product distribution and catalyst life. The idea of short-channel KL zeolite has been previously discussed by Treacy (1999) to minimize the problem of Pt entombment due to metal agglomeration and coking. Furthermore, zeolites with small crystallite size provide advantages over zeolites with large crystallite size by enhancing the ratio of surface area to mass. These advantages are higher diffusion rates and a lower rate of deactivation by pore plugging (Verduijn et al., 2001). Many researchers have focused on synthesis of KL zeolites by conventional hydrothermal treatment (Break and Nancy, 1965; Wortel, 1985; Verduijn, 1987, 1991; Koetsier and Verduijn, 1991). However, in the hydro- thermal treatment, the heat transfer by both convection and conduction results in a slow temperature increase, which lengthens the crystallization time. The use of microwave radiation has been found to be advantageous in the production of micro- porous crystalline materials in comparison to the conventional method. The volu- metric heat generated by microwaves results in a more homogeneous nucleation process and yields crystalline materials in shorter times than the conventional method (Chu et al., 1998). Furthermore, it is a clean and economical heating system (Park and Komarneni, 1998; Romero et al., 2004). To reduce the crystallite size of zeolite, crystallization conditions and composition-dependent parameters were investigated (Renzo, 1998). It was found that the aging process can reduce the size of zeolite crystals in the final product (Gontier and Tuel, 1996; Qinghua et al., 2001). Incorporation of colloidal L zeolite seeds into the gel before crystallization can also reduce the crystallite size (Break and Nancy, 1965). In this contribution, we attempt to evaluate the effects of varying the zeolite crystallite size on the n-octane aromatization over Pt=KL catalysts. First, several KL zeolites were synthesized by using microwave hydrothermal treatment. The effects of synthesis conditions including aging time, amount of barium, and seeding on crys- tallite size of zeolite KL were investigated. The synthesized KL zeolites were charac- terized using X-ray diffraction (XRD), nitrogen adsorption (BET), scanning electron microscopy (SEM), X-ray fluorescence (XRF), Fourier transform-infrared spec- troscopy (FT-IR), and dynamic light scattering spectrometry (DLS). Pt supported 948 S. Trakarnroek et al. on the synthesized KL zeolites was prepared by vapor phase impregnation (VPI) and tested for activity and selectivity of n-octane aromatization at 500C and atmospheric pressure. This impregnation method has been shown to be the most effective to maximize selectivity to benzene from n-hexane (Jacobs et al., 1999, 2001; Jongpatiwut et al., 2003). The fresh catalysts were characterized by means of FT-IR of adsorbed CO, which provides information about the location of the Pt particles and hydrogen chemisorption that quantifies the metal dispersion. Temperature programmed oxidation (TPO) was used to analyze the amount and nature of coke deposited on the catalysts. Experimental Section Materials n-Octane (C8H18) of minimum 99% purity was obtained from Merck. The commercial 2 K-LTL zeolite (HSZ-500, SiO2=Al2O3 ¼ 6, surface area ¼ 280 m =g) was obtained from Tosoh, Japan. Platinum (II) acetylacetonate ([CH3COCH=(COÀ)CH3]2Pt) was obtained from Alfa Aesar. Colloidal silica (40 wt% suspension) was obtained from Aldrich. Potassium hydroxide (KOH) and barium hydroxide (Ba(OH)2) were supplied by Carlo Erba. Aluminium hydroxide (Al(OH)3) of 99.8% purity was acquired from Merck. Synthesis of KL Zeolites The KL zeolites were synthesized from a mixture of silicate and potassium aluminate solutions to attain the following composition: 2.65K2O: 0.0032BaO:0.5Al2O3: 10SiO2: 159H2O. The potassium aluminate solution was prepared by dissolving 2.64 g of Al(OH)3 in 8.15 M of KOH solution. The silicate solution was prepared by mixing colloidal silica with 2.8 mM of Ba(OH)2 solution and stirring for 15 min. The silicate and aluminate solutions were then mixed and stirred vigorously by a mechanical stirrer for different periods of aging time at ambient temperature. After that, the gel mixture was transferred to a microwave vessel and heated using a MARS5 microwave machine up to 170C within 2 min and maintained at that tem- perature for 15–50 h. As a comparison sample, an L zeolite synthesized in a conven- tional manner was aged for 24 h and transferred to a 250 mL Teflon-lined autoclave that was then placed in an oven at 170C and held at that temperature for 96 h. More- over, the effects of adding barium (0–445 ppm) and seeding (0–8 wt%) were studied. The resultant material was washed with deionized water until a pH of 10 was reached. Then, it was centrifuged to separate the solid phase from the solution. The solid product was dried in an oven at 110C overnight and then calcined at 500C in flowing air. The synthesized zeolites are named by their synthesized conditions and composi- tions as listed in Table I. Characterization of the Synthesized KL Zeolites The structures of synthesized KL zeolites were evaluated using a Rigaku X-ray diffractometer, with Cu-K
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