Effect of Modification of Acid Sites Located on the External Surface of a

Acid sites located on the external surface of a MFI-type gallium−silicate crystals were selectively neutralized by a mechanochemical method using ce...
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Ind. Eng. Chem. Res. 1997, 36, 4827-4831

4827

Effect of Modification of Acid Sites Located on the External Surface of a Gallium-Silicate Crystalline Catalyst on Reducing Coke Deposit in Paraffin Aromatization Tomoyuki Inui,* Teruyuki Yamada, Akihiko Matsuoka, and Shu-Bin Pu Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan

Acid sites located on the external surface of a MFI-type gallium-silicate crystals were selectively neutralized by a mechanochemical method using cerium oxide supported on spherical silica particles in micrometer size. The CeO2-modified catalyst exhibited longer catalyst life and higher yield of aromatics than a non-modified catalyst in an octane conversion reaction, and the coke deposited on a modified catalyst was much less than that on a non-modified one. This is attributed to that decrease in acid sites on the external surface of Ga-silicate crystals reduce the deep condensation of aromatic rings. The effect was further enhanced by the combination with Pt modification. The multifunctions of this catalyst are due to the aromatization ability of Ga species incorporated into the silicate framework, neutralization of external acid sites by CeO2 species, and dehydrogenation-cyclization of Pt particles on the external surface of crystals, respectively. Introduction Catalytic conversion of light carbon sources to fuel range and aromatic hydrocarbons are important processes for future petrochemical industries to produce basic feedstocks from inexpensive raw materials (Corma, 1993). From this point of view, in recent years, aromatization of light paraffins, such as propane and light naphtha, on zeolitic catalysts has been investigated extensively, and the main works were summarized in the review by Ono (1992). In sequential studies by Inui and co-workers (Inui and Okazumi, 1984; Inui et al., 1986, 1987a,b), in propane conversion, Pt-modified gallium-silicate (Pt/Ga-silicate) having a pentasil structure isomorphous to ZSM-5 exhibited high catalytic activity, high selectivity to aromatics, and high stability in reaction-regeneration cycles in prolonged operation. The predominant performance of Pt/Ga-silicate was ascribed to its multifunctions, i.e., the aromatization ability of Ga which is incorporated into the silicate framework, and decreases in coke formation during the reaction, and the acceleration of coke combustion in the regeneration course caused by Pt particles dispersed on the external surface of silicate crystals (Inui, 1989a; Inui et al., 1989). However, because of the large amount of aromatics produced in the reaction, the deep condensation of aromatics could not be completely prevented and the suppression of coke deposition was insufficient by merely modification with Pt. The coke deposition on the zeolite catalyst having an MFI structure is limited to the external surface of crystals (Dejaifve et al., 1981), because its 10-oxygen-membered ring three-dimensional medium-pore system without supercage (Meier et al., 1996) is considered to be too narrow for the formation of fused-ring aromatics. Therefore, the inactivation of the external surface of zeolite crystallites, in other words, selective neutralization of the acid sites located on the external surface of the crystals, would be the certain way to improve the catalytic life of the Gasilicate catalyst in paraffin aromatization. * To whom correspondence should be addressed. Tel: +8175-753-5682. Fax: +81-75-771-7285. E-mail: inui@ scl.kyoto-u.ac.jp. S0888-5885(96)00638-0 CCC: $14.00

Up to now, several methods have been tried to modify the external surface of the ZSM-5 catalyst, such as ion exchange (Vinek et al., 1991), impregnation with phosphorus (Kaeding et al., 1981), mixing with MgO (Sotelo et al., 1993), dealumination (Matsuda et al., 1990), chemical vapor deposition (CVD) (Hibino et al., 1991), and mixing with 2,4-dimethylquinoline (Komatsu et al., 1994), to improve the shape selectivity in alkylation or disproportionation of aromatic derivatives. However, the ion exchange or impregnation method is not selective; a solution containing a basic compound can enter the pore channels and cause the decrease in catalytic activity to a large extent, owing to the neutralization of acid sites located even on the internal surface of the crystals. Dealumination might destroy the part of zeolite framework, and CVD might cause the blockage of pore openings and/or channels. The use of 2,4dimethylquinoline is very expensive, and it is limited by the zeolite structure; 2,4-dimethylquinoline can enter the pore channels of large-pore zeolites, such as zeolite beta and Y. Furthermore, the relation between the modification of the external surface and improvement in catalyst life has not been reported. In the present study, neutralization of the acid sites located on the external surface of Ga-silicate was tried by a mechanochemical method, and its effect on the moderation of the catalyst deactivation owing to coke deposition, as well as the combination with Pt modification, is discussed. In this paper, n-octane conversion was selected as the test reaction. Experimental Section Catalyst Preparation. MFI-type Ga-silicate having a Si/Ga atomic ratio of 20 was synthesized by the rapid crystallization method (Inui, 1989b) using Ga2(SO4)3 (Assay 58%, Mitsuwa) as the gallium source. As for the silica source and organic template, a water glass (29 wt % SiO2, Koso) and tetra-n-propylammonium bromide (Tokyo Kasei) were used, respectively. The obtained crystals were washed with distilled water and dried. They were calcined at 540 °C in air for 3.5 h to remove the template and then ion-exchanged twice in 1 N NH4NO3 solution at 80 °C for 1 h, followed by © 1997 American Chemical Society

4828 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 Table 1. Composition of Catalysts Used catalyst 1 2 3 4 a

designation

composition (wt)

H-Ga-silicate Pt/H-Ga-silicatea

70% H-Ga-silicate + 30% SiO2 70% Pt/H-Ga-silicate + 30% SiO2 CeO2/H-Ga-silicate 70% H-Ga-silicate + (3% CeO2/27% SiO2) CeO2/Pt/H-Ga70% Pt/H-Ga-silicate + silicatea (3% CeO2/27% SiO2)

Loading of Pt on H-Ga-silicate was 0.5 wt %.

calcination at the same condition to obtain the protonated form. Method of Modification of the External Surface of Crystals. (1) Modification with Pt. The modification with Pt was carried out by an ordinary ionexchange method in which H-Ga-silicate was treated with an aqueous solution containing Pt(NH3)4Cl2 at 98 °C for 3 h. It was washed with distilled water and dried. Then the thermal decomposition of the platinum ammonium complex was carried out in a 50 mL/min air flow by heating up to 350 °C with a constant heating rate of 3 °C/min and holding at 350 °C for 10 min. Reduction by hydrogen was then carried out in a 50 mL/ min flow of 10% H2/90% N2 by heating up to 400 °C with a constant heating rate of 3 °C/min and holding at that temperature for 30 min. The Pt content in the catalyst was set at 0.5 wt % (Inui et al., 1987a). (2) Modification with Ceria. The procedure of modification with CeO2 was done as follows. Firstly, nonporous microspherical silica particles (Pylosil being 0.5-1 µm in size, Onoda Cement) were impregnated with cerium acetate (Nacalai) aqueous solution. It was dried and then treated at 540 °C in air for 1 h. The obtained CeO2/SiO2 was milled with H-Ga-silicate or Pt/H-Ga-silicate in a ceramic mortar for 30 min (Inui et al., 1996) under the addition of a small amount of ethanol to reduce the surface friction. It was dried, tabletted, crushed, and sieved ranging from 10 to 20 mesh. Finally, it was calcined at 540 °C in air for 2.5 h. Designation and composition of catalysts expressed by weight percentage are described in Table 1. Characterization of Catalysts. X-ray diffraction (XRD) patterns were recorded on a Shimadzu XD-D1 diffractor equipped with Ni-filtered monochromatic Cu KR radiation. Size of crystals and morphology of catalysts were observed by using a Hitachi S-2500CX scanning electron microscope (SEM). BET-surface areas of the catalysts were determined by the BET onepoint method using a Shimadzu Flow Sorb 2300. The acidic property was measured by temperature-programmed desorption (TPD) of preadsorbed NH3 (NH3TPD) using a Rigaku Thermoflex TG 8110 thermogravimeter connected with a TAS 100 thermal analysis station. Samples were pretreated at 550 °C for 30 min in a 50 mL/min N2 flow and then allowed to adsorb NH3 at 80 °C in a 50 mL/min 5% NH3/95% N2 flow. After reaching saturation of NH3 adsorption, N2 was allowed to flow for 1 h to elute reversibly adsorbed NH3. TPD profiles were obtained from the differential of the integral curve of weight loss from 80 to 600 °C at a constant heating rate of 10 °C/min in a 50 mL/min N2 flow. The amount of coke deposited on catalysts after the reaction was measured by temperature-programmed oxidation (TPO) on a Shimadzu DT-40 thermogravimeter with a heating rate of 5 °C/min in a 50 mL/min air flow. The TPO profiles were obtained by a method similar to that described for TPD, and the amount of

Figure 1. SEM photographs of spherical silica (Pylosil) (a) and CeO2/H-Ga-silicate catalyst (catalyst 3) (b).

coke deposited was calculated from the weight loss in the temperature range from 400 to 600 °C. Apparatus and Reaction Method. Reactions were carried out on an ordinary fixed-bed flow-type reaction apparatus. A 0.5 g portion (ca. 0.8 mL) of catalyst was packed in a quartz tubular reactor of 6 mm inner diameter and pretreated in N2 flow at 500 °C for 30 min to standardize the catalyst state by dehydration. The liquid reactant (Guaranteed grade n-octane, Wako) was supplied to an evaporator by a microtube pump and diluted to 20 mol % with N2. The mixed gas was then introduced into the catalyst bed with a constant gaseous hourly space velocity (GHSV). The products in gaseous state were totally analyzed by using one (thermal conductivity detector) TCD-type and two (flame ionization detector) FID-type gas chromatographs; Shimadzu GC-8A equipped with an MS-5A column for N2 and H2, Shimadzu GC-12A with VZ-10 for C1-C4 hydrocarbons, and Shimadzu GC-14A with Hicap-CBP1 for total hydrocarbons. The products were identified by a Shimadzu GC-MS QP-1000EX gas chromatograph-mass spectrometer. Results and Discussion Neutralization of External Acid Sites by Mechanochemical Method. The Ga-silicate synthesized by the rapid crystallization method showed sharp X-ray diffraction (XRD) patterns belonging to the MFI zeolite structure (Meier et al., 1996). The value of the BETsurface area was as high as 310 m2/g, indicating a good crystallinity (Inui et al., 1987a). Figure 1a,b shows the SEM photographs of spherical silica particles and a CeO2-modified H-Ga-silicate catalyst (catalyst 3). Figure 1a shows that Pylosil is composed of mostly uniform size spherical silica ranging from 0.5 to 1.0 µm in diameter. Figure 1b implies that Ga-silicate crystals are sufficiently mixed with silica particles. Therefore, the CeO2 species supported on the spherical silica could contact and roll the external surface of the H-Ga-silicate and be dispersed uniformly onto it by the mechanochemical effect during mixing in the mortar. The CeO2 could also be dissolved slightly in ethanol and transported to the external surface of zeolite crystals. After the milling, no decrease in crystallinity of the Ga-silicate could be detected by XRD or BET-surface area measurement. Figure 2 shows the NH3-TPD profiles of the nonmodified catalyst (catalyst 1) and CeO2-modified catalyst (catalyst 3). As shown, two peaks at lower and higher temperatures are observed, corresponding to weak and strong acid sites, respectively. By the CeO2 modification by the mechanochemical method, a little

Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4829

Figure 4. Change in yield and product distribution with time on stream in n-octane conversion on H-Ga-silicate (catalyst 1). Reactant, 20% n-octane/80% N2; GHSV, 4000 h-1; reaction temperature, 500 °C.

Figure 2. NH3-TPD profiles for H-Ga-silicate (catalyst 1) and CeO2/H-Ga-silicate (catalyst 3).

Figure 3. Yield and distribution of products in 1,3-diisopropylbenzene conversion on H-Ga-silicate (catalyst 1) and CeO2/HGa-slicate (catalyst 3). Reactant, 20% 1,3-diisopropylbenzene/80% N2; GHSV, 1000 h-1; time on stream, 20 min.

decrease in both peaks could be observed. The ratio of the peak area of catalyst 1 to catalyst 3 was 1.00:0.91. It is found that only several percentages of acid sites on H-Ga-silicate was neutralized by CeO2 during milling with spherical silica particles. The external surface area of zeolite particles with 3-5 µm diameter is 10-odd m2/g (Pu and Inui, 1996a), which shares several percentages in the total surface area of H-Gasilicate, having a BET-surface area of 310 m2/g. This fact implies that most of the acid sites on the external surface of H-Ga-silicate crystals were selectively neutralized by the mechanochemical method. In order to inspect the neutralization of external acid sites, 1,3-diisopropylbenzene (1,3-DIPB) conversion was carried out on both catalysts 1 and 3. Since the dimensional diameter of the 1,3-DIPB molecule is considered to be larger than the pore-opening size of the MFI-type zeolite, the reactivity of 1,3-DIPB on a catalyst reflects the external acidity of crystals. The yields of products on non- and CeO2-modified catalysts at 350, 400, and 450 °C are compared in Figure 3, and the length of each bar graph expresses the conversion of 1,3DIPB. As shown, the conversion of 1,3-DIPB on the

CeO2-modified catalyst was much lower than that on non-modified one at any temperature. For example, the conversion at 350 °C on the CeO2-modified catalyst by the mechanochemical method was 3.8%, while the value on the non-modified one was 11.7%. This result indicates that most of the acid sites located on the external surface of H-Ga-silicate were neutralized with CeO2 supported on SiO2 by the mechanochemical neutralization. Compared with all the methods mentioned in the introduction, the mechanochemical method applied in the present study is very easy, selective, and effective to neutralize the external surface. Furthermore, this method can be adopted to a wide variety of zeolites with narrow-, medium-, or large-pore channels (Pu and Inui, 1996b). Prolongation of Catalyst Life by the Modification of the External Surface. Conversion reactions of n-octane on catalysts 1-4 were compared at 500 °C for 20 h on stream. Figures 4-7 show the change in yield and product distribution with an increase of time on stream for catalysts 1-4, respectively. The blank areas in Figures 4-7 represent the percentage of unconverted n-octane. As shown in Figure 4, on H-Ga-silicate (catalyst 1), the selectivity to aromatic hydrocarbons was approximately 20% at the beginning of time on stream and the selectivity to aliphatic hydrocarbons was relatively high. The conversion of n-octane decreased with the increase in time on stream. The degree of decrease in aromatic hydrocarbons was more than that in others. This is ascribed to the decrease in the aromatization function owing to coke formation (Inui et al., 1987a). By the modification with Pt, as shown in Figure 5 the formation of aromatics was enhanced because of the increase in the conversion of octane by the function of dehydrogenation on Pt; however, no significant variation in the product distribution could be observed, and the deactivation behavior was not so evident as Pt nonmodified H-Ga-silicate (catalyst 1). Figure 6 shows the result on the CeO2-modified catalyst (catalyst 3). Compared with the non-modified catalyst (catalyst 1) (Figure 4), the selectivity to aromatics increased to some extent, and distribution of other aliphatic hydrocarbons was similar to catalyst 1. However, the catalyst deactivation was moderated remarkably, and only a little decrease in catalytic activity and scarcely any variation in product distribution could be observed even after 20 h on stream. This result indicates that the deactivation caused by coke

4830 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997

Figure 5. Change in yield and product distribution with time on stream in n-octane conversion on Pt/H-Ga-silicate (catalyst 2). Conditions were same as described in Figure 4.

Figure 6. Change in yield and product distribution with time on stream in n-octane conversion on CeO2/H-Ga-silicate (catalyst 3). Conditions were same as described in Figure 4.

Figure 7. Change in yield and product distribution with time on stream in n-octane conversion on CeO2/Pt/H-Ga-silicate (catalyst 4). Conditions were same as described in Figure 4.

formation mainly occurs on the external surface of crystals, and the neutralization of acid sites on the external surface of crystals with CeO2 suppressed the coke formation. The combination effect of modifications with Pt and CeO2 on the catalyst life was observed prominently. As shown in Figure 7, both the catalytic activity and aromatic selectivity were increased and still almost no deactivation occurred. Platinum has a role of porthole for hydrogen spillover and prevents deep aromatization (Inui et al., 1989). The relation between yield of aromatics and accumulated amount of aromatics produced until each time on stream is plotted in Figure 8. The slope of each line reflects the deactivation rate, in other words, degree

Figure 8. Relation between yield of aromatics and accumulated amount of aromatics formed in n-octane conversion on catalysts. Catalyst: H-Ga-silicate (catalyst 1) (O); Pt/H-Ga-silicate (catalyst 2) (b); CeO2/H-Ga-silicate (catalyst 3) (4); CeO2/Pt/H-Gasilicate (catalyst 4) (2).

Figure 9. TPO profiles of combustion of coke deposited on catalysts. The symbol for each catalyst is the same as those shown in Figure 8.

of further condensation of aromatics. Until 20 h on stream, the accumulated amounts of aromatics produced was 6800 C‚mol/L‚catalyst on H-Ga-silicate (catalyst 1). The value was increased to 9200, 11600, and 13200 on Pt/H-Ga-silicate (catalyst 2), CeO2/H-Ga-silicate (catalyst 3), and CeO2/Pt/H-Ga-silicate (catalyst 4), respectively. The slopes of lines corresponding to nonmodified and Pt-modified catalysts were almost the same. This fact also indicates that although catalytic activity was enhanced by Pt modification, further condensation of aromatics formed could not be prevented effectively. By the neutralization of the external surface with CeO2, the deactivation rate, in other words, aromatics condensation was significantly reduced. It is confirmed that the coke formation owing to aromatics condensation mainly occurs on the external surface of crystals. Suppression of Coke Formation on the External Surface of Crystals. After reacting for 20 h, the coke deposited on the catalysts was analyzed by the TPO method. The TPO profiles from the differential of weight loss in TG measurement in a range from 400 to 600 °C are plotted in Figure 9. The integrated area of each differential curve corresponds to the deposited coke amount (Table 2). As shown in Figure 9, the peaks shifted to lower temperature, in other words, became easier to combust by modifications, indicating the decrease in condensation degree of aromatics. The amount of coke deposited decreased from 3.31 to 2.68 and then to 2.03 wt % by Pt and CeO2 modifications, respectively, and to 1.59 wt % by the composite effect of Pt and CeO2 comodification (Table 2).

Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4831 Table 2. Amount of Coke Deposited on a Catalyst catalyst

designation

1 2 3 4

H-Ga-silicate Pt/H-Ga-silicatea CeO2/H-Ga-silicate CeO2/Pt/H-Ga-silicatea

a

coke

depositeda

(wt %)

3.31 2.68 2.03 1.59

Calculated from the weight loss in the TPO measurement.

Conclusions The catalyst deactivation in n-octane conversion is mainly owing to the deep condensation aromatics on the external surface of H-Ga-silicate crystals. The deactivation and coke deposition on Ga-silicate can be reduced markedly by neutralizing the external acid sites with CeO2 by means of the mechanochemical method and the combination with Pt modification. Pt modification enhanced the catalytic activity and selectivity to aromatics, and CeO2 modification prevented the further condensation of aromatics on the external surface of Ga-silicate. This selective neutralization method could be applied to other zeolitic catalysts and reactions thereon. Acknowledgment The present study was partly supported by Grant-inAid for Developmental Scientific Research (A) (2) no. 02505006 from the Ministry of Education, Science, Sports and Culture, Japan. Literature Cited Corma, A. Transformation of Hydrocarbons on Zeolite Catalysts. Catal. Lett. 1993, 22, 33. Dejaifve, P.; Auroux, A.; Gravelle, P. C.; Ve´drine, J. C.; Gabelica, Z.; Derouane, E. G. Methanol Conversion on Acidic ZSM-5, Offretite, and Mordenite Zeolites: A Comparative Study of the Formation and Stability of Coke Deposits. J. Catal. 1981, 70, 123. Hibino, T.; Niwa, M.; Murakami, Y. Shape-Selectivity over HZSM-5 Modified by Chemical Vapor Deposition of Silicon Alkoxide. J. Catal. 1991, 128, 551. Inui, T. Application of Shape-Selective Catalysts to Cn Chemistry. Stud. Surf. Sci. Catal. 1989a, 44, 189. Inui, T. Zeolite Synthesis; ACS Symposium Series 398; American Chemical Society: Washington, DC, 1989b; p 479. Inui, T.; Okazumi, F. Propane Conversion to Aromatic Hydrocarbons on Pt/H-ZSM-5 Catalysts. J. Catal. 1984, 90, 366. Inui, T.; Makino, Y.; Okazumi, F.; Miyamoto, A. Selective Conversion of Propane into Aromatics on Platinum Ion-Exchanged

Gallium-Silicate Bifunctional Catalysts. J. Chem. Soc., Chem. Commun. 1986, 571. Inui, T.; Makino, Y.; Okazumi, F.; Nagano, S.; Miyamoto, A. Selective Aromatization of Light Paraffins on Platinum-IonExchanged Gallium-Silicate Bifunctional Catalysts. Ind. Eng. Chem. Res. 1987a, 26, 647. Inui, T.; Makino, Y.; Okazumi, F.; Miyamoto, A. Effective Conversion of Paraffins to Aromatics on Pt Ion-Exchanged Ga- and Zn-Silicates. Stud. Surf. Sci. Catal. 1987b, 37, 487. Inui, T.; Kamachi, K.; Ishihara, Y.; Makino, Y.; Matsuda, H. Spillover Effect of Adsorbates on the Reduction of Coke in Paraffin Aromatization over Pt/H-Ga-Silicate Catalysts. Proceedings of the 2nd International Conference on Spillover; Karl-Marx-Universita¨t: Leipzig, Germany, 1989; p 167. Inui, T.; Pu, S. B.; Kugai, J. Selective Neutralization of Acid Sites on the External Surface of H-ZSM-5 Crystallites by a Mechanochemical Method for Methylation of Methylnaphthalene. Appl. Catal., A 1996, 146, 285. Kaeding, W. W.; Chu, C.; Young, L. B.; Weinstein, B.; Butter, S. A. Selective Alkylation of Toluene with Methanol to Produce para-Xylene. J. Catal. 1981, 67, 159. Komatsu, T.; Araki, Y.; Namba, S.; Yashima, T. Selective Formation of 2,6-Dimethylnaphthalene from 2-Methylnaphthalene on ZSM-5 and Metallosilicates with MFI Structure. Stud. Surf. Sci. Catal. 1994, 84, 1821. Matsuda, T.; Yogo, K.; Mogi, Y.; Kikuchi, E. Shape Selective Catalysis by ZSM-5 in Disproportionation of 2-Methylnaphthalene. Chem. Lett. 1990, 1085. Meier, W. M.; Olson, D. H.; Baerlocher, C. Atlas of Zeolite Structure Types, 4th ed.; Elsevier: London, 1996; p 146. Ono, Y. Transformation of Lower Alkanes into Aromatic Hydrocarbons over ZSM-5 Zeolites. Catal. Rev. 1992, 34, 179. Pu, S. B.; Inui, T. Influence of Crystallite Size on Catalytic Performance of HZSM-5 Prepared by Different Methods in 2,7Dimethylnaphthalene Isomerization. Zeolites 1996a, 17, 334. Pu, S. B.; Inui, T. Synthesis of 2,6-Dimethylnaphthalene by Methylation of Methylnaphthalene on Various Medium and Large-Pore Zeolite Catalysts. Appl. Catal., A 1996b, 146, 305. Sotelo, J. L.; Uguina, M. A.; Valverde, J. L.; Serrano, D. P. Kinetic of Toluene Alkylation with Methanol over Mg-Modified ZSM5. Ind. Eng. Chem. Res. 1993, 32, 2548. Vinek, H.; Derewinski, M.; Mirth, G.; Lercher, J. A. Alkylation of Toluene with Methanol over Alkali Exchanged ZSM-5. Appl. Catal. 1991, 68, 277.

Received for review October 10, 1996 Resubmitted for review July 30, 1997 Accepted August 6, 1997X IE960638C

X Abstract published in Advance ACS Abstracts, October 1, 1997.