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Studying the Effect of Barium Modification on the Acidic Properties of Ultrastable HY Zeolite Zaki Shakir Seddigi* Department of Chemistry, Umm Al-Qura UniVersity, Makkah 31261, Saudi Arabia ReceiVed August 16, 2008. ReVised Manuscript ReceiVed October 16, 2008
The effect of barium modification of ultrastable HY zeolite on its acidic properties was investigated using different techniques. Temperature-programmed desorption (TPD) of ammonia results showed the presence of only weak acid sites at 287 °C in HY zeolite. Because of barium modification, the number of acidic sites decreased but the strength of these sites was higher than purer HY zeolite. Magic angle spin nuclear magnetic resonance (MAS NMR) spectra indicated the reduction of acidity after barium impregnation. Reduction of the number of acid sites because of barium modification was also confirmed by the n-hexane cracking reaction test.
Introduction Acid catalysis by different micro- and mesopoprous materials finds application in the transformation of hydrocarbons in refining and petrochemical processes. Acidic-basic properties of catalysts play an important role in reaction mechanisms. Designing a catalyst with appropriate balance of acidic and basic properties can maximize yields of desired products. Many good correlations have been observed between these properties and the catalytic reactivity and selectivity. Because of accumulated information about solid acids and bases, it is possible nowadays to design highly active and selective solid acidic or basic catalysts for particular application. Scheme 1 shows that starting with the same reactants but using catalysts differing in acidity/ basicity, can lead to totally different reaction products.1 The most important application of acid catalysis by zeolites is the transformation of hydrocarbons in different processes, such as catalytic cracking, reforming, hydrocracking, isomerization, hydrodewaxing, alkylation, dealkylation, oligomerization, dehydrogenation, and the synthesis of ethers, such as methyl tertiary butyl ether (MTBE). It is, therefore, important to measure acidity and rank these solids as possible candidates for improving the yields of these processes.2 This demands the characterization of the acidity by direct physicochemical methods. The modification of the acidity of the catalyst using different methods can affect the catalytic activity dramatically. One of these methods is the impregnation of catalysts using metals, such as barium. The concentration of this work is on zeolite Y, which exhibits the FAU (faujasite) structure. This three-dimentional structure leaves large cavities of about 12 Å in diameter, connected by windows about 8 Å wide. The most important use of zeolite Y is as a cracking catalyst. It is used in acidic form in petroleum refinery catalytic cracking units to increase the yield of gasoline and diesel fuel from crude oil feedstock by cracking heavy * To whom correspondence should be addressed. E-mail: zseddigi@ gmail.com. (1) Tanabe, K. Catalysis by Acids and Bases; Imelik, B., Naccache, C., Coudurier, G., Ben Taarit, Y., Vedrine, J., Eds.; Elsevier: Amsterdam, The Netherlands, 1985. (2) Satterfield, C. M. Heterogeneous Catalysis in Industrial Practice, 2nd ed.; McGraw-Hill, Inc.: New York, 1991.
Scheme 1. Alkylation of Toluene with Methanol over Acidic and Basic Catalysts
paraffins into gasoline-grade napthas. Zeolite Y has superseded zeolite X in this use because it is both more active and more stable at high temperatures because of the higher Si/Al ratio. Studying the acidic/basic characteristics of zeolites,3-13 including Y zeolite,14-21 is a subject of continuous interest. The objective of this work is to study the effect of barium (3) Sauer, J.; Horn, H.; Haser, M.; Aldrich, R. Chem. Phys. Lett. 1990, 173, 26. (4) Dwyer, J. Guidelines for Mastering the Properties of Molecular SieVes; Barthomeuf, D., Derouane, E. G., Ho¨lderich, W., Eds.; Nato ASI 221, Plenum: New York, 1990; p 241. (5) Zhang, W.; Smirniotis, P. J. J. Catal. 1999, 182 (2), 400. (6) Savitz, S.; Myers, A.; Gorte, R. J. Phys. Chem. 1999, 103 (18), 3687. (7) Maesen, T.; Hertzenberg, E. J. Catal. 1999, 182 (1), 270. (8) Kunkeler, P.; Zuurdeeg, B.; van der Waal, J. J. Catal. 1998, 180 (2), 234. (9) Patel, A.; Coudurier, G.; Essayem, N.; Vedrine, J. J. Chem. Soc., Faraday Trans. 1997, 93 (2), 347. (10) Jaumain, D.; Su, B. Catal. Today 2002, 73, 187. (11) Stoyanov, S. R.; Gusarov, S.; Kuznicki, S. M.; Kovalenko, A. J. Phys. Chem. 2008, 112 (17), 6794. (12) Carvalho, A. P.; Martins, A.; Silva, J. M.; Pires, J.; Vasques, H.; Brotas de Carvalho, M. Clays Clay Miner. 2003, 51 (3), 340. (13) Castano, P.; Pawelec, B.; Aguayo, A. T.; Arandes, J. M. Ind. Eng. Chem. Res. 2008, 47 (3), 665. (14) Al-khattaf, S. Energy Fuels 2006, 20, 70. (15) Al-khattaf, S. Energy Fuels 2007, 21, 646. (16) Le Van Mao, R.; Al-Yassir, N.; Lu, L.; Vu, N. T.; Fortier, A. Catal. Lett. 2006, 112 (1-2), 13. (17) Arishtirova, K.; Kovacheva, P.; Predoeva, A. Appl. Catal., A 2003, 243 (1), 191. (18) Arishtirova, K.; Kovacheva, P.; Predoeva, A. Appl. Catal., A 2001, 213 (2), 197. (19) Costa, C.; Lopes, J. M.; Lemos, F.; Ramoa Ribeiro, F. Cat. Lett. 1997, 44 (3-4), 255.
10.1021/ef800669s CCC: $40.75 2009 American Chemical Society Published on Web 11/19/2008
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Figure 1. Experimental setup for n-hexane-cracking test.
Figure 2. Thermal analysis of Ba-HY zeolite after calcination at 600 °C overnight.
impregnation on the acidic properties of ultrastable HY zeolite using temperature-programmed desorption (TPD) of ammonia, magic angle spin nuclear magnetic resonance (MAS NMR), and n-hexane cracking. Experimental Section Ion-Exchange Procedure. After calcinations, a sample of HY zeolite was slowly wetted into a slurry form. For every 1 g of sample, a volume of 50 mL of 1 M NH4NO3 solution was added. This solution and the slurry were heated at 80 °C for 1 h with continuous stirring. The zeolite was then filtered and washed thoroughly with deionized water to remove any remaining NH4NO3 or NaNO3. The filtrate was left to dry overnight in a vacuum oven at 110 °C, and then it was calcined at 400 °C for 4 h. The Na content in the sample was checked by flame photometry. (20) Navarro, U.; Trujillo, C. A.; Oviedo, C. A.; Lobo, R. J. Catal. 2002, 211 (1), 64. (21) Makowski, W.; Gil, B.; Majda, D. Catal. Lett. 2008, 120 (1-2), 154.
Impregnation Procedure. The procedure for zeolite impregnation involved the addition of Ba(NO3)4 solution to the HY zeolite powder to first make a slurry and then suspension. The Ba-HY zeolite was analyzed by inductively coupled plasma (ICP) for barium, silicon, and aluminum. The catalyst to 0.25 M Ba(NO3)2 solution ratio was 1:50 by volume. The mixture was then vaporized under vacuum (rotary-vapor apparatus) and calcined in air at 450 °C for 3 h. TPD of Ammonia. TPD of ammonia was conducted on Pulse Chemisorb 2700, supplied by Micromeritics, Inc., Norcross, GA. The instrument applied a dynamic pulse method for chemisorption. In this technique, an inert gas (helium) was continuously passed over the HY zeolite sample and into which small reproducible volumes of chemisorbate (ammonia) gas were introduced. The injections were performed by a built-in calibrated injection loop of a known volume (0.965 cm3). The output signal was recorded by a temperature-regulated thermal conductivity detector. Acidity Assessment by MAS NMR Characterization. The 27Al MAS spectra were recorded on a JEOL 500 instrument at a frequency of 130.20 MHz and 5.5 kHz spinning rate. The pulse width was 6.50 µs (45°), and the delay time was 15 s. The Al
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Seddigi
Figure 3. TPD of ammonia for HY zeolite (solid line) and Ba-HY zeolite (dashed line).
Figure 4. 27Al MAS NMR spectrum of HY zeolite and curve-fitted spectrum (ssb ) spinning sideband).
chemical shifts were referenced against tetramethylsilane (TMS) using the external reference of the solid polysiloxane sample. The 29Si CPMAS spectra was recorded on the same machine at 99.25 MHz frequency and 5.5 kHz spinning, with a contact time of 5 ms. The curve fitting (deconvolution) of overlapping peaks in the 27Al spectra was achieved using the JEOL software and employing Gaussian line shapes. The variables in the program are line-width, area, and chemical shift of the peaks. The variables were adjusted to minimize the root-mean-square error between the observed and calculated spectra, using a method of nonlinear regression. n-Hexane Cracking. Cracking of n-hexane is a widely used approach to test acidity of zeolites. It is also commonly used to study Y zeolite.22,23 The hexane-cracking test was performed in a fixed-bed tubular flow reactor. About 1 g of catalyst was placed at (22) Brait, A.; Seshan, K.; Lercher, J. Appl. Catal., A 1998, 169, 299. (23) Babitz, S.; Williams, B.; Miller, J.; Snurr, R.; Haag, W.; Kung, H. Appl. Catal., A 1999, 179, 71.
the center of the 1 in. diameter reactor in between layers of glass wool. The reactor was heated in a three-zone electric furnace. A syringe pump fed laboratory-grade n-hexane into the reactor. Highpurity nitrogen was used as a diluent. Experiments were performed at 410 °C and a space velocity of 5 h-1. High space velocity was used to minimize thermal cracking. Cracked product was collected in a cooled glass receiver. The product sample was analyzed in a gas chromatograph equipped with a flame ionization detector. The experimental setup is given in Figure 1.
Results and Discussion Acidity Modification. Chemical reagents have been employed to modify the acidity and/or the reactivity of zeolites. Phosphorus, antimony, boron, titania, zirconia, barium, gallium, and metacarborane polymer in various concentrations alter the acidity of zeolites but also their diffusional characteristics.24 For
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Figure 5. 29Si CPMAS spectra of (a) HY zeolite and (b) Ba-HY. Table 1. Results of MAS NMR
HY zeolite Ba-HY
percentage of four-coordinate Al
percentage of five-coordinate Al
percentage of six-coordinate Al
64 60
24 30
12 10
instance, the addition of phosphorus increases the number of acid sites but lowers the intrinsic acidity. To avoid any poisoning effect coming from the presence of sodium during catalytic testing, NaY zeolite should be changed to the acidic form HY through NH4 ion exchange. The NH4Y zeolite was calcined at 600 °C in an oxygen atmosphere to produce the H form of zeolite. Barium was loaded on the HY zeolite through the wet impregnation method. The Ba-impregnated zeolite was calcined at 600 °C overnight to remove nitrate from the catalyst. The chemical analysis of the Ba-HY zeolite showed that 3.5% Ba was impregnated by this method. The TG of the Ba-HY zeolite has shown an initial loss of weight that is characteristic for physisorbed water into the pores of the zeolite structures. The differential thermal analysis (DTA) of Ba-Y zeolite did not exhibit any characteristic endo- or exothermic peaks in the region investigated (up to 1100 °C). This confirms the thermal stability of the modified structure (Figure 2). Acidity Assessment by TPD of Ammonia. Figure 3 depicts the TPD of the NH3 profile for HY and Ba-HY zeolites. The acidity of the synthesized zeolites can be calculated in a semiquantative way using the following equation: acidity of the catalyst )
volume of adsorbed ammonia (cc) weight of the catalyst (g) (1)
Because the volume of the loop size is 0.965 cm3 and the peak area corresponding to this loop size is 4.52 (arbitrary unit), the total volume of NH3 adsorbed by the catalyst can be calculated. The acidity of HY zeolite (Si/Al ) 2.5) calculated using eq 1 is 24.21 cm3/g, whereas the acidity of Ba-HY is 1.21 cm3/g. This substantial decrease in acidity was due to a decreased number of available acidic sites in the case of Ba-HY zeolite.
From Figure 3, the peak temperature for HY zeolite is 287 °C, whereas the one for Ba-HY zeolite is 338 °C. The first two peaks correspond to spillover ammonia and physisorbed ammonia, respectively. The acid strength for the bariummodified HY zeolite was higher than that of HY zeolite. However, for the total acidity, which signifies the number of acid sites, Ba-HY has a much lower value (1.21 cm3/g). Barium has covered about 35% of the active acid sites, thus reducing the number of acid sites in the HY zeolite. Acidity Assessment by MAS NMR. In Y zeolites, it is common to observe extra-framework five- and six-coordinated aluminum in addition to the four-coordinate aluminum in the framework. We observe (Figures 4 and 5 and Table 1) a small increase in the percentage of five-coordinated aluminum in going from HY zeolite to Ba-HY. This is in agreement with the possibility of BaO located on the Lewis acid sites, making it the five-coordinate aluminum in the Ba-HY. This is also corroborated by the 29Si cross-polarization magic angle spinning (CPMAS) spectra of Ba-HY, where we observe an increase in the relative intensity of the high-frequency peaks compared to that in the HY zeolite sample. The high-frequency peaks correspond to Si(2Al) and Si(3Al) sites, and the coordination of Al acidic sites by Lewis bases can shift the signals to high frequency. Acidity Assessment by n-Hexane Cracking. There is evidence for different reaction mechanisms during the cracking of n-hexane on Y zeolite;25 however, the most important thing that we obtain from this test is the strength of acidity of the zeolites. With H-Y zeolite, a hexane conversion of 13.6% was achieved. With barium-modified Y zeolite, a conversion of 20% was obtained. A higher conversion with barium-modified zeolite is due to its stronger acid sites compared to those of H-Y zeolite. The presence of stronger acid sites was indicated by the TPD results, in which it was found that the acid strength for the barium-modified HY zeolite (Figure 3) was higher than that of the HY zeolite. Conclusions Determining the acidic-basic properties is of great importance in understanding the structure-property relationships,
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which is in turn extremely important for the understanding of known solid materials and the development of new materials. The followed impregnation procedure was successful in minimizing the acidity of the ultrastable HY zeolite. TPD of ammonia, MAS NMR, and n-hexane methods were efficient in characterizing the modification of acidity in HY zeoilte by barium.
Seddigi Acknowledgment. The author acknowledges the support of Professor Khalid Khairou, Chairman of the Chemistry Department, Umm Al-Qura University. Support from King Fahd University of Petroleum and Minerals (KFUPM) is also appreciated. In particular, the author thanks Dr. Shakeel Ahmad, Mr. Abdulbari Siddiqui, and Mr. Khursheed Alam (Center for Refining and Petrochemicals, the Research Institute, KFUPM) and Dr. M. Wazeer (Chemistry Department, KFUPM) for the help and very fruitful discussions. EF800669S
(24) Bhatia, S. Zeolite Catalysis: Principles and Applications; CRC Press, Inc.: Boca Raton, FL, 1990.
(25) Williams, B.; Ji, W.; Miller, J.; Snurr, R.; Kung, H. Appl. Catal., A 2000, 203, 179.
