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Ind. Eng. Chem. Res. 2009, 48, 9490–9497
Adsorption/Desorption Studies of 224-Trimethylpentane in β-Zeolite and Mesoporous Materials Using a Tapered Element Oscillating Microbalance (TEOM) Kening Gong,† Tiepan Shi,† Palghat A. Ramachandran,†,§ Keith W. Hutchenson,| and Bala Subramaniam*,† Center for EnVironmentally Beneficial Catalysis, Department of Chemical & Petroleum Engineering, UniVersity of Kansas, Lawrence, Kansas 66045, Department of Energy, EnVironmental & Chemical Engineering, Washington UniVersity in St. Louis, St. Louis, Missouri 63130, DuPont Company, Wilmington, Delaware 19880
The facile desorption of C8 and heavier products from solid acid alkylation catalysts is essential to thwart catalyst deactivation by fouling. To better understand this phenomenon, a tapered element oscillating microbalance (TEOM) was employed to investigate the adsorption/desorption characteristics of 224trimethylpentane (224-TMP), a proxy C8 alkylate product, on β-zeolite and mesoporous materials (T ) 298-473 K, P224-TMP ) 0-0.3 bar). It is found that the 224-TMP desorption rates from saturated β-zeolite (by helium purging) are characterized by a rapid initial “burst” of 224-TMP followed by a rather long desorption phase. Complementary experimental and modeling investigations using pelletized β-zeolites of known sizes indicate that the adsorption rate and initial desorption (during the burst phase) rate of 224-TMP are controlled by meso-/macropore diffusion resistance external to the β-zeolite crystals and that the long transient could be due to pore diffusion resistance within the β-zeolite crystals. In contrast, mesoporous silica materials provide facile pore accessibility for large alkylate molecules such as 224-TMP, as evidenced by complete desorption of 224-TMP even at mild temperatures. These insights provide guidance for rational engineering of stable solid acid alkylation catalysts. 1. Introduction Alkylation of isoparaffins is used commercially to produce nonaromatic gasoline blending material. Conventionally, alkylation units use mineral acids, i.e., sulfuric acid (such as those licensed by DuPont/STRATCO and ExxonMobil) and hydrofluoric acid (such as those licensed by UOP), as catalysts. The main characteristics of these processes are described in detail elsewhere.1 Because of the safety and environmental issues associated with the sulfuric acid and hydrofluoric acid based alkylation units, there has been a strong driving force for developing novel solid acid catalyzed alkylation processes. The grand challenge in this regard is the development of stable and durable solid acid catalysts. Some large-pore zeolites (e.g., Y- and β-zeolites) are known to possess excellent initial activity and C8 alkylates selectivity.2-5 However, they deactivate rapidly due to catalyst fouling. There seems to be a consensus that the slow adsorption/desorption and pore diffusion of the reactants and products (especially the heavy byproducts, such as C12 and C16) are the causative factors for catalyst deactivation in the solid acid catalyzed alkylation.2,3,6 During liquid phase alkylation at subambient temperatures, Sarsani and Subramaniam7 recently reported that β-zeolite catalyst is deactivated even before the desired products (trimethylpentanes or TMPs) transport out of pores, indicating that the slow pore diffusion of the products plays a crucial role in the deactivation process. Yoo et al.5 studied the pore structure effects of different zeolites under mild to relatively severe deactivating conditions (353 K, 20.7 bar, isobutane/olefin molar ratio ) 98, olefin WHSV ) * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +1-785-864-2903. Fax: +1-785-8646051. † University of Kansas. § Washington University in St. Louis. | DuPont Company.
0.1-0.5 h-1) and found that the relatively small compounds formed during the reaction (and therefore the enhanced transport in the pores) contribute to the longer catalyst lifetime observed in β-zeolite and ZSM-12. These results are instructive and clearly point out that a fundamental understanding of the adsorption, desorption, and mass transfer rates of the reactants and products in the microporous and mesoporous catalysts is needed to screen potential solid acid catalysts and rationally determine operating conditions. There are relatively few published reports aimed at investigating the adsorption/desorption and mass transfer phenomena of the reactants and products of alkylation in solid acid catalysts. Employing particles of different sizes (90-230 µm), Simpson and co-workers8,9 found that the alkylation in the large-pore zeolite USHY is severely controlled by intraparticle diffusion of butenes under liquid-phase conditions at 373 K. The authors suggested placing the acid sites as a thin shell very close to the external surface of the particles to eliminate the intraparticle diffusion limitation. Albright10 commented on their work and suggested that the transfer steps of C7-C9 products should be investigated in addition to the diffusion of butenes (and isobutane), because the diffusivities of C7-C9 are relatively small and the accumulated products could possibly cause poreblocking and further catalyst deactivation. However, such insights are not possible from the fixed bed reactor studies such as those employed by Simpson and co-workers.8,9 Platon and Thomson4 used the volumetric method to study butene and isobutane adsorption in β-zeolite, ZSM-5, and sulfated zirconia. They found that the adsorption equilibrium of butene was not attained even after 2 days, while the isobutane adsorption dynamics are relatively fast. The tapered element oscillating microbalance (TEOM) is a valuable tool for fundamental studies of physicochemical behavior within porous catalysts. The advantages of the TEOM
10.1021/ie900334g CCC: $40.75 2009 American Chemical Society Published on Web 07/28/2009
Ind. Eng. Chem. Res., Vol. 48, No. 21, 2009 Table 1. Properties of β-Zeolite properties
β-zeolite
Si/Al ratioa (mol/mol) average crystal sizea (µm) agglomerate size, intergrowna (µm) total surface areab (m2/g) surface area within pores 2 nmb (m2/g) total pore volumeb (cm3/g) pore volume within pores 2 nmb (cm3/g) acidityc (µmol NH3/g catalyst)
13.3 0.6). The steep slopes are caused by capillary condensation of 224-TMP in the mesopores.23 Assuming that the pores are filled with liquid 224-TMP (density ) 0.69 g/cm3) at P f P0 and given that the pore volume of SI 1301 silica support is 1.15 cm3/g, the maximum adsorption capacity of 224-TMP in SI 1301 is estimated to be 0.79 g/g of silica support, which is nearly close to the measured values (approximately 0.7 g/g of silica support). As shown in Figure 3, the experimental results are fitted well with the Brunauer model. The study on intrinsic equilibrium adsorption isotherms of 224-TMP on Davicat SI 1401 silica support reveals that the equilibrium isotherms (T ) 298-373 K, P224-TMP ) 0-0.25 bar) are fitted well by the Langmuir model as shown in Figure 4. The fitted parameters are listed in Table 3. These results demonstrate the similarity (e.g., monolayer coverage in pores) between the zeolite pores and SI 1401 silica support pores. The
Figure 2. Adsorption and desorption equilibrium isotherms of 224-TMP in β-zeolite (T ) 473 K).
Figure 3. Equilibrium adsorption isotherms of 224-TMP in Davicat SI 1301 silica support (Brunauer model with fitting parameters:23 Vm ) 0.036, c ) 16.5, n ) 20.4, and g ) 1785.1).
52 ( 4
saturation adsorption capacity of 224-TMP in SI 1401 silica support is approximately 36% higher than that of β-zeolite, correlating well with the BET characterization results (i.e., the total surface area of SI 1401 silica support is approximately 38% higher than that of β-zeolite). The dramatic difference in the shapes of equilibrium isotherms between SI 1301 and 1401 indicates that the pore size has a significant effect on the adsorption/desorption behavior. The SI 1401 silica support with average pore size of 2.3 nm exhibits a behavior typical of microporous materials wherein the adsorbate size is comparable to pore size and adsorption occurs by micropore filling. In contrast, the SI 1301 silica support with average pore size of 14 nm exhibits behavior typical of mesoporous materials wherein adsorption follows a mechanism of surface layering (i.e., successive layer formation) when the vapor pressure is lower than the threshold pressure for capillary condensation to occur. At sufficiently high adsorbate partial pressure, pore filling by capillary condensation occurs. 4.2. Intrinsic Adsorption/Desorption Dynamics. In a randomly packed bed, the mean pore size of the void channels created by the Davicat SI 1301 and 140 particles (mean size of 70-135 µm) is significantly larger compared to that created by packed β-zeolite crystals (mean size 50 h) in the H-form β-zeolite compared to the Na-form β-zeolite, possibly due to the strong chemisorption of 224-TMP in the H-form β-zeolite. Clearly, the hindered desorption of 224-TMP from β-zeolites and its accumulation in the pores will eventually lead to catalyst deactivation by fouling. It was found that the β-zeolite can be completely regenerated by burning off the 224TMP in air at 773 K for 2 h. The dramatic differences in the shapes of equilibrium adsorption isotherms between nonacidic silica supports Davicat SI 1301 (with average pore size of 14 nm in which capillary condensation occurs) and 1401 (with average pore size of 2.3 nm wherein no capillary condensation occurs) confirm that pore size has a significant effect on the adsorption/desorption behavior. The fact that the mesoporous materials provide good pore accessibility for large alkylate molecules such as 224-TMP seems to explain the longer catalyst lifetime observed by Lyon et al.28 in silica-supported Nafion catalyst. Acknowledgment It is a distinct pleasure and privilege to be able to contribute to this issue honoring Dr. B. D. Kulkarni whose pioneering contributions to the field of catalysis and reaction engineering we greatly admire. This research was supported with funds provided by the Center for Environmentally Beneficial Catalysis under the National Science Foundation Engineering Research Centers Grant (EEC-0310689). We also thank Richard B. Maynard and Dana M. Jones of DuPont for performing the catalyst porosimetry measurements, and Dr. Joe Allison and Dr. Jane Yao of ConocoPhillips for their support on the ammonia TPD characterization of the zeolite samples. Literature Cited (1) Corma, A.; Martinez, A. Chemistry, Catalysts, and Processes for Isoparaffin-Olefin Alkylation - Actual Situation and Future Trends. Catal. ReV. 1993, 35, 483. (2) deJong, K. P.; Mesters, C. M. A. M.; Peferoen, D. G. R.; vanBrugge, P. T. M.; deGroot, C. Paraffin Alkylation Using Zeolite Catalysts in a Slurry Reactor: Chemical Engineering Principles to Extend Catalyst Lifetime. Chem. Eng. Sci. 1996, 51, 2053. (3) Weitkamp, J.; Traa, Y. Isobutane/Butene Alkylation on Solid Catalysts. Where Do We Stand. Catal. Today 1999, 49, 193. (4) Platon, A.; Thomson, W. J. Solid Acid Characteristics and Isobutane/ Butene Alkylation. Appl. Catal. A: Gen. 2005, 282, 93. (5) Yoo, K.; Burckle, E. C.; Smirniotis, P. G. Comparison of Protonated Zeolites with Various Dimensionalities for the Liquid Phase Alkylation of i-Butane with 2-Butene. Catal. Lett. 2001, 74, 85. (6) Nivarthy, G. S.; He, Y. J.; Seshan, K.; Lercher, J. A. Elementary Mechanistic Steps and the Influence of Process Variables in Isobutane Alkylation over H-BEA. J. Catal. 1998, 176, 192. (7) Sarsani, V. S. R.; Subramaniam, B. Isobutane/Butene Alkylation on Microporous and Mesoporous Solid Acid Catalysts: Probing the Pore Transport Effects with Liquid and near Critical Reaction Media. Green Chem. 2009, 11, 102. (8) Simpson, M. F.; Wei, J.; Sundaresan, S. Kinetic Analysis of Isobutane/Butene Alkylation over Ultrastable H-Y Zeolite. Ind. Eng. Chem. Res. 1996, 35, 3861. (9) Simpson, M. F.; Wei, J.; Sundaresan, S. Kinetic Analysis of Isobutane/Butene Alkylations over Ultrastable H-Y Zeolite - Rebuttal. Ind. Eng. Chem. Res. 1997, 36, 2517.
Ind. Eng. Chem. Res., Vol. 48, No. 21, 2009 (10) Albright, L. F. Kinetic Analysis of Isobutane/Butene Alkylations over Ultrastable H-Y Zeolite - Comments. Ind. Eng. Chem. Res. 1997, 36, 2514. (11) Chen, D.; Gronvold, A.; Rebo, H. P.; Moljord, K.; Holmen, A. Catalyst Deactivation Studied by Conventional and Oscillating Microbalance Reactors. Appl. Catal. A: Gen. 1996, 137, L1. (12) Liu, K.; Fung, S. C.; Ho, T. C.; Rumschitzki, D. S. Kinetics of Catalyst Coking in Heptane Reforming over Pt-Re/Al2O3. Ind. Eng. Chem. Res. 1997, 36, 3264. (13) Chen, D.; Rebo, H. P.; Moljord, K.; Holmen, A. Methanol Conversion to Light Olefins over SAPO-34. Sorption, Diffusion, and Catalytic Reactions. Ind. Eng. Chem. Res. 1999, 38, 4241. (14) Chen, D.; Rebo, H. P.; Moljord, K.; Holmen, A. Influence of Coke Deposition on Selectivity in Zeolite Catalysis. Ind. Eng. Chem. Res. 1997, 36, 3473. (15) Hershkowitz, F.; Madiara, P. D. Simultaneous Measurement of Adsorption, Reaction, and Coke Using a Pulsed Microbalance Reactor. Ind. Eng. Chem. Res. 1993, 32, 2969. (16) van Donk, S.; Broersma, A.; Gijzeman, O. L. J.; van Bokhoven, J. A.; Bitter, J. H.; de Jong, K. P. Combined Diffusion, Adsorption, and Reaction Studies of n-Hexane Hydroisomerization over Pt/H-Mordenite in an Oscillating Microbalance. J. Catal. 2001, 204, 272. (17) Zhu, W.; van de Graaf, J. M.; van den Broeke, L. J. P.; Kapteijn, F.; Moulijn, J. A. TEOM: A Unique Technique for Measuring Adsorption Properties. Light Alkanes in Silicalite-1. Ind. Eng. Chem. Res. 1998, 37, 1934. (18) Barrie, P. J.; Lee, C. K.; Gladden, L. F. Adsorption and Desorption Kinetics of Hydrocarbons in FCC Catalysts Studied Using a Tapered Element Oscillating Microbalance (TEOM). Part 2: Numerical Simulations. Chem. Eng. Sci. 2004, 59, 1139. (19) Fung, S. C.; Querini, C. A.; Liu, K.; Rumschitzki, D. S.; Ho, T. C. In Situ Coking Kinetics Obtained from a New Flow Through Microbalance. Stud. Surf. Sci. Catal. 1994, 88, 305.
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(20) Lee, C. K.; Ashtekar, S.; Gladden, L. F.; Barrie, P. J. Adsorption and Desorption Kinetics of Hydrocarbons in FCC Catalysts Studied Using a Tapered Element Oscillating Microbalance (TEOM). Part 1: Experimental Measurements. Chem. Eng. Sci. 2004, 59, 1131. (21) Gong, K.; Subramaniam, B.; Ramachandran, P. A.; Hutchenson, K. W. Tapered Element Oscillating Microbalance (TEOM) Studies of Isobutane, n-Butane and Propane Sorption in β- and Y-Zeolites. AIChE J., submitted for publication. (22) Barcia, P. S.; Silva, J. A. C.; Rodrigues, A. E. Separation by FixedBed Adsorption of Hexane Isomers in Zeolite BETA Pellets. Ind. Eng. Chem. Res. 2006, 45, 4316. (23) Duong, D. D. Adsorption Analysis: Equilibria and Kinetics; Imperial College Press: London, River Edge, NJ, 1998. (24) Ka¨rger, J.; Ruthven, D. M. Diffusion in Zeolites and Other Microporous Solids; Wiley: New York, 1992. (25) Femlab, 3.0; COMSOL AB: Stockholm, Sweden, 2004. (26) Alpay, E.; Chadwick, D.; Kershenbaum, L. S.; Barrie, P. J.; Sivadinarayana, C.; Gladden, L. F. TEOM Analysis of the Equilibria and Kinetics of n-Hexane and n-Heptane Adsorption on FCC Catalyst/Silicalite. Chem. Eng. Sci. 2003, 58, 2777. (27) Ahn, H.; Lee, C. H. Effects of Capillary Condensation on Adsorption and Thermal Desorption Dynamics of Water in Zeolite 13X and Layered Beds. Chem. Eng. Sci. 2004, 59, 2727. (28) Lyon, C. J.; Sarsani, V. S. R.; Subramaniam, B. 1-Butene + Isobutane Reactions on Solid Acid Catalysts in Dense CO2-Based Reaction Media: Experiments and Modeling. Ind. Eng. Chem. Res. 2004, 43, 4809.
ReceiVed for reView February 28, 2009 ReVised manuscript receiVed July 1, 2009 Accepted July 7, 2009 IE900334G
