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Interactions of n-Alkanes within a Restrictive Pore System under Fluid Catalytic Cracking (FCC) Conditions Yong Lu,*,†,‡ Ming-Yuan He,†,§ Xing-Tian Shu,† and Bao-Ning Zong† Research Institute of Petroleum Processing, China Petrochemical Corporation, Beijing 100083, People’s Republic of China, Department of Chemical Engineering, Auburn University, Auburn, Alabama 36849, and Chemistry Department, East China Normal University, Shanghai 200062, People’s Republic of China Received January 22, 2003. Revised Manuscript Received May 15, 2003
Cracking of n-C8 and n-C16 single feedstocks and their mixture was used to test interactions of alkanes, under fluid catalytic cracking (FCC) conditions, within the pores of ferrierite zeolite and MFI-type ZRP-5 zeolites (before and after SiCl4 chemical vapor deposition (SiCl4-CVD) modification). Over ferrierite zeolite and SiCl4-CVD-modified ZRP-5 zeolite, the cracking of n-C16 in a 50/50 (v/v) mixture of n-C8 and n-C16 is greatly suppressed, because of both the restrictive environment (narrowed pore openings) and the presence of n-C8. Comparison of the cracking gas yield and the selectivity to C3 in LPG over ZRP-5 zeolites before and after SiCl4-CVD modification has also been presented.
Introduction The modern fluid catalytic cracking (FCC) process is not only one of the major workhorses in refineries for the production of motor fuel, but it is also an important approach for the production of petrochemical feedstock (e.g., gaseous olefins). Maximizing the production of light cycle oil (LCO) and enhancing the selectivity of propylene in liquefied petroleum gas (LPG) without increasing the total LPG yield is one of the main challenges facing FCC technologies in recent years. MFI-type zeolite is widely used as a catalyst component in catalytic cracking for gaseous olefin production and as an octane-enhancing promoter. Nevertheless, its addition to the FCC catalyst generally decreases the LCO yield.1,2 Recently, Santilli and Zones3 studied the conversion of hexane and hexadecane over ZSM-5 and SSZ-16 zeolites in the presence of H2. In separate experiments, the 10-membered-ring (10-MR) zeolite ZSM-5 converts hexane and hexadecane to similar extents. In mixtures, the presence of hexane has little effect on the hexadecane conversion, but the latter inhibits the hexane conversion. However, over the eightmembered-ring (8-MR) zeolite SSZ-16, the presence of hexadecane results in a slight increase in the hexane conversion and hexane keeps the hexadecane from reacting. Martens and Jacobs4 commented that the * Author to whom correspondence should be addressed. E-mail:
[email protected]. † China Petroleum Corporation. ‡ Auburn University. § East China Normal University. (1) He, M.; Yang, X.; Shu, X.; Luo, J. Phosphorous Containing Zeolite Having MFI Type Structure. U.S. Patent 5,951,963, September 14, 1999. (2) Smith, G. A.; Evans, M. Meeting Changing Gasoline Specifications and Variable Propylene and Butylene Demand through the Use of Additives. Presented at the 1998 NPRA Annual Meeting, San Francisco, CA, March 15-17, 1998; Paper No. AM-98-17. (3) Santilli, D. S.; Zones, S. I. Catal. Lett. 1990, 7, 383-386.
potential of certain zeolites to reverse the reactivity order of some reactants offers new opportunities, e.g., in the catalytic upgrading of petroleum fractions. In this paper, cracking of n-alkane single feedstocks and mixtures was conducted over ferrierite zeolite and MFI-type ZRP-5 zeolites (before and after SiCl4 chemical vapor deposition (SiCl4-CVD) modification). The results show how a reactant that is smaller than other reactants can greatly inhibit the cracking of large molecules under FCC conditions within a restricted pore system. Our effort is targeted at controlling the cracking reactions and, therefore, adjusting the yield structures in the FCC units. A better understanding of the fundamental interactions that occur under FCC conditions can help us to do that. Experimental Section Zeolites. A commercially available MFI-type zeolite, ZRP-5 (SiO2/Al2O3 > 200), developed by the Research Institute of Petroleum Processing (which is affiliated with the China Petrochemical Corporation), and a H+ exchanged ferrierite zeolite (SiO2/Al2O3 ≈ 16) were used in this study. SiCl4-CVD Treatment of Zeolite. Prior to modification by SiCl4-CVD, the ZRP-5 zeolite was calcined at 500 °C for 1 h, placed inside a desiccator, and cooled to room temperature. The zeolite then was put into a saturator that had been filled with SiCl4 gas at room temperature until the sample developed a yellow color. The product was hydrolyzed, dried overnight, and calcined at 500 °C to form SiCl4-CVD-modified zeolite (SiZRP-5). Characterizations. Powder X-ray diffraction (XRD) measurements were performed on a Rigaku model D/MAX-IIIA instrument, using a scanning rate of 0.01°/s and an accelerating voltage of 35 kV. 27Al MAS NMR measurements were (4) Martens, J. A.; Jacobs; P. A. In Zeolite Microporous Solids: Synthesis, Structure, and Reactivity; Derouance, E. G., Lemos, F., Naccache, C., Ribeiro, F. R., Eds.; Kluwer Academic Publishers: Amsterdam, 1992; Vol. 352, pp 511-529.
10.1021/ef030021p CCC: $25.00 © 2003 American Chemical Society Published on Web 06/04/2003
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Table 1. Specific Surface Area, Pore Volume, and Acidic Properties of ZRP-5 Zeolites before and after SiCl4 Chemical Vapor Deposition (SiCl4-CVD) NH3 desorbed in TPD, mmol/g zeolite ZRP-5 Si-ZRP-5
specific surface pore volume, area, m2/g cm3/g 287 278
0.122 0.129
L-peak (Tm, °C)
H-peak (Tm, °C)
0.13 (245) 0.11 (248)
0.20 (380) 0.19 (377)
Table 2. Cracking Results of n-Alkane Single Feedstocks (n-C8 and n-C16) and Their Mixtures over Several Zeolites conversion, wt % ZRP-5 ferrierite Si-ZRP-5
n-C8
n-C16
n-C8/n-C16 (50/50, v/v)
25.8 27.6 26.3
79.0 67.1 70.7
n-C8, 27.0; n-C16, 78.5 n-C8, 28.3; n-C16, 14.3 n-C8, 32.6; n-C16, 30.1
performed on a Bruker model AM-300MAS spectrometer. Brunauer-Emmett-Teller (BET) surface area measurements were conducted using N2 at -196 °C on a Micromeritics model ASAP2400 instrument. Ammonia temperature-programmed desorption (NH3-TPD) was performed on a DuPont model 951 TGA instrument, using highly purified N2 as a carrier gas (50 mL/min). The zeolite sample that was packed into the chamber was purged by a N2 carrier gas at 550 °C to a constant weight and cooled to 150 °C; NH3 gas then was introduced into the chamber, for saturated adsorption at this temperature. Subsequently, such a NH3-saturated zeolite sample was purged with a N2 carrier gas, to remove physically adsorbed NH3, and heated to 650 °C at a rate of 10 °C/min. The specific surface area, the pore volume, and the acidic properties of ZRP-5 zeolites before and after SiCl4-CVD modification are listed in Table 1. Cracking of n-Alkanes. The n-alkane cracking reactions were conducted in a downflow microactivity test (MAT) unit. The unit consisted of a 14 mm (inner diameter) × 188 mm (length) stainless steel tube reactor, a syringe pump, a liquid product collector, a liquid displacement column, and a wet test meter. The zeolite (40-60 mesh) loading was 0.5 g. The amount of n-alkane feedstock was 1.79 g. Cracking reactions were performed at a reactor temperature of 500 °C and at a time-on-stream of 70 s. Each zeolite was tested with three feeds: n-octane (n-C8), n-hexadecane (n-C16), and a 50/50 (by volume) mixture of both n-alkanes. The n-C8 and n-C16 were both 99+% pure. After cracking, the catalyst bed was purged with a highly purified N2 flow (30 mL/min) for 1800 s, to strip entrapped hydrocarbons in the catalyst bed. The effluent was quenched in an ice-salt bath and separated into liquid and gas products. A Shimadzu model GC-9A chromatography system that was equipped with an OV-101 capillary column (50 m × 0.2 mm) and a flame ionization detector was used to analyze the liquid products, and a cracking gas analyzer (Hewlett-Packard model HP5880A) was employed to analyze the cracking gases. The conversions and yields of the cracking gases were calculated to a mass balance of g 95%.
Results and Discussion Cracking results for n-C8 and n-C16 single feedstocks and their mixture over several zeolites are shown in Table 2. Over 10-MR ZRP-5, which is an MFI-type zeolite, conversions of 25.8% for n-C8 and 79.0% for n-C16 were obtained when the two feeds were reacted separately. When the two feeds were reacted simultaneously, considerable conversion of each reactant was obtained, just as in single-feed cracking. The results indicated that the cracking of each reactant is not affected by the presence of another over this 10-MR zeolite, which has a pore opening size of ∼0.55 nm.
However, over ferrierite, which is an 8-MR zeolite with a 0.48 nm pore opening size, the cracking results were quite different. When n-C8 and n-C16 reacted separately, considerable conversion of each reactant was obtained, just as with the ZRP-5 zeolite. In the mixtures, the presence of n-C16 resulted in a slight increase in the conversion of n-C8, but n-C8 strongly suppressed the n-C16 cracking. The same phenomenon has also been observed by Santilli and Zones3 in hydrocracking nhexane and n-hexadecane over 8-MR SSZ-16 zeolite. They explained that this phenomenon occurs when one molecule interferes with the reactivity of another, strictly because of steric constraints that are imposed by the pores, and that this does not happen in an unhindered environment. Obviously, this steric effect, which was first identified in the hydrocracking of mixtures of n-alkanes, was also found under FCC conditions within restrictive pores such as the ferrierite zeolite pore (pore opening size of 0.48 nm). In contrast, the ZRP-5 zeolite (pore opening size of 0.55 nm) does not have such restrictive pores. As a result, the presence of n-C8 did not affect the cracking of n-C16 in the case of the cracking of n-C8/n-C16 mixtures. The aforementioned results also suggest that a value of 0.5 nm for a zeolite pore opening is a critical dimension for such a restricted environment. Further studies were conducted with a Si-ZRP-5 zeolite that had been prepared by SiCl4-CVD. In this case, considerable conversion of each feed component was achieved, just as with both the ZRP-5 and ferrierite zeolites when the alkanes were tested separately. However, when the mixtures of two feeds were reacted, a cracking behavior that was similar to that of the ferrierite zeolite was observed over this Si-ZRP-5 zeolite (see Table 2). According to the results reported by Santilli and Zones,3 such selective cracking of n-C8 in a 50/50 (v/v) mixture of n-C8 and n-C16 is attributed to restrictive pores, because of narrowed pore openings after SiCl4-CVD modification. Indeed, the shape selectivity of CVD zeolites has been greatly improved in sorption separation and catalysis, as a result of the reduced pore opening size. Gao et al.5 indicated that the pore opening size of the ZSM-5 zeolite can be reduced efficiently by ∼0.05 nm using the SiCl4 deposition method, because such modified ZSM-5 zeolites exhibit excellent shape-selective separation properties for pxylene/m-xylene mixtures (kinetic diameters of 0.58/0.63 nm). NH3-TPD and N2 BET measurements show that the specific surface area, the pore volume, and acid sites remained almost unchanged before and after the SiCl4CVD procedure (see Table 1). Moreover, XRD analyses show that crystallinity resistance remained unchanged before and after modification; no aluminum octahedrons were observed in the Si-ZRP-5 zeolite by 27Al magic angle spinning (MAS) NMR. These results indicate that the bulk structure and the properties of the zeolite were not affected during the SiCl4-CVD modification. Correlation of these characterization results with the cracking results over ZRP-5 zeolites, before and after the modification, clearly shows that selective cracking of n-C8 in a 50/50 (v/v) mixture of n-C8 and n-C16 over the Si-ZRP-5 zeolite is definitely not affected by the surface (5) Yue, Y.-H.; Tang, Y.; Liu, Y.; Gao, Z. Ind. Eng. Chem. Res. 1996, 35, 430-433.
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Table 3. Cracking Gas Composition in the Cracking of the 50/50 (v/v) Mixture of n-C8 and n-C16 over ZRP-5 Zeolites before and after SiCl4-CVD Modification gas composition yield, wt % cracking gas yield dry gas propane propylene total butane total butene isobutane isobutene selectivity, wt % C3 in LPG C3) in LPG C4 in LPG C4) in LPG iso-C4 in LPG iso-C4) in LPG
ZRP-5
Si-ZRP-5
23.3 1.8 3.1 8.7 2.0 7.7 0.6 3.4
12.5 1.1 2.4 4.3 1.6 3.2 0.6 1.5
54.9 40.3 45.1 35.7 2.7 16.0
58.4 37.5 41.4 27.9 5.2 13.1
area, the pore volume, or the crystal structure. This, in turn, suggests that pore-opening reduction, which results in a restrictive environment, is the exclusive reason for this phenomenon. The intrinsic attribution to the decrease in n-C16 conversion when a 50/50 (v/v) mixture of n-C8 and n-C16 is cracked in a restrictive environment such as the ferrierite and Si-ZRP-5 zeolite pores is still not clear. Both molecules have similar kinetic diameters, and the n-C16 conversions are very similar in the cases of the cracking of single n-C16 over all three zeolites (see Table 2). Thus, this is probably not attributed to the diffusion limitation of n-C16 into such narrowed pores. Otherwise, over the ferrierite and Si-ZRP-5 zeolites, n-C16 conversions (∼70%) in the cracking of single n-C16 should be reduced to a value that is similar to the conversions of n-C16 (10. This phenomenon does not occur during the cracking of n-C6 over 10-MR ZSM-5 zeolite. Correlated to our study results, perhaps n-C8 causes the n-C16 concentration in the ferrierite and Si-ZRP-5 zeolite pores to decrease in a similar way when these two molecules are both present in the feed. Table 3 compares the yield and selectivity of the cracking gases for ZRP-5 zeolites before and after SiCl4CVD modification. The total yield of the cracking gas is reduced greatly after SiCl4-CVD, because the cracking of n-C16 was suppressed greatly in the case of the cracking of n-C8/n-C16 mixtures. The Si-ZRP-5 zeolite has good selectivity to the sum of the C3 hydrocarbons in LPG; however, the selectivities to propylene and butene all were obviously decreased. These results indicate that the narrowed pore opening favors the production of C3 hydrocarbons. Unfortunately, the narrowed pore opening also favors the hydrogen transfer reaction that results in saturation of the olefins. Conclusions When a 50/50 (v/v) mixture of n-C8 and n-C16 is cracked under fluid catalytic cracking conditions, over ferrierite zeolite and SiCl4-modified ZRP-5 (Si-ZRP-5) zeolite, n-C8 suppresses the cracking of n-C16, because of the restrictive environment that is created as a result of a narrowed pore opening. The pore opening size of 0.5 nm is a critical dimension for the appearance of such a restricted environment in zeolites. Si-ZRP-5 zeolite offers good selectivity to the total C3 hydrocarbons in liquefied petroleum gas (LPG), compared to ZRP-5 zeolite; however, the selectivities to propylene and butene in LPG all were decreased, because of the narrowed pore opening. EF030021P
