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Microporous and Mesoporous Materials xxx (2016) 1e8
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Catalytic conversion of light hydrocarbons to propylene over MFI-zeolite/metal-oxide composites* Shinya Hodoshima*, Azusa Motomiya, Shuhei Wakamatsu, Ryuichi Kanai, Fuyuki Yagi Research & Development Center, Chiyoda Corporation, 3-13 Moriya-cho, Kanagawa-ku, Yokohama, Kanagawa 221-0022, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 September 2015 Received in revised form 23 December 2015 Accepted 24 December 2015 Available online xxx
Zeolite-based composite catalysts for producing propylene efficiently by cracking of light hydrocarbons were developed in the present study to meet future demand for propylene and utilize various light hydrocarbons (e.g., light-naphtha, condensates recovered from natural gas and olefinic hydrocarbons obtained from GTL-process). The composite catalysts, consisting of Al-MFI zeolites containing iron and gallium atoms (FeeGaeAl-MFI) at optimized ratio and aluminum-oxide binder (Al2O3), were employed for cracking of n-hexane or 1-hexene using fixed-bed-type reactor at around 550 C. Since the FeeGaeAlMFI zeolites caused acidity suitable for generating light olefins selectively, high propylene yields (18 e41 wt%) were obtained by suppressing formation of aromatics at mild temperatures. The present catalyst exhibited high propylene productivity and catalytic stability simultaneously in the steamcracking of hydrocarbons at suitable steam ratio. Moreover, in the cracking of n-hexane without dilution over the present catalyst, high propylene yields of ca. 18 wt% were maintained for at least 80 h due to its high resistance to coke formation in spite of severe conditions, suggesting that simplified cracking process for on-purpose propylene production from light hydrocarbons is feasible in fixed-bed mode by using the present composite catalysts. © 2016 Elsevier Inc. All rights reserved.
Keywords: Catalytic cracking Propylene production Light hydrocarbons FeeGaeAl-MFI-zeolite Zeolite/metal-oxide composite
1. Introduction Propylene as a raw material in the petrochemical industry has been increasingly significant because the global demand for propylene has increased at growth rate of 4e5 wt% annually [1]. Since the majority of propylene (ca. 60%) is currently produced as a byproduct by thermal steam-cracking of hydrocarbon feedstock such as naphtha, the thermal crackers can no longer meet the propylene demand. In addition, this technology requires a huge amount of thermal energy (temperature: 850e900 C) as well as its low weight ratio of propylene to ethylene (ca. 0.5). The recent shift to ethane-based feedstock for producing ethylene also might affect the propylene supply, because this technology produces little propylene. Any alternative method for producing propylene efficiently from naphtha, being widely available feedstock, should be thus developed to meet future demand for propylene and reduce energy-consumption in olefin production. Moreover, conversion of various light hydrocarbons (e.g., shale oil, condensates recovered
*
This paper was presented at ZMPC2015. * Corresponding author. Tel.: þ81 45 441 9133; fax: þ81 45 441 9728. E-mail address: [email protected] (S. Hodoshima).
from shale gas and olefinic hydrocarbons obtained from Gas-ToLiquids (GTL) process [2]) into valuable light olefins is also significant task in terms of efficient use of resources. Catalytic cracking of hydrocarbons such as naphtha over solidacid catalysts, e.g., ZSM-5, have been actively investigated as an alternative method for on-purpose propylene production to conventional thermal steam-cracking [3e9]. In particular, catalytic cracking using fixed-bed reactor has the following advantages compared to steam-cracking: (1) high selectivity to propylene (weight ratio of propylene to ethylene > 2.0); (2) being energysaving due to low temperatures (< 650 C). Furthermore, the cracking process in fixed-bed mode is superior to fluidized catalytic cracking (FCC) processes in terms of operational simplicity in reaction unit. However, catalytic cracking process using fixed-bed reactor has not been commercialized, because practical cracking catalyst with excellent stability, being applicable to fixed-bed reactor, has been still undeveloped. From these technological backgrounds, the development of highly active and stable catalyst for the fixed-bed-type cracking of light hydrocarbons to produce propylene efficiently under mild conditions has been focused in the present study. Firstly, the Al-MFI zeolites containing iron and gallium (FeeGaeAl-MFI) were adopted
http://dx.doi.org/10.1016/j.micromeso.2015.12.044 1387-1811/© 2016 Elsevier Inc. All rights reserved.
Please cite this article in press as: S. Hodoshima, et al., Catalytic conversion of light hydrocarbons to propylene over MFI-zeolite/metal-oxide composites, Microporous and Mesoporous Materials (2016), http://dx.doi.org/10.1016/j.micromeso.2015.12.044
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S. Hodoshima et al. / Microporous and Mesoporous Materials xxx (2016) 1e8
as main component of cracking catalyst [10]. It has been recognized that Fe and Ga species as heteroatom could weaken acid strength of MFI zeolites [11] and accelerate dehydrogenation activities for alkanes [12e14], respectively. On the basis of these findings, it was found by our group that the FeeGaeAl-MFI zeolites synthesized at adequate mixed-ratio exhibited both functions derived from Fe and Ga species [10], so that highly selective propylene-generation was attained by suppressing formation of aromatics. Secondly, the composite catalysts for industrial use, consisting of the FeeGaeAlMFI zeolites and metal-oxide binder (e.g., aluminum oxide) [10], were prepared to utilize the proprietary zeolites under practical conditions. This paper presents excellent properties of these catalysts on the experimental basis. 2. Experimental 2.1. Synthesis and characterization of MFI-type zeolites Three kinds of MFI-type zeolites containing some heteroatom such as aluminum, iron and gallium (Al-MFI, FeeAl-MFI and FeeGaeAl-MFI) were hydrothermally synthesized by conventional method. Desired aqueous solution containing Si and Al source, Si, Al and Fe source and Si, Al, Fe and Ga source were prepared as mother gel with the following molar compositions: (1) Si/Al ¼ 25.0; (2) Si/ (Fe þ Al) ¼ 25.0, Fe/(Fe þ Al) ¼ 0.4, Al/(Fe þ Al) ¼ 0.6; (3) Si/ (Fe þ Ga þ Al) ¼ 25.0, Fe/(Fe þ Ga þ Al) ¼ 0.4, Ga/ (Fe þ Ga þ Al) ¼ 0.2, Al/(Fe þ Ga þ Al) ¼ 0.4. Each mother gel was charged into stainless-type autoclave and then heated at 150 C for 24 h under adequate stirring conditions. The obtained sodium-type zeolites were converted into proton-type ones (HeAl-MFI, HeFeeAl-MFI and HeFeeGaeAl-MFI) by ion-exchange with NH4NO3 aqueous solution, dried at 120 C for 3 h and finally calcined at 550 C for 3 h under air stream. These protonated samples (HeAl-MFI, HeFeeAl-MFI and HeFeeGaeAl-MFI (Si/T molar ratio in mother gel: 25.0, T ¼ Al, Fe þ Al, Fe þ Ga þ Al)) were characterized as follows. Both X-ray diffraction (XRD, Rigaku Co. Ltd.) and X-ray fluorescence (XRF, Rigaku Co. Ltd.) measurements were carried out to determine their structure and chemical compositions, respectively. The surface area was estimated by the Langmuir method on the basis of nitrogen adsorption isotherms. The acidity of these samples was investigated by the temperature-programed desorption technique using ammonium probe (NH3-TPD, BEL Japan Co. Ltd.). The NH3-TPD measurement was performed in accordance with the following procedure. Firstly, the sample was pretreated by calcination at 500 C for 1.5 h under He stream (flow rate: 50 mL/min) and then cooled to room temperature. Secondly, NH3 molecules were adsorbed on surface of the sample at 100 C for 0.5 h by switching from He to mixed gas of 5.0% NH3 and He (balance gas) at flow rate of 20 mL/min and then He was introduced at flow rate of 20 mL/min for 40 min to purge remaining ammonium. After the above pretreatment and NH3 adsorption, the NH3-TPD profile was measured by increasing temperature from 100 C to 880 C at heating rate of 10 C/min under He stream including 0.1% Ar as an internal standard (flow rate: 50 mL/min). With respect to desorption peak of NH3 molecules observed at around 350 C in the obtained NH3-TPD profile, both acid amount and acid strength of the zeolite sample were estimated by calculating its peak area and activation energy required for desorption of NH3 molecules [15], respectively. In addition to the above zeolites, sodium-type FeeGaeAl-MFI zeolite with smaller amount of Al, Ga and Fe atoms (molar composition in mother gel: Si/(Fe þ Ga þ Al) ¼ 200, Fe/ (Fe þ Ga þ Al) ¼ 0.4, Ga/(Fe þ Ga þ Al) ¼ 0.2, Al/ (Fe þ Ga þ Al) ¼ 0.4) was also synthesized by the same procedure and characterized by the XRD and XRF measurements.
2.2. Preparation of FeeGaeAl-MFI zeolite/aluminum-oxide composite catalysts Composite catalysts, consisting of the FeeGaeAl-MFI zeolites and aluminum oxide (g-Al2O3) binder, were prepared as follows. Firstly, the sodium-type FeeGaeAl-MFI zeolites with different compositions (molar ratios in mother gel: Si/(Fe þ Ga þ Al) ¼ 25, Fe/(Fe þ Ga þ Al) ¼ 0.4, Ga/(Fe þ Ga þ Al) ¼ 0.2, Al/ (Fe þ Ga þ Al) ¼ 0.4; Si/(Fe þ Ga þ Al) ¼ 200, Fe/ (Fe þ Ga þ Al) ¼ 0.4, Ga/(Fe þ Ga þ Al) ¼ 0.2, Al/ (Fe þ Ga þ Al) ¼ 0.4) were hydrothermally synthesized by the same method described in the Section 2.1. Secondly, after both the zeolite and alumina sol with an average particle size of 5.4 nm (Cataloid AP-1, JGC Catalysts and Chemicals Ltd. [16]) were mixed at weight ratio of 65%-Zeolite/35%-Al2O3, powdery mixture was molded into cylindrical shape with diameter of 1.0 mm4 by extrusion. After these cylindrical samples were converted into protonated ones with NH4NO3 aqueous solution, they were dried at 120 C for 3 h and then calcined at 550 C for 3 h under air stream. In addition, the protonated Fe-Ga-Al-MFI zeolite (molar composition in mother gel: Si/(Fe þ Ga þ Al) ¼ 25, Al/(Fe þ Ga þ Al) ¼ 0.4, Fe/ (Fe þ Ga þ Al) ¼ 0.4, Ga/(Fe þ Ga þ Al) ¼ 0.2) prepared by the above-described method was also formed into cylindrical composite (1.0 mm4) by using the Al2O3 binder. Weight ratios of resultant composites were measured by the XRF technique. Regarding the cylindrical sample with weight ratio of 65%FeeGaeAl-MFI (molar ratios in mother gel: Si/(Fe þ Ga þ Al) ¼ 25, Fe/(Fe þ Ga þ Al) ¼ 0.4, Ga/(Fe þ Ga þ Al) ¼ 0.2, Al/ (Fe þ Ga þ Al) ¼ 0.4)/35%-Al2O3, both observation of its morphology and elemental analysis of silicon and aluminum atoms were carried out by means of scanning electron microscopy equipped with an energy-dispersive fluorescence X-ray spectrometer (SEM-EDX, Hitachi Ltd.). 2.3. Catalytic cracking of liquid hydrocarbons over zeolites and zeolite-based composites Catalytic cracking of n-hexane (n-C6H14) diluted with nitrogen over the synthesized zeolites (HeAl-MFI, HeFeeAl-MFI and HeFeeGaeAl-MFI, Si/T ratio in mother gel: 25.0, T ¼ Al, Fe þ Al, Fe þ Ga þ Al) were performed for 5.0 h under atmospheric pressure by use of a flow-type reactor in fixed-bed mode. The protonated zeolites in powdery state were formed into pellet state, crushed and finally sorted into length of 1.0e2.0 mm. These sieved zeolites of 1.0 mL were charged into the central part of tubular reactor. The normal hexane was employed for reaction test as a model reactant representing paraffinic naphtha feedstock. The n-hexane diluted with nitrogen (molar ratio of N2 to n-C6H14: 15/1, 10/1 and 5/1) was supplied to the catalyst-layer at 550 C with the Liquid Hourly Space Velocity (LHSV) of n-hexane at 1.0 h1. After separating products into gaseous and liquid components through vessel equipped with cooling unit, generation rates of both gaseous and liquid products were measured to check mass balance. Both conversion of n-hexane and yield of each product were calculated based on both generation rates of products and gaschromatographic analysis. Catalytic cracking of n-hexane diluted with nitrogen or steam over the HeFeeGaeAl-MFI/Al2O3 composites (Si/(Fe þ Ga þ Al) ratio in mother gel: 25.0, Mixed weight ratio in preparation step: 65%-Zeolite/35%-Al2O3) were performed for 5.0 h in the fixed-bedtype reactor. After the cylindrical composites were sorted into length of 1.0e2.0 mm, these sieved samples of 1.0 mL were charged into the reactor. Cracking reactions were carried out under the following conditions: Temperature; 550 C, Pressure; 0.1 MPa, Feedstock; n-hexane diluted with nitrogen or steam (molar ratio of
Please cite this article in press as: S. Hodoshima, et al., Catalytic conversion of light hydrocarbons to propylene over MFI-zeolite/metal-oxide composites, Microporous and Mesoporous Materials (2016), http://dx.doi.org/10.1016/j.micromeso.2015.12.044
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N2 to n-C6H14: 15/1, molar ratio of H2O to n-C6H14: 10/1 and 5/1), LHSV of n-hexane; 1.0 h1 (n-C6H14/N2 feedstock), 2.0 h1 (n-C6H14/ H2O feedstock). Catalytic cracking of 1-hexene (1-C6H12) diluted with nitrogen or steam over the HeFeeGaeAl-MFI/Al2O3 composite (Si/(Fe þ Ga þ Al) ratio in mother gel: 25.0, Mixed weight ratio in preparation step: 65%-Zeolite/35%-Al2O3) were also performed for 5.0 h in the fixed-bed-type reactor. The 1-hexene was employed in the present study as a model reactant representing olefinic naphtha feedstock. Cracking reactions were carried out under the following conditions: Temperature; 550 C, Pressure; 0.1 MPa, Charged amount of catalyst: 1.0 mL, Feedstock; 1-hexene diluted with nitrogen or steam (molar ratio of N2 to 1-C6H12: 15/ 1, molar ratio of H2O to 1-C6H12: 5/1), LHSV of 1-hexene; 1.0 h1 (1C6H12/N2 feedstock), 2.0 h1 (1-C6H12/H2O feedstock). Both conversions of feedstock and product yields in cracking reactions over the composite catalysts were evaluated in the same manner as cracking over the MFI-zeolite alone. Catalytic cracking of n-hexane without dilution over HeFeeGaeAl-MFI/Al2O3 composites (Si/(Fe þ Ga þ Al) ratio in mother gel: 25.0 and 200, Mixed weight ratio in preparation step: 65%-Zeolite/35%-Al2O3) were performed for 24 h in the fixed-bedtype reactor under the following conditions: Temperature; 550 C and 565 C, Pressure; 0.1 MPa, Charged amount of catalyst: 2.0 mL, Feedstock; n-hexane without dilution, LHSV of n-hexane; 5.0 h1. Both n-hexane conversion and product yields were evaluated in the same manner as cracking over the MFI-zeolite alone. In addition, reaction tests for longer than 30 h were carried out at the temperature of 565 C to examine lifetime of the composite catalysts (vide infra). 3. Results and discussion 3.1. FeeGaeAl-MFI zeolite as main component for catalytic lighthydrocarbon cracking Zeolites synthesized in the present study and their properties are listed in Table 1. All of resultant zeolites were confirmed by the XRD measurement to show the diffraction patterns corresponding to MFI-type framework. Both molar ratios of silicon atom to heteroatom (Si/Al, Si/(Fe þ Al) and Si/(Fe þ Ga þ Al)) and surface area were almost the same among them. On the basis of the XRF measurement, molar ratios of each heteroatom to total heteroatoms in the HeFeeAl-MFI and the HeFeeGaeAl-MFI zeolites were calculated as follows: HeFeeAl-MFI; Fe/(Fe þ Al) ¼ 0.5, Al/ (Fe þ Al) ¼ 0.5, HeFeeGaeAl-MFI; Fe/(Fe þ Ga þ Al) ¼ 0.4, Ga/ (Fe þ Ga þ Al) ¼ 0.3, Al/(Fe þ Ga þ Al) ¼ 0.3. Fig. 1 shows NH3-TPD profiles of these samples. As seen in Fig. 1, two desorption peaks of NH3 molecules were observed at around 180 C and 350 C, respectively. The peak at around 180 C is the desorption peak derived from NH3 molecules physically adsorbed on zeolites, whereas the peak at around 350 C is definitely associated with NH3 molecules chemically adsorbed on acid sites. Table 2 presents acidity of zeolites estimated by analyzing the high-temperature
3
Fig. 1. NH3-TPD profiles of synthesized MFI-type zeolites.
desorption peak. Both acid amount and acid strength were calculated as a peak area and an activation energy required for desorption of NH3 molecules according to the method proposed by Niwa and Katada et al. [15], respectively. The acid amount was nearly the same among them, whereas the desorption peaks of the HeFeeAlMFI and HeFeeGaeAl-MFI zeolites, closely related to the acid strength, were shifted to the lower temperatures compared to that of the HeAl-MFI zeolite, as shown in the NH3-TPD profiles (Fig. 1). As a consequence, the activation energy required for NH3 desorption (i.e. acid strength) of the HeFeeAl-MFI and HeFeeGaeAl-MFI were slightly smaller than that of the HeAl-MFI zeolite (Table 2). Catalytic cracking of n-hexane diluted with nitrogen (molar ratio of N2 to n-C6H14: 15/1) over the HeAl-MFI, the HeFeeAl-MFI and the HeFeeGaeAl-MFI zeolites were performed at 550 C to compare their catalytic performance under the same conditions. Fig. 2 and Table 3 show product selectivity and catalytic activities, respectively. The product distribution was obviously changed by introducing iron or iron & gallium into the HeAl-MFI zeolite. Though the HeAl-MFI zeolite as a reference exhibited comparatively high selectivity to light olefins, it also gave high selectivity to aromatics such as BTX (benzene, toluene and xylenes). On the other hand, the HeFeeAlMFI and HeFeeGaeAl-MFI remarkably enhanced selectivity to light olefins by suppressing aromatics formation (Fig. 2), so that high propylene yields of 24.7e27.0 wt% were obtained in spite of low temperature of 550 C for cracking and these yields were kept constant during reaction time (5.0 h). In addition, the product selectivity by this HeFeeGaeAl-MFI zeolite was compared to the HeAl-MFI zeolite at the same level of n-hexane conversion. The nhexane cracking over the HeFeeGaeAl-MFI zeolite was conducted at the mixed ratio of N2 to n-C6H14 of 5/1, giving n-hexane conversion of 98.5 wt%. Though selectivity to aromatics increased with the conversion, the HeFeeGaeAl-MFI zeolite still kept higher selectivity to propylene than the HeAl-MFI zeolite (Fig. 2), resulting in high propylene yield of 24.3 wt%. It is well-known that catalytic cracking of paraffinic hydrocarbons over zeolites proceed via light olefins successively to generate aromatics and selectivity to light olefins
Table 1 Compositions and surface area of synthesized MFI-type zeolites. Zeolite species (Si/T ratioa)
Si/T ratiob [mol/mol]
Ratio of each heteroatom [mol/mol] Al/(Fe þ Ga þ Al)
Ga/(Fe þ Ga þ Al)
Fe/(Fe þ Ga þ Al)
HeAl-MFI (25.0) HeFeeAl-MFI (25.0) HeFeeGaeAl-MFI (25.0)
20.0 19.4 19.4
1.0 0.5 0.3
0.0 0.0 0.3
0.0 0.5 0.4
a b c
Surface areac [m2/g]
573 570 587
Si/T molar ratio in mother gel (T ¼ Al, Fe þ Al, Fe þ Ga þ Al). Si/T molar ratio measured by XRF technique (T ¼ Al, Fe þ Al, Fe þ Ga þ Al). Surface area measured by Langmuir method.
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Table 2 Acidity of synthesized MFI-type zeolites. Zeolite species (Si/T ratioa)
Si/T ratiob [mol/mol]
Acid amount [mmol/g]
Acid strength [kJ/mol]
HeAl-MFI (25.0) HeFeeAl-MFI (25.0) HeFeeGaeAl-MFI (25.0)
20.0 19.4 19.4
0.57 0.51 0.52
143.0 138.6 137.1
Conditions of NH3-TPD measurement; Pretreatment: He flow (50 mL/min, 500 C, 1.5 h). NH3 adsorption: He/NH3 (5%) flow (20 mL/min, 100 C, 0.5 h). TPD measurement: He flow (50 mL/min-STP (Ar 0.1%), 100e800 C, 10 C/min). a Si/T molar ratio in mother gel (T ¼ Al, Fe þ Al, Fe þ Ga þ Al). b Si/T molar ratio measured by XRF technique (T ¼ Al, Fe þ Al, Fe þ Ga þ Al).
strongly depends on acidity of zeolites [17]. Iron and gallium atoms in zeolites have been recognized to reduce acid strength of zeolites [11] and accelerate dehydrogenation of alkanes [12e14], respectively. The results in both NH3-TPD measurements and cracking reactions also suggest that the introduction of Fe atoms caused a drastic change of acidity and the combination of Fe and Ga atoms at optimized ratio made it possible to generate propylene further selectively. The HeFeeGaeAl-MFI zeolite with molar composition (Si/(Fe þ Ga þ Al) ¼ 19.4, Fe/(Fe þ Ga þ Al) ¼ 0.4, Ga/ (Fe þ Ga þ Al) ¼ 0.3, Al/(Fe þ Ga þ Al) ¼ 0.3) thus gave the best performance for n-hexane cracking. Catalytic cracking of n-paraffin over zeolite catalysts have been actively investigated to establish efficient methods for on-purpose production of propylene [3e9]. In particular, the MSE-type zeolite (MCM-68) is recognized to have an excellent potential as FCC additives because of its high propylene productivity [9]. Kubota et al. reported that high propylene yield of ca. 40 C-mol% was obtained in n-hexane cracking at 650 C by the La-modified MCM-68 zeolite [9]. Since the propylene yield was ca. 28 C-mol% at 550 C, the FeeGaeAl-MFI zeolite was comparable to the MCM-68. 3.2. HeFeeGaeAl-MFI/aluminum-oxide composite for catalytic light-hydrocarbon cracking Zeolite catalysts are generally formed into shapes suitable for industrial utilization by using metal-oxide binder (e.g., aluminum
oxide) to improve mechanical strength and reduce pressure drop during reaction. In the present study, cylindrical composites with diameter of 1.0 mm4, consisting of the FeeGaeAl-MFI zeolite (Si/ (Fe þ Ga þ Al) ¼ 19.4) and aluminum oxide [18,19], were prepared by the following procedures: (1) After sodium-type zeolite was first converted into proton-type one, this protonated zeolite in powdery state was molded into cylindrical composite using Al2O3; (2) After sodium-type zeolite was first molded into cylindrical composite using Al2O3, this sodium-type zeolite combined with Al2O3 was protonated by ion-exchange. Fig. 3(A) shows the SEM image of the cylindrical composite and Fig. 3(B) and (C) provides the distribution of Si and Al atoms in this composite measured by the SEM-EDX analysis. As obvious from these results, both Si and Al atoms uniformly existed in the present sample. Table 4 compares catalytic activities for n-hexane cracking with respect to the HeFeeGaeAl-MFI alone and the HeFeeGaeAl-MFI/ Al2O3 composites (Weight ratio in obtained sample: 75 wt%/25 wt %). Both composite catalysts exhibited higher n-hexane conversions and apparent yields of light olefins than those in the case of the zeolite alone. Weight of light olefins generated per 1 h with zeolite of 1 g, i.e. space time yield of light olefins, was also enhanced by use of the composite catalyst, suggesting that zeolite species combined with aluminum-oxide binder were well dispersed. However, the composite catalyst prepared by the above method (2) was found to give excellent performance because of its lower aromatics yield. It is preferable in catalytic cracking using fixed-bed reactor to suppress aromatics formation as low as possible to prevent rapid catalyst deactivation due to coking, as described in the Section 3.3. Furthermore, this method is obviously favorable in terms of handling in catalyst preparation. It was thus confirmed on the experimental basis that the composite prepared by the method (2) in the present study could be practical cracking catalyst. Catalytic cracking of n-hexane diluted with steam over the HeFeeGaeAl-MFI/Al2O3 composite (Si/(Fe þ Ga þ Al) ¼ 19.4, Zeolite/Al2O3 ¼ 75 wt%/25 wt%) was attempted at 550 C in fixedbed operation. Fig. 4 shows time courses of n-hexane conversion in the cracking reactions. Under the same conditions (molar ratio of diluent to n-C6H14: 10/1, LHSV of n-C6: 1.0 h1), n-hexane cracking
Fig. 2. Product distribution in catalytic cracking of n-hexane diluted with nitrogen over synthesized MFI-type zeolites. Catalyst; HeAl-MFI (molar composition: Si/Al ¼ 20.0), HeFeeAl-MFI (molar composition: Si/(Fe þ Al) ¼ 19.4, Fe/(Fe þ Al) ¼ 0.5, Al/(Fe þ Al) ¼ 0.5), HeFeeGaeAl-MFI (molar composition: Si/(Fe þ Ga þ Al) ¼ 19.4, Fe/(Fe þ Ga þ Al) ¼ 0.4, Ga/(Fe þ Ga þ Al) ¼ 0.3, Al/(Fe þ Ga þ Al) ¼ 0.3). Reaction conditions; Temperature: 550 C, Pressure: 0.1 MPa, Charged volume of catalyst: 1.0 mL, Feedstock: n-C6H14/N2 (molar ratio of n-C6H14/N2: 1/15, *1/5), LHSV of n-C6H14: 1.0 h1, Reaction time: 5 h.
Please cite this article in press as: S. Hodoshima, et al., Catalytic conversion of light hydrocarbons to propylene over MFI-zeolite/metal-oxide composites, Microporous and Mesoporous Materials (2016), http://dx.doi.org/10.1016/j.micromeso.2015.12.044
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Table 3 Initial activities for cracking of n-hexane over synthesized MFI-type zeolites. Zeolite species (Si/T ratioa)
HeAl-MFI (25.0) HeFeeAl-MFI (25.0) HeFeeGaeAl-MFI (25.0) HeFeeGaeAl-MFI (25.0)
Si/T ratiob [mol/mol]
20.0 19.4 19.4 19.4
Conv. [wt%]
99.9 74.9 71.8 98.5c
Product yield [wt%] Ethylene (C2 ] )
Propylene (C3 ] )
Butenes (C4 ] )
BTX
23.4 8.7 8.3 13.0
16.3 24.7 27.0 24.3
3.9 7.6 7.2 7.4
14.2 7.3 7.1 14.0
Reaction conditions; Feedstock: n-C6H14 diluted with nitrogen (molar ratio of n-C6/N2: 1/15. a Si/T molar ratio in mother gel (T ¼ Al, Fe + Al, Fe + Ga + Al). b Si/T molar ratio measured by XRF technique (T ¼ Al, Fe þ Al, Fe þ Ga þ Al). c 1/5), Temperature: 550 C, Pressure: 0.1 MPa, Charged volume of catalyst: 1.0 mL, LHSV of n-C6: 1.0 h1, Reaction time: 5 h.
in the presence of nitrogen proceeded stably, whereas n-hexane conversion in the presence of steam decreased with reaction time. At the higher LHSV of n-C6 (2.0 h1) and the lower molar-ratio of steam to n-C6 (5/1), constant conversions were obtained during reaction time, as seen in Fig. 4. In other words, its catalytic stability was improved by decreasing concentration of steam supplied to the catalyst. Table 5 compares catalytic performance of the present composite in the n-hexane cracking using different diluents. Both higher propylene yields than 20 wt% and lower aromatics yields than 2.5 wt% were attained in the cracking of n-hexane diluted with steam. Steam is easily adsorbed on strong acid sites of zeolites, so that aromatics formation was quite suppressed compared to cracking reactions using nitrogen diluent (Tables 4 and 5). However, the catalyst in n-hexane cracking at high molar-ratio of steam to nC6 (10/1) was rapidly deactivated in spite of low aromatics yield (2.4 wt%). This rapid decrease in catalytic conversion was probably due to deactivation derived from dealumination. These results indicate that hydrocarbon cracking by steam dilution over the present catalyst makes it possible to give both high propylene yield and low aromatics yield stably by controlling steam concentration carefully.
Olefinic hydrocarbons (C4eC8) are available as byproducts from various processes (e.g., FCC and GTL) [2]. Gas-To-Liquids (GTL) [20], consisting of synthesis gas production process, FischereTropsch (FT) process and hydrocracking process, are technologies to produce synthetic fuels, e.g., diesel oil, from natural gas. In the GTL technologies, since hydrocarbons such as olefinic naphtha, containing hexenes (C6 ] ) and heptenes (C7 ] ), are produced as byproduct, utilization of the GTL-derived hydrocarbons is strongly desired. For example, C6eC7 olefinic hydrocarbons recovered from the FT process, have been converted into light olefins by the SUPERFLEX process using FCC-type cracking unit [21]. From these viewpoints, catalytic cracking of 1-hexene over the present catalyst was investigated as well as n-hexane cracking. The 1-hexene was adopted in the present study as a model reactant representing olefinic hydrocarbon feedstock. It is considered that 1-hexene reacts on zeolites according to two main routes, as illustrated in Fig. 5. Cracking reactions are expected to proceed via light olefins successively to generate aromatics. In addition to this route, 1-hexene itself is expected to be cyclized to cyclohexene and then dehydrogenated to aromatics. The 1-hexene is strongly adsorbed on acid sites of zeolites because of its olefinity and selectivity to light
Fig. 3. SEM image of HeFeeGaeAl-MFI/Al2O3 composite and distribution of silicon and aluminum atoms in composite sample by SEM-EDX analysis. (A): SEM image, (B), (C): Distribution of Si and Al measured by SEM-EDX analysis. Measured catalyst sample; HeFeeGaeAl-MFI/Al2O3 composite (molar composition of zeolite: Si/(Fe þ Ga þ Al) ¼ 19.4, Fe/ (Fe þ Ga þ Al) ¼ 0.4, Ga/(Fe þ Ga þ Al) ¼ 0.3, Al/(Fe þ Ga þ Al) ¼ 0.3, weight ratio of zeolite/Al2O3: 75 wt%/25 wt%).
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Table 4 Initial activities for n-hexane cracking over HeFeeGaeAl-MFI only and HeFeeGaeAl-MFI/Al2O3 composites. Catalyst (Si/T ratioa)
HeFeeGaeAl-MFI (19.4) only HeFeeGaeAl-MFI (19.4)/Al2O3b HeFeeGaeAl-MFI (19.4)/Al2O3c
Zeolite/Al2O3 ratio [wt%/wt%]
Conv. [wt%]
100/0 75/25 75/25
71.8 81.5 80.9
Product yield [wt%] (C2 ] )
(C3 ] )
BTX
8.3 8.9 10.0
27.0 29.1 28.3
7.1 17.5 10.8
Productivity of light olefins [g-(C2 ] þ C3 ] )/g-zeolite h] 0.53 0.62 0.62
Reaction conditions; Feedstock: n-C6H14 diluted with nitrogen (molar ratio of n-C6/N2: 1/15), Temperature: 550 C, Pressure: 0.1 MPa, Charged volume of catalyst: 1.0 mL, LHSV of n-C6: 1.0 h1, Reaction time: 5 h. a Si/T molar ratio measured by XRF technique (T ¼ Fe þ Ga þ Al). b Catalyst sample prepared by the method (1) described in the Section 3.2. c Catalyst sample prepared by the method (2) described in the Section 3.2.
Fig. 5. Reaction routes guessed in 1-hexene cracking over zeolite catalysts.
Fig. 4. Time courses of n-hexane conversion in catalytic cracking of n-hexane diluted with nitrogen or steam over HeFeeGaeAl-MFI/Al2O3 composite. Catalyst; HeFeeGaeAl-MFI/Al2O3 composite (molar composition of zeolite: Si/ (Fe þ Ga þ Al) ¼ 19.4, Fe/(Fe þ Ga þ Al) ¼ 0.4, Ga/(Fe þ Ga þ Al) ¼ 0.3, Al/ (Fe þ Ga þ Al) ¼ 0.3, weight ratio of zeolite/Al2O3: 75 wt%/25 wt%). Reaction conditions; Temperature: 550 C, Pressure: 0.1 MPa, Charged volume of catalyst: 1.0 mL, Feedstock: n-C6H14/N2 (molar ratio of n-C6H14/N2: 1/10), n-C6H14/H2O (molar ratio of n-C6H14/ H2O: 1/10, 1/5), LHSV of n-C6H14: 1.0 h1 (n-C6H14/N2 ¼ 1/10, n-C6H14/H2O ¼ 1/10), 2.0 h1 (n-C6H14/H2O ¼ 1/5).
olefins or aromatics definitely depends on acidity of zeolites. Fig. 6 presents product distribution in catalytic cracking of 1-hexene diluted with nitrogen or steam over the HeFeeGaeAl-MFI/Al2O3 composite at 550 C. Though conversions reached ca. 100 wt% due to high reactivity of 1-hexene, higher selectivity to propylene than 38 wt% were achieved in both cases. Total selectivity to light olefins (ethylene, propylene and butenes) were higher than 70 wt%, indicating that cracking into light olefins is main reaction. In particular, the zeolite catalyst in the presence of steam improved selectivity to light olefins by suppressing aromatics formation, resulting in excellent catalytic performance (Propylene: 38.3 wt%, BTX: 4.5 wt %), as shown in Table 6. It was also confirmed that catalytic activities were constant during reaction time (5 h) by controlling steam concentration. The high productivity of propylene in catalytic
cracking of olefinic light-hydrocarbon over the present catalyst is superior to thermal cracking or other technologies such as FCC process. Moreover, it is noteworthy that the reaction temperature of 550 C was much lower than those (850e900 C) in the thermal cracking. 3.3. Excellent properties of HeFeeGaeAl-MFI/aluminum-oxide composite under practical conditions For the purpose of applying the present catalyst to lighthydrocarbon cracking under practical conditions, cracking of nhexane without dilution was examined at 550e565 C. Compared to the cases using diluent such as nitrogen or steam, catalytic cracking of light-hydrocarbon without dilution is advantageous from the following viewpoints: (1) Saving thermal energy required for heating diluent [10]; (2) No irreversible catalyst deactivation due to the dealumination [7], as observed in Fig. 4. However, hydrocarbon cracking in the absence of diluent requires catalyst with high resistance to coking. Table 7 shows catalytic activities for nhexane cracking in the presence and absence of diluent over the HeFeeGaeAl-MFI/Al2O3 composites with different Si/ (Fe þ Ga þ Al) ratios (19.4 and 121.3). In contrast to the cracking of n-hexane diluted with nitrogen or steam, the composite catalyst with the Si/(Fe þ Ga þ Al) ratio of 19.4 resulted in rapid deactivation due to aromatics or coke formation in cracking of n-hexane without dilution at 550 C and the LHSV of n-hexane in the range of 1.0e2.0 h1. At the LHSV of 5.0 h1 (i.e. shorter contact time),
Table 5 Initial activities for cracking of n-hexane diluted with nitrogen or steam over HeFeeGaeAl-MFI/Al2O3 composite. Catalyst (Si/T ratioa)
HeFeeGaeAl-MFI (19.4)/Al2O3 HeFeeGaeAl-MFI (19.4)/Al2O3 HeFeeGaeAl-MFI (19.4)/Al2O3
Feedstock (mixed ratio [mol/mol])
LHSV of n-C6 [h1]
Conv. [wt%]
n-C6H14/N2 (1/10) n-C6H14/H2O (1/10) n-C6H14/H2O (1/5)
1.0 1.0 2.0
93.8 84.6 77.4
Product yield [wt%] (C2 ] )
(C3 ] )
BTX
13.2 13.6 10.6
30.1 23.9 21.3
17.6 2.4 1.7
Productivity of light olefins [g-(C2 ] þ C3 ] )/g-zeolite h] 0.70 0.61 1.0
Catalyst; HeFeeGaeAl-MFI/Al2O3 composite, Zeolite/Al2O3 ratio: 75 wt%/25 wt%. Reaction conditions; Temperature: 550 C, Pressure: 0.1 MPa, Charged volume of catalyst: 1.0 mL, Reaction time: 5 h. a Si/T molar ratio measured by XRF technique (T ¼ Fe þ Ga þ Al).
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Fig. 6. Product distribution in n-hexane cracking and 1-hexene cracking over HeFeeGaeAl-MFI/Al2O3 composite catalyst. Catalyst; HeFeeGaeAl-MFI/Al2O3 composite (molar composition of zeolite: Si/(Fe þ Ga þ Al) ¼ 19.4, Fe/(Fe þ Ga þ Al) ¼ 0.4, Ga/(Fe þ Ga þ Al) ¼ 0.3, Al/(Fe þ Ga þ Al) ¼ 0.3, weight ratio of zeolite/Al2O3: 75 wt%/25 wt%). Reaction conditions; Temperature: 550 C, Pressure: 0.1 MPa, Charged volume of catalyst: 1.0 mL, Feedstock: n-C6H14/N2 (molar ratio of n-C6H14/N2: 1/15), 1-C6H12/N2 (molar ratio of 1-C6H12/ N2: 1/15), 1-C6H12/H2O (molar ratio of 1-C6H12/H2O: 1/5), LHSV of n-C6H14: 1.0 h1, LHSV of 1-C6H12: 1.0 h1 (1-C6H12/N2 ¼ 1/15), 2.0 h1 (1-C6H12/H2O ¼ 1/5).
aromatics formation still proceeded selectively. In the cracking at higher temperature (565 C) under the same LHSV (5.0 h1), yield of light olefins was slightly increased, whereas aromatics formation was increasingly accelerated (see Table 7). By applying the zeolite catalyst with higher Si/(Fe þ Ga þ Al) ratio of 121.3 (i.e., smaller amount of acid sites) under the same reaction conditions, selectivity to light olefins was obviously enhanced, giving high propylene yield of ca. 18 wt%. Here the space time yield of light olefins was ca. 1.4 [g-(C2 ] þ C3 ] )/g-zeolite h], being ca. twice of the value (0.62) in nitrogen dilution. In cracking of n-hexane without dilution, aromatization proceeds much easier than cracking of n-hexane diluted with nitrogen or steam due to bimolecular reaction of n-hexane and/or re-adsorption of light olefins on acid sites, indicating that aromatics would be generated quite selectively by using zeolites with high acid densities. In other words, zeolites with low acid densities would give high selectivity to light olefins by suppressing aromatics formation [10]. Catalytic stability of the HeFeeGaeAl-MFI/Al2O3 composites with the Si/(Fe þ Ga þ Al) ratios of 19.4 and 121.3 were evaluated by performing n-hexane cracking without dilution at 565 C for long time. Fig. 7 shows time courses of yields of ethylene and propylene. In the catalytic cracking over the sample with the Si/(Fe þ Ga þ Al) ratio of 121.3, both ethylene yield of ca. 8.0 wt% and propylene yield of ca. 18 wt% obtained at initial stage were maintained for at least 80 h in spite of severe conditions to cause coke formation easily. This long lifetime, being applicable to cracking process in fixed-bed mode, was achieved due to its excellent resistance to coke formation. On the other hand, the sample with the Si/(Fe þ Ga þ Al) ratio
of 19.4 resulted in deactivation within 30 h. These results indicate that it is quite significant to yield aromatics as precursor of carbonaceous deposit as low as possible to realize stable performance for long term. Concerning the improvement of catalytic stability in cracking, the application of nano-sized ZSM-5 has been actively studied [4,5,8]. Konno et al. attained both high propylene yield (ca. 33 C-mol%) and excellent stability (ca. 30 h) in n-hexane cracking at 650 C by utilizing ZSM-5 zeolites (Si/Al ratios: 180e192) with smaller crystal size than 200 nm [5], but gradual deactivation was observed after 30 h. In the previous report by our group [10], the FeeGaeAl-MFI (Si/(Fe þ Ga þ Al) ratio: 12.0) with particle size of 570 nm was confirmed to give both high propylene yield (31.2 wt%) and long lifetime (at least 24 h) in n-hexane cracking at 550 C. The stability of the FeeGaeAl-MFI is thus expected to be further improved by controlling its particle size to smaller than 200 nm.
4. Conclusions The composite catalysts for cracking, consisting of FeeGaeAlMFI zeolites and metal-oxide binder (e.g., aluminum oxide), were developed on the basis of original concept to produce propylene efficiently by cracking of light hydrocarbons. Al-MFI-type zeolite containing iron and gallium atoms at optimized ratio was employed as a main component. The FeeGaeAl-MFI zeolites caused a drastic change of acidity, giving high selectivity to propylene by suppressing aromatics formation. The composite catalysts combined with aluminum-oxide binder (FeeGaeAl-MFI/Al2O3) for practical
Table 6 Comparison of initial activities for cracking of n-hexane and 1-hexene over HeFeeGaeAl-MFI/Al2O3 composite. Catalyst (Si/T ratioa)
HeFeeGaeAl-MFI (19.4)/Al2O3 HeFeeGaeAl-MFI (19.4)/Al2O3 HeFeeGaeAl-MFI (19.4)/Al2O3
Feedstock (mixed ratio [mol/mol])
LHSV of C6 [h1]
Conv. [wt%]
n-C6H14/N2 (1/15) 1-C6H12/N2 (1/15) 1-C6H12/H2O (1/5)
1.0 1.0 2.0
80.9 99.9 99.9
Product yield [wt%] (C2 ] )
(C3 ] )
BTX
10.0 19.4 22.1
28.3 40.6 38.3
10.8 15.8 4.5
Productivity of light olefins [g-(C2 ] þ C3 ] )/g-zeolite h] 0.62 1.0 2.0
Catalyst; HeFeeGaeAl-MFI/Al2O3 composites, Zeolite/Al2O3 ratio: 75 wt%/25 wt%. Reaction conditions; Temperature: 550 C, Pressure: 0.1 MPa, Charged volume of catalyst: 1.0 mL, Reaction time: 5 h. a Si/T molar ratio measured by XRF technique (T ¼ Fe þ Ga þ Al).
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Table 7 Initial activities for cracking of n-hexane over HeFeeGaeAl-MFI/Al2O3 composites in the presence and absence of diluent. Catalyst (Si/T ratioa)
HeFeeGaeAl-MFI HeFeeGaeAl-MFI HeFeeGaeAl-MFI HeFeeGaeAl-MFI HeFeeGaeAl-MFI
(19.4)/Al2O3 (19.4)/Al2O3 (19.4)/Al2O3 (19.4)/Al2O3 (121.3)/Al2O3
Feedstock
n-C6H14/N2 (1/15) n-C6H14/H2O (1/5) n-C6H14 only n-C6H14 only n-C6H14 only
LHSV/Temp. [h1]/[ C] 1.0/550 2.0/550 5.0/550 5.0/565 5.0/565
C C C C C
Conv. [wt%]
80.9 77.4 74.7 81.4 67.3
Product yield [wt%] (C2 ] )
(C3 ] )
BTX
10.0 10.6 4.6 5.4 8.2
28.3 23.9 9.0 10.6 18.1
10.8 1.7 15.5 19.4 7.3
Productivity of light olefins [g-(C2 ] þ C3 ] )/g-zeolite h] 0.62 1.0 0.71 0.84 1.4
Catalyst; HeFeeGaeAl-MFI/Al2O3 composites, Zeolite/Al2O3 ratio: 75 wt%/25 wt%. Reaction conditions; Pressure: 0.1 MPa, Charged volume of catalyst: 1.0 mL (n-C6/N2, n-C6/H2O), 2.0 mL (n-C6 only). Reaction time: 5.0 h (n-C6/N2, n-C6/H2O), 24.0 h (n-C6 only). a Si/T molar ratio measured by XRF technique (T ¼ Fe þ Ga þ Al).
Fig. 7. Time course of cracking of n-hexane without dilution over HeFeeGaeAl-MFI/Al2O3 composite catalysts. Catalyst; (A) HeFeeGaeAl-MFI/Al2O3 composites (molar composition of zeolite: Si/(Fe þ Ga þ Al) ¼ 19.4, Fe/(Fe þ Ga þ Al) ¼ 0.4, Ga/(Fe þ Ga þ Al) ¼ 0.3, Al/(Fe þ Ga þ Al) ¼ 0.3, weight ratio of zeolite/Al2O3: 75 wt%/25 wt%). (B) HeFeeGaeAlMFI/Al2O3 composites (molar composition of zeolite: Si/(Fe þ Ga þ Al) ¼ 121.3, Fe/(Fe þ Ga þ Al) ¼ 0.4, Ga/(Fe þ Ga þ Al) ¼ 0.3, Al/(Fe þ Ga þ Al) ¼ 0.3, weight ratio of zeolite/Al2O3: 75 wt%/25 wt%). Reaction conditions; Temperature: 565 C, Pressure: 0.1 MPa, Charged volume of catalyst: 2.0 mL, Feedstock: n-C6H14 only, LHSV of n-C6H14: 5.0 h1.
use were also confirmed to exhibit high activities keeping excellent performance of zeolite species. By applying the zeolite-based catalysts to cracking reactions in fixed-bed mode, selective generation of propylene was efficiently attained at mild temperatures (550e565 C). The present catalysts exhibited both high propylene yields (20e30 wt%) in cracking of nhexane diluted with nitrogen or steam at 550 C. In the steam cracking of n-hexane, its performance was kept stably at suitable steam ratio. In the 1-hexene cracking, the present catalyst gave higher propylene yields than 38 wt%, being superior in propylene productivity to conventional technologies such as thermal cracking. Furthermore, in the catalytic cracking of n-hexane without dilution at 565 C, high propylene yields of ca. 18 wt% were maintained for ca. 80 h in spite of quite severe conditions. This long lifetime was achieved due to its high resistance to coke formation. The supply of hydrocarbon feedstock without dilution was thus confirmed to be advantageous in terms of preventing dealumination-derived deactivation and saving energy. It was suggested on the experimental basis that catalytic cracking of paraffinic and olefinic hydrocarbons in fixed-bed mode is feasible by using the present catalysts as an alternative technology for producing propylene efficiently. Acknowledgments A part of the present study was financially supported by the Japan Oil, Gas and Metals National Corporation (JOGMEC). We would like to express our deep gratitude here. The authors acknowledge Prof. Sachio Asaoka (The University of Kitakyushu) for guidance regarding zeolite synthesis.
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Catalytic conversion of light hydrocarbons to propylene over MFI-zeolite/metal-oxide composites* Shinya Hodoshima*, Azusa Motomiya, Shuhei Wakamatsu, Ryuichi Kanai, Fuyuki Yagi Research & Development Center, Chiyoda Corporation, 3-13 Moriya-cho, Kanagawa-ku, Yokohama, Kanagawa 221-0022, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 September 2015 Received in revised form 23 December 2015 Accepted 24 December 2015 Available online xxx
Zeolite-based composite catalysts for producing propylene efficiently by cracking of light hydrocarbons were developed in the present study to meet future demand for propylene and utilize various light hydrocarbons (e.g., light-naphtha, condensates recovered from natural gas and olefinic hydrocarbons obtained from GTL-process). The composite catalysts, consisting of Al-MFI zeolites containing iron and gallium atoms (FeeGaeAl-MFI) at optimized ratio and aluminum-oxide binder (Al2O3), were employed for cracking of n-hexane or 1-hexene using fixed-bed-type reactor at around 550 C. Since the FeeGaeAlMFI zeolites caused acidity suitable for generating light olefins selectively, high propylene yields (18 e41 wt%) were obtained by suppressing formation of aromatics at mild temperatures. The present catalyst exhibited high propylene productivity and catalytic stability simultaneously in the steamcracking of hydrocarbons at suitable steam ratio. Moreover, in the cracking of n-hexane without dilution over the present catalyst, high propylene yields of ca. 18 wt% were maintained for at least 80 h due to its high resistance to coke formation in spite of severe conditions, suggesting that simplified cracking process for on-purpose propylene production from light hydrocarbons is feasible in fixed-bed mode by using the present composite catalysts. © 2016 Elsevier Inc. All rights reserved.
Keywords: Catalytic cracking Propylene production Light hydrocarbons FeeGaeAl-MFI-zeolite Zeolite/metal-oxide composite
1. Introduction Propylene as a raw material in the petrochemical industry has been increasingly significant because the global demand for propylene has increased at growth rate of 4e5 wt% annually [1]. Since the majority of propylene (ca. 60%) is currently produced as a byproduct by thermal steam-cracking of hydrocarbon feedstock such as naphtha, the thermal crackers can no longer meet the propylene demand. In addition, this technology requires a huge amount of thermal energy (temperature: 850e900 C) as well as its low weight ratio of propylene to ethylene (ca. 0.5). The recent shift to ethane-based feedstock for producing ethylene also might affect the propylene supply, because this technology produces little propylene. Any alternative method for producing propylene efficiently from naphtha, being widely available feedstock, should be thus developed to meet future demand for propylene and reduce energy-consumption in olefin production. Moreover, conversion of various light hydrocarbons (e.g., shale oil, condensates recovered
*
This paper was presented at ZMPC2015. * Corresponding author. Tel.: þ81 45 441 9133; fax: þ81 45 441 9728. E-mail address: [email protected] (S. Hodoshima).
from shale gas and olefinic hydrocarbons obtained from Gas-ToLiquids (GTL) process [2]) into valuable light olefins is also significant task in terms of efficient use of resources. Catalytic cracking of hydrocarbons such as naphtha over solidacid catalysts, e.g., ZSM-5, have been actively investigated as an alternative method for on-purpose propylene production to conventional thermal steam-cracking [3e9]. In particular, catalytic cracking using fixed-bed reactor has the following advantages compared to steam-cracking: (1) high selectivity to propylene (weight ratio of propylene to ethylene > 2.0); (2) being energysaving due to low temperatures (< 650 C). Furthermore, the cracking process in fixed-bed mode is superior to fluidized catalytic cracking (FCC) processes in terms of operational simplicity in reaction unit. However, catalytic cracking process using fixed-bed reactor has not been commercialized, because practical cracking catalyst with excellent stability, being applicable to fixed-bed reactor, has been still undeveloped. From these technological backgrounds, the development of highly active and stable catalyst for the fixed-bed-type cracking of light hydrocarbons to produce propylene efficiently under mild conditions has been focused in the present study. Firstly, the Al-MFI zeolites containing iron and gallium (FeeGaeAl-MFI) were adopted
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as main component of cracking catalyst [10]. It has been recognized that Fe and Ga species as heteroatom could weaken acid strength of MFI zeolites [11] and accelerate dehydrogenation activities for alkanes [12e14], respectively. On the basis of these findings, it was found by our group that the FeeGaeAl-MFI zeolites synthesized at adequate mixed-ratio exhibited both functions derived from Fe and Ga species [10], so that highly selective propylene-generation was attained by suppressing formation of aromatics. Secondly, the composite catalysts for industrial use, consisting of the FeeGaeAlMFI zeolites and metal-oxide binder (e.g., aluminum oxide) [10], were prepared to utilize the proprietary zeolites under practical conditions. This paper presents excellent properties of these catalysts on the experimental basis. 2. Experimental 2.1. Synthesis and characterization of MFI-type zeolites Three kinds of MFI-type zeolites containing some heteroatom such as aluminum, iron and gallium (Al-MFI, FeeAl-MFI and FeeGaeAl-MFI) were hydrothermally synthesized by conventional method. Desired aqueous solution containing Si and Al source, Si, Al and Fe source and Si, Al, Fe and Ga source were prepared as mother gel with the following molar compositions: (1) Si/Al ¼ 25.0; (2) Si/ (Fe þ Al) ¼ 25.0, Fe/(Fe þ Al) ¼ 0.4, Al/(Fe þ Al) ¼ 0.6; (3) Si/ (Fe þ Ga þ Al) ¼ 25.0, Fe/(Fe þ Ga þ Al) ¼ 0.4, Ga/ (Fe þ Ga þ Al) ¼ 0.2, Al/(Fe þ Ga þ Al) ¼ 0.4. Each mother gel was charged into stainless-type autoclave and then heated at 150 C for 24 h under adequate stirring conditions. The obtained sodium-type zeolites were converted into proton-type ones (HeAl-MFI, HeFeeAl-MFI and HeFeeGaeAl-MFI) by ion-exchange with NH4NO3 aqueous solution, dried at 120 C for 3 h and finally calcined at 550 C for 3 h under air stream. These protonated samples (HeAl-MFI, HeFeeAl-MFI and HeFeeGaeAl-MFI (Si/T molar ratio in mother gel: 25.0, T ¼ Al, Fe þ Al, Fe þ Ga þ Al)) were characterized as follows. Both X-ray diffraction (XRD, Rigaku Co. Ltd.) and X-ray fluorescence (XRF, Rigaku Co. Ltd.) measurements were carried out to determine their structure and chemical compositions, respectively. The surface area was estimated by the Langmuir method on the basis of nitrogen adsorption isotherms. The acidity of these samples was investigated by the temperature-programed desorption technique using ammonium probe (NH3-TPD, BEL Japan Co. Ltd.). The NH3-TPD measurement was performed in accordance with the following procedure. Firstly, the sample was pretreated by calcination at 500 C for 1.5 h under He stream (flow rate: 50 mL/min) and then cooled to room temperature. Secondly, NH3 molecules were adsorbed on surface of the sample at 100 C for 0.5 h by switching from He to mixed gas of 5.0% NH3 and He (balance gas) at flow rate of 20 mL/min and then He was introduced at flow rate of 20 mL/min for 40 min to purge remaining ammonium. After the above pretreatment and NH3 adsorption, the NH3-TPD profile was measured by increasing temperature from 100 C to 880 C at heating rate of 10 C/min under He stream including 0.1% Ar as an internal standard (flow rate: 50 mL/min). With respect to desorption peak of NH3 molecules observed at around 350 C in the obtained NH3-TPD profile, both acid amount and acid strength of the zeolite sample were estimated by calculating its peak area and activation energy required for desorption of NH3 molecules [15], respectively. In addition to the above zeolites, sodium-type FeeGaeAl-MFI zeolite with smaller amount of Al, Ga and Fe atoms (molar composition in mother gel: Si/(Fe þ Ga þ Al) ¼ 200, Fe/ (Fe þ Ga þ Al) ¼ 0.4, Ga/(Fe þ Ga þ Al) ¼ 0.2, Al/ (Fe þ Ga þ Al) ¼ 0.4) was also synthesized by the same procedure and characterized by the XRD and XRF measurements.
2.2. Preparation of FeeGaeAl-MFI zeolite/aluminum-oxide composite catalysts Composite catalysts, consisting of the FeeGaeAl-MFI zeolites and aluminum oxide (g-Al2O3) binder, were prepared as follows. Firstly, the sodium-type FeeGaeAl-MFI zeolites with different compositions (molar ratios in mother gel: Si/(Fe þ Ga þ Al) ¼ 25, Fe/(Fe þ Ga þ Al) ¼ 0.4, Ga/(Fe þ Ga þ Al) ¼ 0.2, Al/ (Fe þ Ga þ Al) ¼ 0.4; Si/(Fe þ Ga þ Al) ¼ 200, Fe/ (Fe þ Ga þ Al) ¼ 0.4, Ga/(Fe þ Ga þ Al) ¼ 0.2, Al/ (Fe þ Ga þ Al) ¼ 0.4) were hydrothermally synthesized by the same method described in the Section 2.1. Secondly, after both the zeolite and alumina sol with an average particle size of 5.4 nm (Cataloid AP-1, JGC Catalysts and Chemicals Ltd. [16]) were mixed at weight ratio of 65%-Zeolite/35%-Al2O3, powdery mixture was molded into cylindrical shape with diameter of 1.0 mm4 by extrusion. After these cylindrical samples were converted into protonated ones with NH4NO3 aqueous solution, they were dried at 120 C for 3 h and then calcined at 550 C for 3 h under air stream. In addition, the protonated Fe-Ga-Al-MFI zeolite (molar composition in mother gel: Si/(Fe þ Ga þ Al) ¼ 25, Al/(Fe þ Ga þ Al) ¼ 0.4, Fe/ (Fe þ Ga þ Al) ¼ 0.4, Ga/(Fe þ Ga þ Al) ¼ 0.2) prepared by the above-described method was also formed into cylindrical composite (1.0 mm4) by using the Al2O3 binder. Weight ratios of resultant composites were measured by the XRF technique. Regarding the cylindrical sample with weight ratio of 65%FeeGaeAl-MFI (molar ratios in mother gel: Si/(Fe þ Ga þ Al) ¼ 25, Fe/(Fe þ Ga þ Al) ¼ 0.4, Ga/(Fe þ Ga þ Al) ¼ 0.2, Al/ (Fe þ Ga þ Al) ¼ 0.4)/35%-Al2O3, both observation of its morphology and elemental analysis of silicon and aluminum atoms were carried out by means of scanning electron microscopy equipped with an energy-dispersive fluorescence X-ray spectrometer (SEM-EDX, Hitachi Ltd.). 2.3. Catalytic cracking of liquid hydrocarbons over zeolites and zeolite-based composites Catalytic cracking of n-hexane (n-C6H14) diluted with nitrogen over the synthesized zeolites (HeAl-MFI, HeFeeAl-MFI and HeFeeGaeAl-MFI, Si/T ratio in mother gel: 25.0, T ¼ Al, Fe þ Al, Fe þ Ga þ Al) were performed for 5.0 h under atmospheric pressure by use of a flow-type reactor in fixed-bed mode. The protonated zeolites in powdery state were formed into pellet state, crushed and finally sorted into length of 1.0e2.0 mm. These sieved zeolites of 1.0 mL were charged into the central part of tubular reactor. The normal hexane was employed for reaction test as a model reactant representing paraffinic naphtha feedstock. The n-hexane diluted with nitrogen (molar ratio of N2 to n-C6H14: 15/1, 10/1 and 5/1) was supplied to the catalyst-layer at 550 C with the Liquid Hourly Space Velocity (LHSV) of n-hexane at 1.0 h1. After separating products into gaseous and liquid components through vessel equipped with cooling unit, generation rates of both gaseous and liquid products were measured to check mass balance. Both conversion of n-hexane and yield of each product were calculated based on both generation rates of products and gaschromatographic analysis. Catalytic cracking of n-hexane diluted with nitrogen or steam over the HeFeeGaeAl-MFI/Al2O3 composites (Si/(Fe þ Ga þ Al) ratio in mother gel: 25.0, Mixed weight ratio in preparation step: 65%-Zeolite/35%-Al2O3) were performed for 5.0 h in the fixed-bedtype reactor. After the cylindrical composites were sorted into length of 1.0e2.0 mm, these sieved samples of 1.0 mL were charged into the reactor. Cracking reactions were carried out under the following conditions: Temperature; 550 C, Pressure; 0.1 MPa, Feedstock; n-hexane diluted with nitrogen or steam (molar ratio of
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N2 to n-C6H14: 15/1, molar ratio of H2O to n-C6H14: 10/1 and 5/1), LHSV of n-hexane; 1.0 h1 (n-C6H14/N2 feedstock), 2.0 h1 (n-C6H14/ H2O feedstock). Catalytic cracking of 1-hexene (1-C6H12) diluted with nitrogen or steam over the HeFeeGaeAl-MFI/Al2O3 composite (Si/(Fe þ Ga þ Al) ratio in mother gel: 25.0, Mixed weight ratio in preparation step: 65%-Zeolite/35%-Al2O3) were also performed for 5.0 h in the fixed-bed-type reactor. The 1-hexene was employed in the present study as a model reactant representing olefinic naphtha feedstock. Cracking reactions were carried out under the following conditions: Temperature; 550 C, Pressure; 0.1 MPa, Charged amount of catalyst: 1.0 mL, Feedstock; 1-hexene diluted with nitrogen or steam (molar ratio of N2 to 1-C6H12: 15/ 1, molar ratio of H2O to 1-C6H12: 5/1), LHSV of 1-hexene; 1.0 h1 (1C6H12/N2 feedstock), 2.0 h1 (1-C6H12/H2O feedstock). Both conversions of feedstock and product yields in cracking reactions over the composite catalysts were evaluated in the same manner as cracking over the MFI-zeolite alone. Catalytic cracking of n-hexane without dilution over HeFeeGaeAl-MFI/Al2O3 composites (Si/(Fe þ Ga þ Al) ratio in mother gel: 25.0 and 200, Mixed weight ratio in preparation step: 65%-Zeolite/35%-Al2O3) were performed for 24 h in the fixed-bedtype reactor under the following conditions: Temperature; 550 C and 565 C, Pressure; 0.1 MPa, Charged amount of catalyst: 2.0 mL, Feedstock; n-hexane without dilution, LHSV of n-hexane; 5.0 h1. Both n-hexane conversion and product yields were evaluated in the same manner as cracking over the MFI-zeolite alone. In addition, reaction tests for longer than 30 h were carried out at the temperature of 565 C to examine lifetime of the composite catalysts (vide infra). 3. Results and discussion 3.1. FeeGaeAl-MFI zeolite as main component for catalytic lighthydrocarbon cracking Zeolites synthesized in the present study and their properties are listed in Table 1. All of resultant zeolites were confirmed by the XRD measurement to show the diffraction patterns corresponding to MFI-type framework. Both molar ratios of silicon atom to heteroatom (Si/Al, Si/(Fe þ Al) and Si/(Fe þ Ga þ Al)) and surface area were almost the same among them. On the basis of the XRF measurement, molar ratios of each heteroatom to total heteroatoms in the HeFeeAl-MFI and the HeFeeGaeAl-MFI zeolites were calculated as follows: HeFeeAl-MFI; Fe/(Fe þ Al) ¼ 0.5, Al/ (Fe þ Al) ¼ 0.5, HeFeeGaeAl-MFI; Fe/(Fe þ Ga þ Al) ¼ 0.4, Ga/ (Fe þ Ga þ Al) ¼ 0.3, Al/(Fe þ Ga þ Al) ¼ 0.3. Fig. 1 shows NH3-TPD profiles of these samples. As seen in Fig. 1, two desorption peaks of NH3 molecules were observed at around 180 C and 350 C, respectively. The peak at around 180 C is the desorption peak derived from NH3 molecules physically adsorbed on zeolites, whereas the peak at around 350 C is definitely associated with NH3 molecules chemically adsorbed on acid sites. Table 2 presents acidity of zeolites estimated by analyzing the high-temperature
3
Fig. 1. NH3-TPD profiles of synthesized MFI-type zeolites.
desorption peak. Both acid amount and acid strength were calculated as a peak area and an activation energy required for desorption of NH3 molecules according to the method proposed by Niwa and Katada et al. [15], respectively. The acid amount was nearly the same among them, whereas the desorption peaks of the HeFeeAlMFI and HeFeeGaeAl-MFI zeolites, closely related to the acid strength, were shifted to the lower temperatures compared to that of the HeAl-MFI zeolite, as shown in the NH3-TPD profiles (Fig. 1). As a consequence, the activation energy required for NH3 desorption (i.e. acid strength) of the HeFeeAl-MFI and HeFeeGaeAl-MFI were slightly smaller than that of the HeAl-MFI zeolite (Table 2). Catalytic cracking of n-hexane diluted with nitrogen (molar ratio of N2 to n-C6H14: 15/1) over the HeAl-MFI, the HeFeeAl-MFI and the HeFeeGaeAl-MFI zeolites were performed at 550 C to compare their catalytic performance under the same conditions. Fig. 2 and Table 3 show product selectivity and catalytic activities, respectively. The product distribution was obviously changed by introducing iron or iron & gallium into the HeAl-MFI zeolite. Though the HeAl-MFI zeolite as a reference exhibited comparatively high selectivity to light olefins, it also gave high selectivity to aromatics such as BTX (benzene, toluene and xylenes). On the other hand, the HeFeeAlMFI and HeFeeGaeAl-MFI remarkably enhanced selectivity to light olefins by suppressing aromatics formation (Fig. 2), so that high propylene yields of 24.7e27.0 wt% were obtained in spite of low temperature of 550 C for cracking and these yields were kept constant during reaction time (5.0 h). In addition, the product selectivity by this HeFeeGaeAl-MFI zeolite was compared to the HeAl-MFI zeolite at the same level of n-hexane conversion. The nhexane cracking over the HeFeeGaeAl-MFI zeolite was conducted at the mixed ratio of N2 to n-C6H14 of 5/1, giving n-hexane conversion of 98.5 wt%. Though selectivity to aromatics increased with the conversion, the HeFeeGaeAl-MFI zeolite still kept higher selectivity to propylene than the HeAl-MFI zeolite (Fig. 2), resulting in high propylene yield of 24.3 wt%. It is well-known that catalytic cracking of paraffinic hydrocarbons over zeolites proceed via light olefins successively to generate aromatics and selectivity to light olefins
Table 1 Compositions and surface area of synthesized MFI-type zeolites. Zeolite species (Si/T ratioa)
Si/T ratiob [mol/mol]
Ratio of each heteroatom [mol/mol] Al/(Fe þ Ga þ Al)
Ga/(Fe þ Ga þ Al)
Fe/(Fe þ Ga þ Al)
HeAl-MFI (25.0) HeFeeAl-MFI (25.0) HeFeeGaeAl-MFI (25.0)
20.0 19.4 19.4
1.0 0.5 0.3
0.0 0.0 0.3
0.0 0.5 0.4
a b c
Surface areac [m2/g]
573 570 587
Si/T molar ratio in mother gel (T ¼ Al, Fe þ Al, Fe þ Ga þ Al). Si/T molar ratio measured by XRF technique (T ¼ Al, Fe þ Al, Fe þ Ga þ Al). Surface area measured by Langmuir method.
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Table 2 Acidity of synthesized MFI-type zeolites. Zeolite species (Si/T ratioa)
Si/T ratiob [mol/mol]
Acid amount [mmol/g]
Acid strength [kJ/mol]
HeAl-MFI (25.0) HeFeeAl-MFI (25.0) HeFeeGaeAl-MFI (25.0)
20.0 19.4 19.4
0.57 0.51 0.52
143.0 138.6 137.1
Conditions of NH3-TPD measurement; Pretreatment: He flow (50 mL/min, 500 C, 1.5 h). NH3 adsorption: He/NH3 (5%) flow (20 mL/min, 100 C, 0.5 h). TPD measurement: He flow (50 mL/min-STP (Ar 0.1%), 100e800 C, 10 C/min). a Si/T molar ratio in mother gel (T ¼ Al, Fe þ Al, Fe þ Ga þ Al). b Si/T molar ratio measured by XRF technique (T ¼ Al, Fe þ Al, Fe þ Ga þ Al).
strongly depends on acidity of zeolites [17]. Iron and gallium atoms in zeolites have been recognized to reduce acid strength of zeolites [11] and accelerate dehydrogenation of alkanes [12e14], respectively. The results in both NH3-TPD measurements and cracking reactions also suggest that the introduction of Fe atoms caused a drastic change of acidity and the combination of Fe and Ga atoms at optimized ratio made it possible to generate propylene further selectively. The HeFeeGaeAl-MFI zeolite with molar composition (Si/(Fe þ Ga þ Al) ¼ 19.4, Fe/(Fe þ Ga þ Al) ¼ 0.4, Ga/ (Fe þ Ga þ Al) ¼ 0.3, Al/(Fe þ Ga þ Al) ¼ 0.3) thus gave the best performance for n-hexane cracking. Catalytic cracking of n-paraffin over zeolite catalysts have been actively investigated to establish efficient methods for on-purpose production of propylene [3e9]. In particular, the MSE-type zeolite (MCM-68) is recognized to have an excellent potential as FCC additives because of its high propylene productivity [9]. Kubota et al. reported that high propylene yield of ca. 40 C-mol% was obtained in n-hexane cracking at 650 C by the La-modified MCM-68 zeolite [9]. Since the propylene yield was ca. 28 C-mol% at 550 C, the FeeGaeAl-MFI zeolite was comparable to the MCM-68. 3.2. HeFeeGaeAl-MFI/aluminum-oxide composite for catalytic light-hydrocarbon cracking Zeolite catalysts are generally formed into shapes suitable for industrial utilization by using metal-oxide binder (e.g., aluminum
oxide) to improve mechanical strength and reduce pressure drop during reaction. In the present study, cylindrical composites with diameter of 1.0 mm4, consisting of the FeeGaeAl-MFI zeolite (Si/ (Fe þ Ga þ Al) ¼ 19.4) and aluminum oxide [18,19], were prepared by the following procedures: (1) After sodium-type zeolite was first converted into proton-type one, this protonated zeolite in powdery state was molded into cylindrical composite using Al2O3; (2) After sodium-type zeolite was first molded into cylindrical composite using Al2O3, this sodium-type zeolite combined with Al2O3 was protonated by ion-exchange. Fig. 3(A) shows the SEM image of the cylindrical composite and Fig. 3(B) and (C) provides the distribution of Si and Al atoms in this composite measured by the SEM-EDX analysis. As obvious from these results, both Si and Al atoms uniformly existed in the present sample. Table 4 compares catalytic activities for n-hexane cracking with respect to the HeFeeGaeAl-MFI alone and the HeFeeGaeAl-MFI/ Al2O3 composites (Weight ratio in obtained sample: 75 wt%/25 wt %). Both composite catalysts exhibited higher n-hexane conversions and apparent yields of light olefins than those in the case of the zeolite alone. Weight of light olefins generated per 1 h with zeolite of 1 g, i.e. space time yield of light olefins, was also enhanced by use of the composite catalyst, suggesting that zeolite species combined with aluminum-oxide binder were well dispersed. However, the composite catalyst prepared by the above method (2) was found to give excellent performance because of its lower aromatics yield. It is preferable in catalytic cracking using fixed-bed reactor to suppress aromatics formation as low as possible to prevent rapid catalyst deactivation due to coking, as described in the Section 3.3. Furthermore, this method is obviously favorable in terms of handling in catalyst preparation. It was thus confirmed on the experimental basis that the composite prepared by the method (2) in the present study could be practical cracking catalyst. Catalytic cracking of n-hexane diluted with steam over the HeFeeGaeAl-MFI/Al2O3 composite (Si/(Fe þ Ga þ Al) ¼ 19.4, Zeolite/Al2O3 ¼ 75 wt%/25 wt%) was attempted at 550 C in fixedbed operation. Fig. 4 shows time courses of n-hexane conversion in the cracking reactions. Under the same conditions (molar ratio of diluent to n-C6H14: 10/1, LHSV of n-C6: 1.0 h1), n-hexane cracking
Fig. 2. Product distribution in catalytic cracking of n-hexane diluted with nitrogen over synthesized MFI-type zeolites. Catalyst; HeAl-MFI (molar composition: Si/Al ¼ 20.0), HeFeeAl-MFI (molar composition: Si/(Fe þ Al) ¼ 19.4, Fe/(Fe þ Al) ¼ 0.5, Al/(Fe þ Al) ¼ 0.5), HeFeeGaeAl-MFI (molar composition: Si/(Fe þ Ga þ Al) ¼ 19.4, Fe/(Fe þ Ga þ Al) ¼ 0.4, Ga/(Fe þ Ga þ Al) ¼ 0.3, Al/(Fe þ Ga þ Al) ¼ 0.3). Reaction conditions; Temperature: 550 C, Pressure: 0.1 MPa, Charged volume of catalyst: 1.0 mL, Feedstock: n-C6H14/N2 (molar ratio of n-C6H14/N2: 1/15, *1/5), LHSV of n-C6H14: 1.0 h1, Reaction time: 5 h.
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S. Hodoshima et al. / Microporous and Mesoporous Materials xxx (2016) 1e8
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Table 3 Initial activities for cracking of n-hexane over synthesized MFI-type zeolites. Zeolite species (Si/T ratioa)
HeAl-MFI (25.0) HeFeeAl-MFI (25.0) HeFeeGaeAl-MFI (25.0) HeFeeGaeAl-MFI (25.0)
Si/T ratiob [mol/mol]
20.0 19.4 19.4 19.4
Conv. [wt%]
99.9 74.9 71.8 98.5c
Product yield [wt%] Ethylene (C2 ] )
Propylene (C3 ] )
Butenes (C4 ] )
BTX
23.4 8.7 8.3 13.0
16.3 24.7 27.0 24.3
3.9 7.6 7.2 7.4
14.2 7.3 7.1 14.0
Reaction conditions; Feedstock: n-C6H14 diluted with nitrogen (molar ratio of n-C6/N2: 1/15. a Si/T molar ratio in mother gel (T ¼ Al, Fe + Al, Fe + Ga + Al). b Si/T molar ratio measured by XRF technique (T ¼ Al, Fe þ Al, Fe þ Ga þ Al). c 1/5), Temperature: 550 C, Pressure: 0.1 MPa, Charged volume of catalyst: 1.0 mL, LHSV of n-C6: 1.0 h1, Reaction time: 5 h.
in the presence of nitrogen proceeded stably, whereas n-hexane conversion in the presence of steam decreased with reaction time. At the higher LHSV of n-C6 (2.0 h1) and the lower molar-ratio of steam to n-C6 (5/1), constant conversions were obtained during reaction time, as seen in Fig. 4. In other words, its catalytic stability was improved by decreasing concentration of steam supplied to the catalyst. Table 5 compares catalytic performance of the present composite in the n-hexane cracking using different diluents. Both higher propylene yields than 20 wt% and lower aromatics yields than 2.5 wt% were attained in the cracking of n-hexane diluted with steam. Steam is easily adsorbed on strong acid sites of zeolites, so that aromatics formation was quite suppressed compared to cracking reactions using nitrogen diluent (Tables 4 and 5). However, the catalyst in n-hexane cracking at high molar-ratio of steam to nC6 (10/1) was rapidly deactivated in spite of low aromatics yield (2.4 wt%). This rapid decrease in catalytic conversion was probably due to deactivation derived from dealumination. These results indicate that hydrocarbon cracking by steam dilution over the present catalyst makes it possible to give both high propylene yield and low aromatics yield stably by controlling steam concentration carefully.
Olefinic hydrocarbons (C4eC8) are available as byproducts from various processes (e.g., FCC and GTL) [2]. Gas-To-Liquids (GTL) [20], consisting of synthesis gas production process, FischereTropsch (FT) process and hydrocracking process, are technologies to produce synthetic fuels, e.g., diesel oil, from natural gas. In the GTL technologies, since hydrocarbons such as olefinic naphtha, containing hexenes (C6 ] ) and heptenes (C7 ] ), are produced as byproduct, utilization of the GTL-derived hydrocarbons is strongly desired. For example, C6eC7 olefinic hydrocarbons recovered from the FT process, have been converted into light olefins by the SUPERFLEX process using FCC-type cracking unit [21]. From these viewpoints, catalytic cracking of 1-hexene over the present catalyst was investigated as well as n-hexane cracking. The 1-hexene was adopted in the present study as a model reactant representing olefinic hydrocarbon feedstock. It is considered that 1-hexene reacts on zeolites according to two main routes, as illustrated in Fig. 5. Cracking reactions are expected to proceed via light olefins successively to generate aromatics. In addition to this route, 1-hexene itself is expected to be cyclized to cyclohexene and then dehydrogenated to aromatics. The 1-hexene is strongly adsorbed on acid sites of zeolites because of its olefinity and selectivity to light
Fig. 3. SEM image of HeFeeGaeAl-MFI/Al2O3 composite and distribution of silicon and aluminum atoms in composite sample by SEM-EDX analysis. (A): SEM image, (B), (C): Distribution of Si and Al measured by SEM-EDX analysis. Measured catalyst sample; HeFeeGaeAl-MFI/Al2O3 composite (molar composition of zeolite: Si/(Fe þ Ga þ Al) ¼ 19.4, Fe/ (Fe þ Ga þ Al) ¼ 0.4, Ga/(Fe þ Ga þ Al) ¼ 0.3, Al/(Fe þ Ga þ Al) ¼ 0.3, weight ratio of zeolite/Al2O3: 75 wt%/25 wt%).
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Table 4 Initial activities for n-hexane cracking over HeFeeGaeAl-MFI only and HeFeeGaeAl-MFI/Al2O3 composites. Catalyst (Si/T ratioa)
HeFeeGaeAl-MFI (19.4) only HeFeeGaeAl-MFI (19.4)/Al2O3b HeFeeGaeAl-MFI (19.4)/Al2O3c
Zeolite/Al2O3 ratio [wt%/wt%]
Conv. [wt%]
100/0 75/25 75/25
71.8 81.5 80.9
Product yield [wt%] (C2 ] )
(C3 ] )
BTX
8.3 8.9 10.0
27.0 29.1 28.3
7.1 17.5 10.8
Productivity of light olefins [g-(C2 ] þ C3 ] )/g-zeolite h] 0.53 0.62 0.62
Reaction conditions; Feedstock: n-C6H14 diluted with nitrogen (molar ratio of n-C6/N2: 1/15), Temperature: 550 C, Pressure: 0.1 MPa, Charged volume of catalyst: 1.0 mL, LHSV of n-C6: 1.0 h1, Reaction time: 5 h. a Si/T molar ratio measured by XRF technique (T ¼ Fe þ Ga þ Al). b Catalyst sample prepared by the method (1) described in the Section 3.2. c Catalyst sample prepared by the method (2) described in the Section 3.2.
Fig. 5. Reaction routes guessed in 1-hexene cracking over zeolite catalysts.
Fig. 4. Time courses of n-hexane conversion in catalytic cracking of n-hexane diluted with nitrogen or steam over HeFeeGaeAl-MFI/Al2O3 composite. Catalyst; HeFeeGaeAl-MFI/Al2O3 composite (molar composition of zeolite: Si/ (Fe þ Ga þ Al) ¼ 19.4, Fe/(Fe þ Ga þ Al) ¼ 0.4, Ga/(Fe þ Ga þ Al) ¼ 0.3, Al/ (Fe þ Ga þ Al) ¼ 0.3, weight ratio of zeolite/Al2O3: 75 wt%/25 wt%). Reaction conditions; Temperature: 550 C, Pressure: 0.1 MPa, Charged volume of catalyst: 1.0 mL, Feedstock: n-C6H14/N2 (molar ratio of n-C6H14/N2: 1/10), n-C6H14/H2O (molar ratio of n-C6H14/ H2O: 1/10, 1/5), LHSV of n-C6H14: 1.0 h1 (n-C6H14/N2 ¼ 1/10, n-C6H14/H2O ¼ 1/10), 2.0 h1 (n-C6H14/H2O ¼ 1/5).
olefins or aromatics definitely depends on acidity of zeolites. Fig. 6 presents product distribution in catalytic cracking of 1-hexene diluted with nitrogen or steam over the HeFeeGaeAl-MFI/Al2O3 composite at 550 C. Though conversions reached ca. 100 wt% due to high reactivity of 1-hexene, higher selectivity to propylene than 38 wt% were achieved in both cases. Total selectivity to light olefins (ethylene, propylene and butenes) were higher than 70 wt%, indicating that cracking into light olefins is main reaction. In particular, the zeolite catalyst in the presence of steam improved selectivity to light olefins by suppressing aromatics formation, resulting in excellent catalytic performance (Propylene: 38.3 wt%, BTX: 4.5 wt %), as shown in Table 6. It was also confirmed that catalytic activities were constant during reaction time (5 h) by controlling steam concentration. The high productivity of propylene in catalytic
cracking of olefinic light-hydrocarbon over the present catalyst is superior to thermal cracking or other technologies such as FCC process. Moreover, it is noteworthy that the reaction temperature of 550 C was much lower than those (850e900 C) in the thermal cracking. 3.3. Excellent properties of HeFeeGaeAl-MFI/aluminum-oxide composite under practical conditions For the purpose of applying the present catalyst to lighthydrocarbon cracking under practical conditions, cracking of nhexane without dilution was examined at 550e565 C. Compared to the cases using diluent such as nitrogen or steam, catalytic cracking of light-hydrocarbon without dilution is advantageous from the following viewpoints: (1) Saving thermal energy required for heating diluent [10]; (2) No irreversible catalyst deactivation due to the dealumination [7], as observed in Fig. 4. However, hydrocarbon cracking in the absence of diluent requires catalyst with high resistance to coking. Table 7 shows catalytic activities for nhexane cracking in the presence and absence of diluent over the HeFeeGaeAl-MFI/Al2O3 composites with different Si/ (Fe þ Ga þ Al) ratios (19.4 and 121.3). In contrast to the cracking of n-hexane diluted with nitrogen or steam, the composite catalyst with the Si/(Fe þ Ga þ Al) ratio of 19.4 resulted in rapid deactivation due to aromatics or coke formation in cracking of n-hexane without dilution at 550 C and the LHSV of n-hexane in the range of 1.0e2.0 h1. At the LHSV of 5.0 h1 (i.e. shorter contact time),
Table 5 Initial activities for cracking of n-hexane diluted with nitrogen or steam over HeFeeGaeAl-MFI/Al2O3 composite. Catalyst (Si/T ratioa)
HeFeeGaeAl-MFI (19.4)/Al2O3 HeFeeGaeAl-MFI (19.4)/Al2O3 HeFeeGaeAl-MFI (19.4)/Al2O3
Feedstock (mixed ratio [mol/mol])
LHSV of n-C6 [h1]
Conv. [wt%]
n-C6H14/N2 (1/10) n-C6H14/H2O (1/10) n-C6H14/H2O (1/5)
1.0 1.0 2.0
93.8 84.6 77.4
Product yield [wt%] (C2 ] )
(C3 ] )
BTX
13.2 13.6 10.6
30.1 23.9 21.3
17.6 2.4 1.7
Productivity of light olefins [g-(C2 ] þ C3 ] )/g-zeolite h] 0.70 0.61 1.0
Catalyst; HeFeeGaeAl-MFI/Al2O3 composite, Zeolite/Al2O3 ratio: 75 wt%/25 wt%. Reaction conditions; Temperature: 550 C, Pressure: 0.1 MPa, Charged volume of catalyst: 1.0 mL, Reaction time: 5 h. a Si/T molar ratio measured by XRF technique (T ¼ Fe þ Ga þ Al).
Please cite this article in press as: S. Hodoshima, et al., Catalytic conversion of light hydrocarbons to propylene over MFI-zeolite/metal-oxide composites, Microporous and Mesoporous Materials (2016), http://dx.doi.org/10.1016/j.micromeso.2015.12.044
S. Hodoshima et al. / Microporous and Mesoporous Materials xxx (2016) 1e8
7
Fig. 6. Product distribution in n-hexane cracking and 1-hexene cracking over HeFeeGaeAl-MFI/Al2O3 composite catalyst. Catalyst; HeFeeGaeAl-MFI/Al2O3 composite (molar composition of zeolite: Si/(Fe þ Ga þ Al) ¼ 19.4, Fe/(Fe þ Ga þ Al) ¼ 0.4, Ga/(Fe þ Ga þ Al) ¼ 0.3, Al/(Fe þ Ga þ Al) ¼ 0.3, weight ratio of zeolite/Al2O3: 75 wt%/25 wt%). Reaction conditions; Temperature: 550 C, Pressure: 0.1 MPa, Charged volume of catalyst: 1.0 mL, Feedstock: n-C6H14/N2 (molar ratio of n-C6H14/N2: 1/15), 1-C6H12/N2 (molar ratio of 1-C6H12/ N2: 1/15), 1-C6H12/H2O (molar ratio of 1-C6H12/H2O: 1/5), LHSV of n-C6H14: 1.0 h1, LHSV of 1-C6H12: 1.0 h1 (1-C6H12/N2 ¼ 1/15), 2.0 h1 (1-C6H12/H2O ¼ 1/5).
aromatics formation still proceeded selectively. In the cracking at higher temperature (565 C) under the same LHSV (5.0 h1), yield of light olefins was slightly increased, whereas aromatics formation was increasingly accelerated (see Table 7). By applying the zeolite catalyst with higher Si/(Fe þ Ga þ Al) ratio of 121.3 (i.e., smaller amount of acid sites) under the same reaction conditions, selectivity to light olefins was obviously enhanced, giving high propylene yield of ca. 18 wt%. Here the space time yield of light olefins was ca. 1.4 [g-(C2 ] þ C3 ] )/g-zeolite h], being ca. twice of the value (0.62) in nitrogen dilution. In cracking of n-hexane without dilution, aromatization proceeds much easier than cracking of n-hexane diluted with nitrogen or steam due to bimolecular reaction of n-hexane and/or re-adsorption of light olefins on acid sites, indicating that aromatics would be generated quite selectively by using zeolites with high acid densities. In other words, zeolites with low acid densities would give high selectivity to light olefins by suppressing aromatics formation [10]. Catalytic stability of the HeFeeGaeAl-MFI/Al2O3 composites with the Si/(Fe þ Ga þ Al) ratios of 19.4 and 121.3 were evaluated by performing n-hexane cracking without dilution at 565 C for long time. Fig. 7 shows time courses of yields of ethylene and propylene. In the catalytic cracking over the sample with the Si/(Fe þ Ga þ Al) ratio of 121.3, both ethylene yield of ca. 8.0 wt% and propylene yield of ca. 18 wt% obtained at initial stage were maintained for at least 80 h in spite of severe conditions to cause coke formation easily. This long lifetime, being applicable to cracking process in fixed-bed mode, was achieved due to its excellent resistance to coke formation. On the other hand, the sample with the Si/(Fe þ Ga þ Al) ratio
of 19.4 resulted in deactivation within 30 h. These results indicate that it is quite significant to yield aromatics as precursor of carbonaceous deposit as low as possible to realize stable performance for long term. Concerning the improvement of catalytic stability in cracking, the application of nano-sized ZSM-5 has been actively studied [4,5,8]. Konno et al. attained both high propylene yield (ca. 33 C-mol%) and excellent stability (ca. 30 h) in n-hexane cracking at 650 C by utilizing ZSM-5 zeolites (Si/Al ratios: 180e192) with smaller crystal size than 200 nm [5], but gradual deactivation was observed after 30 h. In the previous report by our group [10], the FeeGaeAl-MFI (Si/(Fe þ Ga þ Al) ratio: 12.0) with particle size of 570 nm was confirmed to give both high propylene yield (31.2 wt%) and long lifetime (at least 24 h) in n-hexane cracking at 550 C. The stability of the FeeGaeAl-MFI is thus expected to be further improved by controlling its particle size to smaller than 200 nm.
4. Conclusions The composite catalysts for cracking, consisting of FeeGaeAlMFI zeolites and metal-oxide binder (e.g., aluminum oxide), were developed on the basis of original concept to produce propylene efficiently by cracking of light hydrocarbons. Al-MFI-type zeolite containing iron and gallium atoms at optimized ratio was employed as a main component. The FeeGaeAl-MFI zeolites caused a drastic change of acidity, giving high selectivity to propylene by suppressing aromatics formation. The composite catalysts combined with aluminum-oxide binder (FeeGaeAl-MFI/Al2O3) for practical
Table 6 Comparison of initial activities for cracking of n-hexane and 1-hexene over HeFeeGaeAl-MFI/Al2O3 composite. Catalyst (Si/T ratioa)
HeFeeGaeAl-MFI (19.4)/Al2O3 HeFeeGaeAl-MFI (19.4)/Al2O3 HeFeeGaeAl-MFI (19.4)/Al2O3
Feedstock (mixed ratio [mol/mol])
LHSV of C6 [h1]
Conv. [wt%]
n-C6H14/N2 (1/15) 1-C6H12/N2 (1/15) 1-C6H12/H2O (1/5)
1.0 1.0 2.0
80.9 99.9 99.9
Product yield [wt%] (C2 ] )
(C3 ] )
BTX
10.0 19.4 22.1
28.3 40.6 38.3
10.8 15.8 4.5
Productivity of light olefins [g-(C2 ] þ C3 ] )/g-zeolite h] 0.62 1.0 2.0
Catalyst; HeFeeGaeAl-MFI/Al2O3 composites, Zeolite/Al2O3 ratio: 75 wt%/25 wt%. Reaction conditions; Temperature: 550 C, Pressure: 0.1 MPa, Charged volume of catalyst: 1.0 mL, Reaction time: 5 h. a Si/T molar ratio measured by XRF technique (T ¼ Fe þ Ga þ Al).
Please cite this article in press as: S. Hodoshima, et al., Catalytic conversion of light hydrocarbons to propylene over MFI-zeolite/metal-oxide composites, Microporous and Mesoporous Materials (2016), http://dx.doi.org/10.1016/j.micromeso.2015.12.044
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S. Hodoshima et al. / Microporous and Mesoporous Materials xxx (2016) 1e8
Table 7 Initial activities for cracking of n-hexane over HeFeeGaeAl-MFI/Al2O3 composites in the presence and absence of diluent. Catalyst (Si/T ratioa)
HeFeeGaeAl-MFI HeFeeGaeAl-MFI HeFeeGaeAl-MFI HeFeeGaeAl-MFI HeFeeGaeAl-MFI
(19.4)/Al2O3 (19.4)/Al2O3 (19.4)/Al2O3 (19.4)/Al2O3 (121.3)/Al2O3
Feedstock
n-C6H14/N2 (1/15) n-C6H14/H2O (1/5) n-C6H14 only n-C6H14 only n-C6H14 only
LHSV/Temp. [h1]/[ C] 1.0/550 2.0/550 5.0/550 5.0/565 5.0/565
C C C C C
Conv. [wt%]
80.9 77.4 74.7 81.4 67.3
Product yield [wt%] (C2 ] )
(C3 ] )
BTX
10.0 10.6 4.6 5.4 8.2
28.3 23.9 9.0 10.6 18.1
10.8 1.7 15.5 19.4 7.3
Productivity of light olefins [g-(C2 ] þ C3 ] )/g-zeolite h] 0.62 1.0 0.71 0.84 1.4
Catalyst; HeFeeGaeAl-MFI/Al2O3 composites, Zeolite/Al2O3 ratio: 75 wt%/25 wt%. Reaction conditions; Pressure: 0.1 MPa, Charged volume of catalyst: 1.0 mL (n-C6/N2, n-C6/H2O), 2.0 mL (n-C6 only). Reaction time: 5.0 h (n-C6/N2, n-C6/H2O), 24.0 h (n-C6 only). a Si/T molar ratio measured by XRF technique (T ¼ Fe þ Ga þ Al).
Fig. 7. Time course of cracking of n-hexane without dilution over HeFeeGaeAl-MFI/Al2O3 composite catalysts. Catalyst; (A) HeFeeGaeAl-MFI/Al2O3 composites (molar composition of zeolite: Si/(Fe þ Ga þ Al) ¼ 19.4, Fe/(Fe þ Ga þ Al) ¼ 0.4, Ga/(Fe þ Ga þ Al) ¼ 0.3, Al/(Fe þ Ga þ Al) ¼ 0.3, weight ratio of zeolite/Al2O3: 75 wt%/25 wt%). (B) HeFeeGaeAlMFI/Al2O3 composites (molar composition of zeolite: Si/(Fe þ Ga þ Al) ¼ 121.3, Fe/(Fe þ Ga þ Al) ¼ 0.4, Ga/(Fe þ Ga þ Al) ¼ 0.3, Al/(Fe þ Ga þ Al) ¼ 0.3, weight ratio of zeolite/Al2O3: 75 wt%/25 wt%). Reaction conditions; Temperature: 565 C, Pressure: 0.1 MPa, Charged volume of catalyst: 2.0 mL, Feedstock: n-C6H14 only, LHSV of n-C6H14: 5.0 h1.
use were also confirmed to exhibit high activities keeping excellent performance of zeolite species. By applying the zeolite-based catalysts to cracking reactions in fixed-bed mode, selective generation of propylene was efficiently attained at mild temperatures (550e565 C). The present catalysts exhibited both high propylene yields (20e30 wt%) in cracking of nhexane diluted with nitrogen or steam at 550 C. In the steam cracking of n-hexane, its performance was kept stably at suitable steam ratio. In the 1-hexene cracking, the present catalyst gave higher propylene yields than 38 wt%, being superior in propylene productivity to conventional technologies such as thermal cracking. Furthermore, in the catalytic cracking of n-hexane without dilution at 565 C, high propylene yields of ca. 18 wt% were maintained for ca. 80 h in spite of quite severe conditions. This long lifetime was achieved due to its high resistance to coke formation. The supply of hydrocarbon feedstock without dilution was thus confirmed to be advantageous in terms of preventing dealumination-derived deactivation and saving energy. It was suggested on the experimental basis that catalytic cracking of paraffinic and olefinic hydrocarbons in fixed-bed mode is feasible by using the present catalysts as an alternative technology for producing propylene efficiently. Acknowledgments A part of the present study was financially supported by the Japan Oil, Gas and Metals National Corporation (JOGMEC). We would like to express our deep gratitude here. The authors acknowledge Prof. Sachio Asaoka (The University of Kitakyushu) for guidance regarding zeolite synthesis.
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Please cite this article in press as: S. Hodoshima, et al., Catalytic conversion of light hydrocarbons to propylene over MFI-zeolite/metal-oxide composites, Microporous and Mesoporous Materials (2016), http://dx.doi.org/10.1016/j.micromeso.2015.12.044