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SiO2 catalyst for efficient synthesis of isoparaffins directly from syngas
Journal of Membrane Science 475 (2015) 22–29
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Development of dual-membrane coated Fe/SiO2 catalyst for efficient synthesis of isoparaffins directly from syngas Yuzhou Jin, Guohui Yang nn, Qingjun Chen nn, Wenqi Niu, Peng Lu, Yoshiharu Yoneyama, Noritatsu Tsubaki n Department of Applied Chemistry, School of Engineering, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 10 April 2014 Received in revised form 5 August 2014 Accepted 4 October 2014 Available online 12 October 2014
A novel dual-membrane coated catalyst (Fe/SiO2-S-Z) with Fe/SiO2 core and Silicalite-1 and H-ZSM-5 zeolite membranes was developed and employed for isoparaffins direct synthesis from syngas via Fischer–Tropsch synthesis (FTS) reaction. The Silicalite-1 zeolite membrane, as a structural membrane, was prepared under a close-to-neutral synthesis condition, which avoided the structure destruction of the core catalyst Fe/SiO2 and favored the in-situ growth of H-ZSM-5 zeolite membrane. In H-ZSM-5 zeolite membrane, as a functional membrane, the micropores and strong acidic sites were preserved in the dual-membrane catalyst, which played important roles in the isoparaffins direct synthesis. Activity tests indicated that the dual-membrane catalyst exhibited an excellent performance for isoparaffins direct synthesis. The isoparaffins selectivity of the dual-membrane catalyst was up to 29.8%, much higher than those of individual core catalyst (12.9%) and physical mixture catalyst of H-ZSM-5 zeolite and Fe/SiO2 catalyst (16.6%). It was found for the first time that the hydrogenation and isomerization of olefins over the dual-membrane catalyst were the main reasons for the high isoparaffins selectivity. In addition, due to the spatial confinement effect of the microporous zeolite membranes, the hydrocracking and isomerization of heavy hydrocarbons (C12 þ ) also contributed to the high selectivity of isoparaffins. & 2014 Elsevier B.V. All rights reserved.
Keywords: Dual-membrane catalyst H-ZSM-5 zeolite membrane Silicalite-1 zeolite membrane Isoparaffins synthesis Fischer–Tropsch Synthesis (FTS)
1. Introduction Fischer–Tropsch synthesis (FTS) is an effective process converting syngas (COþ H2) into a wide range of hydrocarbons that can be used as the substitute of the fossil-based fuels. However, the FTS products are mainly normal aliphatic hydrocarbons, which follow Anderson–Schultz–Flory (ASF) distribution and are only suitable as the synthetic diesel fuel [1–4]. Recently, much attention has been focused on the production of isoparaffins due to their wide foreground as the synthetic gasoline fuel. The catalysts for isoparaffins direct synthesis via FTS are mainly composed of two parts: conventional FTS catalysts and acidic zeolites (such as H-ZSM-5, H-MOR, and H-Beta zeolites) [5–10]. The FTS catalysts can convert syngas to hydrocarbons (mainly long chain normal paraffins, olefins and some oxygenates), and the acidic zeolites can convert these products to isoparaffins by the hydrocracking and isomerization reactions [7–9]. It has been proved that zeolite membrane encapsulated FTS catalysts are more effective for isoparaffins synthesis than the mechanical n
Corresponding author. Tel.: þ 81 76 4456846; fax: þ 81 76 4456848. Corresponding authors. Tel./fax: þ81 76 4456848. E-mail addresses: [email protected] (G. Yang), [email protected] (Q. Chen), [email protected] (N. Tsubaki). nn
http://dx.doi.org/10.1016/j.memsci.2014.10.004 0376-7388/& 2014 Elsevier B.V. All rights reserved.
mixture catalysts [11–13]. During the enforced diffusion in the microporous zeolite membrane, the formed long chain hydrocarbons can be completely cracked and isomerized, which leads to a high isoparaffins selectivity. However, the in-situ synthesis of zeolite membrane over the FTS catalysts (such as Fe/Co supported on SiO2/Al2O3) is not easy to control. The synthesis of the zeolite membrane (such as ZSM-5 zeolite) usually requires a strong alkaline condition [11–14], which may destroy the structure of the FTS catalyst and result in a poor or failing coating of the zeolite membrane. In addition, the coating of zeolite membrane usually causes an increase of methane selectivity due to the different diffusion rate of H2 and CO in the zeolite membrane [15], which is detrimental for the isoparaffins synthesis. In order to decrease the selectivity of methane, a large metal crystalline size or a large amount of Fe or Co loading over the SiO2 or Al2O3 support will be required [16–20]. This will decrease the surface content of Si or Al, and thus further increase the difficulty for the in-situ synthesis of the zeolite membrane. Recently, we have developed a new method to synthesize zeolite membrane encapsulated catalyst under a close-to-neutral condition, where the corrosion or breakage of the core catalyst can be suppressed effectively [21]. The synthesized Pd/SiO2 capsule catalyst had an ideal core–shell structure, and exhibited a high selectivity for DME synthesis from a mixture gas of H2, CO, and CO2. Employing this
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new method and concept, a Silicalite-1 zeolite membrane coated catalyst with a high metal loading might be prepared without the corrosion of the core catalyst. Since Silicalite-1 zeolite exhibits a low activity for hydrocracking and isomerization [22], another zeolite membrane with high activity and selectivity for isoparaffins synthesis is required. Previous work revealed that H-ZSM-5 zeolite is an ideal candidate for the isoparaffins production [10,12,13]. Therefore, a dual-membrane coated catalyst might be developed which exhibited a high performance for isoparaffins direct synthesis from syngas. In this work, we designed a novel dual-membrane coated Fe/SiO2 catalyst (20 wt% Fe loading) and employed it for onestep isoparaffins synthesis via FTS reaction from syngas. The preparation scheme of the dual-membrane catalyst is shown in Fig. 1. One Silicalite-1 zeolite membrane was firstly synthesized on the alkaline-sensitive silica-based core catalyst under a closeto-neutral synthesis condition. The formed Silicalite-1 zeolite membrane can protect the core catalyst from the corrosion and breakage by the strong alkaline solution used for the following H-ZSM-5 zeolite membrane synthesis. Activity tests indicated that the dual-membrane catalyst exhibited a high performance for the isoparaffins synthesis. The isoparaffins selectivity is up to 29% and the methane selectivity is relative low.
2. Experimental
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2.1.3. Synthesis of the dual-membrane coated catalyst Fe/SiO2-S-Z Silicalite-1 zeolite membrane-Fe/SiO2 composites, without calcination, were used here as a new core for the synthesis of the second membrane of H-ZSM-5 zeolite. The new core and H-ZSM-5 zeolite precursor solution (molar ratio of 2TEOS:0.50TPAOH:120H2O: 8EtOH:0.025Al2O3) were mixed and sealed in a Teflon lined autoclave. The hydrothermal synthesis was conducted at a rotation rate of 2 rpm and a temperature of 180 1C for 48 h. In this work, TEOS (tetraethyl orthosilicate) and Al(NO3)3 9H2O were used as Si and Al sources, respectively. TPAOH (tetrapropyl ammonium hydroxide) was used as the template. After the hydrothermal synthesis, the sample was washed, dried and then calcined at 500 1C for 3 h to obtain the dual-membrane coated catalyst Fe/SiO2-S-Z. 2.1.4. Preparation of the physical mixture catalyst The pure H-ZSM-5 zeolite used for the preparation of mixture catalyst was prepared by the same recipe and synthesis procedure as to the dual-membrane catalyst (shown in Section 2.1.3). The only difference in the preparation was that the Silicalite-1 zeolite membrane-Fe/SiO2 composites were not used. The obtained H-ZSM-5 zeolite and the crashed Fe/SiO2 catalyst were mixed well with the weight ratio of 1:15, similar to the zeolite content in dual-membrane catalyst Fe/SiO2-S-Z. After that, the mixture was pressurized, and then granulated into the size range of 0.85–1.7 mm. The mixture catalyst was named as Fe/SiO2-M, where “M” stands for the physical mixture of Fe/SiO2 and H-ZSM-5 zeolite.
2.1. Catalyst preparation 2.2. Catalyst characterization 2.1.1. Preparation of the core catalyst Fe/SiO2 The core catalyst Fe/SiO2 with 20 wt% Fe loading was prepared by an incipient wetness impregnation method. The porous silica pellets (Fuji Silysia Chemical Ltd., Cariact Q-50, specific surface area of 75.0 m2/g, pore volume of 1.02 cm3/g, average pore diameter of 48 nm, and pellet size in 0.85–1.70 mm) were used as support and Fe(NO3)3 9H2O was used as the iron precursor. A desired amount of Fe(NO3)3 9H2O solution was impregnated over the silica pellets with the assistance of ultrasonic. After the impregnation, the sample was first dried at 120 1C overnight, and then calcined in a muffle furnace at 500 1C for 3 h to obtain the core catalyst Fe/SiO2.
2.1.2. Coating of Silicalite-1 zeolite membrane over the core catalyst In order to avoid the structure damage of the core catalyst, the Silicalite-1 zeolite membrane was synthesized under a close-toneutral condition. The core catalyst Fe/SiO2 and the Silicalite-1 zeolite precursor solution (molar ratio of 2TEOS:0.50TPAOH:120H2O:8EtOH:0.25HNO3) were mixed and sealed in a Teflon lined autoclave. The mixture underwent a hydrothermal treatment at 180 1C for 24 h. After the hydrothermal synthesis, the sample was washed and dried to get the Silicalite-1 zeolite membrane-Fe/SiO2 composites. The composites were calcined at 500 1C for 3 h to achieve the Silicalite-1 zeolite membrane coated catalyst, which was denoted as Fe/SiO2-S.
The pore structures of the prepared catalysts are measured by an N2 physisorption method using a NOVA 2200e apparatus. The samples were degassed at 200 1C for 2 h before the analysis. The surface area was calculated by the Brunauer–Emmett–Teller (BET) method and the pore size distribution was obtained by Barrett– Joyner–Halenda (BJH) method using the desorption branch. The morphology of the catalysts was observed by a scanning electron microscope (SEM, JEOL JSM-6360LV) equipped with an energy-diffusive X-ray (EDS) spectroscopy attachment (JED 2300). X-ray diffraction (XRD) patterns of the catalysts were measured using a Rigaku RINT-2400 diffractometer with Cu Kα radiation (λa ¼0.154 nm). The X-ray tube was operated at 40 kV and 40 mA. The acidity properties of the catalysts were measured by temperature-programmed desorption of ammonia (NH3-TPD) using a BELCAT-B-TT (BEL, Japan) instrument. 30 mg catalyst was pretreated at 150 1C for 1 h in Ar flow. After temperature was cooled down to 50 1C, a NH3–He mixture (5% NH3, 20 cm3/min) was purged for 30 min, and then pure He was introduced to purge for 30 min. The NH3-TPD experiment was performed in He flow by raising the temperature from 50 to 800 1C at a rate of 5 1C/min, where the desorbed NH3 was detected by a thermal conductivity detector (TCD). The reduction behaviors of the catalysts were studied by hydrogen temperature-programmed reduction (H2-TPR) (BELCAT-B-TT). Before the measurement, the sample was pretreated at 150 1C for 2 h in argon flow. After that, the sample was cooled down to 50 1C and the purge gas was changed to 5% H2/Ar mixture gas with a flow rate of 30 cm3/min. The temperature was ramped from 50 1C to 800 1C at a heating rate of 5.0 1C/min. The effluent gas was analyzed by a thermal conductivity detector (TCD). 2.3. Isoparaffins synthesis test
Fig. 1. Preparation scheme of the dual-membrane coated Fe/SiO2 catalyst.
Isoparaffins synthesis reaction was carried out in a fixed-bed reactor at 280 1C, 1 MPa and a Wcat: =FðCO þ H2 Þ of 10 g h/mol. Before the reaction, the catalyst was reduced in situ at 300 1C in the flow of syngas (COþH2, H2/CO¼1, 0.1 MPa) for 10 h. During the reaction, an
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ice trap with solvent (n-octane) and inner standard (n-dodecane) was set between reactor and back pressure regulator to capture the heavy hydrocarbons, which was ultimately analyzed by an off-line gas chromatograph (Shimadzu, GC2014, silicone SE-30 column) with a flame ionization detector (FID). The formed n-paraffins, isoparaffins and olefins can be separated by the silicone SE-30 column. The identification of olefins was based on the chromatograph peak variations before and after the olefins esterification by concentrated sulfuric acid. The residual gaseous products released from the reactor were analyzed online by two gas chromatographs. The concentration of CO, CO2, and CH4 was monitored by an online gas chromatograph (Shimadzu, GC-8A, active charcoal column) with a thermal conductivity detector (TCD). The light hydrocarbons (C1–6) including the n-paraffins, isoparaffins and olefins were analyzed with another online gas chromatograph (Shimadzu, GC-14B, J&W Scientific GS-Alumina capillary column) with a FID. Finally, the results of the gas and liquid products were summed to calculate the yields of the olefins, isoparaffins and n-paraffins.
3. Results and discussion 3.1. Structure analysis of the catalysts 3.1.1. Pore structures Pore structures of the naked core catalyst Fe/SiO2 and zeolite membrane coated catalysts were characterized by N2 physisorption tests. The N2 adsorption–desorption isotherms and pore size distributions of the catalysts are shown in Fig. 2. The calculated pore parameters are listed in Table 1. The naked core catalyst exhibited type IV isotherms [23], indicating a typical mesoporous structure. The pore size distribution of the core catalyst was very wide and centered at about 34 nm. Small mesopores centered at 3.9 nm were also observed in this catalyst, which might be formed by the aggregated iron oxide nanoparticles. After the coating of Silicalite-1 zeolite membrane, the isotherm type changed into a combination of type I and type IV. The high N2 adsorption capacity at low relative pressures demonstrated that micropores appeared in Fe/SiO2-S catalyst. The micropores are primarily derived from the Silicalite-1 zeolite membrane [24]. It is clear that the large mesopore and macropore volume decreased significantly after the coating of Silicalite-1 zeolite membrane (Fig. 2(b) and Table 1). This result may be caused by two reasons. One is the dilution effect of the Silicalite-1 zeolite which had no large mesopores and macropores. The other might be that some Silicalite-1 zeolite precursors had deposited in the pores of the core catalyst. The coating of second H-ZSM-5 zeolite membrane further increased the micropore volume and surface area of the catalysts as shown in Fig. 2(a) and Table 1, but did not change pore size distributions significantly. It could be concluded that the Silicalite-1 zeolite membrane acted as the structural membrane to protect the pore structure of core catalyst. It needs to note that small mesopores centered at 3.9 nm increased after the coating of Silicalite-1 and H-ZSM-5 zeolite membranes as shown in Fig. 2(b). The increased small mesopores might be originated from the pores among the stacked Silicalite-1 and H-ZSM-5 zeolite crystallines. 3.1.2. Morphology and composition analysis of the catalysts SEM images of the naked core catalyst Fe/SiO2 and zeolite membranes coated catalysts are presented in Fig. 3. The naked core catalyst showed a smooth external surface. After the growth of Silicalite-1 zeolite membrane, the external surface (Fig. 3(b)) exhibited a relatively rough morphology due to the formation of the Silicalite-1 zeolite crystals. The formed Silicalite-1 zeolite membrane completely enveloped the core catalyst Fe/SiO2, ensuring the
Fig. 2. N2 adsorption–desorption isotherms (a) and pore size distributions (b) of the catalysts.
Table 1 Pore parameters of the catalysts. Sample
BET surface area (m2/g)
Average pore size (nm)
Total pore volume (cm3/g)
Fe/SiO2 Fe/SiO2-S Fe/SiO2-S-Z
58 184 218
3.9, 34.2 4.0, 45.8 3.4, 46.2
0.45 0.18 0.18
minimum damage to the core catalyst in the following hydrothermal synthesis of the H-ZSM-5 zeolite. In addition, the Silicalite-1 zeolite also acted as a layer of seeding, which favored the subsequent growth of H-ZSM-5 zeolite membrane. The SEM image of the external surface of Fe/SiO2-S-Z is shown in Fig. 3(c). It can be observed that an H-ZSM-5 zeolite membrane is successfully coated on Fe/SiO2-S catalyst. The sizes of H-ZSM-5 zeolite crystallines were about 1 μm. Among the stacked zeolite crystallines, there are considerable amount of macropores, which can improve the gas diffusion during the isoparaffins synthesis. The elemental composition on external surfaces of the naked core and dual-membrane catalysts is shown in Fig. 3(d) and (e), respectively. As expected, there are only Fe, Si and O signals in the naked catalyst Fe/SiO2. After the coating of the Silicalite-1 and H-ZSM-5 zeolite membranes, the signal of Fe disappeared. This result suggested that the core catalyst was completely covered by the dual membranes, and there were no obvious pinholes or cracks
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Fig. 3. SEM images (a–c) and EDS results (d, e) of the catalysts: (a, d) external surface of the naked core catalyst Fe/SiO2, (b) external surface of Fe/SiO2-S catalyst, and (c, e) external surface of Fe/SiO2-S-Z catalyst.
in the zeolite membranes. The Si/Al molar ratio of the external surface calculated by the EDS results was 40.2, which is similar to the theoretical value in the H-ZSM-5 zeolite precursor solution.
SEM image and elemental distribution along the cross-section of dual-membrane catalyst Fe/SiO2-S-Z are shown in Fig. 4(a) and (b), respectively. A compact dual-membrane structure can be
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The XRD patterns of pure H-ZSM-5 zeolite and different kind of catalysts are presented in Fig. 5. For naked core catalyst Fe/SiO2, strong diffraction peaks for hematite (Fe2O3) crystalline can be identified. After the coating of Silicalite-1 zeolite, new diffraction peaks appeared (2θ¼5–101 and 21–251), corresponding to the crystalline of Silicalite-1 zeolite. This result confirmed that Silicalite-1 zeolite membrane was formed over the core catalyst. Further coating of the H-ZSM-5 zeolite membrane did not change XRD patterns significantly. This is because H-ZSM-5 zeolite and Silicalite-1 zeolite exhibited the same crystalline structure (MFI structure) [12,13,25,26], which can also be verified by the XRD pattern of the pure H-ZSM-5 zeolite as shown in Fig. 5. The physically mixed catalyst Fe/SiO2-M exhibited the diffraction peaks of H-ZSM-5 zeolite and Fe2O3 crystallines, which were similar to those of the dual-membrane catalyst Fe/SiO2-S-Z. 3.2. Acidity properties analysis of the catalysts
Fig. 4. The cross-sectional SEM image (a) and EDS line analysis along the line (b) of the dual membrane catalyst Fe/SiO2-S-Z.
clearly observed in the Fe/SiO2-S-Z catalyst. The thicknesses of the Silicalite-1 and H-ZSM-5 zeolite membranes are about 6 μm and 8 μm, respectively. It is noted that Al signal was detected in the Silicalite-1 zeolite membrane. Some Al precursor must permeate into the Silicalite-1 zeolite membrane during the synthesis of HZSM-5 zeolite. In addition, some Silicalite-1 zeolite might be dissolved under this strong alkaline condition. The dissolved silica together with introduced Al ion and template agent might form H-ZSM-5 zeolites during the hydrothermal synthesis. Thus, there will be an intermixing area of Silicalte-1 and H-ZSM-5 zeolite in the Silicalite-1 zeolite membrane. The thickness of the intermixing area was about 2 μm according to the SEM-EDS result, and the actual thickness of individual Silicalite-1 membrane decreased to about 4 μm. Although some Silicalite-1 zeolite was dissolved, the left Silicalite-1 zeolite membrane successfully protected the core catalyst from the corrosion or damage by the strong alkaline solution during the synthesis of H-ZSM-5 zeolite membrane. If there is no silicalite-1 membrane, the core catalyst Fe/SiO2 will be damaged and the H-ZSM5 zeolite membrane cannot be coated under the strong alkaline condition. In this work, we tried many times to synthesize the H-ZSM-5 zeolite membrane over 20%Fe/SiO2 core catalyst directly, but all attempts were failed. Therefore, the Silicalite-1 zeolite membrane acted as a structural membrane for protecting the core catalyst and favored the synthesis of H-ZSM-5 zeolite membrane. In the HZSM-5 zeolite membrane, the main elements were Si and Al, and the ratio of Si/Al was almost kept at the same value of 40 along the crosssection of the membrane. This result indicated that a well H-ZSM-5 zeolite membrane was coated in the catalyst.
3.1.3. XRD analysis of the catalysts X-ray diffraction is an effective technology to identify the crystalline structures of the core catalyst and zeolite membranes.
Acidic sites of the zeolites are the active centers for hydrocracking and isomerization. So it is important to measure the acidity property of the dual-membrane catalyst. The NH3-TPD profiles of the core and dual-membrane catalysts are shown in Fig. 6. No obvious NH3 desorption peak was observed in the naked core catalyst Fe/SiO2, which meant that there were no remarkable acidic sites in this catalyst. After the coating of Silicalite-1 and H-ZSM-5 zeolite membranes, two peaks for NH3 desorption appeared. The peak at about 198 1C was attributed to the desorption of NH3 from weak acidic sites. The weak acidic sites probably had no activity for the hydrocracking and isomerization. The peak at about 530 1C could be assigned to the strong Brønsted or Lewis acidic sites, which was originated from the H-ZSM-5 zeolite membrane [5,27,28]. The strong acidic sites were the active centers for the hydrocracking and isomerization of hydrocarbons. The strong acidic sites of the dual-membrane catalyst would play an important role in the isoparaffins synthesis directly from syngas. The H-ZSM-5 zeolite membrane acted as a functional membrane in the isoparaffins synthesis. 3.3. Reduction behavior of the catalysts The reduction behavior of the naked core and dual-membrane catalysts was investigated by H2-TPR, and the results are compared in Fig. 7. There were three reduction peaks for the core catalyst Fe/SiO2. The peak at about 342 1C could be assigned to the
Fig. 5. XRD patterns of H-ZSM-5 zeolite and different kind of catalysts.
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time on stream and reached a stable value after about 5 h on stream. Although possessing the largest surface area (Table 1), the dual-membrane catalyst exhibited a slightly lower steady CO conversion (54.8%) than those of naked core catalyst (59.5%) and physical mixture catalyst (60.0%). This might be partly due to the small diffusion rate of CO in the Silicalite-1 and H-ZSM-5 zeolite membranes. The difficult reduction of iron in the dual-membrane catalyst was also responsible for the relative low activity as indicated from the TPR results.
Fig. 6. NH3-TPD profiles of the naked core and dual-membrane catalysts.
3.4.2. Isoparaffins selectivity over different catalysts The product distributions of different catalysts for isoparaffins synthesis are presented in Table 2 and Fig. 9. The naked core catalyst Fe/SiO2 had a wide product distribution with the carbon number from 1 to 20. Among these products, olefins selectivity is the highest (57.7%), but the selectivity of isoparaffins was very low (12.9%). The methane selectivity was also very low (6.7%) due to the high loading of Fe in the core catalyst. For the physical mixture catalyst Fe/SiO2-M, the isoparaffins selectivity increased slightly to 16.6%. It is worth to note that there were also some long chain hydrocarbons (C13–C19) in the products of Fe/SiO2-M. Thus, the hydrocracking and isomerization of long chain hydrocarbons in Fe/SiO2-M were incomplete. This was mainly attributed to the random diffusion of long chain hydrocarbons in catalyst bed (including the Fe/SiO2 catalyst and H-ZSM-5 zeolite) and only the long chain hydrocarbons diffused to the strong acidic sites of the H-ZSM-5 zeolite could be converted to isoparaffins. In the case of the dual-membrane catalyst, the isoparaffins selectivity was up to 29.8%, which was about two times higher than those of the naked core and physical mixture catalysts. The
Fig. 7. H2-TPR profiles of the naked core and dual-membrane catalysts.
reduction of Fe2O3 to Fe3O4. The other two peaks at about 523 and 680 1C were probably due to the reduction of Fe3O4 to FeO and FeO to metallic Fe, respectively [29–31]. After the coating of the dual membranes, the reduction temperature of Fe2O3 to Fe3O4 increased from 342 to 394 1C and the reduction temperature of Fe3O4 to metallic Fe also increased significantly. Furthermore, there was no obvious boundary for the reduction of Fe3O4 to FeO and FeO to metallic Fe in the dual-membrane catalyst. It can be concluded that the reduction of iron became more difficult after the coating of the Silicalite-1 and H-ZSM-5 zeolite membranes, which might result in a relative lower activity for the isoparaffins synthesis. 3.4. Performance for isoparaffins direct synthesis 3.4.1. Activity for isoparaffins synthesis The dual membrane catalyst Fe/SiO2-S-Z was used for the direct synthesis of isoparaffins from syngas (COþH2) in the fixed bed reactor at the reaction temperature of 280 1C and pressure of 1.0 MPa. For comparison, the naked core catalyst Fe/SiO2 and the physical mixture catalyst Fe/SiO2-M were also tested under the same reaction condition. The activity curves for isoparaffins synthesis are shown in Fig. 8. It can be seen that there is an activation process for each catalyst. The CO conversion increased with the
Fig. 8. Activity curves of the different catalysts for isoparaffins synthesis.
Table 2 Catalytic performances of the different catalysts for isoparaffins synthesis a. Sample
Fe/SiO2 Fe/SiO2-M Fe/SiO2-S-Z
CO conversion (%)
59.5 60.0 54.8
Selectivity (%) CH4b
CO2
Ciso
CQ
6.7 7.0 14.9
32.9 29.9 33.8
12.9 16.6 29.8
57.7 52.5 23.8
Ciso/Cnc
Cole/Cnd
0.83 1.62 1.81
2.68 2.97 0.80
a Reaction conditions: 280 1C, 1.0 MPa, 6 h; W/F¼ 10 g h mol 1; syngas: H2/ CO¼ 1/1. b The calculation of CH4 selectivity is based on all the hydrocarbons. c Ciso/Cn is the ratio of isoparaffins to n-paraffins of C4 þ . d Cole/Cn is the ratio of olefins to n-paraffins of C2 þ .
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Fig. 10. Mechanism scheme of the isoparaffins direct synthesis over the dualmembrane catalyst from syngas.
some contribution to the high selectivity of isoparafins over the dual-membrane catalyst. It can be seen from Table 2 and Fig. 9 that the methane selectivity (14.9%) of the dual-membrane catalyst was higher than those of the naked catalyst and physical mixture catalyst. This might be also attributed to the high H2/CO ratio in the core catalyst due to the faster diffusion of H2 in the zeolite membranes. However, comparing with other zeolite coated catalysts [10–12,32,33], the methane selectivity of the dual-membrane catalyst was much smaller. Recently, many new kinds of catalysts for isoparaffins synthesis have been developed including the hierarchical zeolite supported Ru/Co catalysts [34–37] and weak interaction Ru(Pd)/ zeolite catalysts prepared by a dry sputtering method [28,38]. These catalysts also exhibited high isoparaffins selectivity. But one common feature of these catalysts is that considerable amount of long chain hydrocarbons (C12 þ ) were observed in their products, which were still required for upgrading to produce the gasoline fuel. The dual-membrane coated Fe/SiO2 catalyst, which exhibited high selectivity of isoparaffins, relatively low methane selectivity and completely elimination of the long chain hydrocarbons (C12 þ ), is more promising in the production of synthetic gasoline fuel.
Fig. 9. Hydrocarbon selectivity of (a) core catalyst Fe/SiO2, (b) physical mixture catalyst Fe/SiO2-M and (c) dual-membrane catalyst Fe/SiO2-S-Z.
ratio of isoparaffins to n-paraffins increased significantly from 0.83 (naked catalyst) to 1.81. At the same time, the olefins selectivity decreased from 57.7% (naked catalyst) to 23.8%. From Fig. 9, we can also observe that the increase of isoparaffins selectivity is at the expense of olefins selectivity. It can be deduced that the high isoparaffins selectivity is mainly ascribed to the hydrogenation and isomerization of olefins over the dual-membrane catalyst. Besides, the selectivity of long chain hydrocarbons (C12 þ ) almost decreased to zero (Fig. 9(c)). This result suggested that the formed long chain hydrocarbons (C12 þ ) in the core catalyst (Fe/SiO2) were completely cracked and isomerized in the zeolite membrane. The hydrocracking and isomerization of the long chain hydrocarbon also made
3.4.3. Mechanism of isoparaffins synthesis over dual-membrane coated Fe/SiO2 catalyst For the physical mixture and dual-membrane catalysts, similar core catalyst (Fe/SiO2) and H-ZSM-5 zeolite were employed, but the isoparaffins selectivity was absolutely different. The essential reason is that the dual-membrane catalyst had a spatial restriction effect by the two zeolite membranes. The formed hydrocarbons in the core catalyst Fe/SiO2 had to diffuse through the two zeolite membranes to the exterior of the catalyst. During the enforced diffusion, the long chain hydrocarbon might convert to isoparaffins through hydrocracking and isomerization reactions at the strong acidic sites of the zeolite membranes. In addition, the diffusion rate of H2 in the microporous zeolite membranes is higher than that of CO. So the ratio of H2/CO on the core catalyst was higher than on the external surface of the dual-membrane catalyst. High H2/CO ratio could also improve the hydrogenation and isomerization of olefins. The mechanism scheme of isoparaffins direct synthesis is presented in Fig. 10.
4. Conclusion A novel dual-membrane coated catalyst (Fe/SiO2-S-Z) with Fe/SiO2 core and Silicalite-1 and H-ZSM-5 zeolite membranes was developed and used for isoparaffins synthesis directly from syngas via Fischer–Tropsch reaction. The Silicalite-1 zeolite membrane acted as the structural membrane for protecting the core catalyst while the H-ZSM-5 zeolite membrane acted as the
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functional membrane for isoparaffins synthesis. Activity tests indicated that dual-membrane catalyst exhibited excellent performance for isoparaffins direct synthesis. The isoparaffins selectivity of this catalyst was up to 29.8%, which was much higher than those of individual core catalyst (Fe/SiO2: 12.9%) and physical mixture catalyst (Fe/SiO2-M: 16.6%). The high isoparaffins selectivity was primarily ascribed to the olefins (formed by the core catalyst Fe/SiO2) hydrogenation and isomerization in the dualmembrane catalysts. In addition, due to the spatial confinement effect of the microporous zeolite membranes, the hydrocracking and isomerization of heavy hydrocarbon (C12 þ ) also contributed to the high selectivity of isoparaffins. Due to the high amount of iron loading in the core catalyst, the methane selectivity was obviously restrained comparing with the traditional zeolite membrane coated catalysts. References [1] S.Z. Li, S. Krishnamoorthy, A.W. Li, G.D. Meitzner, E. Iglesia, Promoted ironbased catalysts for the Fischer–Tropsch synthesis: design, synthesis, site densities, and catalytic properties, J. Catal. 206 (2002) 202–217. [2] G.P. Van Der Laan, A. Beenackers, Kinetics and selectivity of the Fischer– Tropsch synthesis: a literature review, Catal. Rev. Sci. Eng. 41 (1999) 255–318. [3] D. Schanke, S. Vada, E.A. Blekkan, A.M. Hilman, A. Hoff, A. Holmen, Study of Ptpromoted cobalt CO hydrogenation catalysts, J. Catal. 156 (1995) 85–95. [4] E. Iglesia, Design, synthesis and use of cobalt-based Fischer–Tropsch synthesis catalysts, Appl. Catal. A 161 (1997) 59–78. [5] X. Huang, B. Hou, J.G. Wang, D.B. Li, L.T. Jia, J.G. Chen, Y.H. Sun, CoZr/H-ZSM-5 hybrid catalysts for synthesis of gasoline-range isoparaffins from syngas, Appl. Catal. A 408 (2011) 38–46. [6] G.H. Yang, C. Xing, W. Hirohama, Y.Z. Jin, C.Y. Zeng, Y. Suehiro, T.J. Wang, Y. Yoneyama, N. Tsubaki, Tandem catalytic synthesis of light isoparaffin from syngas via Fischer–Tropsch synthesis by newly developed core–shell-like zeolite capsule catalysts, Catal. Today 215 (2013) 29–35. [7] T.S. Zhao, J. Chang, Y. Yoneyama, N. Tsubaki, Selective synthesis of middle isoparaffins via a two-stage Fischer–Tropsch reaction: activity investigation for a hybrid catalyst, Ind. Eng. Chem. Res. 44 (2005) 769–775. [8] Y. Yoneyama, J.J. He, Y. Morii, S. Azuma, N. Tsubaki, Direct synthesis of isoparaffin by modified Fischer–Tropsch synthesis using hybrid catalyst of iron catalyst and zeolite, Catal. Today 104 (2005) 37–40. [9] N. Tsubaki, Y. Yoneyama, K. Michiki, K. Fujimoto, Three-component hybrid catalyst for direct synthesis of isoparaffins via modified Fischer–Tropsch synthesis, Catal. Commun. 4 (2003) 108–111. [10] G.H. Yang, Y.S. Tan, Y.Z. Han, J.S. Qiu, N. Tsubaki, Increasing the shell thickness by controlling the core size of zeolite capsule catalyst: application in isoparaffin direct synthesis, Catal. Commun. 9 (2008) 2520–2524. [11] Q.H. Lin, G.H. Yang, X.N. Li, Y. Yoneyama, H.L. Wan, N. Tsubaki, A Catalyst for one-step isoparaffin production via Fischer–Tropsch synthesis: growth of a HMordenite shell encapsulating a fused iron core, ChemCatChem 5 (2013) 3101–3106. [12] G.H. Yang, J.J. He, Y. Zhang, Y. Yoneyama, Y.S. Tan, Y.Z. Han, T. Vitidsant, N. Tsubaki, Design and modification of zeolite capsule catalyst, a confined reaction field, and its application in one-step isoparaffins synthesis from syngas, Energy Fuels 2 (2008) 1463–1468. [13] J. Bao, G.H. Yang, C. Okada, Y. Yoneyama, N. Tsubaki, H-type zeolite coated iron-based multiple-functional catalyst for direct synthesis of middle isoparaffins from syngas, Appl. Catal. A 394 (2011) 195–200. [14] H. Teng, J. Wang, D.M. Chen, P. Liu, X.C. Wang, Silicalite-1 membrane on millimeter-sized HZSM-5 zeolite extrudates: controllable synthesis and catalytic behavior in toluene disproportionation, J. Membr. Sci. 381 (2011) 197–203. [15] R.J. Madon, E. Iglesia, Hydrogen and CO intrapellet diffusion effects in ruthenium-catalyzed hydrocarbon synthesis, J. Catal. 149 (1994) 428–437. [16] A.Y. Khodakov, W. Chu, P. Fongarland, Advances in the development of novel cobalt Fischer–Tropsch catalysts for synthesis of long-chain hydrocarbons and clean fuels, Chem. Rev. 107 (2007) 1692–1744.
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Development of dual-membrane coated Fe/SiO2 catalyst for efficient synthesis of isoparaffins directly from syngas Yuzhou Jin, Guohui Yang nn, Qingjun Chen nn, Wenqi Niu, Peng Lu, Yoshiharu Yoneyama, Noritatsu Tsubaki n Department of Applied Chemistry, School of Engineering, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 10 April 2014 Received in revised form 5 August 2014 Accepted 4 October 2014 Available online 12 October 2014
A novel dual-membrane coated catalyst (Fe/SiO2-S-Z) with Fe/SiO2 core and Silicalite-1 and H-ZSM-5 zeolite membranes was developed and employed for isoparaffins direct synthesis from syngas via Fischer–Tropsch synthesis (FTS) reaction. The Silicalite-1 zeolite membrane, as a structural membrane, was prepared under a close-to-neutral synthesis condition, which avoided the structure destruction of the core catalyst Fe/SiO2 and favored the in-situ growth of H-ZSM-5 zeolite membrane. In H-ZSM-5 zeolite membrane, as a functional membrane, the micropores and strong acidic sites were preserved in the dual-membrane catalyst, which played important roles in the isoparaffins direct synthesis. Activity tests indicated that the dual-membrane catalyst exhibited an excellent performance for isoparaffins direct synthesis. The isoparaffins selectivity of the dual-membrane catalyst was up to 29.8%, much higher than those of individual core catalyst (12.9%) and physical mixture catalyst of H-ZSM-5 zeolite and Fe/SiO2 catalyst (16.6%). It was found for the first time that the hydrogenation and isomerization of olefins over the dual-membrane catalyst were the main reasons for the high isoparaffins selectivity. In addition, due to the spatial confinement effect of the microporous zeolite membranes, the hydrocracking and isomerization of heavy hydrocarbons (C12 þ ) also contributed to the high selectivity of isoparaffins. & 2014 Elsevier B.V. All rights reserved.
Keywords: Dual-membrane catalyst H-ZSM-5 zeolite membrane Silicalite-1 zeolite membrane Isoparaffins synthesis Fischer–Tropsch Synthesis (FTS)
1. Introduction Fischer–Tropsch synthesis (FTS) is an effective process converting syngas (COþ H2) into a wide range of hydrocarbons that can be used as the substitute of the fossil-based fuels. However, the FTS products are mainly normal aliphatic hydrocarbons, which follow Anderson–Schultz–Flory (ASF) distribution and are only suitable as the synthetic diesel fuel [1–4]. Recently, much attention has been focused on the production of isoparaffins due to their wide foreground as the synthetic gasoline fuel. The catalysts for isoparaffins direct synthesis via FTS are mainly composed of two parts: conventional FTS catalysts and acidic zeolites (such as H-ZSM-5, H-MOR, and H-Beta zeolites) [5–10]. The FTS catalysts can convert syngas to hydrocarbons (mainly long chain normal paraffins, olefins and some oxygenates), and the acidic zeolites can convert these products to isoparaffins by the hydrocracking and isomerization reactions [7–9]. It has been proved that zeolite membrane encapsulated FTS catalysts are more effective for isoparaffins synthesis than the mechanical n
Corresponding author. Tel.: þ 81 76 4456846; fax: þ 81 76 4456848. Corresponding authors. Tel./fax: þ81 76 4456848. E-mail addresses: [email protected] (G. Yang), [email protected] (Q. Chen), [email protected] (N. Tsubaki). nn
http://dx.doi.org/10.1016/j.memsci.2014.10.004 0376-7388/& 2014 Elsevier B.V. All rights reserved.
mixture catalysts [11–13]. During the enforced diffusion in the microporous zeolite membrane, the formed long chain hydrocarbons can be completely cracked and isomerized, which leads to a high isoparaffins selectivity. However, the in-situ synthesis of zeolite membrane over the FTS catalysts (such as Fe/Co supported on SiO2/Al2O3) is not easy to control. The synthesis of the zeolite membrane (such as ZSM-5 zeolite) usually requires a strong alkaline condition [11–14], which may destroy the structure of the FTS catalyst and result in a poor or failing coating of the zeolite membrane. In addition, the coating of zeolite membrane usually causes an increase of methane selectivity due to the different diffusion rate of H2 and CO in the zeolite membrane [15], which is detrimental for the isoparaffins synthesis. In order to decrease the selectivity of methane, a large metal crystalline size or a large amount of Fe or Co loading over the SiO2 or Al2O3 support will be required [16–20]. This will decrease the surface content of Si or Al, and thus further increase the difficulty for the in-situ synthesis of the zeolite membrane. Recently, we have developed a new method to synthesize zeolite membrane encapsulated catalyst under a close-to-neutral condition, where the corrosion or breakage of the core catalyst can be suppressed effectively [21]. The synthesized Pd/SiO2 capsule catalyst had an ideal core–shell structure, and exhibited a high selectivity for DME synthesis from a mixture gas of H2, CO, and CO2. Employing this
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new method and concept, a Silicalite-1 zeolite membrane coated catalyst with a high metal loading might be prepared without the corrosion of the core catalyst. Since Silicalite-1 zeolite exhibits a low activity for hydrocracking and isomerization [22], another zeolite membrane with high activity and selectivity for isoparaffins synthesis is required. Previous work revealed that H-ZSM-5 zeolite is an ideal candidate for the isoparaffins production [10,12,13]. Therefore, a dual-membrane coated catalyst might be developed which exhibited a high performance for isoparaffins direct synthesis from syngas. In this work, we designed a novel dual-membrane coated Fe/SiO2 catalyst (20 wt% Fe loading) and employed it for onestep isoparaffins synthesis via FTS reaction from syngas. The preparation scheme of the dual-membrane catalyst is shown in Fig. 1. One Silicalite-1 zeolite membrane was firstly synthesized on the alkaline-sensitive silica-based core catalyst under a closeto-neutral synthesis condition. The formed Silicalite-1 zeolite membrane can protect the core catalyst from the corrosion and breakage by the strong alkaline solution used for the following H-ZSM-5 zeolite membrane synthesis. Activity tests indicated that the dual-membrane catalyst exhibited a high performance for the isoparaffins synthesis. The isoparaffins selectivity is up to 29% and the methane selectivity is relative low.
2. Experimental
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2.1.3. Synthesis of the dual-membrane coated catalyst Fe/SiO2-S-Z Silicalite-1 zeolite membrane-Fe/SiO2 composites, without calcination, were used here as a new core for the synthesis of the second membrane of H-ZSM-5 zeolite. The new core and H-ZSM-5 zeolite precursor solution (molar ratio of 2TEOS:0.50TPAOH:120H2O: 8EtOH:0.025Al2O3) were mixed and sealed in a Teflon lined autoclave. The hydrothermal synthesis was conducted at a rotation rate of 2 rpm and a temperature of 180 1C for 48 h. In this work, TEOS (tetraethyl orthosilicate) and Al(NO3)3 9H2O were used as Si and Al sources, respectively. TPAOH (tetrapropyl ammonium hydroxide) was used as the template. After the hydrothermal synthesis, the sample was washed, dried and then calcined at 500 1C for 3 h to obtain the dual-membrane coated catalyst Fe/SiO2-S-Z. 2.1.4. Preparation of the physical mixture catalyst The pure H-ZSM-5 zeolite used for the preparation of mixture catalyst was prepared by the same recipe and synthesis procedure as to the dual-membrane catalyst (shown in Section 2.1.3). The only difference in the preparation was that the Silicalite-1 zeolite membrane-Fe/SiO2 composites were not used. The obtained H-ZSM-5 zeolite and the crashed Fe/SiO2 catalyst were mixed well with the weight ratio of 1:15, similar to the zeolite content in dual-membrane catalyst Fe/SiO2-S-Z. After that, the mixture was pressurized, and then granulated into the size range of 0.85–1.7 mm. The mixture catalyst was named as Fe/SiO2-M, where “M” stands for the physical mixture of Fe/SiO2 and H-ZSM-5 zeolite.
2.1. Catalyst preparation 2.2. Catalyst characterization 2.1.1. Preparation of the core catalyst Fe/SiO2 The core catalyst Fe/SiO2 with 20 wt% Fe loading was prepared by an incipient wetness impregnation method. The porous silica pellets (Fuji Silysia Chemical Ltd., Cariact Q-50, specific surface area of 75.0 m2/g, pore volume of 1.02 cm3/g, average pore diameter of 48 nm, and pellet size in 0.85–1.70 mm) were used as support and Fe(NO3)3 9H2O was used as the iron precursor. A desired amount of Fe(NO3)3 9H2O solution was impregnated over the silica pellets with the assistance of ultrasonic. After the impregnation, the sample was first dried at 120 1C overnight, and then calcined in a muffle furnace at 500 1C for 3 h to obtain the core catalyst Fe/SiO2.
2.1.2. Coating of Silicalite-1 zeolite membrane over the core catalyst In order to avoid the structure damage of the core catalyst, the Silicalite-1 zeolite membrane was synthesized under a close-toneutral condition. The core catalyst Fe/SiO2 and the Silicalite-1 zeolite precursor solution (molar ratio of 2TEOS:0.50TPAOH:120H2O:8EtOH:0.25HNO3) were mixed and sealed in a Teflon lined autoclave. The mixture underwent a hydrothermal treatment at 180 1C for 24 h. After the hydrothermal synthesis, the sample was washed and dried to get the Silicalite-1 zeolite membrane-Fe/SiO2 composites. The composites were calcined at 500 1C for 3 h to achieve the Silicalite-1 zeolite membrane coated catalyst, which was denoted as Fe/SiO2-S.
The pore structures of the prepared catalysts are measured by an N2 physisorption method using a NOVA 2200e apparatus. The samples were degassed at 200 1C for 2 h before the analysis. The surface area was calculated by the Brunauer–Emmett–Teller (BET) method and the pore size distribution was obtained by Barrett– Joyner–Halenda (BJH) method using the desorption branch. The morphology of the catalysts was observed by a scanning electron microscope (SEM, JEOL JSM-6360LV) equipped with an energy-diffusive X-ray (EDS) spectroscopy attachment (JED 2300). X-ray diffraction (XRD) patterns of the catalysts were measured using a Rigaku RINT-2400 diffractometer with Cu Kα radiation (λa ¼0.154 nm). The X-ray tube was operated at 40 kV and 40 mA. The acidity properties of the catalysts were measured by temperature-programmed desorption of ammonia (NH3-TPD) using a BELCAT-B-TT (BEL, Japan) instrument. 30 mg catalyst was pretreated at 150 1C for 1 h in Ar flow. After temperature was cooled down to 50 1C, a NH3–He mixture (5% NH3, 20 cm3/min) was purged for 30 min, and then pure He was introduced to purge for 30 min. The NH3-TPD experiment was performed in He flow by raising the temperature from 50 to 800 1C at a rate of 5 1C/min, where the desorbed NH3 was detected by a thermal conductivity detector (TCD). The reduction behaviors of the catalysts were studied by hydrogen temperature-programmed reduction (H2-TPR) (BELCAT-B-TT). Before the measurement, the sample was pretreated at 150 1C for 2 h in argon flow. After that, the sample was cooled down to 50 1C and the purge gas was changed to 5% H2/Ar mixture gas with a flow rate of 30 cm3/min. The temperature was ramped from 50 1C to 800 1C at a heating rate of 5.0 1C/min. The effluent gas was analyzed by a thermal conductivity detector (TCD). 2.3. Isoparaffins synthesis test
Fig. 1. Preparation scheme of the dual-membrane coated Fe/SiO2 catalyst.
Isoparaffins synthesis reaction was carried out in a fixed-bed reactor at 280 1C, 1 MPa and a Wcat: =FðCO þ H2 Þ of 10 g h/mol. Before the reaction, the catalyst was reduced in situ at 300 1C in the flow of syngas (COþH2, H2/CO¼1, 0.1 MPa) for 10 h. During the reaction, an
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ice trap with solvent (n-octane) and inner standard (n-dodecane) was set between reactor and back pressure regulator to capture the heavy hydrocarbons, which was ultimately analyzed by an off-line gas chromatograph (Shimadzu, GC2014, silicone SE-30 column) with a flame ionization detector (FID). The formed n-paraffins, isoparaffins and olefins can be separated by the silicone SE-30 column. The identification of olefins was based on the chromatograph peak variations before and after the olefins esterification by concentrated sulfuric acid. The residual gaseous products released from the reactor were analyzed online by two gas chromatographs. The concentration of CO, CO2, and CH4 was monitored by an online gas chromatograph (Shimadzu, GC-8A, active charcoal column) with a thermal conductivity detector (TCD). The light hydrocarbons (C1–6) including the n-paraffins, isoparaffins and olefins were analyzed with another online gas chromatograph (Shimadzu, GC-14B, J&W Scientific GS-Alumina capillary column) with a FID. Finally, the results of the gas and liquid products were summed to calculate the yields of the olefins, isoparaffins and n-paraffins.
3. Results and discussion 3.1. Structure analysis of the catalysts 3.1.1. Pore structures Pore structures of the naked core catalyst Fe/SiO2 and zeolite membrane coated catalysts were characterized by N2 physisorption tests. The N2 adsorption–desorption isotherms and pore size distributions of the catalysts are shown in Fig. 2. The calculated pore parameters are listed in Table 1. The naked core catalyst exhibited type IV isotherms [23], indicating a typical mesoporous structure. The pore size distribution of the core catalyst was very wide and centered at about 34 nm. Small mesopores centered at 3.9 nm were also observed in this catalyst, which might be formed by the aggregated iron oxide nanoparticles. After the coating of Silicalite-1 zeolite membrane, the isotherm type changed into a combination of type I and type IV. The high N2 adsorption capacity at low relative pressures demonstrated that micropores appeared in Fe/SiO2-S catalyst. The micropores are primarily derived from the Silicalite-1 zeolite membrane [24]. It is clear that the large mesopore and macropore volume decreased significantly after the coating of Silicalite-1 zeolite membrane (Fig. 2(b) and Table 1). This result may be caused by two reasons. One is the dilution effect of the Silicalite-1 zeolite which had no large mesopores and macropores. The other might be that some Silicalite-1 zeolite precursors had deposited in the pores of the core catalyst. The coating of second H-ZSM-5 zeolite membrane further increased the micropore volume and surface area of the catalysts as shown in Fig. 2(a) and Table 1, but did not change pore size distributions significantly. It could be concluded that the Silicalite-1 zeolite membrane acted as the structural membrane to protect the pore structure of core catalyst. It needs to note that small mesopores centered at 3.9 nm increased after the coating of Silicalite-1 and H-ZSM-5 zeolite membranes as shown in Fig. 2(b). The increased small mesopores might be originated from the pores among the stacked Silicalite-1 and H-ZSM-5 zeolite crystallines. 3.1.2. Morphology and composition analysis of the catalysts SEM images of the naked core catalyst Fe/SiO2 and zeolite membranes coated catalysts are presented in Fig. 3. The naked core catalyst showed a smooth external surface. After the growth of Silicalite-1 zeolite membrane, the external surface (Fig. 3(b)) exhibited a relatively rough morphology due to the formation of the Silicalite-1 zeolite crystals. The formed Silicalite-1 zeolite membrane completely enveloped the core catalyst Fe/SiO2, ensuring the
Fig. 2. N2 adsorption–desorption isotherms (a) and pore size distributions (b) of the catalysts.
Table 1 Pore parameters of the catalysts. Sample
BET surface area (m2/g)
Average pore size (nm)
Total pore volume (cm3/g)
Fe/SiO2 Fe/SiO2-S Fe/SiO2-S-Z
58 184 218
3.9, 34.2 4.0, 45.8 3.4, 46.2
0.45 0.18 0.18
minimum damage to the core catalyst in the following hydrothermal synthesis of the H-ZSM-5 zeolite. In addition, the Silicalite-1 zeolite also acted as a layer of seeding, which favored the subsequent growth of H-ZSM-5 zeolite membrane. The SEM image of the external surface of Fe/SiO2-S-Z is shown in Fig. 3(c). It can be observed that an H-ZSM-5 zeolite membrane is successfully coated on Fe/SiO2-S catalyst. The sizes of H-ZSM-5 zeolite crystallines were about 1 μm. Among the stacked zeolite crystallines, there are considerable amount of macropores, which can improve the gas diffusion during the isoparaffins synthesis. The elemental composition on external surfaces of the naked core and dual-membrane catalysts is shown in Fig. 3(d) and (e), respectively. As expected, there are only Fe, Si and O signals in the naked catalyst Fe/SiO2. After the coating of the Silicalite-1 and H-ZSM-5 zeolite membranes, the signal of Fe disappeared. This result suggested that the core catalyst was completely covered by the dual membranes, and there were no obvious pinholes or cracks
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Fig. 3. SEM images (a–c) and EDS results (d, e) of the catalysts: (a, d) external surface of the naked core catalyst Fe/SiO2, (b) external surface of Fe/SiO2-S catalyst, and (c, e) external surface of Fe/SiO2-S-Z catalyst.
in the zeolite membranes. The Si/Al molar ratio of the external surface calculated by the EDS results was 40.2, which is similar to the theoretical value in the H-ZSM-5 zeolite precursor solution.
SEM image and elemental distribution along the cross-section of dual-membrane catalyst Fe/SiO2-S-Z are shown in Fig. 4(a) and (b), respectively. A compact dual-membrane structure can be
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The XRD patterns of pure H-ZSM-5 zeolite and different kind of catalysts are presented in Fig. 5. For naked core catalyst Fe/SiO2, strong diffraction peaks for hematite (Fe2O3) crystalline can be identified. After the coating of Silicalite-1 zeolite, new diffraction peaks appeared (2θ¼5–101 and 21–251), corresponding to the crystalline of Silicalite-1 zeolite. This result confirmed that Silicalite-1 zeolite membrane was formed over the core catalyst. Further coating of the H-ZSM-5 zeolite membrane did not change XRD patterns significantly. This is because H-ZSM-5 zeolite and Silicalite-1 zeolite exhibited the same crystalline structure (MFI structure) [12,13,25,26], which can also be verified by the XRD pattern of the pure H-ZSM-5 zeolite as shown in Fig. 5. The physically mixed catalyst Fe/SiO2-M exhibited the diffraction peaks of H-ZSM-5 zeolite and Fe2O3 crystallines, which were similar to those of the dual-membrane catalyst Fe/SiO2-S-Z. 3.2. Acidity properties analysis of the catalysts
Fig. 4. The cross-sectional SEM image (a) and EDS line analysis along the line (b) of the dual membrane catalyst Fe/SiO2-S-Z.
clearly observed in the Fe/SiO2-S-Z catalyst. The thicknesses of the Silicalite-1 and H-ZSM-5 zeolite membranes are about 6 μm and 8 μm, respectively. It is noted that Al signal was detected in the Silicalite-1 zeolite membrane. Some Al precursor must permeate into the Silicalite-1 zeolite membrane during the synthesis of HZSM-5 zeolite. In addition, some Silicalite-1 zeolite might be dissolved under this strong alkaline condition. The dissolved silica together with introduced Al ion and template agent might form H-ZSM-5 zeolites during the hydrothermal synthesis. Thus, there will be an intermixing area of Silicalte-1 and H-ZSM-5 zeolite in the Silicalite-1 zeolite membrane. The thickness of the intermixing area was about 2 μm according to the SEM-EDS result, and the actual thickness of individual Silicalite-1 membrane decreased to about 4 μm. Although some Silicalite-1 zeolite was dissolved, the left Silicalite-1 zeolite membrane successfully protected the core catalyst from the corrosion or damage by the strong alkaline solution during the synthesis of H-ZSM-5 zeolite membrane. If there is no silicalite-1 membrane, the core catalyst Fe/SiO2 will be damaged and the H-ZSM5 zeolite membrane cannot be coated under the strong alkaline condition. In this work, we tried many times to synthesize the H-ZSM-5 zeolite membrane over 20%Fe/SiO2 core catalyst directly, but all attempts were failed. Therefore, the Silicalite-1 zeolite membrane acted as a structural membrane for protecting the core catalyst and favored the synthesis of H-ZSM-5 zeolite membrane. In the HZSM-5 zeolite membrane, the main elements were Si and Al, and the ratio of Si/Al was almost kept at the same value of 40 along the crosssection of the membrane. This result indicated that a well H-ZSM-5 zeolite membrane was coated in the catalyst.
3.1.3. XRD analysis of the catalysts X-ray diffraction is an effective technology to identify the crystalline structures of the core catalyst and zeolite membranes.
Acidic sites of the zeolites are the active centers for hydrocracking and isomerization. So it is important to measure the acidity property of the dual-membrane catalyst. The NH3-TPD profiles of the core and dual-membrane catalysts are shown in Fig. 6. No obvious NH3 desorption peak was observed in the naked core catalyst Fe/SiO2, which meant that there were no remarkable acidic sites in this catalyst. After the coating of Silicalite-1 and H-ZSM-5 zeolite membranes, two peaks for NH3 desorption appeared. The peak at about 198 1C was attributed to the desorption of NH3 from weak acidic sites. The weak acidic sites probably had no activity for the hydrocracking and isomerization. The peak at about 530 1C could be assigned to the strong Brønsted or Lewis acidic sites, which was originated from the H-ZSM-5 zeolite membrane [5,27,28]. The strong acidic sites were the active centers for the hydrocracking and isomerization of hydrocarbons. The strong acidic sites of the dual-membrane catalyst would play an important role in the isoparaffins synthesis directly from syngas. The H-ZSM-5 zeolite membrane acted as a functional membrane in the isoparaffins synthesis. 3.3. Reduction behavior of the catalysts The reduction behavior of the naked core and dual-membrane catalysts was investigated by H2-TPR, and the results are compared in Fig. 7. There were three reduction peaks for the core catalyst Fe/SiO2. The peak at about 342 1C could be assigned to the
Fig. 5. XRD patterns of H-ZSM-5 zeolite and different kind of catalysts.
Y. Jin et al. / Journal of Membrane Science 475 (2015) 22–29
27
time on stream and reached a stable value after about 5 h on stream. Although possessing the largest surface area (Table 1), the dual-membrane catalyst exhibited a slightly lower steady CO conversion (54.8%) than those of naked core catalyst (59.5%) and physical mixture catalyst (60.0%). This might be partly due to the small diffusion rate of CO in the Silicalite-1 and H-ZSM-5 zeolite membranes. The difficult reduction of iron in the dual-membrane catalyst was also responsible for the relative low activity as indicated from the TPR results.
Fig. 6. NH3-TPD profiles of the naked core and dual-membrane catalysts.
3.4.2. Isoparaffins selectivity over different catalysts The product distributions of different catalysts for isoparaffins synthesis are presented in Table 2 and Fig. 9. The naked core catalyst Fe/SiO2 had a wide product distribution with the carbon number from 1 to 20. Among these products, olefins selectivity is the highest (57.7%), but the selectivity of isoparaffins was very low (12.9%). The methane selectivity was also very low (6.7%) due to the high loading of Fe in the core catalyst. For the physical mixture catalyst Fe/SiO2-M, the isoparaffins selectivity increased slightly to 16.6%. It is worth to note that there were also some long chain hydrocarbons (C13–C19) in the products of Fe/SiO2-M. Thus, the hydrocracking and isomerization of long chain hydrocarbons in Fe/SiO2-M were incomplete. This was mainly attributed to the random diffusion of long chain hydrocarbons in catalyst bed (including the Fe/SiO2 catalyst and H-ZSM-5 zeolite) and only the long chain hydrocarbons diffused to the strong acidic sites of the H-ZSM-5 zeolite could be converted to isoparaffins. In the case of the dual-membrane catalyst, the isoparaffins selectivity was up to 29.8%, which was about two times higher than those of the naked core and physical mixture catalysts. The
Fig. 7. H2-TPR profiles of the naked core and dual-membrane catalysts.
reduction of Fe2O3 to Fe3O4. The other two peaks at about 523 and 680 1C were probably due to the reduction of Fe3O4 to FeO and FeO to metallic Fe, respectively [29–31]. After the coating of the dual membranes, the reduction temperature of Fe2O3 to Fe3O4 increased from 342 to 394 1C and the reduction temperature of Fe3O4 to metallic Fe also increased significantly. Furthermore, there was no obvious boundary for the reduction of Fe3O4 to FeO and FeO to metallic Fe in the dual-membrane catalyst. It can be concluded that the reduction of iron became more difficult after the coating of the Silicalite-1 and H-ZSM-5 zeolite membranes, which might result in a relative lower activity for the isoparaffins synthesis. 3.4. Performance for isoparaffins direct synthesis 3.4.1. Activity for isoparaffins synthesis The dual membrane catalyst Fe/SiO2-S-Z was used for the direct synthesis of isoparaffins from syngas (COþH2) in the fixed bed reactor at the reaction temperature of 280 1C and pressure of 1.0 MPa. For comparison, the naked core catalyst Fe/SiO2 and the physical mixture catalyst Fe/SiO2-M were also tested under the same reaction condition. The activity curves for isoparaffins synthesis are shown in Fig. 8. It can be seen that there is an activation process for each catalyst. The CO conversion increased with the
Fig. 8. Activity curves of the different catalysts for isoparaffins synthesis.
Table 2 Catalytic performances of the different catalysts for isoparaffins synthesis a. Sample
Fe/SiO2 Fe/SiO2-M Fe/SiO2-S-Z
CO conversion (%)
59.5 60.0 54.8
Selectivity (%) CH4b
CO2
Ciso
CQ
6.7 7.0 14.9
32.9 29.9 33.8
12.9 16.6 29.8
57.7 52.5 23.8
Ciso/Cnc
Cole/Cnd
0.83 1.62 1.81
2.68 2.97 0.80
a Reaction conditions: 280 1C, 1.0 MPa, 6 h; W/F¼ 10 g h mol 1; syngas: H2/ CO¼ 1/1. b The calculation of CH4 selectivity is based on all the hydrocarbons. c Ciso/Cn is the ratio of isoparaffins to n-paraffins of C4 þ . d Cole/Cn is the ratio of olefins to n-paraffins of C2 þ .
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Y. Jin et al. / Journal of Membrane Science 475 (2015) 22–29
Fig. 10. Mechanism scheme of the isoparaffins direct synthesis over the dualmembrane catalyst from syngas.
some contribution to the high selectivity of isoparafins over the dual-membrane catalyst. It can be seen from Table 2 and Fig. 9 that the methane selectivity (14.9%) of the dual-membrane catalyst was higher than those of the naked catalyst and physical mixture catalyst. This might be also attributed to the high H2/CO ratio in the core catalyst due to the faster diffusion of H2 in the zeolite membranes. However, comparing with other zeolite coated catalysts [10–12,32,33], the methane selectivity of the dual-membrane catalyst was much smaller. Recently, many new kinds of catalysts for isoparaffins synthesis have been developed including the hierarchical zeolite supported Ru/Co catalysts [34–37] and weak interaction Ru(Pd)/ zeolite catalysts prepared by a dry sputtering method [28,38]. These catalysts also exhibited high isoparaffins selectivity. But one common feature of these catalysts is that considerable amount of long chain hydrocarbons (C12 þ ) were observed in their products, which were still required for upgrading to produce the gasoline fuel. The dual-membrane coated Fe/SiO2 catalyst, which exhibited high selectivity of isoparaffins, relatively low methane selectivity and completely elimination of the long chain hydrocarbons (C12 þ ), is more promising in the production of synthetic gasoline fuel.
Fig. 9. Hydrocarbon selectivity of (a) core catalyst Fe/SiO2, (b) physical mixture catalyst Fe/SiO2-M and (c) dual-membrane catalyst Fe/SiO2-S-Z.
ratio of isoparaffins to n-paraffins increased significantly from 0.83 (naked catalyst) to 1.81. At the same time, the olefins selectivity decreased from 57.7% (naked catalyst) to 23.8%. From Fig. 9, we can also observe that the increase of isoparaffins selectivity is at the expense of olefins selectivity. It can be deduced that the high isoparaffins selectivity is mainly ascribed to the hydrogenation and isomerization of olefins over the dual-membrane catalyst. Besides, the selectivity of long chain hydrocarbons (C12 þ ) almost decreased to zero (Fig. 9(c)). This result suggested that the formed long chain hydrocarbons (C12 þ ) in the core catalyst (Fe/SiO2) were completely cracked and isomerized in the zeolite membrane. The hydrocracking and isomerization of the long chain hydrocarbon also made
3.4.3. Mechanism of isoparaffins synthesis over dual-membrane coated Fe/SiO2 catalyst For the physical mixture and dual-membrane catalysts, similar core catalyst (Fe/SiO2) and H-ZSM-5 zeolite were employed, but the isoparaffins selectivity was absolutely different. The essential reason is that the dual-membrane catalyst had a spatial restriction effect by the two zeolite membranes. The formed hydrocarbons in the core catalyst Fe/SiO2 had to diffuse through the two zeolite membranes to the exterior of the catalyst. During the enforced diffusion, the long chain hydrocarbon might convert to isoparaffins through hydrocracking and isomerization reactions at the strong acidic sites of the zeolite membranes. In addition, the diffusion rate of H2 in the microporous zeolite membranes is higher than that of CO. So the ratio of H2/CO on the core catalyst was higher than on the external surface of the dual-membrane catalyst. High H2/CO ratio could also improve the hydrogenation and isomerization of olefins. The mechanism scheme of isoparaffins direct synthesis is presented in Fig. 10.
4. Conclusion A novel dual-membrane coated catalyst (Fe/SiO2-S-Z) with Fe/SiO2 core and Silicalite-1 and H-ZSM-5 zeolite membranes was developed and used for isoparaffins synthesis directly from syngas via Fischer–Tropsch reaction. The Silicalite-1 zeolite membrane acted as the structural membrane for protecting the core catalyst while the H-ZSM-5 zeolite membrane acted as the
Y. Jin et al. / Journal of Membrane Science 475 (2015) 22–29
functional membrane for isoparaffins synthesis. Activity tests indicated that dual-membrane catalyst exhibited excellent performance for isoparaffins direct synthesis. The isoparaffins selectivity of this catalyst was up to 29.8%, which was much higher than those of individual core catalyst (Fe/SiO2: 12.9%) and physical mixture catalyst (Fe/SiO2-M: 16.6%). The high isoparaffins selectivity was primarily ascribed to the olefins (formed by the core catalyst Fe/SiO2) hydrogenation and isomerization in the dualmembrane catalysts. In addition, due to the spatial confinement effect of the microporous zeolite membranes, the hydrocracking and isomerization of heavy hydrocarbon (C12 þ ) also contributed to the high selectivity of isoparaffins. Due to the high amount of iron loading in the core catalyst, the methane selectivity was obviously restrained comparing with the traditional zeolite membrane coated catalysts. References [1] S.Z. Li, S. Krishnamoorthy, A.W. Li, G.D. Meitzner, E. Iglesia, Promoted ironbased catalysts for the Fischer–Tropsch synthesis: design, synthesis, site densities, and catalytic properties, J. Catal. 206 (2002) 202–217. [2] G.P. Van Der Laan, A. Beenackers, Kinetics and selectivity of the Fischer– Tropsch synthesis: a literature review, Catal. Rev. Sci. Eng. 41 (1999) 255–318. [3] D. Schanke, S. Vada, E.A. Blekkan, A.M. Hilman, A. Hoff, A. Holmen, Study of Ptpromoted cobalt CO hydrogenation catalysts, J. Catal. 156 (1995) 85–95. [4] E. Iglesia, Design, synthesis and use of cobalt-based Fischer–Tropsch synthesis catalysts, Appl. Catal. A 161 (1997) 59–78. [5] X. Huang, B. Hou, J.G. Wang, D.B. Li, L.T. Jia, J.G. Chen, Y.H. Sun, CoZr/H-ZSM-5 hybrid catalysts for synthesis of gasoline-range isoparaffins from syngas, Appl. Catal. A 408 (2011) 38–46. [6] G.H. Yang, C. Xing, W. Hirohama, Y.Z. Jin, C.Y. Zeng, Y. Suehiro, T.J. Wang, Y. Yoneyama, N. Tsubaki, Tandem catalytic synthesis of light isoparaffin from syngas via Fischer–Tropsch synthesis by newly developed core–shell-like zeolite capsule catalysts, Catal. Today 215 (2013) 29–35. [7] T.S. Zhao, J. Chang, Y. Yoneyama, N. Tsubaki, Selective synthesis of middle isoparaffins via a two-stage Fischer–Tropsch reaction: activity investigation for a hybrid catalyst, Ind. Eng. Chem. Res. 44 (2005) 769–775. [8] Y. Yoneyama, J.J. He, Y. Morii, S. Azuma, N. Tsubaki, Direct synthesis of isoparaffin by modified Fischer–Tropsch synthesis using hybrid catalyst of iron catalyst and zeolite, Catal. Today 104 (2005) 37–40. [9] N. Tsubaki, Y. Yoneyama, K. Michiki, K. Fujimoto, Three-component hybrid catalyst for direct synthesis of isoparaffins via modified Fischer–Tropsch synthesis, Catal. Commun. 4 (2003) 108–111. [10] G.H. Yang, Y.S. Tan, Y.Z. Han, J.S. Qiu, N. Tsubaki, Increasing the shell thickness by controlling the core size of zeolite capsule catalyst: application in isoparaffin direct synthesis, Catal. Commun. 9 (2008) 2520–2524. [11] Q.H. Lin, G.H. Yang, X.N. Li, Y. Yoneyama, H.L. Wan, N. Tsubaki, A Catalyst for one-step isoparaffin production via Fischer–Tropsch synthesis: growth of a HMordenite shell encapsulating a fused iron core, ChemCatChem 5 (2013) 3101–3106. [12] G.H. Yang, J.J. He, Y. Zhang, Y. Yoneyama, Y.S. Tan, Y.Z. Han, T. Vitidsant, N. Tsubaki, Design and modification of zeolite capsule catalyst, a confined reaction field, and its application in one-step isoparaffins synthesis from syngas, Energy Fuels 2 (2008) 1463–1468. [13] J. Bao, G.H. Yang, C. Okada, Y. Yoneyama, N. Tsubaki, H-type zeolite coated iron-based multiple-functional catalyst for direct synthesis of middle isoparaffins from syngas, Appl. Catal. A 394 (2011) 195–200. [14] H. Teng, J. Wang, D.M. Chen, P. Liu, X.C. Wang, Silicalite-1 membrane on millimeter-sized HZSM-5 zeolite extrudates: controllable synthesis and catalytic behavior in toluene disproportionation, J. Membr. Sci. 381 (2011) 197–203. [15] R.J. Madon, E. Iglesia, Hydrogen and CO intrapellet diffusion effects in ruthenium-catalyzed hydrocarbon synthesis, J. Catal. 149 (1994) 428–437. [16] A.Y. Khodakov, W. Chu, P. Fongarland, Advances in the development of novel cobalt Fischer–Tropsch catalysts for synthesis of long-chain hydrocarbons and clean fuels, Chem. Rev. 107 (2007) 1692–1744.
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