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propane separation on a ferroaluminosilicate levyne zeolite
Microporous and Mesoporous Materials xxx (xxxx) xxx
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Propylene/propane separation on a ferroaluminosilicate levyne zeolite Jung Gi Min, K. Christian Kemp, Suk Bong Hong * Center for Ordered Nanoporous Materials Synthesis, Division of Environmental Science and Engineering, POSTECH, Pohang, 37673, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Propylene/propane separation Levyne zeolite Partial iron substitution Propylene selectivity Durability
The propylene/propane separation properties of levyne zeolites with different framework Si/Al and Fe/Al ratios have been examined and compared with those of the four well-studied zeolite adsorbents (i.e., Na-A, Ca-A, Na-X and ITQ-12) for this separation. Both zeolite framework structure and composition were found to be critical for the propylene uptake, kinetic separation and adsorbent durability. Among the zeolites studied here, the mixed CaNH4 form of a ferroaluminosilcate levyne (Si/Al ¼ 15.5 and Fe/Al ¼ 0.27) showed the highest propylene/ propane selectivity (11). Under vacuum-swing adsorption mode at 298 K and reservoir pressure of 1.2 bar, this partially iron-substituted zeolite was characterized by a higher propylene uptake (ca. 1.0 vs 0.7 mmol g 1) than its aluminosilicate version, which can be attributed to a combination of the molecular sieve effect of its framework topology and relatively weaker acidity. The results of this study demonstrate that, in contrast to widely held belief, zeolite adsorbents selective for propylene/propane separation do not need to be a pure-silica composition.
1. Introduction Propylene is the most important olefin in petrochemical production, except ethylene, because it is an essential building block for manufacturing polypropylene [1]. At present, the most commonly applied industrial processes for producing olefin are catalytic cracking, steam cracking or catalytic dehydrogenation of paraffins. Among these processes, the cracking of C4 hydrocarbon fractions followed by dehy drogenation is most widely utilized for propylene production [2]. However, during the reactions, paraffin with the same carbon number as that of propylene, i.e., propane, is always produced, and so the gas mixture separation is required. Traditionally, propylene/propane sepa ration has relied on the cryogenic distillation and condensation pro cesses that are energy intensive due to the similarity in many physical properties of these two hydrocarbons, e.g., their boiling points, molec ular weights, polarizabilities and kinetic diameters [3]. Adsorptive separation using ordered nanoporous materials such as zeolites and metal-organic frameworks (MOFs) has been considered as the most promising process for seeking alternative olefin/paraffin sep aration technologies due to its cheap cost, low energy consumption and easy operation [4,5]. The mechanisms of olefin/paraffin separation on porous materials can be roughly categorized into two types. The first is the kinetic separation, which makes use of differences in the diffusion rate between olefin and paraffin into the pore system. Here, olefins
generally exhibit faster diffusion rates than the corresponding paraffins due to their slightly smaller kinetic diameters. The second is π-complexation driven separation, which utilizes the interactions be tween transition metal ions, like Agþ and Cuþ, with unsaturated outer s – C double bond of olefins [6]. Recently, orbitals and the C– MOF-supported iron adsorbents have been widely studied in ole fin/paraffin separation, since unsaturated Fe ions can increase the af finity for olefins over paraffins [7–11]. However, in the case of aluminosilicate zeolites, the working capacity of π-complexation driven separation gradually declines with repeated recycling, owing to the strong polarity of the adsorbent which causes oligomerization [12]. There is keen interest in olefin/paraffin separation on small-pore, high- or pure-silica zeolites with different framework topologies, because their 8-ring pores are narrow enough to effectively exclude the paraffin component [13,14]. This is particularly true for separation of propylene and propane with kinetic diameters of 4.7 and 4.3–5.1 Å, respectively, that may be slightly larger than the effective 8-ring pore sizes of many small-pore zeolites [4,5]. The type of extra-framework cations in zeolites are reported to be an important factor for ole fin/paraffin separation, however, the self-diffusion coefficient rates of propylene and propane in LTA-type zeolites, Ca-A and NaCa-A with Si/Al ¼ 1.0, have revealed that both have higher diffusivities than cation-free ITQ-29 with Si/Al ¼ ∞. This result implies that the exchange of divalent cations, like Ca2þ, in small-pore zeolites may result in no
* Corresponding author. E-mail address: [email protected] (S.B. Hong). https://doi.org/10.1016/j.micromeso.2019.109833 Received 14 August 2019; Received in revised form 2 October 2019; Accepted 25 October 2019 1387-1811/© 2019 Elsevier Inc. All rights reserved.
Please cite this article as: Jung Gi Min, Microporous and Mesoporous Materials, https://doi.org/10.1016/j.micromeso.2019.109833
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Table 1 Representative LEV zeolite synthesis conditionsa and results. Run 1 2 3 4 5 6 7 8 9c 10 11 12c 13c 14c 15c
Productb
Gel composition Si/(Al þ Fe)
Fe/(Al þ Fe)
12 8 24 32 40 12 12 12 12 24 24 24 32 32 40
0 0 0 0 0 0.13 0.25 0.50 1.00 0.25 0.50 1.00 0.24 0.50 0.24
Al-LEV Al-LEV þ U Al-LEV Al-LEV Al-LEV þ U FeAl-LEV FeAl-LEV FeAl-LEV U þ Fe-LEV FeAl-LEV FeAl-LEV U þ FeAl-LEV FeAl-LEV FeAl-LEV U þ FeAl-LEV
a The oxide composition of the synthesis mixture is 20AA∙3.2Na2O∙xFe2O3∙ (2.5-x)Al2O3∙60SiO2∙1800H2O, where AA is 1-adamantylamine and x is varied between 0 � x � 2.5. The Si/Al ratio in the mixture was also varied between 8 � Si/Al � 40. b The phase appearing first is the major phase, and U indicates an unknown, but probably dense phase. c A small amount (2 wt% of the raw materials) of seed crystals was added.
Fig. 1. LEV structure and its two different composite building units.
significantly negative effects on propylene/propane separation [15,16]. Levyne (framework type LEV), a naturally occurring zeolite, consists of 17-hedral ([496583]) lev and 10-hedral [4682] double 6-ring (d6r) cages (Fig. 1). The lev cages in this cage-based small-pore zeolites are diagonally connected with one another by sharing a single 8-ring (3.6 � 4.8 Å) [17]. Thus, the two-dimensional, levyne has a large micropore volume and could be useful in the selective separation of propylene and propane. ZK-20, the first synthetic analog of levyne was described by Kerr and Trenton in 1969 [18]. While this Al-rich (Si/Al ¼ 2.0–5.5) zeolite was synthesized using 1-methyl-1-azonia-4-a zabicyclo [2,2,2]octane as an organic structure directing agent (SDA), high-silica (Si/Al > 10) versions of levyne has been subsequently re ported to crystallize in the presence of N-methylquinuclidinium or 1-adamantylamine as an organic SDA [19–21]. Levyne can also be synthesized as a borosilicate composition with milder acidity [22]. However, borosilicate levyne is not thermally stable enough to remain intact during the calcination for organic SDA removal. This led us to focus our attention on the isomorphous substitution of a portion of framework Al by less acidic Fe during the synthesis of levyne. Here we have synthesized a series of ferroaluminosilicate levyne (FeAl-LEV) zeolites with different Fe contents which can maintain the structural integrity during the calcination step. We also demonstrate that the presence of framework Fe atoms and extra-framework Ca2þ ions in levyne allows the resulting material (CaNH4-FeAl-LEV) to be more selective for propylene/propane separation at 298 K and 1.0 bar than any previously reported zeolite adsorbent.
seed crystals. After being mixed at room temperature for 1–3 days, the final gel was filled into Teflon-lined 23-ml autoclaves and then heated at 393–448 K under rotation (60 rpm) for 14–21 days. As-made LEV zeolites were calcined in air at 823 K for 8 h and then refluxed twice in 1.0 M NH4NO3 solutions (1 g solid per 100 mL) at 353 K for 6 h. The resulting NHþ 4 form was refluxed twice in 1.0 M NaNO3 and/or 1.0 M Ca(NO3)2 solutions at 353 K for 6 h, then filtered and dried in ambient conditions overnight. For comparison, three industrially relevant zeolite adsorbents Na-A (LTA; Si/Al ¼ 1.0), Ca-A (LTA; Si/Al ¼ 1.0) and Na-X (FAU; Si/Al ¼ 2.3) were purchased from Sigma-Aldrich. Also, the pure-silica small-pore zeolite ITQ-12 (ITW), which is known as one of the most selective adsorbents for propylene/ propane separation [23], was synthesized and calcined following the procedures in the literature [24]. 2.2. General characterization Powder X-ray diffraction (XRD), SEM, thermal and elemental ana lyses, and N2 sorption measurements were carried out as described in our recent work [25,26]. The 27Al MAS NMR spectra at a spinning rate of 10 kHz were recorded on a Bruker Advance II spectrometer at a27Al frequency of 130.318 MHz with a π/6 rad pulse length of 1.0 μs, a recycle delay of 0.1 s, and an acquisition of ca. 500 pulse transients. X-ray absorption near edge structure (XANES) spectra at the Fe K-edge were collected on the 8C beam line of the Pohang Accelerator Labora tory (PAL, Pohang, Korea) with a monochromator of Si(111) crystal. Energy calibration (E0 ¼ 7112.0 eV) was carried out with Fe foil, and Fe2O3 (99.8%, Aldrich) was utilized as a reference. A set of ultra violet–visible (UV–vis) spectra was collected on a Shimadzu UV-2600 spectrophotometer under diffuse reflectance mode with BaSO4 as a reference. NH3 temperature-programmed desorption (TPD) was measured on a fixed bed, flow-type equipment connected to a Hewlett-Packard 5890 series II gas chromatograph with a thermal conductivity detector, following the procedure given elsewhere [27]. The IR spectra of adsorbed NH3 on zeolites in the 1700 1400 cm 1 region were recorded on a Thermo-Nicolet 6700 FT-IR spectrometer equipped with an MCT detector and a DRIFT cell with ZnSe windows. All the spectra were collected by accumulating 64 scans at a resolution of 4 cm 1. Before IR measurements, a zeolite sample of 10 mg was loaded in a ceramic
2. Experimental section 2.1. Adsorbents synthesis The reagents used included 1-adamantylamine (AA, 97%, Aldrich), sodium hydroxide (NaOH, 50%, Aldrich), aluminium isopropoxide (Al [OCH(CH3)2)3, 98%, Aldrich), iron nitrate nonahydrate (Fe (NO3)3∙9H2O, 98%, Aldrich), fumed silica (SiO2, Aerosil 200, Degusa) and deionized water. The final composition of the synthesis gel can be expressed as 20AA∙3.2Na2O∙xFe2O3∙(2.5-x)Al2O3∙60SiO2∙1800H2O, where x is varied between 0 � x � 2.5. If required, the Si/Al ratio in the synthesis mixture was further varied between 8 and 40. If required, in addition, a small quantity of seed crystals (2 wt% of the raw materials) was added. Here as-made Al-LEV with Si/Al ¼ 12, which was previously synthesized without using seed crystals (run 1 in Table 1), was used as 2
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Table 2 The physical properties of LEV zeolites employed in propylene/propane separation. Adsorbent
Anhydrous unit cell compositiona
Si/Al (Fe/Al) ratio in the producta
Degree of exchangeb (%)
BET surface areac (m2 g 1) Microporous
External
Total
NaNH4-Al-LEV CaNH4-Al-LEV NaNH4-FeAl-LEV CaNH4-FeAl-LEV
Na3.1(NH4)1.2[Al4.3Si49.7O108] Ca1.0(NH4)2.4[Al4.3Si49.7O108] Na3.3(NH4)0.8[Al3.2Fe0.9Si49.9O108] Ca1.0(NH4)2.0[Al3.2Fe0.9Si49.9O108]
11.5 (0) 11.5 (0) 15.5 (0.27) 15.5 (0.27)
72 47 80 49
420 460 430 480
50 40 50 50
470 500 480 520
a b c
Determined by elemental analysis. The numbers in parentheses are the Fe/Al ratios. Defined as (the number of Naþor Caþions per unit cell)/(the sum of Al and Fe atoms per unit cell) � 100%. Determined from N2 adsorption at 77 K.
holder, heated at 498 K under He flow for 3 h, and then cooled to 423 K. A background spectrum was obtained under flowing He at the same temperature and subtracted from the KBr sample spectrum for each measurement. After exposing to 500 ppm NH3 in He flow at 423 K for 30 min, the sample was purged with He for 1 h to remove physisorbed NH3 and heating at different temperatures for 30 min. Finally, the dif ference DRIFT spectra of adsorbed NH3 were obtained. 2.3. Gas adsorption, regeneration and breakthrough experiments Propylene (C3H6, 99.95%) and propane (C3H8, 99.95%) isotherms were volumetrically measured at 298 K and a pressure range from 0 to 1.0 bar using a Setaram PCTPro E&E analyzer. Before the experiments, a sample of 0.1 g was packed in the stainless holder and evacuated under the vacuum to a residual pressure of 10 3 Torr at 498 K for 3 h. Each adsorption point was recorded for a maximum equilibration time of 30 min. The C3H6 and C3H8 adsorption kinetics were carried out at 298 K and a reservoir pressure of 0.1 or 1.2 bar. Adsorption-desorption cycles of C3H6 were repeated 50 times under vacuum-swing adsorption (VSA) mode at 298 K and 1.2 bar. During cycling, the adsorption and vacuum (desorption) time was set as 20 min. The breakthrough experiments were carried out at 298 K using a C3H6/C3H8 (50:50 v/v) gas mixture with a total gas flow rate of 10 cm3 min 1. Prior to the breakthrough experiments, a sample of 0.5 g was charged into a vertical-type fixed bed and then pretreated at 498 K for 6 h under Ar flow (50 cm3 min 1). Further experimental details can be found our recent papers [25,26].
Fig. 2. 27Al MAS NMR spectra of the as-made (top) and Ca2þ-exchanged forms (bottom) of (a) Al-LEV and (b) FeAl-LEV zeolites.
unlike the case of FeAl-LEV from run 8. Therefore, we selected the FeAlLEV and Al-LEV zeolites from run 7 and 1, respectively, as representative samples for the following investigation. Here we will refer to their Ca2þ cation-exchanged form as CaNH4-FeAl-LEV and CaNH4-Al-LEV, respec tively, because their Ca2þ exchange levels are only about 50% (Table 2). SEM images reveal that both of them typically appear as rhombic crystals of ca. 0.5 μm, and powder XRD patterns indicate that they maintain the structural integrity during the initial calcination and suc cessive ion-exchange steps (Fig. S1 in the Supporting Information). It is worth noting that some minor peaks related to the LEV structure, in the powder XRD patterns of the calcined and Ca2þ-exchanged forms, were not detectable, probably due to their low resolution. 27 Al MAS NMR spectra of the as-made and Ca2þ-exchanged forms of the FeAl-LEV and Al-LEV zeolites are shown in Fig. 2. The LEV structure has two distinct tetrahedral sites (T-sites, where T is Si or Al), i.e., T1 in d6rs and T2 in single 6-ring with a multiplicity ratio of 2:1, and as such, an NMR doublet could be expected if Al is incorporated in both sites [17]. Interestingly, the spectra of as-made zeolites show one broad resonance around 53 ppm assigned to tetrahedral Al, while their Ca2þ-exchanged forms showed two narrower peaks around 58 and 55 ppm. This implies that interactions in the as-made, between the occluded SDAs and framework Al, forms may be strong enough to cause severe line broadening of the second-order quadrupolar interaction. The average Al-O-Si angles, calculated using the method of Jacobsen et al. for the tetrahedral 27Al peaks in the spectra of as-made Al-LEV and FeAl-LEV were calculated to be 146.4 and 150.5� , respectively [28]. It has been previously shown that the average T1-O-T and T2-O-T angles for as-made levyne synthesized using N-methylquinuclidinium as an SDA are 148.5 and 155.5� , respectively [20], therefore, we believe that the two peaks around 58 and 55 ppm in Fig. 2 are attributable to Al atoms in the T1 and T2 sites of the LEV framework, respectively [17]. An ideal resonance intensity ratio of 2.0:1.0 would be expected for the statistical distribution of Al atoms over the T1 and T2 sites in the LEV structure [17]. As shown in Fig. 2, however, significantly smaller rela tive intensity ratios (1.1:1.0 and 1.0:1.0, respectively) of the two
3. Results and discussion Table 1 shows the representative synthesis results obtained using AA as an organic SDA and (ferro)aluminosilicate gels with different oxide compositions under the conditions described above. When the Na/Si ratio in the synthetic mixture was fixed to 0.11, the Si/Al ratio leading to pure Al-LEV was found to be narrow (12–32). When substituting Al by Fe in a sodium aluminosilicate gel with Na/Si ¼ 0.11 and Si/Al ¼ 12, in addition, we were able to synthesize FeAl-LEV only at Fe/(Al þ Fe) ra tios lower than 0.5. However, the formation of a dense impurity phase at Fe/(Al þ Fe) > 0.5, even in the presence of 2 wt% Al-LEV crystals as seeds, could not be avoided. A similar trend was observed for syntheses using ferroaluminosilicate gels with Si/(Al þ Fe) > 12, suggesting a cooperative structure-directing role of Al in the crystallization of Fesubstituted LEV zeolites in the presence of AA and Naþ as SDAs. Elemental analysis indicates that the FeAl-LEV zeolite obtained from run 8 in Table 1 has the highest Fe content among the zeolites synthe sized in this study. When converted to the Ca2þ form, however, this FeAl-LEV was characterized by a fairly lower Ca2þ exchange level (35 vs 49%) than that from run 7. Moreover, it has a considerably lower N2 BET surface area (350 vs 520 m2 g 1), which is also true when compared with the value (500 m2 g 1) of Al-LEV from run 1. This led us to conclude that the degree of removal of framework Fe atoms in the FeAl-LEV from run 7 during the initial calcination at 823 K to remove organic SDAs and 2þ the subsequent NHþ ion exchange steps may not be severe, 4 and Ca 3
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larger volume and lower oxidation state than Si4þ will more likely incorporate in the T2 site so as not to destabilize the LEV structure. Fig. 3 displays the Fe K-edge XANES spectra of as-made FeAl-LEV and CaNH4-FeAl-LEV. Both XANES spectra gave a sharp pre-edge peak at ca. 7114 eV corresponding to the 1s → 3d electronic transition, and no noticeable differences in the pre-edge peak intensity are observed. This sharp peak can be attributed to tetrahedrally coordinated Fe3þ ions in the zeolite framework, since the extra-framework Fe3þ ions, which adopt octahedral symmetry, show much weaker intensity due to the symmetrically forbidden A1g → T2g electronic transition [31]. Therefore, most, if not all, of the Fe3þ ions in CaNH4-FeAl-LEV, as well as those in its as-made form, appear to be located in tetrahedral framework posi tions. A similar conclusion can be obtained from the UV–vis data. As shown in Fig. 3, the UV–vis spectra of as-made FeAl-LEV and CaN H4-FeAl-LEV exhibit only one Fe3þ ← O charge-transfer band around 250 nm assignable to isolated framework Fe3þ ions [32]. Fig. 4 compares the propylene and propane adsorption isotherms on NaNH4-Al-LEV, CaNH4-Al-LEV, NaNH4-FeAl-LEV and CaNH4-FeAl-LEV at 298 K and 0–1.0 bar. The propylene uptakes of the Naþ-exchanged form of Al-LEV and FeAl-LEV zeolites at 298 K and 1.0 bar are lower than those of their Ca2þ-exchanged form. This is not unexpected because the number of extra-framework cations per unit cell is larger by ca. 1 in the former materials than in the latter materials (Table 2). An interesting observation is that while CaNH4-FeAl-LEV has a lower propylene uptake (2.2 vs 2.8 mmol g 1) at 298 K and 1.0 bar than CaNH4-Al-LEV, it ex hibits a significantly lower propane uptake (0.2 vs 0.6 mmol g 1). As a result, the CaNH4-FeAl-LEV shows a considerably higher propylene/ propane selectivity (11 vs 5) at 298 K and 1.0 bar than CaNH4-Al-LEV. It is also remarkable that this Fe-containing LEV zeolite is more selective for propylene/propane separation than any of Na-A, Ca-A, Na-X and
Fig. 3. Fe K-edge XANES spectra (left) and UV–vis spectra (right) of (a) asmade FeAl-LEV and (b) CaNH4-FeAl-LEV.
tetrahedral components were calculated from the deconvolution of the 27 Al MAS NMR spectra of CaNH4-Al-LEV and CaNH4-FeAl-LEV. This indicates a preferential substitution of Al into the low-multiplicity T2 site during crystallization, as has been observed for Al atom distribution in other zeolites with multiple T-sites such as ZSM-5 and beta [29,30]. Prior structural analysis has shown that three O-T-O bonds of the high-multiplicity site T1 in the LEV topology are constrained within 4-rings, while only two O-T-O bonds of T2 are [17]. As such, the strain on the T1 site is larger than that on T2 site, and for this reason, Al3þ with a
Fig. 4. Propylene (navy) and propane (green) adsorption isotherms on (a) NaNH4-Al-LEV, (b) CaNH4-Al-LEV, (c) NaNH4-FeAl-LEV and (d) CaNH4-FeAl-LEV at 298 K and 0–1 bar. Each point of the adsorption isotherms was measured for a maximum time of 30 min. Plotted lines are fitted using the dual-site Langmuir (DSL) model. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 4
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Fig. 5. Propylene (navy) and propane (green) adsorption kinetics on (a) NaNH4-Al-LEV, (b) CaNH4-Al-LEV, (c) NaNH4-FeAl-LEV and (d) CaNH4-FeAl-LEV at 298 K at reservoir pressures of 0.1 bar (left) and 1.2 bar (right), respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
ITQ-12 over the pressure range studied here (Figs. S2 and S3). The kinetics of propylene and propane adsorption is another crucial factor in the adsorbents evaluation for this industrially important sep aration. Thus, we measured their adsorption on the Naþ- and Ca2þexchanged forms of Al-LEV and FeAl-LEV as a function of time at 298 K by dosing 0.1 and/or 1.2 bar of propylene from a reservoir to each adsorbent Fig. 5. When the reservoir pressure is 0.1 bar, propylene adsorption on the Naþ form of both zeolites does not reach equilibrium even after 30 min. This suggests that the 8-ring windows in NaNH4-AlLEV and NaNH4-FeAl-LEV are not large enough to allow the smooth diffusion of propylene under the above conditions. As shown in Fig. 5, however, their Ca2þ-exchanged form is characterized by considerably
faster adsorption (equilibrating in 10 min), together with negligible propane uptakes (0.12 and 0.02 mmol g 1, respectively) even after 30 min. Therefore, it is most likely that the type of extra-framework cations in LEV-type zeolites dramatically influences the diffusion rate of propane, compared to prior propane and propylene diffusion exper iments using Ca- and CaNa-A [15], probably due to differences in the cation number per unit cell, location and distribution within these two small-pore zeolites, as well as to those in the shape and size of their 8-ring windows. Fig. 5 also shows that both CaNH4-Al-LEV and CaN H4-FeAl-LEV are still selective for propylene/propane separation even at a reservoir pressure of 1.2 bar, although the adsorption kinetics of pro pylene at this pressure is slower due to its higher oligomerization rate at 5
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Fig. 6. NH3-TPD profiles for (a) CaNH4-Al-LEV and (b) CaNH4-FeAl-LEV.
Fig. 8. Propylene (navy) and propane (green) breakthrough curves on (a) CaNH4-FeAl-LEV and (b) Na-A using a C3H6/C3H8 (50:50 v/v) gas mixture with a total gas flow rate of 10 cm3 min 1 at 298 K and atmospheric pressure. Inset, zoom of the breakthrough for the first 50 s. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
consequence, CaNH4-FeAl-LEV has a higher propylene uptake than that of CaNH4-Al-LEV (ca. 1.0 vs 0.7 mmol g 1), mainly due to differences in the acidity. This can be in line with the fact that the amount (11.0 vs 8.5 wt%) of organic species irreversibly adsorbed during recycling is lower in the former than in the latter (Fig. S5). We also note that the propylene uptake of CaNH4-Al-LEV after 50 adsorption-desorption cy cles under VSA mode is lower than that of Ca-A, Na-X and ITQ-12, but similar to that of Na-A (Fig. S6). The same conclusion was obtained when compared with the regeneration behaviour of AgNH4-FeAl-LEV with Agþ ion exchange level of ca. 50% (Fig. S7). Finally, we examined the dynamic gas separation properties of CaNH4-Al-LEV by breakthrough curve experiments at 298 K and 1.0 bar using an equimolar propylene/propane (50:50 v/v) gas mixture. As shown in Fig. 8, the composition of outlet gas passing through this Fecontaining zeolite consists of pure propane (ca. 30 s) before the break through of propylene. It should be noted that this retention time is two times longer than that (ca. 15 s) of Na-A, which is considerably more selective for this separation than Ca-A, Na-X, or ITQ-12 (Figs. S2 and S3). Therefore, tailoring the composition of zeolite adsorbents might be applicable in the selective separation of propylene and propane, despite the drawback that the propylene oligomerization over zeolite acid sites at the beginning of the process somewhat decreases the maximum adsorption capacity.
Fig. 7. 50 adsorption-desorption cycles under vacuum-swing adsorption (VSA) mode on CaNH4-Al-LEV (navy) and CaNH4-FeAl-LEV (orange) at 298 K and reservoir pressure 1.2 bar. Each adsorption and vacuum (desorption) step was set as 20 min. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
higher pressure. Fig. 6 shows the NH3 TPD profiles on CaNH4-Al-LEV and CaNH4FeAl-LEV. Both profiles exhibit low- and high-temperature desorption peaks, which can be assigned to NH3 desorption from weak and strong acid sites, respectively. As expected from the presence of a portion of framework Fe, less acidic than Al, in CaNH4-FeAl-LEV (Table 2), the temperature maxima (ca. 530 and 720 K, respectively) of both desorp tion peaks from this zeolite is lower by 30 K than those (ca. 560 and 750 K, respectively) of peaks from CaNH4-Al-LEV. The difference DRIFT spectra of these two zeolites after NH3 adsorption at 423 K followed by desorption at temperatures up to 673 K can be found in Fig. S4. When both CaNH4-Al-LEV and CaNH4-FeAl-LEV were exposed to NH3 at 423 K, a new broad band appears around 1460 cm 1, which can be assigned to NH3 adsorption on zeolite Brønsted acid sites [33]. The fact that the rate of the disappearance of this band is faster for CaNH4-FeAl-LEV than that for CaNH4-Al-LEV confirms the weaker acidity of the former. Fig. 7 shows 50 propylene adsorption-desorption cycles under vacuum-swing adsorption (VSA) mode of CaNH4-Al-LEV and CaNH4FeAl-LEV at 298 K at a reservoir pressure 1.2 bar. Each adsorption and vacuum step was set as 20 min. These data reveal that although CaNH4Al-LEV exhibits a higher propylene uptake than CaNH4-FeAl-LEV at the first cycle, a decrease (59 vs 37%) in maximum uptake of propylene on the former is fairly larger than that on the latter. Also, no further decrease in propylene uptake is detectable by the end of recycling. In
4. Conclusions A series of (ferro)aluminosilicate levyne zeolites with different framework Si/Al and Fe/Al ratios have been synthesized using 1-ada mantylamine and Naþ as structure-directing agents in an attempt to test them as adsorbents in propylene/propane separation. The CaNH4 6
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form (CaNH4-FeAl-LEV) of a levyne (Si/Al ¼ 15.5 and Fe/Al ¼ 0.27) was found to show a propylene/propane selectivity of 11 at 298 K and 1.0 bar, the highest selectivity value among the zeolite adsorbents tested for this separation thus far, as well as fast adsorption kinetics. More importantly, the adsorption-desorption recycling experiments under VSA mode reveal that CaNH4-FeAl-LEV has a higher propylene uptake (ca. 1.0 vs 0.7 mmol g 1) than its aluminosilicate version, due to the presence of less acidic Fe than Al in the zeolite framework, allowing this partially iron-substituted zeolite to be potentially useful for selective propylene/propane separation. We hope that our findings will renew interest in the yet largely unexplored zeolites with different structure types and compositions that are applicable to other small olefin/paraffin separations.
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
Declaration of competing interest
[15]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[16] [17] [18] [19] [20] [21] [22] [23] [24]
Acknowledgments This work was supported by the National Creative Research Initiative Program (2012R1A3A-2048833) through the National Research Foun dation of Korea.
[25] [26]
Appendix A. Supplementary data
[27] [28] [29] [30]
Supplementary data to this article can be found online at https://doi. org/10.1016/j.micromeso.2019.109833. References
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[1] W.J. Koros, R. Mahajan, J. Membr. Sci. 175 (2000) 181–196. [2] R. Faiz, K. Li, Chem. Eng. Sci. 73 (2012) 261–284.
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Propylene/propane separation on a ferroaluminosilicate levyne zeolite Jung Gi Min, K. Christian Kemp, Suk Bong Hong * Center for Ordered Nanoporous Materials Synthesis, Division of Environmental Science and Engineering, POSTECH, Pohang, 37673, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Propylene/propane separation Levyne zeolite Partial iron substitution Propylene selectivity Durability
The propylene/propane separation properties of levyne zeolites with different framework Si/Al and Fe/Al ratios have been examined and compared with those of the four well-studied zeolite adsorbents (i.e., Na-A, Ca-A, Na-X and ITQ-12) for this separation. Both zeolite framework structure and composition were found to be critical for the propylene uptake, kinetic separation and adsorbent durability. Among the zeolites studied here, the mixed CaNH4 form of a ferroaluminosilcate levyne (Si/Al ¼ 15.5 and Fe/Al ¼ 0.27) showed the highest propylene/ propane selectivity (11). Under vacuum-swing adsorption mode at 298 K and reservoir pressure of 1.2 bar, this partially iron-substituted zeolite was characterized by a higher propylene uptake (ca. 1.0 vs 0.7 mmol g 1) than its aluminosilicate version, which can be attributed to a combination of the molecular sieve effect of its framework topology and relatively weaker acidity. The results of this study demonstrate that, in contrast to widely held belief, zeolite adsorbents selective for propylene/propane separation do not need to be a pure-silica composition.
1. Introduction Propylene is the most important olefin in petrochemical production, except ethylene, because it is an essential building block for manufacturing polypropylene [1]. At present, the most commonly applied industrial processes for producing olefin are catalytic cracking, steam cracking or catalytic dehydrogenation of paraffins. Among these processes, the cracking of C4 hydrocarbon fractions followed by dehy drogenation is most widely utilized for propylene production [2]. However, during the reactions, paraffin with the same carbon number as that of propylene, i.e., propane, is always produced, and so the gas mixture separation is required. Traditionally, propylene/propane sepa ration has relied on the cryogenic distillation and condensation pro cesses that are energy intensive due to the similarity in many physical properties of these two hydrocarbons, e.g., their boiling points, molec ular weights, polarizabilities and kinetic diameters [3]. Adsorptive separation using ordered nanoporous materials such as zeolites and metal-organic frameworks (MOFs) has been considered as the most promising process for seeking alternative olefin/paraffin sep aration technologies due to its cheap cost, low energy consumption and easy operation [4,5]. The mechanisms of olefin/paraffin separation on porous materials can be roughly categorized into two types. The first is the kinetic separation, which makes use of differences in the diffusion rate between olefin and paraffin into the pore system. Here, olefins
generally exhibit faster diffusion rates than the corresponding paraffins due to their slightly smaller kinetic diameters. The second is π-complexation driven separation, which utilizes the interactions be tween transition metal ions, like Agþ and Cuþ, with unsaturated outer s – C double bond of olefins [6]. Recently, orbitals and the C– MOF-supported iron adsorbents have been widely studied in ole fin/paraffin separation, since unsaturated Fe ions can increase the af finity for olefins over paraffins [7–11]. However, in the case of aluminosilicate zeolites, the working capacity of π-complexation driven separation gradually declines with repeated recycling, owing to the strong polarity of the adsorbent which causes oligomerization [12]. There is keen interest in olefin/paraffin separation on small-pore, high- or pure-silica zeolites with different framework topologies, because their 8-ring pores are narrow enough to effectively exclude the paraffin component [13,14]. This is particularly true for separation of propylene and propane with kinetic diameters of 4.7 and 4.3–5.1 Å, respectively, that may be slightly larger than the effective 8-ring pore sizes of many small-pore zeolites [4,5]. The type of extra-framework cations in zeolites are reported to be an important factor for ole fin/paraffin separation, however, the self-diffusion coefficient rates of propylene and propane in LTA-type zeolites, Ca-A and NaCa-A with Si/Al ¼ 1.0, have revealed that both have higher diffusivities than cation-free ITQ-29 with Si/Al ¼ ∞. This result implies that the exchange of divalent cations, like Ca2þ, in small-pore zeolites may result in no
* Corresponding author. E-mail address: [email protected] (S.B. Hong). https://doi.org/10.1016/j.micromeso.2019.109833 Received 14 August 2019; Received in revised form 2 October 2019; Accepted 25 October 2019 1387-1811/© 2019 Elsevier Inc. All rights reserved.
Please cite this article as: Jung Gi Min, Microporous and Mesoporous Materials, https://doi.org/10.1016/j.micromeso.2019.109833
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Table 1 Representative LEV zeolite synthesis conditionsa and results. Run 1 2 3 4 5 6 7 8 9c 10 11 12c 13c 14c 15c
Productb
Gel composition Si/(Al þ Fe)
Fe/(Al þ Fe)
12 8 24 32 40 12 12 12 12 24 24 24 32 32 40
0 0 0 0 0 0.13 0.25 0.50 1.00 0.25 0.50 1.00 0.24 0.50 0.24
Al-LEV Al-LEV þ U Al-LEV Al-LEV Al-LEV þ U FeAl-LEV FeAl-LEV FeAl-LEV U þ Fe-LEV FeAl-LEV FeAl-LEV U þ FeAl-LEV FeAl-LEV FeAl-LEV U þ FeAl-LEV
a The oxide composition of the synthesis mixture is 20AA∙3.2Na2O∙xFe2O3∙ (2.5-x)Al2O3∙60SiO2∙1800H2O, where AA is 1-adamantylamine and x is varied between 0 � x � 2.5. The Si/Al ratio in the mixture was also varied between 8 � Si/Al � 40. b The phase appearing first is the major phase, and U indicates an unknown, but probably dense phase. c A small amount (2 wt% of the raw materials) of seed crystals was added.
Fig. 1. LEV structure and its two different composite building units.
significantly negative effects on propylene/propane separation [15,16]. Levyne (framework type LEV), a naturally occurring zeolite, consists of 17-hedral ([496583]) lev and 10-hedral [4682] double 6-ring (d6r) cages (Fig. 1). The lev cages in this cage-based small-pore zeolites are diagonally connected with one another by sharing a single 8-ring (3.6 � 4.8 Å) [17]. Thus, the two-dimensional, levyne has a large micropore volume and could be useful in the selective separation of propylene and propane. ZK-20, the first synthetic analog of levyne was described by Kerr and Trenton in 1969 [18]. While this Al-rich (Si/Al ¼ 2.0–5.5) zeolite was synthesized using 1-methyl-1-azonia-4-a zabicyclo [2,2,2]octane as an organic structure directing agent (SDA), high-silica (Si/Al > 10) versions of levyne has been subsequently re ported to crystallize in the presence of N-methylquinuclidinium or 1-adamantylamine as an organic SDA [19–21]. Levyne can also be synthesized as a borosilicate composition with milder acidity [22]. However, borosilicate levyne is not thermally stable enough to remain intact during the calcination for organic SDA removal. This led us to focus our attention on the isomorphous substitution of a portion of framework Al by less acidic Fe during the synthesis of levyne. Here we have synthesized a series of ferroaluminosilicate levyne (FeAl-LEV) zeolites with different Fe contents which can maintain the structural integrity during the calcination step. We also demonstrate that the presence of framework Fe atoms and extra-framework Ca2þ ions in levyne allows the resulting material (CaNH4-FeAl-LEV) to be more selective for propylene/propane separation at 298 K and 1.0 bar than any previously reported zeolite adsorbent.
seed crystals. After being mixed at room temperature for 1–3 days, the final gel was filled into Teflon-lined 23-ml autoclaves and then heated at 393–448 K under rotation (60 rpm) for 14–21 days. As-made LEV zeolites were calcined in air at 823 K for 8 h and then refluxed twice in 1.0 M NH4NO3 solutions (1 g solid per 100 mL) at 353 K for 6 h. The resulting NHþ 4 form was refluxed twice in 1.0 M NaNO3 and/or 1.0 M Ca(NO3)2 solutions at 353 K for 6 h, then filtered and dried in ambient conditions overnight. For comparison, three industrially relevant zeolite adsorbents Na-A (LTA; Si/Al ¼ 1.0), Ca-A (LTA; Si/Al ¼ 1.0) and Na-X (FAU; Si/Al ¼ 2.3) were purchased from Sigma-Aldrich. Also, the pure-silica small-pore zeolite ITQ-12 (ITW), which is known as one of the most selective adsorbents for propylene/ propane separation [23], was synthesized and calcined following the procedures in the literature [24]. 2.2. General characterization Powder X-ray diffraction (XRD), SEM, thermal and elemental ana lyses, and N2 sorption measurements were carried out as described in our recent work [25,26]. The 27Al MAS NMR spectra at a spinning rate of 10 kHz were recorded on a Bruker Advance II spectrometer at a27Al frequency of 130.318 MHz with a π/6 rad pulse length of 1.0 μs, a recycle delay of 0.1 s, and an acquisition of ca. 500 pulse transients. X-ray absorption near edge structure (XANES) spectra at the Fe K-edge were collected on the 8C beam line of the Pohang Accelerator Labora tory (PAL, Pohang, Korea) with a monochromator of Si(111) crystal. Energy calibration (E0 ¼ 7112.0 eV) was carried out with Fe foil, and Fe2O3 (99.8%, Aldrich) was utilized as a reference. A set of ultra violet–visible (UV–vis) spectra was collected on a Shimadzu UV-2600 spectrophotometer under diffuse reflectance mode with BaSO4 as a reference. NH3 temperature-programmed desorption (TPD) was measured on a fixed bed, flow-type equipment connected to a Hewlett-Packard 5890 series II gas chromatograph with a thermal conductivity detector, following the procedure given elsewhere [27]. The IR spectra of adsorbed NH3 on zeolites in the 1700 1400 cm 1 region were recorded on a Thermo-Nicolet 6700 FT-IR spectrometer equipped with an MCT detector and a DRIFT cell with ZnSe windows. All the spectra were collected by accumulating 64 scans at a resolution of 4 cm 1. Before IR measurements, a zeolite sample of 10 mg was loaded in a ceramic
2. Experimental section 2.1. Adsorbents synthesis The reagents used included 1-adamantylamine (AA, 97%, Aldrich), sodium hydroxide (NaOH, 50%, Aldrich), aluminium isopropoxide (Al [OCH(CH3)2)3, 98%, Aldrich), iron nitrate nonahydrate (Fe (NO3)3∙9H2O, 98%, Aldrich), fumed silica (SiO2, Aerosil 200, Degusa) and deionized water. The final composition of the synthesis gel can be expressed as 20AA∙3.2Na2O∙xFe2O3∙(2.5-x)Al2O3∙60SiO2∙1800H2O, where x is varied between 0 � x � 2.5. If required, the Si/Al ratio in the synthesis mixture was further varied between 8 and 40. If required, in addition, a small quantity of seed crystals (2 wt% of the raw materials) was added. Here as-made Al-LEV with Si/Al ¼ 12, which was previously synthesized without using seed crystals (run 1 in Table 1), was used as 2
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Table 2 The physical properties of LEV zeolites employed in propylene/propane separation. Adsorbent
Anhydrous unit cell compositiona
Si/Al (Fe/Al) ratio in the producta
Degree of exchangeb (%)
BET surface areac (m2 g 1) Microporous
External
Total
NaNH4-Al-LEV CaNH4-Al-LEV NaNH4-FeAl-LEV CaNH4-FeAl-LEV
Na3.1(NH4)1.2[Al4.3Si49.7O108] Ca1.0(NH4)2.4[Al4.3Si49.7O108] Na3.3(NH4)0.8[Al3.2Fe0.9Si49.9O108] Ca1.0(NH4)2.0[Al3.2Fe0.9Si49.9O108]
11.5 (0) 11.5 (0) 15.5 (0.27) 15.5 (0.27)
72 47 80 49
420 460 430 480
50 40 50 50
470 500 480 520
a b c
Determined by elemental analysis. The numbers in parentheses are the Fe/Al ratios. Defined as (the number of Naþor Caþions per unit cell)/(the sum of Al and Fe atoms per unit cell) � 100%. Determined from N2 adsorption at 77 K.
holder, heated at 498 K under He flow for 3 h, and then cooled to 423 K. A background spectrum was obtained under flowing He at the same temperature and subtracted from the KBr sample spectrum for each measurement. After exposing to 500 ppm NH3 in He flow at 423 K for 30 min, the sample was purged with He for 1 h to remove physisorbed NH3 and heating at different temperatures for 30 min. Finally, the dif ference DRIFT spectra of adsorbed NH3 were obtained. 2.3. Gas adsorption, regeneration and breakthrough experiments Propylene (C3H6, 99.95%) and propane (C3H8, 99.95%) isotherms were volumetrically measured at 298 K and a pressure range from 0 to 1.0 bar using a Setaram PCTPro E&E analyzer. Before the experiments, a sample of 0.1 g was packed in the stainless holder and evacuated under the vacuum to a residual pressure of 10 3 Torr at 498 K for 3 h. Each adsorption point was recorded for a maximum equilibration time of 30 min. The C3H6 and C3H8 adsorption kinetics were carried out at 298 K and a reservoir pressure of 0.1 or 1.2 bar. Adsorption-desorption cycles of C3H6 were repeated 50 times under vacuum-swing adsorption (VSA) mode at 298 K and 1.2 bar. During cycling, the adsorption and vacuum (desorption) time was set as 20 min. The breakthrough experiments were carried out at 298 K using a C3H6/C3H8 (50:50 v/v) gas mixture with a total gas flow rate of 10 cm3 min 1. Prior to the breakthrough experiments, a sample of 0.5 g was charged into a vertical-type fixed bed and then pretreated at 498 K for 6 h under Ar flow (50 cm3 min 1). Further experimental details can be found our recent papers [25,26].
Fig. 2. 27Al MAS NMR spectra of the as-made (top) and Ca2þ-exchanged forms (bottom) of (a) Al-LEV and (b) FeAl-LEV zeolites.
unlike the case of FeAl-LEV from run 8. Therefore, we selected the FeAlLEV and Al-LEV zeolites from run 7 and 1, respectively, as representative samples for the following investigation. Here we will refer to their Ca2þ cation-exchanged form as CaNH4-FeAl-LEV and CaNH4-Al-LEV, respec tively, because their Ca2þ exchange levels are only about 50% (Table 2). SEM images reveal that both of them typically appear as rhombic crystals of ca. 0.5 μm, and powder XRD patterns indicate that they maintain the structural integrity during the initial calcination and suc cessive ion-exchange steps (Fig. S1 in the Supporting Information). It is worth noting that some minor peaks related to the LEV structure, in the powder XRD patterns of the calcined and Ca2þ-exchanged forms, were not detectable, probably due to their low resolution. 27 Al MAS NMR spectra of the as-made and Ca2þ-exchanged forms of the FeAl-LEV and Al-LEV zeolites are shown in Fig. 2. The LEV structure has two distinct tetrahedral sites (T-sites, where T is Si or Al), i.e., T1 in d6rs and T2 in single 6-ring with a multiplicity ratio of 2:1, and as such, an NMR doublet could be expected if Al is incorporated in both sites [17]. Interestingly, the spectra of as-made zeolites show one broad resonance around 53 ppm assigned to tetrahedral Al, while their Ca2þ-exchanged forms showed two narrower peaks around 58 and 55 ppm. This implies that interactions in the as-made, between the occluded SDAs and framework Al, forms may be strong enough to cause severe line broadening of the second-order quadrupolar interaction. The average Al-O-Si angles, calculated using the method of Jacobsen et al. for the tetrahedral 27Al peaks in the spectra of as-made Al-LEV and FeAl-LEV were calculated to be 146.4 and 150.5� , respectively [28]. It has been previously shown that the average T1-O-T and T2-O-T angles for as-made levyne synthesized using N-methylquinuclidinium as an SDA are 148.5 and 155.5� , respectively [20], therefore, we believe that the two peaks around 58 and 55 ppm in Fig. 2 are attributable to Al atoms in the T1 and T2 sites of the LEV framework, respectively [17]. An ideal resonance intensity ratio of 2.0:1.0 would be expected for the statistical distribution of Al atoms over the T1 and T2 sites in the LEV structure [17]. As shown in Fig. 2, however, significantly smaller rela tive intensity ratios (1.1:1.0 and 1.0:1.0, respectively) of the two
3. Results and discussion Table 1 shows the representative synthesis results obtained using AA as an organic SDA and (ferro)aluminosilicate gels with different oxide compositions under the conditions described above. When the Na/Si ratio in the synthetic mixture was fixed to 0.11, the Si/Al ratio leading to pure Al-LEV was found to be narrow (12–32). When substituting Al by Fe in a sodium aluminosilicate gel with Na/Si ¼ 0.11 and Si/Al ¼ 12, in addition, we were able to synthesize FeAl-LEV only at Fe/(Al þ Fe) ra tios lower than 0.5. However, the formation of a dense impurity phase at Fe/(Al þ Fe) > 0.5, even in the presence of 2 wt% Al-LEV crystals as seeds, could not be avoided. A similar trend was observed for syntheses using ferroaluminosilicate gels with Si/(Al þ Fe) > 12, suggesting a cooperative structure-directing role of Al in the crystallization of Fesubstituted LEV zeolites in the presence of AA and Naþ as SDAs. Elemental analysis indicates that the FeAl-LEV zeolite obtained from run 8 in Table 1 has the highest Fe content among the zeolites synthe sized in this study. When converted to the Ca2þ form, however, this FeAl-LEV was characterized by a fairly lower Ca2þ exchange level (35 vs 49%) than that from run 7. Moreover, it has a considerably lower N2 BET surface area (350 vs 520 m2 g 1), which is also true when compared with the value (500 m2 g 1) of Al-LEV from run 1. This led us to conclude that the degree of removal of framework Fe atoms in the FeAl-LEV from run 7 during the initial calcination at 823 K to remove organic SDAs and 2þ the subsequent NHþ ion exchange steps may not be severe, 4 and Ca 3
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larger volume and lower oxidation state than Si4þ will more likely incorporate in the T2 site so as not to destabilize the LEV structure. Fig. 3 displays the Fe K-edge XANES spectra of as-made FeAl-LEV and CaNH4-FeAl-LEV. Both XANES spectra gave a sharp pre-edge peak at ca. 7114 eV corresponding to the 1s → 3d electronic transition, and no noticeable differences in the pre-edge peak intensity are observed. This sharp peak can be attributed to tetrahedrally coordinated Fe3þ ions in the zeolite framework, since the extra-framework Fe3þ ions, which adopt octahedral symmetry, show much weaker intensity due to the symmetrically forbidden A1g → T2g electronic transition [31]. Therefore, most, if not all, of the Fe3þ ions in CaNH4-FeAl-LEV, as well as those in its as-made form, appear to be located in tetrahedral framework posi tions. A similar conclusion can be obtained from the UV–vis data. As shown in Fig. 3, the UV–vis spectra of as-made FeAl-LEV and CaN H4-FeAl-LEV exhibit only one Fe3þ ← O charge-transfer band around 250 nm assignable to isolated framework Fe3þ ions [32]. Fig. 4 compares the propylene and propane adsorption isotherms on NaNH4-Al-LEV, CaNH4-Al-LEV, NaNH4-FeAl-LEV and CaNH4-FeAl-LEV at 298 K and 0–1.0 bar. The propylene uptakes of the Naþ-exchanged form of Al-LEV and FeAl-LEV zeolites at 298 K and 1.0 bar are lower than those of their Ca2þ-exchanged form. This is not unexpected because the number of extra-framework cations per unit cell is larger by ca. 1 in the former materials than in the latter materials (Table 2). An interesting observation is that while CaNH4-FeAl-LEV has a lower propylene uptake (2.2 vs 2.8 mmol g 1) at 298 K and 1.0 bar than CaNH4-Al-LEV, it ex hibits a significantly lower propane uptake (0.2 vs 0.6 mmol g 1). As a result, the CaNH4-FeAl-LEV shows a considerably higher propylene/ propane selectivity (11 vs 5) at 298 K and 1.0 bar than CaNH4-Al-LEV. It is also remarkable that this Fe-containing LEV zeolite is more selective for propylene/propane separation than any of Na-A, Ca-A, Na-X and
Fig. 3. Fe K-edge XANES spectra (left) and UV–vis spectra (right) of (a) asmade FeAl-LEV and (b) CaNH4-FeAl-LEV.
tetrahedral components were calculated from the deconvolution of the 27 Al MAS NMR spectra of CaNH4-Al-LEV and CaNH4-FeAl-LEV. This indicates a preferential substitution of Al into the low-multiplicity T2 site during crystallization, as has been observed for Al atom distribution in other zeolites with multiple T-sites such as ZSM-5 and beta [29,30]. Prior structural analysis has shown that three O-T-O bonds of the high-multiplicity site T1 in the LEV topology are constrained within 4-rings, while only two O-T-O bonds of T2 are [17]. As such, the strain on the T1 site is larger than that on T2 site, and for this reason, Al3þ with a
Fig. 4. Propylene (navy) and propane (green) adsorption isotherms on (a) NaNH4-Al-LEV, (b) CaNH4-Al-LEV, (c) NaNH4-FeAl-LEV and (d) CaNH4-FeAl-LEV at 298 K and 0–1 bar. Each point of the adsorption isotherms was measured for a maximum time of 30 min. Plotted lines are fitted using the dual-site Langmuir (DSL) model. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 4
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Fig. 5. Propylene (navy) and propane (green) adsorption kinetics on (a) NaNH4-Al-LEV, (b) CaNH4-Al-LEV, (c) NaNH4-FeAl-LEV and (d) CaNH4-FeAl-LEV at 298 K at reservoir pressures of 0.1 bar (left) and 1.2 bar (right), respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
ITQ-12 over the pressure range studied here (Figs. S2 and S3). The kinetics of propylene and propane adsorption is another crucial factor in the adsorbents evaluation for this industrially important sep aration. Thus, we measured their adsorption on the Naþ- and Ca2þexchanged forms of Al-LEV and FeAl-LEV as a function of time at 298 K by dosing 0.1 and/or 1.2 bar of propylene from a reservoir to each adsorbent Fig. 5. When the reservoir pressure is 0.1 bar, propylene adsorption on the Naþ form of both zeolites does not reach equilibrium even after 30 min. This suggests that the 8-ring windows in NaNH4-AlLEV and NaNH4-FeAl-LEV are not large enough to allow the smooth diffusion of propylene under the above conditions. As shown in Fig. 5, however, their Ca2þ-exchanged form is characterized by considerably
faster adsorption (equilibrating in 10 min), together with negligible propane uptakes (0.12 and 0.02 mmol g 1, respectively) even after 30 min. Therefore, it is most likely that the type of extra-framework cations in LEV-type zeolites dramatically influences the diffusion rate of propane, compared to prior propane and propylene diffusion exper iments using Ca- and CaNa-A [15], probably due to differences in the cation number per unit cell, location and distribution within these two small-pore zeolites, as well as to those in the shape and size of their 8-ring windows. Fig. 5 also shows that both CaNH4-Al-LEV and CaN H4-FeAl-LEV are still selective for propylene/propane separation even at a reservoir pressure of 1.2 bar, although the adsorption kinetics of pro pylene at this pressure is slower due to its higher oligomerization rate at 5
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Fig. 6. NH3-TPD profiles for (a) CaNH4-Al-LEV and (b) CaNH4-FeAl-LEV.
Fig. 8. Propylene (navy) and propane (green) breakthrough curves on (a) CaNH4-FeAl-LEV and (b) Na-A using a C3H6/C3H8 (50:50 v/v) gas mixture with a total gas flow rate of 10 cm3 min 1 at 298 K and atmospheric pressure. Inset, zoom of the breakthrough for the first 50 s. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
consequence, CaNH4-FeAl-LEV has a higher propylene uptake than that of CaNH4-Al-LEV (ca. 1.0 vs 0.7 mmol g 1), mainly due to differences in the acidity. This can be in line with the fact that the amount (11.0 vs 8.5 wt%) of organic species irreversibly adsorbed during recycling is lower in the former than in the latter (Fig. S5). We also note that the propylene uptake of CaNH4-Al-LEV after 50 adsorption-desorption cy cles under VSA mode is lower than that of Ca-A, Na-X and ITQ-12, but similar to that of Na-A (Fig. S6). The same conclusion was obtained when compared with the regeneration behaviour of AgNH4-FeAl-LEV with Agþ ion exchange level of ca. 50% (Fig. S7). Finally, we examined the dynamic gas separation properties of CaNH4-Al-LEV by breakthrough curve experiments at 298 K and 1.0 bar using an equimolar propylene/propane (50:50 v/v) gas mixture. As shown in Fig. 8, the composition of outlet gas passing through this Fecontaining zeolite consists of pure propane (ca. 30 s) before the break through of propylene. It should be noted that this retention time is two times longer than that (ca. 15 s) of Na-A, which is considerably more selective for this separation than Ca-A, Na-X, or ITQ-12 (Figs. S2 and S3). Therefore, tailoring the composition of zeolite adsorbents might be applicable in the selective separation of propylene and propane, despite the drawback that the propylene oligomerization over zeolite acid sites at the beginning of the process somewhat decreases the maximum adsorption capacity.
Fig. 7. 50 adsorption-desorption cycles under vacuum-swing adsorption (VSA) mode on CaNH4-Al-LEV (navy) and CaNH4-FeAl-LEV (orange) at 298 K and reservoir pressure 1.2 bar. Each adsorption and vacuum (desorption) step was set as 20 min. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
higher pressure. Fig. 6 shows the NH3 TPD profiles on CaNH4-Al-LEV and CaNH4FeAl-LEV. Both profiles exhibit low- and high-temperature desorption peaks, which can be assigned to NH3 desorption from weak and strong acid sites, respectively. As expected from the presence of a portion of framework Fe, less acidic than Al, in CaNH4-FeAl-LEV (Table 2), the temperature maxima (ca. 530 and 720 K, respectively) of both desorp tion peaks from this zeolite is lower by 30 K than those (ca. 560 and 750 K, respectively) of peaks from CaNH4-Al-LEV. The difference DRIFT spectra of these two zeolites after NH3 adsorption at 423 K followed by desorption at temperatures up to 673 K can be found in Fig. S4. When both CaNH4-Al-LEV and CaNH4-FeAl-LEV were exposed to NH3 at 423 K, a new broad band appears around 1460 cm 1, which can be assigned to NH3 adsorption on zeolite Brønsted acid sites [33]. The fact that the rate of the disappearance of this band is faster for CaNH4-FeAl-LEV than that for CaNH4-Al-LEV confirms the weaker acidity of the former. Fig. 7 shows 50 propylene adsorption-desorption cycles under vacuum-swing adsorption (VSA) mode of CaNH4-Al-LEV and CaNH4FeAl-LEV at 298 K at a reservoir pressure 1.2 bar. Each adsorption and vacuum step was set as 20 min. These data reveal that although CaNH4Al-LEV exhibits a higher propylene uptake than CaNH4-FeAl-LEV at the first cycle, a decrease (59 vs 37%) in maximum uptake of propylene on the former is fairly larger than that on the latter. Also, no further decrease in propylene uptake is detectable by the end of recycling. In
4. Conclusions A series of (ferro)aluminosilicate levyne zeolites with different framework Si/Al and Fe/Al ratios have been synthesized using 1-ada mantylamine and Naþ as structure-directing agents in an attempt to test them as adsorbents in propylene/propane separation. The CaNH4 6
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form (CaNH4-FeAl-LEV) of a levyne (Si/Al ¼ 15.5 and Fe/Al ¼ 0.27) was found to show a propylene/propane selectivity of 11 at 298 K and 1.0 bar, the highest selectivity value among the zeolite adsorbents tested for this separation thus far, as well as fast adsorption kinetics. More importantly, the adsorption-desorption recycling experiments under VSA mode reveal that CaNH4-FeAl-LEV has a higher propylene uptake (ca. 1.0 vs 0.7 mmol g 1) than its aluminosilicate version, due to the presence of less acidic Fe than Al in the zeolite framework, allowing this partially iron-substituted zeolite to be potentially useful for selective propylene/propane separation. We hope that our findings will renew interest in the yet largely unexplored zeolites with different structure types and compositions that are applicable to other small olefin/paraffin separations.
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
Declaration of competing interest
[15]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[16] [17] [18] [19] [20] [21] [22] [23] [24]
Acknowledgments This work was supported by the National Creative Research Initiative Program (2012R1A3A-2048833) through the National Research Foun dation of Korea.
[25] [26]
Appendix A. Supplementary data
[27] [28] [29] [30]
Supplementary data to this article can be found online at https://doi. org/10.1016/j.micromeso.2019.109833. References
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