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Article
Thin Hydrogen-Selective SAPO-34 Zeolite Membranes for Enhanced Conversion and Selectivity in Propane Dehydrogenation Membrane Reactor Seok-Jhin Kim, Yujun Liu, Jason S. Moore, Ravindra S. Dixit, John G. Pendergast, David S. Sholl, Christopher W Jones, and Sankar Nair Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01458 • Publication Date (Web): 06 Jun 2016 Downloaded from http://pubs.acs.org on June 13, 2016
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Chemistry of Materials
The present work first addresses the challenging issue of significantly reducing SAPO-34 membrane thickness to micron levels while maintaining good H2 selectivity. To this end, we re-examined the chemistry of SAPO-34 membrane synthesis. Instead of the common practice of using two structure directing agents (SDAs) - tetraethylammonium hydroxide (TEAOH) and dipropylamine (DPA) - in SAPO-34 membrane synthesis solutions24-27, Zhou28 used only one template (TEAOH) and controlled the pH of the membrane synthesis solution by adjusting the TEAOH/P2O5 ratio, resulting in membranes 3-5 μm thick. However, even though the zeolite film thickness was found to decrease with increasing pH, the use of pH values higher than 9 led to re-dissolution of some of the initially formed SAPO-34 crystals resulting in discontinuous SAPO-34 membrane layers. We describe a systematic synthesis investigation leading to progressively thinner SAPO-34 tubular membranes (as thin as 1 m) on porous ceramic supports. In particular, we are able to form thin SAPO-34 membranes at moderate pH~6 by using the salt form TEABr to independently control the SDA availability, as well as by the use of dip-coated nanoparticle SAPO34 seed crystal layers in contrast to the large (> 1 m) seeds and mechanical seeding (rubbing of substrates with SAPO-34 crystals) used in earlier works. Then we quantify the significant conversion and selectivity improvements obtained in PDH membrane reactors with the use of the above thin SAPO-34 membranes.
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lifted out of the seed suspension and dried at 70°C overnight. SAPO-34 membrane synthesis: Zeolite membranes were synthesized on the inner surfaces of the tubular supports. Usually two seeded supports were placed in an autoclave, which was then filled with the synthesis gel to about 0.5 cm above the top of the supports. The detailed membrane growth conditions for the membranes T1-T5 are described in the Supporting Information. PDH reaction: The Supporting Information describes the packed bed membrane reactor (PBMR) apparatus in detail, along with a schematic (Figure S1). The PDH reaction was also performed in a conventional packed bed reactor (PBR) for comparison. The PBR was made of an impermeable α-alumina tube having the same dimensions as the porous tubes used in the PBMR. The catalyst used in this work was 1% Na2O-doped 20% Cr2O3/80% Al2O3 (Dow Chemical). The same amount of catalyst (1.2 g) was used in both types of reactors. The propane conversion was calculated based on the total propane feed flow rates entering as feed and exiting the reactor in both permeate and retentate streams:
C 3 H 8 1
Si
EXPERIMENTAL SECTION
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SAPO-34 seed synthesis and seed layers: The detailed procedures for the synthesis of SAPO-34 seed nanoparticles (of average sizes ~ 1 mm and ~220 nm) are described in the Supporting Information. The SAPO-34 seeds were deposited on the inner surfaces of porous a-alumina support tubes (Ceramco) by either rub-coating or dipcoating. The tubes are 80 mm long with ID and OD of 8 mm and 11 mm, respectively. The ends of the tubes are glazed with dense glass covering 5 mm length on each end to leave a 70-mm long active section in the middle of the tube. Five types of SAPO-34 membranes (T1-T5) were synthesized. Membrane T1 was seeded by rub-coating with ~1 μm seeds, and membranes T2-T5 were seeded by dip-coating with ~220 nm seeds. Rub-coating was then carried out by rubbing 1.0 wt% seed suspension (~1 μm) evenly onto the inside surface of the tube supports for about 2 min with cotton swabs. The rub-coating process was repeated once and the supports were then dried at 70°C overnight. For dip-coating, the outer surfaces of the supports were wrapped in Teflon tape and they were then then immersed for about 60s in deionized water that contained 1.0 wt% SAPO-34 seeds (~220 nm). The dip-coating process was repeated once to ensure the uniform distribution of seeds on the inner surfaces. The supports were
(1)
The selectivity for gas component i is defined as:
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FCout 3H 8 FCin3H 8
out i in C 3H 8
F
F
Fi in FCout3H 8
(i C3 H 6 , CH 4
)
(2)
The yield for gas component i is calculated by: Yi
C 3 H 8 Si 100
(i C3 H 6 , CH 4
)
(3)
The H2 recovery (RH2) of the membrane is defined by: RH 2
( flow rate of H 2 in permeate) (total flow rate of H 2 in permeate retentate)
(4)
In this work, the above quantities are reported for a reactor operation time of 20 min on-stream for each WHSV.
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Characterization: Scanning Electron Microscopy (SEM) images and energy dispersive X-ray spectroscopy (EDX) analyses (elemental mapping and line scanning) were obtained on a JEOL LEO-1530 at a landing energy of 15 kV using the ‘InLens’ mode detector. The membrane samples were coated with gold to prevent surface charging effects. Before calcination, the integrity of the zeolite membranes was tested via He permeation measurements, for which a transient permeation setup with upstream pressure of 2530 psi at room temperature was employed. All the membranes had very low He permeances (< 1.0×10−10 mol m-2 s-1 Pa-1). After calcination, H2/C3H8 binary gas permeation
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Chemistry of Materials was performed at 25–650 °C. The membrane permeance for component i is defined as:
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2
1
Pm,i
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Qi , Am Pi
(i H 2 , C3 H 8
(5)
5 6
)
where Qi (mol) is the amount of gas permeated over a time period t (s), Am = 17.6 cm2 is the active membrane area, and ∆Pi (Pa) is the transmembrane pressure. The H2/C3H8 permselectivity (aoH2/C3H8) is defined as the ratio of their pure gas permeances:
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Ho 2/C 3H 8 13 15
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support, and small amounts of Si are usually found adhered to alumina particle surfaces deep inside the support. On the other hand, the P profiles are found to be localized to the membrane layer in all cases. The membrane thicknesses shown in Table 1 are those obtained from EDX mapping, which generally agreed with the visually estimated membrane thicknesses obtained from SEM images. To obtain statistically reliable membrane thicknesses, the measurement was made at multiple (2530) points using several EDX/SEM images from at least two membrane samples of each type.
Pm,H2 Pm,C3H8
(6)
The H2/C3H8 separation factor (aH2/C3H8) for the binary mixture is given by
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H 2/C 3H 8 20
( yH 2 / yC 3 H 8 ) permeate ( yH 2 / yC 3H 8 ) feed
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(7)
where yH2 and yC3H8 are mole fractions of H2 and C3H8, respectively.
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Figure 1. SEM images of the seeded surfaces on α-alumina supports; (a) ~1 μm (rub-coating) and (b) ~220 nm (dipcoating). Table 1. Synthesis conditions for SAPO-34 membranes T1-T5 and their average thicknesses.
RESULTS AND DISCUSSION
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Figure 1a shows a SEM image of a typical SAPO-34 seed layer obtained by the mechanical-coating process on the inner surface of α-alumina supports, using a 1 wt% seed suspension of ~1 m SAPO-34 seeds. The seed layer had a smooth surface with no pinholes and cracks observed by SEM in randomly picked areas. Figure 1b shows the SEM pictures of a SAPO-34 seed layer obtained by dip-coating on the inner surface of α-alumina supports, using 1.0 wt% seed suspension with ~220 nm SAPO-34 seeds. The seed particles were well attached to the support surface, and a uniform coverage of seeds was obtained on the support surfaces, thereby providing more favorable growth conditions for continuous thin membranes. Table 1 summarizes the subsequent synthesis conditions for SAPO-34 membranes and the obtained membrane thicknesses (see discussion below). Table S1 (Supporting Information) is an extended version showing permeation data for H2 and C3H8 at 25°C. Figure 2 shows the SEM micrographs of the surface and cross-section of the SAPO-34 zeolite membranes, which consisted of intergrown polycrystalline films. The average crystal grain sizes for the membranes gradually decreased when the membrane thickness decreased from ~6 μm to ~1 μm. EDX analysis and elemental mapping (Figures S2-S3) confirmed the formation of SAPO-34 membrane layers and allowed more accurate measurements of the membrane thickness. Of the four elements Si, Al, P, and O, the P elemental profile provided the most accurate determination of the location of the membrane layer. Al and O profiles are strongly influenced by the underlying alumina
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S.N.
Seed size (nm)
TEAOH: TEABr: H2O Molar Ratio
Initial pH
Membrane thickness (μm)
T1
1000
1.0 : 0.0 : 150
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5.9±0.2
T2
1000
2.0 : 0.0 : 155
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3.2±0.2
T3
220
2.0 : 0.0 : 155
8
2.0±0.3
T4
220
2.0 : 0.0 : 120
8
1.6±0.1
T5
220
1.0 : 1.0 : 120
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1.1±0.1
Membrane T1 was synthesized with two templates (TEAOH and DPA)29, and membrane T2 was prepared with a single template (TEAOH). Compared with membrane T1, membrane T2 was thinner (3-4 μm) and showed higher permeance and selectivity, indicating that the higher synthesis pH of 8 decreased the crystallite size and membrane thickness. Membrane T3 was seeded by dipcoating smaller-sized seeds (~220 nm) than those of T2 (~1 μm), and additional reduction of the membrane thickness was found along with a large permeance increase. The H2/C3H8 selectivity of membrane T3 is the same as that of T1. This result shows that choosing the appropriate seed size can lead to thinner, high-quality SAPO-34 zeolite membranes. Such a result is well known for zeolite MFI membranes31-33 but has not previously been demonstrated for SAPO-34 membranes. Next, membrane T4 was prepared from a more concentrated precursor solution (TEAOH/H2O ratio of 2:120) than that of T3 (2:155). TPAOH serves as a source for OH- ions as well as the TEA+ cationic SDA species. By the use of a synthesis
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Chemistry of Materials
solution containing less water, a well-intergrown zeolite membrane layer with thickness of ∼1.5 μm was formed. This indicates that concentrated solution might lead to faster growth and interlocking of seed-grown crystallites to form a thin membrane. However, further optimization of membranes T3 and T4 proved difficult, since SAPO-34 seeds smaller than 220 nm were difficult to prepare and further increases in concentration of the membrane synthesis solution were accompanied by an increase in pH beyond 9 resulting in a poorly intergrown membrane. Therefore, membrane T5 was prepared using a combination of two templates (TEAOH and TEABr). The addition of TEABr allows decoupling of the SDA concentration and the pH, thereby allowing the growth of thin (~1 m) membranes at moderate pH conditions. Membrane T5 demonstrated further reduction of membrane thickness to about 1 m, and had the highest H2 permeance and selectivity.
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51 Figure 2. SEM images of the secondary grown SAPO-34 zeolite membranes; (a1 and a2) T1; (b1 and b2) T2; (c1 and c2) T3; (d1 and d2) T4; (e1 and e2) T5.
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Figure 3. Separation of H2/C3H8 equimolar mixture by the tubular SAPO-34 zeolite membranes as a function of temperature.
The SAPO-34 membranes (T1-T5) were evaluated in more detail for separation of an equimolar H2/C3H8 mixture over a large temperature range of 23–650°C to determine their applicability in PDH membrane reactors (Figure 3). At 650°C, membrane T5 exhibited the highest H2/C3H8 selectivity of 27 and the highest H2 permeance of 2.3×-10−7 mol.m-2s-1Pa-1. In comparison, membrane T1 had a much lower H2/C3H8 selectivity of 13 and H2 permeance of 1.3×10-7 mol.m-2s-1Pa-1. Note that while the permeance of T5 is about twice that of T1, it is not as high as what might be expected from the 6-fold thickness reduction. There is no in situ growth of SAPO-34 on the outer side of the support (Figure S4), and this factor does not contribute to the transport resistance. External (gas-phase) mass transport resistances were also ruled out by verifying that the permeance did not change upon further increase of the feed and sweep gas velocities. The bare α-alumina supports showed H2 permeances of ~10-6 mol.m-2s-1Pa-1 at 500-650°C, which is only 4-8 times larger than the permeances of MFI and SAPO-34 membranes. Figure S5 shows the corrected SAPO-34 membrane permeance as a function of membrane thickness after the effect of the αalumina support permeance was subtracted using the resistances-in-series model (1/Ptotal = 1/Pmembrane+ 1/Psupport). While the scaling of permeance with membrane thickness is significantly improved after the correction, it still deviates from the ideal scaling. This is possibly due to partial blockage of the support by SAPO-34 infiltration during membrane growth, or amorphous material remaining in the support pores and on the membrane layer even after thorough washing of the membranes. The resulting lower-than-ideal H2 permeance can be expected to affect the H2/C3H8 selectivity as well as the temperature dependence of the permeation characteristics. Therefore, further optimization of the membrane growth techniques and the support microstructure may be needed in future work to obtain the full benefit of the thin SAPO-34 membranes. Nevertheless, for the purposes of this study it is clear that
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Chemistry of Materials the thin SAPO-34 membranes show considerably better performance at temperatures relevant to PDH.
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form the high-temperature propane dehydrogenation (PDH) reaction. The Experimental Section and Supporting Information give full details of the PBMR apparatus (Figure S1), which is based upon packing the PDH catalyst into the zeolite membrane tubes. Figure 4 presents characteristics of the PDH reaction in 70-mm long SAPO-34 PBMRs containing membranes T1-T5 at 600°C for different weight hourly space velocities (WHSVs). For comparison, we also show data from a conventional packed-bed reactor (PBR) as well as from a PBMR containing a zeolite MFI membrane synthesized using previously reported methods.34 The MFI membrane had a H2 permeance of 1.8×10−7 mol.m-2s-1Pa-1 (comparable to membrane T2) and a H2/C3H8 selectivity of 8 (lower than SAPO-34 membranes because of the larger pore size of MFI) at 600°C. During the operation of the PBMR, the reaction product H 2 is transferred from the catalyst bed to the permeate side by selective membrane permeation, resulting in a shift of the PDH equilibrium to the product side and thereby increasing the conversion. Therefore, the propane conversion in all the PBMRs (Figure 4a) is higher than that of the conventional PBR and also higher than the equilibrium conversion. Note that the PBR also shows a conversion higher than equilibrium at lower WHSV values, which is due to the increased effect of thermal cracking side-reactions at larger residence times. A progressive increase in conversion is seen in the PBMRs containing thinner SAPO-34 MRs (T2, T4, and T5) relative to the thicker SAPO-34 membrane (T1) and the zeolite MFI membrane, with the thinnest and most permeable SAPO-34 membrane (T5) exhibiting the best performance in PDH conversion.
31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 Figure 4. Effect of WHSV on (a) propane conversion, (b) propylene selectivity, (c) yield, and (d) H2 recovery in SAPO34 PBMRs, MFI PBMR, and PBR.
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Next, the tubular SAPO-34 zeolite membranes were used in a packed-bed membrane reactor (PBMR) to per-
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Figures 4b and 4c show the propylene selectivity and yield of the PDH reaction. It is seen that the introduction of thinner, selective SAPO-34 membranes (T2-T5) leads to much higher selectivities and yields over the baseline T1 membrane, zeolite MFI membrane, and the PBR. The thinnest (T5) membrane reaches a propylene selectivity of ~85% at a WHSV of 0.5 h-1, due to its efficient removal of hydrogen generated in the PDH reaction and thus increasing the overall reaction selectivity towards PDH over the side-reactions. Figure 4d presents the H2 recovery (fractional H2 removal through the membrane) in the MRs as a function of WHSV. The efficiency of H2 recovery clearly increases from T1 to T5 (i.e., increasing H2 permeance of the membrane), which is consistent with the trend of MR performance enhancement with thinner membranes. Our experimental results clearly indicate that both the H2 selectivity and permeance are critical to the enhancement of PDH PBMR performance, and that the thin SAPO-34 membranes enable large increases in conversion, selectivity, and yield of PBMRs over the conventional PBR. We also note that the above enhancements in performance are measured over an operating time comparable to catalyst cycling times of conventional PBRs and moving-bed reactors used in commercial PDH.
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Chemistry of Materials
CONCLUSION
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Our investigation shows two results of importance in the intensification of olefin production processes. Firstly, micron-thin tubular SAPO-34 zeolite membranes are successfully grown on α-alumina tubular supports by synthesis procedures that independently control pH and precursor (including SDAs) concentrations. The use of appropriately sized seed crystals also leads to thinner SAPO-34 zeolite membranes. The resulting reduction in the SAPO34 membrane thickness leads to a substantial increase in permeance. Further increases in performance should be possible with the use of tubular supports that prevent the occurrence of phenomena such as the infiltration of SAPO-34 or dense material into the support pores during membrane growth. Secondly, the SAPO-34 membranes allow substantial increases in the conversion, olefin selectivity, and olefin yield of PBMRs for high-temperature propane dehydrogenation. Based upon the above results, SAPO-34 membranes should be excellent candidates for further use in the design and fabrication of highly efficient and compact membrane reactors for PDH processes.
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ASSOCIATED CONTENT 24
Supporting Information. Experimental methods, SEM images, EDX elemental mapping data, H2 permeation data, and summary table of membrane synthesis conditions and permeation properties. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION 31
Corresponding Authors
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* S. Nair (
[email protected]) and C. W. Jones (
[email protected])
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Present Addresses
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†
Department of Chemical Engineering, Oklahoma State University, Stillwater, Oklahoma, 74078 USA.
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Author Contributions
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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
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Funding Sources
4
This work was supported by The Dow Chemical Company.
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REFERENCES 49
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(29) Li, S. G.; Carreon, M. A.; Zhang, Y. F.; Funke, H. H.; Noble, R. D.; Falconer, J. L. Scale-up of SAPO-34 Membranes for CO2/CH4 Separation. J. Membr. Sci. 2010, 352, 7-13. (30) van Heyden, H.; Mintova, S.; Bein, T. Nanosized SAPO-34 Synthesized from Colloidal Solutions. Chem. Mater. 2008, 20, 2956-2963. (31) Wong, W. C.; Au, L. T. Y.; Ariso, C. T.; Yeung, K. L. Effects of Synthesis Parameters on the Zeolite Membrane Growth. J. Membr. Sci. 2001, 191, 143-163. (32) Zhang, X. F.; Liu, H. O.; Yeung, K. L. Influence of Seed Size on the Formation and Microstructure of Zeolite Silicalite-1 Membranes by Seeded Growth. Mater. Chem. Phys. 2006, 96, 4250. (33) Shu, X. J.; Wang, X. R.; Kong, Q. Q.; Gu, X. H.; Xu, N. P. High-Flux MFI Zeolite Membrane Supported on YSZ Hollow Fiber for Separation of Ethanol/Water. Ind. Eng. Chem. Res. 2012, 51, 12073-12080. (34) Kim, S. J.; Jones, C. W.; Nair, S.; Liu, Y.; Moore, J. S.; Dixit, R. S.; Pendergast, J. G.; Sarsani, S. Ion Exchange of Zeolite Membranes by A Vacuum ‘Flow-Through’ Technique. Microporous Mesoporous Mater. 2015, 203, 170-177.
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