Article Cite This: Cryst. Growth Des. 2018, 18, 4544−4554
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Highly Oriented Thin Membrane Fabrication with Hierarchically Porous Zeolite Seed Xuguang Liu,†,§ Peipei Ge,† Yiming Zhang,† Baoquan Zhang,†,‡ Zhenhua Yao,∥ Zhiyi Wang,†,§ Xia Li,†,§ and Maocong Hu*,∥ †
College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China State Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China § Guangdong Sitong Group Co., Ltd, Chaozhou 521000, China ∥ Department of Chemical and Materials Engineering, New Jersey Institute of Technology, Newark, New Jersey 07102, United States
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‡
S Supporting Information *
ABSTRACT: Nanosized zeolite is widely used as seed for high quality zeolite membranes fabrication, while its complicated synthesis routine limits large-scale productions. In this work, a non-nanosized cubic hierarchically porous TS1 zeolite (HTS-1), obtained by basic hydrothermal treatment of conventional ellipsoid solid TS-1, is used as seed to prepare highly oriented thin membranes. A capillary condensation phenomenon resulting from the unique hierarchically porous structure benefits gel attachment. Moreover, abundant ledges, kinks, and terraces on the HTS-1 surface promote epitaxial growth of the membrane. In contrast, the solid TS-1 seed induces intergrowth dominantly, which results in a thick TS-1 membrane. The HTS-1 membrane demonstrates superior CO2/N2 separation properties compared to the TS-1 one. It associates with thin oriented membrane morphology, leading to exposure of a high Miller index surface and less diffuse distance and tortuosity. The results suggest beneficial effects of a hierarchically porous TS-1 zeolite seed on the interfacial crystal growth for membrane fabrication. A similar conclusion is applicable to the case of a hierarchically porous zeolite β. This work develops a facile approach to obtain a highly oriented thin zeolite membrane with enhanced separation properties. synthesis.27 Unfortunately, they may be restricted by issues including a special template, low yield, and complicated centrifugal separation for recovery.28−30 Solid phase transformation can overcome these drawbacks and be considered as an alternative to obtain the sub-100 nm zeolite.31,32 However, the resulting zeolite suffering from aggregation would be subjected to dispersion in solvent for subsequent seeding, which limits its potential large-scale applications. The conventional zeolite (typically non-nanosized, i.e. >300 nm) usually shows inferior structure-directing activity compared to sub-100 nm ones during the seeded growth process,33,34 resulting in the formation of a thick zeolite membrane and accordingly poor separation properties. However, its high-yield (>80%), common template (e.g., tetrapropylammonium hydroxide), and simple recovery routine are very attractive for future commercialization of zeolite membrane. Therefore, attempts have been made to enhance the structure-directing activity of the conventional zeolite, leading to an advanced seed material for zeolite membrane production. Hydrothermal
1. INTRODUCTION Zeolite membranes/films are ideal membrane/film materials because they have uniform, molecular-size pores (i.e., 0.3−1.3 nm) and excellent thermal, mechanical, and chemical stability.1−4 Those properties enable their potential in many applications such as separation5 and catalytic reactions.6 Generally, superior separation performance is related to a high quality oriented zeolite membrane with a thin structure.7−10 Such zeolite membranes are usually prepared by a seeded (secondary) growth method.11−13 The process is considered as a typical crystal growth on the interface between the sol−gel solution and porous solid substrate. The seed first deposits on the substrate surface and plays a critical role in directing epitaxial growth to produce membranes.14,15 Therefore, seed characteristics,16,17 seeding method,18,19 synthesis conditions,20 as well as molecular modifiers21,22 are the most important influence factors to harvest a highly oriented thin zeolite membrane.23,24 For instance, a sub-100 nm zeolite seed (523 K), which may be attributed to the different porous structures of TS-1 and HTS-1. As illustrated in
Figure 5. NH3-TPD profiles of TS-1 (thin dash line) and HTS-1 (wide solid line) seed, normalized by sample weight (inset table, desorbed NH3 amount at a different temperature range).
Figure 3 and Table 1, pores of TS-1 are micropores dominantly with a size of 0.50−0.60 nm, while HTS-1 demonstrates a hierarchical pore structure with mesopores between 2 and 50 nm in addition to micropores. The relatively larger size of pores in HTS-1 would facilitate NH3 desorption and accordingly lower the desorption temperature. In addition, HTS-1 has around more than 10% strong acid centers of those from TS-1 (0.133 vs 0.121 mmol/g). It clearly indicates the increase of the silanol group (Si-OH) of HTS-1 compared to TS-1, which is consistent with 29Si MAS NMR analysis. It can be ascribed to the complex hydrothermal reaction(s) over the TS-1 surface in a basic environment. This change is also revealed by zeta potential measurement, in which HTS-1 and TS-1 display a zeta potential of −37 and −26 mV, respectively (Table 1 and Figure S2). 3.2. Seeded Growth of TS-1 and HTS-1 Membrane. The zeolite membranes are successfully obtained through the periodical secondary growth method using TS-1 and HTS-1 as seeds. A similar surface Ti/Si mole ratio (1.0/100) is observed by EDS analysis for both TS-1 and HTS-1 membrane (Figure
Table 1. Texture Properties and Zeta Potential of TS-1 and HTS-1 surface area (m2/g)
pore volume (cm3/g)
sample
micropore
mesopore
total
micropore
mesopore
total
zeta potential (mV)
TS-1 HTS-1
191 140
104 320
295 460
0.17 0.22
0.09 0.35
0.26 0.67
−26 −37
4547
DOI: 10.1021/acs.cgd.8b00553 Cryst. Growth Des. 2018, 18, 4544−4554
Crystal Growth & Design
Article
Figure 6. Cross-sectional liner EDS spectra of TS-1 (left) and HTS-1 (right) membranes.
S3). Trace Al content is detected in the TS-1 membrane, while it is not true in the HTS-1 case. Liner EDS scanning (Figure 6) of
cross-section demonstrates a surface Si-rich layer in both membranes. The layer range of HTS-1 membrane is determined 4548
DOI: 10.1021/acs.cgd.8b00553 Cryst. Growth Des. 2018, 18, 4544−4554
Crystal Growth & Design
Article
to be ca. 5 μm, being less than that (13 μm) of the TS-1 membrane. The Si-rich layers in TS-1 and HTS-1 membrane are further investigated by XRD (Figure 7). Both membranes show typical
XRD peaks from TS-1 zeolite, except sharp peaks from membrane substrate (SiO2/Al2O3). However, significantly diverse in the intensity of feature peaks are observed between the TS-1 and HTS-1 membrane. The latter membrane demonstrates a very strong sharp peak attributed to TS1(303). The peak intensity ratio of TS-1(303) to TS-1(200) for the HTS-1 zeolite membrane is calculated to be 3.9, while that of the TS-1 membrane is estimated to be 1.6. This result obviously reveals the preferable (303) orientation for HTS-1 membrane. The oriented TS-1, as well as other zeolite membranes with MFI structure mainly expose the low miller index crystal surface such as, (100),51 (010),52,53 (001),54 and (101),55 while the HTS-1 membrane unveils higher miller index crystal surface (303). It attributes to the different structure-directing behavior between HTS-1 and TS-1 zeolite seed. Figure 8 demonstrates surface and cross-sectional SEM images of TS-1 and HTS-1 seeded substrate and its membrane. Similar seed distribution is observed on TS-1 and HTS-1 seeded substrate (Figure S4). Therefore, it would not affect subsequent seeded growth of membrane. Both the TS-1 and HTS-1 membrane show a well-intergrowth structure to form a continuous membrane (Figure 8a,c), while membrane thickness is completely different. The TS-1 membrane (Figure 8b) delivers a thickness of ca. 8 μm, while the HTS-1 membrane (Figure 8d,e) displays a highly oriented polycrystal layer with a thickness of only 2−3 μm. An oriented grain boundary (marked
Figure 7. XRD patterns of TS-1 (a), HTS-1 (b), seed layer, TS-1 (c), HTS-1 (d) membrane.
Figure 8. SEM images of surface and cross-sectional view of the TS-1 (a, b) and HTS-1 (c, d, e, f) membrane. 4549
DOI: 10.1021/acs.cgd.8b00553 Cryst. Growth Des. 2018, 18, 4544−4554
Crystal Growth & Design
Article
Figure 9. TEM images of conventional solid zeolite beta (a) and hierarchically porous zeolite beta (b), XRD patterns of zeolite membrane seeded growth with conventional solid zeolite beta and hierarchically porous zeolite beta (c), surface (d), and cross-sectional (e) image of (h0l)-oriented zeolite beta membrane.
with black lines) is also observed in a cross-sectional SEM image of HTS-1 membrane with a magnification of 20 000 (Figure 8f). Such a diverse in membrane orientation is consistent with the above XRD results, revealing the high (303) orientation of the HTS-1 membrane (Figure 7d). A control experiment with a short crystallization time reveals that it is possible to prepare the TS-1 membrane in 2−3 μm thickness, but without continuous morphology. A thickness of ca. 8 μm is necessary to have a continuous TS-1 membrane (i.e., free of defects). On the other hand, the as-synthesized HTS-1 membrane, before activation, shows nondetectable gas permeance (
