Two main types of morphological features resulting in surface roughness were observed on alumina discs used as supports for zeolite membranes. These features can be described as hills and pits and it was shown that defects as cracks formed in the zeolite film at these locations of the support.
Microporous and Mesoporous Materials 308 (2020) 110546 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso Influence of support surface roughness on zeolite membrane quality Ming Zhou *, Mohammad Sadegh Nabavi , Jonas Hedlund Chemical Technology, Luleå University of Technology, SE-971 87, Luleå, Sweden A R T I C L E I N F O A B S T R A C T Keywords: Defects Permporometry Polishing SEM Substrate Two main types of morphological features resulting in surface roughness were observed on alumina discs used as supports for zeolite membranes These features can be described as hills and pits and it was shown that defects as cracks formed in the zeolite film at these locations of the support It was demonstrated that the roughness of the support can be reduced significantly by a polishing strategy developed in this paper Finally, zeolite MFI membranes grown on the polished support shows remarkably improved quality as compared to films grown on non-polished supports Introduction Zeolite membranes are microporous inorganic membranes [1] They have uniform pore size and high chemical stability [2,3] Therefore, these materials can be utilized in a large number of applications where polymeric membranes are not applicable [4] Membrane technologies are suitable for liquid [5–7] and gas separation [8–10], and in reactors [11,12], and as chemical sensors [13] The MFI framework has two types of channels, zigzag channels (5.1 × 5.5 Å) and straight channels (5.3 × 5.6 Å) [14] Based on the Si/Al ratio, MFI is classified as silicalite-1 (Si/Al ratio higher than 200) or ZSM-5 (Si/Al ratio is 10–200) [15] The channel diameter is suitable for separation of hydrocarbons, and consequently this framework attracted much attention in gas separation [16–18] However, it has been reported that defects in the zeolite film may reduce the separation performance [19,20] In a defect free zeolite membrane, permeation would occur only through the zeolite pores However, in reality, membranes contain de fects and permeation also occurs in these defects Open grain boundaries is one of the most common types of defects [21], and pinholes are another type of defect [22,23] and the latter can be reduced by a ho mogeneous seed layer [24] Membranes may also crack during calci nation after synthesis due to two main reasons Firstly, calcination removes the structure-directing agent and any other volatile species left in the micropores This results in contraction of the zeolite crystals, which may result in crack formation Secondly, the thermal expansion mismatch between support and zeolite film can lead to formation of cracks during calcination [25] There are alternatives to regular calcination reported in the literature such as ozonication and rapid thermal processing (RTP) that may be used to reduce the problem [26–28] In present work, the effect of surface roughness of the mac roporous alumina support on the quality of zeolite MFI membrane, i.e cracks formation in the areas of hills and pits is investigated A novel polishing approach that can reduce roughness and avoid cracking in zeolite membranes is reported for the first time Experimental 2.1 Membrane preparation Graded porous alpha-alumina disks (Fraunhofer IKTS, Germany) with a diameter of 25 mm and a total thickness of mm were used as supports, it contains a 40 μm thick top layer with a pore size of 100 nm and a bottom main part with a pore size of μm The washed alumina supports were carefully polished by hand using #4000 SiC paper (average grain size μm, Struers) until a shiny wet surface was observed The alumina discs were first polished on SiC paper supported on a large flat hard surface, and then on a soft rubber surface with a decreasing diameter of ca 23 mm, 18 mm, 12 mm, mm in sequence The reason for this two steps polishing method is that the disk surface is not pure flat, it contains areas of raised ‘plateaus’ and sunken ‘basins’ In the first step, simply polish the surface on a flat SiC paper to remove all the plateaus and hills-shaped rough areas, this process can change all the protruding parts of the support into shiny surface In the second step, the basin-shaped rough area on the disk surface cannot be touched by a flat SiC paper, here a round soft rubber mat (0.8 mm thick) was put under * Corresponding author E-mail address: ming.zhou@ltu.se (M Zhou) https://doi.org/10.1016/j.micromeso.2020.110546 Received 20 June 2020; Received in revised form 23 July 2020; Accepted August 2020 Available online August 2020 1387-1811/© 2021 The Authors Published by Elsevier Inc This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) M Zhou et al Microporous and Mesoporous Materials 308 (2020) 110546 Fig SEM images of a hill a) and a pit b) at the surface of a non-polished alumina disc, cracks in the zeolite film at the foot of a hill c), and at the bottom of a pit d) the SiC paper, which make the paper protruding and touching the basin bottom of the disk surface, in this way the polishing is effective and can change all the lower parts on the support into smooth surface In order to remove the debris formed after polishing, the substrates were auto claved in DI water at 180 ◦ C for 24 h H-ZSM-5 membranes with a thickness of ca 0.5 μm and a Si/Al ratio of 139 [29] were prepared as described in detail earlier [30] and briefly here Prior to film synthesis, the supports were masked as described elsewhere [16] and then seeded with colloidal MFI crystals with 50 nm diameter After seeding, the supports were rinsed with a filtered (0.1 mm) 0.1 M aqueous NH3 solution to remove excess seed crystals The film synthesis was carried out for 70 h at 88 ◦ C in a synthesis solution with a molar composition of 3TPAOH : 25SiO2 : 1450H2O : 100C2H5OH The ethanol released from TEOS hydrolysis form azeotropes with boiling point lower than 100 ◦ C, to reduce turbulent of the synthesis solution, lower temperature was chosen in this study After the synthesis, the membranes were rinsed with a 0.1 M ammonia solution and then calcined for h at 500 ◦ C at a heating rate of 0.2 ◦ C min− and a cooling rate of 0.3 ◦ C min− 2.3 SEM characterization SEM images were recorded using extreme-high-resolution scanning electron microscopy (XHR-SEM), using a Magellan 400 (the FEI Com pany, Eindhoven, the Netherlands) instrument and no conductive coating was applied to the samples 2.4 3D optical surface profile measurement A Wyko 1100NT 3D optical surface profiler was used to measure the surface roughness, using vertical scanning interferometry (VSI) of white light A magnification of 2.5 was used and the data was processed using the Veeco software Results and discussion In the first step, non-polished alumina discs were investigated by SEM to reveal the morphology of the surface Two main types of morphological features were observed and these can be described as hills and pits as shown in Fig 1a) and b), respectively The main dif ference between individual non-polished discs was the density of hills and pits Fig 1c) and d) show SEM images of a ZSM-5 membrane recorded at the location of a hill and pit in the support, respectively It was revealed that cracks mainly formed in the film at the foot of the hills and at the bottom of the pits as indicated by the insets in Fig 1c) and d), respectively Thermal expansion of support and contraction of zeolite membrane upon template removal is not the main cause for these cracks The membrane at hill and pit areas bend more sharply, hence the high curvature in these types of surfaces deviate from being a flat membrane cause the cracks formation For a non-polished support, the thickness of the alumina top layer is 40 μm, Fig a) After polishing of the support, the thickness of the top layer reduced to 30 μm, Fig b) During polishing, the applied pressure (P) had a significant influence on the final smoothness of the alumina 2.2 Permporometry For characterization by permporometry [30], the membranes were mounted in stainless steel cells sealed by graphite gaskets (Eriks, the Netherlands) Then, the cell was heated to 300 ◦ C for h in a flow of pure helium with a heating rate of ◦ C min− followed by natural cooling Permporometry was carried out at room temperature using a total pressure difference across the membrane of atm The permeate was kept at atmospheric pressure The relative pressure of n-hexane was increased stepwise from to ca The system was allowed to reach steady-state at each step A digital flow meter and a soap bubble flow meter were employed to measure the volumetric flow rate of the permeate The defect distribution was calculated as reported previously [20] M Zhou et al Microporous and Mesoporous Materials 308 (2020) 110546 Fig SEM images of the cross section of non-polished a) and polished b) alumina supports, schematic drawing of the polishing process c), camera image of a polished alumina disc d) Fig SEM images and 3D optical surface profiles of non-polished alumina surface a)-c), polished alumina surface with remaining scratches d)-f), polished alumina with smooth surface g)-i) surface The pressure was estimated by simply dividing the force applied on the backside of the support with the area of the whole support By P ˃ 2000 Pa, a smooth surface with some remaining scratches was produced When the pressure was reduced to ca 1000 Pa, a smooth and shiny surface was obtained, as illustrated schematically in Fig c), and by a camera image d) Fig 3a) shows a SEM image of a relatively large hill with a height certainly exceeding μm on a non-polished support The 3D optical M Zhou et al Microporous and Mesoporous Materials 308 (2020) 110546 Fig SEM images of small defect on polished surface a) and b), permporometry data c) of membranes grown on non-polished alumina supports (1), (2) and membranes grown on polished supports (3), (4), schematic illustration of surface roughness induced defects in zeolite membranes d) surface profiles of a non-polished support at low and high magnification are illustrated in Fig 2b) and c), respectively These profiles illustrate the presence of numerous hills higher than 2.0 μm (dark red) and that the average roughness Ra is 907 nm Fig 3d) shows an SEM image of a polished alumina surface using an applied pressure larger than 2000 Pa during polishing Scratches with a width of 3–5 μm (similar to the SiC grain size) can be observed throughout the whole surface The scratches are also observed in the 3D optical profile, Fig 3e) However, the pol ishing reduced the average roughness Ra to 328 nm and removed most of the hills, Fig 3f) By polishing the surface using a pressure of 1000 Pa, a smooth surface without scratches and hills with an average roughness Ra as low as 216 nm was achieved, as shown in Fig 3g), h) and i) This Ra is comparable to the grain size of the alumina support, i.e ~150 nm Even after polishing of the support, a few shallow pits still remained in the top-layer of the support, which resulted in smaller cracks with a width of ca 20 nm in the membrane as illustrated in Fig 4a) and b) Before calcination, the single gas helium permeance of the membrane is < 0.1 × 10− mol m− s− Pa− 1, which means there was not crack in synthesized membrane and proves the cracks are formed during calci nation However, the concentration of these defects is reduced by the polishing as confirmed by permporometry data shown in Fig 4c Line (1) and line (2) are pormporometry curves of membranes grown on nonpolished alumina surfaces The total relative areas of defects > 20 nm are 2.5 × 10− and 1.6 × 10− 5, for these two samples respectively Line (3) and line (4) are permporometry data of membranes grown on pol ished alumina supports The total relative areas of defects > 20 nm are as low as 1.2 × 10− and 0.6 × 10− 5, respectively These two curves are similar to the best results we measured previously for non-polished supports [20] The permporometry results demonstrate that the pol ishing strategy developed in this paper can decrease the amount of larger defects (cracks) in the membrane Conclusion The surface roughness of alumina supports influence the quality of zeolite MFI membranes Micron-sized hills and pits are observed on the supports and cracks in the membrane form at the foot and bottom of these hills and pits, respectively The hills and some pits could be removed by a delicate polishing approach designed in this paper, and the average roughness of the alumina surface could be reduced Zeolite membranes grown on the polished support shows significantly improved quality We anticipate this polishing strategy will have impacts on the fabrication of defect-free zeolite membranes, which will opens up po tential for improved performances in the future CRediT authorship contribution statement Ming Zhou: Conceptualization, Methodology, Data curation, Inves tigation, Methodology, Writing - original draft, Writing - review & editing Mohammad Sadegh Nabavi: Data curation, Methodology, Writing - original draft Jonas Hedlund: Conceptualization, Funding acquisition, Project administration, Supervision, Writing - review & editing Declaration of competing interest 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 Acknowledgments The authors acknowledge Bio4Energy, and The Swedish Research M Zhou et al Microporous and Mesoporous Materials 308 (2020) 110546 Council for financially supporting this work [15] R Szostak, Molecular Sieves-Principles of Synthesis and Identification, Springer, 1989 [16] L Yu, S Fouladvand, M Grahn, J Hedlund, Ultra-thin MFI membranes with different Si/Al ratios for CO2/CH4 separation, Microporous Mesoporous Mater 284 (2019) 258–264 [17] X Feng, Z Zong, S.K Elsaidi, J.B Jasinski, R Krishna, P.K Thallapally, M A Carreon, Kr/Xe 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on polished surface a) and b), permporometry data c) of membranes grown on non-polished alumina supports (1), (2) and membranes grown on polished supports (3), (4), schematic illustration of. .. of surface roughness induced defects in zeolite membranes d) surface profiles of a non-polished support at low and high magnification are illustrated in Fig 2b) and c), respectively These profiles