Oriented MFI membranes by gel-less secondary growth of sub-100nm MFI-nanosheet seed layers
In the present work, we describe the synthesis of ultrathin MFI membranes made by “gelless” secondary growth of MFI nanosheet seed layers on porous supports.1 The synthesized membranes have exceptionally high permeance for para-xylene and n-butane due to a record low thickness of the selective layer.
Following the reports by Pham et al, it was established that the gelless method is a relatively simple and inexpensive method for secondary growth of zeolite seeds.2 For application of this method, a silica nanoparticle layer on the porous support was used as the sacrificial layer for zeolite secondary growth. We prepared Stӧber silica supports and sintered silica fiber (SSF) supports by mechanical pressing of either homemade Stӧber silica particles or quartz wool, respectively. The supports were polished to reduce surface roughness. The SSF supports were then coated with 500 nm diameter homemade Stӧber silica particles by the method of rubbing,3,4 which further served to reduce surface roughness. The final step of support fabrication involved rubbing of 50 nm homemade Stӧber silica particles which act as the silica source as described before.
The supports were coated with a 50-80 nm thick MFI nanosheet coating made by vacuum filtration of a suspension of MFI nanosheets in octanol (synthesis of which has been described previously5) through the porous supports. Following calcination of the supports, they were subjected to gelless secondary growth. This involved impregnation of an SDA solution (typically TPAOH – tetrapropylammonium hydroxide or TPABr – tetrapropylammonium bromide) into the support. The support was then transferred to an autoclave and placed in a static oven at 190⁰C. On completion of the growth, the support was removed, calcined and subjected to permeation testing.
For membranes made on Stӧber silica supports, a membrane thickness of 250 nm was obtained which resulted in a p-xylene permeance of 2.4x10-7 molm-2s-1Pa-1 and a p-xylene/o-xylene separation factor (S.F.) of 500. The permeance is almost twice that reported by Pham et al, who used the gelless method to intergrow coffin-shaped MFI crystals on silica supports. We also obtained 100 nm thick membranes which gave an even higher p-xylene permeance (3x10-7 molm-2s-1Pa-1) but S.F. was only about 20. This is the thinnest MFI membrane reported; though the S.F. is low there may be significant techno-economic benefits from having high p-xylene permeance.
The SSF supports, which we have reported for the first time, have an order of magnitude higher permeance and three times the flexural strength compared to Stӧber silica supports. We have reproducibly obtained p-xylene permeances of 1.7-3.6x10-7 molm-2s-1Pa-1 with a maximum p-xylene/o-xylene S.F. of 185. Two of these membranes were also tested for separation of butane isomers at various temperatures. n-butane permeances were an order of magnitude higher than most reports in literature, with a high n-butane/i-butane S.F. at room temperature.
In conclusion, we report for the first time, high flux MFI membranes for xylene and butane separation with an unprecedented level of control on thickness. SSF allows for high flux and high strength supports on which oriented growth of MFI membranes from nanosheet seeds can be carried out. This is enabled by the gelless secondary growth method, which has several advantages, including simplicity, low chemical use and potential for scale-up, over conventional solution-based growth methods.
(1) Agrawal, K. V.; Topuz, B.; Pham, T. C. T.; Nguyen, T. H.; Sauer, N.; Rangnekar, N.; Zhang, H.; Narasimharao, K.; Basahel, S.; Francis, L. F.; Macosko, C. W.; Al-Thabaiti, S.; Tsapatsis, M.; Yoon, K. B. Adv. Mater. 2015.
(2) Cao, T.; Pham, T.; Nguyen, T. H.; Yoon, K. B. 2013.
(3) Lee, J. S.; Kim, J. H.; Lee, Y. J.; Jeong, N. C.; Yoon, K. B. Angew. Chem. Int. Ed. Engl. 2007, 46, 3087–3090.
(4) Pham, T. C. T.; Kim, H. S.; Yoon, K. B. Science 2011, 334, 1533–1538.
(5) Agrawal, K. V.; Topuz, B.; Jiang, Z.; Nguenkam, K.; Elyassi, B.; Navarro, M.; Francis, L. F.; Tsapatsis, M. AIChE J. 2013, 59, 3458–3467.
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