Growth of High‐Quality, Thickness‐Reduced Zeolite Membranes towards N₂/CH₄ Separation Using High‐Aspect‐Ratio Seeds

Abstract

Greatly improved zeolite membranes were prepared by using high-aspect-ratio zeolite seeds.Slice-shaped seeds with ah igh aspect ratio (AR) facilitated growth of thinner continuous SAPO-34 membranes of muchh igher quality. These membranes showed N 2 permeances as high as (2.87 AE 0.15) 10 À7 mol m À2 s À1 Pa À1at 22 8 8Cw hile maintaining ad ecent N 2 /CH 4 selectivity (9-11.2 for equimolar mixture). On the basis of these thinner high-quality SAPO-34 membranes,fine-tuning the local crystal structure by incorporating more silicon further increased the N 2 permeance by 1.4 times without sacrificing the N 2 /CH 4 selectivity.W ee xpect that application of large AR zeolite seeds might be aviable strategy to growthin high-quality zeolite membranes.Inaddition, finetuning of the crystal structure by changing the crystal composition might be af easible way for further improving the separating performance of high-quality zeolite membranes.According to U.S. natural gas pipeline specifications,i nert gases,s uch as N 2 ,s hould be less than 4%.[1] Currently,N 2 is typically separated by energy-intensive cryogenic distillation (CD) process.M embrane separation technology is am ore energy-efficient alternative for N 2 extraction from natural gas.[2] However,applying it for N 2 /CH 4 separation is challenging because the kinetic diameter difference between N 2 (0.364 nm) and CH 4 (0.38 nm) is less than 0.02 nm and both molecules are classified as weak adsorbates.[3] Thes tate-ofthe-art N 2 -selective polymeric membranes,a ssuming 100 nm membrane thickness,typically exhibit alow N 2 permeance of less than or equal to or 20 GPU (1 GPU = 3.348 10 À10 mol m À2 s À1 Pa À1 )a tN 2 /CH 4 selectivities of 3-5, according to "Robeson" plot. [4] Zeolites are considered as an excellent membrane material because of their molecular-sized pores and high thermal, mechanical and chemical stabilities.G rowth of thin zeolite membranes with high-quality/negligible defects is highly desirable yet very challenging by hydrothermal synthesis. Up to now,several zeolite membranes with framework type of MFI, [5] Y, [6] T, [7] DDR, [8] SSZ-13, [3] and SAPO-34 [3, 9] have been reported to separate CO 2 from CO 2 /CH 4 mixtures.However, when applied for N 2 /CH 4 separation, they suffer from the low N 2 permeance and/or low N 2 /CH 4 separation selectivity.F or example,S SZ-13 and SAPO-34 membranes were prepared and investigated for N 2 /CH 4 mixture separation in ar ecent study.S SZ-13 membrane displayed superior N 2 /CH 4 separation selectivity of about 13 but with al ow N 2 permeance of 2.2 10 À8 mol m À2 s À1 Pa À1 at 20 8 8C.[3] SAPO-34 membranes, which have the CHA structure and pore entrance size similar to the kinetic diameter of CH 4 ,e xhibited much higher N 2 permeance of about 110 À7 mol m À2 s À1 Pa À1.T he reported N 2 /CH 4 selectivities,h owever, were between 5a nd 8. Considering higher N 2 permeance and favorable crystal pore size, SAPO-34 membranes seem to be ag ood candidate for N 2 / CH 4 separation, although their quality st...

Molecular sieve membranes for N2/CH4separation

“…12 In a very elegant work, Yu and Li 's research groups used high-aspect-ratio-SAPO-34 seeds to synthesize via secondary seeded growth thin SAPO-34 membranes for N 2 /CH 4 separation. 40 Figure 3 shows the remarkable control over seeds aspect ratio from ;1 to ;20. These thin seeds were used to synthesize high quality SAPO-34 membranes.…”

“…All reported SAPO-34 membranes for N 2 /CH 4 separation have been successfully grown via secondary seeded growth on porous alumina supports. 12,24,[40][41][42] These porous tubes were typically 6 cm long with effective permeation areas between 6.5 and 7.5 cm 2 . Wu et al 24 prepared SAPO-34 membranes by secondary seeded growth on the inside of seeded, porous Al 2 O 3 tubes (1-cm O.D., 100 nm average pore size, 40-55% porosity).…”

“…Authors claimed that some of these seeds contain the thinnest SAPO-34 crystals reported thus far, which correspond to ;10 unit cells in thickness. 40 These seeds were used to grow SAPO-34 membranes with thicknesses up to ;2.3 lm. Interestingly, the authors found that the seed aspect ratio (AR) not only affected the thickness of membranes but also played a crucial role on the control of defect formation.…”

“…24 On the other hand, AlPO-18 membranes show the highest reported N 2 permeances, at moderate N 2 /CH 4 selectivities. 47 SAPO-34 membranes show the best overall separation performance, with N 2 permeances as high as 2591 GPU 42 and N 2 /CH 4 separation selectivities as high as 11.3 40 In the next paragraphs, the separation mechanisms involved in these molecular sieve membranes for N 2 /CH 4 gas mixtures are discussed and correlated with their observed separation performance.…”

Thin SAPO-34 membranes synthesized in stainless steel autoclaves for N2/CH4 separation

“…Teflon-lined autoclaves have been conventionally employed for SAPO-34 membrane synthesis [8-14, 16-20, 22-29, 31-43]. The reported thinnest SAPO-34 membrane was ~2.3 μm with N 2 permeance of 860 GPUs for N 2 /CH 4 separation [43] and the N 2 permeance of 1200 GPUs for doubled silicon content in the gel to obtain the N 2 /CH 4 selectivity of 9-11.2 by using high-aspect-ratio seeds. Zong et al [42] prepared SAPO-34 membranes by diluted gels with increased crystallization temperature with a N 2 /CH 4 selectivity of 7.2 and N 2 permeance as high as 1,300 GPUs at 23 °C.…”