(177c) Reliable Fabrication and Surface Modification of Beta Zeolite Membrane for Pervaporation of n-Butanol/Water Mixtures

The
depletion of crude oil resources is the main reason for motivating the
different waves of interests in renewable energies including light-voltage,
wind energy, and biofuels [1]. Compared with bioethanol, biobutanol has several
advantages such as higher energy density, lower volatility, higher boiling
point (117.7 ºC) and flash point (29 ºC), plus amenability to the formation of
blends with gasoline in any proportion [2]. Currently, biobutanol is mainly
produced through the fermentation of renewable biomass. In the fermentation
process, the severe inhibition and toxicity of butanol usually lead to a
significant decrease in its yield [3]. To remove butanol from the fermentation
system, several separation techniques such as distillation, adsorption,
liquid-liquid extraction (LLE), gas stripping and pervaporation have been
employed [1]. The pervaporation technique with low energy requirement, easy
operability and harmless to microorganisms is a promising technology for
efficient separation of butanol/water mixtures [4].

The
pervaporation membranes have attracted broad attention recently. Zeolite beta
adopts a truncated bi-pyramidal shape with three-dimensional 12-ring
interconnected channel system with pore diameters of 0.71 * 0.73 nm and 0.56 *
0.56 nm [5]. Zeolite beta membranes have the application potential in both separation
and catalysis. At present, zeolite beta membranes are usually synthesized using
TEAOH as the structure directing agent in either a basic medium (without
fluoride) or a near-neutral medium (with fluoride). In the basic medium, the
synthesis gel is in a liquid state, which can be fully adsorbed into the seed layer
of zeolite beta crystals. Nevertheless, the crystallinity and intergrowth
between vicinal crystals in zeolite beta membranes synthesized as such are not
satisfactory [6]. In contrast to the alkaline medium, zeolite beta membranes
prepared in a near-neutral gel containing fluoride often exhibits a truncated
bi-pyramidal morphology with high crystallinity. The
fluoride ion rich gels possess extremely high viscosity and low fluidity,
therefore, the zeolite beta membranes formed in the near-neutral medium are
generally non-uniform, the adhesion strength of which on their supports are
pretty low.

In the
present work, we propose a two-step method to fabricate zeolite beta membranes on
porous alpha-alumina substrates, in which the secondary growth procedure is
performed in the basic medium without fluoride followed by that in the
near-neutral medium with fluoride. As shown in Figure 1(b), an intergrown and
dense crystal layer was formed with the thickness of ca. 1 µm on the alpha-alumina
supported zeolite beta crystal layer (Figure 1(a)) after the first step. Then,
the thin layer of zeolite beta crystals was put into the near-neutral medium of
synthesis gels. After the second step, the formed zeolite beta membranes
ensured cross-linking with typical truncated bi-pyramidal morphology shown in
Figure 1(c). The total thickness of the membrane was only 2.5~3 µm. As
illustrated in Figure 2, the BEA topology was well-defined reflections from the
characteristic XRD diffraction peaks. The zeolite beta membrane exhibited three
reflections corresponding to (101), (106), and (302),
indicating that the synthesized membrane was preferentially (h0l)-oriented.
The EPMA elemental analysis of the membrane was given in Figure 3. The
concentration of aluminum was gradually decreased from the substrate surface to
the outer surface of the zeolite beta membrane, which demonstrated that there does
not exist a transition layer inside the membrane. The pervaporation fluxes of several
pure components through zeolite beta membranes were checked at 298 K. The separation
of TIPB/ethanol mixtures by pervaporation was also used to evaluate the membrane
quality. It could be concluded that the synthesized zeolite beta membranes by
our two-step method are dense and uniform, especially the preferentially (h0l)-oriented
membranes were well intergrown with little defects.

Figure
1 SEM images of
the seed layer of beta crystals (a), zeolite beta membranes synthesized on the
seed layer under the alkaline synthesis solution without fluoride (b, M1), and
zeolite beta membranes synthesized on M1 under the near-neutral synthesis
solution with fluoride (c, M2). Inset: cross-sectional view of the corresponding
zeolite beta seed layer or membranes.

Figure
2 (a) XRD
patterns of the substrate, seed layer and zeolite beta membranes. (A)
alpha-alumina, (B) the seed layer, (C) zeolite beta membrane synthesized on the
seed layer under the alkaline synthesis solution without fluoride (M1), and (D)
zeolite beta membrane synthesized on M1 under the near-neutral synthesis
solution with fluoride (M2). (b) EPMA elemental analysis of a (h0l)-oriented
zeolite beta membrane. The detection limits for Al, Si, and F are about 0.001
wt%. EPMA elemental profile plotted in mass percentage versus distance with SEM
image of the sample. The dashed line illustrates the EPMA scanning path.

Although
zeolite beta membranes with little defects can be synthesized by the two-step
method, their surface hydrophilicity/hydrophobicity is not satisfactory, and
need to be improved to enhance the pervaporation performance. Mussel-inspired
surface chemistry has attracted great interest due to its simplicity,
versatility and wide applicability [7]. Herein, a facile method was proposed to
functionalize the surface of zeolite beta membranes. A one-step procedure was
developed to improve the surface hydrophilicity of zeolite beta membranes via
co-deposition of polydopamine (PDA) and polyethyleneimine (PEI). Moreover, the
PDA-functionalized surface of zeolite beta membranes could become hydrophobic
after octadecylamine grafting via Schiff base reaction. The change in
surface hydrophilic-hydrophobic properties was checked by contact angle
measurements. As shown in Figure 3, the contact angle of synthesized zeolite
beta membranes was ca. 80º (Figure 3(a)), which was awkward to be
preferentially selective to either butanol or water. After hydrophilic
modification, the average contact angle was decreased to ca. 40º (Figure 3(b)),
leading to an increased hydrophilic surface. While the average contact angle
would be increased to ca. 105º (Figure 3(c)) after hydrophobic modification,
indicating that the surface of zeolite beta membranes became hydrophobic. The
pervaporation performance of zeolite beta membranes before and after surface
modification was demonstrated in Figure 4. The significant improvements in
pervaporation of butanol/water mixtures were achieved by using the
functionalized beta zeolite membranes. With respect to dilute butanol aqueous
solution (5 wt% water, 70 ºC), the separation factor of the zeolite beta
membrane after hydrophilic modification was increased from ca. 19 to over 420
while keeping the total flux at ca. 1.5 kg/ (m² h). The
pervaporation performance of surface hydrophobic modified zeolite beta
membranes was measured for dilute n-butanol aqueous solution (5 wt% n-butanol,
70 ºC). The membrane after surface modification exhibited a total flux of ca. 1.3 kg/ (m² h)
with the separation factor of ca. 100, which increased from ca. 5. These results demonstrated that the
surface property of zeolite beta membrane influenced the
pervaporation performance when the membrane defects was little. Furthermore,
the method we proposed endows the zeolite beta membrane with excellent surface
properties, high separation factor and low flux reduction.
Compared with current literature reports, the pervaporation performance of our
zeolite beta membranes in this report was significantly enhanced for dilute and
concentrated butanol aqueous solutions.

Figure
3 Static water
contact angles of zeolite membranes. (a) without modification, (b) after
hydrophilic modification, (c) after hydrophobic modification.

Figure
4 The
enhancements in the pervaporation performance of beta zeolite membranes after
surface modification. (a) after hydrophilic modification, (b) after hydrophobic
modification.

Acknowledgements

This
research was supported by the National Natural Science Foundation of China
(Grant No. 21136008).

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