(634d) Engineering Innovative Polyelectrolyte Complex Membranes with Enhanced Pervaporation Performance in Ethanol Dehydration

Pervaporation has been a significant technology in
recent years with wide industrial applications in organic dehydration
especially for separating azeotropic, close-boiling, isomeric
and heat
sensitive liquid mixtures. Among these organic-water systems, ethanol
dehydration is one of the most challenging problems as an azeotrope can be
formed when traditional distillation is employed. Complete
separation is hardly to achieve. In comparison,
utilizing membranes for pervaporation can greatly reduce the energy consumption
and realize desired separation effect. Compared with inorganic materials,
organic membranes have already been applied in the water-ethanol separation field
due to their prominent
superiorities such as extensive
availability of raw materials, facile processability, and suitability for large-scale
production.

Conventional hydrophilic polymers used for ethanol dehydration mainly include poly(vinyl alcohol) (PVA), chitosan (CS), polyacrylonitrile
(PAN), polyacrylic acid (PAA), and sodium carboxymethyl
cellulose (CMCNa) [1, 2] etc. Among them, PVA, applied for commercial areas already,
was the most
attractive and economical material, since its high hydrophilicity, facile film-forming ability, thermal and chemical-resistance, and low manufacturing cost. However tiny free volume, low selectivity, and poor stability in aqueous solution
made its pervaporation
performance not as satisfactory as expected.

To
obtain an ideal performance of ethanol dehydration, the difference of physical
and chemical properties in ethanol and water must be fully exploited,
the most distinct of which is polarity. Meanwhile, in order to ensure a higher flux, a novel
organic membrane should possess large free volume, attracting water molecules
and allowing it to pass quickly while rejecting ethanol molecules. Considering
above factors, polymers with higher charge densities are likely to have the
potential to meet these requirements. Focusing on this, the polyelectrolyte complexes (PECs),
which have been paid close attention in recent years, are an promising candidate
to be selected to prepare novel pervaporation membranes for ethanol
dehydration.

PECs are a sort of multicomponent polymers with
many ionizable groups in backbones, which could be applied in many fields
such as gene/DNA delivery, drug release, protein adsorption,
microencapsulation, pervaporation, and fuel cells [3, 4]. Because of
their both good hydrophilicity and ionic cross-linking
structure, the former can ensure a larger flux, while the
latter favors a higher selectivity by limiting the
excessive swelling and the counterions loss. Therefore, they are a kind of
excellent material and very suitable for ethanol
dehydration. Nevertheless, traditional strategies to prepare polyelectrolyte complex membranes (PECMs), i.e., layer-by-layer
assembly and excessive acid blending method [5], could not realize
notable pervaporation performance owing to the underutilization of charges of
ionized groups. Thus, new
methods to synthesize innovative PECM have been constantly explored in
scientific community. 

For the past few years, Zhao et al. [6, 7] proposed a new
route, so-called protection-deprotection, to fabricate soluble and processable
PECs, and homogeneous polyelectrolyte
complex membranes (HPECMs). As it worked out the key
question of PECs, many distinguished properties, i.e.,
no glass transition, high hydrophilicity, tunable surface charge, and
structural stability could be further developed to design novel functional
materials. Along this way, several kinds of needle-like PECs, sizes of which ranged from 300 to 700 nm, and HPECMs
based on CMCNa for polyanions were synthesized [8-12]. Preferable
performances were already obtained in ethanol, isopropanol, acetone, and
dioxane dehydration compared with these of membranes prepared by conventional
methods.

However, in view of particle characteristics, the larger size and
the anisotropy of needle-like PECs cause the nonuniform distribution of long-range arrangement as well as charge densities, which will weaken dehydration performance to a certain extent. In contrast, we speculate that smaller sphere-like PEC
particles can avoid these problems ingeniously. It is believed
that tiny spherical PECs should possess higher charge
density, spatial isotropy, and long-range homogeneity. All of these are beneficial to enhance pervaporation performance of HPECMs. By controlling
operating conditions, such as temperature and stirring speed, the conformation
of polyelectrolytes in solution will change and PECs
with different morphologies can be further generated [13, 14].

Fig. 1.
FESEM image
of HPECM0.31 surface with a magnified top-view insert.

The effect of conformation and morphology of HPECMs on pervaporation
performance in ethanol dehydration was researched. Based
on sodium carboxymethyl cellulose (CMCNa) and poly(diallyldimethylammonium
chloride) (PDDA), a series of sphere-like PEC particles with a diameter between
30-50 nm, and HPECMs with tunable compositions or ionic complexation degrees
(ICD) were prepared, as shown in Fig. 1. The
properties of HPECMs were varied obviously with ICD ranging from 0.23 to 0.57. With the increase of ICD, water contact angles decreased
while the tendency of Zeta
potential was just opposite, indicating that both the hydrophilicity and
charge densities were strengthened. This was exactly contrary to the characteristics of those
needle-like PECs and HPECMs. When subjected to ethanol dehydration, sphere-like
morphology HPECMs revealed more superior
pervaporation performance and desired anti-trade-off
phenomenon, as seen from Fig. 2. The flux increased rapidly, while the water content in permeate remained basically
unchanged and were always kept above 99.30 wt% in the range of 17-70 ºC.
For HPECM0.31 and HPECM0.57, the permeation fluxes were 2.25 kg·m^-2·h^-1
and 2.93 kg·m^-2·h^-1, and
the water concentration in permeate reached up to 99.35 wt% and 99.38 wt%, respectively, with 10 wt% water in feed at 70 ºC. Under the premise of maintaining similar separation effect, the
flux of spherical morphology HPECMs in this work was
nearly as twice as that of needle-like HPECMs and about 10-15 times higher
than that of traditional PVA and CS membranes. Interestingly, the HPECMs flux
of this study was even much bigger than part of inorganic membranes
and hybrid membranes, almost 1-2.5 times higher,
and the selectivity was also outstanding to the majority of them. Moreover, excellent separation effect, good repeatability and
stable performance could be continuously
retained in practical operation. All these results implied that sphere-like
PEC particles and isotropic HPECMs were indeed in
favor of enhancing pervaporation performance in ethanol dehydration.

Fig. 2. Effect
of operating temperature on flux (solid) and water in permeate (hollow) of

HPECM0.31 (triangle) and HPECM0.57 (circle) for
ethanol dehydration with 10 wt% water in feed.

Acknowledgements

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

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