Cellulose Triacetate/Zinc Oxide Membrane for Bioethanol Recovery via Pervaporation ^†

Abstract

In this study, cellulose triacetate (CTA) hybrid membrane is successfully prepared via the phase-inversion method for bioethanol recovery through pervaporation. Nano zinc oxide (ZnO) particles are mixed into the polymer matrices of CTA to enhance the pervaporation membrane’s performance. The fabricated hybrid membrane is characterized using environmental scanning electron microscopy (ESEM) and thermogravimetric analysis (TGA) to reveal the surface morphology and thermal resistance, respectively. The pervaporation performance of the hybrid membrane is assessed for recovering bioethanol from its dilute solution. Pervaporation results show that the hybrid membrane prepared with 3 wt.% ZnO achieved a permeation flux of 1065.71 g/m²h, while the separation factor was around 1038 at 50 °C operating temperature.

1. Introduction

Fortunately, the use of renewable energy resources and environmentally friendly production methods is rising, and biofuels are emerging as a viable alternative to fossil fuels [1]. Bioethanol has been used in the last few years as a substitute for gasoline owing to its low-cost, high-octane rating, and low environmental impact [2]. However, the profitability of bioethanol is considerably reduced due to the difficulties involved in its production and separation. One of the significant bottlenecks among them is product inhibition during the fermentation stage, which minimizes the ethanol titer and productivity [3]. Accordingly, in situ ethanol recovery from fermentation broth can improve ethanol yield and productivity. Recently, pervaporation has been demonstrated to be beneficial for in situ ethanol recovery, which, in turn, enhances product inhibition in the fermenter [4]. Pervaporation is an appealing membrane-based technique owing to its enhanced efficiency, separation selectivity, low energy consumption, and lack of toxicity to microorganisms [5].

The membrane material plays a significant role in the fabrication of pervaporation membranes. It has been seen that combining polymeric and inorganic nanofillers to create hybrid membranes is an easy and cost-effective approach to increase membrane selectivity and overcome trade-off limitations [6,7]. Incorporating suitable nanofillers could improve pervaporation performance by enhancing selective diffusion and sorption, or both. A cellulose derivative, cellulose triacetate (CTA), has gained prominence again as a membrane material because of its superior mechanical characteristics, less fouling affinity, better hydrophilicity, and excellent oxidation stability. Moreover, it can be fabricated into a very dense film, which is requisite for use as PV membranes [8]. As a low-cost, multifunctional inorganic nanofiller, zinc oxide (ZnO) has lately received much attention as a result of its unique chemical and physical properties, such as superior stability, nontoxic nature, and potent bactericide and antibacterial characteristics [9]. Moreover, ZnO nanofiller’s better hydrophilicity has shaped it as a suitable filler for enhancing the hydrophilicity of hybrid membranes [10]. Accordingly, mixing ZnO nanofillers into the CTA matrix for pervaporation is a promising strategy for recovering bioethanol from fermentation broths.

This study employs the phase inversion technique to fabricate a hybrid membrane consisting of cellulose triacetate polymer and ZnO nanofillers. The effects of nano-ZnO fillers on the physicochemical characteristics of the hybrid membrane, such as structural morphology and thermal properties, are also assessed during this study. Furthermore, the PV performance of the fabricated hybrid membrane is evaluated during the recovery of bioethanol from the aqueous solution as a function of feed content.

2. Materials and Methods

Membrane Preparation, Characterization, and Performance

CTA–ZnO hybrid membrane is fabricated via the phase inversion method. First, 3 wt.% ZnO nanoparticles are dispersed in dimethyl sulfoxide (DMSO) solvent for 1 h in an ultrasonic bath operating at 60 °C. Then, the suspension solution of ZnO is stirred for 2 h at 75 °C. Next, CTA solid polymer is added to the suspension of ZnO and stirred for 6 h at 75 °C to obtain a uniform dope solution. The dope solution is placed for 12 h in the fume hood to eradicate air bubbles formed during the stirring process. Then, a fixed amount of dope solution is poured into Petri dishes and left inside the fume hood for 48 h to remove the solvent properly via evaporation at room temperature. Finally, the dried membrane is detached from the Petri dishes and washed with deionized water to remove the remaining solvent.

Morphological assessment of the fabricated hybrid membrane is examined using environmental scanning electron microscopy (ESEM), Thermo Fisher Scientific, Waltham, MS, USA. A thermogravimetric analyzer (TGA), Shimadzu, Kyoto, Japan is used to examine the thermal characteristics of the fabricated hybrid membrane. The pervaporation performance of the CTA–ZnO hybrid membrane in the recovery of bioethanol from the aqueous solution is investigated using a locally fabricated experimental setup.

3. Results and Discussion

3.1. Membrane Characterization

The influence of 3 wt.% ZnO nanoparticles on the morphological characteristics of hybrid CTA–ZnO membrane is assessed by SEM analyses and shown in Figure 1. The CTA–ZnO fabricated hybrid membrane cross-sectional morphology depicts a sponge-like porous structure in Figure 1a. It can be seen that mixing ZnO nanofillers into the CTA matrix results in a surface made of numerous protrusions, as shown in Figure 1b. SEM analysis indicates that CTA membrane mixed with ZnO nanofillers results in a porous and asymmetrical structural membrane.

TGA analysis is carried out to assess the thermal characteristics of fabricated hybrid CTA–ZnO membrane. Figure 2 demonstrates that the thermal degradation of the hybrid membrane occurs in three stages. It can be seen that the hybrid membrane started to degrade at 95 °C and continued to degrade up to 380 °C. The addition of ZnO nanofillers considerably altered the thermal resistance of the fabricated hybrid membrane.

3.2. Pervaporation Performance

The effect of the water content in the feed on the pervaporation performance of the CTA–ZnO hybrid membrane at 50 °C is shown in Figure 3. Results indicate that the permeation flux and the separation factor strongly depend on the feed content. It can be seen that, with increasing water content in the feed, the permeation flux increases considerably because of the improved membrane affinity towards water. In addition, increasing from 10 wt.% to 25 wt.%, the water concentration in the feed reduces the separation factor substantially because of better membrane swelling. Finally, the maximum permeation flux of 1065.71 g/m²h at the concentration of 25 wt.%, while the highest separation factor is around 1038 at 10 wt.% feed concentration.

4. Conclusions

A phase inversion approach is employed to fabricate CTA–ZnO hybrid membrane for bioethanol recovery via pervaporation. It is revealed that adding 3 wt.% ZnO nanofillers to the CTA polymer matrix enhanced the permeation flux. Furthermore, the addition of ZnO nanofillers also enhanced the membranes’ chemical composition, hydrophilicity, and thermal stability. The maximum permeation flux and separation factor achieved by the hybrid membrane at an operating temperature of 50 °C is 1065.71 g/m²h and 1038, respectively.