Abstract:

Objective Mustard gas, as a typical and widely-used chemical warfare agent, poses serious damage to human beings and ecosystem ascribing to their high toxicity and fast-diffusion. How to effectively protect against these hazards is the focus and challenge of the current research. Carbon-based materials, especially activated carbon fibers, have been applied for producing breathable chemical protective clothes, benefitting from their high adsorption capacity, fast adsorption rate, and extensive sources. However, the removal of toxic chemical agents by activated carbon fibers is mainly based on the physical-adsorption process, which is easy to reach adsorption saturation and cause secondary pollutions. Endowing the existing activated carbon fibers with high-efficient catalytic performance against chemical agents seems an effective route to tackle this problem.

Method The commercial activated carbon fibers were first cleaned and then deposited with zirconia sol through suction filtration at ambient temperature. Subsequently, the preprocessed activated carbon fibers (ACF) were sequentially immersed into ZrCl₄ and terephthalic acid solutions at 130 ℃for 15 min to enable the layer-by-layer growth of zirconia-based metal-organic frameworks(Zr-MOF)on the fiber surface. Different consecutive cycles(3, 6, 9, 12, 15, 18 and 21) were performed to prepare various Zr-MOF/ACF composites using the similar procedure. The surface morphologies and micro-structures of the resultant fibrous composites were characterized using scanning electron microscopy(SEM), X-ray diffraction(XRD), and chemisorption-physisorption analysis. Moreover, the degradation and mechanical properties of the composites were tested.

Result It was found that a layer of microscopic zirconium nanoparticles was uniformly loaded on the surface of activated carbon fibers after pre-treated using zirconia sol. After the subsequent layer-by-layer assembly process, the synthesized Zr-MOF particles were loaded onto the fiber surface, and amount of loading was proportional with cyclic number (Fig. 3). By comparing with XRD standard spectrum, it was seen that the characteristic peaks appeared at 2θ= 7.03°, 25.78° were consistent with that of UiO-66, revealing the successful synthesis of Zr-MOF on the fibrous composites(Fig. 4). As presented in the XPS spectrum(Fig. 5), the contents of lattice oxygen, zirconium and C=O increased in line with the cyclic numbers, demonstrating the occurrence of coordination reaction during the layer-by-layer assembly process. The increase of Zr-MOF particles loaded on the fiber surface enabled an obvious decrease of surface area and total pore volume of the resultant fibrous composites(Fig. 6 and Tab. 1). 2-Chloroethyl ehtyl sulfide(CEES)was introduced as the simulator of mustard gas to detect the protective performance against chemical agents of the resultant activated fibrous composites. The in-situ growth of Zr-MOF on the surface of ACF significantly improved the removal performance of CEES, and the degradation of CEES was also significantly improved with the increase of the cyclic numbers(Fig. 7). After 18 cycles, the degradation rate of CEES reached 84.23% after 24 h. After 3 cycles, the removal rate of CEES still reached 60% after 12 h, revealing the favorable recyclability of the obtained Zr-MOF/ACF composites(Fig. 7). In comparison with original ACF, the mechanical strength of the resultant fibrous composites was obviously increased(e.g., 80.99 % for Zr-MOF/ACF15). However, the mechanical performance displayed a declined with the increase of cyclic numbers (Tab. 2).

Conclusion The commercial activated carbon fibers were robustly endowed with high-efficient catalytic performance against chemical agents by a layer-by-layer self-assembly method, and the loading contents could be regulated by increasing the cyclic numbers. The resultant Zr-MOF/ACF composites exhibited outstanding degradation performance against CEES, and the degradation rate could reach 86.02 % for 18 cycles. The removal of CEES by Zr-MOF/ACF composites was proved to be mainly through degradation process. After three times consecutive circulation, the degradation rate of CEES by Zr-MOF/ACF composites could still maintain 63.47 %, demonstrating the favorable recyclability of the resultant activated carbon fiber composites. After the comprehensive consideration of the degradation performance, tensile strength, and sample preparation efficiency, Zr-MOF/ACF-15 with a degradation rate of 83.08 % within 24 h and a tensile strength of 0.4 MPa was preferred. The integration of outstanding degradation performance, enhanced tensile strength, and easy preparation enabled the resultant Zr-MOF/ACF composites with broad application prospect.

Key words: zirconium-based organic framework, layer-by-layer assembly, 2-chloroethyl ethyl sulfide, degradation, activated carbon fiber, chemical warfare agent