# Synthesis of Mesoporous MWCNT/HKUST-1 Composite for Wastewater Treatment
## 1 Introduction  
Metal-organic frameworks (MOFs) have been found to be more versatile than any other class of porous material in applications. This is due to their unique features, such as large surface areas with tunable porosities, open metal sites, and the ability to incorporate specific functionalities and active species without altering the frame topology [1; 2; 3]. These properties have made MOFs an outstanding material for gas storage, the capture of CO\({}_{2}\), and wastewater treatment [4; 5].  
Among these MOFs, the Hong Kong University of Science and Technology (HKUST)-1 MOF has been widely studied for energy-related applications [6]. Recently, the HKUST-1 MOF has also received significant attention in wastewater treatment due to its distinct properties [7]. However, MOF instability with water and poor chemical and thermal stabilities are major challenges in water purification [8; 9]. [PERSON] et al. [10] reported on the effect of water adsorption on the structure of metal-organic frameworks. It was found that Cu\({}_{3}\)(BTC)\({}_{2}\) suffers surface area loss, crystallinity variation, and crystal surface damage. On the other hand, [PERSON] et al. [11] showed that the stretch of the Cu\({}_{2}\)C\({}_{4}\)O\({}_{8}\) cage was as a result of absorbed water molecules near Cu sites. To improve the limiting properties of HKUST-1 MOFs, graphene nanosheets, graphite, and carbon nanotubes have been functionalized and incorporated into the MOF [12; 13; 14; 15].  
Previous studies have shown that MOFs can be synthesized through solvothermal/hydrothermal [16; 14], microwave-assisted [17; 18; 19], sonochemical [20; 21], electrochemical [22; 23; 24], mechanochemical [25; 26], ionothermal [27; 28], dry gel conversion [29; 30], and microfluidic synthesis methods [31; 32]. However,the solvothermal technique is the most commonly used method in the synthesis of HKUST-1. The reason is that the reaction does not require sophisticated equipment, and it is an economically viable process on an industrial scale [33]. In addition, the route is good for growing quality crystals with high levels of crystallinity, phase purity, and large surface areas [33].  
Although remarkable work has been done on the synthesis of HKUST-1 and carbon-based/HKUST-1 composites, very few studies on HKUST-1 and MWCNT/HKUST-1 composites have really resulted in quality crystals with large surface areas. Judging from a literature survey on HKUST-1 and MWCNT/HKUST-1 composites, some studies have reported extremely large surface areas, while others have reported very small values using a solvothermal/hydrothermal synthesis technique. Recently, [PERSON] et al. [34] also attempted to improve the yield and surface area of HKUST-1 using an atmospheric pressure method. However, the typical octahedral crystal structure of HKUST-1 could not be observed. Table 1 summarizes selected results on the BET surface area and pore volume of HKUST-1, SWCNT/HKUST-1, and the MWCNT/HKUST-1 composite using the hydrothermal method.  
It can be noted that only a few reports on MWCNT/HKUST-1 have produced surface areas and pore volumes higher virgin HKUST-1. This could be one of the reasons graphene and SWCNT are commonly used for HKUST-1 carbon-based composites. Thus, more studies are needed on the synthesis and characterization of HKUST-1 and the MWCNT/HKUST-1 composite. In this paper, we investigated the influence of the synthesis process on the crystal structure, surface area, pore volume, and pore size of the MWCNT/HKUST-1 composite, with potential applications in wastewater.