# Electrosynthesis of HKUST-1 with Flow-Reactor Post-Processing
## 3 Results and Discussion
### Flow-Reactor Postprocessing  
An additional processing step is required to improve the product quality as HKUST-1 produced in the batch process is mixed with impurities. In this work, we combined the electrosynthesis system with a flow reactor to enhance the conversion towards HKUST-1. A set of comparison trials were conducted using H\({}_{3}\)BTC concentrations between 0.01 M and 0.1 M, with and without flow reactor post-treatment. The residence time in the flow reactor coils was 6 min at 80 \({}^{\circ}\)C. The PXRD pattern of flow processed electrochemically-synthesised product is similar to the HKUST-1 simulated pattern with some variation in peak intensities (Figure 6). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**H\({}_{3}\)BTC (M)** & **Batch SA (m\({}^{2}\)/g)** & **Flow Processed SA (m\({}^{2}\)/g)** & **\% Difference** \\ \hline
0.01 & 1550 & 1716 & 11 \\
0.05 & 892 & 1625 & 82 \\
0.078 & 481 & 1531 & 218 \\
0.1 & 559 & 1292 & 131 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of Brunauer–Emmett–Teller (BET) surface areas for electrosynthesis from 0.01 M to 0.1 M H\({}_{3}\)BTC in batch and using flow processing. Percentage difference calculated as a percentage of batch surface area. The experimental peak intensities are similar to those demonstrated in our previous work [15], and their deviation from the simulated peak intensities may be explained by the hydration of the sample as HKUST-1 absorbs water from the air [27]. The SEM images demonstrate that electrosynthesis paired with flow processing produced a product with greater purity (Figure 7). Figure 6: Powder X-ray diffraction (PXRD) pattern of flow processed electrochemically synthesised HKUST-1 using 0.05 M H\({}_{3}\)BTC, and a simulated HKUST-1 pattern. Figure 7: SEM images of the product from batch electrosynthesis (**a**,**b**), and electrosynthesis with flow processing (**c**,**d**). Both syntheses used a H\({}_{3}\)BTC concentration of 0.05 M.  
SEM images of products from batch and flow syntheses both presented crystals with typical morphology of HKUST-1, but the batch products also presented another brick-shaped solid phase. Similar impurities were seen in SEM images of all batch products but were more common in experiments using higher concentrations of H\({}_{3}\)BTC. Further characterisation of these byproducts has not been pursued, but as these impurities appear to have a similar brightness to HKUST-1 crystals in SEM images they are likely copper-based. N\({}_{2}\) adsorption-desorption isotherms at 77 K are shown in Figure 8. All products from batch and flow syntheses exhibited a plateau with almost zero N\({}_{2}\) adsorption, indicating their non-porous structure. The adsorption equilibriums were quickly reached in the low relative pressure region, and there were no hysteresis loops shown under high relative pressure. This shape indicates that it is a Type I isotherm suggesting microporous (<2 nm) structure of the samples [19; 28]. Flow processed products also demonstrated improved BET surface areas compared to batch electrosynthesis products (Table 3). At lower H\({}_{3}\)BTC concentrations the difference in surface area between batch and flow electrosynthesis is less as the product is already more pure. Significant differences between batch and flow become more apparent at H\({}_{3}\)BTC concentrations 0.05 M and higher, demonstrating that flow processing is successful in the conversion of undesired byproducts with lower porosity to commercial quality HKUST-1. These results demonstrate that PR electrosynthesis and flow synthesis are complementary techniques with a number of benefits. PR electrosynthesis can produce copper ions without a need for metal salts or additional electrolytes, and any resulting byproducts can be converted to an additional product after processing with the flow reactor. These products required no additional washing and were simply filtered after being collected from the flow reactor.