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Materials 2021, 14, 1786 5 of 20 3. Results and Discussion 3.1. Coordination Polymers Preparation The CP preparation was conceptualized to use greener methodologies, including water as solvent, avoiding washing and crystallization processes with organic solvents, and applying MWAS to allow a faster and more environmentally safe strategy in comparison with the HT synthesis. Since a high content of water molecules in LnCP affects their PL properties, some variations in the synthetic methods, particularly the use of a co-ligand, were employed to achieve lower water content. Two isostructural CPs based in Sm3+ and Tb3+, designated SmCP1 and TbCP1, were prepared following HT and MWAS procedures and two other unprecedent Tb3+ CPs were isolated by HT synthesis (TbCP2 and TbCP3; experimental conditions are summarized in Table 2). The preparation of TbCP3 involved the use of a co-ligand, the phenanthroline (phen). Table 2. Summary of the experimental parameters and conditions used in the synthesis of the four CPs. CP SmCP1 TbCP1 TbCP2 TbCP3 H2 aip 1.0 1.0 1.0 1.0 Equiv Synthesis T/◦ C t/h η/% LnCl3 .6H2 O NaOH phen 0.671.1–HT11072 28 0.64 1.2 MWAS 2 21 0.64 1.2* – HT 110 72 37 0.67 1.1 MWAS 6 16 0.661.0–HT11072 27 0.66 1.2 0.59 HT 110 72 47 * Corresponding to the initial quantity weighted. The pH value of the reactional mixture was adjusted for 3–4 with addition of a 1 M NaOH aqueous solution. As revealed in Table 2, a temperature of 110 ◦C was maintained for all HT and MWAS syntheses. Notably, by using the MWAS process, the SmCP1 and TbCP1 materials were obtained in 2 and 6 h respectively, in contrast to the 72 h required in the conventional HT process, meaning that the MWAS allows significant shortening of reaction time and consequently low energy consumption. However, it was also verified that under the tested MWAS reaction conditions, the products’ yields were slightly lower than that obtained with HT. Most probably, these lower yields are not only directly related to the MW irradiation source, but rather to the lower reaction time and rapid cooling that decrease the production yield of the materials, which may not be beneficial for these Ln materials. In order to study the influence of the pH in the preparation of these materials a specific amount of base (NaOH) was added, for each synthesis. For a set of HT reactions, using Tb3+ as a metal center, three pH ranges (3–4, 7–8 and 10–11) were evaluated. SCXRD analysis revealed that crystalline materials different from the ligand were obtained only using pH = 3–4, indicating that the pH needs to be acidic for the LnCP preparation using H2aip as a ligand. All the materials were characterized by SCXRD, PXRD, SEM/EDX and FT-IR. 3.2. Structures of the Coordination Polymers Suitable crystals of SmCP1 for SCXRD analysis were obtained and collected from the filtered solids (HT synthesis and MWAS), as well as from the filtrate of the MWAS, after 24 h. In the case of TbCP1, the crystals were isolated from the filtered solids in both synthetic methods. The SCXRD analysis revealed three one-dimensional (1D) CPs, SmCP1, TbCP1, TbCP3, and one two-dimensional (2D) CP, TbCP2. SmCP1 and TbCP1 crystal structures revealed the formula {[Ln(Haip)(aip)(H2O)5]· 4H2O}n, with Ln = Sm3+ or Tb3+ for SmCP1 or TbCP1, respectively. The two CPs are isostructural and crystallized in a monoclinic system (space group P21/n) with the respec- tive asymmetric unit (asu) containing a Ln3+ center, two coordinated organic ligands (one fully deprotonated, aip2−, and the other monoprotonated, Haip−), five coordinated andPDF Image | Green Chemistry with Supercritical CO2 and Enzymes
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