Improved Lignin Polyurethane Properties with Lewis Acid Treatment

Chemical modification strategies to improve the mechanical properties of lignin-based polyurethanes are presented. We hypothesized that treatment of lignin with Lewis acids would increase the concentration of hydroxyl groups available to react with diisocyanate monomers. Under the conditions used, hydrogen bromide-catalyzed modification resulted in a 28% increase in hydroxyl group content. Associated increases in hydrophilicity of solvent-cast thin films were also recorded as evidenced by decreases in water contact angle. Polyurethanes were then prepared by first preparing a prepolymer based on mixtures of toluene-2,4-diisocyanate (TDI) and unmodified or modified lignin, then polymerization was completed through addition of polyethylene glycol (PEG), resulting in mass ratios of TDI:lignin:PEG of 43:17:40 in the compositions investigated here. The mixture of TDI and unmodified lignin resulted in a lignin powder at the bottom of the liquid, suggesting it did not react directly with TDI. However, a homogeneous solution resulted when TDI and the hydrogen bromide-treated lignin were mixed, suggesting demethylation indeed increased reactivity and resulted in better integration of lignin into the urethane network. Significant improvements in mechanical properties of modified lignin polyurethanes were observed, with a 6.5-fold increase in modulus, which were attributed to better integration of the modified lignin into the covalent polymer network due to the higher concentration of hydroxyl groups. This research indicates that chemical modification strategies can lead to significant improvements in the properties of lignin-based polymeric materials using a higher fraction of an inexpensive lignin monomer from renewable resources and a lower fraction an expensive, petroleum-derived isocyanate monomer to achieve the required material properties

Polymer-Grafted Lignin Surfactants Prepared via Reversible Addition–Fragmentation Chain-Transfer Polymerization

Kraft lignin grafted with hydrophilic polymers has been prepared using reversible addition–fragmentation chain-transfer (RAFT) polymerization and investigated for use as a surfactant. In this preliminary study, polyacrylamide and poly­(acrylic acid) were grafted from a lignin RAFT macroinitiator at average initiator site densities estimated to be 2 per particle and 17 per particle. The target degrees of polymerization were 50 and 100, but analysis of cleaved polyacrylamide was consistent with a higher average molecular weight, suggesting not all sites were able to participate in the polymerization. All materials were readily soluble in water, and dynamic light scattering data indicate polymer-grafted lignin coexisted in isolated and aggregated forms in aqueous media. The characteristic size was 15–20 nm at low concentrations, and aggregation appeared to be a stronger function of degree of polymerization than graft density. These species were surface active, reducing the surface tension to as low as 60 dyn/cm at 1 mg/mL, and a greater decrease was observed than for polymer-grafted silica nanoparticles, suggesting that the lignin core was also surface active. While these lignin surfactants were soluble in water, they were not soluble in hexanes. Thus, it was unexpected that water-in-oil emulsions formed in all surfactant compositions and solvent ratios tested, with average droplet sizes of 10–20 μm. However, although polymer-grafted lignin has structural features similar to nanoparticles used in Pickering emulsions, its interfacial behavior was qualitatively different. While at air–water interfaces, the hydrophilic grafts promote effective reductions in surface tension, we hypothesize that the low grafting density in these lignin surfactants favors partitioning into the hexanes side of the oil–water interface because collapsed conformations of the polymer grafts improve interfacial coverage and reduce water–hexanes interactions. We propose that polymer-grafted lignin surfactants can be considered as random patchy nanoparticles with mixed hydrophilic and hydrophobic domains that result in unexpected interfacial behaviors. Further studies are necessary to clarify the molecular basis of these phenomena, but grafting of hydrophilic polymers from kraft lignin via radical polymerization could expand the use of this important biopolymer in a broad range of surfactant applications

Electrocatalytic Oxygen Evolution with an Immobilized TAML Activator

Iron complexes of tetra-amido macrocyclic ligands are important members of the suite of oxidation catalysts known as TAML activators. TAML activators are known to be fast homogeneous water oxidation (WO) catalysts, producing oxygen in the presence of chemical oxidants, e.g., ceric ammonium nitrate. These homogeneous systems exhibited low turnover numbers (TONs). Here we demonstrate immobilization on glassy carbon and carbon paper in an ink composed of the prototype TAML activator, carbon black, and Nafion and the subsequent use of this composition in heterogeneous electrocatalytic WO. The immobilized TAML system is shown to readily produce O₂ with much higher TONs than the homogeneous predecessors

Effects of Ligand Chemistry and Geometry on Rare Earth Element Partitioning from Saline Solutions to Functionalized Adsorbents

Rare earth elements (REE) are elements that drive the development of new technologies in many sectors, including green energy. However, the supply chain of REE is subject to a complex set of technical, environmental, and geopolitical constraints. Some of these challenges may be circumvented if REE are recovered from naturally abundant alternative sources, such as saline waters and brines. Here, we synthesized and tested aminated silica gels, functionalized with REE-reactive ligands: diethylenetriaminepentaacetic acid (DTPA), diethylenetriaminepentaacetic dianhydride (DTPADA), phosphonoacetic acid (PAA), and N,N-bisphosphono­(methyl)­glycine (BPG). A suite of characterization techniques and batch adsorption experiments were used to evaluate the properties of the functionalized silica adsorbents and test the REE-uptake chemistry of the adsorbents under environmentally relevant conditions. Results showed that BPG and DTPADA yielded the most REE-reactive adsorbents of those tested. Moreover, the DTPADA adsorbents demonstrated chemical and physical robustness as well as ease of regeneration. However, as in previous studies, amino-poly­(carboxylic acid) adsorbents showed limited uptake at midrange pH and low-sorbate concentrations. This work highlighted the complexity of intermolecular interactions between even moderately sized reactive sites when developing high-capacity, high-selectivity adsorbents. Additional development is required to implement an REE recovery scheme using these materials; however, it is clear that BPG- and DTPADA-based adsorbents offer a highly reactive adsorbent warranting further study

Effects of Ligand Chemistry and Geometry on Rare Earth Element Partitioning from Saline Solutions to Functionalized Adsorbents

Rare earth elements (REE) are elements that drive the development of new technologies in many sectors, including green energy. However, the supply chain of REE is subject to a complex set of technical, environmental, and geopolitical constraints. Some of these challenges may be circumvented if REE are recovered from naturally abundant alternative sources, such as saline waters and brines. Here, we synthesized and tested aminated silica gels, functionalized with REE-reactive ligands: diethylenetriaminepentaacetic acid (DTPA), diethylenetriaminepentaacetic dianhydride (DTPADA), phosphonoacetic acid (PAA), and N,N-bisphosphono­(methyl)­glycine (BPG). A suite of characterization techniques and batch adsorption experiments were used to evaluate the properties of the functionalized silica adsorbents and test the REE-uptake chemistry of the adsorbents under environmentally relevant conditions. Results showed that BPG and DTPADA yielded the most REE-reactive adsorbents of those tested. Moreover, the DTPADA adsorbents demonstrated chemical and physical robustness as well as ease of regeneration. However, as in previous studies, amino-poly­(carboxylic acid) adsorbents showed limited uptake at midrange pH and low-sorbate concentrations. This work highlighted the complexity of intermolecular interactions between even moderately sized reactive sites when developing high-capacity, high-selectivity adsorbents. Additional development is required to implement an REE recovery scheme using these materials; however, it is clear that BPG- and DTPADA-based adsorbents offer a highly reactive adsorbent warranting further study