The novel molecular sieve used in lithium extraction adsorbents achieves precise separation of lithium and magnesium ions through the synergistic effect of structural design and chemical modification. This breakthrough relies primarily on three core technological approaches: molecular sieve pore engineering, surface chemistry manipulation, and functional modification. Combined with electrochemical drive and membrane separation technology, this creates a multi-layered ion separation system.
Molecular sieve pore engineering is the foundation of precise separation. Traditional molecular sieves typically have nanometer-scale pore sizes, but the hydration radii of lithium and magnesium ions vary greatly, making efficient separation difficult based solely on size exclusion. The novel molecular sieve introduces subnanometer pores, such as channels with pore sizes as precise as 0.38 nanometers, to directly block the passage of magnesium ions and their hydrates through steric hindrance while allowing lithium ions to diffuse freely. This pore engineering relies not only on the precise synthesis of the silicon-aluminum framework but also on the formation of a stable spinel structure through the doping of metal ions (such as zinc, copper, and cobalt), further optimizing the pore size and surface charge distribution, thereby enhancing the selective adsorption of lithium ions.
Surface chemistry manipulation is key to improving selectivity. New molecular sieves are modified with specific functional groups on their pore surfaces, such as hydroxyl, carboxyl, or sulfonic acid groups. These groups form weak coordination bonds with lithium ions, enhancing adsorption affinity through chemical reactions. Furthermore, the surface charge distribution is carefully designed to render the molecular sieve negative in solution, thereby repelling positively charged magnesium ions. For example, titanium-based lithium ion sieves utilize a "memory effect" created by lithium vacancies within their layered structure, preferentially adsorbing lithium ions during ion exchange. Aluminum-based layered double hydroxides, on the other hand, utilize the exchangeability of chloride ions between the layers to create adsorption sites with high selectivity for lithium ions.
Functional modification further expands the screening capabilities of molecular sieves. The development of magnetic ion sieves is a prime example. By doping lithium ion sieves with ferroferric oxide nanoparticles, the material is endowed with magnetic properties, facilitating rapid separation and recovery under an applied magnetic field. This design not only improves the recycling rate of lithium extraction adsorbents but also reduces the energy consumption of traditional filtration processes. Furthermore, the introduction of composite membrane technology combines molecular sieves with conductive materials (such as MXene) to form membrane structures with electrochemical actuation capabilities. Under the action of an electric field, lithium ions pass through the membrane at a significantly faster rate, while magnesium ions are blocked due to charge repulsion or size limitations, achieving dynamic screening.
Electrochemically driven technology provides a new impetus for precise screening. Conductive lithium-ion screening membranes, combined with electrochemical switching technology, control ion adsorption and desorption in a constant potential mode. During the adsorption phase, the membrane surface is positively charged, attracting lithium ions from the solution. During the desorption phase, a reverse potential is applied to desorb lithium ions while avoiding material dissolution caused by acid washing. This technology not only improves cycle life but also enables semi-continuous operation, significantly enhancing the feasibility of industrial applications.
The integrated application of membrane separation technology further optimizes screening efficiency. New molecular sieves are combined with polymer materials such as polysulfone to form a membrane structure with selective permeability. Under pressure or electric field drive, lithium ions pass through the membrane at a much higher rate than magnesium ions, which are retained due to their larger size or charge repulsion. This membrane separation technology can be coupled with an adsorption process to form a multi-stage screening system, first concentrating lithium ions through membrane separation, then using molecular sieves for deep purification, significantly reducing the difficulty of subsequent processing.
The application of new molecular sieves in lithium extraction is evolving from single-material innovation to collaborative innovation across materials, processes, and systems. Through multi-metal doping to inhibit dissolution, electrochemical regulation to achieve green regeneration, and the development of three-stage adsorption systems tailored to complex brines, the technical bottlenecks of lithium extraction adsorbents are gradually being overcome. In the future, with the exploration of cutting-edge technologies such as hierarchically porous MOFs and self-healing coatings, new molecular sieves are expected to optimize both efficiency and cost in the industrialization of lithium extraction from salt lakes, providing key technical support for the efficient utilization of global lithium resources.
