Porphyrin-based metal–organic framework/graphene composites for selective yttrium capture: Adsorption properties and mechanistic study

Abstract

In this study, a porous porphyrin-based metal–organic framework/reduced graphene oxide ((Fe–P)n–MOF/graphene) composite was prepared via hydrothermal reduction with tetracarboxyphenyl porphyrin (TCPP) and iron(III) chloride (FeCl3) as the main raw materials. The composite was designed to serve as a selective adsorbent for yttrium ions (Y3+). The adsorption performance of the composite toward Y3+ was investigated. The results indicated that the maximum adsorption capacity of the composite was 102.1 mg/g. The adsorption process followed the quasi-second-order kinetic and Langmuir isotherm models, indicating a monolayer chemical adsorption mechanism. The composite material was comprehensively characterized to analyze its adsorption mechanism. Using density functional theory (DFT) calculations, the electrostatic potential distribution in (Fe–P)n–MOF and the binding energies of its adsorption sites toward metal ions were simulated to further determine the Y3+ adsorption mechanism of (Fe–P)n–MOF. The composite demonstrated excellent selective adsorption of Y3+ from rare-earth leaching solutions and maintained a recovery rate exceeding 90% even after more than five regeneration cycles. Thus, (Fe–P)n–MOF/graphene is a promising Y3+ adsorbent.

1. Introduction

Rare-earth elements (REEs) are indispensable for modern industrial development because doping specialized materials with even small amounts of REEs can significantly enhance their performance, often described as the “Midas touch” effect. Yttrium, a key REE, has been widely used in various fields, including industrial catalysis, medical imaging, cutting-edge technologies, and renewable energy systems. The study of yttrium adsorption and separation holds significant scientific and industrial value. Although ion exchange methods and solvent extraction technologies have been widely explored for yttrium separation, they often encounter challenges, such as high operational costs, complex waste-management procedures, and environmental risks associated with toxic reagents. For example, traditional yttrium separation processes relying on naphthenic acid extractants have become economically unsustainable because of supply shortages and declining quality, further exacerbating cost inefficiency. Adsorption-based methods are being increasingly recognized as promising alternatives to traditional separation processes owing to operational simplicity, cost-effectiveness, and environmental compatibility, particularly for recovering REEs from low-concentration sources. However, commonly used adsorption materials cannot selectively adsorb REEs; thus, the targeted grafting modification of adsorption materials has emerged as a research direction to achieve selective adsorption of REEs.

Metal–organic frameworks (MOFs) are nanoscale adsorption materials consisting of target organic matter and metal-ion clusters, forming a microscopic regular shape. They lose their adsorption performance in strong acidic/basic solutions. Thus, MOF composites are typically prepared to enhance their stability. Carbon-based materials exhibit some level of tolerance to strong acidic and basic environments, allowing composite materials prepared using carbon-based materials and MOFs to exhibit enhanced stability in strong acidic and basic solutions. Graphene is a popular carbon-based material with excellent physical and chemical properties and has been widely studied in the fields of chemical modification and adsorption recovery. Graphene oxide (GO) has abundant active sites on its surface, allowing it to adsorb metal cations either physically or chemically. The extremely high specific surface area of GO provides a large area for interactions with REEs, thereby enhancing the adsorption capacity of GO for REEs. Because of its high stability and abundant oxygen-containing functional groups, GO is widely used as an REE adsorbent material and plays an important role in REE extraction and recovery.

A composite adsorption material with macropores (>50 nm), mesopores (2–50 nm), and micropores (<2 nm) and a high REE adsorption capacity can be synthesized by loading MOF nanoparticles onto a graphene hydrogel. In addition, porphyrins have been selected as organic ligands for MOFs because of their high selectivity and affinity for precious metals. However, the selective Y3+ adsorption ability and adsorption mechanism of porphyrin-based MOF/graphene composites remain unclear; therefore, research in this area will guide the structural optimization of REE adsorption materials. In this study, an iron-based porphyrin MOF ((Fe–P)n–MOF) was first prepared using tetracarboxyphenyl porphyrin (TCPP) and iron(III) chloride (FeCl3) as the main raw materials. The prepared (Fe–P)n–MOF was thereafter combined with GO within a hydrothermal environment.