{
  "abstract": "Recovery of metals from spent homogeneous catalysts is challenging because strong metal-ligand complexation often prevents conventional inorganic precipitation. In this study, ion-pair-based separation processes were developed for the direct recovery of Ni-2,2'-bipyridine (bpy) complexes from spent catalyst solutions. Perchlorate (ClO₄ ⁻) was introduced as a counterion to form the poorly water-soluble ion pair Ni(bpy)₃(ClO₄)₂ (NIP), and ion-pair extraction (IPE) using 1,2-dichloroethane and ion-pair precipitation (IPP) were systematically compared. Alkaline pretreatment was required to adjust the dissolved bpy/Ni ratio to approximately 3, thereby favoring the formation of Ni(bpy)₃ ²⁺. Repeated IPE subsequently achieved >99% Ni recovery from both model and actual spent catalyst solutions. In contrast, IPP strongly depended on the initial Ni concentration and solution composition, requiring increasingly large ClO₄ ⁻/Ni ratios at lower Ni concentrations. PHREEQC modeling showed that the experimentally determined apparent solubility product governed the onset of NIP precipitation, whereas formation of a dissolved Ni(bpy)₃(ClO₄)₂ ion pair limited the Ni recovery by IPP. ATR-FTIR spectroscopy supported the recovery of the Ni-bpy complex containing ClO₄ ⁻ as the counterion. Furthermore, more than 95% of the ClO₄ ⁻ counterions were successively exchanged with Cl⁻ using an anion-exchange resin. These results demonstrate that ion-pair separation enables direct recovery of a functional Ni-bpy complex and provides a strategy for closed-loop recycling of homogeneous catalysts.",
  "abstract_shown": true,
  "affiliations": [
    "Department of Resources and Environmental Engineering, Faculty of Science and Engineering, Waseda University 169-8555 Tokyo Japan tokoro@waseda.jp.",
    "Department of System Innovation, Faculty of Engineering, The University of Tokyo 113-8656 Tokyo Japan.",
    "Next-Generation Technology Design and Creation, Takahata Precision Co., Ltd 160-0023 Tokyo Japan."
  ],
  "article_type": "research article",
  "arxiv": null,
  "author_total": 5,
  "authors": [
    {
      "affs": [
        0
      ],
      "name": "Hisanori Iwai"
    },
    {
      "affs": [
        1
      ],
      "name": "Yutaro Takaya",
      "orcid": "0000-0003-4046-6780"
    },
    {
      "affs": [
        2
      ],
      "name": "Naoki Yokota"
    },
    {
      "affs": [
        2
      ],
      "name": "Yuko Takahashi"
    },
    {
      "affs": [
        0,
        1
      ],
      "name": "Chiharu Tokoro",
      "orcid": "0000-0001-6214-0402"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/5pyfc11afaj69.bib",
  "cited_by": 0,
  "doi": "10.1039/d6ra08431a",
  "fulltext_url": "https://europepmc.org/articles/PMC13687800",
  "id": "5pyfc11afaj69",
  "issns": [
    "2046-2069"
  ],
  "landing_url": "https://europepmc.org/article/MED/42923176",
  "language": "eng",
  "licence": "CC-BY",
  "licence_label": "CC BY",
  "licence_source": "article",
  "licence_url": "https://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": null,
  "native_subjects": [
    [
      "openalex",
      "2210|Extraction and Separation Processes"
    ]
  ],
  "openalex": "W7223171060",
  "pdf_url": "https://europepmc.org/articles/PMC13687800?pdf=render",
  "pmcid": "PMC13687800",
  "pmid": "42923176",
  "published": "2026-10-22",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/5pyfc11afaj69.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:20:33Z",
      "native_id": "MED:42923176",
      "source": "europepmc",
      "url": "https://europepmc.org/article/MED/42923176"
    }
  ],
  "subjects": [
    {
      "field": "chemistry",
      "origin": "issn:2046-2069",
      "subfield": "general"
    }
  ],
  "title": "Ion-pair-assisted recovery of nickel(ii) tris(2,2'-bipyridine) complexes from spent catalysts by extraction and precipitation.",
  "title_text": "Ion-pair-assisted recovery of nickel(ii) tris(2,2'-bipyridine) complexes from spent catalysts by extraction and precipitation.",
  "type": "peer-reviewed",
  "updated_at": "2026-10-30T12:20:33Z",
  "url": "https://research.draconitic.com/paper/5pyfc11afaj69",
  "venue": "RSC advances",
  "versions": []
}