Synthesis and characterization of multi-walled carbon nanotubes-supported dibenzo-14-crown-4 ether with proton ionizable carboxyl sidearm as Li+ adsorbents

• MWCNT-supported crown ether adsorbents were prepared for Li+ recovery.
• Synthesis involved MWCNT oxidation, epoxidation, ring opening, and etherification.
• Functionalization steps were verified through several characterization techniques.
• Two MWCNT adsorbents prepared contained: (1) neutral CE and (2) CE with COOH moiety.
• Type (2): better performance, Langmuir-type adsorption, most selective towards Li+.

A new preparation method for solid-supported crown ethers (CE) as lithium ion (Li+) adsorbents is presented. Hydroxy-dibenzo-14-crown-4 ether (HDB14C4) was immobilized on multi-walled carbon nanotubes (MWCNTs) through the following steps: (1) MWCNTs were oxidized to generate carboxyl groups (COOH) as functionalization sites for (2) epoxide-terminated linkers. The (3) subsequent epoxide ring opening resulted in the attachment of HDB14C4 and generation of a hydroxyl group in which (4) proton ionizable COOH sidearm was etherified as an optional post CE functionalization step. From this synthesis route, two types of adsorbents were produced: type 1 as MWCNTs with neutral HDB14C4 (steps 1–3) and type 2 as MWCNTs with HDB14C4–COOH sidearm (steps 1–4). Functional group titration and gravimetry revealed that the performed reaction steps efficiently modified the MWCNTs which were strongly supported by FTIR and TGA results. Raman and TEM analyses revealed the preservation of structural integrity of the MWCNTs after functionalization. Between the two materials, the presence of a COOH sidearm in CE of type 2 adsorbent significantly enhanced the Li+ uptake at pH ⩾7. A Langmuir-type of Li+ adsorption occurred in type 2 adsorbent. Competitive ion adsorption results revealed that type 2 preferred Li+ uptake than other metal ions as shown in the sequence: Li+ > Na+ > Mg2+ > Ca2+, K+, Sr2+. Overall results suggest that the developed synthesis route can effectively produce solid-supported CEs which can be used for precious metal ions recovery.