Porous and chelated nanostructured multifunctional materials: recoverable and reusable sorbents for extraction of metal ions and catalysts for diverse organic reactions

Inorganic-based sorbents

The use of surfactant-coated mineral oxides (generally alumina and silica) as sorbent materials has received much attention for extraction of various metal ions. These sorbent materials provide high surface areas. The modification of sorbent helps in their selectivity and increased activity. The most commonly used methods of modification are physisorption and chemisorption. However, chemical methods provide high stability and reusability.

Organic-based sorbents

Multiwalled carbon nanotubes (MWCNTs) have high tensile strengths and great thermal and chemical stability. Due to high surface area and large micropore volume, these are good adsorbents for metal ion extraction [ ⁴³ , ⁴⁴ ].

Ghaedi et al. modified MWCNTs with 2-(2-hydroxy-5-nitrophenyl)-4,5-diphenyl imidazole for enrichment of trace amounts of various ions. To obtain highly efficient metal ions, optimization of the influence of variables including pH, amount of ligand and solid phase, type and condition of eluent and common coexisting ions was done. The metal ions (Co ²⁺ , Cu ²⁺ , Ni ²⁺ , Fe ²⁺ , Pb ²⁺ and Zn ²⁺ ) were then desorbed using nitric acid [ ⁶ ].

Soylakab et al. modified MWCNTs with ( E )- N 1-(4-nitro-benzylidene)- N 2-(2-(( E )-4-nitrobenzylideneamino)ethyl) ethane-1,2-diamine (NBNBAEED). Due to the existence of donating nitrogen atom as well as –NH group in NBNBAEED, both the stability and selectivity of its Cu ²⁺ , Zn ²⁺ , Pb ²⁺ and Cd ²⁺ ion complex over other metal ions increased, thus providing fast adsorption and high metal recovery [ ⁴⁵ ].

Reduction reactions

Both metal and metal oxides modified with suitable chelating agents have been used as catalysts in reduction reactions. Du and He immobilized gold nanoparticles into previously fabricated amino-functionalized silica nanoparticles with centre-radially hierarchical mesopores (Au–NH ₂ –HMSNs) and used them as catalyst in catalytic reduction of 2-nitroaniline (Fig.  ² ). The centre-radially hierarchical mesopores enhanced diffusion of guest molecules, thus providing excellent catalytic activity and stability [ ⁴⁷ ].

Fig. 2

Jiang et al. prepared nano-Ni core mesoporous-silica shell (NCMSS) particles using EDTA as the chelating agent (Ni@SiO ₂ /EDTA) and used them as a catalyst for reduction of 4-nitrophenol to 4-aminophenol in the presence of excess amount of NaBH ₄ (Fig.  ³ ). The reaction was found to show pseudo-first order kinetics with respect to 4-nitrophenol. Since only a small amount of catalyst provides great results, NCMSSs have a great potential in catalytic fields in future [ ²⁶ ].

Fig. 3

Sharma et al. immobilized copper(II) acetylacetone complex over amino functionalized silica@magnetite nanoparticles (Cu–acac@Am–Si–Fe ₃ O ₄ ) for catalytic reduction of nitro group using NaBH ₄ as source of hydrogen (Fig.  ⁴ ). They also studied the kinetics for reduction reaction of o -nitroaniline to benzenediamine. Since nano-catalytic system can be easily recovered using magnets, they can be reused for multiple cycles [ ⁴⁸ ].

Fig. 4

Negroa et al. prepared three Fe–N non-noble electrocatalysts using networked graphitic structures as support. They found that catalysts with higher cobalt and iron content showed maximum oxygen reduction reaction (ORR) activity. This electrocatalyst showed promising applications in low-temperature fuel cells [ ⁴⁹ ].

Veerakumara et al. synthesized gold nanoparticles (AuNPs) using highly branched polyethylenimine (PEI) and acylated PEI as the chelating agents. These modified gold nanoparticles were found to be excellent catalyst for selective reduction of nitro compounds in presence of sodium borohydride (Fig.  ⁵ ). This catalyst was found to be stable for a long time without any provided support [ ⁵⁰ ].

Fig. 5

Oh et al. synthesized nitrogen-modified carbon nanofibres by pyrolysis of cobalt, polypyrrole (PPy) using ethylenediamine (ED) as the chelating agent for obtaining stable oxygen reduction catalysts. They found that combined use of PPy and ED increased activity and stability of catalyst by increasing total nitrogen contents [ ⁵¹ ].

Photocatalytic degradation reactions

Yang et al. synthesized YMn ₂ O ₅ nanoparticles using different chelating agents. They found that YMn ₂ O ₅ nanoparticles prepared using citric acid exhibited slightly high photocatalytic activity than the sample prepared using EDTA as the chelating agent. These nanoparticles showed excellent photocatalytic activity for oxidative decomposition of methyl red under UV and visible light irradiation [ ²⁷ ].

Zhanga et al. synthesized Bi ₂ Fe ₄ O ₉ nanoparticles using different chelating agents and observed their photocatalytic activity. The use of EDTA favoured the synthesis of relatively small particle sized Bi ₂ Fe ₄ O ₉ nanoparticles. These nanoparticles were found to exhibit excellent photocatalytic degradation of methyl red [ ⁴⁶ ].

Makarova et al. modified surface of nanocrystalline titanium dioxide with specific chelating agents for photochemical reduction of nitrobenzene. The modification was done using arginine, lauryl sulphate and salicylic acid (Fig.  ⁶ ). It was found that arginine-modified TiO ₂ (TiO ₂ /Arg) was more stable and showed better photocatalytic decomposition as compared to other chelating agents [ ⁵² ].

Fig. 6

Wang et al. synthesized BaTiO ₃ nanoparticles using acetylacetone and citric acid as chelating agents. These nanoparticles were tested for their photocatalytic activities for removal of humic acid from water. The tetragonal phase BaTiO ₃ nanoparticles were found to be excellent photocatalysts for decomposition of organic pollutants [ ⁵³ ].

Habibi and Mokhtari fabricated novel sulphur-modified niobium (V) oxide nanoparticles by a modified sol–gel method. A very stable sol-containing niobium (V) chloride and oxalic acid were used with isopropanol as chelating agent and thiourea as sulphur source. Unlike unmodified niobium (V) nanoparticles, the doped ones induced charge transfer across the niobium pentoxide interface. These nanoparticles can be suitably used as semiconductor photocatalysts and for visible light sensitization [ ⁵⁴ ]. Sun et al. synthesized nano-sized BiVO ₄ by a hydrothermal process in the presence of EDTA as chelating agent. With reduction of hydrothermal temperature and increase in concentration of EDTA, the size of BiVO ₄ nanoparticles decreases. This sample showed a high photocatalytic activity for degradation of phenol solution [ ⁵⁵ ].

Saien and Mesgari prepared nitrogen-doped TiO ₂ nanoparticles using hematoporphyrin as the support material. These nanocomposites exhibited highly efficient visible-light photocatalytic activity for degradation of methyl orange. These nanocomposites can be reused again without any loss in its activity [ ⁵⁶ ].

Hydrogenation/dehydrogenation reactions

Tianbin et al. synthesized Zr-based metal organic frameworks (Zr/MOF) that contained both mesopores and micropores by the reaction of 1,4-benzenedicarboxylic acid (H ₂ BDC) and ZrOCl ₂ ·8H ₂ O using citric acid as the chelating agent and cetyltrimethylammonium bromide (CTAB) as surfactant. The Ru@Zr/MOF (ruthenium nanoparticles embedded on Zr-based MOF) catalyst was prepared in supercritical CO ₂ –methanol solution. This resulted in Ru nanoparticles with small size that were uniformly supported in the Zr–MOF unlike in the case of immobilization where particles were of larger size. This catalyst showed significant catalytic activity in hydrogenation of benzene and its derivatives (Fig.  ⁷ ) [ ⁵⁷ ].

Fig. 7

Qureshi et al. prepared Pd nanoparticles supported on porous silica nanospheres KCC-1. The silica-supported Pd nanocatalysts were prepared by modification of the supports with 3-aminopropyltrimethoxysiliane (3-APTES). 3-APTES acted as binding agent for PdCl ₂ with the aim of obtaining well-spaced and uniformly dispersed Pd atoms on the silica surface. These proved to be efficient, chemoselective catalysts for hydrogenation of alkenes and α,β-unsaturated carbonyl compounds (Fig.  ⁸ ). A comparison between Pd nanoparticles supported on KCC-1 and those prepared on traditional mesoporous silica materials under similar conditions was done. It was found that the nanoparticles on KCC-1 showed excellent catalytic activity and particle dispersion. This catalyst can be reused many times without any significant loss in its catalytic activity [ ⁵⁸ ].

Fig. 8

Oxidation reactions

Sharma and Sharma immobilized palladium complex on silica nanospheres functionalized with 3-aminopropyltriethoxysilane (SiO ₂ @APTES@Pd-FFR). This nano-catalyst was found to be highly selective for oxidative amination of aldehydes (Fig.  ⁹ ). The high activity of this nanocatalyst provided ease in time and cost along with high yield and enhanced dispersion. Since this catalyst provided high turn-over frequencies and can be easily reused without loss in its activity, the catalyst proved to be useful for industrial and environmental applications [ ⁵⁹ ].

Fig. 9

Gholivand and Azadbakht prepared nano-structured Ni(II)–chelidamic acid-modified gold nanoparticles (Ni(II)/CHE/AuNPs) by electrodeposition of Ni(II)/CHE complex on the surface of the AuNP–Au electrode in alkaline solution. This catalyst was tested for electrocatalytic oxidation (Fig.  ¹⁰ ) and determination of methanol. This electrode proved to be an excellent catalyst for oxidation of methanol [ ⁶⁰ ].

Fig. 10

Kumar et al. prepared nano-spinel CuAl ₂ O ₄ and investigated the effect of ethylenediamine addition. This catalyst was found to have high surface area, low activation energy and high porosity. Due to more number of active sites at low temperature, it is highly active for selective oxidation of benzyl alcohol to benzaldehyde (Fig.  ¹¹ ). This explained that addition of ethylenediamine during preparation process enhanced the catalytic activity of copper aluminate [ ⁶¹ ].

Fig. 11

Also, Cruz et al. also used self-prepared copper(II) complexes supported on hybrid mesoporous SBA-15 functionalized with imidazole ionic liquid and an amino chelate ligand, as efficient catalyst for selective oxidation of benzyl alcohol to benzaldehyde in aqueous phase [ ⁶² ].

Pandey et al. prepared stable CuO nanoparticles immobilized on resin in presence of cyclodextrin as an organic support material. These nanoparticles were found to be effective catalyst for liquid phase oxidation of alcohols to aldehydes. The catalytic activity was found to be better than the earlier used Cu(0) nanocomposites [ ⁶³ ].

Coupling reactions

Dutta and Sarkar immobilized palladium nanoparticles on chemically modified silica gel which were stabilized by encapsulation through PEG tentacles. These nanoparticles were prepared by reaction of modified silica gel with potassium tetrachloropalladate(II) in presence of an acyl metal salt of a Fischer carbene complex. This nanocatalyst proved to be an efficient heterogeneous catalyst for Suzuki, Stille and Sonogashira cross-coupling reactions (Fig.  ¹² ) [ ⁶⁴ ].

Fig. 12

He et al. synthesized a novel palladium(II) organometallic catalyst by coordinating Pd(II) ions with PPh ₂ -functionalized mesoporous silica nanospheres. Unlike the ones previously prepared by traditional methods, this catalyst exhibited higher activity and selectivity for water-medium C–C coupling reactions and could be used repeatedly for various cycles [ ⁶⁵ ]. Mondal et al. prepared palladium(II) 2-aminopyridine complex in situ by co-condensation reaction of tetraethylorthosilicate (TEOS) with ((chloromethyl)phenylethyl)trimethoxysilane (CMPE-TMS) using CTAB as the structure directing agent and supported it on mesoporous silica nanospheres (Pd–AMP–MSN). It was shown to have high catalytic activity for Suzuki–Miyaura cross-coupling reactions of aryl iodides, aryl bromides and also aryl chlorides with phenylboronic acids in water medium with high yields (Fig.  ¹³ ) [ ⁶⁶ ].

Fig. 13

Isfahani et al. immobilized palladium nanoparticles on nanosilica triazine dendritic polymer (Pd–nSTDP). These were found to be efficient catalysts for Sonogashira cross coupling of aryl halides with aromatic and aliphatic terminal alkynes where water was used as solvent (Fig.  ¹⁴ ). This catalytic system can also be used for preparation of V- and star-shaped molecules and the catalyst can be recovered and reused [ ⁶⁷ ].

Fig. 14

Vaghei et al. synthesized novel SBA-15/AO/Pd(II) nanocatalyst by grafting of amidoxime on SBA-15 and deposition of palladium chlorides. This catalyst, under mild conditions, was found to have high catalytic activity for the Suzuki–Miyaura cross-coupling reaction of aryl halides with phenylboronic acid (Fig.  ¹⁵ ) [ ⁶⁸ ].

Fig. 15

Naghipour and Fakhri prepared Pd-based nanocatalyst supported on magnetite nanoparticles using chitosan-Schiff base as the chelating agent. This catalyst (Pd/Fe ₃ O ₄ @chitosan-Schiff base) in mild experimental conditions was found to be highly efficient for Suzuki–Miyaura and Heck–Mizoroki C–C coupling reactions. Also, since the catalyst is supported on magnetite nanoparticles, it can be easily recovered magnetically [ ⁶⁹ ].

Miscellaneous reactions

Nyalosaso et al. prepared aluminium-derived silica monodisperse nanospheres (Al–MSS-22) by a one-step synthesis–functionalization method and its surface properties were studied. It was found that this aluminosilicate nanocatalyst showed catalytic activity in various esterification reactions of ethanoic acid with selected alcohols and polyols (Fig.  ¹⁶ ) [ ⁷⁰ ].

Fig. 16

Tang et al. immobilized salicylaldimine transition metal complexes on mesoporous silica nanoparticles (Sal–MSN). These nanoparticles, because of high surface area of support, provide high concentration of catalytically active sites. These nanocatalysts were found to be efficient catalysts for epoxidation of styrene [ ⁷¹ ].

Li et al. used aqueous phosphine–Rh complexes immobilized on non-porous fumed-silica nanoparticles (HRh(CO)(TPPTS) ₃ @SiO ₂ ) for their catalytic activity in higher olefin hydroformylation. They showed that the structure and hydration of support material are the two important factors in hydroformylation reaction. It was found that addition of basic alkali metal salts in the system favoured hydroformylation of 1-hexene [ ⁷² ].

Chen et al. synthesized dialkylaminopyridine-functionalized mesoporous silica nanospheres (DMAP–MSN). This nanocatalyst showed high turnover frequency for Baylis–Hillman, acylation and silylation reactions and could also serve as efficient catalyst for various other nucleophilic reactions (Fig.  ¹⁷ ) [ ⁷³ ].

Fig. 17

Peng et al. synthesized acid–base bi-functionalized mesoporous silica nanoparticles. These nanoparticles were studied for catalytic activity for one-pot conversion of 5-hydroxymethylfurfural (HMF) in an ionic liquid by using cellulose, cellobiose, glucose and fructose as starting materials. It was found that these nanoparticles showed high catalytic activity towards cellulose-to-HMF conversion (Fig.  ¹⁸ ) [ ⁷⁴ ].

Fig. 18

Tang et al. immobilized salicylaldimine cobalt complexes on mesoporous silica nanoparticles (Co–MSN/MAO). These nanocatalysts showed higher activity than existing homogeneous analogue and proved to be highly efficient for catalytic polymerization of 1,3-butadiene [ ⁷⁵ ].

Sharma et al. synthesized zirconium (IV)-modified@amine functionalized silica nanocomposites by grafting of 3-hydroxy-2-methyl-1,4-napthoquinone covalently (Zr(IV)–HMNQ@ASMPs). These catalysts were found to be efficient for Friedel–Crafts, Knoevenagel and Pechmann condensation reactions (Fig.  ¹⁹ ). The catalyst provided high selectivity and could be reused for a number of cycles [ ⁷⁶ ].

Fig. 19

Sharmaet al. immobilized quinolone-2-carboxyaldehyde on amine functionalized silica-coated magnetite nanoparticles. This was followed by metallation with copper acetate (Cu–2QC@Am–SiO ₂ @Fe ₃ O ₄ ). This catalyst showed efficient activity for synthesis of carbamates via C–H activation of formamides (Fig.  ²⁰ ). The catalyst was easily recovered and could be reused for multiple cycles [ ⁷⁷ ].

Fig. 20