Magnetically responsive porous materials for efficient adsorption and desorption processes☆

Peng Tan,Yao Jiang,Xiaoqin Liu*,Linbing Sun*

State Key Laboratory of Materials-Oriented Chemical Engineering,Jiangsu National Synergetic Innovation Center for Advanced Materials(SICAM),College of Chemical Engineering,Nanjing Tech University,Nanjing 210009,China

Keywords:Adsorbents Adsorption Desorption Separation Porous materials Magnetic composites

A B S T R A C T Magnetically responsive porous materials possess unique properties in adsorption processes such as magneticinduced separation and heat generation in alternating magnetic fields,which greatly facilitates recycling procedures,favors long-term operation,and improves desorption rate,making conventional adsorption processes highly efficient.With increasing interest in magnetic adsorbents,great progress has been made in designing and understanding of magnetically responsive porous materials varying from monoliths to nanoscale particles used for adsorption including oil uptake,removal of hazardous substances from water,deep desulfurization of fuels,and CO2 capture in the past few years.Therefore,a review summarizing the advanced strategies of synthesizing these magnetically responsive adsorbents and the utilization of their magnetism in practical applications is highly desired.In this review,we give a comprehensive overview of this emerging field,highlighting the strategies of exquisitely incorporating magnetism to porous materials and subtly exploiting their magnetic responsiveness.Further innovations for fabricating or utilizing magnetic adsorbents are expected to be fueled.The potential opportunities and challenges are also discussed.?2018 The Chemical Industry and Engineering Society of China,and Chemical Industry Press Co.,Ltd.All rights reserved.

1.Introduction

Separating components of chemical mixtures to obtain pure or purer products is vital for chemical engineering,concerning most fundamental aspects of life such as advanced biomedicine,petrochemical industry,environmental remediation,and food safety.In the past decades,various separation progresses have been developed to achieve this goal including distillation,drying,evaporation,and so forth.However,most of the present methods require the input of heat, which accounts for a large percentage of the total energy consumption (e.g., 10%-15% for American) [1]. If alternatives were successfully developed, it could make 80%of these separations 10 times more energy efficient.Adsorption has attracted increasing attention due to its low equipment investment,easy operation,and high energy efficiency[2-9].Adsorption process is a surface phenomenon where the target components are selectively attracted to the surfaces of adsorbents through physical or chemical interactions from the mixtures[10-12].More importantly,highly pure products can be obtained with a much less energy consumption by adsorption compared with that of the widely used distillation.

In consideration that adsorbents are the key factor in adsorption processes,fabricating adsorbents with large capacities,high selectivity,fast adsorption rate,and excellent reusability are highly desired.Recently,there have been successful cases of designing and obtaining excellent adsorbents for separation, such as employing amine-based porous materials for CO2capture [13-15], utilizing uncoordinated metal sites of metal organic frameworks(MOFs)for deep desulfurization[16-19],and introducing inorganic and organic linkers metal coordination networks for separating acetylene capture from ethylene[20-22]. This suggests the great potential of adsorption technology.Nowadays,with increasing diverse demands for completing complex adsorption processes,researchers begin to focus on integrating specific functions into adsorbents in order to improve adsorbents'controllability.However,it is still a great challenge to realize this target because of the unsatisfied responsiveness of adsorbents,which hinders the operation during transportation,use,and recycling[23].In addition,it is also desired to broaden the applications of adsorbents by imparting new properties to them.

Magnetic materials have been widely applied in biomedicine[24-26],magnetic resonance imaging[27-29],and low-temperature refrigeration system through rationally exploiting their magnetic properties [30-32]. The fast responsiveness, tunable structures, and intriguing heat generation of magnetic materials are promising for making adsorption processes highly efficient.One case in point is that magnetic self-supporting porous monoliths are applied for oil/water separation, and the used adsorbents can be conveniently recycled with the assistance of external magnetic fields. The percentage of magnetic nanoparticles in the composites can be well modulated by varying the ratio of precursors.In addition,their capacities and stability as well as the cost are advantageous [33]. Another case is that superparamagnetic porous microspheres are used to remove the trace amount of hazardous substances from water,simplifying the recycling procedures and reducing the investment of time compared with that of the conventional centrifugation or filtration.Superparamagnetism minimizes the magnetic memory effects and favors the dispersion of the microspheres[34].Recently,the utilization of magnetism for adsorption has become diverse beyond recycling.Electromagnetic heating is successfully applied for efficient desorption where magnetic nanoparticles are employed as“nanoheaters”to accelerate the process of heat treating[35-37].This effect can also be used to induce the synthesis of porous materials[38-41].As magnetic materials possess such multiple functions, it is promising that more and more subtle applications would emerge in the future.

In this review,we give an overview of the synthesis,structural and adsorption properties,as well as the prospect of magnetic adsorbents for separation and purification,highlighting the strategies of exquisitely incorporating magnetism to porous materials and subtly exploiting their magnetic responsiveness. The materials are organized by their types, the employed strategies, and the resulting physical/chemical properties. The mechanism and principles of each category are described with some typical examples.In addition,the current research profiles,future opportunities,and potential challenges are discussed as well.This review aims to assist the readers to learn and understand the pioneering works in the field of magnetic adsorbents and fuel the further innovations in this field.

2.Accessible Magnetic Properties

The understanding of magnetic properties is essential to direct the construction and application of magnetic adsorbents. Magnetism is often generated from the unpaired electrons in the orbital shells[42].According to the responsive strength in the external magnetic field,magnetic materials can be classified as diamagnetic (a tendency to oppose an applied magnetic field, and therefore, to be repelled by a magnetic field),paramagnetic(magnetic moments tend to align themselves in the same direction as the applied field,thus reinforcing it),ferromagnetic (even in the absence of an applied field, the magnetic moments of the electrons in the material spontaneously line up parallel to one another),antiferromagnetic(a tendency for the intrinsic magnetic moments of neighboring valence electrons to point in opposite directions), and ferrimagnetic (retaining their magnetization in the absence of a field but with neighboring pairs of electron spins tending to point in opposite directions)substances.Ferromagnetic materials,for example Fe-,Co-,and Ni-based materials[43-46],are paid particularly attention for preparing magnetic adsorbents because of their accessibility,low cost,and suitable magnetic responsiveness.

Nowadays,some magnetic properties have been successfully applied for adsorption.Firstly,the most widely used magnetic property is the motion of magnetic adsorbents responsive to the external magnetic fields.This property can be utilized to achieve the facile separation of adsorbents from liquid phases,simplifying the recycling processes of adsorbents.Secondly,magnetic nanoparticles show superparamagnetic behaviour when their sizes of the nanoparticles are below a critical value, and each nanoparticle becomes a single magnetic domain(Fig.1a)[47].As a result,magnetic shape memory disappears because the generated attraction between nanoparticles immediately disappears once the external magnetic fields are removed.This feature is useful for the repeated aggregation and dispersion of adsorbents. In addition, magnetic nanoparticles show a high percentage of surface atoms,favoring their combination with porous materials.Thirdly,heat generation under alternating magnetic fields with high frequency is also valuable for adsorption.The passing electromagnetic wave exerts torques on the polar molecules and their dipole moments align themselves with the oscillating electric fields of the electromagnetic waves[48].Consequently,the oscillating polar molecules interact with their neighbors,leading to the generation of frictional heat.Thermal treatment is a crucial approach for the synthesis and desorption of adsorbents,which is simple and effective.

For synthesis, with the assistance of electromagnetic heating,solvothermal microenvironments can be constructed where porous crystals prefer to heterogeneously nucleate and rapidly grow around magnetic nanoparticles.As the reaction proceeds,continuous growth of porous crystals on the as-formed seeds leads to the further increase in their yields and sizes(Fig.1b)[49].For desorption,rapid heating is beneficial to facilitate desorption processes. The magnetic-fieldinduced heat can be generated from the internal of magnetic adsorbents,elevating temperatures in short time(Fig.1c)[36].This process is completely different from that of the traditional model where heat is transferred from outside to inside depending on heat transfer coefficients of materials.Fully learning accessible magnetic properties can maximize their effects on adsorption.

Fig.1.(a)Scheme illustrating the facile separation of magnetic adsorbents;(b)Scheme illustrating magnetic induction synthesis of porous materials [49]; (c) Scheme illustrating the desorption of magnetic adsorbents by using magnetic nanoparticles as“nanoheaters”[36].

3.Synthesis of Magnetic Porous Materials

Magnetic porous materials range from centimeter-scale monoliths to nanoscale microspheres.Materials at different scales exhibit distinctive properties,which is capable of meeting diverse demands.Magnetic porous materials can be classified into two categories according to their sizes and pore structures, namely magnetic self-supporting porous monoliths and magnetic microspheres. Firstly, magnetic monoliths have high-porosity and uniformly distributed magnetic components.Magnetic components can be firmly dispersed in frameworks,avoiding the aggregation after regeneration.These advantages ensure magnetic monoliths large surface area and sustainable performance.More importantly,for some applications(e.g.oil uptake),their capacities are obviously higher than that of many other porous materials because of the hierarchical structures.Secondly,the fast development of nanotechnology have pushed the progress of magnetic porous microspheres,especially for core-shell and yolk-shell structures with superparamagnetic cores and porous shells,providing chances for constructing excellent magnetic nanomaterials for adsorption. Superparamagnetism make the microspheres“magnetic memoryless”for redispersion,and various functions can also be integrated into one structure by rationally employing synthetic strategies.Based on the above classification,the synthetic strategies of magnetic adsorbents will be introduced with typical cases and in-depth discussed in this section.

3.1.Magnetic self-supporting porous monoliths

In the past decades, magnetic self-supporting porous monoliths based on metal foams/sponges and ultralow-density porous materials have been developed. The current materials can be classified into two categories according to the magnetic or nonmagnetic supporting frameworks. Firstly, magnetic metal foams/sponges with magnetic supporting frameworks are usually fabricated by constructing pores in magnetic metallic materials composed of mainly or even entirely magnetic building blocks.Various techniques have been developed to improve the porosity of metallic materials, such as spray forming, gas entrapment,electrochemical deposition,and vapor deposition.Metallic materials can either be liquid or solid or even gaseous during synthesis.The porosity obtained by these techniques is relatively high,and pores are usually macropores.Secondly,nonmagnetic ultralow-density porous materials can be employed as the supports,and magnetic components are introduced by in situ formation or post modification. The obtained composites can also possess excellent magnetic responsiveness,and some specific pore structures can be constructed by varying fabricating techniques.The synthesis and resulting textural/chemical properties of the above two categories will be introduced with some typical examples in this section.

3.1.1.Magnetic metal foams

Magnetic metal foams are mainly or completely composed of magnetic building blocks,such as Fe,Co,Ni and their compounds.Naturally,magnetic building blocks are apt to form stable dense solids that exist in mineral resources.For example,magnetite is composed of non-porous Fe3O4crystals with spinel structure.But the dense solids exhibit poor potential for adsorption.Therefore,it is necessary to develop practical strategies to construct desired pore structures during artificial synthesis.

Traditionally,the porosity of metal foams were improved through employing special manufacturing techniques.For example,metallic porous Ni can be obtained by solid-gas eutectic solidification(gasars)[50].The materials were first melted in the hydrogen atmosphere under high pressure,so that the homogeneous melt can be charged with hydrogen.The temperature was then lowered,resulting in the formation of a heterogeneous solid-gas two-phase system.Finally,the hydrogen content near the solidification plane increased,leading to the formation of gas bubbles.The generated bubbles remained near the solidification zone and were entrapped in the solid.In this process,chemical components of the precursors and synthetic conditions collectively determine the final pore structures of metallic porous Ni.Pore sizes obtained by gasars are usually relatively large ranging from 10 μm to 10 mm.It is worth noting that this method is difficult to control the pore size distribution because of the simultaneous formation of small and large pores as well as their coalescence.

In order to controllably obtain desired pores, template-based electro-deposition technique was developed.Polymeric foams with desired pore structures were used as the template.The metals were electrically deposited on the external and internal surfaces, followed by removing the template from the metal/polymer composite by thermal treatment. As a result, a copy of pore structure from the template could be obtained.Based on previous reports,Jiang et al.developed a metal nanocrystal catalyzed electroless deposition technique[51].Silica spheres were modified with thiol first,and then self-assembled into arrays on glass substrates.After that,the arrays were impregnated with Au nanoparticles at the thiol sites that act as catalysts.Finally,the obtained template was immersed in electroless deposition baths,resulting in the formation of metals within the template.Once the silica spheres were etched,uniform and abundant pores were left within Ni foams.The results show that each large cavity formed is connected to the 12 neighbors by small pores of 60-10 nm, and the uniform pores are long-range order(Fig.2a). There are many choices for the template,and the resulting pores are diverse.Since mesopores play an important role in mass transfer for relatively large molecules,hierarchical structures with abundant macropores and mesopores are desired for magnetic metal foams.Kong et al.grew porous Prussian blue (PB)single crystals on the surface of PU,and then transformed PB into hierarchical iron oxide by thermal conversion(Fig.2b)[52].The compounds of the PB family are one kind of porous coordination polymer networks with a typical chemical formula of M3[M'(CN)6]2·nH2O.In this case,K4[Fe(CN)6] was used as the precursor. PB grew on PU sponges by the solvothermal method by its surface interaction with carbamate groups.After hydrolysis and condensation,K4[Fe(CN)6]formed a linear Fe(II)-CN-Fe(III)linkage layer by layer.This allowed the accumulation of PB nanocubes by increasing the growth time. Hierarchical magnetic γ-Fe2O3frameworks were obtained after thermal conversion. Besides,the surface wettability of the obtained materials can be modulated from hydrophilic to hydrophobic via a simple coat of phenolic resins.

In addition to the utilization of different templates, the ligands of transition-metal complexes can also be employed to construct unique pore structures. Tappan et al. developed a facile method by decomposing transition-metal complexes containing high nitrogen energetic ligands[53]. When the metal complexes were burned,metal ions were separated from the ligands and apted to find binding sites to stabilize their electronic structure. Nanostructured Fe foams can be obtained by the dynamically assembly of precursors in a selfpropagating combustion synthesis as shown by Fig.2c.The metal ions were reduced to metals while the nitrogenous ligand acted as the blowing agent during decomposition.The pore structures of the final products can be modulated by varying the preparation conditions(Fig.2c).

In summary,various Fe,Co,Ni-based magnetic metal foams have been fabricated and applied for various applications in the past decades.However,this kind of materials have limited value for adsorption because of their weak mechanical strength, unsuitable pore structure,and poor surface interaction.But the previous works spur the subsequent researches on developing magnetic monolithic composites with enhanced performance for adsorption,which opens up new avenues for obtaining magnetic monolithic adsorbents by different approaches.

3.1.2.Magnetic monolithic composites

In consideration of the disadvantages of magnetic metal foams,it is recognized that the supporting frameworks could be nonmagnetic ultralow-density materials so as to overcome these disadvantages.Interestingly, the obtained materials can also have strong magnetic responsiveness after introducing magnetic components.In addition,these composites exhibit robust structures and tunable surface chemistry.

Most of the early ultralow-density materials were fabricated either from expensive raw materials or by complex preparation processes,which limits their mass production and practical applications.In addition, their random cellular structures decrease the mechanical properties to some degree such as stiffness and strength.Wu et al.reported a facile approach for preparing magnetic superhydrophobic monoliths based on commercial PU sponges[54].Fe3O4nanoparticles were loaded on PU sponges by impregnation. In consideration of the weak host-guest interactions,a SiO2layer was coated on the surface of PU@Fe3O4in case of Fe3O4nanoparticles falling off.The coating process was achieved by chemical vapor deposition in a vacuum desiccator through the hydrolysis of tetraethoxysilane(TEOS).The hydrophobic surface modification was completed by further dip-coating PU@Fe3O4@SiO2in a fluoropolymer aqueous solution as shown by Fig. 3.The obtained PU@Fe3O4@SiO2@FP shows superparamagnetism with a saturation magnetization of 8.37 emu·g-1.The hydrophobic surface modification endows the adsorbents with high selectivity for oil/water separation. Furthermore, the abundant and interconnected pores of the sponges provide large volumes for the storage of oils.

Fig.2.(a)SEM images of Ni foams by using silica spheres as the template[51];(b)Growth of ultralight iron oxide by using PU as the template[52];(c)SEM images of Fe foams with nitrogenous ligand as the blowing agent during decomposition[53].

Similarly, Zhang et al. selected the commercial melamine (MF)sponges as the supports,but modulated the hydrophilic-hydrophobic properties of the obtained materials by further coating a polymer layer[55].Fe3O4nanoparticles were deposited on the surface of melamine sponges,and then poly(sulfobetaine methacrylate)(PSBMA)and polydopamine(PDA)were also coated on the surface.The obtained materials were denoted as MF@Fe3O4@PDA/PSBMA. PDA and PBSMA layers act as the immobilization layer for anchoring Fe3O4nanoparticles,and meanwhile endow the monoliths with unique underwater superoleophobicity.PDA can interact with oil by hydrophobic interactions because of the presence of phenol groups.PSBMA is a hydrophilic polymer that bonds water molecules by electrostatic interactions.As a result,the contact angles are both nearly 0°for water droplet and oil droplet,respectively,indicating the good affinity of MF@Fe3O4@PDA/PSBMA for both water and oil. Notably, when the materials are immersed in liquid/air/water system,the oil contact angle becomes to be 158°.This can be explained by the fact that the bonded water molecules serve as barriers that hinder the immersion of oil droplet.This unique underwater superoleophobility is promising for oil/water separation under both oil-rich and water-rich conditions. As coating polymer layer is a versatile method for the surface modulation, this strategy has attracted increasing attention for fabricating magnetic monoliths.

Fig.3.(a)Schematic illustration for preparing the PU@Fe3O4@SiO2@FP sponges and SEM images of(b)PU,(c)PU@Fe3O4@SiO2,and(d)PU@Fe3O4@SiO2@FP[54].

Du et al.reported a simple vapor deposition process for fabricating magnetic hydrophobic foams [56]. In this case, cobalt nanoparticles from Co(OH)2was loaded as the magnetic component.The hydrophobic polydimethylsiloxane(PDMS)was coated via a vapor deposition process within 15 min at an elevated temperature. The water contact angle of the obtained materials is 157.8°.Interestingly,the conversion of Co(OH)2sheets to nanoparticles results in a high surface roughness of ～6.29.This effect can be used to enhance surface hydrophobicity.

The above reports employed commercial polymer foams as the nonmagnetic supports,but there are also various other ultralight materials that have also been studied.Lee et al.employed siliceous mesocellular foams as the support because of its highly open pores, tunable pore size,and good biocompatibility.Magnetic Fe3O4or γ-Fe3O4nanoparticles were entrapped in the cavities of siliceous mesocellular foams after in situ thermal decomposition. Oleic acid and oleylamine were used to modulate the size of magnetic nanoparticles.The results show that magnetic nanoparticles are confined in the cage-like pores and highly dispersive through the whole foams.Distinctive from the surface modification of polymer sponges/foams, porous silica has abundant silanol groups on their surfaces, thus the hydrophobicity can be achieved by grafting silane coupling agents with hydrophobic groups.

Magnetic carbonaceous monoliths are given particular attention because of their easy preparation and great performance in adsorption.In consideration that there are tremendous carbon-containing frameworks that can be converted into carbonaceous monoliths under rational conditions, it requires a large amount of trials through different materials to find fundamental principles for fabricating specific magnetic carbon-based monoliths. Some common porous carbons have been given prior study.Zhao et al.carbonized alginate hydrogel for electromagnetic absorption[57].NiCl2was added during fabricating precursors where Ni(II) ions were linked to COO-or hydroxyl oxygens of alginate hydrogel in the solution via strong coordination-covalent bonds. The Ni(II)/alginate foams were further calcinated at 900 °C under the argon atmosphere(Fig.4a).The results show that alginate was converted into carbon foams with Ni nanoparticles embedded in the framework.The obtained magnetic monoliths possess a high surface of 451 m2g-1.The content of Ni nanoparticles can reach 42.9%,rendering the materials with a saturation magnetization of 17.0 emu·g-1.More than solely serving as magnetic components, Ni nanoparticles can also contribute to capture adsorbates in some cases(e.g.the removal of aromatic sulfur compounds via metal-sulfur interactions from fuels).This offers chances to make magnetic nanoparticles active to target molecules in adsorption.Carbon nanotubes(CNTs)have also been fabricated as magnetic sponges.CNTs exhibit oleophilic and hydrophobic properties, and are widely used for adsorbing volatile organic compounds and heavy metal ions. Gui et al. synthesized a magnetic CNT sponge by filling CNTs inner cavity with magnetic Fe nanowires(Fig.4b) [58].Fe nanowires are protected by the inert and solid CNT shells.The obtained materials exhibit not only a high saturation magnetism of 21.1 emu·g-1,but also extraordinary stable adsorption performance of 1000 cycles for oil-spill recovery. Wang et al. reported the fabrication of magnetic graphene oxide (GO) which has abundant oxygen-containing groups (e.g., --OH and --COOH) and exhibits good affinity for heavy metal ions [59]. GO was functionalized with polyethylenimine first, and then imparted magnetism to by forming Fe3O4nanoparticles on the surface.The obtained sponges can effectively uptake Cr(VI)ions from aqueous solutions,soil suspension,and even sandy soil.

The polymer sponges above-mentioned as templates or supports can be easily converted into carbonaceous monoliths under suitable conditions,and some intriguing properties can be obtained.Chen et al.reported the construction of ultralight magnetic carbonaceous foams in a facile method by using PU as the template[60].The synthetic process is illustrated in Fig.5a.PU sponges were cut into pieces first,and then treated with the aqueous solution of K2S2O8,cerium(IV)ammonium nitrate,nitric acid,and acrylic acid in sequence to remove homopolymer. The modified sponges were further ion exchanged with aqueous solutions of Fe(NO3)3,Co(NO3)2,or Ni(NO3)2.After calcination,ultralight magnetic Fe3O4/C,Co/C,or Ni/C foams were synthesized.This synthetic method is facile and versatile, and the results show that the foams are robust and can reach a saturation magnetization of 17.2 emu·g-1.The obtained materials possess hierarchical structures from macroscopic to nanometer length scales with an optimal surface area of 93 m2·g-1.Lin et al.grew cobalt-based ZIF-67 on a melamineformaldehyde sponge,and then carbonized the composites at elevated temperatures (Fig.5b)[61].ZIF-67 was selected as the coating layer due to the fact that MOF-derived carbonaceous sponges possess both abundant mesopores and micropores.The results show that compared with parent materials,the main increasement of the total pore volume in the obtained materials lies in micro-and mesopores.In addition,nitrogen atoms are also doped in the carbonaceous frameworks derived from the ligands of ZIF-67.These dispersive nitrogen atoms can act as polar centres,which is beneficial for the enhancement of their surface interactions with adsorbates.

In summary, the fabrication of magnetic monoliths benefits from the rational utilization of nonmagnetic ultralow-density materials.Compared with the traditional metal foams, the magnetic monolithic composites have high porosity, relatively strong mechanical strength, and low cost. On the one hand, nonmagnetic frameworks provide desired and tunable platforms for accommodating adsorbate molecules.On the other hand,the surface hydrophobicity or hydrophilicity of the obtained composites can be easily tuned by coating another hydrophobic/hydrophilic layer or grafting organic molecules with hydrophobic/hydrophilic groups.More importantly, the introducing strategies of magnetic nanoparticles are various,and the resultant materials have excellent magnetic responsiveness.The locations of magnetic nanoparticles can be on the surface,anchored by another layer,or confined in the cavity.These advantages offer the magnetic monolithic composites great potential in adsorption.

3.2.Magnetic porous nanocomposites

Magnetic porous nanocomposites are particularly attractive in adsorption because of their high surface area,fast magnetic responsiveness,and designable structures.During constructing magnetic porous nanocomposites, it is critical to firmly combine the magnetic components with the porous materials.Meanwhile,the adsorption performance of porous materials should be maximized.This requires great efforts to construct desired structures.

Fig.4.(a)Procedure for the preparation of the Ni/carbon foam[57];(b)The macro-and microscopic morphology of magnetic CNT sponges[58].

Wang et al.introduced Fe3O4nanoparticles into the pore channels of the MOF MIL-101(Cr) by a simple reduction-precipitation method(Fig. 6a) [62]. MIL-101(Cr) was first synthesized by the typical solvothermal method followed by immersing in the solution of Fe(III).Fe(III)ions were combined with Cr(III)by metallic bonds and chelated effects on carboxyl groups from MIL-101(Cr). In the presence of Na2SO3,Fe(III)ions were partially reduced to Fe(II)ions,and forming homogeneously Fe3O4nanoparticles with the addition of ammonia solution.The obtained materials have a saturation magnetization value of 15.6 emu·g-1,and exhibit good performance in removing organic dyes from water.However,this method obviously decreases the surface area of MIL-101(Cr)from 3312 to 1790 m2·g-1because Fe3O4nanoparticles partially filled the pores of the parent materials.In order to solve this problem, our group employed a dry-gel conversion method to achieve confined growth of MOFs on the surface of Fe3O4nanoparticles(Fig.6b)[63].During the typical solvothermal method,MOFs prefer to self-seeding nucleation rather than combining with Fe3O4nanoparticles. But through the dry-gel conversion, the two materials can be well combined.The parent materials of MOFs and Fe3O4nanoparticles were physically mixed and separated from the solvent.Fe3O4nanoparticles were pre-synthesized as the seeds,and the growth of MOFs were induced under the vapor of the solvent at elevated temperatures.When solvent molecules penetrate with the solid mixture,small“solution-like phase”composites are formed,leading to the local reaction owing to the mass transfer limitation.As a result,crystal nucleus and growth of MOFs on Fe3O4nanoparticles can be achieved.The obtained materials maintain its surface area at a high level(1036 m2·g-1),which is only slightly lower than the pure MOF HKUST-1(1356 m2·g-1).Interestingly,some mesopores are formed in the composites by the dry-gel conversion.This can be explained by the fact that the rough surface of Fe3O4nanoparticles cannot match perfectly with MOFs.

Fig.5.(a)Illustration of the fabrication of ultralight magnetic foams from a PU sponge[60];(b)Preparation scheme of MOF-derived carbon sponges[61].

Core-shell nanocomposites with magnetic cores and porous shells have attracted great attention as they exhibit high porosities,excellent chemical stability,and easily modifiable internal/external chemical surfaces.Magnetic cores are encapsulated inside to sustain the magnetic responsiveness while the outside porous shells provide active sites for capturing target molecules.Every microsphere plays its individual effect and cooperatively complete adsorption processes.This structure makes it possible to integrate various components into one structure for developing multifunctional and even smart adsorbents. One typical strategy to construct core-shell structure is coating porous silica on Fe3O4nanoparticles. The rapid development of this model benefits from the advances in synthesizing Fe3O4nanoparticles.So far,various methods have been reported to obtain Fe3O4nanoparticles with sizes ranging from several to hundreds nanometers,which is able to meet various practical requirements.In addition,the surface hydrophilic-hydrophobic property of Fe3O4nanoparticles can be modulated via introducing amphoteric surfactants.

Shi's group deposited double silica shells on one Fe2O3core,and then reduced the core into Fe3O4nanoparticles [64]. The first nonporous layer is thin and dense for the protection of magnetic cores,and the second layer is worm-like mesoporous and highly open.The template,n-octadecyltrimethoxysilane, was introduced to the second layer by the in situ method,and then removed by calcination.This strategy can obtain highly uniform magnetic mesoporous microspheres with a saturation magnetization of 27.3 emu·g-1,which offers an easy and efficient way for their separation from a sol or a suspension system. In this case,a large magnetic hysteresis loop of the magnetic microspheres is observed,indicating the existing of magnetic shape memory effects.However,this is not beneficial to a fast redispersion of magnetic nanoparticles in solutions.Superparamagnetism can eliminate the magnetic shape memory effects,but this property only appears when the sizes of magnetic nanoparticles are smaller than the critical dimension.

In consideration of the difficulties of constructing abundant pores if the sizes of adsorbents are too small,Zhao's group developed a facile solvothermal method to synthesize uniform superparamagnetic magnetic nanoparticles with sizes up to 200-300 nm [65]. Fe(III) ions underwent sodium acetate-promoted hydrolysis, and then were partially reduced to Fe3O4nanoparticles by ethylene glycol. Citrate sodium was used to enhance the hydrophilicity by attaching tremendous hydroxyl groups on the surface of magnetic nanoparticles.The obtained nanoparticles are composed of numerous primary magnetite nanocrystals with the sizes of 5-10 nm, which results in their superparamagnetism and high magnetization of 56-82 emu·g-1. In their next work,highly uniform magnetic microspheres were synthesized through two steps (Fig. 7a) [66,67]. A nonporous silica layer was coated to protect magnetic cores followed by another mesoporous silica layer being coated. But the mesopores in the second layer are perpendicularly aligned to the microsphere surface rather than unordered(Fig.7b).This is due to the self-assembly of the template cetyltrimethylammonium bromide.As a result,various active sites,for example metal species,can be introduced to the open pores of the obtained microspheres while maintaining their good magnetic responsiveness(Fig.7c)[68].

Fig.6.(a)Schematic fabrication process of Fe3O4/MIL-101(Cr)composites[62];(b)Synthesis of magnetic HKUST-1 by dry-gel conversion as well as the SEM and TEM images of the obtained materials[63].

In summary, magnetic nanocomposites possess high surface area,fast magnetic responsiveness,and designable structures.The composites that are suitable for adsorption are composed of magnetic nanoparticles and porous materials. Magnetic nanoparticles serve as responsive components while porous materials serve as adsorption components or supports. Magnetic nanoparticles can be combined with porous materials in various forms,such as loaded in the pore channels,embedded inside,or constructed as core-shell structures.This allows the integration of various functions into one structure,achieving facile adsorption processes in complicated environments.It is worth noting that the current fabrication processes of magnetic nanocomposites are complicated, resulting in their high cost for practical applications.In addition,their potential threaten to environment should also be fully taken into consideration.

Fig.7.(a)Schematic fabrication process magnetic microspheres with magnetic cores and double silica shells;(b)TEM images of the obtained materials;(c)The fast responsiveness of the microspheres to the external magnetic field[67].

4.Applications of Magnetic Responsiveness for Adsorption

Magnetic responsiveness is valuable for the synthesis of adsorbents and adsorption processes,such as synthesis of MOFs,water remediation,desulfurization and denitrogenation of fuels,and gas separation.Ingeniously introducing magnetism to adsorbents and utilizing the magnetic properties is beneficial to develop novel and efficient adsorption processes.Presently,the applications of magnetism for adsorption have focused on three aspects,namely magnetic-field-induced synthesis of magnetic adsorbents,separating adsorbents from liquid phases,and efficient desorption. Firstly, during the synthesis of tremendous adsorbents,external heating is required to form desired pore structures especially for the growth of those porous crystals such as zeolites and MOFs.Magnetic nanoparticles can generate heat under alternating magnetic fields,generating heat locally to create solvothermal microenvironments[49].As a result,adsorbents can be produced at orders of magnitude increases in speed compared with conventional solvothermal chemistry. Secondly, magnetic adsorbents can be attracted by the external magnetic field and fixed within a small space. Then the liquid phases can be conveniently removed without solid adsorbents dispersed in.This separation method is time-saving and low-cost compared with the traditional centrifugation or filtration.Thirdly,magnetic-field-induced heating can also be applied in desorption.Adsorbents are generally applied at low temperatures and regenerated at elevated temperatures because adsorption is an exothermic process.Magnetic-field-induced heating is distinctive as the heat is generated from inside to outside.This process is particularly preferred for those adsorbents with low thermal conductivity where the heating time can be greatly reduced.In this section,some typical applications of magnetic adsorbents including oil/water separation,removal of hazardous substances from water,desulfurization of fuels,and CO2capture will be introduced.

4.1.Oil/water separation

Oil-spill accidents can lead to serious environmental problems,causing nearly irreversible damage to ecosystems.It is exceptionally important to propose effective and practical techniques to handle the emergencies. There have been some reports for spilled oil recovery[58],such as physical sorption by porous materials,mechanical recovery by oil skimmers,in situ burning,physical diffusion,and biodegradation.Among these existing methods,physical adsorption by porous materials is given increasing attention because of its simple,fast,and effective features.

In Wang and Zhang's work,PU sponges after integrating magnetism(PU@Fe3O4@SiO2@FP)were used to selectively adsorb floating oils on water [54]. The oil can be rapidly absorbed within several seconds once contacting with the hydrophobic magnetic adsorbents.As shown by Fig.8a,the floating petrol can be completely adsorbed leaving the clean water.The hierarchical and interconnected pores of the sponge offer a large space for the accommodation of oils.The saturated adsorbents remain floating on water. By using an external magnetic field,the adsorbents can be conveniently attracted and recycled.Interestingly, these adsorbents can also be employed to remove heavy oils under water(Fig.8b).As the existence of the air cushion,the adsorbents need an external force to be immersed in water.The ratio of liquid/solid interface is small,indicating the weak interaction between water and the hydrophobic surfaces. Once contacting chloroform under water,the adsorbents rapidly uptake the heavy oil.Besides,the capacities of the adsorbents on 10 kinds of common organic liquids in daily life and industry were evaluated as well.The results show that the adsorbents exhibit high absorbency for all these organic liquids(Fig.8c).The capacities are closely correlated with density,surface tension and viscosity of the oils.After 10 cycles,the capacities can be well maintained.This can be ascribed to the excellent mechanical and chemical stability of the fluoropolymer coating.

Fig.8.Absorption of(a)floating petrol on water surface and(b)chloroform under water as well as(c)different oils and the water shedding angles after they have been used once for oil absorption[54].

Chen et al.studied the performance of ultralight carbon-based magnetic monoliths on oil/water separation. Methyltrichlorosilane was employed as the hydrophobic coating[60].The contact angles of the modified sponges for water and lubricating oil are 152°and 0°,respectively(Fig.9a and b).As shown by Fig.9c,the lubricating oil can be rapidly adsorbed by the obtained adsorbents,and facile recycles can be achieved with the assistance of external magnetic fields.Other 8 oils were also used to evaluate the performance of adsorbents (Fig. 9d).The results show that the adsorbents can adsorb crude oil,bean oil,lubricating oil,and hexane more than 89.3,102.6,101,and 61.8 times its own mass,respectively.These oil-absorption capacities are much higher than many other porous materials such as PU sponges(13-26 times)[33,69,70],organic nanocomposites(2-14 times)[71-73],and 3D macroporous Fe/C(4-10 times)[74].This can be explained by the fact that the oils are mainly stored in the millimeter pores formed by the superoleophilic walls of the interconnected microtubes.Capillaries are proposed to be the force driving the oils into the adsorbents.The capillary flow is further strengthened by the superoleophilic interconnected microtubes when the oils spread into the inner pores of the foams,resulting in high absorption capacities.

The reason that magnetic monoliths have been widely used for oil/water separation probably lies in their excellent adsorption performance.The hierarchical structures,large pore volumes, highly open and interconnected pore channels,hydrophobic surfaces,and fast magnetic responsiveness make magnetic monoliths great candidates for rapid removal of oils on or under water.But before practical applications, the fabricating procedures need to be further simplified for large-scale production.

Fig.9.Contact angle of(a)one water droplet(b)and one oil droplet on the surface of the obtained materials;(c)Adsorption on lubricating oil of the magnetic adsorbents from the surface of water(the oil was dyed blue for observation);(d)Absorption capacities of the magnetic adsorbents for different oils[60].

4.2.Removal of hazardous substances from water

Water is the most essential resource that allows humans to exist.With fast development of productive forces,freshwater systems are seriously threatened by human activities[75].Heavy metal contamination and organic contaminants are considered as two vital problems because of the acute toxic effects on ecosystems and the tendency for bioaccumulation even at relatively low concentration[76].

In Nassar's work, Pb(II) ions were selected as a typical example of heavy metal ions in wastewater, and the adsorption mechanism,kinetics, and thermodynamics on magnetic nanoadsorbents were studied in detail [77]. Commercial Fe3O4nanoparticles were used as nanoadsorbents.The results show that the adsorption isotherms of Pb(II)increase sharply at low equilibrium concentrations,and the maximum capacity can reach 36.0 mg·g-1.This value is obviously higher than many other adsorbents such as goethite(11.04 mg·g-1)[78],diatomite(24.0 mg·g-1)[79],and activated carbon(21.5 mg·g-1)[80].The capacity is pH dependent when pH value is less than 5.5.This suggests that the adsorbent-adsorbate interactions are electrostatic attraction because hydrogen ions can compete with the Pb(II)ions for the active sites.This proposal is supported by the calculated standard enthalpy,which lies in the range of physical adsorption.After 5 cycles,the capacity of the magnetic adsorbents can be well maintained.Beyond electrostatic attraction,the contaminates can also be captured by different interactions[81-84]such as surface site binding[85],magnetic selective adsorption [86], and modified ligand combination [87]. This indicates the great potential of magnetic adsorbents in removing heavy metal ions.

Organic contaminants in water have also seriously harmed the environment.Gong et al. prepared magnetic carbon nanotubes as adsorbents and studied their performance on the removal of cationic dyes[88].The adsorbents were synthesized by in situ introducing magnetic nanoparticles into CNTs. The SEM image (Fig. 10a) shows that iron oxide nanoparticles were successfully coated on the surface of nanotubes,and the corresponding surface area is 62 m2·g-1.Three cationic dyes,Methylene blue(MB),neutral red(NR),and brilliant cresyl blue(BCB),were selected as the typical organic contaminants.As shown by Fig.10b,the equilibrium values can be reached within 1 h,indicating the fast adsorption rates.This is probably caused by the strong adsorptive interactions and accessible active sites.The influence of pH on adsorption capacity was also studied.The results show that the increase of pH from 3 to 7 promotes the adsorption of cationic dyes on magnetic CNTs,and the amounts adsorbed are maintained when pH are beyond 7.This suggests that the surface of magnetic CNTs should be negative in a wide pH range.Organic contaminants in water are complex,usually including polycyclic dyes,malachite green, crystal white,and so forth.Magnetic adsorbents can also been used to remove these contaminants.Some successful cases include magnetic chitosan gel particles,magnetite bearing covalently immobilized copper phthalocyanine dye,magnetic charcoal,and magnetic alginates[89-93].

Fig.10.(a)SEM image and(b)amount adsorbed on different dyes of magnetic CNTs[88].

Magnetic separation offers a common technology for purification of water.By applying rational strategies,various porous materials can be endowed with magnetism.As a result,the design of magnetic adsorbents should be tailored based on the target adsorbates in water.However, magnetic components, usually Fe3O4nanoparticles, are apt to undergo oxidation in humid environments and gradually degenerate their magnetism.Consequently,the protection of magnetic components is vital for their long-term use in industry.In addition,the leach of magnetic nanoparticles from the magnetic adsorbents should also be given full consideration.

4.3.Deep desulfurization of fuels

Traditional hydrodesulfurization is efficient in removing thiols,sulfides and disulfides from fuels, but is less effective in eliminating aromatic sulfur compounds[94-100].With increasingly rigorous regulations,adsorptive desulfurization has attracted increasing attention[101-108].Tremendous efforts have been made to improve adsorption efficiency[109-111].As adsorptive desulfurization proceeds in liquid phases, the separation and recycling of adsorbents would become quite convenient if adsorbents were endowed with magnetism.

Fig.11.(a)Schematic fabrication process of Ag/Fe3O4@mSiO2 and π-complexation;(b)TEM images of Ag/Fe3O4@mSiO2;(c)Adsorption isotherms of different samples;(d)Reusability of Ag/Fe3O4@mSiO2[68].

Our group fabricated a Ag(I)-loaded core-shell adsorbent,and then realized deep desulfurization through π-complexation[68].As shown by Fig.11a,the adsorbents comprise a Fe3O4core and a mesoporous silica shell with Ag(I)impregnated in pore channels, denoted as Ag/Fe3O4@mSiO2. The mechanism of π-complexation is illustrated in Fig. 11b. Ag(I) can form σ-bonds with empty s-orbitals, while its dorbitals can back-donate electron density to the antibonding πorbitals(π*)of the sulfur rings.This specific interaction can realize the selective removal of aromatic sulfur compounds.The adsorption isotherms(Fig.11c)show that Fe3O4nanoparticles exhibit negligible effects on capturing thiophene, and the capacity increases to only 0.042 mmol·g-1after coating mesoporous silica.The introduction of Ag(I)remarkably improves the capacity to 0.147 mmol·g-1,suggesting the great enhancement of surface affinity. This means that πcomplexation plays a vital role in determining the adsorption performance in this case.The capacity of Ag/Fe3O4@mSiO2can be well maintained after 6 cycles (Fig. 11d), indicating its good reusability.However,it is worth noting that Ag(I)is expensive and sensitive to visible light[112],which means that the investment of production should be high.In our subsequent work,mesoporous silica and Ag(I)are replaced with porous carbon shells [113]. The carbon precursor is resorcinol-formaldehyde polymer,which was coated on the surface of Fe3O4nanoparticles and then carbonized at elevated temperatures.The obtained magnetic adsorbents have hierarchical pore structures with dominant micropores range from 0.7 to 1.2 nm and mesopores around 5 to 40 nm.The thickness of carbon shells can be modulated by tuning the concentrations of resorcinol and formaldehyde during coating polymer shells.The capacity of the optimal sample can reach as high as 0.476 mmol·g-1,which is much higher than that of Ag/Fe3O4@mSiO2.Besides, this adsorbent also has comparable ability of removing benzothiophene as well as aromatic nitrogen compounds(indole and quinoline),which property is highly desired in desulfurization processes.

Although the successful application of core-shell adsorbents for deep desulfurization,magnetic adsorbents is still in the early stage in this field,and there are many issues to be solved.As the components of crude fuels are complex,magnetic adsorbents are required to be stable in different conditions.In addition,the magnetic components have negligible effects on adsorption,leaving the capacities of this kind of adsorbents at relatively low level.This greatly weakens their competition.In the next stage,magnetic adsorbents should find a balance between adsorption performance and investment,and the adsorption technique needs to be improved to optimize their performance.

4.4.Efficient carbon dioxide release

Global warming has become an urgent environmental concern,and excessive CO2emission from fossil fuel combustion is considered as a major anthropogenic source of greenhouse gases.Carbon capture is an effective approach to control the concentration of atmospheric CO2[114-120].However,the significant energy penalty for the regeneration of the adsorbent is one of the biggest barriers to the widespread deployment of carbon capture technologies to power plants.Magnetic adsorbents can generate heat for regeneration under alternating magnetic fields with high frequency, where magnetic nanoparticles act as“nanoheaters”that trigger the release of the large majority of adsorbed CO2.

As Mg-MOF-74 is one of the most promising MOF adsorbents for CO2capture,Li et al.introduced Fe3O4nanoparticles with diameters of 10±0.5 nm into Mg-MOF-74 by the one-pot solvothermal reactions(Fig. 12a)[36]. The Fe3O4nanoparticles content ranges from 0.07 to 7.76 wt%for the samples of MFC1-MFC7.Notably,after exposure to a magnetic field of 55.9 mT for 15 min,the temperatures of different samples can be increased by 12-30°C(Fig.12b).The higher Fe3O4nanoparticles content results in the higher ultimate temperature.The heating effects of MFCs are affected by the magnetic density.The stronger magnetic field makes the heating effects quicker and more significant(Fig.12c).The magnetic switching CO2uptakes are shown in Fig.12d and e.The locally generated heat greatly decreases adsorption capacities of different samples,and CO2release can be controlled through tuning Fe3O4nanoparticles content or magnetic field strength.

In traditional regeneration technology,the heat transfers from external sources to internal gas molecules adsorbed. This process is lowefficiency for the adsorbents with poor thermal conductivity,especially when their usages are in large quantity.The magnetic-field-induced heat relies on the localized heating generated by the embedded“nanoheaters”within adsorbents,which may benefit to promote the heat usage.In consideration the diversity of magnetic adsorbents,the localized heat-driven desorption may open up novel avenues for remote adsorbents regeneration.

5.Conclusions

In the past decades,magnetic adsorbents have attracted increasing attention because of their unique properties in adsorption processes such as magnetic-induced separation and heat generation in alternating magnetic fields,which greatly facilitates recycling procedures,favors long-term operation,and improves desorption rate,making conventional adsorption processes highly efficient.In this review,we provide an overview of the synthesis,structural and adsorption properties,as well as the prospect of magnetic adsorbents for separation and purification. The fundamental knowledge of magnetic porous materials was introduced at first.Then the strategies of combining magnetic components and porous materials were concluded.The advantages and disadvantages of the corresponding methods have also been discussed in depth.Finally,the applications of magnetic adsorbents were reviewed.In order to fulfill the requirements of practical applications,some key challenges in this field are proposed as follows:

(1) Adsorption performance should be further improved.In most current reports, magnetic components contribute little to adsorption capacity or selectivity. As a result, the proportion of magnetic components is limited by the trade-off between adsorption performance and magnetic responsiveness.In order to satisfy the requirement of practical applications, the capacity and selectivity of magnetic adsorbents should be further improved.On the other hand,the stability of the magnetic adsorbents should also be given full consideration for repeating usage.

(2) Structural modulation needs to be more controllable.The introduction of magnetic components usually decreases BET surface areas of porous materials because of their undeveloped pore channels.But rational synthetic strategies(e.g.dry-gel conversion) can lead to the formation of mesopores or macropores,which are beneficial to the diffusion of relatively large molecules.However,these results are not predesigned,and the pore volumes formed are not controllable.If the structural modulation of magnetic adsorbents can be achieved during synthesis, it might be possible to greatly enhance the adsorption efficiency.

(3) More efforts should be made to collaboratively utilize distinctive functions of magnetism in one system.Magnetic separation is the most widely used function for magnetic adsorbents, and this function was singly utilized in the most reports.With the ingenious utilization of other aspects of magnetism for adsorption(e.g. magnetic-induced heat), it is possible to collaboratively utilize distinctive functions of magnetism in one system to improve energy efficiency or complete complex specific adsorption processes.