Bimetallic metal–organic frameworks and their derivatives

Bimetallic metal–organic frameworks (MOFs) have two different metal ions in the inorganic nodes. According to the metal distribution, the architecture of bimetallic MOFs can be classified into two main categories namely solid solution and core–shell structures. Various strategies have been developed to prepare bimetallic MOFs with controlled compositions and structures. Bimetallic MOFs show a synergistic effect and enhanced properties compared to their monometallic counterparts and have found many applications in the fields of gas adsorption, catalysis, energy storage and conversion, and luminescence sensing. Moreover, bimetallic MOFs can serve as excellent precursors/templates for the synthesis of functional nanomaterials with controlled sizes, compositions, and structures. Bimetallic MOF derivatives show exposed active sites, good stability and conductivity, enabling them to extend their applications to the catalysis of more challenging reactions and electrochemical energy storage and conversion. This review provides an overview of the significant advances in the development of bimetallic MOFs and their derivatives with special emphases on their preparation and applications.


Introduction
Metal-organic frameworks (MOFs), or porous coordination polymers (PCPs), constructed from inorganic nodes with organic linkers, have emerged as a promising class of materials with high porosity, diverse composition and tuneable pore structures. 1,2 These characteristics have attracted signicant research interest in a variety of elds, such as gas adsorption and separation, 3-5 catalysis, 6-8 luminescence, 9 sensing, 10 biomedicine, 11 and energy. [12][13][14] In order to enhance the catalytic, electronic, and luminescence properties of MOFs, it has been proposed to incorporate second metal ions into the nodes of frameworks for the preparation of bimetallic MOFs. 15,16 The partial substitution by second metal ions in the inorganic nodes or secondary-building units (SBUs) in the framework will allow the bimetallic system to show synergistic effects. In bimetallic MOFs, the proportions of the metals can be adjusted or even controlled, offering the possibility to tune the physicochemical properties of bimetallic MOFs. 17,18 According to the distribution of metal ions, bimetal MOFs can adopt "solid solution" or "core-shell" structures ( Fig. 1). 19 In solid solution bimetallic MOFs, the metals show delocalized or even homogeneous distributions through the whole crystal. Solid solution bimetallic MOFs can be synthesized by direct synthesis, post-synthetic modication and template synthesis. In core-shell bimetallic MOFs, the MOF shell is chemically different from the MOF core while they are integrated into a single architecture. Core-shell bimetallic MOFs can be synthesized through seed-induced growth of a MOF onto the surface of another, post-synthetic exchange of metal ions in the MOF surface, or one-pot synthesis. In this review, we mainly discuss the bimetallic MOFs in which both of metal ions serve as SBUs of MOFs. Bimetallic systems of immobilization of metal ions, metal nanoparticles (NPs), and metal complexes on the inorganic nodes, 20,21 ligands, [22][23][24] or inside MOF cavities [25][26][27] as guests are outside of the scope of this review. 28,29 Bimetallic MOFs with tuneable compositions and structures have shown enhanced properties compared to their monometallic counterparts, exhibiting superior performance in many applications, including gas adsorption, catalysis, energy storage and conversion, and luminescence sensing. 15,[30][31][32] Moreover, bimetallic MOFs can be used as precursors/ templates for the synthesis of a variety of nanostructured materials, including carbon composites, metal compounds, and MOF composites (Fig. 1). 33,34 By selecting appropriate bimetallic MOFs and controlling the post-treatment process (e.g., pyrolysis, hydrolysis, and hydrothermal treatment), the compositions and structures of the afforded nanomaterials can be controlled. Bimetallic MOF-derived nanomaterials exhibit exposed active sites and high stability and conductivity, which can benet their applications in the catalysis of more challenging reactions under harsh conditions and electrochemical energy storage and conversion.
This review provides an overall picture of the signicant advances of bimetallic MOFs (Fig. 2). Synthetic strategies of bimetallic MOFs and their derivatives are summarized. The applications of bimetallic MOFs and their derivatives in various elds including gas adsorption, catalysis, energy storage and conversion, and luminescence sensing are also presented. This review is expected to inspire more efforts in the development of bimetallic MOFs and their derivatives for a variety of applications.

Synthesis of bimetallic MOFs
Many strategies have been developed for the synthesis of bimetallic MOFs. This section focuses on the synthetic strategies of both solid solution and core-shell bimetallic MOFs. For the obtained bimetallic MOFs, using appropriate physicochemical techniques is essential for characterizing the compositions, distributions, and structures of the bimetallic MOFs. Xray diffraction (XRD) and energy-dispersive X-ray (EDX) mapping can be combined to identify the crystalline phases of MOFs. In accordance with Vegard's law, bimetallic MOFs show diffraction peak positions between those of monometallic MOFs. 35,36 Atomic absorption spectroscopy (AAS), inductively coupled plasma (ICP), EDX mapping and aerosol time-of-ight mass spectrometry (ATOFMS) can be coupled to calculate the global and local metal concentrations and distributions. 37,38 Xray absorption techniques, such as X-ray photoelectron spectroscopy (XPS) and X-ray absorption ne structure (XAFS), can determine the location of metals (present as nodes, bonded to the nodes, or present in the pores) in bimetallic MOFs. 39 Extended X-ray absorption ne structure spectroscopy (EXAFS), neutron powder diffraction (NPD) and XPS can determine the spatial arrangements of metals in the SBUs of bimetallic MOFs. [40][41][42] Different techniques should be combined for the accurate characterization of the synthesized bimetallic MOFs.

Solid solution bimetallic MOFs
The term "solid solution" is used somewhat lightly, as in some cases a completely homogeneous distribution of metals may not be present. 16,35,43 In this review, we divide solid solution bimetallic MOFs into two categories based on the spatial arrangements of metals: (1) two different metals are in the same SBU and the mixed metal SBUs are present throughout the MOF structure ( Fig. 3A) and (2) each SBU consists of the same kind of metal and the two different SBUs are well mixed in the MOF structure (Fig. 3B). Two metals with a similar ionic radius and coordination sphere are more likely to exist in the same SBUs. 44 It has been demonstrated that metals mixed in the same SBU can show stronger synergistic interactions compared to metals in different SBUs. 40 However, in many bimetallic MOFs, the specic arrangement of metals in the SBUs throughout the crystal structure remains undetermined. Nevertheless, we generally dene that in solid solution bimetallic MOFs the two metals are well dispersed throughout the MOF crystal. In this section, we focus on the discussion of methodologies to prepare solid solution bimetallic MOFs.

Direct synthesis. Solid solution bimetallic
MOFs can be directly synthesized by mixing all the metal salts during the solvothermal synthesis. [45][46][47][48] Using two metal salts in a one-pot reaction does not guarantee the formation of a solid solution bimetallic MOF due to the varying MOF-forming kinetics of the two metal ions. To achieve a controlled incorporation, delicate control is required to avoid the formation of mixed MOF phases. Parameters including the solubility, reactivity and coordination sphere of metal ions and the pH of the reaction mixture have signicant effects on the nal molar ratio of the metal ions.
Metal ion exchange has focused largely on MOFs that are not considered highly robust, 63,76 while it is also observed on MOFs that are considered to possess high structural stability, such as ZIFs, 38 UiO-66(Zr), 37,80 and MILs. 81 Cohen and co-workers reported the synthesis of bimetallic ZIFs (Mn(II)-based ZIFs) through a postsynthetic exchange approach (Fig. 5A). 38 ZIF-71(Zn) was incubated in a solution of Mn(acetylacetonate) 2 in MeOH to afford ZIF-71(Zn/Mn). Results showed that all of the ZIF-71 particles participated in the exchanged process with 12% of the tetrahedral Zn 2+ centres exchanged for Mn 2+ . ZIF-8 could also exchange $10% of Zn(II) centres for Mn(II). Cohen and co-workers also demonstrated the metal exchange of Zr 4+ in UiO-66(Zr) with Ti 4+ and Hf 4+ . 37 UiO-66(Zr) was exposed to DMF solutions of different Ti 4+ salts, such as TiCp 2 Cl 2 , TiCl 4 (THF) 2 , or TiBr 4 (Cp ¼ h 5 -cyclopentadienyl, THF ¼ tetrahydrofuran) for 5 days at 85 C. The amount of exchanged Ti 4+ depended on the metal salt used. TiCl 4 (THF) 2 showed the highest substitution level (38 wt%), while TiBr 4 showed the lowest level due to its reactivity and instability. UiO-66(Zr) allowed very little metal ion exchange with HfCl 4 at room temperature. Even at elevated temperatures, only $20% of the UiO-66(Zr) particles incorporated Hf 4+ .
One should keep in mind that the postsynthetic exchange of metal ions cannot always allow the replacement of metal ions in the SBUs. The metal ions may gra onto the surface of the inorganic node at a linker vacancy site or deposit on the surface of MOFs in the form of nanoscale metal oxides (Fig. 5B). 82,83 Different analytical methods should be combined to conrm the successful exchange of metal ions in the SBUs of MOFs aer synthesis.
Moreover, metal exchange can also occur between two robust MOF solids. Cohen and co-workers demonstrated that robust MOFs would readily exchange their structural metal ions in a solid-solid postsynthetic exchange process (Fig. 5C). 37 MIL-53(Al)-Br and MIL-53(Fe)-Br as dry solids were mixed and then incubated in water for 5 days at 85 C. ATOFMS showed that 40% of the particles contained both metal ions [i.e., MIL-53(Al/ Fe)].
In addition, mechanical milling has also been demonstrated to be a powerful tool for metal ion exchange between two solid MOFs for the synthesis of solid solution bimetallic MOFs. [84][85][86] Horike, Kitagawa and co-workers demonstrated that solventfree mechanical milling of two MOF crystals could induce the dissolution of each metal ion into the amorphous solid state, which subsequently reconstructed to crystalline structures via vapor treatment (Fig. 5D). 85 The authors prepared nine powder mixtures of Al-ndc and Ga-ndc (ndc ¼ 1,4-naphthalenedicarboxylate) with different molar ratios from 1 : 9 to 9 : 1. The mixtures were treated by ball-milling, followed by exposure to saturated water for 3 days to form solid solution Al/ Ga-ndc materials, as determined by XRD. This method could also be applied for the preparation of other solid solution MOFs, such as Zn/Mg-MOF-74, Zn/Co-ZIF-8, and Zn/Cd-ZIF-8.
2.1.3 Template synthesis. Template methods have been employed for the synthesis of bimetallic MOFs for a good control over the arrangement of metal ions in MOFs. Coskun and co-workers employed a metal-organic polymer with well-dened binding sites for anchoring a secondary metal as a structural template and precursor for the synthesis of bimetallic MOF-74 s (Fig. 6A). 87 1D metal-organic polymer 1, [Zn(H 2 O) 2 (C 8 H 4 O 6 )] n , was synthesized by reacting 2,5-dihydroxy-1,6-benzenedicarboxylate with ZnSO 4 . The 1D polymer possessed well-dened binding sites (hydroxy and carbonyl) for the coordination of secondary metal ions (Mg 2+ or Ni 2+ ). The 1D polymer was suspended in a solution of the secondary metal precursors to get impregnated polymers, followed by transformation into 3D Zn/M-MOF-74 (M ¼ Mg, Ni) under solvothermal conditions. The two different metal ions were homogeneously distributed in MOF-74 crystals with a constant molar ratio of 1 : 1 regardless of the initial metal concentrations in the reaction mixture.
In addition to achieving a better control of the metal arrangement in bimetallic MOFs, templated synthesis can also allow the construction of hollow bimetallic MOFs. Hollow MOFs can not only inherit the merits of MOFs, but also have additional advantages such as facile mass transport and rich active sites. 88 Templated synthesis methods for the synthesis of hollow MOFs can be divided into the exterior-template method and self-template method. Exterior-template synthesis of hollow MOFs usually uses a sacricial template to prefabricate a coreshell intermediate and subsequently removes the template. 89,90 This exterior-template method has been applied for the synthesis of hollow bimetallic MOFs. 91 The self-template method has also been reported for the synthesis of hollow bimetallic MOFs. [92][93][94][95] The self-template method usually goes through a dissolution-regrowth process, leading to the formation of bimetallic MOFs with hollow structures. The selftemplate method does not require a further step to remove the templates, which is simple and facile. For example, Wang and co-workers reported the synthesis of hierarchical Zn/Ni-MOF-2 nanosheet assembled hollow nanocubes (NAHNs) transformed from Zn/Ni-MOF-5 via a dissolution and recrystallization process (Fig. 6B). 93 H 2 BDC, Zn 2+ and Ni 2+ ions were dissolved in a mixed solvent of N,N-dimethylacetamide (DMAC) and ethanol. During the synthesis process, a high concentration of precursors led to a fast formation of cubic crystalline Zn/Ni-MOF-5, which was gradually etched and acted as the template for the growth of thermodynamically favoured Zn/Ni-MOF-2 nanosheets. Xu and co-workers constructed an iron-nickel-based single-crystal open capsular-MOF through a crystal-structure transformation (Fig. 6C). 94 Nonhollow crystals of FeNi-MIL-88B were dispersed in a hot DMF solution of 2,4,6-tris(4-pyridyl) pyridine (tpy), affording uniform capsular-MOF nanocrystals. A detailed study of the growth mechanism revealed that the parent FeNi-MIL-88B with plenty of concave defects dissolved and released Fe 3+ , Ni 2+ , and BDC-NH 2 2À under solvothermal conditions, followed by the construction of FeNi-MIL-88B-tpy on the surface of FeNi-MIL-88B and nal formation of capsular FeNi-MIL-88B-tpy with openings in the nanocrystal walls. FeNi-MIL-88B-tpy with additional size-matching ligands (tpy) in the structure showed enhanced structural stabilities compared to FeNi-MIL-88B, thus driving the dissolution-regrowth process in hot DMF.

Core-shell bimetallic MOFs
In core-shell bimetallic MOFs, the shell and core MOFs are formed from different metal centres. Three well-established strategies have been exploited for fabricating core-shell bimetallic MOFs, namely, seed-induced growth, post-synthetic exchange, and one-pot synthesis. 2.2.1 Seed-induced growth. Seed-induced growth has been demonstrated to be a powerful route for the synthesis of coreshell nanomaterials. 96,97 Two MOFs with similar lattice parameters can be assembled to form core-shell MOFs by epitaxial growth. Kitagawa and co-workers demonstrated the rst synthesis of core-shell bimetallic MOF single crystals by epitaxial growth. 98 The hybrid material consisted of [Zn 2 (ndc) 2 -(dabco)] n (1) as the core crystal and [Cu 2 (ndc) 2 -(dabco)] n (2) as the shell crystal (ndc ¼ 1,4-naphthalenedicarboxylate; dabco ¼ diazabicyclo[2.2.2]octane) and was synthesized by immersing 1 in a solution of CuSO 4 $5H 2 O, H 2 ndc and dabco for the growth of 2. Yamauchi and co-workers reported core-shell bimetallic ZIF materials with Zn-containing ZIF-8 as the core and Cocontaining ZIF-67 as the shell (Fig. 7A). 99 Tang and co-workers  reported a sandwich structure with an inner core and an outer shell composed of MIL-101 with metal nodes of Fe 3+ , Cr 3+ or both. 100 Rosi and co-workers reported a domain building block (DBB) approach to construct a rich library of UiO-67 stratied MOF (sMOF) particles consisting of multiple concentric DBBs with core-shell and multilayered structures, such as UiO-67(Zr) 3UiO-67(Hf), UiO-67(Hf)3UiO-67(Zr), and UiO-67(Zr)3UiO-67(Hf)3UiO-67(Zr). 101 Oh and co-workers further demonstrated that lattice parameter variation of isostructural MOFs could lead to different growth behaviors in an isotropic or anisotropic fashion (Fig. 7B). 102 They employed Fe-MIL-88B as the seed to direct the formation of Ga-MIL-88B and In-MIL-88B with analogous structures but different sizes of metal ions. Ga-MIL-88B isotropically grew on the entire surface of Fe-MIL-88B, while In-MIL-88B anisotropically grew in the c-direction at both tips of Fe-MIL-88B. The different behaviors are due to the size variation of the metal ions. The ion size of Ga(III) is quite similar to that of Fe(III), resulting in a good similarity in the cell parameters for growth in an isotropic manner. However, the ion size of In(III) is larger than that of Fe(III), leading to a signicant increase in the c cell parameters of In-MIL-88B and thus the anisotropic growth of In-MIL-88B in the c-direction.
The epitaxial growth method is usually limited to the synthesis of core-shell MOFs with similar crystallographic parameters. The assembly of two MOFs with different crystal structures would inevitably suffer from the separate growth of MOFs in solution. To address this issue, it has been demonstrated that the use of capping agents can promote a conformal and oriented overgrowth of one MOF on another MOF with different crystal structures. [103][104][105] Tsung and co-workers overgrew uniform ZIF-8 shells on monodisperse UiO-66 microcrystal cores to form UiO-66@ZIF-8 with the assistance of cetyltrimethylammonium bromide (CTAB). 103 The authors proposed that CTAB could adsorb small and uniform ZIF-8 nuclei under sonication and then orient the nuclei to grow into a conformal shell (Fig. 7C) Although capping agents can facilitate the integration of core MOFs and shell MOFs with distinct crystallographic parameters, the capping agent can potentially make the MOF-MOF interface complex and ill dened. In this regard, Zhou and co-workers reported the fabrication of MOF-on-MOF structures with mismatched lattices through retrosynthetic design without the use of capping agents. 107 The authors chose stable PCN-222(Zr) (also known as MOF-545) as the core MOF, which was pretreated with linkers (H 2 BDC) of the shell MOF (MOF-5), resulting in the coordination of H 2 BDC on the surface of PCN-222(Zr). Further adding metal precursors (Zn(NO 3 ) 2 ) allowed the gradual formation of MOF-5 on the surface of PCN-222(Zr). This retrosynthetic method could be successfully applied to the synthesis of other MOF-on-MOF materials with stable MOFs 2.2.2 Post-synthetic exchange. Post-synthetic selective exchange of metal ions in the framework can be used to synthesize core-shell bimetallic MOFs. In the MOF crystal, metal sites in the core and close to the surface show different exibilities and thus distinct reactivities. Therefore, through carefully controlling the post-synthetic metal exchange process, core-shell bimetallic MOFs can be obtained through selective transmetalation. Lah and co-workers reported a kinetically controlled replacement of the metal in the external region of the crystal to obtain core-shell bimetallic MOFs (Fig. 8A). 78 A family of isostructural MOFs, M 6 (BTB) 4 (BP) 3 (where M ¼ Zn(II) (1), Co(II) (2), Cu(II) (3), and Ni(II) (4), BTB ¼ 1,3,5-benzenetribenzoate, and BP ¼ 4,4 0 -dipyridyl), were selected for the transmetalation reactions. Thermodynamically less stable 1 and 2 could undergo transmetalation to more stable 3 and 4, while the reverse transmetalation could not occur. The authors found that the metal sites in the external shell region of the crystal showed higher reactivities, thus reacting faster than those in the more rigid internal core. Therefore, by simply controlling the soaking time, core-shell heterostructures were formed through selective transmetalation.
In the subsequent work, Lah and co-workers further demonstrated the concept of selective transmetalation using two Zn-based MOFs with different ligand lengths, Zn-HKUST-1 and Zn-PMOF-2 (Zn 24 L 8 (H 2 O) 12 , L ¼ 1,3,5-tris(3,5-dicarboxylphenylethynyl)benzene). 108 In the methanol solution of Cu(II), the transmetalation of Zn-HKUST-1 was incomplete even aer three months. Zn-PMOF-2 showed a higher rate and extent in the transmetalation process due to its more exible structure compared with Zn-HKUST-1 (Fig. 8B). For both MOFs, the exchange of the metal ions occurred selectively at the external shell, leading to a core-shell heterostructure.
2.2.3 One-pot synthesis. One-pot synthesis can simplify the synthetic process and reduce steps for the separation and purication of intermediates, which is desirable but challenging. [109][110][111] Core-shell bimetallic MOFs can be prepared in one pot, through mixing all the precursors of core and shell MOFs in the synthesis solution. It is of vital importance to control the nucleation and growth kinetics of the two MOFs in the synthesis solution, allowing the shell MOF to grow exclusively on the surface of the core MOF without self-nucleation. The control of the experimental parameters (e.g., precursors, solvents, temperature, time, and modulators) is necessary to balance the rates of self-nucleation and growth of the core and shell MOFs, allowing them to assemble into a single nanostructure.
Zou and co-workers investigated the reaction kinetics of Co 2+ and Zn 2+ with 2-MeIM for bimetallic Co/Zn ZIFs (Fig. 9A). 112 The authors found that a low Co/Zn ratio resulted in Co-rich cores and Zn-rich shells. The growth went through a nucleationgrowth separated process; Co 2+ with higher reaction kinetics formed the cores rst, followed by the growth of zinc shells. At a high Co/Zn ratio, the nucleation and growth processes were not separated, leading to a quick formation of solid solution Co/ Zn ZIFs. Moreover, when adding Co 2+ rst, ZIF-67@ZIF-8/67 core-shell nanocrystals were obtained with tuneable core/shell thickness ratios. However, exchanging the sequence by adding Zn 2+ rst only formed agglomerates with irregular shapes.
Van Der Voort and co-workers reported egg-yolk bimetallic MOFs with a core of (Cr/V)-MIL-53 and a shell of Cr-MIL-53 using a one-pot microwave (MW) procedure. 113 V 3+ and Cr 3+ salts were mixed with H 2 BDC in demineralized water in a MW for 3 h, resulting in the formation of (Cr/V)-MIL-53@ Cr-MIL-53 (Fig. 9B). For comparison, using a solvothermal method for 96 h resulted in one homogeneous phase of (Cr/V)-MIL-53 with uniform dispersion of V 3+ and Cr 3+ . This work demonstrated that the synthesis time and method had a crucial inuence on the metal dispersion in the crystals.
Kitagawa and co-workers controlled the kinetics of the crystal formation process for the preparation of core-shell MOFs in one pot by using precursors of distinct chemical reactivities (Fig. 9C). 114 The authors employed [M(NO 2 -ip) (bpy)] 0.5DMF$0.5MeOH (MCID-5IG, M ¼ Zn, Mn, and Ni, NO 2 -ip ¼ 5-nitroisophthalate, and bpy ¼ 4,4 0 -bipyridyl) for the study. When using 5-nitroisophthalic acid (H 2 NO 2 -ip) as a precursor, the crystal formation rate of ZnCID-5IG was much quicker than that of MnCID-5IG. The one-pot reaction of H 2 NO 2 -ip and bpy with zinc and manganese perchlorate salts was conducted in a DMF/MeOH solution at 70 C for 3 days, yielding crystals with a Zn-rich inner core and a Mn-rich outer shell. During the synthesis of Zn/MnCID-5IG, the nucleation process was dominated by the Zn rich crystals, which could facilitate the heterogeneous nucleation and growth of Mn-rich crystals. Moreover, additionally adding the precursors of NiCID-5IG with a moderate formation rate could yield Zn/Ni/MnCID-5IG crystals with a sandwich structure.

Applications of bimetallic MOFs
The mixing of metals in the SBUs of a MOF structure is widely recognized as an effective way to achieving optimal catalytic, optical, and electronic properties. A number of compositionproperty relationships have been established for many different MOF families.
The incorporation of second metal ions in the unstable SBUs of MOFs may enhance the stability of MOFs. For example, MOFs constructed from zinc(II) paddlewheel [Zn 2 (OCO) 4 ] SBUs frequently with low stability can show enhanced stability when exchanging the Zn II ion for other metal ions (such as Cu 2+ and Ni 2+ ). 64,[115][116][117] Mg-MOF-74 can gain improved water stability by the incorporation of Ni 2+ or Co 2+ ions that are less likely to hydrolyze than Mg 2+ . 118 The exibility of MOFs can be modulated by adopting different metal ions in the SBUs to meet the requirements of desired applications, such as storage, separation, and sensing. 119 Yeung, Goodwin and co-workers studied the thermal expansion in Zn/Cd ZIF-8. 35 The coefficient of thermal expansion showed a continuous and monotonic decrease with the increase of Cd content in Zn/Cd ZIF-8. These results demonstrated that Cd substitution in ZIF-8 made the framework structure increasingly exible, due to a weaker bonding of Cd 2+ with 2-MeIM compared with Zn 2+ . Serre and co-workers reported the tuning of the breathing behaviour of MIL-53 by cation mixing. 55 MIL-53(Cr-Fe) adopted a hydrated narrow pore (np) form below 343 K, and then changed to a closed pore (cp) form at 343 K and a large pore (lp) form upon further heating above 463 K. This breathing behaviour is different from that of MIL-53(Cr) and MIL-53(Fe), which showed a direct np to lp transformation and a two-step np to cp transformation, respectively.
The electronic properties of MOFs can be tailored through metal node engineering. 120,121 Mixing different metals in the SBUs of MOFs may provide MOFs with excellent electrical conductivity for a variety of applications, such as semiconductors, supercapacitors, thermoelectrics, and resistive sensing. Chen, Shustova and co-workers prepared bimetallic MOFs, Cu 3Ày M y (BTC) 2 (M ¼ Co, Zn), and studied how the substitution of a secondary metal would affect the electronic properties of MOFs. 122 Monometallic M-MOFs (M ¼ Cu and Zn) and bimetallic CuZn-MOFs exhibited zero intensity near the Fermi level which were characteristic of insulators, while CuCo-MOFs exhibited semiconductor behaviour. Microwave conductivity measurements showed that effective conductivities were 0.1 Â 10 À4 and 3.5 Â 10 À4 S cm À1 for Cu 3 (BTC) 2 and Cu 2.4 -Co 0.6 (BTC) 2 , respectively. Density functional theory (DFT) calculations revealed that the Co incorporation into the Cu-BTC matrix resulted in a decrease in the band gap and thus a higher conductivity.
Bimetallic MOFs show enhanced physical and chemical properties due to the synergistic effect of the two metals. The remarkable features of bimetallic MOFs make them suitable for applications in a variety of elds, including gas adsorption, catalysis, energy storage and conversion, and luminescence sensing.

Gas adsorption
Developing efficient porous materials for gas adsorption is fundamentally and industrially important. Bimetallic MOFs with large surface areas, adjustable pore sizes, and open metal sites are promising candidates as adsorbents for gas adsorption. Moreover, the adsorption strength of a specic adsorbate in bimetallic MOFs can be optimized through tuning the compositions of bimetallic MOFs. Many bimetallic MOFs have been designed as excellent adsorbents for gas adsorption.
3.1.1 Hydrogen adsorption. The hydrogen (H 2 ) uptake of bimetallic MOFs is highly related to the structure of MOFs to store H 2 and the unsaturated metal sites to bind with H 2 . 59 Singh and co-workers prepared Co x Zn 100Àx -ZIF-8 with varying Co contents (x ¼ 0, 25, 50, 75, 90 and 100). 123 CoZn-ZIF-8 showed an enhancement of the surface area by $40% and the pore volume by $33% as compared to monometallic Zn-ZIF-8. At 77 K and 1 bar, Co 75 Zn 25 -ZIF-8 showed an enhancement of $23% in the H 2 uptakes as compared to Zn-ZIF-8 (Fig. 10A). The enhanced H 2 uptake capacities of CoZn-ZIF-8 could be attributed to the enhanced pore volume, surface area, microporosity, and heterogeneity in the pores.
Botas and co-workers prepared MOF-74 with different Co contents in the range of 0-100% for H 2 adsorption at 0 C and 10 bar. 51 The H 2 adsorption capacity of ZnCo-MOF-74 generally increased with the increase of Co content. The authors found a correlation between the isosteric heat of adsorption (Q st 0 ) for H 2 and the band gap energy values of Zn 1Àx Co x MOF-74 ( Fig. 10B), implying that H 2 -metal cluster interactions have a more decisive role in H 2 adsorption. Long and co-workers exchanged Mn 2+ ions in Mn 3 [(Mn 4 -Cl) 3 (BTT) 8 (CH 3 OH) 10 ] 2 with selected cations (M ¼ Li + , Cu + , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ ) to tune the interaction strength between H 2 and unsaturated metal centres in the frameworks. 62 All the materials showed similar H 2 uptakes ranging from 2.00 wt% to 2.29 wt% at 77 K and 900 torr. However, the materials showed signicantly different uptake slopes at very low pressure, indicating different metal-hydrogen binding strengths in the materials. The results showed that Mn 2+ , Fe 2+ , and Co 2+ ions yielded the strongest H 2 binding among the cations studied.
3.1.2 Carbon dioxide adsorption. Developing adsorbents with high physicochemical stabilities, high carbon dioxide (CO 2 ) capacity and selectivity, and low regeneration costs is still a challenge. MOFs are promising porous crystalline materials with high porosities, surface areas and open metal sites for CO 2 capture. 124 The incorporation of a second metal in MOFs can improve the stability of MOFs and provide stronger interaction with CO 2 .
Bu, Feng and co-workers reported the cooperative effect of dissimilar metals in the SBUs of MOFs for CO 2 adsorption. 126 The authors prepared a family of isostructural heterometallic MOFs (M II 2 M III (m 3 -OH)(CO 2 ) 6 , CPM-200 series, CPM ¼ crystalline porous materials) with combinations of trivalent (In 3+ , Ga 3+ , Fe 3+ , V 3+ , and Sc 3+ ) and divalent metals (Mg 2+ , Mn 2+ , Co 2+ , and Ni 2+ ) in the SBUs. 126  In addition to the open metal sites, the pore architecture is of vital importance for CO 2 capture performance. Hill and coworkers reported the exchange of Zr in UiO-66 for Ti to yield bimetallic UiO-66 with a smaller pore size and higher adsorption enthalpy for an enhanced CO 2 uptake. 80 With the increase of Ti substitution, the octahedral cages shrank by $1Å and became increasingly broader. The CO 2 uptake at 273 K of UiO-66(Zr 100 ), UiO-66(Ti 32 ) and UiO-66(Ti 56 ) was 2.2, 2.3 and 4 mmol g À1 , respectively. Considering that theoretical enhancement in the CO 2 gravimetric uptake of UiO-66(Ti 100 ) is $19% while UiO-66(Ti 56 ) showed an enhancement of 81% compared with UiO-66(Zr 100 ), the authors proposed that both the decrease of pore sizes and stronger adsorption characteristics of Ti(IV) contributed to the enhanced CO 2 uptake in Ti-exchanged UiO-66.
Moreover, the structural exibility change of bimetallic MOFs will lead to a different CO 2 sorption behaviour compared with their single cation analogues. Serre and co-workers reported different CO 2 sorption isotherms of bimetallic MIL-53(Cr-Fe) compared to monometallic MOFs (Fig. 10E). 55 CO 2 sorption tests at 283 K showed that the np to lp transition of MIL-53(Cr-Fe) occurred at a pressure of $10 bar, which was an intermediate between those of MIL-53(Cr) (3 bar) and MIL-53(Fe) (20 bar

Catalysis
3.2.1 Organocatalysis. The use of the inorganic nodes of MOFs as catalytically active sites has several advantages, including a good dispersion of the inorganic nodes within the framework to well expose the active sites and a well-dened structure of the inorganic nodes for precise structural characterization and computational modeling. 127,128 When using monometallic MOFs as catalysts, the metal ions in the inorganic nodes need to undergo a change of the coordination environment, which may sometimes lead to the collapse of the frameworks. It is a good idea to design a bimetallic MOF, in which one metal contributes to structural stability and the other metal acts as an active centre for catalysis. This design can also allow the isolation and stabilization of active metal sites to achieve high activity, selectivity and stability. [129][130][131][132][133][134][135] For example, Chen and coworkers prepared (Cu x Ru 1Àx ) 3 -(BTC) 2 (CuRhBTC) for propylene hydrogenation. 136 CuRh(33%)BTC exhibited an activity of 1600 mmol g catalyst À1 min À1 for propane production. The CuBTC MOF showed no activity under the reaction conditions, and RhBTC was unstable upon exposure to air. The inactive Cu ions in CuRhBTC played an important role in stabilizing the MOF framework and preventing the reduction of the incorporated Rh 2+ . Although catalytically active metal ions can be introduced through immobilization on the SBUs or organic linkers, the incorporation of a second metal on the SBUs of MOFs for bimetallic MOFs results in distinct catalytic properties compared with other ways. In this regard, Farha, Hupp, Nguyen and co-workers compared the catalytic activity of UiO-66(Zr) functionalized with Ti IV ions as part of the node (Ti-UiO-66), attached to the node (UiO-66-Ti ex ), and on the organic linker (UiO-66-Cat-Ti) in the oxidation of cyclohexene. 137 These three materials had Ti IV ions with different coordination environments at different sites of the UiO-66 support (Fig. 11A). Three UiO-66-based catalysts showed the catalytic activity in the order of Ti-UiO-66 [ UiO-66-Cat-Ti > UiO-66-Ti ex , indicating that tetrahedral Ti IV ions were more active than the more saturated octahedral Ti IV ions.
Monometallic MOF catalysts usually exhibit either unsatisfactory activity or selectivity. Bimetallic MOFs can show a synergistic effect between the two metal ions to activate the reactants and reduce the reaction energy barrier in heterogeneous catalysis. 31 Bimetallic MOFs have shown promising catalytic performances in a variety of reactions, including oxidation reactions, 56,76,138-141 reduction reactions, 142,143 condensation reactions, 144-146 addition reactions, 53,147 and substitution reactions. 148 Zhu, Fan and co-workers applied bimetallic MOF-74(Cu/Co) with different Cu/Co ratios in styrene oxidation with O 2 (Fig. 11B). 139 MOF-74(Cu) showed a low catalytic activity (0.6% conversion) but an absolute selectivity for benzaldehyde. MOF-74(Co) possessed a high catalytic activity (47.3% conversion) but a low selectivity (35%) for benzaldehyde. The incorporation of Co 2+ in MOF-74 (Cu) could signicantly enhance the conversion of styrene. With the increase of Co 2+ content in MOF-74(Cu/Co), the conversion of styrene increased while the selectivity for benzaldehyde decreased. A physical mixture of MOF-74(Cu) and MOF-74(Co) exhibited a lower activity than MOF-74(Cu/Co), indicating the synergistic effect of Cu 2+ and Co 2+ in the same framework.
Garcia, Serre and co-workers synthesized a series of MIL-100(Fe, Ni) materials with different metal ratios as catalysts for the Prins reaction. 144 MIL-100(Fe, Ni) (1-3) samples contained a Ni amount of 1%, 3%, and 5%, respectively. In the Prins reaction between b-pinene and paraformaldehyde, monometallic MIL-100(Fe) only showed a low conversion (<20%). Bimetallic MIL-100(Fe, Ni) afforded higher conversions between 70 and 100%. MIL-100(Fe, Ni) 3 with the highest Ni percentage showed the highest activity, with a total conversion to nopol in 10 h (Fig. 11C). The authors proposed that the incorporation of Ni II could cause the distortion of the framework to facilitate the access to the Fe III sites, thus enhancing the catalytic activity of the catalyst.
Park and co-workers synthesized a solid solution Zn-Co ZIF (CZ-ZIF) for chemical xation of CO 2 . 147 In the cycloaddition reaction of CO 2 with epichlorohydrin (ECH), ZIF-8 showed an excellent ECH conversion of 98% but a low selectivity of 33% towards epichlorohydrin carbonate (ECC). ZIF-67 exhibited high selectivity to ECC (98%) but an inferior conversion of 66%. Solid solution CZ-ZIF afforded both high conversion of ECH (94%) and high selectivity (98%) to the desired ECC. The bimetallic CZ-ZIF could combine the advantageous properties of Zn (high activity) and Co (high selectivity and low leaching), achieving enhanced catalytic performance compared to its monometallic counterparts.
Multicomponent reactions and cascade reactions which require the cooperation of different active sites can be accomplished in bimetallic MOFs. [149][150][151][152] Monge and co-workers demonstrated the control of the catalytic activity and selectivity of bimetallic MOFs in a multicomponent one pot Strecker reaction by modulating specically selected metal ratios (Fig. 11D). 149 In the A 3 reaction between benzaldehyde, trimethylsilyl cyanide (TMSCN), and aniline, monometallic GaPF-1 [Ga(OH)(hpbb)] and InPF-11b [In(O 2 C 2 H 4 ) 0.5 (hpbb)] (H 2 -hpbb ¼ 4,4 0 -(hexauoroisopropylidene) bis(benzoic acid)) gave only aldehyde cyanosilylation and imine as the main product, respectively. Solid-solution bimetallic MOFs [In x Ga 1Àx (O 2 C 4 -H 4 ) 0.5 (hpbb), x ¼ 0.72, 0.55, and 0.28] could selectively yield the nal Strecker product (a-aminonitrile) with different reaction rates. InGaPF-3 with the least amount of In showed the highest reaction rate, with 96% conversion in 0.33 h. Koner and co-workers reported a series of bimetallic MOFs based on copper and alkaline-earth metals (Mg, Ca, Sr, and Ba) to effectively catalyse olen epoxidation and subsequent epoxide ring opening (Fig. 11E). 150 In the tandem reaction, Cu II acted as an active centre in epoxidation reactions and alkaline-earth metal ions served as Lewis acidic sites for epoxide ring opening subsequently. These MOFs with different alkaline-earth metals showed different reaction rates, increasing in the order of Mg < Ca < Sr z Ba. The authors proposed that the increase of the size of the alkaline-earth metals led to a greater number of ligands This journal is © The Royal Society of Chemistry 2020 Chem. Sci., 2020, 11, 5369-5403 | 5379

Perspective
Chemical Science accommodated in the coordination sphere and thus formed more open sites to enhance the catalytic activity.

Photocatalysis.
Bimetallic MOFs can also act as excellent catalysts for photocatalytic reactions, such as organic reactions, water splitting, CO 2 reduction, and organic pollutant degradation.
Ti-based MOFs have shown great promise in photocatalysis. The incorporation of second metal ions in the SBUs of Ti-based MOFs can greatly enhance the photocatalytic activity. Martí-Gastaldo and co-workers reported two Ti-based MOFs (MUV-10, Ti IV 3 M II 3 (m 3 -O) 2 (btc) 4 -(H 2 O) 6 , M ¼ Ca II , Mg II ) by metal doping for photocatalytic H 2 production (Fig. 12A). 153 The authors found that heterometallic clusters served as excellent platforms to manipulate the electronic structure and in turn regulate the photoactivity. MUV-10(Mn) could produce 6500 mmol g À1 of H 2 aer 24 hours under the irradiation of a Xe lamp (300 W), showing two times higher activity than that of MUV-10(Ca). However, few Ti-based MOFs have been reported due to the high reactivity of Ti precursors. Alternatively, Ti can be incorporated into MOFs through post-synthetic exchange methods. For example, Ti-substituted UiO-66 (Zr/Ti) materials have shown enhanced photocatalytic performance compared to monometallic UiO-66(Zr). 154 and Mn x Mg 3Àx O SBUs, the two kinds of metals were in different SBUs and thus showed negligible interactions due to the separation by organic linkers (Fig. 12C). Therefore, these bimetallic MOFs exhibited the highest catalytic rates among their corresponding single component MOFs.
Core-shell MOFs have been successfully applied in photocatalysis, in which the shell MOF can accumulate/recognize reactants and the core MOF with catalytically active sites can transform the reactants to products. 106,107,159 Kitagawa and coworkers applied MIL-101(Cr)@NH 2 -MIL-125(Ti) in the adsorption-photocatalytic removal of Cr VI (Fig. 12D). 106 167,168 and electrochemical CO 2 reduction. 169,170 In bimetallic MOFs, the electron congurations and d-band centres can be tailored through the mix of different metals in the SBUs, offering opportunities to enhance the electrocatalytic performance. 122, 171 Li, Lan and co-workers prepared four isostructural MOFs (NNU-21-24) based on Fe 2 M (M ¼ Fe, Co, Ni, Zn) clusters bridged with biphenyl-3,4 0 ,5-tricarboxylic acid (BPTC) ligands for the electrocatalysis of the OER. 163 Among the four synthesized MOFs, monometallic NNU-21 (Fe 3 -BPTC) showed lower OER performance than bimetallic MOFs (NNU-22 (Fe 2 Co-BPTC), NNU-23 (Fe 2 Ni-BPTC) and NNU-24 (Fe 2 Zn-BPTC)) (Fig. 13A). NNU-23 exhibited the best OER performance with an overpotential of 365 mV at a current density of 10 mA cm À2 in 0.1 M KOH. Moreover, all the catalysts exhibited high stability with almost no decrease of activity aer 2000 cycles. DFT calculation revealed that the incorporation of a second metal (Co, Ni or Zn) into the Fe cluster could induce the d-band centre to be close to the Fermi level, leading to a stronger binding interaction between the O* intermediate and catalysts, thus improving the OER performance. In addition to solid solution bimetallic MOFs, MOF@MOF structures can also show a synergistic effect in electrocatalysis. 172 Zhu, Sun and coworkers grew Fe-MOF NPs onto Ni-MOF nanosheets (Ni-MOF@Fe-MOF) for the OER. 173 The deposition of catalytically inert Fe-MOF on Ni-MOF could enhance the electrocatalytic performance. Ni-MOF@Fe-MOF exhibited an overpotential of 265 mV to reach a current density of 10 mA cm À2 in 1.0 M KOH, which was lower than that of Ni-MOF (370 mV). The enhanced performance of Ni-MOF@Fe-MOF could be attributed to the synergistic effect between Ni active centres and Fe species.
However, bimetallic MOFs usually suffer from poor conductivity, blockage of active metal centres by organic linkers, and low stability, which dramatically limit their utilization as electrocatalysts. To address these challenges, thinning bimetallic MOFs into nanosheets, fabricating an amorphous structure with plenty of defects, and integrating bimetallic MOFs with conductive supports have been proposed to enhance the electrocatalytic performance of bimetallic MOFs.
Thinning bimetallic MOFs into two-dimensional (2D) nanosheets can enhance mass transport and electron transfer and maximize unsaturated metal sites on the surfaces to improve the electrocatalytic performance. 8,160,[174][175][176][177] Liu, Zhao, Tang and co-workers reported ultrathin NiCo bimetallic MOF nanosheets (NiCo-UMOFNs) as effective electrocatalysts for the OER (Fig. 13B). 174 NiCo-UMOFNs on a glassy-carbon electrode showed an overpotential of 250 mV at a current density of 10 mA cm À2 in 1.0 M KOH solution, smaller than those of Ni-UMOFNs (321 mV), Co-UMOFNs (371 mV), bulk NiCo-MOFs (317 mV) and commercial RuO 2 (279 mV). XAFS analysis and DFT calculations demonstrated that the coordinatively unsaturated metal atoms were the dominating active centres and the synergistic effect between Co and Ni further improved the electrocatalytic OER activity. Electron transfer from the Ni 2+ sites to the Co 2+ sites could enhance the interaction between Ni 2+ sites and water, thus offering a lower energy barrier for water oxidation. Oh and co-workers prepared bimetallic conductive 2D MOFs (Co x Ni y -CATs) with varied ratios of Co 2+ to Ni 2+ for the ORR in 0.1 M NaClO 4 and 0.02 M PBS electrolytes. 160 Ni-CAT showed a high onset potential of 0.47 V (vs. the reversible hydrogen electrode, RHE, the same below if not mentioned), but a low diffusion limiting current density at 0.0 V (À3.62 mA cm À2 ). However, Co-CAT displayed a high diffusion limiting current density (À5.59 mA cm À2 ) but a low onset potential (0.42 V). Bimetallic Co 0.27 Ni 0.73 -CAT combined the advantages of Co-CAT (effective adsorption for O 2 ) and Ni-CAT (high conductivity), showing a diffusion-limiting current density (À5.68 mA cm À2 ) and an onset potential (0.46 V). With the increase of the proportion of Ni from 50% to 73%, the onset potential of Co x Ni y -CATs showed a slight increase. Further increasing the proportion of Ni to 86% resulted in a decrease in diffusion-limiting current density. Rotating ring disk electrode (RRDE) investigation showed that the electron transfer number of Co-CAT, Ni-CAT, and Co 0.27 Ni 0.73 -CAT was 3.91, 3.31, and 3.95, respectively. The bimetallic Co 0.27 Ni 0.73 -CAT was more preferable for a fourelectron process. This journal is © The Royal Society of Chemistry 2020 Chem. Sci., 2020, 11, 5369-5403 | 5381

Perspective
Chemical Science In MOFs with good crystallinity, the metal sites are conned in the crystalline frameworks and blocked by the organic linkers, which is not good for electrocatalysis. In this regard, low-crystalline bimetallic MOFs with highly exposed active sites and porous structures hold great promise for electrocatalysis. Mai and co-workers reported the synthesis of low-crystalline bimetallic Fe/Ni-MOF NPs (Fe x Ni y -BDC) for the OER. 164 The addition of Ni precursors in the synthesis solution of Fe-BDC (Fe-MIL-88B) caused mismatched coordination with organic linkers, thus resulting in the loss of the long-range ordered structure and formation of rich defects (Fig. 13C). Fe 1 Ni 2 -BDC showed a small overpotential of 260 mV at a current density of 10 mA cm À2 and a high Faradaic efficiency (FE) of 99.5% for the OER in 1.0 M KOH. The superior catalytic performance of Fe 1 Ni 2 -BDC could be attributed to the exposed active sites, synergistic effect between Fe and Ni, and fast charge transfer in the amorphous structure.
The integration of bimetallic MOFs with functional materials can enhance the characteristics of MOFs with improved electrical conductivity, chemical stability and mechanical strength for electrocatalytic applications. 6,167,[178][179][180] Lu and coworkers reported the in situ growth of amino-functionalized bimetallic NH 2 -MIL-88B(Fe 2 Ni) (NFN-MOF) on 3D microporous nickel foam (NF) as an efficient bifunctional electrocatalyst for overall water splitting. 181 In a 1.0 M KOH electrolyte, the synthesized NFN-MOF/NF exhibited overpotentials of 240 and 87 mV at a current density of 10 mA cm À2 for the OER and HER, respectively. For the overall water splitting with NFN-MOF/NF as both the anode and cathode, a cell voltage of only 1.56 V was required to achieve a current density of 10 mA cm À2 . The excellent catalytic performance of NFN-MOF/NF can be attributed to the synergistic effect between Ni and Fe metal ions in the MOF, and a positive coupling effect between the MOF and NF.

Energy storage and conversion
MOFs with large surface areas, adjustable pore structures, and redox metal centres are promising electrode materials for electrochemical capacitors and rechargeable batteries. 14,[182][183][184][185][186] However, the inherent poor electrical conductivity of MOFs is one of the greatest obstacles to achieve high performance in capacitors. In bimetallic MOFs, the electrochemical properties can be tuned by mixing two different metals in the SBUs. Bimetallic MOFs with enhanced electrical conductivity have been applied as the electrode materials for supercapacitors. [187][188][189][190][191][192][193] (Fig. 14A). The assembled hybrid supercapacitors (Co/Ni-MOF//CNTs-COOH) exhibited an energy density of 49.5 W h kg À1 and a power density of 1450 W kg À1 . The metal ion exchange resulted in an increase of free holes and the interlayer distance and in turn led to an enhancement of the electrical conductivity and surface area, explaining the enhanced performance of bimetallic MOFs for capacitors. Bimetallic MOFs have been investigated as anode and cathode materials for lithium-ion batteries (LIBs). When using bimetallic MOFs as anode materials, the adjustable pores can allow Li ions to be stored and reversibly inserted/extracted, leading to insertion-type electrodes. 194 Some bimetallic MOFs enabling reversible transformation/regeneration can be employed as conversion-type electrodes. 195 For example, Vittal et al. investigated the electrochemical performance of FOR4 (Zn 1.5 Co 1.5 (HCOO) 6 ). 195 The formate-bridged MOFs were converted to lithium formate MOFs upon lithiation and regenerated upon de-lithiation. The reversible conversion reaction led to good cycling stability of FOR4 (Fig. 14B), with a high reversible capacity of 510 mA h g À1 up to 60 cycles at 60 mA g À1 , exceeding that of monometallic CoMOF (FOR3). As cathodes in LIBs, bimetallic MOFs with redox metal centres and good Li-ion mobility within the solid are good candidates, such as K 0.14 Mn 1. 43 197 Bimetallic MOFs can be applied as cathode materials for lithium-oxygen batteries (LOBs). The porous structures and tuneable bimetallic active sites endow bimetallic MOFs with the capabilities to optimize the performance. Lee and co-workers used bimetallic MnCo-MOF-74 materials as cathode catalysts for Li-O 2 batteries. 198 MnCo-MOF-74 delivered a discharge capacity of 11 150 mA h g À1 and excellent cyclability (44 cycles) with a low overpotential at a limited capacity of 1000 mA h g À1 , outperforming the monometallic counterparts, Mn-and Co-MOF-74 (Fig. 14C). The synergistic integration of Mn-and Cometal clusters contributed to the improved efficiency and reversibility.

Luminescence sensing
Luminescent MOFs (LMOFs) are an important sub-category of MOFs. 199 Lanthanide (Ln) cations are particularly attractive for the construction of LMOFs because of their characteristic sharp emission bands and high quantum yields in the near-infrared and visible regions. Typically, Eu 3+ and Tb 3+ are usually used in LnMOFs because of their strong characteristic red emission at 614 nm ( 5 D 0 / 7 F 2 ) and green emission at 541 nm ( 5 D 4 / 7 F 5 ), respectively.
Mixed Ln 3+ ions with different emissions can be integrated into a MOF framework to tune the luminescence properties. 200 Moreover, the metal distribution (solid solution or core-shell) in mixed LnMOFs inuences the luminescence properties. Mahon, Burrows and co-workers studied the effect of metal distribution on the luminescence properties of mixed LnMOFs. 201  showed an emission spectrum with strong bands of Eu 3+ and weak bands of Tb 3+ and H 4 odip. This is due to the energy transfer from Tb 3+ to Eu 3+ , resulting in a quenching of Tb 3+ transitions. Core-shell mixed LnMOFs displayed different emission spectra because the metal energy transfer between Ln 3+ ions was prevented. Gd@Tb@Eu and Tb@Eu@Gd showed emission spectra dominated by Tb 3+ emissions with lower intensity Eu 3+ and ligand transitions. Eu@Gd@Tb exhibited an emission spectrum with only Eu 3+ and ligand transitions present. In all the core-shell structures, the core Ln 3+ emissions were not observable. Therefore, alteration of shell ordering and This journal is © The Royal Society of Chemistry 2020 Chem. Sci., 2020, 11, 5369-5403 | 5383

Perspective
Chemical Science thickness could tailor the luminescence properties of core-shell mixed Ln-MOFs. A variety of bimetallic LnMOFs have acted as ratiometric sensors with high sensitivity and high selectivity toward ionic species, pH, temperature, environmental toxins, explosives, biomolecules, etc. [202][203][204] Bimetallic LnMOFs with dual emissions can be employed as self-referencing luminescence thermometers based on the intensity ratios of two separate transitions. 205 Qian, Chen and co-workers reported luminescent mixed Ln-MOFs, (Eu x Tb 1Àx ) 2 (DMBDC) 3

(H 2 O) 4 $DMF$H 2 O
(Eu x Tb 1Àx -DMBDC, DMBDC ¼ 2,5-dimethoxy-1,4-benzenedicarboxylate, x ¼ 0.0011, 0.0046, and 0.0069) as luminescent thermometers. 206 From 10 to 300 K, the luminescence intensity of Tb 3+ at 545 nm in Tb-DMBDC and Eu 3+ at 613 nm in Eu-DMBDC both decreased gradually as the temperature increases ( Fig. 15A and B). The mixed lanthanide MOFs exhibited a signicantly different temperature-dependent luminescence behaviour. In Eu 0.0069 Tb 0.9931 -DMBDC, the emission intensity of Tb 3+ decreased, while that of Eu 3+ increased with the increase of temperature (Fig. 15C). The emission intensity ratio (I Tb /I Eu ) correlated well with the temperature in the range of 50 to 200 K (Fig. 15D). Moreover, Eu 0.0069 Tb 0.9931 -DMBDC emitted temperature dependent luminescence colours from green-yellow to red at temperatures ranging from 10 to 300 K. The exhibited temperature-dependent emissions and luminescence colours in Eu/Tb-DMBDC may be attributed to the temperature-dependent energy transfer from Tb 3+ to Eu 3+ ions.
Mixed Ln-MOFs have also been studied as sensors for quantitative detection of water and identication of solvents. Humphrey and co-workers synthesized mixed-metal PCM-22 with controlled amounts of Eu 3+ , Gd 3+ and Tb 3+ as sensors for the identication of a wide range of solvents, and especially for trace H 2 O detection in D 2 O (Fig. 15I and J). 208  In conclusion, due to the synergistic effect between the two metals, bimetallic MOFs can exhibit signicantly enhanced physical and chemical features compared with their monometallic counterparts. Therefore, through the rational design of bimetallic MOFs, they can afford excellent performance in gas adsorption, catalysis, energy storage and conversion, luminescence sensing, and so on.

Synthesis of bimetallic MOF derivatives
Since the MOF pyrolysis was reported by Xu and co-workers for porous carbon synthesis in 2008 209 and for metal oxide synthesis in 2010, 210 MOF derivatives have been intensively designed and synthesized. 33,211 Especially, bimetallic MOFs with controllable compositions in the SBUs provide a promising platform for the preparation of a variety of functional materials, such as carbon composites (carbon supported atomically dispersed metals, metal NPs, alloy, oxides, carbides, sulphides, and phosphides), metal compounds (oxides, hydroxides, nitrides, sulphides, and phosphides), and MOF composites, which have a wide variety of physicochemical properties and applications.

Carbon composites
The pyrolysis of MOFs has been adopted as an effective way to obtain carbon composites with various morphologies and compositions. The texture and graphitization degree of carbon supports have a great inuence on the performance of the composites. Carbon supports with a high surface area and hierarchical pores can guarantee the free diffusion of substrates. 212 Graphitized carbon can enhance electrical conductivity and chemical and thermal stability.
ZIFs have been widely investigated as precursors to synthesize carbon-based materials. The pyrolysis of bimetallic ZIFs can realize well-graphitized carbons, high-surface-area carbons, and active metal-N x -C sites at the same time, which cannot be achieved by monometallic ZIFs. For example, the pyrolysis of Zn-based ZIF-8 can afford high-surface-area carbon with high N contents but fail to provide active metal-N x -C sites and wellgraphitized carbon. 213,214 On the other hand, the pyrolysis of Cobased ZIF-67 can give well-graphitized carbon and highly active Co-N x -C sites but only offer carbon with a low surface area and porosity. [215][216][217] In this regard, Xiong, Jiang and co-workers employed a series of bimetallic ZIFs (BMZIFs-n, n represents the ratio of Zn/Co) as precursors to afford Co, N-doped porous carbons (CNCo-n) with a large surface area, high graphitization degree, and highly dispersed N and CoN x active species (Fig. 16A). 218 BMZIFs-n were pyrolyzed at 900 C under N 2 , during which the generated ZnO was reduced by carbon and then evaporated. Aer treating the calcined products in H 2 SO 4 at 80 C to remove the inactive and unstable Co species, CNCo-n could be afforded. The BET surface areas of CNCo-n (n ¼ 0, 0.2, 5, 10, 20, 40, and 100) were 270, 362, 550, 1090, 1225, 1480, and 2184 m 2 g À1 , respectively. Besides, Yamauchi and co-workers thermally transformed core-shell ZIF-8@ZIF-67 into nanoporous nitrogen-doped carbon@graphitic carbon (NC@GC) materials. 99 ZIF-8@ZIF-67(x) (x represents the molar ratio of Co 2+ /Zn 2+ ) materials were thermally treated at 800 C under a N 2 ow, followed by washing with HF solution to remove the residual Zn and unstable Co species. The obtained NC@GC samples showed intermediate surface areas between those of NC and GC derived from ZIF-8 and ZIF-67, respectively. In addition, NC@GC showed mesopores on the particle surface and abundant layered graphitic carbon structures, which were similar to those of GC.
Recently, Xu and co-workers reported that introducing guest Fe ions into a ZIF-8@ZIF-67 precursor led to the self-assembly of open carbon cages into a hydrangea-like 3D superstructure (Fig. 16B). 219 Direct pyrolysis of core-shell ZIF-8@ZIF-67 produced isolated open-wall N-doped carbon cages with insuf-cient contact and total conductivity. The introduction of guest Fe ions into the MOF precursor formed FeCo alloy NPs during the pyrolysis, which could catalyse the growth of carbon nanotubes and thus interconnect neighbouring cages to form a hydrangea-like 3D superstructure.
The employment of bimetallic MOFs as precursors can afford precise size, composition, and structure control of the supported metal/oxide NPs. [220][221][222] This is beneted from the ability to control the compositions and distributions of metal ions in MOFs, which are not achievable by other synthetic methods. The calcination of bimetallic MOFs under an inert atmosphere can afford metal alloy or oxide NPs depending on the reduction potential of the metal ions. 223 Metal ions (e.g., Cu 2+ and Co 2+ ) with a reduction potential of À0.27 V or higher usually form metal NPs aer calcination under an inert atmosphere, whereas metal ions (e.g., Cd 2+ and Mg 2+ ) with reduction potentials lower than À0.27 V form metal oxide NPs (Fig. 16C). For example, Chen and co-workers calcined bimetallic MOFs (Fe 3 [Co(CN) 6 ] 2 ) at 600 C in N 2 , affording FeCo alloy NPs encapsulated in N-doped graphene layers. 220 Gao, Lou and coworkers pyrolyzed Co/Mn-MIL-100 at 800 C in a 5% H 2 /Ar atmosphere, affording a MnO/Co hybrid supported by porous graphitic carbon. 224 The type of metal compound generated from the pyrolysis of bimetallic MOFs can be affected by the elements in the ligands. Carbon atoms can diffuse into the interstices of metal atoms to form bimetallic carbides. [225][226][227] For example, Su and co-workers reported the synthesis of Co/Mo 2 C/Mo 3 Co 3 C@C and Ni/ Mo 2 C@C derived from two bimetallic MOFs (CoMo-MOF and NiMo-MOF) annealed at 900 C in N 2 , respectively. 226 Other metal compounds (metal nitrides, phosphides and sulphides) supported on carbon materials can also be synthesized from bimetallic MOFs if suitable ligands with heteroatoms are employed.
The pyrolysis of MOFs serves as an ideal route for preparing various metal single atoms (SAs) on porous carbon (SAs/C). Delicate control is required to avoid the conversion of metal ions into aggregated NPs at high temperature. Bimetallic MOFs offer a promising platform for the synthesis of SAs/C based on the idea of using a second metal that is easily removed aer pyrolysis to extend the distance between adjacent targeted metals to avoid aggregation during pyrolysis. [236][237][238][239][240] Wu, Li and co-workers demonstrated the concept for the synthesis of Co SAs on N-doped porous carbon (Co SAs/N-C) through the pyrolysis of a bimetallic Zn/Co MOF (Zn/Co-BMOF). 236 The addition of Zn 2+ could dilute the concentration of Co 2+ and extend the adjacent distances between Co atoms. During the pyrolysis process, Zn was evaporated at high temperature, leaving abundant N sites to anchor and stabilize the isolated Co atoms (Fig. 16D). Co SAs/N-C obtained by calcination at 800 and 900 C showed a 4 and 2 metal coordination number with surrounding N atoms, respectively. Further increasing the calcination temperature to 1000 C broke the Co-N bonds and caused the formation of Co NPs. In the sequent work, Wu, Li and co-workers synthesized Fe-Co dual sites embedded on Ndoped porous carbon [(Fe,Co)/N-C] derived from Zn/Co-BMOF encapsulated FeCl 3 . 241 FeCl 3 was immobilized within the pores of Zn/Co-BMOF by a double-solvent method. During the calcination, Fe salts were reduced by carbon and bonded with neighbouring Co atoms to form Fe-Co dual-sites. The Fe species would accelerate the decomposition of MOFs and force the generation of voids inside the N-C support.

Metal compounds
The thermal calcination of bimetallic MOFs in air can lead to their decomposition into their corresponding bimetallic oxides. To achieve a higher surface area and porosity and preserve the original structure of parent MOFs to a certain extent, a two-step Chemical Science annealing (rst in N 2 and then in air) can be applied to synthesize metal oxide nanostructures. The use of bimetallic MOFs as precursors to obtain bimetallic oxides has emerged as a powerful synthetic route, offering the possibility to precisely control the compositions and structures of metal oxides. The achieved success in the control of metal distribution in MOFs can allow the tuning of the composition of the obtained metal oxides. [242][243][244][245] Zhou and co-workers synthesized Zn x -Co 3Àx O 4 (0 < x # 1) hollow polyhedra through calcination of bimetallic Zn-Co-ZIFs. 242 Bimetallic Zn-Co-ZIFs were rst heated in N 2 and then in air at 400 C. The rst thermal treatment in N 2 was of vital importance to partially preserve the porous structure of ZIFs. Monge, Gándara and co-workers demonstrated the control of the composition of multimetal oxides through the use of MOFs constructed from up to four different metal elements (Fig. 17A). 244 Multimetallic MOFs, constructed from a helical inorganic SBU with different cations (Zn 2+ , Co 2+ , Mn 2+ , and Ca 2+ ) and an organic linker H 2 hpbb [4,4 0 -(hexauoroisopropylidene)bis-(benzoic acid)], were subjected to a thermal treatment in air at 800 C to obtain the corresponding multimetal oxides. The metal arrangements and ratios of MOFs could be translated to metal oxides aer calcination.
Metal hydroxides can be fabricated from bimetallic MOF precursors through alkali hydrolysis. [249][250][251][252] Zou, Liu and coworkers reported the synthesis of bimetallic double hydroxides (MDHs) derived from bimetallic NiCo-MOF-74 through an alkaline treatment (Fig. 17C). 250 Ni/Co MOF-74 samples were dispersed in a 2 M KOH aqueous solution under stirring at room temperature, followed by hydrothermal treatment at 120 C for 2 h. The hydrothermal treatment with KOH resulted in the destruction of micron-sized MOF-74 and the formation of nanoscale MDH. Bimetallic nitrides can be prepared from bimetallic MOFs through nitridation under an NH 3 atmosphere. Guo and coworkers synthesized bimetallic CoNi nitrides derived from bimetallic CoNi-MOF-74. 253 CoNi-MOF-74 was rst converted to Ni 1 Co 2 O 4 through calcination at 400 C in air, and then transformed into Co 2 Ni 1 N by nitridation under an NH 3 atmosphere at 500 C (Fig. 17D). The resulting metal nitrides inherited the morphology from their corresponding precursors.
Bimetallic sulphides can be synthesized using bimetallic MOFs as templates via a hydrothermal suldation reaction. 193,[254][255][256] Zou and co-workers reported the synthesis of hollow bimetallic sulphides (M x Co 3Àx S 4 , M ¼ Zn, Ni, and Cu) by solvothermal suldation and thermal annealing from Co-based bimetallic MOFs. 254 MCo-MOFs (M ¼ Zn, Ni, and Cu) were treated in an ethanol solution of thioacetamide (TAA) at 120 C for 4 h, followed by calcination under a N 2 atmosphere at 350 C for 2 h (Fig. 17E). The obtained M x Co 3Àx S 4 preserved the rhombic dodecahedral morphology of MOF precursors, while formed a hollow structure aer the suldation procedure. S 2À ions released from TAA rst reacted with metal ions on the MOF surface to form a thin layer of sulphides, which could promote the outward diffusion of metal ions against the inward diffusion of S 2À ions. Then the inner framework was gradually dissolved and released metal ions to react with S 2À ions on the outer surface, which nally led to the generation of a hollow void inside the shell.
Transition metal phosphides (TMPs) show extremely low cost and expected long-term stability in both acidic and alkaline operating environments. Bimetallic MOFs have been applied for the synthesis of TMPs. [257][258][259][260][261][262] Zhao and co-workers synthesized nickel-cobalt mixed metal phosphide nanotubes (Co x Ni y P, x and y represent the molar ratios of Co and Ni in the MOF precursor, respectively) derived from bimetallic CoNi-MOF-74. 258 Bimetallic MOF-74 materials with different Co/Ni ratios were rst calcined at 350 C in air to afford Co x Ni y O, followed by phosphorization with NaH 2 PO 2 at 300 C in N 2 to afford Co x Ni y P.
As summarized above, various types of metal compounds can be obtained through the treatment of bimetallic MOFs.

MOF composites
Complete decomposition of MOFs usually leads to a dramatic decrease of the surface area and loss of well-dened pore structures. 209,213 Recently, Xu and co-workers have demonstrated a new concept of "quasi-MOFs" fabricated through controlled deligandation of MOFs, which can realize both a porous structure and exposed inorganic nodes. 263 This controlled thermal transformation method has been successfully applied on solid solution bi-and multi-metallic MOFs for the synthesis of MOF immobilized metal NPs or oxides. 264,265 This design takes advantage of different coordination abilities of the metal ions with the organic linkers. The metal ions with weaker coordination with the organic linkers transform into metal NPs/ compounds under certain conditions while the other metal ions remain coordinated with the organic linkers to preserve the framework. Hu and co-workers reported a thermal transformation of a benzimidazole-modied Cu/Co bimetal-MOF into a Cu-nanowire@quasi-MOF composite (Fig. 18A). 264 The Cu/Co bimetal-MOF was calcined at 300 C for 30 min under a N 2 atmosphere to afford a quasi-MOF matrix with a partially preserved MOF structure embedded with Cu nanowires. Under similar calcination conditions, monometallic Cu-MOF transformed into core-shell Cu 2 O@N doped carbon composites. During the calcination process, Cu-ligand bonds with weaker coordination would break and form metastable Cu NPs, while the Co-MOF part with higher stability would serve as a morphology retainer and a nano-channel template to reassemble Cu NPs into Cu nanowires.
Besides thermal transformation, solvent induced transformation is also a powerful strategy for the conversion of bimetallic MOFs to composites of MOFs and metal compounds. Luo and co-workers obtained Zn 1Àx Co x -ZIF@Zn 1Àx Co x O through the transformation of Zn 1Àx Co x -ZIF with AgNO 3 (Fig. 18B). 266 Zn 1Àx Co x -ZIF was dispersed in an aqueous solution of silver nitrate (AgNO 3 ) and stirred for 1 h to obtain Zn 1Àx Co x -ZIF@Zn 1Àx Co x O. AgNO 3 acted as the catalyst to break the coordinative bonds of MOFs and created a hydroxyl rich environment for the formation of Zn 1Àx Co x O on the surface of Zn 1Àx Co x -ZIF.
Selectively removing the core MOFs in single-or multicore-shell bimetallic MOFs can obtain hierarchical-pore or hollow MOF immobilized metal compound composites. In this design, less stable MOFs act as seeds to grow stable MOFs as the shell, followed by selective removal of the core MOFs to generate pores. The generated mesopores/macropores can facilitate the diffusion of substrates and the desorption of products.
For example, Li, Zhong and co-workers used MOF-5 as a template to grow UiO-66(Zr), followed by the removal of the cores to synthesize hierarchical UiO-66(Zr) (H-UiO-66). 267 MOF-5 is stable in certain solvents but sensitive towards moisture and acid, while UiO-66(Zr) is stable even in acidic solutions. Nanosized MOF-5 particles were rst synthesized, followed by the growth of UiO-66(Zr). The obtained material was washed with acid aqueous solution to get the targeted material H-UiO-66(Zr) with Zr species anchoring in the mesopore surface (Fig. 18C). In another system, Wu, Li, and co-workers reported a mild phase transformation of core-shell ZIF-67@ZIF-8 structures into hollow Zn/Co ZIF particles. 268 They treated core-shell ZIF-67@ZIF-8 with Co 2+ in methanol at 120 C for 4 h, resulting in the dissolution of the solid interior and then the formation of a hollow structure with interlaced nanoplates (Fig. 18D). The authors proposed that methanol molecules formed H-bonds in the presence of Co 2+ to break the coordination bonds between Co 2+ and 2-MeIm, driving the structural evolution of ZIF-67. Muhler, Fischer and co-workers reported similar results in parallel. 269 During the epitaxial growth of ZIF-8 on preformed ZIF-67 under solvothermal conditions, ZIF-67 partially dissolved to release cobalt ions to form cobalt hydroxide with a sheet-like structure.
Carbon composites, metal compounds and MOF composites can be derived from bimetallic MOFs through pyrolysis, hydrothermal treatment, and so on. This wide variety of MOF-derived materials can be applied in many applications, which will be introduced in the following section.

Applications of bimetallic MOF derivatives
The applications of the majority of bimetallic MOFs are restricted by their low stability, poor conductivity and blockage of active sites. Bimetallic MOF-derived materials show high stability, high conductivity, and exposed active sites. These properties provide bimetallic MOF derivatives with more opportunities for applications in catalysis under harsh conditions and electrochemical energy storage and conversion.  221 In the hydrogenation of benzonitrile to the corresponding imine, Co-Ni(3 : 1)@C-N exhibited remarkable activity with over 98% yield for the desired product aer 15 h, which was 5 times and 7 times as high as that of Ni@C-N and Co@C-N, respectively (Fig. 19A). Co-Ni@C-N with a Co-Ni molar ratio of 3 : 1 showed the highest activity; further decreasing or increasing the proportion of Co resulted in a decrease of activity.
Bimetallic MOF-derived porous catalysts have also been developed for a variety of oxidation reactions, such as the oxidation of alcohols, 271,276,277 acetone, olen and toluene, 278 and organic pollutant degradation. 279,280 As an example, Han, Yeung and co-workers prepared a binary metal oxide (CeCuO x ) derived from bimetallic MOFs (CeCuBDC) as the catalyst for the oxidation of organic compounds. 278 In the oxidation of toluene to CO 2 , CeCuO x exhibited high catalytic activity, with a T 50 (temperature required for 50% conversion) and T 90 of 150 and 186 C, respectively (Fig. 19B). CeCuO x showed lower activation energy (52.4 kJ mol À1 ) than CeO 2 (57.5 kJ mol À1 ) and CuO (70.5 kJ mol À1 ). The CeCuO x catalyst was also highly active for the oxidation of methanol, acetone and o-xylene, with a T 50 of 80, 87 and 142 C, respectively.
Controlled generation of hydrogen from chemical hydrogen storage materials is of vital importance to the hydrogeneconomy paradigm in the future. [281][282][283][284] Bimetallic MOF-derived nanomaterials exhibit high catalytic performance for the release of hydrogen from chemical hydrogen storage materials such as ammonia borane (NH 3 BH 3 , AB) and lithium borohydride (LiBH 4 ). [285][286][287][288] Chen and co-workers fabricated porous cobalt phosphide supported by carbon-based nanoframeworks (CoP@CNFs), which was derived from Zn/Co-ZIF and was applied for hydrogen generation from AB. 286 Zn/Co-ZIF was calcined in air to get Zn-Co-O@CNF, followed by phosphidation using Na 2 H 2 PO 2 and further acid treatment to get CoP@CNFs. CoP@CNFs gave an initial TOF value of 165.5 mol H 2 mol metal À1 min À1 at 298 K, maintaining 90% of its initial activity aer 4 cycles. Biomass has been an important resource for the production of chemicals, fuels, and energy. 289 Bimetallic MOF-derived nanocatalysts have been applied in effective biomass transformation. [290][291][292][293][294][295] Li, Luque and co-workers applied hollow Fe-Co supported on carbon (FeCo/C) derived from bimetallic Fe/ Co-MIL-45b for chemical conversions of biomass-derived platform chemicals. 290 In the aerobic oxidation of biomassderived 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF), FeCo/C(T) prepared by calcination at different temperatures (T ¼ 500, 600, 700 or 800 C) showed distinct activities. FeCo/C(500) exhibited the highest activity, with a complete conversion and >99% selectivity to DFF at 100 C aer 6 h (Fig. 19C). Jones and co-workers prepared Co/Cu-carbon (Co/ Cu-C) derived from Cu/Co MOF-74 for furfural hydrogenation. 292 The catalyst performance was investigated at 180 C and a weight/volume ow rate (W/F) of 3.63 g cat h mol À1 , and monometallic Co-C showed high conversion (50%) and high selectivity for 2-methylfuran (49%), while Cu-C had low conversion but high selectivity toward furfuryl alcohol. CoCu-C-400 (pyrolyzed at a temperature of 400 C) with a partial Cu shell showed reactivities characteristic of Co-C. CoCu-C-600 with a full Cu shell had reactivities similar to those of Cu-C. Although bimetallic CoCu catalysts did not show activity/selectivity improvements compared to those of monometallic catalysts for furfural conversion, these results have demonstrated synthesis-structure-property correlations, This journal is © The Royal Society of Chemistry 2020 Chem. Sci., 2020, 11, 5369-5403 | 5389

Perspective
Chemical Science which would guide the future development of better catalysts. The conversion of lignin (10-35% by weight in biomass) to value-added chemicals, alternative fuels, and platform compounds is attractive. Bao, Ren and co-workers reported the use of bimetallic-MOF-derived Ni-Co/C materials with varying Ni/Co ratios as catalysts for lignin conversion into monophenols. 294 In the hydrogenolysis of poplar lignin, Ni 0.5 Co 0.5 /C showed superior catalytic efficiency to Ni/C, with a yield of 55.2% to monophenols and a selectivity of 70.3% to guaiacol (Fig. 19D). 5.1.2 Photocatalysis. Bimetallic MOF-derived porous nanomaterials have also been developed for photocatalysis, including photocatalytic degradation of organic pollutants, water splitting and CO 2 reduction.
Photocatalytic degradation of organic pollutants from wastewater is of ecological and environmental importance. Nanomaterials derived from bimetallic MOFs have shown high activity and stability in the degradation of organic pollutants, such as organic dyes and antibiotics. 266,279,[296][297][298][299] Li and coworkers designed ZnO@C-N-Co core-shell nanocomposites derived from Zn/Co ZIF for the degradation of methyl orange (MO). 297 When calcining Zn/Co ZIF at 600 C, ZnO NPs generated from the ZIF-8 shell would aggregate and move to the hollow cavity while the internal Co NPs transferred inversely to the N-C shell, resulting in the unique ZnO@C-N-Co core-shell structure. Under the irradiation of a Xe lamp (300 W) for 2.5 h, ZnO@C-N-Co exhibited a high degradation percentage of MO (99.5%), much higher than that of ZIF-67-600 (29.4%) and ZIF-8-600 (41.9%). The superior performance of ZnO@C-N-Co could be attributed to the synergistic effect between the components, in which the porous carbon shell contributed to the stabilization of ZnO and the adsorption of reactants and Co NPs inhibited the recombination of electrons and holes (Fig. 20A).
Photocatalytic water splitting for hydrogen production is a promising route to convert solar energy into renewable hydrogen energy. 300,301 Bimetallic MOF-derived nanomaterials, such as Fe-Ni-P, 302 CuS/ZnS, 303 Co-doped Zn 1Àx Cd x S, 304 yolkshell CdS microcubes, 305 and NiS/Zn x Cd 1Àx S, 306 have been applied in photocatalytic water splitting for hydrogen production. As an example, Shi, Cheng and co-workers constructed non-noble metal co-catalyst/solid solution heterojunction NiS/ Zn x Cd 1Àx S derived from Ni-Zn x Cd 1Àx -MOF for the production of hydrogen (Fig. 20B). 306 Under visible-light irradiation (l > 420 nm), NiS/Zn 0.5 Cd 0.5 S exhibited a high HER rate of 16.78 mmol g À1 h À1 , much higher than those of NiS/ZnS (0.09 mmol g À1 h À1 ), NiS/ZnS (0.09 mmol g À1 h À1 ) and NiS/CdS (3.52 mmol g À1 h À1 ). The tuning of the Zn/Cd ratio in heterojunction Zn x Cd 1Àx S pointed to an optimized x of 0.5 to achieve a good balance between the light absorption capacity of the catalyst and edge of the conduction band. The co-catalyst NiS could further accelerate the water dissociation kinetics to improve the photocatalytic hydrogen evolution activity. Furthermore, constructing composites of co-catalysts and bimetallic MOF-derived photocatalysts, such as Cu 0.9 Co 2.1 S 4 @MoS 2 , 307 CdS/Zn x Co 3Àx O 4 , 308 Pt-ZnO-Co 3 O 4 , 262 Pt-ZnS-CoS, 262 Pt-Zn 3 P 2 -CoP, 262 and MoS 2 @-ZnCoS, 256 can greatly enhance the activity for photocatalytic hydrogen evolution.
Photocatalytic reduction of CO 2 is considered as an attractive approach to address fossil fuel shortage and carbon emission problems. Ye and co-workers prepared a ZnO@Co 3 O 4 composite derived from ZIF-8@ZIF-67 for CO 2 photoreduction to CH 4 and CO. 309 Under UV-vis irradiation, ZnO@Co 3 O 4 gave a CH 4 generation rate of 0.99 mmol g À1 h À1 , higher than those of ZnO derived from ZIF-8 and Co 3 O 4 derived from ZIF-67 (Fig. 20C). In ZnO@Co 3 O 4 , ZnO played a main role in the catalytic transformation, while Co 3 O 4 effectively protected ZnO from photocorrosion to enhance the photocatalytic stability. Deng, Peng and co-workers used carbonized cobalt composites (C-BMZIFs) derived from bimetallic Zn/Co-ZIF as co-catalysts for photocatalytic CO 2 reduction. 310 Under visible light irradiation with [Ru(bpy) 3 ] 2+ as the photosensitizer and triethanolamine (TEOA) as the electron donor, C-BMZIFs (Zn/Co ¼ 3/1) delivered a high CO yield of 1.1 Â 10 4 mmol g À1 h À1 . Interestingly, decreasing the Zn/Co ratio in the Zn/Co-ZIF precursors could increase the size of Co particles in C-BMZIFs, which could favor the evolution of H 2 against CO, resulting in the decrease of the CO/H 2 ratio in the produced syngas. Other nanomaterials derived from bimetallic MOFs, such as CuNi/C 311 and ZnO/ NiO, 312 have also been designed for photocatalytic reduction of CO 2 . 5.1.3 Electrocatalysis. Functional nanomaterials derived from bimetallic MOFs have been employed as electrocatalysts for efficient and stable electrocatalysis of the ORR, OER, and HER, with applications in proton exchange membrane fuel cells (PEMFCs), alkaline fuel cells (AFCs), metal-air batteries and overall water splitting. [313][314][315] Bimetallic MOF-derived nanomaterials have also been applied in electrochemical CO 2 /N 2 reduction.
The ORR is the key reaction on the cathode of fuel cells and metal-air batteries. Bimetallic MOF derivatives, 244 especially Fe and/or Co in N-doped carbon, 218,236,237,241,316 have been investigated for ORR electrocatalysis. 317 These supported metal catalysts, in the form of atomically dispersed metal sites, metal clusters, and metal NPs have demonstrated activities for the ORR. Deng and co-workers reported the controlled synthesis of NPs, atomic clusters (ACs), and SAs of Co catalysts on N-doped porous carbon derived from ZnCo-ZIFs for the ORR. 318 Three samples with Zn/Co molar ratios of 0 : 1, 2 : 1, and 8 : 1 were synthesized, denoted as Co-ZIF, ZnCo-ZIF-2, and ZnCo-ZIF-8, respectively. Upon pyrolysis at 1173 K in Ar and subsequent reux treatment in H 2 SO 4 solution, Co-ZIF, ZnCo-ZIF-2, and ZnCo-ZIF-8 transformed into Co-NPs@NC, Co-ACs@NC, and Co-SAs@NC nanocomposites, respectively (Fig. 21A). These catalysts showed size-dependent ORR activity in a 0.1 M KOH electrolyte. Co-SAs@NC exhibited a superior activity with a halfwave potential (E 1/2 ) of 0.82 V, outperforming those of Co- ACs@NC (0.81 V) and Co-NPs@NC (0.80 V). The determined electron transfer number of Co-SAs@NC is above 3.9, higher than those of Co-ACs@NC and Co-NPs@NC. These results demonstrated that isolated single Co atoms could provide maximum atom-utilization and be well stabilized by an Ndoped carbon support to show superior reactivity and stability. Besides Fe and Co, Mn-based catalysts are promising alternatives to address the insufficient durability of ORR catalysts. Wang, Wu and co-workers synthesized an atomically dispersed Mn-N-C catalyst for the ORR, obtained by calcination of MnZn-ZIF-8 followed by adsorption and thermal activation processes to increase the density of MnN 4 active sites (Fig. 21B). 239 The 20Mn-NC-second catalyst (where 20 represents the molar percentage of Mn against the total metals for the synthesis of MnZn-ZIF-8; second refers to the second adsorption step) exhibited the most positive E 1/2 of 0.8 V. Mn-NC-rst showed poor ORR activity due to insufficient density of active sites. The 20Mn-NC-second catalyst showed a H 2 O 2 yield of less than 2%, indicating a four-electron reduction pathway. The 20Mn-NCsecond catalyst as a cathode in membrane electrode assemblies for fuel cells showed a power density of up to 0.46 W cm À2 . Efficient OER electrocatalysis is important for various energyrelated processes, such as electrochemical water splitting and rechargeable metal-air batteries. Bimetallic MOF-derived metals/ alloys embedded in carbon, 319 metal compounds 248,261 and their carbon composites 233,320-322 have demonstrated OER activity. 323,324 Xu and co-workers applied FeCo-P/C nanocomposites derived from bimetallic MOFs for the OER. 232 Fe x Co y bimetallic MOFs with various Fe/Co ratios (x ¼ 1, 2 and y ¼ 1, 2) were employed as precursors to synthesize Fe x Co y -P/C. The overpotentials of Fe x -Co y -P/C in 1.0 M KOH at a current density of 10 mA cm À2 showed a trend of Fe 1 Co 1 -P/C (360 mV) < Fe 1 Co 2 -P/C (362 mV) < Fe 2 Co 1 -P/C (368 mV). All the Fe x Co y -P/C materials exhibited a better OER activity than their Fe x Co y /C without P-doping. The authors demonstrated that both P-and Fe-doping could reduce the resistance of charge transfer to accelerate the electron transfer and thus improve the OER activity. Considering that the complete decomposition of MOFs usually leads to a dramatic decrease of the surface area and pore structure, partial decomposition of bimetallic MOFs can preserve the porous structure to a certain extent for effective substrate diffusion while producing active metal NPs. Li and co-workers reported the synthesis of NiCo/Fe 3 O 4 /MOF-74 through controlled pyrolysis of trimetallic NiCoFe-MOF-74 for efficient OER electrocatalysis (Fig. 21C). 265 The obtained NiCo/Fe 3 O 4 /MOF-74 retained 68% of the specic surface area of the pristine NiCoFe-MOF-74 (820 m 2 g À1 ). In Efficient and inexpensive HER electrocatalysis to produce hydrogen from water splitting is of signicant importance. Bimetallic MOF-derived materials, especially Mo-based materials 325 and metal nitrides/phosphides/sulphides, 253,254,258,259,326 have been intensively investigated due to their favourable hydrogen adsorption energy toward the HER. 327 Lou and coworkers synthesized carbon-supported Ni-doped FeP nanocrystals from Ni-doped MIL-88A as HER electrocatalysts. 228 Nidoped MIL-88A was treated with phytic acid and subsequent pyrolysis under Ar and H 2 to obtain Ni-doped FeP/C. The overpotential at 10 mA cm À2 of NFP/C-3 (Ni/Fe atomic ratio of 3) was 72, 117, and 95 mV in 0.5 M H 2 SO 4 (pH ¼ 0), 1.0 M phosphatebuffered saline (pH z 7) and 1.0 M KOH (pH ¼ 14), respectively (Fig. 21D). The NFP/C-3 electrocatalyst also exhibited good stability over the full pH range. The carbon support is important to structural integrity and catalytic performance. NFP-3 without carbon showed degradation of electrocatalytic activity and stability. XPS analysis and DFT calculations demonstrated that Ni doping could weaken the strong hydrogen adsorption on pure FeP to favour the HER.
Highly active and robust bimetallic MOF-derived multifunctional electrocatalysts for the ORR, OER and/or HER can be applied for efficient rechargeable metal-air batteries 185,219,224,315 and overall water splitting. 230,233,258,[328][329][330] Very recently, Xu and co-workers synthesized a capsular carbon embedded with ironnickel phosphide NPs (FeNiP/NCH) through pyrolysis of capsular-MOF followed by phosphidation with melamine. 94 FeNiP/NCH was employed as a trifunctional electrocatalyst for the ORR/OER/HER. In 1.0 M KOH solution, FeNiP/NCH obtained a current density of 10 mA cm À2 at overpotentials of 250 and 216 mV for the OER and HER, respectively. For ORR electrocatalysis in 0.1 M KOH solution, FeNiP/NCH exhibited an E 1/2 of 0.75 V. When FeNiP/NCH was applied as both cathode and anode electrocatalysts for overall water splitting, a small cell voltage of 1.59 V was achieved at 10 mA cm À2 (Fig. 22). When FeNiP/NCH was applied as the air electrode electrocatalyst in a Zn-air battery, a high power density (250 mW cm À2 ) could be achieved. In a galvanostatic discharge-charge cycling test at 5 mA cm À2 , the charge/discharge potentials were 1.89 and 1.23 V with a small voltage gap of 0.66 V. Moreover, no obvious voltage gap change was observed for 500 h, indicating the high stability of FeNiP/NCH. Electrochemical CO 2 reduction is a potentially effective approach to produce synthetic fuels utilizing waste CO 2 . 331 Bimetallic MOF-derived nanomaterials demonstrated impressive FE for CO 2 reduction. 238,240,332,333 Pan, Liu, Jiang and coworkers employed single-atom Ni implanted N doped carbon catalysts (Ni SA -N x -C) with a controlled N coordination number (x ¼ 2, 3, 4) derived from a MgNi-MOF-74 conned polypyrrole (PPy) composite for the electrochemical reduction of CO 2 to CO. 240 The CO FE of Ni SA -N x -C followed a trend of Ni SA -N 2 -C > Ni SA -N 3 -C > Ni SA -N 4 -C. The TOF values for CO production of Ni SA -N 2 -C, Ni SA -N 3 -C and Ni SA -N 4 -C are 1622, 1120, and 225 h À1 at À0.8 V, respectively. Ni SA -N 2 -C possessed the highest CO 2 reduction activity among the Ni SA -N x -C catalysts, attributed to the favorable formation of the COOH* intermediate on low coordinated Ni-N 2 sites. Sun, Han and co-workers applied MOF-derived In-Cu bimetallic oxide catalysts (InCuO-x, where x is the Cu/In molar ratio) for CO 2 electroreduction. 332 InCuO-0.92 showed a highest FE of 92.1% at a potential of À0.8 V, which was 1.1, 1.3, 1.7, and 3.0 times higher than that of InCuO-0.72, InCuO-0.55, InCuO-0.37, and InCuO-0.15, respectively. The increase of x led to enhanced CO 2 reduction activity, indicating the synergistic effect between In oxides and Cu oxides to lead to stronger CO 2 adsorption, a higher electrochemical surface area and lower charge transfer resistance.
Electrochemical N 2 reduction (NNR) under ambient conditions is a promising sustainable route for ammonia synthesis. 334 The related study on bimetallic MOF derivatives for NNR is still in its infancy. Recently, Qin, Cho and co-workers synthesized MoFe embedded in phosphorus-doped carbon microspheres (MoFe-PC) derived from bimetallic MoFe-MOFs for electrocatalytic NNR. 335 193,255 ) and their carbon composites, 342-344 as well as metal@carbon composites [345][346][347] have been employed as electrode materials for supercapacitors.
Superior to monometallic compounds, bimetallic compounds can enable richer redox reactions and higher electrical conductivity owing to charge transfer between different ions. Bimetallic compounds with tuneable compositions reveal facile and reversible faradaic behaviors for excellent electrochemical capacitors. Bimetal oxides derived from bimetallic MOFs have high specic capacities but may suffer from unsatisfactory conductivity. Converting bimetallic MOFs into phosphides and sulphides can improve the electrical conductivity. For example, Xia and co-workers applied cobalt/nickel boride/ sulphide (Co-Ni-B-S) derived from bimetallic Co-Ni MOFs for supercapacitors. 255 A Co-Ni MOF was rst treated with sodium borohydride to lower the valence state of Co/Ni species (Co-Ni-B), followed by a sulfurization process to yield Co-Ni-B-S. When tested for supercapacitors, the Co-Ni-B-S electrode exhibited a high specic capacitance of 1281 F g À1 at 1 A g À1 , with a high retention of 92.1% aer 10 000 cycles (Fig. 23A). The energy density of the Co-Ni-B-S electrode could reach as high as 50.0 Wh kg À1 at a power density of 857.7 W kg À1 , with a capacity retention of 87.7% at a 12 A g À1 aer 5000 cycles.
Metal compounds with complex structures can induce abundant hetero-interfaces and tune the electronic structure to facilitate ion/electron transfer and improve the specic capacity. 245,252,348 Hu, Lou and co-workers fabricated CoO/Co-Cu-S hierarchical tubular heterostructures (HTHSs) for hybrid supercapacitors. 348 CoO/Co-Cu-S was obtained by calcination of polyacrylonitrile (PAN)@MOF-74(Co/Cu) in air followed by annealing with S powder in a N 2 atmosphere. CoO/Co-Cu-S-2 (2 indicates the molar ratio of Co 2+ /Cu 2+ in the precursors) delivered a specic capacity of 320 mA h g À1 at a current density of 2.0 A g À1 , higher than that of CoO/Co-Cu-S-1 (192 mA h g À1 ), CoO/Co-Cu-S-0.5 (167 mA h g À1 ), CoO/Co-Cu-S-2 nanoneedles without a tubular structure (135 mA h g À1 ), monometallic CoO/CoS x nanobers (110 mA h g À1 ) and Cu 1.81 S nanobers (97 mA h g À1 ). A hybrid supercapacitor constructed with CoO/Co-Cu-S-2 and activated carbon electrodes showed a high and stable energy density of 90.7 Wh kg À1 at a power density of 800 W kg À1 .
Constructing metal compound and carbon composites can improve the electrical conductivity, accommodate the volumetric change, and prevent the aggregation and dissolution of the metal compounds, thereby improving the rate capability and cycling stability. Cai, Zhang and co-workers synthesized bimetallic nickel cobalt sulphide embedded nitrogen-doped carbon composites (Ni-Co-S-n/NC, n represents the Ni/Co molar ratio) with a hollow spherical structure for supercapacitors. 344 The Ni-Co-S-0.5/NC composite showed high specic capacities of 543.9 and 366.0C g À1 at 1 and 20 A g À1 , respectively. An asymmetric supercapacitor based on the hollow Ni-Co-S-0.5/NC composite as the cathode exhibited good electrochemical performance with a high energy density of 39.6 W h kg À1 at a power density of 808 W kg À1 . 5.2.2 Batteries. Materials derived from bimetallic MOFs have been investigated in a variety of batteries, including LIBs, 349 sodium-ion batteries (SIBs), 350 LOBs, 351 etc.
For LIBs, bimetallic oxides with complex chemical compositions have been designed to provide enhanced electrical conductivity and rich redox-active sites. 243,[352][353][354] Sun, Wang and co-workers used Ni x Co 3Àx O 4 nanorods derived from Co/Ni-MOF-74 for lithium storage. 243 Co/Ni-MOF-74 A (Co : Ni ¼ 9 : 1) and Co/Ni-MOF-74 B (Co : Ni ¼ 2 : 1) afforded Ni 0.3 Co 2.7 O 4 and NiCo 2 O 4 aer calcination at 450 C in air, respectively. Ni 0.3 Co 2.7 O 4 nanorods exhibited a large reversible Li-storage capacity of 1410 mA h g À1 aer 200 cycles at 100 mA g À1 , which was higher than that of NiCo 2 O 4 (1157 mA h g À1 ) and Co 3 O 4 (625 mA h g À1 ). Ni 0.3 Co 2.7 O 4 also showed large reversible capacities of 812 and 656 mA h g À1 aer 500 cycles at large current densities of 2 and 5 A g À1 , respectively (Fig. 23B). The excellent performances of Ni 0.3 Co 2.7 O 4 could be attributed to the mesoporous nanorod structure and the synergistic effect of two active metal oxides. Hybrid metal oxides with complex structures, such as ZnO/ZnFe 2 O 4 sub-microcubes, 355 Fe 2 O 3 @-NiCo 2 O 4 nanocages, 246 and CuO@NiO hollow spheres, 247 have been designed to accommodate large volume variation and shorten the lithium-ion diffusion length.
Embedding bimetal oxides in porous carbon can accommodate volume change and enhance electron/ion transport to greatly enhance the lithium storage performance. In this regard, bimetallic MOF-derived bimetallic oxide/carbon composites, such as CuCo 2 O 4 /C, 356 ZnFe 2 O 4 /C@N-doped carbon nanotubes, 357 and ZnO/ZnFe 2 O 4 /C, 358 have been designed as anode materials for LIBs. In addition, other bimetal alloys and compounds (sulphides and phosphides) supported on carbon composites 234,235,344,345,359,360 derived from bimetallic MOFs were also reported as anode materials for LIBs.
MOF-derived bimetallic compounds and their carbon composites can also be applied as anodes for SIBs. Bimetallic MOF-derived metal compounds, such as CoFe 2 O 4 (ref. 361) and Co 3 O 4 /ZnO, 362 could deliver high capacities but may suffer from poor rate capability and cycling stability due to a low electrical conductivity and large volume variation during charge/ discharge processes. Bimetallic MOF-derived metal compound/ carbon composites such as (Co 0.5 Ni 0.5 ) 9 S 8 /N-C 231 and Ti-doped-CoO@C, 363 have been demonstrated as high-performance anode materials for SIBs. The porous carbon components can facilitate electron/ion transport and improve the structural integrity, and the metal compound components can deliver high capacities for sodium storage.
Bimetallic MOF-derived materials have also been applied as cathode materials for LOBs. Bimetallic MOF-derived metal compounds and their carbon composites, such as Co-Mn-O nanocubes, 352 Co 3 O 4 @graphitic porous carbon, 364 and ZnO/ ZnFe 2 O 4 /C nanocages, 365 have showed high performance in LOBs. In metal compound/carbon composites, the carbon supports with a large pore volume are benecial for mass transportation and accommodation of discharge products (e.g., LiO 2 ), and the well distributed catalysts accelerate oxygenrelated redox reactions.

Conclusions and prospects
Bimetallic MOFs increase the complexity of MOF materials. Bimetallic MOFs can be divided into two categories according to the metal distribution, namely, solid solution and core-shell bimetallic MOFs. In solid solution bimetallic MOFs, two different metal ions have delocalized or even homogeneous distributions throughout the whole MOF crystals. Direct synthesis, post-synthetic modications, and template synthesis have been applied for the preparation of solid solution bimetallic MOFs. A variety of techniques should be combined to well characterize the composition, location, and arrangement of metals in the synthesized bimetallic MOFs. Many studies lack detailed characterization to identify the metal arrangements within the frameworks, either mixed in the same SBUs or separated in different SBUs. The identication of the metal arrangements in bimetallic MOFs is of vital importance to correlate the metal mixing patterns with properties.
Two MOFs with different metal centres can be assembled into core-shell bimetallic MOFs. Seed-induced growth, postsynthetic exchange, and one-pot synthesis have been developed for the synthesis of core-shell bimetallic MOFs. The assembly of core-shell structures usually leads to distorted interfaces with defects and modied crystal structures and thus induces new mechanical, electronic, and catalytic properties. In future work, detailed characterization of the interfaces is expected, which will improve the fundamental understanding of the assembly process and establish structure-property relationships.
Bimetallic MOFs with complex compositions and structures frequently show superior properties to their monometallic counterparts. Metal substitution in the SBUs can tune the stability, exibility, pore structure, and electronic structure of bimetallic MOFs. The possibility to tune the physical and chemical properties provides bimetallic MOFs with great promise in many applications, including gas adsorption, catalysis, energy storage and conversion, and luminescence sensing. Moreover, bimetallic MOFs with well-dened crystal structures are suitable for atomically precise structural characterization and computational modelling to achieve fundamental understanding of the structure-performance relationship. Despite these advantages, the industrial applications of bimetallic MOFs are restricted due to their low stability, microporous structure, blockage of active sites, and poor conductivity.
Especially, the stability issue of bimetallic MOFs should be addressed if they are to be applied in industrial applications. Much effort has been devoted to improving the stability of MOFs, including direct synthesis of high-valent metalcarboxylate or low-valent metal-azolate MOFs and postsynthetic modication of organic linkers or coating of a protective layer. General and facile strategies are to be developed to be applicable to different bimetallic MOF systems. It is necessary to systematically investigate the chemical, thermal, and mechanical stability of bimetallic MOFs to guide the synthesis of very stable bimetallic MOFs for targeted applications.
Moreover, bimetallic MOFs can serve as excellent precursors/ templates for the synthesis of a variety of nanostructured materials, including carbon composites, metal compounds, and MOF composites. The achieved success in controlling the compositions and distributions of metal ions in bimetallic MOFs can allow the tuning of compositions and structures of the obtained metal atoms/NPs/compounds. The homogeneous doping of heteroatoms (N, P, S, etc.) in the organic linkers can be inherited to the derived carbon materials to stabilize the metal species and facilitate the activity. The different coordination abilities of metal ions with the organic linkers in bimetallic MOFs offer the possibility to selectively transform part of the framework to metal NPs/compounds while preserving the rest of the framework to retain the porosity to a certain extent.
Bimetallic MOF derivatives show exposed active sites, good stability and conductivity, enabling them to extend their applications in the catalysis of more challenging reactions under harsh conditions and electrochemical energy storage and conversion. Despite the achieved success in the transformation of bimetallic MOFs to functional materials, several common drawbacks persist during the transformation process: (1) the poor control over the pore structure of bimetallic MOF-derived materials and (2) the inevitable aggregation of metal NPs/ compounds. To address these issues, template-assisted synthesis and controlled transformation to "quasi-MOFs" are currently emerging to boost the development of bimetallic MOF-derived nanomaterials. Template-assisted synthesis utilizing SiO 2 , etc., can provide robust skeletons to prevent the architecture from collapsing and serious aggregation of metal NPs/compounds during calcination. Besides, controlled transformation into quasi-MOFs can not only allow the exposure of active sites, but also preserve the porous structures to a certain extent. The unique features of quasi-MOFs remain to be explored, which will pave the way to the development of the next-generation of functional materials.
Although many challenges still exist, the rapid development of bimetallic MOFs and their derivatives in recent years will lead to a promising future. Continued research and development in this exciting area can be expected to enable the practical applications of bimetallic MOF-based materials.

Conflicts of interest
There are no conicts to declare.