Flexible side arms of ditopic linker as effective tools to boost proton conductivity of Ni8-pyrazolate metal-organic framework

Jieying Hu,Hu Zhang,Zihao Feng,Qian-Ru Luo,Can-Min Wu,Yuan-Hui Zhong,Jian-Rong Li,Lai-Hon Chung,Wei-Ming Liao,Jun He

School of Chemical Engineering and Light Industry,Guangdong University of Technology,Guangzhou 510006,China

Keywords:Proton conductivity Metal-organic framework Pyrazolate linkers Nickel-oxide cluster Imidazole encapsulation

ABSTRACT Two primitive metal-organic frameworks (MOFs),NiL1 and NiL2,based on Ni8O6-cluster and ditopic pyrazolate linkers,L1 (with rigid alkyne arms) and L2 (with flexible alkyne chains),were prepared.The proton conductivities of these MOFs in pristine form and imidazole-encapsulated forms,Im@NiL1 and Im@NiL2,were measured and compared.Upon introduction of imidazole molecules,the proton conductivity could be increased by 3 to 5 orders of magnitude and reached as high as 1.72×10−2 S/cm (at 98% RH and 80°C).Also,whether imidazole molecules were introduced or not,Ni8O6-based MOFs with L2 in general gave better proton conductivity than those with L1 signifying that flexible side arms indeed assist proton conduction probably via establishment of efficient proton-conducting channels along with formation of highly ordered domains of water/imidazole molecules within the network cavities.Beyond the active Ni8O6-cluster,tuning flexibility of linker pendants serves as an alternative approach to regulate/modulate the proton conductivity of MOFs.

Metal-organic frameworks (MOFs),constructed by metal ions and organic linkers,represent a fast-growing material because of their structural merits such as high crystallinity,tunable porosity,rich guest-host interaction and designable functionality [1–6].Thanks to the unrelenting effort put on MOFs,it was found that hard-soft-acid-base (HSAB) theory applies well to be a guideline for MOF construction [7,8].Specifically,hard donors match hard nodes well (e.g.,common UiO-network [9,10]from hard Zr(IV) ions and carboxylate-based linkers) while soft donors fit soft nodes (e.g.,renowned Zeolitic Imidazolate framework,ZIF [11,12],built from soft imidazolate linkers and Zn2+nodes;M-pyrazolate MOFs [13–16]constructed from relatively soft metal ions like Cu+,Ni2+,Zn2+,etc.and soft pyrazolate ligands).Even though MOFs of UiO-series are research hotspots because of their well-defined structure and ease of post-modification,this class of MOFs still suffers from instability sometimes for the vulnerability of Zr-O cluster towards the complex surrounding environment [8,17].On the other hand,MOFs established from soft precursors feature structural parameters (e.g.,porosity,type of cluster nodes,acidity of the nodes) different from those UiO-frameworks and sometimes express higher stability than UiO-analogues in a wide range of conditions [18,19].These classes of MOFs have been being extensively investigated in the field of porous coordination polymers and their applications.

Among various applications,developing MOFs as proton conductors recently become a research hotspot because of the following reasons: (1) High porosity of MOFs allows introduction of proton-conducting guest ions/molecules to enhance proton conductivity;(2) Rich tunability of linkers confers framework potentials for modulation of stability and functionality.To utilize the unique porosity of MOFs,loading of guest molecules/ions represents one of the most effective strategies to enhance the proton conductivity of the framework materials [20–29].Recently,it has been demonstrated that introduction of imidazole-type molecules into framework successfully improves the proton conductivity of the MOFs by as high as 10−1∼10−3S/cm in the range of 30∼100°C and relative humidity (RH) of 98%∼100% [30–35].Imidazole molecules,encapsulated in voids,can serve as proton carriers themselves or pack orderly to form proton-conducting channels just like the most primitive proton conductor–water molecule does.Hence,imidazole and water molecules carry out proton conduction through the same mechanistic options [36].Interestingly,there arises a new improvement approach in which hydrophobicity of framework assists proton conduction by minimizing host-guest interaction and promotes regular packing of guest molecules into ordered proton-conducting channels for better proton conductivity[37,38].The combination of guest molecules and hydrophobicity of framework may turn to be a novel strategy for rational design of proton conducting MOFs.

When it comes to M-pyrazolate MOFs,those based on facecentered cubic (fcc) [Ni8(OH)4(H2O)2]12+(Ni8O6) cluster and ditopic pyrazolate ligands represent a class of chemically stable materials [8,18,19].Gratifyingly,the Ni8O6cluster in this class of stable MOFs has been found active in both proton and hydroxide conductivity [39,40].However,these works only focused on the function of Ni8O6cluster and omitted the possible contribution from the design of the ditopic pyrazolate linkers.Our group previously reported a Ni8O6-based MOF,NiL1 (Fig.1A for the ligand H2L1;Fig.1C for the structure of NiL1),established from ditopic pyrazolate linkers with specially designed side arms.Considering L1,pyrazolate on both terminals of course serve to form coordination bonds with the clusters while the alkyne side chains were set to control the porosity of the network.As our plan goes,NiL1 exhibited excellent tolerance in a range of conditions (e.g.,solution of extreme pH values and exposure to high temperature) [41].Combining our previous experimental findings and going beyond Ni8O6-cluster,we aim to develop Ni8O6-based MOF as active proton conductor with rational design of the alkyne pendants on ditopic pyrazolyl linker.

Herein,we propose to use L2 (Fig.1A),structurally analogous to L1,as linker to construct Ni8O6-based MOF,NiL2,and compare with NiL1 for proton conductivity of their primitive forms and imidazole-encapsulated forms,Im@NiL1 and Im@NiL2.Looking at L2,the main skeleton is identical to L1,but the pendants are set to bear flexible alkyl chains instead of rigid aromatic side arms (Fig.1A).The flexible alkyl chains within NiL2 network are expected to act as dynamic regulators in the voids and thus facilitate arrangement of imidazole and water molecules into well-organized channels beneficial to proton conduction.This effect is thought to be absent in the network of NiL1 because rigid pendants are fixed in space and fail to complement upcoming imidazole molecules for building efficient proton-conducting channels.This work aims to verify how flexible hydrophobic pendants help proton conduction of MOF when compared with rigid hydrophobic pendants.

Following a similar synthetic strategy of NiL1 [41],the dark yellow crystalline powder of NiL2 was obtainedviasolvothermal reaction between Boc2L2 molecules and Ni(OAc)2·4H2O usingN,N-dimethylacetamide (DMA) and H2O as solvent.Powder X-ray diffraction (PXRD) reveals that NiL2 has a cubic lattice(a=b=c=32.3556 ˚A,Fig.2,details of refinement are included in Supporting information).With reference to previously reported Ni8O6-cluster-based MOFs [18,19,40,42,43],a crystal structure of NiL2 is modelled based on [Ni8(OH)4(H2O)2]node (Fig.1B) and the final network (Fig.1D) is found isoreticular to reported Ni(II)-MOFs having the fcu topology with fcc array of the Ni8O6clusters [44–46].

Elemental analysis of an activated sample of NiL2 suggested the chemical formula to be Ni8(OH)4(H2O)2(L2)3.45(CH3COO)5.1(H2O)16.The high H2O content may originate from atmospheric moisture and the CH3COO−probably come from decomposition of DMA at high temperature.The origin of CH3COO−inside the framework is verified by NMR experiments (Fig.S9 in Supporting information) and these suggest that CH3COO−are coordinating ligands rather than free ions present in channels.In a perfect fcu net,Ni8O6cluster in combination with L2 normally gives a formula of Ni8(OH)4(H2O)2(L2)6in which the Ni8O6cluster is connected to 12 pyrazolate units (provided by 6 ditopic L2).However,the current NiL2 only bears 7 pyrazolate units from 3.5 L2 and the remaining sites are occupied by 5 CH3COO−ligands as reflected by the determined formula.Mixed carboxyl/pyrazolate ligands on Ni8O6cluster have been reported [45]and the linker deficiency in NiL2 most likely results from steric hindrance exerted by the long alkyne side arms which limit the number of linkers approaching the Ni8O6-cluster.It is noted that lower linker deficiency in NiL2 (42.5%)when compared with NiL1 (50%) is likely attributed to less steric bulkiness in L2 than L1.Noteworthy,both NiL1 and NiL2 are stable in a wide range of condition as reflected by their PXRD patterns after being immersed in boiling water,extremely acidic/alkaline solution,and even thermal treatment at 320 °C for 2 h (no significant peaks broadening of PXRD patterns,Fig.S10 in Supporting information).The extraordinary stability of NiL1 and NiL2 highlight their potentials as carriers to house imidazole.

Imidazole molecules were introduced into NiL1 and NiL2 through thermally assisted vaporization of imidazole molecules under reduced pressure to give Im@NiL1 and Im@NiL2 respectively.The similar PXRD patterns of NiL1/NiL2 and Im@NiL1/Im@NiL2 confirm framework integrity of NiL1/NiL2 after introduction of imidazole molecules (Fig.2).From NiL1/NiL2 to Im@NiL1/Im@NiL2,the rise of IR stretching signals at 1068/1057,1324/1327 and 3133/3140 cm−1is indicative of successful introduction of imidazole molecules into NiL1/NiL2 because these three peaks correspond to C−N,C−N−C and N−H stretching of imidazole (Figs.S12 and S14 in Supporting information) [32,35].Importantly,N2adsorption-desorption isotherms at 77 K reveal that both NiL2 and Im@NiL2 display type-I sorption isotherm.The Brunauer-Emmett-Teller (BET) surface area and the pore volume of NiL2 are 1364.6 m2/g and 0.771 cm3/g respectively while those of Im@NiL2 are 49.54 m2/g and 0.217 cm3/g,respectively (Figs.S16-S19 in Supporting information).The decrease in surface area and pore volume from NiL2 to Im@NiL2 supports the encapsulation of imidazole molecules in NiL2.Besides,scanning electron microscope(SEM) images illustrate that the morphology of Im@NiL2 is like that of pristine NiL2 without obvious change of the surface (Fig.S20 in Supporting information),suggesting that there exist no aggregates of imidazole molecules on the surface.The above experimental evidence consolidates the successful encapsulation of imidazole guests in NiL1/NiL2;yet the possibility of trace imidazole molecules left on the surface of framework should not be excluded.

Even though it has been proved that the imidazole molecules are located inside the voids of network,it is necessary to resolve the quantity of imidazole molecules introduced.Thermogravimetric analysis (TGA) of Im@NiL1/Im@NiL2 (Figs.S21 and S22 in Supporting information) showed that there areca.16.4 imidazole molecules per unit of NiL1 andca.16.8 imidazole molecules per unit of NiL2 respectively.Both NiL1 and NiL2 could host similar quantities of imidazole molecules demonstrating comparable guest molecules housing capacity.Water affinity of all four samples were compared by their water contact angles and it was found that Im@NiL1/Im@NiL2 have smaller contact angles than NiL1/NiL2(Fig.S23 in Supporting information) reflecting higher hydrophilicity of Im@NiL1/Im@NiL2 than NiL1/NiL2.Given that the imidazole molecules have been introduced into the cavities inside NiL1/NiL2 successfully,Im@NiL1/Im@NiL2 serves as a good candidate for proton conductivity study.

To investigate how encapsulation of imidazole molecules enhance the proton conductivities of NiL1/NiL2,AC impedance spectroscopy has been conducted on freshly prepared pellets of NiL1/NiL2 and Im@NiL1/Im@NiL2 under different RH (in the range of 30% to 90%) at 90 °C and at different temperatures (in the range of 30 °C to 80 °C) under 98% RH.Nyquist plots of all four samples under different RH (from 30% to 90%) at 90 °C show an increase in proton conductivity (σ) (Fig.S36 in Supporting information).From RH of 30% to 90% at 90 °C,σof NiL1 and NiL2 increase by 2 to 3 orders of magnitude until 10−8S/cm while those of Im@NiL1 and Im@NiL2 increase by 3 to 5 orders of magnitude until 10−5S/cm and 10−3S/cm,respectively (under 90% RH at 90°C,1.17×10−8S/cm for NiL1,8.82×10−8S/cm for NiL2,7.42×10−5S/cm for Im@NiL1 and 1.00×10−3S/cm for Im@NiL2,Table S1 in Supporting information).Enhancement ofσalong with increasing RH is reasonable as water molecules are packed more closely under higher RH and this provides more channels for proton conduction.Noteworthy,when the RH is fixed at 98%,the Nyquist plots of all samples at different temperature (from 30 °C to 80 °C) show a similar trend inσ(Fig.S37 in Supporting information).From 30 °C to 80 °C under RH of 98%,σof NiL1 and NiL2 also increase by 2 to 3 orders of magnitude but reach maximalσof 10−5S/cm while those of Im@NiL1 and Im@NiL2 (Fig.3) increase by 1 to 3 orders of magnitude until 10−3S/cm and 10−2S/cm (under 98% RH at 80 °C,2.46×10−8S/cm for NiL1,1.41×10−5S/cm for NiL2,1.86×10−3S/cm for Im@NiL1 and 1.72×10−2S/cm for Im@NiL2,Table S2 in Supporting information).At fixed 98% RH,water molecules are sufficiently available for proton conduction and elevating temperature indeed facilitate the proton conduction because protons with higher kinetic energy inside the channels are more active.Theσof the imidazole-encapsulated MOFs reported in this work lie in the sample range as expressed by other imidazole-housing MOFs(Table S3 in Supporting information).To our knowledge,our results represent the best example to date considering the boost ofσby introduction of imidazole molecules into Ni-based MOF[47].Also,it is noted that generally higherσof Im@NiL1/Im@NiL2 than NiL1/NiL2 regardless of RH and temperature highlight the supremacy of imidazole molecules in framework as promoter for proton conductivity.

To rationalize the proton conduction mechanisms in these materials,the Arrhenius plots in a linear relationship [ln(σ T)vs.1000T−1]are plotted (Figs.S26,S29,S32 and S34 in Supporting information).Noteworthy,the activation energy (Ea) of NiL1,NiL2,Im@NiL1 and Im@NiL2 under 98% RH are found to be 0.82,1.20,1.29 and 0.30 eV respectively.It is well-established thatEaof 0.1∼0.4 eV corresponds to Grotthuss mechanism where protons are conducted along water channels in a way of proton-jump/hopping whereasEaof>0.4 eV follows the Vehicle mechanism in which protic molecules serve as proton carriers and diffuse for transporting protons [48,49].NiL1,NiL2 and Im@NiL1 follow Vehicle mechanism while Im@NiL2 adopts Grotthuss mechanism for proton conduction.Presumably,flexible alkyl chains in L2 assisted organization of imidazole along with water molecules and gave rise to well-organized imidazole-water channels for efficient proton hopping while rigid pendants in L1 failed to coordinate the ordered arrangement of water and imidazole molecules for efficient proton jump (Fig.4).It is worth-mentioning that L2 itself without introduction of imidazole molecules bearing hydrophobic pendants minimally interact with water molecules effectively promoting formation of ordered domains of water molecules as proton-hopping channels.These are possible rationales to explain the proton conducting mechanisms adopted by all four samples reported in this work.These results undoubtedly stand out the function of flexible pendants on linker for coordinating guest introduction and hence facilitating proton conduction within network.

In summary,Ni8O6-pyrazolate MOFs,NiL1 and NiL2,were prepared and investigated the proton conductivity of their pristine forms and imidazole-encapsulated forms,Im@NiL1 and Im@NiL2.It was found that introduction of imidazole molecules into network of NiL1 and NiL2 effectively boost the proton conductivity for mostly 3 to 5 orders of magnitude to at most 1.72×10−2S/cm(at 98% RH and 80 °C).Reflected by the activation energies,NiL1,NiL2 and Im@NiL1 were rationalized to opt for Vehicle mechanism while Im@NiL2 probably adopted Grotthuss mechanism as major ways for proton conduction.The difference in the protonconducting mechanism probably arises from coordinating packing of imidazole and water molecules by flexible alkyl side arms on L2 during introduction of guest imidazole.This work highlights the success of our strategy on modulating proton conductivity of network through side arms engineering and represents a vivid example of adjusting proton conductivity of MOFs by regulating the microenvironment of voids in MOF.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No.21871061),the Foundation of Basic and Applied Basic Research of Guangdong Province(No.2021A1515010274),the Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (No.2017BT01Z032),the Science and Technology Planning Project of Guangdong Province (No.2021A0505030066),and the Science and Technology Program of Guangzhou (No.201807010026).

Supplementary materials

Supplementary material associated with this article can be found,in the online version,at doi:10.1016/j.cclet.2021.10.042.