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Abstract： In this work， an amino-modified cellulose nanofiber sponge was prepared and used as a support for polyoxometalate （POM） catalysts with a high loading efficiency. Fourier transform infrared spectroscopy， thermogravimetric analysis， and energy-dispersive X-ray spectroscopy revealed that an Anderson-type POM， （NH4）4[CuMo6O18（OH）6]·5H2O was successfully immobilized on the sponge based on electrostatic interactions. Morphological analysis indicated that the POM-loaded sponge retained its porous structure and that the POM was homogeneously distributed on the sponge walls. The POM-loaded sponge exhibited excellent mechanical properties by recovering 79.9% of its original thickness following a 60% compression strain. The POM-loaded sponge was found to effectively catalyze the hydroboration of phenylacetylenes， yielding excellent conversion and regioselectivity of up to 96% and 99%， respectively. Its catalytic activity remained unchanged after five reuse cycles. These findings represent a scalable strategy for immobilizing POMs on porous supports.
Keywords： cellulose nanofiber; polyoxometalates; catalyst immobilization; hydroboration; heterogeneous catalyst
1 Introduction
Based on their unique redox properties， polyoxometalates （POMs） have attracted significant attention in various fields， including oil purification， environmental protection， energy transformation， and catalysis[1-6]. The immobilization of POMs on a proper support enables their convenient recovery and reuse， making them more environmentally friendly and sustainable[7].
POMs can be immobilized on supports through either covalent or non-covalent attachment[8-12]. Covalent bonding is ideal because it provides stronger adhesion[6]. However， it typically requires a complex preparation process[13-14]. In the more feasible non-covalent binding process， POMs are anchored on support materials， such as mesoporous carbon， silica， and cellulose， which are pre-modified with amino functionalities[15-19]. However， these non-covalently POM-loaded materials typically suffer from low support capacity and high leaching of catalysts during separation.
As the most abundant natural macromolecular compound in nature， cellulose has been widely applied in environmental engineering， catalyst immobilization， tissue engineering， and other fields[4， 20-25]. POM-loaded cellulose materials have been prepared[1，19]， however， these materials still face the problems of issues related to low specific surface area and low POM loading capacity. In this paper， we present a polyethylenimine （PEI）-modified nanocellulose sponge for anchoring Anderson-type POMs. The sponge was prepared by taking advantage of chemical crosslinking between cellulose nanofiber （CNF）， g-glycidoxypropyltrimethoxysilane （GPTMS）， and PEI. GPTMS can self-crosslink to form oligomers by after hydrolysis of ethoxy groups， and its Si—OH groups may also react with the hydroxyls of cellulose or bind to CNF via hydrogen bonding[26-27]. On the other hand， amino groups on PEI would react with the epoxy groups on GPTMS[28]. In forming the sponge support， PEI served two purposes： ①to endow the sponge high mechanical strength; ②to provide abundant amino groups for POMs loading. Anderson-type POM （NH4）4[CuMo6O18（OH）6]·5H2O （POM（Cu）） was successfully immobilized on the sponge by electrostatic interactions between the amino groups of PEI and POM anions. The catalytic activity of obtained resulting POM-loaded sponge was analyzed through the hydroboration of phenylacetylenes， which is a common reaction in organic synthesis[29-31].
The proposed process is easily scalable and practical for fixing other anionic POM catalysts. Therefore， it offers a promising method for immobilizing POMs on porous nanocellulose supports. POM-loaded CNF sponges， which have excellent mechanical properties and high loading capacity， have many potential applications in pollutant treatment， organic synthesis， and other fields.
2 Experimental
2.1 Materials
All of the solvents and reagents used in the proposed process are commercially available and were used without further purification. An aqueous suspension of pulp-derived CNFs （30 nm in diameter， several micrometers in length） was obtained from Haojia Cellulose Co.， Ltd. （Tianjin， China）. Branched PEI （molecular weight of 600 g/mol） was supplied by Adamas Reagent Co.， Ltd. （China）. Phenylacetylene and bis（pinacolato）diboron （B2（pin）2） were obtained from Energy Chemical Co.， Ltd. （Shanghai， China）. All other chemical reagents and solvents used in this work were purchased from Sinopharm Chemical Reagent Co.， Ltd. （Shanghai， China）.
2.2 Synthesis of POM（Cu）
Synthesis of POM（Cu） was carried out according to a previously reported procedure[32]. Specifically， 10.0 g of （NH4）6Mo7O24·4H2O was dissolved in 160.0 mL of deionized water and the resulting solution was heated to 100℃. Next， 3.1 g of CuSO4·5H2O was dissolved in 40.0 mL of deionized water and the resulting copper sulfate solution was added drop-by-drop to the hot ammonium molybdate solution with rapid stirring. The resulting mixture was stirred continuously at 100℃ for 1 h. After the reaction was completed， the yellow insoluble precipitate was filtered and the filtrate was mixed with acetonitrile. Light blue particles （POM（Cu）） could be observed after a few days. The phase purity of POM（Cu） was verified through X-Ray Diffraction （XRD） and inductively coupled plasma optical emission spectrometry， and the copper content of the POM（Cu） was 53.4 mg/g， which is almost identical to the theoretical value of 53.7 mg/g. 2.3 Preparation of the CNF-PEI sponge and POM-loaded CNF-PEI sponge
The preparation of the CNF-PEI sponge was carried out based on our previously reported procedure[28]. As illustrated in Fig.1， a predetermined amount of GPTMS （1.2 g） was added to a cellulose suspension （1.2 wt%， 100 g）. The resulting mixture was then stirred with a magnetic stirrer for 1 h at 40℃. Subsequently， PEI aqueous solution （20 wt%， 1.2 g） was added to the mixture and stirring continued for another 1 h at 40℃. The resulting mixture was chilled with liquid nitrogen and freeze-dried at -54℃ for 30 h in a Labconco FD5-3 freeze-dryer （USA）. Finally， the sponge was heated at 100℃ in an air-circulating oven for 30 min to promote additional crosslinking of KH560. The resulting sponge is CNF-PEI sponge， denoted as a CPS.
For preparation of the POM-loaded CPS， 100.0 mg of POM（Cu） was dispersed in 10.0 mL of deionized water. The pH value of the mixture was adjusted within the range of 2.5～3.0 to completely dissolve the POM（Cu）. The CPS was subsequently soaked in the POM（Cu） solution for 24 h. The final POM-loaded CPS was obtained after a sufficient washing procedure and denoted as a CPS/POM（Cu）.
2.4 Catalytic performance test
The general procedure for the hydroboration of alkynes is as follows. A CPS/POM（Cu） （20.0 mg， 0.0012 mmol， 1 mol% （based on Cu））， phenylacetylene （30.7 mg， 0.3 mmol）， B2（pin）2 （114.3 mg， 0.45 mmol）， Na2CO3 （0.06 mmol）， and 2.0 mL of solvent were added to a clean tube in an argon atmosphere. The reaction was maintained at 30℃ with magnetic stirring. Reaction progress was monitored by a SHIMADZU QP-2010 SE GC-MS device （Japan）.
2.5 Characterizations
The phase purity of POM（Cu） was verified through X-Ray Diffraction （XRD， Rigaku D/max 2550PC， Japan） and inductively coupled plasma optical emission spectrometry （Prodigy， USA）.
Fourier transform infrared spectroscopy （FT-IR） analysis was performed using a PerkinElmer Spectrum Two instrument （USA） equipped with an attenuated total reflectance accessory spectrometer in the range of 400～4000 cm-1 with a resolution of 4 cm-1.
Thermogravimetric analysis （TGA） was carried out using a NETZSCH TG209 F3 device （Germany） and the samples were heated at 10℃/min under a nitrogen flow （20 mL/min）.
The microscopic morphologies of the CPS and CPS/POM（Cu） were observed by a Hitachi TM-3030 scanning electron microscope （Japan）.
Brunauer-Emmett-Teller analyses of the sponges were performed using a specific surface area analysis instrument （V-sorb 2800P， China） and the Brunauer-Joyner-Halenda （BJH） formula was used to obtain pore size distributions. Energy-dispersive X-ray spectrometry （EDS） mapping was carried out using a field emission scanning electron microscope （Hitachi S-4800， Japan）.
The mechanical properties of the CPS/POM（Cu） were evaluated using a Changchun Xinke universal testing machine （China） at a compression speed of 2.0 mm/min.
3 Results and discussion
3.1 Characterization of the CPS
The immobilization process can be easily observed. While soaking in the aqueous POM（Cu） solution， the color of the CPS changed from white to light blue after a few minutes （Fig.1）. The synthesis and immobilization of POM（Cu） were confirmed through FT-IR analysis （Fig.2）. The characteristic peaks at 929 and 889 cm-1 correspond to the vibrations of terminal Mo=O units. The peaks at 629 and 568 cm-1 are attributed to the vibrations of Mo—O—Mo groups. These data are consistent with the typical characteristics of Anderson-type POMs[33]. For the CPS/POM（Cu）， all the characteristic peaks of POM（Cu） are present， which confirms that POM（Cu） was successfully loaded onto the CPS.
The POM（Cu） loading onto the CPS was quantified through TGA （Fig.3）. The results indicate that the pure CPS was stable up to 250℃， but a larger mass loss occurred in the CPS/POM（Cu） below 250℃. This phenomenon is attributed to the loss of lattice water in the POM（Cu）[8]. When comparing the differential thermogravimetry （DTG） spectra of the CPS and CPS/POM（Cu）， we can see a new peak at 676℃， which corresponds to the thermal decomposition of POM（Cu）. The CPS/POM（Cu） exhibited a 56.1 wt% residue at 600℃， whereas the pure CPS exhibited a 27.0 wt% residue. The content of POM（Cu） in the CPS/POM（Cu） was determined to be 44.5% based on Eq.（1）.
92.4x+27.0（1-x） =56.1 （1）
Where x is the weight percentage of POM（Cu） in the CPS/POM（Cu）， and the coefficient of 92.4x represents the mass fraction of the non-water components in POM（Cu）.
The microstructures of the CPS and CPS/POM（Cu） were investigated using scanning electron microscopy （SEM）. The CPS exhibited honeycomb-like pores formed by ice templating （Fig.4（a））. The porous morphology of the CPS was maintained after POM（Cu） immobilization （Fig.4（b））， making the resulting POM-loaded sponge suitable for heterogeneous catalysis.
The porous structures of the CPS and CPS/POM（Cu）were further analyzed using the nitrogen sorption technique （Fig.4（c））. The sponges exhibited a typical type-IV isotherm with obvious hysteresis loops， indicating that abundant mesopores existed within the materials[34]. The pore sizes of the sponge were distributed in a narrow range （Fig.4（d））. The porosity of the sponge decreased slightly from 97.6% to 91.3% based on a secondary lyophilization process. The incorporation of the POM（Cu） into the CPS was further verified using EDS mapping （Fig.5）. Mo and Cu elements were distributed uniformly on the surface of the CPS/POM（Cu） and no POM particles were observed on the surface of the sponge. A uniform distribution of POMs facilitates sufficient contact with reactive substrates. This anchoring strategy was determined to be scalable and applicable to anchoring other anionic POM catalysts.
Heterogeneous catalysts should have good mechanical properties to prevent unnecessary material loss during usage. The mechanical properties of the CPS/POM（Cu） were investigated using a universal testing machine and the compression stress-strain curves are summarized in Fig.6. The shape recovery rate R was calculated according to the Eq.（2）.
R（%）=100%-e0N （2）
Where， e0N represents the strain when the detected force reaches 0 N. After the first compression， the sponge recovered to 79.9% of its original thickness. The stress at the maximum deformation was 45.1 kPa. Over 10 pressing and releasing cycles， the sponge demonstrated high mechanical durability， meaning the shape recovery rate and maximum stress did not decrease significantly.
3.2 Catalytic performance of the CPS/POM（Cu）
Our catalytic analysis began with a reaction between phenylacetylene and B2（pin）2 in the presence of the CPS/POM（Cu） （1 mol%） under an argon atmosphere （Fig.7）. The results are summarized in Table 1. Reactions were carried out at 30℃ under various solvent conditions （entries 1～4）. Poor conversion was observed in the DMF solvent， whereas desirable conversions were observed when using ethanol solvents. Additionally， ethanol systems have a beneficial effect on the regioselective hydroboration of alkynes. Na2CO3 provided the best conversion and regioselectivity （entry 1）， and no products were detected in the absence of a base （entry 5）. To confirm the effectiveness of the composition of the POM（Cu） catalyst， we investigated the catalytic properties of other possible substances for this reaction （entries 8～11）. Only the CPS/CuSO4 was found to have weak catalytic activity， but its effect was far weaker than that of the CPS/POM（Cu）. Results of dissolved POM（Cu） showed that immobilization onto the nanocellulose had marginal effects on its catalytic activity （entries 1 and 12， Fig.8（a））. Under optimized conditions， the substrate scope was investigated to assess the effectiveness of the CPS/POM（Cu） for catalyzing alkyne hydroboration reactions （Fig.9 and Table 2）. As shown in Table 2， all of the examined substrates exhibited good conversion from 86% to 92% and high regioselectivity of up to 99%. The selectivity seemed to be slightly compromised by electron-donating substituents such as amino and methoxy groups. Recyclability is an important parameter for evaluating the catalytic properties of a heterogeneous catalyst. The CPS/POM（Cu） can be simply removed from the reaction mixture using tongs without any complex processes， such as filtration or centrifugation， making it easy to reuse. The recyclability of POM（Cu） under optimal reaction conditions was examined. A total of five catalytic trials were carried out. The catalytic efficiency of the last reaction decreased slightly compared to that of the first reaction （Fig.8（a））. However， the conversion and regioselectivity exhibited no significant changes after five cycles （Fig.8（b））. The mass loss of CPS/POM（Cu） after five cycles was quantified through TGA analysis （Fig.8（c））. The content of POM（Cu） on CPS/POM（Cu） only decreased slightly from the original 44.5% to 38.2% at the end the fifth cycle.
To evaluate the chemical stability of the CPS/POM（Cu） catalyst， FT-IR analysis of the used CPS/POM（Cu） catalysts was performed. As show in Fig.8（d）， the chemical composition of the CPS/POM（Cu） remained unchanged after five reaction cycles. The microstructure of the CPS/POM（Cu） after five reaction cycles was also investigated using SEM （Fig.10）. Although the original ordered honeycomb-like structures of the CPS/POM（Cu） became jumbled after long use， the material retained a porous structure for the circulation of reaction substrate.
4 Conclusions
We presented an amino functionalization cellulose nanofiber sponge （CPS） as a support to anchor POM（Cu）. The POM（Cu） catalyst was distributed homogeneously in the CPS. The POM（Cu）-loaded sponge was then used as a catalyst for the hydroboration of phenylacetylene. The conversion of phenylacetylene reached 96% with a regioselectivity of 99%. The POM（Cu）-loaded sponge could be easily recollected and reused with no significant decrease in activity after five reaction cycles. This scalable strategy represents a novel methodology for immobilizing POMs on porous cellulose sponges through high-quantity POM loading. It is also suitable for anchoring other anionic POMs. POM-loaded cellulose sponges exhibit excellent mechanical properties and are potentially useful for pollutant treatment， organic synthesis， and other applications.
Acknowledgments
This work was financially supported by the Fundamental Research Funds for the Central Universities （No. 2232018A3-04， No. 2232018-02， and No. 2232018G-043） and the Program of Introducing Talents of Discipline to Universities （No. 105-07-005735）. References
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Keywords： cellulose nanofiber; polyoxometalates; catalyst immobilization; hydroboration; heterogeneous catalyst
1 Introduction
Based on their unique redox properties， polyoxometalates （POMs） have attracted significant attention in various fields， including oil purification， environmental protection， energy transformation， and catalysis[1-6]. The immobilization of POMs on a proper support enables their convenient recovery and reuse， making them more environmentally friendly and sustainable[7].
POMs can be immobilized on supports through either covalent or non-covalent attachment[8-12]. Covalent bonding is ideal because it provides stronger adhesion[6]. However， it typically requires a complex preparation process[13-14]. In the more feasible non-covalent binding process， POMs are anchored on support materials， such as mesoporous carbon， silica， and cellulose， which are pre-modified with amino functionalities[15-19]. However， these non-covalently POM-loaded materials typically suffer from low support capacity and high leaching of catalysts during separation.
As the most abundant natural macromolecular compound in nature， cellulose has been widely applied in environmental engineering， catalyst immobilization， tissue engineering， and other fields[4， 20-25]. POM-loaded cellulose materials have been prepared[1，19]， however， these materials still face the problems of issues related to low specific surface area and low POM loading capacity. In this paper， we present a polyethylenimine （PEI）-modified nanocellulose sponge for anchoring Anderson-type POMs. The sponge was prepared by taking advantage of chemical crosslinking between cellulose nanofiber （CNF）， g-glycidoxypropyltrimethoxysilane （GPTMS）， and PEI. GPTMS can self-crosslink to form oligomers by after hydrolysis of ethoxy groups， and its Si—OH groups may also react with the hydroxyls of cellulose or bind to CNF via hydrogen bonding[26-27]. On the other hand， amino groups on PEI would react with the epoxy groups on GPTMS[28]. In forming the sponge support， PEI served two purposes： ①to endow the sponge high mechanical strength; ②to provide abundant amino groups for POMs loading. Anderson-type POM （NH4）4[CuMo6O18（OH）6]·5H2O （POM（Cu）） was successfully immobilized on the sponge by electrostatic interactions between the amino groups of PEI and POM anions. The catalytic activity of obtained resulting POM-loaded sponge was analyzed through the hydroboration of phenylacetylenes， which is a common reaction in organic synthesis[29-31].
The proposed process is easily scalable and practical for fixing other anionic POM catalysts. Therefore， it offers a promising method for immobilizing POMs on porous nanocellulose supports. POM-loaded CNF sponges， which have excellent mechanical properties and high loading capacity， have many potential applications in pollutant treatment， organic synthesis， and other fields.
2 Experimental
2.1 Materials
All of the solvents and reagents used in the proposed process are commercially available and were used without further purification. An aqueous suspension of pulp-derived CNFs （30 nm in diameter， several micrometers in length） was obtained from Haojia Cellulose Co.， Ltd. （Tianjin， China）. Branched PEI （molecular weight of 600 g/mol） was supplied by Adamas Reagent Co.， Ltd. （China）. Phenylacetylene and bis（pinacolato）diboron （B2（pin）2） were obtained from Energy Chemical Co.， Ltd. （Shanghai， China）. All other chemical reagents and solvents used in this work were purchased from Sinopharm Chemical Reagent Co.， Ltd. （Shanghai， China）.
2.2 Synthesis of POM（Cu）
Synthesis of POM（Cu） was carried out according to a previously reported procedure[32]. Specifically， 10.0 g of （NH4）6Mo7O24·4H2O was dissolved in 160.0 mL of deionized water and the resulting solution was heated to 100℃. Next， 3.1 g of CuSO4·5H2O was dissolved in 40.0 mL of deionized water and the resulting copper sulfate solution was added drop-by-drop to the hot ammonium molybdate solution with rapid stirring. The resulting mixture was stirred continuously at 100℃ for 1 h. After the reaction was completed， the yellow insoluble precipitate was filtered and the filtrate was mixed with acetonitrile. Light blue particles （POM（Cu）） could be observed after a few days. The phase purity of POM（Cu） was verified through X-Ray Diffraction （XRD） and inductively coupled plasma optical emission spectrometry， and the copper content of the POM（Cu） was 53.4 mg/g， which is almost identical to the theoretical value of 53.7 mg/g. 2.3 Preparation of the CNF-PEI sponge and POM-loaded CNF-PEI sponge
The preparation of the CNF-PEI sponge was carried out based on our previously reported procedure[28]. As illustrated in Fig.1， a predetermined amount of GPTMS （1.2 g） was added to a cellulose suspension （1.2 wt%， 100 g）. The resulting mixture was then stirred with a magnetic stirrer for 1 h at 40℃. Subsequently， PEI aqueous solution （20 wt%， 1.2 g） was added to the mixture and stirring continued for another 1 h at 40℃. The resulting mixture was chilled with liquid nitrogen and freeze-dried at -54℃ for 30 h in a Labconco FD5-3 freeze-dryer （USA）. Finally， the sponge was heated at 100℃ in an air-circulating oven for 30 min to promote additional crosslinking of KH560. The resulting sponge is CNF-PEI sponge， denoted as a CPS.
For preparation of the POM-loaded CPS， 100.0 mg of POM（Cu） was dispersed in 10.0 mL of deionized water. The pH value of the mixture was adjusted within the range of 2.5～3.0 to completely dissolve the POM（Cu）. The CPS was subsequently soaked in the POM（Cu） solution for 24 h. The final POM-loaded CPS was obtained after a sufficient washing procedure and denoted as a CPS/POM（Cu）.
2.4 Catalytic performance test
The general procedure for the hydroboration of alkynes is as follows. A CPS/POM（Cu） （20.0 mg， 0.0012 mmol， 1 mol% （based on Cu））， phenylacetylene （30.7 mg， 0.3 mmol）， B2（pin）2 （114.3 mg， 0.45 mmol）， Na2CO3 （0.06 mmol）， and 2.0 mL of solvent were added to a clean tube in an argon atmosphere. The reaction was maintained at 30℃ with magnetic stirring. Reaction progress was monitored by a SHIMADZU QP-2010 SE GC-MS device （Japan）.
2.5 Characterizations
The phase purity of POM（Cu） was verified through X-Ray Diffraction （XRD， Rigaku D/max 2550PC， Japan） and inductively coupled plasma optical emission spectrometry （Prodigy， USA）.
Fourier transform infrared spectroscopy （FT-IR） analysis was performed using a PerkinElmer Spectrum Two instrument （USA） equipped with an attenuated total reflectance accessory spectrometer in the range of 400～4000 cm-1 with a resolution of 4 cm-1.
Thermogravimetric analysis （TGA） was carried out using a NETZSCH TG209 F3 device （Germany） and the samples were heated at 10℃/min under a nitrogen flow （20 mL/min）.
The microscopic morphologies of the CPS and CPS/POM（Cu） were observed by a Hitachi TM-3030 scanning electron microscope （Japan）.
Brunauer-Emmett-Teller analyses of the sponges were performed using a specific surface area analysis instrument （V-sorb 2800P， China） and the Brunauer-Joyner-Halenda （BJH） formula was used to obtain pore size distributions. Energy-dispersive X-ray spectrometry （EDS） mapping was carried out using a field emission scanning electron microscope （Hitachi S-4800， Japan）.
The mechanical properties of the CPS/POM（Cu） were evaluated using a Changchun Xinke universal testing machine （China） at a compression speed of 2.0 mm/min.
3 Results and discussion
3.1 Characterization of the CPS
The immobilization process can be easily observed. While soaking in the aqueous POM（Cu） solution， the color of the CPS changed from white to light blue after a few minutes （Fig.1）. The synthesis and immobilization of POM（Cu） were confirmed through FT-IR analysis （Fig.2）. The characteristic peaks at 929 and 889 cm-1 correspond to the vibrations of terminal Mo=O units. The peaks at 629 and 568 cm-1 are attributed to the vibrations of Mo—O—Mo groups. These data are consistent with the typical characteristics of Anderson-type POMs[33]. For the CPS/POM（Cu）， all the characteristic peaks of POM（Cu） are present， which confirms that POM（Cu） was successfully loaded onto the CPS.
The POM（Cu） loading onto the CPS was quantified through TGA （Fig.3）. The results indicate that the pure CPS was stable up to 250℃， but a larger mass loss occurred in the CPS/POM（Cu） below 250℃. This phenomenon is attributed to the loss of lattice water in the POM（Cu）[8]. When comparing the differential thermogravimetry （DTG） spectra of the CPS and CPS/POM（Cu）， we can see a new peak at 676℃， which corresponds to the thermal decomposition of POM（Cu）. The CPS/POM（Cu） exhibited a 56.1 wt% residue at 600℃， whereas the pure CPS exhibited a 27.0 wt% residue. The content of POM（Cu） in the CPS/POM（Cu） was determined to be 44.5% based on Eq.（1）.
92.4x+27.0（1-x） =56.1 （1）
Where x is the weight percentage of POM（Cu） in the CPS/POM（Cu）， and the coefficient of 92.4x represents the mass fraction of the non-water components in POM（Cu）.
The microstructures of the CPS and CPS/POM（Cu） were investigated using scanning electron microscopy （SEM）. The CPS exhibited honeycomb-like pores formed by ice templating （Fig.4（a））. The porous morphology of the CPS was maintained after POM（Cu） immobilization （Fig.4（b））， making the resulting POM-loaded sponge suitable for heterogeneous catalysis.
The porous structures of the CPS and CPS/POM（Cu）were further analyzed using the nitrogen sorption technique （Fig.4（c））. The sponges exhibited a typical type-IV isotherm with obvious hysteresis loops， indicating that abundant mesopores existed within the materials[34]. The pore sizes of the sponge were distributed in a narrow range （Fig.4（d））. The porosity of the sponge decreased slightly from 97.6% to 91.3% based on a secondary lyophilization process. The incorporation of the POM（Cu） into the CPS was further verified using EDS mapping （Fig.5）. Mo and Cu elements were distributed uniformly on the surface of the CPS/POM（Cu） and no POM particles were observed on the surface of the sponge. A uniform distribution of POMs facilitates sufficient contact with reactive substrates. This anchoring strategy was determined to be scalable and applicable to anchoring other anionic POM catalysts.
Heterogeneous catalysts should have good mechanical properties to prevent unnecessary material loss during usage. The mechanical properties of the CPS/POM（Cu） were investigated using a universal testing machine and the compression stress-strain curves are summarized in Fig.6. The shape recovery rate R was calculated according to the Eq.（2）.
R（%）=100%-e0N （2）
Where， e0N represents the strain when the detected force reaches 0 N. After the first compression， the sponge recovered to 79.9% of its original thickness. The stress at the maximum deformation was 45.1 kPa. Over 10 pressing and releasing cycles， the sponge demonstrated high mechanical durability， meaning the shape recovery rate and maximum stress did not decrease significantly.
3.2 Catalytic performance of the CPS/POM（Cu）
Our catalytic analysis began with a reaction between phenylacetylene and B2（pin）2 in the presence of the CPS/POM（Cu） （1 mol%） under an argon atmosphere （Fig.7）. The results are summarized in Table 1. Reactions were carried out at 30℃ under various solvent conditions （entries 1～4）. Poor conversion was observed in the DMF solvent， whereas desirable conversions were observed when using ethanol solvents. Additionally， ethanol systems have a beneficial effect on the regioselective hydroboration of alkynes. Na2CO3 provided the best conversion and regioselectivity （entry 1）， and no products were detected in the absence of a base （entry 5）. To confirm the effectiveness of the composition of the POM（Cu） catalyst， we investigated the catalytic properties of other possible substances for this reaction （entries 8～11）. Only the CPS/CuSO4 was found to have weak catalytic activity， but its effect was far weaker than that of the CPS/POM（Cu）. Results of dissolved POM（Cu） showed that immobilization onto the nanocellulose had marginal effects on its catalytic activity （entries 1 and 12， Fig.8（a））. Under optimized conditions， the substrate scope was investigated to assess the effectiveness of the CPS/POM（Cu） for catalyzing alkyne hydroboration reactions （Fig.9 and Table 2）. As shown in Table 2， all of the examined substrates exhibited good conversion from 86% to 92% and high regioselectivity of up to 99%. The selectivity seemed to be slightly compromised by electron-donating substituents such as amino and methoxy groups. Recyclability is an important parameter for evaluating the catalytic properties of a heterogeneous catalyst. The CPS/POM（Cu） can be simply removed from the reaction mixture using tongs without any complex processes， such as filtration or centrifugation， making it easy to reuse. The recyclability of POM（Cu） under optimal reaction conditions was examined. A total of five catalytic trials were carried out. The catalytic efficiency of the last reaction decreased slightly compared to that of the first reaction （Fig.8（a））. However， the conversion and regioselectivity exhibited no significant changes after five cycles （Fig.8（b））. The mass loss of CPS/POM（Cu） after five cycles was quantified through TGA analysis （Fig.8（c））. The content of POM（Cu） on CPS/POM（Cu） only decreased slightly from the original 44.5% to 38.2% at the end the fifth cycle.
To evaluate the chemical stability of the CPS/POM（Cu） catalyst， FT-IR analysis of the used CPS/POM（Cu） catalysts was performed. As show in Fig.8（d）， the chemical composition of the CPS/POM（Cu） remained unchanged after five reaction cycles. The microstructure of the CPS/POM（Cu） after five reaction cycles was also investigated using SEM （Fig.10）. Although the original ordered honeycomb-like structures of the CPS/POM（Cu） became jumbled after long use， the material retained a porous structure for the circulation of reaction substrate.
4 Conclusions
We presented an amino functionalization cellulose nanofiber sponge （CPS） as a support to anchor POM（Cu）. The POM（Cu） catalyst was distributed homogeneously in the CPS. The POM（Cu）-loaded sponge was then used as a catalyst for the hydroboration of phenylacetylene. The conversion of phenylacetylene reached 96% with a regioselectivity of 99%. The POM（Cu）-loaded sponge could be easily recollected and reused with no significant decrease in activity after five reaction cycles. This scalable strategy represents a novel methodology for immobilizing POMs on porous cellulose sponges through high-quantity POM loading. It is also suitable for anchoring other anionic POMs. POM-loaded cellulose sponges exhibit excellent mechanical properties and are potentially useful for pollutant treatment， organic synthesis， and other applications.
Acknowledgments
This work was financially supported by the Fundamental Research Funds for the Central Universities （No. 2232018A3-04， No. 2232018-02， and No. 2232018G-043） and the Program of Introducing Talents of Discipline to Universities （No. 105-07-005735）. References
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