Lei Zhou,Lingling Peng,Xingbang Hu,*

1 College of Chemistry and Chemical Engineering,Yangzhou Polytechnic Institute,Yangzhou 225127,China

2 School of Chemistry and Chemical Engineering,Nanjing University,Nanjing 210093,China

Keywords:Cyclic (alkyl)(amino)carbenes Carbon dioxide Hydrogenation SBA-15 Formate

ABSTRACT Recently,cyclic (alkyl)(amino)carbenes (CAACs) have been widely used as ligands to enhance the catalytic reactivity of center metal,but the problem of recycling this expensive ligand remains to be solved.In this work,the heterogeneous SBA-15-CAAC-Ir catalyst was prepared by a covalent attachment method,and using SBA-15 as the carrier.It shows high reactivity for the hydrogenation of CO2 to formate.After immobilization,the ordered mesoporous structure and the overall rod-like morphology of the original SBA-15 have been preserved very well.Using SBA-15-CAAC-Ir as catalyst,up to 21050 TON can be obtained at 60 °C.In addition,the catalyst can be separated easily by centrifugation,and the catalytic activity of SBA-15-CAAC-Ir can still remain very high after multiple cycles.

1.Introduction

Carbon dioxide (CO2) has stable chemical properties.However,since the industrial revolution,the excessive use of fossil fuels has led to the excessive emission of CO2,which brings a variety of environmental problems such as the greenhouse effect [1-4].During the last few years,the idea of carbon neutrality has attracted widespread attention,so it is very important for the conversion and utilization of CO2[5-9].As shown as Fig.1,using CO2as C1 source,researchers have developed many synthesis routes of different organic chemical products [10-12],including ester,carboxyl or amide products by carboxylation reaction [13],and low carbon organic compounds such as formic acid/formate or methanol by hydrogenation process [14-16].Among them,CO2hydrogenation to formic acid is an efficient and green reaction with 100% atomic utilization.

Because of the higher catalytic efficiency and milder conditions,the homogeneous catalysts were applied to the hydrogenation of CO2for formate earlier than the heterogeneous catalysts.So far,many homogeneous catalysts with different ligands for the hydrogenation of CO2to formic acid have been reported.Among them,noble metals like Ir [17-24],Rh [25-29] and Ru [30-43] have the best catalytic performance.Although non-noble metal-based catalysts such as Mn [44-46],Fe [47-50],Co [51,52] and Cu [53-59]are much cheaper,but their activities are always very low.However,the catalytic mechanism of CO2hydrogenation to formic acid is basically the same regardless of noble metal or non-noble metal,and the interaction mechanism between CO2and metal hydrogen bond(M-H)is the most widely recognized[17,53,54].The performance of homogeneous catalysts is sensitive to the properties of ligands,so homogeneous catalysts are usually not strong enough to withstand harsh reaction environments and are difficult to be separated and reused.On the contrary,heterogeneous catalyst can avoid these shortcomings[60].Especially for noble metals catalyst,heterogeneity is an effective way to recycle the expensive compound.

In 2005,Bertrand’s group has firstly synthesized a novel cyclic(alkyl)(amino) carbenes (CAACs) [61].Structurally,CAACs contain one more σ-donor alkyl group compared with NHCs,which makes the ligand more electronegative.Thus,CAACs are considered to be highly nucleophilic (higher HOMO,stronger σ-donor) and electrophilic (lower LUMO,stronger π-receptor) carbene ligands [62-65].Due to the unique spatial and electronic properties,CAACs can activate various small molecules and stabilize a variety of transition metal.Therefore,the propensity of CAACs to improve the catalytic activity of metal centers has been well acknowledged over the years.For example,CAAC-Pd complexes can be used to promote the α-arylation of propiophenone with aryl chloride under mild conditions [61];CAAC-Ru complexes have been reported as catalysts for olefin re-decomposition [66];CAAC-Fe(CO)4complexes have been used for aryl acetylene dimerization[67];CAAC-Au complexes can efficiently mediate the coupling of terminal alkynes with enamines to give allenes [68].In our previous studies,CAAC-Ir complexes were used as catalysts for ammonia borane hydrolysis and pyrolytic dehydrogenation,with a fast dehydrogenation rate and mild conditions [64].In addition,Bertrand and us ever reported a co-catalytic system of CAAC-Cu/B(C6F5)3-DBU for the hydrogenation of CO2[56],in which CAAC-Cu can activated CO2efficiently and achieved a maximum TON value of 1881.However,almost all catalysts using CAACs as ligands are homogeneous,and the recovery of expensive CAACs ligands remains to be solved.

Fig.1.Roadmap for CO2 conversion to multiple organics.

Herein,we report the heterogeneous SBA-15-CAAC-Ir catalyst synthesized by a covalent attachment method and it shows good activity and recoverability in the hydrogenation of CO2to fomate.The highest TON can be up to 21050 at 60 °C and the catalyst can be recycled multiple times without loss of activity by a simple recovery process.In addition,the structure and properties of the supported SBA-15-CAAC-Ir catalyst was characterized in detail,the reaction conditions and mechanism were explored.

2.Experimental

2.1.Materials

SBA-15,2,6-Diisopropylaniline,2-Ethylbutyraldehyde,3-Chloro-2-methylpropene,Chloro (1,5-cyclooctadiene) iridium(I)dimer,n-Butyllithium,HCl ether solution,(3-Mercaptopropyl)trimethoxysilane,AIBN (2-2′-azoisobutyronitrile),Dimethoxymethane,etc.were purchased from XFNANO,Aldrich Chemical Company,Energy Chemical Company or Tokyo Chemical Industry Company.All reagents were used as received without further purification.The synthesis of catalysts was performed under an atmosphere of N2by using standard schlenk techniques.Solvents were dried by standard methods.

2.2.Preparation of CAAC ligands

CAACEt2and CAACC=Cwere synthesized according to the previously reported procedures [62,63],and the structures of these ligands are shown as Fig.2.The synthetic steps of these ligands are similar,but need to add the n-butyllithium and propylene chloride twice to get the C=C structure.After dried under vacuum,both of CAACEt2and CAACC=Cwere obtained as a white solid.Then,CAACEt2-Ir and CAACC=C-Ir complexes were obtained as yellow solid according to the previously reported procedures[69](Seeing Supplementary Material for details).

Fig.2.Structures of CAACs and CAAC-Ir complexes.

2.3.Preparation of SBA-15-CAAC-Ir

Synthesis of SBA-15-CAAC-Ir was following the steps in Fig.3.(1) SBA-15 should be modified before loading CAACC=C.SBA-15 and (3-Mercaptopropyl) trimethoxysilane were mixed and refluxed in toluene at 110°C for 24 h under a N2atmosphere.Then,the product was purified by Soxhlet extraction with anhydrous dichloromethane,and then dried under vacuum to give SBA-15-SH.(2) SBA-15-SH,CAACC=Cand AIBN (2-2′-azoisobutyronitrile)were mixed and refluxed in toluene at 110 °C for 24 h under a N2atmosphere.The resulting solid was filtered by Soxhlet extraction with anhydrous toluene and then dried under vacuum.The solid powder was labeled as SBA-15-CAAC.(3) The [Ir(cod)Cl]2was fixed on the SBA-15-CAAC by hydrogen removal using super strong base KHMDS at-78°C under a N2atmosphere,and the product was washed by Soxhlet extraction with dichloromethane,and then dried under vacuum.The solid powder was labeled as SBA-15-CAAC-Ir,and the Ir loading amount was determined to be 0.2% (mass) by ICP-AES analysis,which is basically equivalent to the EDS-mapping result.

Fig.3.Synthesis of SBA-15-CAAC-Ir.

2.4.Characterization

1H and13C spectra were recorded on a Bruker Avance 400 spectrometer.Fourier transform-infrared(FT-IR)spectra were obtained with a Nicolet 380 (Thermo Fisher Scientific,USA) spectrometer,using KBr as the blank.X-ray photoelectron spectroscopy (XPS)analysis was performed by PHI 5000 Versa Probe (ULVAC-PHI,JPN).Scanning electron microscopy (SEM) and EDS mapping were performed by Gemini SEM 300 and Oxford X-MAX,respectively.Transmission electron microscopy (TEM) was performed by JEOL JEM-2100.N2and CO2adsorption and desorption isotherms were performed by Micromeritic ASAP-2460.Small-angle X-ray scattering (SAXS) analysis was performed by Bruker D8 ADVANCE.

3.Results and Discussion

3.1.Characterization of SBA-15-CAAC-Ir

In order to determine the structure of the supported catalyst,a variety of characterization was carried out for SBA-15 and SBA-15-CAAC-Ir.Fig.4(a) displayed the SEM images for the pure SBA-15 and the supported SBA-15-CAAC-Ir.SEM images show that both pure SBA-15 molecular sieve and SBA-15-CAAC-Ir catalyst present short rod-like hexagonal structures,so that the surface modification would not change the overall rod-like morphology of SBA-15[70].Moreover,Fig.4(b) displayed the EDS-mapping of SBA-15-CAAC-Ir.It can be seen that the supported catalyst contains not only a large amount of Si from SBA itself,but also a large amount of C and a small amount of Ir,which indicates that the active metal center has been successfully loaded onto the SBA-15.The content of Ir in SBA-15-CAAC-Ir was 0.2%(mass),which was basically consistent with the result of ICP-AES.In addition,TEM was used to further study the pore structure and internal morphology of SBA-15-CAAC-Ir,the result was shown in Fig.5.It can be seen that periodic ordered mesoporous structures can all be observed before or after the modification.These results further enhance the observation effect of SEM images [71].

As shown in Fig.6,FT-IR analysis was performed on CAAC-Ir monomer,SBA-15,SBA-15-SH and SBA-15-CAAC-Ir.On the one hand,an absorption peak of -SH at 2570 cm-1can be observed in the curve of SBA-15-SH,it indicates that the sulfydryl group has been loaded onto SBA-15 successfully.On the other hand,through the immobilization of CAACC=C-Ir complex,absorption peaks at 2920 cm-1and 1450 cm-1can be observed in SBA-15-CAAC-Ir,which were assigned to functional groups of Ar-H and-CH3[72],respectively.Furthermore,the original absorption peak of CAACC=C-Ir at 1730 cm-1and absorption peak of -SH at 2570 cm-1have both disappeared after modification,indicating that the effective reaction between the -SH group and the C=C bond on CAAC [73].

Nitrogen adsorption measurements were used to study the porosities of SBA-15-CAAC-Ir,and the results were shown as Fig.7(a) and (b).The BET specific surface area of pure SBA-15 is 489 m2·g-1,and the specific surface area of the SBA-15-CAAC-Ir can reach 348 m2·g-1after functionalization.The specific surface area decreased slightly because CAAC-Ir has occupied some space.In addition,according to the calculation results of pore size distribution of BJH model [74,75],the mesoporous structure of SBA-15 was not damaged,and the average pore size also decreased slightly from 9.75 nm to 8.60 nm.Furthermore,CO2adsorption and desorption isotherms of SBA-15-CAAC-Ir were displayed in Fig.7(c).Up to 13.1 cm3·g-1CO2was adsorbed,so it can be seen that SBA-15-CAAC-Ir was capable of concentrating CO2,and this characteristic is beneficial for SBA-15-CAAC-Ir to promote the reaction process of CO2hydrogenation [4].

Fig.8(a) displayed the XPS spectra of Ir(I) in the CAAC-Ir and SBA-15-CAAC-Ir.Ir4f7/2peak centered at 61.6 eV and the Ir4f5/2peak centered at 64.3 eV can be observed in both of the two samples,which are typical for Ir(I)compounds[76].Compared with the CAAC-Ir monomer,although the XPS curve peak of the supported catalyst was much weaker due to the Ir content is very low,it can also be concluded that the structure of carbene-Ir is well preserved during the reaction.In addition,Fig.8(b) showed the SAXS analysis results of SBA-15 and SBA-15-CAAC-Ir.Before or after the load,three characteristic peaks can be observed at 0.4°,0.8° and 1.5°,corresponding to the (1 0 0),(1 1 0),and (2 0 0) reflection planes [77],respectively.According to the results,it can be seen that the ordered 2D hexagonal pore structure (space group P6 mm) was preserved very well after functionalization.

Fig.4.(a) SEM images of SBA-15 and SBA-15-CAAC-Ir (b) EDS-Mapping of SBA-15-CAAC-Ir.

Fig.5.TEM images of SBA-15,SBA-15-CAAC-Ir and used SBA-15-CAAC-Ir.

3.2.Catalytic hydrogenation of CO2 to formate

In a typical catalytic reaction,a 50 ml stainless autoclave was charged with the catalyst and 10 mmol base in 10 ml THF.The reactor was pressurized with 6.0 MPa of H2:CO2(2:1) mixture and then heated and stirred.After the reactor was cooled to room temperature and vented,the product was dissolved in water and isopropanol was added as an internal standard.Then,1H NMR spectroscopy was used to estimate the amount of formate [57].The performances of SBA-15-CAAC-Ir on the hydrogenation of CO2to formate were investigated,and the results were shown in Table 1 and Fig.9.According to the Table 1,SBA-15-CAAC-Ir remained the high activity of CAAC-Ir,and the highest TON can be up to 21050 at 60 °C for 24 h.In addition,it can be seen that both of base and solvent have an impact on the process of CO2hydrogenation [78],and the screening results showed that DBU and THF are the most suitable base and solvent,respectively.

Table 1 Hydrogenation of CO2 to formate①

When SBA-15-CAAC-Ir was used as catalyst,the influence of reaction temperature on the hydrogenation of CO2to formate is shown in Fig.9(a).The results indicated from 30 °C to 100 °C,the increase of temperature is beneficial to the reaction,and the highest TON can be obtained at 100 °C.However,when the temperature exceeds 100 °C,the catalytic reactivity decreases rapidly.At 150°C,TON is only 2470,because exorbitant temperature is not conducive to CO2hydrogenation in thermodynamics [79].The influence of reaction time on the hydrogenation of CO2to formate was explored as shown in Fig.9(b).On the whole,with the extension of reaction time,TON increases gradually,indicating that SBA-15-CAAC-Ir maintained good activity during the reaction process.However,when the time exceeds 18 h,the growth rate of TON decreases significantly.This may be due to the increasing concentration of products in the system and the reaction may arrive at equilibrium.

Fig.6.FT-IR spectra of CAAC-Ir,SBA-15,SBA-15-SH and SBA-15-CAAC-Ir.

The hydrogenation of CO2to formate requires the simultaneous activation of inert H2and CO2,which is challenging and often requires high pressure.Study of the catalytic reactivity of SBA-15-CAAC-Ir on the hydrogenation of CO2at different gas partial pressures was shown as Fig.10.The results show that with the increase of H2partial pressure,TON value increases significantly(from 4160 to 8700).Moreover,higher total pressure of system can contribute to the reaction,but the partial pressure of H2is more important,because forming M-H bonds is a key step in this reaction.For example,the TON is lower when the composition of thePCO2/PH2(MPa) goes from 1.5/3 to 4/2.In general,the increase of H2partial pressure obviously plays a key role to improve TON,indicating that the activation of H2is the decisive step in the process of CO2hydrogenation,and it is consistent with previous studies [17,56].

In addition,the recyclability of the catalyst is critical for practical applications.It is well known that most homogeneous catalysts are difficult to be separated from the reaction system,but SBA-15-CAAC-Ir can avoid this problem.The cyclic performance of SBA-15-CAAC-Ir is shown in Fig.11.After the hydrogenation reaction,the catalyst can be obtained again just by centrifugation and reused without further treatment.The results indicated that the catalytic activity of SBA-15-CAAC-Ir can remain very high after five cycles.XRD and SEM were carried out on the reused catalysts as shown in Figs.5 and 8(b),it can be seen that the periodic ordered mesoporous structures have no obvious damage,indicating the good reusability of SBA-15-CAAC-Ir.

3.3.Mechanism of hydrogenation of CO2

Fig.7.(a) N2 adsorption/desorption isotherms for SBA-15 and SBA-15-CAAC-Ir.(b)Pore width distribution determined by BJH method.(c) CO2 adsorption and desorption isotherms of SBA-15-CAAC-Ir.

Fig.8.(a) XPS spectra of Ir(I) in the CAAC-Ir and SBA-15-CAAC-Ir.(b) SAXS patterns of SBA-15,SBA-15-CAAC-Ir and reused SBA-15-CAAC-Ir.

Fig.9.(a) Effects of temperature,24 h.(b) Effects of time,100 °C.Other reaction conditions: Ir 0.0002 mmol,DBU 10 mmol, / (MPa)=2/4.

By now,it is well-known that the formation of metal hydride(M-H) is vital for the hydrogenation of CO2to formate,and then CO2can be inserted into the M-H bonds [53,54].In our previous studies[65],CAAC-Ir-H complex can be obtained by stoichiometric reaction between CAAC-Ir and H2(Figs.S13-S15).Due to the heterogeneity of SBA-15-CAAC-Ir and the instability of CAACM-H complex[56],it is difficult to obtain the active intermediates,but a lot of similar mechanisms have been demonstrated.Zallet al.[54] reported that the copper(I) complex LCu(MeCN)PF6is an active catalyst for CO2hydrogenation in the presence of DBU.They proved that association and deprotonation of H2presumably forms the L-Cu-H complex,which is a transient,steady-state intermediate.Then,reaction of L-Cu-H with CO2forms the κ1-coordinated formate complex,which would be rapidly displaced by excess DBU,facilitating turnover and generating the free formate product.It was found that H2was homolytic in stoichiometric reactions (δ:-7.5 and -16.9 in Fig.S13),so a plausible mechanism for the hydrogenation of CO2catalyzed by the SBA-15-CAAC-Ir was proposed in Fig.12.L-Ir-H2intermediate firstly formed with the splitting of H2,then CO2can insert into the Ir-H bond to obtain L-Ir(-H)-O2CH,and release of DBU formate to complete the catalytic cycle at last.We demonstrate this mechanism by changing the order in which CO2and H2are introduced,and different results appeared.In the case of pre-addition of DBU,H2was first introduced.After a period of reaction (2 h,60 °C),H2was drained and CO2was introduced.In this way,formate products can be obtained(Fig.S12).However,switching the order of the two gases will not result in the target product,and this is consistent with our previous researches [4,65].

Fig.10.Effects of H2 partial pressure (MPa) on hydrogenation of CO2,Normal reaction conditions: Ir 0.0002 mmol,DBU 10 mmol,60 °C,24 h.

Fig.11.Recycling of SBA-15-CAAC-Ir.Normal reaction conditions: Ir 0.0002 mmol,DBU 10 mmol,60 °C,24 h, / (MPa)=2/4.

Fig.12.Plausible mechanism for the hydrogenation of CO2 catalyzed by the SBA-15-CAAC-Ir.

4.Conclusions

In this work,SBA-15-CAAC-Ir heterogeneous catalyst was prepared and applied to catalyze CO2hydrogenation to formate.After functionalization,the overall rod-like morphology and ordered mesoporous structure of SBA-15 have been preserved very well.The experimental results show that SBA-15-CAAC-Ir remains the good activity of CAAC-Ir monomer,and the reaction condition is mild.The highest TON can be up to 21,000 at 60 °C.Moreover,the recovery method of SBA-15-CAAC-Ir is very conveniently,and it can retain high activity after repeated utilization for many times.This provides an effective strategy to recycle the noble metals used for CO2hydrogenation and it can also be used for the recovery of CAACs based catalysts.

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.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Nos.22178159 and 21878141) and Key Research&Development Plan of Jiangsu Province (BE2019095).

Supplementary Material

Supplementary material to this article can be found online at https://doi.org/10.1016/j.cjche.2022.07.029.