Zuojun Wei,Haiyan Liu ,Yidong Chen ,Dechao Guo ,Ruofei Pan ,Yingxin Liu ,*

1 Key Laboratory of Biomass Chemical Engineering of the Ministry of Education,College of Chemical and Biological Engineering,Zhejiang University,38 Zheda Road,Xihu District,Hangzhou 310027,China

2 Research and Development Base of Catalytic Hydrogenation,College of Pharmaceutical Science,Zhejiang University of Technology,18 Chaowang Road,Xiacheng District,Hangzhou 310014,China

Keywords:Catalysis 1,3-Cyclohexanedione Density functional theory Hydrogenation Pd@reduced graphene oxide Resorcinol

A B S T R A C T In our previous work,graphene-supported Pd catalyst(Pd/rGO)exhibited higher activity and selectivity for the liquid phase selective hydrogenation of resorcinolto 1,3-cyclohexanedione compared with other catalysts.In the present study,further experimental and theoretical investigations were conducted to reveal the reaction mechanism and the catalytic mechanism of Pd/rGO for resorcinol hydrogenation.The effects of graphene nanosheet and the solvent on the reaction were investigated,and the pathway for resorcinol hydrogenation was proposed supported by density functional theory(DFT)calculations.The results showed that the excellent selectivity of Pd/rGO to 1,3-cyclohexanedione was attributed to the strong π–π and p–π interactions between the graphene nanosheet and the benzene ring as well as hydroxyl in resorcinol molecule,which was in agreement with our previous speculation.In weak polar aprotic solvents,solvation free energy had less impact to the π–π and p–π interactions mentioned above.In strong polar aprotic solvents and polar protic solvents,however,the influence of solvation free energy was much greater,which led to the decrease in the conversion of resorcinol and the selectivity to 1,3-cyclohexanedione.

1.Introduction

1,3-Cyclohexanedione(1,3-CHD)is an important chemical intermediate for the production of pharmaceuticals,pesticides,cosmetics and polymer additives[1–5].It is generally manufactured by two ways based on the availability of raw materials.One way is through the aldol-condensation method,in which a γ-acyl carboxylic or α,β-unsaturated carboxylic acid ester is used as starting materialand reacts with a ketone in the presence of a strongly basic condensing agent through intramolecular or intermolecular condensation to yield 1,3-CHD[6–8].The other way is through the selective hydrogenation of resorcinol(RES)[9–11].In recent years,1,3-CHD is mainly produced by the latter way owing to its advantages,such as simplicity of operator,high product yield,mild reaction conditions and low cost.RES hydrogenation is generally carried out over conventional Raney Ni or activated carbon supported Pd catalyst(Pd/AC)in an alkaline aqueous solution[9,12–17].In this process,one phenolic hydroxyl group in RES is firstly neutralized with the alkali.After adding 1 mol hydrogen,a stable intermediate called enol 1,3-CHD is formed,and then enol 1,3-CHD is acidized by an acid to obtain the final product 1,3-CHD(Fig.1)[11,12,14,15,17].

Although a 1,3-CHD yield of more than 95%can be achieved by this alkaline hydrogenation process,it generates large amounts of waste water with high salinity due to the alternative use of alkali and acid,and also brings separation problems.Nevertheless,an alkaline atmosphere seems inevitable to achieve a high 1,3-CHD yield.Otherwise,the deep hydrogenation will occur,resulting in byproducts of 1,3-cyclohexanediol or m-hydroxyl cyclohexanone[18].Recently,this perception was altered by our group.We developed a new catalytic system with reduced graphene oxide(rGO)supported Pd(mains)as the catalyst and CH2Cl2as the solvent,by which a 1,3-CHD yield as high as 94.2%was achieved without adding any alkali[19].We proposed that the excellent selectivity of the catalyst system to 1,3-CHD was attributed to the strongπ–πand p–πinteractions between the graphene nanosheet and the benzene ring as well as hydroxyl in RES molecule.Although the speculation was indirectly confirmed by our adsorption experiments and by the Raman spectra determination,which showed that rGO exhibited stronger adsorption towards RES than towards 1,3-CHD and that the rGO surface had more aromatic graphite rings,it seems not feasible to quantitatively determine these π–π and p–π interactions through modern physics and chemical experiments.

Fig.1.Possible existence forms of resorcinol sodium salts during the hydrogenation process in an alkali solution.

In the presentwork,we are going to confirm our previous speculation through density functional theory(DFT)calculations and take deep insights into the effects of the graphene nanosheet and the solvent on the selectivity to 1,3-CHD through further experiments.Besides,the possible pathway for the hydrogenation of RES will also be discussed.

2.Methods

2.1.Experimental

2.1.1.Materials

All reagents were obtained from Sino-pharm Chemical Reagent Co.Ltd.,except PdCl2,which was purchased from Shanghai Jiuling Chemical Co.Ltd.

2.1.2.Preparation of graphene oxide(GO)

Graphene oxide was prepared from natural graphite powder by using a modified Hummers method consisting of two steps of oxidation.In the first pre-oxidation step,concentrated H2SO4(40 ml),K2S2O7(8.4 g)and P2O5(8.4 g)were added into a 500 ml round-bottom flask and maintained at 80°C for 4.5 h.Then the mixture was cooled down to room temperature,diluted with deionized water,left overnight,vacuum- filtered,and washed with deionized water(1.6 L)to obtain the pre-oxidized material.In the second oxidation step,concentrated H2SO4(230 ml)and the pre-oxidized material were added into a 1 L three-neck round-bottom flask and chilled to 0°C.KMnO4(60 g)was then added carefully under continuous stirring to keep the temperature below 10°C for 30 min.Afterwards,the temperature was gradually increased near to 35°C and maintained for 2 h.The mixture was diluted with deionized water(0.5 L)and stirred for 2 h,successively diluted with an additional deionized water(1.5 L),dropwisely added with H2O2(25 ml)and left for 4 days.The precipitate was washed with HCl(1 mol·L-1)and centrifuged for at least three recycles to remove residual metal oxides,and then washed several times with deionized water until the filtrate became neutral.Finally,the brown mixture dispersion in water was sonicated for 30 min,centrifuged,and freezedried to obtain the final GO.

2.1.3.Preparation of Pd/rGO

1 g of GO was dispersed in 200 ml of deionized water by sonication.52 mg of PdCl2was then added and the mixture was rigorously stirred for 1 h.After that,100 ml of NaBH4aqueous solution(7.5 g·L-1)was dropwisely added in and the mixture was incubated at 80°C for 4 h.The resulting Pd/rGO catalyst was then filtrated and dried under vacuum for use.The Pd content in the catalyst was determined to be 3.1 wt%by an Atomic Absorption Spectrometer(AAS,Agilent AA240).

2.1.4.Preparation ofPd/X(X=activated carbon(AC),multi-walled carbon nanotube(MWCNT),and SiO2)

Pd/X was prepared by wetness impregnation method.Taking Pd/AC as an example,1 g of AC was impregnated by PdCl2acidic aqueous solution containing 5 mg·ml-1of Pd(II).The impregnation process was repeated for several times till the calculated Pd content in the final catalyst reached ca.3 wt%.The solid was finally dried under vacuum at 120 °C and reduced by hydrogen at 350 °C for 4 h to obtain the final Pd/AC catalyst.The Pd contents in Pd/AC,Pd/MWCNT and Pd/SiO2were 3.1 wt%,3.0 wt%and 2.9 wt%,respectively,measured by AAS.

2.1.5.Typical procedure for hydrogenation of resorcinol

The hydrogenation of resorcinol was carried out in a 20 ml stainless batch reactor with a magnetic stirrer.In a typical procedure,resorcinol(0.027 mmol),catalyst(50 mg,ca.3 wt%of Pd),and 5 ml of CH2Cl2were added into the reactor.After purging 3 times with H2,the reactor was heated and pressurized with H2to the desired temperature and pressure to initiate the reaction.After the reaction,the reactor was placed in an ice water bath to quench the reaction.The products were analyzed by an Agilent 6820 GC equipped with HP-5 capillary column(30.0 m × 0.25 mm × 0.32 μm)and a flame ionization detector.N-Dodecane was used as an internal standard.

The structures of the reaction products were determined by an Agilent 6890-5973 GC–MS instrument.

2.2.Computational

In our calculation,the optimization of a single model was performed at MP2/6-31+G level[20]and the optimization of combined model of RES,1,3-CHD and rGO was performed at B3LYP/3-21+G level.The calculation of the single point energy of optimized single model and combined model was performed at B3LYP/6-311++G(d,p)level.The effect of the solvent was simulated with the Conductor-like Polarizable Continuum Model(CPCM).All the models mentioned above were built by GaussView 5.08[21],and the calculation was performed using Gaussian 09 program.

Using the wave function film after Geometry optimizations[22]outputted by Gaussian as the input film of Multiwfn 3.3.6 which was compiled by Lu Tian and Chen[23],with its Quantitative molecular surface analysis function,the surface electrostatic potential energy of the interaction between rGO and RES or 1,3-CHD was drawn with VMD[24].The Quantum Theory of Atoms In Molecules(QTAIM)topology analysis[25]was calculated with Multiwfn 3.3.6.

To reveal the reaction mechanism,the optimization of molecule structure,the research of transition state(TS)during RES hydrogenation reaction and the calculation of vibrate frequency were performed at B3LYP/6-31+G level[26,27]based on DFT[28–32].All the stationary points were identified with harmonic vibrational frequencies to ensure that the reactants and the products were with actual frequency whereas the transition states which could be checked with intrinsic reaction coordinate(IRC)calculation whose analytical step was 0.1 Bohr was with only one imaginary frequency.In addition,the single point energy was calculated at B3LYP/6-311+G(d)level.

3.Results and Discussion

3.1.Experimental confirmation

3.1.1.Effect of different supports

Pd manifested superior activity compared with Pt,Ru,Rh in our previous screening work,which was shown in Table A.1.The TEM image illustrated the high stability and dispersion of the Pd particles whose average diameter was merely 3.6 nm,which was shown in Fig.A.1.The Aseries of Pd catalysts supported on rGO,AC,MWCNT and SiO2were prepared and used for the selective hydrogenation of RES to 1,3-CHD,and the main by-products in our case were 3-hydroxycyclohexanone, cyclohexanone and trace cyclohexanol.The performance of the catalysts in a nonaqueous solution CH2Cl2is shown in Table 1(Entries 2,8,9 and 10).It can be seen that the Pd/rGO catalyst showed the highest performance among the catalysts used in this work for RES hydrogenation at the same reaction conditions,with RES conversion and 1,3-CHD selectivity as high as 99.9%and 94.2%,respectively.When using Pd/MWCNT as the catalyst,the conversion of RES could also reach 99.9%,but the selectivity to 1,3-CHD was 52%,lower than that in the case of Pd/rGO.Over Pd/AC and Pd/SiO2,the selectivity to 1,3-CHD was much lower,only 26.6%and 52.3%,respectively.Supports AC and SiO2had higher surface areas than rGO but showed lower selectivity.MWCNT had similar surface property to rGO,while the major difference was that the surface carbon hybrid in MWCNT was mixed with sp2 and sp3,and this structure made the π bond bend,which would increase the surface energy,leading to the decrease in its adsorption property of RES and 1,3-CHD.

3.1.2.Effect of different solvents

The effect of the different solvents on RES hydrogenation was investigated and the results are shown in Table 1.It can be clearly seen from the reaction results in Table 1 that the type of the solvents deeply influenced the reaction.In weak polar solvent CH2Cl2,all the catalysts used in this work showed high activity(Entries 2,8,9 and 10).Particularly,the Pd/rGO catalyst exhibited the highest performance,with 99.9%of a RES conversion and 94.2%of a selectivity to 1,3-CHD.In strong polar solvents,however,the yield of 1,3-CHD was very low regardless of using Pd/rGO or Pd/AC catalysts.In polar solvent CH3CN,the reaction of RES hydrogenation was hard to proceed(Entries 4 and 12),and the conversion of RES was only 8.2%and 3.9%over Pd/rGO and Pd/AC catalysts,respectively even at higher temperature and H2pressure(i.e.60°C and 1 MPa)than in weak polarsolvent CH2Cl2.Although RES conversion could be remarkably increased when raising the temperature and H2pressure to 150°C and 2 MPa(Entries 6 and 14),the selectivity to 1,3-CHD was still low.In other polar solvents 1,4-dioxane and tetrahydrofuran,the hydrogenation reaction was more difficult to proceed(Entries 3,4,11 and 12).In water,the conversion of RES was 46.8%and 39.7%and the selectivity to 1,3-CHD was 46.8%and 39.7%over Pd/rGO and Pd/AC,respectively,at 60°C and 1 MPa(Entries 7 and 15).

The reason why the strong polar solvents caused a negative effect on RES hydrogenation was possibly that the existence of polar solvent(for example acetonitrile)hindered the contact of the catalyst with the substrate,or the polar solvent had some interaction with the substrate or the surface functional group on the catalyst.Because of the hydroxyl group in RES,the solvation effect would be much stronger than the π–π interaction between RES and rGO support which significantly weakens the rGO's selective adsorption to RES.1,3-CHD,the product of hydrogenation,could not leave the catalyst surface rapidly,which led to the significant reduction in conversion and selectivity.The effect of the solvents will be further theoretically investigated in the following work.

Fig.2.The adsorption style of resorcinol and 1,3-cyclohexanedione on graphene nanosheet.

Table 1Hydrogenation of RES to CHD over different Pd-based catalysts and solvents

3.2.DFT calculations

3.2.1.Interaction calculation

The interaction between rGO and RES and 1,3-CHD was theoretically analyzed in order to clarify the catalytic mechanism of rGO supported Pd catalyst for RES hydrogenation.As can be seen from the optimized structure(Fig.2a),the two phenolic hydroxyl groups and the benzene ring in RES were in the same plane.When RES was in contact with rGO,the benzene ring of RES would possibly overlap with the structure of the π-band of rGO and be adsorbed in parallel position.Actually,due to the existence of hydroxyl group it's not exactly adsorbed in parallel position but with slight incline.As shown in Fig.2b,with the hydrogenation reaction proceeding,the plane structure of the substrate was broken,which weakened the adsorption capacity of the support rGO to the substrate.

Using the Quantitative molecular surface analysis function[22]in Multiwfn,the surface electrostatic potential energies between rGO and RES and 1,3-CHD,respectively,were calculated.The corresponding pictures were drawn by VMD[24]as shown in Fig.3.The red point in the picture represented the position of the electrostatic potential maxima whereas the blue one represented the minima.In the red area the electrostatic potential energy was positive while in the blue area it was negative.The interaction between the positive area and negative area could reduce the system energy and stabilize the system.It can be seen from Fig.3a that the distribution of electrostatic potential energy on RES surface was homogeneous,and RES had more overlapped area with rGO.The color of the area near the two carbonyl groups was dark blue which implied that this position was the extreme point of negative electrostatic potential energy.The overlapped area between 1,3-CHD and rGO was shrunk(Fig.3b),indicating that the interaction energy was weakened to a great extent.

Fig.3.Electrostatic surface potentials of resorcinol and 1,3-cyclohexanedione on graphene nanosheet.1 cal=4.1868J.

In topology analysis,the Critical Points(CPs)meant the points whose function gradient norm was 0(not include in finite).In general,the CPs could be divided into 4 categories:(3,-3),(3,-1),(3,+1)and(3,+3)on behalf of the points whose negative eigenvalue quantity ofthe“Hessian”matrix was“all”,2,1 and 0,respectively.In the theory of Bader,(3,-1)was in the chemical bond path or between the atompairs with weak attractive interaction,called Bond Critical Point(BCP).Its value of ρ and the sign of Δ2were closely relative with bond strength and bond type[25].The entire topology path between(3,-3)and(3,+3)was found by using the Newton method.Comparing the quantity of BCP,it could be seen that the bonding number between RES and graphene model was “7”,but the bonding number between 1,3-CHD and graphene was only “3”,indicating that the interaction between RES and graphene was stronger(Fig.4),consistent with the previous conclusion.

Fig.4.Topology analysis of the interactions between resorcinol,1,3-cyclohexanedione and graphene nanosheet.

To verify the above conclusion,after calculating the single point energy of optimized single models and combined models,the interaction energy between the substrate and the support was calculated with the following formula(1):

The computed results are shown in Table 2.The atomunit“a.u.”was equal to 1 Hartree,and 1 Hartree=2625.5 kJ·mol-1.Consequently,there were 4 decimal places when using atom unit so as to improve the data precision,corresponding to the literature[33].

Table 2Interaction energy between substrate and graphene

It could be obviously seen from Table 2 that the interaction energy between RES and rGO was much stronger than that between 1,3-CHD and rGO.With the production of 1,3-CHD in RES hydrogenation reaction,the interaction force between RES and rGO was weakened.It well explained the results of RES hydrogenation with high selectivity to 1,3-CHD when using rGO as the catalyst support.

3.2.2.Effect of solvents

By using the Conductor-like Polarizable Continuum Model(CPCM),the solvation effect was simulated in the cases of CH2Cl2,CHCN3and H2O,respectively.After optimizing all the single models and combined models in CPCM fields,the single point energy was calculated in B3LYP/6-311++G(d,p)level.Then the solvation free energy was calculated with the following formula(2)[34,35]:

In formula(2),ΔEsolvis the solvation free energy,Esolnis the interaction energy between the substrate and rGO with the solvent,and Egasis the interaction energy between the substrate and rGO in the ideal state(0 K,vacuum)which is the same as ΔE in formula(1).

Comparing the ΔEsolvcalculated in Table 3,it could be clearly observed that when using weak polar nonprotic solvent CH2Cl2,the solvation free energy made less impact on the interaction energy of rGO and the substrate,but the impact of polar protic solvent was much stronger,in accordance with the reaction results.The reason was that polar solvent had strong hydrogen-bond interaction with RES and 1,3-CHD which disturbed the π–π interaction and led to the decrease in the activity and selectivity of the catalyst.

3.2.3.Simulation of the hydrogenation route

The environment of hydrogenation in liquid phase wasso complicated that it was hard to obtain the 100%yield of 1,3-CHD which generated in accompany with the by products including dehydration productcyclohexanone,hydrogenation product hydroxycyclohexanone and cyclohexanol.The mechanism of RES hydrogenation was speculated as Fig.5.

All the molecule energies compared to RES are shown in Fig.6.It could be seen that RES was added one hydrogen molecule to form the enolstructure,and became 1,3-CHDin tautomerization reaction passing through transition state 2(TS-2)and transition state 3(TS-3).In addition,1,3-CHD was added two hydrogen molecules to form 3-hydroxycyclohexan-1-one and cyclohexanediol step by step.As can be seen from the energy value,1,3-CHD was the most stable structure and transition state 1(TS-1)had the highest energy,indicating that the first step of hydrogenation reaction needs large amounts of energy which is the rate-determining step.In practice,the hydrogenation process consists of H2adsorption,H2dissociation and two-stage hydrogenation in which the energy barrier is very high.This can also account for the reason why TS-1 had the highest energy.It's common to introduce homogeneous process to search for transition state by one-step hydrogenation which is in agreement with references[36–38].The Intermediate-1&2 and 1,3-CHD had similar energy values,which indicated that this step could be reversible under certain conditions.On the other hand,there're two reasons leading the product to converting deep hydrogenation byproducts:One is that there are a few oxygen containing functional groups on the catalyst surface;the other is that 1,3-CHD has more opportunities to contact the support when its concentration increased in the reaction.

4.Conclusions

The reaction mechanism and the catalytic mechanism of the Pd/rGO catalyst for RES hydrogenation to 1,3-CHD were revealed through experimental and theoretical investigations.The results showed that when RES got close to the rGO surface,the π–π interaction was formed.On Pd active centers,1 mol of hydrogen was added to RES and the product conversed to 1,3-CHD by tautomerism.Due to the disappearanceof big π bond,the interaction between 1,3-CHD and rGO was weakened and 1,3-CHD was easy to leave the support surface,which led to high selectivity to 1,3-CHD.At the same time,RES hydrogenation reaction was obviously affected by the polarity of the solvents.In weak polar aprotic solvent,solvation free energy had less impact on the interaction between the catalyst surface and RES and 1,3-CHD.In polar aprotic and polar protic solvents,however,the influence of solvation free energy was much stronger,which made RES conversion and 1,3-CHD selectivity decrease.Additionally,the reaction pathway of RES hydrogenation was simulated and the results showed that the first step of RES hydrogenation needed much energy,which was the rate-determining step and that the most stable structure was 1,3-CHD,which was formed by tautomerism.

Table 3The effect of solvents on the interaction energy

Fig.5.Pathways of the hydrogenation of resorcinol.

Fig.6.The variation of relative molecular energies during hydrogenation of resorcinol.

Table A.1
Hydrogenation of resorcinol by various noble metal catalysts

Entry Catalyst Reaction time/h Conversion/% 1,3-CHD selectivity/%1 Pd/AC(Pd:5 wt%) 3 82 39 2 Pt/AC(Pd:5 wt%) 2 83 33 3 Ru/AC(Pd:5 wt%) 3 ＜10 ＜10 4 Rh/AC(Ru:5 wt%) 3 12 35

Reaction condition:T=30°C,P=0.1 MPa,RES 0.027 mmol,solvent 5 ml,catalyst 30 mg,metal loadings 5%.

Fig.A.1.TEM image and particle size distribution of 5 wt%Pd/rGO.