energies

Article Selective of to over Ni/CNT in the Absence of External Hydrogen

Changzhou Chen 1,2, Peng Liu 1,2, Minghao Zhou 1,2,3,*, Brajendra K. Sharma 3,* and Jianchun Jiang 1,2,*

1 Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry, Key Laboratory of Biomass Energy and Material, Jiangsu Province, National Engineering Laboratory for Biomass Chemical Utilization, Key and Open Laboratory on Forest Chemical Engineering, SFA, Nanjing 210042, China; [email protected] (C.C.); [email protected] (P.L.) 2 Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University, Nanjing 210037, China 3 Illinois Sustainable Technology Center, Prairie Research Institute, one Hazelwood Dr., Champaign, University of Illinois at Urbana-Champaign, Champaign, IL 61820, USA * Correspondence: [email protected] (M.Z.); [email protected] (B.K.S.); [email protected] (J.J.)

 Received: 7 January 2020; Accepted: 11 February 2020; Published: 14 February 2020 

Abstract: Transfer hydrogenation is a novel and efficient method to realize the hydrogenation in different chemical reactions and exploring a simple heterogeneous catalyst with high activity is crucial. Ni/CNT was synthesized through a traditional impregnation method, and the detailed physicochemical properties were performed by means of XRD, TEM, XPS, BET, and ICP analysis. Through the screening of loading amounts, solvents, reaction temperature, and reaction time, 20% Ni/CNT achieves an almost complete conversion of phenol after 60 min at 220 ◦C in the absence of external hydrogen. Furthermore, the catalytic system is carried out on a variety of phenol derivatives for the generation of corresponding with good to excellent results. The mechanism suggests that the hydrogenation of phenol to cyclohexanone is the first step, while the hydrogenation of cyclohexanone for the generation of cyclohexanol takes place in a successive step. Moreover, Ni/CNT catalyst can be magnetically recovered and reused in the next test for succeeding four times.

Keywords: phenol; hydrogenation; Ni/CNT; cyclohexanol; transfer hydrogenation

1. Introduction Cyclohexanol is an important chemical raw materials, for example, it could be used as the main intermediate for the production of , hexamethylene diamine, cyclohexanone, and , or as an excellent solvent for rubber, resin, nitro fiber, and metal soap [1–5]. Except for this, its downstream product, , can also be converted into various useful chemicals [6–8]. Hence, the production of cyclohexanol has attracted increasing attention, both in industry and in the laboratory, recently. In the past several years, tremendous efforts have been devoted to the hydrogenation of phenol for the generation of cyclohexanol. The main pathways can be classified into two groups, gaseous [9] and liquid phase [10]. In the gaseous phase, a variety of supported noble metals, such as Pd [11,12], Pt [13,14], Ru [15,16], Rh [17,18], have been reported to be effective in the hydrogenation system. For example, Wang et al. reported a hierarchically porous ZSM-5 zeolite with micropore and b-axis-aligned mesopore-supported Ru nanoparticles (Ru/HZSM-5-OM), which were highly effective for the hydrogenation of both phenol and its derivatives to the corresponding cyclohexane [15]. However, elevated temperature and high pressure of hydrogen (150 ◦C, 4 MPa) were required, and carbonaceous deposits in the process would result in the deactivation of catalyst. In the liquid phase, although

Energies 2020, 13, 846; doi:10.3390/en13040846 www.mdpi.com/journal/energies Energies 2020, 13, 846 2 of 12 the hydrogenation process could be performed under a relatively lower temperature [19], the main shortcoming of recent hydrogenation systems is that they could not avoid the presence of hydrogen [20]. For instance, Li et al. developed a series of Pd@FDU-N catalysts for the hydrogenation of phenol in the presence of 0.1 MPa H2 [21]. Therefore, the exploration of a cheap and effective catalyst for the hydrogenation of phenol in the absence of hydrogen is essential. Catalytic transfer hydrogenation (CTH) has been regarded as a good alternative to avoid the application of high-pressure hydrogen [22–25]. Galkin et al. reported that Pd/C could catalyze transfer hydrogenolysis of β-O-4 model compound in lignin employing formic acid as a hydrogen-donor for the generation of acetophenone and phenol derivatives [22]. Paone et al. reported the transfer hydrogenolysis of α-O-4 model compound in lignin conducted on Pd/Fe3O4 under 240 ◦C[23]. Wu et al. found that Ru/C could efficiently catalyze the cleavage of the 4-O-5 aromatic ether bond in a variety of lignin-derived compounds through the transfer hydrogenolytic pathway using isopropanol as the hydrogen-donor solvent [24]. Despite those achievements for the transfer hydrogenolytic transformation of biomass to value-added chemicals, the catalytic hydrogenation without the use of external hydrogen remains a great challenge [25,26]. Carbon nanotubes (CNT) are interesting materials for carrying out chemical reactions in confined spaces, and several organic reactions have been carried out [27–29] inside the nanotubes. In this work, CNT supported nickel catalyst was introduced into the hydrogenation of phenol for the generation of cyclohexanol via a transfer hydrogenolytic route. Through the screening of loading amounts, solvents, temperature and time, herein we reported a mild reaction condition (20% Ni/CNT, iPrOH, 220 ◦C and 60 min) for the hydrogenation of phenol to cyclohexanol. A variety of phenol derivatives were also conducted to verify the high efficiency of this catalytic reaction system. The possible reaction mechanism was finally investigated as well. The detailed physicochemical properties were investigated by means of XRD, TEM, XPS, BET, and ICP analysis.

2. Experimental Materials: CNT was purchased from Aladdin Industrial Inc. Shanghai, China and pre-treated in HNO3 before use [27,28]. Ni(NO3)2.6H2O was provided by Aladdin Industrial Inc. Shanghai, China. Phenol and other derivatives were obtained from tansoole.com. All chemicals were obtained from commercial sources and used without further purification. General procedure for Ni/CNT catalyzed phenol: In one typical catalytic reaction process, 500 mg of phenol and other derivatives, 50 mg of Ni/CNT catalyst, and 10 mL isopropanol were placed in a 25 mL stainless steel reactor. After being sealed, the catalytic reaction was stirred at desired temperature for desired time. The reactor was naturally cooled to room temperature after the reaction. The mixture solution was filtered to collect the catalyst and the filtrate was analyzed by the Gas Chromatograph/Mass Spectrometer (GC/MS, Agilent 7890) utilizing n-dodecane as an internal standard. The collected catalyst was washed with isopropanol three times and dried at 105 ◦C for the next test under the optimal reaction conditions. The conversion and product yields in the liquid phase were calculated according to the following formula, respectively:

mole o f reacted substrate Conversion = 100% total mole o f substrate f eed ×

mole o f cyclohexanone Yield o f cyclohexanone = 100% total mole o f substrate f eed × mole o f cyclohexanol Yield o f cyclohexanol = 100% total mole o f substrate f eed × Catalyst preparation: Ni/CNT was prepared by an impregnation method. In one typical process of 20% Ni/CNT catalyst, Ni(NO3)2.6H2O (0.2 g) was dissolved in 30 mL of deionized water. Then, CNT (2.0 g) was added to the above solution and stirred for 24 h until a uniform dispersion. The obtained suspension was dried for 12 h at 105 ◦C in the oven. Subsequently, the obtained black Energies 2020, 13, 846 3 of 12 Energies 2020, 13, 846 3 of 13

hydrogen atmosphere at 500 °C for 2 h. The preparation of Fe/CNT, Co/CNT, Mo/CNT was the same solid was calcined at 500 ◦C for 2 h in the muffle furnace, and finally reduced in a tube furnace under as Ni/CNT. hydrogen atmosphere at 500 ◦C for 2 h. The preparation of Fe/CNT, Co/CNT, Mo/CNT was the same Catalyst characterization: Powder X-ray diffraction (XRD) was examined on a Bruker D8 as Ni/CNT. Advance X-ray powder diffractometer. Transmission electron microscopy (TEM) images were tested Catalyst characterization: Powder X-ray diffraction (XRD) was examined on a Bruker D8 Advance using a TEM Tecnai G2 20. The X-ray photoelectron spectroscopy (XPS) was carried out on an X-ray powder diffractometer. Transmission electron microscopy (TEM) images were tested using a ESCALAB-250 (Thermo-VG Scientific, USA) spectrometer with Al Kα (1486.6 eV) irradiation source. TEM Tecnai G2 20. The X-ray photoelectron spectroscopy (XPS) was carried out on an ESCALAB-250 The textural properties of the Ni/CNT catalyst were tested by N2 adsorption–desorption isotherms (Thermo-VG Scientific, USA) spectrometer with Al Kα (1486.6 eV) irradiation source. The textural using a COULTER SA 3100 analyzer, and the Brunauer–Emmett–Teller (BET) surface area was properties of the Ni/CNT catalyst were tested by N2 adsorption–desorption isotherms using a COULTER evaluated by the N 2 adsorption–desorption isotherms. SA 3100 analyzer, and the Brunauer–Emmett–Teller (BET) surface area was evaluated by the N 2 adsorption–desorption isotherms. 2.1. Catalyst Characterization 2.1. CatalystFigure Characterization 1 shows the XRD patterns of two different samples, including solo CNT, and the optimal catalystFigure in1 theshows system the XRDfor the patterns hydrogenation of two di ffoferent phenol. samples, The peak including at 26.5 solo° could CNT, be and assigned the optimal to the diffraction peaks of the (002) planes of the graphite-like tube-wall of the CNT [30–32]. Apart from the catalyst in the system for the hydrogenation of phenol. The peak at 26.5◦ could be assigned to the diCNTffraction feature, peaks the of presence the (002) of planes metallic of the Ni graphite-like in the reduced tube-wall 20% Ni/CNT of the CNTcatalyst [30 –was32]. clearly Apart from revealed. the CNTAs could feature, be theclearly presence seen from of metallic the XRD Ni inresults, the reduced only one 20% kind Ni /ofCNT Ni- catalystbased phase was clearlywas observed revealed. for AsNi/CNT could becatalyst. clearly Peaks seen at from 44.5 the°, 51.9 XRD°, and results, 76.4° onlycould one be assigned kind of Ni-based to the diffraction phase was of the observed (111), (200), for and (220) planes of metallic Ni [27], indicating that only metallic nickel species were observed in Ni/CNT catalyst. Peaks at 44.5◦, 51.9◦, and 76.4◦ could be assigned to the diffraction of the (111), (200), andNi/CNT (220) catalyst. planes of metallic Ni [27], indicating that only metallic nickel species were observed in Ni/CNT catalyst.

FigureFigure 1. 1.Characterization Characterization of of the the 20% 20% Ni Ni/CNT/CNT catalyst. catalyst. (a) (a) XRD XRD pattern pattern of of CNT, CNT, (b) (b) XRD XRD pattern pattern of of 20%20% Ni Ni/CNT./CNT.

XPSXPS detection detection was was carried carried out to out analyze to analyze the composition the composition of prepared of 20%prepared Ni/CNT. 20% The Ni/CNT. absorption The peaksabsorption were identified peaks were for identified C, O, and Ni.for AsC, O, for and the presenceNi. As for of the oxygen presence element of oxygen in Figure element2b, perhaps in Fig iture was due to the presence of oxygen-containing groups of CNT during the HNO3 pre-processing [30,31]. 2b, perhaps it was due to the presence of oxygen-containing groups of CNT during the HNO3 pre- Inprocessing addition, the [30,31 oxidation]. In addition,of Ni/CNT t catalysthe oxidation before orof during Ni/CNT the catalyst XPS examination before or could during also the lead XPS to theexamination presence of could oxygen also element. lead to Thethe presence binding energiesof oxygen at 852.9element and. T 856.3he binding eV were energies observed at 852.9 for 20% and Ni/CNT (Figure2a), which corresponded to Ni 0 (2p 3/2) and Ni2+(2p3/2), respectively. The binding 856.3 eV were observed for 20% Ni/CNT (Figure 2a), which corresponded to Ni0 (2p 3/2) and energies at 872.3 and 874.8 eV corresponded to the main lines of Ni0 (2p 1/2) and Ni2+ (2p 1/2). It could Ni2+(2p3/2), respectively. The binding energies at 872.3 and 874.8 eV corresponded to the main lines be found that part of metallic Ni was oxidized before or during the XPS testing, which contributed to of Ni0 (2p 1/2) and Ni2+ (2p 1/2). It could be found that part of metallic Ni was oxidized before or the understanding of Ni2+ detection in the XPS spectra. during the XPS testing, which contributed to the understanding of Ni2+ detection in the XPS spectra.

Energies 2020, 13, 846 4 of 13

Energies 2020, 13, 846 4 of 12 Energies 2020, 13, 846 4 of 13

Figure 2. Characterization of the 20% Ni/CNT catalyst. (a) XPS of Ni 2p, (b) XPS pattern of 20% Ni/CNT. Figure 2. 2. CharacterizationCharacterization of of the the 20% 20% Ni /CNT Ni/CNT catalyst. catalyst. (a) XPS (a) of XPS Ni of2p, Ni (b ) 2p, XPS (b pattern) XPS of pattern 20% Ni of/CNT. 20% As can be seen in Figure 3, most of Ni particles displayed outside the CNT uniformly. However, Ni/CNT. the AsNi canparticle be seen size in and Figure its distribution3, most of Ni varied particles from displayed each other outside mainly the owing CNT uniformly.to the loading However, amount theof NiNiAs particleon can the be surface sizeseen andin of Fig itsCNT.ure distribution 3, We most selected of variedNi 20%particles from Ni/CNT display each catalyst othered outside mainly as an example.the owing CNT to uniformly.The the loadinghistogram However, amount of 20% oftheNi/CNT Ni Ni on particle the in surfacegiven size in and of Fig CNT. itsure distribution 3 Wea, in selected which varied 20%the mean Ni from/CNT size each catalyst was other 14.0 asmainly nm. an example. Taken owing togethe to The the histogram r,loading nanoparticle amount of 20% Ni Niofon /NiCNT the on insurfacethe given surface of in CNT Figureof CNT. performed3a, We in whichselected an theexcellent 20% mean Ni/CNT sizeactivity was catalyst in 14.0 the nm.as hydrogenation an Taken example. together, The of histogramphenol nanoparticle under of 2 Ni 0our% onNi/CNTreaction the surface in system. given of in CNT Figure performed 3a, in which an excellent the mean activity size was in 14.0 the hydrogenationnm. Taken togethe of phenolr, nanoparticle under our Ni reactionon the surface system. of CNT performed an excellent activity in the hydrogenation of phenol under our reaction system.

Figure 3. Characterization of the 20% Ni/CNT catalyst. (a) TEM image of 20% Ni/CNT, (b) Distribution Figure 3. Characterization of the 20% Ni/CNT catalyst. (a) TEM image of 20% Ni/CNT, (b) of Ni particles. Distribution of Ni particles. TableFigure1 show 3. Characterization the chemical of and the physical 20% Ni/CNT properties catalyst. for the (a) di TEMfferent image ratios of 20% of Ni Ni/CNT,/CNT catalysts, (b) Table 1 show the chemical and physical properties for the different ratios of Ni/CNT catalysts, includingDistribution the Ni loadingof Ni particles. amounts, BET surface area, and average Ni-particle size. The ICP results prove including the Ni loading amounts, BET surface area, and average Ni-particle size. The ICP results that the metal loading amounts were in great accordance with the theoretical calculating value with the prove that the metal loading amounts were in great accordance with the theoretical calculating value preparationTable 1 ofshow the catalysts.the chemical The and BET physical surface areaproperties of 5% Nifor/ CNTthe different catalyst ratios was much of Ni/CNT larger thancatalysts, 25% with the preparation of the catalysts. The BET surface area of 5% Ni/CNT catalyst was much larger Niincluding/CNT (range the Ni from loading 149.3 mamounts,2/g to 120.5 BET m 2surface/g), suggesting area, and that average Ni particles Ni-particle could besize. better The dispersed ICP results on than 25% Ni/CNT (range from 149.3 m2/g to 120.5 m2/g), suggesting that Ni particles could be better theprove surface that the of CNT. metal Owing loading to amounts the different were loading in great amounts accordance of Ni with on the the CNT, theoretical the average calculating particle value size dispersed on the surface of CNT. Owing to the different loading amounts of Ni on the CNT, the ofwith Ni /theCNT preparation with different of the metal catalysts amounts. The ranged BET surface from 12.1 area to of 15.6 5% nm Ni (estimated/CNT catalyst from was XRD much in Table larger1). average particle size of Ni/CNT with different2 metal2 amounts ranged from 12.1 to 15.6 nm (estimated Itthan proved 25% thatNi/CNT excessive (range metal from loading 149.3 m on/g CNT to 120 could.5 m inevitably/g), suggesting lead to that metal Ni agglomeration,particles could resultingbe better from XRD in Table 1). It proved that excessive metal loading on CNT could inevitably lead to metal indispersed an increase on ofthe the surface metal particleof CNT. size Owing [32]. to The the TEM different also gave loading the same amounts results of onNi the on distributionthe CNT, the of agglomeration, resulting in an increase of the metal particle size [32]. The TEM also gave the same Niaverage particles particle (Table size1). of Hence, Ni/CNT these with changes different in catalyst metal amount propertiess ranged would from directly 12.1 influence to 15.6 nm the (estimated catalytic results on the distribution of Ni particles (Table 1). Hence, these changes in catalyst properties would activityfrom XRD inthe in Table hydrogenation 1). It proved of phenol, that excessive which will metal be discussedloading on in CNT detail could in the inev followingitably le experiments.ad to metal agglomeration, resulting in an increase of the metal particle size [32]. The TEM also gave the same results on the distribution of Ni particles (Table 1). Hence, these changes in catalyst properties would

Energies 2020, 13, 846 5 of 13 directly influence the catalytic activity in the hydrogenation of phenol, which will be discussed in detail in the following experiments.

Table 1. Chemical and physical properties of CNT-supported Ni catalysts.

Composition (wt.%) a SBET b Average Metal Average Metal Catalysts 2 c d Energies 2020, 13, 846 Ni (m /g) Size (nm) Size (nm) 5 of 12 CNT / 180.6 / / 5% Ni/CNT 4.32 149.3 12.1 12.8 10% Table 1. Chemical and physical properties of CNT-supported Ni catalysts. 10.55 145.6 12.6 13.1 Ni/CNT Composition (wt.%) a b SBET Average Metal Average Metal 15%Catalysts 2 c d 14.66Ni 135.(m 8/g) Size12.8(nm) Size (nm)13.5 Ni/CNT 20% CNT / 180.6 // 5% Ni/CNT19.68 4.32 132. 149.34 12.113.1 12.814.0 Ni/10%CNT Ni /CNT 10.55 145.6 12.6 13.1 25% 15% Ni/CNT24.09 14.66 12 135.80.5 12.815.6 13.516.5 Ni/20%CNT Ni /CNT 19.68 132.4 13.1 14.0

a—25%measured Ni/CNT by ICP analysis 24.09, b—evaluated from 120.5N2 adsorption-desorption 15.6 isotherms, c 16.5—estimated a b by—measured XRD, d-measured by ICP analysis, by TEM—evaluated. from N2 adsorption-desorption isotherms, c—estimated by XRD, d—measured by TEM. 2.2. Activity of Various Catalysts for the Hydrogenation of Phenol 2.2. Activity of Various Catalysts for the Hydrogenation of Phenol First of all, hydrogenation of phenol was chosen as a typical model reaction to explore the catalyticFirst performance of all, hydrogenation over a variety of phenolof catalysts was (Table chosen 2). as When a typical carried model out without reaction any to explorecatalysts the in thecatalytic hydrogenolytic performance system, over the a variety reaction of failed catalysts to transform (Table2). Whenphenol carried to cyclohexanol out without (Table any 2, catalysts entry 1). Meanwhile,in the hydrogenolytic solo CNT system,also show theed reaction a poor failedactivity to transformunder our phenolreaction to condition cyclohexanol (Table (Table 2, entry2, entry 2), 1).indicating Meanwhile, that CNT solo CNTonly se alsorved showed as a support, a poor activityand show undered no our catalytic reaction effect condition in the catalytic (Table2, entryprocess. 2), Thenindicating, we tried that CNT different only servedcatalysts as for a support, the transformation and showed ofno phenol, catalytic including effect in the 10% catalytic Fe/CNT, process. 10% Co/CNT,Then, we tried 10% di Mo/CNTfferent catalysts and 10% for Ni/CNT the transformation (Table 2, entries of phenol, 3–5 including and entry 10% 7). FeIt /CNT, was surprising 10% Co/CNT, to 10%discover Mo/ CNTthat the and supported 10% Ni/CNT Ni catalyst (Table2 exhibited, entries 3–5 good and performance entry 7). It wasin the surprising hydrogenation to discover of phenol that tothe generate supported cyclohexanol Ni catalyst, indicating exhibited tha goodt the performance substrate phenol in the adsorbed hydrogenation on the ofmetal phenol surface to generate prior to thecyclohexanol, reaction decreased indicating the that activation the substrate barrier phenol of hydrogenation adsorbed on the reaction metal surface[33]. The prior catalytic to the reactionactivity coulddecreased be sorted the activation as follows: barrier 10% Ni/CNT of hydrogenation > 10% Co/CNT reaction > 10% [33 ].Mo/CNT The catalytic > 10% activity Fe/CNT. could However, be sorted the lasoading follows: amount 10% Ni of/ CNTNi on> the10% surface Co/CNT of> CNT10% Mocould/CNT also> 10%affect Fe the/CNT. efficiency However, of the loadinghydrogenolytic amount process.of Ni on It the could surface be clearly of CNT seen could in alsoTable aff 2ect that the the effi conversionciency of the increased hydrogenolytic with the process. increased Itcould loading be amountclearly seen of Ni in Table(from2 5%that to the 25%), conversion and the increased trend of withthe cyclohexanol the increased loadingyield show amounted the of Nisame (from result. 5% Meanwhile,to 25%), and we the found trend that of the the cyclohexanol yield of cyclohexanol yield showed reached the 95% same when result. 20% Meanwhile, Ni/CNT was we used found in thatour hydrogenolyticthe yield of cyclohexanol system (Table reached 2, entries 95% when 6–10). 20% On Nithe/CNT base wasof the used results in our above, hydrogenolytic 20% Ni/CNT system was chosen(Table2 ,as entries the most 6–10). suitable On the catalyst base of for the the results subsequent above, 20%exploration. Ni /CNT was chosen as the most suitable catalyst for the subsequent exploration. Table 2. Optimization of catalysts for the hydrogenation of phenol. a. Table 2. Optimization of catalysts for the hydrogenation of phenol a.

b T.( C)/ Yieldb (%) Entry Catalyst T.(◦ °C)/ Con. (%) Yield (%) Entry Catalyst t.(h) Con. (%) t.(h) 2a 2a3a 3a 11 nonenone 180180/4/4 00 0 00 0 22 CNTCNT 180180/4/4 00 0 0 3 10% Fe/CNT 180/4 8 2 0 3 10% Fe/CNT 180/4 8 2 0 4 10% Co/CNT 180/4 48 5 5 54 10%10% Mo Co/CNT/CNT 180180/4/4 4 278 5 45 22 6 5% Ni/CNT 180/4 73 3 66 7 10% Ni/CNT 180/4 81 4 68 8 15% Ni/CNT 180/4 95 3 88 9 20% Ni/CNT 180/4 100 2 95 10 25% Ni/CNT 180/4 100 2 94

a Reaction conditions: 1a (500 mg), catalyst (50 mg), Isopropanol (10 mL). b The conversion and yield were determined by GC/MS with n-dodecane as the internal standard. Energies 2020, 13, 846 6 of 13

5 10% Mo/CNT 180/4 27 4 22 6 5% Ni/CNT 180/4 73 3 66 7 10% Ni/CNT 180/4 81 4 68 8 15% Ni/CNT 180/4 95 3 88 9 20% Ni/CNT 180/4 100 2 95 10 25% Ni/CNT 180/4 100 2 94 a Reaction conditions: 1a (500 mg), catalyst (50 mg), Isopropanol (10 mL). b The conversion and yield were determined by GC/MS with n-dodecane as the internal standard.

2.3. Influence of Hydrogen-Donor Solvents Energies 2020, 13, 846 6 of 12 Alcohol, as an excellent hydrogen-donor solvent, plays an important role in the catalytic transfer hydrogenolytic2.3. Influence process. of Hydrogen-Donor Therefore, Solventswe selected three most common alcohols in our catalytic system. Unfortunately, methanol and ethanol failed the transformation of phenol to cyclohexanol (Table 3, Alcohol, as an excellent hydrogen-donor solvent, plays an important role in the catalytic transfer entries 2hydrogenolytic–3). H2O was process. considered Therefore, as we a greenselected and three environmentally most common alcohols friendly in our catalytic solvent, system. and always applied inUnfortunately, various chemical methanol reaction and ethanols. However, failed the 20% transformation Ni/CNT ofcatalyst phenol show to cyclohexanoled a poor (Table activity3, in the hydrogenationentries 2–3).of phenol H2O wasin aqueous considered medi as aa green (Table and 3, environmentallyentry 4). When friendly isopropanol solvent, was and utilized always in our catalytic appliedsystem, in a various satisfactory chemical result reactions. was However,achieved 20% that Ni cyclohexanol/CNT catalyst showed was obtained a poor activity in the in yield the of 95% (Table 3,hydrogenation entry 1). Therefore, of phenol in isopropanol aqueous media was (Table the3, entry most 4). efficient When isopropanol hydrogen was-donor utilized solvent in our for the catalytic system, a satisfactory result was achieved that cyclohexanol was obtained in the yield of transformatio95% (Tablen of3 phenol, entry 1). to Therefore, cyclohexanol. isopropanol was the most e fficient hydrogen-donor solvent for the transformation of phenol to cyclohexanol. Table 3. Optimization of solvents for the hydrogenation of phenol. a. Table 3. Optimization of solvents for the hydrogenation of phenol a.

b T.( C)/ Yield (%) Entry Solvent T.(◦ °C)/ Con. (%) Yield (%) b Entry Solvent t.(h) Con. (%) 2a 3a t.(h) 2a 3a 1 isopropanol 180/4 100 2 95 21 isopropanol methanol 180180/4/4 10 150 2 3 95 6 32 methanol ethanol 180180/4/4 1315 3 2 6 5 4 H O 180/4 5 0 0 3 ethanol2 180/4 13 2 5 a Reaction conditions: 1a (500 mg), 20% Ni/CNT (50 mg), Solvent (10 mL). b The conversion and yield were 4 H2O 180/4 5 0 0 determined by GC/MS with n-dodecane as the internal standard. a Reaction conditions: 1a (500 mg), 20% Ni/CNT (50 mg), Solvent (10 mL). b The conversion and yield were2.4. determined Influence of by Reaction GC/MS Temperature with n-dodecane and Reaction as Timethe internal standard. In addition, reaction temperature and reaction time were important key factors for the 2.4. Influencetransformation of Reaction of Temperature phenol (Figure and4). As Reaction shown inTime Figure 4a, the conversion of phenol progressively increased when the reaction temperature increased from 160 to 220 C, and phenol could almost In addition, reaction temperature and reaction time were ◦ important key factors for the totally transfer to cyclohexanol at 220 ◦C with a reaction time of 60 min. In the catalytic process, the transformationdominant of products phenol were (Figure cyclohexanol, 4). As shown with a small in Figure amount 4a, of cyclohexanonethe conversion detected of phenol in the GCprogressively/MS. increasedAs when the temperature the reaction continued tem toperature rise, the yield increased of cyclohexanol from 160 kept aroundto 220 95%.°C, Subsequently,and phenol it could could almost totally transferbe found to that cyclohexanol the conversion at of 220 phenol °C increasedwith a reaction with the prolongingtime of 60 of min. the reaction In the time catalytic (Figure process,4b), the dominantand products the phenol were was completely cyclohexanol consumed, with in only a small60 min at amount 220 ◦C. With of cyclohexanone the prolonged reaction detected time, in the the yield of cyclohexanol increased when the reaction time increased from 20 to 60 min. When the GC/MS. As the temperature continued to rise, the yield of cyclohexanol kept around 95%. reaction time exceeded 60 min, the conversion of phenol achieved 100%, and the yield of cyclohexanol Subsequently,kept at it the could highest be point. found Through that the the conversion screening of temperatureof phenol increased and time of with the hydrogenation the prolonging of of the reaction phenol,time (Figure the optimal 4b), temperature and the phenol and time was could completely be summarized consumed as 220 ◦C in and only 60 min, 60 min respectively. at 220 °C. With the prolonged reaction time, the yield of cyclohexanol increased when the reaction time increased from 20 to 60 min. When the reaction time exceeded 60 min, the conversion of phenol achieved 100%, and the yield of cyclohexanol kept at the highest point. Through the screening of temperature and time of the hydrogenation of phenol, the optimal temperature and time could be summarized as 220 °C and 60 min, respectively.

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FigureFigure 4. 4.Influence Influence ofof reactionreaction temperaturetemperature ( a(a)) and and time time ( b(b)) on on the the hydrogenation hydrogenation of of phenol. phenol. ReactionReaction conditions:conditions: PhenolPhenol (500(500 mg),mg), 20%20% NiNi/CNT/CNT (50(50 mg),mg), isopropanolisopropanol (10(10 mL);mL); reactionreaction time,time, 6060 minmin (for(for a);a); reactionreaction temperature, temperature, 220 220◦ °CC (for (for b). b). 2.5. Scope of the Substrates 2.5. Scope of the Substrates Encouraged by the perfect performance of the efficient hydrogenation of phenol, a variety of phenol Encouraged by the perfect performance of the efficient hydrogenation of phenol, a variety of derivatives were tested on 20% Ni/CNT. As shown in Table4, phenol could be completely transformed phenol derivatives were tested on 20% Ni/CNT. As shown in Table 4, phenol could be completely to afford the corresponding cyclohexanol in a yield of 95% without additional hydrogen under the transformed to afford the corresponding cyclohexanol in a yield of 95% without additional hydrogen optimal reaction condition (Table4, entry 1). Methylphenol derivatives gave methylcyclohexanols under the optimal reaction condition (Table 4, entry 1). Methylphenol derivatives gave with a poor conversion and yield under 220 ◦C at a reaction time of 60 min. Therefore, we increased the methylcyclohexanols with a poor conversion and yield under 220 °C at a reaction time of 60 min. temperature from 220 ◦C to 240 ◦C. As excepted, methylphenol derivatives gave methylcyclohexanols Therefore, we increased the temperature from 220 °C to 240 °C. As excepted, methylphenol with both perfect conversion and yield (Table4, entries 2–4). Hydroxy substituted phenol could not derivatives gave methylcyclohexanols with both perfect conversion and yield (Table 4, entries 2–4). achieve the CTH process under optimal conditions as well. Hence, 240 ◦C was employed in our Hydroxy substituted phenol could not achieve the CTH process under optimal conditions as well. reaction system and a 79% yield of cyclohexane-1,2-diol was obtained in the reaction time of 60 min Hence, 240 °C was employed in our reaction system and a 79% yield of cyclohexane-1,2-diol was (Table4, entries 5). Based on the relevant literature [ 34], the reason for the small decrease in the yields of obtained in the reaction time of 60 min (Table 4, entries 5). Based on the relevant literature [34], the corresponding cyclohexanol derivatives probably is the steric effect. Hence, the presence of substituent reason for the small decrease in the yields of corresponding cyclohexanol derivatives probably is the imposes a steric repulsion, which might lower the degree of adsorption and thus hydrogenation rate. steric effect. Hence, the presence of substituent imposes a steric repulsion, which might lower the Taken together, the transfer hydrogenation system was confirmed suitable for the hydrogenation of degree of adsorption and thus hydrogenation rate. Taken together, the transfer hydrogenation system diverse phenol derivatives with good to excellent results. was confirmed suitable for the hydrogenation of diverse phenol derivatives with good to excellent results.

EnergiesEnergiesEnergiesEnergies 20202020,,, 1313 20202020,,, 846846,,,, 13 13 ,,,, 846846 88 ofof 131388 ofof 1313

TableTable Table 4. Hydrogenation 4. 4. HydrogenationHydrogenation of phenol of of phenol phenol with withsubstituted with substituted substituted functional functional functional groups groups groups with with20% with Ni/CNT 20% 20% Ni/CNT Ni/CNT in in in TableTableEnergies 4. 4. 2020HydrogenationHydrogenation, 13, 846 of of phenol phenol with with substituted substituted functional functional groups groups with with 20% 20% Ni/CNT Ni/CNT in in 8 of 12 aa isopropanol.isopropanol.isopropanol.isopropanol.isopropanol.aa a

a Table 4. Hydrogenation of phenol with substituted functional groups with 20% Ni/CNT inYieldYield isopropanol ofof . oo YieldYield ofofYield of EntryEntry EntryEntry SubstrateSubstrateSubstrateSubstrate T(T(ooCC)/)/T(t(min)T(t(min)oCC)/)/t(min)t(min) Conversion(%)Conversion(%)Conversion(%)Conversion(%) product(%)product(%)product(%) Entry Substrate T (◦C)/t (min) Conversion (%) Yieldproduct(%)product(%) of Product (%)

1 220/60 100 1 11 220/60220/60220/60 100 100100 9595 1 220/60 100 9595 95

2 240/60 89 2 22 240/6022 440/600/60 89 8989 80 22 22440/600/60 8989 8080 8080

3 33 3 240240/60/6022 440/600/60 9090 9090 33 22440/600/60 9090 82 8282 8282 EnergiesEnergies 2020EnergiesEnergies, 202013, 846 20202020, 13 ,, , 846 1313,, 846846 9 of 139 of99 13 ofof 1313

4 4 240/60240/60 85 85 44 44 22440/600/6022 440/600/60 8585 8585 78 7878 7878

5 240/60 88 55 55 22440/600/6022 440/600/60 8888 8888 79 7979 7979 aa aa ReactionReactiona ReactionReaction condition:condition:a Reaction condition:condition: 20%20% condition: Ni/CNTNi/CNT 20%20% 20%Ni/CNTNi/CNT (50(50 Ni /mg),mg),CNT (50(50 substrate (50 substrate mg),mg), mg), substratesubstrate substrate (500(500 mg),mg), (500(500 (500 solvent solvent mg), mg), solvent solventsolvent (10(10 mL).mL). (10 (10(10 mL). mL).mL).

2.6.2.6. MechanismMechanism2.6.2.6. MechanismMechanism StudiesStudies StudiesStudies andand thethe andand RecyclabilityRecyclability thethe RecyclabilityRecyclability ofof Ni/CNTNi/CNT ofof Ni/CNTNi/CNT 2.6. Mechanism Studies and the Recyclability of Ni/CNT TThehe mechanismmechanismTThehe mechanismmechanism ofof thethe ofof hydrogenationhydrogenation thethe hydrogenationhydrogenation ofof phenolphenol ofof phenolphenol overover 20%over20%over Ni/CNTNi/CNT20%20% Ni/CNTNi/CNT waswas proposed proposedwaswas proposedproposed inin FigureFigure inin FigureFigure 66.. 66... AccordingAccordingAccordingAccording to toThe the the mechanismto to time time the the timecourses time courses of courses courses the of of phenolhydrogenation phenol of of phenol phenol and and products products and and of products productsphenol (Figure (Figure over (Figure (Figure 4), 4), 20% the the 4), Ni4), hydrogenation hydrogenation/ theCNT the hydrogenation hydrogenation was proposed of of phenol phenol of of in phenol phenol Figure 6. proceededproceededproceededproceededAccording via via cyclohexanone. cyclohexanone. via via to the cyclohexanone. cyclohexanone. time courses In In of order order phenolIn In order order to to and corroborate corroborate to to products corroborate corroborate (Figure this, this, this, this,4we we), the investigated investigated we we hydrogenation investigated investigated the the of activity activity phenolthe the activity activity proceeded of of of of cycyclohexanolclohexanolcycyviaclohexanolclohexanol cyclohexanone. andand cyclohexanone cyclohexanone andand cyclohexanonecyclohexanone In order inin thethe to inin corroborateprocessprocess thethe processprocess ofof transfertransfer this, ofof transfertransfer we hydrogenationhydrogenation investigated hydrogenationhydrogenation.. the WhenWhen activity... WhenWhenWhen solosolo of cyclohexanol cyclohexanol solosolosolo cyclohexanol cyclohexanolcyclohexanolcyclohexanol and waswas carriedcarriedwaswascyclohexanone carriedcarried outout inin outout ourour inin catalytic incatalytic ourour the catalyticcatalytic process system,system, of system,system, transfer nono cyclohexanolcyclohexanol nono hydrogenation. cyclohexanolcyclohexanol waswas transfotransfo waswas When transfotransformedrmed solormed rmed cyclohexanolinintoto cyclohexanonecyclohexanone inintoto cyclohexanonecyclohexanone was carried (Figure(Figure out(Figure(Figure in our 55a).a). TheThe55a).a).catalytic resultsresults TheThe resultsresults aboveabove system, aboveabove showshow no cyclohexanolshow show thethe hydrogenationhydrogenation thethe hydrogenationhydrogenation was transformed ofof phenolphenol ofof phenolphenol toto into cyccyc to cyclohexanonetolohexanollohexanol cyccyclohexanollohexanol viavia cyclohexanonecyclohexanone (Figure viavia cyclohexanonecyclohexanone5a). The underunder results underunder aa above aa transfertransfertransfertransfershow hydrogenationhydrogenation the hydrogenationhydrogenation hydrogenation process.process. process.process. ofIItt cc phenolanan I I betbet cc an anseenseen to bebe cyclohexanol from fromseenseen fromFigurefromFigure FigureFigure via 55bb that,that, cyclohexanone 55bb f fthat,that,romrom fthethefromrom beginningbeginning under thethe beginningbeginning a transfer ofof thethe ofof process,process, hydrogenation thethe process,process, thethe amount amountthetheprocess. amount amount of of Itcyclohexanone cyclohexanone of canof cyclohexanone cyclohexanone be seen from decreased decreased decreasedFigure decreased and and5b that, cyclohexanol cyclohexanoland and from cyclohexanol cyclohexanol the was was beginning observed.was observed. was observed. observed. ofthe Cyclohexanone Cyclohexanone process, Cyclohexanone Cyclohexanone the amount was was was was of completelycompletelycompletelycompletelycyclohexanone consumedconsumed consumedconsumed decreasedafterafter after30after30 mm andinin3030 andand mm cyclohexanolinin cyclohexanol cyclohexanol andand cyclohexanolcyclohexanol was waswas observed. achievedachieved waswas achievedachieved Cyclohexanone inin highhigh inin yieldhighyieldhigh was yieldyield(over(over completely (over (over90%).90%). 90%). 90%). ApartApart consumed ApartApart fromfrom fromfromthis,this,after acetoneacetonethis,this, 30 minacetoneacetone waswas and also alsowaswas cyclohexanol detected detectedalsoalso detecteddetected inin was thethe in achievedin catalyticcatalytic thethe catalyticcatalytic in process.process. high process.process. yield InIn TableTable (over InIn Table Table 5,5, 90%). wewe 5, 5, couldcould Apart wewe couldcould findfind from that thatfindfind this, 95% 95%thatthat ofof95%95% of wasof cyclohexanol,cyclohexanol,cyclohexanol,cyclohexanol,also detected 2%2% ofof 2%2% cyclohexanone,cyclohexanone, in ofof the cyclohexanone,cyclohexanone, catalytic 5.85.8 process. equivalentsequivalents 5.85.8 equivalentsequivalents In Table ofof 5acetoneacetone, weofof acetoneacetone could andand find33.6 33.6 andand equivalents thatequivalents33.633.6 95%equivalentsequivalents of ofof cyclohexanol, isopropanolisopropanol ofof isopropanolisopropanol 2% of werewere wereobservedwereobservedcyclohexanone, observedobserved forfor eacheach forfor 5.8 equivalenteachequivalenteach equivalents equivalentequivalent ofof phenolphenol of ofof acetone phenolphenol consumedconsumed and consumedconsumed 33.6,, whichwhich equivalents,,, whichwhichwhich suggestedsuggested suggestedsuggestedsuggested of isopropanol thatthat the the thatthat hydrogenationhydrogenation thethe were hydrogenationhydrogenation observed ofof for each ofof phenolphenolphenolphenol equivalentwaswas aa was was stoichiometricstoichiometric aa of stoichiometricstoichiometric phenol consumed, reactionreaction reactionreaction andand which eacheach andand suggested phenol eachphenoleach phenolphenol moleculemolecule that moleculemolecule the needed hydrogenationneeded neededneeded sixsix isopropanolisopropanol sixsix of isopropanolisopropanol phenol moleculesmolecules was molecules amolecules stoichiometric forfor forfor thethe generationgenerationthethereaction generationgeneration ofof and cyclohcycloh each ofof cyclohcyclohexanol.exanol. phenolexanol.exanol. Therefore,Therefore, molecule Therefore,Therefore, needed thethe detaileddetailed thethe six detaileddetailed isopropanol mechanismmechanism mechanismmechanism molecules couldcould couldcould bebe for summarizedsummarized the bebe summarized generationsummarized inin FigureFigure of inin cyclohexanol. FigureFigure 6.6. 6.6. TheThe moleculemoleculeTheTheTherefore, moleculemolecule adsorptionadsorption the adsorptionadsorption detailed ofof isopropanolisopropanol ofof mechanism isopropanolisopropanol occurredoccurred could occurredoccurred onon be thethe summarized onon catalystcatalyst thethe catalystcatalyst surfacesurface in Figure surfacesurface toto produceproduce6 . toto The produceproduce molecule H*H* andand H*H* followedfollowed andand adsorption followedfollowed of byby phenolphenolbybyisopropanol phenolphenol adsorptionadsorption adsorptionadsorption occurred (i).(i). Then,Then, (i).(i). on Then,Then, irreversibleirreversible the catalyst irreversibleirreversible phenolphenol surface phenolphenol hydrogenationhydrogenation to produce hydrogenationhydrogenation H* tooktook and followedplacetookplacetook place placeforfor bytthe he forfor phenol generationgeneration tthehe generationgeneration adsorption ofof ofof (i). cyclohexanone,cyclohexanone,cyclohexanone,cyclohexanone,Then, irreversible in in which which in in whichphenol which fourfour fourequivalents hydrogenationfourequivalents equivalentsequivalents of of took H* H* of of w wplace H* H*ereere w w forcocoereerenn thesumedsumed coco generationnnsumedsumed (ii (ii––iv).iv). (ii (ii of – In– Iniv).iv). cyclohexanone, the the In In final final the the final finalstep, step, in step, step,twotwo which twotwo four equivalentsequivalentsequivalentsequivalentsequivalents ofof H*H* of of waswas of H*H* H* employed employedwaswas were employedemployed consumed inin thethe inin catalyticcatalytic (ii–iv). thethe catalyticcatalytic In processprocess the finalprocessprocess toto step, transfertransfer toto two transfertransfer cyclohexanonecyclohexanone equivalents cyclohexanonecyclohexanone of H* toto wascyclohexanolcyclohexanol toto employed cyclohexanolcyclohexanol in the (v).(v). BasedBased(v).(v).catalytic BasedBased onon thethe onon process studystudy thethe studystudy above, toabove, transfer above,above, wewe propose cyclohexanonepropose wewe proposepropose thethe meme thethe tochanismchanism me cyclohexanolmechanismchanism inin twotwo inin (v). steps: steps: twotwo Based steps:steps: (1)(1) CC on onversion onversion(1)(1) the CConversiononversion study of of above, phenolphenol ofof phenol wephenol toto propose toto cyclohexanonecyclohexanonecyclohexanonecyclohexanonetheFigure mechanismFigure overover 5.FigureFigure Time the theover5.over Time course in 5.Ni/CNT5.Ni/CNT theTimetheTime two course Ni/CNT Ni/CNTof steps:coursecourse the catalyst.catalyst. of yield the of (1)of catalyst.catalyst. thetheyield of Conversion (2)(2) cyclohexanolyieldyield T Tofransformationransformation (2) (2)cyclohexanol ofof T T cyclohexanolcyclohexanolransformationransformation of carried phenol carried ofof out carried carriedcyclohexanolcyclohexanol to in of of cyclohexanoneout the cyclohexanolcyclohexanol outoutin transfer the inin thethetransfer fromfrom hydrogenationtransfertransfer fromcyclohexanone overcyclohexanonefrom hydrogenation hydrogenationhydrogenation thecyclohexanonecyclohexanone Ni (a/)CNT. Time ( viaviaa). catalyst. ((Time aa)) . via. via TimeTime ketoketo--enolketoenolketo(2) - course-tautomerism.tautomerism.enol Transformationenolcourse tautomerism.tautomerism. ofcoursecourse the of yield the ofof thetheyield ofof cyclohexanolcyclohexanoneyieldyield of cyclohexanone ofof cyclohexanonecyclohexanone fromcarried carried cyclohexanone out carriedcarried in out the out outin transfer the inin the viathetransfer hydrogenationtransfertransfer keto-enol hydrogenation hydrogenationhydrogenation tautomerism. (b). (b). ( (bb).).

TableTableTable 5. TableTableDistribution 5. 5.Distribution Distribution 5.5. DistributionDistribution of products of of products products ofof productsproducts with with with20% 20% withwith Ni/CNT20% Ni 20%20% Ni/CNT/CNT Ni/CNTNi/CNTin isopropanol. in in isopropanol. isopropanol. inin isopropanol.isopropanol.

CatalystCatalystCatalystCatalystCatalyst TemperatureTemperature/Time Temperature/Time Temperature/TimeTemperature/Time/Time ConversionConversion Conversion ConversionConversion

Ni/CNT o 220o ooC/60min 100% 95% 2% 5.8 equiv 33.6 equiv NiNi/CNT/CNTNi/CNTNi/CNT 220 220 ◦CC/22060/60min min220C/60minC /60min 100% 100%1 00%100% 95% 95%2%95% 2% 5.8 2% equiv5.8 5.8 equiv equivequiv33.6 33.6equiv 33.6 33.6equiv equiv ReactionReactionReactionReactionReaction condition:500 condition:500 condition:500 condition:500condition:500 mg mg phenol mg mgmgphenol (1.0 (1.0 phenolphenol equiv), equiv),(1.0 (1.0 (1.0equiv), 50 50 equiv), mgequiv), mg 20%50 20% mg5050 Ni /mgmgCNT,Ni/CNT,20% 20%20% Ni/CNT, 10mL Ni/CNT,Ni/CNT, 10 isopropanolmL 10 isopropanolmL 1010mL mLisopropanol (40.0 isopropanolisopropanol equiv). (40.0 (40.0equiv). Yield (40.0(40.0 equiv). was equiv).equiv). YielddeterminedYield wasYieldYield determinedwas by waswas determined GC determineddetermined/MS by with GC/MS by n-dodecane GC/MSbyby with GC/MSGC/MS with asn-dodecane the withwith n internal-dodecane nn--dodecanedodecane as standard. the as internal the asas thetheinternal internalstandard.internal standard. standard.standard.

Energies 2020, 13, 846 9 of 12 Energies 2020, 13, 846 9 of 13

Figure 5. Time course of the yield of cyclohexanol carried out in the transfer hydrogenation (a). Figure 5. Time course of the yield of cyclohexanol carried out in the transfer hydrogenation (a). Time EnergiesTime 2020 course, 13, 846 of the yield of cyclohexanone carried out in the transfer hydrogenation (b). 10 of 13 course of the yield of cyclohexanone carried out in the transfer hydrogenation (b).

Table 5. Distribution of products with 20% Ni/CNT in isopropanol.

Catalyst Temperature/Time Conversion

Ni/CNT 220oC/60min 100% 95% 2% 5.8 equiv 33.6 equiv Reaction condition:500 mg phenol (1.0 equiv), 50 mg 20% Ni/CNT, 10mL isopropanol (40.0 equiv). Yield was determined by GC/MS with n-dodecane as the internal standard.

Figure 6. Catalytic pathway in the transfer hydrogenation of phenol.phenol.

In addition, addition, the NiNi/CNT/CNT catalyst could be magnetically recovered after a simple simple process process (washing(washing withwith isopropanol isopropanol and and dried dried over over 105 105◦C) °C) for for the the subsequent subsequent recycling recycling tests. tests. The recyclingThe recycling tests fortests the for Ni the/CNT Ni/CNT catalyst catalyst in the transfer in the transfer hydrogenation hydrogenation of phenol of under phenol the under optimal the reaction optimal condition reaction condition was subsequently evaluated. Ni/CNT maintained its activity for four succeeding runs, and no obvious decrease in cyclohexanol yield was observed, suggesting the good ability of the catalyst (Figure 7b).

Figure 7. (a) Possible catalytic pathway in the transfer hydrogenation of phenol. (b) Result of recycling tests for Ni/CNT catalyst in the transfer hydrogenation of phenol.

3. Conclusions In this paper, phenol was effectively hydrogenated to cyclohexanol by 20% Ni/CNT catalyst under a mild condition (220 °C, 60 min) using isopropanol as the hydrogen-donor solvent. The Ni/CNT catalyst not only had an excellent ability to catalyze the transfer hydrogenolytic cleavage of phenol and diverse phenol derivatives but also could be easily recovered magnetically from the reaction process for the next four recycling tests. Four succeeding recycling tests in the transfer hydrogenation of phenol proved the good stability of the Ni/CNT catalyst. The possible mechanism for the hydrogenation of phenol suggested that the possible pathway might proceed via the formation of cyclohexanone. This work will inspire various studies on the hydrogenation process in the absence of additional hydrogen.

Energies 2020, 13, 846 10 of 13

Figure 6. Catalytic pathway in the transfer hydrogenation of phenol.

In addition, the Ni/CNT catalyst could be magnetically recovered after a simple process (washingEnergies 2020 with, 13, 846isopropanol and dried over 105 °C) for the subsequent recycling tests. The recycling10 of 12 tests for the Ni/CNT catalyst in the transfer hydrogenation of phenol under the optimal reaction condition was subsequently evaluated. Ni/CNT maintained its activity for four succeeding runs, and was subsequently evaluated. Ni/CNT maintained its activity for four succeeding runs, and no obvious no obvious decrease in cyclohexanol yield was observed, suggesting the good ability of the catalyst decrease in cyclohexanol yield was observed, suggesting the good ability of the catalyst (Figure7b). (Figure 7b).

FigureFigure 7. 7. (a(a) )Possible Possible catalytic catalytic pathway pathway in in the the transfer transfer hydrogenation hydrogenation of of phenol phenol.. ( (bb)) Result Result of of recycling recycling teststests for for Ni/CNT Ni/CNT catalyst catalyst in in the the transfer transfer hydrogenation hydrogenation of phenol.

3.3. Conclusions Conclusions IInn this paper,paper, phenolphenol was was e ffeffectivelyectively hydrogenated hydrogenated to cyclohexanolto cyclohexanol by 20% by Ni20%/CNT Ni/CNT catalyst catalyst under undera mild conditiona mild condition (220 ◦C, 60 (220 min) °C, using 60 min) isopropanol using isopropanol as the hydrogen-donor as the hydrogen solvent.-donor The Ni / solvent.CNT catalyst The Ni/CNTnot only catalyst had an excellentnot only abilityhad an to excellent catalyze ability the transfer to catalyze hydrogenolytic the transfer cleavage hydrogenolytic of phenol cleavage and diverse of phenolphenol derivativesand diverse but phenol also could derivat be easilyives but recovered also could magnetically be easily from recovered the reaction magnetically process for from the nextthe reactionfour recycling process tests. for Four the next succeeding four recycling recycling tests. tests Four in the succeeding transfer hydrogenation recycling tests of in phenol the transfer proved hydrogenationthe good stability of phenol of the Niproved/CNT the catalyst. good Thestability possible of the mechanism Ni/CNT catalyst. for the The hydrogenation possible mechanism of phenol forsuggested the hydrogenation that the possible of phenol pathway suggested might proceed that the via pos thesible formation pathway of cyclohexanone. might proceed This via work the formationwill inspire of various cyclohexanone. studies on This the work hydrogenation will inspire process various in studies the absence on the of hydrogenation additional hydrogen. process in the absence of additional hydrogen. Author Contributions: Conceptualization, M.Z. and J.J.; methodology, M.Z.; software, J.J.; validation, M.Z., B.K.S. and J.J.; formal analysis, C.C.; investigation, C.C.; resources, P.L.; data curation, P.L.; writing—original draft preparation, C.C.; writing—review and editing, M.Z.; visualization, M.Z.; supervision, J.J.; project administration, M.Z.; funding acquisition, M.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Fundamental Research Funds of CAF, grant number CAFYBB2018QB007 and the National Natural Science Foundation of China, grant number 31700645. Acknowledgments: Authors are grateful for the financial support from the Fundamental Research Funds of CAF (No. CAFYBB2018QB007) and the National Natural Science Foundation of China (31700645). Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations

CNT Carbon nanotube ICP Inductive Coupled Plasma Emission Spectrometer XRD Powder X-ray diffraction XPS X-Ray photoelectron spectroscopy TEM Transmission electron microscopy BET Brunauer- Emmett- Teller GC/MS Gas Chromatograph/Mass Spectrometer Energies 2020, 13, 846 11 of 12

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