Hydrogenative Cyclization of Levulinic Acid to Γ-Valerolactone with Methanol and Ni-Fe Bimetallic Catalysts

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Article Hydrogenative Cyclization of Levulinic Acid to γ-Valerolactone with Methanol and Ni-Fe Bimetallic Catalysts

1, , 1, 1,2 Ligang Luo * †, Xiao Han † and Qin Zeng 1 College of Life Science, Shanghai Normal University, 100 Guilin Rd, Shanghai 200241, China; [email protected] (X.H.); [email protected] (Q.Z.) 2 The Public Experiment Center, University of Shanghai for Science and Technology, Shanghai 200093, China * Correspondence: [email protected] Ligang Luo and Xiao Han Contributed equally to this work. † Received: 26 August 2020; Accepted: 14 September 2020; Published: 21 September 2020

Abstract: A series of Ni-Fe/SBA-15 catalysts was prepared and tested for the catalytic hydrogenation of levulinic acid to γ-valerolactone, adopting methanol as the only hydrogen donor, and investigating the synergism between Fe and Ni, both supported on SBA-15, towards this reaction. The characterization of the synthesized catalysts was carried out by XRD (X-ray powder diffraction), TEM (transmission electron microscopy), H2-TPD (hydrogen temperature-programmed desorption), XPS (X-ray photoelectron spectroscopy), and in situ FT-IR (Fourier transform–infrared spectroscopy) techniques. H2-TPD and XPS results have shown that electron transfer occurs from Fe to Ni, which is helpful both for the activation of the C=O bond and for the dissociative activation of H2 molecules, also in agreement with the results of the in situ FT-IR spectroscopy. The effect of temperature and reaction time on γ-valerolactone production was also investigated, identifying the best reaction conditions at 200 ◦C and 180 min, allowing for the complete conversion of levulinic acid and the complete selectivity to γ-valerolactone. Moreover, methanol was identified as an efficient hydrogen donor, if used in combination with the Ni-Fe/SBA-15 catalyst. The obtained results are promising, especially if compared with those obtained with the traditional and more expensive molecular hydrogen and noble-based catalysts.

Keywords: biomass ester derivatives; solvothermal processing; levulinic acid; γ-valerolactone; Ni-Fe bimetallic catalysts

1. Introduction The world is highly dependent on the utilization of fossil resources to fulﬁll energy needs for the production of heat and power. Nevertheless, the surplus consumption of fossil fuels is escalating the concentration of atmospheric CO2, which causes serious global warming threats [1,2]. As a solution to the above primary challenges, considerable attention has been paid to the exploitation of renewable resources [1]. In this context, the conversion of the cellulose fraction of biomass into value-added platform chemicals, such as 5-hydroxymethylfurfural (5-HMF) and levulinic acid (LA), has been thoroughly investigated [2,3]. Moreover, the transformation of LA into more added-value bio-fuels and bio-chemicals is strategic, thanks to its reactive keto and carboxylic functional groups, which can be exploited for the synthesis of many valuable bio-chemicals, such as fuel additives, fragrances, solvents, pharmaceuticals, and plasticizers [4]. Among these high-value chemicals, γ-valerolactone (GVL) is receiving considerable attention to synthesize added-value bio-chemicals, such as food additives, solvents, and drug intermediates, as well as new bio-fuels [5,6]. Three pathways have been proposed

Catalysts 2020, 10, 1096; doi:10.3390/catal10091096 www.mdpi.com/journal/catalysts Catalysts 2020, 10, 1096x FOR PEER REVIEW 22 of of 14 such as food additives, solvents, and drug intermediates, as well as new bio-fuels [5,6]. Three pathwaysfor GVL production, have been as proposed reported for in SchemeGVL production,1. The ﬁrst requiresas reported the high-temperaturein Scheme 1. The LAfirst conversion requires the in high-temperaturethe gas phase, occurring LA conversion via its endothermic in the gas dehydrationphase, occurring to angelicalactone, via its endothermic which isdehydration subsequently to angelicalactone,hydrogenated to which GVL. is The subsequently second occurs hydrogenated in the liquid to phase,GVL. The at lower second temperatures, occurs in the where liquid thephase, LA atketo lower group temperatures, is reduced towhere 4-hydroxyvaleric the LA keto group acid, which is reduced is in turnto 4-hydroxyvaleric dehydrated to GVL.acid, Lastly,which is the in thirdturn possibilitydehydrated provides to GVL. Lastly, the esteriﬁcation the third possibility of LA to theprovid lesses reactive the esterification alkyl levulinate, of LA to which the less undergoes reactive alkylhydrogenation levulinate, and which dehydration undergoes to hydrog GVL [6enation]. and dehydration to GVL [6].

SchemeScheme 1. 1.Di Differentﬀerent pathways pathways for for thethe hydrogenationhydrogenation of levulinic levulinic acid acid (LA) (LA) to to γ-valerolactoneγ-valerolactone (GVL). (GVL).

According to the above mechanisms, dehydration is generally favored by the presence of acid catalysts, whereas the hydrogenationhydrogenation is catalyzedcatalyzed byby transitiontransition metals.metals. Many homogeneous and heterogeneous catalysts catalysts have have been been employed employed for for the the hydrogenation hydrogenation of LA of to LA GVL to GVLin the inliquid the liquidphase phase[7]. However, [7]. However, given the given well-known the well-known drawbacks drawbacks of the homogeneous of the homogeneous catalysts (the catalysts possible (the corrosion possible ofcorrosion the equipment, of the equipment, difficult direcycle/reuse,fficult recycle and/reuse, environmental and environmental concerns), concerns), heterogeneous heterogeneous ones have ones beenhave beencertainly certainly preferred preferred [8]. Many [8]. Many noble noble metal metal catalysts catalysts have have been been tested tested for forthis thisreaction, reaction, in particular,in particular, Pt, Pt, Au, Au, Pd, Pd, Rh, Rh, and and Ru, Ru, in in many many cases cases achieving achieving promising promising catalytic catalytic performances performances with respect to GVL [[9–12].9–12]. For For example, example, Son Son et al. ha haveve studied studied the the catalytic catalytic performances performances of of Ru/C, Ru/C, Ru/SBA, Au/ZrC, and Au/ZrO catalysts for the LA hydrogenation, highlighting the good catalytic Ru/SBA, Au/ZrC, and Au/ZrO22 catalysts for the LA hydrogenation, highlighting the good catalytic performances and stability of the Au/ZrO catalyst, which can be reused five times, achieving a high performances and stability of the Au/ZrO2 catalyst, which can be reused five times, achieving a high GVL yield (90 mol %) [10]. [10]. However, However, although although the noble noble metal metal catalysts catalysts show show excellent excellent performances for the hydrogenation of such carbonyl compounds, the high costs limit their further application in the industry. On this basis, non-noble metal catalysts remain the preferred choice for improving the sustainable catalyticcatalytic transformationtransformation ofof LA LA to to GVL GVL [13 [13,14].,14]. In In particular, particular, Ni-based Ni-based catalysts catalysts (Raney-Ni, (Raney- Ni/MoOx/C, Cu/ZrO , NiCu/Al O , and Mo C) are active in this reaction [13–15]. For example, Mohan Ni, Ni/MoOx/C, Cu/ZrO2 2, NiCu/Al2 3 2O3, and2 Mo2C) are active in this reaction [13–15]. For example, et al. have examined the influence of catalysts with Ni supported on SiO , Al O , ZnO, ZrO , TiO , Mohan et al. have examined the influence of catalysts with Ni supported 2on SiO2 23, Al2O3, ZnO,2 ZrO2, and MgO, highlighting the excellent catalytic performances of Ni/SiO towards the LA hydrogenation TiO2, and MgO, highlighting the excellent catalytic performances2 of Ni/SiO2 towards the LA 1 1 hydrogenationto GVL (0.8506 to kg GVL GVL (0.8506 kg catalyst kg GVL− h −kg atcatalyst 250 ◦C)−1 h [15−1 at]. 250 The °C) characterization [15]. The characterization of these catalysts of these by catalystsFT-IR spectroscopy by FT-IR spectroscopy has indicated has that indicated both Lewis that and both Brönsted Lewis acid and sites Brönsted play an acid important sites play role an in importantthe dehydration role in the of the dehydration intermediate of the 4-hydroxypentanoic intermediate 4-hydroxypentanoic acid [16], thus acid highlighting [16], thus highlighting that the use thatof tunable the use bifunctional of tunable bifunctional catalysts is catalysts preferred is forpreferred improving for improving the catalysis the catalysis of this reaction. of this reaction. In this Incontext, this context, the addition the addition of another of another metal metal could coul modifyd modify the electronic the electronic structure structure of Ni, of decreasingNi, decreasing the theinteraction interaction between between active active metal metal and supports, and supports, thus enhancing thus enhancing the reaction the reaction activity [activity17]. For [17]. example, For example,the Cu species the Cu hadspecies an importanthad an important effect oneffect the on chemical the chemical environment environment of Ni of species Ni species and and acted acted as an electronic promoter on Ni surface active sites in Ni-Cu/Al O bimetallic catalysts, achieving the as an electronic promoter on Ni surface active sites in Ni-Cu/Al2 2O33 bimetallic catalysts, achieving the highest GVL yield of 96 mol % (250(250 ◦°C,C, 6.56.5 MPa,MPa, andand 22 h)h) [[18].18]. Regarding the possible hydrogen sources, LA hydrogenation has been carried out mainly with molecular H (0.8–6.5 MPa), both in sources, LA hydrogenation has been carried out mainly with molecular H2 (0.8–6.5 MPa), both in batch autoclaves and and in in continuous continuous flow flow reactors, reactors, generally generally obtaining obtaining high high yields yields and and selectivities selectivities to to GVL [15–17]. However, this reaction requires large quantities of molecular H , whose production GVL [15–17]. However, this reaction requires large quantities of molecular H2, whose2 production is still mostly fossil based, which also raises cost and safety issues. In this context, formic acid and

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Catalysts 2020, 10, x FOR PEER REVIEW 3 of 14 is still mostly fossil based, which also raises cost and safety issues. In this context, formic acid and alcohols havehave been been proposed proposed as as alternative alternative hydrogen hydrogen donors, donors, which which should should help help one to one avoid to avoid the usage the ofusage external of external H2 and, H subsequently,2 and, subsequently, the requirement the requirement for a high-pressure for a high-pressure reactor reactor [19]. Regarding [19]. Regarding formic acid,formic H acid,is generated H2 is generated through through the decarboxylation the decarboxylation route (HCOOH route (HCOOHCO + →H CO), which2 + H is2), generallywhich is 2 → 2 2 slow,generally unless slow, noble unless metals, noble such metals, as Ru, such are as usedRu, are as used catalysts as catalysts [19,20]. [19,20]. However, However, Ruppert Ruppert et al. haveet al. demonstratedhave demonstrated that formic that formic acid canacid be can irreversibly be irreversibly adsorbed adsorbed on the on Ruthe/C Ru/C catalyst, catalyst, hampering hampering the LAthe conversionLA conversion to GVL to GVL [21]. [21]. Furthermore, Furthermore, agglomeration agglomeration and metaland metal leaching leaching generally generally occur, occur, which which could greatlycould greatly affect catalystaffect catalyst stability, stability, thus significantly thus significantly hampering hampering its eff itsective effectiv usee [ 22use]. [22]. Alternatively, Alternatively, it is possibleit is possible to use to use alcohol alcohol as anas an H2 Hsource,2 source, and and perform perform a a hydrogen hydrogen transfer, transfer, which which is is knownknown asas thethe Meerwein–Ponndorf–Verley (MPV) reaction, reaction, which which is is catalyzed catalyzed by by Lewis Lewis acid acid sites, sites, such such as asthose those of ofmetal metal oxides oxides or orzeolites zeolites [23]. [23 ].Two Two mechanisms mechanisms ha haveve been been proposed proposed for for explaining the hydrogenhydrogen transfer in in the the presence presence of of alcohols, alcohols, both both reported reported in Scheme in Scheme 2 [23–25].2[ 23 –In25 the]. Infirst the one first (path one A, (path Scheme A, Scheme2), the MPV2), the reaction MPV reaction is catalyzed is catalyzed by the metal by the cata metallyst, catalyst, which receives which receives the hydrogen the hydrogen from the from alcohol the alcoholdonor, through donor, through β-hydrideβ-hydride elimination, elimination, and the activated and the activated hydrogen hydrogen is then transferred is then transferred from the metal from theto the metal acceptor to the molecule, acceptor molecule,in this case in LA this [24]. case Th LAe second [24]. The one second involves one an involves intermolecular an intermolecular hydrogen hydrogentransfer, which transfer, occurs which by occursadsorption by adsorption of the hydrox of theyl hydroxylgroup of the group alcohol of the an alcohold the carbonyl and the carbonylgroup of groupLA (or ofits LAester) (or on its the ester) catalyst on the surface catalyst [25]. surface This di [25rect]. Thisreaction direct is allowed reaction thanks is allowed to the thanks presence to the of presenceLewis and of Brönsted Lewis and acids, Brönsted both acids,leading both to the leading formation to the of formation a six-membered of a six-membered ring transition ring state. transition Luo state.et al. Luohave et studied al. have studiedthe MPV the reaction MPV reaction of methyl of methyl levulinate levulinate to GVL to GVL in inthe the presence presence of of zeolites, zeolites, confirmingconfirming thatthat thethe mainmain rate-limitingrate-limiting stepstep ofof thisthis reactionreaction isis thethe hydridehydride shiftshift [[25].25].

Scheme 2. Catalytic transfer hydrogenationhydrogenation mechanisms of LA. Many kinds of alcohol have been employed for the MPV reaction, highlighting the higher reaction Many kinds of alcohol have been employed for the MPV reaction, highlighting the higher rates of the secondary alcohols. Chia et al. employed 2-BuOH to study the MPV transfer hydrogenation, reaction rates of the secondary alcohols. Chia et al. employed 2-BuOH to study the MPV transfer achieving a good GVL yield (84.7 mol %), over the ZrO2 catalyst (150 ◦C, 16 h) [24]. A strong correlation hydrogenation, achieving a good GVL yield (84.7 mol %), over the ZrO2 catalyst (150 °C, 16 h) [24]. A has been established between the reducing capacity of alcohols and MPV reduction, according to this strong correlation has been established between the reducing capacity of alcohols and MPV order: MeOH < EtOH < 1-BuOH < 2-BuOH = 2-PrOH [26]. However, by using MeOH or EtOH as reduction, according to this order: MeOH < EtOH < 1-BuOH < 2-BuOH = 2-PrOH [26]. However, by hydrogen donors, more eﬃcient and integrated biomass utilization chains could be created, thanks using MeOH or EtOH as hydrogen donors, more efficient and integrated biomass utilization chains to the high availability of these alcohols from biomass feedstock. Tang et al. have used EtOH as a could be created, thanks to the high availability of these alcohols from biomass feedstock. Tang et al. hydrogen donor, achieving the maximum GVL yield of 64 mol % (250 C, 3 h) [26]. Yi et al. have have used EtOH as a hydrogen donor, achieving the maximum GVL yield◦ of 64 mol % (250 °C, 3 h) reported that no hydrogen transfer observed within 48 h in the presence of EtOH, whilst a GVL yield of [26]. Yi et al. have reported that no hydrogen transfer observed within 48 h in the presence of EtOH, 38 mol % was achieved with 1 MPa H2, after only 2 h [27]. Despite the achieved progress, it is deﬁnitely whilst a GVL yield of 38 mol % was achieved with 1 MPa H2, after only 2 h [27]. Despite the achieved progress, it is definitely still necessary to develop new, efficient and cheap catalytic systems for the LA hydrogenation to GVL, by MPV catalytic transfer, in the presence of short-chain primary alcohols.

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Catalysts 2020, 10, x FOR PEER REVIEW 4 of 14 still necessary to develop new, efficient and cheap catalytic systems for the LA hydrogenation to GVL, by MPVIn this catalytic work, new transfer, bimetallic in the Ni presence-Fe/SBA-15 of short-chain catalysts have primary been alcohols. synthesized and tested for the LA hydrogenationIn this work, to GVL new by bimetallic the MPV Ni-Fe route,/SBA-15 in the catalystspresence haveof MeOH been synthesizedas the only hydrogen and tested donor, for the and LA thehydrogenation oxophilic promotion to GVL by of the Fe MPVwas exploited route, in the to presencedecrease ofthe MeOH reduction as the temperature only hydrogen of the donor, NiO and phase the andoxophilic improve promotion the reducibility of Fe was of exploited mixed oxides. to decrease Ni, Fe the, and reduction bimetallic temperature Ni-Fe catalysts of the NiOwithphase different and Fe/Niimprove molar the ratios reducibility have been of mixed prepared oxides. by impreg Ni, Fe,nation and bimetallic on the SBA-15 Ni-Fe support, catalysts which with di helpsfferent the Fe LA/Ni conversionmolar ratios to have GVL. been Moreover, prepared this by impregnationsupport provides on the high SBA-15 specific support, surface which area helps (ranging the LA between conversion 690 andto GVL. 1040 Moreover, m2/g), narrow this support pore size provides distribution high specific (average surface pore area diameter (ranging between between 5 690 and and 30 1040nm), m2 and/g), highnarrow thermal pore sizestability distribution (wall thicknesses (average pore between diameter 3 and between 5 nm) 5[28,29]. and 30 To nm), investigate and high the thermal relationship stability between(wall thicknesses catalyst structure between 3and and activity, 5 nm) [28characterization,29]. To investigate of the the bimetallic relationship Ni-Fe between catalysts catalyst was performedstructure and by activity,X-ray powder characterization diffraction of (XRD), the bimetallic transmission Ni-Fe catalysts electron was microscopy performed (TEM), by X-ray hydrogen powder diffraction (XRD), transmission electron microscopy (TEM), hydrogen temperature-programmed temperature-programmed desorption (H2-TPD), hydrogen temperature-programmed reduction (H2- TPR),desorption X-ray (Hphotoelectron2-TPD), hydrogen spectroscopy temperature-programmed (XPS), and in situ Fourier reduction transform–infrared (H2-TPR), X-ray photoelectronspectroscopy (FT-IR).spectroscopy Lastly, (XPS), the andeffects in situof the Fourier main transform–infrared reaction parameters spectroscopy on the GVL (FT-IR). production Lastly,the have eff ectsbeen of evaluatedthe main reactionand discussed, parameters starting on from the GVL the results production of preliminary have been catalytic evaluated tests. and discussed, starting from the results of preliminary catalytic tests. 2. Results and Discussion 2. Results and Discussion

2.1.2.1. Characterization Characterization of of Catalysts Catalysts First,First, the the catalysts catalysts have have been been characterized characterized by by the the XRD XRD technique technique to to investigate investigate the the atomic atomic structurestructure of of the the particles particles (Figure (Figure 11).). The XRD pattern patternss of Ni/SBA-15 Ni /SBA-15 catalyst catalyst exhibit exhibit three three diffraction diffraction peaks at 2θ values of 44.2°, 51.8°, and 76.3°, which are assigned to (111), (200), and (220) reflections peaks at 2θ values of 44.2◦, 51.8◦, and 76.3◦, which are assigned to (111), (200), and (220) reflections of metallic Ni, respectively. Fu et al. reported analogous diffraction peaks for the Ni/Al2O3 catalyst at of metallic Ni, respectively. Fu et al. reported analogous diffraction peaks for the Ni/Al2O3 catalyst 2θ angles of 44.3°, 51.7°, and 76.3°, assigned to metallic Ni [15]. Nevertheless, the absence of the at 2θ angles of 44.3◦, 51.7◦, and 76.3◦, assigned to metallic Ni [15]. Nevertheless, the absence of the diffraction peak at 37.3°, due to the NiO species, confirms the complete reduction of the Ni particles diffraction peak at 37.3◦, due to the NiO species, confirms the complete reduction of the Ni particles [15]. [15]. In the case of the monometallic Fe/SBA-15 catalyst, another diffraction peak was found at about In the case of the monometallic Fe/SBA-15 catalyst, another diffraction peak was found at about 44.7◦, 44.7°,which which can be can assigned be assigned to the to (111)the (111) Fe cubic Fe cubic planes planes [30 ].[30]. However, However, in thein the case case of of the the Ni-Fe Ni-Fe/SBA-/SBA-15 15catalysts, catalysts, the the Fe peakFe peak is shifted is shifted to lower to lower 2θ, indicating 2θ, indicating a deep a interaction deep interaction between between Ni and FeNi particlesand Fe particlesto form face-centeredto form face-centered cubic structures cubic structures (fcc) of the (fcc) Ni-Fe of alloy,the Ni-Fe favored alloy, by favored the larger by atomic the larger radius atomic of Fe, radiuswhich of increases Fe, which the increases lattice constants the lattice [16 constants]. [16].

Figure 1. XRD (X-ray powder diﬀraction) patterns of the Ni/SBA-15, 5Ni-1Fe/SBA-15, 5Ni-3Fe/SBA-15, Figure 1. XRD (X-ray powder diffraction) patterns of the Ni/SBA-15, 5Ni-1Fe/SBA-15, 5Ni-3Fe/SBA- 5Ni-5Fe/SBA-15, and Fe/SBA-15 catalysts. 15, 5Ni-5Fe/SBA-15, and Fe/SBA-15 catalysts. TEM characterization of the synthesized catalysts was carried out to better investigate their TEM characterization of the synthesized catalysts was carried out to better investigate their textural properties. As shown in Figure2, Ni particles (black spots) are quite well dispersed on the textural properties. As shown in Figure 2, Ni particles (black spots) are quite well dispersed on the porous SBA-15 support. For Ni/SBA-15, the average Ni particle size is about 20 nm, which is in porous SBA-15 support. For Ni/SBA-15, the average Ni particle size is about 20 nm, which is in agreement with data in the literature [31]. The addition of Fe decreases the average size of Ni particles, which are more uniformly distributed, especially in the case of 5Ni-3Fe/SBA-15 (Ni particle size of about 9 nm), indicating that the Fe doping promotes a more homogeneous Ni dispersion,

Catalysts 2020, 10, 1096 5 of 14 agreement with data in the literature [31]. The addition of Fe decreases the average size of Ni particles, Catalysts 2020, 10, x FOR PEER REVIEW 5 of 14 which are more uniformly distributed, especially in the case of 5Ni-3Fe/SBA-15 (Ni particle size of aboutminimizing 9 nm), the indicating formation that of the aggregates. Fe doping However, promotes when a more both homogeneous metals are Niused dispersion, in the same minimizing amount the(5:5), formation the average of aggregates. particle size However, becomes when higher both (16 metals nm), highlighting are used in the that same the amountNi:Fe molar (5:5), ratio the average should particlebe carefully size becomestuned. Based higher on (16 these nm), preliminary highlighting ch thataracterization the Ni:Fe molar data, ratio the shouldoptimum be carefullyachieved tuned.Ni:Fe Basedmolar onratio these is 5:3. preliminary characterization data, the optimum achieved Ni:Fe molar ratio is 5:3.

Figure 2. TEMTEM (transmission (transmission electron electron microscopy) microscopy) images images of ( ofA) ( ANi/SBA-15,) Ni/SBA-15, (B) (5Ni-1Fe/SBA-15,B) 5Ni-1Fe/SBA-15, (C) (5Ni-3Fe/SBA-15,C) 5Ni-3Fe/SBA-15, and and (D) (5Ni-5Fe/SBA-15D) 5Ni-5Fe/SBA-15 catalysts. catalysts.

To further investigate investigate the the interactions interactions between between Ni Ni and and Fe Fewithin within the theNi-Fe/SBA-15 Ni-Fe/SBA-15 catalyst, catalyst, H2- HTPR2-TPR analysis analysis of both of both monometallic monometallic Ni/SBA-15, Ni/SBA-15, Fe/SBA-15, Fe/SBA-15, and andbimetallic bimetallic Ni-Fe/SBA-15 Ni-Fe/SBA-15 catalysts, catalysts, was wascarried carried out out(Figure (Figure 3). 3For). For the the Ni Ni/SBA-15/SBA-15 catalyst, catalyst, only only one one peak peak was was found found at at 430 430 °C,◦C, due due to the reduction of NiO toto metallicmetallic Ni.Ni. For the monometallicmonometallic FeFe/SBA-15/SBA-15 catalyst, threethree didifferentfferent reductionreduction peaks were detected, at at 370, 370, 550, 550, and and 630 630 °C.◦C. In In this this regard, regard, the the reduction reduction of ofFe Fe2O23O to3 αto-Feα-Fe occurs occurs by bythree three consecutive consecutive steps: steps: Fe2O Fe3-Fe2O33-FeO4-FeO-Fe3O4-FeO-Fe [32]. [Therefore,32]. Therefore, the peak the peak at about at about 370 370°C is◦ Cdue is dueto the to thereduction reduction from from Fe2O Fe3 2toO 3Feto3O Fe4, 3whilstO4, whilst that thatfrom from Fe3O Fe4 to3O FeO4 to FeOoccurs occurs at about at about 550 °C, 550 and◦C, andthat thatat 630 at 630°C corresponds◦C corresponds to the to thereduction reduction from from FeO FeO to to αα-Fe-Fe [32]. [32]. Regarding Regarding the the bimetallic Ni-FeNi-Fe/SBA-15/SBA-15 catalyst, there are two didifferentfferent peaks between 300 and 550 ◦°C,C, whilst no peak was found at 600 °C,◦C, thus confirmingconfirming thethe absence absence of of unalloyed unalloyed Fe Fe in in the the synthesized synthesized bimetallic bimetallic catalyst. catalyst. For For 5Ni-1Fe 5Ni-1Fe/SBA-/SBA-15, the15, the reduction reduction peak peak at 320at 320◦C °C is assignedis assigned to theto the reduction reduction of of Fe 2FeO23Oto3 to Fe Fe3O3O4,4, because because Fe Fe22OO33 isis moremore easily reducible than NiO [[33],33], whilst that at 440440 ◦°CC is attributed to the reduction of NiO. Compared with the monometallic NiNi/SBA-15/SBA-15 catalyst,catalyst, thethe reductionreduction peak of the Ni-FeNi-Fe/SBA-15/SBA-15 catalysts is shifted to higherhigher temperatures,temperatures, corresponding to thethe increase ofof thethe FeFe content,content, andand thethe HH22 consumptionconsumption gradually increases. H2 may be more easily dissociated and activated on Ni, and then overflowed to the adjacent iron oxide to reduce it, thus leading to the formation of the Ni-Fe alloy [34].

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gradually increases. H2 may be more easily dissociated and activated on Ni, and then overﬂowed to theCatalysts adjacent 2020, 10 iron, x FOR oxide PEER to REVIEW reduce it, thus leading to the formation of the Ni-Fe alloy [34]. 6 of 14

Figure 3. H2-TPRH2-TPR (hydrogen (hydrogen temperaturetemperature-programmedprogrammed reduction reduction)) profiles proﬁles of Ni/SBA-15, of Ni/SBA-15, 5Ni- 5Ni-1Fe1Fe/SBA-15,/SBA-15, 5Ni-3Fe/SBA-15, 5Ni-3Fe/SBA-15, and and5Ni-5Fe/SBA-15 5Ni-5Fe/SBA-15 catalysts. catalysts.

H22-TPD proﬁlesprofiles of of the the synthesized synthesized catalysts catalysts are reported are reported in Figure in S2.Figure For Ni S2./SBA-15, For Ni/SBA-15, a desorption a peakdesorption was detected peak was at 100detected◦C, in at agreement 100 °C, in with agreemen the adsorptiont with the of adsorption hydrogen of on hydrogen the surface on of the dispersed surface metalof dispersed particles metal Ni in particles Figure S2. Ni It isin remarkableFigure S2. thatIt is theremarkable hydrogen that desorption the hydrogen peak gradually desorption moves peak to agradually higher temperature moves to a withhigher the temperature corresponding with increase the corresponding of Fe content. increase Moreover, of Fe content. the corresponding Moreover, hydrogenthe corresponding desorption hydrogen peaks for desorption 5Ni-1Fe/ SBA-15peaks for and 5Ni-1Fe/SBA-15 5Ni-3Fe/SBA-15 and are 5Ni-3Fe/SBA-15 located at 105 andare located 125 ◦C, respectively,at 105 and 125 which °C, respectively, is higher than which that ofis higher Ni/SBA-15. than However,that of Ni/SBA-15. 5Ni-5Fe/ SBA-15However, has 5Ni-5Fe/SBA-15 two hydrogen adsorptionhas two hydrogen sites, a low-adsorption (150 ◦C) sites, and a high-temperature low- (150 °C) anddesorption high-temperature peak (300 desorption◦C), the latterpeak deriving(300 °C), fromthe latter the hydrogenderiving from desorption the hydrogen on the desorption surface of Feon orthe from surface the of poor Fe or dispersion from the ofpoor Ni. dispersion In addition, of theNi. hydrogenIn addition, desorption the hydrogen peak fromdesorption Ni/SBA-15 peak to from 5Ni-5Fe Ni/SBA-15/SBA-15 to is shifted5Ni-5Fe/SBA-15 to a higher is temperature, shifted to a indicatinghigher temperature, that the adsorption indicating of hydrogen that the onadsorpti the activeon metalof hydrogen surface ison enhanced. the active The metal desorption surface peak is ofenhanced. Fe/SBA-15 The appears desorption at a higher peak temperatureof Fe/SBA-15 (250 appears◦C), which at a ishigher attributed temperature to the hydrogen (250 °C), desorption which is fromattributed the Fe to surface. the hydrogen The adsorption desorption center from correspondsthe Fe surface. to The the desorptionadsorption peakcenter at corresponds low temperature, to the anddesorption desorption peak temperature at low temperature, of Fe/SBA-15 and is thedesorption highest, meaningtemperature that theof hydrogenFe/SBA-15 desorption is the highest, from themeaning Fe species that the requires hydrogen more desorption energy than from that the from Fe species the Ni requires ones. To more evaluate energy the than dispersion that from of thethe particles,Ni ones. To the evaluate amount the of thedispersion hydrogen of the desorbed particle froms, the the amount catalyst of surfacethe hydrogen was calculated desorbed (Tablefrom the1), andcatalyst it corresponds surface was to thecalculated area of the(Table desorption 1), and peakit corresponds reported in to Figure the area S2. Theof the amount desorption of desorbed peak hydrogenreported in gradually Figure S2. increases The amount with of the desorbed area of the hydrogen desorption gradually peak, indicatingincreases with the increasethe area ofof thethe numberdesorption of active peak, hydrogen indicating on the the increase catalyst surface.of the numb Wither the of increase active hydrogen in Fe content, on the that catalyst is moving surface. from 5Ni-1FeWith the/SBA-15 increase to in 5Ni-3Fe Fe content,/SBA-15, that the is particlemoving dispersionfrom 5Ni-1Fe/SBA-15 increases. However, to 5Ni-3Fe/SBA-15, for the 5Ni-5Fe the /particleSBA-15 catalyst,dispersion the increases. total amount However, of desorbed for the H5Ni-5Fe/SB2 does notA-15 signiﬁcantly catalyst, increase,the total amount at the same of desorbed time showing H2 does a worseningnot significantly of the increase, catalyst dispersion. at the same Moreover, time showing the total a worsening acidity of of catalysts the catalyst increases dispersion. with the Moreover, amount ofthe Fe. total It has acidity been of reported catalysts that, increases when Niwith is dopedthe amount with Fe,of Fe. the It unreduced has been Fereported can act that, as a Lewiswhen Ni acid, is thusdoped increasing with Fe, thethe acid unreduced content ofFe thecan catalyst act as a [35 Lewis]. acid, thus increasing the acid content of the catalyst [35].

Table 1. Chemical properties of the synthesized monometallic Ni/SBA-15, Fe/SBA-15, and bimetallic Ni-Fe/SBA-15 catalysts.

Ni Fe H2 Particle Total Acidity Catalyst (mmol/g) (mmol/g) (μmol/g) a Dispersion (%) b (μmol/g) c Ni/SBA-15 0.85 0 6.3 1.45 28.3 5Ni-1Fe/SBA-15 0.85 0.17 17.1 3.34 30.6 5Ni-3Fe/SBA-15 0.85 0.50 23.6 3.49 65.2 5Ni-5Fe/SBA-15 0.85 0.86 23.8 2.76 61.5 Fe/SBA-15 0 0.86 14.8 - 78.6

a: Amount of H2 desorption calculated by TPD (Figure S2). b: H/(Ni + Fe) ratio. c: calculated by NH3-TPD.

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Table 1. Chemical properties of the synthesized monometallic Ni/SBA-15, Fe/SBA-15, and bimetallic Ni-Fe/SBA-15 catalysts.

Particle Total Acidity Catalyst Ni (mmol/g) Fe (mmol/g) H (µmol/g) a 2 Dispersion (%) b (µmol/g) c Ni/SBA-15 0.85 0 6.3 1.45 28.3 5Ni-1Fe/SBA-15 0.85 0.17 17.1 3.34 30.6 5Ni-3Fe/SBA-15 0.85 0.50 23.6 3.49 65.2 5Ni-5Fe/SBA-15 0.85 0.86 23.8 2.76 61.5 Fe/SBA-15 0 0.86 14.8 - 78.6 a b c : Amount of H2 desorption calculated by TPD (Figure S2). :H/(Ni + Fe) ratio. : calculated by NH3-TPD.

XPS analysis was performed to monitor the change of the oxidation state of the bimetallic catalysts. The XPS patterns of Ni 2p and Fe 2p of 5Ni/SBA-15, 5Ni-1Fe/SBA-15, 5Ni-3Fe/SBA-15, 5Ni-5Fe/SBA-15, and 5Fe/SBA-15 are shown in Figure S1, respectively. For the Ni 2p, Ni 2p 3/2 and 2p1/2 spin orbit doublets were deconvoluted into six peaks within the range 845–885 eV (Figure S1a). For the 5Ni/SBA-15 catalyst, the peaks at 853.2, 856.7, and 861.7eV are assigned to the characteristic 0 2+ 2+ peaks of Ni , Ni , and satellite peaks of Ni in Ni 2P3/2 orbit, respectively [8]. For 5Ni-1Fe/SBA-15, 5Ni-3Fe/SBA-15, and 5Ni-5Fe/SBA-15 catalysts, the characteristic peaks of Ni0, Ni2+, and satellite peaks 2+ of Ni in Ni2P3/2 orbit are progressively shifted to the higher binding energies, respectively, indicating a deeper interaction between Ni and Fe. In detail, the electrons are transferred from the oxophilic site of Fe to Ni, increasing the electron density of the outer layer of Ni, in agreement with the literature [36]. Moreover, the characteristic peaks of 5Ni-3Fe/SBA-15 are shifted greatly (0.6 eV), meaning that the reduction of 5Ni-3Fe/SBA-15 is certainly favored. For the Fe 2p in Figure S1b, two spin orbits are also present as Fe 2p3/2 and Fe 2p1/2. For the Fe/SBA-15 catalyst, the peaks at 707.7, 712.2, and 716.4eV are 0 2+ 3+ assigned to Fe , Fe , and Fe on Fe 2p3/2, respectively [37]. For 5Ni-1Fe/SBA-15, 5Ni-3Fe/SBA-15, and 5Ni-5Fe/SBA-15 catalysts, the characteristic peaks of Fe0 and Fe3+ are shifted towards the low binding energies, respectively, indicating that the electrons are indeed obtained by Fe, decreasing the electron density of the outer layer of Fe, and activating the C=O bond. From the XPS results, it can be concluded that the Lewis acid sites of the catalysts derive from the unsaturated coordination of Fe2+ and Fe3+ ions. Moreover, the characteristic peak area of Fe0 in bimetallic Ni-Fe catalyst increases, indicating that the interaction between Ni and Fe promotes the reduction of iron oxide. The above results conﬁrm that 5Ni-3Fe/SBA-15 is the most promising catalyst to test for the activation of the C=O bond of LA.

2.2. LA to GVL Catalytic Tests Preliminary catalytic tests were carried out to test the effectiveness of the synthesized catalysts towards GVL production. First, GC-MS analysis was carried out to monitor the progress of the reaction (Figure S4). This analysis confirms that methyl levulinate and GVL represent the main intermediate and target product, respectively, indicating that LA could be completely hydrogenated to GVL, by properly tuning the reaction conditions. In this regard, 4-hydroxyvaleric acid, which is the intermediate of interest for the MPV intermolecular hydrogenation mechanism (path B, Scheme2), was not detected in the reaction mixture. In principle, LA is firstly adsorbed on the surface of the catalyst and subsequently esterified with methanol (MeOH) to methyl levulinate, benefiting from the Lewis acidity of the catalyst [22]. To investigate the mechanism of LA adsorption on the catalysts, in situ FT-IR analysis was performed. As shown in Figure4, the LA absorption band in the range 1 1750–1765 cm− is instead absent in Ni/SBA-15 and Ni-Fe/SBA-15, meaning that the physical adsorption of LA on these catalysts is weak. Moreover, in the case of the Ni/SBA-15 catalyst, an absorption band 1 at about 1720 cm− was detected, due to the weak chemical adsorption of C=O stretching vibration of LA, indicating an interaction between the cationic or Lewis acid sites on the surface of Ni/SBA-15 catalyst and the lone pair electron of the carbonyl oxygen of LA [38]. In contrast, no absorption Catalysts 2020, 10, 1096 8 of 14

1 band was found at 1720 cm− for 5Ni-3Fe/SBA-15, whilst a blue shift occurs in the range 1650–1680 1 cm− , which is due to the strong LA chemisorption. The oxophilic promoter of Fe rendered more Lewis active sites for acid adsorption the C=O bond, in agreement with the results of XPS and NH3-TPD characterization. Moreover, the composition of the gas phase was investigated to highlight possible differences between the catalysts towards the potential production of hydrogen (Figure S3). These analyses confirm that H2 and CO2 are the main reaction products of the gas phase. However, for the Ni/SBA-15 catalyst, the H2 selectivity is only 40 mol %, whilst, in the case of 5Ni-3Fe/SBA-15, the addition of Fe significantly improves the H2 selectivity up to 90 mol %. Instead, regarding the 5Ni-5Fe/SBA-15 catalyst, H2 selectivity decreases, probably as a consequence of the larger particle size ofCatalysts Ni, according 2020, 10, x FOR to the PEER results REVIEW of the above-discussed characterization. 8 of 14

Figure 4. InIn situ situ FT-IR FT-IR (Fourier transfor transform–infraredm–infrared spectroscopy) spectrumspectrum of LA and LA absorption on NiNi/SBA-15/SBA-15 and 5Ni-3Fe5Ni-3Fe/SBA-15/SBA-15 (50(50 mLmL∙minmin−1 He,He, at at 200 200 °C,C, 2 2 h). h). · − ◦ The effect effect of temperature, reaction time, and hydrogenhydrogen source on the LA hydrogenation to GVL was investigated. Figure 55aa showsshows thatthat thethe temperaturetemperature hashas anan importantimportant rolerole onon thethe LALA conversionconversion and GVL GVL selectivity. selectivity. Going Going from from 50 50to 100 to 100°C, both◦C, both LA conversion LA conversion and GVL and selectivity GVL selectivity are low, are whilst low, whilstthe catalytic the catalytic performances performances significantly significantly improve improve at 150 at 150°C and,◦C and, even even better, better, at at200 200 °C.◦C. LA LA is is not immediately hydrogenated intointo thethe corresponding alcohol,alcohol, and methyl levulinate is the only detected intermediate, whilstwhilst GVL GVL is is subsequently subsequently produced produced by by cyclization cyclization and and dealcoholation dealcoholation mechanisms mechanisms [39]. The[39]. eTheffect effect of the of reaction the reaction time was time also was considered also considered (Figure5 (Figureb). The LA5b).conversion The LA conversion is almost completeis almost aftercomplete only after 30 min, only and 30 GVLmin, and selectivity GVL selectivity rapidly increases rapidly fromincreases 30 to from 120 min,30 to becoming120 min, becoming complete aftercomplete 180 min.after Lastly,180 min. the Lastly, effect ofthe the effect hydrogen of the sourcehydrogen was source preliminarily was preliminarily investigated investigated by comparing by thecomparing effect of molecularthe effect hydrogenof molecular and MeOHhydrogen for the and GVL MeOH production, for the in theGVL absence production,/presence in of the bimetallicabsence/presence 5Ni-3Fe of/SBA-15 the bimetallic catalyst 5Ni-3Fe/SBA-15 (Figure5c). The catalyst LA conversion (Figure 5c). in MeOH The LA is conversion slightly higher in MeOH than thatis slightly obtained higher with than molecular that obtained hydrogen. with Remarkably, molecular the hydrogen. GVL selectivity Remarkably, is significantly the GVL higher selectivity for the is catalyticsignificantly tests higher carried for out the in catalytic the presence tests ofcarried MeOH, out thus in the confirming presence the of positiveMeOH, thus synergism confirming achieved the thankspositive to synergism the combined achieved use ofthanks the bimetallic to the combined Fe-Ni catalyst use of the and bimetallic MeOH for Fe-Ni promoting catalyst the and selective MeOH hydrogenationfor promoting the of the selective C=O bond. hydrogenation These preliminary of the C=O data bond. confirm These that preliminary the catalytic data performances confirm that of the synthesizedcatalytic performances catalysts are of comparable the synthesized with those cataly of noblests are metal comparable catalysts, suchwith as those ruthenium, of noble which metal has beencatalysts, identified such asas aruthenium, very efficient which catalyst has for been GVL id production.entified as Fora very example, efficient Xiao catalyst et al. reported for GVL an almostproduction. complete For example, LA conversion Xiao et (99.7 al. molreported %) and an a almost complete complete selectivity LA towardsconversion GVL, (99.7 adopting mol %) a and batch a reactor,complete in selectivity the presence towards of Ru GVL,/graphene adopting catalyst a batch (40 reactor, bar H2, in 200 the◦C, presence 8 h) [40 ].of PiskunRu/graphene et al. foundcatalyst a maximum(40 bar H2, LA200 conversion°C, 8 h) [40]. of Piskun 90 mol et %, al. but found a low a selectivitymaximum (66LAmol conversion % of GVL), of 90 using mol %, Ru /butBeta-12.5 a low (45selectivity Bar H2 ,(66 90 ◦molC, 2 % h) of [41 GVL),]. Luo using et al. reportedRu/Beta-12.5 a maximum (45 Bar GVLH2, 90 selectivity °C, 2 h) [41]. of 97.5 Luo mol et %al. at reported complete a LAmaximum conversion, GVL adopting selectivity Ru of/TiO 97.52 as mol a catalyst, % at complete and dioxane LA conversion, as the reaction adopting medium Ru/TiO (4 MPa2 as Ha 2catalyst,, 200 ◦C, 4and h) [dioxane42]. However, as the inreaction all these medium mentioned (4 MPa cases, H pentanoic2, 200 °C, 4 acid h) [42]. was identifiedHowever, as in a all reaction these by-product,mentioned thiscases, compound pentanoic deriving acid was from identified the GVL as ring a reaction opening, by-product, a reaction catalyzedthis compound by highly deriving acidic from supports the GVL [25]. ring opening, a reaction catalyzed by highly acidic supports [25]. Instead, in our case, pentanoic acid was not produced, thanks to the milder acidity of the adopted catalyst, leading to a more selective production of the desired GVL.

Catalysts 2020, 10, 1096 9 of 14

Catalysts 2020, 10, x FOR PEER REVIEW 9 of 14 Instead, in our case, pentanoic acid was not produced, thanks to the milder acidity of the adopted Catalysts 2020, 10, x FOR PEER REVIEW 9 of 14 catalyst, leading to a more selective production of the desired GVL.

Figure 5. (A) The eﬀect of temperature (reaction conditions: 2 g LA, 15 mL MeOH, 0.5 g 5Ni-3Fe/SBA-15,

180 min),Figure (B 5.) reaction(A) The effect time of (reaction temperature conditions: (reaction 2 conditions: g LA, 15 mL 2 g methanol,LA, 15 mL MeOH, 0.5 g 5Ni-3Fe 0.5 g 5Ni-3Fe/SBA-/SBA-15, 200 ◦C) and (Figure15,C) di180ff 5.erentmin), (A) The hydrogen(B) reactioneffect of sources temperaturetime (reaction (H2 and(reaction conditions: MeOH) conditions: (reaction2 g LA, 2 15g conditions:LA,mL 15methanol, mL MeOH, 2 g0.5 LA, 0.5g 5Ni-3Fe/SBA-15, 1g MPa5Ni-3Fe/SBA- H2 or 15 mL 200 °C) and (C) different hydrogen sources (H2 and MeOH) (reaction conditions: 2 g LA, 1 MPa H2 or MeOH,15, 0.5180 gmin), 5Ni-3Fe (B) reaction/SBA-15, time 200 (reaction◦C, 180 conditions: min) on LA 2 conversiong LA, 15 mL andmethanol, GVL selectivity.0.5 g 5Ni-3Fe/SBA-15, 15 mL MeOH, 0.5 g 5Ni-3Fe/SBA-15, 200 °C, 180 min) on LA conversion and GVL selectivity. 200 °C) and (C) different hydrogen sources (H2 and MeOH) (reaction conditions: 2 g LA, 1 MPa H2 or Preliminary15 mL MeOH, recycling 0.5 g 5Ni-3Fe/SBA-15, tests of the most 200 promising°C, 180 min) bimetallicon LA conversion catalyst and (5Ni-3Fe GVL selectivity./SBA-15) were carried out to demonstratePreliminary its recycling stability tests (Figure of the6). Themost 5Ni-3Fe promising/SBA-15 bimetallic catalyst catalyst was (5Ni-3Fe/SBA-15) re-used consecutively were five carried out to demonstrate its stability (Figure 6). The 5Ni-3Fe/SBA-15 catalyst was re-used times underPreliminary the best-identified recycling tests reaction of the conditions. most promising Before bimetallic each cycle catalyst run, the (5Ni-3Fe/SBA-15) spent catalyst was were simply carriedconsecutively out to five demonstrate times under itsthe stabilitybest-identified (Figure reac 6).tion The conditions. 5Ni-3Fe/SBA-15 Before each catalyst cycle run,was the re-used spent washed with ethanol in an ultrasonic cleaner and dried in a vacuum oven at 60 ◦C, thus avoiding consecutivelycatalyst was simply five times washed under with the ethanolbest-identified in an ultr reacasoniction conditions. cleaner and Before dried each in a cyclevacuum run, oven the spent at 60 tedious reactivation procedures. If compared with the performances of the fresh catalyst, those of the catalyst°C, thus wasavoiding simply tedious washed reactivation with ethanol procedures. in an ultr asonicIf compared cleaner with and thedried performances in a vacuum of oven the freshat 60 spent 5Ni-3Fe/SBA-15 catalyst decrease only slightly, even after five recycling tests, thus confirming °C,catalyst, thus avoidingthose of the tedious spent reactivation 5Ni-3Fe/SBA-15 procedures. catalyst If decrease compared only with slightly, the performances even after five of recyclingthe fresh the goodcatalyst,tests, potential thus those confirming of of the this spent the catalytic good 5Ni-3Fe/SBA-15 potential system of for this catalyst GVL catalytic production decrease system only for on GVLslightly, a larger production even scale. after on fivea larger recycling scale. tests, thus confirming the good potential of this catalytic system for GVL production on a larger scale.

Figure 6. Recycling tests of 5Ni-3Fe/SBA-15 catalyst.

Catalysts 2020, 10, 1096 10 of 14

3. Experimental Section

3.1. Materials Levulinic acid, γ-valerolactone, and methyl levulinate were purchased by Shanghai Aladdin Reagent Company (Aladdin, Shanghai, China). Nickel(II) nitrate hexahydrate [(Ni(NO ) 6H O], 3 2· 2 ferric(III) nitrate hexahydrate [Fe(NO ) 9H O], dichloromethane (DCM), and MeOH were purchased 3 3· 2 from Sigma-Aldrich Trading Co., Ltd. (Sigma-Aldrich, Shanghai, China). SBA-15 was produced by Sinopharm Chemical Reagent Co., Ltd. (Sinopharm, Shanghai, China). Double-distilled H2O was used for the synthesis of catalysts.

3.2. Preparation of Catalyst The metal-based catalysts were prepared by the incipient wetness impregnation method [21]. Solutions of nickel(II) nitrate hexahydrate and ferric(III) nitrate hexahydrate were mixed in the appropriate molar ratio (nominal weight content of 5 wt% Ni, Ni:Fe molar ratio = 5:0, 5:1, 5:2, 5:5). Then, 1 g of SBA-15 was added into the 10 mL volume of the metal precursor solution. The resulting slurry was stirred at room temperature for 2 h, then the catalyst was recovered, and dried overnight at 120 ◦C. The calcination was carried out under N2 atmosphere at 450 ◦C (heating rate 5 ◦C/min) for 1 h and the catalyst was reduced at 450 ◦C (heating rate 5 ◦C/min) for 3 h, under H2 ﬂow.

3.3. Characterization of the Catalysts Powder X-ray diffraction (XRD) analysis was carried out by a Rigaku Ultima IV X-ray diffractometer (Rigaku, Tokyo, Japan), utilizing Cu-Kα radiation (λ = 1.5405 Å). The test conditions were tube voltage, 50 kV; tube current, 50 mA; scanning speed, 8◦/min; and scanning range, 10◦–80◦.N2 sorption measurements were carried out at 77 K on a BEL-MAX gas/vapor adsorption instrument (MicrotracBEL Corp., Osaka, Japan). The surface areas were measured by the Brunauer-Emmett-Teller (BET) method. Transmission electron microscopy (TEM) was performed by the JEM-2100 microscope (JEOL, Tokyo, Japan), working at 200 kV. X-ray photoelectron spectroscopy (XPS) characterization was carried out by a Scientific Escalab 250 spectrometer (Thermo, Waltham, MA, USA) with AlKα (hν = 1486.6 eV) radiation, at a pressure of about 1 10 9 Torr. The binding energy values were × − corrected by using the C1s peak (284.6 eV). The total acidity was determined by NH3-TPD technique. The calcined samples were characterized using temperature-programmed reduction by a TP-5080 adsorption instrument (xq-instrument, Tianjin, China), which was equipped with a TCD detector. Hydrogen temperature-programmed desorption (H2-TPD) measurements were performed by an Autochem II-2920 (Norcross, GA, USA) instrument. The total H2 desorption was calculated by measuring the areas of the desorption peaks. The FT-IR spectra were acquired by a Nicolet NEXUS 670 (Thermo, Waltham, MA, USA), which was equipped with an in situ IR cell.

3.4. Catalytic Tests The catalytic hydrogenation of LA was carried out in a 100-mL stainless steel batch autoclave (Parr, Moline, IL, USA). In a typical run, the autoclave was loaded with the catalyst (0.5 g), LA (2 g), and MeOH (15 mL). Then, the reactor was heated up to the set-point temperature, under a constant stirring rate of 1000 rpm. At the end of the reaction, the slurry was recovered and centrifuged to separate the liquid phase from the spent catalyst, and the former was further analyzed by GC-MS and GC-FID techniques.

3.5. Analysis of the Liquid Phase Qualitative analysis of the liquid samples was carried out by a GCMS-QP2010 instrument (Shimadzu, Kyoto, Japan), equipped with an RTX-5MS column (30 m 0.25 mm 0.25 µm). The injector × × temperature was 250 ◦C and a split ratio of 20:1 was employed. The MS operated in EI mode at Catalysts 2020, 10, 1096 11 of 14

70 eV and the ion source temperature was set at 230 ◦C. The temperature program was the following: from 50 ◦C to 250 ◦C at a heating rate of 10 ◦C/min and held for 5 min at 250 ◦C. He was the carrier gas, and a column ﬂow of 1.0 mL/min was used. The quantitative analysis was carried out by a GC-2010 gas-chromatograph (Shimadzu, Kyoto, Japan), equipped with an RTX-50 column (30 m 0.25 mm 0.25 µm) and a ﬂame ionization detector. n-dodecane was used as the internal × × standard. Conversion and selectivity parameters were calculated according to the following equations:

nLA f inal Conversion of LA (mol %) = 1 ) 100 mol % (1) − nLA initial ∗ n Selectivity to GVL (mol %) = GVL 100 mol % (2) nproducts ∗

4. Conclusions γ-valerolactone is an important biomass platform molecule that can be used as a biofuel, pharmaceutical intermediate, food additive, and green solvent. The conversion of levulinic acid into high-value-added γ-valerolactone is an important reaction, and still requires the development of cheap and highly effective catalysts. In this regard, this paper is mainly focused on the synthesis of new Ni-Fe/SBA-15 bimetallic catalysts for improving the hydrogenation of levulinic acid into γ-valerolactone, adopting methanol as the only hydrogen donor. The physicochemical properties of the synthesized bimetallic catalysts were investigated by different techniques, e.g., XRD, TEM, XPS, and H2-TPD. H2-TPD confirmed that the addition of Fe as an oxophilic promoter lowers the reduction temperature of the NiO phase and improves the reducibility of the mixed oxides. At the same time, the presence of Fe favorably increased the Lewis acidity, which is beneficial for the MPV reduction of LA to GVL. Moreover, XPS analysis confirmed an electronic transfer from Fe to Ni, promoting the activation of the C=O bond of LA, and this conclusion is in agreement with the in situ FT-IR characterization. The preliminary catalytic tests have shown that the best bimetallic 5Ni-3Fe/SBA-15 catalyst has promising performances towards GVL synthesis. The effect of temperature and reaction time on GVL production was also considered, achieving complete LA conversion and GVL selectivity, working at 200 ◦C and after 180 min. Lastly, preliminary recycling tests of the best performing Ni-Fe bimetallic catalyst confirmed the possibility of its advantageous re-use, achieving satisfactory performances to γ-valerolactone, even after five catalytic cycles. The good catalytic performances/stability, the low cost, and the easy synthesis/regeneration of our Ni-Fe bimetallic catalysts are key aspects for the real development of γ-valerolactone production on a larger scale, thus filling the still existing gap between the industrial and academic world.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/10/9/1096/s1. Figure S1: XPS profiles of reduced catalysts. Figure S2: H2-TPD profiles of the synthesized SBA-15 supported catalysts. Figure S3: Influence of metals supported with SBA-15 on the amount of hydrogen in gas. Figure S4: GC-MS results for hydrogenation of LA with different reaction times. Author Contributions: Conceptualization, L.L.; methodology, Q.Z. and X.H.; analysis, L.L., Q.Z. and X.H.; writing, original draft preparation, X.H. and Q.Z.; writing, review and editing, L.L.; project administration, L.L. All authors have read and agreed to the published version of the manuscript. Funding: This work is supported by the National Natural Science Foundation of China (Grant No.31801321), and sponsored by Shanghai Sailing Program (Grant No. 19YF1436300) and Natural Science Foundation of Shanghai (Grant No. 18ZR1428100). Conflicts of Interest: The authors declare no conflict of interest. Catalysts 2020, 10, 1096 12 of 14

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