Valerolactone Hydrogenation Into 1,4-Pentanediol in Milder Reaction Conditions

reactions

Article Heterogeneously Catalyzed γ-Valerolactone Hydrogenation into 1,4-Pentanediol in Milder Reaction Conditions

Irina Simakova 1,2,* , Yulia Demidova 1,2, Mikhail Simonov 1,2 , Sergey Prikhod’ko 1,2, Prashant Niphadkar 3, Vijay Bokade 3, Paresh Dhepe 3 and Dmitry Yu. Murzin 4,*

1 Boreskov Institute of Catalysis, pr. Lavrentieva, 5, 630090 Novosibirsk, Russia; [email protected] (Y.D.); [email protected] (M.S.); [email protected] (S.P.) 2 Novosibirsk State University, Pirogova 2, 630090 Novosibirsk, Russia 3 CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India; [email protected] (P.N.); [email protected] (V.B.); [email protected] (P.D.) 4 Johan Gadolin Process Chemistry Centre, Åbo Akademi University, Biskopsgatan 8, FI-20500 Turku/Åbo, Finland * Correspondence: [email protected] (I.S.); dmurzin@abo.ﬁ (D.Y.M.)

Received: 20 September 2020; Accepted: 14 October 2020; Published: 16 October 2020

Abstract: Hydrogenation of γ-valerolactone (GVL) in polar solvents (n-butanol, 1,4-dioxane) to 1,4-pentanediol (PDO) and 2-methyltetrahydrofuran (MTHF) was performed at 363–443 K in a fixed bed reactor under overall H2 pressure of 0.7–1.3 MPa. Preliminary screening in a batch reactior was performed with a series of Ru, Ir, Pt, Co, and Cu catalysts, earlier efficiently applied for levulinic acid hydrogenation to GVL. The fresh catalysts were analyzed by transmission electron microscopy (TEM), X-ray fluorescent analysis (XRF), temperature programmed reduction by H2 (H2-TPR), and N2 physisorption. Cu/SiO2 prepared by reduction of copper hydroxosilicate with chrysocolla mineral structure provided better selectivity of 67% towards PDO at 32% GVL conversion in a continuous flow reactor. This catalyst was applied to study the effect of temperature, hydrogen pressure, and contact time. The main reaction products were PDO, MTHF, and traces of pentanol, while no valeric acid was observed. Activity and selectivity to PDO over Cu/SiO2 did not change over 9 h, indicating a fair resistance of copper to leaching.

Keywords: 1,4-pentanediol; γ-valerolactone; hydrogenation; biodiols; Cu; Co; Ir; Re; Pt; Ru; biomass conversion

1. Introduction Nowadays, lignocellulose-derived carboxylic acids are widely involved as starting materials for the production of commercial chemicals and transport fuel additives [1–3]. Levulinic acid (LA) is considered to be the platform molecule that can be converted into different value-added chemicals [4]. Volatility of fossil fuels prices and their current relatively low levels [5] make alternative, i.e., biomass-derived, fuels not competitive pricewise with counterparts on the basis of petroleum; therefore, production of high value-added chemicals from biomass is deemed to be a more efficient strategy than production of biofuels [6]. Among such high value-added chemicals, bio-based polymers could be mentioned as being a feasible alternative to various synthetic polymers currently produced using predominantly crude oil as a feedstock [7]. Moreover, some of the bio-based polymers such as polylactic acid are biodegradable, which is another advantage of their utilization. On the contrary, production of petroleum-based polymers is largely unsustainable, calling for their efficient recycling. Otherwise, a large increase of waste plastics and, subsequently, environmental pollution are becoming

Reactions 2020, 1, 54–71; doi:10.3390/reactions1020006 www.mdpi.com/journal/reactions Reactions 2020, 3, x FOR PEER REVIEW 2 of 18 Reactions 2020, 1 55 efficient recycling. Otherwise, a large increase of waste plastics and, subsequently, environmental pollution are becoming of tremendous concern [8]. New strategies to replace conventional petroleum- ofbased tremendous polymers concern with [renewable8]. New strategies alternatives to replace are being conventional developed, petroleum-based involving starting polymers material with renewableplatform molecules alternatives with are beingan appropriate developed, involvingmolecular startingstructure material derived platform from moleculesbiomass, withsuch anas appropriateagricultural molecularwastes (wheat structure straw, derived corn stover) from biomass,or forest suchresidues. as agricultural Such biogenic wastes diols, (wheat such straw,as 1,4- cornpentanediol stover) or(PDO), forest can residues. be prepared Such biogenic either from diols, γ-valerolactone such as 1,4-pentanediol (GVL) originating (PDO), can from be preparedLA or its eitheresters fromthroughγ-valerolactone hydrogenation (GVL) or originating directly from from LA LA oresters its esters (Scheme through 1). PDO hydrogenation is considered or directly as an fromalternative LA esters bio-derived (Scheme 1monomer). PDO is to considered produce asbiodegra an alternativedable polyesters bio-derived with monomer a high strength to produce [9], biodegradablewhich are globally polyesters increasingly with a in high demand strength [10]. [9 ],It whichis worth are noting globally that increasingly PDO has wide in demand application [10]. Itareas is worth that notingare also that used PDO as has an wide intermediate application in areasthe production that are also of used fine aschemicals, an intermediate cosmetics, in the or productionpharmaceuticals of ﬁne [11,12]. chemicals, cosmetics, or pharmaceuticals [11,12].

SchemeScheme 1.1. PossiblePossible routes routes of of 1,4-pentanediol 1,4-pentanediol (PDO) (PDO) formation formation from from levulinic levulinic acid acid (LA) (LA) and and γ- γvalerolactone-valerolactone (GVL). (GVL).

CatalyticCatalytic hydrogenationhydrogenation ofof GVLGVL toto PDOPDO waswas conductedconducted selectivelyselectively overover homogeneoushomogeneous catalystscatalysts onon the the basis basis of of Ru Ru [10 ,[10,13,14],13,14], Co [Co15, 16[15,16],], and Feand [17 Fe], for[17], which for almostwhich 100%almost product 100% yieldproduct was yield reported. was However,reported. fromHowever, the viewpoint from the ofviewpoint industrial of implementation,industrial implementation, homogeneous homogeneous catalysts are catalysts inferior are to heterogeneousinferior to heterogeneous ones owing ones todi owingfficulties to difficulties in separating in separating the catalyst the fromcatalyst the from reaction the reaction media. media. Some catalysts based on noble metals (Ru [9,18], Pt [19], Rh-MoOx/SiO [20]) were studied in the liquid-phase Some catalysts based on noble metals (Ru [9,18], Pt [19], Rh-MoOx/SiO2 2 [20]) were studied in the GVLliquid-phase hydrogenation, GVL hydrogenation, not demonstrating not ademonstrati high selectivityng a forhigh the selectivity PDO formation. for the Hydrogenation PDO formation. of GVLHydrogenation over solid catalysts of GVL inover a fixed-bed solid catalysts reactor in under a fixed-bed hydrogen reactor pressure under (0.2–2.0 hydrogen MPa) pressure was reported (0.2–2.0 to proceedMPa) was over reported various to catalysts proceed based over onvarious noble catalyst and non-nobles based on metals noble [21 and]. The non-noble former onesmetals bearing [21]. The Pd andformer Pt, asones well bearing as nickel-based Pd and Pt, catalysts, as well didas nickel-based not result in catalysts, the desired did PDO not yieldresult under in the mild desired reaction PDO conditions.yield under Themild highest reaction yield conditions. of PDO The was highest achieved yield over of PDO 40 wt.% was Cuachieved/ZnO catalyst over 40 atwt.% 140 Cu/ZnO◦C and pressurecatalyst at of 140 1.5 MPa,°C and resulting pressure in of GVL 1.5 MPa, conversion resulting of 82.3%in GVL and conversion selectivity of to82.3% PDO and 99.2%, selectivity while atto higherPDO 99.2%, temperatures, while at 2-methyltetrahydrofuran higher temperatures, 2-methyltetrahydrofuran and 1-pentanol were formed. and 1-pentanol Activity of were Cu/ZnO formed. was stronglyActivity influencedof Cu/ZnO bywas the strongly calcination influenced temperature, by the resulting calcination in atemperature, higher activity resulting upon calcinationin a higher temperatureactivity upon elevation. calcination The temperature use of Cu/MgO elevation. catalysts The with use di offferent Cu/MgO copper catalysts content with was different also found copper to be econtentffective was [22]. also The found optimal to physicalbe effective chemical [22]. The parameters optimal physical of the catalyst chemical and parameters reaction conditions of the catalyst were copperand reaction loading conditions of 18%, reaction were temperaturecopper loading of 473 of K,18%, and reaction hydrogen temperature pressure of of 10 473 MPa. K, Underand hydrogen optimal conditions,pressure of the 10 PDOMPa. yield Under was optimal 94.4% atconditions, 90.5% GVL the conversion. PDO yield The was catalyst 94.4% at displayed 90.5% GVL stable conversion. behavior overThe 10catalyst h and displayed could be reused.stable behavior Copper-based over 10 catalysts h and could have be also reused. been studied Copper-based in [9,18 ,catalysts23]. Cu/ZrO have2 catalyst demonstrated, for example, a high selectivity of 99% and GVL conversion of 97% at 473 K and also been studied in [9,18,23]. Cu/ZrO2 catalyst demonstrated, for example, a high selectivity of 99% hydrogenand GVL pressureconversion of 6 of MPa 97% [9 ].at The 473 main K and drawback hydrogen of GVLpressure conversion of 6 MPa into [9]. PDO The over main copper drawback catalysts of supported on ZnO, MgO, and ZrO was a requirement of high hydrogen pressures (6–10 MPa) and GVL conversion into PDO over 2copper catalysts supported on ZnO, MgO, and ZrO2 was a elevatedrequirement temperatures. of high hydrogen A number ofpressures bifunctional (6–10 Cu-containing MPa) and elevated catalysts temperatures. with the overall A composition number of Cu /Mg AlO (x = 0.5, 1, 1.5, 2) were suggested for hydrogenation of various lactones [24]. The highest bifunctionalx 3-x Cu-containing catalysts with the overall composition Cux/Mg3-xAlO (x = 0.5, 1, 1.5, 2) selectivitywere suggested was observed for hydrogenation for the catalyst of various with the lacton compositiones [24]. The Cu1.5 highest/Mg1.5AlO selectivity (i.e., x =was1.5), observed which was for effective in the hydrogenation of lactones with different carbons in the heterocycle. The authors note the catalyst with the composition Cu1.5/Mg1.5AlO (i.e., x = 1.5), which was effective in the thathydrogenation this catalyst of is lactones featured with not different only by its carbons high e inffi ciency,the heterocycle. but also byThe a authors low cost note and that environmental this catalyst friendliness.is featured not However, only by low its high productivity efficiency, in abut batch also reactor by a low along cost with and high environmental H2 pressure friendliness. was rather unattractive in this case. However, low productivity in a batch reactor along with high H2 pressure was rather unattractive in this Incase. view of the development of a new biorefinery strategy of the one-pot LA hydrogenation into diols,In it view would of bethe reasonable development to selectof a new the biorefinery catalysts, well-known strategy of the as one-pot efficient LA in levulinichydrogenation acid (LA) into hydrogenationdiols, it would tobe GVL, reasonable for studying to select GVL the conversion catalysts, well-known into PDO. Recently, as efficient catalyst in levulinic screening acid in (LA) LA Reactions 2020, 1 56

hydrogenation into GVL over Ru/zeolites and Ru/C[25] with 1,4-dioxane (438 K, H2 pressure ca. 1.6 MPa) displayed high LA conversion and GVL selectivity of such materials in comparison with carbon-based counterparts. Among Ru/zeolites, Ru/H-Beta demonstrated higher LA conversion and an obvious advantage of H-Beta owing to a proper microstructure and acidity [26]. So far, silica supported copper catalytic systems were not studied systematically in GVL conversion, being used, among other catalysts, for comparison with Cu/ZnO in GVL hydrogenation [21], while silica supported catalysts are generally of great interest in a wide range of catalytic reactions. In particular, for the Fischer–Tropsch synthesis [27,28], modified Co/SiO2 were used, while non-promoted Cu/SiO2 catalysts were shown to be active in lactic acid and lactate hydrogenolysis into 1,2-propanediol [29], and Cu/SiO2 promoted catalysts exhibited high activity for oxidation, esters hydrogenolysis, and methanol conversion [30–32]. Herein, a series of supported heterogeneous catalysts were prepared, all of which were known as active and selective in the levulinic acid hydrogenation to GVL, based on both noble metals (i.e., Ir, Pt, and Ru) and non-noble metals (Cu/ZrO2, Cu/SiO2, Co/ZnO2, Co/ZrO2), and screened in GVL hydrogenation to study their ability to convert GVL into PDO. The acidic/basic properties of the support are generally considered crucial for achieving high hydrogenation efficiency or tuning the selectivity of the target products for hydrogenation of levulinic acid to PDO [33]. Therefore, in the current work, for catalysts based on noble metals, acidity was introduced by adding a corresponding promoter such as Re (IrRe/SiO2, IrRe/Al2O3, IrRe/ZrO2, PtRe/SiO2) or by supporting the metal on an acidic support (Ru/H-Beta with the silica to alumina ratio of 30). On the contrary, copper per se is known to be a Lewis acid, not requiring introduction of additional acidity [34]. The influence of reaction variables such as temperature, hydrogen pressure, as well as catalyst stability was studied to show the feasibility of biogenic diols’ synthesis from GVL under rather mild reaction conditions similar to levulinic acid/levulinate hydrogenation into GVL.

2. Materials and Methods

2.1. Materials In this work, reagent grade materials were used, namely, γ-valerolactone (98 wt.%, Sigma Aldrich, St. Louis, MO, USA), silica Aerosil 300, copper nitrate trihydrate (>99 wt.%, Saksk, Russia), cobalt chloride hexahydrate (>99 wt.%, Reactiv, Russia), IrCl3 hydrate (TU 2625-067-00196533-2002 “V.N. Gulidov Krasnoyarsk factory of nonferrous metals, Krasnoyarsk, Russia), rhenium acid HReO4 (TU 38-301-41-137-90 Reachim, Russia), platinum hydrochloric acid hexahydrate H2PtCl6 (TU 2612-034-00205067-2003 AURAT, Russia), zirconia (Acros Organics, Fair Lawn, NJ, USA, SBET = 2 2 2 106 m /g), γ-alumina (SBET = 146 m /g), silica (SBET = 378 m /g), urea (>99 wt.%, Cherkassk, Ukraine), nitrogen ( 99.95%), hydrogen ( 99.95%), and argon ( 99.95%). To prepare substrate solution, all ≥ ≥ ≥ solvents, such as 1,4-dioxane (TU 6-09-659-73) and n-butanol, were puriﬁed by vacuum distillation. For H-Beta zeolite, sodium aluminate (43.8 wt.% Al2O3, 39.0 wt.% Na2O), as well as silica sol (40 wt.% SiO2), tetraethylammoniumhydroxide (TEAOH, aq. 30 wt.% solution), sodium hydroxide (AR), and deionized water, were used. Substrate solutions were prepared and degassed with N2 before experiments.

2.2. Catalyst Preparation To obtain H-Beta zeolite, sodium hydroxide and sodium aluminate dissolved in distilled water were loaded into tetraethyl ammonium hydroxide aqueous solution under vigorous stirring, followed by silica sol gradual addition. A homogeneous gel mixture was obtained after stirring the suspension for 5–6 h. The resulting gel with molar composition of 6.0 (TEA)2O:2.4 Na2O:30.0 SiO2:Al2O3:840.0 H2O was then placed into a stainless-steel autoclave and subjected to hydrothermal crystallization at 413 K for 98 h. Complete crystallization was followed by cooling to 323–333 K, separating the solid Reactions 2020, 1 57 product, washing with distilled water and ethanol, drying overnight at 383 K, and calcining in air for 10 h at 823 K. For preparation of other catalysts, namely, 4%Ru/H-Beta, 4%Ir4%Re/SiO2, 4%Ir4%Re/Al2O3, 4%Pt4%Re/SiO2, 4%Ir/SiO2, 4%Ir4%Re/ZrO2, 30%Cu/ZrO2, 20%Co/ZrO2, and 20%Co/ZnO, incipient wetness impregnation was applied using corresponding aqueous solutions of the metal precursors (0.1 M). Thereafter, the catalysts were dried at 383 K for 17 h, reduced by molecular H2 from 298 up to 673 K (623 K for the zeolite supported Ru) with a ramp rate of 2 K/min. To remove the excess of chloride, the catalysts were treated with H2 over 3 h at 673 K (623 K for Ru/H-beta). A homogenous deposition–precipitation method was used for preparation of copper on silica. An aqueous slurry consisting of Aerosil 300 silica, copper nitrate trihydrate, and urea was heated from 298 K till 363 K and held at 363 K for 20 h. After cooling, the sediment was separated, washed, dried at 393 K, calcined in air at 723 K, and reduced by molecular hydrogen from 298 K up to 653 K with a ramp rate of 2 K/min. More details on Cu/SiO2 catalyst preparation and analysis by differential thermal analysis (DTA), differential thermogravimetric analysis (DTG), and X-ray diffraction analysis (XRD) were reported earlier [29].

2.3. Catalytic Experiments The catalytic experiments on liquid phase hydrogenation of GVL solution in 1,4-dioxane or n-butanol (0.44 M) were performed in a stainless-steel autoclave (150 mL) at 393 K and 3.0 MPa with the catalyst loading of 0.10–0.24 g. Elucidation of the mass transfer limitations done in a similar way as for an even faster hydrogenation of LA to GVL confirmed their absence [26]. Fixed bed GVL hydrogenation (10.1 wt.% in n-butanol or 1,4-dioxane) was carried out in a 3 1 downflow reactor under hydrogen flow (80, 167, or 240 cm min− (NTP, 293 K, 0.1 MPa)) at 363–443 K under hydrogen pressure of 0.7–1.3 MPa. The hydrogenation set-up consisted of a high-pressure pump, a reactor with the thermocouple located in the middle of the catalyst bed, and a cooler. The substrate solution was vaporized in the preheated line and mixed with purified hydrogen, thereafter passing through the catalyst bed. The volatile products were condensed in a trap circumfluous with cold water. Before experiments, catalyst grains (0.5 g, 0.5–0.63 mm) diluted with the inert quartz beads (0.8 g, 0.63–1.6 mm) were loaded to the reactor to avoid overheating because of a high exothermic effect of hydrogenation. Copper hydroxysilicate precursor was preliminary activated in the reductive atmosphere at 653 K for 2 h in a separate reactor, passivated in N2, and reactivated at 523 K prior to GVL hydrogenation [29]. 1 The weight hourly space velocity (WHSV, h− ) was calculated as follows:

v νs ρ ω WHSV = r = · · (1) mcat mcat

3 3 3 where vr—the substrate flow rate, cm /h; νs—the solution flow rate, cm /h; ρ—solution density, g/cm ; ω—substrate concentration, wt.%; and mcat—mass of catalyst, g. The contact time (τ, h) was defined as follows:

τ = 1/WHSV (2)

2.4. Product Analysis Condensed products were analyzed by gas-liquid chromatographic (GLC) analysis (Chromos GC-1000) using BP20 capillary column (60 m/0.25 mm/0.25 µm, flame ionization detector FID). The temperature was increased from 323 to 483 K with a temperature ramp of 10 K/min. The temperature of the detector and the evaporator was 523 K, and the flow rate of the carrier gas (hydrogen) was 20 cm3/min. The reaction components were identified by gas chromatography/mass spectrometry using ZB-Wax capillary column (30 m/0.25 mm/0.25 µm) (VG-7070 Gas Chromatography/Mass Spectrometry (GC/MS)) and VF-5ms capillary column (30 m/0.25 mm/0.25 µm) (Agilent 5973N EI/PCI). Reactions 2020, 1 58

The concentration of a compound i after the reaction was deﬁned in the following way:

Si Qi Ci = P · (3) Sj Qj j · where S —chromatographic peak area of compound i, mV s; and Q —a response factor. i · i Conversion of GVL as well as selectivity to the reaction products were deﬁned from the peak areas using GC considering 100% mass balance for carbon containing reaction components. Conversion was estimated as the mole of consumed substrate per mole of the initial substrate. Selectivity to the corresponding product was calculated as the yield of this product to the overall yield of all products.

2.5. Catalysts Characterization The majority of characterization methods described below, unless specifically mentioned, were performed for the reduced catalysts. Transmission electron microscopy (TEM) was used to evaluate the size of metal nanoparticles (NPs), while the crystallographic structure and metal content were determined by X-ray diffraction analysis (XRD) and X-ray fluorescent analysis (XRF), respectively. Nitrogen physisorption was applied to determine the microtexture parameters. TEM images were obtained with a Hitachi-9000 NAR apparatus. The suspension of the catalyst sample in ethanol was placed on a copper grid, followed by ethanol evaporation. At least 150 nanoparticles from the enlarged images were used to calculate the mean particle size and the standard deviation (SD). The average surface diameter (ds) was calculated according to Equation (4):

P 3 nidi i d = (4) s P 2 nidi i

The average mass diameter (dm) was calculated according to Equation (5):

P 4 nidi i d = (5) m P 3 nidi i where ni—a number of particles with diameter di. The content of elements in the samples was measured by XRF (ARL Advant’X spectrometer) with the powder pellet method using undiluted samples (0.5 g) ground and placed in the die (29 mm diameter). To determine the intensities of the lines for elements, the following excitation conditions were applied: a rhodium anode X-ray tube voltage of 50 kV; current of 40 mA; collimator with a divergence of 0.25◦; LiF200 crystal was used as a monochromator; scintillation counter was used as a detector; and counting time was 12 s. The concentration of elements in the sample was defined using a semi-quantitative method with a QuantAS program for standard-less analysis. The microtextural parameters were estimated by nitrogen physisorption at 77 K (Micromeritics Model ASAP 2400). The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method within the relative partial pressure range of 0.05–0.25. The diffraction patterns were obtained on a high-resolution powder diffractometer Thermo techno ARL X’TRA (Switzerland) equipped with a copper anode (λ = 1.5418 Å) and a solid-state semiconductor detector, allowing operation without filters and a monochromator. The measurements were carried out by scanning in the range of 2θ values between 10 and 70◦ with a step of 0.1◦ and an accumulation time at a point of 10 s. The sizes of the coherently scattering domain (CSD) were determined from broadening of the diffraction peaks. The instrumental broadening of the diffraction lines was taken into account. Such broadening was recorded from the diffraction pattern using the international Reactions 2020, 3, x FOR PEER REVIEW 6 of 18 Reactions 2020, 1 59 pattern using the international standard—α-Al2O3 (SRM 1976). The phase analysis was performed using the The International Centre for Diffraction Data (ICDD) PDF-2 database. standard—α-Al2O3 (SRM 1976). The phase analysis was performed using the The International Centre for Di2.6.ff ractionCopper Surface Data (ICDD) Determination PDF-2 database. 2.6. CopperThe Surface area Determinationof metallic copper surface in copper catalysts was estimated by titration of the preliminary reduced Cu oxide precursor (H2, 653 K) using nitrous oxide injected in a pulse-wise mode The area of metallic copper surface in copper catalysts was estimated by titration of the preliminary [35]. Decomposition of the latter over metal copper resulted in dinitrogen formation, which was reduced Cu oxide precursor (H2, 653 K) using nitrous oxide injected in a pulse-wise mode [35]. quantitatively analyzed by GLC on an LKhM-80 chromatograph (NPO Agropribor, Russia) with a Decomposition of the latter over metal copper resulted in dinitrogen formation, which was quantitatively heat-conductivity detector using He as a carrier gas. Separation was done with a Porapack Q capillary analyzed by GLC on an LKhM-80 chromatograph (NPO Agropribor, Russia) with a heat-conductivity column (71 m/0.5 mm/0.05 mm). Absence of nitrogen formation was an indication to stop injection of detector using He as a carrier gas. Separation was done with a Porapack Q capillary column nitrous oxide pulses. The Cu0 specific surface area was estimated considering the average surface (71 m/0.5 mm/0.05 mm). Absence of nitrogen formation was an indication to stop injection of nitrous area occupied by one Cu atom is 0.0711 nm2 and that a release of N2 molecule corresponds to the oxide pulses. The Cu0 specific surface area was estimated considering the average surface area occupied oxidation of two Cu atoms.2 Therefore, the metal surface S(Cu) can be calculated as follows: by one Cu atom is 0.0711 nm and that a release of N2 molecule corresponds to the oxidation of two Cu atoms. Therefore, the metal surface S(Cu)S(Cu)can = 14.2×10 be calculated−20 𝜈(N 2 as)·N follows:A (6)

whereν(N2) is the number of nitrogen (mol), and NA20 is the Avogadro number. S(Cu) = 14.2 10− ν (N ) N (6) × 2 · A where3. Resultsν(N2) is and the Discussion number of nitrogen (mol), and NA is the Avogadro number.

3. Results3.1. Characterization and Discussion of Catalysts 3.1. CharacterizationGVL hydrogenation of Catalysts was performed out in the presence of catalysts after their preliminary reduction according to the previously developed methods for hydrogenation of various organic GVL hydrogenation was performed out in the presence of catalysts after their preliminary reduction substrates [36–40]. In accordance with the obtained TPR data, reduction of the samples at a according to the previously developed methods for hydrogenation of various organic substrates [36–40]. temperature above 623 K leads to formation of metallic particles for Ru/H-Beta, IrRe/SiO2, IrRe/Al2O3, In accordance with the obtained TPR data, reduction of the samples at a temperature above 623 K leads PtRe/SiO2, Ir/SiO2, IrRe/ZrO2, Cu/ZrO2, Co/ZrO2, and Co/ZnO catalysts. The TEM study showed that to formation of metallic particles for Ru/H-Beta, IrRe/SiO2, IrRe/Al2O3, PtRe/SiO2, Ir/SiO2, IrRe/ZrO2, the samples are characterized by a uniform distribution of particles across the support, and the Cu/ZrO2, Co/ZrO2, and Co/ZnO catalysts. The TEM study showed that the samples are characterized average surface particle diameter ds~0.9–2.2 nm representative for all samples (Figure 1, shown for by a uniform distribution of particles across the support, and the average surface particle diameter Ru/H-Beta, IrRe/ZrO2, IrRe/Al2O3). It was found by XPS (not shown) that the active components of ds~0.9–2.2 nm representative for all samples (Figure1, shown for Ru /H-Beta, IrRe/ZrO2, IrRe/Al2O3). the catalysts are metal particles with a small fraction of the oxidized metal except Cu/SiO2, Cu/ZrO2, It was found by XPS (not shown) that the active components of the catalysts are metal particles with Co/ZrO2, and Co/ZnO, where the fraction of the oxide phase is larger due to easier re-oxidation a small fraction of the oxidized metal except Cu/SiO2, Cu/ZrO2, Co/ZrO2, and Co/ZnO, where the during exposure to air. These samples were transferred to the reactor under an argon flow, fraction of the oxide phase is larger due to easier re-oxidation during exposure to air. These samples subsequently under reductive reaction conditions, the partially oxidized phase of the active were transferred to the reactor under an argon ﬂow, subsequently under reductive reaction conditions, components undergoes complete reduction. the partially oxidized phase of the active components undergoes complete reduction.

Sample Ru/1 100 size - 219 mean - 2 nm SD - 0.3 80 d - 2.1 nm s d - 2.1 nm m

60 Count 40

0 1,01,52,02,53,03,5 size, nm

(a)

Figure 1. Cont. Reactions 2020, 1 60 Reactions 2020, 3, x FOR PEER REVIEW 7 of 18

(b)

Sample IrRe 80 size - 240 mean - 0.9 nm 70 SD - 0.3 d - 1 nm s 60 d - 1.1 nm m

40 Count 30

0 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 size, nm

(c)

FigureFigure 1. 1.Transmission Transmission electron electron microscopy microscopy (TEM) (TEM) images images (left) and (left histograms) and histograms (right) of ( freshright reduced) of fresh

catalysts:reduced (catalysts:a) Ru/H-Beta; (a) Ru/H-Beta; (b) 3%IrRe (/bZrO) 3%IrRe/ZrO2; and (c) 4%Ir4%Re2; and (c) /4%Ir4%Re/AlAl2O3. 2O3.

TheThe catalystcatalyst precursor,precursor, coppercopper hydroxysilicatehydroxysilicate withwith thethe chrysocollachrysocolla structurestructure (CuH)(CuH)[Si4[SiO4O10](OH)](OH)8∙nH2O, was was preliminary preliminary activated activated in ina reductive a reductive atmosphere atmosphere at 653 at K 653 with K a 4 4 10 8· 2 withtemperature a temperature ramp rampof 2 K/min. of 2 K /Themin. specific The speciﬁc surface surface area of area metal of copper metal copper of carefully of carefully reduced reduced Cu/SiO2 2 Cuwas/SiO determined2 was determined to be 15 tom be2/g, 15 while m /g, copper while loading copper loadingaccording according to XRF was to XRF 45.5 waswt.%. 45.5 The wt.%. copper The on 2 coppersilica catalyst on silica after catalyst reduction after reductionwas characterized was characterized by SBET = 520 by m S2BET/g. The= 520 crystal m /g. phase The crystalcomposition phase of compositionthe reduced ofCu/SiO the reduced2 with a minor Cu/SiO excess2 with of a silica minor will excess be disc ofussed silica in will Section be discussed 3.3. Earlier, in it Section was shown 3.3. Earlier,that reduction it was shownof the Cu-Si that oxide reduction precursor of the above Cu-Si 553 oxide K resulted precursor in the above formation 553 Kof resultedhighly dispersed in the formationCu0 particles of highly of 3–8 dispersed nm in size, Cu uniformly0 particles distributed of 3–8 nm inacross size, the uniformly silica surface distributed [41,42]. across the silica surfaceXRF [41, analysis42]. indicated that the desired metal content with 95% confidence interval was achieved in allXRF catalysts, analysis as indicated expected that because the desired of the metal method content of the with metal 95% introduction conﬁdence interval and utilization was achieved of the innon-volatile all catalysts, precursors. as expected because of the method of the metal introduction and utilization of the non-volatile precursors. 3.2. Preliminary Catalyst Screening 3.2. Preliminary Catalyst Screening All prepared catalysts after reductive activation apart from Cu/SiO2 were preliminary tested in GVLAll hydrogenation prepared catalysts in a batch after mode. reductive activation apart from Cu/SiO2 were preliminary tested in GVL hydrogenationThe reaction network in a batch in mode.GVL hydrogenation generally follows the scheme suggested by Sun et al. [21], where, along with the main reaction products PDO and MTHF, other products could be 4- hydroxypentanal, 2-hydroxy-5-methyltetrahydrofuran, 5-hydroxy-2-pentanone, 2,3-dihydro-2- Reactions 2020, 1 61

The reaction network in GVL hydrogenation generally follows the scheme suggested by Sun et al. [21], where, along with the main reaction products PDO and MTHF, other products could Reactions 2020, 3, x FOR PEER REVIEW 8 of 18 be 4-hydroxypentanal, 2-hydroxy-5- methyltetrahydrofuran, 5-hydroxy-2-pentanone, 2,3-dihydro-2- methylfuran,methylfuran, 1-pentanol,1-pentanol, and and valeric valeric acid, acid, formed formed by subsequent by subsequent hydrogenation hydrogenation/dehydration/dehydration reaction (Schemereaction2 (Scheme). This scheme 2). This will scheme be discussed will be discussed in Section in 3.3.2 Section. 3.3.2.

SchemeScheme 2.2. Scheme of catalytic hydrogenation ofof GVL.GVL.

AccordingAccording toto GLC GLC analysis, analysis, the the main main reaction reaction products products in the in currentthe current work work were were MTHF MTHF and PDO, and whilePDO, pentanolwhile pentanol was observed was withobserved content with not exceeding content 0.5%;not exceeding valeric acid 0.5%; and 2,3-dihydromethylfurane valeric acid and 2,3- (DHMF)dihydromethylfurane were observed (DHMF) only inwere experiments observed only with in noble experiments metal catalystswith noble in metal the level catalysts less in 0.6%. the GVLlevel conversionless 0.6%. GVL was conversion surprisingly was very surprisingly low at 393 very K andlow 3.0at 393 MPa, K and with 3.0 the MPa, chosen with substrate the chosen to metalsubstrate ratio to ofmetal 0.7–1.7 ratio mol of 0.7 GVL–1.7/g molMe. GVL/g AbsenceMe. Absence of PDO of over PDO Ru over/H-beta Ru/H-beta was attributed was attributed to a high to a acidityhigh acidity of zeolite of zeolite support, support, which which promotes promotes PDO PDO to MTHF to MTHF consecutive consecutive conversion conversion similar similar to a fastto a intermolecularfast intermolecular esterification-cyclization esterification-cyclization of 3-hydroxypentanoic of 3-hydroxypentanoic acid benefitted acid benefitted by the zeolite by the acidity zeolite in LAacidity hydrogenation in LA hydrogenation to GVL [26 to]. CatalyticGVL [26]. results Catalytic are results presented are inpresented Table S1 in (Supplementary Table S1 (Supplementary Materials), indicatingMaterials), a betterindicating selectivity a better to PDO selectivity in the presence to PDO of in SiO the2 and presence Al2O3 supported of SiO2 and catalysts, Al2O3 particularly supported bearingcatalysts, Ir, particularly IrRe, PtRe, and bearing Cu metals. Ir, IrRe, A PtRe, higher and hydrogen Cu metals. pressure A higher and lower hydrogen temperatures pressure appliedand lower for GVLtemperatures hydrogenation applied promoted for GVL formation hydrogenation of PDO, prom whileoted generation formation of MTHF of PDO, predominately while generation occurred of atMTHF higher predominately temperatures, occurred being more at higher pronounced temperatures, in n-butanol being (Table more S1, pronounced entries 3 and in n 4).-butanol Interestingly, (Table ZrOS1, entries2 as a support 3 and 4). showed Interestingly, a better performanceZrO2 as a support compared showed with aalumina better performance and ZnO among compared supported with copperalumina catalysts and ZnO in theamong liquid-phase supported hydrogenation copper catalysts of GVL in the in ethanolliquid-phase at 200 hydrogenation◦C in a batch reactor of GVL [43 in]. Someethanol contradictions at 200 °C in a with batch the reactor literature [43]. data Some not cont allowingradictions a clear with cut the selection literature of thedata optimal not allowing catalyst a onclear the cut basis selection of the of obtained the optimal results catalyst demonstrate on the basi thats catalytic of the obtained activity results can be demonstrate affected by many that catalytic reaction parametersactivity can suchbe affected as solvent, by many temperature, reaction pressure,parameters the such substrate as solvent, to catalyst temperature, ratio, and pressure, hydrogen the masssubstrate transfer. to catalyst ratio, and hydrogen mass transfer.

3.3. Continuous Hydrogenation Several advantages of the continuous operation compared with the batch mode are apparent, namely, in situ catalyst reduction and regeneration, low dependence on the use of a solvent, and absence of catalyst abrasion. Moreover there is no need for catalyst separation and utilization of high H2 pressure. Reactions 2020, 1 62

3.3. Continuous Hydrogenation Several advantages of the continuous operation compared with the batch mode are apparent, namely,Reactions in2020 situ, 3, xcatalyst FOR PEER reduction REVIEW and regeneration, low dependence on the use of a solvent,9 and of 18 absence of catalyst abrasion. Moreover there is no need for catalyst separation and utilization of high Preliminary catalyst screening in a batch reactor showed feasibility of GVL hydrogenation into H2 pressure. PDO over IrRe and Cu containing catalysts. Considering potential industrial implementation, a less Preliminary catalyst screening in a batch reactor showed feasibility of GVL hydrogenation into expensive Cu/SiO2 catalyst was selected for a further study in continuous GVL hydrogenation into PDO over IrRe and Cu containing catalysts. Considering potential industrial implementation, a PDO. less expensive Cu/SiO2 catalyst was selected for a further study in continuous GVL hydrogenation into PDO. 3.3.1. Effect of Process Parameters 3.3.1. EToffect evaluate of Process the influence Parameters of reaction conditions, the effect of temperature on GVL conversion andTo selectivity evaluate to the PDO influence was studied of reaction first. conditions,Surprisingly, the GVL effect conversion of temperature in 1,4-dioxane on GVL conversion in a continuous and selectivitymode was to very PDO low was compared studied with first. that Surprisingly, in n-butanol, GVL thus conversion the latter was in 1,4-dioxane used further. in This a continuous difference modecan hardly was very be attributed low compared to different with that hydrogen in n-butanol, solubi thuslity theor strong latter wasadsorption used further. of one This of the diff solventserence canblocking hardly the be attributedactive sites. to Apparent differently, hydrogen the effect solubility of the solvent or strong canadsorption tentatively ofbe one ascribed of the to solvents a much blockingmore facile the activeprotonation sites. Apparently,in a polar protic the e ffsolventect of the (i.e., solvent n-butanol) can tentatively in comparison be ascribed with a to low-polar a much moreaprotic facile 1,4-dioxane protonation (Scheme in a polar 2). proticGVL hydrogenation solvent (i.e., n-butanol) was performed in comparison under withthe total a low-polar H2 pressure aprotic not exceeding 1.3 MPa, which is beneficial for the industrial application from the viewpoint of process 1,4-dioxane (Scheme2). GVL hydrogenation was performed under the total H 2 pressure not exceeding 1.3safety MPa, and which low is beneficialproduction for thecosts. industrial It was application observed fromthat theGVL viewpoint concentration of process decreased safety and with low a productiontemperature costs. rise, It while was observedthe PDO yield that GVL onlyconcentration slightly increased. decreased At the with same a temperaturetime, the yield rise, of whileMTHF theincreased PDO yield significantly only slightly (Figure increased. 2). The Atamount the same of valeric time, the acid yield was of very MTHF low, increased being less significantly than 0.05%, (Figureindicating2). The a low amount contribution of valeric of acid 2-hydroxy-5-methylterahydrofuran was very low, being less than 0.05%,e indicatingtransformation a low route contribution through hydrogenolysis of the -CCH3-O- bond at the methyl side (Scheme 2). The reaction mixture contained of 2-hydroxy-5-methylterahydrofurane transformation route through hydrogenolysis of the -CCH3-O- bondonly attraces the methyl of pentanol side (Scheme (<0.5%)2 ).in The this reaction temperature mixture range contained because only dehydration traces of pentanol generally ( < 0.5%)occurs in at thistemperatures temperature higher range than because 473 K, dehydration as discussed generally in [21]. occurs at temperatures higher than 473 K, as discussed in [21].

100 2-MTHF 90 1-POL GVL 80 VA 70 Diol

50 % 40

0 380 390 400 410 420 430 440 Temperature, K

Figure 2. Eﬀect of the reaction temperature on the reaction components’ distribution in hydrogenation

ofFigureγ-valerolactone 2. Effect of (GVL) the reaction over Cu temperature/SiO2. Reaction on the conditions: reaction components’ 10.1% GVL in distribun-butanol,tion GVLin hydrogenation feed rate of 1 3 1 2.1of gγ h-valerolactone− ; catalyst weight (GVL) of over 0.455 Cu/SiO g; H22.pressure Reaction of conditions: 1.3 MPa;H 10.1%2 ﬂow GVL rate in of n 167-butanol, cm min GVL− . feed MTHF, rate 2-methyltetrahydrofuran;of 2.1 g h−1; catalyst weight VA, of valeric 0.455 g; acid. H2 pressure of 1.3 MPa; H2 flow rate of 167 cm3 min−1. MTHF, 2-methyltetrahydrofuran; VA, valeric acid. An increase in temperature led to an increase in GVL conversion, while selectivity to the desired PDO decreasedAn increase and, in temperature simultaneously, led to selectivityan increase toin GVL MHTF conversion, increased while (Figure selectivity3). Therefore, to the desired low temperaturesPDO decreased such and, as below simultaneously, 413 K are preferred selectivity for to the MHTF selective increased formation (Figure of PDO 3). fromTherefore, GVL. Thelow valuestemperatures of the apparent such as activationbelow 413 energy K are preferred will be discussed for the selective below. formation of PDO from GVL. The values of the apparent activation energy will be discussed below. Similar dependences were observed by Sun et al. [21] in continuous GVL hydrogenation over a commercial Cu/ZnO catalyst (N211, Nikki Chemical Co., Ltd., Tokyo, Japan). Selectivity to PDO over Cu/ZnO decreased with increasing temperature, while selectivity to MTHF and 1-pentanol increased. Formation of the latter in these conditions could be attributed to the presence of basic sites in ZnO [21], which was not the case for SiO2 used as a support for Cu in the current study. A lower selectivity Reactions 2020, 3, x FOR PEER REVIEW 10 of 18

to PDO over Cu/SiO2 in the current work compared with that over Cu/ZnO [21] can be attributed to minor acidity of silanol groups in silica, favoring further conversion of PDO to MTHF to a larger extent compared with Cu/ZnO [21]. Reactions 2020, 1 63

Reactions 2020, 3, x FOR PEER REVIEW 10 of 18

80 to PDO over Cu/SiO2 in the current work compared with that over Cu/ZnO [21] can be attributed to minor acidity of silanol groups70 in silica, favoring further conversion of PDO to MTHF to a larger extent compared with Cu/ZnO60 [21].

% 40

80 30 Conv GVL 70 20 Sel PDO Sel MTHF 60 10

50 380 390 400 410 420 430 440

% 40 Temperature, K

FigureFigure 3. GVL 3. conversionGVL 30conversion and selectivity and selectivity to 1,4-pentanediol to 1,4-pentanediol (PDO) and MTHF (PDO) vs. and reaction MTHF temperature vs. reaction Conv GVL 1 over Cu/SiO . Reaction conditions:2 10.1% GVL in n-butanol, GVL feed rate of 2.1 g h ; catalyst weight −1 temperature2 over Cu/SiO . Reaction conditions: 10.1% Sel GVL PDO in n-butanol, GVL− feed rate of 2.1 g h ; 20 3 1 of 0.455catalyst g; H2 weightpressure of of0.455 1.3 g; MPa; H2 pressure H2 ﬂow rateof 1.3 of MPa; 167 cm H2 flow minSel MTHF− rate. of 167 cm3 min−1. 10

Similar dependences were observed2 by Sun et al. [21] in continuous GVL hydrogenation over a Catalytic runs at380 different 390 H flow 400 rates 410 (Figure 420 4) showed 430 440 that GVL hydrogenation rate at the commercial Cu/ZnO catalyst3 (N211,−1 Nikki Chemical Co., Ltd., Tokyo, Japan). Selectivity to PDO over flow rate of 167 cm min resulted in aTemperature, higher GVL K conversion, yielding more PDO (Figure 4). An Cu/ZnOincrease decreased of the flow with rate increasing to 243 cm temperature,3 min−1 did not while affect selectivity GVL and to slightly MTHF andincreased 1-pentanol the PDO increased. formation. Figure 3. GVL conversion and selectivity to 1,4-pentanediol (PDO) and MTHF vs. reaction FormationA tentative of the explanation latter in these for conditions such behavior could beis that attributed initial toelevation the presence of the of flow basic rate sites to in 167 ZnO cm [213 ],min−1 temperature over Cu/SiO2. Reaction conditions: 10.1% GVL in n-butanol, GVL feed rate of 2.1 g h−1; whichincreased was not the the partial case for pressure SiO2 used of hydrogen, as a support while for a Cu large in the excess current of the study. latter A can lower hinder selectivity adsorption to of catalyst weight of 0.455 g; H2 pressure of 1.3 MPa; H2 flow rate of 167 cm3 min−1. PDOGVL over at Cu higher/SiO2 hydrogenin the current flow workrates. compared Earlier, Sun with et al. that [21] over found Cu/ ZnOan optimal [21] can H2 be flow attributed rate of 90 to cm3 minormin acidity−1 for ofGVL silanol hydrogenation groups in silica, on Cu/ZnO. favoring The further differenc conversione could of PDObe attributed to MTHF to to different a larger extentGVL feed Catalytic runs at different H2 flow rates (Figure 4) showed that GVL hydrogenation rate at the comparedrates and with copper Cu/ZnO loadings [21]. as well as differences in the support contribution. flow rate of 167 cm3 min−1 resulted in a higher GVL conversion, yielding more PDO (Figure 4). An Catalytic runs at different H2 flow rates (Figure4) showed that GVL hydrogenation rate at the flow 3 −1 increase of the3 flow1 rate to 243 cm min did not affect GVL and slightly increased the PDO formation. rate of 167 cm min− resulted in a higher GVL conversion, yielding more PDO (Figure4). An increase 3 −1 A tentative explanation for3 such1 behavior is that initial elevation of the flow rate to 167 cm min of the flow rate to 243 cm min− did not affect GVL and slightly increased the PDO formation. increased the partial pressure of hydrogen, while a large excess of the latter can hinder adsorption3 1of A tentative explanation for such behavior is that MTHF initial elevation of the flow rate to 167 cm min− GVL at higher hydrogen flow70 rates. Earlier, Sun et al. [21] found an optimal H2 flow rate of 90 cm3 increased the partial pressure of hydrogen, while Pentanol a large excess of the latter can hinder adsorption of −1 GVL min for GVL hydrogenation60 on Cu/ZnO. The difference could be attributed to different GVL feed3 GVL at higher hydrogen flow rates. Earlier, Sun VA et al. [21] found an optimal H2 flow rate of 90 cm rates1 and copper loadings as well as differences PDO in the support contribution. min− for GVL hydrogenation50 on Cu/ZnO. The difference could be attributed to different GVL feed rates and copper loadings as well as differences in the support contribution. 40 %

30 MTHF 70 20 Pentanol GVL 60 10 VA PDO 50 0 80 167 243 40 3

% WH , cm /h 2 30

Figure 4. Composition20 of the reaction mixture depending on hydrogen flow rate in the hydrogenation

of GVL over Cu/SiO2. Reaction conditions: 10.1% GVL in n-butanol; GVL feed rate of 2.1 g h−1; catalyst 10 weight of 0.455 g; H2 pressure of 1.3 MPa; temperature of 403 K; H2 flow rate of 80, 167, and 240 cm3 min−1. 0 80 167 243 3 WH , cm /h 2

Figure 4. Composition of the reaction mixture depending on hydrogen ﬂow rate in the hydrogenation Figure 4. Composition of the reaction mixture depending on hydrogen flow rate in the hydrogenation1 of GVL over Cu/SiO2. Reaction conditions: 10.1% GVL in n-butanol; GVL feed rate of 2.1 g h− ; catalyst of GVL over Cu/SiO2. Reaction conditions: 10.1% GVL in n-butanol; GVL feed rate of 2.1 g h−1; catalyst3 weight of 0.455 g; H2 pressure of 1.3 MPa; temperature of 403 K; H2 ﬂow rate of 80, 167, and 240 cm weight1 of 0.455 g; H2 pressure of 1.3 MPa; temperature of 403 K; H2 flow rate of 80, 167, and 240 cm3 min− . min−1. Reactions 2020, 3, x FOR PEER REVIEW 11 of 18

The effect of hydrogen pressure on the hydrogenation of GVL to PDO is presented in Figure 5. Reactions 2020, 3, x FOR PEER REVIEW 11 of 18 Similarly to various hydrogenation reactions, an increase of hydrogen pressure is beneficial for the hydrogenation rate, thus GVL conversion and selectivity to PDO and MTHF increased with The effect of hydrogen pressure on the hydrogenation of GVL to PDO is presented in Figure 5. increasing H2 pressure. Moreover, selectivity to the latter products changed in a similar fashion with Similarly to various hydrogenation reactions, an increase of hydrogen pressure is beneficial for the the H2 pressure increase up to 1.3 MPa (Figure 6), demonstrating that their formation rates have Reactionshydrogenation2020, 1 rate, thus GVL conversion and selectivity to PDO and MTHF increased with64 similar reaction orders in hydrogen or both reactions rates have the same preceding rate-determining increasing H2 pressure. Moreover, selectivity to the latter products changed in a similar fashion with step, namely, hydrogenation of GVL into 2-hydroxy-5-methyltetrahydrofurane (Scheme 2). the H2 pressure increase up to 1.3 MPa (Figure 6), demonstrating that their formation rates have TheMaximum eﬀect of PDO hydrogen selectivity pressure of 67% on theobtained hydrogenation at GVL hydrogenation of GVL to PDO over is presentedCu/SiO2 was inFigure achieved5. similar reaction orders in hydrogen or both reactions rates have the same preceding rate-determining Similarlyat GVL conversion to various of hydrogenation 32% at 403 K reactions,and 1.3 MPa, an increasewhile decreasing of hydrogen H2 pressure pressure to is 0.7 beneﬁcial MPa resulted for the in step, namely, hydrogenation of GVL into 2-hydroxy-5-methyltetrahydrofurane (Scheme 2). hydrogenationGVL conversion rate, decrease thus GVL to 9.4%, conversion providing and selectivity a slightly tolower PDO PDO and selectivity MTHF increased of 61% with (Figure increasing 6). Maximum PDO selectivity of 67% obtained at GVL hydrogenation over Cu/SiO2 was achieved H2 pressure. Moreover, selectivity to the latter products changed in a similar fashion with the H2 at GVL conversion of 32% at 403 K and 1.3 MPa, while decreasing H2 pressure to 0.7 MPa resulted in pressure increase up to 1.3 MPa (Figure6), demonstrating that their formation rates have similar GVL conversion decrease to 9.4%, providing a slightly lower PDO selectivity of 61% (Figure 6). reaction orders in hydrogen100 or both reactions rates have the same preceding rate-determining step, namely, hydrogenation of GVL into 2-hydroxy-5-methyltetrahydrofurane (Scheme2).

100 80 MTHF GVL 80 VA

% PDOMTHF 20 PentanolGVL VA

% PDO 20 Pentanol

0 0.7 0.8 0.9 1.0 1.1 1.2 1.3 H pressure, MPa 2 0

0.7 0.8 0.9 1.0 1.1 1.2 1.3 Figure 5. Composition of the reaction mixtureH vs.pressure, hydrog MPaen pressure for hydrogenation of GVL over 2 Cu/SiO2. Reaction conditions: 10.1% GVL in n-butanol; GVL feed rate of 2.1 g h−1; catalyst weight of Figure 5. 3 −1 0.455 g; HComposition2 pressure of of 1.3 the MPa; reaction temperature mixture of vs. 403 hydrogen K; H2 flow pressure rate of for 167 hydrogenation cm min . of GVL over Figure 5. Composition of the reaction mixture vs. hydrogen pressure for hydrogenation1 of GVL over Cu/SiO2. Reaction conditions: 10.1% GVL in n-butanol; GVL feed rate of 2.1 g h− ; catalyst weight of Cu/SiO2. Reaction conditions: 10.1% GVL in n-butanol; GVL feed rate of 2.13 g h−1;1 catalyst weight of 0.455 g; H2 pressure of 1.3 MPa; temperature of 403 K; H2 ﬂow rate of 167 cm min− . 0.455 g; H2 pressure of 1.3 MPa; temperature of 403 K; H2 flow rate of 167 cm3 min−1. 70

60 70 Conv GVL 50 Sel PDO 60 Sel MTHF 40 Conv GVL 50 Sel PDO % Sel MTHF 30 40

% 20 30

10 20 0.7 0.8 0.9 1.0 1.1 1.2 1.3 10 H pressure, MPa 2

Figure 6. Conversion of GVL and0.7 selectivity 0.8 to 0.9 PDO and 1.0 MTHF 1.1 vs. hydrogen 1.2 1.3pressure in hydrogenation Figure 6. Conversion of GVL and selectivityH pressure, to PD MPaO and MTHF vs. hydrogen pressure1 in of GVL over Cu/SiO2. Reaction conditions: 10.1%2 GVL in n-butanol; GVL feed rate of 2.1 g h− ; catalyst hydrogenation of GVL over Cu/SiO2. Reaction conditions: 10.1% GVL in n-butanol; GVL3 feed1 rate of weight of 0.455 g; H2 pressure of 1.3 MPa; temperature of 403 K; H2 ﬂow rate of 167 cm min − . 2.1 g h−1; catalyst weight of 0.455 g; H2 pressure of 1.3 MPa; temperature of 403 K; H2 flow rate of 167 Figure 6. Conversion of GVL and selectivity to PDO and MTHF vs. hydrogen pressure in Maximumcm3 min−1. PDO selectivity of 67% obtained at GVL hydrogenation over Cu/SiO2 was achieved at hydrogenation of GVL over Cu/SiO2. Reaction conditions: 10.1% GVL in n-butanol; GVL feed rate of GVL conversion of 32% at 403 K and 1.3 MPa, while decreasing H2 pressure to 0.7 MPa resulted in 2.1 g h−1; catalyst weight of 0.455 g; H2 pressure of 1.3 MPa; temperature of 403 K; H2 flow rate of 167 GVL conversion decrease to 9.4%, providing a slightly lower PDO selectivity of 61% (Figure6). cm3 min−1. 3.3.2. Reaction Pathways The eﬀect of the contact time was studied at temperatures of 403, 413, 423, and 443 K. It was observed that the GVL content decreased at the expense of the content of the main products when Reactions 2020, 3, x FOR PEER REVIEW 12 of 18

3.3.2. Reaction Pathways The effect of the contact time was studied at temperatures of 403, 413, 423, and 443 K. It was observed that the GVL content decreased at the expense of the content of the main products when Reactionsthe contact2020, 1time increased up to 1.6 h (Figure 7a–d). Figure S1 (Supplementary Material) displays the65 Arrhenius dependence. The apparent activation energy in GVL hydrogenation was estimated to be theca. contact28.5 kJ/mol time (87% increased confidence) up to 1.6 (Figure h (Figure S1),7 confirminga–d). Figure the S1 absence (Supplementary of at least Material) external displaysmass transfer the Arrheniuslimitations. dependence. Taking into The account apparent the absolute activation value energy of the in GVLactivation hydrogenation energy and was the estimated size of catalyst to be ca.particles 28.5 kJ /(0.5–0.63mol (87% mm), conﬁdence) the presence (Figure of S1), internal conﬁrming mass the transfer absence limitations of at least cannot external be mass ruled transfer out. It limitations.follows from Taking Figure into 7a–d account that the the yields absolute of PDO value and of MTHF the activation changed with energy the and contact the sizetime ofdepending catalyst particleson the reaction (0.5–0.63 temperature. mm), the presence The most of internal plausible mass explanation transfer limitations of the observed cannot behaviour be ruled out. is that, It follows while fromPDO Figure and 7MTHFa–d that are the formed yields ofin PDOparallel and ways, MTHF at changed higher withtemperatures, the contact there time dependingis a possibility on the of reactionadditional temperature. MTHF formation The most as plausiblea result of explanation PDO dehydration. of the observed The rate behaviour of this reaction is that, is while apparently PDO andhigher MTHF than are the formed rate of in parallelPDO formation. ways, at higherThis explanation temperatures, agrees there with is a possibilitythe reaction of additionalmechanism MTHFproposed formation by Sun as et a al. result [21], of who PDO considered dehydration. that The GVL rate hydrogenation of this reaction can isapparently occur via four higher reaction than theroutes rate (Scheme of PDO formation.2) including This a parallel explanation transformation agrees with into the PDO reaction and MTHF mechanism with a further proposed conversion by Sun etof al. PDO [21 ],to who MTHF. considered that GVL hydrogenation can occur via four reaction routes (Scheme2) including a parallel transformation into PDO and MTHF with a further conversion of PDO to MTHF.

100 403 K 100 413 K

80 80 MTHF Pentanol 60 MTHF 20 GVL VA Pentanol %

PDO % GVL 40 VA PDO 10 20

0 0

012345 012 τ, h τ, h

(a) (b)

100 100 443 K 423 K MTHF Pentanol 80 GVL 80 VA PDO 60 60 MTHF Pentanol % % 40 GVL 40 VA PDO 20 20

0 0

0123456789 012345 τ, h τ, h

FigureFigure 7. 7.The The composition composition of of the the reaction reaction mixture mixture vs. vs. contactcontact timetime in in hydrogenation hydrogenation of of GVL GVL over over

CuCu/SiO/SiO2 at2 at di differentfferent temperatures temperatures (a) ( 403a) 403 K; ( bK;) 413(b) K;413 (c )K; 423 (c) K; 423 and K; (d and) 443 (d K.) 443 Reaction K. Reacti conditions:on conditions: 10.1% 3 1 GVL10.1% in nGVL-butanol, in n-butanol, catalyst weightcatalyst of weight 0.455 g; of H 0.455pressure g; H of2 pressure 1.3 MPa; of H 1.3flow MPa; rate H of2 167flow cm ratemin of 167. 2 2 · − cm3∙min−1. Considering these routes in detail, the first route (1) includes a direct cleavage of -CC=O-O- bond at theConsidering carbonyl side these of GVL, routes resulting in detail, in 4-hydroxypentanal the first route (1) includes (HPL),whose a direct consecutive cleavage of hydrogenation -CC=O-O- bond resultsat the incarbonyl PDO formation. side of GVL, The resulting other three in 4-hydr routesoxypentanal (2, 3, 4) comprise (HPL), hydrogenation whose consecutive of the -C hydrogenation=O carbonyl bondresults of GVLin PDO into formation. -C-OH group, The resultingother three in theroutes intermediate (2, 3, 4) comprise 2-hydroxy-5-methyltetrahydrofuran hydrogenation of the -C=O (HMTHF).carbonyl Subsequently,bond of GVL the into second -C-OH route (group,2) includes resulting a consecutive in the ringintermediate opening of 2-hydroxy-5- HMTHF to 5-hydroxy-2-pentanonemethyltetrahydrofuran (HPN)(HMTHF). by hydrogenolysisSubsequently, the of thesecond -CC-OH route-O- ( bond2) includes at the a hydroxyl consecutive side ring of HMTHF,opening followedof HMTHF by to hydrogenation 5-hydroxy-2-pentanone to PDO. The (HPN) third by route hydrogenolysis (3) involves of dehydration the -CC-OH-O- of bond HMTHF, at the resulting in 2,3-dihydro-2-methylfuran (DHMF), where the ring -C=C- double bond is further saturated, giving 2-methyltetrahydrofuran (MTHF) followed by the ring hydrogenolysis into 1-pentanol. The fourth route (4) involves the ring opening of HMTHF through the cleavage of -CCH3-O- bond at the methyl side, forming valeric acid (VA), which can be further converted into pentanols. Both VA and Reactions 2020, 1 66

HPN are the ring-opening products of HMTHF; the cleavage of -CCH3-O- bond at the methyl side results in VA, and hydrogenolysis of -CC-OH-O- bond at the hydroxyl side leads to HPN. Note that HPL and HPN could both be PDO intermediates with a higher probability for HPL, considering that the carbonyl group of aldehydes is more prone to hydrogenation than the keto group [44]. PDO can be further transformed to MTHF and pentanol by dehydration and hydrogenation, correspondingly. Thus, PDO and DHMF could both be MTHF intermediates. Analysis of the product distribution depending on the metal applied allowed to suggest that the reaction on Cu/ZnO mainly follows routes (1, 2), while routes (3, 4) are preferred over Ni and Pd catalysts [21]. In the current study, neither VA nor HPN were detected in the reaction mixture during GVL hydrogenation over Cu/SiO2, while pentanol was detected at a low level of 0.5%. These results are in accordance with the product distribution obtained by Sun et al. in hydrogenation of neat GVL over commercial Cu/SiO2 (G-108B, Clariant Catalysts, Japan) [21]. However, even at low temperatures and low GVL conversion, formation of MTHF was observed along with PDO. This is probably a feature of the Cu/SiO2 catalyst applied in this study as the crystal structure of Cu-Si precursor corresponded to the mineral chrysocolla with a negligible amount of over-stoichiometric silica. Subsequently, the reductive activation leads to the formation of highly dispersed and active Cu0 nanoparticles with reversible RedOx properties along with a highly developed surface area of 520 m2/g [29]. This could favor transformations of GVL into PDO, followed by formation of a thermodynamically controlled mixture of PDO and MTHF. The composition of such a mixture is inﬂuenced by formation of water also in routes 2 and 3, giving overall a predominant excess of PDO at low temperatures and a higher content MTHF at higher temperatures. Note that the Cu/SiO2 catalyst applied in this study exhibited unusual bifunctional properties in hydrogenation of methyl lactate, resulting in the 1,2-propanediol yield of 76% [29].

3.3.3. Stability of Cu/SiO2 Catalyst

Separate experiments were carried out in the ﬁxed-bed reactor to study the stability of Cu/SiO2 with time-on-stream (TOS) at the same contact time. According to the experimental data, the content of GVL decreases rather slowly, while the yield of PDO increased slightly (Figure8). The reduction of copper cations in the silicate precursor is irreversible, resulting in the oxygen anions’ removal as water molecules. This explanation is in line with a rather slow catalyst deactivation and apparently high steady state activity. Cu particles oxidized during the reaction can be reduced reversibly after a treatment by molecular hydrogen at 523 K (2 h), thereby restoring catalytic activity, as shown experimentally. Selectivity to PDO was almost the same with TOS indicating no changes of the microstructure of the active copper sites responsible for PDO formation, whereas the total amount of active sites probably decreased as a result of deposition of formed side products. A rather stable catalytic activity can be explained by a partial decoration of the highly dispersed metal particles with silicon oxohydroxide particles of the support, thereby preventing migration of copper particles during hydrogenation. Moreover, according to XRF, Cu0 leaching of ca. 2.5% was observed during 9 h TOS. That could be explained by suppression of GVL transformations to valeric acid (route 3) according to the reaction scheme (Scheme2), as the acids are known to e ﬃciently form complexes with copper facilitating its leaching. Note that the leached Cu might be more active as well, and thus obscure deactivation of the heterogeneous catalyst. Reactions 2020, 3, x FOR PEER REVIEW 14 of 18 Reactions 2020, 1 67

60 MTHF Pentanol 50 GVL VA 40 % PDO

0 34567 Time-on-stream, h

Figure 8. Composition of the reaction mixture vs. time-on-stream in hydrogenation of GVL over Figure 8. Composition of the reaction mixture vs. time-on-stream in hydrogenation1 of GVL over Cu/SiO2. Reaction conditions: 10.1% GVL in n-butanol; GVL feed rate of 2.1 g h− ; catalyst weight of 2 3 −1 1 0.455Cu/SiO g; H.2 Reactionpressure conditions: of 0.9 MPa; 10.1% temperature GVL in of n 403-butanol; K; H2 GVLflow feed rate ofrate 167 of cm 2.1 ming h −; catalyst. weight of 0.455 g; H2 pressure of 0.9 MPa; temperature of 403 K; H2 flow rate of 167 cm3 min−1. 3.4. XRD of Cu/SiO2 before and after the Reaction 3.4. XRD of Cu/SiO2 Before and After the Reaction XRD analysis of Cu/SiO2 before and after the reaction is presented in Table1 and Figure9. The as-preparedXRD analysis and freshly of Cu/SiO reduced2 before at 653 and K Cu after/SiO the2 catalyst reaction showed is presented the presence in Table of a1 highlyand Figure dispersed 9. The 0 crystallographicas-prepared and phase freshly of Cureduced(space at 653 group K Cu/SiO Fm-3m)2 catalyst with the showed elementary the presence cubic cell of parameter a highly dispersed a = b = c crystallographic= 3.613(2) Å, lattice phase angles of Cuα0 =(spaceβ = γ group= 90◦ Fm-3m)close to metallicwith the copperelementary lattice cubic parameters. cell parameter Diffraction a = b = peaksc = 3.613(2) referring Å, tolattice Cu2O angles (space α group = β = Pn-3m)γ = 90° wereclose observedto metallic with copper a significantly lattice parameters. decreased Diffraction value of thepeaks unit referring cell parameter to Cu2O a = (spaceb = c group= 4.269 Pn-3m) Å indicating were observed cationic modification,with a significantly while nodecreased changes value in the of latticethe unit angles cell wereparameter observed a = bα = cβ == 4.269γ = 90 Å◦ indicating. This seems cationic to be typicalmodification, for the Cu-Siwhile catalystno changes obtained in the bylattice reduction angles of were the oxide observed precursor α = β of = copperγ = 90°. hydroxysilicate This seems to be with typical a chrysocolla for the Cu-Si structure. catalyst Coherently obtained 0 scatteringby reduction domains of the (CSD) oxide of Cu precandursor Cu 2ofO phasescopper werehydroxysilicate 6.5 and 2.5 nm,with correspondingly a chrysocolla (Tablestructure.1). AnCoherently intense di ffscatteringraction peak domains with a (CSD) maximum of Cu at0 and 2θ = Cu26.62O◦ phasesin the freshly were 6.5 reduced and 2.5 catalyst nm, correspondingly was related to the(Table quartz 1). phaseAn intense SiO2 (PDF diffraction # 00-046-1045) peak with with a maximum the coherently at 2θ scattering = 26.6° in domain the freshly (CSD) reduced exceeding catalyst 100 0 nm.was The related spent to catalyst the quartz Cu/ phaseSiO2 exhibited SiO2 (PDF presence # 00-046-1045) of Cu withwith thethe elementarycoherently scattering cell parameter domain a = b(CSD)= c =exceeding3.615(1) Å, 100 and nm. the The lattice spent angles catalystα = β Cu/SiO= γ = 902 exhibited◦ typical forpresence the metallic of Cu copper0 with latticethe elementary parameters. cell Theparameter structure a of= b Cu = 2cO = phase3.615(1) was Å, rearrangedand the lattice under angles the influenceα = β = γ of= 90° the typical reaction for media the metallic and the copper value oflattice the lattice parameters. parameter The approaches structure of the Cu value2O phase characteristic was rearranged for Cu2 Ounder (a = bthe= cinfluence= 4.266(2)Å, of theα = reactionβ = γ 0 =media90◦). Theand X-raythe value amorphous of the lattice SiO2 parameterphase has disappearedapproaches the after value the reaction.characteristic The CSDfor Cu values2O (a = of b Cu = c = and4.266(2)Å, Cu2O in α the= β spent= γ = 90°). catalyst Thewere X-ray found amorphous to be 31.0 SiO2 and phase 4.0 has nm disappeared (Table1), respectively, after the reaction. indicating The thatCSD the values copper of particles Cu0 and in Cu these2O samplesin the spent were catalyst covered withwere afound thin film to be of copper31.0 and oxide, 4.0 whilenm (Table during 1), therespectively, reaction, coalescence indicating ofthat metallic the copper copper particles particles in these was observed. samples were The molarcovered ratio with of a the thin metallic film of andcopper oxidized oxide, copper while phases during decreased the reaction, after coalescence the reaction, of indicatingmetallic copper that oxidation particles of was copper observed. particles The occurredmolar ratio along of withthe metallic their sintering. and oxidized Reductive copper regeneration phases decreased of the spent after catalyst the reaction, by thermal indicating treatment that byoxidation molecular of hydrogencopper particles enabled occurred restoration along of with catalytic their activity,sintering. mainly Reductive because regeneration the oxidized of the surface spent coppercatalyst layers by thermal could betreatment fairly well by reducedmolecular under hydrogen appropriate enabled reductive restoration conditions. of catalytic activity, mainly because the oxidized surface copper layers could be fairly well reduced under appropriate reductive conditions.Table 1. X-ray diffraction (XRD) analysis of freshly reduced and spent Cu/SiO2 catalysts. Phase Content, Molar Ratio Unit Cell Parameter a = b = c, Å, CSD, nm Catalyst wt.% of Phases Determined/Standard 0 0 0 0 Cu Cu2O Cu Cu2O Cu /Cu2O Cu Cu2O

Cu/SiO2 fresh 6.5 2.5 30 70 ~1/1 3.613(2)/3.615 4.225(3)/4.269

Cu/SiO2 spent 31.0 4.0 7 93 ~1/6 3.615(1)/3.615 4.266(2)/4.269 Reactions 2020, 1 68 Reactions 2020, 3, x FOR PEER REVIEW 15 of 18

500 I - Cu O SiO 400 2 2 I - Cu O 0 2 I - Cu 0 400 I - Cu 300 ) s 300 / mp

(i 200

I (imp/s) 200

100 100

0 20 30 40ο 50 60 70 0 20 30 40ο 50 60 70 2Θ 2Θ (a) (b)

Figure 9. DiffractionDiﬀraction patternspatterns ofof thethe catalystscatalysts in in comparison comparison with with the the position position and and relative relative intensity intensity of 0 0 ofthe the crystallographic crystallographic phases phases Cu Cuand and Cu Cu2O:2O: (a )(a Cu) Cu/SiO/SiO2 freshly2 freshly reduced; reduced; (b ()b Cu) Cu/SiO/SiO2 spent.2 spent.

4. Conclusions Table 1. X-ray diffraction (XRD) analysis of freshly reduced and spent Cu/SiO2 catalysts. γ Hydrogenation of -valerolactonePhase Content, (GVL) inMolar polar Ratio solvents Unit (n Cell-butanol, Parameter 1,4-dioxane) a = b = c, Å, to CSD, nm 1,4-pentanediolCatalyst (PDO) and 2-methyltetrahydrofuranwt.% (MTHF)of Phases was performedDetermined/Standard at 363–443K in a fixed bed reactor under overallCu0 HCu2 2pressureO Cu0 of 0.7–1.3Cu2O MPa.Cu Preliminary0/Cu2O screeningCu0 in a batchCu reactor2O was performedCu/SiO2 fresh in comparable 6.5 reaction 2.5 conditions 30 70 with a series ~1/1 of Ru, Ir, 3.613(2)/3.615 Pt, Co, and Cu catalysts,4.225(3)/4.269 earlier effiCu/SiOciently2 spent applied for 31.0 levulinic 4.0 acid hydrogenation 7 93 to GVL. ~1/6 Among 3.615(1)/3.615 the catalysts, Cu 4.266(2)/4.269/SiO2 provided better selectivity of 67% towards PDO at 32% GVL conversion in a continuous flow reactor using 4.n-butanol Conclusions as a solvent. This catalyst was applied to study the effect of temperature, hydrogen pressure, and contact time. The main reaction products were PDO, MTHF, and 1-pentanol, while no valeric acid Hydrogenation of γ-valerolactone (GVL) in polar solvents (n-butanol, 1,4-dioxane) to 1,4- was observed. While Cu/SiO activity slightly decreased with time-on-stream (9 h) as a result of a pentanediol (PDO) and 2-methyltetrahydrofuran2 (MTHF) was performed at 363–443K in a fixed bed partial oxidation of metallic copper, it can be restored after treatment by molecular hydrogen at 523 K reactor under overall H2 pressure of 0.7–1.3 MPa. Preliminary screening in a batch reactor was (2 h), demonstrating fair catalyst stability. performed in comparable reaction conditions with a series of Ru, Ir, Pt, Co, and Cu catalysts, earlier Summarizing all experimental observations and literature data, it is inferred that PDO formation efficiently applied for levulinic acid hydrogenation to GVL. Among the catalysts, Cu/SiO2 provided can be selectively achieved over silica supported copper bifunctional catalysts in mild conditions. The better selectivity of 67% towards PDO at 32% GVL conversion in a continuous flow reactor using n- acidity of silica supported catalyst could be balanced by the Cu/Si ratio, Cu loading, and preparation butanol as a solvent. This catalyst was applied to study the effect of temperature, hydrogen pressure, conditions, e.g., temperature of the reductive activation. This finding opens a promising way to and contact time. The main reaction products were PDO, MTHF, and 1-pentanol, while no valeric develop an efficient approach for biogenic diols’ preparation as an alternative to currently applied acid was observed. While Cu/SiO2 activity slightly decreased with time-on-stream (9 h) as a result of apetroleum-based partial oxidation method, of metallic as well copper, as to it integrate can be restored GVL into after PDO treatment stage into by biorefinery molecular strategy hydrogen realizing at 523 Kin (2 mild h), reactiondemonstrating conditions. fair catalyst stability. SupplementarySummarizing Materials: all experimentalThe following observations are available and onlineliterature at http: data,// www.mdpi.comit is inferred that/2624-781X PDO formation/1/2/6/s1, canTable be S1: selectively Liquid-phase achieved GVL hydrogenation over silica supported in a batch copper mode. Reactionbifunctional conditions: catalysts GVL in (6.9 mild mmol), conditions. solvent The(15 mL), acidity mcat =of0.24 silica g, T =supported393 K, PH2 catalyst= 3.0 MPa, could time =be4 h;balanced Figure S1: by Temperature the Cu/Si dependence ratio, Cu inloading, the Arrhenius and 1 preparationcoordinates. Reactionconditions, conditions: e.g., temper 10.1%ature GVL inof nthe-butanol, reductive GVL activation. feed rate 2.1 This g h− finding; catalyst opens weight, a promising 0.455 g; H2 pressure 1.3 MPa; H flow rate 167 cm3 min 1. way to develop an2 efficient approach for− biogenic diols’ preparation as an alternative to currently appliedAuthor Contributions: petroleum-basedConceptualization, method, as I.S.well and as V.B.; to Methodology, integrate GVL D.Y.M.; into Formal PDO Analysis, stage into S.P.; Investigation,biorefinery Y.D. and M.S.; Resources, P.D.; Data Curation, P.N. and Y.D.; Writing—Original Draft Preparation, Y.D. and strategyI.S.; Writing—Review realizing in mild & Editing, reaction D.Y.M.; conditions. Visualization, S.P. and M.S.; Supervision, I.S., P.D., and V.B.; Project Administration, I.S. and V.B.; Funding Acquisition, V.B. and I.S. All authors have read and agreed to the published Supplementaryversion of the manuscript. Materials: The following are available online at www.mdpi.com/xxx/s1, Table S1: Liquid-phase GVLFunding: hydrogenationThis work in was a batch supported mode. by Reaction RFBR Grant conditions: 18-53-45013 GVL (6.9 IND_a. mmol), Funding solvent under (15 mL), INT/ RUSmcat /=RFBR 0.24 /g,P-323 T = 393project K, PH is acknowledged.2 = 3.0 MPa, time = 4 h; Figure S1: Temperature dependence in the Arrhenius coordinates. Reaction conditions: 10.1% GVL in n-butanol, GVL feed rate 2.1 g h−1; catalyst weight, 0.455 g; H2 pressure 1.3 MPa; H2 Acknowledgments: The authors thank engineer O.G. Arkhipova (experimental assistance), E. Gerasimov (TEM), flow rate 167 cm3 min−1. N. Shtertser (H2-TPR), I. Krayevskaya (XRF), and A. Leonova (N2 physisorption) for catalysts’ characterization, and V.A. Utkin, for GC/MS analysis. The authors wish to express their gratitude to T.P. Minyukova for providing Author Contributions: Conceptualization, I.S. and V.B.; Methodology, D.Y.M.; Formal Analysis, S.P.; characterization data of Cu/SiO2 and helpful discussions. Investigation, Y.D. and M.S.; Resources, P.D.; Data Curation, P.N. and Y.D.; Writing—Original Draft Preparation, Y.D. and I.S.; Writing—Review & Editing, D.Y.M.; Visualization, S.P. and M.S.; Supervision, I.S., P.D., and V.B.; Project Administration, I.S. and V.B.; Funding Acquisition, V.B. and I.S. All authors have read and agreed to the published version of the manuscript. Reactions 2020, 1 69

Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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