molecules Article
Selective Oxidation of HMF via Catalytic and Photocatalytic Processes Using Metal-Supported Catalysts Alice Lolli 1 , Valeriia Maslova 1,2 , Danilo Bonincontro 1,2 , Francesco Basile 1 , Simona Ortelli 3 and Stefania Albonetti 1,3, * 1
2 3
*
Department of Industrial Chemistry “Toso Montanari”, Bologna University, Viale Risorgimento 4, 40136 Bologna, Italy;
[email protected] (A.L.);
[email protected] (V.M.);
[email protected] (D.B.);
[email protected] (F.B.) C2P2, UMR 5265, CNRS–Univeristé de Lyon1 UCBL–CPE Lyon, Université de Lyon, 43 Boulevard du 11 Novembre 1918, 69616 Villeurbanne, France ISTEC-CNR, Institute of Science and Technology for Ceramics, National Research Council, Via Granarolo 64, 48018 Faenza, Italy;
[email protected] Correspondence:
[email protected]; Tel.: +39-051-209-3681
Academic Editors: Michela Signoretto and Federica Menegazzo Received: 5 October 2018; Accepted: 25 October 2018; Published: 27 October 2018
Abstract: In this study, 5-hydroxymethylfurfural (HMF) oxidation was carried out via both the catalytic and the photocatalytic approach. Special attention was devoted to the preparation of the TiO2 -based catalysts, since this oxide has been widely used for catalytic and photocatalytic application in alcohol oxidation reactions. Thus, in the catalytic process, the colloidal heterocoagulation of very stable sols, followed by the spray-freeze-drying (SFD) approach, was successfully applied for the preparation of nanostructured porous TiO2 -SiO2 mixed-oxides with high surface areas. The versatility of the process made it possible to encapsulate Pt particles and use this material in the liquid-phase oxidation of HMF. The photocatalytic activity of a commercial titania and a homemade oxide prepared with the microemulsion technique was then compared. The influence of gold, base addition, and oxygen content on product distribution in the photocatalytic process was evaluated. Keywords: 5-hydroxymethyl furfural; spray-freeze drying; photocatalysis; TiO2 ; microemulsion
1. Introduction 5-hydroxymethyl furfural (HMF) is still one of the most-studied platform molecules for the production of fuels and chemicals from renewable biomass sources. This is thanks to its chemical structure, which includes a furan ring, a hydroxyl group, and a formyl group that can undergo different reactions such as reduction, oxidation, and esterification. HMF oxidation has been widely studied over the past two decades using different reaction conditions and catalysts [1–5]. Scheme 1 shows the general HMF oxidation pattern. Indeed, HMF can be transformed in different ways: the carbonyl group can be oxidized to the carboxylic moiety, producing 5-hydroxymethyl-2-furancarboxylic acid (HMFCA); the oxidation of the hydroxymethyl group can then produce FDCA via 5-formyl-2-furancaboxylic acid (FFCA) intermediate formation. Moreover, the formation of 2,5-diformylfuran (DFF) can also be observed, mainly in the absence of an added base and with metals other than Au [6–8].
Molecules 2018, 23, 2792; doi:10.3390/molecules23112792
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Scheme 1. Reaction pathways for 5-hydroxymethylfurfural (HMF) oxidation.
Some characterised by by diols diols or or diacid functionalities are are used used in in the Some HMF HMF derivatives derivatives characterised diacid double double functionalities the polymer polyester production production [9]. As an an example, example, 2,5-furandicarboxylic 2,5-furandicarboxylic acid acid (FDCA), (FDCA), polymer industry industry for for bio bio polyester [9]. As produced is one for the the production of produced from from HMF HMF oxidation, oxidation, is one of of the the most most promising promising intermediates intermediates for production of poly(ethylene furanoate) (PEF), the furan-based analogue to poly(ethylene terephthalate) (PET), poly(ethylene furanoate) (PEF), the furan-based analogue to poly(ethylene terephthalate) (PET), which results have demonstrated which is is the the dominant dominant polymer polymer in in beverage beverage packaging packaging industries. industries. Recent Recent results have demonstrated that poly(ethylene furanoate), when manufactured with bio-sourced ethylene glycol, provides a that poly(ethylene furanoate), when manufactured with bio-sourced ethylene glycol, provides a 100% 100% renewable polymer with an enhanced and carbon permeability compared renewable polymer with an enhanced oxygenoxygen and carbon dioxidedioxide permeability compared to PET, to PET, the despite the an fact that aninincrease in COis is observed for [10,11]. PEF polymer [10,11]. 2 solubility despite fact that increase CO2 solubility observed for PEF polymer Moreover, the Moreover, the chain mobility, reduced due to the suppression of the furan ring-flipping—because chain mobility, reduced due to the suppression of the furan ring-flipping—because of an increased of an increased furan ring hindrance, which for is characteristic the bio-based polymer—affects furan ring hindrance, which is characteristic the bio-basedforpolymer—affects the overall CO2 the overall CO transportation properties. A decrease of the oxygen permeability and higher glass 2 transportation properties. A decrease of the oxygen permeability and higher glass transition transition temperature are present in PEF with respect to its terephtalic acid counterpart [12]. Moreover, temperature are present in PEF with respect to its terephtalic acid counterpart [12]. Moreover, recent recent the enzymatic hydrolysis of PEF powder highlighted thepossibility possibilityofof both both surface studiesstudies on theonenzymatic hydrolysis of PEF powder highlighted the functionalization and the polymer recycling process, thus opening up the prospects for a higher-value functionalization and the polymer recycling process, thus opening up the prospects for a higherapplication of thisof material [13,14].[13,14]. value application this material Today many many companies are very very interested in developing processes for production of FDCA to be used as aa monomer for polyester, monomer for polyester, polyamide, polyamide, and and polyurethane polyurethane synthesis. synthesis. As a result, several patents have been recently published on this subject; among the most recent ones, Dumesic et al. [15] patented the process for FDCA production from C6 sugars, by oxidizing HMF to FDCA with and without separating HMF from the reaction solution containing the by-products. HMF is obtained by dehydrating sugars in a lactone solvent using a Brönsted or Lewis acid catalyst and is oxidised using molecular oxygen oxygen and and aa metal metal supported supportedcatalyst catalystininthe theabsence absenceofofa abase. base.For For this reason, FDCA this reason, FDCA is is extracted the endofofthe thereaction reactionusing usingan anaromatic aromaticsolvent. solvent.Another Anotherpatent, patent, published published in in 2017, extracted at at the end converts HMF metal salt catalyst and water as HMF to to FDCA FDCAwith withmolecular molecularoxygen oxygenusing usinga ahomogeneous homogeneous metal salt catalyst and water solvent [16]. Moreover, FDCA can be produced through an enzymatic pathway starting from glucose as solvent [16]. Moreover, FDCA can be produced through an enzymatic pathway starting from or other or sugar derivatives; the patented enzymes enzymes can perform desired reaction high specificity glucose other sugar derivatives; the patented canthe perform the desiredwith reaction with high and efficiency Other[17]. two-step patented SequeirabyetSequeira al. for the specificity and [17]. efficiency Otherprocesses two-step were processes werebypatented et integrated al. for the process thatprocess generates from HMF aqueous carbohydrate solution and oxidisesand it into FDCA integrated thatHMF generates from aqueous carbohydrate solution oxidises it [18]. into Van Harven et al. Harven [19] produced mixture of 2,4-FDCA and 2,5-FDCA from and a disproportionation FDCA [18]. Van et al. a[19] produced a mixture of 2,4-FDCA 2,5-FDCA from ofa furoic acid salts, obtained from oxidation in an alkaline solutioninand metal-based catalyst. disproportionation of furoic acidfurfural salts, obtained from furfural oxidation an alkaline solution and Other patentscatalyst. on the production of purified and dried FDCA using and different derivatives as the metal-based Other patents on the production of purified driedHMF FDCA using different starting materials as have reported [20,21]. HMF derivatives thebeen starting materials have been reported [20,21]. From the industrial standpoint, some pilot scale scale plants plants have have already already been been developed developed [22,23]. [22,23]. Synvina, the hashas already started a pilot plant for the joint jointventure venturecompany companybetween betweenBASF BASFand andAvantium, Avantium, already started a pilot plant PEF production and is alsoisdeveloping technologies for PEFfor recycling [24]. Metal-supported catalyst for PEF production and also developing technologies PEF recycling [24]. Metal-supported and oxygen mainly used in these pilot-scale Nevertheless, the precise route for the synthesis catalyst andare oxygen are mainly used in these plants. pilot-scale plants. Nevertheless, the precise route for of has not yet been identified, but the current technology for terephthalic theFDCA synthesis of FDCA has not yet been identified, but the current technologyacid for production terephthalicusing acid metal/bromide catalysts is being evaluated. One drawback of these catalytic systems is the use of production using metal/bromide catalysts is being evaluated. One drawback of these catalytic corrosive and dangerousmedia compounds, which make the process polluting. systems ismedia the use of corrosive and dangerous compounds, which make the process polluting. Therefore, the preparation of active and stable metal-supported catalysts, also combing two metals in the form of an alloy, can be of great of interest [25]. The most commonly used monometallic
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Therefore, the preparation of active and stable metal-supported catalysts, also combing two metals in the form of an alloy, can be of great of interest [25]. The most commonly used monometallic supported catalysts are based on Au [26–30], Pd [2,31,32], and Pt [33–37]. The tailoring of metal particles in terms of size and shape can be a useful tool for increasing catalyst activity and stability. AuCu [38–40] and AuPd [5,41–45] bimetallic nanoparticles in the form of alloys and core-shells were found to be active in HMF, showing superior properties compared to their monometallic counterpart. However, the support must also be taken into account in a process development, since it is very well known that a catalyst prepared with the same metal active phase may display different catalytic performances depending on the support used [1,46–49]. Thus, the study of innovative processes aiming at the preparation of metal-supported catalysts, characterized by high surface area and high metal dispersion, is becoming a very important topic. The selection of the method for support preparation is a key factor for catalyst development, since it can affect the thermal and mechanical properties of the material, together with other chemical-physical features such as surface area porosity. As an example, porous ceramics with an open-cell structure are considered suitable catalyst supports; however, some industrial applications might require high temperature-resistant catalysts together with high porosity systems to facilitate mass transfer. Magnesia ceramics prepared with the spray-freeze drying technique made it possible to prepare high-thermal-strength materials, which were also characterized by a large surface area [50]. The spray-freeze drying (SFD) technique is an industrial process which consists of removing water from frozen samples by sublimation and desorption under a vacuum; it has also been applied to the preparation of nanostructured materials because it made it possible to maintain the nanometric size of the phase, while avoiding the agglomeration and segregation of components [51], while at the same time, increasing the stability of the system [52,53]. Moreover, this technique can be used for the homogeneous embedding of active phases into the support, minimizing the possibility of phase separation on a molecular scale. Much effort has been devoted to the formulation of the starting suspension for the identification of the optimal quantity that maximizes stability, safety, and marketability of a given product [54]. In this work, special attention was paid to the development of synthetic procedures for the preparation of high-surface-area supports. At first, the spray-freeze drying approach was used for the preparation of round, highly porous TiO2 -SiO2 grains with a size in the 10–100 µm range; then, a microemulsion procedure was optimized to obtain titania of high surface area and small particle size. Pt-based catalysts supported on a nanostructured TiO2 -SiO2 matrix were prepared by SFD. The support was prepared by heterocoagulation of the nanometric suspension of the oxides together with the metallic salt in a self-assembling approach, which exploits the surface charge of different materials to induce the spontaneous organization of the starting materials [55]. The samples obtained from nanosol heterocoagulation were then used to prepare granules using the spray-freeze-granulator, thus leading to the formation of a micrometric catalyst. Moreover, the possibility to convert the HMF selectively using a photocatalyst active under sunlight at ambient temperature was investigated. First, the effect of titania preparation was studied, comparing the activity of a TiO2 homemade support (TiO2 -m) prepared by microemulsion and commercial titania (TiO2 -c). Microemulsion—which is defined as an isotropic and thermodynamically stable dispersion made up of water, oil, and surfactants whose diameters vary from approximately one to 100 nm [56]—has become one of the most-studied methods for the synthesis of nanomaterials [57]. The preparation of nano-oxides by microemulsion is a very interesting field, which has been widely studied in literature [58–66]. Currently, there is a growing interest in the photocatalytic synthesis of organics, mainly the oxidation of hydroxyl functional groups to aldehydes, because it enables preventing the use of strong chemical oxidants, toxic solvents and by-products, high temperatures, and pressures [67–77]. Therefore, the goal of this paper was to investigate the selective oxidation of HMF by both a heterogeneous batch process and a photocatalytic approach, using Pt- and Au-TiO2 -based supported
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catalysts and devoting special attention to material preparation for promoting the catalytic activity 4 of 24 and selectivity.
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2. Results and Discussion 2. Results and Discussion 2.1. 2.1. HMF HMF Oxidation Oxidation Using Using Heterogeneous Heterogeneous Catalysts Catalysts In the present present study, study,Pt/TiO Pt/TiO2/SiO 2 samples prepared by the spray-freeze drying technique were In the 2 /SiO2 samples prepared by the spray-freeze drying technique were tested in the liquid-phase oxidation of HMF in in base-free base-free conditions. conditions. tested in the liquid-phase oxidation of HMF 2.1.1. 2.1.1. Catalyst Catalyst Preparation Preparation and and Characterisation Characterisation The preliminaryapproach approachtotothe the spray-freeze drying technique for catalyst preparation The preliminary spray-freeze drying technique for catalyst preparation dealsdeals with with the study the reagents as starting materials andoptimization the optimization of amounts their amounts be the study of theofreagents used used as starting materials and the of their to be to used used in the synthesis. From the industrial standpoint, formulation is a key step in the development in the synthesis. From the industrial standpoint, formulation is a key step in the development of new of new materials, theamount relativeofamount of theused reagents used the may affect the chemical-physical materials, since thesince relative the reagents may affect chemical-physical properties of properties of the final solid [54]. the final solid [54]. With this aim, aim, the the measurements measurements of of the the zeta zeta potential potential of of colloidal colloidal SiO SiO22 (LUDOX With this (LUDOX HS-40) HS-40) and and the the water suspension suspension of commercial TiO TiO22 (AEROXIDE wide water of commercial (AEROXIDE P25) P25) have have been been investigated investigated (Figure (Figure 1) 1) in in aa wide range range of of pH. pH.
Figure 1. Zeta potential of SiO2 (x) and TiO2 (•) as a function of pH. Figure 1. Zeta potential of SiO2 (x) and TiO2 (•) as a function of pH.
The results obtained highlighted the different behaviors of the two materials; titania was The results theindifferent the two materials; point titaniaatwas characterized by obtained a positivehighlighted zeta potential the rangebehaviors two–eight,ofhaving its isoelectric 8.5, characterized by a positive zeta potential in the range two–eight, having its isoelectric point at 8.5, while silica showed a negative zeta potential in the full studied range. Moreover, the very low potential while silica showed negative zetaofpotential in the its fullhigh studied range. Moreover, the very low values (between −30aand −50 mV) SiO2 indicated colloidal stability. By exploiting the potential values (between and characterizes −50 mV) of SiO 2 indicated colloidal stability. exploiting opposite superficial charge,−30 which these oxides, its it ishigh possible to design newBy materials by the opposite superficial charge, which characterizes these oxides, it is possible to design new heterocoagulation. The spray-freeze drying of the suspension containing both titania and silica in the materials heterocoagulation. spray-freeze drying of the suspension containing both ratio 1:0.5 by (w/w) made it possibleThe to obtain micrometric round grains as reported in Figure 2. titania and silica in the ratio 1:0.5 (w/w) made it possible to obtain micrometric round grains as reported in Figure 2.
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Figure TiSi (TiO (TiO22:SiO :SiO22 1:0.5 Figure 2. 2. SEM SEM images images of of the the sample sample TiSi 1:0.5 w/w) w/w)characterized characterized both both by by irregular irregular shaped shaped grains spherical granules granules (c,d). (c,d). grains (a,b) (a,b) and and spherical
The use of the same procedure procedure for preparing only TiO22-based materials did not permit obtaining regular round granules; furthermore, furthermore, silica silica was was fundamental fundamental in in the the process process of of heterocoagulation, heterocoagulation, since itithelped thethe mechanical strength of theofgrains. Silica also had also the role promoting helpedincrease increase mechanical strength the grains. Silica hadofthe role of granulation, i.e., the formation of more spherical and and homogeneous granules promoting granulation, i.e., the formation of more spherical homogeneous granules(Figure (Figure 2c,d). 2c,d). Additionally,the theaddition additionofofa avery verysmall small amount SiO 2 helped preparation of these microAdditionally, amount of of SiO in in thethe preparation of these micro-size 2 helped size grains a packed porosity, even though obtainedsamples samplesalso alsocontained contained some some irregularly grains withwith a packed porosity, even though thethe obtained shaped grains typically obtained in the of TiOof2. The of a mixed grains(Figure (Figure2a,b), 2a,b), typically obtained in granulation the granulation TiO2preparation . The preparation of TiO 2-SiO2TiO material zeta potential. In potential. Figure 3 theInmeasurements the zeta potential of a mixed materialthe modified the zeta Figure 3 the of measurements of the 2 -SiO2modified starting materials are reported andare compared thecompared TiO2-SiO2to1.0.5 (TiSi) and platinumzeta potential of starting materials reportedtoand the wt TiO% -SiO 1.0.5 wt % (TiSi) 2 2 containing catalysts (TiPt, catalysts TiSiPt). The isoelectric of the TiSipoint sample 1.9, thus completely and platinum-containing (TiPt, TiSiPt). point The isoelectric of was the TiSi sample was 1.9, changing what was previously with starting materials. The presence of silica, thus completely changing whatobserved was previously observed with starting materials. The although presence in of small it possible to reverse thepossible TiO2 zeta sign,TiO thus demonstrating that TiO 2 silica, amounts, althoughmade in small amounts, made it topotential reverse the zeta potential sign, thus 2 was surrounded by TiO silica NPs.surrounded The addition Pt metal saltsaddition was taken into account; )Pt(NO 3)2 demonstrating that by of silica NPs. The of Pt metal salts (NH was 3taken into 2 was was dissolved the aqueous suspension containing the two oxides before spray-freezing. Pt addition account; (NH3in )Pt(NO ) was dissolved in the aqueous suspension containing the two oxides before 3 2 did not lead to Pt any significant change potential curve. Thiszeta trend was also confirmed by spray-freezing. addition did not lead in to the anyzeta significant change in the potential curve. This trend the preparation of the sample. Thus, and sample. TiSiPt were characterized by very Z potentials, was also confirmed byTiPt the preparation of TiSi the TiPt Thus, TiSi and TiSiPt werelow characterized by indicating the suspension used spray-freeze is spray-freeze stable. very low Zthat potentials, indicating thatfor the suspensiondrying used for drying is stable.
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Figure 3. Zeta potential of suspensions containing SiO2 (x), TiO2 (•, solid line) and Pt containing Figure 3. Zeta (potential suspensions containing 2 (x), TiO2 (•, solid line) and Pt containing mixtures—TiPt •, brokenof line), TiSi (N), and TiSiPt (NSiO , broken line)—as a function of pH. mixtures—TiPt (•, broken line), TiSi (▲), and TiSiPt (▲, broken line)—as a function of pH.
The presence of silica strongly increased the surface area of the material (Table 1). In fact, while 2 /g) presence of silicaarea strongly the surface area of the material (Table In fact, while silica The colloid has a surface of 210increased m2 /g, titania P25 is characterized by a very low 1). value (46 m 2 2 2 silicathe colloid hasofa the surface of ratio 210 mTiO /g,2 titania P25 is characterized by area a very value and mixing two area in the :SiO2 1:0.5 brings the surface uplow to 100 m (46 /g.mIt/g) is 2 and the mixing of the two in the ratio TiO 2 :SiO 2 1:0.5 brings the surface area up to 100 m /g. It is important to note that the spray-freeze drying technique, also, is responsible for the increased surface important to note that the spray-freeze drying technique, also, is responsible for the increased surface area of the sample, which is an important feature to be taken into account in catalyst preparation. As a area of of thefact, sample, which is an important to besuspension taken into account catalyst preparation. As matter the spray-freeze drying of feature titania P25 led to a in significant increase in the 2 a matter of fact, the spray-freeze drying of titania P25 suspension led to a significant increase in the surface area of the sample (59 m /g), despite the fact that grain formation did not occur without silica. surface area of samplereduced (59 m2/g), factofthat grain formation The addition ofthe platinum thedespite surfacethe area Ti and TiSi slightly. did not occur without silica. The addition of platinum reduced the surface area of Ti and TiSi slightly. Table 1. Silica, Platinum content, and surface area of the samples prepared. Table 1. Silica, Platinum content, and surface area of the samples prepared. Sample Name TiO2 :SiO2 (w/w%) Pt (wt %) Surface Area (m2 /g) Sample Name TiO2:SiO2 (w/w%) Pt (wt %) Surface Area (m2/g) SiO2 210 SiO2 210 TiO2 (P25) 46 - 46 Ti TiO2 (P25) - 59 Ti - 59 TiSi 1:0.5100 1:0.5 - 0.5 100 TiPt TiSi 41 0.5 0.5 41 TiSiPt TiPt 1:0.591 TiSiPt 1:0.5 0.5 91
Furthermore, the SEM images in Figure 4 show that grain formation and the macro-porosity of Furthermore, the SEM images in Figure 4 show that grain formation and the macro-porosity of the material are not affected by platinum introduction. However, sample TiPt, which was prepared the material are not affected by platinum introduction. However, sample TiPt, which was prepared without the addition of silica, confirmed once again that only the titania suspension did not permit a without the addition of silica, confirmed once again that only the titania suspension did not permit a proper spray-freeze drying process, preventing homogeneous round grain formation (Figure 4a,b). proper spray-freeze drying process, preventing homogeneous round grain formation (Figure 4a,b). Nevertheless, even in this case, the preservation of the nanostructuring was demonstrated (Figure 4c). Nevertheless, even in this case, the preservation of the nanostructuring was demonstrated (Figure 4c).
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Figure images of (a,b) TiPtTiPt andand (d,e)(d,e) TiSiPt; (c) high SEM image TiPt sample Figure 4. 4.SEM SEM images of (a,b) TiSiPt; (c) magnification high magnification SEM of image of TiPt nanostructuring. sample nanostructuring.
The prepared samples were were treated treated under underH H22 flow flow at 400 ◦°C C to reduce the metal; then, the particle particle size distribution was evaluated through TEM analysis. Figure 5 shows TEM images of the TiPt sample size distribution was evaluated through TEM analysis. Figure 5 shows TEM images of the TiPt showing the presence of small and and well-dispersed metal particles with a anarrow sample showing the presence of small well-dispersed metal particles with narrowparticle particle size distribution distribution centered centered on on 2.2 2.2 nm. nm. Titania Titania particles particles can can be be well well distinguished distinguished from from Pt Pt because because of of their their size seen clearly in in thethe TiSiPt sample (Figure 6) because of its size and and shape. shape.The Thepresence presenceofofsilica silicacan canbebe seen clearly TiSiPt sample (Figure 6) because of round shape. In this case, also,also, metal particles are well dispersed and and havehave a very small diameter (2.8 its round shape. In this case, metal particles are well dispersed a very small diameter nm), meaning thatthat the the presence of silica made it possible totoprepare (2.8 nm), meaning presence of silica made it possible prepareaahomogeneous homogeneousmaterial material with with dispersed Pt nanoparticles.
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11
1.5 1.5
22
2.5 2.5
33
3.5 3.5
44
4.5 4.5 55
Figure TEM images and TiPt particle size distribution. Figure Figure5.5. 5.TEM TEMimages imagesand andTiPt TiPtparticle particlesize sizedistribution. distribution.
11 1.5 1.5 22 2.5 2.5 33 3.5 3.5 44 4.5 4.5 55 5.5 5.5 66
Figure Figure TEM images and TiSiPt particle size distribution. Figure6.6. 6.TEM TEMimages imagesand andTiSiPt TiSiPtparticle particlesize sizedistribution. distribution.
2.1.2. 2.1.2. Catalytic Catalytic Tests Tests The The catalyst catalyst TiSiPt TiSiPt and and the the reference reference material material TiPt TiPt were were tested tested in in the the liquid-phase liquid-phase oxidation oxidation of of HMF HMF in in aa batch batch reactor. reactor. The The preparation preparation of of catalytic catalytic materials materials using using the the spray-freeze spray-freeze drying drying process process
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2.1.2. Catalytic Tests The2018, catalyst TiSiPt the reference Molecules 23, x FOR PEERand REVIEW
material TiPt were tested in the liquid-phase oxidation 9 of of 24 HMF in a batch reactor. The preparation of catalytic materials using the spray-freeze drying process made 2-based micrometric grains with a higher surface areaarea compared to the made itit possible possibletotoobtain obtainTiO TiO micrometric grains with a higher surface compared to 2 -based commercial support. Moreover, thisthis synthetic approach permitted the commercial support. Moreover, synthetic approach permittedthe thepreparation preparationofof small small Pt nanoparticles characterized by a narrow narrow size size distribution distribution and and well welldispersed dispersedthroughout throughoutthe thesupport. support. All these features favorably affected the catalytic activity; the sample TiPt had an interesting catalytic activity activity in in the the absence absence of of a base, base, leading leading to to more more than than 50% 50% HMF HMF conversion conversion and and 10% 10% FDCA FDCA yield. yield. The addition additionof ofsilica silicato tothe thesystem systembrought broughtaalower lowerFDCA FDCAyield yield(4%), (4%), but butaahigher higherHMF HMFconversion. conversion. In the latter case, the product formed most was FFCA (33%); the lower FDCA yield observed may be related to a different reaction rate of FFCA transformation which, in this case, was the rate-limiting step in the process (Figure 7).
◦ C, 10 Catalytic activity activity of of the samples TiSiPt, TiPt. Reaction Reaction conditions: conditions: 66 h, h, 110 110 °C, 10 bar O22,, Figure 7. Catalytic HMF:Pt 1:0.01, Legend: HMF conversion ( ), HMFCA yield ( ), DFF yield ( ), FFCA yield (), and Legend: HMF conversion (■), HMFCA yield(■), DFF yield(■), FFCA yield(■), (). FDCA yield (■).
The conduction conduction of of the the process process without without aa base base slowed slowed down down the the aldehyde aldehyde oxidation; oxidation; in in fact, fact, in in The these conditions, conditions, HMF HMF conversion conversion is is never never complete, complete, and and both both DFF DFF and and FFCA FFCA are are always always present present in in these high amounts. Moreover, HMFCA is formed only in traces, meaning that under the studied conditions high amounts. Moreover, HMFCA is formed only in traces, meaning that under the studied the oxidation the alcoholic group is moregroup favored transformation the aldehyde. of the conditions theofoxidation of the alcoholic is than morethefavored than theoftransformation Subsequently, a base was added to the system to try to promote the catalytic activity and to check aldehyde. if there was a change in the reaction pathway: results were evaluated after the addition of NaOHand andtoa Subsequently, a base was added to the system to try to promote the catalytic activity milderifbase, 8). The reported highlighted the detrimental effect of the 2 CO3 (Figure check theresuch wasasaNa change in the reaction pathway:results results were evaluated after the addition of base addition. Surprisingly, the use of a milder base worsened the carbon balance, confirming the role NaOH and a milder base, such as Na2CO3 (Figure 8). The reported results highlighted the detrimental of OH− in the reaction Surprisingly, medium. Highthe pHuse canof cause HMF degradation, but the a significant amount effect ofgroup the base addition. a milder base worsened carbon balance, − of OH in the in -the presence an active catalyst,High can foster HMF oxidation to HMFCA [30]. confirming thesolution, role of OH group in theofreaction medium. pH can cause HMF degradation, but In both tests, HMF conversion increased, but the side reaction of by-product formation was enhanced. a significant amount of OH in the solution, in the presence of an active catalyst, can foster HMF FFCA wastoobserved highInyield, was detected only when sodium hydroxide oxidation HMFCAin[30]. bothwhile tests, HMFCA HMF conversion increased, but the side reaction of was byused. The basic environment, however, did not stimulate the Cannizzaro reaction on HMF, because no product formation was enhanced. FFCA was observed in high yield, while HMFCA was detected BHMF was observed in the reaction medium. only when sodium hydroxide was used. The basic environment, however, did not stimulate the Cannizzaro reaction on HMF, because no BHMF was observed in the reaction medium.
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Figure addition of of aa different different base. base. Reaction Figure 8. 8. Catalytic Catalytic activity activity of of the the sample sample TiSiPt TiSiPt with with the the addition Reaction ◦ conditions: 6 h, 110 °C, 10 bar O 2 , HMF:Pt:base 1:0.01:2, Legend: HMF conversion (■), conditions: 6 h, 110 C, 10 bar O2 , HMF:Pt:base 1:0.01:2, Legend: HMF conversion (), HMFCA HMFCA yield yield (■), C-loss (■). (),DFF DFFyield yield(■), (),FFCA FFCAyield yield(■), (),FDCA FDCAyield yield(■) (and ) and C-loss ().
Lastly, reaction time timeand andtemperature temperaturewere werestudied, studied,and andthe theincrease increaseofof these parameters Lastly, the reaction these parameters ledled to to enhancement of catalytic the catalytic performances 9). working In fact, working 4 htemperature at a mild an an enhancement of the performances (Figure(Figure 9). In fact, for 4 h at afor mild temperature only 10% HMF conversion and DFF and as FFCA formed as major (70 ◦ C), only (70 10%°C), HMF conversion was obtained,was andobtained, DFF and FFCA formed major products, with ◦ C for 6h products, yieldThe of about 5%. The conducted same process conducted at caused 110 °C for 6h caused aofconversion a yield of with abouta 5%. same process at 110 a conversion more than of more than 50%; was the most important product (33%) by most DFF (17%). The result most 50%; FFCA was theFFCA most important product (33%) followed by DFFfollowed (17%). The interesting interesting result was obtained with a 16h reaction yield; 89% HMF conversion). was obtained with a 16h reaction (29% FDCA yield;(29% 89% FDCA HMF conversion). FFCA formed in FFCA a high formed a highwhile amount while formed 9% yield. absence led to amountin(51%), DFF(51%), formed withDFF a 9% yield.with The aabsence of The HMFCA led of to HMFCA the conclusion the conclusion that reaction mechanisms passed basically via DFF formation, and without the that reaction mechanisms passed basically via DFF formation, and without the addition of the base, addition the base, formed onlyReported in smaller amounts. Reported results HMFCA of formed onlyHMFCA in smaller amounts. results demonstrate once againdemonstrate that when a once basic again that when a basic is not used, no by-products form. environment is not used,environment no by-products form.
Figure 9. 9. Catalytic Catalyticactivity activityof ofthe thesample sampleTiSiPt TiSiPt at at different different reaction reaction times times and and temperatures. temperatures. Reaction Reaction Figure conditions: 10 bar O , HMF:Pt 1:0.01, Legend: HMF conversion ( ), HMFCA yield ( ), DFF yield (), conditions: 10 bar O22, HMF:Pt 1:0.01, Legend: HMF conversion (■), HMFCA yield(■), DFF yield(■), FFCA yield ( ), FDCA yield ( ). FFCA yield(■), and FDCA yield (■).
2.2. Oxidation of HMF by a Photocatalytic Process 2.2. Oxidation of HMF by a Photocatalytic Process The second part of the work was devoted to the preparation of a homemade titania (TiO -m), using The second part of the work was devoted to the preparation of a homemade titania2 (TiO2-m), the microemulsion approach, to be used as the catalyst for the photooxidation of HMF. The effect of using the microemulsion approach, to be used as the catalyst for the photooxidation of HMF. The
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effect of base addition, O2 content, and the presence of gold were studied. The results obtained were base addition, O2those content, and the presence of goldTiO were studied. The results obtained were compared compared with obtained with commercial 2 (DT-51 Millennium chemicals) (TiO2-c). with those obtained with commercial TiO2 (DT-51 Millennium chemicals) (TiO2 -c). 2.2.1. Catalyst Preparation and Characterisation 2.2.1. Catalyst Preparation and Characterisation In order to carry out the photocatalytic oxidation of HMF, some preliminary tests using In order to carry out the photocatalytic oxidation of HMF, some preliminary tests using commercial commercial (TiO2-c) and homemade TiO2 (TiO2-m) were performed. Au nanoparticles were then (TiO2 -c) and homemade TiO2 (TiO2 -m) were performed. Au nanoparticles were then added to added to investigate the possibility to induce the selective conversion of HMF. The samples studied investigate the possibility to induce the selective conversion of HMF. The samples studied are shown are shown in Table 2. in Table 2. Table2.2.Prepared Preparedsamples samples and and main main characterization characterization data data obtained obtained from Table from XRD, XRD, UV-vis UV-vis spectroscopy, spectroscopy, and BET measurements. and BET measurements. Crystallite Size/nm Crystallite Size/nm λ/nm SBET/m2/g SBET /m2 /g λ/nm TiO2 Crystallites Au Crystallites TiO2 Crystallites Au Crystallites TiO2-c 17 371 82 TiO 17 2 -c2-m TiO 8.3 - 413 371 132 82 TiO2 -m 8.3 413 132 Au/TiO2-c 17
