Hydrodeoxygenation of 5-Hydroxymethylfurfural over Alumina

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Hydrodeoxygenation of 5-hydroxymethylfurfural over alumina-supported catalysts in aqueous medium Diana Paola Duarte, Ramiro Martínez, and Luis Javier Hoyos Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b02851 • Publication Date (Web): 20 Nov 2015 Downloaded from http://pubs.acs.org on November 24, 2015

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Industrial & Engineering Chemistry Research

Hydrodeoxygenation of 5-hydroxymethylfurfural over alumina-supported catalysts in aqueous medium Diana P. Duarte,† Ramiro Martínez† and Luis J. Hoyos*,§ †

Universidad Industrial de Santander, Cra. 27 Calle 9, Bucaramanga, Colombia

§

Instituto Colombiano del Petróleo – ICP, Ecopetrol S.A., km 7 vía a Piedecuesta, Piedecuesta,

Colombia Keywords. Hydrodeoxygenation, hydrogenation, decarbonylation, 5-hydroxymethylfurfural, transition metals, alumina.

Abstract. Catalytic activity and selectivity of Ni, Pd and Cu supported on γ-alumina catalysts for the 5-hydroxymethylfurfural hydrodeoxygenation in aqueous phase were studied in this work. The catalysts were prepared by incipient wetness impregnation and characterized by BET surface area, ICP-OES, dynamic chemisorption, temperature-programmed reduction and ATR-IR. The reactions were carried out in a downflow reactor at a hydrogen pressure of 0.41 MPa and a temperature range of 137-189 °C. A variety of valuable compounds were obtained depending on the nature of the metal deposited on the support. Ni and Pd-supported catalysts were selective towards products of the decarbonylation reaction of the HMF carbonyl group, whereas hydrogenolysis products were observed on Cu-supported catalyst. Additionally, the Pd-supported catalyst was quite selective to hydrogenate the furan ring, especially at high temperatures.

1 ACS Paragon Plus Environment


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1. Introduction New policies for environmental protection and economic sustainability have led to the research of renewable sources of fuels and chemicals. Biomass could be used as an alternative resource to fossil oil since it is abundant, sustainable and renewable. Several routes to produce fuels and chemicals from biomass are under intensive research and development efforts.1 Lignocellulosic ethanol, biomass pyrolysis and gasification are among the technologies of interest. However as their development progresses important challenges are appearing. Energy and water management in lignocelullosic ethanol, bio-oil upgrading in biomass pyrolysis, tar treatment and economies of scale in gasification remain as significant scientific and technological issues to solve. In recent years, a growing interest in other routes to produce chemicals and biofuels from catalytic conversion of biomass are being considered. One alternative route for producing biofuels and chemicals is through 5-hydroxymethylfurfural (HMF).2 This chemical platform can be produced from acid-catalyzed dehydration-cyclization reactions of hexoses which are formed by hydrolysis of carbohydrates in aqueous phase.3 HMF has a high reactivity due to its polyfunctionality, i.e. contains an aromatic alcohol, an aromatic aldehyde and a furan ring in the molecule. Reduction, oxidation, condensation and esterification reactions, among others, can be applied to convert HMF into a diversity of compounds for the biofuels, polymers, pharmaceutical and chemical industries. The reduction of HMF to 2,5dimethylfuran (DMF) for use as a transportation fuel,4,5 the partial hydrogenation of HMF to 2,5bis(hydroxymethyl)furan (BHF) for use as a monomer6 and the oxidation of HMF to 2,5furandicarboxylic acid (FDCA) for the synthesis of polyesters and pharmaceutical products2,7 are some examples of chemical transformations of HMF to added value products.


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Therefore, the hydrogenation/hydrogenolysis is a suitable process to convert HMF into useful compounds for the production of biofuels, chemicals and polymers. The HMF hydrogenation has been often studied using bulk and carbon-supported catalysts in autoclave reactors. Schiavo et al.8 reported that the hydrogenation of HMF in aqueous phase over copper chromites and carbonsupported platinum produced 2,5-bis(hydroxymethyl)furan (BHF, eq 1), whereas Raney nickel and carbon-supported palladium caused the hydrogenation of the furan ring producing mainly 2,5-bis(hydroxymethyl)-tetrahydrofuran (BHTF, eq 2). They also reported that the hydrogenation of HMF in acidic aqueous phase led to two other products: 1-hydroxyhexane-2,5-dione and hexane-1,2,5-triol in presence of carbon-supported Pt and Ru catalysts, whereas the Raney Ni and CuCr2O4 catalysts were inactive.8

(1)

(2)

Recently, a catalytic strategy was developed to produce 2,5-dimethylfuran (DMF) from fructose in a two-step process.4 In the first step, HMF is produced by the acid-catalyzed dehydration of fructose in a biphasic reactor using water and 1-butanol as solvents. The aqueous phase is the reactive phase containing the catalyst and the fructose, and 1-butanol is the phase that extracts the HMF product. Subsequently in the second step, the purified organic phase (HMF and 1butanol) is hydrogenolyzed to convert the HMF to DMF (eq 3). They prepared a carbon3 ACS Paragon Plus Environment


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supported Cu:Ru catalyst for the hydrogenolysis of HMF. When the reaction was carried out in a batch reactor the DMF yield was 71%, whereas the DMF yield was 76-79% in a continuous flow reactor.4

(3) Luijkx et al.9 also reported the HMF hydrogenolysis in alcoholic solvents. The reactions were carried out in 1-propanol using Pd/C catalysts. They observed unexpected intermediate compounds and selectivity towards DMF of up to 36%. Besides, the reaction in water at pH 2.5 and 60 °C led to the formation of BHF and 1-hydroxy-2,5-hexanedione, with selectivities of 46% and 28% respectively. The selectivity of DMF in water under the conditions mentioned did not exceed 1%.9 To knowledge of the authors there are few works which make a systematic study of the nature of metals on the hydrogenation of furan-type platform molecules,10-13 and even less carrying out the reaction in aqueous phase.8 In this work, we studied the HMF hydrodeoxygenation in presence of alumina-supported catalysts in aqueous phase. The catalytic activity was evaluated in a downflow reactor in view of a possible application to a large scale biorefinery process. Alumina was chosen as the catalyst support since it has been widely used in the manufacture of many industrial catalysts and its acidity could promote hydrolysis and dehydration reactions. The effect of metals such as Ni, Pd and Cu supported on alumina were evaluated in the HMF hydrodeoxygenation in order to compare their catalytic behavior.


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2. Experimental 2.1. Preparation of catalysts The γ-alumina support used in this study was prepared by calcination of cylindrical shape extrudates of 1mm in diameter using Boehmite Catapal B (Sasol) under air flow at 500 °C overnight. Three monometallic catalysts were prepared by incipient wetness impregnation of the extruded γ-alumina using as precursors: nickel nitrate hexahydrate (99 wt%, Aldrich), copper nitrate trihydrate (99.5 wt%, Aldrich) and palladium acetylacetonate (99 wt%, Aldrich). Each precursor solution was dropped on the calcined extruded support in order to obtain catalysts with nominal content of 10 wt% Ni, 10 wt% Cu and 1 wt% Pd, respectively. The impregnated samples were first dried in an oven at 120 °C overnight and then calcined in a gas stream of a 70:30 mixture of air:N2. Ni and Cu catalysts were calcined at 500 °C for 4 h and Pd catalyst was calcined at 350 °C for 4h. A heating ramp of 10 °C/min was used in all the calcination steps. 2.2. Catalyst characterization methods Surface area (SABET) and pore volume were determined from the nitrogen adsorption/desorption isotherms at 77 K using a Micromeritics ASAP 2020 Instrument. The samples were previously degassed at 350 °C for 12 h. Chemical composition of the catalysts was measured by inductively coupled plasma-optical emission spectroscopy (ICP-OES) with the aim of quantifying the content of Ni, Cu and Pd present in the samples. Prior to the analysis, the calcined catalysts were subjected to an acid digestion treatment with nitric acid. The analysis was carried out in a Perkin Elmer Optima 2100



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DV, equipped with a high-resolution double monochromator with a CCD array detector and using argon as plasma gas. Metal dispersion (D) of the catalysts was determined with a Micromeritics Autochem II 2920 using CO as reactant gas. Prior to the analysis, the calcined catalyst sample was dried at 120 °C using He at a flow rate of 20 cm3/min for 1 h to remove the physically adsorbed water. All catalysts were reduced in a H2 flow of 20 cm3/min, with a heating ramp of 10°C/min and once the reduction temperature was attained the catalyst was kept at this temperature for 2 h. The Ni and Cu-supported catalysts were reduced at 500 °C and the Pd-supported catalyst was reduced at 350 °C. Metal dispersion (D) and active metal particle size (dm) were determined from quantification of the irreversibly chemisorbed gas molecules. Following other authors in order to calculate metal dispersion, it was assumed a CO:metal stoichiometry of 1:1 for nickel and copper catalysts14,15 and 1:1.5 for palladium catalyst16,17. Temperature-programmed reduction (TPR) profiles of the calcined catalysts were obtained using a Micromeritics Autochem II 2920 Instrument. Prior to the analysis, the calcined catalyst samples were pretreated at 120 °C using argon for 1 h. The reduction of the catalysts was carried out under a 5% H2/Ar flow rate of 50 cm3/min, at a heating ramp of 10 °C/min from room temperature to 1000 °C. The hydrogen uptake was monitored by the change in the TCD signal. To determine the hydrogen consumption of the catalysts, a pure copper oxide sample was used to calculate the response factor of the equipment. Elemental analysis of carbon of the spent catalysts after the activity tests were measured on a LECO CS230 carbon determinator. Additionally, infrared spectra (IR) of the spent catalysts were measured on a Shimadzu IR- Prestidge 21 spectrometer equipped with an ATR attachment (Pike-


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Miracle) and a DTGS detector. Samples were grinded in a mortar before the analysis. 5 mg of the sample were pressed to obtain a self-supported wafer. Each spectrum was recorded with a resolution of 8 cm-1 and 32 scans from 4000 to 650 cm-1. All spectra were collected every 15 s. 2.3. Catalysts evaluation The experiments were carried out in a downflow reactor made of stainless steel with a continuous flow of reactants. A schematic flow diagram of the reactor system is illustrated in the supporting information (Scheme S1). The inlet diameter of the reactor is 1.27 cm. In all runs, the catalyst (4 g) was mixed with 2.5 g of 80-mesh sand to prevent poor distribution of the liquid, dried zones, hot spots and external diffusional limitations. The catalyst bed was contained in the tubular reactor by means of two end-plugs of quartz wool. A schematic representation of the packed-bed tubular downflow reactor used for the hydrodeoxygenation of HMF can be found in the supporting information (Scheme S2). Each catalyst was activated in situ with a H2 flow (100 cm3/min) at 500 °C for Ni and Cu catalysts and at 350 °C for the Pd catalyst during 4 h before the HMF hydrogenation tests. An aqueous solution of 5 wt% HMF is fed into the fixed-bed reactor using a Lab Alliance HPLC pump with a liquid feed rate of 0.2 cm3/min and a flow rate of 50 cm3/min of H2 was used in all cases. Pressure in the reactor was 0.41 MPa. The effluent from the reactor was condensed at room temperature in a stainless steel separator. All the experiments were carried out at three temperatures: 137 °C, 155 °C and 189 °C, using the same catalyst in a sequential way. Once the desired reaction temperature was achieved, 4 hours were awaited before product sampling (30 min) and after the reaction temperature was increased to the next level. An aliquot of the liquid aqueous product (3 g) was extracted with dichloromethane (60 cm3).



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Organic products extracted into the dichlomethane were analyzed by GC-MS and GC-FID. The samples were injected in an Agilent 7890A series chromatograph equipped with an Agilent DB5MS column (30 m x 0.25 mm x 0.25 µm) and an Agilent 5975C mass spectrometer. Gas-phase products were analyzed with an Agilent 6890 chromatograph following the standard UOP Method 539. HMF recovery was determined by HPLC analysis of the aqueous sample before and after dichloromethane extraction. The HPLC analysis was carried out in a VWR MerckHitachi L-2000 series LaChrom Elite system equipped with a diode array detector and an L-2400 UV detector. The reverse-phase column used was an Agilent Zorbax Eclipse XDB-C18 (150 mm x 4.6 mm ID, 5 µm particle size). The samples were eluted with 5 mM H2SO4 at 0.6 cm3/min. The injection volume was 20 µL. The wavelength monitored was 282 nm. Several aqueous samples were extracted and then the average recovery of HMF was calculated to be 60%. In the HPLC analysis of the aqueous phase samples after extraction, other compounds different from HMF were not observed. So all yields calculations were based on products concentration detected in the dichloromethane phase after extraction using GC-FID. The quantitative determination of other compounds was done using both external and internal standard methods to calculate the relative response factors of each compound. Dodecane was used as an internal standard.

The

following

compounds

were

used

for

the

calibration

method:

5-

hydroymethylfurfural (HMF), 5-methylfurfural (MFAL), 5-methyl-2-furfuryl alcohol (MFOL), furfural (FAL), 2-furfuryl alcohol (FOL), tetrahydrofurfuryl alcohol (THFOL), 2,5dimethylfuran (DMF), 2,5-dimethyltetrahydrofuran (DMTHF), 2,5-hexanedione (HD), 2,5diformylfuran (DFF) and 2-methylfuran (MF). Other compounds were detected in very low concentrations and the relative response factors of these compounds were calculated using the effective carbon number (ECN) concept.18 In this study the compounds at high selectivities are


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reported and the low concentration compounds are called “others” but most of them were identified by GC-MS. Conversion and yields were calculated with the following equations (eqs 4 and 5):

conversion % =

yield % =

'

∗ 100

∗ 100

(4)

(5)

After the reactions, all catalysts were dried in situ at atmospheric pressure using a N2 flow of 100 cm3/min at 189 °C during 12 h. Then, ATR-IR spectra and carbon content were determined on the spent catalyst.

3. Results and discussion 3.1. Catalyst characterization Table 1 presents the metal content, BET surface area, metal dispersion, active metal particle size and the hydrogen consumption per active metal of the catalysts prepared in this work. The metal content of each catalyst measured by ICP-OES gave a concentration similar to the nominal value. The surface areas of the Ni and Cu-supported catalysts are 85% and 79% of the alumina surface area respectively, while the Pd-supported catalyst presents a surface area similar to the alumina. In the case of Ni and Cu-supported catalysts, these results provide evidence that the surface area of the catalysts decreases due to the partial blocking of the support pores by the metal particles. Other authors have observed a similar behavior with Ni/alumina catalyst



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prepared at high concentrations of Ni.19-21 Pd/alumina catalyst presents a higher metal dispersion and metal surface area given that this catalyst has a lower metal content. The chemisorption data show that metals on the catalysts are well dispersed, presenting metal particle sizes smaller than 4 nm. The dispersion measurements found in this work are similar to those reported elsewhere for catalysts of similar Ni, Pd and Cu content.14,19,22-27 Table 1. Catalyst characterization Metal content (wt %) -

Surface area (m2/g) 213

Da (%) -

dpb (nm) -

H2/Mec (mol/mol) -

Ni/alumina

9.7

180

27.4

3.7

0.71

Pd/alumina

0.7

212

52.2

2.1

-

Cu/alumina

9.5

169

30.0

3.5

0.98

Catalyst alumina

a

D: metal dispersion (%) dp: active metal particle size (nm) c Hydrogen consumption per active metal (Me: Ni, Pd, Cu) presents on the catalysts b

TPR profiles of the catalysts are shown in Figure 1. The profiles of Ni and Pd-supported catalysts were magnified 9 and 30 times, respectively, for comparative purposes. Cu/alumina catalyst presents two overlapping peaks at 217 °C and 241 °C. In the literature the low temperature reduction peak is attributed to the reduction of highly dispersed amorphous CuO species and the high temperature peak to the reduction of highly dispersed bulk CuO.26,28,29 The TPR profile of Pd/alumina catalyst shows a small negative peak at 71 °C, assigned to the decomposition of the β-Pd hydride phase.24,30,31 No reduction peak of PdO is detected at the conditions of the TPR experiment since the reduction of these particles has been reported to be achieved at temperatures below 20 °C.24 Ni/alumina catalyst reduces between 400 and 900 °C. This broad region was deconvolved into three Gaussian-type peaks. The two high temperature


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peaks situated at 693 °C and at 786 °C have been related to Ni2+ ions occupying octahedral and tetrahedral sites in the Ni spinel structure, respectively.32-35 The low temperature peak centered at 576 °C could be ascribed to the reduction of NiO with a strong interaction between metal and support.36-38

Figure 1. TPR profiles of the calcined catalysts supported on alumina The amount of hydrogen consumed per mole of active metal was calculated from TPR data to evaluate the reducibility of the metals present on the alumina-supported catalysts (Table 1). It was assumed that the active metals were present as divalent oxides in the calcined catalysts. The hydrogen consumption per active metal are 0.71 and 0.98 for Ni and Cu based catalysts, respectively. The hydrogen consumption of Pd/alumina is not calculated given that no reduction



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peak is detected. Cu-supported catalyst is totally reduced whereas Ni/alumina has a lower degree of reduction. These results are in agreement with those reported by other authors.39-41 This low degree of reduction of nickel might be explained because some nickel species are incorporated within the support or can be blocked by other metal crystallites, thus making these species non reactive toward hydrogen at the conditions of the TPR experiments.42-44 3.2. Catalysts evaluation The influence of the metal nature supported on alumina in the reactivity of 5hydroxymethylfurfural (HMF) in aqueous medium was studied in this work. All the results of this study can be rationalized according to the chemical reactions network presented in Scheme 1. Scheme 1. Hydrodeoxygenation reaction pathways of 5-hydroxymethylfurfural


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The evolution of HMF conversion on alumina-supported catalysts as a function of time is presented in Figure 2. The reaction progress was monitored at 137 °C during the first 8 h in order to estimate the time necessary to approach to the steady state. The HMF conversion decreases during the first 180 min to achieve a steady state value around 50 wt%, 44 wt% and 39 wt% for the Pd, Ni and Cu-supported catalysts respectively. Consequently, the product sampling was established after 4 h of reaction, time required to reach steady state for all catalysts. The deactivation observed during the first minutes can be ascribed to carbonaceous material deposition on the surface or chemisorbed CO that accumulate on the surface and block a part of the active sites.45

Figure 2. HMF conversion vs. reaction time for the (▲) Ni/alumina, (♦) Pd/alumina and (■) Cu/alumina catalysts evaluated at 137 °C 13 ACS Paragon Plus Environment


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Conversion and product yields obtained at different reaction temperatures with Ni, Pd and Cusupported catalysts are given in Table 2. Some preliminary tests, developed charging the reactor with sand to evaluate the reactivity of HMF in the absence of catalyst, shown that sand was inactive at low temperatures and a 3 wt% yield of 2,5-diformylfuran (DFF) was observed at 189 °C. Other tests were also done charging the reactor with alumina extrudates and no conversion was obtained when alumina was evaluated at 137 °C. As the temperature increased, the conversion of HMF became 19 wt% at 155 °C and 20 wt% at 189 °C. DFF and small amounts of 5-methylfurfural (MFAL) were observed. DFF yield was 7 wt% at 155°C and at 189 °C. The MFAL yield did not exceed 1 wt% at both temperature levels. The results obtained with both sand and alumina are not included in Table 2. DFF is likely produced via dehydrogenation of HMF. This shows that alumina support itself has dehydrogenation sites able to promote this reaction. This result is consistent with Wang et al. results.46 These authors have shown that acetone selectivity in the reaction of isopropanol decomposition is related to basic sites coming from Al vacancies.46 DFF was only observed over alumina and was not detected on the catalysts studied. HMF conversion on the Ni, Pd and Cu-supported catalysts evaluated at 137 °C is in the range of 40 wt% to 50 wt%, being Pd-based catalyst more active. At 155 °C, the HMF conversion is complete on Pd and Ni-supported catalysts and is 94 wt% on Cu/alumina. Unexpectedly, the conversion of HMF on Cu/alumina catalyst decreases to 36 wt% at 189°C. This reduction in conversion is likely caused by partial deactivation of the catalyst after 13.5 h of reaction as it will be shown later.


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Table 2. Conversion and product liquid yield obtained over Ni, Pd and Cu supported on alumina as a function of temperature Temp. Catalyst (°C)

Ni/alumina

Pd/alumina

Cu/alumina

Product liquid yield (wt%) Conv. (wt%) BHFa MFALb MFOLc FOLd THFOLe DMFf DMTHFg Others

137

44

3.6

-

3.2

20.9

3.2

-

-

2.1

155

100

-

-

27.3

41.0

7.6

1.0

0.3

7.7

189

100

1.5

1.2

7.6

39.4

9.9

4.7

1.2

16.5

137

50

3.3

2.2

6.4

14.9

2.4

-

-

5.8

155

100

4.3

-

3.5

8.7

48.6

4.2

1.1

13.6

189

100

-

-

-

3.2

41.5

10.1

2.6

11.5

137

40

-

0.8

-

-

-

-

-

31.0

155

94

5.2

21.3

17.7

2.9

-

-

-

37.8

189

36

-

8.6

4.6

0.7

-

-

-

16.1

a

BHF: 2,5-bis(hydroxymethyl)furan MFAL: 5-methylfurfural c MFOL: 5-methyl-2-furfuryl alcohol d FOL: furfuryl alcohol e THFOL: tetrahydrofurfuryl alcohol f DMF: 2,5-dimethylfuran g DMTHF: 2,5-dimethyltetrahydrofuran b

Furfuryl alcohol (FOL) is the main product observed when the Ni/alumina catalyst is evaluated at the three reaction temperatures. This shows that Ni is able to catalyze the decarbonylation reaction of HMF. Sitthisa et al.10 have obtained furan from furfural using Ni supported on silica at 250 °C and atmospheric pressure. However, some authors observed that HMF hydrogenation to 2,5-bis(hydroxymethyl)furan (BHF) and 2,5-bis(hydroxymethyl)tetrahydrofuran (BHTF) are the main reactions using Ni Raney under milder reaction conditions in aqueous medium.8,47 This might suggest that the activity of nickel for the decarbonylation reaction is faster than the



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hydrogenation reactions to BHF/BHTF when temperature is higher. Previous studies of the reactivity of aldehydes have shown that decarbonylation takes place on metals of Groups 8, 9 and 10 (IUPAC nomenclature of element groups). Decarbonylation reactions of propanal, decanal, acrolein, furfural, among others aldehydes, have been studied using Ni, Pd and Rh based catalysts.48-51 Isotopic labeling studies of aldehydes performed by Barteau et al. have suggested that decarbonylation reaction proceeds through a stable acyl species η1(C)-acyl, as it is observed in Scheme 2(a).52-54 In this mechanism, the C atom of the carbonyl group interacts strongly with the metal causing the C-C scission in this acyl species to release an alkyl fragment one carbon atom shorter than the parent aldehyde and CO.52,53 Thus it is probable that in the decarbonylation reaction of HMF, this acyl species leads to the formation of furfuryl alcohol and CO. The formation of carbon monoxide is also confirmed from analysis of the gas-phase reaction products (data at 189 °C is shown in Table 3). The main gas products from HMF conversion were CO, CO2 and C6+ oxygenated products. On Ni/alumina catalyst, as the reaction temperature is increased other compounds like 5-methyl2-furfuryl alcohol (MFOL), tetrahydrofurfuryl alcohol (THFOL) and 2,5-dimethylfuran (DMF) are also observed. This shows that Ni has also active sites for hydrogenolysis and ring hydrogenation reactions. Even though MFOL can be obtained through MFAL and BHF, the results herein reported on Ni-based catalyst suggest that BHF is the main intermediate in the production of MFOL. In fact, the absence of MFAL at low reaction temperatures indicates that hydrogenation of the HMF carbonyl group occurs faster than hydrogenolysis of the HMF hydroxyl group, as it has also been suggested by Kumalaputri et al.55 Indeed, Nakagawa and Tomishige47 have found that BHF is the main product when HMF is hydrogenated using Raney Ni and Ni/SiO2 catalysts at 40 °C. Mavrikakis and Barteau54 have previously pointed out that the


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preferential carbonyl compounds adsorption mode on the platinum group metals is the η2(C,O)aldehyde configuration (Scheme 2, b). In this way, from this η2(C,O) species BHF could be formed and subsequently this latter compound could be hydrogenolyzed to MFOL. Scheme 2. Proposed oxygenate intermediates species for the conversion of HMF on transition metal surfaces

Table 3. Gas product yield (wt%) from hydrodeoxygenation of HMF evaluated on aluminasupported catalysts at 189 °C Catalyst

Ni/alumina

Pd/alumina

Cu/alumina

CO2

2.8

1.2

0.5

CO

13.2

15.4

0.1

C3

0.1

-

-

nC4

0.1

-

-

C6+

1.4

8.0

0.1



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It is important to note that the MFOL yield decreased from 27 wt% to 7 wt% as the temperature is increased from 155 °C to 189 °C. THFOL and DMF are probably obtained from the hydrogenation of FOL and the hydrogenolysis of MFOL, respectively. The decrease in MFOL yield and the increase in both DMF and other compounds yields when the temperature is increased can be explained by the MFOL hydrogenolysis. Shin and Keane56 have shown from phenol hydrogenation reactions using Ni-supported catalysts that hydrogenolysis of phenol to benzene was only initiated at temperatures as high as 250°C and were also favored by high nickel loadings. Therefore, the MFOL hydrogenolysis is promoted at high temperatures. Additionally, significant amount of other compounds were observed on Ni/alumina at the highest temperature. These compounds correspond to secondary reaction products like 2,5-hexanedione (HD), 2-pentanone and 1,2-pentanediol, among others, which were confirmed by MS (quality index higher than 83%) and their presence indicate that Ni supported on alumina breaks the ether C-O bond of the furan ring.10 Sitthisa et al.10 discussed that these ring-opening compounds are obtained from C-O-C cleavage of the ring since the interaction between furan ring and the Ni atoms is so strong that the ether C-O bond in the ring weakens and then opens the ring. Zhang et al.57 also observed the formation of these compounds in the FOL conversion over a Ru/MnOx catalyst in aqueous phase. They have proposed that the formation of these ring-opening compounds is through the FOL hydrogenolysis to obtain 1-hydroxy-2-pentanone, which is subsequently hydrogenated

and

hydrogenolyzed

to

1,2-pentanediol

and

2-pentanone

respectively.57 It is also important to mention that the ring-opening compounds are only formed in considerable amounts on Ni/alumina and these are favored with high temperatures. The main products observed over Pd/alumina catalyst are FOL at low temperature and THFOL at higher temperatures. As it is known from the literature, Pd-based catalyst favors the 18 ACS Paragon Plus Environment

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hydrogenation of the furan ring.58,59 For example, the formation of tetrahydrofuran (THF) has been reported when the hydrogenation of furfural was carried out using Pd catalysts supported on silica, alumina and activated carbon.60-63 It has been suggested elsewhere that the tendency of Pd to hydrogenate the unsaturated furan ring is a result of the strong interaction between the Pd and the π bonds in the furan.10 This strong interaction is caused apparently by the small d bandwidth of Pd.64 Moreover, Bradley et al.65 showed that the rehybridisation of the C bonding in the furan from sp2 to sp3 pushes the O atom sufficiently far from the Pd surface such that the O-Pd interaction is very small. Due to this repulsion it is unlike that the C-O-C cleavage of the ring and subsequent ring-opening reactions are observed on Pd/alumina. This is in agreement with our results whereby small amounts of 2,5-hexanedione (HD) with yields less than 3 wt% were found in the hydrogenation/hydrogenolysis reactions of HMF on Pd/alumina catalyst. Unlike the Ni-based catalyst, on Pd/alumina the formation of THFOL is remarkably enhanced as the temperature is increased, while the amount of FOL decreases significantly. It suggests that THFOL is obtained from the consecutive hydrogenation reaction of FOL and Pd is active for decarbonylation and the subsequent ring hydrogenation reaction. The other two compounds observed in high concentration are MFOL and DMF. The presence of BHF and MFAL at low temperature indicates these compounds are the intermediates to produce MFOL. Other compounds like 2,5-dimethyltetrahydrofuran

(DMTHF) and 5-methyltetrahydrofuran-2-

methanol (MTHFOL) were observed at 155 °C and 189 °C. Then DMTHF and MTHFOL are likely obtained through the furan-ring hydrogenation of DMF and MFOL, respectively. In summary, Pd favors decarbonylation, ring hydrogenation and hydrogenolysis reactions. Regarding Cu/alumina, Table 2 provides the major products obtained on this catalyst. The main compounds observed in the liquid reaction product are 5-methylfurfural (MFAL) and 5-methyl19 ACS Paragon Plus Environment


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2-furfuryl alcohol (MFOL). This result shows that Cu has the ability to promote hydrogenolysis and hydrogenation reactions. There are numerous reports in the literature showing that Cu is effective in the C=O bond hydrogenation using furfural and HMF as reactants.66-69 In fact, Deutsch and Shanks70 have shown that copper mixed metal oxide catalysts were selective to DMF from MFOL and Bienholz et al.71 and Vasiliadou et al.72 have also reported that this metal is selective to glycerol hydrogenolysis giving rise to propylenglycol. Even though a BHF yield of 5 wt% is obtained at 155 °C, MFOL seems to be produced through reactions involving both the cleavage of the HMF hydroxyl group to obtain MFAL and the hydrogenation of MFAL. Barteau73 suggested from isotopic labeling experiments that aldehydes tend to preferentially adsorb in the η1(O) configuration on Group 11 metals (Scheme 2, c), which would allow the interaction between the Cu surface and the oxygen in the HMF carbonyl group to yield BHF. Several studies have suggested that the methanol decomposition pathway on Cu surfaces involves first the O-H bond scission to form stable alkoxide (CH3O) intermediates.54,74-76 In the case of HMF, it is possible that the first step involves the hydrogen abstraction from the hydroxyl group leading to an adsorbed furoxy intermediate (Scheme 2, d). Thus both the C-O bond scission in the furoxy intermediate and the subsequent hydrogenation of the last furfuryl intermediate are presumed to occur for the formation of MFAL. This latter compound might be hydrogenated to MFOL through the η1(O) aldehyde adsorption configuration. Additionally, the low yields to FOL (up to 2.9 wt%) would confirm the affinity of Cu by both the η1(O) and the furoxy adsorption modes, whereby decarbonylation reaction is not favored on Cu/alumina. Unlike previous reports in which only hydrogenation and hydrogenolysis reaction products are observed on Cu-based catalysts4,8,70,72, in this work other behavior is observed. At 137 °C a large amount of 1-hydroxy-2,5-hexanedione (yield of 27 wt%) is attained, although this compound


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disappears at higher reaction temperatures. Schiavo et al.8 and Nakagawa et al.77 have reported the formation of 1-hydroxy-2,5-hexanedione through the rearrangement of HMF using noblemetal catalysts in aqueous medium catalyzed by acids. When the temperature is increased at 155 °C and 189 °C, the formation of other compounds like furfural, 2,5-hexanedione, 1,2pentanediol, 2-hydroxy-3-methyl-2-cyclopenten-1-one, 2-methyl-2-cyclopenten-1-one and 2methyl-cyclopentanone are observed. These compounds are presented as “others” in Table 2. The cyclic-compounds (yield goes up to 15 wt%) were detected by GC-MS, presenting a quality index higher than 91%. Previous studies reported by Hronec et al.78,79 revealed the formation of cyclic-compounds from FOL using Pt, Pd, Ru or Ni catalysts under similar reaction conditions used herein. They suggested that these cyclic-compounds are the result of the water effect in the reaction medium.78,79 In addition, Ohyama et al.80 pointed out that cyclopentanone derivatives are obtained through intramolecular aldol-condensation of 1-hydroxy-2,5-hexanedione, this latter intermediate compound observed in large amounts at 137 °C. In this study the cyclic-compounds were only detected over Cu/alumina catalyst and these results show that copper has also the capacity to catalyze the formation of cyclic-compounds. 3.3. Carbonaceous material deposition on the catalysts The concentration of carbonaceous material deposited on the spent Ni, Pd and Cu-supported catalysts is presented in Table 4. As the catalyst samples were evaluated at three consecutive temperature conditions without catalyst discharge then the amount of this carbonaceous material corresponds to the total amount deposited during the 13.5 h of contact between HMF and the catalyst. A substantial amount of carbon is found on all catalysts, especially on the alumina support. It is in agreement with other studies showing that alumina support favors coke formation due to the presence of acid sites on the surface of the support.81-83 The presence of 21 ACS Paragon Plus Environment


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metal in the catalyst decreases the amount of carbon deposited on the surface. These high values of carbonaceous deposit might be explained by coke deposition and/or adsorbed HMF/products. Previous studies have shown that furfural and/or furfuryl alcohol have a strong adsorption on copper-based catalysts and these adsorbed compounds are thought to cause deactivation of the catalyst.84-86 Consequently, IR studies were performed with the aim of clarifying the nature of this carbonaceous material. Table 4. Carbonaceous material concentration on spent catalysts and alumina support Support/Catalyst g carbon/ g cat

alumina

Ni/alumina

Pd/alumina

Cu/alumina

0.231

0.173

0.129

0.176

IR spectra of the spent and fresh alumina-supported catalysts are depicted in Figure 3. The broad band centered at 3400 cm-1 present in all calcined and spent catalysts is assigned to the O-H bonds vibrations of adsorbed moisture and OH groups of the alumina support. Likewise, the band at 1640 cm-1 is attributed to O-H deformation modes of water. The IR spectra of the spent catalysts reveal the appearance of bands between 3000 and 2890 cm-1 attributed to aliphatic C-H bands of the asymmetric and symmetric -CH2 and -CH3 stretch modes. In the case of spent Ni and Pd-supported catalysts, the band located at 2860 cm-1 corresponds to the C-H stretching vibration on a CHO group.87 The band at around 1564 cm-1 has been ascribed to C=C bands in aromatic and highly conjugated structures, which some authors have called as “coke band”.88,89 In the region from 1470 to 1250 cm-1, small bands appear on the spent catalysts attributed to C-H deformation modes. In the IR spectra of the spent catalysts, intense bands at 1068 cm-1 and at 1020 cm-1 are also observed. The band situated at 1068 cm-1 is stronger on spent Ni and Pdsupported catalysts while it is weak on the spent Cu/alumina. On the contrary, the band at 1020 22 ACS Paragon Plus Environment

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cm-1 is prominent on spent Cu-based catalyst. Aoshima et al.90 assigned the bands appearing in the region 1030-1070 cm-1 to C-O-C stretching vibration of tetrahydrofuran ring in interaction with acid catalysts. Barsberg and Thygesen91 assigned the band at 1020 cm-1 to furan ring C-H wag vibration of polyfurfuryl alcohol. This suggests that the type of carbonaceous material deposited on the surface of the catalysts depends on the active metal. On spent Ni and Pd based catalysts, this carbonaceous material might be a hydrogenated furan ring oligomer whereas on spent Cu/alumina this material might be of polyfuran type.

Figure 3. IR spectra of fresh and spent catalysts (dot lines are the fresh catalysts and solid lines are the spent catalysts)



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At this point, it is worth to compare our IR results to those obtained on spent catalysts from petroleum fractions hydrotreatment.92,93 On spent petroleum hydrotreatment catalysts, bands have been observed in the regions of 3000-2800 cm-1 and 1700-1300 cm-1.93 The bands in the region of 3000-2800 cm-1 were assigned to the C-H stretch vibrations of hydrogen bonded to aliphatic carbon and the bands in the region of 1700-1300 cm-1 were attributed to C-H, O-H and N-H deformations and C=O and C=C stretch vibrations.93 Also, a band at 1564 cm-1 ascribed to C=C vibrations in highly conjugated structures known as coke band has been observed.94 The intensive bands observed in this work at 1068 cm-1 and 1020 cm-1 (Figure 3) have not been reported on spent petroleum hydrotreatment catalysts. This indicates that the nature of the carbonaceous deposits obtained on the catalyst in rich oxygenate compound environments is different from that obtained in hydrocarbon environments and the former material contains significant amounts of the oxygenated compounds. As it was shown previously, both HMF conversion and product yields decrease on Cu/alumina catalyst when the temperature is increased from 155 °C to 189 °C. It has been reported elsewhere that the Cu-based catalysts are susceptible to deactivation not only by the formation of coke, but also by catalyst poisoning due to adsorption of either reactant or by-products and/or changes in the oxidation state of the copper during the reaction.95-97 The ATR-IR spectra have shown the deposition of oxygenated compounds on the surface of the spent catalysts. These compounds, which are probably produced by oligomerization, might be the main cause of deactivation. Although Ni/alumina also presents high carbon content, no loss of conversion was observed on this catalyst when the temperature was increased. This difference in susceptibility of the catalytic activity toward carbon deposition might be explained by the chemical nature of the deposited carbonaceous material. Polyfuran ring oligomers present on Cu-based catalyst should be more


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strongly adsorbed on the surface and therefore cause more deactivation. Moreover, as it is shown in Table 5, the spent Cu-based catalyst has a lower mass ratio of active metal to aluminum than the fresh catalyst, whereas the fresh and spent Ni and Pd based catalysts have similar mass ratios after reaction. These results indicate that no leaching of Ni and Pd had occurred during the reaction, whereas a small amount of Cu is leached into the aqueous phase. The slight decrease of copper content on the spent Cu/alumina catalyst might be other factor that cause the loss of activity observed on this catalyst.

Table 5. Mass ratio of active metal to aluminum Catalyst (%wt Me/%wt Al) Fresh Spent

Ni/alumina Ni/Al 0.245

Pd/alumina Pd/Al 0.014

Cu/alumina Cu/Al 0.273

0.241

0.014

0.248

4. Conclusions In this work the reactivity of HMF in aqueous medium was studied using alumina-supported transition metal catalysts. FOL and MFOL are the predominant products over Ni-supported catalyst. These compounds are explained by decarbonylation and hydrogenation reactions. Pd/alumina catalyst shows a trend to decarbonylate the HMF carbonyl bond and to hydrogenate the furan ring, especially at high temperatures. Yields towards FOL, THFOL and MTHFOL were found on this catalyst.



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Cu/alumina is active for both hydrogenolysis and hydrogenation reactions. MFAL, MFOL, 1hydroxy-2,5-hexanedione and cyclic-compounds were the main products observed on Cu/alumina catalyst. Results described above indicated that the HMF reaction activity and yields are strongly dependent on transition metal used as active site and that the adsorption mode of HMF on the active site plays a key role determining reactivity and yields. Valuable compounds can be produced using catalysts containing an industrially user-friendly support. This contribution has attempted to show that with the appropriate choice of active transition metal using mild reaction conditions it is possible to produce a fuel with a C/O relation greater than 3.

AUTHOR INFORMATION Corresponding Author *Corresponding author. E-mail address: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources The financial support for the present work was given by Ecopetrol S.A. under the Biofuels research project.


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ACKNOWLEDGMENT The authors acknowledge the support given by Colombian Petroleum Institute of Ecopetrol S.A., Colombia. D.P.D. thanks to Colciencias for its national PhD program support. The authors thank Maribel Castañeda for the technical support to develop the chromatographic method.

ASSOCIATED CONTENT Supporting Information The schematic flow diagram of the reactor system and the schematic representation of the packed-bed tubular downflow reactor.

NOMENCLATURE HMF BHF MFAL MFOL FOL THFOL DMF DMTHF

5-hydroxymethylfurfural 2,5-bis(hydroxymethyl)furan 5-methylfurfural 5-methyl-2-furfuryl alcohol furfuryl alcohol tetrahydrofurfuryl alcohol 2,5-dimethylfuran 2,5-dimethyltetrahydrofuran

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Hydrodeoxygenation of 5-Hydroxymethylfurfural over Alumina

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