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Effect of catalyst and solvent on the furan ring rearrangement to cyclopentanone
Applied Catalysis A: General 437–438 (2012) 104–111
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Effect of catalyst and solvent on the furan ring rearrangement to cyclopentanone Milan Hronec a,∗ , Katarina Fulajtarová a , Tibor Liptaj b a b
Department of Organic Technology, Slovak University of Technology, Radlinského 9, 812 37 Bratislava, Slovakia Central Laboratories, Slovak University of Technology, Radlinského 9, 812 37 Bratislava, Slovakia
a r t i c l e
i n f o
Article history: Received 12 April 2012 Received in revised form 14 May 2012 Accepted 14 June 2012 Available online 23 June 2012 Keywords: Cyclopentanone Furfural Furfuryl alcohol Solvent effect Hydroxy-cyclopentenone Ring rearrangement
a b s t r a c t The effect of solvent and Ni, Pt, Pd, Pt–Ru and Ru catalysts on the products distribution has been investigated in the reaction of furfural, furfuryl alcohol and 2-methyl furan under hydrogen pressure of 30–80 bar and at the reaction temperatures of 160–175 ◦ C. In water as solvent the main reaction pathway is the rearrangement of furfural and furfuryl alcohol to cyclopentanone. In alcohols, the reaction leading to the furan ring rearrangement does not proceed. The distribution of reaction products is influenced by the furfural concentration and acid–base properties of solvent and supported metal catalyst. The important factor influencing the selectivity of the furan ring rearrangement to cyclopentanone is proposed to be stabilization of carbocation by strong binding on the metal surface and by additional interaction with co-adsorbed water and furfural or furfuryl alcohol. In excess of hydrogen this species is created by the scission of the C O bond in the alkoxide or hydroxyalkyl intermediates. A plausible reaction mechanism for the furan ring rearrangement was proposed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The acid-catalyzed dehydration of pentoses and hexoses derived from renewable biomass resources can produce furan derivatives, such as furfural (FA) and 5-hydroxymethylfurfural [1–4]. Furfural is typically used as a precursor in the production of solvents, e.g. 2-methylfuran (2-MeF), furfuryl alcohol (FAL), tetrahydrofurfuryl alcohol (THFA) used in the chemical industry, and recently it is considered as a building block for transportation fuels [5–7]. Recently a number of studies have been published regarding the preparation of these compounds by hydrogenation of furfural on various metal catalysts [8–11]. The main products of furfural hydrogenation arise from the reduction of the C O group and/or the furan ring. Depending on the application of metal catalysts for the hydrogenation of furfural or its primary products, decarbonylation [12] and hydrogenolysis of the etheric C O bond [13] can also proceed. Due to the wide variety of available hydrogenation products of furfural, it is still attractive to design catalysts that are highly selective to the desired products. Previously we have reported [14] that in water as solvent and under hydrogen pressure the catalytic reaction of furfural can lead to unexpected and highly selective transformation to cyclopentanone. This reaction pathway has not been reported so far in the literature dealing with the hydrogenation of furfural in the liquid phase.
In the US Patent [15] is described the multi-step process for the conversion of furfuryl alcohol to cyclopentenone. As shown in Scheme 1 cyclopentenones can be prepared from the corresponding furfuryl alcohols through intermediates, where R1 and R2 are hydrogen, lower alkyl or alkenyl substituents. As it is evident from examples described in the patent, the first step of rearrangement of furfuryl alcohol or its derivatives in an aqueous solution leading to the corresponding hydroxy-cyclopentenones is catalyzed by acids. Similar products can be obtained from substituted furfuryl alcohols in acetone/water mixture in the presence of ZnCl2 catalyst [16]. However, this reaction is characterized by an extremely low rate and poor yields (16–18% after 72 h). Cyclopentanone (CPON ) is a versatile compound used for the synthesis of fungicides, pharmaceuticals, rubber chemicals, flavor and fragrance chemicals. Potentially, it can be used for preparation of polyamides. Cyclopentanone can be prepared by the catalytic cyclization of 1,6-hexanediol or adipic esters [17]. In the present contribution, we have studied the effect of various solvents and Pt, Pd, Ru, Pt–Ru and Ni catalysts on the transformation of furfural, furfuryl alcohol and 2-methylfuran to cyclopentanone. Based on the composition of reaction products, the reaction mechanism of furfural conversion to cyclopentanone was proposed. 2. Experimental 2.1. Chemicals
∗ Corresponding author. Tel.: +421 2 59325328. E-mail address: [email protected] (M. Hronec). 0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2012.06.018
All chemicals were obtained from commercial suppliers and used as provided: furfuryl alcohol (98%), 2-methylfuran
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Scheme 1. Multi-step process of furfuryl alcohols conversion [15].
(99%), tetrahydrofurfuryl alcohol (99%), cyclopentanone (99%), cyclopentanol (99%), D2 O (99.9 atom% D), all from Sigma–Aldrich, 2-propanol (99%), n-butanol (98%) and n-decanol (97%), acetic acid (99.5%), H3 PO4 (85%), Ba(OH)2 ·8H2 O, chemical purity from Microchem Slovakia, palladium (II) chloride, solution, Pd 9.0–11.0 wt.%, dihydrogen hexachloroplatinate (IV) hexahydrate (99.9%), ruthenium (III) chloride hydrate from Alfa Aesar. Furfural (Sigma–Aldrich, 99% assay) was purified by distillation and stored at −15 ◦ C. 2.2. Commercial catalysts 5% Pt/C, 5% Pd/C, 5% Ru/C, all on activated carbon powder, standard reduced, nominally 50% water wet, were purchased from Johnson Matthey Co. The nickel catalyst Actimet C was purchased from BASF, NiSAT® 320 RS and G-134 A from Süd Chemie. The catalyst G-134 A was ground to fine powder and in this form was used for experiments. Prior to the reaction catalysts were reduced in hydrogen (30 bar) for 2 h. 2.3. Preparation of supported catalysts Catalysts supported on activated carbon (Norit), alumina (Eurosupport Czechia s.r.o., Czech Republic) and magnesium oxide were synthesized with the incipient wetness impregnation. Aqueous solutions of H2 PtCl6 , PdCl2 or ruthenium (III) chloride were used as metal precursors in concentrations to obtain the given loading of metal on the catalyst. The bimetallic Pt–Ru catalyst was prepared by co-impregnation. The catalysts were dried at 120 ◦ C for 5 h and then calcined at 300 ◦ C for 5 h and hydrogen reduced at 390 ◦ C for 5 h. Magnesium oxide was prepared by calcination of magnesium ˇ hydroxide (Duslo Sal’a Slovakia) at 500 ◦ C for 3 h. Alumina prior to impregnation was calcined at 1050 ◦ C for 5 h. 2.4. Catalyst characterization The surface areas and pore diameters were determined from BET nitrogen adsorption measurements (Micromeritics ASAP 2020). The samples were degassed at 400 ◦ C for 2 h. The dispersion of metals on the supports was estimated by H2 and CO chemisorption measurements using helium as a carrier gas. The acidity of catalysts was determined from ammonia TPD measurements in the temperature range 100–500 ◦ C in a nitrogen atmosphere. Measurements were carried out in a conventional flow-type apparatus at a heating rate of 20 ◦ C min−1 . 2.5. Catalytic reaction and analysis of reaction products Catalytic experiments were performed using procedure and analytical methods described in our previous paper [14]. For a typical reaction, 20 ml of water, 1.0 g of reactant and given amount of metal catalyst was added to the reactor vessel. After sealing the reactor was several times flushed with low pressure hydrogen and then pressurized with hydrogen usually to 30–80 bar (ambient temperature). The reactor was then heated to the desired temperature and the stirring speed fixed to 1500 rpm to eliminate the diffusion effects. After an appropriate reaction time the reactor was
quickly cooled down, the reactor contents pour out to vial and the catalyst separated from aqueous phase by centrifugation. The quantitative determination of the liquid products concentration was done using gas chromatography by the external standard method using response factors of the corresponding standard compounds. A gas chromatograph–mass spectrometer combination was used to identify the organic compounds. The yields of all reaction products were calculated on the amount of reactant charged into the reactor. The reactions in nitrogen atmosphere were carried out in a 100 ml Teflon lined stainless steel autoclave mixed with a Teflon bar. The autoclave was heated in an oil bath. The composition of reaction mixtures was determined by GC, GC/MS and NMR analysis (Supplementary information).
3. Results The main physicochemical properties of the prepared catalysts are in Table 1. The chemisorption data show high dispersion of the metallic phase of carbon supported platinum and bimetallic Pt–Ru catalysts. A significantly lower dispersion of platinum is observed on the 5%Pt/MgO catalyst, probably related to the lower surface area of the MgO support. Acidity of the Pt/Al2 O3 catalyst, measured by TPD of ammonia is low. As shown in Table 2 all tested commercial catalysts are active, exhibiting nearly complete conversion of furfural derivatives in various solvents, except in n-decanol. However, significantly different is the distribution of reaction products. In water as solvent the 5% Pt/C catalyst and the nickel catalysts NiSAT® 320RS and G134 A are highly active for the rearrangement of furfuryl alcohol to cyclopentanone and cyclopentanol (CPOL ). Using 5% Pt/C catalyst the comprehensive yield of cyclopentanone and cyclopentanol is even higher than in the experiment with furfural as reactant (Table 2, exps. 1 and 3). This difference might be caused by different reactivity of both reactants, but also by the partial decomposition of furfural. It was observed that in the absence of catalyst about 3–5% of furfural is decomposed during heating of its aqueous solution to the reaction temperature. In the absence of metal catalyst furfural and furfuryl alcohol under reaction conditions is not converted to cyclopentanone or cyclopentanol.” Products distribution is significantly different in the presence of 5% Pd/C catalyst. In this case the preferred reaction products are derived from the hydrogenation of the hydroxymethyl group and the furan ring. As was mentioned in our previous study, a key role in determining the selectivity of furfural transformation to cyclopentanone plays water used as solvent. The same is valid for the reaction of furfuryl alcohol in water, which in the presence of platinum and nickel catalysts is converted to cyclopentanone and cyclopentanol with very high selectivity. In contrast to the results in water, in 2-propanol as solvent, the dominant product is tetrahydrofurfuryl alcohol (Table 2, exps. 3 and 5). Similar results were obtained in 2-propanol using the 5%Ru/TiO2 catalyst [18]. Furfural in 2-propanol is hydrogenated to furfuryl alcohol [19] or to products of the carbonyl group and the furan ring hydrogenation [14]. In water as solvent, the main product of furfural conversion is cyclopentanone (Table 2, exp. 1). Furfuryl alcohol in n-decanol is hydrogenated to products of the furan ring and the hydroxymethyl group hydrogenation,
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Table 1 Textural properties of metal supported catalysts. Catalyst
Metal loading (wt.%)
BET surface area (m2 g−1 )
1.4% Pt/C 1% Pt/Al2 O3 1% Pt/MgO 1.4% Pt + 1.4%Ru/C 3% Pd/C
1.4 1.0 1.0 1.4+1.4 3
937 91 36 903 895
Pore diameter (nm) 3.5 9.6 8,7 3.8 3.6
Metal dispersion (%)
Acidity (mmol g−1 )
49 39 19 40 –
– 0.225 – – –
Table 2 Catalytic transformation of furfural derivatives. Exp. no.
1 2a 3 4 5d 6
7b 8c 9d
Solvent (g)
H2 O H2 O H2 O n-Decanol 2-Propanol H2 O/ndecanol (1: 1 vol) aq alc H2 O H2 O H2 O
1.0 g
Conversion (%)
Yield (mol%) CyPON
CyPOL
FAL
THFAL
2-MeF
2-MeTHF
FA 2-MeF FAL FAL FAL FAL
96.5 98.3 100 71.5 100 93.5
51.1 0 55.7 0.9 1.4 7.4
3.8 0 13.6 0.4 1.9 18.8
1.5 0 – – – –
0 0 3.6 8.6 45.5 1.3
4.9 – 1.6 31.1 0 7.2
0.2 24.3 2.0 2.6 18.1 4.3
65.1 26.0 76.4 72.0 66.9 67.2
FAL FAL FAL
99.7 100 100
5.5 22.3 51.1 61.5
8.5 1.3 23.6 21.6
– – – –
5.2 28.3 11.7 5.1
2.0 3.1 0.7 0.4
0.5 17.6 1.4 0.6
73.0 88.5 89.1
Reaction conditions: 0.10 g 5% Pt/C (humidity 18.7%), 20 ml solvent, reaction temperature: 160 ◦ C, hydrogen pressure: 30 bar, reaction time: 60 min. a By reaction was formed 15.7 mol% of pentan-2-one and 9.8 mol% pentan-2-ol (confirmed by GC/MS), the yield calculated on the response factor of n-butanol. b 0.15 g catalyst 5% Pd/C. c 0.15 g catalyst NiSAT® 320 RS. d 0.10 g catalyst G-134 A,; CPON – cyclopentanone; CPOL – cyclopentanol; FA – furfural; FAL – furfuryl alcohol; THFAL – tetrahydrofurfuryl alcohol; 2-MeF – 2-methylfuran; the sum of the yields and unconverted FA. 2-MeTHF – 2-methyltetrahydrofuran,
however, in the mixture of n-decanol/water the preferred reaction is the furan ring rearrangement (Table 2, exps. 4 and 6). A very different distribution of products affords the reaction of 2methylfuran. Under the same reaction conditions the products of reaction are 2-methyltetrahydrofuran (2-MeTHF) and furanic ring opening products, mainly pentan-2-one and pentan-2-ol (Table 2, exp. 2). In the reaction mixture was also detected the presence of 1,4-pentanediol, but cyclopentanone was absent. A very low mass-balance indicates that during this reaction a large amount of unidentified products is formed. According to the literature data [15,16] the rearrangement of the furan ring is an acid catalyzed reaction. The influence of acidity of the reaction medium on the rearrangement of furfural and furfuryl alcohol was studied in water and n-butanol using Pt/C and Pd/C catalysts (Table 3). As it is evident from the results, the addition of acids does not promote the conversion to cyclopentanone, but significantly diminishes the mass balance. In acidified n-butanol (Table 3, exp. 3) the reaction products arise from the C O group and the furan ring hydrogenation. Cyclopentanone is formed only in trace amounts. The presence of acids has a negative influence also on the reaction of furfural and furfuryl alcohol in aqueous solution. In the reactions catalyzed by Pt and Pd catalysts, the formation of products of furan ring rearrangement is suppressed and the mass balance is low (Tables 2 and 3). On the other hand, in water or deuterated water the transformation of furfural to cyclopentanone is highly selective. As can be seen from the results in Table 3 (exps. 1 and 2) in deuterated water the selectivity to cyclopentanone is similar as in water. The GC/MS and NMR analyses have shown (Supplementary data), that products of the furfural rearrangement in deuterated water are derivatives of cyclopentanone deuterated at different carbon atoms, mostly in positions 2 and 5. Cyclopentanone-2,2,5,5-d4 is not the reaction product [20].
When the catalytic reaction of furfural is performed in basic aqueous solution the furan ring rearrangement practically does not proceed. Under these reaction conditions furfural is converted to tetrahydrofurfuryl alcohol. In order to study the influence of the support, a comparison was done among samples of platinum supported on active carbon, Al2 O3 and MgO. As seen from the results in Table 4, the platinum catalyst deposited on slightly acidic alumina is less selective for the conversion of furfural to cyclopentanone than that supported on active carbon. On the other hand, the reaction of furfural in the presence of platinum catalyst deposited on basic MgO affords furfuryl alcohol as the main product. The selectivity of the rearrangement of furfural to cyclopentanone is markedly changed by the adding of 1.4 wt.% of ruthenium onto the 1.4% Pt/C catalyst. In contrast to the monometallic Pt/C catalyst and Ru/C catalyst used at the same furfural concentration and even at higher reaction temperature (Table 5), the ruthenium doped Pt/C catalyst affords different distribution of reaction products and promotes the subsequent hydrogenation of cyclopentanone to cyclopentanol (Table 4, exps. 1 and 2). In the presence of this bimetallic catalyst the ratio of cyclopentanone/cyclopentanol in the reaction mixture reached almost an equilibrium value [14]. Beside solvent and metal catalyst, the selectivity of furfural rearrangement to cyclopentanone can be influenced by the concentration of furfural. To avoid the formation of two-phase reaction mixture due to limited solubility of reaction products in the aqueous phase, the effect of furfural concentration was studied in the range of 2.4–4.8 wt.%. As it is evident from the results in Table 5, even in this narrow concentration range the distribution of reaction products is being dramatically changed. In the presence of nickel catalysts furfural is converted to cyclopentanone with 60–70 mol% yields in the whole concentration range.
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Table 3 Catalytic transformation of 1.0 g of furfural and furfuryl alcohol in 20 ml water and reaction time 30 min. Exp. no.
Catalyst (g)
Additive (g)
Reactant
React. temp. (◦ C)
Conversion (%)
1 2a 3b 4 5c 6c,d 7e 8 9f
5% Pt/C 0.05 5% Pt/C 0.05 5% Pt/C 0.10 5% Pt/C 0.10 5% Pt/C 0.10 5% Pt/C 0.10 5% Pt/C 0.10 5% Pd/C 0.10 5% Pd/C 0.10
– – H3 PO4 (85%) 0.05 H3 PO4 (85%) 0.05 Ba(OH)2 .8H2 O 0.10 AcOH 0.37 H2 SO4 0.05 H3 PO4 (85%) 0.05 –
FA FA FA FA FA FA FAL FAL FAL
160 160 175 100 175 175 160 160 175
100 99.7 100 43.3 100 100 100 97.7 100
a b c d e f
Yield (mol%) CPON
CPOL
FAL
THFAL
2-MeF
2-MeTHF
76.5 75.1 0.6 1.3 1.5 6.8 2.0 6.3 7.8
4.8 6.1 0.8 0 1.7 3.1 0 0.8 23.2
0 0 27.0 1.6 0 0 – – –
0 3.3 7.1 0 30.1 0 0.4 23.2 –
3.4 3.1 36.6 1.1 0 13.4 0.6 0.7 –
0.4 0.8 9.4 0 1.5 6.8 4.0 26.1 –
In 20 ml D2 O. In 20 ml n-butanol. 0.17 g FA. 60 ml water. At 30 bar and 60 min. Prior to hydrogenation the sample was heated at 170 ◦ C for 2 h in nitrogen atmosphere in the absence of metal catalyst (Supplementary information), AcOH–acetic acid.
Table 4 Effect of catalyst support on the furfural transformation. Catalyst (g)
Conversion (%)
1.4% Pt/C 0.18 1.4% Pt + 1.4%Ru/C 0.09 1% Pt/Al2 O3 0.25 1% Pt/MgO 0.25
99.6 100 97.7 97.9
Yield (mol%) CPON
CPOL
FAL
THFAL
2-MeF
2-MeTHF
43.9 3.7 44.7 9.1
16.3 42.7 3.6 0.9
1.9 1.4 10.4 50.4
0.4 9.7 5.8 2.9
7.1 1.0 0 0.3
0.4 8.9 0 0
Reaction conditions: 1.0 g FA, 20 ml water, reaction temperature: 160 ◦ C, hydrogen pressure: 80 bar, reaction time: 30 min.
Table 5 Effect of FA concentration. Catalyst
(g)
FA (g)
React. time (min)
Conversion (%)
5% Pt/C
3% Pd/C
0.05 0.10 0.20 0.10 0.10 0.16
5% Pd/C
0.10
NiSAT® 320 RS
0.10 0.10 0.10
0.5 1.0 3.0a 0.5 1.0 0.5 0.5 0.75 1.0 0.5 1.0 0.5 1.0
30 30 60 30 30 30 30 30 60 30 30 30 30
99.1 100 100 99.2 98.7 98.4 99.8 100 100 98.3 95.4 99.0 100
5% Ru/C
G-134 A
Yield (mol%) CPON
CPOL
FAL
THFAL
2-MeF
2-MeTHF
33.8 40.2 48.8 4.0 56.7 1.2 11.4 13.4 31.1 61.0 67.0 49.0 57.3
25.8 36.2 9.6 41.7 9.6 0.6 1.8 2.4 3.4 17.3 5.0 17.9 6.9
2.6 0 0 0 0 0 0 0 0 1.0 0 0 0
0.7 0.3 0 0.4 6.1 62.1 55.8 36.1 22.6 15.4 10.1 11.5 9.7
0 5.0 0.8 0 3.5 0 0 0 0 0.3 0 2.7 0.2
2.0 9.4 0.8 6.6 3.3 7.0 16.7 14.3 11.2 0 0 0 1.6
Reaction conditions: 20 ml water, reaction temperature: 175 ◦ C, hydrogen pressure: 80 bar. a After reaction oil phase (not analyzed) was present in the reaction mixture.
In the presence of Ru catalyst increases the comprehensive yield of cyclopentanone and cyclopentanol with furfural concentration and sharply is changed the ratio of cyclopentanone/cyclopentanol. At lower concentration of furfural, the equilibrium composition of cyclopentanone/cyclopentanol is achieved in the reaction mixture [14]. However, as the mass balance indicates, a significantly higher amount of undesired products is formed at this concentration. A similar dependence is observed also in the presence of platinum catalyst (the sum of products increases from about 61% to 91%), but using this catalyst the ratio of cyclopentanone/cyclopentanol is changed only slightly. In contrast to Ni, Ru and Pt catalysts, the effect of furfural concentration is very different in the presence of palladium catalysts. While at 2.4% concentration of furfural the main products arise from hydrogenation reactions, at 4.8% concentration about half of
furfural is transformed to cyclopentanone and cyclopentanol. The variation of palladium loading on the catalyst only slightly changes the distribution of reaction products. 4. Discussion Our results pointed out the crucial role played by water in the furan ring rearrangement. These results are in sharp contrast to the previously published studies of furfural hydrogenation. Obviously, depending on the reaction conditions, the furan ring and the carbonyl group hydrogenation or furfural decarbonylation are the preferred reaction pathways. As was mentioned above, depending on the solvent, the main products of reaction are either cyclopentanone or typical products of furfural hydrogenation. One can speculate about the
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intermediates that lead to the furan ring rearrangement and why this reaction is observed only in water. Bradley et al. have shown [21] that the relatively strong adsorption of furfural observed on metals of Group VIII is due to the interaction of the orbitals with the metal d orbitals. This interaction in turn weakens the C O bond, helping stabilizing a di-sigma complex 2 -(C O) aldehyde [22]. When this interaction is strong, what occurs on Ni, it weakens the C O bond enough to open the ring. This should explain the ring opening of furfural over nickelbased catalysts in aqueous phase [7]. The formation of this species has been confirmed on Pd, Pt, Ru metal surfaces using HREELS and proposed as a precursor for aldehyde hydrogenation [23]. At high reaction temperature the 2 -(C O) aldehyde may convert into a more stable acyl surface species, in which the C atom of the carbonyl is strongly attached to the surface, and may be a precursor for the decarbonylation reaction of furfural [12]. The multifunctional compounds, such as furfural, can potentially bind and react on the surface through each individual functional position. However, there is the possibility that multiple binding will strongly affect subsequent surface chemistry. In the case of furfural the presence of oxygenated group on the furan ring strongly influences the reactivity of the ring. Pang and Medlin [24] studied adsorption and reaction of furfural and furfuryl alcohol on Pd (1 1 1) using temperature-programmed desorption and density functional theory (DFT) calculations. These results indicate that both compounds decompose via common surface intermediates. Furfuryl alcohol undergoes an unexpected C O scission in addition to C C scission. This process which is not observed for a simple alcohol [25] yields methylfuran as the product. The observation of C O scission for bifunctional molecules may be related to the stabilization of reaction intermediates due to the strong binding of each functional group with the surface. It was noted [24] that favorable C O scission rates may be related to the orientation of the C O bond of the relevant surface species, which is roughly parallel with the surface. The nature of the first decomposition intermediate which could be created in the course of reaction on the metal surface is debated, proposing formation of the alkoxide or hydroxyalkyl species [12,24–26]. The above discussed studies of furfural and furfuryl alcohol adsorption and decomposition on the metal surface have been conducted in the system, solid catalyst-gaseous reactant. However, as our results pointed out, when the reaction was carried out in the liquid phase a key role in determining the product selectivity played the solvent. In water, efficiency of the novel reaction was considerably enhanced, i.e. the rearrangement of the furan ring to cyclopentanone. Based on the experimental results observed in water and alcohols at different reaction conditions with furfural, furfuryl alcohol and 2-methylfuran using Pt, Pd, Ru and Ni catalysts, we propose the reaction mechanism describing the furan ring rearrangement (Scheme 2). The first step of reaction is the adsorption of furfural on the metal surface. Since the reaction is conducted under high pressure of hydrogen, the metal surface participates on activation of hydrogen. Depending on the metal surface, the hydrogen molecule is dissociated to two hydrogen atoms homolytically (H2 → 2 H) or heterolytically (H2 → H+ + H− ) [27,28]. The coverage of metal surface by furfural and hydrogen atoms depends on the metal and the concentrations of furfural and hydrogen in the liquid phase. Furfural has such an adsorption structure that the molecule lies essentially flat on the surface and the electrons in the furan ring and carbonyl interacts more strongly with the surface than lone pairs in oxygen atoms [24]. This interaction weakens the C O bond, forming the 2 -(C O) furfural species [23]. In the presence of hydrogen this species can be hydrogenated to the alkoxide or hydroxyalkyl intermediates. The studies of gas-phase hydrogenation of furfural suggest that hydrogen attack at oxygen atom is more
Scheme 2. Proposed reaction mechanism for the furan ring rearrangement to cyclopentanone.
favorable than formation of the alkoxide species, proposed by Davis and Barteau [29]. Furfuryl alcohol which can be the primary product of furfural hydrogenation has two possible initial dehydrogenation steps: C H scission to produce hydroxyalkyl species what was found by DFT calculations energetically preferred [24]. However, the investigation of alcohol dehydrogenation has also shown that on various metal surfaces or surface alloy films different selectivities of scission can be observed [30–32]. For example, the O H breaking is promoted by the addition of Zn. As was confirmed experimentally [14], hydrogen pressure significantly influences the selectivity of reaction. Hydrogen pressure changes the ratio of reaction products formed by the furan ring rearrangement [R] and the products formed by hydrogenation (H) of the furan ring, the aldehyde and the hydroxymethyl groups (Table 6). Moreover, at lower hydrogen pressure (11 bar), despite high conversion of furfural, the desired products are produced in very low yields (<3.5 mol%) [14]. It suggests that an excess of surface hydrogen is likely needed to favor the creation of the feasible alkoxide and/or hydroxyalkyl species bound on the metal surface. These species are created by the hydrogenation of 2 -(C O) furfural intermediates on the C or O atoms. The amount of surface hydrogen is determined by the hydrogen pressure and the type and structure of metal surface. As shown in Table 6 and literature [14], the effect of hydrogen pressure on the yield of cyclopentanone and the R/H ratio exhibits the same trend for Pt/C, Ru/C and nickel catalysts. However, in the presence of Pd/C catalyst and at higher hydrogen pressure the preferred reactions are classical hydrogenation reactions of furfural. The different behavior of the Pd/C catalyst can be
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Table 6 Effect of catalyst and solvent on the ratio of reaction products formed by the ring rearrangement (R) and by the hydrogenation (H) of furan ring, aldehyde and hydroxymethyl groups. Catalyst
R/H ratio Watera FAL
5% Pt/C
5% Ru/C 5% Ru/TiO2 3% Pd/C 5% Pd/C G-134A NiSAT® 320 RS R= a b c
9.69
Alcohol b
c
FA
FA
FA
FAL
FA
5.17
11.33
8.23
0.02 (n-decanol, Table 2)
0.006 (n-decanol, Table 2), 0.003 (n-butanol, literature [14]), 0.0 (2-propanol, literature [19])
5.14
6.49
7.82
1.02 6.33 4.83
0.03 0.17 5.72 7.15
0.0d (2-propanol, literature [18])
0.48 13.82 5.39
6.42 4.73 4.67
0.05 (2-propanol, Table 2)
(CPON + CPOL ), H = (FAL + THFAL + 2-MeF + 2-MeTHF). Reaction conditions for 1.0 g of FAL, Table 2; for 1 g of FA, Table 5. Using 0.5 g FA, Table 5. 1.0 g of FA at 160 ◦ C, 30 bar H2 , literature [14].
explained by the high “solubility” of atomic hydrogen in palladium [33]. Similar yields and selectivities obtained in experiments with furfural and furfuryl alcohol suggest that the reaction intermediates responsible for the furan ring rearrangement are the same (Table 6). The higher R/H ratio observed in experiments with furfuryl alcohol, especially in the presence of nickel catalysts, should be ascribed to the preferred C H scission in the initial step of furfuryl alcohol dehydrogenation. Such scission leads to the reaction pathway through hydroxyalkyl species. However, when only comprehensive yields of the desired products are compared, the differences are not so high that this reaction pathway is preferred. The absence of furan in the selected reaction mixtures confirms that at higher hydrogen pressure, the favorable reaction is the consecutive scission of the C O bond in 2 -(C O) furfural intermediate and not the decarbonylation pathway involving formation of 1 -(C)-acyl species. Since the furan ring rearrangement to cyclopentanone proceeds only in the nucleophile, water, it indicates that the reaction intermediate is electrophilic in nature. We propose that the creation of carbocation 1 species through the C O cleavage and its necessary stabilization on the metal surface, are the key steps toward the efficiency of furan ring rearrangement. The support for this assumption provides experimental results conducted with 2-methylfuran (Table 2). Under typical conditions the catalytic reaction of 2-methylfuran leads to 2-methyltetrahydrofuran, as the main reaction product. In interesting amounts are formed also 2-pentanol and 2-pentanone that derive from the ring-opening reactions through C O C cleavage of the ring [13]. The presence of cyclopentanone in the reaction mixture was not detected. This different pattern of reaction products indicates that the preferred reaction of 2-methylfuran in water is the dissociation of the ether C O bond, prior to hydrogenation of the furan ring and the subsequent protonation of oxygen atom. The explanation for the absence of cyclopentanone is that the generation of carbocation 1 species by dissociation of the strong C H bond in the methyl group of 2methylfuran is not likely. It confirms that the inevitable step in the furan ring rearrangement is the formation of carbocation 1 species. However, carbonium ions of formula (I) (Scheme 3) can be also generated by the attack upon the alcoholic function of furfuryl alcohols by a weak Lewis acid (ZnCl2 ) used as electrophilic catalyst [16]. When R3 is an alkyl group the reaction proceeds with extremely low rate and poor yields, even if zinc chloride and the furfuryl alcohol are used in nearly equimolar ratio. The reaction is more favored when R3 is an aromatic substituent. Despite the
low reaction rate and yields, the preferred products of this reaction are derivatives of hydroxy-4-cyclopentenone. In accordance with this mechanism furfuryl alcohol is in acidic aqueous solution (pH ∼ 5) converted to 3-hydroxy-4-cyclopentenone [15]. As is seen from the results in Table 3, adding of acids is not an effective approach to improve the activity toward the furan ring rearrangement. In opposite, the formation of such products sharply decreases and in reaction mixtures increases the amount of products of furfuryl alcohol condensation, oligomerization and ring opening, e.g. levulinic acid. Similar effect on the distribution of reaction products has catalysts prepared on supports with different acid–base properties. Thus, in the presence of platinum catalyst supported on neutral carrier (carbon) the R/H ratio of furfural transformation is 9.85, on slightly acidic alumina the ratio decreases to 2.97 and on basic MgO the products of furfural hydrogenation prevail (Tables 4 and 6). The acid–base properties of the support probably enhance chemisorptions of furfural on the support, and subsequently catalyze the undesired condensation and oligomerization reactions of furfural and furfuryl alcohol. This indicates a low mass balance in these experiments. The mass balance above 90% is for the given catalyst achieved in experiments conducted at nearly optimal conditions [14]. It is well known that in the presence of acid catalysts furfuryl alcohol dissolved in water undergoes condensation and oligomerization reactions [34–36]. The products are conjugated diene structures formed from the condensation of furfuryl alcohol and the carbenium ion. The carbenium ion is created from the initial elimination of a water molecule from furfuryl alcohol through acid catalysis. In water or alcohols furfuryl alcohol in the presence of acids can be also converted to levulinic acid [37]. It is proposed that one reaction pathway leading to levulinic acid takes place via a geminal diol species, 4,5,5-trihydroxypentan-2-one, formed by the addition of two water molecules to furfuryl alcohol, where two of the oxygen atoms from furfuryl alcohol are retained [38]. In these experiments the formation of 3-hydroxy-4-cyclopentenone was not mentioned. The reason for the absence of this compound should be that the reactions were carried out at different reaction conditions, e.g. at inconvenient reaction temperature or pH of the aqueous medium. If we propose that under reaction conditions the same acid catalyzed rearrangement of furfuryl alcohol proceeds, then among reaction products should be present 3-hydroxy-4-cyclopentenone. However, in selected samples the presence of this compound and its hydrogenated derivative was not detected. The argument for the absence of this compound might be that in the presence of metal
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Scheme 3. Simplified reaction scheme of acid catalyzed rearrangement of furfuryl alcohol [16].
Scheme 4. Reaction pathway through 3-hydroxy-4-cyclopentenone.
catalysts and high hydrogen pressure and reaction temperature the hydroxyl group is quickly hydrogenated. To elucidate the possibility that the reaction intermediate is 3-hydroxy-4-cyclopentenone, the experiment with furfuryl alcohol was performed in two reaction steps (Table 3, exp. 9). In the first step the aqueous solution of furfuryl alcohol was heated in nitrogen atmosphere at 170 ◦ C for 2 h. Under these conditions furfuryl alcohol was converted to 3-hydroxy-4-cyclopentenone and to yellowish-brown colored oligomers insoluble in water (Supplementary information). Smaller amount of levulinic acid was also detected. In the second step, after adding of 5%Pt/C catalyst, the aqueous phase was hydrogenated at 175 ◦ C and 80 bar of hydrogen pressure. After these reactions cyclopentanone and cyclopentanol were obtained in the yields 7.84 and 23.15 mol%, respectively. These yields were calculated on the amount of furfuryl alcohol charged into the autoclave in the first reaction step. In comparison to typical experiment performed with furfuryl alcohol at the same reaction conditions (Table 3, exp. 1), the yields of both desired products are only about one third. Moreover, in the two-step experiment (i) the main product of furfuryl alcohol rearrangement is cyclopentanol, and (ii) by condensation of furfuryl alcohol with non-stabilized carbenium ion a large amount of polymeric products is formed. These different findings indicate, that the reaction via 3-hydroxy4-cyclopentenone intermediate is not the major reaction pathway of furan ring rearrangement. On the basis of presented results we assume, that the key step toward the rearrangement of furfural and furfuryl alcohol to cyclopentanone is not only the formation of carbocation 1 but more likely its stabilization. In the metal catalyzed reactions this is achieved by strong binding of the furan ring to the large reservoir of electrons in the catalytic metal (Pt, Pd, Ru, Ni). Moreover, the interaction with co-adsorbed nucleophile, water, and co-adsorbed furfural or furfuryl alcohol may account for the stability and reactivity of carbocation 1. If the reactive carbocation is not stabilized by the action of metal catalyst it undergoes reactions with furfural or furfuryl alcohol leading to the formation of conjugated species [34,35]. This should explain why the yields of products in the reaction of furfuryl alcohol or its derivatives in the presence of
homogeneous acid catalysts which proceed through generating of “free” carbocation are low [16]. As was mentioned above, in the stabilization of the carbocation 1 can participate also furfural or furfuryl alcohol which is adsorbed on the metal surface. This assumption is based on the observation, that in the presence of some catalysts the products distribution is significantly influenced by the initial concentration of furfural in aqueous phase (Tables 5 and 6). For example, in the palladium catalyzed reactions the increase of furfural concentration only from 2.4 wt.% to 4.8 wt.% causes that the R/H ratio is changing dramatically, from 0.17 to 1.02. It means that at higher furfural concentration, probably as a consequence of stabilization of the carbocation 1 by the interaction with co-adsorbed furfural, the furan ring rearrangement is enhanced. The variation of the R/H ratio with the furfural concentration is observed also in the presence of Pt, Ru and Ni catalysts, but in this very narrow concentration range the differences are not very high. The transformation of furfural and furfuryl alcohol to cyclopentanone undergoes only in water as solvent. Water participates on the covering of the metal surface. The heat of adsorption of water (12.4 kcal/mol) is similar that of adsorption of furfural (12.3 kcal/mol) [26]. Interaction of water with metal can influence adsorption characteristics of furfural and furfuryl alcohol, the mode of their bounding on the surface and the concentration of hydrogen dissolved in aqueous layer bound to metal surface [39,40]. Besides influencing the adsorption characteristics of the carbocation 1 stabilized on the metal surface, the water molecule probably attacks the carbocation which is then converted to the intermediate species 2. This transformation probably favors dissociation of the ether C O bridges and the rearrangement of subsequently formed intermediates to cyclopentanone. One of the reaction pathways of furfural transformation to cyclopentanone should take place via 3-hydroxy-4cyclopentenone intermediate (Scheme 4). The first stage of this pathway is the metal catalyzed hydrogenation of furfural to furfuryl alcohol. As was observed experimentally the reaction of furfuryl alcohol at 170 ◦ C in nitrogen atmosphere affords 3hydroxy-4-cyclopentenone also in deuterated water, but not its
M. Hronec et al. / Applied Catalysis A: General 437–438 (2012) 104–111
deuterated isomers (Supplementary information). In the presence of metal catalyst this intermediate product is converted by consecutive reaction to cyclopentanol. Some doubt on this pathway throws the observation, that the main product of this consecutive hydrogenation of 3-hydroxy-4-cyclopentenone is cyclopentanol. Namely, in the majority of experiments performed with both furfural and furfuryl alcohol, the preferred product of furan ring rearrangement is cyclopentanone. The speculation about the direct rearrangement of the carbocation 1 or the species 2 to cyclopentanone promoted by the presence of the nucleophile, water, cannot be also excluded. The elucidation of the major reaction pathways in the process of furan ring rearrangement needs further investigation. Despite these uncertainties it seems most likely that the differences in selectivities of furfural and furfuryl alcohol transformation to cyclopentanone in water are caused by the different reactivity of “free” carbocation created by acid catalyzed reaction and the reactivity of carbocation 1 created and stabilized by the catalytic metal. 5. Conclusions The presented results have shown that under applied reaction conditions furfuryl alcohol dissolved in water can be converted to (i) oligomeric and polymeric compounds with conjugated structures or to levulinic acid, (ii) 3-hydroxy-4cyclopentenone as primary product, (iii) cyclopentanone and (iv) products of hydrogenation of furan ring and hydroxymethyl group. The proportion of these reaction pathways on the transformation of furfuryl alcohol depends mainly on the (a) acidity of reaction medium or catalyst support, (b) metal catalyst, (c) hydrogen pressure, (d) furfuryl alcohol concentration. The furan ring rearrangement to cyclopentanone is the dominant reaction of furfural and furfuryl alcohol in water in the presence of Pt, Pd, Ru or Ni catalysts. This reaction does not proceed with 2-methylfuran as the reactant. Since the furan ring rearrangement proceeds only in the nucleophile, water, we propose that the first step of reaction is the creation of carbocation 1. This species is produced in an excess of hydrogen by the favorable scission of the C O bond in the alkoxide or hydroxyalkyl intermediates, but not by dissociation of strong C H bond in the methyl group of 2methylfuran. However, the crucial factor influencing the selectivity of subsequent reaction seems to be stabilization of carbocation. If the reactive carbocation is not stabilized, it is quickly transformed to precursors of undesirable products. The stabilization of carbocation 1 is achieved by its strong binding on the metal surface and by additional interaction with co-adsorbed water and furfural or furfuryl alcohol. Besides influencing the adsorption characteristics of the carbocation on the metal surface, water can attack the carbocation and thus favor the cleavage of the C O bond and subsequently the rearrangement of formed species to cyclopentanone.
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Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Effect of catalyst and solvent on the furan ring rearrangement to cyclopentanone Milan Hronec a,∗ , Katarina Fulajtarová a , Tibor Liptaj b a b
Department of Organic Technology, Slovak University of Technology, Radlinského 9, 812 37 Bratislava, Slovakia Central Laboratories, Slovak University of Technology, Radlinského 9, 812 37 Bratislava, Slovakia
a r t i c l e
i n f o
Article history: Received 12 April 2012 Received in revised form 14 May 2012 Accepted 14 June 2012 Available online 23 June 2012 Keywords: Cyclopentanone Furfural Furfuryl alcohol Solvent effect Hydroxy-cyclopentenone Ring rearrangement
a b s t r a c t The effect of solvent and Ni, Pt, Pd, Pt–Ru and Ru catalysts on the products distribution has been investigated in the reaction of furfural, furfuryl alcohol and 2-methyl furan under hydrogen pressure of 30–80 bar and at the reaction temperatures of 160–175 ◦ C. In water as solvent the main reaction pathway is the rearrangement of furfural and furfuryl alcohol to cyclopentanone. In alcohols, the reaction leading to the furan ring rearrangement does not proceed. The distribution of reaction products is influenced by the furfural concentration and acid–base properties of solvent and supported metal catalyst. The important factor influencing the selectivity of the furan ring rearrangement to cyclopentanone is proposed to be stabilization of carbocation by strong binding on the metal surface and by additional interaction with co-adsorbed water and furfural or furfuryl alcohol. In excess of hydrogen this species is created by the scission of the C O bond in the alkoxide or hydroxyalkyl intermediates. A plausible reaction mechanism for the furan ring rearrangement was proposed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The acid-catalyzed dehydration of pentoses and hexoses derived from renewable biomass resources can produce furan derivatives, such as furfural (FA) and 5-hydroxymethylfurfural [1–4]. Furfural is typically used as a precursor in the production of solvents, e.g. 2-methylfuran (2-MeF), furfuryl alcohol (FAL), tetrahydrofurfuryl alcohol (THFA) used in the chemical industry, and recently it is considered as a building block for transportation fuels [5–7]. Recently a number of studies have been published regarding the preparation of these compounds by hydrogenation of furfural on various metal catalysts [8–11]. The main products of furfural hydrogenation arise from the reduction of the C O group and/or the furan ring. Depending on the application of metal catalysts for the hydrogenation of furfural or its primary products, decarbonylation [12] and hydrogenolysis of the etheric C O bond [13] can also proceed. Due to the wide variety of available hydrogenation products of furfural, it is still attractive to design catalysts that are highly selective to the desired products. Previously we have reported [14] that in water as solvent and under hydrogen pressure the catalytic reaction of furfural can lead to unexpected and highly selective transformation to cyclopentanone. This reaction pathway has not been reported so far in the literature dealing with the hydrogenation of furfural in the liquid phase.
In the US Patent [15] is described the multi-step process for the conversion of furfuryl alcohol to cyclopentenone. As shown in Scheme 1 cyclopentenones can be prepared from the corresponding furfuryl alcohols through intermediates, where R1 and R2 are hydrogen, lower alkyl or alkenyl substituents. As it is evident from examples described in the patent, the first step of rearrangement of furfuryl alcohol or its derivatives in an aqueous solution leading to the corresponding hydroxy-cyclopentenones is catalyzed by acids. Similar products can be obtained from substituted furfuryl alcohols in acetone/water mixture in the presence of ZnCl2 catalyst [16]. However, this reaction is characterized by an extremely low rate and poor yields (16–18% after 72 h). Cyclopentanone (CPON ) is a versatile compound used for the synthesis of fungicides, pharmaceuticals, rubber chemicals, flavor and fragrance chemicals. Potentially, it can be used for preparation of polyamides. Cyclopentanone can be prepared by the catalytic cyclization of 1,6-hexanediol or adipic esters [17]. In the present contribution, we have studied the effect of various solvents and Pt, Pd, Ru, Pt–Ru and Ni catalysts on the transformation of furfural, furfuryl alcohol and 2-methylfuran to cyclopentanone. Based on the composition of reaction products, the reaction mechanism of furfural conversion to cyclopentanone was proposed. 2. Experimental 2.1. Chemicals
∗ Corresponding author. Tel.: +421 2 59325328. E-mail address: [email protected] (M. Hronec). 0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2012.06.018
All chemicals were obtained from commercial suppliers and used as provided: furfuryl alcohol (98%), 2-methylfuran
M. Hronec et al. / Applied Catalysis A: General 437–438 (2012) 104–111
105
Scheme 1. Multi-step process of furfuryl alcohols conversion [15].
(99%), tetrahydrofurfuryl alcohol (99%), cyclopentanone (99%), cyclopentanol (99%), D2 O (99.9 atom% D), all from Sigma–Aldrich, 2-propanol (99%), n-butanol (98%) and n-decanol (97%), acetic acid (99.5%), H3 PO4 (85%), Ba(OH)2 ·8H2 O, chemical purity from Microchem Slovakia, palladium (II) chloride, solution, Pd 9.0–11.0 wt.%, dihydrogen hexachloroplatinate (IV) hexahydrate (99.9%), ruthenium (III) chloride hydrate from Alfa Aesar. Furfural (Sigma–Aldrich, 99% assay) was purified by distillation and stored at −15 ◦ C. 2.2. Commercial catalysts 5% Pt/C, 5% Pd/C, 5% Ru/C, all on activated carbon powder, standard reduced, nominally 50% water wet, were purchased from Johnson Matthey Co. The nickel catalyst Actimet C was purchased from BASF, NiSAT® 320 RS and G-134 A from Süd Chemie. The catalyst G-134 A was ground to fine powder and in this form was used for experiments. Prior to the reaction catalysts were reduced in hydrogen (30 bar) for 2 h. 2.3. Preparation of supported catalysts Catalysts supported on activated carbon (Norit), alumina (Eurosupport Czechia s.r.o., Czech Republic) and magnesium oxide were synthesized with the incipient wetness impregnation. Aqueous solutions of H2 PtCl6 , PdCl2 or ruthenium (III) chloride were used as metal precursors in concentrations to obtain the given loading of metal on the catalyst. The bimetallic Pt–Ru catalyst was prepared by co-impregnation. The catalysts were dried at 120 ◦ C for 5 h and then calcined at 300 ◦ C for 5 h and hydrogen reduced at 390 ◦ C for 5 h. Magnesium oxide was prepared by calcination of magnesium ˇ hydroxide (Duslo Sal’a Slovakia) at 500 ◦ C for 3 h. Alumina prior to impregnation was calcined at 1050 ◦ C for 5 h. 2.4. Catalyst characterization The surface areas and pore diameters were determined from BET nitrogen adsorption measurements (Micromeritics ASAP 2020). The samples were degassed at 400 ◦ C for 2 h. The dispersion of metals on the supports was estimated by H2 and CO chemisorption measurements using helium as a carrier gas. The acidity of catalysts was determined from ammonia TPD measurements in the temperature range 100–500 ◦ C in a nitrogen atmosphere. Measurements were carried out in a conventional flow-type apparatus at a heating rate of 20 ◦ C min−1 . 2.5. Catalytic reaction and analysis of reaction products Catalytic experiments were performed using procedure and analytical methods described in our previous paper [14]. For a typical reaction, 20 ml of water, 1.0 g of reactant and given amount of metal catalyst was added to the reactor vessel. After sealing the reactor was several times flushed with low pressure hydrogen and then pressurized with hydrogen usually to 30–80 bar (ambient temperature). The reactor was then heated to the desired temperature and the stirring speed fixed to 1500 rpm to eliminate the diffusion effects. After an appropriate reaction time the reactor was
quickly cooled down, the reactor contents pour out to vial and the catalyst separated from aqueous phase by centrifugation. The quantitative determination of the liquid products concentration was done using gas chromatography by the external standard method using response factors of the corresponding standard compounds. A gas chromatograph–mass spectrometer combination was used to identify the organic compounds. The yields of all reaction products were calculated on the amount of reactant charged into the reactor. The reactions in nitrogen atmosphere were carried out in a 100 ml Teflon lined stainless steel autoclave mixed with a Teflon bar. The autoclave was heated in an oil bath. The composition of reaction mixtures was determined by GC, GC/MS and NMR analysis (Supplementary information).
3. Results The main physicochemical properties of the prepared catalysts are in Table 1. The chemisorption data show high dispersion of the metallic phase of carbon supported platinum and bimetallic Pt–Ru catalysts. A significantly lower dispersion of platinum is observed on the 5%Pt/MgO catalyst, probably related to the lower surface area of the MgO support. Acidity of the Pt/Al2 O3 catalyst, measured by TPD of ammonia is low. As shown in Table 2 all tested commercial catalysts are active, exhibiting nearly complete conversion of furfural derivatives in various solvents, except in n-decanol. However, significantly different is the distribution of reaction products. In water as solvent the 5% Pt/C catalyst and the nickel catalysts NiSAT® 320RS and G134 A are highly active for the rearrangement of furfuryl alcohol to cyclopentanone and cyclopentanol (CPOL ). Using 5% Pt/C catalyst the comprehensive yield of cyclopentanone and cyclopentanol is even higher than in the experiment with furfural as reactant (Table 2, exps. 1 and 3). This difference might be caused by different reactivity of both reactants, but also by the partial decomposition of furfural. It was observed that in the absence of catalyst about 3–5% of furfural is decomposed during heating of its aqueous solution to the reaction temperature. In the absence of metal catalyst furfural and furfuryl alcohol under reaction conditions is not converted to cyclopentanone or cyclopentanol.” Products distribution is significantly different in the presence of 5% Pd/C catalyst. In this case the preferred reaction products are derived from the hydrogenation of the hydroxymethyl group and the furan ring. As was mentioned in our previous study, a key role in determining the selectivity of furfural transformation to cyclopentanone plays water used as solvent. The same is valid for the reaction of furfuryl alcohol in water, which in the presence of platinum and nickel catalysts is converted to cyclopentanone and cyclopentanol with very high selectivity. In contrast to the results in water, in 2-propanol as solvent, the dominant product is tetrahydrofurfuryl alcohol (Table 2, exps. 3 and 5). Similar results were obtained in 2-propanol using the 5%Ru/TiO2 catalyst [18]. Furfural in 2-propanol is hydrogenated to furfuryl alcohol [19] or to products of the carbonyl group and the furan ring hydrogenation [14]. In water as solvent, the main product of furfural conversion is cyclopentanone (Table 2, exp. 1). Furfuryl alcohol in n-decanol is hydrogenated to products of the furan ring and the hydroxymethyl group hydrogenation,
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Table 1 Textural properties of metal supported catalysts. Catalyst
Metal loading (wt.%)
BET surface area (m2 g−1 )
1.4% Pt/C 1% Pt/Al2 O3 1% Pt/MgO 1.4% Pt + 1.4%Ru/C 3% Pd/C
1.4 1.0 1.0 1.4+1.4 3
937 91 36 903 895
Pore diameter (nm) 3.5 9.6 8,7 3.8 3.6
Metal dispersion (%)
Acidity (mmol g−1 )
49 39 19 40 –
– 0.225 – – –
Table 2 Catalytic transformation of furfural derivatives. Exp. no.
1 2a 3 4 5d 6
7b 8c 9d
Solvent (g)
H2 O H2 O H2 O n-Decanol 2-Propanol H2 O/ndecanol (1: 1 vol) aq alc H2 O H2 O H2 O
1.0 g
Conversion (%)
Yield (mol%) CyPON
CyPOL
FAL
THFAL
2-MeF
2-MeTHF
FA 2-MeF FAL FAL FAL FAL
96.5 98.3 100 71.5 100 93.5
51.1 0 55.7 0.9 1.4 7.4
3.8 0 13.6 0.4 1.9 18.8
1.5 0 – – – –
0 0 3.6 8.6 45.5 1.3
4.9 – 1.6 31.1 0 7.2
0.2 24.3 2.0 2.6 18.1 4.3
65.1 26.0 76.4 72.0 66.9 67.2
FAL FAL FAL
99.7 100 100
5.5 22.3 51.1 61.5
8.5 1.3 23.6 21.6
– – – –
5.2 28.3 11.7 5.1
2.0 3.1 0.7 0.4
0.5 17.6 1.4 0.6
73.0 88.5 89.1
Reaction conditions: 0.10 g 5% Pt/C (humidity 18.7%), 20 ml solvent, reaction temperature: 160 ◦ C, hydrogen pressure: 30 bar, reaction time: 60 min. a By reaction was formed 15.7 mol% of pentan-2-one and 9.8 mol% pentan-2-ol (confirmed by GC/MS), the yield calculated on the response factor of n-butanol. b 0.15 g catalyst 5% Pd/C. c 0.15 g catalyst NiSAT® 320 RS. d 0.10 g catalyst G-134 A,; CPON – cyclopentanone; CPOL – cyclopentanol; FA – furfural; FAL – furfuryl alcohol; THFAL – tetrahydrofurfuryl alcohol; 2-MeF – 2-methylfuran; the sum of the yields and unconverted FA. 2-MeTHF – 2-methyltetrahydrofuran,
however, in the mixture of n-decanol/water the preferred reaction is the furan ring rearrangement (Table 2, exps. 4 and 6). A very different distribution of products affords the reaction of 2methylfuran. Under the same reaction conditions the products of reaction are 2-methyltetrahydrofuran (2-MeTHF) and furanic ring opening products, mainly pentan-2-one and pentan-2-ol (Table 2, exp. 2). In the reaction mixture was also detected the presence of 1,4-pentanediol, but cyclopentanone was absent. A very low mass-balance indicates that during this reaction a large amount of unidentified products is formed. According to the literature data [15,16] the rearrangement of the furan ring is an acid catalyzed reaction. The influence of acidity of the reaction medium on the rearrangement of furfural and furfuryl alcohol was studied in water and n-butanol using Pt/C and Pd/C catalysts (Table 3). As it is evident from the results, the addition of acids does not promote the conversion to cyclopentanone, but significantly diminishes the mass balance. In acidified n-butanol (Table 3, exp. 3) the reaction products arise from the C O group and the furan ring hydrogenation. Cyclopentanone is formed only in trace amounts. The presence of acids has a negative influence also on the reaction of furfural and furfuryl alcohol in aqueous solution. In the reactions catalyzed by Pt and Pd catalysts, the formation of products of furan ring rearrangement is suppressed and the mass balance is low (Tables 2 and 3). On the other hand, in water or deuterated water the transformation of furfural to cyclopentanone is highly selective. As can be seen from the results in Table 3 (exps. 1 and 2) in deuterated water the selectivity to cyclopentanone is similar as in water. The GC/MS and NMR analyses have shown (Supplementary data), that products of the furfural rearrangement in deuterated water are derivatives of cyclopentanone deuterated at different carbon atoms, mostly in positions 2 and 5. Cyclopentanone-2,2,5,5-d4 is not the reaction product [20].
When the catalytic reaction of furfural is performed in basic aqueous solution the furan ring rearrangement practically does not proceed. Under these reaction conditions furfural is converted to tetrahydrofurfuryl alcohol. In order to study the influence of the support, a comparison was done among samples of platinum supported on active carbon, Al2 O3 and MgO. As seen from the results in Table 4, the platinum catalyst deposited on slightly acidic alumina is less selective for the conversion of furfural to cyclopentanone than that supported on active carbon. On the other hand, the reaction of furfural in the presence of platinum catalyst deposited on basic MgO affords furfuryl alcohol as the main product. The selectivity of the rearrangement of furfural to cyclopentanone is markedly changed by the adding of 1.4 wt.% of ruthenium onto the 1.4% Pt/C catalyst. In contrast to the monometallic Pt/C catalyst and Ru/C catalyst used at the same furfural concentration and even at higher reaction temperature (Table 5), the ruthenium doped Pt/C catalyst affords different distribution of reaction products and promotes the subsequent hydrogenation of cyclopentanone to cyclopentanol (Table 4, exps. 1 and 2). In the presence of this bimetallic catalyst the ratio of cyclopentanone/cyclopentanol in the reaction mixture reached almost an equilibrium value [14]. Beside solvent and metal catalyst, the selectivity of furfural rearrangement to cyclopentanone can be influenced by the concentration of furfural. To avoid the formation of two-phase reaction mixture due to limited solubility of reaction products in the aqueous phase, the effect of furfural concentration was studied in the range of 2.4–4.8 wt.%. As it is evident from the results in Table 5, even in this narrow concentration range the distribution of reaction products is being dramatically changed. In the presence of nickel catalysts furfural is converted to cyclopentanone with 60–70 mol% yields in the whole concentration range.
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Table 3 Catalytic transformation of 1.0 g of furfural and furfuryl alcohol in 20 ml water and reaction time 30 min. Exp. no.
Catalyst (g)
Additive (g)
Reactant
React. temp. (◦ C)
Conversion (%)
1 2a 3b 4 5c 6c,d 7e 8 9f
5% Pt/C 0.05 5% Pt/C 0.05 5% Pt/C 0.10 5% Pt/C 0.10 5% Pt/C 0.10 5% Pt/C 0.10 5% Pt/C 0.10 5% Pd/C 0.10 5% Pd/C 0.10
– – H3 PO4 (85%) 0.05 H3 PO4 (85%) 0.05 Ba(OH)2 .8H2 O 0.10 AcOH 0.37 H2 SO4 0.05 H3 PO4 (85%) 0.05 –
FA FA FA FA FA FA FAL FAL FAL
160 160 175 100 175 175 160 160 175
100 99.7 100 43.3 100 100 100 97.7 100
a b c d e f
Yield (mol%) CPON
CPOL
FAL
THFAL
2-MeF
2-MeTHF
76.5 75.1 0.6 1.3 1.5 6.8 2.0 6.3 7.8
4.8 6.1 0.8 0 1.7 3.1 0 0.8 23.2
0 0 27.0 1.6 0 0 – – –
0 3.3 7.1 0 30.1 0 0.4 23.2 –
3.4 3.1 36.6 1.1 0 13.4 0.6 0.7 –
0.4 0.8 9.4 0 1.5 6.8 4.0 26.1 –
In 20 ml D2 O. In 20 ml n-butanol. 0.17 g FA. 60 ml water. At 30 bar and 60 min. Prior to hydrogenation the sample was heated at 170 ◦ C for 2 h in nitrogen atmosphere in the absence of metal catalyst (Supplementary information), AcOH–acetic acid.
Table 4 Effect of catalyst support on the furfural transformation. Catalyst (g)
Conversion (%)
1.4% Pt/C 0.18 1.4% Pt + 1.4%Ru/C 0.09 1% Pt/Al2 O3 0.25 1% Pt/MgO 0.25
99.6 100 97.7 97.9
Yield (mol%) CPON
CPOL
FAL
THFAL
2-MeF
2-MeTHF
43.9 3.7 44.7 9.1
16.3 42.7 3.6 0.9
1.9 1.4 10.4 50.4
0.4 9.7 5.8 2.9
7.1 1.0 0 0.3
0.4 8.9 0 0
Reaction conditions: 1.0 g FA, 20 ml water, reaction temperature: 160 ◦ C, hydrogen pressure: 80 bar, reaction time: 30 min.
Table 5 Effect of FA concentration. Catalyst
(g)
FA (g)
React. time (min)
Conversion (%)
5% Pt/C
3% Pd/C
0.05 0.10 0.20 0.10 0.10 0.16
5% Pd/C
0.10
NiSAT® 320 RS
0.10 0.10 0.10
0.5 1.0 3.0a 0.5 1.0 0.5 0.5 0.75 1.0 0.5 1.0 0.5 1.0
30 30 60 30 30 30 30 30 60 30 30 30 30
99.1 100 100 99.2 98.7 98.4 99.8 100 100 98.3 95.4 99.0 100
5% Ru/C
G-134 A
Yield (mol%) CPON
CPOL
FAL
THFAL
2-MeF
2-MeTHF
33.8 40.2 48.8 4.0 56.7 1.2 11.4 13.4 31.1 61.0 67.0 49.0 57.3
25.8 36.2 9.6 41.7 9.6 0.6 1.8 2.4 3.4 17.3 5.0 17.9 6.9
2.6 0 0 0 0 0 0 0 0 1.0 0 0 0
0.7 0.3 0 0.4 6.1 62.1 55.8 36.1 22.6 15.4 10.1 11.5 9.7
0 5.0 0.8 0 3.5 0 0 0 0 0.3 0 2.7 0.2
2.0 9.4 0.8 6.6 3.3 7.0 16.7 14.3 11.2 0 0 0 1.6
Reaction conditions: 20 ml water, reaction temperature: 175 ◦ C, hydrogen pressure: 80 bar. a After reaction oil phase (not analyzed) was present in the reaction mixture.
In the presence of Ru catalyst increases the comprehensive yield of cyclopentanone and cyclopentanol with furfural concentration and sharply is changed the ratio of cyclopentanone/cyclopentanol. At lower concentration of furfural, the equilibrium composition of cyclopentanone/cyclopentanol is achieved in the reaction mixture [14]. However, as the mass balance indicates, a significantly higher amount of undesired products is formed at this concentration. A similar dependence is observed also in the presence of platinum catalyst (the sum of products increases from about 61% to 91%), but using this catalyst the ratio of cyclopentanone/cyclopentanol is changed only slightly. In contrast to Ni, Ru and Pt catalysts, the effect of furfural concentration is very different in the presence of palladium catalysts. While at 2.4% concentration of furfural the main products arise from hydrogenation reactions, at 4.8% concentration about half of
furfural is transformed to cyclopentanone and cyclopentanol. The variation of palladium loading on the catalyst only slightly changes the distribution of reaction products. 4. Discussion Our results pointed out the crucial role played by water in the furan ring rearrangement. These results are in sharp contrast to the previously published studies of furfural hydrogenation. Obviously, depending on the reaction conditions, the furan ring and the carbonyl group hydrogenation or furfural decarbonylation are the preferred reaction pathways. As was mentioned above, depending on the solvent, the main products of reaction are either cyclopentanone or typical products of furfural hydrogenation. One can speculate about the
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intermediates that lead to the furan ring rearrangement and why this reaction is observed only in water. Bradley et al. have shown [21] that the relatively strong adsorption of furfural observed on metals of Group VIII is due to the interaction of the orbitals with the metal d orbitals. This interaction in turn weakens the C O bond, helping stabilizing a di-sigma complex 2 -(C O) aldehyde [22]. When this interaction is strong, what occurs on Ni, it weakens the C O bond enough to open the ring. This should explain the ring opening of furfural over nickelbased catalysts in aqueous phase [7]. The formation of this species has been confirmed on Pd, Pt, Ru metal surfaces using HREELS and proposed as a precursor for aldehyde hydrogenation [23]. At high reaction temperature the 2 -(C O) aldehyde may convert into a more stable acyl surface species, in which the C atom of the carbonyl is strongly attached to the surface, and may be a precursor for the decarbonylation reaction of furfural [12]. The multifunctional compounds, such as furfural, can potentially bind and react on the surface through each individual functional position. However, there is the possibility that multiple binding will strongly affect subsequent surface chemistry. In the case of furfural the presence of oxygenated group on the furan ring strongly influences the reactivity of the ring. Pang and Medlin [24] studied adsorption and reaction of furfural and furfuryl alcohol on Pd (1 1 1) using temperature-programmed desorption and density functional theory (DFT) calculations. These results indicate that both compounds decompose via common surface intermediates. Furfuryl alcohol undergoes an unexpected C O scission in addition to C C scission. This process which is not observed for a simple alcohol [25] yields methylfuran as the product. The observation of C O scission for bifunctional molecules may be related to the stabilization of reaction intermediates due to the strong binding of each functional group with the surface. It was noted [24] that favorable C O scission rates may be related to the orientation of the C O bond of the relevant surface species, which is roughly parallel with the surface. The nature of the first decomposition intermediate which could be created in the course of reaction on the metal surface is debated, proposing formation of the alkoxide or hydroxyalkyl species [12,24–26]. The above discussed studies of furfural and furfuryl alcohol adsorption and decomposition on the metal surface have been conducted in the system, solid catalyst-gaseous reactant. However, as our results pointed out, when the reaction was carried out in the liquid phase a key role in determining the product selectivity played the solvent. In water, efficiency of the novel reaction was considerably enhanced, i.e. the rearrangement of the furan ring to cyclopentanone. Based on the experimental results observed in water and alcohols at different reaction conditions with furfural, furfuryl alcohol and 2-methylfuran using Pt, Pd, Ru and Ni catalysts, we propose the reaction mechanism describing the furan ring rearrangement (Scheme 2). The first step of reaction is the adsorption of furfural on the metal surface. Since the reaction is conducted under high pressure of hydrogen, the metal surface participates on activation of hydrogen. Depending on the metal surface, the hydrogen molecule is dissociated to two hydrogen atoms homolytically (H2 → 2 H) or heterolytically (H2 → H+ + H− ) [27,28]. The coverage of metal surface by furfural and hydrogen atoms depends on the metal and the concentrations of furfural and hydrogen in the liquid phase. Furfural has such an adsorption structure that the molecule lies essentially flat on the surface and the electrons in the furan ring and carbonyl interacts more strongly with the surface than lone pairs in oxygen atoms [24]. This interaction weakens the C O bond, forming the 2 -(C O) furfural species [23]. In the presence of hydrogen this species can be hydrogenated to the alkoxide or hydroxyalkyl intermediates. The studies of gas-phase hydrogenation of furfural suggest that hydrogen attack at oxygen atom is more
Scheme 2. Proposed reaction mechanism for the furan ring rearrangement to cyclopentanone.
favorable than formation of the alkoxide species, proposed by Davis and Barteau [29]. Furfuryl alcohol which can be the primary product of furfural hydrogenation has two possible initial dehydrogenation steps: C H scission to produce hydroxyalkyl species what was found by DFT calculations energetically preferred [24]. However, the investigation of alcohol dehydrogenation has also shown that on various metal surfaces or surface alloy films different selectivities of scission can be observed [30–32]. For example, the O H breaking is promoted by the addition of Zn. As was confirmed experimentally [14], hydrogen pressure significantly influences the selectivity of reaction. Hydrogen pressure changes the ratio of reaction products formed by the furan ring rearrangement [R] and the products formed by hydrogenation (H) of the furan ring, the aldehyde and the hydroxymethyl groups (Table 6). Moreover, at lower hydrogen pressure (11 bar), despite high conversion of furfural, the desired products are produced in very low yields (<3.5 mol%) [14]. It suggests that an excess of surface hydrogen is likely needed to favor the creation of the feasible alkoxide and/or hydroxyalkyl species bound on the metal surface. These species are created by the hydrogenation of 2 -(C O) furfural intermediates on the C or O atoms. The amount of surface hydrogen is determined by the hydrogen pressure and the type and structure of metal surface. As shown in Table 6 and literature [14], the effect of hydrogen pressure on the yield of cyclopentanone and the R/H ratio exhibits the same trend for Pt/C, Ru/C and nickel catalysts. However, in the presence of Pd/C catalyst and at higher hydrogen pressure the preferred reactions are classical hydrogenation reactions of furfural. The different behavior of the Pd/C catalyst can be
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Table 6 Effect of catalyst and solvent on the ratio of reaction products formed by the ring rearrangement (R) and by the hydrogenation (H) of furan ring, aldehyde and hydroxymethyl groups. Catalyst
R/H ratio Watera FAL
5% Pt/C
5% Ru/C 5% Ru/TiO2 3% Pd/C 5% Pd/C G-134A NiSAT® 320 RS R= a b c
9.69
Alcohol b
c
FA
FA
FA
FAL
FA
5.17
11.33
8.23
0.02 (n-decanol, Table 2)
0.006 (n-decanol, Table 2), 0.003 (n-butanol, literature [14]), 0.0 (2-propanol, literature [19])
5.14
6.49
7.82
1.02 6.33 4.83
0.03 0.17 5.72 7.15
0.0d (2-propanol, literature [18])
0.48 13.82 5.39
6.42 4.73 4.67
0.05 (2-propanol, Table 2)
(CPON + CPOL ), H = (FAL + THFAL + 2-MeF + 2-MeTHF). Reaction conditions for 1.0 g of FAL, Table 2; for 1 g of FA, Table 5. Using 0.5 g FA, Table 5. 1.0 g of FA at 160 ◦ C, 30 bar H2 , literature [14].
explained by the high “solubility” of atomic hydrogen in palladium [33]. Similar yields and selectivities obtained in experiments with furfural and furfuryl alcohol suggest that the reaction intermediates responsible for the furan ring rearrangement are the same (Table 6). The higher R/H ratio observed in experiments with furfuryl alcohol, especially in the presence of nickel catalysts, should be ascribed to the preferred C H scission in the initial step of furfuryl alcohol dehydrogenation. Such scission leads to the reaction pathway through hydroxyalkyl species. However, when only comprehensive yields of the desired products are compared, the differences are not so high that this reaction pathway is preferred. The absence of furan in the selected reaction mixtures confirms that at higher hydrogen pressure, the favorable reaction is the consecutive scission of the C O bond in 2 -(C O) furfural intermediate and not the decarbonylation pathway involving formation of 1 -(C)-acyl species. Since the furan ring rearrangement to cyclopentanone proceeds only in the nucleophile, water, it indicates that the reaction intermediate is electrophilic in nature. We propose that the creation of carbocation 1 species through the C O cleavage and its necessary stabilization on the metal surface, are the key steps toward the efficiency of furan ring rearrangement. The support for this assumption provides experimental results conducted with 2-methylfuran (Table 2). Under typical conditions the catalytic reaction of 2-methylfuran leads to 2-methyltetrahydrofuran, as the main reaction product. In interesting amounts are formed also 2-pentanol and 2-pentanone that derive from the ring-opening reactions through C O C cleavage of the ring [13]. The presence of cyclopentanone in the reaction mixture was not detected. This different pattern of reaction products indicates that the preferred reaction of 2-methylfuran in water is the dissociation of the ether C O bond, prior to hydrogenation of the furan ring and the subsequent protonation of oxygen atom. The explanation for the absence of cyclopentanone is that the generation of carbocation 1 species by dissociation of the strong C H bond in the methyl group of 2methylfuran is not likely. It confirms that the inevitable step in the furan ring rearrangement is the formation of carbocation 1 species. However, carbonium ions of formula (I) (Scheme 3) can be also generated by the attack upon the alcoholic function of furfuryl alcohols by a weak Lewis acid (ZnCl2 ) used as electrophilic catalyst [16]. When R3 is an alkyl group the reaction proceeds with extremely low rate and poor yields, even if zinc chloride and the furfuryl alcohol are used in nearly equimolar ratio. The reaction is more favored when R3 is an aromatic substituent. Despite the
low reaction rate and yields, the preferred products of this reaction are derivatives of hydroxy-4-cyclopentenone. In accordance with this mechanism furfuryl alcohol is in acidic aqueous solution (pH ∼ 5) converted to 3-hydroxy-4-cyclopentenone [15]. As is seen from the results in Table 3, adding of acids is not an effective approach to improve the activity toward the furan ring rearrangement. In opposite, the formation of such products sharply decreases and in reaction mixtures increases the amount of products of furfuryl alcohol condensation, oligomerization and ring opening, e.g. levulinic acid. Similar effect on the distribution of reaction products has catalysts prepared on supports with different acid–base properties. Thus, in the presence of platinum catalyst supported on neutral carrier (carbon) the R/H ratio of furfural transformation is 9.85, on slightly acidic alumina the ratio decreases to 2.97 and on basic MgO the products of furfural hydrogenation prevail (Tables 4 and 6). The acid–base properties of the support probably enhance chemisorptions of furfural on the support, and subsequently catalyze the undesired condensation and oligomerization reactions of furfural and furfuryl alcohol. This indicates a low mass balance in these experiments. The mass balance above 90% is for the given catalyst achieved in experiments conducted at nearly optimal conditions [14]. It is well known that in the presence of acid catalysts furfuryl alcohol dissolved in water undergoes condensation and oligomerization reactions [34–36]. The products are conjugated diene structures formed from the condensation of furfuryl alcohol and the carbenium ion. The carbenium ion is created from the initial elimination of a water molecule from furfuryl alcohol through acid catalysis. In water or alcohols furfuryl alcohol in the presence of acids can be also converted to levulinic acid [37]. It is proposed that one reaction pathway leading to levulinic acid takes place via a geminal diol species, 4,5,5-trihydroxypentan-2-one, formed by the addition of two water molecules to furfuryl alcohol, where two of the oxygen atoms from furfuryl alcohol are retained [38]. In these experiments the formation of 3-hydroxy-4-cyclopentenone was not mentioned. The reason for the absence of this compound should be that the reactions were carried out at different reaction conditions, e.g. at inconvenient reaction temperature or pH of the aqueous medium. If we propose that under reaction conditions the same acid catalyzed rearrangement of furfuryl alcohol proceeds, then among reaction products should be present 3-hydroxy-4-cyclopentenone. However, in selected samples the presence of this compound and its hydrogenated derivative was not detected. The argument for the absence of this compound might be that in the presence of metal
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Scheme 3. Simplified reaction scheme of acid catalyzed rearrangement of furfuryl alcohol [16].
Scheme 4. Reaction pathway through 3-hydroxy-4-cyclopentenone.
catalysts and high hydrogen pressure and reaction temperature the hydroxyl group is quickly hydrogenated. To elucidate the possibility that the reaction intermediate is 3-hydroxy-4-cyclopentenone, the experiment with furfuryl alcohol was performed in two reaction steps (Table 3, exp. 9). In the first step the aqueous solution of furfuryl alcohol was heated in nitrogen atmosphere at 170 ◦ C for 2 h. Under these conditions furfuryl alcohol was converted to 3-hydroxy-4-cyclopentenone and to yellowish-brown colored oligomers insoluble in water (Supplementary information). Smaller amount of levulinic acid was also detected. In the second step, after adding of 5%Pt/C catalyst, the aqueous phase was hydrogenated at 175 ◦ C and 80 bar of hydrogen pressure. After these reactions cyclopentanone and cyclopentanol were obtained in the yields 7.84 and 23.15 mol%, respectively. These yields were calculated on the amount of furfuryl alcohol charged into the autoclave in the first reaction step. In comparison to typical experiment performed with furfuryl alcohol at the same reaction conditions (Table 3, exp. 1), the yields of both desired products are only about one third. Moreover, in the two-step experiment (i) the main product of furfuryl alcohol rearrangement is cyclopentanol, and (ii) by condensation of furfuryl alcohol with non-stabilized carbenium ion a large amount of polymeric products is formed. These different findings indicate, that the reaction via 3-hydroxy4-cyclopentenone intermediate is not the major reaction pathway of furan ring rearrangement. On the basis of presented results we assume, that the key step toward the rearrangement of furfural and furfuryl alcohol to cyclopentanone is not only the formation of carbocation 1 but more likely its stabilization. In the metal catalyzed reactions this is achieved by strong binding of the furan ring to the large reservoir of electrons in the catalytic metal (Pt, Pd, Ru, Ni). Moreover, the interaction with co-adsorbed nucleophile, water, and co-adsorbed furfural or furfuryl alcohol may account for the stability and reactivity of carbocation 1. If the reactive carbocation is not stabilized by the action of metal catalyst it undergoes reactions with furfural or furfuryl alcohol leading to the formation of conjugated species [34,35]. This should explain why the yields of products in the reaction of furfuryl alcohol or its derivatives in the presence of
homogeneous acid catalysts which proceed through generating of “free” carbocation are low [16]. As was mentioned above, in the stabilization of the carbocation 1 can participate also furfural or furfuryl alcohol which is adsorbed on the metal surface. This assumption is based on the observation, that in the presence of some catalysts the products distribution is significantly influenced by the initial concentration of furfural in aqueous phase (Tables 5 and 6). For example, in the palladium catalyzed reactions the increase of furfural concentration only from 2.4 wt.% to 4.8 wt.% causes that the R/H ratio is changing dramatically, from 0.17 to 1.02. It means that at higher furfural concentration, probably as a consequence of stabilization of the carbocation 1 by the interaction with co-adsorbed furfural, the furan ring rearrangement is enhanced. The variation of the R/H ratio with the furfural concentration is observed also in the presence of Pt, Ru and Ni catalysts, but in this very narrow concentration range the differences are not very high. The transformation of furfural and furfuryl alcohol to cyclopentanone undergoes only in water as solvent. Water participates on the covering of the metal surface. The heat of adsorption of water (12.4 kcal/mol) is similar that of adsorption of furfural (12.3 kcal/mol) [26]. Interaction of water with metal can influence adsorption characteristics of furfural and furfuryl alcohol, the mode of their bounding on the surface and the concentration of hydrogen dissolved in aqueous layer bound to metal surface [39,40]. Besides influencing the adsorption characteristics of the carbocation 1 stabilized on the metal surface, the water molecule probably attacks the carbocation which is then converted to the intermediate species 2. This transformation probably favors dissociation of the ether C O bridges and the rearrangement of subsequently formed intermediates to cyclopentanone. One of the reaction pathways of furfural transformation to cyclopentanone should take place via 3-hydroxy-4cyclopentenone intermediate (Scheme 4). The first stage of this pathway is the metal catalyzed hydrogenation of furfural to furfuryl alcohol. As was observed experimentally the reaction of furfuryl alcohol at 170 ◦ C in nitrogen atmosphere affords 3hydroxy-4-cyclopentenone also in deuterated water, but not its
M. Hronec et al. / Applied Catalysis A: General 437–438 (2012) 104–111
deuterated isomers (Supplementary information). In the presence of metal catalyst this intermediate product is converted by consecutive reaction to cyclopentanol. Some doubt on this pathway throws the observation, that the main product of this consecutive hydrogenation of 3-hydroxy-4-cyclopentenone is cyclopentanol. Namely, in the majority of experiments performed with both furfural and furfuryl alcohol, the preferred product of furan ring rearrangement is cyclopentanone. The speculation about the direct rearrangement of the carbocation 1 or the species 2 to cyclopentanone promoted by the presence of the nucleophile, water, cannot be also excluded. The elucidation of the major reaction pathways in the process of furan ring rearrangement needs further investigation. Despite these uncertainties it seems most likely that the differences in selectivities of furfural and furfuryl alcohol transformation to cyclopentanone in water are caused by the different reactivity of “free” carbocation created by acid catalyzed reaction and the reactivity of carbocation 1 created and stabilized by the catalytic metal. 5. Conclusions The presented results have shown that under applied reaction conditions furfuryl alcohol dissolved in water can be converted to (i) oligomeric and polymeric compounds with conjugated structures or to levulinic acid, (ii) 3-hydroxy-4cyclopentenone as primary product, (iii) cyclopentanone and (iv) products of hydrogenation of furan ring and hydroxymethyl group. The proportion of these reaction pathways on the transformation of furfuryl alcohol depends mainly on the (a) acidity of reaction medium or catalyst support, (b) metal catalyst, (c) hydrogen pressure, (d) furfuryl alcohol concentration. The furan ring rearrangement to cyclopentanone is the dominant reaction of furfural and furfuryl alcohol in water in the presence of Pt, Pd, Ru or Ni catalysts. This reaction does not proceed with 2-methylfuran as the reactant. Since the furan ring rearrangement proceeds only in the nucleophile, water, we propose that the first step of reaction is the creation of carbocation 1. This species is produced in an excess of hydrogen by the favorable scission of the C O bond in the alkoxide or hydroxyalkyl intermediates, but not by dissociation of strong C H bond in the methyl group of 2methylfuran. However, the crucial factor influencing the selectivity of subsequent reaction seems to be stabilization of carbocation. If the reactive carbocation is not stabilized, it is quickly transformed to precursors of undesirable products. The stabilization of carbocation 1 is achieved by its strong binding on the metal surface and by additional interaction with co-adsorbed water and furfural or furfuryl alcohol. Besides influencing the adsorption characteristics of the carbocation on the metal surface, water can attack the carbocation and thus favor the cleavage of the C O bond and subsequently the rearrangement of formed species to cyclopentanone.
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