Top Catal DOI 10.1007/s11244-016-0675-y
ORIGINAL PAPER
A Comparative Catalyst Evaluation for the Selective Oxidative Esterification of Furfural C. Ampelli1 • G. Centi1 • C. Genovese1 • G. Papanikolaou1 • R. Pizzi1 • S. Perathoner1 • R.-J. van Putten2 • K. J. P. Schouten2 • A. C. Gluhoi2 J. C. van der Waal2
•
Ó Springer Science+Business Media New York 2016
Abstract Catalysts based on gold or other metals on different supports were studied in the selective oxidative esterification of furfural with methanol to methyl 2-furoate. Methyl 2-furoate can be used in specialty fragrances, a high added-value application. Catalyst evaluation and selection were performed using the Avantium quick catalyst screening (QCS) platform using industrially relevant conditions. The best performances were exclusively obtained with gold based catalyst supported on ceria or titania, though these catalysts showed rather poor behavior in tests with a different furfural to methanol ratio. Selected gold catalysts were studied subsequently in a conventional lab-scale autoclave under reaction conditions closer to those commonly applied in literature and effects of support type and preparation were included. For each of the two testing conditions a different catalyst was identified as the most optimal, which is explained by the strong chemisorption of furfural on the Au surface. Au/ZrO2 catalysts, identified as optimal in QCS tests, show also high performances (about 99 % yield) in autoclave tests, although a different preparation was employed.
& G. Centi
[email protected] S. Perathoner
[email protected] J. C. van der Waal
[email protected] 1
Section of Industrial Chemistry, Universita` di Messina, ERIC aisbl and CASPE/INSTM, V.le F. Stagno D’Alcontres 31, 98166 Messina, Italy
2
Avantium Chemicals, Zekeringstraat 29, 1014 BV Amsterdam, The Netherlands
Keywords Selective oxidative esterification Furfural Methyl 2-furoate Gold supported catalysts Highthroughput screening
1 Introduction The desire to reduce our carbon footprint and improve sustainability has led to increasing interest in the establishment of a bio-based economy. This development is leading to new models and priorities for the development of bio-refineries and bio-factories [1–8]. For example, the integration of solar energy with CO2 valorization should realize processes with an improved energy and resource efficiency. CO2 is a relevant waste in many biomass processes. For example, over one ton CO2 is emitted per ton of bio-ethanol produced by fermentation [9]. CO2 emitted in the fermentation stage is very pure (over 99.5 %) and it is thus well suited for the conversion to useful chemicals by using solar energy. Renewable methanol produced in this way can be used to valorize some of the secondary products from bio-based processes. The connection of renewable energy sources to bio-based production allows an enhanced carbon-atom economy in biomass transformation and a better process economy via an improvement of energy integration and efficiency. This approach is the basis for the EU project Eco2CO2. This project aims to photoelectrocatalytically convert CO2 to methanol and subsequently use it in the valorization of furfural. The use of lignocellulosic feed stocks, composed of cellulose and hemi-celluloses, leads to the co-production of HMF and furfural, the latter of which is considered a secondary product for valorization [14–18]. The HMF is the starting point for a novel biopolymer polyethylene furanoate (PEF) [10–13]. Interestingly, ethylene glycol can
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also be derived from the hydrogenolysis of C6 sugars with high efficiency [19, 20]. The overall process economics can be potentially increased by using the furfural co-produced and an increased carbon efficiency can be obtained by using CO2-derived methanol [21]. Eerhart studied the economics of a hydrogenation route for furfural to furfuryl methyl ether as potential bio-fuel [10]. Here we studied the oxidative esterification of furfural as possible option for effective furfural valorization: O
O O
+ O2
Catalyst
O OCH3
+ H2 O
CH3OH
The product, methyl 2-furoate, is a specialty fragrance with a market value of around 50–100 US$/kg, i.e. about two orders of magnitude higher than the substrates value. It has a nutty, peppermint, tobacco odor with mushroom undertones. The production of methyl 2-furoate via the catalytic oxidative esterification of furfural has been reported previously, particularly by Pinna et al. [22–27], using gold supported on different oxides (TiO2, CeO2, ZrO2). Gold nanoparticles supported on ceria have been reported by Corma and coworkers [28] for the conversion of HMF to di-methyl 2,5-furandicarboxylate (Me2FDCA) with 99 mol% yield under mild conditions (65–130 °C, 10 bar O2) in the absence of any base. Supported gold nanoparticles on Ce-Zr oxides have been reported by Zhang et al. [29] to have high selectivity (about 99 %) for the oxidative esterification of different benzylic aldehydes. The authors reported that the incorporation of Zr4? into the ceria lattice improves the activation of methanol to methoxy and facilitates the b-H elimination of hemiacetal in the oxidation step. Recently a gold-graphene nanocomposite was reported as a highly efficient catalyst [30]. By comparing different oxides as support for gold nanoparticles, Pinna et al. [25] concluded that the catalytic performances follow the following trend: Au–Zirconia [ Au–Ceria Au–Titania. The state of the art thus suggests that zirconia is the most suitable support regarding activity, selectivity and stability, which is ascribed to highly dispersed Au nanoparticles and the presence of suitable acid–base properties on the support. However, several articles do not support this general view. Pinna et al. [22, 23] mentioned that large gold particles promote the activity in Au–CeO2 catalysts and that sulfonation of a zirconia influences the catalytic performance by modification of the dispersion and the shape of the gold nanoparticles. In contrast, Christensen et al. [35, 36] observed that only supported gold nanoparticles
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below about 10 nm are only active. Smolentseva et al. [31] pointed out that the oxygen storage capacity of the ceria– alumina mixed oxides support determines the catalytic performance for the oxidative esterification of benzyl alcohol and benzaldehyde. And Wang et al. [32] showed that gold supported on MgO, i.e. without oxygen storage capacity and acid sites, also exhibits excellent performance in the oxidative esterification of aldehydes. Suzuki et al. [33] reported excellent results in the oxidative esterification of aldehydes with alcohols using supported gold–nickel oxide nanoparticles. The nanoparticles have a core–shell structure, with the Au nanoparticles at the core and the surface covered by highly oxidized NiOx. Hashmi et al. [34] suggested that mononuclear gold species present in solution, derived from the dissolution of gold nanoparticles, are the active species in the oxidative esterification. Considering this short review of the current state of the art, it is clear that many aspects are still under discussion and further research is required to relate the exact nature of the active sites to the reaction mechanism of the oxidative esterification. From the practical perspective, one may note that all research efforts focuses on the relatively restricted range of gold-supported catalysts. Though high yields are obtained with these catalysts, all studies were performed in rather dilute reaction mixtures and quite far from industrial conditions. Another still open question is thus whether other types of metal catalyst can be active in the reaction is still open. The purpose of this work is to evaluate commercial and research catalysts that are active in the oxidative esterification of furfural in methanol under industrially relevant reaction conditions. Firstly, a wide range of metal-based catalysts was screened to evaluate the potential other metal catalysts besides gold. A large selection of commercial catalysts and catalysts prepared in-house were tested. Secondly, a selection of the best gold catalysts was studied with regard to activity under industrially relevant conditions and compared to typical literature condition for fundamental understanding.
2 Experimental 2.1 Preparation of the Catalysts In addition to commercial samples, as commented in the text, some of the catalysts were prepared using either commercial supports or supports prepared in house. The standard supports based on ceria (CeO2) and zirconia (ZrO2) were purchased from Strem Chemicals Inc. Evonik
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P25 was used as standard Titania (TiO2). AUROliteTM Au/ TiO2 (gold 1 wt% on titanium dioxide extrudates) was acquired from Strem Chemicals Inc. Before testing the catalytic activity, the pellets were crushed in a mortar until a very fine powder was obtained and then sieved to particle size distribution of 120–250 mesh, i.e. 0.063–0.125 mm. The same size of particles was also used for the other samples tested. At this particle size, no issues related to diffusion limitation were observed during the catalytic tests. Distilled water was used in all cases. TiO2 nanoparticles were prepared by a sol–gel method using 10 mL titanium alkoxide as the raw material, mixed with 40 mL 2-propanol in a dry atmosphere. This mixture was added drop wise to a mixture of 10 mL water and 10 mL 2-propanol. The pH was adjusted to approximately 3 by adding hydrochloric acid, leading to the formation of a yellowish transparent gel after 1 h stirring. This gel was then dried at 110 °C for 12 h and calcined in air at 500 °C for 6 h. CeO2 was prepared by a chemical co-precipitation method in water. An aqueous 1 M NaOH solution was added drop wise to a 0.1 M aqueous solution of cerium nitrate [Ce(NO3)36H2O] until a pH of approximately 8 was reached. The resulting solution was stirred for another hour and the precipitate was collected, centrifuged and washed. The washed precipitate was dried in an oven for 12 h at 80 °C, followed by heating at 180 °C for 2 h and annealing at 300 °C for 2 h to enhance the crystallinity. CeO2 and ZrO2 were also synthesized by complexing the metal cations with citrate. After dissolving the salts of cerium [Ce(NO3)6H2O] or zirconium [ZrO(NO3)2nH2O] in water, an aqueous solution of citric acid was added, leading to a final oxide concentration of 40 g L-1. The resulting solution was stirred at 80 °C for 2.5 h to completely dissolve the precursors, followed by stirring at room temperature for at least 12 h. The solvent was subsequently removed on a rotavapor and the solid dried in an oven for 1 h at 80 °C and annealed for 5 h at 500 °C. ZrO2 was also synthesized using a precipitation method. ZrOCl28H2O was dissolved in water, followed by the drop wise addition of an aqueous 5 M NaOH solution under vigorous stirring until a pH of 8.6 was reached. The hydroxide suspension was aged for 20 h at 90 °C. The aged hydroxide was filtered and washed with warm water to remove all chloride. This was confirmed with the AgNO3 test. The solid was then dried at 110 °C for 12 h and calcined at 650 °C for 3 h. Around 1 wt% gold was precipitated on each support by deposition–precipitation (DP) at pH 8.6. The oxide support (2 g) was suspended in 80 mL of an aqueous solution of HAuCl43H2O for 3 h and the pH was adjusted with aqueous NaOH (0.5 M) added slowly up to obtain the pH of 8.6. The sample was washed, filtered and dried at 35 °C
for 12 h, followed by annealing at different temperatures (200, 300, 400 °C) in air to decompose the gold oxide to its metallic form. Photo-deposition (PD) was used as an alternative method for the deposition of gold. The oxide was suspended in water containing methanol as sacrificial donor. The appropriate amount of HAuCl4 was added to the slurry. The pH was adjusted to 7.2 by adding NaOH. PD was carried out in a Pyrex reactor equipped with a quartz window, and photo-irradiation was conducted using a Xe arc-lamp (300 W, Lot Oriel) under N2 atmosphere and constant stirring. After irradiation, the suspension was washed, centrifuged and dried at 110 °C overnight. 2.2 Catalytic Testing Protocols The screening of commercial catalysts was performed using Avantium’s Quick Catalyst Screening (QCS) platform. Simultaneous parallel batch reactions were performed in blocks of 12 stainless steel reactors of about 8 ml, with an effective reaction volume of 1 mL and at pressures as high as 100 bar. In a typical experiment the appropriate amount of catalyst (20 mg) was weighed in each reactor, followed by the addition of 1.00 mL of a 10 wt% furfural solution in methanol. Methanol acts both as a reactant and the solvent. No base is added to the mixture, but calibrated amounts of dioxane as an external standard are added after the reaction was completed. The QCS blocks were pressurized to 8 bar with lean air (8 % O2) and placed in a heating block. The experiments were run at 60, 120 and 160 °C for 1 h with 1000 rpm stirring. The reaction mixtures were analysed on GC-FID using a Trace 1310 GC-FID system (Thermo Scientific), equipped with a TriPlus RSH autosampler. The autoclave experiments were performed in a 250 mL Parr stainless steel autoclave (Teflon-lined). A typical experiment was performed with 20 mmol furfural in 150 mL methanol under 6 bar O2 pressure at 120 °C with 1000 rpm stirring. The reaction was monitored for 6 h by collecting samples in time. Analysis was performed with GC-FID (Finnigan Trace GC Ultra, with a Restek Rx-i-5 column) and GC–MS (Thermo Scientific GC Trace 1310— ISQ MS with a Restek Stabilwax column).
3 Results and Discussion 3.1 Evaluation of Commercial Catalysts and Various Supported Gold Catalysts Under Industrially Relevant Conditions Commercial available catalysts based on different active metals (Co, Cu, Ir, Ni, Pd, Pt, Re, Rh, Ru, Zn) and supports
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Table 1 Reaction conditions used for high-throughput testing with Avantium’s proprietary Quick Catalyst Screening (QCS) platform Solvent
1 mL MeOH
Catalyst loading Gas
20 mg 8 bar lean air (8 % O2)
Temperature
60, 120 or 160 °C
Furfural concentrations
0.85 M furfural (10 wt%)
Reaction time
1h
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Yield methyl 2-furoate , %
30
60°C 120°C 160°C
25
20
15
10
5
0
0
5
10
15
20
25
30
Conversion, %
Fig. 1 Methyl 2-furoate yield versus furfural conversion for all catalysts tested at three reaction temperatures. The dashed line represents 100 % selectivity
20
Yield methyl 2-furoate , %
(zirconia, titania, silica-alumina, silica, graphite, carbon, calcium carbonate, barium carbonate, alumina) were tested using Avantium’s Quick Catalyst Screening (QCS) platform. The reaction conditions are provided in Table 1. The intermediate temperature of 120 °C is the most commonly reported in literature for the oxidative esterification using Au catalysts. The significantly lower and higher temperatures are chosen to accommodate other possible activity regimes for the non-gold commercial catalysts tested. The gold catalysts were prepared in house with an extensive range of supports were investigated: alumina, carbon, CeO2, CeO2–Al2O3, CeO2–ZrO2, Codoped alumina, Fe2O3, Fe-doped alumina, MgAl2O3, MnO2, Mn-doped alumina, Mn–Co-doped alumina, TiO2 (anatase), TiO2 (P25), ZrO2, ZrO2–TiO2, ZrO2–Y2O3. Most of the samples have a gold loading of around 1 wt%. Figure 1 provides an overview of the results for all catalysts evaluated in terms of yield of methyl 2-furoate versus furfural conversion, irrespective of the temperature and the metal catalyst employed. The dashed line represents 100 % selectivity. For the experiments at 60 and 120 °C the selectivity for methyl 2-furoate is consistently close to 100 %. The selectivity decreases at the higher temperature of 160 °C. Different non-gold commercial catalysts, 47 in total, were also tested at the three selected temperatures. The results are summarized in Fig. 2. The best non-gold catalysts contain Pd or Pt, both on CaCO3 as the support. The methyl 2-furoate yields are lower than for the best performing gold catalysts. In addition, these catalysts have the strongly basic CaCO3 as the support and the formation of small amounts of furfuryl alcohol indicates that in this case the reactivity may be related to a non-catalytic Cannizzaro reaction of furfural. In this light, the surprising results of Wang et al. [32] using the strongly basic MgO support is related to a similar Cannizarro reaction. A comparison of the performances of the different Aubased samples (Table 2) in terms of methyl 2-furoate yield is presented in Fig. 3. It is clear that there are sharp differences in the performance depending on the method of preparation, with the methyl 2-furoate yield varying from nearly 0 to about 30 %. The highest yield of methyl 2-furoate was observed for Au/ZrO2 treated at 120 °C. It is
60°C 120°C 160°C
15
10
5
0 Co CoO Cu CuO Ir
Ni NiO Pd Pd/Pt Pt Re Rh Ru ZnO
Catalyst (acve metal component in the starng sample)
Fig. 2 Methyl 2-furoate yield in high-throughput tests and three reaction temperatures with Avantium quick catalyst screening (QCS) platform of different catalysts with the active metal component different from gold
important to note that the results with mixed zirconia-oxide catalysts (R = ZrO2–TiO2, S = ZrO2–TiO2, T = ZrO2– Y2O3) are far worse (yields \3 %). Contrary to what was expected from literature, all the catalysts based on TiO2 (entries O and P) also give significantly lower yields than the best ZrO2 based catalyst. These catalysts were similar to those indicated in literature to give high performances, although under different conditions [35, 36]. Ceria-based catalysts (entries C = CeO2, D = CeO2/Al2O3, E = CeO2/ZrO2) also exhibit relatively low activity with yields less than 13 %. Surprisingly, relatively high yields (22 %) are obtained with catalyst M. This is based on MnO2/Al2O3 as the support and has not been previously reported as an active catalyst for this reaction. Good performances (around 20 % yield) were also obtained with
Top Catal Table 2 Au commercial catalysts used for highthroughput testing with Avantium quick catalyst screening (QCS) platform
CatID
Support
Pretreatment
Catalyst
A
Al2O3
250
1 % Au/Al2O3
B
C
250
Au/C
C
CeO2
250
Au/CeO2
D
CeO2/Al2O3
250
Au/CeO2/Al2O3
E
CeO2/ZrO2
250
1 % Au/CeO2/ZrO2 \ 200 lm
F
Co3O4/Al2O3
G
Fe2O3
250
Au/Fe2O3
H
Fe2O3/Al2O3
250
Au/Fe2O3/Al2O3
I
MgAl2O4
250
Au/MgAl2O4
L
MnO2
250
1 % Au/MnO2
M
MnO2/Al2O3
250
Au/MnO2/Al2O3
N
MnO2/Co3O4/Al2O3
250
Au/MnO2/Co3O4/Al2O3
O
TiO2
250
1 % Au/TiO2 100 % anatase
P
TiO2
Q R
ZrO2 ZrO2–TiO2
250
1 % Au/ZrO2 Au/ZrO2–TiO2 calcined
Au/Co3O4/Al2O3 calcined
Au/TiO2 (P25) calcined
S
ZrO2–TiO2
250
1 % Au/ZrO2–TiO2
T
ZrO2–Y2O3
250
Au/ZrO2–Y2O3
U
ZrO2
Au/ZrO2 calcined
Catalysts were pretreated at 250 °C under H2 before use
Yield methyl 2-furoate , %
30 60°C 120°C 160°C
25 20 15 10 5 0 A
B
C D E F G
H I
L M N O P Q R S
T U Blank
Au catalyst
Fig. 3 Methyl 2-furoate yield in high-throughput tests of different gold-based catalysts (see Table 2) and three reaction temperatures with Avantium quick catalyst screening (QCS) platform
magnesium-aluminate (spinel) as support for the gold nanoparticles (catalyst I). This was also the case for gold on alumina (catalyst A), iron oxide (catalyst G) and Fe2O3/ Al2O3 (catalyst H), with yields of 17–19 %. Catalysts G and H were not previously reported for this reaction. Thus, supports with quite different characteristics in terms of reducibility, oxygen-storage capacity and acid–base properties show good performance under the reaction conditions tested. The gold loading in these samples is similar (around 1 %).
In order to verify the results of QCS screening with gold-based catalysts a new series of catalysts was prepared (Table 3), using ceria and titania supports, either commercial or prepared in house, using the methods provided in Table 3. Gold is added to the supports by deposition– precipitation (DP), the most common method used in literature for depositing gold on oxide supports [37]. Figure 4 summarizes the results obtained at 120 °C for all these catalysts and a selection from those already tested in the QCS screening. The same experimental conditions as used in the catalysts screening program were used. In order to verify whether the oxygen partial pressure using 8 bar lean air (8 % O2, see Table 1) could be responsible for the low performances, additional tests were made using a higher reaction pressure (20 bar of air). However, none of the catalysts prepared on ceria or titania showed appreciable activity in methyl 2-furoate synthesis at both oxygen partial pressures. Only the sample prepared on TiO2 synthesized by sol–gel method and calcined at 300 °C (AT2_300) showed some activity, by increasing the oxygen partial pressure. The yield is around 3 %; about one order of magnitude lower than that of the best catalysts identified in the screening (Figs. 1, 2, 3). The absence of significant activity in the commercial catalysts tested, confirm that only gold-based catalysts are the only catalytically active materials for this reaction. The results of this catalyst evaluation study also indicate that Au/ZrO2 is the preferred catalyst in the oxidative
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Top Catal Table 3 Au-based catalysts synthesized using commercial (Com) or ad-hoc prepared CeO2 and TiO2 supports and used for high-throughput testing with Avantium quick catalyst screening (QCS) platform
CatID
Au loading (%wt)
Support
Calcination (°C)
Catalyst
Method
AC1_200
1.034
CeO2
200
Au/CeO2
Com (STREM)
AC1_400
1.034
CeO2
400
Au/CeO2
Com (STREM)
AC2_200
1.13
CeO2
200
Au/CeO2
Sol–Gel (citrate acid)
AC2_400
1.13
CeO2
400
Au/CeO2
Sol–Gel (citrate acid)
AC3_200
1.45
CeO2
200
Au/CeO2
Precipitation
AC3_400
1.45
CeO2
400
Au/CeO2
Precipitation
AT1_200
1.015
TiO2
200
Au/TiO2
Com (Evonik P25)
AT1_300
1.015
TiO2
300
Au/TiO2
Com (Evonik P25)
AT1_400
1.015
TiO2
400
Au/TiO2
Com (Evonik P25)
AT2_300
1.36
TiO2
300
Au/TiO2
Sol–Gel (isopropoxide)
AT2_400
1.36
TiO2
400
Au/TiO2
Sol–Gel (isopropoxide)
Table 4 Reaction conditions used for tests in oxidative esterification reaction in a lab-scale apparatus based on a 250 mL Parr stainless steel autoclave (Teflon-lined) (apparatus shown in Fig. 5)
30
120°C, 8 bar lean air (8% O2) 120°C, 20 bar air
Yield methyl 2-furoate , %
25
Solvent
150 mL MeOH
Catalyst loading
100 mg
20
Gas
6 bar O2
Temperature
100–140 °C (120 °C typical)
15
Furfural concentrations
0.024 M
Reaction time
0.5–8 h (6 h typical)
10
5
Blank
O
Q
M
AT2_400
Pd-based
AT2_300
AT1_400
AT1_300
AT1_200
AC3_400
AC3_200
AC2_400
AC2_200
AC1_400
AC1_200
0
Fig. 4 Methyl 2-furoate yield in high-throughput tests (Avantium quick catalyst screening—QCS) of different Au catalysts prepared on commercial and synthesized ceria and titania supports (see Table 3). Selected catalysts (Pd-based, M, O, Q—see Figs. 2, 3) were tested again as reference. Results at 120 °C for two different O2 partial pressures are shown
esterification of furfural, in agreement with literature indications [25]. 3.2 Catalytic Testing Under Diluted Conditions The broad high-throughput catalyst evaluation detailed above was performed under relevant industrial reaction conditions, but showed clear differences with results reported in literature. Therefore, a new set of experiments was performed under reaction conditions closer to those in literature (Table 4). Figure 5 reports a scheme of the apparatus used. The main differences are the much lower concentration of furfural and the significantly higher catalyst loading
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compared to the conditions described in Table 1. An extended range of parameters was investigated as indicated in Table 4. The results reported here, however, refer to experiments performed at 120 °C and 6 h reaction time, as the results for the other conditions did not yield additional information. An initial observation is that complete conversion and high yields to methyl 2-furoate, up to [99 % in some cases, could be obtained under these conditions, in agreement to those reported in literature. The first set of experiments was performed using titania and ceria based catalysts using deposition–precipitation (DP) to deposit the gold nanoparticles (Table 3) and AUROliteTM Au/TiO2 (1 wt% gold on titanium dioxide extrudates) as a reference catalyst. In order to verify the role of the method of deposition of gold, a photo-deposition (PD) method was also used to deposit gold on P25 Evonik commercial titania support. The amount of gold deposited in this case is about 0.9 wt%. Selected results obtained at 120 °C and 6 h of reaction time are shown in Fig. 6. The results reported in Fig. 6 clearly show that for both titania and ceria supports good catalytic performances (conversion [90 % and selectivity [80 %) could be obtained. Especially the gold deposited on a commercial ceria sample 6 (AC1_200) calcined at 200 °C shows a
Top Catal Fig. 5 Scheme of the experimental apparatus based on a lab-scale Parr stainless steel autoclave (Teflon-lined) for testing in oxidative esterification reaction
Selecvity to methyl 2-furoate, %
100
6
TiO2
CeO2
90
3 7
2
80
1
5
70
4
60 50 40
1 2 3 4 5
TiO2-based Aurolite AT1_200 (DP) AT1_200 (PD) AT2_300 AT2_400
30 50
60
CeO2-based 6. AC1_200 7. AC1_400 8. AC2_200 9. AC2_400
70
8 9
80
90
100
Furfural conversion, %
Fig. 6 Selectivity to methyl 2-furoate versus furfural conversion at 120 °C (6 h of reaction time) for selected gold catalysts supported on TiO2 and CeO2 oxides. Tests in a lab-scale Parr stainless steel autoclave (Fig. 5) with other reaction conditions indicated in Table 4
good performance with a 98 % yield to methyl 2-furoate. It is worth noting that under the industrially relevant reaction conditions of the initial evaluation, the performances of this catalyst was quite low. Although the specific preparation conditions largely influence the performances, as remarked in Fig. 6, it is concluded from the comparison of
the results in Fig. 4 that operations under diluted conditions (high methanol to furfural ratio) allow good performances, even on catalysts apparently inactive at high furfural to methanol ratios. These results indicate that it is the strong chemisorption of furfural on the catalyst gold surface against the other required components for the reaction, which plays an important role in determining the overall reactivity. At high furfural concentrations catalyst become inactive due to lack of oxygen and/or methanol present on the catalyst surface. The catalytic performances of selected zirconia-based catalysts in the oxidative esterification of furfural at 120 °C (6 h) are reported in Table 5. These results confirm what was observed previously in literature, namely that that Au/ ZrO2 catalysts give the best performances in this reaction (99 % yield). Under more industrially relevant conditions another catalyst (sample Q, Figs. 2, 4), poor at low furfural concentrations, was shown to be best. A gold catalyst on a commercial ZrO2 support prepared with DP, showed poor performances and is not too different from those of a sample prepared by the same deposition procedure of Au, but on a zirconia prepared by precipitation and calcined at 200 °C. Calcining at 400 °C of this support, however, produces a very active and
Table 5 Catalytic performances of selected zirconia-based catalysts in the oxidative esterification of furfural at 120 °C (6 h of reaction time) Catalyst
Preparation
Calcination ( °C)
Au loading (wt%)
Conversion furfural (%)
Selectivity to methyl 2-furoate (%)
ZrO2
Commercial (Strem)
400
–
AZ_400
ZrO2 Strem, Au by DP
400
0.1 ± 0.1
5
10
65
16
AZ1_200
ZrO2 by precip.
200
0.4 ± 0.1
63
10
AZ1_400
Au by DP method
400
0.4 ± 0.1
99
100
Au/ZrO2 (Q)
Commercial (Q sample)
250
0.7 ± 0.1
10
41
Tests in a lab-scale Parr stainless steel autoclave (Fig. 5) with other reaction conditions indicated in Table 4
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selective catalyst. This clearly indicates that the nature of the support also plays a very important role in agreement with literature. The results this study confirm that the performances in the oxidative esterification of furfural to methyl 2-furoate strongly depend on the specific catalyst composition and preparation conditions, but that the performance is also controlled by the chemisorption of the three substrate molecules on the Au surface.
4 Conclusions The screening of different catalysts, based on gold or other metals and different type of supports, and reaction conditions relevant for industrial exploitation (particularly a high furfural to methanol ratio, the latter used both as reactant and solvent), showed that only gold-based catalysts have the better results. A few Pd- and Pt-based catalysts, not previously evidenced in literature, show acceptable performances though at this point it is unclear whether the basic CaCO3 support is here the actual reactive component. Between the gold-based catalysts, Au/ZrO2 showed the best yields to methyl 2-furoate at 120 °C (1 h reaction time). In addition, the gold on oxide supports such as MnO2, MgAl2O4 and Fe2O3, not previously reported as possible supports, also showed interesting properties. On the contrary, some of the materials indicated in literature as highly active catalysts, for example based on gold on titania and ceria, were found to be inactive or have a low activity. For this reason, the performances of gold-based catalysts using titania or ceria (either commercial or synthesized) were also studied in a lab-scale autoclave apparatus with reaction conditions closer to those reported in literature. The results evidence that under these experimental conditions catalysts based on gold supported on titania or ceria could be both highly active and selective to methyl 2-furoate, although with performances highly depending on the specific preparation. The reference standard gold catalyst on TiO2 (AUROliteTM) shows good performances (yield about 70 %), but lower than that of other Au/TiO2 catalysts (yield about 86 %). Au/CeO2 optimal catalyst shows a yield of about 97 %. The same catalysts showed instead a yield of few percentages (about one order of magnitude lower than the best catalyst) in QCS tests. Au/ZrO2 catalysts, identified as optimal under the screening conditions (high furfural to methanol ratio) in the QCS, are is also highly active in lab-scale autoclave apparatus when using low furfural to methanol ratio. The optimal yield in methyl 2-furoate is about 99 %, for a catalyst with a different preparation from that of the catalyst identified as optimal in QCS tests.
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Further studies are in progress to clarify the differences between the performances under the two reaction conditions, although a clear indication is presented that the furfural to methanol ratio and thereby associated sorption on the Au surface is mainly responsible for the differences in the observed catalytic performance. Most likely strong chemisorption of furfural on the Au surface is responsible for the observed differences in catalytic behaviour, which in turn is influenced clearly by specific characteristics of the support. It is thus necessary to give more attention to these aspects and analyse the nature of the active sites on the Au particle and on the support in relationship with the reaction mechanism. This may also help explaining the differences in literature, as commented on in the introduction. Further studies are in progress on these aspects. Acknowledgments This work was realized in the frame of the EU project ‘‘Eco-friendly biorefinery fine chemicals from CO2 photocatalytic reduction’’ (Eco2CO2) (Project 309701), which is gratefully acknowledged. The EU Marie Curie IAPP (Industry-Academia Partnerships and Pathways) project ‘‘Biopolymers and Biofuels from Furan based Building Blocks’’ (BIOFUR, project 324292) of collaboration between Avantium and the University of Messina is also gratefully acknowledged.
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