Selective conversion of concentrated feeds of furfuryl alcohol to alkyl

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Selective conversion of concentrated feeds of furfuryl alcohol to alkyl levulinates catalyzed by metal triflates Alban Chappaz, Jonathan Lai, Karine De Oliveira Vigier, Didier Morvan, Raphael Wischert, Matthieu Corbet, Bertrand Doumert, Xavier Trivelli, Armin Liebens, and François Jerome ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00100 • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 15, 2018

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ACS Sustainable Chemistry & Engineering

Selective conversion of concentrated feeds of furfuryl alcohol to alkyl levulinates catalyzed by metal triflates Alban Chappaz,a Jonathan Lai,b Karine De Oliveira Vigier,a Didier Morvan,c Raphael Wischert,b Matthieu Corbet,b Bertrand Doumertd, Xavier Trivellie, Armin Liebensb and François Jérôme*a. a

Institut de Chimie des Milieux et Matériaux de Poitiers, CNRS/Université de Poitiers, ENSIP, 1

rue Marcel Doré, TSA 41105, 86073 Poitiers Cedex 9, FRANCE. Email : [email protected] b

Eco-Efficient Products and Processes Laboratory, UMI 3464 CNRS/Solvay, 3966 Jin Du Road,

Shanghai 201108, CHINA. c

SOLVAY, 85 rue des frères Perret - BP 62- F - 69192 Saint Fons Cedex, FRANCE.

d

Univ. Lille, CNRS, INRA, Centrale Lille, ENSCL, Univ. Artois, FR 2638 - IMEC - Institut

Michel-Eugène Chevreul, F-59000 Lille, FRANCE. e

Univ. Lille, CNRS, UMR 8576 – UGSF – Unité de Glycobiologie Structurale et Fonctionnelle,

F-59000 Lille, FRANCE.

ACS Paragon Plus Environment

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Abstract: Herein, we describe an efficient and recyclable catalytic system based on metal triflates capable of converting highly concentrated feeds of furfuryl alcohol (30–40 wt. %) to alkyl levulinates in excellent yields (>90%). This constitutes a unique and important advance in the field. Indeed, the dilution of feedstocks represent one of the major bottlenecks in catalysis for the industrial deployment of bio-based fuels and chemicals in our society. The impact of water in the metal triflates catalytic performances is also discussed. A comparison with a commercialized process (SFOS) shows that this catalytic route is in line with industrial requirements in terms of yield, selectivity, reactor productivity and capacity. In particular, unprecedented space time yields up to 200 kg m-3 h−1 were obtained.

Metal triflate with Bi, Al, Ga or Sn as cation are capable of selectively converting high concentrated feed of furfuryl alcohol to alkyl levulinate

KEYWORDS Acid catalysis, furfuryl alcohol, alkyl levulinate, metal triflates, concentred feed, biomass.


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Introduction The catalytic conversion of renewable feedstocks to fuels, fine or specialty chemicals is now the subject of intense research efforts. Although a myriad of reports are published daily on this topic, the emergence of bio-based fuels and chemicals in our society is unfortunately facing important hurdles such as catalyst deactivation and high dilution (typically < 1-5 wt%, often required to better control the selectivity of reactions), thus leading to unacceptable space time yields (i.e. reactor productivity) for industrial implementation.1,2 In many cases, the production cost of bio-based fuels and chemicals is not competitive in comparison to fossil-derived products, for which over the course of several decades, chemistry has already developed very efficient and economically competitive catalytic processes.3 The acid-catalyzed ring opening of furfuryl alcohol (FA) in alcoholic media is a perfect example. This reaction is of high interest and yields alkyl levulinates (AL).4 ALs are nowadays recognized as promising bio-based fuel additives.5-7 In particular, blended with transportation fuels, ALs reduce significantly the formation of soot in engines. ALs are also industrially relevant solvents or intermediates for the manufacture of pesticides, plasticizers or polymers.4-7 The global market of ALs was around 2.5 kt in 2013 with an annual growth estimated at ~6% mostly driven by ethyl levulinate. In contrast to the widely studied route involving 5hydroxymethylfurfural (HMF), ALs or levulinic acid can be produced from FA (industrially available at a price of 1.2€ / kg)8 through a 100% atom economical process. This catalytic reaction is nowadays deployed at an industrial scale in China. However, this process suffers from an overall moderate yield because 20–30% of FA are lost in uncontrolled polymerization side reactions and during purification.9 Furthermore, HCl is used as a catalyst, thus producing salts after neutralization at the end of the reaction.


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Several recent studies have highlighted the possible production of ALs from FA in high yields (> 80%) using either heterogeneous (zeolites,10-15 acidic resins,10,12,16,17 silica-supported –SO3H,12 arylsulfonic acid functionalized mesoporous carbon spheres,18 sulfated oxides,13,19 α-Fe2O3,20 methylimidazolebutylsulfate phosphotungstate,21 Al2O3/SBA-15,22 etc.) or homogeneous acid catalysts (H2SO4,10,23 AlCl3,24 In(OTf)3,15 heteropolyacids,13, etc.). To inhibit the unwanted acidcatalyzed polymerization of FA leading to the formation of tar-like materials, reactions are conducted under diluted conditions, at FA concentrations in a typical range of 0.2-3 wt% (vs. 30– 40 wt. % in the current industrial process). Although good yields of ALs were claimed under these diluted conditions, these processes unfortunately lead to low space time yields, not compatible for an industrial deployment. When the FA concentration was increased to 10 wt. %, the yield to AL dropped unfortunately down to only 40–50% showing the limitation of these catalytic strategies.13,17,19,21 Note that Lange proposed a semi-continuous reactor for which the selectivity to AL was adjusted by tuning the weight hourly space velocity (WHSV).10 However, low WHSV (< 5gFA.gcat.h) were required to maintain the yield to AL higher than 70%. Turning now to the hydrolysis of FA to levulinic acid, Dumesic showed that, in the presence of a HZSM-5 catalyst, utilization of a THF-water mixture allowed the side polymerization of FA to be partly inhibited, affording levulinic acid with 70% yield from a concentration of FA of 10 wt%.25 Beside catalyst and process design, another possibility to increase the space time yield consists in increasing the amount of the catalyst. This strategy has however an important limitation due to the more or less important deactivation of acid catalysts during the reaction. Being able to increase the concentration of FA in alcohol, while preserving a high selectivity to AL, is a very important scientific challenge. Overcoming this hurdle would open the door to more competitive


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catalytic processes in terms of selectivity, yields and space time yield, important criteria for industrial implementation. We herein disclose a catalytic system that enables very high ALs yields from concentrated feeds of FA at levels unreported to date. In particular, in combination with a catalytic amount of water, Lewis acids of the type M(OTf)3 with M = Bi, Ga, and Al and M(OTf)4 with M = Sn afford ALs yields of up to 91% starting from a FA concentration as high as 30 wt. % (space time yield up to 200 kg/m3/h). Last but not least, the generated catalytic species were conveniently recycled after removal of ALs by distillation. These results are an important advancement in the conversion of biomass-derived intermediates to fuel precursors. A comparison of our work with previous data reported in the current literature is provided in Table S1 in order to shed light on the remarkable catalytic performances of these systems. In addition, a comparison with the Société Française d’Organo-Synthèse (SFOS) process commercialized by SOLVAY is also discussed at the end of the manuscript.

Results and Discussion In a first set of experiments, various Lewis acid catalysts were screened. Typically, a solution of FA in n-butanol was heated in batch under reflux in the presence of 1 mol % of a Lewis acid catalyst and 2 wt. % of water (relative to n-butanol). The effect of water is discussed later. A concentration of FA of 10 wt. % was selected initially to screen different acid catalysts. At such concentration, the selectivity to AL is usually rather low ( 4), metal triflates with Bi, Al, and Ga as cation have a pKh value lower than 2.6.28-31 According to the classification of Kobayashi, these metal triflates are more sensitive toward hydrolysis and may thus serve as a reservoir of triflic acid which could explain their highest activities. To check this possibility, triflic acid (TfOH) was tested as a Brønsted acid catalyst (Table 1, entry 7). Under our experimental conditions, TfOH was 13–42 times less active than the corresponding metal triflates and also afforded butyl levulinate in a yield 2 times lower. Therefore, the release of triflic acid cannot explain the superior catalytic activity and selectivity of these metal triflates.


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To support this claim, 2,6-di-tert-butylpyridine (DTP) was added to the reaction media to scavenge potential traces of released TfOH. Because of the steric hindrance of the two tert-butyl groups surrounding the nitrogen atom, DTP is indeed not capable of coordinating metal triflates.32-34 In the presence of 1 eq. of DTP with Bi(OTf)3 as catalyst, no significant change in both rAL and AL yield was observed, thus further supporting that triflic acid is not the catalytic species (Table 1, entry 8). Furthermore, when AL and n-butanol were distilled at the end of the reaction, no trace of triflic acid was detected (monitored by 19F NMR) in the distillate, although the boiling point of TfOH is lower than that of butyl levulinate. This observation agrees well with the absence of triflic acid in the catalytic phase, which is consistent with a previous report of Marks.35 Table 1 does not reveal any correlation between the initial production rate of AL (rAL) and the yield to AL. For instance, the rAL was in a similar range for Sc(OTf)3, Al(OTf)3 and Ga(OTf)3 (60, 98 and 76 h−1, respectively) but the yield in alkyl levulinate was significantly lower in the presence of Sc(OTf)3 (39 vs. 60% with Al(OTf)3 and Ga(OTf)3) (Table 1, entries 3, 10). In addition, butyl levulinate was found even more stable in the presence of Sc(OTf)3 than with Bi(OTf)3, whereas this latter was more selective to butyl levulinate. Hence, effect of reaction rates on the selectivity enhancement to ALs, observed in the presence of M(OTf)3 (M = Bi, Sn, Al and Ga), can be ruled out. The plot of the rAL as a function of the ionic radii of cations revealed, however, a correlation (i.e. the smaller the ionic radius, the higher the rAL), except for Bi(OTf)3 and Sn(OTf)4, the two best catalysts. A similar trend was observed when the rAL was plotted as a function of the charge density (charge divided by ionic volume), both using formal charges (+3 / +4) or DFT-calculated NBO charges of the metal triflates, (i.e. the higher the charge density, the higher the rAL), again


9


except for Bi(OTf)3 and Sn(OTf)4 (Figure S3). These results indicate that the much higher catalytic performances of Bi(OTf)3 and Sn(OTf)4 cannot be ascribed to the ionic radii of Bi and Sn, neither to their charge density, but must have a different reason. Previously, it was shown that small changes in the amount of water in alcohol led to significant differences in the catalytic performance of metal triflates.19,21,23. Hence, the impact of water on catalytic results was assessed. Results are presented in Fig 2.

yield rAL

rAL (h-1)

1000

80%

750

60%

500

40%

250

20%

0

0% 0

0,5

1

5

15

25

50

100

250

500

1000

n H2O (eq. / Bi(OTf)3)

Figure 2. Influence of water on the maximum reaction yield to AL (1 mol% of Bi(OTf)3, 10 wt% of FA in n-butanol, reflux). rAL are provided with an experimental error of about 15%.

A decrease of the amount of water from 1000 to 1 eq (relative to Bi) led to an increase in the yield of butyl levulinate from 60 to 80%. Below one equivalent, the yield of butyl levulinate dropped significantly and, under anhydrous conditions, Bi(OTf)3 gave results similar to those obtained with In and Sc. The activity of Bi(OTf)3 was also maximal at a water content within the range of 1–25 eq (rAL ~ 500 h−1). These results show that the selectivity to alkyl levulinate is optimal at a Bi/H2O molar ratio of 1. A tentative explanation for this result can be proposed on the basis of previous experimental


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AL yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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observations and DFT calculations on metal triflates by the group of Duñach. Indeed, hydration of Bi(OTf)3 and Sn(OTf)4 to form M(OTf)n(OH2) species (M = Sn, Bi, n = 4 for Sn, 3 for Bi) was reported to be strongly favored over hydrolysis.36-39 On the same basis, since no triflic acid was detected in our experiments, alcoholysis of Bi(OTf)3 by n-butanol or furfuryl alcohol can also be excluded. In the case of Bi(OTf)3, it was proposed that Bi3+-H2O association generates an acidic proton.38 In particular, a proton transfer between water molecules of the inner sphere of hydrated Bi(OTf)3 was described, forming a (OTf)3Bi-(H3O+)(-OH) complex. The strongly acidic Bi-(OH2) or Bi-H3O+ species would then be the catalytically active species (so-called Lewis-assisted Brønsted mechanism).36,39 Hence, in the presence of water in n-butanol, Bi(OTf)3 should be written as (OTf)3Bi(-OH)x(H3O+)x with x ranging from 1 to 5.36 Obviously, according to the amount of water present in n-butanol, the exact composition of this complex (i.e. the x value) will vary, and thus its catalytic performance. Unfortunately, we failed in isolating and characterizing such hydrated species, even using different analytical tools such as FT-IR or 1H NMR. In particular, we spent much effort to analyze a n-butanolic solution of Bi(OTf)3 by

209

Bi NMR in the same concentration and same temperature (383K) as in the

catalytic trials. However, no signal was detected because of the very fast relaxation rate of the 9/2-spin nucleus 209Bi with very large Cq. (SI). If hydration of Bi(OTf)3 could be one of the reasons to explain its superior catalytic activity, one may note that the difference of acid strength between metal triflates, which can be affected by solvation with water, could be also another explanation. Solvation of acid sites with water is indeed known to decrease the acid strength, and thus the activity of an acid catalyst. However, between 0 and 25 eq. of water, the reverse was observed in our case (Fig. 2). In addition, the change in the butyl levulinate selectivity as a function of the water content (Fig. 2) suggested a


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change in the chemical composition of Bi(OTf)3, an experimental observation in line with a hydration rather than a solvation. To support the key role played by the Bi-OTf bond in the catalyst performance, BiCl3 was tested as a Lewis acid (Table 1, entry 9). BiCl3 was found 28 times less active than Bi(OTf)3 and afforded butyl levulinate with only 40% yield. Then, one Cl- ligand was replaced by one TfO-. In this context, the (DPPE)BiCl2(OTf) complex was prepared according to the procedure previously reported by Ferguson et al.40 When 1 mol % of (DPPE)BiCl2OTf was used as catalyst, the yield in butyl levulinate was increased to 92%, which supports the important role played by a Bi-OTf bond for the synthesis of AL with high yield from FA (Table 2, entry 1). Note that the selectivity enhancement was not due to the presence of DPPE. Indeed, addition of DPPE on Bi(OTf)3 did not result in a significant improvement of the AL yield.

Table

2.

Catalytic

conversion

of

FA

to

butyl

levulinate

in

the

presence

of

(DPPE)Bi(Cl2)(OTf).a

Entry

FA concentration (wt. %)

Catalyst amount (mol %)b

AL yield (%)

1

10

1

92

2

20

0.5

94

3

30

0.33

71

a

10 wt% of FA in n-butanol, 1 mol% of (DPPE)Bi(Cl2)(OTf), 2wt% of water (relative to nbutanol), reflux; b in all entries, the number of mol of catalyst introduced into the reactor was the same explaining why the mol% of catalyst is decreasing when the concentration of FA


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increases. Note that a decrease of the catalyst loading obviously increases the reaction time but does not change the selectivity Having this highly selective (DPPE)BiCl2OTf catalyst in hand, we then checked its efficiency at higher concentrations of FA (Table 2). Remarkably, starting from a concentration of 20 wt. %, butyl levulinate was still produced in 94% yield, confirming the high selectivity of this complex (Table 2, entry 2). One should note that when the concentration of FA was further increased to 30 wt. %, the yield to butyl levulinate dropped but remained at an acceptable level (71%, Table 2, entry 3). To evaluate the performance of this catalytic pathway, we compared it with the SFOS process operated by a subsidiary of the former Rhône-Poulenc group.9 This process used to convert FA to levulinic acid in an aqueous solution of HCl and was running until the beginning of the 90’s. It was stopped because it became uncompetitive compared to the production of levulinic acid in China, with the same process but with less investment and operational costs. Drawbacks of this process stem essentially from the moderate yield that has a direct impact on the product price. The reactant price indeed accounts for about 70% of the full manufacturing cost. Secondly, the levulinic acid formed had to be distilled at the end of the reaction which required a high level of vacuum (~5 torr). This was extremely energy intensive and was actually the limiting step of the process. Three indicators were used in this study to benchmark our catalytic process: (1) the space time yield (STY), (2) the reaction yield and (3), the E Factor. The STY is defined as the mass of products in kg per m3 of solution and per hour. The E Factor, as defined previously by Prof. R. Sheldon, is the ratio between the mass of waste and the mass of product.41,42 It is important to note that both processes were compared in this work in terms of catalytic performances only. We


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do not aim at providing economics on the present catalytic process since this would require more data not available (or not realistic) at the lab scale. In the case of the SFOS process, FA was added dropwise (over 2 h) to avoid its side polymerization reaction and to maintain a high yield to levulinic acid. The final amount of FA added into the reactor corresponded to 30 wt%. This process affords levulinic acid with 70% yield (full conversion of FA) and is characterized by a STY of 70 kg m-3 h−1. It corresponds to an annual production capacity of 5.6 kt (based on a reactor filled with 10 m3 of solution and running for 8 000 h). The SFOS process generates 0.7 t/tlevulinic

acid

of CO2 since 0.3 t/tlevulinic

acid

of

byproducts is burnt at the end of the process (internal data). Burnt byproducts are “carbon” produced by uncontrolled polymerization of FA. Aqueous effluents are estimated at around 0.065 t/tlevulinic acid and contain 8.5 wt. % of salt, including 5.0 wt. % of chloride. The E factor of the SFOS is thus 0.4 (does not include CO2). To be more realistic in the comparison, the catalytic system Bi(OTf)3/H2O (Bi/H2O molar ratio = 1) was preferred in this study instead of the more exotic (DPPE)BiCl2OTf catalyst. To compare both processes under similar conditions, our experimental procedure was slightly modified. In particular, as in the case of the SFOS process, the FA was poured over a period of 2 h into a batch reactor containing n-butanol, 1 mol % of Bi(OTf)3 (relative to FA) and 1 mol % of H2O. The FA concentration thus corresponds to the total amount of FA added into n-butanol after 2 h. At a concentration of FA of 10 wt. %, the STY of the Bi(OTf)3/H2O catalytic system reached 65 kg m-3 h−1, which is comparable to the SFOS process. However, whereas the yield of levulinic acid was 70% in the case of the SFOS process, the yield of butyl levulinate reached 94% in the presence of the Bi(OTf)3/H2O catalytic system (Figure 3). Considering the possibility of


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recycling Bi(OTf)3 and n-butanol at the end of the reaction, the amount of waste was reduced drastically, leading to an E factor estimated at around 0.06 (vs. 0.4 for the SFOS process). Remarkably, under these conditions, the concentration of FA can be further increased to 20 and 30 wt. % without any significant decrease of the reaction yield (>92%), leading to an increase of the STY to 86 and 182 kg m-3 h−1, respectively (Figure 3). The concentration of FA can be even increased to 40 wt. %, resulting in a further improvement of the STY to 200 kg m3 h−1 but, in this case, the yield of butyl levulinate decreased slightly to 84% (Figure 3).

250

STY (kg.m-3.h-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BL yield (%) Space Time yield (kg/(m3.h))

100%

200

80%

150

60%

100

40%

50

20%

Yield SFOS

AL yield (%)

0

STY SFOS

0% 5 wt% 7,5 wt% 10 wt% 20 wt% 30 wt% 40 wt%

FA concentration (wt%)

Figure 3. Comparison of the Bi(OTf)3/H2O catalytic system with the commercialized SFOS process.

At a FA concentration of 30 wt. %, the Bi(OTf)3/H2O system not only affords butyl levulinate in a quasi-quantitative yield but also exhibits a STY 2.6 times higher than the SFOS process and a E Factor 6.7 times lower. Although the slow addition of FA into the reactor is mandatory in the case of the SFOS process to maintain a high yield to levulinic acid, in our case, the yield to butyl levulinate was only slightly improved from 80 to 90% (Fig 2 vs Fig 3), which further demonstrate the catalytic performances of Bi(OTf)3 in such reaction. An extrapolation of these


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results to the dimension of the batch reactor used in the SFOS process indicated that the annual production capacity could be theoretically improved to 14.5 kt in the presence of the Bi(OTf)3/H2O catalytic system (vs. 5.6 kt for the SFOS process). In our view, one may also expect a reduction of costs due to the easier distillation of alkyl levulinate and the alkyl alcohol (with an alkyl chain shorter than 4 carbon atoms) as compared to that of levulinic acid and water, respectively, in the SFOS process. To finally further highlight the interest on using Bi(OTf)3, similar experiments were conducted with In(OTf)3 which was recently reported by Graham (Figure 4).15 With a pKh of 4,28-31 hydration of In(OTf)3 is less likely to occur. In agreement with our collected results, the STY and yield of butyl levulinate obtained with In(OTf)3 were much lower than those obtained with Bi(OTf)3. With a FA concentration as low as 5 wt. %, the yield of butyl levulinate was even lower (83%) than the yield obtained with Bi(OTf)3 (92%) at a concentration of 30 wt. % (Figure 4). In addition, in line with the literature, the yield in butyl levulinate dropped rapidly down to 50 and 42% when the concentration of FA was increased to 10 and 20 wt. %, respectively, in the presence of In(OTf)3 (Figure 4b). These last results further demonstrate the much improved performances of the Bi(OTf)3 catalyst in the selective conversion of a concentrated feeds of FA to AL.


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(a) Bi(OTf)3

yield STY

100%

200

80%

150

60%

100

40%

50

20%

0

AL yield (%)

STY (kg.m-3.h-1)

250

0% 10

20

30

40


(b) In(OTf)3

STY (kg.m-3.h-1)

250 200

100%

yield STY

80%

150

60%

100

40%

50

20%

0

AL yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0% 5

10

30


Figure 4. Comparison of the (a) Bi(OTf)3/H2O and (b) In(OTf)3/H2O catalytic systems (1 mol% of Bi(OTf)3, 1 mol% of H2O, n-butanol at reflux, addition of FA over a period of 2h). From an environmental point of view, it is noteworthy that bismuth is consider as a safe element because it is non-toxic and non-carcinogenic, mainly due its low solubility in biological fluids.43-45 The LD50 (on rats) values of bismuth compounds indicates they are even less toxic than sodium chloride. Thus, Bi-derived chemicals are widely used for instance in medicine and cosmetic. Furthermore, because bismuth is a co-product of lead, copper and tin refining, it is


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considered as an inexpensive element.43 Its low price, together with its low toxicity, render bismuth as an attractive catalyst for the present application.

Conclusion Bismuth, aluminum, tin, and gallium triflates promote the conversion of concentrated feeds of FA to ALs. Under optimized conditions, butyl levulinate was obtained in up to 91% yield at a concentration of FA as high as 30 wt. %, which corresponds to an unprecedented STY of 182 kg/m3/h. This catalytic system is also compatible with other alkyl alcohols such as methanol, ethanol, or isopropanol. Importantly, a small amount of water in the alcoholic solution of FA seems to play a role for the activity and selectivity of Bi(OTf)3, presumably by changing the chemical composition of Bi(OTf)3. However, further investigations are still needed to clarify the reaction mechanism since water, even in a trace amount, may change also the reaction mechanism by solvation of transition states, by accelerating or inhibiting side reactions, etc. DFT calculations are now the topic of current investigations in our groups to validate these hypotheses. A comparison with the commercialized SFOS process shows that our novel catalytic route is compatible with industrial requirements in terms of yield, productivity, and capacity. This catalytic process leads also to a significantly improved E Factor (= 0.06). Switching from batch to continuous flow conditions would certainly improve again the efficiency of this process. Scale-up, economic study and life cycle assessment should be performed now to assess if this process has the advantage of being much simpler (easier product recovery, catalyst recyclability, recycling of n-butanol) and environmentally safer than the SFOS process.


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Experimental Section Chemicals:

n-butanol

(ACS

reagent

grade,

99.4%),

isopropanol

(Chromasolv),

trifluoromethanesulfonic acid (reagent grade, 98%), furfuryl alcohol (98%), 2,6-di-tertbutylpyridine (97%), bismuth trifluoromethanesulfonate, indium trifluoromethanesulfonate, aluminium trifluoromethanesulfonate, tin chloride pentahydrate, aluminium chloride hexahydrate and iron chloride hexahydrate were purchased from Sigma-Aldrich. Bismuth chloride (98%), gallium trifluoromethanesulfonate (98%) and scandium trifluoromethanesulfonate (98%) were purchased from Strem Chemicals. All chemicals have been used without any further purification, except commercial furfuryl alcohol which was purified by distillation before use. Full characterization 1H,

13

C and

19

F NMR are available on line on the site of suppliers. Otherwise

noted, anhydrous metal triflates were used

Optimized procedure for the catalytic production of butyl levulinate from FA: In a typical experiment, a 100 mL glass reactor containing 1.8 mg of water (0.10 mmol, 1 mol%), 66 mg of Bi(OTf)3 (0.10 mmol, 1mol%) and 5.77 g of butanol were stirred and heated under reflux (117°C) for 5 min. Then, 4 g of a solution of n-butanol containing furfuryl alcohol (10.2 to 40.8 mmol) was introduced into the reactor in one portion. A similar procedure was employed with all other tested metal triflates and chlorides. The reaction (conversion, selectivity and yield) was followed by gas chromatography on a Bruker Scion 456 GC apparatus using a HP5-MS column (30 m x 0.32 mm x 0.1 µm). The temperatures of the detector and the injector were 300°C and 250°C, respectively. The initial temperature of the oven was 50°C and was then increased to 150°C at a heating ramp of 10°C/min and then to 300°C at a heating ramp of 25°C/min.


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Important: for an accurate comparison with the SFOS process, the experimental procedure was slightly modified in order to be under similar conditions in both cases. In this context, the solution of FA in n-butanol (5 to 40% wt%, see Fig. 3) was added dropwise over a period of 2 hours (vs in one portion in the above described experiments).

Recyling experiments: At the end of the first catalytic cycle, n-butanol and butyl levulinate were separated from Bi(OTf)3 by distillation under vacuum (15 mmHg). n-Butanol was first distilled followed by butyl levulinate, leaving a black catalytic residue in the flask. Then, the vacuum was stopped and 5.77 g of butanol containing 2 wt% of water was added to the black catalytic residue and heated again at 117°C under atmospheric pressure before addition (in one portion) of 4 g of a solution of furfuryl alcohol (10 wt%) in n-butanol. Important note: after the 3rd catalytic cycle, the obtained black catalytic residue was re-dissolved in n-butanol and filtered over charcoal to remove tar-like material. Then, n-butanol was removed under vacuum and the “purified” catalytic residue was re-used as collected for further catalytic cycles.

ASSOCIATED CONTENT Supporting Information Available: http://pubs.acs.org A Selected kinetic profile, transposition to other alkyl alcohols and a tentative explanation for the role of the Bi-OH bond on the selectivity of the reaction.

Author

information:

*Corresponding

Author:

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+33-549-454-052;

E-mail:

[email protected]


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Author Contributions: The paper was written through contributions of all authors. All authors have given approval to the final version of the paper. A. Chappaz, J. Lai, K. De Oliveira Vigier, and F. Jérôme contributed equally to catalytic experiments. A. Liebens, M. Corbet, D. Morvan and R. Wischert contributed to rationalize the role of water, in particular, on the structure of metal triflates.

Notes: The authors declare no competing financial interest. Acknowledgement: The authors are grateful to the CNRS, the French Ministry of research and SOLVAY for their financial supports.

Abbreviations: OTf, Triflate; AL, Alkyl levulinate; STY, Space Time Yield; rAG, initial formation rate of alkyl levulinate; SFOS, Société Française d’Organo-Synthèse; FA, Furfuryl alcohol.

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Selective conversion of concentrated feeds of furfuryl alcohol to alkyl

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