Br-Catalyzed Oxidation of 5

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Optimization of Co/Mn/Br-catalyzed oxidation of 5-hydroxymethylfurfural to enhance 2,5-furandicarboxylic acid yield and minimize substrate burning Zuo Xiaobin, Padmesh Venkitasubramanian, Daryle H. Busch, and Bala Subramaniam ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00174 • Publication Date (Web): 18 May 2016 Downloaded from http://pubs.acs.org on May 19, 2016

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ACS Sustainable Chemistry & Engineering

Optimization of Co/Mn/Br-Catalyzed Oxidation of 5-Hydroxymethylfurfural to Enhance 2,5-Furandicarboxylic Acid Yield and Minimize Substrate Burning

Xiaobin Zuo,1 Padmesh Venkitasubramanian,2 Daryle H. Busch,1, 3 and Bala Subramaniam1, 4* 1

Center for Environmentally Beneficial Catalysis, University of Kansas, 1501 Wakarusa Drive, Lawrence, Kansas 66047, United States 2

3

Archer Daniels Midland (ADM) Company, Decatur, Illinois 62521, United States

Department of Chemistry, University of Kansas, 1251 Wescoe Hall Drive, Lawrence, Kansas 66045, United States 4

Department of Chemical and Petroleum Engineering, University of Kansas, 1530 W. 15th Street, Lawrence, Kansas 66045, United States

*Corresponding author: [email protected]; Voice: 785-864-2903; Fax: 785-864-6051

ABSTRACT 2,5-furandicarboxylic acid (FDCA) is a valuable non-phthalate biomass-based plastic precursor with the potential to replace terephthalic acid (TPA) in a variety of polymer applications. In this work, the Co/Mn/Br catalyzed semi-continuous oxidation of 5hydroxymethylfurfural (HMF) to FDCA has been carried out at temperatures lower than those of the traditional Mid-Century (MC) process. As HMF is more susceptible to side reactions (e.g. the over-oxidation to CO and CO2), lower temperatures compared to the MC process are typically used to prevent substrate burning. However, lower temperatures afford much decreased FDCA yield compared to that of TPA in p-xylene oxidation. Therefore, optimization of other operating

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variables such as catalyst composition, water concentration in the acetic acid solvent and pressure are essential to maximize FDCA yield. Using such optimization, we show that the FDCA yield can be enhanced to 90% at 1/0.015/0.5 molar ratio of Co, Mn and Br, 7% (v/v) water, 30 bar (CO2/O2 = 1/1, mol/mol) and 180 oC, the highest value reported for HMF oxidation using Co/Mn/Br catalyst. The use of Zr(IV) as co-catalyst facilitates FDCA formation, but only at lower temperatures (120-160 °C) where the FDCA yield is compromised. These findings broaden the scope of the application of the industrial MC catalytic process for FDCA production. KEYWORDS:

5-hydroxymethylfurfural,

2,5-furandicarboxylic

acid,

semi-continuous

oxidation, Co/Mn/Br catalyst, zirconium

INTRODUCTION During the past decade, there has been a growing interest in the development of technologies that transform biomass-derived feedstock to renewable chemical intermediates that displace petroleum-based chemicals. In this context, carbohydrates are considered as promising feedstock for the production of chemicals with commercial interest.1 One such example involves the conversion of cellulosic biomass-derived fructose or glucose into 5-hydroxymethylfurfural (HMF), a versatile platform chemical, and its subsequent oxidation to 2,5-furandicarboxylic acid (FDCA) which is targeted as a non-phthalate based substitute for terephthalic acid (TPA), used in making polyethylene terephthalate (PET) plastics.2-10 According to a recent report,11 polyethylene furanoate (PEF) plastic, made with ethylene glycol and FDCA, exhibits superior performance compared to PET in many aspects such as (a) improved gas (O2, CO2) and water barrier; (b) enhanced thermal stability; and (c) better mechanical properties. Compared to PET production, PEF production lowers carbon emissions by 70% and NREU (non-renewable energy

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use) by 65%. As a result, FDCA has been identified by DOE as one of the top twelve building blocks for the future green chemicals industry.12, 13 The oxidation of HMF to FDCA was originally carried out in presence of strong oxidants, such as nitric acid or KMnO4.2,14 Apart from environmental concerns, these systems produced only modest FDCA yield owing to substrate destruction under the harsh oxidizing conditions. Alternatively, oxidation with molecular oxygen, a much milder and cleaner oxidant, has been developed, employing noble metals such as platinum,15-20 gold21-27and Pd28-33 as active catalysts. During the past five years, these heterogeneous catalytic systems have been extensively studied and shown to provide nearly quantitative FDCA yield at relatively mild reaction temperatures (65-130 °C). However, because of its low solubility in the reaction medium, FDCA tends to precipitate out in the course of reaction, which might not only deactivate the catalysts by blocking the active sites but also causes separation problems. For this reason, sodium hydroxide is added in some cases to convert the diacid product into its sodium salt, which, after removal of the catalysts, must be treated with a strong acid for recovery of FDCA.18,23,24 More recently, several base-free processes have been reported by using hydrotalcite-supported gold nanoparticles,25 carbon nanotube-supported gold-palladium alloy nanoparticles34 , covalent triazine supported ruthenium35 and magnetic Fe3O4−CoOx36 as catalysts. Nevertheless, even in these cases, the substrate (HMF) concentration needs to be maintained very low to avoid FDCA precipitation. The potential for practical application of these noble metal-based catalytic systems is thus limited by the limited throughput as well as the high price of noble metal catalysts. The Mid-century (MC) oxidation of methylbenzenes to carboxylic acids represents a very important industrial process for the synthesis of polymer intermediates that can be fabricated into heavily used fibers, resins and films.37,38 For example, TPA is prepared in greater than 90% yield

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by liquid phase aerobic oxidation of p-xylene at ca. 200oC in presence of a catalyst composed of cobalt acetate, manganese acetate and hydrogen bromide. The oxidation of HMF with this homogeneous catalytic system has been less reported even though it has two distinct advantages over the noble metal catalytic systems. First, the Co/Mn/Br catalyst is much cheaper and can easily be recovered and recycled. Second, FDCA precipitates out of the reaction mixture at room temperature and can be readily separated from the catalyst via filtration. The similarity of these two reactions (HMF oxidation vs p-xylene oxidation) makes it possible for large scale production of FDCA in existing TPA plants, provided high yield of FDCA can be obtained. Furthermore, one of the most important parameters for polymer grade TPA is < 25 ppm 4carboxylbenzaldehyde (4-CBA), an intermediate oxidation product which causes chain termination during the subsequent polymerization.39 In comparison, the complete conversion of its counterpart 5-formyl-2-furancarboxylic acid (FFCA) to FDCA is much easier due to higher reactivity of FFCA than 4-CBA as well as higher solubility of FDCA in acetic acid compared to TPA,15,40 which alleviates the co-precipitation of the chain terminator with diacid under reaction conditions. The MC oxidation of HMF was first reported by Partenheimer et al. in 2001.41,42 The reaction was performed batch-wise and it was observed that the FDCA yield increased with temperature and catalyst concentration, but not with the addition of zirconium, a very effective co-catalyst in p-xylene oxidation.43-46 Following a three hour batch reaction at 125 °C and 70 bar, the maximum FDCA yield was achieved to be ca. 60% at optimized Co, Mn and Br concentrations. As revealed by Partenheimer et al., the furan ring is more prone to cleavage at high temperatures, leading to the higher yields of CO and CO2.41 Abu-Omar et al. demonstrated the beneficial effect of acid additives on HMF oxidation with Co/Zn/Br catalyst. A 60% FDCA yield was obtained by

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adding 1 wt% trifluoroacetic acid (TFA) at temperature as low as 90 °C. In comparison, 2,5diformylfuran (DFF) was found as the sole oxidation product without TFA.47 The MC oxidation of HMF is a free radical reaction. The synergy between Co and Mn facilitates the rapid generation of bromo radical (via the redox cascade, Scheme 1) which is Co2+ + peroxide (formed via the Haber-Weiss cycle) → Co3+ Co3+ + Mn2+ → Co2+ + Mn3+ Mn3+ + Br- → Mn2+ + Br. Scheme 1 Formation of bromo radical in Co/Mn/Br catalyzed oxidation subsequently involved in the hydrogen abstraction (of the –CH2OH and –CHO groups) of the substrate.37 Thus, the reaction proceeds very rapidly to completion after the initiation step. In this regard, the controlled continuous addition of a given amount of substrate compared to adding all the substrate initially during batch operation can circumvent the large temperature increase caused by the heat of reaction. This in turn mitigates safety concerns and is especially useful to protect compounds that are not stable at high temperatures. We recently studied the semicontinuous oxidation of p-xylene in CO2-based media in the temperature range 120-170 °C. The reaction proceeds very smoothly with up to 97% TPA yield by adding the substrate continuously.45 The use of higher cobalt concentration and lower temperature (compared to industrial process) reduces solvent and substrate burning (to CO and CO2) with no loss of catalytic activity.48 As a superior flame inhibitor to nitrogen, the use of CO2 makes it possible to run the reactions safely with a high percentage of oxygen in the gas phase.49 In this manuscript, we apply these principles (controlled substrate addition and the use of CO2 as diluent) to reduce substrate burning and enhance FDCA yield in Co/Mn/Br catalyst system. We first benchmarked HMF oxidation at 160 oC and compared the results with that of p-xylene oxidation. Then, we

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systematically optimized the reaction system focusing on the effects of catalyst composition, water concentration in the solvent acetic acid, reaction temperature, pressure, and the use of zirconium as co-catalyst. We demonstrate an optimized FDCA yield of ca. 90% (that is comparable to TPA yield in the MC process) by minimizing substrate burning and other side reactions.

EXPERIMENTAL Materials. 5-hydroxymethylfurfural (HMF) used in all the reactions were purchased from Sigma-Aldrich with 99% purity. All the other chemicals (acetic acid, cobalt acetate tetrahydrate, manganese acetate tetrahydrate, hydrobromic acid, zirconium acetate etc) were commercially available and used without further treatment. Industrial grade (≥ 99.9% purity, < 32 ppm H2O, < 20 ppm THC) liquid CO2 and ultra-high purity grade oxygen were purchased from Linweld. Oxidation Experiments. The semi-continuous (or equivalently, semi-batch) oxidation of HMF to 2,5-furandicarboxylic acid (FDCA) was carried out in the 50 mL, stirred, titanium Parr reactor (Figure 1). Typically, either N2 or CO2 was first added to the reactor (to a certain predetermined pressure) containing roughly 30 mL acetic acid solution in which known concentrations of the catalytic components (cobalt acetate, manganese acetate and hydrobromic acid) were dissolved.

The reactor contents were then heated to the reaction temperature

following which O2 was added until the selected final pressure was reached. The partial pressures of O2 and the diluent (N2 or CO2) were thus known. A solution of HMF in acetic acid was subsequently pumped into the reactor at a pre-defined rate to initiate the reaction. The total reactor pressure was maintained constant by continuously supplying fresh O2 from a 40-mL stainless steel reservoir to compensate for the oxygen consumed in the reaction. The pressure

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decrease observed in the external oxygen reservoir was used to monitor the progress of the reaction. Product Analysis. Following the reaction (i.e., after a known amount of the HMF solution was pumped into the reactor and the O2 consumption levels off), the reaction mixture was cooled to room temperature and then the reactor contents were analyzed as follows: The gas phase was sampled and analyzed by gas chromatography (GC) (Shin Carbon ST 100/120 mesh) to determine the yields of CO and CO2 produced by solvent and substrate burning. The insoluble FDCA product was separated from the liquid mixture by filtration and the solid was washed with acetic acid to remove most of the soluble impurities. The resulting white solid was dried in an oven at 100 °C for 2 h to remove absorbed solvent. The reactor was washed with acetic acid and methanol to recover any residual FDCA solid. This extract along with the filtrate that was retained after isolation of the FDCA solid were analyzed by HPLC (C18 ODS-2 column) to determine the composition of the liquid phases. The overall yields of the oxidation products were estimated based on the compositions of the solid and liquid phases. All percentages are expressed as mole percent unless otherwise specified. Safety. The amount of substrate used in the reaction studies was such that the maximum adiabatic temperature rise for total combustion of the substrate (taking into account the heat capacities of the reaction mixture and the solid reactor) was ca. 20 °C. The actual temperature rise observed during most of the reactions was less than 5 ◦C. In addition, the reactor vessel was equipped with a safety release valve that safely exhausts the reactor contents to the building vent via a rupture disk in the event the set safe pressure (200 bar) is exceeded.

RESULTS

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Benchmark of the Semi-Continuous Oxidation at 160 °C. To begin, the semi-continuous oxidation of HMF was benchmarked at 160 °C with N2 as the inert gas. HMF exists as solid at room temperature. It was dissolved in acetic acid and pumped at a fixed rate into the reactor that was preloaded with the catalyst solution. Despite total HMF conversion, the yield of FDCA is only 66% (Figure 2a), much lower than that (ca. 95%, Figure 2a) of TPA during p-xylene oxidation under identical conditions. In addition to 0.4% 5-formylfurancarboxylic acid (FFCA) as intermediate, the reaction produces other byproducts (> 30%) that are not identifiable in the HPLC analysis. On the other hand, solvent and substrate burning, expressed as the ratio of the moles of COx (CO + CO2) produced to the moles of substrate added,50 is more than three times greater than that of p-xylene oxidation (Figure 2b). It is reasonable to assume that a large portion of the unaccounted byproducts is due to over-oxidation of either HMF and/or its intermediate oxidation products to CO and CO2. Effect of Catalyst Composition. The catalytic performance is improved at higher reaction temperature and cobalt concentration. As shown in Table 1, the FDCA yield is increased to ca. 80% by increasing the temperature from 160 oC to 180 oC and doubling the amount of cobalt from 1.1 to 2.2 mmol (entries 1-4). The use of CO2 as the inert gas has a beneficial effect by further elevating the yield to 83.3% (entry 6) at the higher cobalt concentration. It is possible that the interaction of CO2 with O2 leads to the formation of a catalytically active peroxocarbonate species CO42- which in turn may accelerate the key free radical propagation step via hydrogen abstraction, as reported in the p-xylene oxidation.51,52 As a minor component of the MC catalyst, the Mn concentration is maintained at a low level to avoid its precipitation (as MnO2) that might contaminate the solid TPA product.53 Our previous work showed that manganese is very effective in reducing solvent burning.45 The yield

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of CO is decreased by ca. 25% with the addition of a relatively small amount of Mn (Mn/Co = 0.015, mol/mol) that exerts no effect on the TPA yield (entries 1 and 2 in Table 2). Remarkably, Mn plays a more prominent role during HMF oxidation. As shown in Table 2, the reaction without Mn exhibits a long induction period (~ 36 minutes) at 170 °C. The FDCA yield is only 62.4% in the mixture of CO2 and O2 (entry 3). The induction period is shortened to 23 min when the temperature is increased to 180 °C. However, the FDCA yield is still less than 70% (entry 5). In comparison, the reactions in presence of manganese proceed much faster with very short induction time. The FDCA yields exceed 80% at both 170 °C and 180 °C (entries 4 and 6). Furthermore, the addition of the same amount of Mn as in p-xylene oxidation decreases CO formation by ca. 20% at 170 °C (entry 4). In contrast, the corresponding decrease is much less when the temperature is 10 °C higher (entry 6). The reactions were also investigated at higher Mn concentrations. As shown in Figure 3, the FDCA yield is affected little by further increasing the Mn/Co mole ratio to 0.12, although the CO yield decreases. Effect of Water Concentration. As a co-solvent to acetic acid, up to 38% (by volume) water was added during HMF oxidation to investigate its effect on the main reaction as well as side reactions. Although water does not affect HMF conversion (> 99%) for all the reactions studied, it has a large influence on the yields of FDCA and various by-products. As shown in Figure 4a, the FDCA yield is >75% at low water concentrations and reaches a maximum (ca. 83%) with 10% (v/v) water. Then it decreases monotonically with further increase in water content. This inhibition of FDCA yield at high water concentrations is accompanied by a significant increase in the yield of the intermediate product, FFCA. Water also has a marked inhibition effect on CO and CO2 yields, especially when its concentration is > 7% (v/v) (Figure

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4b). In addition, the yield of 5-acetoxymethylfurfural (AcHMF), formed by the reaction between HMF and acetic acid, exhibits a maximum of ca. 3% over a wide range (15-25%, v/v) of water concentrations (Figure 4a). Water has been reported to inhibit TPA yield in the MC oxidation of p-xylene.37 In contrast, our findings show that HMF oxidation benefits from the presence of ca. 10% (v/v) water in the feed mixture, which maximized the FDCA yield by reducing substrate burning, esterification, and possibly other side reactions. Effect of Reaction Temperature. According to Partenheimer’s work,41 the intermediate DFF was the predominant product when the reaction temperature was below 100 °C. To facilitate its conversion to FDCA, HMF oxidation was investigated between 120-200 °C. With the exception of 160 °C, FDCA production is favored at higher temperatures. Up to 80% yield is achieved in the 180-190 °C range (Figure 5a). The reaction at 120 °C also gives 3.7% yield of FFCA, which is totally converted to FDCA when the temperature is > 160 °C. The yield of CO (Figure 5b) increases at a much higher rate than the yield of FDCA, especially from 160 °C to 200 °C. This indicates that a substantial amount of acetic acid is decomposed into gaseous products at higher temperatures. The unusual behavior at 160 °C is also observed in the runs using N2 as the inert gas, where the yield of FDCA at this temperature (66.0%, entry 1, Table 1) is less than those obtained at either 180 °C (73.0%, entry 3, Table 1) or 140 °C (69.8%) under otherwise identical conditions. The FDCA formation is accompanied by many competitive side reactions, appears to be least favored at this temperature. This aspect is further discussed in the discussion section. Effect of Reactor Pressure. In our previous work, the reactor pressure was maintained at 60 bar with an equimolar mixture of CO2 and O2 to ensure a large excess of oxygen with respect to

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p-xylene.45 Given that FDCA formation from HMF consumes only half the stoichiometric amount of oxygen compared to p-xylene oxidation to TPA, the HMF oxidation reaction was carried out at 30-60 bar total pressure (with equimolar CO2 and O2) to investigate the influence of oxygen availability in the liquid phase. As shown in Figure 6, the FDCA yield is inversely related to reactor pressure, increasing from 82.5% to 89.6% when the pressure is decreased from 50 bar to 30 bar. In contrast, the CO yield increases with pressure and levels off at 50 bar. Clearly, excess oxygen in liquid phase promotes substrate burning and lowers the yield of FDCA. Effect of Zirconium as Co-catalyst. Zirconium has been demonstrated as an effective cocatalyst in the MC oxidation of p-xylene.43-46 The introduction of a soluble zirconium salt, e.g. ZrO(OAc)2, is capable of lowering the temperature to ca. 100 °C, which significantly reduces solvent and substrate burning. In a similar manner, zirconium acts as a good activator in HMF oxidation between 120 and 160 °C. The absolute FDCA yield is increased by 5-15% (Figure 7a) with the addition of ZrO(OAc)2 whose molar amount is approximately 1/6 that of cobalt, the optimized Zr/Co ratio reported in p-xylene oxidation.45 This positive effect of zirconium salt was not reported in Partenheimer’s work possibly due to the relatively small amount of zirconium added (Zr/Co < 1/10).41 Unfortunately, zirconium addition does not work well at 180 °C, the optimal reaction temperature. The FDCA yield is decreased from 77.9% to 68.2% (Figure 7a). The decomposition of solvent and substrate becomes more pronounced, as inferred from the surge in CO yield when the temperature rises above 160 °C (Figure 7b). Oxidation of AcHMF. The esterification of HMF with acetic acid has been observed under reaction conditions (vide supra, water concentration effect). In a blank experiment, a certain

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amount of HMF was treated with the mixture of acetic acid, water and catalyst in an inert atmosphere, 60 bar CO2, to avoid substrate oxidation. The 0.5 h reaction at 180 °C affords 61.5% HMF conversion and 54.6% AcHMF yield, respectively. Interestingly, the HMF ester undergoes hydrolysis under the same conditions to yield HMF, however, with much less efficiency. Starting from AcHMF, the reaction gives 23.8% conversion of the ester and 15.8% HMF yield. Table 3 shows the comparison of the oxidation behavior between HMF and AcHMF. The reaction of AcHMF affords lower FDCA yield but much higher CO yield with or without the use of water as co-solvent. The addition of 7% (v/v) water decreases CO formation by ca. 50% during oxidation of these two substrates. One possible oxidation route involves the hydrolysis of AcHMF to yield the intermediate HMF, which is subsequently oxidized into FDCA. If this is the case, the reaction without water should produce much less FDCA. However, the FDCA yield is a little lower with 7% (v/v) water. This suggests that AcHMF oxidation proceeds via a different pathway. Stability of FDCA. The thermal stability of FDCA was investigated under reaction conditions to understand the reason for the lower FDCA yield compared to that of TPA in pxylene oxidation. In a preliminary experiment, 52.3 mg FDCA (Alfa Aesar, 98%) was treated with Co/Mn/Br (1/0.015/0.5, high cobalt concentration as above) at a high temperature of 180 °C and high pressure of 60 bar (molar N2/O2 = 1/1). According to HPLC analysis, 51.2 mg FDCA was recovered after the reaction, corresponding to a 99.9% [51.2/(52.3*0.98)] product recovery. The stability of FDCA was further studied in the presence of HMF, because the free radicals generated by substrate oxidation might attack the product and cause it to decompose, as has been observed in solvent decomposition.54 Thus, two reactions were carried out, based on which the

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recovery of FDCA can be estimated. The first experiment involves the oxidation of 352 mg HMF at 180 oC and 60 bar and yielded 324 mg FDCA. In the second experiment, 352 mg HMF and 100 mg FDCA were subject to oxidation under identical conditions. The 422 mg yield of product suggests that 98 mg (422 mg − 324 mg) FDCA was recovered. Therefore, FDCA is highly stable under reaction conditions with or without HMF oxidation, affording a ≥98% recovery when treated at 180 oC, 60 bar (O2 + N2) and high cobalt catalyst concentration.

DISCUSSION In the MC catalytic system, FDCA is formed via the oxidation of hydroxyl group (of HMF) to aldehyde group followed by the stepwise oxidation of two aldehyde groups. In addition, a variety of side reactions is possible depending on the reaction conditions (Figure 8). First, the conversion of HMF into AcHMF takes place in acetic acid solvent. Second, compared with pxylene oxidation, much higher amounts of CO and CO2 are produced due to the enhanced ring attack as a result of the reduced resonance energy (17 kcal/mol for furan compared to 36 kcal/mol for benzene ring).41 Third, water addition to the two C=C double bonds occurs in acidic medium to give levulinic acid, formic acid and polymeric substances.14 Besides, trace amounts of succinic acid, maleic acid and fumaric acid were identified as other possible by-products associated with furan ring cleavage.41 Finally, self-polymerization of HMF via hydroxyl and/or aldehyde groups leads to the formation of dimers,55 oligomers56 and even polymers known as humins57,58 whose identification and quantification still remain a major challenge. It is noteworthy that FDCA is quite resistant to decomposition at high temperatures and pressures (vi de supra).

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Clearly, the aforementioned side reactions must be minimized to maximize FDCA yield. One way to achieve this is to add a given amount of HMF into the reactor at a controlled rate to reduce temperature overshoot and thereby minimize substrate burning. In a typical semi-batch operation, 5.0 mL of acetic acid solution containing HMF was added at 0.25 mL/min to the reactor containing ca. 30 mL catalyst solution, followed by an additional 10 min of stirring at the reaction temperature. A 40 mL stainless steel reservoir was used to feed fresh O2 continuously to the reactor to replenish the oxygen consumed during the reaction and maintain the reactor pressure. The pressure decrease in the external O2 reservoir is continuously monitored in time to track the overall HMF oxidation rate. Temporal temperature and pressure profiles shown in Figures 9 and 10 provide valuable insights into the reaction behavior and possible pathways. All the runs exhibit at least one temperature spike resulting from the initiation of the highly exothermic oxidation reaction. In general, higher FDCA yields are related to relatively lower temperature increase and steadier O2 consumption from the external reservoir. This is attributed to the rapid conversion of HMF as it is being added to the reactor. The temperature and pressure profiles at different initial reaction temperatures are described in detail as follows. As shown in Figure 9, the reaction at 160 °C is initiated following an approximately 3 min induction period, characterized by a nearly 3 °C increase and relatively rapid reservoir pressure decrease of ca. 1 bar (Figure 9a). However, the reservoir pressure decreases more gradually after a further 1 minute. This is attributed to the relatively slow FDCA formation reaction compared to the combustion reaction which leads to the accumulation of gaseous products CO and CO2 in the gas phase that hinders the replenishment of oxygen from the O2 reservoir. Following 5 min of substrate addition, a second initiation is recorded with similar temperature/pressure pattern,

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which occurs for three more times until oxygen consumption is complete at approximately 22 min. It appears that the oxidations occur following the attainment of certain critical substrate and intermediate concentrations. In other words, the added HMF undergoes oxidation to FDCA via a series of intermediate products. During periods of nearly constant reservoir pressure, by-products that do not consume oxygen might be produced. The induction time is shortened to approximately. 1 min when the temperature is increased to 180 °C. Further, as shown in Figure 9b, a series of reaction initiations (as denoted by the temperature spikes) occur in fairly rapid succession as the HMF is continually added suggesting that the concentrations required for the initiation steps are lower than those at 160 °C. Therefore, the reservoir pressure decreases more uniformly without the intermittent periods of no O2 consumption that were observed at 160 °C. The decrease in the successive induction periods (both at the beginning and following the first initiation) results in the FDCA yield increasing from 67.0% (160 oC) to 77.9% (180 oC). When the Co concentration is doubled at 180 °C, the reaction has almost no induction time and exhibits a fairly smooth decrease in O2 reservoir pressure with no oscillatory temperature fluctuations (Figure 9c). This indicates rapid FDCA formation upon HMF addition. As a result, side reactions are effectively curtailed and the FDCA yield is increased to 83.3%. The use of zirconium as co-catalyst at 160 oC results in similar temperature and reservoir pressure profiles (Figure 9d) with 77.3% FDCA yield. The manganese content provides another way to maximize FDCA yield via the minimization of side reactions. The 170 oC reaction without Mn is very slow with no oxygen consumption during the first 20 min when the substrate is added (Figure 10a). Reaction ‘ignition’ occurs at approximately 35 min as seen from the large temperature spike of ca. 25 oC and concomitant

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decrease (ca. 7 bar) of reservoir O2 pressure in less than 1 min when the majority of oxidation occurs. Although HMF is added continuously to the reactor, the delayed ignition essentially makes it a batch reactor operation. The resulting FDCA yield of 62.4% yield (entry 3 of Table 2) is close to the maximum yield reported in the literature during batch reactor studies.41 Even though no oxygen is consumed during the 35 min induction period, side reactions such as esterification dominates at these conditions to yield AcHMF, which is subsequently oxidized to FDCA (Table 3), albeit in lower yield. Further, the substantial temperature rise results in partial combustion of substrate and solvent into gaseous products, e.g. CO. In sharp contrast, when a small amount of manganese is added, the reaction is initiated after ca. 2 min, followed by uniform oxygen consumption in the course of substrate addition (Figure 10b). The maximum temperature rise is less than 2 oC, much lower than that observed without manganese addition. Consequently, esterification and over-oxidation of HMF are considerably reduced resulting in an enhanced FDCA yield of 81.4% (entry 4 of Table 2). Temperature control is vital to maximize the FDCA yield. The MC process exploits evaporative cooling to control the reactor temperature. Specifically, the reactor operating pressure is chosen such that the acetic acid boils when the reaction temperature is exceeded.39 The latent heat of evaporation is derived from the reaction mixture thereby maintaining the reactor temperature nearly constant. Such ability to control the maximum temperature rise is essential to reduce side reactions, especially burning of HMF that adversely affects selectivity. In general, the boiling point of the solvent increases while its heat of evaporation decreases with an increase in pressure. For better utilization of evaporative cooling, the total pressure should be maintained such that the reaction mixture boils within a predetermined temperature rise upon reaction. As shown in Figure 6, higher than needed pressures are detrimental to FDCA yield, as

16


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inferred from enhanced yields (from ca. 83% to 90%) when the reactor pressure is reduced from 60 bar to 30 bar.

CONCLUSIONS In summary, we have investigated the semi-continuous oxidation of HMF and compared with the results of the well-studied p-xylene oxidation. At 160 °C and 60 bar, HMF oxidation produces approximately 65% FDCA yield, which is considerably lower than that the TPA yield (95% yield) from p-xylene oxidation under identical substrate and catalyst (Co/Mn/Br) concentrations. This is attributed to the higher reactivity of the furan ring (vs benzene ring) and substituted functional groups (–CH2OH and –CHO vs –CH3 group). We have optimized the operating conditions to maximize the FDCA yield by minimizing side reactions such as HMF esterification to AcHMF and its over-oxidation to carbon oxides. The highest FDCA yield, ca. 90%, is obtained under the following conditions: Co/Mn/Br = 1/0.015/0.5, T = 180 °C, P = 30 bar (molar CO2/O2 = 1) and 7% (v/v) water in the reaction mixture. Our ongoing work is focused on obtaining intrinsic kinetic parameters at the optimum conditions, aims at rational process optimization and scale-up.

AUTHOR INFORMATION Corresponding Author * Phone: 785-864-2903. Fax: 785-864-6051. E-mail: [email protected].

ACKNOWLEDGEMENT

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Page 18 of 46

This work was supported by Archer Daniels Midland (ADM) Company, Kansas Bioscience Authority and the United States Department of Agriculture (USDA/NIFA Award 2011-1000630362).

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Table 1. Co/Mn/Br catalyzed oxidation of HMF Co2+

Mn2+

Br-

Inert

T

XHMF

YFDCA a

YFFCA a

YDFF a

CO/HMF

CO2/HMF

mmol

mmol

mmol

gas

(oC)

(%)

(%)

(%)

(%)

(mol/mol)

(mol/mol)

1

1.1

0.033

1.1

N2

160

> 99

66.0

0.4

0

0.106

0.363

2

2.2

0.033

1.1

N2

160

> 99

78.1

0

0

0.111

0.440

3

1.1

0.033

1.1

N2

180

> 99

73.0

0

0

0.174

0.455

4

2.2

0.033

1.1

N2

180

> 99

78.5

0

0

0.189

0.519

5

1.1

0.033

1.1

CO2

180

> 99

77.9

0

0

0.200

------ b

6

2.2

0.033

1.1

CO2

180

> 99

83.3

0

0

0.267

------ b

Entry

Reaction conditions: N2 (CO2)/O2 = 1/1 (mol/mol); P = 60 bar; H2O/HOAC = 7/93 (v/v); 5.0 mL HOAc solution of HMF (13.2 mmol) added at 0.25 mL/min; VT = 35 mL (after the addition of substrate solution); t = 30 min; n = 1200 rpm; a YFDCA: Overall yield of 2,5-furandicarboxylic acid, YFFCA: Overall yield of 5-formylfurancarboxylic acid, YDFF: Overall yield of 2,5diformylfuran; b Reliable analysis not possible when CO2 is used as the inert gas.

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Table 2. HMF oxidation vs p-xylene oxidation: effect of manganese Entry

Substrate

Mn2+

T

Induction

Conversion

Ydiacid a

CO/substrate

(mmol)

(oC)

time

(%)

(%)

(mol/mol)

(min) 1

p-xylene

0

170

99

96.9

0.116

2

p-xylene

0.033

170

99

96.6

0.088

3

HMF

0

170

36

> 99

62.4

0.214

4

HMF

0.033

170

2

> 99

81.4

0.176

5

HMF

0

180

23

> 99

69.6

0.275

6

HMF

0.033

180

99

83.3

0.267

Reaction conditions: Co2+ = 2.2 mmol; Br- = 1.1 mmol; CO2/O2 = 1/1 (mol/mol); P = 60 bar; H2O/HOAC = 7/93 (v/v); 1.6 mL (13.2 mmol) p-xylene added at 0.08 mL/min, 5.0 mL HOAc solution of HMF (13.2 mmol) added at 0.25 mL/min; VT = 35 mL (after the addition of substrate solution); t = 30 min (200 min for entry 3 and 40 min for entry 5); n = 1200 rpm; a Ydiacid: Overall yield of terephthalic acid (p-xylene as substrate) and 2,5-furandicarboxylic acid (HMF as substrate).

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Table 3. HMF oxidation vs AcHMF oxidation Entry

Substrate

H2O/HOAc

Conversion

Y FDCA a

CO/substrate

(v/v)

(%)

(%)

(mol/mol)

1

HMF

7/93

> 99

78.8

0.41

2

HMF

0/100

> 99

75.9

0.97

3

AcHMF

7/93

98.1

67.6

0.63

4

AcHMF

0/100

> 99

70.9

1.19

Reaction conditions: Co2+ = 2.2 mmol; Co/Mn/Br = 1/0.015/0.5 (mol/mol/mol); CO2/O2 = 1/1 (mol/mol); P = 60 bar; T = 180 oC; 5.0 mL HOAc solution of substrate (6.6 mmol) added at 0.25 mL/min; VT = 35 mL (after the addition of substrate solution); t = 30 min; n = 1200 rpm;

a

YFDCA: Overall yield of 2,5-furandicarboxylic acid; yields of 5-formylfurancarboxylic acid (FFCA) and 2,5-diformylfuran (DFF) almost 0 for all the reactions.

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List of figure captions: Figure 1 Schematic diagram of Parr reactor system Figure 2 HMF oxidation vs p-xylene oxidation: product distribution. Reaction conditions: Co2+ = 1.1 mmol; Co/Mn/Br = 1/0.03/1 (mol/mol/mol); N2/O2 = 1/1 (mol/mol); P = 60 bar; T = 160 oC; H2O/HOAC = 7/93 (v/v); 1.6 mL (13.2 mmol) p-xylene added at 0.08 mL/min, 5.0 mL HOAc solution of HMF (13.2 mmol) added at 0.25 mL/min; VT = 35 mL (after the addition of substrate solution); t = 30 min; n = 1200 rpm; Ydiacid: Overall yield of terephthalic acid (p-xylene as substrate) and 2,5-furandicarboxylic acid (HMF as substrate). Figure 3 Effect of Mn2+/Co2+ molar ratio on the oxidation of HMF. Reaction conditions: Co2+ = 2.2 mmol; Br- = 1.1mmol; CO2/O2 = 1/1 (mol/mol); P = 60 bar; T = 170 oC; H2O/HOAC = 7/93 (v/v); 5.0 mL HOAc solution of HMF (13.2 mmol) added at 0.25 mL/min; VT = 35 mL (after the addition of substrate solution); t = 30 min; n = 1200 rpm. Figure 4 Effect of water concentration on the oxidation of HMF. Reaction conditions: Co2+ = 2.2 mmol; Co/Mn/Br = 1/0.015/0.5 (mol/mol/mol); N2/O2 = 1/1 (mol/mol); P = 60 bar; T = 180 oC; 5.0 mL HOAc solution of HMF (13.2 mmol) added at 0.25 mL/min; VT = 35 mL (after the addition of substrate solution); t = 30 min (40 min for the reaction with 38% H2O); n = 1200 rpm. Figure 5 Effect of reaction temperature on the oxidation of HMF. Reaction conditions: Co2+ = 1.1 mmol; Co/Mn/Br = 1/0.03/1 (mol/mol/mol); CO2/O2 = 1/1 (mol/mol); P = 60 bar; H2O/HOAC = 7/93 (v/v); 5.0 mL HOAc solution of HMF (13.2 mmol) added at 0.25

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Page 30 of 46

mL/min; VT = 35 mL (after the addition of substrate solution); t = 30 min; n = 1200 rpm. Figure 6 Effect of reactor pressure on the oxidation of HMF. Reaction conditions: Co2+ = 2.2 mmol; Co/Mn/Br = 1/0.015/0.5 (mol/mol/mol); CO2/O2 = 1/1 (mol/mol); T = 180 oC; H2O/HOAC = 7/93 (v/v); 5.0 mL HOAc solution of HMF (13.2 mmol) added at 0.25 mL/min; VT = 35 mL (after the addition of substrate solution); t = 30 min; n = 1200 rpm. Figure 7 Effect of zirconium on the oxidation of HMF. Reaction conditions: Co2+ = 1.1 mmol; Co/Mn/Br = 1/0.03/1 (mol/mol/mol); Zr4+ = 0.20 mmol; CO2/O2 = 1/1 (mol/mol); P = 60 bar; H2O/HOAC = 7/93 (v/v); 5.0 mL HOAc solution of HMF (13.2 mmol) added at 0.25 mL/min; VT = 35 mL (after the addition of substrate solution); t = 30 min; n = 1200 rpm. Figure 8 Possible reactions in the oxidation of HMF to FDCA Figure 9 Reaction temperature and oxygen reservoir pressure profiles. (a) reaction at 160 oC (Figure 5); (b) reaction at 180 oC (entry 5, Table 1); (c) reaction at 180 oC with doubled cobalt concentration (entry 6, Table 1); (d) reaction at 160 oC with zirconium (Figure 7) Figure 10 Reaction temperature and oxygen reservoir pressure profiles. (a) reaction without manganese (entry 3, Table 2); (b) reaction with manganese (entry 4, Table 2).

30


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Figure 1. Schematic diagram of Parr reactor system

31



100

(a)

90 80

p-xylene oxidation HMF oxidation

70

(mol%)

60 50 40 30 20 10 0 Conversion

Yield of Diacid

Yield of Substrate intermediates unaccounted

0.4

(b) 0.35 0.3

p-xylene oxidation HMF oxidation

0.25 (mol/mol)

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

Page 32 of 46

0.2 0.15 0.1 0.05 0 CO/substrate

CO2/substrate

Figure 2. HMF oxidation vs p-xylene oxidation: product distribution. Reaction conditions: Co2+ = 1.1 mmol; Co/Mn/Br = 1/0.03/1 (mol/mol/mol); N2/O2 = 1/1 (mol/mol); P = 60 bar; T = 160 o

C; H2O/HOAC = 7/93 (v/v); 1.6 mL (13.2 mmol) p-xylene added at 0.08 mL/min, 5.0 mL 32


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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


HOAc solution of HMF (13.2 mmol) added at 0.25 mL/min; VT = 35 mL (after the addition of substrate solution); t = 30 min; n = 1200 rpm; Ydiacid: Overall yield of terephthalic acid (p-xylene as substrate) and 2,5-furandicarboxylic acid (HMF as substrate).

33



85

0.22

80

0.2

75 0.18 70 0.16 65 0.14 60 Yield of FDCA CO/HMF

55

0.12

50 0

0.02

0.04

CO/HMF (mol/mol)

Yield of FDCA (mol%)

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

Page 34 of 46

0.06

0.08

0.1

0.1 0.12

Mn2+/Co2+ (mol/mol) 2+

2+

Figure 3. Effect of Mn /Co molar ratio on the oxidation of HMF. Reaction conditions: Co2+ = 2.2 mmol; Br- = 1.1mmol; CO2/O2 = 1/1 (mol/mol); P = 60 bar; T = 170 oC; H2O/HOAC = 7/93 (v/v); 5.0 mL HOAc solution of HMF (13.2 mmol) added at 0.25 mL/min; VT = 35 mL (after the addition of substrate solution); t = 30 min; n = 1200 rpm.

34


Page 35 of 46

85

12

(a) 10

75 8 70

Yield of FDCA Yield of FFCA Yield of AcHMF

65

6

4 60

Yield of FFCA or AcHMF (mol%)


80

2

55

50

0 0

5

10

15

20

25

30

35

40

Water concentration (v%) 0.9

(b) 0.8 CO/HMF or CO2/HMF (mol/mol)

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


0.7 0.6 0.5 0.4 CO/HMF CO2/HMF

0.3 0.2 0.1 0 0

5

10

15 20 25 Water concentration (v%)

30

35

40

Figure 4. Effect of water concentration on the oxidation of HMF. Reaction conditions: Co2+ = 2.2 mmol; Co/Mn/Br = 1/0.015/0.5 (mol/mol/mol); N2/O2 = 1/1 (mol/mol); P = 60 bar; T = 180

35



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

o

C; 5.0 mL HOAc solution of HMF (13.2 mmol) added at 0.25 mL/min; VT = 35 mL (after the

addition of substrate solution); t = 30 min (40 min for the reaction with 38% H2O); n = 1200 rpm.

36


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Page 37 of 46

85

4

(a) 3.5

80

75 2.5 70 2 65

Yield of FDCA Yield of FFCA

1.5

60

Yield of FFCA (mol%)


3

1 55

0.5

50

0 120

130

140

150

160

170

180

190

200

Reaction temperature (oC) 0.4

(b) 0.35 0.3 CO/HMF (mol/mol)

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


0.25 0.2 0.15

CO/HMF

0.1 0.05 0 120

130

140

150

160

170

Reaction temperature

180

190

200

(oC)

Figure 5. Effect of reaction temperature on the oxidation of HMF. Reaction conditions: Co2+ = 1.1 mmol; Co/Mn/Br = 1/0.03/1 (mol/mol/mol); CO2/O2 = 1/1 (mol/mol); P = 60 bar; 37



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

H2O/HOAC = 7/93 (v/v); 5.0 mL HOAc solution of HMF (13.2 mmol) added at 0.25 mL/min; VT = 35 mL (after the addition of substrate solution); t = 30 min; n = 1200 rpm.

38


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Page 39 of 46

92

0.28 0.27

90

88 0.25 Yield of FDCA CO/HMF

86

0.24 0.23

84

CO/HMF (mol/mol)

0.26 Yield of FDCA (mol%)

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


0.22 82 0.21 80

0.2 30

35

40

45

50

55

60

Reactor pressure (bar)

Figure 6. Effect of reactor pressure on the oxidation of HMF. Reaction conditions: Co2+ = 2.2 mmol; Co/Mn/Br = 1/0.015/0.5 (mol/mol/mol); CO2/O2 = 1/1 (mol/mol); T = 180 oC; H2O/HOAC = 7/93 (v/v); 5.0 mL HOAc solution of HMF (13.2 mmol) added at 0.25 mL/min; VT = 35 mL (after the addition of substrate solution); t = 30 min; n = 1200 rpm.

39



90

(a) 85


80 75 70 65 without zirconium with zirconium

60 55 50 120

130

140

150

160

170

180

Reaction temperature (oC) 0.45

(b) 0.4 0.35 CO/HMF (mol/mol)

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

Page 40 of 46

0.3 0.25 0.2 0.15

without zirconium with zirconium

0.1 0.05 0 120

130

140

150

160

170

180

Reaction temperature (oC)

Figure 7. Effect of zirconium on the oxidation of HMF. Reaction conditions: Co2+ = 1.1 mmol; Co/Mn/Br = 1/0.03/1 (mol/mol/mol); Zr4+ = 0.20 mmol; CO2/O2 = 1/1 (mol/mol); P = 60 bar; 40


Page 41 of 46

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


H2O/HOAC = 7/93 (v/v); 5.0 mL HOAc solution of HMF (13.2 mmol) added at 0.25 mL/min; VT = 35 mL (after the addition of substrate solution); t = 30 min; n = 1200 rpm.

41


riz a lym e

[O ]

se lf-p o

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

tio n


Figure 8. Possible reactions in the oxidation of HMF to FDCA

42


Page 42 of 46

Page 43 of 46

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


(a)

(b)

(d)

(c)

Figure 9. Reaction temperature and oxygen reservoir pressure profiles. (a) reaction at 160 oC (Figure 5); (b) reaction at 180 oC (entry 5, Table 1); (c) reaction at 180 oC with doubled cobalt concentration (entry 6, Table 1); (d) reaction at 160 oC with zirconium (Figure 7).

43



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

(a)

Page 44 of 46

(b)

Figure 10. Reaction temperature and oxygen reservoir pressure profiles. (a) reaction without manganese (entry 3, Table 2); (b) reaction with manganese (entry 4, Table 2).

44


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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


For Table of Contents Use Only

Optimization of Co/Mn/Br-Catalyzed Oxidation of 5-Hydroxymethylfurfural to Enhance 2,5-Furandicarboxylic Acid Yield and Minimize Substrate Burning

Xiaobin Zuo, Padmesh Venkitasubramanian, Daryle H. Busch, and Bala Subramaniam

Synopsis

The Co/Mn/Br-catalyzed oxidation of HMF affords 90% yield of FDCA under optimized conditions.

45



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

Page 46 of 46

Synopsis The Co/Mn/Br catalyzed oxidation of HMF affords 90% yield of FDCA under optimized conditions.


Br-Catalyzed Oxidation of 5

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