Efﬁcient Synthesis of Biobased Furoic Acid from Corncob via Chemoenzymatic Approach

: Valorization of lignocellulosic materials into value-added biobased chemicals is attracting increasing attention in the sustainable chemical industry. As an important building block, furoic acid has been commonly utilized to manufacture polymers, ﬂavors, perfumes, bactericides, fungicides, etc. It is generally produced through the selective oxidation of furfural. In this study, we provide the results of the conversion of biomass-based xylose to furoic acid in a chemoenzymatic cascade reaction with the use of a heterogeneous chemocatalyst and a dehydrogenase biocatalyst. For this purpose, NaOH-treated waste shrimp shell was used as a biobased carrier to prepare high activity and thermostability of biobased solid acid catalysts (Sn-DAT-SS) for the dehydration of corncob-valorized xylose into furfural at 170 ◦ C in 30 min. Subsequently, xylose-derived furfural and its derivative furfuryl alcohol were wholly oxidized into furoic acid with whole cells of E. coli HMFOMUT at 30 ◦ C and pH 7.0. The productivity of furoic acid was 0.35 g furoic acid/(g xylan in corncob). This established chemoenzymatic process could be utilized to efﬁciently valorize biomass into value-added furoic acid.

As an important furan biobased building block platform, furoic acid is commonly utilized to synthesize polymers, flavors, medicines, perfumes, bactericides, and fungicides [16,17]. Industrially, furoic acid is currently manufactured via a Cannizzaro reaction in an aqueous NaOH solution [18]. To avoid the excessive formation of undesirable byproducts, noble metal heterogeneous catalysts have been utilized for the oxidation of furfural into furoic acid [19]. Furfural was selectively oxidized to furoic acid over the Ag 2 O/CuO catalyst [20]. In order to synthesize furoic acid under mild conditions, biocatalytic synthesis has been developed due to high catalytic activity and selectivity [21]. B. cereus cells oxidized furfural into furoic acid with the yield of 95% at 30 • C [22]. Immobilized whole-cell biocatalysts transformed furfural to furoic acid at 30 • C in 24 h [23].
Sequential catalytic reactions play a pivotal role towards sustainable biomass valorization [24]. To efficiently valorize lignocellulosic biomass into furoic acid, the processes involving chemoenzymatically sequential conversion of biomass into furoic acid can been developed in a tandem reaction with catalysis of biomass into furfural by chemocatalysts and bio-oxidization of furfural into furoic acid by biocatalysts. In this work, one sulfonated tin-based Sn-DAT-SS was used as a heterogeneous chemocatalyst for the dehydration of biomass-derived xylose into furfural, and recombinant E. coli HMFOMUT cell was utilized as a dehydrogenase biocatalyst to transform xylose-valorized furfural into furoic acid. A green valorization of low-cost and renewable lignocellulosic materials into furoic acid was conducted in a tandem reaction with Sn-DAT-SS and HMFOMUT cells.

Materials and Reagents
Corncobs were collected from a village in Lianyungang City (Jiangsu province, China), which was composed of 34.1 wt % xylan. Shrimp shells (SSs) were obtained from local seafood market in Changzhou City (Jiangsu province, China). Sodium hydroxide (NaOH), ammonia, SnCl 4 ·5H 2 O, sulfuric acid (H 2 SO 4 ), and ethanol were of chemical grade and bought from Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China).

Synthesis of Furfural from Xylose Hydrolysates via Dehydration by Sn-DAT-SS
Using NaOH-treated waste shrimp shell (SS) powers as a carrier, a sulfonated Sn-DAT-SS catalyst was prepared as previously reported [25]. Xylose-rich hydrolysate (20.0 g/L xylose) from corncob (75 g/L) was prepared using oxalic acid (5.0 wt %) as catalyst as previously reported [26]. Synthesis of furfural was conducted in an autoclave (Dantu Huanqiu Electrical Instrument Co., Zhenjiang, P.R. China). In a typical experiment, 30 mL xylose-rich hydrolysate (20.0 g/L xylose, pH 1.9) was mixed with different loadings of the Sn-DAT-SS catalyst (1-4 wt %). Then, the autoclave was heated through a jacket and quickly raised to the required reaction temperature (160-180 • C) and maintained for a specified reaction duration (10-50 min) by stirring (500 rpm). After the dehydration of xylose, the whole autoclave was quickly placed into an ice-water bath. Furfural yields (%) were calculated according to Equation (1). Three parallel experiments were necessary, and the deviations were lower than 5%.

Biosynthesis of Furoic Acid
Recombinant E. coli HMFOMUT containing dehydrogenase activity were activated for 8 h at 30 • C on Luria-Bertani (LB) medium supplemented with kanamycin (50 mg/L). The cells were then cultured and harvested as previously reported [27].
For the biotransformation of commercial furfuryl alcohol into furoic acid, synthesis of furoic acid was conducted using commercial furfuryl alcohol (75.0 mM) with HMFOMUT cells (50.0 g/L) at 30 • C and pH 7.0.
To improve Sn-DAT-SS's dehydration ability, three parameters (e.g., Sn-DAT-SS loading, dehydration temperature, and dehydration time) were investigated on the influence of furfural formation in the aqueous media. The yields of furfural increased when the loadings of Sn-DAT-SS were raised from 1 to 2 wt %. Using 2 wt % loading of Sn-DAT-SS as a catalyst, the yield of furfural was 41.3%. Upon increasing Sn-DAT-SS's loading from 2 to 4 wt %, no significant change was observed on furfural yields ( Figure 1a). An excessive Sn-DAT-SS dose did not increase the acidity of catalytic media, which could not promote the furfural formation. Hence, the optimum Sn-DAT-SS loading was 2 wt %. The dehydration temperature and duration also have key roles in the valorization of biomass-derived xylose into furfural (Figure 1b). At different dehydration temperatures (160, 170, and 180 • C), the highest furfural yields were obtained as follows: Yield (170 • C, 30 min) = 41.3% > Yield (180 • C, 30 min) = 39.3% > Yield (160 • C, 40 min) = 31.0%. The optimum dehydration temperature and time were 170 • C and 30 min, respectively. A lower dehydration temperature might not supply sufficient energy to dehydrate xylose into furfural. At higher dehydration temperatures, it is evident that furfural shows signs of degrading, resulting in lower yields and the generation of unwanted byproducts [31]. In summary, the optimum Sn-DAT-SS loading, dehydration temperature, and time were 2 wt %, 170 • C, and 30 min, respectively. Under the optimum dehydration conditions, corncob-derived xylose (20.0 g/L) could be dehydrated to furfural (80.0 mM) with the yield of 41.3% (based on the xylan weight in corncob).
The reuse of chemocatalysts is crucial in the industrial production of furfural [14,32]. In order to test the performance stability of shrimp shell-supported tin-based solid acid Sn-DAT-SS catalysts, recovered and repeated reuse of Sn-DAT-SS was carried out for seven batches. As depicted in Figure 2, furfural yield arrived at 41.3% in the first run. In the second run, no significant change was observed in the furfural yield. After the second run, the yields of furfural decreased. From the second to the seventh run, the yields decreased from 41.1% to 30.8%. From the first to the seventh run, the furfural concentration was in the range of 60-80 mM. SO 4 2− /SnO 2 -attapulgite, SO 4 2− /SnO 2 -DM, and Cl0.3-S-R could be reused for five batches [14,33,34]. CSUTS-CSW was reused for seven batches [35]. The Sn-DAT-150 SS demonstrated high stability for the dehydration of xylose to furfural under the synthesis conditions tested. The reuse of chemocatalysts is crucial in the industrial production of furfural [14,32]. In order to test the performance stability of shrimp shell-supported tin-based solid acid Sn-DAT-SS catalysts, recovered and repeated reuse of Sn-DAT-SS was carried out for seven batches. As depicted in Figure 2, furfural yield arrived at 41.3% in the first run. In the second run, no significant change was observed in the furfural yield. After the second run, the yields of furfural decreased. From the second to the seventh run, the yields decreased from 41.1% to 30.8%. From the first to the seventh run, the furfural concentration was in the range of 60-80 mM. SO4 2− /SnO2-attapulgite, SO4 2− /SnO2-DM, and Cl0.3-S-R could be reused for five batches [14,33,34]. CSUTS-CSW was reused for seven batches [35]. The Sn-DAT-150 SS demonstrated high stability for the dehydration of xylose to furfural under the synthesis conditions tested.

Bioconversion of Xylose-Valorized Furfural into Furoic Acid
Whole-cell biocatalysts provide unique advantages and have been widely utilized to efficiently synthesize value-added fine and bulk chemicals [36][37][38]. Biocatalytic temperature and pH have significant influences on the biocatalytic activity [39,40]. Using dilute

Bioconversion of Xylose-Valorized Furfural into Furoic Acid
Whole-cell biocatalysts provide unique advantages and have been widely utilized to efficiently synthesize value-added fine and bulk chemicals [36][37][38]. Biocatalytic temperature and pH have significant influences on the biocatalytic activity [39,40]. Using dilute xylosederived furfural (75.0 mM, pH 7.0) as substrate, the influence of bioreduction temperatures (25-45 • C) on biocatalytic activity was illustrated in Figure 3a. It was observed that the reductase activity clearly increased with the increase in bioreduction temperature from 25 • C to 30 • C. Within 30 • C, a higher temperature could accelerate the biocatalytic activity. The highest reductase activity was observed at 30 • C. Above or below 30 • C, the reductase activity notably dropped (Figure 3a), possibly due to the thermal deactivation of reductase in the HMFOMUT cells during the bioreaction. Various bioreaction pH values in the range of 6-8 were examined for their effects on reductase activity. Low pH (≤6.5) gave a low reaction rate for the oxidation of furfural. Upon raising reaction pH from 6 to 7.0, biocatalytic activity increased. HMFOMUT cells displayed suitable furfural-oxidizing activities only within a narrow pH range (pH 6.5-7.0). The highest reductase activity was reached at pH 7.0 ( Figure 3b). Over pH 7.0, a weak alkaline environment resulted in the decreased biocatalytic activity. Hence, the optimum bioreduction temperature and pH were 30 • C and 7.0, respectively.

Bioconversion of Xylose-Valorized Furfural into Furoic Acid
Whole-cell biocatalysts provide unique advantages and have been widely utilized to efficiently synthesize value-added fine and bulk chemicals [36][37][38]. Biocatalytic temperature and pH have significant influences on the biocatalytic activity [39,40]. Using dilute xylose-derived furfural (75.0 mM, pH 7.0) as substrate, the influence of bioreduction temperatures (25-45 °C) on biocatalytic activity was illustrated in Figure 3a. It was observed that the reductase activity clearly increased with the increase in bioreduction temperature from 25°C to 30 °C. Within 30 °C, a higher temperature could accelerate the biocatalytic activity. The highest reductase activity was observed at 30 °C. Above or below 30 °C, the reductase activity notably dropped (Figure 3a), possibly due to the thermal deactivation of reductase in the HMFOMUT cells during the bioreaction. Various bioreaction pH values in the range of 6-8 were examined for their effects on reductase activity. Low pH (≤ 6.5) gave a low reaction rate for the oxidation of furfural. Upon raising reaction pH from 6 to 7.0, biocatalytic activity increased. HMFOMUT cells displayed suitable furfural-oxidizing activities only within a narrow pH range (pH 6.5-7.0). The highest reductase activity was reached at pH 7.0 (Figure 3b). Over pH 7.0, a weak alkaline environment resulted in the decreased biocatalytic activity. Hence, the optimum bioreduction temperature and pH were 30 °C and 7.0, respectively. HMFOMUT cells were utilized as biocatalysts for transforming dilute xylose-derived furfural (75.0 mM) into furoic acid. Within 5 h, the generation rate of furfuryl alcohol was quicker than that of furoic acid. In 5 h, furfuryl alcohol was achieved at the maximum (61.2 mM), while furoic acid was observed at 4.7 mM. After 5 h, furfuryl alcohol was biotransformed into furoic acid. Upon bioconversion for 60 h, furfural was converted into 72.0 mM furoic acid with the yield of 96% (based on the furfural) (Figure 4a). Upon biotransformation from 60 to 100 h, small amounts of furfural and furfural-derived furfuryl alcohol were oxidized gradually, and the yield of furoic acid increased slowly. In 100 h, furfural and furfural-derived furfuryl alcohol were wholly transformed into furoic acid. Furthermore, synthesis of furoic acid was conducted using commercial furfuryl alcohol (75.0 mM). Within 0-100 h, furfuryl alcohol was linearly transformed into furoic acid (Figure 4b). After 100 h, the furoic acid yield increased slowly. In 120 h, furfuryl alcohol was wholly oxidized into furoic acid. In the range of 20-100 h, furfural formed in the concentration of 1.2-6.3 mM. Furfural was further oxidized to furoic acid within 120 h. Industrially, furoic acid is produced from furfural via the Cannizzaro disproportionation reaction in alkali solution, which needs the addition of H 2 SO 4 to neutralize the reaction mixture, obtaining furfuryl alcohol, furoic acid, and bisulfate salt [17]. Dehydrogenase from C. testosteroni, E. coli, N. corallina, and G. oxydans could be used as biocatalysts for the production of furoic acid under ambient conditions [18,21,27,41,42]. HMFOMUT cells could wholly convert furfural and furfuryl alcohol into furoic acid with high bioreduction activity and adequate selectivity. An attractive alternative exists in the biocatalytic transformation that performs reactions with much higher selectivity under ambient conditions. Efforts with regard to biocatalysis have been made to achieve selective oxidation of furfural to furoic acid.
(75.0 mM). Within 0-100 h, furfuryl alcohol was linearly transformed into furoic acid (Figure 4b). After 100 h, the furoic acid yield increased slowly. In 120 h, furfuryl alcohol was wholly oxidized into furoic acid. In the range of 20-100 h, furfural formed in the concentration of 1.2-6.3 mM. Furfural was further oxidized to furoic acid within 120 h. Industrially, furoic acid is produced from furfural via the Cannizzaro disproportionation reaction in alkali solution, which needs the addition of H2SO4 to neutralize the reaction mixture, obtaining furfuryl alcohol, furoic acid, and bisulfate salt [17]. Dehydrogenase from C. testosteroni, E. coli, N. corallina, and G. oxydans could be used as biocatalysts for the production of furoic acid under ambient conditions [18,21,27,41,42]. HMFOMUT cells could wholly convert furfural and furfuryl alcohol into furoic acid with high bioreduction activity and adequate selectivity. An attractive alternative exists in the biocatalytic transformation that performs reactions with much higher selectivity under ambient conditions. Efforts with regard to biocatalysis have been made to achieve selective oxidation of furfural to furoic acid.

Mass Balance from Corncob to Furans in Chemoenzymatic Cascade Reaction
The calculated mass balance from corncob to furans is illustrated in Figure 5. In 1 L deionized water containing oxalic acid (50.0 g), corncob powders (75.0 g, dry weight) composed of 25.58 g xylan were placed into an autoclave. After incubation for 40 min at 140 °C under the agitation of 500 rpm, the xylose-rich hydrolysate containing 20.0 g xylose was obtained and placed into a 3 L autoclave. The xylose hydrolysate was separated by

Mass Balance from Corncob to Furans in Chemoenzymatic Cascade Reaction
The calculated mass balance from corncob to furans is illustrated in Figure 5. In 1 L deionized water containing oxalic acid (50.0 g), corncob powders (75.0 g, dry weight) composed of 25.58 g xylan were placed into an autoclave. After incubation for 40 min at 140 • C under the agitation of 500 rpm, the xylose-rich hydrolysate containing 20.0 g xylose was obtained and placed into a 3 L autoclave. The xylose hydrolysate was separated by simple filtration, and the corncob residual was collected. In this 3 L autoclave containing 1 L of xylose hydrolysate, Sn-DAT-SS powder (20.0 g) was added to dehydrate xylose into furfural at 170 • C within 30 min. The formed furfural solution composed of 7.68 g furfural was regulated to pH 7.0 and further bioconverted into 8.96 g furoic acid by HMFOMUT cells (50.0 g) at 30 • C for 100 h. The sustainable transformation of renewable and low-cost bioresource into furans has gained increased attention in recent years [43][44][45][46]. One efficient chemoenzymatic valorization of lignocellulosic biomass into furoic acid can be realized in a cascade reaction with a heterogeneous chemocatalyst and HMFOMUT cell biocatalysis. The productivity of furoic acid reached 0.35 g furoic acid/(g xylan in corncob).
Processes 2022, 10, x FOR PEER REVIEW 7 of 9 simple filtration, and the corncob residual was collected. In this 3 L autoclave containing 1 L of xylose hydrolysate, Sn-DAT-SS powder (20.0 g) was added to dehydrate xylose into furfural at 170 °C within 30 min. The formed furfural solution composed of 7.68 g furfural was regulated to pH 7.0 and further bioconverted into 8.96 g furoic acid by HMFOMUT cells (50.0 g) at 30 °C for 100 h. The sustainable transformation of renewable and low-cost bioresource into furans has gained increased attention in recent years [43][44][45][46]. One efficient chemoenzymatic valorization of lignocellulosic biomass into furoic acid can be realized in a cascade reaction with a heterogeneous chemocatalyst and HMFOMUT cell biocatalysis. The productivity of furoic acid reached 0.35 g furoic acid/(g xylan in corncob).

Conclusions
Various homogeneous and heterogeneous catalysts have been widely utilized for the production of furfural. Because of their stability, low corrosion, high catalytic activity, and easy recovery, heterogeneous catalysts have potential in the industrial production of furfural. As a value-added furfural derivative, biobased furoic acid has a market in the phar-

Conclusions
Various homogeneous and heterogeneous catalysts have been widely utilized for the production of furfural. Because of their stability, low corrosion, high catalytic activity, and easy recovery, heterogeneous catalysts have potential in the industrial production of furfural. As a value-added furfural derivative, biobased furoic acid has a market in the pharmaceutical and agrochemical fields. It can be prepared via a biocatalytic process by taking advantage of the biocatalytic oxidation of furfural. In this work, we reported a sustainable process for the chemoenzymatic cascade catalysis of corncob-valorized xylose into furoic acid, by first employing dehydration of xylose with a biocompatible heterogeneous chemocatalyst followed by bio-oxidation with dehydrogenase biocatalyst.
In this work, NaOH-treated waste shrimp shell (SS) was utilized as a biobased carrier to prepare the high activity and thermostability of the Tin-based heterogeneous catalyst Sn-DAT-SS. In the aqueous media, Sn-DAT-SS (2 wt %) could dehydrate corncob-valorized xylose (20.0 g/L xylose) into furfural (80.0 mM) with 41.3% yield (based on the xylan weight in corncob) at 170 • C in 30 min. Using corncob-derived xylose as substrate, the Sn-DAT-SS catalyst could be recovered and reused for seven runs. From the first to the seventh run, the formed furfural could reach 60-80 mM. Using whole cells of E. coli HMFOMUT as biocatalysts, furfural could be wholly oxidized into furoic acid at 30 • C and pH 7.0. During the bioconversion of furfural, a furfural derivative, furfuryl alcohol, was observed. Furfuryl alcohol could be further oxidized into furoic acid. Using 75.0 mM of furfuryl alcohol as substrate, furoic acid could be obtained with 100% yield. A green, low-cost valorization of renewable lignocellulosic materials into furoic acid was conducted in a tandem reaction with Sn-DAT-SS and HMFOMUTS cells. The productivity of furoic acid was 0.35 g furoic acid/g xylan (0.12 g furoic acid/g corncob).
Notably, the toxicity of furfural to the HNFOMUT cell would limit greater concentrations of furoic acid production. In addition, it is necessary to develop an efficient process for the recovery of furoic acid from the aqueous phase containing many different compounds. To synthesize furoic acid via the chemoenzymatic approach in a cost-effective manner, an important challenge is to seek out appropriate heterogeneous chemocatalysts and dehydrogenase biocatalysts. Related work is still ongoing in our laboratory that aims to provide an economic way for the valorization of biomass into value-added biobased chemicals.