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Effect of Metal Chlorides on Glucose Mutarotation and Possible Implications on Humin Formation

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Effect of Metal Chlorides on Glucose Mutarotation and Possible Implications on Humin Formation Reaction Chemistry & Engineering Effect of Metal Chlorides on Glucose Mutarotation and Possible Implications on Humin Formation Journal: Reaction Chemistry & Engineering Manuscript ID RE-COM-10-2018-000233.R1 Article Type: Communication Date Submitted by the 28-Nov-2018 Author: Complete List of Authors: Ramesh, Pranav; Rutgers The State University of New Jersey, Chemical and Biochemical Engineering Kritikos, Athanasios; Rutgers The State University of New Jersey, Chemical and Biochemical Engineering Tsilomelekis, George; Rutgers The State University of New Jersey, Chemical and Biochemical Engineering Page 1 of 5 ReactionPlease doChemistry not adjust & Engineering margins Journal Name COMMUNICATION Effect of Metal Chlorides on Glucose Mutarotation and Possible Implications on Humin Formation a a a Received 00th January 20xx, Pranav Ramesh , Athanasios Kritikos and George Tsilomelekis * Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/ An in-situ Raman spectroscopic kinetic study of the glucose suggested that the possible changes in the anomeric mutarotation reaction is presented herein. The effect of metal equilibrium especially via the stabilization of the α-anomer of chlorides on the ease of ring opening process is discussed. It is glucose, might be responsible for improved selectivity towards 7, 8 shown that SnCl4 facilitates the mutarotation process towards the fructose . This also agrees with the concept of anomeric β-anomer extremely fast, while CrCl3 appears to promote the specificity of enzymes; for instance, immobilized D-glucose formation of the α-anomer of glucose. Infrared spectra of humins isomerase has shown ~40% and ~110% higher conversion rates prepared in different Lewis acids underscore the posibility of starting with α–D-glucose as compared to equilibrated glucose multiple reaction pathways. and β–D-glucose solutions respectively9. Production of 5-HMF in ~60% yields has been reported by coupling homogeneous Lewis with Bronsted acids in a tandem glucose isomerization Introduction and fructose dehydration scheme (scheme S1)10. Converting biomass, a renewable source of carbon, to value- Herein, we report an in-situ Raman kinetic study on the ease of added products is a promising and sustainable approach for glucose mutarotation in the presence of AlCL3, CrCl3 and SnCl4 biofuel/biochemical production with immense benefits to homogeneous Lewis acid catalysts. The rationale behind several industrial sectors. Carbohydrates, aldoses and/or choosing these metal salts lies at the fact that although all three ketoses, derived from the cellulosic/hemicellulosic constituents salts have proven to facilitate glucose isomerization under of non-edible biomass sources comprise excellent candidates various reaction conditions product distribution varies for chemical production but the high oxygen content in their significantly, with AlCl3 and CrCl3 to be more selective. SnCl4 has structure require further downstream process1, 2. The pivotal reported to facilitate glucose isomerization reaction at the 130- impact of the selective conversion of these monosaccharides to 150°C temperature range; however, the process suffers from their dehydration counterparts as intermediates has been significant formation of side reactions (humins) thus sacrificing underscored in recent years3, 4. A characteristic example product selectivity. We believe that metal chlorides as Lewis encompasses the conversion of cellulose derived glucose to 5- acids, can potentially affect product selectivity in biomass Hydroxymethylfurfural (5-HMF) which is considered as one of reactions by altering a) the kinetics of ring opening and b) the the key biomass-derived intermediates. equilibrium level of the mutarotation process as well as by c) Glucose isomerization to fructose is an important intermediate introducing additional pathways towards degradation reaction in the production of 5-Hydroxymethylfurfural from reactions. cellulosic biomass sources and is well established that is Lewis acid catalyzed. The presence of Lewis acids is believed to facilitate glucose ring-opening in aqueous solution followed by Results and discussion, a kinetic relevant intramolecular hydride shift from C2 to the C1 Glucose mutarotation and Raman spectral considerations 5 position of glucose . The ease of the first step, i.e. ring opening, For the quantitative estimation of the anomeric species in the presence of metal chlorides in ionic liquids have been distribution of glucose in aqueous solutions, many methods 6 reported by means of DFT calculations . It has also been have been reported in the open literature. Besides the well- established polarimetric method, molecular spectroscopic techniques have proven to provide comparable and accurate a. Department of Chemical and Biochemical Engineering, Rutgers, The State results11. Vibrational spectroscopies, Raman and FTIR, have University of New Jersey, Piscataway, 08854, NJ, USA This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 1 Please do not adjust margins ReactionPlease doChemistry not adjust & Engineering margins Page 2 of 5 COMMUNICATION Journal Name (d) α-D-Glucoseaqueous│t=0 - 240 min (c) -1 -1 -1 519 cm 518cm -1 . 448 cm 544 cm u . a -1 , 448cm y t i s n -1 e 544cm t n . I u -1 . α-D-Glucoseaqueous│t=0min 515 cm (b) a , y t i s 100 n 푰ퟓퟒퟑ e (e) t n 푰ퟓퟒퟑ + 푰ퟒퟒퟖ I 80 e s o c 60 α-D-Glucose (a) u solid l -1 g 541 cm - α f 40 o 푰ퟓퟒퟑ % 20 푰ퟓퟒퟑ + 푰ퟓퟏퟗ 0 300 350 400 450 500 550 600 0 50 100 150 200 250 -1 Raman Shift, cm Time, min Figure 1: Raman spectrum of α-glucose in a) solid, b) water and c) under mutarotation reaction. Analysis of spectra is shown in d) and e). been routinely used to study also solid-state glucose calculation of the vibrational modes of glucose as well as mutarotation reactions, solvent and cation effects on the experimental Raman/IR and Raman Optical Activity anomeric equilibrium etc. In situ Raman detection of solid state measurements have shown that skeletal endocyclic and glucose mutarotation has been also reported, but a spectral exocyclic modes might coexist with bands that usually are dealt deconvolution using Gaussian peaks required thus making the as “purely” anomeric modes. Corbett et.al.14 studied the analysis time consuming. For Raman spectroscopy, the well- vibrational modes of “wet” α-D-glucose and suggested that the accepted method proposed by Mathlouthi et.al. involves the 538cm-1 band is conformational sensitive and upon interaction -1 -1 ratio of the C2-C1-O1 bending mode of the α- and β- anomer, i.e. with water shifts to 512cm and 522cm . This is further 퐼543/(퐼543 + 퐼520), for the calculation of the anomeric supported by the work of Mathlouthi et.al where modes 12 equilibrium . Although this ratio well estimates the anomeric involving vibration of the CH2 group show shifts in frequencies equilibrium, spectral overlapping limits the accuracy of the more than ~10 cm-1 upon changing D-glucose (or sucrose) method in the whole kinetic profile. This limitation can be concentration15. A deconvolution of the 500-600cm-1 range in overcome by rationalizing the vibrational assignments of the our spectra show four peaks located at 515, 544, 564 and pure aqueous glucose anomers via study of their kinetic 590cm-1 which is in very good agreement with those reported in profiles, as discussed next. literature. Theoretical studies via first principle calculations can In Figure 1a, we show for comparison the Raman spectrum of shed more light into how the interactions of glucose with water crystalline glucose used. The 541cm-1 band is characteristic to molecules can affect the vibrational modes observed. the contribution of C2-C1-O1 bending mode of the α-anomer of In Figure 1c we show the in-situ Raman of the aqueous α-D- glucose. The corresponding C2-C1-O1 bending mode of the β- glucose solution as it undergoes mutarotation in the 300- anomer is expected to give rise at 518cm-1, which is absent in 650cm-1 range. Since all the spectra presented herein our crystalline sample, confirming the purity of the sample correspond to glucose in aqueous solutions, the spectral used. Upon dissolution in water, Figure 1b, characteristic bands contribution from water as well as quartz cuvette used has been of α-glucose (t=0min) become broader and shift slightly to subtracted. In addition, prior to the subtraction, all the spectra higher frequencies. A noticeable change encompasses the have been normalized with respect to the total area of each appearance of a new band at 515cm-1 that almost coexist in spectrum. As the mutarotation reaction progresses, new Raman -1 frequency with the suggested C2-C1-O1 bending mode of the β- bands at 425, 448 and 519cm evolve at the expense of those anomer (518cm-1). Since the spectrum was taken after bands that correspond to the α-anomer of glucose. The band complete glucose dissolution (t=0min indicates the time of located at 448cm-1 (C-C-O skeletal mode as well13, 15), has almost collection of the first spectrum and not the time of mixing) one no spectral overlap from neighbouring bands and thus we have expects a small progress in glucose mutarotation to result in the used this peak as characteristic for the β-anomer instead of the formation of β-anomers that can overlap in the Raman spectra. 520cm-1. As it can be seen in Figure 1d, the relative intensity of -1 -1 However, we support that the 515cm peak is characteristic of the 518 and 544cm bands at early reaction time is almost the α-anomer of glucose since the Raman kinetic data reported identical indicating that the ratio 퐼543/(퐼543 + 퐼520) in Figure 1c shows a slight decrease of the intensity of the underpredicts the % change of the α-anomer at the beginning. 515cm-1 and the appearance of a neighboring band at 519cm-1 This limits significantly our ability to accurately measure the with time. The previous argument is also supported by the clear anomeric concentrations that could potential help us to isosbestic points which appear around the 515cm-1 band understand the catalytic behavior in downstream reactions, confirming a clear transition of α- to β- anomers with time.
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