Cerebrospinal fluid (CSF) analysis is a group of laboratory tests that measure proteins, sugar (glucose), and other chemicals in the fluid that surrounds and protects the brain and spinal cord. How the Test is Performed A sample of CSF is needed. Biomass has great potential as a substitute for fossil feedstock for the renewable production of transportation fuels and industrially important chemicals.
Ethylene glycol is a valuable product used in antifreeze liquids and as a precursor for polymers such as polyethylene terephthalate (PET). The formation of ethylene glycol from (reduced) sugars has been reported since 1935.31–41 Sorbitol, the hydrogenation product of glucose, was often used as feed and catalytically converted into a mixture of glycerol, ethylene glycol and 1,2-propanediol.
In the frame of an industrial collaboration, we thoroughly investigated the bifunctional Ni tungsten carbide system to convert renewable feedstock to ethylene glycol. Besides enabling the use of a highly concentrated glucose feed (final concentration 200 g L?1), the fed batch set-up in our study is also ideal to unambiguously clarify the reaction network of ethylene glycol formation by performing a series of experiments with different substrates.
With the acquired mechanistic understanding, reaction conditions, reactor set-up and catalyst composition were adapted to stimulate retro-aldol, while suppressing glucose hydrogenation and dehydration and polyol hydrogenolysis. In a typical fed-batch experiment, the reactor was loaded with 0.8 g catalyst and 20 ml water. The reaction products were analysed using an Agilent 1200 Series HPLC equipped with a Varian MetaCarb 67C column and a RID.
In this work, we studied the possibility of adapting the cellulose to ethylene glycol reaction system initiated by Zhang and coworkers42,43,50 to a fed-batch reactor using concentrated glucose syrups to achieve high ethylene glycol volume productivities. The proposed benefits of the fed-batch reactor strategy are clear from the data in Table 1: the ethylene glycol yield increases six-fold to 47% at the expense of reduced (and subsequently dehydrated) sugars, while the mass balance increases from 76% to 83%, when compared to the batch reactor experiment. Table 1 shows that sorbitol and erythritol are the main byproducts in the fed-batch reaction. Four different reaction pathways starting from glucose were considered a priori in the study.
Scheme 1 Proposed primary reaction routes for the conversion of glucose to ethylene glycol. As can be seen from Table 1, sorbitol, its dehydration products (sorbitan and isosorbide) and shorter sugar alcohols like erythritol, glycerol and ethylene glycol are the main products of the conversion of glucose in fed-batch mode.
The dehydrated sorbitol products are not very reactive under the applied reaction conditions as reflected by the moderate conversion.
The conversion of the set of reduced sugars (see Table 3) reveals that their stability increases with decreasing carbon chain length: while sorbitol is slowly converted into other products, ethylene glycol – the desired product in this study – is much more stable under the applied reaction conditions.
In parallel, dehydration products like sorbitan isomers (mainly 1,4-sorbitan) are formed as well, further leading to isosorbide and polymerization products as shown in Table 2. Retro-aldol reaction of glucose results in glycol aldehyde and erythrose, which may be cleaved accordingly into two glycol aldehyde molecules. Every aldose leads to detectable amounts of the corresponding sugar alcohol originating from carbonyl hydrogenation.
Carbonyl hydrogenation of galactose to galactitol seems slightly more competitive when compared to that of glucose and mannose, while mannose led to the highest ethylene glycol yield.
The conversion of the C5 aldoses – xylose and arabinose – resulted in the formation of equal amounts of C2 and C3 products.
Generally, the high selectivity for cleaving the ?–? C–C bond with respect to the aldehyde group (e.g.
More specifically, the conversion of fructose results in about 57% of C3 compounds such as glycerol, 1,2-propanediol and n-propanol. In summary, isomerisation during the glucose to ethylene glycol reaction will have a profound influence on the final product distribution.
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Although product distributions varied with the catalyst type, additives like CaO and reaction conditions (pressure, substrate concentration and temperature), 1,2-propanediol always appeared to be one of the most dominant products. Glucose instead of cellulose was used, as highly concentrated cellulosic suspensions are very difficult to handle in state-of-the-art industrial processing. In this way, the conversion of any unstable intermediate is easily assessed because its degradation is avoided as a result of the limited contact time. Ethylene glycol yields up to 66% were ultimately achieved from highly concentrated glucose syrups, while at shorter reaction times, a staggering ethylene glycol productivity of 293 g L?1 h?1 was reached, though somewhat at the expense of the ethylene glycol yield. After flushing with hydrogen, the reactor was pressurized with 6.0 MPa hydrogen and heated to 518 K under constant stirring at 750 ppm. After flushing with hydrogen, the reactor was pressurized with 6.0 MPa hydrogen and heated to 518 K.
In an initial experiment we benchmarked the glucose to ethylene glycol conversion in the fed-batch reactor against that in the reference batch reactor. In 3 hours, 30 ml of a 333 g L?1 aqueous glucose solution was pumped into the reactor vessel bringing the overall feed concentration of glucose to 200 g L?1. To close the mass balance, gas phase and solid residues were analyzed as well, but both pointed to less than 0.5 mol% C content of the input carbon. To explain the tremendous difference between fed-batch and batch, a true understanding of the reaction network was ambiated.
Firstly, since the conversion of a glucose molecule into 3 ethylene glycol molecules formally requires six hydrogen atoms, the reaction in water needs to be performed under hydrogen pressure. This reaction has been reported to produce mainly 1,6-anhydroglucose (AHG) and 5-HMF.56,58,60,61 However, no considerable amounts of AHG or 5-HMF were detected in our batch nor fed-batch reaction, as shown in Table 1. These pathways, visualized in Scheme 1, were further studied in more detail by feeding their corresponding products in the fed-batch reactor. As sorbitol is by far the most dominant byproduct, direct hydrogenation of glucose occurs during reaction.


Product distribution and carbon balance calculated on the total amount of sorbitan isomers, sorbitol and isosorbide in the reactor.b Conversion of sorbitan.
Since no ethylene glycol or other degradation products are detected from the conversion of sorbitan isomers (in the sorbitol dehydration mixture) and isosorbide, it is clear that these compounds are not at play in the formation of ethylene glycol. Such high product stability is of course advantageous for the fed-batch reaction with regard to ethylene glycol production.
Thus, despite some ethylene glycol formation from sorbitol, the hydrogenation of glucose to sorbitol is clearly not a dominant pathway to ethylene glycol in the catalytic system, which is in agreement with the report by Zhao et al.50 Moreover, since sorbitol is significantly more stable than glucose under the applied reaction conditions, its formation through hydrogenation should be avoided in the interest of maximizing the yield of ethylene glycol. As discussed in the previous section, these reduced sugars can undergo unselective hydrogenolysis and dehydration reactions and their stability depends on their chain length, with ethylene glycol as the most stable one. This could indicate that selective C–C splitting is sensitive to the conformation of the hydroxyls in the sugar molecule, which are involved in the coordination with the catalyst site. This is well in agreement with a retro-aldol pathway: the aldoses are cleaved into glyceraldehyde and glycol aldehyde, which yield glycerol and ethylene glycol upon hydrogenation. This is consistent with the report of Zhang and co-workers on the preferential conversion of inuline (= a polyfructan) rich artichoke to 1,2-propanediol.66 In the conversion of erythrulose, C3 compounds are also the main products, but with a lower carbon selectivity of about 23%.
This difference in ratio unambiguously proves that retro-aldol C?–C? cleavage prevails over unselective hydrogenolysis, which should otherwise yield similar ratios for both hexoses. Isomerisation of glucose to fructose will lead to byproducts according to two parallel pathways.
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The system can also track blood glucose trends over time, and can automatically remind users about medication schedules and test strip expiration dates. It was only since the recent discovery by Ji et al.,42,43 who reported the direct catalytic conversion of cellulose to ethylene glycol, that the selective formation of ethylene glycol was conceivable. A fed-batch system was selected as this reactor type simulates the slow release of glucose from cellulose through hydrolysis in a batch reaction, ensuring low glucose concentration. Carbon mass balances approached 90 mol% in most cases after analysis of C in both gas and liquid phases. The dried powder was reduced in a pure hydrogen flow (at 1600 ml g?1 h?1 contact time) according to the reported procedure.
The moment at which the reactor reached a temperature of 518 K was taken as the starting point of the reaction. After reaching the reaction temperature, the sugar solution was fed gradually into the reactor with a Waters 515 HPLC pump.
In addition, a Hewlett Packard 5890 GC, equipped with a HP 7673 auto sampler, a 50 m Poraplot Q column and a FID were used for a better separation of the C1 to C3 compounds such as methanol, ethanol, ethylene glycol and 1,2-propanediol. Table 1 shows that the batch experiment starting from a 200 g L?1 glucose solution results in less than 10% ethylene glycol yield. Afterwards, the fed-batch system was further optimized with regard to temperature, pressure and feed rate. To elucidate the whole reaction network including these products and to track the main reaction pathways towards ethylene glycol formation, these identified chemicals were fed into the reactor under the same reaction conditions of Table 1, albeit at somewhat lower concentration. Moreover, the higher ethylene glycol yield in the fed-batch was accompanied with a decrease in sorbitol yield, when compared to the batch reaction.
For both reactions, low mass balances were measured, probably due to polymerisation reactions, while no other products were detected. For these reactivity reasons, the amount of the corresponding sugar alcohols that are retrieved from these sugars decreases with increasing chain length. Conversion of erythrose (a C4) through a sequential retro-aldol and hydrogenation reaction is expected to yield mainly ethylene glycol. Like the aldoses studied in Table 4, all ketoses are completely converted into more stable reaction products including their respective sugar alcohols. Finally, the conversion of dihydroxyacetone produces almost no smaller fragments (>90% of all carbon remains contained in the C3 fraction), glycerol and 1,2-propanediol being the main products. Since dihydroxyacetone does not have a reactive ? position to the carbonyl, no retro-aldol reaction can take place and any smaller (C1 and C2) compounds are expected to result from hydrogenolysis of glycerol. Useful not only for athletes but also for those with breathing problems, heart issues, or other health concerns, the Pulse Oximeter provides a quick and painless way to track blood oxygen levels. Detailed investigation of the reaction network shows that, under the applied reaction conditions, glucose is converted via a retro-aldol reaction into glycol aldehyde, which is further transformed into ethylene glycol by hydrogenation. In their first publication, they noted a remarkable 61% yield of ethylene glycol with a nickel-promoted tungsten carbide catalyst in a one-pot aqueous batch reaction of 30 min at 518 K.
As such, degradation of thermolabile glucose is prevented, while still allowing the ability to pump in highly concentrated glucose syrups, viz. Preliminary reuse experiments indicated that the catalyst system could be recycled with acceptable losses in ethylene glycol yield and limited W-leaching in the cold reaction filtrate. During the course of the reaction, 30 ml of an aqueous 333 g L?1 (D-)glucose syrup was added at a constant flow rate of 0.167 ml min?1, leading to a total reaction time of 3 hours.
Long-chain polyols (C4, C5 and C6) were additionally quantified after derivatisation of the reaction products via silylation9 and analysed with a Hewlett Packard 5890 Series II GC equipped with a 50 m CP-Sil-5CB column and FID.
The dominant products are reduced C6 sugars such as sorbitol and their dehydrated forms such as sorbitan.
For the network study, 30 ml of a 17 g L?1 substrate solution is gradually fed in the 100 ml reactor over 3 hours with constant feed rate.
Its subsequent hydrogenolysis could lead to ethylene glycol, as was suggested earlier.31–40 If this is the main route to ethylene glycol, the Ni tungsten carbide catalyst should have a certain unique hydrogenolytic activity, mainly producing ethylene glycol instead of 1,2-propanediol and glycerol. Taking into account the mass balance, there will indeed be some char formed in the fed-batch set-up, but additional experiments using 5-HMF and AHG as a feed did exclude their role in ethylene glycol synthesis.
This means that sorbitol or its dehydration products could be key intermediates in the formation of ethylene glycol via (un)selective hydrogenolysis.
It was previously reported62,63 that glycerol can be converted via dehydration to hydroxyacetone and subsequent hydrogenation to 1,2-propanediol.
This is indeed supported by the data in Table 4, where ethylene glycol is the main product. Again, these sugar alcohols will undergo further hydrogenolysis, forming C2 and C3 products.
Since these compounds are almost absent, hydrogenolysis is not a preferred reaction pathway under the applied reaction conditions. Alternatively, fructose undergoes a retro-aldol reaction, eventually leading to C3 products such as glycerol, 1,2-propanediol and n-propanol. In later studies,44,45 the yield was further increased up to 75% by using different carbon supports with a better pore structure. The synthesis of this catalyst is very subtle and the above procedure should be followed punctiliously to obtain reproducible data.


In this way, a total of 10 g glucose and 50 ml water were present in the reactor, corresponding to a 200 g L?1 feed concentration in this work. All reported product yields are expressed as the molar fraction of carbon (mol% C) represented by that product, relative to the total amount of carbon introduced in the reactor. Besides the products listed in Table 1, a significant amount of char was found in the reactor vessel after reaction. Previous research on glucose decomposition in hot-compressed water suggested the occurrence of three other chemical reactions:56,58–60 dehydration to 5-HMF (and others), cleavage to smaller compounds through retro-aldol reaction and aldose–ketose isomerisation. If not, it means that sorbitol (and its dehydrates) is a dead-end product, which renders the fast and selective glucose hydrogenation to be an undesirable competitive pathway.
The conversion of erythritol and xylitol, reduced sugars with a C4 and C5 backbone respectively, results in approximately equal amounts of C2 and C3 compounds. The high conversion of these monosaccharides reflects their high reactivity in the reaction conditions.
The stereoselectivity of the higher threitol content with galactose agrees with the selective retro-aldol C?–C? splitting (to threose and glycol aldehyde), followed by carbonyl hydrogenation. However, closer inspection of the product distribution again indicates that retro-aldol is the dominant C–C bond cleavage mechanism. By comparing the data of erythrose (Table 4) and erythrulose (Table 5) conversion, differences in the product spectrum again support retro-aldol cleavage rather than random tetrol hydrogenolysis: up to 50% of EG was obtained for erythrose compared to only 10% for erythrulose.
Since both of these pathways (summarized as well in Scheme 3) lead to C3 products, glucose isomerisation will always result in an increase in C3 product selectivity. They are formed through a series of unwanted side reactions including hydrogenation, isomerisation, hydrogenolysis and dehydration.
Moreover, such concentrated glucose streams of industrial grade are available today at a reasonable price, whereas in the future, they could become cheaper via the hydrolysis of cellulose in second generation biorefineries. Particular attention should be paid that the catalyst is not exposed to air after the reduction step.
Other addition rates were evaluated as well and they lead to different reaction times, e.g. ICP-OES measurements for W content determination in the reaction filtrate were measured at 224.88 nm. The reaction is thus considered as an undesirable side-reaction that needs to be avoided as much as possible to improve the carbon efficiency.
To distinguish the two possibilities, pure isosorbide and a sorbitol dehydration mixture consisting mainly of sorbitan isomers were fed into the reactor. This product distribution differs from that of the reaction with sorbitol as C3 compounds were analysed as the main products there, with a total selectivity of 53%. Interestingly, the previously reported (Lewis) acid-catalyzed conversion of tetroses to vinyl glyoxal, an interesting precursor for novel polyester building blocks,26,67 was not observed in the applied reaction conditions, possibly due to the preferred retro-aldol reaction instead of the retro-Michael dehydration68 in the presence of the Ni–W2C catalyst at high temperatures. Those findings opened the way for other combinations of hydrogenation catalysts and tungsten-based catalysts.48,49 Unfortunately, low substrate concentration (often less than 10 g L?1 in water) and sometimes complicated catalyst synthesis, in casu nickel tungsten carbide, could hamper industrial upscaling in its present form.
Powder XRD (X-ray diffraction) patterns were recorded on a STOE STADI P Combi diffractometer with an image plate position sensitive detector (IP PSD) in the region 2? = 10 to 60° (?2? = 0.03°). A known route to char is the condensation of sugars and their dehydration products like 5-hydroxymethylfurfural (5-HMF) to oligomeric and polymeric fractions, which eventually become insoluble.
In other words, the catalyst preferably contains no (strong) acidity so as to circumvent such catalyzed dehydration. Because ethylene glycol is not the main product from sorbitol (16% selectivity at 14% yield), its direct formation from sorbitol is not considered as the major pathway.
Instead, glyceraldehyde was mainly hydrogenated to glycerol with some subsequent hydrogenolysis forming 1,2-propanediol and n-propanol.
To assess the relevance of the fed-batch system in biomass conversions, both the influence of the catalyst composition and the reactor setup parameters like temperature, pressure and glucose addition rate were optimized, culminating in ethylene glycol yields up to 66% and separately, volume productivities of nearly 300 gEG L?1 h?1. The mechanism of C–C bond cleavage leading to ethylene glycol formation and the intermediates involved are not yet fully elucidated.
Byproducts are mainly formed through hydrogenation of glucose (and lower monosaccharides) to stable sorbitol (and corresponding polyols) and their unselective C–C hydrogenolysis. The measurements were performed in transmission mode at room temperature using CuK?1 radiation with ? = 1.54056 A selected by means of a Ge(111) monochromator.
After introducing the entire glucose solution, the reactor was cooled and the samples were taken in the same way as in the batch experiment. Volumetric productivity values were calculated by dividing the total weight of ethylene glycol formed by the time of reaction and the total liquid reaction volume (in g L?1 h?1). In addition, the product distribution from reactions with pure sorbitol and shorter reduced sugars like xylitol, erythritol, glycerol and ethylene glycol were examined as well and reported in Table 3. Indeed, if ethylene glycol originated primarily from direct hydrogenolysis of sorbitol, a reaction starting from sorbitol would be expected to yield more ethylene glycol than a reaction starting from glucose, which is not the case (compare data in Table 3vs.Tables 1 and 4). In accordance with the literature, microcrystalline Avicel cellulose (1 wt% in water) was fully converted after 24 hours at 518 K in our setup, yielding 59 wt% ethylene glycol at a carbon mass balance of 78 mol%. Moreover, the data from Table 3 show that sorbitol is converted through two different pathways (presented in Scheme 2). At the end of the reaction, the vessel was cooled and the catalyst was recovered by centrifugation (50 min, 10000 rpm) and subsequent decantation. The issue of char formation emphasises the importance of a low glucose concentration and implies that a batch reactor setup is not ideal to achieve high volume productivities. The dominant pathway is sorbitol C–C and C–O hydrogenolysis, forming mainly C3 compounds such as glycerol, 1,2-propanediol and n-propanol in addition to smaller amounts of ethylene glycol. However, the presence of this reflection entailed no noticeable effect on the activity of the catalyst.
The presence of NiW [00-047-1172] is evidenced by a few reflections with the strongest one at 2? = 43.5°.
The wet catalyst was transferred into the reaction vessel with 20 ml of water for the 2nd run under identical reaction conditions.
It was also partly funded by the Yale Clinical and Translational Science Award (CTSA) grant from the National Center for Research Resources at the National Institutes of Health.Dr. Tamborlane, a member of Yale Medical Group, is chief of pediatric endocrinology at Yale School of Medicine and Yale-New Haven Children’s Hospital.
His major achievements have included pioneering work in the development of insulin pump therapy, continuous glucose monitoring, sensor-augmented pumps, closed loop systems and development of an artificial pancreas. Tamborlane was honored with the Diabetes Technology Leadership award for his extraordinary lifetime accomplishments in diabetes research and technology.



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Comments

  1. 25.03.2014 at 19:13:21


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    Author: SMR
  2. 25.03.2014 at 16:40:44


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