Polyols dehydration

In view of the use of glycerol as a chemical commodity for the production of chemical intermediates, an overview will be made of existing catalytic knowledge. More specifically, glycerol oxidation, dehydration, hydrogenolysis, oligomerization/polymerization, polyol formation, and formation of a few miscellaneous products will be dealt with. 

Concentrated aqueous salt solutions were used for dehydration of carbohydrates catalyzed by RuCh + Ag2S04 ( RUSO4 ) [47]. Such solvents may also help in constracting aqueous-organic biphasic media with good phase separation properties. Selective dehydroxylation of polyols and sugars was achieved in aqueous solutions with the use of anionic rathenium carbonyls, as well [48]. 

Outside of the liver, fructose is channeled into the sugar metabolism by reduction at C-2 to yield sorbitol and subsequent dehydration at C-1 to yield glucose (the polyol pathway not shown). 

Breakdown of diethylene glycol use is as follows unsaturated polyester resins (20%), polyester polyols (15%), antifreeze blending (12%), triethylene and tetraethylene glycol (9%), solvents (9%) morpholine (7%), and natural gas dehydration (4%). Much of the market is captive. The merchant market is small. 

Much work has been done on the incorporation of castor oil into polyurethane formulations, including flexible foams [64], rigid foams [65], and elastomers [66]. Castor oil derivatives have also been investigated, by the isolation of methyl ricinoleate from castor oil, in a fashion similar to that used for the preparation of biodiesel. The methyl ricinoleate is then transesterified to a synthetic triol, and the chain simultaneously extended by homo-polymerization to provide polyols of 1,000, 000 molecular weight. Polyurethane elastomers were then prepared by reaction with MDl. It was determined that lower hardness and tensile/elongation properties could be related to the formation of cyclization products that are common to polyester polyols, or could be due to monomer dehydration, which is a known side reaction of ricinoleic acid [67]. Both side reactions limit the growth of polyol molecular weight. 

In order for a solvent extraction system to be of value, it must be able to separate the phase containing the pollutant from the water. While the polymers can be used to extract contaminants from air, their water solubility precludes separation from groundwater. In the biphasic technique, the separation of the polymer phase from the water is achieved by the well-known physical chemical effect known as salting out. Simply put, inorganic salts are added to the system. The addition has the effect of dehydrating the polyol, making it insoluble and permitting separation. 

Rhenium-catalyzed deoxygenation reactions, on the other hand, can be divided in two topics deoxydehydration and dehydration. These reactions have mostly been applied on biomass-derived substrates such as sugar alcohols and polyols. This procedure lowers the high 0 C ratio present in biomass and as such is of particular interest for the production of both fuels and chemicals from biomass. 

Manufacture of the polyols is usually carried out in the same reactors as for ethoxylates. The first step is to dissolve sodium hydroxide in propylene glycol and warm to 120°C. The required amount is charged to the reactor, dehydrated and padded with nitrogen. Once this is achieved propylene oxide is added as fast as it will react, maintaining the temperature at 120°C until the required molecular weight is reached. Then ethylene oxide is added at a rate, which maintains the temperature at 120°C. When all the ethylene oxide is added, the... 

Experiments on reaction mechanisms showed that urea enhanced the dehydration (weight loss) reactions of monosaccharides in nonacidic environments. Addition of acid to a glucose-urea mixture slowed its dehydration at 108 °C. The addition of sufficient urea to an acid-catalyzed polyol solution appears to prevent, or at least delay considerably, the normal acid dehydration reaction of the polyol. 

Among the simplest of the cyclizations utilized in the formation of C-glycosides is the acid-mediated formation of ethers. Such reactions can be viewed as dehydration between two alcohol units, with the driving force for the reaction being the elimination of water. As shown in Scheme 7.98, Schmidt and Frick [242] formed the perbenzylated polyol by treating the lithiated... 

Protein denaturation has been observed during spray drying due to the dehydration process and the use of excipients to replace hydrogen bonded water is often needed (e.g., sugars, polyols, amino acids) in the protein formulation (3.57.58). 

Acid lability. Several of the polyols although odorless, are acid labile and readily form volatile flavorants at ambient temperature and juice pH (18,28,29) Hotrienol, for example, appears to be formed wholly by acid catalyzed dehydration of dienediol 30a (26,30). Four naturally occurring hydroxyllnalool derivatives l.e. 29,30a,31 and 32 were heated for 15 min at 70°C and pH 3.2 and thirteen volatile monoterpene products (l.e. la,5,6a,9,10,11,12,15, 16,17,18,21 and 22) were identified by headspace analysis (18). 

Furthermore, 1,2-propyleneglycol and other polyols containing vicinal OH-groups could be reacted to valuable aldehydes and ketones. Thereby an acid catalyzed dehydration is subsequently followed by an intramolecular rearrangement according... 

Propoxylated derivatives of sorbitol are used as the polyols in the formation of rigid polyurethane foams. Sorbitol can be dehydrated and esterified with stearic acid to give sorbitol mono- or tristearate. The ester then can be ethoxylated with about 20 ethylene oxide units to give an ethoxylated surfactant. Hydrolysis of sorbitol results with glycerin, which is used in the pharmaceutical and personal care products field [108,109],... 

Mesoporous sulfonie acids are of particular interest for reactions involving molecules that are too large to access the smaller pores of conventional molecular sieves or for the preparation of bulky reaction products. In the hydroxyalkylation of methylfuran with acetone, the ordered mesoporous sulfonic acids surpass large pore zeolites such as H-Beta, with an excellent yield of 82 % for 2,2-bis(5-methyl-furyljpropane instead of 45 % for H-Beta [30]. The activity of the sulfonic MCMs is also clearly superior to that of zeolites Beta and Y in the o-sorbitol dehydration-esterification forming isosorbide dilaurate, or in the liquid biphasic preparation of monoesters starting from various polyols and fatty acids [30]. The catalyst of Stein and co-workers was highly active and selective in the tetrahydropyranylation of ethanol with 2,3-dihydropyran [31]. In this type of reaction also the non-modified H-MCM-41 material is an excellent catalyst [40]. 

In a similar fashion, diols and polyols can be transformed selectively to important chemicals, e. g. carbonyl compounds, dienes and cyclic ethers. Numerous reviews give adequate treatment of the dehydration both of monohydric alcohols [1-11] and of diols and polyols [12-14]. 

One classical polyol dehydration reaction is the transformation of glycerol to acrolein in the presence of fused, powdered KHSO4 [94]. Another important example is the dehydration of substituted 1,2,5-triols (8) to dihydropyran derivatives... 

The presence of a multiplicity, of allyl groups in a polymer results in a thermosetting composition. Thus, the various drying oil alkyds, containing side chains derived from dehydrated castor oil and linseed oil, are cross-linked by polymerization of allylic double bonds. Allyl starch and allyl sucrose are extremely interesting materials prepared by treatment of the polyols with allyl chloride in the presence of base. Coatings prepai ed from these compounds are readily cross-linked by the application of heat. 

In catalytic biomass cmiversion, the dehydration of polyol moieties is a key reaction, forming either an olefin bond, an ether, or a carbonyl group (after tautomerization). This type of reactimi usually requires the aid of acid catalysis. Both Br0nsted and Lewis acids are known to catalyze dehydration. The most famous dehydration in the context of biomass conversion is the formation of HMF from six-carbon sugars. As can be seen in Fig. 12, HMF from fructose (or glucose) preserves the F.C value of 1.17, while the FI index falls from 2 to 1. Similarly, pentoses lead to furfural in aqueous media under acid catalysis. The catalytic formation of HMF and furfural from hexoses and pentoses, respectively, and HMF production directly from cellulose, have been reported frequently over... 

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