Acetals, heterogeneous hydrolysis

C. Moreau, J. Lecomte, S. Mseddi, and N. Zmimita, Stereoelectronic effects in hydrolysis and hydrogenolysis of acetals and thioacetals in the presence of heterogeneous catalysts, J. Mol. Catal. A Chem., 125 (1997) 143-149. 

Most experiments were performed with cotton or cotton linters as highly crystalline celluloses. Table I shows conditions leading to complete dissolution. A minimum amount of an acid which forms a cellulose ester (sulfuric or trifluoromethylsulfuric acid) (Entries 5 7-14) is necessary for the reaction. The dissolution is accelerated by a temperature increase (Entries 10-12 13, 14) and leads to water-soluble cellulose acetate hydrogensulfate. Whereas this primary hydrolysis can be achieved within 1-5 min, the deesterification and complete hydrolysis of the soluble cellulose derivative proved to be much more difficult. This is in contrast to the generally accepted view that the main resistance to the hydrolysis of cellulose lies in the crystalline nature or low accessibility determining the heterogeneous first step of the reaction. 

The gel filtration chromatography is employed as an important technique to isolate heterogeneous LPS extracted from bacterial cells, especially for isolation of truncated LPS without lipid A moiety. The truncated LPS can be obtained by hydrolysis with 1% acetic acid at 100°C for 1 h, or 0.1 M NaOH in 99% ethanol at 37°C for 45 min (Chester and Meadow, 1975 Muller-Seitz et al., 1968 Prehm et al., 1975). The former releases the polysaccharide from the lipid A moiety with exclusive spitting of the extremely acid-labile Kdo linkage (Luderitz et al., 1966 Muller-Seitz et al., 1968) the latter removes ester-linked fatty acids from the lipid A, reducing non-polar interactions between LPS components and facilitating their separation (Chester and Meadow, 1975). 

The term reactive distillation (RD) refers to both catalyzed and uncatalyzed reaction systems. Catalytic distillation systems may use a homogenous or heterogenous catalyst to accelerate the reaction. Reactive distillation is a well-known example of reactive separation process, and is used commercially. The first patent and early journal articles deal mainly with homogenously catalyzed reactions such as esterifications, transesterifications, and hydrolysis.f Heterogenous catalysis with RD is a more recent development. The key advantages for a properly designed RD colunm are complete conversion of reactants and attainment of high selectivity. An example of the benefits of RD is the acid catalyzed production of methyl acetate by... 

Chemical heterogeneities present in soils, sediments, and aquifers undoubtedly have an effect on rates of pollutant degradation. Other sources of surface catalysis not discussed here include Bronsted acidity of surface sites, that become apparent as surfaces become dehydrated (El-Amamy and Mill, 1984). Surface and pore structure may play a role in the catalysis of phosmet hydrolysis by montmorillonite (Sanchez-Camazano and Sanchez-Martin, 1983) and in the catalysis of ethyl acetate hydrolysis by zeolites (Nam-ba et al., 1981). 

The slightly yellow solution is cooied In an ice bath and 60 mL of glacial acetic acid (ca. 1 mol) and 50 mL of water are added. Hydrolysis is achieved by stirring the heterogeneous mixture at room temperature for 2 hr. The now clear solution is poured Into a 2000-mL separatory funnel containing 100 mL of brine and 160 mL of water and is extracted five times with a 50 50 mixture of ether-petroleum ether (35-60°C) (1000-mL total amount). The organic phase Is washed efficiently with 20 mL of 10% hydrochloric acid, 20 mL of water, and two 10-mL portions of brine. 

The key structural feature of POST-1 - the presence of dangling pyridine groups in the channels - affords a unique opportunity to perform asymmetric heterogeneous catalysis. Thus, potentially, any base catalyzed reactions (e.g., esterification or hydrolysis) can be performed with POST-1. Moreover, chiral pores should induce a degree of enantioselectivity in the final product mixture. The catalytic activity of POST-1 in the transesterification reaction was examined. Although the reaction of 16 and ethanol in the presence of POST-1 in carbon tetrachloride produced ethyl acetate in 11% yield, little or no transesterification occured without POST-1 or with the iV-methylated POST-1 (Sect. 2.2). The post chemical modification of the pyridine groups in POST-1 proves the role of free pyridine moiety in transesterification reaction. Transesterification of ester 16 with bulkier alcohols such as isobutanol, neopentanol, and 3,3,3-triphenyl-l-propanol occurs at a much slower rate under otherwise identical reaction conditions. Such size selectivity suggests that catalysis mainly occurs in the channels. 

The most prominent cellulose ester produced on the industrial scale is cellulose acetate. The reaction is usually performed with acetic anhydride and with sulfuric acid as a catalyst. To minimize heterogeneities, acetylation is allowed to run nearly to completion, and subsequently partial ester hydrolysis is initiated by the addition of water until a desirable solubility is achieved that corresponds to a DS of about 2.5. Such higher acyl homologues as propanoyl or butanoyl exhibit more thermoplastic properties. Many specialized esters such as chiral (-)-menthyloxyacetates, furan-2-carboxylates, or crown-ether-containing acylates have been prepared on the laboratory scale and characterized by NMR spectroscopy. Various procedures have been applied, using anhydrides and acyl chlorides as acylating agents in combination with such bases as pyridine, 4-dimethylaminopyridine (DMAP), or iV,iV -carbonyldi-imidazole. The substitution pattern of cellulose acetates has also been modified by postchemical enzymatic deacetylation. Cellulose 6-tosylates have been used as activated intermediates for nucleophihc substitution to afford 6-amino-6-deoxy, 6-deoxy, or 6-deoxy-6-halo-celluloses. ... 

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