Fundamental Research On Green Synthesis And Applications Of Glycerol Cyclic Acetals And Their Derivatives

Glycerol-based cyclic acetals,obtained from acetalization of glycerol with carbonyl compounds（e.g.,ketones or aldehydes）,are value-added fine chemicals that can be used as additives for diesel fuels,solvents,lubricating oil additives and as building blocks for surfactants.It is noteworthy that glycerol acetals,produced from acetalization of glycerol with fatty aldehydes,are useful hydrophobic intermediates for synthesis of cleavable surfactants.Due to the difference in polarity between highly viscous,hydrophilic glycerol and fatty aldehydes,the reaction medium is rendered biphasic,leading to limitations of mass transfer and failure of the reaction.To circumvent this issue,aromatic non-polar solvents（e.g.,benzene and toluene）are preferred as solvents and dehydrating agents.However,the results obtained are not satisfactory,which to some extent limits the further use of glycerol cyclic acetals.From the concept and principles of green chemistry,development of solvent-free processes and use of green and safe solvents in synthetic processes appear to be ideal cases for development of green chemistry and achievement of atomic economy.In this paper,the mild,green and high-efficient synthesis of glycerol cyclic acetals was realized via changing the solvent or reaction conditions,and the applications of glycerol cyclic acetals and their derivatives were further expanded in novel cleavable surfactants and bio-lubricants.Below are the main findings:1.Based on the concept and principles of green chemistry,the mild acetalization reaction of glycerol with n-octanal was realized in the absence of a solvent in this paper,which tackled the problem of low yield of the target product induced by immiscible reagents.The composition and structure of the synthesized products were characterized,indicating that the products were mixtures of cis-and trans-1,3-dioxolanes and cis-and trans-1,3-dioxanes.Different homogeneous acid catalysts were investigated in this reaction and p-dodecylbenzenesulfonic acid（DBSA）displayed superior catalytic performance and was chosen as catalyst in afterward reactions.The effects of reaction variables,including catalyst loading,reaction time,reaction temperature and molar ratio of reagents on acetalization reaction were investigated to optimize the reaction conditions,in which the distributions of the five-membered cyclic acetals and six-membered cyclic acetals were found to be dependent on reaction temperature and time.The optimum reaction conditions were obtained as follows:a reaction temperature of 50°C,a catalyst loading of0.4 mol%,a reaction time of 4 h,and a glycerol/n-octanal molar ratio of 1.1:1.Under the optimum condition,the yield of n-octanal glycerol acetal reached 98.8%and the selectivity in terms of the distribution of the four regioisomers,namely cis-1,3-dioxolane,trans-1,3-dioxolane,cis-1,3-dioxane,trans-1,3-dioxane,was found to be in proportions of 34:30:20:16.2.Water is considered as an ideal green solvent with many excellent properties.In this paper,green and efficient acetalization of glycerol with lauraldehyde was developed via using water as solvent,instead of benzene and toluene used in previous studies.The prepared lauraldehyde glycerol acetal was identified as a mixture of four regioisomers.The reaction parameters with p-dodecylbenzenesulfonic acid（DBSA）as catalyst were subsequently optimized in terms of the yield and isomer distribution of lauraldehyde glycerol acetal.Under the optimum operating conditions,namely,a reaction temperature of120°C,a catalyst loading of 0.6 mol%,a reaction time of 60 min,and a glycerol/lauraldehyde molar ratio of 1.1:1,the yield of lauraldehyde glycerol acetal reached 96.2%and the selectivity in terms of the distribution of the four regioisomers,namely cis-1,3-dioxolane,trans-1,3-dioxolane,cis-1,3-dioxane,trans-1,3-dioxane was found to be in proportions of 27:22:28:23.3.Dehydration acetalization of diglycerol with lauraldehyde for synthesis of diglycerol diacetal in the presence of homogeneous acid catalysts using water as solvent has been studied for the first time.The prepared double long-chain diglycerol diacetal was identified as a mixture of six regioisomers.p-Dodecylbenzenesulfonic acid（DBSA）exhibited excellent activity for this acetalization reaction.The reaction parameters with p-dodecylbenzenesulfonic acid catalyst were subsequently optimized in terms of the yield and isomer distribution of diglycerol diacetal.Under the optimum operating conditions,namely,a reaction temperature of 120°C,a catalyst loading of 0.5 mol%,a reaction time of 2 h,and a diglycerol/lauraldehyde molar ratio of 1:1,the yield of diglycerol diacetal reached 93.9%and the selectivity in terms of the distribution of the six regioisomers was found to be in proportions of24:36:18:11:3:8.4.Glycerol cyclic acetals are important hydrophobic intermediates for synthesis of cleavable surfactants.A new group of cleavable di-anionic surfactants disodium aliphatic glycerol acetal sulfosuccinates have been successfully synthesized starting from esterification reaction of aliphatic glycerol acetals with maleic anhydride,followed by sulfonation with sodium sulfite.The prepared surfactants were characterized by fourier transform infrared spectroscopy（FT-IR）,electrospray ionization mass spectrometry（ESI-MS）and elemental analysis.The surface active properties of the synthesized surfactants were determined in terms of surface tension,critical micelle concentration（CMC）and foaming property.The lauraldehyde glycerol acetal disodium sulfosuccinate could induce a strong decrease of the surface tension of water to a plateau value of about 30 mN/m at 25°C and 28 mN/m at40°C,respectively,and its CMC value（0.11 g/L）is low enough for surfactant applications.5.The tribological performance of diglycerol diacetal was evaluated as potential base oil by means of an Optimol SRV-V oscillating reciprocating friction and wear tester in comparison with commonly used poly-alpha-olefin（PAO 8）.Surprisingly,the tribological test results revealed that diglycerol diacetal displayed excellent lubrication properties.The load-carrying capacity could reach 200 N;the mean friction coefficient was about 0.093 and the value of wear volume of lower disks was 3.19×10^-4 mm³ under lubrication of diglycerol diacetal as neat base oil.Combined with 0.50 wt%T321,the applied load for diglycerol diacetal could be increased to 450 N.The dynamic friction coeffcient curve showed a stable value of about 0.087,and the value of wear volume of lower disks was 2.51×10^-4 mm³.Accordingly,diglycerol diacetal shows superior friction-reducing and anti-wear properties to those of PAO 8 and may thus be good potential lubricating oil for use under practical working conditions.