果糖低温快速热解制备糠醛的机理研究

果糖低温快速热解制备5-羟甲基糠醛(HMF)的过程中,糠醛(FF)是一种重要的副产物。通过Py-GC/MS(快速热解-气相色谱/质谱联用)实验考察果糖低温快速热解过程中FF的形成特性。结果表明,FF的产率和相对含量都随着热解温度的提高先增大后减小,并在350℃时达到最大值,最高相对峰面积含量达到11.6%。此外,通过密度泛函理论计算,研究果糖热解形成FF的四条可能途径,计算结果表明,果糖热解形成FF的最优途径为路径2,即果糖首先经历一个协同的六元环过渡态,C5-C6键断裂的同时C6位羟基上的氢与C4位的羟基发生脱水反应,脱出一分子甲醛和一分子水,生成含C4=C5双键的二氢呋喃中间体,随后C2位上的羟基与C1位上的氢通过一个四元环过渡态又脱出一分子水,生成的烯醇中间体中烯醇氢与C3位的羟基最后经历一个六元环的过渡态再脱出一分子水,最终形成FF。

Mechanistic study on low-temperature fast pyrolysis of fructose to produce furfural

In the low-temperature fast pyrolysis of fructose to produce 5-hydroxymethyl furfural (HMF), furfural (FF) is formed as an important by-product. In this work, pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) were used to reveal the FF formation mechanism from low-temperature fast pyrolysis of fructose. It was found that both the yield and the relative content of FF increased with increasing pyrolysis temperature up to 350℃, but a further increase in the pyrolysis temperature led to a decrease of the FF. The largest peak area of FF reached as high as 11.6%. Four possible pathways are proposed for production of FF from fructose based on the density functional theory (DFT) calculations. The pathway 2 was found to be the optimal route. In this way, the fructose first underwent a six-membered ring (MR) transition state. Then, the C5-C6 bond broke with a simultaneous dehydration of the H of OH at C6 and the OH at C4 to give a dihydrofuran intermediate species containing C4=C5 double bond together with formaldehyde and water. This dihydrofuran intermediate species transformed to an enol intermediate upon a further dehydration of the OH at C2 and the H at C1 through a 4-MR transition state. Finally, the enol intermediate was transformed into FF through a 6-MR transition state and another dehydration process of the OH at C3 and the enol H at C1.