生物油中反应性化合物对羧酸在超临界甲醇中酯化的影响

Effect of the reactive compounds in bio-oils on est erification of the contained carboxylic acids in supercritical methanol

采用乙酸(AC)、丙烯酸(AR)、乙酰丙酮(AA)、糠醛、2-甲氧基苯酚(MP)和水等组分,研究了羧酸在超临界甲醇中的酯化反应,并考察了各种组分对酯化反应的影响。通过两种羧酸单独酯化和共同酯化的特性以及水分影响的考察发现,超临界酯化过程中存在着不同酸的酯交换作用;水分对酯化反应具有明显的抑制作用,但超临界酯化时具有更高的耐水性。水分的抑制作用主要是削弱了羰基正离子的亲电能力,降低酯化反应速率,而不是因为化学反应平衡移动。超临界酯化的高温可加快反应速率,较弱的氢键环境可部分抵消羰基正离子亲电能力的削弱作用。AA和糠醛对酯化反应基本没有影响,但AA自身会被转化为丙酮和乙酸甲酯,而糠醛会发生缩醛化反应。MP对AR的酯化具有促进作用,并抑制聚合,从而可以提高酯化的转化率和选择性。

Using acetic acid （AC）, acrylic acid （AR）, acetyl acetone （AA）, furfural, 2-methoxy phenol （MP） and water as representative compounds and p-toluene sulfonic acid as catalyst, the esterification upgrading was examined in supercritical methanol and under atmosphere pressure. The comparison results of esterification and simultaneous esterification of AC and AR as well as the effect of water show that supercritical esterification is superior to normal liquid-phase esterification in view of acid removal. Furthermore, transesterification reaction could take place when simultaneously esterifying mixed acids in supercritical methanol, leading to relatively higher conversion for the acid like AR which hardly reacts with methanol at normal condition. The reason for low acid conversion in the presence of water is attributed to the retarding effect of water on esterification, rather than a result of equilibrium shifting, since water tends to weaken the electrophilicity of positive carbonyl ion. However, such effect in supercritical methanol is much insignificant because supercritical reaction not only carries out at higher reaction temperature and consequently accelerates the reaction rate, but also provides a weaker hydrogen bond circumstance. Although AA and furfural have unnoticeable effect on the esterification of organic acids, AA itself can be converted into acetone and methyl acetate via a pyrohydrolysis and esterification mechanism. Acetalization reaction of aldehyde with methanol can occur when furfural is present in the esterification. It is found that phenols such as MP can obviously promote the esterification of AR with methanol and inhibit its polymerization, as a result, enhancing the conversion and selectivity of esterification reaction.