Transesterification of triacetin with methanol on Nafion® acid resins

被引:225
作者
Lopez, Dora E. [1 ]
Goodwin, James G., Jr. [1 ]
Bruce, David A. [1 ]
机构
[1] Clemson Univ, Dept Chem & Biomol Engn, Clemson, SC 29634 USA
关键词
biodiesel; triacetin; methanol; transesterification; sulfonic acid resins; Nation (R) SAC-13; Nafion (R) NR50; polymer swelling; H2SO4;
D O I
10.1016/j.jcat.2006.10.027
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
Although homogeneous alkali catalysts (e.g., NaOH) are commonly used to produce biodiesel by transesterification of triglycerides (vegetable oils and animal fats) and methanol, solid acid catalysts, such as acidic resins, are attractive alternatives because they are easy to separate and recover from the product mixture and also show significant activity in the presence of fatty acid impurities, which are common in low-cost feedstocks. To better understand solid acid catalyst performance, a fundamental transesterification kinetic study was carried out using triacetin and methanol on Nafion (R) (perfluorinated-based ion-exchange resin) catalysts. In particular, Nation (R) SAC-13 (silica-supported Nafion) and Nation (R) NR50 (unsupported Nation) were investigated, because both show great promise for biodiesel-forming reactions. The reaction kinetics for a common homogeneous acid catalyst (H2SO4) were also determined for comparison. Liquid-phase reaction was performed at 60 degrees C using a stirred batch reactor. The swelling properties of the resin in solvents of diverse polarity that reflect solutions typically present in a biodiesel synthesis mixture were examined. The initial reaction rate was greatly affected by the extent of swelling of the resin, where, as expected, a greater effect was observed for Nafion (R) NR50 than for the highly dispersed Nafion (R) SAC-13. The reaction orders for triacetin and methanol on Nation (R) SAC-13 were 0.90 and 0.88, respectively, similar to the reaction orders determined for H2SO4 (1.02 and 1.00, respectively). The apparent activation energy for the conversion of triacetin to diacetin was 48.5 kJ/mol for Nafion (R) SAC-13, comparable to that for H2SO4 (46.1 kJ/mol). Selective poisoning of the Bronsted acid sites on Nafion (R) SAC-13 using pyridine before transesterification revealed that only one site was involved in the rate-limiting step. These results suggest that reaction catalyzed by the ion-exchange resin can be considered to follow a mechanism similar to that of the homogeneous catalyzed one, where protonated triglyceride (on the catalyst surface) reaction with methanol is the rate-limiting step. (c) 2006 Elsevier Inc. All rights reserved.
引用
收藏
页码:381 / 391
页数:11
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