Stable catalysis of neutral silica-supported potassium lactate for vapour-phase dehydration of lactic acid to acrylic acid – critical role of the support

Abstract

We have deeply investigated KNO3-derived silica-supported potassium lactate catalysts for vapour-phase dehydration of lactic acid (LA) to acrylic acid (AA) by catalytic testing, IR spectroscopic monitoring, ammonia temperature-programmed desorption, isopropyl...

On the catalytic vapor-phase dehydration of lactic acid to acrylic acid: a systematic review

“…During an LA dehydration reaction, the catalytic stability may be strongly related to the chemical stability of lactate salt as the reaction intermediate or catalytic active species. 22,23 An aqueous 20% LA solution is rather acidic. Its pH or H 0 is estimated to be 1.7 based on pK a = 3.86 at 25 °C.…”

“…Under such an acidic condition, unsupported lactate salt is demonstrated to tend to dehydrate to acrylate salt, which in turn readily polymerizes as follows. 23 n…”

“…In contrast, silica or SiO 2 -Al 2 O 3supported lactate salt is assumed to dehydrate with difficulty because of the interaction between the OH group of the lactate salt and silica or SiO 2 -Al 2 O 3 . 22,23 The good chemical stability of silica or SiO 2 -Al 2 O 3 -supported lactate salt can ensure good catalytic stability. Accordingly, it is advisable that the poor catalytic stability encountered in these studies could get improved by supporting these sulfate salts on inorganic materials such as silica and SiO 2 -Al 2 O 3 .…”

“…21 The reason why the SiO 2 (LSA)supported sodium lactate is unstable is probably that the low surface area of SiO 2 (LSA) limits the interaction between sodium salt and the SiO 2 (LSA) surface so that the in situ generated supported sodium lactate fails to stabilize under the acidic reaction conditions. 23 Under the acidic condition…”

“…When lactate salt dehydration and hydrolysis get controlled in rate and lactate salt dehydration becomes the rate-limiting step in the catalytic process of LA dehydration to AA, it is assumed that resultant acrylate salt timely displaces with excess LA to produce AA and regenerate lactate salt without acrylate salt polymerization, which can secure the content and stability of lactate salt during reaction. 23 It is the case with either the neutral KNO 3 /silica or slightly acidic KNO 3 /SiO 2 -Al 2 O 3 (36) or NaNO 3 /SiO 2 -Al 2 O 3 (36). 22,23 However, the stabilization of the supported lactate salt is affected by the surface acidity.…”