Insights into Dehydration of 1,2,6-Hexanetriol over Model Silica-Alumina Surfaces

Increasing interests in the use of biorenewables feedstocks as alternatives to petroleum sources have promoted many of the recent studies in the literature on the dehydration, hydrogenolysis, decarbonylation and decarboxylation pathways of oxygenated intermediates derived from biomass. The selective dehydration and functionalization of polyols such as 1,2,6-hexanetriol to α,ω-diols are important paths that may enable the production of diacids, diamines, and lactones from renewable sources. Herein we use first-principle theoretical calculations to examine the reaction paths and mechanism involved in the dehydration of 1,2,6-hexanetriol over model silica alumina catalytic surfaces. More specifically, we carry out density function theory (DFT) calculations to examine the reaction pathways and elementary steps in the dehydration of 1,2,6-hexanetriol over a model 2D zeolite structures. Experimental results reported by Nolan and Shanks at ISU show significantly higher selectivities for the conversion of 1,2,6-hexanetriol to THP-2-M than to the more desired 6-hydroxyhexanal product. The results from theoretical calculations indicate that the intrinsic activation barriers for the formation of THP-2-M is 72 kJ/mol whereas the barrier to form 6-hydroxyhexanal is 84 kJ/mol (12 kJ/mol higher) which is consistent with the much higher selectivities to the THP-2-M product reported experimentally. The reaction of 1,2,6-hexanetriol to the THP-2-M proceeds via the protonation and elimination of the OH group at the 2-position of the triol to form a secondary carbenium ion that is stabilized by the direct interaction with the primary OH group at the 6-position to form a stable 6-member ring carbenium ion intermediate. The reaction of 1,2,6-hexanetriol to form the 6-hydroxyhexanal product also proceeds via the protonation and elimination of the OH group at the 2-position. The secondary carbenium ion that forms, however, is now stabilized by a hydride transfer from the 1- to the 2- position on the triol in order to localize the charge on the terminal OH group.

DFT calculations were carried out on all of the other viable dehydration paths including the dehydration of the terminal OH groups at the 1- and 6-positions. The dehydration barriers at the primary 1- and 6- positions of 1,2,6-hexanetriol were all calculated to be at least 9 kJ/mol higher than those at the secondary position. The theoretical results also showed that 1,6 hexanediol can be formed via the dehydration of the primary OH groups along with an OH shift from the secondary carbon to the terminal carbon but requires significantly higher activation barriers. The main paths to THP-2-M and 6-hydroxyhexanal products were thus concluded to occur via the dehydration of the secondary OH intermediate. Theoretical results also indicate that the production of oxepan-3-ol is also possible via the dehydration of both of the terminal 1- and 6- OH species to form oxepan-3-ol with a barrier of 88 kJ/mol which is 16 kJ/mol higher than that to form THP-2-M.

Finally, the ring opening of THP-2-M to hexanal was calculated to be 130 kJ/mol which indicates that THP-2-M is quite stable and does not react further which is consistent with the high selectivities measured experimentally. The mechanistic insights gleaned from this work help to advance our knowledge for the dehydration of polyols found in most biorenewables feeds and can be used to help to provide a more general understanding of the dehydrogenation reaction of polyols over acid catalysts.