(510a) First Principles Analysis of the Selective Hydrogenolysis of Tetrahydrofurfuryl Alcohol Over Rh and ReOx Modified Rh Surfaces

Tetrahydrofurfuryl alcohol (HMTHF), derived  from the acid dehydration of xylose, can be converted over supported metal catalysts to either 1,2-pentanediol or 1,5-pentanediol, important chemical intermediates in polymer industry.  Recent experimental results show that the hydrogenolysis of HMTHF over supported rhodium particles predominantly produces 1, 2-pentanediol whereas the addition of ReOx to Rh results in 95% selectivity to 1,5-pentanediol. Herein we use ab initio density functional theoretical calculations to explore the differences reported over Rh, ReOx and ReOx modified Rh surfaces.

The results indicate that the hydrogenolysis of HMTHF over Rh as well as other metal surfaces take place via metal catalyzed ring opening paths. The activation predominantly proceeds at the C-O bonds comprised of secondary C atoms thus resulting in the formation  1, 2-pentanediol.  The activation of the C-O bond at the more substituted tertiary carbon atom, on the other hand, results in the formation of 1, 5-pentanediol. The activation of the C-O bond appears to proceed via the sequential activation of vicinal C-H bonds followed by the dissociation of the C-O bond to form an oxymetallocycle intermediate.   The barrier to break the C-O bond at a secondary carbon atom appears to be linearly correlated to the binding energy of the CH_x intermediate on the metal surface.  The  barrier to break the C-O bond at a tertiary carbon is about 0.20 eV higher than the barrier to break at the secondary carbon due to the steric effects of the side chain on the tertiary carbon. Higher hydrogen surface coverages that likely form under reaction conditions result in higher activation barriers at both the secondary and tertiary C-O carbon atoms. The selectivity, however, remained about the same.  These results which suggest that 1, 2-pentanediol is the predominant product over Rh are in very good agreement with those found experimentally.

The catalytic conversion of HMTHF over a ReOx modified Rh substrate is rather different than that over Rh as the addition of ReOx results in the formation of acid sites and bifunctional hydrogenolysis of HMTHF. The acid sites aid in the activation of the C-O bonds comprised of tertiary rather than secondary character as they result in the formation of the more stable secondary carbenium ions as opposed to the primary carbenium ions.   The activation barrier for C-O bond cleavage at a secondary carbon is 0.40 eV higher than over the tertiary carbon, due to the lower stability of the primary carbenium  ion that forms C-O bond. The results also show that the hydroxyl group at α-position in HMTHF help to stabilize carbenium ion formation by forming a more stable  transition state structure which lowers the activation barrier at the secondary C-O bond.  The presence of water further stabilizes the secondary carbenium transition states.   The acid-catalyzed mechanism is consistent with the experimental results which show the selective activation of the secondary C-O bond to form 1, 5-pentanediol on the ReO_x modified Rh catalyst.