Theoretical investigation of reaction pathways in the selective hydrogenation of maleic acid to tetrahydrofuran over palladium-rhenium surfaces

Tetrahydrofuran (THF) and 1,4-butanediol (BDO) are important intermediates in the manufacture of polyurethane elastomers and spandex fibers such as Lycra. Commercially, in DuPont's patented process, THF and BDO are produced by the hydrogenation of maleic acid over supported Pd-Re or Ru-Re catalysts. The first step of the hydrogenation reaction involves the saturation of the C=C group of maleic acid to form succinic acid. Subsequently, the two carboxylic acid groups of succinic acid undergo hydrogenolysis to form BDO and THF. Deleterious decomposition reactions result in the formation of straight chain alcohols and acids such as n-butyric acid, n-butanol, n-propanoic acid and n-propanol. While previous experimental studies have established the global pathways for maleic acid hydrogenation to THF, there is still limited understanding of the intrinsic reaction mechanism over a transition metal based catalyst. Also, there is no published work in the literature examining the detailed pathways for decomposition during maleic acid hydrogenation. Developing a fundamental knowledge of the elementary mechanism for maleic acid hydrogenation, and analyzing the decomposition routes is likely to aid in the development of catalytic formulations with improved activity and selectivity.;Towards this goal, we have used density functional theory (DFT) quantum chemical calculations to elucidate the elementary mechanism for the hydrogenation of maleic acid to THF and BDO over Pd-Re surfaces. Reaction coordinate calculations are used to isolate transition states for specific steps that are likely to be kinetically significant along the reaction pathway. Structure-property relationships are developed by correlating the intrinsic barrier for these rate-controlling steps to the fundamental electronic properties of the metal surface. Our study indicates that the hydrogenolysis of carboxylic acids most likely involve rate-determining C-O bond breaking and C-H bond formation steps. The decomposition pathways to the straight chain acid and alcohols are most likely initiated by rate controlling C-H bond activation steps. Our DFT calculations on various Pd-Re surfaces demonstrate that the bond dissociation reactions such as C-O hydrogenolysis and C-H bond breaking are more favored over reactive metals like Re. On the other hand, coupling reactions such as C-H bond formation have lower intrinsic barriers on Pd. This is primarily because metals like rhenium have more vacancies in the d-band and form stronger metal-adsorbate bonds, thus favoring adsorbate-adsorbate bond breaking.;For the maleic acid hydrogenation reaction, our results suggest that the monometallic Pd system should show very poor activity for acid hydrogenolysis, but also exhibit very little acid decomposition. On the other hand, monometallic Re should most likely favor both acid hydrogenolysis and acid decomposition, thus resulting in an overall poor selectivity. The results are consistent with experimental observation. The Pd-Re bimetallic alloyed system appears to offer an optimal balance between bond-breaking and bond-formation activity necessary for the selective hydrogenolysis of maleic acid to BDO and THF.