Effects of phosphorus on bond rupture during hydrodeoxygenation and dehydrogenation reactions on ruthenium

Heterogeneous catalyst;Transition metal phosphide;Surface science;Bio-oil hydrodeoxygenation;Light alkane dehydrogenation;Carboxylic acid;Cyclohexene

Chang, SiWei

关键词: Heterogeneous catalyst;    Transition metal phosphide;    Surface science;    Bio-oil hydrodeoxygenation;    Light alkane dehydrogenation;    Carboxylic acid;    Cyclohexene;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/99325/CHANG-DISSERTATION-2017.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Transition metal phosphide (TMP) catalysts are selective and active towards C-O bond rupture during hydrodeoxygenation (HDO) of oxygenates, making them potential candidates for bio-oil upgrading. However, the mechanism by which the C-X (X = O, C, H) bond ruptures and the corresponding intrinsic barriers (i.e., for C-H, C-C, and C-O bond rupture) between transition metals and TMP catalysts are not well understood. Here, we synthesized and characterized a phosphorus (P) modified Ru(0001) surface by annealing Ru(0001) in the presence of PH3 gas under ultra-high vacuum conditions to produce Px-Ru(0001) (x is the ratio of P atoms to Ru atoms). The P0.43-Ru(0001) surface has a √7 x √7 low energy electron diffraction pattern, which is structurally similar to the (111) facet of well-characterized bulk Ru2P. Temperature programmed desorption measurements of CO and NH3 showed that the addition of P atoms decrease the binding energy of CO by up to 30 kJ mol-1 and NH3 by 14 kJ mol-1 as compared to pristine Ru(0001). This suggests that P atoms decrease the extent of electron exchange between Ru surfaces and adsorbates. We examined the decomposition of C1-C4 carboxylic acid (e.g., formic acid (FA), acetic acid (AA), propionic acid (PA), and butyric acid (BA)) on pristine Ru(0001) and P0.4-Ru(0001) surfaces. Temperature programmed reaction (TPR) and reactive molecular beam scattering (RMBS) experiments were used to determine bond rupture barriers and selectivity of C-O bond rupture compared to C-H/C-C bond rupture. The TPR results showed that longer alkyl carbon chains can promote self-stabilizing lateral interactions between carboxylates via dispersive (van der Waals) interactions, which is evidenced by increases in intrinsic activation energy (Ea) (1-5 kJ mol-1) for R-COOH bond rupture. RMBS of FA demonstrated that apparent activation energies (Eapp) of dehydration and dehydrogenation are greater on P0.43-Ru(0001) by 27 kJ mol-1 and 33 kJ mol-1, respectively, compared to Ru at temperatures greater than 500 K. Additionally, FA decomposition over P0.43-Ru(0001) is more selective toward C-O bond rupture than C-H bond rupture. Moreover, the addition of P atoms to Ru(0001) increases Ea values for all bonds (i.e., C-O, C-H and C-C bonds) by 5-50 kJ mol-1, which suggests that P-atoms decrease Ru surface electron back donation toward all adsorbates and changes the product selectivity by increasing energy barriers for C-O bond rupture more than C-H/C-C bond rupture. Collectively, these data and interpretations led to a proposed a set of elementary steps for carboxylic acid decomposition over Px-Ru(0001) and Ru(0001) surfaces. The results may provide guidance for the design of more selective P-modified transition metal catalysts to surgically cleave C-O bonds and convert biomass derived intermediates into platform chemicals and fuels. TMP catalysts are also active for alkane dehydrogenation, which is especially useful given the recent discovery of large shale-gas reserves. Here, we studied the effect of P atom on light alkane dehydrogenation and coke formation (i.e., a problem common to dehydrogenation catalysts) using Ru(0001). Cyclohexene was used as a probe molecule, and the RMBS of cyclohexene demonstrated that the addition of P atoms enhances the selectivity of cyclohexene dehydrogenation relative to cyclohexene decomposition (i.e., coking) by a factor great than 10 when compared to selectivities on Ru(0001). The change in dehydrogenation selectivity is caused by P atoms decreasing the Eapp for benzene formation by 12 kJ mol-1 while increasing Eapp for coking by 11 kJ mol-1. Additionally, TPR experiments showed that the addition of P atoms to Ru(0001) minimized cyclohexene coking when compared to Ru(0001). Thus, this work shows that TMP catalysts possess enormous potential for use as selective dehydrogenation catalysts.