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Abstract: Selectivity control in heterogeneously catalyzed hydrogenation of conjugated hydrocarbons (ene-yne and diene compounds) is a challenging task. Available studies on the topic mainly encircle 1,3-butadiene as the substrate and palladium as the catalyst, while more elaborated playground molecules and other metals remain largely unexplored. This study investigates the gas-phase hydrogenation of valylene (2-methyl-1-butene-3-yne) and isoprene (2-methyl-1,3-butadiene) over Pd, Pb-poisoned Pd, CO-modified Pd, Cu, Ni, and bimetallic Custs. Chemoselectivity, regioselectivity, full hydrogenation, and Crmation/scission footprints of the catalytic systems at different inlet hydrogen-to-hydrocarbon ratios and conversion degrees have been rationalized. Complementary studies of 3-methylbutyne and 1-penten-4-yne hydrogenation were carried out in order to analyze (i) the impact of isomerization on the observed mono-olefin distribution in valylene/isoprene hydrogenation and (ii) the conjugation issue in partial ene-yne hydrogenation. Our results lead to an improved understanding of hydrogenation of polyunsaturated hydrocarbons and open doors to design more selective heterogeneous catalysts and related processes for this practically important class of reactions. [Copyright &y& Elsevier]

Abstract: Alkyne hydrogenation, a widely used process in industry to purify olefin streams, comprises a prototype reaction to understand selectivity in heterogeneously catalyzed reactions. The selectivity of the reaction on palladium catalysts to the alkene, alkane, or oligomers strongly depends on the state of the (sub)surface; i.e., the occurrence of complex carbide/hydride phases. In practice, hydrogenation reactors in C2 and C3 cuts of steam crackers require continuous CO feeding in order to enhance the alkene selectivity of palladium-supported catalysts. In the present work, we have studied the impact of carbon monoxide on the formation of carbide and hydride phases as a standpoint to derive structure–performance relationships under realistic process conditions. For this purpose, catalytic tests on a standard 1wt.% Pd/Al2O3 and Density Functional Theory on Pd(111) were combined. The influence of: (i) the alkyne (ethyne and propyne), (ii) the hydrogen:alkyne ratio (1–10), (iii) the carbon monoxide:hydrogen ratio (0–0.2), and (iv) the catalyst pretreatment on the product distribution was assessed in a continuous flow fixed-bed reactor at ambient pressure. In absence of CO, subtle changes in the hydrogen:alkyne ratio generate undesired products. Carbon monoxide enables the external control of the catalyst state by suppressing the formation of subsurface hydride and carbide phases, thereby stabilizing a high alkene yield in a broad range of feed hydrogen:alkyne ratios. This scenario contrasts with the more fragile regime of the hydride–carbide phases under CO-free conditions. DFT calculations obtained a single Brønsted–Evans–Polanyi relationship independently of the state of the catalyst (carbide, hydride, CO-covered) and the alkyne–alkene–alkane set (C2, C3). [Copyright &y& Elsevier]

Abstract: The partial hydrogenation of propyne was studied over copper-based catalysts derived from Cu–Al hydrotalcite and malachite precursors and compared with supported systems (Cu/Al2O3 and Cu/SiO2). The as-synthesized samples and the materials derived from calcination and reduction were characterized by XRF, XRD, TGA, TEM, N2 adsorption, H2-TPR, XPS, and N2O pulse chemisorption. Catalytic tests were carried out in a continuous flow-reactor at ambient pressure and 423–523K using H2:C3H4 ratios of 1–12 and were complemented by operando DRIFTS experiments. The propyne conversion and propene selectivity correlated with the copper dispersion, which varied with the type of precursor or support and the calcination and reduction temperatures. The highest exposed copper surface was attained on hydrotalcite-derived catalysts, which displayed C3H6 selectivity up to 80% at full C3H4 conversion and stable performance in long-run tests at T ⩾473K. Both activated Cu–Al hydrotalcites (this work) and Ni–Al hydrotalcites [S. Abelló, D. Verboekend, B. Bridier, J. Pérez-Ramírez, J. Catal. 259 (2008) 85] exhibited a relatively high alkene selectivity under optimal operation conditions, but they present a markedly distinctive catalytic behavior with respect to temperature and hydrogen-to-alkyne ratio. The product distribution was assigned through Density Functional Theory (DFT) simulations to the different stability of subsurface phases (carbides, hydrides) and the energies and barriers for the competing reaction mechanisms. The behavior of Cu in partial alkyne hydrogenation resembles that of Au nanoparticles, while Ni is closer to Pd. [Copyright &y& Elsevier]

Abstract: DFT simulations have uncovered the origin of the highly selective character of gold nanoparticles in hydrogenation of triple bonds. This is ascribed to the better adsorption of Cedges of Au nanoparticles compared with Cthe barriers for hydrogenation of triple and double bonds on gold are comparable, selectivity is determined by the binding energy of the reactants. The situation is less favorable over palladium (commercial hydrorefining catalyst), because both Ce:glyph name="tbnd" />C bonds are adsorbed on the Pd surface. The outcome of the simulations was demonstrated experimentally in mixtures with both propyne and propylene over a Au/CeO2 catalyst, where C3H6 selectivities up to 95% were attained. Our finding opens a new path for chemoselective hydrogenation of molecules containing -yne and -ene groups on gold, with prospective application in petrochemical operations and in the fine chemical industry. [Copyright &y& Elsevier]

[Display omitted] • Impregnated ZnO/ZrO 2 catalysts rival state-of-the-art benchmarks in CO 2 hydrogenation. • Stable methanol productivity > 0.7 g h−1 g cat −1 attained in CO 2 and CO 2 + CO feeds. • Tuning content to 5 mol% Zn on high-surface m -ZrO 2 support maximized Zn dispersion. • Facile vacancy generation and retention of isolated zinc sites during reaction. • Successful translation of performance and architecture from powder to technical form. While impregnation approaches are attractive, scalable routes for preparing supported oxide catalysts, achieving competitive methanol productivities over impregnated ZnO/ZrO 2 remains challenging as they underperform state-of-the art systems with isolated zinc sites. Herein, by targeting the optimal active site structure, ZnO/ZrO 2 systems prepared by impregnation achieve stable methanol space–time yields of 0.73 g MeOH h−1 g cat −1 during CO 2 and hybrid CO-CO 2 hydrogenation at suitable conditions. Notably, controlling the catalyst formulation using 5 mol% Zn and a high surface area m -ZrO 2 support fosters high zinc dispersion. In situ electron paramagnetic resonance and operando X-ray spectroscopy studies affirm the retention of isolated zinc species and facile generation of associated oxygen vacancies during the reaction. Analysis of pore structure and composition within the shaped bodies evidences abundant mesopores and a uniform zinc distribution, ensuring similar performance when translating from powder to technical forms. This work bridges fundamental understanding with practical demonstration of ZnO/ZrO 2 systems. [ABSTRACT FROM AUTHOR]

The hydrolytic hydrogenation of hemicellulose arabinogalactan was investigated in the presence of protonic and Ru (1–5 wt.%)-modified USY zeolites (Si/Al ratio = 15 and 30). The use of the purely acidic materials was effective in depolymerizing the macromolecule into free sugars. While the latter partly dehydrated into 5-hydroxymethylfurfural and furfural, the generation of high molecular-weight compounds (aggregates of sugars and humins) was not favored, in contrast to previous evidences over beta zeolites. Application of the bifunctional Ru/USY catalyst, comprising well-dispersed metallic nanoparticles on the aluminosilicate support, resulted in the formation of galactitol and arabitol, in the suppression of dehydration side products, and further inhibition of polymerization reactions, which only yielded low molecular-weight oligomers. Detailed analysis of the reaction pathways as well as kinetic modeling of hydrolytic hydrogenation was performed with an advanced reaction mechanism. [ABSTRACT FROM AUTHOR]

CeO 2 is a highly selective catalyst for the partial hydrogenation of alkynes. However, due to its limited H 2 splitting ability, a high operating temperature is required for the reaction, hampering the practical exploitation of this abundant oxide. In this work, we demonstrate that gallium promotes the activity of CeO 2 for the semi-hydrogenation of acetylene and methylacetylene, enabling a reduction of the operating temperature to 373 K, while maintaining an outstanding ethylene and propylene selectivity (80–97%), even in the presence of excess alkene in the feed. Oligomers comprised the main secondary product, while the selectivity to the corresponding alkane did not exceed 2%. The characterization of mixed Ce-Ga oxides reveals that the progressive incorporation of gallium into the ceria structure, forming a solid solution, boosts the oxygen storage capacity and the reducibility of the material. This is ascribed to the facilitated H 2 activation on the Ga-promoted samples, as confirmed by in situ infrared spectroscopy and density functional theory simulations. The interplay between the advantage brought by the decreased barrier for H 2 cleavage and the disadvantage due to the increased number of oxygen vacancies governs the reactivity of the CeGaO x catalysts in alkyne hydrogenation. The composition in the optimal catalyst, containing a molar Ce:Ga ratio of 95:5, was extrapolated to other trivalent cations. Indium incorporated in the ceria lattice favored the low-temperature H 2 activation and led to an activity enhancement that surpassed that of gallium. However, aluminum did not form a solid solution with ceria and caused no effect. This work comprises the first application of promoted cerias in hydrogenation catalysis and may prompt further developments in olefin purification. [ABSTRACT FROM AUTHOR]

Abstract: Temporary supply of CO during propyne hydrogenation over a copper-based catalyst derived from a Cu–Al hydrotalcite permanently increased the alkene selectivity from 60% to 92% at 100% alkyne conversion (473K). The superior performance of the CO-modified copper catalyst (higher alkene selectivity and resistance against deactivation by fouling) was further confirmed at lower temperature (373K). Our catalytic results suggested that carbon monoxide in the presence of propyne and hydrogen irreversibly restructures the catalyst surface leading to a smaller copper ensemble that minimizes C–C coupling. Consequently, the enhanced alkene production in the CO-modified catalyst was coupled to a decreased oligomer production and very little propane production. The pretreatment mixture, feed CO concentration, and type of alkyne are important aspects to attain a selective copper catalyst. The CO effect in alkyne hydrogenation over copper radically differs from that over palladium or nickel. On the latter two metals, the increased alkene selectivity in the presence of CO is mostly due to a reduction in the over-hydrogenation channel. In addition, the effect is fully reversible, that is, it vanishes on removing CO from the feed. The permanent modification of the copper ensemble by CO might be advantageous for (chemo) selective hydrogenations of high acetylenic substrates. [Copyright &y& Elsevier]

The gas-phase hydrogenation of ethyne, propyne, and propadiene was investigated over partially reduced Ni–Al mixed oxides derived from takovite, a hydrotalcite-type compound. The unique attributes of the hydrotalcite route leads to more active and selective catalysts compared to conventional Ni/Al2O3 prepared by impregnation. Tuning calcination and reduction conditions of the catalyst precursor is essential to optimize the hydrogenation performance. The best catalyst, calcined and reduced at 773 K, rendered stable propene yields up to ca. 65% and consisted of a Ni(Al)O x solid solution with 55% of the total bulk nickel in reduced form and surface enrichment by aluminum. Sintering of NiO and crystallization of NiAl2O4 at high calcination temperature induce lower activity. The alkyne or diene conversion increases with the percentage of metallic Ni in the samples, while an optimal degree of nickel reduction maximizes the monoalkene selectivity. Below the optimum, oligomer formation is favored and above the optimum, alkane production increases. A similar pattern was found for the H2/HC ratio. The alkene selectivity experiences a dramatic increase in early stages of the reaction, which correlated with the build-up of C-containing species on the catalyst (sub-)surface. These selectivity-enhancing species are formed at specific reaction temperatures, highlighting the relevance of the testing procedure on assessing hydrogenation catalysts. The catalytic performance is strongly influenced by the hydrocarbon substrate. In contrast to propyne and propadiene, ethyne hydrogenation led to a C2H4 yield of only 6%. [Copyright &y& Elsevier]