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The gas-phase hydrogenation of ethyne over Pd/Al 2 O 3 is studied using the Temporal Analysis of Products (TAP) reactor, leading to valuable insights into the reaction mechanism. The exceptional time resolution of this transient technique enables to capture the hydrogenation of the carbon–carbon triple bond via the sequential addition of hydrogen. Hydrogen/deuterium exchange experiments demonstrate the dissociative and reversible adsorption of H 2 on palladium. Conversion and selectivity change at early stages of the reaction as a consequence of the formation of a stable carbide layer. The alkene/alkane ratio was markedly higher in TAP compared to reported flow data, due to the mbar pressure operation of the technique. In this regime, the formation of subsurface hydrogen is unfavorable, enabling the assessment of the sole effect of reactants coverage on the product distribution, that is, in the absence of unselective hydride phases. Mechanistic aspects related to the use of carbon monoxide as a selectivity enhancer are also discussed.

Active and selective: Ceria shows a high activity and selectivity in the gas-phase hydrogenation of alkynes to olefins. This unprecedented behavior has direct impact on the purification of olefin streams and, more importantly, it opens new perspectives for exploring this fascinating oxide as a catalyst for the selective hydrogenation of other functional groups.

Partial hydrogenation of unsaturated hydrocarbons comprises an important family of reactions that are applied in many industrial sectors. Understanding the hydrogenation of polyunsatured and polyunsaturated compounds on heterogeneous catalysts at its molecular level remains a challenging milestone to fine-tune the product distribution. Our study presents a detailed mechanistic analysis of the gas phase hydrogenation of vinylacetylene (1-butene-3-yne) and valylene (2-methyl-1-butene-3-yne) over palladium and copper catalysts. These two metals are selected owing to their pronounced differences in continuous flow catalytic tests at ambient pressure. The chemoselectivity, regioselectivity, isomerization, and oligomerization patterns measured in a broad range of feed hydrogen/hydrocarbon ratios are rationalized by density functional theory simulations. An extended Bronsted–Evans–Polanyi relationship for both substrates and active metals is found for the hydrogenation processes. The identified factors that govern selectivity are similar to those identified for smaller C2 and C3 compounds and, thus, a means of systematization to other polyunsaturated compounds is opened up.

We present 100?% selectivity to p-chloroaniline through the continuous gas-phase (atmospheric pressure; T=393523 K) catalytic hydrogenation of p-chloronitrobenzene over an activated Cu?Al (molar Cu/Al ratio 3:1) hydrotalcite. The synthesis by continuous co-precipitation and application of a Cu?Ni?Al (atomic Cu/Ni/Al ratio 2.75:0.25:1) catalyst is also described. Temperature-programmed reduction in hydrogen and XPS analyses of the ternary catalyst suggest Cu?Ni interaction with no evidence of (bulk) alloy formation (based on XRD). Under the same reaction conditions, Cu?Ni?Al exhibited a higher (by up to a two-fold) hydrogenation rate when compared to Cu?Al, while the exclusive nitro-group reduction was maintained.

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 Cu Ni catalysts. Chemoselectivity, regioselectivity, full hydrogenation, and C C bond formation/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.

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 (473 K). The superior performance of the CO-modified copper catalyst (higher alkene selectivity and resistance against deactivation by fouling) was further confirmed at lower temperature (373 K). 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.

A Lindlar catalyst is a popular heterogeneous catalyst that consists of 5 wt.% palladium supported on porous calcium carbonate and treated with various forms of lead and quinoline. The additives strategically deactivate palladium sites. The catalyst is widely used for the partial hydrogenation of acetylenic compounds in organic syntheses. Alkyne reduction is stereoselective, occurring via syn addition to give the cis-alkene. Even if it has been employed for about 60 years, there is a lack of molecular level understanding of the Lindlar catalyst. We have applied density functional theory simulations to understand the structure and the nature of the interplay between the multiple chemical modifiers in the Lindlar catalyst. Indeed, the poisons influence different parameters controlling selectivity to the alkene: Pb modifies the thermodynamic factor and hinders the formation of hydrides, while quinoline isolates Pd sites thus minimizing oligomerization.

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 1 wt.% Pd/Al 2 O 3 and Density Functional Theory on Pd(1 1 1) 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 Bronsted–Evans–Polanyi relationship independently of the state of the catalyst (carbide, hydride, CO-covered) and the alkyne–alkene–alkane set (C2, C3).

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/Al 2 O 3 and Cu/SiO 2 ). The as-synthesized samples and the materials derived from calcination and reduction were characterized by XRF, XRD, TGA, TEM, N 2 adsorption, H 2 -TPR, XPS, and N 2 O pulse chemisorption. Catalytic tests were carried out in a continuous flow-reactor at ambient pressure and 423–523 K using H 2 :C 3 H 4 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 C 3 H 6 selectivity up to 80% at full C 3 H 4 conversion and stable performance in long-run tests at T ⩾ 473 K. Both activated Cu–Al hydrotalcites (this work) and Ni–Al hydrotalcites [S. Abello, D. Verboekend, B. Bridier, J. Perez-Ramirez, 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.

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)Ox 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%.

CO is widely used as selectivity enhancer in the selective hydrogenation of acetylene to ethylene over palladium catalysts in the petrochemical industry. However, conclusions on its action mechanism from experiments are controversial because of the complexity of the resulting catalytic system. Density functional theory studies over Pd(111) reveal that CO impacts several steps of the reaction mechanism: molecular adsorption of hydrocarbons, dissociative adsorption of H2, and hydrogenation barriers. The most notorious effect attributed to CO, besides partial blocking of active sites, is the diminished binding energy of the alkene to the surface. This leads to a pronounced thermodynamic selectivity mainly due to an electronic contribution that enhances the differential adsorption of the double and triple bonds. Gold nanoparticles attained similar effect without promotion. A simple design rule to identify selectivity enhancers based on the binding energies of reactants and modifier to the surface is proposed.

In partial hydrogenation of highly unsaturated compounds, high-performance heterogeneous catalysts usually consist of multi-metallic systems providing enhanced selectivity. These materials often undergo complex segregation phenomena and to understand their function, a surface-sensitive in situ methodology is crucial. Recently, we reported a novel family of ternary Cu–Ni–Fe catalysts for propyne hydrogenation with exceptional selectivity to propene. Herein, we detail our study on the surface composition and electronic state of two representative samples (Cu2.75Ni0.25Fe and Cu3Fe) using in situ X-ray photoelectron (XPS) and X-ray absorption (XAS) spectroscopies. Surface segregation phenomena during activation of the catalyst precursors (calcination and reduction) and hydrogenation reaction were evaluated. The multiple functions of nickel in the catalyst, which account for the extraordinary alkene selectivity, are unravelled.

The gas-phase hydrogenation of propyne to propene was investigated over a series of heterogeneous catalysts based on Pd (in the absence and the presence of CO), Ni, Au, Cu, and Cu-Ni. Catalytic tests in a continuous flow micro-reactor at ambient pressure and density functional theory (DFT) calculations were combined to understand complex structure-selectivity-activity relationships and their dependence on key experimental variables such as H(2)/alkyne and CO/H(2) ratios. The wide scope of our work enables the identification of similarities and differences between these systems and paves the way to design more selective catalysts.

A new generation of heterogeneous Cu-Ni-Fe catalysts with appropriate metal ratios displayed outstanding alkene selectivity in the gas-phase hydrogenation of propyne (S(C(3)H(6)) up to 100%) and ethyne (S(C(2)H(4)) up to 80%). The design was accomplished by orchestrating key functions in the catalyst: copper is the base hydrogenation metal, nickel increases the hydrogen coverage to minimize oligomerization, and iron acts as structural promoter. In addition to the largely improved alkene selectivity compared to that of the commonly applied Pd catalysts, the ternary Cu-Ni-Fe catalysts promise substantial process advantages, since they do not require CO feeding as selectivity enhancer and they yield high alkene selectivity in a broad window of H(2)/alkyne ratios. The ternary system requires higher operating temperatures compared to those for palladium.