Hydrogenation Of Propene Research Articles

Silica-Supported, Single-Site Sc and Y Alkyls for Catalytic Hydrogenation of Propylene

Single-site Sc and Y on silica catalysts have been prepared by aqueous and organometallic grafting methods. The former yields Y(III) ions with five bonds at an average bond distance of 2.31 A by X-ray absorption spectroscopy. Although the aqueous synthesis gave single-site Y with low coordination number, these were not catalytic for alkane dehydrogenation or olefin hydrogenation. Single-site Sc(III) and Y(III) species were also prepared by grafting Sc(CH2Si(CH3)3)3(THF)2 and Y(CH2Si(CH3)3)3(THF)2, respectively, and these are catalysts for olefin hydrogenation at temperatures from about 60 to 100 °C; however, they were thermally unstable at higher temperatures necessary for alkane dehydrogenation. The structure of the grafted Y complex was determined by X-ray absorption spectroscopy, IR, and NMR. Grafting led to protonolysis of two of the three CH2Si(CH3)3 ligands. Additionally, there was loss of one THF ligand. The EXAFS indicated that there were four Y–ligand bonds in the surface species, two at 2.16 A a...

Parahydrogen-Induced Hyperpolarization inside Meso- and Micropores of Pt-, Rh-, Ir-, and Pd-Containing Solid Catalysts

Homologous series of solid catalysts (silica, SBA-15, dealuminated zeolite DeA–Y, zeolite H,Na–Y) with different pore sizes and loaded with different noble metals (Pt, Rh, Ir, Pd) were applied for the gas phase hydrogenation of propene with para-enriched hydrogen (p-H2) with the aim to produce parahydrogen-induced polarization (PHIP). This hyperpolarization formed by the pairwise incorporation of p-H2 into propene was studied via in situ 1H MAS NMR spectroscopy under continuous-flow conditions. By evaluating the characteristic 1H NMR antiphase signals of the hyperpolarized reaction products, the experimental parameters and properties of noble-metal-containing porous catalysts were investigated, which affect the formation and relaxation of PHIP. For the catalysts under study, small pore diameters and interactions of the hyperpolarized product molecules with nuclei inside the pores, such as framework aluminum atoms and extra-framework sodium cations, were found to enhance the relaxation of the hyperpolariza...

2D Mapping of NMR Signal Enhancement and Relaxation for Heterogeneously Hyperpolarized Propane Gas

Hyperpolarized (HP) propane is a promising contrast agent for magnetic resonance imaging (MRI) of lungs and for studying porous media. The parahydrogen-induced polarization (PHIP) technique is a convenient approach to produce pure propane gas with enhanced proton polarization, when hydrogenation of propylene with parahydrogen is performed over heterogeneous catalysts. Here, we present a new approach of multidimensional mapping of the efficiency of pairwise parahydrogen addition using PHIP-echo pulse sequence. We use this approach to study the performance of three model heterogeneous Rh/TiO2 catalysts in the production of HP propane gas. The three catalysts with 1.0, 13.7, and 23.2 wt % of supported rhodium nanoparticles have been characterized by X-ray photoelectron spectroscopy (XPS) and high resolution transition electron microscopy (HRTEM). By varying the fractions of parahydrogen and propylene in the reactant mixture as well as the gas mixture pressure, 2D maps of PHIP-echo NMR signal and 2D maps of H...

Separation of Anti-Phase Signals Due to Parahydrogen Induced Polarization via 2D Nutation NMR Spectroscopy.

The present work introduces a novel method for the selective detection of 1 H NMR anti-phase signals caused by the pairwise incorporation of parahydrogen into olefins on noble-metal-containing catalysts. Via a two-dimensional (2D) nutation NMR experiment, the anti-phase signals of hyperpolarized 1 H nuclei are separated due to their double nutation frequency compared to that of thermally polarized 1 H nuclei. For demonstrating this approach, parahydrogen induced polarization (PHIP) was achieved via the hydrogenation of propene with parahydrogen on platinum-containing silica and investigated by in situ 1 H MAS NMR spectroscopy under continuous-flow conditions, that is, the hydrogenation reaction was performed inside the magnet of the NMR spectrometer. The 2D nutation NMR experiment described in the present work is useful for the separation of overlapping anti-phase and in-phase signals due to hyperpolarized and thermally polarized 1 H nuclei, respectively, which is important for research in the field of heterogeneous catalysis.

Effect of the flow pattern on catalytic reaction of propylene hydrogenation through structured catalyst bed: Mathematical modeling and experiment observations

This novel study is experimental and modeling. In experimental section a pilot package including trickle bed reactor, automatic dosing pump, measuring instruments, gas chromatograph apparatus and five sample points along the reactor were employed to investigate the performance of propylene hydrogenation through structured catalyst bed under concurrent and countercurrent flow patterns. The experimental setup was configured in two different modes providing countercurrent and concurrent pattern for liquid phase and gas phase flow. First liquid propane containing propylene impurity flew from top of the reactor and hydrogen was injected from bottom as a gas phase counter-currently thereafter liquid and gas contacting phases both flew from bottom side of the reactor concurrently. Propylene content of liquid propane was hydrogenated through structured catalyst bed filling the pilot reactor and sampling has been done after establishing steady state conditions. In modeling a set of 12 partial differential equations (PDEs) were developed for dynamic modeling of current process including mass transfer and momentum transfer equations in liquid and gas phase. Afterward orthogonal collocation method was utilized to solve developed PDEs. Finally the results of mathematical modeling showed good consistency with experimental data. Furthermore, the sample analysis showed nil concentration of propylene in countercurrent flow in several operating condition, however nil concentration of propylene was not reported in concurrent flow pattern even by increasing the liquid flowrate.

Heterogeneous Metal-Free Hydrogenation over Defect-Laden Hexagonal Boron Nitride.

Catalytic hydrogenation is an important process used for the production of everything from foods to fuels. Current heterogeneous implementations of this process utilize metals as the active species. Until recently, catalytic heterogeneous hydrogenation over a metal-free solid was unknown; implementation of such a system would eliminate the health, environmental, and economic concerns associated with metal-based catalysts. Here, we report good hydrogenation rates and yields for a metal-free heterogeneous hydrogenation catalyst as well as its unique hydrogenation mechanism. Catalytic hydrogenation of olefins was achieved over defect-laden h-BN (dh-BN) in a reactor designed to maximize the defects in h-BN sheets. Good yields (>90%) and turnover frequencies (6 × 10–5–4 × 10–3) were obtained for the hydrogenation of propene, cyclohexene, 1,1-diphenylethene, (E)- and (Z)-1,2-diphenylethene, octadecene, and benzylideneacetophenone. Temperature-programmed desorption of ethene over processed h-BN indicates the formation of a highly defective structure. Solid-state NMR (SSNMR) measurements of dh-BN with high and low propene surface coverages show four different binding modes. The introduction of defects into h-BN creates regions of electronic deficiency and excess. Density functional theory calculations show that both the alkene and hydrogen-bond order are reduced over four specific defects: boron substitution for nitrogen (BN), vacancies (VB and VN), and Stone–Wales defects. SSNMR and binding-energy calculations show that VN are most likely the catalytically active sites. This work shows that catalytic sites can be introduced into a material previously thought to be catalytically inactive through the production of defects.

Single-site zinc on silica catalysts for propylene hydrogenation and propane dehydrogenation: Synthesis and reactivity evaluation using an integrated atomic layer deposition-catalysis instrument

Alkyl-zinc and zinc oxide-type sites were synthesized via atomic layer deposition on high-surface-area silica using an integrated atomic layer deposition-catalysis instrument (I-ALD-CAT). One-cycle ALD experiments using diethylzinc (DEZ) afforded Zn/SiO2 systems that provided key insights into the reactivity and stability of Zn sites as a function of dispersion at the submonolayer level. The I-ALD-CAT tool design allowed for systematic comparison of the reactivity of different grafted zinc sites. Open-shell 16-electron, tricoordinate ethyl zinc-silica sites exhibit higher activity in propane hydrogenation-dehydrogenation compared to 18-electron, tetracoordinate zinc oxide-type centers. Silica surface saturation with Zn(II) sites (∼75% of a monolayer) results in facile zinc agglomeration and catalyst deactivation under reaction conditions. Reduced DEZ dosing coupled with thermal substrate pretreatment techniques (e.g., dehydration under vacuum) resulted in increased Zn dispersion and produced Zn/SiO2 catalysts with improved activity and stability under propylene hydrogenation (200°C) and propane dehydrogenation (550°C) conditions.

X–H Bond Activation on Cr(III),O Sites (X = R, H): Key Steps in Dehydrogenation and Hydrogenation Processes

We synthesized isolated Cr(III) sites on SiO2−Al2O3 and Al2O3 by grafting and subsequent controlled thermal treatment of Cr(OSi(OtBu)3)3(THF)2 and Cr(Al(OiPr)4)3 on alumina. CrOx/Al2O3 was obtained from incipient wetness impregnation of Al2O3 with CrO3 in H2O followed by calcination, as carried out for the synthesis of industrial Cr-based dehydrogenation catalysts. These materials were characterized by IR, EPR, XAS, and by the adsorption of the probe molecules CO and pyridine, and the results were compared to previously reported isolated Cr(III)/SiO2. All of these materials were active in propane dehydrogenation at 550 °C, where higher TOFs were obtained for Cr(III)/SiO2-Al2O3 and Cr(III)/Al2O3 than for CrOx/Al2O3 and Cr(III)/SiO2. For mechanistic studies the reverse reaction, propene hydrogenation, was studied. Here, the order of reactivity was opposite that of dehydrogenation, with CrOx/Al2O3 and Cr(III)/SiO2 being more active in hydrogenation than Cr(III)/SiO2-Al2O3 and Cr(III)/Al2O3. Kinetic analysis ...

C-H Activation on Co,O Sites: Isolated Surface Sites versus Molecular Analogs.

The activation and conversion of hydrocarbons is one of the most important challenges in chemistry. Transition-metal ions (V, Cr, Fe, Co, etc.) isolated on silica surfaces are known to catalyze such processes. The mechanisms of these processes are currently unknown but are thought to involve C-H activation as the rate-determining step. Here, we synthesize well-defined Co(II) ions on a silica surface using a metal siloxide precursor followed by thermal treatment under vacuum at 500 °C. We show that these isolated Co(II) sites are catalysts for a number of hydrocarbon conversion reactions, such as the dehydrogenation of propane, the hydrogenation of propene, and the trimerization of terminal alkynes. We then investigate the mechanisms of these processes using kinetics, kinetic isotope effects, isotopic labeling experiments, parahydrogen induced polarization (PHIP) NMR, and comparison with a molecular analog. The data are consistent with all of these reactions occurring by a common mechanism, involving heterolytic C-H or H-H activation via a 1,2 addition across a Co-O bond.

Gas-Phase Hydrogenation with Parahydrogen Over Immobilized Vaska’s Complex

Abstract Generating parahydrogen-induced polarization (PHIP) of nuclear spins with immobilized transition metal complexes as hydrogenation catalysts allows one to produce pure hyperpolarized substances, which can open new revolutionary perspectives for PHIP applications. A major drawback of immobilized complexes is their low stability under reaction conditions. In the present work we studied an immobilized iridium complex, Ir/SiO2 P, synthesized by a covalent anchoring of Vaska’s complex on phospine-modified silica gel. This complex was used to obtain hyperpolarized gasses in the gas phase hydrogenation of propene, propyne and 1-butyne with parahydrogen in PASADENA and ALTADENA experiments. It was found that, in contrast to other immobilized complexes, Ir/SiO2 P is stable under reaction conditions at up to 140°C, and the reduction of iridium does not occur according to XPS analysis. Moreover, the application of Ir/SiO2 P catalyst allowed us to generate continuous flow of hyperpolarized propene and 1-butene with (300–500)-fold NMR signal enhancement which is significantly higher than commonly observed for most supported metal catalysts. The shape of polarized propene signals in PASADENA experiment has indicated that parahydrogen addition to propyne occurs non-stereospecifically, i.e. PHIP was observed for all protons of the vinyl fragment of propene. The analysis of the polarized signals has shown that syn pairwise addition dominates, which was confirmed by spectra simulations. It was found that storage of Ir/SiO2 P under Ar atmosphere leads to a decrease in PHIP amplitude and an increase in the activity of the catalyst. This observation is discussed in terms of the interaction of Ir/SiO2 P with trace amounts of oxygen in Ar, leading to partial oxidation of triphenylphosphine ligand to triphenylphosphine oxide accompanied by the activation of the immobilized complex. It was also found that the interaction of Ir/SiO2 P with alkenes likely leads to formation of stable monohydride complexes, decreasing the production of PHIP in hydrogenations. At the same time, stable substrate complexes are likely formed in alkyne hydrogenations, leading to a significant decrease in the monohydride complex formation and to an increased production of PHIP.

Direct synthesis of propylene oxide in a packed bed membrane reactor

In the present work, the direct synthesis of propylene oxide (PO) in the liquid phase is successfully performed in a packed bed membrane reactor. We show that this engineering device can be implemented under mild reaction conditions (low temperature and pressure) and outside the explosive regime (low hydrogen concentration), making it appealing for industrial applications. The ceramic membrane allows for a separate feed of the reactants, therefore addressing the PO selectivity issue related to the propylene hydrogenation. The reaction is operated in a continuous methanol flow fed with propylene inside the tubular membrane; hydrogen and oxygen are fed through the porous material. We observe that the feeding strategy of the reactants has an important effect on key parameters such as PO selectivity and productivity. By separating propylene from hydrogen, the propane formation was reduced with respect to conventional packed bed reactors. Moreover, the addition of small amounts of NaBr to the reaction medium increases the catalytic activity to PO, but also to propane formation. Therefore, this study provides a good starting point in the design of a membrane reactor device for the direct synthesis of PO where the main limitations such as propylene hydrogenation and water formation can be addressed.

Nature of the Active Sites of VOx/Al2O3 Catalysts for Propane Dehydrogenation

Supported VOx catalysts are promising for use in propane dehydrogenation (PDH) because of the relatively superior activity and stable performance upon regeneration. However, the nature of the active sites and reaction mechanism during PDH over VOx-based catalysts remains elusive. We examined active species by attaining various fractions of V5+, V4+, and V3+ ions by adjusting the surface vanadium density on an alumina support. The results reveal a close relationship between TOF and the fraction of V3+ ion, indicating that V3+ was more active for PDH. In situ diffuse reflectance infrared Fourier transform spectroscopy showed the same strong adsorbed species during both propane dehydrogenation and propylene hydrogenation. The results indicated that such an intermediate may correspond to V species containing a C═C bond, i.e., V–C3H5, and a reaction mechanism was proposed accordingly.

Direct synthesis of propylene oxide in the liquid phase under mild conditions

Here, we study the direct synthesis of propylene oxide (PO) on Pd-based catalysts operating under mild conditions (40°C, 5.5bar) and in a continuous flow microreactor. We show that the PO yield can be improved by a factor of two, with respect to the values present in literature, by using a Pd–Pt/TS-1 catalyst in excess of oxygen. Moreover, we compare the PO reaction with the hydrogen peroxide (H2O2) synthesis, being H2O2 (or OOH species) the intermediate and rate limiting step of the direct PO formation. We found that the optimal conditions for the PO synthesis are not advantageous for the H2O2 productivity. In this respect, the presence of Pt, which improves the PO selectivity by lowering the hydrogenation of propylene, negatively affects the H2O2 productivity due to an acceleration of its side reactions. This effect is more pronounced for the Pd–Au/TS-1 catalyst, which shows a high performance for H2O2 production. However, the PO formation remains relatively poor due to a very fast hydrogenation of propylene to propane. We conclude that the optimization of the H2O2 synthesis is not sufficient to improve the direct PO formation. Indeed, the hydrogenation of propylene needs also to be considered.

The effect of oxidative and reductive treatments of titania-supported metal catalysts on the pairwise hydrogen addition to unsaturated hydrocarbons

Heterogeneous hydrogenation of unsaturated compounds with parahydrogen is a highly promising technique for boosting the sensitivity of magnetic resonance spectroscopy and imaging by hyperpolarizing reaction products in gaseous and liquid phases, and potentially reaction intermediates as well. This demands an efficient heterogeneous catalyst providing both the high selectivity toward pairwise hydrogen addition (i.e., ability to incorporate both H atoms of H2 molecule in the same product molecule) as well as sufficient overall hydrogenation activity. In this work, we studied the influence of oxidative and reductive treatments of the supported metal catalysts on the NMR signal enhancements provided by parahydrogen-induced polarization (PHIP) effects in hydrogenation of propene, propyne, 1,3-butadiene and 1-butyne. The 5wt% titania-supported Pt, Pd, Rh and Ir catalysts used here were characterized by X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). Generally, the preliminarily reduced catalysts were found to be more efficient than the oxidized ones. For instance, while the reduced Ir/TiO2 catalyst provided the intense PHIP NMR signals, its oxidized counterpart showed almost no activity in hydrogenation. For the oxidized Pd/TiO2 catalyst, HRTEM revealed the formation of titania pedestals under large (ca. 5–7nm) PdO nanoparticles. At the same time, the small (ca. 1nm) partially reduced Pdδ+ particles were observed on the facets of TiO2 support. These changes in catalyst structure led to a significant decrease in pairwise hydrogen addition selectivity.

Strong Metal–Support Interactions Enhance the Pairwise Selectivity of Parahydrogen Addition over Ir/TiO2

The effects of strong metal–support interactions (SMSI) on the pairwise selectivity of propene hydrogenation over metal-oxide-supported Ir nanoparticles were investigated using parahydrogen-enhanced NMR spectroscopy. A ∼20-fold increase in the pairwise selectivity was observed following a reduction treatment of the Ir/TiO2 catalyst at 500 °C. Consistent with SMSI, the effects could be completely reversed by oxidation followed by rereduction at 200 °C. Noninteracting supports, such as Al2O3 and SiO2, did not show this behavior. X-ray photoelectron spectroscopy reveals partial reduction of the TiO2 support, and STEM data reveal flattening of Ir particles after high-temperature reduction. The presence of chloride ions during activation was found to further promote pairwise selectivity but only for the Ir/TiO2 catalyst. The results are interpreted in terms of the electronic and possible geometric blocking effects associated with SMSI.

Computational Mechanistic Study of the Hydrogenation and Dehydrogenation Reactions Catalyzed by Cobalt Pincer Complexes

The mechanisms of hydrogenation and dehydrogenation reactions catalyzed by a series of aliphatic PNP cobalt pincer complexes, [(PNRPiPr)CoH]+ (R = H and CH2X; X = H, Me, NH2, OMe, OH, F, and Cl) and [(PNPiPr)CoH], are studied by density functional theory calculations. In the hydrogenation of propylene catalyzed by [(PNRPiPr)CoH]+, a propylene molecule first inserts into the Co–H bond to form a Co–C bond. Then a H2 molecule is inserted into the Co–C bond for the formation and release of propane. The influence of different substituents on the N atom of the pincer ligand for the hydrogenation process is investigated. The relations between the field/inductive effect (σF) and total free energy barriers and the properties of the lowest energy intermediates, including the Co–N bond lengths, the lowest unoccupied molecular orbital energies, and the Wiberg bond indices of Co–N bonds, are analyzed. The results show that σF plays a crucial role in the substituent effect. The mechanism of acceptorless dehydrogenation...

Propylene hydrogenation through structured and conventional catalyst beds: Experiment and modeling

Structured packings have been applied since forty years ago, and their advantages over conventional packings have been proved; however, they are not utilized broadly in industrial process. In order to increase the performance of catalytic reactions, it is possible to produce some solid-phase catalysts in the physical form of structured packings. To reduce operational costs, process industries require structured catalysts that have large specific area, provide high mass transfer coefficient and cause low pressure drop. In this study a pilot trickle bed reactor with 155 cm length and 14 cm diameter, is setup that includes an automatic-stroke dosing pump, supply of hydrogen and liquid propane and necessary instrumentation to measure flowrate, pressure and temperature. Performance of the trickle bed pilot reactor has been studied when it is packed with conventional type and structured-shaped catalysts under different liquid phase flowrates. For each type of catalyst and operating conditions sampling along the reactor is performed at specific sampling points once steady-state conditions established. Composition of samples were analyzed and used as experimental data. Dynamic model of the trickle bed pilot reactor is developed in which mass transfer resistance is considered in liquid phase and assumed to be negligible in gas phase. The outputs of dynamic model at different steady-state conditions are compared with experimental data that shows good agreement between these two that indicates model reliability.

Shaped Ceria Nanocrystals Catalyze Efficient and Selective Para-Hydrogen-Enhanced Polarization.

Intense para-hydrogen-enhanced NMR signals are observed in the hydrogenation of propene and propyne over ceria nanocubes, nano-octahedra, and nanorods. The well-defined ceria shapes, synthesized by a hydrothermal method, expose different crystalline facets with various oxygen vacancy densities, which are known to play a role in hydrogenation and oxidation catalysis. While the catalytic activity of the hydrogenation of propene over ceria is strongly facet-dependent, the pairwise selectivity is low (2.4% at 375 °C), which is consistent with stepwise H atom transfer, and it is the same for all three nanocrystal shapes. Selective semi-hydrogenation of propyne over ceria nanocubes yields hyperpolarized propene with a similar pairwise selectivity of (2.7% at 300 °C), indicating product formation predominantly by a non-pairwise addition. Ceria is also shown to be an efficient pairwise replacement catalyst for propene.

Shaped Ceria Nanocrystals Catalyze Efficient and Selective Para‐Hydrogen‐Enhanced Polarization

AbstractIntense para‐hydrogen‐enhanced NMR signals are observed in the hydrogenation of propene and propyne over ceria nanocubes, nano‐octahedra, and nanorods. The well‐defined ceria shapes, synthesized by a hydrothermal method, expose different crystalline facets with various oxygen vacancy densities, which are known to play a role in hydrogenation and oxidation catalysis. While the catalytic activity of the hydrogenation of propene over ceria is strongly facet‐dependent, the pairwise selectivity is low (2.4 % at 375 °C), which is consistent with stepwise H atom transfer, and it is the same for all three nanocrystal shapes. Selective semi‐hydrogenation of propyne over ceria nanocubes yields hyperpolarized propene with a similar pairwise selectivity of (2.7 % at 300 °C), indicating product formation predominantly by a non‐pairwise addition. Ceria is also shown to be an efficient pairwise replacement catalyst for propene.

Catalyst synthesis and evaluation using an integrated atomic layer deposition synthesis-catalysis testing tool.

An integrated atomic layer deposition synthesis-catalysis (I-ALD-CAT) tool was developed. It combines an ALD manifold in-line with a plug-flow reactor system for the synthesis of supported catalytic materials by ALD and immediate evaluation of catalyst reactivity using gas-phase probe reactions. The I-ALD-CAT delivery system consists of 12 different metal ALD precursor channels, 4 oxidizing or reducing agents, and 4 catalytic reaction feeds to either of the two plug-flow reactors. The system can employ reactor pressures and temperatures in the range of 10(-3) to 1 bar and 300-1000 K, respectively. The instrument is also equipped with a gas chromatograph and a mass spectrometer unit for the detection and quantification of volatile species from ALD and catalytic reactions. In this report, we demonstrate the use of the I-ALD-CAT tool for the synthesis of platinum active sites and Al2O3 overcoats, and evaluation of catalyst propylene hydrogenation activity.