Unprecedented selectivity behavior in the direct dehydrogenation of n-butane to n-butenes with similar active Pt nanoparticle size: unveiling structural and electronic characteristics of supported monometallic catalysts

In this work, supported Pt monometallic catalysts were prepared using oxide and carbon supports by conventional impregnation methods. Similar Pt metallic nanoparticle sizes (mean sizes about 1.8-2 nm) have been obtained using different Pt precursor loadings (0.3 to 5 wt%). For comparison, catalysts with larger nanoparticle sizes were prepared using the liquid phase reduction method. Characterization results indicate different electronic and structural characteristics for the Pt nanoparticles, comparing nanoparticles with similar and different sizes, implying that both the Pt loading and the preparation method affect the formation of different metallic phases. We used the direct dehydrogenation of n-butane to n-butenes reaction as a test reaction to study the catalytic behavior of the Pt nanoparticles obtained at different Pt atomic concentrations. Surprisingly, Pt catalysts with the lowest metallic loading show the highest selectivities to olefins. Besides, Pt catalysts supported on carbon materials showed higher selectivity to butenes than those supported on oxide materials, this was attributed to a higher electron density in the Pt active sites. Likewise, at low Pt loadings, the CNP-supported Pt nanoparticles could be confined at the defect in the nanotube structure as crystalline agglomerates of atoms with few layers or monolayers with very few surface adatom or stepped adatom nanostructures or simply as a group of atoms, thus creating active Pt sites that favor the dehydrogenation reaction over secondary reactions.

MnCe-based catalysts for removal of organic pollutants in urban wastewater by advanced oxidation processes - A critical review

With Advanced oxidation processes (AOPs) widely promoted, MnCe-based catalysts have received extensive attention under the advantages of high efficiency, stability and economy for refractory organic pollutants present in urban wastewater. Driven by multiple factors such as environmental pollution, technological development, and policy promotion, a systematic review of MnCe-based catalysts is urgently needed in the current research situation. This research provides a critical review of MnCe-based catalysts for removal of organic pollutants in urban wastewater by AOPs. It is found that co-precipitation and sol-gel methods are more appropriate methods for catalyst preparation. Among a host of influence factors, catalyst composition and pH are crucial in the catalytic oxidation processes. The synergistic effect of the free radical pathway and surface catalysis results in better pollutants degradation. It is more valuable to utilize multiple systems for oxidation (e.g., photo-Fenton technology) to improve the catalytic efficiency. This review provides theoretical guidance for MnCe-based catalysts and offers a reference direction for future research in the AOPs of organic pollutants removal from urban wastewater.

Non-Classical Deactivation Mechanism in a Supported Intermetallic Catalyst for Propane Dehydrogenation

Platinum-based supported intermetallic alloys (IMAs) demonstrate exceptional performance in catalytic propane dehydrogenation (PDH) primarily because of their remarkable resistance to coke formation. However, these IMAs still encounter a significant hurdle in the form of catalyst deactivation. Understanding the complex deactivation mechanism of supported IMAs, which goes beyond conventional coke deposition, requires meticulous microscopic structural elucidation. In this study, we unravel a nonclassical deactivation mechanism over a PtZn/γ-Al2O3 PDH catalyst, dictated by the PtZn to Pt3Zn nanophase transformation accompanied with dezincification. The physical origin lies in the metal support interaction (MSI) that enables strong chemical bonding between hydroxyl groups on the support and Zn sites on the PtZn phase to selectively remove Zn species followed by the reconstruction towards Pt3Zn phase. Building on these insights, we have devised a solution to circumvent the deactivation by passivating the MSI through surface modification of γ-Al2O3 support. By exchanging protons of hydroxyl groups with potassium ions (K) on the γ-Al2O3 support, such a strategy significantly minimizes the dezincification of PtZn IMA via diminished metal-support bonding, which dramatically reduces the deactivation rate from 0.2044 to 0.0587 h-1. These findings decode the nonclassical PDH deactivation mechanism over supported IMA catalysts and elaborate a new logic for the design of high-performance IMA based PDH catalysts with long-term stability.

Boosting the Stability of Subnanometer Pt Catalysts by the Presence of Framework Indium(III) Sites in Zeolite

Subnanometer metal clusters show advantages over conventional metal nanoparticles in numerous catalytic reactions owing to their high percentage of exposed surface sites, abundance of under-coordinated metal sites and unique electronic structures. However, the applications of subnanometer metal clusters in high-temperature catalytic reactions (>600 °C) are still hindered, because of their low stability under harsh reaction conditions. In this work, we have developed a zeolite-confined bimetallic PtIn catalyst with exceptionally high stability against sintering. A combination of experimental and theoretical studies shows that the isolated framework In(III) species serve as the anchoring sites for Pt species, precluding the migration and sintering of Pt species in the oxidative atmosphere at ≥650 °C. The catalyst comprising subnanometer PtIn clusters exhibits long-term stability of >1000 h during a cyclic reaction-regeneration test for ethane dehydrogenation reaction.

Simultaneous measurement of methane, propane and isobutane using a compact mid-infrared photoacoustic spectrophone

Hydrocarbon gas sensing is a challenging task using laser absorption spectroscopy due to the complex and broad structure of absorption lines. This application requires quick, accurate and highly sensitive detection of hydrocarbon gases concentrations. In this paper, a compact photoacoustic spectrophone was developed to simultaneously measure methane, propane and isobutane. This spectrophone uses wavelength modulation spectroscopy (WMS) with a single acoustic resonator and a single DFB laser emitting at 3368 nm, which greatly reduces the system complexity without using time-division multiplexing technology for multi-gas sensing. Due to the complex and broadband absorption of hydrocarbon gases, a novel signal processing method based on multilinear regression with Ridge regression (MLR-RG) is proposed to reduce the measurement error caused by the nonlinearity of spectra signal. For single gas measurement, the detection limits of methane, propane, and isobutane are determined to be 828 ppb, 419 ppb, and 619 ppb (SNR = 1, integration time = 20 s), respectively. For simultaneous multi-gas sensing in a gaseous mixture, the detection limits of propane and isobutane are determined to be 7 ppb, 68 ppb with an integration time of 860 s, 460 s, respectively. The measurement accuracy of propane and isobutane using MLR-RG is higher than that of ordinary least squares regression and partial least squares regression by 75% and 60%, respectively. The proposed algorithm based on MLR-RG provides a promising approach to process the broad overlapping absorption spectra for accurately retrieving hydrocarbon gases concentrations.

Propane wet reforming over PtSn nanoparticles on γ-Al2O3 for acetone synthesis

Acetone serves as an important solvent and building block for the chemical industry, but the current industrial synthesis of acetone is generally accompanied by the energy-intensive and costly cumene process used for phenol production. Here we propose a sustainable route for acetone synthesis via propane wet reforming at a moderate temperature of 350 oC with the use of platinum-tin nanoparticles supported on γ-aluminium oxide (PtSn/γ-Al2O3) as catalyst. We achieve an acetone productivity of 858.4 μmol/g with a selectivity of 57.8% among all carbon-based products and 99.3% among all liquid products. Detailed spectroscopic and controlled experiments reveal that the acetone is formed through a tandem catalytic process involving propene and isopropanol as intermediates. We also demonstrate facile ketone synthesis via wet reforming with the use of different alkanes (e.g., n-butane, n-pentane, n-hexane, n-heptane, and n-octane) as substrates, proving the wide applicability of this strategy.

Heterolytic C-H Activation Routes in Catalytic Dehydrogenation of Light Alkanes on Lewis Acid-Base Pairs at ZrO2 Surfaces

Alkane dehydrogenation is an enabling route to make alkenes useful as chemical intermediates. This study demonstrates the high reactivity of Lewis acid-base (LAB) site pairs at ZrO2 powders for dehydrogenation of C2-C4 alkanes and the essential requirement for chemical treatments to remove strongly bound H2O and CO2 titrants to avoid the high temperatures required for their desorption and the concomitant loss of active sites through sintering and annealing of ZrO2 crystallites. The energies and free energies of bound intermediates and transition states from density functional theory (DFT), taken together with kinetic analysis and isotopic methods, demonstrated the kinetic relevance and heterolytic character of the first C-H activation at terminal C-atoms for all alkanes with a modest activation barrier (84 kJ mol-1) at essentially bare Zr-O LAB site pairs. β-Hydride elimination from the formed alkyl carbanions lead to their desorption as alkene products in steps that are favored over their parallel C-C cleavage reactions (by 100 kJ mol-1), leading to high dehydrogenation selectivities (>98%) at the temperatures required for practical yields in such endothermic dehydrogenation reactions (700-900 K). The facile recombination of bound proton-hydride pairs then completes a dehydrogenation turnover. These findings provide compelling evidence for the remarkable reactivity and selectivity of LAB sites on earth-abundant oxides and for the need to uncover them through chemical treatments, which combine to give gravimetric dehydrogenation rates that exceed those on the toxic (Cr) or costly (Pt) catalysts used in practice.

Main-group compounds selectively activate natural gas alkanes under room temperature and atmospheric pressure

Most C-H bond activations of natural gas alkanes rely on transition metal complexes. Activations by using main-group systems have been reported but required heating or photo-irradiation under high atmospheric pressure with rather low regioselectivity. Here we report that Lewis acid-carbene adducts facilely undergo oxidative additions to C-H bonds of ethane, propane and n-butane with high selectivity under room temperature and atmospheric pressure. The Lewis acids can be moved by the addition of a base and the carbene-derived products can be easily converted into aldehydes. This work offers a route for main-group element compounds to selectively functionalise C-H bonds of natural gas alkanes and other small molecules.

Silanol-Stabilized Atomically Dispersed Ptδ+-Ox-Sn Active Sites in Protozeolite for Propane Dehydrogenation

Crystalline zeolites have been proven to be excellent supports for confining subnanometric metal catalysts to boost the propane dehydrogenation (PDH) reaction. However, the introduced metallic species may suffer from severe sintering and limited stability during the catalytic process, especially when utilizing an industrial impregnation method for metal incorporation. In this study, we developed a new type of support based on amorphous protozeolite (PZ), taking advantage of its adjustable silanol chemistry and zeolitic microporous characteristic for stabilizing atomically dispersed PtSn catalyst via a simple, cost-effective coimpregnation process. The combination of X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, in situ diffuse reflectance infrared Fourier transform spectroscopy under CO atmosphere, and density functional theory calculations confirmed the formation of highly dispersed active Ptδ+-Ox-Sn species in PtSn/PZ. The PtSn/PZ catalyst exhibited a high propane conversion of 45.4% and a high propylene selectivity of 99% (WHSV= 3.6 h-1, 550 °C), with a high apparent rate coefficient of 565 molC3H6·gPt-1·h-1·bar-1 at a high WHSV of 108 h-1, presenting a top-level performance among the state-of-the-art Pt-based catalysts prepared by in situ synthesis and impregnation methods. The silanol density determined the chemical state of PtSn species, showing a change from atomically dispersed Ptδ+-Ox-Sn sites to PtSn alloy with decreasing silanol density of supports. This work provides a general strategy using silanol-rich amorphous protozeolite as support for stabilizing various metal catalysts by the simple impregnation method and also offers an effective way for fine tailoring the chemical state of metallic species via a silanol-engineered approach.

Modifying the Charge-Density of Tetrahedral Cobalt(II) Centers through Carbon-Layer Modulation Promotes C-H Activation in the Propane Dehydrogenation Reaction (PDH)

The electronic structure of active metal centers plays an indispensable role in regulating catalytic reactivity in heterogeneous catalysis, developing other metals as promoters to decorate electronic state is a common strategy, while non-metal component of carbon as electronic additives to regulate d-band center has rarely been studied in thermal-catalysis field. Herein, we report electron-deficient tetrahedral Co(II) (Td-cobalt(II)) centers through carbon-layer modulation for propane dehydrogenation (PDH). It is indicated that bifunctional sites of both Td-cobalt(II) and metallic-cobalt are designed, and the in situ generated carbon through the disproportionation of CO on metallic-cobalt can cover the inactive metallic-cobalt and tailor d-band of active Td-cobalt(II) simultaneously. More importantly, the pre-deposited carbon-layer is proposed to decrease electron density of Td-cobalt(II) and make d-band center closer to Fermi level, consequently promotes C-H activation in PDH reaction. This study provides new perspective for the utilization of inactive carbon as electronic promoters and unlocks new opportunity to fabricate efficient PDH and other heterogeneous catalysts.

Catalytic Upcycling of Polyolefins

The large production volumes of commodity polyolefins (specifically, polyethylene, polypropylene, polystyrene, and poly(vinyl chloride)), in conjunction with their low unit values and multitude of short-term uses, have resulted in a significant and pressing waste management challenge. Only a small fraction of these polyolefins is currently mechanically recycled, with the rest being incinerated, accumulating in landfills, or leaking into the natural environment. Since polyolefins are energy-rich materials, there is considerable interest in recouping some of their chemical value while simultaneously motivating more responsible end-of-life management. An emerging strategy is catalytic depolymerization, in which a portion of the C-C bonds in the polyolefin backbone is broken with the assistance of a catalyst and, in some cases, additional small molecule reagents. When the products are small molecules or materials with higher value in their own right, or as chemical feedstocks, the process is called upcycling. This review summarizes recent progress for four major catalytic upcycling strategies: hydrogenolysis, (hydro)cracking, tandem processes involving metathesis, and selective oxidation. Key considerations include macromolecular reaction mechanisms relative to small molecule mechanisms, catalyst design for macromolecular transformations, and the effect of process conditions on product selectivity. Metrics for describing polyolefin upcycling are critically evaluated, and an outlook for future advances is described.

Discovery of highly efficient dual-atom catalysts for propane dehydrogenation assisted by machine learning

Propane dehydrogenation (PDH) is a highly efficient approach for industrial production of propylene, and the dual-atom catalysts (DACs) provide new pathways in advancing atomic catalysis for PDH with dual active sites. In this work, we have developed an efficient strategy to identify promising DACs for PDH reaction by combining high-throughput density functional theory (DFT) calculations and the machine-learning (ML) technique. By choosing the γ-Al2O3(100) surface as the substrate to anchor dual metal atoms, 435 kinds of DACs have been considered to evaluate their PDH catalytic activity. Four ML algorithms are employed to predict the PDH activity and determine the relationship between the intrinsic characteristics of DACs and the catalytic activity. The promising catalysts of CuFe, CuCo and CoZn DACs are finally screened out, which are further validated by the whole kinetic reaction calculations, and the highly efficient performance of DACs is attributed to the synergistic effects and interactions between the paired active sites.

Confining platinum clusters in indium-modified ZSM-5 zeolite to promote propane dehydrogenation

Designing highly active and stable catalytic sites is often challenging due to the complex synthesis procedure and the agglomeration of active sites during high-temperature reactions. Here, we report a facile two-step method to synthesize Pt clusters confined by In-modified ZSM-5 zeolite. In-situ characterization confirms that In is located at the extra-framework position of ZSM-5 as In+, and the Pt clusters are stabilized by the In-ZSM-5 zeolite. The resulting Pt clusters confined in In-ZSM-5 show excellent propane conversion, propylene selectivity, and catalytic stability, outperforming monometallic Pt, In, and bimetallic PtIn alloys. The incorporation of In+ in ZSM-5 neutralizes Brønsted acid sites to inhibit side reactions, as well as tunes the electronic properties of Pt clusters to facilitate propane activation and propylene desorption. The strategy of combining precious metal clusters with metal cation-exchanged zeolites opens the avenue to develop stable heterogeneous catalysts for other reaction systems.

Carbon-Boosted and Nitrogen-Stabilized Isolated Single-Atom Sites for Direct Dehydrogenation of Lower Alkanes

Lower olefins are widely used in the chemical industry as basic carbon-based feedstocks. Here, we report the catalytic system featuring isolated single-atom sites of iridium (Ir1) that can function within the entire temperature range of 300-600 °C and transform alkanes with conversions close to thermodynamics-dictated levels. The high turnover frequency values of the Ir1 system are comparable to those of homogeneous catalytic reactions. Experimental data and theoretical calculations both indicate that Ir1 is the primary catalytic site, while the coordinating C and N atoms help to enhance the activity and stability, respectively; all three kinds of elements cooperatively contribute to the high performance of this novel active site. We have further immobilized this catalyst on particulate Al2O3, and we found that the resulting composite system under mimicked industrial conditions could still give high catalytic performances; in addition, we have also developed and established a new scheme of periodical in situ regeneration specifically for this composite particulate catalyst.

Defective TiOx overlayers catalyze propane dehydrogenation promoted by base metals

The industrial catalysts utilized for propane dehydrogenation (PDH) to propylene, an important alternative to petroleum-based cracking processes, either use expensive metals or metal oxides that are environmentally unbenign. We report that a typically less-active oxide, titanium oxide (TiO2), can be combined with earth-abundant metallic nickel (Ni) to form an unconventional Ni@TiOx catalyst for efficient PDH. The catalyst demonstrates a 94% propylene selectivity at 40% propane conversion and superior stability under industrially relevant conditions. Complete encapsulation of Ni nanoparticles was allowed at elevated temperatures (>550°C). A mechanistic study suggested that the defective TiOx overlayer consisting of tetracoordinated Ti sites with oxygen vacancies is catalytically active. Subsurface metallic Ni acts as an electronic promoter to accelerate carbon-hydrogen bond activation and hydrogen (H2) desorption on the TiOx overlayer.

Propane dehydrogenation catalysis of group IIIB and IVB metal hydrides

Catalytic propane dehydrogenation (PDH) has mainly been studied using metal- and metal oxide-based catalysts. Studies on dehydrogenation catalysis by metal hydrides, however, have rarely been reported. In this study, PDH reactions using group IIIB and IVB metal hydride catalysts were investigated under relatively low-temperature conditions of 450 °C. Lanthanum hydride exhibited the lowest activation energy for dehydrogenation and the highest propylene yield. Based on kinetics studies, a comparison between the reported calculation results and isotope experiments, the hydrogen vacancies of metal hydrides were involved in low-temperature PDH reactions.

Photothermal oxidative dehydrogenation of propane to propylene over Cu/BN catalysts

Oxidative dehydrogenation of propane (ODHP) is a reaction with significant practical significance. As for the industrial application of ODHP, it is challenging to achieve high activity and high propylene selectivity simultaneously. In this study, to overcome this obstacle, we designed a series of Cu/BN catalysts with unique morphologies for establishing a photothermal ODHP system with high efficiency and selectivity. Characterization and evaluation results revealed that Cu/BN-NS and Cu/BN-NF with enlarged specific surface areas exhibited higher catalytic activities. The localized surface plasmon resonance (LSPR) effect of Cu nanoparticles further enhanced the photothermal catalytic performances of Cu/BN catalysts under visible light irradiation. To the best of our knowledge, it is the first time to establish a BN-based photothermal ODHP catalytic system. This study is expected to pave pathways to realize high activity and propylene selectivity for the practical application of ODHP.

Catalyst development for O2-assisted oxidative dehydrogenation of propane to propylene

O2-Assisted oxidative dehydrogenation of propane (O2-ODHP) could convert abundant shale gas into propylene as an important chemical raw material, meaning O2-ODHP has practical significance. Thermodynamically, high temperature is beneficial for O2-ODHP; however, high reaction temperature always causes the overoxidation of propylene, leading to a decline in its selectivity. In this regard, it is crucial to achieve low temperatures while maintaining high efficiency and selectivity during O2-ODHP. The use of catalytic technology provides more opportunities for achieving high-efficiency O2-ODHP under mild conditions. Up to now, many kinds of catalytic systems have been elaborately designed, including transition metal oxide catalysts (such as vanadium-based catalysts, molybdenum-based catalysts, etc.), transition metal-based catalysts (such as Pt nanoclusters), rare earth metal oxide catalysts (especially CeO2 related catalysts), and non-metallic catalysts (BN, other B-containing catalysts, and C-based catalysts). In this review, we have summarized the development progress of mainstream catalysts in O2-ODHP, aiming at providing a clear picture to the catalysis community and advancing this research field further.

One-dimensional Ga2O3-Al2O3 nanofibers with unsaturated coordination Ga: Catalytic dehydrogenation of propane under CO2 atmosphere with excellent stability

The pressing demand for propylene has spurred intensive research on the catalytic dehydrogenation of propane to produce propylene. Gallium-based catalysts are regarded as highly promising due to their exceptional dehydrogenation activity in the presence of CO2. However, the inherent coking issue associated with high temperature reactions poses a constraint on the stability development of this process. In this study, we employed the electrospinning method to prepare a range of Ga2O3-Al2O3 mixed oxide one-dimensional nanofiber catalysts with varying molar ratios for CO2 oxidative dehydrogenation of propane (CO2-OPDH). The propane conversion was up to 48.4 % and the propylene selectivity was high as 96.8 % at 500 °C, the ratio of propane to carbon dioxide is 1:2. After 100 h of reaction, the catalyst still maintains approximately 10 % conversion and exhibits a propylene selectivity of around 98 %. The electrospinning method produces one-dimensional nanostructures with a larger specific surface area, unique multi-stage pore structure and low-coordinated Ga3+, which enhances mass transfer and accelerates reaction intermediates. This results in less coking and improved catalyst stability. The high activity of the catalyst is attributed to an abundance of low-coordinated Ga3+ ions associated with weak/medium-strong Lewis acid centers. In situ infrared analysis reveals that the reaction mechanism involves a two-step dehydrogenation via propane isocleavage, with the second dehydrogenation of Ga-OR at the metal-oxygen bond being the decisive speed step.

The nature of crystal facet effect of TiO2-supported Pd/Pt catalysts on selective hydrogenation of cinnamaldehyde: electron transfer process promoted by interfacial oxygen species

Supported noble metal nanocatalysts typically exhibit strong crystal plane dependent catalytic behavior, but their working mechanism is still unclear. Herein, using anatase TiO2 with well-exposed crystal facets of {101}, {100} and {001} as a prototype support, Pd- and Pt-based supported TiO2 nanocatalysts (TiO2-Pd and TiO2-Pt) were prepared by chemical reduction with NaBH4 as reducer, and they showed a distinct metal-dependent crystal facet effect in the selective hydrogenation of cinamaldehyde (CAL). For Pd-based nanocatalysts, most Pd species on the {100} plane of TiO2 are present in the oxidized form with positive charges and unexpectedly show higher reactivity than the Pd species in the zero-valence state on the {101} and {001} planes. On the contrary, Pt species on all three crystal planes of TiO2 show zero-valence state, with relatively low conversion, but much better selectivity for hydrogenation of a CO bond than Pd-based catalysts. Well-designed experiments manipulating the stability and type of surface oxygen species confirmed that the essence of the crystal facet effect of the catalyst support actually creates a unique nanoconfined interface at the molecular level to construct a surface p-band intermediate state (PBIS), which provides a new alternative channel for surface electron transfer and consequently accelerates the reaction kinetics.

Metal-Site Dispersed Zinc-Chromium Oxide Derived from Chromate-Intercalated Layered Hydroxide for Highly Selective Propane Dehydrogenation

Propane dehydrogenation (PDH) is a crucial approach for propylene production. However, commonly used CrOx-based catalysts have issues including easy sintering at elevated reaction temperatures and relying on high acidity supports. In this work, we develop a strategy, to strongly anchor and isolate active sites against their commonly observed aggregation during reactions, by taking advantage of the net trap effect in chromate intercalated Zn-Cr layered hydroxides as precursors. Furthermore, the intercalated chromate overcomes the collapse of traditional layered hydroxides during their transformation to metal oxide, thus exposing more available active sites. A joint fine modulation including crystal structure, surface acidity, specific surface area, and active sites dispersion is performed on the final mixed metal oxides for propane dehydrogenation. As a result, Zn1Cr2-CrO42--MMO delivers attractive propane conversion (~27%) and propylene selectivity (>90%) as compared to other non-noble-metal-based catalysts.

Selective C-H Bond Activation in Propane with Molecular Oxygen over Cu(I)-ZSM-5 at Ambient Conditions

Selective activation of C-H bonds in light alkanes under mild conditions is challenging but holds the promise of efficient upgrading of abundant hydrocarbons. In this work, we report the conversion of propane to propylene with ∼95% selectivity on Cu(I)-ZSM-5 with O2 at room temperature and pressure. The intraporous Cu(I) species was oxidized to Cu(II) during the reaction but could be regenerated with H2 at 220 °C. Diffuse reflectance ultraviolet spectroscopy indicated the presence of both Cu+-O2 and Cu2(μ-O2)2+ species in the zeolite pores during the reaction, and electron paramagnetic resonance results showed that propane activation occurred via a radical-mediated pathway distinct from that with H2O2 as the oxidant. Correlation between spectroscopic and reactivity results on Cu(I)-ZSM-5 with different Cu loadings suggests that the isolated intraporous Cu(I) species is the main active species in propane activation.

Protocol for preparation of electrifiable carbon membrane reactor for non-oxidative alkane dehydrogenation

A membrane reactor (MR) offers a solution to overcome thermodynamic equilibrium limitations by enabling in situ product separation, enhancing product yields and energy efficiency. Here we present a protocol for synthesizing a carbon MR that couples a H2-permeable carbon molecular sieve hollow fiber membrane and a metal supported on zeolite catalyst for non-oxidative propane and ethane dehydrogenation. We describe steps for catalyst preparation, membrane fabrication, and MR construction. The as-developed MR has significant improvements in alkene yield and a record-high stability. For complete details on the use and execution of this protocol, please refer to Liu et al.1.

In Situ Formation of Platinum-Carbon Catalysts in Propane Dehydrogenation

The catalytic production of propylene via propane dehydrogenation (PDH) is a key reaction in the chemical industry. By combining operando transmission electron microscopy with density functional theory analysis, we show that the intercalation and ordering of carbon on Pt interstitials to form Pt-C solid solutions is relevant for increasing propylene production. More specifically, we found that at the point of enhanced propylene formation, the structure of platinum nanoparticles is transformed into a transient caesium chloride-type Pt-C polymorph. At more elevated temperatures, the zincblende and rock salt polymorphs seemingly coexist. When propylene production was highest, multiple crystal structures consisting of Pt and carbon were occasionally found to coexist in one individual nanoparticle, distorting the Pt lattice. Catalyst coking was detected at all stages of the reaction, but did initially not affect all particles. These findings could lead to the development of novel synthesis strategies towards tailoring highly efficient PDH catalysts.

Close Intimacy between PtIn Clusters and Zeolite Channels for Ultrastability toward Propane Dehydrogenation

Propane dehydrogenation (PDH) serves as a pivotal intentional technique to produce propylene. The stability of PDH catalysts is generally restricted by the readsorption of propylene which can subsequently undergo side reactions for coke formation. Herein, we demonstrate an ultrastable PDH catalyst by encapsulating PtIn clusters within silicalite-1 which serves as an efficient promoter for olefin desorption. The mean lifetime of PtIn@S-1 (S-1, silicalite-1) was calculated as 37317 h with high propylene selectivity of >97% at 580 °C with a weight hourly space velocity (WHSV) of 4.7 h-1. With an ultrahigh WHSV of 1128 h-1, which pushed the catalyst away from the equilibrium conversion to 13.3%, PtIn@S-1 substantially outperformed other reported PDH catalysts in terms of mean lifetime (32058 h), reaction rates (3.42 molpropylene gcat-1 h-1 and 341.90 molpropylene gPt-1 h-1), and total turnover number (14387.30 kgpropylene gcat-1). The developed catalyst is likely to lead the way to scalable PDH applications.

Bimetallic CoCu-modified Pt species in S-1 zeolite with enhanced stability for propane dehydrogenation

Propane dehydrogenation (PDH) has been an outstanding technique with a bright prospect, which can meet the growing global demand for propylene. However, undesired side reactions result in the deactivation of the Pt-based catalysts, which contribute to the insufficient lifetime of the catalysts. Herein, we describe a novel catalyst by encapsulating bimetallic CoCu-modified Pt species in S-1 zeolite for efficient dehydrogenation of propane, which synergizes the confinement of zeolites and the geometric and electronic effects on Pt species for enhancing the catalyst stability. The introduction of bimetallic additives efficiently promotes the dispersion of platinum and the electron transfer between Pt species and the additives, which greatly prolongs the lifetime of the catalysts. Particularly, no obvious deactivation is observed on 0.2Pt0.3Co0.5CuK@S-1 after 93 h on stream with a weight hourly space velocity (WHSV) of 5.4 h-1, revealing an ultralow deactivation constant of 0.0011 h-1 (t = 909 h). The formation rate of propylene still maintains at a high value of 407 mol gPt-1 h-1 (WHSV = 21.6 h-1) at 580 ℃ even after on pure propane stream for 42 h. The catalyst with the bimetallic CoCu-modified Pt species in S-1 zeolite reveals ultra-high activity and stability for PDH, which is ascribed to the highly dispersed Pt species and the stabilization effect of bimetallic additives on Pt species.

Silicon Nitride Surface Enabled Propane Dehydrogenation Catalyzed by Supported Organozirconium

Mesoporous silicon nitride (Si3N4) is a nontraditional support for the chemisorption of organometallic complexes with the potential for enhancing catalytic activity through features such as the increased Lewis basicity of nitrogen for heterolytic bond activation, increased ligand donor strength, and metal-ligand orbital overlap. Here, tetrabenzyl zirconium (ZrBn4) was chemisorbed on Si3N4, and the resulting supported organometallic species was characterized by Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), Dynamic Nuclear Polarization-enhanced Solid State Nuclear Magnetic Resonance (DNP-SSNMR), and X-ray Absorption Spectroscopy (XAS). Based on the hypothesis that the nitride might enable facile heterolytic C-H bond activation along the Zr-N bond, this material was found to be a highly active (1.53 molpropene molZr-1 h-1 at 450 °C) and selective (99% to propylene) catalyst for propane dehydrogenation. In contrast, the homologous silica supported complex exhibited negligible activity under these conditions.

Well-Defined Supported ZnOx Species: Synthesis, Structure, and Catalytic Performance in Nonoxidative Dehydrogenation of C3-C4 Alkanes

ConspectusZinc oxide (ZnO) is a multipurpose material and finds its applications in various fields such as rubber manufacturing, medicine, food additives, electronics, etc. It has also been intensively studied in photocatalysis due to its wide band gap and environmental compatibility. Recently, heterogeneous catalysts with supported ZnOx species have attracted more and more attention for the dehydrogenation of propane (PDH) and isobutane (iBDH) present in shale/natural gas. The olefins formed in these reactions are key building blocks of the chemical industry. These reactions are also of academic importance for understanding the fundamentals of the selective activation of C-H bonds. Differently structured ZnOx species supported on zeolites, SiO2, and Al2O3 have been reported to be active for nonoxidative dehydrogenation reactions. However, the structure-activity-selectivity relationships for these catalysts remain elusive. The main difficulty stems from the preparation of catalysts containing only one kind of well-defined ZnOx species.In this Account, we describe the studies on PDH and iBDH over differently structured ZnOx species and highlight our approaches to develop catalysts with controllable ZnOx speciation relevant to their performance. Several methods, including (i) the in situ reaction of gas-phase metallic Zn atoms with OH groups on the surface of supports, (ii) one-pot hydrothermal synthesis, and (iii) impregnation/anchoring methods, have been developed/used for the tailored preparation of supported ZnOx species. The first method allows precise control of the molecular structure of ZnOx through the nature of the defective OH groups on the supports. Using this method, a series of ZnOx species ranging from isolated, binuclear to nanosized ZnOx have been successfully generated on different SiO2-based or ZrO2-based supports as demonstrated by complementary ex/in situ characterization techniques. Based on kinetic studies and detailed characterization results, the intrinsic activity (Zn-related turnover frequency) of ZnOx was found to depend on its speciation. It increases with an increasing number of Zn atoms in a ZnmOn cluster from 1 to a few atoms (less than 10) and then decreases strongly for ZnOx nanoparticles. The latter promote the formation of undesired C1-C2 hydrocarbons and coke, resulting in lower propene selectivity in comparison with the catalysts containing only ZnOx species ranging from isolated to subnanometer ZnmOn clusters. In addition, the strategy for improving the thermal stability of ZnOx species and the consequences of mass-transport limitations for DH reactions were also elucidated. The results obtained allowed us to establish the fundamentals for the targeted preparation of well-structured ZnOx species and the relationships between their structures and the DH performance. This knowledge may inspire further studies in the field of C-H bond activation and other reactions, in which ZnOx species act as catalytically active sites or promoters, such as the dehydroaromatization of light alkanes and the hydrogenation of CO2 to methanol.

Screening and Mechanism Exploration of Non-Noble Metal Ni3M Catalysts for Propane Dehydrogenation: The Excellence of Synergistic Effects

The development of cost-effective and anti-coking catalysts for propane dehydrogenation (PDH) is crucial. Here, non-noble metal-incorporated Ni-based catalysts (Ni3M, M = Sc, Ti, V, Mn, Fe, Co, Cu, Zn, Ga, Zr, Nb, Mo, In, Sn) were employed in the PDH process. The introduction of V, Nb, and Mo, with their strong carbon binding ability, created unique Ni-M cooperative sites, enhancing the catalytic performance. Other non-noble metals influenced the electronic structure of Ni, affecting the overall catalytic behavior. V and Nb exhibited a balanced combination of activity, selectivity, and stability, making them potential catalyst candidates. Microkinetic simulations revealed that Ni3V and Ni3Nb displayed high selectivity toward olefins with low apparent activation energies. This study emphasizes the significance of bimetallic synergy in enhancing PDH performance and provides new directions for the development of efficient alkane dehydrogenation catalyst development.

Tricoordinated Single-Atom Cobalt in Zeolite Boosting Propane Dehydrogenation

Propane dehydrogenation (PDH) reaction has emerged as one of the most promising propylene production routes due to its high selectivity for propylene and good economic benefits. However, the commercial PDH processes usually rely on expensive platinum-based and poisonous chromium oxide based catalysts. The exploration of cost-effective and ecofriendly PDH catalysts with excellent catalytic activity, propylene selectivity, and stability is of great significance yet remains challenging. Here, we discovered a new active center, i.e., an unsaturated tricoordinated cobalt unit (≡Si-O)CoO(O-Mo) in a molybdenum-doped silicalite-1 zeolite, which afforded an unprecedentedly high propylene formation rate of 22.6 molC3H6 gCo-1 h-1 and apparent rate coefficient of 130 molC3H6 gCo-1 h-1 bar-1 with >99% of propylene selectivity at 550 °C. Such activity is nearly one magnitude higher than that of previously reported Co-based catalysts in which cobalt atoms are commonly tetracoordinated, and even superior to that of most of Pt-based catalysts under similar operating conditions. Density functional theory calculations combined with the state-of-the-art characterizations unravel the role of the unsaturated tricoordinated Co unit in facilitating the C-H bond-breaking of propane and propylene desorption. The present work opens new opportunities for future large-scale industrial PDH production based on inexpensive non-noble metal catalysts.

Structured Catalysts and Catalytic Processes: Transport and Reaction Perspectives

The structure of catalysts determines the performance of catalytic processes. Intrinsically, the electronic and geometric structures influence the interaction between active species and the surface of the catalyst, which subsequently regulates the adsorption, reaction, and desorption behaviors. In recent decades, the development of catalysts with complex structures, including bulk, interfacial, encapsulated, and atomically dispersed structures, can potentially affect the electronic and geometric structures of catalysts and lead to further control of the transport and reaction of molecules. This review describes comprehensive understandings on the influence of electronic and geometric properties and complex catalyst structures on the performance of relevant heterogeneous catalytic processes, especially for the transport and reaction over structured catalysts for the conversions of light alkanes and small molecules. The recent research progress of the electronic and geometric properties over the active sites, specifically for theoretical descriptors developed in the recent decades, is discussed at the atomic level. The designs and properties of catalysts with specific structures are summarized. The transport phenomena and reactions over structured catalysts for the conversions of light alkanes and small molecules are analyzed. At the end of this review, we present our perspectives on the challenges for the further development of structured catalysts and heterogeneous catalytic processes.

Overcoming limitations in propane dehydrogenation by codesigning catalyst-membrane systems

Propylene production through propane dehydrogenation (PDH) is endothermic, and high temperatures required to achieve acceptable propane conversions lead to low selectivity and severe carbon-induced deactivation of conventional catalysts. We developed a catalyst-membrane system that removes the hydrogen by-product and can thus achieve propane conversions that exceed equilibrium limits. In this codesigned system, a silica/alumina (SiO2/Al2O3) hollow-fiber hydrogen membrane was packed with a selective platinum-tin (Pt1Sn1/SiO2) PDH catalyst on the tube side with hydrogen diffusing from the tube to the shell side. We demonstrate that the catalyst-membrane system can achieve propane conversions >140% of the nominal equilibrium conversion with a propylene selectivity >98% without deactivation of the system components. We also show that by introducing oxygen on the shell side of the catalyst-membrane system, we can couple the endothermic PDH reaction on the tube side with exothermic hydrogen oxidation on the shell side. This coupling results in higher rates of hydrogen transport, leading to further enhancements in the propane conversion as well as desired thermoneutral system operation.

Stable anchoring of single rhodium atoms by indium in zeolite alkane dehydrogenation catalysts

Maintaining the stability of single-atom catalysts in high-temperature reactions remains extremely challenging because of the migration of metal atoms under these conditions. We present a strategy for designing stable single-atom catalysts by harnessing a second metal to anchor the noble metal atom inside zeolite channels. A single-atom rhodium-indium cluster catalyst is formed inside zeolite silicalite-1 through in situ migration of indium during alkane dehydrogenation. This catalyst demonstrates exceptional stability against coke formation for 5500 hours in continuous pure propane dehydrogenation with 99% propylene selectivity and propane conversions close to the thermodynamic equilibrium value at 550°C. Our catalyst also operated stably at 600°C, offering propane conversions of >60% and propylene selectivity of >95%.

Efficient conversion of propane in a microchannel reactor at ambient conditions

The oxidative dehydrogenation of propane, primarily sourced from shale gas, holds promise in meeting the surging global demand for propylene. However, this process necessitates high operating temperatures, which amplifies safety concerns in its application due to the use of mixed propane and oxygen. Moreover, these elevated temperatures may heighten the risk of overoxidation, leading to carbon dioxide formation. Here we introduce a microchannel reaction system designed for the oxidative dehydrogenation of propane within an aqueous environment, enabling highly selective and active propylene production at room temperature and ambient pressure with mitigated safety risks. A propylene selectivity of over 92% and production rate of 19.57 mmol mCu-2 h-1 are simultaneously achieved. This exceptional performance stems from the in situ creation of a highly active, oxygen-containing Cu catalytic surface for propane activation, and the enhanced propane transfer via an enlarged gas-liquid interfacial area and a reduced diffusion path by establishing a gas-liquid Taylor flow using a custom-made T-junction microdevice. This microchannel reaction system offers an appealing approach to accelerate gas-liquid-solid reactions limited by the solubility of gaseous reactant.

Alkaline-earth ion stabilized sub-nano-platinum tin clusters for propane dehydrogenation

The strong promotion effects of alkali/alkaline earth metals are frequently reported for heterogeneous catalytic processes such as propane dehydrogenation (PDH), but their functioning principles remain elusive. This paper describes the effect of the addition of calcium (Ca) on reducing the deactivation rate of platinum-tin (Pt-Sn) catalyzed PDH from 0.04 h-1 to 0.0098 h-1 at 873 K under a WHSV of 16.5 h-1 of propane. The Pt-Sn-Ca catalyst shows a high propylene selectivity of >96% with a propylene production rate of 41 molC3H6 (gPt h)-1 and ∼1% activity loss after regeneration. The combination of characterization and DFT simulations reveals that Ca acts as a structural promoter favoring the transition of Snn+ in the parent catalyst to Sn0 during reduction, and the latter is an electron donor that increases the electron density of Pt. This greatly suppresses coke formation from deep dehydrogenation. Moreover, it was found that Ca promotes the formation of a highly reactive and sintering-resistant sub-nano Pt-Sn alloy with a diameter of approximately 0.8 nm. These lead to high activity and selectivity for the Pt-Sn-Ca catalyst for PDH.

Homolytic H2 dissociation for enhanced hydrogenation catalysis on oxides

The limited surface coverage and activity of active hydrides on oxide surfaces pose challenges for efficient hydrogenation reactions. Herein, we quantitatively distinguish the long-puzzling homolytic dissociation of hydrogen from the heterolytic pathway on Ga2O3, that is useful for enhancing hydrogenation ability of oxides. By combining transient kinetic analysis with infrared and mass spectroscopies, we identify the catalytic role of coordinatively unsaturated Ga3+ in homolytic H2 dissociation, which is formed in-situ during the initial heterolytic dissociation. This site facilitates easy hydrogen dissociation at low temperatures, resulting in a high hydride coverage on Ga2O3 (H/surface Ga3+ ratio of 1.6 and H/OH ratio of 5.6). The effectiveness of homolytic dissociation is governed by the Ga-Ga distance, which is strongly influenced by the initial coordination of Ga3+. Consequently, by tuning the coordination of active Ga3+ species as well as the coverage and activity of hydrides, we achieve enhanced hydrogenation of CO2 to CO, methanol or light olefins by 4-6 times.

Dual-Atom Catalyst with N-Colligated Zn1Co1 Species as Dominant Active Sites for Propane Dehydrogenation

Dual-atom catalysts (DACs) with paired active sites can provide unique intrinsic properties for heterogeneous catalysis, but the synergy of the active centers remains to be elucidated. Here, we develop a high-performance DAC with Zn1Co1 species anchored on nitrogen-doped carbon (Zn1Co1/NC) as the dominant active site for the propane dehydrogenation (PDH) reaction. It exhibits several times higher turnover frequency (TOF) of C3H8 conversion and enhanced C3H6 selectivity compared to Zn1/NC or Co1/NC with only a single-atom site. Various experimental and theoretical studies suggest that the enhanced PDH performance stems from the promoted activation of the C-H bond of C3H8 triggered by the electronic interaction between Zn1 and Co1 colligated by N species. Moreover, the dynamic sinking of the Zn1 site and rising of the Co1 site, together with the steric effect of the dissociated H species at the bridged N during the PDH reaction, provides a feasible channel for C3H6 desorption through the more exposed Co1 site, thereby boosting the selectivity. This work provides a promising strategy for designing robust hetero DACs to simultaneously increase activity and selectivity in the PDH reaction.

Dealuminated Beta zeolite reverses Ostwald ripening for durable copper nanoparticle catalysts

Copper nanoparticle-based catalysts have been extensively applied in industry, but the nanoparticles tend to sinter into larger ones in the chemical atmospheres, which is detrimental to catalyst performance. In this work, we used dealuminated Beta zeolite to support copper nanoparticles (Cu/Beta-deAl) and showed that these particles become smaller in methanol vapor at 200°C, decreasing from ~5.6 to ~2.4 nanometers in diameter, which is opposite to the general sintering phenomenon. A reverse ripening process was discovered, whereby migratable copper sites activated by methanol were trapped by silanol nests and the copper species in the nests acted as new nucleation sites for the formation of small nanoparticles. This feature reversed the general sintering channel, resulting in robust catalysts for dimethyl oxalate hydrogenation performed with supported copper nanoparticles for use in industry.

Ultramicroporous Lonsdaleite Topology MOF with High Propane Uptake and Propane/Methane Selectivity for Propane Capture from Simulated Natural Gas

Propane (C3H8) is a widely used fuel gas. Metal-organic framework (MOF) physisorbents that are C3H8 selective offer the potential to significantly reduce the energy footprint for capturing C3H8 from natural gas, where C3H8 is typically present as a minor component. Here we report the C3H8 recovery performance of a previously unreported lonsdaleite, lon, topology MOF, a chiral metal-organic material, [Ni(S-IEDC)(bipy)(SCN)]n, CMOM-7. CMOM-7 was prepared from three low-cost precursors: Ni(SCN)2, S-indoline-2-carboxylic acid (S-IDECH), and 4,4'-bipyridine (bipy), and its structure was determined by single crystal X-ray crystallography. Pure gas adsorption isotherms revealed that CMOM-7 exhibited high C3H8 uptake (2.71 mmol g-1) at 0.05 bar, an indication of a higher affinity for C3H8 than both C2H6 and CH4. Dynamic column breakthrough experiments afforded high purity C3H8 capture from a gas mixture comprising C3H8/C2H6/CH4 (v/v/v = 5/10/85). Despite the dilute C3H8 stream, CMOM-7 registered a high dynamic uptake of C3H8 and a breakthrough time difference between C3H8 and C2H6 of 79.5 min g-1, superior to those of previous MOF physisorbents studied under the same flow rate. Analysis of crystallographic data and Grand Canonical Monte Carlo simulations provides insight into the two C3H8 binding sites in CMOM-7, both of which are driven by C-H···π and hydrogen bonding interactions.

Amorphous CeOx Islands on Dealuminated Zeolite Beta to Stabilize Pt Nanoparticles as Efficient and Antisintering Catalysts for Propane Dehydrogenation

Pt-based catalysts have been widely used in propane dehydrogenation due to their superior activation of C-H bonds and weak scission of C-C bonds. However, in the process of repeated calcination to remove deposited coke, the active Pt species tend to sinter, resulting in a significant decline in catalytic activity. In this study, amorphous CeOx islands loaded on dealuminated Beta zeolite were prepared via simple wetness impregnation. Then, partially embedded Pt nanoparticles in CeOx islands were obtained after reduction owing to the affinity of CeOx for Pt. In the propane dehydrogenation reaction, Pt/Ce5-SiBeta with a Ce loading of 4.55 wt % and Pt loading of 0.72 wt % exhibited the highest activity and the lowest inactivation constant at 550 °C. More importantly, due to the anchoring effect of CeOx on Pt, the catalytic activity of Pt could be recovered after a simple calcination-reduction regeneration process, avoiding the chlorination treatment for the redispersion of Pt species used in industry. In addition, to improve the selectivity of the Pt/Ce5-SiBeta catalyst, a PtSn/Ce5-SiBeta catalyst with excellent activity, selectivity, and recycling stability has been prepared by introducing Sn into Pt/Ce5-SiBeta. The use of amorphous CeOx islands to improve the sintering resistance of Pt opens up new prospects for the design of stable industrial dehydrogenation catalysts.

Controlling Metal-Oxide Reducibility for Efficient C-H Bond Activation in Hydrocarbons

Knowing the structure of catalytically active species/phases and providing methods for their purposeful generation are two prerequisites for the design of catalysts with desired performance. Herein, we introduce a simple method for precise preparation of supported/bulk catalysts. It utilizes the ability of metal oxides to dissolve and to simultaneously precipitate during their treatment in an aqueous ammonia solution. Applying this method for a conventional VOx -Al2 O3 catalyst, the concentration of coordinatively unsaturated Al sites was tuned simply by changing the pH value of the solution. These sites affect the strength of V-O-Al bonds of isolated VOx species and thus the reducibility of the latter. This method is also applicable for controlling the reducibility of bulk catalysts as demonstrated for a CeO2 -ZrO2 -Al2 O3 system. The application potential of the developed catalysts was confirmed in the oxidative dehydrogenation of ethylbenzene to styrene with CO2 and in the non-oxidative propane dehydrogenation to propene. Our approach is extendable to the preparation of any metal oxide catalysts dissolvable in an ammonia solution.

Implications of Ga promotion and metal-oxide interface from tailored PtGa propane dehydrogenation catalysts supported on carbon

Propane Dehydrogenation is a key technology, where Pt-based catalysts have widely been investigated in industry and academia, with development exploring the use of promoters (Sn, Zn, Ga, etc.) and additives (Na, K, Ca, Si, etc.) towards improved catalytic performances. Recent studies have focused on the role of Ga promotion: while computations suggest that Ga plays a key role in enhancing catalytic selectivity and stability of PtGa catalysts through Pt-site isolation as well as morphological changes, experimental evidence are lacking because of the use of oxide supports that prevent more detailed investigation. Here, we develop a methodology to generate Pt and PtGa nanoparticles with tailored interfaces on carbon supports by combining surface organometallic chemistry (SOMC) and specific thermolytic molecular precursors containing or not siloxide ligands. This approach enables the preparation of supported nanoparticles, exhibiting or not an oxide interface, suitable for state-of-the art electron microscopy and XANES characterization. We show that the introduction of Ga enables the formation of homogenously alloyed, amorphous PtGa nanoparticles, in sharp contrast to highly crystalline monometallic Pt nanoparticles. Furthermore, the presence of an oxide interface is shown to stabilize the formation of small particles, at the expense of propene selectivity loss (formation of cracking side-products, methane/ethene), explaining the use of additives such as Na, K and Ca in industrial catalysts.

Recent developments in X-ray diffraction/scattering computed tomography for materials science

X-ray diffraction/scattering computed tomography (XDS-CT) methods are a non-destructive class of chemical imaging techniques that have the capacity to provide reconstructions of sample cross-sections with spatially resolved chemical information. While X-ray diffraction CT (XRD-CT) is the most well-established method, recent advances in instrumentation and data reconstruction have seen greater use of related techniques like small angle X-ray scattering CT and pair distribution function CT. Additionally, the adoption of machine learning techniques for tomographic reconstruction and data analysis are fundamentally disrupting how XDS-CT data is processed. The following narrative review highlights recent developments and applications of XDS-CT with a focus on studies in the last five years. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'.

Selective Oxidative Dehydrogenation of Ethane and Propane over Copper-Containing Mordenite: Insights into Reaction Mechanism and Product Protection

Copper(II)-containing mordenite (CuMOR) is capable of activation of C-H bonds in C1 -C3 alkanes, albeit there are remarkable differences between the functionalization of ethane and propane compared to methane. The reaction of ethane and propane with CuMOR results in the formation of ethylene and propylene, while the reaction of methane predominantly yields methanol and dimethyl ether. By combining in situ FTIR and MAS NMR spectroscopies as well as time-resolved Cu K-edge X-ray absorption spectroscopy, the reaction mechanism was derived, which differs significantly for each alkane. The formation of ethylene and propylene proceeds via oxidative dehydrogenation of the corresponding alkanes with selectivity above 95 % for ethane and above 85 % for propane. The formation of stable π-complexes of olefins with CuI sites, formed upon reduction of CuII -oxo species, protects olefins from further oxidation and/or oligomerization. This is different from methane, the activation of which proceeds via oxidative hydroxylation leading to the formation of surface methoxy species bonded to the zeolite framework. Our findings constitute one of the major steps in the direct conversion of alkanes to important commodities and open a novel research direction aiming at the selective synthesis of olefins.

Modeling the Selectivity of Hydrotalcite-Based Catalyst in the Propane Dehydrogenation Reaction

The propylene production processes currently used in the petrochemical industry (fluid catalytic cracking and steam cracking of naphtha and light diesel) are unable to meet the increase of propylene demand for industrial applications. For this reason, alternative processes for propylene production have been investigated, and among the others, the propane dehydrogenation (PDH) process, allowing the production of propylene as a main product, has been industrially implemented (e.g., Catofin and Oleflex processes). The main drawback of such processes is closely linked to the high temperature required to reach a sustainable propane conversion that affects catalyst stability due to coke formation on the catalyst surface. Accordingly, the periodic regeneration of the catalytic bed is required. In this work, the performance in the PDH reaction of different Sn-Pt catalysts, prepared starting by alumina- and hydrotalcite-based supports, is investigated in terms of propane conversion and selectivity to propylene in order to identify a more stable catalyst than the commercial ones. The experimental tests evidenced that the best performance was obtained using the catalyst prepared on commercial pellets of hydrotalcite PURALOX MG70. This catalyst has shown, under pressure conditions of 1 and 5 bar (in order to evaluate the potential future application in integrated membrane reactors), propane conversion values close to the thermodynamic equilibrium ones in all of the investigated temperature ranges (500-600 °C) and the selectivity was always higher than 95%. So, this catalyst was also tested in a stability run, performed at 500 °C and 5 bar: the results highlighted the loss of only 12% in the propane conversion with no changes in the selectivity to propylene. Properly designed experimental tests have also been performed in order to evaluate the kinetic parameters, and the developed mathematical model has been optimized to effectively describe the system behavior and the catalyst deactivation.

Rationalizing kinetic behaviors of isolated boron sites catalyzed oxidative dehydrogenation of propane

Boron-based catalysts exhibit high alkene selectivity in oxidative dehydrogenation of propane (ODHP) but the mechanistic understanding remains incomplete. In this work, we show that the hydroxylation of framework boron species via steaming not only enhances the ODHP rate on both B-MFI and B-BEA, but also impacts the kinetics of the reaction. The altered activity, propane reaction order and the activation energy could be attributed to the hydrolysis of framework [B(OSi≡)3] unit to [B(OSi≡)3-x(OH···O(H)Si≡)x] (x = 1, 2, "···" represents hydrogen bonding). DFT calculations confirm that hydroxylated framework boron sites could stabilize radical species, e.g., hydroperoxyl radical, further facilitating the gas-phase radical mechanism. Variations in the contributions from gas-phase radical mechanisms in ODHP lead to the linear correlation between activation enthalpy and entropy on borosilicate zeolites. Insights gained in this work offer a general mechanistic framework to rationalize the kinetic behavior of the ODHP on boron-based catalysts.

Performance Descriptors for Catalysts Based on Molybdenum, Tungsten, or Rhenium Oxides for Metathesis of Ethylene with 2-Butenes to Propene

The metathesis of ethylene with 2-butenes to propene is an established large-scale process. However, the fundamentals behind in situ transformation of supported WOx , MoOx , or ReOx species into catalytically active metal-carbenes and the intrinsic activity of the latter as well as the role of metathesis-inactive cocatalysts are still unsolved. This is detrimental for catalyst development and process optimization. In this study, we provide the required essentials derived from steady-state isotopic transient kinetic analysis. For the first time, the steady-state concentration, the lifetime, and the intrinsic reactivity of metal carbenes were determined. The obtained results can be directly used for the design and the preparation of metathesis-active catalysts and cocatalysts, thereby opening up possibilities for optimizing propene productivity.

Microwave-assisted, performance-advantaged electrification of propane dehydrogenation

Nonoxidative propane dehydrogenation (PDH) produces on-site propylene for value-added chemicals. While commercial, its modest selectivity and catalyst deactivation hamper the process efficiency and limit operation to lower temperatures. We demonstrate PDH in a microwave (MW)-heated reactor over PtSn/SiO2 catalyst pellets loaded in a SiC monolith acting as MW susceptor and a heat distributor while ensuring comparable conditions with conventional reactors. Time-on-stream experiments show active and stable operation at 500°C without hydrogen addition. Upon increasing temperature or feed partial pressure at high space velocity, catalysts under MWs show resistance in coking and sintering, high activity, and selectivity, starkly contrasting conventional reactors whose catalyst undergoes deactivation. Mechanistic differences in coke formation are exposed. Gas-solid temperature gradients are computationally investigated, and nanoscale temperature inhomogeneities are proposed to rationalize the different performances of the heating modes. The approach highlights the great potential of electrification of endothermic catalytic reactions.

Pore Modulation of Hydrogen-Bonded Organic Frameworks for Efficient Separation of Propylene

Developing hydrogen-bonded organic frameworks (HOFs) that combine functional sites, size control, and storage capability for targeting gas molecule capture is a novel and challenging venture. However, there is a lack of effective strategies to tune the hydrogen-bonded network to achieve high-performance HOFs. Here, a series of HOFs termed as HOF-ZSTU-M (M=1, 2, and 3) with different pore structures are obtained by introducing structure-directing agents (SDAs) into the hydrogen-bonding network of tetrakis (4-carboxyphenyl) porphyrin (TCPP). These HOFs have distinct space configurations with pore channels ranging from discrete to continuous multi-dimensional. Single-crystal X-ray diffraction (SCXRD) analysis reveals a rare diversity of hydrogen-bonding models dominated by SDAs. HOF-ZSTU-2, which forms a strong layered hydrogen-bonding network with ammonium (NH4 + ) through multiple carboxyl groups, has a suitable 1D "pearl-chain" channel for the selective capture of propylene (C3 H6 ). At 298 K and 1 bar, the C3 H6 storage density of HOF-ZSTU-2 reaches 0.6 kg L-1 , representing one of the best C3 H6 storage materials, while offering a propylene/propane (C3 H6 /C3 H8 ) selectivity of 12.2. Theoretical calculations and in situ SCXRD provide a detailed analysis of the binding strength of C3 H6 at different locations in the pearl-chain channel. Dynamic breakthrough tests confirm that HOF-ZSTU-2 can effectively separate C3 H6 from multi-mixtures.

Ti-Doping in Silica-Supported PtZn Propane Dehydrogenation Catalysts: From Improved Stability to the Nature of the Pt-Ti Interaction

Propane dehydrogenation is an important industrial reaction to access propene, the world's second most used polymer precursor. Catalysts for this transformation are required to be long living at high temperature and robust toward harsh oxidative regeneration conditions. In this work, combining surface organometallic chemistry and thermolytic molecular precursor approach, we prepared well-defined silica-supported Pt and alloyed PtZn materials to investigate the effect of Ti-doping on catalytic performances. Chemisorption experiments and density functional calculations reveal a significant change in the electronic structure of the nanoparticles (NPs) due to the Ti-doping. Evaluation of the resulting materials PtZn/SiO₂ and PtZnTi/SiO₂ during long deactivation phases reveal a stabilizing effect of Ti in PtZnTi/SiO₂ with a k_d of 0.015 h^-1 compared to PtZn/SiO₂ with a k_d of 0.022 h^-1 over 108 h on stream. Such a stabilizing effect is also present during a second deactivation phase after applying a regeneration protocol to the materials under O₂ and H₂ at high temperatures. A combined scanning transmission electron microscopy, in situ X-ray absorption spectroscopy, electron paramagnetic resonance, and density functional theory study reveals that this effect is related to a sintering prevention of the alloyed PtZn NPs in PtZnTi/SiO₂ due to a strong interaction of the NPs with Ti sites. However, in contrast to classical strong metal-support interaction, we show that the coverage of the Pt NPs with TiO_x species is not needed to explain the changes in adsorption and reactivity properties. Indeed, the interaction of the Pt NPs with Ti^III sites is enough to decrease CO adsorption and to induce a red-shift of the CO band because of electron transfer from the Ti^III sites to Pt⁰.