RESUMEN
Boron-containing materials, and in particular boron nitride, have recently been identified as highly selective catalysts for the oxidative dehydrogenation of alkanes such as propane. To date, no mechanism exists that can explain both the unprecedented selectivity, the observed surface oxyfunctionalization, and the peculiar kinetic features of this reaction. We combine catalytic activity measurements with quantum chemical calculations to put forward a bold new hypothesis. We argue that the remarkable product distribution can be rationalized by a combination of surface-mediated formation of radicals over metastable sites, and their sequential propagation in the gas phase. Based on known radical propagation steps, we quantitatively describe the oxygen pressure-dependent relative formation of the main product propylene and by-product ethylene. Free radical intermediates most likely differentiate this catalytic system from less selective vanadium-based catalysts.
RESUMEN
Boron-containing materials have recently been identified as highly selective catalysts for the oxidative dehydrogenation (ODH) of alkanes to olefins. It has previously been demonstrated by several spectroscopic characterization techniques that the surface of these boron-containing ODH catalysts oxidize and hydrolyze under reaction conditions, forming an amorphous B2 (OH)x O(3-x/2) (x=0-6) layer. Yet, the precise nature of the active site(s) remains elusive. In this Communication, we provide a detailed characterization of zeolite MCM-22 isomorphously substituted with boron (B-MWW). Using 11 B solid-state NMR spectroscopy, we show that the majority of boron species in B-MWW exist as isolated BO3 units, fully incorporated into the zeolite framework. However, this material shows no catalytic activity for ODH of propane to propene. The catalytic inactivity of B-MWW for ODH of propane falsifies the hypothesis that site-isolated BO3 units are the active site in boron-based catalysts. This observation is at odds with other traditionally studied catalysts like vanadium-based catalysts and provides an important piece of the mechanistic puzzle.
RESUMEN
Boron-based heterogenous catalysts, such as hexagonal boron nitride (h-BN) as well as supported boron oxides, are highly selective catalysts for the oxidative dehydrogenation (ODH) of light alkanes to olefins. Previous catalytic measurements and molecular characterization of boron-based catalysts by 11B solid-state NMR spectroscopy and other techniques suggests that oxidized/hydrolyzed boron clusters are the catalytically active sites for ODH. However, 11B solid-state NMR spectroscopy often suffers from limited resolution because boron-11 is an I = 3/2 half-integer quadrupolar nucleus. Here, ultra-high magnetic field (B 0 = 35.2 T) is used to enhance the resolution of 11B solid-state NMR spectra and unambiguously determine the local structure and connectivity of boron species in h-BN nanotubes used as a ODH catalyst (spent h-BNNT), boron substituted MCM-22 zeolite [B-MWW] and silica supported boron oxide [B/SiO2] before and after use as an ODH catalyst. One-dimensional direct excitation 11B NMR spectra recorded at B 0 = 35.2 T are near isotropic in nature, allowing for the easy identification of all boron species. Two-dimensional 1H-11B heteronuclear correlation NMR spectra aid in the identification of boron species with B-OH functionality. Most importantly, 2D 11B dipolar double-quantum single-quantum homonuclear correlation NMR experiments were used to unambiguously probe boron-boron connectivity within all heterogeneous catalysts. These experiments are practically infeasible at lower, more conventional magnetic fields due to a lack of resolution and reduced NMR sensitivity. The detailed molecular structures determined for the amorphous oxidized/hydrolyzed boron layers on these heterogenous catalysts will aid in the future development of next generation ODH catalysts.
RESUMEN
In recent years, hexagonal boron nitride (hBN) has emerged as an unexpected catalyst for the oxidative dehydrogenation of alkanes. Here, the versatility of hBN was extended to alkane oxidative cracking chemistry by investigating the production of ethylene and propylene from n-butane. Cracking selectivity was primarily controlled by the ratio of n-butane to O2 within the reactant feed. Under O2 -lean conditions, increasing temperature led to increased selectivity to ethylene and propylene and decreased selectivity to COx . In addition to surface-mediated chemistry, homogeneous gas-phase reactions likely contributed to the observed product distribution, and a reaction mechanism was proposed based on these observations. The catalyst showed good stability under oxidative cracking conditions for 100â h time-on-stream while maintaining high selectivity to ethylene and propylene.
RESUMEN
Hexagonal boron nitride (h-BN) and boron nitride nanotubes (BNNT) were recently reported as highly selective catalysts for the oxidative dehydrogenation (ODH) of alkanes to olefins in the gas phase. Previous studies revealed a substantial increase in surface oxygen content after exposure to ODH conditions (heating to ca. 500 °C under a flow of alkane and oxygen); however, the complexity of these materials has thus far precluded an in-depth understanding of the oxygenated surface species. In this contribution, we combine advanced NMR spectroscopy experiments with scanning electron microscopy and soft X-ray absorption spectroscopy to characterize the molecular structure of the oxygen functionalized phase that arises on h-BN and BNNT following catalytic testing for ODH of propane. The pristine BN materials are readily oxidized and hydrolyzed under ODH reaction conditions to yield a phase consisting of three-coordinate boron sites with variable numbers of hydroxyl and bridging oxide groups which is denoted B(OH) xO3- x (where x = 0-3). Evidence for this robust oxide phase revises previous literature hypotheses of hydroxylated BN edges as the active component on h-BN.
RESUMEN
Light olefins such as ethylene and propylene form the foundation of the modern chemical industry, with yearly production volumes well into the hundreds of millions of metric tons. Currently, these light olefins are mainly produced via energy-intensive steam cracking. Alternatively, oxidative dehydrogenation (ODH) of light alkanes to produce olefins allows for lower operation temperatures and extended catalyst lifetimes, potentially leading to valuable process efficiencies. The potential benefits of this route have led to significant research interest due to the wide availability of natural gas from shale deposits. Advances in this area have still not yielded catalysts that are sufficiently selective to olefins for industrial implementation, and ODH still remains a holy grail of selective alkane oxidation research. The main challenge in selective oxidation lies in preventing the overoxidation of the desired product, such as propylene during propane oxidation, to CO and CO2. Research into selective heterogeneous catalysts for the oxidative dehydrogenation of propane has led to the extensive use of vanadium oxide-based catalysts, and studies on the surface mechanism involved have been used to improve the catalytic activity of the material. Despite decades of research, however, selectivity toward propylene has not proven satisfactory at industrially relevant conversions. It is imperative for new catalytic systems that minimize product overoxidation to be developed for future applications of oxidative dehydrogenation processes. While rational catalyst design has been successful in developing homogeneous catalyst systems, its practical use in heterogeneous catalyst development remains modest. The complexity of surfaces with a variety of terminations and bulk structures, let alone their modification by the chemical potential of a reaction mixture, makes heterogeneous catalyst discovery serendipitous in many cases. The catalyst family presented in this Account is no exception. The importance of catalysis research lies in exploring the science behind serendipity. In this Account, we will first present our initial discovery of boron nitride (BN) as an unexpected catalyst for the oxidative dehydrogenation of light alkanes. Beyond its surprising activity, BN also drew interest due to its low selectivity to carbon oxides. This observation made BN distinct from previously studied metal oxide catalysts for selective alkane oxidation. We narrowed down its unique reactivity to the oxygen functionalization of the catalyst surface, particularly the formation of B-O species as probed by various spectroscopic techniques. In investigating the critical role of each of the structural elements during ODH, we discovered that not only BN but an entire class of boron-containing compounds are active and selective for the formation of propylene from propane. All these materials form a complex oxidized surface with a distribution of BO x surface sites. This discovery opens the doors to a new field of boron-based oxidation chemistry that currently has more questions than answers. We aim to make this Account a starting point for the research community to explore these new materials to understand their surface mechanisms and the surface species that offer a unique selectivity toward olefinic products. Effective use of these materials may lead to novel processes for efficient use of abundant light alkane resources by oxidation chemistry.
RESUMEN
Heterogeneous metal oxide catalysts are widely studied for the aerobic oxidations of C1-C4 alkanes to form olefins and oxygenates. In this review, we outline the properties of supported metal oxides, mixed-metal oxides, and zeolites and detail their most common applications as catalysts for partial oxidations of light alkanes. By doing this we establish similarities between different classes of metal oxides and identify common themes in reaction mechanisms and research strategies for catalyst improvement. For example, almost all partial alkane oxidations, regardless of the metal oxide, follow Mars-van Krevelen reaction kinetics, which utilize lattice oxygen atoms to reoxidize the reduced metal centers while the gaseous O2 reactant replenishes these lattice oxygen vacancies. Many of the most-promising metal oxide catalysts include V5+ surface species as a necessary constituent to convert the alkane. Transformations involving sequential oxidation steps (i.e., propane to acrylic acid) require specific reaction sites for each oxidation step and benefit from site isolation provided by spectator species. These themes, and others, are discussed in the text.
