Mechanisms of the Deactivation of SAPO-34 Materials with Different Crystal Sizes Applied as MTO Catalysts

SAPO-34 materials with comparable Brønsted acid site density but different crystal sizes were applied as methanol-to-olefin (MTO) catalysts to elucidate the effect of the crystal size on their deactivation behaviors. ¹³C HPDEC MAS NMR, FTIR, and UV/vis spectroscopy were employed to monitor the formation and nature of organic deposits, and the densities of accessible Brønsted acid sites and active hydrocarbon-pool species were studied as a function of time-on-stream (TOS) by ¹H MAS NMR spectroscopy. The above-mentioned spectroscopic methods gave a very complex picture of the deactivation mechanism consisting of a number of different steps. The most important of these steps is the formation of alkyl aromatics with large alkyl chains improving at first the olefin selectivity, but hindering the reactant diffusion after longer TOS. The hindered reactant diffusion leads to a surplus of retarded olefinic reaction products in the SAPO-34 pores accompanied by their oligomerization and the formation of polycyclic aromatics. Finally, these polycyclic aromatics are responsible for a total blocking of the SAPO-34 pores, making all catalytically active sites inside the pores nonaccessible for further reactants

Improved Postsynthesis Strategy to Sn-Beta Zeolites as Lewis Acid Catalysts for the Ring-Opening Hydration of Epoxides

Nanocrystalline Sn-Beta zeolites have been successfully prepared via an improved two-step postsynthesis strategy, which consists of creating vacant T sites with associated silanol groups by dealumination of parent H-Beta and subsequent dry impregnation of the resulting Si-Beta with organometallic dimethyltin dichloride. Characterization results from UV–vis, XPS, Raman, and ¹¹⁹Sn solid-state MAS NMR reveal that most Sn species have been successfully incorporated into the framework of Beta zeolite through the postsynthesis process and exist as isolated tetrahedral Sn­(IV) in open arrangement. The creation of strong Lewis acid sites upon Sn incorporation is confirmed by FTIR spectroscopy with pyridine adsorption. The Sn-Beta Lewis acid catalysts are applied in the ring-opening hydration of epoxides to the corresponding 1,2-diols under near ambient and solvent-free conditions, and remarkable activity can be obtained. The impacts of Lewis acidity, preparation parameters, and reaction conditions on the catalytic performance of Sn-Beta zeolites are discussed in detail

Mechanistic Insights into One-Step Catalytic Conversion of Ethanol to Butadiene over Bifunctional Zn–Y/Beta Zeolite

Bifunctional Zn–Y/Beta catalyst was applied in the reaction mechanism study of the ethanol to butadiene conversion to clarify the roles of Zn and Y functional sites in each individual reaction step. According to the results of several complementary methods, i.e., ethanol temperature-programmed desorption (TPD), temperature-programmed surface reaction (TPSR), and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), the reaction network consisting of several key steps, i.e., ethanol dehydrogenation, acetaldehyde aldol condensation, and crotonaldehyde reduction, was elucidated. An enolization mechanism was verified to involve in the coupling step. During this reaction, the Lewis acidic Zn and Y species in [Si]­Beta zeolite were both active in the ethanol dehydrogenation, aldol condensation, and Meerwein–Ponndorf–Verley reduction. In this cycle, Zn species exhibited the higher dehydrogenation activity but lower coupling activity than that of Y species. Through the combination of the two species in one catalyst, i.e., Zn–Y/Beta, the synergistic effect of the bifunctional sites could be achieved. Our study provides mechanistic insights into the cascade transformation of ethanol to butadiene and the fundamental guidelines for the rational design of eligible catalysts for the reaction

Zeolite Structural Confinement Effects Enhance One-Pot Catalytic Conversion of Ethanol to Butadiene

The one-pot conversion of ethanol to butadiene is a promising route for butadiene production; however, simultaneous attainment of high butadiene productivity and high butadiene selectivity is challenging. Here, zeolite-confined bicomponent Zn–Y clusters were constructed and applied as robust catalysts for ethanol-to-butadiene conversion with a state-of-the-art butadiene productivity of 2.33 g_BD/g_cat/h and butadiene selectivity of ∼63%. Structural confinement effects are responsible for the enhanced butadiene production efficiency via a multiple-step cascade reaction

Lewis Acid Catalysis Confined in Zeolite Cages as a Strategy for Sustainable Heterogeneous Hydration of Epoxides

We report a heterogeneous catalysis strategy to the sustainable hydration of epoxides by designing robust Lewis acid catalysts confined in zeolite cages as natural shape-selective nanoreactors. In the case of ethylene oxide hydration, Sn-H-SSZ-13 zeolite exhibits remarkable catalytic performance, with an ethylene oxide conversion above 99% and a monoethylene glycol selectivity above 99%, at approaching stoichiometric water/ethylene oxide ratios and near-ambient reaction temperatures. It is revealed by theoretical studies that partially hydroxylated Sn species are the preferred Lewis acid sites for the hydration of ethylene oxide. The concept of Lewis acid catalysis confined in zeolite cages may be applied in the future in the chemical industry to develop energy-saving and environmentally benign processes

Understanding the Early Stages of the Methanol-to-Olefin Conversion on H‑SAPO-34

Little is known on the early stages of the methanol-to-olefin (MTO) conversion over H-SAPO-34, before the steady-state with highly active polymethyl­benzenium cations as most important intermediates is reached. In this work, the formation and evolution of carbenium ions during the early stages of the MTO conversion on a H-SAPO-34 model catalyst were clarified via ¹H MAS NMR and ¹³C MAS NMR. Several initial species (i.e., three-ring compounds, dienes, polymethyl­cyclopentenyl, and polymethyl­cyclohexenyl cations) were, for the first time, directly verified during the MTO conversion. Their detailed evolution network was established from theoretical calculations. On the basis of these results, an olefin-based catalytic cycle is proposed to be the primary reaction pathway during the early stages of the MTO reaction over H-SAPO-34. After that, an aromatic-based cycle may be involved in the MTO conversion for long times on stream

Selective Catalytic Hydrogenolysis of Carbon–Carbon σ Bonds in Primary Aliphatic Alcohols over Supported Metals

The selective scission of chemical bonds is always of great significance in organic chemistry. The cleavage of strong carbon–carbon σ bonds in the unstrained systems remains challenging. Here, we report the selective hydrogenolysis of carbon–carbon σ bonds in primary aliphatic alcohols catalyzed by supported metals under relatively mild conditions. In the case of 1-hexadecanol hydrogenolysis over Ru/TiO₂ as a model reaction system, the selective scission of carbon–carbon bonds over carbon–oxygen bonds is observed, resulting in <i>n</i>-pentadecane as the dominant product with a small quantity of <i>n</i>-hexadecane. Theoretical calculations reveal that the 1-hexadecanol hydrogenolysis on flat Ru (0001) undergoes two parallel pathways: i.e. carbon–carbon bond scission to produce <i>n</i>-pentadecane and carbon–oxygen bond scission to produce <i>n</i>-hexadecane. The removal of adsorbed CO on a flat Ru (0001) surface is a crucial step for the 1-hexadecanol hydrogenolysis. It contributes to the largest energy barrier in <i>n</i>-pentadecane production and also retards the rate for <i>n</i>-hexadecane production by covering the active Ru (0001) surface. The knowledge presented in this work has significance not just for a fundamental understanding of strong carbon–carbon σ bond scission but also for practical biomass conversion to fuels and chemical feedstocks

Effect of <i>n</i>‑Butanol Cofeeding on the Methanol to Aromatics Conversion over Ga-Modified Nano H‑ZSM‑5 and Its Mechanistic Interpretation

Ga-modified nano H-ZSM-5 zeolites with different Ga contents were prepared and applied as methanol-to-aromatics (MTA) catalysts. The Ga introduction can strongly increase the selectivity to aromatics but also decrease the catalyst lifetime simultaneously. Upon the cofeeding of <i>n</i>-butanol with methanol, a significant prolongation of the catalyst lifetime from 18 to ca. 50 h can be achieved. According to several spectroscopic results, e.g., TGA, GC–MS, in situ UV/vis, and solid-state MAS NMR spectroscopy, the addition of <i>n</i>-butanol during the MTA conversion shows no impact on the deactivation mechanism but can influence the dual-cycle mechanism. Namely, <i>n</i>-butanol preferentially adsorbs on Brønsted acid sites over methanol, followed by dehydration into <i>n</i>-butene. The formed <i>n</i>-butene can directly participate in the olefin-based cycle and, therefore, significantly alter the proportions of the dual-cycle mechanism. These results provide mechanistic insights into the roles of <i>n</i>-butanol cofeeding in the MTA conversion and exemplify a simple but efficient strategy to prolonged the catalyst lifetime, which is crucial to the industrial application

Photoprompted Hot Electrons from Bulk Cross-Linked Graphene Materials and Their Efficient Catalysis for Atmospheric Ammonia Synthesis

Ammonia synthesis is the single most important chemical process in industry and has used the successful heterogeneous Haber–Bosch catalyst for over 100 years and requires processing under both high temperature (300–500 °C) and pressure (200–300 atm); thus, it has huge energy costs accounting for about 1–3% of human’s energy consumption. Therefore, there has been a long and vigorous exploration to find a milder alternative process. Here, we demonstrate that by using an iron- and graphene-based catalyst, Fe@3DGraphene, hot (ejected) electrons from this composite catalyst induced by visible light in a wide range of wavelength up to red could efficiently facilitate the activation of N₂ and generate ammonia with H₂ directly at ambient pressure using light (including simulated sun light) illumination directly. No external voltage or electrochemical or any other agent is needed. The production rate increases with increasing light frequency under the same power and with increasing power under the same frequency. The mechanism is confirmed by the detection of the intermediate N₂H₄ and also with a measured apparent activation energy only ∼1/4 of the iron based Haber–Bosch catalyst. Combined with the morphology control using alumina as the structural promoter, the catalyst retains its activity in a 50 h test