Xianfeng Yi

Comprehensive investigation of CO2 adsorption on Mg-Al-CO3 LDH-derived mixed metal oxides

Layered double hydroxides (LDHs) have been intensively studied for high-temperature CO2 capture. However, big differences in the CO2 capture capacity, ranging from 0.28 to 0.6 mmol g−1, have often been reported for the same Mg–Al–CO3 LDH. Furthermore, how the active Mg–O species that are responsible for CO2 adsorption are formed is still unclear. In this work, we have performed a comprehensive investigation on the CO2 adsorption characteristics of Mg–Al–CO3 LDH-derived mixed metal oxides. Based on these results we proposed the possible adsorption sites and the mechanisms for CO2 adsorption. Initially, the effects of synthesis method, Mg : Al ratio, pretreatment conditions, adsorption conditions, and thermal stability on the CO2 adsorption capacity were systematically studied. By carefully examining the structural changes during thermal treatment using X-ray diffraction and solid state NMR, we suggest that the active Mg–O species could be induced either by the substitution of Mg2+ by Al3+ in the periclase MgO lattice, or by the diffusion of Al3+ out of the octahedral brucite layers. This work not only suggests the optimal testing conditions for LDH-derived CO2 adsorbents, but also provides a clearer understanding of the CO2 adsorption sites and mechanisms on LDH-derived mixed oxides and sheds light on the synthesis and utilization of LDH-derived high-temperature CO2 adsorption materials.

Insights of the Crystallization Process of Molecular Sieve AlPO4-5 Prepared by Solvent-Free Synthesis

Crystallization of AlPO4-5 with AFI structure under solvent-free conditions has been investigated. Attention was mainly focused on the characterization of the intermediate phases formed at the early stages during the crystallization. The development in the long-range ordering of the solid phases as a function of crystallization time was monitored by XRD, SEM, IR, UV-Raman, and MAS NMR techniques. Particularly, the UV-Raman spectroscopy was employed to obtain the information on the formation process of the framework. J-HMQC (27)Al/(31)P double-resonance NMR experiments were used to identify the P-O-Al bonded species in the intermediate phases. For the first time the P-O-Al bonded species in the intermediate phases can be correctly described through using this advanced NMR technique. The crystallization under solvent-free conditions appears to follow the pathway: The initial amorphous raw material is converted to an intermediate phase which has four-/six-membered ring species, then gradually transformed into crystalline AlPO4-5. This observation is not consistent with the common idea that the intermediate phase is the semicrystalline intermediates with a three-dimensional structure.

Efficient biomass transformations catalyzed by graphene-like nanoporous carbons functionalized with strong acid ionic liquids and sulfonic groups

Strong acid ionic liquids and sulfonic group bifunctional graphene-like nanoporous carbons (GNC-SO3H-ILs) have been synthesized by treating nitrogen containing graphene-like nanoporous carbons (GNCs) with 1,3-propanesultone, ion exchanging with HSO3CF3 or H2SO4. Introducing nitrogen is important for grafting strong acid ionic liquids and sulfonic group in GNCs, which were synthesized from carbonization of a mixture of dicyandiamide or melamine and glucose. GNC-SO3H-ILs possess abundant nanopores, nanosheet structure, good dispersion and controlled acidity. By themselves, they are capable of enhancing the fast diffusion of reactants and products, while increasing the exposure degree of acidic sites in GNC-SO3H-ILs throughout various reactions. The above characteristics resulted in their much improved catalytic activity in biomass transformations such as the production of biodiesel and depolymerization of crystalline cellulose into sugars, which was even comparable to those of homogeneous ionic liquid and mineral acids.

Slight channel difference influences the reaction pathway of methanol-to-olefins conversion over acidic H-ZSM-22 and H-ZSM-12 zeolites

The methanol to olefins (MTO) process, in which low-value carbon-rich feedstocks are converted to value-added petrochemical products, is one of the most prominent alternatives for the production of light olefins. In order to reveal the confinement effect of zeolites on the catalytic reactions, the MTO mechanisms and reactivity over two unidimensional zeolites (H-ZSM-12 and H-ZSM-22) with a channel difference of only 0.3 A have been systematically explored by DFT calculations in this work. The calculated activation barriers and reaction energies demonstrated that the 0.3 A channel difference between H-ZSM-12 and H-ZSM-22 zeolites results in a dramatic discrepancy in their transition state selectivity associated with the aromatic-based hydrocarbon pool (HCP) mechanism. For the larger H-ZSM-12 zeolite, the formation of pentamethybenzenium cation was favored, which would be the active HCP species in the MTO reaction. For the H-ZSM-22 zeolite with a 0.3 A smaller pore structure, the traditional methylation at the C–H sites of polymethylbenzenes occurred exclusively. When the alternative olefin-based cycle is followed for the MTO reaction, both of the zeolites are active catalysts for the formations of butene and propene. A comparison of the activation barriers for the olefin-based cycle revealed that the larger H-ZSM-12 possesses a higher catalytic activity than the H-ZSM-22 zeolite. Our theoretical results demonstrate that both the aromatic-based cycle and the olefin-based cycle can proceed during the MTO reaction over H-ZSM-12, with the latter cycle being predominant.

Experimental Evidence on the Formation of Ethene through Carbocations in Methanol Conversion over H-ZSM-5 Zeolite

The methanol to olefins conversion over zeolite catalysts is a commercialized process to produce light olefins like ethene and propene but its mechanism is not well understood. We herein investigated the formation of ethene in the methanol to olefins reaction over the H-ZSM-5 zeolite. Three types of ethylcyclopentenyl carbocations, that is, the 1-methyl-3-ethylcyclopentenyl, the 1,4-dimethyl-3-ethylcyclopentenyl, and the 1,5-dimethyl-3-ethylcyclopentenyl cation were unambiguously identified under working conditions by both solid-state and liquid-state NMR spectroscopy as well as GC-MS analysis. These carbocations were found to be well correlated to ethene and lower methylbenzenes (xylene and trimethylbenzene). An aromatics-based paring route provides rationale for the transformation of lower methylbenzenes to ethene through ethylcyclopentenyl cations as the key hydrocarbon-pool intermediates.

Strong or weak acid, which is more efficient for Beckmann rearrangement reaction over solid acid catalysts?

The effects of Bronsted acid strength and pore confinement on the Beckmann rearrangement (BR) reaction over solid acid catalysts have been explored. With the help of catalytic evaluation experiments, it is demonstrated that oximes with different size (cyclohexanone oxime and acetoxime) exhibit quite different BR reactivity dependence on the acid strength over microporous and mesoporous zeolites. In order to reveal the origin of such a difference, electronic structure calculations and kinetic analysis were performed. It was theoretically found that the confinement effect from the microporous zeolite framework has a more significant influence on the rate-determining step of the BR reaction when the oxime reactant is well-confined inside the microporous voids, which in turn controls the BR reactivity.

Interconnected hierarchical HUSY zeolite-loaded Ni nano-particles probed for hydrodeoxygenation of fatty acids, fatty esters, and palm oil

Hierarchical H-style ultra-stable Y (HUSY) zeolites with abundant interconnected mesopores have been prepared using a sequential post-synthesis strategy that includes steaming dealumination and mixed-alkali desilication. The steaming treatment generates a broad size range of intra-mesopores (around 25 and 45 nm) and a moderate Si/Al ratio of 13.4 in the HUSY, which provides optimal material precursors for the ensuing subsequent alkaline desilication. N2 adsorption–desorption isotherms and X-ray diffractometry results indicate that the sample treated with pyridine/sodium hydroxide (HUSY-4) has a larger external surface area and a higher relative crystallinity. Infrared spectra of adsorbed pyridine show that HUSY-4 contains substantial Bronsted acid sites. The 27Al and 29Si nuclear magnetic resonance spectra show that HUSY-4 possesses few extra-framework alumina species. Infrared spectra in a vacuum show that the peak intensities of HUSY-4 in the bridged hydroxyl group (at 3560 and 3631 cm−1) are much stronger than those of the sample treated with tetrapropylammonium hydroxide (HUSY-3), indicating that the framework integrity of HUSY-4 is better. Differences in treatments with tetrapropylammonium hydroxide/sodium hydroxide and pyridine/sodium hydroxide treatments are attributed to the fact that the pyridine molecule (0.54 nm) can pass through the supercages (0.74 nm) to protect the zeolite framework from deep desilication, whereas the tetrapropylammonium hydroxide molecule (0.85 nm) is adsorbed only on the external surface. Eventually, a HUSY zeolite with a high external surface area, inter-connectedness and hierarchical mesopores (10, 25, and 45 nm) is prepared by initial high-temperature steaming, which is followed by desilication using a mixed alkali solution containing pyridine and sodium hydroxide. High-dispersion (5.5%), high-content (35 wt%), small Ni nanoparticles (4.9 ± 1.2 nm) are loaded onto and into the external surface areas and interpores of the hierarchical HUSY by the deposition–precipitation method. The resultant Ni/HUSY-4 shows an ultra-high efficiency in the hydrodeoxygenation of fatty acids, esters, and palm oil, and achieves high initial rates (60 g g−1 h−1) and a high C18 alkane selectivity (96%), which may be attributed to the enhanced Bronsted acid and adjacent Lewis acid (confirmed by the 1H DQ MAS NMR spectrum) together with the substantial dispersive Ni nanoparticles loaded onto/into the interconnected pores of the hierarchical HUSY support.

Preparation of Mesoporous Zeolite ETS‐10 Catalysts for High‐Yield Synthesis of α,β‐Epoxy Ketones

Developing highly active heterogeneous catalysts for the efficient construction of valuable building blocks is of great importance to synthetic chemistry. For this purpose, a mesoporous zeolite ETS‐10 (METS‐10) is synthesized by using a mesoscale silane surfactant as a template and applied to achieve highly efficient syntheses of α,β‐epoxy ketones by employing simple alkenes and aldehydes as starting materials. The high activity of the METS‐10 catalyst is attributed to its unique porous structure and basicity. Electron paramagnetic resonance characterization results and theoretical calculation experimental data reveal that the strong basic sites on METS‐10 catalyst can activate the reaction substrate and intermediate. In addition, the mesopores in METS‐10 catalyst benefit the mass transfer and further improve the catalytic activity.

External or internal surface of H-ZSM-5 zeolite, which is more effective for the Beckmann rearrangement reaction?

Zeolites are effective catalysts for amide formation from oxime through the Beckmann rearrangement (BR) reaction; however, debates about which surface (i.e. external or internal) is more effective for the BR reaction over zeolites still remain. In this contribution, the effective rate constants (keff) are used to quantitatively evaluate the dependence of BR reactivity on the Bronsted acid location in H-ZSM-5 zeolite. Based on our theoretical calculations, it was found that in addition to the dimension size of oxime reactants and reaction temperature, the BR reaction is strongly dependent on the location of Bronsted acid sites. For the cyclohexanone oxime rearrangement, the reaction exclusively occurs on the internal surface of ZSM-5 zeolite at room temperature, while the active sites are those located at the pore mouth or on the external surface when the reaction temperature increases to 598 K. In contrast to cyclohexanone oxime, the Bronsted acid sites on the internal surface are kinetically more effective at room temperature or 598 K for the smaller acetoxime BR reaction.

Zirconium Oxide Supported Palladium Nanoparticles as a Highly Efficient Catalyst in the Hydrogenation–Amination of Levulinic Acid to Pyrrolidones

The selective hydrogenation–amination of levulinic acid into pyrrolidones is regarded as one of the most important reactions to transform biomass‐derived lignocellulose feedstocks into valuable chemicals. Here we report on ZrO2‐supported Pd nanoparticles as a highly active, chemoselective, and reusable catalyst for the hydrogenation–amination of levulinic acid with H2 and various amines under mild reaction conditions. The Pd/ZrO2 catalyst exhibited a marked increase in activity compared with conventional Pd catalysts and a significant enhancement in pyrrolidone selectivity. The excellent catalytic performances are reasonably attributed to the ZrO2 support, which has strong Lewis acidity to enhance the hydrogenation–amination reaction and hinder side reactions.