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An effective carrier concentration n eff is proposed to evaluate the bipolar effect, and the results show good consistency with measured data.
Here, we have explored mechanically embedded three-layer QM/QM/MM ONIOM models for computational studies of binding in Al-substituted zeolites. In all the models considered, the high-level-theory layer consists of the adsorbate molecule and of the framework atoms within the first two coordination spheres of the Al atom and is treated at the M06-2X/6-311G(2df,p) level. For simplicity, flexibility and routine applicability, the outer, low-level-theory layer is treated with the UFF. We have modelled the intermediate-level layer quantum mechanically and investigated the performance of HF theory and of three DFT functionals, B3LYP, M06-2X and ωB97x-D, for different layer sizes and various basis sets, with and without BSSE corrections. We have studied the binding of sixteen probe molecules in H-MFI and compared the computed adsorption enthalpies with published experimental data. We have demonstrated that HF and B3LYP are inadequate for the description of the interactions between the probe molecules and the framework surrounding the metal site of the zeolite on account of their inability to capture dispersion forces. Both M06-2X and ωB97x-D on average converge within ca. 10% of the experimental values. We have further demonstrated transferability of the approach by computing the binding enthalpies of n-alkanes (C1–C8) in H-MFI, H-BEA and H-FAU, with very satisfactory agreement with experiment. The computed entropies of adsorption of n-alkanes in H-MFI are also found to be in good agreement with experimental data. Finally, we compare with published adsorption energies calculated by periodic-DFT for n-C3 to n-C6 alkanes, water and methanol in H-ZSM-5 and find very good agreement.
The adsorption and reaction properties of H-BEA, SnBEA, ZrBEA and siliceous BEA were examined to understand the reaction of 2,5-dimethylfuran (DMF) with ethylene to form p -xylene.
Surface composition of the highly selective Ni–Cu catalysts was characterized by NAP-XPS under the conditions relevant to the HDO reaction.
Wrapping grain boundaries with rGO enhances zT by increasing thermal boundary resistance, R κ , with minimal effect on the electronic transport.
Beyond ZT , ( ZT ) eng was used as a direct indicator for thermoelectric performance to achieve a balance between material tuning and device reliability.
Glucaric acid, a promising bio-based chemical for the production of adipic acid, is produced on a commercial Pt/C catalyst via aerobic oxidation of glucose in water.
The hydrolysis of cellulose and β(1 → 4) oligosaccharides on carbon catalysts is a promising approach for the selective production of glucose from cellulose and its derivatives. In this study, a structure–property relationship was developed for the hydrolysis of water soluble glucan oligomers over three dimensionally ordered mesoporous (3DOm) carbon with sulfonic acid groups. It was found that the number of sulfonic acid groups on 3DOm carbon catalysts and their adsorption properties need to be optimized to maximize the hydrolysis of water soluble glucan oligomers. The adsorption capacity of glucan oligomers on 3DOm carbon increases with their chain length, which can be attributed to two molecular behaviors: (a) long glucan oligomers adsorbed on the carbon surface exhibit a dense packing structure compared with relatively small glucan oligomers; and (b) a part of a glucan oligomer “dangles” off the carbon surface due to its branched structure. Furthermore, it was found that the adsorption enthalpy and entropy increased with increasing the chain length of the glucan oligomers, which favors the hydrolysis reaction to produce glucose over the carbon catalysts.
Catalytic transformation routes for the valorization of biomass-derived 6-amyl-α-pyrone (6PP) were explored for the first time. Ring-opening and decarboxylation of 6PP in water yielded non-2-en-4-one with 95% conversion and nearly 60% selectivity at 498 K, without the requirement of an acid catalyst. The decarboxylated product was further hydrogenated to yield 4-nonanone over a Pd/C catalyst. In order to produce longer chain hydrocarbons of diesel and jet fuel range, C–C coupling via aldol condensation of nonanone with furfural and 5-hydroxymethyl furfural (HMF) was experimented over a mixed oxide (CaO–MgO) catalyst measuring up to 40% and 60% yield of the branched aldol products at 443 K and 393 K respectively.
Levulinic acid (LA) is an important platform intermediate for several high value renewable bioproducts. One-pot synthesis of LA from carbohydrates is disadvantaged by the lack of proximity of the active sites required for sequential isomerization, dehydration and rehydration reactions. Herein, we report an efficient one-pot conversion of C 6 sugars to LA using a carbo-catalyst (ZrHyC), containing Lewis acidic zirconium and Brønsted acidic carboxyl and hydroxyl sites. The proximity of the two acid sites on a hydrothermally prepared carbon support makes the catalyst highly effective enabling a maximum 73% LA yield from glucose conversion. Fructose conversion achieved a higher LA yield (89%) because of a facile reaction without involving an isomerization step. The effects of solvents, the glucose to catalyst ratio, and the reaction time on glucose conversion and LA yield are systemically examined. Detailed characterization of the catalyst using FTIR, Raman spectroscopy, XPS, XRD, FESEM and NH 3 -TPD techniques are presented to elucidate the presence of the two acid sites and zirconium in the +4 oxidation state.
A hierarchical porous metal–organic framework (MOF), PCN-888, containing three types of cavities was utilized to couple two enzymes into a tandem nanoreactor. The largest cavity (6.2 nm) can only accommodate one molecule of glucose oxidase (GOx). The intermediate cavity (5.0 nm) can accommodate one and only one molecule of horseradish peroxidase (HRP). The small cavity (2.0 nm) has sufficient size for neither GOx nor HRP, and remains open as a substrate diffusion pathway. The coupling of the two enzymes can only be achieved through a unique stepwise encapsulation with a specific order (GOx first, followed by HRP). The tandem nanoreactor shows excellent catalytic performances and negligible enzyme leaching. Its catalytic activity is well maintained within several catalytic cycles. Moreover, PCN-888 can provide protection to the encapsulated enzymes against trypsin digestion, indicating the in vitro and in vivo stability of the nanoreactor.
A facile new method for the synthesis of nanostructured Cu 2 O–ZnO and Cu 2 O–ZnO–MgO catalysts has been described.
We report first principles studies of the binary Zn–Sb phases in relation to thermoelectric properties and chemical stability. We identify the unknown structure of the Zn 3 Sb 2 phase using particle swarm optimization, finding a tetragonal structure different from the hexagonal Mg 3 Sb 2 and the hexagonal or cubic Ca 3 Sb 2 phases. All the phases are found to be semiconducting with bandgaps in the range of 0.06–0.77 eV. This semiconducting behavior is understood in Zintl terms as a balance between the Zn:Sb and Sb 3- :½(Sb 2 ) 4- ratios in the stable crystal structures. With the exception of Zn 3 Sb 2 , which has a small gap, all the compounds have electronic properties favorable for thermoelectric performance.
2D zeolite nanosheets can be obtained by shearing or sonication within commercially available telechelic liquid polybutadienes at room temperature.
Straining Bi 2-x Sb x Te 3-y Se y activates additional highly degenerate electronic bands near the Fermi level, providing over two-fold enhancement in thermoelectric power factor.
Herein, Sn was successfully doped into the Sb site of n-type NbCoSb half-Heusler compounds to tune the carrier concentration, and a maximum ZT value of ~0.56 was obtained at 973 K for NbCoSb 1-x Sn x with x = 0.2, an increase of ~40% as compared to that of NbCoSb.
The Brønsted acid site densities of ZSM-5, BEA and single unit cell self-pillared pentasil (SPP) zeolites of varying Si/Al ratios were measured using a new technique, reactive gas chromatography (RGC), which utilizes alkylamine decomposition to selectively count Brønsted acid sites.
A high thermoelectric power factor not only enables a potentially high figure of merit ZT but also leads to a large output power density, and hence it is pivotal to find an effective route to improve the power factor.