Carbon Nanotube/Zeolite Hybrid Catalysts for Glucose Conversion in Water/Oil Emulsions

The isomerization of glucose to fructose and its subsequent dehydration to hydroxymethylfurfural (HMF) have been investigated on nanohybrid catalysts that stabilize emulsions comprising aqueous and organic phases. Significant improvement in catalyst stability was observed when NaX faujasite catalysts were functionalized with multiwalled carbon nanotubes (MWCNT-NaX), with a large fraction of the initial activity and selectivity preserved after several recycles. The combination of MWCNT-NaX, containing Lewis acid sites, and MWCNT-SO₃H, containing Brønsted acid sites, enables glucose isomerization and fructose dehydration at high conversion and HMF selectivity. The use of a water/oil biphasic emulsion favors the continuous separation of the HMF product into the organic phase. Furthermore, selective conversion of HMF into added-value products can be accomplished in the same emulsion by incorporating a metallic function on the amphiphilic nanohybrids in the presence of hydrogen. Depending on the metal used, different final products can be obtained. For example, when Ru was added, the main product was 2,5-hexanedione (47.8 mol %), followed by 2,5-bis­(hydroxymethyl)­furan (15 mol %) and γ-hydroxyvaleric acid (7.8 mol %). When Pd was used, γ-hydroxyvaleric acid (84 mol %) dominated the product distribution, with only small amounts of 2,5-bis­(hydroxymethyl)­furan (2.9 mol %)

Enhanced Activity and Selectivity of Fischer–Tropsch Synthesis Catalysts in Water/Oil Emulsions

Amphiphilic nanohybrid catalysts (Ru particles supported on carbon nanotube–metal oxide hybrids) enable the formation of water-in-oil emulsions and have a positive influence on the Fischer–Tropsch synthesis (FTS) activity and selectivity to desirable products in comparison with those obtained in single-phase solvents under the same reaction conditions. The reaction experiments were conducted at 473 K in a batch reactor that uses H₂/CO syngas as a feed at 2.0–3.5 molar ratio and 4136.85 kPa total pressure. One of the main effects observed when using the biphasic mixture instead of a single solvent is the spontaneous separation of products by solubility differences, which affect mass-transfer-dependent secondary reactions. Another positive effect of using the biphasic system arises from the enhanced FTS activity observed in the presence of condensed aqueous phase. Finally, the presence of an emulsion seems to improve the C₁/C₅₊ product balance, which can be explained by a dual-site model recently proposed in the literature

Propagation of Interfacially Active Carbon Nanohybrids in Porous Media

Interfacially active carbon nanotube hybrids (nanohybrids) exhibit promising properties for potential applications in reservoir systems. They could be used as modifiers of transport properties as well as nanoscale vehicles for catalyst and contrast agents. <i>In situ</i> catalysis might be used to modify interfacial tension and wettability of the rock wall. The main requirements for any of these applications are the ability to form stable dispersions and to effectively propagate through the reservoir porous medium under the temperature and salinity conditions that are typical in commercial operations. In this work, suspensions of purified multi-walled carbon nanotubes (P-MWNTs) in deionized water and high-salinity brine have been prepared using two commercially available polymers, polyvinyl pyrrolidone (PVP) and hydroxyethyl cellulose (HEC-10). Stable dispersions were put in contact with crushed Berea sandstone, quantifying the amount of nanotubes lost from suspension to estimate the adsorption of these nanotubes from suspension onto the walls of the reservoir rocks. Adsorption isotherms were measured from room temperature up to 80 °C from aqueous suspensions with salinities up to 10%. These studies demonstrate that combining these two polymers stabilizes suspensions in high-salinity water and minimizes adsorption on the sand walls. It is proposed that this optimized behavior is due to additive electrostatic and steric repulsions. While the polar PVP helps disaggregation by effectively wrapping individual nanotubes (primary dispersant), the bulky HEC-10 inhibits the reaggregation in saline solutions (secondary dispersant). Column experiments were conducted to study the propagation of these suspensions through porous media. It was found that a small amount of nanohybrids adsorbed to the sand will be able to saturate available adsorption sites, resulting in subsequent injections of nanohybrids to be propagated completely through the column without adsorption. In that sense, we were able to reach 100% of the injected concentration with a low particle concentration of 100 ppm and total particle adsorption to the sand of less than 10% at room temperature

Confirmation of K-Momentum Dark Exciton Vibronic Sidebands Using ¹³C-labeled, Highly Enriched (6,5) Single-walled Carbon Nanotubes

A detailed knowledge of the manifold of both bright and dark excitons in single-walled carbon nanotubes (SWCNTs) is critical to understanding radiative and nonradiative recombination processes. Exciton–phonon coupling opens up additional absorption and emission channels, some of which may “brighten” the sidebands of optically forbidden (dark) excitonic transitions in optical spectra. In this report, we compare ¹²C and ¹³C-labeled SWCNTs that are highly enriched in the (6,5) species to identify both absorptive and emissive vibronic transitions. We find two vibronic sidebands near the bright ¹E₁₁ singlet exciton, one absorptive sideband ∼200 meV above, and one emissive sideband ∼140 meV below, the bright singlet exciton. Both sidebands demonstrate a ∼50 cm^–1 isotope-induced shift, which is commensurate with exciton–phonon coupling involving phonons of A₁^′ symmetry (D band, ω ∼ 1330 cm^–1). Independent analysis of each sideband indicates that both sidebands arise from the same dark exciton level, which lies at an energy approximately 25 meV above the bright singlet exciton. Our observations support the recent prediction of, and mounting experimental evidence for, the dark K-momentum singlet exciton lying ∼25 meV (for the (6,5) SWCNT) above the bright Γ-momentum singlet. This study represents the first use of ¹³C-labeled SWCNTs highly enriched in a single nanotube species to unequivocally confirm these sidebands as vibronic sidebands of the dark K-momentum singlet exciton

Fischer–Tropsch Synthesis Catalyzed by Solid Nanoparticles at the Water/Oil Interface in an Emulsion System

Fischer–Tropsch synthesis (FTS) was carried out in a water/oil mixture medium, using a Ru catalyst supported on a multi-walled carbon nanotube/MgO–Al₂O₃ hybrid as a catalyst support. The nanohybrid particles at the water/oil interface facilitated and stabilized the formation of water-in-oil emulsion, giving rise to an oil/emulsion/water trilayer liquid structure. FTS occurred at the emulsion phase with much higher conversion rates than those in oil single-phase reactions, yielding products with Anderson–Schulz–Flory distribution. Alkane-enriched hydrocarbons migrate to the top oil phase, while short alcohols remain in the bottom water phase. Thus, this multiphase liquid structure facilitates the separation of products according to their solubility in different phases. This significant advantage of combined reaction and separation is unique to the multiphasic system. In addition, differences in solubility could be used to enhance tolerance against impurities and catalyst poisons in the syngas feedstock. As a preliminary case study, hydrochloric acid and pyridine were chosen as model contaminants commonly found in biosyngas. It was found that their presence did not affect the catalytic activity as severely as could be expected in a conventional FTS process. Thus, emulsion-phase FTS could be beneficial to operations where syngas production such as biomass gasification and FTS are integrated. The several advantages of using emulsion systems in FTS are discussed in light of the current results

Different Product Distributions and Mechanistic Aspects of the Hydrodeoxygenation of m‑Cresol over Platinum and Ruthenium Catalysts

Experimental measurements of the conversion of m-cresol over Pt and Ru/SiO₂ catalysts show very different product distributions, even when the reaction is conducted at similarly low conversions and the same operating conditions (300 °C, 1 atm). That is, although ring hydrogenation to 3-methylcyclohexanone is dominant over Pt, deoxygenation to toluene and C–C cleavage to C₁–C₅ hydrocarbons prevail over Ru. For understanding the differences in reaction mechanisms responsible for this contrasting behavior, the conversion of m-cresol over the Pt(111) and Ru(0001) surfaces has been analyzed using density functional theory (DFT) methods. The DFT results show that the direct dehydroxylation of m-cresol is unfavorable over the Pt(111) surface with an energy barrier of 242 kJ/mol. In turn, the calculations suggest that the reaction could proceed through a keto tautomer intermediate, which undergoes hydrogenation of the carbonyl group followed by dehydration to form toluene and water. At the same time, a low energy barrier for the ring hydrogenation path toward 3-methylcyclohexanone compared to the energy barrier for the deoxygenation path toward toluene over the Pt(111) surface is in agreement with the experimental observations, which show that 3-methylcyclohexanone is the dominant product over Pt/SiO₂ at low conversions. By contrast, the direct dehydroxylation of m-cresol becomes more favorable than the tautomerization route over the more oxophilic Ru(0001) surface. In this case, the deoxygenation path exhibits an energy barrier lower than that for the ring hydrogenation, which is also in agreement with experimental results that show higher selectivity to the deoxygenation product toluene. Finally, it is proposed that a partially unsaturated hydrocarbon surface species C₇H₇* is formed during the direct dehydroxylation of m-cresol over Ru(0001), becoming the crucial intermediate for the C–C bond breaking products C₁–C₅ hydrocarbons, which are observed experimentally over the Ru/SiO₂ catalyst

Systems-Level Analysis of Energy and Greenhouse Gas Emissions for Coproducing Biobased Fuels and Chemicals: Implications for Sustainability

In light of advances in the simultaneous production of biobased fuels and chemicals, a prospective well-to-wheel lifecycle assessment (LCA) model of a two-step multistage torrefaction biorefinery is constructed to quantify both lifecycle greenhouse gas (GHG) emissions and energy return on primary fossil energy investment (EROI_fossil) for a transportation-range biofuel product. Coproductsincluding cyclopentanone (CPO), biochar, and a potential net electricity exportare handled via six coproduct scenarios, evaluated across both market-based allocation and displacement methods. Process-scale performance metrics and product distributions are compared across cases to evaluate trade-offs between process and environmental performance; carbon flows are visualized to better explain patterns of carbon yield and waste. LCA results include median GHG values spanning from −30.8 to +36.1 g CO₂e/MJ-fuel and median EROI_fossil values ranging from 1.6 to 12.8 MJ-fuel/MJ-PE_fossil. Sensitivity results for the Market CPO case under market-based allocation display a large dependence on CPO yield, hydrogen consumption and fuel and CPO prices, while exhibiting minimal dependence on liquid fuel yield. Unrealistically low lifecycle GHG and high EROI_fossil values are obtained under displacement for the maximum level of CPO production, prompting a discussion of methodological limitations, especially as they relate to the assignment of system expansion coproduct credit within existing EROI formulations

Systems-Level Analysis of Energy and Greenhouse Gas Emissions for Coproducing Biobased Fuels and Chemicals: Implications for Sustainability

In light of advances in the simultaneous production of biobased fuels and chemicals, a prospective well-to-wheel lifecycle assessment (LCA) model of a two-step multistage torrefaction biorefinery is constructed to quantify both lifecycle greenhouse gas (GHG) emissions and energy return on primary fossil energy investment (EROI_fossil) for a transportation-range biofuel product. Coproductsincluding cyclopentanone (CPO), biochar, and a potential net electricity exportare handled via six coproduct scenarios, evaluated across both market-based allocation and displacement methods. Process-scale performance metrics and product distributions are compared across cases to evaluate trade-offs between process and environmental performance; carbon flows are visualized to better explain patterns of carbon yield and waste. LCA results include median GHG values spanning from −30.8 to +36.1 g CO₂e/MJ-fuel and median EROI_fossil values ranging from 1.6 to 12.8 MJ-fuel/MJ-PE_fossil. Sensitivity results for the Market CPO case under market-based allocation display a large dependence on CPO yield, hydrogen consumption and fuel and CPO prices, while exhibiting minimal dependence on liquid fuel yield. Unrealistically low lifecycle GHG and high EROI_fossil values are obtained under displacement for the maximum level of CPO production, prompting a discussion of methodological limitations, especially as they relate to the assignment of system expansion coproduct credit within existing EROI formulations

Unraveling the ¹³C NMR Chemical Shifts in Single-Walled Carbon Nanotubes: Dependence on Diameter and Electronic Structure

The atomic specificity afforded by nuclear magnetic resonance (NMR) spectroscopy could enable detailed mechanistic information about single-walled carbon nanotube (SWCNT) functionalization as well as the noncovalent molecular interactions that dictate ground-state charge transfer and separation by electronic structure and diameter. However, to date, the polydispersity present in as-synthesized SWCNT populations has obscured the dependence of the SWCNT ¹³C chemical shift on intrinsic parameters such as diameter and electronic structure, meaning that no information is gleaned for specific SWCNTs with unique chiral indices. In this article, we utilize a combination of ¹³C labeling and density gradient ultracentrifugation (DGU) to produce an array of ¹³C-labeled SWCNT populations with varying diameter, electronic structure, and chiral angle. We find that the SWCNT isotropic ¹³C chemical shift decreases systematically with increasing diameter for semiconducting SWCNTs, in agreement with recent theoretical predictions that have heretofore gone unaddressed. Furthermore, we find that the ¹³C chemical shifts for small diameter metallic and semiconducting SWCNTs differ significantly, and that the full-width of the isotropic peak for metallic SWCNTs is much larger than that of semiconducting nanotubes, irrespective of diameter

Hydrodeoxygenation of Phenol over Pd Catalysts. Effect of Support on Reaction Mechanism and Catalyst Deactivation

This work investigates the effect of the type of support (SiO₂, Al₂O₃, TiO₂, ZrO₂, CeO₂, and CeZrO₂) on the performance of Pd-based catalysts for the hydrodeoxygenation of phenol at 573 K using a fixed-bed reactor. Product distribution is significantly affected by the type of support. Benzene was the major product over Pd/TiO₂ and Pd/ZrO₂; on the other hand, cyclohexanone was the main compound over Pd/SiO₂, Pd/Al₂O₃, Pd/CeO₂, and Pd/CeZrO₂. A reaction mechanism based on the tautomerization of phenol was proposed on the basis of DRIFTS experiments and catalytic tests with the intermediate products. The high selectivity to benzene over Pd/TiO₂ and Pd/ZrO₂ catalysts is likely due to the oxophilic sites of this support represented by incompletely coordinated Ti⁴⁺ and Zr⁴⁺ cations in close proximity to the periphery of metal particles. The greater interaction between oxygen in the keto-tautomer intermediate with oxophilic sites promotes the selective hydrogenation of CO bond. Pd/SiO₂, Pd/Al₂O₃, Pd/TiO₂, and Pd/ZrO₂ catalysts significantly deactivated during TOS. However, Pd/CeO₂ and Pd/CeZrO₂ were more stable, and only slight losses in activity were observed. Carbon deposits were not detected by Raman spectroscopy after reaction. DRIFTS experiments under reaction conditions revealed a buildup of phenoxy and intermediate species during reaction. These species remained adsorbed on the Lewis acid sites, blocking those sites and inhibiting further reactant adsorption. The growth of Pd particle size and the reduction in acid site density during HDO of phenol were the primary routes of catalyst deactivation. The higher stability of Pd/CeO₂ and Pd/CeZrO₂ catalysts is likely due to the higher amount of oxygen vacancies of these supports