An official website of the United States government
Ni/SBA-15 meso-structured catalysts modified with chromium and CeO2 (Ni–Cr-CeO2/SBA-15) were utilized to produce hydrogen from glycerol steam reforming (GSR). The catalysts were synthesized by a one-pot hydrothermal process and extensively characterized by analytical techniques such as N2 adsorption–desorption (BET), H2-temperature programmed reduction (H2-TPR), powder X-ray diffraction (PXRD), inductively coupled plasma-optical emission spectrometry (ICP-OES), and transmission electron microscopy (TEM). The low-angle XRD reflections affirmed that the catalysts were crystalline and possessed a 2D-ordered porosity. The BET results depicted that all the catalysts exhibited a good surface area ranging from 633 to 792m2/g, and the pore sizes were consistently in the mesoporous range (between 3 and 5 nm). TEM analysis of both calcined and spent catalysts revealed that the metal active sites were embedded in the hybrid CeO2-SiO2 support. Overall, the Ni-based catalysts exhibited higher glycerol conversion -12Ni-SBA-15–99.9%, 12Ni3CeO2-SBA-15–89.4%, and 8Ni4Cr3CeO2-SBA-15–99.7%. Monometallic 12Ni/SBA-15 performed exceptionally well, while 12Cr/SBA-15 performed poorly with the highest 71.48% CO selectivity. For short-term GSR reactions, CeO2 addition to 12Ni/SBA-15 did not have any effect, whereas Cr addition resulted in a 32% decrease in H2 selectivity. The long-term stability studies of 12Ni-SBA-15 showed H2 selectivity of ~ 64% and ~ 98% glycerol conversion. However, its activity was short-lived. After 20–30 h, the H2 selectivity and conversion dropped precipitously to 40%. The doping of mesoporous Ni/SBA-15 with Cr and CeO2 remarkably enhanced the long-term stability of the catalyst for 12Ni3CeO2-SBA-15, and 8Ni4Cr3CeO2-SBA-15 catalyst which showed ~ 58% H2 selectivity and ~ 100% conversion for the entire 60 h. Interestingly, Cr and CeO2 seem to improve the shelf-life of Ni-SBA-15 via different mechanistic pathways. CeO2 mitigated Ni poisoning through coke oxidation whereas Cr bolstered the catalyst stability via maintaining a well-defined pore size, structural rigidity, and integrity of the heterogeneous framework, thereby restricting structural collapse, and hence retard sintering of the Ni active sites during the long-term 60 h of continuous reaction. Hydrogen generation from renewable biomass like glycerol could potentially serve as a sustainable energy source and could substantially help reduce the carbon footprint of the environment
more »
« less
Single-site, Ni-modified Wells–Dawson-type polyoxometalate for propylene dimerization
Olefin oligomerization is an essential step in the production of liquid fuels, chemicals, and chemical precursors. Here, we report gas phase propylene oligomerization catalyzed by isolated Ni 2+ sites substituted on the lacunary defect of a Wells–Dawson polyoxometalate (K 8 P 2 W 17 O 61 ·Ni 2+ ) supported on SBA-15 (Ni-POM-WD/SBA-15). The Ni-POM-WD/SBA-15 catalyst exhibited high product selectivity for linear propylene dimers (>76%) relative to branched propylene dimers (<24%). The linear dimer selectivity was independent of the overall propylene conversion between 0.6–5% but was dependent on reaction temperature. The propylene dimerization activation energy was measured as 44.5 kJ mol −1 , which is consistent with the reported values for Ni 2+ exchanged-zeolites for propylene oligomerization. Further, the measured dimerization reaction rate order was a strong function of the initial propylene partial pressure and transitioned from second order to first order at higher propylene partial pressures. The catalyst was fully regenerated after reaction by applying a thermal regeneration step in helium. Transient, time-on-stream catalyst performance measurements showed the catalyst had a mean life of ∼0.6–1.15 h during three reaction cycles and had a slightly increased initial propylene consumption rate with each cycle.
more »
« less
- Award ID(s):
- 1647722
- PAR ID:
- 10431160
- Date Published:
- Journal Name:
- Catalysis Science & Technology
- Volume:
- 12
- Issue:
- 19
- ISSN:
- 2044-4753
- Page Range / eLocation ID:
- 5970 to 5981
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
The effect of catalyst hydrophobicity on the kinetics of hydrogenation of aqueous phenol was investigated. The hydrophobicity of a Pd/SBA-15 catalyst was altered by synthesizing an organosilane with biphenylene framework linkers. Partitioning of phenol between the aqueous solution and the pores favors the hydrophobic catalyst by an order of magnitude at room temperature, relative to the hydrophilic catalyst. The rate of hydrogenation at 75 °C is higher in the hydrophobic catalyst, as is the selectivity for the partial hydrogenation product, cyclohexanone. Analysis of kinetic profiles measured using operando 13C NMR reveals that the hydrophobic catalyst has a larger apparent (i.e., composite) adsorption constant for phenol, which results in higher phenol surface coverage and, consequently, faster and more selective hydrogenation to cyclohexanone.more » « less
-
null (Ed.)Alkene oligomerization on heterogeneous Ni-based catalysts has been studied for several decades, with recent attention focused on the preparation, structure and function of Ni active site motifs isolated within microporous and mesoporous supports, including zeolites and metal–organic frameworks (MOFs). This mini-review focuses on the active site requirements and the microscopic kinetic and mechanistic details that become manifested macroscopically as activation and deactivation behavior during oligomerization catalysis and that determine measured reaction rates and selectivity among alkene isomer products. The preponderance of mechanistic evidence is consistent with the coordination–insertion (Cossee–Arlman) cycle for alkene oligomerization prevailing on heterogeneous Ni-exchanged zeolites and MOFs, even when external co-catalysts are not present, as they often are in homogeneous Ni-based oligomerization catalysis. Certain mechanistic features of the coordination–insertion route allow catalyst and active site design strategies to influence product selectivity. Our mini-review provides a critical discussion of reported alkene oligomerization data and the challenges in their measurement and interpretation and concludes with an outlook for future research opportunities to improve our kinetic and mechanistic understanding of alkene chain growth chemistries mediated by Ni-based porous catalysts.more » « less
-
Biomass is a renewable carbon feedstock that can be converted to 5-hydroxymethylfurfural (HMF), a useful platform chemical that can be modified to produce valuable chemicals and fuels. Previous research has shown that high HMF selectivity can be achieved in organic solvents such as dimethyl sulfoxide (DMSO) because of its capability to stabilize HMF in solution, but DMSO is an undesirable solvent to use industrially as product separation from the reaction solution is difficult. Surface functionalization of porous catalysts has been shown as a method to introduce solvent-effects at the surface of heterogeneous catalysts, thus avoiding the need for high boiling solvents like DMSO. Poly(ethylene sulfoxide) (PESO) is added to the surface of sulfonic acid (SA) functionalized SBA-15 silica to obtain the bifunctional catalyst SA-PESO-SBA-15. Co-localization of the sulfoxide polymer with sulfonic acid groups inside the catalyst pores (SA-PESO-SBA-15) increased HMF selectivity to 51% from 26% obtained by monofunctional SA-SBA-15 at 27% fructose conversion in water. Additionally, this bifunctional catalyst performs best in 4 : 1 (w/w) THF : water cosolvent, a more industrially preferred cosolvent system, obtaining 79% HMF selectivity at 87% fructose conversion. Overall, these materials are promising for the selective conversion of fructose to HMF.more » « less
-
Abstract There has been significant interest in developing new catalytic systems to convert linear chain alkanes into olefins and aromatics. In the case of higher alkanes (≥C6), the production of aromatic compounds such as benzene‐toluene‐xylenes is highly desirable. However, as the length of the carbon chain increases, the dehydrogenation process becomes more complex, not only due to the challenges of C−H activation but also the need for selectivity towards the desired products by the possibility of side reactions such as isomerization and cracking. Here, we present a detailed analysis of the dehydroaromatization of n‐hexane, n‐heptane, and n‐octane, using PtSn intermetallic nanoparticles supported on SBA‐15 as the catalyst. Throughin situspectroscopic and kinetic analysis, we have probed the reaction kinetics and catalyst deactivation, and provided a mechanistic understanding of the dehydroaromatization process on the surface of the PtSn intermetallic nanoparticles. Introducing Sn has been shown to be crucial not only for enhancement of catalytic activity, but also for higher aromatics selectivity and stability on stream. Furthermore, the analysis of dehydroaromatization reaction rates of reactant and possible intermediates indicates that the dehydroaromatization of n‐hexane to benzene likely proceeds through initial dehydrogenation steps followed by ring closing.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D2CY01065H
