High-performing Ir- and Pt-containing catalysts based on mesoporous beta zeolite for the selective ring opening of decalin

[EN] Selective ring opening (SRO) of naphthenic molecules resulting from the hydrogenation of polyaromatics is a desirable catalytic route for upgrading low-quality diesel fractions such as hydrotreated light cycle oil (LCO) produced in catalytic crackers. In this work, catalysts based on either Ir or Pt (similar to 3 wt%) dispersed on a Cs+-exchanged mesoporous beta zeolite (beta-meso) obtained by controlled desilication of a commercial beta sample (beta-com, Si/Al = 18) with NaOH in the presence of a CTAB surfactant were prepared and evaluated for the SRO of decalin as a model reactant. In comparison to an equivalent Ir catalyst based on the commercial zeolite (Ir/Cs-beta-com), the Ir catalyst based on mesoporous beta (Ir/Cs-beta-meso) attained higher decalin conversions and yields of target ring opening products (C-10-alkylcycloalkanes, ROP, and C-10-alkanes or open chain decanes, OCD). An unprecedented maximum combined yield of ROP + OCD of 72.6 wt% was achieved over the Ir/Cs-beta-meso catalyst at 89.2% decalin conversion. The improved catalytic performance exhibited by the catalyst based on mesoporous beta zeolite can be mainly ascribed to an increased accessibility of decalin molecules to the active sites and to a faster diffusion of ring opening products, retarding their further conversion into unwanted lighter (C9-) hydrocarbons. On the other hand, the catalyst based on Pt dispersed on Cs-beta-meso was less active and achieved a lower maximum combined yield of ROP + OCD (59.6 wt% at 92.7% conversion) than its homologous catalyst based on Ir. However, compared to the Ir catalyst, less branched ROP and OCD isomers (hence, more desirable from the viewpoint of cetane) were formed on the Pt catalyst. The molecular structure of the main ROP and OCD isomers, as assessed by GC x GC, and the carbon number distribution of C9- products indicated that cleavage of C-C bonds on the Ir-based catalysts predominantly occurred at Ir centers via the (non-selective) dicarbene hydrogenolysis mechanism. Differently, both the hydrogenolysis (via the multiplet or selective mechanism) and the bifunctional (via carbocations) pathways contributed to the breaking of C-C bonds on the Pt-based mesoporous catalyst.M. A. Arribas and A. Martinez acknowledge the MINECO of Spain for financial support through the Severo Ochoa project (SEV2016-0683). N. Suarez and A. Moreno acknowledge Colciencias-Ecopetrol (project 1115-559-36523) and Universidad de Antioquia (Colombia) for financial support. N. Suarez acknowledges Colciencia-Ecopetrol and the Instituto de Tecnologia Quimica (ITQ). The authors are indebted to the Microscopy Service of the Universitat Politecnica de Valencia for its assistance in microscopy characterizationSuarez, N.; Arribas Viana, MDLD.; Moreno, A.; Martinez Feliu, A. (2020). High-performing Ir- and Pt-containing catalysts based on mesoporous beta zeolite for the selective ring opening of decalin. Catalysis Science & Technology. 10(4):1073-1085. https://doi.org/10.1039/c9cy01812cS1073108510

Nature of Active Nickel Sites and Initiation Mechanism for Ethylene Oligomerization on Heterogeneous Ni-beta Catalysts

[EN] Higher olefins produced via ethylene oligomerization are versatile commodity chemicals serving a vast range of industries with large global economic impact. Nickel aluminosilicates are promising candidates to replace the homogeneous catalysts employed in industrial ethylene oligomerization processes. The current poor understanding of the true nature of the active nickel centers and the nickel-mediated oligomerization mechanism in these materials, however, hampers the rational design of improved catalysts. Here we applied in situ time- and temperature-resolved FTIR spectroscopy with simultaneous MS analysis of products to disentangle these fundamental issues using nanocrystalline Ni-beta zeolite as catalyst. We elucidate that isolated Ni2+ cations grafted on acidic silanols are the most likely active species in the working catalysts rather than the generally accepted ion-exchanged nickel cations. On the basis of our results, a plausible initiation mechanism involving a nickel vinyl hydride intermediate from which chain propagation proceeds similarly to the Cossee-Arlman pathway is proposed.This work was supported by the MINECO of Spain through the Severo Ochoa Program for Centers of Excellence (SEV 2016-0683) and ENE2014-5761-R project. The authors extend their acknowledgement to the EU project OCMOL ("Oxidative Coupling of Methane followed by Oligomerization to Liquids", 7th Framework Programme, GA no. 228953)Moussa, S.; Concepción Heydorn, P.; Arribas Viana, MDLD.; Martinez Feliu, A. (2018). Nature of Active Nickel Sites and Initiation Mechanism for Ethylene Oligomerization on Heterogeneous Ni-beta Catalysts. ACS Catalysis. 8(5):3903-3912. https://doi.org/10.1021/acscatal.7b03970S390339128

The nature of active Ni sites and the role of Al species in the oligomerization of ethylene on mesoporous Ni-Al-MCM-41 catalysts

[EN] Although nickel dispersed in mesoporous aluminosilicates are efficient catalysts for the oligomerization of ethylene, the nature of the active nickel sites still remains controversial. Here we applied in situ FTIR-CO spectroscopy during reaction with ethylene combined with reaction kinetics with online MS analysis of reaction products to unravel the nature of the active nickel species in working mesoporous Ni-Al-MCM-41 catalysts. The results revealed that isolated ion-exchanged Ni2+ cations are irreversibly blocked during the initial reaction stages leaving unsaturated Ni2+ cations grafted on silanols and at the surface of small (confined) NiO nanoparticles as the active species in the pseudo-steady state. The low activity of these species in pure silica materials suggested a promotional role of aluminum in the Al-MCM-41 matrix on enhancing the activity of Ni2+ sites, probably through a close interaction between Al species and nearby Ni2+ sites, as inferred from quantitative Al-27 MAS NMR and FTIR spectroscopiesThis work has been financially supported by the Spanish government through the Severo Ochoa Program (SEV2016-0683) and RTI2018-102161 projects. The authors acknowledge the Microscopy Service of the Universitat Politecnica de Valencia for its assistance in the characterization of materials by electron microscopy.Moussa, S.; Concepción Heydorn, P.; Arribas Viana, MDLD.; Martinez Feliu, A. (2020). The nature of active Ni sites and the role of Al species in the oligomerization of ethylene on mesoporous Ni-Al-MCM-41 catalysts. Applied Catalysis A General. 608:1-10. https://doi.org/10.1016/j.apcata.2020.117831S11060