Gamma-valerolactone is a valuable chemical that can be obtained by catalytic methods from nonedible vegetable biomass. However, the demand is felt for more environmentally friendly and cost-effective production processes. The topic is reviewed in this paper from the perspective of catalyst design. Focus is on those systems enabling one-pot reaction sequences in the liquid phase at low energy expenses and combining metal and acid sites, spanning from homogeneous to heterogeneous catalysts. A final section is dedicated to continuous flow applications. The 230 references cover the most significant achievements published in the literature from January 2011 to July 2014 and highlight critical issues and future trends.

Environmentally Friendly Synthesis of γ-Valerolactone by Direct Catalytic Conversion of Renewable Sources

Direct Evidence for the Nonrandom Nature of Al Substitution in Zeolite ZSM‐5: An Investigation by 27Al MAS and MQ MAS NMR

Room temperature activation of methane over a Zn modified H-ZSM-5 zeolite catalyst was investigated by solid-state NMR spectroscopy in combination with other spectroscopic methods and DFT theoretical calculations. We found for the first time that the activation of methane resulted in the preferential formation of surface methoxy intermediates at room temperature, which mediated the formation of methanol and its further conversion to hydrocarbons. Experimental and theoretical calculation results demonstrated that an oxygen-containing dizinc cluster center in an open shell was responsible for homolytic cleavage of the C-H bond of methane at room temperature, leading to the formation of methyl radicals. The zeolite matrix readily trapped the methyl radicals by forming the surface methoxy intermediates for further selective conversion. In parallel to the homolytic cleavage pathway, heterolytic dissociation of methane was also observed on isolated Zn2+ ions at room temperature, which gives rise to zinc methyl species.

Room temperature activation of methane over Zn modified H-ZSM-5 zeolites: Insight from solid-state NMR and theoretical calculations

The interaction of HZSM5 zeolite with n-butane, iso-butane, n-heptane, 1-butene, cis-2-butene, trans-2-butene, iso-butene, benzene, toluene, ortho-xylene, para-xylene, acetonitrile and pivalonitrile has been investigated using FT-IR spectroscopy. These experiments allowed to characterize the hydrogen-bonded species of all adsorbates both with the internal OHs of HZSM5 and with the external OHs. ortho-Xylene and pivalonitrile are unable to enter the zeolite cavities in the given conditions, so that they only interact with the external sites. The other molecules interact with both internal and external sites. However, it is clear that the hindered aromatic molecules do not fully reach the internal sites, part of them being unperturbed. H-bonds apparently involve C–C σ-type orbitals of alkanes and CC π-type orbitals of alkenes and aromatics. The external OHs of ZSM5 are clearly associated with a band near 3746 cm−1, so are certainly terminal. In spite of being terminal, at least part of them are significantly acidic too, nearly like those of silica/aluminas and definitely more than the silanols of pure silica. The internal OHs (3618 cm−1) are more acidic and bridging. Internal defects are characterized by terminal silanols absorbing at 3730 cm−1.

An FT-IR study of the internal and external surfaces of HZSM5 zeolite

The mechanism of alkane dehydrogenation over Zn/ZSM-5 zeolite was studied by means of density functional theory using the cluster modeling approach. Three types of active sites were considered:  Zn2+ ions stabilized in conventional ion-exchange sites (ZnZs), Zn2+ ions stabilized in cation sites with distantly placed aluminum ions (ZnZd), or as binuclear [ZnOZn]2+ cations (ZnOZn). A comparison of the computed energetics of various reaction paths for ethane indicates that the catalytic reaction proceeds most easily over the ZnZd sites. The enhanced Lewis acidity of these sites facilitates the subsequent heterolytic C−H bond cleavage. The most favorable proton-accepting sites are not those of the framework ring to which Zn2+ is attached, but O sites of a neighboring ring. Although the reactivity of isolated Zn2+ ions strongly depends on the distribution of framework aluminum in ZSM-5 zeolite, the adsorption of alkane molecules is only slightly influenced by this factor. After the heterolytic C−H bond cleavag...

J. Steffen

Papers

Synthesis and structure refinement of ZSM—5 single crystals