Ive Hermans

Simple and Scalable Preparation of Highly Active Lewis Acidic Sn‐β

Tin goes in: The solid Lewis acid Sn-zeolite β is obtained in a simple and scalable procedure (see scheme). A high metal content can be obtained, without undesirable side-effects. The space-time-yields of the resulting catalysis are over one order-of-magnitude larger than those of the state-of-the-art materials for the Baeyer–Villiger oxidation of cyclohexanone and the synthesis of ethyl lactate from the triose dihydroxyacetone.

Oxidative Methane Upgrading

The economically viable oxidative upgrading of methane presents one of the most difficult but rewarding challenges within catalysis research. Its potential to revolutionalise the chemical value chain, coupled with the associated supremely challenging scientific aspects, has ensured this topics high popularity over the preceeding decades. Herein, we report a non-exhaustive account of the current developments within the field of oxidative methane upgrading and summarise the pertaining challenges that have yet to be solved.

Catalytic Transfer Hydrogenation/Hydrogenolysis for Reductive Upgrading of Furfural and 5‐(Hydroxymethyl)furfural

The sequential transfer hydrogenation/hydrogenolysis of furfural and 5-hydroxymethylfurfural to 2-methylfuran and 2,5-dimethylfuran was studied over in situ reduced, Fe2 O3 -supported Cu, Ni, and Pd catalysts, with 2-propanol as hydrogen donor. The remarkable activity of Pd/Fe2 O3 in both transfer hydrogenation/hydrogenolysis is attributed to a strong metal-support interaction. Selectivity towards hydrogenation, hydrogenolysis, decarbonylation, and ring-hydrogenation products is shown to strongly depend on the Pd loading. A significant enhancement in yield to 62%, of 2-methylfuran and 2-methyltetrahydrofuran was observed under continuous flow conditions.

NMR signatures of the active sites in Sn-β zeolite.

Dynamic nuclear polarization surface enhanced NMR (DNP-SENS), Mössbauer spectroscopy, and computational chemistry were combined to obtain structural information on the active-site speciation in Sn-β zeolite. This approach unambiguously shows the presence of framework Sn(IV)-active sites in an octahedral environment, which probably correspond to so-called open and closed sites, respectively (namely, tin bound to three or four siloxy groups of the zeolite framework).

Mechanism of the Aerobic Oxidation of α‐Pinene

A combined experimental and theoretical approach is used to study the thermal autoxidation of alpha-pinene. Four different types of peroxyl radicals are generated; the verbenyl peroxyl radical being the most abundant one. The peroxyl radicals propagate a long radical chain, implying that chain termination does not play an important role in the production of the products. Two distinct types of propagation steps are active in parallel: the abstraction of allylic H atoms and the addition to the unsaturated C=C bond. The efficiency for both pathways appears to depend on the structure of the peroxyl radical. The latter step yields the corresponding epoxide product, as well as alkoxyl radicals. Under the investigated reaction conditions the alkoxyl radicals seem to produce both the alcohol and ketone products, the ketone presumably being formed upon the abstraction of the weakly bonded alphaH atom by O2. This mechanism explains the predominantly primary nature of all quantified products. At higher conversion, co-oxidation of the hydroperoxide products constitutes an additional, albeit small, source of alcohol and ketone products.

Continuous D-fructose dehydration to 5- hydroxymethylfurfural under mild conditions.

The dehydration of D-fructose to 5-hydroxymethylfurfural was studied under single-phase conditions in the low boiling solvent 1,4-dioxane at moderate temperatures in the presence of the solid acid-catalyst Amberlyst-15. The reaction was first examined and optimized under batch conditions, where it was found that the yield could be increased up to 75 % by adding small amounts of DMSO. Subsequently, the reaction was performed under continuous flow in a fixed bed reactor. Internal and external mass transfer limitations could be eliminated by changing the particle size and by adjusting the flow rate. Under continuous conditions, the HMF yield could be further increased to 92 %; the space-time yield was found to be 75 times higher compared to the batch case. A long-term stability test (96 h), including solvent regeneration, demonstrated that the catalyst is stable over time. Additionally, it was shown that even small amounts of water have a negative effect on the HMF yield. Overall, the present system shows a good alternative to other systems presented in literature because high space-time yields and selectivities were obtained under relatively mild and continuous conditions.

Selective oxidative dehydrogenation of propane to propene using boron nitride catalysts

Boron nitride catalysis Propene is one of the highest-volume organic chemicals produced. Propene has mainly been made from naphtha, but changes in the global supply chain are creating shortages. Direct conversion from propane, a component of natural gas, via reaction with oxygen is an attractive alternative, but existing approaches produce a large fraction of unwanted CO and CO2. Grant et al. report that boron nitride, normally an unreactive material, has high selectivity to catalyze the production of propene (77%) and ethene (13%). Science, this issue p. 1570 Boron nitride, often considered unreactive, can be a highly active and selective catalyst for propane oxidation to propene. The exothermic oxidative dehydrogenation of propane reaction to generate propene has the potential to be a game-changing technology in the chemical industry. However, even after decades of research, selectivity to propene remains too low to be commercially attractive because of overoxidation of propene to thermodynamically favored CO2. Here, we report that hexagonal boron nitride and boron nitride nanotubes exhibit unique and hitherto unanticipated catalytic properties, resulting in great selectivity to olefins. As an example, at 14% propane conversion, we obtain selectivity of 79% propene and 12% ethene, another desired alkene. Based on catalytic experiments, spectroscopic insights, and ab initio modeling, we put forward a mechanistic hypothesis in which oxygen-terminated armchair boron nitride edges are proposed to be the catalytic active sites.

Post-synthetic preparation of Sn-, Ti- and Zr-beta: a facile route to water tolerant, highly active Lewis acidic zeolites

A two-step procedure for the post-synthetic preparation of Lewis acidic Sn-, Zr- and Ti-zeolite β is reported. Dealumination of a commercially available Al-β zeolite leads to the formation of highly siliceous material containing silanol nests, which can be filled in a second step via the solid-state ion-exchange or impregnation of an appropriate metal precursor. Spectroscopic studies indicate that each metal is subsequently coordinated within the zeolite framework, and that little or no bulk oxides are formed--despite the high metal loadings. The synthesised catalysts demonstrate excellent activity for the isomerisation of glyceraldehyde to dihydroxyacetone, a key model reaction for the upgrading of bio-renewable feedstocks, and the epoxidation of bulky olefins.

Mechanism of the Catalytic Deperoxidation of tert‐Butylhydroperoxide with Cobalt(II) Acetylacetonate

The Co(II)/Co(III)-induced decomposition of hydroperoxides is an important reaction in many industrial processes and is referred to as deperoxidation. In the first step of the so-called Haber-Weiss cycle, alkoxyl radicals and Co(III)-OH species are generated upon the reaction of the Co(II) ion with ROOH. The catalytic cycle is closed upon the regeneration of the Co(II) ion through the reaction of the Co(III)-OH species with a second ROOH molecule, thus producing one equivalent of the peroxyl radicals. Herein, the deperoxidation of tert-butylhydroperoxide by dissolved cobalt(II) acetylacetonate is studied by using UV/Vis spectroscopy in situ with a noninteracting solvent, namely, cyclohexane. Kinetic information extracted from experiments, together with quantum-chemical calculations, led to new mechanistic hypotheses. Even under anaerobic conditions, the Haber-Weiss cycle initiates a radical-chain destruction of ROOH propagated by both alkoxyl and peroxyl radicals. This chain mechanism rationalizes the high deperoxidation rates, which are directly proportional to the cobalt concentration up to approximately 75 μM at 333 K. However, at higher cobalt concentrations, a remarkable decrease of the rate is observed. The hypothesis put forward herein is that this remarkable autoinhibition effect could be explained by the hitherto overlooked chain termination of two Co(III)--OH species. The direct competition between the first-order Haber-Weiss initiation and the second-order termination can indeed explain this peculiar kinetic behavior of this homogeneous deperoxidation system.

Intensification of TEMPO-mediated aerobic alcohol oxidations under three-phase flow conditions

Various homogeneous and heterogeneous catalytic systems based on 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and different (co-)oxidants have been reported for the selective oxidation of alcohols. Herein we report the use of a commercially available silica-immobilized TEMPO catalyst, in combination with catalytic amounts of HNO3 as a NOx source under continuous three-phase flow conditions. First the stability of the catalyst was evaluated with benzyl alcohol and the reaction parameters were optimized. Subsequently different substrates were tested, focusing on the oxidation of renewable substrates like lactic acid and 5-hydroxymethylfurfural (HMF).