Elli Heracleous

ABSTRACT In this study, we report the effect of Cu/Zn/Al chemical composition and Zn and/or Al substitution by Mn and/or Cr in K-promoted Cu-Zn-Al catalysts for the hydrogenation of CO to higher alcohols. In terms of higher alcohols, the synthesis yielded preferentially primary 2-methyl branched alcohols with up to five carbon atoms, namely 2-methyl-1-propanol and 2-methyl-1-butanol, together with ethanol and propanol. Variation of the Cu/Zn/Al chemical composition showed that the catalyst with a Cu/Zn atomic ratio of 1 and low Al content (K-Cu45Zn45Al10) exhibits the optimum performance in terms of both activity and selectivity to higher alcohols. High Al contents on the other hand favor methanol production at the expense of higher alcohols. It is postulated that due to its acidic nature, alumina reduces the basic sites on the catalyst, thereby retarding the C1→C2 step. Substitution of Zn and/or Al by Mn and/or Cr was found to reduce activity by ∼50%, probably due to the lower exposed copper surface area as indicated by the formation of larger CuO crystals. In terms of selectivity, the most appreciable changes were recorded for the K-Cu45Mn45Al10 catalyst, where a 50% increase in higher alcohol formation was measured, rendering the catalyst the most selective among the investigated materials. Characterization of the catalysts provided some insight on the beneficial influence of Zn substitution by Mn in the K-Cu45Mn45Al10 sample. Inverse correlation between acidity and higher alcohols selectivity was evidenced, in accordance with the general notion that higher alcohol formation requires basic sites and indicates that reduced acidity is needed for the aldol-type condensation reactions. The replacement of Mn by Zn in K-Cu-Mn-Al reduced acidity and thus promoted the production of the desired higher alcohol products.

ABSTRACT The role of each structural phase present in highly active and selective Ni0.85Nb0.15 catalyst in ethane oxidative dehydrogenation is investigated. Based on previous studies, Ni0.85Nb0.15 consists of a Ni–Nb solid solution and a Nb-rich amorphous phase. A series of Ni-based mixed oxides with low Nb content simulating the Ni–Nb solid solution and an amorphous Nb-rich sample were synthesized and studied separately. The catalytic tests demonstrated that the introduction of even a tiny amount of Nb (atomic ratio: Nb/(Ni + Nb) = 0.01) leads to a drastic increase in ethylene selectivity (150%) compared to pure NiO. Selectivity gradually increases with increasing Nb loading up to 15%, in parallel with the decrease in oxygen desorbed as measured by O2-TPD. Based on the results of combined catalytic testing and physicochemical characterization, Ni–Nb solid solution can be identified as the key component for the high ethylene selectivity of the Ni0.85Nb0.15 catalyst, as the insertion of even a small amount of Nb drastically decreases the electrophilic oxygen species responsible for the total oxidation reactions. Co-existence of the Ni–Nb solid solution with the amorphous Nb-rich phase does not influence significantly the catalytic properties of the material.

This contribution presents a theoretical study of a multitubular packed-bed membrane reactor for the ethane to ethylene oxidative dehydrogenation reaction over a highly active and selective Ni–Nb–O mixed oxide catalyst. This theoretical study takes into account the radial composition and temperature profiles using a two-dimensional pseudo-homogeneous model on the reaction side. The feasibility and convenience of using this novel design, as well as the influence of the main operating variables on the reactor performance, are analyzed.The introduction of the membrane leads to lower oxygen partial pressures inside the catalyst tubes, which results in an improved selectivity to ethylene (lower heat generation rates) and high effective heat transfer area per unit volume. The multitubular membrane reactor enables significant ethylene productions per tube and milder temperature profiles than a conventional wall-cooled fixed-bed reactor. Operating conditions have to be carefully adjusted to avoid undesired oxygen accumulation inside the tubes. The presence of small amounts of oxygen at the reactor inlet significantly improves the ethylene production rates.

In the present contribution, a theoretical study of a multitubular fixed bed reactor for the ethane to ethylene oxidative dehydrogenation reaction over a highly active and selective Ni–Nb–O mixed oxide catalyst is presented. Two reactor designs are proposed and their performance is analyzed by means of a mathematical model of the catalytic bed.The results suggest that the reactor operation would be feasible, provided that high heat transfer area per unit volume and low oxygen concentrations along the tube are maintained. A two-bed multitubular reactor with intermediate air injection proved to be superior to a single-bed design. In fact, the two-bed configuration offers higher ethylene production rates, due to the increased ethylene selectivity while operation under lower oxygen partial pressures.

In this work, transient and SSITKA experiments with isotopic 18O2 were conducted to study the nature of oxygen species participating in the reaction of ethane oxidative dehydrogenation to ethylene and obtain insight in the mechanistic aspects of the ODH reaction over Ni-based catalysts. The study was performed on NiO, a typical total oxidation catalyst, and a bulk Ni–Nb–O mixed-oxide catalyst (Ni0.85Nb0.15) developed previously [E. Heracleous, A.A. Lemonidou, J. Catal., in press], a very efficient ethane ODH material (46% ethene yield at 400 °C). The results revealed that over both materials, the reaction proceeds via a Mars–van Krevelen-type mechanism, with participation of lattice oxygen anions. However, the 18O2 exchange measurements showed a different distribution of isotopic oxygen species on the two materials. The prevalent formation of cross-labelled oxygen species on NiO indicates that dissociation of oxygen is the fast step of the exchange process, leading to large concentration of intermediate electrophilic oxygen species on the surface, active for the total oxidation of ethane. Larger amounts of doubly exchanged species were observed on the Ni–Nb–O catalyst, indicating that doping with Nb makes diffusion the fast step of the process and suppresses formation of the oxidizing species. Kinetic modeling of ethane ODH over the Ni0.85Nb0.15 catalyst by combined genetic algorithm and nonlinear regression techniques confirmed the above, since the superior model is based on a redox parallel-consecutive reaction network with the participation of two types of active sites: type I, responsible for the ethane ODH and ethene overoxidation reaction, and type II, active for the direct oxidation of ethane to CO2. The kinetic model was able to successfully predict the catalytic performance of the Ni0.85Nb0.15 catalyst in considerably different experimental conditions than the kinetic experiments (high temperature and conversion levels).

The structural and catalytic properties of supported MoO3/Al2O3 catalysts with Mo surface densities, nsns, in the range 1.1–12.5 Mo/nm2 were studied for the oxidative dehydrogenation of ethane by in situ Raman spectroscopy with simultaneous catalytic measurements at temperatures of 400–550 °C. Isolated mono-molybdates (MoO4) and polymolybdates are formed at various proportions (depending on the loading) on the catalyst surface under dehydrated conditions; bulk Al2(MoO4)3 crystals are formed at nsns exceeding the monolayer. Under reactive environment (C2H6/O2), the Raman features attributed to Mo

Two MoO3/γ-Al2O3 catalysts containing 11% (w/w) MoO3 prepared by equilibrium-deposition-filtration at two different pairs of pH and Mo concentration values (pH=3.9, CMo=1.1×10−3 mol L−1 and pH=6.3, CMo=2.7×10−2 mol L−1) were studied for the ethane oxidative dehydrogenation.The two catalysts demonstrated similar ethane conversions in a temperature range 450–600 °C. Selectivity to ethene was 15% higher for the catalyst prepared at relatively low pH than for the sample prepared at a higher pH. The observed difference in ethylene selectivity was related to the different structural characteristics of the molybdena phase in the two catalysts.

In this work, the reactions of ethane and ethene in an oxidizing and non-oxidizing atmosphere over γ-alumina were investigated under temperature-programmed conditions, in an attempt to estimate the possible contribution and functionality of the support in the reaction pathway of ethane ODH over MoO3/Al2O3 catalysts. The results indicate that alumina contributes to the primary deep oxidation and dehydrogenation routes of ethane to COx and coke respectively, which proceed effectively over the acidic OH groups and the Al3+–O2− acidic centers. On the contrary, the formation of ethylene seems to be coupled to the presence of redox sites on the catalytic surface and requires the presence of the molybdena phase. Moreover, the redox sites of the MoOx species were found to unselectively activate the further overoxidation of the olefin to carbon oxides. Therefore, Al2O3 catalyzes the unselective primary oxidation of ethane to carbon oxides, whereas the molybdena phase is involved in the selective oxidative dehydrogenation (ODH) of ethane to ethene and the secondary overoxidation of ethene to COx.

In this work, the temperature-programmed reaction (TP-reaction) technique was employed for the study of the mechanism of ethane oxidative dehydrogenation (ODH) and the identification of primary and secondary steps of the reaction. The reactions of ethane and ethene in an oxidizing and non-oxidizing atmosphere were investigated in the presence and absence of a 20 wt.% MoO3/Al2O3 catalyst, a highly active and selective catalytic material. Experiments were performed at steady flow conditions, with a 15 °C/min linear increase of temperature up to 700 °C, while the reaction effluent was monitored on-line by mass spectroscopy. Thermal reactions of ethane were found to be highly selective to ethene. Ethene seems to be primarily produced through a dehydrogenation mechanism, even in the presence of oxygen in the gas phase. However, oxygen was found to dramatically increase the overall rate of the reaction. Over the 20MoAl catalyst, ethane activation occurred at a significantly lower temperature, while the production of ethene was clearly a result of heterogeneous reactions via the oxidative dehydrogenation route. Surface pathways were found to have a significant role, even at temperatures where homogeneous reactions also occurred, indicating possible occurrence of a homogeneous–heterogeneous reaction scheme at high temperatures. In non-oxidative conditions, lattice oxygen activated ethane at the same temperature as in the presence of oxygen, leading mainly to ethene production, since lattice oxygen was found to be less reactive towards ethene. More labile forms of oxygen, resulting from adsorption of gas-phase oxygen on the surface, are probably responsible for the extensive ethene degradation observed at high conversion levels.

Aspects of the surface pathway of the ethane oxidative dehydrogenation (ODH) were investigated by in situ infrared spectroscopy. Adsorption and surface reactions were studied on MoO3/Al2O3 and Al2O3, in order to investigate contributions of the support as well as selective and unselective routes of the ODH reaction. The interaction of ethane with the oxides was investigated at isothermal conditions (50 °C) in the absence and presence of oxygen by time resolved IR spectroscopy between 50 and 500 °C. Catalytic testing of these materials in ethane oxidative dehydrogenation showed that the molybdena catalyst is highly selective (initial C2H4 selectivity was 96%), while mainly COx was formed with alumina. The strong acid sites on alumina, detected by NH3-TPD, are speculated to be responsible for the unselective conversion of ethane to COx, while the moderate acid strength introduced by molybdena allows selective activation of ethane and inhibits the further oxidation of ethylene produced. The spectroscopic data indicate that the activation of the ethane CH bond proceeds through the formation of alkoxides, which decompose to ethylene and a surface OH group or are oxidized to surface bound oxygenates. MoO3/Al2O3 favors the first route, while pure alumina favors formation of oxygenates and full oxidation.