Guillermo Calleja

Cobalt Doping of the MOF-5 Framework and Its Effect on Gas-Adsorption Properties

Partial isomorphic substitution of Zn in IRMOF metal clusters by cobalt ions is described for the first time. Specifically, different numbers of Co(2+) ions have been incorporated during solvothermal crystallization into the Zn-based MOF-5 (IRMOF-1) framework, which is one of the most studied MOF materials. The amount of Zn that can be substituted seems to be limited, being no more than 25% of total metal content, that is, no more than one Co atom inside every metal cluster formed by four transition-metal ions, on average. Several characterization techniques, including X-ray diffraction, DR UV-visible spectroscopy, N(2) adsorption isotherms, and thermogravimetrical analysis, strongly support the effective incorporation of Co into the material framework. As-synthesized CoMOF-5 has cobalt ions in octahedral coordination, changing to tetrahedral by simple evacuation, presumably by the removal of two diethylformamide molecules per Co ion. Moreover, the H(2), CH(4), and CO(2) uptake of MOF-5 materials systematically increases with the Co content, particularly at high pressure. Such an increase is moderate anyway, considering that Co is incorporated into unexposed metal sites that are less accessible to gas molecules.

Comparison of molecular simulation of adsorption with experiment

Experimental measurements of adsorption yield the surface excess. The Gibbs surface excess is the actual or absolute amount of gas contained in the pores less the amount of gas that would be present in the pores in the absence of gas-solid intermolecular forces. Molecular simulation of adsorption yields the absolute amount adsorbed. Comparison of simulated adsorption isotherms and heats of adsorption with experiment requires a conversion from absolute to excess variables. Molecular simulations of adsorption of methane in slit pores at room temperature show large differences between absolute and excess adsorption. The difference between absolute and excess adsorption may be ignored when the pore volume of the adsorbent is negligible compared to the adsorption second virial coefficient (V≪B1s).

Co/HZSM-5 catalyst for syngas conversion: influence of process variables

The effects of process variables on the activity of a Co/HZSM-5 zeolite bifunctional catalyst for the conversion of syngas to hydrocarbons were studied. The catalyst was prepared by incipient-wetness impregnation from aqueous solutions of cobalt and thorium nitrate, the latter used as promoter. The reactions were carried out in a fixed-bed integral reactor. A simple two-level factorial design was not sufficient to describe the dependence of C6 + yield and C6 + selectivity on the process variables. A composite factorial design led to two response surfaces, describing the dependence of yield and selectivity on temperature (240–320°C), space velocity (0.5–1.3 h−1), COH2 feed ratio (0.5–1.9 molar) and pressure (0.1–4.3 MPa), with a deviation error of ∼20%. The catalyst showed a better selectivity towards higher hydrocarbons than a physical mixture of ZSM-5 zeolite and a conventional Fischer-Tropsch catalyst and only slow deactivation, affecting mainly the C6 + selectivity. This deactivation is probably due to coke deposition on the active surface sites, that is, cobalt particles and acid sites of the zeolite.

Comparative adsorption of phenol, P-nitrophenol and P-hydroxybenzoic acid on activated carbon

Adsorption isotherms of phenol, p-nitrophenol and p-hydroxybenzoic acid at three temperatures in the range 1–40° C on activated carbon have been obtained experimentally. Curve fitting to several theoretical equations shows that the Prausnitz equation gives the lowest deviations (1–3%). An inversion of the adsorption capacity of phenol with temperature has been observed, probably as a consequence of the high microporosity of the carbon and the restricted accessibility of phenol to the smaller pores. Adsorption at 40°C is also accompanied by the degradation of phenol by aerobic micro-organisms (ficomicetus-type fungi). Approximate values of the isosteric heats of adsorption of p-nitrophenol have been calculated, being in the range 5.8–6.5 kcal/mol. Differences in the behaviour of the three adsorbates are commented upon in terms of their physicochemical properties.

Cobalt/HZSM-5 zeolite catalyst for the conversion of syngas to hydrocarbons

Abstract Physical and chemical properties as well as the activity and selectivity of Co/HZSM-5 zeolite bifunctional catalyst have been studied for conversion of synthesis gas. These catalysts were prepared by incipient wetness impregnation with different conditions of drying, calcination and reduction. The cobalt content in the catalyst and the effect of thoria, as promoter, were also studied. The catalysts prepared were characterized and tested in a fixed bed continuous reactor system. The results show that drying is the most important step in the preparation, having more active catalysts with fast drying conditions, probably due to the formation of smaller cobalt particles. The interaction between the metal and the zeolite surface seems to contribute significantly to the catalyst activity. Maximum activity of the catalyst is obtained with 1.5 wt.-% of thorium, resulting in an increase of C6+ selectivity. This is explained in terms of a shift in chemisorption properties of the catalyst. The basic character of the promoter affects the zeolite acidity, decreasing the aromatic proportion in the liquid hydrocarbon fraction. Finally, the bifunctional catalyst selected in this study is compared with a physical mixture of a commercial Fischer-Tropsch catalyst and HZSM-5 zeolite.

Selective production of methanol from syngas over LaTi1−xCuxO3 mixed oxides

Carbon monoxide hydrogenation was studied over partially substituted copper-containing LaTi1−xCuxO3 oxides and on copper supported on La2O3. The unsubstituted (x = 0) oxide was weakly active for CO hydrogenation, whereas all the other oxides were more active and exhibited high selectivity to methanol. Particularly, for substitutions x = 0.5–0.6, where the perovskite structure was observed, CO conversions close to 22% and selectivity close to 80% were found. Cu/La2O3 was, however, less active and yielded CO2 and hydrocarbons as the major products. Using X-ray photoelectron spectroscopy it was determined that under reaction conditions copper exists as reduced species. The L3VV X-ray induced Auger transition at 1849.2 eV observed for a representative LaMn0.5Cu0.5O3 catalyst prereduced and used in CO hydrogenation at 573 K, suggests that Cu+ species dominates in spent catalyst. These Cu+ species are believed to be stable under reaction conditions in the perovskite structure, only a slight Cu enrichment occurs on the topmost layer of catalysts.

Adsorption of phenol and p-nitrophenol on activated carbon: determination of effective diffusion coefficients

Adsorption of phenol and p-nitrophenol in aqueous solution on granular activated carbon has been studied in a stirred-tank system. Equilibrium isotherms and concentration decay curves were obtained experimentally for each adsorbate in the finite batch adsorber, for three temperatures in the range 274-313 K. Effective internal diffusion coefficients, Di, of the two adsorbates were calculated by means of a mathematical model of diffusion-adsorption that takes into account ordinary liquid phase diffusion and surface diffusion mechanisms. Surface diffusion coefficients, DS, were also calculated and correlated with temperature and surface coverage. Isosteric heat of adsorption and activation energy for the adsorption of p-nitrophenol were calculated; values obtained are between those of characteristic physical and chemical adsorption.

Carbon monoxide hydrogenation over Fe/HZSM-5 catalysts. Effect of SiO2/Al2O3 zeolite ratio

Iron catalysts supported on ZSM-5 zeolites of a wide range of silica-to-alumina ratios (29-∞) have been prepared and tested in carbon monoxide hydrogenation. The crystalline phases of the catalysts were characterized by X-ray diffraction and their acidity by infrared spectroscopy of adsorbed pyridine. The catalytic tests were conducted at 533 K, an overall pressure of 21 bar and a feed ratio CO/H2 close to 1. It was found that the selectivity to light olefins (C2–C4) increases in parallel with the increase of the Si/Al ratio of the zeolite. This was explained in terms of the decrease in Brønsted acidity of the catalysts. As a consequence, very high olefin selectivities can be achieved by decreasing the number of strong acid sites in the zeolite structure, but at the expense of high oxygenate formation.

Effects of the Operating Conditions on Coke Deactivation of 5A Molecular Sieve in N-Decane Adsorption/Desorption

The influence of operating conditions (time-on-stream, temperature, pressure and paraffin feed concentration) on the deactivation of a 5A molecular sieve during the adsorption/desorption of n-decane has been studied using a cyclic operating procedure. After 10 adsorption/desorption cycles, the 5A molecular sieve used in this study showed the same deactivation level as an used industrial molecular sieve provided that the deactivation of the zeolite was due to coke deposition by site coverage and pore blockage. The temperature effect was studied in the range of 373–523 K, obtaining a minimum deactivation at 448 K. The pressure does not influence adsorption parameters in the range of 1–3 MPa, but it does affect the nature of the coke deposited on the zeolite. The influence of paraffin concentration can be considered negligible.

A Recyclable Cu-MOF-74 Catalyst for the Ligand-Free O-Arylation Reaction of 4-Nitrobenzaldehyde and Phenol

The activity and recyclability of Cu-MOF-74 as a catalyst was studied for the ligand-free C–O cross-coupling reaction of 4-nitrobenzaldehyde (NB) with phenol (Ph) to form 4-formyldiphenyl ether (FDE). Cu-MOF-74 is characterized by having unsaturated copper sites in a highly porous metal-organic framework. The influence of solvent, reaction temperature, NB/Ph ratio, catalyst concentration, and basic agent (type and concentration) were evaluated. High conversions were achieved at 120 °C, 5 mol % of catalyst, NB/Ph ratio of 1:2, DMF as solvent, and 1 equivalent of K2CO3 base. The activity of Cu-MOF-74 material was higher than other ligand-free copper catalytic systems tested in this study. This catalyst was easily separated and reused in five successive runs, achieving a remarkable performance without significant porous framework degradation. The leaching of copper species in the reaction medium was negligible. The O-arylation between NB and Ph took place only in the presence of Cu-MOF-74 material, being negligible without the solid catalyst. The catalytic advantages of using nanostructured Cu-MOF-74 catalyst were also proven.