Elena Groppo

Response of CPO-27-Ni towards CO, N2 and C2H4

Coordinatively unsaturated Ni(2+) atoms in CPO-27-Ni form linear adducts with molecular nitrogen. The framework responds to the adsorption-modifying vibrational properties and local structure around adsorbing sites. The present paper deals with a fundamental infrared (IR) study of the interaction of gases on a microporous adsorbent metallorganic framework CPO-27-Ni containing, after solvent removal, coordinatively unsaturated Ni(2+) atoms [Dietzel et al., Chem. Commun. 2006, 959]. CO, N(2) and C(2)H(4) have been chosen. Notwithstanding the relative medium (CO and C(2)H(4)) and weak (N(2)) adsorption enthalpies and the low equilibrium pressures adopted (100-10(-3) mbar) the CPO-27-Ni framework responds promptly and reversibly to the adsorption process, modifying significantly both vibrational properties and local structure around Ni(2+) adsorbing sites as determined by a parallel EXAFS investigation locating the N(2) molecule 2.27 +/- 0.03 A apart from Ni(2+). For both N(2) and C(2)H(4), IR spectra have been discussed and carefully compared with literature data. Isosteric heat of adsorption of the Ni(2+)...N(2) complex formation has been evaluated from temperature dependent IR study to be -DeltaH(ads) = 17 kJ mol(-1).

Model oxide supported MoS2 HDS catalysts: structure and surface properties

Supported hydrodesulfidation (HDS) MoS2/SiO2, MoS2/γ-Al2O3 and MoS2/MgO catalysts having a model character have been synthesized by using CS2 as the sulfiding agent and deeply investigated by means of several techniques. XRPD, HRTEM, Raman and UV-Vis methods have been applied to obtain information on the morphology and the structure of the catalysts as well as on the vibrational and spectroscopic properties. It is shown that, when compared with HRTEM results, XRPD, Raman and UV-Vis data give realistic information on the stacking degree, on the particle size distribution and on the heterogeneity of supported MoS2 particles on the various supports. (S K-, Mo L3- and K- edges) EXAFS and XANES spectroscopies have been also used to set up the best sulfidation procedure. UV-vis analysis under controlled atmosphere has been performed to understand the presence of sulfur vacancies and the valence state of Mo ions associated with them. To explore the structure of coordinatively unsaturated Mo sites after reducing or sulfiding treatments (with CS2 or, occasionally, with H2S), in situFTIR of adsorbed CO has been performed. It is demonstrated that CO is a sensitive probe for coordinatively unsaturated sites and that the formation of sulfur vacancies on the MoS2 surface upon reduction in pure H2 at 673 K is accompanied by an increase of the coordinative unsaturation and a decrease of the valence state of a fraction of surface Mo cations, mainly located on corner and edge sites. Furthermore, it is demonstrated that this process can be reversed upon interaction with the sulfiding agent and that this reversible behavior is really mimicking some of the elementary acts occurring in the HDS process. The complexity of the IR results suggests that the adopted reduction procedure in pure H2 at 673 K induces the formation of several types of sulfur vacancies, presumably located in different crystallographic positions. It is also concluded that the sulfiding steps are strongly involving the surface of the support and that reductive treatments at high T in H2 are causing sulfur depletion not only from supported MoS2 particles, but also from the supporting phase. The involvement of the support is particularly relevant for Al2O3 and MgO.

The potential of spectroscopic methods applied to heterogeneous catalysts for olefin polymerization

Heterogeneous Ziegler–Natta and Phillips-type olefin polymerization catalysts have the monopoly of isotactic polypropylene production and a large share in the market of high density polyethylene, respectively. Their high industrial impact and the relatively mild conditions under which they work explain why both catalysts have been the subject of an intense research. The industrially adopted strategy to improve catalysts formulation is still based on a trial-and-error procedure; however, a rational design of new and more efficient catalysts (which is the key to produce polyolefins having a specific architecture) necessarily implies to achieve a detailed understanding of the structure of the active sites at a molecular level. Herein, it is shown that spectroscopic methods have this potential, especially when several complementary techniques are adopted and coupled with theoretical calculations. This is valid for both Phillips-type and Ziegler–Natta catalysts, because most of the problems encountered in their characterization and understanding are common, although for decades they were not considered to be closely related. The main advantages and disadvantages of several spectroscopies in the investigation of both categories of catalysts are critically analyzed, by discussing many examples taken from the recent literature.

Preparation of supported Pd catalysts: from the Pd precursor solution to the deposited Pd2+ phase.

The preparation by the deposition-precipitation method (using Na(2)PdCl(4) as a palladium precursor and Na(2)CO(3) as a basic agent) of Pd catalysts supported on gamma-Al(2)O(3) and on two different types of active carbons has been followed by several techniques (UV-vis, EXAFS, XRPD, and TPR). This work consists of four successive parts: the investigation of (i) the palladium precursor liquid solution (in the absence of substrate), (ii) the solid precipitated phase (in the absence of substrate), (iii) the precipitated Pd(2+)-phase on the supports as a function of Pd loading from 0.5 to 5.0 wt % (i.e., the final catalyst for debenzylation reactions), and (iv) the Pd(0)-phase formed upon reduction in H(2) atmosphere at 393 K. A time/pH-dependent UV-vis experiment indicates that Pd(2+) is present in the mother solution mainly as PdCl(2)(H(2)O)(2)] and [PdCl(H(2)O)(3)](+). Upon progressive addition of NaOH (3.0 < pH < approximately 3.8), the concentration of the two complexes is almost constant and then they rapidly disappear because of the precipitation of an amorphous aggregation of Pd(2+)-polynuclearhydroxo complexes. This phase represents a model material for the active supported phase. Thermal treatments at increasing temperature of this phase cause progressive water loss and resulted in a progressive increase in crystallinity typical of a defective PdO-like phase. The EXAFS spectrum of the final catalysts has been found to be intermediate between that of the unsupported amorphous Pd(2+)-polynuclearhydroxo complexes and that of the PdO-like phase. Independent of the support, EXAFS was not able to evidence any fraction of reduced metallic Pd, meaning that all Pd is in the 2+ oxidation state within the sensitivity of the technique (a few percent). Analogously, independent of the support, XRPD was not able to detect the presence of any crystalline supported phase. The Pd local environment of the as-precipitated samples changes slightly as a function of Pd loading from 0.5 to 2.0 wt %: at higher loadings, no further modification has been observed. After reduction in an H(2) atmosphere, two trends have been observed: (i) the dispersion of Pd nanoparticles tends to decrease with increasing Pd concentration, less significantly on Al(2)O(3)-supported samples and more significantly on carbon-supported ones and (ii) the dispersion depends on the carrier following the sequence Al(2)O(3) >> Cp > Cw according to the increasing palladium-support interaction strength.

Preference towards Five-Coordination in Ti Silicalite-1 upon Molecular Adsorption

. B becomes less pronounced and shifts to higher en-ergies, while C varies depending on the kind of ligand: whenwater (ammonia) is adsorbed it becomes less (more) pro-nounced.Insights into the experimental vtc-XES data can be gainedfrom symmetry arguments because the intensities of the vtc-XES spectral features are related to the matrix elements

Infrared Spectroscopy of Transient Surface Species

A full understanding of a catalytic reaction requires a thorough identification of precursor and intermediate species. In this regard spectroscopies, in general, and infrared (IR) spectroscopy, in particular, can play an important role. However, the spectroscopic identification of precursor and intermediate species is often difficult owing to their transient nature. In this review we show how control of experimental parameters such as temperature, equilibrium pressure, and reactant–catalyst contact time allows experimentalists to alter the rates of dynamic processes and consequently to modify appreciably the relative concentrations of precursor, intermediate, and product species present under reaction conditions. In this context, time-resolved FTIR spectroscopy (with constant temperature and pressure during the experiment) and also temperature-time resolved (with both temperature and time changing simultaneously in a controlled way during the experiment) can be useful for kinetic investigations of several types of reactions. In this review several examples demonstrating the potential value of FTIR spectroscopy for the identification of surface transient species are discussed. These are classified in four main categories: (i) adsorption processes and transformations in the adsorbed state; (ii) proton-catalyzed oligomerization and polymerization of alkenes and unsaturated molecules in protonic zeolites; (iii) oligomerization reactions catalyzed by basic surface sites, and (iv) oligomerization and polymerization of alkenes on catalysts incorporating supported transition metal ions. All these reactions are discussed in the framework of a few common potential energy profiles in which the evolution from the precursor through the intermediate species to the final products is governed by the relative height of the corresponding activation energy barriers.

Spectroscopic Investigation of Heterogeneous Ziegler–Natta Catalysts: Ti and Mg Chloride Tetrahydrofuranates, Their Interaction Compound, and the Role of the Activator

X-ray powder diffraction (XRPD), Infrared, Raman, and UV/Vis spectroscopy have been used to investigate the structural, vibrational, and optical properties of Ti and Mg chloride tetrahydrofuranates as precursors of heterogeneous Ziegler-Natta catalysts for polyethylene production; as well as their interaction compound (pro-catalyst) and the final catalyst obtained after interaction with the AlR(3) activator. Although the structure of the precursors and of the pro-catalyst were well known, that of the catalyst (obtained by reaction of the pro-catalyst with AlR(3)) was not easily obtainable from XRPD data. IR and Raman spectroscopy provided important information on tetrahydrofuran (thf) coordination and on the ν(M-Cl) region; whereas UV/Vis spectroscopy gave the direct proof on both the formal oxidation state and the coordination environment of the active Ti sites. Those presented herein are among the first direct experimental data on the structure of the active Ti sites in Ziegler-Natta catalysts, and can be used to validate the many computational studies that have been increasing exponentially in the last few decades.

Design of high surface area poly(ionic liquid)s to convert carbon dioxide into ethylene carbonate

A series of porous poly(ionic liquid)s (PILs) were synthesized using an innovative method which involves the synthesis of non-ionic co-polymers such as divinylbenzene and vinylimidazole, followed by an alkylation step to introduce the ionic liquid functionality in the polymeric matrix. This synthetic strategy allowed us to obtain tunable imidazolium type PILs having simultaneously high surface area and exposed ionic moieties. A set of PILs was obtained by changing systematically the alkyl chains, the anions and the cross-linking degree. This approach allowed us to elucidate the effect of each synthetic variable on the catalytic performances of PILs towards the carbon dioxide cycloaddition reaction under very mild conditions (room temperature and low pressure). Finally, in situ FTIR spectroscopy allowed us to establish a relationship between the structure of PILs and their catalytic properties.

Enhancing the Initial Rate of Polymerisation of the Reduced Phillips Catalyst by One Order of Magnitude

More than 50 % of the high-density polyethylene produced worldwide is obtained by using the Phillips catalyst, which is prepared by impregnation of an amorphous silica gel with an aqueous solution of an appropriate Cr compound to give a metal loading of about 1 % by weight. After thermal activation, a grafted chromate species is obtained (species 1 in Scheme 1), which can be used directly as the catalyst precursor. In this case, ethylene polymerisation is performed in the temperature range 353–373 K (Scheme 1) and a variable induction period is usually observed that can last from a few minutes to more than 1 hour and during which Cr is slowly reduced to Cr and organic oxygen-containing redox products are desorbed. The mechanism of formation of the reduced state from the chromate precursor in the presence of ethylene is still the subject of ongoing debate, despite well over 30 years of research competently carried out by several research groups. Although there is broad agreement that Cr is an active precursor constituting the catalytic site, a Cr oxidation state has frequently been suggested to be an intermediate species and even cycling between two oxidation states during the catalytic process has frequently been debated in the specialised literature. 13, 14] The above-mentioned induction period becomes a problem when ethylene polymerisation must be performed at a lower temperature (e.g., for production of ultra-high-molecular-weight polyethylene). In these cases, the catalyst is previously reduced by CO (giving species 2 in Scheme 1). Although in this system nearly all of the surface-grafted chromium centres are in the divalent state, only a fraction of them 11a,15] remain highly coordinatively unsaturated. This unsaturation is a key feature for ethylene polymerisation because it confers on the transition-metal ion the ability to bind (simultaneously) both the growing polyethylene chain and the monomer molecule. Ethylene polymerisation readily starts on these sites at room temperature without an induction period. The remaining Cr sites, which become partially buried in the silica framework and surrounded by weak siloxane ligands, are dormant spectators. Recently we showed that amorphous Cr/SiO2 can be an efficient catalyst for reducing nitrogen oxides with CO and (in the case of N2O) the attendant formation of a Cr – oxo complex (species 3 in Scheme 1) was proposed. Based on this experience we realised that controlled oxidation of the reduced Phillips catalyst by N2O could afford a means to increase catalyst activity as the formation of species 3 would help to pull chromium ions out of their dormant sites. Note also that, in contrast to species 1, species 3 retains an open coordination site on the metal cation, which would be ready for inserting an incoming ethylene molecule (Scheme 1). These considerations prompted the study reported herein on ethylene polymerisation on a catalyst (hereafter modified catalyst) obtained by controlled oxidation of the reduced Phillips catalyst (hereafter standard catalyst) with N2O, as described in the Experimental Section. The main finding of our work was a largely increased initial [a] Dr. E. Groppo, Dr. A. Damin, Prof. C. Otero Arean, Prof. A. Zecchina Department of Inorganic, Physical and Material Chemistry NIS Centre of Excellence and INSTM Unit di Torino University of Torino via Quarello 11, 10135 Torino (Italy) Fax: (+39) 011-6707855 E-mail : elena.groppo@unito.it [b] Prof. C. Otero Arean On leave from the Department of Chemistry University of the Balearic Islands, 07122 Palma de Mallorca (Spain) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201101714. Scheme 1. Schematic structure of silica-grafted chromate in the fully oxidised Phillips catalyst 1, the corresponding CO-reduced Phillips catalyst precursor 2 and the hypothesised Cr–oxo surface species 3 on the modified catalyst. Chromium, oxygen and silicon atoms are shown in blue, red and yellow, respectively.

Pd-Supported Catalysts: Evolution of Support Porous Texture along Pd Deposition and Alkali-Metal Doping

Adsorption of N2 at 77 K and scanning electron microscopy have been used to measure the changes in the support morphology, at nano- and microscale level, along the processes involved in the preparation of a supported Pd catalyst: Pd deposition, doping, and thermal treatments. Among the investigated supports, viz., activated carbons, gamma-Al2O3, SiO2, and SiO2-Al2O3 (SA), the SA one was found particularly sensitive to these processes, as a result of its high plasticity and reactivity. Involved processes can be summarized as follows: (i) During the Pd deposition, the support itself is partially dissolved and removed as a result of both the basicity of the precipitating agent and the final washing. (ii) When the undoped sample is thermally treated up to 823 K, only modest phenomena are observed. (iii) Upon doping with potassium carbonate, the support dissolution continues, and the greater the carbonate concentration, the greater the dissolution extent. In this case the dissolved material is not removed, but reprecipitates (partially outside the pores), during the subsequent drying at 393 K. (iv) When doped samples are thermally treated, the reaction between carbonate and support causes the mobilization of the support itself, with sintering phenomena that can reach the total collapse of the porous structure. The starting temperature of the pore collapse decreases with increasing potassium carbonate concentration. The modification of the support influences, directly or indirectly, the surface properties and the availability of Pd particles that can be doped or even covered by materials from support and made more or less accessible or even inaccessible by pore narrowing, widening, or blocking.