Serena Bertarione

Furfuryl alcohol polymerization in H-Y confined spaces: reaction mechanism and structure of carbocationic intermediates.

The acid-catalyzed polymerization and resinification, in the 300-673 K interval, of furfuryl alcohol adsorbed in the framework of a protonic Y zeolite is studied by means of FTIR, Raman, and UV-vis spectroscopies. The idea is that restricted spaces can impose a constraint to the growth of the oligomeric chains, therefore moderating the formation of conjugated sequences responsible for the color of the products and allowing their observation by means of spectroscopic techniques. The detailed study of the evolution of UV-vis, FTIR, and Raman spectra upon dosed amount, contact time, and temperature has allowed the spectroscopic features of some of the single species, either neutral and positively charged (carbocationic intermediates), to be singled out and assigned to understand the mechanism of initiation. The vibrational assignments have been confirmed by computer simulations on model compounds and compared with the results of the mechanistic description of the reaction mechanism made in the past (Choura, et al. Macromolecules 1996, 29, 3839-3850). The spectroscopic methods have been applied in a large temperature range in order to follow also the formation of more complex products into the pores, associated with longer conjugated sequences, gradually filling the open spaces of the zeolite. For samples contacted with furfuryl alcohol at 673 K, this methodology gives information also on the incipient carbonization process, leading to the formation of a carbonaceous replica phase inside the internal porosity of the zeolite.

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.

Micro-FTIR and Micro-Raman Studies of a Carbon Film Prepared from Furfuryl Alcohol Polymerization

The synthesis of a carbon film by the acid-catalyzed polymerization and resinification of furfuryl alcohol with a diluted solution of HCl is studied by combining micro-FTIR and micro-Raman spectroscopies. The detailed study of the evolution of spectra as a function of dosage of furfuryl alcohol and temperature shows that neutral and protonated species are formed at 80 degrees C, while upon gradually increasing the temperature up to 600 degrees C, the viscous polyfurfuryl alcohol resin is transformed into a carbon phase, containing a heterogeneous distribution of pores, with a size in the 100-2000 nm range, as shown by SEM and AFM analyses.

Rapid purification/oxidation of multi-walled carbon nanotubes under 300 kHz-ultrasound and microwave irradiation

The use of ultrasound (US) and microwaves (MW) in the oxidation and purification of multi-walled carbon nanotubes (MWCNTs) was investigated. These techniques, in particular US at a frequency of 300 kHz, strongly accelerate the process and avoid the heavy structural damage, observed at the 20–35 kHz classic range, even at low power. Due to the residual metal catalyst on the head of MWCNTs, MW heating is strongly absorbed, causing the rupture of the tip and the loss of the metal. All our chemico-physical treatment types were performed by suspending the CNTs in a 3 ∶ 1 H2SO4/HNO3 mixture. The resulting samples were investigated by TEM microscopy, TGA analyses and Raman spectroscopy, while the degree of oxidation was estimated by colourimetric analyses.

Temperature resolved FTIR spectroscopy of Cr2+/SiO2catalysts: acetylene and methylacetylene oligomerisation

As is well known, the Cr2+/SiO2 system is an efficient catalyst for ethylene polymerisation already at RT (even if in the industrial process the running temperature is ∼380 K). For this reason it is the ideal system for in situ spectroscopic investigations on a genuinely working catalyst (Adv. Catal., 2001, 46, 265). Many questions are still uncertain as to the initiation, propagation and termination steps of the polymerisation mechanism. On a pre-reduced sample polymerisation commences very quickly and there is no way to record initial stages. When the experiments are performed at RT the phenomenon is too fast and IR spectroscopy, even in the time-resolved mode, failed up to now in the identification of the species formed during the initiation of the ethylene polymerisation step on the Cr2+/SiO2 Phillips catalyst. We present results related to time-resolved FTIR spectroscopy at variable temperature of acetylene and methylacetylene oligomerisation on a model Phillips catalyst. These experiments have been highly informative on the nature of the active sites because we have observed that acetylene and methylacetylene result in the immediate formation of benzene and 1,3,5-trimethylbenzene, respectively, without the evidence of any measurable intermediate product. This implies that the active Cr sites are able to coordinate simultaneously three monomers and thus must exhibit a high unsaturative coordination. The results of these experiments could be an insight of chromium species active in the Phillips catalyst.

The Poly-A Interaction and Interfaces with Carbon Nanotubes

The interaction between a single and a double strand poly-A and single wall carbon nanotubes is studied with SEIRA, SERS, and AFM techniques. After the purification and separation, the sample of nanotube-poly-A contains 1–5 nanotubes covered by poly-A. According to SEIRA and SERS spectroscopy, poly-A in a single strand form preserves its initial conformation with numerous disorders and breaks in a single strand of poly-A at the carbon nanotube surface. After the adsorption on the nanotube surface, the double strand form of poly-A changes its conformation with a partial loss of the pairing between adenine bases. The main cites of the interaction of poly-A with nanotubes are adenine and phosphate groups.

Preparation of Pd/C catalysts : from the Pd-precursor solution to the final systems

The preparation of a Pd/C catalysts for hydrogenation reactions has been followed with different techniques in all its key steps: precursor precipitation from solution, partial reduction from the support, chemical reduction and, to complete the process for sake of study, H2 reduction.

Adsorption of CS2 on MgO microcrystals: formation of a S-doped MgO surface

The interaction of carbon disulfide, CS2, with polycrystalline MgO was investigated by means of infrared and UV–visible spectroscopies and ab initio electronic structure calculations. The aim was to understand the modifications of the surface properties of MgO microcrystals induced by the substitution of O2− with S2− anions. The interaction of CS2 with MgO shows the formation of thiocarbonates followed by O–S exchange reactions and formation of Mg2+–S2− clusters in a MgO matrix. The O–S exchange reaction involves mainly the low-coordinated O2− anions at the steps and corner sites, and only at high temperature are a small number of O2− anions at the (100) terrace sites involved. The replacement of the O with the S atoms at the low-coordinated sites of the MgO surface transforms the reactive polycrystalline material into a chemically inert system.