Rafael A. García

Thermal and catalytic cracking of polyethylene under mild conditions

Abstract Thermal and catalytic cracking of both high and low-density polyethylene (HDPE and LDPE, respectively) under mild conditions have been investigated in order to study the properties of the solid waxy product so obtained. The catalysts employed were n-HZSM-5 and HY zeolites, amorphous silica–alumina, activated carbon, Pd charcoal powder and mesoporous aluminosilicate materials (MCM-41), with and without impregnated Pd. The best results achieved in the LDPE degradation with regard to stability and homogeneity of the waxy product were obtained using MCM-41 as catalyst. The high BET surface area, uniform mesoporosity and medium acid strength of this catalyst promotes the polymer cracking according to a random scission mechanism, as well as, the development of hydrogen transfer reactions which reduce the olefinic character of the solid product. On the other hand, the product obtained from HDPE has a higher homogeneity than that coming from LDPE, leading to a waxy product with better quality for potential applications.

One-step synthesis of a thioester chiral PMO and its use as a catalyst in asymmetric oxidation reactions

The study of key aspects for the heterogenization of chiral homogeneous catalysts is addressed in this work. Two different heterogenization routes were evaluated. On the one hand, the immobilization of a tartaric acid derivative (Sharpless catalyst) was accomplished by following a two-step synthesis methodology. In the first step, the mesoporous support SBA-15 was functionalized with different percentages of mercaptopropyl-(trimethoxy)-silane (MPTMS) through a co-condensation method, obtaining very high degrees of MPTMS incorporation. In the second step, the chiral moiety is anchored onto the previously organically modified silica by means of a thiotransesterification reaction. On the other hand, the second route consisted of the formation of mesoporous structure and chiral ligand immobilization in the walls, simultaneously, in only one step. In this case, the chiral organic functionality was incorporated into the wall silica framework, since the built-organic complex is a bis-silane, obtaining a chiral PMO. Finally, once characterized, the synthesized materials were proved in an asymmetric catalytic test in order to study their enantioselective properties. The chosen reaction was the thioanisole enantioselective sulfoxidation, using cumene hydroperoxide as an oxidant. The materials synthesized by the multi-step method reached 40% enantiomeric excess (ee), while the synthesized one-pot chiral PMO materials reached up to 50% ee. Thereby, materials with the chiral Sharpless ligand incorporated into the three dimensional silica framework were successfully accomplished, as well as its ability to induce higher chirality in the enantiomeric tested reaction than the conventional methods of chiral ligand heterogenization onto a silica framework.

Volatile Organic Compounds Analysis in Breath Air in Healthy Volunteers and Patients Suffering Epidermoid Laryngeal Carcinomas

Exhaled breath contains thousands of gaseous volatile organic compounds (VOCs) that may be used as non-invasive markers of head and neck epidermoid cancer. We hypothesized that solid phase micro-extraction coupled to gas chromatography–mass spectrometry can discriminate patients with epidermoid head and neck cancer from healthy controls by analyzing the gaseous volatile organic compounds, VOC-profile, in exhaled breath, thus identifying some non-invasive biomarkers to be used in early detection. Twenty healthy subjects participated in a cross-sectional study plus 11 patients with epidermoid supraglottic laryngeal cancer. VOCs from T3 supraglottic cancer were clustered distinctly from those of T1 and healthy subjects. Up to seven VOCs were detected differently from healthy volunteers, mainly 2-butanone and ethanol. Thus VOC-patterns of exhaled breath may discriminate patients with epidermoid head and neck cancer from healthy controls.

Modification of chiral dimethyl tartrate through transesterification: Immobilization on POSS and enantioselectivity reversal in sharpless asymmetric epoxidation

Modification of dimethyl tartrate has been investigated through transesterification with aminoalcohols to provide reactive functionalities for the covalent bonding of chiral tartrate to polyhedral oligomeric silsesquioxanes. The transesterification of dimethyl tartrate has been widely studied using different catalytic systems and reaction conditions. Through the proper selection of both the catalytic system and the reaction conditions, it is possible to achieve monosubstituted or bis-substituted tartrate derivatives as sole products. All the intermediate chiral tartrate-derived ligands were successfully used in the homogeneous enantioselective epoxidation of allylic alcohols providing moderate enantiomeric excess over the products. Attached amine groups have been used to support the modified tartrate ligands on to a haloaryl-functionalized silsesquioxane moiety. This final chiral tartrate ligand displays reverse enantioselectivity in the asymmetric epoxidation of allylic alcohols with regard to the starting dimethyl tartrate ligand, both molecules having the same chiral sign. However, the POSS-containing ligand can be easily recovered in almost quantitative yield and reused in asymmetric epoxidation reactions. In addition, recovered silsesquioxane-pendant ligand, though displaying decreasing catalytic activity in recycling epoxidation tests, showed very stable enantioselective behavior.

Catalytic activity of the beta zeolite with enhanced textural properties in the Friedel-Crafts acylation of aromatic compounds

Friedel-Crafts acylation is widely used for the production of aromatic ketones applied as intermediates in both fine chemicals and pharmaceutical industries. The reaction is carried out by using conventional homogenous catalysts, which represents significant technical and environmental problems. The present work reports the results obtained in the Friedel-Crafts acylation of aromatic substrates (anisole and 2-methoxynaphthalene) catalyzed by Beta zeolite obtained by crystallization of silanized seeds. This material exhibits hierarchical porosity and enhanced textural properties. For the anisole acylation, the catalytic activity over the conventional Beta zeolite is slightly higher than with the modified Beta material, probably due to the relatively small size of this substrate and the weaker acidity of the last sample. However, the opposite occurred in the acylation of a bulky substrate (2-methoxynaphthalene), with the modified Beta showing a higher conversion. This result is interpreted due to the presence of a hierarchical porosity in this material, which favors the accessibility to the active sites.

Enantioselective styrene epoxidation using the Jacobsen catalyst immobilized on functionalized SBA-15

The chiral Jacobsen complex has been immobilized by anchoring on organofunctionalized SBA-15 supports. Two types of functionalization agents have been tested: 3-aminopropyltrimethyxysilane and p -aminophenyltrimethoxysilane. The different materials so obtained have been checked as catalysts in the enantioselective styrene epoxidation, several reusing tests being carried out in order to find out the activity and stability of the heterogeneized systems. The best results were obtained with the catalyst prepared by anchoring on a SBA-15 support previously functionalized with aminophenyl groups, since after three reaction tests this system is still active and enantioselective in the epoxidation reaction while it retains 72% of the Mn(salen) complex contained in the raw catalyst. A study of the catalyst stability related with the amount of oxidant and catalyst in the reaction medium is presented. The Jacobsen complex leaching was almost removed by working at both high substrate/oxidant and catalyst/substrate ratios.

Direct synthesis and post-oxidation of SBA-15 and MCM-41 functionalized with butenyl groups

Olefins are interesting compounds to anchor onto the surface of silica supports since their reactivity allow to form a large variety of organic functionalities. In this work, the synthesis of mesostructured silicas (MCM-41 and SBA-15 type materials) functionalized with butenyl groups is studied through the use of post-synthetic grafting or direct-synthesis procedures. Results confirm the higher functionalization degree achieved in the final mesostructured materials when using direct synthesis procedures. Unlike MCM-41 materials, which become microporous when high organic content are attached on, SBA-15 type butenyl functionalized silicas keep the features of a mesoporous solid. Post-synthetic and in-situ oxidations of butenyl groups during the preparation of the mesostructured silicas have been carried out using H2O2 as oxidant, leading to materials functionalized with glycol moieties attached to inorganic mesoporous materials, which are attractive to be used as supports for anchoring different catalytic species.

Friedel-Crafts acylation of aromatic compounds over hybrid zeolitic-mesoporous materials

Abstract Microporous-mesoporous materials were synthesized by a two step process, based on the assembling of ZSM-5 precursors around CTAB micelles. The solids, named HZM, were characterized by XRD, FT-IR, TEM, N 2 adsorption-desorption isotherms, TG and NH 3 TPD analyses, exhibiting properties intermediate of those of Al-MCM-41 and ZSM-5 materials. The HZM samples have been tested in acylation of anisole and 2-methoxynaphtanlene, showing higher catalytic activity than both amorphous mesoporous Al-MCM-41 and crystalline ZSM-5. Selectivity towards the isomers of interest was favoured by the use of acetyl chloride as acylating agent.

Synthesis of Ti-MCM-41 from amorphous SiO2-TiO2 xerogels

A novel method has been developed for the synthesis of Ti-MCM-41 based on the preparation in a first step of an amorphous SiO2-TiO2 xerogel, followed by addition of a cationic surfactant and hydrothermal treatment. Ammonium hydroxide (NH3) and tetrapropylammonium hydroxide (TPAOH) solutions were used as basic agents during both the preparation of the xerogel and its subsequent hydrothermal transformation. The properties of the samples thus obtained strongly depend on the method used to prepare the raw xerogel. When employing NH3 the materials produced present a lack of mesostructure, showing that this basic agent is not suitable for promoting the xerogel reorganization around the surfactant micelles. In contrast, the use of TPAOH leads to Ti-MCM-41 samples with a high degree of mesoscopic ordering and presenting all the Ti atoms incorporated into the walls. This novel method allows Ti atoms to be first stabilized in isolated positions of a silica matrix, which avoids the formation of non-desired TiO2 phases, while the Ti content of the final Ti-MCM-41 is similar to that of the raw xerogel.