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- A variable-temperature diffuse reflectance infrared Fourier transform spectroscopy study of the binding of water and pyridine to the surface of acid-activated metakaolinPublication . Belver, Carolina; Breen, Christopher; Clegg, Francis; Fernandes, César E.; Vicente, Miguel A.Four metakaolins were prepared by heating a Spanish kaolin at 600, 700, 800, and 900 °C for 10 h. Following preliminary optimization, these metakaolins were acid activated in 6 M hydrochloric acid at 90 °C for 6 h; the samples calcined at 600, 700, and 800 °C produced the highest surface area solids and were selected for further study. Variable-temperature diffuse reflectance infrared Fourier transform spectroscopy analysis of the resulting acid-activated metakaolins (AAMKs) identified a wide range of hydrogen bond strengths in adsorbed water at room temperature. Above 300 °C it was possible to fit the broad hydroxyl stretching band to seven contributing components at 3730, 3700, 3655, 3615, 3583, 3424, and 3325 cm-1. As the sample temperature was increased, the 3730 cm-1 band increased in intensity as the water hydrogen bonded to AlOHAl was thermally desorbed. The other six bands decreased in intensity. The spectra of adsorbed pyridine indicated the presence of both Bro¨nsted and Lewis acid sites on the surface of the air-dried AAMKs. Preheating the AAMK at 200 °C prior to pyridine sorption reduced the number of Bro¨nsted acid sites and increased the number of thermally stable Lewis acid sites. A reduction in the amount of adsorbed pyridine after pretreating the AAMK at 400 °C was tentatively attributed to a reduction in surface area. This was reflected in fewer thermally stable Lewis acid sites in the AAMK pretreated at 400 °C compared to the number present in the sample pretreated at 200 °C.
- Catalytic conversion of limonene over acid activated Serra de Dentro (SD) bentonitePublication . Fernandes, C.; Catrinescu, C.; Castilho, P.; Russo, P. A.; Carrott, M. R.; Breen, C.A series of acid-activated clay catalysts were prepared from a purified bentonite, rich in structural iron, collected at Serra de Dentro on the island of Porto Santo. The purified bentonite exhibited a surface area of 130 m2 g 1 which increased to values as high as 500 m2 g 1 following activation with 4M HCl at 95 8C for 30 min (SD-4M-95-30). The ability of the activated samples to convert limonene to p-cymene was evaluated using a reaction time of 15 min at 80 8C. The sample prepared using 3M HCl at 95 8C for 30 min (SD-3M-95-30) offered the optimum combination of surface area (470 m2 g 1 ) and acidity (0.26 mmol g 1 ) and 95% of the limonene was converted to product. About 15% of the product mixture was p-cymene whilst non-volatile products and polymeric species made up 54% of the product mixture. The presence of iron in the octahedral sheet of the SD bentonite appears responsible for the dehydrogenation activity. TG–MS analysis of acid activated samples, saturated with cyclohexylamine, reflected the dehydrogenation capabilities of the catalysts in that SD-3M-95-30 produced the most benzene and aniline as decomposition/ transformation products.
- Influence of exchange cations on the catalytic conversion of limonene over Serra de Dentro (SD) and SAz-1 clays: correlations between acidity and catalytic activity/selectivityPublication . Catrinescu, C.; Fernandes, César; Castilho, P.; Breen, C.A bentonite collected at Serra de Dentro (SD), Porto Santo Island, Portugal, and the source clay SAz-1 (Cheto, Arizona) were ion-exchanged with different cations (Al3+, Ni2+, Cr3+ and Na+ ). Variable temperature diffuse reflectance infrared Fourier transform spectroscopy (VT-DRIFTS) of pyridine treated samples, thermal desorption of cyclohexylamine and real time mass spectrometry of the evolved gases were used to evaluate the acidic properties of the prepared catalysts. The catalytic activity of these ion-exchanged clays was tested in the acid-catalysed conversion of limonene at 150 8C, to yield isomerization products (terpinolene, a-terpinene, g-terpinene and isoterpinolene), disproportionation products ( p cymene and p-menthenes) and high molecular-weight compounds. The possibility of increasing the selectivity toward p-cymene, over a catalyst with substantial Lewis acid character, was particularly envisaged. Catalysts derived from SD were significantly more active than their SAz-1 counterparts. This was mainly attributed to the greater inherent acidity, to the higher structural iron content (providing dehydrogenation activity) and to the higher nitrogen surface area of the starting SD clays. Within the Mn+ -SD series, the order of activity decreased as Ni2+ > Al3+ > Cr3+ > Na+ . A direct comparison between Al3+-SD (with maximized Bro¨nsted acidity) and Ni2+-SD (with predominant Lewis acid character) provided no support for enhanced p-cymene production over Lewis acid sites.
- Argilas modificadas como catalisadores ácidos sólidosPublication . Fernandes, César EsmeraldoImportant bentonitic deposits are present in Porto Santo Island, part of the Madeira Archipelago. Several locations were selected and samples were collected and characterised. The bentonite obtained at Serra de Dentro (SD) was selected for further laboratorial work. The fine fraction of SD bentonite was purified using several methods and the sodium homoionic form was prepared. This was the starting material used in the three generic types of modifications: metal exchange, acid activation and pillaring. These modifications produce materials with markedly different acidic (e.g. Brönsted and/or Lewis acidity), textural (e.g. increase of the surface area and active site accessibility) or structural (e.g. creation of permanent porous structures) properties. The wide range of materials obtained (including reference clays counterparts) was characterised in terms of chemical, structural, textural and catalytic properties. Limonene is an important raw material produced in Portugal, and its aromatisation reaction was chosen for the catalytic characterisation of the clay catalysts prepared.