Nanotubos de titanatos como catalisadores para a reação da acetalização do glicerol com acetona

Abstract

Sodic titanate nanotubes (NaTNTs) have been synthesized by hydrothermal treatment using different synthesis time (24, 48 and 72 h). The as-synthesized solids were submitted to ion-exchange and wet-impregnation to produce protonated titanate nanotubes (HTNTs) titanate nanotubes containing metals (MTNTs). The materials were characterized by XRD, Raman spectroscopy, SEM-EDS, TEM, TEM-HAADF, pyridine adsorption coupled to FTIR and TG analysis, nitrogen adsorption-desorption isotherms and XPS. All solids were evaluated in the acetalization of glycerol reaction with acetone, varying the reaction temperature (50, 80 and 110 ºC), acetone:glycerol composition (1:1, 4:1 and 8:1) and/or mass of catalyst (between 65-260 mg).

The HTNTs exhibited the H2Ti3O7 phase as result of the ion-exchange process that is able to remove Na from NaTNTs. This was confirmed by XRD and Raman measurements. The tubular morphology was obtained for all solids, independently on the synthesis time, as seen by SEM and TEM analysis. The nitrogen adsorption-desorption isotherms displayed type IV curves, characteristic of mesoporous materials having the solids high textural properties. Pyridine adsorption measurements exhibited solids with low to moderate acidity, especially for the solids synthesized with 72 h (HTNT72). The catalytic results showed that HTNT24 and HTNT48 displayed low activity in the reaction due to low amount of moderate acid sites. On the contrary, HTNT72 showed a glycerol conversion of 45.5% and selectivity to solketal and acetal of 98%, due to the high amount of acid sites (0.33 mmol.g-1CAT) with moderate strength. The variation of the reaction conditions, such as temperature and composition revealed the occurrence of parallel reactions, as oxidation and dehydration. The characterizations of spent catalysts illustrated phase transformation as the deactivation phenomenon of HTNT72 at several reaction conditions. The HTNTs showed available physicochemical properties to the application in the acetalization of glycerol with acetone, such as high amount of moderate acid sites, physical stability and textural properties with high surface area. These properties were relevant to perform the reaction, at 50 ºC and molar ratio of acetone:glycerol equal 1, promoting 45.5% of glycerol conversion and selectivity of 98% to solketal and acetal.

In the case of MTNT series (M = Co, Ni and Pt), the highest glycerol conversion to PTNT (ca. 40%), in comparison to CTNT and NTNT tested at 50 ºC, is related to strong interaction of platinum species with the trititanates support, as well as to high surface area and amount of available acid sites. The low performance of CTNT and NTNT is associated to occurrence of leaching of the active species on surface and to low amount of acid sites. When PTNT is applied at 50 ºC and molar ratio of acetone:glycerol equal 4, the catalyst is able to provide higher catalytic performance, with glycerol conversion of 86.5%; however, the selectivity to solketal decreased from 17 to 12.5%, suggesting the occurrence of parallel reactions, as dehydration, due to the highest amount of acetone in the reactional medium. After the reaction, it observed the existence of Na2Ti3O7 semicrystalline phase to all solids, as related by XRD and Raman spectroscopy. Meantime, the broad peaks and vibrational modes, besides the appearance of diffraction peaks, suggested phase or morphological transformation to produce nanosheets and/or disorder in the structure due to chemical adsorption of products come from the acetalization reaction. The TEM, SEM-EDS and TEM-HAADF results suggested the presence of tangled nanotubes containing nanosheets, indicating partial phase transformation to specifics cases of parameters used. The XPS measurements exhibited the presence of Pt species, after the reaction, in the oxidation state of Pt2+ and Pt4+, which are characteristic of PtOx and oxichlorine. The textural properties evidenced decrease in comparison to fresh analysis, indicating that after the reaction, PTNT, CTNT and NTNT suffered modifications in the morphology and/or structure in different conditions of temperature and reactants composition.