"Preparation of Nanostructured Catalysts by Grafting Metal Alkoxides on the Surface of Oxides Supports and their Performances in Some Reactions of Industrial Interest"
Progress in Petrochemical Science , Volume 3, pp 1-11; doi:10.31031/pps.2020.03.000560
Abstract: E Santacesaria1*, M Cozzolino2, R Tesser3 and M Di Serio3 Faculty of Chemistry and Chemical Engineering, Slovenia *Corresponding author: E Santacesaria, Eurochem Engineering srl, Via Codogno 5 (IT-20139) Milano, Italy Submission: April 16, 2020;Published: March 16, 2020 DOI: 10.31031/PPS.2020.03.000560 ISSN 2637-8035Volume3 Issue2 Metal alkoxide grafting technique can be used for changing the acid-base and/or the redox properties of the surface of an oxide rich in hydroxyls. The preparation of catalysts by grafting different commercial available alkoxides, such as: Si, Ti, Zr, and V on the surface of oxides, such as: SiO2, Al2O3 and TiO2, is reviewed. The performances of the acid catalysts were evaluated by adequate test reactions such as: methanol dehydration, skeletal isomerization of 1-butene and alkane isomerization and cracking. The redox properties of vanadium based catalysts, obtained by grafting vanadyl alkoxide on SiO2 and TiO2/SiO2 supports, have been tested in reactions, such as: the SCR of NO with NH3, the Oxidative Dehydrogenation (ODH) of ethanol and methanol to formaldehyde and acetaldehyde, the ODH of propane, isobutane and n-butane. Keywords: Grafting; Metal alkoxides; Silica; Alumina; Titania; Vanadia Metal alkoxides are nowadays largely employed for preparing ceramic materials, for thin coating films and for supports and catalysts preparation using in the different cases the sol-gel technique or the chemical vapour deposition . Although very promising, less popular is the employment of metal alkoxides for preparing supports and catalysts by using the grafting technique. This technique can usefully be used for deeply changing the acid-base and/or the redox properties of the surface of an oxide rich of hydroxyls. As the catalytic properties depends almost exclusively on the surface properties of the solid used as catalyst, modifying opportunely the surface by contacting it with reactive substances like the metal alkoxides can give surprising results in terms of activity and selectivity in different reactions. In particular, the properties of the new surfaces obtained by reacting the superficial hydroxyls with a metal alkoxide, after a stabilizing treatment of hydrolysis and calcinations, are quite different from the original surface. A systematic work has been made on the subject by our research group in the past and a review of the most significant obtained results is reported in this paper. Summary of the properties of some alkoxides affecting the grafting procedure Metal alkoxides are characterised by the presence of M-O-C bonds. Due to the strongly electronegative character of oxygen (3.5 in the Linus Pauling electronegativity scale) the ionic character of the metal-oxygen bond would be preminent. Metal alkoxides exhibit both Lewis and base properties. It could be expected 80% of ionic character for the more electropositive metals (electronegativity in the range 0.9-1.2, that is, alkaly metals, alkaline earths and lanthanons, while, it could be expected 65% of ionic character for metals having electronegativity values in the range 1.3-1.5 such as in the case of aluminium, titanium and zirconium . However, some of the mentioned alkoxides show a fair degree of volatility and solubility in common organic solvents, that is, have properties which can be considered characteristics of covalent compounds. The attenuation of the polarity of the metal-oxygen bond can be attributed to two main factors: The oligomerization of alkoxide complexes [M(OR)n]x depends on a number of factors such as: Bradley  suggested the following rules: Examples of aluminium alkoxides association are reported below: Silicon and Germanium alkoxides are all monomeric (see Mehrotra et al. ). Summary of the properties of some oxide surface that can be used as support for grafting Alumina is an amphoteric oxide with a moderate basic character having a ZPC (Zero Point Charge) = 8-9. According to Peri , on the alumina surface there are 5 different type of hydroxyls A, B, C, D and E, as it can be seen in Figure 1. The difference is given by the number of groups O2- surrounding the hydroxyls and clearly these hydroxyls have different acid-base characters. Figure 1: Different types of hydroxyls on the surface of alumina . Moreover, Tanabe  has demonstrated that on alumina there are both acid sites of Bronsted & Lewis character in an approximately equal amount. However, considering an amphoteric oxide in acid environment the hydroxyls of the support will react as it follows S- OH + H+ A- ↔ S- OH2+ A- (3) On the contrary, in a basic environment the reaction will be: S- OH + B+ OH- ↔ SO- B+ + H2O (4) The hydroxyls on the silica surface are more uniform and have a moderately acid character being the ZPC=1-2. No Lewis acid sites are present. Description of the grafting technique The grafting technique [6-8] consists in putting in contact a metal alkoxide pure or dissolved in a opportune solvent with the surface of an oxide rich of hydroxyls. By grafting a metal alkoxide on a support it is possible: The grafting operation occurs through three different steps that are: A. Grafting: Reaction between the metal alkoxide and the superficial hydroxyls. This reaction can occur with different a stochiometry according to the type of alkoxide employed and the surface density of the hydroxyls: B. Steaming or burning: Have the scope of stabilizing the obtained surface eliminating the organic groups bonded to the grafted metal. C. Calcination: A dehydration occurs and the original oxide surface is more or less coated with another different oxide. Metal alkoxides eventually can also be modified before grafting for a better control of the properties of the heterogenized catalytic site: By changing the alkoxide groups through equilibrium exchange reactions of the type: Me(OR)n + nR’OH ⇄ Me (OR’) + n ROH (6) By introducing other elements changing the electron density on the...
Keywords: nanostructured / Metal Alkoxides / Grafting Technique / surface of an oxide rich / oxide rich of hydroxyls
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