Preparation of Nanostructured Catalysts by Grafting Metal Alkoxides on the Surface of Oxides Supports and their Performances in Some Reactions of Industrial Interest

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: SiO 2 , Al 2 O 3 and TiO 2 , 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 SiO 2 and TiO 2 /SiO 2 supports, have been tested in reactions, such as: the SCR of NO with NH 3 , the Oxidative Dehydrogenation (ODH) of ethanol and methanol to formaldehyde and acetaldehyde, the ODH of propane, isobutane and n-butane.

(i) The inductive effect (electron release) of the alkyl or aryl group bonded to the oxygen.  Examples of aluminium alkoxides association are reported below: (2) Silicon and Germanium alkoxides are all monomeric (see Mehrotra et al. [3]).

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 [4], 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 O 2 surrounding the hydroxyls and clearly these hydroxyls have different acid-base characters. Figure 1: Different types of hydroxyls on the surface of alumina [4].
Moreover, Tanabe [5] 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 On the contrary, in a basic environment the reaction will be: 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][7][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: A. Change of the hydrophilic/hydrophobic character of the surface; B. To put a desired reactive group on the surface of a support; C. To obtain a well dispersed oxide on the surface of another one by using a low ratio alkoxide/support, so modifying the acidbase or redox properties of the surface; E. To anchor another metal on a mono/multilayered already coated system (example vanadia supported on titania coating silica with a monolayer or multilayer.
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:

a.
By changing the alkoxide groups through equilibrium exchange reactions of the type: By introducing other elements changing the electron density on the metal. An example is: So, generating by grafting very strong acid sites.

c.
By condensing a metal alkoxide with another one through a chemical reaction such as, for example, the one originated by the partial hydrolysis. That is, by preparing and then grafting heterometallic alkoxide compounds.
An example of the preparation of hetero-metallic alkoxide, that is, the "active site" is prepared in homogeneous phase and then heterogenized is the one proposed by Rulkens et al. [9]: The main factors influencing the grafting reaction are: a.
The chemical properties and structure of the alkoxide; b.
The chemical properties and the structure of the solid surface; c.
The choice of the alkoxide solvent (polar, apolar, none).
Often the parent alcohol is used as a solvent. This choice, normally,

System Studied and Achieved Results
Many different catalysts have been prepared using the grafting technique starting from some different commercial available alkoxides, such as: silicon, titanium, zirconium and vanadium anchored on the surface of oxides, such as: SiO 2 , Al 2 O 3 and TiO 2 .

Preparation and testing of catalysts with appropriate acid-base properties
The catalyst with acid properties prepared by grafting are summarized in Table 1 together with some peculiar aspect used in the preparation and the reactions test used for their characterization. Cracking and isomerization of n-hexane This reaction occurs in two steps that are:

a. Skeletal isomerisation of 1-butene [7]:
This reaction occurs in two steps that are: The best catalyst obtained by grafting contained 3% of silica corresponding approximately to the coating of a monolayer. The value of ZPC equal to 7.5 for the support decreased to 5.6 after the silica deposition and the intrinsic surface ionization constants strongly decreased too (pK1 from 5.7 to 3.4 and pK2 from 9.3 to 7.6). At last, the hydroxyls density, evaluated by Thermo Gravimetric Analysis, consistently increased from 1.05 to 1.4mmol/g suggesting a combination of the two following reactions stoichiometry: Methanol dehydration to dimethylether [7]: A good performance has been obtained by using a catalyst prepared by grafting zirconium tetra n-butoxide on a γ-alumina of 180m 2 /g. The optimal concentration of zirconium on the support was 0.08mmol/g. By increasing the amount of grafted zirconia the basic character of the catalyst increased and a hydroxyls density reached a maximum in correspondence of the maximum catalytic activity. This suggest a 1:1 stoichiometry for the grafting reaction, that is: Probably, the increase of both the basicity of the active sites and the hydroxyl density is responsible of the activity increase. However,

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despite the very small amount of grafted zirconia the reaction rate is doubled if compared with the activity shown by γ-alumina as it can be appreciated in Figure 3.
A similar behaviour has been obtained by grafting titanium iso-propoxide on a silica support of 450m 2 /g, that is, the activity almost null on silica, strongly increased by increasing the amount of grafted titania until reaching a maximum of 0.95mmol/hg for a catalyst containing 1.8mmol/g of grafted titania corresponding approximately to a mono-layered coating [8]. The specific surface area was slightly reduced from 450 to 400m 2 /g after the grafting procedure and the same occurred for the hydroxyls density decreasing from 2.2 to 1.9mmol/g. Both acid and basic sites are present on the surface that has a cooperative effect in promoting the methanol dehydration reaction.

1)
Cracking and isomerisation of n-hexane [6,8] While, Aluminium Isopropoxide was directly submitted to this reaction, zirconium tetra n-butoxide was preliminarily submitted to an exchange reaction with 1-Octanol, that is: (15) The grafting of the sulphated alkoxides gives place to the following surface reaction:

Preparation and testing of catalysts with redox properties
The catalyst with redox properties, prepared by grafting, are summarized in Table 2 together with some peculiar aspect used in the preparation and the reactions test used for their characterization. (*) Titania grafted on silica surface. The support has been prepared by repeating two or three times the grafting procedure (multi-layers of titania coating silica surface).

The system titania grafted on silica
Titania has been grafted first of all on silica (450m 2 /g) by changing the alkoxide concentration in toluene [8]. properties of these catalysts and their utility as support for preparing vanadium based catalysts prepared by grafting vanadyl alkoxides.
At this purpose, in Figure 4, the Langmuir isotherm related to the titania grafted on silica, starting from a silica of 282m 2 /g and Ti (iso-Propoxide) 4 solution in dioxane, is reported [10]. In this case, the maximum amount of titanium loaded (monolayer) is 1mmol/g instead of 1.8mmol/g obtained on a silica of 450m 2 /g using toluene as solvent. This last silica and the solvent dioxane will be used to prepare the supports for anchoring vanadia.

Figure 4:
A Langmuir isotherm determined for the chemical adsorption of Ti (iso-Propoxide) 4 , dissolved in dioxane, on a commercial silica of 282m 2 /g of specific surface area [10].

The System Vanadia Grafted on Silica/Titania Support
Different vanadium pentoxide based catalysts have been prepared by grafting vanadyl isopropoxide on respectively: silica, A. Silica covered with a monolayer of titania TS1 and silica covered with a multi-layer TS3 (Grafting repeated three times in the same conditions). The amounts of vanadyl alkoxide adsorbed for the different considered supports is compared in Figure 5 [10].
The steep rise of the initial curve related to the TS3 support denotes the strong interactions occurring between vanadyl alkoxide and titania surface with respect to the curve obtained for silica.

Redox reactions test results
The test reactions considered are:

Cyclo-Octene Epoxydation with Cumene Hydroperoxide
The reaction requires redox sites containing titanium [8,11] The catalysts employed in this reaction are the ones prepared by using the support silica with 280m 2 /g coated with increasing amounts of titanium alkoxide dissolved in toluene until reaching the monolayer and a catalyst . The obtained results are reported in Figure 6.

Copyright © E Santacesaria
PPS.MS.ID.000560. 3(2).2020 As it can be seen the activity and selectivity increase with the content of TiO 2 until reaching a maximum for the catalyst charged with a monolayer of titanium alkoxide. This maximum coincides with the maximum of Lewis acid sites concentration on the surface as shown in the work published by Cozzolino et al. [12].

Selective Catalytic Reduction (SCR) of NO x with NH 3
The SCR reaction: Was performed on V 2 O 5 containing catalysts, supported on a TiO 2 surface [13]. Two catalysts have been tested one prepared by grafting vanadyl tri-isopropoxide on a commercial TiO 2 having a specific surface area of 120m 2 /g that has been compared with a catalyst prepared by impregnating vanadium on the same support.
Another catalyst has been prepared starting from TiO 2 /SiO 2 support (a silica coated with multi-layers of TiO 2 repeating three time the grafting procedure of titanium tri-isopropoxide dissolved in toluene on a silica support of 280m 2 /g). The same catalyst has then been compared with a catalyst prepared by impregnation containing the same amount of supported vanadium. The data of conversions in the mentioned reaction have been collected in steady state conditions and are reported in Table 3. As it can be seen, the specific surface area of titania decreased very much as a consequence of the treatments of impregnation, grafting and calcinations. However, although the activity of the catalyst grafted on TiO 2 is lower its selectivity is greater. On the contrary, the activity and selectivity of the catalyst prepared by grafting vanadyl tri-isopropoxide on the described TiO 2 /SiO 2 support are both much greater.
This can be better appreciated from the plots reported in Figure 7.

High temperature ODH (Oxidative dehydrogenation) reactions
The reaction considered in a general approach is: As alkanes have been tested: propane [14], Butane [15] and Isobutane [16] for obtaining the corresponding olefins. The most relevant results have been obtained in the ODH of butane. The best catalysts used for promoting this reaction [15] are reported in Table 4. In all cases reported in Table 4, vanadyl tri-isopropoxide dissolved in dioxane has been grafted on a support prepared by grafting titanium tetra-isopropoxide dissolved in toluene on a commercial silica of 320m 2 /g and repeating three time the grafting procedure to obtain a silica coated with a multilayer of TiO 2 (2.23mmols of Ti anchored/g corresponding to 17.8% of TiO 2 ). The surface area of the TiO 2 -SiO 2 support was 274m 2 /g. The catalytic tests have been performed at atmospheric pressure in a quartz

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tube microreactor containing small amount of catalyst (0.015-0.05 g) and feeding butane diluted with helium and oxygen. The total feed flow rate was kept between 50 and 200cm 3 /minute and the O 2 /butane ratio was kept equal to 1.7.
At the exit the gaseous stream was analyzed by gaschromatography for determining residual oxygen and butane, other formed hydrocarbons, CO and CO 2 . The temperature of the reactor was kept constant, in all cases, at 500 °C. Different runs have been made by using not only the catalysts of Table 4. But also catalyst prepared by impregnation on silica, on titania and on    Table 4 containing increasing amounts of grafted V 2 O 5 .
In Figure 9 the conversions and selectivities obtained for the different tested catalysts of Table 4  In the case of methanol oxidation to formaldehyde [17,18], we can write:  The best catalyst for promoting this reaction resulted the one prepared by grafting vanadyl vanadyl tri-isopropoxide dissolved in dioxane on TiO 2 -SiO 2 (silica coated with a monolayer of titania).

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The titania grafted on the silica support was 7.3w% with a specific At last, on the best catalyst, that is, the one containing 5.37w% of vanadia a kinetic approach has been developed determining the reaction products observed at different reaction temperatures. T the obtained results are reported in Figure 12. As it can be seen, by increasing the temperature the formation of intermediate products as dimethoxy methane and methylformate are observed.
Further increasing the temperature these two products completely disappears, while, the formation of CO 2 increases. More instructive is the evolution of the product distribution as a function of the residence time W/F, at 160 °C, reported in Figure 13. More details about this study can be found in [17] in which a reaction mechanism is proposed and a kinetic model for interpreting all the kinetic runs performed is described. In the case of ethanol oxidation to acetaldehyde [19,20], we can write: Also this reaction is promoted by supported V 2 O 5 and again catalysts prepared by grafting are more selective than the ones prepared by impregnation as it can be appreciated in Figure 14. The conversion and yields obtained at respectively 140 and 160 °C with the vanadium grafted best catalyst are reported in Figures   15&16, while the product distribution obtained at 160 °C is reported in Figure 17. Also in this case, a reaction mechanism has been proposed and a Mars-Krevelen kinetic model has been developed for interpreting all the experimental runs and determining the best fitting parameters [20].   By grafting more titanium alkoxide on silica until reaching a monolayer or repeating more time the grafting procedure depositing multi-layers of TiO 2 on silica a new very interesting support is obtained, because, the specific surface of silica is reduced for a small extent and we have so a large TiO 2 surface that is stable to the heat treatments differently from the anatase surface that in the same conditions is prone to collapse. This support is very suitable to the preparation of vanadium based dispersed catalysts being the vanadium pentoxide-titania interactions very strong [21].
These catalysts have given very promising results in the ODH of hydrocarbons to olefins and to the ODH of methanol and ethanol to the corresponding aldehydes.