471079 Synthesis and Characterization of Au Nanoparticles for Ethanol Oxidation: Effect of Acidity and Support Structure
Gold catalysts have recently turned out to be the most interesting catalysts in alcohol oxidation with environmentally friendly oxidizers such as molecular oxygen. Among different supports for gold, zeolites are promising due to their high thermal stability, presence of large surface area, and micropores that may hinder the sintering of Au. Furthermore, gold catalysts can be used for catalytic oxidation of hydroxyl and carbonyl groups in molecules from biomass.
However, tuning the gold nanoparticles size is one of the challenging steps to maximize the reaction rate . In order to overcome this challenge, efforts have been made to prepare catalysts with gold particle sizes smaller than 10 nm using zeolites (H-Y-12, H-Y-80, H-Beta-25, H-Beta-150 and H-Beta-300) together with Al2O3 and SiO2via deposition-precipitation method. This method has been the most frequently employed method for Au modification and involves the use of tetrachloroauric (III) acid as a precursor of Au. In addition, some alumina, carbon, titania and silica catalysts modified with gold were also tested as commercial catalysts. In this work, was studied. the effect of support structure and acidity, pH and aging time of catalyst preparation step, on gold cluster size
Furthermore, the catalysts were used in gas phase ethanol oxidation utilizing fixed bed tubular reactor. Different reagents like primary linear (1-butanol, 1-octanol)   or aromatic (benzylic alcohol)   alcohols and polyols (glycerol, glycol, glucose)    have been under research. Acetaldehyde and ethyl acetate were the goal products which are needed as intermediates and end products in pharmaceutical and food industry. Activity and stability of these catalysts depends on different parameters including the structure and acidity of the support, as well as on the specific interaction between the gold and the support. The effect of different parameters including Au particle size, type of precursor and reaction temperature on catalytic activity and selectivity were studied.
The catalysts were prepared using deposition-precipitation method by changing the pH between 6.5 and 10.5 and aging time from 3h to one day to achieve catalysts with 1 to 3 wt% Au loading and smaller than 10 nm of Au clusters. Chlorine ions from gold precursor (HAuCl4) responsible for sintering of gold particles were washed away followed by drying of the catalysts at 100°C and calcining at 300°C for 3h. The catalysts were characterized thoroughly by TEM, SEM/EDXA, ICP, nitrogen-physisorption, FTIR, XRD, and XPS.
In order to use the catalysts for the reaction, they were sieved to have the particle sizes between 63 to 125 µm. The catalysts were diluted with sand to avoid pressure drop. The reaction was run at 125 to 250°C. Oxidation reaction was performed by using He gas as a carrier. Organic and inorganic products were detected by utilizing an online GC.
Results and discussions
Au mean sizes and size distributions were mainly affected by the acidity and structure of the catalyst supports. For instance, zeolite gold modified catalysts exhibited bigger gold clusters (6-9 nm) and broader size distributions (2-18 nm). The narrowest size distributions and smallest Au particles were found on Au/Al2O3 which showed 1.5-4 nm range with mean size of 2.1 nm. It should be noted that a slightly wider Au size range and mean compared to Au/Al2O3 characterized Au/SiO2. The influence of acidity on Au particle size was investigated by comparing gold nano sizes on Au/H-Y-12 and Au/H-Y-80 obeying the same preparation conditions. The results suggested that smaller Au particles were formed on Au/H-Y-80 as a less acidic catalyst than Au/H-Y-12. Besides, H-Beta-25 which is more acidic than H-Y-12, formed very large Au particles (>100µm) which are almost non active phase; behaviour of the catalyst in terms of activity and selectivities was the same as proton form of the support without gold. Thus, it was confirmed that high acidic zeolite supports could not be used for formation of Au clusters with less than 10 nm.
Another parameter which affected the Au size was pH of the solution. Au formed on H-Y-80 at pH of 10.5 was about 1.3 nm smaller than Au formed on H-Y-80 at pH of 9. Aging time of the solution also influenced the Au particle size; an increase from 1 to 3 h reduced the Au average size from 9.3 to 6.3 nm.
Selectivities of the catalysts showed that strong acidity of the support is not even needed for this reaction due to the formation of mainly diethyl ether as a side product.
Figure below shows the ethanol conversion as a function of temperature (125-250°C). A comparison of the effect of gold size on activity exhibited that reductions in size from 4 to 2 nm on alumina can double the activity of the catalyst.
Fig. Conversions of gold catalysts at different temperatures
The importance of acidity in ethanol oxidation reaction was investigated in this work by preparing and testing gold catalysts using Y zeolites (H-Y-12 and H-Y-80). TOF of Au/H-Y-80 (80=SiO2/Al2O3molar ratio) was noticeably higher than Au/H-Y-12. Thus, it was concluded that Y zeolite with high acidities are not suitable for this reaction due to showing lower activities.
Besides, among the tested catalysts (Fig), deactivation studies were performed for Au/SiO2which indicated promising results in terms of activity and selectivity. This catalyst was tested at 250°C for fifteen days. Characterizations were performed thoroughly by TEM, EDXA, ICP, nitrogen physisorption and XPS for the spent catalyst. The activity and selectivity results illustrated no deactivation. No sintering (TEM) and change in gold oxidation state (XPS) was observed but a slight reduction in surface area was attributed to the formation of carbon residues on the support mesopores.
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