Abstract
Biodiesel is of high interest due to increased demand for energy with the concern regarding more sustainable production processes. However, an inevitable by-product is glycerol. Hence, the conversion of this by-product to higher-value chemicals, especially 1,3-propanediol (1,3-PDO) via glycerol hydrogenolysis reaction, is one of the most effective pathways towards a profitable process. In general, this process is catalyzed by a highly active Pt-based catalyst supported on γ-Al2O3. However, its low 1,3-PDO selectivity and stability due to surface deactivation of such catalysts remained. This led to the surface modification by WOx to improve both the selectivity by means of the increased Brønsted acidity and the stability in terms of Pt leaching-resistance. Hence, we applied experimental and density functional theory (DFT)-based techniques to study the fundamentals of how WOx modified the catalytic performance in the Pt/γ-Al2O3 catalyst and provided design guidelines. The effects of WOx promoter on improved activity were due to the shifting of the total density of states towards the antibonding region evident by the total density of states (TDOS) profile. On the improved 1,3-PDO selectivity, the main reason was the increasing number of Brønsted acid sites due to the added WOx promoter. Interestingly, the stability improvement was due to the strong metal-support interaction (SMSI) that occurred in the catalyst, like typical high leaching-resistant catalysts. Also, the observed strong metal-support-promoter interaction (SMSPI) is an additional effect preventing leaching. The SMSPI stemmed from additional bonding between the WOx species and the Pt active site, which significantly strengthened Pt adsorption to support and a high electron transfer from both Pt and Al2O3 to WOx promoter. This suggested that the promising promoter for our reaction performed in the liquid phase would improve the stability if SMSI occurred, where the special case of the WOx promoter would even highly improve the stability through SMSPI. Nevertheless, various promoters that can promote SMSPI need investigations.
Highlights
For the used catalyst of the same system, glycerol conversion dropped slightly to 30.0%, and Pt leaching was not detected, whereas 12.1% of W leaching was found. This suggested that the known effect of strong metal-support interaction (SMSI) between Pt and the support caused by the introduction of the W Ox component might play a role in a significant decrease in Pt leaching during hydrogenolysis
Adding WOx to Pt/γ-Al2O3 could improve catalytic activity by preventing Pt from leaching to the liquid phase during hydrogenolysis, where strong evidence on the SMSI that was hypothesized must be gathered
The improved activity can be explained via the total density of states (TDOS) profile of the WOx-modified Pt/γ-Al2O3 surface, suggesting that the WOx promoter lowered the energy gap (Eg) by upshifting the TDOS of the system towards the antibonding region and forming the additional electronic states or interstates between the valence band maximum (VBM) and conduction band minimum (CBM)
Summary
Pt onto the WOx/γ-Al2O3 support to form 5 wt%Pt/ WOx/γ-Al2O3 catalyst was prepared via the wet impregnation method. 0.105 g of the chloroplatinic acid hydrate was used as Pt precursor and homogeneously dissolved in DI water with the ratio of 15 mL DI water/g-Pt-precursor and stirred at room temperature (30 °C) at 500 rpm. 2.0 g of calcined WOx/γ-Al2O3 powder was added to the prepared chloroplatinic acid solution. The mixture was stirred at 500 rpm under the ambient condition (30 °C, 1 atm) for 16 h. The Pt/WOx/γ-Al2O3 catalyst precursor was dried at 110 °C overnight to eliminate the solvent. The dried Pt/WOx/γ-Al2O3 powder was calcined at 300 °C under airflow for 3 h. The loading of Pt and WOx were 5 wt% and 10 wt%, respectively
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