Selective Oxidation Of Glycerol On Supported Noble Metal Catalysts

With the progressive depletion of the fossil energy and the disruption of environment, the use of renewable feedstock is essential to the sustainable development of society. Nowadays, much attention has been devoted to development and application of bio-diesel in view of its extraordinary properties, including renewability, safety, environmental friendly and a good substitute of petro-diesel. Glycerol is the main byproduct during the production of bio-diesel, which is known as transesterification process. A major surplus of glycerol has resulted from the increasing expansion of biodiesel production, which lead to the heterocatalysis oxidation of glycerol into value-added fine chemicals will play a crucial role in future bioindustry. The corresponding oxidation reactions are catalyzed by various supported monometallic or bimetallic catalysts based on metal palladium, platinum, gold or bismuth. Au or Pd based catalysts were mainly invested lately based on the published works, however the activities of which depended strongly on the basicity of the reaction medium. Pt based catalysts exhibit good performance for the oxidation of glycerol in the base-free environment, during which the free oxidation products could be obtained directly without additional neutralization and acidification. Therefore, the aim of this work is to investigate the Pt based catalyst and its reactivity and catalytic mechanism in the glycerol oxidation reaction.A series of monometallic Pt catalysts and bimetallic Pt-M (such as Pt-Cu, Pt-Bi, Pt-Sb) catalysts were prepared by supported on the active carbon or multiwall carbon nanotubes, which were modified either physically or chemically. Meanwhile, various characterizations were taken to investigate the composition and structure of those catalysts and their influence on the reactivity of glycerol oxidation. Main conclusions are as follows:The selective oxidation of glycerol to glyceric acid could be performed successfully on small sized Pt (<6nm) nanoparticles supported on the active carbon in base-free conditions. The high turnover frequency (TOF,223h-1) was achieved after20min of reaction over catalyst Pt-1(3.2nm mean diameter of Pt particle). This performance is at same level as is reported in literatures in a strong base solution and this catalyst can be recycled with a stable activity. Characterizations disclosed that the oxygen groups (C=O) on the surface of active carbon play a essential role in the anchoring of Pt particle on the support; On the other hand, those oxygen groups could also decrease the interaction between the Pt and support, which lead to the migration and aggregation of Pt particles. The H2O2modified active carbon exhibited the best surface structure and chemistry environment which were in good favor of achieving catalyst with the highly dispersed fine Pt particles. Additionally, it was found that the dispersion and size of Pt particles also depended on the amount of reducing agent and the preparation temperature. A series of Pt catalysts on different sized carbon supports were prepared and characterized via SEM, N2-adsorption, TEM and XRD. It is quite interesting to find that the activity of Pt catalysts increased obviously with the decreasing particle size of carbon supports. We believe that the small sized pore structure of active carbon was destroyed by ball milling, which enabled a higher accessibility of substrates to the Pt nanoparticles, thus resulted in the improved reactivity. Therefore, we could draw the conclusion that the highly dispersed and exposed Pt on the support could ensure its effective and sufficient contact with reactants, and enhance the reactivity.Selective oxidation of glycerol with molecular oxygen was studied over different functionalized multiwall carbon nanotubes (MWNTs) supported Pt catalysts in base-free aqueous solution. Characterizations disclose that the stronger chelation of thiol groups (-SH) and Pt leads to its higher effectiveness for the control of dispersion and size of Pt nanoparticles than the hydroxyl and carboxyl groups on the surface of MWNTs. Thus, unique sized and highly dispersed Pt catalysts could be prepared on the surface of S-pretreated MWNTs. And this Pt/S-MWNTs catalyst was much more active (90.4%conversion) and capable for the selective oxidation of glycerol to free GLYA (68.3%selectivity) than Pt/MWNTs, Pt/HNO3-MWNTs and Pt/H2O2-MWNTs in a base-free aqueous solution. In the mixture solution of NaOH/glycerol (2:1), the conversion of glycerol increased quickly than that in base-free condition over Pt/S-MWNTs. The calculated turnover frequency (TOF, at10%conversion of glycerol) of Pt/S-MWNTs reached932.2h"1, which is about3times of that in base-free solution. Unfortunately, the conversion of glycerol increased slightly after3h, and the selectivity of the C-C cleavage products increased quickly to30.4%at6h of the reaction, while the selectivity of GLYA reduced to24.0%. Raman analysis of adsorbed glycerol on Pt catalysts in both base and base-free solutions in couple with the time courses of glycerol oxidation over Pt catalysts disclosed that base promoted the adsorption and activation of glycerol on the surface of Pt, but the main drawback of base is that it catalyzed the cleavage of C—C bonds.A series of carbon supported bimetallic Pt-M catalysts were prepared and used for glycerol oxidation with oxygen in a base-free aqueous solution. Among them, bimetallic Pt-Cu/C, Pt-Bi/C and Pt-Sb/C catalysts were found to be more active and efficient in the selective oxidation of glycerol to either glyceric acid (GLYA) or dihydroxyacetone (DIHA). The performance of monometallic Pt/C catalyst for glycerol oxidation in base-free conditions was greatly improved by addition of Cu.5Pt-Cu/C with the70.8%selectivity of free GLYA at an86.2%conversion of glycerol showed the best performance. Moreover,3Pt-Cu/C catalyst with a lower loading amount of Pt (3.0wt.%) was more active than5Pt/C (5.0wt.%loadings of Pt). Characterizations demonstrated that highly dispersed Pt-Cu nanoparticles with small particle size in dominant alloyed phase of PtCu3were formed in catalyst Pt-Cu/C, which is proposed to contribute to the improved performance. In addition, it was found that the performance of monometallic Pt/C catalyst for glycerol oxidation to DIHA was greatly improved by Bi. Pt-5Bi/C (5.0wt%loadings of Bi) was most selective for the production of DIHA from glycerol (49.0%selectivity of DIHA at a91.5%conversion of glycerol). Either reduced or increased loading amount of Bi leaded to the decreased reactivity and DIHA selectivity. TEM and XRD characterizations indicated that the dispersion of Pt was improved by added Bi, and the superior performance of Pt-5Bi/C catalyst may be attributed to that the presence of the well dispersed Pt in fine grain size which could be achieved when the loading amount of Bi is higher than5%. However, much higher loadings of Bi should be avoided, as the excess amount of Bi could also lead to a sever enwrapping of Pt particles which hindered the accessibility of Pt.Bimetallic Pt-Cu, Pt-Bi and Pt-Sb catalysts which supported on the thiol pretreated multiwall carbon nanotubes (S-MWNTs) were prepared and applied for glycerol oxidation with oxygen in a base-free aqueous solution. It was found that the reactivity of Pt/S-MWNTs was hardly improved by addition of Cu. Higher activity and dihydroxyacetone (DIHA) selectivity were observed when Pt/S-MWNTs was modified with Bi, which was in good accordance with previous result derived from the carbon supported Pt-Bi catalysts. However, it is noteworthy that catalyst Pt-Sb/S-MWNTs was found to be more efficient than Pt-Bi/S-MWNTs for the selective oxidation of glycerol to DIHA. In comparison with Pt/S-MWNTs, the calculated turnover frequency (TOF) over Pt-Bi/S-MWNTs increased to500.8h-1, and the TOF of Pt-Sb/S-MWNTs reached to878.1h-1, which is about2.5times of that of Pt/S-MWNTs. Characterizations suggest that the increased dispersion of Pt in Pt-Bi/S-MWNTs or Pt-Sb/S-MWNTs contributes to the improved activity. Moreover, the glycerol and DIHA oxidation reaction tests suggested that in comparison with Pt-Bi/S-MWNTs, the over-oxidization of DIHA was significantly suppressed over catalyst Pt-Sb/S-MWNTs. The further characterizations via XRD, TEM, STEM and XPS techniques disclosed that Sb homogenously entered into the lattice of Pt particles and formed Pt-Sb alloy in Pt-Sb/S-MWNTs, but Pt particles in Pt-Bi/S-MWNTs were embedded in Bi oxides. The results indicate that this specific structure of Pt-Sb nanoparticles could contribute to the higher stability of DIHA and enhanced performance for selective oxidation of glycerol to DIHA.