Study On One-step Hydrogenation Esterification (OHE) Of Aldehyde And Acid

Biomass is renewable, sustainable and environmentally benign. Consequently it is a potential substitutional energy source for existent fossil fuels. One efficient way for biomass applications is fast pyrolysis to get biomass pyrolysis oil (bio-oil) as liquid fuel.There are large amount of aldehydes and organic acids in bio-oil, and they are acknowledged as the main deleterious components causing corrosiveness and instability of bio-oil. Therefore, the conversions of aldehydes and organic acids should take prior considerations in the upgrading process. In this work, the coupling of aldehyde hydrogenation and acid esterification in the bio-oil upgrading system is proposed for the first time, which is named as "One-step Hydrogenation Esterification" (OHE). The aim is to gain stable and combustible esters. The metal-acid bifunctional catalysts can be utilized in this novel reaction, in which metal sites for aldehyde hydrogenation to alcohol, and acid sites for the oncoming esterification with orgaic acids. In this study, acetaldehyde and acetic acid were utilized as model reactants, and OHE reaction was tested for its feasibility. Studies were also focused on the catalyst for OHE, including the active sites determination, catalyst preparation, active sites modification and their possible interaction mechanisms and catalytic activity enhancement.The catalysts applied in this work were bifunctional which own noble metal sites (Pt) for hydrogenation and acidic sites for esterification. The work started with the preparations of the catalysts 5%Pt/Al2(SiO3)3 and 5%Pt/HZSM-5, and demonstrated the feasibility of OHE of acetaldehyde and acetic acid in our system. The reaction conditions were optimized, and medium conditions (150℃for reaction temperature,15 atm for hydrogen pressure and 0.2 g for catalyst amount) were deemed suitable for high yields of esters.In order to enhance OHE activity, bifunctional mesoporous organic-inorganic hybrid silicas with platinum and propylsulfonic acid group (Pt/SBA15-PrSO3H) were synthesized and tested for the OHE reaction in the following study. Due to its higher surface area (beneficial for Pt dispersion) and superior acidic site capacity, this bifunctional hybrid catalyst has the potential of improving OHE activity relative to previously discussed catalyst systems. The textural properties, strong thiol-bonding interactions, platinum loading, choice of reducing agent, and an apparent positive acid-metal interaction were considered as key factors affecting the catalytic performance. Optimum bifunctionalizing order and procedure were selected for Pt/SBA15-PrSO3H. And it was found that the existence of the propylsulfonic acid groups did not affect the hydrogenation activity of the Pt sites, but the existence of Pt promoted the acidic strength of the propylsulfonic acid groups. The OHE activity of Pt/SBA15-PrSO3H has protential to enhance, and it is observed that hydrogenation is the determining step in OHE reactions over the bifunctional catalyst. Therefore, following studies were engaged in approaches to enhance the hydrogenation performance of metal sites on mesoporous silicas catalysts. High temperature treatment was applied and Pt/SBA15 was tested because of the instability of organosulfonic acid groups in high temperature. Different Pt loading strategies were compared and high temperature treatments under different atmosphere and conditions were tested as activation approaches. It is found that aqueous reduction-deposition by HCHO followed with H2 treatment at 350℃significantly enhanced the acetaldehyde hydrogenation activity for the supported Pt on SBA15. Through catalyst characterization, it is acknowledged that aqueous reduction-deposition by HCHO is beneficial for Pt nanoparticles into the pores of SBA15 with high dispersion, and H2 treatment at 350℃activates more metallic Pt and causes the adsorbed hydrogen more active, which are deemed as the main reasons for acetaldehyde hydrogenation activity enhancement.Additionally, arenesulfonic acid groups (ArSO3H) were utilized to replace propylsulfonic acid groups (PrSO3H) and it led to the increase of the acid strength. Considering the previous discussed strategy for hydrogenation enhancement, a superior catalyst bifunctionalizing approach was proposed with aqueous reduction-deposition by HCHO, H2 treatment at 350℃and postgrafting with arenesulfonic acid groups. It was found that the synthesized Pt/SBA15-ArSO3H has high Pt dispersion and stronger acidic sites and therefore higher OHE activity. Similar as Pt/SBA15-PrSO3H, acidic enhancement led by Pt was also observed for Pt/SBA15-ArSO3H.With regards to the phenomenon of acidity enhancement of organosulfonic acid groups led by Pt, investigations of the detailed mechanism were also done in this work. Catalyst characterizations and molecular modeling were performed trying to explain the acidic enhancement by Pt. Results indicated that there is interaction between Pt and sulfonic acid groups. And it is shown that electron transferring occurs from Pt to sulfonic acid groups, and most of the electrons delocalized on S atoms, which strengthen the bond between S and its surrounding atoms, the S-O bond energy increases and correspondingly the O-H bond energy decreases, which means the acidic proton becomes less restricted and exhibits more acidic.Summarily, the studies in this work proposed OHE reaction for the first time as a novel route for catalytic upgrading of bio-oil, and demonstrated its feasibility catalyzing by metal acid bifunctional catalysts. And bifunctional mesoporous organic-inorganic hybrid silicas with platinum and organosulfonic acid group were applied to catalyze OHE reaction, the OHE activity enhancement was observed and potential for application was exhibited. Also, the acidity enhancement of organosulfonic acid groups led by Pt was observed for the first time and the mechanism was investigated in detail through catalyst characterization and molecular modeling.