Preparation And Characterization Of Nb₂O₅/α-Al₂O₃ Catalysts And Their Applications In Catalytic Hydration Of Ethylene Oxide And Other Acid-Catalyzed Reactions

The production of monoethylene glycol (MEG) by non-catalytic hydration of ethylene oxide (EO) is a well-known process, which is usually performed with a large excess of water, e.g. 20 to 25 moles of water per mole of ethylene oxide, at 140-230 ℃ and 1.5-2.5 MPa to obtain glycol with a selectivity of 88-90%. The raw product of hydration is then completely dehydrated by rectification, which involves large energy consumption and is economically unattractive. Considerable efforts have been paid to explore effective hydration catalysts. A number of materials such as anion exchange resin, quaternary phosphonium halides, polymeric organosilane ammonium salt, and macrocyclic chelating compounds have been employed as hydration catalysts. Although the above catalysts can improve MEG selectivity to some extent, some problems still remain, such as elution of the active species and poor thermal stability. Therefore, development of efficient catalysts that are also insoluble in water, stable and readily recoverable for EO hydration is highly important.Niobic acid (Nb₂O₅·nH₂O)/niobium oxide has received much attention as a new solid acid catalyst because of its high acidity and structural stability in water and its affinity for both water and organic substrate. It has been used as catalyst for some reactions with water participation, such as dehydration, esterification and hydration of low molecular olefins. However, the use of niobium oxide as a catalyst for hydration of ethylene oxide has not been found yet in the literature. In this dissertation, niobium oxide supported on α-alumina or α-alumina coated with MgAl₂O₄ was prepared as catalyst for hydration of EO. Effect of preparation methods and conditions, support modification, promoter addition on the structure, acid properties, adsorption properties of H₂O and EO and catalytic performance was studied by using XRD, TG-DTA, (NH₃, CO₂)-TPD, FT-IR, XPS and catalytic hydration. The catalytic performances of the supported niobia in some other acid catalyzed reactions were also investigated. The following conclusions can be drawn from the dissertation.It was found that the structure and surface acidity of the Nb₂O₅/α-Al₂O₃ was dependent upon the temperature and atmosphere of calcination. Calcination at temperature >300℃ would lead to the complete decomposition of the precursor to Nb₂O₅. The surface acidic sites on the calcined samples studied were all Lewis type, and no Bronsted acid sites were observed. The acidic strength and density of thesamples decreased with increasing calcination temperature. The surface of the catalyst was almost neutral after calcination at temperature above 600°C along with the appearance of TT-Nb2O5 phase. The catalytic test showed that MEG selectivity of 89.6% at EO conversion of 99.8% was achieved over Nb^Os/a-AhOs catalyst under conditions of 160°C, 1.5MPa, LHSV 10-30 h"1 and H2O/EO of 22. The correlation of the catalytic behavior with characterizations indicated that EO conversion was proportional to the surface concentration of acid sites, while the selectivity to MEG and durability of the catalyst decreased with increasing of the acidic strength of the catalyst. The strong acidity of the catalyst may lead to the re-adsorption and secondary reaction of the primary product of MEG, resulting in a decrease of the MEG selectivity and in the formation of polymerized glycols. In terms of MEG yield, the suitable calcination temperature should be between 300 and 500°C.Modification of C1-AI2O3 support with MgAhC^ led to the improvement in mechanical strength but the strength and density of acidic sites of the catalysts were reduced. The catalytic evaluation demonstrated that modification of support with MgAl2O4 resulted in the slight increase in MEG selectivity from 89.6% to 90.5% at MgAhO4 loadings between 1 and 2.5%.The effect of hydrophilic species SnC>2 addition on the structure and H2O adsorption properties and catalytic performance of M^Os/a-AbCb were investigated. It was found that tin was present as P-Sn, SnCh or SnNb2C>6 depending on the loading amount of tin and on treating atmosphere. The different state of tin exhibited the different effect on the acidity and catalytic performance of the catalyst. Catalytic reaction revealed that MEG selectivity was increased from 90.0 of M^OVa-AbCb to 94.0% on keeping EO conversion nearly 100% after doping of suitable amount of tin. On the other hand, a H2O/EO ratio of 15 is enough to achieve a MEG selectivity of 90% for the SnO2 promoted catalyst, compared that of 22 for the non-promoted Nb2Os/a-Al2O3 catalyst to achieve the same MEG selectivity level. The FT-IR of H2O adsorption experiment showed that water molecular was chemisorbed on the surface of Nb2Os/a-Al2O3 catalyst, and the adsorption strength was enhanced with the addition of tin. EO-TPD investigation indicated that two kinds of adsorption species, low temperature adsorption species (LS) and high temperature adsorption species (HS), were found on the surface of T^Os/ct-AbOs. No any EO adsorption was observed on P-Sn and a-Al2O3 support. For TT-M^Os and SnNb2O6, only HS species were found. Therefore, it is of great importance to control the preparation conditions...