Design And Synthesis Of New Amorphous Alloys And Catalytic Properties

Amorphous alloys with short-range ordering and long-range disordering structure have attracted much attention of metallurgists, physicists and chemists owing to their special physical and chemical properties such as unique isotropic structure, high concentration of coordinatively unsaturated sites and corrosion resistance. Along with their environmentally benign nature, amorphous alloys have been regarded as a new category of materials promising in heterogeneous catalysis. However, amorphous alloys prepared via conventional methods tend to be agglomerated and dispersed in size. Moreover, the low surface area and poor thermal stability of these amorphous alloys greatly limit their industrial application. There are two promising ways to overcome those drawbacks, one is to deposit the amorphous alloys on a support with high surface area (such as mesoporous molecular sieves and micro- and mesoporous composite materials), the other is to assemble the amorphous alloy nanoparticles into nanostructures.H₂O₂ has been widely used as "friendly" oxidant. Although there are many ways to produce H₂O₂, by far the dominating process is the anthraquinone autoxidation process, in which the selective hydrogenation of 2-ethylanthraquinone (eAQ) into 2-ethylanthrahydroquinone (H₂eAQ) is one of the key steps. Previous work in our laboratory has demonstrated that ultrafine amorphous alloys exhibited good catalytic behaviors in eAQ hydrogenation. However, their low surface area and poor thermal stability limit their industrial application. Thus, depositing the amorphous alloy on a support and studying their catalytic behavior in eAQ hydrogenation is highly desirable. The enantioselective hydrogenation of prochiral ketone over amorphous alloy catalysts is a new research subject. The unique properties of amorphous alloys are helpful to overcome the shortcomings of the present crystalline heterogeneous enantioselective hydrogenation catalyst, such as low activity, low enantioselectivity and leaching. Based on amorphous alloys, it has great opportunity to obtain new category of carbonyl enantioselective hydrogenation catalyst.1. Mesoporous silica supported Ni-B amorphous alloy catalystsMesoporous silica supported Ni-B amorphous alloy catalysts are prepared via reductant-impregantion method by using mesoporous molecular sieves (HMS, MCM-41 and SBA-15) as supports, which have high surface area, large pore size andnarrow pore size distribution. The effects of pore structure on catalyst texture, particle size and location of Ni-B particles, thermal stability, surface species and chemical states and catalytic behavior in liquid phase hydrogenation of eAQ were studied. It is found that the thermal stabilities of the mesoporous silica supported Ni-B catalysts are much better than ultrafine Ni-B. The location of the Ni-B particles is depended on the pore size of the supports, whereas the uniformity of particle size is related to the interaction between the Ni-B particles and the supports. In Ni-B/SBA-15, the Ni-B particles are highly dispersed in the SBA-15 channels and their particle size is uniform (～ 6 nm). As for Ni-B/MCM-41 and Ni-B/HMS, the Ni-B particles are mainly situated on the outer surface of the supports. For Ni-B/MCM-41, the Ni-B particles are agglomerated and dispersed in size. As for Ni-B/HMS, the Ni-B particles on the surface are homogeneously distributed (-15 run), suggesting the existence of some interaction between Ni-B particles and HMS support.The mesoporous silica supported Ni-B catalysts exhibit remarkable catalytic behavior in eAQ hydrogenation (100% H₂O₂ yield). The best activity and selectivity were achieved over Ni-B/SBA-15 catalyst. It exhibits 100% selectivity to the carbonyl group. Moreover, the catalyst is highly stable, which exhibits great potential in industrial application.2. Designed synthesis of amorphous alloy nanostructuresNi-B@silica is successfully fabricated by microemulsion method in "CTAB-hexanol-H₂O" system. The particle size of Ni-B in Ni-B@silica can be adjusted by varying the concentration of Ni²⁺.1D nanowire, 2D hexagonal nanoarray and 3D gyroidal structure of amorphous alloys were prepared via ultrasound-assisted reductant-infiltration strategy by using mesoporous molecular sieves (MCM-41, SBA-15 and KIT-6) as hard template. It is found that the above nanostructures replicate the mesostructure of the hard template fairly well. The mesopores in MCM-41 are not interconnected, thus only 1D nanowire can be produced. For SBA-15, since its mesopores are interconnected, 2D hexagonal nanoarray can be obtained. As for KIT-6, gyroidal structure can be obtained.Ni-B/PS core-shell and Ni-B hollow spheres are prepared via surface seeding-electroless plating strategy by using polystyrene (PS) spheres as template. It is found that the surface activation of the neutral PS microspheres is a prerequisite for subsequent Ni-B deposition and hollow sphere fabrication. By sequentialimpregnation of Sn²⁺ and Pd²⁺, the surface of the PS spheres can be successfully activated. The thickness of the Ni-B overlayer is dependent on plating time. Among the ways to remove PS template, THF etching is a proper way.The above Ni-B@silica, Ni-B hexgonal nanoarray , Ni-B/PS core-shell and Ni-B hollow sphere exhibit superior catalytic properties to their nanoparticle counterpart in selective hydrogenation of acetophenone to 1-phenylethanol (PE).3. Enantioselective hydrogenation of acetophenone over amorphous alloysIt is found that the activity sequence of ultrafine amorphous alloys in the enantioselective hydrogenation of acetophenone is Pd-B > Ru-B > Ni-B > Co-B > Cu-B > Pt-B, while the carbonyl selectivity sequence is Ni-B > Co-B > Pt-B > Cu-B > Ru-B > Pd-B.As for tartaric acid-modified ultrafine Ni-B amorphous alloy catalyst, the enantioselectivity decreases with the increasing of the reaction temperature ranging from 277 to 348K. The highest enantioselectivity of the reaction is obtained at 277K. When the reaction temperature is increased to 348K, racemic products are produced. The highest optical yield was obtained at 318K. The activity and enantioselectivity of the reaction is higher in polar solvents, such as methanol, ethaol and THF. The highest optical yield was obtained by using ethanol as solvent. Among the three modifying reagents (L-tartaric acid, L-proline and cinchonine), tartaric acid modified Ni-B catalyst exhibits the highest enantioselectivity. The addition of Cr in the Ni-B catalyst or loading Ni-Cr-B over mesoporous molecular sieve MCM-41 can further improve the activity and enantioselectivity of the reaction.Without the modification of amino acid, the amorphous Pd-B catalyst exhibits high activity but low carbonyl selectivity in acetophenone hydrogenation. Large amounts of side-product ethylbenzene (EB) are produced. After amino acid modification, the carbonyl selectivity greatly improves, among which Pro modified catalyst exhibits the highest enantioselectivity and PE yield. Compared with crystalline Pd catalyst, higher enantioselectivity is attained over Pd-B catalysts. When the weight ratio of Pro to Pd-B catalyst is 8, e.e. values as high as 23.3% is obtained over 2D Pd-B hexagonal nanoarray, which is higher than the results reported in literature (22%). The e.e. values can be further improved to 24.1 and 25.2% with the addition of trimethylacetic acid and triethylamine, respectively.4. Designed synthesis of micro- and mesoporus composite materialsANA micro- and meoporous composite material is synthesized without the addition of mesoprous template by using Na₂SiO₃ as silica source, the Na₂SiO₃ leached metallic Al phase in Ni₅₀Al₅₀ as alumina source, the resulting Raney Ni as assisting agents. The sample is fully composed of crystals with uniform diameter of ca. 50 μm. All the crystals have even faces, sharp edges and external morphology of icositetrahedron which consists of 24 well-defined crystal faces. The interiors of the icositetrahedron are fully composed of nanorods with width ranging from 40 nm to 300 nm and length from 60 nm to 2 μm. The sample has a BET specific surface area of 33 m²Â·g^-1, a pore volume of 0.15 cm³Â·g^-1 and a mean pore size of ca. 4.0 nm with narrow distribution. TEM provides direct evidence for the co-existence of micro- and mesopores within the ANA samples. By investigating the reaction products at different growth stages, it is found that "Oriented Aggregation" is the key factor to form the above ANA micro- and mesoporous composite materials.Mesoporous ZSM-5 is synthesized by using carbon nanoparticles and mesoporous carbon molecualr sieves as templates. Unlike the compact crystalline structure of microporous ZSM-5, mesoporous ZSM-5 is composed of nanocrystallites with mesopores. By using mesoporous ZSM-5 as support to deposit amorphous Pd-B, the resulting Pd-B/meso-ZSM-5 catalyst exhibits higher PE yield (18.9%) and enantioselectivity (17.0%) in the enantioselective hydrogenation of acetophenone than microporous ZSM-5 supported Pd-B catalyst (PE yield 4.2%, enantioselectivity 13.6%).