| As an important chemical product, hydrogen peroxide was widely used in many fields, such as the bleaching of the pulp and paper, environmental protection, chemical and food industry for it is an environmentally friendly oxidant. With the increase of the demand, the production of hydrogen peroxide has become more and more important. Among all of the preparation methods, the anthraquinone hydrogenation/oxidation process is by far the most popular route for industrial scale production of hydrogen peroxide. In this famous method, 2-ethylanthraquinone (eAQ) is firstly hydrogenated to 2-ethylanthrahydroquinone (eAQH2) and 2-ethyltetrahydroanthrahydroquinone (H4eAQH2) at the presence of a hydrogenation catalyst. Then, in an oxidation step, eAQH2 and H4eAQH2 are completely oxidized with oxygen-containing gas to yield H2O2, while eAQ and H4eAQ are recovered and go into another cycle. Hydrogenation of eAQ is the key step in this process, for it can determine the yield and the price of the products. The most widely used hydrogenation catalysts, such as Raney Ni catalyst and palladium-based catalysts, cannot satisfy the demand for their shortcomings. Here the modified Raney Ni catalyst and the amorphous Ni-B alloy were firstly introduced to the hydrogenation of anthraquinone and their differences of the hydrogenation properties are compared. According to several characterizations, such as XRD, XPS, SEM and TEM, it shows that their catalytic properties are determined by their structures of the catalysts.1. Rapidly Quenched Raney Ni Catalyst (RQ-Ni)The conventional Raney Ni catalyst was replaced by palladium-based catalyst for its low selectivity in the hydrogenation of anthraquinone. There are two kinds of the unsaturated groups in eAQ molecular: the aromatic rings and the carbonyl groups. Both of them can be hydrogenated in the presence of the Raney Ni catalyst, but the hydrogenation of the aromatic rings is unexpected and should be restrained. One of the resolutions is to prepare the original NiAl alloy by rapid quenching method. Generally, the alloys will be amorphous with the long-range disordering and short-range ordering structure if the melts are quenched at the rate higher than 106 K.s-1, for the atoms or the molecules cannot be arranged regularly. But the NiAl alloys are not amorphous for their inner properties, such as electronegativity or metal size.However, it is also instructive to prepare Raney Ni catalyst by the introduction of the rapid quenching method.The rapidly quenched NiAl alloys mainly contain NiAl3 phase and Ni2Al3 phase, while the amount of the eutectic phase and NiAl phase decreased, which is close to the melt compositions. Comparing with the conventional NiAl alloy, the Ni2Al3 phase in rapidly quenched NiAl alloy is more difficult to be leached by alkali. The crystal size in the rapidly quenched alloy is small for the seed crystals have no time to aggregate during the preparation, so the crystal size in the RQ Ni catalyst is also small; furthermore, hydrogen can be adsorbed on the catalyst surface strongly for there are more crystal defects in RQ Ni catalyst. Both of these reasons are beneficial to increase the selectivity in the hydrogenation of the anthraquinone.Although the content of the Ni2Al3 phase increases when the rapidly quenched NiAl alloy is modified with Cr and Fe, the residual Ni2Al3 phase in the catalyst is less for it can be leached easily in the presence of Cr and Fe. The quenching rate can be controlled by changing the rotation speed of the roller. When the speeds are between 500 and 1000 rpm, the phase compositions are nearly the same, while the active surfaces of the catalyst increase from 11.50 m2·g-1 to 15.05 m2·g-1. It is shown that with the increase of the quenching rate, the hydrogenation activity and selectivity both increase.In order to investigate in detail how the promoters can affect the catalyst, the rapidly quenched NiAl alloys modified with different Mo content are prepared. Besides the NiAl3 phase and Ni2Al3 phase, the Ni16Mo10Al7...
|
PDF Full Text Request