| p-Chlorobenzaldehyde is an important intermediate in the synthesis of a variety of fine chemicals such as dyes, agrochemicals and pharmaceuticals. For industrial p-chlorobenzaldehyde production, soluble acetates of transition metals are used as catalysts, but some of the processes are polluting and expensive. Efforts were put into fabricating solid catalysts, which are capable of producing p-chlorobenzaldehyde through selective oxidation of p-chlorotoluene using molecular oxygen as oxidant. However, both conversion and selectivity are unsatisfactory. According to our previous work, OMS-2 materials with mixed-valence manganese (Mn2+, Mn3+ and Mn4+) and abundant lattice oxygen, showed good performance towards the oxidation of PCT to produce PCB. ZSM-5 zeolites have high surface area, high hydrothermal stability and good ion exchange performance. In addition, its unique three-dimensional microporous channels provide a space limitation for the shape selectivity, and provide a rich in and out channel for the reactants and products. In this paper, metal-doped OMS-2 and ZSM-5 molecular sieve have been prepared to reveal the relationship between the structure, surface properties and catalytic performance was also revealed. Some concrete details as following:At first, a series of manganese oxide octahedral molecular sieves (OMS-2) doped with Co, V, and Ce (Me) were prepared via a one-step reflux method (denoted as Me-M(x), x= Si/M molar ratio). The XRD, SEM, IR, ICP-AES, XPS and TG characterization results showed that Ce mainly enter the tunnel structure of OMS-2 instead of K, Co mainly substitutes for Mn in the framework, and V is likely to substitution into both the Mn and K in the cryptomelane tunnel structure; The length of OMS-2 rods decreases with the decrease of K+mole fraction in the OMS-2 tunnels; Doping V and Ce markedly decreases the crystallization of OMS-2. All the Me-M(x) materials were tested as catalysts for the oxidation of p-chlorotoluene to p-chlorobenzaldehyde with O2, and it is the Ce-M(0.12) that shows the highest activity. Its excellent performance is attributed to the large specific surface area and good mobility of oxygen species.Secondly, it is for the first time the liquid-phase catalytic oxidation of p-chlorotoluene to p-chlorobenzaldehyde over the manganese-containing ZSM-5 that was prepared by a hydrothermal method or an impregnation method (denoted as M-Z(x) or M/Z(y), x=Si/Mn molar ratio, y= the percentage of Mn). The effects of catalyst concentration, water addition, HBr amount, as well as reaction time and temperature on product yield were investigated using M-Z(48) as catalyst. Under the optimized conditions (catalyst 20 mg,p-chlorotoluene 1 mL, solvent (acetic acid) 10 mL, HBr (40 wt.%) 30 mg, H2O 3 g, oxygen flow rate 50 mL/min, time 8 h, temperature 100?), the M-Z(48) catalyst shows a p-chlorotoluene conversion of 93.8% and p-chlorobenzaldehyde selectivity of 90.5%. M/Z(2.0 wt.%) has a similar Mn loading with M-Z(48), while there is a decrease of catalytic activity over M/Z(2.0 wt.%) (Conv.% = 80.3, Sel.%= 76.9). The characterization results show that the manganese ions mainly incorporate in the framework of M-Z (x) while accumulate on the surface of M-Z(x). The structure-activity relationship studies showed that the Mn active sites resulted from manganese incorporation into the ZSM-5 framework play an effective role in p-chlorotoluene oxidation reactions. Moreover, the superior performance of the M-Z(48) catalyst is attributed to its large specific surface area and mild acidity. |
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