Preparation Of Solid Base Catalysts For Toluene Side-chain Alkylation Of Methanol

Styrene is an industrially important monomer that is used for synthesis of various polymers. Industrially, styrene is produced by two subsequent reactions: the alkylation of benzene with ethylene to produce ethylbenzene, and its dehydrogenation to styrene. However, this route has some disadvantages, such as high energy consumption, complicated process, and high cost of production. It is reported that styrene can be directly obtained by side-chain alkylation of toluene with methanol over alkali exchanged zeolite, which may provide an alternative route for the production of styrene. However, the catalytic performance of the tested catalysts is still unsatisfied; the yield of styrene is rather low, which seriously restrict the development toward industry application. It was reported that a suitable catalyst for side-chain alkylation should have the following features: has sufficient basic sites for the dehydrogenation of methanol to formaldehyde, can stabilize adsorbed toluene and polarization of its methyl group, and can also keep good balance for the sorption stoichiometry of the reactants. Currently, it is still a very significant subject to design more efficient catalysts, and to reveal the nature of active sites of the catalysts for understanding the reaction mechanism of the side-chain alkylation.In this work, a series of ion-exchanged Cs X catalysts and modified Cs X catalysts were prepared and their catalytic properties were investigated for the side-chain alkylation of toluene with methanol. The effects of preparation parameters on the Cs contents, distribution states and the acid-base properties were studied by a variety of characterization means. Moreover, the effects of adding promoters like boron phosphate(BPO4), and/or Fe species on the catalytic properties of the Cs X catalyst were also studied. By using in-situ Ft-IR spectra and TPD-Mass technologies, the adsorbed characteristics of methanol on the surface of the Cs X and modified Cs X were determined, and the nature of active sites was discussed by correlated with the catalytic activity and selectivity of the catalysts. The main research contents and the conclusions are as follows: 1. The side-chain alkylation of toluene with methanol over ion-exchanged Cs X catalysts prepared under different conditions.A series of ion-exchanged Cs X catalysts with different Cs contents and acid base properties were prepared by changing the Cs source, the exchange numbers and the exchange temperatures. The Cs contents in Cs X catalysts could be increased by increasing the exchange numbers and the exchange temperatures. It was found that Cs species are distributed uniformly on the surface of zeolite X, mainly in the form of Cs+ ions, Cs2 O clusters as well as some Cs2 Si O3 clusters. A variety of characterization results suggest that the introduction of more Cs contents by ion-exchanging can increase the concentration of basic sites of the catalysts, but does not bring obvious change in the basic strength and the acidic properties of the Cs X catalysts. As shown by the in-situ characterization results of FT-IR and TPD-Mass, an obvious increase in the capacity for the adsorption and activation of methanol to produce surface active intermediates can be observed with the increase of Cs contents in the Cs X catalysts. It is necessary to keep appropriate concentration of basic sites on the surface of the Cs X catalysts in order to form suitable amount of active intermediates like formaldehyde and unidentate formate, which can finally result in the improvement of the utilization rate of methanol for producing more side-chain alkylation products. 2. Side-chain alkylation of toluene with methanol over boron phosphate modified cesium ion-exchanged zeolite X catalysts.In this work, the effects of boron phosphate(BPO4) additive on the catalytic properties of Cs X catalysts were investigated for the side-chain alkylation of toluene with methanol. Both BPO4 and Cs species are highly dispersed on the surface/channel of the BPO4/Cs X zeolites. The addition of a small amount of BPO4 has no obvious effect on the basic strength of catalysts, but decreases the number of basic sites slightly. Meanwhile, more weak Lewis acidic sites appear with the increasing BPO4 loading. Compared with Cs X, the BPO4 modified Cs X catalysts exhibit lower ability for the adsorption/activation of methanol to produce surface intermediates like formaldehyde, unidentate formate and bidentate formate. Catalytic reaction results showed that modification of Cs X with boron phosphate can improve both the conversion of toluene and the selectivity of styrene. The inhibition role in producing the unfavorable bidentate formate might be a key factor for finally resulting in the improvement of the catalytic properties of BPO4/Cs X catalysts for the side-chain alkylation of toluene with methanol. 3. Side-chain alkylation of toluene with methanol over Fe modified cesium ion-exchanged zeolite X catalysts.A series of Fe species modified Cs X zeolites and BPO4/Cs X catalysts were prepared by impregnation method. The modification of Cs X with appropriate amount of Fe(NO3)3 and can improve the catalytic activity for the side-chain alkylation of toluene with methanol, while the Fe Cl3 modified Cs X catalysts show much lower activity under the tested conditions. It was found that introducing a certain amount of Fe Cl3 can lead to the obvious decrease of the basicity/acidity of the Cs X catalyst, thus leading to poor ability for activating reactants. On other hand, the introduced Fe(NO3)3 just decrease the concentration of basic sites slightly, which can inhibit the formation of unfavorable bidentateformate, thus resulting in the improvement of utilization rate of methanol for producing more side-chain alkylation products.Moreover, the catalytic activity of Cs X catalyst can be further enhanced by the comodification Fe(NO3)3 and BPO4. 10.2% conversion of toluene can be achieved over the 0.15Fe/0.75BPO4/Cs X catalyst under optimized conditions. Characterization results show that the addition of Fe(NO3)3 and BPO4 together could adjust the acidity and basicity of the Cs-X catalyst, which meet the requirements of the acid-base properties for the side chain alkylation reaction. Besides, the introduction of a small amount of CO2 into the feed gas can further improve the selectivity of styrene, although rapid deactivation of catalyst can be observed. The addition of CO2 may inhibit the transfer hydrogenation of styrene with H2, thus resulting in the improvement of styrene selectivity.