Selective dehydration of methanol to dimethyl ether on ZSM-5 nanocrystals
Selective dehydration of methanol to dimethyl ether on ZSM-5 nanocrystals
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DOI：
10.1016/j.apcatb.2012.02.017
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发表时间：
2012-05
影响因子：
22.1
通讯作者：
A. Rownaghi;F. Rezaei;Matteo Stante;J. Hedlund
中科院分区：
文献类型：
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作者：
A. Rownaghi;F. Rezaei;Matteo Stante;J. Hedlund
考察了不同晶型和介孔结构的ZSM-5分子筛在甲醇脱水制二甲醚(DME)反应中的催化性能。反应在连续流动固定床反应器中进行，反应温度在180~320℃和1.1bar之间。结果表明，催化剂的晶粒度越小，反应温度越高，甲醇转化率越高。在所研究的温度范围内，均一纳米晶催化剂对甲醇脱水制二甲醚表现出最高的活性和稳定性。另一方面，大晶体的活性和选择性都很低。纳米ZSM-5的高活性和高DME产率是由于纳米ZSM-5样品较小的传质阻力所致，因为纳米ZSM-5的晶体较小。此外，还发现在ZSM-5催化剂上甲醇选择性脱水制二甲醚是基于孔道内产物的选择性，换言之，如果外表面没有酸性中心，则二甲醚的选择性应为100%。结果表明，在甲醇脱水制二甲醚过程中，催化剂的活性与颗粒大小有关，而DME选择性取决于Al分布的均匀性，这决定了催化剂外表面的酸性和小颗粒的低传质阻力。因此，减少催化剂外表面酸性中心的数量和减小催化剂的晶粒度是提高催化剂选择性和活性的关键。
The effects of crystal size and mesoporosity on the performance of various synthesized ZSM-5 zeolites in catalytic dehydration of methanol to dimethyl ether (DME) reaction have been investigated. The reactions were carried out in a continuous flow fixed-bed reactor at temperatures between 180 and 320°C and 1.1bar. It was found that methanol conversion enhances by decreasing crystal size and increasing the reaction temperature. Uniform nanocrystal catalysts showed highest activity and stability for methanol dehydration to dimethyl ether in the temperature range studied. On the other hand, both the activity and selectivity of large crystals were low. The high activity and DME yield for the nanocrystals is due to smaller mass transport resistance for the Nano-ZSM-5 sample, due to the small crystal size. Furthermore, it was found that the selective dehydration of methanol to dimethyl ether on ZSM-5 catalyst is based on the product selectivity inside the pore channels; in other words, the selectivity to dimethyl ether should be 100% if there is no acid site on the external surface. From this evidence, it is concluded that in methanol dehydration to DME, the reaction activity is related to the crystal size, whereas the DME selectivity is determined by the uniformity of Al distribution, which determines acidity of external surface and low mass transport resistance of small crystal size. Therefore, a decrease in the number of acid sites on the external surface and reduction in crystal size are key ways to enhance catalytic selectivity and activity, respectively.