Hydrodesulfurization of 4,6-dimethyldibenzothiophene over noble metals supported on mesoporous zeolites.
Hydrodesulfurization of 4,6-dimethyldibenzothiophene over noble metals supported on mesoporous zeolites.
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DOI：
10.1002/anie.200802540
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发表时间：
2008-10
影响因子：
--
通讯作者：
Yinyong Sun;R. Prins
中科院分区：
文献类型：
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作者：
Yinyong Sun;R. Prins
近年来，汽油和柴油燃料中硫含量的降低一直是人们密切关注的主题，因为出于环境原因，许多国家必须在 2010 年之前将硫含量降至 10 ppm，而对于燃料电池应用，硫含量应低于 0.1 ppm。为了达到如此低的水平，即使是高难熔分子，如 4,6-二甲基二苯并噻吩 (4,6-DMDBT) 也必须进行脱硫。然而，由于与硫原子相邻的甲基的空间位阻，4,6-DM-DBT的脱硫主要发生在分子首先氢化之后。因此，催化剂的加氢能力对于深度加氢脱硫（HDS）至关重要。最近的研究表明，贵金属负载型催化剂在加氢脱硫中比传统金属硫化物具有更好的加氢性能，可用于深度加氢脱硫工艺的第二反应器。不仅活性催化剂，载体对催化剂的催化性能也起着重要作用。酸性载体可以提高二苯并噻吩 (DBT) 和 4,6-DM-DBT 的转化率。对此的一种解释是它们能够实现烷基取代基的脱烷基化和异构化反应，这可能会将难熔组分转化为更具反应性的物质，从而加速 HDS。此外，酸性载体还可以提高催化剂颗粒的催化活性。由于部分电子转移可以从金属颗粒到载体的酸性位点发生，因此所得缺电子金属颗粒被认为通过减少与H2S的相互作用而具有更好的抗硫中毒能力。 [5b,6]这种改进的另一种解释是通过氢原子从金属颗粒溢出到吸附在金属颗粒附近的酸性位点上的芳香族含硫分子而产生第二氢化途径。虽然金属颗粒被硫中毒，但它们仍然可以解离氢分子，因此涉及溢出的氢化途径仍然是可能的。众所周知，沸石具有强酸性、高稳定性和规则的孔排列，因此被应用于许多工业催化反应中。然而，它们的孔径较小，这意味着相对较大的分子如4,6-DM-DBT无法进入孔内；它们只能在沸石的外表面发生反应，无法到达许多活性中心。因此，优选具有强酸性和相对大孔的载体。最近发现的具有分级孔隙率和强酸性的介孔沸石开启了将它们用作 HDS 载体的可能性。然而，到目前为止，它们在 HDS 中用作支撑物的情况尚未见报道。在此，我们报道了介孔 Na-ZSM-5 负载的 Pt、Pd 和 Pt-Pd 催化剂。研究了这些催化剂在4,6-DMDBT HDS中的催化活性和选择性，并对烃类产物和加氢中间体进行了分析。与传统Na-ZSM-5或g-Al2O3负载催化剂相比，介孔Na-ZSM-5负载催化剂表现出更好的加氢脱硫催化性能。介孔 Na-ZSM-5 (MNZ-5) 和 Na-ZSM-5 (NZ-5，参见支持信息中的图 S1) 的粉末 XRD 图案显示出清晰的峰，这是 ZSM-5 沸石结构的特征。 MNZ-5 表现出介孔材料典型的 IV 型 N2 吸附/解吸等温线（参见支持信息中的图 S2a）。相比之下，NZ-5 显示出 I 型等温线，这是微孔材料的典型特征。此外，MNZ-5 观察到以 4.9 nm 为中心的均匀孔隙分布（参见支持信息中的图 S2b）。两个样品的详细吸附数据列于表1中。MNZ-5的BET比表面积和介孔体积分别为579 m g 1 和0.44 cm g ，远高于NZ-5。 2-甲基-2-戊烯（2M2P）的异构化是评估固体酸酸性的良好模型反应。产物中反式-3-甲基-2-戊烯（反式-3M2P，移去一个甲基得到）与反式和顺式-4-甲基-2-戊烯（分别是反式和顺式-4M2P，移去一个H原子得到）的摩尔比反映了固体酸的酸性。摩尔比越高，酸性越强。表1显示MNZ-5在2M2P异构化中比NZ-5产生更高的转化率；这一观察结果可能归因于 MNZ-5 的高 BET 表面积。对两种沸石中Na含量的分析表明，只有90%的Al原子被Na阳离子进行电荷补偿，因此NZ5和MNZ-5中存在约10%的质子，从而表明它们确实是酸性载体。另一方面，g-Al2O3 的转化率较低 [*] Y. Sun 博士、R. Prins 教授博士 化学与生物工程研究所，ETH Z rich 8093 Z rich (瑞士) 传真：(+ 41)44-632-1162 电子邮件：prins@chem.ethz.ch
The reduction of the sulfur content in gasoline and diesel fuel has been a subject of intense investigation in recent years because the sulfur level must in many countries be reduced to 10 ppm by the year 2010 for environmental reasons, while for fuel-cell applications the sulfur content should be below 0.1 ppm. To reach this low level, even highly refractory molecules such as 4,6-dimethyldibenzothiophene (4,6-DMDBT) must be desulfurized. However, because of steric hindrance by the methyl groups adjacent to the sulfur atom, desulfurization of 4,6-DM-DBT mainly takes place after the molecule has first been hydrogenated. Therefore, the hydrogenating ability of the catalyst is of critical importance for deep hydrodesulfurization (HDS). Recent studies have shown that noble-metal-supported catalysts have much better hydrogenation performance than conventional metal sulfides in HDS, and may be used in the second reactor of a deep HDS process. Not only the active catalyst, but also the support plays an important role in the catalytic performance of catalysts. Acidic supports can increase the conversion of dibenzothiophene (DBT) and of 4,6-DM-DBT. One explanation for this is that they enable dealkylation and isomerization reactions of the alkyl substituents, which may transform refractory components into more reactive species and thus accelerate HDS. Moreover, acidic supports may also improve the catalytic activity of the catalyst particles. Since partial electron transfer can occur from the metal particles to acidic sites of the support, the resulting electron-deficient metal particles are deemed to have a better resistance to sulfur poisoning by decreasing the interaction with H2S. [5b,6] Another explanation for this improvement is the creation of a second hydrogenation pathway by spillover of hydrogen atoms from the metal particles to the aromatic sulfurcontaining molecules that are adsorbed on acidic sites in the vicinity of the metal particles. While the metal particles become poisoned by sulfur, they can still dissociate hydrogen molecules, and thus the hydrogenation pathway involving spillover would still be possible. It is well known that zeolites possess strong acidity, high stability, and a regular pore array, and are for these reasons applied in many industrial catalytic reactions. However, their small pore size means that relatively large molecules such as 4,6-DM-DBT cannot enter the pores; they can only react on the outer surface of the zeolites and cannot reach many active centers. A support with strong acidity and relatively large pores would, therefore, be preferred. The recent discovery of mesoporous zeolites with their hierarchical porosity and strong acidity opens the possibility of using them as supports in HDS. However, until now their use as a support in HDS has not been reported. Herein we report on Pt, Pd, and Pt-Pd catalysts supported on mesoporous Na-ZSM-5. The catalytic activity and selectivity of these catalysts were studied in the HDS of 4,6-DMDBT, and the hydrocarbon products as well as the hydrogenated intermediates were analyzed. Compared with conventional Na-ZSM-5or g-Al2O3-supported catalysts, the mesoporous Na-ZSM-5-supported catalysts exhibited much better catalytic performance for hydrodesulfurization. The powder XRD patterns of mesoporous Na-ZSM-5 (MNZ-5) and Na-ZSM-5 (NZ-5, see Figure S1 in the Supporting Information) show well-resolved peaks which are characteristic of the ZSM-5 zeolite structure. MNZ-5 exhibited a type IV N2 adsorption/desorption isotherm (see Figure S2 a in the Supporting Information) typical for mesoporous materials. In contrast, NZ-5 showed a type I isotherm, which is typical of microporous materials. Moreover, a uniform pore distribution centered at around 4.9 nm was observed for MNZ-5 (see Figure S2b in the Supporting Information). The detailed sorption data of both samples are listed in Table 1. The BET special surface area and mesoporous volume for MNZ-5 are 579 m g 1 and 0.44 cm g , respectively, much higher than those of NZ-5. The isomerization of 2-methyl-2-pentene (2M2P) is a good model reaction to evaluate the acidity of solid acids. The molar ratio of trans-3-methyl-2-pentene (trans-3M2P, obtained by shift of a methyl group) to transand cis-4methyl-2-pentene (transand cis-4M2P, respectively, obtained by shift of an H atom) in the product reflects the acidity of solid acids. The higher the molar ratio is, the stronger the acidity is. Table 1 shows that MNZ-5 resulted in a higher conversion in the isomerization of 2M2P than did NZ-5; this observation may be attributed to the high BET surface area of MNZ-5. The analysis of the Na content in the two zeolites showed that only 90 % of the Al atoms were charge-compensated by Na cations, so that about 10 % protons existed in NZ5 and MNZ-5, thus suggesting that they are indeed acidic supports. g-Al2O3, on the other hand, gave a lower conversion [*] Dr. Y. Sun, Prof. Dr. R. Prins Institute for Chemical and Bioengineering, ETH Z rich 8093 Z rich (Switzerland) Fax: (+ 41)44-632-1162 E-mail: prins@chem.ethz.ch