亚微米ZSM - 5负载Cr催化剂上 - 省略 - CO - 2气氛下脱氢制乙烯 - 英文 - 程彦虎 - 图文

1246 Yanhu Cheng et al. / Chinese Journal of Catalysis 36 (2015) 1242–1248

Table 3

Reaction data for ZSM-5-supported chromium oxide catalysts obtained at 10 min and 6 h. Catalyst

3Cr/NaZSM-5-60 3Cr/NaZSM-5-100 3Cr/NaZSM-5-160 3Cr/HZSM-5-160

Conversion (%) 10 min 64.1 65.9 65.5 59.1

6 h 57.9 60.5 61.3 57.2

CH4 10 min 24.1 25.6 24.3 26.3

6 h 18.0 19.9 20.9 23.2

Selectivity (%)

C2H4 10 min 6 h 75.7 81.8 74.2 79.9 75.4 78.9 73.4 76.2

C3H6 10 min 0.2 0.1 0.3 0.3

Yield (%) 10 min 48.5 48.9 49.4 43.4

6 h 47.4 48.3 48.4 43.7

6 h 0.2 0.1 0.2 0.3

reaction processes. However, several groups have reported that coordinatively unsaturated Cr(III) formed from the reduc-tion of higher-valence states (i.e. Cr(VI)) is more active, wheth-er applied to the non-oxidative dehydrogenation reaction or oxidative dehydrogenation with CO2. As a result, the amount of reducible Cr(VI) on the calcined samples is crucial for the de-hydrogenation reaction [17,32]. H2 consumption results allow one to estimate the amount of redox-active Cr(VI) species, and it can be seen from Table 2 that the amount of reducible Cr(VI) on the freshly calcined samples increases with the Si/Al ratio, which is in agreement with their increasing dehydrogenation activity. These results confirm that the reducible Cr(VI) makes an important contribution to the dehydrogenation reaction. This is also the reason why the activity of the 3Cr/NaZSM-5 is higher than that of the 3Cr/HZSM-5.

The type of Cr(VI) species is considered to be another poss-ible factor in the dehydrogenation reaction. Kumar et al. [34] reported that isolated chromium species are more active for the dehydrogenation reaction than crystalline α-Cr2O3, whereas oligomeric chromium species are more active than isolated chromium species. We can see from Fig. 5 that dispersed Cr(VI) species are present in the form of polymeric chromates and monochromates on the surface of Cr/NaZSM-5 while crystal-line Cr2O3 was formed on the Cr/HZSM-5. This may be the rea-son why 3Cr/HZSM-5-160 exhibits relatively low activity even though it has a higher amount of reducible Cr(VI) as compared with 3Cr/NaZSM-5-60.

The effect of the Cr loading on the dehydrogenation activity was also investigated. The ethane conversion was found to increase with increasing extents of Cr loading and then de-creased as the Cr content was further increased, such that mass fraction of 3% Cr2O3 is optimal.

To investigate the stability of the 3Cr/NaZSM-5-160 cata-lyst, the dehydrogenation reaction was run continuously for 50 h, with the results shown in Fig. 7. The catalyst is relatively stable; the ethylene yield over the catalyst is maintained at about 46% without any obvious deactivation over the 50 h, although the activity does drop slowly. This is quite different

90Conversion/Selectivity/Yield (%)807060(1)5040(3)0102030Time on stream (h)

4050 C3H6 10 min 1.3 0.3 0.2 0.3 0.2

6 h 0.8 0.3 0.2 0.2 0.2

Yield (%) 10 min 19.5 46.5 49.1 49.4 49.4

6 h 12.1 45.0 47.7 48.4 48.0

(2)Fig. 7. (1) Ethane conversion, (2) ethylene selectivity and (3) ethylene yield over 3Cr/NaZSM-5-160 as functions of reaction time.

from the behavior of other commonly studied Cr-containing catalysts, over which the ethylene yields are reported to have dropped quickly within 6 h. 3.4. Effect of CO2 partial pressure

The effect of CO2 partial pressure on the dehydrogenation of ethane was also investigated, and the results are summarized in Table 4. The promotional effect of CO2 on the reaction is quite evident. The initial ethane conversion increases quickly from 20.4% with increasing CO2/C2H6 ratios until it reaches its peak at 65.5% when the CO2/C2H6 ratio equals 5, after which the conversion decreases slightly with further increases in the CO2/C2H6 ratio. The effect of CO2 can be attributed to the re-verse water-gas shift reaction, which accelerates the formation of the dehydrogenation products by transforming H2 and CO2 into CO and H2O. This is demonstrated by the results of the H2/CO2 reaction over 3Cr/NaZSM-5-160. Obviously, the cata-lyst is very active for the reverse water-gas shift reaction, with a CO2 conversion of 22.6% at 650 °C.

The stability of the catalysts was also improved greatly by

Table 4

Reaction data for 3Cr/NaZSM-5-160 under different CO2 partial pressures obtained at 10 min and 6 h. C2H6/CO2 ratio ∞ 1/1 1/3 1/5 1/7

Conversion (%) 10 min 20.4 55.2 60.5 65.5 64.9

6 h 12.5 52.6 57.8 61.3 60.3

CH4 10 min 3.1 15.4 18.7 24.3 23.7

Selectivity (%) C2H4 10 min 6 h 95.6 96.8 84.3 85.5 81.1 82.5 75.4 78.9 76.2 79.5

6 h

2.4 14.2 17.3 20.9 20.3

Yanhu Cheng et al. / Chinese Journal of Catalysis 36 (2015) 1242–1248 1247

the addition of CO2. About 40% of the initial activity was lost within 6 h in the absence of CO2. However, when 15% CO2 was introduced, only 5% loss was observed during the same period. The enhanced stability can be explained primarily by two ef-fects: (1) CO2 promotes the Cr6+/Cr3+ reaction through the oxi-dation of reduced Cr species, Cr(III)Ox–1 + CO2 → Cr(VI)Ox + CO, and regenerates the active species [6,7,17,35–37], and (2) CO2 eliminates the formation of coke by the Boudouard reaction, CO2 + C → 2CO, and thus exposes more active sites to the reac-tants [6,7,17,35–37]. This can be confirmed by thermogravi-metric tests that show that the amount of coke deposited on the catalyst after 6 h is 3.4% in the absence of CO2, a value that is higher than the 3.0% amount obtained in the presence of CO2. The fact that the Boudouard reaction proceeds is also evident from the observation that the molar ratio of CO formed to CO2 converted is approximately 1.4 during the reaction, since this ratio should be equal to unity if the Boudouard reaction does not occur. 4. Conclusions

Catalysts composed of submicron ZSM-5-supported chro-mium oxide particles were prepared by an incipient wetness method, and their catalytic performance for ethane dehydro-genation in the presence of CO2 was compared. The results show that chromium oxide supported on submicron Na-type ZSM-5 with a high Si/Al ratio is an excellent catalyst for the oxidative dehydrogenation of ethane with CO2. High activity as well as high stability can be obtained over this catalyst. The promotional effect of CO2 on dehydrogenation can also be ob-served on this catalyst and is attributed to the reverse wa-ter-gas shift reaction. Characterization by laser Raman, UV-Vis DRS, XPS and H2-TPR revealed that improved dispersion of Cr species can be achieved in submicron catalysts, resulting in a greater quantity of the reducible Cr(VI) species that play a key role in the ethane dehydrogenation reaction. The type of Cr(VI)

species present is another possible factor affecting the dehy-drogenation reaction. References

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Graphical Abstract Chin. J. Catal., 2015, 36: 1242–1248 doi: 10.1016/S1872-2067(15)60893-2Oxidative dehydrogenation of ethane with CO2 over Cr supported on submicron ZSM-5 zeolite Yanhu Cheng, Fan Zhang, Yi Zhang, Changxi Miao, Weiming Hua, Yinghong Yue *, Zi Gao Fudan University; Shanghai Research Institute of Petrochemical Technology, SINOPEC C2H6CrOConversion/Selectivity/Yield (%)1008060402000C2H4 selectivityC2H6 conversionC2H4 yieldSubmicron ZSM-5 ZeoliteC2H410203040Time on stream (h)50 Submicron ZSM-5 supported chromium oxide catalysts were prepared and exhibit both high activity and stability, with an ethane conver-sion of ～65% and ethylene yield of ～49% without any obvious trend of deactivation in 50 h.

1248 Yanhu Cheng et al. / Chinese Journal of Catalysis 36 (2015) 1242–1248

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亚微米ZSM-5负载Cr催化剂上乙烷CO2气氛下脱氢制乙烯

程彦虎a, 张 帆a, 张 翼a, 缪长喜b, 华伟明a, 乐英红a,*, 高 滋a

a

复旦大学化学系上海市分子催化与功能材料重点实验室, 上海200433

b

中石化上海石油化工研究院, 上海201208

摘要: 石油资源的日趋短缺使天然气和页岩气的开发利用受到重视, 因而低碳烷烃脱氢制取低碳烯烃也随之引起了人们越来越多的关注. 由于乙烷纯脱氢反应的平衡收率低, 能耗高, 而氧气氧化脱氢又易将乙烷深度氧化为CO2或CO, 因此开发具有反应条件温和、装置投资和操作费用低等优势的CO2气氛下乙烷脱氢的技术路线日益得到重视. CrOx是该反应理想的催化剂之一, CO2的加入可使CrOx对乙烷脱氢的催化活性提升3倍, 然而受困于CrOx过小的比表面积, 通常将CrOx制备成负载型催化剂使用. CrOx的常见载体有Al2O3, ZrO2和SiO2等氧化物及MCM-41, SBA-15, SBA-1和MSU-x等介孔硅材料, ZSM-5作为载体负载CrOx用于低碳烷烃脱氢的研究则较少, 所得结果也不甚理想. 我们采用亚微米尺寸的ZSM-5作为载体制备了负载型CrOx催化剂, 研究了其在CO2气氛下催化乙烷脱氢反应, 发现该催化剂具有非常优异的脱氢活性, 高硅铝比和Na型的ZSM-5作载体对反应更加有利, 而且在反应进行50 h后, 催化剂依然保持很好的活性和很高的乙烯收率, 这是在一般负载型CrOx催化剂上所不能实现的. X射线光电子能谱(XPS)表征发现, Na型ZSM-5载体制得的催化剂具有更高的Cr6+/Cr3+比. 一般认为, Cr6+是Cr系催化剂进行低碳烷烃脱氢反应时的活性位(或活性位前驱体), 因此可以初步判定, Na型载体具有很好催化效果的原因可能是由它制得的催化剂具有更多的反应活性位. 程序升温还原(H2-TPR)表征结果证实了这一点, Na型载体明显具有更高的H2消耗量; 也就是说, Na型载体制得的催化剂具有更多的可还原Cr物种, 即脱氢活性位. 进一步表征发现, 反应活性还与Cr物种存在形式有关. 文献报道, 低聚态的Cr物种和孤立态的Cr物种比Cr2O3有更好的催化活性. 通过漫反射紫外-可见光谱(UV-Vis)对Cr物种的存在形态进行表征后发现, Na型载体上Cr主要以四配位形式存在, 而在H型载体上出现了对应于六配位的Cr物种; 激光Raman表征结果表明, Na型载体上出现的都是低聚态Cr物种和孤立态Cr物种, 而H型载体上出现了明显的对应于α-Cr2O3的峰, 说明相较于H型载体, Na型载体更有利于Cr组分分散, 这也是Na型ZSM-5载体催化剂具有更高活性的原因之一.

CO2引入后对乙烷脱氢反应具有明显的促进作用, 特别是在CO2/C2H6 = 5时, 催化剂上C2H6转化率是非CO2气氛下的3.2倍; 同时, CO2的引入也提高了脱氢反应的稳定性. 在非CO2气氛下, 反应进行6 h后, C2H6转化率降低到初活性的60%左右, 而在CO2/C2H6 = 5时, 相同时间内催化剂活性下降仅有5%左右. 实验分析了CO2对脱氢反应具有促进作用的原因. 在脱氢反应温度650 oC下, CO2/H2 = 1时进行了逆水煤气反应测试, 发现CO2的转化率达到22.5%, 说明引入CO2后可以通过逆水煤气反应有效地消耗掉乙烷脱氢反应生成的H2, 从而促进反应向脱氢方向进行; CO2的引入也可以促进Cr物种的CrOx/CrOx?1循环, 从而提高催化剂效率, 减缓催化剂失活; CO2还可与反应中生成的积碳类物质发生Boudouard反应, 将反应活性位暴露出来, 从而提高催化剂的稳定性. CO2气氛下反应6 h后催化剂的积碳量为3.0%, 低于非CO2气氛下的3.4%, 同时在脱氢反应中生成的CO量与消耗掉的CO2量的比值约为1.4, 也有力地说明Boudouard反应的存在.

关键词: 脱氢反应; 乙烷; ZSM-5分子筛; 亚微米; 二氧化碳

收稿日期: 2015-01-28. 接受日期: 2015-05-11. 出版日期: 2015-08-20.

*通讯联系人. 电话: (021)65642409; 传真: (021)65641740; 电子信箱: yhyue@fuan.edu.cn

基金来源: 国家自然科学基金(20773027, 20773028和21273043); 上海市科学技术委员会(08DZ2270500).

本文的英文电子版由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).