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摘要: 分别采用共沉淀法和水热法制备了ZnCrOx复合金属氧化物和HZSM-5沸石,通过物理混合得到双功能催化剂,实现了合成气一步高选择性制备异构烷烃。采用XRD、TEM、氮吸附和NH3-TPD等技术对催化剂进行了表征,考察了双功能催化剂中HZSM-5沸石组分硅铝比以及ZnCrOx/HZSM-5质量比(OX/ZEO mass ratio)对合成气催化转化反应性能的影响。结果表明,随着HZSM-5硅铝比的增加,催化剂酸密度下降,CO转化率略有下降,产物中C5+选择性显著提高,异构烷烃比例不断增加。此外,在保证CO转化率的前提下提高双功能催化剂中ZnCrOx组分的比例,产物中C5+的选择性也显著上升。在400 ℃、2.0 MPa、进料空速(GHSV)为3600 mL/(h·gcat)的条件下,合成气(H2/CO(volume ratio)=2)转化率达到35%,C5+选择性超过44%,且C5+中异戊烷比例高达65%。Abstract: ZnCrOx composite oxide and HZSM-5 zeolite were prepared by using the coprecipitation and hydrothermal methods, respectively; after that, a bi-functional ZnCrOx/HZSM-5 catalyst was obtained through physical mixing of ZnCrOx with HZSM-5 and used in the direct synthesis of isoalkanes from syngas. The ZnCrOx/HZSM-5 catalyst was characterized by XRD, TEM, N2 sorption, and NH3-TPD and the effects of Si/Al ratio in HZSM-5 and the mass ratio of ZnCrOx to HZSM-5 (OX/ZEO mass ratio) on the catalytic performance of ZnCrOx/HZSM-5 in syngas conversion were investigated. The results indicated that with the increase of Si/Al ratio in HZSM-5, the catalyst acid density is decreased, resulting in a lower CO conversion but higher selectivity to C5+ products and higher isoparaffin fraction. Moreover, the selectivity to C5+ products is significantly increased by increasing the proportion of ZnCrOx components in the bifunctional catalyst without losing CO conversion. For the syngas conversion over ZnCrOx/HZSM-5 catalyst under the conditions of 400 ℃, 2.0 MPa, gas hourly space velocity (GHSV) of 3600 mL/(h·gcat), the conversion of syngas reaches 35%, with a selectivity of 44% to C5+ products and an isopentane fraction up to 65% in the C5+ products.
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Key words:
- bifunctional catalyst /
- syngas /
- acid-catalyzed /
- isopentane
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图 4 不同温度下ZnCrOx/ HZSM-5(160)的催化反应性能
Figure 4 Catalytic performance of ZnCrOx/ HZSM-5(160) in syngas conversion at different temperatures reaction conditions: 0.5 g ZnCrOx/HZSM-5
(mass ratio = 1:1), H2/CO(volume ratio)=2, p=2.0 MPa, GHSV=3600 mL/(h·gcat), TOS= 4 h, C2-4=: C2-4 olefins, C2-4o: C2-4 alkanes, C5+: hydrocarbons with more than 5 carbons except aromatics, Ar: aromatics
表 1 ZnCrOx和HZSM-5样品的物理化学性质
Table 1 Textural properties of the ZnCrOx and HZSM-5 samplesa
Sample ABET/(m2·g-1) Amicro/(m2·g-1) Ameso/(m2·g-1) vpore/(mL·g-1) vmicro/(mL·g-1) ZnCrOx 88 0 88 0.407 0 HZSM-5(20) 342 289 53 0.223 0.135 HZSM-5(40) 356 303 53 0.312 0.135 HZSM-5(80) 375 296 79 0.233 0.126 HZSM-5(160) 363 283 80 0.226 0.121 a: obtained by N2 physisorption 表 2 HZSM-5样品的表面酸性质
Table 2 Surface acidity of H-ZSM-5 samplesa
Sample t1/℃ Weak acidity/ (mmol·g-1) Weak acidity density /(μmol·m-2) t2 /℃ Strong acidity/ (mmol·g-1) Strong acidity density/ (μmol·m-2) HZSM-5(20) 172 0.41 1.19 267 0.24 0.70 HZSM-5(40) 169 0.28 0.77 346 0.24 0.67 HZSM-5(80) 158 0.13 0.34 333 0.13 0.33 HZSM-5(160) 152 0.07 0.20 326 0.09 0.25 a: obtained by NH3-TPD 表 3 不同硅铝比ZnCrOx/HZSM-5的催化反应性能
Table 3 Catalytic performance of ZnCrOx/ HZSM-5 with different Si/Al ratios
Catalyst CO
conversion x/%CO2
selectivity s/%ZnCrOx/HZSM-5(20) 39.9 42.0 ZnCrOx/HZSM-5(40) 38.8 41.8 ZnCrOx/HZSM-5(80) 37.3 40.6 ZnCrOx/HZSM-5(160) 36.9 42.6 ZnCrOx 5.6 42.9 reaction conditions: 0.5 g ZnCrOx/HZSM-5(mass ratio=1:1), H2/CO(volume ratio)=2, 400 ℃, 2.0 MPa, GHSV=3600 mL/(h·gcat), TOS= 8 h 表 4 不同硅铝比ZnCrOx/HZSM-5催化反应产物中的C5+分布
Table 4 C5+ distribution of hybrid ZnCrOx/HZSM-5 with different Si/Al ratios
Catalyst C5+ distribution/% i/n ratio i-C5 n-C5 others ZnCrOx/HZSM-5(20) 48.7 13.9 37.4 3.5 ZnCrOx/HZSM-5(40) 50.2 10.1 39.7 5.0 ZnCrOx/HZSM-5(80) 56.6 9.7 33.7 5.8 ZnCrOx/HZSM-5(160) 63.8 6.8 29.4 9.3 reaction conditions: 0.5 g ZnCrOx/HZSM-5(mass ratio=1:1), H2/CO(volume ratio)=2, t=400 ℃, p=2.0 MPa, GHSV=3600 mL/(h·gcat), TOS=8 h 表 5 不同OX/ZEO质量比ZnCrOx/HZSM-5的催化反应性能
Table 5 Catalytic performance of ZnCrOx/HZSM-5(160) with different OX/ZEO mass ratios
OX/ZEO (mass ratio) CO conversion x/% CO2 selectivity s/% 9:1 24.8 37.8 4:1 27.4 37.7 2:1 35.5 41.0 1:1 36.9 42.6 1:2 37.1 43.3 1:4 36.5 45.3 reaction conditions: 0.5 g ZnCrOx/HZSM-5(160), H2/CO(volume ratio)= 2, 400 ℃, 2.0 MPa, GHSV=3600 mL/(h·gcat), TOS=8 h 表 6 不同质量比ZnCrOx/HZSM-5催化反应产物中的C5+分布
Table 6 C5+ distribution for syngas conversion over hybrid ZnCrOx/HZSM-5 with different OX/ZEO mass ratios
OX/ZEO (mass ratio) C5+ distribution/% i/n ratio i-C5 n-C5 others 9:1 47.1 1.8 51.1 25.8 4:1 62.0 10.4 27.6 6.0 2:1 65.4 6.9 27.7 9.4 1:1 63.8 6.8 29.4 9.3 1:2 55.9 3.5 40.6 15.9 1:4 52.9 5.1 42.1 10.5 reaction conditions: 0.5 g ZnCrOx/HZSM-5(160), H2/CO(volume ratio)=2, t=400 ℃, p=2.0 MPa, GHSV=3600 mL/(h·gcat), TOS=8 h -
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