Effect of Lewis acid sites of FER zeolite on catalytic transformation of isobutene
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摘要: 采用不同浓度六氟硅酸铵（AHFS）对FER型分子筛进行同晶置换改性，制备了一系列具有不同酸类型、酸密度及可接近性等特征的改性FER分子筛样品，运用XRD、N2吸附-脱附等温线等方法对其织构性质进行表征，通过NH3-TPD和Py-FTIR关联改性FER分子筛的酸性质，并结合反应评价系统探究了改性FER分子筛酸中心与丁烯（1-丁烯、异丁烯）骨架异构化反应间的构效关系。结果表明，根据产物收率确定，以1-丁烯和异丁烯分别作为原料时其骨架异构化的最佳反应温度均为350 ℃，以异丁烯作为原料时副反应更为明显。在脱铝过程中，由于脱铝剂和分子筛中的非骨架铝羟基物种的相互作用，产生了强弱不同的两种新Lewis酸中心，在反应过程中上述两种Lewis酸中心促进了异丁烯齐聚-裂化反应的进行，进而降低了主反应的选择性。Abstract: A series of modified FER zeolite samples with different acid types, acid density and accessibility were prepared by using different concentrations of ammonium hexafluorosilicate (AHFS) for isomorphous substitution. The texture properties were characterized by XRD and N2 adsorption isotherms. And the acid properties were characterized by NH3-TPD and Py-FTIR. The results showed that when 1-butene and isobutene were respectively used as raw materials, the optimal reaction temperature for skeleton isomerization was 350 ℃, and the side reaction was more obvious when isobutene was used as raw material. During the dealumination process, two new Lewis acid sites with different strength were generated, due to the interaction between the dealumination agent and the extra framework aluminum hydroxyl species on the zeolite. During the reaction, the above two Lewis acid sites promoted the oligomerization-cracking of isobutene, thereby, reducing the selectivity of the main reaction.
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Key words:
- zeolite /
- catalysis /
- FER /
- butene /
- Lewis acid
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图 3 四种丁烯在不同温度下的热力学平衡图
Figure 3 Thermodynamic equilibrium diagrams of four butenes at different temperatures
图 4 不同反应温度产物中四种丁烯总含量对比
Figure 4 Comparison of four butenes content in products at different reaction temperatures
图 5 HZSM-35以及不同浓度AHFS改性后样品XRD谱图
Figure 5 XRD patterns of the parent and AHFS modified HZSM-35 zeolites
图 6 HZSM-35以及不同浓度AHFS改性后样品的SEM照片
Figure 6 SEM images of the parent and AHFS modified HZSM-35 zeolites
(a)/(d): Z-35; (b)/(e): Z-35AHFS(0.1); (c)/(f): Z-35AHFS(0.2)
图 7 HZSM-35以及不同浓度AHFS改性后样品NH3-TPD谱图
Figure 7 NH3-TPD patterns of the parent and AHFS modified HZSM-35 zeolites
图 8 HZSM-35以及不同浓度AHFS改性后样品的红外光谱谱图
Figure 8 FT-IR spectra of the parent and AHFS modified HZSM-35 zeolites
图 9 HZSM-35以及不同浓度AHFS改性后样品在不同脱附温度下的吡啶红外光谱谱图
Figure 9 Py-FTIR spectra of the parent and AHFS modified HZSM-35 zeolites after desorption at different temperatures
(black: background, red: desorption at 150 ℃, blue: desorption at 400 ℃)
图 10 HZSM-35以及不同浓度AHFS改性后样品异丁烯骨架异构化反应评价
Figure 10 Evaluation of isomerization reaction of isobutene at parent and AHFS modified HZSM-35 zeolites
(a): isobutene conversion; (b): n-butene selectivity reaction conditions: 350 ℃, 0.1 MPa, WHSV = 6 h−1
图 11 HZSM-35以及不同浓度AHFS改性后样品催化异丁烯反应产物分布
Figure 11 Product distribution of isomerization reaction of isobutene at parent and AHFS modified HZSM-35 zeolites
图 12 HZSM-35以及不同浓度AHFS改性后样品催化异丁烯反应25 h后的TG曲线
Figure 12 TG diagram of the isobutene reaction at parent and AHFS modified HZSM-35 zeolites after 25 h
表 1 不同温度下正丁烯骨架异构化和异丁烯骨架异构化反应评价表
Table 1 Evaluation of isomerization reaction of n-butene and isobutene at different temperatures
t/℃ n-butene isomerization isobutene isomerization conv.
x/ %selec.
s/%yield.
y/%conv.
x/%selec.
s/%yield
y/%300 15.67 73.71 11.55 29.66 74.58 22.12 325 31.79 70.62 22.45 44.35 68.10 30.34 350 49.99 64.97 32.48 58.28 55.34 32.25 375 60.12 53.06 31.90 68.96 43.53 30.02 410 67.67 40.74 27.57 81.00 27.00 21.87 reaction conditions: 0.1 MPa, WHSV = 6 h−1, reaction time: 8 h 
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表 2 两个反应的四种丁烯异构体产物分布
Table 2 Distribution of four butene isomer products for two reactions
Sample 1-butene
/%cis-2-
butene
/%tran-2-
butene
/%Isobutene
/%Thermodynamic equilibrium 8.34 14.39 23.64 53.67 Isobutene isomerization 7.77 14.72 21.12 56.40 n-butene isomerization 11.53 19.03 31.71 37.73 reaction conditions: 350 ℃, 0.1 MPa, WHSV = 6 h−1, reaction time: 8 h
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表 3 不同浓度AHFS处理HZSM-35分子筛的织构性质
Table 3 Basic physical parameters of the parent and AHFS modified HZSM-35 zeolites
Sample SAR SBET/(m2∙g−1) Smicro/(m2∙g−1) vtotal/(cm3∙g−1) vmicro/(cm3∙g−1) (vtotal− vmicro)/(cm3∙g−1) Pore size/Å Z-35 8.91 298 245 0.318 0.129 0.189 5.216 Z-35AHSF(0.1) 11.80 282 228 0.292 0.120 0.172 5.278 Z-35AHSF(0.2) 12.35 288 223 0.363 0.117 0.246 5.670
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