Effect of framework Al siting on catalytic performance in methanol to aromatics over ZSM-5 zeolites
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摘要: 采用水热合成法，在合成过程中通过添加矿化剂、尿素和改变硅源，制备了不同骨架铝落位的ZSM-5分子筛。通过SEM、XRD、BET、XRF、MAS NMR、NH3-TPD和Py-FTIR等表征手段对分子筛的形貌、织构、骨架铝落位和酸性进行了系统研究，同时考察了不同ZSM-5分子筛催化剂甲醇制芳烃的催化性能。研究结果表明，制备的ZSM-5分子筛均具有结晶度高和形貌均一等特点，但在骨架铝落位和酸性方面存在显著差异。椭球状ZSM-5分子筛的骨架铝主要分布于直通孔道或正弦孔道中，并表现出较多的酸性位。块状分子筛中骨架铝主要落位在孔道交叉处，且具有较低的强酸量。在甲醇制芳烃反应中，骨架铝主要位于直通或正弦孔道并表现出较多酸性位的椭球状ZSM-5分子筛催化剂具有较高的活性稳定性和芳烃选择性。Abstract: ZSM-5 zeolites with different framework Al (AlF) siting were hydrothermally synthesized by adding mineralizer, urea, or by changing the silicon source. The morphology, textural properties, AlF siting, and acidity of different ZSM-5 zeolites were characterized using SEM, XRD, BET, XRF, MAS NMR, NH3-TPD, and Py-IR. Furthermore, the conversion of methanol to aromatics (MTA) was used to investigate the catalytic performance of different catalysts.The results suggested that different ZSM-5 zeolites were highly crystalline with a uniform morphology, but there were large differences in AlF siting and acidity. The AlF in the ellipsoidal ZSM-5 sample was mainly distributed in straight or sinusoidal channels and displayed more acidic sites. AlF of bulk ZSM-5 was mainly located at the intersection of channels, and it showed the lowest amount of strong acid sites. The ellipsoidal ZSM-5 catalyst in which AlF was mainly located in straight or sinusoidal channels exhibited more acidic sites and higher stability and aromatic selectivity during the MTA reaction.
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Key words:
- ZSM-5 zeolites /
- framework Al siting /
- methanol to aromatics
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图 2 HA (a)、HB (b)和HC ((c)、(d))分子筛样品的SEM照片
Figure 2 SEM images of HA (a), HB (b) and HC ((c) and (d))
图 3 不同ZSM-5分子筛样品的N2吸附-脱附曲线
Figure 3 N2 adsorption-desorption isotherms of different ZSM-5 samples
图 4 不同ZSM-5分子筛样品的27Al MAS NMR谱图
Figure 4 27Al MAS NMR spectra of different ZSM-5 zeolite samples
图 5 不同ZSM-5分子筛样品的NH3-TPD谱图
Figure 5 NH3-TPD spectra of different ZSM-5 zeolite samples
图 6 不同ZSM-5分子筛200 ℃ (a)和350 ℃ (b)的Py-FTIR谱图
Figure 6 Py-FTIR spectra of different ZSM-5 zeolite samples at 200 ℃ (a) and 350 ℃ (b)
图 7 不同ZSM-5分子筛催化剂上甲醇转化率随反应时间的变化
Figure 7 Change of methanol conversion with reaction time over different ZSM-5 zeolite catalysts
图 8 不同ZSM-5分子筛催化剂上芳烃选择性随反应时间的变化
Figure 8 Change of aromatics selectivity with reaction time over different ZSM-5 zeolite catalysts
图 9 ZSM-5分子筛上甲醇转化制芳烃反应路径示意图
Figure 9 MTA reaction routes over ZSM-5 zeolites catalysts
表 1 不同形貌ZSM-5分子筛的硅铝比和织构性质
Table 1 Si/Al ratios and textural properties of different ZSM-5 samples
Sample Si/Ala Relative crystallinity /%b BET surface area A/(m2·g-1)c Microporous area A/(m2·g-1)c External surface area A/(m2·g-1)c HA 43.16 94 393.85 318.30 75.55 HB 44.78 100 378.85 314.80 64.04 HC 41.06 91 403.05 334.80 68.25 a: determined by XRF; b: calculated from XRD patterns; c: obtained by N2-adsorption at -196 ℃ 
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表 2 不同ZSM-5分子筛样品中的骨架铝落位
Table 2 Framework Al (AlF) siting of different ZSM-5 zeolite samples
Sample AlF distribution /% Al(54) Al(56) HA 15.96 27.65 HB 32.63 19.63 HC 38.63 12.19
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表 3 NH3-TPD和Py-FTIR表征的不同ZSM-5分子筛的酸性质
Table 3 Acidic properties of various ZSM-5 zeolites measured by NH3-TPD and Py-FTIR
Sample Acidity /(μmol·g-1) a Acidity /(μmolPy·g-1) b weak strong Brønsted Lewis 200 ℃ 350 ℃ 200 ℃ 350 ℃ HA 166.22 171.97 155.49 138.11 13.38 9.15 HB 149.54 185.73 148.23 131.04 10.90 8.11 HC 154.93 149.72 143.62 118.59 13.57 12.80 a: determined by NH3-TPD; b: calculated from Py-FTIR spectra
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表 4 不同ZSM-5分子筛催化剂上产物分布
Table 4 Product distribution of different ZSM-5 zeolite catalysts
Catalyst Product distribution /% a CH4 C2-5- C2-5= C5+ non-aromatics BTEX b C9+ aromatics HA 1.20 34.69 5.69 21.10 23.33 13.99 HB 2.08 38.97 5.74 23.12 19.86 10.23 HC 2.81 35.35 6.31 27.69 14.66 13.18 a: analyzed at time-on-stream of 12 h; b: benzene, toluene, ethylbenzene and xylene
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