壳层厚度对骨架Fe@HZSM-5核壳催化剂费托合成催化性能的影响

doi:10.6023/A20120563

摘要/Abstract

在不同的水热合成时间下, 以铁铝合金为铁源和铝源, 四丙基氢氧化铵为分子筛的模板剂和抽提合金中的铝的碱, 一步制得了以骨架铁为核、不同厚度的HZSM-5分子筛为壳的Raney Fe@HZSM-5催化剂. 采用元素分析、氮物理吸附、X射线粉末衍射、氨脱附、扫描电子显微镜等手段, 考察了水热时间对催化剂基本物化性质的影响. 随着水热时间的延长, HZSM-5分子筛壳层不断增厚, 结晶度不断增大, 但分子筛组成基本不变, 酸量与分子筛壳层厚度正相关. 在费托合成反应中, Raney Fe@HZSM-5核壳催化剂上的CO转化率和汽油段产物选择性随分子筛壳层厚度呈火山型变化趋势, 说明反应需要适宜的酸量, 酸量过低或过高均不利于得到高的催化活性及汽油段产物选择性. 在水热合成时间为4 d制得的Raney Fe@HZSM-5核壳催化剂上, 当CO转化率为92%时, C₅~C₁₁汽油段产物选择性可达71%, 异正比为1.9. 当合成气中的n(H₂)/n(CO)比从2降为1时, 汽油段产物选择性和异正比进一步提高至73%和2.1, 显示了将该催化剂用于煤基或生物质基合成气转化为高辛烷值汽油的良好潜力.

关键词: 骨架Fe, HZSM-5, 核壳结构, 费托合成, 汽油

Using the Raney Fe-Al alloy as the precursor for Fe and Al, and tetrapropylammonium hydroxide (TPAOH) as both the base for the leaching of the alloy and the template for the synthesis of HZSM-5, we synthesized the skeletal Fe core-HZSM-5 shell catalysts (Raney Fe@HZSM-5) via a facile one-pot hydrothermal strategy. The thickness of the zeolite shell was adjusted by varying the hydrothermal time. The catalysts were characterized by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), N₂ physisorption, X-ray powder diffraction (XRD), temperature-programmed desorption of NH₃ (NH₃-TPD), and scanning electron microscopy (SEM). It was identified that the Raney Fe-Al alloy was more suitable than the rapidly quenched Fe-Al alloy as the precursor to the skeletal Fe core, since the skeletal Fe catalyst alkali-leached from the former produced more long-chain hydrocarbons during the Fischer-Tropsch synthesis (FTS) reaction for successive cracking and isomerization. A complete HZSM-5 shell was obtained only after above 24 h of hydrothermal treatment. With the hydrothermal time from 2 d to 8 d, the thickness of the compact HZSM-5 shell increased from 1.3 μm to 9.7 μm, the relative crystallinity increased steadily, while the SiO₂/Al₂O₃ ratio remained essentially constant, and the amount of the acid sites is parallel to the thickness of the zeolite shell. In the FTS reaction using syngas with the H₂/CO molar ratio of 2 and at 543 K and 2.0 MPa, the CO conversion and the selectivity to the gasoline fraction (C₅~C₁₁ hydrocarbons) evolved in a volcanic trend with the shell thickness, suggesting the existence of an optimal amount of the acid sites, while less or more is adverse to the activity and selectivity to the gasoline fraction. The Raney Fe@HZSM-5 catalyst hydrothermally treated for 4 d exhibited the highest CO conversion of 92% and the selectivity to the gasoline fraction of 71%, along with an iso-paraffin to n-paraffin ratio of 1.9. At the n(H₂)/n(CO) ratio of 1, the selectivity to the gasoline fraction and the iso-paraffin to n-paraffin ratio were further improved to 73% and 2.1, respectively, which shows promise for this catalyst to transform the coal- or biomass- derived syngas to high-octane number gasoline.

Key words: skeletal Fe, HZSM-5, core-shell structure, Fischer-Tropsch synthesis, gasoline

引用此文

孙冬, 孙博, 裴燕, 闫世润, 范康年, 乔明华, 张晓昕, 宗保宁. 壳层厚度对骨架Fe@HZSM-5核壳催化剂费托合成催化性能的影响[J]. 化学学报, 2021, 79(6): 771-777.

Dong Sun, Bo Sun, Yan Pei, Shirun Yan, Kangnian Fan, Minghua Qiao, Xiaoxin Zhang, Baoning Zong. Effect of Shell Thickness on Skeletal Fe@HZSM-5 Core-Shell Catalysts for Fischer-Tropsch Synthesis[J]. Acta Chimica Sinica, 2021, 79(6): 771-777.

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图/表 8

图1. Raney Fe催化剂和RQ Fe催化剂在FTS反应中的(a) CO转化率和(b, c) 产物选择性

Figure 1. (a) CO conversions and (b, c) hydrocarbon distributions over the Raney Fe and RQ Fe catalysts in the FTS reaction

图2. 不同水热时间得到的Fe@Z-5催化剂的SEM照片. (a) 水热2 h, (b) 水热12 h, (c) 水热24 h

Figure 2. SEM images of the Fe@Z-5 catalysts after (a) 2 h, (b) 12 h, and (c) 24 h of hydrothermal treatment

图3. 不同水热时间的Fe@Z-5催化剂的XRD谱

Figure 3. XRD patterns of the Fe@Z-5 catalysts with different hydrothermal time ? HZSM-5, □ γ-Fe2O3

图4. (a) Raney Fe和(b) Fe@Z-5-4d核壳催化剂的SEM照片

Figure 4. SEM images of the (a) Raney Fe and (b) Fe@Z-5-4d catalysts

图5. 不同水热时间的Fe@Z-5催化剂的剖面SEM照片. (a) Fe@Z-5-2d, (b) Fe@Z-5-4d, (c) Fe@Z-5-6d, (d) Fe@Z-5-8d

Figure 5. Cross-sectional SEM images of the Fe@Z-5 catalysts with different hydrothermal time. (a) Fe@Z-5-2d, (b) Fe@Z-5-4d, (c) Fe@Z-5-6d, and (d) Fe@Z-5-8d

Catalyst	S_BET^a/(m²•g^-1)	V_pore/(cm³•g^-1)	d_pore/nm	w(Zeolite)/%	SiO₂/Al₂O₃^b (molar ratio)	Relative crystallinity/%	NH₃ uptake ^c/(µmol•g^-1)
Fe@Z-5-2d	100	0.12	6.54	7.9	62	73	38.5
Fe@Z-5-4d	116	0.08	6.47	12.5	63	76	56.1
Fe@Z-5-6d	121	0.10	6.39	20.8	63	89	77.6
Fe@Z-5-8d	132	0.14	6.08	25.1	63	100	94.5

表1. 不同水热时间的Fe@Z-5催化剂的物理化学性质

Table 1. The physicochemical properties of the Fe@Z-5 catalysts with different hydrothermal time

Catalyst	X_CO/%	S_CO2/%	S_hydrocarbon^b/%				C_i/C_n^c	O/P^d
Catalyst	X_CO/%	S_CO2/%	CH₄	C₂~C₄	C₅~C₁₁	C₁₂₊	C_i/C_n^c	O/P^d
Fe@Z-5-2d	71	18	11	23	61	5.3	0.7	0.4
Fe@Z-5-4d	92	13	7.7	21	71	—	1.9	0.5
Fe@Z-5-6d	87	16	12	24	64	—	2.1	0.3
Fe@Z-5-8d	77	13	16	30	55	—	2.4	0.3

表2. 不同水热时间的Fe@Z-5系列催化剂的费托合成催化性能a

Table 2. Catalytic performances of the Fe@Z-5 catalysts with different hydrothermal time in the FTS reactiona

图6. 不同H2/CO物质的量比下150 h反应后Fe@Z-5-4d催化剂上的FTS结果

Figure 6. The FTS results over the Fe@Z-5-4d catalyst after 150 h on stream using syngas with different H2/CO molar ratios

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