Manipulation of hydroisomerization performance on Pt/ZSM-23 by introducing Al2O3
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摘要: 采用双模板剂合成了ZSM-23分子筛，并制备ZSM-23-Al2O3复合载体，通过引入Al2O3调控Pt/ZSM-23催化剂的正十六烷(n-C16)加氢异构性能。采用XRD、SEM、TEM、N2物理吸附-脱附及NH3-TPD等方法探讨Al2O3对催化剂结构性质和化学性质的影响。结果表明，引入Al2O3可以提高Pt在复合载体上的分散度，降低催化剂的酸性位点浓度，有效调控了催化剂的金属-酸性浓度平衡(CPt/CA)。适宜的金属-酸性浓度平衡有利于提高对异构产物的选择性并抑制裂化副反应。Al2O3同时对ZSM-23晶粒起到了分散作用，改善了其分子筛纳米晶粒团聚现象，增加了ZSM-23中孔口的暴露数量，促进反应物和中间体的扩散。复合载体催化剂呈现了出色的异构选择性，其中，Pt/(9Z-1Al)催化剂由于具有适宜的金属-酸性浓度平衡呈现最高的异构产物收率，可达60%，明显高于Pt/ZSM-23催化剂的异构体收率(42%)。反应温度低于310 ℃时，Pt/ZSM-23催化剂的吸附模型符合孔口吸附，而高温时则为锁钥吸附。引入Al2O3后ZSM-23有更多相邻孔口暴露，吸附模型以锁钥择形为主，导致了7/8Me-C15异构产物的大量生成。Abstract: ZSM-23 zeolite was successfully synthesized in a dual-template system, and ZSM-23-Al2O3 composites with different ratios were also prepared. The hydroisomerization performance of Pt/ZSM-23 catalyst was manipulated by introducing Al2O3, and the influence of Al2O3 on physicochemical properties was investigated by XRD, SEM, TEM, N2 physical adsorption-desorption and NH3-TPD characterizations. The results showed that Al2O3 improved the dispersion of Pt, significantly reduced the acid sites concentration of the catalyst, and regulated the metal-acid balance in quantitative. The suitable metal-acid balance concentration could improve the selectivity of isomers and suppress the cracking reactions. Meanwhile, Al2O3 dispersed the ZSM-23 grains, which improved the dispersion and increased the number of exposed pores in ZSM-23. Thus the diffusion efficiency of reactants and intermediates could be promoted and the isomer products selectivity could be improved. All composite catalysts showed high selectivity of isomer products, among which, Pt/(9Z-1Al) had the highest yield of isomer products due to its suitable metal-acid concentration balance, reached 60% at 340 ℃, which was a significant improvement compared with Pt/ZSM-23 (42%). When the reaction temperature was lower than 310 ℃, the pore mouth mechanism dominated in Pt/ZSM-23, while the key-lock mechanism was significantly strengthened at higher reaction temperature. After the introduction of Al2O3, more adjacent pores in ZSM-23 were exposed and the key-lock mechanism became the domination, which led to a large number of 7/8Me-C15 isomers.
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Key words:
- ZSM-23 /
- Al2O3 /
- hydroisomerization /
- n-hexadecane
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图 2 ZSM-23、Al2O3及复合载体的SEM照片和相应的Al元素Mapping照片
Figure 2 SEM images of ZSM-23, Al2O3 and ZSM-23-Al2O3 composites, and the corresponding Al element mapping images
图 3 （a） Pt/ZSM-23、Pt/Al2O3及其复合载体催化剂的N2物理吸附-脱附等温线和 （b） NLDFT孔径分布
Figure 3 (a) N2 physical adsorption-desorption isotherms and (b) NLDFT pore size distributions of Pt/ZSM-23, Pt/Al2O3, and Pt catalysts supported on ZSM-23-Al2O3
图 5 Pt/ZSM-23、Pt/Al2O3及其复合载体催化剂的TEM图片、金属粒径分布图（（a）−（e））以及ZSM-23和γ-Al2O3的Zeta电位变化曲线（f）
Figure 5 TEM images and metal particle size distributions of Pt/ZSM-23, Pt/Al2O3 and the composite catalysts ((a)−(e)) and Zeta potential curves of ZSM-23 and γ-Al2O3 (f)
图 6 正十六烷在不同催化剂作用下的加氢异构化
Figure 6 Hydroisomerization of n-C16 over various catalysts: (a) Conversion of n-C16, (b) Selectivity of i-C16, and ((c)−(f)) yield of various products, including (c) total i-C16, (d) mono-branched, (e) multi-branched, and (f) cracked fractions
图 7 不同催化剂作用下多支链异构产物（MuB）与单支链异构产物（MB）比值随n-C16转化率的变化（a）及不同催化剂对应的异构体产物分布随温度的变化（b）−（f）
Figure 7 Ratio values of multi-branched (MuB) to mono-branched (MB) i-C16 versus conversion of n-C16 (a) and i-C16 product distribution with temperature (b)−(f) of catalysts
表 1 Pt/ZSM-23、Pt/Al2O3及其复合载体催化剂的结构性质
Table 1 Textural properties of Pt/ZSM-23, Pt/Al2O3, and Pt catalysts supported ZSM-23-Al2O3
Sample SBETa /
(m2·g−1)Smicrob /
(m2·g−1)Sextc /
(m2·g−1)vtotald /
(cm3·g−1)Pt/ZSM-23 221 119 102 0.78 Pt/(9Z-1Al) 219 95 124 0.79 Pt/(7Z-3Al) 189 68 121 0.70 Pt/(5Z-5Al) 137 21 116 0.60 Pt/Al2O3 60 0 60 0.09 a BET method, b t-plot method, c Sext = SBET−Smicro,
d vtotal determined at p/p0 = 0.99
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表 2 由NH3-TPD确定的催化剂酸性位浓度
Table 2 Concentration of acid sites of catalysts determined by NH3-TPD
Sample (Si/Al2)a Acidic sites/ (NH3 mmol·g−1)b weak medium strong total Pt/ZSM-23 79 0.124 0.061 0.108 0.293 Pt/(9Z-1Al) 52 0.110 0.033 0.069 0.213 Pt/(7Z-3Al) 29 0.096 0.029 0.062 0.187 Pt/(5Z-5Al) 21 0.076 0.020 0.050 0.146 Pt/Al2O3 − 0.041 0.006 0.019 0.066 a determined by ICP-AES, b determined by NH3-TPD and calculated by the deconvolution method
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表 3 催化剂的金属担载特性及酸性质
Table 3 Metal loading and acid properties of catalysts
Sample Pt wa/% dPtb /nm Dc /% CPtd /(μmol·g−1) CA /(μmol·g−1) CPt/CA Pt/ZSM-23 0.60 2.28 50 15 169 0.09 Pt/(9Z-1Al) 0.64 2.20 51 17 102 0.17 Pt/(7Z-3Al) 0.69 2.13 53 19 91 0.21 Pt/(5Z-5Al) 0.73 1.95 58 22 70 0.31 Pt/Al2O3 0.82 1.50 75 32 25 1.27 a determined by ICP-AES, b average size with standard deviations analyzed by more than 200 particles from TEM images, tolerance scope was ± 0.49 nm, which was obtained via normally distributed data statistics, c D = 1.13/dPt size[27], d CPt denotes density of surface Pt atoms, calculated by Pt loading and dispersion
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