A comparative synthesis of ZSM-5 with ethanol or TPABr template: distinction of Brønsted/Lewis acidity ratio and its impact on n-hexane cracking†,Catalysis Science & Technology

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A comparative synthesis of ZSM-5 with ethanol or TPABr template: distinction of Brønsted/Lewis acidity ratio and its impact on n-hexane cracking†
Catalysis Science & Technology ( IF 4.4 ) Pub Date : 2018-02-27 00:00:00 , DOI: 10.1039/c7cy02418e
Tong Ma _{1,

2,

3,

4,

5} , Luoming Zhang _{1,

2,

3,

4,

5} , Yu Song _{1,

2,

3,

4,

5} , Yunshan Shang _{1,

2,

3,

4,

5} , Yanliang Zhai _{1,

2,

3,

4,

5} , Yanjun Gong _{1,

2,

3,

4,

5}

Affiliation

Two kinds of ZSM-5 zeolites with various SiO₂/Al₂O₃ ratios were synthesized using ethanol (EtOH) or tetrapropylammonium bromide (TPABr) as the template respectively and their physicochemical properties were investigated extensively. Different ZSM-5 morphologies were observed for each kind of sample, the former with coffin-shaped single crystals and the latter with aggregates assembled from nanocrystals. The desilication process occurred and intensified at a later period of crystallization for ZSM-5 by EtOH with increasing SiO₂/Al₂O₃ ratio and/or increasing alkalinity, due to its weaker pore-filling effect. Specifically, ZSM-5 zeolites by EtOH had a large amount of Brønsted acid sites (BAS) and a much less amount of Lewis acid sites (LAS), leading to an ultra-high B/L ratio (45–66); while those by TPA⁺ with comparable acidity possessed a usual B/L ratio (6–8). Owing to no octahedrally coordinated Al in both kinds of ZSM-5, the amount of LAS should be attributed to the tri-coordinated Al “defect” sites, in accordance with the extra silanols determined by OH-IR. Further, ZSM-5 by EtOH exhibited fewer internal defective Al sites, which was responsible for its small number of LAS. Applied to n-hexane cracking, the ZSM-5 zeolite by EtOH had high activity with twice the lifetime (60 vs. 35 h) compared to that by TPA⁺ with a similar SiO₂/Al₂O₃ ratio. The ratio of accumulated amount of coke to converted n-hexane was obtained as a measure of average coke selectivity (S_c). The ZSM-5 zeolites by EtOH displayed less than half the S_c value (0.45) in comparison with those by TPA⁺ (1.08), due to the big difference in LAS, regardless of their SiO₂/Al₂O₃ ratio, morphological structure and reaction conditions. Thus, for ZSM-5 possessing a high B/L ratio improves the catalytic activity and decreases coke deposit through suppressing the dehydrogenation process of the reaction intermediate to form coke precursor.

中文翻译：

ZSM-5的比较合成用乙醇或TPABr模板：的布朗斯台德/路易斯酸性比的区别及其对冲击Ñ正己烷裂化†

以乙醇（EtOH）或溴化四丙铵（TPABr）为模板分别合成了两种SiO ₂ / Al ₂ O ₃比的ZSM-5分子筛，对其理化性质进行了广泛的研究。每种样品观察到不同的ZSM-5形态，前者具有棺材状单晶，而后者具有由纳米晶体组装而成的聚集体。随着SiO ₂ / Al ₂ O _3的增加，EtOH在ZSM-5结晶的后期发生了脱硅过程并加剧了硅化过程。由于其较弱的孔填充作用，因此具有较高的比例和/或增加碱度。特别是，EtOH制备的ZSM-5沸石具有大量的布朗斯台德酸位点（BAS）和少量的路易斯酸位点（LAS），从而导致超高的B / L比（45-66）。TPA ⁺酸度可比的那些具有通常的B / L比（6-8）。由于在两种ZSM-5中均没有八面体配位的Al，因此应根据OH-IR确定的多余硅烷醇，将LAS的含量归因于三配位的Al“缺陷”位点。此外，EtOH的ZSM-5表现出较少的内部缺陷Al位点，这是其少量LAS的原因。施加到Ñ正己烷裂化，所述ZSM-5沸石由EtOH中具有高的活性，寿命的两倍（60对比35 h）相比，TPA ⁺具有相似的SiO ₂ / Al ₂ O ₃比。焦炭累积量的经转换的比率Ñ，得到平均焦炭选择性（的量度-己烷小号_Ç）。与TPA ⁺（1.08）相比，EtOH制备的ZSM-5沸石的S _c值（0.45）不到一半，这是因为LAS的差异很大，而与SiO ₂ / Al ₂ O ₃无关比例，形态结构和反应条件。因此，对于具有高B / L比的ZSM-5，通过抑制反应中间体形成焦炭前体的脱氢过程，提高了催化活性并减少了焦炭沉积。

更新日期：2018-02-27

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