Effect of silicon source on synthesis and physicochemical properties of MCM-22 zeolite
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摘要: 在动态水热条件下，研究了硅溶胶、白炭黑、硅酸及硅胶为硅源时对MCM-22分子筛合成及物化性质的影响。以硅溶胶、白炭黑、硅酸三种硅源均可合成出高结晶度且无杂晶的片状MCM-22分子筛，其平均粒径分别为190、220和750 nm。硅源影响分子筛的聚集形态，三种硅源分别形成晶粒分散、晶粒半分散及晶粒聚集形态。三组样品的酸强度分布基本一致，都具有较多的中强酸分布，由硅溶胶和硅酸所得MCM-22分子筛在中强酸范围具有更高的B/L酸比值，以白炭黑合成的分子筛总酸量最高。NMR结果表明，样品中的铝以骨架铝为主，不存在明显的非骨架铝。由于硅胶对合成体系中游离水的吸附作用，水热反应难以发生，不能得到MCM-22分子筛，硅胶作为分子筛合成硅源时需要选择合适的反应条件。Abstract: The effects of silicon source on the synthesis and physicochemical properties of MCM-22 zeolite were studied by using silica sol, fumed silica, silicic acid and silica gel as synthesis materials under dynamic hydrothermal conditions. MCM-22 zeolite with high crytallinity and purity was synthesized by using silica sol, silica, silicic acid and silica gel as silicon sources. The average sizes of zeolite particle are 190, 220 and 750 nm, respectively. Three silicon sources can affect the aggregate morphology of the zeolites, which are grain dispersion, grain semi dispersion and grain aggregation. The distribution of acid strength of the three groups of samples are almost same, all of them have more medium and strong acidity, and the total acid content of zeolite synthesized by fumed silica is the highest. MCM-22 obtained from silica sol and silicic acid has higher B/L acid ratios in the range of medium strong acid. The results of NMR show that the main aluminum in the samples is skeleton aluminum without obvious non-skeleton aluminum. MCM-22 zeolite can not be obtained when silica gel is used as silicon source because silica gel can adsorb the free water in the preparation system, suppressing the hydrothermal reaction. The reaction conditions should be carefully set up when silica gel is used as silicon source to prepare zeolite.
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Key words:
- silica sol /
- fumed silica /
- silicic acid /
- zeolite /
- MCM-22
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图 1 采用不同硅源合成样品的XRD谱图
Figure 1 XRD patterns of the as-synthesized samples using different silicon sources
图 2 采用不同硅源合成样品的SEM(左列)与TEM(右列)照片
Figure 2 SEM (left column) and TEM (right column) images of the as-synthesized samples
(a1) and (a2), (b1) and (b2), (c1) and (c2) by sillica sol, fumed sillica and silicic acid, respectively
图 3 以硅溶胶，白炭黑，硅酸为硅源合成的MCM-22分子筛的N2吸附-脱附曲线
Figure 3 N2 adsorption-desorption isotherms of the MCM-22 synthesized with silica sol, fumed silica and silicic acid, respectively
图 4 以不同硅源合成的MCM-22分子筛的NMR 29Si(a)及27Al谱图(b)
Figure 4 29Si MAS NMR(a) and 27Al MAS NMR(b) of the MCM-22 synthesized using different silicon sources
表 1 不同硅源合成的MCM-22分子筛的比表面积和孔容
Table 1 Surface area and pore volume of the MCM-22 synthesized using different silicon sources
Silicon source Product Size d/nm Specific surface area S/(m2·g-1) Pore volume v/(mL·g-1) Stotal Sinner Souter vmicropore vtotal Sillica sol MCM-22 190 562 408 154 0.179 0.435 Fumed sillica MCM-22 220 522 395 127 0.180 0.455 Silicic acid MCM-22 750 486 377 109 0.174 0.478 
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表 2 不同硅源合成的MCM-22分子筛的SiO2/Al2O3比及样品收率
Table 2 SiO2/Al2O3 ratios of the MCM-22 synthesized using different silicon sources
Silicon source Feed Sillica sol Fumed sillica Silica gel SiO2/Al2O3 30 24.5 25.2 25.7 Yield w/% 71.4 69.7 70.3
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表 3 由不同硅源合成样品的酸强度分布
Table 3 Acid strength distribution of the samples synthesized using different silicon sources
Acid strength
distributionMCM-22
by silica solMCM-22
by fumed silicaMCM-22
by silicic acidcontent B/L content B/L content B/L Total acid(160℃)/(mmol·g-1) 0.95 2.92 1.10 1.99 0.91 3.27 160-250℃(weak acid) 19.4% 1.14 18.3% 0.71 17.4% 2.18 250-450℃(medium acid) 37.6% 6.25 36.1% 1.58 38.2% 5.33 450℃(strong acid) 43.0% 2.96 45.6% 4.17 44.40% 2.71
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