微波辅助水热合成条件对直接合成纳米HZSM-5晶体性质的影响

利用微波辅助水热合成法直接制备了纳米HZSM-5晶体.采用XRD、FT-IR、SEM、BET和NH3-TPD等手段对合成样品进行了分析表征,研究了晶化温度和时间对合成产物晶体性质的影响.结果表明,晶化温度和时间对微波辅助水热直接合成产物微观形貌、晶粒尺寸和分散度影响明显.较低的晶化温度和较短的晶化时间均难以形成形貌规则的HZSM-5晶体.随着晶化温度的升高,合成样品逐渐变为球形晶粒、晶粒尺寸逐渐增大、分散度逐渐提高,继续提高晶化温度达180 ℃时,晶粒长大使比表面积稍有降低;随着晶化时间的延长,样品的微孔和介孔增多,比表面积和孔容逐渐增大,继续延长晶化时间,晶体内微孔可能的收缩和晶粒的长大使得样品孔容和比表面积减小.160 ℃和1.5 h条件下制备的HZSM-5分子筛晶体形貌呈球形,晶粒尺寸约为60 nm,分散程度较好;其比表面积、孔容和平均孔径分别为398.45 m2·g-1、0.63 cm3·g-1和6.27 nm;晶体表面具有弱酸特征.

Influence of Microwave Hydrothermal Synthesis Conditions on Crystal Properties of HZSM-5 from Direct Synthesis

Nano HZSM-5 crystal was synthesized directly by microwave-assisted hydrothermal route.The obtained products were characterized by XRD, FT-IR, SEM, BET and NH3-TPD techniques, and the effect of various crystallization temperatures and time on the crystal properties of HZSM-5 were investigated.The results indicate that crystallization temperature and time has significant impact on the micromorphology, crystal size and dispersibility of the synthesized products.HZSM-5 crystals with regular morphology are hard to form at lower crystallization temperature or shorter crystallization time.As crystallization temperature increases, the synthetic sample gradually changes into spherical grains, the grain size increases and the degree of dispersion improved.When the crystallization temperature increases to 180 ℃, the grain size became larger and the specific surface area decreases slightly.With the extension of the crystallization time, the micropores, mesopores, specific surface area and pore volume of the samples increases.When the crystallization time was further prolonged, the possible shrinkage of the micropores in the crystal and the growth of the grains made the pore volume and specific surface area of the sample decrease.The HZSM-5 molecular sieve crystals with spherical crystal size about 60 nm and excellent dispersibility was synthesized at 160 ℃ for 1.5 h by microwave hydrothermal route, and the obtained HZSM-5 possessed distinct physicochemical properties including high specific surface area of 398.45 m2·g-1, pore volume of 0.63 cm3·g-1, average pore diameter of 6.27 nm and the weak acidity on the surface of crystal.