摘要
本文分别以介孔材料SBA-15和改性SBA-15(SBA-15-NH2)为载体,用浸渍法和原位合成法制备了一系列负载型磷钨酸催化剂,运用傅里叶变换红外光谱(FT-IR)、透射电镜(TEM)、X-射线衍射(XRD)、N2-吸附脱附、NH3-TPD等分析手段对所制备的催化剂进行表征,并且通过催化合成苹果酯的反应评价催化剂的催化活性、选择性及重复使用性能。
以SBA-15为载体,采用浸渍法制备了一系列不同负载量的负载磷钨酸催化剂HPW/SBA-15,并对其进行FT-IR、TEM、XRD、N2-吸附脱附及NH3-TPD分析。结果表明,浸渍法制备的催化剂中磷钨酸仍然保持Keggin型结构,磷钨酸成功进入SBA-15孔道内部,在高负载量下,磷钨酸在SBA-15孔道内部发生晶相聚集,介孔孔道发生堵塞使比表面积、孔容和孔径明显减小, HPW/SBA-15的酸性较纯磷钨酸略有下降。此外,浸渍法制备的HPW/SBA-15催化剂在催化苹果酯合成反应中显示出了很高的活性及较好的选择性,但是重复使用效果不佳,在最佳工艺条件下重复使用6次以后,反应转化率仅为30%。
分别采用灼烧和溶剂萃取去除模板法制备SBA-15,使用氨丙基三甲氧基硅烷(APTES)对SBA-15孔道表面进行氨基改性,然后采用浸渍法制备催化剂HPW/NH2/SBA-15,FT-IR、TEM、XRD、N2-吸附脱附及NH3-TPD分析结果表明,磷钨酸在载体表面高度分散没有晶相产生,同时仍然保持了Keggin型结构特征。溶剂萃取去模板法制备的SBA-15表面比灼烧法制备的载体富含羟基,因此能嫁接更多的氨基,进而能负载更多的磷钨酸,并且磷钨酸分散性更好。该法制备的催化剂显示出了较好的重复使用性能,重复使用6次,反应转化率一直维持在50%左右,但是由于氨基与磷钨酸较强的化学作用导致催化剂酸性降低,进而导致催化剂的活性不高。
利用原位合成法在SBA-15的合成过程中加入磷钨酸制备负载催化剂DS-HPW/SBA-15,FT-IR、TEM、XRD、N2-吸附脱附及NH3-TPD分析结果表明,该法制备的催化剂中磷钨酸高度分散没有晶相峰产生,依然保持了Keggin型结构特征峰,在较低负载量时催化剂仍然保持了SBA-15长程有序的规整孔道结构,在负载量达到33.3%时,催化剂高度有序的结构遭到破坏。溶脱实验表明,该催化剂具有较好的水热稳定性,同时,在催化苹果酯合成反应中显示出了高活性和较好的重复使用性,重复使用6次以后,反应转化率为80.3%。
12-Tungstophosphoric acid was supported on SBA-15 and amine-modified SBA-15 by impregnation and direct synthesis . The structure and properties of the catalysts were characterized by FT-IR, XRD, N2 adsorption-desorption, TEM, and NH3–TPD. And the catalytic activity, selectivity and reusability was evaluated by catalytic synthesis of fructone.
A series of different loading of catalysts expressed as HPW/SBA-15 were prepared by impregnating SBA-15 with HPW, and were characterized by FT-IR, XRD, N2 adsorption-desorption, TEM, and NH3–TPD. The result showed that the Keggin structure of the heteropolyanions was Maintained, but the Acid of the catalysts was reduced. And the BET surface area, pore volume and pore diameter decreased beacause the tungstophosphoric acid entered the pore, and in the high-loading amount aggregation occurred. The catalysts exhibited excellent catalytic activity and selectivity except for reusability, and the reaction conversion dropped to 30% after reused for six times.
Templates were Removed by calcination and extraction, and the prepared SBA-15 was modified by APTES, and 12-tungstophosphoric acid was supported on amine-modified SBA-15 by impregnation. The structure and properties of the catalyst were characterized by FT-IR, XRD, N2 adsorption-desorption, TEM, and NH3–TPD. The result showed that the Keggin structure of the heteropolyanions was maintained and the tungstophosphoric acid dispersed highly. The surface of SBA-15 prepared by extracting template contains more silanol groups and can combine more amino and phosphotungstic acids. The catalysts exhibited excellent reusability but low activity beacause the acidity of the catalysts were reduced.
SBA-15 ordered mesoporous silica materials containing of HPW heteropoly acid were synthesized via involving HPW in the SBA -15 synthesis process. The results of FT-IR, XRD, N2 adsorption-desorption, TEM, and NH3–TPD showed that the Keggin structure of the heteropolyanions was maintained and the tungstophosphoric acid dispersed highly. The catalysts maintained the ordered mesoporous structure in the low-loading amount, but the ordered structure damaged in the loading amount of 33.3%. The catalysts exhibited excellent catalytic activity, selectivity and reusability, and the reaction conversion reached up to 80.3% after reused for six times.
引文
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