奈米非晶態CoNiB雙金屬觸媒的製備與氫化探討

陳世文(Shr-Wen Chen)

指導教授 ： 陳吟足

國立中央大學/工學院/化學工程與材料工程學系/碩士(2006年)

摘要

本研究採化學還原法製備CoNiB觸媒，並引入水溶性高分子PVP，製備PVP-CoNiB觸媒。藉液相丁醛氫化模式反應有系統探討CoNiB及PVP-CoNiB觸媒製備變因，以ICP、BET、TEM、XRD、DSC、XPS瞭解其物理性質，藉糠醛(含呋喃環外羰基及環內烯基鍵)、巴豆醛(含共軛烯基-羰基鍵)及檸檬醛(含共軛烯基-羰基鍵及一孤立烯基鍵)的選擇性氫化反應瞭解雙金屬觸媒之催化特性，並與活性及選擇性都優於倫尼金屬之NiB與CoB觸媒比較。
CoNiB與PVP-CoNiB觸媒最佳製備條件為以醋酸鹽類為前驅鹽，金屬莫耳比例Co/Ni = 5/5，硼氫化鈉水溶液進料速率2.6 ml/min，水溶性高分子PVP添加莫耳比例為PVP/Metal = 2~5 。
CoNiB雙金屬觸媒粒徑及粒徑分佈(3.8~5.6 nm)都比NiB(15~50 nm)及CoB(5.5~9.4 nm)觸媒小，添加PVP高分子穩定劑能讓PVP-CoNiB觸媒的粒徑更小更均一。CoNiB雙金屬觸媒整體組成中金屬對硼之元素比值，與NiB及CoB單金屬觸媒比值相近，引入高分子PVP並不影響PVP-CoNiB組成中金屬對硼之元素比值。CoNiB與PVP-CoNiB雙金屬觸媒組成分佈均勻，表面Co/Ni比例與整體組成相近。觸媒經不同溫度熱處理之XRD分析與DSC晶化分析發現，CoNiB並非為CoB與NiB的物理混合物。CoNiB、PVP-CoNiB皆為非晶態觸媒，PVP-CoNiB觸媒熱穩定性最佳，CoNiB觸媒與CoB觸媒次之，NiB觸媒最差。
以最佳比例之CoNiB(5/5)觸媒於丁醛、巴豆醛、糠醛及檸檬醛等液相氫化反應，活性皆優於NiB、CoB觸媒，且引入高分子PVP，以莫耳比PVP/Me = 2~5即達保護觸媒效果，避免顆粒聚集成長，製
得顆粒更小更均一的PVP-CoNiB觸媒，活性優於CoNiB觸媒。
丁醛與糠醛氫化反應，皆屬單一官能基C=O鍵的氫化。CoNiB觸媒活性為NiB及CoB觸媒的2~3倍，PVP-CoNiB觸媒又為CoNiB觸媒的1.7~2倍。糠醛氫化反應中，PVP-CoNiB觸媒對糠醇選擇率更高於CoNiB觸媒，幾無開環產物。
巴豆醛及檸檬醛選擇性氫化反應，皆屬共軛C=C/C=O鍵的選擇性氫化。於相同反應條件下，CoB觸媒活性不佳，CoNiB及PVP-CoNiB觸媒的催化特性較偏向NiB觸媒，都優先選擇氫化共軛C=C/C=O中的C=C鍵成丁醛及香茅醛，CoNiB觸媒活性為NiB觸媒的2倍，PVP-CoNiB觸媒又為CoNiB觸媒的1.6~1.8倍以上。
檸檬醛氫化反應，反應溶劑影響觸媒活性也影響產物的產率，以環己烷為溶劑，對香茅醛產率較高，但活性不佳；以甲醇或乙醇為溶劑，活性較佳，但對香茅醛產率較低。若以香茅醛為主要產物，則以乙醇為溶劑，在低溫(30℃)下進行反應可獲得香茅醛最大產率(86%)；若以香茅醛與香茅醇為主要產物，則以甲醇為溶劑，在短時間內即可獲得最大香茅醛與香茅醇產率(97%)。

關鍵字

觸媒

並列摘要

The PVP-stabilized CoNiB catalysts were prepared using the chemical reduction method with NaBH4, dissolving the water-soluble polymer of polyvinylpyrrolidone (PVP) in the precursor salt solution as a protective reagent. The PVP-CoNiB catalysts were characterized and examined for their catalysis on the hydrogenation of furfural, crotonaldehyde and citral. PVP polymer could adsorb on the nano-particles of CoNiB via a weak coordination bonding and stabilize it; the molecular weight of PVP about 10,000 was suitable, and the optimal quantity of PVP (PVP/Ni+Co) in the salt solution for preparing catalysts was around 2~10. The PVP-CoNiB samples were characterized by XRD as an amorphous structure and by TEM with a particle size distribution in the range of 3–5 nm. On catalysis, the PVP-CoNiB catalyst was significantly more active and slightly more selective than NiB for hydrogenating furfural to furfuryl alcohol and crotonaldehyde to butyraldehyde. A good yield of citronellal about 87% could be obtained by reducing citral in ethanol at a low reaction temperature of 30oC over the PVP-CoNiB catalyst.

並列關鍵字

catalyst

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