介孔洞SBA-15負載之鎢鋯氧化物對正戊烷異構化反應活性之研究

Abstract

文獻上鎢鋯氧化物可以在低溫的反應條件之下，對於烷類的異構化反應扮演有效的固體酸的角色。然而，鎢鋯氧化物其本身的比表面積通常只有50 m2/g。而介孔洞材料SBA-15具有高比表面積 ( ~800 m2/g) 以及均勻的六角柱狀規則孔洞，再加上其本身的高水熱穩定性，使得它成為一個很好的觸媒載體。為了增加鎢鋯氧化物的比表面積，本篇論文利用不同方式的沉積沉澱法，將鎢鋯氧化物負載於介孔洞的SBA-15材料上，用來比較它們對於直鏈戊烷異構化反應的反應活性。探討不同的反應條件所合成的鎢鋯氧化物複合材料，譬如將鎢鋯氧化物負載於SBA-15的方式、氧化鎢和氧化鋯以及SBA-15三者之間的成分比例、此複合材料之鍛燒溫度，以及白金促進劑的添加與否。由反應的測試可知，利用超臨界乙醇共沉澱法所合成之鎢鋯氧化物複合材料，由低角度X光繞射得知其仍然保有原來SBA-15的規則六角柱狀排列。而且從氮氣吸附脫附實驗可知得到高表面積的觸媒材料。然而，若是以直接含浸法將鎢鋯氧化物負載於介孔洞SBA-15上，所合成的混合金屬氧化物顆粒會阻塞並填滿介孔洞SBA-15之孔道。所合成的複合材料對於直鏈戊烷異構化反應的反應過程發現，1wt%的白金當作促進劑添加到此複合材料以及反應過程中所供應的氫氣，對於此異構化反應是很必須要的兩個要素。以超臨界乙醇共沉澱法將鎢鋯氧化物負載於介孔洞SBA-15材料，其成分比例當氧化鎢對於氧化鋯對於SBA-15的重量比為1:2:2的時候，所得的複合材料對直鏈戊烷異構化反應有最大的反應活性。反應的主產物為異戊烷，其選擇率約70 %，而選擇率高達95 %以上。異構化反應過程之選擇率以及觸媒反應的失活狀態受到反應過程中所添加之氫氣含量而有所改變。

Zirconia loaded with tungsten oxide (abbreviated as WZ) has been reported to be an efficient solid catalyst at relatively low temperatures in the skeletal isomerization of alkane or benzoylation of toluene, which are usually catalyzed by strong acids. In order to increase the surface area of tungstated zirconia, which is usually around 50 m2/g, mesoporous SBA-15 with a uniform hexagonal array of channel-shaped mesopores and very high surface area (typically around 800 m2/g) was used as the support for preparing supported WZ catalysts. In the present study, the supported WZ catalysts were prepared by depositing precipitation methods, and the catalytic activities of the resultant materials were compared in n-pentane isomerization reaction. Factors of catalyst preparation such as the method of supporting WZ on SBA-15, the weight ratios of tungsten and zirconia to SBA-15, the calcination temperature of the composite materials, and the addition of Pt were examined. The catalyst prepared through supercritical ethanol was found to retain well ordered porous structure based on small angle X-ray diffraction measurements and high surface area by nitrogen sorption experiments. On the contrary, SBA-15 supported tungstated zirconia prepared by direct evaporation was found to have the mixed oxide particles block the pores of SBA-15. The impregnation of 1% Pt on the materials before used as the catalyst in n-pentane isomerization and the inlet of hydrogen flow during reaction were found to be essential. The SBA-15 supported WZ catalyst with the weight ratio of WO3 : ZrO2 : SBA-15 in 1: 2: 2 prepared by supercritical ethanol treatment showed the highest isomerization activity than the others. The main product of isomerization reaction was iso-pentane. The selectivity of the isomerization reaction and the deactivation of catalysts were also found to vary with the feed of hydrogen gas.