丙烯在氧氣及氫氣共存下一步合成環氧丙烷

Abstract

本研究以Au/TS-1觸媒催化丙烯氣體進行環氧化反應，反應器進口通入O2及H2，奈米金顆粒催化兩氣體反應生成H2O2，然後以TS-1上的活性位置，催化H2O2氧化丙烯形成環氧丙烷。TS-1沸石擔體是由水熱法合成，並以沉澱沉積法擔載奈米金粒子。本研究目的為透過減少TS-1的顆粒大小，縮短內部孔徑長度，避免反應物因為在TS-1孔洞中的滯留時間過長而過度氧化，進而提高環氧丙烷生成比例；然而，較小尺度的TS-1顆粒卻存在極為顯著的團聚(aggregation)情況，因此，透過Tween 80的添加改善顆粒間的聚集現象，提高內部質傳，並進一步探討對Au/TS-1催化活性的影響。 本研究內容主要針對以下四個部分做探討：(1)探討金擔載量和GHSV (Gas Hourly Space Velocity)的變化對Au/TS-1活性的影響，以及在不同金擔載量下的金粒子活性差異。(2)透過改變水熱溶液中TPAOH用量，製備小顆粒的TS-1擔體，探討TS-1尺度差異對催化活性的影響。(3)透過硝酸預處理程序清除擔體表面Octahedral的Ti。(4)添加Tween 80改善擔體顆粒聚集的現象。 由第一部分研究結果顯示，金擔載量越高 (高於0.2 wt%)，越容易發生全氧化反應生成CO2，不利於環氧丙烷的生成；另一方面，當金擔載量低於0.2 wt%，金粒子的活性皆很相近。藉由降低GHSV，能提高丙烯的轉化率，進而提高環氧丙烷的產率，但當GHSV低於11200 mLkgcat-1h-1後，環氧丙烷的產率即無法再增加，因為GHSV較低，代表氣體滯留時間較長，產物較易進行全氧化反應生成CO2，造成環氧丙烷產率無法再提升。本研究發現在金擔載量0.2 wt%、GHSV=11200 mLkgcat-1h-1時，Au/TS-1觸媒能有最佳的產率6.26 %。 在第二部分，將水熱溶液中TPAOH的含量提高2倍，即可製備出顆粒大小僅原本一半的TS-1擔體 (命名為TS-1-2T)，由鑑定結果顯示，TS-1-2T之MFI結構、表面積、孔洞體積、骨架內Ti含量和整體Si/Ti莫耳比等性質，相較於TS-1沒有明顯差異，但是Octehedral Ti含量卻提高。在活性測試中， Au/TS-1-2T的環氧丙烷選擇率明顯較Au/TS-1來得低，很可能是因為表面上有較多的Ti以Octahedal形式存在，是促使副反應發生的活性位置，因此產生較多副產物。 在第三部分，為減少TS-1-2T中Octahedral的Ti含量，以2M硝酸在80℃高溫下清洗擔體 (命名TS-1-2T-HNO3)。由鑑定結果顯示，硝酸預處理程序確實有效降低擔體內部Octahedral的Ti含量。經由反應活性測試，在相同的丙烯轉化率下，Au/TS-1-2T-HNO3的環氧丙烷選擇率和氫氣使用效率皆明顯優於Au/TS-1和Au/TS-1-2T，且幾乎沒有副產物Acrolein的生成，顯示octahedral Ti的移除確實可有效避免副產物的產生，此外，這也意謂較短的擔體內通道能有效提升環氧丙烷的選擇率。然而，由於Au/TS-1-2T-HNO3的丙烯轉化率較低，造成其環氧丙烷產率仍低於Au/TS-1觸媒，透過降低GHSV(提高反應物於Au/ TS-1-2T-HNO3內部滯留時間)，不僅無法有效提升丙烯轉化率，環氧丙烷選擇率亦隨之減少。目前Au/TS-1-2T-HNO3的環氧丙烷選擇率雖優於Au/TS-1，但產率仍低，因此如何有效提升Au/TS-1-2T-HNO3的丙烯轉化率將是下一階段的研究目標。 在第四部分，透過Tween 80的添加，可以有效改善擔體顆粒聚集的現象，將200μm左右的顆粒團聚切割成50μm左右的團聚，並且增加顆粒之間的孔隙度，使得反應物較易接觸到觸媒表面。從實驗結果發現，有加Tween 80的觸媒，丙烯的轉化率都較低，但副產物的生成量皆明顯較高。由於Tween 80在鍛燒過程中燃燒造成熱量大量釋放，可能導致擔體內部tetrahedral Ti被移除於結構之外，形成defect sites，進而促使副產物的生成。整體而言，雖然添加Tween 80能有效改善顆粒團聚的問題，但觸媒表面上的defect sites將更嚴重的影響觸媒活性。

The research is mainly about one-step synthesis of propylene oxide from propene in the presence of oxygen and hydrogen, with Au/TS-1 as catalysts. The TS-1 zeolite is synthesized by hydrothermal method, followed by deposition-precipitation method to load gold nano-particles onto TS-1. In order to avoid complete oxidation of propylene, the objective of this study is to decrease the space time of reactants inside the channels of zeolite by reducing the particle sizes of TS-1. However, the aggregation of smaller TS-1 is severe. Therefore, Tween 80 is added to increase porosity among TS-1 particles and promote mass transfer inside the zeolite support. The result of this study is divided into the following parts: (1) The effect of gold loading and GHSV (Gas Hourly Space Velocity) on the activity of Au/TS-1 catalysts. (2) Preparation of TS-1 with smaller sizes by adjusting the TPAOH amount in hydrothermal solution. (3) Elimination of octahedral Ti from the surface of TS-1 support by HNO3 pretreatment. (4) Addition of Tween 80 to decrease the aggregation of TS-1 particles. From the experimental results of the first part, as gold loading became higher than 0.2%, less amount of propylene oxide was produced. In addition, more amounts of reactants were completely oxidized to CO2; as gold loading was less than 0.2 wt%, the activities of Au/TS-1 maintained almost unchanged with the decreasing of gold loadings. On the other hand, the propylene conversion as well as the yield of propylene oxide raised as GHSV decreased. However, as GHSV became lower than 11200 mLkgcat-1h-1, the yield of propylene oxide could not increase anymore. The reason is that propylene tended to completely oxidized as the contact time between the reactant gas and Au/TS-1 became longer. The best propylene oxide yield 6.26% could be achieved when the gold loading was 0.2 wt% and the GHSV was 11200 mLkgcat-1h-1. In the second part of our research, TS-1 with sizes only half of fresh one could be prepared by increasing two times amount of TPAOH in the hydrothermal sol-gel solution (designated as TS-1-2T). Based on the characterization results, the properties of MFI structure, surface area, pore volume, ratio of framework Ti as well as bulk Si/Ti molar ratio maintained unchanged. However, the increase of Octahedral Ti content was significant, which is considered as active sites for side reaction. In the activity test, the selectivity of propylene oxide of Au/TS-1-2T was apparently lower than that of Au/TS-1. In the third part of our research, HNO3 pretreatment was used to eliminate octahedral Ti contents. The as-pretreated support is designated as TS-1-2T-HNO3. From the activity result, the Au/TS-1-2T-HNO3 catalyst possessed much better propylene oxide selectivity and H2 efficiency than Au/TS-1 catalyst under the same conversion of propene, which suggested that the shorter channel of TS-1-2T was contributive to partial oxidation of propylene. However, the yield of propylene oxide was still smaller than Au/TS-1, owing to low propylene conversion. By decreasing GHSV, not only the propylene conversion but also the selectivity of propylene oxide could not be effectively promoted. Therefore, although Au/TS-1-2T-HNO3 possessed better propylene oxide selectivity than Au/TS-1, further works is required to improve the conversion of propene. In the last part, the addition of Tween 80 could successfully reduce the aggregation of TS-1 particles. From the characterization results, some cracks were found and aggregation of particles (~200 μm) was divided into smaller parts, together with more amount of porosity formed among particles. However, the activity results showed that the catalysts with addition of Tween 80 possessed lower propene conversion, accompanied with higher selectivity of side products. There might have been some defect sites formed during calcinations process, caused by enormous heat release from the combustion of Tween 80. Therefore, even though the addition of Tween 80 could reduce the aggregation problem among particles and improve mass transfer inside catalysts, the formation of defect sites might be more deleterious to activity of catalysts.