以加氫脫氧、加氫裂化及加氫異構化從模型化合物和蓖麻油中生產生物燃料和生物噴氣燃料

Production of Biofuels and Bio Jet-Fuels from Model Compounds and Castor Oil via Hydrodeoxygenation, Hydrocracking and Hydroisomerization

索夫(Bhattacharjee, Saurav)

指導教授 ： 談駿嵩

國立清華大學/工學院/化學工程學系/博士(2021年)

摘要

在過去的幾十年，從非食物競爭來源之生物燃料和生物噴氣燃料作為日益減少的化石燃料儲備替代品一直在激增。然而，生物油中的大量含氧基團導致生物燃料的能量密度低、黏度高和穩定性差等不良燃料特性。加氫脫氧 (HDO) 是一種用於降低生物油氧含量的燃料升級過程，目前已成功將生物油直接轉化為燃料範圍的碳氫化合物。 HDO 傳統上涉及使用硫化的 Ni、Mo、Co 或負載的貴金屬（如 Pt 或 Pd）作為催化劑。然而，這些催化劑的穩定性差、HDO 選擇性、硫污染敏感性和高成本是 HDO 工藝的挑戰。因此，開發高選擇性但廉價且穩定的 HDO 催化劑是當前所需。此外，傳統的 HDO 反應是能源和成本密集型的，由於溫度、H2 壓力和時間等反應條件苛刻，因此需要更環保的操作條件來使 HDO 過程可進行。本論文中提出關於生產生物燃料和生物噴氣燃料的研究，試圖透過以下策略解決這些問題：
(1) 使用功能性和更環保的溶劑系統
(2) 透過選擇金屬、催化劑載體和催化劑合成方法開發和表徵新型多相催化劑
(3) 優化更適當的反應條件以最優化所需的產物選擇性
根據先前研究，在使用 Fe/SBA-15 催化劑對油酸進行加氫處理後，透過部分抑制涉及去除 CO2/CO 的競爭性脫羧/脫羰途徑，含有加壓 CO2更環保的己烷溶劑系統在最優化 HDO 選擇性方面非常有效，由勒夏特列原理，加壓的 CO2 進一步降低黏度和顆粒內擴散阻力，同時增加了催化劑的分散性、H2 溶解度和界面質傳速率。根據本研究結果，油酸轉化率為 100.0%，對 HDO 產物十八烷 (C18) 的選擇性最大為 83.3%，對脫羧/脫羰產物十七烷 (C17) 的選擇性最大為 8.7%。
在本論文中，透過使用開孔、3 維中孔泡沫 (MCF) 載體以及添加 Pd 和 Ni 作為功能金屬，結合濕浸漬法 (WI) 合成三金屬 Fe- Pd-Ni 催化劑。使用MCF作為載體材料，可以完全切斷脫羧/脫羰途徑，因為MCF的大孔徑和籠狀結構減少了顆粒內擴散阻力，從而減少了反應過程中的停留時間。在 278 ℃、40 bar H2 和 20 bar CO2 壓力下反應 4 小時，使用 Fe-Pd-Ni/MCF 由結果可得油酸轉化率為 100.0%，對 C18 的選擇性為 93.0%。由於 H2 在 Pd-Ni 合金奈米粒子 (NPs) 表面上的吸附和解離能力提高，使得H2 溢出到活性 Fe0 的表面上，因此提高了Fe-Pd-Ni/MCF的活性，並加快了反應速率。
蓖麻油在黏度和極性方面與其他生物油不同，因為它的主要成分是蓖麻油酸，一種羥基化蓖麻油酸的甘油三酯，這代表具有良好的冷流動性和潤滑性的蓖麻油的生物燃料。因此，冷榨蓖麻油被用作本研究中的原料。在溫度275 ℃、H2壓力40 bar 和 CO2壓力20 bar操作條件下，使用 Fe-Pd-Ni/MCF反應4 小時，獲得了 95.8% 的蓖麻油轉化率，對 C18 的選擇性為 65.9%。為了進一步提高蓖麻油的轉化率和 HDO 選擇性，故引入 TiO2 以開發熱穩定的 SiO2-TiO2 混合載體材料，能夠誘導強烈的金屬-載體相互作用。在WI上使用一步驟溶膠-凝膠催化劑合成方法，製備具有良好分散活性位點的SiO2-TiO2包封催化劑。使用 Fe-Ni-Pd@SiO2-TiO2 催化劑的蓖麻油 HDO 在溫度275 ℃、H2壓力40 bar 和 CO2壓力20 bar操作條件下，對 C18 產生 89.3% 的選擇性，持續反應 4 小時。透過以 33.3% Pd 濃度的 Au 取代 Fe-Ni-Pd@SiO2-TiO2 中的 Ni，結果產生明顯改善。使用 Fe-Au-Pd@SiO2-TiO2，在溫度250 ℃、H2壓力40 bar 和 CO2壓力20 bar條件下，即使使用高蓖麻油與催化劑的比例，也能在 3 小時內實現蓖麻油完全轉化，對 C18 的選擇性為 96.0%，重量比為 18.4。這種十八烷選擇性比使用 Fe-Ni-Pd@SiO2-TiO2 催化劑所得的選擇性高 72.6%，在相同的反應條件下，蓖麻油與催化劑的比率僅為 9.2。催化劑揭示了 Fe-Au-Pd@SiO2-TiO2 中不同類型的活性位點，協助催化蓖麻油的 HDO。單晶 X-射線衍射將 Au 中心四方雙錐晶體系統分配給 Fe-Au-Pd。 Fe-Au-Pd@SiO2-TiO2 具有由 Fe-Au-Pd 奈米粒子驅動所展現出優異的 H2 溢出能力，以及由 Pd-Au 相互作用通過電荷轉移產生的大量孤立的缺電子 Pdδ+ 表面物質。 Fe-Au-Pd@SiO2-TiO2 中的 Fe 組分以單獨的 Fe、雙金屬 Fe-Au/Fe-Pd 或三金屬 Fe-Au-Pd 奈米粒子的形式存在，這些Fe0奈米顆粒表面是主要的 HDO 位點。在一步法合成中可以實現高實際金屬負載量（Fe 為 96.7%），這為 HDO 提供了豐富的活性位點， Fe-Ni-Pd@SiO2-TiO2 和 Fe-Au-Pd@SiO2-TiO2 在至少五次循環中都具有活性和熱穩定性。
使用一步法合成的 Al 改性 MCF 包封的 Ni-Pd 催化劑（Ni-Pd@Al-MCF），透過同時加氫裂化和加氫異構化將來自蓖麻油的 HDO 的 C18 轉化為生物噴氣燃料。由實驗結果得知，雖然 Ni-Pd 負責加氫/脫氫，但酸性 Al-MCF 載體會催化加氫裂化及加氫異構化。由於生物噴氣燃料必須符合嚴格的監管標準 (ASTM D1655) ，因此加氫裂化和加氫異構化的最佳平衡至關重要。使用模型 C18，在溫度227 ℃、H2壓力57 bar 和CO2 壓力 20 bar下持續反應 2.5 小時，轉化率為 99.4%，對液體噴氣燃料產品 (LJFP) 的選擇性為 98.2%。本研究中的 C7-C17 與異正烷烴得到 1.1 的烷烴比，使用加壓的 CO2-己烷-水官能和更環保的溶劑系統，它不僅增加了顆粒內擴散速率，而且釋放了碳酸，碳酸在 C18 的酸催化加氫裂化/加氫異構化中發揮促進劑的作用。為了進行結果驗證，使用來自蓖麻油的 C18，獲得了 100% 的轉化率和 98.8% 的 LJFP 選擇性，且 Ni-Pd@Al-MCF 至少穩定了 8 個循環。
綜上所述，本論文中的研究提供了新的方法來提高 HDO 和加氫裂化/加氫異構化過程的可持續性和效率，參考模型油酸、C18、生物油以及蓖麻油，用於生產清潔燃燒的生物燃料和生物噴射燃料。透過溶劑選擇更環保的反應系統、比文獻中的反應條件有更適合、高實用金屬負載量的一步法直接催化劑合成，並以廉價過渡金屬及Fe 作為主要 HDO 位點的高度穩定之新型催化劑，此為本論文的突出發現，並與當前可比較技術水平相較之下有了顯著的進步。

關鍵字

加氫脫氧 ； 生物燃料 ； 生物噴氣燃料 ； 蓖麻油 ； 加壓二氧化碳 ； 加氫裂化 ； 加氫異構化 ； 多相催化

並列摘要

Exploration of biofuels and bio-jet fuels obtained from non-food competing sources as an alternative to rapidly dwindling fossils fuel reserves has been on the surge over the last few decades. However, large number of oxygen-containing groups in bio-oils result into undesirable fuel properties such as low energy density, high viscosity and poor stability in biofuels. Hydrodeoxygenation (HDO), a fuel upgrading process for lowering the oxygen content of bio-oils, has been investigated with success for converting bio-oils directly into fuel range hydrocarbons. HDO has traditionally involved the use of sulfided Ni, Mo, Co or supported noble metals such as Pt or Pd as catalysts. The poor stability, HDO selectivity, sulfur contamination susceptibility and high costs of these catalysts are, however, challenging impediments to the HDO process. Therefore, there is an immediate need to develop highly selective yet cheap and stable catalysts for HDO. Further, traditional HDO reactions being energy and cost intensive, due to severe reaction conditions of temperature, H2 pressure and time, greener operating conditions are desirable to make HDO processes sustainable. The research presented in this thesis for the production of biofuels and bio jet-fuels attempts to address these concerns through the following strategies:
(1) the use of functional and greener solvent systems
(2) development and characterization of novel heterogeneous catalysts through choice of metals, catalyst support and catalyst synthesis methods
(3) optimization of milder reaction conditions to maximize desired product selectivity
A previous publication by the author served as the prequel to the research presented in this thesis. A greener solvent system of hexane containing pressurized CO2 was reported as very efficient in maximizing HDO selectivity during the hydrotreatment of oleic acid using a Fe/SBA-15 catalyst through partial suppression of the competitive decarboxylation/decarbonylation pathways involving the removal of CO2/CO by obeying Le Chatelier’s principle. Pressurized CO2 further lowered viscosity and intraparticle diffusion resistance while increasing catalyst dispersion, H2 solubility and interphase mass transfer rate. 100.0% conversion of oleic acid with a maximum 83.3% yield of the HDO product, octadecane (C18) and 8.7% yield of the decarboxylation/decarbonylation product, heptadecane (C17) was reported.
In this thesis, the HDO selectivity during the hydrotreatment of oleic acid was further enhanced by the use of an open cell, 3-dimesional mesocellular foam (MCF) support and through the addition of Pd and Ni as functional metals to develop a trimetallic Fe-Pd-Ni catalyst synthesized using wet impregnation (WI). Using MCF as the support material, decarboxylation/decarbonylation pathway could be completely cut-off as the large pore size and cage-like structure of MCF curtailed intraparticle diffusion resistance, thereby reducing residence time during reaction. At 278 ℃, 40 bars H2 and 20 bars CO2 pressure for 4 h of reaction, 100.0% conversion of oleic acid with 93.0% yield of C18 was reported using Fe-Pd-Ni/MCF. An enhanced H2 spillover onto the surface of active Fe0 species as a result of an improved adsorption and dissociation capability of H2 on the surface of Pd-Ni alloy nanoparticles (NPs) was responsible for the activity of Fe-Pd-Ni/MCF in enhancing reaction rate.
Castor oil differs from other bio-oils in terms of viscosity and polarity owing to its major constituent ricinolein, a triglyceride of hydroxylated ricinoleic acid. This characterizes biofuels derived from castor oil with good cold flow and lubricity. Cold pressed castor oil was therefore identified and used as a real time feedstock in this research. At 275 ℃, 40 bars H2 and 20 bars CO2 pressure for 4 h, 95.8% conversion of castor oil with 65.9% selectivity towards C18 was obtained using Fe-Pd-Ni/MCF. To further enhance the conversion and HDO selectivity of castor oil, TiO2 was introduced to develop a thermally stable SiO2-TiO2 hybrid support material with the ability to induce strong metal-support interactions. A one-pot sol-gel catalyst synthesis method was used over WI, to produce a SiO2-TiO2 encapsulated catalyst with well-dispersed active sites. HDO of castor oil using a Fe-Ni-Pd@SiO2-TiO2 catalyst could produce 89.3% selectivity towards C18 at 275 ℃ 40 bar H2 and 20 bars CO2 pressure for 4 h. By substituting Ni in Fe-Ni-Pd@SiO2-TiO2 with Au at 33.3% of Pd concentration, results were substantially improved. Using Fe-Au-Pd@SiO2-TiO2, at 250 ℃, 40 bars H2 and 20 bars CO2 pressure, a complete conversion of castor oil with 96.0% selectivity towards C18 was obtained in 3 h even with a high castor oil to catalyst ratio (weight/weight) of 18.4. This C18 selectivity was 72.6% higher than what was obtained using a Fe-Ni-Pd@SiO2-TiO2 catalyst with only half the castor oil to catalyst ratio of 9.2 at the same reaction conditions. Catalyst characterization revealed different types of active sites in Fe-Au-Pd@SiO2-TiO2, acting synergistically to catalyse the HDO of castor oil. Single crystal X-ray diffraction assigned an Au centric tetragonal dipyramidal crystal system to the Fe-Au-Pd ensemble. Fe-Au-Pd@SiO2-TiO2 possessed impressive H2 spillover capability driven by the Fe-Au-Pd nanoparticles and a large number of isolated electron-deficient Pdδ+ surface species arising out of Pd-Au interaction via charge transfer. The Fe component in Fe-Au-Pd@SiO2-TiO2 existed either as individual Fe, bimetallic Fe-Au/Fe-Pd or trimetallic Fe-Au-Pd nanoparticles. The Fe0 surface of these nanoparticles was the primary HDO site. High practical metal loading (96.7% for Fe) could be achieved in one-pot synthesis, that provided abundant active sites for HDO. Both Fe-Ni-Pd@SiO2-TiO2 and Fe-Au-Pd@SiO2-TiO2 were active and thermally stable over at least five cycles.
The conversion of derivates obtained from HDO of castor oil to bio jet-fuel was carried out via simultaneous hydrogenation/dehydrogenation and hydrocracking/hydroisomerization, using a one-pot synthesized Al modified MCF encapsulated Ni-Pd catalyst (Ni-Pd@Al-MCF). It was observed that while Ni-Pd was responsible for hydrogenation/dehydrogenation, the acidic Al-MCF support catalysed hydrocracking/hydroisomerization. Since bio jet-fuels must comply with stringent regulatory standards (ASTM D1655), optimal balancing of hydrocracking and hydroisomerization was crucial. Using model C18, at 227 ℃, 57 bars H2 and 20 bars CO2 pressure for 2.5 h, 99.4% conversion with 98.2% selectivity towards liquid jet-fuel products (LJFPs), C7-C17 in this study, and a moderate iso:n-alkane ratio of 1.1 was obtained. A pressurized CO2-hexane-water functional and greener solvent system was used, which not only increased intraparticle diffusion rate but also released carbonic acid that acted as a promoter in the acid-catalysed hydrocracking/hydroisomerization of C18. As a validation, upon using the derivates obtained from HDO of castor oil, 100% conversion with 98.8% selectivity towards LJFPs was obtained. Ni-Pd@Al-MCF was stable for at least eight cycles.
To summarize, the research presented in this thesis provides novel approaches for increasing sustainability and efficiency of HDO and hydrocracking/hydroisomerization processes with reference to model oleic acid, C18 and a real time bio-oil, castor oil for producing clean burning biofuel and bio jet-fuel. By utilizing greener reaction systems through solvent selection, milder than reported reaction conditions, one-pot direct catalyst synthesis with high practical metal loading and highly stable novel catalysts with cheap transition metal, Fe as the primary HDO site, the salient findings of this thesis indeed present a significant advance over comparable current state of art.

並列關鍵字

Hydrodeoxygenation ； Biofuel ； Bio jet-fuel ； Castor oil ； Pressurized CO2 ； Hydrocracking ； Hydroisomerization ； Heterogeneous catalysis

參考文獻

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