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Liquid-phase deoxygenation of free fatty acids to hydrocarbons using supported palladium catalysts.

機(jī)譯:使用負(fù)載的鈀催化劑將游離脂肪酸液相脫氧為烴。

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摘要

Hydrocarbon biofuels that are drop-in replacements for traditional petroleum-derived liquid fuels can be produced from edible and inedible fats and oils (triglyceride sources) via thermocatalytic processes. Liquid-phase deoxygenation of stearic acid (SA) in dodecane at 300°C and 15 atm was employed to screen supported noble metal catalysts for decarboxylation of free fatty acids to hydrocarbons. Commercial samples of Pt/C, Pd/C (4), Pd/Al2O3, and Pd/SiO2 catalysts and an in-house prepared Pd/SiO2 catalyst (each containing 5 wt.% metal) were screened under flowing 0, 5, and 10% H 2 (balance He). Under flowing He, most of the catalysts studied failed to achieve 100% SA conversion after 4 h under reaction conditions due to rapid deactivation. The exception was a uniformly impregnated Pd/C catalyst that gave >99% conversion in ∼1 h with 99% CO2 selectivity. All of the catalysts were far more stable under H2 yielding nearly complete SA conversion after 4 h; however, they differed markedly in their CO2 selectivities. Pd/SiO2 and Pt/C catalysts were selective toward decarbonylation (CO production), and Pd/C and Pd/Al2O 3 catalysts were selective toward decarboxylation. Even under H 2, the uniformly impregnated Pd/C catalyst was the most active and selective for the hydrogen-neutral decarboxylation pathway.;Semi-batch deoxygenation of SA employing this 5 wt.% Pd/C catalyst was investigated further using on-line quadrupole mass spectrometry. With fresh catalyst, SA deoxygenation under He occurred rapidly with very high CO 2 selectivity; however, reuse of the catalyst showed an orders of magnitude loss of decarboxylation activity and high decarbonylation selectivity. Experiments employing smaller amounts of fresh catalyst evidenced that decarboxylation activity under He is limited to ∼220 turnovers. Attempts to reactivate the used Pd/C catalyst by H2 treatment were only modestly effective. Increased catalyst lifetime (>2200 turnovers) was achieved by employing a H2-containing purge gas; however, the decarboxylation rate decreases with increasing H2 partial pressure resulting in lower CO 2 selectivity. Increasing the initial SA concentration also inhibited decarboxylation, substantially prolonging the batch time and yielding lower overall CO2 selectivity. The origin of this effect was traced to catalyst poisoning by endogenous CO from the decarbonylation pathway. Catalyst poisoning experiments demonstrated that CO strongly inhibits the decarboxylation pathway and that the inhibitory effects of CO and H2 are additive. Under conditions of strong decarboxylation inhibition, the decarbonylation rate was unaffected, and we infer that decarboxylation occurs over different catalytic sites than decarbonylation. An elementary reaction sequence for Pd-catalyzed decarboxylation is proposed which accounts for our observations.;Fed-batch deoxygenation of SA and oleic acid was demonstrated in a 50-mL stirred autoclave reactor with continuous feeding for run times up to 24 h. The maximum quasisteady state decarboxylation rate observed under 5% H 2 was 0.43 mmol/gcat˙min (0.078 s-1 turnover frequency). When higher H2 partial pressures were employed, an abrupt switchover in product selectivity from CO2 to CO was observed. Higher CO selectivity leads to increased H2 consumption due to hydrogenation of heptadecene, the primary product of the decarbonylation pathway. The on-stream time at which this switchover occurs was found to increase with decreasing H2 pressure. We infer that the switchover phenomenon arises from H2 inhibition of the decarboxylation pathway resulting in SA accumulation. SA accumulation increases the decarbonylation rate leading to further inhibition of the decarboxylation pathway by endogenous CO. Parametric studies involving SA feed rate, H2 partial pressure and exogenous CO partial pressure support the proposed switchover mechanism. Inhibition of decarboxylation activity was reversible at least in the short term by lowering the H2 or CO partial pressure or stopping SA injection; however, if a catalyst was aged >10 h under reaction conditions favoring decarbonylation, decarboxylation activity could not be recovered.
機(jī)譯:烴類生物燃料是傳統(tǒng)石油衍生液體燃料的直接替代品,可通過熱催化工藝從食用和不可食用的油脂(甘油三酸酯來源)中生產(chǎn)。硬脂酸(SA)在十二烷中于300°C和15 atm的液相脫氧用于篩選負(fù)載型貴金屬催化劑,以將游離脂肪酸脫羧成烴。在流動(dòng)0、5、5、5和5的條件下,篩選了Pt / C,Pd / C(4),Pd / Al2O3和Pd / SiO2催化劑以及內(nèi)部制備的Pd / SiO2催化劑(每種金屬含量為5 wt。和10%H 2(平衡He)。在流動(dòng)的氦氣下,由于快速失活,大多數(shù)研究的催化劑在反應(yīng)條件下4 h后未能達(dá)到100%的SA轉(zhuǎn)化率。唯一的例外是均勻浸漬的Pd / C催化劑,在約1小時(shí)內(nèi)轉(zhuǎn)化率> 99%,CO2選擇性為99%。在H2下,所有催化劑都更加穩(wěn)定,在4 h后幾乎可以完全轉(zhuǎn)化為SA。但是,它們的二氧化碳選擇性明顯不同。 Pd / SiO2和Pt / C催化劑對脫羰具有選擇性(生成CO),Pd / C和Pd / Al2O 3催化劑對脫羧具有選擇性。即使在H 2下,均勻浸漬的Pd / C催化劑對于氫中性脫羧途徑也是最具活性和選擇性的。;使用這種5 wt%Pd / C催化劑對SA的半間歇脫氧進(jìn)行了進(jìn)一步的在線研究四極質(zhì)譜。使用新鮮催化劑,在He下SA脫氧迅速發(fā)生,具有很高的CO 2選擇性。然而,催化劑的再利用顯示出脫羧活性和高脫羰選擇性的數(shù)量級損失。使用少量新鮮催化劑的實(shí)驗(yàn)表明,在He下的脫羧活性被限制在220左右。嘗試通過H2處理來重新活化用過的Pd / C催化劑只是適度有效。通過使用含H2的吹掃氣可以延長催化劑的使用壽命(> 2200周轉(zhuǎn))。然而,脫羧速率隨H2分壓的增加而降低,導(dǎo)致較低的CO 2選擇性。增加初始SA濃度也抑制了脫羧,大大延長了分批時(shí)間并降低了總的CO2選擇性。這種作用的起源可追溯到脫羰途徑中的內(nèi)源性CO引起的催化劑中毒。催化劑中毒實(shí)驗(yàn)表明,CO強(qiáng)烈抑制脫羧途徑,并且CO和H2的抑制作用是累加的。在強(qiáng)抑制脫羧的條件下,脫羰速率不受影響,我們推斷脫羧發(fā)生在與脫羰不同的催化位點(diǎn)上。提出了Pd催化脫羧的基本反應(yīng)順序,這解釋了我們的觀察結(jié)果。在50 mL攪拌高壓釜反應(yīng)器中證明了SA和油酸的分批進(jìn)料脫氧,連續(xù)進(jìn)料時(shí)間長達(dá)24 h。在5%H 2下觀察到的最大擬穩(wěn)態(tài)脫羧速率為0.43mmol / gcat·min(0.078s-1轉(zhuǎn)換頻率)。當(dāng)使用較高的H2分壓時(shí),觀察到產(chǎn)物選擇性從CO2突然轉(zhuǎn)變?yōu)镃O。較高的CO選擇性會(huì)由于脫碳途徑的主要產(chǎn)物七癸烯的氫化而導(dǎo)致H2消耗增加。人們發(fā)現(xiàn),隨著H2壓力的降低,這種切換發(fā)生的在流時(shí)間會(huì)增加。我們推斷,轉(zhuǎn)換現(xiàn)象是由于H2抑制了脫羧途徑而導(dǎo)致SA積累。 SA積累會(huì)增加脫羰速率,從而導(dǎo)致內(nèi)源CO進(jìn)一步抑制脫羧途徑。涉及SA進(jìn)料速率,H2分壓和外源CO分壓的參數(shù)研究支持了擬議的轉(zhuǎn)換機(jī)制。至少在短期內(nèi),通過降低H2或CO分壓或停止SA注入,脫羧活性的抑制作用是可逆的。然而,如果催化劑在有利于脫羰的反應(yīng)條件下老化> 10 h,則脫羧活性無法恢復(fù)。

著錄項(xiàng)

  • 作者

    Immer, Jeremy Glen.;

  • 作者單位

    North Carolina State University.;

  • 授予單位 North Carolina State University.;
  • 學(xué)科 Alternative Energy.;Engineering Chemical.
  • 學(xué)位 Ph.D.
  • 年度 2010
  • 頁碼 215 p.
  • 總頁數(shù) 215
  • 原文格式 PDF
  • 正文語種 eng
  • 中圖分類
  • 關(guān)鍵詞

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