以表面改質鉬基觸媒由合成氣產製烷類及醇類之研究

Abstract

本研究利用鉬基(Mo-based) 觸媒針對合成氣進行轉化。研究中以氧化鋁(gama-alumina, γ-Al2O3)為載體，於還原狀態下將Mo-based觸媒以硫化氫(hydrogen sulfide, H2S) 硫化為硫化鉬(molybdenum disulfide, MoS2)，使成為MoS2/γ-Al2O3觸媒；亦以分子篩(Zeolite Socony Mobil – 5, ZSM-5) 做為觸媒載體，並於還原狀態下將Mo-based 觸媒以甲烷(methane, CH4) 碳化為碳化鉬(bata-molybdenum carbide, β-Mo2C)，使成為β-Mo2C/ZSM-5觸媒。應用此等觸媒探討不同反應狀態下觸媒對合成氣轉化效率之影響。 此外，本研究進一步選擇以鹼金族-鉀(potassium, K)及過渡金屬-釩(vanadium, V) 做為觸媒改質劑，將已合成之β-Mo2C/ZSM-5觸媒表面改質，探討對合成氣轉化率(XCO)及產物（如：烷類或醇類(alcohol, AOH)特別是乙醇(EtOH)）產率(yield, Y)及選擇率(selectivity, S)之影響。 以MoS2做為催化劑，分別於不同反應溫度(T)、合成氣進料mole比(H2/CO)、進氣流量(QG)及設定壓力(PST)之條件下，探討MoS2觸媒對於合成氣轉製醇類之催化效能。 結果顯示T = 523 K為使用MoS2觸媒作為催化劑時較適之反應溫度，配合H2/CO = 2以及較低之QG (300 cm3 min-1)和較高之PST (540 psig)可得到可接受度高之合成氣轉製效能（XCO = 9.6%，醇類產率(YAOH) = 9.6%，醇類選擇率(SAOH) = 65.9%）。 此外，當停留時間較短時(< 3.5 sec)，其合成氣轉化反應對於CO之反應階數n = 0、1或2之XCO幾乎無差異皆符合線性分佈（r2 = 0.938，0.945及0.951）；而當停留時間延長時(~40 sec)，反應階數n = 0、1或2之XCO即呈現明顯差異。此顯示反應在停留時間較短時，可簡化其反應階數為n = 0，可有利闡釋其反應機制。 以β-Mo2C做為反應系統之催化劑，配合同為Mo-based之MoS2之實驗結果，可知MoS2觸媒之XCO (0.6 - 9.6%)遠低於β-Mo2C觸媒(XCO = 10.6 - 22.4%)。其中合成氣於β-Mo2C觸媒轉製反應下，以T = 573 K時可獲得可接受度高之結果(XCO = 17.3%, YAOH = 4.7%)。而以K以及V 對β-Mo2C觸媒進行改質後，於K/Mo = 0.52及V/Mo = 0.049時，其觸媒可明顯提升合成氣轉化反應之XCO (28.8 及 26.2%)以及YAOH (18.1 及 12.0%)。 進一步以K及V同時對β-Mo2C觸媒表面改質為V-K-Mo2C觸媒進行合成氣轉化，可得到較K-Mo2C及V-Mo2C觸媒提升之結果，當V/K/Mo = 0.1/1/2時，且其主要生成產物為EtOH，其XCO = 35.1%，YAOH = 25.4%，SEtOH = 39.9%。 針對V-K-Mo2C觸媒轉製合成氣之特性，本研究進一步探討於催化系統添加水份之影響，由研究結果可知其XCO = 35.7%及YAOH = 26.8%皆獲得提升。而受到來自於添加水份所提供之H及O原子，以至於提升轉化程序中較高碳數之醇類生成，而使得EtOH於產物比例上降低，其SEtOH = 35.8%。 由Anderson-Schulz-Flory (ASF)產物分佈（mole分率(Mn) vs.碳數(n)）分析，可知β-Mo2C觸媒之產物無論是烷類或是醇類，皆是以線性分佈為主；而K-Mo2C、V-Mo2C及V-K-Mo2C觸媒之產物，則僅有烷類仍維持線性分佈，而醇類之ASF分佈於C1 (甲醇，MeOH)處發生明顯偏移。結果顯示經改質後之β-Mo2C觸媒，對於催化生成醇類的機制上發生了變化，且C1與C2-C4存在不同之反應機制，以至於發生此偏移。

Effects of reaction conditions on the production of alcohols (AOHs) and alkanes (Alk) from CO and H2, which can be obtained from the gasification of biomass, using molybdenum (Mo) based catalysts such as molybdenum sulfide (MoS2), molybdenum carbides (β-Mo2C) and potassium and vanadium modified β-Mo2C of K-Mo2C, V-Mo2C and V-K-Mo2C were studied. A high-pressure fixed packed bed (HPFPB) was employed to carry out the reaction. The results indicate that the conversion of CO (XCO, in C%) and specific production rates of alcohol (SPRAOH) and alkane (SPRAlk) (g h-1 gcat-1) are highly dependent on temperature (T). In T = 423–573 K, maximum yield of alcohols (YAOH, in C%) of 5.0% and SPRAOH of 7.9 mg h-1 gcat-1 occur at T = 523 K over MoS2 catalyst. In the meantime, well performance gives the selectivity of ethanol (SEtOH) of 52.0 C%. For the studies on varying H2/CO mole ratio (MH/C) from 1 to 4 at 523 K, the appropriate MH/C to produce EtOH is 2, giving higher ratios of SPRAOH/SPRAlk of 2.08 and YAOH/YAlk of 2.50 than those with other MH/C. As for varying the total gas flow rates (QG) of 300, 450, 600 to 900 cm3 min−1 tested at T = 523 K and MH/C = 2, the lower QG provides longer reaction time (or gaseous retention time, tR) thus offering higher XCO, however lower productivity. For setting pressure (PST) = 225–540 psi, a supply of higher pressure is equivalent to providing a larger amount of reactants into the reaction system. This thus suggests that the use of higher PST whould give both higher XCO and productivity. Effects of modification of β-Mo2C catalyst using potassium (K) and vanadium (V) on AOHs and Alks from the hydrogenation of CO were studied. Both K and V promoters show the effectiveness on the XCO, SPRAOH, (g gcat-1 h-1), and SAOH (in C%). At the K/Mo weight ratio of 0.52, the contributions of XCO and YAOH are as high as 28.8% and 18.1%, respectively. At the V/Mo weight ratio of 0.049, the XCO and YAOH are 26.2 and 12.0%, respectively. The corresponding XCO and YAOH for the case using β-Mo2C are 21.5% and 6.6%, respectively. The comparison indicates that both K and V are promoters for enhancing the XCO and YAOH, with K exhibiting more vigorous effect. The V-K co-modified β-Mo2C catalyst with V/K/Mo = 0.1/1/2 gives XCO of 35.1% and YAOH of 25.4% with SEtOH as high as 39.9%. The results reveal the synergistic enhancement of K and V on the XCO and YAOH, especially on the SEtOH. The presence of water induces the water-gas shift reaction (WGS) of CO which further contributes the formation of reactive radicals, thus enhancing the yields of alcohols. The XCO and YAOH can reach as high as 35.7% and 26.8%. The SEtOH may be retained at satisfactory value of 35.79%. Some ethanols are converted to higher alcohols of propanol and butanol due to the presence of H2O which contains H and O. The deviation of methanol (MeOH) from the linear Anderson-Schulz-Flory (ASF) distribution for the cases using K-Mo2C, V-Mo2C and V-K-Mo2C catalysts indicates that the reaction mechanisms of the chain-propagation of alcohols are different from those using sole β-Mo2C. This may be due to the functions of V or/and K on the mechanisms, such as the insertion and re-adsorption. For the alkanes, the ASF distributions are linear for the un-modified and modified β-Mo2C catalysts, revealing that reaction mechanisms of alkanes are not altered by the K and V added on the catalysts.