Multiple Alcohol Liquid Phase Reforming Catalysts

With decreased crude-oil reserves, enhanced demand for fuels worldwide, increased climate concerns about the use of fossil-based energy carriers, the focus has recently turned towards improved utilization of renewable energy resources. Biomass has attracted much attention, because it is an abundant and carbon-neutral renewable energy resource, which can be used for the production of biofuels and valuable chemicals.Recently, Dumesic and coworkers reported a process to generate hydrogen, alkanes and syngas by aqueous-phase reforming (APR) of oxygenated hydrocarbon, such as ethylene glycol, glycerol, glucose, and sorbitol. Compared with the existed steam reforming process, the APR process has the advantages of higher energy efficiency, lower operating temperature, and broader range of safe liquid feedstocks.Ni shows high activity for C-C bond cleavage and moderate activity for water-gas shift (WGS) reaction among the VIIIB metals, and is less costly than Pt, so it has attracted much attention in the APR of polyols. Co-base catalysts, which also have cost superiority and moderate activity for C-C bond cleavage and WGS reaction, haven't been exploited in the field of APR of polyols. In the present work, we carried out the APR of ethylene glycol and glycerol over the Ni-based and Co-based catalysts, and obtained the following results.1. Aqueous phase reforming of ethylene glycol to H2 over Sn-RQ NiMo catalystsThe rapidly quenched skeletal Ni-Mo (RQ Ni50-xMox) catalysts were prepared by alkali leaching of RQ Ni50-xMoxAl50 alloy and investigated in the APR of ethylene glycol. Addition of Mo to the RQ Ni catalyst improved the activity remarkably, but the H2 selectivity decreased with the increment of Mo content. Ni2Al3 phase was easier to be leached at the presence of Mo, which resulted in smaller Ni crystallite size. The reason for the decreased H2 selectivity is that Mo can improve the activity of the methanation reaction. It has been reported that addition of Sn to Raney Ni catalyst can improve the H2 selectivity but suppress the activity. Thus, we modified the RQ Ni47.8Mo2.2 catalyst with SnCl4, SnCl2 and Sn(n-C4H9)4. The addition of Sn to RQ catalyst drastically enhanced the H2 selectivity, and SnCl4 has the best modification efficiency. The selectivity to H2 reached 91.1%at Sn/Ni atomic ratio of 2%. Moreover, when the Sn/Ni atomic ratio was 5%, the selectivity to H2 was above 95%, while the formation of alkanes was substantially suppressed. H2-TPD results showed that the number of desorption peaks reduced after Sn modification, suggesting that the active sites over the catalysts became more uniform, which may have decreased the adsorption modes of the reactant and suppressed side reactions. According to the kinetic results of the APR of ethylene glycol, we concluded that the addition of Sn to RQ Ni47.8Mo2.2 improved the H2 selectivity by promoting the WGS reaction while suppressing the methanation reaction. For the catalysts modified by the same Sn species, higher dispersion of Sn in the catalysts resulted in higher catalytic activity. The stability tests over Sn(IV)-RQ Ni47.8Mo2.2 and Sn(IV)-RQ Ni catalysts showed that Sn(IV)-RQ Ni47.8Mo2.2 showed higher activity and stability than Sn(IV)-RQ Ni catalyst, and its H2 selectivity catalyst was above 99% during 100 h reaction, indicating that this catalyst is a good candidate in fuel-cell system.2. Aqueous phase reforming of ethylene glycol to H2 over CONi/Al2O3 catalystsDumesic et al. found that the Ni/Al2O3 catalyst deactivated rapidly under the APR condition. However, bimetallic supported Co-Ni catalysts displayed an excellent catalytic performance in carbon dioxide reforming of methane. We prepared the CoNi/y-Al2O3 catalysts by the coimpregnation method and studied their catalytic performance in the APR of ethylene glycol. We obtained the best acitivity over Co67Ni33 catalyst with H2 and alkane selectivity of 47% and 33%, respectively. The formation of spinel-type NixCo3-xO4 phase after calcination reduced the metal-support interaction, thus leading to higher reduction degree of bimetallic CoNi catalysts. The electron transfer of Co→Ni was found in the CoNi catalysts by XAFS, and the highest transfer degree was obtained on Co67Ni33 catalyst. The stability tests of Co100, Co67Ni33 and Ni100 catalysts showed that Co67Ni33 catalyst displayed the best activity and stability. Both Co100 and Ni100 catalysts deactivated quickly, while Co67Ni33 remained 86%of its initial activity after 150 h time on stream. After reaction the Co 100 catalyst was seriously sintered (dCo> 120 nm). The BET surface area of the Ni100 catalyst decreased drastically from 101 to 40 m2·g-1, indicating that the destruction of the structure resulted in the decreased activity. On the contrary, Co67Ni33 catalyst retained the structure and the metal dispersion after reaction. In addition, we added the third metal to Co67Ni33 catalyst to improve the H2 selectivity. The addition of Sn increased the H2 selectivity remarkably but drastically suppressed the activity. Adding Au to Co67Ni33 enhanced the activity but hardly influenced the selectivity. The addition of Pt can improve both H2 selectivity and activity.3. Aqueous phase reforming of glycerol over CoNi/Al2O3 catalystsWe carried out the APR of glycerol on the CoNi/Al2O3 catalysts, which displayed an excellent catalytic performance in the APR of ethylene glycol. The Co/Ni ratio had a great influence on the activity, and the best activity and H2 selectivity were obtained on the Co50Ni50 catalyst. However, we found that the Co67Ni33 catalyst have superior stability to Co50Ni50. The characterization results showed that after reaction Co67Ni33 catalyst had smaller crystallite size and higher active surface area than Co50Ni50 catalyst. Since acid species were formed in the reaction, the leaching of the metal is another reason for the decreased activity. ICP results showed that the leaching of Co is much more serious than that of Ni, and the leaching of Co in Co67Ni33 is higher than that in the Co50Ni50 catalyst. Thus the Co/Ni ratio of the Co67Ni33 and Co50Ni50 catalysts after reaction turned into 60/40 and 39/61, respectively. The reaction pathway of glycerol was discussed based on the APR of methanol, ethanol, ethylene glycol and 1,2-dipropanol. The reaction pathway of glycerol can be divided into two parts:(1) Glycerol can be converted into H2, CO2 and alkanes by C-C cleavage followed by WGS or methanation/FTS reaction. (2) Glycerol can be converted into 1,2-dipropanol by dehydration to acetol and subsequent hydrogenation. 1,2-dipropanol will undergo further reaction to produce gas products and ethanol at high conversion levels. In addition, ethanol can be transformed into acetic acid under the reaction condition.We also studied the catalytic conversion of glycerol into glycol by the APR process. We first investigated the APR of glycerol on M/Y-Al2O3 (M= Ru, Cu, Fe, Co, Ni) catalysts. For the Fe catalyst, no glycol was found in the APR of glycerol. Both Ni and Ru catalysts have good glycerol conversion but poor glycol selectivity. Since Ni and Ru show high activity to C-C cleavage, the selectivity to gas products is much higher than that of other catalysts. Co and Cu have low activity, and the main product on these two catalysts is acetol. The poor ability to convert acetol to 1,2-dipropanol may be attributed to the poor reforming activity of Co and Cu towards glycerol. Adding proper amount of Ni to the Co and Cu catalysts can improve the glycol yield, while suppressing the side reactions. The optimized content of Ni in CuNi and CoNi catalysts is 75% and 10%, respectively.4. Aqueous phase reforming of ethylene glycol to H2 over Co/ZnO catalystsCo/ZnO catalysts with different Co/Zn ratio were prepared by coprecipitation method and their catalytic performances were evaluated in the APR of ethylene glycol. The spinel type of ZnCo2O4 was formed after calcination at 723 K. After reduction, the Co/ZnO catalysts are composed of fcc Co and ZnO. XPS results shows that surface Co0 content increased with the increment of Co/Zn ratio, which is in good agreement with the reduction degree. When the Co/Zn ratio increased from 1/3 to 2/1, the H2 selectivity decreased from 89% to 52%, and the TOF of H2 decreased from 101.4 h"1 to 27.9 h-1. Since Co is a common catalyst in FTS reaction, Co/ZnO catalyst produced the alkenes in the APR of ethylene glycol. The liquid products contained methanol, ethanol, and acetic acid. Based on the results of the APR of methanol, ethanol and acetic acid on Co/ZnO-21 catalyst, we concluded that besides the reaction pathways of ethylene glycol with water over metallic catalysts proposed in the literature, reaction pathways from ethanol to acetic acid and 2-propanol and via the FTS reaction to alkenes should be taken into account over the Co/ZnO catalysts in APR of ethylene glycol.