Aqueous-phase Reforming Of Ethylene Glycol Research Articles

An environmentally benign and catalytically efficient non-pyrophoric Ni catalyst for aqueous-phase reforming of ethylene glycol

A non-pyrophoric Ni catalyst (NP Ni) was prepared by alkali leaching of a Ni50Al50 alloy using only ∼ 1/10 of the amount of NaOH required for the preparation of the conventional Raney Ni catalyst. Characterizations reveal that the as-prepared NP Ni catalyst can be looked at as a Ni–Al(OH)3 composite catalyst with Ni in the metallic state and Al(OH)3 in forms of gibbsite and bayerite. After 100 h on stream in aqueous-phase reforming (APR) of ethylene glycol, phase transformation of gibbsite and bayerite to flake-like boehmite occurred, along with the growth of Ni crystallites and partial oxidation of metallic Ni to Ni(OH)2. Under identical reaction conditions for APR of ethylene glycol, the NP Ni catalyst is about 40–52% more active than Raney Ni in terms of the conversion of ethylene glycol to gas products, which is attributed to the stabilizing effect of hydrated alumina on Ni crystallites. The higher selectivity toward H2 and the lower concentration of CO in the product gas on the NP Ni catalyst are attributed to the activation of water by hydrated alumina which is beneficial to the water-gas shift reaction.

Characterization and catalytic properties of Sn-modified rapidly quenched skeletal Ni catalysts in aqueous-phase reforming of ethylene glycol

Skeletal Ni (RQ Ni) catalyst was prepared by alkali leaching of rapidly quenched Ni 50Al 50 alloy. By impregnating RQ Ni with SnCl 4 followed by thermal treatment in inert atmosphere, homogeneous Sn-modified skeletal Ni (RQ Ni–Sn) catalysts were obtained. It was found that thermal treatment induces alloying of the deposited metallic Sn with Ni, forming Ni 3Sn alloy segregated on the catalyst surface. In aqueous-phase reforming of ethylene glycol, the RQ Ni catalyst was less selective in producing H 2 but more selective in producing alkanes than the Raney Ni catalyst described in the literature. This is attributed to the expansion of the lattice of RQ Ni favoring the dissociation of CO and, consequently, the methanation reaction consuming H 2. Moreover, the structural difference influences the reaction pathway, with the undesired C O cleavage pathway obstructed on RQ Ni. Modification of RQ Ni with Sn drastically improves the H 2 selectivity. On the RQ Ni 80Sn 20 catalyst, alkane production was virtually retarded, whereas H 2 selectivity as high as 98 mol% was achieved at high conversion. Based on the characterizations and previous findings, it is suggested that Sn may block the active sites for CO adsorption and/or dissociation, thus suppressing the undesired methanation reaction. On the other hand, bifunctional Ni–Sn ensembles may form, in which Sn facilitates H 2O dissociation while neighboring Ni adsorbs CO, thus promoting the desired water–gas shift reaction, leading to more H 2.

Aqueous-phase reforming of ethylene glycol over supported Pt and Pd bimetallic catalysts

More than 130 Pt and Pd bimetallic catalysts were screened for hydrogen production by aqueous-phase reforming (APR) of ethylene glycol solutions using a high-throughput reactor. Promising catalysts were characterized by CO chemisorption and tested further in a fixed bed reactor. Bimetallic PtNi, PtCo, PtFe and PdFe catalysts were significantly more active per gram of catalyst and had higher turnover frequencies for hydrogen production (TOFH2) than monometallic Pt and Pd catalysts. The PtNi/Al2O3 and PtCo/Al2O3 catalysts, with Pt to Co or Ni atomic ratios ranging from 1:1 to 1:9, had TOFH2 values (based on CO chemisorption uptake) equal to 2.8–5.2min−1 at 483K for APR of ethylene glycol solutions, compared to 1.9min−1 for Pt/Al2O3 under similar reaction conditions. A Pt1Fe9/Al2O3 catalyst showed TOFH2 values of 0.3–4.3min−1 at 453–483K, about three times higher than Pt/Al2O3 under identical reaction conditions. A Pd1Fe9/Al2O3 catalyst had values of TOFH2 equal to 1.4 and 4.3min−1 at temperatures of 453 and 483K, respectively, and these values are 39–46 times higher than Pd/Al2O3 at the same reaction conditions. Catalysts consisting of Pd supported on high surface area Fe2O3 (Nanocat) showed the highest turnover frequencies for H2 production among those catalysts tested, with values of TOFH2 equal to 14.6, 39.1 and 60.1min−1 at temperatures of 453, 483 and 498K, respectively. These results suggest that the activity of Pt-based catalysts for APR can be increased by alloying Pt with a metal (Ni or Co) that decreases the strengths with which CO and hydrogen interact with the surface (because these species inhibit the reaction), thereby increasing the fraction of catalytic sites available for reaction with ethylene glycol. The activity of Pd-based catalysts for APR can be increased by adding a water-gas shift promoter (e.g. Fe2O3).

Kinetics of Aqueous-Phase Reforming of Oxygenated Hydrocarbons: Pt/Al2O3 and Sn-Modified Ni Catalysts

Reaction kinetics studies were conducted on the aqueous-phase reforming of ethylene glycol to produce hydrogen at temperatures near 500 K over Pt/Al2O3 and Raney NiSn catalysts. Ethylene glycol reforming proceeds through similar mechanisms over Pt and NiSn catalysts, involving initial dehydrogenation of ethylene glycol, followed by C−C bond cleavage and water−gas shift. The initial dehydrogenation of ethylene glycol appears to be kinetically significant over Pt/Al2O3, whereas the subsequent rate of C−C cleavage appears to be kinetically significant over R−Ni14Sn. The reforming reaction is fractional order in the ethylene glycol concentration, because of the strong adsorption of the oxygenated reactant, and negative order in the system pressure, through product inhibition by adsorption of H2 and/or CO at high pressures. High selectivity for hydrogen production is achieved for gas-phase products over Pt/Al2O3, whereas the addition of Sn is necessary to avoid alkane formation by methanation over Ni-based catalysts. The rate of methane formation increases at high system pressures, suggesting that the R−Ni14Sn catalyst is still vulnerable to methanation at high H2 and CO2 partial pressures. The reforming of ethylene glycol is accompanied by significant production of acetic acid through bifunctional dehydrogenation/isomerization and dehydration/hydrogenation routes over the metal and support. These bifunctional routes can be used to produce long-chain alkanes or partially reduced chemical intermediates by appropriate use of catalyst−support combinations, catalyst modifiers, and process conditions.

Aqueous-Phase Reforming of Ethylene Glycol Over Supported Platinum Catalysts

Aqueous-phase reforming of 10 wt% ethylene glycol solutions was studied at temperatures of 483 and 498 K over Pt-black and Pt supported on TiO2, Al2O3, carbon, SiO2, SiO2-Al2O3, ZrO2, CeO2, and ZnO. High activity for the production of H2 by aqueous-phase reforming was observed over Pt-black and over Pt supported on TiO2, carbon, and Al2O3 (i.e., turnover frequencies near 8-15 min-1 at 498 K); moderate catalytic activity for the production of hydrogen is demonstrated by Pt supported on SiO2-Al2O3 and ZrO2 (turnover frequencies near 5 min-1); and lower catalytic activity is exhibited by Pt supported on CeO2, ZnO, and SiO2 (H2 turnover frequencies lower than about 2 min-1). Pt supported on Al2O3, and to a lesser extent ZrO2, exhibits high selectivity for production of H2 and CO2 from aqueous-phase reforming of ethylene glycol. In contrast, Pt supported on carbon, TiO2, SiO2-Al2O3 and Pt-black produce measurable amounts of gaseous alkanes and liquid-phase compounds that would lead to alkanes at higher conversions (e.g., ethanol, acetic acid, acetaldehyde). The total rate of formation of these byproducts is about 1-3 min-1 at 498 K. An important bifunctional route for the formation of liquid-phase alkane-precursor compounds over less selective catalysts involves dehydration reactions on the catalyst support (or in the aqueous reforming solution) followed by hydrogenation reactions on Pt.

Aqueous-phase reforming of ethylene glycol on silica-supported metal catalysts

Reaction kinetic studies were conducted for aqueous-phase reforming of ethylene glycol over silica-supported Ni, Pd, Pt, Ru, Rh and Ir catalysts at temperatures of 483 and 498K and at a total pressure of 22bar. The rates of production of hydrogen, carbon dioxide, carbon monoxide, methane, ethane and higher paraffins were measured for conversion of an aqueous feed solution containing 10wt.% ethylene glycol. It was observed that the overall catalytic activity for ethylene glycol reforming, as measured by the rate of CO2 production at 483K, decreases in the following order for silica-supported metals: Pt∼Ni>Ru>Rh∼Pd>Ir. Silica supported Rh, Ru and Ni showed a low selectivity for production of H2 and a high selectivity for alkane production. In addition, Ni/SiO2 showed significant deactivation at the higher temperature of 498K. Silica-supported Pt and Pd catalysts exhibited relatively high selectivities for production of H2, with low rates of alkane production. It appears that catalysts based on Pt and Pd may be promising materials for the selective production of hydrogen by aqueous-phase reforming of oxygenated hydrocarbons, such as ethylene glycol.

Pd-MCM-41/Al2O3 Catalyst for Hydrogenation of aromatics

A comparison of Pt-M/Al2O3 catalysts (M = Ni, Co, none) for the aqueous phase reforming (APR) of different polyols that can be obtained from glucose degradation was made. A standard monometallic Pt/Al2O3 catalyst was prepared by incipient wetness impregnation (IWI) of platinum on an alumina support. The bimetallic catalysts had Ni or Co promoters incorporated by the urea matrix combustion method (UCM) and Pt by IWI technique (PtNi and PtCo catalysts).The catalysts were characterized by ICP/MS analysis, CO/chemisorption, TPR, XPS, SEM-EPMA and XRD. The catalytic activity was assessed with the tests of APR of an aqueous solution of 10 %wt of ethylene glycol (EG), glycerol (Gly) or sorbitol (Sorb), in a tubular fixed bed reactor at 498 K, 2.2 MPa, WHSV = 2.3–2.5 h−1, 3 cm3min-1 He carrier. Monitored variables were: conversion to carbon product in the gas phase, hydrogen and methane yield, selectivities, TOF and coke content on the spent catalysts.PtNi and PtCo catalysts had better hydrogen yield and stability during the experiments than the un-promoted Pt catalyst, (H2 yield = 24.4, 21.2 and 15% for PtNi, 17.9 and 15.1 for PtCo and 13.7, 10.8 and 7.5% for Pt after 8 h on stream in APR of EG, Gly or Sorb solution, respectively). Especially PtNi showed excellent yield and selectivity to hydrogen and a good stability, steadily generating hydrogen for long times. Co addition mainly helped in keeping low levels of coke and a low selectivity to methane.