Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE
Kinetics, Catalysis, and Reaction Engineering
Rare Earth / Transition Metal Oxides for Syngas Tar Reforming - a Model Compound Study Jaren Lee, Rui Li, Mike Janik, and Kerry M. Dooley Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00682 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Industrial & Engineering Chemistry Research
Rare Earth / Transition Metal Oxides for Syngas Tar Reforming - a Model Compound Study Jaren Lee,a Rui Li,a Michael J. Janikb and Kerry M. Dooleya* a
Dept. of Chemical Engineering, Louisiana State University, Baton Rouge, LA 70803.
b
Dept. of Chemical Engineering, Pennsylvania State University, University Park, PA 16802
KEYWORDS Manganese-doped cerium oxide; palladium-doped cerium oxide; tar reforming; sulfur tolerance; effects of water
ABSTRACT
A major problem in biomass or coal gasification is removal of syngas byproducts such as H2S, NH3 and tars (heavy hydrocarbons) that cause catalyst deactivation and clogging problems downstream. Rare earth oxides (REOs) doped with transition metals (TMs) are promising catalysts for tar reforming. With propane as a model compound, we compared such catalysts to a typical supported Ni catalyst, and also to recent DFT results modeling these systems. The REO/TM catalysts are active over the range 920-1000 K, with no significant deactivation in nonsulfur containing feeds. In particular, a Mn/CeO2 catalyst showed good reforming activity with low carbon, CO2 and CH4 yields. This catalyst also maintained some activity in the presence of 1 ACS Paragon Plus Environment
Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 29
40 ppm H2S. Kinetics calculations showed that most such catalysts have near zero-order kinetics with respect to water, making them usable with a variety of gasifier effluents. Characterization of used catalysts by multiple techniques suggests that the metal-doped REOs do not undergo much (if any) phase separation in extended use under tar reforming conditions, with Mn- and Ladoped CeO2 being especially stable.
1.
INTRODUCTION Syngas from biomass gasification can contribute to renewable energy resources. The energy
stored in syngas can be transformed into electricity with turbines, or catalytically into liquid fuels such as methanol or diesel.1-2 Biomass can be reacted with steam and (sometimes) air into a syngas containing H2, CO, CO2, H2O, N2, CH4 and other light hydrocarbons, and undesirable impurities such as H2S, NH3 and tars (benzene and higher hydrocarbons). In order to meet the environmental emissions requirements, such applications have constraints on the gas concentrations of tars, sulfur and nitrogen.2-6 Gasifier effluents with high concentrations of these impurities are well known to cause fouling and blockage in downstream processes.1-2 Most tar removal takes place above 823 K in an oxidizing environment and above 923 K in a syngas environment. While higher temperatures make tar removal easier, if the temperature is above that of the gasifier the cost of utilities will be high. One goal in developing tar reforming catalysts is to reduce the need for reactor temperatures greater than 1173 K. Two catalytic reactors in series (e.g., one for initial sulfur removal and the second to crack/reform tars) can effectively condition syngas for downstream operations. But in tar reforming, supported-Ni catalysts show low sulfur tolerance except at high temperature (e.g., 1123 K).7 Rare earth oxides
2 ACS Paragon Plus Environment
Page 3 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Industrial & Engineering Chemistry Research
(REOs) (e.g., Ce/LaOx) doped with transition metals (e.g., Mn, Fe) have shown promising results for tar reforming in the presence of sulfur.8 Most reforming research work uses naphthalene or toluene as the model tar compounds, as we also have done previously.3 However, in this work, we use propane to investigate the reforming and cracking activity of TM / REO catalysts at 923–998K. Propane was chosen here as a model tar compound because it has both C-C and C-H bonds and still is simple enough to model adsorption and reaction on a simple surface such as a metal-doped CeO2 (111) using the DFT+U method. The computational work was done in parallel with this study. Our previous DFT work has suggested Mn/CeO2 as an encouraging catalyst for catalytic oxidation of alkanes.9-11 The oxygen vacancy formation energy of an M-doped ceria surface was determined to be a good descriptor for methane activation, with Mn/CeO2 showing a good balance of activity for C-H bond activation and favorability for re-oxidation.11 For Mn/CeO2, the reducibility descriptor and C-H activation energy are very similar to Pd-doped ceria; Pd-doped ceria is an active alkane catalytic combustion catalyst.12-14 We have also utilized DFT+U methods combined with XANES studies to determine that Mn in Mn/CeO2 is present in Mn2+ and Mn3+ states,9 and that Mn/CeO2 shows viable elementary step energetics to present high activity for propane reforming.10 Herein we test a series of TM/REO catalysts, including Mn/CeO2, for their activity and product selectivity in propane reforming. We also include measurements of all the important ancillary reactions such as coking, water-gas shift, and methanation. Comparison on these bases is made to a Ni-based steam reforming catalyst of commercial composition. Catalysts are characterized post-mortem by nitrogen adsorption, X-ray diffraction, hydrogen pulse chemisorption, X-ray absorption near edge spectroscopy (XANES), Raman and TEM.
3 ACS Paragon Plus Environment
Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
2.
Page 4 of 29
EXPERIMENTAL SECTION 2.1. Reforming of propane. Details of the catalyst syntheses are given in the Supporting
Information. They are designated by their elemental ratios (except for oxygen and Al, where present). All catalysts were cleaned in N2 at 723 K just prior to reaction. These elemental ratios were chosen to maximize the TM dopant amounts without generating a second XRD-visible oxide phase, at least under initial (fresh catalyst) conditions.8 The reactor setup included five mass flow controllers to prepare the gas mixture, a Harvard 944 infusion pump to supply the water by syringe, a ½” stainless steel reactor tube packed with α-alumina layered above and below the catalyst (11.4 g α-Al2O3, determined to be inactive) and either 0.2 (kinetics experiments) or 1 (in generating used material for characterizations) g of catalyst, and an online Agilent 6890N GC-MS to analyze the gas composition. The reactor tube was heated by a furnace (Teco F-5-1000, 320 watt) whose temperature was controlled by a Eurotherm 818P PID controller. Additional K thermocouples were placed on heated lines carrying the gases to and from the reactor. We explored two reactor feeds, characteristic of either an air-blown or a steam-blown gasifier effluent. One feed was 53 vol% CO, 3.3% C3H8, 10% H2O, 28% H2, with 1.4% CH4 and C2’s.
The balance was N2, and the GHSV
(mL/min/gcat) was 34000, at ambient conditions. The water volume fraction increased to 20.7% at a GHSV of 40000 to simulate a steam-blown feed. The other compounds were kept in the same ratio as before. Two automated sampling valves (Valco) injected 1 mL samples into the GC, which by using switching valves and four columns (Agilent Technologies/Wasson Instruments) in two ovens separated all compounds from H2 to H2O and C3H8 in ~40 min.15
4 ACS Paragon Plus Environment
Page 5 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Industrial & Engineering Chemistry Research
While the exact reaction pathway is undetermined, and undoubtedly includes many simultaneous microkinetic reactions, for the purposes of quantification we employed five reactions to represent the primary reforming and cracking activity:15 + ↔ +
ξ1
+ ↔ + 3
ξ2
+ ↔ +
ξ3
+ ↔ +
ξ4
+ ↔ 2
ξ5
These reactions are consistent with both the product distribution, and with the assumptions of several previous works.2-3, 16-17 The ξ’s are the molar extents of reactions for these five reactions in gmol/min, and they were calculated by solving the component mass balances simultaneously, using not only the compositions but also the measured effluent flow rate.18 The yields Y of the products on an elemental carbon basis, the selectivity S(C2) to C2 products (from propane), and the propane conversion X(C3) were calculated from the following equations: =
+ + 3 + + 2
=
+ 2 + + 3 + + 2
=
−
+ + 3 + + 2
=
×
=
−
For catalysts where Y(C) is insignificant, the carbon balance is perfectly satisfied. Obviously it cannot be satisfied if large amounts of coke are being formed. The presence of large amounts of coke on those catalysts in
