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Extramural Research

2001 Progress Report: Federal Demonstration Partnership (FDP) solid-catalyzed reactions in supercritical reaction media

EPA Grant Number: R826034
Title: Federal Demonstration Partnership (FDP) solid-catalyzed reactions in supercritical reaction media
Investigators: Subramaniam, Bala
Institution: University of Kansas
EPA Project Officer: Karn, Barbara
Project Period: October 1, 1998 through September 30, 2001 (Extended to May 20, 2003)
Project Period Covered by this Report: October 1, 2000 through September 30, 2001
Project Amount: $125,000
RFA: Technology for a Sustainable Environment (1998)
Research Category: Nanotechnology , Pollution Prevention/Sustainable Development

Description:

Objective:

The objectives of this research project are to: (1) develop a solid-acid catalyzed 1-butene/isobutane and skeletal isomerization processes with enhanced catalyst activity and product selectivity; and (2) demonstrate the safe operation (characterized by stable catalyst activity and excellent temperature control) of solid catalyzed hydrogenations in supercritical CO2-based reaction media.

Progress Summary:

Objective 1. Steady C8 alkylates production activity during experimental runs lasting up to 2 days was demonstrated during the alkylation of isobutene, with 1-butene over silica-supported Nafion® catalyst particles suspended in a CO2-based supercritical reaction mixture in a slurry reactor. At a butene space velocity of 0.05 h-1, 368 K (1.1 Tc), molar feed I/O ratio of 5 with 70 mole percent CO2 in feed, pressure-tuning studies revealed that although the butene conversion was relatively insensitive to pressure at 80 percent between 80 (approximately 1.1 Pc) and 167 bar (approximately 2.3 Pc), the C8 alkylates selectivity decreased fourfold from approximately 30 percent at 80 bar to 7 percent at 167 bar. The overall C8 selectivity decreased from approximately 60 percent to 30 percent in the same pressure range, with heavier (C12 and higher) products being formed in denser supercritical reaction mixtures. The pressure-tuning studies clearly show that milder supercritical pressures provide the optimum combination of liquid-like densities and gas-like transport properties to desorb the C8 products and transport them out of the catalyst pores before they are transformed to heavier products. Currently, we are performing modeling studies to better understand the pressure-tuning effects. Such an understanding should aid in rational process design and development.

Similar trends were observed with heteropolyacids (HPAs), although the C8 alkylates activity generally was lower on the HPAs. Based on the steady conversion data, we developed a complementary mathematical model of the process to estimate effective rate constants on the various catalysts, and successfully predicted the pressure tuning effects on conversion and product selectivity. The model results clearly show that with the rational design of catalyst and tailoring parameters such as acidity and pore structure, it should be possible to further enhance the C8 alkylates selectivity. Thus, CO2-based supercritical reaction mixtures generally offer an excellent opportunity for developing environmentally benign alternatives to conventional processes that employ mineral acids.

The demonstration of extended butene conversion (80 percent) and C8 selectivity (approximately 74 percent, with the alkylates constituting approximately 40 percent of the total C8 compounds) at a relatively mild pressure (80 bar at 568 K), low I/O ratio (5), and reasonable CO2 dilution (70 percent) is a significant advance over previous efforts.

Objective 2. The hydrogenation of cyclohexene to cyclohexane over Pd/C was performed in a fixed-bed reactor employing scCO2 to solubilize the reaction mixture consisting of the reactants (cyclohexene and hydrogen) and the product (cyclohexane) in a single supercritical phase surrounding the solid catalyst. The reaction was performed at a near critical temperature of 70°C and at a pressure of 138 bar verified experimentally to permit operation in a single-phase. For an olefin space velocity of 20 h-1, excellent temperature control around the set point (70°C), and stable catalyst activity were demonstrated at cyclohexene conversion exceeding 80 percent throughout a 22-hour run. The "hot spot" temperature in the bed was about 16°C when the fixed-bed reactor was operated at adiabatic conditions. Further, the reaction was parametrically insensitive to temperature between 70°C and 78°C. Reducing the operating pressure led to an increase in hot spot temperature because of the lowering of the heat capacity of the reaction medium. This indicates that the near-critical reaction medium (possibly aided by the reactor material) has sufficient heat capacity to effectively remove the heat of reaction.

The selective hydrogenation of toluene to methylcyclohexene over Ru/Al2O3 was conducted in a fixed-bed reactor (60°C, 138 bar) using scCO2 as the solvent. It was shown that selectivities greater than 20 percent toward the cycloalkenes could be obtained without the addition of any modifiers. This increase in selectivity is attributed to the increased solubility and pore-diffusivity afforded by the supercritical reaction medium.

The important issue of possible deactivation of noble metal catalysts by CO formed from the reverse water gas shift reaction between CO2 and H2 was investigated using high-pressure transmission fourier transform infrared (FTIR) spectroscopy. It was shown that CO could be formed on Pd/Al2O3 when exposed to CO2 (95 percent) and H2 (5 percent) at the reaction condition 70°C, 138 bar. This adsorbed CO evolved with time and was insignificant at short residence times, implying that short residence time continuous reactors are preferred over batch reactors (with residence times > 20 minutes) to minimize the effects of possible catalyst deactivation by CO.

The equipment infrastructure, and the insights provided by the foregoing results on how to operate an exothermic reaction in scCO2 with tight temperature control and stable catalyst activity pave the way for systematic fundamental investigations of fixed-bed hydrogenations of functional groups on supported catalysts. Clearly, such investigations are essential for rational design and scaleup of scCO2-based hydrogenations.

Future Activities:

Future activities are to systematically address the following issues: What are the adsorbed surface species on a supported noble metal hydrogenation catalyst (such as Pt and Ru) when exposed to a CO2+H2 mixture at supercritical conditions? Are the adsorbed species formed in the homogeneous phase, or are they catalyzed by the presence of the noble metal? What effect does the catalyst support (such as C, Al2O3, SiO2) have, if any, on the formation of the surface species? Are there hydrogenation catalysts (such as Ni) that either do not promote, or are less affected by the formation of surface deactivating species? How does replacing CO2 with other solvents (such as ethane or propane) affect the formation of the surface species? We plan to employ FTIR spectroscopy to probe the surface species during reaction conditions.


Journal Articles on this Report : 5 Displayed | Download in RIS Format

Other project views: All 26 publications 7 publications in selected types All 6 journal articles

Type Citation Project Document Sources
Journal Article Arunajatesan V, Subramaniam B, Hutchenson KW, Herkes FE. Fixed-bed hydrogenation of organic compounds in supercritical carbon dioxide. Chemical Engineering Science 2001;56(4):1363-1369.
abstract available   full text available
R826034 (2000)
R826034 (2001)
  • Full-text: Science Direct - Full Text HTML
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  • Abstract: Science Direct - Abstract
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  • Other: ScienceDirect - Full Text PDF
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  • Journal Article Arunajatesan V, Wilson KA, Subramaniam B. Pressure-tuning the effective diffusivity of near-critical reaction mixtures in mesoporous catalysts. Industrial & Engineering Chemistry Research 2003;42(12):2639-2643.
    abstract available  
    R826034 (2001)
  • Abstract: ACS -Abstract
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  • Journal Article Clark MC, Subramaniam B. Kinetics on a supported catalyst at supercritical, nondeactivating conditions. AIChE Journal 1999;45(7):1559-1565.
    abstract available   full text available
    R826034 (2001)
    R824729 (Final)
  • Abstract: Wiley-Abstract
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  • Journal Article Subramaniam B. Enhancing the stability of porous catalysts with supercritical reaction media. Applied Catalysis A: General 2001;212(1-2):199-213.
    abstract available   full text available
    R826034 (2000)
    R826034 (2001)
  • Abstract: Science Direct - Abstract
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  • Journal Article Subramaniam B, Lyon CJ, Arunajatesan V. Environmentally benign multiphase catalysis with dense phase carbon dioxide. Applied Catalysis B: Environmental 2002;37(4):279-292.
    abstract available  
    R826034 (2001)
  • Abstract: ScienceDirect - Abstract
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  • Supplemental Keywords:

    supercritical carbon dioxide, acid catalyst, hydrogenation, 1-butene/isobutane alkylation., RFA, Industry Sectors, Scientific Discipline, Sustainable Industry/Business, Chemical Engineering, cleaner production/pollution prevention, Environmental Chemistry, Manufacturing - NAIC 31-33, Sustainable Environment, Technology for Sustainable Environment, Environmental Engineering, Accommodation and Food Services - NAIC 72, Federal Demostration Program, aldehydes, cleaner production, sustainable development, waste minimization, waste reduction, environmentally conscious manufacturing, Federal Demonstration Partnership, solid-catalyzed reactions, catalysts, green process systems, alkylation reaction, isomerization, pollution prevention, source reduction, supercritical reaction media, catalysis, green chemistry

    Relevant Websites:

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    Progress and Final Reports:
    Original Abstract
    2000 Progress Report

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    The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.

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