Bala Subramaniam

Catalytic oxidations in carbon dioxide-based reaction media, including novel CO2-expanded phases

Abstract Environmentally benign oxidations with dense CO2 (either near-critical, ncCO2, or supercritical carbon dioxide, scCO2) as solvent media have been receiving increased attention during the last decade. This paper reviews catalytic oxidations in dense CO2 with emphasis on reported homogeneous systems in scCO2, most of which involve transition metal catalysts and dioxygen or organic peroxides as oxidant. Based on recent work in our laboratory, we offer some perspective and provide examples to demonstrate that scCO2 can be adapted to a broader range of homogeneous oxidations including those that utilize CH3ReO3 as catalyst and t-BuOOH as terminal oxidant, and the oxidation of substituted phenols by dioxygen using Co(salen) complex as catalyst. The advantages of using scCO2 include the total replacement of organic solvents with environmentally benign CO2, the complete miscibility of the oxidants such as O2 in scCO2 eliminating interphase transport limitations, and the resistance of CO2 to oxidation. However, the scCO2-based oxidation has limitations including low reaction rates, inadequate solubilities of a number of transition metal catalysts in CO2 necessitating high process pressures on the order of hundreds of bars, and the lack of pressure-tunability of the dielectric constant of the reaction medium. We present a new process developed in our laboratory in which the conventional solvent medium is only partially replaced by dense CO2. We term this a CO2-expanded solvent medium, which offers several advantages as follows: solvent replacement with dense CO2 by up to 80 mol%, representing a substantial reduction in solvent usage; maintenance of the solubilities of the catalyst and substrate in the reaction mixture while enhancing the miscibility of dioxygen therein; lower process pressures on the order of tens of bars; and pressure-tunable dielectric constants making it possible to realize an optimum reaction medium between scCO2 and neat solvent limits. We distinguish CO2-expanded phases from the traditional concept of a ‘co-solvent’ for a CO2 based system in the following way. To produce a CO2 expanded organic solvent medium, we start with the organic solvent and increase its volume by the addition of CO2, whereas relatively small amounts of ‘co-solvents’ have traditionally been added to dense CO2 phases to improve solubilities of certain compounds. We present examples that show enhanced oxidation rates compared to either neat organic solvent or scCO2 for organic substrates (alkenes and phenols) in CO2-expanded media using dioxygen and metal complexes of both Schiff base and porphyrin ligands. Further, the selectivity toward desired products (alkenes to epoxides; phenols to quinones) is also improved over either neat solvents or scCO2. The CO2-expanded solvents thus offer excellent potential for exploitation in catalytic oxidations.

Enhancing the stability of porous catalysts with supercritical reaction media

Abstract Adsorption/desorption and pore-transport are key parameters influencing the activity and product selectivity in porous catalysts. With conventional reaction media (gas or liquid phase), one of these parameters is generally favorable while the other is not. For instance, while desorption of heavy hydrocarbons from the catalyst is usually the rate-limiting step in gas-phase reactions, transport of the reactants/products is the limiting step in liquid-phase reaction media. With conventional media, it is difficult to achieve the desired combination of fluid properties for optimum system performance. In contrast, density and transport properties can be continuously pressure-tuned in the near-critical region to obtain unique fluid properties (e.g. gas-like transport properties, liquid-like solvent power and heat capacities), that have been exploited in several ways such as (a) the in situ extraction of heavy hydrocarbons (i.e. coke precursors) from the catalyst surface and their transport out of the pores before they are transformed to consolidated coke; (b) complete miscibility of reactants such as hydrogen in the reaction mixture and enhanced pore-transport of these reactants to the catalyst surface, thereby promoting desired reaction pathways; and (c) control of temperature rise in exothermic reactions. Experimental and theoretical investigations are presented to demonstrate beneficial pressure-tuning effects on catalyst activity and product selectivity during continuous processing of a variety of reactions such as these: geometric isomerization and alkylation on solid acid catalysts; Fischer–Tropsch (FT) synthesis on supported Fe catalysts; and fixed-bed hydrogenation on supported catalysts. The possibility to perform solid acid catalysis with extended activity (an environmentally safer alternative to liquid acid processes) and fixed-bed hydrogenations with tunable selectivity and controlled temperature rise (preferred over slurry phase operation) makes supercritical reaction media particularly appealing alternatives to conventional reactor operation.

Environmentally benign multiphase catalysis with dense phase carbon dioxide

Abstract Environmental concerns stemming from the use of conventional solvents and from hazardous waste generation have propelled research efforts aimed at developing benign chemical processing techniques that either eliminate or significantly mitigate pollution at the source. This paper provides an overview of heterogeneous and homogeneous catalysis in dense phase CO 2 , considered a green solvent. In addition to solvent replacement, the demonstrated advantages of using dense phase CO 2 include the enhanced miscibility of reactants, such as O 2 and H 2 which eliminate interphase transport limitations, and the chemical inertness of CO 2 . Further, the physicochemical properties of CO 2 -based reaction media can be pressure-tuned to obtain unique fluid properties (e.g. gas-like transport properties, liquid-like solvent power and heat capacities). The advantages of CO 2 -based reaction media for optimizing catalyst activity and product selectivity are highlighted for a variety of reactions including alkylation on solid-acid catalysts, hydrogenation on supported noble metal catalysts and a broad range of homogeneous oxidations with transition metal catalysts and dioxygen as an oxidant. Through these examples, the need is emphasized for a systematic approach to research and development of supercritical carbon dioxide based processes, taking into account conventional multiphase reaction engineering principles, catalytic chemistry and phase behavior.

Lattice-matched bimetallic CuPd-graphene nanocatalysts for facile conversion of biomass-derived polyols to chemicals.

A bimetallic nanocatalyst with unique surface configuration displays extraordinary performance for converting biomass-derived polyols to chemicals, with potentially much broader applications in the design of novel catalysts for several reactions of industrial relevance. The synthesis of nanostructured metal catalysts containing a large population of active surface facets is critical to achieve high activity and selectivity in catalytic reactions. Here, we describe a new strategy for synthesizing copper-based nanocatalysts on reduced graphene oxide support in which the catalytically active {111} facet is achieved as the dominant surface by lattice-match engineering. This method yields highly active Cu-graphene catalysts (turnover frequency = 33-114 mol/g atom Cu/h) for converting biopolyols (glycerol, xylitol, and sorbitol) to value-added chemicals, such as lactic acid and other useful co-products consisting of diols and linear alcohols. Palladium incorporation in the Cu-graphene system in trace amounts results in a tandem synergistic system in which the hydrogen generated in situ from polyols is used for sequential hydrogenolysis of the feedstock itself. Furthermore, the Pd addition remarkably enhances the overall stability of the nanocatalysts. The insights gained from this synthetic methodology open new vistas for exploiting graphene-based supports to develop novel and improved metal-based catalysts for a variety of heterogeneous catalytic reactions.

Fixed-bed hydrogenation of organic compounds in supercritical carbon dioxide

Abstract The Pd/C hydrogenation of cyclohexene to cyclohexane was performed in a continuous fixed-bed reactor employing CO 2 to solubilize the reaction mixture in a single supercritical (sc) phase surrounding the solid catalyst. Employing an equimolar feed of reactants (cyclohexene and hydrogen) in 90% CO 2 and an olefin space velocity of 20 h −1 , excellent temperature control and stable catalyst activity were demonstrated at 343 K and 13.6 MPa , which represent dense supercritical operating conditions. Total cyclohexane selectivity was observed with its productivity being approximately 16 kg/kg cat/h. Gradual catalyst deactivation (2% loss in cyclohexene conversion per hour) occurred with the as-received cyclohexene feed that typically contained 180 ppm organic peroxides. When these peroxides were mitigated to 6 ppm or less by pretreating the cyclohexene feed through an alumina trap, the catalyst activity was stable with no measurable losses in either surface area or pore volume. Based on prior studies in our laboratory, it has been well established that organic peroxides can catalyze the formation of olefinic oligomers that can adsorb strongly on the catalyst surface and cause deactivation by fouling. No CO was detected in the reactor effluent and no H 2 was observed during the post-run depressurization step, indicating that neither the reverse water–gas shift activity between CO 2 and H 2 nor the formation of Pd formate complexes is significant enough at our operating conditions to deactivate the Pd sites. This conclusion is also supported by the fact that no measurable loss of hydrogenation activity was observed after prolonged (∼35 h ) catalyst exposure to CO 2 +H 2 at reaction conditions. These insights on how to operate an exothermic reaction in scCO 2 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 scCO 2 -based hydrogenations.

Towards highly selective ethylene epoxidation catalysts using hydrogen peroxide and tungsten- or niobium-incorporated mesoporous silicate (KIT-6)

Significant ethylene epoxidation activity was observed over Nb- and W-incorporated KIT-6 materials with aqueous hydrogen peroxide (H2O2) as the oxidant and methanol as solvent under mild operating conditions (35 °C and 50 bar) where CO2 formation is avoided. The Nb-KIT-6 materials generally show greater epoxidation activity compared to the W-KIT-6 materials. Further, the ethylene oxide (EO) productivity observed with these materials [30–800 mg EO h−1 (g metal)−1] is of the same order of magnitude as that of the conventional silver (Ag)-based gas phase ethylene epoxidation process. Our results reveal that the framework-incorporated metal species, rather than the extra-framework metal oxide species, are mainly responsible for the observed epoxidation activity. However, the tetrahedrally coordinated framework metal species also introduce Lewis acidity that promotes their solvolysis (which in turn results in their gradual leaching) as well as H2O2 decomposition. These results and mechanistic insights provide rational guidance for developing catalysts with improved leaching resistance and minimal H2O2 decomposition.

Liquid phase oxidation of p-xylene to terephthalic acid at medium-high temperatures: multiple benefits of CO2-expanded liquids

The Co/Mn/Br catalyzed oxidation of p-xylene to terephthalic acid (TPA) is demonstrated in CO2-expanded solvents at temperatures lower than those of the traditional Mid-Century (MC) process. As compared with the traditional air (N2/O2) oxidation system, the reaction with CO2/O2 mixture at 160 °C and using an additional inert gas (N2 or CO2) pressure of 100 bar increases both the yield of TPA and the purity of solid TPA via a more efficient conversion of the intermediates, 4-carboxybenzaldehyde and p-toluic acid. At the same time, the amount of yellow colored by-products in the solid TPA product is also lessened, as determined by spectroscopic analysis. Equally important, the decomposition or burning of the solvent, acetic acid, monitored in terms of the yield of the gaseous products, CO and CO2, is reduced by ca. 20% based on labeled CO2 experiments. These findings broaden the versatility of this new class of reaction media in homogeneous catalytic oxidations by maximizing the utilization of feedstock carbon for desired products while simultaneously reducing carbon emissions.

Autoxidation of 2,6-di-tert-butylphenol with cobalt Schiff base catalysts by oxygen in CO2-expanded liquids

CO2-expanded acetonitrile and methylene chloride have been used in this first detailed study of catalytic O2 oxidations in these remarkably effective reaction media. The autoxidation of 2,6-di-tert-butylphenol (DTBP) with the cobalt Schiff-base (Co(salen*) in these so-called CO2-expanded liquids (CXLs) has been extensively studied using precisely controlled and monitored batch reactions. The dependence of conversion, selectivity and turn-over-frequency on various reaction parameters, including temperature, [O2], [catalyst], and solvent composition has been evaluated. The rates of O2-oxidation in CXLs are typically 1–2 orders of magnitude greater than those obtained with either the neat organic solvent or supercritical CO2 as reaction media. In keeping with the proposed mechanism, the dependence of both the selectivity and conversion on O2 concentration and catalyst concentration indicates that the O2 adduct, and not free O2, serves as oxidant in two critical steps in these systems. The increase in conversion with increasing temperature supports formation of the phenoxy radical as the rate determining step. In contrast, the temperature independence of selectivity is as expected for two competing radical coupling reactions. The balance between O2 solubility and mixed-solvent dielectric constant determines some of the benefits of the CXLs. Because of the greatly increased solubility of O2 in CXLs, the conversion in those media is substantially greater than that in either scCO2 or the neat organic solvent. However, conversion eventually decreases with increasing CO2 content of the solvent because of the decreasing dielectric constant of the medium. The solubilities of O2 and Co(salen*) have been determined in CXLs based on methylene chloride.

Numerical simulation of a periodic flow reversal reactor for sulfur dioxide oxidation

Abstract A one-dimensional, heterogeneous model was employed to simulate the performance of an adiabatic, periodic flow reversal reactor for the reversible catalytic oxidation of sulfur dioxide to sulfur trioxide. For a given set of operating conditions, variation of the feed gas temperature over a sufficiently wide range allows one to understand the qualitative effects of changes in the reactor operating variables (inlet gas velocity, flow reversal frequency and catalyst pellet size) on reactor performance. In general, operating conditions that cause reaction rate limitations give rise to bell-shaped temperature profiles, which facilitate better enthalpy trapping in the reactor. The temperature profiles are plateau-shaped when operating conditions are such that thermodynamic limitations to sulfur dioxide conversion prevail in the central portion of the reactor. In such a case, sulfur dioxide oxidation is primarily confined to the cooler entrance and exit regions of the reactor. For typical operating conditions, overall sulfur dioxide conversion remains high (> 80%) even when feed temperatures are within 10 K of reaction extinction. While the maximum catalyst temperature, the average catalyst temperature and the sulfur dioxide conversion decrease consistently as an extinction point is approached, these reactor performance measures vary non-monotonically away from an extinction point. Because the catalyst bed acts as a heat reservoir, the reaction extinction process is relatively slow. Hence, if detected sufficiently early, reaction extinction can be thwarted by suitable changes in operating conditions. These qualitative results are applicable in general to reversible exothermic reactions occurring in adiabatic flow reversal reactors.

Isomerization of 1-Hexene over Pt/γ-Al2O3 catalyst: Reaction mixture density and temperature effects on catalyst effectiveness factor, coke laydown, and catalyst micromeritics

Abstract The coking of a highly porous commercial Pt/γ-Al 2 O 3 catalyst via an olefinic precursor, viz., 1-hexene, was investigated in a CSTR over a range of temperatures and pressures that encompass low density subcritical as well as dense supercritical regions of operation. Temporal changes in catalyst effectiveness factor, a measure of catalyst activity, are presented for coking in subcritical and supercritical reaction mixtures at three different reactor temperatures between 1.01 and 1.10 T c The coke laydown during each run along with coked catalyst characteristics such as BET surface area, pore volume, and pore volume distribution are also reported. The maintenance or decay of catalyst activity is dependent upon the relative rates of coke formation and of coke extraction through the catalyst pores. Each of these rates is dictated by density and temperature. At each temperature studied, there is insignificant coke extraction in subcritical reaction mixtures and hence catalyst effectiveness factor decreases with time. Also, when 1-hexene isomerization is kinetically controlled, there is volume loss in both high activity (20–80 A) and low activity (100–600 A) pores indicating coke laydown throughout the catalyst pore structure. In low to moderate supercritical density reaction mixtures, the effectiveness factor decreases rapidly accompanied by reduced coke laydown and virtually no volume loss in the high activity pores. This is attributed to extensive pore mouth plugging of the high activity pores caused by increased coke formation rates relative to coke extraction rates. In dense supercritical reaction mixtures (>1.7 ϱ c , the enhanced solubilities of the coke compounds in the reaction mixture alleviate pore mouth plugging of the high activity pores, as evidenced by an increase in the effectiveness factor and by extensive volume and surface area losses in the high activity pores. At the highest density studied (2.53 ϱ c , while effectiveness factors are smaller than subcritical values at 1.01 T c , up to two-fold enhancements in temporal effectiveness factors are seen at 1.10 T c . A theoretical model is needed to better understand the density and temperature effects on the physicochemical processes underlying coke deposition with simultaneous extraction in porous catalysts.