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2005 Progress Report: Generation of Hydrogen Peroxide in CO2 from H2 and O2 and Subsequent Green Oxidations

EPA Grant Number: R831533
Title: Generation of Hydrogen Peroxide in CO2 from H2 and O2 and Subsequent Green Oxidations
Investigators: Beckman, Eric J.
Institution: University of Pittsburgh - Main Campus
EPA Project Officer: Bauer, Diana
Project Period: February 18, 2004 through February 17, 2007
Project Period Covered by this Report: February 18, 2004 through February 17, 2005
Project Amount: $375,000
RFA: Technology for a Sustainable Environment (2003)
Research Category: Pollution Prevention/Sustainable Development

Description:

Objective:

Hydrogen peroxide is generally considered to be a green oxidant, given that it is relatively non-toxic and breaks down in the environment to benign fragments. However, the process by which 95 percent of the world’s hydrogen peroxide is generated (the sequential hydrogenation and oxidation of an alkyl-anthraquinone) is far from benign. The current process consumes significant quantities of energy and produces a number of significant waste streams owing to degradation of both the anthraquinone and the organic solvent required to conduct the process. These problems not only render hydrogen peroxide production less sustainable than it should be, but also raise the cost for producing H2O2, meaning that it is not typically used in commodity chemical processing. The direct route to H2O2, the reaction of H2 and O2, has previously been investigated by various large companies, but has not moved beyond the pilot stage at this point owing to either safety concerns or low productivity. As such, we proposed to investigate the direct reaction of H2 and O2 to make H2O2 in liquid CO2, a benign solvent with some unique advantages to the proposed process. Carbon dioxide, unlike typical organic solvents, cannot be oxidized during the process, and also enlarges the non-flammable regime for O2/H2 mixtures. Following an understanding of the kinetics of H2O2 formation, we also propose to evaluate the use of H2O2 generated in this way to perform in-situ oxidations of important substrates such as propylene, benzene, and cyclohexane.

Progress Summary:

Selection of “Indicator” Compound

An “indicator” compound was used to measure the amount of in situ generated H2O2 in compressed CO2 without the need to titrate samples. This “indicator” had to meet the following criteria:

  1. It should be easily oxidized by H2O2 under mild reaction conditions with TS-1 as the catalyst. One problem associated with using H2O2 as a selective oxidant in carrying out green oxidation is the presence of water in H2O2 solution, because water usually can cause the deactivation of many catalysts, especially those used in organic oxidations. The discovery of TS-1 removed this obstacle because the hydrophobic nature of TS-1’s micro-pores favors the diffusion of organic substrates to the active site and protects the active site from deactivation by water. This makes TS-1 one of the most studied catalysts in carrying out green oxidation reactions with H2O2. Therefore, proving that precious metal loaded TS-1 is an effective catalyst in direct synthesis of H2O2 has special meaning in future applications.
  2. The indicator should not be oxidized by oxygen alone or hydrogenated; this is obvious because the mixture of O2 and H2 is used for direct synthesis of H2O2.
  3. The selectivity to its oxide should be high at reasonable H2O2 concentration because the formation of more than one product will make the measurement complicated.
  4. The “indicator” should be converted to its oxide proportional to the amount of H2O2 added.
  5. The “indicator” and its oxide should both be soluble in CO2.

This proposed “indicator” compound can be selected from a list of various alcohols, tertiary amines and sulfides. Ultimately, pyridine was selected as the “indicator” according to the above mentioned criteria.

Direct Synthesis of H2O2 from O2 and H2 in CO2

The direct synthesis of H2O2 was conducted using a mixture of O2 and H2 as the starting reactants over precious metal loaded TS-1 with pyridine as the “indicator” in compressed CO2. Here, precious metal loaded TS-1 is a bifunctional catalyst: the precious metal functioned as the catalyst for direct synthesis of H2O2 from O2 and H2, while TS-1 functioned as the catalyst for the oxidation of pyridine by in situ generated H2O2.

In agreement with the results of several previous studies, we found that the selectivity of the reaction increased as the O2/H2 molar ratio dropped (see Figure 1). In the rest of this study, the O2/H2 molar ratio was kept at 1 in order to maximize the formation of H2O2 in CO2.

The effect of H2 concentration on direct synthesis of H2O2 has not been well explored in the literature. The results shown in Figure 1 disclose the effect of H2 concentration (defined as the amount of H2 added to the reactor over the net volume of the reactor, mM) on H2 conversion, H2O2 selectivity and H2O2 yield at constant O2/H2 ratio. It is concluded that with the increase of H2 concentration in the reactor, H2 conversion, H2O2 selectivity and H2O2 yield all increased until H2 concentration reached 500 mM. When H2 concentration was increased from 250 mM to 500 mM, H2 conversion only increased about 20 percent (from 52.5% to 62.6%), while H2O2 selectivity almost doubled (from 22.4% to 39.3%). This in turn resulted in the increase of H2O2 yield from 11.8 percent to 24.6 percent. Figure 1 also showed that when H2 concentration was below 500 mM, the relationship between H2O2 yield and H2 concentration was linear, implying that the reaction rate for the direct synthesis of H2O2 from O2 and H2 in CO2 was first order with respect to H2 concentration. Further increase to H2 concentration led to a decrease in H2O2 selectivity at similar H2 conversions. The possible reason is that, at higher H2 concentration, the existence of significant amounts of H2 favored the hydrogenation of H2O2 over the Pd/TS-1 catalyst. Therefore, part of the in situ generated H2O2 could be hydrogenated before it can react with the “indicator.”

A survey of the literature showed that high palladium (≥ 1% Pd) contents were preferred by many researchers in carrying out direct synthesis of H2O2 from O2 and H2. Few investigators examined the direct synthesis by using lower Pd content catalysts (≤ 1%Pd). From an economic viewpoint, for a similar H2O2 yield, the lower the Pd content in the catalyst, the more competitive the catalyst. In this study, a series of catalysts with Pd contents ranging from 0.2%~1.0% were used to conduct direct synthesis of H2O2 in CO2 and the results are given in Figure 2. With the increase of Pd content in the catalyst, H2 conversion increased while H2O2 selectivity decreased steadily. The combination of these changes led to almost no change in H2O2 yield until the Pd content reached 0.6 percent. Further increasing Pd content in the catalyst resulted in a decrease in H2O2 yield. In order to explore the reason behind this phenomenon, H2O2 decomposition experiments were conducted using catalysts with different Pd contents. By comparing with TS-1 alone, we found that Pd on TS-1 can cause significant decomposition of H2O2. The amounts of H2O2 decomposed by Pd/TS-1 increased with the increase in Pd contents in the catalysts. 1.0% Pd/TS-1 could decompose about 50 percent of H2O2 under the given experimental conditions. Therefore, the decrease in H2O2 yield when using the higher Pd loaded catalyst was likely because the higher Pd loaded catalyst could cause the partial decomposition of in situ generated H2O2 before it could react with the “indicator.” Finally, the lower selectivities observed upon raising Pd content could be due to hydrogenation of H2O2 as well.

Figure 1. Effect of H2 Concentration on the Direct Synthesis of H2O2 in CO2

Experimental conditions: O2/H2=1, indicator=6.2 mmol, water=27.8 mmol, 0.35% Pd/TS-1=0.05 g; P=125 bar, T=60ºC, reaction time=5 hours

Figure 2. Effect of Pd Content on the Direct Synthesis of H2O2 in CO2

Experimental conditions: H2=12.4 mmol, O2/H2=1, indicator=6.2 mmol, water=27.8 mmol, Pd/TS-1= 0.05 g; P=125 bar, T=60ºC, reaction time=5 hours

Platinum (Pt) has the ability to promote the direct synthesis of H2O2 from O2 and H2 over Pd catalysts; the optimal amount of Pt is typically about one-tenth that of Pd. In this study, the influence of adding Pt (Pt/Pd = 0.1 by weight) to Pd/TS-1 catalysts was investigated, the results given in Figure 3 supported the previous literature conclusions. It can be seen from Figure 3 that the addition of Pt to the Pd/TS-1 catalyst had significant influence on direct synthesis of H2O2 in compressed CO2, especially when Pd content was less than 0.6 percent. This influence was mainly on H2O2 selectivity, while for H2 conversion, it was minor. The combination of these effects led to an increase in H2O2 yield. For example, adding 0.02% Pt to 0.2% Pd/TS-1 could increase H2O2 selectivity from 43.8 percent to 56.1 percent with almost no change in H2 conversion, which, in turn, resulted in the increase in H2O2 yield from 23.9 percent to 31.7 percent. As pointed out by Meiers, et al., during the preparation of Pd/TS-1, the interaction of the Pd precursor, [Pd(NH3)4]2+, with TS-1 (support) created a new Pd species, Pd(II), with a binding energy in the range of 337.2~337.8 eV. This new Pd species was neither Pd0 (binding energy in the range of 335.3~335.5 eV) nor PdO (binding energy: 336.1~336.2 eV), and it played an important role in epoxidation of propylene by the mixture of O2 and H2 (via the in situ generation of H2O2). Adding small amounts of Pt to Pd/TS-1 could dramatically increase the fraction of Pd(II) sites, thus leading to an increase in H2O2 yield. However, one should be aware that the addition of Pt to Pd/TS-1 affects not only the Pd oxidation state but also particle features. A further increase in Pt content could only slightly increase the fraction of Pd(II), but significantly changed the surface morphology of the Pd aggregates from primarily needle-shaped to a mixture of needle-shaped crystallites and the undesirable spherical crystallites.

Figure 3. Effect of Adding Pt to the Pd/TS-1 Catalysts in Direct Synthesis of H2O2

Experimental conditions: H2=12.4 mmol, O2/H2=1, indicator=6.2 mmol, water=27.8 mmol, catalyst=0.05 g; P=125 bar, T=60ºC, reaction time=5 hours

Finally, we have begun work on the oxidation of commercially important substrates, including propylene, benzene, and cyclohexane. One set of results (for the oxidation of propylene to propylene oxide) is shown in Figure 4–as can be seen, use of in-situ generated H2O2 can allow for high selectivity to the desired product.

Figure 4. Synthesis of Propylene Oxide (PO) From Propylene, Hydrogen, and Oxygen at 125 Bar and 60ºC in CO2. Effect of use of weak bases as promoters on selectivity to PO using Pd/Pt/TS-1 catalyst.

Future Activities:

We propose to continue our work on oxidation of commercially important substrates using the in-situ H2O2 system.

Journal Articles:

No journal articles submitted with this report: View all 6 publications for this project

Supplemental Keywords:

green oxidations, hydrogen peroxide, supercritical carbon dioxide, , INTERNATIONAL COOPERATION, Sustainable Industry/Business, Scientific Discipline, RFA, POLLUTION PREVENTION, Technology for Sustainable Environment, Sustainable Environment, Chemical Engineering, Energy, Chemicals Management, Environmental Chemistry, energy conservation, adipic acid, green oxidant, Propylene oxide, green chemistry, in situ H2O2 generation

Progress and Final Reports:
Original Abstract
2006 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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