The gasoline we buy is one-third a mixture of aromatics, and benzene is one of them. Only benzene is known to be cancer causing, and it’s very difficult to remove. Our catalyst opened a whole new way to do that—and probably a very inexpensive way. We could keep the cost of gasoline down, and a big environmental and health problem would be solved.Tobin Marks, co-corresponding author

Marks is the Vladimir N. Ipatieff Research Professor of Chemistry in the Weinberg College of Arts and Sciences at Northwestern and professor of materials science and engineering in the McCormick School of Engineering and Applied Science.

Benzene, a clear, colorless, flammable liquid with a gasoline-like odor that can volatilize to vapors in air, is classified as a known carcinogen based on occupational studies in adults that demonstrated increased incidence of several types of leukemia in exposed adults. Benzene has also been shown to be genotoxic (cause damage to DNA) in experimental animal studies.

Currently in the US, there are significant concentrations of benzene in ambient air; mobile sources account for more than 70% of ambient exposure, according to the US Environmental Protection Agency (EPA). EPA’s 2007 Mobile Source Air Toxics (MSAT) rules reduce hazardous air pollutants (air toxics), emitted by cars and trucks. Air toxics include benzene and other hydrocarbons such as 1,3-butadiene, formaldehyde, acetaldehyde, acrolein, and naphthalene. Each refinery and importer must meet specific compliance baselines for conventional and reformulated gasoline.

Beginning in 2011, the MSAT2 rules required refiners to meet an annual average gasoline benzene content standard of 0.62 vol% on all gasoline, both reformulated and conventional, nationwide. (According to refinery batch reports, average gasoline benzene was still around 1 vol% in 2009.)

Gasoline benzene content based on 2009 refinery batch data. Source: EPA. Click to enlarge.

In addition to the 0.62 vol% standard, refiners had to meet a maximum average benzene standard of 1.3 vol% beginning on 1 July 1 2012. A refinery’s or importer’s actual annual average gasoline benzene levels may not exceed this maximum average standard. The regulations included a nationwide averaging, banking, and trading (ABT) program—e.g., underperforming refiners can purchase benzene credits earned by overperforming refiners.

The strict government limitations on carcinogenic benzene content in gasoline thus make the hydrogenation of arenes—aromatic hydrocarbons commonly based on benzene rings—an important element in refinery processes. The most frequently utilized methods refineries use to remove benzene from the gasoline pool are:

• Pre-fractionation of naphtha to eliminate the C₆ and/or C₇ fractions in the reformer feed;

• extractive distillation;

• liquid−liquid extraction; and

• zeolite-catalyzed alkylation with light olefins to form alkylbenzenes.

All of these are energy-intensive, poorly selective for benzene in the presence of other arenes, and/or produce undesired byproducts, the researchers said. While conventional hydrogenation catalysts typically display higher selectivities for substituted arenes, these substituted arenes have high octane numbers and are desired in gasoline to compensate for the benzene removal.

Thus, selective benzene hydrogenation presents itself as an efficient, scalable means to remove benzene from fuels without eliminating other octane-enhancing aromatics and would represent a significant advance. Although several catalysts are known to exhibit hydrogenation activity differences for neat benzene vs neat toluene, few studies have focused on benzene-selective hydrogenation in mixtures of aromatics.

Herein, we report a detailed study of competitive arene hydrogenation mediated by organozirconium complexes on a series of Brønsted acidic oxides. It will be seen that integrated solid-state NMR spectroscopy, extended X-ray absorption fine structure (EXAFS), reaction kinetic, and density functional theory (DFT) quantum chemical analysis affords unique insight into the nature of these molecule-derived surface electrophiles, leading to the discovery of new catalysts with enhanced activity and distinctive selectivity for the hydrogenation of benzene vs toluene and other arenes under mild conditions.

—Gu et al.

The Northwestern-led team studied the arene hydrogenation activity of a series of organometallic catalysts chemisorbed on several strongly Brønsted acidic metal oxides. Organozirconium catalysts exhibited the highest activities.

In addition to Marks, other authors of the paper are Weixing Gu, Madelyn Marie Stalzer, Christopher P. Nicholas, Alak Bhattacharyya, Alessandro Motta, James R. Gallagher, Guanghui Zhang, Jeffrey T. Miller, Takeshi Kobayashi, Marek Pruski and Massimiliano Delferro.

The US Department of Energy (grant DE-FG02-86ER13511) supported the research.

Resources

• Weixing Gu, Madelyn Marie Stalzer, Christopher P. Nicholas, Alak Bhattacharyya, Alessandro Motta, James R. Gallagher, Guanghui Zhang, Jeffrey T. Miller, Takeshi Kobayashi, Marek Pruski, Massimiliano Delferro, and Tobin J. Marks (2015) “Benzene Selectivity in Competitive Arene Hydrogenation: Effects of Single-Site Catalyst···Acidic Oxide Surface Binding Geometry” Journal of the American Chemical Society 137 (21), 6770-6780 doi: 10.1021/jacs.5b03254