Hydroformylation’s Diamond Jubilee

Everybody likes a birthday celebration. Not always the part about growing older, but the part about getting better.

The hydroformylation reaction is a good example of improvement with age. This year marks the 75th anniversary of the discovery of this important industrial catalytic process for producing aldehydes. The aldehydes serve as chemical intermediates for making products as varied as detergents, paints, and even automobile windshields. Rather than retiring the process and putting it out to pasture, chemical companies are keeping hydroformylation busy at work, and finding new applications to boot—a little extra icing on the cake.

Hydroformylation is not the sexiest chemistry when it comes right down to it, but it’s essential to today’s society, observed George G. Stanley, a chemistry professor and hydroformylation expert at Louisiana State University. Stanley was a coorganizer of a hydroformylation birthday party symposium sponsored by the Division of Industrial & Engineering Chemistry at the American Chemical Society meeting in New Orleans earlier this month.

“It’s great to have a bunch of hydroformylation geeks in the audience,” Stanley chortled as he kicked off the symposium with his opening remarks. The chemists and chemical engineers in attendance responded with honks of laughter.

Hydroformylation, also known as oxo chemistry, was discovered in 1938 by German chemist Otto Roelen and first commercialized by German chemical company Ruhrchemie, Stanley told C&EN. The reaction involves the catalytic addition of carbon monoxide and hydrogen (a mixture known as synthesis gas) across the double bond of an olefin to make an aldehyde. Roelen, who died in 1993 at age 95, had previously worked with Franz Fischer and Hans Tropsch in the early development of Fischer-Tropsch chemistry, which converts synthesis gas into fuels and chemicals.

“We thought the 75th anniversary of hydroformylation would be a perfect time to host a symposium on some of its history, as well as the recent research work in this industrially important area,” Stanley added. “New Orleans is the perfect location, because three of the world’s major hydroformylation plants are within an hour’s drive of the city.”

One of them is the first commercial hydroformylation plant constructed outside Germany, built in Baton Rouge, La., in the late 1940s by what is now Exxon­Mobil Chemical, Stanley related. The plant was expanded in the 1990s and is still operating, he said. The other facilities are the Dow Chemical (formerly Union Carbide) facility in Taft, La., and Shell’s plant in Geismar, La.

Developing catalyst technology is at the heart of hydroformylation chemistry, Stanley pointed out. Roelen’s original process used a cobalt carbonyl catalyst, HCo(CO)₄, which operates under high pressure and temperature. Shell chemists soon discovered that a phosphine-modified catalyst, HCo(CO)₃(PR₃), operates under lower pressure and gives considerably better linear-to-branched aldehyde selectivity.

By the mid-1970s, chemical companies discovered that rhodium carbonyl triphenylphosphine catalysts were more active, provided better selectivity for desired aldehydes, ran at even lower pressure to reduce capital and operating costs, and made it easier to recycle the catalyst. Catalyst technology took another leap forward beginning in the 1980s with the advent of bidentate phosphine ligands, which have even higher reactivity and controllable selectivity for different aldehydes.

In hydroformylation, the typical olefins used are ethylene and propylene, which lead to propionaldehyde and butyraldehydes, respectively. Most companies start with propylene, which gives a product mixture of n-butyraldehyde and its branched isomer, isobutyraldehyde. But generally any olefin will work for hydroformylation, including longer chain olefins, branched olefins, and cyclic versions such as styrene and cyclohexene.

The aldehyde producers tend to focus on maximizing production of n-butyraldehyde, which in most cases provides more economical downstream products. The global aldehyde market is immense, in the ballpark of 25 billion lb per year, with the butyraldehyde isomers making up about 80% of the total.

At the New Orleans symposium, Mark Bolinger of Shell Technology Center, in Houston, provided an overview of hydroformylation chemistry stemming from the Shell Higher Olefins Process. Shell’s process is a highly integrated method for producing linear olefins from ethylene. One line of products relies on converting the olefins into aldehydes via hydroformylation and on into plasticizer and surfactant alcohols by hydrogenation. The alcohols are sold directly as Shell’s Neodol detergent alcohols and as the Linevol line of plasticizers, which are used to control the flexibility of plastics. Shell also treats the alcohols further with ethylene oxide to make Neodol ethoxylates, which are nonionic surfactants.

The original cobalt catalysts made aldehydes in up to a 4:1 normal-to-branched ratio, Bolinger noted. Shell’s more thermally stable cobalt triorganophosphine catalyst now achieves a ratio of 8:1 at lower pressure, and it’s easier to recycle. Instead of doing the hydrogenation of aldehydes to make alcohols in a separate step, like other companies do, Shell does it all in the same reactor owing to the higher hydrogenation activity of its catalyst.

Shell got into the hydroformylation business in 1963, Bolinger said, originally producing n-butanol and 2-ethylhexanol from propylene via butyraldehyde. The company later began producing higher olefins and higher alcohols. Bolinger estimated that in 50 years Shell has produced some 47 billion lb of plasticizer and surfactant alcohols.

“We have sold all of that,” he said proudly. “Hydroformylation has been good to us. It’s 50 years young at Shell, still going strong, and it doesn’t look like that is going to change.”

Like Shell, Eastman Chemical, another company using hydroformylation, has focused on continually searching for phosphine-based catalysts that are more active, more selective, and longer lived, noted Thomas A. Puckette, a chemist in Eastman’s process research group, in Longview, Texas.

“But at Eastman, we do things a little different,” Puckette told C&EN. “We want a rhodium catalyst system that can easily be tuned so we can make substantial quantities of both normal and iso isomers to meet our business needs as demanded by seasonal markets.”

The two isomers go into different products, and Eastman’s demand for them has changed over time, Puckette explained. Early in his career at Eastman, Puckette became part of a team of five young researchers working on a project to make new bidentate phosphine ligands. The coworkers became known as the BISBI Boys for their work on a particular bisphosphine ligand called BISBI.

“We take a fairly Edisonian trial-and-error approach, making ligands, trying them, using what we learned to modify our syntheses, and trying again,” Puckette related. “We made this one compound, and lo and behold when we tested it the selectivity for n-butyraldehyde was very high, well above the industry standard to that point with monodentate triphenylphosphine catalysts.

“Obviously we recognized this as having value to the industry,” Puckette continued. “Ultimately, Eastman commercially developed the rhodium BISBI system, the first ‘high ratio’ normal-to-iso oxo catalyst, and used it for almost 20 years before the business needs changed.”

The company adjusted to its shifting needs by developing a new set of catalysts with halophosphite ligands. Puckette and his colleagues stumbled into discovering the catalysts when they purchased a commercially available phosphorus-based antioxidant called Ethanox 398 to try as a ligand. They got surprisingly good results, he said, and then made a large number of analogs and tested them.

The halophosphite ligands exhibit a unique ability to produce a variable mixture of linear and branched aldehyde products by altering reaction temperature, ligand concentration, or CO pressure, Puckette said. They also are more resistant to poisoning by traditional hydroformylation impurities such as acetylene. “We commercialized a halophosphite catalyst in 2002, and it has stood the test of time,” Puckette noted.

n-Butyraldehyde has two major downstream outlets that are about 90% of its use: hydrogenation to n-butanol and conversion to 2-ethylhexanol via an aldol condensation process, Puckette noted. Both alcohols are used as intermediates to make a range of products. The largest uses of n-butanol include making the polymer building block butyl acrylate and solvents such as butyl acetate. The most important use of 2-ethylhexanol is to prepare phthalate diester plasticizers. Another important n-butyraldehyde derivative is polyvinyl butyral, a specialty polymer used as a binding layer between sheets of glass in automobile windshields.

For isobutyraldehyde, it is a completely different story, Puckette said. Eastman uses it to make neopentyl glycol, which goes into polyester and urethane applications. Another outlet is an isobutyraldehyde trimer, which goes by the name of Texanol and is used as a coalescing aid in latex paints. Because more paint is sold and used in the warmer summer months than in the cooler winter months, Eastman’s demand for isobutyraldehyde fluctuates. “For that reason, it’s been very helpful for us to vary the normal-to-iso ratio a little bit throughout the course of the year,” Puckette said.

In the pharmaceutical arena, researchers look at hydroformylation chemistry from a different perspective, noted Joseph R. Martinelli, a senior research scientist at Eli Lilly & Co. “Hydroformylation is a powerful transformation,” Martinelli told C&EN, “but we have recognized, and this thought has echoed throughout pharma, that the method is underutilized.”

A Couple of the main reasons are safety and cost, which go hand in hand, Martinelli explained. “In the commodity chemicals industry, processes for one specific transformation operate on a huge scale,” he said. “But in pharma we don’t have that luxury. Given the nature of the chemicals we are making, we have to have manufacturing facilities that are flexible and can be switched to make different products.”

That flexibility ties back into safety issues with large batch reactors that don’t have high pressure tolerances, Martinelli noted. There’s an investment cost associated with upgrading that equipment, he said.

The advent of continuous-flow reactions during the past five years, however, is bringing about a change. “Flow chemistry allows us to mitigate the safety concerns of batch reactions through low-cost specialized equipment running at smaller scale with high throughput,” Martinelli said. “We like to refer to it as flow-enabled technology, because you can access hydroformylation and other transformations that we avoided before for safety reasons and had to design around.”

In New Orleans, Martinelli described Lilly’s efforts to use rhodium-catalyzed hydroformylation to develop an alternative route to an aldehyde-containing side chain in an active pharmaceutical ingredient. The goal is to install the aldehyde at the desired position on a cyclohexene ring with the desired stereochemistry.

“It’s a high-risk, high-reward situation,” Martinelli said. “It’s very challenging research, but the benefit could be very big in terms of improving synthetic efficiency. Anytime we can reduce the number of reaction steps, it speeds innovation by saving time, reducing waste, and reducing overhead costs. It’s greener chemistry.”

These efficiency innovations are much more difficult to achieve for established commodity chemical hydroformylation processes. In fact, seeking out game-changing advances in industrial hydroformylation technology borders on the impossible, commented Mick Brammer, a research scientist at Dow Chemical in Freeport, Texas.

Dow, along with its licensing partner Davy Process Technology, is the global leader in hydroformylation technology, Brammer pointed out. Dow’s LP Oxo Selector technology uses rhodium catalysts to produce butyraldehydes and the corresponding alcohols.

“The current rhodium-catalyzed hydroformylation technology is highly optimized and sets the bar quite high for anyone wishing to develop disruptive alternatives,” Brammer said. But recently the company challenged him to do just that—go in search of ways to change the game in industrial hydroformylation.

The challenge was daunting, yet intriguing, Brammer said. He started out studying the dollars and cents—the economics—of the industrial process and how changing certain aspects might affect Dow’s ability to remain on top of the hydroformylation leaderboard.

For example, he wondered what if the rhodium catalyst were cheaper, or if a cheaper catalyst like iron could be used instead. He concluded that the catalyst and ligand costs are not really a key issue. Most companies lease rhodium from a precious-metal supplier rather than buying it, he pointed out. And although phosphine ligands are usually expensive, they last a long time. The metal and ligand costs are each just a fraction of a cent per pound of product and hard to improve upon, he said.

Brammer also considered increasing the normal-to-iso ratio and substantially increasing the hydroformylation reaction rate. But neither of those would be a game changer, he determined. Dow’s n-butyraldehyde yield is already high, and increasing the reaction rate would mean production plants would have to be modified to handle extra capacity, which would take extra capital costs.

He also investigated the possibility of reducing the cost of olefin and syngas feedstocks, which make up about 85% of the cost of making aldehydes via hydroformylation. There isn’t a more economical alternative to syngas, he noted. But propylene demand is increasing, he added, and the supply is going down as a result of the shift to using more natural gas in the chemical industry instead of petroleum.

This imbalance has prompted Dow and other companies to take steps to ensure their propylene supply. “If propylene prices stabilize or even decline as a result, that will make it even harder to disrupt current hydroformylation technology,” Brammer said.

In the end, Brammer thinks that Dow and other companies are very good at what they do and are naturally keeping the current hydroformylation processes optimized. Their efforts to ensure the hydroformylation business stays healthy will have an intangible benefit, allowing this important industrial process to continue to celebrate many more happy birthdays.