SCIENCE & TECHNOLOGY HELPING HANDS Solid acid catalysts are sparking improvements in alkylation and isomerization reactions. alkylate properties and the benefits of solid acid catalysis.
FROM
THE
ACS
M E E T I N G
ALKYLATE RISING Solid acid catalysis is leading the way toward environmentally friendlier gasoline and cleaner air STEPHEN K. RITTER, C&EN WASHINGTON
S
TEPS ALREADY TAKEN TO CON-
trol vehicle emissions—such as the use of reformulated gasolines—have had a significant effect on air pollution. But stricter regulations and the expected growth in the number of automobiles worldwide in the coming years, especially in developing countries, means that much more needs to be done. There are problems with both methyl tert-butyl ether (MTBE) and ethanol, the two primary oxygenates used in reformulated gasolines. Deservedly or not, MTBE is perceived as toxic, and the additive is now notorious for leaching from spilled gasoline or from leaky underground storage tanks, leading in some cases to smelly and bad-tasting drinking water. Ethanol is a good alternative, but because it is hygroscopic, it currently must be shipped separately to distributors before it is mixed with gasoline, which increases costs. A different strategy for cleaner burning fuels is to use more alkylate, a high-octane gasoline blend that is often called "the perfect gasoline." Methods for improving alkylate production—primarily switching from liquid acid catalysts to solid acid catalysts— were the main topic of a symposium held by the Division of Petroleum Chemistry at HTTP://PUBS.ACS.ORG/CEN
the recent American Chemical Society national meeting in Chicago. The symposium included new results on solid-acidcatalyzed isomerization reactions, which are key to providing feedstocks and intermediates to a variety of industry sectors, including gasoline, petrochemicals, pharmaceuticals, and fine chemicals. The symposium was cosponsored by ACS Corporation Associates and several companies with the goal of encouraging greater participation by industrial chemists. Indeed, many of the symposium speakers were industrial chemists representing petroleum and petrochemical companies, catalyst producers, and refinery equipment manufacturers such as ExxonMobil, W.R. Grace, Honeywell, Lubrizol, Stratco, and UOP. The presentations centered on catalyst development and provided a review of new reactor systems and processes that are being readied for full-scale alkylate production. "Solid-acid-catalyzed alkylation and isomerization represent a rapidly advancing area of catalytic science with significant environmental and economic impact," noted symposium organizer Karl W Plumlee, director of engineering at W R. Grace's Davison Division. In the lead talk at the symposium, Plumlee gave an overview of
ALKYLATE IS A LIQUID made up of C 7 to C 9 alkanes such as trimethylpentanes and dimethylhexanes, Plumlee explained. It is low in undesirable components such as sulfur, benzene, and total aromatics, and allows environmentally undesirable highvapor-pressure paraffins and olefins to be eliminated from gasoline. 'Alkylate is the cleanest gasoline blending stream produced in a refinery" Plumlee said, "and is considered an essential requirement for producing environmentally sound reformulated gasoline." It makes up about 13%, or more than 12 million barrels per day, of the current North American fuel market, he added. "Continued regulatory constraints on gasoline will increase alkylated contribution to the gasoline pool in the coming years," he noted. Alkylate has been blended into gasoline for decades to improve octane and thus the antiknock properties of gasoline, Plumlee said. It is created in refineries primarily by liquid-acid-catalyzed alkylation of an olefin or a mixture of olefins, usually butene, with an isoparaffin, such as isobutane. Use of leaded gasoline in the U.S. was phased out by 1986, and the tetraethyl lead used as the octane enhancer prior to that was replaced primarily by alkylate. Following Clean Air Act legislation in 1990, some areas of the U.S. were required to add oxygenates such as MTBE or ethanol to gasoline to make it burn cleaner. However, California and New York have banned MTBE, effective in 2004. Some refineries in California reportedly are making reformulated gasoline that meets national air quality standards with 20 to 25% alkylate and no oxygenate rather than 12 to 15% alkylate used with MTBE, according to industry sources. With the expected phaseout of MTBE, alkylate is being advocated as the best solution to improve gasoline without using oxygenates. A KEY PROCESS chemistry step addressed at the symposium was the switch from traditional liquid acid catalysts to acid catalysts on solid supports. Concentrated hydrofluoric acid and sulfuric acid have been reliable standards for alkylation reactions for more than 50 years, Plumlee pointed out. Although considered safe among refinery operations, he said, liquid catalysts are hazardous to handle, transC&EN
/ OCTOBER
1, 2001
63
SCIENCE & TECHNOLOGY port, and store; have additional environmental and economic concerns associated with catalyst regeneration; and lead to waste by-products that in turn pose safety and environmental problems. In addition, permits for new H F production facilities are no longer being issued. Solid acid catalysts have been investigated as alternatives to liquid catalysts for nearly 30 years, Plumlee noted. Some of these catalysts include A1C13; platinum compounds; heteropolyacids, such as tungstates; and liquid acids immobilized on silica, polymers, or other solid supports. Natural or artificial zeolites also have been used. Solid catalysts can improve selectivity and reduce production costs, he said, but they tend to deactivate rapidly under alkylation reaction conditions. The problem is buildup of heavier hydrocarbons (C 12 -C 16 ), known as coke, on the active sites of the catalyst surface. The heavy hydrocarbons also tend to plug the pore structure of solid catalysts, reducing access to acidic sites. Burning off the heavy hydrocarbons by high-temperature oxidation—the traditional regeneration approach for solid catalysts—turns out to be too harsh and can quickly destroy alkylation catalyst activity, Plumlee said. And
attempts to dissolve the hydrocarbons in a solvent take too long. To get around the deactivation dilemma, several companies have been working to develop new catalysts and process technology For example, U O P has a new proprietary catalyst developed for alkylation and has developed a new reactor design and process, called Alkylene. The Alkylene process was described at the symposium by Steven M. Black, UOP's project manager for alkylation technologies. The company first looked at finding a catalyst with sufficient acidity and PLUMLEE an optimal pore size and surface area that could be optimized for alkylation reaction conditions, Black explained. IMPORTANT FACTORS to consider include the isobutane-to-olefin ratio. Higher isobutane concentration results in higher product quality since olefin polymerization side reactions are reduced, he noted. Lower reaction temperature also improves the octane of the product and prevents coking and cracking, he said, and refriger-
ALKYLENE UOP's alkylate reactor design continuously reactivates proprietary solid acid catalyst Products
Residual light hydrocarbons
Fractionation section
• Liquefied petroleum gas
lsobutane/H2 Catalyst reactivation zone
Alkylate
i Hot ' reactivation 1 vessel
Olefin feed
Feed pretreatment
i
Isobutane recycle
ation typically is required to optimize reaction temperature, if economically feasible. Catalyst contact time is important to obtain high olefin conversion as well, but extended contact can lead to secondary isomerization and cracking reactions, Black continued. Also important is having a large catalyst surface area Liquid catalysts supply more than enough surface area, he said, but when a solid acid catalyst is considered, it must have a large number of active sites constandy available. Zeolites and alumina generally fit the bill, he noted, as do some polymer substrates. The Alkylene solution to avoid deactivation was to develop a proprietary solid catalyst, called HAL-100, that can be recycled and cleaned in the reactor by flushing it with a stream of hydrogen or a hydrogen/isobutane mixture, Black said. Hydrogen saturates the heavy hydrocarbons, which are desorbed and transferred from the catalyst. The catalyst reactivation method required a new reactor design. UOP settled on a "transport" reactor that continuously delivers freshly reactivated catalyst to the reactants. Purified fresh olefin is mixed with fresh and recycled isobutane and injected into the bottom of a vertical reactor, he explained. Catalyst is introduced at the bottom of the reactor, where it contacts the isobutane/olefin mixture. Reactants and catalyst flow up the reactor riser at a rate of about 1 foot per second as the reaction occurs. At the end of the reactor, the effluent flows out the top to a fractionator where it is separated into alkylate, liquefied petroleum gas (propane and butane), and residual light hydrocarbons such as methane and ethane brought in with the hydrogen and olefin feeds. Alkylate yield by volume is about 3% higher than for liquid acid technologies, Black said. Meanwhile, the catalyst enters a cold reactivation zone that is flushed with a stream of hydrogen-saturated isobutane, he noted. The catalyst flows by gravity back to the bottom of the reactor, where it begins the reaction cycle again. A portion of the catalyst is continuously sent to a hot re-
"Solid-acid-catalyzed alkylation and isomerization represent a rapidly advancing area of catalytic science with significant environmental and economic impact." 64
C&EN
/
OCTOBER
1.
2001
HTTP://PUBS.ACS.ORG/CEN
Developed by chemists for chemists, Coherent Synthesis™ from Personal Chemistry is winning more and more favourable personal reactions every day. This complete range of knowledge-based systems, optimized reaction kits and software allows you to perform rapid chemical synthesis with consistent, reproducible results and precise control of reactions. By allowing more creative, spontaneous ways of working, it is dramatically cutting lead optimization times. As part of this integrated approach to chemical synthesis, SmithReactionKit™ provides a shortcut to successful results when testing new chemical transformations in drug development. SmithReactionKit also eliminates time-consuming manual preparation steps like weighing, dispensing and programming. The result? Higher overall productivity and increased likelihood of success. For more great reactions, visit our website.
SmithProcessVials™ I For safe and convenient chemistry I
A reaction. "SmithReactionKir allows the chemist to rapidly optimize 10 reactions in parallel, resulting in significantly reduced development times."
SmithCreator ™ The ideal personal system for testing new chemistries with simplicity and speed
Noel Wilson, Research Associate, Boehringer-lngelheim, Ridgefield, USA
y
^i
SmithWorkflowManager™ | Integrates planning, execution and documentation of your reactions
reactionkit Su
*uk. coupling
S
^ith
r
nk ^tion
C *
• «MOQO
^Biill
*X
SmithSynthesizer™ Fully-automated for highest productivity
Coherent Synthesis systems include Single Mode Resonator and Dynamic Field Tuning technology, and are free to run patented Pd-catalysed reactions
www.personalchemistry.com
chemistry
SCIENCE & TECHNOLOGY activation vessel where it is flushed with hydrogen and then returned to the vertical reactor. The catalyst has been tested to more than 9,000 cycles over nine months of continuous operation, Black said, maintaining a more than 95% olefin conversion rate—results that are comparable to those of an H F or H 2 S 0 4 liquid acid reactor. ALKYLENE IS designed to be used as a new system, or it can be used to revamp an existing H F alkylation facility with minimal replacement of equipment, Black said. MTBE producers will also like the process, he predicted, because existing facilities can be reconfigured to accommodate alkylate production. Cost analysis indicates that the system can provide more than 10% savings in capital and production costs over a comparable sulfuric acid plant, Black noted. Thus far, several engineering studies have been completed, and one European customer has chosen to implement the process. A different approach to improving solid acid catalysis and mitigating the problem of catalyst deactivation is the use of supercritical carbon dioxide as the reaction medium. Carbon dioxide has moderate
critical properties (71.8 bar, 31.1 °C), and the supercritical C 0 2 region provides a continuum of properties, noted symposium speaker Bala Subramaniam, professor and chair of the department of chemical and petroleum engineering at the University of Kansas. "There is a window near the critical point where there are unique combinations of fluid properties," Subramaniam said. For example, the density within the window is liquidlike and suitable for solvating heavy hydrocarbons. The supercritical fluid also retains the diffusivity properties of a gas that allows the transport of heavy hydrocarbons or their precursors out of the catalyst pores before they consolidate and foul the catalyst. "By tuning the pressure, one can determine the optimum combination of gaslike and liquidlike properties that is essential for in situ decoking," he added. Supercritical C 0 2 properties can be further enhanced by adding auxiliaries, such as cosolvents, he noted. At first, Subramaniam's group studied alkylation reactions using zeolite catalysts in a lab-scale supercritical C 0 2 system.
Although supercritical C 0 2 was found to be good at dissolving the heavy coke material on the outer catalyst surface, difficulties remained in removing the material from inside the small pores (5 to 10 A) of the catalyst. Subramaniam and graduate student Christopher J. Lyon next tried Nafion on silica—which has a larger mean pore size in the range of 50 to 70 A—as the catalyst. Nafion is a DuPont perfluoropolymer that has sulfonic acid groups along its backbone; the silica-supported form is now marketed by Engelhard. The acidity of Nafion is similar to sulfuric acid, Subramaniam pointed out, and there have been some reports of using Nafion as an alkylation catalyst. These attempts were not successful, he noted, because the catalyst tends to rapidly deactivate in the liquid phase. AS A TEST of the supercritical C 0 2 system, Subramaniam and Lyon reacted isobutane with 1-butene in a 5:1 ratio in a slurry reactor using silica-supported Nafion particles suspended in supercritical C 0 2 . Typically a 10- to 20-fold excess of isobu-
SeXY
CH2NH2
Building new blocks for chemical construction. Every m o n t h .
www.maybridge.com 66
C&EN
/
OCTOBER
1 , 20
Bringing life to drug discovery
MAYBRIDGE HTTP://PUBS. ACS.ORG/CEN
nated in the reaction vessel. The Kansas researchers tested this idea by running daylong cycles of alkylation, then stopping the feed of reactants and increasing the C 0 2 pressure. After four days of cycling, the catalyst activity was retained. By alternating the reaction/regeneration cycle between a pair of reactors, an uninterrupted production process could be established, Subramaniam said. "These results imply that C 0 2 based reaction mixtures and solid acids tailored for optimum pore structure and acidity offer an excellent opportunity for developing environmentally benign S U P E R C R I T I C A L Alkylation reactor in alternatives to conventional alkylSubramaniam's lab at Kansas pumps an ation," he concluded. The reacisobutane/butene mixture (in red bottle) into a tion system should be amenable mechanically stirred reactor (under pressure to other solid-acid-catalyzed reacgauge, right center) where it contacts a silications and isomerization reactions, supported Nafion catalyst suspended in he said. The next steps will be to supercritical C0 2 at 80 bar and 95 °C. The work on further enhancing the C 8 reactor effluent flows to a gas chromatograph alkylate selectivity by designing (bottom left) for product analysis. supported Nafion catalysts with tailored pore size and acidity in collaboratane is needed in conventional alkylation, tion with Engelhard researchers. Subramaniam explained, but in this system part of the isobutane is effectively replaced with C 0 2 . This saves on costs, A RELATED TALK on supercritical fluids has a lower flammability hazard, and lowwas presented in the Division of Fuel ers environmental concerns, he said. Chemistry by Daniel M. Ginosar of Idaho National Engineering & Environmental Operating at 95 °C, reaction conditions Laboratory (C&EN, Sept. 17, page 55). A were optimized by tuning the pressure of former Ph.D. student in Subramaniam's the reactor to 80 bar. Under those condigroup, Ginosar also has been working to tions, C 8 alkylate selectivity was maximized address industry concerns over solid acid at 30% with a butene conversion rate of catalyst regeneration. 80%. On-line gas chromatographic analysis of the effluent was used to look at In a small continuous-flow lab reactor, butene conversion and product composiGinosar and coworkers used a zeolite to tion. The researchers were able to maintain catalyze the reaction of isobutane and transsteady C 8 alkylate production for up to two 2-butene in a 20:1 ratio. Once the catalyst days before the catalyst began to show signs bed was deactivated, alkylation producof deactivation—compared with a few tion was stopped and the reaction system hours operating at subcritical conditions. flushed with supercritical fluid. After the catalyst was rejuvenated, production con"In terms of product quality, these are tinued. Weeklong runs of the alkylation/ not the most ideal results," Subramaniam regeneration cycle with 90% recovery of noted, "but certainly in terms of changing catalyst activity have been achieved, Ginofrom a microporous to a mesoporous catsar reported. alyst, we see dramatic improvement in conversion and selectivity Clearly, milder The role of gasoline blends in reducing supercritical pressures provide the optiair pollution has many possibilities. mum combination of liquidlike densities Although several competing strategies and gaslike transport properties to effecunderway include developing engines with tively remove C 8 products and coke prebetter fuel efficiency that are powered by cursors from the catalyst pores." fuel cells or batteries, the widespread use of these electric or hybrid vehicles is still The initial results led to the hypothesis years away or may never happen. Thus, that catalyst pores could periodically be developing cleaner burning fuels by solid cleaned by dissolving the heavier hydroacid catalysis appears to be a more feasible carbons at high pressure before catalyst immediate fix to meet stringent clean gasoreactivity is significantly diminished. In this line requirements. • way, the catalyst could be directly rejuveHTTP://PUBS.ACS.ORG/CEN
Flash Purification has never been easier.
Introducing
FlashMaster Personal A low cost, single user flash chromatography system.
Features Include: Disposable, Pre-Packed Columns Internal Pump Precise Flow Control Varying Sample Size Accommodation Compact & Portable Design Optional Fraction Collector
J-ISIES CHROMATOGRAPHY 800-874-6244 www.joneschrom.com C&EN
/ OCTOBER
1. 2001
67
