| MECHANISM OF ACTION |
Based on mechanistic studies, author explains that the newly discovered methyl mercaptan-to-formaldehyde reaction proceeds by way of an S--O exchange step. Upon adsorption on the catalyst surface, methyl groups in the sulfur compounds bind to lattice oxygen atoms--in effect bridging sulfur and oxygen. The additional 20 kcal per mol strength of C--O bonds relative to S--O bonds drives the reaction selectively to form surface methoxy groups, which then lose a methyl hydrogen atom--just as methanol does--to make formaldehyde. The reaction proceeds by way of a mechanism first described some 50 years ago, author notes. A key feature of the mechanism is that adsorbates react with oxygen in the lattice, which is then replenished by gas-phase oxygen. |
| TECHNOLOGY |
CATALYTIC CLEANUP AT THE PULP MILL New procedure converts methanol and mercaptan waste to formaldehyde MITCH JACOBY, C&EN CHIGAGO Alchemists are going to turn green with envy. Turning matter--any kind of matter--into gold is quite a feat. But turning garbage into gold? That's a real trick. Okay, the gold isn't really gold. But the garbage is honest-to-goodness stinky garbage. Author developed a new catalytic process that converts waste streams from pulp and paper mills into valuable products. Not only does the technology provide the U.S. paper industry with a new and efficient option for complying with recently enacted environmental legislation, it enables them to do so at a profit. At more than 150 pulp mills in the U.S., and many more abroad, wood is converted into pulp for papermaking through kraft processing--high-temperature and high-pressure procedures in which wood chips are digested in caustic sulfurous solutions. The process, which has been used for over a century, generates large quantities of unwanted methanol and mercaptans. For many years, the process waste was often discharged into streams and rivers. "The methanol caused fish in the streams to die, and residents living near the mills had to put up with extremely foul odors of mercaptans and ammonia," says author. To alleviate the environmental problems, the pulp and paper industry voluntarily adopted practices of incinerating the waste or feeding it to large holding ponds where bacteria consume the undesirable products. The trouble with those methods, author notes, is that they emit carbon dioxide, a greenhouse gas, and sulfur dioxide, which leads to acid rain. "This approach has converted a water pollution problem to an air pollution problem," he says. Dissatisfied with those options, author turned to catalysis for a more effective treatment procedure. He discovered and patented a supported vanadia-titania catalyst that selectively oxidizes methanol and mercaptans in pulp mill waste streams to formaldehyde via a nonpolluting route with a yield and selectivity of roughly 90%. Formaldehyde is an attractive product because the material is widely used to make high-strength adhesives, scratch-resistant coatings, and particle-board adhesives, and is used in other applications. Collected from digesters, evaporators, and other mill sources, typical waste streams contain 40 to 50% methanol and about the same amount of water. Methanol alone accounts for 70 to 80% of the total volatile organic emissions from these types of mills. Some sulfur compounds, such as CH3SH, CH3SCH3, and CH3SSCH3, are present in the stream in the 1 to 5% range, as are low levels of terpenes, pinenes, and related wood and plant oil components. Small amounts of ammonia are also present. Since many of today's paper companies plant more trees than they harvest, the process is sustainable. Although many major mills in the U.S. had begun treating these waste products decades before such action was mandated last April by EPA, the new methanol-emissions regulations have led additional pulp mill operators to incinerate the waste. But because the stream does not burn particularly well, the waste is often supplemented with natural gas, which increases the cost of the operation and leads to emissions of CO2, SO2, and nitrogen oxides. Other procedures for improving the combustibility of the waste have also been examined. "It just didn't make sense to me to pay to burn the methanol if a suitable heterogeneous catalyst could be found to convert the waste into a valuable product." "Initially, I was concerned that the enormous amount of water would flood the catalyst surface and that the sulfur compounds would poison it," he notes. But based on earlier work at University. Author predicted that a supported V2O5/TiO2 catalyst had the best chance of surviving the harsh environment. Similar catalysts are used in the presence of 10% steam and 1,000 ppm of SO2 to selectively reduce ammonia and nitrogen oxides in power plant emissions. In initial microreactor studies at University, author found that the catalyst was not deactivated by sulfur or flooded by steam. Rather, methanol was converted to formaldehyde, and the sulfur components yielded CO2 and SO2. But upon further study, he found that mercaptans are converted to formaldehyde en route to forming CO2. "That was an amazingly unexpected surprise," author exclaims, "and the Co.'s people were pleasantly shocked by the discovery." At that point, author fine-tuned the catalyst to minimize overoxidation of formaldehyde to CO2. SO RATHER THAN laying out money to comply with EPA regulations, author emphasizes, his process provides an environmentally sound alternative for meeting emissions standards and turning a profit at the same time Summarizing the method's advantages, author points out that, except for small quantities of CO2 from an unwanted overoxidation reaction, all the carbon in the waste stream is tied up in formaldehyde--a valuable material used in large quantities by companies Ammonia, also present in the waste stream, is converted to nitrogen and water. SO2, which is produced by incineration, biodigestion, or catalytic conversion in roughly equal volumes, can be captured readily in the new procedure and used in mill operations, such as wood digestion, or sold as a commodity. According to author options for using sulfur dioxide include oxidizing SO2 to SO3 and making sulfuric acid--also utilized in large quantities by the paper industry--or making calcium sulfate (gypsum), which is turned into construction wallboard (drywall). Another benefit of the catalytic treatment is that it does not emit nitrogen oxides--unlike incinerating the waste at high temperature (1,000 °C). Author also points out that the technology is green chemistry. "There are no fossil fuels involved," he says. And since many of today's paper companies plant more trees than they harvest, the process is sustainable. In addition, the terpenes and related compounds have commercial value. Those materials are separated from the waste by adsorbing them on activated carbon. Author stresses that the new method "has the potential to change the paradigm of pollution control. Companies should no longer assume that they have to operate at an economic deficit to solve pollution problems and meet EPA requirements." Several pulp and paper manufacturers have expressed interest in the cleanup technology, author notes, adding that an agreement is being discussed with a major company to begin manufacturing the catalyst. |
| UPDATE | 03.02 |
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