The Changing Landscape of Hydrocarbon Feedstocks for Chemical Production Implications for Catalysis: Proceedings of a Workshop (2016) / Chapter Skim
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4 Catalytic Conversion of Light Alkanes
4 Catalytic Conversion of Light Alkanes
Pages 37-70
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From page 37... ...
Alexis Bell, the Dow Professor of Sustainable Energy at the University of California, Berkeley, and faculty senior scientist at the Lawrence Berkeley National Laboratory, described some of the lessons learned from theory and experiment about methane, ethane, and propane conversion over heterogeneous catalysts, and Shannon Stahl from the University of Wisconsin–Madison discussed the use of homogeneous catalysts to activate the carbon–hydrogen bond. Each presentation was followed by a brief discussion period.
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From page 38... ...
NOTE: FCC = fluid catalytic cracking; MTA = million tons per annum. SOURCE: Bricker, 2016.
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From page 39... ...
Another feature of PDH technology is that coke formation is unavoidable, leading to a catalyst life of days and the need for frequent regeneration. As in all catalytic–process technology, he added, the process and catalyst are intertwined and cannot be separated.
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From page 40... ...
Today, two PDH technologies -- UOP's Oleflex process and the Lummus Catofin process -- dominate propylene production. In total, there are now 22 PDH units in operation worldwide, and unit capacities of the newest facilities have been increasing to as large as 1 million tons per year.
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From page 41... ...
Important advances have been made with two other routes to ethylene -- catalytic ethane dehydrogenation and ethane oxidative dehydrogenation -- but there are no commercial units that he is aware of that either use or plan to use these technologies in the near future, largely because the economics of ethane stream cracking are hard to beat. Steam crackers, explained Bricker, not only have a low cost of production and
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From page 42... ...
. Oxidative dehydrogenation, he noted, could have a much lower energy and carbon dioxide footprint if done with high selectivity and at high ethane-conversion rates, and would likely enable a continuous production process.
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From page 43... ...
However, his group is now working with high-velocity reactor designs to see if it can figure out how to manage heat flow at an industrial scale. Another interesting approach for oxidative dehydrogenation used sulfur as a mild oxidant in combination with a molten salt catalyst (Gaspar et al., 1974)
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From page 44... ...
He noted that his company has made a significant effort aimed at improving downstream separations. HETEROGENEOUS CATALYSIS: LESSONS LEARNED FROM EXPERIMENT AND THEORY Chemists have developed a number of different routes by which natural gas can be converted to chemicals using catalysis (see Figure 4-6)
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From page 45... ...
. Several years ago, Bell and his colleagues began to look at the effect of platinum particle size and the concentration of tin on coke formation and to identify the mechanism of coke formation and its influence on platinum nanoparticles.
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From page 46... ...
, and they demonstrated that carbon deposition induces step formation that then serves as additional nucleation sites for carbon formation. FIGURE 4-7 The effect of platinum particle size on carbon accumulation.
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From page 47... ...
SOURCE: Peng et al., 2012. Researchers have also studied oxidative dehydrogenation of ethane and propane by vanadium catalysts dispersed onto alumina.
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From page 48... ...
. At the same time, steps decrease the activation energy for methane dissociation, so what is good for catalytic activity is bad for coke formation, Bell explained.
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From page 49... ...
Looking to the future, Bell listed several goals, including • identify catalysts that operate at high temperature and are resistant to coke formation; • identify single-site catalysts that enable the continuous conversion of methane to methanol; • identify catalysts than can promote the oxidative dehydrogenation of alkanes to alkenes selectively; and • understand the nature of oxygen species and what controls their activity. During the ensuing discussion, Bala Subramaniam from the University of Kansas noted that because catalysts will increase the rate of conversion but not the equilibrium, process engineering in combination with catalyst development could be a powerful complement.
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From page 50... ...
Bricker then asked if the tin in the platinum–tin catalysts was exerting a geometric or electronic effect, and Bell replied that quantum mechanical calculations show it to be an electronic effect in addition to the geometric effect of impacting nucleation sites. HOMOGENEOUS CATALYSIS FOR CARBON– HYDROGEN BOND ACTIVATION Homogeneous catalysts, said Shannon Stahl, have been used in numerous major industrial processes, and he wondered if the dividing line between homogeneous and heterogeneous catalysis is meaningful or if it is an artifact of the silos that separate chemistry and chemical engineering departments at most universities.
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From page 51... ...
. One of the features of homogeneous catalysis that distinguishes it from heterogeneous catalysis is that soluble molecular complexes can often activate specific carbon–hydrogen bonds, not necessarily the weakest one.
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From page 52... ...
The question, said Stahl, is whether this type of system could be used to couple two methane molecules to produce ethane, which would then undergo oxidative dehydrogenation on the same palladium catalyst to yield ethylene. In such a scheme, oxygen would act as a hydrogen acceptor in the ethane-to-ethylene reaction.
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From page 53... ...
The subjects discussed by the four groups included light alkanes to alkenes and dienes, light alkanes to aromatics, emerging opportunities for novel approaches to natural gas conversion, and activation of natural gas using nontraditional oxidants. As was the case with the first set of working group discussions, each group, after hearing a short introductory presentation, was asked to answer a set of questions over the course of their of deliberation.
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From page 54... ...
2. What are the top two to three well-established research approaches to making alkenes and dienes or aromatics or for activating natural gas with nontraditional oxidants viable, and what are the challenges associated with them?
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From page 55... ...
There is a desire, she said, to exploit new processes for butadiene production, and she suggested two potential routes: ethylene dimerization followed by oxidative dehydrogenation and a one-step oxidation of butane to butadiene. She also highlighted the lack of research on alternative oxidants for oxidative dehydrogenation, including the use of carbon dioxide as a mild oxidant (Ascoop et al., 2016; Koirala et al., 2015; Porosoff et al., 2015)
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From page 56... ...
For PDH, one challenge is to develop enough of a knowledge base to enable the rational design of selective and stable catalysts, which Marks added is an overarching theme for the entire workshop. Another challenge is to achieve similar selectivity but with a lower carbon footprint than oxidative dehydrogenation, and to do so with simpler reactors requiring smaller capital expenditures.
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From page 57... ...
The group noted that the national laboratories have facilities to test novel reactor designs safely and that researchers could collaborate with those laboratories when it comes to testing design prototypes. When incorporating oxidative dehydrogenation chemistry with other processes, or considering the use of alternative oxidants, Marks added that there is value for researchers to think about scalability and environmental viability.
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From page 58... ...
deactivation requires continuous regeneration using UOP's moving bed reactor and catalyst regenerator design incorporated in its Oleflex process, but the robust catalyst has a substantial lifetime. An alternative process, Chevron's Aromax® process, uses a platinum cluster-zeolite catalyst to produce benzene and toluene, but it is more suited to converting larger alkanes, such as hexane and heptane, into aromatics.
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From page 59... ...
Another avenue for research would be to attempt to tailor metalcontaining catalytic sites on or in a molecular sieve framework, either as single sites or multi-atom clusters. He also suggested a research effort aimed at understanding the chemistry of catalyst synthesis and at relating catalytic activity, selectivity, and stability to structure using theory and spectroscopy with functioning catalysts.
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From page 60... ...
• Determine the nature and mechanism of coke formation and devise strategies for limiting the sites at which coke is able to form or directing coke to form away from the active sites. • Characterize the location of active sites in zeolite structures, their stability in the presence of reagents at process-relevant tempera tures, and any factors that might increase the lifetime of the active sites.
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From page 61... ...
At 1,273 K, this fuel cell produces ethylene almost exclusively, with only trace amounts of ethane, carbon monoxide, and carbon dioxide. There are also published reports of electrocatalytic conversion of methane to methanol (Fan, 2015; Lee and Hibino, 2011; Spinner and Mustain, 2013)
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From page 62... ...
Turning to the subject of biocatalysis, Koffas said that methane is an excellent source of carbon and energy for microorganisms known as methanotrophs, which historically have been used for producing feedgrade biomass. These bacteria are capable, he explained, of converting methane into protein, alkanes, alcohols, sugars, dicarboxylic acids, and other higher-value chemicals such as carotenoid pigments and vitamins.
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From page 63... ...
As far as specifics, the working group voiced interest in these electrochemical processes but the concern was that these technologies may not be viable at an industrial scale because of the difficulty in scaling the electrocatalytic systems and operating them at scale. Another barrier to commercial viability is the high expected cost of building industrial scale electrocatalytic reactors, whether they are fuel cells or systems based on non-Faradaic electrochemical modification of catalytic activity.
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From page 64... ...
Another confounding issue for biological systems is the potential impact of natural gas impurities on the microorganisms. One potential advantage of biosynthetic approaches to alkane modification is the possibility of making materials not currently accessible in high volumes or entirely new materials for which markets could be developed.
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From page 65... ...
As an example, converting methane to syngas for the production of methanol and other chemicals produces between 0.5 and 1 ton of carbon dioxide per ton of methane. Aside from the issue of carbon dioxide emissions, McFarland said there is another reason to look at alternative oxidants for hydrocarbon conversion, which is to make the best use of the chemical potential stored in the carbon–hydrogen bond.
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From page 66... ...
Discussion Group rapporteur James Stevens, recently retired as the Dow Distinguished Fellow at The Dow Chemical Company, began the discussion with the comment that over the years he had seen numerous examples of methane activation with nontraditional oxidants involving the conversion of methane to methyl-X, where X is a leaving group, and that if he were to poll the workshop participants, each one could probably identify one leaving group that someone in industry or academia had tried and FIGURE 4-13 Methane conversion to methyl halide using a molten salt configuration. SOURCE: McFarland, 2016.
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From page 67... ...
With regard to well-established research approaches for activating natural gas with nontraditional oxidants, this group pointed out that processes using halogens to make, for example, methyl chloride and methyl bromide, which would serve as intermediates to make valueadded hydrocarbon products have been piloted by numerous companies. The use of bromine as a nontraditional oxidant for methane coupling has several advantages, particularly because the heat of reaction of methane with bromine is much lower than that with oxygen, while still being an exothermic reaction, which has the potential to make the bromination reaction more selective.
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From page 68... ...
Molten halide salt processes and solvents could offer an approach to recycle reactants and dissipate heat. Research opportunities the group discussed as potential routes for overcoming obstacles for the use of nontraditional oxidants for natural gas included developing a more extensive knowledge base about strong electrolytes and identifying novel reactor and process materials for dealing with corrosive conditions.
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From page 69... ...
Tobin Marks added developing more robust catalysts, and Lercher said studying carbon–hydrogen bond activation with the goal of creating more active and selective catalysts capable of operating at lower temperature. Moving forward, Mark Barteau from the University of Michigan suggested that the catalyst and process should be thought about as an integrated system.
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