December 19, 2011
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[It seems that 2011 was the year of propylene in Patent Watch. Six reviews covered patents on processes for propylene production or the manufacture of propylene derivatives. Here they are again—Ed.]
Make propylene from inexpensive ethane. For many years, the US petrochemical industry enjoyed an enviable advantage because of the relatively low cost of ethane versus the cost of naphtha in Europe and Asia. For most of the 2000s, this advantage disappeared; natural gas prices spiked along with the price of petroleum.
Now, however, the abundance of shale gas in the United States has caused gas prices, including ethane, to fall relative to oil prices. This occurrence restored the competitiveness of the US ethylene and ethylene derivatives businesses and is driving cracker operators to use more and more ethane instead of heavier, more expensive feedstocks. An unintended consequence is that less propylene is being produced because ethane crackers make very little of this olefin. Consequently, propylene prices have risen.
J.-A. Chodorge and C. Dupraz disclose a method that makes propylene from ethane. Their route includes steam cracking ethane to make ethylene, dimerizing some of the ethylene to 1-butene, hydroisomerizing the 1-butene to 2-butene, and finally metathesizing 2-butene with additional ethylene to make propylene.
The ethylene dimerization step is carried out at a pressure of 2.3 MPa and a temperature of 50 °C. The catalyst consists of an alkyl titanate, an ether, and an aluminum compound. The hydroisomerization step is carried out at a hydrogen pressure of 2.3 MPa and a temperature of 80 °C with a Pd/Al2O3 catalyst.
In the patent’s lone example, feed and production rates are expressed in megatonnes (Mt) per year. The ethylene feed, 243 Mt/year, was fed into the dimerization and hydroisomerization reactors. The effluent consisted of 220 Mt/year of n-butenes, 18 Mt/year of hexenes, and 4 Mt/year of unreacted ethylene. The 2-butene/1-butene ratio was 12:1.
The metathesis reaction was carried out with a rhenium oxide–based catalyst at 35 °C. The feed to the metathesis reactor consisted of the 220 Mt/year of n-butenes from the hydroisomerization reaction supplemented with additional C4 feed (198 Mt/year) derived from a steam cracker and 164 Mt/year of ethylene. The product stream from the metathesis reactor contained 475 Mt/year of propylene, 99 Mt/year of C4 raffinate, and a purge stream (5.5 Mt/year, mostly ethylene). Losses were 2.4 Mt/year. Propylene yield was limited by the reaction equilibrium. (IFP Energies Nouvelles [Rueil Malmaison, France]. US Patent 7,868,216, Jan. 11, 2011; Jeffrey S. Plotkin)
Produce ethylene and propylene with a durable zeolite catalyst. Catalytic cracking of low-value olefins such as butenes to higher value olefins such as ethylene and propylene would be economical if robust catalysts were developed. The challenge to developing such catalysts is that when activity is high, catalyst activity often quickly degrades because coke deposits on the catalyst surface. When activity is low, ethylene and propylene yields are poor. Y. Takamatsu and K. Nomura developed catalysts that can crack butenes to ethylene and propylene in good yield and maintain fairly high activities over time.
The catalysts in this invention are based on hydrogen ion–exchange types of ZSM-5/SiO2 zeolites with 230 ppmw sodium. The zeolite is treated with AgNO3 and ion-exchanged for 2 h to add 0.084 wt% silver to the catalyst. The catalyst is placed in a tubular reactor, and C4 raffinate-2 (the C4 fraction from a naphtha cracker after butadiene and isobutylene are removed) is passed through the catalyst bed at a weight hourly space velocity of 7.25 h–1, a temperature of 550 °C, and a gauge pressure of 0.1 MPa. After 2 h, C4 olefin conversion is 67.97%, and ethylene and propylene yields are 4.95% and 23.1%, respectively. After 48 h, C4 olefin conversion is 60.01%, and ethylene and propylene yields are 3.07% and 21.34%, respectively.
In a test of a similar catalyst with 2200 ppmw sodium, C4 olefin conversion is 57.74%, and ethylene and propylene yields are 2.79% and 19.44%, respectively after 2 h. After 48 h, however the catalyst degrades substantially, and C4 olefin conversion is only 31.63% with very low ethylene and propylene yields. (Asahi Kasei Chemicals [Tokyo]. US Patent 7,893,311, Feb. 22, 2011; Jeffrey S. Plotkin)
These cracking catalysts give ethylene and propylene equally. Catalytically cracking hydrocarbon feedstocks to give light olefins is a long-term goal of industry. The conventional process for manufacturing the two largest-volume petrochemical building blocks, ethylene and propylene, is noncatalytic steam cracking of natural gas liquids, naphtha, or gas oil.
Historically, the goal of steam cracking was to maximize the yield of ethylene; propylene was considered a byproduct. For example, naphtha steam cracking typically gives an ethylene/propylene ratio of 2:1. Over the past few years, however, propylene demand has been strong, and its price is often higher than ethylene. Consequently, techniques to increase propylene yield from steam crackers are now of keen interest.
J.-s. Choi and colleagues disclose a family of catalysts that produce ethylene and propylene in almost equal amounts in good yield. The catalysts have excellent thermal stability and can be regenerated by burning off coke deposits. The catalysts consist of chromium, zirconium, and in some cases phosphorus oxides; some of the catalysts also contain cesium or titanium. Seemingly, the best results were obtained from catalyst composition CrZr4Ce4P11.13Ox.
To test the catalysts, a quartz reactor tube (¼” outside diam) is filled with catalyst to a height of 5 cm. The hexane feedstock is pumped into a vaporizer at a rate of 2.75 mL/h. Water is pumped into a separate vaporizer at 0.92 mL/h. The vaporizer temperatures are maintained at 400 and 500 °C, respectively. The two gases are mixed well and fed to the reactor tube at 800 °C. Hexane conversion is 83.09%, the ethylene/propylene ratio is 1.1:1, and the combined yield of ethylene and propylene is 45.67%.
A key challenge to commercializing this type of catalytic process is demonstrating sufficiently long-lasting catalytic activity and life. The inventors state that the catalysts can be reused after burning the coke off the catalyst, but they supply no data. (LG Chem [Seoul]. US Patent 7,935,654; May 3, 2011; Jeffrey S. Plotkin)
Here’s a biobased route to propylene glycol. Green chemicals and polymers continue to generate increasing interest and activity in the chemical industry. It is clear that a certain percentage of end users, primarily in the packaging industry, are willing to pay a premium for so-called “green” polymers and plastics. The question is: Can viable green chemical and plastics industries be built on the premise of premium pricing?
In the long run, green products must compete on price with conventional hydrocarbon-based chemicals and plastics. There is therefore a need to develop new processes and catalysts that can transform biobased feedstocks into monomers and intermediates with very high selectivity.
G. J. Suppes, W. R. Sutterlin, and M. Dasari describe a process and catalyst system for converting biodiesel glycerol to propylene glycol in good selectivity. The invention is based on the idea that glycerol can be first dehydrated to hydroxyacetone (“acetol”), which can then be hydrogenated to propylene glycol.
The inventors conducted two experiments to support the process:
- Glycerol was converted to acetol over a copper chromite [(CuO)x(Cr2O3)y] catalyst at 180–220 °C and 34–96 MPa pressure. Hydrogen was not used in this step. The acetol was formed in this step, but no data are given as to selectivity or conversion.
- The catalyst used in the first step was used to hydrogenate the acetol at a temperature of 180–220 °C and a pressure of 1–25 bar. Again, no yield data are given.
In separate experiments, glycerol was converted directly to propylene glycol with the same catalyst. The inventors found that glycerol water content plays a major role in propylene glycol yield. If the water content is 80%, conversion is only 33.5% and propylene glycol selectivity is only 64.8%. If the water content is reduced to 20%, however, glycerol conversion increases to 54.8% and selectivity to 85.0%. (The Curators of the University of Missouri [Columbia]. US Patent 7,943,805, May 17, 2011; Jeffrey S. Plotkin)
Convert byproduct glycerol to propylene glycol. Renewables-to-chemicals process technologies are receiving much attention in established industrial laboratories, biotech startups, government labs, and academia. Whereas many innovative processes for taking biofeedstocks to chemicals and polymers are available, these processes will be sustainable in the long term only if their economics are competitive with existing petrochemicals-based processes.
One route with potential attractive economics is the conversion of biodiesel-derived glycerol to C3-based chemicals because the price of propylene, the key C3 petrochemical building-block, has risen dramatically. High propylene prices are being driven by supply-side and demand-side trends.
On the supply side, US steam crackers are cracking more ethane to take advantage of low ethane pricing that is driven by the abundance of shale gas. The result is that less byproduct propylene is being made.
On the demand side, the demand for polypropylene is growing strongly and is increasing the demand for propylene. Limited supply coupled with strong demand is pushing propylene prices to historic highs. Glycerol, a byproduct of biodiesel manufacture, presents a unique opportunity to convert a relatively low-priced C3 feedstock to C3 compounds ordinarily made from costly propylene.
H. Kouno, S. Ozawa, and N. Yoshimura disclose catalysts and operating conditions that foster the liquid-phase conversion of glycerol to propylene glycol in high yields. The catalysts contain zinc oxide, silica, and copper oxide.
Of the patent’s 42 examples, the first one is representative of the efficacy of the invention. Glycerol (24 g), distilled water (6 g), and the catalyst system are sealed in a 100-mL autoclave. The catalyst system consists of two materials, 1.0 g F10G (50 wt% CuxO and 50 wt% ZnO) and 0.20 g E35S (67 wt% CuxO, 27 wt% of SiO2, and 6 wt% binder). (The inventors do not specify the oxidation state of copper in the oxide.) After the autoclave is flushed with nitrogen, it is filled with hydrogen and pressurized to 10 MPa at room temperature. It is then heated to 200 °C and stirred at 450 rpm for 12 h.
Glycerol conversion is 91.1%, and the propylene glycol yield is 86%. A key finding is that it is not necessary to vaporize glycerol to effect the hydrogenolysis, a potentially significant cost savings. (Mitsui Chemicals [Minato-ku Japan]. US Patent 8,053,608, Nov. 8, 2011; Jeffrey S. Plotkin)
A little hydrogen boosts selectivity to propylene oxide. Propylene oxide (PO) process technology has undergone dramatic changes over the past several years. Beginning in 1974, the PO process that was generally considered to be the low-cost method was the so-called propylene oxide–styrene monomer (POSM) process. In it, 2.3 tons of styrene is produced for each ton of PO. Therefore, POSM technology users are in the styrene business whether they want to be or not.
This type of process is called a “2-for-1” process—nice in concept, but in practice it only works if the markets for both products are growing at about the same rate. In POSM, styrene was often in oversupply, resulting in very low prices and causing a loss in overall profitability for POSM producers.
Over the past several years, two new processes for making PO with no coproduct have been commercialized. One is Sumitomo’s cumene-based process, and the other is based on H2O2. The “holy grail” of the PO business, however, is a process for direct oxidation of propylene by oxygen to PO. This route has eluded commercialization because of low yields.
M. Haruta and colleagues found that the direct oxidation of propylene with oxygen can be carried out with very high selectivity if a catalytic amount of hydrogen is used. To demonstrate their method, the inventors first passed propylene, oxygen, and argon (1:1:17 vol/vol/vol) over a potassium-modified gold-containing titanium silicalite catalyst (Au/TS-1-K1) with a gaseous hourly space velocity of 4000 h–1 at 200 °C. After 2 h, propylene conversion was only 0.33%, and selectivity to PO was 11.8%. CO was the main product (56.5% selectivity).
The experiment was repeated with 1 vol% hydrogen added to the gas mixture. Propylene conversion and PO selectivity increased dramatically to 2.3% and 70%, respectively, and CO selectivity decreased to 24%. This process has promise, but propylene conversion must be increased to make it commercially viable. (Tokyo Metropolitan University and National Institute of Advanced Industrial Science and Technology [Tokyo]; US Patent 7,973,184, July 5, 2011; Jeffrey S. Plotkin)