Isobutene Production

Figure 3. Catalytic activity of NaHY fv = initial rate for isobutene production) as a function of n Na " ions exchanged per U.C.

Fig. 4. Isobutane dehydrogenation. Turnover frequeneies for isobutene production over Pt-Sn/Al203 catalysts as a function of Sn/Pt atomic ratio. Catalysts prepared by I coimpregnation SI APt/Al203 blank SI without hydrogen.

In many of the processes where ion exchange resin catalysts have proved valuable some additional factor has played a role in allowing technology to evolve, and in some instances the selectivity achieved is not well understood, but is accepted. In the case of pure isobutene production from cleavage of MTBE (see above) the sulphonic acid resin used is specifically designed to minimise other known side reactions (Figure 6.30). These are... 

ABSTRACT. This contribution reviews the most recent progress on the development of novel FCC catalyst. The following subjects are considered catalyst preparation and structure (matrix, binder, zeolite component), catalytic selectivity (stability, mesoporosity, nonframework alumina, hydrogen transfer), isobutene production for reformulated gasolines and FCC additives (CO promoters, 80, transfer catalysts, and octane additives). 

Major methods for isobutene production are from a C4 stream of a steam cracker, from a catalytic cracker butene-butane stream, through dehydration of tert-butanol (which is obtained from a propene oxide process) and through isomerisation of n-butane to isobutene and subsequent dehydrogenation to isobutene (Obenaus et al. 2000 van Leeuwen et al. 2012 Romanow-Garcia et al. 2007). 

On the other hand, a biological route for isobutene production has a distinct advantage over biobutanol fermentation. During the fermentation process not liquid products but instead gaseous isobutene is formed. This means that it can be relatively easily recovered from the fermenter together with CO2. Furthermore, due to the low solubility of isobutene in water, there are no problems with product toxicity, unlike butanol or isobutanol (Lee et al. 2008 van Leeuwen et al. 2012). 

When considering the knowledge base of biobutanol production, enormous amounts of time and effort have already been invested into research and development, there is an abtmdance of scientific and industrial resources, and there already are production plants in place, while sustainable industrial-scale production is plausible in the foreseeable future. On the other hand, research and know-how on industrial bio-isobutene production is practically stiU in its infancy. 

A cell-free isobutene-forming system from disrupted R. minuta cells was also created (Fujii et al. 1988). Isobutene was produced from a-ketoisocaproate, isovaleryl-CoA and isovalerate where isovalerate gave the best isobutene formation rate of these three with 9.1 nL/L/h. Adding NADPH to the reaction mixture proportionally increased the rate of isobutene production, until a concentration of around 0.1 mM, as did increasing the concentratiOTi of isovalerate until 30 mM. EDTA had no inhibitory effect (Fujii et al. 1988). The formation of isobutene was inhibited by some redox reagents and completely eliminated by the presence of carbon monoxide (Fujii et al. 1989b). 

Differently than with living cells, the optimal pH for isobutene production was between pH 7.3 and 8.0, while no formation of isobutene occurred above pH 8.5 or below pH 6.5. The optimum temperature was, as in the cell system, cs. 25 °C (Fujii et al. 1988). 

The productivity is low—the highest achieved isobutene production rate is 0.45 mg/L/h, but at least 1 g/L/h is required for commercial production of low-value chemicals (Fujii et al. 1987 Gogerty and Bobik 2010). 

This pathway was also used in a patent by Marliere (2010), which describes how MDDs from S. cerevisiae and 10 other microorganisms were analysed for isobutene synthesis. From the tested genes, MDD from Picrophilus torridus (an obligate aerobic archaeon) expressed in E. coli achieved the highest isobutene production rate. 

Additional MDDs were assayed using Th. acidophilum MDD. Special focus was on Streptococcus genus. Enzymes that showed especially greater efficiency in isobutene production are from S. gallolyticus, S. sp. M143, S. salivarius and S. mitis, achieving approximately two-fold increase in isobutene production than the Th. acidophilum MDD alone (Marliere et al. 2013). 

Substrate selection and raw biomass processing, especially in the context of lignocellulosic biomass utilisation, have already been addressed in detail in the previous chapters. Here, we will focus on specificities in terms of isobutene production. In order to achieve sustainable and economically viable isobutene production, future isobutene processes should also be focused on using lignocellulosic biomass as a substrate. 

Isobutene production could use more dilute hydrolysate substrate than the ones in ethanol fermentation. This is because isobutene concentraticMi in the off-gas will not depend on substrate cmicentratiOTi, and the risk of product separation problems due to contamination is much smaller for isobutene (van Leeuwen et al. 2012). 

There are very limited information on this topic, since most of the experiments on bio-isobutene production are lab scale and did not reach the phase of commercial productivity. This is why there is almost no data on isobutene fermentation in different bioreactor operation modes. Still, some considerations can be made theoretically. 

Besides these drawbacks, fermentative isobutene production has a clear advantage over butanol, considering the relatively simple separation methods and circumvention of product toxicity. Also, direct fermentative production seems to be more favourable than fermentative production of isobutanol followed by chemical dehydration (van Leeuwen et al. 2012). 

Fujii T, Ogawa T, Fukuda H (1987) Isobutene production by Rhodotorula minuta. Appl Microbiol Biotechnol 25(5) 430-433... 

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