Formation Of Isobutene Research Articles

Highly selective formation of isobutene over Zn modified m-ZrO2 catalyst from syngas

The direct synthesis of isobutene from syngas is a very promising route, but there has been a bottleneck in achieving high isobutene selectivity. Here, the efficient synthesis of isobutene from syngas was achieved over a series of Zn modified m-ZrO2 catalysts. Zn species played a key role in improving the physicochemical properties of the m-ZrO2 catalyst. The structure and physicochemical properties of the catalysts were revealed by multiple characterization technique. The characterization results revealed that the introduction of Zn species significantly improved the intensity of Zr3+ and the number of oxygen vacancies. Meanwhile, it also improved the acid-base properties of the m-ZrO2 catalyst. These excellent properties were beneficial to the formation of isobutene from syngas. In CO hydrogenation, 2 % Zn/m-ZrO2 catalyst achieved 93.3 % selectivity for isobutene in C4 hydrocarbons. This enhancement of isobutene selectivity and catalytic activity is very helpful for the development of novel efficient catalysts for the direct synthesis isobutene from syngas.

Hydrothermally fabricated Yb2O3 catalyst for vapor-phase dehydration of 3-methyl-1,3-butanediol to isoprene

Several rare-earth oxides, prepared through a hydrothermal process, were employed as catalysts for vapor-phase dehydration of 3-methyl-1,3-butanediol (3MBDO) to isoprene. Ytterbium oxide (Yb2O3) emerged as the most efficient catalyst for the dehydration of 3MBDO to isoprene. The hydrothermal time and calcination temperature influenced the performance of Yb2O3. The reaction temperature and contact time strongly affect the dehydration of 3MBDO and unsaturated alcohols, while the major side reaction of isobutene formation via decomposition of 3-methyl-3-butene-1-ol was mainly influenced by the reaction temperature. The highest isoprene yield of 92% was achieved at 450 °C and a long contact time of 3.75 h. The poisoning experiment using CO2, 2,6-, and 3,5-dimethylpyridine revealed the importance of base and acid sites of Yb2O3 in the dehydration of 3MBDO, indicating the dehydration of 3MBDO to isoprene proceeded via an acid-base concerted mechanism.

Effect of crystal structure of ZrO2 catalyst on isobutene synthesis from CO hydrogenation

ZrO2 catalysts with different crystal structures show different catalytic performance in isobutene synthesis from CO hydrogenation reaction. Although monoclinic ZrO2 has the best catalytic performance in isobutene synthesis from syngas, its isosynthesis active sites are still not well understood. To better understand the critical parameters that influence syngas to isobutene reactions over ZrO2 catalysts, we prepared a series of ZrO2 catalysts with distinct crystal structures and investigated their catalytic performance of CO hydrogenation to isobutene. Compared with tetragonal and amorphous ZrO2 catalysts, there are more coordinatively unsaturated Zr and O sites on the surface of monoclinic ZrO2 catalyst. The coordinatively unsaturated Zr sites are the active sites of CO adsorption and activation, which is beneficial to CO conversion. The coordinatively unsaturated O sites provide more basic sites for isobutene formation. Furthermore, the coordinatively unsaturated Zr and O sites on monoclinic ZrO2 catalyst surface may inhibit electron transfer to formate species formed during reaction, resulting in weak adsorption on catalyst surface of formate species. The weakly adsorbed formate species on the surface of monoclinic ZrO2 catalyst is favorable for the synthesis of isobutene from CO hydrogenation reaction.

Effect of Diffusion Constraints and ZnOx Speciation on Nonoxidative Dehydrogenation of Propane and Isobutane over ZnO-Containing Catalysts

Heterogeneously catalyzed gas–solid-phase reactions generally suffered from diffusion limitations in large-scale processes or in academic studies when zeolites were used as catalysts or supports. Here, we elucidated the effects of diffusion of reactants/products in nonoxidative propane (PDH) and isobutane dehydrogenation (iBDH) reactions on the performance of catalysts possessing differently structured ZnOx species on (S-1), dealuminated beta (deAl beta), and ZrO2. The catalysts were prepared through physically mixing ZnO and the support. Force-field molecular dynamics simulations revealed that the effectiveness factor η is larger than 0.99 in the PDH reaction over all catalysts and in the iBDH reaction over the ZnO-deAl beta catalyst, thus suggesting that mass transport limitations do not play any significant role. However, the iBDH reaction over S-1-based catalysts suffers from some diffusion limitations (0.35 < η < 0.9). Such conditions are favorable for cracking reactions responsible for isobutene selectivity loss. To compare intrinsic catalyst activity in the PDH and iBDH reactions over the ZnOx/S-1 catalyst, molecular-level insights into individual reaction pathways were derived from density functional theory calculations. The nature of active ZnOx sites was investigated by X-ray absorption spectroscopy and was established to depend on the kind of support material. Binuclear ZnOx species are formed inside small S-1 pores or on the surface of ZrO2, while three-dimensional multinuclear ZnOx clusters are generated in the β zeolite with larger pores. The latter show higher Zn-related activity in the PDH reaction under conditions free of any diffusion constraints. The developed ZnO–deAl beta showed the space–time yield of propene or isobutene formation of 2 kgC3H6 kgcat–1 h–1 or 6.3 kgi-C4H8 kgcat–1 h–1 at 550 °C and about 70–80% equilibrium alkane conversion with an olefin selectivity of about 90%. The activity values are higher than those reported for the state-of-the-art non-noble metal oxide catalysts tested at the same or even higher temperatures.

Comparative catalytic study on butene/isobutane alkylation over LaX and CeX zeolites: The influence of calcination atmosphere

Lanthanum-containing (LaX) and cerium-containing X zeolites (CeX) were prepared by a double-exchange, double-calcination method. By changing the calcination atmospheres between nitrogen and air, the CeIV contents in CeX zeolites were adjusted and their impacts on physicochemical properties and catalytic performance in isobutane alkylation were established. The crystallinity of CeX zeolite was found to be negatively correlated with the CeIV content. This i s believed to be due to the water formed during the oxidation of CeIII, which facilitates the framework dealumination. As a consequence, calcining in air resulted in a great elimination of strong Brønsted acid sites while under nitrogen protection, this phenomenon was mostly hindered and the sample's acidity was preserved. When tested in a continuously flowed slurry reactor, the catalyst lifetime for isobutane alkylation was found to be linearly related to the strong Brønsted acid concentration. In addition, Ce3+ was found more benefit for the hydride transfer compared with La3+, which is ascribed to the stronger polarization effect on the CH bond of isobutane. Moreover, the decline of hydride transfer activity can be slowed down by the catalytic cracking of the bulky molecules. Based on the product distribution, a new catalytic cycle of dimethylhexanes (DMHs) involving a direct formation of isobutene rather than tert-butyl carbocation was proposed in isobutane alkylation.

Mechanism and Kinetics of Acetone Conversion to Isobutene over Isolated Hf Sites Grafted to Silicalite-1 and SiO2.

Isolated hafnium (Hf) sites were prepared on Silicalite-1 and SiO2 and investigated for acetone conversion to isobutene. Characterization by IR, 1H MAS NMR, and UV-vis spectroscopy suggests that Hf atoms are bonded to the support via three O atoms and have one hydroxyl group, i.e, (≡SiO)3Hf-OH. In the case of Hf/Silicalite-1, Hf-OH groups hydrogen bond with adjacent Si-OH to form (≡SiO)3Hf-OH···HO-Si≡ complexes. The turnover frequency for isobutene formation from acetone is 4.5 times faster over Hf/Silicalite-1 than Hf/SiO2. Lewis acidic Hf sites promote the aldol condensation of acetone to produce mesityl oxide (MO), which is the precursor to isobutene. For Hf/SiO2, both Hf sites and Si-OH groups are responsible for the decomposition of MO to isobutene and acetic acid, whereas for Hf/Silicalite-1, the (≡SiO)3Hf-OH···HO-Si≡ complex is the active site. Measured reaction kinetics show that the rate of isobutene formation over Hf/SiO2 and Hf/Silicalite-1 is nearly second order in acetone partial pressure, suggesting that the rate-limiting step involves formation of the C-C bond between two acetone molecules. The rate expression for isobutene formation predicts a second order dependence in acetone partial pressure at low partial pressures and a decrease in order with increasing acetone partial pressure, in good agreement with experimental observation. The apparent activation energy for isobutene formation from acetone over Hf/SiO2 is 116.3 kJ/mol, while that for Hf/Silicalite-1 is 79.5 kJ/mol. The lower activation energy for Hf/Silicalite-1 is attributed to enhanced adsorption of acetone and formation of a C-C bond favored by the H-bonding interaction between Hf-OH and an adjacent Si-OH group.

Isobutene production in Synechocystis sp. PCC 6803 by introducing α-ketoisocaproate dioxygenase from Rattus norvegicus

Cyanobacteria can be utilized as a platform for direct phototrophic conversion of CO2 to produce several types of carbon-neutral biofuels. One promising compound to be produced photobiologically in cyanobacteria is isobutene. As a volatile compound, isobutene will quickly escape the cells without building up to toxic levels in growth medium or get caught in the membranes. Unlike liquid biofuels, gaseous isobutene may be collected from the headspace and thus avoid the costly extraction of a chemical from culture medium or from cells. Here we investigate a putative synthetic pathway for isobutene production suitable for a photoautotrophic host. First, we expressed α-ketoisocaproate dioxygenase from Rattus norvegicus (RnKICD) in Escherichia coli. We discovered isobutene formation with the purified RnKICD with the rate of 104.6 ​± ​9 ​ng (mg protein)-1 min-1 using α-ketoisocaproate as a substrate. We further demonstrate isobutene production in the cyanobacterium Synechocystis sp. PCC 6803 by introducing the RnKICD enzyme. Synechocystis strain heterologously expressing the RnKICD produced 91 ​ng ​l−1 OD750−1 ​h−1. Thus, we demonstrate a novel sustainable platform for cyanobacterial production of an important building block chemical, isobutene. These results indicate that RnKICD can be used to further optimize the synthetic isobutene pathway by protein and metabolic engineering efforts.

Fabrication of hierarchically porous MgFe2O4/N-doped carbon composites for oxidative dehydrogenation of isobutane

MgFe2O4/N-doped carbon (MgFe2O4/N-C) composites with a hierarchically porous structure were successfully synthesized by the one-pot method using MgO as the template and were investigated for the oxidative dehydrogenation of isobutane with CO2. The catalysts consisting of MgFe2O4 nanoparticles supported on the active carbon (MgFe2O4/AC) and pure MgFe2O4 phase were also prepared for comparison. A series of characterizations with XRD, TEM, XPS and Raman spectroscopy were performed for these materials in order to explore the relationship between the structure and catalytic performance. The high surface area with multiple types of pore structures showed up to 15% iron addition in MgFe2O4/N-C. Furthermore, the effects of acid and base properties on the catalysts were investigated by CO2-TPD and NH3-TPD analyses. Although the coexistence of basic and acidic sites in MgFe2O4/N-C and MgFe2O4/AC samples was observed, the amounts and distribution of the acid and base strengths were very different. The adsorption–desorption of isobutane or preadsorbed CO2 was utilized to examine the conversion of reactants. It was found that the lower alkali and higher acid contents may have been responsible for the increased catalytic activity. The isobutene formation rate on MgFe2O4/N-C exceeded that obtained when using the active carbon as a support and was 50%.

Ga(N,P) Growth on Si and Decomposition Studies of the N–P Precursor Di-tert-butylaminophosphane (DTBAP)

III/V semiconductors containing small amounts of nitrogen (dilute nitrides) are promising for applications such as lasers and solar cells. Metal–organic vapor-phase epitaxy (MOVPE) is a widely used technique for growing III/V semiconductors on an industrial scale, and the growth of dilute nitrides with this method is promising for later successful market entry. The main issues of dilute nitrides are carbon incorporation and low nitrogen incorporation efficiency of the conventional N precursors. Due to the high N incorporation efficiency and the low decomposition temperature of the As and N precursor di-tert-butylaminoarsane (DTBAA), a similar P- and N-containing precursor, di-tert-butylaminophosphane (DTBAP), was synthesized and purified on a laboratory scale. Growth studies using this precursor were carried out in this work realizing Ga(N,P)/GaP multi quantum wells on Si and GaP substrates. The structures show evidence of N incorporation, and good layer structures were confirmed by high-resolution X-ray diffraction. Following the influence of different growth parameters on the N incorporation, the growth rate and surface morphology were characterized to set a foundation for possible growth applications in the future. DTBAP shows many advantages over the conventional N source 1,1-dimethylhydrazine (UDMHy) such as a much lower decomposition temperature of 310 °C and the realization of Ga(N,P) layers grown at temperatures as low as 475 °C with a high N incorporation of over 10%. Furthermore, the gas-phase decomposition of DTBAP has been studied with a real-time fast Fourier transform quadrupole ion trap mass spectrometer attached inline to the MOVPE reactor. The decomposition of DTBAP behaves very similarly to the As analogue DTBAA. On the one hand, the tert-butyl groups attached to DTBAP decompose radically, leading to the formation of isobutane, and decompose, on the other hand, by β-H elimination, leading to the formation of isobutene. Furthermore, the decomposition products indicate a direct cleavage of the P–N bond of the molecule, resulting in the formation of aminyl radicals (NH2•). The formation of NH2• explains the high N incorporation efficiency of DTBAP at low temperatures as well as its limitations due to loss of NH3 at higher temperatures.

Ethene and butene oligomerization over isostructural H-SAPO-5 and H-SSZ-24: Kinetics and mechanism

Brønsted-acidic zeolite and zeotype materials are potential catalysts for the conversion of ethene to higher alkenes. In this study, two materials with AFI structure but different acid strength, H-SAPO-5 and H-SSZ-24, were subject to studies of ethene, cis-2-butene and ethene-butene mixture conversion under conditions where C 3 -C 5 alkene formation is thermodynamically favoured over higher hydrocarbons (673–823 K, 1 atm). Ethene and cis-2-butene partial pressures were varied in the range 9–60 and 0.9–8.1 kPa, respectively, and contact times were varied in the range 3.78–756 and 0.573–76.4 s.µmol H+/cm 3 over H-SAPO-5 and H-SSZ-24, respectively. Less than 1% conversion of ethene and less than 10% conversion of butene was obtained in the range of conditions used for elucidation of rate parameters. The ethene conversion rates were more than an order of magnitude higher over the more acidic H-SSZ-24 than over H-SAPO-5 (6.5 vs. 0.3 mmol/mol H + .s at 748 K, P ethene = 33 kPa), with corresponding lower reaction order in ethene (1.5 vs. 2.0 at 673 K) and lower apparent activation energy (52 vs. 80 kJ/mol at 698–823 K). Propene selectivity was substantially higher over H-SSZ-24 than over H-SAPO-5 (68% vs. 36% at 0.5% ethene conversion). A similar difference in apparent reaction rates was observed for cis-2-butene conversion over the two catalysts, and for co-feeds of ethene and cis-2-butene. However, the cis-2-butene conversion to C 3 -C 5 alkenes was found to be severely influenced by thermodynamic limitations, impeding a detailed kinetic analysis, and leading predominantly to isobutene formation at the highest temperatures. Reactor with three zones; representing the three measurement regimes. On the side; three typical conversion - selectivity graphs for H-SAPO-5 and H-SSZ-24, representing C3-C5 alkene selectivity in the three regimes.

Decomposition Mechanisms of Di-tert-butylaminoarsane (DTBAA)

III–V semiconductors containing small amounts of nitrogen (“dilute nitrides”) are very promising material systems for optoelectronic applications. Devices based on dilute nitrides currently suffer from problematic C incorporation. To overcome this problem, a novel nitrogen (N) and arsenic (As) precursor for metal–organic vapor phase epitaxy (MOVPE) of the dilute nitride di-tert-butylaminoarsane (DTBAA) has been introduced. DTBAA in comparison to the commonly used 1,1-dimethylhydrazine (UDMHy) showed a significantly improved N incorporation efficiency. The molecule exhibits no strong carbon (C)–N bond, and the C is only present in large alkyl groups which form fewer C radicals since β-H elimination is the dominating decomposition process. This should significantly lower the problematic C incorporation in dilute nitrides and lead to highly efficient devices. To understand the high N incorporation efficiency as well as the As incorporation, the gas-phase decomposition of this novel precursor has been studied with a real time Fourier transform (FT) quadrupole ion trap mass spectrometer (iTrap) from Carl Zeiss SMT GmbH in a horizontal Aixtron Aix 200 GFR MOVPE reactor. Formation of isobutane and isobutene proves a radical cleavage and β-H-elimination as decomposition processes of the tert-butyl groups attached to the molecule. Furthermore, the appearance of ammonia (NH3) has been detected. This indicates a direct cleavage of the As–N bond of the molecule, resulting in the formation of an aminyl radical (NH2•). The formation of NH2• explains the high N incorporation efficiency of DTBAA as well as its limitations due to desorption of NH3 at higher temperatures.

Surface acetone reactions on ZnxZryOz: A DRIFTS-MS study

We report a systematic DRIFTS-MS study of surface acetone aldolization and self-deoxygenation reactions on ZrO2 and Zn1Zr10Oz, using temperature programmed surface reactions of both acetone reactant and possible intermediate compounds including diacetone alcohol, mesityl oxide, phorone, and isophorone. DRIFTS was used to monitor the evolution of surface species while MS was used to track corresponding gas phase products, in an attempt to identify the distinct reaction pathways observed on the two catalysts as well as the role zinc plays in the corresponding reaction pathways. It is found that aldol condensation (or aldolization) of acetone occurs readily at low temperature (298 K), forming mainly acetone dimers (i.e., diacetone alcohol, DAA/mesityl oxide, MSO) on both ZrO2 and Zn1Zr10Oz with the latter being more active. At elevated temperatures, secondary MSO reactions are diverted, mainly caused by the different Lewis basicity on the two catalysts. Over ZrO2, MSO reacts with activated acetone (acetone enolate) to form a surface 4,4-dimethyl-2,6-heptadione (phorone-C), which is further converted into a stable isophorone intermediate. At temperatures above 473 K, polymerization/oligomerization of isophorone along with basic hydroxyl promoted decomposition reactions occur, leading to the formation of heavier carbonaceous surface species and gas-phase side products (methane and CO2). The addition of zinc balances the surface acid-base properties on ZnxZryOz (i.e., Zn1Zr10Oz) via not only enhancing the surface Lewis basicity but passivating the strong acidity. This balance, especially the enhanced surface Lewis basicity largely promotes the α-H abstraction of MSO to form MSO enolate on the Zn1Zr10Oz, which thus favors the aldolization of MSO enolate and acetone, a reaction tends to form 2,6-dimethyl-2,5-heptadien-4-one (phorone-A). Phorone-A then readily decomposes to produce isobutene as a major product at temperature as low as 373 K. Note that, despite the facile formation of isobutene at low temperatures, a high temperature (673 K) is still necessary to release the strongly adsorbed formate/carbonate/acetate species via CO2 formation.

Kinetic analysis of distinct product generation in oxidative pyrolysis of four octane isomers

Abstract The molecular structures of hydrocarbon fuels are known to have a substantial impact on their combustion properties. However, the relationship between the fuel structure and thermal decomposition intermediate products, which determine the global combustion behaviors, is not as well known. In this study, four octane isomers, n-octane, 2,5-dimethylhexane, 2,2,4-trimethylpentane (iso-octane), and 2,2,3,3-tetramethylbutane, are selected as the model compounds to illustrate the distinct product generation in the oxidative pyrolysis of large hydrocarbons with substantially different molecular structures. Both experimental and kinetic analysis show that all the octane isomers lead to the formation of a similar group of stable products, with the major ones being ethene, methane, propene, and isobutene. The distributions of these products vary from fuel to fuel; n-octane produces primarily ethene, while isobutene formation increases with increasing branching in the fuel molecular structure. The most branched isomer, 2,2,3,3-tetramethylbutane, produces predominately isobutene. Lumped, two-step reaction schemes are proposed for each octane isomer. Together with a detailed foundational fuel chemistry model, it is shown that the reaction models accurately predict the formation and subsequent consumption of the intermediate products for all octane isomers studied.

Selective conversion of acetone to isobutene and acetic acid on aluminosilicates: Kinetic coupling between acid-catalyzed and radical-mediated pathways

Solid Brønsted acids catalyze aldol condensations that form CC bonds and remove O-atoms from oxygenate reactants, but sequential β-scission reactions also cleave CC bonds, leading to isobutene and acetic acid products for acetone reactants. The elementary steps and site requirements that cause these selectivities to depend sensitively on Al content and framework type in aluminosilicate solid acids remain speculative. Acetone reactions on microporous and mesoporous aluminosilicates (FER, TON, MFI, BEA, MCM-41) showed highest β-scission selectivities on MFI and BEA; they increased as the Al content and the intracrystalline density of active protons decreased. The effects of acetone and H2O pressure on turnover rates and selectivities indicate that an equilibrated pool of reactive C6 ketols and alkenones are present at pseudo-steady-state concentrations during catalysis and that they act as intermediates in β-scission routes. Two distinct C6 β-scission pathways contribute to the formation of isobutene and acetic acid: (i) a minor H2O-mediated route involving β-scission of C6 ketols on protons and (ii) the predominant anhydrous path, in which H-transfer forms unsaturated C6 enols at protons and these enols propagate radical chains mediated by transition states stabilized by van der Waals contacts within vicinal microporous voids. This latter route is consistent with coupled-cluster free energy estimates for these unsaturated C6 enols and their respective free radicals. It accounts for β-scission selectivities that increase with decreasing proton density, a finding that precludes the sole involvement of acid sites and requires instead the kinetic coupling between reactions at protons and propagation steps mediated by transition states confined within proximate voids, even when such voids lack a specific binding site. These mechanistic interpretations also account for the observed effects of residence time, of the loss of active protons by deactivation, of acetone and H2O pressures, and of aluminosilicate framework structure on selectivity. These mechanistic insights also demonstrate the ability of voids to stabilize transition states that mediate homogeneous reactions by mere confinement, even in the absence of chemical binding onto specific sites, as well as the essential requirement of intimate proximity for the effective kinetic coupling between reactions on protons and in proton-free voids, a process mediated by the diffusion of very reactive and unstable intermediates present at very low local concentrations within microporous frameworks.

Consequence of heterogeneity of active sites for reactivity mechanism of n-butane isomerization over SO42−/ZrO2-Al2O3 catalyst

The relationship between the heterogeneity of active sites and isomerization mechanism of n-butane over alumina-promoted sulfated zirconia (SZA) was elucidated by two series of catalysts with different surface properties. The incomplete removal of coke deposition leads to a decrease in overall activity without the reduction of the rate of isobutane formation. Subjecting the fresh SZA catalyst to the thermal treatment at 650°C causes a striking decrease in activity, resulting from the decrease in the superacidity and oxidizability of the catalyst. While the thermal treatment at 600°C only selectively suppresses the side reactions. It is found that the peaks in NH3-TPD profiles at 500–700°C are caused by a redox reaction between strongly adsorbed ammonia and superacid sites by in situ NH3-TPD-MS measurements. Thus, this peak reflects the strength of superacidity and oxidizability. At the beginning of the reaction, the effective oxidative and protonative activation routes result in the higher concentration of intermediates, which facilitates the “dimerization-isomerization-cracking” reactions. The incomplete removal of coke and the appropriate decrease in the density of sulfate species on the surface selectively promoted the monomolecular pathway.

Direct synthesis of iso-butane from synthesis gas or CO2 over CuZnZrAl/Pd-β hybrid catalyst

The effect of various factors on the catalytic performance of iso-butane formation over CuZnZrAl/Pd-β hybrid catalyst via synthesis gas or CO2 hydrogenation has been deeply investigated in this work. It was interesting to note that the iso-butane/n-butane ratio value was much higher than that of thermodynamic equilibrium (about 1/1), whose value was directly related to the reaction condition using this hybrid catalyst. In order to further clearly clarify this finding, various experimental reaction factors were selected to investigate the formation of iso-butane. The results revealed that increasing temperature, H2/COx, CO2/COx, and/or Pd loading possessed an inhibiting effect on the iso-butane yield. High selectivity of iso-butane could be achieved by increasing the reaction pressure, W/F and the weight ratio of CuZnZrAl methanol catalyst to Pd-β catalyst. It is also noted that the addition of water seriously suppressed the reaction activity, resulting in the low ratio of iso-butane/n-butane. A possible reaction route was elucidated based on the latest results. This might shed light on the development of a high efficient catalyst for iso-butane production from synthesis gas or CO2 hydrogenation.

Study of butanol conversion to butenes over H-ZSM-5: Effect of chemical structure on activity, selectivity and reaction pathways

To evaluate the viability of the use of butanol as a green chemical key molecule, the effects of temperature and site time on the transformation of the three butanol isomers, 1-butanol, 2-butanol and iso-butanol, towards butenes over H-ZSM-5 have been studied in search of the most promising isomer. Under dehydration conditions, 2-butanol is by far the most active and intermediate ether formation is only observed for 1-butanol. On the other hand, only isobutanol allows the direct formation of isobutene, the most valuable of the butene isomers. Hence, it is especially interesting to further stimulate the biomass derived isobutanol production, e.g. via genetic modification of appropriate microorganisms, in order to allow thereafter the formation of green drop-in isobutene upon dehydration with H-ZSM-5 in existing refineries. Due to the large potential of isobutanol, a reaction path analysis via ab-initio based microkinetic simulations is conducted for this molecule. Comparing these results with simulations on 1-butanol indicates that the shift is occurring due to a shift of the dominant reaction pathway towards the direct dehydration via anti elimination. A Gibbs free energy analysis shows that the large distortion of the transition state for isobutanol etherification renders this path far less favorable, resulting in the lack of formation of the di-alkyl ether, whilst the increased degree of substitution of the alkyl chain in isobutanol is found to promote the direct dehydration path, leading to an increase of the overall activity of isobutanol as compared to 1-butanol.

Monomolecular Skeletal Isomerization of 1-Butene over Selective Zeolite Catalysts

The mechanism of the 1-butene skeletal isomerization catalyzed by zeolites has remained elusive. We present direct evidence that even the initial isobutene formation over H-ferrierite, the best-known isomerization catalyst, is monomolecular in nature, whereas a bimolecular pathway is significant over the unselective H-ZSM-5. We also report that medium-pore high-silica H-HPM-1 outperforms H-ferrierite in selectively forming isobutene. This new catalyst displays a high activity and selectivity from the onset of the reaction, as well as an excellent resistance to deactivation, thanks to its anomalously weak acidity and low acid site density, together with an ability to effectively isolate reactant molecules from one another.

Mechanism of n-butane skeletal isomerization on H-mordenite and Pt/H-mordenite

Kinetics and isotope labeling experiments were used to investigate the reaction pathways of n-butane on H-mordenite and Pt/H-mordenite at atmospheric pressure and temperatures of 543–583K. Butenes, either formed on the catalyst or present in the feed, controlled the relative rates of mono- and bimolecular reaction pathways. The true activation energy for isobutane formation was found to be 120–134kJ/mol. The reaction order for isobutane formation with respect to n-butene on Pt/H-mordenite was 1.0–1.2, consistent with a predominately monomolecular route of formation. An order close to 2 for disproportionation products indicated a bimolecular route of formation. An increase of the butene concentration from less than 20ppm to about 120ppm greatly increased the rate of bimolecular skeletal isomerization, as determined from conversion of 1,4-13C2-n-butane. The findings explain how reaction conditions affect product selectivity and clarify the controversy around butane isomerization on solid acids.

The Putative mevalonate diphosphate decarboxylase from Picrophilus torridus is in reality a mevalonate-3-kinase with high potential for bioproduction of isobutene.

Mevalonate diphosphate decarboxylase (MVD) is an ATP-dependent enzyme that catalyzes the phosphorylation/decarboxylation of (R)-mevalonate-5-diphosphate to isopentenyl pyrophosphate in the mevalonate (MVA) pathway. MVD is a key enzyme in engineered metabolic pathways for bioproduction of isobutene, since it catalyzes the conversion of 3-hydroxyisovalerate (3-HIV) to isobutene, an important platform chemical. The putative homologue from Picrophilus torridus has been identified as a highly efficient variant in a number of patents, but its detailed characterization has not been reported. In this study, we have successfully purified and characterized the putative MVD from P. torridus. We discovered that it is not a decarboxylase per se but an ATP-dependent enzyme, mevalonate-3-kinase (M3K), which catalyzes the phosphorylation of MVA to mevalonate-3-phosphate. The enzyme's potential in isobutene formation is due to the conversion of 3-HIV to an unstable 3-phosphate intermediate that undergoes consequent spontaneous decarboxylation to form isobutene. Isobutene production rates were as high as 507 pmol min−1 g cells−1 using Escherichia coli cells expressing the enzyme and 2,880 pmol min−1 mg protein−1 with the purified histidine-tagged enzyme, significantly higher than reported previously. M3K is a key enzyme of the novel MVA pathway discovered very recently in Thermoplasma acidophilum. We suggest that P. torridus metabolizes MVA by the same pathway.