Zeolite Species Research Articles

Removal of Fe<sup>2+</sup> and Mn<sup>2+</sup> from Water by Using Mining Rock Wastes and Their Synthesized Zeolites

The process of using mining rock waste as raw material to synthesis zeolite species is a viable alternative. Such process is minimizing serious impact of mining wastes on the environment and solving environmental problems such as water purification. In the present study, faujasite (NaX) and sodalite – cancrinite (sod-can) zeolites were successfully synthesized by hydrothermal crystallization using green siltstone, mudstone and black shale of mining rock wastes from Abu Tartur phosphate mine, western desert, Egypt. The raw materials as well as synthesized zeolites were characterized by means of X-ray Diffraction (XRD), Fourier Transform Infrared (FTIR), X-ray Fluorescence (XRF), Scanning Electron Microscopy (SEM), Thermogravimetry (TG), Differential Thermal Analysis (DTA) and Brunauer–Emmett–Teller (BET). Raw material (black shale) and synthesized zeolites (Na-X) faujasite were examined as heavy metal (Fe and Mn) adsorbent from aqueous solution. The results show that Na-X faujasite is very faster and higher in the up taking of Fe²⁺ and Mn²⁺ ions relative to the black shale absorbent or even the synthesized sodalite-cancrinite. Fe²⁺ and Mn²⁺ adsorption onto black shale absorbent and synthesized faujasite followed Langmuir isotherm, however, Freundlich isotherm model is accepted in the case of sodalite-cancrinite adsorbent. The adsorption kinetics of Fe²⁺ and Mn²⁺ onto the studied samples follow pseudo-second order model indicating chemical interaction (chemisorption) adsorption process.

Zeolites as multifunctional additives stabilize high-voltage Li-batteries based on LiNi0.5Mn1.5O4 cathodes, mechanistic studies

The work reported herein discusses the improved electrochemical and thermal behavior of LiNi0.5Mn1.5O4 (LNMO) spinel cathodes via surface engineering using a series of zeolites. The limiting issues of these high voltage electrodes are phase transition during Li-ions intercalation/de-intercalation processes, weakening the active material's structure. Besides, it initiates harmful interfacial side reactions, including solution species oxidation and Ni & Mn dissolution, affecting their long-term cycling stability severely and detrimentally. Therefore, we propose a zeolite-based surface modification of LNMO involving a simple surface coating strategy that includes liquid-phase (ethanol) mixing followed by heat treatment at 200 °C under nitrogen gas flow. The cathodes comprising LNMO coated with 2 wt% zeolites exhibited significantly improved cycling stability than the reference cathodes with the uncoated material. Furthermore, we discovered that the zeolite species adsorbed to the LNMO surface act as buffer interphase that enhance the electrodes' redox kinetics, trapping dissolved-TMs ions and serving as local Li+-ions reservoirs. Pouch cells containing graphite anodes and zeolite-coated LNMO cathodes demonstrated impressively improved electrochemical behavior in capacity retention during prolonged cycling, enhanced rate capability, lower voltage hysteresis, and direct current internal resistance (DCIR) evolution. The zeolite-based surface coating participates in (i) lowering HF formation in battery solutions by absorbing trace water, (ii) HF scavenging, and (iii) lowering TMs cations dissolution. Furthermore, Si and Al constituents of the zeolites can deposit on Li-anodes and possibly increase their stability, later established by additional electrochemical studies of full-pouch cells comprising uncoated LNMO cathodes vs. zeolite-coated graphite anodes. Other pivotal findings of this work are the coherent structural, morphological, and thermal stabilization of zeolite-coated LNMO cathodes during prolonged cycling experiments.

Understanding formation thermodynamics of structurally diverse zeolite oligomers with first principles calculations.

The mechanisms of many zeolitic processes, including nucleation and interzeolite transformation, are not fully understood owing to complex growth mixtures that obfuscate in situ monitoring of molecular events. In this work, we provide insights into zeolite chemistry by investigating the formation thermodynamics of small zeolitic species using first principles calculations. We systematically study how formation energies of pure-silicate and aluminosilicate species differ by structure type and size, temperature, and the presence of alkali or alkaline earth metal cations (Na+, K+, and Ca2+). Highly condensed (cage-like) species are found to be strongly preferred to simple rings in the pure-silicate system, and this thermodynamic preference increases with temperature. Introducing aluminum leads to more favorable formation thermodynamics for all species. Moreover, for species with a low Si/Al ratio (≤2), a thermodynamic preference does not exist among structure types; instead, a pool of diverse aluminosilicate structures compete in formation. Metal cation effects strongly depend on the presence of aluminum, cage size, cation type, and location, since each of these factors can alter electrostatic interactions between cations and zeolitic species. We reveal that confined metal cations may destabilize pure-silicate cages due to localized interactions; conversely, they stabilize aluminosilicates due to strong cation-framework attractions in sufficiently large cages. Importantly, this work rationalizes a series of experimental observations and can potentially guide efforts for controlling zeolite nucleation/crystallization processes.

Regulation of the nature and sites of copper species in CuNaY zeolites for ethylene and ethane separation

Relationship between the states of Cu species and ethylene adsorption capacity.

Tuning the Mechanism of H/D Exchange for Isobutane on H-BEA by Loading Zn Species in Zeolite.

Kinetics of H/D hydrogen exchange between deuterated isobutane-d10 and Brønsted acid sites (BAS) of three zeolite samples (H-BEA, ZnO/H-BEA, Zn2+ /H-BEA) were monitored with 1 H MAS NMR in situ at 343-468 K. The regioselective H/D exchange in the methyl groups detected on H-BEA can be rationalized in terms of the mechanism of indirect exchange, which involves protonation of the intermediate olefin and further hydride abstraction from the other alkane molecule by the formed carbenium ion. Loading of Zn species in the zeolite results in a decrease of the rate and an increase of the activation energy of the exchange. The loaded Zn species provide the tuning effect on the reaction occurrence, changing the mechanism from the indirect one to the mechanism of the direct exchange.

Stepwise conversion of methane to methanol over Cu-mordenite prepared by supercritical and aqueous ion exchange routes and quantification of active Cu species by H2-TPR

Copper-exchanged mordenite prepared by supercritical ion exchange (SCIE) and aqueous ion exchange (AIE) were investigated in stepwise conversion of methane to methanol. Increasing the oxygen activation temperature and methane reaction time enhances the methanol yield of copper-exchanged mordenite prepared by SCIE (Cu-MORS). The reducibility of Cu-MORS was compared with those of Cu-MORA prepared by aqueous ion exchange (AIE) using H2-TPR. It was demonstrated for the first time that deconvoluted H2-TPR profile coupled with effects of Cu loading and oxygen activation temperature on methanol yield data can be used to distinguish the active Cu sites from inactive ones based on their reduction temperature. The copper species responsible for methane activation were found to be reduced below 150 °C by H2 in both Cu-MORS and Cu-MORA. From the stoichiometry of the reaction of H2 with Cu2+ species, the average number of copper atoms of active sites were calculated as 2.07 and 2.80 for Cu-MORS and Cu-MORA, respectively. Differences in structure of copper species caused by the synthesis routes were also detected by in-situ FTIR upon NO adsorption indicating a higher susceptibility of Cu-MORS towards autoreduction. The results demonstrated the potential of TPR based methods to identify copper active sites and suggested the importance of site selective ion exchange in order to controllably synthesize active Cu species in zeolites.

Novel and emerging concepts related to cationic species in zeolites: Characterization, chemistry and catalysis

This review surveys our efforts to form and characterize cationic species located in the micropores of various zeolites, both for metal and non-metal cations, with specific emphasis on their novel, emerging properties, and reactivity. Herein, it is described how we realized synthesis of such species with a focus on both cation identity as well as zeolite architecture, and synthesis methods. Well-defined cationic species could be produced by careful choice of metal incorporation methods, Al distribution tuning and synthesis parameters. Previously unknown phenomena in zeolite and cation-zeolite chemistry were discovered by employing these well-defined systems and will be covered in this review article. Methodology to dramatically improve stability of cations in zeolite for adsorption and catalytic applications, and usefulness of these materials in elucidating hitherto unknown catalytic intermediates and mechanisms will be discussed.

Effect of hydrolysis of tetraethyl orthosilicate (TEOS) on titanium distribution of TS-1 zeolite

The hydrolysis process of tetraethyl orthosilicate (TEOS) was tracked by FT-IR analysis, and the effect of hydrolysis time on the distribution of titanium species in TS-1 zeolites was discussed through XRF and UV-vis. The catalytic oxidation performance of TS-1 zeolite was tested by 1-hexene epoxidation as a model reaction. The results show that moderate hydrolysis of tetraethyl orthosilicate can effectively inhibit the formation of extra-framework Ti and increase the content of framework Ti. However, shorter or longer hydrolysis time is not conducive to the entry of titanium into the skeleton and reduce the catalytic performance of TS-1 zeolites. TS-1 prepared with 2 h TEOS hydrolysis time has the best catalytic performance for epoxidation of 1-hexene. Under the reaction conditions of atmospheric pressure and 60 °C, the conversion of 1-hexene can reach 37.5% and the selectivity of 1,2-epoxy hexane can maintain at about 85%, and the result has a guiding role for the development of olefin epoxidation catalyst.

Monomeric Fe Species in Square Planar Geometry Active for Low Temperature NH3-SCR of NO

Monomeric Fe species in zeolites are considered to be the active sites for the low temperature activity toward ammonia-assisted selective catalytic reduction of nitrogen oxides (NH3-SCR) in exhaust gases. Herein, we report on a preparation method to synthesize single-site Fe/ZSM-5 by combination of dealumination with the use of a bulky iron complex to introduce Fe. Transient in situ XAS experiments at the Fe K-edge under dynamic NH3-SCR reaction conditions demonstrated the involvement of all iron atoms in the redox cycle and, with that, suggest that Fe species in the studied Fe/ZSM-5 catalyst are monomers. Simulation of experimental X-ray absorption data revealed that the Fe species in the sample obtained by this approach adopt a square planar geometry, which changes into fivefold coordination at low temperature by strongly binding either NO or NH3 as the ligand. The presence of monomeric Fe species in this Fe/ZSM-5 conveys molecular level insights into the temperature-dependent NH3-SCR activity and might prove useful in the study of other reactions over monomeric Fe species, such as methane partial oxidation or dehydroaromatization reactions.

Possible formation pathways for zeolites in closed-basin lakes on noachian Mars: Insights from geochemical modeling

Zeolites are among the most common and widespread authigenic silicate minerals found in saline-alkaline paleolacustrine environments on Earth. Analcime, a Na-zeolite, has been detected on crustal outcrops on Mars using orbital spectral image data. Identifying other zeolite species is complicated by the lack of diagnostic absorption features in spectral data and spectral similarity to some polyhydrated sulfates. Therefore, analcime is the only zeolite species so far unambiguously identified as present on Mars. The presence of certain zeolite mineral species or assemblages in a closed basin paleolake can be used to track hydrological changes in that basin. This paper discusses the application of geochemical modeling to identify zeolite phases that could have formed in closed lake basins of late Noachian Mars. The composition of the high-silica Buckskin sample from Gale crater is used as a starting material because 1) most zeolite-rich assemblages in closed hydrologic systems on Earth originate from the decomposition of high-silica volcanic glass, and 2) geochemical modeling studies are already available for the more common lower silica basalts on Mars. This study, along with previous studies involving basaltic starting materials, allow a comparison between terrestrial and Martian data and an exploration of the differences in secondary mineral precipitation with substrates of different silica contents. The EQ3/6 geochemical modeling code was adopted, and Late Noachian Mars was assumed to be warm and semi-arid, with an atmosphere with 1.3 bars of PCO2 and current Mars PO2 (14.7 μbar). The Buckskin sample was reacted with three different starting solutions including two different rainfall chemistries and an acidic H2SO4 − HCl solution, which could have formed through volcanic degassing. The mineral formation and re-dissolution patterns under different Water/Rock (W/R) ratios are also explored.The results show that near neutral and acidic solutions in closed-basin lakes filled with high-silica volcaniclastic materials could eventually produce zeolite minerals like those in saline-alkaline lakes on Earth. Analcime forms only in water-limited environments (low W/R ratio), while chabazite is present even at relatively high W/R ratio. Clinoptilolite transforms to analcime with an increase in pH, with decreased W/R ratio, and with time. Analcime was found to be the most stable zeolite mineral over the greatest range of conditions in this environment, associated with increased pH, consistent with its presence as a very common constituent of saline-alkaline lake deposits on Earth. The study shows that the mineral precipitation patterns associated with the dissolution of high-silica materials through acid weathering are comparable to the mineral precipitation patterns following the dissolution of basaltic rocks by acid weathering under 0 − 25oC conditions or hydrothermal conditions as described in previous laboratory, field, and geochemical modeling studies.

Metal Sites in Zeolites: Synthesis, Characterization, and Catalysis.

Zeolites with ordered microporous systems, distinct framework topologies, good spatial nanoconfinement effects, and superior (hydro)thermal stability are an ideal scaffold for planting diverse active metal species, including single sites, clusters, and nanoparticles in the framework and framework-associated sites and extra-framework positions, thus affording the metal-in-zeolite catalysts outstanding activity, unique shape selectivity, and enhanced stability and recyclability in the processes of Brønsted acid-, Lewis acid-, and extra-framework metal-catalyzed reactions. Especially, thanks to the advances in zeolite synthesis and characterization techniques in recent years, zeolite-confined extra-framework metal catalysts (denoted as metal@zeolite composites) have experienced rapid development in heterogeneous catalysis, owing to the combination of the merits of both active metal sites and zeolite intrinsic properties. In this review, we will present the recent developments of synthesis strategies for incorporating and tailoring of active metal sites in zeolites and advanced characterization techniques for identification of the location, distribution, and coordination environment of metal species in zeolites. Furthermore, the catalytic applications of metal-in-zeolite catalysts are demonstrated, with an emphasis on the metal@zeolite composites in hydrogenation, dehydrogenation, and oxidation reactions. Finally, we point out the current challenges and future perspectives on precise synthesis, atomic level identification, and practical application of the metal-in-zeolite catalyst system.

Natural zeolites as host matrices for the development of low-cost and stable thermochemical energy storage materials

Advanced thermal energy storage technologies based on physical adsorption and chemical reactions of thermochemical materials (TCMs) are capable of storing large shares of renewable energy with high energy density. Further research and development is required to improve the performance and reduce the cost of these materials. A promising approach to developing low-cost TCM is to use natural zeolite adsorbents as host matrices in the development of salt-loaded composite TCM. In this study, the thermal properties of various species of low-cost zeolites from natural deposits across Canada were investigated. Two high purity crystal (HPC) zeolites from the Trans Canada (TC-HPC) and Juniper Creek (J-HPC) deposits in British Columbia were determined to have the highest water uptake capacity (0.145 g/g and 0.113 g/g, respectively) and enthalpy of adsorption (408 J/g and 304 J/g, respectively). Despite having approximately half of the water uptake capacity and adsorption enthalpy of the commercially available synthetic zeolite 13X, the cost of thermal energy storage ($CAD/kWhth) of the natural zeolites was determined to be 72–79% lower than that of the synthetic zeolite. Repeated adsorption and desorption experiments demonstrated the hydrothermal stability of the HPC zeolites over multiple charge and discharge cycles. Overall, the experimental results and cost analysis indicate that Canadian HPC zeolites are promising alternatives to synthetic zeolites in the pursuit of low-cost and stable TCM.

Direct methane to methanol stepwise conversion over Cu-oxo species in zeolites – Insights on the Cu-zeolite activation in air or helium from in situ UV-Vis analyses

Among the plentiful scientific research focused to find new routes to substitute stronger unsustainable chemical processes, the direct conversion (DC) of methane to methanol (MTM) over exchanged Cu-oxo cations in zeolites has gained great attention. Within that process, the paper proposed that the first activation step of the Cu-zeolite to catalyze the cycling DC-MTM be done with air instead industrial oxidant gases. Thus, Cu-oxo exchanged ferrierites, as model Cu-zeolites, were prepared and evaluated in the DC-MTM at 210 ºC. XRD, SEM/EDS, H2-TPR, FTIR of in situ adsorbed CO and pyridine were used to characterize the activated Cu-ferrierites. In situ DRS-UV-Vis evidenced the generation of Cu-oxo species after activation in air or He and their subsequent consumption during methane reaction. The air and He-activated Cu-ferrierites showed a close catalytic behavior that was supported by their similar profile to the formed Cu-oxo clusters. Thus, being more environmentally friendly to select air as an activation agent, instead of the use of pure O2, inert gases, or even N2O.

Advanced Transmission Electron Microscopy for Identification of Atomic‐Scale Configurations of Zeolite‐Supported Metal Catalysts

AbstractZeolite‐supported metal catalysts have important applications in various catalytic reactions, but unveiling their atomic‐scale structures is always a grand challenge. The developed transmission electron microscopy (TEM) characterization techniques, e.g., integrated differential‐phase contrast (iDPC) scanning transmission electron microscopy (STEM) is emerging as a powerful tool for visualizing the electron beam‐sensitive materials at atomic resolution, thus, providing new opportunities for imaging the exact configurations of metal species in the various channels of zeolites. This emerging topic summarizes recent progress utilizing iDPC‐STEM‐related technique for determination of the structures of metal species in zeolites that should offer insights into the structure‐property relationships of zeolite‐supported metal catalysts. A brief perspective about the new TEM technique is also presented, showing prospects in this field that will stimulate further scientific research in zeolite‐supported metal catalysts.

A New Study on the Eastern Flank of the Loma Blanca Deposit (Cuba) to Establish the Mineralogical, Chemical, and Pozzolanic Properties of Zeolitised Tuffs

The geological nature of the territory of the Republic of Cuba has favoured the formation of large and varied deposits of volcanic tuffs enriched by various species of zeolites. Today, new zeolite deposits continue to be discovered in the country. This work aims to present the results of a study carried out in an unexplored area that is located approximately 1.2 km east of the Loma Blanca deposit, outside the mining operation limits. To carry out this research and to establish a qualitative comparison between both sample populations, four samples were taken from the study area, and another four were taken from the Loma Blanca deposit. The characterisation of the samples was performed by XRD, SEM, and XRF. The pozzolan quality was determined by the pozzolanicity test (PT) and quality chemical analysis (QCA). Finally, a study of the mechanical strength (MST) was performed at 7, 28, and 90 days, using mortar specimens made with PC/ZT: 75–25% and PC/ZT: 70–30%, respectively. The results of the studies using XRD, SEM, and XRF indicated that both groups of samples had a similar complex mineralogical composition, consisting mainly of mordenite and clinoptilolite accompanied by secondary phases such as quartz and amorphous materials in the form of altered glass. The pozzolanicity test showed that both the samples from the study area and those from the Loma Blanca deposit behaved like typical pozzolans, which is a trend that can be seen in the high values of mechanical strength to compression up to 72 MPa for the PC/ZT: 75–25% formulation and 66 MPa for the PC/ZT: 70–30%. The results obtained establish that the zeolite varieties detected in the study area are similar to those of the Loma Blanca deposit, which could have a positive impact on the increase in current reserves, especially for manufacturing pozzolanic cements with properties that contribute to the preservation of the environment.

Catalytic Activation of Polyethylene Model Compounds Over Metal-Exchanged Beta Zeolites.

Decomposition of polymers by heterogeneous catalysts presents a promising approach for reuse of waste plastics. We demonstrated non-hydrogenative decomposition of model polyolefins over proton-form and metal (Cu, Ni) ion-exchanged beta (BEA) zeolites at moderate temperatures (around 300 °C). Near complete polyolefin decomposition was observed in batch reactions monitored by thermogravimetric analysis, while decomposition at partial conversion was studied in flow reactions. Ni-exchanged zeolites produced H2 at substantially higher rates (>10x) than other catalysts while also uniquely resisting deactivation over time. Application of the delplot formalism offered insights into the reaction network for polyolefin decomposition over Ni/BEA most notably that H2 is solely a primary product. We deduce that H2 production is catalyzed by activation of C-H bonds at ionic Ni sites, and H2 prevents buildup of polyaromatic coke species in Ni-exchanged zeolites that deactivate Cu-exchanged and protonic zeolites.

A novel material for passive NOx adsorber: Ce-based BEA zeolite

Passive NOx adsorbers (PNAs) were proposed to address the NOx emissions during the cold start phase. Here we show a novel Ce-based BEA zeolite, as a noble-metal-free passive NOx adsorber. The NOx adsorption capacity of Ce/BEA reaches 36 μmol/g in the feed gas close to realistic exhaust conditions, and the NOx desorption temperature, which is around 290 °C, is ideal for diesel exhaust after-treatment systems. Ce/BEA also behaves notable stability of high temperature CO exposure conditions. Multiple characterizations were performed to explore the NOx adsorption chemistry of Ce/BEA. The Ce(Ⅳ) species in the BEA zeolite serves as the active center for NOx adsorption. The bidentate nitrate species is responsible for the observed NOx storage capacity, and the active oxygen around Ce(Ⅳ) plays a critical role in its formation. Considering the significantly better cost efficiency of Ce compared to Pd, Ce/BEA presents an enormous potential for the PNA applications and provides a novel formulation for the noble-metal choice of PNA materials.

A novel conversion of Ti-bearing blast furnace slag into Ti-containing zeolites: Comparison study between FAU and MFI type zeolites

In this work, the synthesis of FAU and MFI type Ti-containing zeolites from Ti-bearing blast furnace slag was first achieved via a facile hydrothermal method. The synthesized zeolites were identified to be Ti-NaX and Ti-NaZSM-5 zeolites, with excellent specific surface area of 663.2 and 325.2 m2/g, respectively. Ti species in the Ti-NaX zeolite contained the framework Ti species and amorphous extraframework Ti species, while the Ti species in Ti-NaZSM-5 zeolite were in the form of the two species of above and another anatase TiO2. To investigate the potential application of the synthesized zeolites in photocatalysis field, an exploratory study was carried out by degradation of methyl orange under UV irradiation. As demonstrated, the Ti-NaZSM-5 zeolite showed higher photocatalytic performance and was more suitable to be the support of the TiO2 photocatalyst than the Ti-NaX zeolite. Innovative conversion of TBFS into Ti-containing zeolite materials does provide not only a novel and low-cost approach to waste management, but also a promising material candidate for catalytic oxidation and environmental purification.

Enhancement of the strong Brønsted acidity and mesoporosity: Zr4+ promoted framework modification of Zeolite Y

Brønsted acid of zeolite Y plays an important role in catalytic cracking reaction. However the proportion of strong Brønsted acid in conventional zeolite Y is small which limits the efficiency of transformation. This work described a way to achieve the goal of significant increasing the density of strong Brønsted acid via introducing Zr4+ into the framework of zeolite Y. The density of strong Brønsted acidity increased from 0.085 (HY) to 0.208 mmol/g ([Zr,Al]-HY). Systematic characterization results showed that the introduction of Zr4+ could produce more Si(1Al) but less Si(3Al) and Si(2Al) species in zeolite which was responsible for the high density of Brønsted acid. And abundant mesopores were also formed during the Zr4+ implanting. For the catalytic cracking results of n-octane, the conversion on [Zr,Al]-HY was 1.67 times of that on conventional HY. The apparent activation energy of cracking n-octane on [Zr,Al]-HY was reduced to 73.3%, which indicated that the increase of strong Brønsted acid had significant effect on catalytic cracking. On the other hand, a large number of mesoporous structures of [Zr,Al]-HY provided a good foundation for the catalytic cracking of steric hindered 1.3.5-triisopropylbenzene (1,3,5-TIPB).

Production of butadiene and lower olefins via oxidative conversion of n-butane over Ni-Bi-O/zeolite catalysts

The on-purpose production of butadiene and light olefins via oxidative conversion of n-butane was investigated over nanocomposites of Ni-Bi-Si oxides comprising Bi12SiO20/Bi2SiO5 phase regulated with silica of Beta, USY and ZSM-5 zeolites. Different ratios of Bi12SiO20 and Bi2SiO5 phases were obtained depending on the zeolite species and its SiO2/Al2O3 ratio. The Ni-Bi-Si oxides were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectroscopy, etc. to examine the electronic state and oxygen mobility in specific Bi-Si oxides; Bi2SiO5 and Bi12SiO20. The active species-zeolites interaction controlled the reducibility of Ni-Bi-O system and acidity of the catalysts. Ni-Bi-O/ZSM-5(1500) catalyst exhibited high selectivity for the oxidative conversion of n-butane to ethylene and propylene (60.7 wt. %) at 16 % conversion. Similarly, the olefinic selectivity over NiO-Bi12SiO20-Bi2SiO5/ZSM-5(280) catalyst at 20 % conversion, was 87 % (ethylene and propylene = 28.3 %, C4 olefins = 58.6 % including BD = 20 %) at 500°C and feed ratio of O2/n-butane = 2.0.