Direct Observation of Acetyl Group Formation from the Reaction of CO with Methylated H-MOR by in Situ Diffuse Reflectance Infrared Spectroscopy

Methanol to olefins (MTO) is an important and economical process for methanol to olefins conversion, which can potentially lower the dependence of petrochemicals on the expensive liquid hydrocarbon feeds. The mechanism of the first C−C bond formation in MTO reaction is poorly understood despite numerous previous studies. The direct observation of acetyl group formation by the reaction between intrazeolitic Brønsted methyl groups and CO by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) is reported. It was found that only under high CO pressure could the acetyl group accumulate to substantial amount for spectroscopic observation; at low CO pressure, other competing reactions would either consume the active Brønsted methyl groups or react with the acetyl groups. Such direct observation further confirms the recent theoretical simulation results. We also postulate that this reaction may be one of the possible pathways for the formation of the first carbon−carbon species in the methanol to olefins process

Direct Conversion of Syngas to Olefins over a Hybrid CrZn Mixed Oxide/SAPO-34 Catalyst: Incorporation of Dopants for Increased Olefin Yield Stability

A bifunctional catalyst was developed utilizing a physical mixture of a CrZn-based mixed metal oxide and zeotype SAPO-34 for the direct conversion of syngas to short-chain olefins. A series of promoted CrZn-M (M = Fe, Ga, Al) mixed oxide catalysts were synthesized by coprecipitation and calcined at different temperatures. CrZn-Fe-SAPO-34 catalysts calcined at 400 °C selectively converted syngas to C2–C4 olefins, while maintaining high CO conversion and olefin stability over time. The high olefin yield is ascribed to the stabilization effect of iron on inversed spinel phase ZnCr2O4 and to reduction of the detrimental ZnO phase formed during syngas conditions. At a higher calcination temperature of 600 °C, the stabilization effect is less pronounced. Ga and Al-doped CrZn oxides enabled high and stable olefin selectivity of the hybrid catalysts CrZn-Ga-SAPO-34 and CrZn-Al-SAPO-34, regardless the applied calcination temperature. Spectroscopy analysis demonstrated that these promoters are able to scavenge free ZnO formed on the catalyst, thus stabilizing the inversed spinel. This work demonstrates that a rational design of mixed metal oxide components of the hybrid catalyst process is required to maximize olefin yield and catalyst stability. The selection of dopants capable of stabilizing an inversed spinel phase and scavenging detrimental ZnO is a critical step in successful catalyst design

Effect of Cage Size on the Selective Conversion of Methanol to Light Olefins

Zeolites that contain eight-membered ring pores but different cavity geometries (LEV, CHA, and AFX structure types) are synthesized at similar Si/Al ratios and crystal sizes. These materials are tested as catalysts for the selective conversion of methanol to light olefins. At 400 °C, atmospheric pressure, and 100% conversion of methanol, the ethylene selectivity decreases as the cage size increases. Variations in the Si/Al ratio of the LEV and CHA show that the maximum selectivity occurs at Si/Al = 15–18. Because lower Si/Al ratios tend to produce faster deactivation rates and poorer selectivities, reactivity comparisons between frameworks are performed with solids having a ratio Si/Al = 15–18. With LEV and AFX, the data are the first from materials with this high Si/Al. At similar Si/Al and primary crystallite size, the propylene selectivity for the material with the CHA structure exceeds those from either the LEV or AFX structure. The AFX material gives the shortest reaction lifetime, but has the lowest amount of carbonaceous residue after reaction. Thus, there appears to be an intermediate cage size for maximizing the production of light olefins and propylene selectivities equivalent to or exceeding ethylene selectivities

In Situ Generation of Radical Coke and the Role of Coke-Catalyst Contact on Coke Oxidation

A thermogravimetric analyzer (TGA) equipped for flowing hydrocarbon gases allowed in situ deposition of coke on catalyst and support samples with excellent coke-catalyst contact. The coke deposition on the catalysts and supports, which occurs via a gas phase radical mechanism, depends on the reaction time, temperature, hydrocarbon concentration, and sample external surface area but not on the chemical composition of the support under the conditions used. The coke samples, including in situ generated samples and an industrial coke sample, are characterized quantitatively by both deconvolution of Raman spectra and temperature-programmed oxidation (TPO) analyses. Thermal aging of coke is shown to be effective in increasing the hardness of the coke samples. Ceria dispersed on α-alumina, used as a model catalyst for coke oxidation, allows coke oxidation at lower temperatures. Using these catalysts, coke deposited in situ is shown to oxidize similarly to ground (tight contact conditions) coked catalyst samples, suggesting that in situ coke deposition in the TGA can be used to generate samples with realistic coke-catalyst contacting, as might be found in an industrial reactor or catalyst bed. In situ coking is also observed to be reproducible and reliable as compared to loose and tight contact methodologies

Kinetics of Direct Olefin Synthesis from Syngas over Mixed Beds of Zn–Zr Oxides and SAPO-34

A packed bed containing a physical mixture of both Zn–Zr mixed oxide catalyst and SAPO-34 converts syngas directly into a mixture of C2–C5 olefins and paraffins. Specifically, the mixed oxide catalyst is responsible for intermediate oxygenate synthesis from syngas while the molecular sieve catalyzes olefin synthesis from the oxygenate intermediates. Kinetic measurements with cofed propylene over each catalyst independently confirm olefin hydrogenation activity over both components of the composite bed. The addition of either water or CO to the feed drops the activity of propylene hydrogenation over the Zn–Zr oxide. In sum, under reaction conditions of syngas feed and produced water, olefin hydrogenation predominantly occurs over the SAPO-34 catalyst, rather than over the catalyst responsible for hydrogenating CO into oxygenate intermediates. A developed kinetic model consistent with this conclusion describes measurements at differing feed compositions, temperatures, space velocities, and bed catalyst mixing ratios. Technoeconomic analysis of the process indicates that the olefin-to-paraffin ratio is a key performance metric for commercial scale syngas conversion and highlights the importance of considering olefin hydrogenation rates over the molecular sieve component

Effect of Heteroatom Concentration in SSZ-13 on the Methanol-to-Olefins Reaction

SSZ-13 materials have been synthesized with varying amounts of Al to produce samples with different concentrations of Brønsted acid sites, and consequently, these SSZ-13 materials contain increasing numbers of paired Al heteroatoms with increasing Al content. These materials were then characterized and tested as catalysts for the methanol-to-olefins (MTO) reaction at 400 °C and 100% methanol conversion under atmospheric pressure. A SAPO-34 sample was also synthesized and tested for comparison. SSZ-13 materials exhibited significant differences in MTO reactivity as Si/Al ratios varied. Reduced Al content (higher Si/Al ratio) and, consequently, fewer paired Al sites led to more stable light olefin selectivities, with a reduced initial transient period, lower initial propane selectivities, and longer catalyst lifetime. To further support the importance of paired Al sites in the formation of propane during this initial transient period, a series of experiments was conducted wherein an H-SSZ-13 sample was exchanged with Cu²⁺, steamed, and then back-exchanged to the H form. The H-SSZ-13 sample exhibited high initial propane selectivity, while the steamed H-SSZ-13, the Cu²⁺-exchanged SSZ-13 sample, and the steamed Cu-SSZ-13 sample did not, as expected since steaming selectively removes paired Al sites and Cu²⁺ exchanges onto these sites. However, when it was back-exchanged to the proton form, the steamed Cu-SSZ-13 sample still exhibited the high initial alkane selectivity and transient period typical of the higher Al content materials. This is attributed to protection of paired Al sites during steaming via the Cu²⁺ cation. Post-reaction coke analyses reveal that the degree of methylation for each aromatic species increases with increasing Si/Al in SSZ-13. Further, SAPO-34 produces more polycyclic species than SSZ-13 samples. From these data, the paired Al site content appears to be correlated with both MTO reaction behavior and coke species formation in SSZ-13 samples

Anticoking Performance of Electrodeposited Mn/MnO Surface Coating on Fe–Ni–Cr Alloy during Steam Cracking

Manganese electrodeposition and anodization are performed on an Fe–Ni–Cr alloy (Incoloy 800H) to form an Mn/MnO surface coating after thermal pretreatment. The Mn/MnO-coated alloy is coked under simulated steam cracking conditions in ethylene-steam, and its anticoking performance is compared with pretreated, uncoated alloys. The mass of deposited coke during repeated coking cycles is measured by thermogravimetric analysis (TGA) and also determined from the measured CO/CO2 concentrations during decoking with air. Compared to the uncoated alloys that have Cr2O3-rich surfaces, the Mn/MnO-coated alloy shows 30–40% less deposited coke and a peak coke oxidation temperature reduced by about 100 °C. The Mn/MnO surface coating is hypothesized to reduce coke deposition by limiting the amount of Fe/Ni species on the surface and by catalyzing coke gasification/oxidation reactions via the formation of catalytically active Mn3+ species

Formation and Oxidation/Gasification of Carbonaceous Deposits: A Review

A wide variety of hydrocarbon processes, catalytic or noncatalytic, involve the formation of carbon deposits, either on catalysts or on reactor (or engine/exhaust) surfaces. Therefore, researchers have developed a large array of catalysts to aid the combustion of these deposits. Recently, the mechanism of catalytic carbon oxidation and/or gasification has been the focus of research in an attempt to design better catalysts for carbon removal. With this approach, understanding the mechanism of formation of different types of carbon deposits is desired. Efforts undertaken for studying oxidation or gasification of various forms of carbon deposits are discussed in this review, along with the techniques used to study the mechanism of oxidation/gasification. The kinetics of catalyzed and noncatalytic carbon oxidation are described in detail. The effect of reactive gases such as NO_<i>x</i>, water vapor, CO₂, and SO₂ on the gasification behavior of carbon deposits is also discussed. Reaction rates of oxidation/gasification of carbon under different operating conditions have been calculated, allowing for a comprehensive overview of carbon removal reactivity