Direct partial oxidation of methane over ZSM-5 catalyst: Zn-ZSM-5 catalyst studies

Extended studies on Zn-ZSM-5 catalyst for the production of liquid hydrocarbons in the direct partial oxidation (DPO) of CH₄ with O₂ are reported. Previously, it was reported that metal-containing ZSM-5 catalysts could produce C₅₊ hydrocarbons from pure CH₄/O₂ feeds without feed additives. Zn-ZSM-5 produced the highest C₅₊ yields of the catalysts tested. This work shows that the method of introducing Zn onto the catalyst, ion-exchange versus impregnation, does not significantly alter C₅₊ yields if low Zn content is maintained ( 0.4–0.5 wt%). Liquid hydrocarbon yields in this system doubled after 8 h on stream while overall C₂₊ yields increased by over 300%. Mechanistic implications of these findings are discussed. Finally, processing a natural gas feed over Zn-ZSM-5 gave higher C₅₊ yields over CH₄ feed but these yields were not improved over previously published results using HZSM-5.     

Evidence for a cation intermediate during methanol dehydration on Pt(110)

NiB amorphous alloy and Ni catalysts supported on HMCM-22, HZSM-5, HY, -Al₂O₃ and SiO₂ were prepared, respectively, by the chemical reduction method and the standard incipient wetness impregnation method. These catalysts were examined for catalytic performance in the two-step conversion of CH₄ to produce hydrogen and higher hydrocarbons. All catalysts give similar methane conversion and yields of hydrogen and H-deficient carbon-containing species in step I. In the subsequent hydrogenation step (step II), they have similar carbon conversion, however, the yield of C₂ and C₃ hydrocarbons depends greatly on the nature of the metal particles and support acidity. Supported NiB amorphous alloy catalysts offer higher yield of C₂ and C₃ hydrocarbons than the corresponding Ni catalysts, due to their unique properties: nanoscale size, long-range disorder in structure, and electron-deficiency. Of the zeolite supported catalysts, HMCM-22 and HZSM-5 supported catalysts produce higher yield of C₂ and C₃ hydrocarbons than the zeolite HY supported catalyst because of stronger acidity of the supports. A NiB/HMCM-22 catalyst shows a rather slow deactivation during a multiple reaction cycles test. High temperature favors CH₄ decomposition and H₂ production in step I, but makes the subsequent hydrogenation of carbon formed from CH₄ decomposition difficult. The nature of the carbons formed from CH₄ decomposition was also studied by XPS and TEM combined with H₂-TPSR.     

The effect of synthesis gas composition on the Fischer–Tropsch synthesis over Co/γ-Al2O3 and Co–Re/γ-Al2O3 catalysts

The Fischer–Tropsch synthesis over Co/γ-Al₂O₃ and Co–Re/γ-Al₂O₃ was investigated in a fixed-bed reactor at 20 bar and 483 K using feed gases with molar H₂/CO ratios of 2.1, 1.5 and 1.0 simulating synthesis gas derived from biomass. With lower H₂/CO ratios in the feed, the CO conversion and the CH₄ selectivity decreased, while the C₅₊ selectivity and olefin/paraffin ratio for C₂–C₄ increased slightly. The water–gas shift activity was low for both catalysts, resulting in high molar usage ratios of H₂/CO (close to 2.0), even at the lower inlet ratios (i.e. 1.5 and 1.0). For both catalysts, the drop in the production rate of hydrocarbons when shifting from an inlet ratio of 2.1 to 1.5 was significant mainly because the H₂/CO usage ratio did not follow the change in the inlet ratio. The hydrocarbon selectivities were rather similar for inlet H₂/CO ratios of 2.1 and 1.5, while significantly deviating from those for an inlet ratio of 1.0. With the studied catalysts, it is possible to utilize the advantages of an inlet ratio of 1.0 (higher selectivity to C₅₊, lower selectivity to CH₄, no water–gas shifting of the bio-syngas needed prior to the FT reactor) if a low syngas conversion is accepted.     

Higher Hydrocarbon Production through Oxidative Coupling of Methane Combined with Hydrogenation of Carbon Oxides

In the production of higher hydrocarbons, combining oxidative coupling of methane (OCM) with hydrogenation of the formed carbon oxides in a separate reactor provides an alternative to the currently applied methane conversion to syngas followed by Fischer‐Tropsch synthesis. The effects of CH₄:O₂ feed ratio in the OCM reactor and partial pressures of H₂ or/and H₂O in the hydrogenation reactor were analyzed to maximize production of C₂₊ hydrocarbons and reduce CO_x formation. The highest C₂₊ yield was achieved with low CH₄:O₂ feed ratio for OCM and removal of the formed water before entering the hydrogenation reactor.     

Bifunctional Catalysis of Mo/HZSM-5 in the Dehydroaromatization of Methane to Benzene and Naphthalene XAFS/TG/DTA/MASS/FTIR Characterization and Supporting Effects

The direct conversion of methane to aromatics such as benzene and naphthalene has been studied on a series of Mo-supported catalysts using HZSM-5, FSM-16, mordenite, USY, SiO₂, and Al₂O₃as the supporting materials. Among all the supports used, the HZSM-5-supported Mo catalysts exhibit the highest yield of aromatic products, achieving over 70% total selectivity of the hydrocarbons on a carbon basis at 5–12% methane conversion at 973 K and 1 atm. By contrast, less than 20% of the converted methane is transformed to hydrocarbon products on the other Mo-supported catalysts, which are drastically deactivated, owing to serious coke formation. The XANES/EXAFS and TG/DTA/mass studies reveal that the zeolite-supported Mo oxide is endothermally converted with methane around 955 K to molybdenum carbide (Mo₂C) cluster (Mo-C, C.N.=1,R=2.09 Å; Mo-Mo, C.N.=2.3–3.5;R=2.98 Å), which initiates the methane aromatization yielding benzene and naphthalene at 873–1023 K. Although both Mo₂C and HZSM-5 support alone have a very low activity for the reaction, physically mixed hybrid catalysts consisting of 3 wt% Mo/SiO₂+HZSM-5 and Mo₂C+HZSM-5 exhibited a remarkable promotion to enhance the yields of benzene and naphthalene over 100–300 times more than either component alone. On the other hand, it was demonstrated by the IR measurement in pyridine adsorption that the Mo/HZSM-5 catalysts having the optimum SiO₂/Al₂O₃ratios, around 40, show maximum Brönsted acidity among the catalysts with SiO₂/Al₂O₃ratios of 20–1900. There is a close correlation between the activity of benzene formation in methane aromatization and the Brönsted acidity of Mo/HZSM-5, but not Lewis aciditiy. It was found that maximum benzene formation was obtained on the Moz/HZSM-5 having SiO₂/Al₂O₃ratios of 20–49, but substantially poor activities on those with SiO₂/Al₂O₃ratios smaller and higher than 40. The results suggest that methane is dissociated on the molybdenum carbide cluster supported on HZSM-5 having optimum Brönsted acidity to form CH_x(x>1) and C₂-species as the primary intermediates which are oligomerized subsequently to aromatics such as benzene and naphthalene at the interface of Mo₂C and HZSM-5 zeolite having the optimum Brönsted acidity. The bifunctional catalysis of Mo/HZSM for methane conversion towards aromatics is discussed by analogy with the promotion mechanism on the Pt/Al₂O₃catalyst for the dehydro-aromatization of alkanes.     

Selective catalytic reduction of nitrogen monoxide with methane over impregnated In/HZSM-5 in the presence of excess oxygen

In/HZSM-5 catalyst prepared by the impregnation method was active for NO reduction with methane. Complete reduction of NO was obtained at 450°C over an In/HZSM-5 catalyst. The presence of oxygen in the feed greatly enhanced the NO reduction activity of In/HZSM-5. Co/HZSM-5 and Ga/HZSM-5 were less effective than In/HZSM-5. Cu/HZSM-5, In/Na-ZSM-5 and In₂O₃/Al₂O₃ were ineffective for NO reduction with CH₄. The NO reduction activity was proportional to the level of indium impregnated onto HZSM-5 but excess amounts of indium were detrimental to the catalytic activity. Phase analysis by XRD measurements demonstrated that there was a threshold value in the indium content, i.e., the maximum dispersion capacity of indium oxides. It is concluded that highly dispersed indium species are the active centers for the selective catalytic reduction of NO with CH₄.     

Combined Single-Pass Conversion of Methane Via Oxidative Coupling and Dehydroaromatization

A new reaction mode, i.e., the combined single-pass conversion of methane via oxidative coupling (OCM) over mixed metal oxide (SLC) catalysts and dehydroaromatization (MDA) over Mo/HZSM-5 catalysts, is reported. With the assistance of an OCM reaction over SLC catalysts in the top layer of the reactor, the deactivation resistance of Mo/HZSM-5 catalysts is remarkably enhanced. Under the selected reaction conditions, the CH₄ conversion decreased from 18 to 1% and the aromatics yield decreased from 12.8 to 0.1%, respectively, after running the reaction for 960min on both 6Mo/HZSM-5 and SLC-6Mo/HZSM-5 catalyst system without O₂ in the feed. On the other hand, for the SLC-6Mo/HZSM-5 catalyst system with O₂ in the feed, the deactivation was improved greatly, and after 960min onstream the CH₄ conversion and aromatics yield were still as high as 12.0 and 8.0%, respectively. The promotion effect mainly appears to be associated with in situ formation of CO₂ in the OCM layer, which reacts with coke via the reverse Boudouard reaction.     

Theoretical analysis of oxidative coupling of methane and Fischer Tropsch synthesis in two consecutive reactors: Comparison of fixed bed and membrane reactor

In this paper, theoretical performance of Fischer Tropsch (FT) synthesis is analyzed where its feed comes from an oxidative coupling of methane (OCM) reactor. In this model based analysis, two consecutive reactors are intended that first reactor is OCM and second reactor is FT and FT reactor performance is compared in two conditions of fixed bed and membrane reactor (MR). The parameters concerned, were CH₄/O₂ ratio, contact time, temperature, and amount of N₂ in OCM feed. High CH₄/O₂ ratio gave low yield of C₂₊ in OCM due to insufficient oxygen, but favored FT reaction due to more yield of C₅₊ and other products. Therefore, it was concluded that production and yield of C₅₊ could be more by use of these configurations.     

Cerium promoted Pd/HZSM-5 catalyst for methane combustion

A series of Pd/HZSM-5 (Si/Al₂ = 165) catalysts without and with additives of oxides of La, Ce, Sm, Nd and Tb were prepared by the impregnation method, and characterized by XRD, Raman spectra, N₂-adsorption, CO-chemisorption, O₂-TPD and CH₄-TPR techniques. The catalysts were investigated for low-temperature CH₄ combustion, and CeO₂ was found to have a significant promoting effect on the activity of Pd/HZSM-5. Pd–Ce/HZSM-5 showed the best methane combustion activity and the improved thermal/hydrothermal reaction stability among tested catalysts. The characterization results of catalysts indicated that CeO₂ can effectively promote the formation of crystalline PdO and weaken the bond strength of Pd–O on Pd–Ce/HZSM-5, resulting in that Pd–Ce/HZSM-5 possessed lower temperatures for oxygen desorption and CH₄ reduction than Pd/HZSM-5. This could be ascribed to the covalent property and large oxygen storage/supplying capacity of CeO₂. It is believed that more active PdO species on Pd/HZSM-5 for low-temperature methane combustion process could be effectively promoted due to the introduction of CeO₂.     

Simulating and Optimizing Auto-Thermal Reforming of Methane to Synthesis Gas Using a Non-Dominated Sorting Genetic Algorithm II Method

Conventional synthesis gas production plants consist of a natural gas steam reforming to CO + 3H₂ on Ni catalysts in a furnace. An alternative method for highly endothermic steam reforming is auto-thermal reforming. In this work, synthesis gas production by auto-thermal reforming was simulated based on a heterogeneous and one-dimensional model in two cases. The first case was the auto-thermal reformer of Dias and Assaf's study. The present work was validated by the reported experimental results. The second case was the fixed-bed catalytic auto-thermal reactor operated at high pressure, which was suitable for methanol production and Fischer–Tropsch reactions (baseline case). Then, the effect of operating variables on the system behavior was studied. Finally, Pareto-optimal solutions were determined by non-dominated sorting genetic algorithm II. The objectives included obtaining a H₂/CO ratio of 2 in the produced synthesis gas and the maximum methane conversion. The adjustable parameters were the feed temperature, mass flux, and O₂/CH₄ and H₂O/CH₄ ratios in the feed.     

Promoting effect of propane traces on the activity of Pt/γ-Al2O3 and Pt–Sn/γ–Al2O3 catalysts in CH4–SO2–O2 reactions

A strong promoting effect of the presence of C₃H₈ or C₃H₆ was determined for the CH₄–SO₂–O₂ reaction, over pre-sulfated 1%Pt/γ–Al₂O₃ and pre-sulfated 1%Pt–2%Sn/γ–Al₂O₃ catalysts. These results suggest that over 1%Pt–2%Sn/γ–Al₂O₃ catalysts, small amounts of propane or propylene in the gas feed may eliminate methane emissions at low temperatures from lean-burn NGV exhausts.     

Methane-Propylene homologation and reactivity of surface carbon on modified Ni-Al2O3 catalysts

Modified Ni catalysts supported on alumina and reduced in CH₄ have been investigated for the synthesis of C₄ hydrocarbons from CH₄ and C₃H₆. Addition of K or P to the Ni/Al₂O₃ catalyst increased C₄ selectivity and C₃H₆ conversion. A maximum selectivity to the desired C₄ product of 9 mol % was obtained at 350°C and 101 kPa with a feed gas composition of 90 mol% CH₄/10 mol% C₃H₆, but the catalyst activity declined rapidly with time-on-stream. Large amounts of unreactive carbon were deposited on the catalyst surface following reduction in CH₄ and reaction in CH₄/C₃H₆. However, the relative amount of a much more reactive species identified from Temperature-Programmed-Surface-Reaction that was formed in the presence of CH₄ and C₃H₆, is shown to correlate with the catalyst C₄ yield. Both the C₄ yield and the relative amount of this low temperature carbonaceous species increased in the order Ni<Ni/P<Ni/K.     

Microwave discharge-assisted NO reduction by CH4 over Co/HZSM-5 and Ni/HZSM-5 under O2 excess

Microwave discharge-assisted reduction of NO by CH₄ in the presence of excess O₂ over Co/HZSM-5 and Ni/HZSM-5 catalysts was studied. By comparing the activities of the catalysts in the microwave discharge mode with that in the conventional reaction mode, it is demonstrated that microwave discharge enhanced greatly the conversion of NO to N₂, and expanded the reaction temperature range of the catalysts. For the Co/HZSM-5 catalyst, the conversion of NO to N₂ increased by 30%, and the optimum temperature decreased by 200°C. With the Ni/HZSM-5 catalyst, the highest activity was close to 100%, and the optimum temperature decreased by 325°C. The conversion of CH₄ also increased in the microwave discharge mode over both of the catalysts.     

CrHZSM-5 Zeolites – Highly Efficient Catalysts for Catalytic Cracking of Isobutane to Produce Light Olefins

The CrHZSM-5 catalysts with trace amount of Cr were firstly used for catalytic cracking of isobutane, and the effect of Cr-loading on the catalytic performances of CrHZSM-5 catalysts for the cracking of isobutane was also studied. The results suggested that when the loading of Cr in the CrHZSM-5 catalysts was less than 0.038 mmol/g Cr, especially at Cr loading of 0.004 mmol/g, both the reactivity of isobutane cracking and the selectivity to light olefins of CrHZSM-5 samples were greatly enhanced compared with the unpromoted HZSM-5, and very high yields of olefins(C₂+C₃) and ethylene were obtained. For instance, the yield of olefins(C₂+C₃) and ethylene reached 56.1% and 30.8%, respectively, at 625 °C when 0.004 mmol/g Cr was loaded on HZSM-5 sample.     

Catalytic cracking of isobutane over HZSM-5, FeHZSM-5 and CrHZSM-5 catalysts with different SiO<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> ratios

Several systems of HZSM-5, FeHZSM-5 and CrHZSM-5 zeolite catalysts with different ratios of SiO₂/Al₂O₃ (25,38,50,80, and 150) were prepared and they were characterized by means of X-ray diffraction (XRD), UV–Vis, NH₃-TPD and BET techniques. The results indicated that, compared with uncalcined HZSM-5 zeolites, the total acid amounts, acidic site density and acidic strength of HZSM-5, FeHZSM-5 and CrHZSM-5 zeolite catalysts obviously decreased, while those of weak acid amounts obviously enhanced with the decrease of SiO₂/Al₂O₃ molar ratio. When the ratio of SiO₂/Al₂O₃ is less than 50, the three systems of HZSM-5, FeHZSM-5 and CrHZSM-5 zeolite catalysts with same ratio of SiO₂/Al₂O₃ gave similar and high isobutane conversions. However, when the ratio of SiO₂/Al₂O₃ was equal to or greater than 80, these three systems of catalysts possessed different altering tendencies of isobutane conversions, thus their isobutene conversions were different. High yields of light olefins were obtained over the FeHZSM-5 and CrHZSM-5 zeolite catalysts with high ratio of SiO₂/Al₂O₃ (≥80). The ratio of SiO₂/Al₂O₃ has large effects on the surface area, and acidic characteristics of HZSM-5, FeHZSM-5 and CrHZSM-5 zeolites catalysts, and thus further affect their catalytic performances for isobutane cracking. That is the nature of SiO₂/Al₂O₃ ratio effect on the catalytic performances.     

Conversion of light paraffins for preparing small olefins over ZSM-5 zeolites

The conversion of C₃-C₉ paraffins to small olefins over ZSM-5 zeolite is investigated. The small olefins are primary products and are usually converted into other more stable secondary products such as aromatics on the ZSM-5 zeolites. Thermally treated HZSM-5, K/HZSM-5 and Ba/HZSM-5 catalysts were developed and favourable oxidative conditions were introduced for the conversion process to maximize selective conversion of light paraffins to small olefins at the relatively low temperature of 873 K. The role of K and Ba is to minimize bimolecular hydrogen transfer reactions and enhance the dehydrogenation activity of the catalysts. Meanwhile, the oxygen in the gas phase is effective to improve the olefin selectivity and yield. C₂-C₄ olefin selectivities of 70.4 and 66.8% have been obtained for propane andn-hexane feed-stocks, respectively, at a temperature of 873 K.     

Conversion of C8 aromatics and n-pentane over Ga2O3/HZSM-5 mechanically mixed catalysts

Ga₂O₃/HZSM-5 catalysts prepared as intimate physical mixtures of Ga₂O₃ with HZSM-5 have been studied for the conversion of C₈ aromatics and the aromatization of n-pentane. Hydrogen reduction of the mechanical mixture is a necessary step in the formation of an active catalyst and the reduction process has been characterized by IR spectroscopy. Reduced materials exhibit superior activity and selectivity compared to mechanical mixtures which have not be reduced.     

Reduction of NO by CH4 with Microwave Heating

The reduction of NO by CH₄ in the presence of excess O₂ over Co/HZSM-5, Ni/HZSM-5 and Mn/HZSM-5 catalysts with microwave heating was studied. By comparing the activities of the catalysts in the microwave heating mode with that in the conventional reaction mode, it was demonstrated that microwave heating could greatly reduce the reaction temperature, and could clearly expand the temperature window of the catalysts. Especially for the Co/HZSM-5 catalyst, the maximum conversion of NO to N₂ in the conventional reaction mode was consistent with that in the microwave heating mode. However, the temperature window for the maximum conversion in the microwave heating mode was from 260 to 360 °C instead of a temperature of 420 °C in the conventional reaction mode. The results suggest that microwave heating has a novel effect in the reduction of NO.     

Conversion of natural gas to C2 product,hydrogen and carbon black using a catalytic plasma reaction

In the microwave and RF plasma catalytic reaction at room temperature, the decomposition of natural gas over Pd–NiO/γ-Al₂O₃ was carried out. The decomposition of methane is caused by collision by excitation of unstable electronic state. Measuring the flow rate and plasma power can represent kinetic data and mechanism. The conversion of C₂ hydrocarbons was increased from 47% to 63.7% in the microwave plasma catalytic reaction within electric field. Comparing the activities of catalysts, Pd–NiO/γ-Al₂O₃ bimetallic catalyst was more active than Pt–Sn/γ-Al₂O₃ catalyst because of modifying the surface of catalysts by carbon formation. In RF plasma catalytic reaction, we obtained high C₂ yield of 72%, in which the conversion and selectivity of C₂ hydrocarbons were related to the applied power and feed rate of natural gas.     

NO2 formation and its effect on the selective catalytic reduction of NO over Co/ZSM-5

Catalytic performance of Co/ZSM-5 with different metal loadings and of HZSM-5 was compared in the NO + O₂, C₃H₈ + O₂, and NO + C₃H₈ + O₂ reactions. It was found that Co/ZSM-5 catalysts containing only isolated cobalt ions in cationic positions are inactive in NO₂ formation. To achieve appreciable NO conversion in the SCR process over these catalysts higher reaction temperatures are required. These results make it possible to suggest that NO₂ formation is not a prerequisite for the SCR of NO with hydrocarbons over Co/ZSM-5. With increasing Co loading, however, Co/ZSM-5 begins to exhibit activity in NO₂ formation. This is explained by the formation of cobalt oxide particles on the zeolite carrier, which are active in the NO₂ formation. Increase in NO₂ formation strongly enhances catalytic activity in SCR of NO at lower reaction temperatures. Comparison of the C₃H₈ conversion in the C₃H₈ + O₂ and C₃H₈ + O₂ + NO reactions provides evidence that NO₂ activates hydrocarbon molecules resulting in the formation of the reaction intermediates of the SCR process.On leave from N.D. Zelinskii Institute of Organic Chemistry, Leninskii Pr. 47, Moscow, Russia.