Kinetics and Catalysis

Oxide vanadium–titanium catalysts modified by phosphorus additives (20V2O5–(80 –n)TiO2–nP2O5, n = 1, 3, 5, 10, and 15 wt %) are studied in methylpyrazine ammoxidation. Two regions of compositions are found corresponding to radically different catalytic properties, namely, catalysts with a low (≤5 wt % P2O5) and high (≥10 wt % P2O5) concentration of the additive. In the first case, the introduction of phosphorus is accompanied by a gradual increase in the activity. In the second case, an increase in the additive concentration results in a decrease in the activity and selectivity to the target product, pyrazineamide, and a simultaneous increase in the selectivities to by-products, pyrazine and carbon oxides. The catalysts are characterized by X-ray diffraction analysis, differential dissolution, IR, and NMR spectroscopic data. As in the binary system, the active sites of the samples with a low concentration of phosphorus contain V5+ cations in a strongly distorted octahedral oxygen environment, which are strongly bound to a support due to the formation of V–O–Ti bonds. The catalytic properties of the samples containing ≥10 wt % P2O5 are due to the presence of the phase of a triple V–P–Ti compound with an atomic ratio V : P : Ti approximately equal to 1 : 1 : 1. The V5+ cations in this compound occur in a weakly distorted tetrahedral oxygen environment and are bound to the tetrahedral P5+ cations.

The influence of preirradiation with an electron beam from a linear resonance electron accelerator in flowing argon on the structure and properties of the 1% Pt/H pentasil catalyst was studied. The support structure was studied by x-ray diffraction and low-temperature nitrogen adsorption. No changes in the crystal structure and pentasil specific surface area were observed in the irradiation dose range from 120 to 900 Mrad at an average electron energy of 7.7 MeV. The electron beam treatment resulted in a considerable increase in the catalytic activity of the Pt/H pentasil in gas-phase toluene hydrogenation as a model reaction. The results obtained by IR spectroscopy of adsorbed CO and X-ray diffraction data suggested that the increase in the catalyst activity after electron beam irradiation is due to changes in the size and charge of the Pt particles.

Cobalt 4-octasulfophenyltetrapyrazinoporphyrazine is an efficient catalyst for the oxidation of diethylamine and cysteine with atmospheric oxygen. This complex activates the substrate through its one-electron oxidation, not an oxygen molecule. The role of oxygen is the oxidation of the intermediate anionic complex (catalyst regeneration step).

The effect of equipment corrosion products (transition metal cations) on the physicochemical and catalytic properties of a homogeneous Pd(II)+HPA-x (Mo–V–P heteropoly acid containing x vanadium atoms) catalyst developed for the two-stage oxidation of n-butene to methyl ethyl ketone (MEK) with oxygen has been studied. The thermal stability of a solution of a catalyst based on HPA-x in the presence of transition metal cations has been determined. The composition of the two-component catalyst recommended for pilot testing of the MEK process has been optimized.

The CoAl hydroxides with Co/Al ratios of 2 and 4 were synthesized using traditional coprecipitation and a mechanochemical method. The structural properties of the samples at all stages of catalyst preparation, the transformations occurred on the reduction of cobalt from corresponding oxides, the textural characteristics of calcined and reduced systems, and the size, morphology, and composition of particles formed after high-temperature treatments were studied in detail. It was established that the synthesis procedure of CoAl hydroxides has a significant effect on the phase composition and properties of the resulting systems. The layered double hydroxide phase was formed only when the coprecipitation method was used. The mechanochemical approach made it possible to obtain materials with a higher specific surface area. According to TEM data, the samples synthesized by coprecipitation (after oxidative and reductive treatments) had a core–shell structure, where the core included metallic Co atoms and the shell mainly consisted of CoAl spinel. The samples prepared by the mechanochemical method had highly dispersed Co nanoparticles on their surface. The catalysts based on CoAl systems prepared by the mechanochemical method were more active in the hydrogenation of furfural: its conversion reached 97% on a sample with the ratio Co/Al = 4. In this case, the selectivity for furfuryl alcohol formation on the studied catalysts was almost 100% regardless of the synthesis procedure and Co/Al ratio.

Bimetallic membrane catalysts based on the Mo2C–WC binary carbides were obtained. Their structural characteristics and catalytic properties in a reaction of dry reforming of methane were compared with those of monocarbide catalysts. The advantage of the use of the mixed carbide catalysts was established.

Experimental and computational studies of soot formation during the pyrolysis and oxidative pyrolysis of hydrocarbons with multiple bonds have been performed on ethylene and acetylene as examples. The experiments have been carried out behind reflected shock waves over the temperature range from 1700 to 2800 K at a pressure of 3 to 5 bar. The process of soot formation was recorded using the absorption-emission method, which enables one to simultaneously measure the mass of soot particles per unit volume and their temperature. The experimental data are interpreted within a new kinetic mechanism that takes into account the nucleation of soot particles from both polyaromatic hydrocarbon fragments and unsaturated aliphatic hydrocarbons. The proposed kinetic model of soot formation closely reproduces the time dependence of the soot yield and the temperature of soot particles measured in the present work, as well as the concentrations of several key components formed in the early stages of pyrolysis and oxidation of acetylene and ethylene, reported in a number of works by other authors.

Under the conditions of a joint reaction of propane and n-heptane at temperatures of 460–520°C and a pressure of 0.35 MPa, the conversion of propane and the concentration of C7+ aromatization products on platinum-containing catalysts modified by Group III (Ga) and Group IV (Ge, Ti, and Zr) elements were higher than those on an unmodified Pt/Al2O3 sample. This is explained by a change in the aprotic acidity of the catalysts as a result of the support modification. The sample with the addition of gallium was most active. A plausible reason for this is the conversion of hydrocarbons at active sites that consist of Pt and Ga, which were formed upon catalyst activation. It is believed that gallium adjacent to platinum in an ionic form on the support surface acts as an aprotic acid site.

Quá trình dehydrocondensation xúc tác của metan để tạo ra các hợp chất thơm như benzen và naphthalene đã được nghiên cứu trên các xúc tác carbide molybdenum được hỗ trợ trên các vật liệu xốp vi và xốp meso như HZSM-5 (0.6 nm) và FSM-16 (2.7 nm). Các xúc tác Mo được hỗ trợ trên H-ZSM-5 với kích thước mao quản thích hợp (0.6 nm) và tỷ lệ Si/Al (20-70) thể hiện năng suất cao hơn (90-150 nmol/g-cat/s) và độ chọn lọc (cao hơn 74% dựa trên carbon) trong việc chuyển đổi metan thành các sản phẩm thơm như benzen và naphthalene ở 973 K và 1 atm, mặc dù chúng bị suy giảm mạnh do sự hình thành coke đáng kể. Đã chứng minh rằng việc thêm CO/CO2 vào metan làm cải thiện hiệu suất của xúc tác bằng cách giữ cho tỷ lệ chuyển đổi metan và độ chọn lọc tạo ra benzen ở mức cao hơn trong thời gian hoạt động dài. Oxy thu được từ sự phân ly CO và CO2 ức chế polycondensation của các sản phẩm thơm và sự hình thành coke trong quá trình chuyển đổi metan. Các nghiên cứu XAFS và TG/DTA/spetrometric khối lượng cho thấy rằng oxide molybdenum hỗ trợ zeolit được chuyển đổi theo kiểu thu nhiệt dưới tác dụng của metan khoảng 955 K thành các hạt molybdenum carbide (Mo2C) kích thước nano (Mo-C, số phối trí = 1,R- 2.09 å; Mo-Mo, số phối trí = 2.3–3.5;R = 2.98 å). Hình ảnh SEM cho thấy rằng các hạt carbide molybdenum có cấu trúc nano được phân tán cao trên và bên trong các tinh thể HZSM-5. Mặt khác, các phép đo IR của sự hấp phụ pyridine đã chứng minh rằng các xúc tác Mo/HZSM-5 với tỷ lệ SiO2/Al2O3 tối ưu khoảng 40 cho thấy độ axit Brönsted tối đa trong số các xúc tác với tỷ lệ SiO2/Al2O3 từ 20–1900. Có một mối tương quan gần gũi giữa hoạt động hình thành benzen trong quá trình aromat hóa metan và độ axit Brönsted của HZSM-5 do xúc tác hai chức năng.