Elucidating the deactivation mechanism of beta zeolite catalyzed linear alkylbenzene production with oxygenated organic compound contaminated feedstocks

Zeolite catalyzed alkylation of benzene with long-chain α-olefins is a promising method for the detergent industry. Considering the long-chain α-olefins from Fischer-Tropsch synthesis always contain some oxygenated organic compounds, the effect of which on the alkylation of benzene with 1-dodecene was comprehensively investigated over beta zeolite herein. n-heptanol, n-heptaldehyde and n-heptanoic acid were selected as the model oxygenated organic compounds, and it was revealed that an obvious decrease of lifetime occurred when only trace amount of oxygenated organic compounds were added into the feedstocks. The deactivated catalyst was difficult to regenerate by extraction with hot benzene or coke-burning. A series of characterization tests complementary with DFT calculations revealed that the deactivation was mainly caused by the firm adsorption of oxygenated organic compounds on the acid sites. Further, comparison with the open-framework MWW zeolite revealed a similar effect of oxygenated organic compounds and deactivation mechanism for both beta and MWW, but beta is less sensitive to the oxygenated organic compounds. The main reason lies in the three-dimensional framework of beta, wherein the much higher adsorption energy of 1-dodecene makes it difficult to be replaced by oxygenated organic compounds. Additionally, beta could be regenerated more easily by extraction with hot benzene compared with MWW. But coke-burning caused a sharp decrease of its lifetime, which is mainly due to the decreased acid sites after calcination.

Molecular Views on Mechanisms of Brønsted Acid-Catalyzed Reactions in Zeolites

The Brønsted acidity of proton-exchanged zeolites has historically led to the most impactful applications of these materials in heterogeneous catalysis, mainly in the fields of transformations of hydrocarbons and oxygenates. Unravelling the mechanisms at the atomic scale of these transformations has been the object of tremendous efforts in the last decades. Such investigations have extended our fundamental knowledge about the respective roles of acidity and confinement in the catalytic properties of proton exchanged zeolites. The emerging concepts are of general relevance at the crossroad of heterogeneous catalysis and molecular chemistry. In the present review, emphasis is given to molecular views on the mechanism of generic transformations catalyzed by Brønsted acid sites of zeolites, combining the information gained from advanced kinetic analysis, in situ, and operando spectroscopies, and quantum chemistry calculations. After reviewing the current knowledge on the nature of the Brønsted acid sites themselves, and the key parameters in catalysis by zeolites, a focus is made on reactions undergone by alkenes, alkanes, aromatic molecules, alcohols, and polyhydroxy molecules. Elementary events of C-C, C-H, and C-O bond breaking and formation are at the core of these reactions. Outlooks are given to take up the future challenges in the field, aiming at getting ever more accurate views on these mechanisms, and as the ultimate goal, to provide rational tools for the design of improved zeolite-based Brønsted acid catalysts.

Elucidation of radical- and oxygenate-driven paths in zeolite-catalysed conversion of methanol and methyl chloride to hydrocarbons

Understanding hydrocarbon generation in the zeolite-catalysed conversions of methanol and methyl chloride requires advanced spectroscopic approaches to distinguish the complex mechanisms governing C-C bond formation, chain growth and the deposition of carbonaceous species. Here operando photoelectron photoion coincidence (PEPICO) spectroscopy enables the isomer-selective identification of pathways to hydrocarbons of up to C₁₄ in size, providing direct experimental evidence of methyl radicals in both reactions and ketene in the methanol-to-hydrocarbons reaction. Both routes converge to C₅ molecules that transform into aromatics. Operando PEPICO highlights distinctions in the prevalence of coke precursors, which is supported by electron paramagnetic resonance measurements, providing evidence of differences in the representative molecular structure, density and distribution of accumulated carbonaceous species. Radical-driven pathways in the methyl chloride-to-hydrocarbons reaction(s) accelerate the formation of extended aromatic systems, leading to fast deactivation. By contrast, the generation of alkylated species through oxygenate-driven pathways in the methanol-to-hydrocarbons reaction extends the catalyst lifetime. The findings demonstrate the potential of the presented methods to provide valuable mechanistic insights into complex reaction networks.

Catalytic co-pyrolysis of herb residue and polypropylene for pyrolysis products upgrading and diversification using nickel-X/biochar and ZSM-5 (X = iron, cobalt, copper)

It is very important to find cheap and efficient catalysts for catalytic co-pyrolysis. Catalytic co-pyrolysis of herb residue (HR) and reused polypropylene (PP) using Ni-X/biochar and ZSM-5 (X = Fe, Co, Cu) was performed to produce pyrolysis oil, pyrolysis gas and carbon nanotubes (CNTs) in a two-stage fixed bed reactor. Bimetallic biochar catalysts exhibited higher catalytic activity due to their higher specific surface area (S_BET) and more strong acid sites. NiCu/biochar significantly increased the yield of pyrolysis oil by enhancing Fischer-Tropsch synthesis. In addition, the stronger secondary cracking capacity of NiCu/biochar resulted in the highest content of hydrocarbons (80.47%) and C6-C11(61.10%), while the availability of higher content of carbon source gas also facilitated the formation of CNTs and H₂ at back-end. The cheap and efficient NiCu/biochar catalyst has great potential in the application of catalytic pyrolysis, which is conducive to the large-scale promotion of biomass pyrolysis technology.

Insights into the alkylation of benzene with olefins: effect of chain length of the olefins

Herein, the structure–reactivity relationship has been elucidated from a different perspective by using olefins with different sizes.

A new mechanistic proposal for the aromatic cycle of the MTO process based on a computational investigation for H-SSZ-13

The paring mechanism of the aromatic cycle of the hydrocarbon pool is reinvestigated based on the heptamethylbenzenium cation adsorbed within H-SSZ-13 using quantum chemical calculations.

Elucidation of the mechanism and structure–reactivity relationship in zeolite catalyzed alkylation of benzene with propylene

The intrinsic mechanism and structure–reactivity relationship were systemically elucidated using the alkylation of benzene with propylene as a model reaction based on the periodic DFT calculations.

First-principles microkinetic analysis of Lewis acid sites in Zn-ZSM-5 for alkane dehydrogenation and its implication to methanol-to-aromatics conversion

Stabilities and dehydrogenation activities of butane and cyclohexane on four different Zn sites in ZSM-5 zeolite were theoretically revealed. ZnOH⁺ was identified as the most active site at low temperature and the activity increases with the sequence of dehydrogenation.

Theoretical investigation of the side-chain mechanism of the MTO process over H-SSZ-13 using DFT and ab initio calculations

The side-chain mechanism of the methanol-to-olefins process over the H-SSZ-13 acidic zeolite was investigated using periodic density functional theory with corrections from highly accurate ab intio calculations on large cluster models.

Catalytic co-pyrolysis of cellulose and polypropylene over all-silica mesoporous catalyst MCM-41 and Al-MCM-41

Fast pyrolysis is one of the most economical and efficient technologies to convert biomass to bio-oil and valuable chemical products. Co-pyrolysis with hydrogen rich materials such as plastics over zeolite catalysts is one of the significant solutions to various problems of bio-oil such as high oxygen content, low heat value and high acid content. This paper studied pyrolysis of cellulose and polypropylene (PP) separately and co-pyrolysis of cellulose and PP over MCM-41 and Al-MCM-41. The pyrolysis over different heating rates (10K/min, 20K/min, 30K/min) was studied by Thermogravimetry Analysis (TGA) and kinetic parameters were obtained by Coats-Redfern method and isoconversion method. TG and DTG data shows that the two catalysts advance the pyrolysis reaction of PP significantly and reduce its peak temperature of DTG curve from 458°C to 341°C. The activation energy of pyrolysis of PP also has a remarkable reduction over the two catalysts. Py-GC/MS method was used to obtain the product distribution of pyrolysis of cellulose and PP separately and co-pyrolysis of cellulose and PP over MCM-41 and Al-MCM-41 at constant temperature of 650°C. Experiment results proved that co-pyrolysis with PP bring significant changes to the product distribution of cellulose. Oxygenated compounds such as furans are decreased, while yields of olefins and aromatics increase greatly. The yield of furans increases with the catalysis of MCM-41 as for the pyrolysis of cellulose and co-pyrolysis, while the yield of olefins and aromatics both experience significant growth over Al-MCM-41, which can be explained by the abundant acid centers in Al-MCM-41.

Reaction mechanism for the conversion of methanol to olefins over H-ITQ-13 zeolite: a density functional theory study

For MTO over H-ITQ-13, the alkene cycle takes priority over the aromatic one, giving a high selectivity to propene.

Methylation of olefins with ketene in zeotypes and its implications for the direct conversion of syngas to light olefins: a periodic DFT study

Theoretical calculations suggested that the associative pathway other than the dissociative pathway dominates the methylation of tetramethylethene with ketene in H-SAPO-34, and the former pathway is more sensitive to acid strength than the latter one in CHA-structured zeotypes.

Kinetics and thermodynamics of polymethylbenzene formation over zeolites with different pore sizes for understanding the mechanisms of methanol to olefin conversion – a computational study

The most kinetically and thermodynamically favored intermediates from methylation and geminal methylation.

Conversion of methanol to olefins over Al-rich ZSM-5 modified with alkaline earth metal oxides

Aluminum-rich H-ZSM-5 zeolites modified with alkaline earth metal oxides of Mg, Ca and Ba were applied in the conversion of methanol to olefins (MTO).

A low-temperature approach to synthesize low-silica SAPO-34 nanocrystals and their application in the methanol-to-olefins (MTO) reaction

A facile low-temperature approach has been developed to synthesize low-silica SAPO-34 nanocrystals with excellent catalytic performance in the MTO reaction.

Methanol-to-olefins process over zeolite catalysts with DDR topology: effect of composition and structural defects on catalytic performance

The effect of physicochemical properties on catalyst deactivation, overall olefin selectivity and ethylene/propylene ratio during the methanol-to-olefins (MTO) reaction is presented for two zeolites with the DDR topology, Sigma-1 and ZSM-58.

Oxygen-containing coke species in zeolite-catalyzed conversion of methanol to hydrocarbons

Oxygen-containing coke species are identified during methanol-to-hydrocarbons reactions, and their influence on the deactivation of catalyst is investigated.

Equilibrium analysis of methylbenzene intermediates for a methanol-to-olefins process

An Anderson–Schulz–Flory distribution in an MTO process comes from a thermodynamic equilibrium.