Investigations into the Mechanisms of Zeolite-Catalyzed Transalkylation of <i>iso</i>-Propylbenzene with Toluene

The catalytic properties of four large-pore (H–Y, H-beta, H-mordenite, and H-UZM-35) and three medium-pore (H-NU-87, H-TNU-9, and H-ZSM-5) zeolites for the transalkylation of <i>iso</i>-propylbenzene (<i>i</i>PB) with toluene are investigated. Among the zeolite catalysts employed here, H-UZM-35 with a 12 × 10 × 10-ring channel system was found to exhibit a comparable cymenes yield to that of H-beta with a 12 × 12 × 12-ring channel system, the most widely studied catalyst for this reaction. GC-MS analysis reveals that monomethylated 2,2,-diphenylpropane species, whose existence has not been experimentally verified yet, are serving as the main reaction intermediates of the bimolecular <i>i</i>PB-toluene transalkylation. Also, the intrazeolitic buildup of dimethylated 2,2-diphenylpropane and 2-methylphenyl-2-<i>iso</i>-propylphenylpropane species, which must be involved in the formation of 2-tolylpropanylium cations and thus in the simultaneous consumption and production of the reactant molecules (i.e., toluene and <i>i</i>PB), was observed. The formation of these three different groups of diphenylpropane species, which has been further supported by the DFT calculation results, allowed us to propose a new bimolecular reaction mechanism for this transalkylation. To our knowledge, our study is the first example where the repetitive mechanism is ascertained in zeolite-catalyzed reactions

Unveiling the Structural Characteristics of Intergrowth Zeolites Synthesized in the Presence of Isopropylimidazolium-Based Cations and Fluoride Anions

Here, we present the synthesis of RTH/ITE and MEL/MFI intergrowth zeolites using 2-isopropylimidazolium-based cations as organic structure-directing agents (OSDAs) in concentrated fluoride media and their local structural properties. Phase selectivity in the synthesis of zeolite intergrowths was found to differ according to the concentration of OSDA cations and fluoride anions in the synthesis mixture as well as to the type of OSDA employed. Molecular modeling results suggest that the crystallization of intergrowth zeolites in fluoride media may be kinetically rather than thermodynamically controlled, as in ordered zeolites. Cs-corrected STEM analysis of MEL/MFI crystals synthesized at HF/OSDA = 2.0 in the presence of 2-isopropyl-1,3-dipropylimidazolium ions as an OSDA indicates the existence of previously unobserved MEL-MFI intergrowth along the [100] direction, leading to a partial blockage of MEL 10-ring channels

Monomolecular Skeletal Isomerization of 1‑Butene over Selective Zeolite Catalysts

The mechanism of the 1-butene skeletal isomerization catalyzed by zeolites has remained elusive. We present direct evidence that even the initial isobutene formation over H-ferrierite, the best-known isomerization catalyst, is monomolecular in nature, whereas a bimolecular pathway is significant over the unselective H-ZSM-5. We also report that medium-pore high-silica H-HPM-1 outperforms H-ferrierite in selectively forming isobutene. This new catalyst displays a high activity and selectivity from the onset of the reaction, as well as an excellent resistance to deactivation, thanks to its anomalously weak acidity and low acid site density, together with an ability to effectively isolate reactant molecules from one another

MSE-Type Zeolites: A Promising Catalyst for the Conversion of Ethene to Propene

The direct conversion of ethene to propene (ETP) is a potentially important route for the selective production of the latter olefin. Here we report that after some time on stream, H-UZM-35, an MSE-type large-pore zeolite, shows much better propene yield than H-SSZ-13, the best catalyst for the ETP reaction thus far. The key to this improvement is the presence of large cylindrical cages in H-UZM-35 that allows the easy formation of isopropylnaphthalene-based reaction centers for ETP catalysis, while being relatively resistant to coke formation. Mild dealumination was found to further mitigate catalyst deactivation

Crystallization Mechanism of Cage-Based, Small-Pore Molecular Sieves: A Case Study of CHA and LEV Structures

Here we have investigated the crystallization mechanisms of SSZ-13 and SAPO-34 with the CHA topology and NU-3 and SAPO-35 with the LEV topology using ¹H–¹³C CP MAS NMR and IR spectroscopies. The nucleation of these cage-based, small-pore molecular sieves begins with the formation of large 20-hedral <i>cha</i> or 17-hedral <i>lev</i> cages, with incorporation of organic structure-directing agents (SDAs) alone or together with inorganic cations, in both aluminosilicate and silicoaluminophosphate compositions. The next two steps are the construction of multiple-<i>cha</i> or multiple-<i>lev</i> cages in an appropriate arrangement by sharing 8-rings and their subsequent coupling to form smaller 8-hedral double 6-ring units, leading to viable nuclei of CHA or LEV molecular sieves. The initial formation of the large cages, especially in the presence of large SDA molecules, can in our view be generalized to other cage-based zeolite systems

Zeolites ZSM-25 and PST-20: Selective Carbon Dioxide Adsorbents at High Pressures

Natural gas upgrading requires the separation of its major components, predominantly CH₄ and CO₂, at the high pressures found at the wellhead. Here we demonstrate that the Na⁺-tetraethylammonium form of ZSM-25 and PST-20, the fourth and fifth generations of the RHO family of zeolites with embedded isoreticular structures, can be efficiently employed to separate CO₂ from CH₄ over a wide temperature and pressure range (298–373 K and 0–25 bar). Both zeolites exhibit impressive CO₂/CH₄ selectivity (>20) at 298 K and 25 bar, relatively large adsorption capacity (4.0 and 3.6 mmol g^–1, respectively) even at 373 K and 25 bar, and long-term durability even in the presence of H₂O. These superior separation properties could be linked to the limited framework flexibilities of ZSM-25 and PST-20 mainly arising from the larger ratio of embedded to scaffold cages in the higher generation of the RHO family

Synthesis and Crystal Growth Mechanism of PST-2: An Aluminosilicate SBS/SBT Zeolite Intergrowth

The synthesis of PST-2, an aluminosilicate zeolite intergrowth of cage-based, large-pore SBS and SBT topologies, and its intergrowth characteristics are presented. With the Si/Al ratio and crystallization inorganic structure-directing agent in zeolite synthesis mixtures fixed to 8.0 and Cs+ ions, respectively, pure PST-2 is obtained at 120 °C using tetraalkylammonium ions with C/N+ ratios of 5–9 as a charge density mismatch (CDM) organic structure-directing agent (OSDA). More interestingly, the intergrowth ratio between SBS and SBT in PST-2 was found to vary notably not only with the type of CDM OSDA employed but also with the crystallization time, unlike the case of other well-known zeolite intergrowths such as β and MFI/MEL. When tetraethylammonium ions are used as a CDM OSDA at 100 °C in the presence of Cs+, the SBS portion in PST-2 decreases from over 60% to less than 45% with increasing crystallization time from 2.5 to 14 days, suggesting that SBS formation is kinetically more favorable than SBT formation. A thorough characterization of changes in the crystallite dimension of PST-2 with crystallization time, together with those in the chemical composition, allowed us to propose a plausible crystal growth mechanism of this large-pore zeolite intergrowth

Hydrothermal Aging Enhances Nitrogen Oxide Reduction over Iron-Exchanged Zeolites at 150 °C

Ammonia selective catalytic reduction (NH3-SCR) over copper- and iron-exchanged zeolites is a state-of-the-art technology for removal of nitrogen oxides (NOx, NO, and NO2) from exhaust emissions but suffers from poor low-temperature (i.e., 150 °C) activity. Here we show that hydrothermal aging of Fe-beta, Fe-ZSM-5, and Fe-ferrierite at 650 °C or higher leads to a remarkable increase in NOx conversion from ∼30 to ∼80% under fast NH3-SCR conditions at 150 °C. The practical relevance of this finding becomes more evident as an aged Fe-beta/fresh Cu-SSZ-13 composite catalyst exhibits ∼90% conversion. We propose that a neutral heteronuclear bis-μ-oxo ironaluminum dimer might be created within iron zeolites during hydrothermal aging and catalyze ammonium nitrate reduction by NO at 150 °C. Density functional theory calculations reveal that the activation free energy (125 versus 147 kJ mol–1) for the reaction of NO with adsorbed NO3– species, the rate-determining step of ammonium nitrate reduction, is considerably lower on the bis-μ-oxo ironaluminum site than on the well-known mononuclear iron–oxo cation site, thus greatly enhancing the overall SCR activity

Zeolite-Catalyzed Disproportionation of <i>iso</i>-Propylbenzene: Identification of Reaction Intermediates and Mechanism

The catalytic properties of a series of large-pore (H–Y, H-beta, H-mordenite, and H-UZM-35) and medium-pore (H-NU-87, H-TNU-9, and H-ZSM-5) zeolites are compared in <i>iso</i>-propylbenzene (<i>i</i>PB) disproportionation. Among the zeolite catalysts studied here, H-UZM-35 with a three-dimensional framework consisting of one type of straight 12-ring channels and two types of tortuous 10-ring channels was found to show a di-<i>iso</i>-propylbenzenes (D<i>i</i>PBs) yield comparable to that of H-beta with two intersecting 12-ring channels, the best catalyst tested for this reaction to date. Gas chromatography–mass spectrometry analysis of used zeolite catalysts demonstrates that while mono-<i>iso</i>-propylated 2,2-diphenylpropane derivatives serve as real reaction intermediates of <i>i</i>PB disproportionation over large-pore zeolites, mono-<i>iso</i>-propenylated 2,2-diphenylpropane species, which contain a double bond in the alkyl chain, are intermediates of its side reaction. Unlike that of other aromatic hydrocarbons such as <i>m</i>-xylene, ethylbenzene, and <i>n</i>-propylbenzene, the formation of di-<i>iso</i>-propylated derivatives was not observed as reaction intermediates. A new bimolecular diphenylpropane-mediated reaction pathway, which includes both intermediates of main and side reactions of <i>i</i>PB disproportionation, is proposed based on the experimental and theoretical results

<i>n</i>‑Propylbenzene Disproportionation: An Efficient Tool for Assessing the Framework Topology of Large-Pore Zeolites

The mechanisms of <i>n</i>-propylbenzene (<i>n</i>PB) disproportionation over various large-pore (LaNa-Y, H-Y, H-mordenite, and H-beta) and medium-pore (H-NU-87, H-TNU-9, and H-ZSM-5) zeolites were investigated. The GC–MS results from the used zeolite catalysts demonstrate that monopropylated 1,1-diphenylpropane and dipropylated 1,1-diphenylpropane derivatives are serving as the main reaction intermediates of bimolecular <i>n</i>PB disproportionation, whereas 1-phenyl-2-propylphenylpropane, 1-propylphenyl-2-phenylpropane, and 1-propylphenyl-2-propylphenylpropane species are the intermediates of some particular side reactions. It was found that the intrazeolitic buildup of these five different groups of diphenylpropane-based species is strongly influenced by the size and shape of zeolite void spaces, as was also corroborated by DFT calculation results. This allows <i>n</i>PB disproportionation to be quite useful for estimating the framework topology of zeolites with unknown structures, especially those of large-pore materials