Properties of Desilicated ZSM-5, ZSM-12, MCM-22 and ZSM-12/MCM-41 Derivatives in Isomerization of α-Pinene

Limited information is available on the isomerization of α-pinene proceeding on zeolites and related desilicated materials. We wish therefore to report on the title reaction proceeding over ZSM-5, ZSM-12 and MCM-22 type zeolites, parent and modified by the sodium hydroxide treatment. The NaOH solutions of various concentrations (0.05–1 M) were used as a desilicating agent. Such treatments with basic solutions were applied: (i) under atmospheric pressure, and (ii) under hydrothermal conditions. It was shown that ZSM-12 was more resistant towards the basic solutions treatment, and its structure was retained over a whole range of NaOH concentrations studied. Nitrogen sorption revealed strong influence of the desilication process on the pore structure of modified materials—the mesopores system was formed in the zeolite crystals. Finally, catalytic studies were carried out using ZSM-5, ZSM-12, MCM-22, their desilicated derivatives and a ZSM-12/MCM-41 composite material. Catalytic properties of the samples studied were affected to a large extent by the NaOH treatment.     

不同结构分子筛的甲醇制丙烯催化性能

在常压、空速为1.5 h^-1、反应温度为450℃条件下,考察了4种具有不同拓扑结构的分子筛(SAPO-34、ZSM-48、ZSM-5和beta)在甲醇转化制丙烯(MTP)反应中的催化性能,并对催化剂的积炭失活行为进行了研究。结果表明,从8元环到12元环,分子筛孔口尺寸越小,低碳烯烃(乙烯+丙烯)选择性越高,积炭失活速率也越快。孔道尺寸越大,丙烯/乙烯(P/E)比越高,但产物分布向C₄以上组分偏移,丙烯选择性降低。10元环分子筛具有较高的丙烯选择性,但催化剂的积炭失活速率随孔道体系的不同有很大差异。一维直通孔道的ZSM-48容易积炭失活,而具有三维交叉孔结构的ZSM-5表现出了优异的抗积炭失活性能。不同结构分子筛在MTP反应中催化性能的差异主要归因于分子筛的过渡态择形和产物择形作用的不同。     

Methanol-to-Olefins Reaction over Large-Pore Zeolites: Impact of Pore Structure on Catalytic Performance

In the methanol-to-olefins (MTO) reaction, product flexibility is important for meeting the fluctuating chemicals demands. In this paper, the catalytic performance of recently reported zeolites having 12-ring pores, such as boroaluminosilicate CON-type zeolite ([Al,B]-CON), MCM-68, and YNU-5 zeolites, in the MTO reaction was compared to understand the relationship between zeolite framework and product selectivity. The catalytic performance of the synthesized zeolites was compared with that of Beta, mordenite, and ZSM-5. The highest selectivities for propylene and butylenes were achieved over MCM-68. The highest propylene-to-ethylene ratio in the products was obtained over [Al,B]-CON. ZSM-5 and [Al,B]-CON exhibited a superior ability to suppress coke formation compared to Beta, mordenite, MCM-68, and YNU-5.     

1H broad-line and MAS NMR: application to the study of acid sites of desilicated zeolite ZSM-5

Broad-line¹H NMR study of desilicated zeolite ZSM-5 was carried out as a function of the number of adsorbed water molecules in amounts lower than or equal to that of the Brønsted acid sites. The dissociation coefficient of the acid OH groups, currently associated with the acid strength, was shown not to be affected by the selective removal of Si from the zeolite ZSM-5 framework which resulted in more Brønsted acid sites per unit surface area. On the other hand, by using MAS NMR, bridging Brønsted acid sites hydrogen-bonded to the zeolite framework were identified on the anhydrous surface of the desilicated ZSM-5. Moreover, MAS NMR spectra of the desilicated zeolite partially rehydrated showed the presence of some Lewis acid sites.     

Synthesis of Dihydropyrimidinones Using Large Pore Zeolites

Abstract  

A series of dihydropyrimidin-2(1H)-one (DHPM) belongs to one of the important class of therapeutic and pharmacological active compound, were synthesized through the multicomponent reactions (MCRs) of aldehydes, ethyl acetoacetate and urea, followed by the heterogeneous catalyzed Biginelli reaction. In the present endeavour, medium (ZSM-5) and large pore zeolites (Y, BEA and MOR) as well as dealuminated zeolites BEA, were studied as catalysts. An excellent activity for DHPMs synthesis is achieved by optimizing accessibility of the reactants to the active sites and the surface polarity of zeolite catalysts. Moreover, the mechanism of Biginelli reaction was studied by means of GAUSSVIEW energy calculations of adsorbed acylimine intermediate on zeolite by using the density functional method (DFT).     

Comparison of n-Dodecane Hydroisomerization Performance Over Pt Supported ZSM-48 and ZSM-22

ZSM-48 and ZSM-22 zeolites with similar Si/Al ratio were synthesized and modified by alkali treatment. Moreover, n-dodecane hydroisomerization performance of Pt supported ZSM-22 and ZSM-48 were investigated. The catalytic results showed that the activity and the isomers selectivity of n-dodecane hydroisomerization could be improved by alkali treatment. The isomers distributions were distinct for Pt/ZSM-48 and Pt/ZSM-22. Mono-branched isomers near the end of the chains were more prone to be generated on Pt/ZSM-22 catalyst, which suggested “pore-mouth” catalysis model dominating the hydroisomerization catalysis. However, di-branched isomers and mono-branched isomers with methyl near the middle of the carbon chains were favorable to be formed over Pt/ZSM-48 catalyst according to the “key-lock” catalysis model. Moreover, more central-branched isomers were formed at high reaction temperature (>?320 °C) especially for Pt/ HZSM-22.

Graphic Abstract

     

Pt-loaded zeolites for reducing exhaust gas emissions at low temperatures and in lean conditions

In this study, pure and platinum-loaded zeolites, ZSM-5, Beta, zeolite Y and Ferrierite, were examined for the reduction of NO with propene in lean conditions and at low temperatures. The studies were carried out by utilising the FT-IR technique both in determination of surface species as well as concentrations in the gas flow at reactor outlet. The maximum in the intermediate formation can be observed at the light-off temperatures over all studied catalyst materials. The maximum conversions of NO were reached with 1 wt% Pt-loaded Beta and Y zeolites in excess oxygen. The lowest light-off temperatures of NO as well as propene can be detected also with Beta and Y zeolite catalysts.     

Crystal-size–dependent external surface diffusion barriers in Pt/ZSM-5 catalyzed n-pentane isomerization

Surface barriers persist in the molecular transport across external crystal surface of zeolites, which brings strong influences on zeolite catalysis. This study probes the role of crystal-size–dependent surface barriers in zeolite catalysis, with n-pentane isomerization catalyzed by Pt/ZSM-5 as a model reaction system. Chemical liquid deposition of SiO₂ is performed to reduce surface barriers on ZSM-5 crystals with different sizes. After SiO₂ deposition, surface barriers on both small (266 nm) and large (1304 nm) crystals are significantly reduced, while the improvement in catalytic activity for the large crystals (76.1%–129.1%) is much higher than that for the small ones (−17.4% to 16.7%). For the large ZSM-5 crystals, diffusion limitation is stronger, and thus the effect of surface barriers on catalytic activity is more important. This study reveals the unique role of crystal-size–dependent surface barriers in zeolite catalysis, and understanding this is important for the optimal design of zeolite catalysts.     

Hydrothermal stability of different zeolites in supercritical water: Implication for synthesis of supported catalysts by supercritical water impregnation

Supercritical water (SCW) impregnation is an efficient and feasible method that has been used to prepare highly dispersed supported catalysts, but few studies have investigated the stability of support materials in supercritical water. Thus, our aim was to investigate the hydrothermal stability of zeolite supports (ZSM-5, TS-1, ZSM-35, HY, 13X, Beta, SAPO-11 and SAPO-34) as model compounds in supercritical water. Results showed that almost all of zeolites suffered from crystallinity change, structural properties degradation, obvious desilication and dealumination. The decrease of surface areas and the collapse of crystalline structures in HY, 13X, Beta, SAPO-11 and SAPO-34 were more serious compared to ZSM-5, ZSM-35 and TS-1. The micropore areas and acidity of all SCW-treated zeolites were reduced. 13X with lower Si/Al ratio had higher hydrothermal stability than HY due to the formation of extra-framework Al (EFAL). EFAl also generated strong Lewis acid sites determined by ammonia temperature-programmed desorption and ²⁷Al magic angle spinning nuclear magnetic resonance. Desilication and dealumination were simultaneous, and led to the increase of framework Si/Al ratio. ZSM zeolites (ZSM-5, ZSM-35 and TS-1) had higher hydrothermal stability than HY, 13X, Beta, SAPO-11 and SAPO-34 in SCW.     

The Effect of Zeolite Structure on the Activity and Selectivity in p-Xylene Alkylation with Isopropyl Alcohol

The effect of zeolite architecture and channel dimensionality on p-xylene conversion and selectivity to 1-isopropyl-2,5-dimethyl-benzene was investigated in p-xylene alkylation with isopropyl alcohol over novel zeolites SSZ-33 and SSZ-35. Catalytic behavior of these zeolites was compared with those of zeolites Beta, mordenite, ZSM-11 and ZSM-5. It was found that p-xylene conversion increases with increasing pore size and connectivity of the channel system of individual zeolites with the exception of SSZ-35, which possesses a system of one-dimensional 10-ring channels that periodically open into wide, shallow cavities circumscribed by 18-rings. SSZ-35 exhibited the highest conversion among all zeolites at the reaction temperature of 150 °C and also the highest selectivity to 1-isopropyl-2,5-dimethyl-benzene. Molecular modeling confirmed the dimensions of the 18-ring cages are optimal for the formation of this alkylation product.     

Promising zeolite-type hydrocarbon trap catalyst by a knowledge-based combinatorial approach

Libraries consisting of more than 100 zeolite samples were prepared and examined for developing a promising HC trap catalyst. Parallel adsorptions of toluene onto the catalyst samples were conducted over a 10 × 10 array reactor under dry and wet conditions with or without a heating process three knowledge-based conditions for developing an automotive catalyst during the cold-start period. FAU and BEA type zeolites revealed a high performance of toluene adsorption under the dry condition. However, FAU type zeolite significantly decreased the amount of toluene adsorbed in the presence of water in the feed gas stream, mainly due to the hydrophobicity of the catalyst surface. Over Beta type zeolites, the toluene adsorbed was found to be considerably preserved, even after forced desorption temperature-ramping to the warm-up condition of an automotive engine. Li, K, or Ag ion-exchanged Beta zeolites seem to be particularly promising as an HC trap catalyst.     

Heterogeneous Catalytic Transformation of Citronellal to Menthol in a Single Step on Ir-Beta Zeolite Catalysts

The synthesis of menthol from citronellal was studied on bifunctional Ir/Beta zeolite catalysts, prepared via impregnation of Beta zeolite with different concentrations of Ir. The results show that Beta zeolites, impregnated with Ir, catalyze the one-pot full conversion of citronellal to menthol by consecutive acid-catalyzed cyclization and Ir-catalyzed hydrogenation. The best results (95% selectivity for the menthol isomers, of which 75% is the desired (±)-menthol) were recorded on 3% Ir/Beta zeolite catalyst. In contrast with earlier studies, 0.05 g Ir/Beta catalyst per mL of substrate leads to complete and selective substrate transformation. The textural properties, acidity and hydrogenation function were studied by liquid N₂ adsorption–desorption, FT-IR, XPS and hydrogen chemisorption techniques. To clarify the mechanism of the cyclization of citronellal, tests were also performed using zeolites with different acidity and loaded with other metals (Pt, Pd, Rh, Ru).     

不同方法合成β沸石的FCC反应性能

报道了不同方法合成β沸石的FCC反应性能,并考察了β沸石的硅铝比对FCC反应产品分布的影响。结果表明：β沸石应用于FCC,与高硅ZSM-5相比,具有提高汽油产率及辛烷值的功能;另外β沸石较高硅ZSM-5表现出更强的异构化功能,具有提高异丁烷和轻汽油异构烯烃（或叔碳烯烃）产率的作用,可丰富烷基化技术和醚化技术的原料来源。     

The function of zeolite on Pt autoreduction and dispersion in Pt/L and Pt/β catalysts

TPR, CO-FTIR and¹²⁹Xe NMR spectroscopic techniques were used to measure the distribution of platinum species after the calcination of Pt/L and Pt/ zeolites. Autoreduction which occurred in Pt/ zeolite was avoided in the channel of L zeolite. Pt particles dispersed well and exhibited excellent reactivity for the aromatization ofn-hexane in L zeolite.     

Yield of gasoline-range hydrocarbons as a function of uniform ZSM-5 crystal size

Uniform ZSM-5 nanocrystals were synthesized by a single-templating procedure. The samples were then characterized by a variety of physical techniques such as XRD, SEM, BET, ICP and TPD. The dehydration of methanol over synthesized ZSM-5 zeolite was studied in a fixed-bed continuous flow reactor at 370 °C and WHSV of 2.6 gg⁻¹ h under ambient pressure. The effect of crystal size of zeolite catalysts on product distribution in methanol dehydration reaction was investigated. Good correlation was observed between catalytic performance, product distribution and size of ZSM-5 crystals. It was found that the decrease in crystal size significantly influences light olefins (ethylene and propylene) and paraffins (C₁–C₄) selectivity in methanol dehydration reaction. Furthermore, nanocrystal ZSM-5 showed long-term catalytic stability compared with conventional ZSM-5 provided that the reaction activity is strongly dependent on the crystal size in methanol dehydration process. The results indicated that crystal size significantly affects the catalyst lifetime and hydrocarbon distributions in product stream. Based on the obtained results, it is concluded that the use of uniform ZSM-5 nanocrystals improves the yield of propylene and alkyl aromatics in methanol conversion reaction at mild conditions.     

Synthesis and catalytic performance of a bi-phase core-shell zeolite composite

A zeolite composite Y/Beta with core-shell structure was synthesized by adding tetraethylammonium bromide (TEABr) exchanged NaY zeolite into the pre-reacted mixture used to prepare Beta zeolite. The composite was characterized by XRD, N₂ adsorption, SEM, and FTIR spectra of pyridine. The results show that the composite is composed of a core zeolite Y and a shell of intergrown zeolite Beta crystals, representing dual microporous structures of both Y and Beta zeolites and a new mesoporous structure. The composite has a high activity in n-octane catalytic cracking because of the formation of intergrowths and the change of acidity due to the distorted interface and surface defects.     

Structural features of binary microporous zeolite composite Y-Beta and its hydrocracking performance

Y-Beta zeolite composites denoted as HS-FBZ (High-Silica-Faujasite-Beta-Zeolite) were synthesized hydrothermally using high silica Y as precursor and characterized by XRD, N₂ adsorption, SEM, HRTEM, NH₃-TPD and FT-IR spectra of pyridine. The results reveal that the different ratios of Y and Beta zeolites in the composites induced the change of surface areas, pore diameters and acid properties of HS-FBZ, and the extended Beta layers on the zeolite Y surface were observed. The composite exhibited an excellent hydrocracking activity for heavy oil. Compared with the corresponding physical mixture of Y and Beta zeolites, the zeolite composite as a hydrocracking catalyst for vacuum gas oil increased the heavy naphtha yield by 5.61 wt% and the jet fuel yield by 11.52 wt%.     

Physical Properties of Nutritive Shortenings Produced from Regioselective Hardening of Soybean Oil with Pt Containing Zeolite

Soybean oil was partially hydrogenated using Pt supported in microporous zeolite ZSM-5 and on mesoporous alumina at various IV. Their fatty acid and triacylglycerol composition were determined with GC and HPLC, respectively, and their physical characteristics were monitored by the slip melting point, solid fat content, melting and crystallization thermograms, polymorphism behavior, and the crystal and solid fat network formation. Both the chemical and physical properties were compared with commercial fat samples. Usage of Pt instead of Ni results in a significant reduction in trans fatty acids in the hardened fat. Moreover, the catalyst support of Pt, viz. zeolite ZSM-5 versus γ-alumina, markedly affects the TAG composition. Pt/alumina fats contain large amounts of SSS and polyunsaturates (PUFA), making them unsuitable for shortening application. Because of the (regio)selective hydrogenation property of Pt/ZSM-5, sn-2 unsaturates are hydrogenated faster, yielding an enrichment of intermediately reduced TAG. In addition, this unique fat composition shows a high nutritional added-value (high content of oleate, very low content of trans fatty acids, and low content of cholesterol-raising palmitate and myristate) and high thermal stability (very low in linolenate). Moreover, their melting characteristics perfectly match those of commercial shortenings. Pt/zeolite hardened soybean oil contains spherulitic crystals with orthorhombic β′ molecular packing, arranged in an open, flexible solid network, in accordance with their high plasticity.     

Hierarchical ZSM-5 zeolite designed by combining desilication and dealumination with related study of n-heptane cracking performance

The effect of the basic (NaOH) and/or acid (citric acid and EDTA-2Na) treatment of ZSM-5 zeolite has been studied comparing the structural and acidic features and their catalytic performance in n-heptane cracking. The properties of the catalysts have been elucidated using XRD, N₂ low-temperature sorption, ²⁷Al and ²⁹Si NMR, pyridine adsorbed FTIR, NH₃–TPD, SEM and TEM analysis. The results showed that the degree of desilication and dealumination of ZSM-5 zeolites was greatly dependent on the agents. NaOH obviously created new mesopores on parent ZSM-5 zeolites by desilication. Citric acid contributed to the removal of nonframework Al species, causing the increase of micropore surface area. EDTA-2Na promoted desilication and simultaneously converted part of removed framework Al species into nonframework Al species. The treatment of ZSM-5 combined with those three agents was very effective to obtain a hierarchical structure with partial breakdown of the crystallites and high acid amounts of both Brönsted and Lewis acid sites. Catalytic tests showed that the post-treated ZSM-5 catalysts had higher activity and stability than parent ZSM-5 catalyst at the same reaction temperature. The synergetic effect of Brönsted acid and Lewis acid of ZSM-5 catalyst (Z5-ACE) probably facilitated n-heptane conversion, while more clean micropore and newly created mesopores facilitated the slight increase of olefin selectivity and suppressing the formation of coke deposition in its inherent micropores to some extent.     

Oligomerization of α-octene catalyzed by zeolites

The oligomerization of octene-1 was studied in the presence of zeolites of different structural types (e.g., Y, Beta, ZSM-12, and ZSM-5). It was shown that at 150–200°C, wide-pore zeolite catalysts Y, Beta, and ZSM-12 exhibit high catalytic activity in the reaction. The conversion of catalyzed octene can be as high as 96%, and the yield of oligomers is 88–100 wt %. Zeolite ZSM-5 at 150–180°C has low activity and mainly catalyzes the isomerization of octene-1. As the temperature rises to 250°C, low molecular weight oligomers resulting from the cracking process are the main products of this reaction. The activity and selectivity of zeolite catalysts in the oligomerization of octene-1 are conditioned by their acidic properties and structural characteristics, as well as by the reaction conditions. It was revealed that the main oligomerization products are octene dimers having an alkylnaphthene structure and containing unsaturated hydrocarbons with tri- and tetra-substituted double bonds in amounts of 2.2–3.2%. The properties of the octene oligomers synthesized in the presence of zeolite Beta are similar to the characteristics of hydrogenated poly-α-olefins, the oligomerization of which was performed on AlCl₃ complexes.