Cumene Cracking Research Articles

Role of Iridium in n‐Pentane Isomerization, Cyclohexane Ring Opening, and Cumene Cracking Over Ir/Pt‐HZSM5

AbstractProducing aromatic compounds sustainably from renewable sources is essential to reduce reliance on fossil fuels. This study aims to elucidate the multifunctional behavior of Ir on Pt‐HZSM5, and their influence on the catalytic activity of n‐pentane isomerization, cyclohexane ring opening, and cumene cracking. The results showed that Ir created another active and Lewis's acid site, which increased the acid sites (H+), thus enhancing the catalytic performance of Pt‐HZSM5. Moreover, Ir inhibited the atomic hydrogen bonding interaction and hence improved the main reaction and suppressed the side reaction. Therefore, the mechanism of the main reaction over Ir/Pt‐HZSM5 can be described as the following steps: (i) Interaction of reactant with H+; (ii) Reaction of carbenium ion intermediate; and (iii) Interaction of a carbenium ion intermediate with H−, followed by desorption. This study provides a comprehensive understanding of the intricate catalytic pathways, offering insights into the design of efficient multifunctional catalysts for various petrochemical applications.

Synthesis of amorphous silica-alumina with enhanced specific surface area and acidity by pH-swing method and its catalytic activity in cumene cracking

High-performance amorphous silica-alumina (ASA) is an important acid material in the chemical industry. In this work, we successfully synthesized mesoporous ASA with enhanced specific surface area, Lewis acidity and total acidity by pH-swing method. The morphology and structure of ASA were systematically investigated. With the increase of SiO2 content from 20.1% to 67.0%, the morphology of ASA varied considerably from fibrous to granular, and the specific surface area decreased from 436 to 306 m2 g−1. Meanwhile, the gradual increase of surface silanol groups facilitated the formation of pseudo-bridged silanol groups and promoted the formation of Brønsted acid sites (BAS). Furthermore, the concentration of BAS was found to increase with increasing tetra- and penta-coordinated aluminum and decrease with increasing octa-coordinated aluminum. In addition, the acidity-activity relationship was also explored by applying the as-synthesized ASA samples directly to the cracking reaction of cumene. The results showed that the activity of ASA was proportional to the concentration of BAS. The pH-swing method can controllably synthesize ASA materials with different structures and acidity for petrochemical industry.

Plate-Like ZSM-5 Zeolites as Robust Catalysts for the Cracking of Hydrocarbons.

Zeolites with good acid site accessibility and high diffusion rates are highly desirable catalysts, especially when dealing with bulk molecules. In this work, ZSM-5 zeolites with similar Si/Al ratios but different thicknesses along the b-axis (from ∼30 nm to ∼5 μm), namely, two plate-like ZSM-5 zeolites and two reference zeolites have been prepared and the impacts of b-axis thickness on the surface properties and catalytic cracking performances are explored. Comprehensive physiochemical studies demonstrate that reducing the b-axis thickness of the zeolite crystals endows the samples with better acid site accessibility and more external surface acid sites. Two model compounds with different molecule sizes, namely, 1,3,5-triisopropylbenzene (TIPB) and cumene, are selected to explore the catalytic cracking performances of the as-synthesized samples. The results reveal that decreasing the b-axis thickness of zeolite crystals can effectually promote the catalytic activity and stability in catalytic cracking reactions. For TIPB cracking, the greatly enhanced catalytic activity is ascribed to the enhanced acid site accessibility in plate-like ZSM-5 zeolites, and for cumene cracking, the improved catalytic stability is ascribed to the shortened diffusion length of plate-like zeolites.

Synthesis of Amorphous Silica-Alumina with Enhanced Specific Surface Area and Acidity by Ph-Swing Method and its Catalytic Activity in Cumene Cracking

Synthesis of Amorphous Silica-Alumina with Enhanced Specific Surface Area and Acidity by Ph-Swing Method and its Catalytic Activity in Cumene Cracking

Catalysis of water molecules acting as Brönsted acids at Lewis acid sites on niobium oxide

The effects of water addition on cumene cracking over niobium oxide and H-ZSM-5 are investigated in order to clarify the behavior of water molecules on solid acid catalysts. The activity of niobium oxide was improved in the presence of water vapor although H-ZSM-5 showed constant activity. When heavy water was introduced into the reaction over niobium oxide, the hydrogen atoms were substituted with the deuterium atoms in about 85 % of the produced benzene. The heavy water molecule on niobium oxide acts as the catalyst for cumene cracking, resulting in the formation of benzene containing D atoms. In addition, the Lewis acid on the niobium oxide was observed to change reversibly to the Brönsted acid in the presence of water using IR of adsorbed pyridine. We propose a mechanism that water behaves as Brönsted acid, which was clarified by repeating adsorption and desorption on Lewis acid sites of niobium oxide.

Organic-Free Synthesis of Zeolite Y with High Si/Al Ratios: Combined Strategy of In Situ Hydroxyl Radical Assistance and Post-Synthesis Treatment.

Zeolite Y, with a high SiO2 /Al2 O3 ratio (SAR), plays an important role in fluidized catalytic cracking processes. However, in situ synthesis of zeolite Y with high SARs remains a challenge because of kinetic limitations. Here, zeolite Y with an SAR of 6.35 is synthesized by a hydroxyl radical assisted route. Density-functional theory (DFT) calculations suggest that hydroxyl radicals preferentially enhanced the formation of Si-O-Si bonds, thus leading to an increased SAR. To further increase the SAR, a dealumination process was carried out using citric acid, with a subsequent second-step hydrothermal crystallization, giving an SAR of up to 7.5 while maintaining good crystallinity and high product yield. The resultant zeolite Y shows good performance in cumene cracking. Introduced here is a new strategy for synthesizing high SAR zeolite Y, which is widely used in commercial applications.

Catalytic nanosponges of acidic aluminosilicates for plastic degradation and CO2 to fuel conversion

The synthesis of solid acids with strong zeolite-like acidity and textural properties like amorphous aluminosilicates (ASAs) is still a challenge. In this work, we report the synthesis of amorphous “acidic aluminosilicates (AAS)”, which possesses Brønsted acidic sites like in zeolites and textural properties like ASAs. AAS catalyzes different reactions (styrene oxide ring-opening, vesidryl synthesis, Friedel−Crafts alkylation, jasminaldehyde synthesis, m-xylene isomerization, and cumene cracking) with better performance than state-of-the-art zeolites and amorphous aluminosilicates. Notably, AAS efficiently converts a range of waste plastics to hydrocarbons at significantly lower temperatures. A Cu-Zn-Al/AAS hybrid shows excellent performance for CO2 to fuel conversion with 79% selectivity for dimethyl ether. Conventional and DNP-enhanced solid-state NMR provides a molecular-level understanding of the distinctive Brønsted acidic sites of these materials. Due to their unique combination of strong acidity and accessibility, AAS will be a potential alternative to zeolites.

Strategy towards enhanced performance of zeolite catalysts: Raising effective diffusion coefficient versus reducing diffusion length

In the present study, two strategies for improving the catalyst efficiency either by (1) raising the effective diffusion coefficient or (2) reducing the diffusion length of zeolite catalysts are presented. These two strategies are exemplified by the preparation of (i) ordered mesoporous material with zeotype secondary building units and (ii) self-assembled nanosized zeolites. The textural properties, acidity, and the results from the cumene cracking show that the strength and amount of Brønsted acid sites of the mesoporous materials are inferior, due to their amorphous pore walls. Even though the lifespan of ordered mesoporous material with zeotype secondary building units during cumene cracking can be improved due to the effective diffusion coefficient enhancement, the strength and amount of Brønsted acid sites related to the reaction are not akin to conventional microporous zeolites. While the self-assembled nanosized zeolites with short diffusion length shows enhanced catalytic performance by diffusion controlled reactions.

Crystallization of Mordenite Platelets using Cooperative Organic Structure-Directing Agents.

Organic structure-directing agents (OSDAs) are exploited in the crystallization of microporous materials to tailor the physicochemical properties of the resulting zeolite for applications ranging from separations to catalysis. The rational design of these OSDAs often entails the identification of molecules with a geometry that is commensurate with the channels and cages of the target zeolite structure. Syntheses tend to employ only a single OSDA, but there are a few examples where two or more organics operate synergistically to yield a desired product. Using a combination of state-of-the-art characterization techniques and molecular modeling, we show that the coupling of N,N,N-trimethyl-1,1-adamantammonium and 1,2-hexanediol, each yielding distinct zeolites when used alone, results in the cooperative direction of a third structure, HOU-4, with the mordenite framework type (MOR). Rietveld refinement using synchrotron X-ray diffraction data reveals the spatial arrangement of the organics in the HOU-4 crystals, with amines located in the large channels and alcohols oriented in the side pockets lining the one-dimensional pores. These results are in excellent agreement with molecular dynamics calculations, which predict similar spatial distributions of organics with an energetically favorable packing density that agrees with experimental measurements of OSDA loading, as well as with solid-state two-dimensional 27Al{29Si}, 27Al{1H}, and 13C{1H} NMR correlation spectra, which establish the proximities and interactions of occluded OSDAs. A combination of high-resolution transmission electron microscopy and atomic force microscopy is used to quantify the size of the HOU-4 crystals, which exhibit a platelike morphology, and to index the crystal facets. Our findings reveal that the combined OSDAs work in tandem to produce ultrathin, nonfaulted HOU-4 crystals that exhibit improved catalytic activity for cumene cracking in comparison to mordenite crystals prepared via conventional syntheses. This novel demonstration of cooperativity highlights the potential possibilities for expanding the use of dual structure-directing agents in zeolite synthesis.

Mechanistic insights into structural and surface variations in Y-type zeolites upon interaction with binders

Zeolite Y is one of the most important solid acid catalysts that find immense applications in catalytic processes such as fluid catalytic cracking (FCC), acylation and cumene cracking. The technical grades of FCC catalyst require specifications that are met by dispersing the zeolite in amorphous binders. On contact with binders, zeolite properties in composite catalysts alter. This article reports an insight into structural and surface modifications of Y zeolites caused by binders’ incorporation. Experiments employing various characterization techniques, including solid state MAS-NMR and FTIR reveal alumina incorporation adjusts zeolite framework in comparison with silica binder. Incorporation of alumina binder into zeolite Y polarizes the framework by extra-framework alumina species (such as Al3+, AlO+ and Al(OH)2+) coming from the binder. The polarization, apparent with USY and REY, is considered as the operating mechanism for distortion of zeolite framework, involving the attraction of aluminum species from the binder to the framework of zeolite and its subsequent framework structure reorientation. Compensating framework Na+ cations restrain the polarization of NaY framework due to its deficiency of protons, revealing Brönsted acid sites as active centers for the zeolite framework polarization. The resulting alumina bound catalysts are amorphized by layers of alumina materials that implicate the Brönsted acidity as a function of its weight in the catalyst. Catalytic reaction reveals that polarization of the zeolite framework by extra-framework alumina species decreases the transformation of paraffin to carbenium ion, increasing the hydrogen transfer reaction pathway.

Combined alkali dissolution and re-assembly approach toward ZSM-5 mesostructures with extended lifetime in cumene cracking

The basis and contribution of mesopores created in ZSM-5 structures at different treatment conditions are systematically investigated. The results reveal that the mesopores originated from the alkali dissolution of pristine ZSM-5 are mainly intracrystalline and they contribute to excessive Brønsted acid sites, while the mesopores originated from the re-assembly of alkali dissolved aluminosilicate species possess Lewis acid sites. These ZSM-5 mesostructures showed an extended lifespan during the cracking of cumene (88.0%) in comparison to the pristine ZSM-5 (27.0%) after 460 min. The zeolite mesostructures obtained in this study could be used as a base for further design of new porous materials with desired acidic properties.

Mesoporous EU-1 zeolite with enhanced accessibility and diffusion for bulky molecular reaction

Mesoporous EU-1 zeolites with adjustable mesoporosity have been prepared by employing silanized silica into the starting synthesis gel, based on the hinder effect of organic moiety to crystal growth. The synthesized mesoporous EU-1 zeolites were characterized by XRD, NMR, SEM/TEM and N2 adsorption/desorption at 77K, showing the persistence of inherent microporous framework structure of EU-1 zeolite and the existence of additional mesopores within the range of 3–10nm. The generation of mesoporosities is from the intercrystalline voids of nanocrystal aggregates. The introduction of mesopores in EU-1 zeolite increased the adsorption of mesitylene with the kinetics diameter larger than the micropore size, and made it possible for the diffusion pattern of n-heptane from one-dimensional in microporous EU-1 to three-dimensional in mesoporous EU-1. This result together with the enhanced accessibility of acidic sites of mesoporous EU-1 improved greatly the catalytic activity and product yields of cumene cracking reaction involving bulky molecules.

Influence of post-synthetic treatments of aluminum-rich ZSM-5 on the catalytic cracking of bulky hydrocarbons at low temperature

The influence of post-synthetic modifications of Al-rich ZSM-5 (Si/Al ratio ca. 10) involving alkaline and subsequent acid treatments on the textural, acidic and catalytic properties in the cracking of bulky hydrocarbon molecules at low temperature is reported. It was found that the optimized base treatment, i.e. 0.5 M NaOH should be used for the substantial generation of intracrystal mesoporosity while preserving the intrinsic zeolite properties of ZSM-5 zeolite. A subsequent strong acid washing, i.e. 0.5 M HCl is required to completely remove Al-rich debris formed during the base treatment, thereby improving the textural and acidic properties. The catalytic performance of the parent and treated ZSM-5 materials was evaluated in the gas phase cracking of 1,3,5-triisopropylbenzene (TIPB) and cumene as test reactions. The results showed that under the applied cracking conditions, the introduction of mesoporosity by the optimized base–acid treatments on the one hand increased the accessibility of Bronsted acid sites, producing a higher TIPB conversion (82.5% as compared to 61.9% of the parent ZSM-5), on the other hand, it promoted the transformation of pre-cracking products, leading to a higher selectivity to deep cracking products, i.e. the selectivity of cumene and benzene of 22.7% as compared to 17.6% of the parent ZSM-5. A similar conclusion has been reached in the cracking of cumene where the presence of considerable mesoporosity indeed improved cumene reactivity of the alkaline and acid treated ZSM-5 as its density of strong zeolite Bronsted sites was mainly retained.

Influence of initial Si/Al ratios on the structural, acidic and catalytic properties of nanosized-ZSM-5/SBA-15 analog composites prepared from ZSM-5 precursors

Nano-ZSM-5/SBA-15 analog composites (ZSC) were prepared in a two-step process from ZSM-5 precursors with different Si/Al molar ratios (10–50) via high-temperature synthesis in mildly acidic media (200 °C, pH 3.5) aiming to evaluate the influence of the initial Si/Al ratio on their structural, acidic and catalytic properties. The resulting materials were characterized by SAXS, XRD, FTIR, TEM, N2 sorption, 27Al solid state-NMR, NH3-TPD, FTIR spectroscopy of adsorbed pyridine, AAS and ICP-AES. Under the applied synthesis conditions, a ZSC material with controlled distribution of nano-ZSM-5 and SBA-15 analog phases can be prepared from ZSM-5 precursors by adjusting the initial Si/Al ratio in the range of 20–30. Increasing the initial Si/Al ratio to 50, only ZSM-5 nanocrystals were obtained whereas reducing the initial Si/Al ratio to 10 led to the formation of a disordered mesoporous SBA-15 analog. The total acidity increases with the crystallinity of the ZSM-5 phase as varying the Si/Al ratio from 10 to 30 despite the decreased amount of incorporated aluminum. However, the acidity declines slightly when raising the Si/Al ratio to 50 because of the low incorporated aluminum. The catalytic performance of the ZSC materials compared to the reference materials, i.e. purely mesoporous Al-SBA-15 and purely microporous H-ZSM-5 was assessed in the gas phase cracking of cumene and 1,3,5-tri-isopropylbenzene (TIPB) as test reactions. The results show that a balanced ratio of nano-ZSM-5 and SBA-15 analog phases obtained by tuning the initial Si/Al ratio is crucial to achieve superior catalytic performance of the ZSC materials in the cracking of both cumene and TIPB.

Compensation between activation entropy and enthalpy in reactions of aromatic hydrocarbons catalyzed by solid acids

Reaction rates of toluene disproportionation (A) and cumene cracking (B) normalized by the number of Brønsted acid sites were analyzed on aluminosilicates. The activation entropy showed linear and compensatory relationship against the activation enthalpy. The slope of entropy-enthalpy plot was in the order of (B)>(A)>small alkane cracking, whereas the intercept on the horizontal axis was in the order of propane and isobutane cracking>linear C4–8 alkanes and iso-pentane cracking>(A)>(B). The former is consistent with the bulkiness of reactant, while the latter is consistent with intrinsic difficulty of formation of intermediate cations.

Facile tailoring of hierarchical mesoporous AlSBA-15 by ionic liquid and their applications in heterogeneous catalysis

Significant enhancement in the performance of incorporation of high content of aluminum within hierarchical mesoporous SBA-15 has been achieved by direct route using urea tetrachloroaluminate ionic liquid as novel aluminum source. The fabricated materials were fully characterized by N2 sorption isotherms, powder X-ray diffraction (XRD), FT-IR, 27Al MAS NMR, XRF, HRTEM and FESEM. The acidic properties of these materials have been examined using NH3-TPD. The catalytic performance was investigated using cumene cracking and the esterification reaction as a probe molecules to estimate the acidity of the material. It is worth noting that the ionic liquid with accomplished Al–O–Si bonds is an efficient precursor to synthesis AlSBA-15 with high aluminum content (nSi/mAl = 7) without destroying the structural order of the material in acidic medium. 27Al MAS NMR spectra of AlSBA-15 show that all aluminum species were incorporated into the SBA-15 framework with nSi/mAl ratio up to 7. Overall, this work emphasize that the AlSBA-15 materials contain Bronsted and Lewis acid sites with medium acidity which makes them adequate to be adopted as acid catalysts in heterogeneous catalysis.

Synthesis, Characterization and Catalytic Performance of Well-Ordered Crystalline Heteroatom Mesoporous MCM-41

Mesoporous heteroatom molecular sieve MCM-41 bulk crystals with the crystalline phase were synthesized via a one-step hydrothermal method using an ionic complex as template. The ionic complex template was formed by interaction between cetyltrimethylammonium ions and metal complex ion [M(EDTA)]2− (M = Co or Ni)]. The materials were characterized by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, N2 adsorption-desorption isotherms, and X-ray absorption fine structure spectroscopy. The results showed that the materials possess a highly-ordered mesoporous structure with a crystalline phase and possess highly uniform ordered arrangement channels. The structure is in the vertical cross directions with a crystalline size of about 12 µm and high specific surface areas. The metal atoms were incorporated into the zeolite frameworks in the form of octahedral coordinate and have a uniform distribution in the materials. The amount of metal complexes formed by metal ion and EDTA is an essential factor for the formation of the vertical cross structure. Compared to Si-MCM-41, the samples exhibited better conversion and higher selectivity for cumene cracking.

Two-stage dual template alkali metal-free synthesis of micro-mesoporous aluminosilicates based on ZSM-5 zeolite precursors

Carrying out the dual template synthesis (in the presence of molecular and micellar templates) in the alkali metal-free reaction media (pH 11.7–12) at a relatively low-temperature hydrothermal treatment (100–120 °C, duration 3–5 days) allowed to obtain micro-mesoporous aluminosilicates combining the properties of zeolite ZSM-5 and mesoporous molecular sieve AlSi-MCM-41. X-ray amorphous sol-precursors of ZSM-5 zeolite obtained by preliminary hydrothermal treatment of alkali metal-free reaction mixture at 160 °C and containing secondary building units of zeolite were used as initial substances for synthesis of micro-mesoporous aluminosilicates. During the joint formation of the structure of zeolite and mesoporous molecular sieve in one reaction mixture and in contiguous conditions close to optimal for their individual formation, composites zeolite/mesoporous molecular sieve were formed. The structural and sorption characteristics and acidity of the obtained composites differ from mechanical mixture of two phases. X-ray amorphous micro-mesoporous aluminosilicate with secondary building units and precursors of ZSM-5 in mesostructure is characterized by higher concentration and strength of Bronsted and Lewis acid sites as well as a higher catalytic activity in the reaction of cracking of cumene compared to AlSi-MCM-41 of similar composition. This aluminosilicate also exhibits a higher total selectivity toward the major products of reaction, in comparison with ZSM-5.

Preparation of Alumina Pillared Montmorillonite Clays Employing Ultrasonics and Their Catalytic Properties

<p>An accelerated diffusion of alumina precursor species in-between the gallery region of montmorillonite clays by the use of ultrasonic technique is described. This method offers a faster, efficient and attractive route for the synthesis of alumina-pillared clay catalysts. Out of the several preparative parameters studied, the role of pre-exchanged cations influenced the optimum duration of ultrasonic agitation and treatment for the pillaring process. By comparing ultrasonic route and conventional preparation procedure, it is revealed that uniform spacing of alumina pillars in the clay means homogeneously distributed acid centers. The influence of calcination temperature on the acidity and hence the catalytic activity is described. The choice of three different montmorillonite clays with different cation exchange capacities enables one to tune the pore dimensions of the pillared clays. The catalytic applications include studies on cumene cracking, dehydration of propan-2-ol and alkylation of benzene with linear olefins. </p>

Preparation of cylindrical extrudates of mesoporous silica-alumina without use of binder and their activity in cracking of cumene

A series of mesoporous silica–aluminas (MSA) containing 24–59 wt% Al2O3 was synthesized from sodium metasilicate and aluminum nitrate and extruded in 1/16″ cylinders without use of a binder. This was followed by slow hydrogel drying, extraction by water and calcination at 500 °C. This procedure gave stable hard extrudates and the absence of binder allowed to keep the surface area, mesoporosity and cracking activity of MSA at the original high values. The resistance to crushing was much better as compared to commercial molecular sieve 13X. The extrudates have average pore size around 3.5 nm. Their properties were affected by the contents of Al2O3 and residual sodium. The MSA containing 50 wt% Al2O3 possessed the maximum values of surface area, pore volume and activity in cumene cracking. The calcined extrudates showed good water sorption capacity and were stable in aqueous environment. This predestinates the MSA extrudates as the shaped mesoporous acidic carriers for preparation of different supported catalysts.