Regulation of Framework Aluminum Siting and Acid Distribution in H-MCM-22 by Boron Incorporation and Its Effect on the Catalytic Performance in Methanol to Hydrocarbons

Boron-incorporated nanosized SUZ-4 zeolite for DME carbonylation

Boron-incorporated nanosized HB-SUZ-4 showcased a noteworthy 24% boost in dimethyl ether carbonylation, with an elevation in methyl acetate selectivity from 91.8% to 96.0%. The improved performance is attributed to shortened diffusion lengths along the 8-member ring channels, decreased Brønsted acidity in the 10-member ring channels, and Lewis acid sites stabilizing CO.

2-Isopropyl-1,3-dimethylimidazolium as a versatile structure-directing agent in the synthesis of zeolites

An organic cation lacking specificity in its structure-directing action offers the possibility, through the screening of other structure-directing parameters, to synthesize a variety of zeolites. In this work we show that the organic structure-directing agent 2-isopropyl-1,3-dimethylimidazolium (2iPr13DMI) can produce up to seven different zeolite phases depending on water concentration, the presence of inorganic impurities, crystallization temperature and time, and germanium molar fraction. The obtained phases are very different in terms of pore system, connectivity of the zeolite structure and structural units. At the pure SiO2 side, ZSM-12 and SSZ-35 dominate, with ZSM-12 being favored by the presence of potassium impurities and by less concentrated conditions. The introduction of Ge at low levels favors SSZ-35 over ZSM-12 and as the Ge fraction increases it successively affords CSV, -CLO and two distinct UOS zeolites, HPM-11 and HPM-6. These two zeolites have the same topology but distinct chemical compositions and display powder X-ray diffraction patterns that are much different from each other and from that of as-synthesized IM-16 (UOS reference material). They also show different symmetry at 96 K. Rietveld refinements of the three as-made UOS materials mentioned are provided. HPM-6 and HPM-11 are produced in distinct, non-adjacent crystallization fields. The frequent cocrystallization of the chiral STW zeolite, however, did not afford its synthesis as a pure phase. Molecular mechanics simulations of the location of the organic cation and host-guest interactions fail to explain the observed trends, but also considering the intrinsic stability of the zeolites and the effect of germanium help to rationalize the results. The study is completed by DFT calculations of the NMR chemical shifts of 13C in UOS (helping to understand splittings in the spectrum) and 19F in CSV (supporting the location of fluoride inside the new [4452], which is an incomplete double 4-ring).

Advanced zeolite and ordered mesoporous silica-based catalysts for the conversion of CO2 to chemicals and fuels

For many years, capturing, storing or sequestering CO₂ from concentrated emission sources or from air has been a powerful technique for reducing atmospheric CO₂. Moreover, the use of CO₂ as a C1 building block to mitigate CO₂ emissions and, at the same time, produce sustainable chemicals or fuels is a challenging and promising alternative to meet global demand for chemicals and energy. Hence, the chemical incorporation and conversion of CO₂ into valuable chemicals has received much attention in the last decade, since CO₂ is an abundant, inexpensive, nontoxic, nonflammable, and renewable one-carbon building block. Nevertheless, CO₂ is the most oxidized form of carbon, thermodynamically the most stable form and kinetically inert. Consequently, the chemical conversion of CO₂ requires highly reactive, rich-energy substrates, highly stable products to be formed or harder reaction conditions. The use of catalysts constitutes an important tool in the development of sustainable chemistry, since catalysts increase the rate of the reaction without modifying the overall standard Gibbs energy in the reaction. Therefore, special attention has been paid to catalysis, and in particular to heterogeneous catalysis because of its environmentally friendly and recyclable nature attributed to simple separation and recovery, as well as its applicability to continuous reactor operations. Focusing on heterogeneous catalysts, we decided to center on zeolite and ordered mesoporous materials due to their high thermal and chemical stability and versatility, which make them good candidates for the design and development of catalysts for CO₂ conversion. In the present review, we analyze the state of the art in the last 25 years and the potential opportunities for using zeolite and OMS (ordered mesoporous silica) based materials to convert CO₂ into valuable chemicals essential for our daily lives and fuels, and to pave the way towards reducing carbon footprint. In this review, we have compiled, to the best of our knowledge, the different reactions involving catalysts based on zeolites and OMS to convert CO₂ into cyclic and dialkyl carbonates, acyclic carbamates, 2-oxazolidones, carboxylic acids, methanol, dimethylether, methane, higher alcohols (C₂₊OH), C₂₊ (gasoline, olefins and aromatics), syngas (RWGS, dry reforming of methane and alcohols), olefins (oxidative dehydrogenation of alkanes) and simple fuels by photoreduction. The use of advanced zeolite and OMS-based materials, and the development of new processes and technologies should provide a new impulse to boost the conversion of CO₂ into chemicals and fuels.

Regulation of the Si/Al ratios and Al distributions of zeolites and their impact on properties

Zeolites are typically a class of crystalline microporous aluminosilicates that are constructed by SiO₄ and AlO₄ tetrahedra. Because of their unique porous structures, strong Brönsted acidity, molecular-level shape selectivity, exchangeable cations, and high thermal/hydrothermal stability, zeolites are widely used as catalysts, adsorbents, and ion-exchangers in industry. The activity, selectivity, and stability/durability of zeolites in applications are closely related to their Si/Al ratios and Al distributions in the framework. In this review, we discussed the basic principles and the state-of-the-art methodologies for regulating the Si/Al ratios and Al distributions of zeolites, including seed-assisted recipe modification, interzeolite transformation, fluoride media, and usage of organic structure-directing agents (OSDAs), etc. The conventional and newly developed characterization methods for determining the Si/Al ratios and Al distributions were summarized, which include X-ray fluorescence spectroscopy (XRF), solid state ²⁹Si/²⁷Al magic-angle-spinning nuclear magnetic resonance spectroscopy (²⁹Si/²⁷Al MAS NMR), Fourier-transform infrared spectroscopy (FT-IR), etc. The impact of Si/Al ratios and Al distributions on the catalysis, adsorption/separation, and ion-exchange performance of zeolites were subsequently demonstrated. Finally, we presented a perspective on the precise control of the Si/Al ratios and Al distributions of zeolites and the corresponding challenges.

Synthesis strategies to control the Al distribution in zeolites: thermodynamic and kinetic aspects

The activity and selectivity of acid-catalyzed chemistry is highly dependent on the Brønsted and Lewis acid sites generated by Al substitutions in a zeolite framework with the desired pore architecture. The siting of two Al atoms in close proximity in the framework of high-silica zeolites can also play a decisive role in improving the performance of redox catalysts by producing exchangeable positions for extra-framework multivalent cations. Thus, considerable attention has been devoted to controlling the Al incorporation through direct synthesis approaches and post-synthesis treatments to optimize the performance as (industrial) solid catalysts and to develop new acid- and redox-catalyzed reactions. This Feature Article highlights bottom-up synthetic strategies to fine-tune the Al incorporation in zeolites, interpreted with respect to thermodynamic and kinetic aspects. They include (i) variation in extra-framework components in zeolite synthesis, (ii) isomorphous substitution of other heteroatoms in the zeolite framework, and (iii) control over the (alumino)silicate network in the initial synthesis mixture via in situ and ex situ methods. Most synthetic approaches introduced here tentatively showed that the energy barriers associated with Al incorporation in zeolites can be variable during zeolite crystallization processes, occurring in complex media with multiple chemical interactions. Although the generic interpretation of each strategy and underlying crystallization mechanism remains largely unknown (and often limited to a specific framework), this review will provide guidance on more efficient methods to prepare fine-tuned zeolites with desired chemical properties.

Hydrogenation of Carbon Dioxide to Value-Added Liquid Fuels and Aromatics over Fe-Based Catalysts Based on the Fischer–Tropsch Synthesis Route

Hydrogenation of CO2 to value-added chemicals and fuels not only effectively alleviates climate change but also reduces over-dependence on fossil fuels. Therefore, much attention has been paid to the chemical conversion of CO2 to value-added products, such as liquid fuels and aromatics. Recently, efficient catalysts have been developed to face the challenge of the chemical inertness of CO2 and the difficulty of C–C coupling. Considering the lack of a detailed summary on hydrogenation of CO2 to liquid fuels and aromatics via the Fischer–Tropsch synthesis (FTS) route, we conducted a comprehensive and systematic review of the research progress on the development of efficient catalysts for hydrogenation of CO2 to liquid fuels and aromatics. In this work, we summarized the factors influencing the catalytic activity and stability of various catalysts, the strategies for optimizing catalytic performance and product distribution, the effects of reaction conditions on catalytic performance, and possible reaction mechanisms for CO2 hydrogenation via the FTS route. Furthermore, we also provided an overview of the challenges and opportunities for future research associated with hydrogenation of CO2 to liquid fuels and aromatics.

Preparation of sodium silicate/red mud-based ZSM-5 with glucose as a second template for catalytic cracking of waste plastics into useful chemicals

ZSM-5 was economically synthesized with red mud (RM) and industrial sodium silicate (ISS) in a tetrapropylammonium bromide (TPABr)-glucose dual-template system. The roles of glucose, Fe and Ca in RM on the formation of ZSM-5 were investigated. The catalytic performances of the resultant ZSM-5 were tested by cracking waste plastics. It was found that the formation of ZSM-5 was attributed to a synergistic effect between TPABr and glucose. The addition of glucose decreased the pH value in the crystallization solution and thus promoted the crystallization effect. Glucose acted as a hard template to generate mesopores. Fe atoms were partly distributed in the framework and partly adsorbed in the pores of ZSM-5, and helped to generate more Lewis acid sites. Ca atoms were mainly adsorbed in the pores of ZSM-5, and showed an inhibitory effect on the formation of zeolites. The synthesized ZSM-5 showed a weakly acidic and mesoporous structure and achieved an enhanced effect on producing gaseous products (gas yield: 85.3%), especially light olefins (C^{[double bond, length as m-dash]} _2-4) (selectivity: 77.1%) from cracking of low density polyethylene at 500 °C. The long-term cracking experiment showed that the synthesized ZSM-5 is superior in converting waste plastics to light olefins (ethylene and propene) than the commercial ZSM-5.

Unraveling templated-regulated distribution of isolated SiO4 tetrahedra in silicoaluminophosphate zeolites with high-throughput computations

Silicoaluminophosphate (SAPO) zeolites are well-known catalytic materials because of the mild acidity originating from the isolated SiO₄ tetrahedra in their frameworks. Regulating the distribution of isolated SiO₄ tetrahedra in SAPO zeolites is formidably challenging because SiO₄ tetrahedra tend to agglomerate to form Si islands and the isolated SiO₄ tetrahedra are difficult to determine using conventional characterization techniques. Here we synthesized Si-island-free SAPO-35 zeolites by using N-methylpiperidine as a new template, which exhibited excellent thermal stability compared to conventional SAPO-35 zeolites and a substantially improved methanol-to-olefins catalytic lifetime even comparable to that of commercial SAPO-34 zeolites. More strikingly, with the aid of high-throughput computations on 44 697 structure models combined with various state-of-the-art characterization techniques, for the first time, we reveal that the host–guest interactions between template molecules and SAPO frameworks determine the specific distributions of isolated SiO₄ tetrahedra, which are responsible for the improvement in the chemical properties of zeolites. Our work provides an insight into the template-based regulation of isolated SiO₄ tetrahedra in SAPO zeolites, which opens a new avenue in the discovery of promising zeolite catalysts with optimal SiO₄ distribution.

Anchoring Boron Atom to the Specific Tetrahedral Sites of Borosilicate MFI by Imidazolium-based Molecules

Regulating heteroatom substituted tetrahedral sites (T sites) within a zeolite crystal significantly influences the zeolite properties. Herein, the as-synthesized MFI zeolite with tailored boron location at various T sites is...

Conversion of methanol to hydrocarbons over H-MCM-22 zeolite: deactivation behaviours related to acid density and distribution

The deactivation of H-MCM-22 zeolites with different Si/Al ratios can be roughly divided into three stages: first the rapid deactivation of the supercages, the second reaction with slow coking and the deactivation stage with rapid coking mainly on the external pockets.

Selective regulation of n-dodecane isomerization and cracking performance in Pt/beta catalysts via orientation control of Brønsted acid site distribution

The isomerization and cracking performance of n-dodecane have been successfully regulated by the orientation control of Brønsted acid site distribution in Beta zeolites.

Catalytic Performances of Cu/MCM-22 Zeolites with Different Cu Loadings in NH3-SCR

The NH₃-SCR activities and hydrothermal stabilities of five xCu/MCM-22 zeolites with different Cu loadings (x = 2–10 wt%) prepared by incipient wetness impregnation method were systematically investigated. The physicochemical properties of xCu/MCM-22 zeolites were analyzed by XRD, nitrogen physisorption, ICP-AES, SEM, NH₃-TPD, UV-vis, H₂-TPR and XPS experiments. The Cu species existing in xCu/MCM-22 are mainly isolated Cu²⁺, CuO_x and unreducible copper species. The concentrations of both isolated Cu²⁺ and CuO_x species in xCu/MCM-22 increase with Cu contents, but the increment of CuO_x species is more distinct, especially in high Cu loadings (>4 wt%). NH₃-SCR experimental results demonstrated that the activity of xCu/MCM-22 is sensitive to Cu content at low Cu loadings (≤4 wt%). When the Cu loading exceeds 4 wt%, the NH₃-SCR activity of xCu/MCM-22 is irrelevant to Cu content due to the severe pore blockage effects caused by aggregated CuO_x species. Among the five xCu/MCM-22 zeolites, 4Cu/MCM-22 with moderate Cu content has the best NH₃-SCR performance, which displays higher than 80% NO_x conversions in a wide temperature window (160–430 °C). Furthermore, the hydrothermal aging experiments (xCu/MCM-22 was treated at 750 °C for 10 h under 10% water vapor atmosphere) illustrated that all the xCu/MCM-22 zeolites exhibit high hydrothermal stability in NH₃-SCR reactions.

Modification of MCM-22 Zeolite and Its Derivatives with Iron for the Application in N2O Decomposition

Layered 2D zeolite MCM-22 and its delaminated derivative, ITQ-2, were modified with iron, by different methods (ion-exchange and direct synthesis), and with the use of different precursors (FeSO4∙7H2O, Fe(NO3)3∙9H2O, and [Fe3(OCOCH3)7∙OH∙2H2O]NO3 oligocations. The applied modifications were aimed at optimization of iron form in the samples (aggregation, amount, location, and reducibility), in order to achieve the highest catalytic activity in the N2O decomposition. The synthesis of the samples was verified with the use of XRD (X-Ray Diffraction), N2-sorption and ICP-OES (Inductively Coupled Plasma Optical Emission Spectroscopy) techniques, while the form of iron in the samples was investigated by UV–vis-DRS (UV–vis diffuse reflectance spectroscopy), H2-TPR (Hydrogen Temperature-Programmed Reduction) and HRTEM (High-Resolution Transmission Electron Microscopy). The highest activity in the N2O decomposition presented the sample Fe(O,IE)MCM-22, prepared by ion-exchange of MCM-22 with Fe3(III) oligocations. This activity was related to the oligomeric FexOy species (the main form of iron in the sample) and the higher loading of active species (in comparison to the modification with FeSO4∙7H2O).

Selective active site placement in Lewis acid zeolites and implications for catalysis of oxygenated compounds

The selective incorporation of isolated framework Lewis acid sites at specific crystallographic positions in high-silica zeolites was achieved by applying a rationalized post-synthetic grafting methodology. The removal of framework Ge atoms from a Ge-BEC zeolite with low concentrations of Ge in the framework (Si/Ge ∼ 150) followed by grafting allows the synthesis of Sn-BEC zeolites with Sn atoms positionally biased into the double-4-ring (D4R) crystallographic positions of the BEC framework. Spectroscopic characterization using solid-state nuclear magnetic resonance (NMR) coupled with theoretical calculations revealed that Sn atoms preferentially form open Sn sites in the D4R of Sn-BEC. This observation was supported by IR spectra of adsorbed deuterated acetonitrile (CD₃CN), a known titrant of Sn sites in zeolites. The catalytic implications of selective incorporation of open Sn sites in Sn-BEC were probed using the Meerwein–Ponndorf–Verley–Oppenauer (MPVO) reaction. Although the MPVO turnover rates normalized by the total number of open Sn sites were comparable on Sn-BEC and a conventional Sn-Beta catalyst synthesized in fluoride media (Sn-Beta(F)), Sn-BEC demonstrated higher per gram reaction rates because of its larger fraction of open sites compared to Sn-Beta(F). These results highlight the advantage of placing active sites in targeted locations within a zeolite structure. The methodology presented here to selectively place catalytic active sites via sacrificial heteroatoms, such as Ge, can be generalized for the design of many other tetrahedrally-coordinated metal-containing zeolites.

The selective incorporation of isolated framework Lewis acid sites in specific crystallographic positions in high-silica zeolites was achieved successfully by applying a rationalized post-synthetic grafting methodology.

Research on the Wear Behavior of the Fixed Cone Liner of a Cone Crusher Based on the Discrete Element Method

Taking reducing the wear of the fixed cone liner of a cone crusher as the starting point, the movement and geometry parameters of the cone crusher are studied using the discrete element method. To improve the service life and working efficiency of the whole cone crusher. The UG model and discrete element Yade model of the cone crusher are established, and the different shapes of the tin ore are represented using Yade's preprocessor through eight different ways of particle combination and superposition. The static friction coefficient between the manganese ore and the cone crusher is studied and calibrated using the slope method. The relative error between the Yade and test results is 1.58%, and the calibration result is 0.44. The repose angle of the manganese ore is studied using the collapse method. The repose angle increases with the increase of the static friction coefficient and the dynamic friction coefficient, but the change trend is different. The effect of the dynamic friction coefficient on the repose angle is obviously greater than that of the static friction coefficient. The dynamic friction coefficient obtained by Yade is 0.042. Taking the swing distance, rotating speed, and bottom angle of the fixed cone as the orthogonal experimental factors of Yade, the regression equation of the fixed cone liner was obtained through the nonlinear processing of SPSS 25.0. According to Matlab R2017b, the influences of the swinging distance, rotating speed, and bottom angle of the fixed cone on the fixed cone liner are obtained. According to Yade's research results, the order of the influence degree of liner wear is: the rotating speed of moving cone, the swinging distance of the fixed cone, and the base angle of fixed rotation. When the swinging distance of the fixed cone is 146 mm, the rotating speed of the moving cone is 198 rpm, the fixed rotation bottom angle is 28°, and the minimum value of the liner wear is 23 mm. Yade's results are consistent with the change trend of the wear amount of the bushing obtained from the test. The research results show the correctness of using the Yade method to study the wear of the fixed cone liner of a cone crusher, which provides a theoretical basis for reducing the wear of the fixed cone liner of a cone crusher, and puts forward a new method to study the wear of relevant parts of a fixed cone crusher. At the same time, the research results are of great significance for achieving energy-saving in mining enterprises.

B-MWW Zeolite: The Case Against Single-Site Catalysis

Boron-containing materials have recently been identified as highly selective catalysts for the oxidative dehydrogenation (ODH) of alkanes to olefins. It has previously been demonstrated by several spectroscopic characterization techniques that the surface of these boron-containing ODH catalysts oxidize and hydrolyze under reaction conditions, forming an amorphous B₂ (OH)_x O_(3-x/2) (x=0-6) layer. Yet, the precise nature of the active site(s) remains elusive. In this Communication, we provide a detailed characterization of zeolite MCM-22 isomorphously substituted with boron (B-MWW). Using ¹¹ B solid-state NMR spectroscopy, we show that the majority of boron species in B-MWW exist as isolated BO₃ units, fully incorporated into the zeolite framework. However, this material shows no catalytic activity for ODH of propane to propene. The catalytic inactivity of B-MWW for ODH of propane falsifies the hypothesis that site-isolated BO₃ units are the active site in boron-based catalysts. This observation is at odds with other traditionally studied catalysts like vanadium-based catalysts and provides an important piece of the mechanistic puzzle.

Deboronation-assisted construction of defective Ti(OSi)3OH species in MWW-type titanosilicate and their enhanced catalytic performance

Regulating the state of titanium species via the deboronation-assisted route is a facile strategy to construct highly efficient titanosilicate catalysts.

Boric acid treated HZSM-5 for improved catalyst activity in non-oxidative methane dehydroaromatization

The dehydroaromatization (DHA) reaction of methane under non-oxidative conditions is carried out over a molybdenum catalyst supported on HZSM-5 and boric acid (BA) treated HZSM-5 at 700 °C and atmospheric pressure.

Methanol to olefins over H-RUB-13 zeolite: regulation of framework aluminum siting and acid density and their relationship to the catalytic performance

Incorporating boron promotes siting of more aluminum atoms at the sites accessible to MTO and then enhances the catalytic performance of H-RUB-13 in MTO.

Excellent performance of one-pot synthesized Fe-containing MCM-22 zeolites for the selective catalytic reduction of NOx with NH3

One-pot synthesized OP-Fe/M22 zeolites exhibit excellent performances in NH₃-SCR reactions.

Boosting the turnover number of core–shell Al-ZSM-5@B-ZSM-5 zeolite for methanol to propylene reaction by modulating its gradient acid site distribution and low consumption diffusion

Hierarchical Al-ZSM-5@B-ZSM-5 core–shell zeolite was prepared to boost turnover number of MTP reaction.

Brønsted/Lewis acid sites synergistically promote the initial C-C bond formation in the MTO reaction

The Lewis acid site combined with a Brønsted acid site in zeolite catalysts facilitates first C–C bond formation in the initiation step of the MTO reaction.

The methanol-to-olefin (MTO) reaction is an active field of research due to conflicting mechanistic proposals for the initial carbon–carbon (C–C) bond formation. Herein, a new methane–formaldehyde pathway, a Lewis acid site combined with a Brønsted acid site in zeolite catalysts can readily activate dimethyl ether (DME) to form ethene, is identified theoretically. The mechanism involves a hydride transfer from Al–OCH₃ on the Lewis acid site to the methyl group of the protonated methanol molecule on the adjacent Brønsted acid site leading to synchronous formation of methane and Al–COH₂⁺ (which can be considered as formaldehyde (HCHO) adsorbed on the Al³⁺ Lewis acid sites). The strong electrophilic character of the Al–COH₂⁺ intermediate can strongly accelerate the C–C bond formation with CH₄, as indicated by the significant decrease of activation barriers in the rate-determining-step of the catalytic processes. These results highlight a synergy of extra-framework aluminum (EFAl) Lewis and Brønsted sites in zeolite catalysts that facilitates initial C–C bond formation in the initiation step of the MTO reaction via the Al–COH₂⁺ intermediate.

Site-Directed Synthesis of Cobalt Oxide Clusters in a Metal-Organic Framework

Direct control over structure and location of catalytic species deposited on amorphous supports represents a formidable challenge in heterogeneous catalysis. In contrast, a structurally well-defined, crystalline metal-organic framework (MOF) can be rationally designed using postsynthetic techniques to allow for desired structural or locational changes of deposited metal ions. Herein, naphthalene dicarboxylate linkers are incorporated in the MOF, NU-1000, to block the small cavities where few-atom clusters of cobalt oxide preferentially grow, inducing catalyst deposition toward hitherto ill-favored grafting sites orientated toward NU-1000s mesoporous channels. Despite the different cobalt oxide location, the resulting material is still an active propane oxidative dehydrogenation catalyst at low temperature, reaching a turnover frequency of 0.68 ± 0.05 h^-1 at 230 °C and confirming the utility of MOFs as crystalline supports to guide rational design of catalysts.

Reaction Mechanism for Direct Cyclization of Linear C5 , C6 , and C7 Alkenes over H-ITQ-13 Zeolite Investigated Using Density Functional Theory

Although dienes or trienes have been shown to be possible precursors for cyclization, direct cyclization of alkenes or alkoxides has not been systematically studied yet. Thus, the reaction mechanism of cyclization of linear alkenes over H-ITQ-13 was investigated here by density functional theory considering dispersive interactions (DFT-D). The similar free energy of different linear alkoxides of the same carbon number suggests that they can co-exist in the H-ITQ-13 intersection at 673.15 K during the methanol to olefins (MTO) process. The formation of linear alkenes by olefins methylation with methoxyl groups (ZOCH₃ ), trimethyloxonium ions (TMO⁺ ), and methanol are kinetically more favorable than by dimerization of olefins. Linear alkoxides or alkenes prefer direct cyclization to cycloalkanes rather than hydride transfer to diene. This study provides new insight into the alkene cyclization and aromatization mechanisms in MTO process.

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.