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A series of Beta zeolite catalysts with controllable particle sizes and similar Si/Al ratios were prepared by introducing imidazole (IMD) into the synthesis system and used for a liquid-phase transalkylation of diethylbenzene (DEB) with benzene (Bz). Beta zeolite catalyst prepared with IMD/SiO2 molar ratio of 0.5 possessed the smallest particle size (mean particle size of 119 nm) and the highest DEB conversion of 79.16%, under the reaction conditions: 260 oC, 4.0 MPa, WHSV (diethylbenzene) of 2.0 h-1, W(benzene)/W(diethylbenzene) of 3/1, time on stream of 5 h. Then, for the purpose of elucidating the catalytic roles of the acid sites with different strength, types and positions in Beta zeolite in the transalkylation reaction, we carried out Na+ loading on the prepared Beta zeolite to eliminate strong acid sites, and poisoned Bronsted acid sites and external acid sites by using 2,4-dimethylquinoline (2,4-DMQ) and triphenylphosphorus (TPP) as toxic reagents, respectively. The results suggested that the moderate and strong Bronsted acid sites of Beta-IMD0.5 were the active sites for the liquid-phase transalkylation of DEB with Bz and their utilization rate showed a trend of increased first and then decreased. Significantly, 31% of the external Bronsted acid sites of Beta-IMD0.5 contributed 81% of DEB conversion. The external acid sites had more contribution to the reaction, due to the short diffusion path and fine accessibility.

Crystallographic sites of Bronsted acids (Si-OH-Al) in zeolites, which are closely associated with the Al sites, play a significant and unique role in the catalytic application, especially when they are distributed in open channel systems or confined in cavities with small pore openings. In this article, we unraveled constrained Al crystallographic sites in FER-type zeolites containing the distinct local environments (10-ring channels and ferrierite cavities) by Rietveld refinement against the powder X-ray diffraction data. Final refinement demonstrates that regardless of the types of structure-directing agents and synthetic medium utilized, T1 and/or T3 are Al-rich positions, which are further confirmed by theoretical calculations. This new finding of constrained Al sites in the FER-type zeolite can well explain its limited catalytic activity in the DME carbonylation reaction.

This article presents a simplified model predictive control (MPC) method for single-phase four-level hybrid-clamped converters (1P-4L-HCCs), achieving high-quality current control and strong capacitor voltage balancing. Different from a conventional MPC, the proposed method aims to dramatically reduce switching states (control voltage vectors) in the cost function optimization of MPC. It utilizes only 16 switching states, compared to 64 in conventional methods, resulting in reduced execution time and significantly increased control bandwidth. Steady-state and dynamic performances are evaluated by simulated and experimental test, and the main results are provided to verify the effectiveness of the proposed MPC method.

The organic structure-directing agent (OSDA)-free synthesis of MCM-49 is strongly desired because OSDA makes synthesis cost expensive and environment unfriendly. In this work, an efficient, eco-friendly and simple route to synthesize MCM-49 zeolite is developed, via seed-assistance, without deliberately adding OSDA, with ultraphonic aging treatment. For comparison, the effect of microwave aging, mechanical stirring aging and without any aging processes on the synthesis is investigated. Due to the acoustic cavitation, the ultraphonic aging treatment shows superiority over the others. Ultraphonic aging not only keeps the MCM-49 seed integrity but also promotes both the silica species and aluminum species to the solid phase, which is beneficial for the growth of MCM-49 and the suppression of MOR formation in the absence of OSDA. 27Al MAS NMR data show that the seed-assisted MCM-49 almost has no extra-framework aluminum, but dealumination will happen upon ammonium exchange. 23Na MAS NMR results demonstrate that the interaction between Na ions and framework of the seed-assisted MWW product is strengthened after calcination, which makes the exchange of Na species difficult. Alternatively, ammonium exchange upon the as-made sample is an effective method to remove Na species, which improves the micropore volume, external surface area, and acid amount of the seed-assisted MCM-49.

This work deals with the sizing of submodule capacitors for modular multilevel converters (MMCs) used for variable speed drives for medium voltage and high voltage electric machines. Variable speed drives are required to run over a wide frequency range from zero to a few hundred Hz or even over 1 kHz. Under low frequency operation (

This article presents the design and development of a 7-kV dc bus, 1-MVA, 0–1000 Hz modular multilevel converter (MMC) prototype for high speed medium-voltage variable frequency drives. The system is designed based on 1.7-kV silicon carbide (SiC) mosfet s. First, the full in-depth design of the prototype is provided, including the submodules, arm inductors, and system integration. Then, the unique control scheme and the control hardware design are presented to achieve the full-frequency range operation. The power loss based on Si and SiC devices is analyzed. Finally, experimental results are presented to demonstrate the performance of both the SiC-based submodule and the full-scale MMC system. Compared with the traditional Si-based solution, the established SiC MMC can reduce the drive system volume by 75% and achieve 325% power density, and the peak energy efficiency reaches around 99.35%.

When a four-level hybrid clamped converter (4L-HCC) operates at low frequencies, the high load current causes significant voltage fluctuations among the capacitors in the dc link. A mathematical model is developed to clarify the mechanism of excessive capacitor voltage fluctuations. An improved common-mode voltage injection algorithm is proposed and integrated to the overall control method. With the applied algorithm, the voltage fluctuation minimization of the dc-link capacitors and the dynamic voltage balancing capability are investigated. Both simulation and experimental results validate the effectiveness of the proposed algorithm, with a voltage fluctuation reduction over 50%. The 4L-HCC with the proposed algorithm achieves much faster dynamic response to converge the unbalanced capacitor voltages and the reduced output current harmonics compared with the conventional common-mode voltage injection algorithm.

The internal capacitor voltages of a four-level hybrid clamped converter (4L-HCC) can have serious instability issues without proper control, limiting its application in variable speed drives for transportation electrification. This article presents a new integrated control scheme for an electrical machine drive fed by the 4L-HCC. Compared with the traditional PWM control, the proposed method effectively minimizes the ac voltage fluctuations and dc voltage deviations for all dc-link and flying capacitors. The advantages of the proposed control method include easy implementation, accurate current and speed control, and strong capacitor-voltage-balancing capability. Four-quadrant operation with a high dynamic response is achieved over the full-speed range of the system. Simulation and experimental results are presented to validate the effectiveness of the proposed integrated method.

An optimized model predictive control (MPC) method is proposed based on the selected switching states to control the four-level hybrid-clamped converter. The proposed method utilizes 64, instead of 512, switching states, compared with a conventional MPC, which greatly eases the calculation burden while achieving the high-quality regulation of output currents and the balance of capacitor voltages. The steady-state performance at both high and low frequencies is investigated, and results are compared with those by the traditional carrier phase-shifted pulsewidth modulation method. The dynamic response to the current or frequency step change is also evaluated. Both simulation and experimental results validate the feasibility and effectiveness of the optimized MPC method.

Errors in the injected high-frequency circulating currents are originally investigated for modular multilevel converters (MMCs) in low-speed drive operation. The mathematical analysis reveals that the magnitude and phase errors in the injected circulating currents cause serious energy imbalance in the MMC arms, resulting in large voltage fluctuations in the submodules. Furthermore, these fluctuations get larger when the circulating currents are injected at higher frequencies, which reduces the drive system stability and damages the submodule capacitors. A remedy approach is proposed to minimise the error effects by adjusting the circulating current command. A full-scale system comprising a 1 MVA MMC and a 1 MW induction machine is simulated to verify the theoretical analysis. The experimental investigation of a down-scale MMC feeding an inductor is also conducted to validate the theoretical results.

Solar photovoltaic (PV) energy sources are rapidly becoming more popular. The PV system output power relies on the applied current or voltage nonlinearity and there exist a unique point called the ...

Low frequency operation in the modular multilevel converters (MMCs) is enabled by applying a reference common mode voltage and circulating currents at a frequency much higher than the desired output frequency. The frequency of injected common-mode signals is limited for maintaining a good performance in the circulating current control. Firstly, this paper theoretically analyzes the effect of increasing the frequency of injected common-mode signals by evaluating the error between reference and actual circulating currents. Secondly, we introduce an additional parameter in the reference circulating current to enhance the suppression of MMC's capacitor voltage fluctuations. Intensive computer simulations are developed for a seven-level MMC driving a medium-voltage induction motor drive system to verify the circulating current error analysis and assess the capacitor voltage fluctuations. Finally, the proposed technique for suppressing the capacitor voltage fluctuations is experimentally validated on a 600-V three-level MMC feeding an inductive load. The voltage fluctuations are reduced by more than 50% (63.6%) when the proposed technique is applied with a high common-mode frequency.

ZSM-11 zeolite is an important microporous material that has wide application prospects in various petrochemical and chemical processes. Herein, we reported a simple method to significantly reduce the crystallization time of ZSM-11 zeolite by adding N-methyl-2-pyrrolidone (NMP) as the ‘promoter’. The effects of various synthetic parameters on the NMP-assisted synthesis of ZSM-11 zeolite were also investigated. Subsequently, characterization techniques such as SEM, XRD, XRF, FT-IR, TEM-EDS, TG and 13C NMR were used to unveil the acceleration mechanism of NMP. It was revealed that NMP can facilitate the bonding of TBA+ with aluminosilicate by breaking the hydration layer around them, thus reducing the apparent nucleation activation energy and accelerating the nucleation process. Meanwhile, we used N-ethyl-2-pyrrolidone (NEP) instead of NMP as the promoter to verify the acceleration mechanism. In addition, the fast-synthesized ZSM-11 zeolite exhibited comparable composition, hydrothermal stability, and catalytic performance to conventionally synthesized counterparts.

In this article, a double-vector-based model predictive control (MPC) method with efficient computation for single-phase five-level active neutral-point clamped (5L-ANPC) converters is proposed. It uses double voltage vectors per control cycle rather than the single in the conventional MPC method, resulting in reduced output current ripples and fixed switching frequency. First, the MPC method based on the Lyapunov function is optimized to select two voltage sectors rather than 25, reducing the calculation complexity significantly. Then, two current tracking algorithms based on double voltage vectors are presented, which are using the current cost function and current slope, respectively. Finally, a fast voltage balancing scheme for the dc-link and flying capacitors is developed and embedded in the MPC method. Both simulated and experimental results are used to validate the correctness and feasibility of the proposed MPC method.

Without active control, the voltage fluctuations of the dc-link and flying capacitors in a 4-level hybrid-clamped converter (4L-HCC) become excessively large in low-frequency operation. This paper studies the switching state redundancy inherent in a 4L-HCC and proposes a new capacitor voltage balancing and stabilizing method to minimize the dc-link and flying capacitor voltage fluctuations. A straightforward algorithm is developed based on a logic table to select the suitable switching states. Verified by simulation and experimental testing results, the new method and algorithm virtually eliminate all the big voltage fluctuations of capacitors at low and zero frequencies, greatly enhancing the robustness and reliability of a 4L-HCC. The capacitor size can be reduced by more than 80% compared with the conventional method.

Ferrierite (FER) zeolite has been successfully synthesized within 2–3 h through microwave assisted crystallization without using organic structure directing agent. Crystallization temperature and seed content were found to be two of the key factors. XRD patterns and SEM images of the samples taken at different periods of time revealed that seeds were only partially dissolved during the crystallization process and silicate (alumino) species in the gel grew upon the surface of the partially dissolved seed. A core-shell zeolite growth model was proposed. Compared with original seed zeolite, as-synthesized FER zeolite had more regular morphology and larger particle size. The FER crystal size could be controlled in the range of 0.4–3.0 μm by adjusting the particle size of the seeds. FER obtained through microwave assisted rapid synthesis showed similar catalytic performance in 1-butene skeletal isomerization with the FER obtained through conventional hydrothermal synthesis.

A procedure to calculate maximum torque operating conditions for the inverter-fed induction machine is proposed in this paper. The proposed procedure minimizes the finite-element analysis computation load by identifying the variation range of stator current and slip frequency for the entire torque-speed map. Thus, the total calculation time for machine steady-state performance estimation can be significantly reduced. The effectiveness of the proposed procedure is validated by an experiment.

Introduction of mesopore is critical for applications where mass-transport limitations within microporous networks, especially for zeolite with one-dimensional microporous network, hinder their performance. Generally, the creation of mesopore in zeolite through a direct synthesis route is strongly dependent on complex and expensive organic molecules, which limits their commercial application. Here, we successfully developed a facile synthesis route for preparing ZSM-48 zeolite (*MRE topology) with ultralarge mesoporosity in which typical 1,6-hexylenediamine worked as an organic structure-directing agent, innovatively assisted by a simple crystal growth modifier (tetraethylammonium bromide, TEABr). The working mechanism of TEABr during crystallization was revealed and proposed on the basis of TEM, thermal gravimetric mass spectrum, and 13C cross-polarization magic angle spinning NMR characterization results. In the process, TEA+ ions preferentially interacted with the solid during the induction period, which effectively suppressed the aggregation of ZSM-48 primary nanorods. As a result, ultralarge mesoporosity of 0.97 cm3·g-1 was constructed through the stacking of the nanorods. Interestingly, TEA+ ions only took part in the crystallization process and did not occlude in the pores of the final zeolites indicating its potential in recyclability. Moreover, similar synthesis strategy could be applied for the preparation of hierarchical ferrierite zeolites, implying the universality of this strategy. Compared with a conventional sample, ZSM-48 zeolite with ultralarge mesoporosity showed superior catalytic stability in the m-xylene isomerization reaction due to its significantly enhanced diffusion and mass transfer capability, which will greatly promote the practical application of ZSM-48 zeolite.