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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.

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.

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.

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.

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.

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.

Hollow zeolites are widely investigated as catalysts and nanoreactors in the petrochemical and fine chemical industries because they offer fast mass transfer along with a hollow core for encapsulation of functional components. However, the synthesis of hollow zeolites still remains a significant challenge due to the complexity and restrictions of currently available synthetic strategies. Herein, a direct and efficient approach for constructing hollow zeolite beta with simultaneously a low Si/Al ratio (∼19) and high yield (above 89 wt%), based on some cost-effective and recyclable pentacyclic lactams, is reported. Meanwhile, a unique formation mechanism involving the selective decomposition of the tetraethylammonium (TEA+) cations locating at internal Al sites, and dissolution and recrystallization of the internal activated aluminosilicate species from the core to outer surface is proposed for the first time. In addition, the hollow beta catalyst exhibits better catalytic performance in the alkylation of benzene with isobutylene compared with the conventional solid beta sample.

The crystal size of zeolites has an enormous effect on zeolite properties and catalytic performance. Herein, a direct hydrothermal synthesis method to control the size of FER zeolites, by using piperidine (PI) as the structure directing agent (SDA) assisted by cetyltrimethyl ammonium bromide (CTAB), is developed. All the resultant FER samples possess high crystallinity, while their crystal sizes range from ∼100 nm to ∼2 μm. The effect of CTAB in the synthesis gel on the physicochemical properties, especially the crystal sizes, and the 1-butene skeletal isomerization performance of FER zeolites was investigated. The addition of CTAB has an obvious effect on the formation of FER zeolites, and the hydrophobic tail of CTAB can restrict the excessive growth of FER leading to the formation of highly crystallized FER with various crystal sizes. The nano-scale FER shows higher initial 1-butene conversion and better catalytic stability than micron-sized FER, which is mainly related to the superior accessibility of acid sites, larger external surface area, and less diffusion limitation due to the decrease of the crystal size. This novel synthesis strategy provides an effective way to regulate the size and performance of FER zeolites, and a new function of CTAB in the synthesis of zeolites is developed.

Modular multilevel converter (MMC) is a popular topology in the medium-/high-voltage power conversion due to its modular structure, equal voltage stress, small line-line filter and low-distortion current. It can be operated as an active rectifier and achieve controllable dc output voltage and adjustable active/reactive power. However, dc-link voltage sensors must be installed to realize the feedback control of the system. To reduce the costs and improve the performance of MMC-based rectifier, this paper presents a sensor-less control strategy, which removes the traditional dc-link voltage sensors. An accurate prediction model of dc-link voltage is developed as well as a complete implementing method. The value of the output DC voltage is obtained through the prediction algorithm. The pre-charging, active-power and reactive-power mode of the MMC-based rectifier is extensively studied to validate its effectiveness. The proposed method not only reduces the system device cost but presents excellent steady-state and dynamic performance.

Due to the complexity of structure, operating principle, and control purposes, it is challenging to design and analyze a brushless doubly-fed machine viewing from its overall performance across the entire operating range. This paper elaborates on an efficiency mapping method of a dual-converter-driven brushless doubly-fed machine using finite element method and data post-processing. The efficiency mapping method can characterize various electromagnetic properties over the operating range and provide insightful information for the design and control development of brushless doubly-fed machines.

Modular multilevel converters (MMC) are popular for high-voltage, high-power electric energy conversion. To improve the reliability of MMCs, fault diagnosis and tolerant control have been studied by many researchers. Fault reconfiguration, one important step of the fault tolerant control, has become a challenge, especially under high-voltage operation. Arm current overshoot occurs during the reconfiguration if the MMC does not provide redundant submodules (SMs). Existing publications focus on the fault detection and localization. This work investigates the reconfiguration and proposes a suppression control strategy for the overcurrent. The strategy does not require redundant SMs. The proposed method is simulated in Matlab/Simulink and validated by experiments based on a three-phase MMC using an RL load.

The four-level hybrid clamped converter (4L-HCC) is emerging as a new multilevel topology for medium/high-voltage motor variable speed drives. However, the internal capacitor voltage has unstable issues at extra low frequencies, limiting the 4L-HCC’s application. This paper presents an integrated control scheme of the 4L-HCC and an electrical machine. Compared with the traditional PWM method, the proposed method can effectively suppress the excessive voltage ripples at low speed. The proposed control has features of easy implementation, accurate/fast current and torque response, and strong capacitor voltage balancing capability. Four-quadrant operation with high constant torque current is realized. Simulated and experimental results are used to validate the effectiveness of the proposed integrated method.

The existing concept of aviation turboelectric distributed propulsion for passenger aircraft relies on direct current (DC) for power transmission and distribution. The mass and efficiency performance of DC-based propulsion systems becomes challenging as the system power rating increases. This work proposes a turboelectric distributed propulsion based on brushless doubly-fed machines, which will minimize the power conversion, enhance the mechanical reliability, and strengthen the fault-tolerance capability. This paper presents the background of the proposed turboelectric distributed propulsion along with the system modeling, control algorithm development, and preliminary simulation results of a full flight profile.

In this paper, a dv/dt filter is designed for the modular multilevel converters (MMCs) based variable frequency motor drives. This concept utilizes the inherent arm inductors of the MMC as a part of the output low-pass filter, thus eliminating the need for an add-on inductor. Particularly, a mathematical model is proposed to optimize the filter loss and volume for a given dv/dt requirements. It turns out that with SiC-MOSFET-based high switching frequency operation, the dv/dt filter improves the output power quality and alleviates the voltage stress in motor drive applications. For applications with various requirements on filter volume and loss, weighted-sum optimal solutions are presented. Simulation results based on a seven-level MMC are provided to validate the theoretical analysis.