Steam-assisted Crystallization Method Research Articles

Hydrogenation of CO2 to p-xylene over ZnZrOx/hollow tubular HZSM-5 tandem catalyst

The conversion of CO2 into specific aromatics by modulating the morphology of zeolites is a promising strategy. HZSM-5 zeolite with hollow tubular morphology is reported. The morphology of zeolite was precisely controlled, and the acid sites on its outer surface were passivated by steam-assisted crystallization method, so that the zeolite exhibits higher aromatic selectivity than sheet HZSM-5 zeolite and greater p-xylene selectivity than chain HZSM-5 zeolite. The tandem catalyst was formed by combining hollow tubular HZSM-5 zeolites with ZnZrOx metal oxides. The para-selectivity of p-xylene reached 76.2% at reaction temperature of 320 °C, pressure of 3.0 MPa, and a flow rate of 2400 mL g−1 h−1 with an H2/CO2 molar ratio of 3/1. Further research indicates that the high selectivity of p-xylene is due to the pore structure of hollow tubular HZSM-5 zeolite, which is conducive to the formation of p-xylene. Moreover, the passivation of the acid site located on the outer surface of zeolite effectively prevents the isomerization of p-xylene. The reaction mechanism of CO2 hydrogenation over the tandem catalyst was investigated using in-situ diffuse reflectance Fourier transform infrared spectroscopy and density functional theory. The results showed that the CO2 to p-xylene followed a methanol-mediated route over ZnZrOx/hollow tubular HZSM-5 tandem catalysts. In addition, the catalyst showed no significant deactivation in the 100 h stability test. This present study provides an effective strategy for the design of catalysts aimed at selectively preparing aromatics through CO2 hydrogenation.

Supercapacitive performance of cobalt-loaded amorphous zeolite for energy storage applications

Co-loaded amorphous zeolite has been prepared, characterized and tested as an electrochemical supercapacitor. Amorphous zeolite (AZ) and H-amorphous zeolite were prepared using the steam assisted crystallization method; the H+ was added on AZ via ion exchange with NH4NO3 to evaluate the supercapacitive difference between AZ and H-AZ. Co was loaded into the AZ and H-AZ by the impregnation method to enhance the redox properties. The physico-chemical properties of the AZ, H-AZ, Co/AZ and Co/H-AZ were investigated using various analytical characterization techniques, namely X-ray Diffraction (XRD), Fourier Transform Infrared (FTIR), High Resolution Scanning Electron Microscopy (HRSEM), N2 physisorption and X-ray Photoelectron Spectroscopy (XPS). The materials were subsequently drop-cast into a Ni foam and evaluated for supercapacitive properties. Notably, the resulting electrode materials exhibit supercapacitive behaviour that is effective over a potential window from −0.1 to +0.7 V in a potassium hydroxide (1 M KOH) aqueous electrolyte. A relatively high specific capacitance of 550 F/g at 1 A/g was calculated from Galvanometric Charge Discharge (GCD) analysis for the Co/H-AZ electrode with a Coulombic efficiency of 98% after 2000 cycles and a capacitance retention of 91.6%, while the value calculated for Co/AZ was 400 F/g with a Coulombic efficiency of 96% after 2000 cycles and a capacitance retention of 75.5%. The energy density and power density for Co/H-AZ were 19.09 W h/kg and 250 W/kg respectively, while the energy density and power density for Co/AZ were 13.88 W/kg and 248 W h/kg respectively. The difference in the performance of Co/H-AZ and Co/AZ can be attributed to the effect of H+ ions at the electrode/electrolyte interface.

Effect of SiO2/Al2O3 ratio on the electrochemical performance of amorphous zeolite loaded with cobalt oxide grown via steam-assisted crystallization method.

Improving the performance of a supercapacitor is one of the main approaches to solve the energy shortage problem. Electrode material is one of the key components limiting the efficiency of a supercapacitor. Discovering, tuning, and improving electrode materials are very important. This work reports the effect of SiO2/Al2O3 ratio on electrochemical performances of amorphous zeolites ZSM5 (AZ) and H-ZSM5 (H-AZ) loaded with cobalt oxide. Two SiO2/Al2O3 ratios (1 = 6.2 and 2 = 8.3) of AZ1, AZ2 and H-AZ1, H-AZ2 were synthesized by a facile impregnation method. Then, controlled masses of cobalt oxide were introduced to enhance the supercapacitive performances of the amorphous zeolite. Investigation of the SiO2/Al2O3 ratio in the cobalt oxide/zeolite composite (Co/AZ and Co/H-AZ) was carried out to unveil its effect on the electrochemical properties. Worthy of note is the fact that the resulting electrode materials exhibited supercapacitive behavior that is effective over a potential window ranging from 0 to 0.5 V in potassium hydroxide (1 M KOH) aqueous electrolyte. Results from Galvanometry Charging and Discharging (GCD) analyses show that the modified Ni-foam electrodes loaded with Co/H-AZ1 and Co/H-AZ2 are capable of delivering a relatively high specific capacity from 45.97 mA h g-1 to a high value of 72.5 mA h g-1 at 1 A g-1 and Ni-foam electrodes loaded with Co/AZ1 and Co/AZ2 exhibited values from 26 mA h g-1 to 52.83 mA h g-1 respectively. It is clearly shown that, when the mass ratio SiO2/Al2O3 increases, the specific capacity increases as well. It was also noticed that after 2000 cycles, Co/H-AZ1 and Co/AZ1 have a poor coulombic efficiency while Co/H-AZ2 and Co/AZ2 exhibited 98% for coulombic efficiency. Finally, this study shows that to fabricate high performance supercapacitors with amorphous zeolite loaded with cobalt oxide, one should keep the ratio of SiO2/Al2O3 as high as possible during synthesis.

In-situ preparation of hierarchical Beta zeolite by steam- assisted crystallization method using meso-macroporous silica

In this paper, a series of meso-macroporous silica (MMS) were prepared by phase separation, then mic-meso-macroporous Beta zeolite was prepared by steam-assisted crystallization (SAC) method using the meso-macroporous silica as substrate. The factors such as the amount of water, crystallization time, aluminum source, amount of template agent during the SAC method were investigated. Various characterization methods such as BET, XRD, SEM were used to investigate the physical and chemical properties of the prepared materials. The results showed that the meso-macroporous silica could be transformed into Beta zeolite through the SAC method while the macroporous structure was still maintained.

Steam-assisted synthesis of SAPO-34@ZSM-5 core-shell zeolite for enhancing the synergies of n-hexane-Methanol Co-Reaction to light olefins

An alternative and promising route for the production of light olefins which realizes the objective of effective resource usage is the coupling conversion of n -hexane-methanol. The SAPO-34@ZSM-5 core-shell composite has been designed and fabricated by the steam-assisted crystallization (SAC) method for boosting the n -hexane-methanol synergies. SAC avoids the undesired phase transformation/dissolution of SAPO-34 caused by exposure to harsh basic conditions of the ZSM-5 precursor solution. In comparison with the nanosized ZSM-5 or physical mixture, the SAPO-34@ZSM-5 core-shell zeolite demonstrates a 32–38 wt% increase in the light olefin selectivity, a 28–67 wt% increase in the n -hexane conversion, and an 89–143% improvement in the catalyst lifetime under identical reaction conditions. With the aid of GC-MS analysis of carbonaceous deposits in spent catalysts, higher polymethylbenzenes concentration and lower polycyclic aromatic hydrocarbon (PHA) concentration are obtained from the extracted species of spent SAPO-34@ZSM-5 in comparison with that of other samples, which suggests the aromatic-based route to light olefins is enhanced and the carbonaceous deposition is slowed down. The core-shell configuration of the SAPO-34@ZSM-5 composite diminishes the diffusion rate of methanol towards the SAPO-34 core layer and increases the coupling of n -hexane derivatives with methanol in the hydrocarbon pool (HCP) route. The decreased number of acidic sites on the external surface of the SAPO-34 core layer may also be accounted for the inhibited carbon deposition. • SAPO-34@ZSM-5 is synthesized firstly by a steam-assisted crystallization method. • Core-shell structure of SAPO-34@ZSM-5 enhances the synergistic effect of reactants. • Core-shell structure of SAPO-34@ZSM-5 slows down coking inside the SAPO-34 core layer. • A synergistic path for n-hexane-methanol co-reaction on SAPO-34@ZSM-5 is proposed.

Methanol aromatization over Zn-modified HZSM-5 catalysts derived from ZIF-8

Methanol to aromatics (MTA) is an effective way to produce aromatics by non-petroleum route. The acidity and structure of zeolite catalysts are the key to aromatics yield and stability of catalysts in MTA reaction. Here, Zn-modified HZSM-5 catalysts were prepared by physical mixing method (PM), in-situ hydrothermal synthesis method (SH) and steam-assisted crystallization method (SAC) using ZIF-8 as sacrificial template and Zn source. XRD, SEM, TEM, N2 adsorption-desorption, ICP-OES, NH3-TPD, Py-IR, XPS, H2-TPR, TG characterization methods were used to investigate the physical and chemical properties of the catalysts. The influence of preparation methods of catalysts, the addition amount of ZIF-8, different Zn sources and reaction conditions on the products distribution in MTA reaction were investigated. The results showed that the crystallinity of ZnZ8(SAC)/Z5 catalyst was improved and the hierarchical pore structure was obtained successfully after introduced ZIF-8 by steam-assisted crystallization method. Among them, ZnZ8 mainly exists as ZnOH+ species, which improved the utilization rate of Zn atoms and more L acid sites and moderate acid concentration were formed. The addition amount of ZIF-8 can affect the crystalline size, pore structure and the concentration of ZnOH+ species of ZnZ8(SAC)/Z5. An appropriate ratio of B/L acid concentration was formed when adding an appropriate amount of ZIF-8. Compared with the traditional Z5 catalyst modified with Zn(NO3)2 as Zn source (1% ZnZN(SAC)/Z5), there are smaller particle size, larger specific surface area and mesopore volume as well as suitable acid distribution in 1% ZnZ8(SAC)/Z5 catalyst. The preparation method of catalyst, the addition amount of ZIF-8, different Zn sources and reaction conditions have a significant effect on the selectivity of BTX.

Utilization of zeolite H-MOR based on bagasse ash silica as a catalyst for the hydrolysis reaction of cassava peel cellulose for glucose production

Bagasse and cassava peel are waste that can be increased economic value. In this study, we have successfully synthesized zeolite H-MOR from sugarcane bagasse ash (SCBA) silica using the Steam-Assisted Crystallization (SAC) method at 170°C for 120 hours and determined its catalytic activity on the hydrolysis of cassava peel starch to produce glucose. H-MOR synthesized using LUDOX (commercial silica) was used as a comparison. The results showed that the MOR zeolite synthesized using SCBA and LUDOX silica had a crystallinity of 92.12% and 81.17%, respectively. The cellulose content in cassava peel flour is 57.8%. Optimization of the catalytic test variable showed that the hydrolysis of cellulose occurred at 140°C for 4 hours with a 1: 1 ratio of catalyst and substrate. The glucose concentrations obtained from cellulose hydrolysis using zeolite H-MOR catalyst from SCBA and Ludox were 398.5 ppm and 237.45 ppm with conversion degrees of 60.4% and 61.2%, respectively.

Rational Design of Pomegranate-like Base-Acid Bifunctional β Zeolite by Steam-Assisted Crystallization for the Tandem Deacetalization-Knoevenagel Condensation.

A highly crystalline pomegranate-like base-acid bifunctional beta zeolite was successfully synthesized by the steam-assisted crystallization method using a basic nitrided N-beta as the starting material. The secondary crystal growth of a beta zeolite generating acid functionality occurred over the outer surface and intercrystalline void spaces of the N-beta zeolite. The pomegranate-like N-beta@H-beta zeolite had a high surface area and base-acid dual functionality because of the well-connected framework topologies of the H-beta and N-beta crystallites. The N-beta@H-beta zeolite exhibited a superior yield of benzylidenemalononitrile during the tandem deacetalization-Knoevenagel condensation of benzaldehyde dimethyl acetal and malononitrile compared to H-beta, N-beta, and their physical mixture. This is likely due to the isolated and balanced activity of the base- and acid-catalyzed reactions.

Dichloromethane catalytic combustion over Co3O4 catalysts supported on MFI type zeolites

Three different MFI type zeolites Silicalite-1, ZSM-5 and TS-1 as supports were synthesized by steam-assisted crystallization method. Then, the as-synthesized zeolites were used to prepare Co 3 O 4 supported MFI type zeolite catalysts by wet impregnation. The as-prepared Co 3 O 4 supported MFI type zeolite catalysts possessed high specific surface area, abundant pore structure and surface acidity. Finally, the as-prepared catalysts were used for dichloromethane (DCM) catalytic combustion and the results showed that Co 3 O 4 supported MFI type zeolite catalysts had better performance than pure Co 3 O 4 . The synergistic catalytic effects between MFI type zeolite and Co 3 O 4 effectively reduced the formation of polychlorinated by-products and suppressed the deactivation of catalyst during the catalytic combustion of DCM. And Co 3 O 4 /ZSM-5 catalyst exhibited the best performance due to its proper Brønsted acid sites amount and the T 90 (the reaction temperature at which the dichloromethane conversion reached 90%) was 370 °C at 1000 ppm DCM, 10% O 2 /N 2 and the gas hourly space velocity of 30,000 mL/(g∙h). • The Co 3 O 4 supported MFI type zeolites were synthesized by impregnation method. • The zeolite as support restrained the deactivation of Co 3 O 4 catalyst during DCM catalytic combustion. • The surface acid sites of zeolite restrained the polychlorinated byproducts formation. • Co 3 O 4 /ZSM-5 exhibited the best performance for the catalytic combustion of DCM.

Tungstophosphoric acid incorporated hierarchical HZSM-5 catalysts for direct synthesis of dimethyl ether

HZSM-5 zeolites are active materials in dimethyl ether (DME) production with high surface acidity. In this study, hierarchical HZSM-5 catalysts were synthesized with steam-assisted crystallization (SAC) method and then in order to increase its surface acidity, TPA was loaded into the HZSM-5 catalyst having various mass ratios (5, 10, 25%) by wet impregnation method. Synthesized catalysts were characterized by N2 physisorption (BET analysis), X-Ray diffraction and pyridine adsorbed diffuse reflectance FTIR spectroscopy techniques. Characterization analysis of tungstophosphoric acid (TPA) impregnated catalysts indicated that hierarchical HZSM-5 possesses mesoporous structures. The average pore size distribution of TPA impregnated HZSM-5 catalysts were between 17 and 20 nm. TPA impregnation promoted Brønsted acid sites of the catalyst, which favors methanol dehydration reaction. Activity tests have been performed at reaction temperatures of 200–300 °C at 50 bar reaction pressure in the presence of admixed catalysts (physically mixed commercial HifuelR-120 and HZSM-5 based catalysts with a weight ratio of 1:1). Results revealed that the increase in the amount of heteropoly acid has enhanced DME selectivity and CO conversion. Maximum DME selectivity of 57% and CO conversion of 46% were achieved in the presence of the 25TPA@HZSM-5 catalyst at the optimum reaction temperature of 275 °C. TGA analysis result of spent catalysts presented the highest amount of coke over HZSM-5. TPA incorporation decreased coke formation due to suppression of the Lewis acid site, which is responsible for the coke formation.

Steam-Assisted in Situ Prepared TS-1 with Hierarchical Pores and Tunable Acid Sites Grown on Carbon Nanotubes Decorated Nickel Foam

The TS-1 particles grown on carbon nanotubes decorated nickel foam (denoted as MTS-1S) are successfully synthesized by a steam-assisted crystallization method that does not waste the gels and is mo...

Fast synthesis of SSZ-13 zeolite by steam-assisted crystallization method

Abstract The crystallization time of SSZ-13 was shortened from several days to 6 h without seeding or pre-aging at conventional synthetic temperature. The fast synthesis was realized by converting a clear-sol derived dry gel with the assistance of steam at 160 °C with a high yield (86%). The crystals of fast synthesized SSZ-13 appeared much smaller in size (333 ± 52 nm) compared to its bulk conventional counterpart. The effects of synthetic parameters such as silica sources, water to dry-gel ratio, aluminum content and inorganic cations were investigated. It was concluded that appropriate chosen of silica source and water to dry-gel ratio was the key to accelerate the formation of SSZ-13. The fast-synthesized SSZ-13 was tested with the reaction of methanol to olefins and showed a more than twofold increase in catalyst lifetime and higher selectivity to olefins than that of the conventional SSZ-13.

Hierarchical mesoporous ZSM-5 supported nickel catalyst for the catalytic hydrodeoxygenation of anisole to cyclohexane

Upgrading of bio-oil through catalytic hydrotreating was investigated with anisole as a model compound. Hierarchical ZSM-5 zeolites with nano-crystalline structure were successfully synthesized by a segmented steam-assisted crystallization method by tuning the rate of crystallization during the phase transforming between the amorphous structure and crystalline structure without using the organic template. The diffusional properties of ZSM-5 samples were investigated by catalytic cracking of 1,3,5-TIPB. Ni/hierarchical ZSM-5 catalysts displayed an excellent performance in the hydrodeoxygenation (HDO) of biomass-derived anisole to cyclohexane in comparison with conventional Ni/ZSM-5 catalyst. The activity and selectivity were significantly improved by enhancing the accessibility of acidities. Anisole HDO mainly follows the anisole to benzene with metal sites by direct deoxygenation, then cyclohexane formed by hydrogenation.

Efficient synthesis of high silica SSZ-13 zeolite via a steam-assisted crystallization process

High silica SSZ-13 zeolite was synthesized by an efficient and green steam-assisted crystallization (SAC) method under a low alkalinity and low organic templates amount system. The as-prepared samples were characterized by XRD, SEM, N2 adsorption–desorption, TG–DTG and NH3-TPD. The results showed that the SAC method can not only remarkably improve zeolite yield but also enhance the crystallization rate of SSZ-13 zeolite compare to conventional hydrothermal route. Meanwhile, it was also found that the various content of the organic structure directing agent (N,N,N-trimethyladamantammonium hydroxide, TMAdaOH) in the dry gel can adjust flexibly the crystal size, morphology and acidity of samples. The zeolite samples with smaller particles and more strong acidity amount were more likely obtained under the higher TMAdaOH/SiO2 ratio (0.2) condition. In addition, the catalytic evaluation in methanol-to-olefins (MTO) reaction showed that the high silica SSZ-13 catalysts synthesized by SAC method exhibited longer lifetime and comparative selectivity to ethylene and propene than those of the SSZ-13s obtained by conventional hydrothermal route. Thus, the SAC route is believed to be a competitive strategy to synthesize high silica SSZ-13 zeolites with improved MTO catalytic performance.

Direct conversion of syngas to isoparaffins over hierarchical beta zeolite supported cobalt catalyst for Fischer-Tropsch synthesis

A hierarchical micro-meso-macroporous Beta zeolite (HB) was developed by a steam-assisted crystallization method and was employed for cobalt Fischer-Tropsch synthesis. Meso-macroporous silica (HS), alumina modified meso-macroporous silica (HAS) and conventional Beta zeolite (CB) were used for comparison. Structural characterization indicated that the as-synthesized HB exhibited nanosized Beta zeolite crystallites and a hierarchical micro-meso-macroporous structure. The HB supported catalyst showed very high isoparaffins selectivity because of the isomerization reaction at the strong acidic sites of the Beta zeolite, while due to its good macroporous structure, which is in favor of reducing the internal diffusion limitations, the methane selectivity was kept as low as HAS supported catalysts. In this paper, we use large particle catalysts for evaluation, which are very close to industrial condition. And the conclusions obtained may be very important for the design of new Fischer-Tropsch catalysts.

Transformation behavior of gibbsite to boehmite by steam-assisted synthesis

Conversion of gibbsite to boehmite as a precursor for many aluminum oxide materials was widely concerned. Herein, the steam-assisted crystallization (SAC) method was conducted to synthesis the boehmite from gibbsite. The transformation behavior of gibbsite to boehmite treated at 155, 165, 175, and 185 °C by SAC method was studied by XRD and SEM analysis. The treated temperature and time were the main factor determining the transformation of morphologies and crystal phase of gibbsite to boehmite. Boehmite was appeared at 165 °C and 12 h for the SAC method but at 175 °C and 12 h for the hydrothermal method. Lower temperature and less time were required for conversion of gibbsite to boehmite by SAC compared to hydrothermal method. The advantageous transformation of gibbsite to boehmite is ascribed to the steam activation. Thus, a novel and effective SAC method was presented to synthesis of boehmite from gibbsite.

Sustainable Synthesis of Hierarchically Porous Silicalite-1 Zeolite by Steam-assisted Crystallization of Solid Raw Materials Without Secondary Templates

Hierarchical silicalite-1 zeolites were obtained from the direct conversion of a mixture of ground solid raw materials via a steam-assisted crystallization(SAC) method without involvement of any mesoscale template. Only a trace amount of water was necessary during the crystallization, implying that the amount of water can be dramatically reduced, which still offers easy separation and high yields. The simple procedure involved only grinding and heating, which not only saves resources and energy, but also significantly reduces the discharge of eco-friendly synthesis of zeolites for practical applications. Compared to conventional bulk silicalite-1, the nanosized hierarchical zeolites with MFI structure show enhanced removal capabilities for methylene blue owing to their hierarchical porosity.

Butylene catalytic cracking to propylene over a hierarchical HZSM-5 zeolite: Location of acid sites controlling the reaction pathway

A hierarchical ZSM-5 zeolite was prepared by steam-assisted crystallization (SAC) method and then the acidity of zeolites was modified by using organic silane compounds (catalytic cracking of silane, or the CCS method). The obtained zeolite was finally applied in the process of butylene catalytic cracking to produce propylene. High yield of propylene, 40 wt%, was achieved and it was much higher than that achieved on other prepared zeolites. This rise in propylene yield is mainly ascribed to the specific location of framework aluminum of zeolite in crystal sphere. Compared with the ZSM-5 zeolite synthesized by hydrothermal method, the ZSM-5 zeolite prepared from SAC method shows more acid sites in straight and sinusoidal channels rather than the intersection of channels, which is beneficial to reinforcing the reaction pathway of producing propylene and simultaneously suppressing the main side reactions in butylene catalytic cracking. And since the external acid sites of zeolite were covered by using the CCS method, the main side reactions were further inhibited. These results also help to elucidate the catalytic behavior of acid sites in different locations of HZSM-5 in butylene catalytic cracking and better understand the processing of SAC method.

Propene epoxidation with H2 and O2 on Au/TS-1 catalyst: Cost-effective synthesis of small-sized mesoporous TS-1 and its unique performance

Designing cost-effective titanium silicalite-1 (TS-1) with enhanced mass transfer ability is of prime scientific and industrial importance. In this work, a novel small-sized mesoporous TS-1 (STS-1) material is first synthesized in cheap TPABr system using steam-assisted crystallization method. Compared with cheap microporous TS-1 prepared by classical hydrothermal method (HTS-1) with particle size of 950 nm, STS-1 shows mesoporous character of 13 nm and reduced average particle size of 590 nm. Moreover, effect of TPABr concentration is systematically investigated, and both of insufficient and overmuch TPABr concentrations are unfavorable because of low crystallinity and exorbitant price, respectively. Importantly, the catalytic performance of cost-effective Au/STS-1 for direct propene epoxidation with H2 and O2 is much better than that of Au/HTS-1, and comparable to the performance of the expensive Au/TS-1. By multi-characterizations and activation energy analysis, the underlying mechanism for the enhanced performance is further discussed. This work sheds new light on the design and synthesis of highly effective and cost-efficient TS-1 catalysts.

From nano- to macro-engineering of ZSM-11 onto thin-felt stainless-steel-fiber: Steam-assisted crystallization synthesis and methanol-to-propylene performance

• Thin-felt ZSM-11/SS-fiber is fabricated via a combined washcoating and SAC method. • Pre-aging treatment of the synthesis sol can facilitate the subsequent SAC growth. • The ZSM-11/SS-fiber catalyst shows the stability improvement for the MTP reaction. • The enhanced mass transfer by microstructured design is paramount for the stability improvement. Thin-felt ZSM-11/stainless-steel(SS)-fiber composites are successfully prepared via a combined washcoating and steam-assisted crystallization (SAC) method. Pre-aging of synthesis sol at a lower temperature of 80 °C can facilitate the zeolite nucleation and subsequent SAC growth at 180 °C. Such synthesis approach realizes the flexible tuning of acidity via adjusting SiO 2 /Al 2 O 3 molar ratio in a wide range, including but not limited from 105 to 425. The as-synthesized ZSM-11/SS-fiber composites are employed as catalysts for the methanol-to-propylene reaction, and the promising thin-felt catalyst with SiO 2 /Al 2 O 3 molar ratio of 200 shows remarkable stability improvement in comparison with its powdered counterpart, due to enhanced zeolite utilization efficiency and heat/mass transfer by the microfibrous-structured design.