HMF Yield Research Articles

Improved surface acidity of niobium doped tungstated-zirconia solid acid catalyst over production of 5-hydroxymethylfurfural

The 5-hydroxymethylfurfural (5-HMF) acts as an important chemical intermediate to bridge the biomass resources and industrial applications, which shows the potential for green development. However, the performance of biomass materials conversion to 5-HMF is still limited in the green solvent. Herein, an effective approach is reported to prepare the highly efficient solid acid catalysts, NbOx/WOy-ZrO2, to improve fructose conversion. It is found that the introduction of Nb results in the generation of the niobium oxides, which improves acid sites and tunes the ratios of Brønsted acid and Lewis acid on the surface of the WOy-ZrO2 support. With the acidity improvement and increasing acid sites of the NbOx/WOy-ZrO2, the highest fructose conversion is 99% in water. Meanwhile, the 5-HMF yield and the selectivity are also as high as 50.1% and 50.7% under the reaction temperature of 180 °C for a short reaction time of 30 min. The proposed NbOx/WOy-ZrO2 catalyst strategy will not only open a new way for designing the solid acid catalysts to achieve high performance of the 5-HMF in the water, but also promote the green production of biomass and sustainable development in the future.

Sulfonic acid functionalized β zeolite as efficient bifunctional solid acid catalysts for the synthesis of 5-hydroxymethylfurfural from cellulose

Introduction of the sulfonic acid group into H-β zeolite to prepare β-SO3H bifunctional catalysts for the efficient synthesis of 5-hydroxymethylfurfural (HMF) from cellulose. Catalysts characterization, such as XRD, ICP-OES, SEM (Mapping), FTIR, XPS, N2 adsorption-desorption isotherm, NH3-TPD, Py-FTIR demonstrate the sulfonic acid group was successfully grafted onto the β zeolite. A superior HMF yield (59.4 %) and cellulose conversion (89.4 %) was obtained in the H2O(NaCl)/THF biphasic system under 200 °C for 3 h with β-SO3H(3) zeolite as catalyst. More valuable, β-SO3H(3) zeolite converts other sugars and obtains ideal HMF yield, including fructose (95.5 %), glucose (86.5 %), sucrose (76.8 %), maltose (71.5 %), cellobiose (67.0 %), starch (68.1 %), glucan (64.4 %) and also converts plant material (25.1 % for moso bamboo and 18.7 % for wheat straw) with great HMF yield. β-SO3H(3) zeolite catalyst keeps an appreciable recyclability after 5 cycles. Moreover, in the presence of β-SO3H(3) zeolite catalyst, the by-products during the production of HMF from cellulose were detected, and the possible conversion pathway of cellulose to HMF was proposed. The β-SO3H bifunctional catalyst has excellent potential for the biorefinery of high value platform compound from carbohydrates.

A facile and practical conversion of carbohydrates into HMF using metal chlorides in [DMSO][CholineCl] deep eutectic solvent

Biofuels are a type of renewable energy that can become more vital in our lives. There are several advantages to switching to alternative biofuels and green techniques for producing these biofuels from biomass feedstock. 5-Hydroxymethylfurfural (HMF) is a potential precursor for biofuel and is synthesized from biomass sources. DMSO is specifically known as the most effective solvent for the conversion of biomass into HMF, leading to high yields. Due to the high miscibility of DMSO with various organic solvents, isolating HMF from the reaction mixture continues to be complicated. The use of a deep eutectic solvent (DES) in the biomass conversion process is attracting much attention. We have developed the synthesis of HMF from glucose, fructose, and cellulose using metal chloride in a [DMSO][CholineCl] deep eutectic solvent, which was easily obtained by combining DMSO and choline chloride. The yields of HMF under optimal conditions from glucose, fructose, and cellulose were 40%, 93%, and 15%, respectively. The HMF product can be isolated from [DMSO][CholineCl] deep eutectic solvent mixture by the liquid-liquid extraction using organic solvents with high purity.

Insights into solvent effect on selective production of furfural and 5-hydroxymethylfurfural from fructose

Selective transformation of biomass-based feedstocks into value-added energy chemical is one of the most critical steps in biorefinery process. Herein, we gave insight into solvent effect on selective production of furfural or 5-hydroxymethylfurfural (HMF) from fructose with the combination of experiments, nuclear magnetic resonance (NMR) and molecular dynamics (MD) simulation. It was found that the selective formation of furfural and HMF was mainly dominated by solvent polarity and basicity, and solvent properties could change the reaction pathway of fructose. High polarity-high basicity solvent promoted HMF formation, while high polarity-low basicity solvent was conductive to furfural formation. The preferential arrangement of solvents around fructose tautomers, especially on different C-OH, consequently led to the different products. High yields of furfural (76.2%) and HMF (90.5%) were obtained in the solvents of γ-butyrolactone (GBL)–H2O and dimethyl sulfoxide (DMSO)–H2O.

Effect of salt addition on cellulose conversion into 5-hydroxymethylfurfural in metal chloride aqueous solution

Metal chloride aqueous solution is an effective method to promote the conversion of cellulose into 5-hydroxymethylfurfural (HMF). In this work, the influences of sodium chloride (NaCl) and lithium chloride (LiCl) on the conversion of cellulose into HMF in ferric chloride (FeCl3) aqueous solution were investigated. The conversion of glucose and fructose in FeCl3–NaCl and FeCl3–LiCl aqueous solutions were studied, and the interaction between active species and cellobiose was characterized by UV spectrum and electrospray ion mass spectrometry to explore salt addition on the reaction of cellulose conversion and the active specie. Results showed that the yield of HMF converted from cellulose in FeCl3 aqueous solution was low, while the yield of HMF was significantly increased when NaCl or LiCl added to the system. In FeCl3–NaCl aqueous solutions, the yield of HMF reach a maximum of 40.0% at a concentration of 0.02 M FeCl3, and hydrolysis of cellulose was the key step in the conversion of cellulose; In FeCl3–LiCl aqueous solutions, the yield of HMF reach a maximum of 36.7% at a concentration of 0.06 M FeCl3, and isomerization of glucose was the controlling step. The characterization results illustrated that the addition of NaCl or LiCl changed the active species in FeCl3 aqueous solutions, and [Fe(OH)2]+ and [FeCl2(OH) + Li]+ were the active species in FeCl3–NaCl and FeCl3–LiCl aqueous solution for cellulose conversion, respectively.

Sulfonated ionic liquid immobilized SBA-16 as an active solid acid catalyst for the synthesis of biofuel precursor 5-hydroxymethylfurfural from fructose

The catalytic conversion of biomass-derived chemicals to 5-hydroxymethylfurfural (HMF) is of high current interest. In this work, an efficient solid acid catalyst was synthesized by anchoring sulfonic imidazolium-based ionic liquid (IL) over the surface of mercaptopropyl-modified SBA-16 via thiol-ene click reaction. Various characterization techniques exhibited that the mesostructure of SBA-16 was preserved after the immobilization of acidic IL on its surface. The catalytic activity of the synthesized catalyst was evaluated for the dehydration of fructose to HMF. The influence of different reaction parameters on the catalyst efficiency was examined and optimized. A maximum HMF yield of 98% was attained after 30 min of reaction at 120 °C in dimethyl sulfoxide (DMSO) with a catalyst load of 15 mg. Importantly, the catalyst was simply recoverable and can be reused for five successive runs.

One-pot conversion of glucose to 5-hydroxymethylfurfural under aqueous conditions using acid/base bifunctional mesoporous silica catalyst

One-pot conversion of carbohydrate-derived biomass compounds to 5-hydroxymethylfurfural (HMF) using an environmentally green solvent such as water is meritorious, yet challenging, which requires the strategic design of catalysts. We prepare an ordered bifunctional organocatalyst (MS-AB) containing 3-aminopropylacetic acid grafted within the silica nanocage that converts glucose-to-HMF, exhibiting the highest HMF yield (60 wt.%/85.7 mol%) in water and under microwave conditions. Moreover, the calculated activation energies (150–170 °C) for glucose isomerization to fructose, vis-à-vis dehydration to HMF were 59.06 and 138.05 kJ mol−1, respectively which can even be comparable with the metal-catalyzed reactions. Further evidence was shown from the rate constant values, indicating fructose dehydration is faster (rate constant: k2: 1.92 x 10−2 to 1.13 x 10−1 min−1) than glucose isomerization (rate constant: k1: 2.5 to 5.3 x 10−3 min−1). Considering the advantages associated with MS-AB, the results might showcase the potential of organocatalyst for sustainable utilization of lignocellulosic biomass to platform chemicals.

Optimization Study of Glucose Synthesis to 5-Hydroxymethylfurfural (5-HMF) by Using Low Transition-Temperature Mixtures (LTTM)

5-Hydroxymethylfurfural (5-HMF) is a potential value-added product gaining popularity due to its wide range of applications. Glucose is widely used for 5-HMF production because it is abundant and more cost-effective than other resources. In the current research, a combination of microwave irradiation and low transition-temperature mixture (LTTM) for 5-HMF production was sustainably created. A preliminary study was conducted to derive 5-HMF using LTTM as a green solvent in various ratios of glucose: LTTM (MA/ChCl): water (10:100:15) and (10:48:100) via microwave heating at 90 °C for 10–60 min reaction time. The product analysis revealed that the 10:100:15 combination ratio of glucose: LTTM: water produced more 5-HMF (20.5%) than the 10:48:100 (0.28%) ratio. Following the preliminary results, an optimization study was conducted, focusing on the reaction temperature, LTTM mass, and water to assess the impact on 5-HMF yield and glucose conversion, using CCD in Design-Expert software. The quadratic model fit for 5-HMF yield and the 2FI model for glucose conversion yielded R2 values of 0.9861 and 0.8610, respectively. Both responses had a significant p value of 0.0061 for glucose conversion and <0.0001 for 5-HMF yield. An optimum point was achieved at 100 °C, 100 g of LTTM, and 20 g of water, with an expected 5-HMF yield and glucose conversion of 45.77% and 69.03%, respectively.

Co-production of 5-hydroxymethyl furfural and furfural from holocellulose over UiO-66-derived acid-base catalysts

UiO-66-derived H2N-UiO-66-SO3H with acid-base active sites was used for the co-production of 5-hydroxymethyl furfural (5-HMF) and furfural (FUR) in the direct conversion of holocellulose composed of cellulose and hemicellulose. The prepared catalyst was further characterized by various techniques, confirming its formation, structure, and composition. H2N-UiO-66-SO3H displayed excellent performance for the selective conversion of holocellulose to 5-HMF and FUR, achieving yields of 39.1 % and 28.2 %, respectively. The reusability of the catalyst was also investigated by direct use after recycling without any pretreatment. The results indicated that H2N-UiO-66-SO3H had good stability, and after four cycles, the yields of 5-HMF and FUR decreased only slightly to 37.2 % and 27.1 %, respectively. The recyclable catalyst H2N-UiO-66-SO3H for the effective conversion of holocellulose to 5-HMF and FUR demonstrated application potential for the utilization of lignocellulosic biomass.

Phosphotungstic acid immobilized over SnO2 mesoporous material as a heterogeneous catalyst for fructose to 5-hydroxymethylfurfural conversion

5-Hydroxymethylfurfural (HMF) is regarded as an essential platform chemical due to its flexible nature that could be converted into a variety of value-added compounds including production of liquid fuels and fine chemicals. We have developed a phosphotungstic acid (PTA) immobilized over SnO2 mesoporous material by hydrothermally employing Pluronic P123 as a structure directing agent (SDA) to obtain a heterogeneous catalyst for fructose to HMF conversion, and the catalyst is designated as MSnPTA. The powder X-ray diffraction patterns in low angle region, type IV isotherm and fringes of the catalyst confirmed the mesoporosity of the material. The catalyst is comprised of BET surface area of 85.86 m2/g and average pore diameter of 7.9 nm. Spherical shape through worm like structure formed small particles facilitated the large surface area of the MSnPTA catalyst that prefer to occur the catalytic reaction. The uniform elemental distribution of PTA over SnO2 ascribed more active sites, resulting in high acidity of the catalyst as measured by TPD of ammonia, which increased the conversion of fructose to high yield of HMF in a short reaction time. The yield of the HMF increased as increasing the amount of catalyst and temperature as expected.

Titanium oxide/nickel foam (TiO2/NF) catalyzed the conversion of glucose to 5-hydroxymethylfurfural in subcritical solvothermal liquefaction

The glucose conversion into 5-hydroxymethylfurfural (HMF) typically required Lewis and Brønsted acid catalysts. The mesoporous TiO2 supported on nickel foam (NF) was synthesized using modified sol–gel dip-coating methods. The prepared catalysts were characterized to observe their physical and physicochemical properties. The catalyst characterization showed that TiO2 is successfully incorporated onto the NF surface. Nitrogen adsorption/desorption isotherms and NH3-TPD analysis showed that the TiO2/NF catalysts have a large specific surface area and high surface acidity, which can enhance glucose dehydration. The liquid products from the solvothermal reaction were analyzed using GC–MS analysis. Under the optimal reaction conditions, about 46% of HMF yield was obtained, and the liquid products contained furan derivatives and phenolic compounds. The solid TiO2/NF catalyst allowed easy catalyst recovery from liquid products. The reusability of the TiO2/NF catalysts is still acceptable and shows the TiO2/NF catalyst's potential for usage in HMF production.

A new method for conversion of fructose and glucose to 5-hydroxymethylfurfural by magnetic mesoporous of SBA-16 was modified to sulfonic acid as Lewis's acid catalysts

The sustainable process and direct synthesis of 5-hydroxymethylfurfural (5-HMF), a valuable renewable biofuel and biochemical, was systematically studied using inexpensive catalysts and recyclable magnetic nanocatalysts. Fe3O4@SBA-16-SO3H was obtained by the synthesis of SBA-16 and was modified by MPTES and H2O2. After that, Fe3O4 was synthesized via chemical precipitation and mixed with SBA-16-SO3H for easy separation and recovery (using five cycles of catalysts with excellent yield for the synthesis of 5-HMF) and a new magnetic heterogeneous catalyst for the synthesis of 5-HMF that was found to have 95% glucose and fructose conversion and 97% yield of 5-HMF, respectively. The prepared catalyst was characterized by FT-IR, EDAX, XRD, FE-SEM, TEM, VSM, BET, and TGA techniques. The experiments were carried out with good yields and implied that the catalytic method was effective, convenient, and promising for the synthesis of 5-HMF. The present study opens a new avenue for the renewable one-pot synthesis of 5-HMF, a precious product. The efficient conversion of biomass into value-added compounds under mild and safer conditions is a cornerstone of our current research efforts to meet the needs of the chemical industry with sustainable development.

Effective one-pot chemoenzymatic cascade catalysis of biobased feedstock for synthesizing 2,5-diformylfuran in a sustainable reaction system

2,5-Diformylfuran, which can be prepared via the oxidation of biobased HMF, has received considerable attention because of its potential applications in producing furan-based chemicals and functional materials, such as biofuels, polymers, fluorescent material, vitrimers, surfactants, antifungal agents and medicines. This work aimed to develop an efficient one-pot process for chemoenzymatic transformation of biobased substrate to 2,5-diformylfuran with deep eutectic solvent (DES) Betaine:Lactic acid ([BA][LA]) catalyst and oxidase biocatalyst in [BA][LA]–H2O. Using waste bread (50 g/L) and D-fructose (18.0 g/L) as feedstocks in [BA][LA]–H2O (15:85, vol/vol), the yields of HMF were 32.8% (15 min) and 91.6% (90 min) at 150 °C, respectively. These prepared HMF could be biologically oxidized to 2,5-diformylfuran by Escherichia coli pRSFDuet-GOase, achieving a productivity of 0.631 g 2,5-diformylfuran/(g fructose) and 0.323 g 2,5-diformylfuran/(g bread) after 6 h under the mild performance condition. This bioresourced intermediate 2,5-diformylfuran was effectively synthesized from biobased feedstock in an environmentally-friendly system.

Synthesis of 5-hydroxymethylfurfural from cellulose using silica-amorphous carbon and choline chloride:CrCl3

The silica-amorphous carbon bearing SO3H group was synthesised by incomplete carbonisation of glucose and sulfonation with concentrated H2SO4 and the structure was determined by FTIR, XRD, and EDX. In this study, their catalytic activities were investigated in the synthesis of 5-hydroxymethylfurfural (HMF) from cellulose with choline chloride:CrCl3 deep eutectic solvent (DES). The results showed the highest yield of HMF at a reaction temperature of 140ºC for 1 h with 162 mg cellulose, 6 mmol [EMIM]Cl, 10 mg silica-amorphous carbon and 50% mol DES ChCl:CrCl3. In particular, the catalyst was able to recover and reuse three times with a small decrease in yield for the conversion of cellulose to HMF.

Quantification of the Microwave Effect in the Synthesis of 5-Hydroxymethylfurfural over Sulfonated MIL-101(Cr)

The potential benefits of microwave irradiation for fructose dehydration into 5 hydroxymethylfurfural (5-HMF) have been quantified over a sulfonated metal–organic framework (MOF), MIL 101(Cr)-SO3H. The effects of temperature (140–170 °C), batch time (5–300 min), and catalyst-to-substrate ratio (0.1–0.01 g/g) were systematically mapped. After 10 min of microwave (MW) irradiation at 140 °C in a DMSO–acetone reaction medium, practically complete fructose conversion was obtained with a 70% yield of 5-HMF. Without MW, i.e., using conventional heating (CH) at the same conditions, the fructose conversion was limited to 13% without any 5-HMF yield. Rather, 90 min of CH was required to reach a similarly high conversion and yield. The profound impact of moving from CH towards MW conditions on the reaction kinetics, also denoted as the microwave effect, has been quantified through kinetic modeling via a change in the Gibbs free energy of the transition state. The modeling results revealed an eight-fold rate coefficient enhancement for fructose dehydration owing to MW irradiation, while the temperature dependence of the various reaction steps almost completely disappeared in the investigated range of operating conditions.

Conversion of Glucose to 5-Hydroxymethylfurfural Using Consortium Catalyst in a Biphasic System and Mechanistic Insights

We found an effective catalytic consortium capable of converting glucose to 5-hydroxymethylfurfural (HMF) in high yields (50%). The reaction consists of a consortium of a Lewis acid (NbCl5) and a Brønsted acid (p-sulfonic acid calix[4]arene (CX4SO3H)), in a microwave-assisted reactor and in a biphasic system. The best result for the conversion of glucose to HMF (yield of 50%) was obtained with CX4SO3H/NbCl5 (5 wt%/7.5 wt%), using water/NaCl and MIBK (1:3), at 150 °C, for 17.5 min. The consortium catalyst recycling was tested, allowing its reuse for up to seven times, while maintaining the HMF yield constant. Additionally, it proposed a catalytic cycle by converting glucose to HMF, highlighting the following two key points: the isomerization of glucose into fructose, in the presence of Lewis acid (NbCl5), and the conversion of fructose into HMF, in the presence of CX4SO3H/NbCl5. A mechanism for the conversion of glucose to HMF was proposed and validated.

Microwave-assisted alkali pre-treatment medium for fractionation of rice straw and catalytic conversion to value-added 5-hydroxymethyl furfural and lignin production

In the current study, the use of microwave-assisted sodium hydroxide medium (MWSH) for pre-treatment and saccharification of rice straw to obtain sugar syrup for the production of 5-hydroxymethyl furfural (5-HMF) was investigated. The optimization of the MWSH pre-treatment was carried out using central composite methodology, resulting in a maximum reducing sugar yield of 350 mg/g of treated rice straw (TRS) and a glucose yield of 255 mg/g of TRS under the conditions of a microwave power of 681 W, a NaOH concentration of 0.54 M, and a pre-treatment time of 3 min. Additionally, the microwave assisted transformation of sugar syrup with titanium magnetic silica nanoparticle as catalyst, producing 41.1 % yield of 5-HMF from the sugar syrup after 30 min microwave irradiation at 120 °C with catalyst loading of 2.0:200 (w/v)). The structural characterization of the lignin was analysed using 1H NMR techniques, and the surface carbon (C1s spectra) and oxygen (O1s spectra) composition changes of the rice straw during pre-treatment were analysed using X-ray photoelectron spectroscopy. The rice straw based bio-refinery process which contains MWSH pretreatment followed by dehydration of sugars achieved high efficiency of 5-HMF production.

Niobium oxide-phosphorylated carbon xerogel composites as solid acid catalysts for cascade conversion of glucose to 5-hydroxymethylfurfural (HMF) in pure water

The cascade conversion of glucose to a key platform commodity, 5-hydroxymethylfurfural (HMF), was studied as a sustainable method for producing platform molecules from biomass. Bifunctional catalysts were prepared by the deposition/precipitation of Nb2O5 (containing NbO4 Lewis and Nb-OH Brønsted acid sites) onto the surface of phosphorylated carbon xerogels (CX_P) with Brønsted acid sites (BA) in the form of oxygen and phosphorus functionalities. The number of Lewis acid (LA) sites in the composites was tuned by varying the Nb loading (0–60%), and Brønsted acidity was modified by thermal (post)treatments. Catalytic tests carried out in pure water revealed that the LA sites played an important role in selective glucose conversion, enhancing the isomerization of glucose to fructose, whereas BA sites, in the absence of LA sites, catalyzed the degradation of glucose to humin. For bifunctional catalysts, the glucose conversion was proportional to the total Brønsted acidity of the catalysts. Moreover, the glucose conversion to fructose increased with higher Nb loadings, while the selectivity to HMF decreased, resulting in similar HMF yields obtained on the composites. The drop in selectivity to HMF was attributed to the degradation of HMF to humin catalyzed by high concentrations of BA sites. After the removal of strong BA sites originating from Nb2O5, the selectivity to HMF significantly increased, leading to the maximum HMF yield of 31% obtained by a composite containing 60% Nb2O5 and treated at 300 °C in air. Thus, a catalyst with poor Brønsted acidity of Nb2O5 origin (Nb-OH) was shown to be preferable to achieve moderate HMF yield without humin formation in an environmentally friendly and cost-effective process. It was also shown that the type and amount of BA sites from the carbon component affected the yield of HMF. BA in the form of oxygen functionalities increased the selectivity to HMF, whereas larger concentrations of P-OH showed the opposite effect.

Catalyst-free synthesis of 5-hydroxymethylfurfural from fructose by extractive reaction in supercritical CO2 – subcritical H2O two-phase system

An extractive reaction configuration using supercritical carbon dioxide (scCO2) as the extracting solvent was tested for the production of 5-hydroxymethyl furfural (HMF) from a 5 wt% fructose aqueous feed. In this configuration, extraction of HMF by scCO2 prevents HMF degradation in the aqueous phase. Because of water co-extraction by scCO2, the volume of the reactional mixture was maintained by continuous injection of water. Reaction was operated in a 90 mL high pressure reactor, where an HMF maximum yield of 62.4 % was achieved at 160 °C and 25 MPa, with a CO2 flow rate of 20 g.min−1 for 420 min. This is the first time that HMF is reportedly produced with such a yield by a catalyst-and organic solvent-free process. Besides, the separation efficiency reached 97.3 % and the relative purity of HMF in the extract was 95.8 wt%. Therefore, this configuration avoids post reactional purification which is needed in conventional batch processes or in extractive reaction processes using organic solvents. Based on kinetic and thermodynamic studies, modeling of the extractive reaction process was developed to perform a sensitivity analysis for CO2 flow rate and extraction efficiency, upon the HMF yield. As an example, it was shown that for 800 min reaction duration, a CO2 flow rate of 100 g.min−1 or an extraction efficiency increase by a 10-fold factor could theoretically led to HMF yields of 73.0 % and 73.7 %, respectively.

Efficient one-pot synthesis of 5-hydroxymethylfurfural from functionalized microcrystalline cellulose: An alternative approach to strong acidic/basic catalysis

Selective cellulose transformation into the 5-hydroxymethylfurfural (HMF), a versatile platform chemical, has been researched using various catalysts, while desirable efficiency remains a challenge. This study revealed an alternative conversion approach to strong acidic/basic catalysis, an efficient one-pot synthesis of HMF without external catalyst from microcrystalline cellulose (MCC), functionalized via milling and subsequent oxidation pretreatment. The outstanding HMF yield (∼55%) was obtained under moderate reaction conditions (3 h, 190 °C), demonstrating the significant impact of functionalized MCC’s characteristics (e.g., reduced crystallinity index (CI), degree of polymerization (DP), and the formed –COOH) and the reaction conditions (e.g., the role of NaCl and H2O/THF ratio) for HMF synthesis. The formed –COOH group demonstrated a dual-function i) in-situ destruction of MCC’s β-1,4-glycosidic linkages during pretreatment ii) catalyzing hydrolysis/dehydration reactions of carbohydrates to form HMF. Furthermore, chloride ions could selectively promote consecutive reactions. The promising findings in this study provide a greener route to produce HMF from cellulose without using acid/base catalysts through cellulose structure functionalization by green/simple methods.