Hydrodeoxygenation Products Research Articles

Metal Oxide-Promoted Hydrodeoxygenation Activity of Platinum in Pt-MOx/Al2O3 Catalysts for Green Diesel Production

Catalytic deoxygenation of fatty acids into renewable hydrocarbons (green diesel) was investigated over 4Pt-8MOx/Al2O3 (M = Mo, Re, W, and Sn) catalysts prepared by the wet impregnation method. Platinum deposited on MOx-modified γ-Al2O3 showed higher catalytic hydrodeoxygenation activity than that of the “neat” Pt/Al2O3 catalyst. The promotional effect of metal oxides (MOx) decreased in the following order: MoOx > ReOx > WOx > SnOx. Characterization studies revealed that metal oxides affect the textural and electronic properties of Pt. Supported Pt facilitated the reduction of these metal oxides. Synergy and electronic contact between Pt and MOx determined the catalytic deoxygenation performance. Fatty acid conversion increased with increasing metallic nature (decreasing binding energy) of Pt. Hydrodeoxygenation product selectivity correlated with the extent of metal oxide reduction. Among the catalysts, 4Pt-8MoOx/Al2O3 had the optimum dispersion, electron-rich Pt, and reduced Mo5+ species, enabling quant...

Effect of Surface Hydrophobicity of Pd/Al2O3 on Vanillin Hydrodeoxygenation in a Water/Oil System

Pickering emulsions stabilized by solid catalysts have received an increasing amount of attention as a reaction platform because of potential applications in the chemical industry. Here, we report a surface modification strategy for controlling the catalytic performance of Pd/Al2O3 during vanillin hydrodeoxygenation (HDO) in a water/decalin emulsion environment. A series of organophosphonic acids (PAs) were applied as modifiers to control the surface hydrophobicity, thereby determining the continuous and dispersed emulsion phases. The PAs also introduced Bronsted acid sites that promoted vanillin HDO. Reaction studies showed that the yield of the HDO product p-creosol was improved from 20% to as high as 90% upon PA modification after a reaction duration of 1 h at 50 °C. This improvement greatly depended on the surface hydrophobicity, which influenced the selectivity to different reaction pathways as well as the emulsion structures. Statistical analysis of the kinetic data confirmed the hydrophobic/hydroph...

Hydrodeoxygenation of methyl laurate over the sulfur-free Ni/γ-Al2O3 catalysts

A series of sulfur-free Ni/γ-Al2O3 catalysts with different metal Ni loading amount were prepared by impregnation method. The prepared catalysts and corresponding precursors were investigated through XRD, BET, H2-TPD, and H2-TPR analysis methods. The effect of Ni loading on catalytic performance of Ni/γ-Al2O3 catalysts was evaluated by methyl laurate catalytic hydrodeoxygenation (HDO). Results indicate that the catalytic HDO performance of the prepared Ni/γ-Al2O3 catalysts was directly related to hydrogen storage properties, hydrogen desorption properties, dispersion of the active component Ni, and so on. The 10 wt.% Ni/γ-Al2O3 catalyst exhibits the best HDO performance, and the methyl laurate conversion can reach 91.1% when the reaction temperature is 400 °C and pressure is 2 MPa. The HDO products are alkanes, such as C<10, C10, C11, C12 alkane, C12 alcohol, and C12 acid, especially. The main product C11 alkane with one carbon less is formed by decarbonylation/decarboxylation, and selectivity can reach 68.6%. The effects of reaction conditions on HDO reaction of oils were studied, for example, reaction pressure and H2/Oil ratio. The catalyst retains comparable activity upon regeneration through calcinations and H2 reduction.

Ni/HZSM-5 catalyst preparation by deposition-precipitation. Part 2. Catalytic hydrodeoxygenation reactions of lignin model compounds in organic and aqueous systems

Nickel metal supported on HZSM-5 (zeolite) is a promising catalyst for lignin depolymerization. In this work, the ability of catalysts prepared via deposition-precipitation (DP) to perform hydrodeoxygenation (HDO) on two lignin model compounds in organic and aqueous solvents was evaluated; guaiacol in dodecane and 2-phenoxy-1-phenylethanol (PPE) in aqueous solutions. All Ni/HZSM-5 catalysts were capable of guaiacol HDO into cyclohexane at 523 K. The role of the HZSM-5 acid sites was confirmed by comparison with Ni/SiO2 (inert support) which exhibited incomplete deoxygenation of guaiacol due to the inability to perform the cyclohexanol dehydration step. The catalyst prepared with 15 wt% Ni, a DP time of 16 h, and a calcination temperature of 673 K (Ni(15)/HZSM-5 DP16_Cal673), performed the guaiacol conversion with the greatest selectivity towards HDO products, with an intrinsic rate ratio (HDO rate to conversion rate) of 0.31, and 90% selectivity to cyclohexane. Catalytic activity and selectivity of Ni/HZSM-5 (15 wt%) in aqueous environments (water and 0.1 M NaOH solution) was confirmed using PPE reactions at 523 K. After 30 min reaction time in water, Ni/HZSM-5 exhibited ∼100% conversion of PPE, and good yield of the desired products; ethylbenzene and phenol (∼35% and 23% of initial carbon, respectively). Ni/HZSM-5 in NaOH solution resulted in significantly higher ring saturation compared to the Ni/HZSM-5 in water or the NaOH solution control.

Investigation on hydroisomerization and hydrocracking of C15–C18n-alkanes utilizing a hollow tubular Ni-Mo/SAPO-11 catalyst with high selectivity of jet fuel

This paper focused on the isomerization and cracking of n-alkanes to synthesize bio-jet fuel. A series of SAPO-11 supports with different Si/Al molar ratio (0.2, 0.4, 0.6) were prepared, characterized, and evaluated by the hydroisomerization on hydrodeoxygenation product of castor oil. As a result, a Ni-Mo/SAPO-11(0.4) catalyst with hollow tubular structure and moderate acidic strength exhibited optimum conversion (97.2%) and jet fuel selectivity (81.6%) in the reaction. Subsequently, Ni-Mo/SAPO-11(0.4) was selected to carry out further experiments on studying the hydroisomerization and hydrocracking of long chain n-alkanes. The results show that increasing temperature may promote hydroisomerization and hydrocracking. Temperature is more favorable for the reaction with the length of the carbon chain decreasing, no matter conversion, hydrocracking degree or isomerization tendency. The product selectivity of different n-alkanes is associated with the carbon number. In reaction to n-C18, Ni-Mo/SAPO(0.4) surpassed the other two catalysts in the conversion and i/n ratio at 623 K which is the optimum temperature.

Measurement of HDO Products Using GC-TCD: Towards Obtaining Reliable Analytical Data

Abstract This paper reported the method development and validation of a gas chromatography with thermal conductivity detector (GC-TCD) method for the measurement of the gaseous products of hydrodeoxygenation (HDO). The method validation parameters include selectivity, precision (repeatability and reproducibility), accuracy, linearity, limit of detection (LoD), limit of quantitation (LoQ), and robustness. The results showed that the developed method was able to separate the target components (H2, CO2, CH4 and CO) from their mixtures without any special sample treatment. The validated method was selective, precise, accurate, and robust. Application of the developed and validated GC-TCD method to the measurement of by-product components of HDO of bio-oil revealed a good performance with relative standard deviation (RSD) less than 1.0% for all target components, implying that the process of method development and validation provides a trustworthy way of obtaining reliable analytical data.

Production of Alcohols from Cellulose by Supercritical Methanol Depolymerization and Hydrodeoxygenation

The reaction pathway and products of cellulose supercritical methanol depolymerization and hydrodeoxygenation (SCM-DHDO) were investigated. Monoalcohols, diols, alcohol ethers, and methyl esters we...

Investigating the effect of Fe as a poison for catalytic HDO over sulfided NiMo alumina catalysts

The effect of iron (Fe) as poison present in renewable feeds was studied during hydrodeoxygenation (HDO) over molybdenum based sulfided catalysts. The study was carried out at 6 MPa and 325 °C in batch reaction conditions. Different concentrations of Fe in the feed were tested over MoS2/Al2O3 and NiMoS/Al2O3. A notable drop in activity for the conversion of oxygenates was observed for both catalyst systems with an increased concentration of Fe in the feed. However, the changes in selectivity of products was opposite for unpromoted and Ni-promoted catalysts. In the case of the NiMoS catalyst, at higher Fe concentration, the decarbonated product (C17 hydrocarbons) decreased while the direct hydrodeoxygenation product (C18 hydrocarbons) increased. On the contrary, for the base catalyst (MoS2), there was a decrease in the yield of direct hydrodeoxygenation (C18 hydrocarbons) products and an increase in yield of decarbonated products (C17 hydrocarbons). These sulfided catalysts have different sites for these two different reaction routes and they interacted differently with Fe during the deactivation process. With surface deposition of Fe, the ability of these catalysts to create active sites i.e. via sulfur vacancies deteriorated. TEM-EDX results suggested that the effect of Ni as a promoter for the decarbonation route was nullified and a resultant FeMo phase explains the drop in activity and change in selectivity.

Synthesis of green diesel through hydrolysis and hydrodeoxygenation reaction from waste cooking oil using NiMo/Al2O3 catalyst

Green diesel is a second generation of biofuel that has a potential to answer the energy needs in Indonesia, also in the world. The reactions used to produce green diesel are hydrolysis and hydrodeoxygenation by using NiMo/Al2O3 catalyst. Hydrolysis will change the triglycerides in the raw material, which is waste cooking oil into free fatty acids (FFA) and then converted into green diesel through hydrodeoxygenation. Hydrolysis of waste cooking oil carried out at temperature of 200°C and pressure of 16 bar for 3 hours with water and oil volume ratio of 1:1. FFA product of hydrolysis is as much as 73.89%. For hydrodeoxygenation, variation in operating condition used are 375°C with pressure of 12 bar and 400°C with pressure of 15 bar. Characterizations of green diesel have shown that both of hydrodeoxygenation products have met the specifications of diesel, except the acid number with optimum operating condition at 400°C and pressure of 15 bar. These conditions with NiMo/Al2O3 activated led to dominant yield of 68.87% of diesel fraction, selectivity of 69.58%, and conversion of 82.15%.

The comparison of Co, Ni, Mo, CoMo and NiMo sulfided catalysts in rapeseed oil hydrodeoxygenation

The Al2O3 supported monometallic Co, Ni and Mo and bimetallic CoMo and NiMo catalysts were compared in rapeseed oil hydrodeoxygenation (HDO) reaction after in situ sulfidation. The reaction was described by five pseudo-first order rate constants (k 1–k 5) for the simplified reaction scheme: triglycerides (Tgs) to octadecane (k 1); Tgs to oxygenates (Oxs; i.e., sum of fatty acids, fatty alcohols, and esters of fatty acids and fatty alcohols) (k 2); Tgs to heptadecane (k 3); Oxs to octadecane (k 4), and Oxs to heptadecane (k 5). The empirical pseudo-first order rate constant of the hydrocarbons (Hcs) product formation (k Hc ) increased in the order Ni/Al2O3 ~ Co/Al2O3 < CoMo/Al2O3 ~ Mo/Al2O3 ≪ NiMo/Al2O3 showing hence a significant synergy between Ni and Mo. All monometallic catalysts exhibited k 1 and k 3 practically zero and the reaction proceeded essentially through the formation of the oxygenated reaction intermediates (high k 2). The Co/Al2O3 and Ni/Al2O3 catalyzed selectively hydrodecarboxylation (HDC) of fatty acids (high k 5). Over Mo/Al2O3, the HDO pathway, however, was nearly the exclusive one (high k4). CoMo/Al2O3 and NiMo/Al2O3 catalysts yielded both HDO and HDC products suggesting partial synergy in the relative selectivity HDO/HDC between Co(Ni) and Mo.

Understanding trends in hydrodeoxygenation reactivity of metal and bimetallic alloy catalysts from ethanol reaction on stepped surface

Platform molecules obtained from biomass are often found enriched with oxygen and require a suitable deoxygenation process for their conversion into high value chemicals and fuels. In this study, a combined density functional theory (DFT) and microkinetic model (MKM) based approach was utilized to understand the catalytic activity and selectivity trends observed for the hydrodeoxygenation (HDO) reactions of biomass-derived platform molecules on supported metal catalysts. This understanding was developed on modeling the CO cleavage reaction of ethanol molecule over the stepped (211) surface of the metal catalysts using both carbon and oxygen binding energies as the descriptors. The two descriptor MKM showed that the activity for ethanol conversion to ethane at 523K for a range of metal catalysts follows the order; Co>Ru>Ir>Rh>Ni>Fe>Pt>Pd>Cu>Re>Ag>Au. In general, as observed in the experiments of previous reports, the CC cleavage in ethanol to produce methane shows higher turnover frequencies (TOFs) with negligible ethane formation. In contrast, since the focus of the MKM developed here was on the stepped surfaces, significant conversion towards the formation of ethane was calculated. The selectivity towards the HDO product was further improved on reducing the reaction temperature to 373K. A range of metal catalysts viz. Cu, Pt, Rh, Ir, Ru, Ni and Co was observed to yield ethane selectively at 373K, albeit with significantly reduced conversion. While the shift in selectivity towards the HDO product was at the expense of catalyst reactivity, general conclusions on the design of a selective metal HDO catalyst were drawn, further extending it to the design of a bimetallic alloy. In the screening of metal alloys, three specific catalysts (Co3Ni, Co3Fe and Ni3Fe) were observed to show the maximum turnover of ethane (10−3s−1) with high selectivity. More specifically, the MKM developed for both metal and bimetallic catalysts provided insights into the reactivity trends observed in recent experimental HDO studies on biomass-derived model phenolic compounds.

Levulinic acid hydrodeoxygenation, decarboxylation and oligmerization over NiMo/Al2O3 catalyst to bio-based value-added chemicals: Modelling of mass transfer, thermodynamics and micro-kinetics

Levulinic acid (LA) hydrotreatment at solvent-free conditions was studied in a slurry reactor over the sulphide form of NiMo/Al2O3 catalyst. The influence of process parameters on the H2 equilibrium solubility in the liquid (calculated by Soave–Redlich–Kwong, EOS), on the external and internal mass transfer limitations, adsorption and desorption rates and the rates of surface and homognenous reactions were determined experimentally and computationally by utilizing a newly-developed micro-kinetic model. Main catalytic hydrodeoxygenation (HDO) product γ-valerolactone (GVL) was formed on NiMoSx phase (surface sites concentration of 0.33μmolm−2 was determined) exclusively by LA hydrogenation to hydroxypentanoic acid, and its subsequent intramolecular esterification, while cyclisation of LA to angelica lactones and their saturation was negligible. Activation energies determined by the regression analysis for the rate-determining step of catalytic LA HDO (Ea 39kJmol−1) and competitive LA decarboxylation in bulk liquid (Ea 134kJmol−1) show, that the temperature increase from 225°C to 275°C accelerated the decarboxylation rates much faster than HDO, which reflected in the drop of selectivity towards GVL formation from 90 to 42%, while its yield was not significantly affected (2.5±3molL−1 within 60min at final temperature). LA oligomerization by aldol addition and subsequent dehydrative cyclisation is reported for the first time and its rate was independent of the catalyst loading or H2/N2 pressure. Internal mass transfer had no influence on the global reaction rate, while the external mass transfer was eliminated at high agitation rate.

Renewable aromatic production through hydrodeoxygenation of model bio-oil over mesoporous Ni/SBA-15 and Co/SBA-15

In present work, ordered mesoporous material SBA-15 was synthesized and used as the support for the dispersion of the nickel and cobalt active metal. XRD, N2 sorption, ICP, H2-TPR and TEM techniques were used for the Ni/SBA-15 and Co/SBA-15 catalysts characterization. Both Ni/SBA-15 and Co/SBA-15 catalysts were tested for the hydrogenation of the typical oxygen-containing molecule present in bio-oil in order to produce the renewable aromatic. Ni/SBA-15 catalyst exhibit much higher catalytic performance as compared with Co/SBA-15 catalyst for the hydrodeoxygenation reaction performance of anisole, but it is also obvious that the Ni/SBA-15 catalyst would inevitably lead to the emergence of partial hydrodeoxygenation product methoxycyclohexane. On the contrary, the direct hydrogenolysis of methoxy-functional group from anisole molecule could prevail over the Co/SBA-15, leading to much higher aromatic selectivity over the Co/SBA-15. Co/SBA-15 could selectively hydrogenolyse the CO bond and could act as the potential hydrodeoxygenation catalyst for aromatic production from the bio-oils because of its high efficiency in the removal of oxygen by a direct hydrogenolysis route.

Hydrodeoxygenation of oleic acid in hexane containing pressurized CO2 using Fe/SBA-15 as catalyst

The hydrotreatment of oleic acid in a green reaction media to produce the major hydrodeoxygenation product octadecane using a 6 wt% nano Fe loaded Fe/SBA-15 as catalyst was demonstrated in this study. The data show that hexane containing pressurized CO2 could dictate the selectivity of the hydrotreatment of oleic acid with high selectivity for hydrodeoxygenation over decarboxylation/decarbonylation. In addition, hexane containing pressurized CO2 could offer beneficial effects including reduction of viscosity of the solution and increase of H2 solubility. As a result, uniform dispersion of the catalyst occurred in the solution and the diffusion resistances were reduced as well due to the increase in mass transfer. At the best operating condition of 290 °C, 6 h, 30 bars H2 and 40 bars CO2, an octadecane yield of 83.3% and a heptadecane yield of 8.7% were observed when compared to 65% and 15%, which were the yields in the absence of CO2. The effects of three independent operation variables namely, temperature, CO2 pressure and time and their interactions on the hydrotreatment of oleic acid were studied using a central composite design and response surface methodology. The results showed that all these three variables were significant factors for increasing the yield of octadecane while CO2 pressure was the most significant variable in decreasing the yield of heptadecane, the major decarboxylation/decarbonylation product.

Mechanistic analysis of the role of metal oxophilicity in the hydrodeoxygenation of anisole

A combined experimental and theoretical comparative study of the hydrodeoxygenation (HDO) of anisole was conducted over Pt, Ru, and Fe metals. In the experimental part, an inert silica support was used to directly compare the catalytic activity and selectivity of the three metals at 375°C under H2 flow at atmospheric pressure. In parallel, for density functional theory (DFT) calculations the close-packed Pt(111), Ru(0001), and Fe(110) surfaces were employed to compare the possible mechanisms on these metals. It was observed that over Pt/SiO2 and Ru/SiO2 catalysts, both phenol and benzene were the major products in a phenol/benzene ratio that decreased with the level of conversion. By contrast, over the Fe/SiO2 catalyst, no phenol formation was detected, even at low conversions. The DFT results show that over all the three metal surfaces the dehydrogenation at the CH3 side group occurs before the CO bond breaking. This removal of H atoms from the CH3 group facilitates the activation of the aliphatic CalkylO bond. Therefore, it can be concluded that a common intermediate for the three metals is a surface phenoxy and the significant differences between the three metals are related to the reactivity of this surface phenoxy. That is, over Pt(111) and Ru(0001) the phenoxy intermediate is hydrogenated to phenol, which in turn, can undergo further HDO to form benzene. This result is in agreement with the experiments over Pt/SiO2 and Ru/SiO2 catalysts. Over these catalysts, both phenol and benzene are major products, with the selectivity to benzene increasing with conversion at the expense of phenol. In contrast, over the Fe(110) surface, the strong metal oxophilicity makes the direct cleavage of the CO bond in the surface phenoxy easier than hydrogenation to phenol. Thus, it is predicted that phenol is not formed over iron, but only benzene should be observed as HDO product at all conversion levels, which is also in agreement with the experimental observations.

Hydrodeoxygenation of p-Cresol on MoS2/Amorphous Carbon Composites Synthesized by a One-Step Hydrothermal Method: The Effect of Water on Their Activity and Structure

Associated with the carbonization of glucose in the hydrothermal process, MoS2/amorphous carbon composites were synthesized by a one-step hydrothermal method and characterized by various technologies. Introducing carbon into MoS2 increased its surface area and enlarged the edge-to-corner ratio of the MoS2 slab of the MoS2 phase, which contributed to a high deoxygenation degree in the hydrodeoxygenation (HDO) of p-cresol, but excessive carbon covered some MoS2 edges that acted as active sites for HDO reaction and lowered the conversion. The presence of water modified the surface structure of MoS2 and had a great effect on the HDO activity and product distribution. During the HDO reaction, sulfur–oxygen exchange at the edges of MoS2 resulted in the loss of S atoms and the intensity decrease of the (002) diffraction peak for MoS2 phase, which was accelerated by increasing the amount of added water. Carbon in MoS2 only acted as a support and had no preventing effect on water. When the added water was 0.2 g, i.e., water/p-cresol molar ratio was 0.5:1, after reaction at 275 °C for 8 h, toluene selectivity and deoxygenation degree increased to 91.3% and 94.2%, respectively.

Titania-supported gold-based nanoparticles efficiently catalyze the hydrodeoxygenation of guaiacol

Well-defined Au, Rh and AuRh nanoparticles were synthesized by colloidal chemical co-reduction, immobilized on rutile titania nanorods, and evaluated in the gas-phase hydrodeoxygenation (HDO) of guaiacol, an important product of lignin pyrolysis. Au/TiO2 appears active, stable and selective for the HDO of guaiacol to phenol in broad temperature and conversion ranges, making this model catalyst the first reported example of gold efficiency for the hydroprocessing of a lignin derivative. Furthermore, upon nanoalloying Au with Rh, the main HDO product at 280°C switches from phenol to cyclohexane, which is tentatively ascribed to a cooperative effect.

Effect of support on the NiMo phase and its catalytic hydrodeoxygenation of triglycerides

The support effect on the NiMo phase was studied by using two kinds of mesoporous materials, SAPO-11 and Al2O3, which were applied on the hydroconversion of jatropha oil. It was shown that the amounts of strong acidic sites of the NiMo/SAPO-11 catalyst (610μmol/g) was higher than that of NiMo/Al2O3 catalyst (190μmol/g), resulting into the different NiMo phase formed on the respective SAPO-11 and Al2O3. The NiMo/SAPO-11 catalyst with larger NiMoO4− and MoO3 sulfided phases showed higher selectivity for decarboxylation reaction (41.2%), while the NiMo/Al2O3 catalyst with more Mo4+ polymeric octahedral sulfided phases led to main hydrodeoxygenation products (62.5%).

Production of Hydrocarbons via Hydrodeoxygenation of Lignin-Derived Phenolic Compounds

Lignin is the component with the highest carbon content in biomass. The transformation of lignin to high-grade liquid fuels can be achieved via hydrodeoxygenation(HDO) of phenolic intermediates derived from the products of lignin depolymerization. The octane numbers of the hydrodeoxygenation products of phenolic intermediates are quite high. They have vapor pressures and carbon atom number (C6 similar to C10) within the range of gasoline. Thus, these hydrodeoxygenation products would be the most desirable components for a fungible liquid transportation fuel. This is very meaningful to application of lignin. Recently, researches about the hydrodeoxygenation of phenolic compounds develop rapidly. In this paper, the HDO reactions of phenolic compounds using sulfided Mo-based catalyst, noble metal catalyst and inexpensive non-sulfided catalyst are reviewed in detail. It is found that most of the investigated catalysts are bifunctional catalysts, combining the hydrogenation function of active metal with hydrolysis and dehydration of support. The catalytic mechanism for the HDO of phenolic compounds is sketched, and the effects of catalyst supporter on the catalytic activity are also discussed. Furthermore, the current technique challenges are summarized, and future technologic explorations for the efficient hydrodeoxygenation of lignin-derived phenolic compounds are proposed.

Structure Analysis of Pine Bark-, Residue-, and Stem-Derived Light Oil and Its Hydrodeoxygenation Products

Three constituents of loblolly pine (stem, residue, and bark) were independently pyrolyzed to produce light oil. The chemical structures of the light oils were analyzed using GC-MS, 1H NMR, 13C NMR, and HSQC-NMR. The experimental results indicate that levoglucosan observed in the light oil is the major product from cellulose pyrolysis. Furanic and phenolic monomers are derived from hemicellulose and lignin, respectively. The light oil hydrodeoxygenation reaction study was carried out using carbon-supported ruthenium as a catalyst. Light oils from all three constituents were independently upgraded under 8 MPa hydrogen gas at 300 °C for 2 h. After the upgrading process, the aromatic rings were hydrogenated and the oxygen-containing functional groups were extensively removed. According to 1H NMR, the percentage of the CHnO peak area for stem, residue, and bark decreases by 90.03%, 77.84%, and 94.98%, respectively, compared to that for the pre-HDO light oil After upgrading, the carbon yields are 83.31% for the bark and approximately 100% for both the stem and the residue. The results indicate that ruthenium can hydrogenate carbonyl (C═O) bonds, furan ring, and aromatic ring as well as cleave the aliphatic C–O and C–C bonds by hydrogenolysis. However, it cannot cleave the ether and ester type bonds.