JP7589350B2 - A solution used to prepare a catalyst consisting of molybdenum (Mo) and a group VIII metal for synthesizing a catalyst for hydrodesulfurization (HDS) - Google Patents

Description

The present invention relates generally to the field of organic and inorganic chemistry and to the preparation of an impregnation solution for the preparation of a catalyst comprising molybdenum (Mo) and a Group VIII metal for the synthesis of a catalyst for hydrodesulfurization (HDS) processes.

In many countries, environmentally friendly fuels are now being produced and used. As a result, fuel quality improvement standards have become stricter. Therefore, reducing the sulfur content in fuel has become even more important. In 2024, Thailand will change its diesel oil quality standard from Euro 4 to Euro 5, reducing the sulfur content in fuel from 50 ppm to 10 ppm. Thus, Thailand's fuel quality standards will continue to gradually reduce the sulfur content in fuel, following the trends of fuel standards in other countries around the world.

To reduce the sulfur content in diesel fuel, oil refineries use a process called hydrodesulfurization (HDS) to remove sulfur compounds in diesel fuel using hydrogen gas so that the amount does not exceed the value set by the diesel fuel standard. Hydrogen gas reacts with the sulfur compounds in diesel fuel at a temperature of 320-400° C. and a pressure of 3-7 MPa. This results in products based on hydrocarbons containing sulfur atoms that do not exceed the value specified by the diesel fuel standard. The sulfur atoms are removed as hydrogen sulfide gas (H ₂ S). This HDS process requires a catalyst to accelerate the reaction. Thailand needs to improve diesel fuel quality standards to meet the global demand for more environmentally friendly fuels, which results in a greater demand for this type of catalyst, both in quantity and cost value.

This HDS process has been shown to be effective in removing sulfur from sulfur-containing compounds. However, when considering the structure of sulfur compounds in diesel fuel, complex sulfur compounds also coexist in the fuel, most of which are dibenzothiophene derivatives (DBT) with one or two alkyl groups at the 4- and/or 6-positions. This is a structure of sulfur compounds that is difficult to remove by this process, because the alkyl groups prevent the sulfur molecules from binding to the active sites on the catalyst.

Therefore, it is very important to react a large amount of hydrogen gas with the catalyst at high temperatures when reducing the sulfur content in diesel fuel to less than 10 ppm by HDS process. The reason is that the sulfur residues in diesel fuel are complex and do not easily react with hydrogen gas under the normal conditions used in HDS process according to Euro 4 fuel standard. Therefore, there is a need for more efficient and innovative catalysts that can reduce the amount of sulfur compounds in diesel fuel by HDS process at normal reaction temperature and pressure to meet Euro 5 fuel quality standard. This is because of plant equipment constraints that cannot tolerate anything other than normal operating conditions.

The catalysts for HDS process commonly used in refineries and petrochemical industries are nickel molybdenum sulfide supported on gamma alumina (Ni-Mo-S/γ-Al ₂ O ₃ ) or cobalt molybdenum sulfide supported on gamma alumina (Co-Mo-S/γ-Al ₂ O ₃ ). Depending on the formulation shown in the patent, the catalyst efficiency can be improved by different processes such as the selection of the type of active metal, the type of additive, and the improvement of the synthesis method (which may include changing the chemical composition of the solution (impregnation solution) used for catalyst preparation). In addition, the improvement of the support, such as the improvement of the porous structure of the support, the improvement of the catalyst sintering (drying process), and the improvement of the catalyst activation process, are also reflected in the efficiency of the catalyst, resulting in a more efficient removal of sulfur compounds in diesel fuel. Details of the catalyst improvement from the patent research are given below:

JP-A-61-114737 describes the synthesis of a catalyst (supported on an alumina or silica support) using a solution containing a nitrogen-containing organic compound (nitrogen-containing ligand) and an active metal. The catalyst is then dried at a temperature not exceeding 200°C.

U.S. Patent No. US 6,329,314 describes a process for activating catalysts containing Group VIII and Group VI metals by mixing them under certain conditions with a petroleum-based liquid containing thiophenes and nitrogen compounds.

U.S. Pat. No. 5,032,565 describes a process for reducing a catalyst containing a Group VIII metal by reacting it with a reducing agent such as an alcohol or polyhydric alcohol.

US Patent No. 6,540,908 describes the preparation of sulfide catalysts, which consists of a process of mixing alumina with an organic compound containing nitrogen and carbonyl groups. This is followed by a procedure to convert the catalyst to the sulfide form.

US Patent No. 3,114,701 describes immersing alumina in multiple cycles in an aqueous solution of dissolved nickel nitrate and ammonium molybdate to ultimately achieve a catalyst containing 4-10% nickel and 19-30% molybdenum by weight.

U.S. Patent No. 6,872,678 describes the synthesis of a catalyst containing a compound of a Group VIB metal, a compound of a Group VIII metal, and an organic sulfur additive. The catalyst is then activated using a combination of a petroleum-based organic liquid and hydrogen gas or in a continuous process.

From the patent search mentioned above, these are mainly related to catalyst synthesis processes aimed at improving catalyst efficiency. However, in the synthesis of such catalysts, the solution used to prepare the catalyst (impregnation solution) is one of the most important elements of catalyst synthesis. The impregnation solution used for catalyst preparation contains active species that provide the main function of the catalyst upon completion of synthesis. From the patent search, the details of the impregnation solution used to prepare the catalyst are disclosed as follows:

U.S. Patent No. US 3,232,887 describes an impregnation solution for preparing a catalyst that contains an acidic organic or inorganic compound, such as phosphoric acid, that acts as a stabilizer, and has a phosphorus to molybdenum molar ratio (P/Mo) in the range of 0.25 to 2.5.

US Patent No. US 3,840,472 describes an impregnation solution for preparing a catalyst, which contains molybdenum oxide, at least one compound from a Group VIII metal, and phosphoric acid dissolved in water, with a molar ratio P/Mo in the range of 0.065 to 2.5.

U.S. Pat. No. 5,332,709 describes an aqueous solution for catalyst preparation. It consists of a Group VIB metal, a Group VIII metal, a phosphorus-containing inorganic acid, and a reducing agent in an amount sufficient to dissolve the Group VIB and Group VIII metals and stabilize the solution. The reducing agent may be selected from hydrazine and hydroxylamine compounds.

U.S. Patent No. US 9,364,816 describes an aqueous impregnation solution for preparing a catalyst that contains a phosphorus compound, a compound of a Group VI metal and a Group VIII metal, the concentration of the Group VI metal being greater than 5.6 mol/L.

From the patent search mentioned above, most of them refer to the preparation of catalyst impregnation solution, and each patent focuses on different types of compounds and the ratio of each compound. There are very few patents that describe controlling the structure of active species in the impregnation solution to a certain structure to increase the catalytic efficiency. In this regard, from the patent search, a similarity is found in US Pat. No. 7,427,578, which describes how to control the structure of active species in the impregnation solution by controlling the pH of the impregnation solution in the range of 2-5 using phosphoric acid. Thus, when the impregnation solution is examined using Raman spectroscopy technique, the Raman spectrum of the solution appears at wavelengths of 965-975 cm ^-1 .

However, only the position of the Raman spectrum of the solution does not indicate the specific structure of the active species in the impregnation solution used in the catalyst preparation, because the active species structure is very important for the properties of the impregnation solution and therefore affects the efficiency of the catalyst synthesized from this impregnation solution. Therefore, the present invention focuses on controlling the structure of the decaying heptamolybdate ion (Mo ₇ O ₂₄ ^6- ) in both quality and quantity by controlling the pH and the potential difference (Eh) of the redox reaction to be in a specific range. This achieves the structure of the active species in the impregnation solution as described above. Also, the ratio of the height of the Raman spectrum obtained from the impregnation solution for catalyst preparation to the height of the spectrum of ammonium heptamolybdate (NH ₄ ) ₆ Mo ₇ O ₂₄ must be within a controlled range. Furthermore, in order to increase the catalytic efficiency of the catalyst prepared from the impregnation solution according to the present invention for use in the removal of sulfur compounds by hydrogen gas process, various compounds in the impregnation solution must be controlled within a specific range.

The present invention relates to a solution consisting of molybdenum (Mo) and a group 8 transition metal, used for catalyst preparation for hydrodesulfurization processes. By using water as a solvent, the solution is a mixture of molybdenum trioxide, a carbonate compound of a group 8 transition metal, phosphoric acid, a reducing agent, and an organic acid.

The present invention provides a solution preparation method for synthesizing high performance catalysts used to reduce sulfur compounds in diesel according to Euro 5 standards. The structure of the active species in the impregnation solution and the interaction of the solution compounds consisting of molybdenum (Mo), cobalt (Co), phosphorus (P), organic acids and a specific ratio of reducing agent during the solution preparation method must be controlled. In particular, the pH and redox potential (Eh) of the impregnation solution must be controlled during preparation. The redox potential (Eh) is a measure of the tendency of a chemical species to accept (reduce) or donate (oxidize) electrons. These controlled factors have a significant impact on the quality of the impregnation solution and the performance of the catalysts synthesized using the solution prepared according to the present invention.

The catalyst synthesized using the solution of the present invention is a Co-Mo catalyst that has little interaction with the support, and therefore a very low deactivation rate over the life of the catalyst, and has high catalytic performance for hydrodesulfurization reactions. From the perspective of energy efficiency, the initial operating temperature is lower than other commercially available Co-Mo catalysts, and the temperature range from the beginning to the end of operation can be increased, so the catalyst life can be extended.

Raman spectroscopy spectrum of a reference standard solution: ammonium heptamolybdate tetrahydrate ((NH ₄ ) ₆ Mo ₇ O _{24.4H 2} O) 25.3 g in 50 mL of distilled water (pH adjusted to 0.5 using citric acid). Sulfur content in hydrodesulfurized diesel at various temperatures from catalysts synthesized using the solution according to the present invention. I/I ₀ ratio of solutions at different pH and sulfur content in hydrodesulfurized diesel at 350° C. from catalysts synthesized using solutions of different pH. I/ _I0 ratio and viscosity of solutions at various reduction-oxidation potentials (Eh) Stability test of catalysts in hydrodesulfurization process: (a) catalysts synthesized using the solution according to the present invention, (b) commercial catalysts.

The present invention relates to an impregnation solution for preparing a catalyst used in a hydrodesulfurization process. The impregnation solution consists of molybdenum trioxide, a carbonate compound of a group VIII metal, phosphoric acid, a reducing agent, an organic acid, and water as a solvent. The properties of the impregnation solution are listed as follows:

the molar ratio of molybdenum (Mo) to group VIII metal is in the range of 2.00-2.22; the molar ratio of phosphorus (P) to group VIII metal is in the range of 0.20-0.40; the molar ratio of organic acid to group VIII metal is in the range of 1.00-1.08; the pH value of the impregnation solution is in the range of 0.5-1; the redox potential value (Eh) of the impregnation solution is in the range of 500-530 mV, which is controlled using a reducing agent from which ascorbic acid is selected; the structure of the active clusters in the impregnation solution can be investigated using Raman spectroscopy, which shows a Raman spectrum at a wave number of about 945 cm ^-1 . The intensity ratio at this position of the impregnated solution to the Raman spectrum at this position of the reference standard ammonium heptamolybdate solution ((NH ₄ ) ₆ Mo ₇ O ₂₄ ) (equimolar with respect to the molybdenum of the impregnated solution) is 0.04-0.06.

Preferably, the Group VIII metal carbonate compound for the impregnation solution according to the present disclosure is selected from cobalt carbonate, nickel carbonate, or a mixture of these compounds.

Preferably, the organic acid for the impregnation solution according to the present disclosure is selected from citric acid, malic acid, or a mixture of these acids.

Preferably, the redox potential value (Eh) for the impregnation solution according to the present disclosure is in the range of 515-520 mV.

In another aspect, the present invention also relates to catalyst preparation by an impregnation method using an impregnation solution on an inorganic porous oxide as a catalyst support.

Preferably, the inorganic porous oxide as the catalyst support according to the present disclosure is selected from alumina, silica, silica-alumina, zeolite, or a mixture of these materials.

Preferably, the pore size of the inorganic porous oxide as the catalyst support according to the present disclosure is in the range of 85 to 160 angstroms.

Structural control of active clusters in the impregnation solution is an important concept for constructing a specific structure of active clusters for catalyst preparation in the present invention. By having the active clusters have a specific structure in the impregnation solution, the excellent performance of the catalysts synthesized according to the present invention is improved compared to conventional catalysts of the same type.

Experimental data shows that the performance of catalyst for hydrodesulfurization process can be improved by controlling the pH and Eh of the impregnation solution within a certain range and controlling the molar ratio of Group VIII metal, phosphoric acid, and organic acid as disclosed in the present invention. Figure 1 shows the Raman spectrum of ammonium heptamolybdate (25.3 g) (pH controlled at 0.5 using citric acid) dissolved in distilled water (50 mL) (using a high-resolution Raman microspectrometer (HORIBA)). The impregnation solution was dropped onto a sample holder, and then the Raman spectrum was measured. The Raman spectrum at a wave number of about 945 cm ^-1 is due to the symmetric stretching vibration (ν _S (Mo=O)) of the bond between molybdenum and oxygen in Mo ₇ O ₂₄ ^6- , and the spectrum at a wave number of about 900 cm ^-1 is due to the asymmetric stretching vibration (ν _AS (Mo=O)) of the bond between molybdenum and oxygen. Furthermore, the Raman spectrum of the impregnating solution according to the present invention shows a spectrum that is significantly weaker than the intensities of the ammonium heptamolybdate solution at these two positions, suggesting that the Mo ₇ O ₂₄ ^6- structure is partially decomposed in the impregnating solution, resulting in a decrease in the valence of the molybdenum structure. Thus, the impregnating solution according to the present invention is composed of a majority of the Mo ₇ O ₂₄ ^6- structure and molybdenum structures of various valences. The present invention focuses on the control of the decomposed molybdenum structures in terms of the quality, quantity, and mixture of molybdenum structures of various valences in the impregnating solution. The decrease in the Raman spectrum intensity at wave numbers of about 945 and 900 cm ^-1 of the impregnating solution according to the present invention suggests that the molybdenum structure has changed as follows:

1. The structural transformation of _{Mo 7} O ₂₄ ^6- from Mo=O to Mo-O-organic acid brings Mo ₇ O ₂₄ ^6- and group VIII metal closer, which increases the chance of Mo, group VIII metal, and organic acid forming clusters. This highly stabilized cluster can become the dispersed active sites on the catalyst after the impregnation process. Thus, the decrease in the spectral intensity at wavenumbers of about 945 and 900 cm ^-1 suggests the increase in the active sites on the catalyst and thus the improved performance of the catalyst synthesized according to the present invention.

2. ^The partial decomposition of _Mo7O246- structure leading to a smaller valence of _the molybdenum structure improved the dispersion of _MoS2 on the catalyst after the sulfidation process.

3. The reducing agent used in the impregnation solution partially converted Mo= ^O of _Mo7O246- to Mo _- OH, which increased the electron density of the molybdenum structure to contain more anions. This phenomenon can strengthen the electrostatic interaction between the molybdenum structure and the positively charged gamma-alumina support in the acidic aqueous solution, which can further fix the clusters of Mo, Group VIII metal, and organic acid on the gamma-alumina support.

As mentioned above, the position and intensity of the Raman spectrum of the impregnating solution is important for the amount and distribution of active sites on the catalyst. A large amount of active sites and good distribution on the catalyst significantly improves the performance of the catalyst. Therefore, controlling the position and intensity of the Raman spectrum of the impregnating solution is a very important factor for the catalytic hydrodesulfurization performance of the catalyst synthesized according to the present invention.

Control of the pH and Eh of the impregnation solution is also important for the partial conversion of Mo ₇ O ₂₄ ^6- , which can be evaluated from the intensity ratio (I/I ₀ ) of the Raman spectra at a wavenumber of about 945 cm ^-1 of the impregnation solution (I) and the ammonium heptamolybdate solution (I ₀ ) as the reference standard. The catalytic hydrodesulfurization performance of the catalysts synthesized according to the present invention is shown as an example in the detailed description.

The amount of various valence molybdenum structures decomposed from the Mo ₇ O ₂₄ ^6- structure, which coexists with the majority Mo ₇ O ₂₄ ^6- structure in the impregnation solution according to the present invention, is controlled by controlling the pH and Eh of the impregnation solution and the amount of each compound to the specific values disclosed in the present invention.

A catalyst is prepared according to the present invention. 64.17 g of molybdenum trioxide (manufactured by Merck) and 26.7 g of cobalt (II) carbonate (manufactured by ACROS Organics, Thermo Fisher Scientific brand) are added to 300 mL of distilled water. Additionally, 3.75 mL of 85% phosphoric acid (manufactured by Merck) and 50 g of citric acid (manufactured by Merck) are added. The solution is heated at 85° C. with stirring. After 1 hour and 30 minutes, 0.63 g of ascorbic acid (manufactured by VETEC) is added, followed by natural cooling to room temperature. The final solution volume is 156 mL.

The final solution was impregnated into an alumina support (JGC Catalysts and Chemicals) to prepare a HDS catalyst. The support had a surface area, pore volume, and pore diameter of 302 m ² /g, 0.83 mL/g, and 10.2 nm, respectively. HDS experiments were carried out using this catalyst and straight run gas oil (SRGO) as the feed. The sulfur and nitrogen concentrations in the SRGO were 6,670 and 100 ppm, respectively. The hydrogen partial pressure was 44 bar, and the volume ratio of H ₂ to SRGO was 205. The liquid hourly space velocity (LHSV) of the SRGO was 1 h ^-1 . The experiment was carried out for 9 days. The oils produced at reaction temperatures of 340° C., 350° C., and 360° C. were collected and analyzed for their sulfur content.

Figure 2 shows the sulfur content of the oil produced collected at each reaction temperature. The results clearly show that the catalyst according to the present invention has high performance for hydrodesulfurization reaction. Diesel oil with sulfur content of less than 10 ppm (EURO5 standard) is produced at 350°C. Moreover, the high sulfur level obtained at the reaction temperature of 340°C during the first three days is probably due to the change in catalyst structure, which is the nature of this uncalcined catalyst.

The effect of solution pH on the Mo ₇ O ₂₄ ^6- structure in the impregnation solution according to the invention and the HDS activity of the resulting catalyst is demonstrated. The impregnation solution had a Mo/group (VIII) metal molar ratio of 2.0, a P/group (VIII) metal molar ratio of 0.25 and an organic acid/group (VIII) metal molar ratio of 1.08. Changing the pH of the solution from 0.5 to 2 and 3 (913 pH meter, Metrohm) changed the redox potential (Eh, RM-30P ORP meter, DKK-TOA) from 529 to 379 and 311 mV, respectively. Figure 3 clearly shows the effect of pH on the I/I ₀ ratio of the solution and the HDS activity of the resulting catalyst at 350°C. The Mo ₇ O ₂₄ ^6- structure in the impregnation solution is not significantly affected by pH, as the value of I/I ₀ ratio does not change significantly. The HDS activity of the solution with pH less than 1 is better than that of pH 2 and 3. Therefore, the pH of the impregnation solution should be controlled below 1 to obtain a good HDS catalyst.

Effect of redox potential (Eh) on Mo ₇ O ₂₄ ^6- structure in the impregnation solution according to the present invention. The concentrations of Mo, group (VIII) metals, and P in the solution were the same as those described in Example 2, but the pH was controlled to be less than 1. The redox potential (Eh) of the solution was varied in the range of 500-800 mV. Figure 4 shows the effect of redox potential (Eh) on the I/I ₀ ratio of the solution. Table 1 shows the sulfur content in the diesel oil product at 350°C and the difference in redox potential (Eh) of the impregnation solution.

From this result, it is clear that the height of the Raman spectroscopy peak at 945 cm ^-1 decreases with a decrease in the redox potential (Eh). As a result, the I/I ₀ ratio decreases. This suggests that the redox potential (Eh) has a significant effect on the Mo ₇ O ₂₄ ^6- structure. Considering the HDS activity, as shown in Table 1, the performance of the catalyst is highly dependent on the redox potential (Eh).

In order to obtain a good impregnation solution with a suitable Mo ₇ O ₂₄ ^6- structure, the redox potential (Eh) and I/Io ratio should be within the ranges of 500-530 mV and 0.04-0.06, respectively. A catalyst prepared in such a controlled solution can have high HDS activity. If the redox potential (Eh) of the impregnation solution is out of the range, the HDS activity of the resulting catalyst will be relatively low. However, the viscosity of the solution increases with the decrease in the redox potential (Eh). Thus, the viscosity of a solution with an Eh of less than 500 mV and a pH of less than 1 is too high to prepare a catalyst.

Effect of the molar ratio of components in the impregnation solution for the present invention Apart from the pH and the redox potential value Eh, the components in the impregnation solution are also very important in the catalyst preparation, since they affect the structure of the active species in the impregnation solution. In order to obtain the active species with the proper structure for the catalyst preparation, the amounts of the components need to be suitable, as described in the detailed description.

Table 2 shows the effect of molar ratio of components in the impregnation solution and sulfur content on the hydrodesulfurization process using catalysts prepared from the impregnation solution at 340, 350, and 360°C with hydrogen gas. The results show that the molar ratio of components in the impregnation solution must be controlled within a certain range to obtain a high-performance catalyst. If the molar ratio of components is too high or too low, the performance of the catalyst may be reduced.

A 110-day stability study comparing this HDS catalyst prepared from the example with a commercial catalyst is carried out to determine the deactivation rate under practical conditions ( the sulfur content in the product was kept below 10 ppm by increasing the reaction temperature ).

Figure 5 shows that the start temperature for both catalysts was 340°C, and then the reaction temperature was increased to maintain sulfur levels in the product below 10 ppm according to the Euro 5 standard. The catalyst of the present invention had a relatively high deactivation rate at the start of the run (SOR) of 340°C, which was typical behavior for this catalyst type II. Then, after 7 days, the reaction temperature was increased to 350°C, whereas the commercial catalyst started at 354°C.

From the results, it can be concluded that this catalyst of the present invention has high performance compared to the commercial catalyst. However, after 110 days, the reaction temperature was up to 356°C. The calculated deactivation rate was 1.6°C/month for 110 days. However, it will be significantly lower over the entire life (more than about 2 years). A lower deactivation rate means less frequent catalyst replacement. This is one of the opportunities to reduce the production cost of Euro 5 diesel.

The best method of the present invention is as described in the detailed description of the invention.

Claims (8)

A solution used to prepare a catalyst comprising molybdenum trioxide, a carbonate compound of a Group VIII metal, phosphoric acid, a reducing agent, an organic acid, and water, said solution having the following characteristics:
the molar ratio of molybdenum to Group VIII metal is within the range of 2.00 to 2.22;
a molar ratio of phosphorus to Group VIII metal in the range of 0.20 to 0.40;
the molar ratio of organic acid to Group VIII metal is in the range of 1.00 to 1.08;
the pH value of the solution is in the range of 0.5 to 1;
The redox potential value controlled by using ascorbic acid as a reducing agent is in the range of 500-530 mV. 2. The solution used for preparing the catalyst ^according to claim 1, wherein the solution with _molybdenum oxoanion structure ( _Mo7O246- ) in the solution is detected by Raman spectroscopy at a wave number of about 945 cm ^- 1, and the ratio of the height of the spectrum of the molybdenum oxoanion from the solution to the height of the molybdenum oxoanion from ammonium heptamolybdate (as a reference standard) is in the range of 0.04 to 0.06 when the molybdenum content in the solution is equimolar. The solution used to prepare the catalyst of claim 1, wherein the Group VIII metal carbonate compound is selected from the group consisting of cobalt carbonate, nickel carbonate, or a mixture thereof. The solution used to prepare the catalyst of claim 1, wherein the organic acid is selected from the group consisting of citric acid, malic acid, or a mixture thereof. The solution used to prepare the catalyst according to claim 1, wherein the redox potential value is in the range of 515-520 mV. 1. A catalyst preparation process comprising the step of impregnating an inorganic porous oxide support with a solution comprising molybdenum trioxide, a carbonate compound of a Group VIII metal, phosphoric acid, a reducing agent, and an organic acid, the solution including water as a solvent and having the following characteristics:
the molar ratio of molybdenum to Group VIII metal is within the range of 2.00 to 2.22;
a molar ratio of phosphorus to Group VIII metal in the range of 0.20 to 0.40;
the molar ratio of organic acid to Group VIII metal is in the range of 1.00 to 1.08;
the pH value of the solution is in the range of 0.5 to 1;
The oxidation-reduction potential value controlled by using ascorbic acid as a reducing agent is in the range of 500-530 mV. 7. The catalyst preparation process of claim 6, wherein the inorganic porous oxide support is selected from the group consisting of alumina, silica, silica-alumina, zeolites, or mixtures thereof. The catalyst preparation process according to claim 6 or 7, wherein the pore size of the inorganic porous oxide support is in the range of 85-160 Å.

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