Catalysis over zinc-incorporated berlinite (ZnAlPO4) of the methoxycarbonylation of 1,6-hexanediamine with dimethyl carbonate to form dimethylhexane-1,6-dicarbamate
© Sun et al 2007
Received: 29 June 2007
Accepted: 07 November 2007
Published: 07 November 2007
The alkoxycarbonylation of diamines with dialkyl carbonates presents promising route for the synthesis of dicarbamates, one that is potentially 'greener' owing to the lack of a reliance on phosgene. While a few homogeneous catalysts have been reported, no heterogeneous catalyst could be found in the literature for use in the synthesis of dicarbamates from diamines and dialkyl carbonates. Because heterogeneous catalysts are more manageable than homogeneous catalysts as regards separation and recycling, in our study, we hydrothermally synthesized and used pure berlinite (AlPO4) and zinc-incorporated berlinite (ZnAlPO4) as heterogeneous catalysts in the production of dimethylhexane-1,6-dicarbamate from 1,6-hexanediamine (HDA) and dimethyl carbonate (DMC). The catalysts were characterized by means of XRD, FT-IR and XPS. Various influencing factors, such as the HDA/DMC molar ratio, reaction temperature, reaction time, and ZnAlPO4/HDA ratio, were investigated systematically.
The XRD characterization identified a berlinite structure associated with both the AlPO4 and ZnAlPO4 catalysts. The FT-IR result confirmed the incorporation of zinc into the berlinite framework for ZnAlPO4. The XPS measurement revealed that the zinc ions in the ZnAlPO4 structure possessed a higher binding energy than those in ZnO, and as a result, a greater electron-attracting ability. It was found that ZnAlPO4 catalyzed the formation of dimethylhexane-1,6-dicarbamate from the methoxycarbonylation of HDA with DMC, while no activity was detected on using AlPO4. Under optimum reaction conditions (i.e. a DMC/HDA molar ratio of 8:1, reaction temperature of 349 K, reaction time of 8 h, and ZnAlPO4/HDA ratio of 5 (mg/mmol)), a yield of up to 92.5% of dimethylhexane-1,6-dicarbamate (with almost 100% conversion of HDA) was obtained. Based on these results, a possible mechanism for the methoxycarbonylation over ZnAlPO4 was also proposed.
As a heterogeneous catalyst ZnAlPO4 berlinite is highly active and selective for the methoxycarbonylation of HDA with DMC. We propose that dimethylhexane-1,6-dicarbamate is formed via a catalytic cycle, which involves activation of the DMC by a key active intermediate species, formed from the coordination of the carbonyl oxygen with Zn(II), as well as a reaction intermediate formed from the nucleophilic attack of the amino group on the carbonyl carbon.
Organic carbamates are important raw materials and/or intermediates in the production of fine chemicals , such as pesticides, herbicides, pharmaceuticals and dyestuffs [2, 3], in addition to being employed as amine protecting groups in organic synthesis  and linkers in combinatorial synthesis [1, 4].
The conventional process for the production of organic carbamates is based mainly on the reaction between alcohols and toxic isocyanates . This approach is not environmentally friendly, because of the need to input high amounts of energy, the use of toxic phosgene as a raw material and also the formation of corrosive hydrochloric acid as a side product [2, 6]. Several phosgene-free processes for the production of organic carbamates, such as reductive carbonylation of nitro compounds , oxidative carbonylation of amines , and alcoholysis of substituted urea [9–11], have been explored. Before these processes acquire a real industrial application, much work has to be carried out in order to reduce the severity of the reaction conditions employed, which presently require high temperatures and pressures, and the use of expensive noble metal catalysts. Recently, an alternative route to organic carbamates via the alkoxycarbonylation of amines with alkyl carbonates [12–14] – especially dimethyl carbonate (DMC) – under relatively mild reaction conditions and using cheap catalysts, has been receiving considerable attention. This route appears promising, also given that DMC is presently produced on a large scale by the oxidative carbonylation of methanol . In addition, DMC is easy to handle, cheap, nontoxic and clean, with the methoxycarbonylation reaction yielding methanol as main byproduct. Thus, if the methoxycarbonylation reaction between amines and DMC is coupled with the oxidative carbonylation of alcohols, it is hoped that a green, "zero emission" process for the production of organic carbamates, with could be achieved.
Dialkyl carbamates are of significant importance in industry because their thermolysis results in diisocyanates, which are precursors of polyurethane . A few reports have dealt with the synthesis of dialkyl carbamates from the alkoxycarbonylation reaction between diamines and DMC using homogeneous catalysts [17, 18]. However, difficulties associated with homogeneous catalysis systems are well known, notably those relating to the separation and recycling of the catalyst that result in loss of the latter and also product contamination. These issues can be overcome easily if a heterogeneous, rather than homogeneous, catalyst is employed. Regarding the synthesis of dialkyl carbamates from the reaction between diamines and DMC, to the best of our knowledge, no heterogeneous catalyst has been hitherto reported.
Berlinite is the most stable and nonporous phase of all polymorphs of aluminophosphates . Its main potential application can be seen in the field of functional materials, such as high-performance sealants of corrosion- and wear-resistant coatings [20, 21], acoustic wave devices, memory glass  and piezoelectric materials . While porous aluminophosphates and their transition metal-incorporated derivatives were widely used as catalysts [24, 25], a catalytic application of berlinite was never found.
In this study, we address for the first time the preparation of a zinc-incorporated berlinite, namely ZnAlPO4 – using 1,6-hexanediamine (HDA) as a structural template – and its application as a heterogeneous catalyst in the production of dimethylhexane-1,6-dicarbamate via the methoxycarbonylation between HDA and DMC under mild conditions. Factors influencing the reaction were studied systematically, with a possible reaction mechanism also proposed.
2.1. Synthesis of the ZnAlPO4catalyst
Zn(OAc)2 (A.R., Shanghai Zhenxin chemical reagent factory), Al(OAc)3 (A.R., Beijing chemical reagent company), and H3PO4 (85%, Tianjin Kermel chemical reagent company) were used as inorganic resource species and HDA (A.R., Shanghai Lingfeng chemicals company) as the structure-directing template.
The ZnAlPO4 catalyst was synthesized as follows: Al(OAc)3 was first hydrolyzed at room temperature for 2 h and then mixed homogeneously with an aqueous solution of Zn(OAc)2 and H3PO4. The resulting mixture was then aged at room temperature for 2 h, before the addition of HDA at 273 K under vigorous stirring, resulting in a gel with a molar composition of 0.05 Zn:1.78 Al:2.0 P:1.0 HDA:50 H2O. The gel was stirred at 273 K for 3 h and then charged into a Teflon-lined autoclave. After hydrothermal crystallization at 453 K for 24 h, the ZnAlPO4 solid specimen was recovered by filtration, washed with deionized water until neutrality, and dried. To remove the HDA template, the ZnAlPO4 was calcined at 823 K for 8 h.
As a comparison, an AlPO4 catalyst was also prepared using the same procedure as that used for ZnAlPO4, but in the absence of Zn(OAc)2.
2.2. Characterization of the AlPO4 and ZnAlPO4catalysts
X-ray diffraction (XRD) spectroscopy was performed on a Brucker D8 Advance diffractometer with Cu Kα1 radiation. Scans were made over a range of 2θ = 6–80°, at a rate of 1°/min.
Fourier transform infrared (FT-IR) spectroscopy was conducted on a Varian 3100 spectrometer, and operated in transmission mode at a resolution of 4 cm-1. The catalyst specimen was mixed with KBr in a weight ratio of 1:200, and pressed into pellet to be used in the measurement. The spectrum was recorded as an accumulation of the results of 125 scans, with that of dry KBr subtracted as the background.
X-ray photoelectron spectroscopy (XPS) was carried out on a Phi Quantum 2000 Scanning ESCA Microprobe with Al Kα radiation. A C1s binding energy of 284.6 eV was used as a reference.
2.3. Methoxycarbonylation of HDA with DMC
The methoxycarbonylation of HDA with DMC was performed under an N2 atmosphere in a 250 mL three-neck flask equipped with a condenser and a magnetic stirrer. In the typical procedure, certain amounts of the reactants, HDA and DMC, and the AlPO4 or ZnAlPO4 catalyst were added into the three-neck flask at room temperature, before the introduction of a nitrogen flow to drive out the air contained in the flask. The DMC acted as both reactant and solvent. The reaction was carried out at a target temperature, and then, to study the effect of reaction time, the reaction product was sampled periodically. Unless otherwise stated, the reaction can be assumed to have been carried out at 353 K for 8 h, employing 200 mmol of HDA, 1.0 g of ZnAlPO4 catalyst and a DMC/HDA molar ratio of 8:1. After the reaction, the catalyst was separated from the reaction products by filtration. The filtrate was first vacuum-distillated to remove the unreacted DMC, and dried at 373 K, resulting in a solid. The solid was then dissolved in methanol and the resulting solution subjected to chromatographic analysis. The qualitative analysis was performed on a Thermo-Finnigan LCQ-Advantage LC/MSn. The quantitative analysis was carried out on an Agilent 1100 Series HPLC instrument equipped with a 125 × 4.0 mm Hypersil ODS (C-18) column and an ultraviolet detector. The analysis conditions were flow phase V(methanol):V(water) = 60:40, a flow rate of 1.0 ml/min, column temperature of 298 K, and ultraviolet wavelength of 208 nm. The content of dimethylhexane-1,6-dicarbamate in the reaction product was determined using an external standard method and calculated using the equation W sp = W st ·A sp /A st × 100%, where sp and st refer respectively to specimen and standard. The conversion of HDA and the yield of dimethylhexane-1,6-dicarbamate were calculated based on the quantity of converted HDA. The selectivities to the components in the reaction product were calculated using the area normalization method.
3. Results and discussion
3.1. Structural characterization of the catalyst
The XPS measurement shows that the atomic composition of the surface of ZnAlPO4 is composed of Zn:Al:P:O = 1.0 2.9:3.2:15.6. The binding energy of Zn2p3/2 for ZnAlPO4 was found to be 1022.3 eV, slightly higher than that for ZnO (1021.8 ± 0.3 eV ). This indicates that Zn(II) ions are incorporated into the berlinite framework, and possess a higher tendency to draw electrons as compared to those in ZnO.
3.2. The reaction between HDA and DMC
AlPO4 and ZnAlPO4 had been employed as the catalyst for the reaction between HDA and DMC. We found that AlPO4 presented no catalytic activity, while ZnAlPO4 did have a catalytic effecr. It was found that, in addition to the main product, dimethylhexane-1,6-dicarbamate (dicarbamate), two by-products, methyl-6-amino-hexane-carbamate (monocarbamate) and methyl-6-methylamino-hexane-carbamate (N-methylated-carbamate) were also formed using the ZnAlPO4 catalyst and reaction conditions tested in this work. In the following sections, the factors influencing the reaction between HDA and DMC, including DMC/HDA molar ratio, reaction temperature, reaction time and ZnAlPO4/HDA ratio (mg/mmol), are examined.
3.2.1. Effect of DMC/HDA molar ratio
3.2.2. Effect of reaction temperature
3.2.3. Effect of reaction time
3.2.4. Effect of the ZnAlPO4/HDA ratio (mg/mmol)
3.3. Mechanistic consideration to the methoxycarbonylation of HDA with DMC over the ZnAlPO4catalyst
Hitherto only the use of homogeneous catalysts has been reported in the methoxycarbonylation of diamines with dialkyl carbonates. In this study we employed zinc-incorporated berlinite (ZnAlPO4) as a heterogeneous catalyst for the production dimethylhexane-1,6-dicarbamate from the methoxycarbonylation of HDA with DMC. The structure of berlinite is known to be derived from that of quartz, in that Si(IV) is replaced alternately with P(V) and Al(III), with a neutral framework. When Zn(II) ions are incorporated into the framework of berlinite, P(V) and/or Al(III) are replaced, resulting in oxygen vacancies in close vicinity to Zn(II). This enables zinc ions in ZnAlPO4 to acquire a higher electron attracting ability than those in ZnO, as evidenced by the XPS results.
The alkoxycarbonylation of amines with dialkyl carbonate results from the nucleophilic attack of the amine on the carbonyl carbon of dialkyl carbonate. While an amine of significantly higher basicity than the leaving group (alkoxyl) is required [6, 31–33], the alkoxycarbonylation reaction depends largely upon the reactivity of dialkyl carbonate. Factors that increase the electrophilicity of the carbonyl carbon may therefore promote the reaction, i.e. an electron-withdrawing effect exerted on the carbonyl group facilitates its reaction with nucleophilic amine [6, 14, 31, 34]. Baba  studied the methoxycarbonylation of diamines with DMC using Zn(OAc)2·2H2O as a catalyst. It was identified that the transformation from the bidentate to the monodentate coordination mode of CH3COO- anions to zinc ionshelped activate the carbonyl groups of DMC, owing to the coordinated DMC behaving as an active intermediate species, able to react with amines. For the methoxycarbonylation of HDA with DMC over ZnAlPO4, an active intermediate species resulting by the coordination of the carbonyl oxygen of DMC to Zn(II) is believed to form. This deduction is based on the following considerations: 1) It was found in this work that the methoxycarbonylation of HDA with DMC occurred only in the presence of ZnAlPO4, and not in the presence of the AlPO4, revealing that that either HDA or DMC had been activated in some manner over the catalyst; 2) Although as a potential chelating N-donor species [13, 35], the aliphatic diamine, may coordinate Zn(II), the basicity of a HDA coordinated to Zn(II) would be expected to be no more than that of free HDA. Thus, the possibility of HDA coordinating Zn(II) and acting as an active species can be excluded. In fact, transition metal ions often possess high oxophilicity in the presence of both a N-donor and O-donor species ; 3) The unusually high ability of Zn(II) in ZnAlPO4 to attract electrons facilitates the activation of DMC by partially transferring an electron from the DMC carboxyl oxygen to Zn(II).
The ZnAlPO4 material prepared in this work was identified as having a berlinite structure. The FT-IR characterization of the material indicated that zinc had been incorporated into the aluminophosphate framework. Examination by XPS indicated that the zinc ions in ZnAlPO4 had a higher binding energy than those in ZnO, suggesting a larger electron withdrawing tendency in the former. As a heterogeneous catalyst, ZnAlPO4 exhibits very high activity and selectivity under mild reaction conditions for the methoxycarbonylation of HDA with DMC to form dimethylhexane-1,6-dicarbamate. A yield of up to 92.5% of dimethylhexane-1,6-dicarbamate with nearly 100% conversion of HDA was obtained at 349 K, over 8 h, with DMC/HDA = 8 and ZnAlPO4/HDA = 5 (mg/mmol).
The mechanism for the methoxycarbonylation may have resulted from a catalytic cycle involving a key active intermediate species – formed from the coordination of the carbonyl oxygen with Zn(II) – that activates the DMC, in addition to a reaction intermediate formed from the nucleophilic attack of the amino group on the carbonyl carbon.
We are grateful for the financial support provided by the Chinese Ministry of Education's 'Program for New Century Excellent Talents in University', and the 'Program for Lotus Scholar in Hunan Province, P. R. China'.
- Greene TW, Wuts PGM: Protective Groups in Organic Synthesis. 1999, Wiley, New York, 503-550. 3View ArticleGoogle Scholar
- Adames P, Baron FA: Esters of Carbamic Acid. Chem Rev. 1965, 65: 567-10.1021/cr60237a002.View ArticleGoogle Scholar
- Aresta M, Dibenedetto A, Quaranta E: Reaction of aromatic diamines with diphenylcarbonate catalyzed by phosphorous acids: a new clean synthetic route to mono- and dicarbamates. Tetrahedron. 1998, 54: 1415-Google Scholar
- Fordon KJ, Crane CG, Burrows CJ: Cyclization of bis-urethanes as a new method for the synthesis of 5-substituted, 6-membered cyclic ureas. Tetrahedron Lett. 1994, 35: 6215-10.1016/S0040-4039(00)73394-6.View ArticleGoogle Scholar
- Shapiro SL, Bandurco V, Freedman L: Reaction of t-Ethinyl Alcohols with Aryl Isocyanates. J Org Chem. 1961, 26: 3710-10.1021/jo01068a021.View ArticleGoogle Scholar
- Baba T, Kobayashi A, Kawanami Y, Inazu K, et al: Characteristics of methoxycarbonylation of aromatic diamine with dimethyl carbonate to dicarbamate using zinc acetate catalyst. Gree Chem. 2005, 7: 159-10.1039/b413334j.View ArticleGoogle Scholar
- Paul F, Fischer J, Ochsenbein P, Osborn JA: The Palladium-Catalyzed Carbonylation of Nitrobenzene into Phenyl Isocyanate: The Structural Characterization of a Metallacylic Intermediate. Organometallics. 1998, 17: 2199-10.1021/om9708485.View ArticleGoogle Scholar
- Wan B, Liao SJ, Yu D: Polymer-supported palladium-manganese bimetallic catalyst for the oxidative carbonylation of amines to carbamate esters. Appl Catal A: Gen. 1999, 183: 81-10.1016/S0926-860X(99)00062-9.View ArticleGoogle Scholar
- Kaminshkia NV, Kostic NM: Alcoholysis of urea catalyzed by palladium(II) Complexes. Inorg Chem. 1998, 37: 4302-10.1021/ic980065r.View ArticleGoogle Scholar
- D'Addona D, Bochet CG: Preparation of Carbamates from Amines and Alcohols under Mild Conditions. Tetrahedron. Lett. 2001, 42: 5227-10.1016/S0040-4039(01)00991-1.View ArticleGoogle Scholar
- Jacobson RA: Carbamic Esters from Urea. J Am Chem Soc. 1938, 60: 1742-10.1021/ja01275a011.View ArticleGoogle Scholar
- Inesi A, Mucciante V, Rossi L: Efficient Carbamate Synthesis via a Three-Component Coupling of an Amine, CO2, and Alkyl Halides in the Presence of Cs2CO3 and Tetrabutylammonium Iodide. J Org Chem. 2001, 66: 1035-10.1021/jo001140u.View ArticleGoogle Scholar
- Monica D, Eugenio Q: Carbomethoxylating Reactivity of Methyl phenyl Carbonate Toward Aromatic Amines in the Presence of Group 3 metal (Sc, La) Triflate Catalysts. J Catal. 2004, 228: 36-10.1016/j.jcat.2004.08.004.View ArticleGoogle Scholar
- Carloni S, Devos DE, Jacobs PA, Maggi R, Sartori G, Raffaella S: Catalytic Activity of MCM-41-TBD in the Selective Preparation of Carbamates and Unsymmetrical Alkyl Carbonates from Diethyl Carbonate. J Catal. 2002, 205: 199-10.1006/jcat.2001.3439.View ArticleGoogle Scholar
- Jung KT, Bell AT: An in Situ Infrared Study of Dimethyl Carbonate Synthesis from Carbon Dioxide and Methanol over Zirconia. J Catal. 2001, 204: 339-10.1006/jcat.2001.3411.View ArticleGoogle Scholar
- Mason R, Charles L: Non-phosgene Route to the Manufacture of Organic Isocyanates. US Patent. 2004, 6: 781010-Google Scholar
- Baba T, Fujiwara M, Oosaku A, Kobayashi A, Deleon RG: Catalytic Synthesis of N-alkyl Carbamates by Methoxycarbonylation of Alkylamines with Dimethyl Carbonate Using Pb(NO3)2. Appl Catal A: Gen. 2002, 227: 1-10.1016/S0926-860X(01)00939-5.View ArticleGoogle Scholar
- Deleon RG, Kobayashi A, Yamauchi T, Ooishi J, Baba T, Sasaki M, Hiarata F: Catalytic Methoxycarbonylation of 1,6-hexanediamine with Dimethyl Carbonate to Dimethylhexane-1,6-dicarbamate Using Bi(NO3)3. Appl Catal A: Gen. 2002, 225: 43-10.1016/S0926-860X(01)00692-5.View ArticleGoogle Scholar
- Rokita M, Handke M, Mozgawa W: Spectroscopic Studies of Polymorphs of AlPO4 and SiO2. J Mol Struct. 1998, 450: 213-10.1016/S0022-2860(98)00430-X.View ArticleGoogle Scholar
- Vippola M, Vuorinen J, Vuoristo P, Lepisto T, Mantyla T: Thermal Analysis of Plasma Sprayed Oxide Coatings Sealed with Aluminium Phosphate. J Eur Ceram Soc. 2002, 22: 1937-10.1016/S0955-2219(01)00522-2.View ArticleGoogle Scholar
- Vippola M, Ahmaniemi S, Keranen J, Vuoristo P, Lepisto T, Mantyla T, Olsson E: Aluminum Phosphate Sealed Alumina Coating: Characterization of Microstructure. Mater Sci Eng, A. 2002, 323: 1-10.1016/S0921-5093(01)01367-3.View ArticleGoogle Scholar
- Christie DM, Troullier N, Chelikowsky JR: Electronic and Structural Properties of α-Berlinite (AlPO4). Solid State Commun. 1996, 98 (10): 923-10.1016/0038-1098(96)00766-1.View ArticleGoogle Scholar
- Sreeram AN, Hobbs LW, Bordes N, Ewing RC: Nucl Instrum. Irradiation-induced Amorphization of AlPO4. Methods Phys Res, Sect B. 1996, 116: 126-10.1016/0168-583X(96)00022-5.Google Scholar
- Chen JD, Sheldon RA: Selective Oxidation of Hydrocarbons with O2 over Chromium Aluminophosphate-5 Molecular-Sieve. J Catal. 1995, 153: 1-10.1006/jcat.1995.1101.View ArticleGoogle Scholar
- Modén B, Zhan BZ, Dakka J, Santiesteban JG, glesia E: Kinetics and Mechanism of Cyclohexane oxidation on MnAPO-5 Catalysts. J Catal. 2006, 239: 390-10.1016/j.jcat.2006.02.006.View ArticleGoogle Scholar
- Rokita M, Handke M, Mozgawa W: The AlPO4 Polymorphs Structure in the Light of Raman and Spectroscopy Studies. J Mol Struct. 2000, 555: 351-10.1016/S0022-2860(00)00620-7.View ArticleGoogle Scholar
- Munoz T, Prakash AM, Kevan L, Balkus KJ: Synthesis and Characterization of CuAPO-5 Molecular Sieves: Evidence for the Framework Incorporation of Cu(II) Ions. J Phys Chem. 1998, 102: 1379-View ArticleGoogle Scholar
- Pawlig O, Trettin R: In-Situ DRIFT Spectroscopic Investigation on the Chemical Evolution of Zinc Phosphate Acid-Base Cement. Chem Mater. 2000, 12: 1279-10.1021/cm991148o.View ArticleGoogle Scholar
- Frost RL, Weier ML: Vibrational Spectroscopy of Natural Augelite. J Mol Struct. 2004, 697: 207-10.1016/j.molstruc.2004.04.012.View ArticleGoogle Scholar
- Wagner CD, Zatko DA, Raymond RH: Use of the Oxygen KLL Auger Lines in Identification of Surface Chemical States by Electron Spectroscopy for Chemical Analysis. Anal Chem. 1980, 52: 1445-10.1021/ac50059a017.View ArticleGoogle Scholar
- Shivarkar AB, Gupte SP, Chaudhari RV: Carbamate Synthesis via Transfunctionalization of Substituted Ureas and Carbonates. J Mol Catal A: Chem. 2004, 223: 85-View ArticleGoogle Scholar
- Gresser MJ, Jencks WP: Ester Aminolysis. Structure-reactivity Relationships and the Rate-determining Step in the Aminolysis of Substituted Diphenyl Carbonates. J Am Chem Soc. 1977, 99: 6963-10.1021/ja00463a032.View ArticleGoogle Scholar
- Castro EA: Kinetics and Mechanisms of Reactions of Thiol, Thiono, and Dithio Analogues of Carboxylic Esters with Nucleophiles. Chem Rev. 1999, 99: 3505-10.1021/cr990001d.View ArticleGoogle Scholar
- Barcelo G, Grenouillat D, Senet JP, Sennyey G: Penta alkylguanidines as Etherification and Esterification Catalysts. Tetrahedron. 1990, 46: 1839-10.1016/S0040-4020(01)89753-2.View ArticleGoogle Scholar
- Distaso M, Quaranta E: Highly Selective Carbamation of Aliphatic Diamines under Mild Conditions using Sc(OTf)3 as Catalyst and Dimethyl Carbonate as a Phosgene Substitute. Appl Catal, B. 2006, 66: 72-10.1016/j.apcatb.2006.02.019.View ArticleGoogle Scholar
- Aresta M, Quaranta E: Mechanistic Studies on the Role of Carbon Dioxide in the Synthesis of Methylcarbamates from Amines and Dimethylcarbonate in the Presence of CO2. Tetrahedron. 1991, 47: 9489-10.1016/S0040-4020(01)80894-2.View ArticleGoogle Scholar
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