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1.
Yunzhe Zhou Dr. Cheng Chen Prof. Rajamani Krishna Dr. Zhenyu Ji Prof. Dr. Daqiang Yuan Prof. Dr. Mingyan Wu 《Angewandte Chemie (International ed. in English)》2023,62(25):e202305041
Hydrogen-bonded organic frameworks (HOFs) show great potential in energy-saving C2H6/C2H4 separation, but there are few examples of one-step acquisition of C2H4 from C2H6/C2H4 because it is still difficult to achieve the reverse-order adsorption of C2H6 and C2H4. In this work, we boost the C2H6/C2H4 separation performance in two graphene-sheet-like HOFs by tuning pore polarization. Upon heating, an in situ solid phase transformation can be observed from HOF-NBDA(DMA) (DMA=dimethylamine cation) to HOF-NBDA , accompanied with transformation of the electronegative skeleton into neutral one. As a result, the pore surface of HOF-NBDA has become nonpolar, which is beneficial to selectively adsorbing C2H6. The difference in the capacities for C2H6 and C2H4 is 23.4 cm3 g−1 for HOF-NBDA , and the C2H6/C2H4 uptake ratio is 136 %, which are much higher than those for HOF-NBDA(DMA) (5.0 cm3 g−1 and 108 % respectively). Practical breakthrough experiments demonstrate HOF-NBDA could produce polymer-grade C2H4 from C2H6/C2H4 (1/99, v/v) mixture with a high productivity of 29.2 L kg−1 at 298 K, which is about five times as high as HOF-NBDA(DMA) (5.4 L kg−1). In addition, in situ breakthrough experiments and theoretical calculations indicate the pore surface of HOF-NBDA is beneficial to preferentially capture C2H6 and thus boosts selective separation of C2H6/C2H4. 相似文献
2.
Dr. Gang-Ding Wang Dr. Yong-Zhi Li Prof. Rajamani Krishna Prof. Wen-Yan Zhang Prof. Lei Hou Prof. Yao-Yu Wang Prof. Zhonghua Zhu 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2024,136(15):e202319978
Ethylene (C2H4) purification and propylene (C3H6) recovery are highly relevant in polymer synthesis, yet developing physisorbents for these industrial separation faces the challenges of merging easy scalability, economic feasibility, high moisture stability with great separation efficiency. Herein, we reported a robust and scalable MOF (MAC-4) for simultaneous recovery of C3H6 and C2H4. Through creating nonpolar pores decorated by accessible N/O sites, MAC-4 displays top-tier uptakes and selectivities for C2H6 and C3H6 over C2H4 at ambient conditions. Molecular modelling combined with infrared spectroscopy revealed that C2H6 and C3H6 molecules were trapped in the framework with stronger contacts relative to C2H4. Breakthrough experiments demonstrated exceptional separation performance for binary C2H6/C2H4 and C3H6/C2H4 as well as ternary C3H6/C2H6/C2H4 mixtures, simultaneously affording record productivities of 27.4 and 36.2 L kg−1 for high-purity C2H4 (≥99.9 %) and C3H6 (≥99.5 %). MAC-4 was facilely prepared at deckgram-scale under reflux condition within 3 hours, making it as a smart MOF to address challenging gas separations. 相似文献
3.
The thermal reaction of Ru3(CO)12 with ethacrynic acid, 4‐[bis(2‐chlorethyl)amino]benzenebutanoic acid (chlorambucil), or 4‐phenylbutyric acid in refluxing solvents, followed by addition of two‐electron donor ligands (L), gives the diruthenium complexes Ru2(CO)4(O2CR)2L2 ( 1 : R = CH2O‐C6H2Cl2‐COC(CH2)C2H5, L = C5H5N; 2 : R = CH2O‐C6H2Cl2‐COC(CH2)C2H5, L = PPh3; 3 : R = C3H6‐C6H4‐N(C2H4‐Cl)2, L = C5H5N; 4 : R = C3H6‐C6H4‐N(C2H4‐Cl)2, L = PPh3; 5 : R = C3H6‐C6H5, L = C5H5N; 6 : R = C3H6‐C6H5, L = PPh3). The single‐crystal structure analyses of 2 , 3 , 5 and 6 reveal a dinuclear Ru2(CO)4 sawhorse structure, the diruthenium backbone being bridged by the carboxylato ligands, while the two L ligands occupy the axial positions of the diruthenium unit. 相似文献
4.
Active Sites Decorated Nonpolar Pore-Based MOF for One-step Acquisition of C2H4 and Recovery of C3H6
Dr. Gang-Ding Wang Dr. Yong-Zhi Li Dr. Wen-Juan Shi Prof. Lei Hou Prof. Yao-Yu Wang Prof. Zhonghua Zhu 《Angewandte Chemie (International ed. in English)》2023,62(43):e202311654
Herein, a 2-fold interpenetrated metal-organic framework (MOF) Zn-BPZ-TATB with accessible N/O active sites in nonpolar pore surfaces was reported for one-step C2H4 purification from C2H6 or C3H6 mixtures as well as recovery of C3H6 from C2H6/C3H6/C2H4 mixtures. The MOF exhibits the favorable C2H6 and C3H6 uptakes (>100 cm3 g−1 at 298 K under 100 kPa) as well as selective adsorption of C2H6 and C3H6 over C2H4. The C3H6- and C2H6-selective feature were investigated detailedly by experimental tests as well as sorption kinetic studyies. Molecular modelling revealed the multiple interactions between C3H6 or C2H6 molecules and methyl groups as well as triazine rings in pores. Zn-BPZ-TATB not only can directly generate 323.4 L kg−1 and 15.4 L kg−1 of high-purity (≥99.9 %) C2H4 from C3H6/C2H4 and C2H6/C2H4 mixtures, but also provide a large high-purity (≥99.5 %) C3H6 recovery capacity of 60.1 L kg−1 from C3H6/C2H4 mixtures. More importantly, the high-purity C3H6 (≥99.5 %) and C2H4 (≥99.9 %) with the productivities of 38.2 and 12.7 L kg−1 can be simultaneously obtained from C2H6/C3H6/C2H4 mixtures through a single adsorption/desorption cycle. 相似文献
5.
Andrew J. Bilbo Clay M. Sharts 《Journal of polymer science. Part A, Polymer chemistry》1967,5(11):2891-2898
Polymers [N(PN)4(C6H5)6N?P(C6H5)2(CH2)4P(C6H5)2]x and [N(PN)4(C6H5)6N?P–(C6H5)2C6H4C6H4P(C6H5)2]x have been formed by thermal copolymerization of trans-2,6-diazidohexaphenylcyclophosphonitrile [N3(PN)4(C6H5)6N3] with either 1,4-bis-(diphenylphosphino)butane [(C6H5)2P(CH2)4P(C6H5)2] or 4,4′-bis(diphenylphosphino)-biphenyl [(C6H5)2C6H4C6H4P(C6H5)2]. The maximum molecular weights obtained were about 10,000. A polymer endcapped with triphenyl phosphine was stable to 400°C. 相似文献
6.
X.-T. Mu Y.-S. Ouyang L.-M. Pei Z.-X. Peng S.-Q. Shao S.-M. Wang H. Xiong Y. Xiao Q.-Y. Yang 《Materials Today Chemistry》2022
Adsorptive separation of C2H6 from C2H4 by adsorbents is an energy-efficient and promising method to boost the polymer grades C2H4 production. However, that C2H6 and C2H4 display very similar physical properties, making their separation extremely challenging. In this work, by regulating the pore environment in a family of chitosan-based carbon materials (C-CTS-1, C-CTS-2, C-CTS-4, and C-CTS-6)- we target ultrahigh C2H6 uptake and C2H6/C2H4 separation, which exceeds most benchmark carbon materials. Explicitly, the C2H6 uptake of C-CTS-2 (166 cm3/g at 100 kPa and 298 K) has the second-highest adsorption capacity among all the porous materials. In addition, C-CTS-2 gives C2H6/C2H4 selectivity of 1.75 toward a 1:15 mixture of C2H6/C2H4. Notably, the adsorption enthalpies for C2H6 in C-CTS-2 are low (21.3 kJ/mol), which will facilitate regeneration in mild conditions. Furthermore, C2H6/C2H4 separation performance was confirmed by binary breakthrough experiments. Under different ethane/ethylene ratios, C-CTS-X extracts a low ethane concentration from an ethane/ethylene mixture and produces high-purity C2H4 in one step. Spectroscopic measurement and diffraction analysis provide critical insight into the adsorption/separation mechanism. The nitrogen functional groups on the surface play a vital role in improving C2H6/C2H4 selectivity, and the adsorption capacities depend on the pore size and micropore volume. Moreover, these robust porous materials exhibit outstanding stability (up to 800 °C) and can be easily prepared on a large scale (kg) at a low cost (~$26 per kg), which is very significant for potential industrial applications. 相似文献
7.
Wansheng Liu Shubo Geng Ning Li Sa Wang Shuping Jia Fazheng Jin Ting Wang Katherine A. Forrest Prof. Tony Pham Prof. Peng Cheng Prof. Yao Chen Prof. Jian-Gong Ma Prof. Zhenjie Zhang 《Angewandte Chemie (International ed. in English)》2023,62(8):e202217662
Two C2H6-selective metal-organic framework (MOF) adsorbents with ultrahigh stability, high surface areas, and suitable pore size have been designed and synthesized for one-step separation of ethane/ethylene (C2H6/C2H4) under humid conditions to produce polymer-grade pure C2H4. Experimental results reveal that these two MOFs not only adsorb a high amount of C2H6 but also display good C2H6/C2H4 selectivity verified by fixed bed column breakthrough experiments. Most importantly, the good water stability and hydrophobic pore environments make these two MOFs capable of efficiently separating C2H6/C2H4 under humid conditions, exhibiting the benchmark performance among all reported adsorbents for separation of C2H6/C2H4 under humid conditions. Moreover, the affinity sites and their static adsorption energies were successfully revealed by single crystal data and computation studies. Adsorbents described in this work can be used to address major chemical industrial challenges. 相似文献
8.
M. Helena Garcia Paulo J. Mendes A. Romão Dias 《Journal of organometallic chemistry》2005,690(17):4063-4071
The family of organometallic Co(III) benzonitrile derivatives of general formula [CoCp(dppe)(p-NCR)][PF6]2 (R = C6H4NMe2, C6H4NH2, C6H4OMe, C6H4C6H5, C6H5, C6H4C6H4NO2, and C6H4NO2) have been synthesized. Spectroscopic and electrochemical data were analyzed in order to evaluate the extent of electronic coupling between the organometallic fragment and the nitrile ligands. An attempt of correlation between NMR spectroscopic data and the second-order non-linear optical properties is presented, based on this work and available published data for related η5-monocyclopentadienyliron, ruthenium and nickel complexes. 相似文献
9.
Dimerization reactions of diphenyldiazomethane have been applied to the polycondensation of six bisdiazobenzyl arylenes, namely 1,4- and 1,3-bis(α-diazobenzyl)-benzenes C6H5CN2? (C6H4)? CN2C6H5; 1,4- and 1,3-bis(α-diazo-p-methoxybenzyl)-benzenes, p,p′-MeO? C6H4? CN2? (C6H4)? CN2C6H4? OMe; 4,4′-bis(α-diazobenzyl)-diphenylmethane, C6H5CN2? (C6H4CH2C6H4)? CN2C6H5; and 4,4′-bis(α-diazobenyl)-diphenyl ether, C6H5CN2? (C6H4? O? C6H4)CN2C6H5. Depending on the nature of the catalysts, polyene-arylenes (? C(Ar)?C(Ar)? C6H4)n, and polyazine-arylenes, (? C(Ar)?N? N? C(Ar)? C6H4? )n, can be obtained selectively by acid-catalyzed decomposition of these bisdiazoalkanes at room temperature. With perchloric acid and with arylsulfonic acids in strong polar media, polyene-arylenes are formed. On the other hand, boron trifluoride and arylsulfonic acids in solvents of low dielectric constant afford polyazine-arylenes. Less selective is the thermal decomposition at 75°C in toluene solution; it gives a polymer containing about 90% azine and 10% olefinic groups. All these polymers are soluble in common solvents. Their molecular weight vary from 3 200 to 5 000, i.e., X?n from 12 to 20. The polyene-arylenes are very stable and decompose only around 500°C; the polyazine-arylenes are less stable and decompose around 370°C by losing nitrogen. 相似文献
10.
The aromatic character of divalent three, five and seven-membered rings C2H2M, C4H4M and C6H6M(M=C, Si, Ge, Sn and Pb) is investigated through magnetic and geometric criteria by Density Functional Theory (DFT)method using 6-311++G(3df,2p) basis set of the GAUSSIAN 98 program. The result of Nucleus-independent Chemical Shifts (NICS)(0.5) calculations show an aromatic character for singlet state of C2H2M(M=C, Si, Ge, Sn and Sn) and nonaromatic character for triplet states of C2H2M(except M=Ge and Pb). NICS (0.5) calculations show nonaromatic character for the singlet state of C4H4C and antiaromatic character for C4H4M(M=Si, Ge, Sn and Pb). In contrast, NICS (0.5) calculations indicate antiaromatic character for the triplet state of C4H4C and nonaromatic character to C4H4M(M=Si, Ge, Sn and Pb). NICS (0.5) calculations show a slightly homoaromatic character for the singlet state of C6H6M and anti-aromatic character for triplet state of C6H6M. 相似文献
11.
Fabio Marchetti Guido Pampaloni Calogero Pinzino 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(41):13962-13969
The 1:3 reactions of the alkoxy arenes 1,4‐(MeO)2C6H4 and 1,4‐F2‐2,5‐(MeO)2C6H2 with TaF5 in chloroform at 40–50 °C resulted in formation in about 35 % yield of the long‐lived radical cation salts [1,4‐(MeO)2C6H4][Ta2F11] ( 2 a ) and [1,4‐F2‐2,5‐(MeO)2C6H2][Ta2F11] ( 2 b ), respectively. The non‐alkoxy‐substituted [arene][M2X11] [M=Ta, X=F: arene=C6H5Me ( 2 c ), 1,4‐C6H4Me2 ( 2 d ), C6H5F ( 2 e ), C6H5NO2 ( 2 f ); M=Nb, X=F: arene=C6H5Me ( 4 a ), 1,4‐C6H4Me2 ( 4 b ), C6H5F ( 4 c ), C6H5NO2 ( 4 d ); M=Ta, X=Cl: arene=1,4‐C6H4Me2 ( 5 )] were obtained from the 3:1 reactions of MX5 with the appropriate arene in chloroform at temperatures in the range 40–90 °C. Compounds 2 – 5 were detected by EPR spectroscopy (in CHCl3) at room temperature, and their gas‐phase structures were optimized by DFT calculations. Formation of the MIV species [MX4(NCMe)2] [M=Ta, X=F ( 3 a ); M=Nb, X=F ( 3 b ); M=Ta, X=Cl ( 3 c )] was ascertained by EPR spectroscopy on solutions obtained by treatment of the reaction mixtures with acetonitrile. Non‐selective reactions occurred upon combination of 1,4‐F2‐2,5‐(MeO)2C6H2 with AgNbF6 (in CH2Cl2) and 1,4‐(MeO)2C6H4 with SbF5. 相似文献
12.
Inorganic-organic hybrid membranes containing silica as the structure matrix, poly(N-vinylpyrrolidone) (PVP) as the organic mediating agent and silver ions as olefinic carriers were prepared using sol–gel method and dip-coating process. The structure and permeances of the membranes for N2, He, C2H4, C2H6 at different temperatures indicated that defect-free membranes were obtained and the transportation of the C2H4 through the membranes followed the dissolution and diffusion mechanism. Ideal separation factors of C2H4/C2H6 through the membranes were evaluated at the temperature of 298, 373 and 423 K respectively using mixture gas of 50% C2H4-50% C2H6. The results showed that the ideal separation factors of C2H4/C2H6 through the membranes were obviously greater than the ratio of PC2H4/PC2H6 obtained from the single gas measurement due to the hindering effect by the adsorbed C2H4. The ideal separation factors of C2H4/C2H6 increased with temperature and reached 10 at 423 K, which suggested that C2H4 and C2H6 could be separated at lower humidity compared to the reported organic polymer/silver salt membranes in which humidified gases and higher silver loading were usually used. The transport of C2H4 in the inorganic-organic hybrid membrane was proposed to follow the hopping mechanism, that is, olefins moved across the fixed silver sites. 相似文献
13.
Complexes of the Alkali Metal Tetraphenylborates with Macrocyclic Crown Ethers Alkali metal tetraphenylborates, MB(C6H5)4 (M = Li to Cs), react in tetrahydrofuran with macrocyclic crown ethers to give complexes of the general formula MB(C6H5)4(crown)m(THF)n. Suitable single crystals for X‐ray structure analysis were grown from a solvent mixture of tetrahydrofuran and n‐hexane. The salt like complexes [Li(12‐crown‐4)(thf)][B(C6H5)4] ( 1 ), [Na(15‐crown‐5)(thf)][B(C6H5)4] ( 2 ), and [Cs(18‐crown‐6)2][B(C6H5)4] · THF ( 6 ), the mononuclear molecular complexes [KB(C6H5)4(18‐crown‐6)(thf)] ( 3 ), [RbB(C6H5)4(18‐crown‐6)] ( 4 ), and [CsB(C6H5)4(18‐crown‐6)] · THF ( 5 ), and the compound [CsB(C6H5)4(18‐crown‐6)]2[Cs(18‐crown‐6)2][B(C6H5)4] ( 7 ), which contains a binuclear molecule ([CsB(C6H5)4(18‐crown‐6)]2) beside a [Cs(18‐crown‐6)2]+ cation and a [B(C6H5)4]? anion, are described. All compounds are charactarized by infrared spectra, elemental analysis, NMR‐spectroscopy, and X‐ray single crystal structure analysis. 相似文献
14.
The interaction of some transition metal halides with o-mercaptophenol o-Mercaptophenol reacts with WCl6, TiCl4, ZrCl4, NbCl5 and TaCl5 giving the corresponding tris-chelat-komplexe W(C6H4OS)3, H2[M(C6H4OS)3] (M = Ti, Zr), H[M(C6H4OS)3] (M = Nb, Ta). (C5H5)2TiCl2 and (C5H5)2ZrBr2give in presence of triethylamine the compounds (C5H5)2M(C6H4OS) (M = Ti, Zr). By reaction of nickel(II) acetyl-acetonate with o-mercaptophenol the polymeric octahedral complex nickel-bis-(o-hydroxy-thiophenolate) results. 相似文献
15.
Dr. Yingxiang Ye Yi Xie Dr. Yanshu Shi Dr. Lingshan Gong Dr. Joshua Phipps Prof. Abdullah M. Al-Enizi Prof. Ayman Nafady Prof. Banglin Chen Prof. Shengqian Ma 《Angewandte Chemie (International ed. in English)》2023,62(21):e202302564
Developing adsorptive separation processes based on C2H6-selective sorbents to replace energy-intensive cryogenic distillation is a promising alternative for C2H4 purification from C2H4/C2H6 mixtures, which however remains challenging. During our studies on two isostructural metal–organic frameworks ( Ni-MOF 1 and Ni-MOF 2 ), we found that Ni-MOF 2 exhibited significantly higher performance for C2H6/C2H4 separation than Ni-MOF-1 , as clearly established by gas sorption isotherms and breakthrough experiments. Density-Functional Theory (DFT) studies showed that the unblocked unique aromatic pore surfaces within Ni-MOF 2 induce more and stronger C−H⋅⋅⋅π with C2H6 over C2H4 while the suitable pore spaces enforce its high C2H6 uptake capacity, featuring Ni-MOF 2 as one of the best porous materials for this very important gas separation. It generates 12 L kg−1 of polymer-grade C2H4 product from equimolar C2H6/C2H4 mixtures at ambient conditions. 相似文献
16.
Takashi Sakai Akitoshi Yamawaki Hiroshi Ito Masanori Utaka Akira Takeda 《Journal of heterocyclic chemistry》1986,23(4):1199-1201
A novel, convenient synthetic method of 5-atyl-2,2-dimethyl-3(2H)-furanones (aryl = C6H5, 2-CH3C6H4, 3-CH3C6H4, 4-CH3C6H4, 2-CIC6H4, 4-CIC6H4, 2,4-Cl2C6H3) is described. It involves the Claisen-Schmidt condensation (potassium hydroxide/ethanol) of aromatic aldehydes with 3-hydroxy-3-methyl-2-butanone to give enones, whose bromination followed by alkaline hydrolysis (sodium hydroxide/ethanol) affords 3(2H)-furanone derivatives in 54–64% overall yields. The procedure is also applicable to nicotinaldehyde and furfural, although the yields are not satisfactory. 相似文献
17.
About ScienceDirect 《Journal of organometallic chemistry》1973,50(2):353-380
For the compounds C6H5C6H4YC6H4C6H5 and C6H5C6H4YC6H4C6H4YC6H4C6H5 where Y is either Si(CH3)2 or CH2, reduction potentials and p-band positions are reported. These data as well as the UV data for several phenyl derivatives are consistent with silicon blocking, to a large extent, conjugation (little or no through conjugation) of biphenyl moieties separated by Si(CH3)2groups. 相似文献
18.
《Polyhedron》2003,22(25-26):3333-3337
Low temperature reactions of dilute solutions of 1,5-dichloro-1,1,3,3,5,5-hexamethyltrisiloxane with boronic acids {RB(OH)2; R=nBu, C6H4Me-2, C6H4Me-3, C6H4Me-4, C6H4OMe-3, C6H4OMe-4, C6H4Br-2, C6H4Br-3, C6H4Br-4} in the presence of a twofold excess of Et3N afforded the 8-membered ring products, cyclo-boratetrasiloxanes, (RBO)(Me2SiO)3, in moderate yields. New compounds were colourless oils and were characterised by elemental analysis, NMR (1H, 11B, 13C, 29Si), IR and MS. The cyclo-boratetrasiloxanes are weakly Lewis acidic, with acceptor number (AN) values of ∼30, but do not form adducts with amines. 相似文献
19.
Calculations of the C3H6 · LiH, C4H8 · M+, and C4H8 · MH systems and of C2H2 · MH complexes (M = Li or Na) were carried out by the unrestricted Hartree-Fock-Roothaan (UHF) method with partial optimization of the geometry using fixed geometric parameters of the C3H6 and C4H8 molecules. The standard 3-21G and 6-31G* basis sets were used. Unlike the C3H6 · LiH structure, the C4H8 · M+ and C4H8 · MH systems are typical complexes. It was found that the C4H8 · M+, C4H8 · MH, and C2H2 · MH complexes are similar in coordination of M+ ions and MH molecules by carbon atoms in spite of considerable differences in the interatomic distances (–1 A) between these atoms in the C4H8 and C2H2 molecules. The heats of formation (Q), which were calculated in the UHF/6-31G* approximation and using second- and fourth-order Möller-Plesset perturbation theory taking into account the electron correlation energy in the MP2/6-31G*. MP4(SDQ)/6-31G*, and MP4(SDTQ)/6-31G* approximations, satisfy the following relationships: Q(C2H3 · MH) < Q(C4H8 · MH) < Q(C4H8 · M+). It was observed that in going from Li to Na the corresponding values of Q tend to decrease.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya. No. 7, pp. 1636–1640, July, 1996. 相似文献
20.
Dr. Changhua Song Dr. Fang Zheng Dr. Ying Liu Prof. Qiwei Yang Prof. Zhiguo Zhang Prof. Qilong Ren Prof. Zongbi Bao 《Angewandte Chemie (International ed. in English)》2023,62(51):e202313855
The one-step purification of ethylene (C2H4) from mixtures containing ethane (C2H6) and acetylene (C2H2) is an industrially important yet challenging process. In this work, we present a site-engineering strategy aimed at manipulating the spatial distribution of binding sites within a confined pore space. We realized successfully by incorporating nitrogen-containing heterocycles, such as indole-5-carboxylic acid (Ind), benzimidazole-5-carboxylic acid (Bzz), and indazole-5-carboxylic acid (Izo), into the robust MOF-808 platform via post-synthetic modification. The resulting functionalized materials, namely MOF-808-Ind, MOF-808-Bzz, and MOF-808-Izo, demonstrated significantly improved selectivity for C2H2 and C2H6 over C2H4. MOF-808-Bzz with two uniformly distributed nitrogen binding sites gave the optimal geometry for selective ethane trapping through multiple strong C−H⋅⋅⋅N hydrogen bonds, leading to the highest C2H2/C2H4 and C2H6/C2H4 combined selectivities among known MOFs. Column breakthrough experiments validated its ability to purify C2H4 from ternary C2H2/C2H4/C2H6 mixtures in a single step. 相似文献