Assessng the use of NMR chemcal shfts for predcton of VLE n nondeal bnary lqud mtures Q. Zhu, G. D. Moggrdge, T. Dalton, J. Cooper, M.D. Mantle, L. F. Gladden, C. D Agostno* *Correspondng Author: Dr Carmne D Agostno Department of Chemcal Engneerng & Botechnology Unversty of Cambrdge Pembroke Street Cambrdge CB2 3RA, UK Emal: cd419@cam.ac.uk Tel: +44(0)1223-761629 1
Abstract A method of estmatng vapour lqud equlbrum (VLE) usng NMR chemcal shft data has been proposed by Xu et al. (12). Ths method s based on the concept that the average local composton around each speces s determned by the thermodynamcs of the system, and also determnes the screenng of the NMR actve groups wthn that molecule, and so ther NMR chemcal shfts. Xu et al. s method has been replcated and verfed; results are confrmed to be accurate for alcohol + hydrocarbon mtures, gvng VLE predctons of comparable accuracy to the UNIFAC, generally consdered the best predctve actvty coeffcent model avalable. However, for more strongly non-deal mtures, the method becomes less relable, gvng sgnfcantly less accurate predctons of total pressure than UNIFAC. Several causes for ths are dentfed. The model proposed by Xu et al. (12) s unable to ft mnma or mama n chemcal shfts, whch are observed epermentally n some bnary mtures. Dfferent NMR resonances wthn the same molecule lead to dfferent predctons of VLE, clearly an un-physcal result. The thermodynamcs of strongly non-deal mtures are determned by more comple nteractons than a smple descrpton of average local composton around each component n the mtures, for eample strong and drectonal hydrogen bonds. Dfferent groups wthn the same molecule may have dfferent local compostons n ther mmedate vcnty; for eample n the case of alcohol + water mtures, one would epect a clusterng of water molecules around the hydroyl group but not the alphatc group. Hence, the concept of a smple local composton model s not vald for these more comple cases, and t s therefore not surprsng that a model based on ths smple concept s often not effectve n predctng VLE. Keywords: VLE, Wlson model, NMR spectroscopy, UNIFAC 2
Letter to the Edtor Knowledge of vapour-lqud equlbrum (VLE) data s essental n many technologcal applcatons, ncludng modellng of dstllaton columns and other separaton processes. In a recent paper, Xu et al. (12) reported that by measurng 1 H NMR chemcal shfts t s possble to accurately predct VLE data n alcohol + hydrocarbon bnary mtures. Startng from Wlson s local composton model (Wlson, 1964), they proposed an epresson relatng the measured chemcal shft of component, of the two components, accordng to:, obs, to the compostons, and 0 j j, obs (1) j j j j j where 0 and nfnte dluton, respectvely, and are the chemcal shft of the pure component and that at j s a Wlson parameter. Therefore, by measurng the NMR chemcal shfts of component and j as a functon of composton, and usng Equaton (1), and ts equvalent for component j, t s possble to obtan the Wlson parameters, and, whch can be used to calculate the actvty coeffcents, and j (Gothard et al., 1976), whch can then be used for VLE predctons. Xu et al. (12) used ths approach to calculate VLE data for several bnary mtures of alcohol + hydrocarbon mtures and good agreement between the predcted VLE and epermental data reported n the lterature was demonstrated. However, the proposed method rases some mportant questons that were not addressed n the orgnal paper. The method descrbed was proposed as an alternatve for the predcton of VLE data n cases for whch the data s dffcult to measure drectly. However, no dscusson of the range of valdty and applcablty was gven; wthout knowledge of ths, t s not possble to know a pror f the predcted VLE s accurate, and so the method s of lttle practcal value. Another ssue s whch chemcal shft should to be used n Equaton (1), and whether usng the chemcal shfts of dfferent groups n the same molecule wll gve the same result. Consder, for eample, a smple case dscussed by Xu et al. (12), a mture of methanol and benzene; although there s no problem about what chemcal shft needs to be used for benzene (snce t has only a sngle 1 H NMR resonance), a queston arses as to whether the chemcal shft of the methyl group of methanol could be used n Equaton (1), nstead of the chemcal shft of the OH group used by Xu et al. (12), and whether the same result s obtaned n ether case. We therefore carred out 1 H NMR chemcal shfts measurements for a varety of bnary mtures, n order to assess and ratonalse the range of valdty of the proposed model. We started by reproducng the results obtaned by Xu et j j 3
al. for 1-heanol + n-heane; predctons of pressure for ths system were very good and consstent wth the work of Xu et al. (12). We then appled the same approach to other types of bnary mtures. We frst focused on alcohol + water mtures and found that the model predcts well the VLE data for aqueous methanol mtures; predctons become less accurate for aqueous ethanol mtures. In the case of aqueous 1-propanol and 2-propanol, the predctons become rather poor. Ths trend of decreasng accuracy wth ncreasng sze of alphatc group of the alcohol reflects the etent of nondealty of the mtures. Indeed, beyond the propanols, full mscblty of alcohol and water does not occur; 1-butanol + water shows partal mscblty at room temperature. In addton, a dfference between the predctons based on the OH and the alphatc chemcal shft of the alcohol was also observed, partcularly for aqueous propanol mtures. Ths calls nto queston the valdty of the model, snce t s based on the average local composton surroundng each component of the mture, whch should be ndependent of the NMR resonance beng consdered. It seems that use of the OH chemcal shft of methanol and ethanol descrbes slghtly better the thermodynamcs of the system. Because n such cases the domnant nteracton n alcohol + water mture s that of hydrogen bondng, t s reasonable that use of the OH resonance of the alcohol gves a better predcton. For the aqueous propanols both analyses show a poor ft to epermental VLE data. An eample of VLE predctons s shown n Fgure 1, where we compare epermental VLE data wth those predcted by usng the model proposed by Xu et al. as well as UNIFAC, whch s a standard predctve tool for VLE data. The orgnal verson of the UNIFAC method was used (Fredenslund et al., 1975), wth nteracton parameters taken from Hansen et al. (1991) and Wttg et al. (03). 4
Total Pressure pressure [mm [mmhg] Total pressure [mmhg] Pressure [mm Hg] Total Pressure pressure [mm [mmhg] Total Pressure pressure [mm [mmhg] Hg] 1 1 100 (a) 65 60 55 50 (b) 80 45 60 0 Alcohol 35 30 25 (c) 35 30 25 Alcohol 55 50 45 35 30 (d) Alcohol 25 Alcohol Fgure 1. VLE data as total pressure for varous alcohol + water mtures usng the OH alcohol chemcal shft: (a) methanol + water; (b) ethanol + water; (c) 1-propanol + water; (d) 2-propanol + water. The symbols are: ( ) epermental values; ( ) predctons usng the chemcal shft data reported n ths work; ( ) predctons from UNIFAC. The epermental data for methanol + water are taken from Koner et al. (1980); the epermental data for the other alcohol + water mtures are taken from Hu et al. (03). We have also carred out eperments on the acetone + chloroform system, whch shows a negatve devaton from Raoult s law (see supportng nformaton S1). Agan, VLE predctons usng Xu et al. s chemcal shft method are not very accurate and more accurate predcton can be obtaned usng UNIFAC method. Another ssue that we have found s that some mtures, such as acetone + water, show a mamum n chemcal shft versus composton, whch cannot be ftted by usng the model proposed by Xu et al. (see supportng nformaton S2). In summary, we have confrmed that the model proposed by Xu et al. (12), summarsed as Equaton (1), gves relable predctons of VLE for alcohol + 5
hydrocarbon mtures. However, our results demonstrate that ths model s not of general valdty, wth predctons of total pressure becomng less accurate for ncreasngly non-deal mtures. The frst lmt s n the form of Equaton (1), whch cannot predct chemcal shft profles showng mama or mnma. Secondly, even when ecellent fts to the chemcal shft data can be acheved (for eample n the case of chloroform + acetone mtures), VLE predctons are often poor, partcularly for hghly non-deal mtures. Thrdly, sgnfcantly dfferent predctons are obtaned usng the chemcal shfts of dfferent resonances from the same molecule. Our results clearly suggest that f a relatonshp ests between NMR chemcal shft and local composton, t s the case that dfferent groups wthn the same molecule (e.g., alphatc and OH moetes n alcohols) have dfferent local compostons. Ths results n the varaton of chemcal shft as a functon of composton beng dfferent for the dfferent moetes wthn the same molecule. Ths s unsurprsng gven the dfferent energy of nteractons between water and the dfferent moetes of alcohols, alphatc or OH. Dfferent local compostons surroundng dfferent groups wthn a molecule are mplct n group contrbuton methods for predctng VLE, such as UNIFAC. It s possble that Xu et al. s model could be refned by applcaton of such a group contrbuton method, but ths would only be practcal n cases where the NMR chemcal shft of all groups could be observed. We have confrmed that Xu et al. (12) model s effectve for predctng VLE of alcohol-hydrocarbon mtures (although t s no more accurate than UNIFAC). However we have shown that the method does not have general valdty for other bnary lqud mtures. From a practcal pont of vew, undermnes ts value, snce one cannot know a pror whether Xu et al. s model wll be accurate for VLE or not. Establshed methods such as UNIFAC reman more relable for the predcton of VLE n the absence of epermental data. Nonetheless Xu et al. s fndngs have consderable nterest snce they rase (and partally answer) the queston of the relatonshp between local composton n actvty coeffcent models and ts effect on chemcal shft n NMR measurements. Based on our fndngs, t could be of nterest to etend the approach proposed by Xu et al. to predct UNIFAC group contrbuton parameters. These are related to ndvdual groups wthn a molecule, and so t s reasonable to assume that these should be related more drectly to the chemcal shfts of those same groups. Acknowledgments C. D Agostno would lke to acknowledge Wolfson College, Cambrdge, for supportng hs research actvtes. 6
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