Changhong Yu,Litao Chen*,Baojiang Sun

School of Petroleum Engineering,China University of Petroleum(East China),Qingdao 266580,China

Keywords:Clathrate hydrate Cage occupancy Experimental characterization PXRD Raman NMR

ABSTRACT Study on the microscopic structure of clathrate hydrate has made significant progress in the past decades.This review aims to summarize the state of the art of the experimental characterization of guest molecular occupancy in clathrate hydrate cages,which is an important area of the microscopic structures.The characterizing method and features of different guest molecular,such as hydrocarbon,carbon dioxide,hydrogen and inhibitor/promoter,in different hydrate cages have been extensively reviewed.A comprehensive use of advanced technologies such as X-ray diffraction,Raman spectroscopy and nuclear magnetic resonance may provide better understanding on the compositions and microscopic mechanisms of clathrate hydrate.

1.Introduction

Clathrate hydrate is a type of non-stoichiometric cage-like crystalline compound formed by water molecules and other low molecular weight molecules such as methane,ethane,carbon dioxide or hydrogen under certain temperature and pressure conditions [1-3].Water molecules(host)form hydrogen bond linked cage-like framework in which other low molecular weight molecules(guest)are enclathrated in.It is believed that,the guest molecule in hydrate cages can freely rotate and vibrate,but the translational motion of guest molecules is limited[1-3].

In 1811,Sir Humphrey Davy first reported the discovery of gas hydrate.He noticed that an ice-like solid formed in the solution of water and chlorine gas (known as oxymuriatic gas at that time)at temperatures above the ice point [2].And in the late 19th century,Villard first confirmed the existence of hydrocarbon hydrates including methane hydrate,ethane hydrate and propane hydrate[2].In 1951,von Stackelberg confirmed the hydrate of structure II which was presented by Claussen.Then in 1952,the hydrate of structure I was also confirmed by Claussen et al.[2]For hydrate mixture,the coexistence of structure I and structure II hydrates was first studied by Holder in 1976 [2,3].Ripmeester et al.reported the structure H hydrate in 1987[4].

The lattices of sI and sII hydrate have two types of cages with different sizes(see in Fig.1).For sI hydrate,the small 512cage is composed of twelve pentagonal faces.The large tetradecahedral 51262cage contains twelve pentagonal faces and two hexagonal faces.For sII hydrate,the small 512cage also exists in addition to the large quasi-spherical 51264cage which is composed of twelve pentagonal faces and four hexagonal faces,but the 512cage in sII hydrate is smaller than that in sI hydrate in diameter[2].The sH hydrate comprises three different cages including the small 512cage,middle sized 435663cages(cages which contain three square,six pentagonal and three hexagonal faces)and large 51268cages(cages which contain twelve pentagonal and eight hexagonal faces)[2].

In general cases,one cage can only accommodate one guest molecule(two or more small molecules like hydrogen molecules can also be accommodated under high pressure).It is known that clathrate hydrate is a non-stoichiometric crystalline compound,and some cages are possible to be vacant when the occupancy of guests are sufficient enough to stabilize the hydrate lattice[3].

The type of hydrate structure shows strong dependence on the size of guest molecules.For example,the size of CH4molecule is suitable for both the large and small cages in sI hydrate,while C3H8molecules are too large to occupy the cages of sI hydrate but can occupy the large cages of sII hydrate.Therefore the common structure of CH4hydrate is sI,while the common structure of C3H8hydrate is sII.The sI hydrate is formed with guest molecules of which the diameters are between 0.42 nm and 0.6 nm(0.42 nm ＜d ＜0.6 nm)such as CH4[1].The small 512cage in sII hydrate is smaller than that in sI hydrate in diameter[2,6],and for small molecules(d ＜0.42 nm)such as N2,they can stabilize the small cage of sII hydrate as single guests and therefore form the sII hydrate.It is very difficult for very small molecules such as H2to form hydrate as single component unless at very high pressure [5].However,H2can form sII hydrate with the help of hydrate promoters such as THF under relatively low pressures[7].The middle sized single molecules(0.6 nm ＜d ＜0.7 nm)such as C3H8will also form sII hydrate because they can only occupy the large cages of sII hydrate.The large molecules(d ＞0.7 nm)such as cyclohexane with accompany of small molecules may form sH hydrate(Table 1)[1].

Fig.1.The cages in gas clathrate hydrates:(a)pentagonal dodecahedron (512),(b)tetrakaidecahedron (51262),(c)hexakaidecahedron (51264),(d)irregular dodecahedron(435663),and(e)icosahedron(51268).(Sloan[1],1998).

For gas mixture,the competition among guest molecules in cage occupancy may make difference in hydrate structures.The structure type of the gas mixture formed hydrate may differ from the structure types of single gas formed hydrate.The early studies on the natural airhydrate(N2+O2hydrate)showed that the structure of mixed gas hydrate is of sII which is in agreement with the structure type of hydrate with the single guest N2or O2[8].However,the result of the first single crystal diffraction studies by Kirchner et al.[9]on CH4,C3H8,CH4/C3H8,and adamantane gas hydrates shows that the pure CH4hydrate and adamantane hydrate are both of sI,while the mixed gas CH4+adamantane hydrate was found to be of hexagonal structure.And it was found that most cages are fully occupied by guest molecules for most hydrates expect the 512cages(60%occupied)for the acetylene hydrate.

Besides,some previous work showed that the composition of gas mixture may also result in different hydrate structures.By using a combination of NMR and Raman spectroscopy on the CH4+C2H6hydrate,Subramanian et al.[10]observed different structure types(sI or sII)by changing the composition of CH4+C2H6gas mixture.The formation of sII hydrate in system with two sI hydrate formers(CH4and C2H6)may be caused by the reduced competition between guest molecules on large cage occupancy and the abundance of small cages for sII hydrate.

The occupancy preferences of guest molecules within the hydrate cages make effect in hydrate structures.Therefore,it is of great significance to study the occupancy of guest molecules within the hydrate cages.Some previous work on the guest molecular cage occupancy and distribution has been done.Udachin et al.[11]used single X-ray diffraction to study the cage occupancy of sI,sII,sH hydrate.It was found the small cages of CH4hydrate are weakly occupied(about 5%),which wasn't observed in the previous NMR measurement.The tilt angle between the C--C bond of C2H6and the equatorial plane of large cages is determined to be at 23°,which is in good agreement with the result of NMR measurement.Besides,some other detailed structural information like lattice parameters is also reported.Also by SXRD,the occupancy of cages and the guest distribution in CO2hydrate were studied by Udachin et al.[12]By analyzing the absolute occupancy,the hydrate composition was determined as CO2·6.20(15)H2O.For the position of guest molecules,it was confirmed that CO2molecule is offcenter in the large cage and has a bimodal distribution of out-of-plane orientations for the long axis of CO2.Similar results on the cage occupancy for sI CH4+CO2hydrate can also be obtained in the early Raman studies[13].

It is widely accepted that there are three common types of clathrate hydrate structures,which include cubic structure I(sI),cubic structure II(sII)and hexagonal structure H (sH)(see in Fig.2).But it has been reported that the semi-clathrate hydrate such as TBAB hydrate(see in Fig.3)with the composition C16H36N+·38H2O can also accommodate small gas molecules,which indicates that the semi-clathrate hydrate may also be a potential material with gas storage capability[14].Therefore,some uncommon types of hydrate structures(semi-clathrate structures,Jeffrey's structures III-VII)are also mentioned in this review to give a comprehensive introduction of recent studies on cage occupancy of hydrate.

As a fundamental scientific issue in the area of hydrate research,the studies on the guest molecular occupancy in hydrate cages are of great significances in the fields of gas storage and separation,natural gas hydrates exploitation and flow assurance in offshore oil and gas industries.With the application of advanced characterizing technologies including Raman spectroscopy,nuclear magnetic resonance (NMR)measurements,X-ray diffraction,Neutron diffraction and Infrared spectroscopy(IR),cage occupancy of different components in hydrate cages has been comprehensively characterized.This review summarizes the recent results on cage occupancy and guest distributions of gas hydrate.A better understanding on the cage occupancy will be helpful in the explanation of both thermodynamic and kinetic properties.

2.Studies on Guest Molecular Occupancy in Hydrate Cages

Several advance technologies including Raman spectroscopy,nuclear magnetic resonance (NMR)spectroscopy,X-ray diffraction(XRD),Neutron diffraction and Infrared spectroscopy(IR)have been used to study cage occupancy in clathrate hydrate cages.Featured spectra of guest(host)molecules at different vibrating modes can be used to determine the occupancy in different hydrate cages.Among the technologies,Raman spectroscopy is often considered as the primary method for cage occupancy study since it is simpler and less resource intensive.NMR spectroscopy allows the implementation of quantitative analysis on cage occupancy of guest molecules.X-ray diffraction is usually considered as the primary method to identify of the type of hydrate structures.As a powerful tool for crystallographic study,XRD can provide the detailed information on the positions of host (guest)molecules.With the molecule position data,occupancy in hydrate cages can be estimated.For single crystal XRD,to make the analysis of hydrate structure composition more accurate,high quality single crystal which is very difficult to form is required[3].

These techniques for microscopic structural characterization have been widely used in hydrate studies and are usually used in combination with each other.This review covers the microscopic composition study on hydrate systems with different guest components including hydrocarbon,CO2,H2and semi-clathrate hydrate,natural gas hydrate,inhibitor/promoter containing/formed hydrate and so on.

2.1.Hydrocarbon hydrate

Hydrocarbon hydrate is the most important system in cage occupancy and composition studies.Flow assurance and natural gas hydrate resource exploitation is the most demanded and promising area of hydrate research,respectively.In addition,hydrocarbon mixtureseparation and natural gas storage by hydrate are also promising technologies.All of these need a better understanding on the cage occupancy and composition of hydrocarbon hydrate.The strong dependence of basic properties such as heat capacity and dissociation pressure on hydrate structures requires detailed cage occupancy information.Other properties related to cage occupancy such as density and composition of hydrate also matter to flow assurance and gas storage.Therefore,it is of great significance to study the occupancy of guest molecules in hydrocarbon hydrates to provide fundamental knowledge for the advances of study in these areas.The structures of mixture formed hydrate and the structure transitions have been widely studied for hydrocarbon hydrate in the past ten years.Changyu Sun et al.[15]used Raman spectroscopy(see in Fig.4)to characterize the structure transition of CH4+C2H6+THF formed hydrate.The coexistence of sI hydrate dominated by ethane and sII hydrate dominated by THF was observed when the concentration of THF was 0.06 mol fraction in the aqueous solution.However,only sII hydrate existed when THF concentration was increased to 0.10 mol fraction.Higher THF concentration may inhibit the formation of sI ethane dominated hydrate and reduce the occupancy of ethane in cages.These features can be used to improve the separation efficiency of ethane from methane-ethane mixtures.Hiroshi Ohno et al.[16]used Raman spectroscopy to determine the hydrate structure in CH4+C2H6system and study the transformation between metastable structures and stable structures.The coexistence of sI and sII hydrate was observed in both the 65%and 93%(CH4)mixture formed hydrate.It was found that the transition rate of metastable to stable hydrate structure is up to the gas composition in system.The structure transition varies from hours to a week for different CH4fractions.The higher CH4concentration,the faster transformation of metastable to stable structures.Besides,the addition of kinetic inhibitor PVCap was found to affect the transformation process of metastable structures by promoting the metastable transformation at 65%CH4concentration and reducing it at 93%CH4concentration.Some evidence shows that the pore structure will lead to the preferential occupation for the heavy hydrocarbon molecules within the hydrate cages,which can be applied for the development of hydrate-based technique for gas transportation and storage.To evaluate its effect,the NMR spectroscopy can provide quantitative analysis on cage occupancy of guests for hydrates.Therefore,Yutaek Seo et al.[17]used13C NMR(see in Fig.5)to study the structure and composition of hydrate formed from natural gas mixtures both in bulk water and silica gel.The natural gas mixture contains CH4(89.86%),C2H6(6.40%),C3H8(2.71%),and isobutane(1.03%).It was found that,a sII hydrate was formed in the early stage and a sI hydrate was formed in the later stage,dominated by composition of residual gas mixtures.Heavy hydrocarbon molecules only occupy the large cage while methane molecules occupy both large and small cages in sI and sII hydrate.The presence of silica gel enhanced the preference of heavy hydrocarbon occupancy,about 73%of the large cages in sII hydrate were occupied by heavy hydrocarbon molecules(C2H6,C3H8,isobutane),and CH4only occupied 19%of the large cages.The occupancy in bulk water was 28%and 68%,respectively.The preference of cage occupancy may be an advantage for gas mixture separation but may also be an important characteristic for gas storage and transportation.As an important area of the hydrate-forming condition which can affect the behavior of guest molecules,the different composition of hydrate-forming hydrocarbon mixture will contribute to the different hydrate structure.So some work on the characterization of the guest occupancy within the hydrate cages has been done.Yutaek Seo et al.[18]used13C NMR to study the structure of hydrate formed from natural gas in the presence of large molecule guest substance neohexane(NH).The synthetic natural gas contains CH4(89.86 mol%),C2H6(6.40 mol%),C3H8(2.71 mol%),and iso-butane(1.03 mol%).When the concentration of NH is below 1.7 wt%,the structure of synthetic natural gas hydrate was found to be sII,and NH only served as an inhibitor.And when the concentration of NH is in the range of 7.8 wt%-14.5 wt%,they observed the coexistence of sH and sII hydrate.And with the more addition of NH,the proportion of sH hydrate gradually increased.For sH hydrate,the cages were occupied only by CH4and NH,and other gas components didn't participate in the formation of sH hydrate.Guests C2H6,C3H8and iso-butane were only found in large cages of sII hydrate.It is true that NMR spectroscopy is an effective way to characterize the hydrate structure quantitatively,but more detail information can be obtained by a combination of characterization methods.Apart from the13C NMR measurements,also by using PXRD,Kida et al.[19]studied guest occupancy within the hydrate cages and the effect of hydrate guest composition on the lattice parameters and density for sII CH4+butane(n-butane and i-butane)double hydrates.It was found that the large cages in the sII hydrate were almost 100%occupied by methane and butane,and the guest butane molecules prefer to occupy the large cages of sII hydrate.When the butane composition increases,the guest occupancy of small cages will decrease.The lattice constant will be in the range of about 1.715-1.721 nm when the butane composition varies in the range of 20.9%-37.8%.And it was found that the lattice constants will also increase with the increase of the butane composition,which indicates that the change of guest distributions in sII CH4+butane hydrates will lead to the expansion of hydrate lattice.By analyzing the lattice parameters and guest occupancy,the densities were estimated,and it was found that the lattice expansion may lead to the lower density of hydrate.This study provides a better understanding on the effect of guest composition on hydrate lattice.

Table 1 Structural and cage information for common hydrate structure(sI,sII,sH)(Koh and Sloan[6],2007)

Fig.2.Common clathrate hydrate structures(sI,sII sH)(Kwon et al.[6]2007).

Fig.3.The structure around the TBA+cation located at the centre of four cages；two 512 cages and two 51262.The butyl groups have two possible positions,with occupancy factors of 50%each.The 512 cages for TBAB hydrate are empty.Therefore,small gas molecules could be encaged,as shown by the shaded areas(Shimada et al.[14]).

Fig.4.Raman spectra of the C-H stretching in the cages of mixed(CH4+C2H6+THF)hydrate(Sun et al.[15]2010).

Fig.5.13C MAS NMR spectra of the coexisting sI CH4hydrate and sII mixed hydrate.(Seo et al.[17]2009).

For hydrates formed by some hydrocarbons with structural isomers,the different host-guest interaction caused by the structural isomers may contribute to the difference in guest occupancy within the hydrate cages.Thus,it is of great significance to characterize the hydrocarbon(with isomers)hydrate to estimate their possible applications.Takeya et al.[20]used PXRD,Raman spectroscopy and MD simulation to study the distribution of guest butane molecules (iso-C4H10and n-C4H10)in sII hydrate.According to the result of PXRD on simple iso-C4H10hydrate,n-C4H10+CH4hydrate and iso-C4H10+CH4hydrate,they confirmed the formation of sII hydrate.And they found that the lattice constants of these three hydrates are almost the same at the temperature of 100 K,and the simple iso-C4H10hydrate has the smallest thermal expansivity.For n-C4H10+CH4hydrate,82% of the large cages were occupied by n-C4H10,and 18%of the large cages and 97%of small cages were occupied by CH4.For iso-C4H10+CH4hydrate,99%of the large cages were occupied by iso-C4H10,and 1%of the large cages and 90%of small cages were occupied by CH4.According to the results of Raman spectroscopy and MD simulation,it was found that the n-C4H10molecules are more favorable to become gauche conformation in 51264cages.Besides,by using the Rietveld analysis and direct-space method,they found that there is no distortion in the empty small 512cage for simple iso-C4H10hydrate,and spherical extention of the dynamic disorder for iso-C4H10and gauche-n-C4H10was also found in 51264cages.Therefore,these isomers of guests were engaged in the cages with the same way.Compared with the C4H10molecules in 51264cages,the CH4molecules in small 512cages have little effect on the structure of iso-C4H10hydrate.

From the above result,it can be seen that the guest occupancy within the hydrocarbon hydrate cages can be affected by many factors including the gas composition,the effect of structural isomers and the presence of pore structures.And a summary can be made that the type and composition of guest substances is the dominating factor for the stable hydrate structures.Thermodynamic preference may be the intrinsic reason of the transition between hydrate structures.Besides,some external factors such as the presence of silica gel pores in the hydrate system can also cause the preference of the guest molecules on the cage occupancy.

The formation and dissociation process of hydrates plays a fundamental role for the study on hydrates,and its mechanism are of importance to many applications of gas hydrate.For example,the hydrate stability is the key in the field of hydrate-based transportation technique.And some hydrate development methods like thermal methods,depressurization and inhibitor method are all based on the hydrate dissociation process.And hydrate formation and dissociation also plays an important role in the field of flow assurance.For the insight fundamental mechanism of hydrate formation and dissociation process,the information on the hydrate structure and the structural changes which matters the gas storage capability and stability of hydrate is required.By analyzing the temperature or time-dependent cage occupancy,the formation and dissociation rate of hydrate cages can be obtained to study the kinetic properties of hydrates microscopically.Structural characterization is powerful method for the study of kinetic process during hydrate formation or dissociation.Arvind Gupta et al.[21]used temperature-dependent13C NMR(see in Fig.6)to study the dissociation mechanism of sI CH4hydrate.By measuring the cage occupancy and pressure,it was found that large to small cage occupancy ration remained the same during the sI CH4hydrate dissociation.Therefore it was concluded that the hydrate dissociate as a whole and the decomposition of large and small cages in sI CH4hydrate is at the same rate,which is different from the earlier formation of small cages during the formation of CH4hydrate.However,the cage decomposing rate is guest molecule dependent.Steven F.Dec et al.[22]used Timeresolved13C NMR to study the structural changes during the dissociation process of sI CH4+C2H6hydrate.The result showed that CH4occupy both the large and small cages while C2H6occupy large cages.And it was found that the decomposition speed of ethane occupied sI large cages is much higher than that of methane occupied sI small cages.Masato Kida et al.[23]used PXRD(see in Fig.7)and13C NMR spectroscopy to characterize the structure change during the dissociation process of CH4+C2H6hydrate at the temperature below 223 K.The coexistence of sI and sII hydrate was confirmed by PXRD.It was found that the decomposition of cages with guests CH4is at the same rate with that of cages with guests C2H6at 198 K and 203 K.However,when the temperature is higher than 208 K,the cages with guests CH4will decompose faster than cages with guests C2H6.In this way,the re-formation of sI C2H6hydrate in a large amount was observed.

Fig.6.Time-resolved 13C MAS NMR during sI CH4hydrate dissociation；The temperature changed from 269 to 271 K.；time resolution was 5.2048 s(Gupta et al.[21]2007).

Fig.7.PXRD profile of the CH4+C2H6mixed gas hydrate at 146 K.(Kida et al.[23],2010).

It should be notable that,during the hydrate dissociation process,the self-preservation phenomenon may be observed.And it may be reasonable to suppose that the self-preservation will be more significant at lower temperatures,and therefore the dissociation rate of hydrate will decrease with the decreased temperature at atmospheric pressure.To study the microscopic mechanisms and control factors of the selfpreservation effect during the hydrate dissociation process,the analysis on the guest-host interactions and guest occupancy is required.Zhong et al.[24]used in situ Raman spectroscopic analysis to study the selfpreservation phenomenon and structural transition during the dissociation process of pure CH4hydrate and CH4+C2H6hydrate at atmospheric pressure below the temperature of the ice point.By analyzing the AL/ASvalue which represents the ratio on occupancy of guest molecules in large cages to small cages and the stable C-H region Raman peaks,it was found that when the temperature is in the range of 268.15 to 270.15 K,the self-preservation phenomenon of sI CH4hydrate is very obvious.And for CH4+C2H6hydrate,the self-preservation phenomenon was found when the temperature is below 271.15 K.and when the temperature rises from 269.15 K to 271.15 K,by analyzing the shift of Raman peak and the changes of relative peak intensities,it was found that the hydrate structure will transit from sI to sII.Besides,further studies indicated that the selectivity for self-preservation which increases with the lower temperature may contribute to the structural transition.Thus,it can also be seen that the temperature or time-dependent structural characterization technique is an effective way to study kinetic process of hydrocarbon hydrate formation or dissociation,and this kind of technique has been widely adopted to study the overall kinetics process of hydrate by analyzing the structural change.

During the hydrate production process,when the development methods like thermal methods,depressurization and inhibitor method which are all based on the decomposition process of hydrate driven by external simulations are limited by the environment conditions,it has been reported that the replacement technique for the development of CH4from natural CH4hydrate by using alternate media such as CO2may be an optional way.Specially,for the purposed of natural gas production from hydrate reservoirs,the structural transition and the behaviors of guest molecules within the hydrate cages during the replacement process of CH4hydrate with CO2has been extensively studied.Youngjune Park et al.[25]used FT-Raman spectrometer and13C NMR to characterize the swapping process in CH4hydrate(sI,sII,and sH)+CO2system.The replacement processes between CO2/N2+CO2and sII CH4+C2H6hydrate/sI CH4hydrate/sH CH4+isopentane hydrate were studied.It was found that the CH4hydrate structure change from sII and sH to sI,and the addition of N2in the system can promote the replacement between CH4and CO2.In the swapping process,the preference on the guest occupancy for hydrate cages may play an important role,and both qualitative and quantitative measurements should be carried out.Kyuchul Shin et al.[26]used13C NMR and Raman spectroscopy to analyze the structural transformation and guest behaviors of hydrate during the swapping process in CO2atmosphere.They first synthesized sH CH4+isopentane hydrate,then put the hydrate into the CO2system and CO2+N2system.The structural transformation of sH to sI was observed.And it was found that the N2in the system will attack the CH4in small cages of sH hydrate and promote the structural transformation.Minjun Cha et al.[27]used13C NMR to study the occupancy of CH4molecules in CH4hydrate during the swapping process in CO2+N2system.According to the result,they evaluated the replacement effect.The replacement ratio in small cages is higher than that in large cages,but the amount of replaced CH4molecules in large cages is more than that in small cages.

According to the studies on the swapping process between CO2molecules and CH4hydrates,it can be natural to assume that the structural transition which is caused by the affinity on the specific-cage occupation of guests will reduce the number of the small cage sites.The addition of N2molecules which can attack the CH4within small cages can contribute to this structural transition and lead to a considerable CH4recovery rate.

For structural characterization technique,the Raman spectroscopy,when used alone,is recommended for its simple application in monitoring hydrate formation although the results need to be calibrated by quantitative method.To make the result obtained from Raman spectroscopy more accurate,some extra work is required.Qin and Kuhs et al.[28]provide a calibration procedure to make quantitative analysis on the cage occupancy of guest molecules,the composition and the hydrate number of the gas hydrates by Raman spectroscopy in deuterated and hydrogenated sI CH4hydrate system,sI CO2hydrate system and sI C2H6hydrate system.According to comparing the results of the Raman spectroscopy with the absolute cage fillings established by synchrotron X-ray powder diffraction,it was found that the Raman scattering cross sections of the CH4molecules engaged in 512cages and 51262cages are different,which matters the determination of hydration number.By comparing the Raman spectroscopy of sI gas hydrate to the mixed system,they derived the empirical Raman quantification factor.And with some additional assumptions,the Raman quantification factor can be used to determine the cage occupancy of guest molecules,the composition and the hydrate number of the gas hydrates,which is more accurate compared with the available values for calibration.

For multi-component clathrate hydrate from gas mixture system,the formation of hydrate may be different from the hydrate formation in the single-guest hydrate system due to the thermodynamics and kinetic reasons.Though it's true that the occupancy of each guest within the different hydrate cages for the multi-component gas system can also be studied microscopically by in-situ Raman spectroscopy,the single structural characterization may have some limitations to get a more accurate quantitative result in detail.Therefore,a combination of various techniques is the general trend for structure study.Aims to design method for CH4+C2H6+C3H8storage and transport facilities,Rajnish Kumar et al.[29]used a combination of Raman spectroscopy,PXRD and13C NMR to determine the phase composition of hydrate and cage occupancy of guest molecules in CH4+C2H6+C3H8hydrate system.The formation of sII hydrate was determined by PXRD at 2.8 MPa and 270 K.The space group of the sII hydrate is Fd3m,unit cell constant is 1.723 nm,and unit cell volume of 5.115 nm3.13C NMR measurement shows the large cages are almost fully occupied by a combination of all three gases while the small cages were about 90%occupied by methane.The Raman spectra gave similar cage occupancy information to that from NMR.Thus,the combination of various structural characterization technique can be adopted to give a verification on the results as well as more structural information in detail.

Besides,progress of new methods and in new field has also been made.To analyze the occupation of guest molecules within the hydrate cages,NMR measurement is an effective method which can give a quantitative result.And NMR chemical shifts are regarded as the key information for NMR measurements which are used to evaluate the electronic environment of guest molecules as well as the host-guest interactions.So it is of great significance to study the NMR chemical shifts caused by the host-guest interactions.Faisal Masato Kida et al.[30]used13C NMR spectra to study the13C chemical shifts of guest C3H8molecules in the C3H8hydrate.They reported the inversion of13C chemical shifts caused by propane-water molecule interaction.It was speculated that the inversion may result from the difference of the deshielding effect of methyl carbons and methylene carbon in guest C3H8molecules in the hydrate.Similarly,Raman spectroscopy has been used to analyze vibration modes of guest molecules enclathrated in hydrate cages,and by distinguish the difference of Raman frequencies for different cages,Raman bands can be used to identify the formation of hydrate and to investigate cage occupancy of hydrates.So some work has been done to analyze the vibration modes for Raman spectroscopy.Al-Otaibi et al.[31]used Raman spectroscopy to analyze the structure of hydrate formed in C2H6+C3H8system.With the frequency peaks of C--C stretching,it was determined that the C2H6molecules only occupy the large cages in the formed sII hydrate.Hiroshi Ohno et al.[32]used Raman spectroscopy to study the vibration mode of CH4in sI,sII and sH hydrates.The three structures hydrate was synthesized from CH4(sI),CH4+C2H6(sII),and CH4+deuterated methylcyclohexane(sH),respectively.Raman bands of CH4in sH with the presence of large melocule guest substance was firstly reported.The C--H symmetric stretching vibration mode of guest CH4molecules differs among structures even for the same host cages.The Raman spectroscopy and NMR spectroscopy are both powerful technique on the determination of guest occupancy which is an important part for hydrate structure.Hence,these studies can give the insight understanding on the nature of host-guest interactions for hydrates and the mechanism of the guest occupation within the hydrate cages(Table 2).

2.2.CO2hydrate

As a greenhouse gas,CO2was found to contribute to about 64%of the greenhouse effect,and CH4is also a greenhouse gas with greenhouse effect which is 21 times greater than that of CO2and contributes about 18% of the greenhouse effect.The hydrate formation can be adopted for the gas storage and separation to reduce the greenhouse effect,which can be an important application for the hydrate-based technique.And gas hydrate can also be a promising media for the transportation of CO2which is needed for industrial uses.For these practical applications of gas hydrates,the deep understanding on structural identification and cage occupancy of guests within the hydrate cages should be of particular significance.

Therefore,studies on the CO2hydrate structure mainly focus on the cage occupancy in hydrate formed from CO2containing mixtures.And the structural characterization methods are very effective methods to give the molecule-scale structural information which will help understand the occupation of guest molecules.

Table 2 Studies on clathrate hydrates for hydrocarbon/hydrocarbon+other substances mixture systems

For CO2+CH4hydrate,the studies on the occupancy of guest molecules within the hydrate cages may be instructive for the control of greenhouse effect.By using PXRD,Yun-Je Lee et al.[33]determined the structures of CO2+CH4(50:50)gas formed hydrate in pure water and THF aqueous solution are sI and sII,respectively.Raman spectroscopy was used to study the CO2occupancy in hydrate cages.For sI CO2+CH4hydrate,the space group is Pm3n,and the unit cell parameter a=1.1869(2)nm,and CO2molecules will preferentially occupy the large 51262cages.For sII CO2+CH4+THF hydrate,the large cages were fully occupied by THF molecules,the space group is Fd3m,the unit cell parameter is a=1.7224(2)nm,and CO2molecules can only occupy the small cages.

In this study,the measurements on the hydrate with the presence of THF molecules should be notable.In fact,THF molecules alone can form sII hydrate and can only the large 51264cages of sII hydrate due to the large molecular size.And it was found that the THF molecules can also form sII hydrate with the participation of small guest molecules such as CH4and CO2.According to the performance of gas separation and thermodynamic stability of the hydrate,THP may be a good hydrate promoter for the technology on hydrate-based gas separation.So some hydrate promoters such as THF or tetrahydropyran(THP)were employed in many studies on the guest occupancy for hydrates formed in the CO2+CH4system to assess its gas storage and separation effect.Base on the phase equilibrium data,Iino et al.[34]used PXRD to characterize the CO2+THP hydrate and CH4+THP hydrate.The structure of these hydrates was found to be sII.And by using Rietveld method,the occupancy of CO2and CH4were studied.Assuming guest molecules to be in the center of small cages,the occupancy of CH4in small 512cages was determined to be 98%and 100%,and the occupancy of CO2in 512cages was found to be 59%.The results of PXRD and phase equilibrium data indicate that CH4molecules are easier to form hydrate compared with CO2molecules.Litao Chen et al.[35]used Raman spectroscopy to study the CO2characters in hydrate cages.By exposing THF hydrate in CO2atmosphere,THF+CO2hydrate in which CO2only occupy the small cage of sII hydrate is prepared.The Fermi dyad peaks of CO2in small cage are determined to locate at 1275 cm-1and 1382 cm-1.Compared with feature peaks in large cage,CO2does not follow the loose cage-tight cage model.Explanations are speculated as the linear guest molecule does not completely sample the space of non-spherical cages and the H-bonding may contribute to the guesthost interactions.

According to the studies above,it can be seen that the CH4may be much easier to form hydrate than CO2based on the cage occupancy information of the CH4+THP hydrate and CO2+THP hydrate,but CO2molecules may have higher affinity on the large 51262cages according the guest occupation for CH4+CO2hydrate.

For the combustion flue gas mixture,N2and CO2are the major components,and SO2,NOx,CO,O2are minor components.Also for the control of the greenhouse effect which is related with the CO2emission,based on the higher affinity of CO2over other gases on the hydrate cages,the hydrate formation can be an effective way to separate CO2from the flue gas.In this way,the hydrate will be CO2-enriched and the concentration of CO2in flue gas will decrease significantly.Thus,some work has been done to study the structure of hydrate with guest flue gas molecules.Yohan Lee et al.[36]adopted Powder X-ray diffraction and Raman spectroscopy to determine the structure of CO2+N2hydrates.The formation of sI CO2+N2hydrate was observed.The cubic space group with a lattice parameter a=1.191 nm is Pm3n.Besides,it was confirmed that the hydrate structure will not change when the proportion of gas composition changes in CO2+N2system.For the CO2+SO2and CO2+H2S mixture formed hydrate,Litao Chen et al.[37]used Raman spectroscopy to study the CO2occupancy in sI and sII hydrate.Raman peaks of CO2in gaseous,solid,large cage if sI and small cage of sII were experimentally determined.By giving the structural information of hydrates with guest CO2molecules,these studies may also provide insight understanding on the mechanism of hydrate formation technique for the greenhouse effect control.

Table 3 Studies on clathrate hydrates for CO2/CO2+other substances mixture systems

It has been determined that the presence of pore structure may lead to the preference on cage occupation for guest molecules in hydrates.And compared with the hydrate formation in the bulk water,the hydrate formation in pore structures such as silica gel pores may be with an enhanced separation effect and a higher formation rate.Hence,the characterization on the guest occupancy within the hydrate cages should be carried out.By PXRD and Raman spectra,Sungwon Park et al.[38]determined the hydrate structure formed from H2(60%)+CO2(40%)and H2(60%)+CO2(40%)+THF system in silica gel pores.The hydrate structure formed from H2(60%)+CO2(40%)system was sI,and when THF was added,the formation of sII hydrate was observed.Yutaek Seo et al.[39]used13C NMR spectra to study the composition of CO2hydrate which is formed from H2+CO2mixtures in silica gel for the purpose of CO2separation.It was found that composition of sI hydrate formed in the silica gel pore is(0.14H2/1.86CO2)S·(6CO2)L·46H2O with 93%of small cages and all large cages occupied by CO2molecules.From the result of guest occupancy within the hydrate cages,it was shown that the separation effect in CO2+H2by the formation of CO2hydrate is very significant.Therefore,the hydrate-based separation of CO2in silica gel pores may be a promising method for the precombustion CO2capture in CO2+H2gas system.

Similar to hydrocarbon hydrate,the structure and guest occupancy of hydrate formed from CO2containing gas mixtures may also be dominated by the guest compositions.Everett et al.[40]used high-resolution neutron diffraction to study the structure and guest occupancy of binary CH4+CO2hydrates.The 5 hydrate samples with nominal gas amounts CH4(100%),CO2(25%)+CH4(75%),CO2(50%)+CH4(50%),CO2(75%)+CH4(25%)and CO2(100%)were studied.The different composition for the gas mixture was found to lead to the different cage occupancy for the CO2+CH4hydrate.And it was found that the lattice constants of hydrates in CO2-rich system are smaller because of the affinity on the interaction between guest CO2and host H2O molecules.So the unit-cell volume is smaller for CO2-rich hydrate.And the change of cage occupancy and energy landscape with the influence of composition was also studied by analysis of experimental nuclear scattering densities.The results are instructive for mechanisms on the structures of mixed CH4/CO2gas hydrate for different gas compositions.Yohan Lee et al.[41]used Powder X-ray diffraction and Raman spectra to determine the hydrate structure in CO2+N2+neohexane(NH)system.Gas mixture composition may affect the hydrate structure at stable.For hydrate structurein CO2(10%)+N2(90%)+NH system,the unit cell space group is P6/mmm and lattice parameters are a=1.225 nm and c=1.010 nm.For CO2(20%)+N2(80%)+NH system,lattice parameters are a=1.225 nm and c=1.014 nm.Therefore the hydrate structure is sH at low CO2concentration.The coexistence of sI and sH hydrates was determined for the CO2(40%)+N2(60%)system,while only sI for CO2-rich gas mixtures.For cubic sI hydrate in CO2-rich(60%and 80%of CO2)system,the space group is Pm3n,and the lattice parameter of a=1.188 nm and 1.195 nm.It can be concluded that the hydrate structure transit from sH to sI when the CO2concentration is increased.The cage occupancy and selectivity of CO2molecules in difference cages was analyzed with Raman spectra.And by these studies on the guest occupancy for CO2containing hydrate,it was confirmed again that the composition of the guest gas mixture plays an important role on the guest occupancy within the hydrate cages(Table 3).

2.3.Hydrogen hydrate

It is well known that H2is a clean and efficient energy of which the combustion products will not cause any pollution and greenhouse effect.Therefore the storage,separation and transportation on H2is of great concern for the developments of industrial technology.The hydrate crystallization methods may be an optional way.Some work on the H2hydrate by structural characterization methods has been done to study the guest occupancy within the hydrate cages.

Study on pure hydrogen hydrate has made significant progress.Jianwei Wang et al.[42]used MD simulation and the calculated Raman spectroscopy to analyze the Raman shift of hydrate cages with guest Hydrogen Clusters.In their work,the Raman peak at 4120-4125 cm-1represents the small cages with single H2molecule.And Raman peak at 4125-4150 cm-1represents the large cages with one to four H2molecules.The Raman peaks which represent the small cages with double H2molecules were found at above 4155 cm-1.Focusing on the occupancy of pure H2hydrate,this study may be beneficial for the better understanding on the occupation H2clusters in the H2hydrate cages,the electronic environment of guest molecules,the host-guest interactions as well as the vibration modes of Raman spectroscopy.

Though H2alone can form hydrate,for the application of H2containing hydrate formation method,there is a big limitation for the pure H2hydrate that the pure H2hydrate is usually generated only under the extremely high pressure.So the pure H2hydrate formation method is very hard to be applied for the H2separation,storage and transportation.Therefore,the application of H2containing hydrate formation method requires the participation of other molecules.

The CO2molecules together with H2can form sII hydrate,which can be designed as a double-effect technique(CO2capture and H2separation for gas mixture).But the pressure required to apply this technique is still very high even if CO2is engaged in the hydrate cages.The addition of C3H8may reduce the hydrate formation pressure significantly.Rajnish Kumar et al.[43]used Raman spectroscopy,PXRD,1H and13C NMR and a Fourier transform infrared spectrometer(FTIR)(see in Fig.8)to determine the hydrate structure and the cage occupancy of H2molecules in CO2+H2system and CO2+H2+C3H8system.Structure I hydrate was observed in the binary gas system.H2molecules including single H2and bimolecular H2molecules mainly occupy the small cages.With the addition of C3H8,pure structure II hydrate was observed in the ternary gas system.With all large cages shared by CO2and C3H8and part of the small cages occupied by CO2,H2can only occupy some small cages.In this study,the FTIR measurement can exactly solve the difficulties on the distinguishing the large and small cages with guest CO2which can't be shown directly by Raman spectroscopy and NMR measurements.

Fig.8.FTIR spectrum of CO2in small cages of CO2/C3H8hydrate；(a)the CO2peak in small and large cages of H2+CO2hydrate(b)the CO2peak in small and large cages of H2+CO2+C3H8hydrate.(Kumar et al.[43]2009).

In many cases for H2containing hydrate,a hydrate promoter which can reduce the equilibrium pressure of hydrate formation will be employed to stabilize the hydrate structure.To evaluate the storage or separation effect,it is of great significance to characterize the guest occupancy within the hydrate cages.

For hydrogen hydrate,efforts have been put on the characterization of hydrogen in cages of hydrate formed from various mixtures.THF has been widely used as a hydrogen hydrate promoter.It has been continuously studied in the past ten years.Takuo Okuchi et al.[44]used1H NMR to study the hydrogen storage capacity of hydrate.They first synthesized THF hydrates of the stoichiometric sII.Then the hydrogen storage capacity of hydrate in the tube with H2under pressures was evaluated by the Raman peak.The absorption process all finished in 80 h,which indicated that hydrogen-free THF hydrate can efficiently absorb the H2.

The occupancy of H2molecules affected by the composition of hydrate-forming mixture is also of great concern.Some studied has been done to analyze the influence on the H2occupancy by changing the mole fraction of THF in the system.Shunsuke Hashimoto et al.[45]used Raman spectroscopy to determine the cage occupancy of guest molecules in THF+H2hydrate system at different THF mole fractions.It was found that large cages are fully occupied by THF molecules,and H2only occupy the small cages of sII hydrate.Besides,the cage occupancy of guest molecules remained the same with the change of THF mole fractions from 0.01 to 0.13.Takeshi Sugahara et al.[46]used Raman microprobe spectroscopy and powder X-ray diffraction to analyze the distribution of guest H2molecules and study the H2storage capability of H2+THF hydrate.In their study,they adopted a new method using THF and powdered ice to synthesize the hydrate.The result shows that when the concentration of THF in the system is below 0.01 mol%,H2molecules can occupy the large cage of the hydrate.In this way,the H2storage capability can be improved.According to the studies on THF concentration-dependent guest occupancy for H2+THF hydrate,a conclusion can be made that the influence of the composition for THF+H2system is significant when THF mole fraction is below 0.01,whereas the effect is very weak when the THF mole fraction is more than 0.01.

Apart from THF,other help substance has also been studied as a hydrogen hydrate promoter.Timothy A.Strobel et al.[47]used Highresolution powder neutron diffraction (PND)measurements (see in Fig.9)to verify the structure of the binary cyclohexanone(CHONE)+H2clathrate hydrate(see in Fig.10).The formation of sII hydrate was observed.It was found that CHONE molecules completely occupy the large cages whereas H2molecules only occupy small cages.And they found that H2molecules can improve the stability of the sII lattice.Timothy A.Strobel et al.[48]used X-ray diffraction and Raman spectra to determine the structure of sH hydrate formed in H2+large molecule substances (methyl tert-butyl ether (MTBE),methylcyclohexane(MCH),2,2,3-trimethylbutane(2,2,3-TMB),1,1-dimethylcyclohexane(1,1-DMCH))system.The formation of sH hydrate was observed and the space group is P6/mmm and unit cell parameters a=1.2203(9)nm,c=0.9894(8)nm.It was found that H2molecules can occupy the small cages of sH hydrate when large cages are stabilized by large molecules.Besides,the sH hydrate was found to have higher(40%)hydrogen storage capability compared with sII binary hydrate.Timothy A.Strobe et al.[49]used High-pressure Raman spectroscopy,theoretical storage considerations and direct gas release measurements to study the hydrogen storage capability of hydrates in sII,semi-clathrates,and Jeffrey's structures.Raman peaks were assigned for hydrogen in both small and large cages of sII hydrate.Ji-Ho Yoon et al.[50]used X-ray diffraction,1H NMR and Raman spectroscopy to determine the structure of binary H2+1,4-dioxane hydrate and study the distribution of guest H2molecules.The hydrate structure was found to be sII,and H2molecules occupied the small cages of hydrate.

The large molecule substances mentioned above such as CHONE can't form hydrate alone,and the H2can work as help molecules to stabilize the small cage,therefore the large molecule can only occupy large cages,and the small cages will be used for the H2storage.The characterization on the guest occupancy within the H2hydrate cages is helpful for the understanding of the mechanism of H2storage.And it is also implied that the H2molecules can also stabilize the hydrate with different cage size or cage type which can provide higher H2storage potential(Table 4).

2.4.Inhibitor/promoter containing system

Because of the unique chemical and physical properties of sH hydrates which were first discovered by Ripmeester et al.,some research on the sH hydrate has been done.The addition of some large molecules resulting in the formation of sH hydrate can lower the hydrate equilibrium point of some hydrate such as the H2hydrate mentioned above.And for its applications on energy gas storage,sH hydrate was found to have 25%higher gas storage capability than sII hydrate.Recent studies have focused more on the structure characterization and the behavior of guest molecules within the sH hydrate cages.The characterization methods are wildly adopted for these studies.Some substances can be promotors for hydrate formation,such as the LMGS in sH hydrate,and therefore the implement of hydrate-based gas storage technique can be available.The formation of hydrate with different guest substances which can work as sH hydrate formers has been studied.Robin Susilo et al.[51]used1H and13C NMR spectra to study the hydrogen bonds in sH hydrate formed in the tert-butylmethylether(TBME)+CH4system and neohexane(NH)+CH4system.By measuring the NMR relaxation time,the host-guest interaction of the two hydrates was analyzed.For TBME+CH4hydrate system,the hydrogen bonds between the H2O molecules and ether oxygen atoms of TBME were detected.And if temperature is increased,the hydrogen bonds will have less lifetime.But compared with TBME,the NH molecules didn't have hydrogen bonds with H2O molecules.For the applications of sH hydrate formation,the key is to find a effective sH hydrate formers.So the performance of sH hydrates with different guest large molecule substances should be compared.Robin Susilo et al.[52]used PXRD,13C NMR,and Raman spectroscopy to study the sH CH4hydrates formed by the addition of large molecule sH hydrate formers:methylcyclohexane(MCH),tert-butyl methyl ether(TBME),and 2,2-dimetylbutane(NH).It was found that the methane occupancy depends on the type of LMGS following the order of TBME ＜MCH ＜NH,and increases with pressure.In this study,though both Raman and NMR can identify methane and LMGS in hydrate,only NMR can give the cage occupancy values of sH hydrate,which indicates that Raman spectroscopy is more suitable for the qualitative studies and the quantitative data can be given by NMR measurements.Takeya et al.[53]adopted PXRD combined with the ab initio methodology(direct-space technique and Rietveld refinement)to determine structure and the cage occupancy of the guest molecules for sI CO2hydrate,sI C2H6hydrate,sII C3H8hydrate,sH MCH+CH4hydrate,sH NH+CH4hydrate and sH TBME+CH4hydrate.And the results of single crystal X-ray diffraction and13C NMR spectroscopy were used to verify the combined method.It was found that the result of PXRD combined with the ab initio methodology is in good agreement with results of single crystal X-ray diffraction and13C NMR spectroscopy.So it can be concluded that the single structural characterization method combined with the proper adoption of the ab initio methodology can also get an accurate result.

Fig.9.PND pattern of H2+CHONE hydrate at 0.1 MPa and 20 K.Rwp=5.34%,χ2=4.53.Tick marks:upper=ice,lower=sII.Inset:Rwpas a function of H2fractional occupancy in small cages.(Strobel et al.[47]2007).

Fig.10.Structure of H2+CHONE hydrate:the disordered CHONE molecule in 51264 cages(left:)；the disordered H2in small cages(right).(Strobel et al.[47]2007).

Table 4 Studies on clathrate hydrates for H2/H2+other substances mixture systems

Distinguish by the solubility of the sH hydrate formers in water,the substances like MCH,NH and TBME are all very weakly miscible or even immiscible in water,and the practical applications such as the gas storage capability and efficiency and the hydrate formation rate may be limited by their low solubility in water.For this case,there have been a breakthrough on the study of water-soluble sH hydrate formers.Woongchul Shin et al.[54]used13C NMR,Raman spectroscopy,and XRD to study the structure of CH4hydrates formed in CH4+sH hydrate formers system,and evaluate of the effect of new hydrate formers(1-methylpiperidine,2-methylpiperidine,3-methylpiperidine,4-methylpiperidine,hexamethyleneimine).These five hydrate formers all have good solubility in water.In the study,The effect of previous sH hydrate promoter methylcyclohexane(MCH)was also compared.For MCH+CH4system,the coexistence of sH hydrate and a considerable amount of sI hydrate can be observed.For new hydrate formers+CH4system,sH hydrate was found to have high purity.It is indicated that the new hydrate formers have higher efficiency on the formation of sH hydrate.They found that 1-methylpiperidine seemed to have best effect among the five new formers.

According to the studies on the effect of hydrate formers which also work as hydrate promoters for hydrate formation,several structural characterization methods are adopted for the insight understanding on the structure and guest occupancy.The unique occupation only within the large cages for these guest sH hydrate former molecules is caused by their large molecular size which can't be accommodated by other smaller cages.Therefore,the guest molecular size plays a dominant role which on both the hydrate structure and the occupancy of guest within the hydrate cage in these studies.

Besides sH hydrate,the formation of sII alcohol hydrate can also be promoted by some introducing some small molecular help gas.Alcohols,which are usually considered as the inhibitor of hydrate formation,can't form pure hydrate at slightly high temperatures,but the binary hydrate with guest alcohol molecule can generate with the participation of help gas molecules such as CH4.The further understanding on the hostguest interaction and guest occupancy can be obtained by the adoption of structural characterization methods.Youngjune Park et al.[55]used Powder X-ray diffraction and13C NMR to determine the structure of the clathrate hydrate formed in tert-butyl alcohol(TBA)+CH4system.The formation of sII hydrate in TBA+CH4system was observed.According to the fact that clathrate structure doesn't show up when there is only TBA without CH4,it was confirmed that the addition of CH4can promote the formation of sII clathrate hydrate in TBA system.Keita Yasuda et al.[56]used Raman spectroscopic measurements and Powder X-ray diffraction to determine structure of the CH4+C2H5OH hydrate.The formation of sII hydrate with cages occupied by guest CH4and C2H5OH molecules was identified by PXRD and the occupancy of guest molecules were also estimated from Raman spectra.Both C2H5OH and CH4were observed in large 51264cages even if the C2H5OH component in the system is sufficient enough.For alcohol hydrate,the guest-host interaction with the effect of hydroxyl in guest molecules may be different from other hydrates.Konstantin Udachin et al.[57]used Single crystal X-ray diffraction and MD simulation to study the structure and hydrogen bond of sII CH4+1-propanol hydrate.The hydrate composition formula is C4.57H48.30O18,which can be described as 1.57(CH4)·C3H7OH·17H2O.Based on the result of X-ray diffraction,the shortest distance between the H2O oxygen atom and the oxygen atom of 1-propanol was measured.And the shortest distance between the H2O oxygen atom and hydroxyl hydrogen atom of 1-propanol was also measured.The distances can be used to characterize the hydrogen bond of hydrate.The result of MD simulation(see in Fig.11)showed that guest-host hydrogen bonding will lead to the individual cage distortions,which can't be observed by X-ray diffraction.So a combination of MD simulation or other characterizations can also work as a way to give a detail information on hydrate structures.

Fig.11.Overlaid snapshot of a 1-propanol molecule in a sII large cage during a 250 ps simulation from MD simulations at 100 K.The distortion of the hexagonal face of the cage and the gauche conformation of the guest are seen(Udachin et al.[57]2011).

The hydrate structure and guest occupancy affected by temperature for alcohol hydrate has also been studied.Takeya et al.[58]adopted powder X-ray diffraction to characterize the sII binary CH4+propanol(1-propanol,2-propanol)hydrates.It was found that the structure of both CH4+1-propanol hydrate and CH4+2-propanol hydrate are of sII at about 200 K.And when the temperature is below 110 K,CH4+2-propanol hydrate will transit from cubic sII to a less symmetrical crystal structure with tetragonal lattice.Relatively,the hydrate unit cell will also transit from 16(512)·8(51264)·136H2O to 8(512)·4(51264)·136H2O.And by increasing the temperature to 200 K,the structure will become cubic sII again which indicates that the structure transition is reversible.Because of the transition,the sizes of 512cages and 51264cages are enlarged.The structure of CH4+1-propanol hydrate was found to be still sII when the temperature was changed between 200 K and 93 K.The cage occupancy and guest distribution were studied.The cage occupancy of guest molecules almost remains the same.By analyzing the guest distribution,it was found that the deformation of the cages of CH4+2-propanol hydrate may be caused by the restriction of the motion of guest 2-propanol in 51264cages.It can be seen that some external conditions may affect the hydrate structures such as the structural symmetry,and the PXRD can provide detail information on hydrate crystal structure.

Table 5 Studies on clathrate hydrates for inhibitor/promoter containing system

For the flow assurance in the pipelines which is related with the economic effect and safety,the hydrate inhibitors especially the kinetics inhibitors have been widely used.And to understand the inhibition mechanism and evaluate the inhibition effect of hydrate inhibitors,the influence of different inhibitors on the hydrate structure and cage occupancy has been studied by structural characterization techniques.Nagu Daraboina et al.[59]used Powder X-ray Diffraction,Raman Spectroscopy and13C NMR Spectroscopy to determine the structure and guest occupancy of hydrate formed from CH4+C2H6+C3H8system,and the inhibition of two kinetic inhibitors(polyvinylpyrrolidone(PVP),H1W85281)and one biological inhibitor(antifreeze protein type III(AFP-III)).And the cage occupancy affected by inhibitor was also studied.In the system inhibited by PVP or H1W85281,the coexistence of sI and sII was observed,and almost all the large cages were occupied.In the system inhibited by AFP-III,only sII hydrate was observed,and about 10%of large cages were empty.Besides,it was found that both of the two kinds of inhibitors can lower the occupancy of CH4in large cages by 25%.

According to the XRD graphs on pure CH4hydrate and CH4hydrate inhibited by PVCap with the presence of ironic liquid N-(2-hydroxyethyl)-N-methylpyrrolidinium tetrafluoroborate ([HEMP][BF4]),Seong-Pil Kang et al.[60]studied the influence of IL inhibitor on CH4hydrate structure.It was found that the two peaks in graphs coincide with each other and therefore this inhibitor doesn't affect the single-component hydrate structure when they interrupted the nucleation and growth process.And further studies on the synergy inhibition effect for the gas mixture system have also been done,by13C NMR spectroscopy and Raman spectra,Seong-Pil Kang et al.[61]determined the hydrate structure formed in CH4inhibited system and synthesized natural gas(CH4+C2H6+C3H8+iso-C4H10)inhibited system.The inhibition of only Poly(N-vinylcaprolactam)(PVCap)and PVCap+N-(2-hydroxyethyl)-N-methylpyrrolidinium tetrafluoroborate ([HEMP][BF4])was studied.The results show that both PVCap and PVCap +[HEMP][BF4]can delay the formation of sI hydrate but cannot prevent it.For synthesized natural gas hydrate system,when there was only PVCap,the coexistence of sI and sII was observed.And it was found that most of the hydrates are sII with the addition of PVCap +[HEMP][BF4].By comparing the sI to sII hydrate amount ratio,the synergistic effect of PVCap and [HEMP][BF4]on hydrate inhibition can be inferred.Therefore,it can be seen that the hydrate structure and guest occupancy may also be affected by the addition of hydrate inhibitors(Table 5).

2.5.Semi-clathrate hydrate

For semi-clathrate hydrate,part of the cage is broken to trap the large guest molecules,and this characteristic may contribute to the higher gas storage capability.So the semi-clathrate hydrate can be a promising material for gas separation and storage.To determine these uncommon hydrate structure and the guest distribution,the adoption of structural characterization method is required.The single crystal X-ray diffraction is the most frequently adopted and powerful method to give structural information in detail.

TBAB hydrate was the most extensively studied semi-clathrate hydrate.For its applications,the capture of CO2and the storage capability on H2or CH4are of great concern.Li et al.[62]used Powder X-ray Diffraction to determine the structures of hydrate in cyclopentane(CP)+mixed gas+TBAB system.The initial mixed gas contained H2(24%)and CO2(76%).The phenomenon of a large amount of CO2can occupy the large cages was observed.According to the analysis of the guest distribution,it was inferred that CP and TBAB may have a synergic effect on CO2capture by promoting the formation of the semi-clathrate hydrate in CP+mixed gas+TBAB system.This study may be instructive for the technique which is related to the capture of CO2and the purification of H2.On the other hand,the formation of TBAB hydrate can also be applied for the hydrate-based control on the greenhouse effect,so the storage capability for greenhouse gas such as CO2or CH4has been studied.Muromachi et al.[63]used single crystal X-ray diffraction，NMR and MD simulation to characterize the new crystal structure of the tetra-nbutylammonium bromide(TBAB)+CO2clathrate hydrate (see in Fig.12).The TBAB hydrate is used for CO2capture.It was found the stability of the ionic clathrate hydrate structure was affected by the distribution of guest gas molecules.The occupancy of CO2guest molecules in each type of cages was measured by13C NMR spectra.Further work on the TBAB+CH4hydrate has also been done,and a comparison on the cage selectivity between CO2and CH4was given.Muromachi et al.[64]used single crystal X-ray diffraction and MD simulation to study the hydrate structure in TBAB+CH4system(see in Fig.13)and analyze the cage occupancy selectivity of CH4molecules.The TBAB+CH4hydrate structure was found to be Jeffrey's type-IV structure with Pmma space group and the unit cell parameters a=2.10329 nm,b=1.25972 nm,c=1.20333 nm.By comparing the guest distributions of TBAB+CH4hydrate and TBAB+CO2hydrate,it was found that CH4shows strong selectivity on two kinds of more regular quasi-spherical cages(CH4occupancy:DB1(0.989)and DB2(0.993))while CO2prefers to occupy cages with the irregular shape(DA).

Fig.12.The highly distorted DAand regular shaped DBdodecahedral cages in the TBAB+CO2structure(Muromachi et al.[63]2014).

Fig.13.Three D cages in the TBAB+CH4hydrate.(a)A view from the b axis.(b)A view from the a axis(Muromachi et al.[64],2016).

Similar to THF,TBAB hydrate is also considered as an available media for hydrate-based H2storage technique.And to analyze the effect of the THF concentration on the H2storage capability,Raman spectroscopy was used by Hashimoto et al.[65]to study the cage occupancy of H2in H2+TBAB hydrate at different TBAB mole fraction range from 0.006 to 0.070.It was found that the concentration of TBAB doesn't affect the cage occupancy of H2under the experimental condition and H2molecules only occupy small cages in H2+TBAB hydrate.

Some efforts have been made to characterize the uncommon hydrate structure with various guest molecules.Besides TBAB,some other substances forming semi-clathrate have also been studied.It is notable that the ionic hydrate with guest TBA or TBP cation was found to have high thermodynamic stability which may be caused by the interaction between cations and host H2O molecules.Therefore,great interest has been generated for the structural characterization on hydrate with guest substances of this family such as TBA acrylate,TBPB and TBPOH.

Sanehiro Muromachi et al.[66]adopted single crystal X-ray diffraction measurements to analyze the structure of tetra-n-butylphosphonium bromide (TBPB)semi-clathrate hydrate.The TBPB hydrate has an orthorhombic structure with the space group Pmma,and unit cell parameters a=2.1065(5),b=1.2657(3)and c=1.1992(3)nm,which is different with the most stable tetragonal TBAB hydrate.The chemical formula is TBPB·38H2O.The hydrate corresponds to Jeffrey's type-IV structure.In their work,they gave a comparison between the structures of TBPB hydrate and TBAB hydrate and found that the most stable structure of the TBPB hydrate is not tetragonal but orthorhombic.Sanehiro Muromachi et al.[67]used NMR and single crystal X-ray diffraction measurements to analyze the structure of the tetra-n-butylammonium(TBA)acrylate semi-clathrate hydrate(see in Fig.14).The TBA acrylate hydrate has a tetragonal structure with the space group P42/n,and unit cell parameters a=3.3076(7)nm,b=3.3076(7)nm and c=1.2170(2)nm.The hydrate corresponds to Jeffrey's type III structure.They also measured the distribution of guests.The TBA+was found to occupy two kinds of fused cages.And they found that acrylate anion occupied the cage and was next to TBA+.Takayuki Kobori et al.[68]used single crystal X-ray diffraction to determine the structure of tetra-n-butylphosphonium hydroxide(TBPOH)hydrate(see in Figs.15,16).The structure of this hydrate was found to be Jeffrey's type I structure.The ionic clathrate hydrate has a cubic structure with the space group,and unit cell constant is 2.45191(13)nm.The chemical formula is TBPOH·29.6H2O.They also compared the structure of TBPOH hydrate with that of tetra-nbutylammonium fluoride (TBAF)hydrate.V.Yu.Komarov et al.[69]used single crystal X-ray diffraction to study the structure of (C4H9)4NF·29.7H2O ionic clathrate hydrate formed from binary tetrabutylammonium fluoride((C4H9)4NF)+H2O system.At the temperature of 150 K,it was found that the hydrate has a cubic structure with space group and the unit cell parameter a=2.4375(3)nm.The hydrate structure is similar to sI and is a sI superstructure with eight-time unit cell.Based on these studies on guest occupancy and distribution,the hydrate with uncommon structure such as semi-clathrate hydrate structure may be a potential material for hydrate-based gas separation,storage and transportation techniques(Table 6).

Fig.14.Occupancy of acrylate anion in the D cage neighboring the fused 3 T+P cage(Muromachi et al.[67]2015).

2.6.Natural gas hydrate

Fig.15.Two types of morphology for TBPOH hydrate crystals.(a)Rectangular columnar shape(T=276.1 K；xTBPOH=0.0072),(b)Polyhedral shape(T=285.0 K；xTBPOH=0.0161)(Kobori et al.[68]2015).

The natural gas hydrates in the earth has been regarded as a great source for energy supply.And the exploitation for natural gas hydrate has raised great concern.The cage occupancy for hydrates can be regarded as a ratio which is related with the gas storage capability of hydrate,and can vary with the change of gas component.Therefore,the cage occupancy of guest molecules for natural gas hydrate is of great importance to estimate the capability of natural gas hydrate reservoir.Detailed experimental studies on the natural gas hydrate samples by structural characterization techniques have been carried out in the past decade years.And the structure of naturally occurring gas hydrate from various locations has been characterized in the lab.K.C.Hester et al.[70]used Raman spectra to analyze the composition and structure of the 12 hydrate samples from southern Hydrate Ridge,Oregon.The results show that the structure of hydrate is sI.The CH4molecules were found to be the main guests and very small quantities of guest H2S molecules were also detected in three of the samples.Besides,the cage occupancy of CH4was also measured,and the large to small cage occupancy ratios change within the range of 1.01-1.30,which is in good agreement with laboratory measurements.Masato Kida et al.[71]used Gas chromatography,powder X-ray diffraction,13C NMR and Raman spectra to determine the composition and structure of hydrate in pores of marine sediments recovered from eastern Nankai Trough area off of Japan.By gas chromatography,they found that CH4is the main gas composition of the hydrate samples.And according to the result of PXRD,the hydrate was found to be sI hydrate with the lattice constants 1.183-1.207 nm.The results of13C NMR and Raman spectra confirmed that CH4molecules are trapped in the sI hydrate lattice.Based on the Raman data,the average cage occupancy of CH4in large cages 0.97,and the cage occupancy of CH4in small cages is 0.83.Besides,they also determined the hydration numbers which are between 6.1-6.2.

Konstantin A.Udachin et al.[72]analyzed the single-crystal X-ray diffraction data to determine the structures of two hydrate samples from Cascadia Margin.The structure of the natural occurring CH4hydrate sample is sI with cubic lattice.And the structure of CH4+C2H6+C3H8hydrate was found to be cubic sII.Besides,they also measured the occupancy and positions of guest molecules.

Fig.16.Cage framework of the TBPOH hydrate.The four T cages accommodate the TBP cation,and a water molecule was found in the D cage with the occupancy of 0.71(Kobori et al.[68]2015).

Table 6 Studies on clathrate hydrates for semi-clathrate hydrate systems

The gas hydrate samples recovered from various locations in the Gulf of Mexico(GOM)was found to be different in their structure,and some studies has been done on the structural characterization.Stephan A.Klapp et al.[73]used X-ray diffraction to determine the structure of hydrate samples recovered from GOM.The hydrate from the Green Canyon,the northern of GOM is sII hydrate,and the guest components of the hydrate contain CH4(about 70%)and other hydrocarbons(about 30%).For hydrate from the Chapopote asphalt volcano,the southern of GOM,the structure is sI,and the proportion of CH4in the guest components is more than 98%.By Raman spectroscopy and X-ray diffraction,Stephan A.Klapp et al.[74]also determined the structure of another three hydrate samples recovered from the Chapopote Knoll,the southern Gulf of Mexico.Two hydrate samples were found to be sI CH4hydrate.And the structure of another sample was the coexistence of sI and sII.They analyzed the formation of the coexistence of sI and sII hydrate.

The studies on the structural of hydrates recovered in china have also made some significant progress.Changling Liu et al.[75]used Raman spectroscopy and X-ray diffraction to determine the structure of the hydrate recovered from the Pearl River Mouth(PRM)basin.The hydrate was found to be sI CH4hydrate with cubic lattice,and the lattice constant is 1.19338 nm.The 99.5% of large cages and 91.4% of small cages were occupied by CH4.Zhengquan Lu et al.[76]used the Raman spectra to determine the structure of the hydrate recovered from the Qilian Mountain permafrost.The hydrate structure was found to be sII.CH4is the main gas composition of the sII hydrate,and C2H6,C3H8,CO2were also found in the hydrate(Table 7).

Table 7 Studies on clathrate hydrates for natural gas hydrate systems

2.7.Others

Besides the hydrates we have listed above,the studies on the structure for other hydrates with special guest molecules have been done.

THF is well-known as a hydrate promoter.For hydrate formation,THF alone can form sII hydrate with guest THF molecules occupying the large cages,and the guest THF within hydrate cages weakly interacts with the host H2O molecules.to study the host-guest interaction,the structure of hydrate formed at atmosphere pressure has been characterized.P.S.R.Prasad et al.[77]used Laser Raman spectroscopy and Fourier transform infrared(FTIR-NIR)to determine the structure of THF clathrate hydrate and study the vibration mode of the bonds in guest molecules.At 64 cm-1in Raman spectra,the mode which shows the lattice of sII hydrate was observed.The NIR second order mode at 4295 cm-1will have some changes representing the ring breathing mode(C--C--C--C stretching mode)in Raman spectra.The ring breathing mode showed asymmetric behavior at the temperature below 120 K.They reasoned that asymmetric behavior may result from the different host-guest interactions.Heiko Conrad et al.[78]used X-ray Raman scattering measurements to study the changes in the local molecular structure of THF clathrate hydrate at different temperatures.In this way,the stochastic hydrate formation model was supported.

Jeffery A.Greathouse and Randall T.Cygan et al.[79]used Raman spectroscopy MD simulation and Calculated Density Functional Theory(DFT)vibrational spectra to determine the 1,1-dichloro-1-fluoroethane(R141b)clathrate hydrate structure and study dynamics of guest R141b molecules.The result shows that the crystal structure corresponds to the structure II.And it was found by MD simulation that R141b molecules can rotate freely in the host water cage,though there is hydrogen bonding between host and guest.

By analyzing the data of time-dependent neutron diffraction and MD simulation,Zhu et al.[80]determine the distribution of CO molecules to study encapsulation kinetics and dynamics.They confirmed that when the CO is saturated in the system,the sII CO hydrate is more stable than sI CO hydrate,and sI hydrate which is initially crystallized sII hydrate will form in a time-evolving sequence.Because of the difference between 51264cages and 512cages in CO binding energy,a crossover of binding energy was found which indicates that when the guest occupancy is beyond the critical point,the 51264cages are energetically favored to be formed compared with the 51262cages for the double occupancy of CO hydrate.Besides,the results of MD simulation show that the guest-host and guest-guest interactions play an important role in the double occupancy of 51264in for CO hydrate.

It was found that the noble gas atoms such as Ne can also form hydrates.Xiaohui Yu et al.[81]used in situ neutron diffraction to study the Neon hydrate formed at 480 MPa,260 to 70 K.According to the result,they confirmed that Ne atoms can work as guests which are enclathrated by host H2O molecules in Neon hydrate.The structure of the hydrate synthesized in their study is ice-II structure.The guest Ne atoms were found to occupy the centers of H2O channels.And because there is no direct bond between guests and hosts,the Ne atoms have high freedom of movement.The host lattice itself has also been studied in the past ten years.

The structural characterization performed by Yu et al.[81]confirmed that the Ne clathrate hydrate has ice-like structure at low temperatures.And by adopting Ne hydrate as initial material,a breakthrough has been made in the hydrate formation method.Falenty et al.[82]used neutron diffraction to study structure and the properties of ice XVI formed by continuous vacuum pumping for 5 days on the small particles of sII Ne hydrate.They found that the ice XVI will exhibit negative thermal expansion when the temperature is below about 55 K,and it may be more stable mechanically.Compared with the guest-filled hydrate,the ice XVI has larger lattice constants at low temperatures.

The structural characterization for hydrates with guest hydrofluorocarbons has also been performed to study the behavior of guest molecules and the host-guest interactions.Takeya and Ripmeester et al.[83]used temperature-dependent Powder X-ray diffraction(PXRD)to characterize the dissociation process of hydrates at the temperature below 273 K and analyze the effect of different guest molecules.For sI hydrate with only one kind of guest molecules CH4/CH3F/CF4/CO2and sII hydrate with guest O2/N2/Ar/Kr,self-preservation phenomenon of hydrate was observed in dissociation process.For sI hydrate with guest C2H6/CH2F2/CHF3/Xe/H2S and sII C3H8hydrate,the phenomenon didn't show up.It was concluded that the self-preservation of hydrate is up to the type of guest molecules and the host-guest interaction.Takeya et al.[84]used PXRD and19F NMR spectroscopy to characterize the structure of hydrofluorocarbons (CH3F,CH2F2,and CHF3)hydrate and tetrafluoromethane(CF4)hydrate.According to the results,the hydrates were found of sI.And By the direct-space technique and Rietveld refinement,they studied the guest distribution and dynamic disorder related with reorientation.It was found that 512and 51262cages were almost fully occupied by the guest CH3F molecules,but for guests larger than CH3F,the 512cages will be partially occupied.And with the size of guest molecule increasing,the cage occupancy became smaller.For the CF4molecules,the 512cages are empty,and CF4molecules were isotropically distributed within the 51262cages.The expansion of the CF4hydrate unit cell was observed because of the expansion of the 51262cages.And hydrofluorocarbon molecules are located near the equatorial area of the cages,and the long axis of the guests was found to extend along the equatorial area of 51262cages.Though the dipole moments and van der Waals sizes for guest CH3F,CH2F2,and CHF3molecules are different,because of the different number of fluorine atoms in each kind of guest molecule,there is almost no difference on the size of the unit cell for these hydrates at the temperature of 93 K.Besides,based on the Restriction on guest dynamics caused by 51262cages,they found that the host lattice may be slightly affected by the type of guest molecule.

Fig.17.Framework structures of three types of clathrasils:(a)sI,(b)sII,(C)sH(Momma et al.[85],2011).

Table 8 Studies on clathrate hydrates for other hydrate systems

Silica clathrate compounds(clathrasils)are similar to clathrate hydrates in structure with the cage-like voids occupied by guest substances.And one natural occurring silica clathrate mineral(melanophlogite)with the sI-type framework(see in Fig.17)has been reported in the previous studies.In recent years,a great breakthrough has been made on the new structure of natural occurring silica clathrate mineral.Momma et al.[85]adopted Raman spectroscopy,PXRD and single crystal X-Ray diffraction to characterize the structure of two new silica clathrate minerals.According to the results,the silica clathrate minerals were found to be isostructural with the sH and sII hydrates(see in Fig.17),and the structure and the ideal molecular formulas for the three clathrasils were determined as cubic sI(2M12·6M14·46H2O),cubic sII(16M12·8M16·136H2O)and hexagonal sH(M18·2M12·2M12·34H2O),where the Mxrepresents the guest position in X-faced polyhedral cage.The chemical composition was studied by Raman spectra,and the occupation of hydrocarbon gases such as CH4,C2H6,C3H8and i-C4H10was also detected,which indicates that silica clathrate minerals also have gas storage capability.Besides further studies by PXRD and single crystal X-Ray diffraction gave the detailed crystal structural information(Table 8).

3.Conclusions

In this review,we have summarized the recent studies on guest occupancy and compositions in hydrate systems with different components.The information about hydrate structures in each study can be available by the application of structural characterization techniques such as PXRD,Raman,NMR etc.For these experimental structural characterization methods,Raman spectroscopy is often considered as the primary method for cage occupancy study,while NMR spectroscopy allows the implementation of quantitative analysis on cage occupancy of guest molecules.X-ray diffraction is usually considered as the primary method to identify of the type of hydrate structure.As a powerful tool for crystallographic study,XRD can provide the detailed information on the positions of host (guest)molecules.With the molecule position data,occupancy in hydrate cages can be estimated.The IR and Neutron diffraction measurements are also effective ways to characterize the hydrate structure under some experimental conditions.By using the structural characterization methods,these studies on the characterization for guest occupancy within the hydrate cages can give insight understanding for the application of many hydrate-based techniques in the fields of gas storage and separation,exploitation of natural gas hydrates and flow assurance.