Ultra-rapid uptake and highly stable storage of methane as combustible ice

The natural as a key player in the current energy landscape tenders rare the development of new, robust gas storage technologies. Gas hydrate combustible ice based Solidified Natural Gas (SNG) technology realizes compact and safe long term storage of natural gas eco-friendly water the major raw material Yet, its practicality has been limited by problems in forming natural gas hydrates at a rapid rate and then ensuring prolonged stability of the same. We report on 1,3-Dioxolane (DIOX) as a dual-action chemical promoter for methane sII hydrate formation, offering elite thermodynamic and kinetic enhancement ability. A small amount (300 ppm) of kinetic promoter L-tryptophan added to the scheme further helps in achieving ultra-rapid hydrate formation rates. Mixed methane-DIOX hydrate pellet stored at atmospheric pressure and 20 conventional freezer for 8 days remains highly stable, thereby demonstrating industrial applicability, scalability and ease of operation. Abstract Ever-increasing natural gas (NG) consumption trends due to its cleanest tag and abundant availability point towards an inevitable transition into an NG dominated economy. Solidified 26 Natural Gas (SNG) storage via combustible ice or clathrate hydrates presents an economically sound prospect, promising high volume density, and long-term storage. Here we establish 1,3-dioxolane (DIOX), as a highly efficient dual-action (thermodynamic and 29 kinetic promoter) additive for clathrate (methane

Natural Gas (SNG) storage via combustible ice or clathrate hydrates presents an 27 economically sound prospect, promising high volume density, and long-term storage. Here 28 we establish 1,3-dioxolane (DIOX), as a highly efficient dual-action (thermodynamic and 29 kinetic promoter) additive for clathrate (methane sII) hydrate formation. By synergistically 30 combining a small concentration (300 ppm) of kinetic promoter L-tryptophan with DIOX, we 31 further demonstrate ultra-rapid hydrate formation with a methane uptake of 83.81 (±0.77) 32 volume of gas/volume of hydrate (v/v) within 15 minutes. To the best of our knowledge, this 33 is the fastest reaction time ever reported for sII hydrates related to SNG technology and 34 represents a 147% increase in the hydrate formation rate compared to the standard water- 35 DIOX system. Mixed methane-DIOX hydrates in pelletized form also exhibit incredible 36 stability when stored at atmospheric pressure and moderate temperature of 268.15 K, thereby 37 showcasing potential to be industrially adoptable for the development of a large-scale NG 38 storage system. As economic progress and population growth drive global energy demand, fossil fuels 3 continue to remain strategically important and natural gas (NG) will play a vital role in 4 perpetuating the same. 1 Exploration of abundant NG reserves available in unconventional 5 form (shale, hydrates and coal-bed) coupled with NG being the cleanest burning fossil fuel 6 compared to gasoline and coal, makes this resource-which is primarily methane, 2 7 economically competitive and environment friendly. 2018 witnessed a 4.6% increase in NG 8 consumption with projected 0.9% average annual increase over the next decade. 1 Increased 9 demand and reliance on NG imports would mean the requirement of the development of safe 10 and sound large-scale gas storage technology by NG importing countries to cater for energy 11 security, resilience and redundancy. While liquefied natural gas (LNG) is the best option to 12 transport NG where pipeline is not possible, it is not considered suitable for long-term storage Nature has been storing methane gas in the form of natural gas hydrates for millions of years 20 albeit in a slow manner, presenting itself today as a huge energy resource. 8-10 Gas hydrates 21 are crystalline inclusion compounds where under suitable conditions cages made of water 22 molecules may host guest gas molecules within. 11 Gas hydrates made of methane or natural 23 gas are also known as combustible ice. With the appropriate tuning of formation conditions, 24 and the identification of suitable promoters, we can accelerate the formation of gas hydrates. 25 Thus, NG stored in hydrate form -Solidified Natural Gas (SNG), has re-emerged as an option 26 for large volume, and long term storage. 12-14 Tetrahydrofuran (THF) has been proven to be a 27 stable thermodynamic promoter for H 2 storage via clathrate hydrate formation. 15,16 Recently, 28 THF has also been demonstrated to perform as a dual-action (thermodynamic and kinetic 29 promotion) promoter for methane storage. 17 sII methane hydrate forms rapidly in presence of 30 THF over a wide temperature range 17 and has been shown to be stable at near ambient 31 pressure of 0.15 MPa and at 271.5 K. 18

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Despite the use of THF in many industrial chemical processes, 19, 20 its place in hydrate 33 formation has often been questioned, given its carcinogenicity, 21 high volatility 22, 23 and 34 corrosive nature 24 that impedes its adoption for large scale technology deployment. Thus, 35 there is a need to identify a cleaner alternative to THF without compromising the vital dual 36 functionality. 1,3-dioxolane (DIOX) is a heterocyclic compound closely related to THF, 37 where the carbon atom at 3-position of THF has been replaced with an oxygen atom. 25 It has 38 similar water solubility as THF, but is less volatile and less toxic (Table S1 in the Supporting 39 Information (SI)). DIOX by itself, stabilizes the sII hydrate. 26 There is only one phase 40 equilibrium study on methane-DIOX system, where 5.0 mol% of DIOX was found to be 1 optimal as a thermodynamic promoter among an investigated concentration range of 0.99 2 mol% to 20.02 mol%. 27

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Here we report rapid methane uptake for mixed methane-DIOX hydrate formation by means 4 of a detailed kinetic study of hydrate formation, and elucidate the mechanism for this rapid 5 enhancement by combining with crystal morphology observations. Further we report ultra-6 rapid mixed methane-DIOX hydrate formation at mild operating conditions with 7 exceptionally high gas uptake, achieved in conjugation with L-tryptophan. The hydrate 8 formed is also analyzed using Powder X-Ray Diffraction (p-XRD) characterization technique 9 for structure identification. Finally, this study documents the first production of sII methane-10 DIOX hydrate pellet along with the monitoring of its stability over an eight-day period. pure methane hydrate at 283.15 K is 7.2 MPa, 28 the same for a methane-DIOX system is less 20 than 1.0 MPa. This shift augurs well for DIOX, as thermodynamic promotion ability directly 21 co-relates to hydrate formation at milder operating conditions compared to pure methane (sI) 22 hydrates. Average equilibrium pressures for the methane-DIOX/water system based on two 23 individual measurements each at the three temperatures studied, can be found in Table S2 in 24 the SI.
1 Figure 1 Three phase (H-Lw-V) equilibrium points for methane-water 29 and methane-2 DIOX/water systems 27 along with experimental data obtained in the present sudy for 3 methane-DIOX/water system containing stoichiometric concentration of DIOX (5.56 mol%).

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We present hydrate growth (gas uptake) curve for the methane-DIOX/water system operated 6 under quiescent (unstirred) condition in Figure 2a.   Table S3 in the supporting information.

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Visual images captured during a typical hydrate formation experiment for the methane-24 DIOX/water system is presented in Figure 2b to provide insight into the physical hydrate 25 growth patterns. We have also presented the video of the visual observation of hydrate 26 formation in the supporting information (Video SV1). For the methane-DIOX/water system 27 under quiescent growth condition, hydrate masses propagate from the three-phase (solid-28 liquid-gas) interface points on either side of the reactor (as observed through the viewing 29 window, see visual image at t = 7 minutes), inwards towards the center, where they 30 eventually meet and thereon, grow as bulk hydrates. Interestingly, at about 7 minutes in 31 Figure 2b, there is visibly significant amount of hydrates in the reactor but the methane 32 uptake is only about 9.12 v/v [11.1 (±0.76)%] of the total methane uptake. 88.9 (±0.76)% of 33 the methane uptake happens after 7 minutes of the formation experiment. These calculations 34 are based on the assumption that the methane uptake at the 35 minute mark is the total 35 methane uptake, since beyond this point (Figure 2b), the hydrate growth observed is minimal. 36 It should be noted that this particular pattern of methane uptake and hydrate growth is 37 consistent for all the experiments as seen by the very small standard error in Figure 2a.  Complete Raman spectrum obtained for hydrates formed from methane-DIOX/water system 10 at 283.15 K and an initial pressure of 7.2 MPa -DIOX concentration used is 5.56 mol%.

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For a comparison, we also conducted experiments using the traditional stirred tank operation 12 mode for the growth phase and the results are presented as Figure S2 in supporting 13 information. As observed in Figure S2, in-situ Raman spectroscopy study we provide the first complete Raman spectrum and 9 associated analysis for mixed methane-DIOX hydrates. The detailed analysis of the observed 10 Raman spectra has been presented in the supporting information (refer to Page S5 to S9).

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Comparison with methane-THF mixed hydrate system hydrate formation experiments can be found in Table S4. THF.

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The kinetic gas uptake data in Figure 3a  with unknown relevance to humans 21 (refer Table S1 for details of the properties represented 21 in Figure 3c). Lower toxicity and non-carcinogenicity of DIOX implies obvious safety 22 benefits whereas lower volatility of the same indicates both safety and possible recyclability 23 advantages due to lower promoter loss in between repeat cycles. Taking all available 24 information into account, the DIOX/water system thus far appears to be an attractive 25 alternative to the rather more toxic THF to be applied for SNG technology for gas storage 26 application.

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We observed a distinctive pattern of hydrate growth behavior for the methane-DIOX/water 29 system. When hydrates nucleate, all three types of molecules present in the system (water, 30 methane and DIOX) participate in the ensuing hydrate formation phenomenon. However, 31 once hydrate nucleation takes place, sluggish gas uptake kinetics is observed for an initial hydrate. This preferential DIOX enclathration stage which occurs at the beginning of the 38 hydrate growth process has thus been termed by us as Step 1 of the growth mechanism for 39 mixed methane-DIOX hydrate system.

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In Figure S3, we present the visual images of the methane-DIOX hydrate system 41 accompanied by the quantified methane uptake volume and percentage for the accompanying 1 time periods for both quiescent and stirred growth regimes. As seen in Figure S3a, at about 7 2 min, while there is visibly lots of hydrates in reactor, the average methane uptake at this stage 3 is only about 9.12 v/v [11.1 (±0.76)%] of the total methane uptake.
Step 1 of growth can also 4 be observed for the stirred system (see Figure S3b); at 2 minutes of hydrate growth (this is 5 the time about when the stirring of contents ceased, refer to Video SV2), the reactor already 6 appears to contain considerable amount of hydrate mass, while the average methane uptake 7 for this system at this point in time is only 12.56 v/v [15.2 (±0.94)%] of the total methane 8 uptake]. If we assume that the enclathration (or cage loading) of DIOX and Methane occur at 9 the same rate into the large (for DIOX) and small (for methane) cages, i.e. 1:2 10 (DIOX:methane), the methane uptake presented in Step 1 in Figure S3 (refer to the bar 11 charts) would correspond to a conversion of ~2.62 ml (for quiescent growth) and 3.53 ml 12 (for stirred growth) of the 32.4 ml solution present in the reactor. However, on the contrary, 13 we observe a considerable amount of hydrate mass in the reactor for both operating modes 14 (quiescent and stirred).

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In Step 2 of the hydrate growth process, accelerated enclathration of methane molecules in 16 the hydrate structure (small cages of sII hydrate) is taking place. Visual images coupled with 17 methane uptake presented in Figure S3 indicates the occupation of methane molecules in the 18 small cages of the formed sII hydrates during Step 2 of hydrate growth. Quantifiably, on an 19 average, 88.9 (±0.76)% and 84.8 (±0.94)% of the total methane uptake into the hydrate phase, 20 for quiescent growth and stirred growth respectively, happened during Step 2. The stirred 21 system ensures better gas-liquid contact due to rigorous mixing right after nucleation, it is 22 plausible to expect that the transition from Step 1 to Step 2 would be much quicker for mixed 23 methane-DIOX hydrate growth under fully stirred condition as compared to that under 24 quiescent condition. 25 In-situ Raman Spectroscopy experiments were independently carried out on the mixed 26 methane-DIOX hydrate system for an independent perspective on the proposed two-step 27 hydrate growth mechanism. A detailed discussion on the in-situ Raman Spectroscopy study 28 carried out as part of the current work has been presented in the supporting information 29 (Pages S5 to S9). Time dependent Raman spectra obtained at various time intervals, for the 30 first 30 minutes of mixed methane-DIOX hydrate growth at 7.2 MPa pressure and 283.2 K, is 31 presented as Figure S5. As already mentioned previously, when hydrates nucleate, we expect 32 both DIOX and CH 4 molecules to start moving into the hydrate structure and stabilize the 33 hydrate cages, i.e. both DIOX and CH 4 trigger hydrate nucleation and initial growth. This is 34 in fact, something we have also observed in our prior work on a similar mixed methane-THF

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(sII) hydrate system. 18, 37 As seen in Figure   and are discussed in detail in supporting information. We see from Figure S5b  for similar time periods. On the other hand, the various peak intensities for DIOX in 5 12 6 4 4 cages do not see a significant increase during the course of this period, alluding to the fact 5 that a significant percentage of the DIOX enclathration in hydrates may already be getting 6 completed during a short initial hydrate growth period, right after nucleation. Independent 7 conclusions drawn from the in-situ Raman spectroscopy study thus appear to align with the 8 observed two-step growth mechanism for mixed methane-DIOX hydrate formation.

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Thus, based on methane uptake, visual observations, in-situ Raman spectra analysis for 10 methane loading in hydrate structure, we present a schematic to describe the two-step hydrate 11 growth for the mixed methane-DIOX hydrate system in Figure 4. The visual observation of 12 the reactor contents, methane uptake and Raman spectra for DIOX and methane signals in sII 13 hydrate structure for 2 minutes and 30 minutes post hydrate nucleation are also presented in 14 Figure 4 to support the proposed mechanism of hydrate growth. The two-step hydrate growth 15 mechanism comprises a preferential DIOX over methane enclathration step (Step 1) and a 16 subsequent rapid and sustained methane enclathration step ( Step 2). The methane uptake formation comprising a preferential DIOX over methane enclathration step (Step 1) and a 24 subsequent rapid and sustained methane enclathration step ( Step 2).

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We further examined the DIOX/water system to form sII hydrates at ultra-rapid rates. To 27 obtain ultra-rapid rates of hydrate formation, we explored the use of small concentrations of 28 bio-friendly amino acids as kinetic promoters. Recently, amino acids have gained promise in 1 the applicability to kinetically promote gas hydrate growth formation, owing to the peculiar 2 morphology pattern for crystal formation. 38 We studied the possibility of enhancing the 3 kinetics of quiescent hydrate growth for the methane-DIOX system by introducing a 4 hydrophobic amino acid and benign kinetic promoter, L-tryptophan into the mixture.   Figure 5a represents gas uptake (quiescent hydrate growth) obtained for a methane-2 DIOX/water system containing 300 ppm L-tryptophan and compares it with the gas uptake 3 (quiescent hydrate growth) for the standard system in the present study, i.e. "the methane-4 DIOX/water system without any additional kinetic promoter". As observed, the presence of 5 L-tryptophan greatly boosts hydrate formation kinetics by inducing ultra-rapid hydrate 6 growth even under quiescent operation. Figure 5b presents a comparison of t 90 and methane 7 uptake rate between the standard case and the solution with 300 ppm L-tryptophan. With L-8 tryptophan present, mixed methane-DIOX hydrate formation reaches 90% completion (t 90 ) in 9 12.11 (±0.79) minutes after nucleation (also see Table S5 in the SI), which is faster by a 10 factor of 2.7 times compared to the standard system without any L-tryptophan. A normalized 11 gas uptake rate comparison for the t 90 periods between the two systems, also presented in  Table S6 in the SI. With regards to 33 hydrate growth pattern of guest molecules, the methane-DIOX system with L-tryptophan also 34 appears to follow the two-step mechanism presented for methane-DIOX system. This can be 35 clearly seen from the initial slow methane uptake rates in Figure 5e (<5 minutes) while 36 Videos SV3 and SV4 reveal the presence of considerable amount of hydrates 5 minute post 37 nucleation, following which an ultra-rapid and sustained methane uptake can be observed in   Figure S7, while the various peak intensities for DIOX trapped in the large cages of 2 sII hydrate do not see as much of a significant increase over the same time period. This then 3 indicates that majority of the DIOX enclathration into hydrate phase would then be occurring 4 during an initial short hydrate growth period immediately following hydrate nucleation, 5 whereas methane enclathration into the hydrate phase happens throughout the hydrate growth 6 process. Coupling the gas uptake trends obtained along with the evolution of time dependent 7 in-situ Raman spectra observed, for hydrate formation from the methane-DIOX/water/L-8 tryptophan system leads us to the conclusion that similar to the methane-DIOX/water 9 systems, for the systems containing L-tryptophan, hydrate formation follows the distinct two-10 step hydrate growth mechanism. This includes, a) a preferential DIOX over methane 11 enclathration step (Step 1) right after hydrate nucleation where majority of the large cages of 12 formed sII hydrate get filled with DIOX; only a small amount of methane molecules get 13 enclatharted into the hydrate phase during this period occupying a few small cages of said 14 hydrate, and b) a rapid and sustained methane enclathration step ( Step 2), which sees the 15 majority of methane uptake into the small cages of formed sII hydrate taking place. 16 We also tested the recyclability of the DIOX/water/L demonstrate the strong recyclability characteristics of the DIOX/water/L-tryptophan system.

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Individual kinetic performance parameters of the two sets of multiple cycle experiments (1C1 25 to 1C10 and 2C1 to 2C7), performed for the DIOX/water/L-tryptophan system, can be found 26 in the supporting information as Table S7.

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Two supplemental studies were conducted to culminate the investigation on mixed methane-28 DIOX hydrate formation in presence of L-tryptophan. For the first, the effect of L-tryptophan 29 concentration on mixed methane-DIOX hydrate formation kinetics was considered, with the 30 conclusion that 300 ppm is the optimum L-tryptophan concentration needed for the current 31 experimental investigation. The results and relevant discussion are presented in the 32 supporting information ( Figure S3 and Table S7). The second supplemental study involved 33 comparing the kinetic performance parameters of mixed methane-DIOX hydrate formation 34 and mixed methane-THF hydrate formation, each in presence of 300 ppm L-tryptophan. The 35 overall conclusion stemming from this study is that given the initial driving force for hydrate 36 formation is kept constant, the methane-DIOX/water/L-tryptophan system kinetically 37 outperforms the methane-THF/water/L-tryptophan system, with a significant edge obtained in 38 the t 90 period, for the system containing DIOX and L-tryptophan. The results and relevant 39 discussion are presented in the supporting information ( Figure S4 and Table S8). effect to remain stable, which happens at 253.15 K at atmospheric pressure for sI hydrates 12 . 7 We investigated the stability of mixed methane-DIOX hydrate pellets, stored at atmospheric  Thus for the first time, we demonstrate highly stable storage of mixed methane-DIOX 2 hydrate pellets at atmospheric pressure. Through the identification of DIOX as a dual-action 3 promoter and synergistically combining with small ampounts of L-tryptophan, we thus 4 address both the bottlenecks pertaining to SNG technology, a) ensuring ultra-rapid hydrate 5 formation at moderate pressure and temperature conditions and b) ensuring highly stable 6 storage of formed hydrates at moderate pressure and temperature conditions.  Table S10 in the for the methane-DIOX hydrate system, wherein the introduction of hydrophobic methane into 17 the system catalyzes initial formation of sII hydrate with a preferential incorporation of 18 DIOX, followed by the incorporation of methane into the molecular water (host) framework.

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Further, by synergistically combining with small concentration (300 ppm) of kientic promoter 20 L-tryptophan, we establish ultra-rapid rates of hydrate formation for methane-DIOX/water

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Experimental Setup 38 The experimental setup employed in this study is the same as detailed elsewhere in the 39 literature. 30,41 Briefly, a high-pressure stainless-steel autoclave with an internal volume of 1~142 ml was used as the reactor vessel. The vessel was fitted with two acrylic viewing 2 windows (3 cm diameter) at the front and back to observe morphological visual changes of 3 contents inside the reactor and a cooling jacket which allowed for coolant circulation from an 4 external chiller so as to regulate the reactor's internal temperature. Highly sensitive pressure 5 transducer (PT) and thermocouple mounts were employed to monitor the pressure and 6 temperature inside the system at all times. The PT and thermocouple were connected to a 7 Data Acquisition (DAQ) system which recorded the requisite data at twenty second intervals.

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Stirring was provided using a 3 cm stirrer bar, controlled by a magnetic stirring plate 9 positioned underneath the reactor.  stirring was initiated to once again induce hydrate formation. When nucleation was observed, 40 stirring was stopped and the reactor was depressurised to the estimated equilibrium pressure. 41 Thereafter, the system was left to reach a state of equilibrium such that an infinitesimally 1 small amount of hydrate crystals remained stable for a sufficiently long period of time; 2 typically any time period upwards of 5 hours. If hydrate crystals were not observed to persist 3 for a sufficiently long period, i.e. complete hydrate dissociation was observed to have taken 4 place inside 5 hours; the reactor was pressurized once again to reform hydrates and 5 subsequently depressurized, this time to a pressure higher than the earlier attempts. If hydrate 6 crystals were observed to persist for a sufficiently long period, the reactor temperature and 7 pressure were noted and the reactor was depressurized slightly, by 20-50 kPa, and allowed to 8 equilibrate for at least another 5 hours. If hydrates continued to persist, the above step was 9 repeated. Otherwise, the last recorded reactor temperature and pressure where infinitesimally 10 small amounts of hydrates were observed to persist for a sufficiently long period of time was 11 noted to be the equilibrium temperature and pressure respectively. Since the temperature of

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After the internal temperature of the reactor had reached the desired set value, the reactor was 23 pressurized with pure methane gas to the predetermined experimental pressure over a period 24 3 -5 minutes so as to ensure the reactor temperature stays stable. The reactor was then sealed 25 tightly and no further gas injection to the reactor was allowed to take place during the 26 experiment, thus essentially making the operation a batch one. Pressure and temperature data was recorded over the course of hydrate formation using a data 10 acquisition system. Pressure drop inside the system was used to calculate the amount of gas 11 consumed due to hydrate formation. As hydrate formation proceeds, pressure inside the 12 system drops as more and more gas is incorporated into the solid hydrate phase. (schematic representation shown in Figure S1 in the Supporting Information (SI)) consists of 38 a stainless steel horizontal cylinder, divided into two halves. The first half (right hand side 39 section) of the prototype is the hydrate formation zone which is designed to form hydrates at (if any) that will be recycled for successive formation trials. form pure methane (sI) hydrates at these conditions (refer Figure 1). Prior to the pelletization compressibility factor, Z, can be calculated using the Pitzer correlation; 30 V R is the volume of 23 the gas phase inside the crystallizer, R is the universal gas constant, P and T are the measured 24 pressure and temperature of the reactor at time "0" (start of the experiment) and any time "t" 25 (during the experiment) respectively. The normalized molar gas uptake (mmol of gas consumed/mol of water) was calculated in the 28 following manner, where Δn methane uptake is the number of moles of methane participating in 29 hydrate formation at any given point of time "t" during the experiment and n water is the total 30 number of moles of water used for hydrate formation.

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---Eq 2 The normalized molar gas uptake (mmol of gas consumed/mol of water) can be converted 33 into the volumetric gas storage capacity (volume of gas (STP) /volume of hydrate) by 34 multiplying with a coefficient of proportionality "K" as shown below.  The normalized gas uptake rate (NR) was computed by fitting the normalized molar gas 18 uptake over a specific interval of time (R t ), 30 then multiplying the result by unit conversion 19 coefficient, K (Eq 4). The overall hydrate formation productivity is taken from the point of 20 nucleation or a deflection point, characteristic of a distinct change in hydrate productivity.

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Procedure followed for Powder X-ray Diffraction (p-XRD) Analysis

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Powder X-ray Diffraction (p-XRD) characterization was used to obtain information about the 24 crystal structure of the resulting mixed methane-DIOX hydrate. The p-XRD measurements 25 were carried out using a BRUKER D8 Advance model (40 kV, 30 mA) capable of analysing 26 the hydrate samples at atmospheric pressure and low temperature conditions. 18 Mixed 27 methane-DIOX hydrates were synthesised using a high pressure reactor as discussed earlier.

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After completion of hydrate formation (as indicated from the ceasing of pressure drop), the 1 excess gas in the reactor was quickly vented out, the reactor was opened up and formed 2 hydrates were instantaneously quenched using liquid nitrogen. This allowed easy recovery of 3 the hydrates while ensuring that they remained stable at atmospheric pressure conditions. The 4 hydrate samples were then ground to a uniform powder using a mortar and pestle in liquid 5 nitrogen environment and quickly transferred to the p-XRD unit for subsequent analysis.

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Each p-XRD pattern was collected with a total 2ϴ scan time of 1.5 minutes and 2ϴ range of Data Acquisition (DAQ) system which records the requisite data at twenty second intervals.

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The high pressure crystallizer is also coupled with a Raman probe which is responsible for 20 providing the in-situ Raman spectrum. A dispersive laser Raman spectrometer was used for 21 real-time Raman characterization and analysis of mixed methane-DIOX hydrate formation.

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Specific details of the spectrometer used may also be found in the literature. 18,46 23 The in-situ Raman spectroscopy experiments were conducted in fully stirred operation (600 target masses of 26.37 g and 6.39 g for water and DIOX respectively, was loaded into the 28 crystallizer for in-situ Raman spectroscopy measurements. For mixed methane-DIOX hydrate 29 formation in presence of L-tryptophan, the solution also contained 300 ppm L-tryptophan as a 30 kinetic promoter, calculated relative to the total weight of the liquid used to make up the 31 solution. Once solution loading was complete, the crystallizer was tightly closed and allowed 32 to reach the desired experimental temperature (making use of the external refrigerator) prior 33 to injection of methane gas. The location of the Raman probe was pre-fixed before the 34 solution was introduced into the crystallizer and was chosen so as to ensure that the probe 35 was as close to the solution interface as possible whilst also being fully submerged in the 36 solution. The eventual selected location of the Raman probe in the present study was 37 consistent with that used in a previous study published by our group and thus representative 38 schematic illustration of the same is already available in the literature. 46 A 3 cm stirrer bar 39 controlled using a magnetic stirring plate positioned underneath the crystallizer was used to 40 provide agitation to the system. Care was taken to ensure that the stirrer bar does not interfere 41 with the Raman probe present inside the system. Once the desired experimental temperature 1 had been reached, the crystallizer was flushed with methane gas through rapid pressurization 2 and depressurization cycles to remove any air present inside the system, following which 3 methane gas injection was carried out slowly up till the desired experimental pressure making 4 sure that the temperature of the system stays close to the desired experimental temperature.

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When methane gas pressurization up to the desired experimental pressure was completed, the 6 system was isolated, i.e. the gas inlet valve was closed (refer schematic available in the 7 literature) 46 and data acquisition was started. Additionally, at this point, both stirring of the 8 system (600 rpm) and Raman signal acquisition were also simultaneously initiated. The real 9 time Raman spectrometer was set to record Raman spectra at 20 second intervals throughout 10 the hydrate formation process. Hydrate nucleation was determined using three simultaneous 11 markers; the familiar characteristic pressure drop and temperature spike of hydrate 12 nucleation, as well as the appearances of characteristic Raman spectra signatures for one or 13 more of hydrate guests (methane/DIOX) incorporated into the hydrate structure.

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GB and MNG performed the p-XRD characterization. GB and YZ performed the in-situ 25 Raman study and analysis. GB and MNG performed the stability study. GB and MNG wrote 26 the original manuscript draft. PL edited the manuscript and contributed to the final version.

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All authors read the final manuscript and consented to the submission.