Small Molecule Cyclic Amide and Urea Based Thickeners for Organic

Jun 17, 2016 - ... Urea Based Thickeners for Organic and sc-CO2/Organic Solutions ... intermolecular interactions necessary for increasing solution vi...
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Small Molecule Cyclic Amide and Urea Based Thickeners for Organic and sc-CO2/Organic Solutions Mark D. Doherty,*,† Jason J. Lee,‡ Aman Dhuwe,‡ Michael J. O’Brien,† Robert J. Perry,† Eric J. Beckman,‡ and Robert M. Enick‡ †

GE Global Research, 1 Research Circle, Niskayuna, New York 12309, United States Department of Chemical and Petroleum Engineering, University of Pittsburgh, 940 Benedum Hall, 3700 O’Hara Street, Pittsburgh, Pennsylvania 15261, United States

‡

S Supporting Information *

ABSTRACT: A series of cyclic amide and urea materials were prepared and screened as small molecule thickeners for organic solvents, dense CO2, and mixtures thereof. In addition to a cyclohexane or benzene core, both of which are mildly CO2-phobic, these molecules contained either ester, amide, or urea groups responsible for establishing intermolecular interactions necessary for increasing solution viscosity. These groups also function to connect siloxane or heavily acetylated CO2-philic segments to the cyclic core of the thickener molecule. Many of these compounds were shown to thicken conventional organic liquids (e.g., toluene, hexane), usually after heating and cooling the mixture. A propyltris(trimethylsiloxy)silane-functionalized benzene trisurea material was also shown to thicken compressed liquid propane and butane. Attaining solubility and self-assembly in CO2 proved more challenging, however. Several ester, amide, and urea containing compounds were discovered that are soluble in dense CO2 at low loadings (0.5−2 wt %). For linear siloxane segments, increasing the number of silicon atoms provides greater solubility in dense CO2. Branched siloxane segments were shown to have superior solubility characteristics in dense CO2 to linear siloxanes of similar silicon content. However, only the propyltris(trimethylsiloxy)silane-functionalized benzene trisurea and trisureas functionalized with varying proportions of propyltris(trimethylsiloxy)silane and propyl-poly(dimethylsiloxane)-butyl groups exhibited remarkable viscosity increases (e.g., 3−300-fold at 0.5−2.0 wt %) in CO2, although high concentrations of an organic cosolvent (18−48 wt %) such as hexane were required to attain dissolution in CO2.

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INTRODUCTION In 2014 CO2 miscible and immiscible enhanced oil recovery (EOR) production from 117 projects accounted for more than 335,000 barrels of oil per day (BOPD) or 3.8% of the total 8.7 million BOPD oil production in the United States.1 Despite more than 40 years of success, CO2 EOR does not recover all of the oil from a formation regardless of whether the reservoir has been previously waterflooded and instead produces only 5− 20% of the original oil in place (OOIP).2,3 Furthermore, the recovery of these 335,000 plus BOPD requires the injection of approximately 3,500 Mscf/d of CO24 or over 10,000 scf of CO2 per barrel of oil. At typical reservoir conditions this corresponds to a utilization ratio of approximately 6 barrels of dense CO2 per barrel of oil recovered.2 Two fundamental reasons behind this poor oil recovery are related to the density and viscosity of CO2 at typical reservoir conditions. The low density of high pressure CO2 relative to oil results in gravity override of the CO2 and reduced recovery of oil from the lower portions of the reservoir. At typical CO2 flooding conditions the viscosity of liquid or supercritical carbon dioxide is approximately 0.03−0.10 cP, while the oil viscosity typically varies from 0.5−5 cP.5 This discrepancy leads to an unfavorable mobility ratio which results in viscous fingering of the dense CO2 around the oil it is intended to displace. This leads to early CO2 breakthrough, high CO2 utilization ratios, low oil production rates, and low percent OOIP recovery.2 The low viscosity of dense carbon dioxide also promotes CO2 flow into low permeability zones which have © 2016 American Chemical Society

been effectively waterflooded causing the majority of the CO2 to bypass much of the recoverable oil. These mobility control issues are widely acknowledged to be the most pressing concerns associated with CO2 flooding.2 The current state-of-the-art technique to reduce the unfavorable mobility of CO2 through porous media is referred to as water-alternating-gas (WAG) injection in which the initial CO2 injection is followed by alternating slugs of water (brine) and CO2. This technique does not change the inherent viscosity of CO2; rather it increases the water saturation and therefore decreases the CO2 saturation within the pores of the formation. This decreased CO2 saturation also causes a reduction in the permeability of CO2 relative to that of the oil it is intended to displace which inhibits viscous fingering. While this technique generally yields more oil than continuous injection of CO2, approximately 30−65% of the OOIP is left behind. Furthermore, this process requires the installation of water injection, production, collection, and processing facilities for the disposal and/or treatment of large volumes of produced water.2 An alternate approach to decrease the mobility of dense CO2 is to employ an additive which will dissolve in CO2 at reservoir conditions and increase the viscosity of the CO2-rich solution to a value that is comparable to or slightly higher than that of the oil that is being displaced. Initial attempts to thicken dense Received: April 11, 2016 Revised: June 6, 2016 Published: June 17, 2016 5601

DOI: 10.1021/acs.energyfuels.6b00859 Energy Fuels 2016, 30, 5601−5610

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solution while incorporating the desired viscosity increasing intermolecular interactions. Many of these fluorinated materials are both environmentally persistent, and all are prohibitively expensive for use as an EOR CO2 thickener at the 1−5 wt % ranges reported above. Given this background, our initial attempts to thicken dense CO2 using nonfluorous associating small molecules focused on end-functionalized polydimethylsiloxane (PDMS) oligomers.18 These materials contained a short linear or branched PDMS core (2 to 30 −Si(CH3)2O− repeat units) with various aromatic end groups (phenyl, naphthyl, anthracenyl, etc.) responsible for generating the necessary intermolecular interactions via π−π stacking. Despite exhibiting good solubility in dense CO2 at 1 wt %, none of these additives increased the solution viscosity. Replacing the end groups with aromatic amides resulted in materials which were capable of gelling hexanes at ∼1 wt % and which are still soluble in high pressure CO 2 -rich solutions. For example, a material featuring anthraquinone amide end groups was shown to increase the solution viscosity at 25 °C and 8200 psi by up to 6× at 10 wt % with the addition of 20 wt % hexanes as a cosolvent.19 In order to maximize the intermolecular interactions between thickener molecules provided by the amide groups while increasing CO2 solubility, we redesigned the structure of the thickener according to three simple design principles as shown in Figure 1. First, the thickener should contain a mildly CO2-

CO2 focused on the addition of high molecular weight polymeric materials.2 Groups at Chevron and the New Mexico Petroleum Recovery Research Center studied the use of high molecular weight polydimethylsiloxane (PDMS: solubility parameter 98:2), 16 (A:B = 83:17), and 17 (A:B = 91:9), and these mixtures were used without further purification in the synthesis of 6, 7, and 31−33 (see below). Replacing the cyclohexane core with an aromatic moiety such as a benzene ring also yields materials which thicken and/or gel a variety of organic solvents, albeit at slightly higher concentrations than analogous thickeners with cyclohexane cores.20−23,26 One benzene triester and several benzene di- and 5604

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Energy & Fuels Scheme 1. Synthesis of cis-1,3,5-Cyclohexanetricarboxamides

Scheme 2. Synthesis of trans-1,2Cyclohexanedicarboxamides

aminodecyl functionalized siloxanes to study the impact of a longer hydrocarbon linker, while keeping a relatively short (10−15 OSiMe2 repeat units) siloxane fragment compared to diamide 9. The aminodecyl functionalized siloxanes were themselves prepared by hydrosilylation of 9-decenenitrile with HSiMe2(OSiMe2)nSiMe2Bu (n = 10 or 15) using Karstedt’s catalyst followed by reduction27 with lithium aluminum hydride. The synthesis for compounds 35 and 36 is identical to that used for the remaining urea containing materials until the point at which the aminosilicone is added to the in situ generated benzene triisocyanate. For these materials, a mixture of the aminopropyl end-functionalized siloxanes depicted in Scheme 7 is added with the stoichiometry indicated. Ability To Thicken Organic Liquids. Screening experiments were performed in hexanes and toluene at room temperature and pressure to evaluate each compound’s ability to thicken organic solvents, and the results are presented in Table 1. For these experiments, 1 and 5 wt % solutions of each compound were prepared in sealed glass vials and heated to obtain homogeneous solutions. After having been cooled to room temperature, the vials were tilted and visually compared with vials containing neat hexanes or toluene to arrive at the results given in Table 1. While the capability of any particular compound to thicken hexanes and/or toluene was indeed encouraging, it was not a prerequisite for further testing in dense CO2. CO2 Solubility Measurements. Compounds 1−11 and 18−37 were subsequently evaluated for solubility in dense CO2 at 1 wt % at temperatures ranging from 25 to 100 °C and pressures up to 10,000 psi using a high pressure windowed cell as described in the Experimental Section, and the results are also displayed in Table 1. Cloud point pressures and relative viscosities for those materials which exhibited solubility in CO2 are collected in Table 2. The physical appearance of the

tricarboxamides were prepared by reaction of the appropriate acid chloride with the corresponding siloxane or acetate functionalized amines or alcohols as depicted in Schemes 4, 5, and 6. As before, commercially available aminopropyl endfunctionalized siloxanes were used to prepare amides 18−20, 22, and 23, while pentaacetylated D-glucamine was used to prepare tricarboxamide 21. Diamide 24 featuring a styrenic linking group was prepared from the aniline functionalized siloxane isolated following hydrosilylation of 4-vinylaniline with HSiMe2(OSiMe2)24OSiMe2Bu using Karstedt’s catalyst. Commercially available carbinol end-functionalized siloxane was used to prepared benzene triester 25. In an effort to increase the number of hydrogen bonding groups relative to the amides described already, several benzene tris- and bisureas were prepared according to previously published methods26 and as depicted in Schemes 7 and 8. The desired acid chloride is first converted to the corresponding di- or triacyl azide which after an aqueous workup is subjected to a Curtius rearrangement to yield the desired di- or triisocyanate. Ureas 26, 27, 31−33, and 37 were prepared by the addition of the corresponding aminopropylsiloxane, while 28 and 34 are derived from pentaacetylated D-glucamine and 3,3-dimethylbutylamine, respectively. Compounds 29 and 30 were prepared from the corresponding 5605

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Energy & Fuels Scheme 3. Synthesis of Branched Aminopropyl Siloxanes 15−17

Scheme 4. Synthesis of Benzene Tricarboxamides 18−21

Scheme 5. Synthesis of Benzene Dicarboxamides 22−24

compounds in Table 1 varied significantly, from low viscosity liquids to waxy solids to free-flowing powders. This variation in turn suggests a variety in the strength of self-interaction in the compounds. Hence, at first glance the physical state of these materials is a good indicator of solubility at 1 wt % in dense CO2. With the exception of acetylated benzene tricarboxamide 21, any of the materials which are classified as a solid (see Table 1) were found to be insoluble as neat compounds at 1 wt %. Tricarboxamide 21 also happens to be the only acetylated compound which was found to be soluble in dense CO2 with a cloud point28 of 3180 psi at 1 wt % at 25 °C. Across all core structures and linking group architectures, the most commonly used CO2-philic group is that derived from a linear monoaminopropyl terminated siloxane containing

approximately 13 silicon atoms. This includes cyclohexane amides 2 and 11, benzene amides 18, 22, and 23, and the urea 27. Of these, the benzene 1,2-dicarboxamide 22 is the only material which is soluble at 1 wt % with a moderate cloud point pressure of 4050 psi at 40 °C. Changing the orientation of the amides as with 1,4-dicarboxamide 23 or adding a third substituent as in the 1,3,5-tricarboxamide 18 resulted in greatly reduced solubility. Replacing the central aromatic ring with a cyclohexane ring as for cis,cis-1,3,5-tricarboxamide 2 does not improve solubility of the neat material. However, addition of 5606

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Energy & Fuels Scheme 6. Synthesis of Benzene Triester 25

Scheme 7. Synthesis of Benzene Trisureas 26−36

toluene as a cosolvent provides a homogeneous solution in CO2 at 25 °C containing 5 wt % 2 with 10 wt % toluene with a moderate cloud point of 3900 psi. Triester analogue 25 is a low viscosity liquid material which is soluble in dense CO2 at 1 wt % with cloud points of 3900, 4130, and 4460 psi at 25, 40, and 60 °C, respectively. Although the CO2-philic group in 25 contains an extra ethylene oxide unit between the siloxane groups and the benzene ring as compared to 18, the drastic difference in the observed solubility of the two compounds is likely due to a lack of hydrogen bond donor groups in triester 25 that are present in tricarboxamide 18. Perhaps not surprisingly, adding more hydrogen bond donor groups as exemplified by the trisurea 27 does not improve solubility in dense CO2. Increasing the number of silicon atoms in the siloxane moiety has a significant impact on solubility in dense CO2. For instance, doubling the number of silicon atoms in a linear siloxane moiety from 13 to 26 results in increased solubility in dense CO2. Typically, increasing the molecular weight of an oligomeric material in CO2 decreases its solubility at constant temperature and pressure owing to the impact of chain length on the entropy of mixing (decrease). However, in these multifunctional materials, increasing the length of the silicone chain, while lowering the entropy of mixing, enhances the enthalpy of mixing via the addition of “CO2-philic” functional groups. Much like 2, cyclohexane tricarboxamide 3 requires 10 wt % toluene as cosolvent to achieve a homogeneous solution; however, the observed cloud point is ∼1100 psi lower than that for 2. Similarly, benzene tricarboxamide 19 exhibits a cloud point of 4500 psi at 1 wt % at 40 °C, whereas 18 is insoluble at all temperatures examined. The related cyclohexane dicarboxamide 9 which contains 27 silicon atoms in a linear siloxane displays an even lower cloud point of 1800 psi at 25 °C and 1 wt %. One material for which this does not hold is dicarboxamide 24 which is insoluble in dense CO2 despite having 26 silicon atoms in each siloxane arm. In this case, the replacement of the propyl connecting group with an ethylphenyl group is likely responsible for the poor solubility of 24 as the extended π-system increases intermolecular π−π interactions too much. The identity and composition of the connecting group does not always have such a strong impact on solubility as illustrated by comparing 27 and 29. Increasing the length of this connecting group from propyl to decyl without significantly altering the number of silicon atoms does not significantly alter the cloud points of these two compounds in CO2. Introducing branching into the structure of the siloxane moieties was also shown to have a significant impact on CO2 solubility. The addition of linear or cyclic branches to the siloxane chains of cyclohexane tricarboxamides 6 and 7 resulted in improved solubility at 1 wt % at 25 °C with cloud points of

2600 and 2800 psi, respectively, and is consistent with CO2 solubility results previously reported29 for branched vs linear polymers. These are the lowest pressures observed for the cyclohexane tricarboxamides examined. A similar effect was observed with linear and cyclic branched siloxane functionalized benzene trisureas 31, 32, and 33 which readily dissolved in dense CO2 at 1 wt % at 25 °C with cloud points of 1600, 3000, and 1500 psi, respectively. Branched trisurea 26 exhibits an even lower cloud point of 1280 psi at 25 °C at 1.6 wt %; however, a large volume (48.4 wt %) of hexanes is required as a 5607

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Energy & Fuels Scheme 8. Synthesis of Benzene Bisurea 37

Table 2. Cloud Point Pressures and Relative Viscosities of Compounds Determined To Be Soluble in Dense CO2a

Table 1. Organic Liquid Thickening Capability and CO2 Solubility Data for Compounds 1−11 and 18−34a thickens organic solvents hexanes

compd 1 2 3 4 5 6 7 8 9 10 11 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

physical state powder clear gel soft gel waxy solid powder tacky gum viscous liquid powder liquid waxy solid viscous liquid clear gel soft gel powder powder clear gel tacky solid clear gel liquid powder waxy solid powder tacky solid rubbery solid liquid viscous liquid viscous liquid powder clear gel clear gel powder

1 wt % 21

toluene

5 wt % 21

1 wt % 5 wt % 21

21

soluble in dense CO2 at 1 wt %

G N N N N N N

G N N S N N N

G N N N N N N

G N N S N N N

N N N N N Y Y

hazy G N N N

hazy G N N N

G N N N

G N N N

N Y N Y

N N N I N N

N hazy S N I N N

N N N N N N

N N N N N N

N Y N Y Y N

N N V S N S

N N hazy G I N S

N N S N N S

N N S N S S

N Y N N N N

N

N

N

N

N

N N

N N

N N

N N

Y Y

N

N

N

S

Y

I S S S

I M S S

I S S S

I V M S

N N N N

a

compd

temp (°C)

concn (wt %)

cosolvent; concn (wt %)

cloud point pressure (psi)

relative viscosity

2 2 3 6 7 9 9 11 19 21 22 22 25 25 25 26 26 26 27 29 31 32 33 35 35 36 Bu3SnF Bu3SnF Bu3SnF Bu3SnF Bu3SnF Bu3SnF

25 40 25 25 25 25 40 25 40 25 40 60 25 40 60 25 25 25 81 90 25 25 25 25 25 25 25 40 60 25 40 60

5.0 1.0 5.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.6 1.3 0.5 0.5 0.5 1.0 1.0 1.0 1.5 1.0 2.0 0.5 0.5 0.5 1.0 1.0 1.0

toluene; 10.0

3900 4100 2820 2600 2800 950 1800 3900 4500 3180 4050 4125 3900 4130 4460 1280 8800 ∼800 8300 8610 1600 3000 1500 4670

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 300 100 14 1 1 1 1.2 1 30 5 3 2.2 1.9 1.2 110 42 20

toluene; 10.0

hexanes; 48.4 hexanes; 38.7 hexanes; 39.5

hexanes; hexanes; hexanes; hexanes; hexanes; hexanes; hexanes; hexanes; hexanes;

18.5 19.0 18.0 40.0 40.0 40.0 40.0 40.0 40.0

2100 2850 2640 2570 6700 5040 4780

Shear rate ∼7000/(relative viscosity) s−1.25

requires less cosolvent than 26 to attain a single phase solution with a cloud point of 4760 psi at 1.5 wt % with 18.5 wt % hexanes at 25 °C. Similarly, 36 produces a single phase solution at 2 wt % 36, 18 wt % hexanes, and 80 wt % CO2 with a cloud point of 2100 psi at 25 °C. CO2 Viscosity Measurements. Relative viscosity data shown in Table 2 was generated using close-clearance falling ball viscometry, where a Pyrex sphere was placed within the sample volume of a hollow quartz tube contained within the high pressure cell used for solubility measurements as described in detail elsewhere.3,5,25 By comparison of the terminal velocity of the Pyrex sphere in a thickened solution to that obtained in the absence of thickener one arrives at a ratio of terminal velocities which is approximately equal to the ratio of viscosities of the two solutions. This assumes the solution in question is Newtonian and that the change in CO2 density induced by the presence of the thickener is small.3 If the terminal velocity of the ball in pure CO2 is ∼1 cm s−1, then the surface areaaveraged shear rate of the falling ball for this viscometer can be approximated as 7000/(relative viscosity) s−1. The relative viscosities recorded in Table 2 were measured at the concentration and temperature indicated and at a pressure approximately 1000 psi above the cloud point pressure to ensure a single phase solution was examined. Table 2 also

a

Y = yes, N = no, S = slightly thicker, M = moderately thicker, V = very thick, G = gel, I = insoluble.

cosolvent to obtain a single phase CO2-rich solution. Mixed trisurea 35, which contains both propyltris(trimethylsiloxy)silane and propyl-poly(dimethylsiloxane)-butyl groups, actually 5608

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(7.5 wt % 35 in hexanes), and 80 wt % CO2 yields a single phase solution at 25 °C which exhibits a moderate increase in solution viscosity by a factor of 30. A similar mixture consisting of 2 wt % 36, 18 wt % hexanes (10 wt % 36 in hexanes), and 80 wt % CO2 produced a single phase solution at 25 °C with a viscosity 3 times higher than in the absence of thickener. Contrary to trisurea 26, these CO2-hexanes solutions of 35 and 36 did not require a heating and cooling cycle to achieve a homogeneous CO2-rich solution.

contains CO2-thickening data for one of the most efficient small molecule thickeners for light alkanes (pentane−ethane)13,30 that has ever been reported, tributyltin fluoride, which also requires a cosolvent for dissolution in CO2. Of all of the compounds listed as being soluble in dense CO2 at 1 wt % in Table 1 (6, 7, 9, 11, 19, 21, 22, 25, and 31−33), only 32 was observed to increase the viscosity of CO2, albeit by a disappointingly low value of 20% (see Table 2). Six other compounds which the laboratory screening experiments in hexanes and toluene identified as effective thickeners were also tested in dense CO2 with hexanes or toluene as a cosolvent. Although cyclohexane tricarboxamides 2 and 3 did not thicken hexanes or toluene at 5 wt %, both were shown to thicken toluene at 33 wt %. Despite the increased solubility of 2 and 3 at 5 wt % with 10 wt % toluene as cosolvent, neither compound thickened the CO2 rich solution at 25 or 40 °C. Branched benzene trisurea 26 exhibited only slight increases in viscosity at both 1 and 5 wt % in toluene. These same concentrations in hexanes showed significant increases in viscosity to generate a thick flowable liquid at 1 wt % and a hazy gel at 5 wt % which does not move upon inversion of the vial. Examination of a 1.6 wt % 26 and 48.4 wt % hexanes mixture (3.2 wt % 26 in hexanes) in dense CO2 provided a single phase solution at 25 °C which exhibited a remarkable increase in solution viscosity by a factor of 300. Addition of CO2 to this solution to give a total composition of 1.3 wt % 26, 38.7 wt % hexanes, and 60 wt % CO2 (still 3.2 wt % 26 in hexanes) decreased the solubility of the thickener as the cloud point increases to 8800 psi at 25 °C. This also results in a decrease in the observed relative viscosity of the solution which now is only increased by a factor of 100. Reducing the concentration of 26 to 0.5 wt % with 39.5 wt % hexanes (1.25 wt % 26 in hexanes) yields a solution with a much lower cloud point of ca. 800 psi at 25 °C; however, the relative viscosity of this composition is only 14 times higher than that measured for the corresponding 40 wt % hexanes and 60 wt % CO2 solution in the absence of 26. It should be noted that all compositions required a heating (70 °C) and cooling cycle to achieve dissolution of 26. Because of the efficacy of 26 in thickening toluene, hexanes, and CO2hexanes solutions, it was also assessed in liquid butane, propane, and ethane. At a concentration of 1.5 wt %, 26 thickened liquid butane by a factor of 280, 46, 12, and 1.8 at temperatures of 25, 40, 60, and 80 °C, respectively. However, at a concentration of 1.5 wt % in propane, 26 induced only 1.5-, 1.3-, 1.3-, and 1.2-fold increases in viscosity at 25, 40, 60, and 80 °C, respectively. 26 was ethane-insoluble. In an effort to increase the solubility of 26 by disrupting some of the crystallinity, one-third of the aminopropyltris(trimethylsiloxy)silane employed in Scheme 7 was replaced by the aminopropyl end-functionalized siloxane used to generate 27. The resulting benzene trisurea 35 contains on average two branched siloxane substituents like those present in 26 and one linear siloxane substituent like those present in 27. Similarly, on average 36 consists of one branched siloxane substituent and two linear siloxane substituents. While this structural change was successful in disrupting the crystallinity as evidenced by the fact that these materials are clear gels rather than solids, neither of these materials are soluble in dense CO2 in the absence of a cosolvent. Trisurea 35 produces very thick flowable solutions in both hexanes and toluene at 5 wt %, while 36 gives a moderately thicker solution at 5 wt % in toluene and only slightly thicker solutions in hexanes and toluene at 1 wt %. As such, a mixture consisting of 1.5 wt % 35, 18.5 wt % hexanes

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CONCLUSION A wide variety of cyclic amide and urea materials were synthesized and tested for their ability to thicken organic solvents (hexanes and toluene) and dense CO2. Three structural classes in particular were explored. First, cyclohexane amide materials 1−11 were synthesized by reaction of the appropriate amine and carboxylic acid or acid chloride in the presence of base. These materials each contain two or three amide linking groups attached to siloxane or acetylated CO2philic groups and were modeled after the analogous alkyl amides20−22 which are known to thicken a wide variety of polar and nonpolar organic solvents. A second class of materials, 18−25, featuring an aromatic benzene ring at its core was synthesized in a similar fashion. Many of these compounds also contain two or three amide linking groups attached to siloxane or acetylated CO2-philic groups; however, the aromatic core of these molecules can also contribute to intermolecular interactions necessary to increase solvent viscosity. One instance, compound 25, contains three ester linking groups which lack the hydrogen bond donor groups necessary to form intermolecular interactions. Aromatic ureas, 26−36, were prepared by reaction of the appropriate amine with the in situ generated benzene di- or triisocyanate and make up the third and final class of materials examined. Here, the siloxane or acetylated CO2-philic groups are connected to an aromatic core via two or three urea linkages which contain additional hydrogen bond donor and acceptor groups relative to the amides described above. Several of these materials, 6, 7, 9, 11, 19, 21, 22, 25, and 31− 33, were found to be sufficiently soluble in dense CO2 at 1 wt % at 25 °C to generate transparent single phase solutions. In general, CO2-philic segments containing branched siloxane groups were more soluble than linear siloxane groups with a similar number of silicon atoms (cf. 27 vs 31 and 33). For linear siloxane based CO2-philic segments, increasing the number of silicon atoms significantly increased the solubility of the resultant material in dense CO2 (cf. 2 vs 3 or 18 vs 19). Laboratory screening experiments in hexanes and toluene identified several materials which were capable of thickening or gelling these solvents at 1, 5, or in two instances 33 wt %. Based on these results, the addition of hexanes or toluene as a cosolvent enabled five additional compounds, 2, 3, 26, 35, and 36, to achieve homogeneous single phase solutions in dense CO2. Urea linkages outperformed amide and ester linking groups in terms of establishing the intermolecular interactions required to thicken dense CO2-rich solutions as 26, 32, 35, and 36 are all benzene trisureas. This is not surprising considering the urea linkage contains twice as many hydrogen bond donor groups as the amides, while the ester contains none. Considering that at typical CO2 flooding conditions the viscosity of liquid or supercritical carbon dioxide is approximately 0.03−0.10 cP while the oil viscosity typically varies from 0.5−5 cP5 even moderate increases in the CO2 viscosity 5609

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could overcome many of the issues associated with viscous fingering and CO2 flow into low permeability zones. Use of a 1.5 wt % solution of hybrid benzene trisurea 35 with 18.5 wt % cosolvent above approximately 5000 psi would increase the solution viscosity to approximately 0.9−3 cP which would be sufficient to inhibit viscous fingering during the recovery of many lighter oils. Similarly, the use of a solution containing 1.3 wt % 26 and 38.7 wt % cosolvent above approximately 9000 psi would increase the solution viscosity to approximately 3−10 cP which would increase recovery of moderately viscous oils, albeit at a pressure that is above the pressure that can be maintained in the formation. Increasing the amount of thickener and cosolvent to 1.6 wt % 26 and 48.4 wt % cosolvent above 1500 psi would increase the solution viscosity even further to 9−30 cP enabling more efficient recovery of some the heaviest oils! Unfortunately, the large cosolvent fraction required to obtain viscous CO2 rich solutions limits the practical utility of these thickeners. Despite this limitation, these materials compare favorably with one of the most effective small molecule thickeners for light alkanes ever reported: tributyltin fluoride, which also requires high concentrations of a cosolvent to thicken CO2. In addition, these urea based materials do not contain environmentally harmful fluorinated segments or toxic organotin moieties which are frequently found in other small molecule thickeners.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b00859. Experimental procedures and characterization data for compounds 3−7, 10, 11, 16, 17, 19−21, 24, and 27−36 (PDF)

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

Disclosure: This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government, any agency thereof, nor any of their employees makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed or represents that its use would not infringe privately owned rights. References herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency − Energy (ARPA-E), U.S. Department of Energy, under Award Number DE-AR0000292. We wish to thank Dr. Tom Early for 29Si NMR analyses. 5610

DOI: 10.1021/acs.energyfuels.6b00859 Energy Fuels 2016, 30, 5601−5610