Effect of Surface Chemistry on Confined Phase Behavior in

Subscriber access provided by UNIV OF DURHAM

Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage

The Effect of Surface Chemistry on Confined Phase Behavior in Nanoporous Media: An Experimental and Molecular Modeling Study Evan Lowry, and Mohammad Piri Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00986 • Publication Date (Web): 14 Jul 2018 Downloaded from http://pubs.acs.org on July 19, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

The Eect of Surface Chemistry on Conned Phase Behavior in Nanoporous Media: An Experimental and Molecular Modeling Study Evan Lowry∗ and Mohammad Piri

College of Engineering and Applied Sciences, University of Wyoming, Laramie WY E-mail: [email protected]

Abstract It is well accepted that nanopore size is a controlling parameter in determining the phase behavior of conned adsorbate molecules. Despite this knowledge, the quantitative eect of surface chemistry on the conned phase behavior is a factor that remains obfuscated. Obtaining a complete understanding of the variables controlling conned phase behavior is a critical step in developing more complete equations of state for predictive modeling. To this end, a combined experimental and molecular modeling study was conducted to investigate the eects of surface chemistry and wetting on the conned phase behavior of propane and n-butane in modied and unmodied silica MCM-41. Isotherms were measured in four types of silica MCM-41 modied with varying sizes of alkyl groups to determine the eects of increasing surface modication. Results showed that increased pore surface coverage of carbon resulted in a notable change in the capillary condensation pressures, adsorption enthalpy, and conned critical temperature of the adsorbate. Correlations between the surface coverage of carbon and the conned critical temperature were presented and supported by thermodynamic arguments. The primary conclusions were partially supported by hybrid molecular dynamics-Monte 1

ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 33

Carlo simulations of propane adsorption in models of the four types of experimental adsorbents. Several dierences were noted and explained between the experimental and modeling results. Energetic heterogeneity on the surface of the modied MCM41 adsorbents as well as dierences in adsorbate entropy induced by surface features and chemistry were suggested as primary driving factors for the observed trends. The results of this work have direct implications for improving understanding of conned phase behavior in materials of varying surface chemistry.

Introduction The eects of decreasing pore sizes within microporous adsorbents have been investigated at length in recent literature.

14

This phenomenon is critically relevant to a number of indus-

trial applications and fundamental research areas. Careful characterization of the type and extent of the eects of connement on phase behavior is vital for engineering design in areas such as catalysis, oil and gas applications, and air pollution remediation.

511

Previous studies

have suggested that connement induced phase behavoir, also termed and associated with capillary condensation, appears when pore sizes are in the range of 2 to 100 nm and when the pore diameters approach the mean free path of the adsorbate molecules.

1,2

This behavior

results in a rst, or nearly-rst, order phase transition that often occurs at lower pressures than the bulk vapor-liquid phase transition.

12

Travalloni and colleagues produced a series

of modied equations of state based on previous knowledge regarding capillary condensation.

1315

The fundamental variables in their models were based on the relationship between

the molecular diameter and the pore radius with the eects of uid-pore wall interactions increasing as the pore size decreased. This correlative observation has been upheld in molecular dynamics and Monte Carlo simulation studies.

1620

Using an empirical approach, Tan

and Piri devised an equation of state for conned uids based on coupling of PC-SAFT and the Young-Laplace equations.

21

Despite these notable eorts, there remains a large amount

of uncertainty regarding the key variables controlling phase behavior in connement.

2


Page 3 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Silica based adsorbents have received much attention in research associated with capillary condensation primarily due to the relative ease of production via chemical templating and the regular, nanometric pore sizes associated with such materials.

22

Adsorbents such as

MCM-41 and SBA-15 have been routinely used to investigate connement phenomena.

23,24

Morishige et al. investigated the so-called capillary critical point of argon, nitrogen, ethylene, and carbon dioxide in MCM-41. They showed that the capillary critical temperature of these conned gases is much lower than the bulk critical point.

3

In subsequent studies, the

hysteresis temperature was dierentiated from the conned critical temperature by noting a change in the linearity of the plot of logarithmic pressure versus temperature over a series of measured isotherms.

4

The hysteresis temperature, conned critical temperature, and dif-

ferential enthalpy of adsorption were observed to be dependent on the pore size in a number of other studies as well.

2527

Although there is a relatively signicant amount of data available regarding the impact of pore size on capillary condensation, the eects of variation in adsorbate-adsorbent interactions are still poorly understood.

The eects of wettability on the qualitative shape of

adsorption isotherms were codied in a recent IUPAC report.

28

Isotherms are generally clas-

sied by type. A wetting isotherm (Type IV) is indicated by a downward concave shape in the low pressure region whereas non-wetting behavior (Type V) is classied by an isotherm that exhibits upward concavity in the low pressure region. Despite having evidence of the eects of surface chemistry on capillary condensation via isotherm shape, there remains very little explanation of the fundamentals governing this phenomena.

Attempts to model the

eects of these interactions have been made by Gubbins et al. by introducing a wetting parameter to the traditional expressions for the grand partition function. This parameter is based on the ratio between the adsorbate-adsorbent and the adsorbate-adsorbate interactions. Although the parameter is rather esoteric, it was shown to t experimental data fairly well for adsorption in carbon nanotubes with a variety of adsorbates and for MCM-41 with the adsorption of water.

29

Other studies have focused on the industrial applications of surface

3


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 33

modication in MCM-41. One such study showed that a hybrid organic/inorganic MCM41 matrix enhanced the loading and release characteristics of ibuprofen.

The discussion

incorporated both an investigation of pore size and surface modication with aminopropyl groups.

30

The outcome showed that the surface chemistry leveraged rst order eects on the

loading and desorption while the pore size was not a signicant controlling factor.

Mello

et al. demonstrated that amino-modied MCM-41 served as a more ecient low-pressure adsorbent for CO2 capture due to increased enthalpy of adsorption compared to unmodied MCM-41.

31

The surface groups induced chemisorption at lower pressure that enhanced the

low pressure loading capacity of the modied MCM-41 material. Xu et al. demonstrated that adsorption capacity and kinetics are directly aected by the degree of surface modication in a silica MCM-41, which was modied with polyethylenimine (PEI) to varying degrees. It was observed that although the PEI decreased the pore size, it lead to greater loading capacity for CO2 compared to pure MCM-41 and pure PEI adsorbents.

32

Several Monte Carlo studies have also investigated the eects of interactions between surface and adsorbate. Puibasset et al. performed simulations in the grand canonical ensemble to determine the phase transition of a Lennard-Jones uid within three dierent pore types. Using a regular, geometrically undulated, and chemically undulated pore, the adsorption behavior and coexistence regions were extracted from simulation. Results indicated that the geometrical undulation did not signicantly impact the phase behavior of the uid while chemical undulations resulted in a larger hysteresis region as well as creating apparent intermediate phases within the vapor-liquid coexistence region.

33,34

Other simulations showed

that changes in the attractive parameters to make the solid less wetting resulted in a shift of the phase behavior towards the bulk behavior.

35,36

Experimentally, most previous inves-

tigations of surface-uid interactions have focused on maximizing the adsorption capacity for the removal of VOCs from industrial waste euent streams.

3741

Both Kim et al. and

Wang et al. demonstrated that MCM-41 and SBA-15 modied with organic surface groups showed superior adsorption performance with common industrial solvents including hexane,

4


Page 5 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

gasoline, benzene, and toluene.

37,38

In spite of the aforementioned studies, there is still a lack of understanding regarding the fundamental mechanisms and driving factors that govern the interplay between adsorbentadsorbate interactions and capillary condensation. To this end, experiments and simulations were conducted to develop a better understanding of these complex interactions. following section, the experimental methods and theory is presented.

In the

This is followed by

detailed presentation and discussion of the results, which leads to the major conclusions of this work.

Methods Preparation of adsorbents and uids Four types of adsorbents were synthesized based on standard templated mesoporous silica MCM-41.

Pure silica MCM-41 as well as silica MCM-41 possessing surface modication

with C1 , C8 , and C18 alkyl functionalities were synthesized by Galantreo, LTD. The adsorbents were modied such that all had nearly the same pore diameter and resulted in alkyl bonding densities between 0.475 and 1.44

µmol/m2 .

Elemental analysis was provided by the

manufacturer and was also independently conrmed using electron dispersive spectroscopy (EDS). EDS was conducted with a Bruker X-Flash detector with energy resolution of 121 eV. Measurements were taken over multiple sample particles with accelerating voltage of 10 kV and current of 0.4 nA. Spectra were acquired and subjected to matrix corrections before integrating and determining the averages. The results from BET and the average wt% of carbon from elemental analysis for each adsorbent are listed in Table 1. Pure n-Butane and Propane (99.995% purity) were obtained for use as the uid phases during all experiments. The gravimetric adsorption apparatus was designed and fabricated to produce highly accurate temperature and mass measurements.

42,43

Dierential balances

manufactured by Mettler Toledo were used to read the mass of the adsorption cells to an

5


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 33

Table 1: Results of BET analysis for silica MCM-41 with various surface modiers MCM-41 Surface Modier

2 Surface Area (m /g)

None

C1

C8

C18

596

345

183

122

◦

Pore Diameter (A)

40

36

42

38

Pore Volume (cc/g)

0.49

0.34

0.22

0.15

wt% C

0

6.15

12.3

17.3

accuracy of 0.00001 g. Temperature was controlled within 0.1 K using a Thermotron environmental chamber. High precision Rosemount pressure transducers monitored the positive pressure of the uid in contact with the titanium adsorption cell while Leybold vacuum gauges were used to read values below atmospheric pressure. Data was digitally logged and recorded for averaging after the experiment. More details about the apparatus used in this study can be found elsewhere.

43

Experimental Procedure Four adsorption cells were rst cleaned and weighed before packing with each adsorbent and weighing again. The adsorption cells were installed into the apparatus and the system was pressure tested at 400 psi to eliminate leaks.

Special care was taken not to increase the

temperature to a point where the surface functional groups may be chemically altered or removed from the pore surfaces. Jaroniec et al. performed a systematic study of the surface modication eects in silica MCM-41 using organosilane modication reactions.

Thermo-

gravimetric analysis showed that, unlike standard MCM-41, the organo-modied MCM-41

◦ had excellent thermal stability up to 100 C at which point the organosilane bonds began to decompose.

44

◦ Therefore, the system was regulated at 50 C for two weeks while under

high vacuum to remove contaminants from the surface of the adsorbents. The vacuum level

◦ reached to 1 mbar at which point it became stable. The initial isotherms at 16 C for propane in the unmodied MCM-41 were compared to those recently published by Barsotti et al.

45

The isotherms were found to match nearly identically and therefore it was concluded that no

6


Page 7 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

signicant adsorbed water remained within the system. Experiments were conducted at multiple dierent temperatures with propane and n-butane. In order to construct each isotherm, the pressure, temperature, and mass were recorded in real time while small amounts of uid were injected to each adsorption cell. The pressure response was monitored for stability that was taken as an indicator of thermodynamic equilibrium. Doses of uid were progressively administered to each cell until after the bulk condensation point of the uid was observed. At this point, the desorption isotherm was established by progressively placing the system under short periods of vacuum pressure to remove incremental amounts of uid. The mass and pressure data points on the isotherm were extracted by taking an average over 100 time-series data points prior to the time at which a dose was administered. By doing this, each isotherm point was taken as close to equilibrium as possible.

Between experimental

temperatures, the system was placed under high vacuum (1 mbar) to return the adsorbents as close to the initial conditions as possible.

NVT-GCMC Simulation In an attempt to verify results obtained from experiments at the molecular level, coupled grand canonical Monte Carlo (GCMC) and molecular dynamics (NVT) simulation was used to study the adsorption of propane on several dierent models of silica MCM-41. GCMC simulation uses traditional Monte Carlo moves as well as particle insertion-deletion steps in order to match the model system to the thermodynamic characteristics of an imaginary reservoir.

The grand canonical ensemble xes the chemical potential (µ), system volume,

and temperature during particle insertion-deletion steps and follows the metropolis criterion for determining probabilities in Monte Carlo moves. predicting phase behavior and adsorption.

47,48

46

GCMC has been traditionally used for

By coupling GCMC with NVT simulation,

particles are allowed to translate and rotate according to the traditional molecular dynamics framework between GCMC moves. This allows the system to reach a minimum energy state more quickly and provides for more complex particle-surface interactions.

7


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 33

Four dierent model pores were prepared to mimic the substrates used in the experimental work. Initally, a base silica MCM-41 pore was prepared as described previously.

49

An algorithm was used to place dierent surface modifying groups within the pores. First, under-coordinated silica was removed from the interior surface of the pore.

Next, each

under-coordinated surface oxygen atom was considered for bonding to a surface modifying group.

The surface modifying molecule with appropriate chain length was progressively

grown, starting from an oxygen atom at the surface of the pore toward the central pore axis. Surface modifying groups were placed randomly with the constraint that each progressive addition must be a maximal distance from other nearby groups.

The algorithm was

terminated once the desired level of surface carbon was reached as determined by matching the characterization data in Table 1 as closely as possible. This procedure was used to create four pore types: unmodied MCM-41 and MCM-41 with methyl, octyl, and octadecyl group surface modications. It was desirable to use a very simple model to approximate the adsorbents to aid in determining the controlling eects in the adsorption process. As a result, charge eects were not considered and only Lennard-Jones dispersive forces were used for simulation using a united-atom approach. The TRAPPE-UA model was used for the alkyl surface groups as well as for the propane adsorbate molecules.

50

Once modied, each model was subjected to a 5 ns

NVT equilibration sequence with a timestep of 1 fs and temperature coupling every 10 steps to allow the surface alkyl chain molecules to achieve a minimum energy conguration. The void volume of each model was calculated using simulated helium porosimetry with a probe ◦

radius of 1.2

A. 51 To simulate adsorption, a hybrid NVT-GCMC framework was implemented

using the LAMMPS platform.

52

This framework allows the surface groups and the adsorbate

molecules to undergo translational and rotational motions between Monte Carlo steps. Each chemical potential and temperature condition was simulated for over 1×10

6

Monte Carlo

steps until equilibrium was reached, as implied by stability of the total inserted particles. Isotherms, as well as thermodynamic data, were extracted from the raw simulation data and

8


Page 9 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

averaged over the last 50,000 simulation steps. Only dispersion interactions were considered in the simulations and consequently surface hydrogen atoms were ignored.

Results and Discussion Isotherms were extracted from the raw data and are reported in terms of the average uid density within the pore. This quantity was calculated by rst determining the pore volume from BET data in Table 1. Next, the amount of bulk phase uid was subtracted from the total amount of uid prior to dividing by the pore volume.

The bulk phase density was

extracted from data published by NIST at the equilibrium pressure for each point.

53

By

reporting the amount adsorbed in terms of average uid density within the pores, direct comparisons were possible between each isotherm.

9


Langmuir

1.25

1.25

T = 2 0C

T = 8 0C

1 Pore Fluid Density (g/cc)

Pore Fluid Density (g/cc)

1

0.75

0.5

0.75

0.5

0.25

0.25

0

0 0

0.2

0.4

0.6

0.8

1

0

0.2

0.4

P/P0 1.25

1.25

T = 16 0C

0.6 P/P0

0.8

1

0.8

1

0.8

1

T = 24 0C



1

0.75

0.5

0.75

0.5

0.25

0.25

0

0 0

0.2

0.4

0.6

0.8

1

0

0.2

0.4

P/P0 1.25

1.25

T = 28 0C

0.6 P/P0

T = 32 0C



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 33

0.75

0.5

0.25

0.75

0.5

0.25

0

0 0

0.2

0.4

0.6

0.8

1

0

0.2

0.4

P/P0

Figure 1:

0.6 P/P0

Propane isotherms for standard, C1, C8, and C18 modied silica MCM-41.

Isotherms for MCM-41-C8, MCM-41-C18, and MCM-41 are shifted vertically by 0.15, 0.3, and 0.45 respectively. MCM-41 isotherms are denoted by triangles, C1 modied MCM-41 is denoted by circles, C8 modied MCM-41 is denoted by squares, C18 modied MCM-41 is denoted by pentagons. Open symbols denote desorption data.

10


Page 11 of 33

T = 2 0C

T = 8 0C

1.25



1.25

1

0.75

0.5

0.25

1

0.75

0.5

0.25

0

0 0

0.2

0.4

0.6

0.8

1

0

0.2

0.4

P/P0 1.25

T = 16 0C

0.6 P/P0

0.8

1

0.8

1

0.8

1

T = 18 0C

1.25



1

0.75

0.5

0.25

1

0.75

0.5

0.25

0

0 0

0.2

0.4

0.6

0.8

1

0

0.2

0.4

P/P0

T = 24 0C

1

0.75

0.5

0.25

0.6 P/P0

T = 32 0C

1.25


1.25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

1

0.75

0.5

0.25

0

0 0

0.2

0.4

0.6

0.8

1

0

0.2

0.4

P/P0

11


0.6 P/P0

Langmuir

1.25

T = 40 0C

T = 45 0C

0.7 0.6 Pore Fluid Density (g/cc)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 33

0.75

0.5

0.5 0.4 0.3 0.2

0.25 0.1 0

0 0

Figure 2:

0.2

0.4 P/P0

0.6

0.8

0

0.2

0.4 P/P0

0.6

0.8

n-Butane isotherms for standard, C1, C8, and C18 modied silica MCM-41.

Isotherms for MCM-41-C8, MCM-41-C18, and MCM-41 are shifted vertically by 0.15, 0.3, and 0.45 respectively. MCM-41 isotherms are denoted by triangles, C1 modied MCM-41 is denoted by circles, C8 modied MCM-41 is denoted by squares, C18 modied MCM-41 is denoted by pentagons. Open symbols denote desorption data.

On rst observation, it is clear that tests conducted on the unmodied, C1 modied, and C8 modied MCM-41 adsorbents resulted in Type IV isotherms for both propane and n-butane.

All isotherms showed a downward concavity which indicates favorable or wet-

ting adsorption.

28

The C18 modied MCM-41 did not present classically Type IV isotherm

behavior. Instead, the C18 MCM-41 isotherms appeared to induce continuous condensation along the lower region of the isotherm prior to reaching a capillary saturation plateau. For all isotherms in the modied MCM-41, the general trend appeared to be a decrease in the capillary condensation pressure as the degree of surface modication of the MCM-41 increased.

40

This is attributable to the increased adsorbent-adsorbate interactions resulting

from the alkyl surface groups.

Additionally, the unmodied MCM-41 induced the largest

average density in the adsorbed uid within the pores followed by the C18, C8, and C1 modied MCM-41 adsorbents. This observation is agreement with the previous literature.

54,55

One hypothesis for the lack of a distinct capillary condensation point in the C18 modied MCM-41 is that the adsorbate experienced premature condensation during the pore lling region across the entire pressure range preceding the pore saturation plateau and bulk

12


Page 13 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

condensation. One surprising result is the fact that the unmodied MCM-41 substrate resulted in the lowest capillary condensation pressure for both the propane and n-butane adsorbates. Similar observations were reported by Zhao and Lu for adsorption of benzene on MCM-41 and silylated MCM-41

55

and two other studies with toluene and triuoromethane adsorbates.

54,56

The explanation provided by Zhao and Lu was related to increased diusive resistance in the pore-lling process due to the addition of methyl group surface modication.

55

This

is a plausable explanation for the dierence observed between the capillary condensation pressures in the modied MCM-41 and those of the unmodied MCM-41 in Figures 1 and 2. Another contributing factor could be related to the smoothness of the interior pore surface in the modied versus unmodied MCM-41.

57

Non-uniform modication of the surface with

alkyl groups may have resulted in a less homogeneous adsorption surface that did not favor uniform adsorbate nucleation and simultaneous onset of capillary condensation in all areas of the adsorbent.

These hypotheses are additionally supported by a qualitative review of

the slope of the condensation step for each adsorbent. Isotherms in the modied MCM-41 adsorbents appear to have a progressively smaller slope over the condensation region. This would indicate that capillary condensation in the modied substrates continuously occurred over a wider pressure range than in the unmodied MCM-41. Hysteresis was not pronounced for the propane isotherms but was clearly present in experiments with n-butane. For n-butane, the C18 modied MCM-41 displayed a hysteresis

◦ region at 2 C which extended to low pressure, further supporting the hypothesis of adsorbate trapping and impediment within the long alkyl chains on the surface. Hysteresis rapidly dis-

◦ appeared for the C8 and C18 modied MCM-41 after 8 C. The disappearance of hysteresis seemed to be weakly correlated to the degree of surface modication as well. The unmodied MCM-41 displayed some amount of hysteresis even above the estimated pore critical temperature. In order to evaluate the driving thermodynamic factors for the qualitative observations

13


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 33

from the isotherms, the dierential enthalpy of adsorption was calculated between two subcritical temperatures for each adsorbent-adsorbate pair. Classically, the dierential enthalpy of adsorption may be calculated using the following thermodynamic relation:

( where

P∗

∆ads hT,Γ ∂ ln(P ∗ ))Γ = − ∂T RT 2

(1)

is the equilibrium pressure divided by the standard pressure. By integrating

over paths of constant loading (Γ), the dierential enthalpy may be estimated from several adsorption isotherms.

It is important to note that the estimate is only a good approxi-

mation over the low loading region of the adsorption isotherm since it is considered to be thermodynamically reversible. As such, the data in Figures 3 and 4 are only reported for low loading.

14


50

50

40

40

30

30

20

20

10

10

0

0 1

-ΔadshT,Γ (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir


Page 15 of 33

1.5

2

50

50

40

40

30

30

20

20

10

10

0

0.5

1

1.5

2

0

0.5

1

1.5

2

0 0

0.5

1

1.5

2

Loading (mmol/g)

Figure 3:

0

Loading (mmol/g)

Dierential enthalpy of adsorption for propane in (a) MCM-41, (b) MCM-41-

C1, (c) MCM-41-C8,(d) MCM-41-C18.

Dashed line indicates the bulk value for standard

enthalpy of vaporization of propane. The open symbols are the values obtained from GCMC simulation of propane adsorption in the four model adsorbents (with linear t line).

15


Page 16 of 33

60

60

50

50

40

40

30

30

20

20

10

10

0 0.6 0.8


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Langmuir

1

1.2 1.4

0 0.6 0.8

60 50

60 50

40

40

30

30

20

20

10

10

0 0.6 0.8

1

1.2 1.4

0 0.6 0.8

Loading (mmol/g)

1

1

1.2 1.4

1.2 1.4

Loading (mmol/g)

Figure 4: Dierential enthalpy of adsorption for n-butane in (a) MCM-41, (b) MCM-41C1, (c) MCM-41-C8,(d) MCM-41-C18.

Dashed line indicates the bulk value for standard

enthalpy of vaporization of n-butane

For both propane and n-butane, the unmodied MCM-41 resulted in the largest values of

∆ads hT,Γ

at loading prior to capillary condensation.

This indicates that the adsorbate

experienced stronger physisorption on the unmodied substrate resulting in a larger reduction in entropy (as

∆ads s0T,Γ = ∆ads hT,Γ − Rln(P/P ∗ ))

the other modied adsorbents.

of the conned uid compared to

The lower entropy was a result of favorable siting of the

16


Page 17 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

adsorbate molecules in the pores of the unmodied surfaces.

Although counter-intuitive,

this phenomena could be explained by the smoothness of the interior pore surface of the unmodied substrate. The uniform decline with increased loading, which is observable in all cases, is indicative of a energetically heterogeneous adsorption and is due to the progressive lling of high energy surface adsorption sites. A coupled eect of larger decrease in entropy and greater change in enthalpy upon adsorption in the unmodied MCM-41 is the best explaination for the lower condensation pressures observed in the experimental isotherms for the unmodied MCM-41.

58

The results of the GCMC simulation are also shown in Figure 3.

The simulations did not match the experimental data for the unmodied MCM-41. For the modied adsorbents, the modeling results showed better agreement with the experimental counterparts but were not able to account for the signicant heterogeneity that was present in the experimental systems.

Conned Critical Temperature The depression of the critical point in connement is a phenomena that has been well documented in literature.

3,4,2527

As such, the conned critical point of propane and n-butane

were calculated in all types of adsorbent material used. The method of Nardon and Larher was used to calculate the conned critical temperature (Tcp ) based on the inverse slope of pressure versus loading over the capillary condensation region of the isotherm.

3,59

The inverse

slope changes drastically once the critical point has been reached, allowing for the estimation of the critical point from multiple isotherms over a temperature range that includes the critical point.

59

Only the points that were decidedly above the critical point, based on the

inverse slope of the capillary condensation region, were used to t the supercritical line. The

Tcp

estimates from the desorption branch for the n-butane isotherms rendered values

that were typically within 1

◦

C of the adsorption branch.

The critical temperatures of

propane and n-butane in all the adsorbent types are shown in Figure 5. From the gure, it is clear that there is a trend of decreasing conned critical temperature with increasing

17


Langmuir

surface modication.

40 n-Butane (Adsorption) n-Butane (Desorption) Propane (Adsorption)

35

30

Tcp (oC)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 33

25

20

15

10 0

5

10 wt% Carbon

15

Figure 5: Conned critical temperature of propane and n-butane in silica MCM-41 with as a function of the amount of surface modication (surface bonded carbon). Dotted lines are shown for reference.

For both propane and n-butane,

Tcp

decreases with increase in surface modication.

This observation is consistent with previous GCMC simulation results. followed a linear trend described by Equation 2 while Equation 3 where

Tcp

35,49

Tcp

for propane

for n-butane was described by

Xcarbon represents the wt% of surface carbon and Tcp,M CM −41 is the conned

critical temperature for unmodied MCM-41.

Tcp = Tcp,M CM −41 − 0.565Xcarbon

(2)

Tcp = Tcp,M CM −41 − 0.982Xcarbon

(3)

From Figure 5, there appears to be a correlation between the slope of the decline in

Tcp

with increased modication and the size of the n-alkane adsorbate. Although more adsorption

18


Page 19 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

data for larger n-alkanes would be needed to conrm a general trend, this observation may possess a qualitative explanation. One possible hypothesis is that n-butane presents a steeper decline in

Tcp

as compared to propane due to the additional post-adsorption entropy retained

through conformational isomerism. This additional, albeit small, rotational entropy retained by n-butane may become much more signicant in connement where translational motion is restricted. As a result, n-butane may be advanced toward conned criticality more quickly with the introduction of surface modifying alkyl groups.

NVT-GCMC Simulations Propane adsorption was simulated over a range of temperature and chemical potential values in the four types of model pores shown in Figure 6.

A hybrid NVT-GCMC scheme was

used that allowed the surface groups and the adsorbate molecules to move throughout the simulations. The raw data from the simulations were used to extract the grand potential using Equations 4 and 5.

20,49,60

Figure 6: Model silica MCM-41 pores with (a) no surface modication, (b) C1 surface groups, (c) C8 surface groups, (d) C18 surface groups. Pores are shown during an NVT equilibration sequence.

19


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

∂φ G

∂µ

= −hN i

Page 20 of 33

(4)

T,V

∂φ /T G = hE − µN i ∂1/T µ,V

(5)

The grand potential was used to locate the phase transition point using the relation in Equation 6.

(

∂φG )T,µ = 0 ∂N

(6)

The critical temperature can be estimated from the grand potential using methods previously presented in literature.

60

The grand potential is related to the average pressure for

a homogeneous uid by the relation in Equation 7 and therefore is a useful way to present isotherm data from GCMC simulation.

φG = −hP iV

(7)

Figure 7 shows simulated propane isotherms for all pore types. It can be noted that, with the exception of the unmodied MCM-41 model, the simulated isotherms follow the same trend as the experimental data.

The capillary condensation point occurs at lower grand

potential values for the pore systems with more alkyl group surface modication.

20


Page 21 of 33

0.8

Fluid Density (g/cc)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

0.6 0.8 0.6

0.4 0.4 0.2

0.2

0 0

50 100 150 200 250 MCM-41 MCM-41-C1 MCM-41-C8 MCM-41-C18

0 0

500

1000 -Φ/(RT)

1500

2000

Figure 7: Propane isotherms at 210 K from GCMC simulation with four types of modied and unmodied silica MCM-41.

Isotherms are plotted in terms of the normalized grand

potential

The density of the uid is larger in the C1 modied MCM-41 compared to the C8 and C18 modied MCM-41, likely due to better packing congurations achieved with the lack of the long alkyl surface modiers. The larger values of normalized grand potential in the vapor adsorption region of the unmodied MCM-41 simulated isotherms indicates a combination of higher entropy of the adsorbate and lower surface interaction with the substrate which is dierent from what was observed experimentally. The fact that the simulated isotherms for MCM-41 resulted in the largest capillary condensation pressure of the four model pores, in contrast to the experiment, indicates that the potentials used to model unmodied MCM-41 may not adequately represent the real material. This discrepancy is also apparent in Figure 3 where the dierential enthalpy of adsorption does not match well with the experimental data

21


Langmuir

for the unmodied MCM-41. This might be attributed to the eects of Coulomb charge on the surface of the unmodied MCM-41 on in the adsorption process which could not be adequately represented by the relatively simple Lennard-Jones model employed here.

26 NVT-GCMC Experiment

24

22

Tcp (oC)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 33

20

18

16

14

12 0

5

10 wt% Carbon

15

Figure 8: Conned critical temperature of propane estimated by GCMC in four types of model pores with varying surface modication. Experimental results are shown for reference

The computed conned critical temperatures of propane are presented in Figure 8 as a function of the pore type. The experimental values are shown in the plot for reference. Both experimental and simulated data show the same, nearly linear, negatively sloped trend. The simulations produced generally lower values of Tcp for the modied MCM-41 models. This is likely due to the lack of heterogeneity in the model that was present in the actual substrates. The magnitude of this discrepancy increased for the more modied surfaces indicating that signicant heterogeneity may have been present in the modied MCM-41 materials used

22


Page 23 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

in the experiments.

This heterogeneity, likely caused by uneven modication of the pore

surfaces, would result in large energetic dierences in the adsorption surface. The simulated value of Tcp for the unmodied silica MCM-41 was interestingly similar to the experimental value despite the dierences in the qualitative isotherm behavior and the dierential enthalpy of adsorption.

Conclusions In this study, both experimental and modeling techniques were employed to investigate the eects of surface chemistry on the adsorption and thermodynamic behavior of propane and n-butane. Four types of MCM-41 were created with varying degrees of surface modication. The MCM-41 was modied using silylation reactions to bond methyl, octyl, and octadecyl groups to the surface of pure MCM-41. These adsorbents were used to measure adsorption isotherms of both propane and n-butane at multiple temperatures.

Additionally, hybrid

NVT-GCMC modeling was used to simulate adsorption in four model pores that were analogous to the experimental adsorbents using a united-atom, Lennard-Jones interaction scheme. The surface modication directly impacted the shape of the isotherms as well as the dierential enthalpy of adsorption. Interestingly, the unmodied MCM-41 presented the lowest capillary condensation pressures and highest dierential enthalpy of adsorption, pointing to energetically favorable adsorption behavior and larger reductions in entropy upon adsorption. These ndings were explained by diusive resistance caused by the surface modifying groups and heterogeneity caused by non-uniform modication of the surface.

The combi-

nation of these two factors resulted in less reduction in entropy upon adsorption as well as causing capillary condensation to occur over a wider pressure range as compared to the unmodied MCM-41. The experimental and simulated isotherms were used to calculate the conned critical temperature (Tcp ) of both uids. Both calculations supported a negatively sloped trend of

Tcp

with increasing alkyl surface modication. The slope of this trend also

23


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 33

appeared to be correlated to the adsorbate size, however more data are needed to conrm a more general behavior. The results of the simulations did not closely agree with their experimental counterparts for the unmodied MCM-41 but showed encouragingly similar results for the modied substrates in terms of qualitative capillary condensation pressure behavior and the calculated dierential enthalpy of adsorption. These results suggest that impacts of surface charge eects are non-negligible for adsorption on unmodied MCM-41, even when the adsorbate is a non-polar molecule. The results presented in this work have direct implications for developing better understanding regarding the eects of surface chemistry on conned phase behavior.

Acknowledgments The authors gratefully acknowledge the nancial support of Saudi Aramco and the School of Energy Resources at the University of Wyoming.

References (1) Barsotti, E.; Tan, S. P.; Saraji, S.; Piri, M.; Chen, J.-H. A review on capillary condensation in nanoporous media: Implications for hydrocarbon recovery from tight reservoirs.

Fuel 2016, 184, 344361. (2) Jin, L.; Ma, Y.; Jamili, A. Investigating The Eect of Pore Proximity on Phase Behavior And Fluid Properties in Shale Formations. 2013.

(3) Morishige, K.; Fujii, H.; Uga, M.; Kinukawa, D. Capillary Critical Point of Argon, Nitrogen, Oxygen, Ethylene, and Carbon Dioxide in MCM-41.

Langmuir

1997,

13,

34943498.

(4) Morishige, K.; Nakamura, Y. Nature of Adsorption and Desorption Branches in Cylindrical Pores.

Langmuir 2004, 20, 45034506. 24


Page 25 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(5) Jia, M.; Seifert, A.; Thiel, W. R. Mesoporous MCM-41 Materials Modied with Oxodiperoxo Molybdenum Complexes: clooctene.

Ecient Catalysts for the Epoxidation of Cy-

Chem. Mater. 2003, 15, 21742180.

(6) Gelb, L. D.; Gubbins, K. E.; Radhakrishnan, R.; Sliwinska-Bartkowiak, M. Phase separation in conned systems.

Rep. Prog. Phys. 2000, 63, 727.

(7) Ye, G.; Zhou, X.; Yuan, W.; Coppens, M.-O. Probing pore blocking eects on multiphase reactions within porous catalyst particles using a discrete model.

AIChE J. 2016,

62, 451460. (8) Arogundade, O.; Sohrabi, M. A Review of Recent Developments and Challenges in Shale Gas Recovery. 2012.

(9) Chen, J.-H.; Mehmani, A.; Li, B.; Georgi, D.; Jin, G. Estimation of Total Hydrocarbon in the Presence of Capillary Condensation for Unconventional Shale Reservoirs. 2013.

(10) Nojabaei, B.; Johns, R. T.; Chu, L. Eect of Capillary Pressure on Phase Behavior in Tight Rocks and Shales.

SPE Reservoir Evaluation & Engineering 2013, 16, 281289.

(11) Shim, W. G.; Lee, J. W.; Moon, H. Heterogeneous Adsorption Characteristics of Volatile Organic Compounds (VOCs) on MCM-48.

Separation Science and Technology

2006,

41, 36933719. (12) Thommes, M.; Findenegg, G. H. Pore Condensation and Critical-Point Shift of a Fluid in Controlled-Pore Glass.

Langmuir 1994, 10, 42704277.

(13) Travalloni, L.; Castier, M.; Tavares, F. W.; Sandler, S. I. Thermodynamic modeling of conned uids using an extension of the generalized van der Waals theory.

Chemical

Engineering Science 2010, 65, 30883099. (14) Travalloni, L.; Castier, M.; Tavares, F. W. Phase equilibrium of uids conned in porous

25


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 33

media from an extended Peng-Robinson equation of state.

Fluid Phase Equilibria 2014,

362, 335341. (15) Travalloni, L.; Castier, M.; Tavares, F. W.; Sandler, S. I. Critical behavior of pure conned uids from an extension of the van der Waals equation of state.

The Journal

of Supercritical Fluids 2010, 55, 455461. (16) Pitakbunkate, T.; Balbuena, P. B.; Moridis, G. J.; Blasingame, T. A.; others, Eect of connement on pressure/volume/temperature properties of hydrocarbons in shale reservoirs.

SPE Journal 2016, 21, 621634.

(17) Zarragoicoechea, G. J.; Kuz, V. A. Critical shift of a conned uid in a nanopore.

Fluid

Phase Equilibria 2004, 220, 79. (18) Jin, B.; Nasrabadi, H. Phase behavior of multi-component hydrocarbon systems in nano-pores using gauge-GCMC molecular simulation.

Fluid Phase Equilibria

2016,

425, 324334. (19) Shirono, K.; Daiguji, H. Molecular simulation of the phase behavior of water conned in silica nanopores.

The Journal of Physical Chemistry C

2007, 111, 79387946.

(20) Page, K. S.; Monson, P. A. Monte Carlo calculations of phase diagrams for a uid conned in a disordered porous material.

Physical Review E 1996, 54, 6557.

(21) Tan, S. P.; Piri, M. Equation-of-state modeling of associating-uids phase equilibria in nanopores.

Fluid Phase Equilibria 2015, 405, 157166.

(22) Hanrahan, J. P.;

Copley, M. P.;

Ziegler, K. J.;

Spalding, T. R.;

Morris, M. A.;

Steytler, D. C.; Heenan, R. K.; Schweins, R.; Holmes, J. D. Pore Size Engineering in Mesoporous Silicas Using Supercritical CO2.

Langmuir 2005, 21, 41634167.

(23) Neimark, A. V.; Ravikovitch, P. I. Capillary condensation in MMS and pore structure characterization.

Microporous and Mesoporous Materials 2001, 44, 697707. 26


Page 27 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(24) Gor, G. Y.; Paris, O.; Prass, J.; Russo, P. A.; Ribeiro Carrott, M. M. L.; Neimark, A. V. Adsorption of n-Pentane on Mesoporous Silica and Adsorbent Deformation.

Langmuir

2013, 29, 86018608. (25) Qiao, S. Z.; Bhatia, S. K.; Nicholson, D. Study of Hexane Adsorption in Nanoporous MCM-41 Silica.

Langmuir 2004, 20, 389395.

(26) Russo, P. A.; Ribeiro Carrott, M. M. L.; Carrott, P. J. M. Trends in the condensation/evaporation and adsorption enthalpies of volatile organic compounds on mesoporous silica materials.

Microporous and Mesoporous Materials 2012, 151, 223230.

(27) Tanchoux, N.; Trens, P.; Maldonado, D.; Di Renzo, F.; Fajula, F. The adsorption of hexane over MCM-41 type materials.

Colloids and Surfaces A: Physicochemical and

Engineering Aspects 2004, 246, 18. (28) Thommes, M.; Kaneko, K.; Neimark, A. V.; Olivier, J. P.; Rodriguez-Reinoso, F.; Rouquerol, J.; Sing, K. S. Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report).

Pure and Applied

Chemistry 2015, 87, 10511069. (29) Gubbins, K. E.; Long, Y.; Asliwinska-Bartkowiak, M. Thermodynamics of conned nano-phases.

The Journal of Chemical Thermodynamics 2014, 74, 169183.

(30) Munoz, B.; Ramila, A.; Perez-ParienteJ.,; Diaz, I.; Vallet-Regi, M. MCM-41 Organic Modication as Drug Delivery Rate Regulator.

Chem. Mater. 2003, 15, 500503.

(31) Mello, M. R.; Phanon, D.; Silveira, G. Q.; Llewellyn, P. L.; Ronconi, C. M. Aminemodied MCM-41 mesoporous silica for carbon dioxide capture.

Microporous and Meso-

porous Materials 2011, 143, 174179. (32) Xu, X.; Song, C.; Andresen, J. M.; Miller, B. G.; Scaroni, A. W. Preparation and characterization of novel CO2 molecular basket adsorbents based on polymer-modied

27


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

mesoporous molecular sieve MCM-41.

Page 28 of 33

Microporous and Mesoporous Materials

2003,

62, 2945. (33) Puibasset, J. Grand Potential, Helmholtz Free Energy, and Entropy Calculation in Heterogeneous Cylindrical Pores by the Grand Canonical Monte Carlo Simulation Method.

J. Phys. Chem. B 2005, 109, 480487. (34) Puibasset, J. Capillary Condensation in a Geometrically and a Chemically Heterogeneous Pore: A Molecular Simulation Study.

J. Phys. Chem. B 2005, 109, 47004706.

(35) Singh, S. K.; Saha, A. K.; Singh, J. K. Molecular Simulation Study of Vapor- Liquid Critical Properties of a Simple Fluid in Attractive Slit Pores: Crossover from 3D to 2D.

The Journal of Physical Chemistry B 2010, 114, 42834292.

(36) Singh, S. K.; Sinha, A.; Deo, G.; Singh, J. K. Vapor- liquid phase coexistence, critical properties, and surface tension of conned alkanes.

C

The Journal of Physical Chemistry

2009, 113, 71707180.

(37) Kim, S.-H.; Park, J.-H.; Lee, C.-Y. Surface-functionalized mesoporous silica nanoparticles as sorbents for BTEX.

J Porous Mater 2013, 20, 10871093.

(38) Wang, H.; Wang, T.; Han, L.; Tang, M.; Zhong, J.; Huang, W.; Chen, R. VOC adsorption and desorption behavior of hydrophobic, functionalized SBA-15.

Journal of

Materials Research 2016, 31, 516525. (39) Dou, B.; Hu, Q.; Li, J.; Qiao, S.; Hao, Z. Adsorption performance of VOCs in ordered mesoporous silicas with dierent pore structures and surface chemistry.

Journal

of Hazardous Materials 2011, 186, 16151624. (40) Hu, Q.; Li, J. J.; Hao, Z. P.; Li, L. D.; Qiao, S. Z. Dynamic adsorption of volatile organic compounds on organofunctionalized SBA-15 materials.

Journal 2009, 149, 281288. 28


Chemical Engineering

Page 29 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(41) Zandavi, S. H.; Ward, C. A. Contact angles and surface properties of nanoporous materials.

Journal of Colloid and Interface Science 2013, 407, 255264.

(42) Barsotti, E.; Saraji, S.; Piri, M. Nanocondensation Apparatus. 2017; US 15/588,094.

(43) Barsotti, E.; Saraji, S.; Tan, S. P.; Piri, M. Capillary Condensation of Binary and Ternary Mixtures of n-Pentane, Isopentane, and CO2 in Nanopores: An Experimental Study on the Eects of Composition and Equilibrium.

Langmuir 2018, 34, 19671980.

(44) Jaroniec, C. P.; Kruk, M.; Jaroniec, M.; Sayari, A. Tailoring Surface and Structural Properties of MCM-41 Silicas by Bonding Organosilanes.

J. Phys. Chem. B 1998, 102,

55035510.

(45) Barsotti, E.; Tan, S. P.; Piri, M.; Chen, J.-H. Phenomenological Study of Conned Criticality: Insights from the Capillary Condensation of Propane, n-Butane, and nPentane in Nanopores.

Langmuir 2018, 34, 44734483.

(46) Adams, D. J. Grand canonical ensemble Monte Carlo for a Lennard-Jones uid.

Molec-

ular Physics 1975, 29, 307311. (47) Peterson, B. K.; Gubbins, K. E. Phase transitions in a cylindrical pore: grand canonical Monte Carlo, mean eld theory and the Kelvin equation.

Molecular Physics 1987, 62,

215226.

(48) Orkoulas, G.; Panagiotopoulos, A. Z. Phase behavior of the restricted primitive model and square-well uids from Monte Carlo simulations in the grand canonical ensemble.

The Journal of chemical physics 1999, 110, 15811590. (49) Lowry, E.; Piri, M. Eects of chemical and physical heterogeneity on conned phase behavior in nanopores.

Microporous and Mesoporous Materials 2018, 263, 5361.

29


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 33

(50) Martin, M. G.; Siepmann, J. I. Transferable potentials for phase equilibria. 1. Unitedatom description of n-alkanes.

The Journal of Physical Chemistry B 1998, 102, 2569

2577.

(51) Willems, T. F.; Rycroft, C. H.; Kazi, M.; Meza, J. C.; Haranczyk, M. Algorithms and tools for high-throughput geometry-based analysis of crystalline porous materials.

Microporous and Mesoporous Materials 2012, 149, 134141. (52) Plimpton, S. Fast Parallel Algorithms for Short-Range Molecular Dynamics.

Journal

of Computational Physics 1995, 117, 119. (53) Linstrom, P. J.; Mallard, W. G. The NIST Chemistry WebBook: A Chemical Data Resource on the Internet.

J. Chem. Eng. Data 2001, 46, 10591063.

(54) Russo, P. A.; Carrott, M. M. L. R.; Carrott, P. J. Hydrocarbons adsorption on templated mesoporous materials: eect of the pore size, geometry and surface chemistry.

New Journal of Chemistry 2011, 35, 407416. (55) Zhao, X. S.; Ma, Q.; Lu, G. Q. M. VOC Removal: Hydrophobic Zeolites and Activated Carbon.

Comparison of MCM-41 with

Energy Fuels 1998, 12, 10511054.

(56) Schreiber, A.; Bock, H.; Schoen, M.; Findenegg, G. H. Eect of surface modication on the pore condensation of uids: experimental results and density functional theory.

Molecular Physics 2002, 100, 20972107. (57) Long, Y.; Xu, T.; Sun, Y.; Dong, W. Adsorption behavior on defect structure of mesoporous molecular sieve MCM-41.

Langmuir 1998, 14, 61736178.

(58) Pascal, T. A.; Goddard, W. A.; Jung, Y. Entropy and the driving force for the lling of carbon nanotubes with water.

(59) Nardon, Y.;

PNAS 2011, 108, 1179411798.

Larher, Y. Critical temperatures of two-dimensional condensation of

30


Page 31 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

methane and rare gases on the cleavage face of layer-like halides.

Surface Science 1974,

42, 299305. (60) Panagiotopoulos, A. Z. Molecular simulation of phase coexistence: Finite-size eects and determination of critical parameters for two-and three-dimensional Lennard-Jones uids.

International journal of thermophysics 1994, 15, 10571072.

31


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

For Table of Contents Only

32


Page 32 of 33

Page 33 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

n-Butane isotherms in MCM-41 modified with different n-alkane surface groups 80x38mm (150 x 150 DPI)


Effect of Surface Chemistry on Confined Phase Behavior in

Recommend Documents