there is a dissertation called Hysteresis phenomena in mesoporous materials. Probably it will help you to explain your problem

http://www.uni-leipzig.de/~gfp/sites/default/files/27-dr._sergej_naumov/dissertation.pdf

Hystersis indicates many ideas, including:

1) Langmuir model is not practical and only realistic

2) Adsorption occurs through chemisorption. The adsorbed species is not same as the free species, and the vacant site is not same as occupied site

2) at equilibrium, rate of adsorption equals rate of desorption. Otherwise, they have different.

4) Not all sites are the same (contrary to Langmuir)

5) Not all adsorptions are the same (contrary to Langmuir)

Good summary by Prof. Hilal above..

You can get wealth of information from the shape of hysteresis curves. Refer any text book on porous materials.

A plausible explanation for these phenomena is that the pore diameter at the interior of the mesochannels is smaller than other parts. Therefore for desorption, it is necessary that the pressure reduces more than the values which were applied in the adsorption to expel out the N2 from mesochannels. You can imagine something like potter or cruse for estimation the structure of pores.

Hello Bayu

Minor correction on my earlier answer (Langmuir ..... is only idealistic not realistic)

Some more ideas now:

You are using modified ZSM-5 zeolite. Normally ZSM-5 has medium size pores. The modified system needs to be known.

However, hysteresis may indicate that: Adsorption affects the modified zeolite framework.

With reference to your question: meso-porous has pores between ~5 nm - ~50 nm

H-ZSM-5 has smaller pores (less than 1 nm pore sizes). Therefore I would not agree with your original conclusions (unless if modification is too hugely effective that it totally changed the ZSM-5 into something else)..I do not see zeolite structure having mesoporoes.

My suggestion to you is to recheck the actual structure of the material you have if you really believe in mesoporous nature. If it is really zeolite nature, then exclude mesoporous nature, and think in other ideas.

Best wishes.

Hysteresis loops occur when the mechanism of mesopore filling by capillary condensation varies from mesopore emptying. I have these loops in my activated carbon samples as well. And I got to know that usually porous materials like activated carbons do not show a plateau (or flat plane) in high relative pressure region i.e. closer to 1.

An understanding of the adsoption process in a porous material starts with understanding the Kelvin equation. The Kelvin equation describes the relative pressure at which a meniscus of a given curvature is stable (will not grow or shrink). For a convex meniscus (condensed material on the same side as the radius of curvature eg a droplet) the material is only stable above supersaturation, but for a concave meniscus such as in a porous material that is wet, the liquid phase is stable below saturation. So as you do a adsorption isotherm you are increasing the relative vapour pressure up to saturation and (hopefully) allowing for the liquid phase to grow to reach equilibrium at each stage. During this process you are filling the pores and the level to which they fill is determined by the radius of curvature for the liquid in each pore. Note they only see how the curvature changes with a small change in volume - so they experience the local effect on meniscus curvature. When decreasing the supersaturation you are emptying the pores and again the curvature follows the Kelvin equation but it can also see only the local effects on the curvature. So if you have no contact angle hysteresis and perfectly cylindrical pores you might see no hysteresis. But pores are complex shapes and the local shape is what determines the equilibrium for each pore. So during filling and emptying the meniscus will get stuck where the pore widens and this will different whether your filling or emptying the pore for a non cylindrical pore. To see this draw a pore with a diamond shape cross section. You can see that there are two locations where a meniscus of a given contact angle and radius are stable...the lower one is relevant to filling and the upper one is relevant to emptying. Other effects can also contribute such as contact angle hysteresis and local variations in contact angle.

I totally agree and appreciate the comments and explanation by Vincent Graig..

I just add hereafter additional refs by Benoit Coasne at al which give useful and recent papers on this subject.

Molecular Simulation of Nitrogen Adsorption in Nanoporous Silica

Benoit Coasne, et al LANGMUIR 2010, 26(13) 10872-10881

Intrusion and Retraction of Fluids in Nanopores: Effect of Morphological Heterogeneity

Benoit Coasne, et al JOURNAL OF PHYSICAL CHEMISTRY C 2009, 113 (5 ) 1953-1962 Adsorption of simple fluid on silica surface and nanopore: Effect of surface chemistry and pore shape Benoit Coasne, et al LANGMUIR 2008, 24 (14) 7285-7293

Molecular simulation of adsorption and intrusion in nanopores

Benoit Coasne, et al. ADSORPTION-JOURNAL OF THE INTERNATIONAL ADSORPTION SOCIETY 2008, 14 ( 2-3) 215-221

Effect of morphological defects on gas adsorption in nanoporous silicas Benoit Coasne, et al. JOURNAL OF PHYSICAL CHEMISTRY C 2007, 111 (43) 15759-15770

Gas adsorption in mesoporous micelle-templated silicas: MCM-41, MCM-48, and SBA-15

Benoit Coasne, et al. LANGMUIR 2006, 22 (26 ) 11097-11105

incent

Molecular Simulation of Nitrogen Adsorption in Nanoporous Silica

Benoit Coasne, et al LANGMUIR 2010, 26(13) 10872-10881

Intrusion and Retraction of Fluids in Nanopores: Effect of Morphological Heterogeneity

Benoit Coasne, et al JOURNAL OF PHYSICAL CHEMISTRY C 2009, 113 (5 ) 1953-1962 Adsorption of simple fluid on silica surface and nanopore: Effect of surface chemistry and pore shape Benoit Coasne, et al LANGMUIR 2008, 24 (14) 7285-7293

Molecular simulation of adsorption and intrusion in nanopores

Benoit Coasne, et al. ADSORPTION-JOURNAL OF THE INTERNATIONAL ADSORPTION SOCIETY 2008, 14 ( 2-3) 215-221

Effect of morphological defects on gas adsorption in nanoporous silicas Benoit Coasne, et al. JOURNAL OF PHYSICAL CHEMISTRY C 2007, 111 (43) 15759-15770

Gas adsorption in mesoporous micelle-templated silicas: MCM-41, MCM-48, and SBA-15

Benoit Coasne, et al. LANGMUIR 2006, 22 (26 ) 11097-11105

An Excellent explanation about the origin of the 5 types of hysteresis loops can be found in the following book:

Principles and Practice of Heterogeneous Catalysis by: J. M. Thomas, W. J. Thomas

I strongly recommend download this book and read the pages 275-285

http://en.bookfi.org/book/1398234

Hysteresis appears in gas adsorption in meso pores if the curvature of the meniscus with filling is different from the curvature upon emptying (see also Vincent). For instance, close cylindrical pore no hysteresis (filling and emptying spherical), open cylindrical pore hysteresis (Cylindrical filling/ spherical emptying) . For micropores the Kelvin equation cannot be used, here the filling also depends on the pore size because the pores are only a little larger than the N2 molecules. The theory is more complicated than for mesopores.

Simple treatments: the book of S.J. Gregg en K.S.W. Sing, Adsorption, Surface Area and Porosity (1982), 2e ed Academic Press, London. Aimed at determination of surface area and pore distribution. Classical, but not yet specific attention to gas adsorption on zeolites.

See for zeolites the classical paper by A. Saito and H.C. Foley Microporous Materials 3 (1995) 531.

More modern approaches: The DFT (density functional theory) and the Monte Carlo simulation method have largely been applied to the characterization of micro- and mesoporous carbons, silicas and zeolites.

L.D. Gelb, K.E. Gubbins, R. Radhakrsihnan, and M. Sliwinska-Bartowiak, Rep. Prog.

Phys. 62, 1573 (1999). zeolites

N.A Seaton, J.P.R.B. Walton and N. Quirke, Carbon 27, 853 (1989).

A. Vishnyakov, P. Ravikovitch, and A.V. Neimark, Langmuir 16, 2311 (2000).

A.V. Neimark, P.I. Ravikovitch, M. Gruen, F. Schueth, and K.K. Unger, J. Colloid

Interface Sci, 207, 159 (1998). zeolites

P.I. Ravikovitch, D. Wie, W.T. Church, G.L. Haller, and A.V. Neimark, H. Phys. Chem. B 101, 3671 (1997). zeolites

It is very interesting discussion. Just one thing I would like to add and to know, as every solid particle have different surface patterns. And hysteresis phenomena is observed due to such differences in different particles. As far as I know, most of all the solids are shows hysteresis. Is there any solid and catalytic material reported where hysteresis are not observed for adsorption phenomena? Thanks.

because there is the phenomenon of capillary condensation when the pose are filling.

The adsorption is gas and by capillary gas condenses then the desorption is slower and the hysteresis loop is formed

Depends on what pressure hysteresis occurs: if we are talking of mesopores it will occur at mid relative pressures (0.3 - 0.6) and it occurs, as discussed above, because paths for filling and emptying of the mesopore are different.

However, I have seen hysteresis occur at low relative pressure (0.1 - 0.2, with N2 or Ar ar 77 / 87K) and is because sorption did not reach equilibrium during adsorption stage (low temperature makes diffusion and mobility of molecules slower), so equipment did not equilibrate completely before moving to the next adsorption point.

I'm sure if you post a picture and conditions of measurement everyone will be eager to comment and keep this interesting discussion!

In general if your measurements are correct and you collect the true equilibrium of your system you have hysteresis for two reasons. Tensile strength effect (TSE) which is common for microporous zeolites and silica and it is because of ink bottle shape of the pores and small size of pore throats. The other reason which is signature for mesoporous materials and addresses your question is capillary condensation. Molecules adsorb on a surface because of the force field exerted on them by the pore walls. In mesoporous materials moelcules get adsorbed layer by layer (fill higher energy sites near pore wall then low energy sites away from wall). When molecules accumulated on two opposing walls get close enough to each other they collapse into a thermodynamically lower energy state. This process is called capillary condensation. When you reduce pressure during desorption the molecules at low energy do not tent to leave their place, so you need higher gradient of chemical potential  (or equivalently pressure drop) to pull the adsorbed molecules out of their sites. So you see that the same molecules desorb at lower pressure. This gap is between equilibrium adsorption and desorption pressures is the hysteresis.

very nice and informative discusions

Dear David Fairen-Jimenez 

I could not understand how you say that microporous materials will not show hysteresis?

I have run my sample (Activated Carbon) and i got the pore volume 2.315 cm3/g, BET Surface area 1865 m2/g, Adsorption average pore width (4V/A by BET):49.6310 Å,  BJH Adsorption average pore diameter (4V/A):62.004 Å,  BJH Desorption average pore diameter (4V/A):60.972 Å, all the above measurements is coming in mesoporous rang and i have attached the adsorption-desorption isotherm  Please some body guide me that this is mesoporous or micro? 

t-Plot Information

t-Plot Micropore Area:143.8034 m²/g

t-Plot External Surface Area:1721.9698 m²/g

Single point adsorption total pore volume of pores less than 3859.479 Å diameter at P/Po = 0.995023085: 2.315004 cm³/g

t-Plot micropore volume:0.049727 cm³/g

mesopore volume :2.266 cm³/g

But what will be the isotherm type IV or V, and also the type of hysteresis loop of type H3 or H4

guide me please

thanks

Ashfaq, your sample  represents type IV isotherm , and  type of hysteresis loop is H3.

Vincents answer is quite to the point. Somewhat more simplified one considers the adsorbed N2 layer above monolayer coverage as a liquid nitrogen layer. The vapour pressure of liquids depends on the curvature of the surface and in pores the surface of the N2-liquid is curved. When the curvature of the adsorption branch differs from that of the desorption branch hysteresis occurs.

For zeolites the BJH method will give you a pseudo pore size and pore size distribution; so the values have to be considered with reservation. A better method for zeolites is the method of Saito and Foley. (A. Saito and H.C. Foley Microporous Materials 3 (1995) 531. Horvath-Kawazoe/Saito-Foley method).

By shape of your hysteresis you can have nice info about the kind of pores you have (spherical,cylindrical,etc), which your zeolites must have. I strongly suggest you to read the technical report of IUPAC by M. Thommes. It will explain you very clearly yhe adsoprtion phenomena. By yhe way, it is recognized that N2 is npt the best candidate to perform adsorptions but to overcome, do not use BJH, but rather DFT methods that take inyo account the interaction btw substrate and molecules.

Characteristic features of the Type IV isotherm are its hysteresis loop.A mesoporous material having monolayer-mutilayer adsorption with a capillary condensation will generate a hysteresis loop due to the difference in the in curvature of the meniscus on adsorption  (cylindrical) and desorption (spherical) between the adsorption and desorption.This is common to many industrial adsorbent. (Sing et al. 1985,Marsh and Rodriguez-Reinoso, 2006) .  read up more from these reference cheers.

Does capillary condensation and pore filling mean the same thing?

Capillary condensation is in general reseved for meso-pore filling and application of the Kelvin eq., whereas pore filling can apply to both micro-pore and meso-pore filling. Micropore filling cannot be described by the Kelvin eq. because this eq. applies to macroscopic thermodynamic properties, which are assumed to hold for the adsobed layer in mesopores, but not for the adsorbate in micropores. In modern literature both micro- and meso-pore filling are described by density functional theory.

Why does Satish Kumar say Ashfaq's plot is type IV? Type IV should have a plateau as shown in the attached picture from IAPUC

I have a question... Can all isotherms have hysteresis loop? Or hysteresis is specific to type IV and V? my samples exhibit BET surface of 400-500 m2/g and external surface area and mesopore volume about 10-20 m2/g and 0.04-0.08 cm3/g ,respectively. my samples show a small hysteresis, so I am confused that what is the type of isotherms? I or IV??

I agree with Wang Ge and Ali that it is not a type IV adsorption isotherm.