Determination of acidity in porous aluminosilicate:A short review
Contents
1.Introduction – Theories
2.Surface acidity in ...
Fig. Lewis acidity in zeolites
Zeolites are porous alumino-silicates with well defied pore architecture. In zeolites, sili...
equivalent to the number of acid sites on the zeolite.
ZO-NH4 + K+ (aq) ------> ZO-K + NH4+ (aq)

3.1.2. Titration of Aque...
3.2.2. The HR acidity function
This is similar to the H0 acidity function but the indicators used are Arylmethanols.
Arylm...
3.3. Temperature programmed Desorption of bases.
3.3.1. TPD of NH3
This method is extensively practiced to quantify the ac...
3.4. Micro-calorimetry

For a hypothetical reaction M+H+ ----> MH+-ve of the enthalpy change in isolation from its
surroun...
3.5. IR – Spectroscopy of chemisorbed pyridine
Pyridine forms the stoichiometric adsorption complex on the acid sites. Thi...
3.7.2 .Cumenecracking
Cumenehas a very simple cracking scheme due to the fact that the benzene ring is not attacked under
...
get accurate picture about the acidity of zeolites. Model reaction can give more information about the
actual behaviour of...
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Determination of acidity in porous aluminosilicate

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Merits and demerits methods for determination of acidity in porous silicates.

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Determination of acidity in porous aluminosilicate

  1. 1. Determination of acidity in porous aluminosilicate:A short review Contents 1.Introduction – Theories 2.Surface acidity in zeolites 3.Measurement of Strength and number of surface acid sites 3.1.Aqueous method 3.2.Amine titration, Adsorption indicators 3.3.Temperature programmed desorption of bases 3.4.Calorimetric method 3.5.Infrared spectroscopy 3.6.NMR 3.7.Model reactions for determination of surface acidity 4.Conclusive remarks 1. Introduction The concept of acid/base is well studied centuries before, and very well understood for the conventional acid/base, for solid acids ambiguity still exists. To characterize the acidity of solid acids no single method is agreed as standard and good method in literature ie., there is difference of opinion among research groups. In this short review we shall discuss few methods and their limitations. In this review we will be focusing on acidity determination of zeolites, broadly it may also applicable to other solid acids. 1.1. Theories on acids and bases After Arrhenius theory made an attempt to explain the acid base concept many theories were put foreword to explain the acid/base concept. Brönsted Lowry and Lewis theories are well accepted among other theories. According to Arrhenius theory acid is a substance donates hydrogen (H+) ions to the solution and base are substance which donates hydroxyl (OH-) ions to the solution. According to Brönsted Lowry theory acid are substance which donates hydrogen (H+)ions to the solution and base are substance which accepts hydrogen (H+)ions from the solution. Lewis acid theory says that acid are substance which accepts electrons and substance which donates electrons are called as base. These donation or acceptance of proton is usually happens both in presence of medium or in the absence of medium. Usually acid base are standardized and referred to water as medium. Conventional acids are active only in the solution phase. Solid acids viz., zeolites, SAPOs, clays; pillared clays, Ion-exchange resins, oxides; hallides, sulphated-oxides, mixed oxides etc., are active even in solid state and at wide range of temperatures. 2. Surface acidity in zeolites Fig. Formal charge on frame work Fig. Brönsted acidity
  2. 2. Fig. Lewis acidity in zeolites Zeolites are porous alumino-silicates with well defied pore architecture. In zeolites, silicon is bonded tetrahedrally with oxygen is the primary building unit repeats over a longer range, aluminium replaces tetrahedral silicon creating a charge imbalance. With increase in the number of aluminium in the framework of zeolite, formal negative charge on the framework increases. Cations are present inside the pores of the zelolite to balance the charge imbalance. These cations can be easily exchanged with different cations. If cations are exchanged with proton, Brönsted acidity in the zeolite can be generated. But preferred method is to exchange the cation with ammonium ion and subsequent calcination to yield acid form of zeolite. Acidic Proton will be attached to oxygen bridging the silicon and aluminium. Catalytic activity of zeolites increases with increase in the aluminium. Strength of acid site strength increases with decrease in the aluminium content and conversely acid site strength decreases with increase in aluminium content. This can be explained with change in the net electronegativity of oxygen connected to proton. With decrease in the Al content net electro-negativity of the oxygen containing proton increases, which can be readily reacted to the incoming reactant therefore the acid site strength increases. With increase in aluminium content acid sites increases, so the activity of the zeolite but the net electro-negativity of the bridging oxygen decreases this stabilizes the proton. Which makes the donation of proton difficult compared to high silica counterparts, hence the acidic strength per site decreases. 2.1 Zeolites v/s Conventional acids As we all aware about proton donation in conventional acids, it depends on the basicity of the given acid, for a monobasic, dibasic and tribasic acid, the proton donation sites are one, two and three per molecule respectively. Conventional acids are active only in the aqueous phase. Unlike conventional acids, zeolites are not a single proton donation site per molecule. Zeolite is a collection of proton donor sites with a continuous framework. Zeolites can donate protons in solid phase! Activity of zeolites increases linearly with increase in 'Al' in the frame work. Post synthesis treatment can be utilized to tailor the activity of catalyst to a specific reaction while this is not possible for conventional acids. 3. Measurement of Strength and number of surface acid sites 3.1. Aqueous methods Aqueous method of determination of strength and concentration of acid sites for conventional acids are very well established and widely accepted. For solid acids indirect determination of number and acid site strength can be done as they are insoluble in water. Ion-exchange and titration of aqueous slurry of catalyst are being followed. 3.1.1. Ion exchange methods for total acid sites Proton form of zeolite after activation to remove any physisorbed components, it can be neutralized with NH3 gas. Ammonium forms of zeolite are ion exchanged with excess of K+. Amount of K+ left in the solution can be estimated and the amount of K+ ion taken-up by the zeolite can be calculated. As the number of proton donating site is equivalent to the amount of NH4+ and the amount of NH4+ is equivalent to the amount of K+ uptake by zeolite, therefore the amount of K+ taken up is
  3. 3. equivalent to the number of acid sites on the zeolite. ZO-NH4 + K+ (aq) ------> ZO-K + NH4+ (aq) 3.1.2. Titration of Aqueous slurries of the acidic solids. For chemists titration is a well mastered technique for the determination of acid concentration in aqueous or non-aqueous solution. Similar to conventional acids, aqueous slurries of solid acids decreases the pH of solution, in a typical estimation it can be titrated with a standard NaOH solution in small increments to either an indicator end point or preferably a potentiometer to get an agreeable good end point. Ex 1: ZO-H + NaOH(aq)<------> ZO-Na + H2O Fluoride ion reacts with bridging hydroxyl group of the zeolite frame work and pH of the solution increases this resulting basic solution can be titrated with standard HCl. Ex 2: ZO-H + F-(aq) <------> Z-F + OH- 3.1.3. Limitations of aqueous methods Interaction of zeolite with base in aqueous phase is not quantitative; this is one of the major drawbacks of this method. In slurries of solid acids in water, acid site reaction may not behave in a regular manner that found on solid acids. 3.2. Amine titration, Adsorption indicators 3.2.1. H0 acidity function A weak base reacts with water to a minute extent. When acid is added to water containing weak base more of the base is converted to its conjugated form this can be called as the ionization of the base. The extent to which in solution reaction occurs between a base (B) and a Brönsted acid to its conjugate acid (BH+) [for Lewis acid A to AB form] is determined by Hammett acidity function H 0= − log a H (A ) fB f BH (AB ) a H ,f B ,f BH ,are solution phase activity coefficients. It is assumed that at the point of colour change H0 is equal to pKA of the indicator For heterogeneous systems the indicator in the basic form (I) is adsorbed and is converted into its conjugated (IH+) by the reaction with the surface acid sites. H0 is redefined as H 0 = − log a H fI f IH C H 0 =pK IH − log IH CI a H ,f B ,f BH ,are solution phase activity coefficients. C IH is experimentally determined ionization ratio . CI pKIH is the -ve log of equilibrium constant for the reaction I + , H+ , IH+. Assuming its value for a given indicator is identical for both in solution and on solids, H0function is applied to solids.
  4. 4. 3.2.2. The HR acidity function This is similar to the H0 acidity function but the indicators used are Arylmethanols. Arylmethanols are able to react with H+ in the following manner R+ + H2O ROH + H+ Similar to H0, HR is defined as CR H R =pK R − log C ROH CR is experimentally determined ionization ratio . C ROH The reaction is more specific and the molecular structure is of the HR indicators are more Uniform than that of the H0 indicators 3.2.3. Amine titration – using H0 or HR indicators Dehydrated catalyst shall be transferred to airtight vials followed by the addition of petroleum ether through the septum to avoid any further moisture interference. Calculated amount of n-butyl amine (based on expected acidity present on solid acid) diluted with petroleum ether will be added. After attaining equilibrium (usually 6-12 h without sonication to avoid possible disintegration of indicator and acid sites) 0.1% indicator in dry benzene will be added. Indicator reacts with the excess of amine and changes the colour this is end point at specific concentration of n-butylamine is considered to be the acid site concentration on solid acid. Dehydrated catalyst + Petroleum ether Required amount of 0.3-0.5M n-butyl amine in petroleum ether After Equilibration 0.1% in dry benzene is added 3.2.4. Limitation 1) Major drawback of this method is to assume the indicator adsorbed behaves similar as in solution. 2) Assuming PKIH value for a given indicator is identical for both in solution and on solids. 3) Acid strength determined and the activity of the catalyst cannot be co-related with the activity of the catalyst. 4) Acid site strength determined, and the type of acid site catalyzing a given reaction deferrers to great extent. Acidity strength sequence determined by H0 Mounted superacids> clays > mixed oxides > zeolites Acidity sequence based on n-paraffins cracking activity Mounted superacids> zeolites > mixed oxides ≈ clays Solution phase acidities are not the intrinsic property of molecules; it’s a relative proton- donating ability of the molecule to the referance base with in the medium of reference base.
  5. 5. 3.3. Temperature programmed Desorption of bases. 3.3.1. TPD of NH3 This method is extensively practiced to quantify the acid sites. Ammonia being basic chemically adsorbed on the acid sites of the solid acid. At higher temperature ammonia is desorbed and the amount of ammonia desorbed is a direct measure of acid sites on the surface. In a typical experiment catalyst surface will be saturate with ammonia under certain adsorption conditions, followed by linear ramp of temperature in a flowing inert gas stream or connected to vacuum. Desorbed ammonia can be monitored through TCD, or Mass-Spectroscopy, or absorption followed by titration. . 3.3.2. Limitations of TPD of NH3 Results depend on the measurement conditions used. Adsorption of ammonia is not specific to Brönsted site. Eg- CaO adsorbs ammonia strongly than USY. Adsorption on non Brönsted sites may be stronger than Brönsted sites. Desorption temperature from acid sites, strongly depends on the experimental conditions. No information is available about the type of acid sites. 3.3.3. TPD of Amines Similar to ammonia alkyl amines it forms a stoichiometric adsorption complex. Unlike ammonia it does not desorb but reacts at narrow range of temperature which gives products similar to Hoffmann elimination. This can be identified by mass spectra. Ex: ethylamine, n-propylamine, isopropylamine and ter-butylamine. Adsorption complexes could be easily identified by their reaction to olefins and ammonia in a specific temperature range in TPD. HRNH2 + ZOH ----> HRNH3+....ZOHRNH3+....ZO- ----> R + NH3 + ZOH Ex: isopropyl amine reacts to give propene, ammonia between 575 and 650K As the reaction occurs only at the Brönsted sites, its contributions to the total acidity can be determined. Ex: TPD of NH3 on CaO gave similar specific coverage as that of a zeolite whereas no reaction of isopropylamine. Unlike the TPD of ammonia desorption temp is not only identical under vacuum and with carrier gas but also for different compositions (H-[Ga] ZSM-5, H-[Fe] ZSM-5). Using the amines of different sizes the concentration of Brönsted acid sites in each of the components of a fluid catalytic cracking catalyst containing H-ZSM-5, H-Y and amorphous silica-alumina could be determined. By choosing the larger amines acidity inside the pores and on the external surface can be distinguished.
  6. 6. 3.4. Micro-calorimetry For a hypothetical reaction M+H+ ----> MH+-ve of the enthalpy change in isolation from its surroundings is called as proton affinity, corresponding ΔG is referred as gas phase basicity. For the reaction M + NH+<----> N + MH+ measurement of difference in equilibrium constant will give the difference in the gas phase basicity of M & N.By knowing the absolute proton affinity of M, proton affinity of N can be determined.During quantification of Brönsted sites if the one of the base is water then it is known as pKa scale. It is the proton donating ability of the molecule to the reference base water in the medium water.Enthalpy of protonation and the free energy change during protonaton is molecule specific and specific tomedium. Ex: proton affinity of Cl- is 1393 kJ/mol and that of water is 724 kJ/mol. At equilibrium HCl + H2 O <---> H3 O+ +Cl- is 669 kJ/mol but in solution phase is -42 kJ/mol If proton affinities are measured in solution phase its interaction with base or even acid leading to erroneous results. Hence solution phase measurement is not the intrinsic proton affinity measured. Measurement of acidity in gas phase will avoid the solvent effect.It provides the intrinsic measure of acidity.Enthalpy change during protonation of pyridine on zeolite Brönsted sites in gas phase is 200kJ/mol where as in aqueous phase is approximately 20kJ/mol. This difference will have a tremendous impact on reaction Kinetics. Proton affinity of zeolite frame work anion is less than that of very strong acids.On this basis Zeolites is considered as super acids Zeolite CF3 SO3HSO4ICF3COO- - 1200 kJ/mol 1280 kJ/mol 1296 kJ/mol 1312 kJ/mol 1351 kJ/mol A typical experimental method involves dosing of aliquots of reference bases on to solid held at given temperature. The resulting heat of adsorption pulse is collectedby calvet type of thermocouple and integrated. Dosing is continued until saturation coverage is reached. Enthalpy of adsorption versus coverage is plotted.Micro-Calorimetric adsorption studies on zeolite ZSM-5 showed the presence of identical strength acid sites. Enthalpy of adsorption is constant for a given molecule up to stoichiometric coverage on Al sites and further drastically decreases.
  7. 7. 3.5. IR – Spectroscopy of chemisorbed pyridine Pyridine forms the stoichiometric adsorption complex on the acid sites. This method exploits the change in chemical nature of the adsorbed species. Wavenumber of pyridine adsorbed on Brönsted sites and Lewis sites vary because of difference in bonding nature. On Brönsted sites pyridine accepts proton forming pyridinium ion whereas at Lewis sites it donates the lone pair of electron to form an adduct. Hence difference in the absorption frequencies can be observed and physisorbed pyridine IR absorption doesn’t fall in this region. - Brönsted = 1515-1565 cm-1 - Lewis = 1435-1470 cm-1 Based on this distinction of the type of acid site can be quantified by this method Quantification of the number of acid sites present can be obtained from the integrated band areas for a particular vibrational band of the probe molecule using Lambert-Beer law. Experimental procedure used for this method varies among research groups. Generally pyridine (vapours/liquid) is adsorbed on the zeolite surface and heated at 150- 200°C to remove the physisorbed pyridine. It can be done either insitu orexsitu. 3.6. Nuclear Magnetic Resonance It provides the direct way to quantify the number and type of OH groups present in Zeolites. Chemical shift are shown to increase with increase in mean electro-negativity of the zeolite framework It has no limitation of the previousspectroscopic techniques based on probe molecules and medium 3.7. Model Reactions as Acidity Probes Methods used during the acidity determination of the zeolite may not be identical to the catalytic reaction conditions. Therefore it is better to use test reactionsto understand better about the acid sites. There are many probe reacts to characterize the acid sites. Here a few reactions which can be more useful than the others are mentioned. 3.7.1. n-Hexane cracking n-Hexane cracking can be used to determination the concentration of acid sites as the cracking activity shows the linear dependence on the number of acid sites.
  8. 8. 3.7.2 .Cumenecracking Cumenehas a very simple cracking scheme due to the fact that the benzene ring is not attacked under these conditions. Therefore, the primary products in cumene cracking are propene and benzene. Additional products of this reaction are diisopropylbenzene, toluene, ethyltoluene, ethylene, ethane, butenes, ethyl- and propylbenzenes, cymene (p-methylcumene), methane, and isobutane. Thus, a thorough kinetic analysis is needed to distinguish between all possible products and to obtain the correct activity, which can be related to the number and/or strength of the acid sites.Only a part of the Brønsted acid sites is claimed to be active in cumene conversion, and some by-products are formed on Lewis acid sites. Therefore, this reaction can also be used to differentiate between Lewis and Brønsted acid sites. 3.7.3. Toluene alkylation with methanol. Toluene alkylation over acid sites alkylates the aromatic ring giving xylenes. In presence of basic sites ethylbenzene is formed due to the side chain alkylation of toluene. Ethylbezene concentration increase is a direct measure of amount of basic sites. Toluene alkylation with methanol can be done to detect the presence of basic site. 3.7.4. Isomerization/Disproportionation of cyclohexene The reaction of cyclohexene has two different possible reaction pathways (Above scheme). The first is the (monomolecular) isomerization to yield methylcyclopentenes. The second pathway is the (bimolecular) formation of cyclohexane by hydride transfer from a feed molecule. The formation of methylcyclopentane is a secondary reaction of methylcyclopentenes with a feed molecule. By-products in this test reaction are C12 components formed by dimerization (saturated and unsaturated) and coke. For the hydride transfer reaction requires active sites with next-nearest Al neighbours. Rate Decreases with decrease in the Al content due to the smaller number of adjacent sites. On the contrary, the isomerization proceeds on a single site and, therefore, a comparison between hydride transfer and isomerization (i.e., cyclohexane v/smethylcyclopentene formation) can be used for the evaluation of the density and the strength of the acid sites in a zeolite. 4. Conclusive remarks In acid solutions, solvation effects are important for proton transfer steps. Describing the acidic solids acidity by solution phase analogies will be inappropriate.Inappropriate comparisons of the zeolite acid sites with solution phase acids are misleading.Adsorption of ammonia is not specific to Brönstedsites and desorption temperature depends on the experimental condition TPD results may be helpful only with careful interpretation of data.TPD of amines may give clearerpicture about the Brönsted site on the zeolite.Micro-calorimetric studies have shown the absence of the weak and moderate strength acid sites on zeolites.No single method can give all the details of the acidity of zeolite.TPD of amines, IR spectroscopy, micro-calorimetry&1H MASNMR can be used collectivelyto
  9. 9. get accurate picture about the acidity of zeolites. Model reaction can give more information about the actual behaviour of the catalyst performance. References: 1) Peter A. Jacobs, Characterization of heterogeneous catalysts chemical industries series/15 2) Louis P. Hammett; Chem.Rev. 1934,(16) 67-79 3) KunyuanWang, Xiangsheng Wang, Gang Li ; microporous and mesoporous materials2006(94) 325-239 4) C. A. Emeis; J. Catal. 1993 (141) 347-354 5) R.J.Gorte; Catal.Lett.1999 (62) 1-13 6) W.E. Farneth; R.J.Gorte, Chem. Rev. 1995 (95) 615-635 7) Gorte, R J;Catal.Lett., 1999(62)1-13 8) Parrillo;D J, Lee, C; Gorte, R J; Applied Catalysis A: General, 1994(93)67-74 9) Parrillo,D J; Gorte, R J; Catal.Lett 1992(16)17-25 10) Johannes A. Lercher; AndereasJentys; Axel Brait, Doi:10.1007/3829_2007_017 Graphs are reproduced from referencesgenerously. Scholarly article submitted as a part of Orientation Programme exercise to Dr. B. Vishvanathan NCCR, IITM Chennai by -Janardhan H. L. Research fellow Poornaprajna Institute of Scientific Research Deavanahalli, Bangalore – 562110.

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