Variable-Temperature Diffuse Reflectance Fourier Transform Infrared

Downloaded by UNIV OF LEEDS on June 18, 2016 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0234.ch013

13 Variable-Temperature Diffuse Reflectance Fourier Transform Infrared Spectroscopic Studies of Amine Desorption from a Siliceous Surface Donald E. Leyden and Kristina G. Proctor 1

2

Condensed Matter Sciences Laboratory, Department of Chemistry, Colorado State University, Fort Collins, CO 80523

Variable-temperature diffuse reflectance infrared Fourier transform spectroscopy was used in conjunction with pyridine desorption studies to assess the acidity of a siliceous surface. An amorphous, porous silica substrate was investigated. The results contribute to an understanding of the acidic strength and the distribution of acidic sites on this material. A hydrogen-bonding interaction was observed between pyridine and the surface. Isothermal rate constants and an activation energy for the desorption process are reported and can be used as direct measures of surface site acidity.

SILICEOUS SUBSTRATES ARE USED EXTENSIVELY in industry

and applied

research. These materials can undergo a variety of chemical modifications that make them useful for applications such as catalysis and chromatography. A n increased understanding of the nature of surface reactions and reaction products would facilitate current and future applications of these C u r r e n t address: Philip Morris U.S.A., P.O. Box 26583, Richmond, V A 23261. Current address: Department of Chemistry, University of Southern Colorado, Pueblo, C O 81001.

2

0065-2393/94/0234-0257$08.00/0 © 1994 American Chemical Society

Bergna; The Colloid Chemistry of Silica Advances in Chemistry; American Chemical Society: Washington, DC, 1994.


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substrates. Acquisition of such information requires innovative surface characterization methods. One characteristic of interest is the surface propensity toward adsorption, hydrogen bonding, and acid-base interactions with chemical reagents. For example, surface silylation by alkoxysilanes may involve hydrolysis of the alkoxy groups by surface-adsorbed water. The overall rate of reaction (from dry, aprotic solvents such as toluene) may be limited by the rate of hydrolysis, which is likely dependent on the fraction of alkoxy silane associated with the substrate surface. A convenient measure of the distribution of a solute between the solvent and substrate is the chromatographic capacity factor (k') (I). Figure 1 shows a plot of the relative rate of hydrolysis for several alkoxysilanes (n-octyltriethoxysilane, 3-mercaptopropyltriethoxysilane, 3-cyanopropyltriethoxysilane, and 3aminopropyltriethoxysilane) on the surface of controlled-pore glass versus the amount of silane adsorbed. The hydrolysis rate is followed by measuring the amount of ethanol produced in toluene solution. The amount of adsorbed silane is represented by the quantity S[/c'/(l + k')], where S is the total amount of alkoxysilane in solution (2). The rate of hydrolysis of the silane alkoxy groups is linearly related to the fraction of the reagent adsorbed to the substrate for three of the compounds. However, in 3-aminopropyltriethoxysilane the rate of reaction is much faster than that predicted by the simple adsorption model. Knowledge of such differences is of considerable importance to a better understanding of the nature of surface substrate reactions. Another area of interest is the acidity of siliceous substrates. Current methods of surface characterization provide a variety of information about the acidity of a surface. Visible indicators covering a range of p K values may be used to estimate the acidity as defined by the Hammett acidity function (3, 4). This method is vague in interpretation and can only provide a measure of relative acidic strength. Other methods involving the adsorption and desorption of gaseous bases can also assess relative acidic strengths. However, assignments to specific surface sites are subject to ambiguity of interpretation. These methods include differential thermal analysis (DTA) (5), thermal gravimetric analysis (TGA) (6), and titration calorimetry (7). The catalytic efficiency of a material can also be used to assess a general measure of surface acidity (8, 9). Investigations of the acidity of specific surface sites may be accomplished by studies coordinated with spectroscopic methods, such as infrared (IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, or mass spectrometry (MS). Surface characterization with Fourier transform infrared (FTIR) spectroscopy can provide quantitative results with experimental methods that are easily performed. However, the transmission sampling techniques traditionally employed for infrared studies may introduce experimental artifacts on the analyzed surface (10, a


13.

L E Y D E N & PROCTOR

Amine Desorption from a Siliceous Surface

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S » [k'/(1+k'>] Figure 1. Relative amount of ethanol produced versus amount of silane adsorbed on the substrate, calculated from the chromatographic capacity factor k'.

11). T o minimize this problem, self-supporting pellets are usually pre pared, and the minimum pressure required to form a mechanically stable pellet is used. Although much information related to surface acidity has been obtained with transmission techniques, there remains some question regarding the extent of surface alteration due to sampling procedures. Diffuse reflectance FTIR (DRIFT) spectroscopy provides an alterna tive to transmission infrared spectroscopy with respect to sampling procedures. D R I F T spectroscopy requires dispersion of the sample i n a finely ground, nonabsorbing matrix such as K C l or K B r . The integrity of the sample surface is ensured because no pressure is used i n the preparation. Variable-temperature D R I F T (VT-DRIFT) spectroscopy can be performed with commercially available, heatable-evacuable sample cells that can be interfaced for computer temperature control (12). Gaseous base adsorption and desorption processes can be followed directly; thus, specific surface sites can be identified and quantified and their acidic strength can be assessed. Previously, such results could be obtained only by combined methods such as L R - T G A (13) or I R - T P D - M S (TPD, temperature-programmed desorption) (14). Such combinations require variable-temperature experiments of independently prepared samples or elaborate instrumental design for measurements taken from the


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T H E C O L L O I D CHEMISTRY O F

SILICA

same sample. V T - D R I F T spectroscopy provides a direct and independent means for the characterization of acidic surfaces in their native form. This chapter describes the results of the acidity characterization of a selected silica surface with V T - D R I F T spectroscopy. Examples of the capabilities of the method are demonstrated by the qualitative determination of the adsorption and thermal desorption characteristics of pyridine on amorphous, porous silica gel. Procedures for the determination of isothermal desorption rate constants and activation energy of desorption are presented and discussed as a means of assessing acid site strength.


Experimental Details Materials. Amorphous silica gel was used (J.T. Baker), 290-m /g surface area, 6 0 - 2 0 0 mesh, and 126-À mean pore diameter. Pyridine (Baker, reagent grade) was used as received. 2

Instrumentation. Spectra were acquired with a Nicolet 60SX FTIR spectrometer, continuously purged with dry air and equipped with a liquid-nitrogencooled, wideband mercury-cadmium telluride detector. Coaddition of 100 interferometer scans at 8 - c m resolution was employed. The location of absorption maxima was confirmed by spectra taken at l - c m resolution. A l l spectra were converted into K u b e l k a - M u n k units prior to use. Integration of peak areas was accomplished by using software available on the Nicolet 60SX. A l l peak areas were normalized to the 1 8 7 0 - c m S i - O - S i combination band (15). The diffuse reflectance accessory (model D R A - 2 C N , Harrick Scientific) was modified with a three-dimensional translational stage to optimally position the sample for maximum radiation throughput (15). The sample cell (model H V C - D R P , Harrick Scientific) was heated by a resistive heater contained within a post that housed a sample cup. The base of the sample cell also contained an external connector for evacuation and a second port, which was sealed with a septum and used for the introduction of pyridine by microsyringe. The sample cell cover contained a channel and connectors for water cooling and ZnSe windows (12 mm in diameter) for the IR radiation. The base and cover of the cell were sealed vacuumright with an O-ring. Temperature of the sample cell was monitored and controlled by an interface to an Apple I I computer. The temperature of the sample cell was sensed by an internal Fe-constantan thermocouple connected to a digital thermometer (Omega Engineering, model 199AIC-D). The digital value of the temperature was available as a binary decimal and was read by an input-output card (John Bell Engineering, model 79-295) in an expansion slot of the computer. The cell temperature was controlled to 1 °C by using software written in Applesoft and assembly languages and a signal to an exterior switch card that controlled the on-off status of a variablevoltage transformer. -1

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Procedures. Prior to use, silica samples were calcined at 500 °C for 5 h in an open-ended tube furnace and stored in a desiccator after rehydroxylation. Samples were dispersed (15% w/w) in finely ground K C l by mixing in a Wig-L-Bug capsule without the grinding ball (Crescent Dental Manufacturing). Dispersions containing 4 - 6 mg of sample were spread over a bed of K C l in the sample cup and flattened by light compression with a smooth object. Samples were heated to 200 °C under



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slight vacuum (100-150 mmHg, or 1 3 - 2 0 kPa) to remove physisorbed water then cooled to room temperature for the adsorption of pyridine. L i q u i d pyridine was introduced into the evacuated cell containing the sample at 25 °C by injection with a Hamilton microsyringe (0.5 ^L). A volume of 3 μL resulted in saturation of the sample surface sites. Equilibrium occurred within 10 min under these conditions. The isothermal desorption of pyridine from silica gel was followed after adsorption of the base at room temperature. The sample cell was evacuated at room temperature to remove excess pyridine, and the temperature was then quickly ramped (30 s) and maintained at the desired temperature for the desorption. Spectra were recorded at intervals of 1, 5, 10, 20, and 30 min during the progression of 5-h desorption studies conducted at 50, 60, 70, 80, and 90 °C. The possibility of interaction of pyridine with the dispersion matrix material was investigated by collecting spectra of pure K C l and of pyridine adsorbed on pure K C l before and after evacuation. A l l traces of pyridine were removed from the K C l spectrum following a few minutes of evacuation.

Results and

Discussion

The surface of amorphous silica gel was qualitatively characterized by the adsorption of pyridine. Because pyridine is a weak base (pK 5.25), it selectively interacts with the more acidic surface sites. This interaction is relevant because metal oxide promoted catalysis is believed to occur by mechanisms involving acid-induced intermediate species (e.g., carbonium ions). Ring vibrations of pyridine give rise to infrared absorptions i n the 1 4 0 0 - 1 7 0 0 - c m region. The absorption bands of pyridinium ion, covalently bonded pyridine, and hydrogen-bonded pyridine were assigned by Parry (16). These species are formed upon interaction with Brônsted acid, Lewis acid, and hydrogen-bonding sites, respectively. The absorption bands of these interactions are distinguishable and can serve as a tool for the qualitative identification of surface acidic sites. The V T - D R I F T spectrum of silica gel in the silanol absorption region before the adsorption of pyridine is shown in Figure 2. The absorption band at 3740 c m was assigned to silanols that do not hydrogen-bond to other silanols or to surface-adsorbed water (17). These sites are referred to as unassociated silanol sites. The absorption at 3740 c m is slightly downshifted from that assigned to a freely vibrating silanol at 3750 c m (17). The other absorption maxima observed at 3655 c m and 3450 c m arise from surface silanols that hydrogen-bond to each other and are hydrogen-bonded by surface-adsorbed water, respectively (17). These two groups of silanols are collectively referred to as associated silanols. The V T - D R I F T spectrum of pyridine adsorbed on unmodified silica gel is shown in Figure 3, Absorption maxima are observed at 1595, 1485, and 1445 c m . According to the assignments made by Parry (16), these bands indicate hydrogen-bond formation. The absorptions result from inplane C - C stretching modes 8a, 19a, and 19b, respectively (18). The acidic strength of the surface sites is insufficient to generate pyridinium a

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Variable-Temperature Diffuse Reflectance Fourier Transform Infrared

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