I know only about a few liberal economists who directly adhered to philosophical materialism, while not all were vocal about it (Locke, Adam Smith, Voltaire, David Friedman*). I am not completely sure for Alain, and Raymond Boudon. Mises defended a methodological dualism (cf. Theory and history, 1957).
Two issues are related to whether liberal economy theory tends to be materialist or not: objective vs. subjective value, and utilitarianism vs. jusnaturalism.
- The objective vs. subjective value theory debate.
As explained in a comment of another question, "Adam Smith defined a labor theory of value attributing a property directly to the material, the more modern approach is to reject the property of the material and instead attribute a subjective utility that depends on the individual or where a trade could increase value despite nothing material happening." (haxor789). This is the objective vs. subjective value theory debate.
- The utilitarianism vs. jusnaturalism debate
As far as I know, Mandeville (a precursor of liberal economy, according to Hayek), Bright, Cobden, Ricardo, Mill, Dunoyer, Say, Bastiat and Mises are considered utilitarians; Milton and David Friedman** are considered direct utilitarians; Hayek, Hume, Smith, Ferguson are considered indirect utilitarians.
Utilitarianism is composed of hedonism (Wikipedia: Hedonism refers to a family of theories, all of which have in common that pleasure plays a central role in them), which itself has to see with epicurianism, a materialist philosophy.
- Materialism and the two debates
I define materialism as the belief that gives prevalence to the physical, as opposed to the ideas or the spirits.
Overall, I would consider the utilitarian economists cited above as materialists, for the theoretical link utilitarianism has with materialism on the one hand, and for the importance it gives to the material consequences on the other.
In contrast, I would consider Rothbard and Pascal Salin, and other Austrian economists, for whom liberal economics is good for it simply being moral and fair, as not materialists, and maybe as idealists. This is the utilitarianism vs. jusnaturalism debate.
- Liberal economy against materialism
As of today, I feel materialism is frowned upon in liberal economy theory circles; a situation that may have to see in part with liberal economy theory being gradually adopted by (religious) conservatives (jusnaturalism over utilitarianism because of its historical link with Christianity); and also the existence of what I would call a "hippie trend" which exists in the libertarian movement, as represented by Nick Gillespie the editor-in-chief of Reason magazine (see one of their last reporting "Welcome to the psychedelic renaissance" for an illustration) (jusnaturalism over utilitarianism so that one should not think too much critically about the beliefs and practices of people, especially irrational ones, especially their own hippie beliefs).
I don't think that liberal economy theory adopting the subjective value theory (Austrian economics) makes it non-materialist.
Nonetheless, it seems to me that liberal economy theory is materialist in theory, but also historically (as shown in the question).
Is liberal economics theory materialist?
*A kind of common-sense materialism, I suppose. "It is that our knowledge of moral facts comes in the same way as our knowledge of physical facts and so has the same epistemological status—a reasonable, although not in either case certain, basis for belief." (Chapter 61, The machinary of freedom, 3rd edition, David Friedman, 2014)
**"As a moral philosopher I am a libertarian, insofar as I am anything. As an economist I am a utilitarian. One could describe most of this book as a utilitarian approach to libertarianism, but only by using “utilitarian” in a very general sense. My approach would be more accurately described as consequentialist." (Chapter 43, The machinary of freedom, 3rd edition, David Friedman, 2014).
See also here for a synthesis of D. Friedman's arguments in favor of utilitarianism (in French).